1

Chapter ndashI

Section ndash A

Literature Survey of Molybdenum and Tungsten Heteropolyoxometalates

1 A1 Introduction to polyoxometalates-

The term polyoxometalate (abbreviated as POM) is applied to an extremely

large group of generally anionic clusters with frameworks built from transition

metal oxoanions linked by shared oxide ions Generally polyoxometalates are

the polyoxoanions of the early transition elements especially vanadium

molybdenum and tungsten Research activity concerning practical

applications of polyoxometalates especially in heterogeneous amp

homogeneous catalysis and in medicine as antiviral and antitumoral agents

[1]Historically the first example of POM is ammonium phosphomolybdate

containing the [PMo12O40]3minus ion was discovered in 1826 [2]The structure of

the related phosphotungstate anion was determined in 1934 and is generally

called the Keggin structure for its discoverer [3] In the period following this

other fundamental structures eg the Wells-Dawson ion were discovered

and their chemistry and applications as catalysts were determined With this

work still continues new areas of interest have emerged for example

The discovery of large highly symmetric polyoxomolybdates such as

the wheel-shaped molybdenum blue anions

Numerous hybrid organicinorganic materials that contain POM cores

New potential applications based on unusual magnetic and optical

properties of some POMs

Potential medicinal applications in particular anti-tumoral and anti-viral

Two kinds of polyoxoanions are known those exemplified by the silicates

and oxoanions of neighboring main group elements and these of the early

transition elements of group V and VI Polyoxometalates are predominantly

characterized by Mo6 octahedra with short lsquoterminalrsquo M=0 bonds that tend to

result in closed discrete structure with such bonds directed outwards [4] One

of the most challenge to chemistry is to prepare compounds amp materials

2

including those with network structures having desirable or predictable

properties such as mesoporsity (well defined cavities and channels)

electronic and ionic transport ferro as well as ferri-magnetism luminescence

and catalytic activity Transition metal oxide compounds are of special interest

in that respect For example the deeply colored mixed valence hydrogen

molybdenum bronzes with their unusual property of high conductivity and

wide range of composition play an important role in technology industrial

chemical process and materials science Their fields of applications range

from electrochemical elements hydrogenation amp dehydrogenation catalysts

superconductors passive electrochromic display devices to lsquosmartrsquo windows

[5]

The enormous variation in topology size electronic properties and

elemental composition that is unique to polyoxometalates provides the basis

for an expanding research effort into their chemistry amp their applications in

areas which include catalysis materials chemistry and biochemistry It is

neither practical nor appropriate to attempt a complete review of the

heteropolyoxometalates and the methods by which they have been prepared

Pope has noted that heteropoly anions have been prepared with more than 65

elements as the central atom while the elements that serve as peripheral

metal atoms appear to be more restrictive apparently requiring certain ionic

radius amp charge and ability to form d-p peripheral metal oxygen bonds [6]

Probably more heterogeneous catalytic studies have involved heteropoly

anions with phosphorous as central atom amp keggin structure than any other

heteropolyoxometalates Almost inevitably the peripheral metal atoms are

tungsten molybdenum and vanadium or combinations of these

eg PW12O403- PMo12O40

3- PW12-x MoxO403- PW12-xVxO40

(3+x)- PM012-

xVxO40(3+x)-

In addition to the aforementioned phosphorous compounds those with

keggin structure and silicon as a central atom have also been studied for their

catalytic properties As will be discussed later the use of these materials

undoubtedly relates to their relatively high stabilities [7]

Polyoxometalates have been traditionally the subject of study of

molecular inorganic chemistry Yet these polynuclear molecules

reminiscent of oxide clusters present a wide range of structures and with

3

them ideal frameworks for the deployment of useful magnetic

electroionic catalytic bioactive and photochemical properties With this in

mind a new trend towards the application of these remarkable species in

materials science is beginning to develop The applications of polyoxo-

metalates are

i) Their use as clusters with inherently useful properties on themselves a

line which has produced fundamental studies of their magnetic electronic or

photoelectrochemical properties and has shown these clusters

as models for quantum-sized oxides

ii) The encapsulation or instigation of polyoxometalates in

to organic polymeric or inorganic matrices or substrates opens

a whole new field within the area of hybrid materials for harnessing the

multifunctional properties of these versatile species in a wide variety of

applications ranging from catalysis to energy storage to biomedicine

Polyoxometalates have been known and used in the chemistry

laboratory for nearly two hundred years but only after the second

half of the 20th century we have been able to fully perceive the richness of

their chemistry structure and activity Modern techniques such as X-ray

crystallography or NMR and entire areas such as magnetochemistry or

electrochemistry have allowed a whole generation of contemporary

chemists to build and make known a complete body of understanding on the

structure bonding and properties of these fascinating cluster molecules

Several enlightening reviews and compendiums have been published [8-11]

With the turn of the century the coming age of materials

science and the advent of nanotechnology polyoxometalates are beginning to

be considered as unique chemical species that could turn from very special

molecules to very useful materials With sizes just one order of magnitude

smaller than the smallest of living biological structures such as the Rhinovirus

(approx20nm) they are not colloids but soluble polynuclear species Indeed

one of the main reasons why polyoxometalates have not been considered

in the past for design of functional materials is precisely because their

molecular nature makes them soluble in water and common organic solvents

Yet they not only share structural and topological features with related

transition metal oxides but also resemble them concerning their redox

4

electron transfer or ion transport behavior In all these respects polyoxo-

metalates can be generically considered as the perfect models for quantum-

sized transition metal oxide nanoparticals [12 13] For example

the electrochemical or photochemical injection of electrons in heteropolyions

(HPA) with the induction of thermally activated delocalization between metal

centers and IVCT (InterValence Charge Transfer Bands) leading to change

in color closely parallel the corresponding electrochromic properties of the

corresponding oxides upon doping

Contrary to ever-smaller nanostructures and quantum dots designed

bymeans of physical methods following a top-down approach

polyoxometalates represents a very significant example of the bottom-up

pproach Chemists use to build polynuclear and supramolecular structures

with collective properties The control of size and structure in

polyoxometalates is based on now well known acid hydrolysis and

condensation reactions driven by the very rich acid base chemistry of

some transition metal cations primarily W Mo and VBut this framework

chemistry of isopolytungstates molybdates and vanadates is remarkably

broadened up when other elements come to add richness through structural

and chemical multiplicity within the field of heteropolyanions

In addition the remarkable stability of many of these clusters makes

possible an extensive redox chemistry leading to wealth of ldquobluesrdquo reduced

species where thermally activated delocalization of electrons and a variety

of spin states make for a remarkable landscape of electronic and magnetic

states in small clusters that become in this way ideal models for the study of

spin interactions [14 -18]

Very recently this rich oxygen-metal chemistry has been broadened

with specific and unique examples that include for the first time anions such

as S-2 or MeO- [1920] or iron ions as the framework-building metals The

control of the extent of condensation and the isolation of new larger clusters

formed through a building-block approach making use of smaller fragments

to assemble larger units have also been a tremendous source of development

in very recent years especially for vanadium species[2122]

At this point the field of polyoxometalate chemistry has

broadened so much in variety of elements and structures properties and

5

applications that the reference books in the field would certainly welcome

substantial additions and revision

The intrinsic properties of polyoxometalates are of interest in themselves

not only from a fundamental point of view but also to make of them

materials of interest in various applications Beyond their traditional interest as

catalysts polyoxometalates constitutes base materials for electrochromics

energy storage and conversion devices (batteries super capacitors and

fuel cells) sensors or biomedical applications Many of the applications of

polyoxometalate clusters as materials require their use in the form of

membranes or electrodes that is in the form of solid insoluble material or

coatings There is therefore a main strategic line of work that has centered

on the inclusion or integration of polyoxometalates in all sorts of substrates

polymeric inorganic or mineraland their combination with surfactants or

organic carriers

Heteropolyanions are negatively-charged clusters of corner-sharing and

edge shairing early transition metal MO6 octahedra and heteroatom

XO4 tetrahedra where the tetrahedra are usually located the interior

of the cluster[23] The geometry composition and charge of these clusters

are varied through synthesis parameters and cluster properties are

highly tunable as a function of these characteristics Heteropolyanions have

been employed in a range of applications that include virus-binding

inorganic drugs [24] homogenious and heterogeneous catalysts [25 26]

electro-optic and electrochromic materials [27 28] metal and protein binding

[29] and as building blocks for nanostructuring of materials [30] The α-Keggin

geometry which was first structurally characterized in 1933 by JF Keggin

[31] the phosphotungstic acid (H3PW12O40) is one of the most

widely recognized and thoroughly studied heteropolyanion geometries[32]

1A2 Fundamental concept of polyoxometalate structures-

Keggin structure is the best known structural form for heteropoly acids It

is the structural form of αndashKeggin anions which have a general formula of

[XM12O40]n- where X is the hetero atom (most commonly are P5+ Si4+ or B3+)

M is the addenda atom (most common are molybdenum and tungsten) and O

6

represents oxygen[33] The structure self assembles in acidic aqueous

solution and is the most stable structure of polyoxometalate catalysts

Fig 1A1 Keggin structure

The first α-Keggin anion ammonium phosphomolybdate

((NH4)3[PMo12O40]) was first reported by Berzelius in 1826 In 1892

Blomstrand proposed the structure of phosphomolybdic acid and other poly-

acids as a chain or ring configuration Alfred Werner using the coordination

compounds ideas of Copaux attempted to explain the structure of

silicotungstic acid He assumed a central group [SiO4]4- ion enclosed by four

[RW2O6]+ where R is a unipositive ion The [RW2O6]

+ are linked to the central

group by primary valences Two more R2W2O7 groups were linked to the

central group by secondary valences This proposal accounted for the

characteristics of most poly-acids but not all

In 1928 Linus Pauling proposed a structure for α-Keggin anions

consisting of a tetrahedral central ion [XO4] n-8 caged by twelve WO6

octahedral In this proposed structure three of the oxygen on each of the

octahedral shared electrons with three neighboring octahedral As a result 18

oxygen atoms were used as bridging atoms between the metal atoms The

remaining oxygen atoms bonded to a proton This structure explained many

characteristics that were observed such as basicities of alkali metal salts and

the hydrated of some of the salts However the structure could not explain the

structure of dehydrated acids

JF Keggin with the use of X-ray diffraction experimentally determined

the structure of α-Keggin anions in 1934 The Keggin structure accounts for

both the hydrated and dehydrated α-Keggin anions without a need for

7

significant structural change The Keggin structure is the widely accepted

structure for the α-Keggin anions [34] For example the α-Keggin anion of

phosphotungustic acid is shown in Fig1A2

The structure is composed of one heteroatm surrounded by four

oxygen to form a tetrahedronThe heteroatom is located centrally and caged

by 12 octahedral WO3 units linked to one another by the neighboring oxygen

atoms There are a total of 24 bridging oxygen atoms that link the 12 addenda

atoms The metal centres in the 12 octahedra are arranged on a sphere

almost equidistant from each other in four M3O13 units giving the complete

structure an overall tetrahedral symmetry The bond length between atoms

varies depending on the heteroatom (X) and the addenda atoms (M)

Fig 1A2 α-Keggin anion of phosphotungustic acid ( PW12O40 ) 3-

For the 12ndashphosphotungstic acid Keggin determined the bond length

between the heteroatom and each the four central oxygen atoms to be 15 Adeg

The bond length form the central oxygen to the addenda atoms is 243 Adeg

The bond length between the addenda atoms and each of the bridging

oxygen is 19 Adeg The remaining 12 oxygen atoms that are each double

bonded to an addenda atom have a bond length of 170 Adeg The octahedra

are therefore distorted This structure allows the molecule to hydrate and

dehydrate without significant structural changes and the molecule is thermally

stable in the solid state for use in vapor phase reactions at high temperatures

(400-500 degC)[35-36]

8

Including the original Keggin structure there are 5 isomers designated

by the prefixes α- β- γ- δ- and ε- The original Keggin structure is designated

α- These isomers are sometimes termed Baker Baker-Figgis or rotational

isomers [37]These involve different rotational orientations of the Mo3O13 units

which lowers the symmetry of the overall structure Lacunary Keggin

structures

The term lacunary is applied to ions which have a fragment missing

sometimes called defect structures Examples are the (XM11O39)nminus and

(XM9O34)nminus formed by the removal from the Keggin structure of sufficient Mo

and O atoms to eliminate 1 or 3 adjacent MO6 octahedra The Dawson

structure X2M18O62nminus is made up of two Keggin lacunary fragments with 3

missing octahedra Some structural types are found in many different

compounds The first known example of this was the Keggin ion whose

structure was found to be common to both molybdates and tungstates with

different central hetero atoms Examples of some fundamental

polyoxometalate structures are shown below The Lindquist ion (Fig1A4) is

an iso-polyoxometalate the other three are hetero-polyoxometalates The

Keggin and Dawson structures (Fig1A3 and (Fig1A5) have tetrahedrally

coordinated hetero-atoms eg P or Si Anderson structure has an octahedral

central atom egAl

Fig 1A3 Keggin structure XM12O40nminus

9

Fig 1A 4 Lindquist structure M6O19nminus

Fig 1A 5 Dawson structure X2M18O62nminus

In general α-Keggin anions are synthesized in acidic solutions For

example 12-Phosphotungstic acid is formed by condensing phosphate ion

10

with tungstate ions The heteropolyacid that is formed has the Keggin

structure

[PO4]3- + 12 [WO4]

2- + 27 H+ rarr H3PW12O40 + 12 H2O ----------- 11

α-Keggin anions have been used as catalyst in hydration polymerization and

oxidation reaction as catalysts

The metal atoms that make up the framework (termed addenda atoms)

are typically Mo W and V When more than one element is present the

cluster is called a mixed addendaclusterThe ligands coordinated to metal

atoms that together form the bridged framework are usually oxide ions but

other elements such as S and Br have been substituted for some of the oxide

ions (Note that sulfur substituted POM is often termed a

polyoxothiometalates) Another development is the use of other ligands eg

nitrosy and alkoxy to replace oxide ions The typical framework building

blocks are polyhedral units with 4 5 6 or 7 coordinate metal centers These

units usually share edges andor vertices The most common unit for

polymolybdates is the octahedral MoO6 unit which is a distorted octahedron

where the Mo atom moves off centre to give one short Mo-O bond In some

polymolybdates there are pentagonal bipyramidal units and these are key

building blocks in the molybdenum bluesHetero atoms are present in many

polyoxometalates Many different elements can act as hetero-atoms

Examples of various coordination numbers around the hetero-atom are

known

4 co-ordinate (tetrahedral) in Keggin Dawson and Lindquist structures

(eg PO4 SiO4 AsO4)

6 co-ordinate (octahedral) in Anderson structure (eg Al(OH)6 TeO6

8 co-ordinate (square antiprism) in ((CeO8)W10O28)8minus

12 co-ordinate (icosahedral) in (UO12)Mo12O30 8minus

11

Often the hetero-atom is centrally located in the anion (eg Keggin

structure) or in a structure fragment eg the 2 phosphorus atoms in the

Dawson ion are central to the two symmetric fragments There are similarities

to clathrate structures The Keggin ion can be formulated as PO4 2minusand

M12O36 and the Dawson as (XO42-)2 and M18O54Structural isomerism is

common For example the Keggin structure has 5 isomers which can be

considered to contain one or more of the four M3O13 units being rotated

through 60degMany compounds share the same framework architectures or

frameworks derived from a larger framework with one or more addenda atoms

and oxide ions removed to give defect structure usually called a lacunary

structure An example of a compound with a Dawson lacunary structure is

As2W15O56Some cage structures containing ions are known eg an example

is the vanadate cage V18O42 containing a Clminus ion [38] This has 5 co-

ordinates square pyramidal vanadium units linked together

Fig 1A 6 - H4V18O42 cage containing Cl

1A3 General Properties of polyoxometalates -

Typically polyoxoanions are water and air stable species of large size

(6-25 Adeg) and high ionic weight In aqueous solution they are subject to

decomposition by hydroxide ions eg

[PW12O40]3- + 23 OH- HPO4

2- + 12WO42- + 11 H2O -------- 12

12

Although the PH at which such reactions are rapid can very widely

depending upon the polyanion involved Polyanions are often much stable

towards the H3O+ ions and numerous crystalline heteropoly acids are known

Such acids may be extremely soluble in water and polar solvents (giving

solutions with densities in excess of 4 gcm3) and have large dissociation

constants (PK lt 0)

Crystalline heteropoly acids and salts are frequently highly hydrated with

up to 50 molecules of water per anion Much of this water is zeolite in nature

and crystal composition can vary accordingly On the other hand the cation

anion stiochiometry is always well defined as the anion structure and

composition Finally many polyanions are powerful oxidizing agents and

undergo multiple reversible one or two electron reductions leading to intensely

colored mixed valence species known as heteropoly blues Polyanions are

known which can accept as many as 32 electrons without major structural

change

1A4 Chemistry of Molybdenum and Tungusten Heteropolyoxometalates

A photochromic monolayer film of phosphomolybdic acid (denoted as

PMo12) was fabricated by self-assembly approach UV-visible spectrum and

AFM observation show that the monolayer film is composed of aggregated

PMo12 molecules The monolayer film shows good photochromic properties

with enough stability and reversibility The colour change of the monolayer

after UV-irradiation can be captured by a microscope equipped with a color

CCD camera Photochromic response of the monolayer film can be doubled

after being modified by an amine monolayer [39]

Series of vanadium substituted molybdo Keggin HPA with 12 or 3

adjacent vanadium atoms were prepared These materials were supported

on carbon cloth electrodes and hot pressed onto Nafion with an ETEK

electrode used as a standard on the opposite side The MEArsquos were run at

temperatureslt100 oC with the HPA electrode as either the anode or the

cathode Stable polarisation curves are obtained for an HPA based cathode

with reasonable current densities at 80 oC [40]

A novel mixed-valence polyoxomolybdenum anion was synthesized

hydrothermally from molybdenum oxidemolybdenum metalboric and

phosphoric acids12-phenyldiphosphonicacidand imidazole (ImH) and was

13

structurally characterized as an imidazolium saltOne-and two-dimensional

structures of this anion and additional molybdenum diphosphonate linkers

were assembled as wellThey were structurally characterized as their

pyridinium(pyH) salts [41]

Phosphomolybdic acidpolyvinylpyrrolidone hybrid films were found to

show visible light photochromism It is identified that the intra-supramolecular

charge transfer between the inorganic and organic molecules is responsible

for the visible-light coloration Interestingly the films show photo-memory and

thermal activation The films show a small change in absorbance after being

irradiated with visible light for a short time and the coloration can be

enhanced greatly by subsequent thermal treatment Electrical measurements

indicate that the conductivity of the film increases after the brief irradiation

which promotes transfer of the electrons induced by the thermal treatment

[42]

In this work major effort was concentrated on passive thermal control

coatings based on photochromic and thermochromic materials The inorganic

photochromic materials were based on tungsten and molybdenum oxide films

and the organic photochromic materials included spiropyrans and

spirooxazines In addition photochromic composite organic-inorganic films

and thermochromic vanadium oxide films were prepared The samples were

synthesized using sputtering sol-gel process and thermal oxidation [43]

Polyoxometalates a class of oxidatively robust inorganic oxidants and

oxidation catalysts are currently under investigation at the Forest Products

Laboratory and at Emory University as an alternative to chlorinebased

chemicals in the bleaching of soft Woodkraft and other pulps Although

polyoxometalate salts are used in a number of industrial processes the

feasibility of using these salts and oxygen in the commercial bleaching of

chemical pulps was only recently demonstratedA clear advantage of

polyoxometalates over oxygen alone hydrogen peroxide or ozone is their

inherently high selectivity for the residual lignin in softwood kraft pulps The

goal of ongoing research is to develop a highly selective energy efficient

oxygen based polyoxometalate delignification and bleaching technology

compatible with mill closure [44]

14

Tungsten oxide exhibits pronounced photochromism upon bandgap

photoexcitation which makes it attractive and promising for applications in

many areas Some advances have been achieved during the past decades

The research on nanocrystalline films and single crystals indicates the critical

importance of defects in tungsten oxide to its photochromism Based on

energy-band engineering of semiconductors enhancement of photochromism

has been achieved for instance extension of the photoresponse from UV to

visible light by cathodic polarization improved change in absorption before

and after coloration through modification by a noble metal or another metal-

oxide semiconductor and increased photochromic reversibility via

hybridization with organic amines Nanocrystalline oxide films exhibit

controllable wettability which is coherent in nature with photochromism [45]

Polyoxometalates represent a diverse range of molecular clusters with

an almost unmatched range of physical properties and the ability to form

structures that can bridge several length scalesThe new building block

principles that have been discovered are beginning to allow the design of

complex clusters with desired properties and structures and several structural

types and novel physical properties are examinedIn this critical review the

synthetic and design approaches to the many polyoxometalate cluster types

are presented encompassing all the sub-types of polyoxometalates including

isopolyoxometa- lates heteropolyoxometalates and reduced molybdenum

blue systems As well as the fundamental structure and bonding aspectsthe

final section is devoted to discussing these clusters in the context of

contemporary and emerging interdisciplinary interests from areas as diverse

as antiviral agentsbiological ion transport modelsand materialsscience [46]

Keggin type molybdovanadophosphoric heteropoly acids were prepare

d by a novel environmentally benign method and their catalytic performances

were evaluated via hydroxylation of benzene to phenol with hydrogen

peroxide as oxidant in a mixed solvent of glacial acetic acid and acetonitrile

Various reaction parameters such as reaction time reaction temperature

ratio of benzene to hydrogen peroxide concentration of aqueous hydrogen

peroxide ratio of glacial acetic acid to acetonitrile in solvent and catalyst

concentration were changed to obtain an optimal reaction conditions

15

Molybdovanadophosphoric heteropoly acids are revealed to be highly

efficient catalyst for hydroxylation of benzene [47]

Thin films from the system (As2S3)Tl were deposited by thermal

evaporation on Si graphite and optical glass substratesFrom transmission

and reflection measurements of the thin films the refractive index (n) film

thickness(d) optical band gap(Eg) optical oscillator energy(Eo) and

dispersion energy(Ed) before and after exposure to light were determined

The results for optical parameters were analyzed using the Wemple - Di

Domenico single oscillator model and Lorenc-Lorenc equation It was found

that Eg decreases while n E0 and Ed increase for as deposited films

decreases while n E0 and Ed increase for as deposited films with increasing

of Tlconcentration passing through a maximum at 6 at of Tl After exposure

to light n E0 Ed increase and Eg decreases for all compositions

investigated The maximum change in n (Dn = 016 at l = 6328 nm) was

observed for thin As38S56Tl6 films From infrared spectroscopy measurements

of bulk glasses and thin films we could conclude that when up to 6 at of

thallium is introduced As-S-As chains break and a ternary TlAsS2

compound appears at 10 at Tl [48]

The organo - inorganic hybrid materialconsisting of Poly (34 Ethylene

Dioxythiophene) (PEDOT) doped with phosphomolybdate cluster anions

[PMo12O40]3-has been synthesized by direct insitu oxidative polymerization of

34-Ethylene Dioxythiophene (EDOT) with phosphomolybdic acid

(H3PMo12O40) Its characterization is investigated by Fourier Transform

Infrared Spectroscopy (FT-IR) and Scanning Electron Microscopy (SEM) The

hybrid material presents predominantly high electronic conductivities of

around 20 and 70 S cm1at 300 and 400 K respectively [49]

Heteropolyacids (HPAs) are known to be excellent re-dox catalysts In

combination with TiO2 HPAs can be used as photocatalysts active in visible

light The HPA accepts electron and get reduced to heteropolyblue (HPB)

That can absorb light in the visible range HPA can be incorporated onto the

external surface or in the pores of zeolite based composite photocatalysts

have been designed by incorporation of HPA semiconductor TiO2 and

transition metal cobalt on zeolite This composite metallozeolite photocatalyst

is efficient in photoreduction of methyl orange (MO) in visible light to the tune

16

of 411 mg of MO photo reducedg TiO2 This catalyst also shows encouraging

results for hydrogen evolution from water to the tune of 2730 micromolhg TiO

[50]

A new class of proton conducting glass membranes based on heteropo

lyacids such as phosphotungstic acid (PWA) as electrolytes for low

temperature H2O2 fuel cells was investigated Parameters for a single fuel cell

with a catalyst electrode of 015 mgcm2 of PtC and a glass composite

membrane were characterized by electrochemical measurements at open

circuit potential conditions The performance of the membrane electrode

assemblies (MEA) was systematically studied as an effect of SiO2 and P2O5

concentrations in the glass composite membrane and the MEA was found to

exhibit a maximum power density of 162 mW cm2 for an H2O2 fuel cell at

30 degC and 30 relative humidity (RH) [51]

Two new photochromic inorganic-organic hybrid materials formed from

Keggin type Polyoxometalates (POMs) and metronidazole (C6H9N3O3 MNZ)

formulated as H3PMo12O40bull3 MNZ3H2O (1) and H3PW12O40bull3MNZ3H2O(2)

were synthesized and characterized by elemental analysis IR spectra

electronic spectra electron spin resonance (ESR) spectra and TG-DTA

Reflectance spectra show the presence of weak inter molecular charge

transfer between the organic and inorganic moieties in the solid state The

photochromic properties were studied by solid diffuse reflectance spectra and

ESR spectra and the photochromic reactions were found to exhibit first-order

kinetics TG-DTA showed that two hybrid materials have similar thermal

behavior [52]

Heteropolycompounds (HPCs) have been a matter of interest in basic

and applied science for more than a century From their first synthesis many

advances have been made to promote the use of HPCs in different ways in

science and technology The aim of this article is to review the main structural

characteristics of heteropolycompounds of the Keggin type (12

tungstophosphoric12-molybdophosphoric12-tungstosilicic acid alkaline and

alkaline earth salts of12 tungstophosphoric acid and gels doped with HPCs)to

understand and explain their different activities such as high proton

conductivity and catalytic biochemical and biomedical activities [53]

17

A solid hybrid molecular material containing 1-butyl 3-methyl

imidazolium cations and Keggin anions of phosphotungstic acid has been

synthesized It is fully characterized by CHN analysis FTIR XRD UV-Vis-NIR

DRS 31P MAS NMR TGA and SEM The FTIR spectrum of the compound

shows the fingerprint vibrational bands of both Keggin molecular anions and

imidazolium cations The aromatic CndashH stretch region (2700ndash3250 cmndash1) of

imidazolium cation is split due to the interaction between the ring CndashH and

bulky Keggin anion The red-shift in the UV-Vis spectra and the downfield 31P

MAS NMR chemical shift also confirm the electrostatic interaction between

the ions in the compound Near IR spectral region (1000ndash2500 nm) shows the

elimination of water in the compound which is hydrophobic [54]

Transport coefficient measurements (electrical conductivity

thermoelectric power and Hall coefficient) have been performed on a

compact Tl033MoO3 polycrystalline compound in a wide temperature range

(200ndash400 K) Experimental results are interpreted with the help of a p-type

semiconductor model with two inverted deep levels near the midgap The

valence band and the conduction band are assumed to be formed from the

dxy orbitals of molybdenum atoms in the Mo6O22 cluster leading to narrow π-

bonding bands The donor and acceptor levels may be formed from

nonbonding dxy orbitals arising respectively from anionic and Tl+ defects

Electron paramagnetic resonance and magnetic measurements are in good

agreement with the theoretical band semiconductor model which has been

retained [55]

The optical properties of GaInTlAs epilayers grown at low temperature

~230degC by solid-source molecular-beam epitaxy on InP substrates were

characterized using optical absorptionand photoluminescence techniques

Optical absorption measurements a room temperature show a gap shrinkage

toward lower energies from 071 to 061 and 053 eV when the Tl content

increases from 0 to 4and 8in good agreement with theoretical

predictionsLow-temperature photoluminescence band-gap signals from

GaInAs and GaInTlAs layers are only obtained after rapid thermal annealing

performed inorder to improve the electronic quality of the layersA band gap

decrease as much as 41 meV for GaInTlAs with 19 Tl incorporation is

measured by photoluminescence at 8K [56]

18

The electrochromic performance of all solid ndash state cells employing

phosphotungstic acid and phosphomolybdic acid is reported These cells

employ SnO2 as the viewing electrode and graphite as the back electrodeThe

cells in the bleached state can be made white to red and become black in the

coloured state [57]

Various organic compounds were oxidized by molecular oxygen in the

presence of a catalytic amount of mixed addenda heteropolyoxometalates

containing molybdenum and vanadium The catalytic activity of the

Molybdovanadophosphate was found to be greatly enhanced by supporting

on charcoal The supported catalyst has high catalytic activity for oxidative

dehydrogenation of benzylic and allylic alcohols to the corresponding

aldehydes and ketones (46-92) nevertheless the nonsupported catalyst

was inactive for the same oxidations under these conditions 236Trimethyl

phenol was selectively oxidized to trimethyl-p benzoquinone which is

precursor of VitE in the presence of a catalytic amount of

molybdophosphate In addition the aerobic oxidation of amines alkyl-

substituted phenols and alkanes were also examined [58]

The reaction of Tl2CO3 with 111555-hexafluoro-24-pentanedione

and diglyme CH3O(CH2CH2O)2CH3 or tetraglyme CH3O(CH2CH2O)4CH3 in

dichloromethane yields the anhydrous thermally and air stable volatile Tl

diglyme and Tl tetraglyme adducts They have been characterized by single

crystal X-ray diffraction elemental analysis 1H and 13C NMR IR and mass

spectroscopy Thermal and mass-transport properties have been investigated

using thermo gravimetric and differential scanning calorimetric

measurements There is evidence that both precursors are very low melting

and volatile and can be used as liquid Tl sources Both adducts have been

successfully applied to metalndashorganic chemical vapor deposition of thallium

containing films [59]

Large size and high quality single crystals of quasi-two-dimensional

thallium molybdenum purple bronze TlMo6O17 have been grown by electrolytic

reduction of molten salt of Tl2CO3-MoO3 The crystal structure is trigonal with

space group P3m1 determined by X-ray diffraction and four-circle single crystal

diffraction The lattice parameters of the unit cell are a = b = 55282 Adeg and c

= 136991 Adeg The temperature dependence of resistivity and magnetic

19

susceptibility confirmed that a metal-to-metal transition occurs near 110 K

[60]

Hall coefficient and dc conductivity studies were made on p-type

Pb08Sn02Te thin films doped with different concentrations of thallium in the

temperature range 77 to 500 K The Hall coefficient and Hall mobility are

found to decrease with an increase in the doping concentration of thallium

Hall coefficient data have been analyzed in the light of a double valence-band

model Various band parameters such as valence band separation population

ratio mobility ratio and effective mass ratio have been calculated Hall

mobility data have been analyzed in the light of lattice and defect limited

scattering mechanisms [61]

Proton conducting composites of heteropolyacid hydrates phosphomol

ybdic acid H3PMo12O40 nH2O(PMA) phosphotungstic acid H3PW12O40 nH2O

( PTA) and salt hydrate like NiCl2 6H2O were prepared

with insulating Al2O3 as despersoidThe ionic conductivity peaks at two

concentrations of Al2O3 indicating two percolation thresholds for percolation

thresholds for proton conduction Two separate experiments were carried out

to check the existence of such percolation thresholds viz the volta battery

experiment involving the measurement of emf of an electrochemical cell

with composites of different compositions used as electrolyte and the

composition vs conductivity measured by the complex impedance

spectroscopy The presence of two maxima has been attributed to two

different percolation thresholds for the two possible mobile protonic

species H+ + (H3O+) and OH arising from the hydrates [62]

1 A5 Applications of Heteropolyoxometalates-

Applications of heteropolyanions centre depend on their redox properties

their high charges and ionic weights An enormous patent and journal

literature is devoted to the applications of heteropolyanions

1 Analysis-

The formation and subsequent precipitation or reduction of

[XMO12O40]n- anions form the basis of gravimetric and colorimetric analytical

methods for P As Si or Geeither separately or in combination [63 64]

20

2 Biochemical applications-

lsquoPhosphotungstic Acidsrsquo have for decades been used as precipitants

for proteins and as analytical reagents for proteinsalkaloidsand purines eg

the [P2M18O62]6- anions for colorimetric determination of uric acid [65] and

cholesterol The acid H3P12O40 either in aqueous or ethanolic solution is also

widely used as a non specific electron dense stain for electron spectroscopy

The dyestuffs industry has for many years used heteropolymolybdates and

tungstates to form color lakes and toners from basic dyes Large

heteropolyanions exhibit antiviral antitumoral properties at non-cytotoxic

doses in vitro and in vivo and are protein inhibitors of cellular bacterial and

viral DNA RNA polymerizes [66]

3 Catalysis

Heteropoly acids and salts have been used as heterogeneous catalysts

for a broad variety of reactions and compilations of such applications up to

1973 are available Examples include oxidation of propylene and isobutylene

to acrylic acid methacrylic acids and ammoxidation of acrylonitrile olefin

polymerization and epoxidation Much of current activity concerning

heterogeneous catalysis by heteropoly compounds is being carried out in

Japan [67 68]

4 Other Applications

Insoluble salts of Heteropolyanions especially ammonium

molybdophosphates have been used and are commercially available as ion-

exchange materials [69]Recent work in this area includes thin layer

chromatography of amino acids ion selective membranes [70] and the

preparation of new ion exchangers based on heteropolyanions Crystalline12-

tungstophosphoric and 12-molybdophosphoric acids are excellent protonic

conductors Heteropolyacids are electrochromic in the solid state as a

consequence of heteropolyblue formation Heteropolyblue formation has also

been used to detect alcohol or carboxylic acid radicals generated by radiolysis

of aqueous solutions Potential applications of heteropoly complexes as flame

retardants and smoke suppressants or as corrosion inhibitors and conversion

coatings on steel and aluminium are reported [71] Some potential ldquogreenrdquo

applications have been reported eg non-chlorine based wood pulp

bleaching process and a method of decontaminating water Some structures

21

containing transition metal atoms with unpaired electrons have unusual

magnetic properties and are being investigated as nano computer storage

devices Some compounds exhibit luminescence There are many reported

potential medicinal applications eg anti tumoral and anti-viral There have

been reports on the role of weak or non bonding interactions on the crystal

engineering of hybrid polyoxometalates

Spherical nonporous polyoxomolybdate based capsules of different

types containing more than 100 metal atoms reported by Achim Muller and his

group have versatile unique properties regarding their assembly to vesicles

and the chemistry which can be done inside the pores and cavities A discrete

polyoxometalate Lindquist ion of the form W6O192minus was successfully imaged

recently for the first time within the capillary of a carbon nanotube following

steric locking of the anion with the tubule In situ relaxation of the anion in its

equatorial plain was demonstrated [72]

22

Section-B

Litreture Survey on Chromism in Transition Metal

Oxides

1 B1 Chromism in Transition Metal Oxides

Chromism is a reversible change in a substances colour resulting from

a process caused by some form of stimulus Many materials are chromic

including inorganic and organic compounds and conducting polymers and the

property can result from many different mechanisms Several transition metal

oxides show EC properties The most popular are from the VI - B oxides In

this group WO3 and MoO3 are the most thoroughly studied cathodic EC

materials which can be electrochemically coloured and bleached when used

as the cathode in electrochemical cells Cathodic EC materials also include

V2O3 TiO2 and Nb2O5 Another distinguishable group is anodic EC material

including VIII oxides like IrOx nH2O Rh2O3 nH2O NiO nH2O etc which can be

anodicaly coloured in the electrochemical process when used as anode

There are several types of chromism which are discussed as below

B11 Photochromism

Photochromism is the reversible transformation of a chemical species

between two forms by the absorption of electromagnetic radiation where

the two forms have different absorption spectra [7374]

Trivially this can be described as a reversible change of color upon

exposure to light The phenomenon was discovered in the late 1880s

including work by Markwald who studied the reversible change of color of 23

44-tetrachloronaphthalen-1(4H)-one in the solid state He labeled this

phenomenon phototropy and this name was used until the 1950s

when Yehuda Hirshberg of the Weizmann Institute of Science in Israel

proposed the term photochromism [75] Photochromism can take place

in both organic and inorganic compounds and also has its place in biological

systems (for example retinal in the vision process)

Photochromism does not have a rigorous definition but is usually used

to describe compounds that undergo a reversible photochemical reaction

where an absorption band in the visible part of the electromagnetic spectrum

23

changes dramatically in strength or wavelength In many cases an

absorbance band is present in only one form The degree of change required

for a photochemical reaction to be dubbed photochromic is that which

appears dramatic by eye but in essence there is no dividing line between

photochromic reactions and other photochemistry Therefore while the

transcis isomerization of azobenzene is considered a photochromic reaction

the analogous reaction of stilbene is not Since photochromism is just a

special case of a photochemical reaction almost any photochemical reaction

type may be used to produce photochromism with appropriate molecular desi

gnSome of the most common processes involved in photochromism are peric

yclic reactions cis-trans somerizations intramolecular hydrogen transfer

intramolecular group transfers dissociation processes and electron transfers

(oxidation-reduction)

Another some what arbitrary requirement of photochromism is that

it requires the two states of the molecule to be thermally stable under

ambient conditions for a reasonable time All the same nitrospiropyran (which

back-isomerizes in the dark over ~10 minutes at room temperature) is

considered photochromic All photochromic molecules back-isomerize to their

more stable form at some rate and this back-isomerization is accelerated by

heating There is therefore a close relationship between photochromic and the

rmochromic compounds The timescale of thermal back-isomerization is

important for applications and may be molecularly engineered

Photochromic compounds considered to be thermally stable include some

diarylethenes which do not back isomerize even after heating at 800C for 3

months

Since photochromic chromophores are dyes and operate according to

well-known reactions their molecular engineering to fine-tune their properties

can be achieved relatively easily using known design models quantum

mechanics calculations and experimentation In particular the tuning of

absorbance bands to particular parts of the spectrum and the engineering

of thermal stability have received much attention

Sometimes and particularly in the dye industry the term irreversible

photochromic is used to describe materials that undergo a permanent color

change upon exposure to Ultraviolet or visible light radiation Because by

24

definition photochromics are reversible there is technically no such thing as a

n irreversible photochromic this is loose usage and these compounds

are better referred to as photochangable or photoreactive dyes

Apart from the qualities already mentioned several other properties of

photochromics are important for their use These include

Quantum yield of the photochemical reaction

This determined the efficiency of the photochromic change with respect

to the amount of light absorbed The quantum yield of isomerization

can be strongly dependent on conditions

Fatigue resistance In photochromic materials fatigue refers to the

loss of reversibility by processes such as photodegradation

photobleaching photooxidation and other side reactions All

photochromics suffer fatigue to some extent and its rate is strongly

dependent on the activating light and the conditions of the sample

Photostationary state Photochromic materials have two states and

their interconversion can be controlled using different wavelengths of

light Excitation with any given wavelength of light will result in a

mixture of the two states at a particular ratio called the photo-

stationary state In a perfect system there would exist wavelengths

that can be used to provide 10 and 01 ratios of the isomers

but in real systems this is not possible since the active

absorbance bands always overlap to some extent

Polarity and solubility In order to incorporate photochromics in

working systems they suffer the same issues as other dyes They are

often charged in one or more state leading to very high polarity and

possible large changes in polarity They also often contain large

conjugated systems that limit their solubility

Photochromic complexes

A photochromic complex is a kind of chemical compound that has

photoresponsive Parts on its ligand These complexes have a specific

structure photoswitchable organic compounds are attached to

metalcomplexes For the photocontrollable parts thermally and

photochemically stable chromophores (azobenzene diarylethene

25

spiropyran etc) are usually used And for the metal complexes a wide

variety of compounds that have various functions (redox response

luminescence magnetism etc) are applied The photochromic parts and

metal parts are so close that they can affect each others molecular

orbitals The physical properties of these compounds shown by parts of

them (ie chromophores or metals) thus can be controlled by switching

their other sites by external stimuli For example photoisomerization

behaviors of some complexes can be switched by oxidation and reduction of

their metal parts Some other compounds can be changed in their

luminescence behavior magnetic interaction of metal sites or stability of

metal-to-ligand coordination by photoisomerization of their photochromic

parts

Photochemistry of Polyoxometalates

The photochemistry of polyoxometalates is of great interest to inorganic

chemistsMore than 80 years agoit was found that the R-Keggin tungstate

H3[PW12O40] was reduced photochemically to yield a blue-colored species

which was reoxidized by air and by various other oxidizing agents such as

Fe3+AgNO3and H2O2 [7879]The photoredox reactions of H4[SiW12O40]and

H3[PW12O40] proceeded effectively in the presence of primary and secondary

alcohols their ethers and aldehydes and proteinsbut less effectively in the

presence of tertiary alcoholsketonesestersthe fatty acids above formic

acidand simple amines[8081] The basic photoredox reaction involving

ethanol is illustrated by eq 13

2 H3PW12O40 + H3CCH2OH h ν ν ν ν 2 H4PW12O40 + H3CCHO ------- 13

2 H4PW12O40 +12 O2 2 H3PW12O40 + H2O ------ 14

In this reactionone molecule of ethanol photochemically reduces two

molecules of H3PW12O40 and is itself oxidized to acetaldehyde In the

presence of air the thermal oxidation of the reduced species takes place at

room temperature(eq 14)The reduced polyoxometalates which are the so-

calledldquoheteropolybluesrdquo have been used for the colorimetric analysis of the

26

elements P Si As and Ge and for the determination of uric acidsugarand

other biological compounds [8283] Piperidinium metavanadate also

undergoes photoinduced coloration from white to black followed by a

reversible color change in the presence of oxidizing agentsHowever

ammonium metavanadates ([NH4][VO3]) exhibits no photoinduced

coloration[84] The early photoredox reactions of the R-Keggin

polyoxometalates H4[SiW12O40] and H3PW12O40 were carried out in the

presence of photographic paper however the limited number of the

structurally well-characterized compounds available for study delayed the

development of modern cluster-compound photochemistry until the discovery

of photochromism in alkylammonium polyoxo- molybdate solids[8586] A

photochromic or electrochromic material is one whose light-absorbing

properties are altered upon optical excitation or reduction under the influence

of an externally applied electric field respectively The induced coloration

remains even after the excitation source has been removed These materials

are of technological interest because they return to their original state either

thermally upon irradiation with light of a frequency corresponding to the

induced absorption or electrochemically upon reversing the polarity of the

externally applied electric field Thus photochromic and electrochromic

materials behave in a reversible manner Polyoxometalates exhibit significant

photo-and electrochromism which makes them suitable as nanocomposite

molecular devices and as models for probing the physical properties of infinite

metal oxides Since the metal ions in the oxidized polyoxometalates have d0

electronic configurations the only absorption band which occurs in the UV-vis

range of the electronic spectra is due to the oxygen-to-metal (O-M) ligand-to-

metal charge transfer (LMCT)Upon irradiation electrons are promoted from

the low-energy electronic states which are mainly comprised of oxygen 2p

orbitals (the valence band in the band model)to the high-energy electronic

states which are mainly comprised of metal d orbitalrsquos (the conduction band

in the band model)The fundamental transitions in polyoxometalate lattices

are depicted schematically in Fig1B1

27

Fig1B1 Simple model showing the electronic transitions in the

polyoxometalates containing electron donar and acceptor (a)generation

of charge carriers(b)electron and hole trapping(c)electron release due

to stimulation(d)recombination

between electron and hole Electrons are e-and holes are h+

In the polyoxometalates containing heteroatoms and especially in

mixed metal polyoxometalates the charge carriers which are created by the

light or electric field may be trapped in electron traps and hole traps These

traps provide states of localized energy in the O-M LMCT energy gap due to

the heteroatoms or counter cations which correspond to impurities or lattice

defects in the band model If the trap depth ∆E is large compared to kT the

probability for thermal escape from the trap will be negligibly small and

metastable situation will existThe trapped carriers can be released by thermal

or optical stimulationIn the case of thermal stimulation the irradiated

polyoxometalate is heated until the energy barrier ∆E can be overcome The

trapped electron (or hole) then can escape from the trap and nonradiatively

recombine with the trapped hole (or electron)Under optical stimulation the

energy of an incident photon is used to overcome ∆E The relaxation

processes of the OndashM LMCT excitation energy include both the nonradiative

recombination of electrons and holes within the energy gap and the

intramolecular energy transfer leading to a charge-transfer emission This

intramolecular energy transfer corresponds to the O-M LMCT energy gap and

occurs via radiative recombination and sensitized emission from the

28

heteroatoms or cations If several energy levels based on the hetero atoms or

counter cations act as energy acceptors within the O-M LMCTenergy gap the

energy transfer occurs from the O-M LMCT states to these levels followed by

the nonradiative or radiative deactivation of the excitation energy It should be

noted that the O-M LMCT states also can be generated by the application of

very high electric fields to the polyoxometalate solids as demonstrated by the

observation of electroluminescence[87]If an external electric field with a

potential more negative than the energy levels of the vacant orbitals involved

in the O-M LMCT transition is applied to a polyoxometalate on the electrode

surface an electrochemical reduction occurs via the injection of electrons

from the electrode in to the vacant levels of the polyoxometalate as shown in

Fig1B 2

Fig1B2 - Energy scheme for the electrochromism of polyoxometalates

a)electrochemical reduction (b) electrochemical oxidation

Electrons injected in to the high-energy levels also may be trapped by

electron traps in a process analogous to that which occurs during LMCT

photoexcitation of the polyoxometalates These electrons are returned to the

electrode by electrochemical oxidation at an electrode potential more positive

than the energy levels for the d1 electron states The d1electrons in the O-M

LMCTstates facilitate the absorption of visible light via intervalence charge

transfer among metal centers and d-d transitions The same type of transition

may be possible for the d1electron captured by the electron traps too In

addition to searching for new photosensitive polyoxometalates with the

29

potential for having practical application there is now a need to elucidate the

fundamental photo-and electrochemical coloration processes by studying

electron transfer within the polyoxometalate lattices in conjunction with their

crystal structures So far few polyoxometalates exhibit a perfect reversibility

of coloration The irreversibility of the color change arises from as yet

uncharacterized side reactions during both the coloration and decoloration of

the polyoxometalates

Many metal oxides including aluminum titanium vanadium niobium

molybdenum and tungsten oxides are photochromic when they contain

impurities or dopants This coloration has been interpreted on the basis of

electron trapping at appropriate lattice sites within the crystals as shown in

Fig1B1 where the O-M LMCT transition corresponds to the transition

between the valence and conduction bands for the infinite metal-oxide lattice

[88-91]

B12 Applications of Photochromic materials

Sunglasses

One of the most famous reversible photochromic applications is color

changing lenses for sunglasses as found in eyeglasses The largest limitation

in using PC technology is that the materials cannot be made stable enough

to withstand thousands of hours of outdoor exposure so long-term outdoor

applications are not appropriate at this time The switching speed of

photochromic dyes is highly sensitive to the rigidity of the environment around

the dye As result they switch most rapidly in solution and slowest in the rigid

environment like a polymer lens Recently it has been reported that attaching

flexible low Tg polymers (for example siloxanes or poly (butyl acrylate) to the

dyes allows them to switch much more rapidly in a rigid lens [76] Some

spirooxazines with siloxane polymers attached switch at near solution like

speeds even though they are in a rigid lens matrix

Supramolecular chemistry

Photochromic units have been employed extensively in supramolecular

chemistry Their ability to give a light controlled reversible shape change

means that they can be used to make or break molecular recognition motifs

30

or to cause a consequent shape change in their surroundings Thus

photochromic units have been demonstrated as components of molecular

switches The coupling of photochromic units to enzymes or enzyme cofactors

even provides the ability to reversibly turn enzymes on and off

by altering their shape or orientation in such a way that their functions

are either working or broken

Data storage

The possibility of using photochromic compounds for data storage was

first suggested in 1956 by Yehuda Hirshberg[77] Since that time there have

been many investigations by various academic and commercial groups

particularly in the area of 3D optical data storage which promises discs that

can hold a terabyte of data Initially issues with thermal back-reactions

and destructive reading dogged these studies but more recently more stable

systems have been developed

Novelty items

Reversible photochromics are also found in applications such as toys

cosmeticsclothing and industrial applications If necessary they can be made

to change between desired colors by combination with a permanent pigment

A large number of inorganic compounds exhibit photochromism

These solids often have large band gaps of the order of 3 - 12 eV and

excitation of these solids leads to the formation of metastable centers that

absorb visible light giving rise to their colour They can return to their ground

state by heating or by optical excitation within the colour-centre band In most

cases the photochromism is a structure sensitive phenomenon involving

localized defect impurities or dislocations Some of these inorganic

compounds have the potential for a number of different uses Photochromic

compounds have a number of useful applications These can be divided

according to the most important property that is being used (Table 11) [92]

31

Table 11 Applications of Photochromic materials

Applications Depending Upon

Sensitivity to

Radiation

Reversibility Thermal Chemical or

Physical Properties

Self-developing

photography

Chemical switches for

computers

Temperature indicators

Protective

materials

Data displays

Heat-sensitive recording

media

Optical signal

processing

Photomasking and

photoresist technology

Reusable data storage

media

Anaytical reagents

Photochromic

microimages

Photopolymerisation

Information encoding

and steganography

Photocontractile

polymers and the

photoviscosity effect

Control of light

intensity

Q-switches

Pyroelectric

photochromic materials

B13 Thermochromism

Thermochromism is the reversible colour change of a substance

induced by temperature change A large variety of substances organic

inorganic organometallic supramolecular and polymeric systems exhibit this

phenomenon Examples of these include bianthrones cobalt

hexacyanoferrate the zirconocene complex of 1 4-diphenyl-1 3-butadiene

and poly (3-alkylthiophene) The organic 99-bixanthenylidene is colourless at

90 K yellow-green at 298 K and dark-blue when melted at 592 K Heating

conducting polymers can cause them to change colour This is achieved by

causing conformational changes to the polymer backbone resulting in a

change in the band gap of the polymer It has been reported that regioregular

32

P3HT reversibly changes colour upon heating to 220ordmC due to temperature-

dependent conformation changes Thermally cross linked polymer undergoes

the same colour change but it is much less reversible [93] Other forms of

thermochromism may be commercially important eg to give a visual

indication of temperature changes

B14 Electrochromism

Electrochromism describes a phenomenon of material color change in

a persistent but reversible manner produced by electrochemically induced

oxidation-reduction reactions Electrochromic materials can be applied to

various kinds of products such as smart windows and display devices Among

those applications there have been lots of efforts to develop electrochromic

display devices (ECDs) Especially flexible display devices are now attracting

much attention worldwide since they can facilitate new technological demands

such as bending and folding of paper-like displays High electrochromic

efficiency short response time long operating life time and reduction of

energy consumption are the most important requisites of the materials for the

paperlike displays [94 95] Among those properties the operation life time is

the most important barrier to overcome for a realization of ECDs There are

two types of electrochromic material a) inorganic transition metal oxides

(TMOs) b) organic polymer materials The TMOs have been studied longer

than the organic materials that they have been studied since 1960s [9697]

Electrochromism describes a reversible color change of material

produced by electrochemically induced oxidation-reduction reactions It is one

of several types of chromism of materials As thermochromism and

photochromism mean material color changes made by heat and light

respectively electrochromism refers that the color change is caused by an

electric potential In most cases the color change in electrochromism can be

driven by rather low electrical potential of the order of a fraction of volt or a

few volts [94-96] The color change of material means variation in

transmittance andor reflectance change in visible range which is originated

from different electronic absorption bands according to a switching between

oxidation and reduction state of material When electric potential is applied on

electrochromic material forced oxidation or reduction is derived and the

33

individual color is originated from the corresponding oxidation or reduction

state of the material For electrochromic materials the characteristic color

change is reversible since the oxidation and reduction state can be converted

reversibly by switching potential

Application fields

The application area for the electrochromism is rather broad that it

covers from smart window glazing and optical modulators to information

displays [98-102] The smart windows are typical examples The

electrochromic property is used to control the amount of light and heat to pass

through the windows Usually the electrochromic material is in form of thin film

coated on a window glass The transmittance modulation has also been

applied at the automobiles to automatically tint rear-view mirrors in various

lighting conditions The electrochromic application fields are illustrated in Fig

1B3

Since the smart windows control the transmittance of heat as well as

the transmittance of visible light the working definition of electrochromism has

now been extended to include devices for modulation of radiation in the near

infrared thermal infrared and microwave regions When color for

electrochromic materials is used this can now mean a response by detectors

at these wavelengths and not just by the human eyes Nowadays

electrochromic material draws much attention as being used in the display

devices Electrochromic display device (ECD) is being considered as one of

the candidates for the conventional liquid crystal display (LCD) since ECD

has many advantages over LCDs Among these advantages the most

important are low energy consumption wider viewing angle high contrast

rate and possibility to achieve multiple colors with a single material [103]

When a new redox state of electrochromic materialis established by

the applied electric pulse then it is maintained after the potential is switched

off This means the colored or bleached state of the material can be sustained

for a considerable time without applying electrical power This is so-called

ldquocolor memory effectrdquo of electrochromic material Because of the color

memory effect energy consumption for the electrochromic display device

could be drastically reduced and this would be a big advantage over other

34

emissive devices The low power consumption is especially advantageous

when it is applied to mobile devices with limited power source The possibility

for a flexible display is another attraction for electrochromic material

Information displays

Real-view mirrors for automobiles

Fig 1B4 Application fields of electrochromic devices Smart windows information displays and real-view mirrors for automobiles

35

Using polymeric electrochromic material and plastic substrate with a

transparent conductive coating it is possible to build all-plastic flexible display

device There are lots of papers and patents about the flexible display devices

[104105] In these cases gel-type electrolyte is also needed The gel-type

electrolyte immobilizes liquid electrolyte in the polymer network [106107]

Recently ITO-coated polymer films are readily available which would provide

the plastic substrate for a flexible device The flexible electrochromic displays

are frequently tried with the plastic substrate flexible electrochromic material

and gel-type electrolyte The flexible electrochromic displays would facilitate

increasing technical demands for foldable display devices

Metal Ion Electrochromism

Many transition metal oxides are capable of redox reactions that result in

colour change Metal oxide films are commonly prepared as thin layers of

either tungsten nickel molybdenum or other metal compounds by a number

of techniques These include sol-gel electrochemical by dc or rfreactive

sputtering techniques electron-beam evaporation by anodic or cathodic

electrodeposition or by solution dipping of the electrochromic metal

compounds (or compounds that can be changed into these metal compounds)

onto optically transparent electrodes (OTE) [108 -114] Their electrochromism

is derived from the colour change associated with a change in the oxidation

state of the metal anion The behaviour of these materials is dependent upon

pH moisture and exposure to the atmosphere [115] Generally the switching

rates of these films is somewhat slow with typical switching times of about 15

- 60 seconds to achieve 100 conversion to either coloured or bleached state

[116 -120] An Example of this includes nickel oxide which changes from

transparent (pale green) to brownblack taking about 30 seconds to do so

[121] Other examples include [(NH4)5Ru]2(pyrazine)5+ and [(NH4)5Ru]2(44-

bipyridine)5+ whose electrochromism is significantly different due to the effect

of the ligand [122] Table 12 below gives some examples of metal oxide films

with electrochromic properties

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

2

including those with network structures having desirable or predictable

properties such as mesoporsity (well defined cavities and channels)

electronic and ionic transport ferro as well as ferri-magnetism luminescence

and catalytic activity Transition metal oxide compounds are of special interest

in that respect For example the deeply colored mixed valence hydrogen

molybdenum bronzes with their unusual property of high conductivity and

wide range of composition play an important role in technology industrial

chemical process and materials science Their fields of applications range

from electrochemical elements hydrogenation amp dehydrogenation catalysts

superconductors passive electrochromic display devices to lsquosmartrsquo windows

[5]

The enormous variation in topology size electronic properties and

elemental composition that is unique to polyoxometalates provides the basis

for an expanding research effort into their chemistry amp their applications in

areas which include catalysis materials chemistry and biochemistry It is

neither practical nor appropriate to attempt a complete review of the

heteropolyoxometalates and the methods by which they have been prepared

Pope has noted that heteropoly anions have been prepared with more than 65

elements as the central atom while the elements that serve as peripheral

metal atoms appear to be more restrictive apparently requiring certain ionic

radius amp charge and ability to form d-p peripheral metal oxygen bonds [6]

Probably more heterogeneous catalytic studies have involved heteropoly

anions with phosphorous as central atom amp keggin structure than any other

heteropolyoxometalates Almost inevitably the peripheral metal atoms are

tungsten molybdenum and vanadium or combinations of these

eg PW12O403- PMo12O40

3- PW12-x MoxO403- PW12-xVxO40

(3+x)- PM012-

xVxO40(3+x)-

In addition to the aforementioned phosphorous compounds those with

keggin structure and silicon as a central atom have also been studied for their

catalytic properties As will be discussed later the use of these materials

undoubtedly relates to their relatively high stabilities [7]

Polyoxometalates have been traditionally the subject of study of

molecular inorganic chemistry Yet these polynuclear molecules

reminiscent of oxide clusters present a wide range of structures and with

3

them ideal frameworks for the deployment of useful magnetic

electroionic catalytic bioactive and photochemical properties With this in

mind a new trend towards the application of these remarkable species in

materials science is beginning to develop The applications of polyoxo-

metalates are

i) Their use as clusters with inherently useful properties on themselves a

line which has produced fundamental studies of their magnetic electronic or

photoelectrochemical properties and has shown these clusters

as models for quantum-sized oxides

ii) The encapsulation or instigation of polyoxometalates in

to organic polymeric or inorganic matrices or substrates opens

a whole new field within the area of hybrid materials for harnessing the

multifunctional properties of these versatile species in a wide variety of

applications ranging from catalysis to energy storage to biomedicine

Polyoxometalates have been known and used in the chemistry

laboratory for nearly two hundred years but only after the second

half of the 20th century we have been able to fully perceive the richness of

their chemistry structure and activity Modern techniques such as X-ray

crystallography or NMR and entire areas such as magnetochemistry or

electrochemistry have allowed a whole generation of contemporary

chemists to build and make known a complete body of understanding on the

structure bonding and properties of these fascinating cluster molecules

Several enlightening reviews and compendiums have been published [8-11]

With the turn of the century the coming age of materials

science and the advent of nanotechnology polyoxometalates are beginning to

be considered as unique chemical species that could turn from very special

molecules to very useful materials With sizes just one order of magnitude

smaller than the smallest of living biological structures such as the Rhinovirus

(approx20nm) they are not colloids but soluble polynuclear species Indeed

one of the main reasons why polyoxometalates have not been considered

in the past for design of functional materials is precisely because their

molecular nature makes them soluble in water and common organic solvents

Yet they not only share structural and topological features with related

transition metal oxides but also resemble them concerning their redox

4

electron transfer or ion transport behavior In all these respects polyoxo-

metalates can be generically considered as the perfect models for quantum-

sized transition metal oxide nanoparticals [12 13] For example

the electrochemical or photochemical injection of electrons in heteropolyions

(HPA) with the induction of thermally activated delocalization between metal

centers and IVCT (InterValence Charge Transfer Bands) leading to change

in color closely parallel the corresponding electrochromic properties of the

corresponding oxides upon doping

Contrary to ever-smaller nanostructures and quantum dots designed

bymeans of physical methods following a top-down approach

polyoxometalates represents a very significant example of the bottom-up

pproach Chemists use to build polynuclear and supramolecular structures

with collective properties The control of size and structure in

polyoxometalates is based on now well known acid hydrolysis and

condensation reactions driven by the very rich acid base chemistry of

some transition metal cations primarily W Mo and VBut this framework

chemistry of isopolytungstates molybdates and vanadates is remarkably

broadened up when other elements come to add richness through structural

and chemical multiplicity within the field of heteropolyanions

In addition the remarkable stability of many of these clusters makes

possible an extensive redox chemistry leading to wealth of ldquobluesrdquo reduced

species where thermally activated delocalization of electrons and a variety

of spin states make for a remarkable landscape of electronic and magnetic

states in small clusters that become in this way ideal models for the study of

spin interactions [14 -18]

Very recently this rich oxygen-metal chemistry has been broadened

with specific and unique examples that include for the first time anions such

as S-2 or MeO- [1920] or iron ions as the framework-building metals The

control of the extent of condensation and the isolation of new larger clusters

formed through a building-block approach making use of smaller fragments

to assemble larger units have also been a tremendous source of development

in very recent years especially for vanadium species[2122]

At this point the field of polyoxometalate chemistry has

broadened so much in variety of elements and structures properties and

5

applications that the reference books in the field would certainly welcome

substantial additions and revision

The intrinsic properties of polyoxometalates are of interest in themselves

not only from a fundamental point of view but also to make of them

materials of interest in various applications Beyond their traditional interest as

catalysts polyoxometalates constitutes base materials for electrochromics

energy storage and conversion devices (batteries super capacitors and

fuel cells) sensors or biomedical applications Many of the applications of

polyoxometalate clusters as materials require their use in the form of

membranes or electrodes that is in the form of solid insoluble material or

coatings There is therefore a main strategic line of work that has centered

on the inclusion or integration of polyoxometalates in all sorts of substrates

polymeric inorganic or mineraland their combination with surfactants or

organic carriers

Heteropolyanions are negatively-charged clusters of corner-sharing and

edge shairing early transition metal MO6 octahedra and heteroatom

XO4 tetrahedra where the tetrahedra are usually located the interior

of the cluster[23] The geometry composition and charge of these clusters

are varied through synthesis parameters and cluster properties are

highly tunable as a function of these characteristics Heteropolyanions have

been employed in a range of applications that include virus-binding

inorganic drugs [24] homogenious and heterogeneous catalysts [25 26]

electro-optic and electrochromic materials [27 28] metal and protein binding

[29] and as building blocks for nanostructuring of materials [30] The α-Keggin

geometry which was first structurally characterized in 1933 by JF Keggin

[31] the phosphotungstic acid (H3PW12O40) is one of the most

widely recognized and thoroughly studied heteropolyanion geometries[32]

1A2 Fundamental concept of polyoxometalate structures-

Keggin structure is the best known structural form for heteropoly acids It

is the structural form of αndashKeggin anions which have a general formula of

[XM12O40]n- where X is the hetero atom (most commonly are P5+ Si4+ or B3+)

M is the addenda atom (most common are molybdenum and tungsten) and O

6

represents oxygen[33] The structure self assembles in acidic aqueous

solution and is the most stable structure of polyoxometalate catalysts

Fig 1A1 Keggin structure

The first α-Keggin anion ammonium phosphomolybdate

((NH4)3[PMo12O40]) was first reported by Berzelius in 1826 In 1892

Blomstrand proposed the structure of phosphomolybdic acid and other poly-

acids as a chain or ring configuration Alfred Werner using the coordination

compounds ideas of Copaux attempted to explain the structure of

silicotungstic acid He assumed a central group [SiO4]4- ion enclosed by four

[RW2O6]+ where R is a unipositive ion The [RW2O6]

+ are linked to the central

group by primary valences Two more R2W2O7 groups were linked to the

central group by secondary valences This proposal accounted for the

characteristics of most poly-acids but not all

In 1928 Linus Pauling proposed a structure for α-Keggin anions

consisting of a tetrahedral central ion [XO4] n-8 caged by twelve WO6

octahedral In this proposed structure three of the oxygen on each of the

octahedral shared electrons with three neighboring octahedral As a result 18

oxygen atoms were used as bridging atoms between the metal atoms The

remaining oxygen atoms bonded to a proton This structure explained many

characteristics that were observed such as basicities of alkali metal salts and

the hydrated of some of the salts However the structure could not explain the

structure of dehydrated acids

JF Keggin with the use of X-ray diffraction experimentally determined

the structure of α-Keggin anions in 1934 The Keggin structure accounts for

both the hydrated and dehydrated α-Keggin anions without a need for

7

significant structural change The Keggin structure is the widely accepted

structure for the α-Keggin anions [34] For example the α-Keggin anion of

phosphotungustic acid is shown in Fig1A2

The structure is composed of one heteroatm surrounded by four

oxygen to form a tetrahedronThe heteroatom is located centrally and caged

by 12 octahedral WO3 units linked to one another by the neighboring oxygen

atoms There are a total of 24 bridging oxygen atoms that link the 12 addenda

atoms The metal centres in the 12 octahedra are arranged on a sphere

almost equidistant from each other in four M3O13 units giving the complete

structure an overall tetrahedral symmetry The bond length between atoms

varies depending on the heteroatom (X) and the addenda atoms (M)

Fig 1A2 α-Keggin anion of phosphotungustic acid ( PW12O40 ) 3-

For the 12ndashphosphotungstic acid Keggin determined the bond length

between the heteroatom and each the four central oxygen atoms to be 15 Adeg

The bond length form the central oxygen to the addenda atoms is 243 Adeg

The bond length between the addenda atoms and each of the bridging

oxygen is 19 Adeg The remaining 12 oxygen atoms that are each double

bonded to an addenda atom have a bond length of 170 Adeg The octahedra

are therefore distorted This structure allows the molecule to hydrate and

dehydrate without significant structural changes and the molecule is thermally

stable in the solid state for use in vapor phase reactions at high temperatures

(400-500 degC)[35-36]

8

Including the original Keggin structure there are 5 isomers designated

by the prefixes α- β- γ- δ- and ε- The original Keggin structure is designated

α- These isomers are sometimes termed Baker Baker-Figgis or rotational

isomers [37]These involve different rotational orientations of the Mo3O13 units

which lowers the symmetry of the overall structure Lacunary Keggin

structures

The term lacunary is applied to ions which have a fragment missing

sometimes called defect structures Examples are the (XM11O39)nminus and

(XM9O34)nminus formed by the removal from the Keggin structure of sufficient Mo

and O atoms to eliminate 1 or 3 adjacent MO6 octahedra The Dawson

structure X2M18O62nminus is made up of two Keggin lacunary fragments with 3

missing octahedra Some structural types are found in many different

compounds The first known example of this was the Keggin ion whose

structure was found to be common to both molybdates and tungstates with

different central hetero atoms Examples of some fundamental

polyoxometalate structures are shown below The Lindquist ion (Fig1A4) is

an iso-polyoxometalate the other three are hetero-polyoxometalates The

Keggin and Dawson structures (Fig1A3 and (Fig1A5) have tetrahedrally

coordinated hetero-atoms eg P or Si Anderson structure has an octahedral

central atom egAl

Fig 1A3 Keggin structure XM12O40nminus

9

Fig 1A 4 Lindquist structure M6O19nminus

Fig 1A 5 Dawson structure X2M18O62nminus

In general α-Keggin anions are synthesized in acidic solutions For

example 12-Phosphotungstic acid is formed by condensing phosphate ion

10

with tungstate ions The heteropolyacid that is formed has the Keggin

structure

[PO4]3- + 12 [WO4]

2- + 27 H+ rarr H3PW12O40 + 12 H2O ----------- 11

α-Keggin anions have been used as catalyst in hydration polymerization and

oxidation reaction as catalysts

The metal atoms that make up the framework (termed addenda atoms)

are typically Mo W and V When more than one element is present the

cluster is called a mixed addendaclusterThe ligands coordinated to metal

atoms that together form the bridged framework are usually oxide ions but

other elements such as S and Br have been substituted for some of the oxide

ions (Note that sulfur substituted POM is often termed a

polyoxothiometalates) Another development is the use of other ligands eg

nitrosy and alkoxy to replace oxide ions The typical framework building

blocks are polyhedral units with 4 5 6 or 7 coordinate metal centers These

units usually share edges andor vertices The most common unit for

polymolybdates is the octahedral MoO6 unit which is a distorted octahedron

where the Mo atom moves off centre to give one short Mo-O bond In some

polymolybdates there are pentagonal bipyramidal units and these are key

building blocks in the molybdenum bluesHetero atoms are present in many

polyoxometalates Many different elements can act as hetero-atoms

Examples of various coordination numbers around the hetero-atom are

known

4 co-ordinate (tetrahedral) in Keggin Dawson and Lindquist structures

(eg PO4 SiO4 AsO4)

6 co-ordinate (octahedral) in Anderson structure (eg Al(OH)6 TeO6

8 co-ordinate (square antiprism) in ((CeO8)W10O28)8minus

12 co-ordinate (icosahedral) in (UO12)Mo12O30 8minus

11

Often the hetero-atom is centrally located in the anion (eg Keggin

structure) or in a structure fragment eg the 2 phosphorus atoms in the

Dawson ion are central to the two symmetric fragments There are similarities

to clathrate structures The Keggin ion can be formulated as PO4 2minusand

M12O36 and the Dawson as (XO42-)2 and M18O54Structural isomerism is

common For example the Keggin structure has 5 isomers which can be

considered to contain one or more of the four M3O13 units being rotated

through 60degMany compounds share the same framework architectures or

frameworks derived from a larger framework with one or more addenda atoms

and oxide ions removed to give defect structure usually called a lacunary

structure An example of a compound with a Dawson lacunary structure is

As2W15O56Some cage structures containing ions are known eg an example

is the vanadate cage V18O42 containing a Clminus ion [38] This has 5 co-

ordinates square pyramidal vanadium units linked together

Fig 1A 6 - H4V18O42 cage containing Cl

1A3 General Properties of polyoxometalates -

Typically polyoxoanions are water and air stable species of large size

(6-25 Adeg) and high ionic weight In aqueous solution they are subject to

decomposition by hydroxide ions eg

[PW12O40]3- + 23 OH- HPO4

2- + 12WO42- + 11 H2O -------- 12

12

Although the PH at which such reactions are rapid can very widely

depending upon the polyanion involved Polyanions are often much stable

towards the H3O+ ions and numerous crystalline heteropoly acids are known

Such acids may be extremely soluble in water and polar solvents (giving

solutions with densities in excess of 4 gcm3) and have large dissociation

constants (PK lt 0)

Crystalline heteropoly acids and salts are frequently highly hydrated with

up to 50 molecules of water per anion Much of this water is zeolite in nature

and crystal composition can vary accordingly On the other hand the cation

anion stiochiometry is always well defined as the anion structure and

composition Finally many polyanions are powerful oxidizing agents and

undergo multiple reversible one or two electron reductions leading to intensely

colored mixed valence species known as heteropoly blues Polyanions are

known which can accept as many as 32 electrons without major structural

change

1A4 Chemistry of Molybdenum and Tungusten Heteropolyoxometalates

A photochromic monolayer film of phosphomolybdic acid (denoted as

PMo12) was fabricated by self-assembly approach UV-visible spectrum and

AFM observation show that the monolayer film is composed of aggregated

PMo12 molecules The monolayer film shows good photochromic properties

with enough stability and reversibility The colour change of the monolayer

after UV-irradiation can be captured by a microscope equipped with a color

CCD camera Photochromic response of the monolayer film can be doubled

after being modified by an amine monolayer [39]

Series of vanadium substituted molybdo Keggin HPA with 12 or 3

adjacent vanadium atoms were prepared These materials were supported

on carbon cloth electrodes and hot pressed onto Nafion with an ETEK

electrode used as a standard on the opposite side The MEArsquos were run at

temperatureslt100 oC with the HPA electrode as either the anode or the

cathode Stable polarisation curves are obtained for an HPA based cathode

with reasonable current densities at 80 oC [40]

A novel mixed-valence polyoxomolybdenum anion was synthesized

hydrothermally from molybdenum oxidemolybdenum metalboric and

phosphoric acids12-phenyldiphosphonicacidand imidazole (ImH) and was

13

structurally characterized as an imidazolium saltOne-and two-dimensional

structures of this anion and additional molybdenum diphosphonate linkers

were assembled as wellThey were structurally characterized as their

pyridinium(pyH) salts [41]

Phosphomolybdic acidpolyvinylpyrrolidone hybrid films were found to

show visible light photochromism It is identified that the intra-supramolecular

charge transfer between the inorganic and organic molecules is responsible

for the visible-light coloration Interestingly the films show photo-memory and

thermal activation The films show a small change in absorbance after being

irradiated with visible light for a short time and the coloration can be

enhanced greatly by subsequent thermal treatment Electrical measurements

indicate that the conductivity of the film increases after the brief irradiation

which promotes transfer of the electrons induced by the thermal treatment

[42]

In this work major effort was concentrated on passive thermal control

coatings based on photochromic and thermochromic materials The inorganic

photochromic materials were based on tungsten and molybdenum oxide films

and the organic photochromic materials included spiropyrans and

spirooxazines In addition photochromic composite organic-inorganic films

and thermochromic vanadium oxide films were prepared The samples were

synthesized using sputtering sol-gel process and thermal oxidation [43]

Polyoxometalates a class of oxidatively robust inorganic oxidants and

oxidation catalysts are currently under investigation at the Forest Products

Laboratory and at Emory University as an alternative to chlorinebased

chemicals in the bleaching of soft Woodkraft and other pulps Although

polyoxometalate salts are used in a number of industrial processes the

feasibility of using these salts and oxygen in the commercial bleaching of

chemical pulps was only recently demonstratedA clear advantage of

polyoxometalates over oxygen alone hydrogen peroxide or ozone is their

inherently high selectivity for the residual lignin in softwood kraft pulps The

goal of ongoing research is to develop a highly selective energy efficient

oxygen based polyoxometalate delignification and bleaching technology

compatible with mill closure [44]

14

Tungsten oxide exhibits pronounced photochromism upon bandgap

photoexcitation which makes it attractive and promising for applications in

many areas Some advances have been achieved during the past decades

The research on nanocrystalline films and single crystals indicates the critical

importance of defects in tungsten oxide to its photochromism Based on

energy-band engineering of semiconductors enhancement of photochromism

has been achieved for instance extension of the photoresponse from UV to

visible light by cathodic polarization improved change in absorption before

and after coloration through modification by a noble metal or another metal-

oxide semiconductor and increased photochromic reversibility via

hybridization with organic amines Nanocrystalline oxide films exhibit

controllable wettability which is coherent in nature with photochromism [45]

Polyoxometalates represent a diverse range of molecular clusters with

an almost unmatched range of physical properties and the ability to form

structures that can bridge several length scalesThe new building block

principles that have been discovered are beginning to allow the design of

complex clusters with desired properties and structures and several structural

types and novel physical properties are examinedIn this critical review the

synthetic and design approaches to the many polyoxometalate cluster types

are presented encompassing all the sub-types of polyoxometalates including

isopolyoxometa- lates heteropolyoxometalates and reduced molybdenum

blue systems As well as the fundamental structure and bonding aspectsthe

final section is devoted to discussing these clusters in the context of

contemporary and emerging interdisciplinary interests from areas as diverse

as antiviral agentsbiological ion transport modelsand materialsscience [46]

Keggin type molybdovanadophosphoric heteropoly acids were prepare

d by a novel environmentally benign method and their catalytic performances

were evaluated via hydroxylation of benzene to phenol with hydrogen

peroxide as oxidant in a mixed solvent of glacial acetic acid and acetonitrile

Various reaction parameters such as reaction time reaction temperature

ratio of benzene to hydrogen peroxide concentration of aqueous hydrogen

peroxide ratio of glacial acetic acid to acetonitrile in solvent and catalyst

concentration were changed to obtain an optimal reaction conditions

15

Molybdovanadophosphoric heteropoly acids are revealed to be highly

efficient catalyst for hydroxylation of benzene [47]

Thin films from the system (As2S3)Tl were deposited by thermal

evaporation on Si graphite and optical glass substratesFrom transmission

and reflection measurements of the thin films the refractive index (n) film

thickness(d) optical band gap(Eg) optical oscillator energy(Eo) and

dispersion energy(Ed) before and after exposure to light were determined

The results for optical parameters were analyzed using the Wemple - Di

Domenico single oscillator model and Lorenc-Lorenc equation It was found

that Eg decreases while n E0 and Ed increase for as deposited films

decreases while n E0 and Ed increase for as deposited films with increasing

of Tlconcentration passing through a maximum at 6 at of Tl After exposure

to light n E0 Ed increase and Eg decreases for all compositions

investigated The maximum change in n (Dn = 016 at l = 6328 nm) was

observed for thin As38S56Tl6 films From infrared spectroscopy measurements

of bulk glasses and thin films we could conclude that when up to 6 at of

thallium is introduced As-S-As chains break and a ternary TlAsS2

compound appears at 10 at Tl [48]

The organo - inorganic hybrid materialconsisting of Poly (34 Ethylene

Dioxythiophene) (PEDOT) doped with phosphomolybdate cluster anions

[PMo12O40]3-has been synthesized by direct insitu oxidative polymerization of

34-Ethylene Dioxythiophene (EDOT) with phosphomolybdic acid

(H3PMo12O40) Its characterization is investigated by Fourier Transform

Infrared Spectroscopy (FT-IR) and Scanning Electron Microscopy (SEM) The

hybrid material presents predominantly high electronic conductivities of

around 20 and 70 S cm1at 300 and 400 K respectively [49]

Heteropolyacids (HPAs) are known to be excellent re-dox catalysts In

combination with TiO2 HPAs can be used as photocatalysts active in visible

light The HPA accepts electron and get reduced to heteropolyblue (HPB)

That can absorb light in the visible range HPA can be incorporated onto the

external surface or in the pores of zeolite based composite photocatalysts

have been designed by incorporation of HPA semiconductor TiO2 and

transition metal cobalt on zeolite This composite metallozeolite photocatalyst

is efficient in photoreduction of methyl orange (MO) in visible light to the tune

16

of 411 mg of MO photo reducedg TiO2 This catalyst also shows encouraging

results for hydrogen evolution from water to the tune of 2730 micromolhg TiO

[50]

A new class of proton conducting glass membranes based on heteropo

lyacids such as phosphotungstic acid (PWA) as electrolytes for low

temperature H2O2 fuel cells was investigated Parameters for a single fuel cell

with a catalyst electrode of 015 mgcm2 of PtC and a glass composite

membrane were characterized by electrochemical measurements at open

circuit potential conditions The performance of the membrane electrode

assemblies (MEA) was systematically studied as an effect of SiO2 and P2O5

concentrations in the glass composite membrane and the MEA was found to

exhibit a maximum power density of 162 mW cm2 for an H2O2 fuel cell at

30 degC and 30 relative humidity (RH) [51]

Two new photochromic inorganic-organic hybrid materials formed from

Keggin type Polyoxometalates (POMs) and metronidazole (C6H9N3O3 MNZ)

formulated as H3PMo12O40bull3 MNZ3H2O (1) and H3PW12O40bull3MNZ3H2O(2)

were synthesized and characterized by elemental analysis IR spectra

electronic spectra electron spin resonance (ESR) spectra and TG-DTA

Reflectance spectra show the presence of weak inter molecular charge

transfer between the organic and inorganic moieties in the solid state The

photochromic properties were studied by solid diffuse reflectance spectra and

ESR spectra and the photochromic reactions were found to exhibit first-order

kinetics TG-DTA showed that two hybrid materials have similar thermal

behavior [52]

Heteropolycompounds (HPCs) have been a matter of interest in basic

and applied science for more than a century From their first synthesis many

advances have been made to promote the use of HPCs in different ways in

science and technology The aim of this article is to review the main structural

characteristics of heteropolycompounds of the Keggin type (12

tungstophosphoric12-molybdophosphoric12-tungstosilicic acid alkaline and

alkaline earth salts of12 tungstophosphoric acid and gels doped with HPCs)to

understand and explain their different activities such as high proton

conductivity and catalytic biochemical and biomedical activities [53]

17

A solid hybrid molecular material containing 1-butyl 3-methyl

imidazolium cations and Keggin anions of phosphotungstic acid has been

synthesized It is fully characterized by CHN analysis FTIR XRD UV-Vis-NIR

DRS 31P MAS NMR TGA and SEM The FTIR spectrum of the compound

shows the fingerprint vibrational bands of both Keggin molecular anions and

imidazolium cations The aromatic CndashH stretch region (2700ndash3250 cmndash1) of

imidazolium cation is split due to the interaction between the ring CndashH and

bulky Keggin anion The red-shift in the UV-Vis spectra and the downfield 31P

MAS NMR chemical shift also confirm the electrostatic interaction between

the ions in the compound Near IR spectral region (1000ndash2500 nm) shows the

elimination of water in the compound which is hydrophobic [54]

Transport coefficient measurements (electrical conductivity

thermoelectric power and Hall coefficient) have been performed on a

compact Tl033MoO3 polycrystalline compound in a wide temperature range

(200ndash400 K) Experimental results are interpreted with the help of a p-type

semiconductor model with two inverted deep levels near the midgap The

valence band and the conduction band are assumed to be formed from the

dxy orbitals of molybdenum atoms in the Mo6O22 cluster leading to narrow π-

bonding bands The donor and acceptor levels may be formed from

nonbonding dxy orbitals arising respectively from anionic and Tl+ defects

Electron paramagnetic resonance and magnetic measurements are in good

agreement with the theoretical band semiconductor model which has been

retained [55]

The optical properties of GaInTlAs epilayers grown at low temperature

~230degC by solid-source molecular-beam epitaxy on InP substrates were

characterized using optical absorptionand photoluminescence techniques

Optical absorption measurements a room temperature show a gap shrinkage

toward lower energies from 071 to 061 and 053 eV when the Tl content

increases from 0 to 4and 8in good agreement with theoretical

predictionsLow-temperature photoluminescence band-gap signals from

GaInAs and GaInTlAs layers are only obtained after rapid thermal annealing

performed inorder to improve the electronic quality of the layersA band gap

decrease as much as 41 meV for GaInTlAs with 19 Tl incorporation is

measured by photoluminescence at 8K [56]

18

The electrochromic performance of all solid ndash state cells employing

phosphotungstic acid and phosphomolybdic acid is reported These cells

employ SnO2 as the viewing electrode and graphite as the back electrodeThe

cells in the bleached state can be made white to red and become black in the

coloured state [57]

Various organic compounds were oxidized by molecular oxygen in the

presence of a catalytic amount of mixed addenda heteropolyoxometalates

containing molybdenum and vanadium The catalytic activity of the

Molybdovanadophosphate was found to be greatly enhanced by supporting

on charcoal The supported catalyst has high catalytic activity for oxidative

dehydrogenation of benzylic and allylic alcohols to the corresponding

aldehydes and ketones (46-92) nevertheless the nonsupported catalyst

was inactive for the same oxidations under these conditions 236Trimethyl

phenol was selectively oxidized to trimethyl-p benzoquinone which is

precursor of VitE in the presence of a catalytic amount of

molybdophosphate In addition the aerobic oxidation of amines alkyl-

substituted phenols and alkanes were also examined [58]

The reaction of Tl2CO3 with 111555-hexafluoro-24-pentanedione

and diglyme CH3O(CH2CH2O)2CH3 or tetraglyme CH3O(CH2CH2O)4CH3 in

dichloromethane yields the anhydrous thermally and air stable volatile Tl

diglyme and Tl tetraglyme adducts They have been characterized by single

crystal X-ray diffraction elemental analysis 1H and 13C NMR IR and mass

spectroscopy Thermal and mass-transport properties have been investigated

using thermo gravimetric and differential scanning calorimetric

measurements There is evidence that both precursors are very low melting

and volatile and can be used as liquid Tl sources Both adducts have been

successfully applied to metalndashorganic chemical vapor deposition of thallium

containing films [59]

Large size and high quality single crystals of quasi-two-dimensional

thallium molybdenum purple bronze TlMo6O17 have been grown by electrolytic

reduction of molten salt of Tl2CO3-MoO3 The crystal structure is trigonal with

space group P3m1 determined by X-ray diffraction and four-circle single crystal

diffraction The lattice parameters of the unit cell are a = b = 55282 Adeg and c

= 136991 Adeg The temperature dependence of resistivity and magnetic

19

susceptibility confirmed that a metal-to-metal transition occurs near 110 K

[60]

Hall coefficient and dc conductivity studies were made on p-type

Pb08Sn02Te thin films doped with different concentrations of thallium in the

temperature range 77 to 500 K The Hall coefficient and Hall mobility are

found to decrease with an increase in the doping concentration of thallium

Hall coefficient data have been analyzed in the light of a double valence-band

model Various band parameters such as valence band separation population

ratio mobility ratio and effective mass ratio have been calculated Hall

mobility data have been analyzed in the light of lattice and defect limited

scattering mechanisms [61]

Proton conducting composites of heteropolyacid hydrates phosphomol

ybdic acid H3PMo12O40 nH2O(PMA) phosphotungstic acid H3PW12O40 nH2O

( PTA) and salt hydrate like NiCl2 6H2O were prepared

with insulating Al2O3 as despersoidThe ionic conductivity peaks at two

concentrations of Al2O3 indicating two percolation thresholds for percolation

thresholds for proton conduction Two separate experiments were carried out

to check the existence of such percolation thresholds viz the volta battery

experiment involving the measurement of emf of an electrochemical cell

with composites of different compositions used as electrolyte and the

composition vs conductivity measured by the complex impedance

spectroscopy The presence of two maxima has been attributed to two

different percolation thresholds for the two possible mobile protonic

species H+ + (H3O+) and OH arising from the hydrates [62]

1 A5 Applications of Heteropolyoxometalates-

Applications of heteropolyanions centre depend on their redox properties

their high charges and ionic weights An enormous patent and journal

literature is devoted to the applications of heteropolyanions

1 Analysis-

The formation and subsequent precipitation or reduction of

[XMO12O40]n- anions form the basis of gravimetric and colorimetric analytical

methods for P As Si or Geeither separately or in combination [63 64]

20

2 Biochemical applications-

lsquoPhosphotungstic Acidsrsquo have for decades been used as precipitants

for proteins and as analytical reagents for proteinsalkaloidsand purines eg

the [P2M18O62]6- anions for colorimetric determination of uric acid [65] and

cholesterol The acid H3P12O40 either in aqueous or ethanolic solution is also

widely used as a non specific electron dense stain for electron spectroscopy

The dyestuffs industry has for many years used heteropolymolybdates and

tungstates to form color lakes and toners from basic dyes Large

heteropolyanions exhibit antiviral antitumoral properties at non-cytotoxic

doses in vitro and in vivo and are protein inhibitors of cellular bacterial and

viral DNA RNA polymerizes [66]

3 Catalysis

Heteropoly acids and salts have been used as heterogeneous catalysts

for a broad variety of reactions and compilations of such applications up to

1973 are available Examples include oxidation of propylene and isobutylene

to acrylic acid methacrylic acids and ammoxidation of acrylonitrile olefin

polymerization and epoxidation Much of current activity concerning

heterogeneous catalysis by heteropoly compounds is being carried out in

Japan [67 68]

4 Other Applications

Insoluble salts of Heteropolyanions especially ammonium

molybdophosphates have been used and are commercially available as ion-

exchange materials [69]Recent work in this area includes thin layer

chromatography of amino acids ion selective membranes [70] and the

preparation of new ion exchangers based on heteropolyanions Crystalline12-

tungstophosphoric and 12-molybdophosphoric acids are excellent protonic

conductors Heteropolyacids are electrochromic in the solid state as a

consequence of heteropolyblue formation Heteropolyblue formation has also

been used to detect alcohol or carboxylic acid radicals generated by radiolysis

of aqueous solutions Potential applications of heteropoly complexes as flame

retardants and smoke suppressants or as corrosion inhibitors and conversion

coatings on steel and aluminium are reported [71] Some potential ldquogreenrdquo

applications have been reported eg non-chlorine based wood pulp

bleaching process and a method of decontaminating water Some structures

21

containing transition metal atoms with unpaired electrons have unusual

magnetic properties and are being investigated as nano computer storage

devices Some compounds exhibit luminescence There are many reported

potential medicinal applications eg anti tumoral and anti-viral There have

been reports on the role of weak or non bonding interactions on the crystal

engineering of hybrid polyoxometalates

Spherical nonporous polyoxomolybdate based capsules of different

types containing more than 100 metal atoms reported by Achim Muller and his

group have versatile unique properties regarding their assembly to vesicles

and the chemistry which can be done inside the pores and cavities A discrete

polyoxometalate Lindquist ion of the form W6O192minus was successfully imaged

recently for the first time within the capillary of a carbon nanotube following

steric locking of the anion with the tubule In situ relaxation of the anion in its

equatorial plain was demonstrated [72]

22

Section-B

Litreture Survey on Chromism in Transition Metal

Oxides

1 B1 Chromism in Transition Metal Oxides

Chromism is a reversible change in a substances colour resulting from

a process caused by some form of stimulus Many materials are chromic

including inorganic and organic compounds and conducting polymers and the

property can result from many different mechanisms Several transition metal

oxides show EC properties The most popular are from the VI - B oxides In

this group WO3 and MoO3 are the most thoroughly studied cathodic EC

materials which can be electrochemically coloured and bleached when used

as the cathode in electrochemical cells Cathodic EC materials also include

V2O3 TiO2 and Nb2O5 Another distinguishable group is anodic EC material

including VIII oxides like IrOx nH2O Rh2O3 nH2O NiO nH2O etc which can be

anodicaly coloured in the electrochemical process when used as anode

There are several types of chromism which are discussed as below

B11 Photochromism

Photochromism is the reversible transformation of a chemical species

between two forms by the absorption of electromagnetic radiation where

the two forms have different absorption spectra [7374]

Trivially this can be described as a reversible change of color upon

exposure to light The phenomenon was discovered in the late 1880s

including work by Markwald who studied the reversible change of color of 23

44-tetrachloronaphthalen-1(4H)-one in the solid state He labeled this

phenomenon phototropy and this name was used until the 1950s

when Yehuda Hirshberg of the Weizmann Institute of Science in Israel

proposed the term photochromism [75] Photochromism can take place

in both organic and inorganic compounds and also has its place in biological

systems (for example retinal in the vision process)

Photochromism does not have a rigorous definition but is usually used

to describe compounds that undergo a reversible photochemical reaction

where an absorption band in the visible part of the electromagnetic spectrum

23

changes dramatically in strength or wavelength In many cases an

absorbance band is present in only one form The degree of change required

for a photochemical reaction to be dubbed photochromic is that which

appears dramatic by eye but in essence there is no dividing line between

photochromic reactions and other photochemistry Therefore while the

transcis isomerization of azobenzene is considered a photochromic reaction

the analogous reaction of stilbene is not Since photochromism is just a

special case of a photochemical reaction almost any photochemical reaction

type may be used to produce photochromism with appropriate molecular desi

gnSome of the most common processes involved in photochromism are peric

yclic reactions cis-trans somerizations intramolecular hydrogen transfer

intramolecular group transfers dissociation processes and electron transfers

(oxidation-reduction)

Another some what arbitrary requirement of photochromism is that

it requires the two states of the molecule to be thermally stable under

ambient conditions for a reasonable time All the same nitrospiropyran (which

back-isomerizes in the dark over ~10 minutes at room temperature) is

considered photochromic All photochromic molecules back-isomerize to their

more stable form at some rate and this back-isomerization is accelerated by

heating There is therefore a close relationship between photochromic and the

rmochromic compounds The timescale of thermal back-isomerization is

important for applications and may be molecularly engineered

Photochromic compounds considered to be thermally stable include some

diarylethenes which do not back isomerize even after heating at 800C for 3

months

Since photochromic chromophores are dyes and operate according to

well-known reactions their molecular engineering to fine-tune their properties

can be achieved relatively easily using known design models quantum

mechanics calculations and experimentation In particular the tuning of

absorbance bands to particular parts of the spectrum and the engineering

of thermal stability have received much attention

Sometimes and particularly in the dye industry the term irreversible

photochromic is used to describe materials that undergo a permanent color

change upon exposure to Ultraviolet or visible light radiation Because by

24

definition photochromics are reversible there is technically no such thing as a

n irreversible photochromic this is loose usage and these compounds

are better referred to as photochangable or photoreactive dyes

Apart from the qualities already mentioned several other properties of

photochromics are important for their use These include

Quantum yield of the photochemical reaction

This determined the efficiency of the photochromic change with respect

to the amount of light absorbed The quantum yield of isomerization

can be strongly dependent on conditions

Fatigue resistance In photochromic materials fatigue refers to the

loss of reversibility by processes such as photodegradation

photobleaching photooxidation and other side reactions All

photochromics suffer fatigue to some extent and its rate is strongly

dependent on the activating light and the conditions of the sample

Photostationary state Photochromic materials have two states and

their interconversion can be controlled using different wavelengths of

light Excitation with any given wavelength of light will result in a

mixture of the two states at a particular ratio called the photo-

stationary state In a perfect system there would exist wavelengths

that can be used to provide 10 and 01 ratios of the isomers

but in real systems this is not possible since the active

absorbance bands always overlap to some extent

Polarity and solubility In order to incorporate photochromics in

working systems they suffer the same issues as other dyes They are

often charged in one or more state leading to very high polarity and

possible large changes in polarity They also often contain large

conjugated systems that limit their solubility

Photochromic complexes

A photochromic complex is a kind of chemical compound that has

photoresponsive Parts on its ligand These complexes have a specific

structure photoswitchable organic compounds are attached to

metalcomplexes For the photocontrollable parts thermally and

photochemically stable chromophores (azobenzene diarylethene

25

spiropyran etc) are usually used And for the metal complexes a wide

variety of compounds that have various functions (redox response

luminescence magnetism etc) are applied The photochromic parts and

metal parts are so close that they can affect each others molecular

orbitals The physical properties of these compounds shown by parts of

them (ie chromophores or metals) thus can be controlled by switching

their other sites by external stimuli For example photoisomerization

behaviors of some complexes can be switched by oxidation and reduction of

their metal parts Some other compounds can be changed in their

luminescence behavior magnetic interaction of metal sites or stability of

metal-to-ligand coordination by photoisomerization of their photochromic

parts

Photochemistry of Polyoxometalates

The photochemistry of polyoxometalates is of great interest to inorganic

chemistsMore than 80 years agoit was found that the R-Keggin tungstate

H3[PW12O40] was reduced photochemically to yield a blue-colored species

which was reoxidized by air and by various other oxidizing agents such as

Fe3+AgNO3and H2O2 [7879]The photoredox reactions of H4[SiW12O40]and

H3[PW12O40] proceeded effectively in the presence of primary and secondary

alcohols their ethers and aldehydes and proteinsbut less effectively in the

presence of tertiary alcoholsketonesestersthe fatty acids above formic

acidand simple amines[8081] The basic photoredox reaction involving

ethanol is illustrated by eq 13

2 H3PW12O40 + H3CCH2OH h ν ν ν ν 2 H4PW12O40 + H3CCHO ------- 13

2 H4PW12O40 +12 O2 2 H3PW12O40 + H2O ------ 14

In this reactionone molecule of ethanol photochemically reduces two

molecules of H3PW12O40 and is itself oxidized to acetaldehyde In the

presence of air the thermal oxidation of the reduced species takes place at

room temperature(eq 14)The reduced polyoxometalates which are the so-

calledldquoheteropolybluesrdquo have been used for the colorimetric analysis of the

26

elements P Si As and Ge and for the determination of uric acidsugarand

other biological compounds [8283] Piperidinium metavanadate also

undergoes photoinduced coloration from white to black followed by a

reversible color change in the presence of oxidizing agentsHowever

ammonium metavanadates ([NH4][VO3]) exhibits no photoinduced

coloration[84] The early photoredox reactions of the R-Keggin

polyoxometalates H4[SiW12O40] and H3PW12O40 were carried out in the

presence of photographic paper however the limited number of the

structurally well-characterized compounds available for study delayed the

development of modern cluster-compound photochemistry until the discovery

of photochromism in alkylammonium polyoxo- molybdate solids[8586] A

photochromic or electrochromic material is one whose light-absorbing

properties are altered upon optical excitation or reduction under the influence

of an externally applied electric field respectively The induced coloration

remains even after the excitation source has been removed These materials

are of technological interest because they return to their original state either

thermally upon irradiation with light of a frequency corresponding to the

induced absorption or electrochemically upon reversing the polarity of the

externally applied electric field Thus photochromic and electrochromic

materials behave in a reversible manner Polyoxometalates exhibit significant

photo-and electrochromism which makes them suitable as nanocomposite

molecular devices and as models for probing the physical properties of infinite

metal oxides Since the metal ions in the oxidized polyoxometalates have d0

electronic configurations the only absorption band which occurs in the UV-vis

range of the electronic spectra is due to the oxygen-to-metal (O-M) ligand-to-

metal charge transfer (LMCT)Upon irradiation electrons are promoted from

the low-energy electronic states which are mainly comprised of oxygen 2p

orbitals (the valence band in the band model)to the high-energy electronic

states which are mainly comprised of metal d orbitalrsquos (the conduction band

in the band model)The fundamental transitions in polyoxometalate lattices

are depicted schematically in Fig1B1

27

Fig1B1 Simple model showing the electronic transitions in the

polyoxometalates containing electron donar and acceptor (a)generation

of charge carriers(b)electron and hole trapping(c)electron release due

to stimulation(d)recombination

between electron and hole Electrons are e-and holes are h+

In the polyoxometalates containing heteroatoms and especially in

mixed metal polyoxometalates the charge carriers which are created by the

light or electric field may be trapped in electron traps and hole traps These

traps provide states of localized energy in the O-M LMCT energy gap due to

the heteroatoms or counter cations which correspond to impurities or lattice

defects in the band model If the trap depth ∆E is large compared to kT the

probability for thermal escape from the trap will be negligibly small and

metastable situation will existThe trapped carriers can be released by thermal

or optical stimulationIn the case of thermal stimulation the irradiated

polyoxometalate is heated until the energy barrier ∆E can be overcome The

trapped electron (or hole) then can escape from the trap and nonradiatively

recombine with the trapped hole (or electron)Under optical stimulation the

energy of an incident photon is used to overcome ∆E The relaxation

processes of the OndashM LMCT excitation energy include both the nonradiative

recombination of electrons and holes within the energy gap and the

intramolecular energy transfer leading to a charge-transfer emission This

intramolecular energy transfer corresponds to the O-M LMCT energy gap and

occurs via radiative recombination and sensitized emission from the

28

heteroatoms or cations If several energy levels based on the hetero atoms or

counter cations act as energy acceptors within the O-M LMCTenergy gap the

energy transfer occurs from the O-M LMCT states to these levels followed by

the nonradiative or radiative deactivation of the excitation energy It should be

noted that the O-M LMCT states also can be generated by the application of

very high electric fields to the polyoxometalate solids as demonstrated by the

observation of electroluminescence[87]If an external electric field with a

potential more negative than the energy levels of the vacant orbitals involved

in the O-M LMCT transition is applied to a polyoxometalate on the electrode

surface an electrochemical reduction occurs via the injection of electrons

from the electrode in to the vacant levels of the polyoxometalate as shown in

Fig1B 2

Fig1B2 - Energy scheme for the electrochromism of polyoxometalates

a)electrochemical reduction (b) electrochemical oxidation

Electrons injected in to the high-energy levels also may be trapped by

electron traps in a process analogous to that which occurs during LMCT

photoexcitation of the polyoxometalates These electrons are returned to the

electrode by electrochemical oxidation at an electrode potential more positive

than the energy levels for the d1 electron states The d1electrons in the O-M

LMCTstates facilitate the absorption of visible light via intervalence charge

transfer among metal centers and d-d transitions The same type of transition

may be possible for the d1electron captured by the electron traps too In

addition to searching for new photosensitive polyoxometalates with the

29

potential for having practical application there is now a need to elucidate the

fundamental photo-and electrochemical coloration processes by studying

electron transfer within the polyoxometalate lattices in conjunction with their

crystal structures So far few polyoxometalates exhibit a perfect reversibility

of coloration The irreversibility of the color change arises from as yet

uncharacterized side reactions during both the coloration and decoloration of

the polyoxometalates

Many metal oxides including aluminum titanium vanadium niobium

molybdenum and tungsten oxides are photochromic when they contain

impurities or dopants This coloration has been interpreted on the basis of

electron trapping at appropriate lattice sites within the crystals as shown in

Fig1B1 where the O-M LMCT transition corresponds to the transition

between the valence and conduction bands for the infinite metal-oxide lattice

[88-91]

B12 Applications of Photochromic materials

Sunglasses

One of the most famous reversible photochromic applications is color

changing lenses for sunglasses as found in eyeglasses The largest limitation

in using PC technology is that the materials cannot be made stable enough

to withstand thousands of hours of outdoor exposure so long-term outdoor

applications are not appropriate at this time The switching speed of

photochromic dyes is highly sensitive to the rigidity of the environment around

the dye As result they switch most rapidly in solution and slowest in the rigid

environment like a polymer lens Recently it has been reported that attaching

flexible low Tg polymers (for example siloxanes or poly (butyl acrylate) to the

dyes allows them to switch much more rapidly in a rigid lens [76] Some

spirooxazines with siloxane polymers attached switch at near solution like

speeds even though they are in a rigid lens matrix

Supramolecular chemistry

Photochromic units have been employed extensively in supramolecular

chemistry Their ability to give a light controlled reversible shape change

means that they can be used to make or break molecular recognition motifs

30

or to cause a consequent shape change in their surroundings Thus

photochromic units have been demonstrated as components of molecular

switches The coupling of photochromic units to enzymes or enzyme cofactors

even provides the ability to reversibly turn enzymes on and off

by altering their shape or orientation in such a way that their functions

are either working or broken

Data storage

The possibility of using photochromic compounds for data storage was

first suggested in 1956 by Yehuda Hirshberg[77] Since that time there have

been many investigations by various academic and commercial groups

particularly in the area of 3D optical data storage which promises discs that

can hold a terabyte of data Initially issues with thermal back-reactions

and destructive reading dogged these studies but more recently more stable

systems have been developed

Novelty items

Reversible photochromics are also found in applications such as toys

cosmeticsclothing and industrial applications If necessary they can be made

to change between desired colors by combination with a permanent pigment

A large number of inorganic compounds exhibit photochromism

These solids often have large band gaps of the order of 3 - 12 eV and

excitation of these solids leads to the formation of metastable centers that

absorb visible light giving rise to their colour They can return to their ground

state by heating or by optical excitation within the colour-centre band In most

cases the photochromism is a structure sensitive phenomenon involving

localized defect impurities or dislocations Some of these inorganic

compounds have the potential for a number of different uses Photochromic

compounds have a number of useful applications These can be divided

according to the most important property that is being used (Table 11) [92]

31

Table 11 Applications of Photochromic materials

Applications Depending Upon

Sensitivity to

Radiation

Reversibility Thermal Chemical or

Physical Properties

Self-developing

photography

Chemical switches for

computers

Temperature indicators

Protective

materials

Data displays

Heat-sensitive recording

media

Optical signal

processing

Photomasking and

photoresist technology

Reusable data storage

media

Anaytical reagents

Photochromic

microimages

Photopolymerisation

Information encoding

and steganography

Photocontractile

polymers and the

photoviscosity effect

Control of light

intensity

Q-switches

Pyroelectric

photochromic materials

B13 Thermochromism

Thermochromism is the reversible colour change of a substance

induced by temperature change A large variety of substances organic

inorganic organometallic supramolecular and polymeric systems exhibit this

phenomenon Examples of these include bianthrones cobalt

hexacyanoferrate the zirconocene complex of 1 4-diphenyl-1 3-butadiene

and poly (3-alkylthiophene) The organic 99-bixanthenylidene is colourless at

90 K yellow-green at 298 K and dark-blue when melted at 592 K Heating

conducting polymers can cause them to change colour This is achieved by

causing conformational changes to the polymer backbone resulting in a

change in the band gap of the polymer It has been reported that regioregular

32

P3HT reversibly changes colour upon heating to 220ordmC due to temperature-

dependent conformation changes Thermally cross linked polymer undergoes

the same colour change but it is much less reversible [93] Other forms of

thermochromism may be commercially important eg to give a visual

indication of temperature changes

B14 Electrochromism

Electrochromism describes a phenomenon of material color change in

a persistent but reversible manner produced by electrochemically induced

oxidation-reduction reactions Electrochromic materials can be applied to

various kinds of products such as smart windows and display devices Among

those applications there have been lots of efforts to develop electrochromic

display devices (ECDs) Especially flexible display devices are now attracting

much attention worldwide since they can facilitate new technological demands

such as bending and folding of paper-like displays High electrochromic

efficiency short response time long operating life time and reduction of

energy consumption are the most important requisites of the materials for the

paperlike displays [94 95] Among those properties the operation life time is

the most important barrier to overcome for a realization of ECDs There are

two types of electrochromic material a) inorganic transition metal oxides

(TMOs) b) organic polymer materials The TMOs have been studied longer

than the organic materials that they have been studied since 1960s [9697]

Electrochromism describes a reversible color change of material

produced by electrochemically induced oxidation-reduction reactions It is one

of several types of chromism of materials As thermochromism and

photochromism mean material color changes made by heat and light

respectively electrochromism refers that the color change is caused by an

electric potential In most cases the color change in electrochromism can be

driven by rather low electrical potential of the order of a fraction of volt or a

few volts [94-96] The color change of material means variation in

transmittance andor reflectance change in visible range which is originated

from different electronic absorption bands according to a switching between

oxidation and reduction state of material When electric potential is applied on

electrochromic material forced oxidation or reduction is derived and the

33

individual color is originated from the corresponding oxidation or reduction

state of the material For electrochromic materials the characteristic color

change is reversible since the oxidation and reduction state can be converted

reversibly by switching potential

Application fields

The application area for the electrochromism is rather broad that it

covers from smart window glazing and optical modulators to information

displays [98-102] The smart windows are typical examples The

electrochromic property is used to control the amount of light and heat to pass

through the windows Usually the electrochromic material is in form of thin film

coated on a window glass The transmittance modulation has also been

applied at the automobiles to automatically tint rear-view mirrors in various

lighting conditions The electrochromic application fields are illustrated in Fig

1B3

Since the smart windows control the transmittance of heat as well as

the transmittance of visible light the working definition of electrochromism has

now been extended to include devices for modulation of radiation in the near

infrared thermal infrared and microwave regions When color for

electrochromic materials is used this can now mean a response by detectors

at these wavelengths and not just by the human eyes Nowadays

electrochromic material draws much attention as being used in the display

devices Electrochromic display device (ECD) is being considered as one of

the candidates for the conventional liquid crystal display (LCD) since ECD

has many advantages over LCDs Among these advantages the most

important are low energy consumption wider viewing angle high contrast

rate and possibility to achieve multiple colors with a single material [103]

When a new redox state of electrochromic materialis established by

the applied electric pulse then it is maintained after the potential is switched

off This means the colored or bleached state of the material can be sustained

for a considerable time without applying electrical power This is so-called

ldquocolor memory effectrdquo of electrochromic material Because of the color

memory effect energy consumption for the electrochromic display device

could be drastically reduced and this would be a big advantage over other

34

emissive devices The low power consumption is especially advantageous

when it is applied to mobile devices with limited power source The possibility

for a flexible display is another attraction for electrochromic material

Information displays

Real-view mirrors for automobiles

Fig 1B4 Application fields of electrochromic devices Smart windows information displays and real-view mirrors for automobiles

35

Using polymeric electrochromic material and plastic substrate with a

transparent conductive coating it is possible to build all-plastic flexible display

device There are lots of papers and patents about the flexible display devices

[104105] In these cases gel-type electrolyte is also needed The gel-type

electrolyte immobilizes liquid electrolyte in the polymer network [106107]

Recently ITO-coated polymer films are readily available which would provide

the plastic substrate for a flexible device The flexible electrochromic displays

are frequently tried with the plastic substrate flexible electrochromic material

and gel-type electrolyte The flexible electrochromic displays would facilitate

increasing technical demands for foldable display devices

Metal Ion Electrochromism

Many transition metal oxides are capable of redox reactions that result in

colour change Metal oxide films are commonly prepared as thin layers of

either tungsten nickel molybdenum or other metal compounds by a number

of techniques These include sol-gel electrochemical by dc or rfreactive

sputtering techniques electron-beam evaporation by anodic or cathodic

electrodeposition or by solution dipping of the electrochromic metal

compounds (or compounds that can be changed into these metal compounds)

onto optically transparent electrodes (OTE) [108 -114] Their electrochromism

is derived from the colour change associated with a change in the oxidation

state of the metal anion The behaviour of these materials is dependent upon

pH moisture and exposure to the atmosphere [115] Generally the switching

rates of these films is somewhat slow with typical switching times of about 15

- 60 seconds to achieve 100 conversion to either coloured or bleached state

[116 -120] An Example of this includes nickel oxide which changes from

transparent (pale green) to brownblack taking about 30 seconds to do so

[121] Other examples include [(NH4)5Ru]2(pyrazine)5+ and [(NH4)5Ru]2(44-

bipyridine)5+ whose electrochromism is significantly different due to the effect

of the ligand [122] Table 12 below gives some examples of metal oxide films

with electrochromic properties

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

3

them ideal frameworks for the deployment of useful magnetic

electroionic catalytic bioactive and photochemical properties With this in

mind a new trend towards the application of these remarkable species in

materials science is beginning to develop The applications of polyoxo-

metalates are

i) Their use as clusters with inherently useful properties on themselves a

line which has produced fundamental studies of their magnetic electronic or

photoelectrochemical properties and has shown these clusters

as models for quantum-sized oxides

ii) The encapsulation or instigation of polyoxometalates in

to organic polymeric or inorganic matrices or substrates opens

a whole new field within the area of hybrid materials for harnessing the

multifunctional properties of these versatile species in a wide variety of

applications ranging from catalysis to energy storage to biomedicine

Polyoxometalates have been known and used in the chemistry

laboratory for nearly two hundred years but only after the second

half of the 20th century we have been able to fully perceive the richness of

their chemistry structure and activity Modern techniques such as X-ray

crystallography or NMR and entire areas such as magnetochemistry or

electrochemistry have allowed a whole generation of contemporary

chemists to build and make known a complete body of understanding on the

structure bonding and properties of these fascinating cluster molecules

Several enlightening reviews and compendiums have been published [8-11]

With the turn of the century the coming age of materials

science and the advent of nanotechnology polyoxometalates are beginning to

be considered as unique chemical species that could turn from very special

molecules to very useful materials With sizes just one order of magnitude

smaller than the smallest of living biological structures such as the Rhinovirus

(approx20nm) they are not colloids but soluble polynuclear species Indeed

one of the main reasons why polyoxometalates have not been considered

in the past for design of functional materials is precisely because their

molecular nature makes them soluble in water and common organic solvents

Yet they not only share structural and topological features with related

transition metal oxides but also resemble them concerning their redox

4

electron transfer or ion transport behavior In all these respects polyoxo-

metalates can be generically considered as the perfect models for quantum-

sized transition metal oxide nanoparticals [12 13] For example

the electrochemical or photochemical injection of electrons in heteropolyions

(HPA) with the induction of thermally activated delocalization between metal

centers and IVCT (InterValence Charge Transfer Bands) leading to change

in color closely parallel the corresponding electrochromic properties of the

corresponding oxides upon doping

Contrary to ever-smaller nanostructures and quantum dots designed

bymeans of physical methods following a top-down approach

polyoxometalates represents a very significant example of the bottom-up

pproach Chemists use to build polynuclear and supramolecular structures

with collective properties The control of size and structure in

polyoxometalates is based on now well known acid hydrolysis and

condensation reactions driven by the very rich acid base chemistry of

some transition metal cations primarily W Mo and VBut this framework

chemistry of isopolytungstates molybdates and vanadates is remarkably

broadened up when other elements come to add richness through structural

and chemical multiplicity within the field of heteropolyanions

In addition the remarkable stability of many of these clusters makes

possible an extensive redox chemistry leading to wealth of ldquobluesrdquo reduced

species where thermally activated delocalization of electrons and a variety

of spin states make for a remarkable landscape of electronic and magnetic

states in small clusters that become in this way ideal models for the study of

spin interactions [14 -18]

Very recently this rich oxygen-metal chemistry has been broadened

with specific and unique examples that include for the first time anions such

as S-2 or MeO- [1920] or iron ions as the framework-building metals The

control of the extent of condensation and the isolation of new larger clusters

formed through a building-block approach making use of smaller fragments

to assemble larger units have also been a tremendous source of development

in very recent years especially for vanadium species[2122]

At this point the field of polyoxometalate chemistry has

broadened so much in variety of elements and structures properties and

5

applications that the reference books in the field would certainly welcome

substantial additions and revision

The intrinsic properties of polyoxometalates are of interest in themselves

not only from a fundamental point of view but also to make of them

materials of interest in various applications Beyond their traditional interest as

catalysts polyoxometalates constitutes base materials for electrochromics

energy storage and conversion devices (batteries super capacitors and

fuel cells) sensors or biomedical applications Many of the applications of

polyoxometalate clusters as materials require their use in the form of

membranes or electrodes that is in the form of solid insoluble material or

coatings There is therefore a main strategic line of work that has centered

on the inclusion or integration of polyoxometalates in all sorts of substrates

polymeric inorganic or mineraland their combination with surfactants or

organic carriers

Heteropolyanions are negatively-charged clusters of corner-sharing and

edge shairing early transition metal MO6 octahedra and heteroatom

XO4 tetrahedra where the tetrahedra are usually located the interior

of the cluster[23] The geometry composition and charge of these clusters

are varied through synthesis parameters and cluster properties are

highly tunable as a function of these characteristics Heteropolyanions have

been employed in a range of applications that include virus-binding

inorganic drugs [24] homogenious and heterogeneous catalysts [25 26]

electro-optic and electrochromic materials [27 28] metal and protein binding

[29] and as building blocks for nanostructuring of materials [30] The α-Keggin

geometry which was first structurally characterized in 1933 by JF Keggin

[31] the phosphotungstic acid (H3PW12O40) is one of the most

widely recognized and thoroughly studied heteropolyanion geometries[32]

1A2 Fundamental concept of polyoxometalate structures-

Keggin structure is the best known structural form for heteropoly acids It

is the structural form of αndashKeggin anions which have a general formula of

[XM12O40]n- where X is the hetero atom (most commonly are P5+ Si4+ or B3+)

M is the addenda atom (most common are molybdenum and tungsten) and O

6

represents oxygen[33] The structure self assembles in acidic aqueous

solution and is the most stable structure of polyoxometalate catalysts

Fig 1A1 Keggin structure

The first α-Keggin anion ammonium phosphomolybdate

((NH4)3[PMo12O40]) was first reported by Berzelius in 1826 In 1892

Blomstrand proposed the structure of phosphomolybdic acid and other poly-

acids as a chain or ring configuration Alfred Werner using the coordination

compounds ideas of Copaux attempted to explain the structure of

silicotungstic acid He assumed a central group [SiO4]4- ion enclosed by four

[RW2O6]+ where R is a unipositive ion The [RW2O6]

+ are linked to the central

group by primary valences Two more R2W2O7 groups were linked to the

central group by secondary valences This proposal accounted for the

characteristics of most poly-acids but not all

In 1928 Linus Pauling proposed a structure for α-Keggin anions

consisting of a tetrahedral central ion [XO4] n-8 caged by twelve WO6

octahedral In this proposed structure three of the oxygen on each of the

octahedral shared electrons with three neighboring octahedral As a result 18

oxygen atoms were used as bridging atoms between the metal atoms The

remaining oxygen atoms bonded to a proton This structure explained many

characteristics that were observed such as basicities of alkali metal salts and

the hydrated of some of the salts However the structure could not explain the

structure of dehydrated acids

JF Keggin with the use of X-ray diffraction experimentally determined

the structure of α-Keggin anions in 1934 The Keggin structure accounts for

both the hydrated and dehydrated α-Keggin anions without a need for

7

significant structural change The Keggin structure is the widely accepted

structure for the α-Keggin anions [34] For example the α-Keggin anion of

phosphotungustic acid is shown in Fig1A2

The structure is composed of one heteroatm surrounded by four

oxygen to form a tetrahedronThe heteroatom is located centrally and caged

by 12 octahedral WO3 units linked to one another by the neighboring oxygen

atoms There are a total of 24 bridging oxygen atoms that link the 12 addenda

atoms The metal centres in the 12 octahedra are arranged on a sphere

almost equidistant from each other in four M3O13 units giving the complete

structure an overall tetrahedral symmetry The bond length between atoms

varies depending on the heteroatom (X) and the addenda atoms (M)

Fig 1A2 α-Keggin anion of phosphotungustic acid ( PW12O40 ) 3-

For the 12ndashphosphotungstic acid Keggin determined the bond length

between the heteroatom and each the four central oxygen atoms to be 15 Adeg

The bond length form the central oxygen to the addenda atoms is 243 Adeg

The bond length between the addenda atoms and each of the bridging

oxygen is 19 Adeg The remaining 12 oxygen atoms that are each double

bonded to an addenda atom have a bond length of 170 Adeg The octahedra

are therefore distorted This structure allows the molecule to hydrate and

dehydrate without significant structural changes and the molecule is thermally

stable in the solid state for use in vapor phase reactions at high temperatures

(400-500 degC)[35-36]

8

Including the original Keggin structure there are 5 isomers designated

by the prefixes α- β- γ- δ- and ε- The original Keggin structure is designated

α- These isomers are sometimes termed Baker Baker-Figgis or rotational

isomers [37]These involve different rotational orientations of the Mo3O13 units

which lowers the symmetry of the overall structure Lacunary Keggin

structures

The term lacunary is applied to ions which have a fragment missing

sometimes called defect structures Examples are the (XM11O39)nminus and

(XM9O34)nminus formed by the removal from the Keggin structure of sufficient Mo

and O atoms to eliminate 1 or 3 adjacent MO6 octahedra The Dawson

structure X2M18O62nminus is made up of two Keggin lacunary fragments with 3

missing octahedra Some structural types are found in many different

compounds The first known example of this was the Keggin ion whose

structure was found to be common to both molybdates and tungstates with

different central hetero atoms Examples of some fundamental

polyoxometalate structures are shown below The Lindquist ion (Fig1A4) is

an iso-polyoxometalate the other three are hetero-polyoxometalates The

Keggin and Dawson structures (Fig1A3 and (Fig1A5) have tetrahedrally

coordinated hetero-atoms eg P or Si Anderson structure has an octahedral

central atom egAl

Fig 1A3 Keggin structure XM12O40nminus

9

Fig 1A 4 Lindquist structure M6O19nminus

Fig 1A 5 Dawson structure X2M18O62nminus

In general α-Keggin anions are synthesized in acidic solutions For

example 12-Phosphotungstic acid is formed by condensing phosphate ion

10

with tungstate ions The heteropolyacid that is formed has the Keggin

structure

[PO4]3- + 12 [WO4]

2- + 27 H+ rarr H3PW12O40 + 12 H2O ----------- 11

α-Keggin anions have been used as catalyst in hydration polymerization and

oxidation reaction as catalysts

The metal atoms that make up the framework (termed addenda atoms)

are typically Mo W and V When more than one element is present the

cluster is called a mixed addendaclusterThe ligands coordinated to metal

atoms that together form the bridged framework are usually oxide ions but

other elements such as S and Br have been substituted for some of the oxide

ions (Note that sulfur substituted POM is often termed a

polyoxothiometalates) Another development is the use of other ligands eg

nitrosy and alkoxy to replace oxide ions The typical framework building

blocks are polyhedral units with 4 5 6 or 7 coordinate metal centers These

units usually share edges andor vertices The most common unit for

polymolybdates is the octahedral MoO6 unit which is a distorted octahedron

where the Mo atom moves off centre to give one short Mo-O bond In some

polymolybdates there are pentagonal bipyramidal units and these are key

building blocks in the molybdenum bluesHetero atoms are present in many

polyoxometalates Many different elements can act as hetero-atoms

Examples of various coordination numbers around the hetero-atom are

known

4 co-ordinate (tetrahedral) in Keggin Dawson and Lindquist structures

(eg PO4 SiO4 AsO4)

6 co-ordinate (octahedral) in Anderson structure (eg Al(OH)6 TeO6

8 co-ordinate (square antiprism) in ((CeO8)W10O28)8minus

12 co-ordinate (icosahedral) in (UO12)Mo12O30 8minus

11

Often the hetero-atom is centrally located in the anion (eg Keggin

structure) or in a structure fragment eg the 2 phosphorus atoms in the

Dawson ion are central to the two symmetric fragments There are similarities

to clathrate structures The Keggin ion can be formulated as PO4 2minusand

M12O36 and the Dawson as (XO42-)2 and M18O54Structural isomerism is

common For example the Keggin structure has 5 isomers which can be

considered to contain one or more of the four M3O13 units being rotated

through 60degMany compounds share the same framework architectures or

frameworks derived from a larger framework with one or more addenda atoms

and oxide ions removed to give defect structure usually called a lacunary

structure An example of a compound with a Dawson lacunary structure is

As2W15O56Some cage structures containing ions are known eg an example

is the vanadate cage V18O42 containing a Clminus ion [38] This has 5 co-

ordinates square pyramidal vanadium units linked together

Fig 1A 6 - H4V18O42 cage containing Cl

1A3 General Properties of polyoxometalates -

Typically polyoxoanions are water and air stable species of large size

(6-25 Adeg) and high ionic weight In aqueous solution they are subject to

decomposition by hydroxide ions eg

[PW12O40]3- + 23 OH- HPO4

2- + 12WO42- + 11 H2O -------- 12

12

Although the PH at which such reactions are rapid can very widely

depending upon the polyanion involved Polyanions are often much stable

towards the H3O+ ions and numerous crystalline heteropoly acids are known

Such acids may be extremely soluble in water and polar solvents (giving

solutions with densities in excess of 4 gcm3) and have large dissociation

constants (PK lt 0)

Crystalline heteropoly acids and salts are frequently highly hydrated with

up to 50 molecules of water per anion Much of this water is zeolite in nature

and crystal composition can vary accordingly On the other hand the cation

anion stiochiometry is always well defined as the anion structure and

composition Finally many polyanions are powerful oxidizing agents and

undergo multiple reversible one or two electron reductions leading to intensely

colored mixed valence species known as heteropoly blues Polyanions are

known which can accept as many as 32 electrons without major structural

change

1A4 Chemistry of Molybdenum and Tungusten Heteropolyoxometalates

A photochromic monolayer film of phosphomolybdic acid (denoted as

PMo12) was fabricated by self-assembly approach UV-visible spectrum and

AFM observation show that the monolayer film is composed of aggregated

PMo12 molecules The monolayer film shows good photochromic properties

with enough stability and reversibility The colour change of the monolayer

after UV-irradiation can be captured by a microscope equipped with a color

CCD camera Photochromic response of the monolayer film can be doubled

after being modified by an amine monolayer [39]

Series of vanadium substituted molybdo Keggin HPA with 12 or 3

adjacent vanadium atoms were prepared These materials were supported

on carbon cloth electrodes and hot pressed onto Nafion with an ETEK

electrode used as a standard on the opposite side The MEArsquos were run at

temperatureslt100 oC with the HPA electrode as either the anode or the

cathode Stable polarisation curves are obtained for an HPA based cathode

with reasonable current densities at 80 oC [40]

A novel mixed-valence polyoxomolybdenum anion was synthesized

hydrothermally from molybdenum oxidemolybdenum metalboric and

phosphoric acids12-phenyldiphosphonicacidand imidazole (ImH) and was

13

structurally characterized as an imidazolium saltOne-and two-dimensional

structures of this anion and additional molybdenum diphosphonate linkers

were assembled as wellThey were structurally characterized as their

pyridinium(pyH) salts [41]

