Chapter ndash V

155

Synthesis Characterization and Applications of CeO2 Nanoparticles

51 Introduction

Ceria (CeO2) is an oxide with important applications in areas of catalysis

electrochemistry photochemistry and materials science [1-5] Also it is highly

efficient ultraviolet (UV) absorber to protect light-sensitive materials as a coating

material for protection of corrosion of metals as an oxidation catalyst and as a

counter electrode for electrochemical devices [6-9] The physical properties of

CeO2 are represented in Table 51 Cerium oxide has outstanding physical and

chemical properties therefore it is used as LPG sensor as well as electrolyte

materials for solid fuel cells [10-13] Recently Zhang et al has reported the CeO2

nanocrystal microsphers as a novel adsorbent for the removal of Cr (VI) from

waste water [4] In its most stable phase bulk CeO2 adopts a fluorite-type crystal

structure in which each metal cation is surrounded by eight oxygen atoms The

band gap of pure ceria is 5 eV but crystal defects or impurities can transform the

material in a good n-type semiconductor [14] Experimental and theoretical studies

indicate that bulk CeO2 is not a fully ionic oxide [15] Experiments of

photoelectron spectroscopies and optical reflectivity measurements show a strong

hybridization of the metal and oxygen orbitals and the valence band although

dominated by O 2p character still contains a significant amount of metal character

[16] Thus the charge on the metal cations is probably much smaller than the

formal value of +4 frequently assigned and CeO2 is best described as an

ionocovalent compound or covalent insulator One of the most interesting

properties of ceria is its ability to undergo a facile conversion between +4 and +3

formal oxidation states Because of this ceria is a key component in the so-called

three-way catalysts (TWC) commonly used to reduce the emissions of CO NOx

and hydrocarbons from automobile exhaust or is used as a base material of

electrolytes and electrodes in solid oxide fuel cells [17-20] Ceria-supported noble

metal catalysts are capable of storing oxygen under oxidizing conditions and

Chapter ndash V

156

releasing oxygen under reducing conditions through a transformation between

Ce4+

and Ce3+

oxidation states [21-23]

Table 51 Properties of CeO2

Molecular formula CeO2

Molar mass 172115 gmol

Appearance white or pale yellow solid

slightly hygroscopic

Density 765 gcm3 solid

7215 gcm3 fluorite phase

Melting point 2400degC

Boiling point 3500degC

Solubility in water Insoluble

In the area of catalysis nanoparticles of ceria have been studied since the

early 1970s but they were poorly characterized In recent years substantial

progress has been made thanks the use of better synthetic methods and

sophisticated techniques for characterizing structural and electronic properties

Only for the largest nanoparticles was the fluorite structure clearly observed

Small nanoparticles exhibited a nearly amorphous structure [24-29] In general

the energy required to reduce the CenO2n systems increased with particle size but

large fluctuations were also observed The reduction of the ceria nanoparticles was

structure sensitive being easier in systems that had a low degree of crystallinity

Several of these theoretical predictions have been verified by subsequent

experimental studies as we will see bellow

Chapter ndash V

156

releasing oxygen under reducing conditions through a transformation between

Ce4+

and Ce3+

oxidation states [21-23]

Table 51 Properties of CeO2

Molecular formula CeO2

Molar mass 172115 gmol

Appearance white or pale yellow solid

slightly hygroscopic

Density 765 gcm3 solid

7215 gcm3 fluorite phase

Melting point 2400degC

Boiling point 3500degC

Solubility in water Insoluble

In the area of catalysis nanoparticles of ceria have been studied since the

early 1970s but they were poorly characterized In recent years substantial

progress has been made thanks the use of better synthetic methods and

sophisticated techniques for characterizing structural and electronic properties

Only for the largest nanoparticles was the fluorite structure clearly observed

Small nanoparticles exhibited a nearly amorphous structure [24-29] In general

the energy required to reduce the CenO2n systems increased with particle size but

large fluctuations were also observed The reduction of the ceria nanoparticles was

structure sensitive being easier in systems that had a low degree of crystallinity

Several of these theoretical predictions have been verified by subsequent

experimental studies as we will see bellow

Chapter ndash V

157

It is not easy to find synthetic methods that allow the preparation of ceria

nanoparticles that are small and have a narrow distribution of sizes This makes

difficult a direct comparison between experiment and theory [30] But

experimentally it is known that very small particles of ceria may deviate from the

fluorite structure of the bulk oxide For particles that are a little bit larger

measurements of XAS Raman and XRD would suggest the existence of local

distortions on the cubic fluorite structure as a consequence of defects in the oxide

lattice [31-34] Depending on the method of preparation and particularly of the

Ce oxidation state of the precursor salt the content of O vacancies and

concomitant presence of Ce3+

in a ceria nanoparticle can change this has been

shown by using Raman and XRD Since Ce3+

is significantly bigger than Ce4+

(atomic sizes 114 and 097 Aring respectively) the presence of O vacancies increases

the size of the unit cell and can distort it In addition to O vacancies other

structural imperfections as well as surface effects can be present in a ceria

nanoparticle introducing strain in the lattice The O vacancies and defects present

in ceria nanoparticles can lead to special electronic properties introducing

electronic states within the band gap of the oxide [35-39] Ceria particles with

diameters of less than 10 nm have a substantially higher electronic conductivity

than bulk ceria [40-43]

Bulk ceria is able to absorb and store hydrogen Ceria nanoparticles have

the same property The absorption of hydrogen causes an expansion in the lattice

constant of the oxide detected by using XRD [44] Theoretical calculations

indicate that the H atoms do not remain at a high symmetry position in the center

of the cavities of the ceria lattice but instead move toward the O sites forming

hydroxyl species [45-49] These species can be seen as the precursors for the

removal of oxygen during a reduction process Results of temperature

programmed reduction and time-resolved XRD indicate that ceria nanoparticles

are reduced at temperatures that are lower than those seen for the reduction of bulk

ceria This is consistent with the prediction of theoretical studies During the

Chapter ndash V

158

reduction process before the appearance of Ce2O3 there is a substantial expansion

in the unit cell of the CeO2 nanoparticles as a consequence of the embedding of

hydrogen and the formation of O vacancies [50]

The fine powder of CeO2 has been prepared by many methods including

forced hydrolysis sol-gel hydrothermal coprecipitation surfactant templating

method and spray pyrolysis [51-57] However microwave synthesis is very

beneficial to find a fast simple and energy efficient approach to produce fine

CeO2 nanoparticles [20 58-63] It is relatively new method to produce inorganic

compounds for materials processing to enhance the material properties as well as

economic advantages through energy saving and acceleration of product

development [64-65]

In the present work we have studied the structural optical and electrical

properties of CeO2 nanoparticles This material was characterized by using

UV-Visible FTIR XRD SEM EDAX TEM TGA-DTA and electrical

conductivity The CeO2 nanoparticles have been investigated for gas sensing

properties for LPG gas catalytic activity for the oxidative regeneration of ketones

and novel adsorbents for wastewater treatment

52 Materials and Methods

All the chemicals used for the preparation were of analytical grade It

includes Cerium nitrate (Ce (NO3)3 6H2O) propylene glycol and ammonia All the

solutions were prepared in millipore water obtained from ultra pure water system

Monodispersed nanocrystalline CeO2 powder was prepared by controlled addition

of aqueous ammonia to a mixture of 01M aqueous solution of cerium nitrate and

propylene glycol until the solution reached pH = 10 The ratio of concentration of

propylene glycol to cerium nitrate solution was kept 11 The special arrangement

was made to add drop wise aqueous ammonia into the solution with constant

stirring After complete precipitation the precipitated hydroxide was washed with

distilled water Then pure hydroxide in a glass beaker was placed in a microwave

Chapter ndash V

159

oven (in put power 600W) about 30 minutes with on-off cycle The Phase purity

and the degree of crystallinity of the resulting CeO2 sample were monitored by

XRD analysis The schematic flow chart of preparation of nanocrystalline CeO2 is

represented in Fig51

Aqueous ammonia

darr

Ce(NO3)3 6H2O + Propylene glycol

darr Centrifugation and washing

Microwave treatment

darr

CeO2 nanocrystalline particles

Fig51 Schematic Flow Chart for the preparation of nanocrystalline CeO2

The powder Xndashray diffraction patterns were recorded on Philips PW-1710

X-ray diffractometer by using Cr Kα radiation The lattice parameters were

calculated using high angle reflection of XRD by using the following formula

--- 51

Crystallite size was calculated by using the Scherrerrsquos formula having

wavelength of X-ray (Cr Kα line 228 Aring)

The UV-Visible Spectra were recorded on UV-Visible-NIR

Spectrophotometer (UV-3600 Shimadzu Japan)

Chapter ndash V

160

FTIR spectrum was recorded with a Perkin Elmer-USA in the range of

wavenumber from 4000-400 cm-1

The SEM micrographs of the samples were obtained using the scanning

electron microscope (Model JEOL-JSM 6360) Elemental analysis was carried

out by using the energy dispersive X-ray spectroscopy equipped with Scanning

electron microscopy

TEM study of the powder sample was carried out by using model Philips

CM 12 transmission electron microscope with Energy Dispersive Spectroscopy

(EDS) detector for microanalysis (IIT-Madras)

The thermal behavior of the nanocrystalline CeO2 powder is studied upto 10

to1000degC in nitrogen using SDT Q600 V209 Build 20 Instrument

The oxidation of oxime was carried out at 353 K in presence of cerium

oxide as catalyst ethyl acetate as a solvent and H2O2 as the oxidant [19] The

liquid phase reaction was carried out in 25 ml round bottom flask equipped with a

reflux condenser In catalytic oxidation reaction oxime (10 mmol) H2O2

(20 mmol) along with catalyst (1 by wt of the substrate) and the content were

heated in pre-heated oil bath at 353 K

NOH

R

O

R

H2O

2

CeO2

Oxime Ketone

Where R = CH3 C6H5

One hundred milligrams of CeO2 nanocrystal microspheres was suspended

in 100 mL of rhodamine B aqueous solution with a concentration of 20 mg L-1

in a

flask under stirring The flask was covered with carbon paper to prevent the

Chapter ndash V

161

photodegradation of Rh B At different intervals 5mL of the suspensions was

collected filtered through a 045μm membrane and finally analyzed by a

UV-visible spectrophotometer immediately [22]

53 Results and Discussion

531 UV-Visible Spectroscopic Analysis

The absorption spectra of CeO2 nanoparticles dispersed in ethanol solution

is shown in Fig52 The absorption edge of the CeO2 nanoparticles synthesized

a) by microwave (without annealed) annealed at b) 200degC and c) 300degC occurs at

around 400 nm and its band gap energy is estimated to be 31 eV A sharp band is

an indicative of narrow distribution of particles within the matrix The accurate

band gap of material was 31 eV as obtained by plotting (αhυ)2

against hυ shown

in Fig53

Fig 52 UV-Visible absorption spectrum of CeO2 nanoparticles synthesized

a) by microwave (without annealed) annealed at b) 200degC and c) 300degC

Chapter ndash V

162

In semiconductors the band gap will be increased as particle size is

decreased As particles shrink in size there is a dramatic change in valence band

and conduction bands as the continuous density of states in bulk is replaced with a

set of discrete energy levels This leads to interesting optical properties In the

visible region colour can changes with size Since with increase in size energy

decreases the nanoparticles which have size less than the bulk compound shows

blue shift in the UV-Visible spectra of nanoparticles This blue shift may be

considered as one of the confirmatory test for the existence of nanoparticles

Fig53 Variation of (αhυ) 2

vs Photon Energy (eV) of CeO2 nanoparticle synthesized

a) by microwave (without annealed) annealed at b)200degC and c) 300degC

532 FTIR Analysis

The FT-IR spectrum of the obtained ceria nanoparticles is shown in

Fig 54 Three strong absorption peaks were observed at 3381 1558 and

1372 cm-1

The former was attributed to the stretching band of hydroxyl group and

the others were attributed to the antisymmetric and symmetric stretching band of

Chapter ndash V

163

the glycol group respectively In the present synthesis propylene glycol works as

a protective agent against particle growth and since the stretching band of the

carbonyl around 1700 cm-1

was not observed in the FT-IR spectrum it could be

that the surface of the ceria nanoparticles was covered with glycol species In

addition it has also been reported that non-stoichiometric cerium oxide

synthesized by an inert gas condensation process was brownish yellow

Fig54 FTIR spectrum of CeO2 nanoparticles

533 The XRD Analysis

Fig55 shows the X-ray diffractograms of the as prepared powder of CeO2

nanoparticles a) by microwave (without annealed) annealed at b)200degC and

c) 300degC The CeO2 nanoparticles are crystalline in nature with d values 313

270 191 and 163 Aring The synthesized samples by microwave confirmed the

Chapter ndash V

164

formation of face centered cubic structure (FCC) of CeO2 nanoparticles The

determined characteristics 2θ values and [hkl] planes are 4287deg [111] 5017deg

[200] 7386deg [220] and 89deg [311] respectively (JCPDS card No 81-0792) and

lattice parameter is 542 Aring The average particle size of CeO2 nanoparticles was

found to about 10 nm The particle size of CeO2 nanoparticles obtained by

microwave is less than that of annealed at 200degC and 300degC However the lattice

parameter is found to increase with reduction temperature indicating an expansion

in the FCC lattice

Fig 55 XRD patterns of the CeO2 nanoparticles synthesized a) by microwave

(without annealed) annealed at b) 200degC and c) 300degC

To know the correct structure in cubic the indexing pattern of cubic crystal

was studied in detail A cubic crystal gives diffraction lines whose sin2θ values

Chapter ndash V

165

satisfy the equation obtained by combining the Braggrsquos law with the plane-spacing

equation for the cubic system as

sin2θ (h

2+k

2+l

2) = sin

2θ s = λ

2 4 a

2 --- 52

Since the sum (s) =

(h

2+k

2+l

2) is always integral and λ

2 4a

2 is a constant for any

one pattern of a cubic substance is one of finding a set of integers (s) which will

yield a constant quotient when divided one by one into the observed sin2θ values

Once the proper integers (s) are found the indices (hkl) of each line can be written

down by inspection The proper set of integers (s) is not hard to find because there

are only a few possible sets [30] Each of the four common cubic lattice types has

a characteristic sequence of diffraction lines described by their sequential values

Simple cubic 1 2 3 4 5 6 7 8 9 10 11 12 13 14 16 -------

Body-centered cubic 2 4 6 8 10 12 14 16 -------

Face-centered cubic 3 4 8 11 12 16 --------

In our case the crystal system was found to be face centered cubic (FCC) because

the integers(s) 3 4 8 11 which matches the FCC diffractions lines

Fig 56 Fluorite structure of CeO2

Chapter ndash V

166

The unit cell of ceria is shown in Fig56 In the face centered cubic (FCC)

structure of ceria Ce4+

ions form a cubic close packing arrangement and all the

tetrahedral sites are occupied by the oxide ions whereas the octahedral sites remain

vacant The unit cell of ceria can be considered as a simple cube in which the face

center positions and corners are occupied by Ce4+

ions The tetrahedral sites can be

visualized by dividing the cube into eight smaller cubes The body center positions

of all the small cubes are occupied by oxide ions and the alternate corners are

occupied by Ce4+

ions [31]

Fig57 βcos θ λ vs sin θ λ for CeO2 nanocrystals

The particle size of CeO2 nanoparticles was calculated by using Scherrerrsquos

formula The lattice strain and crystalline size was calculated from the following

equation [18]

β cos θ λ = (1 ε) + (τ sin θ λ) --- 53

Chapter ndash V

167

where β is the measured FWHM (Full Width at Half Maximum) in radian θ is the

Bragg angle of the peak λ is wavelength of X-ray (Cr Kα line 228 Aring) ε is the

effective particle size and τ is the effective strain The average particle size can be

estimated from the extrapolation of the Williamson-Hall plot shown in Fig 57

and the particle size was obtained 9 nm based on the intercept inverse

ie 1 ε = 0011 X 108

which yields ε = 909 X 10-9

m or 9 nm

534 Energy Dispersive Analysis of X-Rays (EDAX)

The cerium oxide nanoparticles were characterized by energy dispersive

X- ray analysis (EDAX) which is attachment of SEM The sample for EDAX was

prepared by depositing a very thin layer of cerium oxide nanoparticles on glass

slide The dispersive X ndash ray analysis was conducted by focusing an electron beam

on several different sectioned regions of copper nanoparticles

Fig58 EDAX spectrum of CeO2 nanoparticles

Chapter ndash V

168

The EDAX spectrum of cerium oxide nano particles synthesized by

microwave assisted sol-gel method is shown in Fig58 the composition of sample

shows the presense of cerium and oxygen elements The spectrum peak reveals the

presence of Ce and O at 4837 and 0525 keV respectively The atomic of Ce

and O is 3597 and 6403 respectively which confirms the presence of Ce and O in

the powder The present composition of Ce and O reveals that the formation of

non-stoichometric CeO2 which is superior for adsorption of toxic ions from

wastewater

535 Scanning Electron Microscopic (SEM) Analysis

The model JEOL JSM-6360 was used for the determination of morphology

of nanoparticle The cerium oxide nanoparticles were dispersed in isopropyl

alcohol and sonicated for frac12 hour One or two drop of sonicated solution was kept

on 1x1 cm glass plate dried naturally and coated the platinum The SEM images

are formed by scanning narrow beam of electrons across the surface of specimen

Fig59 (a) SEM micrograph of CeO2 nanoparticles by microwave

(without annealed)

Chapter ndash V

169

Collecting and processing emitting electron and displaying them on cathode ray

tubecomputer monitor Using a visual raster which is synchronized with the

beams scanned The dried thin films samples of cerium oxide nanoparticles were

scanned by SEM The SEM images of cerium oxide synthesized by microwave

(without annealed) and annealed at 300degC are shown in Figs59 (a) and (b) From

SEM images revealed that there were various sizes of particles in the as-prepared

sample The large particles are composed of small crystallites and show particle

aggregates of irregular shapes The particles are spherical in shapes which are

good agreement with XRD results of particles size These observations were

consistent with similar observations reported in the literature [42ndash44]

Fig59 (b) SEM micrograph of CeO2 nanoparticles annealed at 300degC

536 Transmission Electron Microscopic (TEM) Analysis

In case of cerium oxide the transmission electron micrograph was obtained

by employing Philips microscopy IIT Madras TEM is analogous to the optical

microscope It provides very high resolution which can reach approximately

Chapter ndash V

170

01 nm in the case of lattice images Consequently very high magnification can be

obtained TEM is used to examine very thin section through materials of the

surface A transmission electron microscope Philips 120 kV is equipped with CCD

camera This instrument has the resolution of 036 nm with 42 to 120 kV operating

voltage and magnifies object upto 6 lakhs times in high resolution mode Samples

for TEM examinations were prepared by placing a drop of sample suspension on a

copper grade coated with carbon film and were allowed to dry in air A typical

TEM image of CeO2 powders synthesized by cerium nitrate salts by microwave

assisted sol-gel method (without calcination) is presented in Fig 510 The mean

diameter of the particles as measured is about 10 nm These results are in good

agreement with that of estimated by X-ray line broadening Also the synthesized

Fig510 TEM image of the CeO2 nanoparticles

nanopowder is well crystallized without defects non-agglomerated with narrow

size distributions The selected area electron diffraction (SAED) pattern shown in

the Fig511 it also supports the d values obtained from XRD with rings which

can be assigned [111] [200] [220] and [311] diffractions of CeO2 structure

Chapter ndash V

171

Fig511 SAED image of the CeO2 nanoparticles

537 Thermo-Gravimetric and Differential Thermal Analysis

Fig512 shows the TG-DTA curves for the dried ceria precursor prepared

by the Ce(IV) methods The dried samples may consist of three portions of

materials (A) crystalline ceria CeO2 (B) crystalline ceria with structural water

CeO22H2O and (C) amorphous cerium hydroxide Ce(OH)4 The endothermic

peaks in 64degC could be due to crystallization of the amorphous portion in the

sample Ce(OH)4 to crystalline CeO2 The larger endothermic peak for the sample

indicates that the former contains more amorphous phase than the latter The total

weight loss up to 800 degC for the sample is about 1685 (or 194 for n which is

the number of water molecules per mole of cerium in the dried sample) This is

consistent with the change of [Ce (OH)4 + CeO22H2O] to [CeO2] The two

endothermic peaks at about 313 and 436degC in figure can be attributed to the loss of

the structural water molecules The less obvious endothermic peaks for the

structural water loss for this sample also agree with the weight loss data These

results suggest that the Ce(IV) derived sample contains primarily Ce(OH)4 and

CeO22H2O

Chapter ndash V

172

Fig 512 Weight loss of CeO2 nanoparticles as a function of temperature

538 Electrical Conductivity of Semiconductors CeO2

Semiconductors find wide appilication particularly in electronics Their

application is based on certain properties that will be an electrical conductivity of

semiconductors When the temperature is not very low the semiconductor will

conduct electricity due to the movement of the electrons in the conduction band

and that of holes in the valence band The conductivity of nano CeO2 thick film is

represented in Fig513 From the figure the almost linear relationship was found

in the measured temperature range of 200-500oC If it assumed that conduction in

crystalline ceria is essentially an n-type electronic process then using a simple

model we can express the conductivity-temperature relation by the equation

σ = σo e-Ea kT

where σo is a constant Ea is thermal activation energy T is the

Kelvin temperature and k is Boltzmanns constant Thus an Arrhenius plot of

conductivity and temperature should be linear over certain temperature range

Although the results in Fig513 are curved a linear section can be drawn above

280oC which gives activation energy of 236 eV

Chapter ndash V

173

Fig513 Electrical conductivity of CeO2 nanoparticles as a function of temperature

539 Gas Sensing Performance

5391 LPG Gas Sensing

Fig514 shows the variation of response of CeO2 fired at various

temperature The gas response increases with temperature from 50 to 150degC and

then decreases with a further increase in temperature In present work every time

prior to exposing the CeO2 film to LPG it was allowed to stabilize at an operating

temperature for 10 min and the stabilized resistance was taken as Ra After

exposing the film to the LPG gas the changed resistance was taken as Rg LPG

reacts with surface oxygen ions of the film Oxidation of film decreases the

number of free carriers Therefore resistance of the film increases with gas The

result of reaction of LPG with polycrystalline CeO2 is adsorbed CO2 and H2O on

the surface of the oxide The Ce harrLPG interactions on CeO2 are strong and Ce

sites probably get oxidized

Chapter ndash V

174

Fig 514 Variation of gas responses with operating temperature for LPG gas

5393 Effect of Gas Concentration (active region)

The variation of sensor response of CeO2 thick film sample with LPG gas

concentration at operating temperature 150degC is represented in Fig 515

Fig 515 Variation in sensor response with LPG gas concentration

Chapter ndash V

175

It is clear from the figure that the gas response goes on increasing with gas

concentration upto 600 ppm The rate of increase in response was relatively large

upto 1000 ppm and saturated beyond 600 ppm The monolayer of the gas

molecules formed on the surface covers the whole surface of the film The gas

molecules from that layer would reach the surface active sites of the film The

excess gas molecules remain idle and would not reach the surface active sites of

the sensor Thus the active region of the sensor would be up to 600 ppm

5394 Response and Recovery Time

The response of CeO2 thick film sensor was found to be quick (~ 28 s) to

600 ppm of while the recovery was fast (~ 10 s) (Fig 516) The fast response

may be attributed to faster oxidation of the gas The negligible quantity of the

surface reaction product and its high volatility explains its fast response and quick

recovery to its initial chemical status

Fig 516 Response and recovery time

Chapter ndash V

176

5395 Oxygen Adsorption-Desorption Mechanism

O2 (air) + 2e- 2O

-(film surface) --- 56

At higher temperature the atmospheric oxygen O2 adsorbs on the surface of the

thick film It captures the electrons (Fig517) from conduction band It would

result in decreasing conductivity of the film When LPG reacts with the adsorbed

oxygen on the surface of the film it gets oxidized to CO2 and H2O by following

series of intermediate stages This liberates free electrons in the conduction band

The final reaction takes place as

C4H10(gas) + 13Ominus

(filmsurface) rarr 4CO2(gas) + 5H2O(gas) + 13eminus

(condband) --- 57

Fig 517 Sensing mechanism on the surface of the film

This shows n-type conduction mechanism Thus generated electrons

contribute to a sudden increase in conductance of the thick film The mass of

Ce and O in sample was not as per the stoichiometric proportion and sample was

Chapter ndash V

177

observed to be the oxygen deficient This deficiency gets reduced due to

adsorption of atmosphericmolecular oxygen This helps in decreasing electronic

conductivity of the film Upon exposure LPG molecules got oxidized with the

adsorbed oxygen ions by following the series of intermediate stages producing

CO2 and H2O This results in evolving oxygen as electrically neutral atoms

trapping behind the negative charges (electrons) Upon exposure the energy

released in decomposition of LPG molecules would be sufficient for trapped

electrons to jump into the conduction band of activated CeO2 resulting in increase

in the conductivity of the film The drastic increase in conductivity of the sensor

could be attributed to the adsorptionndashdesorption mechanism resulted from the

electronic defects [26]

5310 Catalytic Activity

The utility of this catalytic system deprotection of several oximes were

studied (Table 52) The catalyst was also tested for reusability and it was found

that it could be reused without loss of initial activity for at least four cycles [19]

TLC analysis indicated the formation of only one product of corresponding

carbonyls FT-IR and NMR analysis showed that the oximes were converted into

carbonyls For checking the reusability (after each run) the catalyst was recovered

by simple filtration and washed several times with acetone and then dried at

373 K The catalyst thus obtained was used for the subsequent runs without any

further modification both activated and deactivated oximes were oxidized in better

yield more than ZrO2 catalyst as shown in Table 52

Chapter ndash V

178

Table 52 Oxidative Deoximation of Carbonyls on CeO2

Entry

Substrate

Product

Time

(h)

Yield

()

1

NOH

O

15

62

2

NOH

Cl

O

Cl

15

58

3

NOH

O

25

73

4

NOHCl

OCl

40

57

5

NOH

Cl

O

Cl

30

59

Solvent- Ethyl acetate Catalyst- ZrO2 Temperature- 353 K

Chapter ndash V

179

5311 Adsorption of Rhodamine B (Rh B)

The removal of dye pollutants are also of great importance for wastewater

treatment Rhodamine B a common cationic dye used in textile industry is one of

the most notorious contaminants in aquatic environments because of it huge

amounts slow biodegradation and toxicity In this study synthesized CeO2

nanocrystals are further used to remove Rh B The absorption spectrum of Rh B

solution was characterized by its characteristic absorption at 555 nm which was

attributed to the chromophore containing azo linkage of the dye molecules [22]

Fig 518 Absorption spectra of aqueous solution of Rh B in the presence of CeO2

nanocrystals at different times

UV-Visible absorption spectra represented in Fig518 confirms that over 93 of

Rh B in 100 mL of 20 mg L-1

Rh B aqueous solution could be removed with 100

mg of the as-prepared CeO2 nanocrystalline within 60 min The reasons may

account for the efficient removal of Rh B with CeO2 nanocrystals The first reason

Chapter ndash V

180

should be their to facilitate the adsorption of Rh B The second one is attributed to

the oxygen vacancies on the surface of CeO2 nanocrystals These abundant surface

oxygen vacancies could produce strong electrostatic attraction with the cationic

groups of Rh B as well as hydrogen bonding with the nitrogen atoms of Rh B

Chapter ndash V

181

Conclusions

Microwave synthesis is very beneficial to find a fast simple and energy

efficient approach to produce fine CeO2 nanoparticles It is relatively new method

to produce inorganic compounds for materials processing to enhance the material

properties as well as economic advantages through energy saving and acceleration

of product development Cerium oxide has outstanding physical and chemical

properties therefore it is used as LPG sensor In the area of catalysis nanoparticles

of ceria have been studied Also it is concluded as

1] The CeO2 nanoparticles were prepared by simple microwave assisted sol-gel

method

2] The well spherical and narrow size distribution with 10 nm size particles were

obtained by this method

3] It is very simple time as well as energy saving technique

4] The XRD pattern shows the CeO2 nanoparticles exists in face centered cubic

5] Scanning electron micrographs indicates that grains are uniformaly distributed

6] Transmission electron micrographs indicates that the mean diameter of the

particles as measured is about 10 nm

7] The conductivity of nano CeO2 thick film is represents the almost linear

relationship was found in the measured temperature range of 200-5000C It is

clear that the conduction in crystalline ceria is essentially an n-type electronic

process

8] The CeO2 nanoparticles in the thick film form prepared by screen printing

technique showed better response to LPG gas at 250degC temperature at 600 ppm

concentration of LPG gas

9] Catalyst can be prepared from inexpensive precursors using the microwave

assisted sol-gel technique is promising for the oxidative regeneration of ketones