Phosphomolybdic acidpolyvinylpyrrolidone hybrid films were found to

show visible light photochromism It is identified that the intra-supramolecular

charge transfer between the inorganic and organic molecules is responsible

for the visible-light coloration Interestingly the films show photo-memory and

thermal activation The films show a small change in absorbance after being

irradiated with visible light for a short time and the coloration can be

enhanced greatly by subsequent thermal treatment Electrical measurements

indicate that the conductivity of the film increases after the brief irradiation

which promotes transfer of the electrons induced by the thermal treatment

[42]

In this work major effort was concentrated on passive thermal control

coatings based on photochromic and thermochromic materials The inorganic

photochromic materials were based on tungsten and molybdenum oxide films

and the organic photochromic materials included spiropyrans and

spirooxazines In addition photochromic composite organic-inorganic films

and thermochromic vanadium oxide films were prepared The samples were

synthesized using sputtering sol-gel process and thermal oxidation [43]

Polyoxometalates a class of oxidatively robust inorganic oxidants and

oxidation catalysts are currently under investigation at the Forest Products

Laboratory and at Emory University as an alternative to chlorinebased

chemicals in the bleaching of soft Woodkraft and other pulps Although

polyoxometalate salts are used in a number of industrial processes the

feasibility of using these salts and oxygen in the commercial bleaching of

chemical pulps was only recently demonstratedA clear advantage of

polyoxometalates over oxygen alone hydrogen peroxide or ozone is their

inherently high selectivity for the residual lignin in softwood kraft pulps The

goal of ongoing research is to develop a highly selective energy efficient

oxygen based polyoxometalate delignification and bleaching technology

compatible with mill closure [44]

14

Tungsten oxide exhibits pronounced photochromism upon bandgap

photoexcitation which makes it attractive and promising for applications in

many areas Some advances have been achieved during the past decades

The research on nanocrystalline films and single crystals indicates the critical

importance of defects in tungsten oxide to its photochromism Based on

energy-band engineering of semiconductors enhancement of photochromism

has been achieved for instance extension of the photoresponse from UV to

visible light by cathodic polarization improved change in absorption before

and after coloration through modification by a noble metal or another metal-

oxide semiconductor and increased photochromic reversibility via

hybridization with organic amines Nanocrystalline oxide films exhibit

controllable wettability which is coherent in nature with photochromism [45]

Polyoxometalates represent a diverse range of molecular clusters with

an almost unmatched range of physical properties and the ability to form

structures that can bridge several length scalesThe new building block

principles that have been discovered are beginning to allow the design of

complex clusters with desired properties and structures and several structural

types and novel physical properties are examinedIn this critical review the

synthetic and design approaches to the many polyoxometalate cluster types

are presented encompassing all the sub-types of polyoxometalates including

isopolyoxometa- lates heteropolyoxometalates and reduced molybdenum

blue systems As well as the fundamental structure and bonding aspectsthe

final section is devoted to discussing these clusters in the context of

contemporary and emerging interdisciplinary interests from areas as diverse

as antiviral agentsbiological ion transport modelsand materialsscience [46]

Keggin type molybdovanadophosphoric heteropoly acids were prepare

d by a novel environmentally benign method and their catalytic performances

were evaluated via hydroxylation of benzene to phenol with hydrogen

peroxide as oxidant in a mixed solvent of glacial acetic acid and acetonitrile

Various reaction parameters such as reaction time reaction temperature

ratio of benzene to hydrogen peroxide concentration of aqueous hydrogen

peroxide ratio of glacial acetic acid to acetonitrile in solvent and catalyst

concentration were changed to obtain an optimal reaction conditions

15

Molybdovanadophosphoric heteropoly acids are revealed to be highly

efficient catalyst for hydroxylation of benzene [47]

Thin films from the system (As2S3)Tl were deposited by thermal

evaporation on Si graphite and optical glass substratesFrom transmission

and reflection measurements of the thin films the refractive index (n) film

thickness(d) optical band gap(Eg) optical oscillator energy(Eo) and

dispersion energy(Ed) before and after exposure to light were determined

The results for optical parameters were analyzed using the Wemple - Di

Domenico single oscillator model and Lorenc-Lorenc equation It was found

that Eg decreases while n E0 and Ed increase for as deposited films

decreases while n E0 and Ed increase for as deposited films with increasing

of Tlconcentration passing through a maximum at 6 at of Tl After exposure

to light n E0 Ed increase and Eg decreases for all compositions

investigated The maximum change in n (Dn = 016 at l = 6328 nm) was

observed for thin As38S56Tl6 films From infrared spectroscopy measurements

of bulk glasses and thin films we could conclude that when up to 6 at of

thallium is introduced As-S-As chains break and a ternary TlAsS2

compound appears at 10 at Tl [48]

The organo - inorganic hybrid materialconsisting of Poly (34 Ethylene

Dioxythiophene) (PEDOT) doped with phosphomolybdate cluster anions

[PMo12O40]3-has been synthesized by direct insitu oxidative polymerization of

34-Ethylene Dioxythiophene (EDOT) with phosphomolybdic acid

(H3PMo12O40) Its characterization is investigated by Fourier Transform

Infrared Spectroscopy (FT-IR) and Scanning Electron Microscopy (SEM) The

hybrid material presents predominantly high electronic conductivities of

around 20 and 70 S cm1at 300 and 400 K respectively [49]

Heteropolyacids (HPAs) are known to be excellent re-dox catalysts In

combination with TiO2 HPAs can be used as photocatalysts active in visible

light The HPA accepts electron and get reduced to heteropolyblue (HPB)

That can absorb light in the visible range HPA can be incorporated onto the

external surface or in the pores of zeolite based composite photocatalysts

have been designed by incorporation of HPA semiconductor TiO2 and

transition metal cobalt on zeolite This composite metallozeolite photocatalyst

is efficient in photoreduction of methyl orange (MO) in visible light to the tune

16

of 411 mg of MO photo reducedg TiO2 This catalyst also shows encouraging

results for hydrogen evolution from water to the tune of 2730 micromolhg TiO

[50]

A new class of proton conducting glass membranes based on heteropo

lyacids such as phosphotungstic acid (PWA) as electrolytes for low

temperature H2O2 fuel cells was investigated Parameters for a single fuel cell

with a catalyst electrode of 015 mgcm2 of PtC and a glass composite

membrane were characterized by electrochemical measurements at open

circuit potential conditions The performance of the membrane electrode

assemblies (MEA) was systematically studied as an effect of SiO2 and P2O5

concentrations in the glass composite membrane and the MEA was found to

exhibit a maximum power density of 162 mW cm2 for an H2O2 fuel cell at

30 degC and 30 relative humidity (RH) [51]

Two new photochromic inorganic-organic hybrid materials formed from

Keggin type Polyoxometalates (POMs) and metronidazole (C6H9N3O3 MNZ)

formulated as H3PMo12O40bull3 MNZ3H2O (1) and H3PW12O40bull3MNZ3H2O(2)

were synthesized and characterized by elemental analysis IR spectra

electronic spectra electron spin resonance (ESR) spectra and TG-DTA

Reflectance spectra show the presence of weak inter molecular charge

transfer between the organic and inorganic moieties in the solid state The

photochromic properties were studied by solid diffuse reflectance spectra and

ESR spectra and the photochromic reactions were found to exhibit first-order

kinetics TG-DTA showed that two hybrid materials have similar thermal

behavior [52]

Heteropolycompounds (HPCs) have been a matter of interest in basic

and applied science for more than a century From their first synthesis many

advances have been made to promote the use of HPCs in different ways in

science and technology The aim of this article is to review the main structural

characteristics of heteropolycompounds of the Keggin type (12

tungstophosphoric12-molybdophosphoric12-tungstosilicic acid alkaline and

alkaline earth salts of12 tungstophosphoric acid and gels doped with HPCs)to

understand and explain their different activities such as high proton

conductivity and catalytic biochemical and biomedical activities [53]

17

A solid hybrid molecular material containing 1-butyl 3-methyl

imidazolium cations and Keggin anions of phosphotungstic acid has been

synthesized It is fully characterized by CHN analysis FTIR XRD UV-Vis-NIR

DRS 31P MAS NMR TGA and SEM The FTIR spectrum of the compound

shows the fingerprint vibrational bands of both Keggin molecular anions and

imidazolium cations The aromatic CndashH stretch region (2700ndash3250 cmndash1) of

imidazolium cation is split due to the interaction between the ring CndashH and

bulky Keggin anion The red-shift in the UV-Vis spectra and the downfield 31P

MAS NMR chemical shift also confirm the electrostatic interaction between

the ions in the compound Near IR spectral region (1000ndash2500 nm) shows the

elimination of water in the compound which is hydrophobic [54]

Transport coefficient measurements (electrical conductivity

thermoelectric power and Hall coefficient) have been performed on a

compact Tl033MoO3 polycrystalline compound in a wide temperature range

(200ndash400 K) Experimental results are interpreted with the help of a p-type

semiconductor model with two inverted deep levels near the midgap The

valence band and the conduction band are assumed to be formed from the

dxy orbitals of molybdenum atoms in the Mo6O22 cluster leading to narrow π-

bonding bands The donor and acceptor levels may be formed from

nonbonding dxy orbitals arising respectively from anionic and Tl+ defects

Electron paramagnetic resonance and magnetic measurements are in good

agreement with the theoretical band semiconductor model which has been

retained [55]

The optical properties of GaInTlAs epilayers grown at low temperature

~230degC by solid-source molecular-beam epitaxy on InP substrates were

characterized using optical absorptionand photoluminescence techniques

Optical absorption measurements a room temperature show a gap shrinkage

toward lower energies from 071 to 061 and 053 eV when the Tl content

increases from 0 to 4and 8in good agreement with theoretical

predictionsLow-temperature photoluminescence band-gap signals from

GaInAs and GaInTlAs layers are only obtained after rapid thermal annealing

performed inorder to improve the electronic quality of the layersA band gap

decrease as much as 41 meV for GaInTlAs with 19 Tl incorporation is

measured by photoluminescence at 8K [56]

18

The electrochromic performance of all solid ndash state cells employing

phosphotungstic acid and phosphomolybdic acid is reported These cells

employ SnO2 as the viewing electrode and graphite as the back electrodeThe

cells in the bleached state can be made white to red and become black in the

coloured state [57]

Various organic compounds were oxidized by molecular oxygen in the

presence of a catalytic amount of mixed addenda heteropolyoxometalates

containing molybdenum and vanadium The catalytic activity of the

Molybdovanadophosphate was found to be greatly enhanced by supporting

on charcoal The supported catalyst has high catalytic activity for oxidative

dehydrogenation of benzylic and allylic alcohols to the corresponding

aldehydes and ketones (46-92) nevertheless the nonsupported catalyst

was inactive for the same oxidations under these conditions 236Trimethyl

phenol was selectively oxidized to trimethyl-p benzoquinone which is

precursor of VitE in the presence of a catalytic amount of

molybdophosphate In addition the aerobic oxidation of amines alkyl-

substituted phenols and alkanes were also examined [58]

The reaction of Tl2CO3 with 111555-hexafluoro-24-pentanedione

and diglyme CH3O(CH2CH2O)2CH3 or tetraglyme CH3O(CH2CH2O)4CH3 in

dichloromethane yields the anhydrous thermally and air stable volatile Tl

diglyme and Tl tetraglyme adducts They have been characterized by single

crystal X-ray diffraction elemental analysis 1H and 13C NMR IR and mass

spectroscopy Thermal and mass-transport properties have been investigated

using thermo gravimetric and differential scanning calorimetric

measurements There is evidence that both precursors are very low melting

and volatile and can be used as liquid Tl sources Both adducts have been

successfully applied to metalndashorganic chemical vapor deposition of thallium

containing films [59]

Large size and high quality single crystals of quasi-two-dimensional

thallium molybdenum purple bronze TlMo6O17 have been grown by electrolytic

reduction of molten salt of Tl2CO3-MoO3 The crystal structure is trigonal with

space group P3m1 determined by X-ray diffraction and four-circle single crystal

diffraction The lattice parameters of the unit cell are a = b = 55282 Adeg and c

= 136991 Adeg The temperature dependence of resistivity and magnetic

19

susceptibility confirmed that a metal-to-metal transition occurs near 110 K

[60]

Hall coefficient and dc conductivity studies were made on p-type

Pb08Sn02Te thin films doped with different concentrations of thallium in the

temperature range 77 to 500 K The Hall coefficient and Hall mobility are

found to decrease with an increase in the doping concentration of thallium

Hall coefficient data have been analyzed in the light of a double valence-band

model Various band parameters such as valence band separation population

ratio mobility ratio and effective mass ratio have been calculated Hall

mobility data have been analyzed in the light of lattice and defect limited

scattering mechanisms [61]

Proton conducting composites of heteropolyacid hydrates phosphomol

ybdic acid H3PMo12O40 nH2O(PMA) phosphotungstic acid H3PW12O40 nH2O

( PTA) and salt hydrate like NiCl2 6H2O were prepared

with insulating Al2O3 as despersoidThe ionic conductivity peaks at two

concentrations of Al2O3 indicating two percolation thresholds for percolation

thresholds for proton conduction Two separate experiments were carried out

to check the existence of such percolation thresholds viz the volta battery

experiment involving the measurement of emf of an electrochemical cell

with composites of different compositions used as electrolyte and the

composition vs conductivity measured by the complex impedance

spectroscopy The presence of two maxima has been attributed to two

different percolation thresholds for the two possible mobile protonic

species H+ + (H3O+) and OH arising from the hydrates [62]

1 A5 Applications of Heteropolyoxometalates-

Applications of heteropolyanions centre depend on their redox properties

their high charges and ionic weights An enormous patent and journal

literature is devoted to the applications of heteropolyanions

1 Analysis-

The formation and subsequent precipitation or reduction of

[XMO12O40]n- anions form the basis of gravimetric and colorimetric analytical

methods for P As Si or Geeither separately or in combination [63 64]

20

2 Biochemical applications-

lsquoPhosphotungstic Acidsrsquo have for decades been used as precipitants

for proteins and as analytical reagents for proteinsalkaloidsand purines eg

the [P2M18O62]6- anions for colorimetric determination of uric acid [65] and

cholesterol The acid H3P12O40 either in aqueous or ethanolic solution is also

widely used as a non specific electron dense stain for electron spectroscopy

The dyestuffs industry has for many years used heteropolymolybdates and

tungstates to form color lakes and toners from basic dyes Large

heteropolyanions exhibit antiviral antitumoral properties at non-cytotoxic

doses in vitro and in vivo and are protein inhibitors of cellular bacterial and

viral DNA RNA polymerizes [66]

3 Catalysis

Heteropoly acids and salts have been used as heterogeneous catalysts

for a broad variety of reactions and compilations of such applications up to

1973 are available Examples include oxidation of propylene and isobutylene

to acrylic acid methacrylic acids and ammoxidation of acrylonitrile olefin

polymerization and epoxidation Much of current activity concerning

heterogeneous catalysis by heteropoly compounds is being carried out in

Japan [67 68]

4 Other Applications

Insoluble salts of Heteropolyanions especially ammonium

molybdophosphates have been used and are commercially available as ion-

exchange materials [69]Recent work in this area includes thin layer

chromatography of amino acids ion selective membranes [70] and the

preparation of new ion exchangers based on heteropolyanions Crystalline12-

tungstophosphoric and 12-molybdophosphoric acids are excellent protonic

conductors Heteropolyacids are electrochromic in the solid state as a

consequence of heteropolyblue formation Heteropolyblue formation has also

been used to detect alcohol or carboxylic acid radicals generated by radiolysis

of aqueous solutions Potential applications of heteropoly complexes as flame

retardants and smoke suppressants or as corrosion inhibitors and conversion

coatings on steel and aluminium are reported [71] Some potential ldquogreenrdquo

applications have been reported eg non-chlorine based wood pulp

bleaching process and a method of decontaminating water Some structures

21

containing transition metal atoms with unpaired electrons have unusual

magnetic properties and are being investigated as nano computer storage

devices Some compounds exhibit luminescence There are many reported

potential medicinal applications eg anti tumoral and anti-viral There have

been reports on the role of weak or non bonding interactions on the crystal

engineering of hybrid polyoxometalates

Spherical nonporous polyoxomolybdate based capsules of different

types containing more than 100 metal atoms reported by Achim Muller and his

group have versatile unique properties regarding their assembly to vesicles

and the chemistry which can be done inside the pores and cavities A discrete

polyoxometalate Lindquist ion of the form W6O192minus was successfully imaged

recently for the first time within the capillary of a carbon nanotube following

steric locking of the anion with the tubule In situ relaxation of the anion in its

equatorial plain was demonstrated [72]

22

Section-B

Litreture Survey on Chromism in Transition Metal

Oxides

1 B1 Chromism in Transition Metal Oxides

Chromism is a reversible change in a substances colour resulting from

a process caused by some form of stimulus Many materials are chromic

including inorganic and organic compounds and conducting polymers and the

property can result from many different mechanisms Several transition metal

oxides show EC properties The most popular are from the VI - B oxides In

this group WO3 and MoO3 are the most thoroughly studied cathodic EC

materials which can be electrochemically coloured and bleached when used

as the cathode in electrochemical cells Cathodic EC materials also include

V2O3 TiO2 and Nb2O5 Another distinguishable group is anodic EC material

including VIII oxides like IrOx nH2O Rh2O3 nH2O NiO nH2O etc which can be

anodicaly coloured in the electrochemical process when used as anode

There are several types of chromism which are discussed as below

B11 Photochromism

Photochromism is the reversible transformation of a chemical species

between two forms by the absorption of electromagnetic radiation where

the two forms have different absorption spectra [7374]

Trivially this can be described as a reversible change of color upon

exposure to light The phenomenon was discovered in the late 1880s

including work by Markwald who studied the reversible change of color of 23

44-tetrachloronaphthalen-1(4H)-one in the solid state He labeled this

phenomenon phototropy and this name was used until the 1950s

when Yehuda Hirshberg of the Weizmann Institute of Science in Israel

proposed the term photochromism [75] Photochromism can take place

in both organic and inorganic compounds and also has its place in biological

systems (for example retinal in the vision process)

Photochromism does not have a rigorous definition but is usually used

to describe compounds that undergo a reversible photochemical reaction

where an absorption band in the visible part of the electromagnetic spectrum

23

changes dramatically in strength or wavelength In many cases an

absorbance band is present in only one form The degree of change required

for a photochemical reaction to be dubbed photochromic is that which

appears dramatic by eye but in essence there is no dividing line between

photochromic reactions and other photochemistry Therefore while the

transcis isomerization of azobenzene is considered a photochromic reaction

the analogous reaction of stilbene is not Since photochromism is just a

special case of a photochemical reaction almost any photochemical reaction

type may be used to produce photochromism with appropriate molecular desi

gnSome of the most common processes involved in photochromism are peric

yclic reactions cis-trans somerizations intramolecular hydrogen transfer

intramolecular group transfers dissociation processes and electron transfers

(oxidation-reduction)

Another some what arbitrary requirement of photochromism is that

it requires the two states of the molecule to be thermally stable under

ambient conditions for a reasonable time All the same nitrospiropyran (which

back-isomerizes in the dark over ~10 minutes at room temperature) is

considered photochromic All photochromic molecules back-isomerize to their

more stable form at some rate and this back-isomerization is accelerated by

heating There is therefore a close relationship between photochromic and the

rmochromic compounds The timescale of thermal back-isomerization is

important for applications and may be molecularly engineered

Photochromic compounds considered to be thermally stable include some

diarylethenes which do not back isomerize even after heating at 800C for 3

months

Since photochromic chromophores are dyes and operate according to

well-known reactions their molecular engineering to fine-tune their properties

can be achieved relatively easily using known design models quantum

mechanics calculations and experimentation In particular the tuning of

absorbance bands to particular parts of the spectrum and the engineering

of thermal stability have received much attention

Sometimes and particularly in the dye industry the term irreversible

photochromic is used to describe materials that undergo a permanent color

change upon exposure to Ultraviolet or visible light radiation Because by

24

definition photochromics are reversible there is technically no such thing as a

n irreversible photochromic this is loose usage and these compounds

are better referred to as photochangable or photoreactive dyes

Apart from the qualities already mentioned several other properties of

photochromics are important for their use These include

Quantum yield of the photochemical reaction

This determined the efficiency of the photochromic change with respect

to the amount of light absorbed The quantum yield of isomerization

can be strongly dependent on conditions

Fatigue resistance In photochromic materials fatigue refers to the

loss of reversibility by processes such as photodegradation

photobleaching photooxidation and other side reactions All

photochromics suffer fatigue to some extent and its rate is strongly

dependent on the activating light and the conditions of the sample

Photostationary state Photochromic materials have two states and

their interconversion can be controlled using different wavelengths of

light Excitation with any given wavelength of light will result in a

mixture of the two states at a particular ratio called the photo-

stationary state In a perfect system there would exist wavelengths

that can be used to provide 10 and 01 ratios of the isomers

but in real systems this is not possible since the active

absorbance bands always overlap to some extent

Polarity and solubility In order to incorporate photochromics in

working systems they suffer the same issues as other dyes They are

often charged in one or more state leading to very high polarity and

possible large changes in polarity They also often contain large

conjugated systems that limit their solubility

Photochromic complexes

A photochromic complex is a kind of chemical compound that has

photoresponsive Parts on its ligand These complexes have a specific

structure photoswitchable organic compounds are attached to

metalcomplexes For the photocontrollable parts thermally and

photochemically stable chromophores (azobenzene diarylethene

25

spiropyran etc) are usually used And for the metal complexes a wide

variety of compounds that have various functions (redox response

luminescence magnetism etc) are applied The photochromic parts and

metal parts are so close that they can affect each others molecular

orbitals The physical properties of these compounds shown by parts of

them (ie chromophores or metals) thus can be controlled by switching

their other sites by external stimuli For example photoisomerization

behaviors of some complexes can be switched by oxidation and reduction of

their metal parts Some other compounds can be changed in their

luminescence behavior magnetic interaction of metal sites or stability of

metal-to-ligand coordination by photoisomerization of their photochromic

parts

Photochemistry of Polyoxometalates

The photochemistry of polyoxometalates is of great interest to inorganic

chemistsMore than 80 years agoit was found that the R-Keggin tungstate

H3[PW12O40] was reduced photochemically to yield a blue-colored species

which was reoxidized by air and by various other oxidizing agents such as

Fe3+AgNO3and H2O2 [7879]The photoredox reactions of H4[SiW12O40]and

H3[PW12O40] proceeded effectively in the presence of primary and secondary

alcohols their ethers and aldehydes and proteinsbut less effectively in the

presence of tertiary alcoholsketonesestersthe fatty acids above formic

acidand simple amines[8081] The basic photoredox reaction involving

ethanol is illustrated by eq 13

2 H3PW12O40 + H3CCH2OH h ν ν ν ν 2 H4PW12O40 + H3CCHO ------- 13

2 H4PW12O40 +12 O2 2 H3PW12O40 + H2O ------ 14

In this reactionone molecule of ethanol photochemically reduces two

molecules of H3PW12O40 and is itself oxidized to acetaldehyde In the

presence of air the thermal oxidation of the reduced species takes place at

room temperature(eq 14)The reduced polyoxometalates which are the so-

calledldquoheteropolybluesrdquo have been used for the colorimetric analysis of the

26

elements P Si As and Ge and for the determination of uric acidsugarand

other biological compounds [8283] Piperidinium metavanadate also

undergoes photoinduced coloration from white to black followed by a

reversible color change in the presence of oxidizing agentsHowever

ammonium metavanadates ([NH4][VO3]) exhibits no photoinduced

coloration[84] The early photoredox reactions of the R-Keggin

polyoxometalates H4[SiW12O40] and H3PW12O40 were carried out in the

presence of photographic paper however the limited number of the

structurally well-characterized compounds available for study delayed the

development of modern cluster-compound photochemistry until the discovery

of photochromism in alkylammonium polyoxo- molybdate solids[8586] A

photochromic or electrochromic material is one whose light-absorbing

properties are altered upon optical excitation or reduction under the influence

of an externally applied electric field respectively The induced coloration

remains even after the excitation source has been removed These materials

are of technological interest because they return to their original state either

thermally upon irradiation with light of a frequency corresponding to the

induced absorption or electrochemically upon reversing the polarity of the

externally applied electric field Thus photochromic and electrochromic

materials behave in a reversible manner Polyoxometalates exhibit significant

photo-and electrochromism which makes them suitable as nanocomposite

molecular devices and as models for probing the physical properties of infinite

metal oxides Since the metal ions in the oxidized polyoxometalates have d0

electronic configurations the only absorption band which occurs in the UV-vis

range of the electronic spectra is due to the oxygen-to-metal (O-M) ligand-to-

metal charge transfer (LMCT)Upon irradiation electrons are promoted from

the low-energy electronic states which are mainly comprised of oxygen 2p

orbitals (the valence band in the band model)to the high-energy electronic

states which are mainly comprised of metal d orbitalrsquos (the conduction band

in the band model)The fundamental transitions in polyoxometalate lattices

are depicted schematically in Fig1B1

27

Fig1B1 Simple model showing the electronic transitions in the

polyoxometalates containing electron donar and acceptor (a)generation

of charge carriers(b)electron and hole trapping(c)electron release due

to stimulation(d)recombination

between electron and hole Electrons are e-and holes are h+

In the polyoxometalates containing heteroatoms and especially in

mixed metal polyoxometalates the charge carriers which are created by the

light or electric field may be trapped in electron traps and hole traps These

traps provide states of localized energy in the O-M LMCT energy gap due to

the heteroatoms or counter cations which correspond to impurities or lattice

defects in the band model If the trap depth ∆E is large compared to kT the

probability for thermal escape from the trap will be negligibly small and

metastable situation will existThe trapped carriers can be released by thermal

or optical stimulationIn the case of thermal stimulation the irradiated

polyoxometalate is heated until the energy barrier ∆E can be overcome The

trapped electron (or hole) then can escape from the trap and nonradiatively

recombine with the trapped hole (or electron)Under optical stimulation the

energy of an incident photon is used to overcome ∆E The relaxation

processes of the OndashM LMCT excitation energy include both the nonradiative

recombination of electrons and holes within the energy gap and the

intramolecular energy transfer leading to a charge-transfer emission This

intramolecular energy transfer corresponds to the O-M LMCT energy gap and

occurs via radiative recombination and sensitized emission from the

28

heteroatoms or cations If several energy levels based on the hetero atoms or

counter cations act as energy acceptors within the O-M LMCTenergy gap the

energy transfer occurs from the O-M LMCT states to these levels followed by

the nonradiative or radiative deactivation of the excitation energy It should be

noted that the O-M LMCT states also can be generated by the application of

very high electric fields to the polyoxometalate solids as demonstrated by the

observation of electroluminescence[87]If an external electric field with a

potential more negative than the energy levels of the vacant orbitals involved

in the O-M LMCT transition is applied to a polyoxometalate on the electrode

surface an electrochemical reduction occurs via the injection of electrons

from the electrode in to the vacant levels of the polyoxometalate as shown in

Fig1B 2

Fig1B2 - Energy scheme for the electrochromism of polyoxometalates

a)electrochemical reduction (b) electrochemical oxidation

Electrons injected in to the high-energy levels also may be trapped by

electron traps in a process analogous to that which occurs during LMCT

photoexcitation of the polyoxometalates These electrons are returned to the

electrode by electrochemical oxidation at an electrode potential more positive

than the energy levels for the d1 electron states The d1electrons in the O-M

LMCTstates facilitate the absorption of visible light via intervalence charge

transfer among metal centers and d-d transitions The same type of transition

may be possible for the d1electron captured by the electron traps too In

addition to searching for new photosensitive polyoxometalates with the

29

potential for having practical application there is now a need to elucidate the

fundamental photo-and electrochemical coloration processes by studying

electron transfer within the polyoxometalate lattices in conjunction with their

crystal structures So far few polyoxometalates exhibit a perfect reversibility

of coloration The irreversibility of the color change arises from as yet

uncharacterized side reactions during both the coloration and decoloration of

the polyoxometalates

Many metal oxides including aluminum titanium vanadium niobium

molybdenum and tungsten oxides are photochromic when they contain

impurities or dopants This coloration has been interpreted on the basis of

electron trapping at appropriate lattice sites within the crystals as shown in

Fig1B1 where the O-M LMCT transition corresponds to the transition

between the valence and conduction bands for the infinite metal-oxide lattice

[88-91]

B12 Applications of Photochromic materials

Sunglasses

One of the most famous reversible photochromic applications is color

changing lenses for sunglasses as found in eyeglasses The largest limitation

in using PC technology is that the materials cannot be made stable enough

to withstand thousands of hours of outdoor exposure so long-term outdoor

applications are not appropriate at this time The switching speed of

photochromic dyes is highly sensitive to the rigidity of the environment around

the dye As result they switch most rapidly in solution and slowest in the rigid

environment like a polymer lens Recently it has been reported that attaching

flexible low Tg polymers (for example siloxanes or poly (butyl acrylate) to the

dyes allows them to switch much more rapidly in a rigid lens [76] Some

spirooxazines with siloxane polymers attached switch at near solution like

speeds even though they are in a rigid lens matrix

Supramolecular chemistry

Photochromic units have been employed extensively in supramolecular

chemistry Their ability to give a light controlled reversible shape change

means that they can be used to make or break molecular recognition motifs

30

or to cause a consequent shape change in their surroundings Thus

photochromic units have been demonstrated as components of molecular

switches The coupling of photochromic units to enzymes or enzyme cofactors

even provides the ability to reversibly turn enzymes on and off

by altering their shape or orientation in such a way that their functions

are either working or broken

Data storage

The possibility of using photochromic compounds for data storage was

first suggested in 1956 by Yehuda Hirshberg[77] Since that time there have

been many investigations by various academic and commercial groups

particularly in the area of 3D optical data storage which promises discs that

can hold a terabyte of data Initially issues with thermal back-reactions

and destructive reading dogged these studies but more recently more stable

systems have been developed

Novelty items

Reversible photochromics are also found in applications such as toys

cosmeticsclothing and industrial applications If necessary they can be made

to change between desired colors by combination with a permanent pigment

A large number of inorganic compounds exhibit photochromism

These solids often have large band gaps of the order of 3 - 12 eV and

excitation of these solids leads to the formation of metastable centers that

absorb visible light giving rise to their colour They can return to their ground

state by heating or by optical excitation within the colour-centre band In most

cases the photochromism is a structure sensitive phenomenon involving

localized defect impurities or dislocations Some of these inorganic

compounds have the potential for a number of different uses Photochromic

compounds have a number of useful applications These can be divided

according to the most important property that is being used (Table 11) [92]

31

Table 11 Applications of Photochromic materials

Applications Depending Upon

Sensitivity to

Radiation

Reversibility Thermal Chemical or

Physical Properties

Self-developing

photography

Chemical switches for

computers

Temperature indicators

Protective

materials

Data displays

Heat-sensitive recording

media

Optical signal

processing

Photomasking and

photoresist technology

Reusable data storage

media

Anaytical reagents

Photochromic

microimages

Photopolymerisation

Information encoding

and steganography

Photocontractile

polymers and the

photoviscosity effect

Control of light

intensity

Q-switches

Pyroelectric

photochromic materials

B13 Thermochromism

Thermochromism is the reversible colour change of a substance

induced by temperature change A large variety of substances organic

inorganic organometallic supramolecular and polymeric systems exhibit this

phenomenon Examples of these include bianthrones cobalt

hexacyanoferrate the zirconocene complex of 1 4-diphenyl-1 3-butadiene

and poly (3-alkylthiophene) The organic 99-bixanthenylidene is colourless at

90 K yellow-green at 298 K and dark-blue when melted at 592 K Heating

conducting polymers can cause them to change colour This is achieved by

causing conformational changes to the polymer backbone resulting in a

change in the band gap of the polymer It has been reported that regioregular

32

P3HT reversibly changes colour upon heating to 220ordmC due to temperature-

dependent conformation changes Thermally cross linked polymer undergoes

the same colour change but it is much less reversible [93] Other forms of

thermochromism may be commercially important eg to give a visual

indication of temperature changes

B14 Electrochromism

Electrochromism describes a phenomenon of material color change in

a persistent but reversible manner produced by electrochemically induced

oxidation-reduction reactions Electrochromic materials can be applied to

various kinds of products such as smart windows and display devices Among

those applications there have been lots of efforts to develop electrochromic

display devices (ECDs) Especially flexible display devices are now attracting

much attention worldwide since they can facilitate new technological demands

such as bending and folding of paper-like displays High electrochromic

efficiency short response time long operating life time and reduction of

energy consumption are the most important requisites of the materials for the

paperlike displays [94 95] Among those properties the operation life time is

the most important barrier to overcome for a realization of ECDs There are

two types of electrochromic material a) inorganic transition metal oxides

(TMOs) b) organic polymer materials The TMOs have been studied longer

than the organic materials that they have been studied since 1960s [9697]

Electrochromism describes a reversible color change of material

produced by electrochemically induced oxidation-reduction reactions It is one

of several types of chromism of materials As thermochromism and

photochromism mean material color changes made by heat and light

respectively electrochromism refers that the color change is caused by an

electric potential In most cases the color change in electrochromism can be

driven by rather low electrical potential of the order of a fraction of volt or a

few volts [94-96] The color change of material means variation in

transmittance andor reflectance change in visible range which is originated

from different electronic absorption bands according to a switching between

oxidation and reduction state of material When electric potential is applied on

electrochromic material forced oxidation or reduction is derived and the

33

individual color is originated from the corresponding oxidation or reduction

state of the material For electrochromic materials the characteristic color

change is reversible since the oxidation and reduction state can be converted

reversibly by switching potential

Application fields

The application area for the electrochromism is rather broad that it

covers from smart window glazing and optical modulators to information

displays [98-102] The smart windows are typical examples The

electrochromic property is used to control the amount of light and heat to pass

through the windows Usually the electrochromic material is in form of thin film

coated on a window glass The transmittance modulation has also been

applied at the automobiles to automatically tint rear-view mirrors in various

lighting conditions The electrochromic application fields are illustrated in Fig

1B3

Since the smart windows control the transmittance of heat as well as

the transmittance of visible light the working definition of electrochromism has

now been extended to include devices for modulation of radiation in the near

infrared thermal infrared and microwave regions When color for

electrochromic materials is used this can now mean a response by detectors

at these wavelengths and not just by the human eyes Nowadays

electrochromic material draws much attention as being used in the display

devices Electrochromic display device (ECD) is being considered as one of

the candidates for the conventional liquid crystal display (LCD) since ECD

has many advantages over LCDs Among these advantages the most

important are low energy consumption wider viewing angle high contrast

rate and possibility to achieve multiple colors with a single material [103]

When a new redox state of electrochromic materialis established by

the applied electric pulse then it is maintained after the potential is switched

off This means the colored or bleached state of the material can be sustained

for a considerable time without applying electrical power This is so-called

ldquocolor memory effectrdquo of electrochromic material Because of the color

memory effect energy consumption for the electrochromic display device

could be drastically reduced and this would be a big advantage over other

34

emissive devices The low power consumption is especially advantageous

when it is applied to mobile devices with limited power source The possibility

for a flexible display is another attraction for electrochromic material

Information displays

Real-view mirrors for automobiles

Fig 1B4 Application fields of electrochromic devices Smart windows information displays and real-view mirrors for automobiles

35

Using polymeric electrochromic material and plastic substrate with a

transparent conductive coating it is possible to build all-plastic flexible display

device There are lots of papers and patents about the flexible display devices

[104105] In these cases gel-type electrolyte is also needed The gel-type

electrolyte immobilizes liquid electrolyte in the polymer network [106107]

Recently ITO-coated polymer films are readily available which would provide

the plastic substrate for a flexible device The flexible electrochromic displays

are frequently tried with the plastic substrate flexible electrochromic material

and gel-type electrolyte The flexible electrochromic displays would facilitate

increasing technical demands for foldable display devices

Metal Ion Electrochromism

Many transition metal oxides are capable of redox reactions that result in

colour change Metal oxide films are commonly prepared as thin layers of

either tungsten nickel molybdenum or other metal compounds by a number

of techniques These include sol-gel electrochemical by dc or rfreactive

sputtering techniques electron-beam evaporation by anodic or cathodic

electrodeposition or by solution dipping of the electrochromic metal

compounds (or compounds that can be changed into these metal compounds)

onto optically transparent electrodes (OTE) [108 -114] Their electrochromism

is derived from the colour change associated with a change in the oxidation

state of the metal anion The behaviour of these materials is dependent upon

pH moisture and exposure to the atmosphere [115] Generally the switching

rates of these films is somewhat slow with typical switching times of about 15

- 60 seconds to achieve 100 conversion to either coloured or bleached state

[116 -120] An Example of this includes nickel oxide which changes from

transparent (pale green) to brownblack taking about 30 seconds to do so

[121] Other examples include [(NH4)5Ru]2(pyrazine)5+ and [(NH4)5Ru]2(44-

bipyridine)5+ whose electrochromism is significantly different due to the effect

of the ligand [122] Table 12 below gives some examples of metal oxide films

with electrochromic properties

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

4

electron transfer or ion transport behavior In all these respects polyoxo-

metalates can be generically considered as the perfect models for quantum-

sized transition metal oxide nanoparticals [12 13] For example

the electrochemical or photochemical injection of electrons in heteropolyions

(HPA) with the induction of thermally activated delocalization between metal

centers and IVCT (InterValence Charge Transfer Bands) leading to change

in color closely parallel the corresponding electrochromic properties of the

corresponding oxides upon doping

Contrary to ever-smaller nanostructures and quantum dots designed

bymeans of physical methods following a top-down approach

polyoxometalates represents a very significant example of the bottom-up

pproach Chemists use to build polynuclear and supramolecular structures

with collective properties The control of size and structure in

polyoxometalates is based on now well known acid hydrolysis and

condensation reactions driven by the very rich acid base chemistry of

some transition metal cations primarily W Mo and VBut this framework

chemistry of isopolytungstates molybdates and vanadates is remarkably

broadened up when other elements come to add richness through structural

and chemical multiplicity within the field of heteropolyanions

In addition the remarkable stability of many of these clusters makes

possible an extensive redox chemistry leading to wealth of ldquobluesrdquo reduced

species where thermally activated delocalization of electrons and a variety

of spin states make for a remarkable landscape of electronic and magnetic

states in small clusters that become in this way ideal models for the study of

spin interactions [14 -18]

Very recently this rich oxygen-metal chemistry has been broadened

with specific and unique examples that include for the first time anions such

as S-2 or MeO- [1920] or iron ions as the framework-building metals The

control of the extent of condensation and the isolation of new larger clusters

formed through a building-block approach making use of smaller fragments

to assemble larger units have also been a tremendous source of development

in very recent years especially for vanadium species[2122]

At this point the field of polyoxometalate chemistry has

broadened so much in variety of elements and structures properties and

5

applications that the reference books in the field would certainly welcome

substantial additions and revision

The intrinsic properties of polyoxometalates are of interest in themselves

not only from a fundamental point of view but also to make of them

materials of interest in various applications Beyond their traditional interest as

catalysts polyoxometalates constitutes base materials for electrochromics

energy storage and conversion devices (batteries super capacitors and

fuel cells) sensors or biomedical applications Many of the applications of

polyoxometalate clusters as materials require their use in the form of

membranes or electrodes that is in the form of solid insoluble material or

coatings There is therefore a main strategic line of work that has centered

on the inclusion or integration of polyoxometalates in all sorts of substrates

polymeric inorganic or mineraland their combination with surfactants or

organic carriers

Heteropolyanions are negatively-charged clusters of corner-sharing and

edge shairing early transition metal MO6 octahedra and heteroatom

XO4 tetrahedra where the tetrahedra are usually located the interior

of the cluster[23] The geometry composition and charge of these clusters

are varied through synthesis parameters and cluster properties are

highly tunable as a function of these characteristics Heteropolyanions have

been employed in a range of applications that include virus-binding

inorganic drugs [24] homogenious and heterogeneous catalysts [25 26]

electro-optic and electrochromic materials [27 28] metal and protein binding

[29] and as building blocks for nanostructuring of materials [30] The α-Keggin

geometry which was first structurally characterized in 1933 by JF Keggin

[31] the phosphotungstic acid (H3PW12O40) is one of the most

widely recognized and thoroughly studied heteropolyanion geometries[32]

1A2 Fundamental concept of polyoxometalate structures-

Keggin structure is the best known structural form for heteropoly acids It

is the structural form of αndashKeggin anions which have a general formula of

[XM12O40]n- where X is the hetero atom (most commonly are P5+ Si4+ or B3+)

M is the addenda atom (most common are molybdenum and tungsten) and O

6

represents oxygen[33] The structure self assembles in acidic aqueous

solution and is the most stable structure of polyoxometalate catalysts

Fig 1A1 Keggin structure

The first α-Keggin anion ammonium phosphomolybdate

((NH4)3[PMo12O40]) was first reported by Berzelius in 1826 In 1892

Blomstrand proposed the structure of phosphomolybdic acid and other poly-

acids as a chain or ring configuration Alfred Werner using the coordination

compounds ideas of Copaux attempted to explain the structure of

silicotungstic acid He assumed a central group [SiO4]4- ion enclosed by four

[RW2O6]+ where R is a unipositive ion The [RW2O6]

+ are linked to the central

group by primary valences Two more R2W2O7 groups were linked to the

central group by secondary valences This proposal accounted for the

characteristics of most poly-acids but not all

In 1928 Linus Pauling proposed a structure for α-Keggin anions

consisting of a tetrahedral central ion [XO4] n-8 caged by twelve WO6

octahedral In this proposed structure three of the oxygen on each of the

octahedral shared electrons with three neighboring octahedral As a result 18

oxygen atoms were used as bridging atoms between the metal atoms The

remaining oxygen atoms bonded to a proton This structure explained many

characteristics that were observed such as basicities of alkali metal salts and

the hydrated of some of the salts However the structure could not explain the

structure of dehydrated acids

JF Keggin with the use of X-ray diffraction experimentally determined

the structure of α-Keggin anions in 1934 The Keggin structure accounts for

both the hydrated and dehydrated α-Keggin anions without a need for

7

significant structural change The Keggin structure is the widely accepted

structure for the α-Keggin anions [34] For example the α-Keggin anion of

phosphotungustic acid is shown in Fig1A2

The structure is composed of one heteroatm surrounded by four

oxygen to form a tetrahedronThe heteroatom is located centrally and caged

by 12 octahedral WO3 units linked to one another by the neighboring oxygen

atoms There are a total of 24 bridging oxygen atoms that link the 12 addenda

atoms The metal centres in the 12 octahedra are arranged on a sphere

almost equidistant from each other in four M3O13 units giving the complete

structure an overall tetrahedral symmetry The bond length between atoms

varies depending on the heteroatom (X) and the addenda atoms (M)

Fig 1A2 α-Keggin anion of phosphotungustic acid ( PW12O40 ) 3-

For the 12ndashphosphotungstic acid Keggin determined the bond length

between the heteroatom and each the four central oxygen atoms to be 15 Adeg

The bond length form the central oxygen to the addenda atoms is 243 Adeg

The bond length between the addenda atoms and each of the bridging

oxygen is 19 Adeg The remaining 12 oxygen atoms that are each double

bonded to an addenda atom have a bond length of 170 Adeg The octahedra

are therefore distorted This structure allows the molecule to hydrate and

dehydrate without significant structural changes and the molecule is thermally

stable in the solid state for use in vapor phase reactions at high temperatures

(400-500 degC)[35-36]

8

Including the original Keggin structure there are 5 isomers designated

by the prefixes α- β- γ- δ- and ε- The original Keggin structure is designated

α- These isomers are sometimes termed Baker Baker-Figgis or rotational

isomers [37]These involve different rotational orientations of the Mo3O13 units

which lowers the symmetry of the overall structure Lacunary Keggin

structures

The term lacunary is applied to ions which have a fragment missing

sometimes called defect structures Examples are the (XM11O39)nminus and

(XM9O34)nminus formed by the removal from the Keggin structure of sufficient Mo

and O atoms to eliminate 1 or 3 adjacent MO6 octahedra The Dawson

structure X2M18O62nminus is made up of two Keggin lacunary fragments with 3

missing octahedra Some structural types are found in many different

compounds The first known example of this was the Keggin ion whose

structure was found to be common to both molybdates and tungstates with

different central hetero atoms Examples of some fundamental

polyoxometalate structures are shown below The Lindquist ion (Fig1A4) is

an iso-polyoxometalate the other three are hetero-polyoxometalates The

Keggin and Dawson structures (Fig1A3 and (Fig1A5) have tetrahedrally

coordinated hetero-atoms eg P or Si Anderson structure has an octahedral

central atom egAl

Fig 1A3 Keggin structure XM12O40nminus

9

Fig 1A 4 Lindquist structure M6O19nminus

Fig 1A 5 Dawson structure X2M18O62nminus

In general α-Keggin anions are synthesized in acidic solutions For

example 12-Phosphotungstic acid is formed by condensing phosphate ion

10

with tungstate ions The heteropolyacid that is formed has the Keggin

structure

[PO4]3- + 12 [WO4]

2- + 27 H+ rarr H3PW12O40 + 12 H2O ----------- 11

α-Keggin anions have been used as catalyst in hydration polymerization and

oxidation reaction as catalysts

The metal atoms that make up the framework (termed addenda atoms)

are typically Mo W and V When more than one element is present the

cluster is called a mixed addendaclusterThe ligands coordinated to metal

atoms that together form the bridged framework are usually oxide ions but

other elements such as S and Br have been substituted for some of the oxide

ions (Note that sulfur substituted POM is often termed a

polyoxothiometalates) Another development is the use of other ligands eg

nitrosy and alkoxy to replace oxide ions The typical framework building

blocks are polyhedral units with 4 5 6 or 7 coordinate metal centers These

units usually share edges andor vertices The most common unit for

polymolybdates is the octahedral MoO6 unit which is a distorted octahedron

where the Mo atom moves off centre to give one short Mo-O bond In some

polymolybdates there are pentagonal bipyramidal units and these are key

building blocks in the molybdenum bluesHetero atoms are present in many

polyoxometalates Many different elements can act as hetero-atoms

Examples of various coordination numbers around the hetero-atom are

known

4 co-ordinate (tetrahedral) in Keggin Dawson and Lindquist structures

(eg PO4 SiO4 AsO4)

6 co-ordinate (octahedral) in Anderson structure (eg Al(OH)6 TeO6

8 co-ordinate (square antiprism) in ((CeO8)W10O28)8minus

12 co-ordinate (icosahedral) in (UO12)Mo12O30 8minus

11

Often the hetero-atom is centrally located in the anion (eg Keggin

structure) or in a structure fragment eg the 2 phosphorus atoms in the

Dawson ion are central to the two symmetric fragments There are similarities

to clathrate structures The Keggin ion can be formulated as PO4 2minusand

M12O36 and the Dawson as (XO42-)2 and M18O54Structural isomerism is

common For example the Keggin structure has 5 isomers which can be

considered to contain one or more of the four M3O13 units being rotated

through 60degMany compounds share the same framework architectures or

frameworks derived from a larger framework with one or more addenda atoms

and oxide ions removed to give defect structure usually called a lacunary

structure An example of a compound with a Dawson lacunary structure is

As2W15O56Some cage structures containing ions are known eg an example

is the vanadate cage V18O42 containing a Clminus ion [38] This has 5 co-

ordinates square pyramidal vanadium units linked together

Fig 1A 6 - H4V18O42 cage containing Cl

1A3 General Properties of polyoxometalates -

Typically polyoxoanions are water and air stable species of large size

(6-25 Adeg) and high ionic weight In aqueous solution they are subject to

decomposition by hydroxide ions eg

[PW12O40]3- + 23 OH- HPO4

2- + 12WO42- + 11 H2O -------- 12

12

Although the PH at which such reactions are rapid can very widely

depending upon the polyanion involved Polyanions are often much stable

towards the H3O+ ions and numerous crystalline heteropoly acids are known

Such acids may be extremely soluble in water and polar solvents (giving

solutions with densities in excess of 4 gcm3) and have large dissociation

constants (PK lt 0)

Crystalline heteropoly acids and salts are frequently highly hydrated with

up to 50 molecules of water per anion Much of this water is zeolite in nature

and crystal composition can vary accordingly On the other hand the cation

anion stiochiometry is always well defined as the anion structure and

composition Finally many polyanions are powerful oxidizing agents and

undergo multiple reversible one or two electron reductions leading to intensely

colored mixed valence species known as heteropoly blues Polyanions are

known which can accept as many as 32 electrons without major structural

change

1A4 Chemistry of Molybdenum and Tungusten Heteropolyoxometalates

A photochromic monolayer film of phosphomolybdic acid (denoted as

PMo12) was fabricated by self-assembly approach UV-visible spectrum and

AFM observation show that the monolayer film is composed of aggregated

PMo12 molecules The monolayer film shows good photochromic properties

with enough stability and reversibility The colour change of the monolayer

after UV-irradiation can be captured by a microscope equipped with a color

CCD camera Photochromic response of the monolayer film can be doubled

after being modified by an amine monolayer [39]

Series of vanadium substituted molybdo Keggin HPA with 12 or 3

adjacent vanadium atoms were prepared These materials were supported

on carbon cloth electrodes and hot pressed onto Nafion with an ETEK

electrode used as a standard on the opposite side The MEArsquos were run at

temperatureslt100 oC with the HPA electrode as either the anode or the

cathode Stable polarisation curves are obtained for an HPA based cathode

with reasonable current densities at 80 oC [40]

A novel mixed-valence polyoxomolybdenum anion was synthesized

hydrothermally from molybdenum oxidemolybdenum metalboric and

phosphoric acids12-phenyldiphosphonicacidand imidazole (ImH) and was

13

structurally characterized as an imidazolium saltOne-and two-dimensional

structures of this anion and additional molybdenum diphosphonate linkers

were assembled as wellThey were structurally characterized as their

pyridinium(pyH) salts [41]

Phosphomolybdic acidpolyvinylpyrrolidone hybrid films were found to

show visible light photochromism It is identified that the intra-supramolecular

charge transfer between the inorganic and organic molecules is responsible

for the visible-light coloration Interestingly the films show photo-memory and

thermal activation The films show a small change in absorbance after being

irradiated with visible light for a short time and the coloration can be

enhanced greatly by subsequent thermal treatment Electrical measurements

indicate that the conductivity of the film increases after the brief irradiation

which promotes transfer of the electrons induced by the thermal treatment

[42]

In this work major effort was concentrated on passive thermal control

coatings based on photochromic and thermochromic materials The inorganic

photochromic materials were based on tungsten and molybdenum oxide films

and the organic photochromic materials included spiropyrans and

spirooxazines In addition photochromic composite organic-inorganic films

and thermochromic vanadium oxide films were prepared The samples were

synthesized using sputtering sol-gel process and thermal oxidation [43]

Polyoxometalates a class of oxidatively robust inorganic oxidants and

oxidation catalysts are currently under investigation at the Forest Products

Laboratory and at Emory University as an alternative to chlorinebased

chemicals in the bleaching of soft Woodkraft and other pulps Although

polyoxometalate salts are used in a number of industrial processes the

feasibility of using these salts and oxygen in the commercial bleaching of

chemical pulps was only recently demonstratedA clear advantage of

polyoxometalates over oxygen alone hydrogen peroxide or ozone is their

inherently high selectivity for the residual lignin in softwood kraft pulps The

goal of ongoing research is to develop a highly selective energy efficient

oxygen based polyoxometalate delignification and bleaching technology

compatible with mill closure [44]

14

Tungsten oxide exhibits pronounced photochromism upon bandgap

photoexcitation which makes it attractive and promising for applications in

many areas Some advances have been achieved during the past decades

The research on nanocrystalline films and single crystals indicates the critical

importance of defects in tungsten oxide to its photochromism Based on

energy-band engineering of semiconductors enhancement of photochromism

has been achieved for instance extension of the photoresponse from UV to

visible light by cathodic polarization improved change in absorption before

and after coloration through modification by a noble metal or another metal-

oxide semiconductor and increased photochromic reversibility via

hybridization with organic amines Nanocrystalline oxide films exhibit

controllable wettability which is coherent in nature with photochromism [45]

Polyoxometalates represent a diverse range of molecular clusters with

an almost unmatched range of physical properties and the ability to form

structures that can bridge several length scalesThe new building block

principles that have been discovered are beginning to allow the design of

complex clusters with desired properties and structures and several structural

types and novel physical properties are examinedIn this critical review the

synthetic and design approaches to the many polyoxometalate cluster types

are presented encompassing all the sub-types of polyoxometalates including

isopolyoxometa- lates heteropolyoxometalates and reduced molybdenum

blue systems As well as the fundamental structure and bonding aspectsthe

final section is devoted to discussing these clusters in the context of

contemporary and emerging interdisciplinary interests from areas as diverse

as antiviral agentsbiological ion transport modelsand materialsscience [46]

Keggin type molybdovanadophosphoric heteropoly acids were prepare

d by a novel environmentally benign method and their catalytic performances

were evaluated via hydroxylation of benzene to phenol with hydrogen

peroxide as oxidant in a mixed solvent of glacial acetic acid and acetonitrile

Various reaction parameters such as reaction time reaction temperature

ratio of benzene to hydrogen peroxide concentration of aqueous hydrogen

peroxide ratio of glacial acetic acid to acetonitrile in solvent and catalyst

concentration were changed to obtain an optimal reaction conditions

15

Molybdovanadophosphoric heteropoly acids are revealed to be highly

efficient catalyst for hydroxylation of benzene [47]

Thin films from the system (As2S3)Tl were deposited by thermal

evaporation on Si graphite and optical glass substratesFrom transmission

and reflection measurements of the thin films the refractive index (n) film

thickness(d) optical band gap(Eg) optical oscillator energy(Eo) and

dispersion energy(Ed) before and after exposure to light were determined

The results for optical parameters were analyzed using the Wemple - Di

Domenico single oscillator model and Lorenc-Lorenc equation It was found

that Eg decreases while n E0 and Ed increase for as deposited films

decreases while n E0 and Ed increase for as deposited films with increasing

of Tlconcentration passing through a maximum at 6 at of Tl After exposure

to light n E0 Ed increase and Eg decreases for all compositions

investigated The maximum change in n (Dn = 016 at l = 6328 nm) was

observed for thin As38S56Tl6 films From infrared spectroscopy measurements

of bulk glasses and thin films we could conclude that when up to 6 at of

thallium is introduced As-S-As chains break and a ternary TlAsS2

compound appears at 10 at Tl [48]

The organo - inorganic hybrid materialconsisting of Poly (34 Ethylene

Dioxythiophene) (PEDOT) doped with phosphomolybdate cluster anions

[PMo12O40]3-has been synthesized by direct insitu oxidative polymerization of

34-Ethylene Dioxythiophene (EDOT) with phosphomolybdic acid

(H3PMo12O40) Its characterization is investigated by Fourier Transform

Infrared Spectroscopy (FT-IR) and Scanning Electron Microscopy (SEM) The

hybrid material presents predominantly high electronic conductivities of

around 20 and 70 S cm1at 300 and 400 K respectively [49]

Heteropolyacids (HPAs) are known to be excellent re-dox catalysts In

combination with TiO2 HPAs can be used as photocatalysts active in visible

light The HPA accepts electron and get reduced to heteropolyblue (HPB)

That can absorb light in the visible range HPA can be incorporated onto the

external surface or in the pores of zeolite based composite photocatalysts

have been designed by incorporation of HPA semiconductor TiO2 and

transition metal cobalt on zeolite This composite metallozeolite photocatalyst

is efficient in photoreduction of methyl orange (MO) in visible light to the tune

16

of 411 mg of MO photo reducedg TiO2 This catalyst also shows encouraging

results for hydrogen evolution from water to the tune of 2730 micromolhg TiO

[50]

A new class of proton conducting glass membranes based on heteropo

lyacids such as phosphotungstic acid (PWA) as electrolytes for low

temperature H2O2 fuel cells was investigated Parameters for a single fuel cell

with a catalyst electrode of 015 mgcm2 of PtC and a glass composite

membrane were characterized by electrochemical measurements at open

circuit potential conditions The performance of the membrane electrode

assemblies (MEA) was systematically studied as an effect of SiO2 and P2O5

concentrations in the glass composite membrane and the MEA was found to

exhibit a maximum power density of 162 mW cm2 for an H2O2 fuel cell at

30 degC and 30 relative humidity (RH) [51]

Two new photochromic inorganic-organic hybrid materials formed from

Keggin type Polyoxometalates (POMs) and metronidazole (C6H9N3O3 MNZ)

formulated as H3PMo12O40bull3 MNZ3H2O (1) and H3PW12O40bull3MNZ3H2O(2)

were synthesized and characterized by elemental analysis IR spectra

electronic spectra electron spin resonance (ESR) spectra and TG-DTA

Reflectance spectra show the presence of weak inter molecular charge

transfer between the organic and inorganic moieties in the solid state The

photochromic properties were studied by solid diffuse reflectance spectra and

ESR spectra and the photochromic reactions were found to exhibit first-order

kinetics TG-DTA showed that two hybrid materials have similar thermal

behavior [52]

Heteropolycompounds (HPCs) have been a matter of interest in basic

and applied science for more than a century From their first synthesis many

advances have been made to promote the use of HPCs in different ways in

science and technology The aim of this article is to review the main structural

characteristics of heteropolycompounds of the Keggin type (12

tungstophosphoric12-molybdophosphoric12-tungstosilicic acid alkaline and

alkaline earth salts of12 tungstophosphoric acid and gels doped with HPCs)to

understand and explain their different activities such as high proton

conductivity and catalytic biochemical and biomedical activities [53]

17

A solid hybrid molecular material containing 1-butyl 3-methyl

imidazolium cations and Keggin anions of phosphotungstic acid has been

synthesized It is fully characterized by CHN analysis FTIR XRD UV-Vis-NIR

DRS 31P MAS NMR TGA and SEM The FTIR spectrum of the compound

shows the fingerprint vibrational bands of both Keggin molecular anions and

imidazolium cations The aromatic CndashH stretch region (2700ndash3250 cmndash1) of

imidazolium cation is split due to the interaction between the ring CndashH and

bulky Keggin anion The red-shift in the UV-Vis spectra and the downfield 31P

MAS NMR chemical shift also confirm the electrostatic interaction between

the ions in the compound Near IR spectral region (1000ndash2500 nm) shows the

elimination of water in the compound which is hydrophobic [54]

Transport coefficient measurements (electrical conductivity

thermoelectric power and Hall coefficient) have been performed on a

compact Tl033MoO3 polycrystalline compound in a wide temperature range

(200ndash400 K) Experimental results are interpreted with the help of a p-type

semiconductor model with two inverted deep levels near the midgap The

valence band and the conduction band are assumed to be formed from the

dxy orbitals of molybdenum atoms in the Mo6O22 cluster leading to narrow π-

bonding bands The donor and acceptor levels may be formed from

nonbonding dxy orbitals arising respectively from anionic and Tl+ defects

Electron paramagnetic resonance and magnetic measurements are in good

agreement with the theoretical band semiconductor model which has been

retained [55]

The optical properties of GaInTlAs epilayers grown at low temperature

~230degC by solid-source molecular-beam epitaxy on InP substrates were

characterized using optical absorptionand photoluminescence techniques

Optical absorption measurements a room temperature show a gap shrinkage

toward lower energies from 071 to 061 and 053 eV when the Tl content

increases from 0 to 4and 8in good agreement with theoretical

predictionsLow-temperature photoluminescence band-gap signals from

GaInAs and GaInTlAs layers are only obtained after rapid thermal annealing

performed inorder to improve the electronic quality of the layersA band gap

decrease as much as 41 meV for GaInTlAs with 19 Tl incorporation is

measured by photoluminescence at 8K [56]

18

The electrochromic performance of all solid ndash state cells employing

phosphotungstic acid and phosphomolybdic acid is reported These cells

employ SnO2 as the viewing electrode and graphite as the back electrodeThe

cells in the bleached state can be made white to red and become black in the

coloured state [57]

Various organic compounds were oxidized by molecular oxygen in the

presence of a catalytic amount of mixed addenda heteropolyoxometalates

containing molybdenum and vanadium The catalytic activity of the

Molybdovanadophosphate was found to be greatly enhanced by supporting

on charcoal The supported catalyst has high catalytic activity for oxidative

dehydrogenation of benzylic and allylic alcohols to the corresponding

aldehydes and ketones (46-92) nevertheless the nonsupported catalyst

was inactive for the same oxidations under these conditions 236Trimethyl

phenol was selectively oxidized to trimethyl-p benzoquinone which is

precursor of VitE in the presence of a catalytic amount of

molybdophosphate In addition the aerobic oxidation of amines alkyl-

substituted phenols and alkanes were also examined [58]

The reaction of Tl2CO3 with 111555-hexafluoro-24-pentanedione

and diglyme CH3O(CH2CH2O)2CH3 or tetraglyme CH3O(CH2CH2O)4CH3 in

dichloromethane yields the anhydrous thermally and air stable volatile Tl

diglyme and Tl tetraglyme adducts They have been characterized by single

crystal X-ray diffraction elemental analysis 1H and 13C NMR IR and mass

spectroscopy Thermal and mass-transport properties have been investigated

using thermo gravimetric and differential scanning calorimetric

measurements There is evidence that both precursors are very low melting

and volatile and can be used as liquid Tl sources Both adducts have been

successfully applied to metalndashorganic chemical vapor deposition of thallium

containing films [59]

Large size and high quality single crystals of quasi-two-dimensional

thallium molybdenum purple bronze TlMo6O17 have been grown by electrolytic

reduction of molten salt of Tl2CO3-MoO3 The crystal structure is trigonal with

space group P3m1 determined by X-ray diffraction and four-circle single crystal

diffraction The lattice parameters of the unit cell are a = b = 55282 Adeg and c

= 136991 Adeg The temperature dependence of resistivity and magnetic

19

susceptibility confirmed that a metal-to-metal transition occurs near 110 K

[60]

Hall coefficient and dc conductivity studies were made on p-type

Pb08Sn02Te thin films doped with different concentrations of thallium in the

temperature range 77 to 500 K The Hall coefficient and Hall mobility are

found to decrease with an increase in the doping concentration of thallium

Hall coefficient data have been analyzed in the light of a double valence-band

model Various band parameters such as valence band separation population

ratio mobility ratio and effective mass ratio have been calculated Hall

mobility data have been analyzed in the light of lattice and defect limited

scattering mechanisms [61]

Proton conducting composites of heteropolyacid hydrates phosphomol

ybdic acid H3PMo12O40 nH2O(PMA) phosphotungstic acid H3PW12O40 nH2O

( PTA) and salt hydrate like NiCl2 6H2O were prepared

with insulating Al2O3 as despersoidThe ionic conductivity peaks at two

concentrations of Al2O3 indicating two percolation thresholds for percolation

thresholds for proton conduction Two separate experiments were carried out

to check the existence of such percolation thresholds viz the volta battery

experiment involving the measurement of emf of an electrochemical cell

with composites of different compositions used as electrolyte and the

composition vs conductivity measured by the complex impedance

spectroscopy The presence of two maxima has been attributed to two

different percolation thresholds for the two possible mobile protonic

species H+ + (H3O+) and OH arising from the hydrates [62]

1 A5 Applications of Heteropolyoxometalates-

Applications of heteropolyanions centre depend on their redox properties

their high charges and ionic weights An enormous patent and journal

literature is devoted to the applications of heteropolyanions

1 Analysis-

The formation and subsequent precipitation or reduction of

[XMO12O40]n- anions form the basis of gravimetric and colorimetric analytical

methods for P As Si or Geeither separately or in combination [63 64]

20

2 Biochemical applications-

lsquoPhosphotungstic Acidsrsquo have for decades been used as precipitants

for proteins and as analytical reagents for proteinsalkaloidsand purines eg

the [P2M18O62]6- anions for colorimetric determination of uric acid [65] and

cholesterol The acid H3P12O40 either in aqueous or ethanolic solution is also

widely used as a non specific electron dense stain for electron spectroscopy

The dyestuffs industry has for many years used heteropolymolybdates and

tungstates to form color lakes and toners from basic dyes Large

heteropolyanions exhibit antiviral antitumoral properties at non-cytotoxic

doses in vitro and in vivo and are protein inhibitors of cellular bacterial and

viral DNA RNA polymerizes [66]

3 Catalysis

Heteropoly acids and salts have been used as heterogeneous catalysts

for a broad variety of reactions and compilations of such applications up to

1973 are available Examples include oxidation of propylene and isobutylene

to acrylic acid methacrylic acids and ammoxidation of acrylonitrile olefin

polymerization and epoxidation Much of current activity concerning

heterogeneous catalysis by heteropoly compounds is being carried out in

Japan [67 68]

4 Other Applications

Insoluble salts of Heteropolyanions especially ammonium

molybdophosphates have been used and are commercially available as ion-

exchange materials [69]Recent work in this area includes thin layer

chromatography of amino acids ion selective membranes [70] and the

preparation of new ion exchangers based on heteropolyanions Crystalline12-

tungstophosphoric and 12-molybdophosphoric acids are excellent protonic

conductors Heteropolyacids are electrochromic in the solid state as a

consequence of heteropolyblue formation Heteropolyblue formation has also

been used to detect alcohol or carboxylic acid radicals generated by radiolysis

of aqueous solutions Potential applications of heteropoly complexes as flame

retardants and smoke suppressants or as corrosion inhibitors and conversion

coatings on steel and aluminium are reported [71] Some potential ldquogreenrdquo

applications have been reported eg non-chlorine based wood pulp

bleaching process and a method of decontaminating water Some structures

21

containing transition metal atoms with unpaired electrons have unusual

magnetic properties and are being investigated as nano computer storage

devices Some compounds exhibit luminescence There are many reported

potential medicinal applications eg anti tumoral and anti-viral There have

been reports on the role of weak or non bonding interactions on the crystal

engineering of hybrid polyoxometalates

Spherical nonporous polyoxomolybdate based capsules of different

types containing more than 100 metal atoms reported by Achim Muller and his

group have versatile unique properties regarding their assembly to vesicles

and the chemistry which can be done inside the pores and cavities A discrete

polyoxometalate Lindquist ion of the form W6O192minus was successfully imaged

recently for the first time within the capillary of a carbon nanotube following

steric locking of the anion with the tubule In situ relaxation of the anion in its

equatorial plain was demonstrated [72]

22

Section-B

Litreture Survey on Chromism in Transition Metal

Oxides

1 B1 Chromism in Transition Metal Oxides

Chromism is a reversible change in a substances colour resulting from

a process caused by some form of stimulus Many materials are chromic

including inorganic and organic compounds and conducting polymers and the

property can result from many different mechanisms Several transition metal

oxides show EC properties The most popular are from the VI - B oxides In

this group WO3 and MoO3 are the most thoroughly studied cathodic EC

materials which can be electrochemically coloured and bleached when used

as the cathode in electrochemical cells Cathodic EC materials also include

V2O3 TiO2 and Nb2O5 Another distinguishable group is anodic EC material

including VIII oxides like IrOx nH2O Rh2O3 nH2O NiO nH2O etc which can be

anodicaly coloured in the electrochemical process when used as anode

There are several types of chromism which are discussed as below

B11 Photochromism

Photochromism is the reversible transformation of a chemical species

between two forms by the absorption of electromagnetic radiation where

the two forms have different absorption spectra [7374]

Trivially this can be described as a reversible change of color upon

exposure to light The phenomenon was discovered in the late 1880s

including work by Markwald who studied the reversible change of color of 23

44-tetrachloronaphthalen-1(4H)-one in the solid state He labeled this

phenomenon phototropy and this name was used until the 1950s

when Yehuda Hirshberg of the Weizmann Institute of Science in Israel

proposed the term photochromism [75] Photochromism can take place

in both organic and inorganic compounds and also has its place in biological

systems (for example retinal in the vision process)

Photochromism does not have a rigorous definition but is usually used

to describe compounds that undergo a reversible photochemical reaction

where an absorption band in the visible part of the electromagnetic spectrum

23

changes dramatically in strength or wavelength In many cases an

absorbance band is present in only one form The degree of change required

for a photochemical reaction to be dubbed photochromic is that which

appears dramatic by eye but in essence there is no dividing line between

photochromic reactions and other photochemistry Therefore while the

transcis isomerization of azobenzene is considered a photochromic reaction

the analogous reaction of stilbene is not Since photochromism is just a

special case of a photochemical reaction almost any photochemical reaction

type may be used to produce photochromism with appropriate molecular desi

gnSome of the most common processes involved in photochromism are peric

yclic reactions cis-trans somerizations intramolecular hydrogen transfer

intramolecular group transfers dissociation processes and electron transfers

(oxidation-reduction)

Another some what arbitrary requirement of photochromism is that

it requires the two states of the molecule to be thermally stable under

ambient conditions for a reasonable time All the same nitrospiropyran (which

back-isomerizes in the dark over ~10 minutes at room temperature) is

considered photochromic All photochromic molecules back-isomerize to their

more stable form at some rate and this back-isomerization is accelerated by

heating There is therefore a close relationship between photochromic and the

rmochromic compounds The timescale of thermal back-isomerization is

important for applications and may be molecularly engineered

Photochromic compounds considered to be thermally stable include some

diarylethenes which do not back isomerize even after heating at 800C for 3

months

Since photochromic chromophores are dyes and operate according to

well-known reactions their molecular engineering to fine-tune their properties

can be achieved relatively easily using known design models quantum

mechanics calculations and experimentation In particular the tuning of

absorbance bands to particular parts of the spectrum and the engineering

of thermal stability have received much attention

Sometimes and particularly in the dye industry the term irreversible

photochromic is used to describe materials that undergo a permanent color

change upon exposure to Ultraviolet or visible light radiation Because by

24

definition photochromics are reversible there is technically no such thing as a

n irreversible photochromic this is loose usage and these compounds

are better referred to as photochangable or photoreactive dyes

Apart from the qualities already mentioned several other properties of

photochromics are important for their use These include

Quantum yield of the photochemical reaction

This determined the efficiency of the photochromic change with respect

to the amount of light absorbed The quantum yield of isomerization

can be strongly dependent on conditions

Fatigue resistance In photochromic materials fatigue refers to the

loss of reversibility by processes such as photodegradation

photobleaching photooxidation and other side reactions All

photochromics suffer fatigue to some extent and its rate is strongly

dependent on the activating light and the conditions of the sample

Photostationary state Photochromic materials have two states and

their interconversion can be controlled using different wavelengths of

light Excitation with any given wavelength of light will result in a

mixture of the two states at a particular ratio called the photo-

stationary state In a perfect system there would exist wavelengths

that can be used to provide 10 and 01 ratios of the isomers

but in real systems this is not possible since the active

absorbance bands always overlap to some extent

Polarity and solubility In order to incorporate photochromics in

working systems they suffer the same issues as other dyes They are

often charged in one or more state leading to very high polarity and

possible large changes in polarity They also often contain large

conjugated systems that limit their solubility

Photochromic complexes

A photochromic complex is a kind of chemical compound that has

photoresponsive Parts on its ligand These complexes have a specific

structure photoswitchable organic compounds are attached to

metalcomplexes For the photocontrollable parts thermally and

photochemically stable chromophores (azobenzene diarylethene

25

spiropyran etc) are usually used And for the metal complexes a wide

variety of compounds that have various functions (redox response

luminescence magnetism etc) are applied The photochromic parts and

metal parts are so close that they can affect each others molecular

orbitals The physical properties of these compounds shown by parts of

them (ie chromophores or metals) thus can be controlled by switching

their other sites by external stimuli For example photoisomerization

behaviors of some complexes can be switched by oxidation and reduction of

their metal parts Some other compounds can be changed in their

luminescence behavior magnetic interaction of metal sites or stability of

metal-to-ligand coordination by photoisomerization of their photochromic

parts

Photochemistry of Polyoxometalates

The photochemistry of polyoxometalates is of great interest to inorganic

chemistsMore than 80 years agoit was found that the R-Keggin tungstate

H3[PW12O40] was reduced photochemically to yield a blue-colored species

which was reoxidized by air and by various other oxidizing agents such as

Fe3+AgNO3and H2O2 [7879]The photoredox reactions of H4[SiW12O40]and

H3[PW12O40] proceeded effectively in the presence of primary and secondary

alcohols their ethers and aldehydes and proteinsbut less effectively in the

presence of tertiary alcoholsketonesestersthe fatty acids above formic

acidand simple amines[8081] The basic photoredox reaction involving

ethanol is illustrated by eq 13

2 H3PW12O40 + H3CCH2OH h ν ν ν ν 2 H4PW12O40 + H3CCHO ------- 13

2 H4PW12O40 +12 O2 2 H3PW12O40 + H2O ------ 14

In this reactionone molecule of ethanol photochemically reduces two

molecules of H3PW12O40 and is itself oxidized to acetaldehyde In the

presence of air the thermal oxidation of the reduced species takes place at

room temperature(eq 14)The reduced polyoxometalates which are the so-

calledldquoheteropolybluesrdquo have been used for the colorimetric analysis of the

26

elements P Si As and Ge and for the determination of uric acidsugarand

other biological compounds [8283] Piperidinium metavanadate also

undergoes photoinduced coloration from white to black followed by a

reversible color change in the presence of oxidizing agentsHowever

ammonium metavanadates ([NH4][VO3]) exhibits no photoinduced

coloration[84] The early photoredox reactions of the R-Keggin

polyoxometalates H4[SiW12O40] and H3PW12O40 were carried out in the

presence of photographic paper however the limited number of the

structurally well-characterized compounds available for study delayed the

development of modern cluster-compound photochemistry until the discovery

of photochromism in alkylammonium polyoxo- molybdate solids[8586] A

photochromic or electrochromic material is one whose light-absorbing

properties are altered upon optical excitation or reduction under the influence

of an externally applied electric field respectively The induced coloration

remains even after the excitation source has been removed These materials

are of technological interest because they return to their original state either

thermally upon irradiation with light of a frequency corresponding to the

induced absorption or electrochemically upon reversing the polarity of the

externally applied electric field Thus photochromic and electrochromic

materials behave in a reversible manner Polyoxometalates exhibit significant

photo-and electrochromism which makes them suitable as nanocomposite

molecular devices and as models for probing the physical properties of infinite

metal oxides Since the metal ions in the oxidized polyoxometalates have d0

electronic configurations the only absorption band which occurs in the UV-vis

range of the electronic spectra is due to the oxygen-to-metal (O-M) ligand-to-

metal charge transfer (LMCT)Upon irradiation electrons are promoted from

the low-energy electronic states which are mainly comprised of oxygen 2p

orbitals (the valence band in the band model)to the high-energy electronic

states which are mainly comprised of metal d orbitalrsquos (the conduction band

in the band model)The fundamental transitions in polyoxometalate lattices

are depicted schematically in Fig1B1

27

Fig1B1 Simple model showing the electronic transitions in the

polyoxometalates containing electron donar and acceptor (a)generation

of charge carriers(b)electron and hole trapping(c)electron release due

to stimulation(d)recombination

between electron and hole Electrons are e-and holes are h+

In the polyoxometalates containing heteroatoms and especially in

mixed metal polyoxometalates the charge carriers which are created by the

light or electric field may be trapped in electron traps and hole traps These

traps provide states of localized energy in the O-M LMCT energy gap due to

the heteroatoms or counter cations which correspond to impurities or lattice

defects in the band model If the trap depth ∆E is large compared to kT the

probability for thermal escape from the trap will be negligibly small and

metastable situation will existThe trapped carriers can be released by thermal

or optical stimulationIn the case of thermal stimulation the irradiated

polyoxometalate is heated until the energy barrier ∆E can be overcome The

trapped electron (or hole) then can escape from the trap and nonradiatively

recombine with the trapped hole (or electron)Under optical stimulation the

energy of an incident photon is used to overcome ∆E The relaxation

processes of the OndashM LMCT excitation energy include both the nonradiative

recombination of electrons and holes within the energy gap and the

intramolecular energy transfer leading to a charge-transfer emission This

intramolecular energy transfer corresponds to the O-M LMCT energy gap and

occurs via radiative recombination and sensitized emission from the

28

heteroatoms or cations If several energy levels based on the hetero atoms or

counter cations act as energy acceptors within the O-M LMCTenergy gap the

energy transfer occurs from the O-M LMCT states to these levels followed by

the nonradiative or radiative deactivation of the excitation energy It should be

noted that the O-M LMCT states also can be generated by the application of

very high electric fields to the polyoxometalate solids as demonstrated by the

observation of electroluminescence[87]If an external electric field with a

potential more negative than the energy levels of the vacant orbitals involved

in the O-M LMCT transition is applied to a polyoxometalate on the electrode

surface an electrochemical reduction occurs via the injection of electrons

from the electrode in to the vacant levels of the polyoxometalate as shown in

Fig1B 2

Fig1B2 - Energy scheme for the electrochromism of polyoxometalates

a)electrochemical reduction (b) electrochemical oxidation

Electrons injected in to the high-energy levels also may be trapped by

electron traps in a process analogous to that which occurs during LMCT

photoexcitation of the polyoxometalates These electrons are returned to the

electrode by electrochemical oxidation at an electrode potential more positive

than the energy levels for the d1 electron states The d1electrons in the O-M

LMCTstates facilitate the absorption of visible light via intervalence charge

transfer among metal centers and d-d transitions The same type of transition

may be possible for the d1electron captured by the electron traps too In

addition to searching for new photosensitive polyoxometalates with the

29

potential for having practical application there is now a need to elucidate the

fundamental photo-and electrochemical coloration processes by studying

electron transfer within the polyoxometalate lattices in conjunction with their

crystal structures So far few polyoxometalates exhibit a perfect reversibility

of coloration The irreversibility of the color change arises from as yet

uncharacterized side reactions during both the coloration and decoloration of

the polyoxometalates

Many metal oxides including aluminum titanium vanadium niobium

molybdenum and tungsten oxides are photochromic when they contain

impurities or dopants This coloration has been interpreted on the basis of

electron trapping at appropriate lattice sites within the crystals as shown in

Fig1B1 where the O-M LMCT transition corresponds to the transition

between the valence and conduction bands for the infinite metal-oxide lattice

[88-91]

B12 Applications of Photochromic materials

Sunglasses

One of the most famous reversible photochromic applications is color

changing lenses for sunglasses as found in eyeglasses The largest limitation

in using PC technology is that the materials cannot be made stable enough

to withstand thousands of hours of outdoor exposure so long-term outdoor

applications are not appropriate at this time The switching speed of

photochromic dyes is highly sensitive to the rigidity of the environment around

the dye As result they switch most rapidly in solution and slowest in the rigid

environment like a polymer lens Recently it has been reported that attaching

flexible low Tg polymers (for example siloxanes or poly (butyl acrylate) to the

dyes allows them to switch much more rapidly in a rigid lens [76] Some

spirooxazines with siloxane polymers attached switch at near solution like

speeds even though they are in a rigid lens matrix

Supramolecular chemistry

Photochromic units have been employed extensively in supramolecular

chemistry Their ability to give a light controlled reversible shape change

means that they can be used to make or break molecular recognition motifs

30

or to cause a consequent shape change in their surroundings Thus

photochromic units have been demonstrated as components of molecular

switches The coupling of photochromic units to enzymes or enzyme cofactors

even provides the ability to reversibly turn enzymes on and off

by altering their shape or orientation in such a way that their functions

are either working or broken

Data storage

The possibility of using photochromic compounds for data storage was

first suggested in 1956 by Yehuda Hirshberg[77] Since that time there have

been many investigations by various academic and commercial groups

particularly in the area of 3D optical data storage which promises discs that

can hold a terabyte of data Initially issues with thermal back-reactions

and destructive reading dogged these studies but more recently more stable

systems have been developed

Novelty items

Reversible photochromics are also found in applications such as toys

cosmeticsclothing and industrial applications If necessary they can be made

to change between desired colors by combination with a permanent pigment

A large number of inorganic compounds exhibit photochromism

These solids often have large band gaps of the order of 3 - 12 eV and

excitation of these solids leads to the formation of metastable centers that

absorb visible light giving rise to their colour They can return to their ground

state by heating or by optical excitation within the colour-centre band In most

cases the photochromism is a structure sensitive phenomenon involving

localized defect impurities or dislocations Some of these inorganic

compounds have the potential for a number of different uses Photochromic

compounds have a number of useful applications These can be divided

according to the most important property that is being used (Table 11) [92]

31

Table 11 Applications of Photochromic materials

Applications Depending Upon

Sensitivity to

Radiation

Reversibility Thermal Chemical or

Physical Properties

Self-developing

photography

Chemical switches for

computers

Temperature indicators

Protective

materials

Data displays

Heat-sensitive recording

media

Optical signal

processing

Photomasking and

photoresist technology

Reusable data storage

media

Anaytical reagents

Photochromic

microimages

Photopolymerisation

Information encoding

and steganography

Photocontractile

polymers and the

photoviscosity effect

Control of light

intensity

Q-switches

Pyroelectric

photochromic materials

B13 Thermochromism

Thermochromism is the reversible colour change of a substance

induced by temperature change A large variety of substances organic

inorganic organometallic supramolecular and polymeric systems exhibit this

phenomenon Examples of these include bianthrones cobalt

hexacyanoferrate the zirconocene complex of 1 4-diphenyl-1 3-butadiene

and poly (3-alkylthiophene) The organic 99-bixanthenylidene is colourless at

90 K yellow-green at 298 K and dark-blue when melted at 592 K Heating

conducting polymers can cause them to change colour This is achieved by

causing conformational changes to the polymer backbone resulting in a

change in the band gap of the polymer It has been reported that regioregular

32

P3HT reversibly changes colour upon heating to 220ordmC due to temperature-

dependent conformation changes Thermally cross linked polymer undergoes

the same colour change but it is much less reversible [93] Other forms of

thermochromism may be commercially important eg to give a visual

indication of temperature changes

B14 Electrochromism

Electrochromism describes a phenomenon of material color change in

a persistent but reversible manner produced by electrochemically induced

oxidation-reduction reactions Electrochromic materials can be applied to

various kinds of products such as smart windows and display devices Among

those applications there have been lots of efforts to develop electrochromic

display devices (ECDs) Especially flexible display devices are now attracting

much attention worldwide since they can facilitate new technological demands

such as bending and folding of paper-like displays High electrochromic

efficiency short response time long operating life time and reduction of

energy consumption are the most important requisites of the materials for the

paperlike displays [94 95] Among those properties the operation life time is

the most important barrier to overcome for a realization of ECDs There are

two types of electrochromic material a) inorganic transition metal oxides

(TMOs) b) organic polymer materials The TMOs have been studied longer

than the organic materials that they have been studied since 1960s [9697]

Electrochromism describes a reversible color change of material

produced by electrochemically induced oxidation-reduction reactions It is one

of several types of chromism of materials As thermochromism and

photochromism mean material color changes made by heat and light

respectively electrochromism refers that the color change is caused by an

electric potential In most cases the color change in electrochromism can be

driven by rather low electrical potential of the order of a fraction of volt or a

few volts [94-96] The color change of material means variation in

transmittance andor reflectance change in visible range which is originated

from different electronic absorption bands according to a switching between

oxidation and reduction state of material When electric potential is applied on

electrochromic material forced oxidation or reduction is derived and the

33

individual color is originated from the corresponding oxidation or reduction

state of the material For electrochromic materials the characteristic color

change is reversible since the oxidation and reduction state can be converted

reversibly by switching potential

Application fields

The application area for the electrochromism is rather broad that it

covers from smart window glazing and optical modulators to information

displays [98-102] The smart windows are typical examples The

electrochromic property is used to control the amount of light and heat to pass

through the windows Usually the electrochromic material is in form of thin film

coated on a window glass The transmittance modulation has also been

applied at the automobiles to automatically tint rear-view mirrors in various

lighting conditions The electrochromic application fields are illustrated in Fig

1B3

Since the smart windows control the transmittance of heat as well as

the transmittance of visible light the working definition of electrochromism has

now been extended to include devices for modulation of radiation in the near

infrared thermal infrared and microwave regions When color for

electrochromic materials is used this can now mean a response by detectors

at these wavelengths and not just by the human eyes Nowadays

electrochromic material draws much attention as being used in the display

devices Electrochromic display device (ECD) is being considered as one of

the candidates for the conventional liquid crystal display (LCD) since ECD

has many advantages over LCDs Among these advantages the most

important are low energy consumption wider viewing angle high contrast

rate and possibility to achieve multiple colors with a single material [103]

When a new redox state of electrochromic materialis established by

the applied electric pulse then it is maintained after the potential is switched

off This means the colored or bleached state of the material can be sustained

for a considerable time without applying electrical power This is so-called

ldquocolor memory effectrdquo of electrochromic material Because of the color

memory effect energy consumption for the electrochromic display device

could be drastically reduced and this would be a big advantage over other

34

emissive devices The low power consumption is especially advantageous

when it is applied to mobile devices with limited power source The possibility

for a flexible display is another attraction for electrochromic material

Information displays

Real-view mirrors for automobiles

Fig 1B4 Application fields of electrochromic devices Smart windows information displays and real-view mirrors for automobiles

35

Using polymeric electrochromic material and plastic substrate with a

transparent conductive coating it is possible to build all-plastic flexible display

device There are lots of papers and patents about the flexible display devices

[104105] In these cases gel-type electrolyte is also needed The gel-type

electrolyte immobilizes liquid electrolyte in the polymer network [106107]

Recently ITO-coated polymer films are readily available which would provide

the plastic substrate for a flexible device The flexible electrochromic displays

are frequently tried with the plastic substrate flexible electrochromic material

and gel-type electrolyte The flexible electrochromic displays would facilitate

increasing technical demands for foldable display devices

Metal Ion Electrochromism

Many transition metal oxides are capable of redox reactions that result in

colour change Metal oxide films are commonly prepared as thin layers of

either tungsten nickel molybdenum or other metal compounds by a number

of techniques These include sol-gel electrochemical by dc or rfreactive

sputtering techniques electron-beam evaporation by anodic or cathodic

electrodeposition or by solution dipping of the electrochromic metal

compounds (or compounds that can be changed into these metal compounds)

onto optically transparent electrodes (OTE) [108 -114] Their electrochromism

is derived from the colour change associated with a change in the oxidation

state of the metal anion The behaviour of these materials is dependent upon

pH moisture and exposure to the atmosphere [115] Generally the switching

rates of these films is somewhat slow with typical switching times of about 15

- 60 seconds to achieve 100 conversion to either coloured or bleached state

[116 -120] An Example of this includes nickel oxide which changes from

transparent (pale green) to brownblack taking about 30 seconds to do so

[121] Other examples include [(NH4)5Ru]2(pyrazine)5+ and [(NH4)5Ru]2(44-

bipyridine)5+ whose electrochromism is significantly different due to the effect

of the ligand [122] Table 12 below gives some examples of metal oxide films

with electrochromic properties

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

5

applications that the reference books in the field would certainly welcome

substantial additions and revision

The intrinsic properties of polyoxometalates are of interest in themselves

not only from a fundamental point of view but also to make of them

materials of interest in various applications Beyond their traditional interest as

catalysts polyoxometalates constitutes base materials for electrochromics

energy storage and conversion devices (batteries super capacitors and

fuel cells) sensors or biomedical applications Many of the applications of

polyoxometalate clusters as materials require their use in the form of

membranes or electrodes that is in the form of solid insoluble material or

coatings There is therefore a main strategic line of work that has centered

on the inclusion or integration of polyoxometalates in all sorts of substrates

polymeric inorganic or mineraland their combination with surfactants or

organic carriers

Heteropolyanions are negatively-charged clusters of corner-sharing and

edge shairing early transition metal MO6 octahedra and heteroatom

XO4 tetrahedra where the tetrahedra are usually located the interior

of the cluster[23] The geometry composition and charge of these clusters

are varied through synthesis parameters and cluster properties are

highly tunable as a function of these characteristics Heteropolyanions have

been employed in a range of applications that include virus-binding

inorganic drugs [24] homogenious and heterogeneous catalysts [25 26]

electro-optic and electrochromic materials [27 28] metal and protein binding

[29] and as building blocks for nanostructuring of materials [30] The α-Keggin

geometry which was first structurally characterized in 1933 by JF Keggin

[31] the phosphotungstic acid (H3PW12O40) is one of the most

widely recognized and thoroughly studied heteropolyanion geometries[32]

1A2 Fundamental concept of polyoxometalate structures-

Keggin structure is the best known structural form for heteropoly acids It

is the structural form of αndashKeggin anions which have a general formula of

[XM12O40]n- where X is the hetero atom (most commonly are P5+ Si4+ or B3+)

M is the addenda atom (most common are molybdenum and tungsten) and O

6

represents oxygen[33] The structure self assembles in acidic aqueous

solution and is the most stable structure of polyoxometalate catalysts

Fig 1A1 Keggin structure

The first α-Keggin anion ammonium phosphomolybdate

((NH4)3[PMo12O40]) was first reported by Berzelius in 1826 In 1892

Blomstrand proposed the structure of phosphomolybdic acid and other poly-

acids as a chain or ring configuration Alfred Werner using the coordination

compounds ideas of Copaux attempted to explain the structure of

silicotungstic acid He assumed a central group [SiO4]4- ion enclosed by four

[RW2O6]+ where R is a unipositive ion The [RW2O6]

+ are linked to the central

group by primary valences Two more R2W2O7 groups were linked to the

central group by secondary valences This proposal accounted for the

characteristics of most poly-acids but not all

In 1928 Linus Pauling proposed a structure for α-Keggin anions

consisting of a tetrahedral central ion [XO4] n-8 caged by twelve WO6

octahedral In this proposed structure three of the oxygen on each of the

octahedral shared electrons with three neighboring octahedral As a result 18

oxygen atoms were used as bridging atoms between the metal atoms The

remaining oxygen atoms bonded to a proton This structure explained many

characteristics that were observed such as basicities of alkali metal salts and

the hydrated of some of the salts However the structure could not explain the

structure of dehydrated acids

JF Keggin with the use of X-ray diffraction experimentally determined

the structure of α-Keggin anions in 1934 The Keggin structure accounts for

both the hydrated and dehydrated α-Keggin anions without a need for

7

significant structural change The Keggin structure is the widely accepted

structure for the α-Keggin anions [34] For example the α-Keggin anion of

phosphotungustic acid is shown in Fig1A2

The structure is composed of one heteroatm surrounded by four

oxygen to form a tetrahedronThe heteroatom is located centrally and caged

by 12 octahedral WO3 units linked to one another by the neighboring oxygen

atoms There are a total of 24 bridging oxygen atoms that link the 12 addenda

atoms The metal centres in the 12 octahedra are arranged on a sphere

almost equidistant from each other in four M3O13 units giving the complete

structure an overall tetrahedral symmetry The bond length between atoms

varies depending on the heteroatom (X) and the addenda atoms (M)

Fig 1A2 α-Keggin anion of phosphotungustic acid ( PW12O40 ) 3-

For the 12ndashphosphotungstic acid Keggin determined the bond length

between the heteroatom and each the four central oxygen atoms to be 15 Adeg

The bond length form the central oxygen to the addenda atoms is 243 Adeg

The bond length between the addenda atoms and each of the bridging

oxygen is 19 Adeg The remaining 12 oxygen atoms that are each double

bonded to an addenda atom have a bond length of 170 Adeg The octahedra

are therefore distorted This structure allows the molecule to hydrate and

dehydrate without significant structural changes and the molecule is thermally

stable in the solid state for use in vapor phase reactions at high temperatures

(400-500 degC)[35-36]

8

Including the original Keggin structure there are 5 isomers designated

by the prefixes α- β- γ- δ- and ε- The original Keggin structure is designated

α- These isomers are sometimes termed Baker Baker-Figgis or rotational

isomers [37]These involve different rotational orientations of the Mo3O13 units

which lowers the symmetry of the overall structure Lacunary Keggin

structures

The term lacunary is applied to ions which have a fragment missing

sometimes called defect structures Examples are the (XM11O39)nminus and

(XM9O34)nminus formed by the removal from the Keggin structure of sufficient Mo

and O atoms to eliminate 1 or 3 adjacent MO6 octahedra The Dawson

structure X2M18O62nminus is made up of two Keggin lacunary fragments with 3

missing octahedra Some structural types are found in many different

compounds The first known example of this was the Keggin ion whose

structure was found to be common to both molybdates and tungstates with

different central hetero atoms Examples of some fundamental

polyoxometalate structures are shown below The Lindquist ion (Fig1A4) is

an iso-polyoxometalate the other three are hetero-polyoxometalates The

Keggin and Dawson structures (Fig1A3 and (Fig1A5) have tetrahedrally

coordinated hetero-atoms eg P or Si Anderson structure has an octahedral

central atom egAl

Fig 1A3 Keggin structure XM12O40nminus

9

Fig 1A 4 Lindquist structure M6O19nminus

Fig 1A 5 Dawson structure X2M18O62nminus

In general α-Keggin anions are synthesized in acidic solutions For

example 12-Phosphotungstic acid is formed by condensing phosphate ion

10

with tungstate ions The heteropolyacid that is formed has the Keggin

structure

[PO4]3- + 12 [WO4]

2- + 27 H+ rarr H3PW12O40 + 12 H2O ----------- 11

α-Keggin anions have been used as catalyst in hydration polymerization and

oxidation reaction as catalysts

The metal atoms that make up the framework (termed addenda atoms)

are typically Mo W and V When more than one element is present the

cluster is called a mixed addendaclusterThe ligands coordinated to metal

atoms that together form the bridged framework are usually oxide ions but

other elements such as S and Br have been substituted for some of the oxide

ions (Note that sulfur substituted POM is often termed a

polyoxothiometalates) Another development is the use of other ligands eg

nitrosy and alkoxy to replace oxide ions The typical framework building

blocks are polyhedral units with 4 5 6 or 7 coordinate metal centers These

units usually share edges andor vertices The most common unit for

polymolybdates is the octahedral MoO6 unit which is a distorted octahedron

where the Mo atom moves off centre to give one short Mo-O bond In some

polymolybdates there are pentagonal bipyramidal units and these are key

building blocks in the molybdenum bluesHetero atoms are present in many

polyoxometalates Many different elements can act as hetero-atoms

Examples of various coordination numbers around the hetero-atom are

known

4 co-ordinate (tetrahedral) in Keggin Dawson and Lindquist structures

(eg PO4 SiO4 AsO4)

6 co-ordinate (octahedral) in Anderson structure (eg Al(OH)6 TeO6

8 co-ordinate (square antiprism) in ((CeO8)W10O28)8minus

12 co-ordinate (icosahedral) in (UO12)Mo12O30 8minus

11

Often the hetero-atom is centrally located in the anion (eg Keggin

structure) or in a structure fragment eg the 2 phosphorus atoms in the

Dawson ion are central to the two symmetric fragments There are similarities

to clathrate structures The Keggin ion can be formulated as PO4 2minusand

M12O36 and the Dawson as (XO42-)2 and M18O54Structural isomerism is

common For example the Keggin structure has 5 isomers which can be

considered to contain one or more of the four M3O13 units being rotated

through 60degMany compounds share the same framework architectures or

frameworks derived from a larger framework with one or more addenda atoms

and oxide ions removed to give defect structure usually called a lacunary

structure An example of a compound with a Dawson lacunary structure is

As2W15O56Some cage structures containing ions are known eg an example

is the vanadate cage V18O42 containing a Clminus ion [38] This has 5 co-

ordinates square pyramidal vanadium units linked together

Fig 1A 6 - H4V18O42 cage containing Cl

1A3 General Properties of polyoxometalates -

Typically polyoxoanions are water and air stable species of large size

(6-25 Adeg) and high ionic weight In aqueous solution they are subject to

decomposition by hydroxide ions eg

[PW12O40]3- + 23 OH- HPO4

2- + 12WO42- + 11 H2O -------- 12

12

Although the PH at which such reactions are rapid can very widely

depending upon the polyanion involved Polyanions are often much stable

towards the H3O+ ions and numerous crystalline heteropoly acids are known

Such acids may be extremely soluble in water and polar solvents (giving

solutions with densities in excess of 4 gcm3) and have large dissociation

constants (PK lt 0)

Crystalline heteropoly acids and salts are frequently highly hydrated with

up to 50 molecules of water per anion Much of this water is zeolite in nature

and crystal composition can vary accordingly On the other hand the cation

anion stiochiometry is always well defined as the anion structure and

composition Finally many polyanions are powerful oxidizing agents and

undergo multiple reversible one or two electron reductions leading to intensely

colored mixed valence species known as heteropoly blues Polyanions are

known which can accept as many as 32 electrons without major structural

change

1A4 Chemistry of Molybdenum and Tungusten Heteropolyoxometalates

A photochromic monolayer film of phosphomolybdic acid (denoted as

PMo12) was fabricated by self-assembly approach UV-visible spectrum and

AFM observation show that the monolayer film is composed of aggregated

PMo12 molecules The monolayer film shows good photochromic properties

with enough stability and reversibility The colour change of the monolayer

after UV-irradiation can be captured by a microscope equipped with a color

CCD camera Photochromic response of the monolayer film can be doubled

after being modified by an amine monolayer [39]

Series of vanadium substituted molybdo Keggin HPA with 12 or 3

adjacent vanadium atoms were prepared These materials were supported

on carbon cloth electrodes and hot pressed onto Nafion with an ETEK

electrode used as a standard on the opposite side The MEArsquos were run at

temperatureslt100 oC with the HPA electrode as either the anode or the

cathode Stable polarisation curves are obtained for an HPA based cathode

with reasonable current densities at 80 oC [40]

A novel mixed-valence polyoxomolybdenum anion was synthesized

hydrothermally from molybdenum oxidemolybdenum metalboric and

phosphoric acids12-phenyldiphosphonicacidand imidazole (ImH) and was

13

structurally characterized as an imidazolium saltOne-and two-dimensional

structures of this anion and additional molybdenum diphosphonate linkers

were assembled as wellThey were structurally characterized as their

pyridinium(pyH) salts [41]

Phosphomolybdic acidpolyvinylpyrrolidone hybrid films were found to

show visible light photochromism It is identified that the intra-supramolecular

charge transfer between the inorganic and organic molecules is responsible

for the visible-light coloration Interestingly the films show photo-memory and

thermal activation The films show a small change in absorbance after being

irradiated with visible light for a short time and the coloration can be

enhanced greatly by subsequent thermal treatment Electrical measurements

indicate that the conductivity of the film increases after the brief irradiation

which promotes transfer of the electrons induced by the thermal treatment

[42]

In this work major effort was concentrated on passive thermal control

coatings based on photochromic and thermochromic materials The inorganic

photochromic materials were based on tungsten and molybdenum oxide films

and the organic photochromic materials included spiropyrans and

spirooxazines In addition photochromic composite organic-inorganic films

and thermochromic vanadium oxide films were prepared The samples were

synthesized using sputtering sol-gel process and thermal oxidation [43]

Polyoxometalates a class of oxidatively robust inorganic oxidants and

oxidation catalysts are currently under investigation at the Forest Products

Laboratory and at Emory University as an alternative to chlorinebased

chemicals in the bleaching of soft Woodkraft and other pulps Although

polyoxometalate salts are used in a number of industrial processes the

feasibility of using these salts and oxygen in the commercial bleaching of

chemical pulps was only recently demonstratedA clear advantage of

polyoxometalates over oxygen alone hydrogen peroxide or ozone is their

inherently high selectivity for the residual lignin in softwood kraft pulps The

goal of ongoing research is to develop a highly selective energy efficient

oxygen based polyoxometalate delignification and bleaching technology

compatible with mill closure [44]

14

Tungsten oxide exhibits pronounced photochromism upon bandgap

photoexcitation which makes it attractive and promising for applications in

many areas Some advances have been achieved during the past decades

The research on nanocrystalline films and single crystals indicates the critical

importance of defects in tungsten oxide to its photochromism Based on

energy-band engineering of semiconductors enhancement of photochromism

has been achieved for instance extension of the photoresponse from UV to

visible light by cathodic polarization improved change in absorption before

and after coloration through modification by a noble metal or another metal-

oxide semiconductor and increased photochromic reversibility via

hybridization with organic amines Nanocrystalline oxide films exhibit

controllable wettability which is coherent in nature with photochromism [45]

Polyoxometalates represent a diverse range of molecular clusters with

an almost unmatched range of physical properties and the ability to form

structures that can bridge several length scalesThe new building block

principles that have been discovered are beginning to allow the design of

complex clusters with desired properties and structures and several structural

types and novel physical properties are examinedIn this critical review the

synthetic and design approaches to the many polyoxometalate cluster types

are presented encompassing all the sub-types of polyoxometalates including

isopolyoxometa- lates heteropolyoxometalates and reduced molybdenum

blue systems As well as the fundamental structure and bonding aspectsthe

final section is devoted to discussing these clusters in the context of

contemporary and emerging interdisciplinary interests from areas as diverse

as antiviral agentsbiological ion transport modelsand materialsscience [46]

Keggin type molybdovanadophosphoric heteropoly acids were prepare

d by a novel environmentally benign method and their catalytic performances

were evaluated via hydroxylation of benzene to phenol with hydrogen

peroxide as oxidant in a mixed solvent of glacial acetic acid and acetonitrile

Various reaction parameters such as reaction time reaction temperature

ratio of benzene to hydrogen peroxide concentration of aqueous hydrogen

peroxide ratio of glacial acetic acid to acetonitrile in solvent and catalyst

concentration were changed to obtain an optimal reaction conditions

15

Molybdovanadophosphoric heteropoly acids are revealed to be highly

efficient catalyst for hydroxylation of benzene [47]

Thin films from the system (As2S3)Tl were deposited by thermal

evaporation on Si graphite and optical glass substratesFrom transmission

and reflection measurements of the thin films the refractive index (n) film

thickness(d) optical band gap(Eg) optical oscillator energy(Eo) and

dispersion energy(Ed) before and after exposure to light were determined

The results for optical parameters were analyzed using the Wemple - Di

Domenico single oscillator model and Lorenc-Lorenc equation It was found

that Eg decreases while n E0 and Ed increase for as deposited films

decreases while n E0 and Ed increase for as deposited films with increasing

of Tlconcentration passing through a maximum at 6 at of Tl After exposure

to light n E0 Ed increase and Eg decreases for all compositions

investigated The maximum change in n (Dn = 016 at l = 6328 nm) was

observed for thin As38S56Tl6 films From infrared spectroscopy measurements

of bulk glasses and thin films we could conclude that when up to 6 at of

thallium is introduced As-S-As chains break and a ternary TlAsS2

compound appears at 10 at Tl [48]

The organo - inorganic hybrid materialconsisting of Poly (34 Ethylene

Dioxythiophene) (PEDOT) doped with phosphomolybdate cluster anions

[PMo12O40]3-has been synthesized by direct insitu oxidative polymerization of

34-Ethylene Dioxythiophene (EDOT) with phosphomolybdic acid

(H3PMo12O40) Its characterization is investigated by Fourier Transform

Infrared Spectroscopy (FT-IR) and Scanning Electron Microscopy (SEM) The

hybrid material presents predominantly high electronic conductivities of

around 20 and 70 S cm1at 300 and 400 K respectively [49]

Heteropolyacids (HPAs) are known to be excellent re-dox catalysts In

combination with TiO2 HPAs can be used as photocatalysts active in visible

light The HPA accepts electron and get reduced to heteropolyblue (HPB)

That can absorb light in the visible range HPA can be incorporated onto the

external surface or in the pores of zeolite based composite photocatalysts

have been designed by incorporation of HPA semiconductor TiO2 and

transition metal cobalt on zeolite This composite metallozeolite photocatalyst

is efficient in photoreduction of methyl orange (MO) in visible light to the tune

16

of 411 mg of MO photo reducedg TiO2 This catalyst also shows encouraging

results for hydrogen evolution from water to the tune of 2730 micromolhg TiO

[50]

A new class of proton conducting glass membranes based on heteropo

lyacids such as phosphotungstic acid (PWA) as electrolytes for low

temperature H2O2 fuel cells was investigated Parameters for a single fuel cell

with a catalyst electrode of 015 mgcm2 of PtC and a glass composite

membrane were characterized by electrochemical measurements at open

circuit potential conditions The performance of the membrane electrode

assemblies (MEA) was systematically studied as an effect of SiO2 and P2O5

concentrations in the glass composite membrane and the MEA was found to

exhibit a maximum power density of 162 mW cm2 for an H2O2 fuel cell at

30 degC and 30 relative humidity (RH) [51]

Two new photochromic inorganic-organic hybrid materials formed from

Keggin type Polyoxometalates (POMs) and metronidazole (C6H9N3O3 MNZ)

formulated as H3PMo12O40bull3 MNZ3H2O (1) and H3PW12O40bull3MNZ3H2O(2)

were synthesized and characterized by elemental analysis IR spectra

electronic spectra electron spin resonance (ESR) spectra and TG-DTA

Reflectance spectra show the presence of weak inter molecular charge

transfer between the organic and inorganic moieties in the solid state The

photochromic properties were studied by solid diffuse reflectance spectra and

ESR spectra and the photochromic reactions were found to exhibit first-order

kinetics TG-DTA showed that two hybrid materials have similar thermal

behavior [52]

Heteropolycompounds (HPCs) have been a matter of interest in basic

and applied science for more than a century From their first synthesis many

advances have been made to promote the use of HPCs in different ways in

science and technology The aim of this article is to review the main structural

characteristics of heteropolycompounds of the Keggin type (12

tungstophosphoric12-molybdophosphoric12-tungstosilicic acid alkaline and

alkaline earth salts of12 tungstophosphoric acid and gels doped with HPCs)to

understand and explain their different activities such as high proton

conductivity and catalytic biochemical and biomedical activities [53]

17

A solid hybrid molecular material containing 1-butyl 3-methyl

imidazolium cations and Keggin anions of phosphotungstic acid has been

synthesized It is fully characterized by CHN analysis FTIR XRD UV-Vis-NIR

DRS 31P MAS NMR TGA and SEM The FTIR spectrum of the compound

shows the fingerprint vibrational bands of both Keggin molecular anions and

imidazolium cations The aromatic CndashH stretch region (2700ndash3250 cmndash1) of

imidazolium cation is split due to the interaction between the ring CndashH and

bulky Keggin anion The red-shift in the UV-Vis spectra and the downfield 31P

MAS NMR chemical shift also confirm the electrostatic interaction between

the ions in the compound Near IR spectral region (1000ndash2500 nm) shows the

elimination of water in the compound which is hydrophobic [54]

Transport coefficient measurements (electrical conductivity

thermoelectric power and Hall coefficient) have been performed on a

compact Tl033MoO3 polycrystalline compound in a wide temperature range

(200ndash400 K) Experimental results are interpreted with the help of a p-type

semiconductor model with two inverted deep levels near the midgap The

valence band and the conduction band are assumed to be formed from the

dxy orbitals of molybdenum atoms in the Mo6O22 cluster leading to narrow π-

bonding bands The donor and acceptor levels may be formed from

nonbonding dxy orbitals arising respectively from anionic and Tl+ defects

Electron paramagnetic resonance and magnetic measurements are in good

agreement with the theoretical band semiconductor model which has been

retained [55]

The optical properties of GaInTlAs epilayers grown at low temperature

~230degC by solid-source molecular-beam epitaxy on InP substrates were

characterized using optical absorptionand photoluminescence techniques

Optical absorption measurements a room temperature show a gap shrinkage

toward lower energies from 071 to 061 and 053 eV when the Tl content

increases from 0 to 4and 8in good agreement with theoretical

predictionsLow-temperature photoluminescence band-gap signals from

GaInAs and GaInTlAs layers are only obtained after rapid thermal annealing

performed inorder to improve the electronic quality of the layersA band gap

decrease as much as 41 meV for GaInTlAs with 19 Tl incorporation is

measured by photoluminescence at 8K [56]

18

The electrochromic performance of all solid ndash state cells employing

phosphotungstic acid and phosphomolybdic acid is reported These cells

employ SnO2 as the viewing electrode and graphite as the back electrodeThe

cells in the bleached state can be made white to red and become black in the

coloured state [57]

Various organic compounds were oxidized by molecular oxygen in the

presence of a catalytic amount of mixed addenda heteropolyoxometalates

containing molybdenum and vanadium The catalytic activity of the

Molybdovanadophosphate was found to be greatly enhanced by supporting

on charcoal The supported catalyst has high catalytic activity for oxidative

dehydrogenation of benzylic and allylic alcohols to the corresponding

aldehydes and ketones (46-92) nevertheless the nonsupported catalyst

was inactive for the same oxidations under these conditions 236Trimethyl

phenol was selectively oxidized to trimethyl-p benzoquinone which is

precursor of VitE in the presence of a catalytic amount of

molybdophosphate In addition the aerobic oxidation of amines alkyl-

substituted phenols and alkanes were also examined [58]

The reaction of Tl2CO3 with 111555-hexafluoro-24-pentanedione

and diglyme CH3O(CH2CH2O)2CH3 or tetraglyme CH3O(CH2CH2O)4CH3 in

dichloromethane yields the anhydrous thermally and air stable volatile Tl

diglyme and Tl tetraglyme adducts They have been characterized by single

crystal X-ray diffraction elemental analysis 1H and 13C NMR IR and mass

spectroscopy Thermal and mass-transport properties have been investigated

using thermo gravimetric and differential scanning calorimetric

measurements There is evidence that both precursors are very low melting

and volatile and can be used as liquid Tl sources Both adducts have been

successfully applied to metalndashorganic chemical vapor deposition of thallium

containing films [59]

Large size and high quality single crystals of quasi-two-dimensional

thallium molybdenum purple bronze TlMo6O17 have been grown by electrolytic

reduction of molten salt of Tl2CO3-MoO3 The crystal structure is trigonal with

space group P3m1 determined by X-ray diffraction and four-circle single crystal

diffraction The lattice parameters of the unit cell are a = b = 55282 Adeg and c

= 136991 Adeg The temperature dependence of resistivity and magnetic

19

susceptibility confirmed that a metal-to-metal transition occurs near 110 K

[60]

Hall coefficient and dc conductivity studies were made on p-type

Pb08Sn02Te thin films doped with different concentrations of thallium in the

temperature range 77 to 500 K The Hall coefficient and Hall mobility are

found to decrease with an increase in the doping concentration of thallium

Hall coefficient data have been analyzed in the light of a double valence-band

model Various band parameters such as valence band separation population

ratio mobility ratio and effective mass ratio have been calculated Hall

mobility data have been analyzed in the light of lattice and defect limited

scattering mechanisms [61]

Proton conducting composites of heteropolyacid hydrates phosphomol

ybdic acid H3PMo12O40 nH2O(PMA) phosphotungstic acid H3PW12O40 nH2O

( PTA) and salt hydrate like NiCl2 6H2O were prepared

with insulating Al2O3 as despersoidThe ionic conductivity peaks at two

concentrations of Al2O3 indicating two percolation thresholds for percolation

thresholds for proton conduction Two separate experiments were carried out

to check the existence of such percolation thresholds viz the volta battery

experiment involving the measurement of emf of an electrochemical cell

with composites of different compositions used as electrolyte and the

composition vs conductivity measured by the complex impedance

spectroscopy The presence of two maxima has been attributed to two

different percolation thresholds for the two possible mobile protonic

species H+ + (H3O+) and OH arising from the hydrates [62]

1 A5 Applications of Heteropolyoxometalates-

Applications of heteropolyanions centre depend on their redox properties

their high charges and ionic weights An enormous patent and journal

literature is devoted to the applications of heteropolyanions

1 Analysis-

The formation and subsequent precipitation or reduction of

[XMO12O40]n- anions form the basis of gravimetric and colorimetric analytical

methods for P As Si or Geeither separately or in combination [63 64]

20

2 Biochemical applications-

lsquoPhosphotungstic Acidsrsquo have for decades been used as precipitants

for proteins and as analytical reagents for proteinsalkaloidsand purines eg

the [P2M18O62]6- anions for colorimetric determination of uric acid [65] and

cholesterol The acid H3P12O40 either in aqueous or ethanolic solution is also

widely used as a non specific electron dense stain for electron spectroscopy

The dyestuffs industry has for many years used heteropolymolybdates and

tungstates to form color lakes and toners from basic dyes Large

heteropolyanions exhibit antiviral antitumoral properties at non-cytotoxic

doses in vitro and in vivo and are protein inhibitors of cellular bacterial and

viral DNA RNA polymerizes [66]

3 Catalysis

Heteropoly acids and salts have been used as heterogeneous catalysts

for a broad variety of reactions and compilations of such applications up to

1973 are available Examples include oxidation of propylene and isobutylene

to acrylic acid methacrylic acids and ammoxidation of acrylonitrile olefin

polymerization and epoxidation Much of current activity concerning

heterogeneous catalysis by heteropoly compounds is being carried out in

Japan [67 68]

4 Other Applications

Insoluble salts of Heteropolyanions especially ammonium

molybdophosphates have been used and are commercially available as ion-

exchange materials [69]Recent work in this area includes thin layer

chromatography of amino acids ion selective membranes [70] and the

preparation of new ion exchangers based on heteropolyanions Crystalline12-

tungstophosphoric and 12-molybdophosphoric acids are excellent protonic

conductors Heteropolyacids are electrochromic in the solid state as a

consequence of heteropolyblue formation Heteropolyblue formation has also

been used to detect alcohol or carboxylic acid radicals generated by radiolysis

of aqueous solutions Potential applications of heteropoly complexes as flame

retardants and smoke suppressants or as corrosion inhibitors and conversion

coatings on steel and aluminium are reported [71] Some potential ldquogreenrdquo

applications have been reported eg non-chlorine based wood pulp

bleaching process and a method of decontaminating water Some structures

21

containing transition metal atoms with unpaired electrons have unusual

magnetic properties and are being investigated as nano computer storage

devices Some compounds exhibit luminescence There are many reported

potential medicinal applications eg anti tumoral and anti-viral There have

been reports on the role of weak or non bonding interactions on the crystal

engineering of hybrid polyoxometalates

Spherical nonporous polyoxomolybdate based capsules of different

types containing more than 100 metal atoms reported by Achim Muller and his

group have versatile unique properties regarding their assembly to vesicles

and the chemistry which can be done inside the pores and cavities A discrete

polyoxometalate Lindquist ion of the form W6O192minus was successfully imaged

recently for the first time within the capillary of a carbon nanotube following

steric locking of the anion with the tubule In situ relaxation of the anion in its

equatorial plain was demonstrated [72]

22

Section-B

Litreture Survey on Chromism in Transition Metal

Oxides

1 B1 Chromism in Transition Metal Oxides

Chromism is a reversible change in a substances colour resulting from

a process caused by some form of stimulus Many materials are chromic

including inorganic and organic compounds and conducting polymers and the

property can result from many different mechanisms Several transition metal

oxides show EC properties The most popular are from the VI - B oxides In

this group WO3 and MoO3 are the most thoroughly studied cathodic EC

materials which can be electrochemically coloured and bleached when used

as the cathode in electrochemical cells Cathodic EC materials also include

V2O3 TiO2 and Nb2O5 Another distinguishable group is anodic EC material

including VIII oxides like IrOx nH2O Rh2O3 nH2O NiO nH2O etc which can be

anodicaly coloured in the electrochemical process when used as anode

There are several types of chromism which are discussed as below

B11 Photochromism

Photochromism is the reversible transformation of a chemical species

between two forms by the absorption of electromagnetic radiation where

the two forms have different absorption spectra [7374]

Trivially this can be described as a reversible change of color upon

exposure to light The phenomenon was discovered in the late 1880s

including work by Markwald who studied the reversible change of color of 23

44-tetrachloronaphthalen-1(4H)-one in the solid state He labeled this

phenomenon phototropy and this name was used until the 1950s

when Yehuda Hirshberg of the Weizmann Institute of Science in Israel

proposed the term photochromism [75] Photochromism can take place

in both organic and inorganic compounds and also has its place in biological

systems (for example retinal in the vision process)

Photochromism does not have a rigorous definition but is usually used

to describe compounds that undergo a reversible photochemical reaction

where an absorption band in the visible part of the electromagnetic spectrum

23

changes dramatically in strength or wavelength In many cases an

absorbance band is present in only one form The degree of change required

for a photochemical reaction to be dubbed photochromic is that which

appears dramatic by eye but in essence there is no dividing line between

photochromic reactions and other photochemistry Therefore while the

transcis isomerization of azobenzene is considered a photochromic reaction

the analogous reaction of stilbene is not Since photochromism is just a

special case of a photochemical reaction almost any photochemical reaction

type may be used to produce photochromism with appropriate molecular desi

gnSome of the most common processes involved in photochromism are peric

yclic reactions cis-trans somerizations intramolecular hydrogen transfer

intramolecular group transfers dissociation processes and electron transfers

(oxidation-reduction)

Another some what arbitrary requirement of photochromism is that

it requires the two states of the molecule to be thermally stable under

ambient conditions for a reasonable time All the same nitrospiropyran (which

back-isomerizes in the dark over ~10 minutes at room temperature) is

considered photochromic All photochromic molecules back-isomerize to their

more stable form at some rate and this back-isomerization is accelerated by

heating There is therefore a close relationship between photochromic and the

rmochromic compounds The timescale of thermal back-isomerization is

important for applications and may be molecularly engineered

Photochromic compounds considered to be thermally stable include some

diarylethenes which do not back isomerize even after heating at 800C for 3

months

Since photochromic chromophores are dyes and operate according to

well-known reactions their molecular engineering to fine-tune their properties

can be achieved relatively easily using known design models quantum

mechanics calculations and experimentation In particular the tuning of

absorbance bands to particular parts of the spectrum and the engineering

of thermal stability have received much attention

Sometimes and particularly in the dye industry the term irreversible

photochromic is used to describe materials that undergo a permanent color

change upon exposure to Ultraviolet or visible light radiation Because by

24

definition photochromics are reversible there is technically no such thing as a

n irreversible photochromic this is loose usage and these compounds

are better referred to as photochangable or photoreactive dyes

Apart from the qualities already mentioned several other properties of

photochromics are important for their use These include

Quantum yield of the photochemical reaction

This determined the efficiency of the photochromic change with respect

to the amount of light absorbed The quantum yield of isomerization

can be strongly dependent on conditions

Fatigue resistance In photochromic materials fatigue refers to the

loss of reversibility by processes such as photodegradation

photobleaching photooxidation and other side reactions All

photochromics suffer fatigue to some extent and its rate is strongly

dependent on the activating light and the conditions of the sample

Photostationary state Photochromic materials have two states and

their interconversion can be controlled using different wavelengths of

light Excitation with any given wavelength of light will result in a

mixture of the two states at a particular ratio called the photo-

stationary state In a perfect system there would exist wavelengths

that can be used to provide 10 and 01 ratios of the isomers

but in real systems this is not possible since the active

absorbance bands always overlap to some extent

Polarity and solubility In order to incorporate photochromics in

working systems they suffer the same issues as other dyes They are

often charged in one or more state leading to very high polarity and

possible large changes in polarity They also often contain large

conjugated systems that limit their solubility

Photochromic complexes

A photochromic complex is a kind of chemical compound that has

photoresponsive Parts on its ligand These complexes have a specific

structure photoswitchable organic compounds are attached to

metalcomplexes For the photocontrollable parts thermally and

photochemically stable chromophores (azobenzene diarylethene

25

spiropyran etc) are usually used And for the metal complexes a wide

variety of compounds that have various functions (redox response

luminescence magnetism etc) are applied The photochromic parts and

metal parts are so close that they can affect each others molecular

orbitals The physical properties of these compounds shown by parts of

them (ie chromophores or metals) thus can be controlled by switching

their other sites by external stimuli For example photoisomerization

behaviors of some complexes can be switched by oxidation and reduction of

their metal parts Some other compounds can be changed in their

luminescence behavior magnetic interaction of metal sites or stability of

metal-to-ligand coordination by photoisomerization of their photochromic

parts

Photochemistry of Polyoxometalates

The photochemistry of polyoxometalates is of great interest to inorganic

chemistsMore than 80 years agoit was found that the R-Keggin tungstate

H3[PW12O40] was reduced photochemically to yield a blue-colored species

which was reoxidized by air and by various other oxidizing agents such as

Fe3+AgNO3and H2O2 [7879]The photoredox reactions of H4[SiW12O40]and

H3[PW12O40] proceeded effectively in the presence of primary and secondary

alcohols their ethers and aldehydes and proteinsbut less effectively in the

presence of tertiary alcoholsketonesestersthe fatty acids above formic

acidand simple amines[8081] The basic photoredox reaction involving

ethanol is illustrated by eq 13

2 H3PW12O40 + H3CCH2OH h ν ν ν ν 2 H4PW12O40 + H3CCHO ------- 13

2 H4PW12O40 +12 O2 2 H3PW12O40 + H2O ------ 14

In this reactionone molecule of ethanol photochemically reduces two

molecules of H3PW12O40 and is itself oxidized to acetaldehyde In the

presence of air the thermal oxidation of the reduced species takes place at

room temperature(eq 14)The reduced polyoxometalates which are the so-

calledldquoheteropolybluesrdquo have been used for the colorimetric analysis of the

26

elements P Si As and Ge and for the determination of uric acidsugarand

other biological compounds [8283] Piperidinium metavanadate also

undergoes photoinduced coloration from white to black followed by a

reversible color change in the presence of oxidizing agentsHowever

ammonium metavanadates ([NH4][VO3]) exhibits no photoinduced

coloration[84] The early photoredox reactions of the R-Keggin

polyoxometalates H4[SiW12O40] and H3PW12O40 were carried out in the

presence of photographic paper however the limited number of the

structurally well-characterized compounds available for study delayed the

development of modern cluster-compound photochemistry until the discovery

of photochromism in alkylammonium polyoxo- molybdate solids[8586] A

photochromic or electrochromic material is one whose light-absorbing

properties are altered upon optical excitation or reduction under the influence

of an externally applied electric field respectively The induced coloration

remains even after the excitation source has been removed These materials

are of technological interest because they return to their original state either

thermally upon irradiation with light of a frequency corresponding to the

induced absorption or electrochemically upon reversing the polarity of the

externally applied electric field Thus photochromic and electrochromic

materials behave in a reversible manner Polyoxometalates exhibit significant

photo-and electrochromism which makes them suitable as nanocomposite

molecular devices and as models for probing the physical properties of infinite

metal oxides Since the metal ions in the oxidized polyoxometalates have d0

electronic configurations the only absorption band which occurs in the UV-vis

range of the electronic spectra is due to the oxygen-to-metal (O-M) ligand-to-

metal charge transfer (LMCT)Upon irradiation electrons are promoted from

the low-energy electronic states which are mainly comprised of oxygen 2p

orbitals (the valence band in the band model)to the high-energy electronic

states which are mainly comprised of metal d orbitalrsquos (the conduction band

in the band model)The fundamental transitions in polyoxometalate lattices

are depicted schematically in Fig1B1

27

Fig1B1 Simple model showing the electronic transitions in the

polyoxometalates containing electron donar and acceptor (a)generation

of charge carriers(b)electron and hole trapping(c)electron release due

to stimulation(d)recombination

between electron and hole Electrons are e-and holes are h+

In the polyoxometalates containing heteroatoms and especially in

mixed metal polyoxometalates the charge carriers which are created by the

light or electric field may be trapped in electron traps and hole traps These

traps provide states of localized energy in the O-M LMCT energy gap due to

the heteroatoms or counter cations which correspond to impurities or lattice

defects in the band model If the trap depth ∆E is large compared to kT the

probability for thermal escape from the trap will be negligibly small and

metastable situation will existThe trapped carriers can be released by thermal

or optical stimulationIn the case of thermal stimulation the irradiated

polyoxometalate is heated until the energy barrier ∆E can be overcome The

trapped electron (or hole) then can escape from the trap and nonradiatively

recombine with the trapped hole (or electron)Under optical stimulation the

energy of an incident photon is used to overcome ∆E The relaxation

processes of the OndashM LMCT excitation energy include both the nonradiative

recombination of electrons and holes within the energy gap and the

intramolecular energy transfer leading to a charge-transfer emission This

intramolecular energy transfer corresponds to the O-M LMCT energy gap and

occurs via radiative recombination and sensitized emission from the

28

heteroatoms or cations If several energy levels based on the hetero atoms or

counter cations act as energy acceptors within the O-M LMCTenergy gap the

energy transfer occurs from the O-M LMCT states to these levels followed by

the nonradiative or radiative deactivation of the excitation energy It should be

noted that the O-M LMCT states also can be generated by the application of

very high electric fields to the polyoxometalate solids as demonstrated by the

observation of electroluminescence[87]If an external electric field with a

potential more negative than the energy levels of the vacant orbitals involved

in the O-M LMCT transition is applied to a polyoxometalate on the electrode

surface an electrochemical reduction occurs via the injection of electrons

from the electrode in to the vacant levels of the polyoxometalate as shown in

Fig1B 2

Fig1B2 - Energy scheme for the electrochromism of polyoxometalates

a)electrochemical reduction (b) electrochemical oxidation

Electrons injected in to the high-energy levels also may be trapped by

electron traps in a process analogous to that which occurs during LMCT

photoexcitation of the polyoxometalates These electrons are returned to the

electrode by electrochemical oxidation at an electrode potential more positive

than the energy levels for the d1 electron states The d1electrons in the O-M

LMCTstates facilitate the absorption of visible light via intervalence charge

transfer among metal centers and d-d transitions The same type of transition

may be possible for the d1electron captured by the electron traps too In

addition to searching for new photosensitive polyoxometalates with the

29

potential for having practical application there is now a need to elucidate the

fundamental photo-and electrochemical coloration processes by studying

electron transfer within the polyoxometalate lattices in conjunction with their

crystal structures So far few polyoxometalates exhibit a perfect reversibility

of coloration The irreversibility of the color change arises from as yet

uncharacterized side reactions during both the coloration and decoloration of

the polyoxometalates

Many metal oxides including aluminum titanium vanadium niobium

molybdenum and tungsten oxides are photochromic when they contain

impurities or dopants This coloration has been interpreted on the basis of

electron trapping at appropriate lattice sites within the crystals as shown in

Fig1B1 where the O-M LMCT transition corresponds to the transition

between the valence and conduction bands for the infinite metal-oxide lattice

[88-91]

B12 Applications of Photochromic materials

Sunglasses

One of the most famous reversible photochromic applications is color

changing lenses for sunglasses as found in eyeglasses The largest limitation

in using PC technology is that the materials cannot be made stable enough

to withstand thousands of hours of outdoor exposure so long-term outdoor

applications are not appropriate at this time The switching speed of

photochromic dyes is highly sensitive to the rigidity of the environment around

the dye As result they switch most rapidly in solution and slowest in the rigid

environment like a polymer lens Recently it has been reported that attaching

flexible low Tg polymers (for example siloxanes or poly (butyl acrylate) to the

dyes allows them to switch much more rapidly in a rigid lens [76] Some

spirooxazines with siloxane polymers attached switch at near solution like

speeds even though they are in a rigid lens matrix

Supramolecular chemistry

Photochromic units have been employed extensively in supramolecular

chemistry Their ability to give a light controlled reversible shape change

means that they can be used to make or break molecular recognition motifs

30

or to cause a consequent shape change in their surroundings Thus

photochromic units have been demonstrated as components of molecular

switches The coupling of photochromic units to enzymes or enzyme cofactors

even provides the ability to reversibly turn enzymes on and off

by altering their shape or orientation in such a way that their functions

are either working or broken

Data storage

The possibility of using photochromic compounds for data storage was

first suggested in 1956 by Yehuda Hirshberg[77] Since that time there have

been many investigations by various academic and commercial groups

particularly in the area of 3D optical data storage which promises discs that

can hold a terabyte of data Initially issues with thermal back-reactions

and destructive reading dogged these studies but more recently more stable

systems have been developed

Novelty items

Reversible photochromics are also found in applications such as toys

cosmeticsclothing and industrial applications If necessary they can be made

to change between desired colors by combination with a permanent pigment

A large number of inorganic compounds exhibit photochromism

These solids often have large band gaps of the order of 3 - 12 eV and

excitation of these solids leads to the formation of metastable centers that

absorb visible light giving rise to their colour They can return to their ground

state by heating or by optical excitation within the colour-centre band In most

cases the photochromism is a structure sensitive phenomenon involving

localized defect impurities or dislocations Some of these inorganic

compounds have the potential for a number of different uses Photochromic

compounds have a number of useful applications These can be divided

according to the most important property that is being used (Table 11) [92]

31

Table 11 Applications of Photochromic materials

Applications Depending Upon

Sensitivity to

Radiation

Reversibility Thermal Chemical or

Physical Properties

Self-developing

photography

Chemical switches for

computers

Temperature indicators

Protective

materials

Data displays

Heat-sensitive recording

media

Optical signal

processing

Photomasking and

photoresist technology

Reusable data storage

media

Anaytical reagents

Photochromic

microimages

Photopolymerisation

Information encoding

and steganography

Photocontractile

polymers and the

photoviscosity effect

Control of light

intensity

Q-switches

Pyroelectric

photochromic materials

B13 Thermochromism

Thermochromism is the reversible colour change of a substance

induced by temperature change A large variety of substances organic

inorganic organometallic supramolecular and polymeric systems exhibit this

phenomenon Examples of these include bianthrones cobalt

hexacyanoferrate the zirconocene complex of 1 4-diphenyl-1 3-butadiene

and poly (3-alkylthiophene) The organic 99-bixanthenylidene is colourless at

90 K yellow-green at 298 K and dark-blue when melted at 592 K Heating

conducting polymers can cause them to change colour This is achieved by

causing conformational changes to the polymer backbone resulting in a

change in the band gap of the polymer It has been reported that regioregular

32

P3HT reversibly changes colour upon heating to 220ordmC due to temperature-

dependent conformation changes Thermally cross linked polymer undergoes

the same colour change but it is much less reversible [93] Other forms of

thermochromism may be commercially important eg to give a visual

indication of temperature changes

B14 Electrochromism

Electrochromism describes a phenomenon of material color change in

a persistent but reversible manner produced by electrochemically induced

oxidation-reduction reactions Electrochromic materials can be applied to

various kinds of products such as smart windows and display devices Among

those applications there have been lots of efforts to develop electrochromic

display devices (ECDs) Especially flexible display devices are now attracting

much attention worldwide since they can facilitate new technological demands

such as bending and folding of paper-like displays High electrochromic

efficiency short response time long operating life time and reduction of

energy consumption are the most important requisites of the materials for the

paperlike displays [94 95] Among those properties the operation life time is

the most important barrier to overcome for a realization of ECDs There are

two types of electrochromic material a) inorganic transition metal oxides

(TMOs) b) organic polymer materials The TMOs have been studied longer

than the organic materials that they have been studied since 1960s [9697]

Electrochromism describes a reversible color change of material

produced by electrochemically induced oxidation-reduction reactions It is one

of several types of chromism of materials As thermochromism and

photochromism mean material color changes made by heat and light

respectively electrochromism refers that the color change is caused by an

electric potential In most cases the color change in electrochromism can be

driven by rather low electrical potential of the order of a fraction of volt or a

few volts [94-96] The color change of material means variation in

transmittance andor reflectance change in visible range which is originated

from different electronic absorption bands according to a switching between

oxidation and reduction state of material When electric potential is applied on

electrochromic material forced oxidation or reduction is derived and the

33

individual color is originated from the corresponding oxidation or reduction

state of the material For electrochromic materials the characteristic color

change is reversible since the oxidation and reduction state can be converted

reversibly by switching potential

Application fields

The application area for the electrochromism is rather broad that it

covers from smart window glazing and optical modulators to information

displays [98-102] The smart windows are typical examples The

electrochromic property is used to control the amount of light and heat to pass

through the windows Usually the electrochromic material is in form of thin film

coated on a window glass The transmittance modulation has also been

applied at the automobiles to automatically tint rear-view mirrors in various

lighting conditions The electrochromic application fields are illustrated in Fig

1B3

Since the smart windows control the transmittance of heat as well as

the transmittance of visible light the working definition of electrochromism has

now been extended to include devices for modulation of radiation in the near

infrared thermal infrared and microwave regions When color for

electrochromic materials is used this can now mean a response by detectors

at these wavelengths and not just by the human eyes Nowadays

electrochromic material draws much attention as being used in the display

devices Electrochromic display device (ECD) is being considered as one of

the candidates for the conventional liquid crystal display (LCD) since ECD

has many advantages over LCDs Among these advantages the most

important are low energy consumption wider viewing angle high contrast

rate and possibility to achieve multiple colors with a single material [103]

When a new redox state of electrochromic materialis established by

the applied electric pulse then it is maintained after the potential is switched

off This means the colored or bleached state of the material can be sustained

for a considerable time without applying electrical power This is so-called

ldquocolor memory effectrdquo of electrochromic material Because of the color

memory effect energy consumption for the electrochromic display device

could be drastically reduced and this would be a big advantage over other

34

emissive devices The low power consumption is especially advantageous

when it is applied to mobile devices with limited power source The possibility

for a flexible display is another attraction for electrochromic material

Information displays

Real-view mirrors for automobiles

Fig 1B4 Application fields of electrochromic devices Smart windows information displays and real-view mirrors for automobiles

35

Using polymeric electrochromic material and plastic substrate with a

transparent conductive coating it is possible to build all-plastic flexible display

device There are lots of papers and patents about the flexible display devices

[104105] In these cases gel-type electrolyte is also needed The gel-type

electrolyte immobilizes liquid electrolyte in the polymer network [106107]

Recently ITO-coated polymer films are readily available which would provide

the plastic substrate for a flexible device The flexible electrochromic displays

are frequently tried with the plastic substrate flexible electrochromic material

and gel-type electrolyte The flexible electrochromic displays would facilitate

increasing technical demands for foldable display devices

Metal Ion Electrochromism

Many transition metal oxides are capable of redox reactions that result in

colour change Metal oxide films are commonly prepared as thin layers of

either tungsten nickel molybdenum or other metal compounds by a number

of techniques These include sol-gel electrochemical by dc or rfreactive

sputtering techniques electron-beam evaporation by anodic or cathodic

electrodeposition or by solution dipping of the electrochromic metal

compounds (or compounds that can be changed into these metal compounds)

onto optically transparent electrodes (OTE) [108 -114] Their electrochromism

is derived from the colour change associated with a change in the oxidation

state of the metal anion The behaviour of these materials is dependent upon

pH moisture and exposure to the atmosphere [115] Generally the switching

rates of these films is somewhat slow with typical switching times of about 15

- 60 seconds to achieve 100 conversion to either coloured or bleached state

[116 -120] An Example of this includes nickel oxide which changes from

transparent (pale green) to brownblack taking about 30 seconds to do so

[121] Other examples include [(NH4)5Ru]2(pyrazine)5+ and [(NH4)5Ru]2(44-

bipyridine)5+ whose electrochromism is significantly different due to the effect

of the ligand [122] Table 12 below gives some examples of metal oxide films

with electrochromic properties

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

6

represents oxygen[33] The structure self assembles in acidic aqueous

solution and is the most stable structure of polyoxometalate catalysts

Fig 1A1 Keggin structure

The first α-Keggin anion ammonium phosphomolybdate

((NH4)3[PMo12O40]) was first reported by Berzelius in 1826 In 1892

Blomstrand proposed the structure of phosphomolybdic acid and other poly-

acids as a chain or ring configuration Alfred Werner using the coordination

compounds ideas of Copaux attempted to explain the structure of

silicotungstic acid He assumed a central group [SiO4]4- ion enclosed by four

[RW2O6]+ where R is a unipositive ion The [RW2O6]

+ are linked to the central

group by primary valences Two more R2W2O7 groups were linked to the

central group by secondary valences This proposal accounted for the

characteristics of most poly-acids but not all

In 1928 Linus Pauling proposed a structure for α-Keggin anions

consisting of a tetrahedral central ion [XO4] n-8 caged by twelve WO6

octahedral In this proposed structure three of the oxygen on each of the

octahedral shared electrons with three neighboring octahedral As a result 18

oxygen atoms were used as bridging atoms between the metal atoms The

remaining oxygen atoms bonded to a proton This structure explained many

characteristics that were observed such as basicities of alkali metal salts and

the hydrated of some of the salts However the structure could not explain the

structure of dehydrated acids

JF Keggin with the use of X-ray diffraction experimentally determined

the structure of α-Keggin anions in 1934 The Keggin structure accounts for

both the hydrated and dehydrated α-Keggin anions without a need for

7

significant structural change The Keggin structure is the widely accepted

structure for the α-Keggin anions [34] For example the α-Keggin anion of

phosphotungustic acid is shown in Fig1A2

The structure is composed of one heteroatm surrounded by four

oxygen to form a tetrahedronThe heteroatom is located centrally and caged

by 12 octahedral WO3 units linked to one another by the neighboring oxygen

atoms There are a total of 24 bridging oxygen atoms that link the 12 addenda

atoms The metal centres in the 12 octahedra are arranged on a sphere

almost equidistant from each other in four M3O13 units giving the complete

structure an overall tetrahedral symmetry The bond length between atoms

varies depending on the heteroatom (X) and the addenda atoms (M)

Fig 1A2 α-Keggin anion of phosphotungustic acid ( PW12O40 ) 3-

For the 12ndashphosphotungstic acid Keggin determined the bond length

between the heteroatom and each the four central oxygen atoms to be 15 Adeg

The bond length form the central oxygen to the addenda atoms is 243 Adeg

The bond length between the addenda atoms and each of the bridging

oxygen is 19 Adeg The remaining 12 oxygen atoms that are each double

bonded to an addenda atom have a bond length of 170 Adeg The octahedra

are therefore distorted This structure allows the molecule to hydrate and

dehydrate without significant structural changes and the molecule is thermally

stable in the solid state for use in vapor phase reactions at high temperatures

(400-500 degC)[35-36]

8

Including the original Keggin structure there are 5 isomers designated

by the prefixes α- β- γ- δ- and ε- The original Keggin structure is designated

α- These isomers are sometimes termed Baker Baker-Figgis or rotational

isomers [37]These involve different rotational orientations of the Mo3O13 units

which lowers the symmetry of the overall structure Lacunary Keggin

structures

The term lacunary is applied to ions which have a fragment missing

sometimes called defect structures Examples are the (XM11O39)nminus and

(XM9O34)nminus formed by the removal from the Keggin structure of sufficient Mo

and O atoms to eliminate 1 or 3 adjacent MO6 octahedra The Dawson

structure X2M18O62nminus is made up of two Keggin lacunary fragments with 3

missing octahedra Some structural types are found in many different

compounds The first known example of this was the Keggin ion whose

structure was found to be common to both molybdates and tungstates with

different central hetero atoms Examples of some fundamental

polyoxometalate structures are shown below The Lindquist ion (Fig1A4) is

an iso-polyoxometalate the other three are hetero-polyoxometalates The

Keggin and Dawson structures (Fig1A3 and (Fig1A5) have tetrahedrally

coordinated hetero-atoms eg P or Si Anderson structure has an octahedral

central atom egAl

Fig 1A3 Keggin structure XM12O40nminus

9

Fig 1A 4 Lindquist structure M6O19nminus

Fig 1A 5 Dawson structure X2M18O62nminus

In general α-Keggin anions are synthesized in acidic solutions For

example 12-Phosphotungstic acid is formed by condensing phosphate ion

10

with tungstate ions The heteropolyacid that is formed has the Keggin

structure

[PO4]3- + 12 [WO4]

2- + 27 H+ rarr H3PW12O40 + 12 H2O ----------- 11

α-Keggin anions have been used as catalyst in hydration polymerization and

oxidation reaction as catalysts

The metal atoms that make up the framework (termed addenda atoms)

are typically Mo W and V When more than one element is present the

cluster is called a mixed addendaclusterThe ligands coordinated to metal

atoms that together form the bridged framework are usually oxide ions but

other elements such as S and Br have been substituted for some of the oxide

ions (Note that sulfur substituted POM is often termed a

polyoxothiometalates) Another development is the use of other ligands eg

nitrosy and alkoxy to replace oxide ions The typical framework building

blocks are polyhedral units with 4 5 6 or 7 coordinate metal centers These

units usually share edges andor vertices The most common unit for

polymolybdates is the octahedral MoO6 unit which is a distorted octahedron

where the Mo atom moves off centre to give one short Mo-O bond In some

polymolybdates there are pentagonal bipyramidal units and these are key

building blocks in the molybdenum bluesHetero atoms are present in many

polyoxometalates Many different elements can act as hetero-atoms

Examples of various coordination numbers around the hetero-atom are

known

4 co-ordinate (tetrahedral) in Keggin Dawson and Lindquist structures

(eg PO4 SiO4 AsO4)

6 co-ordinate (octahedral) in Anderson structure (eg Al(OH)6 TeO6

8 co-ordinate (square antiprism) in ((CeO8)W10O28)8minus

12 co-ordinate (icosahedral) in (UO12)Mo12O30 8minus

11

Often the hetero-atom is centrally located in the anion (eg Keggin

structure) or in a structure fragment eg the 2 phosphorus atoms in the

Dawson ion are central to the two symmetric fragments There are similarities

to clathrate structures The Keggin ion can be formulated as PO4 2minusand

M12O36 and the Dawson as (XO42-)2 and M18O54Structural isomerism is

common For example the Keggin structure has 5 isomers which can be

considered to contain one or more of the four M3O13 units being rotated

through 60degMany compounds share the same framework architectures or

frameworks derived from a larger framework with one or more addenda atoms

and oxide ions removed to give defect structure usually called a lacunary

structure An example of a compound with a Dawson lacunary structure is

As2W15O56Some cage structures containing ions are known eg an example

is the vanadate cage V18O42 containing a Clminus ion [38] This has 5 co-

ordinates square pyramidal vanadium units linked together

Fig 1A 6 - H4V18O42 cage containing Cl

1A3 General Properties of polyoxometalates -

Typically polyoxoanions are water and air stable species of large size

(6-25 Adeg) and high ionic weight In aqueous solution they are subject to

decomposition by hydroxide ions eg

[PW12O40]3- + 23 OH- HPO4

2- + 12WO42- + 11 H2O -------- 12

12

Although the PH at which such reactions are rapid can very widely

depending upon the polyanion involved Polyanions are often much stable

towards the H3O+ ions and numerous crystalline heteropoly acids are known

Such acids may be extremely soluble in water and polar solvents (giving

solutions with densities in excess of 4 gcm3) and have large dissociation

constants (PK lt 0)

Crystalline heteropoly acids and salts are frequently highly hydrated with

up to 50 molecules of water per anion Much of this water is zeolite in nature

and crystal composition can vary accordingly On the other hand the cation

anion stiochiometry is always well defined as the anion structure and

composition Finally many polyanions are powerful oxidizing agents and

undergo multiple reversible one or two electron reductions leading to intensely

colored mixed valence species known as heteropoly blues Polyanions are

known which can accept as many as 32 electrons without major structural

change

1A4 Chemistry of Molybdenum and Tungusten Heteropolyoxometalates

A photochromic monolayer film of phosphomolybdic acid (denoted as

PMo12) was fabricated by self-assembly approach UV-visible spectrum and

AFM observation show that the monolayer film is composed of aggregated

PMo12 molecules The monolayer film shows good photochromic properties

with enough stability and reversibility The colour change of the monolayer

after UV-irradiation can be captured by a microscope equipped with a color

CCD camera Photochromic response of the monolayer film can be doubled

after being modified by an amine monolayer [39]

Series of vanadium substituted molybdo Keggin HPA with 12 or 3

adjacent vanadium atoms were prepared These materials were supported

on carbon cloth electrodes and hot pressed onto Nafion with an ETEK

electrode used as a standard on the opposite side The MEArsquos were run at

temperatureslt100 oC with the HPA electrode as either the anode or the

cathode Stable polarisation curves are obtained for an HPA based cathode

with reasonable current densities at 80 oC [40]

A novel mixed-valence polyoxomolybdenum anion was synthesized

hydrothermally from molybdenum oxidemolybdenum metalboric and

phosphoric acids12-phenyldiphosphonicacidand imidazole (ImH) and was

13

structurally characterized as an imidazolium saltOne-and two-dimensional

structures of this anion and additional molybdenum diphosphonate linkers

were assembled as wellThey were structurally characterized as their

pyridinium(pyH) salts [41]

Phosphomolybdic acidpolyvinylpyrrolidone hybrid films were found to

show visible light photochromism It is identified that the intra-supramolecular

charge transfer between the inorganic and organic molecules is responsible

for the visible-light coloration Interestingly the films show photo-memory and

thermal activation The films show a small change in absorbance after being

irradiated with visible light for a short time and the coloration can be

enhanced greatly by subsequent thermal treatment Electrical measurements

indicate that the conductivity of the film increases after the brief irradiation

which promotes transfer of the electrons induced by the thermal treatment

[42]

In this work major effort was concentrated on passive thermal control

coatings based on photochromic and thermochromic materials The inorganic

photochromic materials were based on tungsten and molybdenum oxide films

and the organic photochromic materials included spiropyrans and

spirooxazines In addition photochromic composite organic-inorganic films

and thermochromic vanadium oxide films were prepared The samples were

synthesized using sputtering sol-gel process and thermal oxidation [43]

Polyoxometalates a class of oxidatively robust inorganic oxidants and

oxidation catalysts are currently under investigation at the Forest Products

Laboratory and at Emory University as an alternative to chlorinebased

chemicals in the bleaching of soft Woodkraft and other pulps Although

polyoxometalate salts are used in a number of industrial processes the

feasibility of using these salts and oxygen in the commercial bleaching of

chemical pulps was only recently demonstratedA clear advantage of

polyoxometalates over oxygen alone hydrogen peroxide or ozone is their

inherently high selectivity for the residual lignin in softwood kraft pulps The

goal of ongoing research is to develop a highly selective energy efficient

oxygen based polyoxometalate delignification and bleaching technology

compatible with mill closure [44]

14

Tungsten oxide exhibits pronounced photochromism upon bandgap

photoexcitation which makes it attractive and promising for applications in

many areas Some advances have been achieved during the past decades

The research on nanocrystalline films and single crystals indicates the critical

importance of defects in tungsten oxide to its photochromism Based on

energy-band engineering of semiconductors enhancement of photochromism

has been achieved for instance extension of the photoresponse from UV to

visible light by cathodic polarization improved change in absorption before

and after coloration through modification by a noble metal or another metal-

oxide semiconductor and increased photochromic reversibility via

hybridization with organic amines Nanocrystalline oxide films exhibit

controllable wettability which is coherent in nature with photochromism [45]

Polyoxometalates represent a diverse range of molecular clusters with

an almost unmatched range of physical properties and the ability to form

structures that can bridge several length scalesThe new building block

principles that have been discovered are beginning to allow the design of

complex clusters with desired properties and structures and several structural

types and novel physical properties are examinedIn this critical review the

synthetic and design approaches to the many polyoxometalate cluster types

are presented encompassing all the sub-types of polyoxometalates including

isopolyoxometa- lates heteropolyoxometalates and reduced molybdenum

blue systems As well as the fundamental structure and bonding aspectsthe

final section is devoted to discussing these clusters in the context of

contemporary and emerging interdisciplinary interests from areas as diverse

as antiviral agentsbiological ion transport modelsand materialsscience [46]

Keggin type molybdovanadophosphoric heteropoly acids were prepare

d by a novel environmentally benign method and their catalytic performances

were evaluated via hydroxylation of benzene to phenol with hydrogen

peroxide as oxidant in a mixed solvent of glacial acetic acid and acetonitrile

Various reaction parameters such as reaction time reaction temperature

ratio of benzene to hydrogen peroxide concentration of aqueous hydrogen

peroxide ratio of glacial acetic acid to acetonitrile in solvent and catalyst

concentration were changed to obtain an optimal reaction conditions

15

Molybdovanadophosphoric heteropoly acids are revealed to be highly

efficient catalyst for hydroxylation of benzene [47]

Thin films from the system (As2S3)Tl were deposited by thermal

evaporation on Si graphite and optical glass substratesFrom transmission

and reflection measurements of the thin films the refractive index (n) film

thickness(d) optical band gap(Eg) optical oscillator energy(Eo) and

dispersion energy(Ed) before and after exposure to light were determined

The results for optical parameters were analyzed using the Wemple - Di

Domenico single oscillator model and Lorenc-Lorenc equation It was found

that Eg decreases while n E0 and Ed increase for as deposited films

decreases while n E0 and Ed increase for as deposited films with increasing

of Tlconcentration passing through a maximum at 6 at of Tl After exposure

to light n E0 Ed increase and Eg decreases for all compositions

investigated The maximum change in n (Dn = 016 at l = 6328 nm) was

observed for thin As38S56Tl6 films From infrared spectroscopy measurements

of bulk glasses and thin films we could conclude that when up to 6 at of

thallium is introduced As-S-As chains break and a ternary TlAsS2

compound appears at 10 at Tl [48]

The organo - inorganic hybrid materialconsisting of Poly (34 Ethylene

Dioxythiophene) (PEDOT) doped with phosphomolybdate cluster anions

[PMo12O40]3-has been synthesized by direct insitu oxidative polymerization of

34-Ethylene Dioxythiophene (EDOT) with phosphomolybdic acid

(H3PMo12O40) Its characterization is investigated by Fourier Transform

Infrared Spectroscopy (FT-IR) and Scanning Electron Microscopy (SEM) The

hybrid material presents predominantly high electronic conductivities of

around 20 and 70 S cm1at 300 and 400 K respectively [49]

Heteropolyacids (HPAs) are known to be excellent re-dox catalysts In

combination with TiO2 HPAs can be used as photocatalysts active in visible

light The HPA accepts electron and get reduced to heteropolyblue (HPB)

That can absorb light in the visible range HPA can be incorporated onto the

external surface or in the pores of zeolite based composite photocatalysts

have been designed by incorporation of HPA semiconductor TiO2 and

transition metal cobalt on zeolite This composite metallozeolite photocatalyst

is efficient in photoreduction of methyl orange (MO) in visible light to the tune

16

of 411 mg of MO photo reducedg TiO2 This catalyst also shows encouraging

results for hydrogen evolution from water to the tune of 2730 micromolhg TiO

[50]

A new class of proton conducting glass membranes based on heteropo

lyacids such as phosphotungstic acid (PWA) as electrolytes for low

temperature H2O2 fuel cells was investigated Parameters for a single fuel cell

with a catalyst electrode of 015 mgcm2 of PtC and a glass composite

membrane were characterized by electrochemical measurements at open

circuit potential conditions The performance of the membrane electrode

assemblies (MEA) was systematically studied as an effect of SiO2 and P2O5

concentrations in the glass composite membrane and the MEA was found to

exhibit a maximum power density of 162 mW cm2 for an H2O2 fuel cell at

30 degC and 30 relative humidity (RH) [51]

Two new photochromic inorganic-organic hybrid materials formed from

Keggin type Polyoxometalates (POMs) and metronidazole (C6H9N3O3 MNZ)

formulated as H3PMo12O40bull3 MNZ3H2O (1) and H3PW12O40bull3MNZ3H2O(2)

were synthesized and characterized by elemental analysis IR spectra

electronic spectra electron spin resonance (ESR) spectra and TG-DTA

Reflectance spectra show the presence of weak inter molecular charge

transfer between the organic and inorganic moieties in the solid state The

photochromic properties were studied by solid diffuse reflectance spectra and

ESR spectra and the photochromic reactions were found to exhibit first-order

kinetics TG-DTA showed that two hybrid materials have similar thermal

behavior [52]

Heteropolycompounds (HPCs) have been a matter of interest in basic

and applied science for more than a century From their first synthesis many

advances have been made to promote the use of HPCs in different ways in

science and technology The aim of this article is to review the main structural

characteristics of heteropolycompounds of the Keggin type (12

tungstophosphoric12-molybdophosphoric12-tungstosilicic acid alkaline and

alkaline earth salts of12 tungstophosphoric acid and gels doped with HPCs)to

understand and explain their different activities such as high proton

conductivity and catalytic biochemical and biomedical activities [53]

17

A solid hybrid molecular material containing 1-butyl 3-methyl

imidazolium cations and Keggin anions of phosphotungstic acid has been

synthesized It is fully characterized by CHN analysis FTIR XRD UV-Vis-NIR

DRS 31P MAS NMR TGA and SEM The FTIR spectrum of the compound

shows the fingerprint vibrational bands of both Keggin molecular anions and

imidazolium cations The aromatic CndashH stretch region (2700ndash3250 cmndash1) of

imidazolium cation is split due to the interaction between the ring CndashH and

bulky Keggin anion The red-shift in the UV-Vis spectra and the downfield 31P

MAS NMR chemical shift also confirm the electrostatic interaction between

the ions in the compound Near IR spectral region (1000ndash2500 nm) shows the

elimination of water in the compound which is hydrophobic [54]

Transport coefficient measurements (electrical conductivity

thermoelectric power and Hall coefficient) have been performed on a

compact Tl033MoO3 polycrystalline compound in a wide temperature range

(200ndash400 K) Experimental results are interpreted with the help of a p-type

semiconductor model with two inverted deep levels near the midgap The

valence band and the conduction band are assumed to be formed from the

dxy orbitals of molybdenum atoms in the Mo6O22 cluster leading to narrow π-

bonding bands The donor and acceptor levels may be formed from

nonbonding dxy orbitals arising respectively from anionic and Tl+ defects

Electron paramagnetic resonance and magnetic measurements are in good

agreement with the theoretical band semiconductor model which has been

retained [55]

The optical properties of GaInTlAs epilayers grown at low temperature

~230degC by solid-source molecular-beam epitaxy on InP substrates were

characterized using optical absorptionand photoluminescence techniques

Optical absorption measurements a room temperature show a gap shrinkage

toward lower energies from 071 to 061 and 053 eV when the Tl content

increases from 0 to 4and 8in good agreement with theoretical

predictionsLow-temperature photoluminescence band-gap signals from

GaInAs and GaInTlAs layers are only obtained after rapid thermal annealing

performed inorder to improve the electronic quality of the layersA band gap

decrease as much as 41 meV for GaInTlAs with 19 Tl incorporation is

measured by photoluminescence at 8K [56]

18

The electrochromic performance of all solid ndash state cells employing

phosphotungstic acid and phosphomolybdic acid is reported These cells

employ SnO2 as the viewing electrode and graphite as the back electrodeThe

cells in the bleached state can be made white to red and become black in the

coloured state [57]

Various organic compounds were oxidized by molecular oxygen in the

presence of a catalytic amount of mixed addenda heteropolyoxometalates

containing molybdenum and vanadium The catalytic activity of the

Molybdovanadophosphate was found to be greatly enhanced by supporting

on charcoal The supported catalyst has high catalytic activity for oxidative

dehydrogenation of benzylic and allylic alcohols to the corresponding

aldehydes and ketones (46-92) nevertheless the nonsupported catalyst

was inactive for the same oxidations under these conditions 236Trimethyl

phenol was selectively oxidized to trimethyl-p benzoquinone which is

precursor of VitE in the presence of a catalytic amount of

molybdophosphate In addition the aerobic oxidation of amines alkyl-

substituted phenols and alkanes were also examined [58]

The reaction of Tl2CO3 with 111555-hexafluoro-24-pentanedione

and diglyme CH3O(CH2CH2O)2CH3 or tetraglyme CH3O(CH2CH2O)4CH3 in

dichloromethane yields the anhydrous thermally and air stable volatile Tl

diglyme and Tl tetraglyme adducts They have been characterized by single

crystal X-ray diffraction elemental analysis 1H and 13C NMR IR and mass

spectroscopy Thermal and mass-transport properties have been investigated

using thermo gravimetric and differential scanning calorimetric

measurements There is evidence that both precursors are very low melting

and volatile and can be used as liquid Tl sources Both adducts have been

successfully applied to metalndashorganic chemical vapor deposition of thallium

containing films [59]

Large size and high quality single crystals of quasi-two-dimensional

thallium molybdenum purple bronze TlMo6O17 have been grown by electrolytic

reduction of molten salt of Tl2CO3-MoO3 The crystal structure is trigonal with

space group P3m1 determined by X-ray diffraction and four-circle single crystal

diffraction The lattice parameters of the unit cell are a = b = 55282 Adeg and c

= 136991 Adeg The temperature dependence of resistivity and magnetic

19

susceptibility confirmed that a metal-to-metal transition occurs near 110 K

[60]

Hall coefficient and dc conductivity studies were made on p-type

Pb08Sn02Te thin films doped with different concentrations of thallium in the

temperature range 77 to 500 K The Hall coefficient and Hall mobility are

found to decrease with an increase in the doping concentration of thallium

Hall coefficient data have been analyzed in the light of a double valence-band

model Various band parameters such as valence band separation population

ratio mobility ratio and effective mass ratio have been calculated Hall

mobility data have been analyzed in the light of lattice and defect limited

scattering mechanisms [61]

Proton conducting composites of heteropolyacid hydrates phosphomol

ybdic acid H3PMo12O40 nH2O(PMA) phosphotungstic acid H3PW12O40 nH2O

( PTA) and salt hydrate like NiCl2 6H2O were prepared

with insulating Al2O3 as despersoidThe ionic conductivity peaks at two

concentrations of Al2O3 indicating two percolation thresholds for percolation

thresholds for proton conduction Two separate experiments were carried out

to check the existence of such percolation thresholds viz the volta battery

experiment involving the measurement of emf of an electrochemical cell

with composites of different compositions used as electrolyte and the

composition vs conductivity measured by the complex impedance

spectroscopy The presence of two maxima has been attributed to two

different percolation thresholds for the two possible mobile protonic

species H+ + (H3O+) and OH arising from the hydrates [62]

1 A5 Applications of Heteropolyoxometalates-

Applications of heteropolyanions centre depend on their redox properties

their high charges and ionic weights An enormous patent and journal

literature is devoted to the applications of heteropolyanions

1 Analysis-

The formation and subsequent precipitation or reduction of

[XMO12O40]n- anions form the basis of gravimetric and colorimetric analytical

methods for P As Si or Geeither separately or in combination [63 64]

20

2 Biochemical applications-

lsquoPhosphotungstic Acidsrsquo have for decades been used as precipitants

for proteins and as analytical reagents for proteinsalkaloidsand purines eg

the [P2M18O62]6- anions for colorimetric determination of uric acid [65] and

cholesterol The acid H3P12O40 either in aqueous or ethanolic solution is also

widely used as a non specific electron dense stain for electron spectroscopy

The dyestuffs industry has for many years used heteropolymolybdates and

tungstates to form color lakes and toners from basic dyes Large

heteropolyanions exhibit antiviral antitumoral properties at non-cytotoxic

doses in vitro and in vivo and are protein inhibitors of cellular bacterial and

viral DNA RNA polymerizes [66]

3 Catalysis

Heteropoly acids and salts have been used as heterogeneous catalysts

for a broad variety of reactions and compilations of such applications up to

1973 are available Examples include oxidation of propylene and isobutylene

to acrylic acid methacrylic acids and ammoxidation of acrylonitrile olefin

polymerization and epoxidation Much of current activity concerning

heterogeneous catalysis by heteropoly compounds is being carried out in

Japan [67 68]

4 Other Applications

Insoluble salts of Heteropolyanions especially ammonium

molybdophosphates have been used and are commercially available as ion-

exchange materials [69]Recent work in this area includes thin layer

chromatography of amino acids ion selective membranes [70] and the

preparation of new ion exchangers based on heteropolyanions Crystalline12-

tungstophosphoric and 12-molybdophosphoric acids are excellent protonic

conductors Heteropolyacids are electrochromic in the solid state as a

consequence of heteropolyblue formation Heteropolyblue formation has also

been used to detect alcohol or carboxylic acid radicals generated by radiolysis

of aqueous solutions Potential applications of heteropoly complexes as flame

retardants and smoke suppressants or as corrosion inhibitors and conversion

coatings on steel and aluminium are reported [71] Some potential ldquogreenrdquo

applications have been reported eg non-chlorine based wood pulp

bleaching process and a method of decontaminating water Some structures

21

containing transition metal atoms with unpaired electrons have unusual

magnetic properties and are being investigated as nano computer storage

devices Some compounds exhibit luminescence There are many reported

potential medicinal applications eg anti tumoral and anti-viral There have

been reports on the role of weak or non bonding interactions on the crystal

engineering of hybrid polyoxometalates

Spherical nonporous polyoxomolybdate based capsules of different

types containing more than 100 metal atoms reported by Achim Muller and his

group have versatile unique properties regarding their assembly to vesicles

and the chemistry which can be done inside the pores and cavities A discrete

polyoxometalate Lindquist ion of the form W6O192minus was successfully imaged

recently for the first time within the capillary of a carbon nanotube following

steric locking of the anion with the tubule In situ relaxation of the anion in its

equatorial plain was demonstrated [72]

22

Section-B

Litreture Survey on Chromism in Transition Metal

Oxides

1 B1 Chromism in Transition Metal Oxides

Chromism is a reversible change in a substances colour resulting from

a process caused by some form of stimulus Many materials are chromic

including inorganic and organic compounds and conducting polymers and the

property can result from many different mechanisms Several transition metal

oxides show EC properties The most popular are from the VI - B oxides In

this group WO3 and MoO3 are the most thoroughly studied cathodic EC

materials which can be electrochemically coloured and bleached when used

as the cathode in electrochemical cells Cathodic EC materials also include

V2O3 TiO2 and Nb2O5 Another distinguishable group is anodic EC material

including VIII oxides like IrOx nH2O Rh2O3 nH2O NiO nH2O etc which can be

anodicaly coloured in the electrochemical process when used as anode

There are several types of chromism which are discussed as below

B11 Photochromism

Photochromism is the reversible transformation of a chemical species

between two forms by the absorption of electromagnetic radiation where

the two forms have different absorption spectra [7374]

Trivially this can be described as a reversible change of color upon

exposure to light The phenomenon was discovered in the late 1880s

including work by Markwald who studied the reversible change of color of 23

44-tetrachloronaphthalen-1(4H)-one in the solid state He labeled this

phenomenon phototropy and this name was used until the 1950s

when Yehuda Hirshberg of the Weizmann Institute of Science in Israel

proposed the term photochromism [75] Photochromism can take place

in both organic and inorganic compounds and also has its place in biological

systems (for example retinal in the vision process)

Photochromism does not have a rigorous definition but is usually used

to describe compounds that undergo a reversible photochemical reaction

where an absorption band in the visible part of the electromagnetic spectrum

23

changes dramatically in strength or wavelength In many cases an

absorbance band is present in only one form The degree of change required

for a photochemical reaction to be dubbed photochromic is that which

appears dramatic by eye but in essence there is no dividing line between

photochromic reactions and other photochemistry Therefore while the

transcis isomerization of azobenzene is considered a photochromic reaction

the analogous reaction of stilbene is not Since photochromism is just a

special case of a photochemical reaction almost any photochemical reaction

type may be used to produce photochromism with appropriate molecular desi

gnSome of the most common processes involved in photochromism are peric

yclic reactions cis-trans somerizations intramolecular hydrogen transfer

intramolecular group transfers dissociation processes and electron transfers

(oxidation-reduction)

Another some what arbitrary requirement of photochromism is that

it requires the two states of the molecule to be thermally stable under

ambient conditions for a reasonable time All the same nitrospiropyran (which

back-isomerizes in the dark over ~10 minutes at room temperature) is

considered photochromic All photochromic molecules back-isomerize to their

more stable form at some rate and this back-isomerization is accelerated by

heating There is therefore a close relationship between photochromic and the

rmochromic compounds The timescale of thermal back-isomerization is

important for applications and may be molecularly engineered

Photochromic compounds considered to be thermally stable include some

diarylethenes which do not back isomerize even after heating at 800C for 3

months

Since photochromic chromophores are dyes and operate according to

well-known reactions their molecular engineering to fine-tune their properties

can be achieved relatively easily using known design models quantum

mechanics calculations and experimentation In particular the tuning of

absorbance bands to particular parts of the spectrum and the engineering

of thermal stability have received much attention

Sometimes and particularly in the dye industry the term irreversible

photochromic is used to describe materials that undergo a permanent color

change upon exposure to Ultraviolet or visible light radiation Because by

24

definition photochromics are reversible there is technically no such thing as a

n irreversible photochromic this is loose usage and these compounds

are better referred to as photochangable or photoreactive dyes

Apart from the qualities already mentioned several other properties of

photochromics are important for their use These include

Quantum yield of the photochemical reaction

This determined the efficiency of the photochromic change with respect

to the amount of light absorbed The quantum yield of isomerization

can be strongly dependent on conditions

Fatigue resistance In photochromic materials fatigue refers to the

loss of reversibility by processes such as photodegradation

photobleaching photooxidation and other side reactions All

photochromics suffer fatigue to some extent and its rate is strongly

dependent on the activating light and the conditions of the sample

Photostationary state Photochromic materials have two states and

their interconversion can be controlled using different wavelengths of

light Excitation with any given wavelength of light will result in a

mixture of the two states at a particular ratio called the photo-

stationary state In a perfect system there would exist wavelengths

that can be used to provide 10 and 01 ratios of the isomers

but in real systems this is not possible since the active

absorbance bands always overlap to some extent

Polarity and solubility In order to incorporate photochromics in

working systems they suffer the same issues as other dyes They are

often charged in one or more state leading to very high polarity and

possible large changes in polarity They also often contain large

conjugated systems that limit their solubility

Photochromic complexes

A photochromic complex is a kind of chemical compound that has

photoresponsive Parts on its ligand These complexes have a specific

structure photoswitchable organic compounds are attached to

metalcomplexes For the photocontrollable parts thermally and

photochemically stable chromophores (azobenzene diarylethene

25

spiropyran etc) are usually used And for the metal complexes a wide

variety of compounds that have various functions (redox response

luminescence magnetism etc) are applied The photochromic parts and

metal parts are so close that they can affect each others molecular

orbitals The physical properties of these compounds shown by parts of

them (ie chromophores or metals) thus can be controlled by switching

their other sites by external stimuli For example photoisomerization

behaviors of some complexes can be switched by oxidation and reduction of

their metal parts Some other compounds can be changed in their

luminescence behavior magnetic interaction of metal sites or stability of

metal-to-ligand coordination by photoisomerization of their photochromic

parts

Photochemistry of Polyoxometalates

The photochemistry of polyoxometalates is of great interest to inorganic

chemistsMore than 80 years agoit was found that the R-Keggin tungstate

H3[PW12O40] was reduced photochemically to yield a blue-colored species

which was reoxidized by air and by various other oxidizing agents such as

Fe3+AgNO3and H2O2 [7879]The photoredox reactions of H4[SiW12O40]and

H3[PW12O40] proceeded effectively in the presence of primary and secondary

alcohols their ethers and aldehydes and proteinsbut less effectively in the

presence of tertiary alcoholsketonesestersthe fatty acids above formic

acidand simple amines[8081] The basic photoredox reaction involving

ethanol is illustrated by eq 13

2 H3PW12O40 + H3CCH2OH h ν ν ν ν 2 H4PW12O40 + H3CCHO ------- 13

2 H4PW12O40 +12 O2 2 H3PW12O40 + H2O ------ 14

In this reactionone molecule of ethanol photochemically reduces two

molecules of H3PW12O40 and is itself oxidized to acetaldehyde In the

presence of air the thermal oxidation of the reduced species takes place at

room temperature(eq 14)The reduced polyoxometalates which are the so-

calledldquoheteropolybluesrdquo have been used for the colorimetric analysis of the

26

elements P Si As and Ge and for the determination of uric acidsugarand

other biological compounds [8283] Piperidinium metavanadate also

undergoes photoinduced coloration from white to black followed by a

reversible color change in the presence of oxidizing agentsHowever

ammonium metavanadates ([NH4][VO3]) exhibits no photoinduced

coloration[84] The early photoredox reactions of the R-Keggin

polyoxometalates H4[SiW12O40] and H3PW12O40 were carried out in the

presence of photographic paper however the limited number of the

structurally well-characterized compounds available for study delayed the

development of modern cluster-compound photochemistry until the discovery

of photochromism in alkylammonium polyoxo- molybdate solids[8586] A

photochromic or electrochromic material is one whose light-absorbing

properties are altered upon optical excitation or reduction under the influence

of an externally applied electric field respectively The induced coloration

remains even after the excitation source has been removed These materials

are of technological interest because they return to their original state either

thermally upon irradiation with light of a frequency corresponding to the

induced absorption or electrochemically upon reversing the polarity of the

externally applied electric field Thus photochromic and electrochromic

materials behave in a reversible manner Polyoxometalates exhibit significant

photo-and electrochromism which makes them suitable as nanocomposite

molecular devices and as models for probing the physical properties of infinite

metal oxides Since the metal ions in the oxidized polyoxometalates have d0

electronic configurations the only absorption band which occurs in the UV-vis

range of the electronic spectra is due to the oxygen-to-metal (O-M) ligand-to-

metal charge transfer (LMCT)Upon irradiation electrons are promoted from

the low-energy electronic states which are mainly comprised of oxygen 2p

orbitals (the valence band in the band model)to the high-energy electronic

states which are mainly comprised of metal d orbitalrsquos (the conduction band

in the band model)The fundamental transitions in polyoxometalate lattices

are depicted schematically in Fig1B1

27

Fig1B1 Simple model showing the electronic transitions in the

polyoxometalates containing electron donar and acceptor (a)generation

of charge carriers(b)electron and hole trapping(c)electron release due

to stimulation(d)recombination

between electron and hole Electrons are e-and holes are h+

In the polyoxometalates containing heteroatoms and especially in

mixed metal polyoxometalates the charge carriers which are created by the

light or electric field may be trapped in electron traps and hole traps These

traps provide states of localized energy in the O-M LMCT energy gap due to

the heteroatoms or counter cations which correspond to impurities or lattice

defects in the band model If the trap depth ∆E is large compared to kT the

probability for thermal escape from the trap will be negligibly small and

metastable situation will existThe trapped carriers can be released by thermal

or optical stimulationIn the case of thermal stimulation the irradiated

polyoxometalate is heated until the energy barrier ∆E can be overcome The

trapped electron (or hole) then can escape from the trap and nonradiatively

recombine with the trapped hole (or electron)Under optical stimulation the

energy of an incident photon is used to overcome ∆E The relaxation

processes of the OndashM LMCT excitation energy include both the nonradiative

recombination of electrons and holes within the energy gap and the

intramolecular energy transfer leading to a charge-transfer emission This

intramolecular energy transfer corresponds to the O-M LMCT energy gap and

occurs via radiative recombination and sensitized emission from the

28

heteroatoms or cations If several energy levels based on the hetero atoms or

counter cations act as energy acceptors within the O-M LMCTenergy gap the

energy transfer occurs from the O-M LMCT states to these levels followed by

the nonradiative or radiative deactivation of the excitation energy It should be

noted that the O-M LMCT states also can be generated by the application of

very high electric fields to the polyoxometalate solids as demonstrated by the

observation of electroluminescence[87]If an external electric field with a

potential more negative than the energy levels of the vacant orbitals involved

in the O-M LMCT transition is applied to a polyoxometalate on the electrode

surface an electrochemical reduction occurs via the injection of electrons

from the electrode in to the vacant levels of the polyoxometalate as shown in

Fig1B 2

Fig1B2 - Energy scheme for the electrochromism of polyoxometalates

a)electrochemical reduction (b) electrochemical oxidation

Electrons injected in to the high-energy levels also may be trapped by

electron traps in a process analogous to that which occurs during LMCT

photoexcitation of the polyoxometalates These electrons are returned to the

electrode by electrochemical oxidation at an electrode potential more positive

than the energy levels for the d1 electron states The d1electrons in the O-M

LMCTstates facilitate the absorption of visible light via intervalence charge

transfer among metal centers and d-d transitions The same type of transition

may be possible for the d1electron captured by the electron traps too In

addition to searching for new photosensitive polyoxometalates with the

29

potential for having practical application there is now a need to elucidate the

fundamental photo-and electrochemical coloration processes by studying

electron transfer within the polyoxometalate lattices in conjunction with their

crystal structures So far few polyoxometalates exhibit a perfect reversibility

of coloration The irreversibility of the color change arises from as yet

uncharacterized side reactions during both the coloration and decoloration of

the polyoxometalates

Many metal oxides including aluminum titanium vanadium niobium

molybdenum and tungsten oxides are photochromic when they contain

impurities or dopants This coloration has been interpreted on the basis of

electron trapping at appropriate lattice sites within the crystals as shown in

Fig1B1 where the O-M LMCT transition corresponds to the transition

between the valence and conduction bands for the infinite metal-oxide lattice

[88-91]

B12 Applications of Photochromic materials

Sunglasses

One of the most famous reversible photochromic applications is color

changing lenses for sunglasses as found in eyeglasses The largest limitation

in using PC technology is that the materials cannot be made stable enough

to withstand thousands of hours of outdoor exposure so long-term outdoor

applications are not appropriate at this time The switching speed of

photochromic dyes is highly sensitive to the rigidity of the environment around

the dye As result they switch most rapidly in solution and slowest in the rigid

environment like a polymer lens Recently it has been reported that attaching

flexible low Tg polymers (for example siloxanes or poly (butyl acrylate) to the

dyes allows them to switch much more rapidly in a rigid lens [76] Some

spirooxazines with siloxane polymers attached switch at near solution like

speeds even though they are in a rigid lens matrix

Supramolecular chemistry

Photochromic units have been employed extensively in supramolecular

chemistry Their ability to give a light controlled reversible shape change

means that they can be used to make or break molecular recognition motifs

30

or to cause a consequent shape change in their surroundings Thus

photochromic units have been demonstrated as components of molecular

switches The coupling of photochromic units to enzymes or enzyme cofactors

even provides the ability to reversibly turn enzymes on and off

by altering their shape or orientation in such a way that their functions

are either working or broken

Data storage

The possibility of using photochromic compounds for data storage was

first suggested in 1956 by Yehuda Hirshberg[77] Since that time there have

been many investigations by various academic and commercial groups

particularly in the area of 3D optical data storage which promises discs that

can hold a terabyte of data Initially issues with thermal back-reactions

and destructive reading dogged these studies but more recently more stable

systems have been developed

Novelty items

Reversible photochromics are also found in applications such as toys

cosmeticsclothing and industrial applications If necessary they can be made

to change between desired colors by combination with a permanent pigment

A large number of inorganic compounds exhibit photochromism

These solids often have large band gaps of the order of 3 - 12 eV and

excitation of these solids leads to the formation of metastable centers that

absorb visible light giving rise to their colour They can return to their ground

state by heating or by optical excitation within the colour-centre band In most

cases the photochromism is a structure sensitive phenomenon involving

localized defect impurities or dislocations Some of these inorganic

compounds have the potential for a number of different uses Photochromic

compounds have a number of useful applications These can be divided

according to the most important property that is being used (Table 11) [92]

31

Table 11 Applications of Photochromic materials

Applications Depending Upon

Sensitivity to

Radiation

Reversibility Thermal Chemical or

Physical Properties

Self-developing

photography

Chemical switches for

computers

Temperature indicators

Protective

materials

Data displays

Heat-sensitive recording

media

Optical signal

processing

Photomasking and

photoresist technology

Reusable data storage

media

Anaytical reagents

Photochromic

microimages

Photopolymerisation

Information encoding

and steganography

Photocontractile

polymers and the

photoviscosity effect

Control of light

intensity

Q-switches

Pyroelectric

photochromic materials

B13 Thermochromism

Thermochromism is the reversible colour change of a substance

induced by temperature change A large variety of substances organic

inorganic organometallic supramolecular and polymeric systems exhibit this

phenomenon Examples of these include bianthrones cobalt

hexacyanoferrate the zirconocene complex of 1 4-diphenyl-1 3-butadiene

and poly (3-alkylthiophene) The organic 99-bixanthenylidene is colourless at

90 K yellow-green at 298 K and dark-blue when melted at 592 K Heating

conducting polymers can cause them to change colour This is achieved by

causing conformational changes to the polymer backbone resulting in a

change in the band gap of the polymer It has been reported that regioregular

32

P3HT reversibly changes colour upon heating to 220ordmC due to temperature-

dependent conformation changes Thermally cross linked polymer undergoes

the same colour change but it is much less reversible [93] Other forms of

thermochromism may be commercially important eg to give a visual

indication of temperature changes

B14 Electrochromism

Electrochromism describes a phenomenon of material color change in

a persistent but reversible manner produced by electrochemically induced

oxidation-reduction reactions Electrochromic materials can be applied to

various kinds of products such as smart windows and display devices Among

those applications there have been lots of efforts to develop electrochromic

display devices (ECDs) Especially flexible display devices are now attracting

much attention worldwide since they can facilitate new technological demands

such as bending and folding of paper-like displays High electrochromic

efficiency short response time long operating life time and reduction of

energy consumption are the most important requisites of the materials for the

paperlike displays [94 95] Among those properties the operation life time is

the most important barrier to overcome for a realization of ECDs There are

two types of electrochromic material a) inorganic transition metal oxides

(TMOs) b) organic polymer materials The TMOs have been studied longer

than the organic materials that they have been studied since 1960s [9697]

Electrochromism describes a reversible color change of material

produced by electrochemically induced oxidation-reduction reactions It is one

of several types of chromism of materials As thermochromism and

photochromism mean material color changes made by heat and light

respectively electrochromism refers that the color change is caused by an

electric potential In most cases the color change in electrochromism can be

driven by rather low electrical potential of the order of a fraction of volt or a

few volts [94-96] The color change of material means variation in

transmittance andor reflectance change in visible range which is originated

from different electronic absorption bands according to a switching between

oxidation and reduction state of material When electric potential is applied on

electrochromic material forced oxidation or reduction is derived and the

33

individual color is originated from the corresponding oxidation or reduction

state of the material For electrochromic materials the characteristic color

change is reversible since the oxidation and reduction state can be converted

reversibly by switching potential

Application fields

The application area for the electrochromism is rather broad that it

covers from smart window glazing and optical modulators to information

displays [98-102] The smart windows are typical examples The

electrochromic property is used to control the amount of light and heat to pass

through the windows Usually the electrochromic material is in form of thin film

coated on a window glass The transmittance modulation has also been

applied at the automobiles to automatically tint rear-view mirrors in various

lighting conditions The electrochromic application fields are illustrated in Fig

1B3

Since the smart windows control the transmittance of heat as well as

the transmittance of visible light the working definition of electrochromism has

now been extended to include devices for modulation of radiation in the near

infrared thermal infrared and microwave regions When color for

electrochromic materials is used this can now mean a response by detectors

at these wavelengths and not just by the human eyes Nowadays

electrochromic material draws much attention as being used in the display

devices Electrochromic display device (ECD) is being considered as one of

the candidates for the conventional liquid crystal display (LCD) since ECD

has many advantages over LCDs Among these advantages the most

important are low energy consumption wider viewing angle high contrast

rate and possibility to achieve multiple colors with a single material [103]

When a new redox state of electrochromic materialis established by

the applied electric pulse then it is maintained after the potential is switched

off This means the colored or bleached state of the material can be sustained

for a considerable time without applying electrical power This is so-called

ldquocolor memory effectrdquo of electrochromic material Because of the color

memory effect energy consumption for the electrochromic display device

could be drastically reduced and this would be a big advantage over other

34

emissive devices The low power consumption is especially advantageous

when it is applied to mobile devices with limited power source The possibility

for a flexible display is another attraction for electrochromic material

Information displays

Real-view mirrors for automobiles

Fig 1B4 Application fields of electrochromic devices Smart windows information displays and real-view mirrors for automobiles

35

Using polymeric electrochromic material and plastic substrate with a

transparent conductive coating it is possible to build all-plastic flexible display

device There are lots of papers and patents about the flexible display devices

[104105] In these cases gel-type electrolyte is also needed The gel-type

electrolyte immobilizes liquid electrolyte in the polymer network [106107]

Recently ITO-coated polymer films are readily available which would provide

the plastic substrate for a flexible device The flexible electrochromic displays

are frequently tried with the plastic substrate flexible electrochromic material

and gel-type electrolyte The flexible electrochromic displays would facilitate

increasing technical demands for foldable display devices

Metal Ion Electrochromism

Many transition metal oxides are capable of redox reactions that result in

colour change Metal oxide films are commonly prepared as thin layers of

either tungsten nickel molybdenum or other metal compounds by a number

of techniques These include sol-gel electrochemical by dc or rfreactive

sputtering techniques electron-beam evaporation by anodic or cathodic

electrodeposition or by solution dipping of the electrochromic metal

compounds (or compounds that can be changed into these metal compounds)

onto optically transparent electrodes (OTE) [108 -114] Their electrochromism

is derived from the colour change associated with a change in the oxidation

state of the metal anion The behaviour of these materials is dependent upon

pH moisture and exposure to the atmosphere [115] Generally the switching

rates of these films is somewhat slow with typical switching times of about 15

- 60 seconds to achieve 100 conversion to either coloured or bleached state

[116 -120] An Example of this includes nickel oxide which changes from

transparent (pale green) to brownblack taking about 30 seconds to do so

[121] Other examples include [(NH4)5Ru]2(pyrazine)5+ and [(NH4)5Ru]2(44-

bipyridine)5+ whose electrochromism is significantly different due to the effect

of the ligand [122] Table 12 below gives some examples of metal oxide films

with electrochromic properties

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

7

significant structural change The Keggin structure is the widely accepted

structure for the α-Keggin anions [34] For example the α-Keggin anion of

phosphotungustic acid is shown in Fig1A2

The structure is composed of one heteroatm surrounded by four

oxygen to form a tetrahedronThe heteroatom is located centrally and caged

by 12 octahedral WO3 units linked to one another by the neighboring oxygen

atoms There are a total of 24 bridging oxygen atoms that link the 12 addenda

atoms The metal centres in the 12 octahedra are arranged on a sphere

almost equidistant from each other in four M3O13 units giving the complete

structure an overall tetrahedral symmetry The bond length between atoms

varies depending on the heteroatom (X) and the addenda atoms (M)

Fig 1A2 α-Keggin anion of phosphotungustic acid ( PW12O40 ) 3-

For the 12ndashphosphotungstic acid Keggin determined the bond length

between the heteroatom and each the four central oxygen atoms to be 15 Adeg

The bond length form the central oxygen to the addenda atoms is 243 Adeg

The bond length between the addenda atoms and each of the bridging

oxygen is 19 Adeg The remaining 12 oxygen atoms that are each double

bonded to an addenda atom have a bond length of 170 Adeg The octahedra

are therefore distorted This structure allows the molecule to hydrate and

dehydrate without significant structural changes and the molecule is thermally

stable in the solid state for use in vapor phase reactions at high temperatures

(400-500 degC)[35-36]

8

Including the original Keggin structure there are 5 isomers designated

by the prefixes α- β- γ- δ- and ε- The original Keggin structure is designated

α- These isomers are sometimes termed Baker Baker-Figgis or rotational

isomers [37]These involve different rotational orientations of the Mo3O13 units

which lowers the symmetry of the overall structure Lacunary Keggin

structures

The term lacunary is applied to ions which have a fragment missing

sometimes called defect structures Examples are the (XM11O39)nminus and

(XM9O34)nminus formed by the removal from the Keggin structure of sufficient Mo

and O atoms to eliminate 1 or 3 adjacent MO6 octahedra The Dawson

structure X2M18O62nminus is made up of two Keggin lacunary fragments with 3

missing octahedra Some structural types are found in many different

compounds The first known example of this was the Keggin ion whose

structure was found to be common to both molybdates and tungstates with

different central hetero atoms Examples of some fundamental

polyoxometalate structures are shown below The Lindquist ion (Fig1A4) is

an iso-polyoxometalate the other three are hetero-polyoxometalates The

Keggin and Dawson structures (Fig1A3 and (Fig1A5) have tetrahedrally

coordinated hetero-atoms eg P or Si Anderson structure has an octahedral

central atom egAl

Fig 1A3 Keggin structure XM12O40nminus

9

Fig 1A 4 Lindquist structure M6O19nminus

Fig 1A 5 Dawson structure X2M18O62nminus

In general α-Keggin anions are synthesized in acidic solutions For

example 12-Phosphotungstic acid is formed by condensing phosphate ion

10

with tungstate ions The heteropolyacid that is formed has the Keggin

structure

[PO4]3- + 12 [WO4]

2- + 27 H+ rarr H3PW12O40 + 12 H2O ----------- 11

α-Keggin anions have been used as catalyst in hydration polymerization and

oxidation reaction as catalysts

The metal atoms that make up the framework (termed addenda atoms)

are typically Mo W and V When more than one element is present the

cluster is called a mixed addendaclusterThe ligands coordinated to metal

atoms that together form the bridged framework are usually oxide ions but

other elements such as S and Br have been substituted for some of the oxide

ions (Note that sulfur substituted POM is often termed a

polyoxothiometalates) Another development is the use of other ligands eg

nitrosy and alkoxy to replace oxide ions The typical framework building

blocks are polyhedral units with 4 5 6 or 7 coordinate metal centers These

units usually share edges andor vertices The most common unit for

polymolybdates is the octahedral MoO6 unit which is a distorted octahedron

where the Mo atom moves off centre to give one short Mo-O bond In some

polymolybdates there are pentagonal bipyramidal units and these are key

building blocks in the molybdenum bluesHetero atoms are present in many

polyoxometalates Many different elements can act as hetero-atoms

Examples of various coordination numbers around the hetero-atom are

known

4 co-ordinate (tetrahedral) in Keggin Dawson and Lindquist structures

(eg PO4 SiO4 AsO4)

6 co-ordinate (octahedral) in Anderson structure (eg Al(OH)6 TeO6

8 co-ordinate (square antiprism) in ((CeO8)W10O28)8minus

12 co-ordinate (icosahedral) in (UO12)Mo12O30 8minus

11

Often the hetero-atom is centrally located in the anion (eg Keggin

structure) or in a structure fragment eg the 2 phosphorus atoms in the

Dawson ion are central to the two symmetric fragments There are similarities

to clathrate structures The Keggin ion can be formulated as PO4 2minusand

M12O36 and the Dawson as (XO42-)2 and M18O54Structural isomerism is

common For example the Keggin structure has 5 isomers which can be

considered to contain one or more of the four M3O13 units being rotated

through 60degMany compounds share the same framework architectures or

frameworks derived from a larger framework with one or more addenda atoms

and oxide ions removed to give defect structure usually called a lacunary

structure An example of a compound with a Dawson lacunary structure is

As2W15O56Some cage structures containing ions are known eg an example

is the vanadate cage V18O42 containing a Clminus ion [38] This has 5 co-

ordinates square pyramidal vanadium units linked together

Fig 1A 6 - H4V18O42 cage containing Cl

1A3 General Properties of polyoxometalates -

Typically polyoxoanions are water and air stable species of large size

(6-25 Adeg) and high ionic weight In aqueous solution they are subject to

decomposition by hydroxide ions eg

[PW12O40]3- + 23 OH- HPO4

2- + 12WO42- + 11 H2O -------- 12

12

Although the PH at which such reactions are rapid can very widely

depending upon the polyanion involved Polyanions are often much stable

towards the H3O+ ions and numerous crystalline heteropoly acids are known

Such acids may be extremely soluble in water and polar solvents (giving

solutions with densities in excess of 4 gcm3) and have large dissociation

constants (PK lt 0)

Crystalline heteropoly acids and salts are frequently highly hydrated with

up to 50 molecules of water per anion Much of this water is zeolite in nature

and crystal composition can vary accordingly On the other hand the cation

anion stiochiometry is always well defined as the anion structure and

composition Finally many polyanions are powerful oxidizing agents and

undergo multiple reversible one or two electron reductions leading to intensely

colored mixed valence species known as heteropoly blues Polyanions are

known which can accept as many as 32 electrons without major structural

change

1A4 Chemistry of Molybdenum and Tungusten Heteropolyoxometalates

A photochromic monolayer film of phosphomolybdic acid (denoted as

PMo12) was fabricated by self-assembly approach UV-visible spectrum and

AFM observation show that the monolayer film is composed of aggregated

PMo12 molecules The monolayer film shows good photochromic properties

with enough stability and reversibility The colour change of the monolayer

after UV-irradiation can be captured by a microscope equipped with a color

CCD camera Photochromic response of the monolayer film can be doubled

after being modified by an amine monolayer [39]

Series of vanadium substituted molybdo Keggin HPA with 12 or 3

adjacent vanadium atoms were prepared These materials were supported

on carbon cloth electrodes and hot pressed onto Nafion with an ETEK

electrode used as a standard on the opposite side The MEArsquos were run at

temperatureslt100 oC with the HPA electrode as either the anode or the

cathode Stable polarisation curves are obtained for an HPA based cathode

with reasonable current densities at 80 oC [40]

A novel mixed-valence polyoxomolybdenum anion was synthesized

hydrothermally from molybdenum oxidemolybdenum metalboric and

phosphoric acids12-phenyldiphosphonicacidand imidazole (ImH) and was

13

structurally characterized as an imidazolium saltOne-and two-dimensional

structures of this anion and additional molybdenum diphosphonate linkers

were assembled as wellThey were structurally characterized as their

pyridinium(pyH) salts [41]

Phosphomolybdic acidpolyvinylpyrrolidone hybrid films were found to

show visible light photochromism It is identified that the intra-supramolecular

charge transfer between the inorganic and organic molecules is responsible

for the visible-light coloration Interestingly the films show photo-memory and

thermal activation The films show a small change in absorbance after being

irradiated with visible light for a short time and the coloration can be

enhanced greatly by subsequent thermal treatment Electrical measurements

indicate that the conductivity of the film increases after the brief irradiation

which promotes transfer of the electrons induced by the thermal treatment

[42]

In this work major effort was concentrated on passive thermal control

coatings based on photochromic and thermochromic materials The inorganic

photochromic materials were based on tungsten and molybdenum oxide films

and the organic photochromic materials included spiropyrans and

spirooxazines In addition photochromic composite organic-inorganic films

and thermochromic vanadium oxide films were prepared The samples were

synthesized using sputtering sol-gel process and thermal oxidation [43]

Polyoxometalates a class of oxidatively robust inorganic oxidants and

oxidation catalysts are currently under investigation at the Forest Products

Laboratory and at Emory University as an alternative to chlorinebased

chemicals in the bleaching of soft Woodkraft and other pulps Although

polyoxometalate salts are used in a number of industrial processes the

feasibility of using these salts and oxygen in the commercial bleaching of

chemical pulps was only recently demonstratedA clear advantage of

polyoxometalates over oxygen alone hydrogen peroxide or ozone is their

inherently high selectivity for the residual lignin in softwood kraft pulps The

goal of ongoing research is to develop a highly selective energy efficient

oxygen based polyoxometalate delignification and bleaching technology

compatible with mill closure [44]

14

Tungsten oxide exhibits pronounced photochromism upon bandgap

photoexcitation which makes it attractive and promising for applications in

many areas Some advances have been achieved during the past decades

The research on nanocrystalline films and single crystals indicates the critical

importance of defects in tungsten oxide to its photochromism Based on

energy-band engineering of semiconductors enhancement of photochromism

has been achieved for instance extension of the photoresponse from UV to

visible light by cathodic polarization improved change in absorption before

and after coloration through modification by a noble metal or another metal-

oxide semiconductor and increased photochromic reversibility via

hybridization with organic amines Nanocrystalline oxide films exhibit

controllable wettability which is coherent in nature with photochromism [45]

Polyoxometalates represent a diverse range of molecular clusters with

an almost unmatched range of physical properties and the ability to form

structures that can bridge several length scalesThe new building block

principles that have been discovered are beginning to allow the design of

complex clusters with desired properties and structures and several structural

types and novel physical properties are examinedIn this critical review the

synthetic and design approaches to the many polyoxometalate cluster types

are presented encompassing all the sub-types of polyoxometalates including

isopolyoxometa- lates heteropolyoxometalates and reduced molybdenum

blue systems As well as the fundamental structure and bonding aspectsthe

final section is devoted to discussing these clusters in the context of

contemporary and emerging interdisciplinary interests from areas as diverse

as antiviral agentsbiological ion transport modelsand materialsscience [46]

Keggin type molybdovanadophosphoric heteropoly acids were prepare

d by a novel environmentally benign method and their catalytic performances

were evaluated via hydroxylation of benzene to phenol with hydrogen

peroxide as oxidant in a mixed solvent of glacial acetic acid and acetonitrile

Various reaction parameters such as reaction time reaction temperature

ratio of benzene to hydrogen peroxide concentration of aqueous hydrogen

peroxide ratio of glacial acetic acid to acetonitrile in solvent and catalyst

concentration were changed to obtain an optimal reaction conditions

15

Molybdovanadophosphoric heteropoly acids are revealed to be highly

efficient catalyst for hydroxylation of benzene [47]

Thin films from the system (As2S3)Tl were deposited by thermal

evaporation on Si graphite and optical glass substratesFrom transmission

and reflection measurements of the thin films the refractive index (n) film

thickness(d) optical band gap(Eg) optical oscillator energy(Eo) and

dispersion energy(Ed) before and after exposure to light were determined

The results for optical parameters were analyzed using the Wemple - Di

Domenico single oscillator model and Lorenc-Lorenc equation It was found

that Eg decreases while n E0 and Ed increase for as deposited films

decreases while n E0 and Ed increase for as deposited films with increasing

of Tlconcentration passing through a maximum at 6 at of Tl After exposure

to light n E0 Ed increase and Eg decreases for all compositions

investigated The maximum change in n (Dn = 016 at l = 6328 nm) was

observed for thin As38S56Tl6 films From infrared spectroscopy measurements

of bulk glasses and thin films we could conclude that when up to 6 at of

thallium is introduced As-S-As chains break and a ternary TlAsS2

compound appears at 10 at Tl [48]

The organo - inorganic hybrid materialconsisting of Poly (34 Ethylene

Dioxythiophene) (PEDOT) doped with phosphomolybdate cluster anions

[PMo12O40]3-has been synthesized by direct insitu oxidative polymerization of

34-Ethylene Dioxythiophene (EDOT) with phosphomolybdic acid

(H3PMo12O40) Its characterization is investigated by Fourier Transform

Infrared Spectroscopy (FT-IR) and Scanning Electron Microscopy (SEM) The

hybrid material presents predominantly high electronic conductivities of

around 20 and 70 S cm1at 300 and 400 K respectively [49]

Heteropolyacids (HPAs) are known to be excellent re-dox catalysts In

combination with TiO2 HPAs can be used as photocatalysts active in visible

light The HPA accepts electron and get reduced to heteropolyblue (HPB)

That can absorb light in the visible range HPA can be incorporated onto the

external surface or in the pores of zeolite based composite photocatalysts

have been designed by incorporation of HPA semiconductor TiO2 and

transition metal cobalt on zeolite This composite metallozeolite photocatalyst

is efficient in photoreduction of methyl orange (MO) in visible light to the tune

16

of 411 mg of MO photo reducedg TiO2 This catalyst also shows encouraging

results for hydrogen evolution from water to the tune of 2730 micromolhg TiO

[50]

A new class of proton conducting glass membranes based on heteropo

lyacids such as phosphotungstic acid (PWA) as electrolytes for low

temperature H2O2 fuel cells was investigated Parameters for a single fuel cell

with a catalyst electrode of 015 mgcm2 of PtC and a glass composite

membrane were characterized by electrochemical measurements at open

circuit potential conditions The performance of the membrane electrode

assemblies (MEA) was systematically studied as an effect of SiO2 and P2O5

concentrations in the glass composite membrane and the MEA was found to

exhibit a maximum power density of 162 mW cm2 for an H2O2 fuel cell at

30 degC and 30 relative humidity (RH) [51]

Two new photochromic inorganic-organic hybrid materials formed from

Keggin type Polyoxometalates (POMs) and metronidazole (C6H9N3O3 MNZ)

formulated as H3PMo12O40bull3 MNZ3H2O (1) and H3PW12O40bull3MNZ3H2O(2)

were synthesized and characterized by elemental analysis IR spectra

electronic spectra electron spin resonance (ESR) spectra and TG-DTA

Reflectance spectra show the presence of weak inter molecular charge

transfer between the organic and inorganic moieties in the solid state The

photochromic properties were studied by solid diffuse reflectance spectra and

ESR spectra and the photochromic reactions were found to exhibit first-order

kinetics TG-DTA showed that two hybrid materials have similar thermal

behavior [52]

Heteropolycompounds (HPCs) have been a matter of interest in basic

and applied science for more than a century From their first synthesis many

advances have been made to promote the use of HPCs in different ways in

science and technology The aim of this article is to review the main structural

characteristics of heteropolycompounds of the Keggin type (12

tungstophosphoric12-molybdophosphoric12-tungstosilicic acid alkaline and

alkaline earth salts of12 tungstophosphoric acid and gels doped with HPCs)to

understand and explain their different activities such as high proton

conductivity and catalytic biochemical and biomedical activities [53]

17

A solid hybrid molecular material containing 1-butyl 3-methyl

imidazolium cations and Keggin anions of phosphotungstic acid has been

synthesized It is fully characterized by CHN analysis FTIR XRD UV-Vis-NIR

DRS 31P MAS NMR TGA and SEM The FTIR spectrum of the compound

shows the fingerprint vibrational bands of both Keggin molecular anions and

imidazolium cations The aromatic CndashH stretch region (2700ndash3250 cmndash1) of

imidazolium cation is split due to the interaction between the ring CndashH and

bulky Keggin anion The red-shift in the UV-Vis spectra and the downfield 31P

MAS NMR chemical shift also confirm the electrostatic interaction between

the ions in the compound Near IR spectral region (1000ndash2500 nm) shows the

elimination of water in the compound which is hydrophobic [54]

Transport coefficient measurements (electrical conductivity

thermoelectric power and Hall coefficient) have been performed on a

compact Tl033MoO3 polycrystalline compound in a wide temperature range

(200ndash400 K) Experimental results are interpreted with the help of a p-type

semiconductor model with two inverted deep levels near the midgap The

valence band and the conduction band are assumed to be formed from the

dxy orbitals of molybdenum atoms in the Mo6O22 cluster leading to narrow π-

bonding bands The donor and acceptor levels may be formed from

nonbonding dxy orbitals arising respectively from anionic and Tl+ defects

Electron paramagnetic resonance and magnetic measurements are in good

agreement with the theoretical band semiconductor model which has been

retained [55]

The optical properties of GaInTlAs epilayers grown at low temperature

~230degC by solid-source molecular-beam epitaxy on InP substrates were

characterized using optical absorptionand photoluminescence techniques

Optical absorption measurements a room temperature show a gap shrinkage

toward lower energies from 071 to 061 and 053 eV when the Tl content

increases from 0 to 4and 8in good agreement with theoretical

predictionsLow-temperature photoluminescence band-gap signals from

GaInAs and GaInTlAs layers are only obtained after rapid thermal annealing

performed inorder to improve the electronic quality of the layersA band gap

decrease as much as 41 meV for GaInTlAs with 19 Tl incorporation is

measured by photoluminescence at 8K [56]

18

The electrochromic performance of all solid ndash state cells employing

phosphotungstic acid and phosphomolybdic acid is reported These cells

employ SnO2 as the viewing electrode and graphite as the back electrodeThe

cells in the bleached state can be made white to red and become black in the

coloured state [57]

Various organic compounds were oxidized by molecular oxygen in the

presence of a catalytic amount of mixed addenda heteropolyoxometalates

containing molybdenum and vanadium The catalytic activity of the

Molybdovanadophosphate was found to be greatly enhanced by supporting

on charcoal The supported catalyst has high catalytic activity for oxidative

dehydrogenation of benzylic and allylic alcohols to the corresponding

aldehydes and ketones (46-92) nevertheless the nonsupported catalyst

was inactive for the same oxidations under these conditions 236Trimethyl

phenol was selectively oxidized to trimethyl-p benzoquinone which is

precursor of VitE in the presence of a catalytic amount of

molybdophosphate In addition the aerobic oxidation of amines alkyl-

substituted phenols and alkanes were also examined [58]

The reaction of Tl2CO3 with 111555-hexafluoro-24-pentanedione

and diglyme CH3O(CH2CH2O)2CH3 or tetraglyme CH3O(CH2CH2O)4CH3 in

dichloromethane yields the anhydrous thermally and air stable volatile Tl

diglyme and Tl tetraglyme adducts They have been characterized by single

crystal X-ray diffraction elemental analysis 1H and 13C NMR IR and mass

spectroscopy Thermal and mass-transport properties have been investigated

using thermo gravimetric and differential scanning calorimetric

measurements There is evidence that both precursors are very low melting

and volatile and can be used as liquid Tl sources Both adducts have been

successfully applied to metalndashorganic chemical vapor deposition of thallium

containing films [59]

Large size and high quality single crystals of quasi-two-dimensional

thallium molybdenum purple bronze TlMo6O17 have been grown by electrolytic

reduction of molten salt of Tl2CO3-MoO3 The crystal structure is trigonal with

space group P3m1 determined by X-ray diffraction and four-circle single crystal

diffraction The lattice parameters of the unit cell are a = b = 55282 Adeg and c

= 136991 Adeg The temperature dependence of resistivity and magnetic

19

susceptibility confirmed that a metal-to-metal transition occurs near 110 K

[60]

Hall coefficient and dc conductivity studies were made on p-type

Pb08Sn02Te thin films doped with different concentrations of thallium in the

temperature range 77 to 500 K The Hall coefficient and Hall mobility are

found to decrease with an increase in the doping concentration of thallium

Hall coefficient data have been analyzed in the light of a double valence-band

model Various band parameters such as valence band separation population

ratio mobility ratio and effective mass ratio have been calculated Hall

mobility data have been analyzed in the light of lattice and defect limited

scattering mechanisms [61]

Proton conducting composites of heteropolyacid hydrates phosphomol

ybdic acid H3PMo12O40 nH2O(PMA) phosphotungstic acid H3PW12O40 nH2O

( PTA) and salt hydrate like NiCl2 6H2O were prepared

with insulating Al2O3 as despersoidThe ionic conductivity peaks at two

concentrations of Al2O3 indicating two percolation thresholds for percolation

thresholds for proton conduction Two separate experiments were carried out

to check the existence of such percolation thresholds viz the volta battery

experiment involving the measurement of emf of an electrochemical cell

with composites of different compositions used as electrolyte and the

composition vs conductivity measured by the complex impedance

spectroscopy The presence of two maxima has been attributed to two

different percolation thresholds for the two possible mobile protonic

species H+ + (H3O+) and OH arising from the hydrates [62]

1 A5 Applications of Heteropolyoxometalates-

Applications of heteropolyanions centre depend on their redox properties

their high charges and ionic weights An enormous patent and journal

literature is devoted to the applications of heteropolyanions

1 Analysis-

The formation and subsequent precipitation or reduction of

[XMO12O40]n- anions form the basis of gravimetric and colorimetric analytical

methods for P As Si or Geeither separately or in combination [63 64]

20

2 Biochemical applications-

lsquoPhosphotungstic Acidsrsquo have for decades been used as precipitants

for proteins and as analytical reagents for proteinsalkaloidsand purines eg

the [P2M18O62]6- anions for colorimetric determination of uric acid [65] and

cholesterol The acid H3P12O40 either in aqueous or ethanolic solution is also

widely used as a non specific electron dense stain for electron spectroscopy

The dyestuffs industry has for many years used heteropolymolybdates and

tungstates to form color lakes and toners from basic dyes Large

heteropolyanions exhibit antiviral antitumoral properties at non-cytotoxic

doses in vitro and in vivo and are protein inhibitors of cellular bacterial and

viral DNA RNA polymerizes [66]

3 Catalysis

Heteropoly acids and salts have been used as heterogeneous catalysts

for a broad variety of reactions and compilations of such applications up to

1973 are available Examples include oxidation of propylene and isobutylene

to acrylic acid methacrylic acids and ammoxidation of acrylonitrile olefin

polymerization and epoxidation Much of current activity concerning

heterogeneous catalysis by heteropoly compounds is being carried out in

Japan [67 68]

4 Other Applications

Insoluble salts of Heteropolyanions especially ammonium

molybdophosphates have been used and are commercially available as ion-

exchange materials [69]Recent work in this area includes thin layer

chromatography of amino acids ion selective membranes [70] and the

preparation of new ion exchangers based on heteropolyanions Crystalline12-

tungstophosphoric and 12-molybdophosphoric acids are excellent protonic

conductors Heteropolyacids are electrochromic in the solid state as a

consequence of heteropolyblue formation Heteropolyblue formation has also

been used to detect alcohol or carboxylic acid radicals generated by radiolysis

of aqueous solutions Potential applications of heteropoly complexes as flame

retardants and smoke suppressants or as corrosion inhibitors and conversion

coatings on steel and aluminium are reported [71] Some potential ldquogreenrdquo

applications have been reported eg non-chlorine based wood pulp

bleaching process and a method of decontaminating water Some structures

21

containing transition metal atoms with unpaired electrons have unusual

magnetic properties and are being investigated as nano computer storage

devices Some compounds exhibit luminescence There are many reported

potential medicinal applications eg anti tumoral and anti-viral There have

been reports on the role of weak or non bonding interactions on the crystal

engineering of hybrid polyoxometalates

Spherical nonporous polyoxomolybdate based capsules of different

types containing more than 100 metal atoms reported by Achim Muller and his

group have versatile unique properties regarding their assembly to vesicles

and the chemistry which can be done inside the pores and cavities A discrete

polyoxometalate Lindquist ion of the form W6O192minus was successfully imaged

recently for the first time within the capillary of a carbon nanotube following

steric locking of the anion with the tubule In situ relaxation of the anion in its

equatorial plain was demonstrated [72]

22

Section-B

Litreture Survey on Chromism in Transition Metal

Oxides

1 B1 Chromism in Transition Metal Oxides

Chromism is a reversible change in a substances colour resulting from

a process caused by some form of stimulus Many materials are chromic

including inorganic and organic compounds and conducting polymers and the

property can result from many different mechanisms Several transition metal

oxides show EC properties The most popular are from the VI - B oxides In

this group WO3 and MoO3 are the most thoroughly studied cathodic EC

materials which can be electrochemically coloured and bleached when used

as the cathode in electrochemical cells Cathodic EC materials also include

V2O3 TiO2 and Nb2O5 Another distinguishable group is anodic EC material

including VIII oxides like IrOx nH2O Rh2O3 nH2O NiO nH2O etc which can be

anodicaly coloured in the electrochemical process when used as anode

There are several types of chromism which are discussed as below

B11 Photochromism

Photochromism is the reversible transformation of a chemical species

between two forms by the absorption of electromagnetic radiation where

the two forms have different absorption spectra [7374]

Trivially this can be described as a reversible change of color upon

exposure to light The phenomenon was discovered in the late 1880s

including work by Markwald who studied the reversible change of color of 23

44-tetrachloronaphthalen-1(4H)-one in the solid state He labeled this

phenomenon phototropy and this name was used until the 1950s

when Yehuda Hirshberg of the Weizmann Institute of Science in Israel

proposed the term photochromism [75] Photochromism can take place

in both organic and inorganic compounds and also has its place in biological

systems (for example retinal in the vision process)

Photochromism does not have a rigorous definition but is usually used

to describe compounds that undergo a reversible photochemical reaction

where an absorption band in the visible part of the electromagnetic spectrum

23

changes dramatically in strength or wavelength In many cases an

absorbance band is present in only one form The degree of change required

for a photochemical reaction to be dubbed photochromic is that which

appears dramatic by eye but in essence there is no dividing line between

photochromic reactions and other photochemistry Therefore while the

transcis isomerization of azobenzene is considered a photochromic reaction

the analogous reaction of stilbene is not Since photochromism is just a

special case of a photochemical reaction almost any photochemical reaction

type may be used to produce photochromism with appropriate molecular desi

gnSome of the most common processes involved in photochromism are peric

yclic reactions cis-trans somerizations intramolecular hydrogen transfer

intramolecular group transfers dissociation processes and electron transfers

(oxidation-reduction)

Another some what arbitrary requirement of photochromism is that

it requires the two states of the molecule to be thermally stable under

ambient conditions for a reasonable time All the same nitrospiropyran (which

back-isomerizes in the dark over ~10 minutes at room temperature) is

considered photochromic All photochromic molecules back-isomerize to their

more stable form at some rate and this back-isomerization is accelerated by

heating There is therefore a close relationship between photochromic and the

rmochromic compounds The timescale of thermal back-isomerization is

important for applications and may be molecularly engineered

Photochromic compounds considered to be thermally stable include some

diarylethenes which do not back isomerize even after heating at 800C for 3

months

Since photochromic chromophores are dyes and operate according to

well-known reactions their molecular engineering to fine-tune their properties

can be achieved relatively easily using known design models quantum

mechanics calculations and experimentation In particular the tuning of

absorbance bands to particular parts of the spectrum and the engineering

of thermal stability have received much attention

Sometimes and particularly in the dye industry the term irreversible

photochromic is used to describe materials that undergo a permanent color

change upon exposure to Ultraviolet or visible light radiation Because by

24

definition photochromics are reversible there is technically no such thing as a

n irreversible photochromic this is loose usage and these compounds

are better referred to as photochangable or photoreactive dyes

Apart from the qualities already mentioned several other properties of

photochromics are important for their use These include

Quantum yield of the photochemical reaction

This determined the efficiency of the photochromic change with respect

to the amount of light absorbed The quantum yield of isomerization

can be strongly dependent on conditions

Fatigue resistance In photochromic materials fatigue refers to the

loss of reversibility by processes such as photodegradation

photobleaching photooxidation and other side reactions All

photochromics suffer fatigue to some extent and its rate is strongly

dependent on the activating light and the conditions of the sample

Photostationary state Photochromic materials have two states and

their interconversion can be controlled using different wavelengths of

light Excitation with any given wavelength of light will result in a

mixture of the two states at a particular ratio called the photo-

stationary state In a perfect system there would exist wavelengths

that can be used to provide 10 and 01 ratios of the isomers

but in real systems this is not possible since the active

absorbance bands always overlap to some extent

Polarity and solubility In order to incorporate photochromics in

working systems they suffer the same issues as other dyes They are

often charged in one or more state leading to very high polarity and

possible large changes in polarity They also often contain large

conjugated systems that limit their solubility

Photochromic complexes

A photochromic complex is a kind of chemical compound that has

photoresponsive Parts on its ligand These complexes have a specific

structure photoswitchable organic compounds are attached to

metalcomplexes For the photocontrollable parts thermally and

photochemically stable chromophores (azobenzene diarylethene

25

spiropyran etc) are usually used And for the metal complexes a wide

variety of compounds that have various functions (redox response

luminescence magnetism etc) are applied The photochromic parts and

metal parts are so close that they can affect each others molecular

orbitals The physical properties of these compounds shown by parts of

them (ie chromophores or metals) thus can be controlled by switching

their other sites by external stimuli For example photoisomerization

behaviors of some complexes can be switched by oxidation and reduction of

their metal parts Some other compounds can be changed in their

luminescence behavior magnetic interaction of metal sites or stability of

metal-to-ligand coordination by photoisomerization of their photochromic

parts

Photochemistry of Polyoxometalates

The photochemistry of polyoxometalates is of great interest to inorganic

chemistsMore than 80 years agoit was found that the R-Keggin tungstate

H3[PW12O40] was reduced photochemically to yield a blue-colored species

which was reoxidized by air and by various other oxidizing agents such as

Fe3+AgNO3and H2O2 [7879]The photoredox reactions of H4[SiW12O40]and

H3[PW12O40] proceeded effectively in the presence of primary and secondary

alcohols their ethers and aldehydes and proteinsbut less effectively in the

presence of tertiary alcoholsketonesestersthe fatty acids above formic

acidand simple amines[8081] The basic photoredox reaction involving

ethanol is illustrated by eq 13

2 H3PW12O40 + H3CCH2OH h ν ν ν ν 2 H4PW12O40 + H3CCHO ------- 13

2 H4PW12O40 +12 O2 2 H3PW12O40 + H2O ------ 14

In this reactionone molecule of ethanol photochemically reduces two

molecules of H3PW12O40 and is itself oxidized to acetaldehyde In the

presence of air the thermal oxidation of the reduced species takes place at

room temperature(eq 14)The reduced polyoxometalates which are the so-

calledldquoheteropolybluesrdquo have been used for the colorimetric analysis of the

26

elements P Si As and Ge and for the determination of uric acidsugarand

other biological compounds [8283] Piperidinium metavanadate also

undergoes photoinduced coloration from white to black followed by a

reversible color change in the presence of oxidizing agentsHowever

ammonium metavanadates ([NH4][VO3]) exhibits no photoinduced

coloration[84] The early photoredox reactions of the R-Keggin

polyoxometalates H4[SiW12O40] and H3PW12O40 were carried out in the

presence of photographic paper however the limited number of the

structurally well-characterized compounds available for study delayed the

development of modern cluster-compound photochemistry until the discovery

of photochromism in alkylammonium polyoxo- molybdate solids[8586] A

photochromic or electrochromic material is one whose light-absorbing

properties are altered upon optical excitation or reduction under the influence

of an externally applied electric field respectively The induced coloration

remains even after the excitation source has been removed These materials

are of technological interest because they return to their original state either

thermally upon irradiation with light of a frequency corresponding to the

induced absorption or electrochemically upon reversing the polarity of the

externally applied electric field Thus photochromic and electrochromic

materials behave in a reversible manner Polyoxometalates exhibit significant

photo-and electrochromism which makes them suitable as nanocomposite

molecular devices and as models for probing the physical properties of infinite

metal oxides Since the metal ions in the oxidized polyoxometalates have d0

electronic configurations the only absorption band which occurs in the UV-vis

range of the electronic spectra is due to the oxygen-to-metal (O-M) ligand-to-

metal charge transfer (LMCT)Upon irradiation electrons are promoted from

the low-energy electronic states which are mainly comprised of oxygen 2p

orbitals (the valence band in the band model)to the high-energy electronic

states which are mainly comprised of metal d orbitalrsquos (the conduction band

in the band model)The fundamental transitions in polyoxometalate lattices

are depicted schematically in Fig1B1

27

Fig1B1 Simple model showing the electronic transitions in the

polyoxometalates containing electron donar and acceptor (a)generation

of charge carriers(b)electron and hole trapping(c)electron release due

to stimulation(d)recombination

between electron and hole Electrons are e-and holes are h+

In the polyoxometalates containing heteroatoms and especially in

mixed metal polyoxometalates the charge carriers which are created by the

light or electric field may be trapped in electron traps and hole traps These

traps provide states of localized energy in the O-M LMCT energy gap due to

the heteroatoms or counter cations which correspond to impurities or lattice

defects in the band model If the trap depth ∆E is large compared to kT the

probability for thermal escape from the trap will be negligibly small and

metastable situation will existThe trapped carriers can be released by thermal

or optical stimulationIn the case of thermal stimulation the irradiated

polyoxometalate is heated until the energy barrier ∆E can be overcome The

trapped electron (or hole) then can escape from the trap and nonradiatively

recombine with the trapped hole (or electron)Under optical stimulation the

energy of an incident photon is used to overcome ∆E The relaxation

processes of the OndashM LMCT excitation energy include both the nonradiative

recombination of electrons and holes within the energy gap and the

intramolecular energy transfer leading to a charge-transfer emission This

intramolecular energy transfer corresponds to the O-M LMCT energy gap and

occurs via radiative recombination and sensitized emission from the

28

heteroatoms or cations If several energy levels based on the hetero atoms or

counter cations act as energy acceptors within the O-M LMCTenergy gap the

energy transfer occurs from the O-M LMCT states to these levels followed by

the nonradiative or radiative deactivation of the excitation energy It should be

noted that the O-M LMCT states also can be generated by the application of

very high electric fields to the polyoxometalate solids as demonstrated by the

observation of electroluminescence[87]If an external electric field with a

potential more negative than the energy levels of the vacant orbitals involved

in the O-M LMCT transition is applied to a polyoxometalate on the electrode

surface an electrochemical reduction occurs via the injection of electrons

from the electrode in to the vacant levels of the polyoxometalate as shown in

Fig1B 2

Fig1B2 - Energy scheme for the electrochromism of polyoxometalates

a)electrochemical reduction (b) electrochemical oxidation

Electrons injected in to the high-energy levels also may be trapped by

electron traps in a process analogous to that which occurs during LMCT

photoexcitation of the polyoxometalates These electrons are returned to the

electrode by electrochemical oxidation at an electrode potential more positive

than the energy levels for the d1 electron states The d1electrons in the O-M

LMCTstates facilitate the absorption of visible light via intervalence charge

transfer among metal centers and d-d transitions The same type of transition

may be possible for the d1electron captured by the electron traps too In

addition to searching for new photosensitive polyoxometalates with the

29

potential for having practical application there is now a need to elucidate the

fundamental photo-and electrochemical coloration processes by studying

electron transfer within the polyoxometalate lattices in conjunction with their

crystal structures So far few polyoxometalates exhibit a perfect reversibility

of coloration The irreversibility of the color change arises from as yet

uncharacterized side reactions during both the coloration and decoloration of

the polyoxometalates

Many metal oxides including aluminum titanium vanadium niobium

molybdenum and tungsten oxides are photochromic when they contain

impurities or dopants This coloration has been interpreted on the basis of

electron trapping at appropriate lattice sites within the crystals as shown in

Fig1B1 where the O-M LMCT transition corresponds to the transition

between the valence and conduction bands for the infinite metal-oxide lattice

[88-91]

B12 Applications of Photochromic materials

Sunglasses

One of the most famous reversible photochromic applications is color

changing lenses for sunglasses as found in eyeglasses The largest limitation

in using PC technology is that the materials cannot be made stable enough

to withstand thousands of hours of outdoor exposure so long-term outdoor

applications are not appropriate at this time The switching speed of

photochromic dyes is highly sensitive to the rigidity of the environment around

the dye As result they switch most rapidly in solution and slowest in the rigid

environment like a polymer lens Recently it has been reported that attaching

flexible low Tg polymers (for example siloxanes or poly (butyl acrylate) to the

dyes allows them to switch much more rapidly in a rigid lens [76] Some

spirooxazines with siloxane polymers attached switch at near solution like

speeds even though they are in a rigid lens matrix

Supramolecular chemistry

Photochromic units have been employed extensively in supramolecular

chemistry Their ability to give a light controlled reversible shape change

means that they can be used to make or break molecular recognition motifs

30

or to cause a consequent shape change in their surroundings Thus

photochromic units have been demonstrated as components of molecular

switches The coupling of photochromic units to enzymes or enzyme cofactors

even provides the ability to reversibly turn enzymes on and off

by altering their shape or orientation in such a way that their functions

are either working or broken

Data storage

The possibility of using photochromic compounds for data storage was

first suggested in 1956 by Yehuda Hirshberg[77] Since that time there have

been many investigations by various academic and commercial groups

particularly in the area of 3D optical data storage which promises discs that

can hold a terabyte of data Initially issues with thermal back-reactions

and destructive reading dogged these studies but more recently more stable

systems have been developed

Novelty items

Reversible photochromics are also found in applications such as toys

cosmeticsclothing and industrial applications If necessary they can be made

to change between desired colors by combination with a permanent pigment

A large number of inorganic compounds exhibit photochromism

These solids often have large band gaps of the order of 3 - 12 eV and

excitation of these solids leads to the formation of metastable centers that

absorb visible light giving rise to their colour They can return to their ground

state by heating or by optical excitation within the colour-centre band In most

cases the photochromism is a structure sensitive phenomenon involving

localized defect impurities or dislocations Some of these inorganic

compounds have the potential for a number of different uses Photochromic

compounds have a number of useful applications These can be divided

according to the most important property that is being used (Table 11) [92]

31

Table 11 Applications of Photochromic materials

Applications Depending Upon

Sensitivity to

Radiation

Reversibility Thermal Chemical or

Physical Properties

Self-developing

photography

Chemical switches for

computers

Temperature indicators

Protective

materials

Data displays

Heat-sensitive recording

media

Optical signal

processing

Photomasking and

photoresist technology

Reusable data storage

media

Anaytical reagents

Photochromic

microimages

Photopolymerisation

Information encoding

and steganography

Photocontractile

polymers and the

photoviscosity effect

Control of light

intensity

Q-switches

Pyroelectric

photochromic materials

B13 Thermochromism

Thermochromism is the reversible colour change of a substance

induced by temperature change A large variety of substances organic

inorganic organometallic supramolecular and polymeric systems exhibit this

phenomenon Examples of these include bianthrones cobalt

hexacyanoferrate the zirconocene complex of 1 4-diphenyl-1 3-butadiene

and poly (3-alkylthiophene) The organic 99-bixanthenylidene is colourless at

90 K yellow-green at 298 K and dark-blue when melted at 592 K Heating

conducting polymers can cause them to change colour This is achieved by

causing conformational changes to the polymer backbone resulting in a

change in the band gap of the polymer It has been reported that regioregular

32

P3HT reversibly changes colour upon heating to 220ordmC due to temperature-

dependent conformation changes Thermally cross linked polymer undergoes

the same colour change but it is much less reversible [93] Other forms of

thermochromism may be commercially important eg to give a visual

indication of temperature changes

B14 Electrochromism

Electrochromism describes a phenomenon of material color change in

a persistent but reversible manner produced by electrochemically induced

oxidation-reduction reactions Electrochromic materials can be applied to

various kinds of products such as smart windows and display devices Among

those applications there have been lots of efforts to develop electrochromic

display devices (ECDs) Especially flexible display devices are now attracting

much attention worldwide since they can facilitate new technological demands

such as bending and folding of paper-like displays High electrochromic

efficiency short response time long operating life time and reduction of

energy consumption are the most important requisites of the materials for the

paperlike displays [94 95] Among those properties the operation life time is

the most important barrier to overcome for a realization of ECDs There are

two types of electrochromic material a) inorganic transition metal oxides

(TMOs) b) organic polymer materials The TMOs have been studied longer

than the organic materials that they have been studied since 1960s [9697]

Electrochromism describes a reversible color change of material

produced by electrochemically induced oxidation-reduction reactions It is one

of several types of chromism of materials As thermochromism and

photochromism mean material color changes made by heat and light

respectively electrochromism refers that the color change is caused by an

electric potential In most cases the color change in electrochromism can be

driven by rather low electrical potential of the order of a fraction of volt or a

few volts [94-96] The color change of material means variation in

transmittance andor reflectance change in visible range which is originated

from different electronic absorption bands according to a switching between

oxidation and reduction state of material When electric potential is applied on

electrochromic material forced oxidation or reduction is derived and the

33

individual color is originated from the corresponding oxidation or reduction

state of the material For electrochromic materials the characteristic color

change is reversible since the oxidation and reduction state can be converted

reversibly by switching potential

Application fields

The application area for the electrochromism is rather broad that it

covers from smart window glazing and optical modulators to information

displays [98-102] The smart windows are typical examples The

electrochromic property is used to control the amount of light and heat to pass

through the windows Usually the electrochromic material is in form of thin film

coated on a window glass The transmittance modulation has also been

applied at the automobiles to automatically tint rear-view mirrors in various

lighting conditions The electrochromic application fields are illustrated in Fig

1B3

Since the smart windows control the transmittance of heat as well as

the transmittance of visible light the working definition of electrochromism has

now been extended to include devices for modulation of radiation in the near

infrared thermal infrared and microwave regions When color for

electrochromic materials is used this can now mean a response by detectors

at these wavelengths and not just by the human eyes Nowadays

electrochromic material draws much attention as being used in the display

devices Electrochromic display device (ECD) is being considered as one of

the candidates for the conventional liquid crystal display (LCD) since ECD

has many advantages over LCDs Among these advantages the most

important are low energy consumption wider viewing angle high contrast

rate and possibility to achieve multiple colors with a single material [103]

When a new redox state of electrochromic materialis established by

the applied electric pulse then it is maintained after the potential is switched

off This means the colored or bleached state of the material can be sustained

for a considerable time without applying electrical power This is so-called

ldquocolor memory effectrdquo of electrochromic material Because of the color

memory effect energy consumption for the electrochromic display device

could be drastically reduced and this would be a big advantage over other

34

emissive devices The low power consumption is especially advantageous

when it is applied to mobile devices with limited power source The possibility

for a flexible display is another attraction for electrochromic material

Information displays

Real-view mirrors for automobiles

Fig 1B4 Application fields of electrochromic devices Smart windows information displays and real-view mirrors for automobiles

35

Using polymeric electrochromic material and plastic substrate with a

transparent conductive coating it is possible to build all-plastic flexible display

device There are lots of papers and patents about the flexible display devices

[104105] In these cases gel-type electrolyte is also needed The gel-type

electrolyte immobilizes liquid electrolyte in the polymer network [106107]

Recently ITO-coated polymer films are readily available which would provide

the plastic substrate for a flexible device The flexible electrochromic displays

are frequently tried with the plastic substrate flexible electrochromic material

and gel-type electrolyte The flexible electrochromic displays would facilitate

increasing technical demands for foldable display devices

Metal Ion Electrochromism

Many transition metal oxides are capable of redox reactions that result in

colour change Metal oxide films are commonly prepared as thin layers of

either tungsten nickel molybdenum or other metal compounds by a number

of techniques These include sol-gel electrochemical by dc or rfreactive

sputtering techniques electron-beam evaporation by anodic or cathodic

electrodeposition or by solution dipping of the electrochromic metal

compounds (or compounds that can be changed into these metal compounds)

onto optically transparent electrodes (OTE) [108 -114] Their electrochromism

is derived from the colour change associated with a change in the oxidation

state of the metal anion The behaviour of these materials is dependent upon

pH moisture and exposure to the atmosphere [115] Generally the switching

rates of these films is somewhat slow with typical switching times of about 15

- 60 seconds to achieve 100 conversion to either coloured or bleached state

[116 -120] An Example of this includes nickel oxide which changes from

transparent (pale green) to brownblack taking about 30 seconds to do so

[121] Other examples include [(NH4)5Ru]2(pyrazine)5+ and [(NH4)5Ru]2(44-

bipyridine)5+ whose electrochromism is significantly different due to the effect

of the ligand [122] Table 12 below gives some examples of metal oxide films

with electrochromic properties

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

8

Including the original Keggin structure there are 5 isomers designated

by the prefixes α- β- γ- δ- and ε- The original Keggin structure is designated

α- These isomers are sometimes termed Baker Baker-Figgis or rotational

isomers [37]These involve different rotational orientations of the Mo3O13 units

which lowers the symmetry of the overall structure Lacunary Keggin

structures

The term lacunary is applied to ions which have a fragment missing

sometimes called defect structures Examples are the (XM11O39)nminus and

(XM9O34)nminus formed by the removal from the Keggin structure of sufficient Mo

and O atoms to eliminate 1 or 3 adjacent MO6 octahedra The Dawson

structure X2M18O62nminus is made up of two Keggin lacunary fragments with 3

missing octahedra Some structural types are found in many different

compounds The first known example of this was the Keggin ion whose

structure was found to be common to both molybdates and tungstates with

different central hetero atoms Examples of some fundamental

polyoxometalate structures are shown below The Lindquist ion (Fig1A4) is

an iso-polyoxometalate the other three are hetero-polyoxometalates The

Keggin and Dawson structures (Fig1A3 and (Fig1A5) have tetrahedrally

coordinated hetero-atoms eg P or Si Anderson structure has an octahedral

central atom egAl

Fig 1A3 Keggin structure XM12O40nminus

9

Fig 1A 4 Lindquist structure M6O19nminus

Fig 1A 5 Dawson structure X2M18O62nminus

In general α-Keggin anions are synthesized in acidic solutions For

example 12-Phosphotungstic acid is formed by condensing phosphate ion

10

with tungstate ions The heteropolyacid that is formed has the Keggin

structure

[PO4]3- + 12 [WO4]

2- + 27 H+ rarr H3PW12O40 + 12 H2O ----------- 11

α-Keggin anions have been used as catalyst in hydration polymerization and

oxidation reaction as catalysts

The metal atoms that make up the framework (termed addenda atoms)

are typically Mo W and V When more than one element is present the

cluster is called a mixed addendaclusterThe ligands coordinated to metal

atoms that together form the bridged framework are usually oxide ions but

other elements such as S and Br have been substituted for some of the oxide

ions (Note that sulfur substituted POM is often termed a

polyoxothiometalates) Another development is the use of other ligands eg

nitrosy and alkoxy to replace oxide ions The typical framework building

blocks are polyhedral units with 4 5 6 or 7 coordinate metal centers These

units usually share edges andor vertices The most common unit for

polymolybdates is the octahedral MoO6 unit which is a distorted octahedron

where the Mo atom moves off centre to give one short Mo-O bond In some

polymolybdates there are pentagonal bipyramidal units and these are key

building blocks in the molybdenum bluesHetero atoms are present in many

polyoxometalates Many different elements can act as hetero-atoms

Examples of various coordination numbers around the hetero-atom are

known

4 co-ordinate (tetrahedral) in Keggin Dawson and Lindquist structures

(eg PO4 SiO4 AsO4)

6 co-ordinate (octahedral) in Anderson structure (eg Al(OH)6 TeO6

8 co-ordinate (square antiprism) in ((CeO8)W10O28)8minus

12 co-ordinate (icosahedral) in (UO12)Mo12O30 8minus

11

Often the hetero-atom is centrally located in the anion (eg Keggin

structure) or in a structure fragment eg the 2 phosphorus atoms in the

Dawson ion are central to the two symmetric fragments There are similarities

to clathrate structures The Keggin ion can be formulated as PO4 2minusand

M12O36 and the Dawson as (XO42-)2 and M18O54Structural isomerism is

common For example the Keggin structure has 5 isomers which can be

considered to contain one or more of the four M3O13 units being rotated

through 60degMany compounds share the same framework architectures or

frameworks derived from a larger framework with one or more addenda atoms

and oxide ions removed to give defect structure usually called a lacunary

structure An example of a compound with a Dawson lacunary structure is

As2W15O56Some cage structures containing ions are known eg an example

is the vanadate cage V18O42 containing a Clminus ion [38] This has 5 co-

ordinates square pyramidal vanadium units linked together

Fig 1A 6 - H4V18O42 cage containing Cl

1A3 General Properties of polyoxometalates -

Typically polyoxoanions are water and air stable species of large size

(6-25 Adeg) and high ionic weight In aqueous solution they are subject to

decomposition by hydroxide ions eg

[PW12O40]3- + 23 OH- HPO4

2- + 12WO42- + 11 H2O -------- 12

12

Although the PH at which such reactions are rapid can very widely

depending upon the polyanion involved Polyanions are often much stable

towards the H3O+ ions and numerous crystalline heteropoly acids are known

Such acids may be extremely soluble in water and polar solvents (giving

solutions with densities in excess of 4 gcm3) and have large dissociation

constants (PK lt 0)

Crystalline heteropoly acids and salts are frequently highly hydrated with

up to 50 molecules of water per anion Much of this water is zeolite in nature

and crystal composition can vary accordingly On the other hand the cation

anion stiochiometry is always well defined as the anion structure and

composition Finally many polyanions are powerful oxidizing agents and

undergo multiple reversible one or two electron reductions leading to intensely

colored mixed valence species known as heteropoly blues Polyanions are

known which can accept as many as 32 electrons without major structural

change

1A4 Chemistry of Molybdenum and Tungusten Heteropolyoxometalates

A photochromic monolayer film of phosphomolybdic acid (denoted as

PMo12) was fabricated by self-assembly approach UV-visible spectrum and

AFM observation show that the monolayer film is composed of aggregated

PMo12 molecules The monolayer film shows good photochromic properties

with enough stability and reversibility The colour change of the monolayer

after UV-irradiation can be captured by a microscope equipped with a color

CCD camera Photochromic response of the monolayer film can be doubled

after being modified by an amine monolayer [39]

Series of vanadium substituted molybdo Keggin HPA with 12 or 3

adjacent vanadium atoms were prepared These materials were supported

on carbon cloth electrodes and hot pressed onto Nafion with an ETEK

electrode used as a standard on the opposite side The MEArsquos were run at

temperatureslt100 oC with the HPA electrode as either the anode or the

cathode Stable polarisation curves are obtained for an HPA based cathode

with reasonable current densities at 80 oC [40]

A novel mixed-valence polyoxomolybdenum anion was synthesized

hydrothermally from molybdenum oxidemolybdenum metalboric and

phosphoric acids12-phenyldiphosphonicacidand imidazole (ImH) and was

13

structurally characterized as an imidazolium saltOne-and two-dimensional

structures of this anion and additional molybdenum diphosphonate linkers

were assembled as wellThey were structurally characterized as their

pyridinium(pyH) salts [41]

Phosphomolybdic acidpolyvinylpyrrolidone hybrid films were found to

show visible light photochromism It is identified that the intra-supramolecular

charge transfer between the inorganic and organic molecules is responsible

for the visible-light coloration Interestingly the films show photo-memory and

thermal activation The films show a small change in absorbance after being

irradiated with visible light for a short time and the coloration can be

enhanced greatly by subsequent thermal treatment Electrical measurements

indicate that the conductivity of the film increases after the brief irradiation

which promotes transfer of the electrons induced by the thermal treatment

[42]

In this work major effort was concentrated on passive thermal control

coatings based on photochromic and thermochromic materials The inorganic

photochromic materials were based on tungsten and molybdenum oxide films

and the organic photochromic materials included spiropyrans and

spirooxazines In addition photochromic composite organic-inorganic films

and thermochromic vanadium oxide films were prepared The samples were

synthesized using sputtering sol-gel process and thermal oxidation [43]

Polyoxometalates a class of oxidatively robust inorganic oxidants and

oxidation catalysts are currently under investigation at the Forest Products

Laboratory and at Emory University as an alternative to chlorinebased

chemicals in the bleaching of soft Woodkraft and other pulps Although

polyoxometalate salts are used in a number of industrial processes the

feasibility of using these salts and oxygen in the commercial bleaching of

chemical pulps was only recently demonstratedA clear advantage of

polyoxometalates over oxygen alone hydrogen peroxide or ozone is their

inherently high selectivity for the residual lignin in softwood kraft pulps The

goal of ongoing research is to develop a highly selective energy efficient

oxygen based polyoxometalate delignification and bleaching technology

compatible with mill closure [44]

14

Tungsten oxide exhibits pronounced photochromism upon bandgap

photoexcitation which makes it attractive and promising for applications in

many areas Some advances have been achieved during the past decades

The research on nanocrystalline films and single crystals indicates the critical

importance of defects in tungsten oxide to its photochromism Based on

energy-band engineering of semiconductors enhancement of photochromism

has been achieved for instance extension of the photoresponse from UV to

visible light by cathodic polarization improved change in absorption before

and after coloration through modification by a noble metal or another metal-

oxide semiconductor and increased photochromic reversibility via

hybridization with organic amines Nanocrystalline oxide films exhibit

controllable wettability which is coherent in nature with photochromism [45]

Polyoxometalates represent a diverse range of molecular clusters with

an almost unmatched range of physical properties and the ability to form

structures that can bridge several length scalesThe new building block

principles that have been discovered are beginning to allow the design of

complex clusters with desired properties and structures and several structural

types and novel physical properties are examinedIn this critical review the

synthetic and design approaches to the many polyoxometalate cluster types

are presented encompassing all the sub-types of polyoxometalates including

isopolyoxometa- lates heteropolyoxometalates and reduced molybdenum

blue systems As well as the fundamental structure and bonding aspectsthe

final section is devoted to discussing these clusters in the context of

contemporary and emerging interdisciplinary interests from areas as diverse

as antiviral agentsbiological ion transport modelsand materialsscience [46]

Keggin type molybdovanadophosphoric heteropoly acids were prepare

d by a novel environmentally benign method and their catalytic performances

were evaluated via hydroxylation of benzene to phenol with hydrogen

peroxide as oxidant in a mixed solvent of glacial acetic acid and acetonitrile

Various reaction parameters such as reaction time reaction temperature

ratio of benzene to hydrogen peroxide concentration of aqueous hydrogen

peroxide ratio of glacial acetic acid to acetonitrile in solvent and catalyst

concentration were changed to obtain an optimal reaction conditions

15

Molybdovanadophosphoric heteropoly acids are revealed to be highly

efficient catalyst for hydroxylation of benzene [47]

Thin films from the system (As2S3)Tl were deposited by thermal

evaporation on Si graphite and optical glass substratesFrom transmission

and reflection measurements of the thin films the refractive index (n) film

thickness(d) optical band gap(Eg) optical oscillator energy(Eo) and

dispersion energy(Ed) before and after exposure to light were determined

The results for optical parameters were analyzed using the Wemple - Di

Domenico single oscillator model and Lorenc-Lorenc equation It was found

that Eg decreases while n E0 and Ed increase for as deposited films

decreases while n E0 and Ed increase for as deposited films with increasing

of Tlconcentration passing through a maximum at 6 at of Tl After exposure

to light n E0 Ed increase and Eg decreases for all compositions

investigated The maximum change in n (Dn = 016 at l = 6328 nm) was

observed for thin As38S56Tl6 films From infrared spectroscopy measurements

of bulk glasses and thin films we could conclude that when up to 6 at of

thallium is introduced As-S-As chains break and a ternary TlAsS2

compound appears at 10 at Tl [48]

The organo - inorganic hybrid materialconsisting of Poly (34 Ethylene

Dioxythiophene) (PEDOT) doped with phosphomolybdate cluster anions

[PMo12O40]3-has been synthesized by direct insitu oxidative polymerization of

34-Ethylene Dioxythiophene (EDOT) with phosphomolybdic acid

(H3PMo12O40) Its characterization is investigated by Fourier Transform

Infrared Spectroscopy (FT-IR) and Scanning Electron Microscopy (SEM) The

hybrid material presents predominantly high electronic conductivities of

around 20 and 70 S cm1at 300 and 400 K respectively [49]

Heteropolyacids (HPAs) are known to be excellent re-dox catalysts In

combination with TiO2 HPAs can be used as photocatalysts active in visible

light The HPA accepts electron and get reduced to heteropolyblue (HPB)

That can absorb light in the visible range HPA can be incorporated onto the

external surface or in the pores of zeolite based composite photocatalysts

have been designed by incorporation of HPA semiconductor TiO2 and

transition metal cobalt on zeolite This composite metallozeolite photocatalyst

is efficient in photoreduction of methyl orange (MO) in visible light to the tune

16

of 411 mg of MO photo reducedg TiO2 This catalyst also shows encouraging

results for hydrogen evolution from water to the tune of 2730 micromolhg TiO

[50]

A new class of proton conducting glass membranes based on heteropo

lyacids such as phosphotungstic acid (PWA) as electrolytes for low

temperature H2O2 fuel cells was investigated Parameters for a single fuel cell

with a catalyst electrode of 015 mgcm2 of PtC and a glass composite

membrane were characterized by electrochemical measurements at open

circuit potential conditions The performance of the membrane electrode

assemblies (MEA) was systematically studied as an effect of SiO2 and P2O5

concentrations in the glass composite membrane and the MEA was found to

exhibit a maximum power density of 162 mW cm2 for an H2O2 fuel cell at

30 degC and 30 relative humidity (RH) [51]

Two new photochromic inorganic-organic hybrid materials formed from

Keggin type Polyoxometalates (POMs) and metronidazole (C6H9N3O3 MNZ)

formulated as H3PMo12O40bull3 MNZ3H2O (1) and H3PW12O40bull3MNZ3H2O(2)

were synthesized and characterized by elemental analysis IR spectra

electronic spectra electron spin resonance (ESR) spectra and TG-DTA

Reflectance spectra show the presence of weak inter molecular charge

transfer between the organic and inorganic moieties in the solid state The

photochromic properties were studied by solid diffuse reflectance spectra and

ESR spectra and the photochromic reactions were found to exhibit first-order

kinetics TG-DTA showed that two hybrid materials have similar thermal

behavior [52]

Heteropolycompounds (HPCs) have been a matter of interest in basic

and applied science for more than a century From their first synthesis many

advances have been made to promote the use of HPCs in different ways in

science and technology The aim of this article is to review the main structural

characteristics of heteropolycompounds of the Keggin type (12

tungstophosphoric12-molybdophosphoric12-tungstosilicic acid alkaline and

alkaline earth salts of12 tungstophosphoric acid and gels doped with HPCs)to

understand and explain their different activities such as high proton

conductivity and catalytic biochemical and biomedical activities [53]

17

A solid hybrid molecular material containing 1-butyl 3-methyl

imidazolium cations and Keggin anions of phosphotungstic acid has been

synthesized It is fully characterized by CHN analysis FTIR XRD UV-Vis-NIR

DRS 31P MAS NMR TGA and SEM The FTIR spectrum of the compound

shows the fingerprint vibrational bands of both Keggin molecular anions and

imidazolium cations The aromatic CndashH stretch region (2700ndash3250 cmndash1) of

imidazolium cation is split due to the interaction between the ring CndashH and

bulky Keggin anion The red-shift in the UV-Vis spectra and the downfield 31P

MAS NMR chemical shift also confirm the electrostatic interaction between

the ions in the compound Near IR spectral region (1000ndash2500 nm) shows the

elimination of water in the compound which is hydrophobic [54]

Transport coefficient measurements (electrical conductivity

thermoelectric power and Hall coefficient) have been performed on a

compact Tl033MoO3 polycrystalline compound in a wide temperature range

(200ndash400 K) Experimental results are interpreted with the help of a p-type

semiconductor model with two inverted deep levels near the midgap The

valence band and the conduction band are assumed to be formed from the

dxy orbitals of molybdenum atoms in the Mo6O22 cluster leading to narrow π-

bonding bands The donor and acceptor levels may be formed from

nonbonding dxy orbitals arising respectively from anionic and Tl+ defects

Electron paramagnetic resonance and magnetic measurements are in good

agreement with the theoretical band semiconductor model which has been

retained [55]

The optical properties of GaInTlAs epilayers grown at low temperature

~230degC by solid-source molecular-beam epitaxy on InP substrates were

characterized using optical absorptionand photoluminescence techniques

Optical absorption measurements a room temperature show a gap shrinkage

toward lower energies from 071 to 061 and 053 eV when the Tl content

increases from 0 to 4and 8in good agreement with theoretical

predictionsLow-temperature photoluminescence band-gap signals from

GaInAs and GaInTlAs layers are only obtained after rapid thermal annealing

performed inorder to improve the electronic quality of the layersA band gap

decrease as much as 41 meV for GaInTlAs with 19 Tl incorporation is

measured by photoluminescence at 8K [56]

18

The electrochromic performance of all solid ndash state cells employing

phosphotungstic acid and phosphomolybdic acid is reported These cells

employ SnO2 as the viewing electrode and graphite as the back electrodeThe

cells in the bleached state can be made white to red and become black in the

coloured state [57]

Various organic compounds were oxidized by molecular oxygen in the

presence of a catalytic amount of mixed addenda heteropolyoxometalates

containing molybdenum and vanadium The catalytic activity of the

Molybdovanadophosphate was found to be greatly enhanced by supporting

on charcoal The supported catalyst has high catalytic activity for oxidative

dehydrogenation of benzylic and allylic alcohols to the corresponding

aldehydes and ketones (46-92) nevertheless the nonsupported catalyst

was inactive for the same oxidations under these conditions 236Trimethyl

phenol was selectively oxidized to trimethyl-p benzoquinone which is

precursor of VitE in the presence of a catalytic amount of

molybdophosphate In addition the aerobic oxidation of amines alkyl-

substituted phenols and alkanes were also examined [58]

The reaction of Tl2CO3 with 111555-hexafluoro-24-pentanedione

and diglyme CH3O(CH2CH2O)2CH3 or tetraglyme CH3O(CH2CH2O)4CH3 in

dichloromethane yields the anhydrous thermally and air stable volatile Tl

diglyme and Tl tetraglyme adducts They have been characterized by single

crystal X-ray diffraction elemental analysis 1H and 13C NMR IR and mass

spectroscopy Thermal and mass-transport properties have been investigated

using thermo gravimetric and differential scanning calorimetric

measurements There is evidence that both precursors are very low melting

and volatile and can be used as liquid Tl sources Both adducts have been

successfully applied to metalndashorganic chemical vapor deposition of thallium

containing films [59]

Large size and high quality single crystals of quasi-two-dimensional

thallium molybdenum purple bronze TlMo6O17 have been grown by electrolytic

reduction of molten salt of Tl2CO3-MoO3 The crystal structure is trigonal with

space group P3m1 determined by X-ray diffraction and four-circle single crystal

diffraction The lattice parameters of the unit cell are a = b = 55282 Adeg and c

= 136991 Adeg The temperature dependence of resistivity and magnetic

19

susceptibility confirmed that a metal-to-metal transition occurs near 110 K

[60]

Hall coefficient and dc conductivity studies were made on p-type

Pb08Sn02Te thin films doped with different concentrations of thallium in the

temperature range 77 to 500 K The Hall coefficient and Hall mobility are

found to decrease with an increase in the doping concentration of thallium

Hall coefficient data have been analyzed in the light of a double valence-band

model Various band parameters such as valence band separation population

ratio mobility ratio and effective mass ratio have been calculated Hall

mobility data have been analyzed in the light of lattice and defect limited

scattering mechanisms [61]

Proton conducting composites of heteropolyacid hydrates phosphomol

ybdic acid H3PMo12O40 nH2O(PMA) phosphotungstic acid H3PW12O40 nH2O

( PTA) and salt hydrate like NiCl2 6H2O were prepared

with insulating Al2O3 as despersoidThe ionic conductivity peaks at two

concentrations of Al2O3 indicating two percolation thresholds for percolation

thresholds for proton conduction Two separate experiments were carried out

to check the existence of such percolation thresholds viz the volta battery

experiment involving the measurement of emf of an electrochemical cell

with composites of different compositions used as electrolyte and the

composition vs conductivity measured by the complex impedance

spectroscopy The presence of two maxima has been attributed to two

different percolation thresholds for the two possible mobile protonic

species H+ + (H3O+) and OH arising from the hydrates [62]

1 A5 Applications of Heteropolyoxometalates-

Applications of heteropolyanions centre depend on their redox properties

their high charges and ionic weights An enormous patent and journal

literature is devoted to the applications of heteropolyanions

1 Analysis-

The formation and subsequent precipitation or reduction of

[XMO12O40]n- anions form the basis of gravimetric and colorimetric analytical

methods for P As Si or Geeither separately or in combination [63 64]