10] This study reveals that these CeO2 nanocrystals are novel adsorbents for

wastewater treatment

Chapter ndash V

182

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56 (2007) 90

[2] C Lettmann K Hildenbrand H Kisch W Macyk W F Maier

Appl Catal B 32 (2001) 215

[3] X Zhang Z H Ai F L Jia L Z Zhang J Phys Chem C 112 (2008) 747

[4] N Du H Zhang BD Chen X Y Ma D RYang J Phys ChemC

111 (2007) 12677

[5] F Zhang S Yang H Chen X Yu Cera Intern 30 (2004) 997

[6] F Gu SF Wang MK Lu GJ Zhou D Xu DR Langmuir J SciIR Iran

20 (2004) 3528

[7] H Xiao Z Ai L Zhang JPhys Chem 113 (2009) 16625

[8] L Combemale G Caboche D Stuerga D Chaumont MicroRese

Bull 40 (2005) 529

[9] M Zawadzki Jof Alloys and Comp 454 (2008) 347

[10] A V Patil C G Dighavkar S K Sonawane S J Patil R Y Borse

Sens and Transducers 108 (2009) 189

[11] AV Patil CG Dighavkar RY Borse Sens and Transducers

101 (2009) 96

[12] AV Patil CG Dighavkar SK Sonawane S J Patil RY Borse Sensand

Transducers 9 (2010) 11

[13] DR Patil LA Patil Talanta 77 (2009)1409

[14] R S Blackburn Environ Sci Tech 38 (2004) 4905

[15] GNaja CMustin JBerthelin BVolesky J Colloide Inter Sci

292 (2005) 537

[16] VKGupta IAli Environ Sci Tech 42 (2008) 766

[17] A Mittal VK Gupta A Malviya and JMittal J Hazard Mater

151 (2008) 821

[18] VKGupta IAli VKSaini J Colloid Interface Sci 315(2007) 87

Chapter ndash V

183

[19] S S Deshpande S U Sonavane R V Jayaram Cat Com 9 (2008) 639

[20] KM Garadkar BS Shirke YB Patil D R Patil Sens and Transducers

110 (2009) 17

[21] VKGupta AMittal VGajbe JMittal J Colloid Inter Sci

319 (2008) 30

[22] T A Khan I Ali V V Singh S Sharma J Envir Protec Sci 3 (2009)11

[23] A Hosseinnia M Keyanpour-Rad M Pazouki World Applied Sci J

8 (2010) 1327

[24] MI Baraton L Merhari J Am Ceram Soc 75 (2004) 1587

[25] CWang B Xu XWang J Zhao J Solid State Chem 178 (2005) 3500

[26] H Xiao Z Ai L Zhang J Phys Chem C doi 10 1021 JP9050269 (2009)

[27] A M Thompson Oxides of the Rare Earths Wiley New York (1978)

[28] H Noumlremberg G A D Briggs Phys Rev Lett 79 (1997) 4222

[29] A Hosseinnia M Keyanpour-Rad M Pazouki World Appl Sci J

8 (2010) 1327

[30] BD Cullity SR Stock Elements of X-Ray Diffraction 3rd edn

(Prentice Hall Upper Saddle River NJ) (2001) 07458

[31] G Ranga Rao B Gopal Mishra Bul Cat Soc of Ind 2 (2003)122

[32] A K Bhattacharyal T K Naiya S N Mondal S K Das Chem Eng J

137 (2008) 529

[33] G Liu J A Rodriguez Z Chang J Hrbek C H F Peden

J Phys Chem B 108 (2004) 2931

[34] P F Ji J L Zhang F Chen M Anpo J Phys Chem C 112 (2008) 17809

[35] A Trovarelli Catal Rev SciEng 38 (1996) 439

[36] B Li D M Pan J S Zheng Y J Cheng X Y Ma F Huang L Zhang

Langmuir 24 (2008) 9630

[37] M Fernaacutendez-Garcıacutea A A B Martıacutenez-Arias A Hungrıacutea Iglesias-Juez

J C Conesa J Soria J Phys Chem Chem Phys 4 (2002) 2473

[38] H M Chen J H He J Phys Chem C 112 (2008) 17540

Chapter ndash V

184

[39] F Zhang S Chan W Spanier J E Apak E Jin Q Robinson

R D Herman I P Appl Phys Lett 80 (2002) 127

[40] G Adachi TMasui In Catalysis by Ceria and Related Materials

ATrovarelli Ed Imperial College Press London (2002)

[41] J A Rodriguez X Wang J C Hanson G Liu A Iglesias-Juez

MFernaacutendez-Garcıacutea J Chem Phys 119 (2003) 5659

[42] S Eck C Castellarin-Cudia S Surnev M G Ramsey F PNetzer

Surf Sci 520 (2002) 173

[43] R B Yu L Yan P Zheng JChen X X R J Phys ChemC

112 (2008) 19896

[44] Y Nagai T Yamamoto T Tanaka SYoshida T Nonaka T Okamoto

A Suda M Sugiura Catal Today 74 (2002) 225

[45] Y M Chiang E B Lavik I Kosacki H L Tuller JY Ying

J Electroceram 1 (1997) 7

[46] D R Patil L A Patil G H Jain M S Wagh S A Patil

Sens and Transducers 74 (2006) 874

[47] K Sohlberg S K Pantelides S J Pennycook J Am Chem Soc

123 (2001) 6609

[48] L S Zhong J S Hu A M Cao Q Liu W G Song LWan

J Chem Mater 19 (2007) 1648

[49] J B Fei Y Cui X H Yan Y Yang K W Wang QHe J B Li

AdV Mater 20 (2008) 452

[50] P V Kamat J Phys Chem B 106 (2002) 7729

[51] R D Robinson J E Spanier F Zhang SW Chan I P Herman J Appl

Phys 92 (2002) 1936

[52] S W Cao Y J Zhu J Phys Chem C 112 (2008) 6253

[53] R Georgekutty M K Seery S C Pillai J Phys Chem C

112 (2008) 13563

[54] J S Hu L S Zhong W G Song L J Wan Adv Mater 20 (2008)1

Chapter ndash V

185

[55] HXu F L Jia Z H Ai L Z Zhang Cryst Growth Des (2007)

[56] H M Jia W J Xiao L Z Zhang Z F Zheng J PhysChem C

112 (2008) 1379

[57] Z H Ai H Y Xiao T Mei J Liu L Z Zhang K J Deng J R Qiu

J PhysChem C 112 (2008) 11929

[58] L M Ma W X Zhang EnViron Sci Technol 42 (2008) 5384

[59] Z H Ai Y Chen L Z Zhang J R Qiu EnViron Sci Technol

42 (2008) 69550

[60] Z Y Zhong J Ho J Teo S C Shen AGedanken ChemMater

19 (2007) 4776

[61] S C Kuiry S D Patil S Deshpande S Seal J Phys ChemB

109 (2005) 6936

[62] D R Patil L A Patil Sens IEEE 7 (2007) 434

[63] P Jasinski Anderson H U Sens Actuators B 95 (2003) 73

[64] J P Nair E Wachtel I Lubomirsky J Fleig JMaier AdVMater

15 (2003) 2077

[65] R J Qi Y J Zhu G F Cheng Y H Huang Nanotechnology

16 (2005) 2502

Chapter ndash V

158

reduction process before the appearance of Ce2O3 there is a substantial expansion

in the unit cell of the CeO2 nanoparticles as a consequence of the embedding of

hydrogen and the formation of O vacancies [50]

The fine powder of CeO2 has been prepared by many methods including

forced hydrolysis sol-gel hydrothermal coprecipitation surfactant templating

method and spray pyrolysis [51-57] However microwave synthesis is very

beneficial to find a fast simple and energy efficient approach to produce fine

CeO2 nanoparticles [20 58-63] It is relatively new method to produce inorganic

compounds for materials processing to enhance the material properties as well as

economic advantages through energy saving and acceleration of product

development [64-65]

In the present work we have studied the structural optical and electrical

properties of CeO2 nanoparticles This material was characterized by using

UV-Visible FTIR XRD SEM EDAX TEM TGA-DTA and electrical

conductivity The CeO2 nanoparticles have been investigated for gas sensing

properties for LPG gas catalytic activity for the oxidative regeneration of ketones

and novel adsorbents for wastewater treatment

52 Materials and Methods

All the chemicals used for the preparation were of analytical grade It

includes Cerium nitrate (Ce (NO3)3 6H2O) propylene glycol and ammonia All the

solutions were prepared in millipore water obtained from ultra pure water system

Monodispersed nanocrystalline CeO2 powder was prepared by controlled addition

of aqueous ammonia to a mixture of 01M aqueous solution of cerium nitrate and

propylene glycol until the solution reached pH = 10 The ratio of concentration of

propylene glycol to cerium nitrate solution was kept 11 The special arrangement

was made to add drop wise aqueous ammonia into the solution with constant

stirring After complete precipitation the precipitated hydroxide was washed with

distilled water Then pure hydroxide in a glass beaker was placed in a microwave

Chapter ndash V

159

oven (in put power 600W) about 30 minutes with on-off cycle The Phase purity

and the degree of crystallinity of the resulting CeO2 sample were monitored by

XRD analysis The schematic flow chart of preparation of nanocrystalline CeO2 is

represented in Fig51

Aqueous ammonia

darr

Ce(NO3)3 6H2O + Propylene glycol

darr Centrifugation and washing

Microwave treatment

darr

CeO2 nanocrystalline particles

Fig51 Schematic Flow Chart for the preparation of nanocrystalline CeO2

The powder Xndashray diffraction patterns were recorded on Philips PW-1710

X-ray diffractometer by using Cr Kα radiation The lattice parameters were

calculated using high angle reflection of XRD by using the following formula

--- 51

Crystallite size was calculated by using the Scherrerrsquos formula having

wavelength of X-ray (Cr Kα line 228 Aring)

The UV-Visible Spectra were recorded on UV-Visible-NIR

Spectrophotometer (UV-3600 Shimadzu Japan)

Chapter ndash V

160

FTIR spectrum was recorded with a Perkin Elmer-USA in the range of

wavenumber from 4000-400 cm-1

The SEM micrographs of the samples were obtained using the scanning

electron microscope (Model JEOL-JSM 6360) Elemental analysis was carried

out by using the energy dispersive X-ray spectroscopy equipped with Scanning

electron microscopy

TEM study of the powder sample was carried out by using model Philips

CM 12 transmission electron microscope with Energy Dispersive Spectroscopy

(EDS) detector for microanalysis (IIT-Madras)

The thermal behavior of the nanocrystalline CeO2 powder is studied upto 10

to1000degC in nitrogen using SDT Q600 V209 Build 20 Instrument

The oxidation of oxime was carried out at 353 K in presence of cerium

oxide as catalyst ethyl acetate as a solvent and H2O2 as the oxidant [19] The

liquid phase reaction was carried out in 25 ml round bottom flask equipped with a

reflux condenser In catalytic oxidation reaction oxime (10 mmol) H2O2

(20 mmol) along with catalyst (1 by wt of the substrate) and the content were

heated in pre-heated oil bath at 353 K

NOH

R

O

R

H2O

2

CeO2

Oxime Ketone

Where R = CH3 C6H5

One hundred milligrams of CeO2 nanocrystal microspheres was suspended

in 100 mL of rhodamine B aqueous solution with a concentration of 20 mg L-1

in a

flask under stirring The flask was covered with carbon paper to prevent the

Chapter ndash V

161

photodegradation of Rh B At different intervals 5mL of the suspensions was

collected filtered through a 045μm membrane and finally analyzed by a

UV-visible spectrophotometer immediately [22]

53 Results and Discussion

531 UV-Visible Spectroscopic Analysis

The absorption spectra of CeO2 nanoparticles dispersed in ethanol solution

is shown in Fig52 The absorption edge of the CeO2 nanoparticles synthesized

a) by microwave (without annealed) annealed at b) 200degC and c) 300degC occurs at

around 400 nm and its band gap energy is estimated to be 31 eV A sharp band is

an indicative of narrow distribution of particles within the matrix The accurate

band gap of material was 31 eV as obtained by plotting (αhυ)2

against hυ shown

in Fig53

Fig 52 UV-Visible absorption spectrum of CeO2 nanoparticles synthesized

a) by microwave (without annealed) annealed at b) 200degC and c) 300degC

Chapter ndash V

162

In semiconductors the band gap will be increased as particle size is

decreased As particles shrink in size there is a dramatic change in valence band

and conduction bands as the continuous density of states in bulk is replaced with a

set of discrete energy levels This leads to interesting optical properties In the

visible region colour can changes with size Since with increase in size energy

decreases the nanoparticles which have size less than the bulk compound shows

blue shift in the UV-Visible spectra of nanoparticles This blue shift may be

considered as one of the confirmatory test for the existence of nanoparticles

Fig53 Variation of (αhυ) 2

vs Photon Energy (eV) of CeO2 nanoparticle synthesized

a) by microwave (without annealed) annealed at b)200degC and c) 300degC

532 FTIR Analysis

The FT-IR spectrum of the obtained ceria nanoparticles is shown in

Fig 54 Three strong absorption peaks were observed at 3381 1558 and

1372 cm-1

The former was attributed to the stretching band of hydroxyl group and

the others were attributed to the antisymmetric and symmetric stretching band of

Chapter ndash V

163

the glycol group respectively In the present synthesis propylene glycol works as

a protective agent against particle growth and since the stretching band of the

carbonyl around 1700 cm-1

was not observed in the FT-IR spectrum it could be

that the surface of the ceria nanoparticles was covered with glycol species In

addition it has also been reported that non-stoichiometric cerium oxide

synthesized by an inert gas condensation process was brownish yellow

Fig54 FTIR spectrum of CeO2 nanoparticles

533 The XRD Analysis

Fig55 shows the X-ray diffractograms of the as prepared powder of CeO2

nanoparticles a) by microwave (without annealed) annealed at b)200degC and

c) 300degC The CeO2 nanoparticles are crystalline in nature with d values 313

270 191 and 163 Aring The synthesized samples by microwave confirmed the

Chapter ndash V

164

formation of face centered cubic structure (FCC) of CeO2 nanoparticles The

determined characteristics 2θ values and [hkl] planes are 4287deg [111] 5017deg

[200] 7386deg [220] and 89deg [311] respectively (JCPDS card No 81-0792) and

lattice parameter is 542 Aring The average particle size of CeO2 nanoparticles was

found to about 10 nm The particle size of CeO2 nanoparticles obtained by

microwave is less than that of annealed at 200degC and 300degC However the lattice

parameter is found to increase with reduction temperature indicating an expansion

in the FCC lattice

Fig 55 XRD patterns of the CeO2 nanoparticles synthesized a) by microwave

(without annealed) annealed at b) 200degC and c) 300degC

To know the correct structure in cubic the indexing pattern of cubic crystal

was studied in detail A cubic crystal gives diffraction lines whose sin2θ values

Chapter ndash V

165

satisfy the equation obtained by combining the Braggrsquos law with the plane-spacing

equation for the cubic system as

sin2θ (h

2+k

2+l

2) = sin

2θ s = λ

2 4 a

2 --- 52

Since the sum (s) =

(h

2+k

2+l

2) is always integral and λ

2 4a

2 is a constant for any

one pattern of a cubic substance is one of finding a set of integers (s) which will

yield a constant quotient when divided one by one into the observed sin2θ values

Once the proper integers (s) are found the indices (hkl) of each line can be written

down by inspection The proper set of integers (s) is not hard to find because there

are only a few possible sets [30] Each of the four common cubic lattice types has

a characteristic sequence of diffraction lines described by their sequential values

Simple cubic 1 2 3 4 5 6 7 8 9 10 11 12 13 14 16 -------

Body-centered cubic 2 4 6 8 10 12 14 16 -------

Face-centered cubic 3 4 8 11 12 16 --------

In our case the crystal system was found to be face centered cubic (FCC) because

the integers(s) 3 4 8 11 which matches the FCC diffractions lines

Fig 56 Fluorite structure of CeO2

Chapter ndash V

166

The unit cell of ceria is shown in Fig56 In the face centered cubic (FCC)

structure of ceria Ce4+

ions form a cubic close packing arrangement and all the

tetrahedral sites are occupied by the oxide ions whereas the octahedral sites remain

vacant The unit cell of ceria can be considered as a simple cube in which the face

center positions and corners are occupied by Ce4+

ions The tetrahedral sites can be

visualized by dividing the cube into eight smaller cubes The body center positions

of all the small cubes are occupied by oxide ions and the alternate corners are

occupied by Ce4+

ions [31]

Fig57 βcos θ λ vs sin θ λ for CeO2 nanocrystals

The particle size of CeO2 nanoparticles was calculated by using Scherrerrsquos

formula The lattice strain and crystalline size was calculated from the following

equation [18]

β cos θ λ = (1 ε) + (τ sin θ λ) --- 53

Chapter ndash V

167

where β is the measured FWHM (Full Width at Half Maximum) in radian θ is the

Bragg angle of the peak λ is wavelength of X-ray (Cr Kα line 228 Aring) ε is the

effective particle size and τ is the effective strain The average particle size can be

estimated from the extrapolation of the Williamson-Hall plot shown in Fig 57

and the particle size was obtained 9 nm based on the intercept inverse

ie 1 ε = 0011 X 108

which yields ε = 909 X 10-9

m or 9 nm

534 Energy Dispersive Analysis of X-Rays (EDAX)

The cerium oxide nanoparticles were characterized by energy dispersive

X- ray analysis (EDAX) which is attachment of SEM The sample for EDAX was

prepared by depositing a very thin layer of cerium oxide nanoparticles on glass

slide The dispersive X ndash ray analysis was conducted by focusing an electron beam

on several different sectioned regions of copper nanoparticles

Fig58 EDAX spectrum of CeO2 nanoparticles

Chapter ndash V

168

The EDAX spectrum of cerium oxide nano particles synthesized by

microwave assisted sol-gel method is shown in Fig58 the composition of sample

shows the presense of cerium and oxygen elements The spectrum peak reveals the

presence of Ce and O at 4837 and 0525 keV respectively The atomic of Ce

and O is 3597 and 6403 respectively which confirms the presence of Ce and O in

the powder The present composition of Ce and O reveals that the formation of

non-stoichometric CeO2 which is superior for adsorption of toxic ions from

wastewater

535 Scanning Electron Microscopic (SEM) Analysis

The model JEOL JSM-6360 was used for the determination of morphology

of nanoparticle The cerium oxide nanoparticles were dispersed in isopropyl

alcohol and sonicated for frac12 hour One or two drop of sonicated solution was kept

on 1x1 cm glass plate dried naturally and coated the platinum The SEM images

are formed by scanning narrow beam of electrons across the surface of specimen

Fig59 (a) SEM micrograph of CeO2 nanoparticles by microwave

(without annealed)

Chapter ndash V

169

Collecting and processing emitting electron and displaying them on cathode ray

tubecomputer monitor Using a visual raster which is synchronized with the

beams scanned The dried thin films samples of cerium oxide nanoparticles were

scanned by SEM The SEM images of cerium oxide synthesized by microwave

(without annealed) and annealed at 300degC are shown in Figs59 (a) and (b) From

SEM images revealed that there were various sizes of particles in the as-prepared

sample The large particles are composed of small crystallites and show particle

aggregates of irregular shapes The particles are spherical in shapes which are

good agreement with XRD results of particles size These observations were

consistent with similar observations reported in the literature [42ndash44]

Fig59 (b) SEM micrograph of CeO2 nanoparticles annealed at 300degC

536 Transmission Electron Microscopic (TEM) Analysis

In case of cerium oxide the transmission electron micrograph was obtained

by employing Philips microscopy IIT Madras TEM is analogous to the optical

microscope It provides very high resolution which can reach approximately

Chapter ndash V

170

01 nm in the case of lattice images Consequently very high magnification can be

obtained TEM is used to examine very thin section through materials of the

surface A transmission electron microscope Philips 120 kV is equipped with CCD

camera This instrument has the resolution of 036 nm with 42 to 120 kV operating

voltage and magnifies object upto 6 lakhs times in high resolution mode Samples

for TEM examinations were prepared by placing a drop of sample suspension on a

copper grade coated with carbon film and were allowed to dry in air A typical

TEM image of CeO2 powders synthesized by cerium nitrate salts by microwave

assisted sol-gel method (without calcination) is presented in Fig 510 The mean

diameter of the particles as measured is about 10 nm These results are in good

agreement with that of estimated by X-ray line broadening Also the synthesized

Fig510 TEM image of the CeO2 nanoparticles

nanopowder is well crystallized without defects non-agglomerated with narrow

size distributions The selected area electron diffraction (SAED) pattern shown in

the Fig511 it also supports the d values obtained from XRD with rings which

can be assigned [111] [200] [220] and [311] diffractions of CeO2 structure

Chapter ndash V

171

Fig511 SAED image of the CeO2 nanoparticles

537 Thermo-Gravimetric and Differential Thermal Analysis

Fig512 shows the TG-DTA curves for the dried ceria precursor prepared

by the Ce(IV) methods The dried samples may consist of three portions of

materials (A) crystalline ceria CeO2 (B) crystalline ceria with structural water

CeO22H2O and (C) amorphous cerium hydroxide Ce(OH)4 The endothermic

peaks in 64degC could be due to crystallization of the amorphous portion in the

sample Ce(OH)4 to crystalline CeO2 The larger endothermic peak for the sample

indicates that the former contains more amorphous phase than the latter The total

weight loss up to 800 degC for the sample is about 1685 (or 194 for n which is

the number of water molecules per mole of cerium in the dried sample) This is

consistent with the change of [Ce (OH)4 + CeO22H2O] to [CeO2] The two

endothermic peaks at about 313 and 436degC in figure can be attributed to the loss of

the structural water molecules The less obvious endothermic peaks for the

structural water loss for this sample also agree with the weight loss data These

results suggest that the Ce(IV) derived sample contains primarily Ce(OH)4 and

CeO22H2O

Chapter ndash V

172

Fig 512 Weight loss of CeO2 nanoparticles as a function of temperature

538 Electrical Conductivity of Semiconductors CeO2

Semiconductors find wide appilication particularly in electronics Their

application is based on certain properties that will be an electrical conductivity of

semiconductors When the temperature is not very low the semiconductor will

conduct electricity due to the movement of the electrons in the conduction band

and that of holes in the valence band The conductivity of nano CeO2 thick film is

represented in Fig513 From the figure the almost linear relationship was found

in the measured temperature range of 200-500oC If it assumed that conduction in

crystalline ceria is essentially an n-type electronic process then using a simple

model we can express the conductivity-temperature relation by the equation

σ = σo e-Ea kT

where σo is a constant Ea is thermal activation energy T is the

Kelvin temperature and k is Boltzmanns constant Thus an Arrhenius plot of

conductivity and temperature should be linear over certain temperature range

Although the results in Fig513 are curved a linear section can be drawn above

280oC which gives activation energy of 236 eV

Chapter ndash V

173

Fig513 Electrical conductivity of CeO2 nanoparticles as a function of temperature

539 Gas Sensing Performance

5391 LPG Gas Sensing

Fig514 shows the variation of response of CeO2 fired at various

temperature The gas response increases with temperature from 50 to 150degC and

then decreases with a further increase in temperature In present work every time

prior to exposing the CeO2 film to LPG it was allowed to stabilize at an operating

temperature for 10 min and the stabilized resistance was taken as Ra After

exposing the film to the LPG gas the changed resistance was taken as Rg LPG

reacts with surface oxygen ions of the film Oxidation of film decreases the

number of free carriers Therefore resistance of the film increases with gas The

result of reaction of LPG with polycrystalline CeO2 is adsorbed CO2 and H2O on

the surface of the oxide The Ce harrLPG interactions on CeO2 are strong and Ce

sites probably get oxidized

Chapter ndash V

174

Fig 514 Variation of gas responses with operating temperature for LPG gas

5393 Effect of Gas Concentration (active region)

The variation of sensor response of CeO2 thick film sample with LPG gas

concentration at operating temperature 150degC is represented in Fig 515

Fig 515 Variation in sensor response with LPG gas concentration

Chapter ndash V

175

It is clear from the figure that the gas response goes on increasing with gas

concentration upto 600 ppm The rate of increase in response was relatively large

upto 1000 ppm and saturated beyond 600 ppm The monolayer of the gas

molecules formed on the surface covers the whole surface of the film The gas

molecules from that layer would reach the surface active sites of the film The

excess gas molecules remain idle and would not reach the surface active sites of

the sensor Thus the active region of the sensor would be up to 600 ppm

5394 Response and Recovery Time

The response of CeO2 thick film sensor was found to be quick (~ 28 s) to

600 ppm of while the recovery was fast (~ 10 s) (Fig 516) The fast response

may be attributed to faster oxidation of the gas The negligible quantity of the

surface reaction product and its high volatility explains its fast response and quick

recovery to its initial chemical status

Fig 516 Response and recovery time

Chapter ndash V

176

5395 Oxygen Adsorption-Desorption Mechanism

O2 (air) + 2e- 2O

-(film surface) --- 56

At higher temperature the atmospheric oxygen O2 adsorbs on the surface of the

thick film It captures the electrons (Fig517) from conduction band It would

result in decreasing conductivity of the film When LPG reacts with the adsorbed

oxygen on the surface of the film it gets oxidized to CO2 and H2O by following

series of intermediate stages This liberates free electrons in the conduction band

The final reaction takes place as

C4H10(gas) + 13Ominus

(filmsurface) rarr 4CO2(gas) + 5H2O(gas) + 13eminus

(condband) --- 57

Fig 517 Sensing mechanism on the surface of the film

This shows n-type conduction mechanism Thus generated electrons

contribute to a sudden increase in conductance of the thick film The mass of

Ce and O in sample was not as per the stoichiometric proportion and sample was

Chapter ndash V

177

observed to be the oxygen deficient This deficiency gets reduced due to

adsorption of atmosphericmolecular oxygen This helps in decreasing electronic

conductivity of the film Upon exposure LPG molecules got oxidized with the

adsorbed oxygen ions by following the series of intermediate stages producing

CO2 and H2O This results in evolving oxygen as electrically neutral atoms

trapping behind the negative charges (electrons) Upon exposure the energy

released in decomposition of LPG molecules would be sufficient for trapped

electrons to jump into the conduction band of activated CeO2 resulting in increase

in the conductivity of the film The drastic increase in conductivity of the sensor

could be attributed to the adsorptionndashdesorption mechanism resulted from the

electronic defects [26]

5310 Catalytic Activity

The utility of this catalytic system deprotection of several oximes were

studied (Table 52) The catalyst was also tested for reusability and it was found

that it could be reused without loss of initial activity for at least four cycles [19]

TLC analysis indicated the formation of only one product of corresponding

carbonyls FT-IR and NMR analysis showed that the oximes were converted into

carbonyls For checking the reusability (after each run) the catalyst was recovered

by simple filtration and washed several times with acetone and then dried at

373 K The catalyst thus obtained was used for the subsequent runs without any

further modification both activated and deactivated oximes were oxidized in better

yield more than ZrO2 catalyst as shown in Table 52

Chapter ndash V

178

Table 52 Oxidative Deoximation of Carbonyls on CeO2

Entry

Substrate

Product

Time

(h)

Yield

()

1

NOH

O

15

62

2

NOH

Cl

O

Cl

15

58

3

NOH

O

25

73

4

NOHCl

OCl

40

57

5

NOH

Cl

O

Cl

30

59

Solvent- Ethyl acetate Catalyst- ZrO2 Temperature- 353 K

Chapter ndash V

179

5311 Adsorption of Rhodamine B (Rh B)

The removal of dye pollutants are also of great importance for wastewater

treatment Rhodamine B a common cationic dye used in textile industry is one of

the most notorious contaminants in aquatic environments because of it huge

amounts slow biodegradation and toxicity In this study synthesized CeO2

nanocrystals are further used to remove Rh B The absorption spectrum of Rh B

solution was characterized by its characteristic absorption at 555 nm which was

attributed to the chromophore containing azo linkage of the dye molecules [22]

Fig 518 Absorption spectra of aqueous solution of Rh B in the presence of CeO2

nanocrystals at different times

UV-Visible absorption spectra represented in Fig518 confirms that over 93 of

Rh B in 100 mL of 20 mg L-1

Rh B aqueous solution could be removed with 100

mg of the as-prepared CeO2 nanocrystalline within 60 min The reasons may

account for the efficient removal of Rh B with CeO2 nanocrystals The first reason