20

2 Biochemical applications-

lsquoPhosphotungstic Acidsrsquo have for decades been used as precipitants

for proteins and as analytical reagents for proteinsalkaloidsand purines eg

the [P2M18O62]6- anions for colorimetric determination of uric acid [65] and

cholesterol The acid H3P12O40 either in aqueous or ethanolic solution is also

widely used as a non specific electron dense stain for electron spectroscopy

The dyestuffs industry has for many years used heteropolymolybdates and

tungstates to form color lakes and toners from basic dyes Large

heteropolyanions exhibit antiviral antitumoral properties at non-cytotoxic

doses in vitro and in vivo and are protein inhibitors of cellular bacterial and

viral DNA RNA polymerizes [66]

3 Catalysis

Heteropoly acids and salts have been used as heterogeneous catalysts

for a broad variety of reactions and compilations of such applications up to

1973 are available Examples include oxidation of propylene and isobutylene

to acrylic acid methacrylic acids and ammoxidation of acrylonitrile olefin

polymerization and epoxidation Much of current activity concerning

heterogeneous catalysis by heteropoly compounds is being carried out in

Japan [67 68]

4 Other Applications

Insoluble salts of Heteropolyanions especially ammonium

molybdophosphates have been used and are commercially available as ion-

exchange materials [69]Recent work in this area includes thin layer

chromatography of amino acids ion selective membranes [70] and the

preparation of new ion exchangers based on heteropolyanions Crystalline12-

tungstophosphoric and 12-molybdophosphoric acids are excellent protonic

conductors Heteropolyacids are electrochromic in the solid state as a

consequence of heteropolyblue formation Heteropolyblue formation has also

been used to detect alcohol or carboxylic acid radicals generated by radiolysis

of aqueous solutions Potential applications of heteropoly complexes as flame

retardants and smoke suppressants or as corrosion inhibitors and conversion

coatings on steel and aluminium are reported [71] Some potential ldquogreenrdquo

applications have been reported eg non-chlorine based wood pulp

bleaching process and a method of decontaminating water Some structures

21

containing transition metal atoms with unpaired electrons have unusual

magnetic properties and are being investigated as nano computer storage

devices Some compounds exhibit luminescence There are many reported

potential medicinal applications eg anti tumoral and anti-viral There have

been reports on the role of weak or non bonding interactions on the crystal

engineering of hybrid polyoxometalates

Spherical nonporous polyoxomolybdate based capsules of different

types containing more than 100 metal atoms reported by Achim Muller and his

group have versatile unique properties regarding their assembly to vesicles

and the chemistry which can be done inside the pores and cavities A discrete

polyoxometalate Lindquist ion of the form W6O192minus was successfully imaged

recently for the first time within the capillary of a carbon nanotube following

steric locking of the anion with the tubule In situ relaxation of the anion in its

equatorial plain was demonstrated [72]

22

Section-B

Litreture Survey on Chromism in Transition Metal

Oxides

1 B1 Chromism in Transition Metal Oxides

Chromism is a reversible change in a substances colour resulting from

a process caused by some form of stimulus Many materials are chromic

including inorganic and organic compounds and conducting polymers and the

property can result from many different mechanisms Several transition metal

oxides show EC properties The most popular are from the VI - B oxides In

this group WO3 and MoO3 are the most thoroughly studied cathodic EC

materials which can be electrochemically coloured and bleached when used

as the cathode in electrochemical cells Cathodic EC materials also include

V2O3 TiO2 and Nb2O5 Another distinguishable group is anodic EC material

including VIII oxides like IrOx nH2O Rh2O3 nH2O NiO nH2O etc which can be

anodicaly coloured in the electrochemical process when used as anode

There are several types of chromism which are discussed as below

B11 Photochromism

Photochromism is the reversible transformation of a chemical species

between two forms by the absorption of electromagnetic radiation where

the two forms have different absorption spectra [7374]

Trivially this can be described as a reversible change of color upon

exposure to light The phenomenon was discovered in the late 1880s

including work by Markwald who studied the reversible change of color of 23

44-tetrachloronaphthalen-1(4H)-one in the solid state He labeled this

phenomenon phototropy and this name was used until the 1950s

when Yehuda Hirshberg of the Weizmann Institute of Science in Israel

proposed the term photochromism [75] Photochromism can take place

in both organic and inorganic compounds and also has its place in biological

systems (for example retinal in the vision process)

Photochromism does not have a rigorous definition but is usually used

to describe compounds that undergo a reversible photochemical reaction

where an absorption band in the visible part of the electromagnetic spectrum

23

changes dramatically in strength or wavelength In many cases an

absorbance band is present in only one form The degree of change required

for a photochemical reaction to be dubbed photochromic is that which

appears dramatic by eye but in essence there is no dividing line between

photochromic reactions and other photochemistry Therefore while the

transcis isomerization of azobenzene is considered a photochromic reaction

the analogous reaction of stilbene is not Since photochromism is just a

special case of a photochemical reaction almost any photochemical reaction

type may be used to produce photochromism with appropriate molecular desi

gnSome of the most common processes involved in photochromism are peric

yclic reactions cis-trans somerizations intramolecular hydrogen transfer

intramolecular group transfers dissociation processes and electron transfers

(oxidation-reduction)

Another some what arbitrary requirement of photochromism is that

it requires the two states of the molecule to be thermally stable under

ambient conditions for a reasonable time All the same nitrospiropyran (which

back-isomerizes in the dark over ~10 minutes at room temperature) is

considered photochromic All photochromic molecules back-isomerize to their

more stable form at some rate and this back-isomerization is accelerated by

heating There is therefore a close relationship between photochromic and the

rmochromic compounds The timescale of thermal back-isomerization is

important for applications and may be molecularly engineered

Photochromic compounds considered to be thermally stable include some

diarylethenes which do not back isomerize even after heating at 800C for 3

months

Since photochromic chromophores are dyes and operate according to

well-known reactions their molecular engineering to fine-tune their properties

can be achieved relatively easily using known design models quantum

mechanics calculations and experimentation In particular the tuning of

absorbance bands to particular parts of the spectrum and the engineering

of thermal stability have received much attention

Sometimes and particularly in the dye industry the term irreversible

photochromic is used to describe materials that undergo a permanent color

change upon exposure to Ultraviolet or visible light radiation Because by

24

definition photochromics are reversible there is technically no such thing as a

n irreversible photochromic this is loose usage and these compounds

are better referred to as photochangable or photoreactive dyes

Apart from the qualities already mentioned several other properties of

photochromics are important for their use These include

Quantum yield of the photochemical reaction

This determined the efficiency of the photochromic change with respect

to the amount of light absorbed The quantum yield of isomerization

can be strongly dependent on conditions

Fatigue resistance In photochromic materials fatigue refers to the

loss of reversibility by processes such as photodegradation

photobleaching photooxidation and other side reactions All

photochromics suffer fatigue to some extent and its rate is strongly

dependent on the activating light and the conditions of the sample

Photostationary state Photochromic materials have two states and

their interconversion can be controlled using different wavelengths of

light Excitation with any given wavelength of light will result in a

mixture of the two states at a particular ratio called the photo-

stationary state In a perfect system there would exist wavelengths

that can be used to provide 10 and 01 ratios of the isomers

but in real systems this is not possible since the active

absorbance bands always overlap to some extent

Polarity and solubility In order to incorporate photochromics in

working systems they suffer the same issues as other dyes They are

often charged in one or more state leading to very high polarity and

possible large changes in polarity They also often contain large

conjugated systems that limit their solubility

Photochromic complexes

A photochromic complex is a kind of chemical compound that has

photoresponsive Parts on its ligand These complexes have a specific

structure photoswitchable organic compounds are attached to

metalcomplexes For the photocontrollable parts thermally and

photochemically stable chromophores (azobenzene diarylethene

25

spiropyran etc) are usually used And for the metal complexes a wide

variety of compounds that have various functions (redox response

luminescence magnetism etc) are applied The photochromic parts and

metal parts are so close that they can affect each others molecular

orbitals The physical properties of these compounds shown by parts of

them (ie chromophores or metals) thus can be controlled by switching

their other sites by external stimuli For example photoisomerization

behaviors of some complexes can be switched by oxidation and reduction of

their metal parts Some other compounds can be changed in their

luminescence behavior magnetic interaction of metal sites or stability of

metal-to-ligand coordination by photoisomerization of their photochromic

parts

Photochemistry of Polyoxometalates

The photochemistry of polyoxometalates is of great interest to inorganic

chemistsMore than 80 years agoit was found that the R-Keggin tungstate

H3[PW12O40] was reduced photochemically to yield a blue-colored species

which was reoxidized by air and by various other oxidizing agents such as

Fe3+AgNO3and H2O2 [7879]The photoredox reactions of H4[SiW12O40]and

H3[PW12O40] proceeded effectively in the presence of primary and secondary

alcohols their ethers and aldehydes and proteinsbut less effectively in the

presence of tertiary alcoholsketonesestersthe fatty acids above formic

acidand simple amines[8081] The basic photoredox reaction involving

ethanol is illustrated by eq 13

2 H3PW12O40 + H3CCH2OH h ν ν ν ν 2 H4PW12O40 + H3CCHO ------- 13

2 H4PW12O40 +12 O2 2 H3PW12O40 + H2O ------ 14

In this reactionone molecule of ethanol photochemically reduces two

molecules of H3PW12O40 and is itself oxidized to acetaldehyde In the

presence of air the thermal oxidation of the reduced species takes place at

room temperature(eq 14)The reduced polyoxometalates which are the so-

calledldquoheteropolybluesrdquo have been used for the colorimetric analysis of the

26

elements P Si As and Ge and for the determination of uric acidsugarand

other biological compounds [8283] Piperidinium metavanadate also

undergoes photoinduced coloration from white to black followed by a

reversible color change in the presence of oxidizing agentsHowever

ammonium metavanadates ([NH4][VO3]) exhibits no photoinduced

coloration[84] The early photoredox reactions of the R-Keggin

polyoxometalates H4[SiW12O40] and H3PW12O40 were carried out in the

presence of photographic paper however the limited number of the

structurally well-characterized compounds available for study delayed the

development of modern cluster-compound photochemistry until the discovery

of photochromism in alkylammonium polyoxo- molybdate solids[8586] A

photochromic or electrochromic material is one whose light-absorbing

properties are altered upon optical excitation or reduction under the influence

of an externally applied electric field respectively The induced coloration

remains even after the excitation source has been removed These materials

are of technological interest because they return to their original state either

thermally upon irradiation with light of a frequency corresponding to the

induced absorption or electrochemically upon reversing the polarity of the

externally applied electric field Thus photochromic and electrochromic

materials behave in a reversible manner Polyoxometalates exhibit significant

photo-and electrochromism which makes them suitable as nanocomposite

molecular devices and as models for probing the physical properties of infinite

metal oxides Since the metal ions in the oxidized polyoxometalates have d0

electronic configurations the only absorption band which occurs in the UV-vis

range of the electronic spectra is due to the oxygen-to-metal (O-M) ligand-to-

metal charge transfer (LMCT)Upon irradiation electrons are promoted from

the low-energy electronic states which are mainly comprised of oxygen 2p

orbitals (the valence band in the band model)to the high-energy electronic

states which are mainly comprised of metal d orbitalrsquos (the conduction band

in the band model)The fundamental transitions in polyoxometalate lattices

are depicted schematically in Fig1B1

27

Fig1B1 Simple model showing the electronic transitions in the

polyoxometalates containing electron donar and acceptor (a)generation

of charge carriers(b)electron and hole trapping(c)electron release due

to stimulation(d)recombination

between electron and hole Electrons are e-and holes are h+

In the polyoxometalates containing heteroatoms and especially in

mixed metal polyoxometalates the charge carriers which are created by the

light or electric field may be trapped in electron traps and hole traps These

traps provide states of localized energy in the O-M LMCT energy gap due to

the heteroatoms or counter cations which correspond to impurities or lattice

defects in the band model If the trap depth ∆E is large compared to kT the

probability for thermal escape from the trap will be negligibly small and

metastable situation will existThe trapped carriers can be released by thermal

or optical stimulationIn the case of thermal stimulation the irradiated

polyoxometalate is heated until the energy barrier ∆E can be overcome The

trapped electron (or hole) then can escape from the trap and nonradiatively

recombine with the trapped hole (or electron)Under optical stimulation the

energy of an incident photon is used to overcome ∆E The relaxation

processes of the OndashM LMCT excitation energy include both the nonradiative

recombination of electrons and holes within the energy gap and the

intramolecular energy transfer leading to a charge-transfer emission This

intramolecular energy transfer corresponds to the O-M LMCT energy gap and

occurs via radiative recombination and sensitized emission from the

28

heteroatoms or cations If several energy levels based on the hetero atoms or

counter cations act as energy acceptors within the O-M LMCTenergy gap the

energy transfer occurs from the O-M LMCT states to these levels followed by

the nonradiative or radiative deactivation of the excitation energy It should be

noted that the O-M LMCT states also can be generated by the application of

very high electric fields to the polyoxometalate solids as demonstrated by the

observation of electroluminescence[87]If an external electric field with a

potential more negative than the energy levels of the vacant orbitals involved

in the O-M LMCT transition is applied to a polyoxometalate on the electrode

surface an electrochemical reduction occurs via the injection of electrons

from the electrode in to the vacant levels of the polyoxometalate as shown in

Fig1B 2

Fig1B2 - Energy scheme for the electrochromism of polyoxometalates

a)electrochemical reduction (b) electrochemical oxidation

Electrons injected in to the high-energy levels also may be trapped by

electron traps in a process analogous to that which occurs during LMCT

photoexcitation of the polyoxometalates These electrons are returned to the

electrode by electrochemical oxidation at an electrode potential more positive

than the energy levels for the d1 electron states The d1electrons in the O-M

LMCTstates facilitate the absorption of visible light via intervalence charge

transfer among metal centers and d-d transitions The same type of transition

may be possible for the d1electron captured by the electron traps too In

addition to searching for new photosensitive polyoxometalates with the

29

potential for having practical application there is now a need to elucidate the

fundamental photo-and electrochemical coloration processes by studying

electron transfer within the polyoxometalate lattices in conjunction with their

crystal structures So far few polyoxometalates exhibit a perfect reversibility

of coloration The irreversibility of the color change arises from as yet

uncharacterized side reactions during both the coloration and decoloration of

the polyoxometalates

Many metal oxides including aluminum titanium vanadium niobium

molybdenum and tungsten oxides are photochromic when they contain

impurities or dopants This coloration has been interpreted on the basis of

electron trapping at appropriate lattice sites within the crystals as shown in

Fig1B1 where the O-M LMCT transition corresponds to the transition

between the valence and conduction bands for the infinite metal-oxide lattice

[88-91]

B12 Applications of Photochromic materials

Sunglasses

One of the most famous reversible photochromic applications is color

changing lenses for sunglasses as found in eyeglasses The largest limitation

in using PC technology is that the materials cannot be made stable enough

to withstand thousands of hours of outdoor exposure so long-term outdoor

applications are not appropriate at this time The switching speed of

photochromic dyes is highly sensitive to the rigidity of the environment around

the dye As result they switch most rapidly in solution and slowest in the rigid

environment like a polymer lens Recently it has been reported that attaching

flexible low Tg polymers (for example siloxanes or poly (butyl acrylate) to the

dyes allows them to switch much more rapidly in a rigid lens [76] Some

spirooxazines with siloxane polymers attached switch at near solution like

speeds even though they are in a rigid lens matrix

Supramolecular chemistry

Photochromic units have been employed extensively in supramolecular

chemistry Their ability to give a light controlled reversible shape change

means that they can be used to make or break molecular recognition motifs

30

or to cause a consequent shape change in their surroundings Thus

photochromic units have been demonstrated as components of molecular

switches The coupling of photochromic units to enzymes or enzyme cofactors

even provides the ability to reversibly turn enzymes on and off

by altering their shape or orientation in such a way that their functions

are either working or broken

Data storage

The possibility of using photochromic compounds for data storage was

first suggested in 1956 by Yehuda Hirshberg[77] Since that time there have

been many investigations by various academic and commercial groups

particularly in the area of 3D optical data storage which promises discs that

can hold a terabyte of data Initially issues with thermal back-reactions

and destructive reading dogged these studies but more recently more stable

systems have been developed

Novelty items

Reversible photochromics are also found in applications such as toys

cosmeticsclothing and industrial applications If necessary they can be made

to change between desired colors by combination with a permanent pigment

A large number of inorganic compounds exhibit photochromism

These solids often have large band gaps of the order of 3 - 12 eV and

excitation of these solids leads to the formation of metastable centers that

absorb visible light giving rise to their colour They can return to their ground

state by heating or by optical excitation within the colour-centre band In most

cases the photochromism is a structure sensitive phenomenon involving

localized defect impurities or dislocations Some of these inorganic

compounds have the potential for a number of different uses Photochromic

compounds have a number of useful applications These can be divided

according to the most important property that is being used (Table 11) [92]

31

Table 11 Applications of Photochromic materials

Applications Depending Upon

Sensitivity to

Radiation

Reversibility Thermal Chemical or

Physical Properties

Self-developing

photography

Chemical switches for

computers

Temperature indicators

Protective

materials

Data displays

Heat-sensitive recording

media

Optical signal

processing

Photomasking and

photoresist technology

Reusable data storage

media

Anaytical reagents

Photochromic

microimages

Photopolymerisation

Information encoding

and steganography

Photocontractile

polymers and the

photoviscosity effect

Control of light

intensity

Q-switches

Pyroelectric

photochromic materials

B13 Thermochromism

Thermochromism is the reversible colour change of a substance

induced by temperature change A large variety of substances organic

inorganic organometallic supramolecular and polymeric systems exhibit this

phenomenon Examples of these include bianthrones cobalt

hexacyanoferrate the zirconocene complex of 1 4-diphenyl-1 3-butadiene

and poly (3-alkylthiophene) The organic 99-bixanthenylidene is colourless at

90 K yellow-green at 298 K and dark-blue when melted at 592 K Heating

conducting polymers can cause them to change colour This is achieved by

causing conformational changes to the polymer backbone resulting in a

change in the band gap of the polymer It has been reported that regioregular

32

P3HT reversibly changes colour upon heating to 220ordmC due to temperature-

dependent conformation changes Thermally cross linked polymer undergoes

the same colour change but it is much less reversible [93] Other forms of

thermochromism may be commercially important eg to give a visual

indication of temperature changes

B14 Electrochromism

Electrochromism describes a phenomenon of material color change in

a persistent but reversible manner produced by electrochemically induced

oxidation-reduction reactions Electrochromic materials can be applied to

various kinds of products such as smart windows and display devices Among

those applications there have been lots of efforts to develop electrochromic

display devices (ECDs) Especially flexible display devices are now attracting

much attention worldwide since they can facilitate new technological demands

such as bending and folding of paper-like displays High electrochromic

efficiency short response time long operating life time and reduction of

energy consumption are the most important requisites of the materials for the

paperlike displays [94 95] Among those properties the operation life time is

the most important barrier to overcome for a realization of ECDs There are

two types of electrochromic material a) inorganic transition metal oxides

(TMOs) b) organic polymer materials The TMOs have been studied longer

than the organic materials that they have been studied since 1960s [9697]

Electrochromism describes a reversible color change of material

produced by electrochemically induced oxidation-reduction reactions It is one

of several types of chromism of materials As thermochromism and

photochromism mean material color changes made by heat and light

respectively electrochromism refers that the color change is caused by an

electric potential In most cases the color change in electrochromism can be

driven by rather low electrical potential of the order of a fraction of volt or a

few volts [94-96] The color change of material means variation in

transmittance andor reflectance change in visible range which is originated

from different electronic absorption bands according to a switching between

oxidation and reduction state of material When electric potential is applied on

electrochromic material forced oxidation or reduction is derived and the

33

individual color is originated from the corresponding oxidation or reduction

state of the material For electrochromic materials the characteristic color

change is reversible since the oxidation and reduction state can be converted

reversibly by switching potential

Application fields

The application area for the electrochromism is rather broad that it

covers from smart window glazing and optical modulators to information

displays [98-102] The smart windows are typical examples The

electrochromic property is used to control the amount of light and heat to pass

through the windows Usually the electrochromic material is in form of thin film

coated on a window glass The transmittance modulation has also been

applied at the automobiles to automatically tint rear-view mirrors in various

lighting conditions The electrochromic application fields are illustrated in Fig

1B3

Since the smart windows control the transmittance of heat as well as

the transmittance of visible light the working definition of electrochromism has

now been extended to include devices for modulation of radiation in the near

infrared thermal infrared and microwave regions When color for

electrochromic materials is used this can now mean a response by detectors

at these wavelengths and not just by the human eyes Nowadays

electrochromic material draws much attention as being used in the display

devices Electrochromic display device (ECD) is being considered as one of

the candidates for the conventional liquid crystal display (LCD) since ECD

has many advantages over LCDs Among these advantages the most

important are low energy consumption wider viewing angle high contrast

rate and possibility to achieve multiple colors with a single material [103]

When a new redox state of electrochromic materialis established by

the applied electric pulse then it is maintained after the potential is switched

off This means the colored or bleached state of the material can be sustained

for a considerable time without applying electrical power This is so-called

ldquocolor memory effectrdquo of electrochromic material Because of the color

memory effect energy consumption for the electrochromic display device

could be drastically reduced and this would be a big advantage over other

34

emissive devices The low power consumption is especially advantageous

when it is applied to mobile devices with limited power source The possibility

for a flexible display is another attraction for electrochromic material

Information displays

Real-view mirrors for automobiles

Fig 1B4 Application fields of electrochromic devices Smart windows information displays and real-view mirrors for automobiles

35

Using polymeric electrochromic material and plastic substrate with a

transparent conductive coating it is possible to build all-plastic flexible display

device There are lots of papers and patents about the flexible display devices

[104105] In these cases gel-type electrolyte is also needed The gel-type

electrolyte immobilizes liquid electrolyte in the polymer network [106107]

Recently ITO-coated polymer films are readily available which would provide

the plastic substrate for a flexible device The flexible electrochromic displays

are frequently tried with the plastic substrate flexible electrochromic material

and gel-type electrolyte The flexible electrochromic displays would facilitate

increasing technical demands for foldable display devices

Metal Ion Electrochromism

Many transition metal oxides are capable of redox reactions that result in

colour change Metal oxide films are commonly prepared as thin layers of

either tungsten nickel molybdenum or other metal compounds by a number

of techniques These include sol-gel electrochemical by dc or rfreactive

sputtering techniques electron-beam evaporation by anodic or cathodic

electrodeposition or by solution dipping of the electrochromic metal

compounds (or compounds that can be changed into these metal compounds)

onto optically transparent electrodes (OTE) [108 -114] Their electrochromism

is derived from the colour change associated with a change in the oxidation

state of the metal anion The behaviour of these materials is dependent upon

pH moisture and exposure to the atmosphere [115] Generally the switching

rates of these films is somewhat slow with typical switching times of about 15

- 60 seconds to achieve 100 conversion to either coloured or bleached state

[116 -120] An Example of this includes nickel oxide which changes from

transparent (pale green) to brownblack taking about 30 seconds to do so

[121] Other examples include [(NH4)5Ru]2(pyrazine)5+ and [(NH4)5Ru]2(44-

bipyridine)5+ whose electrochromism is significantly different due to the effect

of the ligand [122] Table 12 below gives some examples of metal oxide films

with electrochromic properties

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

9

Fig 1A 4 Lindquist structure M6O19nminus

Fig 1A 5 Dawson structure X2M18O62nminus

In general α-Keggin anions are synthesized in acidic solutions For

example 12-Phosphotungstic acid is formed by condensing phosphate ion

10

with tungstate ions The heteropolyacid that is formed has the Keggin

structure

[PO4]3- + 12 [WO4]

2- + 27 H+ rarr H3PW12O40 + 12 H2O ----------- 11

α-Keggin anions have been used as catalyst in hydration polymerization and

oxidation reaction as catalysts

The metal atoms that make up the framework (termed addenda atoms)

are typically Mo W and V When more than one element is present the

cluster is called a mixed addendaclusterThe ligands coordinated to metal

atoms that together form the bridged framework are usually oxide ions but

other elements such as S and Br have been substituted for some of the oxide

ions (Note that sulfur substituted POM is often termed a

polyoxothiometalates) Another development is the use of other ligands eg

nitrosy and alkoxy to replace oxide ions The typical framework building

blocks are polyhedral units with 4 5 6 or 7 coordinate metal centers These

units usually share edges andor vertices The most common unit for

polymolybdates is the octahedral MoO6 unit which is a distorted octahedron

where the Mo atom moves off centre to give one short Mo-O bond In some

polymolybdates there are pentagonal bipyramidal units and these are key

building blocks in the molybdenum bluesHetero atoms are present in many

polyoxometalates Many different elements can act as hetero-atoms

Examples of various coordination numbers around the hetero-atom are

known

4 co-ordinate (tetrahedral) in Keggin Dawson and Lindquist structures

(eg PO4 SiO4 AsO4)

6 co-ordinate (octahedral) in Anderson structure (eg Al(OH)6 TeO6

8 co-ordinate (square antiprism) in ((CeO8)W10O28)8minus

12 co-ordinate (icosahedral) in (UO12)Mo12O30 8minus

11

Often the hetero-atom is centrally located in the anion (eg Keggin

structure) or in a structure fragment eg the 2 phosphorus atoms in the

Dawson ion are central to the two symmetric fragments There are similarities

to clathrate structures The Keggin ion can be formulated as PO4 2minusand

M12O36 and the Dawson as (XO42-)2 and M18O54Structural isomerism is

common For example the Keggin structure has 5 isomers which can be

considered to contain one or more of the four M3O13 units being rotated

through 60degMany compounds share the same framework architectures or

frameworks derived from a larger framework with one or more addenda atoms

and oxide ions removed to give defect structure usually called a lacunary

structure An example of a compound with a Dawson lacunary structure is

As2W15O56Some cage structures containing ions are known eg an example

is the vanadate cage V18O42 containing a Clminus ion [38] This has 5 co-

ordinates square pyramidal vanadium units linked together

Fig 1A 6 - H4V18O42 cage containing Cl

1A3 General Properties of polyoxometalates -

Typically polyoxoanions are water and air stable species of large size

(6-25 Adeg) and high ionic weight In aqueous solution they are subject to

decomposition by hydroxide ions eg

[PW12O40]3- + 23 OH- HPO4

2- + 12WO42- + 11 H2O -------- 12

12

Although the PH at which such reactions are rapid can very widely

depending upon the polyanion involved Polyanions are often much stable

towards the H3O+ ions and numerous crystalline heteropoly acids are known

Such acids may be extremely soluble in water and polar solvents (giving

solutions with densities in excess of 4 gcm3) and have large dissociation

constants (PK lt 0)

Crystalline heteropoly acids and salts are frequently highly hydrated with

up to 50 molecules of water per anion Much of this water is zeolite in nature

and crystal composition can vary accordingly On the other hand the cation

anion stiochiometry is always well defined as the anion structure and

composition Finally many polyanions are powerful oxidizing agents and

undergo multiple reversible one or two electron reductions leading to intensely

colored mixed valence species known as heteropoly blues Polyanions are

known which can accept as many as 32 electrons without major structural

change

1A4 Chemistry of Molybdenum and Tungusten Heteropolyoxometalates

A photochromic monolayer film of phosphomolybdic acid (denoted as

PMo12) was fabricated by self-assembly approach UV-visible spectrum and

AFM observation show that the monolayer film is composed of aggregated

PMo12 molecules The monolayer film shows good photochromic properties

with enough stability and reversibility The colour change of the monolayer

after UV-irradiation can be captured by a microscope equipped with a color

CCD camera Photochromic response of the monolayer film can be doubled

after being modified by an amine monolayer [39]

Series of vanadium substituted molybdo Keggin HPA with 12 or 3

adjacent vanadium atoms were prepared These materials were supported

on carbon cloth electrodes and hot pressed onto Nafion with an ETEK

electrode used as a standard on the opposite side The MEArsquos were run at

temperatureslt100 oC with the HPA electrode as either the anode or the

cathode Stable polarisation curves are obtained for an HPA based cathode

with reasonable current densities at 80 oC [40]

A novel mixed-valence polyoxomolybdenum anion was synthesized

hydrothermally from molybdenum oxidemolybdenum metalboric and

phosphoric acids12-phenyldiphosphonicacidand imidazole (ImH) and was

13

structurally characterized as an imidazolium saltOne-and two-dimensional

structures of this anion and additional molybdenum diphosphonate linkers

were assembled as wellThey were structurally characterized as their

pyridinium(pyH) salts [41]

Phosphomolybdic acidpolyvinylpyrrolidone hybrid films were found to

show visible light photochromism It is identified that the intra-supramolecular

charge transfer between the inorganic and organic molecules is responsible

for the visible-light coloration Interestingly the films show photo-memory and

thermal activation The films show a small change in absorbance after being

irradiated with visible light for a short time and the coloration can be

enhanced greatly by subsequent thermal treatment Electrical measurements

indicate that the conductivity of the film increases after the brief irradiation

which promotes transfer of the electrons induced by the thermal treatment

[42]

In this work major effort was concentrated on passive thermal control

coatings based on photochromic and thermochromic materials The inorganic

photochromic materials were based on tungsten and molybdenum oxide films

and the organic photochromic materials included spiropyrans and

spirooxazines In addition photochromic composite organic-inorganic films

and thermochromic vanadium oxide films were prepared The samples were

synthesized using sputtering sol-gel process and thermal oxidation [43]

Polyoxometalates a class of oxidatively robust inorganic oxidants and

oxidation catalysts are currently under investigation at the Forest Products

Laboratory and at Emory University as an alternative to chlorinebased

chemicals in the bleaching of soft Woodkraft and other pulps Although

polyoxometalate salts are used in a number of industrial processes the

feasibility of using these salts and oxygen in the commercial bleaching of

chemical pulps was only recently demonstratedA clear advantage of

polyoxometalates over oxygen alone hydrogen peroxide or ozone is their

inherently high selectivity for the residual lignin in softwood kraft pulps The

goal of ongoing research is to develop a highly selective energy efficient

oxygen based polyoxometalate delignification and bleaching technology

compatible with mill closure [44]

14

Tungsten oxide exhibits pronounced photochromism upon bandgap

photoexcitation which makes it attractive and promising for applications in

many areas Some advances have been achieved during the past decades

The research on nanocrystalline films and single crystals indicates the critical

importance of defects in tungsten oxide to its photochromism Based on

energy-band engineering of semiconductors enhancement of photochromism

has been achieved for instance extension of the photoresponse from UV to

visible light by cathodic polarization improved change in absorption before

and after coloration through modification by a noble metal or another metal-

oxide semiconductor and increased photochromic reversibility via

hybridization with organic amines Nanocrystalline oxide films exhibit

controllable wettability which is coherent in nature with photochromism [45]

Polyoxometalates represent a diverse range of molecular clusters with

an almost unmatched range of physical properties and the ability to form

structures that can bridge several length scalesThe new building block

principles that have been discovered are beginning to allow the design of

complex clusters with desired properties and structures and several structural

types and novel physical properties are examinedIn this critical review the

synthetic and design approaches to the many polyoxometalate cluster types

are presented encompassing all the sub-types of polyoxometalates including

isopolyoxometa- lates heteropolyoxometalates and reduced molybdenum

blue systems As well as the fundamental structure and bonding aspectsthe

final section is devoted to discussing these clusters in the context of

contemporary and emerging interdisciplinary interests from areas as diverse

as antiviral agentsbiological ion transport modelsand materialsscience [46]

Keggin type molybdovanadophosphoric heteropoly acids were prepare

d by a novel environmentally benign method and their catalytic performances

were evaluated via hydroxylation of benzene to phenol with hydrogen

peroxide as oxidant in a mixed solvent of glacial acetic acid and acetonitrile

Various reaction parameters such as reaction time reaction temperature

ratio of benzene to hydrogen peroxide concentration of aqueous hydrogen

peroxide ratio of glacial acetic acid to acetonitrile in solvent and catalyst

concentration were changed to obtain an optimal reaction conditions

15

Molybdovanadophosphoric heteropoly acids are revealed to be highly

efficient catalyst for hydroxylation of benzene [47]

Thin films from the system (As2S3)Tl were deposited by thermal

evaporation on Si graphite and optical glass substratesFrom transmission

and reflection measurements of the thin films the refractive index (n) film

thickness(d) optical band gap(Eg) optical oscillator energy(Eo) and

dispersion energy(Ed) before and after exposure to light were determined

The results for optical parameters were analyzed using the Wemple - Di

Domenico single oscillator model and Lorenc-Lorenc equation It was found

that Eg decreases while n E0 and Ed increase for as deposited films

decreases while n E0 and Ed increase for as deposited films with increasing

of Tlconcentration passing through a maximum at 6 at of Tl After exposure

to light n E0 Ed increase and Eg decreases for all compositions

investigated The maximum change in n (Dn = 016 at l = 6328 nm) was

observed for thin As38S56Tl6 films From infrared spectroscopy measurements

of bulk glasses and thin films we could conclude that when up to 6 at of

thallium is introduced As-S-As chains break and a ternary TlAsS2

compound appears at 10 at Tl [48]

The organo - inorganic hybrid materialconsisting of Poly (34 Ethylene

Dioxythiophene) (PEDOT) doped with phosphomolybdate cluster anions

[PMo12O40]3-has been synthesized by direct insitu oxidative polymerization of

34-Ethylene Dioxythiophene (EDOT) with phosphomolybdic acid

(H3PMo12O40) Its characterization is investigated by Fourier Transform

Infrared Spectroscopy (FT-IR) and Scanning Electron Microscopy (SEM) The

hybrid material presents predominantly high electronic conductivities of

around 20 and 70 S cm1at 300 and 400 K respectively [49]

Heteropolyacids (HPAs) are known to be excellent re-dox catalysts In

combination with TiO2 HPAs can be used as photocatalysts active in visible

light The HPA accepts electron and get reduced to heteropolyblue (HPB)

That can absorb light in the visible range HPA can be incorporated onto the

external surface or in the pores of zeolite based composite photocatalysts

have been designed by incorporation of HPA semiconductor TiO2 and

transition metal cobalt on zeolite This composite metallozeolite photocatalyst

is efficient in photoreduction of methyl orange (MO) in visible light to the tune

16

of 411 mg of MO photo reducedg TiO2 This catalyst also shows encouraging

results for hydrogen evolution from water to the tune of 2730 micromolhg TiO

[50]

A new class of proton conducting glass membranes based on heteropo

lyacids such as phosphotungstic acid (PWA) as electrolytes for low

temperature H2O2 fuel cells was investigated Parameters for a single fuel cell

with a catalyst electrode of 015 mgcm2 of PtC and a glass composite

membrane were characterized by electrochemical measurements at open

circuit potential conditions The performance of the membrane electrode

assemblies (MEA) was systematically studied as an effect of SiO2 and P2O5

concentrations in the glass composite membrane and the MEA was found to

exhibit a maximum power density of 162 mW cm2 for an H2O2 fuel cell at

30 degC and 30 relative humidity (RH) [51]

Two new photochromic inorganic-organic hybrid materials formed from

Keggin type Polyoxometalates (POMs) and metronidazole (C6H9N3O3 MNZ)

formulated as H3PMo12O40bull3 MNZ3H2O (1) and H3PW12O40bull3MNZ3H2O(2)

were synthesized and characterized by elemental analysis IR spectra

electronic spectra electron spin resonance (ESR) spectra and TG-DTA

Reflectance spectra show the presence of weak inter molecular charge

transfer between the organic and inorganic moieties in the solid state The

photochromic properties were studied by solid diffuse reflectance spectra and

ESR spectra and the photochromic reactions were found to exhibit first-order

kinetics TG-DTA showed that two hybrid materials have similar thermal

behavior [52]

Heteropolycompounds (HPCs) have been a matter of interest in basic

and applied science for more than a century From their first synthesis many

advances have been made to promote the use of HPCs in different ways in

science and technology The aim of this article is to review the main structural

characteristics of heteropolycompounds of the Keggin type (12

tungstophosphoric12-molybdophosphoric12-tungstosilicic acid alkaline and

alkaline earth salts of12 tungstophosphoric acid and gels doped with HPCs)to

understand and explain their different activities such as high proton

conductivity and catalytic biochemical and biomedical activities [53]

17

A solid hybrid molecular material containing 1-butyl 3-methyl

imidazolium cations and Keggin anions of phosphotungstic acid has been

synthesized It is fully characterized by CHN analysis FTIR XRD UV-Vis-NIR

DRS 31P MAS NMR TGA and SEM The FTIR spectrum of the compound

shows the fingerprint vibrational bands of both Keggin molecular anions and

imidazolium cations The aromatic CndashH stretch region (2700ndash3250 cmndash1) of

imidazolium cation is split due to the interaction between the ring CndashH and

bulky Keggin anion The red-shift in the UV-Vis spectra and the downfield 31P

MAS NMR chemical shift also confirm the electrostatic interaction between

the ions in the compound Near IR spectral region (1000ndash2500 nm) shows the

elimination of water in the compound which is hydrophobic [54]

Transport coefficient measurements (electrical conductivity

thermoelectric power and Hall coefficient) have been performed on a

compact Tl033MoO3 polycrystalline compound in a wide temperature range

(200ndash400 K) Experimental results are interpreted with the help of a p-type

semiconductor model with two inverted deep levels near the midgap The

valence band and the conduction band are assumed to be formed from the

dxy orbitals of molybdenum atoms in the Mo6O22 cluster leading to narrow π-

bonding bands The donor and acceptor levels may be formed from

nonbonding dxy orbitals arising respectively from anionic and Tl+ defects

Electron paramagnetic resonance and magnetic measurements are in good

agreement with the theoretical band semiconductor model which has been

retained [55]

The optical properties of GaInTlAs epilayers grown at low temperature

~230degC by solid-source molecular-beam epitaxy on InP substrates were

characterized using optical absorptionand photoluminescence techniques

Optical absorption measurements a room temperature show a gap shrinkage

toward lower energies from 071 to 061 and 053 eV when the Tl content

increases from 0 to 4and 8in good agreement with theoretical

predictionsLow-temperature photoluminescence band-gap signals from

GaInAs and GaInTlAs layers are only obtained after rapid thermal annealing

performed inorder to improve the electronic quality of the layersA band gap

decrease as much as 41 meV for GaInTlAs with 19 Tl incorporation is

measured by photoluminescence at 8K [56]

18

The electrochromic performance of all solid ndash state cells employing

phosphotungstic acid and phosphomolybdic acid is reported These cells

employ SnO2 as the viewing electrode and graphite as the back electrodeThe

cells in the bleached state can be made white to red and become black in the

coloured state [57]

Various organic compounds were oxidized by molecular oxygen in the

presence of a catalytic amount of mixed addenda heteropolyoxometalates

containing molybdenum and vanadium The catalytic activity of the

Molybdovanadophosphate was found to be greatly enhanced by supporting

on charcoal The supported catalyst has high catalytic activity for oxidative

dehydrogenation of benzylic and allylic alcohols to the corresponding

aldehydes and ketones (46-92) nevertheless the nonsupported catalyst

was inactive for the same oxidations under these conditions 236Trimethyl

phenol was selectively oxidized to trimethyl-p benzoquinone which is

precursor of VitE in the presence of a catalytic amount of

molybdophosphate In addition the aerobic oxidation of amines alkyl-

substituted phenols and alkanes were also examined [58]

The reaction of Tl2CO3 with 111555-hexafluoro-24-pentanedione

and diglyme CH3O(CH2CH2O)2CH3 or tetraglyme CH3O(CH2CH2O)4CH3 in

dichloromethane yields the anhydrous thermally and air stable volatile Tl

diglyme and Tl tetraglyme adducts They have been characterized by single

crystal X-ray diffraction elemental analysis 1H and 13C NMR IR and mass

spectroscopy Thermal and mass-transport properties have been investigated

using thermo gravimetric and differential scanning calorimetric

measurements There is evidence that both precursors are very low melting

and volatile and can be used as liquid Tl sources Both adducts have been

successfully applied to metalndashorganic chemical vapor deposition of thallium

containing films [59]

Large size and high quality single crystals of quasi-two-dimensional

thallium molybdenum purple bronze TlMo6O17 have been grown by electrolytic

reduction of molten salt of Tl2CO3-MoO3 The crystal structure is trigonal with

space group P3m1 determined by X-ray diffraction and four-circle single crystal

diffraction The lattice parameters of the unit cell are a = b = 55282 Adeg and c

= 136991 Adeg The temperature dependence of resistivity and magnetic

19

susceptibility confirmed that a metal-to-metal transition occurs near 110 K

[60]

Hall coefficient and dc conductivity studies were made on p-type

Pb08Sn02Te thin films doped with different concentrations of thallium in the

temperature range 77 to 500 K The Hall coefficient and Hall mobility are

found to decrease with an increase in the doping concentration of thallium

Hall coefficient data have been analyzed in the light of a double valence-band

model Various band parameters such as valence band separation population

ratio mobility ratio and effective mass ratio have been calculated Hall

mobility data have been analyzed in the light of lattice and defect limited

scattering mechanisms [61]

Proton conducting composites of heteropolyacid hydrates phosphomol

ybdic acid H3PMo12O40 nH2O(PMA) phosphotungstic acid H3PW12O40 nH2O

( PTA) and salt hydrate like NiCl2 6H2O were prepared

with insulating Al2O3 as despersoidThe ionic conductivity peaks at two

concentrations of Al2O3 indicating two percolation thresholds for percolation

thresholds for proton conduction Two separate experiments were carried out

to check the existence of such percolation thresholds viz the volta battery

experiment involving the measurement of emf of an electrochemical cell

with composites of different compositions used as electrolyte and the

composition vs conductivity measured by the complex impedance

spectroscopy The presence of two maxima has been attributed to two

different percolation thresholds for the two possible mobile protonic

species H+ + (H3O+) and OH arising from the hydrates [62]

1 A5 Applications of Heteropolyoxometalates-

Applications of heteropolyanions centre depend on their redox properties

their high charges and ionic weights An enormous patent and journal

literature is devoted to the applications of heteropolyanions

1 Analysis-

The formation and subsequent precipitation or reduction of

[XMO12O40]n- anions form the basis of gravimetric and colorimetric analytical

methods for P As Si or Geeither separately or in combination [63 64]

20

2 Biochemical applications-

lsquoPhosphotungstic Acidsrsquo have for decades been used as precipitants

for proteins and as analytical reagents for proteinsalkaloidsand purines eg

the [P2M18O62]6- anions for colorimetric determination of uric acid [65] and

cholesterol The acid H3P12O40 either in aqueous or ethanolic solution is also

widely used as a non specific electron dense stain for electron spectroscopy

The dyestuffs industry has for many years used heteropolymolybdates and

tungstates to form color lakes and toners from basic dyes Large

heteropolyanions exhibit antiviral antitumoral properties at non-cytotoxic

doses in vitro and in vivo and are protein inhibitors of cellular bacterial and

viral DNA RNA polymerizes [66]

3 Catalysis

Heteropoly acids and salts have been used as heterogeneous catalysts

for a broad variety of reactions and compilations of such applications up to

1973 are available Examples include oxidation of propylene and isobutylene

to acrylic acid methacrylic acids and ammoxidation of acrylonitrile olefin

polymerization and epoxidation Much of current activity concerning

heterogeneous catalysis by heteropoly compounds is being carried out in

Japan [67 68]

4 Other Applications

Insoluble salts of Heteropolyanions especially ammonium

molybdophosphates have been used and are commercially available as ion-

exchange materials [69]Recent work in this area includes thin layer

chromatography of amino acids ion selective membranes [70] and the

preparation of new ion exchangers based on heteropolyanions Crystalline12-

tungstophosphoric and 12-molybdophosphoric acids are excellent protonic

conductors Heteropolyacids are electrochromic in the solid state as a

consequence of heteropolyblue formation Heteropolyblue formation has also

been used to detect alcohol or carboxylic acid radicals generated by radiolysis

of aqueous solutions Potential applications of heteropoly complexes as flame

retardants and smoke suppressants or as corrosion inhibitors and conversion

coatings on steel and aluminium are reported [71] Some potential ldquogreenrdquo

applications have been reported eg non-chlorine based wood pulp

bleaching process and a method of decontaminating water Some structures

21

containing transition metal atoms with unpaired electrons have unusual

magnetic properties and are being investigated as nano computer storage

devices Some compounds exhibit luminescence There are many reported

potential medicinal applications eg anti tumoral and anti-viral There have

been reports on the role of weak or non bonding interactions on the crystal

engineering of hybrid polyoxometalates

Spherical nonporous polyoxomolybdate based capsules of different

types containing more than 100 metal atoms reported by Achim Muller and his

group have versatile unique properties regarding their assembly to vesicles

and the chemistry which can be done inside the pores and cavities A discrete

polyoxometalate Lindquist ion of the form W6O192minus was successfully imaged

recently for the first time within the capillary of a carbon nanotube following

steric locking of the anion with the tubule In situ relaxation of the anion in its

equatorial plain was demonstrated [72]

22

Section-B

Litreture Survey on Chromism in Transition Metal

Oxides

1 B1 Chromism in Transition Metal Oxides

Chromism is a reversible change in a substances colour resulting from

a process caused by some form of stimulus Many materials are chromic

including inorganic and organic compounds and conducting polymers and the

property can result from many different mechanisms Several transition metal

oxides show EC properties The most popular are from the VI - B oxides In

this group WO3 and MoO3 are the most thoroughly studied cathodic EC

materials which can be electrochemically coloured and bleached when used

as the cathode in electrochemical cells Cathodic EC materials also include

V2O3 TiO2 and Nb2O5 Another distinguishable group is anodic EC material

including VIII oxides like IrOx nH2O Rh2O3 nH2O NiO nH2O etc which can be

anodicaly coloured in the electrochemical process when used as anode

There are several types of chromism which are discussed as below

B11 Photochromism

Photochromism is the reversible transformation of a chemical species

between two forms by the absorption of electromagnetic radiation where

the two forms have different absorption spectra [7374]

Trivially this can be described as a reversible change of color upon

exposure to light The phenomenon was discovered in the late 1880s

including work by Markwald who studied the reversible change of color of 23

44-tetrachloronaphthalen-1(4H)-one in the solid state He labeled this

phenomenon phototropy and this name was used until the 1950s

when Yehuda Hirshberg of the Weizmann Institute of Science in Israel

proposed the term photochromism [75] Photochromism can take place

in both organic and inorganic compounds and also has its place in biological

systems (for example retinal in the vision process)

Photochromism does not have a rigorous definition but is usually used

to describe compounds that undergo a reversible photochemical reaction

where an absorption band in the visible part of the electromagnetic spectrum

23

changes dramatically in strength or wavelength In many cases an

absorbance band is present in only one form The degree of change required

for a photochemical reaction to be dubbed photochromic is that which

appears dramatic by eye but in essence there is no dividing line between

photochromic reactions and other photochemistry Therefore while the

transcis isomerization of azobenzene is considered a photochromic reaction

the analogous reaction of stilbene is not Since photochromism is just a

special case of a photochemical reaction almost any photochemical reaction

type may be used to produce photochromism with appropriate molecular desi

gnSome of the most common processes involved in photochromism are peric

yclic reactions cis-trans somerizations intramolecular hydrogen transfer

intramolecular group transfers dissociation processes and electron transfers

(oxidation-reduction)

Another some what arbitrary requirement of photochromism is that

it requires the two states of the molecule to be thermally stable under

ambient conditions for a reasonable time All the same nitrospiropyran (which

back-isomerizes in the dark over ~10 minutes at room temperature) is

considered photochromic All photochromic molecules back-isomerize to their

more stable form at some rate and this back-isomerization is accelerated by

heating There is therefore a close relationship between photochromic and the

rmochromic compounds The timescale of thermal back-isomerization is

important for applications and may be molecularly engineered

Photochromic compounds considered to be thermally stable include some

diarylethenes which do not back isomerize even after heating at 800C for 3

months

Since photochromic chromophores are dyes and operate according to

well-known reactions their molecular engineering to fine-tune their properties

can be achieved relatively easily using known design models quantum

mechanics calculations and experimentation In particular the tuning of

absorbance bands to particular parts of the spectrum and the engineering

of thermal stability have received much attention

Sometimes and particularly in the dye industry the term irreversible

photochromic is used to describe materials that undergo a permanent color

change upon exposure to Ultraviolet or visible light radiation Because by

24

definition photochromics are reversible there is technically no such thing as a

n irreversible photochromic this is loose usage and these compounds

are better referred to as photochangable or photoreactive dyes

Apart from the qualities already mentioned several other properties of

photochromics are important for their use These include

Quantum yield of the photochemical reaction

This determined the efficiency of the photochromic change with respect

to the amount of light absorbed The quantum yield of isomerization

can be strongly dependent on conditions

Fatigue resistance In photochromic materials fatigue refers to the

loss of reversibility by processes such as photodegradation

photobleaching photooxidation and other side reactions All

photochromics suffer fatigue to some extent and its rate is strongly

dependent on the activating light and the conditions of the sample

Photostationary state Photochromic materials have two states and

their interconversion can be controlled using different wavelengths of

light Excitation with any given wavelength of light will result in a

mixture of the two states at a particular ratio called the photo-

stationary state In a perfect system there would exist wavelengths

that can be used to provide 10 and 01 ratios of the isomers

but in real systems this is not possible since the active

absorbance bands always overlap to some extent

Polarity and solubility In order to incorporate photochromics in

working systems they suffer the same issues as other dyes They are

often charged in one or more state leading to very high polarity and

possible large changes in polarity They also often contain large

conjugated systems that limit their solubility

Photochromic complexes

A photochromic complex is a kind of chemical compound that has

photoresponsive Parts on its ligand These complexes have a specific

structure photoswitchable organic compounds are attached to

metalcomplexes For the photocontrollable parts thermally and

photochemically stable chromophores (azobenzene diarylethene

25

spiropyran etc) are usually used And for the metal complexes a wide

variety of compounds that have various functions (redox response

luminescence magnetism etc) are applied The photochromic parts and

metal parts are so close that they can affect each others molecular

orbitals The physical properties of these compounds shown by parts of

them (ie chromophores or metals) thus can be controlled by switching

their other sites by external stimuli For example photoisomerization

behaviors of some complexes can be switched by oxidation and reduction of

their metal parts Some other compounds can be changed in their

luminescence behavior magnetic interaction of metal sites or stability of

metal-to-ligand coordination by photoisomerization of their photochromic

parts

Photochemistry of Polyoxometalates

The photochemistry of polyoxometalates is of great interest to inorganic

chemistsMore than 80 years agoit was found that the R-Keggin tungstate

H3[PW12O40] was reduced photochemically to yield a blue-colored species

which was reoxidized by air and by various other oxidizing agents such as

Fe3+AgNO3and H2O2 [7879]The photoredox reactions of H4[SiW12O40]and

H3[PW12O40] proceeded effectively in the presence of primary and secondary

alcohols their ethers and aldehydes and proteinsbut less effectively in the

presence of tertiary alcoholsketonesestersthe fatty acids above formic

acidand simple amines[8081] The basic photoredox reaction involving

ethanol is illustrated by eq 13

2 H3PW12O40 + H3CCH2OH h ν ν ν ν 2 H4PW12O40 + H3CCHO ------- 13

2 H4PW12O40 +12 O2 2 H3PW12O40 + H2O ------ 14

In this reactionone molecule of ethanol photochemically reduces two

molecules of H3PW12O40 and is itself oxidized to acetaldehyde In the

presence of air the thermal oxidation of the reduced species takes place at

room temperature(eq 14)The reduced polyoxometalates which are the so-

calledldquoheteropolybluesrdquo have been used for the colorimetric analysis of the

26

elements P Si As and Ge and for the determination of uric acidsugarand

other biological compounds [8283] Piperidinium metavanadate also

undergoes photoinduced coloration from white to black followed by a

reversible color change in the presence of oxidizing agentsHowever

ammonium metavanadates ([NH4][VO3]) exhibits no photoinduced

coloration[84] The early photoredox reactions of the R-Keggin

polyoxometalates H4[SiW12O40] and H3PW12O40 were carried out in the

presence of photographic paper however the limited number of the

structurally well-characterized compounds available for study delayed the

development of modern cluster-compound photochemistry until the discovery

of photochromism in alkylammonium polyoxo- molybdate solids[8586] A

photochromic or electrochromic material is one whose light-absorbing

properties are altered upon optical excitation or reduction under the influence

of an externally applied electric field respectively The induced coloration

remains even after the excitation source has been removed These materials

are of technological interest because they return to their original state either

thermally upon irradiation with light of a frequency corresponding to the

induced absorption or electrochemically upon reversing the polarity of the

externally applied electric field Thus photochromic and electrochromic

materials behave in a reversible manner Polyoxometalates exhibit significant

photo-and electrochromism which makes them suitable as nanocomposite

molecular devices and as models for probing the physical properties of infinite

metal oxides Since the metal ions in the oxidized polyoxometalates have d0

electronic configurations the only absorption band which occurs in the UV-vis

range of the electronic spectra is due to the oxygen-to-metal (O-M) ligand-to-

metal charge transfer (LMCT)Upon irradiation electrons are promoted from

the low-energy electronic states which are mainly comprised of oxygen 2p

orbitals (the valence band in the band model)to the high-energy electronic

states which are mainly comprised of metal d orbitalrsquos (the conduction band

in the band model)The fundamental transitions in polyoxometalate lattices

are depicted schematically in Fig1B1

27

Fig1B1 Simple model showing the electronic transitions in the

polyoxometalates containing electron donar and acceptor (a)generation

of charge carriers(b)electron and hole trapping(c)electron release due

to stimulation(d)recombination

between electron and hole Electrons are e-and holes are h+

In the polyoxometalates containing heteroatoms and especially in

mixed metal polyoxometalates the charge carriers which are created by the

light or electric field may be trapped in electron traps and hole traps These

traps provide states of localized energy in the O-M LMCT energy gap due to

the heteroatoms or counter cations which correspond to impurities or lattice

defects in the band model If the trap depth ∆E is large compared to kT the

probability for thermal escape from the trap will be negligibly small and

metastable situation will existThe trapped carriers can be released by thermal

or optical stimulationIn the case of thermal stimulation the irradiated

polyoxometalate is heated until the energy barrier ∆E can be overcome The

trapped electron (or hole) then can escape from the trap and nonradiatively

recombine with the trapped hole (or electron)Under optical stimulation the

energy of an incident photon is used to overcome ∆E The relaxation

processes of the OndashM LMCT excitation energy include both the nonradiative

recombination of electrons and holes within the energy gap and the

intramolecular energy transfer leading to a charge-transfer emission This

intramolecular energy transfer corresponds to the O-M LMCT energy gap and

occurs via radiative recombination and sensitized emission from the

28

heteroatoms or cations If several energy levels based on the hetero atoms or

counter cations act as energy acceptors within the O-M LMCTenergy gap the

energy transfer occurs from the O-M LMCT states to these levels followed by

the nonradiative or radiative deactivation of the excitation energy It should be

noted that the O-M LMCT states also can be generated by the application of

very high electric fields to the polyoxometalate solids as demonstrated by the

observation of electroluminescence[87]If an external electric field with a

potential more negative than the energy levels of the vacant orbitals involved

in the O-M LMCT transition is applied to a polyoxometalate on the electrode

surface an electrochemical reduction occurs via the injection of electrons

from the electrode in to the vacant levels of the polyoxometalate as shown in

Fig1B 2

Fig1B2 - Energy scheme for the electrochromism of polyoxometalates

a)electrochemical reduction (b) electrochemical oxidation

Electrons injected in to the high-energy levels also may be trapped by

electron traps in a process analogous to that which occurs during LMCT

photoexcitation of the polyoxometalates These electrons are returned to the

electrode by electrochemical oxidation at an electrode potential more positive

than the energy levels for the d1 electron states The d1electrons in the O-M

LMCTstates facilitate the absorption of visible light via intervalence charge

transfer among metal centers and d-d transitions The same type of transition

may be possible for the d1electron captured by the electron traps too In

addition to searching for new photosensitive polyoxometalates with the

29

potential for having practical application there is now a need to elucidate the

fundamental photo-and electrochemical coloration processes by studying

electron transfer within the polyoxometalate lattices in conjunction with their

crystal structures So far few polyoxometalates exhibit a perfect reversibility

of coloration The irreversibility of the color change arises from as yet

uncharacterized side reactions during both the coloration and decoloration of

the polyoxometalates

Many metal oxides including aluminum titanium vanadium niobium

molybdenum and tungsten oxides are photochromic when they contain

impurities or dopants This coloration has been interpreted on the basis of

electron trapping at appropriate lattice sites within the crystals as shown in

Fig1B1 where the O-M LMCT transition corresponds to the transition

between the valence and conduction bands for the infinite metal-oxide lattice

[88-91]

B12 Applications of Photochromic materials

Sunglasses

One of the most famous reversible photochromic applications is color

changing lenses for sunglasses as found in eyeglasses The largest limitation

in using PC technology is that the materials cannot be made stable enough

to withstand thousands of hours of outdoor exposure so long-term outdoor

applications are not appropriate at this time The switching speed of

photochromic dyes is highly sensitive to the rigidity of the environment around

the dye As result they switch most rapidly in solution and slowest in the rigid

environment like a polymer lens Recently it has been reported that attaching

flexible low Tg polymers (for example siloxanes or poly (butyl acrylate) to the

dyes allows them to switch much more rapidly in a rigid lens [76] Some

spirooxazines with siloxane polymers attached switch at near solution like

speeds even though they are in a rigid lens matrix

Supramolecular chemistry

Photochromic units have been employed extensively in supramolecular

chemistry Their ability to give a light controlled reversible shape change

means that they can be used to make or break molecular recognition motifs

30

or to cause a consequent shape change in their surroundings Thus

photochromic units have been demonstrated as components of molecular

switches The coupling of photochromic units to enzymes or enzyme cofactors

even provides the ability to reversibly turn enzymes on and off

by altering their shape or orientation in such a way that their functions

are either working or broken

Data storage

The possibility of using photochromic compounds for data storage was

first suggested in 1956 by Yehuda Hirshberg[77] Since that time there have

been many investigations by various academic and commercial groups

particularly in the area of 3D optical data storage which promises discs that

can hold a terabyte of data Initially issues with thermal back-reactions

and destructive reading dogged these studies but more recently more stable

systems have been developed

Novelty items

Reversible photochromics are also found in applications such as toys

cosmeticsclothing and industrial applications If necessary they can be made

to change between desired colors by combination with a permanent pigment

A large number of inorganic compounds exhibit photochromism

These solids often have large band gaps of the order of 3 - 12 eV and

excitation of these solids leads to the formation of metastable centers that

absorb visible light giving rise to their colour They can return to their ground

state by heating or by optical excitation within the colour-centre band In most

cases the photochromism is a structure sensitive phenomenon involving

localized defect impurities or dislocations Some of these inorganic

compounds have the potential for a number of different uses Photochromic

compounds have a number of useful applications These can be divided

according to the most important property that is being used (Table 11) [92]

31

Table 11 Applications of Photochromic materials

Applications Depending Upon

Sensitivity to

Radiation

Reversibility Thermal Chemical or

Physical Properties

Self-developing

photography

Chemical switches for

computers

Temperature indicators

Protective

materials

Data displays

Heat-sensitive recording

media

Optical signal

processing

Photomasking and

photoresist technology

Reusable data storage

media

Anaytical reagents

Photochromic

microimages

Photopolymerisation

Information encoding

and steganography

Photocontractile

polymers and the

photoviscosity effect

Control of light

intensity

Q-switches

Pyroelectric

photochromic materials

B13 Thermochromism

Thermochromism is the reversible colour change of a substance

induced by temperature change A large variety of substances organic

inorganic organometallic supramolecular and polymeric systems exhibit this

phenomenon Examples of these include bianthrones cobalt

hexacyanoferrate the zirconocene complex of 1 4-diphenyl-1 3-butadiene

and poly (3-alkylthiophene) The organic 99-bixanthenylidene is colourless at

90 K yellow-green at 298 K and dark-blue when melted at 592 K Heating

conducting polymers can cause them to change colour This is achieved by

causing conformational changes to the polymer backbone resulting in a

change in the band gap of the polymer It has been reported that regioregular

32

P3HT reversibly changes colour upon heating to 220ordmC due to temperature-

dependent conformation changes Thermally cross linked polymer undergoes

the same colour change but it is much less reversible [93] Other forms of

thermochromism may be commercially important eg to give a visual

indication of temperature changes

B14 Electrochromism

Electrochromism describes a phenomenon of material color change in

a persistent but reversible manner produced by electrochemically induced

oxidation-reduction reactions Electrochromic materials can be applied to

various kinds of products such as smart windows and display devices Among

those applications there have been lots of efforts to develop electrochromic

display devices (ECDs) Especially flexible display devices are now attracting

much attention worldwide since they can facilitate new technological demands

such as bending and folding of paper-like displays High electrochromic

efficiency short response time long operating life time and reduction of

energy consumption are the most important requisites of the materials for the

paperlike displays [94 95] Among those properties the operation life time is

the most important barrier to overcome for a realization of ECDs There are

two types of electrochromic material a) inorganic transition metal oxides

(TMOs) b) organic polymer materials The TMOs have been studied longer

than the organic materials that they have been studied since 1960s [9697]

Electrochromism describes a reversible color change of material

produced by electrochemically induced oxidation-reduction reactions It is one

of several types of chromism of materials As thermochromism and

photochromism mean material color changes made by heat and light

respectively electrochromism refers that the color change is caused by an

electric potential In most cases the color change in electrochromism can be

driven by rather low electrical potential of the order of a fraction of volt or a

few volts [94-96] The color change of material means variation in

transmittance andor reflectance change in visible range which is originated

from different electronic absorption bands according to a switching between

oxidation and reduction state of material When electric potential is applied on

electrochromic material forced oxidation or reduction is derived and the

33

individual color is originated from the corresponding oxidation or reduction

state of the material For electrochromic materials the characteristic color

change is reversible since the oxidation and reduction state can be converted

reversibly by switching potential

Application fields

The application area for the electrochromism is rather broad that it

covers from smart window glazing and optical modulators to information

displays [98-102] The smart windows are typical examples The

electrochromic property is used to control the amount of light and heat to pass

through the windows Usually the electrochromic material is in form of thin film

coated on a window glass The transmittance modulation has also been

applied at the automobiles to automatically tint rear-view mirrors in various

lighting conditions The electrochromic application fields are illustrated in Fig

1B3

Since the smart windows control the transmittance of heat as well as

the transmittance of visible light the working definition of electrochromism has

now been extended to include devices for modulation of radiation in the near

infrared thermal infrared and microwave regions When color for

electrochromic materials is used this can now mean a response by detectors

at these wavelengths and not just by the human eyes Nowadays

electrochromic material draws much attention as being used in the display

devices Electrochromic display device (ECD) is being considered as one of

the candidates for the conventional liquid crystal display (LCD) since ECD

has many advantages over LCDs Among these advantages the most

important are low energy consumption wider viewing angle high contrast

rate and possibility to achieve multiple colors with a single material [103]

When a new redox state of electrochromic materialis established by

the applied electric pulse then it is maintained after the potential is switched

off This means the colored or bleached state of the material can be sustained

for a considerable time without applying electrical power This is so-called

ldquocolor memory effectrdquo of electrochromic material Because of the color

memory effect energy consumption for the electrochromic display device

could be drastically reduced and this would be a big advantage over other

34

emissive devices The low power consumption is especially advantageous

when it is applied to mobile devices with limited power source The possibility

for a flexible display is another attraction for electrochromic material

Information displays

Real-view mirrors for automobiles

Fig 1B4 Application fields of electrochromic devices Smart windows information displays and real-view mirrors for automobiles

35

Using polymeric electrochromic material and plastic substrate with a

transparent conductive coating it is possible to build all-plastic flexible display

device There are lots of papers and patents about the flexible display devices

[104105] In these cases gel-type electrolyte is also needed The gel-type

electrolyte immobilizes liquid electrolyte in the polymer network [106107]

Recently ITO-coated polymer films are readily available which would provide

the plastic substrate for a flexible device The flexible electrochromic displays

are frequently tried with the plastic substrate flexible electrochromic material

and gel-type electrolyte The flexible electrochromic displays would facilitate

increasing technical demands for foldable display devices

Metal Ion Electrochromism

Many transition metal oxides are capable of redox reactions that result in

colour change Metal oxide films are commonly prepared as thin layers of

either tungsten nickel molybdenum or other metal compounds by a number

of techniques These include sol-gel electrochemical by dc or rfreactive

sputtering techniques electron-beam evaporation by anodic or cathodic

electrodeposition or by solution dipping of the electrochromic metal

compounds (or compounds that can be changed into these metal compounds)

onto optically transparent electrodes (OTE) [108 -114] Their electrochromism

is derived from the colour change associated with a change in the oxidation

state of the metal anion The behaviour of these materials is dependent upon

pH moisture and exposure to the atmosphere [115] Generally the switching

rates of these films is somewhat slow with typical switching times of about 15

- 60 seconds to achieve 100 conversion to either coloured or bleached state

[116 -120] An Example of this includes nickel oxide which changes from

transparent (pale green) to brownblack taking about 30 seconds to do so

[121] Other examples include [(NH4)5Ru]2(pyrazine)5+ and [(NH4)5Ru]2(44-

bipyridine)5+ whose electrochromism is significantly different due to the effect

of the ligand [122] Table 12 below gives some examples of metal oxide films

with electrochromic properties

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

10

with tungstate ions The heteropolyacid that is formed has the Keggin

structure

[PO4]3- + 12 [WO4]

2- + 27 H+ rarr H3PW12O40 + 12 H2O ----------- 11

α-Keggin anions have been used as catalyst in hydration polymerization and

oxidation reaction as catalysts

The metal atoms that make up the framework (termed addenda atoms)

are typically Mo W and V When more than one element is present the

cluster is called a mixed addendaclusterThe ligands coordinated to metal

atoms that together form the bridged framework are usually oxide ions but

other elements such as S and Br have been substituted for some of the oxide

ions (Note that sulfur substituted POM is often termed a

polyoxothiometalates) Another development is the use of other ligands eg

nitrosy and alkoxy to replace oxide ions The typical framework building

blocks are polyhedral units with 4 5 6 or 7 coordinate metal centers These

units usually share edges andor vertices The most common unit for

polymolybdates is the octahedral MoO6 unit which is a distorted octahedron

where the Mo atom moves off centre to give one short Mo-O bond In some

polymolybdates there are pentagonal bipyramidal units and these are key

building blocks in the molybdenum bluesHetero atoms are present in many

polyoxometalates Many different elements can act as hetero-atoms

Examples of various coordination numbers around the hetero-atom are

known

4 co-ordinate (tetrahedral) in Keggin Dawson and Lindquist structures

(eg PO4 SiO4 AsO4)

6 co-ordinate (octahedral) in Anderson structure (eg Al(OH)6 TeO6

8 co-ordinate (square antiprism) in ((CeO8)W10O28)8minus

12 co-ordinate (icosahedral) in (UO12)Mo12O30 8minus

11

Often the hetero-atom is centrally located in the anion (eg Keggin

structure) or in a structure fragment eg the 2 phosphorus atoms in the

Dawson ion are central to the two symmetric fragments There are similarities

to clathrate structures The Keggin ion can be formulated as PO4 2minusand

M12O36 and the Dawson as (XO42-)2 and M18O54Structural isomerism is

common For example the Keggin structure has 5 isomers which can be

considered to contain one or more of the four M3O13 units being rotated

through 60degMany compounds share the same framework architectures or

frameworks derived from a larger framework with one or more addenda atoms

and oxide ions removed to give defect structure usually called a lacunary

structure An example of a compound with a Dawson lacunary structure is

As2W15O56Some cage structures containing ions are known eg an example

is the vanadate cage V18O42 containing a Clminus ion [38] This has 5 co-

ordinates square pyramidal vanadium units linked together

Fig 1A 6 - H4V18O42 cage containing Cl

1A3 General Properties of polyoxometalates -

Typically polyoxoanions are water and air stable species of large size

(6-25 Adeg) and high ionic weight In aqueous solution they are subject to

decomposition by hydroxide ions eg

[PW12O40]3- + 23 OH- HPO4

2- + 12WO42- + 11 H2O -------- 12

12

Although the PH at which such reactions are rapid can very widely

depending upon the polyanion involved Polyanions are often much stable

towards the H3O+ ions and numerous crystalline heteropoly acids are known

Such acids may be extremely soluble in water and polar solvents (giving

solutions with densities in excess of 4 gcm3) and have large dissociation

constants (PK lt 0)

Crystalline heteropoly acids and salts are frequently highly hydrated with

up to 50 molecules of water per anion Much of this water is zeolite in nature

and crystal composition can vary accordingly On the other hand the cation

anion stiochiometry is always well defined as the anion structure and

composition Finally many polyanions are powerful oxidizing agents and

undergo multiple reversible one or two electron reductions leading to intensely

colored mixed valence species known as heteropoly blues Polyanions are

known which can accept as many as 32 electrons without major structural

change

1A4 Chemistry of Molybdenum and Tungusten Heteropolyoxometalates

A photochromic monolayer film of phosphomolybdic acid (denoted as

PMo12) was fabricated by self-assembly approach UV-visible spectrum and

AFM observation show that the monolayer film is composed of aggregated

PMo12 molecules The monolayer film shows good photochromic properties

with enough stability and reversibility The colour change of the monolayer

after UV-irradiation can be captured by a microscope equipped with a color

CCD camera Photochromic response of the monolayer film can be doubled

after being modified by an amine monolayer [39]

Series of vanadium substituted molybdo Keggin HPA with 12 or 3

adjacent vanadium atoms were prepared These materials were supported

on carbon cloth electrodes and hot pressed onto Nafion with an ETEK

electrode used as a standard on the opposite side The MEArsquos were run at

temperatureslt100 oC with the HPA electrode as either the anode or the

cathode Stable polarisation curves are obtained for an HPA based cathode

with reasonable current densities at 80 oC [40]

A novel mixed-valence polyoxomolybdenum anion was synthesized

hydrothermally from molybdenum oxidemolybdenum metalboric and

phosphoric acids12-phenyldiphosphonicacidand imidazole (ImH) and was

13

structurally characterized as an imidazolium saltOne-and two-dimensional

structures of this anion and additional molybdenum diphosphonate linkers

were assembled as wellThey were structurally characterized as their

pyridinium(pyH) salts [41]

Phosphomolybdic acidpolyvinylpyrrolidone hybrid films were found to

show visible light photochromism It is identified that the intra-supramolecular

charge transfer between the inorganic and organic molecules is responsible

for the visible-light coloration Interestingly the films show photo-memory and

thermal activation The films show a small change in absorbance after being

irradiated with visible light for a short time and the coloration can be

enhanced greatly by subsequent thermal treatment Electrical measurements

indicate that the conductivity of the film increases after the brief irradiation

which promotes transfer of the electrons induced by the thermal treatment

[42]

In this work major effort was concentrated on passive thermal control

coatings based on photochromic and thermochromic materials The inorganic

photochromic materials were based on tungsten and molybdenum oxide films

and the organic photochromic materials included spiropyrans and

spirooxazines In addition photochromic composite organic-inorganic films

and thermochromic vanadium oxide films were prepared The samples were

synthesized using sputtering sol-gel process and thermal oxidation [43]

Polyoxometalates a class of oxidatively robust inorganic oxidants and

oxidation catalysts are currently under investigation at the Forest Products

Laboratory and at Emory University as an alternative to chlorinebased

chemicals in the bleaching of soft Woodkraft and other pulps Although

polyoxometalate salts are used in a number of industrial processes the

feasibility of using these salts and oxygen in the commercial bleaching of

chemical pulps was only recently demonstratedA clear advantage of

polyoxometalates over oxygen alone hydrogen peroxide or ozone is their

inherently high selectivity for the residual lignin in softwood kraft pulps The

goal of ongoing research is to develop a highly selective energy efficient

oxygen based polyoxometalate delignification and bleaching technology

compatible with mill closure [44]

14

Tungsten oxide exhibits pronounced photochromism upon bandgap

photoexcitation which makes it attractive and promising for applications in

many areas Some advances have been achieved during the past decades

The research on nanocrystalline films and single crystals indicates the critical

importance of defects in tungsten oxide to its photochromism Based on

energy-band engineering of semiconductors enhancement of photochromism

has been achieved for instance extension of the photoresponse from UV to

visible light by cathodic polarization improved change in absorption before

and after coloration through modification by a noble metal or another metal-

oxide semiconductor and increased photochromic reversibility via

hybridization with organic amines Nanocrystalline oxide films exhibit

controllable wettability which is coherent in nature with photochromism [45]

Polyoxometalates represent a diverse range of molecular clusters with

an almost unmatched range of physical properties and the ability to form

structures that can bridge several length scalesThe new building block

principles that have been discovered are beginning to allow the design of

complex clusters with desired properties and structures and several structural

types and novel physical properties are examinedIn this critical review the

synthetic and design approaches to the many polyoxometalate cluster types

are presented encompassing all the sub-types of polyoxometalates including

isopolyoxometa- lates heteropolyoxometalates and reduced molybdenum

blue systems As well as the fundamental structure and bonding aspectsthe

final section is devoted to discussing these clusters in the context of

contemporary and emerging interdisciplinary interests from areas as diverse

as antiviral agentsbiological ion transport modelsand materialsscience [46]

Keggin type molybdovanadophosphoric heteropoly acids were prepare

d by a novel environmentally benign method and their catalytic performances

were evaluated via hydroxylation of benzene to phenol with hydrogen

peroxide as oxidant in a mixed solvent of glacial acetic acid and acetonitrile

Various reaction parameters such as reaction time reaction temperature

ratio of benzene to hydrogen peroxide concentration of aqueous hydrogen

peroxide ratio of glacial acetic acid to acetonitrile in solvent and catalyst

concentration were changed to obtain an optimal reaction conditions

15

Molybdovanadophosphoric heteropoly acids are revealed to be highly

efficient catalyst for hydroxylation of benzene [47]

Thin films from the system (As2S3)Tl were deposited by thermal

evaporation on Si graphite and optical glass substratesFrom transmission

and reflection measurements of the thin films the refractive index (n) film

thickness(d) optical band gap(Eg) optical oscillator energy(Eo) and

dispersion energy(Ed) before and after exposure to light were determined

The results for optical parameters were analyzed using the Wemple - Di

Domenico single oscillator model and Lorenc-Lorenc equation It was found

that Eg decreases while n E0 and Ed increase for as deposited films

decreases while n E0 and Ed increase for as deposited films with increasing

of Tlconcentration passing through a maximum at 6 at of Tl After exposure

to light n E0 Ed increase and Eg decreases for all compositions

investigated The maximum change in n (Dn = 016 at l = 6328 nm) was

observed for thin As38S56Tl6 films From infrared spectroscopy measurements

of bulk glasses and thin films we could conclude that when up to 6 at of

thallium is introduced As-S-As chains break and a ternary TlAsS2

compound appears at 10 at Tl [48]

The organo - inorganic hybrid materialconsisting of Poly (34 Ethylene

Dioxythiophene) (PEDOT) doped with phosphomolybdate cluster anions

[PMo12O40]3-has been synthesized by direct insitu oxidative polymerization of

34-Ethylene Dioxythiophene (EDOT) with phosphomolybdic acid

(H3PMo12O40) Its characterization is investigated by Fourier Transform

Infrared Spectroscopy (FT-IR) and Scanning Electron Microscopy (SEM) The

hybrid material presents predominantly high electronic conductivities of

around 20 and 70 S cm1at 300 and 400 K respectively [49]

Heteropolyacids (HPAs) are known to be excellent re-dox catalysts In

combination with TiO2 HPAs can be used as photocatalysts active in visible

light The HPA accepts electron and get reduced to heteropolyblue (HPB)

That can absorb light in the visible range HPA can be incorporated onto the

external surface or in the pores of zeolite based composite photocatalysts

have been designed by incorporation of HPA semiconductor TiO2 and

transition metal cobalt on zeolite This composite metallozeolite photocatalyst

is efficient in photoreduction of methyl orange (MO) in visible light to the tune

16

of 411 mg of MO photo reducedg TiO2 This catalyst also shows encouraging

results for hydrogen evolution from water to the tune of 2730 micromolhg TiO

[50]

A new class of proton conducting glass membranes based on heteropo

lyacids such as phosphotungstic acid (PWA) as electrolytes for low

temperature H2O2 fuel cells was investigated Parameters for a single fuel cell

with a catalyst electrode of 015 mgcm2 of PtC and a glass composite

membrane were characterized by electrochemical measurements at open

circuit potential conditions The performance of the membrane electrode

assemblies (MEA) was systematically studied as an effect of SiO2 and P2O5

concentrations in the glass composite membrane and the MEA was found to

exhibit a maximum power density of 162 mW cm2 for an H2O2 fuel cell at

30 degC and 30 relative humidity (RH) [51]

Two new photochromic inorganic-organic hybrid materials formed from

Keggin type Polyoxometalates (POMs) and metronidazole (C6H9N3O3 MNZ)

formulated as H3PMo12O40bull3 MNZ3H2O (1) and H3PW12O40bull3MNZ3H2O(2)

were synthesized and characterized by elemental analysis IR spectra

electronic spectra electron spin resonance (ESR) spectra and TG-DTA

Reflectance spectra show the presence of weak inter molecular charge

transfer between the organic and inorganic moieties in the solid state The

photochromic properties were studied by solid diffuse reflectance spectra and

ESR spectra and the photochromic reactions were found to exhibit first-order

kinetics TG-DTA showed that two hybrid materials have similar thermal

behavior [52]

Heteropolycompounds (HPCs) have been a matter of interest in basic

and applied science for more than a century From their first synthesis many

advances have been made to promote the use of HPCs in different ways in

science and technology The aim of this article is to review the main structural

characteristics of heteropolycompounds of the Keggin type (12

tungstophosphoric12-molybdophosphoric12-tungstosilicic acid alkaline and

alkaline earth salts of12 tungstophosphoric acid and gels doped with HPCs)to

understand and explain their different activities such as high proton

conductivity and catalytic biochemical and biomedical activities [53]

17

A solid hybrid molecular material containing 1-butyl 3-methyl

imidazolium cations and Keggin anions of phosphotungstic acid has been

synthesized It is fully characterized by CHN analysis FTIR XRD UV-Vis-NIR

DRS 31P MAS NMR TGA and SEM The FTIR spectrum of the compound

shows the fingerprint vibrational bands of both Keggin molecular anions and

imidazolium cations The aromatic CndashH stretch region (2700ndash3250 cmndash1) of

imidazolium cation is split due to the interaction between the ring CndashH and

bulky Keggin anion The red-shift in the UV-Vis spectra and the downfield 31P

MAS NMR chemical shift also confirm the electrostatic interaction between

the ions in the compound Near IR spectral region (1000ndash2500 nm) shows the

elimination of water in the compound which is hydrophobic [54]

Transport coefficient measurements (electrical conductivity

thermoelectric power and Hall coefficient) have been performed on a

compact Tl033MoO3 polycrystalline compound in a wide temperature range

(200ndash400 K) Experimental results are interpreted with the help of a p-type

semiconductor model with two inverted deep levels near the midgap The

valence band and the conduction band are assumed to be formed from the

dxy orbitals of molybdenum atoms in the Mo6O22 cluster leading to narrow π-

bonding bands The donor and acceptor levels may be formed from

nonbonding dxy orbitals arising respectively from anionic and Tl+ defects

Electron paramagnetic resonance and magnetic measurements are in good

agreement with the theoretical band semiconductor model which has been

retained [55]

The optical properties of GaInTlAs epilayers grown at low temperature

~230degC by solid-source molecular-beam epitaxy on InP substrates were

characterized using optical absorptionand photoluminescence techniques

Optical absorption measurements a room temperature show a gap shrinkage

toward lower energies from 071 to 061 and 053 eV when the Tl content

increases from 0 to 4and 8in good agreement with theoretical

predictionsLow-temperature photoluminescence band-gap signals from

GaInAs and GaInTlAs layers are only obtained after rapid thermal annealing

performed inorder to improve the electronic quality of the layersA band gap

decrease as much as 41 meV for GaInTlAs with 19 Tl incorporation is

measured by photoluminescence at 8K [56]

18

The electrochromic performance of all solid ndash state cells employing

phosphotungstic acid and phosphomolybdic acid is reported These cells

employ SnO2 as the viewing electrode and graphite as the back electrodeThe

cells in the bleached state can be made white to red and become black in the

coloured state [57]

Various organic compounds were oxidized by molecular oxygen in the

presence of a catalytic amount of mixed addenda heteropolyoxometalates

containing molybdenum and vanadium The catalytic activity of the

Molybdovanadophosphate was found to be greatly enhanced by supporting

on charcoal The supported catalyst has high catalytic activity for oxidative

dehydrogenation of benzylic and allylic alcohols to the corresponding

aldehydes and ketones (46-92) nevertheless the nonsupported catalyst

was inactive for the same oxidations under these conditions 236Trimethyl

phenol was selectively oxidized to trimethyl-p benzoquinone which is

precursor of VitE in the presence of a catalytic amount of

molybdophosphate In addition the aerobic oxidation of amines alkyl-

substituted phenols and alkanes were also examined [58]

The reaction of Tl2CO3 with 111555-hexafluoro-24-pentanedione

and diglyme CH3O(CH2CH2O)2CH3 or tetraglyme CH3O(CH2CH2O)4CH3 in

dichloromethane yields the anhydrous thermally and air stable volatile Tl

diglyme and Tl tetraglyme adducts They have been characterized by single

crystal X-ray diffraction elemental analysis 1H and 13C NMR IR and mass

spectroscopy Thermal and mass-transport properties have been investigated

using thermo gravimetric and differential scanning calorimetric

measurements There is evidence that both precursors are very low melting

and volatile and can be used as liquid Tl sources Both adducts have been

successfully applied to metalndashorganic chemical vapor deposition of thallium

containing films [59]

Large size and high quality single crystals of quasi-two-dimensional

thallium molybdenum purple bronze TlMo6O17 have been grown by electrolytic

reduction of molten salt of Tl2CO3-MoO3 The crystal structure is trigonal with

space group P3m1 determined by X-ray diffraction and four-circle single crystal

diffraction The lattice parameters of the unit cell are a = b = 55282 Adeg and c

= 136991 Adeg The temperature dependence of resistivity and magnetic

19

susceptibility confirmed that a metal-to-metal transition occurs near 110 K

[60]

Hall coefficient and dc conductivity studies were made on p-type

Pb08Sn02Te thin films doped with different concentrations of thallium in the

temperature range 77 to 500 K The Hall coefficient and Hall mobility are

found to decrease with an increase in the doping concentration of thallium

Hall coefficient data have been analyzed in the light of a double valence-band

model Various band parameters such as valence band separation population

ratio mobility ratio and effective mass ratio have been calculated Hall

mobility data have been analyzed in the light of lattice and defect limited

scattering mechanisms [61]

Proton conducting composites of heteropolyacid hydrates phosphomol

ybdic acid H3PMo12O40 nH2O(PMA) phosphotungstic acid H3PW12O40 nH2O

( PTA) and salt hydrate like NiCl2 6H2O were prepared

with insulating Al2O3 as despersoidThe ionic conductivity peaks at two

concentrations of Al2O3 indicating two percolation thresholds for percolation

thresholds for proton conduction Two separate experiments were carried out

to check the existence of such percolation thresholds viz the volta battery

experiment involving the measurement of emf of an electrochemical cell

with composites of different compositions used as electrolyte and the

composition vs conductivity measured by the complex impedance

spectroscopy The presence of two maxima has been attributed to two

different percolation thresholds for the two possible mobile protonic

species H+ + (H3O+) and OH arising from the hydrates [62]

1 A5 Applications of Heteropolyoxometalates-

Applications of heteropolyanions centre depend on their redox properties

their high charges and ionic weights An enormous patent and journal

literature is devoted to the applications of heteropolyanions

1 Analysis-

The formation and subsequent precipitation or reduction of

[XMO12O40]n- anions form the basis of gravimetric and colorimetric analytical

methods for P As Si or Geeither separately or in combination [63 64]

20

2 Biochemical applications-

lsquoPhosphotungstic Acidsrsquo have for decades been used as precipitants

for proteins and as analytical reagents for proteinsalkaloidsand purines eg

the [P2M18O62]6- anions for colorimetric determination of uric acid [65] and

cholesterol The acid H3P12O40 either in aqueous or ethanolic solution is also

widely used as a non specific electron dense stain for electron spectroscopy

The dyestuffs industry has for many years used heteropolymolybdates and

tungstates to form color lakes and toners from basic dyes Large

heteropolyanions exhibit antiviral antitumoral properties at non-cytotoxic

doses in vitro and in vivo and are protein inhibitors of cellular bacterial and

viral DNA RNA polymerizes [66]

3 Catalysis

Heteropoly acids and salts have been used as heterogeneous catalysts

for a broad variety of reactions and compilations of such applications up to

1973 are available Examples include oxidation of propylene and isobutylene

to acrylic acid methacrylic acids and ammoxidation of acrylonitrile olefin

polymerization and epoxidation Much of current activity concerning

heterogeneous catalysis by heteropoly compounds is being carried out in

Japan [67 68]

4 Other Applications

Insoluble salts of Heteropolyanions especially ammonium

molybdophosphates have been used and are commercially available as ion-

exchange materials [69]Recent work in this area includes thin layer

chromatography of amino acids ion selective membranes [70] and the

preparation of new ion exchangers based on heteropolyanions Crystalline12-

tungstophosphoric and 12-molybdophosphoric acids are excellent protonic

conductors Heteropolyacids are electrochromic in the solid state as a

consequence of heteropolyblue formation Heteropolyblue formation has also

been used to detect alcohol or carboxylic acid radicals generated by radiolysis

of aqueous solutions Potential applications of heteropoly complexes as flame

retardants and smoke suppressants or as corrosion inhibitors and conversion

coatings on steel and aluminium are reported [71] Some potential ldquogreenrdquo

applications have been reported eg non-chlorine based wood pulp

bleaching process and a method of decontaminating water Some structures

21

containing transition metal atoms with unpaired electrons have unusual

magnetic properties and are being investigated as nano computer storage

devices Some compounds exhibit luminescence There are many reported

potential medicinal applications eg anti tumoral and anti-viral There have

been reports on the role of weak or non bonding interactions on the crystal

engineering of hybrid polyoxometalates

Spherical nonporous polyoxomolybdate based capsules of different

types containing more than 100 metal atoms reported by Achim Muller and his

group have versatile unique properties regarding their assembly to vesicles

and the chemistry which can be done inside the pores and cavities A discrete

polyoxometalate Lindquist ion of the form W6O192minus was successfully imaged

recently for the first time within the capillary of a carbon nanotube following

steric locking of the anion with the tubule In situ relaxation of the anion in its

equatorial plain was demonstrated [72]

22

Section-B

Litreture Survey on Chromism in Transition Metal

Oxides

1 B1 Chromism in Transition Metal Oxides

Chromism is a reversible change in a substances colour resulting from

a process caused by some form of stimulus Many materials are chromic

including inorganic and organic compounds and conducting polymers and the

property can result from many different mechanisms Several transition metal

oxides show EC properties The most popular are from the VI - B oxides In

this group WO3 and MoO3 are the most thoroughly studied cathodic EC

materials which can be electrochemically coloured and bleached when used

as the cathode in electrochemical cells Cathodic EC materials also include

V2O3 TiO2 and Nb2O5 Another distinguishable group is anodic EC material

including VIII oxides like IrOx nH2O Rh2O3 nH2O NiO nH2O etc which can be

anodicaly coloured in the electrochemical process when used as anode

There are several types of chromism which are discussed as below

B11 Photochromism

Photochromism is the reversible transformation of a chemical species

between two forms by the absorption of electromagnetic radiation where

the two forms have different absorption spectra [7374]

Trivially this can be described as a reversible change of color upon

exposure to light The phenomenon was discovered in the late 1880s

including work by Markwald who studied the reversible change of color of 23

44-tetrachloronaphthalen-1(4H)-one in the solid state He labeled this

phenomenon phototropy and this name was used until the 1950s

when Yehuda Hirshberg of the Weizmann Institute of Science in Israel

proposed the term photochromism [75] Photochromism can take place

in both organic and inorganic compounds and also has its place in biological

systems (for example retinal in the vision process)

Photochromism does not have a rigorous definition but is usually used

to describe compounds that undergo a reversible photochemical reaction

where an absorption band in the visible part of the electromagnetic spectrum

23

changes dramatically in strength or wavelength In many cases an

absorbance band is present in only one form The degree of change required

for a photochemical reaction to be dubbed photochromic is that which

appears dramatic by eye but in essence there is no dividing line between

photochromic reactions and other photochemistry Therefore while the

transcis isomerization of azobenzene is considered a photochromic reaction

the analogous reaction of stilbene is not Since photochromism is just a

special case of a photochemical reaction almost any photochemical reaction

type may be used to produce photochromism with appropriate molecular desi

gnSome of the most common processes involved in photochromism are peric

yclic reactions cis-trans somerizations intramolecular hydrogen transfer

intramolecular group transfers dissociation processes and electron transfers

(oxidation-reduction)

Another some what arbitrary requirement of photochromism is that

it requires the two states of the molecule to be thermally stable under

ambient conditions for a reasonable time All the same nitrospiropyran (which

back-isomerizes in the dark over ~10 minutes at room temperature) is

considered photochromic All photochromic molecules back-isomerize to their

more stable form at some rate and this back-isomerization is accelerated by

heating There is therefore a close relationship between photochromic and the

rmochromic compounds The timescale of thermal back-isomerization is

important for applications and may be molecularly engineered

Photochromic compounds considered to be thermally stable include some

diarylethenes which do not back isomerize even after heating at 800C for 3

months

Since photochromic chromophores are dyes and operate according to

well-known reactions their molecular engineering to fine-tune their properties

can be achieved relatively easily using known design models quantum

mechanics calculations and experimentation In particular the tuning of

absorbance bands to particular parts of the spectrum and the engineering

of thermal stability have received much attention

Sometimes and particularly in the dye industry the term irreversible

photochromic is used to describe materials that undergo a permanent color

change upon exposure to Ultraviolet or visible light radiation Because by

24

definition photochromics are reversible there is technically no such thing as a

n irreversible photochromic this is loose usage and these compounds

are better referred to as photochangable or photoreactive dyes

Apart from the qualities already mentioned several other properties of

photochromics are important for their use These include

Quantum yield of the photochemical reaction

This determined the efficiency of the photochromic change with respect

to the amount of light absorbed The quantum yield of isomerization

can be strongly dependent on conditions

Fatigue resistance In photochromic materials fatigue refers to the

loss of reversibility by processes such as photodegradation

photobleaching photooxidation and other side reactions All

photochromics suffer fatigue to some extent and its rate is strongly

dependent on the activating light and the conditions of the sample

Photostationary state Photochromic materials have two states and

their interconversion can be controlled using different wavelengths of

light Excitation with any given wavelength of light will result in a

mixture of the two states at a particular ratio called the photo-

stationary state In a perfect system there would exist wavelengths

that can be used to provide 10 and 01 ratios of the isomers

but in real systems this is not possible since the active

absorbance bands always overlap to some extent

Polarity and solubility In order to incorporate photochromics in

working systems they suffer the same issues as other dyes They are

often charged in one or more state leading to very high polarity and

possible large changes in polarity They also often contain large

conjugated systems that limit their solubility

Photochromic complexes

A photochromic complex is a kind of chemical compound that has

photoresponsive Parts on its ligand These complexes have a specific

structure photoswitchable organic compounds are attached to

metalcomplexes For the photocontrollable parts thermally and

photochemically stable chromophores (azobenzene diarylethene

25

spiropyran etc) are usually used And for the metal complexes a wide

variety of compounds that have various functions (redox response

luminescence magnetism etc) are applied The photochromic parts and

metal parts are so close that they can affect each others molecular

orbitals The physical properties of these compounds shown by parts of

them (ie chromophores or metals) thus can be controlled by switching

their other sites by external stimuli For example photoisomerization

behaviors of some complexes can be switched by oxidation and reduction of

their metal parts Some other compounds can be changed in their

luminescence behavior magnetic interaction of metal sites or stability of

metal-to-ligand coordination by photoisomerization of their photochromic

parts

Photochemistry of Polyoxometalates

The photochemistry of polyoxometalates is of great interest to inorganic

chemistsMore than 80 years agoit was found that the R-Keggin tungstate

H3[PW12O40] was reduced photochemically to yield a blue-colored species

which was reoxidized by air and by various other oxidizing agents such as

Fe3+AgNO3and H2O2 [7879]The photoredox reactions of H4[SiW12O40]and

H3[PW12O40] proceeded effectively in the presence of primary and secondary

alcohols their ethers and aldehydes and proteinsbut less effectively in the

presence of tertiary alcoholsketonesestersthe fatty acids above formic

acidand simple amines[8081] The basic photoredox reaction involving

ethanol is illustrated by eq 13

2 H3PW12O40 + H3CCH2OH h ν ν ν ν 2 H4PW12O40 + H3CCHO ------- 13

2 H4PW12O40 +12 O2 2 H3PW12O40 + H2O ------ 14

In this reactionone molecule of ethanol photochemically reduces two

molecules of H3PW12O40 and is itself oxidized to acetaldehyde In the

presence of air the thermal oxidation of the reduced species takes place at

room temperature(eq 14)The reduced polyoxometalates which are the so-

calledldquoheteropolybluesrdquo have been used for the colorimetric analysis of the

26

elements P Si As and Ge and for the determination of uric acidsugarand

other biological compounds [8283] Piperidinium metavanadate also

undergoes photoinduced coloration from white to black followed by a

reversible color change in the presence of oxidizing agentsHowever

ammonium metavanadates ([NH4][VO3]) exhibits no photoinduced

coloration[84] The early photoredox reactions of the R-Keggin

polyoxometalates H4[SiW12O40] and H3PW12O40 were carried out in the

presence of photographic paper however the limited number of the

structurally well-characterized compounds available for study delayed the

development of modern cluster-compound photochemistry until the discovery

of photochromism in alkylammonium polyoxo- molybdate solids[8586] A

photochromic or electrochromic material is one whose light-absorbing

properties are altered upon optical excitation or reduction under the influence

of an externally applied electric field respectively The induced coloration

remains even after the excitation source has been removed These materials

are of technological interest because they return to their original state either

thermally upon irradiation with light of a frequency corresponding to the

induced absorption or electrochemically upon reversing the polarity of the

externally applied electric field Thus photochromic and electrochromic

materials behave in a reversible manner Polyoxometalates exhibit significant

photo-and electrochromism which makes them suitable as nanocomposite

molecular devices and as models for probing the physical properties of infinite

metal oxides Since the metal ions in the oxidized polyoxometalates have d0

electronic configurations the only absorption band which occurs in the UV-vis

range of the electronic spectra is due to the oxygen-to-metal (O-M) ligand-to-

metal charge transfer (LMCT)Upon irradiation electrons are promoted from

the low-energy electronic states which are mainly comprised of oxygen 2p

orbitals (the valence band in the band model)to the high-energy electronic

states which are mainly comprised of metal d orbitalrsquos (the conduction band

in the band model)The fundamental transitions in polyoxometalate lattices

are depicted schematically in Fig1B1

27

Fig1B1 Simple model showing the electronic transitions in the

polyoxometalates containing electron donar and acceptor (a)generation

of charge carriers(b)electron and hole trapping(c)electron release due

to stimulation(d)recombination

between electron and hole Electrons are e-and holes are h+

In the polyoxometalates containing heteroatoms and especially in

mixed metal polyoxometalates the charge carriers which are created by the

light or electric field may be trapped in electron traps and hole traps These

traps provide states of localized energy in the O-M LMCT energy gap due to

the heteroatoms or counter cations which correspond to impurities or lattice

defects in the band model If the trap depth ∆E is large compared to kT the

probability for thermal escape from the trap will be negligibly small and

metastable situation will existThe trapped carriers can be released by thermal

or optical stimulationIn the case of thermal stimulation the irradiated

polyoxometalate is heated until the energy barrier ∆E can be overcome The

trapped electron (or hole) then can escape from the trap and nonradiatively

recombine with the trapped hole (or electron)Under optical stimulation the

energy of an incident photon is used to overcome ∆E The relaxation

processes of the OndashM LMCT excitation energy include both the nonradiative

recombination of electrons and holes within the energy gap and the

intramolecular energy transfer leading to a charge-transfer emission This

intramolecular energy transfer corresponds to the O-M LMCT energy gap and

occurs via radiative recombination and sensitized emission from the

28

heteroatoms or cations If several energy levels based on the hetero atoms or

counter cations act as energy acceptors within the O-M LMCTenergy gap the

energy transfer occurs from the O-M LMCT states to these levels followed by

the nonradiative or radiative deactivation of the excitation energy It should be

noted that the O-M LMCT states also can be generated by the application of

very high electric fields to the polyoxometalate solids as demonstrated by the

observation of electroluminescence[87]If an external electric field with a

potential more negative than the energy levels of the vacant orbitals involved

in the O-M LMCT transition is applied to a polyoxometalate on the electrode

surface an electrochemical reduction occurs via the injection of electrons

from the electrode in to the vacant levels of the polyoxometalate as shown in

Fig1B 2

Fig1B2 - Energy scheme for the electrochromism of polyoxometalates

a)electrochemical reduction (b) electrochemical oxidation

Electrons injected in to the high-energy levels also may be trapped by

electron traps in a process analogous to that which occurs during LMCT

photoexcitation of the polyoxometalates These electrons are returned to the

electrode by electrochemical oxidation at an electrode potential more positive

than the energy levels for the d1 electron states The d1electrons in the O-M

LMCTstates facilitate the absorption of visible light via intervalence charge

transfer among metal centers and d-d transitions The same type of transition

may be possible for the d1electron captured by the electron traps too In

addition to searching for new photosensitive polyoxometalates with the

29

potential for having practical application there is now a need to elucidate the

fundamental photo-and electrochemical coloration processes by studying

electron transfer within the polyoxometalate lattices in conjunction with their

crystal structures So far few polyoxometalates exhibit a perfect reversibility

of coloration The irreversibility of the color change arises from as yet

uncharacterized side reactions during both the coloration and decoloration of

the polyoxometalates

Many metal oxides including aluminum titanium vanadium niobium

molybdenum and tungsten oxides are photochromic when they contain

impurities or dopants This coloration has been interpreted on the basis of

electron trapping at appropriate lattice sites within the crystals as shown in

Fig1B1 where the O-M LMCT transition corresponds to the transition

between the valence and conduction bands for the infinite metal-oxide lattice

[88-91]

B12 Applications of Photochromic materials

Sunglasses

One of the most famous reversible photochromic applications is color

changing lenses for sunglasses as found in eyeglasses The largest limitation

in using PC technology is that the materials cannot be made stable enough

to withstand thousands of hours of outdoor exposure so long-term outdoor

applications are not appropriate at this time The switching speed of

photochromic dyes is highly sensitive to the rigidity of the environment around

the dye As result they switch most rapidly in solution and slowest in the rigid

environment like a polymer lens Recently it has been reported that attaching

flexible low Tg polymers (for example siloxanes or poly (butyl acrylate) to the

dyes allows them to switch much more rapidly in a rigid lens [76] Some

spirooxazines with siloxane polymers attached switch at near solution like

speeds even though they are in a rigid lens matrix

Supramolecular chemistry

Photochromic units have been employed extensively in supramolecular

chemistry Their ability to give a light controlled reversible shape change

means that they can be used to make or break molecular recognition motifs

30

or to cause a consequent shape change in their surroundings Thus

photochromic units have been demonstrated as components of molecular

switches The coupling of photochromic units to enzymes or enzyme cofactors

even provides the ability to reversibly turn enzymes on and off

by altering their shape or orientation in such a way that their functions

are either working or broken

Data storage

The possibility of using photochromic compounds for data storage was

first suggested in 1956 by Yehuda Hirshberg[77] Since that time there have

been many investigations by various academic and commercial groups

particularly in the area of 3D optical data storage which promises discs that

can hold a terabyte of data Initially issues with thermal back-reactions

and destructive reading dogged these studies but more recently more stable

systems have been developed

Novelty items

Reversible photochromics are also found in applications such as toys

cosmeticsclothing and industrial applications If necessary they can be made

to change between desired colors by combination with a permanent pigment

A large number of inorganic compounds exhibit photochromism

These solids often have large band gaps of the order of 3 - 12 eV and

excitation of these solids leads to the formation of metastable centers that

absorb visible light giving rise to their colour They can return to their ground

state by heating or by optical excitation within the colour-centre band In most

cases the photochromism is a structure sensitive phenomenon involving

localized defect impurities or dislocations Some of these inorganic

compounds have the potential for a number of different uses Photochromic

compounds have a number of useful applications These can be divided

according to the most important property that is being used (Table 11) [92]

31

Table 11 Applications of Photochromic materials

Applications Depending Upon

Sensitivity to

Radiation

Reversibility Thermal Chemical or

Physical Properties

Self-developing

photography

Chemical switches for

computers

Temperature indicators

Protective

materials

Data displays

Heat-sensitive recording

media

Optical signal

processing

Photomasking and

photoresist technology

Reusable data storage

media

Anaytical reagents

Photochromic

microimages

Photopolymerisation

Information encoding

and steganography

Photocontractile

polymers and the

photoviscosity effect

Control of light

intensity

Q-switches

Pyroelectric

photochromic materials

B13 Thermochromism

Thermochromism is the reversible colour change of a substance

induced by temperature change A large variety of substances organic

inorganic organometallic supramolecular and polymeric systems exhibit this

phenomenon Examples of these include bianthrones cobalt

hexacyanoferrate the zirconocene complex of 1 4-diphenyl-1 3-butadiene

and poly (3-alkylthiophene) The organic 99-bixanthenylidene is colourless at

90 K yellow-green at 298 K and dark-blue when melted at 592 K Heating

conducting polymers can cause them to change colour This is achieved by

causing conformational changes to the polymer backbone resulting in a

change in the band gap of the polymer It has been reported that regioregular

32

P3HT reversibly changes colour upon heating to 220ordmC due to temperature-

dependent conformation changes Thermally cross linked polymer undergoes

the same colour change but it is much less reversible [93] Other forms of

thermochromism may be commercially important eg to give a visual

indication of temperature changes

B14 Electrochromism

Electrochromism describes a phenomenon of material color change in

a persistent but reversible manner produced by electrochemically induced

oxidation-reduction reactions Electrochromic materials can be applied to

various kinds of products such as smart windows and display devices Among

those applications there have been lots of efforts to develop electrochromic

display devices (ECDs) Especially flexible display devices are now attracting

much attention worldwide since they can facilitate new technological demands

such as bending and folding of paper-like displays High electrochromic

efficiency short response time long operating life time and reduction of

energy consumption are the most important requisites of the materials for the

paperlike displays [94 95] Among those properties the operation life time is

the most important barrier to overcome for a realization of ECDs There are

two types of electrochromic material a) inorganic transition metal oxides

(TMOs) b) organic polymer materials The TMOs have been studied longer

than the organic materials that they have been studied since 1960s [9697]

Electrochromism describes a reversible color change of material

produced by electrochemically induced oxidation-reduction reactions It is one

of several types of chromism of materials As thermochromism and

photochromism mean material color changes made by heat and light

respectively electrochromism refers that the color change is caused by an

electric potential In most cases the color change in electrochromism can be

driven by rather low electrical potential of the order of a fraction of volt or a

few volts [94-96] The color change of material means variation in

transmittance andor reflectance change in visible range which is originated

from different electronic absorption bands according to a switching between

oxidation and reduction state of material When electric potential is applied on

electrochromic material forced oxidation or reduction is derived and the

33

individual color is originated from the corresponding oxidation or reduction

state of the material For electrochromic materials the characteristic color

change is reversible since the oxidation and reduction state can be converted

reversibly by switching potential

Application fields

The application area for the electrochromism is rather broad that it

covers from smart window glazing and optical modulators to information

displays [98-102] The smart windows are typical examples The

electrochromic property is used to control the amount of light and heat to pass

through the windows Usually the electrochromic material is in form of thin film

coated on a window glass The transmittance modulation has also been

applied at the automobiles to automatically tint rear-view mirrors in various

lighting conditions The electrochromic application fields are illustrated in Fig

1B3

Since the smart windows control the transmittance of heat as well as

the transmittance of visible light the working definition of electrochromism has

now been extended to include devices for modulation of radiation in the near

infrared thermal infrared and microwave regions When color for

electrochromic materials is used this can now mean a response by detectors

at these wavelengths and not just by the human eyes Nowadays

electrochromic material draws much attention as being used in the display

devices Electrochromic display device (ECD) is being considered as one of

the candidates for the conventional liquid crystal display (LCD) since ECD

has many advantages over LCDs Among these advantages the most

important are low energy consumption wider viewing angle high contrast

rate and possibility to achieve multiple colors with a single material [103]

When a new redox state of electrochromic materialis established by

the applied electric pulse then it is maintained after the potential is switched

off This means the colored or bleached state of the material can be sustained

for a considerable time without applying electrical power This is so-called

ldquocolor memory effectrdquo of electrochromic material Because of the color

memory effect energy consumption for the electrochromic display device

could be drastically reduced and this would be a big advantage over other

34

emissive devices The low power consumption is especially advantageous

when it is applied to mobile devices with limited power source The possibility

for a flexible display is another attraction for electrochromic material

Information displays

Real-view mirrors for automobiles

Fig 1B4 Application fields of electrochromic devices Smart windows information displays and real-view mirrors for automobiles

35

Using polymeric electrochromic material and plastic substrate with a

transparent conductive coating it is possible to build all-plastic flexible display

device There are lots of papers and patents about the flexible display devices

[104105] In these cases gel-type electrolyte is also needed The gel-type

electrolyte immobilizes liquid electrolyte in the polymer network [106107]

Recently ITO-coated polymer films are readily available which would provide

the plastic substrate for a flexible device The flexible electrochromic displays

are frequently tried with the plastic substrate flexible electrochromic material

and gel-type electrolyte The flexible electrochromic displays would facilitate

increasing technical demands for foldable display devices

Metal Ion Electrochromism

Many transition metal oxides are capable of redox reactions that result in

colour change Metal oxide films are commonly prepared as thin layers of

either tungsten nickel molybdenum or other metal compounds by a number

of techniques These include sol-gel electrochemical by dc or rfreactive

sputtering techniques electron-beam evaporation by anodic or cathodic

electrodeposition or by solution dipping of the electrochromic metal

compounds (or compounds that can be changed into these metal compounds)

onto optically transparent electrodes (OTE) [108 -114] Their electrochromism

is derived from the colour change associated with a change in the oxidation

state of the metal anion The behaviour of these materials is dependent upon

pH moisture and exposure to the atmosphere [115] Generally the switching

rates of these films is somewhat slow with typical switching times of about 15

- 60 seconds to achieve 100 conversion to either coloured or bleached state

[116 -120] An Example of this includes nickel oxide which changes from

transparent (pale green) to brownblack taking about 30 seconds to do so

[121] Other examples include [(NH4)5Ru]2(pyrazine)5+ and [(NH4)5Ru]2(44-

bipyridine)5+ whose electrochromism is significantly different due to the effect

of the ligand [122] Table 12 below gives some examples of metal oxide films

with electrochromic properties

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

11

Often the hetero-atom is centrally located in the anion (eg Keggin

structure) or in a structure fragment eg the 2 phosphorus atoms in the

Dawson ion are central to the two symmetric fragments There are similarities

to clathrate structures The Keggin ion can be formulated as PO4 2minusand

M12O36 and the Dawson as (XO42-)2 and M18O54Structural isomerism is

common For example the Keggin structure has 5 isomers which can be

considered to contain one or more of the four M3O13 units being rotated

through 60degMany compounds share the same framework architectures or

frameworks derived from a larger framework with one or more addenda atoms

and oxide ions removed to give defect structure usually called a lacunary

structure An example of a compound with a Dawson lacunary structure is

As2W15O56Some cage structures containing ions are known eg an example

is the vanadate cage V18O42 containing a Clminus ion [38] This has 5 co-

ordinates square pyramidal vanadium units linked together

Fig 1A 6 - H4V18O42 cage containing Cl

1A3 General Properties of polyoxometalates -

Typically polyoxoanions are water and air stable species of large size

(6-25 Adeg) and high ionic weight In aqueous solution they are subject to

decomposition by hydroxide ions eg

[PW12O40]3- + 23 OH- HPO4

2- + 12WO42- + 11 H2O -------- 12

12

Although the PH at which such reactions are rapid can very widely

depending upon the polyanion involved Polyanions are often much stable

towards the H3O+ ions and numerous crystalline heteropoly acids are known

Such acids may be extremely soluble in water and polar solvents (giving

solutions with densities in excess of 4 gcm3) and have large dissociation

constants (PK lt 0)

Crystalline heteropoly acids and salts are frequently highly hydrated with

up to 50 molecules of water per anion Much of this water is zeolite in nature

and crystal composition can vary accordingly On the other hand the cation

anion stiochiometry is always well defined as the anion structure and

composition Finally many polyanions are powerful oxidizing agents and

undergo multiple reversible one or two electron reductions leading to intensely

colored mixed valence species known as heteropoly blues Polyanions are

known which can accept as many as 32 electrons without major structural

change

1A4 Chemistry of Molybdenum and Tungusten Heteropolyoxometalates

A photochromic monolayer film of phosphomolybdic acid (denoted as

PMo12) was fabricated by self-assembly approach UV-visible spectrum and

AFM observation show that the monolayer film is composed of aggregated

PMo12 molecules The monolayer film shows good photochromic properties

with enough stability and reversibility The colour change of the monolayer

after UV-irradiation can be captured by a microscope equipped with a color

CCD camera Photochromic response of the monolayer film can be doubled

after being modified by an amine monolayer [39]

Series of vanadium substituted molybdo Keggin HPA with 12 or 3

adjacent vanadium atoms were prepared These materials were supported

on carbon cloth electrodes and hot pressed onto Nafion with an ETEK

electrode used as a standard on the opposite side The MEArsquos were run at

temperatureslt100 oC with the HPA electrode as either the anode or the

cathode Stable polarisation curves are obtained for an HPA based cathode

with reasonable current densities at 80 oC [40]

A novel mixed-valence polyoxomolybdenum anion was synthesized

hydrothermally from molybdenum oxidemolybdenum metalboric and

phosphoric acids12-phenyldiphosphonicacidand imidazole (ImH) and was

13

structurally characterized as an imidazolium saltOne-and two-dimensional

structures of this anion and additional molybdenum diphosphonate linkers

were assembled as wellThey were structurally characterized as their

pyridinium(pyH) salts [41]

Phosphomolybdic acidpolyvinylpyrrolidone hybrid films were found to

show visible light photochromism It is identified that the intra-supramolecular

charge transfer between the inorganic and organic molecules is responsible

for the visible-light coloration Interestingly the films show photo-memory and

thermal activation The films show a small change in absorbance after being

irradiated with visible light for a short time and the coloration can be

enhanced greatly by subsequent thermal treatment Electrical measurements

indicate that the conductivity of the film increases after the brief irradiation

which promotes transfer of the electrons induced by the thermal treatment

[42]

In this work major effort was concentrated on passive thermal control

coatings based on photochromic and thermochromic materials The inorganic

photochromic materials were based on tungsten and molybdenum oxide films

and the organic photochromic materials included spiropyrans and

spirooxazines In addition photochromic composite organic-inorganic films

and thermochromic vanadium oxide films were prepared The samples were

synthesized using sputtering sol-gel process and thermal oxidation [43]

Polyoxometalates a class of oxidatively robust inorganic oxidants and

oxidation catalysts are currently under investigation at the Forest Products

Laboratory and at Emory University as an alternative to chlorinebased

chemicals in the bleaching of soft Woodkraft and other pulps Although

polyoxometalate salts are used in a number of industrial processes the

feasibility of using these salts and oxygen in the commercial bleaching of

chemical pulps was only recently demonstratedA clear advantage of

polyoxometalates over oxygen alone hydrogen peroxide or ozone is their

inherently high selectivity for the residual lignin in softwood kraft pulps The

goal of ongoing research is to develop a highly selective energy efficient

oxygen based polyoxometalate delignification and bleaching technology

compatible with mill closure [44]

14

Tungsten oxide exhibits pronounced photochromism upon bandgap

photoexcitation which makes it attractive and promising for applications in

many areas Some advances have been achieved during the past decades

The research on nanocrystalline films and single crystals indicates the critical

importance of defects in tungsten oxide to its photochromism Based on

energy-band engineering of semiconductors enhancement of photochromism

has been achieved for instance extension of the photoresponse from UV to

visible light by cathodic polarization improved change in absorption before

and after coloration through modification by a noble metal or another metal-

oxide semiconductor and increased photochromic reversibility via

hybridization with organic amines Nanocrystalline oxide films exhibit

controllable wettability which is coherent in nature with photochromism [45]

Polyoxometalates represent a diverse range of molecular clusters with

an almost unmatched range of physical properties and the ability to form

structures that can bridge several length scalesThe new building block

principles that have been discovered are beginning to allow the design of

complex clusters with desired properties and structures and several structural

types and novel physical properties are examinedIn this critical review the

synthetic and design approaches to the many polyoxometalate cluster types

are presented encompassing all the sub-types of polyoxometalates including

isopolyoxometa- lates heteropolyoxometalates and reduced molybdenum

blue systems As well as the fundamental structure and bonding aspectsthe

final section is devoted to discussing these clusters in the context of

contemporary and emerging interdisciplinary interests from areas as diverse

as antiviral agentsbiological ion transport modelsand materialsscience [46]

Keggin type molybdovanadophosphoric heteropoly acids were prepare

d by a novel environmentally benign method and their catalytic performances

were evaluated via hydroxylation of benzene to phenol with hydrogen

peroxide as oxidant in a mixed solvent of glacial acetic acid and acetonitrile

Various reaction parameters such as reaction time reaction temperature

ratio of benzene to hydrogen peroxide concentration of aqueous hydrogen

peroxide ratio of glacial acetic acid to acetonitrile in solvent and catalyst

concentration were changed to obtain an optimal reaction conditions

15

Molybdovanadophosphoric heteropoly acids are revealed to be highly

efficient catalyst for hydroxylation of benzene [47]

Thin films from the system (As2S3)Tl were deposited by thermal

evaporation on Si graphite and optical glass substratesFrom transmission

and reflection measurements of the thin films the refractive index (n) film

thickness(d) optical band gap(Eg) optical oscillator energy(Eo) and

dispersion energy(Ed) before and after exposure to light were determined

The results for optical parameters were analyzed using the Wemple - Di

Domenico single oscillator model and Lorenc-Lorenc equation It was found

that Eg decreases while n E0 and Ed increase for as deposited films

decreases while n E0 and Ed increase for as deposited films with increasing

of Tlconcentration passing through a maximum at 6 at of Tl After exposure

to light n E0 Ed increase and Eg decreases for all compositions

investigated The maximum change in n (Dn = 016 at l = 6328 nm) was

observed for thin As38S56Tl6 films From infrared spectroscopy measurements

of bulk glasses and thin films we could conclude that when up to 6 at of

thallium is introduced As-S-As chains break and a ternary TlAsS2

compound appears at 10 at Tl [48]

The organo - inorganic hybrid materialconsisting of Poly (34 Ethylene

Dioxythiophene) (PEDOT) doped with phosphomolybdate cluster anions

[PMo12O40]3-has been synthesized by direct insitu oxidative polymerization of

34-Ethylene Dioxythiophene (EDOT) with phosphomolybdic acid

(H3PMo12O40) Its characterization is investigated by Fourier Transform

Infrared Spectroscopy (FT-IR) and Scanning Electron Microscopy (SEM) The

hybrid material presents predominantly high electronic conductivities of

around 20 and 70 S cm1at 300 and 400 K respectively [49]

Heteropolyacids (HPAs) are known to be excellent re-dox catalysts In

combination with TiO2 HPAs can be used as photocatalysts active in visible

light The HPA accepts electron and get reduced to heteropolyblue (HPB)

That can absorb light in the visible range HPA can be incorporated onto the

external surface or in the pores of zeolite based composite photocatalysts

have been designed by incorporation of HPA semiconductor TiO2 and

transition metal cobalt on zeolite This composite metallozeolite photocatalyst

is efficient in photoreduction of methyl orange (MO) in visible light to the tune

16

of 411 mg of MO photo reducedg TiO2 This catalyst also shows encouraging

results for hydrogen evolution from water to the tune of 2730 micromolhg TiO

[50]

A new class of proton conducting glass membranes based on heteropo

lyacids such as phosphotungstic acid (PWA) as electrolytes for low

temperature H2O2 fuel cells was investigated Parameters for a single fuel cell

with a catalyst electrode of 015 mgcm2 of PtC and a glass composite

membrane were characterized by electrochemical measurements at open

circuit potential conditions The performance of the membrane electrode

assemblies (MEA) was systematically studied as an effect of SiO2 and P2O5

concentrations in the glass composite membrane and the MEA was found to

exhibit a maximum power density of 162 mW cm2 for an H2O2 fuel cell at

30 degC and 30 relative humidity (RH) [51]

Two new photochromic inorganic-organic hybrid materials formed from

Keggin type Polyoxometalates (POMs) and metronidazole (C6H9N3O3 MNZ)

formulated as H3PMo12O40bull3 MNZ3H2O (1) and H3PW12O40bull3MNZ3H2O(2)

were synthesized and characterized by elemental analysis IR spectra

electronic spectra electron spin resonance (ESR) spectra and TG-DTA

Reflectance spectra show the presence of weak inter molecular charge

transfer between the organic and inorganic moieties in the solid state The

photochromic properties were studied by solid diffuse reflectance spectra and

ESR spectra and the photochromic reactions were found to exhibit first-order

kinetics TG-DTA showed that two hybrid materials have similar thermal

behavior [52]

Heteropolycompounds (HPCs) have been a matter of interest in basic

and applied science for more than a century From their first synthesis many

advances have been made to promote the use of HPCs in different ways in

science and technology The aim of this article is to review the main structural

characteristics of heteropolycompounds of the Keggin type (12

tungstophosphoric12-molybdophosphoric12-tungstosilicic acid alkaline and

alkaline earth salts of12 tungstophosphoric acid and gels doped with HPCs)to

understand and explain their different activities such as high proton

conductivity and catalytic biochemical and biomedical activities [53]

17

A solid hybrid molecular material containing 1-butyl 3-methyl

imidazolium cations and Keggin anions of phosphotungstic acid has been

synthesized It is fully characterized by CHN analysis FTIR XRD UV-Vis-NIR

DRS 31P MAS NMR TGA and SEM The FTIR spectrum of the compound

shows the fingerprint vibrational bands of both Keggin molecular anions and

imidazolium cations The aromatic CndashH stretch region (2700ndash3250 cmndash1) of

imidazolium cation is split due to the interaction between the ring CndashH and

bulky Keggin anion The red-shift in the UV-Vis spectra and the downfield 31P

MAS NMR chemical shift also confirm the electrostatic interaction between

the ions in the compound Near IR spectral region (1000ndash2500 nm) shows the

elimination of water in the compound which is hydrophobic [54]

Transport coefficient measurements (electrical conductivity

thermoelectric power and Hall coefficient) have been performed on a

compact Tl033MoO3 polycrystalline compound in a wide temperature range

(200ndash400 K) Experimental results are interpreted with the help of a p-type

semiconductor model with two inverted deep levels near the midgap The

valence band and the conduction band are assumed to be formed from the

dxy orbitals of molybdenum atoms in the Mo6O22 cluster leading to narrow π-

bonding bands The donor and acceptor levels may be formed from

nonbonding dxy orbitals arising respectively from anionic and Tl+ defects

Electron paramagnetic resonance and magnetic measurements are in good

agreement with the theoretical band semiconductor model which has been

retained [55]

The optical properties of GaInTlAs epilayers grown at low temperature

~230degC by solid-source molecular-beam epitaxy on InP substrates were

characterized using optical absorptionand photoluminescence techniques

Optical absorption measurements a room temperature show a gap shrinkage

toward lower energies from 071 to 061 and 053 eV when the Tl content

increases from 0 to 4and 8in good agreement with theoretical

predictionsLow-temperature photoluminescence band-gap signals from

GaInAs and GaInTlAs layers are only obtained after rapid thermal annealing

performed inorder to improve the electronic quality of the layersA band gap

decrease as much as 41 meV for GaInTlAs with 19 Tl incorporation is

measured by photoluminescence at 8K [56]

18

The electrochromic performance of all solid ndash state cells employing

phosphotungstic acid and phosphomolybdic acid is reported These cells

employ SnO2 as the viewing electrode and graphite as the back electrodeThe

cells in the bleached state can be made white to red and become black in the

coloured state [57]

Various organic compounds were oxidized by molecular oxygen in the

presence of a catalytic amount of mixed addenda heteropolyoxometalates

containing molybdenum and vanadium The catalytic activity of the

Molybdovanadophosphate was found to be greatly enhanced by supporting

on charcoal The supported catalyst has high catalytic activity for oxidative

dehydrogenation of benzylic and allylic alcohols to the corresponding

aldehydes and ketones (46-92) nevertheless the nonsupported catalyst

was inactive for the same oxidations under these conditions 236Trimethyl

phenol was selectively oxidized to trimethyl-p benzoquinone which is

precursor of VitE in the presence of a catalytic amount of

molybdophosphate In addition the aerobic oxidation of amines alkyl-

substituted phenols and alkanes were also examined [58]

The reaction of Tl2CO3 with 111555-hexafluoro-24-pentanedione

and diglyme CH3O(CH2CH2O)2CH3 or tetraglyme CH3O(CH2CH2O)4CH3 in

dichloromethane yields the anhydrous thermally and air stable volatile Tl

diglyme and Tl tetraglyme adducts They have been characterized by single

crystal X-ray diffraction elemental analysis 1H and 13C NMR IR and mass

spectroscopy Thermal and mass-transport properties have been investigated

using thermo gravimetric and differential scanning calorimetric

measurements There is evidence that both precursors are very low melting

and volatile and can be used as liquid Tl sources Both adducts have been

successfully applied to metalndashorganic chemical vapor deposition of thallium

containing films [59]

Large size and high quality single crystals of quasi-two-dimensional

thallium molybdenum purple bronze TlMo6O17 have been grown by electrolytic

reduction of molten salt of Tl2CO3-MoO3 The crystal structure is trigonal with

space group P3m1 determined by X-ray diffraction and four-circle single crystal

diffraction The lattice parameters of the unit cell are a = b = 55282 Adeg and c

= 136991 Adeg The temperature dependence of resistivity and magnetic

19

susceptibility confirmed that a metal-to-metal transition occurs near 110 K

[60]

Hall coefficient and dc conductivity studies were made on p-type

Pb08Sn02Te thin films doped with different concentrations of thallium in the

temperature range 77 to 500 K The Hall coefficient and Hall mobility are

found to decrease with an increase in the doping concentration of thallium

Hall coefficient data have been analyzed in the light of a double valence-band

model Various band parameters such as valence band separation population

ratio mobility ratio and effective mass ratio have been calculated Hall

mobility data have been analyzed in the light of lattice and defect limited

scattering mechanisms [61]

Proton conducting composites of heteropolyacid hydrates phosphomol

ybdic acid H3PMo12O40 nH2O(PMA) phosphotungstic acid H3PW12O40 nH2O

( PTA) and salt hydrate like NiCl2 6H2O were prepared

with insulating Al2O3 as despersoidThe ionic conductivity peaks at two

concentrations of Al2O3 indicating two percolation thresholds for percolation

thresholds for proton conduction Two separate experiments were carried out

to check the existence of such percolation thresholds viz the volta battery

experiment involving the measurement of emf of an electrochemical cell

with composites of different compositions used as electrolyte and the

composition vs conductivity measured by the complex impedance

spectroscopy The presence of two maxima has been attributed to two

different percolation thresholds for the two possible mobile protonic

species H+ + (H3O+) and OH arising from the hydrates [62]

1 A5 Applications of Heteropolyoxometalates-

Applications of heteropolyanions centre depend on their redox properties

their high charges and ionic weights An enormous patent and journal

literature is devoted to the applications of heteropolyanions

1 Analysis-

The formation and subsequent precipitation or reduction of

[XMO12O40]n- anions form the basis of gravimetric and colorimetric analytical

methods for P As Si or Geeither separately or in combination [63 64]

20

2 Biochemical applications-

lsquoPhosphotungstic Acidsrsquo have for decades been used as precipitants

for proteins and as analytical reagents for proteinsalkaloidsand purines eg

the [P2M18O62]6- anions for colorimetric determination of uric acid [65] and

cholesterol The acid H3P12O40 either in aqueous or ethanolic solution is also

widely used as a non specific electron dense stain for electron spectroscopy

The dyestuffs industry has for many years used heteropolymolybdates and

tungstates to form color lakes and toners from basic dyes Large

heteropolyanions exhibit antiviral antitumoral properties at non-cytotoxic

doses in vitro and in vivo and are protein inhibitors of cellular bacterial and

viral DNA RNA polymerizes [66]

3 Catalysis

Heteropoly acids and salts have been used as heterogeneous catalysts

for a broad variety of reactions and compilations of such applications up to

1973 are available Examples include oxidation of propylene and isobutylene

to acrylic acid methacrylic acids and ammoxidation of acrylonitrile olefin

polymerization and epoxidation Much of current activity concerning

heterogeneous catalysis by heteropoly compounds is being carried out in

Japan [67 68]

4 Other Applications

Insoluble salts of Heteropolyanions especially ammonium

molybdophosphates have been used and are commercially available as ion-

exchange materials [69]Recent work in this area includes thin layer

chromatography of amino acids ion selective membranes [70] and the

preparation of new ion exchangers based on heteropolyanions Crystalline12-

tungstophosphoric and 12-molybdophosphoric acids are excellent protonic

conductors Heteropolyacids are electrochromic in the solid state as a

consequence of heteropolyblue formation Heteropolyblue formation has also

been used to detect alcohol or carboxylic acid radicals generated by radiolysis

of aqueous solutions Potential applications of heteropoly complexes as flame

retardants and smoke suppressants or as corrosion inhibitors and conversion

coatings on steel and aluminium are reported [71] Some potential ldquogreenrdquo

applications have been reported eg non-chlorine based wood pulp

bleaching process and a method of decontaminating water Some structures

21

containing transition metal atoms with unpaired electrons have unusual

magnetic properties and are being investigated as nano computer storage

devices Some compounds exhibit luminescence There are many reported

potential medicinal applications eg anti tumoral and anti-viral There have

been reports on the role of weak or non bonding interactions on the crystal

engineering of hybrid polyoxometalates

Spherical nonporous polyoxomolybdate based capsules of different

types containing more than 100 metal atoms reported by Achim Muller and his

group have versatile unique properties regarding their assembly to vesicles

and the chemistry which can be done inside the pores and cavities A discrete

polyoxometalate Lindquist ion of the form W6O192minus was successfully imaged

recently for the first time within the capillary of a carbon nanotube following

steric locking of the anion with the tubule In situ relaxation of the anion in its

equatorial plain was demonstrated [72]

22

Section-B

Litreture Survey on Chromism in Transition Metal

Oxides

1 B1 Chromism in Transition Metal Oxides

Chromism is a reversible change in a substances colour resulting from

a process caused by some form of stimulus Many materials are chromic

including inorganic and organic compounds and conducting polymers and the

property can result from many different mechanisms Several transition metal

oxides show EC properties The most popular are from the VI - B oxides In

this group WO3 and MoO3 are the most thoroughly studied cathodic EC

materials which can be electrochemically coloured and bleached when used

as the cathode in electrochemical cells Cathodic EC materials also include

V2O3 TiO2 and Nb2O5 Another distinguishable group is anodic EC material

including VIII oxides like IrOx nH2O Rh2O3 nH2O NiO nH2O etc which can be

anodicaly coloured in the electrochemical process when used as anode

There are several types of chromism which are discussed as below

B11 Photochromism

Photochromism is the reversible transformation of a chemical species

between two forms by the absorption of electromagnetic radiation where

the two forms have different absorption spectra [7374]

Trivially this can be described as a reversible change of color upon

exposure to light The phenomenon was discovered in the late 1880s

including work by Markwald who studied the reversible change of color of 23

44-tetrachloronaphthalen-1(4H)-one in the solid state He labeled this

phenomenon phototropy and this name was used until the 1950s

when Yehuda Hirshberg of the Weizmann Institute of Science in Israel

proposed the term photochromism [75] Photochromism can take place

in both organic and inorganic compounds and also has its place in biological

systems (for example retinal in the vision process)

Photochromism does not have a rigorous definition but is usually used

to describe compounds that undergo a reversible photochemical reaction

where an absorption band in the visible part of the electromagnetic spectrum

23

changes dramatically in strength or wavelength In many cases an

absorbance band is present in only one form The degree of change required

for a photochemical reaction to be dubbed photochromic is that which

appears dramatic by eye but in essence there is no dividing line between

photochromic reactions and other photochemistry Therefore while the

transcis isomerization of azobenzene is considered a photochromic reaction

the analogous reaction of stilbene is not Since photochromism is just a

special case of a photochemical reaction almost any photochemical reaction

type may be used to produce photochromism with appropriate molecular desi

gnSome of the most common processes involved in photochromism are peric

yclic reactions cis-trans somerizations intramolecular hydrogen transfer

intramolecular group transfers dissociation processes and electron transfers

(oxidation-reduction)

Another some what arbitrary requirement of photochromism is that

it requires the two states of the molecule to be thermally stable under

ambient conditions for a reasonable time All the same nitrospiropyran (which

back-isomerizes in the dark over ~10 minutes at room temperature) is

considered photochromic All photochromic molecules back-isomerize to their

more stable form at some rate and this back-isomerization is accelerated by

heating There is therefore a close relationship between photochromic and the

rmochromic compounds The timescale of thermal back-isomerization is

important for applications and may be molecularly engineered

Photochromic compounds considered to be thermally stable include some

diarylethenes which do not back isomerize even after heating at 800C for 3

months

Since photochromic chromophores are dyes and operate according to

well-known reactions their molecular engineering to fine-tune their properties

can be achieved relatively easily using known design models quantum

mechanics calculations and experimentation In particular the tuning of

absorbance bands to particular parts of the spectrum and the engineering

of thermal stability have received much attention

Sometimes and particularly in the dye industry the term irreversible

photochromic is used to describe materials that undergo a permanent color

change upon exposure to Ultraviolet or visible light radiation Because by

24

definition photochromics are reversible there is technically no such thing as a

n irreversible photochromic this is loose usage and these compounds

are better referred to as photochangable or photoreactive dyes

Apart from the qualities already mentioned several other properties of

photochromics are important for their use These include

Quantum yield of the photochemical reaction

This determined the efficiency of the photochromic change with respect

to the amount of light absorbed The quantum yield of isomerization

can be strongly dependent on conditions

Fatigue resistance In photochromic materials fatigue refers to the

loss of reversibility by processes such as photodegradation

photobleaching photooxidation and other side reactions All

photochromics suffer fatigue to some extent and its rate is strongly

dependent on the activating light and the conditions of the sample

Photostationary state Photochromic materials have two states and

their interconversion can be controlled using different wavelengths of

light Excitation with any given wavelength of light will result in a

mixture of the two states at a particular ratio called the photo-

stationary state In a perfect system there would exist wavelengths

that can be used to provide 10 and 01 ratios of the isomers

but in real systems this is not possible since the active

absorbance bands always overlap to some extent

Polarity and solubility In order to incorporate photochromics in

working systems they suffer the same issues as other dyes They are

often charged in one or more state leading to very high polarity and

possible large changes in polarity They also often contain large

conjugated systems that limit their solubility

Photochromic complexes

A photochromic complex is a kind of chemical compound that has

photoresponsive Parts on its ligand These complexes have a specific

structure photoswitchable organic compounds are attached to

metalcomplexes For the photocontrollable parts thermally and

photochemically stable chromophores (azobenzene diarylethene

25

spiropyran etc) are usually used And for the metal complexes a wide

variety of compounds that have various functions (redox response

luminescence magnetism etc) are applied The photochromic parts and

metal parts are so close that they can affect each others molecular

orbitals The physical properties of these compounds shown by parts of

them (ie chromophores or metals) thus can be controlled by switching

their other sites by external stimuli For example photoisomerization

behaviors of some complexes can be switched by oxidation and reduction of

their metal parts Some other compounds can be changed in their

luminescence behavior magnetic interaction of metal sites or stability of

metal-to-ligand coordination by photoisomerization of their photochromic

parts

Photochemistry of Polyoxometalates

The photochemistry of polyoxometalates is of great interest to inorganic

chemistsMore than 80 years agoit was found that the R-Keggin tungstate

H3[PW12O40] was reduced photochemically to yield a blue-colored species

which was reoxidized by air and by various other oxidizing agents such as

Fe3+AgNO3and H2O2 [7879]The photoredox reactions of H4[SiW12O40]and

H3[PW12O40] proceeded effectively in the presence of primary and secondary

alcohols their ethers and aldehydes and proteinsbut less effectively in the

presence of tertiary alcoholsketonesestersthe fatty acids above formic

acidand simple amines[8081] The basic photoredox reaction involving

ethanol is illustrated by eq 13

2 H3PW12O40 + H3CCH2OH h ν ν ν ν 2 H4PW12O40 + H3CCHO ------- 13

2 H4PW12O40 +12 O2 2 H3PW12O40 + H2O ------ 14

In this reactionone molecule of ethanol photochemically reduces two

molecules of H3PW12O40 and is itself oxidized to acetaldehyde In the

presence of air the thermal oxidation of the reduced species takes place at

room temperature(eq 14)The reduced polyoxometalates which are the so-

calledldquoheteropolybluesrdquo have been used for the colorimetric analysis of the

26

elements P Si As and Ge and for the determination of uric acidsugarand

other biological compounds [8283] Piperidinium metavanadate also

undergoes photoinduced coloration from white to black followed by a

reversible color change in the presence of oxidizing agentsHowever

ammonium metavanadates ([NH4][VO3]) exhibits no photoinduced

coloration[84] The early photoredox reactions of the R-Keggin

polyoxometalates H4[SiW12O40] and H3PW12O40 were carried out in the

presence of photographic paper however the limited number of the

structurally well-characterized compounds available for study delayed the

development of modern cluster-compound photochemistry until the discovery

of photochromism in alkylammonium polyoxo- molybdate solids[8586] A

photochromic or electrochromic material is one whose light-absorbing

properties are altered upon optical excitation or reduction under the influence

of an externally applied electric field respectively The induced coloration

remains even after the excitation source has been removed These materials

are of technological interest because they return to their original state either

thermally upon irradiation with light of a frequency corresponding to the

induced absorption or electrochemically upon reversing the polarity of the

externally applied electric field Thus photochromic and electrochromic

materials behave in a reversible manner Polyoxometalates exhibit significant

photo-and electrochromism which makes them suitable as nanocomposite

molecular devices and as models for probing the physical properties of infinite

metal oxides Since the metal ions in the oxidized polyoxometalates have d0

electronic configurations the only absorption band which occurs in the UV-vis

range of the electronic spectra is due to the oxygen-to-metal (O-M) ligand-to-

metal charge transfer (LMCT)Upon irradiation electrons are promoted from

the low-energy electronic states which are mainly comprised of oxygen 2p

orbitals (the valence band in the band model)to the high-energy electronic

states which are mainly comprised of metal d orbitalrsquos (the conduction band

in the band model)The fundamental transitions in polyoxometalate lattices

are depicted schematically in Fig1B1

27

Fig1B1 Simple model showing the electronic transitions in the

polyoxometalates containing electron donar and acceptor (a)generation

of charge carriers(b)electron and hole trapping(c)electron release due

to stimulation(d)recombination

between electron and hole Electrons are e-and holes are h+

In the polyoxometalates containing heteroatoms and especially in

mixed metal polyoxometalates the charge carriers which are created by the

light or electric field may be trapped in electron traps and hole traps These

traps provide states of localized energy in the O-M LMCT energy gap due to

the heteroatoms or counter cations which correspond to impurities or lattice

defects in the band model If the trap depth ∆E is large compared to kT the

probability for thermal escape from the trap will be negligibly small and

metastable situation will existThe trapped carriers can be released by thermal

or optical stimulationIn the case of thermal stimulation the irradiated

polyoxometalate is heated until the energy barrier ∆E can be overcome The

trapped electron (or hole) then can escape from the trap and nonradiatively

recombine with the trapped hole (or electron)Under optical stimulation the

energy of an incident photon is used to overcome ∆E The relaxation

processes of the OndashM LMCT excitation energy include both the nonradiative

recombination of electrons and holes within the energy gap and the

intramolecular energy transfer leading to a charge-transfer emission This

intramolecular energy transfer corresponds to the O-M LMCT energy gap and

occurs via radiative recombination and sensitized emission from the

28

heteroatoms or cations If several energy levels based on the hetero atoms or

counter cations act as energy acceptors within the O-M LMCTenergy gap the

energy transfer occurs from the O-M LMCT states to these levels followed by

the nonradiative or radiative deactivation of the excitation energy It should be

noted that the O-M LMCT states also can be generated by the application of

very high electric fields to the polyoxometalate solids as demonstrated by the

observation of electroluminescence[87]If an external electric field with a

potential more negative than the energy levels of the vacant orbitals involved

in the O-M LMCT transition is applied to a polyoxometalate on the electrode

surface an electrochemical reduction occurs via the injection of electrons

from the electrode in to the vacant levels of the polyoxometalate as shown in

Fig1B 2

Fig1B2 - Energy scheme for the electrochromism of polyoxometalates

a)electrochemical reduction (b) electrochemical oxidation

Electrons injected in to the high-energy levels also may be trapped by

electron traps in a process analogous to that which occurs during LMCT

photoexcitation of the polyoxometalates These electrons are returned to the

electrode by electrochemical oxidation at an electrode potential more positive

than the energy levels for the d1 electron states The d1electrons in the O-M

LMCTstates facilitate the absorption of visible light via intervalence charge

transfer among metal centers and d-d transitions The same type of transition

may be possible for the d1electron captured by the electron traps too In

addition to searching for new photosensitive polyoxometalates with the

29

potential for having practical application there is now a need to elucidate the

fundamental photo-and electrochemical coloration processes by studying

electron transfer within the polyoxometalate lattices in conjunction with their

crystal structures So far few polyoxometalates exhibit a perfect reversibility

of coloration The irreversibility of the color change arises from as yet

uncharacterized side reactions during both the coloration and decoloration of

the polyoxometalates

Many metal oxides including aluminum titanium vanadium niobium

molybdenum and tungsten oxides are photochromic when they contain

impurities or dopants This coloration has been interpreted on the basis of

electron trapping at appropriate lattice sites within the crystals as shown in

Fig1B1 where the O-M LMCT transition corresponds to the transition

between the valence and conduction bands for the infinite metal-oxide lattice

[88-91]

B12 Applications of Photochromic materials

Sunglasses

One of the most famous reversible photochromic applications is color

changing lenses for sunglasses as found in eyeglasses The largest limitation

in using PC technology is that the materials cannot be made stable enough

to withstand thousands of hours of outdoor exposure so long-term outdoor

applications are not appropriate at this time The switching speed of

photochromic dyes is highly sensitive to the rigidity of the environment around

the dye As result they switch most rapidly in solution and slowest in the rigid

environment like a polymer lens Recently it has been reported that attaching

flexible low Tg polymers (for example siloxanes or poly (butyl acrylate) to the

dyes allows them to switch much more rapidly in a rigid lens [76] Some

spirooxazines with siloxane polymers attached switch at near solution like

speeds even though they are in a rigid lens matrix

Supramolecular chemistry

Photochromic units have been employed extensively in supramolecular

chemistry Their ability to give a light controlled reversible shape change

means that they can be used to make or break molecular recognition motifs

30

or to cause a consequent shape change in their surroundings Thus

photochromic units have been demonstrated as components of molecular

switches The coupling of photochromic units to enzymes or enzyme cofactors

even provides the ability to reversibly turn enzymes on and off

by altering their shape or orientation in such a way that their functions

are either working or broken

Data storage

The possibility of using photochromic compounds for data storage was

first suggested in 1956 by Yehuda Hirshberg[77] Since that time there have

been many investigations by various academic and commercial groups

particularly in the area of 3D optical data storage which promises discs that

can hold a terabyte of data Initially issues with thermal back-reactions

and destructive reading dogged these studies but more recently more stable

systems have been developed

Novelty items

Reversible photochromics are also found in applications such as toys

cosmeticsclothing and industrial applications If necessary they can be made

to change between desired colors by combination with a permanent pigment

A large number of inorganic compounds exhibit photochromism

These solids often have large band gaps of the order of 3 - 12 eV and

excitation of these solids leads to the formation of metastable centers that

absorb visible light giving rise to their colour They can return to their ground

state by heating or by optical excitation within the colour-centre band In most

cases the photochromism is a structure sensitive phenomenon involving

localized defect impurities or dislocations Some of these inorganic

compounds have the potential for a number of different uses Photochromic

compounds have a number of useful applications These can be divided

according to the most important property that is being used (Table 11) [92]

31

Table 11 Applications of Photochromic materials

Applications Depending Upon

Sensitivity to

Radiation

Reversibility Thermal Chemical or

Physical Properties

Self-developing

photography

Chemical switches for

computers

Temperature indicators

Protective

materials

Data displays

Heat-sensitive recording

media

Optical signal

processing

Photomasking and

photoresist technology

Reusable data storage

media

Anaytical reagents

Photochromic

microimages

Photopolymerisation

Information encoding

and steganography

Photocontractile

polymers and the

photoviscosity effect

Control of light

intensity

Q-switches

Pyroelectric

photochromic materials

B13 Thermochromism

Thermochromism is the reversible colour change of a substance

induced by temperature change A large variety of substances organic

inorganic organometallic supramolecular and polymeric systems exhibit this

phenomenon Examples of these include bianthrones cobalt

hexacyanoferrate the zirconocene complex of 1 4-diphenyl-1 3-butadiene

and poly (3-alkylthiophene) The organic 99-bixanthenylidene is colourless at

90 K yellow-green at 298 K and dark-blue when melted at 592 K Heating

conducting polymers can cause them to change colour This is achieved by

causing conformational changes to the polymer backbone resulting in a

change in the band gap of the polymer It has been reported that regioregular

32

P3HT reversibly changes colour upon heating to 220ordmC due to temperature-

dependent conformation changes Thermally cross linked polymer undergoes

the same colour change but it is much less reversible [93] Other forms of

thermochromism may be commercially important eg to give a visual

indication of temperature changes

B14 Electrochromism

Electrochromism describes a phenomenon of material color change in

a persistent but reversible manner produced by electrochemically induced

oxidation-reduction reactions Electrochromic materials can be applied to

various kinds of products such as smart windows and display devices Among

those applications there have been lots of efforts to develop electrochromic

display devices (ECDs) Especially flexible display devices are now attracting

much attention worldwide since they can facilitate new technological demands

such as bending and folding of paper-like displays High electrochromic

efficiency short response time long operating life time and reduction of

energy consumption are the most important requisites of the materials for the

paperlike displays [94 95] Among those properties the operation life time is

the most important barrier to overcome for a realization of ECDs There are

two types of electrochromic material a) inorganic transition metal oxides

(TMOs) b) organic polymer materials The TMOs have been studied longer

than the organic materials that they have been studied since 1960s [9697]

Electrochromism describes a reversible color change of material

produced by electrochemically induced oxidation-reduction reactions It is one

of several types of chromism of materials As thermochromism and

photochromism mean material color changes made by heat and light

respectively electrochromism refers that the color change is caused by an

electric potential In most cases the color change in electrochromism can be

driven by rather low electrical potential of the order of a fraction of volt or a

few volts [94-96] The color change of material means variation in

transmittance andor reflectance change in visible range which is originated

from different electronic absorption bands according to a switching between

oxidation and reduction state of material When electric potential is applied on

electrochromic material forced oxidation or reduction is derived and the

33

individual color is originated from the corresponding oxidation or reduction

state of the material For electrochromic materials the characteristic color

change is reversible since the oxidation and reduction state can be converted

reversibly by switching potential

Application fields

The application area for the electrochromism is rather broad that it

covers from smart window glazing and optical modulators to information

displays [98-102] The smart windows are typical examples The

electrochromic property is used to control the amount of light and heat to pass

through the windows Usually the electrochromic material is in form of thin film

coated on a window glass The transmittance modulation has also been

applied at the automobiles to automatically tint rear-view mirrors in various

lighting conditions The electrochromic application fields are illustrated in Fig

1B3

Since the smart windows control the transmittance of heat as well as

the transmittance of visible light the working definition of electrochromism has

now been extended to include devices for modulation of radiation in the near

infrared thermal infrared and microwave regions When color for

electrochromic materials is used this can now mean a response by detectors

at these wavelengths and not just by the human eyes Nowadays

electrochromic material draws much attention as being used in the display

devices Electrochromic display device (ECD) is being considered as one of

the candidates for the conventional liquid crystal display (LCD) since ECD

has many advantages over LCDs Among these advantages the most

important are low energy consumption wider viewing angle high contrast

rate and possibility to achieve multiple colors with a single material [103]

When a new redox state of electrochromic materialis established by

the applied electric pulse then it is maintained after the potential is switched

off This means the colored or bleached state of the material can be sustained

for a considerable time without applying electrical power This is so-called

ldquocolor memory effectrdquo of electrochromic material Because of the color

memory effect energy consumption for the electrochromic display device

could be drastically reduced and this would be a big advantage over other

34

emissive devices The low power consumption is especially advantageous

when it is applied to mobile devices with limited power source The possibility

for a flexible display is another attraction for electrochromic material

Information displays

Real-view mirrors for automobiles

Fig 1B4 Application fields of electrochromic devices Smart windows information displays and real-view mirrors for automobiles

35

Using polymeric electrochromic material and plastic substrate with a

transparent conductive coating it is possible to build all-plastic flexible display

device There are lots of papers and patents about the flexible display devices

[104105] In these cases gel-type electrolyte is also needed The gel-type

electrolyte immobilizes liquid electrolyte in the polymer network [106107]

Recently ITO-coated polymer films are readily available which would provide

the plastic substrate for a flexible device The flexible electrochromic displays

are frequently tried with the plastic substrate flexible electrochromic material

and gel-type electrolyte The flexible electrochromic displays would facilitate

increasing technical demands for foldable display devices

Metal Ion Electrochromism

Many transition metal oxides are capable of redox reactions that result in

colour change Metal oxide films are commonly prepared as thin layers of

either tungsten nickel molybdenum or other metal compounds by a number

of techniques These include sol-gel electrochemical by dc or rfreactive

sputtering techniques electron-beam evaporation by anodic or cathodic

electrodeposition or by solution dipping of the electrochromic metal

compounds (or compounds that can be changed into these metal compounds)

onto optically transparent electrodes (OTE) [108 -114] Their electrochromism

is derived from the colour change associated with a change in the oxidation

state of the metal anion The behaviour of these materials is dependent upon

pH moisture and exposure to the atmosphere [115] Generally the switching

rates of these films is somewhat slow with typical switching times of about 15

- 60 seconds to achieve 100 conversion to either coloured or bleached state

[116 -120] An Example of this includes nickel oxide which changes from

transparent (pale green) to brownblack taking about 30 seconds to do so

[121] Other examples include [(NH4)5Ru]2(pyrazine)5+ and [(NH4)5Ru]2(44-

bipyridine)5+ whose electrochromism is significantly different due to the effect

of the ligand [122] Table 12 below gives some examples of metal oxide films

with electrochromic properties

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

12

Although the PH at which such reactions are rapid can very widely

depending upon the polyanion involved Polyanions are often much stable

towards the H3O+ ions and numerous crystalline heteropoly acids are known

Such acids may be extremely soluble in water and polar solvents (giving

solutions with densities in excess of 4 gcm3) and have large dissociation

constants (PK lt 0)

Crystalline heteropoly acids and salts are frequently highly hydrated with

up to 50 molecules of water per anion Much of this water is zeolite in nature

and crystal composition can vary accordingly On the other hand the cation

anion stiochiometry is always well defined as the anion structure and

composition Finally many polyanions are powerful oxidizing agents and

undergo multiple reversible one or two electron reductions leading to intensely

colored mixed valence species known as heteropoly blues Polyanions are

known which can accept as many as 32 electrons without major structural

change

1A4 Chemistry of Molybdenum and Tungusten Heteropolyoxometalates

A photochromic monolayer film of phosphomolybdic acid (denoted as

PMo12) was fabricated by self-assembly approach UV-visible spectrum and

AFM observation show that the monolayer film is composed of aggregated

PMo12 molecules The monolayer film shows good photochromic properties

with enough stability and reversibility The colour change of the monolayer

after UV-irradiation can be captured by a microscope equipped with a color

CCD camera Photochromic response of the monolayer film can be doubled

after being modified by an amine monolayer [39]

Series of vanadium substituted molybdo Keggin HPA with 12 or 3

adjacent vanadium atoms were prepared These materials were supported

on carbon cloth electrodes and hot pressed onto Nafion with an ETEK

electrode used as a standard on the opposite side The MEArsquos were run at

temperatureslt100 oC with the HPA electrode as either the anode or the

cathode Stable polarisation curves are obtained for an HPA based cathode

with reasonable current densities at 80 oC [40]

A novel mixed-valence polyoxomolybdenum anion was synthesized

hydrothermally from molybdenum oxidemolybdenum metalboric and

phosphoric acids12-phenyldiphosphonicacidand imidazole (ImH) and was

13

structurally characterized as an imidazolium saltOne-and two-dimensional

structures of this anion and additional molybdenum diphosphonate linkers

were assembled as wellThey were structurally characterized as their

pyridinium(pyH) salts [41]

Phosphomolybdic acidpolyvinylpyrrolidone hybrid films were found to

show visible light photochromism It is identified that the intra-supramolecular

charge transfer between the inorganic and organic molecules is responsible

for the visible-light coloration Interestingly the films show photo-memory and

thermal activation The films show a small change in absorbance after being

irradiated with visible light for a short time and the coloration can be

enhanced greatly by subsequent thermal treatment Electrical measurements

indicate that the conductivity of the film increases after the brief irradiation

which promotes transfer of the electrons induced by the thermal treatment

[42]

In this work major effort was concentrated on passive thermal control

coatings based on photochromic and thermochromic materials The inorganic

photochromic materials were based on tungsten and molybdenum oxide films

and the organic photochromic materials included spiropyrans and

spirooxazines In addition photochromic composite organic-inorganic films

and thermochromic vanadium oxide films were prepared The samples were

synthesized using sputtering sol-gel process and thermal oxidation [43]

Polyoxometalates a class of oxidatively robust inorganic oxidants and

oxidation catalysts are currently under investigation at the Forest Products

Laboratory and at Emory University as an alternative to chlorinebased

chemicals in the bleaching of soft Woodkraft and other pulps Although

polyoxometalate salts are used in a number of industrial processes the

feasibility of using these salts and oxygen in the commercial bleaching of

chemical pulps was only recently demonstratedA clear advantage of

polyoxometalates over oxygen alone hydrogen peroxide or ozone is their

inherently high selectivity for the residual lignin in softwood kraft pulps The

goal of ongoing research is to develop a highly selective energy efficient

oxygen based polyoxometalate delignification and bleaching technology

compatible with mill closure [44]

14

Tungsten oxide exhibits pronounced photochromism upon bandgap

photoexcitation which makes it attractive and promising for applications in

many areas Some advances have been achieved during the past decades

The research on nanocrystalline films and single crystals indicates the critical

importance of defects in tungsten oxide to its photochromism Based on

energy-band engineering of semiconductors enhancement of photochromism

has been achieved for instance extension of the photoresponse from UV to

visible light by cathodic polarization improved change in absorption before

and after coloration through modification by a noble metal or another metal-

oxide semiconductor and increased photochromic reversibility via

hybridization with organic amines Nanocrystalline oxide films exhibit

controllable wettability which is coherent in nature with photochromism [45]

Polyoxometalates represent a diverse range of molecular clusters with

an almost unmatched range of physical properties and the ability to form

structures that can bridge several length scalesThe new building block

principles that have been discovered are beginning to allow the design of

complex clusters with desired properties and structures and several structural

types and novel physical properties are examinedIn this critical review the

synthetic and design approaches to the many polyoxometalate cluster types

are presented encompassing all the sub-types of polyoxometalates including

isopolyoxometa- lates heteropolyoxometalates and reduced molybdenum

blue systems As well as the fundamental structure and bonding aspectsthe

final section is devoted to discussing these clusters in the context of

contemporary and emerging interdisciplinary interests from areas as diverse

as antiviral agentsbiological ion transport modelsand materialsscience [46]

Keggin type molybdovanadophosphoric heteropoly acids were prepare

d by a novel environmentally benign method and their catalytic performances

were evaluated via hydroxylation of benzene to phenol with hydrogen

peroxide as oxidant in a mixed solvent of glacial acetic acid and acetonitrile

Various reaction parameters such as reaction time reaction temperature

ratio of benzene to hydrogen peroxide concentration of aqueous hydrogen

peroxide ratio of glacial acetic acid to acetonitrile in solvent and catalyst

concentration were changed to obtain an optimal reaction conditions

15

Molybdovanadophosphoric heteropoly acids are revealed to be highly

efficient catalyst for hydroxylation of benzene [47]

Thin films from the system (As2S3)Tl were deposited by thermal

evaporation on Si graphite and optical glass substratesFrom transmission

and reflection measurements of the thin films the refractive index (n) film

thickness(d) optical band gap(Eg) optical oscillator energy(Eo) and

dispersion energy(Ed) before and after exposure to light were determined

The results for optical parameters were analyzed using the Wemple - Di

Domenico single oscillator model and Lorenc-Lorenc equation It was found

that Eg decreases while n E0 and Ed increase for as deposited films

decreases while n E0 and Ed increase for as deposited films with increasing

of Tlconcentration passing through a maximum at 6 at of Tl After exposure

to light n E0 Ed increase and Eg decreases for all compositions

investigated The maximum change in n (Dn = 016 at l = 6328 nm) was

observed for thin As38S56Tl6 films From infrared spectroscopy measurements

of bulk glasses and thin films we could conclude that when up to 6 at of

thallium is introduced As-S-As chains break and a ternary TlAsS2

compound appears at 10 at Tl [48]

The organo - inorganic hybrid materialconsisting of Poly (34 Ethylene

Dioxythiophene) (PEDOT) doped with phosphomolybdate cluster anions

[PMo12O40]3-has been synthesized by direct insitu oxidative polymerization of

34-Ethylene Dioxythiophene (EDOT) with phosphomolybdic acid

(H3PMo12O40) Its characterization is investigated by Fourier Transform

Infrared Spectroscopy (FT-IR) and Scanning Electron Microscopy (SEM) The

hybrid material presents predominantly high electronic conductivities of

around 20 and 70 S cm1at 300 and 400 K respectively [49]

Heteropolyacids (HPAs) are known to be excellent re-dox catalysts In

combination with TiO2 HPAs can be used as photocatalysts active in visible

light The HPA accepts electron and get reduced to heteropolyblue (HPB)

That can absorb light in the visible range HPA can be incorporated onto the

external surface or in the pores of zeolite based composite photocatalysts

have been designed by incorporation of HPA semiconductor TiO2 and

transition metal cobalt on zeolite This composite metallozeolite photocatalyst

is efficient in photoreduction of methyl orange (MO) in visible light to the tune

16

of 411 mg of MO photo reducedg TiO2 This catalyst also shows encouraging

results for hydrogen evolution from water to the tune of 2730 micromolhg TiO

[50]

A new class of proton conducting glass membranes based on heteropo

lyacids such as phosphotungstic acid (PWA) as electrolytes for low

temperature H2O2 fuel cells was investigated Parameters for a single fuel cell

with a catalyst electrode of 015 mgcm2 of PtC and a glass composite

membrane were characterized by electrochemical measurements at open

circuit potential conditions The performance of the membrane electrode

assemblies (MEA) was systematically studied as an effect of SiO2 and P2O5

concentrations in the glass composite membrane and the MEA was found to

exhibit a maximum power density of 162 mW cm2 for an H2O2 fuel cell at

30 degC and 30 relative humidity (RH) [51]

Two new photochromic inorganic-organic hybrid materials formed from

Keggin type Polyoxometalates (POMs) and metronidazole (C6H9N3O3 MNZ)

formulated as H3PMo12O40bull3 MNZ3H2O (1) and H3PW12O40bull3MNZ3H2O(2)

were synthesized and characterized by elemental analysis IR spectra

electronic spectra electron spin resonance (ESR) spectra and TG-DTA

Reflectance spectra show the presence of weak inter molecular charge

transfer between the organic and inorganic moieties in the solid state The

photochromic properties were studied by solid diffuse reflectance spectra and

ESR spectra and the photochromic reactions were found to exhibit first-order

kinetics TG-DTA showed that two hybrid materials have similar thermal

behavior [52]

Heteropolycompounds (HPCs) have been a matter of interest in basic

and applied science for more than a century From their first synthesis many

advances have been made to promote the use of HPCs in different ways in

science and technology The aim of this article is to review the main structural

characteristics of heteropolycompounds of the Keggin type (12

tungstophosphoric12-molybdophosphoric12-tungstosilicic acid alkaline and

alkaline earth salts of12 tungstophosphoric acid and gels doped with HPCs)to

understand and explain their different activities such as high proton

conductivity and catalytic biochemical and biomedical activities [53]

17

A solid hybrid molecular material containing 1-butyl 3-methyl

imidazolium cations and Keggin anions of phosphotungstic acid has been

synthesized It is fully characterized by CHN analysis FTIR XRD UV-Vis-NIR

DRS 31P MAS NMR TGA and SEM The FTIR spectrum of the compound

shows the fingerprint vibrational bands of both Keggin molecular anions and

imidazolium cations The aromatic CndashH stretch region (2700ndash3250 cmndash1) of

imidazolium cation is split due to the interaction between the ring CndashH and

bulky Keggin anion The red-shift in the UV-Vis spectra and the downfield 31P

MAS NMR chemical shift also confirm the electrostatic interaction between

the ions in the compound Near IR spectral region (1000ndash2500 nm) shows the

elimination of water in the compound which is hydrophobic [54]

Transport coefficient measurements (electrical conductivity

thermoelectric power and Hall coefficient) have been performed on a

compact Tl033MoO3 polycrystalline compound in a wide temperature range

(200ndash400 K) Experimental results are interpreted with the help of a p-type

semiconductor model with two inverted deep levels near the midgap The

valence band and the conduction band are assumed to be formed from the

dxy orbitals of molybdenum atoms in the Mo6O22 cluster leading to narrow π-

bonding bands The donor and acceptor levels may be formed from

nonbonding dxy orbitals arising respectively from anionic and Tl+ defects

Electron paramagnetic resonance and magnetic measurements are in good

agreement with the theoretical band semiconductor model which has been

retained [55]

The optical properties of GaInTlAs epilayers grown at low temperature

~230degC by solid-source molecular-beam epitaxy on InP substrates were

characterized using optical absorptionand photoluminescence techniques

Optical absorption measurements a room temperature show a gap shrinkage

toward lower energies from 071 to 061 and 053 eV when the Tl content

increases from 0 to 4and 8in good agreement with theoretical

predictionsLow-temperature photoluminescence band-gap signals from

GaInAs and GaInTlAs layers are only obtained after rapid thermal annealing

performed inorder to improve the electronic quality of the layersA band gap

decrease as much as 41 meV for GaInTlAs with 19 Tl incorporation is

measured by photoluminescence at 8K [56]

18

The electrochromic performance of all solid ndash state cells employing

phosphotungstic acid and phosphomolybdic acid is reported These cells

employ SnO2 as the viewing electrode and graphite as the back electrodeThe

cells in the bleached state can be made white to red and become black in the

coloured state [57]

Various organic compounds were oxidized by molecular oxygen in the

presence of a catalytic amount of mixed addenda heteropolyoxometalates

containing molybdenum and vanadium The catalytic activity of the

Molybdovanadophosphate was found to be greatly enhanced by supporting

on charcoal The supported catalyst has high catalytic activity for oxidative

dehydrogenation of benzylic and allylic alcohols to the corresponding

aldehydes and ketones (46-92) nevertheless the nonsupported catalyst

was inactive for the same oxidations under these conditions 236Trimethyl

phenol was selectively oxidized to trimethyl-p benzoquinone which is

precursor of VitE in the presence of a catalytic amount of

molybdophosphate In addition the aerobic oxidation of amines alkyl-

substituted phenols and alkanes were also examined [58]

The reaction of Tl2CO3 with 111555-hexafluoro-24-pentanedione

and diglyme CH3O(CH2CH2O)2CH3 or tetraglyme CH3O(CH2CH2O)4CH3 in

dichloromethane yields the anhydrous thermally and air stable volatile Tl

diglyme and Tl tetraglyme adducts They have been characterized by single

crystal X-ray diffraction elemental analysis 1H and 13C NMR IR and mass

spectroscopy Thermal and mass-transport properties have been investigated

using thermo gravimetric and differential scanning calorimetric

measurements There is evidence that both precursors are very low melting

and volatile and can be used as liquid Tl sources Both adducts have been

successfully applied to metalndashorganic chemical vapor deposition of thallium

containing films [59]

Large size and high quality single crystals of quasi-two-dimensional

thallium molybdenum purple bronze TlMo6O17 have been grown by electrolytic

reduction of molten salt of Tl2CO3-MoO3 The crystal structure is trigonal with

space group P3m1 determined by X-ray diffraction and four-circle single crystal

diffraction The lattice parameters of the unit cell are a = b = 55282 Adeg and c

= 136991 Adeg The temperature dependence of resistivity and magnetic

19

susceptibility confirmed that a metal-to-metal transition occurs near 110 K

[60]

Hall coefficient and dc conductivity studies were made on p-type

Pb08Sn02Te thin films doped with different concentrations of thallium in the

temperature range 77 to 500 K The Hall coefficient and Hall mobility are

found to decrease with an increase in the doping concentration of thallium

Hall coefficient data have been analyzed in the light of a double valence-band

model Various band parameters such as valence band separation population

ratio mobility ratio and effective mass ratio have been calculated Hall

mobility data have been analyzed in the light of lattice and defect limited

scattering mechanisms [61]

Proton conducting composites of heteropolyacid hydrates phosphomol

ybdic acid H3PMo12O40 nH2O(PMA) phosphotungstic acid H3PW12O40 nH2O

( PTA) and salt hydrate like NiCl2 6H2O were prepared

with insulating Al2O3 as despersoidThe ionic conductivity peaks at two

concentrations of Al2O3 indicating two percolation thresholds for percolation

thresholds for proton conduction Two separate experiments were carried out

to check the existence of such percolation thresholds viz the volta battery

experiment involving the measurement of emf of an electrochemical cell

with composites of different compositions used as electrolyte and the

composition vs conductivity measured by the complex impedance

spectroscopy The presence of two maxima has been attributed to two

different percolation thresholds for the two possible mobile protonic

species H+ + (H3O+) and OH arising from the hydrates [62]

1 A5 Applications of Heteropolyoxometalates-

Applications of heteropolyanions centre depend on their redox properties

their high charges and ionic weights An enormous patent and journal

literature is devoted to the applications of heteropolyanions

1 Analysis-

The formation and subsequent precipitation or reduction of

[XMO12O40]n- anions form the basis of gravimetric and colorimetric analytical

methods for P As Si or Geeither separately or in combination [63 64]

20

2 Biochemical applications-

lsquoPhosphotungstic Acidsrsquo have for decades been used as precipitants

for proteins and as analytical reagents for proteinsalkaloidsand purines eg

the [P2M18O62]6- anions for colorimetric determination of uric acid [65] and

cholesterol The acid H3P12O40 either in aqueous or ethanolic solution is also

widely used as a non specific electron dense stain for electron spectroscopy

The dyestuffs industry has for many years used heteropolymolybdates and

tungstates to form color lakes and toners from basic dyes Large

heteropolyanions exhibit antiviral antitumoral properties at non-cytotoxic

doses in vitro and in vivo and are protein inhibitors of cellular bacterial and

viral DNA RNA polymerizes [66]

3 Catalysis

Heteropoly acids and salts have been used as heterogeneous catalysts

for a broad variety of reactions and compilations of such applications up to

1973 are available Examples include oxidation of propylene and isobutylene

to acrylic acid methacrylic acids and ammoxidation of acrylonitrile olefin

polymerization and epoxidation Much of current activity concerning

heterogeneous catalysis by heteropoly compounds is being carried out in

Japan [67 68]

4 Other Applications

Insoluble salts of Heteropolyanions especially ammonium

molybdophosphates have been used and are commercially available as ion-

exchange materials [69]Recent work in this area includes thin layer

chromatography of amino acids ion selective membranes [70] and the

preparation of new ion exchangers based on heteropolyanions Crystalline12-

tungstophosphoric and 12-molybdophosphoric acids are excellent protonic

conductors Heteropolyacids are electrochromic in the solid state as a

consequence of heteropolyblue formation Heteropolyblue formation has also

been used to detect alcohol or carboxylic acid radicals generated by radiolysis

of aqueous solutions Potential applications of heteropoly complexes as flame

retardants and smoke suppressants or as corrosion inhibitors and conversion

coatings on steel and aluminium are reported [71] Some potential ldquogreenrdquo

applications have been reported eg non-chlorine based wood pulp

bleaching process and a method of decontaminating water Some structures

21

containing transition metal atoms with unpaired electrons have unusual

magnetic properties and are being investigated as nano computer storage

devices Some compounds exhibit luminescence There are many reported

potential medicinal applications eg anti tumoral and anti-viral There have

been reports on the role of weak or non bonding interactions on the crystal

engineering of hybrid polyoxometalates

Spherical nonporous polyoxomolybdate based capsules of different

types containing more than 100 metal atoms reported by Achim Muller and his

group have versatile unique properties regarding their assembly to vesicles

and the chemistry which can be done inside the pores and cavities A discrete

polyoxometalate Lindquist ion of the form W6O192minus was successfully imaged

recently for the first time within the capillary of a carbon nanotube following

steric locking of the anion with the tubule In situ relaxation of the anion in its

equatorial plain was demonstrated [72]

22

Section-B

Litreture Survey on Chromism in Transition Metal

Oxides

1 B1 Chromism in Transition Metal Oxides

Chromism is a reversible change in a substances colour resulting from

a process caused by some form of stimulus Many materials are chromic

including inorganic and organic compounds and conducting polymers and the

property can result from many different mechanisms Several transition metal

oxides show EC properties The most popular are from the VI - B oxides In

this group WO3 and MoO3 are the most thoroughly studied cathodic EC

materials which can be electrochemically coloured and bleached when used

as the cathode in electrochemical cells Cathodic EC materials also include

V2O3 TiO2 and Nb2O5 Another distinguishable group is anodic EC material

including VIII oxides like IrOx nH2O Rh2O3 nH2O NiO nH2O etc which can be

anodicaly coloured in the electrochemical process when used as anode

There are several types of chromism which are discussed as below

B11 Photochromism

Photochromism is the reversible transformation of a chemical species

between two forms by the absorption of electromagnetic radiation where

the two forms have different absorption spectra [7374]

Trivially this can be described as a reversible change of color upon

exposure to light The phenomenon was discovered in the late 1880s

including work by Markwald who studied the reversible change of color of 23

44-tetrachloronaphthalen-1(4H)-one in the solid state He labeled this

phenomenon phototropy and this name was used until the 1950s

when Yehuda Hirshberg of the Weizmann Institute of Science in Israel

proposed the term photochromism [75] Photochromism can take place

in both organic and inorganic compounds and also has its place in biological

systems (for example retinal in the vision process)

Photochromism does not have a rigorous definition but is usually used

to describe compounds that undergo a reversible photochemical reaction

where an absorption band in the visible part of the electromagnetic spectrum

23

changes dramatically in strength or wavelength In many cases an

absorbance band is present in only one form The degree of change required

for a photochemical reaction to be dubbed photochromic is that which

appears dramatic by eye but in essence there is no dividing line between

photochromic reactions and other photochemistry Therefore while the

transcis isomerization of azobenzene is considered a photochromic reaction

the analogous reaction of stilbene is not Since photochromism is just a

special case of a photochemical reaction almost any photochemical reaction

type may be used to produce photochromism with appropriate molecular desi

gnSome of the most common processes involved in photochromism are peric

yclic reactions cis-trans somerizations intramolecular hydrogen transfer

intramolecular group transfers dissociation processes and electron transfers

(oxidation-reduction)

Another some what arbitrary requirement of photochromism is that

it requires the two states of the molecule to be thermally stable under

ambient conditions for a reasonable time All the same nitrospiropyran (which

back-isomerizes in the dark over ~10 minutes at room temperature) is

considered photochromic All photochromic molecules back-isomerize to their

more stable form at some rate and this back-isomerization is accelerated by

heating There is therefore a close relationship between photochromic and the

rmochromic compounds The timescale of thermal back-isomerization is

important for applications and may be molecularly engineered

Photochromic compounds considered to be thermally stable include some

diarylethenes which do not back isomerize even after heating at 800C for 3

months

Since photochromic chromophores are dyes and operate according to

well-known reactions their molecular engineering to fine-tune their properties

can be achieved relatively easily using known design models quantum

mechanics calculations and experimentation In particular the tuning of

absorbance bands to particular parts of the spectrum and the engineering

of thermal stability have received much attention

Sometimes and particularly in the dye industry the term irreversible

photochromic is used to describe materials that undergo a permanent color

change upon exposure to Ultraviolet or visible light radiation Because by

24

definition photochromics are reversible there is technically no such thing as a

n irreversible photochromic this is loose usage and these compounds

are better referred to as photochangable or photoreactive dyes

Apart from the qualities already mentioned several other properties of

photochromics are important for their use These include

Quantum yield of the photochemical reaction

This determined the efficiency of the photochromic change with respect

to the amount of light absorbed The quantum yield of isomerization

can be strongly dependent on conditions

Fatigue resistance In photochromic materials fatigue refers to the

loss of reversibility by processes such as photodegradation

photobleaching photooxidation and other side reactions All

photochromics suffer fatigue to some extent and its rate is strongly

dependent on the activating light and the conditions of the sample

Photostationary state Photochromic materials have two states and

their interconversion can be controlled using different wavelengths of

light Excitation with any given wavelength of light will result in a

mixture of the two states at a particular ratio called the photo-

stationary state In a perfect system there would exist wavelengths

that can be used to provide 10 and 01 ratios of the isomers

but in real systems this is not possible since the active

absorbance bands always overlap to some extent

Polarity and solubility In order to incorporate photochromics in

working systems they suffer the same issues as other dyes They are

often charged in one or more state leading to very high polarity and

possible large changes in polarity They also often contain large

conjugated systems that limit their solubility

Photochromic complexes

A photochromic complex is a kind of chemical compound that has

photoresponsive Parts on its ligand These complexes have a specific

structure photoswitchable organic compounds are attached to

metalcomplexes For the photocontrollable parts thermally and

photochemically stable chromophores (azobenzene diarylethene

25

spiropyran etc) are usually used And for the metal complexes a wide

variety of compounds that have various functions (redox response

luminescence magnetism etc) are applied The photochromic parts and

metal parts are so close that they can affect each others molecular

orbitals The physical properties of these compounds shown by parts of

them (ie chromophores or metals) thus can be controlled by switching

their other sites by external stimuli For example photoisomerization

behaviors of some complexes can be switched by oxidation and reduction of

their metal parts Some other compounds can be changed in their

luminescence behavior magnetic interaction of metal sites or stability of

metal-to-ligand coordination by photoisomerization of their photochromic

parts

Photochemistry of Polyoxometalates

The photochemistry of polyoxometalates is of great interest to inorganic

chemistsMore than 80 years agoit was found that the R-Keggin tungstate

H3[PW12O40] was reduced photochemically to yield a blue-colored species

which was reoxidized by air and by various other oxidizing agents such as

Fe3+AgNO3and H2O2 [7879]The photoredox reactions of H4[SiW12O40]and

H3[PW12O40] proceeded effectively in the presence of primary and secondary

alcohols their ethers and aldehydes and proteinsbut less effectively in the

presence of tertiary alcoholsketonesestersthe fatty acids above formic

acidand simple amines[8081] The basic photoredox reaction involving

ethanol is illustrated by eq 13

2 H3PW12O40 + H3CCH2OH h ν ν ν ν 2 H4PW12O40 + H3CCHO ------- 13

2 H4PW12O40 +12 O2 2 H3PW12O40 + H2O ------ 14

In this reactionone molecule of ethanol photochemically reduces two

molecules of H3PW12O40 and is itself oxidized to acetaldehyde In the

presence of air the thermal oxidation of the reduced species takes place at

room temperature(eq 14)The reduced polyoxometalates which are the so-

calledldquoheteropolybluesrdquo have been used for the colorimetric analysis of the

26

elements P Si As and Ge and for the determination of uric acidsugarand

other biological compounds [8283] Piperidinium metavanadate also

undergoes photoinduced coloration from white to black followed by a

reversible color change in the presence of oxidizing agentsHowever

ammonium metavanadates ([NH4][VO3]) exhibits no photoinduced

coloration[84] The early photoredox reactions of the R-Keggin

polyoxometalates H4[SiW12O40] and H3PW12O40 were carried out in the

presence of photographic paper however the limited number of the

structurally well-characterized compounds available for study delayed the

development of modern cluster-compound photochemistry until the discovery

of photochromism in alkylammonium polyoxo- molybdate solids[8586] A

photochromic or electrochromic material is one whose light-absorbing

properties are altered upon optical excitation or reduction under the influence

of an externally applied electric field respectively The induced coloration

remains even after the excitation source has been removed These materials

are of technological interest because they return to their original state either

thermally upon irradiation with light of a frequency corresponding to the

induced absorption or electrochemically upon reversing the polarity of the

externally applied electric field Thus photochromic and electrochromic

materials behave in a reversible manner Polyoxometalates exhibit significant

photo-and electrochromism which makes them suitable as nanocomposite

molecular devices and as models for probing the physical properties of infinite

metal oxides Since the metal ions in the oxidized polyoxometalates have d0

electronic configurations the only absorption band which occurs in the UV-vis

range of the electronic spectra is due to the oxygen-to-metal (O-M) ligand-to-

metal charge transfer (LMCT)Upon irradiation electrons are promoted from

the low-energy electronic states which are mainly comprised of oxygen 2p

orbitals (the valence band in the band model)to the high-energy electronic

states which are mainly comprised of metal d orbitalrsquos (the conduction band

in the band model)The fundamental transitions in polyoxometalate lattices

are depicted schematically in Fig1B1

27

Fig1B1 Simple model showing the electronic transitions in the

polyoxometalates containing electron donar and acceptor (a)generation

of charge carriers(b)electron and hole trapping(c)electron release due

to stimulation(d)recombination

between electron and hole Electrons are e-and holes are h+

In the polyoxometalates containing heteroatoms and especially in

mixed metal polyoxometalates the charge carriers which are created by the

light or electric field may be trapped in electron traps and hole traps These

traps provide states of localized energy in the O-M LMCT energy gap due to

the heteroatoms or counter cations which correspond to impurities or lattice

defects in the band model If the trap depth ∆E is large compared to kT the

probability for thermal escape from the trap will be negligibly small and

metastable situation will existThe trapped carriers can be released by thermal

or optical stimulationIn the case of thermal stimulation the irradiated

polyoxometalate is heated until the energy barrier ∆E can be overcome The

trapped electron (or hole) then can escape from the trap and nonradiatively

recombine with the trapped hole (or electron)Under optical stimulation the

energy of an incident photon is used to overcome ∆E The relaxation

processes of the OndashM LMCT excitation energy include both the nonradiative

recombination of electrons and holes within the energy gap and the

intramolecular energy transfer leading to a charge-transfer emission This

intramolecular energy transfer corresponds to the O-M LMCT energy gap and

occurs via radiative recombination and sensitized emission from the

28

heteroatoms or cations If several energy levels based on the hetero atoms or

counter cations act as energy acceptors within the O-M LMCTenergy gap the

energy transfer occurs from the O-M LMCT states to these levels followed by

the nonradiative or radiative deactivation of the excitation energy It should be

noted that the O-M LMCT states also can be generated by the application of

very high electric fields to the polyoxometalate solids as demonstrated by the

observation of electroluminescence[87]If an external electric field with a

potential more negative than the energy levels of the vacant orbitals involved

in the O-M LMCT transition is applied to a polyoxometalate on the electrode

surface an electrochemical reduction occurs via the injection of electrons

from the electrode in to the vacant levels of the polyoxometalate as shown in

Fig1B 2

Fig1B2 - Energy scheme for the electrochromism of polyoxometalates

a)electrochemical reduction (b) electrochemical oxidation

Electrons injected in to the high-energy levels also may be trapped by

electron traps in a process analogous to that which occurs during LMCT

photoexcitation of the polyoxometalates These electrons are returned to the

electrode by electrochemical oxidation at an electrode potential more positive

than the energy levels for the d1 electron states The d1electrons in the O-M

LMCTstates facilitate the absorption of visible light via intervalence charge

transfer among metal centers and d-d transitions The same type of transition

may be possible for the d1electron captured by the electron traps too In

addition to searching for new photosensitive polyoxometalates with the

29

potential for having practical application there is now a need to elucidate the

fundamental photo-and electrochemical coloration processes by studying

electron transfer within the polyoxometalate lattices in conjunction with their

crystal structures So far few polyoxometalates exhibit a perfect reversibility

of coloration The irreversibility of the color change arises from as yet

uncharacterized side reactions during both the coloration and decoloration of

the polyoxometalates

Many metal oxides including aluminum titanium vanadium niobium

molybdenum and tungsten oxides are photochromic when they contain

impurities or dopants This coloration has been interpreted on the basis of

electron trapping at appropriate lattice sites within the crystals as shown in

Fig1B1 where the O-M LMCT transition corresponds to the transition

between the valence and conduction bands for the infinite metal-oxide lattice

[88-91]

B12 Applications of Photochromic materials

Sunglasses

One of the most famous reversible photochromic applications is color

changing lenses for sunglasses as found in eyeglasses The largest limitation

in using PC technology is that the materials cannot be made stable enough

to withstand thousands of hours of outdoor exposure so long-term outdoor

applications are not appropriate at this time The switching speed of

photochromic dyes is highly sensitive to the rigidity of the environment around

the dye As result they switch most rapidly in solution and slowest in the rigid

environment like a polymer lens Recently it has been reported that attaching

flexible low Tg polymers (for example siloxanes or poly (butyl acrylate) to the

dyes allows them to switch much more rapidly in a rigid lens [76] Some

spirooxazines with siloxane polymers attached switch at near solution like

speeds even though they are in a rigid lens matrix

Supramolecular chemistry

Photochromic units have been employed extensively in supramolecular

chemistry Their ability to give a light controlled reversible shape change

means that they can be used to make or break molecular recognition motifs

30

or to cause a consequent shape change in their surroundings Thus

photochromic units have been demonstrated as components of molecular

switches The coupling of photochromic units to enzymes or enzyme cofactors

even provides the ability to reversibly turn enzymes on and off

by altering their shape or orientation in such a way that their functions

are either working or broken

Data storage

The possibility of using photochromic compounds for data storage was

first suggested in 1956 by Yehuda Hirshberg[77] Since that time there have

been many investigations by various academic and commercial groups

particularly in the area of 3D optical data storage which promises discs that

can hold a terabyte of data Initially issues with thermal back-reactions

and destructive reading dogged these studies but more recently more stable

systems have been developed

Novelty items

Reversible photochromics are also found in applications such as toys

cosmeticsclothing and industrial applications If necessary they can be made

to change between desired colors by combination with a permanent pigment

A large number of inorganic compounds exhibit photochromism

These solids often have large band gaps of the order of 3 - 12 eV and

excitation of these solids leads to the formation of metastable centers that

absorb visible light giving rise to their colour They can return to their ground

state by heating or by optical excitation within the colour-centre band In most

cases the photochromism is a structure sensitive phenomenon involving

localized defect impurities or dislocations Some of these inorganic

compounds have the potential for a number of different uses Photochromic

compounds have a number of useful applications These can be divided

according to the most important property that is being used (Table 11) [92]

31

Table 11 Applications of Photochromic materials

Applications Depending Upon

Sensitivity to

Radiation

Reversibility Thermal Chemical or

Physical Properties

Self-developing

photography

Chemical switches for

computers

Temperature indicators

Protective

materials

Data displays

Heat-sensitive recording

media

Optical signal

processing

Photomasking and

photoresist technology

Reusable data storage

media

Anaytical reagents

Photochromic

microimages

Photopolymerisation

Information encoding

and steganography

Photocontractile

polymers and the

photoviscosity effect

Control of light

intensity

Q-switches

Pyroelectric

photochromic materials

B13 Thermochromism

Thermochromism is the reversible colour change of a substance

induced by temperature change A large variety of substances organic

inorganic organometallic supramolecular and polymeric systems exhibit this

phenomenon Examples of these include bianthrones cobalt

hexacyanoferrate the zirconocene complex of 1 4-diphenyl-1 3-butadiene

and poly (3-alkylthiophene) The organic 99-bixanthenylidene is colourless at

90 K yellow-green at 298 K and dark-blue when melted at 592 K Heating

conducting polymers can cause them to change colour This is achieved by

causing conformational changes to the polymer backbone resulting in a

change in the band gap of the polymer It has been reported that regioregular

32

P3HT reversibly changes colour upon heating to 220ordmC due to temperature-

dependent conformation changes Thermally cross linked polymer undergoes

the same colour change but it is much less reversible [93] Other forms of

thermochromism may be commercially important eg to give a visual

indication of temperature changes

B14 Electrochromism

Electrochromism describes a phenomenon of material color change in

a persistent but reversible manner produced by electrochemically induced

oxidation-reduction reactions Electrochromic materials can be applied to

various kinds of products such as smart windows and display devices Among

those applications there have been lots of efforts to develop electrochromic

display devices (ECDs) Especially flexible display devices are now attracting

much attention worldwide since they can facilitate new technological demands

such as bending and folding of paper-like displays High electrochromic

efficiency short response time long operating life time and reduction of

energy consumption are the most important requisites of the materials for the

paperlike displays [94 95] Among those properties the operation life time is

the most important barrier to overcome for a realization of ECDs There are

two types of electrochromic material a) inorganic transition metal oxides

(TMOs) b) organic polymer materials The TMOs have been studied longer

than the organic materials that they have been studied since 1960s [9697]

Electrochromism describes a reversible color change of material

produced by electrochemically induced oxidation-reduction reactions It is one

of several types of chromism of materials As thermochromism and

photochromism mean material color changes made by heat and light

respectively electrochromism refers that the color change is caused by an

electric potential In most cases the color change in electrochromism can be

driven by rather low electrical potential of the order of a fraction of volt or a

few volts [94-96] The color change of material means variation in

transmittance andor reflectance change in visible range which is originated

from different electronic absorption bands according to a switching between

oxidation and reduction state of material When electric potential is applied on

electrochromic material forced oxidation or reduction is derived and the

33

individual color is originated from the corresponding oxidation or reduction

state of the material For electrochromic materials the characteristic color

change is reversible since the oxidation and reduction state can be converted

reversibly by switching potential

Application fields

The application area for the electrochromism is rather broad that it

covers from smart window glazing and optical modulators to information

displays [98-102] The smart windows are typical examples The

electrochromic property is used to control the amount of light and heat to pass

through the windows Usually the electrochromic material is in form of thin film

coated on a window glass The transmittance modulation has also been

applied at the automobiles to automatically tint rear-view mirrors in various

lighting conditions The electrochromic application fields are illustrated in Fig

1B3

Since the smart windows control the transmittance of heat as well as

the transmittance of visible light the working definition of electrochromism has

now been extended to include devices for modulation of radiation in the near

infrared thermal infrared and microwave regions When color for

electrochromic materials is used this can now mean a response by detectors

at these wavelengths and not just by the human eyes Nowadays

electrochromic material draws much attention as being used in the display

devices Electrochromic display device (ECD) is being considered as one of

the candidates for the conventional liquid crystal display (LCD) since ECD

has many advantages over LCDs Among these advantages the most

important are low energy consumption wider viewing angle high contrast

rate and possibility to achieve multiple colors with a single material [103]

When a new redox state of electrochromic materialis established by

the applied electric pulse then it is maintained after the potential is switched

off This means the colored or bleached state of the material can be sustained

for a considerable time without applying electrical power This is so-called

ldquocolor memory effectrdquo of electrochromic material Because of the color

memory effect energy consumption for the electrochromic display device

could be drastically reduced and this would be a big advantage over other

34

emissive devices The low power consumption is especially advantageous

when it is applied to mobile devices with limited power source The possibility

for a flexible display is another attraction for electrochromic material

Information displays

Real-view mirrors for automobiles

Fig 1B4 Application fields of electrochromic devices Smart windows information displays and real-view mirrors for automobiles

35

Using polymeric electrochromic material and plastic substrate with a

transparent conductive coating it is possible to build all-plastic flexible display

device There are lots of papers and patents about the flexible display devices

[104105] In these cases gel-type electrolyte is also needed The gel-type

electrolyte immobilizes liquid electrolyte in the polymer network [106107]

Recently ITO-coated polymer films are readily available which would provide

the plastic substrate for a flexible device The flexible electrochromic displays

are frequently tried with the plastic substrate flexible electrochromic material

and gel-type electrolyte The flexible electrochromic displays would facilitate

increasing technical demands for foldable display devices

Metal Ion Electrochromism

Many transition metal oxides are capable of redox reactions that result in

colour change Metal oxide films are commonly prepared as thin layers of

either tungsten nickel molybdenum or other metal compounds by a number

of techniques These include sol-gel electrochemical by dc or rfreactive

sputtering techniques electron-beam evaporation by anodic or cathodic

electrodeposition or by solution dipping of the electrochromic metal

compounds (or compounds that can be changed into these metal compounds)

onto optically transparent electrodes (OTE) [108 -114] Their electrochromism

is derived from the colour change associated with a change in the oxidation

state of the metal anion The behaviour of these materials is dependent upon

pH moisture and exposure to the atmosphere [115] Generally the switching

rates of these films is somewhat slow with typical switching times of about 15

- 60 seconds to achieve 100 conversion to either coloured or bleached state

[116 -120] An Example of this includes nickel oxide which changes from

transparent (pale green) to brownblack taking about 30 seconds to do so

[121] Other examples include [(NH4)5Ru]2(pyrazine)5+ and [(NH4)5Ru]2(44-

bipyridine)5+ whose electrochromism is significantly different due to the effect

of the ligand [122] Table 12 below gives some examples of metal oxide films

with electrochromic properties

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

13

structurally characterized as an imidazolium saltOne-and two-dimensional

structures of this anion and additional molybdenum diphosphonate linkers

were assembled as wellThey were structurally characterized as their

pyridinium(pyH) salts [41]

Phosphomolybdic acidpolyvinylpyrrolidone hybrid films were found to

show visible light photochromism It is identified that the intra-supramolecular

charge transfer between the inorganic and organic molecules is responsible

for the visible-light coloration Interestingly the films show photo-memory and

thermal activation The films show a small change in absorbance after being

irradiated with visible light for a short time and the coloration can be

enhanced greatly by subsequent thermal treatment Electrical measurements

indicate that the conductivity of the film increases after the brief irradiation

which promotes transfer of the electrons induced by the thermal treatment

[42]

In this work major effort was concentrated on passive thermal control

coatings based on photochromic and thermochromic materials The inorganic

photochromic materials were based on tungsten and molybdenum oxide films

and the organic photochromic materials included spiropyrans and

spirooxazines In addition photochromic composite organic-inorganic films

and thermochromic vanadium oxide films were prepared The samples were

synthesized using sputtering sol-gel process and thermal oxidation [43]

Polyoxometalates a class of oxidatively robust inorganic oxidants and

oxidation catalysts are currently under investigation at the Forest Products

Laboratory and at Emory University as an alternative to chlorinebased

chemicals in the bleaching of soft Woodkraft and other pulps Although

polyoxometalate salts are used in a number of industrial processes the

feasibility of using these salts and oxygen in the commercial bleaching of

chemical pulps was only recently demonstratedA clear advantage of

polyoxometalates over oxygen alone hydrogen peroxide or ozone is their

inherently high selectivity for the residual lignin in softwood kraft pulps The

goal of ongoing research is to develop a highly selective energy efficient

oxygen based polyoxometalate delignification and bleaching technology

compatible with mill closure [44]

14

Tungsten oxide exhibits pronounced photochromism upon bandgap

photoexcitation which makes it attractive and promising for applications in

many areas Some advances have been achieved during the past decades

The research on nanocrystalline films and single crystals indicates the critical

importance of defects in tungsten oxide to its photochromism Based on

energy-band engineering of semiconductors enhancement of photochromism

has been achieved for instance extension of the photoresponse from UV to

visible light by cathodic polarization improved change in absorption before

and after coloration through modification by a noble metal or another metal-

oxide semiconductor and increased photochromic reversibility via

hybridization with organic amines Nanocrystalline oxide films exhibit

controllable wettability which is coherent in nature with photochromism [45]

Polyoxometalates represent a diverse range of molecular clusters with

an almost unmatched range of physical properties and the ability to form

structures that can bridge several length scalesThe new building block

principles that have been discovered are beginning to allow the design of

complex clusters with desired properties and structures and several structural

types and novel physical properties are examinedIn this critical review the

synthetic and design approaches to the many polyoxometalate cluster types

are presented encompassing all the sub-types of polyoxometalates including

isopolyoxometa- lates heteropolyoxometalates and reduced molybdenum

blue systems As well as the fundamental structure and bonding aspectsthe

final section is devoted to discussing these clusters in the context of

contemporary and emerging interdisciplinary interests from areas as diverse

as antiviral agentsbiological ion transport modelsand materialsscience [46]

Keggin type molybdovanadophosphoric heteropoly acids were prepare

d by a novel environmentally benign method and their catalytic performances

were evaluated via hydroxylation of benzene to phenol with hydrogen

peroxide as oxidant in a mixed solvent of glacial acetic acid and acetonitrile

Various reaction parameters such as reaction time reaction temperature

ratio of benzene to hydrogen peroxide concentration of aqueous hydrogen

peroxide ratio of glacial acetic acid to acetonitrile in solvent and catalyst

concentration were changed to obtain an optimal reaction conditions

15

Molybdovanadophosphoric heteropoly acids are revealed to be highly

efficient catalyst for hydroxylation of benzene [47]

Thin films from the system (As2S3)Tl were deposited by thermal

evaporation on Si graphite and optical glass substratesFrom transmission

and reflection measurements of the thin films the refractive index (n) film

thickness(d) optical band gap(Eg) optical oscillator energy(Eo) and

dispersion energy(Ed) before and after exposure to light were determined

The results for optical parameters were analyzed using the Wemple - Di

Domenico single oscillator model and Lorenc-Lorenc equation It was found

that Eg decreases while n E0 and Ed increase for as deposited films

decreases while n E0 and Ed increase for as deposited films with increasing

of Tlconcentration passing through a maximum at 6 at of Tl After exposure

to light n E0 Ed increase and Eg decreases for all compositions

investigated The maximum change in n (Dn = 016 at l = 6328 nm) was

observed for thin As38S56Tl6 films From infrared spectroscopy measurements

of bulk glasses and thin films we could conclude that when up to 6 at of

thallium is introduced As-S-As chains break and a ternary TlAsS2

compound appears at 10 at Tl [48]

The organo - inorganic hybrid materialconsisting of Poly (34 Ethylene

Dioxythiophene) (PEDOT) doped with phosphomolybdate cluster anions

[PMo12O40]3-has been synthesized by direct insitu oxidative polymerization of

34-Ethylene Dioxythiophene (EDOT) with phosphomolybdic acid

(H3PMo12O40) Its characterization is investigated by Fourier Transform

Infrared Spectroscopy (FT-IR) and Scanning Electron Microscopy (SEM) The

hybrid material presents predominantly high electronic conductivities of

around 20 and 70 S cm1at 300 and 400 K respectively [49]

Heteropolyacids (HPAs) are known to be excellent re-dox catalysts In

combination with TiO2 HPAs can be used as photocatalysts active in visible

light The HPA accepts electron and get reduced to heteropolyblue (HPB)

That can absorb light in the visible range HPA can be incorporated onto the

external surface or in the pores of zeolite based composite photocatalysts

have been designed by incorporation of HPA semiconductor TiO2 and

transition metal cobalt on zeolite This composite metallozeolite photocatalyst

is efficient in photoreduction of methyl orange (MO) in visible light to the tune

16

of 411 mg of MO photo reducedg TiO2 This catalyst also shows encouraging

results for hydrogen evolution from water to the tune of 2730 micromolhg TiO

[50]

A new class of proton conducting glass membranes based on heteropo

lyacids such as phosphotungstic acid (PWA) as electrolytes for low

temperature H2O2 fuel cells was investigated Parameters for a single fuel cell

with a catalyst electrode of 015 mgcm2 of PtC and a glass composite

membrane were characterized by electrochemical measurements at open

circuit potential conditions The performance of the membrane electrode

assemblies (MEA) was systematically studied as an effect of SiO2 and P2O5

concentrations in the glass composite membrane and the MEA was found to

exhibit a maximum power density of 162 mW cm2 for an H2O2 fuel cell at

30 degC and 30 relative humidity (RH) [51]

Two new photochromic inorganic-organic hybrid materials formed from

Keggin type Polyoxometalates (POMs) and metronidazole (C6H9N3O3 MNZ)

formulated as H3PMo12O40bull3 MNZ3H2O (1) and H3PW12O40bull3MNZ3H2O(2)

were synthesized and characterized by elemental analysis IR spectra

electronic spectra electron spin resonance (ESR) spectra and TG-DTA

Reflectance spectra show the presence of weak inter molecular charge

transfer between the organic and inorganic moieties in the solid state The

photochromic properties were studied by solid diffuse reflectance spectra and

ESR spectra and the photochromic reactions were found to exhibit first-order

kinetics TG-DTA showed that two hybrid materials have similar thermal

behavior [52]

Heteropolycompounds (HPCs) have been a matter of interest in basic

and applied science for more than a century From their first synthesis many

advances have been made to promote the use of HPCs in different ways in

science and technology The aim of this article is to review the main structural

characteristics of heteropolycompounds of the Keggin type (12

tungstophosphoric12-molybdophosphoric12-tungstosilicic acid alkaline and

alkaline earth salts of12 tungstophosphoric acid and gels doped with HPCs)to

understand and explain their different activities such as high proton

conductivity and catalytic biochemical and biomedical activities [53]

17

A solid hybrid molecular material containing 1-butyl 3-methyl

imidazolium cations and Keggin anions of phosphotungstic acid has been

synthesized It is fully characterized by CHN analysis FTIR XRD UV-Vis-NIR

DRS 31P MAS NMR TGA and SEM The FTIR spectrum of the compound

shows the fingerprint vibrational bands of both Keggin molecular anions and

imidazolium cations The aromatic CndashH stretch region (2700ndash3250 cmndash1) of

imidazolium cation is split due to the interaction between the ring CndashH and

bulky Keggin anion The red-shift in the UV-Vis spectra and the downfield 31P

MAS NMR chemical shift also confirm the electrostatic interaction between

the ions in the compound Near IR spectral region (1000ndash2500 nm) shows the

elimination of water in the compound which is hydrophobic [54]

Transport coefficient measurements (electrical conductivity

thermoelectric power and Hall coefficient) have been performed on a

compact Tl033MoO3 polycrystalline compound in a wide temperature range

(200ndash400 K) Experimental results are interpreted with the help of a p-type

semiconductor model with two inverted deep levels near the midgap The

valence band and the conduction band are assumed to be formed from the

dxy orbitals of molybdenum atoms in the Mo6O22 cluster leading to narrow π-

bonding bands The donor and acceptor levels may be formed from

nonbonding dxy orbitals arising respectively from anionic and Tl+ defects

Electron paramagnetic resonance and magnetic measurements are in good

agreement with the theoretical band semiconductor model which has been

retained [55]

The optical properties of GaInTlAs epilayers grown at low temperature

~230degC by solid-source molecular-beam epitaxy on InP substrates were

characterized using optical absorptionand photoluminescence techniques

Optical absorption measurements a room temperature show a gap shrinkage

toward lower energies from 071 to 061 and 053 eV when the Tl content

increases from 0 to 4and 8in good agreement with theoretical

predictionsLow-temperature photoluminescence band-gap signals from

GaInAs and GaInTlAs layers are only obtained after rapid thermal annealing

performed inorder to improve the electronic quality of the layersA band gap

decrease as much as 41 meV for GaInTlAs with 19 Tl incorporation is

measured by photoluminescence at 8K [56]

18

The electrochromic performance of all solid ndash state cells employing

phosphotungstic acid and phosphomolybdic acid is reported These cells

employ SnO2 as the viewing electrode and graphite as the back electrodeThe

cells in the bleached state can be made white to red and become black in the

coloured state [57]

Various organic compounds were oxidized by molecular oxygen in the

presence of a catalytic amount of mixed addenda heteropolyoxometalates

containing molybdenum and vanadium The catalytic activity of the

Molybdovanadophosphate was found to be greatly enhanced by supporting

on charcoal The supported catalyst has high catalytic activity for oxidative

dehydrogenation of benzylic and allylic alcohols to the corresponding

aldehydes and ketones (46-92) nevertheless the nonsupported catalyst

was inactive for the same oxidations under these conditions 236Trimethyl

phenol was selectively oxidized to trimethyl-p benzoquinone which is

precursor of VitE in the presence of a catalytic amount of

molybdophosphate In addition the aerobic oxidation of amines alkyl-

substituted phenols and alkanes were also examined [58]

The reaction of Tl2CO3 with 111555-hexafluoro-24-pentanedione

and diglyme CH3O(CH2CH2O)2CH3 or tetraglyme CH3O(CH2CH2O)4CH3 in

dichloromethane yields the anhydrous thermally and air stable volatile Tl

diglyme and Tl tetraglyme adducts They have been characterized by single

crystal X-ray diffraction elemental analysis 1H and 13C NMR IR and mass

spectroscopy Thermal and mass-transport properties have been investigated

using thermo gravimetric and differential scanning calorimetric

measurements There is evidence that both precursors are very low melting

and volatile and can be used as liquid Tl sources Both adducts have been

successfully applied to metalndashorganic chemical vapor deposition of thallium

containing films [59]

Large size and high quality single crystals of quasi-two-dimensional

thallium molybdenum purple bronze TlMo6O17 have been grown by electrolytic

reduction of molten salt of Tl2CO3-MoO3 The crystal structure is trigonal with

space group P3m1 determined by X-ray diffraction and four-circle single crystal

diffraction The lattice parameters of the unit cell are a = b = 55282 Adeg and c

= 136991 Adeg The temperature dependence of resistivity and magnetic

19

susceptibility confirmed that a metal-to-metal transition occurs near 110 K

[60]

Hall coefficient and dc conductivity studies were made on p-type

Pb08Sn02Te thin films doped with different concentrations of thallium in the

temperature range 77 to 500 K The Hall coefficient and Hall mobility are

found to decrease with an increase in the doping concentration of thallium

Hall coefficient data have been analyzed in the light of a double valence-band

model Various band parameters such as valence band separation population

ratio mobility ratio and effective mass ratio have been calculated Hall

mobility data have been analyzed in the light of lattice and defect limited

scattering mechanisms [61]

Proton conducting composites of heteropolyacid hydrates phosphomol

ybdic acid H3PMo12O40 nH2O(PMA) phosphotungstic acid H3PW12O40 nH2O

( PTA) and salt hydrate like NiCl2 6H2O were prepared

with insulating Al2O3 as despersoidThe ionic conductivity peaks at two

concentrations of Al2O3 indicating two percolation thresholds for percolation

thresholds for proton conduction Two separate experiments were carried out

to check the existence of such percolation thresholds viz the volta battery

experiment involving the measurement of emf of an electrochemical cell

with composites of different compositions used as electrolyte and the

composition vs conductivity measured by the complex impedance

spectroscopy The presence of two maxima has been attributed to two

different percolation thresholds for the two possible mobile protonic

species H+ + (H3O+) and OH arising from the hydrates [62]

1 A5 Applications of Heteropolyoxometalates-

Applications of heteropolyanions centre depend on their redox properties

their high charges and ionic weights An enormous patent and journal

literature is devoted to the applications of heteropolyanions

1 Analysis-

The formation and subsequent precipitation or reduction of

[XMO12O40]n- anions form the basis of gravimetric and colorimetric analytical

methods for P As Si or Geeither separately or in combination [63 64]

20

2 Biochemical applications-

lsquoPhosphotungstic Acidsrsquo have for decades been used as precipitants

for proteins and as analytical reagents for proteinsalkaloidsand purines eg

the [P2M18O62]6- anions for colorimetric determination of uric acid [65] and

cholesterol The acid H3P12O40 either in aqueous or ethanolic solution is also

widely used as a non specific electron dense stain for electron spectroscopy

The dyestuffs industry has for many years used heteropolymolybdates and

tungstates to form color lakes and toners from basic dyes Large

heteropolyanions exhibit antiviral antitumoral properties at non-cytotoxic

doses in vitro and in vivo and are protein inhibitors of cellular bacterial and

viral DNA RNA polymerizes [66]

3 Catalysis

Heteropoly acids and salts have been used as heterogeneous catalysts

for a broad variety of reactions and compilations of such applications up to

1973 are available Examples include oxidation of propylene and isobutylene

to acrylic acid methacrylic acids and ammoxidation of acrylonitrile olefin

polymerization and epoxidation Much of current activity concerning

heterogeneous catalysis by heteropoly compounds is being carried out in

Japan [67 68]

4 Other Applications

Insoluble salts of Heteropolyanions especially ammonium

molybdophosphates have been used and are commercially available as ion-

exchange materials [69]Recent work in this area includes thin layer

chromatography of amino acids ion selective membranes [70] and the

preparation of new ion exchangers based on heteropolyanions Crystalline12-

tungstophosphoric and 12-molybdophosphoric acids are excellent protonic

conductors Heteropolyacids are electrochromic in the solid state as a

consequence of heteropolyblue formation Heteropolyblue formation has also

been used to detect alcohol or carboxylic acid radicals generated by radiolysis

of aqueous solutions Potential applications of heteropoly complexes as flame

retardants and smoke suppressants or as corrosion inhibitors and conversion

coatings on steel and aluminium are reported [71] Some potential ldquogreenrdquo

applications have been reported eg non-chlorine based wood pulp

bleaching process and a method of decontaminating water Some structures

21

containing transition metal atoms with unpaired electrons have unusual

magnetic properties and are being investigated as nano computer storage

devices Some compounds exhibit luminescence There are many reported

potential medicinal applications eg anti tumoral and anti-viral There have

been reports on the role of weak or non bonding interactions on the crystal

engineering of hybrid polyoxometalates

Spherical nonporous polyoxomolybdate based capsules of different

types containing more than 100 metal atoms reported by Achim Muller and his

group have versatile unique properties regarding their assembly to vesicles

and the chemistry which can be done inside the pores and cavities A discrete

polyoxometalate Lindquist ion of the form W6O192minus was successfully imaged

recently for the first time within the capillary of a carbon nanotube following

steric locking of the anion with the tubule In situ relaxation of the anion in its

equatorial plain was demonstrated [72]

22

Section-B

Litreture Survey on Chromism in Transition Metal

Oxides

1 B1 Chromism in Transition Metal Oxides

Chromism is a reversible change in a substances colour resulting from

a process caused by some form of stimulus Many materials are chromic

including inorganic and organic compounds and conducting polymers and the

property can result from many different mechanisms Several transition metal

oxides show EC properties The most popular are from the VI - B oxides In

this group WO3 and MoO3 are the most thoroughly studied cathodic EC

materials which can be electrochemically coloured and bleached when used

as the cathode in electrochemical cells Cathodic EC materials also include

V2O3 TiO2 and Nb2O5 Another distinguishable group is anodic EC material

including VIII oxides like IrOx nH2O Rh2O3 nH2O NiO nH2O etc which can be

anodicaly coloured in the electrochemical process when used as anode

There are several types of chromism which are discussed as below

B11 Photochromism

Photochromism is the reversible transformation of a chemical species

between two forms by the absorption of electromagnetic radiation where

the two forms have different absorption spectra [7374]

Trivially this can be described as a reversible change of color upon

exposure to light The phenomenon was discovered in the late 1880s

including work by Markwald who studied the reversible change of color of 23

44-tetrachloronaphthalen-1(4H)-one in the solid state He labeled this

phenomenon phototropy and this name was used until the 1950s

when Yehuda Hirshberg of the Weizmann Institute of Science in Israel

proposed the term photochromism [75] Photochromism can take place

in both organic and inorganic compounds and also has its place in biological

systems (for example retinal in the vision process)

Photochromism does not have a rigorous definition but is usually used

to describe compounds that undergo a reversible photochemical reaction

where an absorption band in the visible part of the electromagnetic spectrum

23

changes dramatically in strength or wavelength In many cases an

absorbance band is present in only one form The degree of change required

for a photochemical reaction to be dubbed photochromic is that which

appears dramatic by eye but in essence there is no dividing line between

photochromic reactions and other photochemistry Therefore while the

transcis isomerization of azobenzene is considered a photochromic reaction

the analogous reaction of stilbene is not Since photochromism is just a

special case of a photochemical reaction almost any photochemical reaction

type may be used to produce photochromism with appropriate molecular desi

gnSome of the most common processes involved in photochromism are peric

yclic reactions cis-trans somerizations intramolecular hydrogen transfer

intramolecular group transfers dissociation processes and electron transfers

(oxidation-reduction)

Another some what arbitrary requirement of photochromism is that

it requires the two states of the molecule to be thermally stable under

ambient conditions for a reasonable time All the same nitrospiropyran (which

back-isomerizes in the dark over ~10 minutes at room temperature) is

considered photochromic All photochromic molecules back-isomerize to their

more stable form at some rate and this back-isomerization is accelerated by

heating There is therefore a close relationship between photochromic and the

rmochromic compounds The timescale of thermal back-isomerization is

important for applications and may be molecularly engineered

Photochromic compounds considered to be thermally stable include some

diarylethenes which do not back isomerize even after heating at 800C for 3

months

Since photochromic chromophores are dyes and operate according to

well-known reactions their molecular engineering to fine-tune their properties

can be achieved relatively easily using known design models quantum

mechanics calculations and experimentation In particular the tuning of

absorbance bands to particular parts of the spectrum and the engineering

of thermal stability have received much attention

Sometimes and particularly in the dye industry the term irreversible

photochromic is used to describe materials that undergo a permanent color

change upon exposure to Ultraviolet or visible light radiation Because by

24

definition photochromics are reversible there is technically no such thing as a

n irreversible photochromic this is loose usage and these compounds

are better referred to as photochangable or photoreactive dyes

Apart from the qualities already mentioned several other properties of

photochromics are important for their use These include

Quantum yield of the photochemical reaction

This determined the efficiency of the photochromic change with respect

to the amount of light absorbed The quantum yield of isomerization

can be strongly dependent on conditions

Fatigue resistance In photochromic materials fatigue refers to the

loss of reversibility by processes such as photodegradation

photobleaching photooxidation and other side reactions All

photochromics suffer fatigue to some extent and its rate is strongly

dependent on the activating light and the conditions of the sample

Photostationary state Photochromic materials have two states and

their interconversion can be controlled using different wavelengths of

light Excitation with any given wavelength of light will result in a

mixture of the two states at a particular ratio called the photo-

stationary state In a perfect system there would exist wavelengths

that can be used to provide 10 and 01 ratios of the isomers

but in real systems this is not possible since the active

absorbance bands always overlap to some extent

Polarity and solubility In order to incorporate photochromics in

working systems they suffer the same issues as other dyes They are

often charged in one or more state leading to very high polarity and

possible large changes in polarity They also often contain large

conjugated systems that limit their solubility

Photochromic complexes

A photochromic complex is a kind of chemical compound that has

photoresponsive Parts on its ligand These complexes have a specific

structure photoswitchable organic compounds are attached to

metalcomplexes For the photocontrollable parts thermally and

photochemically stable chromophores (azobenzene diarylethene

25

spiropyran etc) are usually used And for the metal complexes a wide

variety of compounds that have various functions (redox response

luminescence magnetism etc) are applied The photochromic parts and

metal parts are so close that they can affect each others molecular

orbitals The physical properties of these compounds shown by parts of

them (ie chromophores or metals) thus can be controlled by switching

their other sites by external stimuli For example photoisomerization

behaviors of some complexes can be switched by oxidation and reduction of

their metal parts Some other compounds can be changed in their

luminescence behavior magnetic interaction of metal sites or stability of

metal-to-ligand coordination by photoisomerization of their photochromic

parts

Photochemistry of Polyoxometalates

The photochemistry of polyoxometalates is of great interest to inorganic

chemistsMore than 80 years agoit was found that the R-Keggin tungstate

H3[PW12O40] was reduced photochemically to yield a blue-colored species

which was reoxidized by air and by various other oxidizing agents such as

Fe3+AgNO3and H2O2 [7879]The photoredox reactions of H4[SiW12O40]and

H3[PW12O40] proceeded effectively in the presence of primary and secondary

alcohols their ethers and aldehydes and proteinsbut less effectively in the

presence of tertiary alcoholsketonesestersthe fatty acids above formic

acidand simple amines[8081] The basic photoredox reaction involving

ethanol is illustrated by eq 13

2 H3PW12O40 + H3CCH2OH h ν ν ν ν 2 H4PW12O40 + H3CCHO ------- 13

2 H4PW12O40 +12 O2 2 H3PW12O40 + H2O ------ 14

In this reactionone molecule of ethanol photochemically reduces two

molecules of H3PW12O40 and is itself oxidized to acetaldehyde In the

presence of air the thermal oxidation of the reduced species takes place at

room temperature(eq 14)The reduced polyoxometalates which are the so-

calledldquoheteropolybluesrdquo have been used for the colorimetric analysis of the

26

elements P Si As and Ge and for the determination of uric acidsugarand

other biological compounds [8283] Piperidinium metavanadate also

undergoes photoinduced coloration from white to black followed by a

reversible color change in the presence of oxidizing agentsHowever

ammonium metavanadates ([NH4][VO3]) exhibits no photoinduced

coloration[84] The early photoredox reactions of the R-Keggin

polyoxometalates H4[SiW12O40] and H3PW12O40 were carried out in the

presence of photographic paper however the limited number of the

structurally well-characterized compounds available for study delayed the

development of modern cluster-compound photochemistry until the discovery

of photochromism in alkylammonium polyoxo- molybdate solids[8586] A

photochromic or electrochromic material is one whose light-absorbing

properties are altered upon optical excitation or reduction under the influence

of an externally applied electric field respectively The induced coloration

remains even after the excitation source has been removed These materials

are of technological interest because they return to their original state either

thermally upon irradiation with light of a frequency corresponding to the

induced absorption or electrochemically upon reversing the polarity of the

externally applied electric field Thus photochromic and electrochromic

materials behave in a reversible manner Polyoxometalates exhibit significant

photo-and electrochromism which makes them suitable as nanocomposite

molecular devices and as models for probing the physical properties of infinite

metal oxides Since the metal ions in the oxidized polyoxometalates have d0

electronic configurations the only absorption band which occurs in the UV-vis

range of the electronic spectra is due to the oxygen-to-metal (O-M) ligand-to-

metal charge transfer (LMCT)Upon irradiation electrons are promoted from

the low-energy electronic states which are mainly comprised of oxygen 2p

orbitals (the valence band in the band model)to the high-energy electronic

states which are mainly comprised of metal d orbitalrsquos (the conduction band

in the band model)The fundamental transitions in polyoxometalate lattices

are depicted schematically in Fig1B1

27

Fig1B1 Simple model showing the electronic transitions in the

polyoxometalates containing electron donar and acceptor (a)generation

of charge carriers(b)electron and hole trapping(c)electron release due

to stimulation(d)recombination

between electron and hole Electrons are e-and holes are h+

In the polyoxometalates containing heteroatoms and especially in

mixed metal polyoxometalates the charge carriers which are created by the

light or electric field may be trapped in electron traps and hole traps These

traps provide states of localized energy in the O-M LMCT energy gap due to

the heteroatoms or counter cations which correspond to impurities or lattice

defects in the band model If the trap depth ∆E is large compared to kT the

probability for thermal escape from the trap will be negligibly small and

metastable situation will existThe trapped carriers can be released by thermal

or optical stimulationIn the case of thermal stimulation the irradiated

polyoxometalate is heated until the energy barrier ∆E can be overcome The

trapped electron (or hole) then can escape from the trap and nonradiatively

recombine with the trapped hole (or electron)Under optical stimulation the

energy of an incident photon is used to overcome ∆E The relaxation

processes of the OndashM LMCT excitation energy include both the nonradiative

recombination of electrons and holes within the energy gap and the

intramolecular energy transfer leading to a charge-transfer emission This

intramolecular energy transfer corresponds to the O-M LMCT energy gap and

occurs via radiative recombination and sensitized emission from the

28

heteroatoms or cations If several energy levels based on the hetero atoms or

counter cations act as energy acceptors within the O-M LMCTenergy gap the

energy transfer occurs from the O-M LMCT states to these levels followed by

the nonradiative or radiative deactivation of the excitation energy It should be

noted that the O-M LMCT states also can be generated by the application of

very high electric fields to the polyoxometalate solids as demonstrated by the

observation of electroluminescence[87]If an external electric field with a

potential more negative than the energy levels of the vacant orbitals involved

in the O-M LMCT transition is applied to a polyoxometalate on the electrode

surface an electrochemical reduction occurs via the injection of electrons

from the electrode in to the vacant levels of the polyoxometalate as shown in

Fig1B 2

Fig1B2 - Energy scheme for the electrochromism of polyoxometalates

a)electrochemical reduction (b) electrochemical oxidation

Electrons injected in to the high-energy levels also may be trapped by

electron traps in a process analogous to that which occurs during LMCT

photoexcitation of the polyoxometalates These electrons are returned to the

electrode by electrochemical oxidation at an electrode potential more positive

than the energy levels for the d1 electron states The d1electrons in the O-M

LMCTstates facilitate the absorption of visible light via intervalence charge

transfer among metal centers and d-d transitions The same type of transition

may be possible for the d1electron captured by the electron traps too In

addition to searching for new photosensitive polyoxometalates with the

29

potential for having practical application there is now a need to elucidate the

fundamental photo-and electrochemical coloration processes by studying

electron transfer within the polyoxometalate lattices in conjunction with their

crystal structures So far few polyoxometalates exhibit a perfect reversibility

of coloration The irreversibility of the color change arises from as yet

uncharacterized side reactions during both the coloration and decoloration of

the polyoxometalates

Many metal oxides including aluminum titanium vanadium niobium

molybdenum and tungsten oxides are photochromic when they contain

impurities or dopants This coloration has been interpreted on the basis of

electron trapping at appropriate lattice sites within the crystals as shown in

Fig1B1 where the O-M LMCT transition corresponds to the transition

between the valence and conduction bands for the infinite metal-oxide lattice

[88-91]

B12 Applications of Photochromic materials

Sunglasses

One of the most famous reversible photochromic applications is color

changing lenses for sunglasses as found in eyeglasses The largest limitation

in using PC technology is that the materials cannot be made stable enough

to withstand thousands of hours of outdoor exposure so long-term outdoor

applications are not appropriate at this time The switching speed of

photochromic dyes is highly sensitive to the rigidity of the environment around

the dye As result they switch most rapidly in solution and slowest in the rigid

environment like a polymer lens Recently it has been reported that attaching

flexible low Tg polymers (for example siloxanes or poly (butyl acrylate) to the

dyes allows them to switch much more rapidly in a rigid lens [76] Some

spirooxazines with siloxane polymers attached switch at near solution like

speeds even though they are in a rigid lens matrix

Supramolecular chemistry

Photochromic units have been employed extensively in supramolecular

chemistry Their ability to give a light controlled reversible shape change

means that they can be used to make or break molecular recognition motifs

30

or to cause a consequent shape change in their surroundings Thus

photochromic units have been demonstrated as components of molecular

switches The coupling of photochromic units to enzymes or enzyme cofactors

even provides the ability to reversibly turn enzymes on and off

by altering their shape or orientation in such a way that their functions

are either working or broken

Data storage

The possibility of using photochromic compounds for data storage was

first suggested in 1956 by Yehuda Hirshberg[77] Since that time there have

been many investigations by various academic and commercial groups

particularly in the area of 3D optical data storage which promises discs that

can hold a terabyte of data Initially issues with thermal back-reactions

and destructive reading dogged these studies but more recently more stable

systems have been developed

Novelty items

Reversible photochromics are also found in applications such as toys

cosmeticsclothing and industrial applications If necessary they can be made

to change between desired colors by combination with a permanent pigment

A large number of inorganic compounds exhibit photochromism

These solids often have large band gaps of the order of 3 - 12 eV and

excitation of these solids leads to the formation of metastable centers that

absorb visible light giving rise to their colour They can return to their ground

state by heating or by optical excitation within the colour-centre band In most

cases the photochromism is a structure sensitive phenomenon involving

localized defect impurities or dislocations Some of these inorganic

compounds have the potential for a number of different uses Photochromic

compounds have a number of useful applications These can be divided

according to the most important property that is being used (Table 11) [92]

31

Table 11 Applications of Photochromic materials

Applications Depending Upon

Sensitivity to

Radiation

Reversibility Thermal Chemical or

Physical Properties

Self-developing

photography

Chemical switches for

computers

Temperature indicators

Protective

materials

Data displays

Heat-sensitive recording

media

Optical signal

processing

Photomasking and

photoresist technology

Reusable data storage

media

Anaytical reagents

Photochromic

microimages

Photopolymerisation

Information encoding

and steganography

Photocontractile

polymers and the

photoviscosity effect

Control of light

intensity

Q-switches

Pyroelectric

photochromic materials

B13 Thermochromism

Thermochromism is the reversible colour change of a substance

induced by temperature change A large variety of substances organic

inorganic organometallic supramolecular and polymeric systems exhibit this

phenomenon Examples of these include bianthrones cobalt

hexacyanoferrate the zirconocene complex of 1 4-diphenyl-1 3-butadiene

and poly (3-alkylthiophene) The organic 99-bixanthenylidene is colourless at

90 K yellow-green at 298 K and dark-blue when melted at 592 K Heating

conducting polymers can cause them to change colour This is achieved by

causing conformational changes to the polymer backbone resulting in a

change in the band gap of the polymer It has been reported that regioregular

32

P3HT reversibly changes colour upon heating to 220ordmC due to temperature-

dependent conformation changes Thermally cross linked polymer undergoes

the same colour change but it is much less reversible [93] Other forms of

thermochromism may be commercially important eg to give a visual

indication of temperature changes

B14 Electrochromism

Electrochromism describes a phenomenon of material color change in

a persistent but reversible manner produced by electrochemically induced

oxidation-reduction reactions Electrochromic materials can be applied to

various kinds of products such as smart windows and display devices Among

those applications there have been lots of efforts to develop electrochromic

display devices (ECDs) Especially flexible display devices are now attracting

much attention worldwide since they can facilitate new technological demands

such as bending and folding of paper-like displays High electrochromic

efficiency short response time long operating life time and reduction of

energy consumption are the most important requisites of the materials for the

paperlike displays [94 95] Among those properties the operation life time is

the most important barrier to overcome for a realization of ECDs There are

two types of electrochromic material a) inorganic transition metal oxides

(TMOs) b) organic polymer materials The TMOs have been studied longer

than the organic materials that they have been studied since 1960s [9697]

Electrochromism describes a reversible color change of material

produced by electrochemically induced oxidation-reduction reactions It is one

of several types of chromism of materials As thermochromism and

photochromism mean material color changes made by heat and light

respectively electrochromism refers that the color change is caused by an

electric potential In most cases the color change in electrochromism can be

driven by rather low electrical potential of the order of a fraction of volt or a

few volts [94-96] The color change of material means variation in

transmittance andor reflectance change in visible range which is originated

from different electronic absorption bands according to a switching between

oxidation and reduction state of material When electric potential is applied on

electrochromic material forced oxidation or reduction is derived and the

33

individual color is originated from the corresponding oxidation or reduction

state of the material For electrochromic materials the characteristic color

change is reversible since the oxidation and reduction state can be converted

reversibly by switching potential

Application fields

The application area for the electrochromism is rather broad that it

covers from smart window glazing and optical modulators to information

displays [98-102] The smart windows are typical examples The

electrochromic property is used to control the amount of light and heat to pass

through the windows Usually the electrochromic material is in form of thin film

coated on a window glass The transmittance modulation has also been

applied at the automobiles to automatically tint rear-view mirrors in various

lighting conditions The electrochromic application fields are illustrated in Fig

1B3

Since the smart windows control the transmittance of heat as well as

the transmittance of visible light the working definition of electrochromism has

now been extended to include devices for modulation of radiation in the near

infrared thermal infrared and microwave regions When color for

electrochromic materials is used this can now mean a response by detectors

at these wavelengths and not just by the human eyes Nowadays

electrochromic material draws much attention as being used in the display

devices Electrochromic display device (ECD) is being considered as one of

the candidates for the conventional liquid crystal display (LCD) since ECD

has many advantages over LCDs Among these advantages the most

important are low energy consumption wider viewing angle high contrast

rate and possibility to achieve multiple colors with a single material [103]

When a new redox state of electrochromic materialis established by

the applied electric pulse then it is maintained after the potential is switched

off This means the colored or bleached state of the material can be sustained

for a considerable time without applying electrical power This is so-called

ldquocolor memory effectrdquo of electrochromic material Because of the color

memory effect energy consumption for the electrochromic display device

could be drastically reduced and this would be a big advantage over other

34

emissive devices The low power consumption is especially advantageous

when it is applied to mobile devices with limited power source The possibility

for a flexible display is another attraction for electrochromic material

Information displays

Real-view mirrors for automobiles

Fig 1B4 Application fields of electrochromic devices Smart windows information displays and real-view mirrors for automobiles

35

Using polymeric electrochromic material and plastic substrate with a

transparent conductive coating it is possible to build all-plastic flexible display

device There are lots of papers and patents about the flexible display devices

[104105] In these cases gel-type electrolyte is also needed The gel-type

electrolyte immobilizes liquid electrolyte in the polymer network [106107]

Recently ITO-coated polymer films are readily available which would provide

the plastic substrate for a flexible device The flexible electrochromic displays

are frequently tried with the plastic substrate flexible electrochromic material

and gel-type electrolyte The flexible electrochromic displays would facilitate

increasing technical demands for foldable display devices

Metal Ion Electrochromism

Many transition metal oxides are capable of redox reactions that result in

colour change Metal oxide films are commonly prepared as thin layers of

either tungsten nickel molybdenum or other metal compounds by a number

of techniques These include sol-gel electrochemical by dc or rfreactive

sputtering techniques electron-beam evaporation by anodic or cathodic

electrodeposition or by solution dipping of the electrochromic metal

compounds (or compounds that can be changed into these metal compounds)

onto optically transparent electrodes (OTE) [108 -114] Their electrochromism

is derived from the colour change associated with a change in the oxidation

state of the metal anion The behaviour of these materials is dependent upon

pH moisture and exposure to the atmosphere [115] Generally the switching

rates of these films is somewhat slow with typical switching times of about 15

- 60 seconds to achieve 100 conversion to either coloured or bleached state

[116 -120] An Example of this includes nickel oxide which changes from

transparent (pale green) to brownblack taking about 30 seconds to do so

[121] Other examples include [(NH4)5Ru]2(pyrazine)5+ and [(NH4)5Ru]2(44-

bipyridine)5+ whose electrochromism is significantly different due to the effect

of the ligand [122] Table 12 below gives some examples of metal oxide films

with electrochromic properties

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

14

Tungsten oxide exhibits pronounced photochromism upon bandgap

photoexcitation which makes it attractive and promising for applications in

many areas Some advances have been achieved during the past decades

The research on nanocrystalline films and single crystals indicates the critical

importance of defects in tungsten oxide to its photochromism Based on

energy-band engineering of semiconductors enhancement of photochromism

has been achieved for instance extension of the photoresponse from UV to

visible light by cathodic polarization improved change in absorption before

and after coloration through modification by a noble metal or another metal-

oxide semiconductor and increased photochromic reversibility via

hybridization with organic amines Nanocrystalline oxide films exhibit

controllable wettability which is coherent in nature with photochromism [45]

Polyoxometalates represent a diverse range of molecular clusters with

an almost unmatched range of physical properties and the ability to form

structures that can bridge several length scalesThe new building block

principles that have been discovered are beginning to allow the design of

complex clusters with desired properties and structures and several structural

types and novel physical properties are examinedIn this critical review the

synthetic and design approaches to the many polyoxometalate cluster types

are presented encompassing all the sub-types of polyoxometalates including

isopolyoxometa- lates heteropolyoxometalates and reduced molybdenum

blue systems As well as the fundamental structure and bonding aspectsthe

final section is devoted to discussing these clusters in the context of

contemporary and emerging interdisciplinary interests from areas as diverse

as antiviral agentsbiological ion transport modelsand materialsscience [46]

Keggin type molybdovanadophosphoric heteropoly acids were prepare

d by a novel environmentally benign method and their catalytic performances

were evaluated via hydroxylation of benzene to phenol with hydrogen

peroxide as oxidant in a mixed solvent of glacial acetic acid and acetonitrile

Various reaction parameters such as reaction time reaction temperature

ratio of benzene to hydrogen peroxide concentration of aqueous hydrogen

peroxide ratio of glacial acetic acid to acetonitrile in solvent and catalyst

concentration were changed to obtain an optimal reaction conditions

15

Molybdovanadophosphoric heteropoly acids are revealed to be highly

efficient catalyst for hydroxylation of benzene [47]

Thin films from the system (As2S3)Tl were deposited by thermal

evaporation on Si graphite and optical glass substratesFrom transmission

and reflection measurements of the thin films the refractive index (n) film

thickness(d) optical band gap(Eg) optical oscillator energy(Eo) and

dispersion energy(Ed) before and after exposure to light were determined

The results for optical parameters were analyzed using the Wemple - Di

Domenico single oscillator model and Lorenc-Lorenc equation It was found

that Eg decreases while n E0 and Ed increase for as deposited films

decreases while n E0 and Ed increase for as deposited films with increasing

of Tlconcentration passing through a maximum at 6 at of Tl After exposure

to light n E0 Ed increase and Eg decreases for all compositions

investigated The maximum change in n (Dn = 016 at l = 6328 nm) was

observed for thin As38S56Tl6 films From infrared spectroscopy measurements

of bulk glasses and thin films we could conclude that when up to 6 at of

thallium is introduced As-S-As chains break and a ternary TlAsS2

compound appears at 10 at Tl [48]

The organo - inorganic hybrid materialconsisting of Poly (34 Ethylene

Dioxythiophene) (PEDOT) doped with phosphomolybdate cluster anions

[PMo12O40]3-has been synthesized by direct insitu oxidative polymerization of

34-Ethylene Dioxythiophene (EDOT) with phosphomolybdic acid

(H3PMo12O40) Its characterization is investigated by Fourier Transform

Infrared Spectroscopy (FT-IR) and Scanning Electron Microscopy (SEM) The

hybrid material presents predominantly high electronic conductivities of

around 20 and 70 S cm1at 300 and 400 K respectively [49]

Heteropolyacids (HPAs) are known to be excellent re-dox catalysts In

combination with TiO2 HPAs can be used as photocatalysts active in visible

light The HPA accepts electron and get reduced to heteropolyblue (HPB)

That can absorb light in the visible range HPA can be incorporated onto the

external surface or in the pores of zeolite based composite photocatalysts

have been designed by incorporation of HPA semiconductor TiO2 and

transition metal cobalt on zeolite This composite metallozeolite photocatalyst

is efficient in photoreduction of methyl orange (MO) in visible light to the tune

16

of 411 mg of MO photo reducedg TiO2 This catalyst also shows encouraging

results for hydrogen evolution from water to the tune of 2730 micromolhg TiO

[50]

A new class of proton conducting glass membranes based on heteropo

lyacids such as phosphotungstic acid (PWA) as electrolytes for low

temperature H2O2 fuel cells was investigated Parameters for a single fuel cell

with a catalyst electrode of 015 mgcm2 of PtC and a glass composite

membrane were characterized by electrochemical measurements at open

circuit potential conditions The performance of the membrane electrode

assemblies (MEA) was systematically studied as an effect of SiO2 and P2O5

concentrations in the glass composite membrane and the MEA was found to

exhibit a maximum power density of 162 mW cm2 for an H2O2 fuel cell at

30 degC and 30 relative humidity (RH) [51]

Two new photochromic inorganic-organic hybrid materials formed from

Keggin type Polyoxometalates (POMs) and metronidazole (C6H9N3O3 MNZ)

formulated as H3PMo12O40bull3 MNZ3H2O (1) and H3PW12O40bull3MNZ3H2O(2)

were synthesized and characterized by elemental analysis IR spectra

electronic spectra electron spin resonance (ESR) spectra and TG-DTA

Reflectance spectra show the presence of weak inter molecular charge

transfer between the organic and inorganic moieties in the solid state The

photochromic properties were studied by solid diffuse reflectance spectra and

ESR spectra and the photochromic reactions were found to exhibit first-order

kinetics TG-DTA showed that two hybrid materials have similar thermal

behavior [52]

Heteropolycompounds (HPCs) have been a matter of interest in basic

and applied science for more than a century From their first synthesis many

advances have been made to promote the use of HPCs in different ways in

science and technology The aim of this article is to review the main structural

characteristics of heteropolycompounds of the Keggin type (12

tungstophosphoric12-molybdophosphoric12-tungstosilicic acid alkaline and

alkaline earth salts of12 tungstophosphoric acid and gels doped with HPCs)to

understand and explain their different activities such as high proton

conductivity and catalytic biochemical and biomedical activities [53]

17

A solid hybrid molecular material containing 1-butyl 3-methyl

imidazolium cations and Keggin anions of phosphotungstic acid has been

synthesized It is fully characterized by CHN analysis FTIR XRD UV-Vis-NIR

DRS 31P MAS NMR TGA and SEM The FTIR spectrum of the compound

shows the fingerprint vibrational bands of both Keggin molecular anions and

imidazolium cations The aromatic CndashH stretch region (2700ndash3250 cmndash1) of

imidazolium cation is split due to the interaction between the ring CndashH and

bulky Keggin anion The red-shift in the UV-Vis spectra and the downfield 31P

MAS NMR chemical shift also confirm the electrostatic interaction between

the ions in the compound Near IR spectral region (1000ndash2500 nm) shows the

elimination of water in the compound which is hydrophobic [54]

Transport coefficient measurements (electrical conductivity

thermoelectric power and Hall coefficient) have been performed on a

compact Tl033MoO3 polycrystalline compound in a wide temperature range

(200ndash400 K) Experimental results are interpreted with the help of a p-type

semiconductor model with two inverted deep levels near the midgap The

valence band and the conduction band are assumed to be formed from the

dxy orbitals of molybdenum atoms in the Mo6O22 cluster leading to narrow π-

bonding bands The donor and acceptor levels may be formed from

nonbonding dxy orbitals arising respectively from anionic and Tl+ defects

Electron paramagnetic resonance and magnetic measurements are in good

agreement with the theoretical band semiconductor model which has been

retained [55]

The optical properties of GaInTlAs epilayers grown at low temperature

~230degC by solid-source molecular-beam epitaxy on InP substrates were

characterized using optical absorptionand photoluminescence techniques

Optical absorption measurements a room temperature show a gap shrinkage

toward lower energies from 071 to 061 and 053 eV when the Tl content

increases from 0 to 4and 8in good agreement with theoretical

predictionsLow-temperature photoluminescence band-gap signals from

GaInAs and GaInTlAs layers are only obtained after rapid thermal annealing

performed inorder to improve the electronic quality of the layersA band gap

decrease as much as 41 meV for GaInTlAs with 19 Tl incorporation is

measured by photoluminescence at 8K [56]

18

The electrochromic performance of all solid ndash state cells employing

phosphotungstic acid and phosphomolybdic acid is reported These cells

employ SnO2 as the viewing electrode and graphite as the back electrodeThe

cells in the bleached state can be made white to red and become black in the

coloured state [57]

Various organic compounds were oxidized by molecular oxygen in the

presence of a catalytic amount of mixed addenda heteropolyoxometalates

containing molybdenum and vanadium The catalytic activity of the

Molybdovanadophosphate was found to be greatly enhanced by supporting

on charcoal The supported catalyst has high catalytic activity for oxidative

dehydrogenation of benzylic and allylic alcohols to the corresponding

aldehydes and ketones (46-92) nevertheless the nonsupported catalyst

was inactive for the same oxidations under these conditions 236Trimethyl

phenol was selectively oxidized to trimethyl-p benzoquinone which is

precursor of VitE in the presence of a catalytic amount of

molybdophosphate In addition the aerobic oxidation of amines alkyl-

substituted phenols and alkanes were also examined [58]

The reaction of Tl2CO3 with 111555-hexafluoro-24-pentanedione

and diglyme CH3O(CH2CH2O)2CH3 or tetraglyme CH3O(CH2CH2O)4CH3 in

dichloromethane yields the anhydrous thermally and air stable volatile Tl

diglyme and Tl tetraglyme adducts They have been characterized by single

crystal X-ray diffraction elemental analysis 1H and 13C NMR IR and mass

spectroscopy Thermal and mass-transport properties have been investigated

using thermo gravimetric and differential scanning calorimetric

measurements There is evidence that both precursors are very low melting

and volatile and can be used as liquid Tl sources Both adducts have been

successfully applied to metalndashorganic chemical vapor deposition of thallium

containing films [59]

Large size and high quality single crystals of quasi-two-dimensional

thallium molybdenum purple bronze TlMo6O17 have been grown by electrolytic

reduction of molten salt of Tl2CO3-MoO3 The crystal structure is trigonal with

space group P3m1 determined by X-ray diffraction and four-circle single crystal

diffraction The lattice parameters of the unit cell are a = b = 55282 Adeg and c

= 136991 Adeg The temperature dependence of resistivity and magnetic

19

susceptibility confirmed that a metal-to-metal transition occurs near 110 K

[60]

Hall coefficient and dc conductivity studies were made on p-type

Pb08Sn02Te thin films doped with different concentrations of thallium in the

temperature range 77 to 500 K The Hall coefficient and Hall mobility are

found to decrease with an increase in the doping concentration of thallium

Hall coefficient data have been analyzed in the light of a double valence-band

model Various band parameters such as valence band separation population

ratio mobility ratio and effective mass ratio have been calculated Hall

mobility data have been analyzed in the light of lattice and defect limited

scattering mechanisms [61]

Proton conducting composites of heteropolyacid hydrates phosphomol

ybdic acid H3PMo12O40 nH2O(PMA) phosphotungstic acid H3PW12O40 nH2O

( PTA) and salt hydrate like NiCl2 6H2O were prepared

with insulating Al2O3 as despersoidThe ionic conductivity peaks at two

concentrations of Al2O3 indicating two percolation thresholds for percolation

thresholds for proton conduction Two separate experiments were carried out

to check the existence of such percolation thresholds viz the volta battery

experiment involving the measurement of emf of an electrochemical cell

with composites of different compositions used as electrolyte and the

composition vs conductivity measured by the complex impedance

spectroscopy The presence of two maxima has been attributed to two

different percolation thresholds for the two possible mobile protonic

species H+ + (H3O+) and OH arising from the hydrates [62]

1 A5 Applications of Heteropolyoxometalates-

Applications of heteropolyanions centre depend on their redox properties

their high charges and ionic weights An enormous patent and journal

literature is devoted to the applications of heteropolyanions

1 Analysis-

The formation and subsequent precipitation or reduction of

[XMO12O40]n- anions form the basis of gravimetric and colorimetric analytical

methods for P As Si or Geeither separately or in combination [63 64]

20

2 Biochemical applications-

lsquoPhosphotungstic Acidsrsquo have for decades been used as precipitants

for proteins and as analytical reagents for proteinsalkaloidsand purines eg

the [P2M18O62]6- anions for colorimetric determination of uric acid [65] and

cholesterol The acid H3P12O40 either in aqueous or ethanolic solution is also

widely used as a non specific electron dense stain for electron spectroscopy

The dyestuffs industry has for many years used heteropolymolybdates and

tungstates to form color lakes and toners from basic dyes Large

heteropolyanions exhibit antiviral antitumoral properties at non-cytotoxic

doses in vitro and in vivo and are protein inhibitors of cellular bacterial and

viral DNA RNA polymerizes [66]

3 Catalysis

Heteropoly acids and salts have been used as heterogeneous catalysts

for a broad variety of reactions and compilations of such applications up to

1973 are available Examples include oxidation of propylene and isobutylene

to acrylic acid methacrylic acids and ammoxidation of acrylonitrile olefin

polymerization and epoxidation Much of current activity concerning

heterogeneous catalysis by heteropoly compounds is being carried out in

Japan [67 68]

4 Other Applications

Insoluble salts of Heteropolyanions especially ammonium

molybdophosphates have been used and are commercially available as ion-

exchange materials [69]Recent work in this area includes thin layer

chromatography of amino acids ion selective membranes [70] and the

preparation of new ion exchangers based on heteropolyanions Crystalline12-

tungstophosphoric and 12-molybdophosphoric acids are excellent protonic

conductors Heteropolyacids are electrochromic in the solid state as a

consequence of heteropolyblue formation Heteropolyblue formation has also

been used to detect alcohol or carboxylic acid radicals generated by radiolysis

of aqueous solutions Potential applications of heteropoly complexes as flame

retardants and smoke suppressants or as corrosion inhibitors and conversion

coatings on steel and aluminium are reported [71] Some potential ldquogreenrdquo

applications have been reported eg non-chlorine based wood pulp

bleaching process and a method of decontaminating water Some structures

21

containing transition metal atoms with unpaired electrons have unusual

magnetic properties and are being investigated as nano computer storage

devices Some compounds exhibit luminescence There are many reported

potential medicinal applications eg anti tumoral and anti-viral There have

been reports on the role of weak or non bonding interactions on the crystal

engineering of hybrid polyoxometalates

Spherical nonporous polyoxomolybdate based capsules of different

types containing more than 100 metal atoms reported by Achim Muller and his

group have versatile unique properties regarding their assembly to vesicles

and the chemistry which can be done inside the pores and cavities A discrete

polyoxometalate Lindquist ion of the form W6O192minus was successfully imaged

recently for the first time within the capillary of a carbon nanotube following

steric locking of the anion with the tubule In situ relaxation of the anion in its

equatorial plain was demonstrated [72]

22

Section-B

Litreture Survey on Chromism in Transition Metal

Oxides

1 B1 Chromism in Transition Metal Oxides

Chromism is a reversible change in a substances colour resulting from

a process caused by some form of stimulus Many materials are chromic

including inorganic and organic compounds and conducting polymers and the

property can result from many different mechanisms Several transition metal

oxides show EC properties The most popular are from the VI - B oxides In

this group WO3 and MoO3 are the most thoroughly studied cathodic EC

materials which can be electrochemically coloured and bleached when used

as the cathode in electrochemical cells Cathodic EC materials also include

V2O3 TiO2 and Nb2O5 Another distinguishable group is anodic EC material

including VIII oxides like IrOx nH2O Rh2O3 nH2O NiO nH2O etc which can be

anodicaly coloured in the electrochemical process when used as anode

There are several types of chromism which are discussed as below

B11 Photochromism

Photochromism is the reversible transformation of a chemical species

between two forms by the absorption of electromagnetic radiation where

the two forms have different absorption spectra [7374]

Trivially this can be described as a reversible change of color upon

exposure to light The phenomenon was discovered in the late 1880s

including work by Markwald who studied the reversible change of color of 23

44-tetrachloronaphthalen-1(4H)-one in the solid state He labeled this

phenomenon phototropy and this name was used until the 1950s

when Yehuda Hirshberg of the Weizmann Institute of Science in Israel

proposed the term photochromism [75] Photochromism can take place

in both organic and inorganic compounds and also has its place in biological

systems (for example retinal in the vision process)

Photochromism does not have a rigorous definition but is usually used

to describe compounds that undergo a reversible photochemical reaction

where an absorption band in the visible part of the electromagnetic spectrum

23

changes dramatically in strength or wavelength In many cases an

absorbance band is present in only one form The degree of change required

for a photochemical reaction to be dubbed photochromic is that which

appears dramatic by eye but in essence there is no dividing line between

photochromic reactions and other photochemistry Therefore while the

transcis isomerization of azobenzene is considered a photochromic reaction

the analogous reaction of stilbene is not Since photochromism is just a

special case of a photochemical reaction almost any photochemical reaction

type may be used to produce photochromism with appropriate molecular desi

gnSome of the most common processes involved in photochromism are peric

yclic reactions cis-trans somerizations intramolecular hydrogen transfer

intramolecular group transfers dissociation processes and electron transfers

(oxidation-reduction)

Another some what arbitrary requirement of photochromism is that

it requires the two states of the molecule to be thermally stable under

ambient conditions for a reasonable time All the same nitrospiropyran (which

back-isomerizes in the dark over ~10 minutes at room temperature) is

considered photochromic All photochromic molecules back-isomerize to their

more stable form at some rate and this back-isomerization is accelerated by

heating There is therefore a close relationship between photochromic and the

rmochromic compounds The timescale of thermal back-isomerization is

important for applications and may be molecularly engineered

Photochromic compounds considered to be thermally stable include some

diarylethenes which do not back isomerize even after heating at 800C for 3

months

Since photochromic chromophores are dyes and operate according to

well-known reactions their molecular engineering to fine-tune their properties

can be achieved relatively easily using known design models quantum

mechanics calculations and experimentation In particular the tuning of

absorbance bands to particular parts of the spectrum and the engineering

of thermal stability have received much attention

Sometimes and particularly in the dye industry the term irreversible

photochromic is used to describe materials that undergo a permanent color

change upon exposure to Ultraviolet or visible light radiation Because by

24

definition photochromics are reversible there is technically no such thing as a

n irreversible photochromic this is loose usage and these compounds

are better referred to as photochangable or photoreactive dyes

Apart from the qualities already mentioned several other properties of

photochromics are important for their use These include

Quantum yield of the photochemical reaction

This determined the efficiency of the photochromic change with respect

to the amount of light absorbed The quantum yield of isomerization

can be strongly dependent on conditions

Fatigue resistance In photochromic materials fatigue refers to the

loss of reversibility by processes such as photodegradation

photobleaching photooxidation and other side reactions All

photochromics suffer fatigue to some extent and its rate is strongly

dependent on the activating light and the conditions of the sample

Photostationary state Photochromic materials have two states and

their interconversion can be controlled using different wavelengths of

light Excitation with any given wavelength of light will result in a

mixture of the two states at a particular ratio called the photo-

stationary state In a perfect system there would exist wavelengths

that can be used to provide 10 and 01 ratios of the isomers

but in real systems this is not possible since the active

absorbance bands always overlap to some extent

Polarity and solubility In order to incorporate photochromics in

working systems they suffer the same issues as other dyes They are

often charged in one or more state leading to very high polarity and

possible large changes in polarity They also often contain large

conjugated systems that limit their solubility

Photochromic complexes

A photochromic complex is a kind of chemical compound that has

photoresponsive Parts on its ligand These complexes have a specific

structure photoswitchable organic compounds are attached to

metalcomplexes For the photocontrollable parts thermally and

photochemically stable chromophores (azobenzene diarylethene

25

spiropyran etc) are usually used And for the metal complexes a wide

variety of compounds that have various functions (redox response

luminescence magnetism etc) are applied The photochromic parts and

metal parts are so close that they can affect each others molecular

orbitals The physical properties of these compounds shown by parts of

them (ie chromophores or metals) thus can be controlled by switching

their other sites by external stimuli For example photoisomerization

behaviors of some complexes can be switched by oxidation and reduction of

their metal parts Some other compounds can be changed in their

luminescence behavior magnetic interaction of metal sites or stability of

metal-to-ligand coordination by photoisomerization of their photochromic

parts

Photochemistry of Polyoxometalates

The photochemistry of polyoxometalates is of great interest to inorganic

chemistsMore than 80 years agoit was found that the R-Keggin tungstate

H3[PW12O40] was reduced photochemically to yield a blue-colored species

which was reoxidized by air and by various other oxidizing agents such as

Fe3+AgNO3and H2O2 [7879]The photoredox reactions of H4[SiW12O40]and

H3[PW12O40] proceeded effectively in the presence of primary and secondary

alcohols their ethers and aldehydes and proteinsbut less effectively in the

presence of tertiary alcoholsketonesestersthe fatty acids above formic

acidand simple amines[8081] The basic photoredox reaction involving

ethanol is illustrated by eq 13

2 H3PW12O40 + H3CCH2OH h ν ν ν ν 2 H4PW12O40 + H3CCHO ------- 13

2 H4PW12O40 +12 O2 2 H3PW12O40 + H2O ------ 14

In this reactionone molecule of ethanol photochemically reduces two

molecules of H3PW12O40 and is itself oxidized to acetaldehyde In the

presence of air the thermal oxidation of the reduced species takes place at

room temperature(eq 14)The reduced polyoxometalates which are the so-

calledldquoheteropolybluesrdquo have been used for the colorimetric analysis of the