Chapter ndash V

180

should be their to facilitate the adsorption of Rh B The second one is attributed to

the oxygen vacancies on the surface of CeO2 nanocrystals These abundant surface

oxygen vacancies could produce strong electrostatic attraction with the cationic

groups of Rh B as well as hydrogen bonding with the nitrogen atoms of Rh B

Chapter ndash V

181

Conclusions

Microwave synthesis is very beneficial to find a fast simple and energy

efficient approach to produce fine CeO2 nanoparticles It is relatively new method

to produce inorganic compounds for materials processing to enhance the material

properties as well as economic advantages through energy saving and acceleration

of product development Cerium oxide has outstanding physical and chemical

properties therefore it is used as LPG sensor In the area of catalysis nanoparticles

of ceria have been studied Also it is concluded as

1] The CeO2 nanoparticles were prepared by simple microwave assisted sol-gel

method

2] The well spherical and narrow size distribution with 10 nm size particles were

obtained by this method

3] It is very simple time as well as energy saving technique

4] The XRD pattern shows the CeO2 nanoparticles exists in face centered cubic

5] Scanning electron micrographs indicates that grains are uniformaly distributed

6] Transmission electron micrographs indicates that the mean diameter of the

particles as measured is about 10 nm

7] The conductivity of nano CeO2 thick film is represents the almost linear

relationship was found in the measured temperature range of 200-5000C It is

clear that the conduction in crystalline ceria is essentially an n-type electronic

process

8] The CeO2 nanoparticles in the thick film form prepared by screen printing

technique showed better response to LPG gas at 250degC temperature at 600 ppm

concentration of LPG gas

9] Catalyst can be prepared from inexpensive precursors using the microwave

assisted sol-gel technique is promising for the oxidative regeneration of ketones

10] This study reveals that these CeO2 nanocrystals are novel adsorbents for

wastewater treatment

Chapter ndash V

182

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56 (2007) 90

[2] C Lettmann K Hildenbrand H Kisch W Macyk W F Maier

Appl Catal B 32 (2001) 215

[3] X Zhang Z H Ai F L Jia L Z Zhang J Phys Chem C 112 (2008) 747

[4] N Du H Zhang BD Chen X Y Ma D RYang J Phys ChemC

111 (2007) 12677

[5] F Zhang S Yang H Chen X Yu Cera Intern 30 (2004) 997

[6] F Gu SF Wang MK Lu GJ Zhou D Xu DR Langmuir J SciIR Iran

20 (2004) 3528

[7] H Xiao Z Ai L Zhang JPhys Chem 113 (2009) 16625

[8] L Combemale G Caboche D Stuerga D Chaumont MicroRese

Bull 40 (2005) 529

[9] M Zawadzki Jof Alloys and Comp 454 (2008) 347

[10] A V Patil C G Dighavkar S K Sonawane S J Patil R Y Borse

Sens and Transducers 108 (2009) 189

[11] AV Patil CG Dighavkar RY Borse Sens and Transducers

101 (2009) 96

[12] AV Patil CG Dighavkar SK Sonawane S J Patil RY Borse Sensand

Transducers 9 (2010) 11

[13] DR Patil LA Patil Talanta 77 (2009)1409

[14] R S Blackburn Environ Sci Tech 38 (2004) 4905

[15] GNaja CMustin JBerthelin BVolesky J Colloide Inter Sci

292 (2005) 537

[16] VKGupta IAli Environ Sci Tech 42 (2008) 766

[17] A Mittal VK Gupta A Malviya and JMittal J Hazard Mater

151 (2008) 821

[18] VKGupta IAli VKSaini J Colloid Interface Sci 315(2007) 87

Chapter ndash V

183

[19] S S Deshpande S U Sonavane R V Jayaram Cat Com 9 (2008) 639

[20] KM Garadkar BS Shirke YB Patil D R Patil Sens and Transducers

110 (2009) 17

[21] VKGupta AMittal VGajbe JMittal J Colloid Inter Sci

319 (2008) 30

[22] T A Khan I Ali V V Singh S Sharma J Envir Protec Sci 3 (2009)11

[23] A Hosseinnia M Keyanpour-Rad M Pazouki World Applied Sci J

8 (2010) 1327

[24] MI Baraton L Merhari J Am Ceram Soc 75 (2004) 1587

[25] CWang B Xu XWang J Zhao J Solid State Chem 178 (2005) 3500

[26] H Xiao Z Ai L Zhang J Phys Chem C doi 10 1021 JP9050269 (2009)

[27] A M Thompson Oxides of the Rare Earths Wiley New York (1978)

[28] H Noumlremberg G A D Briggs Phys Rev Lett 79 (1997) 4222

[29] A Hosseinnia M Keyanpour-Rad M Pazouki World Appl Sci J

8 (2010) 1327

[30] BD Cullity SR Stock Elements of X-Ray Diffraction 3rd edn

(Prentice Hall Upper Saddle River NJ) (2001) 07458

[31] G Ranga Rao B Gopal Mishra Bul Cat Soc of Ind 2 (2003)122

[32] A K Bhattacharyal T K Naiya S N Mondal S K Das Chem Eng J

137 (2008) 529

[33] G Liu J A Rodriguez Z Chang J Hrbek C H F Peden

J Phys Chem B 108 (2004) 2931

[34] P F Ji J L Zhang F Chen M Anpo J Phys Chem C 112 (2008) 17809

[35] A Trovarelli Catal Rev SciEng 38 (1996) 439

[36] B Li D M Pan J S Zheng Y J Cheng X Y Ma F Huang L Zhang

Langmuir 24 (2008) 9630

[37] M Fernaacutendez-Garcıacutea A A B Martıacutenez-Arias A Hungrıacutea Iglesias-Juez

J C Conesa J Soria J Phys Chem Chem Phys 4 (2002) 2473

[38] H M Chen J H He J Phys Chem C 112 (2008) 17540

Chapter ndash V

184

[39] F Zhang S Chan W Spanier J E Apak E Jin Q Robinson

R D Herman I P Appl Phys Lett 80 (2002) 127

[40] G Adachi TMasui In Catalysis by Ceria and Related Materials

ATrovarelli Ed Imperial College Press London (2002)

[41] J A Rodriguez X Wang J C Hanson G Liu A Iglesias-Juez

MFernaacutendez-Garcıacutea J Chem Phys 119 (2003) 5659

[42] S Eck C Castellarin-Cudia S Surnev M G Ramsey F PNetzer

Surf Sci 520 (2002) 173

[43] R B Yu L Yan P Zheng JChen X X R J Phys ChemC

112 (2008) 19896

[44] Y Nagai T Yamamoto T Tanaka SYoshida T Nonaka T Okamoto

A Suda M Sugiura Catal Today 74 (2002) 225

[45] Y M Chiang E B Lavik I Kosacki H L Tuller JY Ying

J Electroceram 1 (1997) 7

[46] D R Patil L A Patil G H Jain M S Wagh S A Patil

Sens and Transducers 74 (2006) 874

[47] K Sohlberg S K Pantelides S J Pennycook J Am Chem Soc

123 (2001) 6609

[48] L S Zhong J S Hu A M Cao Q Liu W G Song LWan

J Chem Mater 19 (2007) 1648

[49] J B Fei Y Cui X H Yan Y Yang K W Wang QHe J B Li

AdV Mater 20 (2008) 452

[50] P V Kamat J Phys Chem B 106 (2002) 7729

[51] R D Robinson J E Spanier F Zhang SW Chan I P Herman J Appl

Phys 92 (2002) 1936

[52] S W Cao Y J Zhu J Phys Chem C 112 (2008) 6253

[53] R Georgekutty M K Seery S C Pillai J Phys Chem C

112 (2008) 13563

[54] J S Hu L S Zhong W G Song L J Wan Adv Mater 20 (2008)1

Chapter ndash V

185

[55] HXu F L Jia Z H Ai L Z Zhang Cryst Growth Des (2007)

[56] H M Jia W J Xiao L Z Zhang Z F Zheng J PhysChem C

112 (2008) 1379

[57] Z H Ai H Y Xiao T Mei J Liu L Z Zhang K J Deng J R Qiu

J PhysChem C 112 (2008) 11929

[58] L M Ma W X Zhang EnViron Sci Technol 42 (2008) 5384

[59] Z H Ai Y Chen L Z Zhang J R Qiu EnViron Sci Technol

42 (2008) 69550

[60] Z Y Zhong J Ho J Teo S C Shen AGedanken ChemMater

19 (2007) 4776

[61] S C Kuiry S D Patil S Deshpande S Seal J Phys ChemB

109 (2005) 6936

[62] D R Patil L A Patil Sens IEEE 7 (2007) 434

[63] P Jasinski Anderson H U Sens Actuators B 95 (2003) 73

[64] J P Nair E Wachtel I Lubomirsky J Fleig JMaier AdVMater

15 (2003) 2077

[65] R J Qi Y J Zhu G F Cheng Y H Huang Nanotechnology

16 (2005) 2502

Chapter ndash V

159

oven (in put power 600W) about 30 minutes with on-off cycle The Phase purity

and the degree of crystallinity of the resulting CeO2 sample were monitored by

XRD analysis The schematic flow chart of preparation of nanocrystalline CeO2 is

represented in Fig51

Aqueous ammonia

darr

Ce(NO3)3 6H2O + Propylene glycol

darr Centrifugation and washing

Microwave treatment

darr

CeO2 nanocrystalline particles

Fig51 Schematic Flow Chart for the preparation of nanocrystalline CeO2

The powder Xndashray diffraction patterns were recorded on Philips PW-1710

X-ray diffractometer by using Cr Kα radiation The lattice parameters were

calculated using high angle reflection of XRD by using the following formula

--- 51

Crystallite size was calculated by using the Scherrerrsquos formula having

wavelength of X-ray (Cr Kα line 228 Aring)

The UV-Visible Spectra were recorded on UV-Visible-NIR

Spectrophotometer (UV-3600 Shimadzu Japan)

Chapter ndash V

160

FTIR spectrum was recorded with a Perkin Elmer-USA in the range of

wavenumber from 4000-400 cm-1

The SEM micrographs of the samples were obtained using the scanning

electron microscope (Model JEOL-JSM 6360) Elemental analysis was carried

out by using the energy dispersive X-ray spectroscopy equipped with Scanning

electron microscopy

TEM study of the powder sample was carried out by using model Philips

CM 12 transmission electron microscope with Energy Dispersive Spectroscopy

(EDS) detector for microanalysis (IIT-Madras)

The thermal behavior of the nanocrystalline CeO2 powder is studied upto 10

to1000degC in nitrogen using SDT Q600 V209 Build 20 Instrument

The oxidation of oxime was carried out at 353 K in presence of cerium

oxide as catalyst ethyl acetate as a solvent and H2O2 as the oxidant [19] The

liquid phase reaction was carried out in 25 ml round bottom flask equipped with a

reflux condenser In catalytic oxidation reaction oxime (10 mmol) H2O2

(20 mmol) along with catalyst (1 by wt of the substrate) and the content were

heated in pre-heated oil bath at 353 K

NOH

R

O

R

H2O

2

CeO2

Oxime Ketone

Where R = CH3 C6H5

One hundred milligrams of CeO2 nanocrystal microspheres was suspended

in 100 mL of rhodamine B aqueous solution with a concentration of 20 mg L-1

in a

flask under stirring The flask was covered with carbon paper to prevent the

Chapter ndash V

161

photodegradation of Rh B At different intervals 5mL of the suspensions was

collected filtered through a 045μm membrane and finally analyzed by a

UV-visible spectrophotometer immediately [22]

53 Results and Discussion

531 UV-Visible Spectroscopic Analysis

The absorption spectra of CeO2 nanoparticles dispersed in ethanol solution

is shown in Fig52 The absorption edge of the CeO2 nanoparticles synthesized

a) by microwave (without annealed) annealed at b) 200degC and c) 300degC occurs at

around 400 nm and its band gap energy is estimated to be 31 eV A sharp band is

an indicative of narrow distribution of particles within the matrix The accurate

band gap of material was 31 eV as obtained by plotting (αhυ)2

against hυ shown

in Fig53

Fig 52 UV-Visible absorption spectrum of CeO2 nanoparticles synthesized

a) by microwave (without annealed) annealed at b) 200degC and c) 300degC

Chapter ndash V

162

In semiconductors the band gap will be increased as particle size is

decreased As particles shrink in size there is a dramatic change in valence band

and conduction bands as the continuous density of states in bulk is replaced with a

set of discrete energy levels This leads to interesting optical properties In the

visible region colour can changes with size Since with increase in size energy

decreases the nanoparticles which have size less than the bulk compound shows

blue shift in the UV-Visible spectra of nanoparticles This blue shift may be

considered as one of the confirmatory test for the existence of nanoparticles

Fig53 Variation of (αhυ) 2

vs Photon Energy (eV) of CeO2 nanoparticle synthesized

a) by microwave (without annealed) annealed at b)200degC and c) 300degC

532 FTIR Analysis

The FT-IR spectrum of the obtained ceria nanoparticles is shown in

Fig 54 Three strong absorption peaks were observed at 3381 1558 and

1372 cm-1

The former was attributed to the stretching band of hydroxyl group and

the others were attributed to the antisymmetric and symmetric stretching band of

Chapter ndash V

163

the glycol group respectively In the present synthesis propylene glycol works as

a protective agent against particle growth and since the stretching band of the

carbonyl around 1700 cm-1

was not observed in the FT-IR spectrum it could be

that the surface of the ceria nanoparticles was covered with glycol species In

addition it has also been reported that non-stoichiometric cerium oxide

synthesized by an inert gas condensation process was brownish yellow

Fig54 FTIR spectrum of CeO2 nanoparticles

533 The XRD Analysis

Fig55 shows the X-ray diffractograms of the as prepared powder of CeO2

nanoparticles a) by microwave (without annealed) annealed at b)200degC and

c) 300degC The CeO2 nanoparticles are crystalline in nature with d values 313

270 191 and 163 Aring The synthesized samples by microwave confirmed the

Chapter ndash V

164

formation of face centered cubic structure (FCC) of CeO2 nanoparticles The

determined characteristics 2θ values and [hkl] planes are 4287deg [111] 5017deg

[200] 7386deg [220] and 89deg [311] respectively (JCPDS card No 81-0792) and

lattice parameter is 542 Aring The average particle size of CeO2 nanoparticles was

found to about 10 nm The particle size of CeO2 nanoparticles obtained by

microwave is less than that of annealed at 200degC and 300degC However the lattice

parameter is found to increase with reduction temperature indicating an expansion

in the FCC lattice

Fig 55 XRD patterns of the CeO2 nanoparticles synthesized a) by microwave

(without annealed) annealed at b) 200degC and c) 300degC

To know the correct structure in cubic the indexing pattern of cubic crystal

was studied in detail A cubic crystal gives diffraction lines whose sin2θ values

Chapter ndash V

165

satisfy the equation obtained by combining the Braggrsquos law with the plane-spacing

equation for the cubic system as

sin2θ (h

2+k

2+l

2) = sin

2θ s = λ

2 4 a

2 --- 52

Since the sum (s) =

(h

2+k

2+l

2) is always integral and λ

2 4a

2 is a constant for any

one pattern of a cubic substance is one of finding a set of integers (s) which will

yield a constant quotient when divided one by one into the observed sin2θ values

Once the proper integers (s) are found the indices (hkl) of each line can be written

down by inspection The proper set of integers (s) is not hard to find because there

are only a few possible sets [30] Each of the four common cubic lattice types has

a characteristic sequence of diffraction lines described by their sequential values

Simple cubic 1 2 3 4 5 6 7 8 9 10 11 12 13 14 16 -------

Body-centered cubic 2 4 6 8 10 12 14 16 -------

Face-centered cubic 3 4 8 11 12 16 --------

In our case the crystal system was found to be face centered cubic (FCC) because

the integers(s) 3 4 8 11 which matches the FCC diffractions lines

Fig 56 Fluorite structure of CeO2

Chapter ndash V

166

The unit cell of ceria is shown in Fig56 In the face centered cubic (FCC)

structure of ceria Ce4+

ions form a cubic close packing arrangement and all the

tetrahedral sites are occupied by the oxide ions whereas the octahedral sites remain

vacant The unit cell of ceria can be considered as a simple cube in which the face

center positions and corners are occupied by Ce4+

ions The tetrahedral sites can be

visualized by dividing the cube into eight smaller cubes The body center positions

of all the small cubes are occupied by oxide ions and the alternate corners are

occupied by Ce4+

ions [31]

Fig57 βcos θ λ vs sin θ λ for CeO2 nanocrystals

The particle size of CeO2 nanoparticles was calculated by using Scherrerrsquos

formula The lattice strain and crystalline size was calculated from the following

equation [18]

β cos θ λ = (1 ε) + (τ sin θ λ) --- 53

Chapter ndash V

167

where β is the measured FWHM (Full Width at Half Maximum) in radian θ is the

Bragg angle of the peak λ is wavelength of X-ray (Cr Kα line 228 Aring) ε is the

effective particle size and τ is the effective strain The average particle size can be

estimated from the extrapolation of the Williamson-Hall plot shown in Fig 57

and the particle size was obtained 9 nm based on the intercept inverse

ie 1 ε = 0011 X 108

which yields ε = 909 X 10-9

m or 9 nm

534 Energy Dispersive Analysis of X-Rays (EDAX)

The cerium oxide nanoparticles were characterized by energy dispersive

X- ray analysis (EDAX) which is attachment of SEM The sample for EDAX was

prepared by depositing a very thin layer of cerium oxide nanoparticles on glass

slide The dispersive X ndash ray analysis was conducted by focusing an electron beam

on several different sectioned regions of copper nanoparticles

Fig58 EDAX spectrum of CeO2 nanoparticles

Chapter ndash V

168

The EDAX spectrum of cerium oxide nano particles synthesized by

microwave assisted sol-gel method is shown in Fig58 the composition of sample

shows the presense of cerium and oxygen elements The spectrum peak reveals the

presence of Ce and O at 4837 and 0525 keV respectively The atomic of Ce

and O is 3597 and 6403 respectively which confirms the presence of Ce and O in

the powder The present composition of Ce and O reveals that the formation of

non-stoichometric CeO2 which is superior for adsorption of toxic ions from

wastewater

535 Scanning Electron Microscopic (SEM) Analysis

The model JEOL JSM-6360 was used for the determination of morphology

of nanoparticle The cerium oxide nanoparticles were dispersed in isopropyl

alcohol and sonicated for frac12 hour One or two drop of sonicated solution was kept

on 1x1 cm glass plate dried naturally and coated the platinum The SEM images

are formed by scanning narrow beam of electrons across the surface of specimen

Fig59 (a) SEM micrograph of CeO2 nanoparticles by microwave

(without annealed)

Chapter ndash V

169

Collecting and processing emitting electron and displaying them on cathode ray

tubecomputer monitor Using a visual raster which is synchronized with the

beams scanned The dried thin films samples of cerium oxide nanoparticles were

scanned by SEM The SEM images of cerium oxide synthesized by microwave

(without annealed) and annealed at 300degC are shown in Figs59 (a) and (b) From

SEM images revealed that there were various sizes of particles in the as-prepared

sample The large particles are composed of small crystallites and show particle

aggregates of irregular shapes The particles are spherical in shapes which are

good agreement with XRD results of particles size These observations were

consistent with similar observations reported in the literature [42ndash44]

Fig59 (b) SEM micrograph of CeO2 nanoparticles annealed at 300degC

536 Transmission Electron Microscopic (TEM) Analysis

In case of cerium oxide the transmission electron micrograph was obtained

by employing Philips microscopy IIT Madras TEM is analogous to the optical

microscope It provides very high resolution which can reach approximately

Chapter ndash V

170

01 nm in the case of lattice images Consequently very high magnification can be

obtained TEM is used to examine very thin section through materials of the

surface A transmission electron microscope Philips 120 kV is equipped with CCD

camera This instrument has the resolution of 036 nm with 42 to 120 kV operating

voltage and magnifies object upto 6 lakhs times in high resolution mode Samples

for TEM examinations were prepared by placing a drop of sample suspension on a

copper grade coated with carbon film and were allowed to dry in air A typical

TEM image of CeO2 powders synthesized by cerium nitrate salts by microwave

assisted sol-gel method (without calcination) is presented in Fig 510 The mean

diameter of the particles as measured is about 10 nm These results are in good

agreement with that of estimated by X-ray line broadening Also the synthesized

Fig510 TEM image of the CeO2 nanoparticles

nanopowder is well crystallized without defects non-agglomerated with narrow

size distributions The selected area electron diffraction (SAED) pattern shown in

the Fig511 it also supports the d values obtained from XRD with rings which

can be assigned [111] [200] [220] and [311] diffractions of CeO2 structure

Chapter ndash V

171

Fig511 SAED image of the CeO2 nanoparticles

537 Thermo-Gravimetric and Differential Thermal Analysis

Fig512 shows the TG-DTA curves for the dried ceria precursor prepared

by the Ce(IV) methods The dried samples may consist of three portions of

materials (A) crystalline ceria CeO2 (B) crystalline ceria with structural water

CeO22H2O and (C) amorphous cerium hydroxide Ce(OH)4 The endothermic

peaks in 64degC could be due to crystallization of the amorphous portion in the

sample Ce(OH)4 to crystalline CeO2 The larger endothermic peak for the sample

indicates that the former contains more amorphous phase than the latter The total

weight loss up to 800 degC for the sample is about 1685 (or 194 for n which is

the number of water molecules per mole of cerium in the dried sample) This is

consistent with the change of [Ce (OH)4 + CeO22H2O] to [CeO2] The two

endothermic peaks at about 313 and 436degC in figure can be attributed to the loss of

the structural water molecules The less obvious endothermic peaks for the

structural water loss for this sample also agree with the weight loss data These

results suggest that the Ce(IV) derived sample contains primarily Ce(OH)4 and

CeO22H2O

Chapter ndash V

172

Fig 512 Weight loss of CeO2 nanoparticles as a function of temperature

538 Electrical Conductivity of Semiconductors CeO2

Semiconductors find wide appilication particularly in electronics Their

application is based on certain properties that will be an electrical conductivity of

semiconductors When the temperature is not very low the semiconductor will

conduct electricity due to the movement of the electrons in the conduction band

and that of holes in the valence band The conductivity of nano CeO2 thick film is

represented in Fig513 From the figure the almost linear relationship was found

in the measured temperature range of 200-500oC If it assumed that conduction in

crystalline ceria is essentially an n-type electronic process then using a simple

model we can express the conductivity-temperature relation by the equation

σ = σo e-Ea kT

where σo is a constant Ea is thermal activation energy T is the

Kelvin temperature and k is Boltzmanns constant Thus an Arrhenius plot of

conductivity and temperature should be linear over certain temperature range

Although the results in Fig513 are curved a linear section can be drawn above

280oC which gives activation energy of 236 eV

Chapter ndash V

173

Fig513 Electrical conductivity of CeO2 nanoparticles as a function of temperature

539 Gas Sensing Performance

5391 LPG Gas Sensing

Fig514 shows the variation of response of CeO2 fired at various

temperature The gas response increases with temperature from 50 to 150degC and

then decreases with a further increase in temperature In present work every time

prior to exposing the CeO2 film to LPG it was allowed to stabilize at an operating

temperature for 10 min and the stabilized resistance was taken as Ra After

exposing the film to the LPG gas the changed resistance was taken as Rg LPG

reacts with surface oxygen ions of the film Oxidation of film decreases the

number of free carriers Therefore resistance of the film increases with gas The

result of reaction of LPG with polycrystalline CeO2 is adsorbed CO2 and H2O on

the surface of the oxide The Ce harrLPG interactions on CeO2 are strong and Ce

sites probably get oxidized

Chapter ndash V

174

Fig 514 Variation of gas responses with operating temperature for LPG gas

5393 Effect of Gas Concentration (active region)

The variation of sensor response of CeO2 thick film sample with LPG gas

concentration at operating temperature 150degC is represented in Fig 515

Fig 515 Variation in sensor response with LPG gas concentration

Chapter ndash V

175

It is clear from the figure that the gas response goes on increasing with gas

concentration upto 600 ppm The rate of increase in response was relatively large

upto 1000 ppm and saturated beyond 600 ppm The monolayer of the gas

molecules formed on the surface covers the whole surface of the film The gas

molecules from that layer would reach the surface active sites of the film The

excess gas molecules remain idle and would not reach the surface active sites of

the sensor Thus the active region of the sensor would be up to 600 ppm

5394 Response and Recovery Time

The response of CeO2 thick film sensor was found to be quick (~ 28 s) to

600 ppm of while the recovery was fast (~ 10 s) (Fig 516) The fast response

may be attributed to faster oxidation of the gas The negligible quantity of the

surface reaction product and its high volatility explains its fast response and quick

recovery to its initial chemical status

Fig 516 Response and recovery time

Chapter ndash V

176

5395 Oxygen Adsorption-Desorption Mechanism

O2 (air) + 2e- 2O

-(film surface) --- 56

At higher temperature the atmospheric oxygen O2 adsorbs on the surface of the

thick film It captures the electrons (Fig517) from conduction band It would

result in decreasing conductivity of the film When LPG reacts with the adsorbed

oxygen on the surface of the film it gets oxidized to CO2 and H2O by following

series of intermediate stages This liberates free electrons in the conduction band

The final reaction takes place as

C4H10(gas) + 13Ominus

(filmsurface) rarr 4CO2(gas) + 5H2O(gas) + 13eminus

(condband) --- 57

Fig 517 Sensing mechanism on the surface of the film

This shows n-type conduction mechanism Thus generated electrons

contribute to a sudden increase in conductance of the thick film The mass of

Ce and O in sample was not as per the stoichiometric proportion and sample was

Chapter ndash V

177

observed to be the oxygen deficient This deficiency gets reduced due to

adsorption of atmosphericmolecular oxygen This helps in decreasing electronic

conductivity of the film Upon exposure LPG molecules got oxidized with the

adsorbed oxygen ions by following the series of intermediate stages producing

CO2 and H2O This results in evolving oxygen as electrically neutral atoms

trapping behind the negative charges (electrons) Upon exposure the energy

released in decomposition of LPG molecules would be sufficient for trapped

electrons to jump into the conduction band of activated CeO2 resulting in increase

in the conductivity of the film The drastic increase in conductivity of the sensor

could be attributed to the adsorptionndashdesorption mechanism resulted from the

electronic defects [26]

5310 Catalytic Activity

The utility of this catalytic system deprotection of several oximes were

studied (Table 52) The catalyst was also tested for reusability and it was found

that it could be reused without loss of initial activity for at least four cycles [19]

TLC analysis indicated the formation of only one product of corresponding

carbonyls FT-IR and NMR analysis showed that the oximes were converted into

carbonyls For checking the reusability (after each run) the catalyst was recovered

by simple filtration and washed several times with acetone and then dried at

373 K The catalyst thus obtained was used for the subsequent runs without any

further modification both activated and deactivated oximes were oxidized in better

yield more than ZrO2 catalyst as shown in Table 52

Chapter ndash V

178

Table 52 Oxidative Deoximation of Carbonyls on CeO2

Entry

Substrate

Product

Time

(h)

Yield

()

1

NOH

O

15

62

2

NOH

Cl

O

Cl

15

58

3

NOH

O

25

73

4

NOHCl

OCl

40

57

5

NOH

Cl

O

Cl

30

59

Solvent- Ethyl acetate Catalyst- ZrO2 Temperature- 353 K

Chapter ndash V

179

5311 Adsorption of Rhodamine B (Rh B)

The removal of dye pollutants are also of great importance for wastewater

treatment Rhodamine B a common cationic dye used in textile industry is one of

the most notorious contaminants in aquatic environments because of it huge

amounts slow biodegradation and toxicity In this study synthesized CeO2

nanocrystals are further used to remove Rh B The absorption spectrum of Rh B

solution was characterized by its characteristic absorption at 555 nm which was

attributed to the chromophore containing azo linkage of the dye molecules [22]

Fig 518 Absorption spectra of aqueous solution of Rh B in the presence of CeO2

nanocrystals at different times

UV-Visible absorption spectra represented in Fig518 confirms that over 93 of

Rh B in 100 mL of 20 mg L-1

Rh B aqueous solution could be removed with 100

mg of the as-prepared CeO2 nanocrystalline within 60 min The reasons may

account for the efficient removal of Rh B with CeO2 nanocrystals The first reason