26

elements P Si As and Ge and for the determination of uric acidsugarand

other biological compounds [8283] Piperidinium metavanadate also

undergoes photoinduced coloration from white to black followed by a

reversible color change in the presence of oxidizing agentsHowever

ammonium metavanadates ([NH4][VO3]) exhibits no photoinduced

coloration[84] The early photoredox reactions of the R-Keggin

polyoxometalates H4[SiW12O40] and H3PW12O40 were carried out in the

presence of photographic paper however the limited number of the

structurally well-characterized compounds available for study delayed the

development of modern cluster-compound photochemistry until the discovery

of photochromism in alkylammonium polyoxo- molybdate solids[8586] A

photochromic or electrochromic material is one whose light-absorbing

properties are altered upon optical excitation or reduction under the influence

of an externally applied electric field respectively The induced coloration

remains even after the excitation source has been removed These materials

are of technological interest because they return to their original state either

thermally upon irradiation with light of a frequency corresponding to the

induced absorption or electrochemically upon reversing the polarity of the

externally applied electric field Thus photochromic and electrochromic

materials behave in a reversible manner Polyoxometalates exhibit significant

photo-and electrochromism which makes them suitable as nanocomposite

molecular devices and as models for probing the physical properties of infinite

metal oxides Since the metal ions in the oxidized polyoxometalates have d0

electronic configurations the only absorption band which occurs in the UV-vis

range of the electronic spectra is due to the oxygen-to-metal (O-M) ligand-to-

metal charge transfer (LMCT)Upon irradiation electrons are promoted from

the low-energy electronic states which are mainly comprised of oxygen 2p

orbitals (the valence band in the band model)to the high-energy electronic

states which are mainly comprised of metal d orbitalrsquos (the conduction band

in the band model)The fundamental transitions in polyoxometalate lattices

are depicted schematically in Fig1B1

27

Fig1B1 Simple model showing the electronic transitions in the

polyoxometalates containing electron donar and acceptor (a)generation

of charge carriers(b)electron and hole trapping(c)electron release due

to stimulation(d)recombination

between electron and hole Electrons are e-and holes are h+

In the polyoxometalates containing heteroatoms and especially in

mixed metal polyoxometalates the charge carriers which are created by the

light or electric field may be trapped in electron traps and hole traps These

traps provide states of localized energy in the O-M LMCT energy gap due to

the heteroatoms or counter cations which correspond to impurities or lattice

defects in the band model If the trap depth ∆E is large compared to kT the

probability for thermal escape from the trap will be negligibly small and

metastable situation will existThe trapped carriers can be released by thermal

or optical stimulationIn the case of thermal stimulation the irradiated

polyoxometalate is heated until the energy barrier ∆E can be overcome The

trapped electron (or hole) then can escape from the trap and nonradiatively

recombine with the trapped hole (or electron)Under optical stimulation the

energy of an incident photon is used to overcome ∆E The relaxation

processes of the OndashM LMCT excitation energy include both the nonradiative

recombination of electrons and holes within the energy gap and the

intramolecular energy transfer leading to a charge-transfer emission This

intramolecular energy transfer corresponds to the O-M LMCT energy gap and

occurs via radiative recombination and sensitized emission from the

28

heteroatoms or cations If several energy levels based on the hetero atoms or

counter cations act as energy acceptors within the O-M LMCTenergy gap the

energy transfer occurs from the O-M LMCT states to these levels followed by

the nonradiative or radiative deactivation of the excitation energy It should be

noted that the O-M LMCT states also can be generated by the application of

very high electric fields to the polyoxometalate solids as demonstrated by the

observation of electroluminescence[87]If an external electric field with a

potential more negative than the energy levels of the vacant orbitals involved

in the O-M LMCT transition is applied to a polyoxometalate on the electrode

surface an electrochemical reduction occurs via the injection of electrons

from the electrode in to the vacant levels of the polyoxometalate as shown in

Fig1B 2

Fig1B2 - Energy scheme for the electrochromism of polyoxometalates

a)electrochemical reduction (b) electrochemical oxidation

Electrons injected in to the high-energy levels also may be trapped by

electron traps in a process analogous to that which occurs during LMCT

photoexcitation of the polyoxometalates These electrons are returned to the

electrode by electrochemical oxidation at an electrode potential more positive

than the energy levels for the d1 electron states The d1electrons in the O-M

LMCTstates facilitate the absorption of visible light via intervalence charge

transfer among metal centers and d-d transitions The same type of transition

may be possible for the d1electron captured by the electron traps too In

addition to searching for new photosensitive polyoxometalates with the

29

potential for having practical application there is now a need to elucidate the

fundamental photo-and electrochemical coloration processes by studying

electron transfer within the polyoxometalate lattices in conjunction with their

crystal structures So far few polyoxometalates exhibit a perfect reversibility

of coloration The irreversibility of the color change arises from as yet

uncharacterized side reactions during both the coloration and decoloration of

the polyoxometalates

Many metal oxides including aluminum titanium vanadium niobium

molybdenum and tungsten oxides are photochromic when they contain

impurities or dopants This coloration has been interpreted on the basis of

electron trapping at appropriate lattice sites within the crystals as shown in

Fig1B1 where the O-M LMCT transition corresponds to the transition

between the valence and conduction bands for the infinite metal-oxide lattice

[88-91]

B12 Applications of Photochromic materials

Sunglasses

One of the most famous reversible photochromic applications is color

changing lenses for sunglasses as found in eyeglasses The largest limitation

in using PC technology is that the materials cannot be made stable enough

to withstand thousands of hours of outdoor exposure so long-term outdoor

applications are not appropriate at this time The switching speed of

photochromic dyes is highly sensitive to the rigidity of the environment around

the dye As result they switch most rapidly in solution and slowest in the rigid

environment like a polymer lens Recently it has been reported that attaching

flexible low Tg polymers (for example siloxanes or poly (butyl acrylate) to the

dyes allows them to switch much more rapidly in a rigid lens [76] Some

spirooxazines with siloxane polymers attached switch at near solution like

speeds even though they are in a rigid lens matrix

Supramolecular chemistry

Photochromic units have been employed extensively in supramolecular

chemistry Their ability to give a light controlled reversible shape change

means that they can be used to make or break molecular recognition motifs

30

or to cause a consequent shape change in their surroundings Thus

photochromic units have been demonstrated as components of molecular

switches The coupling of photochromic units to enzymes or enzyme cofactors

even provides the ability to reversibly turn enzymes on and off

by altering their shape or orientation in such a way that their functions

are either working or broken

Data storage

The possibility of using photochromic compounds for data storage was

first suggested in 1956 by Yehuda Hirshberg[77] Since that time there have

been many investigations by various academic and commercial groups

particularly in the area of 3D optical data storage which promises discs that

can hold a terabyte of data Initially issues with thermal back-reactions

and destructive reading dogged these studies but more recently more stable

systems have been developed

Novelty items

Reversible photochromics are also found in applications such as toys

cosmeticsclothing and industrial applications If necessary they can be made

to change between desired colors by combination with a permanent pigment

A large number of inorganic compounds exhibit photochromism

These solids often have large band gaps of the order of 3 - 12 eV and

excitation of these solids leads to the formation of metastable centers that

absorb visible light giving rise to their colour They can return to their ground

state by heating or by optical excitation within the colour-centre band In most

cases the photochromism is a structure sensitive phenomenon involving

localized defect impurities or dislocations Some of these inorganic

compounds have the potential for a number of different uses Photochromic

compounds have a number of useful applications These can be divided

according to the most important property that is being used (Table 11) [92]

31

Table 11 Applications of Photochromic materials

Applications Depending Upon

Sensitivity to

Radiation

Reversibility Thermal Chemical or

Physical Properties

Self-developing

photography

Chemical switches for

computers

Temperature indicators

Protective

materials

Data displays

Heat-sensitive recording

media

Optical signal

processing

Photomasking and

photoresist technology

Reusable data storage

media

Anaytical reagents

Photochromic

microimages

Photopolymerisation

Information encoding

and steganography

Photocontractile

polymers and the

photoviscosity effect

Control of light

intensity

Q-switches

Pyroelectric

photochromic materials

B13 Thermochromism

Thermochromism is the reversible colour change of a substance

induced by temperature change A large variety of substances organic

inorganic organometallic supramolecular and polymeric systems exhibit this

phenomenon Examples of these include bianthrones cobalt

hexacyanoferrate the zirconocene complex of 1 4-diphenyl-1 3-butadiene

and poly (3-alkylthiophene) The organic 99-bixanthenylidene is colourless at

90 K yellow-green at 298 K and dark-blue when melted at 592 K Heating

conducting polymers can cause them to change colour This is achieved by

causing conformational changes to the polymer backbone resulting in a

change in the band gap of the polymer It has been reported that regioregular

32

P3HT reversibly changes colour upon heating to 220ordmC due to temperature-

dependent conformation changes Thermally cross linked polymer undergoes

the same colour change but it is much less reversible [93] Other forms of

thermochromism may be commercially important eg to give a visual

indication of temperature changes

B14 Electrochromism

Electrochromism describes a phenomenon of material color change in

a persistent but reversible manner produced by electrochemically induced

oxidation-reduction reactions Electrochromic materials can be applied to

various kinds of products such as smart windows and display devices Among

those applications there have been lots of efforts to develop electrochromic

display devices (ECDs) Especially flexible display devices are now attracting

much attention worldwide since they can facilitate new technological demands

such as bending and folding of paper-like displays High electrochromic

efficiency short response time long operating life time and reduction of

energy consumption are the most important requisites of the materials for the

paperlike displays [94 95] Among those properties the operation life time is

the most important barrier to overcome for a realization of ECDs There are

two types of electrochromic material a) inorganic transition metal oxides

(TMOs) b) organic polymer materials The TMOs have been studied longer

than the organic materials that they have been studied since 1960s [9697]

Electrochromism describes a reversible color change of material

produced by electrochemically induced oxidation-reduction reactions It is one

of several types of chromism of materials As thermochromism and

photochromism mean material color changes made by heat and light

respectively electrochromism refers that the color change is caused by an

electric potential In most cases the color change in electrochromism can be

driven by rather low electrical potential of the order of a fraction of volt or a

few volts [94-96] The color change of material means variation in

transmittance andor reflectance change in visible range which is originated

from different electronic absorption bands according to a switching between

oxidation and reduction state of material When electric potential is applied on

electrochromic material forced oxidation or reduction is derived and the

33

individual color is originated from the corresponding oxidation or reduction

state of the material For electrochromic materials the characteristic color

change is reversible since the oxidation and reduction state can be converted

reversibly by switching potential

Application fields

The application area for the electrochromism is rather broad that it

covers from smart window glazing and optical modulators to information

displays [98-102] The smart windows are typical examples The

electrochromic property is used to control the amount of light and heat to pass

through the windows Usually the electrochromic material is in form of thin film

coated on a window glass The transmittance modulation has also been

applied at the automobiles to automatically tint rear-view mirrors in various

lighting conditions The electrochromic application fields are illustrated in Fig

1B3

Since the smart windows control the transmittance of heat as well as

the transmittance of visible light the working definition of electrochromism has

now been extended to include devices for modulation of radiation in the near

infrared thermal infrared and microwave regions When color for

electrochromic materials is used this can now mean a response by detectors

at these wavelengths and not just by the human eyes Nowadays

electrochromic material draws much attention as being used in the display

devices Electrochromic display device (ECD) is being considered as one of

the candidates for the conventional liquid crystal display (LCD) since ECD

has many advantages over LCDs Among these advantages the most

important are low energy consumption wider viewing angle high contrast

rate and possibility to achieve multiple colors with a single material [103]

When a new redox state of electrochromic materialis established by

the applied electric pulse then it is maintained after the potential is switched

off This means the colored or bleached state of the material can be sustained

for a considerable time without applying electrical power This is so-called

ldquocolor memory effectrdquo of electrochromic material Because of the color

memory effect energy consumption for the electrochromic display device

could be drastically reduced and this would be a big advantage over other

34

emissive devices The low power consumption is especially advantageous

when it is applied to mobile devices with limited power source The possibility

for a flexible display is another attraction for electrochromic material

Information displays

Real-view mirrors for automobiles

Fig 1B4 Application fields of electrochromic devices Smart windows information displays and real-view mirrors for automobiles

35

Using polymeric electrochromic material and plastic substrate with a

transparent conductive coating it is possible to build all-plastic flexible display

device There are lots of papers and patents about the flexible display devices

[104105] In these cases gel-type electrolyte is also needed The gel-type

electrolyte immobilizes liquid electrolyte in the polymer network [106107]

Recently ITO-coated polymer films are readily available which would provide

the plastic substrate for a flexible device The flexible electrochromic displays

are frequently tried with the plastic substrate flexible electrochromic material

and gel-type electrolyte The flexible electrochromic displays would facilitate

increasing technical demands for foldable display devices

Metal Ion Electrochromism

Many transition metal oxides are capable of redox reactions that result in

colour change Metal oxide films are commonly prepared as thin layers of

either tungsten nickel molybdenum or other metal compounds by a number

of techniques These include sol-gel electrochemical by dc or rfreactive

sputtering techniques electron-beam evaporation by anodic or cathodic

electrodeposition or by solution dipping of the electrochromic metal

compounds (or compounds that can be changed into these metal compounds)

onto optically transparent electrodes (OTE) [108 -114] Their electrochromism

is derived from the colour change associated with a change in the oxidation

state of the metal anion The behaviour of these materials is dependent upon

pH moisture and exposure to the atmosphere [115] Generally the switching

rates of these films is somewhat slow with typical switching times of about 15

- 60 seconds to achieve 100 conversion to either coloured or bleached state

[116 -120] An Example of this includes nickel oxide which changes from

transparent (pale green) to brownblack taking about 30 seconds to do so

[121] Other examples include [(NH4)5Ru]2(pyrazine)5+ and [(NH4)5Ru]2(44-

bipyridine)5+ whose electrochromism is significantly different due to the effect

of the ligand [122] Table 12 below gives some examples of metal oxide films

with electrochromic properties

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

15

Molybdovanadophosphoric heteropoly acids are revealed to be highly

efficient catalyst for hydroxylation of benzene [47]

Thin films from the system (As2S3)Tl were deposited by thermal

evaporation on Si graphite and optical glass substratesFrom transmission

and reflection measurements of the thin films the refractive index (n) film

thickness(d) optical band gap(Eg) optical oscillator energy(Eo) and

dispersion energy(Ed) before and after exposure to light were determined

The results for optical parameters were analyzed using the Wemple - Di

Domenico single oscillator model and Lorenc-Lorenc equation It was found

that Eg decreases while n E0 and Ed increase for as deposited films

decreases while n E0 and Ed increase for as deposited films with increasing

of Tlconcentration passing through a maximum at 6 at of Tl After exposure

to light n E0 Ed increase and Eg decreases for all compositions

investigated The maximum change in n (Dn = 016 at l = 6328 nm) was

observed for thin As38S56Tl6 films From infrared spectroscopy measurements

of bulk glasses and thin films we could conclude that when up to 6 at of

thallium is introduced As-S-As chains break and a ternary TlAsS2

compound appears at 10 at Tl [48]

The organo - inorganic hybrid materialconsisting of Poly (34 Ethylene

Dioxythiophene) (PEDOT) doped with phosphomolybdate cluster anions

[PMo12O40]3-has been synthesized by direct insitu oxidative polymerization of

34-Ethylene Dioxythiophene (EDOT) with phosphomolybdic acid

(H3PMo12O40) Its characterization is investigated by Fourier Transform

Infrared Spectroscopy (FT-IR) and Scanning Electron Microscopy (SEM) The

hybrid material presents predominantly high electronic conductivities of

around 20 and 70 S cm1at 300 and 400 K respectively [49]

Heteropolyacids (HPAs) are known to be excellent re-dox catalysts In

combination with TiO2 HPAs can be used as photocatalysts active in visible

light The HPA accepts electron and get reduced to heteropolyblue (HPB)

That can absorb light in the visible range HPA can be incorporated onto the

external surface or in the pores of zeolite based composite photocatalysts

have been designed by incorporation of HPA semiconductor TiO2 and

transition metal cobalt on zeolite This composite metallozeolite photocatalyst

is efficient in photoreduction of methyl orange (MO) in visible light to the tune

16

of 411 mg of MO photo reducedg TiO2 This catalyst also shows encouraging

results for hydrogen evolution from water to the tune of 2730 micromolhg TiO

[50]

A new class of proton conducting glass membranes based on heteropo

lyacids such as phosphotungstic acid (PWA) as electrolytes for low

temperature H2O2 fuel cells was investigated Parameters for a single fuel cell

with a catalyst electrode of 015 mgcm2 of PtC and a glass composite

membrane were characterized by electrochemical measurements at open

circuit potential conditions The performance of the membrane electrode

assemblies (MEA) was systematically studied as an effect of SiO2 and P2O5

concentrations in the glass composite membrane and the MEA was found to

exhibit a maximum power density of 162 mW cm2 for an H2O2 fuel cell at

30 degC and 30 relative humidity (RH) [51]

Two new photochromic inorganic-organic hybrid materials formed from

Keggin type Polyoxometalates (POMs) and metronidazole (C6H9N3O3 MNZ)

formulated as H3PMo12O40bull3 MNZ3H2O (1) and H3PW12O40bull3MNZ3H2O(2)

were synthesized and characterized by elemental analysis IR spectra

electronic spectra electron spin resonance (ESR) spectra and TG-DTA

Reflectance spectra show the presence of weak inter molecular charge

transfer between the organic and inorganic moieties in the solid state The

photochromic properties were studied by solid diffuse reflectance spectra and

ESR spectra and the photochromic reactions were found to exhibit first-order

kinetics TG-DTA showed that two hybrid materials have similar thermal

behavior [52]

Heteropolycompounds (HPCs) have been a matter of interest in basic

and applied science for more than a century From their first synthesis many

advances have been made to promote the use of HPCs in different ways in

science and technology The aim of this article is to review the main structural

characteristics of heteropolycompounds of the Keggin type (12

tungstophosphoric12-molybdophosphoric12-tungstosilicic acid alkaline and

alkaline earth salts of12 tungstophosphoric acid and gels doped with HPCs)to

understand and explain their different activities such as high proton

conductivity and catalytic biochemical and biomedical activities [53]

17

A solid hybrid molecular material containing 1-butyl 3-methyl

imidazolium cations and Keggin anions of phosphotungstic acid has been

synthesized It is fully characterized by CHN analysis FTIR XRD UV-Vis-NIR

DRS 31P MAS NMR TGA and SEM The FTIR spectrum of the compound

shows the fingerprint vibrational bands of both Keggin molecular anions and

imidazolium cations The aromatic CndashH stretch region (2700ndash3250 cmndash1) of

imidazolium cation is split due to the interaction between the ring CndashH and

bulky Keggin anion The red-shift in the UV-Vis spectra and the downfield 31P

MAS NMR chemical shift also confirm the electrostatic interaction between

the ions in the compound Near IR spectral region (1000ndash2500 nm) shows the

elimination of water in the compound which is hydrophobic [54]

Transport coefficient measurements (electrical conductivity

thermoelectric power and Hall coefficient) have been performed on a

compact Tl033MoO3 polycrystalline compound in a wide temperature range

(200ndash400 K) Experimental results are interpreted with the help of a p-type

semiconductor model with two inverted deep levels near the midgap The

valence band and the conduction band are assumed to be formed from the

dxy orbitals of molybdenum atoms in the Mo6O22 cluster leading to narrow π-

bonding bands The donor and acceptor levels may be formed from

nonbonding dxy orbitals arising respectively from anionic and Tl+ defects

Electron paramagnetic resonance and magnetic measurements are in good

agreement with the theoretical band semiconductor model which has been

retained [55]

The optical properties of GaInTlAs epilayers grown at low temperature

~230degC by solid-source molecular-beam epitaxy on InP substrates were

characterized using optical absorptionand photoluminescence techniques

Optical absorption measurements a room temperature show a gap shrinkage

toward lower energies from 071 to 061 and 053 eV when the Tl content

increases from 0 to 4and 8in good agreement with theoretical

predictionsLow-temperature photoluminescence band-gap signals from

GaInAs and GaInTlAs layers are only obtained after rapid thermal annealing

performed inorder to improve the electronic quality of the layersA band gap

decrease as much as 41 meV for GaInTlAs with 19 Tl incorporation is

measured by photoluminescence at 8K [56]

18

The electrochromic performance of all solid ndash state cells employing

phosphotungstic acid and phosphomolybdic acid is reported These cells

employ SnO2 as the viewing electrode and graphite as the back electrodeThe

cells in the bleached state can be made white to red and become black in the

coloured state [57]

Various organic compounds were oxidized by molecular oxygen in the

presence of a catalytic amount of mixed addenda heteropolyoxometalates

containing molybdenum and vanadium The catalytic activity of the

Molybdovanadophosphate was found to be greatly enhanced by supporting

on charcoal The supported catalyst has high catalytic activity for oxidative

dehydrogenation of benzylic and allylic alcohols to the corresponding

aldehydes and ketones (46-92) nevertheless the nonsupported catalyst

was inactive for the same oxidations under these conditions 236Trimethyl

phenol was selectively oxidized to trimethyl-p benzoquinone which is

precursor of VitE in the presence of a catalytic amount of

molybdophosphate In addition the aerobic oxidation of amines alkyl-

substituted phenols and alkanes were also examined [58]

The reaction of Tl2CO3 with 111555-hexafluoro-24-pentanedione

and diglyme CH3O(CH2CH2O)2CH3 or tetraglyme CH3O(CH2CH2O)4CH3 in

dichloromethane yields the anhydrous thermally and air stable volatile Tl

diglyme and Tl tetraglyme adducts They have been characterized by single

crystal X-ray diffraction elemental analysis 1H and 13C NMR IR and mass

spectroscopy Thermal and mass-transport properties have been investigated

using thermo gravimetric and differential scanning calorimetric

measurements There is evidence that both precursors are very low melting

and volatile and can be used as liquid Tl sources Both adducts have been

successfully applied to metalndashorganic chemical vapor deposition of thallium

containing films [59]

Large size and high quality single crystals of quasi-two-dimensional

thallium molybdenum purple bronze TlMo6O17 have been grown by electrolytic

reduction of molten salt of Tl2CO3-MoO3 The crystal structure is trigonal with

space group P3m1 determined by X-ray diffraction and four-circle single crystal

diffraction The lattice parameters of the unit cell are a = b = 55282 Adeg and c

= 136991 Adeg The temperature dependence of resistivity and magnetic

19

susceptibility confirmed that a metal-to-metal transition occurs near 110 K

[60]

Hall coefficient and dc conductivity studies were made on p-type

Pb08Sn02Te thin films doped with different concentrations of thallium in the

temperature range 77 to 500 K The Hall coefficient and Hall mobility are

found to decrease with an increase in the doping concentration of thallium

Hall coefficient data have been analyzed in the light of a double valence-band

model Various band parameters such as valence band separation population

ratio mobility ratio and effective mass ratio have been calculated Hall

mobility data have been analyzed in the light of lattice and defect limited

scattering mechanisms [61]

Proton conducting composites of heteropolyacid hydrates phosphomol

ybdic acid H3PMo12O40 nH2O(PMA) phosphotungstic acid H3PW12O40 nH2O

( PTA) and salt hydrate like NiCl2 6H2O were prepared

with insulating Al2O3 as despersoidThe ionic conductivity peaks at two

concentrations of Al2O3 indicating two percolation thresholds for percolation

thresholds for proton conduction Two separate experiments were carried out

to check the existence of such percolation thresholds viz the volta battery

experiment involving the measurement of emf of an electrochemical cell

with composites of different compositions used as electrolyte and the

composition vs conductivity measured by the complex impedance

spectroscopy The presence of two maxima has been attributed to two

different percolation thresholds for the two possible mobile protonic

species H+ + (H3O+) and OH arising from the hydrates [62]

1 A5 Applications of Heteropolyoxometalates-

Applications of heteropolyanions centre depend on their redox properties

their high charges and ionic weights An enormous patent and journal

literature is devoted to the applications of heteropolyanions

1 Analysis-

The formation and subsequent precipitation or reduction of

[XMO12O40]n- anions form the basis of gravimetric and colorimetric analytical

methods for P As Si or Geeither separately or in combination [63 64]

20

2 Biochemical applications-

lsquoPhosphotungstic Acidsrsquo have for decades been used as precipitants

for proteins and as analytical reagents for proteinsalkaloidsand purines eg

the [P2M18O62]6- anions for colorimetric determination of uric acid [65] and

cholesterol The acid H3P12O40 either in aqueous or ethanolic solution is also

widely used as a non specific electron dense stain for electron spectroscopy

The dyestuffs industry has for many years used heteropolymolybdates and

tungstates to form color lakes and toners from basic dyes Large

heteropolyanions exhibit antiviral antitumoral properties at non-cytotoxic

doses in vitro and in vivo and are protein inhibitors of cellular bacterial and

viral DNA RNA polymerizes [66]

3 Catalysis

Heteropoly acids and salts have been used as heterogeneous catalysts

for a broad variety of reactions and compilations of such applications up to

1973 are available Examples include oxidation of propylene and isobutylene

to acrylic acid methacrylic acids and ammoxidation of acrylonitrile olefin

polymerization and epoxidation Much of current activity concerning

heterogeneous catalysis by heteropoly compounds is being carried out in

Japan [67 68]

4 Other Applications

Insoluble salts of Heteropolyanions especially ammonium

molybdophosphates have been used and are commercially available as ion-

exchange materials [69]Recent work in this area includes thin layer

chromatography of amino acids ion selective membranes [70] and the

preparation of new ion exchangers based on heteropolyanions Crystalline12-

tungstophosphoric and 12-molybdophosphoric acids are excellent protonic

conductors Heteropolyacids are electrochromic in the solid state as a

consequence of heteropolyblue formation Heteropolyblue formation has also

been used to detect alcohol or carboxylic acid radicals generated by radiolysis

of aqueous solutions Potential applications of heteropoly complexes as flame

retardants and smoke suppressants or as corrosion inhibitors and conversion

coatings on steel and aluminium are reported [71] Some potential ldquogreenrdquo

applications have been reported eg non-chlorine based wood pulp

bleaching process and a method of decontaminating water Some structures

21

containing transition metal atoms with unpaired electrons have unusual

magnetic properties and are being investigated as nano computer storage

devices Some compounds exhibit luminescence There are many reported

potential medicinal applications eg anti tumoral and anti-viral There have

been reports on the role of weak or non bonding interactions on the crystal

engineering of hybrid polyoxometalates

Spherical nonporous polyoxomolybdate based capsules of different

types containing more than 100 metal atoms reported by Achim Muller and his

group have versatile unique properties regarding their assembly to vesicles

and the chemistry which can be done inside the pores and cavities A discrete

polyoxometalate Lindquist ion of the form W6O192minus was successfully imaged

recently for the first time within the capillary of a carbon nanotube following

steric locking of the anion with the tubule In situ relaxation of the anion in its

equatorial plain was demonstrated [72]

22

Section-B

Litreture Survey on Chromism in Transition Metal

Oxides

1 B1 Chromism in Transition Metal Oxides

Chromism is a reversible change in a substances colour resulting from

a process caused by some form of stimulus Many materials are chromic

including inorganic and organic compounds and conducting polymers and the

property can result from many different mechanisms Several transition metal

oxides show EC properties The most popular are from the VI - B oxides In

this group WO3 and MoO3 are the most thoroughly studied cathodic EC

materials which can be electrochemically coloured and bleached when used

as the cathode in electrochemical cells Cathodic EC materials also include

V2O3 TiO2 and Nb2O5 Another distinguishable group is anodic EC material

including VIII oxides like IrOx nH2O Rh2O3 nH2O NiO nH2O etc which can be

anodicaly coloured in the electrochemical process when used as anode

There are several types of chromism which are discussed as below

B11 Photochromism

Photochromism is the reversible transformation of a chemical species

between two forms by the absorption of electromagnetic radiation where

the two forms have different absorption spectra [7374]

Trivially this can be described as a reversible change of color upon

exposure to light The phenomenon was discovered in the late 1880s

including work by Markwald who studied the reversible change of color of 23

44-tetrachloronaphthalen-1(4H)-one in the solid state He labeled this

phenomenon phototropy and this name was used until the 1950s

when Yehuda Hirshberg of the Weizmann Institute of Science in Israel

proposed the term photochromism [75] Photochromism can take place

in both organic and inorganic compounds and also has its place in biological

systems (for example retinal in the vision process)

Photochromism does not have a rigorous definition but is usually used

to describe compounds that undergo a reversible photochemical reaction

where an absorption band in the visible part of the electromagnetic spectrum

23

changes dramatically in strength or wavelength In many cases an

absorbance band is present in only one form The degree of change required

for a photochemical reaction to be dubbed photochromic is that which

appears dramatic by eye but in essence there is no dividing line between

photochromic reactions and other photochemistry Therefore while the

transcis isomerization of azobenzene is considered a photochromic reaction

the analogous reaction of stilbene is not Since photochromism is just a

special case of a photochemical reaction almost any photochemical reaction

type may be used to produce photochromism with appropriate molecular desi

gnSome of the most common processes involved in photochromism are peric

yclic reactions cis-trans somerizations intramolecular hydrogen transfer

intramolecular group transfers dissociation processes and electron transfers

(oxidation-reduction)

Another some what arbitrary requirement of photochromism is that

it requires the two states of the molecule to be thermally stable under

ambient conditions for a reasonable time All the same nitrospiropyran (which

back-isomerizes in the dark over ~10 minutes at room temperature) is

considered photochromic All photochromic molecules back-isomerize to their

more stable form at some rate and this back-isomerization is accelerated by

heating There is therefore a close relationship between photochromic and the

rmochromic compounds The timescale of thermal back-isomerization is

important for applications and may be molecularly engineered

Photochromic compounds considered to be thermally stable include some

diarylethenes which do not back isomerize even after heating at 800C for 3

months

Since photochromic chromophores are dyes and operate according to

well-known reactions their molecular engineering to fine-tune their properties

can be achieved relatively easily using known design models quantum

mechanics calculations and experimentation In particular the tuning of

absorbance bands to particular parts of the spectrum and the engineering

of thermal stability have received much attention

Sometimes and particularly in the dye industry the term irreversible

photochromic is used to describe materials that undergo a permanent color

change upon exposure to Ultraviolet or visible light radiation Because by

24

definition photochromics are reversible there is technically no such thing as a

n irreversible photochromic this is loose usage and these compounds

are better referred to as photochangable or photoreactive dyes

Apart from the qualities already mentioned several other properties of

photochromics are important for their use These include

Quantum yield of the photochemical reaction

This determined the efficiency of the photochromic change with respect

to the amount of light absorbed The quantum yield of isomerization

can be strongly dependent on conditions

Fatigue resistance In photochromic materials fatigue refers to the

loss of reversibility by processes such as photodegradation

photobleaching photooxidation and other side reactions All

photochromics suffer fatigue to some extent and its rate is strongly

dependent on the activating light and the conditions of the sample

Photostationary state Photochromic materials have two states and

their interconversion can be controlled using different wavelengths of

light Excitation with any given wavelength of light will result in a

mixture of the two states at a particular ratio called the photo-

stationary state In a perfect system there would exist wavelengths

that can be used to provide 10 and 01 ratios of the isomers

but in real systems this is not possible since the active

absorbance bands always overlap to some extent

Polarity and solubility In order to incorporate photochromics in

working systems they suffer the same issues as other dyes They are

often charged in one or more state leading to very high polarity and

possible large changes in polarity They also often contain large

conjugated systems that limit their solubility

Photochromic complexes

A photochromic complex is a kind of chemical compound that has

photoresponsive Parts on its ligand These complexes have a specific

structure photoswitchable organic compounds are attached to

metalcomplexes For the photocontrollable parts thermally and

photochemically stable chromophores (azobenzene diarylethene

25

spiropyran etc) are usually used And for the metal complexes a wide

variety of compounds that have various functions (redox response

luminescence magnetism etc) are applied The photochromic parts and

metal parts are so close that they can affect each others molecular

orbitals The physical properties of these compounds shown by parts of

them (ie chromophores or metals) thus can be controlled by switching

their other sites by external stimuli For example photoisomerization

behaviors of some complexes can be switched by oxidation and reduction of

their metal parts Some other compounds can be changed in their

luminescence behavior magnetic interaction of metal sites or stability of

metal-to-ligand coordination by photoisomerization of their photochromic

parts

Photochemistry of Polyoxometalates

The photochemistry of polyoxometalates is of great interest to inorganic

chemistsMore than 80 years agoit was found that the R-Keggin tungstate

H3[PW12O40] was reduced photochemically to yield a blue-colored species

which was reoxidized by air and by various other oxidizing agents such as

Fe3+AgNO3and H2O2 [7879]The photoredox reactions of H4[SiW12O40]and

H3[PW12O40] proceeded effectively in the presence of primary and secondary

alcohols their ethers and aldehydes and proteinsbut less effectively in the

presence of tertiary alcoholsketonesestersthe fatty acids above formic

acidand simple amines[8081] The basic photoredox reaction involving

ethanol is illustrated by eq 13

2 H3PW12O40 + H3CCH2OH h ν ν ν ν 2 H4PW12O40 + H3CCHO ------- 13

2 H4PW12O40 +12 O2 2 H3PW12O40 + H2O ------ 14

In this reactionone molecule of ethanol photochemically reduces two

molecules of H3PW12O40 and is itself oxidized to acetaldehyde In the

presence of air the thermal oxidation of the reduced species takes place at

room temperature(eq 14)The reduced polyoxometalates which are the so-

calledldquoheteropolybluesrdquo have been used for the colorimetric analysis of the

26

elements P Si As and Ge and for the determination of uric acidsugarand

other biological compounds [8283] Piperidinium metavanadate also

undergoes photoinduced coloration from white to black followed by a

reversible color change in the presence of oxidizing agentsHowever

ammonium metavanadates ([NH4][VO3]) exhibits no photoinduced

coloration[84] The early photoredox reactions of the R-Keggin

polyoxometalates H4[SiW12O40] and H3PW12O40 were carried out in the

presence of photographic paper however the limited number of the

structurally well-characterized compounds available for study delayed the

development of modern cluster-compound photochemistry until the discovery

of photochromism in alkylammonium polyoxo- molybdate solids[8586] A

photochromic or electrochromic material is one whose light-absorbing

properties are altered upon optical excitation or reduction under the influence

of an externally applied electric field respectively The induced coloration

remains even after the excitation source has been removed These materials

are of technological interest because they return to their original state either

thermally upon irradiation with light of a frequency corresponding to the

induced absorption or electrochemically upon reversing the polarity of the

externally applied electric field Thus photochromic and electrochromic

materials behave in a reversible manner Polyoxometalates exhibit significant

photo-and electrochromism which makes them suitable as nanocomposite

molecular devices and as models for probing the physical properties of infinite

metal oxides Since the metal ions in the oxidized polyoxometalates have d0

electronic configurations the only absorption band which occurs in the UV-vis

range of the electronic spectra is due to the oxygen-to-metal (O-M) ligand-to-

metal charge transfer (LMCT)Upon irradiation electrons are promoted from

the low-energy electronic states which are mainly comprised of oxygen 2p

orbitals (the valence band in the band model)to the high-energy electronic

states which are mainly comprised of metal d orbitalrsquos (the conduction band

in the band model)The fundamental transitions in polyoxometalate lattices

are depicted schematically in Fig1B1

27

Fig1B1 Simple model showing the electronic transitions in the

polyoxometalates containing electron donar and acceptor (a)generation

of charge carriers(b)electron and hole trapping(c)electron release due

to stimulation(d)recombination

between electron and hole Electrons are e-and holes are h+

In the polyoxometalates containing heteroatoms and especially in

mixed metal polyoxometalates the charge carriers which are created by the

light or electric field may be trapped in electron traps and hole traps These

traps provide states of localized energy in the O-M LMCT energy gap due to

the heteroatoms or counter cations which correspond to impurities or lattice

defects in the band model If the trap depth ∆E is large compared to kT the

probability for thermal escape from the trap will be negligibly small and

metastable situation will existThe trapped carriers can be released by thermal

or optical stimulationIn the case of thermal stimulation the irradiated

polyoxometalate is heated until the energy barrier ∆E can be overcome The

trapped electron (or hole) then can escape from the trap and nonradiatively

recombine with the trapped hole (or electron)Under optical stimulation the

energy of an incident photon is used to overcome ∆E The relaxation

processes of the OndashM LMCT excitation energy include both the nonradiative

recombination of electrons and holes within the energy gap and the

intramolecular energy transfer leading to a charge-transfer emission This

intramolecular energy transfer corresponds to the O-M LMCT energy gap and

occurs via radiative recombination and sensitized emission from the

28

heteroatoms or cations If several energy levels based on the hetero atoms or

counter cations act as energy acceptors within the O-M LMCTenergy gap the

energy transfer occurs from the O-M LMCT states to these levels followed by

the nonradiative or radiative deactivation of the excitation energy It should be

noted that the O-M LMCT states also can be generated by the application of

very high electric fields to the polyoxometalate solids as demonstrated by the

observation of electroluminescence[87]If an external electric field with a

potential more negative than the energy levels of the vacant orbitals involved

in the O-M LMCT transition is applied to a polyoxometalate on the electrode

surface an electrochemical reduction occurs via the injection of electrons

from the electrode in to the vacant levels of the polyoxometalate as shown in

Fig1B 2

Fig1B2 - Energy scheme for the electrochromism of polyoxometalates

a)electrochemical reduction (b) electrochemical oxidation

Electrons injected in to the high-energy levels also may be trapped by

electron traps in a process analogous to that which occurs during LMCT

photoexcitation of the polyoxometalates These electrons are returned to the

electrode by electrochemical oxidation at an electrode potential more positive

than the energy levels for the d1 electron states The d1electrons in the O-M

LMCTstates facilitate the absorption of visible light via intervalence charge

transfer among metal centers and d-d transitions The same type of transition

may be possible for the d1electron captured by the electron traps too In

addition to searching for new photosensitive polyoxometalates with the

29

potential for having practical application there is now a need to elucidate the

fundamental photo-and electrochemical coloration processes by studying

electron transfer within the polyoxometalate lattices in conjunction with their

crystal structures So far few polyoxometalates exhibit a perfect reversibility

of coloration The irreversibility of the color change arises from as yet

uncharacterized side reactions during both the coloration and decoloration of

the polyoxometalates

Many metal oxides including aluminum titanium vanadium niobium

molybdenum and tungsten oxides are photochromic when they contain

impurities or dopants This coloration has been interpreted on the basis of

electron trapping at appropriate lattice sites within the crystals as shown in

Fig1B1 where the O-M LMCT transition corresponds to the transition

between the valence and conduction bands for the infinite metal-oxide lattice

[88-91]

B12 Applications of Photochromic materials

Sunglasses

One of the most famous reversible photochromic applications is color

changing lenses for sunglasses as found in eyeglasses The largest limitation

in using PC technology is that the materials cannot be made stable enough

to withstand thousands of hours of outdoor exposure so long-term outdoor

applications are not appropriate at this time The switching speed of

photochromic dyes is highly sensitive to the rigidity of the environment around

the dye As result they switch most rapidly in solution and slowest in the rigid

environment like a polymer lens Recently it has been reported that attaching

flexible low Tg polymers (for example siloxanes or poly (butyl acrylate) to the

dyes allows them to switch much more rapidly in a rigid lens [76] Some

spirooxazines with siloxane polymers attached switch at near solution like

speeds even though they are in a rigid lens matrix

Supramolecular chemistry

Photochromic units have been employed extensively in supramolecular

chemistry Their ability to give a light controlled reversible shape change

means that they can be used to make or break molecular recognition motifs

30

or to cause a consequent shape change in their surroundings Thus

photochromic units have been demonstrated as components of molecular

switches The coupling of photochromic units to enzymes or enzyme cofactors

even provides the ability to reversibly turn enzymes on and off

by altering their shape or orientation in such a way that their functions

are either working or broken

Data storage

The possibility of using photochromic compounds for data storage was

first suggested in 1956 by Yehuda Hirshberg[77] Since that time there have

been many investigations by various academic and commercial groups

particularly in the area of 3D optical data storage which promises discs that

can hold a terabyte of data Initially issues with thermal back-reactions

and destructive reading dogged these studies but more recently more stable

systems have been developed

Novelty items

Reversible photochromics are also found in applications such as toys

cosmeticsclothing and industrial applications If necessary they can be made

to change between desired colors by combination with a permanent pigment

A large number of inorganic compounds exhibit photochromism

These solids often have large band gaps of the order of 3 - 12 eV and

excitation of these solids leads to the formation of metastable centers that

absorb visible light giving rise to their colour They can return to their ground

state by heating or by optical excitation within the colour-centre band In most

cases the photochromism is a structure sensitive phenomenon involving

localized defect impurities or dislocations Some of these inorganic

compounds have the potential for a number of different uses Photochromic

compounds have a number of useful applications These can be divided

according to the most important property that is being used (Table 11) [92]

31

Table 11 Applications of Photochromic materials

Applications Depending Upon

Sensitivity to

Radiation

Reversibility Thermal Chemical or

Physical Properties

Self-developing

photography

Chemical switches for

computers

Temperature indicators

Protective

materials

Data displays

Heat-sensitive recording

media

Optical signal

processing

Photomasking and

photoresist technology

Reusable data storage

media

Anaytical reagents

Photochromic

microimages

Photopolymerisation

Information encoding

and steganography

Photocontractile

polymers and the

photoviscosity effect

Control of light

intensity

Q-switches

Pyroelectric

photochromic materials

B13 Thermochromism

Thermochromism is the reversible colour change of a substance

induced by temperature change A large variety of substances organic

inorganic organometallic supramolecular and polymeric systems exhibit this

phenomenon Examples of these include bianthrones cobalt

hexacyanoferrate the zirconocene complex of 1 4-diphenyl-1 3-butadiene

and poly (3-alkylthiophene) The organic 99-bixanthenylidene is colourless at

90 K yellow-green at 298 K and dark-blue when melted at 592 K Heating

conducting polymers can cause them to change colour This is achieved by

causing conformational changes to the polymer backbone resulting in a

change in the band gap of the polymer It has been reported that regioregular

32

P3HT reversibly changes colour upon heating to 220ordmC due to temperature-

dependent conformation changes Thermally cross linked polymer undergoes

the same colour change but it is much less reversible [93] Other forms of

thermochromism may be commercially important eg to give a visual

indication of temperature changes

B14 Electrochromism

Electrochromism describes a phenomenon of material color change in

a persistent but reversible manner produced by electrochemically induced

oxidation-reduction reactions Electrochromic materials can be applied to

various kinds of products such as smart windows and display devices Among

those applications there have been lots of efforts to develop electrochromic

display devices (ECDs) Especially flexible display devices are now attracting

much attention worldwide since they can facilitate new technological demands

such as bending and folding of paper-like displays High electrochromic

efficiency short response time long operating life time and reduction of

energy consumption are the most important requisites of the materials for the

paperlike displays [94 95] Among those properties the operation life time is

the most important barrier to overcome for a realization of ECDs There are

two types of electrochromic material a) inorganic transition metal oxides

(TMOs) b) organic polymer materials The TMOs have been studied longer

than the organic materials that they have been studied since 1960s [9697]

Electrochromism describes a reversible color change of material

produced by electrochemically induced oxidation-reduction reactions It is one

of several types of chromism of materials As thermochromism and

photochromism mean material color changes made by heat and light

respectively electrochromism refers that the color change is caused by an

electric potential In most cases the color change in electrochromism can be

driven by rather low electrical potential of the order of a fraction of volt or a

few volts [94-96] The color change of material means variation in

transmittance andor reflectance change in visible range which is originated

from different electronic absorption bands according to a switching between

oxidation and reduction state of material When electric potential is applied on

electrochromic material forced oxidation or reduction is derived and the

33

individual color is originated from the corresponding oxidation or reduction

state of the material For electrochromic materials the characteristic color

change is reversible since the oxidation and reduction state can be converted

reversibly by switching potential

Application fields

The application area for the electrochromism is rather broad that it

covers from smart window glazing and optical modulators to information

displays [98-102] The smart windows are typical examples The

electrochromic property is used to control the amount of light and heat to pass

through the windows Usually the electrochromic material is in form of thin film

coated on a window glass The transmittance modulation has also been

applied at the automobiles to automatically tint rear-view mirrors in various

lighting conditions The electrochromic application fields are illustrated in Fig

1B3

Since the smart windows control the transmittance of heat as well as

the transmittance of visible light the working definition of electrochromism has

now been extended to include devices for modulation of radiation in the near

infrared thermal infrared and microwave regions When color for

electrochromic materials is used this can now mean a response by detectors

at these wavelengths and not just by the human eyes Nowadays

electrochromic material draws much attention as being used in the display

devices Electrochromic display device (ECD) is being considered as one of

the candidates for the conventional liquid crystal display (LCD) since ECD

has many advantages over LCDs Among these advantages the most

important are low energy consumption wider viewing angle high contrast

rate and possibility to achieve multiple colors with a single material [103]

When a new redox state of electrochromic materialis established by

the applied electric pulse then it is maintained after the potential is switched

off This means the colored or bleached state of the material can be sustained

for a considerable time without applying electrical power This is so-called

ldquocolor memory effectrdquo of electrochromic material Because of the color

memory effect energy consumption for the electrochromic display device

could be drastically reduced and this would be a big advantage over other

34

emissive devices The low power consumption is especially advantageous

when it is applied to mobile devices with limited power source The possibility

for a flexible display is another attraction for electrochromic material

Information displays

Real-view mirrors for automobiles

Fig 1B4 Application fields of electrochromic devices Smart windows information displays and real-view mirrors for automobiles

35

Using polymeric electrochromic material and plastic substrate with a

transparent conductive coating it is possible to build all-plastic flexible display

device There are lots of papers and patents about the flexible display devices

[104105] In these cases gel-type electrolyte is also needed The gel-type

electrolyte immobilizes liquid electrolyte in the polymer network [106107]

Recently ITO-coated polymer films are readily available which would provide

the plastic substrate for a flexible device The flexible electrochromic displays

are frequently tried with the plastic substrate flexible electrochromic material

and gel-type electrolyte The flexible electrochromic displays would facilitate

increasing technical demands for foldable display devices

Metal Ion Electrochromism

Many transition metal oxides are capable of redox reactions that result in

colour change Metal oxide films are commonly prepared as thin layers of

either tungsten nickel molybdenum or other metal compounds by a number

of techniques These include sol-gel electrochemical by dc or rfreactive

sputtering techniques electron-beam evaporation by anodic or cathodic

electrodeposition or by solution dipping of the electrochromic metal

compounds (or compounds that can be changed into these metal compounds)

onto optically transparent electrodes (OTE) [108 -114] Their electrochromism

is derived from the colour change associated with a change in the oxidation

state of the metal anion The behaviour of these materials is dependent upon

pH moisture and exposure to the atmosphere [115] Generally the switching

rates of these films is somewhat slow with typical switching times of about 15

- 60 seconds to achieve 100 conversion to either coloured or bleached state

[116 -120] An Example of this includes nickel oxide which changes from

transparent (pale green) to brownblack taking about 30 seconds to do so

[121] Other examples include [(NH4)5Ru]2(pyrazine)5+ and [(NH4)5Ru]2(44-

bipyridine)5+ whose electrochromism is significantly different due to the effect

of the ligand [122] Table 12 below gives some examples of metal oxide films

with electrochromic properties

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

16

of 411 mg of MO photo reducedg TiO2 This catalyst also shows encouraging

results for hydrogen evolution from water to the tune of 2730 micromolhg TiO

[50]

A new class of proton conducting glass membranes based on heteropo

lyacids such as phosphotungstic acid (PWA) as electrolytes for low

temperature H2O2 fuel cells was investigated Parameters for a single fuel cell

with a catalyst electrode of 015 mgcm2 of PtC and a glass composite

membrane were characterized by electrochemical measurements at open

circuit potential conditions The performance of the membrane electrode

assemblies (MEA) was systematically studied as an effect of SiO2 and P2O5

concentrations in the glass composite membrane and the MEA was found to

exhibit a maximum power density of 162 mW cm2 for an H2O2 fuel cell at

30 degC and 30 relative humidity (RH) [51]

Two new photochromic inorganic-organic hybrid materials formed from

Keggin type Polyoxometalates (POMs) and metronidazole (C6H9N3O3 MNZ)

formulated as H3PMo12O40bull3 MNZ3H2O (1) and H3PW12O40bull3MNZ3H2O(2)

were synthesized and characterized by elemental analysis IR spectra

electronic spectra electron spin resonance (ESR) spectra and TG-DTA

Reflectance spectra show the presence of weak inter molecular charge

transfer between the organic and inorganic moieties in the solid state The

photochromic properties were studied by solid diffuse reflectance spectra and

ESR spectra and the photochromic reactions were found to exhibit first-order

kinetics TG-DTA showed that two hybrid materials have similar thermal

behavior [52]

Heteropolycompounds (HPCs) have been a matter of interest in basic

and applied science for more than a century From their first synthesis many

advances have been made to promote the use of HPCs in different ways in

science and technology The aim of this article is to review the main structural

characteristics of heteropolycompounds of the Keggin type (12

tungstophosphoric12-molybdophosphoric12-tungstosilicic acid alkaline and

alkaline earth salts of12 tungstophosphoric acid and gels doped with HPCs)to

understand and explain their different activities such as high proton

conductivity and catalytic biochemical and biomedical activities [53]

17

A solid hybrid molecular material containing 1-butyl 3-methyl

imidazolium cations and Keggin anions of phosphotungstic acid has been

synthesized It is fully characterized by CHN analysis FTIR XRD UV-Vis-NIR

DRS 31P MAS NMR TGA and SEM The FTIR spectrum of the compound

shows the fingerprint vibrational bands of both Keggin molecular anions and

imidazolium cations The aromatic CndashH stretch region (2700ndash3250 cmndash1) of

imidazolium cation is split due to the interaction between the ring CndashH and

bulky Keggin anion The red-shift in the UV-Vis spectra and the downfield 31P

MAS NMR chemical shift also confirm the electrostatic interaction between

the ions in the compound Near IR spectral region (1000ndash2500 nm) shows the

elimination of water in the compound which is hydrophobic [54]

Transport coefficient measurements (electrical conductivity

thermoelectric power and Hall coefficient) have been performed on a

compact Tl033MoO3 polycrystalline compound in a wide temperature range

(200ndash400 K) Experimental results are interpreted with the help of a p-type

semiconductor model with two inverted deep levels near the midgap The

valence band and the conduction band are assumed to be formed from the

dxy orbitals of molybdenum atoms in the Mo6O22 cluster leading to narrow π-

bonding bands The donor and acceptor levels may be formed from

nonbonding dxy orbitals arising respectively from anionic and Tl+ defects

Electron paramagnetic resonance and magnetic measurements are in good

agreement with the theoretical band semiconductor model which has been

retained [55]

The optical properties of GaInTlAs epilayers grown at low temperature

~230degC by solid-source molecular-beam epitaxy on InP substrates were

characterized using optical absorptionand photoluminescence techniques

Optical absorption measurements a room temperature show a gap shrinkage

toward lower energies from 071 to 061 and 053 eV when the Tl content

increases from 0 to 4and 8in good agreement with theoretical

predictionsLow-temperature photoluminescence band-gap signals from

GaInAs and GaInTlAs layers are only obtained after rapid thermal annealing

performed inorder to improve the electronic quality of the layersA band gap

decrease as much as 41 meV for GaInTlAs with 19 Tl incorporation is

measured by photoluminescence at 8K [56]

18

The electrochromic performance of all solid ndash state cells employing

phosphotungstic acid and phosphomolybdic acid is reported These cells

employ SnO2 as the viewing electrode and graphite as the back electrodeThe

cells in the bleached state can be made white to red and become black in the

coloured state [57]

Various organic compounds were oxidized by molecular oxygen in the

presence of a catalytic amount of mixed addenda heteropolyoxometalates

containing molybdenum and vanadium The catalytic activity of the

Molybdovanadophosphate was found to be greatly enhanced by supporting

on charcoal The supported catalyst has high catalytic activity for oxidative

dehydrogenation of benzylic and allylic alcohols to the corresponding

aldehydes and ketones (46-92) nevertheless the nonsupported catalyst

was inactive for the same oxidations under these conditions 236Trimethyl

phenol was selectively oxidized to trimethyl-p benzoquinone which is

precursor of VitE in the presence of a catalytic amount of

molybdophosphate In addition the aerobic oxidation of amines alkyl-

substituted phenols and alkanes were also examined [58]

The reaction of Tl2CO3 with 111555-hexafluoro-24-pentanedione

and diglyme CH3O(CH2CH2O)2CH3 or tetraglyme CH3O(CH2CH2O)4CH3 in

dichloromethane yields the anhydrous thermally and air stable volatile Tl

diglyme and Tl tetraglyme adducts They have been characterized by single

crystal X-ray diffraction elemental analysis 1H and 13C NMR IR and mass

spectroscopy Thermal and mass-transport properties have been investigated

using thermo gravimetric and differential scanning calorimetric

measurements There is evidence that both precursors are very low melting

and volatile and can be used as liquid Tl sources Both adducts have been

successfully applied to metalndashorganic chemical vapor deposition of thallium

containing films [59]

Large size and high quality single crystals of quasi-two-dimensional

thallium molybdenum purple bronze TlMo6O17 have been grown by electrolytic

reduction of molten salt of Tl2CO3-MoO3 The crystal structure is trigonal with

space group P3m1 determined by X-ray diffraction and four-circle single crystal

diffraction The lattice parameters of the unit cell are a = b = 55282 Adeg and c

= 136991 Adeg The temperature dependence of resistivity and magnetic

19

susceptibility confirmed that a metal-to-metal transition occurs near 110 K

[60]

Hall coefficient and dc conductivity studies were made on p-type

Pb08Sn02Te thin films doped with different concentrations of thallium in the

temperature range 77 to 500 K The Hall coefficient and Hall mobility are

found to decrease with an increase in the doping concentration of thallium

Hall coefficient data have been analyzed in the light of a double valence-band

model Various band parameters such as valence band separation population

ratio mobility ratio and effective mass ratio have been calculated Hall

mobility data have been analyzed in the light of lattice and defect limited

scattering mechanisms [61]

Proton conducting composites of heteropolyacid hydrates phosphomol

ybdic acid H3PMo12O40 nH2O(PMA) phosphotungstic acid H3PW12O40 nH2O

( PTA) and salt hydrate like NiCl2 6H2O were prepared

with insulating Al2O3 as despersoidThe ionic conductivity peaks at two

concentrations of Al2O3 indicating two percolation thresholds for percolation

thresholds for proton conduction Two separate experiments were carried out

to check the existence of such percolation thresholds viz the volta battery

experiment involving the measurement of emf of an electrochemical cell

with composites of different compositions used as electrolyte and the

composition vs conductivity measured by the complex impedance

spectroscopy The presence of two maxima has been attributed to two

different percolation thresholds for the two possible mobile protonic

species H+ + (H3O+) and OH arising from the hydrates [62]

1 A5 Applications of Heteropolyoxometalates-

Applications of heteropolyanions centre depend on their redox properties

their high charges and ionic weights An enormous patent and journal

literature is devoted to the applications of heteropolyanions

1 Analysis-

The formation and subsequent precipitation or reduction of

[XMO12O40]n- anions form the basis of gravimetric and colorimetric analytical

methods for P As Si or Geeither separately or in combination [63 64]

20

2 Biochemical applications-

lsquoPhosphotungstic Acidsrsquo have for decades been used as precipitants

for proteins and as analytical reagents for proteinsalkaloidsand purines eg

the [P2M18O62]6- anions for colorimetric determination of uric acid [65] and

cholesterol The acid H3P12O40 either in aqueous or ethanolic solution is also

widely used as a non specific electron dense stain for electron spectroscopy

The dyestuffs industry has for many years used heteropolymolybdates and

tungstates to form color lakes and toners from basic dyes Large

heteropolyanions exhibit antiviral antitumoral properties at non-cytotoxic

doses in vitro and in vivo and are protein inhibitors of cellular bacterial and

viral DNA RNA polymerizes [66]

3 Catalysis

Heteropoly acids and salts have been used as heterogeneous catalysts

for a broad variety of reactions and compilations of such applications up to

1973 are available Examples include oxidation of propylene and isobutylene

to acrylic acid methacrylic acids and ammoxidation of acrylonitrile olefin

polymerization and epoxidation Much of current activity concerning

heterogeneous catalysis by heteropoly compounds is being carried out in

Japan [67 68]

4 Other Applications

Insoluble salts of Heteropolyanions especially ammonium

molybdophosphates have been used and are commercially available as ion-

exchange materials [69]Recent work in this area includes thin layer

chromatography of amino acids ion selective membranes [70] and the

preparation of new ion exchangers based on heteropolyanions Crystalline12-

tungstophosphoric and 12-molybdophosphoric acids are excellent protonic

conductors Heteropolyacids are electrochromic in the solid state as a

consequence of heteropolyblue formation Heteropolyblue formation has also

been used to detect alcohol or carboxylic acid radicals generated by radiolysis

of aqueous solutions Potential applications of heteropoly complexes as flame

retardants and smoke suppressants or as corrosion inhibitors and conversion

coatings on steel and aluminium are reported [71] Some potential ldquogreenrdquo

applications have been reported eg non-chlorine based wood pulp

bleaching process and a method of decontaminating water Some structures

21

containing transition metal atoms with unpaired electrons have unusual

magnetic properties and are being investigated as nano computer storage

devices Some compounds exhibit luminescence There are many reported

potential medicinal applications eg anti tumoral and anti-viral There have

been reports on the role of weak or non bonding interactions on the crystal

engineering of hybrid polyoxometalates

Spherical nonporous polyoxomolybdate based capsules of different

types containing more than 100 metal atoms reported by Achim Muller and his

group have versatile unique properties regarding their assembly to vesicles

and the chemistry which can be done inside the pores and cavities A discrete

polyoxometalate Lindquist ion of the form W6O192minus was successfully imaged

recently for the first time within the capillary of a carbon nanotube following

steric locking of the anion with the tubule In situ relaxation of the anion in its

equatorial plain was demonstrated [72]

22

Section-B

Litreture Survey on Chromism in Transition Metal

Oxides

1 B1 Chromism in Transition Metal Oxides

Chromism is a reversible change in a substances colour resulting from

a process caused by some form of stimulus Many materials are chromic

including inorganic and organic compounds and conducting polymers and the

property can result from many different mechanisms Several transition metal

oxides show EC properties The most popular are from the VI - B oxides In

this group WO3 and MoO3 are the most thoroughly studied cathodic EC

materials which can be electrochemically coloured and bleached when used

as the cathode in electrochemical cells Cathodic EC materials also include

V2O3 TiO2 and Nb2O5 Another distinguishable group is anodic EC material

including VIII oxides like IrOx nH2O Rh2O3 nH2O NiO nH2O etc which can be

anodicaly coloured in the electrochemical process when used as anode

There are several types of chromism which are discussed as below

B11 Photochromism

Photochromism is the reversible transformation of a chemical species

between two forms by the absorption of electromagnetic radiation where

the two forms have different absorption spectra [7374]

Trivially this can be described as a reversible change of color upon

exposure to light The phenomenon was discovered in the late 1880s

including work by Markwald who studied the reversible change of color of 23

44-tetrachloronaphthalen-1(4H)-one in the solid state He labeled this

phenomenon phototropy and this name was used until the 1950s

when Yehuda Hirshberg of the Weizmann Institute of Science in Israel

proposed the term photochromism [75] Photochromism can take place

in both organic and inorganic compounds and also has its place in biological

systems (for example retinal in the vision process)

Photochromism does not have a rigorous definition but is usually used

to describe compounds that undergo a reversible photochemical reaction

where an absorption band in the visible part of the electromagnetic spectrum

23

changes dramatically in strength or wavelength In many cases an

absorbance band is present in only one form The degree of change required

for a photochemical reaction to be dubbed photochromic is that which

appears dramatic by eye but in essence there is no dividing line between

photochromic reactions and other photochemistry Therefore while the

transcis isomerization of azobenzene is considered a photochromic reaction

the analogous reaction of stilbene is not Since photochromism is just a

special case of a photochemical reaction almost any photochemical reaction

type may be used to produce photochromism with appropriate molecular desi

gnSome of the most common processes involved in photochromism are peric

yclic reactions cis-trans somerizations intramolecular hydrogen transfer

intramolecular group transfers dissociation processes and electron transfers

(oxidation-reduction)

Another some what arbitrary requirement of photochromism is that

it requires the two states of the molecule to be thermally stable under

ambient conditions for a reasonable time All the same nitrospiropyran (which

back-isomerizes in the dark over ~10 minutes at room temperature) is

considered photochromic All photochromic molecules back-isomerize to their

more stable form at some rate and this back-isomerization is accelerated by

heating There is therefore a close relationship between photochromic and the

rmochromic compounds The timescale of thermal back-isomerization is

important for applications and may be molecularly engineered

Photochromic compounds considered to be thermally stable include some

diarylethenes which do not back isomerize even after heating at 800C for 3

months

Since photochromic chromophores are dyes and operate according to

well-known reactions their molecular engineering to fine-tune their properties

can be achieved relatively easily using known design models quantum

mechanics calculations and experimentation In particular the tuning of

absorbance bands to particular parts of the spectrum and the engineering

of thermal stability have received much attention

Sometimes and particularly in the dye industry the term irreversible

photochromic is used to describe materials that undergo a permanent color

change upon exposure to Ultraviolet or visible light radiation Because by

24

definition photochromics are reversible there is technically no such thing as a

n irreversible photochromic this is loose usage and these compounds

are better referred to as photochangable or photoreactive dyes

Apart from the qualities already mentioned several other properties of

photochromics are important for their use These include

Quantum yield of the photochemical reaction

This determined the efficiency of the photochromic change with respect

to the amount of light absorbed The quantum yield of isomerization

can be strongly dependent on conditions

Fatigue resistance In photochromic materials fatigue refers to the

loss of reversibility by processes such as photodegradation

photobleaching photooxidation and other side reactions All

photochromics suffer fatigue to some extent and its rate is strongly

dependent on the activating light and the conditions of the sample

Photostationary state Photochromic materials have two states and

their interconversion can be controlled using different wavelengths of

light Excitation with any given wavelength of light will result in a

mixture of the two states at a particular ratio called the photo-

stationary state In a perfect system there would exist wavelengths

that can be used to provide 10 and 01 ratios of the isomers

but in real systems this is not possible since the active

absorbance bands always overlap to some extent

Polarity and solubility In order to incorporate photochromics in

working systems they suffer the same issues as other dyes They are

often charged in one or more state leading to very high polarity and

possible large changes in polarity They also often contain large

conjugated systems that limit their solubility

Photochromic complexes

A photochromic complex is a kind of chemical compound that has

photoresponsive Parts on its ligand These complexes have a specific

structure photoswitchable organic compounds are attached to

metalcomplexes For the photocontrollable parts thermally and

photochemically stable chromophores (azobenzene diarylethene

25

spiropyran etc) are usually used And for the metal complexes a wide

variety of compounds that have various functions (redox response

luminescence magnetism etc) are applied The photochromic parts and

metal parts are so close that they can affect each others molecular

orbitals The physical properties of these compounds shown by parts of

them (ie chromophores or metals) thus can be controlled by switching

their other sites by external stimuli For example photoisomerization

behaviors of some complexes can be switched by oxidation and reduction of

their metal parts Some other compounds can be changed in their

luminescence behavior magnetic interaction of metal sites or stability of

metal-to-ligand coordination by photoisomerization of their photochromic

parts

Photochemistry of Polyoxometalates

The photochemistry of polyoxometalates is of great interest to inorganic

chemistsMore than 80 years agoit was found that the R-Keggin tungstate

H3[PW12O40] was reduced photochemically to yield a blue-colored species

which was reoxidized by air and by various other oxidizing agents such as

Fe3+AgNO3and H2O2 [7879]The photoredox reactions of H4[SiW12O40]and

H3[PW12O40] proceeded effectively in the presence of primary and secondary

alcohols their ethers and aldehydes and proteinsbut less effectively in the

presence of tertiary alcoholsketonesestersthe fatty acids above formic

acidand simple amines[8081] The basic photoredox reaction involving

ethanol is illustrated by eq 13

2 H3PW12O40 + H3CCH2OH h ν ν ν ν 2 H4PW12O40 + H3CCHO ------- 13

2 H4PW12O40 +12 O2 2 H3PW12O40 + H2O ------ 14

In this reactionone molecule of ethanol photochemically reduces two

molecules of H3PW12O40 and is itself oxidized to acetaldehyde In the

presence of air the thermal oxidation of the reduced species takes place at

room temperature(eq 14)The reduced polyoxometalates which are the so-

calledldquoheteropolybluesrdquo have been used for the colorimetric analysis of the

26

elements P Si As and Ge and for the determination of uric acidsugarand

other biological compounds [8283] Piperidinium metavanadate also

undergoes photoinduced coloration from white to black followed by a

reversible color change in the presence of oxidizing agentsHowever

ammonium metavanadates ([NH4][VO3]) exhibits no photoinduced

coloration[84] The early photoredox reactions of the R-Keggin

polyoxometalates H4[SiW12O40] and H3PW12O40 were carried out in the

presence of photographic paper however the limited number of the

structurally well-characterized compounds available for study delayed the

development of modern cluster-compound photochemistry until the discovery

of photochromism in alkylammonium polyoxo- molybdate solids[8586] A

photochromic or electrochromic material is one whose light-absorbing

properties are altered upon optical excitation or reduction under the influence

of an externally applied electric field respectively The induced coloration

remains even after the excitation source has been removed These materials

are of technological interest because they return to their original state either

thermally upon irradiation with light of a frequency corresponding to the

induced absorption or electrochemically upon reversing the polarity of the

externally applied electric field Thus photochromic and electrochromic

materials behave in a reversible manner Polyoxometalates exhibit significant

photo-and electrochromism which makes them suitable as nanocomposite

molecular devices and as models for probing the physical properties of infinite

metal oxides Since the metal ions in the oxidized polyoxometalates have d0

electronic configurations the only absorption band which occurs in the UV-vis

range of the electronic spectra is due to the oxygen-to-metal (O-M) ligand-to-

metal charge transfer (LMCT)Upon irradiation electrons are promoted from

the low-energy electronic states which are mainly comprised of oxygen 2p

orbitals (the valence band in the band model)to the high-energy electronic

states which are mainly comprised of metal d orbitalrsquos (the conduction band

in the band model)The fundamental transitions in polyoxometalate lattices

are depicted schematically in Fig1B1

27

Fig1B1 Simple model showing the electronic transitions in the

polyoxometalates containing electron donar and acceptor (a)generation

of charge carriers(b)electron and hole trapping(c)electron release due

to stimulation(d)recombination

between electron and hole Electrons are e-and holes are h+

In the polyoxometalates containing heteroatoms and especially in

mixed metal polyoxometalates the charge carriers which are created by the

light or electric field may be trapped in electron traps and hole traps These

traps provide states of localized energy in the O-M LMCT energy gap due to

the heteroatoms or counter cations which correspond to impurities or lattice

defects in the band model If the trap depth ∆E is large compared to kT the

probability for thermal escape from the trap will be negligibly small and

metastable situation will existThe trapped carriers can be released by thermal

or optical stimulationIn the case of thermal stimulation the irradiated

polyoxometalate is heated until the energy barrier ∆E can be overcome The

trapped electron (or hole) then can escape from the trap and nonradiatively

recombine with the trapped hole (or electron)Under optical stimulation the

energy of an incident photon is used to overcome ∆E The relaxation

processes of the OndashM LMCT excitation energy include both the nonradiative

recombination of electrons and holes within the energy gap and the

intramolecular energy transfer leading to a charge-transfer emission This

intramolecular energy transfer corresponds to the O-M LMCT energy gap and

occurs via radiative recombination and sensitized emission from the

28

heteroatoms or cations If several energy levels based on the hetero atoms or

counter cations act as energy acceptors within the O-M LMCTenergy gap the

energy transfer occurs from the O-M LMCT states to these levels followed by

the nonradiative or radiative deactivation of the excitation energy It should be

noted that the O-M LMCT states also can be generated by the application of

very high electric fields to the polyoxometalate solids as demonstrated by the

observation of electroluminescence[87]If an external electric field with a

potential more negative than the energy levels of the vacant orbitals involved

in the O-M LMCT transition is applied to a polyoxometalate on the electrode

surface an electrochemical reduction occurs via the injection of electrons

from the electrode in to the vacant levels of the polyoxometalate as shown in

Fig1B 2

Fig1B2 - Energy scheme for the electrochromism of polyoxometalates

a)electrochemical reduction (b) electrochemical oxidation

Electrons injected in to the high-energy levels also may be trapped by

electron traps in a process analogous to that which occurs during LMCT

photoexcitation of the polyoxometalates These electrons are returned to the

electrode by electrochemical oxidation at an electrode potential more positive

than the energy levels for the d1 electron states The d1electrons in the O-M

LMCTstates facilitate the absorption of visible light via intervalence charge

transfer among metal centers and d-d transitions The same type of transition

may be possible for the d1electron captured by the electron traps too In

addition to searching for new photosensitive polyoxometalates with the

29

potential for having practical application there is now a need to elucidate the

fundamental photo-and electrochemical coloration processes by studying

electron transfer within the polyoxometalate lattices in conjunction with their

crystal structures So far few polyoxometalates exhibit a perfect reversibility

of coloration The irreversibility of the color change arises from as yet

uncharacterized side reactions during both the coloration and decoloration of

the polyoxometalates

Many metal oxides including aluminum titanium vanadium niobium

molybdenum and tungsten oxides are photochromic when they contain

impurities or dopants This coloration has been interpreted on the basis of

electron trapping at appropriate lattice sites within the crystals as shown in

Fig1B1 where the O-M LMCT transition corresponds to the transition

between the valence and conduction bands for the infinite metal-oxide lattice

[88-91]

B12 Applications of Photochromic materials

Sunglasses

One of the most famous reversible photochromic applications is color

changing lenses for sunglasses as found in eyeglasses The largest limitation

in using PC technology is that the materials cannot be made stable enough

to withstand thousands of hours of outdoor exposure so long-term outdoor

applications are not appropriate at this time The switching speed of

photochromic dyes is highly sensitive to the rigidity of the environment around

the dye As result they switch most rapidly in solution and slowest in the rigid

environment like a polymer lens Recently it has been reported that attaching

flexible low Tg polymers (for example siloxanes or poly (butyl acrylate) to the

dyes allows them to switch much more rapidly in a rigid lens [76] Some

spirooxazines with siloxane polymers attached switch at near solution like

speeds even though they are in a rigid lens matrix

Supramolecular chemistry

Photochromic units have been employed extensively in supramolecular

chemistry Their ability to give a light controlled reversible shape change

means that they can be used to make or break molecular recognition motifs

30

or to cause a consequent shape change in their surroundings Thus

photochromic units have been demonstrated as components of molecular

switches The coupling of photochromic units to enzymes or enzyme cofactors

even provides the ability to reversibly turn enzymes on and off

by altering their shape or orientation in such a way that their functions

are either working or broken

Data storage

The possibility of using photochromic compounds for data storage was

first suggested in 1956 by Yehuda Hirshberg[77] Since that time there have

been many investigations by various academic and commercial groups

particularly in the area of 3D optical data storage which promises discs that

can hold a terabyte of data Initially issues with thermal back-reactions

and destructive reading dogged these studies but more recently more stable

systems have been developed

Novelty items

Reversible photochromics are also found in applications such as toys

cosmeticsclothing and industrial applications If necessary they can be made

to change between desired colors by combination with a permanent pigment

A large number of inorganic compounds exhibit photochromism

These solids often have large band gaps of the order of 3 - 12 eV and

excitation of these solids leads to the formation of metastable centers that

absorb visible light giving rise to their colour They can return to their ground

state by heating or by optical excitation within the colour-centre band In most

cases the photochromism is a structure sensitive phenomenon involving

localized defect impurities or dislocations Some of these inorganic

compounds have the potential for a number of different uses Photochromic

compounds have a number of useful applications These can be divided

according to the most important property that is being used (Table 11) [92]

31

Table 11 Applications of Photochromic materials

Applications Depending Upon

Sensitivity to

Radiation

Reversibility Thermal Chemical or

Physical Properties

Self-developing

photography

Chemical switches for

computers

Temperature indicators

Protective

materials

Data displays

Heat-sensitive recording

media

Optical signal

processing

Photomasking and

photoresist technology

Reusable data storage

media

Anaytical reagents

Photochromic

microimages

Photopolymerisation

Information encoding

and steganography

Photocontractile

polymers and the

photoviscosity effect

Control of light

intensity

Q-switches

Pyroelectric

photochromic materials

B13 Thermochromism

Thermochromism is the reversible colour change of a substance

induced by temperature change A large variety of substances organic

inorganic organometallic supramolecular and polymeric systems exhibit this

phenomenon Examples of these include bianthrones cobalt

hexacyanoferrate the zirconocene complex of 1 4-diphenyl-1 3-butadiene

and poly (3-alkylthiophene) The organic 99-bixanthenylidene is colourless at

90 K yellow-green at 298 K and dark-blue when melted at 592 K Heating

conducting polymers can cause them to change colour This is achieved by

causing conformational changes to the polymer backbone resulting in a

change in the band gap of the polymer It has been reported that regioregular

32

P3HT reversibly changes colour upon heating to 220ordmC due to temperature-

dependent conformation changes Thermally cross linked polymer undergoes

the same colour change but it is much less reversible [93] Other forms of

thermochromism may be commercially important eg to give a visual

indication of temperature changes

B14 Electrochromism

Electrochromism describes a phenomenon of material color change in

a persistent but reversible manner produced by electrochemically induced

oxidation-reduction reactions Electrochromic materials can be applied to

various kinds of products such as smart windows and display devices Among

those applications there have been lots of efforts to develop electrochromic

display devices (ECDs) Especially flexible display devices are now attracting

much attention worldwide since they can facilitate new technological demands

such as bending and folding of paper-like displays High electrochromic

efficiency short response time long operating life time and reduction of

energy consumption are the most important requisites of the materials for the

paperlike displays [94 95] Among those properties the operation life time is

the most important barrier to overcome for a realization of ECDs There are

two types of electrochromic material a) inorganic transition metal oxides

(TMOs) b) organic polymer materials The TMOs have been studied longer

than the organic materials that they have been studied since 1960s [9697]

Electrochromism describes a reversible color change of material

produced by electrochemically induced oxidation-reduction reactions It is one

of several types of chromism of materials As thermochromism and

photochromism mean material color changes made by heat and light

respectively electrochromism refers that the color change is caused by an

electric potential In most cases the color change in electrochromism can be

driven by rather low electrical potential of the order of a fraction of volt or a

few volts [94-96] The color change of material means variation in

transmittance andor reflectance change in visible range which is originated

from different electronic absorption bands according to a switching between

oxidation and reduction state of material When electric potential is applied on

electrochromic material forced oxidation or reduction is derived and the

33

individual color is originated from the corresponding oxidation or reduction

state of the material For electrochromic materials the characteristic color

change is reversible since the oxidation and reduction state can be converted

reversibly by switching potential

Application fields

The application area for the electrochromism is rather broad that it

covers from smart window glazing and optical modulators to information

displays [98-102] The smart windows are typical examples The

electrochromic property is used to control the amount of light and heat to pass

through the windows Usually the electrochromic material is in form of thin film

coated on a window glass The transmittance modulation has also been

applied at the automobiles to automatically tint rear-view mirrors in various

lighting conditions The electrochromic application fields are illustrated in Fig

1B3

Since the smart windows control the transmittance of heat as well as

the transmittance of visible light the working definition of electrochromism has

now been extended to include devices for modulation of radiation in the near

infrared thermal infrared and microwave regions When color for

electrochromic materials is used this can now mean a response by detectors

at these wavelengths and not just by the human eyes Nowadays

electrochromic material draws much attention as being used in the display

devices Electrochromic display device (ECD) is being considered as one of

the candidates for the conventional liquid crystal display (LCD) since ECD

has many advantages over LCDs Among these advantages the most

important are low energy consumption wider viewing angle high contrast

rate and possibility to achieve multiple colors with a single material [103]

When a new redox state of electrochromic materialis established by

the applied electric pulse then it is maintained after the potential is switched

off This means the colored or bleached state of the material can be sustained

for a considerable time without applying electrical power This is so-called

ldquocolor memory effectrdquo of electrochromic material Because of the color

memory effect energy consumption for the electrochromic display device

could be drastically reduced and this would be a big advantage over other

34

emissive devices The low power consumption is especially advantageous

when it is applied to mobile devices with limited power source The possibility

for a flexible display is another attraction for electrochromic material

Information displays

Real-view mirrors for automobiles

Fig 1B4 Application fields of electrochromic devices Smart windows information displays and real-view mirrors for automobiles

35

Using polymeric electrochromic material and plastic substrate with a

transparent conductive coating it is possible to build all-plastic flexible display

device There are lots of papers and patents about the flexible display devices

[104105] In these cases gel-type electrolyte is also needed The gel-type

electrolyte immobilizes liquid electrolyte in the polymer network [106107]

Recently ITO-coated polymer films are readily available which would provide

the plastic substrate for a flexible device The flexible electrochromic displays

are frequently tried with the plastic substrate flexible electrochromic material

and gel-type electrolyte The flexible electrochromic displays would facilitate

increasing technical demands for foldable display devices

Metal Ion Electrochromism

Many transition metal oxides are capable of redox reactions that result in

colour change Metal oxide films are commonly prepared as thin layers of

either tungsten nickel molybdenum or other metal compounds by a number

of techniques These include sol-gel electrochemical by dc or rfreactive

sputtering techniques electron-beam evaporation by anodic or cathodic

electrodeposition or by solution dipping of the electrochromic metal

compounds (or compounds that can be changed into these metal compounds)

onto optically transparent electrodes (OTE) [108 -114] Their electrochromism

is derived from the colour change associated with a change in the oxidation

state of the metal anion The behaviour of these materials is dependent upon

pH moisture and exposure to the atmosphere [115] Generally the switching

rates of these films is somewhat slow with typical switching times of about 15

- 60 seconds to achieve 100 conversion to either coloured or bleached state

[116 -120] An Example of this includes nickel oxide which changes from

transparent (pale green) to brownblack taking about 30 seconds to do so

[121] Other examples include [(NH4)5Ru]2(pyrazine)5+ and [(NH4)5Ru]2(44-

bipyridine)5+ whose electrochromism is significantly different due to the effect

of the ligand [122] Table 12 below gives some examples of metal oxide films

with electrochromic properties

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

17

A solid hybrid molecular material containing 1-butyl 3-methyl

imidazolium cations and Keggin anions of phosphotungstic acid has been

synthesized It is fully characterized by CHN analysis FTIR XRD UV-Vis-NIR

DRS 31P MAS NMR TGA and SEM The FTIR spectrum of the compound

shows the fingerprint vibrational bands of both Keggin molecular anions and

imidazolium cations The aromatic CndashH stretch region (2700ndash3250 cmndash1) of

imidazolium cation is split due to the interaction between the ring CndashH and

bulky Keggin anion The red-shift in the UV-Vis spectra and the downfield 31P

MAS NMR chemical shift also confirm the electrostatic interaction between

the ions in the compound Near IR spectral region (1000ndash2500 nm) shows the

elimination of water in the compound which is hydrophobic [54]

Transport coefficient measurements (electrical conductivity

thermoelectric power and Hall coefficient) have been performed on a

compact Tl033MoO3 polycrystalline compound in a wide temperature range

(200ndash400 K) Experimental results are interpreted with the help of a p-type

semiconductor model with two inverted deep levels near the midgap The

valence band and the conduction band are assumed to be formed from the

dxy orbitals of molybdenum atoms in the Mo6O22 cluster leading to narrow π-

bonding bands The donor and acceptor levels may be formed from

nonbonding dxy orbitals arising respectively from anionic and Tl+ defects

Electron paramagnetic resonance and magnetic measurements are in good

agreement with the theoretical band semiconductor model which has been

retained [55]

The optical properties of GaInTlAs epilayers grown at low temperature

~230degC by solid-source molecular-beam epitaxy on InP substrates were

characterized using optical absorptionand photoluminescence techniques

Optical absorption measurements a room temperature show a gap shrinkage

toward lower energies from 071 to 061 and 053 eV when the Tl content

increases from 0 to 4and 8in good agreement with theoretical

predictionsLow-temperature photoluminescence band-gap signals from

GaInAs and GaInTlAs layers are only obtained after rapid thermal annealing

performed inorder to improve the electronic quality of the layersA band gap

decrease as much as 41 meV for GaInTlAs with 19 Tl incorporation is

measured by photoluminescence at 8K [56]

18

The electrochromic performance of all solid ndash state cells employing

phosphotungstic acid and phosphomolybdic acid is reported These cells

employ SnO2 as the viewing electrode and graphite as the back electrodeThe

cells in the bleached state can be made white to red and become black in the

coloured state [57]

Various organic compounds were oxidized by molecular oxygen in the

presence of a catalytic amount of mixed addenda heteropolyoxometalates

containing molybdenum and vanadium The catalytic activity of the

Molybdovanadophosphate was found to be greatly enhanced by supporting

on charcoal The supported catalyst has high catalytic activity for oxidative

dehydrogenation of benzylic and allylic alcohols to the corresponding

aldehydes and ketones (46-92) nevertheless the nonsupported catalyst

was inactive for the same oxidations under these conditions 236Trimethyl

phenol was selectively oxidized to trimethyl-p benzoquinone which is

precursor of VitE in the presence of a catalytic amount of

molybdophosphate In addition the aerobic oxidation of amines alkyl-

substituted phenols and alkanes were also examined [58]

The reaction of Tl2CO3 with 111555-hexafluoro-24-pentanedione

and diglyme CH3O(CH2CH2O)2CH3 or tetraglyme CH3O(CH2CH2O)4CH3 in

dichloromethane yields the anhydrous thermally and air stable volatile Tl

diglyme and Tl tetraglyme adducts They have been characterized by single

crystal X-ray diffraction elemental analysis 1H and 13C NMR IR and mass

spectroscopy Thermal and mass-transport properties have been investigated

using thermo gravimetric and differential scanning calorimetric

measurements There is evidence that both precursors are very low melting

and volatile and can be used as liquid Tl sources Both adducts have been

successfully applied to metalndashorganic chemical vapor deposition of thallium

containing films [59]

Large size and high quality single crystals of quasi-two-dimensional

thallium molybdenum purple bronze TlMo6O17 have been grown by electrolytic

reduction of molten salt of Tl2CO3-MoO3 The crystal structure is trigonal with

space group P3m1 determined by X-ray diffraction and four-circle single crystal

diffraction The lattice parameters of the unit cell are a = b = 55282 Adeg and c

= 136991 Adeg The temperature dependence of resistivity and magnetic

19

susceptibility confirmed that a metal-to-metal transition occurs near 110 K

[60]

Hall coefficient and dc conductivity studies were made on p-type

Pb08Sn02Te thin films doped with different concentrations of thallium in the

temperature range 77 to 500 K The Hall coefficient and Hall mobility are

found to decrease with an increase in the doping concentration of thallium

Hall coefficient data have been analyzed in the light of a double valence-band

model Various band parameters such as valence band separation population

ratio mobility ratio and effective mass ratio have been calculated Hall

mobility data have been analyzed in the light of lattice and defect limited

scattering mechanisms [61]

Proton conducting composites of heteropolyacid hydrates phosphomol

ybdic acid H3PMo12O40 nH2O(PMA) phosphotungstic acid H3PW12O40 nH2O

( PTA) and salt hydrate like NiCl2 6H2O were prepared

with insulating Al2O3 as despersoidThe ionic conductivity peaks at two

concentrations of Al2O3 indicating two percolation thresholds for percolation

thresholds for proton conduction Two separate experiments were carried out

to check the existence of such percolation thresholds viz the volta battery

experiment involving the measurement of emf of an electrochemical cell

with composites of different compositions used as electrolyte and the

composition vs conductivity measured by the complex impedance

spectroscopy The presence of two maxima has been attributed to two

different percolation thresholds for the two possible mobile protonic

species H+ + (H3O+) and OH arising from the hydrates [62]

1 A5 Applications of Heteropolyoxometalates-

Applications of heteropolyanions centre depend on their redox properties

their high charges and ionic weights An enormous patent and journal

literature is devoted to the applications of heteropolyanions

1 Analysis-

The formation and subsequent precipitation or reduction of

[XMO12O40]n- anions form the basis of gravimetric and colorimetric analytical

methods for P As Si or Geeither separately or in combination [63 64]

20

2 Biochemical applications-

lsquoPhosphotungstic Acidsrsquo have for decades been used as precipitants

for proteins and as analytical reagents for proteinsalkaloidsand purines eg

the [P2M18O62]6- anions for colorimetric determination of uric acid [65] and

cholesterol The acid H3P12O40 either in aqueous or ethanolic solution is also

widely used as a non specific electron dense stain for electron spectroscopy

The dyestuffs industry has for many years used heteropolymolybdates and

tungstates to form color lakes and toners from basic dyes Large

heteropolyanions exhibit antiviral antitumoral properties at non-cytotoxic

doses in vitro and in vivo and are protein inhibitors of cellular bacterial and

viral DNA RNA polymerizes [66]

3 Catalysis

Heteropoly acids and salts have been used as heterogeneous catalysts

for a broad variety of reactions and compilations of such applications up to

1973 are available Examples include oxidation of propylene and isobutylene

to acrylic acid methacrylic acids and ammoxidation of acrylonitrile olefin

polymerization and epoxidation Much of current activity concerning

heterogeneous catalysis by heteropoly compounds is being carried out in

Japan [67 68]

4 Other Applications

Insoluble salts of Heteropolyanions especially ammonium

molybdophosphates have been used and are commercially available as ion-

exchange materials [69]Recent work in this area includes thin layer

chromatography of amino acids ion selective membranes [70] and the

preparation of new ion exchangers based on heteropolyanions Crystalline12-

tungstophosphoric and 12-molybdophosphoric acids are excellent protonic

conductors Heteropolyacids are electrochromic in the solid state as a

consequence of heteropolyblue formation Heteropolyblue formation has also

been used to detect alcohol or carboxylic acid radicals generated by radiolysis

of aqueous solutions Potential applications of heteropoly complexes as flame

retardants and smoke suppressants or as corrosion inhibitors and conversion

coatings on steel and aluminium are reported [71] Some potential ldquogreenrdquo

applications have been reported eg non-chlorine based wood pulp

bleaching process and a method of decontaminating water Some structures

21

containing transition metal atoms with unpaired electrons have unusual

magnetic properties and are being investigated as nano computer storage

devices Some compounds exhibit luminescence There are many reported

potential medicinal applications eg anti tumoral and anti-viral There have

been reports on the role of weak or non bonding interactions on the crystal

engineering of hybrid polyoxometalates

Spherical nonporous polyoxomolybdate based capsules of different

types containing more than 100 metal atoms reported by Achim Muller and his

group have versatile unique properties regarding their assembly to vesicles

and the chemistry which can be done inside the pores and cavities A discrete

polyoxometalate Lindquist ion of the form W6O192minus was successfully imaged

recently for the first time within the capillary of a carbon nanotube following

steric locking of the anion with the tubule In situ relaxation of the anion in its

equatorial plain was demonstrated [72]

22

Section-B

Litreture Survey on Chromism in Transition Metal

Oxides

1 B1 Chromism in Transition Metal Oxides

Chromism is a reversible change in a substances colour resulting from

a process caused by some form of stimulus Many materials are chromic

including inorganic and organic compounds and conducting polymers and the

property can result from many different mechanisms Several transition metal

oxides show EC properties The most popular are from the VI - B oxides In

this group WO3 and MoO3 are the most thoroughly studied cathodic EC

materials which can be electrochemically coloured and bleached when used

as the cathode in electrochemical cells Cathodic EC materials also include

V2O3 TiO2 and Nb2O5 Another distinguishable group is anodic EC material

including VIII oxides like IrOx nH2O Rh2O3 nH2O NiO nH2O etc which can be

anodicaly coloured in the electrochemical process when used as anode

There are several types of chromism which are discussed as below

B11 Photochromism

Photochromism is the reversible transformation of a chemical species

between two forms by the absorption of electromagnetic radiation where

the two forms have different absorption spectra [7374]

Trivially this can be described as a reversible change of color upon

exposure to light The phenomenon was discovered in the late 1880s

including work by Markwald who studied the reversible change of color of 23

44-tetrachloronaphthalen-1(4H)-one in the solid state He labeled this

phenomenon phototropy and this name was used until the 1950s

when Yehuda Hirshberg of the Weizmann Institute of Science in Israel

proposed the term photochromism [75] Photochromism can take place

in both organic and inorganic compounds and also has its place in biological

systems (for example retinal in the vision process)

Photochromism does not have a rigorous definition but is usually used

to describe compounds that undergo a reversible photochemical reaction

where an absorption band in the visible part of the electromagnetic spectrum

23

changes dramatically in strength or wavelength In many cases an

absorbance band is present in only one form The degree of change required

for a photochemical reaction to be dubbed photochromic is that which

appears dramatic by eye but in essence there is no dividing line between

photochromic reactions and other photochemistry Therefore while the

transcis isomerization of azobenzene is considered a photochromic reaction

the analogous reaction of stilbene is not Since photochromism is just a

special case of a photochemical reaction almost any photochemical reaction

type may be used to produce photochromism with appropriate molecular desi

gnSome of the most common processes involved in photochromism are peric

yclic reactions cis-trans somerizations intramolecular hydrogen transfer

intramolecular group transfers dissociation processes and electron transfers

(oxidation-reduction)

Another some what arbitrary requirement of photochromism is that

it requires the two states of the molecule to be thermally stable under

ambient conditions for a reasonable time All the same nitrospiropyran (which

back-isomerizes in the dark over ~10 minutes at room temperature) is

considered photochromic All photochromic molecules back-isomerize to their

more stable form at some rate and this back-isomerization is accelerated by

heating There is therefore a close relationship between photochromic and the

rmochromic compounds The timescale of thermal back-isomerization is

important for applications and may be molecularly engineered

Photochromic compounds considered to be thermally stable include some

diarylethenes which do not back isomerize even after heating at 800C for 3

months

Since photochromic chromophores are dyes and operate according to

well-known reactions their molecular engineering to fine-tune their properties

can be achieved relatively easily using known design models quantum

mechanics calculations and experimentation In particular the tuning of

absorbance bands to particular parts of the spectrum and the engineering

of thermal stability have received much attention

Sometimes and particularly in the dye industry the term irreversible

photochromic is used to describe materials that undergo a permanent color

change upon exposure to Ultraviolet or visible light radiation Because by

24

definition photochromics are reversible there is technically no such thing as a

n irreversible photochromic this is loose usage and these compounds

are better referred to as photochangable or photoreactive dyes

Apart from the qualities already mentioned several other properties of

photochromics are important for their use These include

Quantum yield of the photochemical reaction

This determined the efficiency of the photochromic change with respect

to the amount of light absorbed The quantum yield of isomerization

can be strongly dependent on conditions

Fatigue resistance In photochromic materials fatigue refers to the

loss of reversibility by processes such as photodegradation

photobleaching photooxidation and other side reactions All

photochromics suffer fatigue to some extent and its rate is strongly

dependent on the activating light and the conditions of the sample

Photostationary state Photochromic materials have two states and

their interconversion can be controlled using different wavelengths of

light Excitation with any given wavelength of light will result in a

mixture of the two states at a particular ratio called the photo-

stationary state In a perfect system there would exist wavelengths

that can be used to provide 10 and 01 ratios of the isomers

but in real systems this is not possible since the active

absorbance bands always overlap to some extent

Polarity and solubility In order to incorporate photochromics in

working systems they suffer the same issues as other dyes They are

often charged in one or more state leading to very high polarity and

possible large changes in polarity They also often contain large

conjugated systems that limit their solubility

Photochromic complexes

A photochromic complex is a kind of chemical compound that has

photoresponsive Parts on its ligand These complexes have a specific

structure photoswitchable organic compounds are attached to

metalcomplexes For the photocontrollable parts thermally and

photochemically stable chromophores (azobenzene diarylethene

25

spiropyran etc) are usually used And for the metal complexes a wide

variety of compounds that have various functions (redox response

luminescence magnetism etc) are applied The photochromic parts and

metal parts are so close that they can affect each others molecular

orbitals The physical properties of these compounds shown by parts of

them (ie chromophores or metals) thus can be controlled by switching

their other sites by external stimuli For example photoisomerization

behaviors of some complexes can be switched by oxidation and reduction of

their metal parts Some other compounds can be changed in their

luminescence behavior magnetic interaction of metal sites or stability of

metal-to-ligand coordination by photoisomerization of their photochromic

parts

Photochemistry of Polyoxometalates

The photochemistry of polyoxometalates is of great interest to inorganic

chemistsMore than 80 years agoit was found that the R-Keggin tungstate

H3[PW12O40] was reduced photochemically to yield a blue-colored species

which was reoxidized by air and by various other oxidizing agents such as

Fe3+AgNO3and H2O2 [7879]The photoredox reactions of H4[SiW12O40]and

H3[PW12O40] proceeded effectively in the presence of primary and secondary

alcohols their ethers and aldehydes and proteinsbut less effectively in the

presence of tertiary alcoholsketonesestersthe fatty acids above formic

acidand simple amines[8081] The basic photoredox reaction involving

ethanol is illustrated by eq 13

2 H3PW12O40 + H3CCH2OH h ν ν ν ν 2 H4PW12O40 + H3CCHO ------- 13

2 H4PW12O40 +12 O2 2 H3PW12O40 + H2O ------ 14

In this reactionone molecule of ethanol photochemically reduces two

molecules of H3PW12O40 and is itself oxidized to acetaldehyde In the

presence of air the thermal oxidation of the reduced species takes place at

room temperature(eq 14)The reduced polyoxometalates which are the so-

calledldquoheteropolybluesrdquo have been used for the colorimetric analysis of the

26

elements P Si As and Ge and for the determination of uric acidsugarand

other biological compounds [8283] Piperidinium metavanadate also

undergoes photoinduced coloration from white to black followed by a

reversible color change in the presence of oxidizing agentsHowever

ammonium metavanadates ([NH4][VO3]) exhibits no photoinduced

coloration[84] The early photoredox reactions of the R-Keggin

polyoxometalates H4[SiW12O40] and H3PW12O40 were carried out in the

presence of photographic paper however the limited number of the

structurally well-characterized compounds available for study delayed the

development of modern cluster-compound photochemistry until the discovery

of photochromism in alkylammonium polyoxo- molybdate solids[8586] A

photochromic or electrochromic material is one whose light-absorbing

properties are altered upon optical excitation or reduction under the influence

of an externally applied electric field respectively The induced coloration

remains even after the excitation source has been removed These materials

are of technological interest because they return to their original state either

thermally upon irradiation with light of a frequency corresponding to the

induced absorption or electrochemically upon reversing the polarity of the

externally applied electric field Thus photochromic and electrochromic

materials behave in a reversible manner Polyoxometalates exhibit significant

photo-and electrochromism which makes them suitable as nanocomposite

molecular devices and as models for probing the physical properties of infinite

metal oxides Since the metal ions in the oxidized polyoxometalates have d0

electronic configurations the only absorption band which occurs in the UV-vis

range of the electronic spectra is due to the oxygen-to-metal (O-M) ligand-to-

metal charge transfer (LMCT)Upon irradiation electrons are promoted from

the low-energy electronic states which are mainly comprised of oxygen 2p

orbitals (the valence band in the band model)to the high-energy electronic

states which are mainly comprised of metal d orbitalrsquos (the conduction band

in the band model)The fundamental transitions in polyoxometalate lattices

are depicted schematically in Fig1B1

27

Fig1B1 Simple model showing the electronic transitions in the

polyoxometalates containing electron donar and acceptor (a)generation

of charge carriers(b)electron and hole trapping(c)electron release due

to stimulation(d)recombination

between electron and hole Electrons are e-and holes are h+

In the polyoxometalates containing heteroatoms and especially in

mixed metal polyoxometalates the charge carriers which are created by the

light or electric field may be trapped in electron traps and hole traps These

traps provide states of localized energy in the O-M LMCT energy gap due to

the heteroatoms or counter cations which correspond to impurities or lattice

defects in the band model If the trap depth ∆E is large compared to kT the

probability for thermal escape from the trap will be negligibly small and

metastable situation will existThe trapped carriers can be released by thermal

or optical stimulationIn the case of thermal stimulation the irradiated

polyoxometalate is heated until the energy barrier ∆E can be overcome The

trapped electron (or hole) then can escape from the trap and nonradiatively

recombine with the trapped hole (or electron)Under optical stimulation the

energy of an incident photon is used to overcome ∆E The relaxation

processes of the OndashM LMCT excitation energy include both the nonradiative

recombination of electrons and holes within the energy gap and the

intramolecular energy transfer leading to a charge-transfer emission This

intramolecular energy transfer corresponds to the O-M LMCT energy gap and

occurs via radiative recombination and sensitized emission from the

28

heteroatoms or cations If several energy levels based on the hetero atoms or

counter cations act as energy acceptors within the O-M LMCTenergy gap the

energy transfer occurs from the O-M LMCT states to these levels followed by

the nonradiative or radiative deactivation of the excitation energy It should be

noted that the O-M LMCT states also can be generated by the application of

very high electric fields to the polyoxometalate solids as demonstrated by the

observation of electroluminescence[87]If an external electric field with a

potential more negative than the energy levels of the vacant orbitals involved

in the O-M LMCT transition is applied to a polyoxometalate on the electrode

surface an electrochemical reduction occurs via the injection of electrons

from the electrode in to the vacant levels of the polyoxometalate as shown in

Fig1B 2

Fig1B2 - Energy scheme for the electrochromism of polyoxometalates

a)electrochemical reduction (b) electrochemical oxidation

Electrons injected in to the high-energy levels also may be trapped by

electron traps in a process analogous to that which occurs during LMCT

photoexcitation of the polyoxometalates These electrons are returned to the

electrode by electrochemical oxidation at an electrode potential more positive

than the energy levels for the d1 electron states The d1electrons in the O-M

LMCTstates facilitate the absorption of visible light via intervalence charge

transfer among metal centers and d-d transitions The same type of transition

may be possible for the d1electron captured by the electron traps too In

addition to searching for new photosensitive polyoxometalates with the

29

potential for having practical application there is now a need to elucidate the

fundamental photo-and electrochemical coloration processes by studying

electron transfer within the polyoxometalate lattices in conjunction with their

crystal structures So far few polyoxometalates exhibit a perfect reversibility

of coloration The irreversibility of the color change arises from as yet

uncharacterized side reactions during both the coloration and decoloration of

the polyoxometalates

Many metal oxides including aluminum titanium vanadium niobium

molybdenum and tungsten oxides are photochromic when they contain

impurities or dopants This coloration has been interpreted on the basis of

electron trapping at appropriate lattice sites within the crystals as shown in

Fig1B1 where the O-M LMCT transition corresponds to the transition

between the valence and conduction bands for the infinite metal-oxide lattice

[88-91]

B12 Applications of Photochromic materials

Sunglasses

One of the most famous reversible photochromic applications is color

changing lenses for sunglasses as found in eyeglasses The largest limitation

in using PC technology is that the materials cannot be made stable enough

to withstand thousands of hours of outdoor exposure so long-term outdoor

applications are not appropriate at this time The switching speed of

photochromic dyes is highly sensitive to the rigidity of the environment around

the dye As result they switch most rapidly in solution and slowest in the rigid

environment like a polymer lens Recently it has been reported that attaching

flexible low Tg polymers (for example siloxanes or poly (butyl acrylate) to the

dyes allows them to switch much more rapidly in a rigid lens [76] Some

spirooxazines with siloxane polymers attached switch at near solution like

speeds even though they are in a rigid lens matrix

Supramolecular chemistry

Photochromic units have been employed extensively in supramolecular

chemistry Their ability to give a light controlled reversible shape change

means that they can be used to make or break molecular recognition motifs

30

or to cause a consequent shape change in their surroundings Thus

photochromic units have been demonstrated as components of molecular

switches The coupling of photochromic units to enzymes or enzyme cofactors

even provides the ability to reversibly turn enzymes on and off

by altering their shape or orientation in such a way that their functions

are either working or broken

Data storage

The possibility of using photochromic compounds for data storage was

first suggested in 1956 by Yehuda Hirshberg[77] Since that time there have

been many investigations by various academic and commercial groups

particularly in the area of 3D optical data storage which promises discs that

can hold a terabyte of data Initially issues with thermal back-reactions

and destructive reading dogged these studies but more recently more stable

systems have been developed

Novelty items

Reversible photochromics are also found in applications such as toys

cosmeticsclothing and industrial applications If necessary they can be made

to change between desired colors by combination with a permanent pigment

A large number of inorganic compounds exhibit photochromism

These solids often have large band gaps of the order of 3 - 12 eV and

excitation of these solids leads to the formation of metastable centers that

absorb visible light giving rise to their colour They can return to their ground

state by heating or by optical excitation within the colour-centre band In most

cases the photochromism is a structure sensitive phenomenon involving

localized defect impurities or dislocations Some of these inorganic

compounds have the potential for a number of different uses Photochromic

compounds have a number of useful applications These can be divided

according to the most important property that is being used (Table 11) [92]

31

Table 11 Applications of Photochromic materials

Applications Depending Upon

Sensitivity to

Radiation

Reversibility Thermal Chemical or

Physical Properties

Self-developing

photography

Chemical switches for

computers

Temperature indicators

Protective

materials

Data displays

Heat-sensitive recording

media

Optical signal

processing

Photomasking and

photoresist technology

Reusable data storage

media

Anaytical reagents

Photochromic

microimages

Photopolymerisation

Information encoding

and steganography

Photocontractile

polymers and the

photoviscosity effect

Control of light

intensity

Q-switches

Pyroelectric

photochromic materials

B13 Thermochromism

Thermochromism is the reversible colour change of a substance

induced by temperature change A large variety of substances organic

inorganic organometallic supramolecular and polymeric systems exhibit this

phenomenon Examples of these include bianthrones cobalt

hexacyanoferrate the zirconocene complex of 1 4-diphenyl-1 3-butadiene

and poly (3-alkylthiophene) The organic 99-bixanthenylidene is colourless at

90 K yellow-green at 298 K and dark-blue when melted at 592 K Heating

conducting polymers can cause them to change colour This is achieved by

causing conformational changes to the polymer backbone resulting in a

change in the band gap of the polymer It has been reported that regioregular

32

P3HT reversibly changes colour upon heating to 220ordmC due to temperature-

dependent conformation changes Thermally cross linked polymer undergoes

the same colour change but it is much less reversible [93] Other forms of

thermochromism may be commercially important eg to give a visual

indication of temperature changes

B14 Electrochromism

Electrochromism describes a phenomenon of material color change in

a persistent but reversible manner produced by electrochemically induced

oxidation-reduction reactions Electrochromic materials can be applied to

various kinds of products such as smart windows and display devices Among

those applications there have been lots of efforts to develop electrochromic

display devices (ECDs) Especially flexible display devices are now attracting

much attention worldwide since they can facilitate new technological demands

such as bending and folding of paper-like displays High electrochromic

efficiency short response time long operating life time and reduction of

energy consumption are the most important requisites of the materials for the

paperlike displays [94 95] Among those properties the operation life time is

the most important barrier to overcome for a realization of ECDs There are

two types of electrochromic material a) inorganic transition metal oxides

(TMOs) b) organic polymer materials The TMOs have been studied longer

than the organic materials that they have been studied since 1960s [9697]

Electrochromism describes a reversible color change of material

produced by electrochemically induced oxidation-reduction reactions It is one

of several types of chromism of materials As thermochromism and

photochromism mean material color changes made by heat and light

respectively electrochromism refers that the color change is caused by an

electric potential In most cases the color change in electrochromism can be

driven by rather low electrical potential of the order of a fraction of volt or a

few volts [94-96] The color change of material means variation in

transmittance andor reflectance change in visible range which is originated

from different electronic absorption bands according to a switching between

oxidation and reduction state of material When electric potential is applied on

electrochromic material forced oxidation or reduction is derived and the

33

individual color is originated from the corresponding oxidation or reduction

state of the material For electrochromic materials the characteristic color

change is reversible since the oxidation and reduction state can be converted

reversibly by switching potential

Application fields

The application area for the electrochromism is rather broad that it

covers from smart window glazing and optical modulators to information

displays [98-102] The smart windows are typical examples The

electrochromic property is used to control the amount of light and heat to pass

through the windows Usually the electrochromic material is in form of thin film

coated on a window glass The transmittance modulation has also been

applied at the automobiles to automatically tint rear-view mirrors in various

lighting conditions The electrochromic application fields are illustrated in Fig

1B3

Since the smart windows control the transmittance of heat as well as

the transmittance of visible light the working definition of electrochromism has

now been extended to include devices for modulation of radiation in the near

infrared thermal infrared and microwave regions When color for

electrochromic materials is used this can now mean a response by detectors

at these wavelengths and not just by the human eyes Nowadays

electrochromic material draws much attention as being used in the display

devices Electrochromic display device (ECD) is being considered as one of

the candidates for the conventional liquid crystal display (LCD) since ECD

has many advantages over LCDs Among these advantages the most

important are low energy consumption wider viewing angle high contrast

rate and possibility to achieve multiple colors with a single material [103]

When a new redox state of electrochromic materialis established by

the applied electric pulse then it is maintained after the potential is switched

off This means the colored or bleached state of the material can be sustained

for a considerable time without applying electrical power This is so-called

ldquocolor memory effectrdquo of electrochromic material Because of the color

memory effect energy consumption for the electrochromic display device

could be drastically reduced and this would be a big advantage over other

34

emissive devices The low power consumption is especially advantageous

when it is applied to mobile devices with limited power source The possibility

for a flexible display is another attraction for electrochromic material

Information displays

Real-view mirrors for automobiles

Fig 1B4 Application fields of electrochromic devices Smart windows information displays and real-view mirrors for automobiles

35

Using polymeric electrochromic material and plastic substrate with a

transparent conductive coating it is possible to build all-plastic flexible display

device There are lots of papers and patents about the flexible display devices

[104105] In these cases gel-type electrolyte is also needed The gel-type

electrolyte immobilizes liquid electrolyte in the polymer network [106107]

Recently ITO-coated polymer films are readily available which would provide

the plastic substrate for a flexible device The flexible electrochromic displays

are frequently tried with the plastic substrate flexible electrochromic material

and gel-type electrolyte The flexible electrochromic displays would facilitate

increasing technical demands for foldable display devices

Metal Ion Electrochromism

Many transition metal oxides are capable of redox reactions that result in

colour change Metal oxide films are commonly prepared as thin layers of

either tungsten nickel molybdenum or other metal compounds by a number

of techniques These include sol-gel electrochemical by dc or rfreactive

sputtering techniques electron-beam evaporation by anodic or cathodic

electrodeposition or by solution dipping of the electrochromic metal

compounds (or compounds that can be changed into these metal compounds)

onto optically transparent electrodes (OTE) [108 -114] Their electrochromism

is derived from the colour change associated with a change in the oxidation

state of the metal anion The behaviour of these materials is dependent upon

pH moisture and exposure to the atmosphere [115] Generally the switching

rates of these films is somewhat slow with typical switching times of about 15

- 60 seconds to achieve 100 conversion to either coloured or bleached state

[116 -120] An Example of this includes nickel oxide which changes from

transparent (pale green) to brownblack taking about 30 seconds to do so

[121] Other examples include [(NH4)5Ru]2(pyrazine)5+ and [(NH4)5Ru]2(44-

bipyridine)5+ whose electrochromism is significantly different due to the effect

of the ligand [122] Table 12 below gives some examples of metal oxide films

with electrochromic properties

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

18

The electrochromic performance of all solid ndash state cells employing

phosphotungstic acid and phosphomolybdic acid is reported These cells

employ SnO2 as the viewing electrode and graphite as the back electrodeThe

cells in the bleached state can be made white to red and become black in the

coloured state [57]

Various organic compounds were oxidized by molecular oxygen in the

presence of a catalytic amount of mixed addenda heteropolyoxometalates

containing molybdenum and vanadium The catalytic activity of the

Molybdovanadophosphate was found to be greatly enhanced by supporting

on charcoal The supported catalyst has high catalytic activity for oxidative

dehydrogenation of benzylic and allylic alcohols to the corresponding

aldehydes and ketones (46-92) nevertheless the nonsupported catalyst

was inactive for the same oxidations under these conditions 236Trimethyl

phenol was selectively oxidized to trimethyl-p benzoquinone which is

precursor of VitE in the presence of a catalytic amount of

molybdophosphate In addition the aerobic oxidation of amines alkyl-

substituted phenols and alkanes were also examined [58]

The reaction of Tl2CO3 with 111555-hexafluoro-24-pentanedione

and diglyme CH3O(CH2CH2O)2CH3 or tetraglyme CH3O(CH2CH2O)4CH3 in

dichloromethane yields the anhydrous thermally and air stable volatile Tl

diglyme and Tl tetraglyme adducts They have been characterized by single

crystal X-ray diffraction elemental analysis 1H and 13C NMR IR and mass

spectroscopy Thermal and mass-transport properties have been investigated

using thermo gravimetric and differential scanning calorimetric

measurements There is evidence that both precursors are very low melting

and volatile and can be used as liquid Tl sources Both adducts have been

successfully applied to metalndashorganic chemical vapor deposition of thallium

containing films [59]

Large size and high quality single crystals of quasi-two-dimensional

thallium molybdenum purple bronze TlMo6O17 have been grown by electrolytic

reduction of molten salt of Tl2CO3-MoO3 The crystal structure is trigonal with

space group P3m1 determined by X-ray diffraction and four-circle single crystal

diffraction The lattice parameters of the unit cell are a = b = 55282 Adeg and c

= 136991 Adeg The temperature dependence of resistivity and magnetic

19

susceptibility confirmed that a metal-to-metal transition occurs near 110 K

[60]

Hall coefficient and dc conductivity studies were made on p-type

Pb08Sn02Te thin films doped with different concentrations of thallium in the

temperature range 77 to 500 K The Hall coefficient and Hall mobility are

found to decrease with an increase in the doping concentration of thallium

Hall coefficient data have been analyzed in the light of a double valence-band

model Various band parameters such as valence band separation population

ratio mobility ratio and effective mass ratio have been calculated Hall

mobility data have been analyzed in the light of lattice and defect limited

scattering mechanisms [61]

Proton conducting composites of heteropolyacid hydrates phosphomol

ybdic acid H3PMo12O40 nH2O(PMA) phosphotungstic acid H3PW12O40 nH2O

( PTA) and salt hydrate like NiCl2 6H2O were prepared

with insulating Al2O3 as despersoidThe ionic conductivity peaks at two

concentrations of Al2O3 indicating two percolation thresholds for percolation

thresholds for proton conduction Two separate experiments were carried out

to check the existence of such percolation thresholds viz the volta battery

experiment involving the measurement of emf of an electrochemical cell

with composites of different compositions used as electrolyte and the

composition vs conductivity measured by the complex impedance

spectroscopy The presence of two maxima has been attributed to two

different percolation thresholds for the two possible mobile protonic

species H+ + (H3O+) and OH arising from the hydrates [62]

1 A5 Applications of Heteropolyoxometalates-

Applications of heteropolyanions centre depend on their redox properties

their high charges and ionic weights An enormous patent and journal

literature is devoted to the applications of heteropolyanions

1 Analysis-

The formation and subsequent precipitation or reduction of

[XMO12O40]n- anions form the basis of gravimetric and colorimetric analytical

methods for P As Si or Geeither separately or in combination [63 64]

20

2 Biochemical applications-

lsquoPhosphotungstic Acidsrsquo have for decades been used as precipitants

for proteins and as analytical reagents for proteinsalkaloidsand purines eg

the [P2M18O62]6- anions for colorimetric determination of uric acid [65] and

cholesterol The acid H3P12O40 either in aqueous or ethanolic solution is also

widely used as a non specific electron dense stain for electron spectroscopy

The dyestuffs industry has for many years used heteropolymolybdates and

tungstates to form color lakes and toners from basic dyes Large

heteropolyanions exhibit antiviral antitumoral properties at non-cytotoxic

doses in vitro and in vivo and are protein inhibitors of cellular bacterial and

viral DNA RNA polymerizes [66]

3 Catalysis

Heteropoly acids and salts have been used as heterogeneous catalysts

for a broad variety of reactions and compilations of such applications up to

1973 are available Examples include oxidation of propylene and isobutylene

to acrylic acid methacrylic acids and ammoxidation of acrylonitrile olefin

polymerization and epoxidation Much of current activity concerning

heterogeneous catalysis by heteropoly compounds is being carried out in

Japan [67 68]

4 Other Applications

Insoluble salts of Heteropolyanions especially ammonium

molybdophosphates have been used and are commercially available as ion-

exchange materials [69]Recent work in this area includes thin layer

chromatography of amino acids ion selective membranes [70] and the

preparation of new ion exchangers based on heteropolyanions Crystalline12-

tungstophosphoric and 12-molybdophosphoric acids are excellent protonic

conductors Heteropolyacids are electrochromic in the solid state as a

consequence of heteropolyblue formation Heteropolyblue formation has also

been used to detect alcohol or carboxylic acid radicals generated by radiolysis

of aqueous solutions Potential applications of heteropoly complexes as flame

retardants and smoke suppressants or as corrosion inhibitors and conversion

coatings on steel and aluminium are reported [71] Some potential ldquogreenrdquo

applications have been reported eg non-chlorine based wood pulp

bleaching process and a method of decontaminating water Some structures

21

containing transition metal atoms with unpaired electrons have unusual

magnetic properties and are being investigated as nano computer storage

devices Some compounds exhibit luminescence There are many reported

potential medicinal applications eg anti tumoral and anti-viral There have

been reports on the role of weak or non bonding interactions on the crystal

engineering of hybrid polyoxometalates

Spherical nonporous polyoxomolybdate based capsules of different

types containing more than 100 metal atoms reported by Achim Muller and his

group have versatile unique properties regarding their assembly to vesicles

and the chemistry which can be done inside the pores and cavities A discrete

polyoxometalate Lindquist ion of the form W6O192minus was successfully imaged

recently for the first time within the capillary of a carbon nanotube following

steric locking of the anion with the tubule In situ relaxation of the anion in its

equatorial plain was demonstrated [72]

22

Section-B

Litreture Survey on Chromism in Transition Metal

Oxides

1 B1 Chromism in Transition Metal Oxides

Chromism is a reversible change in a substances colour resulting from

a process caused by some form of stimulus Many materials are chromic

including inorganic and organic compounds and conducting polymers and the

property can result from many different mechanisms Several transition metal

oxides show EC properties The most popular are from the VI - B oxides In

this group WO3 and MoO3 are the most thoroughly studied cathodic EC

materials which can be electrochemically coloured and bleached when used

as the cathode in electrochemical cells Cathodic EC materials also include

V2O3 TiO2 and Nb2O5 Another distinguishable group is anodic EC material

including VIII oxides like IrOx nH2O Rh2O3 nH2O NiO nH2O etc which can be

anodicaly coloured in the electrochemical process when used as anode

There are several types of chromism which are discussed as below

B11 Photochromism

Photochromism is the reversible transformation of a chemical species

between two forms by the absorption of electromagnetic radiation where

the two forms have different absorption spectra [7374]

Trivially this can be described as a reversible change of color upon

exposure to light The phenomenon was discovered in the late 1880s

including work by Markwald who studied the reversible change of color of 23

44-tetrachloronaphthalen-1(4H)-one in the solid state He labeled this

phenomenon phototropy and this name was used until the 1950s

when Yehuda Hirshberg of the Weizmann Institute of Science in Israel

proposed the term photochromism [75] Photochromism can take place

in both organic and inorganic compounds and also has its place in biological

systems (for example retinal in the vision process)

Photochromism does not have a rigorous definition but is usually used

to describe compounds that undergo a reversible photochemical reaction

where an absorption band in the visible part of the electromagnetic spectrum

23

changes dramatically in strength or wavelength In many cases an

absorbance band is present in only one form The degree of change required

for a photochemical reaction to be dubbed photochromic is that which

appears dramatic by eye but in essence there is no dividing line between

photochromic reactions and other photochemistry Therefore while the

transcis isomerization of azobenzene is considered a photochromic reaction

the analogous reaction of stilbene is not Since photochromism is just a

special case of a photochemical reaction almost any photochemical reaction

type may be used to produce photochromism with appropriate molecular desi

gnSome of the most common processes involved in photochromism are peric

yclic reactions cis-trans somerizations intramolecular hydrogen transfer

intramolecular group transfers dissociation processes and electron transfers

(oxidation-reduction)

Another some what arbitrary requirement of photochromism is that

it requires the two states of the molecule to be thermally stable under

ambient conditions for a reasonable time All the same nitrospiropyran (which

back-isomerizes in the dark over ~10 minutes at room temperature) is

considered photochromic All photochromic molecules back-isomerize to their

more stable form at some rate and this back-isomerization is accelerated by

heating There is therefore a close relationship between photochromic and the

rmochromic compounds The timescale of thermal back-isomerization is

important for applications and may be molecularly engineered

Photochromic compounds considered to be thermally stable include some

diarylethenes which do not back isomerize even after heating at 800C for 3

months

Since photochromic chromophores are dyes and operate according to

well-known reactions their molecular engineering to fine-tune their properties

can be achieved relatively easily using known design models quantum

mechanics calculations and experimentation In particular the tuning of

absorbance bands to particular parts of the spectrum and the engineering

of thermal stability have received much attention

Sometimes and particularly in the dye industry the term irreversible

photochromic is used to describe materials that undergo a permanent color

change upon exposure to Ultraviolet or visible light radiation Because by

24

definition photochromics are reversible there is technically no such thing as a

n irreversible photochromic this is loose usage and these compounds

are better referred to as photochangable or photoreactive dyes

Apart from the qualities already mentioned several other properties of

photochromics are important for their use These include

Quantum yield of the photochemical reaction

This determined the efficiency of the photochromic change with respect

to the amount of light absorbed The quantum yield of isomerization

can be strongly dependent on conditions

Fatigue resistance In photochromic materials fatigue refers to the

loss of reversibility by processes such as photodegradation

photobleaching photooxidation and other side reactions All

photochromics suffer fatigue to some extent and its rate is strongly

dependent on the activating light and the conditions of the sample

Photostationary state Photochromic materials have two states and

their interconversion can be controlled using different wavelengths of

light Excitation with any given wavelength of light will result in a

mixture of the two states at a particular ratio called the photo-

stationary state In a perfect system there would exist wavelengths

that can be used to provide 10 and 01 ratios of the isomers

but in real systems this is not possible since the active

absorbance bands always overlap to some extent

Polarity and solubility In order to incorporate photochromics in

working systems they suffer the same issues as other dyes They are

often charged in one or more state leading to very high polarity and

possible large changes in polarity They also often contain large

conjugated systems that limit their solubility

Photochromic complexes

A photochromic complex is a kind of chemical compound that has

photoresponsive Parts on its ligand These complexes have a specific

structure photoswitchable organic compounds are attached to

metalcomplexes For the photocontrollable parts thermally and

photochemically stable chromophores (azobenzene diarylethene

25

spiropyran etc) are usually used And for the metal complexes a wide

variety of compounds that have various functions (redox response

luminescence magnetism etc) are applied The photochromic parts and

metal parts are so close that they can affect each others molecular

orbitals The physical properties of these compounds shown by parts of

them (ie chromophores or metals) thus can be controlled by switching

their other sites by external stimuli For example photoisomerization

behaviors of some complexes can be switched by oxidation and reduction of

their metal parts Some other compounds can be changed in their

luminescence behavior magnetic interaction of metal sites or stability of

metal-to-ligand coordination by photoisomerization of their photochromic

parts

Photochemistry of Polyoxometalates

The photochemistry of polyoxometalates is of great interest to inorganic

chemistsMore than 80 years agoit was found that the R-Keggin tungstate

H3[PW12O40] was reduced photochemically to yield a blue-colored species

which was reoxidized by air and by various other oxidizing agents such as

Fe3+AgNO3and H2O2 [7879]The photoredox reactions of H4[SiW12O40]and

H3[PW12O40] proceeded effectively in the presence of primary and secondary

alcohols their ethers and aldehydes and proteinsbut less effectively in the

presence of tertiary alcoholsketonesestersthe fatty acids above formic

acidand simple amines[8081] The basic photoredox reaction involving

ethanol is illustrated by eq 13

2 H3PW12O40 + H3CCH2OH h ν ν ν ν 2 H4PW12O40 + H3CCHO ------- 13

2 H4PW12O40 +12 O2 2 H3PW12O40 + H2O ------ 14

In this reactionone molecule of ethanol photochemically reduces two

molecules of H3PW12O40 and is itself oxidized to acetaldehyde In the

presence of air the thermal oxidation of the reduced species takes place at

room temperature(eq 14)The reduced polyoxometalates which are the so-

calledldquoheteropolybluesrdquo have been used for the colorimetric analysis of the

26

elements P Si As and Ge and for the determination of uric acidsugarand

other biological compounds [8283] Piperidinium metavanadate also

undergoes photoinduced coloration from white to black followed by a

reversible color change in the presence of oxidizing agentsHowever

ammonium metavanadates ([NH4][VO3]) exhibits no photoinduced

coloration[84] The early photoredox reactions of the R-Keggin

polyoxometalates H4[SiW12O40] and H3PW12O40 were carried out in the

presence of photographic paper however the limited number of the

structurally well-characterized compounds available for study delayed the

development of modern cluster-compound photochemistry until the discovery

of photochromism in alkylammonium polyoxo- molybdate solids[8586] A

photochromic or electrochromic material is one whose light-absorbing

properties are altered upon optical excitation or reduction under the influence

of an externally applied electric field respectively The induced coloration

remains even after the excitation source has been removed These materials

are of technological interest because they return to their original state either

thermally upon irradiation with light of a frequency corresponding to the

induced absorption or electrochemically upon reversing the polarity of the

externally applied electric field Thus photochromic and electrochromic

materials behave in a reversible manner Polyoxometalates exhibit significant

photo-and electrochromism which makes them suitable as nanocomposite

molecular devices and as models for probing the physical properties of infinite

metal oxides Since the metal ions in the oxidized polyoxometalates have d0

electronic configurations the only absorption band which occurs in the UV-vis

range of the electronic spectra is due to the oxygen-to-metal (O-M) ligand-to-

metal charge transfer (LMCT)Upon irradiation electrons are promoted from

the low-energy electronic states which are mainly comprised of oxygen 2p

orbitals (the valence band in the band model)to the high-energy electronic

states which are mainly comprised of metal d orbitalrsquos (the conduction band

in the band model)The fundamental transitions in polyoxometalate lattices

are depicted schematically in Fig1B1

27

Fig1B1 Simple model showing the electronic transitions in the

polyoxometalates containing electron donar and acceptor (a)generation

of charge carriers(b)electron and hole trapping(c)electron release due

to stimulation(d)recombination

between electron and hole Electrons are e-and holes are h+

In the polyoxometalates containing heteroatoms and especially in

mixed metal polyoxometalates the charge carriers which are created by the

light or electric field may be trapped in electron traps and hole traps These

traps provide states of localized energy in the O-M LMCT energy gap due to

the heteroatoms or counter cations which correspond to impurities or lattice

defects in the band model If the trap depth ∆E is large compared to kT the

probability for thermal escape from the trap will be negligibly small and

metastable situation will existThe trapped carriers can be released by thermal

or optical stimulationIn the case of thermal stimulation the irradiated

polyoxometalate is heated until the energy barrier ∆E can be overcome The

trapped electron (or hole) then can escape from the trap and nonradiatively

recombine with the trapped hole (or electron)Under optical stimulation the

energy of an incident photon is used to overcome ∆E The relaxation

processes of the OndashM LMCT excitation energy include both the nonradiative

recombination of electrons and holes within the energy gap and the

intramolecular energy transfer leading to a charge-transfer emission This

intramolecular energy transfer corresponds to the O-M LMCT energy gap and

occurs via radiative recombination and sensitized emission from the

28

heteroatoms or cations If several energy levels based on the hetero atoms or

counter cations act as energy acceptors within the O-M LMCTenergy gap the

energy transfer occurs from the O-M LMCT states to these levels followed by

the nonradiative or radiative deactivation of the excitation energy It should be

noted that the O-M LMCT states also can be generated by the application of

very high electric fields to the polyoxometalate solids as demonstrated by the

observation of electroluminescence[87]If an external electric field with a

potential more negative than the energy levels of the vacant orbitals involved

in the O-M LMCT transition is applied to a polyoxometalate on the electrode

surface an electrochemical reduction occurs via the injection of electrons

from the electrode in to the vacant levels of the polyoxometalate as shown in

Fig1B 2

Fig1B2 - Energy scheme for the electrochromism of polyoxometalates

a)electrochemical reduction (b) electrochemical oxidation

Electrons injected in to the high-energy levels also may be trapped by

electron traps in a process analogous to that which occurs during LMCT

photoexcitation of the polyoxometalates These electrons are returned to the

electrode by electrochemical oxidation at an electrode potential more positive

than the energy levels for the d1 electron states The d1electrons in the O-M

LMCTstates facilitate the absorption of visible light via intervalence charge

transfer among metal centers and d-d transitions The same type of transition

may be possible for the d1electron captured by the electron traps too In

addition to searching for new photosensitive polyoxometalates with the

29

potential for having practical application there is now a need to elucidate the

fundamental photo-and electrochemical coloration processes by studying

electron transfer within the polyoxometalate lattices in conjunction with their

crystal structures So far few polyoxometalates exhibit a perfect reversibility

of coloration The irreversibility of the color change arises from as yet

uncharacterized side reactions during both the coloration and decoloration of

the polyoxometalates

Many metal oxides including aluminum titanium vanadium niobium

molybdenum and tungsten oxides are photochromic when they contain

impurities or dopants This coloration has been interpreted on the basis of

electron trapping at appropriate lattice sites within the crystals as shown in

Fig1B1 where the O-M LMCT transition corresponds to the transition

between the valence and conduction bands for the infinite metal-oxide lattice

[88-91]