Chapter ndash V

180

should be their to facilitate the adsorption of Rh B The second one is attributed to

the oxygen vacancies on the surface of CeO2 nanocrystals These abundant surface

oxygen vacancies could produce strong electrostatic attraction with the cationic

groups of Rh B as well as hydrogen bonding with the nitrogen atoms of Rh B

Chapter ndash V

181

Conclusions

Microwave synthesis is very beneficial to find a fast simple and energy

efficient approach to produce fine CeO2 nanoparticles It is relatively new method

to produce inorganic compounds for materials processing to enhance the material

properties as well as economic advantages through energy saving and acceleration

of product development Cerium oxide has outstanding physical and chemical

properties therefore it is used as LPG sensor In the area of catalysis nanoparticles

of ceria have been studied Also it is concluded as

1] The CeO2 nanoparticles were prepared by simple microwave assisted sol-gel

method

2] The well spherical and narrow size distribution with 10 nm size particles were

obtained by this method

3] It is very simple time as well as energy saving technique

4] The XRD pattern shows the CeO2 nanoparticles exists in face centered cubic

5] Scanning electron micrographs indicates that grains are uniformaly distributed

6] Transmission electron micrographs indicates that the mean diameter of the

particles as measured is about 10 nm

7] The conductivity of nano CeO2 thick film is represents the almost linear

relationship was found in the measured temperature range of 200-5000C It is

clear that the conduction in crystalline ceria is essentially an n-type electronic

process

8] The CeO2 nanoparticles in the thick film form prepared by screen printing

technique showed better response to LPG gas at 250degC temperature at 600 ppm

concentration of LPG gas

9] Catalyst can be prepared from inexpensive precursors using the microwave

assisted sol-gel technique is promising for the oxidative regeneration of ketones

10] This study reveals that these CeO2 nanocrystals are novel adsorbents for

wastewater treatment

Chapter ndash V

182

References

[1] A Ramesh H Hasegawa T Maki KUeda Sep Purif Tech

56 (2007) 90

[2] C Lettmann K Hildenbrand H Kisch W Macyk W F Maier

Appl Catal B 32 (2001) 215

[3] X Zhang Z H Ai F L Jia L Z Zhang J Phys Chem C 112 (2008) 747

[4] N Du H Zhang BD Chen X Y Ma D RYang J Phys ChemC

111 (2007) 12677

[5] F Zhang S Yang H Chen X Yu Cera Intern 30 (2004) 997

[6] F Gu SF Wang MK Lu GJ Zhou D Xu DR Langmuir J SciIR Iran

20 (2004) 3528

[7] H Xiao Z Ai L Zhang JPhys Chem 113 (2009) 16625

[8] L Combemale G Caboche D Stuerga D Chaumont MicroRese

Bull 40 (2005) 529

[9] M Zawadzki Jof Alloys and Comp 454 (2008) 347

[10] A V Patil C G Dighavkar S K Sonawane S J Patil R Y Borse

Sens and Transducers 108 (2009) 189

[11] AV Patil CG Dighavkar RY Borse Sens and Transducers

101 (2009) 96

[12] AV Patil CG Dighavkar SK Sonawane S J Patil RY Borse Sensand

Transducers 9 (2010) 11

[13] DR Patil LA Patil Talanta 77 (2009)1409

[14] R S Blackburn Environ Sci Tech 38 (2004) 4905

[15] GNaja CMustin JBerthelin BVolesky J Colloide Inter Sci

292 (2005) 537

[16] VKGupta IAli Environ Sci Tech 42 (2008) 766

[17] A Mittal VK Gupta A Malviya and JMittal J Hazard Mater

151 (2008) 821

[18] VKGupta IAli VKSaini J Colloid Interface Sci 315(2007) 87

Chapter ndash V

183

[19] S S Deshpande S U Sonavane R V Jayaram Cat Com 9 (2008) 639

[20] KM Garadkar BS Shirke YB Patil D R Patil Sens and Transducers

110 (2009) 17

[21] VKGupta AMittal VGajbe JMittal J Colloid Inter Sci

319 (2008) 30

[22] T A Khan I Ali V V Singh S Sharma J Envir Protec Sci 3 (2009)11

[23] A Hosseinnia M Keyanpour-Rad M Pazouki World Applied Sci J

8 (2010) 1327

[24] MI Baraton L Merhari J Am Ceram Soc 75 (2004) 1587

[25] CWang B Xu XWang J Zhao J Solid State Chem 178 (2005) 3500

[26] H Xiao Z Ai L Zhang J Phys Chem C doi 10 1021 JP9050269 (2009)

[27] A M Thompson Oxides of the Rare Earths Wiley New York (1978)

[28] H Noumlremberg G A D Briggs Phys Rev Lett 79 (1997) 4222

[29] A Hosseinnia M Keyanpour-Rad M Pazouki World Appl Sci J

8 (2010) 1327

[30] BD Cullity SR Stock Elements of X-Ray Diffraction 3rd edn

(Prentice Hall Upper Saddle River NJ) (2001) 07458

[31] G Ranga Rao B Gopal Mishra Bul Cat Soc of Ind 2 (2003)122

[32] A K Bhattacharyal T K Naiya S N Mondal S K Das Chem Eng J

137 (2008) 529

[33] G Liu J A Rodriguez Z Chang J Hrbek C H F Peden

J Phys Chem B 108 (2004) 2931

[34] P F Ji J L Zhang F Chen M Anpo J Phys Chem C 112 (2008) 17809

[35] A Trovarelli Catal Rev SciEng 38 (1996) 439

[36] B Li D M Pan J S Zheng Y J Cheng X Y Ma F Huang L Zhang

Langmuir 24 (2008) 9630

[37] M Fernaacutendez-Garcıacutea A A B Martıacutenez-Arias A Hungrıacutea Iglesias-Juez

J C Conesa J Soria J Phys Chem Chem Phys 4 (2002) 2473

[38] H M Chen J H He J Phys Chem C 112 (2008) 17540

Chapter ndash V

184

[39] F Zhang S Chan W Spanier J E Apak E Jin Q Robinson

R D Herman I P Appl Phys Lett 80 (2002) 127

[40] G Adachi TMasui In Catalysis by Ceria and Related Materials

ATrovarelli Ed Imperial College Press London (2002)

[41] J A Rodriguez X Wang J C Hanson G Liu A Iglesias-Juez

MFernaacutendez-Garcıacutea J Chem Phys 119 (2003) 5659

[42] S Eck C Castellarin-Cudia S Surnev M G Ramsey F PNetzer

Surf Sci 520 (2002) 173

[43] R B Yu L Yan P Zheng JChen X X R J Phys ChemC

112 (2008) 19896

[44] Y Nagai T Yamamoto T Tanaka SYoshida T Nonaka T Okamoto

A Suda M Sugiura Catal Today 74 (2002) 225

[45] Y M Chiang E B Lavik I Kosacki H L Tuller JY Ying

J Electroceram 1 (1997) 7

[46] D R Patil L A Patil G H Jain M S Wagh S A Patil

Sens and Transducers 74 (2006) 874

[47] K Sohlberg S K Pantelides S J Pennycook J Am Chem Soc

123 (2001) 6609

[48] L S Zhong J S Hu A M Cao Q Liu W G Song LWan

J Chem Mater 19 (2007) 1648

[49] J B Fei Y Cui X H Yan Y Yang K W Wang QHe J B Li

AdV Mater 20 (2008) 452

[50] P V Kamat J Phys Chem B 106 (2002) 7729

[51] R D Robinson J E Spanier F Zhang SW Chan I P Herman J Appl

Phys 92 (2002) 1936

[52] S W Cao Y J Zhu J Phys Chem C 112 (2008) 6253

[53] R Georgekutty M K Seery S C Pillai J Phys Chem C

112 (2008) 13563

[54] J S Hu L S Zhong W G Song L J Wan Adv Mater 20 (2008)1

Chapter ndash V

185

[55] HXu F L Jia Z H Ai L Z Zhang Cryst Growth Des (2007)

[56] H M Jia W J Xiao L Z Zhang Z F Zheng J PhysChem C

112 (2008) 1379

[57] Z H Ai H Y Xiao T Mei J Liu L Z Zhang K J Deng J R Qiu

J PhysChem C 112 (2008) 11929

[58] L M Ma W X Zhang EnViron Sci Technol 42 (2008) 5384

[59] Z H Ai Y Chen L Z Zhang J R Qiu EnViron Sci Technol

42 (2008) 69550

[60] Z Y Zhong J Ho J Teo S C Shen AGedanken ChemMater

19 (2007) 4776

[61] S C Kuiry S D Patil S Deshpande S Seal J Phys ChemB

109 (2005) 6936

[62] D R Patil L A Patil Sens IEEE 7 (2007) 434

[63] P Jasinski Anderson H U Sens Actuators B 95 (2003) 73

[64] J P Nair E Wachtel I Lubomirsky J Fleig JMaier AdVMater

15 (2003) 2077

[65] R J Qi Y J Zhu G F Cheng Y H Huang Nanotechnology

16 (2005) 2502

Chapter ndash V

160

FTIR spectrum was recorded with a Perkin Elmer-USA in the range of

wavenumber from 4000-400 cm-1

The SEM micrographs of the samples were obtained using the scanning

electron microscope (Model JEOL-JSM 6360) Elemental analysis was carried

out by using the energy dispersive X-ray spectroscopy equipped with Scanning

electron microscopy

TEM study of the powder sample was carried out by using model Philips

CM 12 transmission electron microscope with Energy Dispersive Spectroscopy

(EDS) detector for microanalysis (IIT-Madras)

The thermal behavior of the nanocrystalline CeO2 powder is studied upto 10

to1000degC in nitrogen using SDT Q600 V209 Build 20 Instrument

The oxidation of oxime was carried out at 353 K in presence of cerium

oxide as catalyst ethyl acetate as a solvent and H2O2 as the oxidant [19] The

liquid phase reaction was carried out in 25 ml round bottom flask equipped with a

reflux condenser In catalytic oxidation reaction oxime (10 mmol) H2O2

(20 mmol) along with catalyst (1 by wt of the substrate) and the content were

heated in pre-heated oil bath at 353 K

NOH

R

O

R

H2O

2

CeO2

Oxime Ketone

Where R = CH3 C6H5

One hundred milligrams of CeO2 nanocrystal microspheres was suspended

in 100 mL of rhodamine B aqueous solution with a concentration of 20 mg L-1

in a

flask under stirring The flask was covered with carbon paper to prevent the

Chapter ndash V

161

photodegradation of Rh B At different intervals 5mL of the suspensions was

collected filtered through a 045μm membrane and finally analyzed by a

UV-visible spectrophotometer immediately [22]

53 Results and Discussion

531 UV-Visible Spectroscopic Analysis

The absorption spectra of CeO2 nanoparticles dispersed in ethanol solution

is shown in Fig52 The absorption edge of the CeO2 nanoparticles synthesized

a) by microwave (without annealed) annealed at b) 200degC and c) 300degC occurs at

around 400 nm and its band gap energy is estimated to be 31 eV A sharp band is

an indicative of narrow distribution of particles within the matrix The accurate

band gap of material was 31 eV as obtained by plotting (αhυ)2

against hυ shown

in Fig53

Fig 52 UV-Visible absorption spectrum of CeO2 nanoparticles synthesized

a) by microwave (without annealed) annealed at b) 200degC and c) 300degC

Chapter ndash V

162

In semiconductors the band gap will be increased as particle size is

decreased As particles shrink in size there is a dramatic change in valence band

and conduction bands as the continuous density of states in bulk is replaced with a

set of discrete energy levels This leads to interesting optical properties In the

visible region colour can changes with size Since with increase in size energy

decreases the nanoparticles which have size less than the bulk compound shows

blue shift in the UV-Visible spectra of nanoparticles This blue shift may be

considered as one of the confirmatory test for the existence of nanoparticles

Fig53 Variation of (αhυ) 2

vs Photon Energy (eV) of CeO2 nanoparticle synthesized

a) by microwave (without annealed) annealed at b)200degC and c) 300degC

532 FTIR Analysis

The FT-IR spectrum of the obtained ceria nanoparticles is shown in

Fig 54 Three strong absorption peaks were observed at 3381 1558 and

1372 cm-1

The former was attributed to the stretching band of hydroxyl group and

the others were attributed to the antisymmetric and symmetric stretching band of

Chapter ndash V

163

the glycol group respectively In the present synthesis propylene glycol works as

a protective agent against particle growth and since the stretching band of the

carbonyl around 1700 cm-1

was not observed in the FT-IR spectrum it could be

that the surface of the ceria nanoparticles was covered with glycol species In

addition it has also been reported that non-stoichiometric cerium oxide

synthesized by an inert gas condensation process was brownish yellow

Fig54 FTIR spectrum of CeO2 nanoparticles

533 The XRD Analysis

Fig55 shows the X-ray diffractograms of the as prepared powder of CeO2

nanoparticles a) by microwave (without annealed) annealed at b)200degC and

c) 300degC The CeO2 nanoparticles are crystalline in nature with d values 313

270 191 and 163 Aring The synthesized samples by microwave confirmed the

Chapter ndash V

164

formation of face centered cubic structure (FCC) of CeO2 nanoparticles The

determined characteristics 2θ values and [hkl] planes are 4287deg [111] 5017deg

[200] 7386deg [220] and 89deg [311] respectively (JCPDS card No 81-0792) and

lattice parameter is 542 Aring The average particle size of CeO2 nanoparticles was

found to about 10 nm The particle size of CeO2 nanoparticles obtained by

microwave is less than that of annealed at 200degC and 300degC However the lattice

parameter is found to increase with reduction temperature indicating an expansion

in the FCC lattice

Fig 55 XRD patterns of the CeO2 nanoparticles synthesized a) by microwave

(without annealed) annealed at b) 200degC and c) 300degC

To know the correct structure in cubic the indexing pattern of cubic crystal

was studied in detail A cubic crystal gives diffraction lines whose sin2θ values

Chapter ndash V

165

satisfy the equation obtained by combining the Braggrsquos law with the plane-spacing

equation for the cubic system as

sin2θ (h

2+k

2+l

2) = sin

2θ s = λ

2 4 a

2 --- 52

Since the sum (s) =

(h

2+k

2+l

2) is always integral and λ

2 4a

2 is a constant for any

one pattern of a cubic substance is one of finding a set of integers (s) which will

yield a constant quotient when divided one by one into the observed sin2θ values

Once the proper integers (s) are found the indices (hkl) of each line can be written

down by inspection The proper set of integers (s) is not hard to find because there

are only a few possible sets [30] Each of the four common cubic lattice types has

a characteristic sequence of diffraction lines described by their sequential values

Simple cubic 1 2 3 4 5 6 7 8 9 10 11 12 13 14 16 -------

Body-centered cubic 2 4 6 8 10 12 14 16 -------

Face-centered cubic 3 4 8 11 12 16 --------

In our case the crystal system was found to be face centered cubic (FCC) because

the integers(s) 3 4 8 11 which matches the FCC diffractions lines

Fig 56 Fluorite structure of CeO2

Chapter ndash V

166

The unit cell of ceria is shown in Fig56 In the face centered cubic (FCC)

structure of ceria Ce4+

ions form a cubic close packing arrangement and all the

tetrahedral sites are occupied by the oxide ions whereas the octahedral sites remain

vacant The unit cell of ceria can be considered as a simple cube in which the face

center positions and corners are occupied by Ce4+

ions The tetrahedral sites can be

visualized by dividing the cube into eight smaller cubes The body center positions

of all the small cubes are occupied by oxide ions and the alternate corners are

occupied by Ce4+

ions [31]

Fig57 βcos θ λ vs sin θ λ for CeO2 nanocrystals

The particle size of CeO2 nanoparticles was calculated by using Scherrerrsquos

formula The lattice strain and crystalline size was calculated from the following

equation [18]

β cos θ λ = (1 ε) + (τ sin θ λ) --- 53

Chapter ndash V

167

where β is the measured FWHM (Full Width at Half Maximum) in radian θ is the

Bragg angle of the peak λ is wavelength of X-ray (Cr Kα line 228 Aring) ε is the

effective particle size and τ is the effective strain The average particle size can be

estimated from the extrapolation of the Williamson-Hall plot shown in Fig 57

and the particle size was obtained 9 nm based on the intercept inverse

ie 1 ε = 0011 X 108

which yields ε = 909 X 10-9

m or 9 nm

534 Energy Dispersive Analysis of X-Rays (EDAX)

The cerium oxide nanoparticles were characterized by energy dispersive

X- ray analysis (EDAX) which is attachment of SEM The sample for EDAX was

prepared by depositing a very thin layer of cerium oxide nanoparticles on glass

slide The dispersive X ndash ray analysis was conducted by focusing an electron beam

on several different sectioned regions of copper nanoparticles

Fig58 EDAX spectrum of CeO2 nanoparticles

Chapter ndash V

168

The EDAX spectrum of cerium oxide nano particles synthesized by

microwave assisted sol-gel method is shown in Fig58 the composition of sample

shows the presense of cerium and oxygen elements The spectrum peak reveals the

presence of Ce and O at 4837 and 0525 keV respectively The atomic of Ce

and O is 3597 and 6403 respectively which confirms the presence of Ce and O in

the powder The present composition of Ce and O reveals that the formation of

non-stoichometric CeO2 which is superior for adsorption of toxic ions from

wastewater

535 Scanning Electron Microscopic (SEM) Analysis

The model JEOL JSM-6360 was used for the determination of morphology

of nanoparticle The cerium oxide nanoparticles were dispersed in isopropyl

alcohol and sonicated for frac12 hour One or two drop of sonicated solution was kept

on 1x1 cm glass plate dried naturally and coated the platinum The SEM images

are formed by scanning narrow beam of electrons across the surface of specimen

Fig59 (a) SEM micrograph of CeO2 nanoparticles by microwave

(without annealed)

Chapter ndash V

169

Collecting and processing emitting electron and displaying them on cathode ray

tubecomputer monitor Using a visual raster which is synchronized with the

beams scanned The dried thin films samples of cerium oxide nanoparticles were

scanned by SEM The SEM images of cerium oxide synthesized by microwave

(without annealed) and annealed at 300degC are shown in Figs59 (a) and (b) From

SEM images revealed that there were various sizes of particles in the as-prepared

sample The large particles are composed of small crystallites and show particle

aggregates of irregular shapes The particles are spherical in shapes which are

good agreement with XRD results of particles size These observations were

consistent with similar observations reported in the literature [42ndash44]

Fig59 (b) SEM micrograph of CeO2 nanoparticles annealed at 300degC

536 Transmission Electron Microscopic (TEM) Analysis

In case of cerium oxide the transmission electron micrograph was obtained

by employing Philips microscopy IIT Madras TEM is analogous to the optical

microscope It provides very high resolution which can reach approximately

Chapter ndash V

170

01 nm in the case of lattice images Consequently very high magnification can be

obtained TEM is used to examine very thin section through materials of the

surface A transmission electron microscope Philips 120 kV is equipped with CCD

camera This instrument has the resolution of 036 nm with 42 to 120 kV operating

voltage and magnifies object upto 6 lakhs times in high resolution mode Samples

for TEM examinations were prepared by placing a drop of sample suspension on a

copper grade coated with carbon film and were allowed to dry in air A typical

TEM image of CeO2 powders synthesized by cerium nitrate salts by microwave

assisted sol-gel method (without calcination) is presented in Fig 510 The mean

diameter of the particles as measured is about 10 nm These results are in good

agreement with that of estimated by X-ray line broadening Also the synthesized

Fig510 TEM image of the CeO2 nanoparticles

nanopowder is well crystallized without defects non-agglomerated with narrow

size distributions The selected area electron diffraction (SAED) pattern shown in

the Fig511 it also supports the d values obtained from XRD with rings which

can be assigned [111] [200] [220] and [311] diffractions of CeO2 structure

Chapter ndash V

171

Fig511 SAED image of the CeO2 nanoparticles

537 Thermo-Gravimetric and Differential Thermal Analysis

Fig512 shows the TG-DTA curves for the dried ceria precursor prepared

by the Ce(IV) methods The dried samples may consist of three portions of

materials (A) crystalline ceria CeO2 (B) crystalline ceria with structural water

CeO22H2O and (C) amorphous cerium hydroxide Ce(OH)4 The endothermic

peaks in 64degC could be due to crystallization of the amorphous portion in the

sample Ce(OH)4 to crystalline CeO2 The larger endothermic peak for the sample

indicates that the former contains more amorphous phase than the latter The total

weight loss up to 800 degC for the sample is about 1685 (or 194 for n which is

the number of water molecules per mole of cerium in the dried sample) This is

consistent with the change of [Ce (OH)4 + CeO22H2O] to [CeO2] The two

endothermic peaks at about 313 and 436degC in figure can be attributed to the loss of

the structural water molecules The less obvious endothermic peaks for the

structural water loss for this sample also agree with the weight loss data These

results suggest that the Ce(IV) derived sample contains primarily Ce(OH)4 and

CeO22H2O

Chapter ndash V

172

Fig 512 Weight loss of CeO2 nanoparticles as a function of temperature

538 Electrical Conductivity of Semiconductors CeO2

Semiconductors find wide appilication particularly in electronics Their

application is based on certain properties that will be an electrical conductivity of

semiconductors When the temperature is not very low the semiconductor will

conduct electricity due to the movement of the electrons in the conduction band

and that of holes in the valence band The conductivity of nano CeO2 thick film is

represented in Fig513 From the figure the almost linear relationship was found

in the measured temperature range of 200-500oC If it assumed that conduction in

crystalline ceria is essentially an n-type electronic process then using a simple

model we can express the conductivity-temperature relation by the equation

σ = σo e-Ea kT

where σo is a constant Ea is thermal activation energy T is the

Kelvin temperature and k is Boltzmanns constant Thus an Arrhenius plot of

conductivity and temperature should be linear over certain temperature range

Although the results in Fig513 are curved a linear section can be drawn above

280oC which gives activation energy of 236 eV

Chapter ndash V

173

Fig513 Electrical conductivity of CeO2 nanoparticles as a function of temperature

539 Gas Sensing Performance

5391 LPG Gas Sensing

Fig514 shows the variation of response of CeO2 fired at various

temperature The gas response increases with temperature from 50 to 150degC and

then decreases with a further increase in temperature In present work every time

prior to exposing the CeO2 film to LPG it was allowed to stabilize at an operating

temperature for 10 min and the stabilized resistance was taken as Ra After

exposing the film to the LPG gas the changed resistance was taken as Rg LPG

reacts with surface oxygen ions of the film Oxidation of film decreases the

number of free carriers Therefore resistance of the film increases with gas The

result of reaction of LPG with polycrystalline CeO2 is adsorbed CO2 and H2O on

the surface of the oxide The Ce harrLPG interactions on CeO2 are strong and Ce

sites probably get oxidized

Chapter ndash V

174

Fig 514 Variation of gas responses with operating temperature for LPG gas

5393 Effect of Gas Concentration (active region)

The variation of sensor response of CeO2 thick film sample with LPG gas

concentration at operating temperature 150degC is represented in Fig 515

Fig 515 Variation in sensor response with LPG gas concentration

Chapter ndash V

175

It is clear from the figure that the gas response goes on increasing with gas

concentration upto 600 ppm The rate of increase in response was relatively large

upto 1000 ppm and saturated beyond 600 ppm The monolayer of the gas

molecules formed on the surface covers the whole surface of the film The gas

molecules from that layer would reach the surface active sites of the film The

excess gas molecules remain idle and would not reach the surface active sites of

the sensor Thus the active region of the sensor would be up to 600 ppm

5394 Response and Recovery Time

The response of CeO2 thick film sensor was found to be quick (~ 28 s) to

600 ppm of while the recovery was fast (~ 10 s) (Fig 516) The fast response

may be attributed to faster oxidation of the gas The negligible quantity of the

surface reaction product and its high volatility explains its fast response and quick

recovery to its initial chemical status

Fig 516 Response and recovery time

Chapter ndash V

176

5395 Oxygen Adsorption-Desorption Mechanism

O2 (air) + 2e- 2O

-(film surface) --- 56

At higher temperature the atmospheric oxygen O2 adsorbs on the surface of the

thick film It captures the electrons (Fig517) from conduction band It would

result in decreasing conductivity of the film When LPG reacts with the adsorbed

oxygen on the surface of the film it gets oxidized to CO2 and H2O by following

series of intermediate stages This liberates free electrons in the conduction band

The final reaction takes place as

C4H10(gas) + 13Ominus

(filmsurface) rarr 4CO2(gas) + 5H2O(gas) + 13eminus

(condband) --- 57

Fig 517 Sensing mechanism on the surface of the film

This shows n-type conduction mechanism Thus generated electrons

contribute to a sudden increase in conductance of the thick film The mass of

Ce and O in sample was not as per the stoichiometric proportion and sample was

Chapter ndash V

177

observed to be the oxygen deficient This deficiency gets reduced due to

adsorption of atmosphericmolecular oxygen This helps in decreasing electronic

conductivity of the film Upon exposure LPG molecules got oxidized with the

adsorbed oxygen ions by following the series of intermediate stages producing

CO2 and H2O This results in evolving oxygen as electrically neutral atoms

trapping behind the negative charges (electrons) Upon exposure the energy

released in decomposition of LPG molecules would be sufficient for trapped

electrons to jump into the conduction band of activated CeO2 resulting in increase

in the conductivity of the film The drastic increase in conductivity of the sensor

could be attributed to the adsorptionndashdesorption mechanism resulted from the

electronic defects [26]

5310 Catalytic Activity

The utility of this catalytic system deprotection of several oximes were

studied (Table 52) The catalyst was also tested for reusability and it was found

that it could be reused without loss of initial activity for at least four cycles [19]

TLC analysis indicated the formation of only one product of corresponding

carbonyls FT-IR and NMR analysis showed that the oximes were converted into

carbonyls For checking the reusability (after each run) the catalyst was recovered

by simple filtration and washed several times with acetone and then dried at

373 K The catalyst thus obtained was used for the subsequent runs without any

further modification both activated and deactivated oximes were oxidized in better

yield more than ZrO2 catalyst as shown in Table 52

Chapter ndash V

178

Table 52 Oxidative Deoximation of Carbonyls on CeO2

Entry

Substrate

Product

Time

(h)

Yield

()

1

NOH

O

15

62

2

NOH

Cl

O

Cl

15

58

3

NOH

O

25

73

4

NOHCl

OCl

40

57

5

NOH

Cl

O

Cl

30

59

Solvent- Ethyl acetate Catalyst- ZrO2 Temperature- 353 K

Chapter ndash V

179

5311 Adsorption of Rhodamine B (Rh B)

The removal of dye pollutants are also of great importance for wastewater

treatment Rhodamine B a common cationic dye used in textile industry is one of

the most notorious contaminants in aquatic environments because of it huge

amounts slow biodegradation and toxicity In this study synthesized CeO2

nanocrystals are further used to remove Rh B The absorption spectrum of Rh B

solution was characterized by its characteristic absorption at 555 nm which was

attributed to the chromophore containing azo linkage of the dye molecules [22]

Fig 518 Absorption spectra of aqueous solution of Rh B in the presence of CeO2

nanocrystals at different times

UV-Visible absorption spectra represented in Fig518 confirms that over 93 of

Rh B in 100 mL of 20 mg L-1

Rh B aqueous solution could be removed with 100

mg of the as-prepared CeO2 nanocrystalline within 60 min The reasons may

account for the efficient removal of Rh B with CeO2 nanocrystals The first reason

Chapter ndash V

180

should be their to facilitate the adsorption of Rh B The second one is attributed to

the oxygen vacancies on the surface of CeO2 nanocrystals These abundant surface

oxygen vacancies could produce strong electrostatic attraction with the cationic

groups of Rh B as well as hydrogen bonding with the nitrogen atoms of Rh B

Chapter ndash V

181

Conclusions

Microwave synthesis is very beneficial to find a fast simple and energy

efficient approach to produce fine CeO2 nanoparticles It is relatively new method

to produce inorganic compounds for materials processing to enhance the material

properties as well as economic advantages through energy saving and acceleration

of product development Cerium oxide has outstanding physical and chemical

properties therefore it is used as LPG sensor In the area of catalysis nanoparticles

of ceria have been studied Also it is concluded as

1] The CeO2 nanoparticles were prepared by simple microwave assisted sol-gel

method

2] The well spherical and narrow size distribution with 10 nm size particles were

obtained by this method

3] It is very simple time as well as energy saving technique

4] The XRD pattern shows the CeO2 nanoparticles exists in face centered cubic

5] Scanning electron micrographs indicates that grains are uniformaly distributed

6] Transmission electron micrographs indicates that the mean diameter of the

particles as measured is about 10 nm

7] The conductivity of nano CeO2 thick film is represents the almost linear

relationship was found in the measured temperature range of 200-5000C It is

clear that the conduction in crystalline ceria is essentially an n-type electronic

process

8] The CeO2 nanoparticles in the thick film form prepared by screen printing

technique showed better response to LPG gas at 250degC temperature at 600 ppm

concentration of LPG gas

9] Catalyst can be prepared from inexpensive precursors using the microwave

assisted sol-gel technique is promising for the oxidative regeneration of ketones