B12 Applications of Photochromic materials

Sunglasses

One of the most famous reversible photochromic applications is color

changing lenses for sunglasses as found in eyeglasses The largest limitation

in using PC technology is that the materials cannot be made stable enough

to withstand thousands of hours of outdoor exposure so long-term outdoor

applications are not appropriate at this time The switching speed of

photochromic dyes is highly sensitive to the rigidity of the environment around

the dye As result they switch most rapidly in solution and slowest in the rigid

environment like a polymer lens Recently it has been reported that attaching

flexible low Tg polymers (for example siloxanes or poly (butyl acrylate) to the

dyes allows them to switch much more rapidly in a rigid lens [76] Some

spirooxazines with siloxane polymers attached switch at near solution like

speeds even though they are in a rigid lens matrix

Supramolecular chemistry

Photochromic units have been employed extensively in supramolecular

chemistry Their ability to give a light controlled reversible shape change

means that they can be used to make or break molecular recognition motifs

30

or to cause a consequent shape change in their surroundings Thus

photochromic units have been demonstrated as components of molecular

switches The coupling of photochromic units to enzymes or enzyme cofactors

even provides the ability to reversibly turn enzymes on and off

by altering their shape or orientation in such a way that their functions

are either working or broken

Data storage

The possibility of using photochromic compounds for data storage was

first suggested in 1956 by Yehuda Hirshberg[77] Since that time there have

been many investigations by various academic and commercial groups

particularly in the area of 3D optical data storage which promises discs that

can hold a terabyte of data Initially issues with thermal back-reactions

and destructive reading dogged these studies but more recently more stable

systems have been developed

Novelty items

Reversible photochromics are also found in applications such as toys

cosmeticsclothing and industrial applications If necessary they can be made

to change between desired colors by combination with a permanent pigment

A large number of inorganic compounds exhibit photochromism

These solids often have large band gaps of the order of 3 - 12 eV and

excitation of these solids leads to the formation of metastable centers that

absorb visible light giving rise to their colour They can return to their ground

state by heating or by optical excitation within the colour-centre band In most

cases the photochromism is a structure sensitive phenomenon involving

localized defect impurities or dislocations Some of these inorganic

compounds have the potential for a number of different uses Photochromic

compounds have a number of useful applications These can be divided

according to the most important property that is being used (Table 11) [92]

31

Table 11 Applications of Photochromic materials

Applications Depending Upon

Sensitivity to

Radiation

Reversibility Thermal Chemical or

Physical Properties

Self-developing

photography

Chemical switches for

computers

Temperature indicators

Protective

materials

Data displays

Heat-sensitive recording

media

Optical signal

processing

Photomasking and

photoresist technology

Reusable data storage

media

Anaytical reagents

Photochromic

microimages

Photopolymerisation

Information encoding

and steganography

Photocontractile

polymers and the

photoviscosity effect

Control of light

intensity

Q-switches

Pyroelectric

photochromic materials

B13 Thermochromism

Thermochromism is the reversible colour change of a substance

induced by temperature change A large variety of substances organic

inorganic organometallic supramolecular and polymeric systems exhibit this

phenomenon Examples of these include bianthrones cobalt

hexacyanoferrate the zirconocene complex of 1 4-diphenyl-1 3-butadiene

and poly (3-alkylthiophene) The organic 99-bixanthenylidene is colourless at

90 K yellow-green at 298 K and dark-blue when melted at 592 K Heating

conducting polymers can cause them to change colour This is achieved by

causing conformational changes to the polymer backbone resulting in a

change in the band gap of the polymer It has been reported that regioregular

32

P3HT reversibly changes colour upon heating to 220ordmC due to temperature-

dependent conformation changes Thermally cross linked polymer undergoes

the same colour change but it is much less reversible [93] Other forms of

thermochromism may be commercially important eg to give a visual

indication of temperature changes

B14 Electrochromism

Electrochromism describes a phenomenon of material color change in

a persistent but reversible manner produced by electrochemically induced

oxidation-reduction reactions Electrochromic materials can be applied to

various kinds of products such as smart windows and display devices Among

those applications there have been lots of efforts to develop electrochromic

display devices (ECDs) Especially flexible display devices are now attracting

much attention worldwide since they can facilitate new technological demands

such as bending and folding of paper-like displays High electrochromic

efficiency short response time long operating life time and reduction of

energy consumption are the most important requisites of the materials for the

paperlike displays [94 95] Among those properties the operation life time is

the most important barrier to overcome for a realization of ECDs There are

two types of electrochromic material a) inorganic transition metal oxides

(TMOs) b) organic polymer materials The TMOs have been studied longer

than the organic materials that they have been studied since 1960s [9697]

Electrochromism describes a reversible color change of material

produced by electrochemically induced oxidation-reduction reactions It is one

of several types of chromism of materials As thermochromism and

photochromism mean material color changes made by heat and light

respectively electrochromism refers that the color change is caused by an

electric potential In most cases the color change in electrochromism can be

driven by rather low electrical potential of the order of a fraction of volt or a

few volts [94-96] The color change of material means variation in

transmittance andor reflectance change in visible range which is originated

from different electronic absorption bands according to a switching between

oxidation and reduction state of material When electric potential is applied on

electrochromic material forced oxidation or reduction is derived and the

33

individual color is originated from the corresponding oxidation or reduction

state of the material For electrochromic materials the characteristic color

change is reversible since the oxidation and reduction state can be converted

reversibly by switching potential

Application fields

The application area for the electrochromism is rather broad that it

covers from smart window glazing and optical modulators to information

displays [98-102] The smart windows are typical examples The

electrochromic property is used to control the amount of light and heat to pass

through the windows Usually the electrochromic material is in form of thin film

coated on a window glass The transmittance modulation has also been

applied at the automobiles to automatically tint rear-view mirrors in various

lighting conditions The electrochromic application fields are illustrated in Fig

1B3

Since the smart windows control the transmittance of heat as well as

the transmittance of visible light the working definition of electrochromism has

now been extended to include devices for modulation of radiation in the near

infrared thermal infrared and microwave regions When color for

electrochromic materials is used this can now mean a response by detectors

at these wavelengths and not just by the human eyes Nowadays

electrochromic material draws much attention as being used in the display

devices Electrochromic display device (ECD) is being considered as one of

the candidates for the conventional liquid crystal display (LCD) since ECD

has many advantages over LCDs Among these advantages the most

important are low energy consumption wider viewing angle high contrast

rate and possibility to achieve multiple colors with a single material [103]

When a new redox state of electrochromic materialis established by

the applied electric pulse then it is maintained after the potential is switched

off This means the colored or bleached state of the material can be sustained

for a considerable time without applying electrical power This is so-called

ldquocolor memory effectrdquo of electrochromic material Because of the color

memory effect energy consumption for the electrochromic display device

could be drastically reduced and this would be a big advantage over other

34

emissive devices The low power consumption is especially advantageous

when it is applied to mobile devices with limited power source The possibility

for a flexible display is another attraction for electrochromic material

Information displays

Real-view mirrors for automobiles

Fig 1B4 Application fields of electrochromic devices Smart windows information displays and real-view mirrors for automobiles

35

Using polymeric electrochromic material and plastic substrate with a

transparent conductive coating it is possible to build all-plastic flexible display

device There are lots of papers and patents about the flexible display devices

[104105] In these cases gel-type electrolyte is also needed The gel-type

electrolyte immobilizes liquid electrolyte in the polymer network [106107]

Recently ITO-coated polymer films are readily available which would provide

the plastic substrate for a flexible device The flexible electrochromic displays

are frequently tried with the plastic substrate flexible electrochromic material

and gel-type electrolyte The flexible electrochromic displays would facilitate

increasing technical demands for foldable display devices

Metal Ion Electrochromism

Many transition metal oxides are capable of redox reactions that result in

colour change Metal oxide films are commonly prepared as thin layers of

either tungsten nickel molybdenum or other metal compounds by a number

of techniques These include sol-gel electrochemical by dc or rfreactive

sputtering techniques electron-beam evaporation by anodic or cathodic

electrodeposition or by solution dipping of the electrochromic metal

compounds (or compounds that can be changed into these metal compounds)

onto optically transparent electrodes (OTE) [108 -114] Their electrochromism

is derived from the colour change associated with a change in the oxidation

state of the metal anion The behaviour of these materials is dependent upon

pH moisture and exposure to the atmosphere [115] Generally the switching

rates of these films is somewhat slow with typical switching times of about 15

- 60 seconds to achieve 100 conversion to either coloured or bleached state

[116 -120] An Example of this includes nickel oxide which changes from

transparent (pale green) to brownblack taking about 30 seconds to do so

[121] Other examples include [(NH4)5Ru]2(pyrazine)5+ and [(NH4)5Ru]2(44-

bipyridine)5+ whose electrochromism is significantly different due to the effect

of the ligand [122] Table 12 below gives some examples of metal oxide films

with electrochromic properties

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

19

susceptibility confirmed that a metal-to-metal transition occurs near 110 K

[60]

Hall coefficient and dc conductivity studies were made on p-type

Pb08Sn02Te thin films doped with different concentrations of thallium in the

temperature range 77 to 500 K The Hall coefficient and Hall mobility are

found to decrease with an increase in the doping concentration of thallium

Hall coefficient data have been analyzed in the light of a double valence-band

model Various band parameters such as valence band separation population

ratio mobility ratio and effective mass ratio have been calculated Hall

mobility data have been analyzed in the light of lattice and defect limited

scattering mechanisms [61]

Proton conducting composites of heteropolyacid hydrates phosphomol

ybdic acid H3PMo12O40 nH2O(PMA) phosphotungstic acid H3PW12O40 nH2O

( PTA) and salt hydrate like NiCl2 6H2O were prepared

with insulating Al2O3 as despersoidThe ionic conductivity peaks at two

concentrations of Al2O3 indicating two percolation thresholds for percolation

thresholds for proton conduction Two separate experiments were carried out

to check the existence of such percolation thresholds viz the volta battery

experiment involving the measurement of emf of an electrochemical cell

with composites of different compositions used as electrolyte and the

composition vs conductivity measured by the complex impedance

spectroscopy The presence of two maxima has been attributed to two

different percolation thresholds for the two possible mobile protonic

species H+ + (H3O+) and OH arising from the hydrates [62]

1 A5 Applications of Heteropolyoxometalates-

Applications of heteropolyanions centre depend on their redox properties

their high charges and ionic weights An enormous patent and journal

literature is devoted to the applications of heteropolyanions

1 Analysis-

The formation and subsequent precipitation or reduction of

[XMO12O40]n- anions form the basis of gravimetric and colorimetric analytical

methods for P As Si or Geeither separately or in combination [63 64]

20

2 Biochemical applications-

lsquoPhosphotungstic Acidsrsquo have for decades been used as precipitants

for proteins and as analytical reagents for proteinsalkaloidsand purines eg

the [P2M18O62]6- anions for colorimetric determination of uric acid [65] and

cholesterol The acid H3P12O40 either in aqueous or ethanolic solution is also

widely used as a non specific electron dense stain for electron spectroscopy

The dyestuffs industry has for many years used heteropolymolybdates and

tungstates to form color lakes and toners from basic dyes Large

heteropolyanions exhibit antiviral antitumoral properties at non-cytotoxic

doses in vitro and in vivo and are protein inhibitors of cellular bacterial and

viral DNA RNA polymerizes [66]

3 Catalysis

Heteropoly acids and salts have been used as heterogeneous catalysts

for a broad variety of reactions and compilations of such applications up to

1973 are available Examples include oxidation of propylene and isobutylene

to acrylic acid methacrylic acids and ammoxidation of acrylonitrile olefin

polymerization and epoxidation Much of current activity concerning

heterogeneous catalysis by heteropoly compounds is being carried out in

Japan [67 68]

4 Other Applications

Insoluble salts of Heteropolyanions especially ammonium

molybdophosphates have been used and are commercially available as ion-

exchange materials [69]Recent work in this area includes thin layer

chromatography of amino acids ion selective membranes [70] and the

preparation of new ion exchangers based on heteropolyanions Crystalline12-

tungstophosphoric and 12-molybdophosphoric acids are excellent protonic

conductors Heteropolyacids are electrochromic in the solid state as a

consequence of heteropolyblue formation Heteropolyblue formation has also

been used to detect alcohol or carboxylic acid radicals generated by radiolysis

of aqueous solutions Potential applications of heteropoly complexes as flame

retardants and smoke suppressants or as corrosion inhibitors and conversion

coatings on steel and aluminium are reported [71] Some potential ldquogreenrdquo

applications have been reported eg non-chlorine based wood pulp

bleaching process and a method of decontaminating water Some structures

21

containing transition metal atoms with unpaired electrons have unusual

magnetic properties and are being investigated as nano computer storage

devices Some compounds exhibit luminescence There are many reported

potential medicinal applications eg anti tumoral and anti-viral There have

been reports on the role of weak or non bonding interactions on the crystal

engineering of hybrid polyoxometalates

Spherical nonporous polyoxomolybdate based capsules of different

types containing more than 100 metal atoms reported by Achim Muller and his

group have versatile unique properties regarding their assembly to vesicles

and the chemistry which can be done inside the pores and cavities A discrete

polyoxometalate Lindquist ion of the form W6O192minus was successfully imaged

recently for the first time within the capillary of a carbon nanotube following

steric locking of the anion with the tubule In situ relaxation of the anion in its

equatorial plain was demonstrated [72]

22

Section-B

Litreture Survey on Chromism in Transition Metal

Oxides

1 B1 Chromism in Transition Metal Oxides

Chromism is a reversible change in a substances colour resulting from

a process caused by some form of stimulus Many materials are chromic

including inorganic and organic compounds and conducting polymers and the

property can result from many different mechanisms Several transition metal

oxides show EC properties The most popular are from the VI - B oxides In

this group WO3 and MoO3 are the most thoroughly studied cathodic EC

materials which can be electrochemically coloured and bleached when used

as the cathode in electrochemical cells Cathodic EC materials also include

V2O3 TiO2 and Nb2O5 Another distinguishable group is anodic EC material

including VIII oxides like IrOx nH2O Rh2O3 nH2O NiO nH2O etc which can be

anodicaly coloured in the electrochemical process when used as anode

There are several types of chromism which are discussed as below

B11 Photochromism

Photochromism is the reversible transformation of a chemical species

between two forms by the absorption of electromagnetic radiation where

the two forms have different absorption spectra [7374]

Trivially this can be described as a reversible change of color upon

exposure to light The phenomenon was discovered in the late 1880s

including work by Markwald who studied the reversible change of color of 23

44-tetrachloronaphthalen-1(4H)-one in the solid state He labeled this

phenomenon phototropy and this name was used until the 1950s

when Yehuda Hirshberg of the Weizmann Institute of Science in Israel

proposed the term photochromism [75] Photochromism can take place

in both organic and inorganic compounds and also has its place in biological

systems (for example retinal in the vision process)

Photochromism does not have a rigorous definition but is usually used

to describe compounds that undergo a reversible photochemical reaction

where an absorption band in the visible part of the electromagnetic spectrum

23

changes dramatically in strength or wavelength In many cases an

absorbance band is present in only one form The degree of change required

for a photochemical reaction to be dubbed photochromic is that which

appears dramatic by eye but in essence there is no dividing line between

photochromic reactions and other photochemistry Therefore while the

transcis isomerization of azobenzene is considered a photochromic reaction

the analogous reaction of stilbene is not Since photochromism is just a

special case of a photochemical reaction almost any photochemical reaction

type may be used to produce photochromism with appropriate molecular desi

gnSome of the most common processes involved in photochromism are peric

yclic reactions cis-trans somerizations intramolecular hydrogen transfer

intramolecular group transfers dissociation processes and electron transfers

(oxidation-reduction)

Another some what arbitrary requirement of photochromism is that

it requires the two states of the molecule to be thermally stable under

ambient conditions for a reasonable time All the same nitrospiropyran (which

back-isomerizes in the dark over ~10 minutes at room temperature) is

considered photochromic All photochromic molecules back-isomerize to their

more stable form at some rate and this back-isomerization is accelerated by

heating There is therefore a close relationship between photochromic and the

rmochromic compounds The timescale of thermal back-isomerization is

important for applications and may be molecularly engineered

Photochromic compounds considered to be thermally stable include some

diarylethenes which do not back isomerize even after heating at 800C for 3

months

Since photochromic chromophores are dyes and operate according to

well-known reactions their molecular engineering to fine-tune their properties

can be achieved relatively easily using known design models quantum

mechanics calculations and experimentation In particular the tuning of

absorbance bands to particular parts of the spectrum and the engineering

of thermal stability have received much attention

Sometimes and particularly in the dye industry the term irreversible

photochromic is used to describe materials that undergo a permanent color

change upon exposure to Ultraviolet or visible light radiation Because by

24

definition photochromics are reversible there is technically no such thing as a

n irreversible photochromic this is loose usage and these compounds

are better referred to as photochangable or photoreactive dyes

Apart from the qualities already mentioned several other properties of

photochromics are important for their use These include

Quantum yield of the photochemical reaction

This determined the efficiency of the photochromic change with respect

to the amount of light absorbed The quantum yield of isomerization

can be strongly dependent on conditions

Fatigue resistance In photochromic materials fatigue refers to the

loss of reversibility by processes such as photodegradation

photobleaching photooxidation and other side reactions All

photochromics suffer fatigue to some extent and its rate is strongly

dependent on the activating light and the conditions of the sample

Photostationary state Photochromic materials have two states and

their interconversion can be controlled using different wavelengths of

light Excitation with any given wavelength of light will result in a

mixture of the two states at a particular ratio called the photo-

stationary state In a perfect system there would exist wavelengths

that can be used to provide 10 and 01 ratios of the isomers

but in real systems this is not possible since the active

absorbance bands always overlap to some extent

Polarity and solubility In order to incorporate photochromics in

working systems they suffer the same issues as other dyes They are

often charged in one or more state leading to very high polarity and

possible large changes in polarity They also often contain large

conjugated systems that limit their solubility

Photochromic complexes

A photochromic complex is a kind of chemical compound that has

photoresponsive Parts on its ligand These complexes have a specific

structure photoswitchable organic compounds are attached to

metalcomplexes For the photocontrollable parts thermally and

photochemically stable chromophores (azobenzene diarylethene

25

spiropyran etc) are usually used And for the metal complexes a wide

variety of compounds that have various functions (redox response

luminescence magnetism etc) are applied The photochromic parts and

metal parts are so close that they can affect each others molecular

orbitals The physical properties of these compounds shown by parts of

them (ie chromophores or metals) thus can be controlled by switching

their other sites by external stimuli For example photoisomerization

behaviors of some complexes can be switched by oxidation and reduction of

their metal parts Some other compounds can be changed in their

luminescence behavior magnetic interaction of metal sites or stability of

metal-to-ligand coordination by photoisomerization of their photochromic

parts

Photochemistry of Polyoxometalates

The photochemistry of polyoxometalates is of great interest to inorganic

chemistsMore than 80 years agoit was found that the R-Keggin tungstate

H3[PW12O40] was reduced photochemically to yield a blue-colored species

which was reoxidized by air and by various other oxidizing agents such as

Fe3+AgNO3and H2O2 [7879]The photoredox reactions of H4[SiW12O40]and

H3[PW12O40] proceeded effectively in the presence of primary and secondary

alcohols their ethers and aldehydes and proteinsbut less effectively in the

presence of tertiary alcoholsketonesestersthe fatty acids above formic

acidand simple amines[8081] The basic photoredox reaction involving

ethanol is illustrated by eq 13

2 H3PW12O40 + H3CCH2OH h ν ν ν ν 2 H4PW12O40 + H3CCHO ------- 13

2 H4PW12O40 +12 O2 2 H3PW12O40 + H2O ------ 14

In this reactionone molecule of ethanol photochemically reduces two

molecules of H3PW12O40 and is itself oxidized to acetaldehyde In the

presence of air the thermal oxidation of the reduced species takes place at

room temperature(eq 14)The reduced polyoxometalates which are the so-

calledldquoheteropolybluesrdquo have been used for the colorimetric analysis of the

26

elements P Si As and Ge and for the determination of uric acidsugarand

other biological compounds [8283] Piperidinium metavanadate also

undergoes photoinduced coloration from white to black followed by a

reversible color change in the presence of oxidizing agentsHowever

ammonium metavanadates ([NH4][VO3]) exhibits no photoinduced

coloration[84] The early photoredox reactions of the R-Keggin

polyoxometalates H4[SiW12O40] and H3PW12O40 were carried out in the

presence of photographic paper however the limited number of the

structurally well-characterized compounds available for study delayed the

development of modern cluster-compound photochemistry until the discovery

of photochromism in alkylammonium polyoxo- molybdate solids[8586] A

photochromic or electrochromic material is one whose light-absorbing

properties are altered upon optical excitation or reduction under the influence

of an externally applied electric field respectively The induced coloration

remains even after the excitation source has been removed These materials

are of technological interest because they return to their original state either

thermally upon irradiation with light of a frequency corresponding to the

induced absorption or electrochemically upon reversing the polarity of the

externally applied electric field Thus photochromic and electrochromic

materials behave in a reversible manner Polyoxometalates exhibit significant

photo-and electrochromism which makes them suitable as nanocomposite

molecular devices and as models for probing the physical properties of infinite

metal oxides Since the metal ions in the oxidized polyoxometalates have d0

electronic configurations the only absorption band which occurs in the UV-vis

range of the electronic spectra is due to the oxygen-to-metal (O-M) ligand-to-

metal charge transfer (LMCT)Upon irradiation electrons are promoted from

the low-energy electronic states which are mainly comprised of oxygen 2p

orbitals (the valence band in the band model)to the high-energy electronic

states which are mainly comprised of metal d orbitalrsquos (the conduction band

in the band model)The fundamental transitions in polyoxometalate lattices

are depicted schematically in Fig1B1

27

Fig1B1 Simple model showing the electronic transitions in the

polyoxometalates containing electron donar and acceptor (a)generation

of charge carriers(b)electron and hole trapping(c)electron release due

to stimulation(d)recombination

between electron and hole Electrons are e-and holes are h+

In the polyoxometalates containing heteroatoms and especially in

mixed metal polyoxometalates the charge carriers which are created by the

light or electric field may be trapped in electron traps and hole traps These

traps provide states of localized energy in the O-M LMCT energy gap due to

the heteroatoms or counter cations which correspond to impurities or lattice

defects in the band model If the trap depth ∆E is large compared to kT the

probability for thermal escape from the trap will be negligibly small and

metastable situation will existThe trapped carriers can be released by thermal

or optical stimulationIn the case of thermal stimulation the irradiated

polyoxometalate is heated until the energy barrier ∆E can be overcome The

trapped electron (or hole) then can escape from the trap and nonradiatively

recombine with the trapped hole (or electron)Under optical stimulation the

energy of an incident photon is used to overcome ∆E The relaxation

processes of the OndashM LMCT excitation energy include both the nonradiative

recombination of electrons and holes within the energy gap and the

intramolecular energy transfer leading to a charge-transfer emission This

intramolecular energy transfer corresponds to the O-M LMCT energy gap and

occurs via radiative recombination and sensitized emission from the

28

heteroatoms or cations If several energy levels based on the hetero atoms or

counter cations act as energy acceptors within the O-M LMCTenergy gap the

energy transfer occurs from the O-M LMCT states to these levels followed by

the nonradiative or radiative deactivation of the excitation energy It should be

noted that the O-M LMCT states also can be generated by the application of

very high electric fields to the polyoxometalate solids as demonstrated by the

observation of electroluminescence[87]If an external electric field with a

potential more negative than the energy levels of the vacant orbitals involved

in the O-M LMCT transition is applied to a polyoxometalate on the electrode

surface an electrochemical reduction occurs via the injection of electrons

from the electrode in to the vacant levels of the polyoxometalate as shown in

Fig1B 2

Fig1B2 - Energy scheme for the electrochromism of polyoxometalates

a)electrochemical reduction (b) electrochemical oxidation

Electrons injected in to the high-energy levels also may be trapped by

electron traps in a process analogous to that which occurs during LMCT

photoexcitation of the polyoxometalates These electrons are returned to the

electrode by electrochemical oxidation at an electrode potential more positive

than the energy levels for the d1 electron states The d1electrons in the O-M

LMCTstates facilitate the absorption of visible light via intervalence charge

transfer among metal centers and d-d transitions The same type of transition

may be possible for the d1electron captured by the electron traps too In

addition to searching for new photosensitive polyoxometalates with the

29

potential for having practical application there is now a need to elucidate the

fundamental photo-and electrochemical coloration processes by studying

electron transfer within the polyoxometalate lattices in conjunction with their

crystal structures So far few polyoxometalates exhibit a perfect reversibility

of coloration The irreversibility of the color change arises from as yet

uncharacterized side reactions during both the coloration and decoloration of

the polyoxometalates

Many metal oxides including aluminum titanium vanadium niobium

molybdenum and tungsten oxides are photochromic when they contain

impurities or dopants This coloration has been interpreted on the basis of

electron trapping at appropriate lattice sites within the crystals as shown in

Fig1B1 where the O-M LMCT transition corresponds to the transition

between the valence and conduction bands for the infinite metal-oxide lattice

[88-91]

B12 Applications of Photochromic materials

Sunglasses

One of the most famous reversible photochromic applications is color

changing lenses for sunglasses as found in eyeglasses The largest limitation

in using PC technology is that the materials cannot be made stable enough

to withstand thousands of hours of outdoor exposure so long-term outdoor

applications are not appropriate at this time The switching speed of

photochromic dyes is highly sensitive to the rigidity of the environment around

the dye As result they switch most rapidly in solution and slowest in the rigid

environment like a polymer lens Recently it has been reported that attaching

flexible low Tg polymers (for example siloxanes or poly (butyl acrylate) to the

dyes allows them to switch much more rapidly in a rigid lens [76] Some

spirooxazines with siloxane polymers attached switch at near solution like

speeds even though they are in a rigid lens matrix

Supramolecular chemistry

Photochromic units have been employed extensively in supramolecular

chemistry Their ability to give a light controlled reversible shape change

means that they can be used to make or break molecular recognition motifs

30

or to cause a consequent shape change in their surroundings Thus

photochromic units have been demonstrated as components of molecular

switches The coupling of photochromic units to enzymes or enzyme cofactors

even provides the ability to reversibly turn enzymes on and off

by altering their shape or orientation in such a way that their functions

are either working or broken

Data storage

The possibility of using photochromic compounds for data storage was

first suggested in 1956 by Yehuda Hirshberg[77] Since that time there have

been many investigations by various academic and commercial groups

particularly in the area of 3D optical data storage which promises discs that

can hold a terabyte of data Initially issues with thermal back-reactions

and destructive reading dogged these studies but more recently more stable

systems have been developed

Novelty items

Reversible photochromics are also found in applications such as toys

cosmeticsclothing and industrial applications If necessary they can be made

to change between desired colors by combination with a permanent pigment

A large number of inorganic compounds exhibit photochromism

These solids often have large band gaps of the order of 3 - 12 eV and

excitation of these solids leads to the formation of metastable centers that

absorb visible light giving rise to their colour They can return to their ground

state by heating or by optical excitation within the colour-centre band In most

cases the photochromism is a structure sensitive phenomenon involving

localized defect impurities or dislocations Some of these inorganic

compounds have the potential for a number of different uses Photochromic

compounds have a number of useful applications These can be divided

according to the most important property that is being used (Table 11) [92]

31

Table 11 Applications of Photochromic materials

Applications Depending Upon

Sensitivity to

Radiation

Reversibility Thermal Chemical or

Physical Properties

Self-developing

photography

Chemical switches for

computers

Temperature indicators

Protective

materials

Data displays

Heat-sensitive recording

media

Optical signal

processing

Photomasking and

photoresist technology

Reusable data storage

media

Anaytical reagents

Photochromic

microimages

Photopolymerisation

Information encoding

and steganography

Photocontractile

polymers and the

photoviscosity effect

Control of light

intensity

Q-switches

Pyroelectric

photochromic materials

B13 Thermochromism

Thermochromism is the reversible colour change of a substance

induced by temperature change A large variety of substances organic

inorganic organometallic supramolecular and polymeric systems exhibit this

phenomenon Examples of these include bianthrones cobalt

hexacyanoferrate the zirconocene complex of 1 4-diphenyl-1 3-butadiene

and poly (3-alkylthiophene) The organic 99-bixanthenylidene is colourless at

90 K yellow-green at 298 K and dark-blue when melted at 592 K Heating

conducting polymers can cause them to change colour This is achieved by

causing conformational changes to the polymer backbone resulting in a

change in the band gap of the polymer It has been reported that regioregular

32

P3HT reversibly changes colour upon heating to 220ordmC due to temperature-

dependent conformation changes Thermally cross linked polymer undergoes

the same colour change but it is much less reversible [93] Other forms of

thermochromism may be commercially important eg to give a visual

indication of temperature changes

B14 Electrochromism

Electrochromism describes a phenomenon of material color change in

a persistent but reversible manner produced by electrochemically induced

oxidation-reduction reactions Electrochromic materials can be applied to

various kinds of products such as smart windows and display devices Among

those applications there have been lots of efforts to develop electrochromic

display devices (ECDs) Especially flexible display devices are now attracting

much attention worldwide since they can facilitate new technological demands

such as bending and folding of paper-like displays High electrochromic

efficiency short response time long operating life time and reduction of

energy consumption are the most important requisites of the materials for the

paperlike displays [94 95] Among those properties the operation life time is

the most important barrier to overcome for a realization of ECDs There are

two types of electrochromic material a) inorganic transition metal oxides

(TMOs) b) organic polymer materials The TMOs have been studied longer

than the organic materials that they have been studied since 1960s [9697]

Electrochromism describes a reversible color change of material

produced by electrochemically induced oxidation-reduction reactions It is one

of several types of chromism of materials As thermochromism and

photochromism mean material color changes made by heat and light

respectively electrochromism refers that the color change is caused by an

electric potential In most cases the color change in electrochromism can be

driven by rather low electrical potential of the order of a fraction of volt or a

few volts [94-96] The color change of material means variation in

transmittance andor reflectance change in visible range which is originated

from different electronic absorption bands according to a switching between

oxidation and reduction state of material When electric potential is applied on

electrochromic material forced oxidation or reduction is derived and the

33

individual color is originated from the corresponding oxidation or reduction

state of the material For electrochromic materials the characteristic color

change is reversible since the oxidation and reduction state can be converted

reversibly by switching potential

Application fields

The application area for the electrochromism is rather broad that it

covers from smart window glazing and optical modulators to information

displays [98-102] The smart windows are typical examples The

electrochromic property is used to control the amount of light and heat to pass

through the windows Usually the electrochromic material is in form of thin film

coated on a window glass The transmittance modulation has also been

applied at the automobiles to automatically tint rear-view mirrors in various

lighting conditions The electrochromic application fields are illustrated in Fig

1B3

Since the smart windows control the transmittance of heat as well as

the transmittance of visible light the working definition of electrochromism has

now been extended to include devices for modulation of radiation in the near

infrared thermal infrared and microwave regions When color for

electrochromic materials is used this can now mean a response by detectors

at these wavelengths and not just by the human eyes Nowadays

electrochromic material draws much attention as being used in the display

devices Electrochromic display device (ECD) is being considered as one of

the candidates for the conventional liquid crystal display (LCD) since ECD

has many advantages over LCDs Among these advantages the most

important are low energy consumption wider viewing angle high contrast

rate and possibility to achieve multiple colors with a single material [103]

When a new redox state of electrochromic materialis established by

the applied electric pulse then it is maintained after the potential is switched

off This means the colored or bleached state of the material can be sustained

for a considerable time without applying electrical power This is so-called

ldquocolor memory effectrdquo of electrochromic material Because of the color

memory effect energy consumption for the electrochromic display device

could be drastically reduced and this would be a big advantage over other

34

emissive devices The low power consumption is especially advantageous

when it is applied to mobile devices with limited power source The possibility

for a flexible display is another attraction for electrochromic material

Information displays

Real-view mirrors for automobiles

Fig 1B4 Application fields of electrochromic devices Smart windows information displays and real-view mirrors for automobiles

35

Using polymeric electrochromic material and plastic substrate with a

transparent conductive coating it is possible to build all-plastic flexible display

device There are lots of papers and patents about the flexible display devices

[104105] In these cases gel-type electrolyte is also needed The gel-type

electrolyte immobilizes liquid electrolyte in the polymer network [106107]

Recently ITO-coated polymer films are readily available which would provide

the plastic substrate for a flexible device The flexible electrochromic displays

are frequently tried with the plastic substrate flexible electrochromic material

and gel-type electrolyte The flexible electrochromic displays would facilitate

increasing technical demands for foldable display devices

Metal Ion Electrochromism

Many transition metal oxides are capable of redox reactions that result in

colour change Metal oxide films are commonly prepared as thin layers of

either tungsten nickel molybdenum or other metal compounds by a number

of techniques These include sol-gel electrochemical by dc or rfreactive

sputtering techniques electron-beam evaporation by anodic or cathodic

electrodeposition or by solution dipping of the electrochromic metal

compounds (or compounds that can be changed into these metal compounds)

onto optically transparent electrodes (OTE) [108 -114] Their electrochromism

is derived from the colour change associated with a change in the oxidation

state of the metal anion The behaviour of these materials is dependent upon

pH moisture and exposure to the atmosphere [115] Generally the switching

rates of these films is somewhat slow with typical switching times of about 15

- 60 seconds to achieve 100 conversion to either coloured or bleached state

[116 -120] An Example of this includes nickel oxide which changes from

transparent (pale green) to brownblack taking about 30 seconds to do so

[121] Other examples include [(NH4)5Ru]2(pyrazine)5+ and [(NH4)5Ru]2(44-

bipyridine)5+ whose electrochromism is significantly different due to the effect

of the ligand [122] Table 12 below gives some examples of metal oxide films

with electrochromic properties

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

20

2 Biochemical applications-

lsquoPhosphotungstic Acidsrsquo have for decades been used as precipitants

for proteins and as analytical reagents for proteinsalkaloidsand purines eg

the [P2M18O62]6- anions for colorimetric determination of uric acid [65] and

cholesterol The acid H3P12O40 either in aqueous or ethanolic solution is also

widely used as a non specific electron dense stain for electron spectroscopy

The dyestuffs industry has for many years used heteropolymolybdates and

tungstates to form color lakes and toners from basic dyes Large

heteropolyanions exhibit antiviral antitumoral properties at non-cytotoxic

doses in vitro and in vivo and are protein inhibitors of cellular bacterial and

viral DNA RNA polymerizes [66]

3 Catalysis

Heteropoly acids and salts have been used as heterogeneous catalysts

for a broad variety of reactions and compilations of such applications up to

1973 are available Examples include oxidation of propylene and isobutylene

to acrylic acid methacrylic acids and ammoxidation of acrylonitrile olefin

polymerization and epoxidation Much of current activity concerning

heterogeneous catalysis by heteropoly compounds is being carried out in

Japan [67 68]

4 Other Applications

Insoluble salts of Heteropolyanions especially ammonium

molybdophosphates have been used and are commercially available as ion-

exchange materials [69]Recent work in this area includes thin layer

chromatography of amino acids ion selective membranes [70] and the

preparation of new ion exchangers based on heteropolyanions Crystalline12-

tungstophosphoric and 12-molybdophosphoric acids are excellent protonic

conductors Heteropolyacids are electrochromic in the solid state as a

consequence of heteropolyblue formation Heteropolyblue formation has also

been used to detect alcohol or carboxylic acid radicals generated by radiolysis

of aqueous solutions Potential applications of heteropoly complexes as flame

retardants and smoke suppressants or as corrosion inhibitors and conversion

coatings on steel and aluminium are reported [71] Some potential ldquogreenrdquo

applications have been reported eg non-chlorine based wood pulp

bleaching process and a method of decontaminating water Some structures

21

containing transition metal atoms with unpaired electrons have unusual

magnetic properties and are being investigated as nano computer storage

devices Some compounds exhibit luminescence There are many reported

potential medicinal applications eg anti tumoral and anti-viral There have

been reports on the role of weak or non bonding interactions on the crystal

engineering of hybrid polyoxometalates

Spherical nonporous polyoxomolybdate based capsules of different

types containing more than 100 metal atoms reported by Achim Muller and his

group have versatile unique properties regarding their assembly to vesicles

and the chemistry which can be done inside the pores and cavities A discrete

polyoxometalate Lindquist ion of the form W6O192minus was successfully imaged

recently for the first time within the capillary of a carbon nanotube following

steric locking of the anion with the tubule In situ relaxation of the anion in its

equatorial plain was demonstrated [72]

22

Section-B

Litreture Survey on Chromism in Transition Metal

Oxides

1 B1 Chromism in Transition Metal Oxides

Chromism is a reversible change in a substances colour resulting from

a process caused by some form of stimulus Many materials are chromic

including inorganic and organic compounds and conducting polymers and the

property can result from many different mechanisms Several transition metal

oxides show EC properties The most popular are from the VI - B oxides In

this group WO3 and MoO3 are the most thoroughly studied cathodic EC

materials which can be electrochemically coloured and bleached when used

as the cathode in electrochemical cells Cathodic EC materials also include

V2O3 TiO2 and Nb2O5 Another distinguishable group is anodic EC material

including VIII oxides like IrOx nH2O Rh2O3 nH2O NiO nH2O etc which can be

anodicaly coloured in the electrochemical process when used as anode

There are several types of chromism which are discussed as below

B11 Photochromism

Photochromism is the reversible transformation of a chemical species

between two forms by the absorption of electromagnetic radiation where

the two forms have different absorption spectra [7374]

Trivially this can be described as a reversible change of color upon

exposure to light The phenomenon was discovered in the late 1880s

including work by Markwald who studied the reversible change of color of 23

44-tetrachloronaphthalen-1(4H)-one in the solid state He labeled this

phenomenon phototropy and this name was used until the 1950s

when Yehuda Hirshberg of the Weizmann Institute of Science in Israel

proposed the term photochromism [75] Photochromism can take place

in both organic and inorganic compounds and also has its place in biological

systems (for example retinal in the vision process)

Photochromism does not have a rigorous definition but is usually used

to describe compounds that undergo a reversible photochemical reaction

where an absorption band in the visible part of the electromagnetic spectrum

23

changes dramatically in strength or wavelength In many cases an

absorbance band is present in only one form The degree of change required

for a photochemical reaction to be dubbed photochromic is that which

appears dramatic by eye but in essence there is no dividing line between

photochromic reactions and other photochemistry Therefore while the

transcis isomerization of azobenzene is considered a photochromic reaction

the analogous reaction of stilbene is not Since photochromism is just a

special case of a photochemical reaction almost any photochemical reaction

type may be used to produce photochromism with appropriate molecular desi

gnSome of the most common processes involved in photochromism are peric

yclic reactions cis-trans somerizations intramolecular hydrogen transfer

intramolecular group transfers dissociation processes and electron transfers

(oxidation-reduction)

Another some what arbitrary requirement of photochromism is that

it requires the two states of the molecule to be thermally stable under

ambient conditions for a reasonable time All the same nitrospiropyran (which

back-isomerizes in the dark over ~10 minutes at room temperature) is

considered photochromic All photochromic molecules back-isomerize to their

more stable form at some rate and this back-isomerization is accelerated by

heating There is therefore a close relationship between photochromic and the

rmochromic compounds The timescale of thermal back-isomerization is

important for applications and may be molecularly engineered

Photochromic compounds considered to be thermally stable include some

diarylethenes which do not back isomerize even after heating at 800C for 3

months

Since photochromic chromophores are dyes and operate according to

well-known reactions their molecular engineering to fine-tune their properties

can be achieved relatively easily using known design models quantum

mechanics calculations and experimentation In particular the tuning of

absorbance bands to particular parts of the spectrum and the engineering

of thermal stability have received much attention

Sometimes and particularly in the dye industry the term irreversible

photochromic is used to describe materials that undergo a permanent color

change upon exposure to Ultraviolet or visible light radiation Because by

24

definition photochromics are reversible there is technically no such thing as a

n irreversible photochromic this is loose usage and these compounds

are better referred to as photochangable or photoreactive dyes

Apart from the qualities already mentioned several other properties of

photochromics are important for their use These include

Quantum yield of the photochemical reaction

This determined the efficiency of the photochromic change with respect

to the amount of light absorbed The quantum yield of isomerization

can be strongly dependent on conditions

Fatigue resistance In photochromic materials fatigue refers to the

loss of reversibility by processes such as photodegradation

photobleaching photooxidation and other side reactions All

photochromics suffer fatigue to some extent and its rate is strongly

dependent on the activating light and the conditions of the sample

Photostationary state Photochromic materials have two states and

their interconversion can be controlled using different wavelengths of

light Excitation with any given wavelength of light will result in a

mixture of the two states at a particular ratio called the photo-

stationary state In a perfect system there would exist wavelengths

that can be used to provide 10 and 01 ratios of the isomers

but in real systems this is not possible since the active

absorbance bands always overlap to some extent

Polarity and solubility In order to incorporate photochromics in

working systems they suffer the same issues as other dyes They are

often charged in one or more state leading to very high polarity and

possible large changes in polarity They also often contain large

conjugated systems that limit their solubility

Photochromic complexes

A photochromic complex is a kind of chemical compound that has

photoresponsive Parts on its ligand These complexes have a specific

structure photoswitchable organic compounds are attached to

metalcomplexes For the photocontrollable parts thermally and

photochemically stable chromophores (azobenzene diarylethene

25

spiropyran etc) are usually used And for the metal complexes a wide

variety of compounds that have various functions (redox response

luminescence magnetism etc) are applied The photochromic parts and

metal parts are so close that they can affect each others molecular

orbitals The physical properties of these compounds shown by parts of

them (ie chromophores or metals) thus can be controlled by switching

their other sites by external stimuli For example photoisomerization

behaviors of some complexes can be switched by oxidation and reduction of

their metal parts Some other compounds can be changed in their

luminescence behavior magnetic interaction of metal sites or stability of

metal-to-ligand coordination by photoisomerization of their photochromic

parts

Photochemistry of Polyoxometalates

The photochemistry of polyoxometalates is of great interest to inorganic

chemistsMore than 80 years agoit was found that the R-Keggin tungstate

H3[PW12O40] was reduced photochemically to yield a blue-colored species

which was reoxidized by air and by various other oxidizing agents such as

Fe3+AgNO3and H2O2 [7879]The photoredox reactions of H4[SiW12O40]and

H3[PW12O40] proceeded effectively in the presence of primary and secondary

alcohols their ethers and aldehydes and proteinsbut less effectively in the

presence of tertiary alcoholsketonesestersthe fatty acids above formic

acidand simple amines[8081] The basic photoredox reaction involving

ethanol is illustrated by eq 13

2 H3PW12O40 + H3CCH2OH h ν ν ν ν 2 H4PW12O40 + H3CCHO ------- 13

2 H4PW12O40 +12 O2 2 H3PW12O40 + H2O ------ 14

In this reactionone molecule of ethanol photochemically reduces two

molecules of H3PW12O40 and is itself oxidized to acetaldehyde In the

presence of air the thermal oxidation of the reduced species takes place at

room temperature(eq 14)The reduced polyoxometalates which are the so-

calledldquoheteropolybluesrdquo have been used for the colorimetric analysis of the

26

elements P Si As and Ge and for the determination of uric acidsugarand

other biological compounds [8283] Piperidinium metavanadate also

undergoes photoinduced coloration from white to black followed by a

reversible color change in the presence of oxidizing agentsHowever

ammonium metavanadates ([NH4][VO3]) exhibits no photoinduced

coloration[84] The early photoredox reactions of the R-Keggin

polyoxometalates H4[SiW12O40] and H3PW12O40 were carried out in the

presence of photographic paper however the limited number of the

structurally well-characterized compounds available for study delayed the

development of modern cluster-compound photochemistry until the discovery

of photochromism in alkylammonium polyoxo- molybdate solids[8586] A

photochromic or electrochromic material is one whose light-absorbing

properties are altered upon optical excitation or reduction under the influence

of an externally applied electric field respectively The induced coloration

remains even after the excitation source has been removed These materials

are of technological interest because they return to their original state either

thermally upon irradiation with light of a frequency corresponding to the

induced absorption or electrochemically upon reversing the polarity of the

externally applied electric field Thus photochromic and electrochromic

materials behave in a reversible manner Polyoxometalates exhibit significant

photo-and electrochromism which makes them suitable as nanocomposite

molecular devices and as models for probing the physical properties of infinite

metal oxides Since the metal ions in the oxidized polyoxometalates have d0

electronic configurations the only absorption band which occurs in the UV-vis

range of the electronic spectra is due to the oxygen-to-metal (O-M) ligand-to-

metal charge transfer (LMCT)Upon irradiation electrons are promoted from

the low-energy electronic states which are mainly comprised of oxygen 2p

orbitals (the valence band in the band model)to the high-energy electronic

states which are mainly comprised of metal d orbitalrsquos (the conduction band

in the band model)The fundamental transitions in polyoxometalate lattices

are depicted schematically in Fig1B1

27

Fig1B1 Simple model showing the electronic transitions in the

polyoxometalates containing electron donar and acceptor (a)generation

of charge carriers(b)electron and hole trapping(c)electron release due

to stimulation(d)recombination

between electron and hole Electrons are e-and holes are h+

In the polyoxometalates containing heteroatoms and especially in

mixed metal polyoxometalates the charge carriers which are created by the

light or electric field may be trapped in electron traps and hole traps These

traps provide states of localized energy in the O-M LMCT energy gap due to

the heteroatoms or counter cations which correspond to impurities or lattice

defects in the band model If the trap depth ∆E is large compared to kT the

probability for thermal escape from the trap will be negligibly small and

metastable situation will existThe trapped carriers can be released by thermal

or optical stimulationIn the case of thermal stimulation the irradiated

polyoxometalate is heated until the energy barrier ∆E can be overcome The

trapped electron (or hole) then can escape from the trap and nonradiatively

recombine with the trapped hole (or electron)Under optical stimulation the

energy of an incident photon is used to overcome ∆E The relaxation

processes of the OndashM LMCT excitation energy include both the nonradiative

recombination of electrons and holes within the energy gap and the

intramolecular energy transfer leading to a charge-transfer emission This

intramolecular energy transfer corresponds to the O-M LMCT energy gap and

occurs via radiative recombination and sensitized emission from the

28

heteroatoms or cations If several energy levels based on the hetero atoms or

counter cations act as energy acceptors within the O-M LMCTenergy gap the

energy transfer occurs from the O-M LMCT states to these levels followed by

the nonradiative or radiative deactivation of the excitation energy It should be

noted that the O-M LMCT states also can be generated by the application of

very high electric fields to the polyoxometalate solids as demonstrated by the

observation of electroluminescence[87]If an external electric field with a

potential more negative than the energy levels of the vacant orbitals involved

in the O-M LMCT transition is applied to a polyoxometalate on the electrode

surface an electrochemical reduction occurs via the injection of electrons

from the electrode in to the vacant levels of the polyoxometalate as shown in

Fig1B 2

Fig1B2 - Energy scheme for the electrochromism of polyoxometalates

a)electrochemical reduction (b) electrochemical oxidation

Electrons injected in to the high-energy levels also may be trapped by

electron traps in a process analogous to that which occurs during LMCT

photoexcitation of the polyoxometalates These electrons are returned to the

electrode by electrochemical oxidation at an electrode potential more positive

than the energy levels for the d1 electron states The d1electrons in the O-M

LMCTstates facilitate the absorption of visible light via intervalence charge

transfer among metal centers and d-d transitions The same type of transition

may be possible for the d1electron captured by the electron traps too In

addition to searching for new photosensitive polyoxometalates with the

29

potential for having practical application there is now a need to elucidate the

fundamental photo-and electrochemical coloration processes by studying

electron transfer within the polyoxometalate lattices in conjunction with their

crystal structures So far few polyoxometalates exhibit a perfect reversibility

of coloration The irreversibility of the color change arises from as yet

uncharacterized side reactions during both the coloration and decoloration of

the polyoxometalates

Many metal oxides including aluminum titanium vanadium niobium

molybdenum and tungsten oxides are photochromic when they contain

impurities or dopants This coloration has been interpreted on the basis of

electron trapping at appropriate lattice sites within the crystals as shown in

Fig1B1 where the O-M LMCT transition corresponds to the transition

between the valence and conduction bands for the infinite metal-oxide lattice

[88-91]

B12 Applications of Photochromic materials

Sunglasses

One of the most famous reversible photochromic applications is color

changing lenses for sunglasses as found in eyeglasses The largest limitation

in using PC technology is that the materials cannot be made stable enough

to withstand thousands of hours of outdoor exposure so long-term outdoor

applications are not appropriate at this time The switching speed of

photochromic dyes is highly sensitive to the rigidity of the environment around

the dye As result they switch most rapidly in solution and slowest in the rigid

environment like a polymer lens Recently it has been reported that attaching

flexible low Tg polymers (for example siloxanes or poly (butyl acrylate) to the

dyes allows them to switch much more rapidly in a rigid lens [76] Some

spirooxazines with siloxane polymers attached switch at near solution like

speeds even though they are in a rigid lens matrix

Supramolecular chemistry

Photochromic units have been employed extensively in supramolecular

chemistry Their ability to give a light controlled reversible shape change

means that they can be used to make or break molecular recognition motifs

30

or to cause a consequent shape change in their surroundings Thus

photochromic units have been demonstrated as components of molecular

switches The coupling of photochromic units to enzymes or enzyme cofactors

even provides the ability to reversibly turn enzymes on and off

by altering their shape or orientation in such a way that their functions

are either working or broken

Data storage

The possibility of using photochromic compounds for data storage was

first suggested in 1956 by Yehuda Hirshberg[77] Since that time there have

been many investigations by various academic and commercial groups

particularly in the area of 3D optical data storage which promises discs that

can hold a terabyte of data Initially issues with thermal back-reactions

and destructive reading dogged these studies but more recently more stable

systems have been developed

Novelty items

Reversible photochromics are also found in applications such as toys

cosmeticsclothing and industrial applications If necessary they can be made

to change between desired colors by combination with a permanent pigment

A large number of inorganic compounds exhibit photochromism

These solids often have large band gaps of the order of 3 - 12 eV and

excitation of these solids leads to the formation of metastable centers that

absorb visible light giving rise to their colour They can return to their ground

state by heating or by optical excitation within the colour-centre band In most

cases the photochromism is a structure sensitive phenomenon involving

localized defect impurities or dislocations Some of these inorganic

compounds have the potential for a number of different uses Photochromic

compounds have a number of useful applications These can be divided

according to the most important property that is being used (Table 11) [92]

31

Table 11 Applications of Photochromic materials

Applications Depending Upon

Sensitivity to

Radiation

Reversibility Thermal Chemical or

Physical Properties

Self-developing

photography

Chemical switches for

computers

Temperature indicators

Protective

materials

Data displays

Heat-sensitive recording

media

Optical signal

processing

Photomasking and

photoresist technology

Reusable data storage

media

Anaytical reagents

Photochromic

microimages

Photopolymerisation

Information encoding

and steganography

Photocontractile

polymers and the

photoviscosity effect

Control of light

intensity

Q-switches

Pyroelectric

photochromic materials

B13 Thermochromism

Thermochromism is the reversible colour change of a substance

induced by temperature change A large variety of substances organic

inorganic organometallic supramolecular and polymeric systems exhibit this

phenomenon Examples of these include bianthrones cobalt

hexacyanoferrate the zirconocene complex of 1 4-diphenyl-1 3-butadiene

and poly (3-alkylthiophene) The organic 99-bixanthenylidene is colourless at

90 K yellow-green at 298 K and dark-blue when melted at 592 K Heating

conducting polymers can cause them to change colour This is achieved by

causing conformational changes to the polymer backbone resulting in a

change in the band gap of the polymer It has been reported that regioregular

32

P3HT reversibly changes colour upon heating to 220ordmC due to temperature-

dependent conformation changes Thermally cross linked polymer undergoes

the same colour change but it is much less reversible [93] Other forms of

thermochromism may be commercially important eg to give a visual

indication of temperature changes

B14 Electrochromism

Electrochromism describes a phenomenon of material color change in

a persistent but reversible manner produced by electrochemically induced

oxidation-reduction reactions Electrochromic materials can be applied to

various kinds of products such as smart windows and display devices Among

those applications there have been lots of efforts to develop electrochromic

display devices (ECDs) Especially flexible display devices are now attracting

much attention worldwide since they can facilitate new technological demands

such as bending and folding of paper-like displays High electrochromic

efficiency short response time long operating life time and reduction of

energy consumption are the most important requisites of the materials for the

paperlike displays [94 95] Among those properties the operation life time is

the most important barrier to overcome for a realization of ECDs There are

two types of electrochromic material a) inorganic transition metal oxides

(TMOs) b) organic polymer materials The TMOs have been studied longer

than the organic materials that they have been studied since 1960s [9697]

Electrochromism describes a reversible color change of material

produced by electrochemically induced oxidation-reduction reactions It is one

of several types of chromism of materials As thermochromism and

photochromism mean material color changes made by heat and light

respectively electrochromism refers that the color change is caused by an

electric potential In most cases the color change in electrochromism can be

driven by rather low electrical potential of the order of a fraction of volt or a

few volts [94-96] The color change of material means variation in

transmittance andor reflectance change in visible range which is originated

from different electronic absorption bands according to a switching between

oxidation and reduction state of material When electric potential is applied on

electrochromic material forced oxidation or reduction is derived and the

33

individual color is originated from the corresponding oxidation or reduction

state of the material For electrochromic materials the characteristic color

change is reversible since the oxidation and reduction state can be converted

reversibly by switching potential

Application fields

The application area for the electrochromism is rather broad that it

covers from smart window glazing and optical modulators to information

displays [98-102] The smart windows are typical examples The

electrochromic property is used to control the amount of light and heat to pass

through the windows Usually the electrochromic material is in form of thin film

coated on a window glass The transmittance modulation has also been

applied at the automobiles to automatically tint rear-view mirrors in various

lighting conditions The electrochromic application fields are illustrated in Fig

1B3

Since the smart windows control the transmittance of heat as well as

the transmittance of visible light the working definition of electrochromism has

now been extended to include devices for modulation of radiation in the near

infrared thermal infrared and microwave regions When color for

electrochromic materials is used this can now mean a response by detectors

at these wavelengths and not just by the human eyes Nowadays

electrochromic material draws much attention as being used in the display

devices Electrochromic display device (ECD) is being considered as one of

the candidates for the conventional liquid crystal display (LCD) since ECD

has many advantages over LCDs Among these advantages the most

important are low energy consumption wider viewing angle high contrast

rate and possibility to achieve multiple colors with a single material [103]

When a new redox state of electrochromic materialis established by

the applied electric pulse then it is maintained after the potential is switched

off This means the colored or bleached state of the material can be sustained

for a considerable time without applying electrical power This is so-called

ldquocolor memory effectrdquo of electrochromic material Because of the color

memory effect energy consumption for the electrochromic display device

could be drastically reduced and this would be a big advantage over other

34

emissive devices The low power consumption is especially advantageous

when it is applied to mobile devices with limited power source The possibility

for a flexible display is another attraction for electrochromic material

Information displays

Real-view mirrors for automobiles

Fig 1B4 Application fields of electrochromic devices Smart windows information displays and real-view mirrors for automobiles

35

Using polymeric electrochromic material and plastic substrate with a

transparent conductive coating it is possible to build all-plastic flexible display

device There are lots of papers and patents about the flexible display devices

[104105] In these cases gel-type electrolyte is also needed The gel-type

electrolyte immobilizes liquid electrolyte in the polymer network [106107]

Recently ITO-coated polymer films are readily available which would provide

the plastic substrate for a flexible device The flexible electrochromic displays

are frequently tried with the plastic substrate flexible electrochromic material

and gel-type electrolyte The flexible electrochromic displays would facilitate

increasing technical demands for foldable display devices

Metal Ion Electrochromism

Many transition metal oxides are capable of redox reactions that result in

colour change Metal oxide films are commonly prepared as thin layers of

either tungsten nickel molybdenum or other metal compounds by a number

of techniques These include sol-gel electrochemical by dc or rfreactive

sputtering techniques electron-beam evaporation by anodic or cathodic

electrodeposition or by solution dipping of the electrochromic metal

compounds (or compounds that can be changed into these metal compounds)

onto optically transparent electrodes (OTE) [108 -114] Their electrochromism

is derived from the colour change associated with a change in the oxidation

state of the metal anion The behaviour of these materials is dependent upon

pH moisture and exposure to the atmosphere [115] Generally the switching

rates of these films is somewhat slow with typical switching times of about 15

- 60 seconds to achieve 100 conversion to either coloured or bleached state

[116 -120] An Example of this includes nickel oxide which changes from

transparent (pale green) to brownblack taking about 30 seconds to do so

[121] Other examples include [(NH4)5Ru]2(pyrazine)5+ and [(NH4)5Ru]2(44-

bipyridine)5+ whose electrochromism is significantly different due to the effect

of the ligand [122] Table 12 below gives some examples of metal oxide films

with electrochromic properties

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

21

containing transition metal atoms with unpaired electrons have unusual

magnetic properties and are being investigated as nano computer storage

devices Some compounds exhibit luminescence There are many reported

potential medicinal applications eg anti tumoral and anti-viral There have

been reports on the role of weak or non bonding interactions on the crystal

engineering of hybrid polyoxometalates

Spherical nonporous polyoxomolybdate based capsules of different

types containing more than 100 metal atoms reported by Achim Muller and his

group have versatile unique properties regarding their assembly to vesicles

and the chemistry which can be done inside the pores and cavities A discrete

polyoxometalate Lindquist ion of the form W6O192minus was successfully imaged

recently for the first time within the capillary of a carbon nanotube following

steric locking of the anion with the tubule In situ relaxation of the anion in its

equatorial plain was demonstrated [72]

22

Section-B

Litreture Survey on Chromism in Transition Metal

Oxides

1 B1 Chromism in Transition Metal Oxides

Chromism is a reversible change in a substances colour resulting from

a process caused by some form of stimulus Many materials are chromic

including inorganic and organic compounds and conducting polymers and the

property can result from many different mechanisms Several transition metal

oxides show EC properties The most popular are from the VI - B oxides In

this group WO3 and MoO3 are the most thoroughly studied cathodic EC

materials which can be electrochemically coloured and bleached when used

as the cathode in electrochemical cells Cathodic EC materials also include

V2O3 TiO2 and Nb2O5 Another distinguishable group is anodic EC material

including VIII oxides like IrOx nH2O Rh2O3 nH2O NiO nH2O etc which can be

anodicaly coloured in the electrochemical process when used as anode

There are several types of chromism which are discussed as below

B11 Photochromism

Photochromism is the reversible transformation of a chemical species

between two forms by the absorption of electromagnetic radiation where

the two forms have different absorption spectra [7374]

Trivially this can be described as a reversible change of color upon

exposure to light The phenomenon was discovered in the late 1880s

including work by Markwald who studied the reversible change of color of 23

44-tetrachloronaphthalen-1(4H)-one in the solid state He labeled this

phenomenon phototropy and this name was used until the 1950s

when Yehuda Hirshberg of the Weizmann Institute of Science in Israel

proposed the term photochromism [75] Photochromism can take place

in both organic and inorganic compounds and also has its place in biological

systems (for example retinal in the vision process)

Photochromism does not have a rigorous definition but is usually used

to describe compounds that undergo a reversible photochemical reaction

where an absorption band in the visible part of the electromagnetic spectrum

23

changes dramatically in strength or wavelength In many cases an

absorbance band is present in only one form The degree of change required

for a photochemical reaction to be dubbed photochromic is that which

appears dramatic by eye but in essence there is no dividing line between

photochromic reactions and other photochemistry Therefore while the

transcis isomerization of azobenzene is considered a photochromic reaction

the analogous reaction of stilbene is not Since photochromism is just a

special case of a photochemical reaction almost any photochemical reaction

type may be used to produce photochromism with appropriate molecular desi

gnSome of the most common processes involved in photochromism are peric

yclic reactions cis-trans somerizations intramolecular hydrogen transfer

intramolecular group transfers dissociation processes and electron transfers

(oxidation-reduction)

Another some what arbitrary requirement of photochromism is that

it requires the two states of the molecule to be thermally stable under

ambient conditions for a reasonable time All the same nitrospiropyran (which

back-isomerizes in the dark over ~10 minutes at room temperature) is

considered photochromic All photochromic molecules back-isomerize to their

more stable form at some rate and this back-isomerization is accelerated by

heating There is therefore a close relationship between photochromic and the

rmochromic compounds The timescale of thermal back-isomerization is

important for applications and may be molecularly engineered

Photochromic compounds considered to be thermally stable include some

diarylethenes which do not back isomerize even after heating at 800C for 3

months

Since photochromic chromophores are dyes and operate according to

well-known reactions their molecular engineering to fine-tune their properties

can be achieved relatively easily using known design models quantum

mechanics calculations and experimentation In particular the tuning of

absorbance bands to particular parts of the spectrum and the engineering

of thermal stability have received much attention

Sometimes and particularly in the dye industry the term irreversible

photochromic is used to describe materials that undergo a permanent color

change upon exposure to Ultraviolet or visible light radiation Because by

24

definition photochromics are reversible there is technically no such thing as a

n irreversible photochromic this is loose usage and these compounds

are better referred to as photochangable or photoreactive dyes

Apart from the qualities already mentioned several other properties of

photochromics are important for their use These include

Quantum yield of the photochemical reaction

This determined the efficiency of the photochromic change with respect

to the amount of light absorbed The quantum yield of isomerization

can be strongly dependent on conditions

Fatigue resistance In photochromic materials fatigue refers to the

loss of reversibility by processes such as photodegradation

photobleaching photooxidation and other side reactions All

photochromics suffer fatigue to some extent and its rate is strongly

dependent on the activating light and the conditions of the sample

Photostationary state Photochromic materials have two states and

their interconversion can be controlled using different wavelengths of

light Excitation with any given wavelength of light will result in a

mixture of the two states at a particular ratio called the photo-

stationary state In a perfect system there would exist wavelengths

that can be used to provide 10 and 01 ratios of the isomers

but in real systems this is not possible since the active

absorbance bands always overlap to some extent

Polarity and solubility In order to incorporate photochromics in

working systems they suffer the same issues as other dyes They are

often charged in one or more state leading to very high polarity and

possible large changes in polarity They also often contain large

conjugated systems that limit their solubility

Photochromic complexes

A photochromic complex is a kind of chemical compound that has

photoresponsive Parts on its ligand These complexes have a specific

structure photoswitchable organic compounds are attached to

metalcomplexes For the photocontrollable parts thermally and

photochemically stable chromophores (azobenzene diarylethene

25

spiropyran etc) are usually used And for the metal complexes a wide

variety of compounds that have various functions (redox response

luminescence magnetism etc) are applied The photochromic parts and

metal parts are so close that they can affect each others molecular

orbitals The physical properties of these compounds shown by parts of

them (ie chromophores or metals) thus can be controlled by switching

their other sites by external stimuli For example photoisomerization

behaviors of some complexes can be switched by oxidation and reduction of

their metal parts Some other compounds can be changed in their

luminescence behavior magnetic interaction of metal sites or stability of

metal-to-ligand coordination by photoisomerization of their photochromic

parts

Photochemistry of Polyoxometalates

The photochemistry of polyoxometalates is of great interest to inorganic

chemistsMore than 80 years agoit was found that the R-Keggin tungstate

H3[PW12O40] was reduced photochemically to yield a blue-colored species

which was reoxidized by air and by various other oxidizing agents such as

Fe3+AgNO3and H2O2 [7879]The photoredox reactions of H4[SiW12O40]and

H3[PW12O40] proceeded effectively in the presence of primary and secondary

alcohols their ethers and aldehydes and proteinsbut less effectively in the

presence of tertiary alcoholsketonesestersthe fatty acids above formic

acidand simple amines[8081] The basic photoredox reaction involving

ethanol is illustrated by eq 13

2 H3PW12O40 + H3CCH2OH h ν ν ν ν 2 H4PW12O40 + H3CCHO ------- 13

2 H4PW12O40 +12 O2 2 H3PW12O40 + H2O ------ 14

In this reactionone molecule of ethanol photochemically reduces two

molecules of H3PW12O40 and is itself oxidized to acetaldehyde In the

presence of air the thermal oxidation of the reduced species takes place at

room temperature(eq 14)The reduced polyoxometalates which are the so-

calledldquoheteropolybluesrdquo have been used for the colorimetric analysis of the

26

elements P Si As and Ge and for the determination of uric acidsugarand

other biological compounds [8283] Piperidinium metavanadate also

undergoes photoinduced coloration from white to black followed by a

reversible color change in the presence of oxidizing agentsHowever

ammonium metavanadates ([NH4][VO3]) exhibits no photoinduced

coloration[84] The early photoredox reactions of the R-Keggin

polyoxometalates H4[SiW12O40] and H3PW12O40 were carried out in the

presence of photographic paper however the limited number of the

structurally well-characterized compounds available for study delayed the

development of modern cluster-compound photochemistry until the discovery

of photochromism in alkylammonium polyoxo- molybdate solids[8586] A

photochromic or electrochromic material is one whose light-absorbing

properties are altered upon optical excitation or reduction under the influence

of an externally applied electric field respectively The induced coloration

remains even after the excitation source has been removed These materials

are of technological interest because they return to their original state either

thermally upon irradiation with light of a frequency corresponding to the

induced absorption or electrochemically upon reversing the polarity of the

externally applied electric field Thus photochromic and electrochromic

materials behave in a reversible manner Polyoxometalates exhibit significant

photo-and electrochromism which makes them suitable as nanocomposite

molecular devices and as models for probing the physical properties of infinite

metal oxides Since the metal ions in the oxidized polyoxometalates have d0

electronic configurations the only absorption band which occurs in the UV-vis

range of the electronic spectra is due to the oxygen-to-metal (O-M) ligand-to-

metal charge transfer (LMCT)Upon irradiation electrons are promoted from

the low-energy electronic states which are mainly comprised of oxygen 2p

orbitals (the valence band in the band model)to the high-energy electronic

states which are mainly comprised of metal d orbitalrsquos (the conduction band

in the band model)The fundamental transitions in polyoxometalate lattices

are depicted schematically in Fig1B1

27

Fig1B1 Simple model showing the electronic transitions in the

polyoxometalates containing electron donar and acceptor (a)generation

of charge carriers(b)electron and hole trapping(c)electron release due

to stimulation(d)recombination

between electron and hole Electrons are e-and holes are h+

In the polyoxometalates containing heteroatoms and especially in

mixed metal polyoxometalates the charge carriers which are created by the

light or electric field may be trapped in electron traps and hole traps These

traps provide states of localized energy in the O-M LMCT energy gap due to

the heteroatoms or counter cations which correspond to impurities or lattice

defects in the band model If the trap depth ∆E is large compared to kT the

probability for thermal escape from the trap will be negligibly small and

metastable situation will existThe trapped carriers can be released by thermal

or optical stimulationIn the case of thermal stimulation the irradiated

polyoxometalate is heated until the energy barrier ∆E can be overcome The

trapped electron (or hole) then can escape from the trap and nonradiatively

recombine with the trapped hole (or electron)Under optical stimulation the

energy of an incident photon is used to overcome ∆E The relaxation

processes of the OndashM LMCT excitation energy include both the nonradiative

recombination of electrons and holes within the energy gap and the

intramolecular energy transfer leading to a charge-transfer emission This

intramolecular energy transfer corresponds to the O-M LMCT energy gap and

occurs via radiative recombination and sensitized emission from the

28

heteroatoms or cations If several energy levels based on the hetero atoms or

counter cations act as energy acceptors within the O-M LMCTenergy gap the

energy transfer occurs from the O-M LMCT states to these levels followed by

the nonradiative or radiative deactivation of the excitation energy It should be

noted that the O-M LMCT states also can be generated by the application of

very high electric fields to the polyoxometalate solids as demonstrated by the

observation of electroluminescence[87]If an external electric field with a

potential more negative than the energy levels of the vacant orbitals involved

in the O-M LMCT transition is applied to a polyoxometalate on the electrode

surface an electrochemical reduction occurs via the injection of electrons

from the electrode in to the vacant levels of the polyoxometalate as shown in

Fig1B 2

Fig1B2 - Energy scheme for the electrochromism of polyoxometalates

a)electrochemical reduction (b) electrochemical oxidation

Electrons injected in to the high-energy levels also may be trapped by

electron traps in a process analogous to that which occurs during LMCT

photoexcitation of the polyoxometalates These electrons are returned to the

electrode by electrochemical oxidation at an electrode potential more positive

than the energy levels for the d1 electron states The d1electrons in the O-M

LMCTstates facilitate the absorption of visible light via intervalence charge

transfer among metal centers and d-d transitions The same type of transition

may be possible for the d1electron captured by the electron traps too In

addition to searching for new photosensitive polyoxometalates with the

29

potential for having practical application there is now a need to elucidate the

fundamental photo-and electrochemical coloration processes by studying

electron transfer within the polyoxometalate lattices in conjunction with their

crystal structures So far few polyoxometalates exhibit a perfect reversibility

of coloration The irreversibility of the color change arises from as yet

uncharacterized side reactions during both the coloration and decoloration of

the polyoxometalates

Many metal oxides including aluminum titanium vanadium niobium

molybdenum and tungsten oxides are photochromic when they contain

impurities or dopants This coloration has been interpreted on the basis of

electron trapping at appropriate lattice sites within the crystals as shown in

Fig1B1 where the O-M LMCT transition corresponds to the transition

between the valence and conduction bands for the infinite metal-oxide lattice

[88-91]

B12 Applications of Photochromic materials

Sunglasses

One of the most famous reversible photochromic applications is color

changing lenses for sunglasses as found in eyeglasses The largest limitation

in using PC technology is that the materials cannot be made stable enough

to withstand thousands of hours of outdoor exposure so long-term outdoor

applications are not appropriate at this time The switching speed of

photochromic dyes is highly sensitive to the rigidity of the environment around

the dye As result they switch most rapidly in solution and slowest in the rigid

environment like a polymer lens Recently it has been reported that attaching

flexible low Tg polymers (for example siloxanes or poly (butyl acrylate) to the

dyes allows them to switch much more rapidly in a rigid lens [76] Some

spirooxazines with siloxane polymers attached switch at near solution like

speeds even though they are in a rigid lens matrix

Supramolecular chemistry

Photochromic units have been employed extensively in supramolecular

chemistry Their ability to give a light controlled reversible shape change

means that they can be used to make or break molecular recognition motifs

30

or to cause a consequent shape change in their surroundings Thus

photochromic units have been demonstrated as components of molecular

switches The coupling of photochromic units to enzymes or enzyme cofactors

even provides the ability to reversibly turn enzymes on and off

by altering their shape or orientation in such a way that their functions

are either working or broken

Data storage

The possibility of using photochromic compounds for data storage was

first suggested in 1956 by Yehuda Hirshberg[77] Since that time there have

been many investigations by various academic and commercial groups

particularly in the area of 3D optical data storage which promises discs that

can hold a terabyte of data Initially issues with thermal back-reactions

and destructive reading dogged these studies but more recently more stable

systems have been developed

Novelty items

Reversible photochromics are also found in applications such as toys

cosmeticsclothing and industrial applications If necessary they can be made

to change between desired colors by combination with a permanent pigment

A large number of inorganic compounds exhibit photochromism

These solids often have large band gaps of the order of 3 - 12 eV and

excitation of these solids leads to the formation of metastable centers that

absorb visible light giving rise to their colour They can return to their ground

state by heating or by optical excitation within the colour-centre band In most

cases the photochromism is a structure sensitive phenomenon involving

localized defect impurities or dislocations Some of these inorganic

compounds have the potential for a number of different uses Photochromic

compounds have a number of useful applications These can be divided

according to the most important property that is being used (Table 11) [92]

31

Table 11 Applications of Photochromic materials

Applications Depending Upon

Sensitivity to

Radiation

Reversibility Thermal Chemical or

Physical Properties

Self-developing

photography

Chemical switches for

computers

Temperature indicators

Protective

materials

Data displays

Heat-sensitive recording

media

Optical signal

processing

Photomasking and

photoresist technology

Reusable data storage

media

Anaytical reagents

Photochromic

microimages

Photopolymerisation

Information encoding

and steganography

Photocontractile

polymers and the

photoviscosity effect

Control of light

intensity

Q-switches

Pyroelectric

photochromic materials

B13 Thermochromism

Thermochromism is the reversible colour change of a substance

induced by temperature change A large variety of substances organic

inorganic organometallic supramolecular and polymeric systems exhibit this

phenomenon Examples of these include bianthrones cobalt

hexacyanoferrate the zirconocene complex of 1 4-diphenyl-1 3-butadiene

and poly (3-alkylthiophene) The organic 99-bixanthenylidene is colourless at

90 K yellow-green at 298 K and dark-blue when melted at 592 K Heating

conducting polymers can cause them to change colour This is achieved by

causing conformational changes to the polymer backbone resulting in a

change in the band gap of the polymer It has been reported that regioregular

32

P3HT reversibly changes colour upon heating to 220ordmC due to temperature-

dependent conformation changes Thermally cross linked polymer undergoes

the same colour change but it is much less reversible [93] Other forms of

thermochromism may be commercially important eg to give a visual

indication of temperature changes

B14 Electrochromism

Electrochromism describes a phenomenon of material color change in

a persistent but reversible manner produced by electrochemically induced

oxidation-reduction reactions Electrochromic materials can be applied to

various kinds of products such as smart windows and display devices Among

those applications there have been lots of efforts to develop electrochromic

display devices (ECDs) Especially flexible display devices are now attracting

much attention worldwide since they can facilitate new technological demands

such as bending and folding of paper-like displays High electrochromic

efficiency short response time long operating life time and reduction of

energy consumption are the most important requisites of the materials for the

paperlike displays [94 95] Among those properties the operation life time is

the most important barrier to overcome for a realization of ECDs There are

two types of electrochromic material a) inorganic transition metal oxides

(TMOs) b) organic polymer materials The TMOs have been studied longer

than the organic materials that they have been studied since 1960s [9697]

Electrochromism describes a reversible color change of material

produced by electrochemically induced oxidation-reduction reactions It is one

of several types of chromism of materials As thermochromism and

photochromism mean material color changes made by heat and light

respectively electrochromism refers that the color change is caused by an

electric potential In most cases the color change in electrochromism can be

driven by rather low electrical potential of the order of a fraction of volt or a

few volts [94-96] The color change of material means variation in

transmittance andor reflectance change in visible range which is originated

from different electronic absorption bands according to a switching between

oxidation and reduction state of material When electric potential is applied on

electrochromic material forced oxidation or reduction is derived and the

33

individual color is originated from the corresponding oxidation or reduction

state of the material For electrochromic materials the characteristic color

change is reversible since the oxidation and reduction state can be converted

reversibly by switching potential

Application fields

The application area for the electrochromism is rather broad that it

covers from smart window glazing and optical modulators to information

displays [98-102] The smart windows are typical examples The

electrochromic property is used to control the amount of light and heat to pass

through the windows Usually the electrochromic material is in form of thin film

coated on a window glass The transmittance modulation has also been

applied at the automobiles to automatically tint rear-view mirrors in various

lighting conditions The electrochromic application fields are illustrated in Fig

1B3

Since the smart windows control the transmittance of heat as well as

the transmittance of visible light the working definition of electrochromism has

now been extended to include devices for modulation of radiation in the near

infrared thermal infrared and microwave regions When color for

electrochromic materials is used this can now mean a response by detectors

at these wavelengths and not just by the human eyes Nowadays

electrochromic material draws much attention as being used in the display

devices Electrochromic display device (ECD) is being considered as one of

the candidates for the conventional liquid crystal display (LCD) since ECD

has many advantages over LCDs Among these advantages the most

important are low energy consumption wider viewing angle high contrast

rate and possibility to achieve multiple colors with a single material [103]

When a new redox state of electrochromic materialis established by

the applied electric pulse then it is maintained after the potential is switched

off This means the colored or bleached state of the material can be sustained

for a considerable time without applying electrical power This is so-called

ldquocolor memory effectrdquo of electrochromic material Because of the color

memory effect energy consumption for the electrochromic display device

could be drastically reduced and this would be a big advantage over other

34

emissive devices The low power consumption is especially advantageous

when it is applied to mobile devices with limited power source The possibility

for a flexible display is another attraction for electrochromic material

Information displays

Real-view mirrors for automobiles

Fig 1B4 Application fields of electrochromic devices Smart windows information displays and real-view mirrors for automobiles

35

Using polymeric electrochromic material and plastic substrate with a

transparent conductive coating it is possible to build all-plastic flexible display

device There are lots of papers and patents about the flexible display devices

[104105] In these cases gel-type electrolyte is also needed The gel-type

electrolyte immobilizes liquid electrolyte in the polymer network [106107]

Recently ITO-coated polymer films are readily available which would provide

the plastic substrate for a flexible device The flexible electrochromic displays

are frequently tried with the plastic substrate flexible electrochromic material

and gel-type electrolyte The flexible electrochromic displays would facilitate

increasing technical demands for foldable display devices

Metal Ion Electrochromism

Many transition metal oxides are capable of redox reactions that result in

colour change Metal oxide films are commonly prepared as thin layers of

either tungsten nickel molybdenum or other metal compounds by a number

of techniques These include sol-gel electrochemical by dc or rfreactive

sputtering techniques electron-beam evaporation by anodic or cathodic

electrodeposition or by solution dipping of the electrochromic metal

compounds (or compounds that can be changed into these metal compounds)

onto optically transparent electrodes (OTE) [108 -114] Their electrochromism

is derived from the colour change associated with a change in the oxidation

state of the metal anion The behaviour of these materials is dependent upon

pH moisture and exposure to the atmosphere [115] Generally the switching

rates of these films is somewhat slow with typical switching times of about 15

- 60 seconds to achieve 100 conversion to either coloured or bleached state

[116 -120] An Example of this includes nickel oxide which changes from

transparent (pale green) to brownblack taking about 30 seconds to do so

[121] Other examples include [(NH4)5Ru]2(pyrazine)5+ and [(NH4)5Ru]2(44-

bipyridine)5+ whose electrochromism is significantly different due to the effect

of the ligand [122] Table 12 below gives some examples of metal oxide films

with electrochromic properties

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

22

Section-B

Litreture Survey on Chromism in Transition Metal

Oxides

1 B1 Chromism in Transition Metal Oxides

Chromism is a reversible change in a substances colour resulting from

a process caused by some form of stimulus Many materials are chromic

including inorganic and organic compounds and conducting polymers and the

property can result from many different mechanisms Several transition metal

oxides show EC properties The most popular are from the VI - B oxides In

this group WO3 and MoO3 are the most thoroughly studied cathodic EC

materials which can be electrochemically coloured and bleached when used

as the cathode in electrochemical cells Cathodic EC materials also include

V2O3 TiO2 and Nb2O5 Another distinguishable group is anodic EC material

including VIII oxides like IrOx nH2O Rh2O3 nH2O NiO nH2O etc which can be

anodicaly coloured in the electrochemical process when used as anode

There are several types of chromism which are discussed as below

B11 Photochromism

Photochromism is the reversible transformation of a chemical species

between two forms by the absorption of electromagnetic radiation where

the two forms have different absorption spectra [7374]

Trivially this can be described as a reversible change of color upon

exposure to light The phenomenon was discovered in the late 1880s

including work by Markwald who studied the reversible change of color of 23

44-tetrachloronaphthalen-1(4H)-one in the solid state He labeled this

phenomenon phototropy and this name was used until the 1950s

when Yehuda Hirshberg of the Weizmann Institute of Science in Israel

proposed the term photochromism [75] Photochromism can take place

in both organic and inorganic compounds and also has its place in biological

systems (for example retinal in the vision process)

Photochromism does not have a rigorous definition but is usually used

to describe compounds that undergo a reversible photochemical reaction

where an absorption band in the visible part of the electromagnetic spectrum

23

changes dramatically in strength or wavelength In many cases an

absorbance band is present in only one form The degree of change required

for a photochemical reaction to be dubbed photochromic is that which

appears dramatic by eye but in essence there is no dividing line between

photochromic reactions and other photochemistry Therefore while the

transcis isomerization of azobenzene is considered a photochromic reaction

the analogous reaction of stilbene is not Since photochromism is just a

special case of a photochemical reaction almost any photochemical reaction

type may be used to produce photochromism with appropriate molecular desi

gnSome of the most common processes involved in photochromism are peric

yclic reactions cis-trans somerizations intramolecular hydrogen transfer

intramolecular group transfers dissociation processes and electron transfers

(oxidation-reduction)

Another some what arbitrary requirement of photochromism is that

it requires the two states of the molecule to be thermally stable under

ambient conditions for a reasonable time All the same nitrospiropyran (which

back-isomerizes in the dark over ~10 minutes at room temperature) is

considered photochromic All photochromic molecules back-isomerize to their

more stable form at some rate and this back-isomerization is accelerated by

heating There is therefore a close relationship between photochromic and the

rmochromic compounds The timescale of thermal back-isomerization is

important for applications and may be molecularly engineered

Photochromic compounds considered to be thermally stable include some

diarylethenes which do not back isomerize even after heating at 800C for 3

months

Since photochromic chromophores are dyes and operate according to

well-known reactions their molecular engineering to fine-tune their properties

can be achieved relatively easily using known design models quantum

mechanics calculations and experimentation In particular the tuning of

absorbance bands to particular parts of the spectrum and the engineering

of thermal stability have received much attention

Sometimes and particularly in the dye industry the term irreversible

photochromic is used to describe materials that undergo a permanent color

change upon exposure to Ultraviolet or visible light radiation Because by

24

definition photochromics are reversible there is technically no such thing as a

n irreversible photochromic this is loose usage and these compounds

are better referred to as photochangable or photoreactive dyes

Apart from the qualities already mentioned several other properties of

photochromics are important for their use These include

Quantum yield of the photochemical reaction

This determined the efficiency of the photochromic change with respect

to the amount of light absorbed The quantum yield of isomerization

can be strongly dependent on conditions

Fatigue resistance In photochromic materials fatigue refers to the

loss of reversibility by processes such as photodegradation

photobleaching photooxidation and other side reactions All

photochromics suffer fatigue to some extent and its rate is strongly

dependent on the activating light and the conditions of the sample

Photostationary state Photochromic materials have two states and

their interconversion can be controlled using different wavelengths of

light Excitation with any given wavelength of light will result in a

mixture of the two states at a particular ratio called the photo-

stationary state In a perfect system there would exist wavelengths

that can be used to provide 10 and 01 ratios of the isomers

but in real systems this is not possible since the active

absorbance bands always overlap to some extent

Polarity and solubility In order to incorporate photochromics in

working systems they suffer the same issues as other dyes They are

often charged in one or more state leading to very high polarity and

possible large changes in polarity They also often contain large

conjugated systems that limit their solubility

Photochromic complexes

A photochromic complex is a kind of chemical compound that has

photoresponsive Parts on its ligand These complexes have a specific

structure photoswitchable organic compounds are attached to

metalcomplexes For the photocontrollable parts thermally and

photochemically stable chromophores (azobenzene diarylethene

25

spiropyran etc) are usually used And for the metal complexes a wide

variety of compounds that have various functions (redox response

luminescence magnetism etc) are applied The photochromic parts and

metal parts are so close that they can affect each others molecular

orbitals The physical properties of these compounds shown by parts of

them (ie chromophores or metals) thus can be controlled by switching

their other sites by external stimuli For example photoisomerization

behaviors of some complexes can be switched by oxidation and reduction of

their metal parts Some other compounds can be changed in their

luminescence behavior magnetic interaction of metal sites or stability of

metal-to-ligand coordination by photoisomerization of their photochromic

parts

Photochemistry of Polyoxometalates

The photochemistry of polyoxometalates is of great interest to inorganic

chemistsMore than 80 years agoit was found that the R-Keggin tungstate

H3[PW12O40] was reduced photochemically to yield a blue-colored species

which was reoxidized by air and by various other oxidizing agents such as

Fe3+AgNO3and H2O2 [7879]The photoredox reactions of H4[SiW12O40]and

H3[PW12O40] proceeded effectively in the presence of primary and secondary

alcohols their ethers and aldehydes and proteinsbut less effectively in the

presence of tertiary alcoholsketonesestersthe fatty acids above formic

acidand simple amines[8081] The basic photoredox reaction involving

ethanol is illustrated by eq 13

2 H3PW12O40 + H3CCH2OH h ν ν ν ν 2 H4PW12O40 + H3CCHO ------- 13

2 H4PW12O40 +12 O2 2 H3PW12O40 + H2O ------ 14

In this reactionone molecule of ethanol photochemically reduces two

molecules of H3PW12O40 and is itself oxidized to acetaldehyde In the

presence of air the thermal oxidation of the reduced species takes place at

room temperature(eq 14)The reduced polyoxometalates which are the so-

calledldquoheteropolybluesrdquo have been used for the colorimetric analysis of the

26

elements P Si As and Ge and for the determination of uric acidsugarand

other biological compounds [8283] Piperidinium metavanadate also

undergoes photoinduced coloration from white to black followed by a

reversible color change in the presence of oxidizing agentsHowever

ammonium metavanadates ([NH4][VO3]) exhibits no photoinduced

coloration[84] The early photoredox reactions of the R-Keggin

polyoxometalates H4[SiW12O40] and H3PW12O40 were carried out in the

presence of photographic paper however the limited number of the

structurally well-characterized compounds available for study delayed the

development of modern cluster-compound photochemistry until the discovery

of photochromism in alkylammonium polyoxo- molybdate solids[8586] A

photochromic or electrochromic material is one whose light-absorbing

properties are altered upon optical excitation or reduction under the influence

of an externally applied electric field respectively The induced coloration

remains even after the excitation source has been removed These materials

are of technological interest because they return to their original state either

thermally upon irradiation with light of a frequency corresponding to the

induced absorption or electrochemically upon reversing the polarity of the

externally applied electric field Thus photochromic and electrochromic

materials behave in a reversible manner Polyoxometalates exhibit significant

photo-and electrochromism which makes them suitable as nanocomposite

molecular devices and as models for probing the physical properties of infinite

metal oxides Since the metal ions in the oxidized polyoxometalates have d0

electronic configurations the only absorption band which occurs in the UV-vis

range of the electronic spectra is due to the oxygen-to-metal (O-M) ligand-to-

metal charge transfer (LMCT)Upon irradiation electrons are promoted from

the low-energy electronic states which are mainly comprised of oxygen 2p

orbitals (the valence band in the band model)to the high-energy electronic

states which are mainly comprised of metal d orbitalrsquos (the conduction band

in the band model)The fundamental transitions in polyoxometalate lattices

are depicted schematically in Fig1B1

27

Fig1B1 Simple model showing the electronic transitions in the

polyoxometalates containing electron donar and acceptor (a)generation

of charge carriers(b)electron and hole trapping(c)electron release due

to stimulation(d)recombination

between electron and hole Electrons are e-and holes are h+

In the polyoxometalates containing heteroatoms and especially in

mixed metal polyoxometalates the charge carriers which are created by the

light or electric field may be trapped in electron traps and hole traps These

traps provide states of localized energy in the O-M LMCT energy gap due to

the heteroatoms or counter cations which correspond to impurities or lattice

defects in the band model If the trap depth ∆E is large compared to kT the

probability for thermal escape from the trap will be negligibly small and

metastable situation will existThe trapped carriers can be released by thermal

or optical stimulationIn the case of thermal stimulation the irradiated

polyoxometalate is heated until the energy barrier ∆E can be overcome The

trapped electron (or hole) then can escape from the trap and nonradiatively

recombine with the trapped hole (or electron)Under optical stimulation the

energy of an incident photon is used to overcome ∆E The relaxation

processes of the OndashM LMCT excitation energy include both the nonradiative

recombination of electrons and holes within the energy gap and the

intramolecular energy transfer leading to a charge-transfer emission This

intramolecular energy transfer corresponds to the O-M LMCT energy gap and

occurs via radiative recombination and sensitized emission from the

28

heteroatoms or cations If several energy levels based on the hetero atoms or

counter cations act as energy acceptors within the O-M LMCTenergy gap the

energy transfer occurs from the O-M LMCT states to these levels followed by

the nonradiative or radiative deactivation of the excitation energy It should be

noted that the O-M LMCT states also can be generated by the application of

very high electric fields to the polyoxometalate solids as demonstrated by the

observation of electroluminescence[87]If an external electric field with a

potential more negative than the energy levels of the vacant orbitals involved

in the O-M LMCT transition is applied to a polyoxometalate on the electrode

surface an electrochemical reduction occurs via the injection of electrons

from the electrode in to the vacant levels of the polyoxometalate as shown in

Fig1B 2

Fig1B2 - Energy scheme for the electrochromism of polyoxometalates

a)electrochemical reduction (b) electrochemical oxidation

Electrons injected in to the high-energy levels also may be trapped by

electron traps in a process analogous to that which occurs during LMCT

photoexcitation of the polyoxometalates These electrons are returned to the

electrode by electrochemical oxidation at an electrode potential more positive

than the energy levels for the d1 electron states The d1electrons in the O-M

LMCTstates facilitate the absorption of visible light via intervalence charge

transfer among metal centers and d-d transitions The same type of transition

may be possible for the d1electron captured by the electron traps too In

addition to searching for new photosensitive polyoxometalates with the

29

potential for having practical application there is now a need to elucidate the

fundamental photo-and electrochemical coloration processes by studying

electron transfer within the polyoxometalate lattices in conjunction with their

crystal structures So far few polyoxometalates exhibit a perfect reversibility

of coloration The irreversibility of the color change arises from as yet

uncharacterized side reactions during both the coloration and decoloration of

the polyoxometalates

Many metal oxides including aluminum titanium vanadium niobium

molybdenum and tungsten oxides are photochromic when they contain

impurities or dopants This coloration has been interpreted on the basis of

electron trapping at appropriate lattice sites within the crystals as shown in

Fig1B1 where the O-M LMCT transition corresponds to the transition

between the valence and conduction bands for the infinite metal-oxide lattice

[88-91]

B12 Applications of Photochromic materials

Sunglasses

One of the most famous reversible photochromic applications is color

changing lenses for sunglasses as found in eyeglasses The largest limitation

in using PC technology is that the materials cannot be made stable enough

to withstand thousands of hours of outdoor exposure so long-term outdoor

applications are not appropriate at this time The switching speed of

photochromic dyes is highly sensitive to the rigidity of the environment around

the dye As result they switch most rapidly in solution and slowest in the rigid

environment like a polymer lens Recently it has been reported that attaching

flexible low Tg polymers (for example siloxanes or poly (butyl acrylate) to the

dyes allows them to switch much more rapidly in a rigid lens [76] Some

spirooxazines with siloxane polymers attached switch at near solution like

speeds even though they are in a rigid lens matrix

Supramolecular chemistry

Photochromic units have been employed extensively in supramolecular

chemistry Their ability to give a light controlled reversible shape change

means that they can be used to make or break molecular recognition motifs

30

or to cause a consequent shape change in their surroundings Thus

photochromic units have been demonstrated as components of molecular

switches The coupling of photochromic units to enzymes or enzyme cofactors

even provides the ability to reversibly turn enzymes on and off

by altering their shape or orientation in such a way that their functions

are either working or broken

Data storage

The possibility of using photochromic compounds for data storage was

first suggested in 1956 by Yehuda Hirshberg[77] Since that time there have

been many investigations by various academic and commercial groups

particularly in the area of 3D optical data storage which promises discs that

can hold a terabyte of data Initially issues with thermal back-reactions

and destructive reading dogged these studies but more recently more stable

systems have been developed

Novelty items

Reversible photochromics are also found in applications such as toys

cosmeticsclothing and industrial applications If necessary they can be made

to change between desired colors by combination with a permanent pigment

A large number of inorganic compounds exhibit photochromism

These solids often have large band gaps of the order of 3 - 12 eV and

excitation of these solids leads to the formation of metastable centers that

absorb visible light giving rise to their colour They can return to their ground

state by heating or by optical excitation within the colour-centre band In most

cases the photochromism is a structure sensitive phenomenon involving

localized defect impurities or dislocations Some of these inorganic

compounds have the potential for a number of different uses Photochromic

compounds have a number of useful applications These can be divided

according to the most important property that is being used (Table 11) [92]

31

Table 11 Applications of Photochromic materials

Applications Depending Upon

Sensitivity to

Radiation

Reversibility Thermal Chemical or

Physical Properties

Self-developing

photography

Chemical switches for

computers

Temperature indicators

Protective

materials

Data displays

Heat-sensitive recording

media

Optical signal

processing

Photomasking and

photoresist technology

Reusable data storage

media

Anaytical reagents

Photochromic

microimages

Photopolymerisation

Information encoding

and steganography

Photocontractile

polymers and the

photoviscosity effect

Control of light

intensity

Q-switches

Pyroelectric

photochromic materials

B13 Thermochromism

Thermochromism is the reversible colour change of a substance

induced by temperature change A large variety of substances organic

inorganic organometallic supramolecular and polymeric systems exhibit this

phenomenon Examples of these include bianthrones cobalt

hexacyanoferrate the zirconocene complex of 1 4-diphenyl-1 3-butadiene

and poly (3-alkylthiophene) The organic 99-bixanthenylidene is colourless at

90 K yellow-green at 298 K and dark-blue when melted at 592 K Heating

conducting polymers can cause them to change colour This is achieved by

causing conformational changes to the polymer backbone resulting in a

change in the band gap of the polymer It has been reported that regioregular

32

P3HT reversibly changes colour upon heating to 220ordmC due to temperature-

dependent conformation changes Thermally cross linked polymer undergoes

the same colour change but it is much less reversible [93] Other forms of

thermochromism may be commercially important eg to give a visual

indication of temperature changes

B14 Electrochromism

Electrochromism describes a phenomenon of material color change in

a persistent but reversible manner produced by electrochemically induced

oxidation-reduction reactions Electrochromic materials can be applied to

various kinds of products such as smart windows and display devices Among

those applications there have been lots of efforts to develop electrochromic

display devices (ECDs) Especially flexible display devices are now attracting

much attention worldwide since they can facilitate new technological demands

such as bending and folding of paper-like displays High electrochromic

efficiency short response time long operating life time and reduction of

energy consumption are the most important requisites of the materials for the

paperlike displays [94 95] Among those properties the operation life time is

the most important barrier to overcome for a realization of ECDs There are

two types of electrochromic material a) inorganic transition metal oxides

(TMOs) b) organic polymer materials The TMOs have been studied longer

than the organic materials that they have been studied since 1960s [9697]

Electrochromism describes a reversible color change of material

produced by electrochemically induced oxidation-reduction reactions It is one

of several types of chromism of materials As thermochromism and

photochromism mean material color changes made by heat and light

respectively electrochromism refers that the color change is caused by an

electric potential In most cases the color change in electrochromism can be

driven by rather low electrical potential of the order of a fraction of volt or a

few volts [94-96] The color change of material means variation in

transmittance andor reflectance change in visible range which is originated

from different electronic absorption bands according to a switching between

oxidation and reduction state of material When electric potential is applied on

electrochromic material forced oxidation or reduction is derived and the

33

individual color is originated from the corresponding oxidation or reduction

state of the material For electrochromic materials the characteristic color

change is reversible since the oxidation and reduction state can be converted

reversibly by switching potential

Application fields

The application area for the electrochromism is rather broad that it

covers from smart window glazing and optical modulators to information

displays [98-102] The smart windows are typical examples The

electrochromic property is used to control the amount of light and heat to pass

through the windows Usually the electrochromic material is in form of thin film

coated on a window glass The transmittance modulation has also been

applied at the automobiles to automatically tint rear-view mirrors in various

lighting conditions The electrochromic application fields are illustrated in Fig

1B3

Since the smart windows control the transmittance of heat as well as

the transmittance of visible light the working definition of electrochromism has

now been extended to include devices for modulation of radiation in the near

infrared thermal infrared and microwave regions When color for

electrochromic materials is used this can now mean a response by detectors

at these wavelengths and not just by the human eyes Nowadays

electrochromic material draws much attention as being used in the display

devices Electrochromic display device (ECD) is being considered as one of

the candidates for the conventional liquid crystal display (LCD) since ECD

has many advantages over LCDs Among these advantages the most

important are low energy consumption wider viewing angle high contrast

rate and possibility to achieve multiple colors with a single material [103]

When a new redox state of electrochromic materialis established by

the applied electric pulse then it is maintained after the potential is switched

off This means the colored or bleached state of the material can be sustained

for a considerable time without applying electrical power This is so-called

ldquocolor memory effectrdquo of electrochromic material Because of the color

memory effect energy consumption for the electrochromic display device

could be drastically reduced and this would be a big advantage over other

34

emissive devices The low power consumption is especially advantageous

when it is applied to mobile devices with limited power source The possibility

for a flexible display is another attraction for electrochromic material

Information displays

Real-view mirrors for automobiles

Fig 1B4 Application fields of electrochromic devices Smart windows information displays and real-view mirrors for automobiles

35

Using polymeric electrochromic material and plastic substrate with a

transparent conductive coating it is possible to build all-plastic flexible display

device There are lots of papers and patents about the flexible display devices

[104105] In these cases gel-type electrolyte is also needed The gel-type

electrolyte immobilizes liquid electrolyte in the polymer network [106107]

Recently ITO-coated polymer films are readily available which would provide

the plastic substrate for a flexible device The flexible electrochromic displays

are frequently tried with the plastic substrate flexible electrochromic material

and gel-type electrolyte The flexible electrochromic displays would facilitate

increasing technical demands for foldable display devices

Metal Ion Electrochromism

Many transition metal oxides are capable of redox reactions that result in

colour change Metal oxide films are commonly prepared as thin layers of

either tungsten nickel molybdenum or other metal compounds by a number

of techniques These include sol-gel electrochemical by dc or rfreactive

sputtering techniques electron-beam evaporation by anodic or cathodic

electrodeposition or by solution dipping of the electrochromic metal

compounds (or compounds that can be changed into these metal compounds)

onto optically transparent electrodes (OTE) [108 -114] Their electrochromism

is derived from the colour change associated with a change in the oxidation

state of the metal anion The behaviour of these materials is dependent upon

pH moisture and exposure to the atmosphere [115] Generally the switching

rates of these films is somewhat slow with typical switching times of about 15

- 60 seconds to achieve 100 conversion to either coloured or bleached state

[116 -120] An Example of this includes nickel oxide which changes from

transparent (pale green) to brownblack taking about 30 seconds to do so

[121] Other examples include [(NH4)5Ru]2(pyrazine)5+ and [(NH4)5Ru]2(44-

bipyridine)5+ whose electrochromism is significantly different due to the effect

of the ligand [122] Table 12 below gives some examples of metal oxide films

with electrochromic properties

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

23

changes dramatically in strength or wavelength In many cases an

absorbance band is present in only one form The degree of change required

for a photochemical reaction to be dubbed photochromic is that which

appears dramatic by eye but in essence there is no dividing line between

photochromic reactions and other photochemistry Therefore while the

transcis isomerization of azobenzene is considered a photochromic reaction

the analogous reaction of stilbene is not Since photochromism is just a

special case of a photochemical reaction almost any photochemical reaction

type may be used to produce photochromism with appropriate molecular desi

gnSome of the most common processes involved in photochromism are peric

yclic reactions cis-trans somerizations intramolecular hydrogen transfer

intramolecular group transfers dissociation processes and electron transfers

(oxidation-reduction)

Another some what arbitrary requirement of photochromism is that

it requires the two states of the molecule to be thermally stable under

ambient conditions for a reasonable time All the same nitrospiropyran (which

back-isomerizes in the dark over ~10 minutes at room temperature) is

considered photochromic All photochromic molecules back-isomerize to their

more stable form at some rate and this back-isomerization is accelerated by

heating There is therefore a close relationship between photochromic and the

rmochromic compounds The timescale of thermal back-isomerization is

important for applications and may be molecularly engineered

Photochromic compounds considered to be thermally stable include some

diarylethenes which do not back isomerize even after heating at 800C for 3

months

Since photochromic chromophores are dyes and operate according to

well-known reactions their molecular engineering to fine-tune their properties

can be achieved relatively easily using known design models quantum

mechanics calculations and experimentation In particular the tuning of

absorbance bands to particular parts of the spectrum and the engineering

of thermal stability have received much attention

Sometimes and particularly in the dye industry the term irreversible

photochromic is used to describe materials that undergo a permanent color

change upon exposure to Ultraviolet or visible light radiation Because by

24

definition photochromics are reversible there is technically no such thing as a

n irreversible photochromic this is loose usage and these compounds

are better referred to as photochangable or photoreactive dyes

Apart from the qualities already mentioned several other properties of

photochromics are important for their use These include

Quantum yield of the photochemical reaction

This determined the efficiency of the photochromic change with respect

to the amount of light absorbed The quantum yield of isomerization

can be strongly dependent on conditions

Fatigue resistance In photochromic materials fatigue refers to the

loss of reversibility by processes such as photodegradation

photobleaching photooxidation and other side reactions All

photochromics suffer fatigue to some extent and its rate is strongly

dependent on the activating light and the conditions of the sample

Photostationary state Photochromic materials have two states and

their interconversion can be controlled using different wavelengths of

light Excitation with any given wavelength of light will result in a

mixture of the two states at a particular ratio called the photo-

stationary state In a perfect system there would exist wavelengths

that can be used to provide 10 and 01 ratios of the isomers

but in real systems this is not possible since the active

absorbance bands always overlap to some extent

Polarity and solubility In order to incorporate photochromics in

working systems they suffer the same issues as other dyes They are

often charged in one or more state leading to very high polarity and

possible large changes in polarity They also often contain large

conjugated systems that limit their solubility

Photochromic complexes

A photochromic complex is a kind of chemical compound that has

photoresponsive Parts on its ligand These complexes have a specific

structure photoswitchable organic compounds are attached to

metalcomplexes For the photocontrollable parts thermally and

photochemically stable chromophores (azobenzene diarylethene

25

spiropyran etc) are usually used And for the metal complexes a wide

variety of compounds that have various functions (redox response

luminescence magnetism etc) are applied The photochromic parts and

metal parts are so close that they can affect each others molecular

orbitals The physical properties of these compounds shown by parts of

them (ie chromophores or metals) thus can be controlled by switching

their other sites by external stimuli For example photoisomerization

behaviors of some complexes can be switched by oxidation and reduction of

their metal parts Some other compounds can be changed in their

luminescence behavior magnetic interaction of metal sites or stability of

metal-to-ligand coordination by photoisomerization of their photochromic

parts

Photochemistry of Polyoxometalates

The photochemistry of polyoxometalates is of great interest to inorganic

chemistsMore than 80 years agoit was found that the R-Keggin tungstate

H3[PW12O40] was reduced photochemically to yield a blue-colored species

which was reoxidized by air and by various other oxidizing agents such as

Fe3+AgNO3and H2O2 [7879]The photoredox reactions of H4[SiW12O40]and

H3[PW12O40] proceeded effectively in the presence of primary and secondary

alcohols their ethers and aldehydes and proteinsbut less effectively in the

presence of tertiary alcoholsketonesestersthe fatty acids above formic

acidand simple amines[8081] The basic photoredox reaction involving

ethanol is illustrated by eq 13

2 H3PW12O40 + H3CCH2OH h ν ν ν ν 2 H4PW12O40 + H3CCHO ------- 13

2 H4PW12O40 +12 O2 2 H3PW12O40 + H2O ------ 14

In this reactionone molecule of ethanol photochemically reduces two

molecules of H3PW12O40 and is itself oxidized to acetaldehyde In the

presence of air the thermal oxidation of the reduced species takes place at

room temperature(eq 14)The reduced polyoxometalates which are the so-

calledldquoheteropolybluesrdquo have been used for the colorimetric analysis of the

26

elements P Si As and Ge and for the determination of uric acidsugarand

other biological compounds [8283] Piperidinium metavanadate also

undergoes photoinduced coloration from white to black followed by a

reversible color change in the presence of oxidizing agentsHowever

ammonium metavanadates ([NH4][VO3]) exhibits no photoinduced

coloration[84] The early photoredox reactions of the R-Keggin

polyoxometalates H4[SiW12O40] and H3PW12O40 were carried out in the

presence of photographic paper however the limited number of the

structurally well-characterized compounds available for study delayed the

development of modern cluster-compound photochemistry until the discovery

of photochromism in alkylammonium polyoxo- molybdate solids[8586] A

photochromic or electrochromic material is one whose light-absorbing

properties are altered upon optical excitation or reduction under the influence

of an externally applied electric field respectively The induced coloration

remains even after the excitation source has been removed These materials

are of technological interest because they return to their original state either

thermally upon irradiation with light of a frequency corresponding to the

induced absorption or electrochemically upon reversing the polarity of the

externally applied electric field Thus photochromic and electrochromic

materials behave in a reversible manner Polyoxometalates exhibit significant

photo-and electrochromism which makes them suitable as nanocomposite

molecular devices and as models for probing the physical properties of infinite

metal oxides Since the metal ions in the oxidized polyoxometalates have d0

electronic configurations the only absorption band which occurs in the UV-vis

range of the electronic spectra is due to the oxygen-to-metal (O-M) ligand-to-

metal charge transfer (LMCT)Upon irradiation electrons are promoted from

the low-energy electronic states which are mainly comprised of oxygen 2p

orbitals (the valence band in the band model)to the high-energy electronic

states which are mainly comprised of metal d orbitalrsquos (the conduction band

in the band model)The fundamental transitions in polyoxometalate lattices

are depicted schematically in Fig1B1

27

Fig1B1 Simple model showing the electronic transitions in the

polyoxometalates containing electron donar and acceptor (a)generation

of charge carriers(b)electron and hole trapping(c)electron release due

to stimulation(d)recombination

between electron and hole Electrons are e-and holes are h+

In the polyoxometalates containing heteroatoms and especially in

mixed metal polyoxometalates the charge carriers which are created by the

light or electric field may be trapped in electron traps and hole traps These

traps provide states of localized energy in the O-M LMCT energy gap due to

the heteroatoms or counter cations which correspond to impurities or lattice

defects in the band model If the trap depth ∆E is large compared to kT the

probability for thermal escape from the trap will be negligibly small and

metastable situation will existThe trapped carriers can be released by thermal

or optical stimulationIn the case of thermal stimulation the irradiated

polyoxometalate is heated until the energy barrier ∆E can be overcome The

trapped electron (or hole) then can escape from the trap and nonradiatively

recombine with the trapped hole (or electron)Under optical stimulation the

energy of an incident photon is used to overcome ∆E The relaxation

processes of the OndashM LMCT excitation energy include both the nonradiative

recombination of electrons and holes within the energy gap and the

intramolecular energy transfer leading to a charge-transfer emission This

intramolecular energy transfer corresponds to the O-M LMCT energy gap and

occurs via radiative recombination and sensitized emission from the

28

heteroatoms or cations If several energy levels based on the hetero atoms or

counter cations act as energy acceptors within the O-M LMCTenergy gap the

energy transfer occurs from the O-M LMCT states to these levels followed by

the nonradiative or radiative deactivation of the excitation energy It should be

noted that the O-M LMCT states also can be generated by the application of

very high electric fields to the polyoxometalate solids as demonstrated by the

observation of electroluminescence[87]If an external electric field with a

potential more negative than the energy levels of the vacant orbitals involved

in the O-M LMCT transition is applied to a polyoxometalate on the electrode

surface an electrochemical reduction occurs via the injection of electrons

from the electrode in to the vacant levels of the polyoxometalate as shown in

Fig1B 2

Fig1B2 - Energy scheme for the electrochromism of polyoxometalates

a)electrochemical reduction (b) electrochemical oxidation

Electrons injected in to the high-energy levels also may be trapped by

electron traps in a process analogous to that which occurs during LMCT

photoexcitation of the polyoxometalates These electrons are returned to the

electrode by electrochemical oxidation at an electrode potential more positive

than the energy levels for the d1 electron states The d1electrons in the O-M

LMCTstates facilitate the absorption of visible light via intervalence charge

transfer among metal centers and d-d transitions The same type of transition

may be possible for the d1electron captured by the electron traps too In

addition to searching for new photosensitive polyoxometalates with the

29

potential for having practical application there is now a need to elucidate the

fundamental photo-and electrochemical coloration processes by studying

electron transfer within the polyoxometalate lattices in conjunction with their

crystal structures So far few polyoxometalates exhibit a perfect reversibility

of coloration The irreversibility of the color change arises from as yet

uncharacterized side reactions during both the coloration and decoloration of

the polyoxometalates

Many metal oxides including aluminum titanium vanadium niobium

molybdenum and tungsten oxides are photochromic when they contain

impurities or dopants This coloration has been interpreted on the basis of

electron trapping at appropriate lattice sites within the crystals as shown in

Fig1B1 where the O-M LMCT transition corresponds to the transition

between the valence and conduction bands for the infinite metal-oxide lattice

[88-91]

B12 Applications of Photochromic materials

Sunglasses

One of the most famous reversible photochromic applications is color

changing lenses for sunglasses as found in eyeglasses The largest limitation

in using PC technology is that the materials cannot be made stable enough

to withstand thousands of hours of outdoor exposure so long-term outdoor

applications are not appropriate at this time The switching speed of

photochromic dyes is highly sensitive to the rigidity of the environment around

the dye As result they switch most rapidly in solution and slowest in the rigid

environment like a polymer lens Recently it has been reported that attaching

flexible low Tg polymers (for example siloxanes or poly (butyl acrylate) to the

dyes allows them to switch much more rapidly in a rigid lens [76] Some

spirooxazines with siloxane polymers attached switch at near solution like

speeds even though they are in a rigid lens matrix

Supramolecular chemistry

Photochromic units have been employed extensively in supramolecular

chemistry Their ability to give a light controlled reversible shape change

means that they can be used to make or break molecular recognition motifs

30

or to cause a consequent shape change in their surroundings Thus

photochromic units have been demonstrated as components of molecular

switches The coupling of photochromic units to enzymes or enzyme cofactors

even provides the ability to reversibly turn enzymes on and off

by altering their shape or orientation in such a way that their functions

are either working or broken

Data storage

The possibility of using photochromic compounds for data storage was

first suggested in 1956 by Yehuda Hirshberg[77] Since that time there have

been many investigations by various academic and commercial groups

particularly in the area of 3D optical data storage which promises discs that

can hold a terabyte of data Initially issues with thermal back-reactions

and destructive reading dogged these studies but more recently more stable

systems have been developed

Novelty items

Reversible photochromics are also found in applications such as toys

cosmeticsclothing and industrial applications If necessary they can be made

to change between desired colors by combination with a permanent pigment

A large number of inorganic compounds exhibit photochromism

These solids often have large band gaps of the order of 3 - 12 eV and

excitation of these solids leads to the formation of metastable centers that

absorb visible light giving rise to their colour They can return to their ground

state by heating or by optical excitation within the colour-centre band In most

cases the photochromism is a structure sensitive phenomenon involving

localized defect impurities or dislocations Some of these inorganic

compounds have the potential for a number of different uses Photochromic

compounds have a number of useful applications These can be divided

according to the most important property that is being used (Table 11) [92]

31

Table 11 Applications of Photochromic materials

Applications Depending Upon

Sensitivity to

Radiation

Reversibility Thermal Chemical or

Physical Properties

Self-developing

photography

Chemical switches for

computers

Temperature indicators

Protective

materials

Data displays

Heat-sensitive recording

media

Optical signal

processing

Photomasking and

photoresist technology

Reusable data storage

media

Anaytical reagents

Photochromic

microimages

Photopolymerisation

Information encoding

and steganography

Photocontractile

polymers and the

photoviscosity effect

Control of light

intensity

Q-switches

Pyroelectric

photochromic materials

B13 Thermochromism

Thermochromism is the reversible colour change of a substance

induced by temperature change A large variety of substances organic

inorganic organometallic supramolecular and polymeric systems exhibit this

phenomenon Examples of these include bianthrones cobalt

hexacyanoferrate the zirconocene complex of 1 4-diphenyl-1 3-butadiene

and poly (3-alkylthiophene) The organic 99-bixanthenylidene is colourless at

90 K yellow-green at 298 K and dark-blue when melted at 592 K Heating

conducting polymers can cause them to change colour This is achieved by

causing conformational changes to the polymer backbone resulting in a

change in the band gap of the polymer It has been reported that regioregular

32

P3HT reversibly changes colour upon heating to 220ordmC due to temperature-

dependent conformation changes Thermally cross linked polymer undergoes

the same colour change but it is much less reversible [93] Other forms of

thermochromism may be commercially important eg to give a visual

indication of temperature changes

B14 Electrochromism

Electrochromism describes a phenomenon of material color change in

a persistent but reversible manner produced by electrochemically induced

oxidation-reduction reactions Electrochromic materials can be applied to

various kinds of products such as smart windows and display devices Among

those applications there have been lots of efforts to develop electrochromic

display devices (ECDs) Especially flexible display devices are now attracting

much attention worldwide since they can facilitate new technological demands

such as bending and folding of paper-like displays High electrochromic

efficiency short response time long operating life time and reduction of

energy consumption are the most important requisites of the materials for the

paperlike displays [94 95] Among those properties the operation life time is

the most important barrier to overcome for a realization of ECDs There are

two types of electrochromic material a) inorganic transition metal oxides

(TMOs) b) organic polymer materials The TMOs have been studied longer

than the organic materials that they have been studied since 1960s [9697]

Electrochromism describes a reversible color change of material

produced by electrochemically induced oxidation-reduction reactions It is one

of several types of chromism of materials As thermochromism and

photochromism mean material color changes made by heat and light

respectively electrochromism refers that the color change is caused by an

electric potential In most cases the color change in electrochromism can be

driven by rather low electrical potential of the order of a fraction of volt or a

few volts [94-96] The color change of material means variation in

transmittance andor reflectance change in visible range which is originated

from different electronic absorption bands according to a switching between

oxidation and reduction state of material When electric potential is applied on

electrochromic material forced oxidation or reduction is derived and the

33

individual color is originated from the corresponding oxidation or reduction

state of the material For electrochromic materials the characteristic color

change is reversible since the oxidation and reduction state can be converted

reversibly by switching potential

Application fields

The application area for the electrochromism is rather broad that it

covers from smart window glazing and optical modulators to information

displays [98-102] The smart windows are typical examples The

electrochromic property is used to control the amount of light and heat to pass

through the windows Usually the electrochromic material is in form of thin film

coated on a window glass The transmittance modulation has also been

applied at the automobiles to automatically tint rear-view mirrors in various

lighting conditions The electrochromic application fields are illustrated in Fig

1B3

Since the smart windows control the transmittance of heat as well as

the transmittance of visible light the working definition of electrochromism has

now been extended to include devices for modulation of radiation in the near

infrared thermal infrared and microwave regions When color for

electrochromic materials is used this can now mean a response by detectors

at these wavelengths and not just by the human eyes Nowadays

electrochromic material draws much attention as being used in the display

devices Electrochromic display device (ECD) is being considered as one of

the candidates for the conventional liquid crystal display (LCD) since ECD

has many advantages over LCDs Among these advantages the most

important are low energy consumption wider viewing angle high contrast

rate and possibility to achieve multiple colors with a single material [103]

When a new redox state of electrochromic materialis established by

the applied electric pulse then it is maintained after the potential is switched

off This means the colored or bleached state of the material can be sustained

for a considerable time without applying electrical power This is so-called

ldquocolor memory effectrdquo of electrochromic material Because of the color

memory effect energy consumption for the electrochromic display device

could be drastically reduced and this would be a big advantage over other

34

emissive devices The low power consumption is especially advantageous

when it is applied to mobile devices with limited power source The possibility

for a flexible display is another attraction for electrochromic material

Information displays

Real-view mirrors for automobiles

Fig 1B4 Application fields of electrochromic devices Smart windows information displays and real-view mirrors for automobiles

35

Using polymeric electrochromic material and plastic substrate with a

transparent conductive coating it is possible to build all-plastic flexible display

device There are lots of papers and patents about the flexible display devices

[104105] In these cases gel-type electrolyte is also needed The gel-type

electrolyte immobilizes liquid electrolyte in the polymer network [106107]

Recently ITO-coated polymer films are readily available which would provide

the plastic substrate for a flexible device The flexible electrochromic displays

are frequently tried with the plastic substrate flexible electrochromic material

and gel-type electrolyte The flexible electrochromic displays would facilitate

increasing technical demands for foldable display devices

Metal Ion Electrochromism

Many transition metal oxides are capable of redox reactions that result in

colour change Metal oxide films are commonly prepared as thin layers of

either tungsten nickel molybdenum or other metal compounds by a number

of techniques These include sol-gel electrochemical by dc or rfreactive

sputtering techniques electron-beam evaporation by anodic or cathodic

electrodeposition or by solution dipping of the electrochromic metal

compounds (or compounds that can be changed into these metal compounds)

onto optically transparent electrodes (OTE) [108 -114] Their electrochromism

is derived from the colour change associated with a change in the oxidation

state of the metal anion The behaviour of these materials is dependent upon

pH moisture and exposure to the atmosphere [115] Generally the switching

rates of these films is somewhat slow with typical switching times of about 15

- 60 seconds to achieve 100 conversion to either coloured or bleached state

[116 -120] An Example of this includes nickel oxide which changes from

transparent (pale green) to brownblack taking about 30 seconds to do so

[121] Other examples include [(NH4)5Ru]2(pyrazine)5+ and [(NH4)5Ru]2(44-

bipyridine)5+ whose electrochromism is significantly different due to the effect

of the ligand [122] Table 12 below gives some examples of metal oxide films

with electrochromic properties

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

24

definition photochromics are reversible there is technically no such thing as a

n irreversible photochromic this is loose usage and these compounds

are better referred to as photochangable or photoreactive dyes

Apart from the qualities already mentioned several other properties of

photochromics are important for their use These include

Quantum yield of the photochemical reaction

This determined the efficiency of the photochromic change with respect

to the amount of light absorbed The quantum yield of isomerization

can be strongly dependent on conditions

Fatigue resistance In photochromic materials fatigue refers to the

loss of reversibility by processes such as photodegradation

photobleaching photooxidation and other side reactions All

photochromics suffer fatigue to some extent and its rate is strongly

dependent on the activating light and the conditions of the sample

Photostationary state Photochromic materials have two states and

their interconversion can be controlled using different wavelengths of

light Excitation with any given wavelength of light will result in a

mixture of the two states at a particular ratio called the photo-

stationary state In a perfect system there would exist wavelengths

that can be used to provide 10 and 01 ratios of the isomers

but in real systems this is not possible since the active

absorbance bands always overlap to some extent

Polarity and solubility In order to incorporate photochromics in

working systems they suffer the same issues as other dyes They are

often charged in one or more state leading to very high polarity and

possible large changes in polarity They also often contain large

conjugated systems that limit their solubility

Photochromic complexes

A photochromic complex is a kind of chemical compound that has

photoresponsive Parts on its ligand These complexes have a specific

structure photoswitchable organic compounds are attached to

metalcomplexes For the photocontrollable parts thermally and

photochemically stable chromophores (azobenzene diarylethene

25

spiropyran etc) are usually used And for the metal complexes a wide

variety of compounds that have various functions (redox response

luminescence magnetism etc) are applied The photochromic parts and

metal parts are so close that they can affect each others molecular

orbitals The physical properties of these compounds shown by parts of

them (ie chromophores or metals) thus can be controlled by switching

their other sites by external stimuli For example photoisomerization

behaviors of some complexes can be switched by oxidation and reduction of

their metal parts Some other compounds can be changed in their

luminescence behavior magnetic interaction of metal sites or stability of

metal-to-ligand coordination by photoisomerization of their photochromic

parts

Photochemistry of Polyoxometalates

The photochemistry of polyoxometalates is of great interest to inorganic

chemistsMore than 80 years agoit was found that the R-Keggin tungstate

H3[PW12O40] was reduced photochemically to yield a blue-colored species

which was reoxidized by air and by various other oxidizing agents such as

Fe3+AgNO3and H2O2 [7879]The photoredox reactions of H4[SiW12O40]and

H3[PW12O40] proceeded effectively in the presence of primary and secondary

alcohols their ethers and aldehydes and proteinsbut less effectively in the

presence of tertiary alcoholsketonesestersthe fatty acids above formic

acidand simple amines[8081] The basic photoredox reaction involving

ethanol is illustrated by eq 13

2 H3PW12O40 + H3CCH2OH h ν ν ν ν 2 H4PW12O40 + H3CCHO ------- 13

2 H4PW12O40 +12 O2 2 H3PW12O40 + H2O ------ 14

In this reactionone molecule of ethanol photochemically reduces two

molecules of H3PW12O40 and is itself oxidized to acetaldehyde In the

presence of air the thermal oxidation of the reduced species takes place at

room temperature(eq 14)The reduced polyoxometalates which are the so-

calledldquoheteropolybluesrdquo have been used for the colorimetric analysis of the