10] This study reveals that these CeO2 nanocrystals are novel adsorbents for

wastewater treatment

Chapter ndash V

182

References

[1] A Ramesh H Hasegawa T Maki KUeda Sep Purif Tech

56 (2007) 90

[2] C Lettmann K Hildenbrand H Kisch W Macyk W F Maier

Appl Catal B 32 (2001) 215

[3] X Zhang Z H Ai F L Jia L Z Zhang J Phys Chem C 112 (2008) 747

[4] N Du H Zhang BD Chen X Y Ma D RYang J Phys ChemC

111 (2007) 12677

[5] F Zhang S Yang H Chen X Yu Cera Intern 30 (2004) 997

[6] F Gu SF Wang MK Lu GJ Zhou D Xu DR Langmuir J SciIR Iran

20 (2004) 3528

[7] H Xiao Z Ai L Zhang JPhys Chem 113 (2009) 16625

[8] L Combemale G Caboche D Stuerga D Chaumont MicroRese

Bull 40 (2005) 529

[9] M Zawadzki Jof Alloys and Comp 454 (2008) 347

[10] A V Patil C G Dighavkar S K Sonawane S J Patil R Y Borse

Sens and Transducers 108 (2009) 189

[11] AV Patil CG Dighavkar RY Borse Sens and Transducers

101 (2009) 96

[12] AV Patil CG Dighavkar SK Sonawane S J Patil RY Borse Sensand

Transducers 9 (2010) 11

[13] DR Patil LA Patil Talanta 77 (2009)1409

[14] R S Blackburn Environ Sci Tech 38 (2004) 4905

[15] GNaja CMustin JBerthelin BVolesky J Colloide Inter Sci

292 (2005) 537

[16] VKGupta IAli Environ Sci Tech 42 (2008) 766

[17] A Mittal VK Gupta A Malviya and JMittal J Hazard Mater

151 (2008) 821

[18] VKGupta IAli VKSaini J Colloid Interface Sci 315(2007) 87

Chapter ndash V

183

[19] S S Deshpande S U Sonavane R V Jayaram Cat Com 9 (2008) 639

[20] KM Garadkar BS Shirke YB Patil D R Patil Sens and Transducers

110 (2009) 17

[21] VKGupta AMittal VGajbe JMittal J Colloid Inter Sci

319 (2008) 30

[22] T A Khan I Ali V V Singh S Sharma J Envir Protec Sci 3 (2009)11

[23] A Hosseinnia M Keyanpour-Rad M Pazouki World Applied Sci J

8 (2010) 1327

[24] MI Baraton L Merhari J Am Ceram Soc 75 (2004) 1587

[25] CWang B Xu XWang J Zhao J Solid State Chem 178 (2005) 3500

[26] H Xiao Z Ai L Zhang J Phys Chem C doi 10 1021 JP9050269 (2009)

[27] A M Thompson Oxides of the Rare Earths Wiley New York (1978)

[28] H Noumlremberg G A D Briggs Phys Rev Lett 79 (1997) 4222

[29] A Hosseinnia M Keyanpour-Rad M Pazouki World Appl Sci J

8 (2010) 1327

[30] BD Cullity SR Stock Elements of X-Ray Diffraction 3rd edn

(Prentice Hall Upper Saddle River NJ) (2001) 07458

[31] G Ranga Rao B Gopal Mishra Bul Cat Soc of Ind 2 (2003)122

[32] A K Bhattacharyal T K Naiya S N Mondal S K Das Chem Eng J

137 (2008) 529

[33] G Liu J A Rodriguez Z Chang J Hrbek C H F Peden

J Phys Chem B 108 (2004) 2931

[34] P F Ji J L Zhang F Chen M Anpo J Phys Chem C 112 (2008) 17809

[35] A Trovarelli Catal Rev SciEng 38 (1996) 439

[36] B Li D M Pan J S Zheng Y J Cheng X Y Ma F Huang L Zhang

Langmuir 24 (2008) 9630

[37] M Fernaacutendez-Garcıacutea A A B Martıacutenez-Arias A Hungrıacutea Iglesias-Juez

J C Conesa J Soria J Phys Chem Chem Phys 4 (2002) 2473

[38] H M Chen J H He J Phys Chem C 112 (2008) 17540

Chapter ndash V

184

[39] F Zhang S Chan W Spanier J E Apak E Jin Q Robinson

R D Herman I P Appl Phys Lett 80 (2002) 127

[40] G Adachi TMasui In Catalysis by Ceria and Related Materials

ATrovarelli Ed Imperial College Press London (2002)

[41] J A Rodriguez X Wang J C Hanson G Liu A Iglesias-Juez

MFernaacutendez-Garcıacutea J Chem Phys 119 (2003) 5659

[42] S Eck C Castellarin-Cudia S Surnev M G Ramsey F PNetzer

Surf Sci 520 (2002) 173

[43] R B Yu L Yan P Zheng JChen X X R J Phys ChemC

112 (2008) 19896

[44] Y Nagai T Yamamoto T Tanaka SYoshida T Nonaka T Okamoto

A Suda M Sugiura Catal Today 74 (2002) 225

[45] Y M Chiang E B Lavik I Kosacki H L Tuller JY Ying

J Electroceram 1 (1997) 7

[46] D R Patil L A Patil G H Jain M S Wagh S A Patil

Sens and Transducers 74 (2006) 874

[47] K Sohlberg S K Pantelides S J Pennycook J Am Chem Soc

123 (2001) 6609

[48] L S Zhong J S Hu A M Cao Q Liu W G Song LWan

J Chem Mater 19 (2007) 1648

[49] J B Fei Y Cui X H Yan Y Yang K W Wang QHe J B Li

AdV Mater 20 (2008) 452

[50] P V Kamat J Phys Chem B 106 (2002) 7729

[51] R D Robinson J E Spanier F Zhang SW Chan I P Herman J Appl

Phys 92 (2002) 1936

[52] S W Cao Y J Zhu J Phys Chem C 112 (2008) 6253

[53] R Georgekutty M K Seery S C Pillai J Phys Chem C

112 (2008) 13563

[54] J S Hu L S Zhong W G Song L J Wan Adv Mater 20 (2008)1

Chapter ndash V

185

[55] HXu F L Jia Z H Ai L Z Zhang Cryst Growth Des (2007)

[56] H M Jia W J Xiao L Z Zhang Z F Zheng J PhysChem C

112 (2008) 1379

[57] Z H Ai H Y Xiao T Mei J Liu L Z Zhang K J Deng J R Qiu

J PhysChem C 112 (2008) 11929

[58] L M Ma W X Zhang EnViron Sci Technol 42 (2008) 5384

[59] Z H Ai Y Chen L Z Zhang J R Qiu EnViron Sci Technol

42 (2008) 69550

[60] Z Y Zhong J Ho J Teo S C Shen AGedanken ChemMater

19 (2007) 4776

[61] S C Kuiry S D Patil S Deshpande S Seal J Phys ChemB

109 (2005) 6936

[62] D R Patil L A Patil Sens IEEE 7 (2007) 434

[63] P Jasinski Anderson H U Sens Actuators B 95 (2003) 73

[64] J P Nair E Wachtel I Lubomirsky J Fleig JMaier AdVMater

15 (2003) 2077

[65] R J Qi Y J Zhu G F Cheng Y H Huang Nanotechnology

16 (2005) 2502

Chapter ndash V

161

photodegradation of Rh B At different intervals 5mL of the suspensions was

collected filtered through a 045μm membrane and finally analyzed by a

UV-visible spectrophotometer immediately [22]

53 Results and Discussion

531 UV-Visible Spectroscopic Analysis

The absorption spectra of CeO2 nanoparticles dispersed in ethanol solution

is shown in Fig52 The absorption edge of the CeO2 nanoparticles synthesized

a) by microwave (without annealed) annealed at b) 200degC and c) 300degC occurs at

around 400 nm and its band gap energy is estimated to be 31 eV A sharp band is

an indicative of narrow distribution of particles within the matrix The accurate

band gap of material was 31 eV as obtained by plotting (αhυ)2

against hυ shown

in Fig53

Fig 52 UV-Visible absorption spectrum of CeO2 nanoparticles synthesized

a) by microwave (without annealed) annealed at b) 200degC and c) 300degC

Chapter ndash V

162

In semiconductors the band gap will be increased as particle size is

decreased As particles shrink in size there is a dramatic change in valence band

and conduction bands as the continuous density of states in bulk is replaced with a

set of discrete energy levels This leads to interesting optical properties In the

visible region colour can changes with size Since with increase in size energy

decreases the nanoparticles which have size less than the bulk compound shows

blue shift in the UV-Visible spectra of nanoparticles This blue shift may be

considered as one of the confirmatory test for the existence of nanoparticles

Fig53 Variation of (αhυ) 2

vs Photon Energy (eV) of CeO2 nanoparticle synthesized

a) by microwave (without annealed) annealed at b)200degC and c) 300degC

532 FTIR Analysis

The FT-IR spectrum of the obtained ceria nanoparticles is shown in

Fig 54 Three strong absorption peaks were observed at 3381 1558 and

1372 cm-1

The former was attributed to the stretching band of hydroxyl group and

the others were attributed to the antisymmetric and symmetric stretching band of

Chapter ndash V

163

the glycol group respectively In the present synthesis propylene glycol works as

a protective agent against particle growth and since the stretching band of the

carbonyl around 1700 cm-1

was not observed in the FT-IR spectrum it could be

that the surface of the ceria nanoparticles was covered with glycol species In

addition it has also been reported that non-stoichiometric cerium oxide

synthesized by an inert gas condensation process was brownish yellow

Fig54 FTIR spectrum of CeO2 nanoparticles

533 The XRD Analysis

Fig55 shows the X-ray diffractograms of the as prepared powder of CeO2

nanoparticles a) by microwave (without annealed) annealed at b)200degC and

c) 300degC The CeO2 nanoparticles are crystalline in nature with d values 313

270 191 and 163 Aring The synthesized samples by microwave confirmed the

Chapter ndash V

164

formation of face centered cubic structure (FCC) of CeO2 nanoparticles The

determined characteristics 2θ values and [hkl] planes are 4287deg [111] 5017deg

[200] 7386deg [220] and 89deg [311] respectively (JCPDS card No 81-0792) and

lattice parameter is 542 Aring The average particle size of CeO2 nanoparticles was

found to about 10 nm The particle size of CeO2 nanoparticles obtained by

microwave is less than that of annealed at 200degC and 300degC However the lattice

parameter is found to increase with reduction temperature indicating an expansion

in the FCC lattice

Fig 55 XRD patterns of the CeO2 nanoparticles synthesized a) by microwave

(without annealed) annealed at b) 200degC and c) 300degC

To know the correct structure in cubic the indexing pattern of cubic crystal

was studied in detail A cubic crystal gives diffraction lines whose sin2θ values

Chapter ndash V

165

satisfy the equation obtained by combining the Braggrsquos law with the plane-spacing

equation for the cubic system as

sin2θ (h

2+k

2+l

2) = sin

2θ s = λ

2 4 a

2 --- 52

Since the sum (s) =

(h

2+k

2+l

2) is always integral and λ

2 4a

2 is a constant for any

one pattern of a cubic substance is one of finding a set of integers (s) which will

yield a constant quotient when divided one by one into the observed sin2θ values

Once the proper integers (s) are found the indices (hkl) of each line can be written

down by inspection The proper set of integers (s) is not hard to find because there

are only a few possible sets [30] Each of the four common cubic lattice types has

a characteristic sequence of diffraction lines described by their sequential values

Simple cubic 1 2 3 4 5 6 7 8 9 10 11 12 13 14 16 -------

Body-centered cubic 2 4 6 8 10 12 14 16 -------

Face-centered cubic 3 4 8 11 12 16 --------

In our case the crystal system was found to be face centered cubic (FCC) because

the integers(s) 3 4 8 11 which matches the FCC diffractions lines

Fig 56 Fluorite structure of CeO2

Chapter ndash V

166

The unit cell of ceria is shown in Fig56 In the face centered cubic (FCC)

structure of ceria Ce4+

ions form a cubic close packing arrangement and all the

tetrahedral sites are occupied by the oxide ions whereas the octahedral sites remain

vacant The unit cell of ceria can be considered as a simple cube in which the face

center positions and corners are occupied by Ce4+

ions The tetrahedral sites can be

visualized by dividing the cube into eight smaller cubes The body center positions

of all the small cubes are occupied by oxide ions and the alternate corners are

occupied by Ce4+

ions [31]

Fig57 βcos θ λ vs sin θ λ for CeO2 nanocrystals

The particle size of CeO2 nanoparticles was calculated by using Scherrerrsquos

formula The lattice strain and crystalline size was calculated from the following

equation [18]

β cos θ λ = (1 ε) + (τ sin θ λ) --- 53

Chapter ndash V

167

where β is the measured FWHM (Full Width at Half Maximum) in radian θ is the

Bragg angle of the peak λ is wavelength of X-ray (Cr Kα line 228 Aring) ε is the

effective particle size and τ is the effective strain The average particle size can be

estimated from the extrapolation of the Williamson-Hall plot shown in Fig 57

and the particle size was obtained 9 nm based on the intercept inverse

ie 1 ε = 0011 X 108

which yields ε = 909 X 10-9

m or 9 nm

534 Energy Dispersive Analysis of X-Rays (EDAX)

The cerium oxide nanoparticles were characterized by energy dispersive

X- ray analysis (EDAX) which is attachment of SEM The sample for EDAX was

prepared by depositing a very thin layer of cerium oxide nanoparticles on glass

slide The dispersive X ndash ray analysis was conducted by focusing an electron beam

on several different sectioned regions of copper nanoparticles

Fig58 EDAX spectrum of CeO2 nanoparticles

Chapter ndash V

168

The EDAX spectrum of cerium oxide nano particles synthesized by

microwave assisted sol-gel method is shown in Fig58 the composition of sample

shows the presense of cerium and oxygen elements The spectrum peak reveals the

presence of Ce and O at 4837 and 0525 keV respectively The atomic of Ce

and O is 3597 and 6403 respectively which confirms the presence of Ce and O in

the powder The present composition of Ce and O reveals that the formation of

non-stoichometric CeO2 which is superior for adsorption of toxic ions from

wastewater

535 Scanning Electron Microscopic (SEM) Analysis

The model JEOL JSM-6360 was used for the determination of morphology

of nanoparticle The cerium oxide nanoparticles were dispersed in isopropyl

alcohol and sonicated for frac12 hour One or two drop of sonicated solution was kept

on 1x1 cm glass plate dried naturally and coated the platinum The SEM images

are formed by scanning narrow beam of electrons across the surface of specimen

Fig59 (a) SEM micrograph of CeO2 nanoparticles by microwave

(without annealed)

Chapter ndash V

169

Collecting and processing emitting electron and displaying them on cathode ray

tubecomputer monitor Using a visual raster which is synchronized with the

beams scanned The dried thin films samples of cerium oxide nanoparticles were

scanned by SEM The SEM images of cerium oxide synthesized by microwave

(without annealed) and annealed at 300degC are shown in Figs59 (a) and (b) From

SEM images revealed that there were various sizes of particles in the as-prepared

sample The large particles are composed of small crystallites and show particle

aggregates of irregular shapes The particles are spherical in shapes which are

good agreement with XRD results of particles size These observations were

consistent with similar observations reported in the literature [42ndash44]

Fig59 (b) SEM micrograph of CeO2 nanoparticles annealed at 300degC

536 Transmission Electron Microscopic (TEM) Analysis

In case of cerium oxide the transmission electron micrograph was obtained

by employing Philips microscopy IIT Madras TEM is analogous to the optical

microscope It provides very high resolution which can reach approximately

Chapter ndash V

170

01 nm in the case of lattice images Consequently very high magnification can be

obtained TEM is used to examine very thin section through materials of the

surface A transmission electron microscope Philips 120 kV is equipped with CCD

camera This instrument has the resolution of 036 nm with 42 to 120 kV operating

voltage and magnifies object upto 6 lakhs times in high resolution mode Samples

for TEM examinations were prepared by placing a drop of sample suspension on a

copper grade coated with carbon film and were allowed to dry in air A typical

TEM image of CeO2 powders synthesized by cerium nitrate salts by microwave

assisted sol-gel method (without calcination) is presented in Fig 510 The mean

diameter of the particles as measured is about 10 nm These results are in good

agreement with that of estimated by X-ray line broadening Also the synthesized

Fig510 TEM image of the CeO2 nanoparticles

nanopowder is well crystallized without defects non-agglomerated with narrow

size distributions The selected area electron diffraction (SAED) pattern shown in

the Fig511 it also supports the d values obtained from XRD with rings which

can be assigned [111] [200] [220] and [311] diffractions of CeO2 structure

Chapter ndash V

171

Fig511 SAED image of the CeO2 nanoparticles

537 Thermo-Gravimetric and Differential Thermal Analysis

Fig512 shows the TG-DTA curves for the dried ceria precursor prepared

by the Ce(IV) methods The dried samples may consist of three portions of

materials (A) crystalline ceria CeO2 (B) crystalline ceria with structural water

CeO22H2O and (C) amorphous cerium hydroxide Ce(OH)4 The endothermic

peaks in 64degC could be due to crystallization of the amorphous portion in the

sample Ce(OH)4 to crystalline CeO2 The larger endothermic peak for the sample

indicates that the former contains more amorphous phase than the latter The total

weight loss up to 800 degC for the sample is about 1685 (or 194 for n which is

the number of water molecules per mole of cerium in the dried sample) This is

consistent with the change of [Ce (OH)4 + CeO22H2O] to [CeO2] The two

endothermic peaks at about 313 and 436degC in figure can be attributed to the loss of

the structural water molecules The less obvious endothermic peaks for the

structural water loss for this sample also agree with the weight loss data These

results suggest that the Ce(IV) derived sample contains primarily Ce(OH)4 and

CeO22H2O

Chapter ndash V

172

Fig 512 Weight loss of CeO2 nanoparticles as a function of temperature

538 Electrical Conductivity of Semiconductors CeO2

Semiconductors find wide appilication particularly in electronics Their

application is based on certain properties that will be an electrical conductivity of

semiconductors When the temperature is not very low the semiconductor will

conduct electricity due to the movement of the electrons in the conduction band

and that of holes in the valence band The conductivity of nano CeO2 thick film is

represented in Fig513 From the figure the almost linear relationship was found

in the measured temperature range of 200-500oC If it assumed that conduction in

crystalline ceria is essentially an n-type electronic process then using a simple

model we can express the conductivity-temperature relation by the equation

σ = σo e-Ea kT

where σo is a constant Ea is thermal activation energy T is the

Kelvin temperature and k is Boltzmanns constant Thus an Arrhenius plot of

conductivity and temperature should be linear over certain temperature range

Although the results in Fig513 are curved a linear section can be drawn above

280oC which gives activation energy of 236 eV

Chapter ndash V

173

Fig513 Electrical conductivity of CeO2 nanoparticles as a function of temperature

539 Gas Sensing Performance

5391 LPG Gas Sensing

Fig514 shows the variation of response of CeO2 fired at various

temperature The gas response increases with temperature from 50 to 150degC and

then decreases with a further increase in temperature In present work every time

prior to exposing the CeO2 film to LPG it was allowed to stabilize at an operating

temperature for 10 min and the stabilized resistance was taken as Ra After

exposing the film to the LPG gas the changed resistance was taken as Rg LPG

reacts with surface oxygen ions of the film Oxidation of film decreases the

number of free carriers Therefore resistance of the film increases with gas The

result of reaction of LPG with polycrystalline CeO2 is adsorbed CO2 and H2O on

the surface of the oxide The Ce harrLPG interactions on CeO2 are strong and Ce

sites probably get oxidized

Chapter ndash V

174

Fig 514 Variation of gas responses with operating temperature for LPG gas

5393 Effect of Gas Concentration (active region)

The variation of sensor response of CeO2 thick film sample with LPG gas

concentration at operating temperature 150degC is represented in Fig 515

Fig 515 Variation in sensor response with LPG gas concentration

Chapter ndash V

175

It is clear from the figure that the gas response goes on increasing with gas

concentration upto 600 ppm The rate of increase in response was relatively large

upto 1000 ppm and saturated beyond 600 ppm The monolayer of the gas

molecules formed on the surface covers the whole surface of the film The gas

molecules from that layer would reach the surface active sites of the film The

excess gas molecules remain idle and would not reach the surface active sites of

the sensor Thus the active region of the sensor would be up to 600 ppm

5394 Response and Recovery Time

The response of CeO2 thick film sensor was found to be quick (~ 28 s) to

600 ppm of while the recovery was fast (~ 10 s) (Fig 516) The fast response

may be attributed to faster oxidation of the gas The negligible quantity of the

surface reaction product and its high volatility explains its fast response and quick

recovery to its initial chemical status

Fig 516 Response and recovery time

Chapter ndash V

176

5395 Oxygen Adsorption-Desorption Mechanism

O2 (air) + 2e- 2O

-(film surface) --- 56

At higher temperature the atmospheric oxygen O2 adsorbs on the surface of the

thick film It captures the electrons (Fig517) from conduction band It would

result in decreasing conductivity of the film When LPG reacts with the adsorbed

oxygen on the surface of the film it gets oxidized to CO2 and H2O by following

series of intermediate stages This liberates free electrons in the conduction band

The final reaction takes place as

C4H10(gas) + 13Ominus

(filmsurface) rarr 4CO2(gas) + 5H2O(gas) + 13eminus

(condband) --- 57

Fig 517 Sensing mechanism on the surface of the film

This shows n-type conduction mechanism Thus generated electrons

contribute to a sudden increase in conductance of the thick film The mass of

Ce and O in sample was not as per the stoichiometric proportion and sample was

Chapter ndash V

177

observed to be the oxygen deficient This deficiency gets reduced due to

adsorption of atmosphericmolecular oxygen This helps in decreasing electronic

conductivity of the film Upon exposure LPG molecules got oxidized with the

adsorbed oxygen ions by following the series of intermediate stages producing

CO2 and H2O This results in evolving oxygen as electrically neutral atoms

trapping behind the negative charges (electrons) Upon exposure the energy

released in decomposition of LPG molecules would be sufficient for trapped

electrons to jump into the conduction band of activated CeO2 resulting in increase

in the conductivity of the film The drastic increase in conductivity of the sensor

could be attributed to the adsorptionndashdesorption mechanism resulted from the

electronic defects [26]

5310 Catalytic Activity

The utility of this catalytic system deprotection of several oximes were

studied (Table 52) The catalyst was also tested for reusability and it was found

that it could be reused without loss of initial activity for at least four cycles [19]

TLC analysis indicated the formation of only one product of corresponding

carbonyls FT-IR and NMR analysis showed that the oximes were converted into

carbonyls For checking the reusability (after each run) the catalyst was recovered

by simple filtration and washed several times with acetone and then dried at

373 K The catalyst thus obtained was used for the subsequent runs without any

further modification both activated and deactivated oximes were oxidized in better

yield more than ZrO2 catalyst as shown in Table 52

Chapter ndash V

178

Table 52 Oxidative Deoximation of Carbonyls on CeO2

Entry

Substrate

Product

Time

(h)

Yield

()

1

NOH

O

15

62

2

NOH

Cl

O

Cl

15

58

3

NOH

O

25

73

4

NOHCl

OCl

40

57

5

NOH

Cl

O

Cl

30

59

Solvent- Ethyl acetate Catalyst- ZrO2 Temperature- 353 K

Chapter ndash V

179

5311 Adsorption of Rhodamine B (Rh B)

The removal of dye pollutants are also of great importance for wastewater

treatment Rhodamine B a common cationic dye used in textile industry is one of

the most notorious contaminants in aquatic environments because of it huge

amounts slow biodegradation and toxicity In this study synthesized CeO2

nanocrystals are further used to remove Rh B The absorption spectrum of Rh B

solution was characterized by its characteristic absorption at 555 nm which was

attributed to the chromophore containing azo linkage of the dye molecules [22]

Fig 518 Absorption spectra of aqueous solution of Rh B in the presence of CeO2

nanocrystals at different times

UV-Visible absorption spectra represented in Fig518 confirms that over 93 of

Rh B in 100 mL of 20 mg L-1

Rh B aqueous solution could be removed with 100

mg of the as-prepared CeO2 nanocrystalline within 60 min The reasons may

account for the efficient removal of Rh B with CeO2 nanocrystals The first reason

Chapter ndash V

180

should be their to facilitate the adsorption of Rh B The second one is attributed to

the oxygen vacancies on the surface of CeO2 nanocrystals These abundant surface

oxygen vacancies could produce strong electrostatic attraction with the cationic

groups of Rh B as well as hydrogen bonding with the nitrogen atoms of Rh B

Chapter ndash V

181

Conclusions

Microwave synthesis is very beneficial to find a fast simple and energy

efficient approach to produce fine CeO2 nanoparticles It is relatively new method

to produce inorganic compounds for materials processing to enhance the material

properties as well as economic advantages through energy saving and acceleration

of product development Cerium oxide has outstanding physical and chemical

properties therefore it is used as LPG sensor In the area of catalysis nanoparticles

of ceria have been studied Also it is concluded as

1] The CeO2 nanoparticles were prepared by simple microwave assisted sol-gel

method

2] The well spherical and narrow size distribution with 10 nm size particles were

obtained by this method

3] It is very simple time as well as energy saving technique

4] The XRD pattern shows the CeO2 nanoparticles exists in face centered cubic

5] Scanning electron micrographs indicates that grains are uniformaly distributed

6] Transmission electron micrographs indicates that the mean diameter of the

particles as measured is about 10 nm

7] The conductivity of nano CeO2 thick film is represents the almost linear

relationship was found in the measured temperature range of 200-5000C It is

clear that the conduction in crystalline ceria is essentially an n-type electronic

process

8] The CeO2 nanoparticles in the thick film form prepared by screen printing

technique showed better response to LPG gas at 250degC temperature at 600 ppm

concentration of LPG gas

9] Catalyst can be prepared from inexpensive precursors using the microwave

assisted sol-gel technique is promising for the oxidative regeneration of ketones

10] This study reveals that these CeO2 nanocrystals are novel adsorbents for

wastewater treatment

Chapter ndash V

182

References

[1] A Ramesh H Hasegawa T Maki KUeda Sep Purif Tech

56 (2007) 90

[2] C Lettmann K Hildenbrand H Kisch W Macyk W F Maier

Appl Catal B 32 (2001) 215

[3] X Zhang Z H Ai F L Jia L Z Zhang J Phys Chem C 112 (2008) 747

[4] N Du H Zhang BD Chen X Y Ma D RYang J Phys ChemC

111 (2007) 12677

[5] F Zhang S Yang H Chen X Yu Cera Intern 30 (2004) 997

[6] F Gu SF Wang MK Lu GJ Zhou D Xu DR Langmuir J SciIR Iran

20 (2004) 3528

[7] H Xiao Z Ai L Zhang JPhys Chem 113 (2009) 16625

[8] L Combemale G Caboche D Stuerga D Chaumont MicroRese

Bull 40 (2005) 529

[9] M Zawadzki Jof Alloys and Comp 454 (2008) 347

[10] A V Patil C G Dighavkar S K Sonawane S J Patil R Y Borse

Sens and Transducers 108 (2009) 189

[11] AV Patil CG Dighavkar RY Borse Sens and Transducers

101 (2009) 96

[12] AV Patil CG Dighavkar SK Sonawane S J Patil RY Borse Sensand

Transducers 9 (2010) 11

[13] DR Patil LA Patil Talanta 77 (2009)1409

[14] R S Blackburn Environ Sci Tech 38 (2004) 4905

[15] GNaja CMustin JBerthelin BVolesky J Colloide Inter Sci

292 (2005) 537

[16] VKGupta IAli Environ Sci Tech 42 (2008) 766

[17] A Mittal VK Gupta A Malviya and JMittal J Hazard Mater

151 (2008) 821

[18] VKGupta IAli VKSaini J Colloid Interface Sci 315(2007) 87

Chapter ndash V

183

[19] S S Deshpande S U Sonavane R V Jayaram Cat Com 9 (2008) 639

[20] KM Garadkar BS Shirke YB Patil D R Patil Sens and Transducers

110 (2009) 17

[21] VKGupta AMittal VGajbe JMittal J Colloid Inter Sci

319 (2008) 30

[22] T A Khan I Ali V V Singh S Sharma J Envir Protec Sci 3 (2009)11

[23] A Hosseinnia M Keyanpour-Rad M Pazouki World Applied Sci J

8 (2010) 1327

[24] MI Baraton L Merhari J Am Ceram Soc 75 (2004) 1587

[25] CWang B Xu XWang J Zhao J Solid State Chem 178 (2005) 3500

[26] H Xiao Z Ai L Zhang J Phys Chem C doi 10 1021 JP9050269 (2009)

[27] A M Thompson Oxides of the Rare Earths Wiley New York (1978)

[28] H Noumlremberg G A D Briggs Phys Rev Lett 79 (1997) 4222

[29] A Hosseinnia M Keyanpour-Rad M Pazouki World Appl Sci J

8 (2010) 1327

[30] BD Cullity SR Stock Elements of X-Ray Diffraction 3rd edn

(Prentice Hall Upper Saddle River NJ) (2001) 07458

[31] G Ranga Rao B Gopal Mishra Bul Cat Soc of Ind 2 (2003)122

[32] A K Bhattacharyal T K Naiya S N Mondal S K Das Chem Eng J

137 (2008) 529

[33] G Liu J A Rodriguez Z Chang J Hrbek C H F Peden

J Phys Chem B 108 (2004) 2931

[34] P F Ji J L Zhang F Chen M Anpo J Phys Chem C 112 (2008) 17809

[35] A Trovarelli Catal Rev SciEng 38 (1996) 439

[36] B Li D M Pan J S Zheng Y J Cheng X Y Ma F Huang L Zhang

Langmuir 24 (2008) 9630

[37] M Fernaacutendez-Garcıacutea A A B Martıacutenez-Arias A Hungrıacutea Iglesias-Juez

J C Conesa J Soria J Phys Chem Chem Phys 4 (2002) 2473

[38] H M Chen J H He J Phys Chem C 112 (2008) 17540

Chapter ndash V

184

[39] F Zhang S Chan W Spanier J E Apak E Jin Q Robinson

R D Herman I P Appl Phys Lett 80 (2002) 127

[40] G Adachi TMasui In Catalysis by Ceria and Related Materials

ATrovarelli Ed Imperial College Press London (2002)

[41] J A Rodriguez X Wang J C Hanson G Liu A Iglesias-Juez

MFernaacutendez-Garcıacutea J Chem Phys 119 (2003) 5659

[42] S Eck C Castellarin-Cudia S Surnev M G Ramsey F PNetzer

Surf Sci 520 (2002) 173

[43] R B Yu L Yan P Zheng JChen X X R J Phys ChemC

112 (2008) 19896

[44] Y Nagai T Yamamoto T Tanaka SYoshida T Nonaka T Okamoto

A Suda M Sugiura Catal Today 74 (2002) 225

[45] Y M Chiang E B Lavik I Kosacki H L Tuller JY Ying

J Electroceram 1 (1997) 7

[46] D R Patil L A Patil G H Jain M S Wagh S A Patil

Sens and Transducers 74 (2006) 874

[47] K Sohlberg S K Pantelides S J Pennycook J Am Chem Soc

123 (2001) 6609

[48] L S Zhong J S Hu A M Cao Q Liu W G Song LWan

J Chem Mater 19 (2007) 1648

[49] J B Fei Y Cui X H Yan Y Yang K W Wang QHe J B Li

AdV Mater 20 (2008) 452

[50] P V Kamat J Phys Chem B 106 (2002) 7729

[51] R D Robinson J E Spanier F Zhang SW Chan I P Herman J Appl

Phys 92 (2002) 1936

[52] S W Cao Y J Zhu J Phys Chem C 112 (2008) 6253

[53] R Georgekutty M K Seery S C Pillai J Phys Chem C

112 (2008) 13563

[54] J S Hu L S Zhong W G Song L J Wan Adv Mater 20 (2008)1

Chapter ndash V

185

[55] HXu F L Jia Z H Ai L Z Zhang Cryst Growth Des (2007)

[56] H M Jia W J Xiao L Z Zhang Z F Zheng J PhysChem C

112 (2008) 1379

[57] Z H Ai H Y Xiao T Mei J Liu L Z Zhang K J Deng J R Qiu

J PhysChem C 112 (2008) 11929

[58] L M Ma W X Zhang EnViron Sci Technol 42 (2008) 5384

[59] Z H Ai Y Chen L Z Zhang J R Qiu EnViron Sci Technol

42 (2008) 69550

[60] Z Y Zhong J Ho J Teo S C Shen AGedanken ChemMater

19 (2007) 4776

[61] S C Kuiry S D Patil S Deshpande S Seal J Phys ChemB

109 (2005) 6936

[62] D R Patil L A Patil Sens IEEE 7 (2007) 434

[63] P Jasinski Anderson H U Sens Actuators B 95 (2003) 73

[64] J P Nair E Wachtel I Lubomirsky J Fleig JMaier AdVMater

15 (2003) 2077

[65] R J Qi Y J Zhu G F Cheng Y H Huang Nanotechnology

16 (2005) 2502

Chapter ndash V

162

In semiconductors the band gap will be increased as particle size is

decreased As particles shrink in size there is a dramatic change in valence band

and conduction bands as the continuous density of states in bulk is replaced with a

set of discrete energy levels This leads to interesting optical properties In the

visible region colour can changes with size Since with increase in size energy

decreases the nanoparticles which have size less than the bulk compound shows

blue shift in the UV-Visible spectra of nanoparticles This blue shift may be

considered as one of the confirmatory test for the existence of nanoparticles

Fig53 Variation of (αhυ) 2

vs Photon Energy (eV) of CeO2 nanoparticle synthesized

a) by microwave (without annealed) annealed at b)200degC and c) 300degC

532 FTIR Analysis

The FT-IR spectrum of the obtained ceria nanoparticles is shown in

Fig 54 Three strong absorption peaks were observed at 3381 1558 and

1372 cm-1

The former was attributed to the stretching band of hydroxyl group and

the others were attributed to the antisymmetric and symmetric stretching band of

Chapter ndash V

163

the glycol group respectively In the present synthesis propylene glycol works as

a protective agent against particle growth and since the stretching band of the

carbonyl around 1700 cm-1

was not observed in the FT-IR spectrum it could be

that the surface of the ceria nanoparticles was covered with glycol species In

addition it has also been reported that non-stoichiometric cerium oxide

synthesized by an inert gas condensation process was brownish yellow

Fig54 FTIR spectrum of CeO2 nanoparticles

533 The XRD Analysis

Fig55 shows the X-ray diffractograms of the as prepared powder of CeO2

nanoparticles a) by microwave (without annealed) annealed at b)200degC and

c) 300degC The CeO2 nanoparticles are crystalline in nature with d values 313

270 191 and 163 Aring The synthesized samples by microwave confirmed the

Chapter ndash V

164

formation of face centered cubic structure (FCC) of CeO2 nanoparticles The

determined characteristics 2θ values and [hkl] planes are 4287deg [111] 5017deg

[200] 7386deg [220] and 89deg [311] respectively (JCPDS card No 81-0792) and

lattice parameter is 542 Aring The average particle size of CeO2 nanoparticles was

found to about 10 nm The particle size of CeO2 nanoparticles obtained by

microwave is less than that of annealed at 200degC and 300degC However the lattice

parameter is found to increase with reduction temperature indicating an expansion

in the FCC lattice

Fig 55 XRD patterns of the CeO2 nanoparticles synthesized a) by microwave

(without annealed) annealed at b) 200degC and c) 300degC

To know the correct structure in cubic the indexing pattern of cubic crystal

was studied in detail A cubic crystal gives diffraction lines whose sin2θ values

Chapter ndash V

165

satisfy the equation obtained by combining the Braggrsquos law with the plane-spacing

equation for the cubic system as

sin2θ (h

2+k

2+l

2) = sin

2θ s = λ

2 4 a

2 --- 52

Since the sum (s) =

(h

2+k

2+l

2) is always integral and λ

2 4a

2 is a constant for any

one pattern of a cubic substance is one of finding a set of integers (s) which will

yield a constant quotient when divided one by one into the observed sin2θ values

Once the proper integers (s) are found the indices (hkl) of each line can be written

down by inspection The proper set of integers (s) is not hard to find because there

are only a few possible sets [30] Each of the four common cubic lattice types has

a characteristic sequence of diffraction lines described by their sequential values

Simple cubic 1 2 3 4 5 6 7 8 9 10 11 12 13 14 16 -------

Body-centered cubic 2 4 6 8 10 12 14 16 -------

Face-centered cubic 3 4 8 11 12 16 --------

In our case the crystal system was found to be face centered cubic (FCC) because

the integers(s) 3 4 8 11 which matches the FCC diffractions lines

Fig 56 Fluorite structure of CeO2

Chapter ndash V

166

The unit cell of ceria is shown in Fig56 In the face centered cubic (FCC)

structure of ceria Ce4+

ions form a cubic close packing arrangement and all the

tetrahedral sites are occupied by the oxide ions whereas the octahedral sites remain

vacant The unit cell of ceria can be considered as a simple cube in which the face

center positions and corners are occupied by Ce4+

ions The tetrahedral sites can be

visualized by dividing the cube into eight smaller cubes The body center positions

of all the small cubes are occupied by oxide ions and the alternate corners are

occupied by Ce4+

ions [31]

Fig57 βcos θ λ vs sin θ λ for CeO2 nanocrystals

The particle size of CeO2 nanoparticles was calculated by using Scherrerrsquos

formula The lattice strain and crystalline size was calculated from the following

equation [18]

β cos θ λ = (1 ε) + (τ sin θ λ) --- 53

Chapter ndash V

167

where β is the measured FWHM (Full Width at Half Maximum) in radian θ is the

Bragg angle of the peak λ is wavelength of X-ray (Cr Kα line 228 Aring) ε is the

effective particle size and τ is the effective strain The average particle size can be

estimated from the extrapolation of the Williamson-Hall plot shown in Fig 57

and the particle size was obtained 9 nm based on the intercept inverse

ie 1 ε = 0011 X 108

which yields ε = 909 X 10-9

m or 9 nm

534 Energy Dispersive Analysis of X-Rays (EDAX)

The cerium oxide nanoparticles were characterized by energy dispersive

X- ray analysis (EDAX) which is attachment of SEM The sample for EDAX was

prepared by depositing a very thin layer of cerium oxide nanoparticles on glass

slide The dispersive X ndash ray analysis was conducted by focusing an electron beam

on several different sectioned regions of copper nanoparticles

Fig58 EDAX spectrum of CeO2 nanoparticles

Chapter ndash V

168

The EDAX spectrum of cerium oxide nano particles synthesized by

microwave assisted sol-gel method is shown in Fig58 the composition of sample

shows the presense of cerium and oxygen elements The spectrum peak reveals the

presence of Ce and O at 4837 and 0525 keV respectively The atomic of Ce

and O is 3597 and 6403 respectively which confirms the presence of Ce and O in

the powder The present composition of Ce and O reveals that the formation of

non-stoichometric CeO2 which is superior for adsorption of toxic ions from

wastewater

535 Scanning Electron Microscopic (SEM) Analysis

The model JEOL JSM-6360 was used for the determination of morphology

of nanoparticle The cerium oxide nanoparticles were dispersed in isopropyl

alcohol and sonicated for frac12 hour One or two drop of sonicated solution was kept

on 1x1 cm glass plate dried naturally and coated the platinum The SEM images

are formed by scanning narrow beam of electrons across the surface of specimen

Fig59 (a) SEM micrograph of CeO2 nanoparticles by microwave

(without annealed)

Chapter ndash V

169

Collecting and processing emitting electron and displaying them on cathode ray

tubecomputer monitor Using a visual raster which is synchronized with the

beams scanned The dried thin films samples of cerium oxide nanoparticles were

scanned by SEM The SEM images of cerium oxide synthesized by microwave

(without annealed) and annealed at 300degC are shown in Figs59 (a) and (b) From

SEM images revealed that there were various sizes of particles in the as-prepared

sample The large particles are composed of small crystallites and show particle

aggregates of irregular shapes The particles are spherical in shapes which are

good agreement with XRD results of particles size These observations were

consistent with similar observations reported in the literature [42ndash44]

Fig59 (b) SEM micrograph of CeO2 nanoparticles annealed at 300degC

536 Transmission Electron Microscopic (TEM) Analysis

In case of cerium oxide the transmission electron micrograph was obtained

by employing Philips microscopy IIT Madras TEM is analogous to the optical

microscope It provides very high resolution which can reach approximately

Chapter ndash V

170

01 nm in the case of lattice images Consequently very high magnification can be

obtained TEM is used to examine very thin section through materials of the

surface A transmission electron microscope Philips 120 kV is equipped with CCD

camera This instrument has the resolution of 036 nm with 42 to 120 kV operating

voltage and magnifies object upto 6 lakhs times in high resolution mode Samples

for TEM examinations were prepared by placing a drop of sample suspension on a

copper grade coated with carbon film and were allowed to dry in air A typical

TEM image of CeO2 powders synthesized by cerium nitrate salts by microwave

assisted sol-gel method (without calcination) is presented in Fig 510 The mean

diameter of the particles as measured is about 10 nm These results are in good

agreement with that of estimated by X-ray line broadening Also the synthesized

Fig510 TEM image of the CeO2 nanoparticles

nanopowder is well crystallized without defects non-agglomerated with narrow

size distributions The selected area electron diffraction (SAED) pattern shown in

the Fig511 it also supports the d values obtained from XRD with rings which

can be assigned [111] [200] [220] and [311] diffractions of CeO2 structure

Chapter ndash V

171

Fig511 SAED image of the CeO2 nanoparticles

537 Thermo-Gravimetric and Differential Thermal Analysis

Fig512 shows the TG-DTA curves for the dried ceria precursor prepared

by the Ce(IV) methods The dried samples may consist of three portions of

materials (A) crystalline ceria CeO2 (B) crystalline ceria with structural water

CeO22H2O and (C) amorphous cerium hydroxide Ce(OH)4 The endothermic

peaks in 64degC could be due to crystallization of the amorphous portion in the

sample Ce(OH)4 to crystalline CeO2 The larger endothermic peak for the sample

indicates that the former contains more amorphous phase than the latter The total

weight loss up to 800 degC for the sample is about 1685 (or 194 for n which is

the number of water molecules per mole of cerium in the dried sample) This is

consistent with the change of [Ce (OH)4 + CeO22H2O] to [CeO2] The two

endothermic peaks at about 313 and 436degC in figure can be attributed to the loss of

the structural water molecules The less obvious endothermic peaks for the

structural water loss for this sample also agree with the weight loss data These

results suggest that the Ce(IV) derived sample contains primarily Ce(OH)4 and

CeO22H2O

Chapter ndash V

172

Fig 512 Weight loss of CeO2 nanoparticles as a function of temperature

538 Electrical Conductivity of Semiconductors CeO2

Semiconductors find wide appilication particularly in electronics Their

application is based on certain properties that will be an electrical conductivity of

semiconductors When the temperature is not very low the semiconductor will

conduct electricity due to the movement of the electrons in the conduction band

and that of holes in the valence band The conductivity of nano CeO2 thick film is

represented in Fig513 From the figure the almost linear relationship was found

in the measured temperature range of 200-500oC If it assumed that conduction in

crystalline ceria is essentially an n-type electronic process then using a simple

model we can express the conductivity-temperature relation by the equation

σ = σo e-Ea kT

where σo is a constant Ea is thermal activation energy T is the

Kelvin temperature and k is Boltzmanns constant Thus an Arrhenius plot of

conductivity and temperature should be linear over certain temperature range

Although the results in Fig513 are curved a linear section can be drawn above

280oC which gives activation energy of 236 eV

Chapter ndash V

173

Fig513 Electrical conductivity of CeO2 nanoparticles as a function of temperature

539 Gas Sensing Performance

5391 LPG Gas Sensing

Fig514 shows the variation of response of CeO2 fired at various

temperature The gas response increases with temperature from 50 to 150degC and

then decreases with a further increase in temperature In present work every time

prior to exposing the CeO2 film to LPG it was allowed to stabilize at an operating

temperature for 10 min and the stabilized resistance was taken as Ra After

exposing the film to the LPG gas the changed resistance was taken as Rg LPG

reacts with surface oxygen ions of the film Oxidation of film decreases the

number of free carriers Therefore resistance of the film increases with gas The

result of reaction of LPG with polycrystalline CeO2 is adsorbed CO2 and H2O on

the surface of the oxide The Ce harrLPG interactions on CeO2 are strong and Ce

sites probably get oxidized

Chapter ndash V

174

Fig 514 Variation of gas responses with operating temperature for LPG gas

5393 Effect of Gas Concentration (active region)

The variation of sensor response of CeO2 thick film sample with LPG gas

concentration at operating temperature 150degC is represented in Fig 515

Fig 515 Variation in sensor response with LPG gas concentration

Chapter ndash V

175

It is clear from the figure that the gas response goes on increasing with gas

concentration upto 600 ppm The rate of increase in response was relatively large

upto 1000 ppm and saturated beyond 600 ppm The monolayer of the gas

molecules formed on the surface covers the whole surface of the film The gas

molecules from that layer would reach the surface active sites of the film The

excess gas molecules remain idle and would not reach the surface active sites of

the sensor Thus the active region of the sensor would be up to 600 ppm

5394 Response and Recovery Time

The response of CeO2 thick film sensor was found to be quick (~ 28 s) to

600 ppm of while the recovery was fast (~ 10 s) (Fig 516) The fast response

may be attributed to faster oxidation of the gas The negligible quantity of the

surface reaction product and its high volatility explains its fast response and quick

recovery to its initial chemical status

Fig 516 Response and recovery time

Chapter ndash V

176

5395 Oxygen Adsorption-Desorption Mechanism

O2 (air) + 2e- 2O

-(film surface) --- 56

At higher temperature the atmospheric oxygen O2 adsorbs on the surface of the

thick film It captures the electrons (Fig517) from conduction band It would

result in decreasing conductivity of the film When LPG reacts with the adsorbed

oxygen on the surface of the film it gets oxidized to CO2 and H2O by following

series of intermediate stages This liberates free electrons in the conduction band

The final reaction takes place as

C4H10(gas) + 13Ominus

(filmsurface) rarr 4CO2(gas) + 5H2O(gas) + 13eminus

(condband) --- 57

Fig 517 Sensing mechanism on the surface of the film

This shows n-type conduction mechanism Thus generated electrons

contribute to a sudden increase in conductance of the thick film The mass of

Ce and O in sample was not as per the stoichiometric proportion and sample was

Chapter ndash V

177

observed to be the oxygen deficient This deficiency gets reduced due to

adsorption of atmosphericmolecular oxygen This helps in decreasing electronic

conductivity of the film Upon exposure LPG molecules got oxidized with the

adsorbed oxygen ions by following the series of intermediate stages producing

CO2 and H2O This results in evolving oxygen as electrically neutral atoms

trapping behind the negative charges (electrons) Upon exposure the energy

released in decomposition of LPG molecules would be sufficient for trapped

electrons to jump into the conduction band of activated CeO2 resulting in increase

in the conductivity of the film The drastic increase in conductivity of the sensor

could be attributed to the adsorptionndashdesorption mechanism resulted from the

electronic defects [26]

5310 Catalytic Activity

The utility of this catalytic system deprotection of several oximes were

studied (Table 52) The catalyst was also tested for reusability and it was found

that it could be reused without loss of initial activity for at least four cycles [19]

TLC analysis indicated the formation of only one product of corresponding

carbonyls FT-IR and NMR analysis showed that the oximes were converted into

carbonyls For checking the reusability (after each run) the catalyst was recovered

by simple filtration and washed several times with acetone and then dried at

373 K The catalyst thus obtained was used for the subsequent runs without any

further modification both activated and deactivated oximes were oxidized in better

yield more than ZrO2 catalyst as shown in Table 52

Chapter ndash V

178

Table 52 Oxidative Deoximation of Carbonyls on CeO2

Entry

Substrate

Product

Time

(h)

Yield

()

1

NOH

O

15

62

2

NOH

Cl

O

Cl

15

58

3

NOH

O

25

73

4

NOHCl

OCl

40

57

5

NOH

Cl

O

Cl

30

59

Solvent- Ethyl acetate Catalyst- ZrO2 Temperature- 353 K

Chapter ndash V

179

5311 Adsorption of Rhodamine B (Rh B)

The removal of dye pollutants are also of great importance for wastewater

treatment Rhodamine B a common cationic dye used in textile industry is one of

the most notorious contaminants in aquatic environments because of it huge

amounts slow biodegradation and toxicity In this study synthesized CeO2

nanocrystals are further used to remove Rh B The absorption spectrum of Rh B

solution was characterized by its characteristic absorption at 555 nm which was

attributed to the chromophore containing azo linkage of the dye molecules [22]

Fig 518 Absorption spectra of aqueous solution of Rh B in the presence of CeO2

nanocrystals at different times

UV-Visible absorption spectra represented in Fig518 confirms that over 93 of

Rh B in 100 mL of 20 mg L-1

Rh B aqueous solution could be removed with 100

mg of the as-prepared CeO2 nanocrystalline within 60 min The reasons may

account for the efficient removal of Rh B with CeO2 nanocrystals The first reason

Chapter ndash V

180

should be their to facilitate the adsorption of Rh B The second one is attributed to

the oxygen vacancies on the surface of CeO2 nanocrystals These abundant surface

oxygen vacancies could produce strong electrostatic attraction with the cationic

groups of Rh B as well as hydrogen bonding with the nitrogen atoms of Rh B

Chapter ndash V

181

Conclusions

Microwave synthesis is very beneficial to find a fast simple and energy

efficient approach to produce fine CeO2 nanoparticles It is relatively new method

to produce inorganic compounds for materials processing to enhance the material

properties as well as economic advantages through energy saving and acceleration

of product development Cerium oxide has outstanding physical and chemical

properties therefore it is used as LPG sensor In the area of catalysis nanoparticles

of ceria have been studied Also it is concluded as

1] The CeO2 nanoparticles were prepared by simple microwave assisted sol-gel

method

2] The well spherical and narrow size distribution with 10 nm size particles were

obtained by this method

3] It is very simple time as well as energy saving technique

4] The XRD pattern shows the CeO2 nanoparticles exists in face centered cubic

5] Scanning electron micrographs indicates that grains are uniformaly distributed

6] Transmission electron micrographs indicates that the mean diameter of the

particles as measured is about 10 nm

7] The conductivity of nano CeO2 thick film is represents the almost linear

relationship was found in the measured temperature range of 200-5000C It is

clear that the conduction in crystalline ceria is essentially an n-type electronic

process

8] The CeO2 nanoparticles in the thick film form prepared by screen printing

technique showed better response to LPG gas at 250degC temperature at 600 ppm

concentration of LPG gas

9] Catalyst can be prepared from inexpensive precursors using the microwave

assisted sol-gel technique is promising for the oxidative regeneration of ketones

10] This study reveals that these CeO2 nanocrystals are novel adsorbents for

wastewater treatment

Chapter ndash V

182

References

[1] A Ramesh H Hasegawa T Maki KUeda Sep Purif Tech

56 (2007) 90

[2] C Lettmann K Hildenbrand H Kisch W Macyk W F Maier

Appl Catal B 32 (2001) 215

[3] X Zhang Z H Ai F L Jia L Z Zhang J Phys Chem C 112 (2008) 747

[4] N Du H Zhang BD Chen X Y Ma D RYang J Phys ChemC

111 (2007) 12677

[5] F Zhang S Yang H Chen X Yu Cera Intern 30 (2004) 997

[6] F Gu SF Wang MK Lu GJ Zhou D Xu DR Langmuir J SciIR Iran

20 (2004) 3528

[7] H Xiao Z Ai L Zhang JPhys Chem 113 (2009) 16625

[8] L Combemale G Caboche D Stuerga D Chaumont MicroRese

Bull 40 (2005) 529

[9] M Zawadzki Jof Alloys and Comp 454 (2008) 347

[10] A V Patil C G Dighavkar S K Sonawane S J Patil R Y Borse

Sens and Transducers 108 (2009) 189

[11] AV Patil CG Dighavkar RY Borse Sens and Transducers

101 (2009) 96

[12] AV Patil CG Dighavkar SK Sonawane S J Patil RY Borse Sensand

Transducers 9 (2010) 11

[13] DR Patil LA Patil Talanta 77 (2009)1409

[14] R S Blackburn Environ Sci Tech 38 (2004) 4905

[15] GNaja CMustin JBerthelin BVolesky J Colloide Inter Sci

292 (2005) 537

[16] VKGupta IAli Environ Sci Tech 42 (2008) 766

[17] A Mittal VK Gupta A Malviya and JMittal J Hazard Mater

151 (2008) 821

[18] VKGupta IAli VKSaini J Colloid Interface Sci 315(2007) 87

Chapter ndash V

183

[19] S S Deshpande S U Sonavane R V Jayaram Cat Com 9 (2008) 639

[20] KM Garadkar BS Shirke YB Patil D R Patil Sens and Transducers

110 (2009) 17

[21] VKGupta AMittal VGajbe JMittal J Colloid Inter Sci

319 (2008) 30

[22] T A Khan I Ali V V Singh S Sharma J Envir Protec Sci 3 (2009)11

[23] A Hosseinnia M Keyanpour-Rad M Pazouki World Applied Sci J

8 (2010) 1327

[24] MI Baraton L Merhari J Am Ceram Soc 75 (2004) 1587

[25] CWang B Xu XWang J Zhao J Solid State Chem 178 (2005) 3500

[26] H Xiao Z Ai L Zhang J Phys Chem C doi 10 1021 JP9050269 (2009)

[27] A M Thompson Oxides of the Rare Earths Wiley New York (1978)

[28] H Noumlremberg G A D Briggs Phys Rev Lett 79 (1997) 4222

[29] A Hosseinnia M Keyanpour-Rad M Pazouki World Appl Sci J

8 (2010) 1327

[30] BD Cullity SR Stock Elements of X-Ray Diffraction 3rd edn

(Prentice Hall Upper Saddle River NJ) (2001) 07458

[31] G Ranga Rao B Gopal Mishra Bul Cat Soc of Ind 2 (2003)122

[32] A K Bhattacharyal T K Naiya S N Mondal S K Das Chem Eng J

137 (2008) 529

[33] G Liu J A Rodriguez Z Chang J Hrbek C H F Peden

J Phys Chem B 108 (2004) 2931

[34] P F Ji J L Zhang F Chen M Anpo J Phys Chem C 112 (2008) 17809

[35] A Trovarelli Catal Rev SciEng 38 (1996) 439

[36] B Li D M Pan J S Zheng Y J Cheng X Y Ma F Huang L Zhang

Langmuir 24 (2008) 9630

[37] M Fernaacutendez-Garcıacutea A A B Martıacutenez-Arias A Hungrıacutea Iglesias-Juez

J C Conesa J Soria J Phys Chem Chem Phys 4 (2002) 2473

[38] H M Chen J H He J Phys Chem C 112 (2008) 17540

Chapter ndash V

184

[39] F Zhang S Chan W Spanier J E Apak E Jin Q Robinson

R D Herman I P Appl Phys Lett 80 (2002) 127

[40] G Adachi TMasui In Catalysis by Ceria and Related Materials

ATrovarelli Ed Imperial College Press London (2002)

[41] J A Rodriguez X Wang J C Hanson G Liu A Iglesias-Juez

MFernaacutendez-Garcıacutea J Chem Phys 119 (2003) 5659

[42] S Eck C Castellarin-Cudia S Surnev M G Ramsey F PNetzer

Surf Sci 520 (2002) 173

[43] R B Yu L Yan P Zheng JChen X X R J Phys ChemC

112 (2008) 19896

[44] Y Nagai T Yamamoto T Tanaka SYoshida T Nonaka T Okamoto

A Suda M Sugiura Catal Today 74 (2002) 225

[45] Y M Chiang E B Lavik I Kosacki H L Tuller JY Ying

J Electroceram 1 (1997) 7

[46] D R Patil L A Patil G H Jain M S Wagh S A Patil

Sens and Transducers 74 (2006) 874

[47] K Sohlberg S K Pantelides S J Pennycook J Am Chem Soc

123 (2001) 6609

[48] L S Zhong J S Hu A M Cao Q Liu W G Song LWan

J Chem Mater 19 (2007) 1648

[49] J B Fei Y Cui X H Yan Y Yang K W Wang QHe J B Li

AdV Mater 20 (2008) 452

[50] P V Kamat J Phys Chem B 106 (2002) 7729

[51] R D Robinson J E Spanier F Zhang SW Chan I P Herman J Appl

Phys 92 (2002) 1936

[52] S W Cao Y J Zhu J Phys Chem C 112 (2008) 6253

[53] R Georgekutty M K Seery S C Pillai J Phys Chem C

112 (2008) 13563

[54] J S Hu L S Zhong W G Song L J Wan Adv Mater 20 (2008)1

Chapter ndash V

185

[55] HXu F L Jia Z H Ai L Z Zhang Cryst Growth Des (2007)