26

elements P Si As and Ge and for the determination of uric acidsugarand

other biological compounds [8283] Piperidinium metavanadate also

undergoes photoinduced coloration from white to black followed by a

reversible color change in the presence of oxidizing agentsHowever

ammonium metavanadates ([NH4][VO3]) exhibits no photoinduced

coloration[84] The early photoredox reactions of the R-Keggin

polyoxometalates H4[SiW12O40] and H3PW12O40 were carried out in the

presence of photographic paper however the limited number of the

structurally well-characterized compounds available for study delayed the

development of modern cluster-compound photochemistry until the discovery

of photochromism in alkylammonium polyoxo- molybdate solids[8586] A

photochromic or electrochromic material is one whose light-absorbing

properties are altered upon optical excitation or reduction under the influence

of an externally applied electric field respectively The induced coloration

remains even after the excitation source has been removed These materials

are of technological interest because they return to their original state either

thermally upon irradiation with light of a frequency corresponding to the

induced absorption or electrochemically upon reversing the polarity of the

externally applied electric field Thus photochromic and electrochromic

materials behave in a reversible manner Polyoxometalates exhibit significant

photo-and electrochromism which makes them suitable as nanocomposite

molecular devices and as models for probing the physical properties of infinite

metal oxides Since the metal ions in the oxidized polyoxometalates have d0

electronic configurations the only absorption band which occurs in the UV-vis

range of the electronic spectra is due to the oxygen-to-metal (O-M) ligand-to-

metal charge transfer (LMCT)Upon irradiation electrons are promoted from

the low-energy electronic states which are mainly comprised of oxygen 2p

orbitals (the valence band in the band model)to the high-energy electronic

states which are mainly comprised of metal d orbitalrsquos (the conduction band

in the band model)The fundamental transitions in polyoxometalate lattices

are depicted schematically in Fig1B1

27

Fig1B1 Simple model showing the electronic transitions in the

polyoxometalates containing electron donar and acceptor (a)generation

of charge carriers(b)electron and hole trapping(c)electron release due

to stimulation(d)recombination

between electron and hole Electrons are e-and holes are h+

In the polyoxometalates containing heteroatoms and especially in

mixed metal polyoxometalates the charge carriers which are created by the

light or electric field may be trapped in electron traps and hole traps These

traps provide states of localized energy in the O-M LMCT energy gap due to

the heteroatoms or counter cations which correspond to impurities or lattice

defects in the band model If the trap depth ∆E is large compared to kT the

probability for thermal escape from the trap will be negligibly small and

metastable situation will existThe trapped carriers can be released by thermal

or optical stimulationIn the case of thermal stimulation the irradiated

polyoxometalate is heated until the energy barrier ∆E can be overcome The

trapped electron (or hole) then can escape from the trap and nonradiatively

recombine with the trapped hole (or electron)Under optical stimulation the

energy of an incident photon is used to overcome ∆E The relaxation

processes of the OndashM LMCT excitation energy include both the nonradiative

recombination of electrons and holes within the energy gap and the

intramolecular energy transfer leading to a charge-transfer emission This

intramolecular energy transfer corresponds to the O-M LMCT energy gap and

occurs via radiative recombination and sensitized emission from the

28

heteroatoms or cations If several energy levels based on the hetero atoms or

counter cations act as energy acceptors within the O-M LMCTenergy gap the

energy transfer occurs from the O-M LMCT states to these levels followed by

the nonradiative or radiative deactivation of the excitation energy It should be

noted that the O-M LMCT states also can be generated by the application of

very high electric fields to the polyoxometalate solids as demonstrated by the

observation of electroluminescence[87]If an external electric field with a

potential more negative than the energy levels of the vacant orbitals involved

in the O-M LMCT transition is applied to a polyoxometalate on the electrode

surface an electrochemical reduction occurs via the injection of electrons

from the electrode in to the vacant levels of the polyoxometalate as shown in

Fig1B 2

Fig1B2 - Energy scheme for the electrochromism of polyoxometalates

a)electrochemical reduction (b) electrochemical oxidation

Electrons injected in to the high-energy levels also may be trapped by

electron traps in a process analogous to that which occurs during LMCT

photoexcitation of the polyoxometalates These electrons are returned to the

electrode by electrochemical oxidation at an electrode potential more positive

than the energy levels for the d1 electron states The d1electrons in the O-M

LMCTstates facilitate the absorption of visible light via intervalence charge

transfer among metal centers and d-d transitions The same type of transition

may be possible for the d1electron captured by the electron traps too In

addition to searching for new photosensitive polyoxometalates with the

29

potential for having practical application there is now a need to elucidate the

fundamental photo-and electrochemical coloration processes by studying

electron transfer within the polyoxometalate lattices in conjunction with their

crystal structures So far few polyoxometalates exhibit a perfect reversibility

of coloration The irreversibility of the color change arises from as yet

uncharacterized side reactions during both the coloration and decoloration of

the polyoxometalates

Many metal oxides including aluminum titanium vanadium niobium

molybdenum and tungsten oxides are photochromic when they contain

impurities or dopants This coloration has been interpreted on the basis of

electron trapping at appropriate lattice sites within the crystals as shown in

Fig1B1 where the O-M LMCT transition corresponds to the transition

between the valence and conduction bands for the infinite metal-oxide lattice

[88-91]

B12 Applications of Photochromic materials

Sunglasses

One of the most famous reversible photochromic applications is color

changing lenses for sunglasses as found in eyeglasses The largest limitation

in using PC technology is that the materials cannot be made stable enough

to withstand thousands of hours of outdoor exposure so long-term outdoor

applications are not appropriate at this time The switching speed of

photochromic dyes is highly sensitive to the rigidity of the environment around

the dye As result they switch most rapidly in solution and slowest in the rigid

environment like a polymer lens Recently it has been reported that attaching

flexible low Tg polymers (for example siloxanes or poly (butyl acrylate) to the

dyes allows them to switch much more rapidly in a rigid lens [76] Some

spirooxazines with siloxane polymers attached switch at near solution like

speeds even though they are in a rigid lens matrix

Supramolecular chemistry

Photochromic units have been employed extensively in supramolecular

chemistry Their ability to give a light controlled reversible shape change

means that they can be used to make or break molecular recognition motifs

30

or to cause a consequent shape change in their surroundings Thus

photochromic units have been demonstrated as components of molecular

switches The coupling of photochromic units to enzymes or enzyme cofactors

even provides the ability to reversibly turn enzymes on and off

by altering their shape or orientation in such a way that their functions

are either working or broken

Data storage

The possibility of using photochromic compounds for data storage was

first suggested in 1956 by Yehuda Hirshberg[77] Since that time there have

been many investigations by various academic and commercial groups

particularly in the area of 3D optical data storage which promises discs that

can hold a terabyte of data Initially issues with thermal back-reactions

and destructive reading dogged these studies but more recently more stable

systems have been developed

Novelty items

Reversible photochromics are also found in applications such as toys

cosmeticsclothing and industrial applications If necessary they can be made

to change between desired colors by combination with a permanent pigment

A large number of inorganic compounds exhibit photochromism

These solids often have large band gaps of the order of 3 - 12 eV and

excitation of these solids leads to the formation of metastable centers that

absorb visible light giving rise to their colour They can return to their ground

state by heating or by optical excitation within the colour-centre band In most

cases the photochromism is a structure sensitive phenomenon involving

localized defect impurities or dislocations Some of these inorganic

compounds have the potential for a number of different uses Photochromic

compounds have a number of useful applications These can be divided

according to the most important property that is being used (Table 11) [92]

31

Table 11 Applications of Photochromic materials

Applications Depending Upon

Sensitivity to

Radiation

Reversibility Thermal Chemical or

Physical Properties

Self-developing

photography

Chemical switches for

computers

Temperature indicators

Protective

materials

Data displays

Heat-sensitive recording

media

Optical signal

processing

Photomasking and

photoresist technology

Reusable data storage

media

Anaytical reagents

Photochromic

microimages

Photopolymerisation

Information encoding

and steganography

Photocontractile

polymers and the

photoviscosity effect

Control of light

intensity

Q-switches

Pyroelectric

photochromic materials

B13 Thermochromism

Thermochromism is the reversible colour change of a substance

induced by temperature change A large variety of substances organic

inorganic organometallic supramolecular and polymeric systems exhibit this

phenomenon Examples of these include bianthrones cobalt

hexacyanoferrate the zirconocene complex of 1 4-diphenyl-1 3-butadiene

and poly (3-alkylthiophene) The organic 99-bixanthenylidene is colourless at

90 K yellow-green at 298 K and dark-blue when melted at 592 K Heating

conducting polymers can cause them to change colour This is achieved by

causing conformational changes to the polymer backbone resulting in a

change in the band gap of the polymer It has been reported that regioregular

32

P3HT reversibly changes colour upon heating to 220ordmC due to temperature-

dependent conformation changes Thermally cross linked polymer undergoes

the same colour change but it is much less reversible [93] Other forms of

thermochromism may be commercially important eg to give a visual

indication of temperature changes

B14 Electrochromism

Electrochromism describes a phenomenon of material color change in

a persistent but reversible manner produced by electrochemically induced

oxidation-reduction reactions Electrochromic materials can be applied to

various kinds of products such as smart windows and display devices Among

those applications there have been lots of efforts to develop electrochromic

display devices (ECDs) Especially flexible display devices are now attracting

much attention worldwide since they can facilitate new technological demands

such as bending and folding of paper-like displays High electrochromic

efficiency short response time long operating life time and reduction of

energy consumption are the most important requisites of the materials for the

paperlike displays [94 95] Among those properties the operation life time is

the most important barrier to overcome for a realization of ECDs There are

two types of electrochromic material a) inorganic transition metal oxides

(TMOs) b) organic polymer materials The TMOs have been studied longer

than the organic materials that they have been studied since 1960s [9697]

Electrochromism describes a reversible color change of material

produced by electrochemically induced oxidation-reduction reactions It is one

of several types of chromism of materials As thermochromism and

photochromism mean material color changes made by heat and light

respectively electrochromism refers that the color change is caused by an

electric potential In most cases the color change in electrochromism can be

driven by rather low electrical potential of the order of a fraction of volt or a

few volts [94-96] The color change of material means variation in

transmittance andor reflectance change in visible range which is originated

from different electronic absorption bands according to a switching between

oxidation and reduction state of material When electric potential is applied on

electrochromic material forced oxidation or reduction is derived and the

33

individual color is originated from the corresponding oxidation or reduction

state of the material For electrochromic materials the characteristic color

change is reversible since the oxidation and reduction state can be converted

reversibly by switching potential

Application fields

The application area for the electrochromism is rather broad that it

covers from smart window glazing and optical modulators to information

displays [98-102] The smart windows are typical examples The

electrochromic property is used to control the amount of light and heat to pass

through the windows Usually the electrochromic material is in form of thin film

coated on a window glass The transmittance modulation has also been

applied at the automobiles to automatically tint rear-view mirrors in various

lighting conditions The electrochromic application fields are illustrated in Fig

1B3

Since the smart windows control the transmittance of heat as well as

the transmittance of visible light the working definition of electrochromism has

now been extended to include devices for modulation of radiation in the near

infrared thermal infrared and microwave regions When color for

electrochromic materials is used this can now mean a response by detectors

at these wavelengths and not just by the human eyes Nowadays

electrochromic material draws much attention as being used in the display

devices Electrochromic display device (ECD) is being considered as one of

the candidates for the conventional liquid crystal display (LCD) since ECD

has many advantages over LCDs Among these advantages the most

important are low energy consumption wider viewing angle high contrast

rate and possibility to achieve multiple colors with a single material [103]

When a new redox state of electrochromic materialis established by

the applied electric pulse then it is maintained after the potential is switched

off This means the colored or bleached state of the material can be sustained

for a considerable time without applying electrical power This is so-called

ldquocolor memory effectrdquo of electrochromic material Because of the color

memory effect energy consumption for the electrochromic display device

could be drastically reduced and this would be a big advantage over other

34

emissive devices The low power consumption is especially advantageous

when it is applied to mobile devices with limited power source The possibility

for a flexible display is another attraction for electrochromic material

Information displays

Real-view mirrors for automobiles

Fig 1B4 Application fields of electrochromic devices Smart windows information displays and real-view mirrors for automobiles

35

Using polymeric electrochromic material and plastic substrate with a

transparent conductive coating it is possible to build all-plastic flexible display

device There are lots of papers and patents about the flexible display devices

[104105] In these cases gel-type electrolyte is also needed The gel-type

electrolyte immobilizes liquid electrolyte in the polymer network [106107]

Recently ITO-coated polymer films are readily available which would provide

the plastic substrate for a flexible device The flexible electrochromic displays

are frequently tried with the plastic substrate flexible electrochromic material

and gel-type electrolyte The flexible electrochromic displays would facilitate

increasing technical demands for foldable display devices

Metal Ion Electrochromism

Many transition metal oxides are capable of redox reactions that result in

colour change Metal oxide films are commonly prepared as thin layers of

either tungsten nickel molybdenum or other metal compounds by a number

of techniques These include sol-gel electrochemical by dc or rfreactive

sputtering techniques electron-beam evaporation by anodic or cathodic

electrodeposition or by solution dipping of the electrochromic metal

compounds (or compounds that can be changed into these metal compounds)

onto optically transparent electrodes (OTE) [108 -114] Their electrochromism

is derived from the colour change associated with a change in the oxidation

state of the metal anion The behaviour of these materials is dependent upon

pH moisture and exposure to the atmosphere [115] Generally the switching

rates of these films is somewhat slow with typical switching times of about 15

- 60 seconds to achieve 100 conversion to either coloured or bleached state

[116 -120] An Example of this includes nickel oxide which changes from

transparent (pale green) to brownblack taking about 30 seconds to do so

[121] Other examples include [(NH4)5Ru]2(pyrazine)5+ and [(NH4)5Ru]2(44-

bipyridine)5+ whose electrochromism is significantly different due to the effect

of the ligand [122] Table 12 below gives some examples of metal oxide films

with electrochromic properties

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

25

spiropyran etc) are usually used And for the metal complexes a wide

variety of compounds that have various functions (redox response

luminescence magnetism etc) are applied The photochromic parts and

metal parts are so close that they can affect each others molecular

orbitals The physical properties of these compounds shown by parts of

them (ie chromophores or metals) thus can be controlled by switching

their other sites by external stimuli For example photoisomerization

behaviors of some complexes can be switched by oxidation and reduction of

their metal parts Some other compounds can be changed in their

luminescence behavior magnetic interaction of metal sites or stability of

metal-to-ligand coordination by photoisomerization of their photochromic

parts

Photochemistry of Polyoxometalates

The photochemistry of polyoxometalates is of great interest to inorganic

chemistsMore than 80 years agoit was found that the R-Keggin tungstate

H3[PW12O40] was reduced photochemically to yield a blue-colored species

which was reoxidized by air and by various other oxidizing agents such as

Fe3+AgNO3and H2O2 [7879]The photoredox reactions of H4[SiW12O40]and

H3[PW12O40] proceeded effectively in the presence of primary and secondary

alcohols their ethers and aldehydes and proteinsbut less effectively in the

presence of tertiary alcoholsketonesestersthe fatty acids above formic

acidand simple amines[8081] The basic photoredox reaction involving

ethanol is illustrated by eq 13

2 H3PW12O40 + H3CCH2OH h ν ν ν ν 2 H4PW12O40 + H3CCHO ------- 13

2 H4PW12O40 +12 O2 2 H3PW12O40 + H2O ------ 14

In this reactionone molecule of ethanol photochemically reduces two

molecules of H3PW12O40 and is itself oxidized to acetaldehyde In the

presence of air the thermal oxidation of the reduced species takes place at

room temperature(eq 14)The reduced polyoxometalates which are the so-

calledldquoheteropolybluesrdquo have been used for the colorimetric analysis of the

26

elements P Si As and Ge and for the determination of uric acidsugarand

other biological compounds [8283] Piperidinium metavanadate also

undergoes photoinduced coloration from white to black followed by a

reversible color change in the presence of oxidizing agentsHowever

ammonium metavanadates ([NH4][VO3]) exhibits no photoinduced

coloration[84] The early photoredox reactions of the R-Keggin

polyoxometalates H4[SiW12O40] and H3PW12O40 were carried out in the

presence of photographic paper however the limited number of the

structurally well-characterized compounds available for study delayed the

development of modern cluster-compound photochemistry until the discovery

of photochromism in alkylammonium polyoxo- molybdate solids[8586] A

photochromic or electrochromic material is one whose light-absorbing

properties are altered upon optical excitation or reduction under the influence

of an externally applied electric field respectively The induced coloration

remains even after the excitation source has been removed These materials

are of technological interest because they return to their original state either

thermally upon irradiation with light of a frequency corresponding to the

induced absorption or electrochemically upon reversing the polarity of the

externally applied electric field Thus photochromic and electrochromic

materials behave in a reversible manner Polyoxometalates exhibit significant

photo-and electrochromism which makes them suitable as nanocomposite

molecular devices and as models for probing the physical properties of infinite

metal oxides Since the metal ions in the oxidized polyoxometalates have d0

electronic configurations the only absorption band which occurs in the UV-vis

range of the electronic spectra is due to the oxygen-to-metal (O-M) ligand-to-

metal charge transfer (LMCT)Upon irradiation electrons are promoted from

the low-energy electronic states which are mainly comprised of oxygen 2p

orbitals (the valence band in the band model)to the high-energy electronic

states which are mainly comprised of metal d orbitalrsquos (the conduction band

in the band model)The fundamental transitions in polyoxometalate lattices

are depicted schematically in Fig1B1

27

Fig1B1 Simple model showing the electronic transitions in the

polyoxometalates containing electron donar and acceptor (a)generation

of charge carriers(b)electron and hole trapping(c)electron release due

to stimulation(d)recombination

between electron and hole Electrons are e-and holes are h+

In the polyoxometalates containing heteroatoms and especially in

mixed metal polyoxometalates the charge carriers which are created by the

light or electric field may be trapped in electron traps and hole traps These

traps provide states of localized energy in the O-M LMCT energy gap due to

the heteroatoms or counter cations which correspond to impurities or lattice

defects in the band model If the trap depth ∆E is large compared to kT the

probability for thermal escape from the trap will be negligibly small and

metastable situation will existThe trapped carriers can be released by thermal

or optical stimulationIn the case of thermal stimulation the irradiated

polyoxometalate is heated until the energy barrier ∆E can be overcome The

trapped electron (or hole) then can escape from the trap and nonradiatively

recombine with the trapped hole (or electron)Under optical stimulation the

energy of an incident photon is used to overcome ∆E The relaxation

processes of the OndashM LMCT excitation energy include both the nonradiative

recombination of electrons and holes within the energy gap and the

intramolecular energy transfer leading to a charge-transfer emission This

intramolecular energy transfer corresponds to the O-M LMCT energy gap and

occurs via radiative recombination and sensitized emission from the

28

heteroatoms or cations If several energy levels based on the hetero atoms or

counter cations act as energy acceptors within the O-M LMCTenergy gap the

energy transfer occurs from the O-M LMCT states to these levels followed by

the nonradiative or radiative deactivation of the excitation energy It should be

noted that the O-M LMCT states also can be generated by the application of

very high electric fields to the polyoxometalate solids as demonstrated by the

observation of electroluminescence[87]If an external electric field with a

potential more negative than the energy levels of the vacant orbitals involved

in the O-M LMCT transition is applied to a polyoxometalate on the electrode

surface an electrochemical reduction occurs via the injection of electrons

from the electrode in to the vacant levels of the polyoxometalate as shown in

Fig1B 2

Fig1B2 - Energy scheme for the electrochromism of polyoxometalates

a)electrochemical reduction (b) electrochemical oxidation

Electrons injected in to the high-energy levels also may be trapped by

electron traps in a process analogous to that which occurs during LMCT

photoexcitation of the polyoxometalates These electrons are returned to the

electrode by electrochemical oxidation at an electrode potential more positive

than the energy levels for the d1 electron states The d1electrons in the O-M

LMCTstates facilitate the absorption of visible light via intervalence charge

transfer among metal centers and d-d transitions The same type of transition

may be possible for the d1electron captured by the electron traps too In

addition to searching for new photosensitive polyoxometalates with the

29

potential for having practical application there is now a need to elucidate the

fundamental photo-and electrochemical coloration processes by studying

electron transfer within the polyoxometalate lattices in conjunction with their

crystal structures So far few polyoxometalates exhibit a perfect reversibility

of coloration The irreversibility of the color change arises from as yet

uncharacterized side reactions during both the coloration and decoloration of

the polyoxometalates

Many metal oxides including aluminum titanium vanadium niobium

molybdenum and tungsten oxides are photochromic when they contain

impurities or dopants This coloration has been interpreted on the basis of

electron trapping at appropriate lattice sites within the crystals as shown in

Fig1B1 where the O-M LMCT transition corresponds to the transition

between the valence and conduction bands for the infinite metal-oxide lattice

[88-91]

B12 Applications of Photochromic materials

Sunglasses

One of the most famous reversible photochromic applications is color

changing lenses for sunglasses as found in eyeglasses The largest limitation

in using PC technology is that the materials cannot be made stable enough

to withstand thousands of hours of outdoor exposure so long-term outdoor

applications are not appropriate at this time The switching speed of

photochromic dyes is highly sensitive to the rigidity of the environment around

the dye As result they switch most rapidly in solution and slowest in the rigid

environment like a polymer lens Recently it has been reported that attaching

flexible low Tg polymers (for example siloxanes or poly (butyl acrylate) to the

dyes allows them to switch much more rapidly in a rigid lens [76] Some

spirooxazines with siloxane polymers attached switch at near solution like

speeds even though they are in a rigid lens matrix

Supramolecular chemistry

Photochromic units have been employed extensively in supramolecular

chemistry Their ability to give a light controlled reversible shape change

means that they can be used to make or break molecular recognition motifs

30

or to cause a consequent shape change in their surroundings Thus

photochromic units have been demonstrated as components of molecular

switches The coupling of photochromic units to enzymes or enzyme cofactors

even provides the ability to reversibly turn enzymes on and off

by altering their shape or orientation in such a way that their functions

are either working or broken

Data storage

The possibility of using photochromic compounds for data storage was

first suggested in 1956 by Yehuda Hirshberg[77] Since that time there have

been many investigations by various academic and commercial groups

particularly in the area of 3D optical data storage which promises discs that

can hold a terabyte of data Initially issues with thermal back-reactions

and destructive reading dogged these studies but more recently more stable

systems have been developed

Novelty items

Reversible photochromics are also found in applications such as toys

cosmeticsclothing and industrial applications If necessary they can be made

to change between desired colors by combination with a permanent pigment

A large number of inorganic compounds exhibit photochromism

These solids often have large band gaps of the order of 3 - 12 eV and

excitation of these solids leads to the formation of metastable centers that

absorb visible light giving rise to their colour They can return to their ground

state by heating or by optical excitation within the colour-centre band In most

cases the photochromism is a structure sensitive phenomenon involving

localized defect impurities or dislocations Some of these inorganic

compounds have the potential for a number of different uses Photochromic

compounds have a number of useful applications These can be divided

according to the most important property that is being used (Table 11) [92]

31

Table 11 Applications of Photochromic materials

Applications Depending Upon

Sensitivity to

Radiation

Reversibility Thermal Chemical or

Physical Properties

Self-developing

photography

Chemical switches for

computers

Temperature indicators

Protective

materials

Data displays

Heat-sensitive recording

media

Optical signal

processing

Photomasking and

photoresist technology

Reusable data storage

media

Anaytical reagents

Photochromic

microimages

Photopolymerisation

Information encoding

and steganography

Photocontractile

polymers and the

photoviscosity effect

Control of light

intensity

Q-switches

Pyroelectric

photochromic materials

B13 Thermochromism

Thermochromism is the reversible colour change of a substance

induced by temperature change A large variety of substances organic

inorganic organometallic supramolecular and polymeric systems exhibit this

phenomenon Examples of these include bianthrones cobalt

hexacyanoferrate the zirconocene complex of 1 4-diphenyl-1 3-butadiene

and poly (3-alkylthiophene) The organic 99-bixanthenylidene is colourless at

90 K yellow-green at 298 K and dark-blue when melted at 592 K Heating

conducting polymers can cause them to change colour This is achieved by

causing conformational changes to the polymer backbone resulting in a

change in the band gap of the polymer It has been reported that regioregular

32

P3HT reversibly changes colour upon heating to 220ordmC due to temperature-

dependent conformation changes Thermally cross linked polymer undergoes

the same colour change but it is much less reversible [93] Other forms of

thermochromism may be commercially important eg to give a visual

indication of temperature changes

B14 Electrochromism

Electrochromism describes a phenomenon of material color change in

a persistent but reversible manner produced by electrochemically induced

oxidation-reduction reactions Electrochromic materials can be applied to

various kinds of products such as smart windows and display devices Among

those applications there have been lots of efforts to develop electrochromic

display devices (ECDs) Especially flexible display devices are now attracting

much attention worldwide since they can facilitate new technological demands

such as bending and folding of paper-like displays High electrochromic

efficiency short response time long operating life time and reduction of

energy consumption are the most important requisites of the materials for the

paperlike displays [94 95] Among those properties the operation life time is

the most important barrier to overcome for a realization of ECDs There are

two types of electrochromic material a) inorganic transition metal oxides

(TMOs) b) organic polymer materials The TMOs have been studied longer

than the organic materials that they have been studied since 1960s [9697]

Electrochromism describes a reversible color change of material

produced by electrochemically induced oxidation-reduction reactions It is one

of several types of chromism of materials As thermochromism and

photochromism mean material color changes made by heat and light

respectively electrochromism refers that the color change is caused by an

electric potential In most cases the color change in electrochromism can be

driven by rather low electrical potential of the order of a fraction of volt or a

few volts [94-96] The color change of material means variation in

transmittance andor reflectance change in visible range which is originated

from different electronic absorption bands according to a switching between

oxidation and reduction state of material When electric potential is applied on

electrochromic material forced oxidation or reduction is derived and the

33

individual color is originated from the corresponding oxidation or reduction

state of the material For electrochromic materials the characteristic color

change is reversible since the oxidation and reduction state can be converted

reversibly by switching potential

Application fields

The application area for the electrochromism is rather broad that it

covers from smart window glazing and optical modulators to information

displays [98-102] The smart windows are typical examples The

electrochromic property is used to control the amount of light and heat to pass

through the windows Usually the electrochromic material is in form of thin film

coated on a window glass The transmittance modulation has also been

applied at the automobiles to automatically tint rear-view mirrors in various

lighting conditions The electrochromic application fields are illustrated in Fig

1B3

Since the smart windows control the transmittance of heat as well as

the transmittance of visible light the working definition of electrochromism has

now been extended to include devices for modulation of radiation in the near

infrared thermal infrared and microwave regions When color for

electrochromic materials is used this can now mean a response by detectors

at these wavelengths and not just by the human eyes Nowadays

electrochromic material draws much attention as being used in the display

devices Electrochromic display device (ECD) is being considered as one of

the candidates for the conventional liquid crystal display (LCD) since ECD

has many advantages over LCDs Among these advantages the most

important are low energy consumption wider viewing angle high contrast

rate and possibility to achieve multiple colors with a single material [103]

When a new redox state of electrochromic materialis established by

the applied electric pulse then it is maintained after the potential is switched

off This means the colored or bleached state of the material can be sustained

for a considerable time without applying electrical power This is so-called

ldquocolor memory effectrdquo of electrochromic material Because of the color

memory effect energy consumption for the electrochromic display device

could be drastically reduced and this would be a big advantage over other

34

emissive devices The low power consumption is especially advantageous

when it is applied to mobile devices with limited power source The possibility

for a flexible display is another attraction for electrochromic material

Information displays

Real-view mirrors for automobiles

Fig 1B4 Application fields of electrochromic devices Smart windows information displays and real-view mirrors for automobiles

35

Using polymeric electrochromic material and plastic substrate with a

transparent conductive coating it is possible to build all-plastic flexible display

device There are lots of papers and patents about the flexible display devices

[104105] In these cases gel-type electrolyte is also needed The gel-type

electrolyte immobilizes liquid electrolyte in the polymer network [106107]

Recently ITO-coated polymer films are readily available which would provide

the plastic substrate for a flexible device The flexible electrochromic displays

are frequently tried with the plastic substrate flexible electrochromic material

and gel-type electrolyte The flexible electrochromic displays would facilitate

increasing technical demands for foldable display devices

Metal Ion Electrochromism

Many transition metal oxides are capable of redox reactions that result in

colour change Metal oxide films are commonly prepared as thin layers of

either tungsten nickel molybdenum or other metal compounds by a number

of techniques These include sol-gel electrochemical by dc or rfreactive

sputtering techniques electron-beam evaporation by anodic or cathodic

electrodeposition or by solution dipping of the electrochromic metal

compounds (or compounds that can be changed into these metal compounds)

onto optically transparent electrodes (OTE) [108 -114] Their electrochromism

is derived from the colour change associated with a change in the oxidation

state of the metal anion The behaviour of these materials is dependent upon

pH moisture and exposure to the atmosphere [115] Generally the switching

rates of these films is somewhat slow with typical switching times of about 15

- 60 seconds to achieve 100 conversion to either coloured or bleached state

[116 -120] An Example of this includes nickel oxide which changes from

transparent (pale green) to brownblack taking about 30 seconds to do so

[121] Other examples include [(NH4)5Ru]2(pyrazine)5+ and [(NH4)5Ru]2(44-

bipyridine)5+ whose electrochromism is significantly different due to the effect

of the ligand [122] Table 12 below gives some examples of metal oxide films

with electrochromic properties

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

26

elements P Si As and Ge and for the determination of uric acidsugarand

other biological compounds [8283] Piperidinium metavanadate also

undergoes photoinduced coloration from white to black followed by a

reversible color change in the presence of oxidizing agentsHowever

ammonium metavanadates ([NH4][VO3]) exhibits no photoinduced

coloration[84] The early photoredox reactions of the R-Keggin

polyoxometalates H4[SiW12O40] and H3PW12O40 were carried out in the

presence of photographic paper however the limited number of the

structurally well-characterized compounds available for study delayed the

development of modern cluster-compound photochemistry until the discovery

of photochromism in alkylammonium polyoxo- molybdate solids[8586] A

photochromic or electrochromic material is one whose light-absorbing

properties are altered upon optical excitation or reduction under the influence

of an externally applied electric field respectively The induced coloration

remains even after the excitation source has been removed These materials

are of technological interest because they return to their original state either

thermally upon irradiation with light of a frequency corresponding to the

induced absorption or electrochemically upon reversing the polarity of the

externally applied electric field Thus photochromic and electrochromic

materials behave in a reversible manner Polyoxometalates exhibit significant

photo-and electrochromism which makes them suitable as nanocomposite

molecular devices and as models for probing the physical properties of infinite

metal oxides Since the metal ions in the oxidized polyoxometalates have d0

electronic configurations the only absorption band which occurs in the UV-vis

range of the electronic spectra is due to the oxygen-to-metal (O-M) ligand-to-

metal charge transfer (LMCT)Upon irradiation electrons are promoted from

the low-energy electronic states which are mainly comprised of oxygen 2p

orbitals (the valence band in the band model)to the high-energy electronic

states which are mainly comprised of metal d orbitalrsquos (the conduction band

in the band model)The fundamental transitions in polyoxometalate lattices

are depicted schematically in Fig1B1

27

Fig1B1 Simple model showing the electronic transitions in the

polyoxometalates containing electron donar and acceptor (a)generation

of charge carriers(b)electron and hole trapping(c)electron release due

to stimulation(d)recombination

between electron and hole Electrons are e-and holes are h+

In the polyoxometalates containing heteroatoms and especially in

mixed metal polyoxometalates the charge carriers which are created by the

light or electric field may be trapped in electron traps and hole traps These

traps provide states of localized energy in the O-M LMCT energy gap due to

the heteroatoms or counter cations which correspond to impurities or lattice

defects in the band model If the trap depth ∆E is large compared to kT the

probability for thermal escape from the trap will be negligibly small and

metastable situation will existThe trapped carriers can be released by thermal

or optical stimulationIn the case of thermal stimulation the irradiated

polyoxometalate is heated until the energy barrier ∆E can be overcome The

trapped electron (or hole) then can escape from the trap and nonradiatively

recombine with the trapped hole (or electron)Under optical stimulation the

energy of an incident photon is used to overcome ∆E The relaxation

processes of the OndashM LMCT excitation energy include both the nonradiative

recombination of electrons and holes within the energy gap and the

intramolecular energy transfer leading to a charge-transfer emission This

intramolecular energy transfer corresponds to the O-M LMCT energy gap and

occurs via radiative recombination and sensitized emission from the

28

heteroatoms or cations If several energy levels based on the hetero atoms or

counter cations act as energy acceptors within the O-M LMCTenergy gap the

energy transfer occurs from the O-M LMCT states to these levels followed by

the nonradiative or radiative deactivation of the excitation energy It should be

noted that the O-M LMCT states also can be generated by the application of

very high electric fields to the polyoxometalate solids as demonstrated by the

observation of electroluminescence[87]If an external electric field with a

potential more negative than the energy levels of the vacant orbitals involved

in the O-M LMCT transition is applied to a polyoxometalate on the electrode

surface an electrochemical reduction occurs via the injection of electrons

from the electrode in to the vacant levels of the polyoxometalate as shown in

Fig1B 2

Fig1B2 - Energy scheme for the electrochromism of polyoxometalates

a)electrochemical reduction (b) electrochemical oxidation

Electrons injected in to the high-energy levels also may be trapped by

electron traps in a process analogous to that which occurs during LMCT

photoexcitation of the polyoxometalates These electrons are returned to the

electrode by electrochemical oxidation at an electrode potential more positive

than the energy levels for the d1 electron states The d1electrons in the O-M

LMCTstates facilitate the absorption of visible light via intervalence charge

transfer among metal centers and d-d transitions The same type of transition

may be possible for the d1electron captured by the electron traps too In

addition to searching for new photosensitive polyoxometalates with the

29

potential for having practical application there is now a need to elucidate the

fundamental photo-and electrochemical coloration processes by studying

electron transfer within the polyoxometalate lattices in conjunction with their

crystal structures So far few polyoxometalates exhibit a perfect reversibility

of coloration The irreversibility of the color change arises from as yet

uncharacterized side reactions during both the coloration and decoloration of

the polyoxometalates

Many metal oxides including aluminum titanium vanadium niobium

molybdenum and tungsten oxides are photochromic when they contain

impurities or dopants This coloration has been interpreted on the basis of

electron trapping at appropriate lattice sites within the crystals as shown in

Fig1B1 where the O-M LMCT transition corresponds to the transition

between the valence and conduction bands for the infinite metal-oxide lattice

[88-91]

B12 Applications of Photochromic materials

Sunglasses

One of the most famous reversible photochromic applications is color

changing lenses for sunglasses as found in eyeglasses The largest limitation

in using PC technology is that the materials cannot be made stable enough

to withstand thousands of hours of outdoor exposure so long-term outdoor

applications are not appropriate at this time The switching speed of

photochromic dyes is highly sensitive to the rigidity of the environment around

the dye As result they switch most rapidly in solution and slowest in the rigid

environment like a polymer lens Recently it has been reported that attaching

flexible low Tg polymers (for example siloxanes or poly (butyl acrylate) to the

dyes allows them to switch much more rapidly in a rigid lens [76] Some

spirooxazines with siloxane polymers attached switch at near solution like

speeds even though they are in a rigid lens matrix

Supramolecular chemistry

Photochromic units have been employed extensively in supramolecular

chemistry Their ability to give a light controlled reversible shape change

means that they can be used to make or break molecular recognition motifs

30

or to cause a consequent shape change in their surroundings Thus

photochromic units have been demonstrated as components of molecular

switches The coupling of photochromic units to enzymes or enzyme cofactors

even provides the ability to reversibly turn enzymes on and off

by altering their shape or orientation in such a way that their functions

are either working or broken

Data storage

The possibility of using photochromic compounds for data storage was

first suggested in 1956 by Yehuda Hirshberg[77] Since that time there have

been many investigations by various academic and commercial groups

particularly in the area of 3D optical data storage which promises discs that

can hold a terabyte of data Initially issues with thermal back-reactions

and destructive reading dogged these studies but more recently more stable

systems have been developed

Novelty items

Reversible photochromics are also found in applications such as toys

cosmeticsclothing and industrial applications If necessary they can be made

to change between desired colors by combination with a permanent pigment

A large number of inorganic compounds exhibit photochromism

These solids often have large band gaps of the order of 3 - 12 eV and

excitation of these solids leads to the formation of metastable centers that

absorb visible light giving rise to their colour They can return to their ground

state by heating or by optical excitation within the colour-centre band In most

cases the photochromism is a structure sensitive phenomenon involving

localized defect impurities or dislocations Some of these inorganic

compounds have the potential for a number of different uses Photochromic

compounds have a number of useful applications These can be divided

according to the most important property that is being used (Table 11) [92]

31

Table 11 Applications of Photochromic materials

Applications Depending Upon

Sensitivity to

Radiation

Reversibility Thermal Chemical or

Physical Properties

Self-developing

photography

Chemical switches for

computers

Temperature indicators

Protective

materials

Data displays

Heat-sensitive recording

media

Optical signal

processing

Photomasking and

photoresist technology

Reusable data storage

media

Anaytical reagents

Photochromic

microimages

Photopolymerisation

Information encoding

and steganography

Photocontractile

polymers and the

photoviscosity effect

Control of light

intensity

Q-switches

Pyroelectric

photochromic materials

B13 Thermochromism

Thermochromism is the reversible colour change of a substance

induced by temperature change A large variety of substances organic

inorganic organometallic supramolecular and polymeric systems exhibit this

phenomenon Examples of these include bianthrones cobalt

hexacyanoferrate the zirconocene complex of 1 4-diphenyl-1 3-butadiene

and poly (3-alkylthiophene) The organic 99-bixanthenylidene is colourless at

90 K yellow-green at 298 K and dark-blue when melted at 592 K Heating

conducting polymers can cause them to change colour This is achieved by

causing conformational changes to the polymer backbone resulting in a

change in the band gap of the polymer It has been reported that regioregular

32

P3HT reversibly changes colour upon heating to 220ordmC due to temperature-

dependent conformation changes Thermally cross linked polymer undergoes

the same colour change but it is much less reversible [93] Other forms of

thermochromism may be commercially important eg to give a visual

indication of temperature changes

B14 Electrochromism

Electrochromism describes a phenomenon of material color change in

a persistent but reversible manner produced by electrochemically induced

oxidation-reduction reactions Electrochromic materials can be applied to

various kinds of products such as smart windows and display devices Among

those applications there have been lots of efforts to develop electrochromic

display devices (ECDs) Especially flexible display devices are now attracting

much attention worldwide since they can facilitate new technological demands

such as bending and folding of paper-like displays High electrochromic

efficiency short response time long operating life time and reduction of

energy consumption are the most important requisites of the materials for the

paperlike displays [94 95] Among those properties the operation life time is

the most important barrier to overcome for a realization of ECDs There are

two types of electrochromic material a) inorganic transition metal oxides

(TMOs) b) organic polymer materials The TMOs have been studied longer

than the organic materials that they have been studied since 1960s [9697]

Electrochromism describes a reversible color change of material

produced by electrochemically induced oxidation-reduction reactions It is one

of several types of chromism of materials As thermochromism and

photochromism mean material color changes made by heat and light

respectively electrochromism refers that the color change is caused by an

electric potential In most cases the color change in electrochromism can be

driven by rather low electrical potential of the order of a fraction of volt or a

few volts [94-96] The color change of material means variation in

transmittance andor reflectance change in visible range which is originated

from different electronic absorption bands according to a switching between

oxidation and reduction state of material When electric potential is applied on

electrochromic material forced oxidation or reduction is derived and the

33

individual color is originated from the corresponding oxidation or reduction

state of the material For electrochromic materials the characteristic color

change is reversible since the oxidation and reduction state can be converted

reversibly by switching potential

Application fields

The application area for the electrochromism is rather broad that it

covers from smart window glazing and optical modulators to information

displays [98-102] The smart windows are typical examples The

electrochromic property is used to control the amount of light and heat to pass

through the windows Usually the electrochromic material is in form of thin film

coated on a window glass The transmittance modulation has also been

applied at the automobiles to automatically tint rear-view mirrors in various

lighting conditions The electrochromic application fields are illustrated in Fig

1B3

Since the smart windows control the transmittance of heat as well as

the transmittance of visible light the working definition of electrochromism has

now been extended to include devices for modulation of radiation in the near

infrared thermal infrared and microwave regions When color for

electrochromic materials is used this can now mean a response by detectors

at these wavelengths and not just by the human eyes Nowadays

electrochromic material draws much attention as being used in the display

devices Electrochromic display device (ECD) is being considered as one of

the candidates for the conventional liquid crystal display (LCD) since ECD

has many advantages over LCDs Among these advantages the most

important are low energy consumption wider viewing angle high contrast

rate and possibility to achieve multiple colors with a single material [103]

When a new redox state of electrochromic materialis established by

the applied electric pulse then it is maintained after the potential is switched

off This means the colored or bleached state of the material can be sustained

for a considerable time without applying electrical power This is so-called

ldquocolor memory effectrdquo of electrochromic material Because of the color

memory effect energy consumption for the electrochromic display device

could be drastically reduced and this would be a big advantage over other

34

emissive devices The low power consumption is especially advantageous

when it is applied to mobile devices with limited power source The possibility

for a flexible display is another attraction for electrochromic material

Information displays

Real-view mirrors for automobiles

Fig 1B4 Application fields of electrochromic devices Smart windows information displays and real-view mirrors for automobiles

35

Using polymeric electrochromic material and plastic substrate with a

transparent conductive coating it is possible to build all-plastic flexible display

device There are lots of papers and patents about the flexible display devices

[104105] In these cases gel-type electrolyte is also needed The gel-type

electrolyte immobilizes liquid electrolyte in the polymer network [106107]

Recently ITO-coated polymer films are readily available which would provide

the plastic substrate for a flexible device The flexible electrochromic displays

are frequently tried with the plastic substrate flexible electrochromic material

and gel-type electrolyte The flexible electrochromic displays would facilitate

increasing technical demands for foldable display devices

Metal Ion Electrochromism

Many transition metal oxides are capable of redox reactions that result in

colour change Metal oxide films are commonly prepared as thin layers of

either tungsten nickel molybdenum or other metal compounds by a number

of techniques These include sol-gel electrochemical by dc or rfreactive

sputtering techniques electron-beam evaporation by anodic or cathodic

electrodeposition or by solution dipping of the electrochromic metal

compounds (or compounds that can be changed into these metal compounds)

onto optically transparent electrodes (OTE) [108 -114] Their electrochromism

is derived from the colour change associated with a change in the oxidation

state of the metal anion The behaviour of these materials is dependent upon

pH moisture and exposure to the atmosphere [115] Generally the switching

rates of these films is somewhat slow with typical switching times of about 15

- 60 seconds to achieve 100 conversion to either coloured or bleached state

[116 -120] An Example of this includes nickel oxide which changes from

transparent (pale green) to brownblack taking about 30 seconds to do so

[121] Other examples include [(NH4)5Ru]2(pyrazine)5+ and [(NH4)5Ru]2(44-

bipyridine)5+ whose electrochromism is significantly different due to the effect

of the ligand [122] Table 12 below gives some examples of metal oxide films

with electrochromic properties

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

27

Fig1B1 Simple model showing the electronic transitions in the

polyoxometalates containing electron donar and acceptor (a)generation

of charge carriers(b)electron and hole trapping(c)electron release due

to stimulation(d)recombination

between electron and hole Electrons are e-and holes are h+

In the polyoxometalates containing heteroatoms and especially in

mixed metal polyoxometalates the charge carriers which are created by the

light or electric field may be trapped in electron traps and hole traps These

traps provide states of localized energy in the O-M LMCT energy gap due to

the heteroatoms or counter cations which correspond to impurities or lattice

defects in the band model If the trap depth ∆E is large compared to kT the

probability for thermal escape from the trap will be negligibly small and

metastable situation will existThe trapped carriers can be released by thermal

or optical stimulationIn the case of thermal stimulation the irradiated

polyoxometalate is heated until the energy barrier ∆E can be overcome The

trapped electron (or hole) then can escape from the trap and nonradiatively

recombine with the trapped hole (or electron)Under optical stimulation the

energy of an incident photon is used to overcome ∆E The relaxation

processes of the OndashM LMCT excitation energy include both the nonradiative

recombination of electrons and holes within the energy gap and the

intramolecular energy transfer leading to a charge-transfer emission This

intramolecular energy transfer corresponds to the O-M LMCT energy gap and

occurs via radiative recombination and sensitized emission from the

28

heteroatoms or cations If several energy levels based on the hetero atoms or

counter cations act as energy acceptors within the O-M LMCTenergy gap the

energy transfer occurs from the O-M LMCT states to these levels followed by

the nonradiative or radiative deactivation of the excitation energy It should be

noted that the O-M LMCT states also can be generated by the application of

very high electric fields to the polyoxometalate solids as demonstrated by the

observation of electroluminescence[87]If an external electric field with a

potential more negative than the energy levels of the vacant orbitals involved

in the O-M LMCT transition is applied to a polyoxometalate on the electrode

surface an electrochemical reduction occurs via the injection of electrons

from the electrode in to the vacant levels of the polyoxometalate as shown in

Fig1B 2

Fig1B2 - Energy scheme for the electrochromism of polyoxometalates

a)electrochemical reduction (b) electrochemical oxidation

Electrons injected in to the high-energy levels also may be trapped by

electron traps in a process analogous to that which occurs during LMCT

photoexcitation of the polyoxometalates These electrons are returned to the

electrode by electrochemical oxidation at an electrode potential more positive

than the energy levels for the d1 electron states The d1electrons in the O-M

LMCTstates facilitate the absorption of visible light via intervalence charge

transfer among metal centers and d-d transitions The same type of transition

may be possible for the d1electron captured by the electron traps too In

addition to searching for new photosensitive polyoxometalates with the

29

potential for having practical application there is now a need to elucidate the

fundamental photo-and electrochemical coloration processes by studying

electron transfer within the polyoxometalate lattices in conjunction with their

crystal structures So far few polyoxometalates exhibit a perfect reversibility

of coloration The irreversibility of the color change arises from as yet

uncharacterized side reactions during both the coloration and decoloration of

the polyoxometalates

Many metal oxides including aluminum titanium vanadium niobium

molybdenum and tungsten oxides are photochromic when they contain

impurities or dopants This coloration has been interpreted on the basis of

electron trapping at appropriate lattice sites within the crystals as shown in

Fig1B1 where the O-M LMCT transition corresponds to the transition

between the valence and conduction bands for the infinite metal-oxide lattice

[88-91]

B12 Applications of Photochromic materials

Sunglasses

One of the most famous reversible photochromic applications is color

changing lenses for sunglasses as found in eyeglasses The largest limitation

in using PC technology is that the materials cannot be made stable enough

to withstand thousands of hours of outdoor exposure so long-term outdoor

applications are not appropriate at this time The switching speed of

photochromic dyes is highly sensitive to the rigidity of the environment around

the dye As result they switch most rapidly in solution and slowest in the rigid

environment like a polymer lens Recently it has been reported that attaching

flexible low Tg polymers (for example siloxanes or poly (butyl acrylate) to the

dyes allows them to switch much more rapidly in a rigid lens [76] Some

spirooxazines with siloxane polymers attached switch at near solution like

speeds even though they are in a rigid lens matrix

Supramolecular chemistry

Photochromic units have been employed extensively in supramolecular

chemistry Their ability to give a light controlled reversible shape change

means that they can be used to make or break molecular recognition motifs

30

or to cause a consequent shape change in their surroundings Thus

photochromic units have been demonstrated as components of molecular

switches The coupling of photochromic units to enzymes or enzyme cofactors

even provides the ability to reversibly turn enzymes on and off

by altering their shape or orientation in such a way that their functions

are either working or broken

Data storage

The possibility of using photochromic compounds for data storage was

first suggested in 1956 by Yehuda Hirshberg[77] Since that time there have

been many investigations by various academic and commercial groups

particularly in the area of 3D optical data storage which promises discs that

can hold a terabyte of data Initially issues with thermal back-reactions

and destructive reading dogged these studies but more recently more stable

systems have been developed

Novelty items

Reversible photochromics are also found in applications such as toys

cosmeticsclothing and industrial applications If necessary they can be made

to change between desired colors by combination with a permanent pigment

A large number of inorganic compounds exhibit photochromism

These solids often have large band gaps of the order of 3 - 12 eV and

excitation of these solids leads to the formation of metastable centers that

absorb visible light giving rise to their colour They can return to their ground

state by heating or by optical excitation within the colour-centre band In most

cases the photochromism is a structure sensitive phenomenon involving

localized defect impurities or dislocations Some of these inorganic

compounds have the potential for a number of different uses Photochromic

compounds have a number of useful applications These can be divided

according to the most important property that is being used (Table 11) [92]

31

Table 11 Applications of Photochromic materials

Applications Depending Upon

Sensitivity to

Radiation

Reversibility Thermal Chemical or

Physical Properties

Self-developing

photography

Chemical switches for

computers

Temperature indicators

Protective

materials

Data displays

Heat-sensitive recording

media

Optical signal

processing

Photomasking and

photoresist technology

Reusable data storage

media

Anaytical reagents

Photochromic

microimages

Photopolymerisation

Information encoding

and steganography

Photocontractile

polymers and the

photoviscosity effect

Control of light

intensity

Q-switches

Pyroelectric

photochromic materials

B13 Thermochromism

Thermochromism is the reversible colour change of a substance

induced by temperature change A large variety of substances organic

inorganic organometallic supramolecular and polymeric systems exhibit this

phenomenon Examples of these include bianthrones cobalt

hexacyanoferrate the zirconocene complex of 1 4-diphenyl-1 3-butadiene

and poly (3-alkylthiophene) The organic 99-bixanthenylidene is colourless at

90 K yellow-green at 298 K and dark-blue when melted at 592 K Heating

conducting polymers can cause them to change colour This is achieved by

causing conformational changes to the polymer backbone resulting in a

change in the band gap of the polymer It has been reported that regioregular

32

P3HT reversibly changes colour upon heating to 220ordmC due to temperature-

dependent conformation changes Thermally cross linked polymer undergoes

the same colour change but it is much less reversible [93] Other forms of

thermochromism may be commercially important eg to give a visual

indication of temperature changes

B14 Electrochromism

Electrochromism describes a phenomenon of material color change in

a persistent but reversible manner produced by electrochemically induced

oxidation-reduction reactions Electrochromic materials can be applied to

various kinds of products such as smart windows and display devices Among

those applications there have been lots of efforts to develop electrochromic

display devices (ECDs) Especially flexible display devices are now attracting

much attention worldwide since they can facilitate new technological demands

such as bending and folding of paper-like displays High electrochromic

efficiency short response time long operating life time and reduction of

energy consumption are the most important requisites of the materials for the

paperlike displays [94 95] Among those properties the operation life time is

the most important barrier to overcome for a realization of ECDs There are

two types of electrochromic material a) inorganic transition metal oxides

(TMOs) b) organic polymer materials The TMOs have been studied longer

than the organic materials that they have been studied since 1960s [9697]

Electrochromism describes a reversible color change of material

produced by electrochemically induced oxidation-reduction reactions It is one

of several types of chromism of materials As thermochromism and

photochromism mean material color changes made by heat and light

respectively electrochromism refers that the color change is caused by an

electric potential In most cases the color change in electrochromism can be

driven by rather low electrical potential of the order of a fraction of volt or a

few volts [94-96] The color change of material means variation in

transmittance andor reflectance change in visible range which is originated

from different electronic absorption bands according to a switching between

oxidation and reduction state of material When electric potential is applied on

electrochromic material forced oxidation or reduction is derived and the

33

individual color is originated from the corresponding oxidation or reduction

state of the material For electrochromic materials the characteristic color

change is reversible since the oxidation and reduction state can be converted

reversibly by switching potential

Application fields

The application area for the electrochromism is rather broad that it

covers from smart window glazing and optical modulators to information

displays [98-102] The smart windows are typical examples The

electrochromic property is used to control the amount of light and heat to pass

through the windows Usually the electrochromic material is in form of thin film

coated on a window glass The transmittance modulation has also been

applied at the automobiles to automatically tint rear-view mirrors in various

lighting conditions The electrochromic application fields are illustrated in Fig

1B3

Since the smart windows control the transmittance of heat as well as

the transmittance of visible light the working definition of electrochromism has

now been extended to include devices for modulation of radiation in the near

infrared thermal infrared and microwave regions When color for

electrochromic materials is used this can now mean a response by detectors

at these wavelengths and not just by the human eyes Nowadays

electrochromic material draws much attention as being used in the display

devices Electrochromic display device (ECD) is being considered as one of

the candidates for the conventional liquid crystal display (LCD) since ECD

has many advantages over LCDs Among these advantages the most

important are low energy consumption wider viewing angle high contrast

rate and possibility to achieve multiple colors with a single material [103]

When a new redox state of electrochromic materialis established by

the applied electric pulse then it is maintained after the potential is switched

off This means the colored or bleached state of the material can be sustained

for a considerable time without applying electrical power This is so-called

ldquocolor memory effectrdquo of electrochromic material Because of the color

memory effect energy consumption for the electrochromic display device

could be drastically reduced and this would be a big advantage over other

34

emissive devices The low power consumption is especially advantageous

when it is applied to mobile devices with limited power source The possibility

for a flexible display is another attraction for electrochromic material

Information displays

Real-view mirrors for automobiles

Fig 1B4 Application fields of electrochromic devices Smart windows information displays and real-view mirrors for automobiles

35

Using polymeric electrochromic material and plastic substrate with a

transparent conductive coating it is possible to build all-plastic flexible display

device There are lots of papers and patents about the flexible display devices

[104105] In these cases gel-type electrolyte is also needed The gel-type

electrolyte immobilizes liquid electrolyte in the polymer network [106107]

Recently ITO-coated polymer films are readily available which would provide

the plastic substrate for a flexible device The flexible electrochromic displays

are frequently tried with the plastic substrate flexible electrochromic material

and gel-type electrolyte The flexible electrochromic displays would facilitate

increasing technical demands for foldable display devices

Metal Ion Electrochromism

Many transition metal oxides are capable of redox reactions that result in

colour change Metal oxide films are commonly prepared as thin layers of

either tungsten nickel molybdenum or other metal compounds by a number

of techniques These include sol-gel electrochemical by dc or rfreactive

sputtering techniques electron-beam evaporation by anodic or cathodic

electrodeposition or by solution dipping of the electrochromic metal

compounds (or compounds that can be changed into these metal compounds)

onto optically transparent electrodes (OTE) [108 -114] Their electrochromism

is derived from the colour change associated with a change in the oxidation

state of the metal anion The behaviour of these materials is dependent upon

pH moisture and exposure to the atmosphere [115] Generally the switching

rates of these films is somewhat slow with typical switching times of about 15

- 60 seconds to achieve 100 conversion to either coloured or bleached state

[116 -120] An Example of this includes nickel oxide which changes from

transparent (pale green) to brownblack taking about 30 seconds to do so

[121] Other examples include [(NH4)5Ru]2(pyrazine)5+ and [(NH4)5Ru]2(44-

bipyridine)5+ whose electrochromism is significantly different due to the effect

of the ligand [122] Table 12 below gives some examples of metal oxide films

with electrochromic properties

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

28

heteroatoms or cations If several energy levels based on the hetero atoms or

counter cations act as energy acceptors within the O-M LMCTenergy gap the

energy transfer occurs from the O-M LMCT states to these levels followed by

the nonradiative or radiative deactivation of the excitation energy It should be

noted that the O-M LMCT states also can be generated by the application of

very high electric fields to the polyoxometalate solids as demonstrated by the

observation of electroluminescence[87]If an external electric field with a

potential more negative than the energy levels of the vacant orbitals involved

in the O-M LMCT transition is applied to a polyoxometalate on the electrode

surface an electrochemical reduction occurs via the injection of electrons

from the electrode in to the vacant levels of the polyoxometalate as shown in

Fig1B 2

Fig1B2 - Energy scheme for the electrochromism of polyoxometalates

a)electrochemical reduction (b) electrochemical oxidation

Electrons injected in to the high-energy levels also may be trapped by

electron traps in a process analogous to that which occurs during LMCT

photoexcitation of the polyoxometalates These electrons are returned to the

electrode by electrochemical oxidation at an electrode potential more positive

than the energy levels for the d1 electron states The d1electrons in the O-M

LMCTstates facilitate the absorption of visible light via intervalence charge

transfer among metal centers and d-d transitions The same type of transition

may be possible for the d1electron captured by the electron traps too In

addition to searching for new photosensitive polyoxometalates with the

29

potential for having practical application there is now a need to elucidate the

fundamental photo-and electrochemical coloration processes by studying

electron transfer within the polyoxometalate lattices in conjunction with their

crystal structures So far few polyoxometalates exhibit a perfect reversibility

of coloration The irreversibility of the color change arises from as yet

uncharacterized side reactions during both the coloration and decoloration of

the polyoxometalates

Many metal oxides including aluminum titanium vanadium niobium

molybdenum and tungsten oxides are photochromic when they contain

impurities or dopants This coloration has been interpreted on the basis of

electron trapping at appropriate lattice sites within the crystals as shown in

Fig1B1 where the O-M LMCT transition corresponds to the transition

between the valence and conduction bands for the infinite metal-oxide lattice

[88-91]

B12 Applications of Photochromic materials

Sunglasses

One of the most famous reversible photochromic applications is color

changing lenses for sunglasses as found in eyeglasses The largest limitation

in using PC technology is that the materials cannot be made stable enough

to withstand thousands of hours of outdoor exposure so long-term outdoor

applications are not appropriate at this time The switching speed of

photochromic dyes is highly sensitive to the rigidity of the environment around

the dye As result they switch most rapidly in solution and slowest in the rigid

environment like a polymer lens Recently it has been reported that attaching

flexible low Tg polymers (for example siloxanes or poly (butyl acrylate) to the

dyes allows them to switch much more rapidly in a rigid lens [76] Some

spirooxazines with siloxane polymers attached switch at near solution like

speeds even though they are in a rigid lens matrix

Supramolecular chemistry

Photochromic units have been employed extensively in supramolecular

chemistry Their ability to give a light controlled reversible shape change

means that they can be used to make or break molecular recognition motifs

30

or to cause a consequent shape change in their surroundings Thus

photochromic units have been demonstrated as components of molecular

switches The coupling of photochromic units to enzymes or enzyme cofactors

even provides the ability to reversibly turn enzymes on and off

by altering their shape or orientation in such a way that their functions

are either working or broken

Data storage

The possibility of using photochromic compounds for data storage was

first suggested in 1956 by Yehuda Hirshberg[77] Since that time there have

been many investigations by various academic and commercial groups

particularly in the area of 3D optical data storage which promises discs that

can hold a terabyte of data Initially issues with thermal back-reactions

and destructive reading dogged these studies but more recently more stable

systems have been developed

Novelty items

Reversible photochromics are also found in applications such as toys

cosmeticsclothing and industrial applications If necessary they can be made

to change between desired colors by combination with a permanent pigment

A large number of inorganic compounds exhibit photochromism

These solids often have large band gaps of the order of 3 - 12 eV and

excitation of these solids leads to the formation of metastable centers that

absorb visible light giving rise to their colour They can return to their ground

state by heating or by optical excitation within the colour-centre band In most

cases the photochromism is a structure sensitive phenomenon involving

localized defect impurities or dislocations Some of these inorganic

compounds have the potential for a number of different uses Photochromic

compounds have a number of useful applications These can be divided

according to the most important property that is being used (Table 11) [92]

31

Table 11 Applications of Photochromic materials

Applications Depending Upon

Sensitivity to

Radiation

Reversibility Thermal Chemical or

Physical Properties

Self-developing

photography

Chemical switches for

computers

Temperature indicators

Protective

materials

Data displays

Heat-sensitive recording

media

Optical signal

processing

Photomasking and

photoresist technology

Reusable data storage

media

Anaytical reagents

Photochromic

microimages

Photopolymerisation

Information encoding

and steganography

Photocontractile

polymers and the

photoviscosity effect

Control of light

intensity

Q-switches

Pyroelectric

photochromic materials

B13 Thermochromism

Thermochromism is the reversible colour change of a substance

induced by temperature change A large variety of substances organic

inorganic organometallic supramolecular and polymeric systems exhibit this

phenomenon Examples of these include bianthrones cobalt

hexacyanoferrate the zirconocene complex of 1 4-diphenyl-1 3-butadiene

and poly (3-alkylthiophene) The organic 99-bixanthenylidene is colourless at

90 K yellow-green at 298 K and dark-blue when melted at 592 K Heating

conducting polymers can cause them to change colour This is achieved by

causing conformational changes to the polymer backbone resulting in a

change in the band gap of the polymer It has been reported that regioregular

32

P3HT reversibly changes colour upon heating to 220ordmC due to temperature-

dependent conformation changes Thermally cross linked polymer undergoes

the same colour change but it is much less reversible [93] Other forms of

thermochromism may be commercially important eg to give a visual

indication of temperature changes

B14 Electrochromism

Electrochromism describes a phenomenon of material color change in

a persistent but reversible manner produced by electrochemically induced

oxidation-reduction reactions Electrochromic materials can be applied to

various kinds of products such as smart windows and display devices Among

those applications there have been lots of efforts to develop electrochromic

display devices (ECDs) Especially flexible display devices are now attracting

much attention worldwide since they can facilitate new technological demands

such as bending and folding of paper-like displays High electrochromic

efficiency short response time long operating life time and reduction of

energy consumption are the most important requisites of the materials for the

paperlike displays [94 95] Among those properties the operation life time is

the most important barrier to overcome for a realization of ECDs There are

two types of electrochromic material a) inorganic transition metal oxides

(TMOs) b) organic polymer materials The TMOs have been studied longer

than the organic materials that they have been studied since 1960s [9697]

Electrochromism describes a reversible color change of material

produced by electrochemically induced oxidation-reduction reactions It is one

of several types of chromism of materials As thermochromism and

photochromism mean material color changes made by heat and light

respectively electrochromism refers that the color change is caused by an

electric potential In most cases the color change in electrochromism can be

driven by rather low electrical potential of the order of a fraction of volt or a

few volts [94-96] The color change of material means variation in

transmittance andor reflectance change in visible range which is originated

from different electronic absorption bands according to a switching between

oxidation and reduction state of material When electric potential is applied on

electrochromic material forced oxidation or reduction is derived and the

33

individual color is originated from the corresponding oxidation or reduction

state of the material For electrochromic materials the characteristic color

change is reversible since the oxidation and reduction state can be converted

reversibly by switching potential

Application fields

The application area for the electrochromism is rather broad that it

covers from smart window glazing and optical modulators to information

displays [98-102] The smart windows are typical examples The

electrochromic property is used to control the amount of light and heat to pass

through the windows Usually the electrochromic material is in form of thin film

coated on a window glass The transmittance modulation has also been

applied at the automobiles to automatically tint rear-view mirrors in various

lighting conditions The electrochromic application fields are illustrated in Fig

1B3

Since the smart windows control the transmittance of heat as well as

the transmittance of visible light the working definition of electrochromism has

now been extended to include devices for modulation of radiation in the near

infrared thermal infrared and microwave regions When color for

electrochromic materials is used this can now mean a response by detectors

at these wavelengths and not just by the human eyes Nowadays

electrochromic material draws much attention as being used in the display

devices Electrochromic display device (ECD) is being considered as one of

the candidates for the conventional liquid crystal display (LCD) since ECD

has many advantages over LCDs Among these advantages the most

important are low energy consumption wider viewing angle high contrast

rate and possibility to achieve multiple colors with a single material [103]

When a new redox state of electrochromic materialis established by

the applied electric pulse then it is maintained after the potential is switched

off This means the colored or bleached state of the material can be sustained

for a considerable time without applying electrical power This is so-called

ldquocolor memory effectrdquo of electrochromic material Because of the color

memory effect energy consumption for the electrochromic display device

could be drastically reduced and this would be a big advantage over other

34

emissive devices The low power consumption is especially advantageous

when it is applied to mobile devices with limited power source The possibility

for a flexible display is another attraction for electrochromic material

Information displays

Real-view mirrors for automobiles

Fig 1B4 Application fields of electrochromic devices Smart windows information displays and real-view mirrors for automobiles

35

Using polymeric electrochromic material and plastic substrate with a

transparent conductive coating it is possible to build all-plastic flexible display

device There are lots of papers and patents about the flexible display devices

[104105] In these cases gel-type electrolyte is also needed The gel-type

electrolyte immobilizes liquid electrolyte in the polymer network [106107]

Recently ITO-coated polymer films are readily available which would provide

the plastic substrate for a flexible device The flexible electrochromic displays

are frequently tried with the plastic substrate flexible electrochromic material

and gel-type electrolyte The flexible electrochromic displays would facilitate

increasing technical demands for foldable display devices

Metal Ion Electrochromism

Many transition metal oxides are capable of redox reactions that result in

colour change Metal oxide films are commonly prepared as thin layers of

either tungsten nickel molybdenum or other metal compounds by a number

of techniques These include sol-gel electrochemical by dc or rfreactive

sputtering techniques electron-beam evaporation by anodic or cathodic

electrodeposition or by solution dipping of the electrochromic metal

compounds (or compounds that can be changed into these metal compounds)

onto optically transparent electrodes (OTE) [108 -114] Their electrochromism

is derived from the colour change associated with a change in the oxidation

state of the metal anion The behaviour of these materials is dependent upon

pH moisture and exposure to the atmosphere [115] Generally the switching

rates of these films is somewhat slow with typical switching times of about 15

- 60 seconds to achieve 100 conversion to either coloured or bleached state

[116 -120] An Example of this includes nickel oxide which changes from

transparent (pale green) to brownblack taking about 30 seconds to do so

[121] Other examples include [(NH4)5Ru]2(pyrazine)5+ and [(NH4)5Ru]2(44-

bipyridine)5+ whose electrochromism is significantly different due to the effect

of the ligand [122] Table 12 below gives some examples of metal oxide films

with electrochromic properties

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

29

potential for having practical application there is now a need to elucidate the

fundamental photo-and electrochemical coloration processes by studying

electron transfer within the polyoxometalate lattices in conjunction with their

crystal structures So far few polyoxometalates exhibit a perfect reversibility

of coloration The irreversibility of the color change arises from as yet

uncharacterized side reactions during both the coloration and decoloration of

the polyoxometalates

Many metal oxides including aluminum titanium vanadium niobium

molybdenum and tungsten oxides are photochromic when they contain

impurities or dopants This coloration has been interpreted on the basis of

electron trapping at appropriate lattice sites within the crystals as shown in

Fig1B1 where the O-M LMCT transition corresponds to the transition

between the valence and conduction bands for the infinite metal-oxide lattice

[88-91]

B12 Applications of Photochromic materials

Sunglasses

One of the most famous reversible photochromic applications is color

changing lenses for sunglasses as found in eyeglasses The largest limitation

in using PC technology is that the materials cannot be made stable enough

to withstand thousands of hours of outdoor exposure so long-term outdoor

applications are not appropriate at this time The switching speed of

photochromic dyes is highly sensitive to the rigidity of the environment around

the dye As result they switch most rapidly in solution and slowest in the rigid

environment like a polymer lens Recently it has been reported that attaching

flexible low Tg polymers (for example siloxanes or poly (butyl acrylate) to the

dyes allows them to switch much more rapidly in a rigid lens [76] Some

spirooxazines with siloxane polymers attached switch at near solution like

speeds even though they are in a rigid lens matrix

Supramolecular chemistry

Photochromic units have been employed extensively in supramolecular

chemistry Their ability to give a light controlled reversible shape change

means that they can be used to make or break molecular recognition motifs

30

or to cause a consequent shape change in their surroundings Thus

photochromic units have been demonstrated as components of molecular

switches The coupling of photochromic units to enzymes or enzyme cofactors

even provides the ability to reversibly turn enzymes on and off

by altering their shape or orientation in such a way that their functions

are either working or broken

Data storage

The possibility of using photochromic compounds for data storage was

first suggested in 1956 by Yehuda Hirshberg[77] Since that time there have

been many investigations by various academic and commercial groups

particularly in the area of 3D optical data storage which promises discs that

can hold a terabyte of data Initially issues with thermal back-reactions

and destructive reading dogged these studies but more recently more stable

systems have been developed

Novelty items

Reversible photochromics are also found in applications such as toys

cosmeticsclothing and industrial applications If necessary they can be made

to change between desired colors by combination with a permanent pigment

A large number of inorganic compounds exhibit photochromism

These solids often have large band gaps of the order of 3 - 12 eV and

excitation of these solids leads to the formation of metastable centers that

absorb visible light giving rise to their colour They can return to their ground

state by heating or by optical excitation within the colour-centre band In most

cases the photochromism is a structure sensitive phenomenon involving

localized defect impurities or dislocations Some of these inorganic

compounds have the potential for a number of different uses Photochromic

compounds have a number of useful applications These can be divided

according to the most important property that is being used (Table 11) [92]

31

Table 11 Applications of Photochromic materials

Applications Depending Upon

Sensitivity to

Radiation

Reversibility Thermal Chemical or

Physical Properties

Self-developing

photography

Chemical switches for

computers

Temperature indicators

Protective

materials

Data displays

Heat-sensitive recording

media

Optical signal

processing

Photomasking and

photoresist technology

Reusable data storage

media

Anaytical reagents

Photochromic

microimages

Photopolymerisation

Information encoding

and steganography

Photocontractile

polymers and the

photoviscosity effect

Control of light

intensity

Q-switches

Pyroelectric

photochromic materials

B13 Thermochromism

Thermochromism is the reversible colour change of a substance

induced by temperature change A large variety of substances organic

inorganic organometallic supramolecular and polymeric systems exhibit this

phenomenon Examples of these include bianthrones cobalt

hexacyanoferrate the zirconocene complex of 1 4-diphenyl-1 3-butadiene

and poly (3-alkylthiophene) The organic 99-bixanthenylidene is colourless at

90 K yellow-green at 298 K and dark-blue when melted at 592 K Heating

conducting polymers can cause them to change colour This is achieved by

causing conformational changes to the polymer backbone resulting in a

change in the band gap of the polymer It has been reported that regioregular

32

P3HT reversibly changes colour upon heating to 220ordmC due to temperature-

dependent conformation changes Thermally cross linked polymer undergoes

the same colour change but it is much less reversible [93] Other forms of

thermochromism may be commercially important eg to give a visual

indication of temperature changes

B14 Electrochromism

Electrochromism describes a phenomenon of material color change in

a persistent but reversible manner produced by electrochemically induced

oxidation-reduction reactions Electrochromic materials can be applied to

various kinds of products such as smart windows and display devices Among

those applications there have been lots of efforts to develop electrochromic

display devices (ECDs) Especially flexible display devices are now attracting

much attention worldwide since they can facilitate new technological demands

such as bending and folding of paper-like displays High electrochromic

efficiency short response time long operating life time and reduction of

energy consumption are the most important requisites of the materials for the

paperlike displays [94 95] Among those properties the operation life time is

the most important barrier to overcome for a realization of ECDs There are

two types of electrochromic material a) inorganic transition metal oxides

(TMOs) b) organic polymer materials The TMOs have been studied longer

than the organic materials that they have been studied since 1960s [9697]

Electrochromism describes a reversible color change of material

produced by electrochemically induced oxidation-reduction reactions It is one

of several types of chromism of materials As thermochromism and

photochromism mean material color changes made by heat and light

respectively electrochromism refers that the color change is caused by an

electric potential In most cases the color change in electrochromism can be

driven by rather low electrical potential of the order of a fraction of volt or a

few volts [94-96] The color change of material means variation in

transmittance andor reflectance change in visible range which is originated

from different electronic absorption bands according to a switching between

oxidation and reduction state of material When electric potential is applied on

electrochromic material forced oxidation or reduction is derived and the

33

individual color is originated from the corresponding oxidation or reduction

state of the material For electrochromic materials the characteristic color

change is reversible since the oxidation and reduction state can be converted

reversibly by switching potential

Application fields

The application area for the electrochromism is rather broad that it

covers from smart window glazing and optical modulators to information

displays [98-102] The smart windows are typical examples The

electrochromic property is used to control the amount of light and heat to pass

through the windows Usually the electrochromic material is in form of thin film

coated on a window glass The transmittance modulation has also been

applied at the automobiles to automatically tint rear-view mirrors in various

lighting conditions The electrochromic application fields are illustrated in Fig

1B3

Since the smart windows control the transmittance of heat as well as

the transmittance of visible light the working definition of electrochromism has

now been extended to include devices for modulation of radiation in the near

infrared thermal infrared and microwave regions When color for

electrochromic materials is used this can now mean a response by detectors

at these wavelengths and not just by the human eyes Nowadays

electrochromic material draws much attention as being used in the display

devices Electrochromic display device (ECD) is being considered as one of

the candidates for the conventional liquid crystal display (LCD) since ECD

has many advantages over LCDs Among these advantages the most

important are low energy consumption wider viewing angle high contrast

rate and possibility to achieve multiple colors with a single material [103]

When a new redox state of electrochromic materialis established by

the applied electric pulse then it is maintained after the potential is switched

off This means the colored or bleached state of the material can be sustained

for a considerable time without applying electrical power This is so-called

ldquocolor memory effectrdquo of electrochromic material Because of the color

memory effect energy consumption for the electrochromic display device

could be drastically reduced and this would be a big advantage over other

34

emissive devices The low power consumption is especially advantageous

when it is applied to mobile devices with limited power source The possibility

for a flexible display is another attraction for electrochromic material

Information displays

Real-view mirrors for automobiles

Fig 1B4 Application fields of electrochromic devices Smart windows information displays and real-view mirrors for automobiles

35

Using polymeric electrochromic material and plastic substrate with a

transparent conductive coating it is possible to build all-plastic flexible display

device There are lots of papers and patents about the flexible display devices

[104105] In these cases gel-type electrolyte is also needed The gel-type

electrolyte immobilizes liquid electrolyte in the polymer network [106107]

Recently ITO-coated polymer films are readily available which would provide

the plastic substrate for a flexible device The flexible electrochromic displays

are frequently tried with the plastic substrate flexible electrochromic material

and gel-type electrolyte The flexible electrochromic displays would facilitate

increasing technical demands for foldable display devices

Metal Ion Electrochromism

Many transition metal oxides are capable of redox reactions that result in

colour change Metal oxide films are commonly prepared as thin layers of

either tungsten nickel molybdenum or other metal compounds by a number

of techniques These include sol-gel electrochemical by dc or rfreactive

sputtering techniques electron-beam evaporation by anodic or cathodic

electrodeposition or by solution dipping of the electrochromic metal

compounds (or compounds that can be changed into these metal compounds)

onto optically transparent electrodes (OTE) [108 -114] Their electrochromism

is derived from the colour change associated with a change in the oxidation

state of the metal anion The behaviour of these materials is dependent upon

pH moisture and exposure to the atmosphere [115] Generally the switching

rates of these films is somewhat slow with typical switching times of about 15

- 60 seconds to achieve 100 conversion to either coloured or bleached state

[116 -120] An Example of this includes nickel oxide which changes from

transparent (pale green) to brownblack taking about 30 seconds to do so

[121] Other examples include [(NH4)5Ru]2(pyrazine)5+ and [(NH4)5Ru]2(44-

bipyridine)5+ whose electrochromism is significantly different due to the effect

of the ligand [122] Table 12 below gives some examples of metal oxide films

with electrochromic properties

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

30

or to cause a consequent shape change in their surroundings Thus

photochromic units have been demonstrated as components of molecular

switches The coupling of photochromic units to enzymes or enzyme cofactors

even provides the ability to reversibly turn enzymes on and off

by altering their shape or orientation in such a way that their functions

are either working or broken

Data storage

The possibility of using photochromic compounds for data storage was

first suggested in 1956 by Yehuda Hirshberg[77] Since that time there have

been many investigations by various academic and commercial groups

particularly in the area of 3D optical data storage which promises discs that

can hold a terabyte of data Initially issues with thermal back-reactions

and destructive reading dogged these studies but more recently more stable

systems have been developed

Novelty items

Reversible photochromics are also found in applications such as toys

cosmeticsclothing and industrial applications If necessary they can be made

to change between desired colors by combination with a permanent pigment

A large number of inorganic compounds exhibit photochromism

These solids often have large band gaps of the order of 3 - 12 eV and

excitation of these solids leads to the formation of metastable centers that

absorb visible light giving rise to their colour They can return to their ground

state by heating or by optical excitation within the colour-centre band In most

cases the photochromism is a structure sensitive phenomenon involving

localized defect impurities or dislocations Some of these inorganic

compounds have the potential for a number of different uses Photochromic

compounds have a number of useful applications These can be divided

according to the most important property that is being used (Table 11) [92]

31

Table 11 Applications of Photochromic materials

Applications Depending Upon

Sensitivity to

Radiation

Reversibility Thermal Chemical or

Physical Properties

Self-developing

photography

Chemical switches for

computers

Temperature indicators

Protective

materials

Data displays

Heat-sensitive recording

media

Optical signal

processing

Photomasking and

photoresist technology

Reusable data storage

media

Anaytical reagents

Photochromic

microimages

Photopolymerisation

Information encoding

and steganography

Photocontractile

polymers and the

photoviscosity effect

Control of light

intensity

Q-switches

Pyroelectric

photochromic materials

B13 Thermochromism

Thermochromism is the reversible colour change of a substance

induced by temperature change A large variety of substances organic

inorganic organometallic supramolecular and polymeric systems exhibit this

phenomenon Examples of these include bianthrones cobalt

hexacyanoferrate the zirconocene complex of 1 4-diphenyl-1 3-butadiene

and poly (3-alkylthiophene) The organic 99-bixanthenylidene is colourless at

90 K yellow-green at 298 K and dark-blue when melted at 592 K Heating

conducting polymers can cause them to change colour This is achieved by

causing conformational changes to the polymer backbone resulting in a

change in the band gap of the polymer It has been reported that regioregular

32

P3HT reversibly changes colour upon heating to 220ordmC due to temperature-

dependent conformation changes Thermally cross linked polymer undergoes

the same colour change but it is much less reversible [93] Other forms of

thermochromism may be commercially important eg to give a visual

indication of temperature changes

B14 Electrochromism

Electrochromism describes a phenomenon of material color change in

a persistent but reversible manner produced by electrochemically induced

oxidation-reduction reactions Electrochromic materials can be applied to

various kinds of products such as smart windows and display devices Among

those applications there have been lots of efforts to develop electrochromic

display devices (ECDs) Especially flexible display devices are now attracting

much attention worldwide since they can facilitate new technological demands

such as bending and folding of paper-like displays High electrochromic

efficiency short response time long operating life time and reduction of

energy consumption are the most important requisites of the materials for the

paperlike displays [94 95] Among those properties the operation life time is

the most important barrier to overcome for a realization of ECDs There are

two types of electrochromic material a) inorganic transition metal oxides

(TMOs) b) organic polymer materials The TMOs have been studied longer

than the organic materials that they have been studied since 1960s [9697]

Electrochromism describes a reversible color change of material

produced by electrochemically induced oxidation-reduction reactions It is one

of several types of chromism of materials As thermochromism and

photochromism mean material color changes made by heat and light

respectively electrochromism refers that the color change is caused by an

electric potential In most cases the color change in electrochromism can be

driven by rather low electrical potential of the order of a fraction of volt or a

few volts [94-96] The color change of material means variation in

transmittance andor reflectance change in visible range which is originated

from different electronic absorption bands according to a switching between

oxidation and reduction state of material When electric potential is applied on

electrochromic material forced oxidation or reduction is derived and the

33

individual color is originated from the corresponding oxidation or reduction

state of the material For electrochromic materials the characteristic color

change is reversible since the oxidation and reduction state can be converted

reversibly by switching potential

Application fields

The application area for the electrochromism is rather broad that it

covers from smart window glazing and optical modulators to information

displays [98-102] The smart windows are typical examples The

electrochromic property is used to control the amount of light and heat to pass

through the windows Usually the electrochromic material is in form of thin film

coated on a window glass The transmittance modulation has also been

applied at the automobiles to automatically tint rear-view mirrors in various

lighting conditions The electrochromic application fields are illustrated in Fig

1B3

Since the smart windows control the transmittance of heat as well as

the transmittance of visible light the working definition of electrochromism has

now been extended to include devices for modulation of radiation in the near

infrared thermal infrared and microwave regions When color for

electrochromic materials is used this can now mean a response by detectors

at these wavelengths and not just by the human eyes Nowadays

electrochromic material draws much attention as being used in the display

devices Electrochromic display device (ECD) is being considered as one of

the candidates for the conventional liquid crystal display (LCD) since ECD

has many advantages over LCDs Among these advantages the most

important are low energy consumption wider viewing angle high contrast

rate and possibility to achieve multiple colors with a single material [103]

When a new redox state of electrochromic materialis established by

the applied electric pulse then it is maintained after the potential is switched

off This means the colored or bleached state of the material can be sustained

for a considerable time without applying electrical power This is so-called

ldquocolor memory effectrdquo of electrochromic material Because of the color

memory effect energy consumption for the electrochromic display device

could be drastically reduced and this would be a big advantage over other

34

emissive devices The low power consumption is especially advantageous

when it is applied to mobile devices with limited power source The possibility

for a flexible display is another attraction for electrochromic material

Information displays

Real-view mirrors for automobiles

Fig 1B4 Application fields of electrochromic devices Smart windows information displays and real-view mirrors for automobiles

35

Using polymeric electrochromic material and plastic substrate with a

transparent conductive coating it is possible to build all-plastic flexible display

device There are lots of papers and patents about the flexible display devices

[104105] In these cases gel-type electrolyte is also needed The gel-type

electrolyte immobilizes liquid electrolyte in the polymer network [106107]

Recently ITO-coated polymer films are readily available which would provide

the plastic substrate for a flexible device The flexible electrochromic displays

are frequently tried with the plastic substrate flexible electrochromic material

and gel-type electrolyte The flexible electrochromic displays would facilitate

increasing technical demands for foldable display devices

Metal Ion Electrochromism

Many transition metal oxides are capable of redox reactions that result in

colour change Metal oxide films are commonly prepared as thin layers of

either tungsten nickel molybdenum or other metal compounds by a number

of techniques These include sol-gel electrochemical by dc or rfreactive

sputtering techniques electron-beam evaporation by anodic or cathodic

electrodeposition or by solution dipping of the electrochromic metal

compounds (or compounds that can be changed into these metal compounds)

onto optically transparent electrodes (OTE) [108 -114] Their electrochromism

is derived from the colour change associated with a change in the oxidation

state of the metal anion The behaviour of these materials is dependent upon

pH moisture and exposure to the atmosphere [115] Generally the switching

rates of these films is somewhat slow with typical switching times of about 15

- 60 seconds to achieve 100 conversion to either coloured or bleached state

[116 -120] An Example of this includes nickel oxide which changes from

transparent (pale green) to brownblack taking about 30 seconds to do so

[121] Other examples include [(NH4)5Ru]2(pyrazine)5+ and [(NH4)5Ru]2(44-

bipyridine)5+ whose electrochromism is significantly different due to the effect

of the ligand [122] Table 12 below gives some examples of metal oxide films

with electrochromic properties

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

31

Table 11 Applications of Photochromic materials

Applications Depending Upon

Sensitivity to

Radiation

Reversibility Thermal Chemical or

Physical Properties

Self-developing

photography

Chemical switches for

computers

Temperature indicators

Protective

materials

Data displays

Heat-sensitive recording

media

Optical signal

processing

Photomasking and

photoresist technology

Reusable data storage

media

Anaytical reagents

Photochromic

microimages

Photopolymerisation

Information encoding

and steganography

Photocontractile

polymers and the

photoviscosity effect

Control of light

intensity

Q-switches

Pyroelectric

photochromic materials

B13 Thermochromism

Thermochromism is the reversible colour change of a substance

induced by temperature change A large variety of substances organic

inorganic organometallic supramolecular and polymeric systems exhibit this

phenomenon Examples of these include bianthrones cobalt

hexacyanoferrate the zirconocene complex of 1 4-diphenyl-1 3-butadiene

and poly (3-alkylthiophene) The organic 99-bixanthenylidene is colourless at

90 K yellow-green at 298 K and dark-blue when melted at 592 K Heating

conducting polymers can cause them to change colour This is achieved by

causing conformational changes to the polymer backbone resulting in a

change in the band gap of the polymer It has been reported that regioregular

32

P3HT reversibly changes colour upon heating to 220ordmC due to temperature-

dependent conformation changes Thermally cross linked polymer undergoes

the same colour change but it is much less reversible [93] Other forms of

thermochromism may be commercially important eg to give a visual

indication of temperature changes

B14 Electrochromism

Electrochromism describes a phenomenon of material color change in

a persistent but reversible manner produced by electrochemically induced

oxidation-reduction reactions Electrochromic materials can be applied to

various kinds of products such as smart windows and display devices Among

those applications there have been lots of efforts to develop electrochromic

display devices (ECDs) Especially flexible display devices are now attracting

much attention worldwide since they can facilitate new technological demands

such as bending and folding of paper-like displays High electrochromic

efficiency short response time long operating life time and reduction of

energy consumption are the most important requisites of the materials for the

paperlike displays [94 95] Among those properties the operation life time is

the most important barrier to overcome for a realization of ECDs There are

two types of electrochromic material a) inorganic transition metal oxides

(TMOs) b) organic polymer materials The TMOs have been studied longer

than the organic materials that they have been studied since 1960s [9697]

Electrochromism describes a reversible color change of material

produced by electrochemically induced oxidation-reduction reactions It is one

of several types of chromism of materials As thermochromism and

photochromism mean material color changes made by heat and light

respectively electrochromism refers that the color change is caused by an

electric potential In most cases the color change in electrochromism can be

driven by rather low electrical potential of the order of a fraction of volt or a

few volts [94-96] The color change of material means variation in

transmittance andor reflectance change in visible range which is originated

from different electronic absorption bands according to a switching between

oxidation and reduction state of material When electric potential is applied on

electrochromic material forced oxidation or reduction is derived and the

33

individual color is originated from the corresponding oxidation or reduction

state of the material For electrochromic materials the characteristic color

change is reversible since the oxidation and reduction state can be converted

reversibly by switching potential

Application fields

The application area for the electrochromism is rather broad that it

covers from smart window glazing and optical modulators to information

displays [98-102] The smart windows are typical examples The

electrochromic property is used to control the amount of light and heat to pass

through the windows Usually the electrochromic material is in form of thin film

coated on a window glass The transmittance modulation has also been

applied at the automobiles to automatically tint rear-view mirrors in various

lighting conditions The electrochromic application fields are illustrated in Fig

1B3

Since the smart windows control the transmittance of heat as well as

the transmittance of visible light the working definition of electrochromism has

now been extended to include devices for modulation of radiation in the near

infrared thermal infrared and microwave regions When color for

electrochromic materials is used this can now mean a response by detectors

at these wavelengths and not just by the human eyes Nowadays

electrochromic material draws much attention as being used in the display

devices Electrochromic display device (ECD) is being considered as one of

the candidates for the conventional liquid crystal display (LCD) since ECD

has many advantages over LCDs Among these advantages the most

important are low energy consumption wider viewing angle high contrast

rate and possibility to achieve multiple colors with a single material [103]

When a new redox state of electrochromic materialis established by

the applied electric pulse then it is maintained after the potential is switched

off This means the colored or bleached state of the material can be sustained

for a considerable time without applying electrical power This is so-called

ldquocolor memory effectrdquo of electrochromic material Because of the color

memory effect energy consumption for the electrochromic display device

could be drastically reduced and this would be a big advantage over other

34

emissive devices The low power consumption is especially advantageous

when it is applied to mobile devices with limited power source The possibility

for a flexible display is another attraction for electrochromic material

Information displays

Real-view mirrors for automobiles

Fig 1B4 Application fields of electrochromic devices Smart windows information displays and real-view mirrors for automobiles

35

Using polymeric electrochromic material and plastic substrate with a

transparent conductive coating it is possible to build all-plastic flexible display

device There are lots of papers and patents about the flexible display devices

[104105] In these cases gel-type electrolyte is also needed The gel-type

electrolyte immobilizes liquid electrolyte in the polymer network [106107]

Recently ITO-coated polymer films are readily available which would provide

the plastic substrate for a flexible device The flexible electrochromic displays

are frequently tried with the plastic substrate flexible electrochromic material

and gel-type electrolyte The flexible electrochromic displays would facilitate

increasing technical demands for foldable display devices

Metal Ion Electrochromism

Many transition metal oxides are capable of redox reactions that result in

colour change Metal oxide films are commonly prepared as thin layers of

either tungsten nickel molybdenum or other metal compounds by a number

of techniques These include sol-gel electrochemical by dc or rfreactive

sputtering techniques electron-beam evaporation by anodic or cathodic

electrodeposition or by solution dipping of the electrochromic metal

compounds (or compounds that can be changed into these metal compounds)

onto optically transparent electrodes (OTE) [108 -114] Their electrochromism

is derived from the colour change associated with a change in the oxidation

state of the metal anion The behaviour of these materials is dependent upon

pH moisture and exposure to the atmosphere [115] Generally the switching

rates of these films is somewhat slow with typical switching times of about 15

- 60 seconds to achieve 100 conversion to either coloured or bleached state

[116 -120] An Example of this includes nickel oxide which changes from

transparent (pale green) to brownblack taking about 30 seconds to do so

[121] Other examples include [(NH4)5Ru]2(pyrazine)5+ and [(NH4)5Ru]2(44-

bipyridine)5+ whose electrochromism is significantly different due to the effect

of the ligand [122] Table 12 below gives some examples of metal oxide films

with electrochromic properties

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

32

P3HT reversibly changes colour upon heating to 220ordmC due to temperature-

dependent conformation changes Thermally cross linked polymer undergoes

the same colour change but it is much less reversible [93] Other forms of

thermochromism may be commercially important eg to give a visual

indication of temperature changes

B14 Electrochromism

Electrochromism describes a phenomenon of material color change in

a persistent but reversible manner produced by electrochemically induced

oxidation-reduction reactions Electrochromic materials can be applied to

various kinds of products such as smart windows and display devices Among

those applications there have been lots of efforts to develop electrochromic

display devices (ECDs) Especially flexible display devices are now attracting

much attention worldwide since they can facilitate new technological demands

such as bending and folding of paper-like displays High electrochromic

efficiency short response time long operating life time and reduction of

energy consumption are the most important requisites of the materials for the

paperlike displays [94 95] Among those properties the operation life time is

the most important barrier to overcome for a realization of ECDs There are

two types of electrochromic material a) inorganic transition metal oxides

(TMOs) b) organic polymer materials The TMOs have been studied longer

than the organic materials that they have been studied since 1960s [9697]

Electrochromism describes a reversible color change of material

produced by electrochemically induced oxidation-reduction reactions It is one

of several types of chromism of materials As thermochromism and

photochromism mean material color changes made by heat and light

respectively electrochromism refers that the color change is caused by an

electric potential In most cases the color change in electrochromism can be

driven by rather low electrical potential of the order of a fraction of volt or a

few volts [94-96] The color change of material means variation in

transmittance andor reflectance change in visible range which is originated

from different electronic absorption bands according to a switching between

oxidation and reduction state of material When electric potential is applied on

electrochromic material forced oxidation or reduction is derived and the

33

individual color is originated from the corresponding oxidation or reduction

state of the material For electrochromic materials the characteristic color

change is reversible since the oxidation and reduction state can be converted

reversibly by switching potential

Application fields

The application area for the electrochromism is rather broad that it

covers from smart window glazing and optical modulators to information

displays [98-102] The smart windows are typical examples The

electrochromic property is used to control the amount of light and heat to pass

through the windows Usually the electrochromic material is in form of thin film

coated on a window glass The transmittance modulation has also been

applied at the automobiles to automatically tint rear-view mirrors in various

lighting conditions The electrochromic application fields are illustrated in Fig

1B3

Since the smart windows control the transmittance of heat as well as

the transmittance of visible light the working definition of electrochromism has

now been extended to include devices for modulation of radiation in the near

infrared thermal infrared and microwave regions When color for

electrochromic materials is used this can now mean a response by detectors

at these wavelengths and not just by the human eyes Nowadays

electrochromic material draws much attention as being used in the display

devices Electrochromic display device (ECD) is being considered as one of

the candidates for the conventional liquid crystal display (LCD) since ECD

has many advantages over LCDs Among these advantages the most

important are low energy consumption wider viewing angle high contrast

rate and possibility to achieve multiple colors with a single material [103]

When a new redox state of electrochromic materialis established by

the applied electric pulse then it is maintained after the potential is switched

off This means the colored or bleached state of the material can be sustained

for a considerable time without applying electrical power This is so-called

ldquocolor memory effectrdquo of electrochromic material Because of the color

memory effect energy consumption for the electrochromic display device

could be drastically reduced and this would be a big advantage over other

34

emissive devices The low power consumption is especially advantageous

when it is applied to mobile devices with limited power source The possibility

for a flexible display is another attraction for electrochromic material

Information displays

Real-view mirrors for automobiles

Fig 1B4 Application fields of electrochromic devices Smart windows information displays and real-view mirrors for automobiles

35

Using polymeric electrochromic material and plastic substrate with a

transparent conductive coating it is possible to build all-plastic flexible display

device There are lots of papers and patents about the flexible display devices

[104105] In these cases gel-type electrolyte is also needed The gel-type

electrolyte immobilizes liquid electrolyte in the polymer network [106107]

Recently ITO-coated polymer films are readily available which would provide

the plastic substrate for a flexible device The flexible electrochromic displays

are frequently tried with the plastic substrate flexible electrochromic material

and gel-type electrolyte The flexible electrochromic displays would facilitate

increasing technical demands for foldable display devices

Metal Ion Electrochromism

Many transition metal oxides are capable of redox reactions that result in

colour change Metal oxide films are commonly prepared as thin layers of

either tungsten nickel molybdenum or other metal compounds by a number

of techniques These include sol-gel electrochemical by dc or rfreactive

sputtering techniques electron-beam evaporation by anodic or cathodic

electrodeposition or by solution dipping of the electrochromic metal

compounds (or compounds that can be changed into these metal compounds)

onto optically transparent electrodes (OTE) [108 -114] Their electrochromism

is derived from the colour change associated with a change in the oxidation

state of the metal anion The behaviour of these materials is dependent upon

pH moisture and exposure to the atmosphere [115] Generally the switching

rates of these films is somewhat slow with typical switching times of about 15

- 60 seconds to achieve 100 conversion to either coloured or bleached state

[116 -120] An Example of this includes nickel oxide which changes from

transparent (pale green) to brownblack taking about 30 seconds to do so

[121] Other examples include [(NH4)5Ru]2(pyrazine)5+ and [(NH4)5Ru]2(44-

bipyridine)5+ whose electrochromism is significantly different due to the effect

of the ligand [122] Table 12 below gives some examples of metal oxide films

with electrochromic properties

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

33

individual color is originated from the corresponding oxidation or reduction

state of the material For electrochromic materials the characteristic color

change is reversible since the oxidation and reduction state can be converted

reversibly by switching potential

Application fields

The application area for the electrochromism is rather broad that it

covers from smart window glazing and optical modulators to information

displays [98-102] The smart windows are typical examples The

electrochromic property is used to control the amount of light and heat to pass

through the windows Usually the electrochromic material is in form of thin film

coated on a window glass The transmittance modulation has also been

applied at the automobiles to automatically tint rear-view mirrors in various

lighting conditions The electrochromic application fields are illustrated in Fig

1B3

Since the smart windows control the transmittance of heat as well as

the transmittance of visible light the working definition of electrochromism has

now been extended to include devices for modulation of radiation in the near

infrared thermal infrared and microwave regions When color for

electrochromic materials is used this can now mean a response by detectors

at these wavelengths and not just by the human eyes Nowadays

electrochromic material draws much attention as being used in the display

devices Electrochromic display device (ECD) is being considered as one of

the candidates for the conventional liquid crystal display (LCD) since ECD

has many advantages over LCDs Among these advantages the most

important are low energy consumption wider viewing angle high contrast

rate and possibility to achieve multiple colors with a single material [103]

When a new redox state of electrochromic materialis established by

the applied electric pulse then it is maintained after the potential is switched

off This means the colored or bleached state of the material can be sustained

for a considerable time without applying electrical power This is so-called

ldquocolor memory effectrdquo of electrochromic material Because of the color

memory effect energy consumption for the electrochromic display device

could be drastically reduced and this would be a big advantage over other

34

emissive devices The low power consumption is especially advantageous

when it is applied to mobile devices with limited power source The possibility

for a flexible display is another attraction for electrochromic material

Information displays

Real-view mirrors for automobiles

Fig 1B4 Application fields of electrochromic devices Smart windows information displays and real-view mirrors for automobiles

35

Using polymeric electrochromic material and plastic substrate with a

transparent conductive coating it is possible to build all-plastic flexible display

device There are lots of papers and patents about the flexible display devices

[104105] In these cases gel-type electrolyte is also needed The gel-type

electrolyte immobilizes liquid electrolyte in the polymer network [106107]

Recently ITO-coated polymer films are readily available which would provide

the plastic substrate for a flexible device The flexible electrochromic displays

are frequently tried with the plastic substrate flexible electrochromic material

and gel-type electrolyte The flexible electrochromic displays would facilitate

increasing technical demands for foldable display devices

Metal Ion Electrochromism

Many transition metal oxides are capable of redox reactions that result in

colour change Metal oxide films are commonly prepared as thin layers of

either tungsten nickel molybdenum or other metal compounds by a number

of techniques These include sol-gel electrochemical by dc or rfreactive

sputtering techniques electron-beam evaporation by anodic or cathodic

electrodeposition or by solution dipping of the electrochromic metal

compounds (or compounds that can be changed into these metal compounds)

onto optically transparent electrodes (OTE) [108 -114] Their electrochromism

is derived from the colour change associated with a change in the oxidation

state of the metal anion The behaviour of these materials is dependent upon

pH moisture and exposure to the atmosphere [115] Generally the switching

rates of these films is somewhat slow with typical switching times of about 15

- 60 seconds to achieve 100 conversion to either coloured or bleached state

[116 -120] An Example of this includes nickel oxide which changes from

transparent (pale green) to brownblack taking about 30 seconds to do so

[121] Other examples include [(NH4)5Ru]2(pyrazine)5+ and [(NH4)5Ru]2(44-

bipyridine)5+ whose electrochromism is significantly different due to the effect

of the ligand [122] Table 12 below gives some examples of metal oxide films

with electrochromic properties

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

34

emissive devices The low power consumption is especially advantageous

when it is applied to mobile devices with limited power source The possibility

for a flexible display is another attraction for electrochromic material

Information displays

Real-view mirrors for automobiles

Fig 1B4 Application fields of electrochromic devices Smart windows information displays and real-view mirrors for automobiles

35

Using polymeric electrochromic material and plastic substrate with a

transparent conductive coating it is possible to build all-plastic flexible display

device There are lots of papers and patents about the flexible display devices

[104105] In these cases gel-type electrolyte is also needed The gel-type

electrolyte immobilizes liquid electrolyte in the polymer network [106107]

Recently ITO-coated polymer films are readily available which would provide

the plastic substrate for a flexible device The flexible electrochromic displays

are frequently tried with the plastic substrate flexible electrochromic material

and gel-type electrolyte The flexible electrochromic displays would facilitate

increasing technical demands for foldable display devices

Metal Ion Electrochromism

Many transition metal oxides are capable of redox reactions that result in

colour change Metal oxide films are commonly prepared as thin layers of

either tungsten nickel molybdenum or other metal compounds by a number

of techniques These include sol-gel electrochemical by dc or rfreactive

sputtering techniques electron-beam evaporation by anodic or cathodic

electrodeposition or by solution dipping of the electrochromic metal

compounds (or compounds that can be changed into these metal compounds)

onto optically transparent electrodes (OTE) [108 -114] Their electrochromism

is derived from the colour change associated with a change in the oxidation

state of the metal anion The behaviour of these materials is dependent upon

pH moisture and exposure to the atmosphere [115] Generally the switching

rates of these films is somewhat slow with typical switching times of about 15

- 60 seconds to achieve 100 conversion to either coloured or bleached state

[116 -120] An Example of this includes nickel oxide which changes from

transparent (pale green) to brownblack taking about 30 seconds to do so

[121] Other examples include [(NH4)5Ru]2(pyrazine)5+ and [(NH4)5Ru]2(44-

bipyridine)5+ whose electrochromism is significantly different due to the effect

of the ligand [122] Table 12 below gives some examples of metal oxide films

with electrochromic properties

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

35

Using polymeric electrochromic material and plastic substrate with a

transparent conductive coating it is possible to build all-plastic flexible display

device There are lots of papers and patents about the flexible display devices

[104105] In these cases gel-type electrolyte is also needed The gel-type

electrolyte immobilizes liquid electrolyte in the polymer network [106107]

Recently ITO-coated polymer films are readily available which would provide

the plastic substrate for a flexible device The flexible electrochromic displays

are frequently tried with the plastic substrate flexible electrochromic material

and gel-type electrolyte The flexible electrochromic displays would facilitate

increasing technical demands for foldable display devices

Metal Ion Electrochromism

Many transition metal oxides are capable of redox reactions that result in

colour change Metal oxide films are commonly prepared as thin layers of

either tungsten nickel molybdenum or other metal compounds by a number

of techniques These include sol-gel electrochemical by dc or rfreactive

sputtering techniques electron-beam evaporation by anodic or cathodic

electrodeposition or by solution dipping of the electrochromic metal

compounds (or compounds that can be changed into these metal compounds)

onto optically transparent electrodes (OTE) [108 -114] Their electrochromism

is derived from the colour change associated with a change in the oxidation

state of the metal anion The behaviour of these materials is dependent upon

pH moisture and exposure to the atmosphere [115] Generally the switching

rates of these films is somewhat slow with typical switching times of about 15

- 60 seconds to achieve 100 conversion to either coloured or bleached state

[116 -120] An Example of this includes nickel oxide which changes from

transparent (pale green) to brownblack taking about 30 seconds to do so

[121] Other examples include [(NH4)5Ru]2(pyrazine)5+ and [(NH4)5Ru]2(44-

bipyridine)5+ whose electrochromism is significantly different due to the effect

of the ligand [122] Table 12 below gives some examples of metal oxide films

with electrochromic properties

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

36

Table 12 Some examples of electrochromic metal oxides

Metal Oxide Reaction Colour Change

Cobalt Oxide 3CoO + 2OH Co3O4 + H2O +

2e-

green brown

Indium Tin

Oxide

In2O3 + 2x (Li + + e -) Li2x InIII

(1-

x)InIxO3

colourless pale

blue

Iridium Oxide Ir (OH)3 IrO2bullH2O + H+ + e- colourless

bluegrey

Molybdenum

Trioxide

MoO3 + x(Li+ + e-) LixMoVI (1-x)

MoVxO3

colourless blue

Nickel Oxide NiOxHy [NiII(1-z)NiIIIz]OxH(y-z) +

zH+ +ze-

colourless

brownblack

Tungsten

Trioxide

WO3 + x(Li+ + e-) LixW VI(1-

x)W VxO3

very pale blue

blue

Vanadium

Pentoxide

LixV2O5 V2O5 + x(Li+ + e-) very pale blue

(brownyellow)

Cerium Oxide CeO2 + x(Li+ + e-) LixCeO2 yellow very

pale

Manganese

Oxide

MnO2 + ze- + zH+ MnO(2-z)

(OH)

yellow brown

Niobium

Pentoxide

Nb2O5 + x(Li+ + e-) LixNb2O5 colourless pale

blue

Ruthenium

Dioxide

RuO2bull2H2O+H2O+e-

frac12(Ru2O3bull5H2O) + OH-

(blue brown)

black

For inorganic electrochromic material tungsten oxide (WO3) is most

typical The electrochromism actually was first discovered in WO3 films it still

remains most frequently studied material and as a consequence most feasible

candidate among inorganic electrochromic materials for the devices The

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

37

electrochemically induced oxidation and reduction state in WO3 film can be

represented by a simple reaction equation as eq15

WO3 + x Mrsquo+ + x e- Mrsquo x WO3 -------- 15

Bleached state Colored state (dark blue)

Mrsquo+ denotes metal ions such as H+ Li+ Na+ and K+ The left side of the

equation represents bleached state where the material becomes optically

transparent and the right side is colored state with dark blue color

Electrochromic color change could also be observed from other transition

metal oxides such as WO3 MoO3 V2O5 LiO Nb2O5 etc Since the color

change of material comes from non-stoichiometric redox state many

transition metal oxides which tend to have non-stoichiometric state are

electrochromic in nature Transition metal oxides films can be made by

several processing technique such as vacuum evaporation sputtering spray

pyrolysis chemical bath deposition and sol-gel chemical method [123-125]

For a low cost production of electrochromic film on the large area

substrate for the smart windows of buildingschemical bath deposition would

be most preferred In the current nanoscience and technology era the

transition metal oxides (TMOs) constitute a fascinating and promising

class of inorganic solids that have received substantial attention of solid

state materials chemists due to their novel material characteristics Because

of the extensive studies on the material the transition metal oxides are still

widely used to smart window system and transmission modulation devices

The electrochromic mechanism and kinetics are relatively well understood for

the transition metal oxides

1B2 Aim and object of the research work

Saving energy in the building sector and automotive industry is a major

global socio-economic target in energy efficiency as well as from

environmental viewpoint Substantial savings in energy consumption can be r

ealized through an optimal solar radiations management with the emerging s

mart photonics in minimizing the usage of air-conditioning systems With

worldwide asymp 2 billions m2 of smart photonics coated glass windows energy

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

38

saving in the two mentioned air-conditioning segments ie buildings and

cars has been estimated to be approx 1 billion GJ and CO2 atmospheric

emissions would be reduced by approx 100 millions of tons The

global production of glass which could be solar regulated to minimize the air c

onditioning using emerging smart nano-photonics could be a part of 1

billion m2year with about 25 for building and ~11 for automotive industry

Examples of these smart photonics include electrochromic Transition Metal

oxide based devices These smart windows can be tuned to be transparent

or dark in a reversible manner Due to such a significant optical modulation

this later nanotechnology with a well established scientific platform could

play a key role in energy management in both automotive and architectural

sectorsas mentioned previously To set the scene one has to note

that heating cooling lighting ventilation and powering of buildings and

automotives account for more than the half of the total energy consumption

worldwide and hence responsible for more energy consumption than

any other end-user sector such as industrial production

Worldwide research is conducted on advanced electrochromic devices

for obtaining this optical modulation function through the action of electrical v

oltage pulses of few voltsThe electrochromic device comprises generally five

superimposed thin layers on a transparent substrate (glass or polyester foil)

or in between two such materials The outermost layers deposited on glasses

consist of transparent electrical conductors (for example tin doped indium

oxide) The three layers in between are made of porous tungsten oxide

(WO3) a transparent ion conductor (electrolyte) and porous nickel oxide

(NiO) in general When an electrical voltage is applied over the outer layers

electrical charge is shuttled between the porous oxide layers whose

transparency thereby is changed so that the overall light throughput of the

device is altered The function is similar to that of an electrical ldquothin film

batteryrdquo whose charging state manifests itself in optical absorption

Therefore electrochromic smart windows can be used to achieve a

combination of enhanced indoor comfort and energy efficiency in buildings

and automobiles If the device is based on flexible foils it can be used in

visors for motorcycle helmets and in sky goggles Other applications concern

information displays and surfaces with variable heat emission [125]

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

39

Phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)

are extensively studied inorganic EC material due to its outstanding

electrochromic properties Amongst the different deposition techniques

chemical bath depositon method becomes simple and cost effective among

researchers for producing EC and IS films because of the inexpensive

deposition equipment and a wide choice of precursors The central idea of this

work is to test the applicability of simple and inexpensive chemical bath

depositon method for the synthesis of Tl doped Phosphotungstic acid

(H3PW12O40) and phosphomolybdic acid (H3PMo12O40) thin films To our

knowledge chemical bath depositon method has not previously been

used to obtain electrochromic Tl doped Phosphotungstic acid( H3PW12O40)

and phosphomolybdic acid ( H3PMo12O40) thin films Chemical bath deposition

has many attractive features and have the benefit of being easily realizable

from the point of view of industrialization especially on large area devices

with the required electrochromic properties Because of its simplicity low cost

and feasibility In recent years chemical bath deposition thin films are playing

important role in energy conversions solar selective coatings Optoelectronic

devices gas and humidity sensors etc

From the literature survey [xyz] it was found that there are two types

of electrochromic material a) inorganic transition metal oxides b) polymers

such as polyaniline Ever since the discovery of electrochromism in transition

metal oxidesalmost all efforts have been devoted to the inorganic materials

In recent years however polymer materials are gaining attentions because

of the possibility of being applied to the flexible display devices From

previous research works It could be said that conducting polymers such

as polyaniline and polypyrrole are more suitable material for the

electrochromic displays since they exhibit faster response and longer

operating life than the inorganic material

However it still has problems for the display applications The

response times of polymeric materials could reach down to 10 ms which is

short enough for a display device application Therefore we prapose to use

inorganic transition metal oxides for preparing electrochromic thin films

As a result of the literature survey it can be stated that a considerable i

mprovement in chemical stability and electrochromic property of

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

40

phophotungstic acid and phosphomolybdic acid is necessary after doping the

thallium It could also be understood from the results of many research

workers that they have prepared composite electrochromic thin films using

organic polymers such as polyacrylamide polyvinyl alcohol etc

Hence it was planed to synthesize Tl doped Phosphotungstic acid

(Tl3PW12O40) and Tl doped phosphomolybdic acid (Tl3PMo12O40) thin films by

using chemical bath depositon technique and to test the applicability of

this technique to produce high quality EC material Based on afore-mentioned

points the present work is systematically planned and presented chapter

wise in the thesis

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

41

References

[1] Introduction to Polyoxometalate Chemistry From Topology via Self-

Assembly to applicationsMTPope Department of Chemistry

Georgetown University Washington DC 20057 USA

[2] MT Pope A Muumlller Polyoxometalate Chemistry An Old Field with

New Dimensions in Several Disciplines Angew Chem Int Ed Engl

30 (1991) 34

[3] The Structure and Formula of 12-Phosphotungstic Acid JF Keggin

Proc Roy Soc A 144 85 (1934) 75

[4] Supramolecular Inorganic Chemistry Small Guests in Small and Large

Hosts A Muumlller H Reuter S Dillinger Angew Chem Int Ed Engl

34 (1995) 2328

[5] MT Pope ldquoHeteropoly and Isopoly Oxometalatesrdquo Springer Verlag

New York (1983)

[6] MT Pope Inorganic Chemistry Concepts 8 Heteropoly and Isopoly

oxometalates Springer-Verlag Heidelberg (1983) 101

[7] MT Pope A Muumlller Polyoxometalates From Platonic Solids to Antimdash

retroviral Activity Kluwer Academic Publications The Netherlands

(1994) 262

[8] Baker LCW ldquoAdvances in The Chemistry of Heteropoly Electrolytes

and Their Pertinence for Coordination Chemistryrdquo Ed

Kirschner S McMillan New York (1961)604

[9] Pope MT Heteropoly and IsopolyOxometalatesSpringer Verlag

(1983)

[10] Chemical Reviews special issue January February all chapters

(1998)

[11] Gomez-Romero P N Casan-Pastor J Phys Chem 100 (1996)

12448

[12] Gomez-Romero P Solid State Ionics 243(1997) 101

[13] Baker LCW VE Simmons-Baker SH Wasfi J AmChem Soc 94

(1972) 5499

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

42

[14] Casantilde-Pastor N Doctoral Dissertation Georgetown University

1988 Diss Abst Internat B 50 (1989)1397

[15] Kozik M N Casan-Pastor C F Hammer and LCWBaker

J Am Chem Soc 110 7697 (1988)

[16] CasantildePastor N and LCW Baker J Am Chem Soc 114 (1992)10384

[17] Casan-Pastor N J Bas-Serra E Coronado G Pourroy and LCW

Baker J Am Chem Soc114 (1992)10380

[18] Marrot J MA Pilette F Scheresse and E Cadot Inorg Chem 42

(2003)3609

[19] Bino A M Ardon D Lee B Spingler and S J Lippard

J Am Chem Soc 142 (2002) 4578

[20] Muller A F Peters MT Pope and D Gatteschi

Chem Rev 98 (1998)239

[21] Liu T E Diemann H Liu A WM Dress and AMuller

Nature 426 59(2003)

[22] M T Pope Heteropoly and Isopoly Oxometalates Springer-

Verlag New York 1983

[23] J T Rhule C L Hill D A Judd Chem Rev 98 (1998) 327

[24] I V Kozhevnikov Chem Rev 98 (1998) 171

[25] N Mizuno M Misono Chem Rev 98 (1998) 199

[26] T Yamase Chem Rev 98 (1998) 307

[27] M Sadakane E Steckhan Chem Rev 98 (1998) 219

[28] D E Katsoulis Chem Rev 98 (1998) 359

[29] E Coronado C J Gomez-Garcia Chem Rev 1998

[30] J F Keggin Nature 131(1933)908

[31] Y P Jeannin Chem Rev 98 (1998) 51

[32] JC Bailar Jr The Chemistry of the Coordination Compounds

Reinhold Publishing Corporation (1956) 472

[33] JF Keggin Proc Roy Soc A 144 (1934)75

[34] GM Brown MR Noe-Spirlet WR Bursing HA Levy Acta Cryst

B33 (1977) 1038

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

43

[35] Y Izumi K Urabe M Onaka Zeolite Clay and Heteropoly Acid in

Organic Reactions Kodansha Ltd Tokoyo (1992) 100

[36] LCW Baker JS Figgis Journal of the American Chemical Society

92(12) (1970) 3794

[37] Polyoxometalates in Medicine Jeffrey T Rhule Craig L Hill and

Deborah A Judd Chem Rev 98 (1) (1998) 327

[38] Guangjin Zhang Tao He Ying Ma Zhaohui Chen Wensheng Yang

and Jiannian Yao Physical Chemistry Chemical Physics 51313

(2004)2751

[39] Andrew M Herring John A Turner Steven F Dec Bradford

Limoges Fanqin Meng Mary Ann Sweikart Jennifer L Malers and

James L Horan National Renewable Energy Laboratory

Golden CO 80401

[40] Nathalie Calinand Slavi CSevov Inorganic ChemistryVol42 No22

(2003) 7304

[41] Guangjin Zhang Wensheng Yang Jiannian Yao

Journal Advanced functional materials 15 (8) (2005) 1255

[42] Mo Yeon- Gon Thesis (PhD) The University of Nebraska - Lincoln

Source DAI- B 6010 (2000) 5180

[43] I A Weinstock R H Atalla and R S Reiner

Proceedings of 1995 International environmental conference

May 7-10 Atlanta GA Book 2 (1995)1197

[44] Tao He and Jiannian Yao J Mater Chem 17 (2007) 4547

[45] De-Liang LongEric Burkholder and Leroy Cronin ChemSocRev 36

(2007)105

[46] Zhang Fumin Guo Maiping Ge Hanqing and Wang Jun)

Chin J Chem Eng 15(6) (2007) 895

[47] K Petkov R Todorov M Kincl L Tichy Journal of Optoelectronics

and Advanced Materials Vol 7 No 5 (2005) 2587

[48] AVadivel Murugan CW Kwon GCampet and BBKale J Active

and Passive ElecComp Vol26(2) (2003)81

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

44

[49] Sadhana S Rayalu Nidhi Dubey Ravikrishna V Chatti Meenal V

JoshiNitin K Labhsetwar and Sukumar Devotta Current Science 93

NO 10(2005) 1376

[50] T Uma and M Nogami Journal of New Materials for Electrochemical

Systems 10 (2007) 75

[51] KU Zongjun JIN Surong J of Wuhan University of Technology-

Mater Sci Ed Vol23 (3) (2008) 367

[52] UBMiocMRTodorovicMDavidovic PhColomban IHolclajtner-

Antunovic Solid State Ionics176(2005)3005

[53] T Rajkumar and G Ranga Rao J Chem Sci Vol 120 No 6 (2008)

587

[54] MGanne A Jouanneaux MMorsli and AConan Phys Rev B 39

(1989) 3735

[55] ASibai JOlivaresGGuillot and GBremond J of Applied Physics 94

(2003) 2403 [56] B Tell F Wudl Jof Applied Phy50(9)(1979) 5944

[57] S Fujibayashi K Nakayama M Hamamoto S Sakaguchi

Y Nishiyama Y Ishii J Mole Cat A Chemical 110 (1996) 105

[58] G Malandrino Anna M Borzigrave F Castelli Ignazio LFragalagrave Walter

Dastrugrave R Gobetto Patrizia Rossi and Paolo Dapporto Dalton Trans

(2003) 369

[59] R Xionga M Tianb H Liua W Tanga M Jinga JSunaQ Koua

DTiana and Jing

Shia Materials Science and Engineering B Vol 87(2) (2001) 191

[60] C Jagadish A L Dawarand P C Mathur Volume 23(3) (1988) 1002

[61] N Laxmi and S Chandra Bulof Mat Sci25 (3)(2002) 197

[62] Clabaugh WS JacksonAJResNatBurStand62 (1959)201

[63] Simon SJ BoltzDF AnalChem 47 (1975) 1758

[64] GeisingerKRBatsakisJGBauerRCAmJClinPath 72

(1979)330

[65] Chermann JC Sinoussi F Jatmin C BiochemBiophysRes

Commun 65 (1975) 1229

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

45

[66] Giordano N Caporali G Ferlazz N USPatent3226(1965) 421

[67] KlinkenbergJW(to Shell Oil Co)USPatent 2982(1961) 799

[68] ShengMN ZajecekJGAdvanChemSer 57 (1968) 418

[69] SmitJVan RNature181 (1958)1530

[70] Guilbault GG BrignacPJAnalChimActa 56 (1971) 139

[71] Tell B Wagner SApplPhysLetter 33 (1978) 837

[72] Chemical Reviews Thematic issue on photochromism

Vol100 Issue 5 (2000)

[73] PhotochromismMolecules and Systems (Heinz Durr and Henri Bouas-

Laurent) ISBN978-0444513229

[74] Nature Materials 4 (2005) 249

[75] Macromolecules 39 (2006) 1391

[76] Australian Journal of Chemistry 58 (2005) 825

[77] Rindel M S African J Sci 11 (1916) 362

[78] Sheppard S E Eberlin L W US Patent 1934 (1933) 451

[79] Chalkley L J Phys Chem 56 (1952) 1084

[80] Chalkley L J Opt Sci Am 44 (1954) 699

[81] Vogel A I A Text Book of Quantitative Inorganic Analysis Wiley

and Sons New York (1966)

[82] Wu H J Biol Chem 43 (1920) 189

[83] Baudisch O Gates F L J Am Chem Soc 56 (1934) 373

[84] Yamase T Ikawa T Kokado H Inoue E Chem Lett (1973) 615

[85] Arnaud-Neu F Schwing-Weill M-J Bull Soc Chim Fr (1973) 3225

[86] Yamase T Uheda K J Electrochem Soc 140 (1993) 2378

[87] Deb S K Forrestal J L Photochromism Brown G H Ed

Wiley New York (1971) 342

[88] Faughnan B W Staebler D L Kiss Z T In Applied Solid States

Science Wolke R Ed Academic Press New York (1971)107

[89] Exelby R Grinten R Chem Rev 65 (1965) 247

[90] Faughnan B W Crandall R S Heyman R P RCA Rev

Electrochem Soc (1975)

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

46

[91] GH Brown Photochromism John Wiley amp Sons Inc (1971)

[92] KA Murray AB Holmes SC Moratti G Rumbles J Mater Chem

9 (1999)2109

[93] M Mastragostino In B Scrosati Editor Applications of Electroactive

Polymers Chapman amp Hall London (1993) 223

[94] P R Somani and S Radhakrishnan Materials Chemistry and

Physics 77 (2002)117

[95] C G Granqvist Solar Energy Materials amp Solar Cells 60 (2000) 201

[96] A Seeboth J Schneider and A Patzak Solar Energy Materials amp

Solar Cells 60 (2000)263

[97] C G Granqvist Journal of the European Ceramic Society 25 (2005)

2907

[98] J Livage and D Ganguli Solar Energy Materials amp Solar Cells 68

(2001) 365

[99] G-L Chen US PATENT 20050141074 A1 (2005)

[100] W L Tonar J S Anderson J S Forgette and K B Kar US Patent

20050094279 A1 (2005)

[101] httpwwwsage-eccom SAGE Electronics Inc (2005)

[102] P Bonhocircte E Gogniat F Campus

and M Graumltzel Displays 20 (1999)137

[103] F Michalak and M D Aldebert Solid State Ionics 85 (1996) 265

[104] P J Martin and M D Pasquela US Patent 6456418 (2001)

[105] D V Varaprasad M Zhao C A Dornan A Agrawal P-

W Allemand and N R Lynam US Patent 6136 (2002)161

[106] J P Coleman A T Lynch P Madhukar and J H Wagenknecht

Solar Energy Materials amp Solar Cells 56 (1999) 395

[107] C Xu and M Taya Canadian Patent CA 2451615 A1 (2003)

[108] PMS Monk RJ Mortimer DR Rosseinsky Electrochromism

Fundamentals and Applications VCH Inc Weinheim (1995)

[109] BW Faughnan RS Crandall PM Heyman RCA Rev 36 (1975)

177

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

47

[110] H Inaba M Iwaku K Nakase H Yasukawa I Seo N Oyama

Electrochim Acta 40 (1995)227

[111] SA Sapp GA Sotzing JR Reynolds Chem Mater10 (1998)2101

[112] SK Deb Solar Energy Mater Solar cells 25 (1992) 327

[113] MS Habib SP Maheswari Solar Energy Mater Solar cells 25

(1992)195

[114] C Arbizzani M Mastragostino L MeneghelloM Morselli AJZanelli J

Appl Electrochem 26 (1996) 121

[115] Q Pei G Yu C Zhang Y Yang AJ Heeger J Science 269

(1995)1086

[116] M Granstom O Inganas Adv Mater 7 (1995)1012

[117] J Scarminio A Urbano BJ GardesJ Of Mater Sci Lett 11

(1992)562

[118] DH Oh SG Boxer J Am Chem Soc 112 (1990)8161

[119] S Papaefthimiou G Leftheriotis and P Yianoulis Thin Solid Films 343-

344 (1999)183

[120] N A OBrien J Gordon H Mathew and B P Hichwa Thin Solid Films

345 (1999) 312

[121] P S Patil S H Mujawar A I Inamdar and S B Sadale Thin Solid Fil

ms 250 (2005) 117

[122] T Ivanova K Gesheva F Hamelman G Popkirov M Abrashev M G

anchev and E Tzvetkova Vacuum 76 (2004)195

[123] CG Granqvist Handbook of inorganic Electrochromic Materials

Elsevier Amsterdam (1995)

[124] CG Granqvist MH Francombe JL Vossen (Eds) Physics of Thin Film

Academic San Diego 70 (1993) 301

[125] CG Granqvist Solid State Ionics 60 (1993) 213

48

48