[56] H M Jia W J Xiao L Z Zhang Z F Zheng J PhysChem C

112 (2008) 1379

[57] Z H Ai H Y Xiao T Mei J Liu L Z Zhang K J Deng J R Qiu

J PhysChem C 112 (2008) 11929

[58] L M Ma W X Zhang EnViron Sci Technol 42 (2008) 5384

[59] Z H Ai Y Chen L Z Zhang J R Qiu EnViron Sci Technol

42 (2008) 69550

[60] Z Y Zhong J Ho J Teo S C Shen AGedanken ChemMater

19 (2007) 4776

[61] S C Kuiry S D Patil S Deshpande S Seal J Phys ChemB

109 (2005) 6936

[62] D R Patil L A Patil Sens IEEE 7 (2007) 434

[63] P Jasinski Anderson H U Sens Actuators B 95 (2003) 73

[64] J P Nair E Wachtel I Lubomirsky J Fleig JMaier AdVMater

15 (2003) 2077

[65] R J Qi Y J Zhu G F Cheng Y H Huang Nanotechnology

16 (2005) 2502

Chapter ndash V

163

the glycol group respectively In the present synthesis propylene glycol works as

a protective agent against particle growth and since the stretching band of the

carbonyl around 1700 cm-1

was not observed in the FT-IR spectrum it could be

that the surface of the ceria nanoparticles was covered with glycol species In

addition it has also been reported that non-stoichiometric cerium oxide

synthesized by an inert gas condensation process was brownish yellow

Fig54 FTIR spectrum of CeO2 nanoparticles

533 The XRD Analysis

Fig55 shows the X-ray diffractograms of the as prepared powder of CeO2

nanoparticles a) by microwave (without annealed) annealed at b)200degC and

c) 300degC The CeO2 nanoparticles are crystalline in nature with d values 313

270 191 and 163 Aring The synthesized samples by microwave confirmed the

Chapter ndash V

164

formation of face centered cubic structure (FCC) of CeO2 nanoparticles The

determined characteristics 2θ values and [hkl] planes are 4287deg [111] 5017deg

[200] 7386deg [220] and 89deg [311] respectively (JCPDS card No 81-0792) and

lattice parameter is 542 Aring The average particle size of CeO2 nanoparticles was

found to about 10 nm The particle size of CeO2 nanoparticles obtained by

microwave is less than that of annealed at 200degC and 300degC However the lattice

parameter is found to increase with reduction temperature indicating an expansion

in the FCC lattice

Fig 55 XRD patterns of the CeO2 nanoparticles synthesized a) by microwave

(without annealed) annealed at b) 200degC and c) 300degC

To know the correct structure in cubic the indexing pattern of cubic crystal

was studied in detail A cubic crystal gives diffraction lines whose sin2θ values

Chapter ndash V

165

satisfy the equation obtained by combining the Braggrsquos law with the plane-spacing

equation for the cubic system as

sin2θ (h

2+k

2+l

2) = sin

2θ s = λ

2 4 a

2 --- 52

Since the sum (s) =

(h

2+k

2+l

2) is always integral and λ

2 4a

2 is a constant for any

one pattern of a cubic substance is one of finding a set of integers (s) which will

yield a constant quotient when divided one by one into the observed sin2θ values

Once the proper integers (s) are found the indices (hkl) of each line can be written

down by inspection The proper set of integers (s) is not hard to find because there

are only a few possible sets [30] Each of the four common cubic lattice types has

a characteristic sequence of diffraction lines described by their sequential values

Simple cubic 1 2 3 4 5 6 7 8 9 10 11 12 13 14 16 -------

Body-centered cubic 2 4 6 8 10 12 14 16 -------

Face-centered cubic 3 4 8 11 12 16 --------

In our case the crystal system was found to be face centered cubic (FCC) because

the integers(s) 3 4 8 11 which matches the FCC diffractions lines

Fig 56 Fluorite structure of CeO2

Chapter ndash V

166

The unit cell of ceria is shown in Fig56 In the face centered cubic (FCC)

structure of ceria Ce4+

ions form a cubic close packing arrangement and all the

tetrahedral sites are occupied by the oxide ions whereas the octahedral sites remain

vacant The unit cell of ceria can be considered as a simple cube in which the face

center positions and corners are occupied by Ce4+

ions The tetrahedral sites can be

visualized by dividing the cube into eight smaller cubes The body center positions

of all the small cubes are occupied by oxide ions and the alternate corners are

occupied by Ce4+

ions [31]

Fig57 βcos θ λ vs sin θ λ for CeO2 nanocrystals

The particle size of CeO2 nanoparticles was calculated by using Scherrerrsquos

formula The lattice strain and crystalline size was calculated from the following

equation [18]

β cos θ λ = (1 ε) + (τ sin θ λ) --- 53

Chapter ndash V

167

where β is the measured FWHM (Full Width at Half Maximum) in radian θ is the

Bragg angle of the peak λ is wavelength of X-ray (Cr Kα line 228 Aring) ε is the

effective particle size and τ is the effective strain The average particle size can be

estimated from the extrapolation of the Williamson-Hall plot shown in Fig 57

and the particle size was obtained 9 nm based on the intercept inverse

ie 1 ε = 0011 X 108

which yields ε = 909 X 10-9

m or 9 nm

534 Energy Dispersive Analysis of X-Rays (EDAX)

The cerium oxide nanoparticles were characterized by energy dispersive

X- ray analysis (EDAX) which is attachment of SEM The sample for EDAX was

prepared by depositing a very thin layer of cerium oxide nanoparticles on glass

slide The dispersive X ndash ray analysis was conducted by focusing an electron beam

on several different sectioned regions of copper nanoparticles

Fig58 EDAX spectrum of CeO2 nanoparticles

Chapter ndash V

168

The EDAX spectrum of cerium oxide nano particles synthesized by

microwave assisted sol-gel method is shown in Fig58 the composition of sample

shows the presense of cerium and oxygen elements The spectrum peak reveals the

presence of Ce and O at 4837 and 0525 keV respectively The atomic of Ce

and O is 3597 and 6403 respectively which confirms the presence of Ce and O in

the powder The present composition of Ce and O reveals that the formation of

non-stoichometric CeO2 which is superior for adsorption of toxic ions from

wastewater

535 Scanning Electron Microscopic (SEM) Analysis

The model JEOL JSM-6360 was used for the determination of morphology

of nanoparticle The cerium oxide nanoparticles were dispersed in isopropyl

alcohol and sonicated for frac12 hour One or two drop of sonicated solution was kept

on 1x1 cm glass plate dried naturally and coated the platinum The SEM images

are formed by scanning narrow beam of electrons across the surface of specimen

Fig59 (a) SEM micrograph of CeO2 nanoparticles by microwave

(without annealed)

Chapter ndash V

169

Collecting and processing emitting electron and displaying them on cathode ray

tubecomputer monitor Using a visual raster which is synchronized with the

beams scanned The dried thin films samples of cerium oxide nanoparticles were

scanned by SEM The SEM images of cerium oxide synthesized by microwave

(without annealed) and annealed at 300degC are shown in Figs59 (a) and (b) From

SEM images revealed that there were various sizes of particles in the as-prepared

sample The large particles are composed of small crystallites and show particle

aggregates of irregular shapes The particles are spherical in shapes which are

good agreement with XRD results of particles size These observations were

consistent with similar observations reported in the literature [42ndash44]

Fig59 (b) SEM micrograph of CeO2 nanoparticles annealed at 300degC

536 Transmission Electron Microscopic (TEM) Analysis

In case of cerium oxide the transmission electron micrograph was obtained

by employing Philips microscopy IIT Madras TEM is analogous to the optical

microscope It provides very high resolution which can reach approximately

Chapter ndash V

170

01 nm in the case of lattice images Consequently very high magnification can be

obtained TEM is used to examine very thin section through materials of the

surface A transmission electron microscope Philips 120 kV is equipped with CCD

camera This instrument has the resolution of 036 nm with 42 to 120 kV operating

voltage and magnifies object upto 6 lakhs times in high resolution mode Samples

for TEM examinations were prepared by placing a drop of sample suspension on a

copper grade coated with carbon film and were allowed to dry in air A typical

TEM image of CeO2 powders synthesized by cerium nitrate salts by microwave

assisted sol-gel method (without calcination) is presented in Fig 510 The mean

diameter of the particles as measured is about 10 nm These results are in good

agreement with that of estimated by X-ray line broadening Also the synthesized

Fig510 TEM image of the CeO2 nanoparticles

nanopowder is well crystallized without defects non-agglomerated with narrow

size distributions The selected area electron diffraction (SAED) pattern shown in

the Fig511 it also supports the d values obtained from XRD with rings which

can be assigned [111] [200] [220] and [311] diffractions of CeO2 structure

Chapter ndash V

171

Fig511 SAED image of the CeO2 nanoparticles

537 Thermo-Gravimetric and Differential Thermal Analysis

Fig512 shows the TG-DTA curves for the dried ceria precursor prepared

by the Ce(IV) methods The dried samples may consist of three portions of

materials (A) crystalline ceria CeO2 (B) crystalline ceria with structural water

CeO22H2O and (C) amorphous cerium hydroxide Ce(OH)4 The endothermic

peaks in 64degC could be due to crystallization of the amorphous portion in the

sample Ce(OH)4 to crystalline CeO2 The larger endothermic peak for the sample

indicates that the former contains more amorphous phase than the latter The total

weight loss up to 800 degC for the sample is about 1685 (or 194 for n which is

the number of water molecules per mole of cerium in the dried sample) This is

consistent with the change of [Ce (OH)4 + CeO22H2O] to [CeO2] The two

endothermic peaks at about 313 and 436degC in figure can be attributed to the loss of

the structural water molecules The less obvious endothermic peaks for the

structural water loss for this sample also agree with the weight loss data These

results suggest that the Ce(IV) derived sample contains primarily Ce(OH)4 and

CeO22H2O

Chapter ndash V

172

Fig 512 Weight loss of CeO2 nanoparticles as a function of temperature

538 Electrical Conductivity of Semiconductors CeO2

Semiconductors find wide appilication particularly in electronics Their

application is based on certain properties that will be an electrical conductivity of

semiconductors When the temperature is not very low the semiconductor will

conduct electricity due to the movement of the electrons in the conduction band

and that of holes in the valence band The conductivity of nano CeO2 thick film is

represented in Fig513 From the figure the almost linear relationship was found

in the measured temperature range of 200-500oC If it assumed that conduction in

crystalline ceria is essentially an n-type electronic process then using a simple

model we can express the conductivity-temperature relation by the equation

σ = σo e-Ea kT

where σo is a constant Ea is thermal activation energy T is the

Kelvin temperature and k is Boltzmanns constant Thus an Arrhenius plot of

conductivity and temperature should be linear over certain temperature range

Although the results in Fig513 are curved a linear section can be drawn above

280oC which gives activation energy of 236 eV

Chapter ndash V

173

Fig513 Electrical conductivity of CeO2 nanoparticles as a function of temperature

539 Gas Sensing Performance

5391 LPG Gas Sensing

Fig514 shows the variation of response of CeO2 fired at various

temperature The gas response increases with temperature from 50 to 150degC and

then decreases with a further increase in temperature In present work every time

prior to exposing the CeO2 film to LPG it was allowed to stabilize at an operating

temperature for 10 min and the stabilized resistance was taken as Ra After

exposing the film to the LPG gas the changed resistance was taken as Rg LPG

reacts with surface oxygen ions of the film Oxidation of film decreases the

number of free carriers Therefore resistance of the film increases with gas The

result of reaction of LPG with polycrystalline CeO2 is adsorbed CO2 and H2O on

the surface of the oxide The Ce harrLPG interactions on CeO2 are strong and Ce

sites probably get oxidized

Chapter ndash V

174

Fig 514 Variation of gas responses with operating temperature for LPG gas

5393 Effect of Gas Concentration (active region)

The variation of sensor response of CeO2 thick film sample with LPG gas

concentration at operating temperature 150degC is represented in Fig 515

Fig 515 Variation in sensor response with LPG gas concentration

Chapter ndash V

175

It is clear from the figure that the gas response goes on increasing with gas

concentration upto 600 ppm The rate of increase in response was relatively large

upto 1000 ppm and saturated beyond 600 ppm The monolayer of the gas

molecules formed on the surface covers the whole surface of the film The gas

molecules from that layer would reach the surface active sites of the film The

excess gas molecules remain idle and would not reach the surface active sites of

the sensor Thus the active region of the sensor would be up to 600 ppm

5394 Response and Recovery Time

The response of CeO2 thick film sensor was found to be quick (~ 28 s) to

600 ppm of while the recovery was fast (~ 10 s) (Fig 516) The fast response

may be attributed to faster oxidation of the gas The negligible quantity of the

surface reaction product and its high volatility explains its fast response and quick

recovery to its initial chemical status

Fig 516 Response and recovery time

Chapter ndash V

176

5395 Oxygen Adsorption-Desorption Mechanism

O2 (air) + 2e- 2O

-(film surface) --- 56

At higher temperature the atmospheric oxygen O2 adsorbs on the surface of the

thick film It captures the electrons (Fig517) from conduction band It would

result in decreasing conductivity of the film When LPG reacts with the adsorbed

oxygen on the surface of the film it gets oxidized to CO2 and H2O by following

series of intermediate stages This liberates free electrons in the conduction band

The final reaction takes place as

C4H10(gas) + 13Ominus

(filmsurface) rarr 4CO2(gas) + 5H2O(gas) + 13eminus

(condband) --- 57

Fig 517 Sensing mechanism on the surface of the film

This shows n-type conduction mechanism Thus generated electrons

contribute to a sudden increase in conductance of the thick film The mass of

Ce and O in sample was not as per the stoichiometric proportion and sample was

Chapter ndash V

177

observed to be the oxygen deficient This deficiency gets reduced due to

adsorption of atmosphericmolecular oxygen This helps in decreasing electronic

conductivity of the film Upon exposure LPG molecules got oxidized with the

adsorbed oxygen ions by following the series of intermediate stages producing

CO2 and H2O This results in evolving oxygen as electrically neutral atoms

trapping behind the negative charges (electrons) Upon exposure the energy

released in decomposition of LPG molecules would be sufficient for trapped

electrons to jump into the conduction band of activated CeO2 resulting in increase

in the conductivity of the film The drastic increase in conductivity of the sensor

could be attributed to the adsorptionndashdesorption mechanism resulted from the

electronic defects [26]

5310 Catalytic Activity

The utility of this catalytic system deprotection of several oximes were

studied (Table 52) The catalyst was also tested for reusability and it was found

that it could be reused without loss of initial activity for at least four cycles [19]

TLC analysis indicated the formation of only one product of corresponding

carbonyls FT-IR and NMR analysis showed that the oximes were converted into

carbonyls For checking the reusability (after each run) the catalyst was recovered

by simple filtration and washed several times with acetone and then dried at

373 K The catalyst thus obtained was used for the subsequent runs without any

further modification both activated and deactivated oximes were oxidized in better

yield more than ZrO2 catalyst as shown in Table 52

Chapter ndash V

178

Table 52 Oxidative Deoximation of Carbonyls on CeO2

Entry

Substrate

Product

Time

(h)

Yield

()

1

NOH

O

15

62

2

NOH

Cl

O

Cl

15

58

3

NOH

O

25

73

4

NOHCl

OCl

40

57

5

NOH

Cl

O

Cl

30

59

Solvent- Ethyl acetate Catalyst- ZrO2 Temperature- 353 K

Chapter ndash V

179

5311 Adsorption of Rhodamine B (Rh B)

The removal of dye pollutants are also of great importance for wastewater

treatment Rhodamine B a common cationic dye used in textile industry is one of

the most notorious contaminants in aquatic environments because of it huge

amounts slow biodegradation and toxicity In this study synthesized CeO2

nanocrystals are further used to remove Rh B The absorption spectrum of Rh B

solution was characterized by its characteristic absorption at 555 nm which was

attributed to the chromophore containing azo linkage of the dye molecules [22]

Fig 518 Absorption spectra of aqueous solution of Rh B in the presence of CeO2

nanocrystals at different times

UV-Visible absorption spectra represented in Fig518 confirms that over 93 of

Rh B in 100 mL of 20 mg L-1

Rh B aqueous solution could be removed with 100

mg of the as-prepared CeO2 nanocrystalline within 60 min The reasons may

account for the efficient removal of Rh B with CeO2 nanocrystals The first reason

Chapter ndash V

180

should be their to facilitate the adsorption of Rh B The second one is attributed to

the oxygen vacancies on the surface of CeO2 nanocrystals These abundant surface

oxygen vacancies could produce strong electrostatic attraction with the cationic

groups of Rh B as well as hydrogen bonding with the nitrogen atoms of Rh B

Chapter ndash V

181

Conclusions

Microwave synthesis is very beneficial to find a fast simple and energy

efficient approach to produce fine CeO2 nanoparticles It is relatively new method

to produce inorganic compounds for materials processing to enhance the material

properties as well as economic advantages through energy saving and acceleration

of product development Cerium oxide has outstanding physical and chemical

properties therefore it is used as LPG sensor In the area of catalysis nanoparticles

of ceria have been studied Also it is concluded as

1] The CeO2 nanoparticles were prepared by simple microwave assisted sol-gel

method

2] The well spherical and narrow size distribution with 10 nm size particles were

obtained by this method

3] It is very simple time as well as energy saving technique

4] The XRD pattern shows the CeO2 nanoparticles exists in face centered cubic

5] Scanning electron micrographs indicates that grains are uniformaly distributed

6] Transmission electron micrographs indicates that the mean diameter of the

particles as measured is about 10 nm

7] The conductivity of nano CeO2 thick film is represents the almost linear

relationship was found in the measured temperature range of 200-5000C It is

clear that the conduction in crystalline ceria is essentially an n-type electronic

process

8] The CeO2 nanoparticles in the thick film form prepared by screen printing

technique showed better response to LPG gas at 250degC temperature at 600 ppm

concentration of LPG gas

9] Catalyst can be prepared from inexpensive precursors using the microwave

assisted sol-gel technique is promising for the oxidative regeneration of ketones

10] This study reveals that these CeO2 nanocrystals are novel adsorbents for

wastewater treatment

Chapter ndash V

182

References

[1] A Ramesh H Hasegawa T Maki KUeda Sep Purif Tech

56 (2007) 90

[2] C Lettmann K Hildenbrand H Kisch W Macyk W F Maier

Appl Catal B 32 (2001) 215

[3] X Zhang Z H Ai F L Jia L Z Zhang J Phys Chem C 112 (2008) 747

[4] N Du H Zhang BD Chen X Y Ma D RYang J Phys ChemC

111 (2007) 12677

[5] F Zhang S Yang H Chen X Yu Cera Intern 30 (2004) 997

[6] F Gu SF Wang MK Lu GJ Zhou D Xu DR Langmuir J SciIR Iran

20 (2004) 3528

[7] H Xiao Z Ai L Zhang JPhys Chem 113 (2009) 16625

[8] L Combemale G Caboche D Stuerga D Chaumont MicroRese

Bull 40 (2005) 529

[9] M Zawadzki Jof Alloys and Comp 454 (2008) 347

[10] A V Patil C G Dighavkar S K Sonawane S J Patil R Y Borse

Sens and Transducers 108 (2009) 189

[11] AV Patil CG Dighavkar RY Borse Sens and Transducers

101 (2009) 96

[12] AV Patil CG Dighavkar SK Sonawane S J Patil RY Borse Sensand

Transducers 9 (2010) 11

[13] DR Patil LA Patil Talanta 77 (2009)1409

[14] R S Blackburn Environ Sci Tech 38 (2004) 4905

[15] GNaja CMustin JBerthelin BVolesky J Colloide Inter Sci

292 (2005) 537

[16] VKGupta IAli Environ Sci Tech 42 (2008) 766

[17] A Mittal VK Gupta A Malviya and JMittal J Hazard Mater

151 (2008) 821

[18] VKGupta IAli VKSaini J Colloid Interface Sci 315(2007) 87

Chapter ndash V

183

[19] S S Deshpande S U Sonavane R V Jayaram Cat Com 9 (2008) 639

[20] KM Garadkar BS Shirke YB Patil D R Patil Sens and Transducers

110 (2009) 17

[21] VKGupta AMittal VGajbe JMittal J Colloid Inter Sci

319 (2008) 30

[22] T A Khan I Ali V V Singh S Sharma J Envir Protec Sci 3 (2009)11

[23] A Hosseinnia M Keyanpour-Rad M Pazouki World Applied Sci J

8 (2010) 1327

[24] MI Baraton L Merhari J Am Ceram Soc 75 (2004) 1587

[25] CWang B Xu XWang J Zhao J Solid State Chem 178 (2005) 3500

[26] H Xiao Z Ai L Zhang J Phys Chem C doi 10 1021 JP9050269 (2009)

[27] A M Thompson Oxides of the Rare Earths Wiley New York (1978)

[28] H Noumlremberg G A D Briggs Phys Rev Lett 79 (1997) 4222

[29] A Hosseinnia M Keyanpour-Rad M Pazouki World Appl Sci J

8 (2010) 1327

[30] BD Cullity SR Stock Elements of X-Ray Diffraction 3rd edn

(Prentice Hall Upper Saddle River NJ) (2001) 07458

[31] G Ranga Rao B Gopal Mishra Bul Cat Soc of Ind 2 (2003)122

[32] A K Bhattacharyal T K Naiya S N Mondal S K Das Chem Eng J

137 (2008) 529

[33] G Liu J A Rodriguez Z Chang J Hrbek C H F Peden

J Phys Chem B 108 (2004) 2931

[34] P F Ji J L Zhang F Chen M Anpo J Phys Chem C 112 (2008) 17809

[35] A Trovarelli Catal Rev SciEng 38 (1996) 439

[36] B Li D M Pan J S Zheng Y J Cheng X Y Ma F Huang L Zhang

Langmuir 24 (2008) 9630

[37] M Fernaacutendez-Garcıacutea A A B Martıacutenez-Arias A Hungrıacutea Iglesias-Juez

J C Conesa J Soria J Phys Chem Chem Phys 4 (2002) 2473

[38] H M Chen J H He J Phys Chem C 112 (2008) 17540

Chapter ndash V

184

[39] F Zhang S Chan W Spanier J E Apak E Jin Q Robinson

R D Herman I P Appl Phys Lett 80 (2002) 127

[40] G Adachi TMasui In Catalysis by Ceria and Related Materials

ATrovarelli Ed Imperial College Press London (2002)

[41] J A Rodriguez X Wang J C Hanson G Liu A Iglesias-Juez

MFernaacutendez-Garcıacutea J Chem Phys 119 (2003) 5659

[42] S Eck C Castellarin-Cudia S Surnev M G Ramsey F PNetzer

Surf Sci 520 (2002) 173

[43] R B Yu L Yan P Zheng JChen X X R J Phys ChemC

112 (2008) 19896

[44] Y Nagai T Yamamoto T Tanaka SYoshida T Nonaka T Okamoto

A Suda M Sugiura Catal Today 74 (2002) 225

[45] Y M Chiang E B Lavik I Kosacki H L Tuller JY Ying

J Electroceram 1 (1997) 7

[46] D R Patil L A Patil G H Jain M S Wagh S A Patil

Sens and Transducers 74 (2006) 874

[47] K Sohlberg S K Pantelides S J Pennycook J Am Chem Soc

123 (2001) 6609

[48] L S Zhong J S Hu A M Cao Q Liu W G Song LWan

J Chem Mater 19 (2007) 1648

[49] J B Fei Y Cui X H Yan Y Yang K W Wang QHe J B Li

AdV Mater 20 (2008) 452

[50] P V Kamat J Phys Chem B 106 (2002) 7729

[51] R D Robinson J E Spanier F Zhang SW Chan I P Herman J Appl

Phys 92 (2002) 1936

[52] S W Cao Y J Zhu J Phys Chem C 112 (2008) 6253

[53] R Georgekutty M K Seery S C Pillai J Phys Chem C

112 (2008) 13563

[54] J S Hu L S Zhong W G Song L J Wan Adv Mater 20 (2008)1

Chapter ndash V

185

[55] HXu F L Jia Z H Ai L Z Zhang Cryst Growth Des (2007)

[56] H M Jia W J Xiao L Z Zhang Z F Zheng J PhysChem C

112 (2008) 1379

[57] Z H Ai H Y Xiao T Mei J Liu L Z Zhang K J Deng J R Qiu

J PhysChem C 112 (2008) 11929

[58] L M Ma W X Zhang EnViron Sci Technol 42 (2008) 5384

[59] Z H Ai Y Chen L Z Zhang J R Qiu EnViron Sci Technol

42 (2008) 69550

[60] Z Y Zhong J Ho J Teo S C Shen AGedanken ChemMater

19 (2007) 4776

[61] S C Kuiry S D Patil S Deshpande S Seal J Phys ChemB

109 (2005) 6936

[62] D R Patil L A Patil Sens IEEE 7 (2007) 434

[63] P Jasinski Anderson H U Sens Actuators B 95 (2003) 73

[64] J P Nair E Wachtel I Lubomirsky J Fleig JMaier AdVMater

15 (2003) 2077

[65] R J Qi Y J Zhu G F Cheng Y H Huang Nanotechnology

16 (2005) 2502

Chapter ndash V

164

formation of face centered cubic structure (FCC) of CeO2 nanoparticles The

determined characteristics 2θ values and [hkl] planes are 4287deg [111] 5017deg

[200] 7386deg [220] and 89deg [311] respectively (JCPDS card No 81-0792) and

lattice parameter is 542 Aring The average particle size of CeO2 nanoparticles was

found to about 10 nm The particle size of CeO2 nanoparticles obtained by

microwave is less than that of annealed at 200degC and 300degC However the lattice

parameter is found to increase with reduction temperature indicating an expansion

in the FCC lattice

Fig 55 XRD patterns of the CeO2 nanoparticles synthesized a) by microwave

(without annealed) annealed at b) 200degC and c) 300degC

To know the correct structure in cubic the indexing pattern of cubic crystal

was studied in detail A cubic crystal gives diffraction lines whose sin2θ values

Chapter ndash V

165

satisfy the equation obtained by combining the Braggrsquos law with the plane-spacing

equation for the cubic system as

sin2θ (h

2+k

2+l

2) = sin

2θ s = λ

2 4 a

2 --- 52

Since the sum (s) =

(h

2+k

2+l

2) is always integral and λ

2 4a

2 is a constant for any

one pattern of a cubic substance is one of finding a set of integers (s) which will

yield a constant quotient when divided one by one into the observed sin2θ values

Once the proper integers (s) are found the indices (hkl) of each line can be written

down by inspection The proper set of integers (s) is not hard to find because there

are only a few possible sets [30] Each of the four common cubic lattice types has

a characteristic sequence of diffraction lines described by their sequential values

Simple cubic 1 2 3 4 5 6 7 8 9 10 11 12 13 14 16 -------

Body-centered cubic 2 4 6 8 10 12 14 16 -------

Face-centered cubic 3 4 8 11 12 16 --------

In our case the crystal system was found to be face centered cubic (FCC) because

the integers(s) 3 4 8 11 which matches the FCC diffractions lines

Fig 56 Fluorite structure of CeO2

Chapter ndash V

166

The unit cell of ceria is shown in Fig56 In the face centered cubic (FCC)

structure of ceria Ce4+

ions form a cubic close packing arrangement and all the

tetrahedral sites are occupied by the oxide ions whereas the octahedral sites remain

vacant The unit cell of ceria can be considered as a simple cube in which the face

center positions and corners are occupied by Ce4+

ions The tetrahedral sites can be

visualized by dividing the cube into eight smaller cubes The body center positions

of all the small cubes are occupied by oxide ions and the alternate corners are

occupied by Ce4+

ions [31]

Fig57 βcos θ λ vs sin θ λ for CeO2 nanocrystals

The particle size of CeO2 nanoparticles was calculated by using Scherrerrsquos

formula The lattice strain and crystalline size was calculated from the following

equation [18]

β cos θ λ = (1 ε) + (τ sin θ λ) --- 53

Chapter ndash V

167

where β is the measured FWHM (Full Width at Half Maximum) in radian θ is the

Bragg angle of the peak λ is wavelength of X-ray (Cr Kα line 228 Aring) ε is the

effective particle size and τ is the effective strain The average particle size can be

estimated from the extrapolation of the Williamson-Hall plot shown in Fig 57

and the particle size was obtained 9 nm based on the intercept inverse

ie 1 ε = 0011 X 108

which yields ε = 909 X 10-9

m or 9 nm

534 Energy Dispersive Analysis of X-Rays (EDAX)

The cerium oxide nanoparticles were characterized by energy dispersive

X- ray analysis (EDAX) which is attachment of SEM The sample for EDAX was

prepared by depositing a very thin layer of cerium oxide nanoparticles on glass

slide The dispersive X ndash ray analysis was conducted by focusing an electron beam

on several different sectioned regions of copper nanoparticles

Fig58 EDAX spectrum of CeO2 nanoparticles

Chapter ndash V

168

The EDAX spectrum of cerium oxide nano particles synthesized by

microwave assisted sol-gel method is shown in Fig58 the composition of sample

shows the presense of cerium and oxygen elements The spectrum peak reveals the

presence of Ce and O at 4837 and 0525 keV respectively The atomic of Ce

and O is 3597 and 6403 respectively which confirms the presence of Ce and O in

the powder The present composition of Ce and O reveals that the formation of

non-stoichometric CeO2 which is superior for adsorption of toxic ions from

wastewater

535 Scanning Electron Microscopic (SEM) Analysis

The model JEOL JSM-6360 was used for the determination of morphology

of nanoparticle The cerium oxide nanoparticles were dispersed in isopropyl

alcohol and sonicated for frac12 hour One or two drop of sonicated solution was kept

on 1x1 cm glass plate dried naturally and coated the platinum The SEM images

are formed by scanning narrow beam of electrons across the surface of specimen

Fig59 (a) SEM micrograph of CeO2 nanoparticles by microwave

(without annealed)

Chapter ndash V

169

Collecting and processing emitting electron and displaying them on cathode ray

tubecomputer monitor Using a visual raster which is synchronized with the

beams scanned The dried thin films samples of cerium oxide nanoparticles were

scanned by SEM The SEM images of cerium oxide synthesized by microwave

(without annealed) and annealed at 300degC are shown in Figs59 (a) and (b) From

SEM images revealed that there were various sizes of particles in the as-prepared

sample The large particles are composed of small crystallites and show particle

aggregates of irregular shapes The particles are spherical in shapes which are

good agreement with XRD results of particles size These observations were

consistent with similar observations reported in the literature [42ndash44]

Fig59 (b) SEM micrograph of CeO2 nanoparticles annealed at 300degC

536 Transmission Electron Microscopic (TEM) Analysis

In case of cerium oxide the transmission electron micrograph was obtained

by employing Philips microscopy IIT Madras TEM is analogous to the optical

microscope It provides very high resolution which can reach approximately

Chapter ndash V

170

01 nm in the case of lattice images Consequently very high magnification can be

obtained TEM is used to examine very thin section through materials of the

surface A transmission electron microscope Philips 120 kV is equipped with CCD

camera This instrument has the resolution of 036 nm with 42 to 120 kV operating

voltage and magnifies object upto 6 lakhs times in high resolution mode Samples

for TEM examinations were prepared by placing a drop of sample suspension on a

copper grade coated with carbon film and were allowed to dry in air A typical

TEM image of CeO2 powders synthesized by cerium nitrate salts by microwave

assisted sol-gel method (without calcination) is presented in Fig 510 The mean

diameter of the particles as measured is about 10 nm These results are in good

agreement with that of estimated by X-ray line broadening Also the synthesized

Fig510 TEM image of the CeO2 nanoparticles

nanopowder is well crystallized without defects non-agglomerated with narrow

size distributions The selected area electron diffraction (SAED) pattern shown in

the Fig511 it also supports the d values obtained from XRD with rings which

can be assigned [111] [200] [220] and [311] diffractions of CeO2 structure

Chapter ndash V

171

Fig511 SAED image of the CeO2 nanoparticles

537 Thermo-Gravimetric and Differential Thermal Analysis

Fig512 shows the TG-DTA curves for the dried ceria precursor prepared

by the Ce(IV) methods The dried samples may consist of three portions of

materials (A) crystalline ceria CeO2 (B) crystalline ceria with structural water

CeO22H2O and (C) amorphous cerium hydroxide Ce(OH)4 The endothermic

peaks in 64degC could be due to crystallization of the amorphous portion in the

sample Ce(OH)4 to crystalline CeO2 The larger endothermic peak for the sample

indicates that the former contains more amorphous phase than the latter The total

weight loss up to 800 degC for the sample is about 1685 (or 194 for n which is

the number of water molecules per mole of cerium in the dried sample) This is

consistent with the change of [Ce (OH)4 + CeO22H2O] to [CeO2] The two

endothermic peaks at about 313 and 436degC in figure can be attributed to the loss of

the structural water molecules The less obvious endothermic peaks for the

structural water loss for this sample also agree with the weight loss data These

results suggest that the Ce(IV) derived sample contains primarily Ce(OH)4 and

CeO22H2O

Chapter ndash V

172

Fig 512 Weight loss of CeO2 nanoparticles as a function of temperature

538 Electrical Conductivity of Semiconductors CeO2

Semiconductors find wide appilication particularly in electronics Their

application is based on certain properties that will be an electrical conductivity of

semiconductors When the temperature is not very low the semiconductor will

conduct electricity due to the movement of the electrons in the conduction band

and that of holes in the valence band The conductivity of nano CeO2 thick film is

represented in Fig513 From the figure the almost linear relationship was found

in the measured temperature range of 200-500oC If it assumed that conduction in

crystalline ceria is essentially an n-type electronic process then using a simple

model we can express the conductivity-temperature relation by the equation

σ = σo e-Ea kT

where σo is a constant Ea is thermal activation energy T is the

Kelvin temperature and k is Boltzmanns constant Thus an Arrhenius plot of

conductivity and temperature should be linear over certain temperature range

Although the results in Fig513 are curved a linear section can be drawn above

280oC which gives activation energy of 236 eV

Chapter ndash V

173

Fig513 Electrical conductivity of CeO2 nanoparticles as a function of temperature

539 Gas Sensing Performance

5391 LPG Gas Sensing

Fig514 shows the variation of response of CeO2 fired at various

temperature The gas response increases with temperature from 50 to 150degC and

then decreases with a further increase in temperature In present work every time

prior to exposing the CeO2 film to LPG it was allowed to stabilize at an operating

temperature for 10 min and the stabilized resistance was taken as Ra After

exposing the film to the LPG gas the changed resistance was taken as Rg LPG

reacts with surface oxygen ions of the film Oxidation of film decreases the

number of free carriers Therefore resistance of the film increases with gas The

result of reaction of LPG with polycrystalline CeO2 is adsorbed CO2 and H2O on

the surface of the oxide The Ce harrLPG interactions on CeO2 are strong and Ce

sites probably get oxidized

Chapter ndash V

174

Fig 514 Variation of gas responses with operating temperature for LPG gas

5393 Effect of Gas Concentration (active region)

The variation of sensor response of CeO2 thick film sample with LPG gas

concentration at operating temperature 150degC is represented in Fig 515

Fig 515 Variation in sensor response with LPG gas concentration

Chapter ndash V

175

It is clear from the figure that the gas response goes on increasing with gas

concentration upto 600 ppm The rate of increase in response was relatively large

upto 1000 ppm and saturated beyond 600 ppm The monolayer of the gas

molecules formed on the surface covers the whole surface of the film The gas

molecules from that layer would reach the surface active sites of the film The

excess gas molecules remain idle and would not reach the surface active sites of

the sensor Thus the active region of the sensor would be up to 600 ppm

5394 Response and Recovery Time

The response of CeO2 thick film sensor was found to be quick (~ 28 s) to

600 ppm of while the recovery was fast (~ 10 s) (Fig 516) The fast response

may be attributed to faster oxidation of the gas The negligible quantity of the

surface reaction product and its high volatility explains its fast response and quick

recovery to its initial chemical status

Fig 516 Response and recovery time

Chapter ndash V

176

5395 Oxygen Adsorption-Desorption Mechanism

O2 (air) + 2e- 2O

-(film surface) --- 56

At higher temperature the atmospheric oxygen O2 adsorbs on the surface of the

thick film It captures the electrons (Fig517) from conduction band It would

result in decreasing conductivity of the film When LPG reacts with the adsorbed

oxygen on the surface of the film it gets oxidized to CO2 and H2O by following

series of intermediate stages This liberates free electrons in the conduction band

The final reaction takes place as

C4H10(gas) + 13Ominus

(filmsurface) rarr 4CO2(gas) + 5H2O(gas) + 13eminus

(condband) --- 57

Fig 517 Sensing mechanism on the surface of the film

This shows n-type conduction mechanism Thus generated electrons

contribute to a sudden increase in conductance of the thick film The mass of

Ce and O in sample was not as per the stoichiometric proportion and sample was

Chapter ndash V

177

observed to be the oxygen deficient This deficiency gets reduced due to

adsorption of atmosphericmolecular oxygen This helps in decreasing electronic

conductivity of the film Upon exposure LPG molecules got oxidized with the

adsorbed oxygen ions by following the series of intermediate stages producing

CO2 and H2O This results in evolving oxygen as electrically neutral atoms

trapping behind the negative charges (electrons) Upon exposure the energy

released in decomposition of LPG molecules would be sufficient for trapped

electrons to jump into the conduction band of activated CeO2 resulting in increase

in the conductivity of the film The drastic increase in conductivity of the sensor

could be attributed to the adsorptionndashdesorption mechanism resulted from the

electronic defects [26]

5310 Catalytic Activity

The utility of this catalytic system deprotection of several oximes were

studied (Table 52) The catalyst was also tested for reusability and it was found

that it could be reused without loss of initial activity for at least four cycles [19]

TLC analysis indicated the formation of only one product of corresponding

carbonyls FT-IR and NMR analysis showed that the oximes were converted into

carbonyls For checking the reusability (after each run) the catalyst was recovered

by simple filtration and washed several times with acetone and then dried at

373 K The catalyst thus obtained was used for the subsequent runs without any

further modification both activated and deactivated oximes were oxidized in better

yield more than ZrO2 catalyst as shown in Table 52

Chapter ndash V

178

Table 52 Oxidative Deoximation of Carbonyls on CeO2

Entry

Substrate

Product

Time

(h)

Yield

()

1

NOH

O

15

62

2

NOH

Cl

O

Cl

15

58

3

NOH

O

25

73

4

NOHCl

OCl

40

57

5

NOH

Cl

O

Cl

30

59

Solvent- Ethyl acetate Catalyst- ZrO2 Temperature- 353 K

Chapter ndash V

179

5311 Adsorption of Rhodamine B (Rh B)

The removal of dye pollutants are also of great importance for wastewater

treatment Rhodamine B a common cationic dye used in textile industry is one of

the most notorious contaminants in aquatic environments because of it huge

amounts slow biodegradation and toxicity In this study synthesized CeO2

nanocrystals are further used to remove Rh B The absorption spectrum of Rh B

solution was characterized by its characteristic absorption at 555 nm which was

attributed to the chromophore containing azo linkage of the dye molecules [22]

Fig 518 Absorption spectra of aqueous solution of Rh B in the presence of CeO2

nanocrystals at different times

UV-Visible absorption spectra represented in Fig518 confirms that over 93 of

Rh B in 100 mL of 20 mg L-1

Rh B aqueous solution could be removed with 100

mg of the as-prepared CeO2 nanocrystalline within 60 min The reasons may

account for the efficient removal of Rh B with CeO2 nanocrystals The first reason

Chapter ndash V

180

should be their to facilitate the adsorption of Rh B The second one is attributed to

the oxygen vacancies on the surface of CeO2 nanocrystals These abundant surface

oxygen vacancies could produce strong electrostatic attraction with the cationic

groups of Rh B as well as hydrogen bonding with the nitrogen atoms of Rh B

Chapter ndash V

181

Conclusions

Microwave synthesis is very beneficial to find a fast simple and energy

efficient approach to produce fine CeO2 nanoparticles It is relatively new method

to produce inorganic compounds for materials processing to enhance the material

properties as well as economic advantages through energy saving and acceleration

of product development Cerium oxide has outstanding physical and chemical

properties therefore it is used as LPG sensor In the area of catalysis nanoparticles

of ceria have been studied Also it is concluded as

1] The CeO2 nanoparticles were prepared by simple microwave assisted sol-gel

method

2] The well spherical and narrow size distribution with 10 nm size particles were

obtained by this method

3] It is very simple time as well as energy saving technique

4] The XRD pattern shows the CeO2 nanoparticles exists in face centered cubic

5] Scanning electron micrographs indicates that grains are uniformaly distributed

6] Transmission electron micrographs indicates that the mean diameter of the

particles as measured is about 10 nm

7] The conductivity of nano CeO2 thick film is represents the almost linear

relationship was found in the measured temperature range of 200-5000C It is

clear that the conduction in crystalline ceria is essentially an n-type electronic

process

8] The CeO2 nanoparticles in the thick film form prepared by screen printing

technique showed better response to LPG gas at 250degC temperature at 600 ppm

concentration of LPG gas

9] Catalyst can be prepared from inexpensive precursors using the microwave

assisted sol-gel technique is promising for the oxidative regeneration of ketones

10] This study reveals that these CeO2 nanocrystals are novel adsorbents for

wastewater treatment

Chapter ndash V

182

References

[1] A Ramesh H Hasegawa T Maki KUeda Sep Purif Tech

56 (2007) 90

[2] C Lettmann K Hildenbrand H Kisch W Macyk W F Maier

Appl Catal B 32 (2001) 215

[3] X Zhang Z H Ai F L Jia L Z Zhang J Phys Chem C 112 (2008) 747

[4] N Du H Zhang BD Chen X Y Ma D RYang J Phys ChemC

111 (2007) 12677

[5] F Zhang S Yang H Chen X Yu Cera Intern 30 (2004) 997

[6] F Gu SF Wang MK Lu GJ Zhou D Xu DR Langmuir J SciIR Iran

20 (2004) 3528

[7] H Xiao Z Ai L Zhang JPhys Chem 113 (2009) 16625

[8] L Combemale G Caboche D Stuerga D Chaumont MicroRese

Bull 40 (2005) 529

[9] M Zawadzki Jof Alloys and Comp 454 (2008) 347

[10] A V Patil C G Dighavkar S K Sonawane S J Patil R Y Borse

Sens and Transducers 108 (2009) 189

[11] AV Patil CG Dighavkar RY Borse Sens and Transducers

101 (2009) 96

[12] AV Patil CG Dighavkar SK Sonawane S J Patil RY Borse Sensand

Transducers 9 (2010) 11

[13] DR Patil LA Patil Talanta 77 (2009)1409

[14] R S Blackburn Environ Sci Tech 38 (2004) 4905

[15] GNaja CMustin JBerthelin BVolesky J Colloide Inter Sci

292 (2005) 537

[16] VKGupta IAli Environ Sci Tech 42 (2008) 766

[17] A Mittal VK Gupta A Malviya and JMittal J Hazard Mater

151 (2008) 821

[18] VKGupta IAli VKSaini J Colloid Interface Sci 315(2007) 87

Chapter ndash V

183

[19] S S Deshpande S U Sonavane R V Jayaram Cat Com 9 (2008) 639

[20] KM Garadkar BS Shirke YB Patil D R Patil Sens and Transducers

110 (2009) 17

[21] VKGupta AMittal VGajbe JMittal J Colloid Inter Sci

319 (2008) 30

[22] T A Khan I Ali V V Singh S Sharma J Envir Protec Sci 3 (2009)11

[23] A Hosseinnia M Keyanpour-Rad M Pazouki World Applied Sci J

8 (2010) 1327

[24] MI Baraton L Merhari J Am Ceram Soc 75 (2004) 1587

[25] CWang B Xu XWang J Zhao J Solid State Chem 178 (2005) 3500

[26] H Xiao Z Ai L Zhang J Phys Chem C doi 10 1021 JP9050269 (2009)

[27] A M Thompson Oxides of the Rare Earths Wiley New York (1978)

[28] H Noumlremberg G A D Briggs Phys Rev Lett 79 (1997) 4222

[29] A Hosseinnia M Keyanpour-Rad M Pazouki World Appl Sci J

8 (2010) 1327

[30] BD Cullity SR Stock Elements of X-Ray Diffraction 3rd edn

(Prentice Hall Upper Saddle River NJ) (2001) 07458

[31] G Ranga Rao B Gopal Mishra Bul Cat Soc of Ind 2 (2003)122

[32] A K Bhattacharyal T K Naiya S N Mondal S K Das Chem Eng J

137 (2008) 529

[33] G Liu J A Rodriguez Z Chang J Hrbek C H F Peden

J Phys Chem B 108 (2004) 2931

[34] P F Ji J L Zhang F Chen M Anpo J Phys Chem C 112 (2008) 17809

[35] A Trovarelli Catal Rev SciEng 38 (1996) 439

[36] B Li D M Pan J S Zheng Y J Cheng X Y Ma F Huang L Zhang

Langmuir 24 (2008) 9630

[37] M Fernaacutendez-Garcıacutea A A B Martıacutenez-Arias A Hungrıacutea Iglesias-Juez

J C Conesa J Soria J Phys Chem Chem Phys 4 (2002) 2473

[38] H M Chen J H He J Phys Chem C 112 (2008) 17540

Chapter ndash V

184

[39] F Zhang S Chan W Spanier J E Apak E Jin Q Robinson

R D Herman I P Appl Phys Lett 80 (2002) 127

[40] G Adachi TMasui In Catalysis by Ceria and Related Materials

ATrovarelli Ed Imperial College Press London (2002)

[41] J A Rodriguez X Wang J C Hanson G Liu A Iglesias-Juez

MFernaacutendez-Garcıacutea J Chem Phys 119 (2003) 5659

[42] S Eck C Castellarin-Cudia S Surnev M G Ramsey F PNetzer

Surf Sci 520 (2002) 173

[43] R B Yu L Yan P Zheng JChen X X R J Phys ChemC

112 (2008) 19896

[44] Y Nagai T Yamamoto T Tanaka SYoshida T Nonaka T Okamoto

A Suda M Sugiura Catal Today 74 (2002) 225

[45] Y M Chiang E B Lavik I Kosacki H L Tuller JY Ying

J Electroceram 1 (1997) 7

[46] D R Patil L A Patil G H Jain M S Wagh S A Patil

Sens and Transducers 74 (2006) 874

[47] K Sohlberg S K Pantelides S J Pennycook J Am Chem Soc

123 (2001) 6609

[48] L S Zhong J S Hu A M Cao Q Liu W G Song LWan

J Chem Mater 19 (2007) 1648

[49] J B Fei Y Cui X H Yan Y Yang K W Wang QHe J B Li

AdV Mater 20 (2008) 452

[50] P V Kamat J Phys Chem B 106 (2002) 7729

[51] R D Robinson J E Spanier F Zhang SW Chan I P Herman J Appl

Phys 92 (2002) 1936

[52] S W Cao Y J Zhu J Phys Chem C 112 (2008) 6253

[53] R Georgekutty M K Seery S C Pillai J Phys Chem C

112 (2008) 13563

[54] J S Hu L S Zhong W G Song L J Wan Adv Mater 20 (2008)1

Chapter ndash V

185

[55] HXu F L Jia Z H Ai L Z Zhang Cryst Growth Des (2007)

[56] H M Jia W J Xiao L Z Zhang Z F Zheng J PhysChem C

112 (2008) 1379

[57] Z H Ai H Y Xiao T Mei J Liu L Z Zhang K J Deng J R Qiu

J PhysChem C 112 (2008) 11929

[58] L M Ma W X Zhang EnViron Sci Technol 42 (2008) 5384

[59] Z H Ai Y Chen L Z Zhang J R Qiu EnViron Sci Technol

42 (2008) 69550

[60] Z Y Zhong J Ho J Teo S C Shen AGedanken ChemMater

19 (2007) 4776

[61] S C Kuiry S D Patil S Deshpande S Seal J Phys ChemB

109 (2005) 6936

[62] D R Patil L A Patil Sens IEEE 7 (2007) 434

[63] P Jasinski Anderson H U Sens Actuators B 95 (2003) 73

[64] J P Nair E Wachtel I Lubomirsky J Fleig JMaier AdVMater

15 (2003) 2077

[65] R J Qi Y J Zhu G F Cheng Y H Huang Nanotechnology

16 (2005) 2502

Chapter ndash V

165

satisfy the equation obtained by combining the Braggrsquos law with the plane-spacing

equation for the cubic system as

sin2θ (h

2+k

2+l

2) = sin

2θ s = λ

2 4 a

2 --- 52

Since the sum (s) =

(h

2+k

2+l

2) is always integral and λ

2 4a

2 is a constant for any

one pattern of a cubic substance is one of finding a set of integers (s) which will

yield a constant quotient when divided one by one into the observed sin2θ values

Once the proper integers (s) are found the indices (hkl) of each line can be written

down by inspection The proper set of integers (s) is not hard to find because there

are only a few possible sets [30] Each of the four common cubic lattice types has

a characteristic sequence of diffraction lines described by their sequential values

Simple cubic 1 2 3 4 5 6 7 8 9 10 11 12 13 14 16 -------

Body-centered cubic 2 4 6 8 10 12 14 16 -------

Face-centered cubic 3 4 8 11 12 16 --------

In our case the crystal system was found to be face centered cubic (FCC) because

the integers(s) 3 4 8 11 which matches the FCC diffractions lines

Fig 56 Fluorite structure of CeO2

Chapter ndash V

166

The unit cell of ceria is shown in Fig56 In the face centered cubic (FCC)

structure of ceria Ce4+

ions form a cubic close packing arrangement and all the

tetrahedral sites are occupied by the oxide ions whereas the octahedral sites remain

vacant The unit cell of ceria can be considered as a simple cube in which the face

center positions and corners are occupied by Ce4+

ions The tetrahedral sites can be

visualized by dividing the cube into eight smaller cubes The body center positions

of all the small cubes are occupied by oxide ions and the alternate corners are

occupied by Ce4+

ions [31]

Fig57 βcos θ λ vs sin θ λ for CeO2 nanocrystals

The particle size of CeO2 nanoparticles was calculated by using Scherrerrsquos

formula The lattice strain and crystalline size was calculated from the following

equation [18]

β cos θ λ = (1 ε) + (τ sin θ λ) --- 53

Chapter ndash V

167

where β is the measured FWHM (Full Width at Half Maximum) in radian θ is the

Bragg angle of the peak λ is wavelength of X-ray (Cr Kα line 228 Aring) ε is the

effective particle size and τ is the effective strain The average particle size can be

estimated from the extrapolation of the Williamson-Hall plot shown in Fig 57

and the particle size was obtained 9 nm based on the intercept inverse

ie 1 ε = 0011 X 108

which yields ε = 909 X 10-9

m or 9 nm

534 Energy Dispersive Analysis of X-Rays (EDAX)

The cerium oxide nanoparticles were characterized by energy dispersive

X- ray analysis (EDAX) which is attachment of SEM The sample for EDAX was

prepared by depositing a very thin layer of cerium oxide nanoparticles on glass

slide The dispersive X ndash ray analysis was conducted by focusing an electron beam

on several different sectioned regions of copper nanoparticles

Fig58 EDAX spectrum of CeO2 nanoparticles

Chapter ndash V

168

The EDAX spectrum of cerium oxide nano particles synthesized by

microwave assisted sol-gel method is shown in Fig58 the composition of sample

shows the presense of cerium and oxygen elements The spectrum peak reveals the

presence of Ce and O at 4837 and 0525 keV respectively The atomic of Ce

and O is 3597 and 6403 respectively which confirms the presence of Ce and O in

the powder The present composition of Ce and O reveals that the formation of

non-stoichometric CeO2 which is superior for adsorption of toxic ions from

wastewater

535 Scanning Electron Microscopic (SEM) Analysis

The model JEOL JSM-6360 was used for the determination of morphology

of nanoparticle The cerium oxide nanoparticles were dispersed in isopropyl

alcohol and sonicated for frac12 hour One or two drop of sonicated solution was kept

on 1x1 cm glass plate dried naturally and coated the platinum The SEM images

are formed by scanning narrow beam of electrons across the surface of specimen

Fig59 (a) SEM micrograph of CeO2 nanoparticles by microwave

(without annealed)

Chapter ndash V

169

Collecting and processing emitting electron and displaying them on cathode ray

tubecomputer monitor Using a visual raster which is synchronized with the

beams scanned The dried thin films samples of cerium oxide nanoparticles were

scanned by SEM The SEM images of cerium oxide synthesized by microwave

(without annealed) and annealed at 300degC are shown in Figs59 (a) and (b) From

SEM images revealed that there were various sizes of particles in the as-prepared

sample The large particles are composed of small crystallites and show particle

aggregates of irregular shapes The particles are spherical in shapes which are

good agreement with XRD results of particles size These observations were

consistent with similar observations reported in the literature [42ndash44]

Fig59 (b) SEM micrograph of CeO2 nanoparticles annealed at 300degC

536 Transmission Electron Microscopic (TEM) Analysis

In case of cerium oxide the transmission electron micrograph was obtained

by employing Philips microscopy IIT Madras TEM is analogous to the optical

microscope It provides very high resolution which can reach approximately

Chapter ndash V

170

01 nm in the case of lattice images Consequently very high magnification can be

obtained TEM is used to examine very thin section through materials of the

surface A transmission electron microscope Philips 120 kV is equipped with CCD

camera This instrument has the resolution of 036 nm with 42 to 120 kV operating

voltage and magnifies object upto 6 lakhs times in high resolution mode Samples

for TEM examinations were prepared by placing a drop of sample suspension on a

copper grade coated with carbon film and were allowed to dry in air A typical

TEM image of CeO2 powders synthesized by cerium nitrate salts by microwave

assisted sol-gel method (without calcination) is presented in Fig 510 The mean

diameter of the particles as measured is about 10 nm These results are in good

agreement with that of estimated by X-ray line broadening Also the synthesized

Fig510 TEM image of the CeO2 nanoparticles

nanopowder is well crystallized without defects non-agglomerated with narrow

size distributions The selected area electron diffraction (SAED) pattern shown in

the Fig511 it also supports the d values obtained from XRD with rings which

can be assigned [111] [200] [220] and [311] diffractions of CeO2 structure

Chapter ndash V

171

Fig511 SAED image of the CeO2 nanoparticles

537 Thermo-Gravimetric and Differential Thermal Analysis

Fig512 shows the TG-DTA curves for the dried ceria precursor prepared

by the Ce(IV) methods The dried samples may consist of three portions of

materials (A) crystalline ceria CeO2 (B) crystalline ceria with structural water

CeO22H2O and (C) amorphous cerium hydroxide Ce(OH)4 The endothermic

peaks in 64degC could be due to crystallization of the amorphous portion in the

sample Ce(OH)4 to crystalline CeO2 The larger endothermic peak for the sample

indicates that the former contains more amorphous phase than the latter The total

weight loss up to 800 degC for the sample is about 1685 (or 194 for n which is

the number of water molecules per mole of cerium in the dried sample) This is

consistent with the change of [Ce (OH)4 + CeO22H2O] to [CeO2] The two

endothermic peaks at about 313 and 436degC in figure can be attributed to the loss of

the structural water molecules The less obvious endothermic peaks for the

structural water loss for this sample also agree with the weight loss data These

results suggest that the Ce(IV) derived sample contains primarily Ce(OH)4 and

CeO22H2O

Chapter ndash V

172

Fig 512 Weight loss of CeO2 nanoparticles as a function of temperature

538 Electrical Conductivity of Semiconductors CeO2

Semiconductors find wide appilication particularly in electronics Their

application is based on certain properties that will be an electrical conductivity of

semiconductors When the temperature is not very low the semiconductor will

conduct electricity due to the movement of the electrons in the conduction band

and that of holes in the valence band The conductivity of nano CeO2 thick film is

represented in Fig513 From the figure the almost linear relationship was found

in the measured temperature range of 200-500oC If it assumed that conduction in

crystalline ceria is essentially an n-type electronic process then using a simple

model we can express the conductivity-temperature relation by the equation

σ = σo e-Ea kT

where σo is a constant Ea is thermal activation energy T is the

Kelvin temperature and k is Boltzmanns constant Thus an Arrhenius plot of

conductivity and temperature should be linear over certain temperature range

Although the results in Fig513 are curved a linear section can be drawn above

280oC which gives activation energy of 236 eV

Chapter ndash V

173

Fig513 Electrical conductivity of CeO2 nanoparticles as a function of temperature

539 Gas Sensing Performance

5391 LPG Gas Sensing

Fig514 shows the variation of response of CeO2 fired at various

temperature The gas response increases with temperature from 50 to 150degC and

then decreases with a further increase in temperature In present work every time

prior to exposing the CeO2 film to LPG it was allowed to stabilize at an operating

temperature for 10 min and the stabilized resistance was taken as Ra After

exposing the film to the LPG gas the changed resistance was taken as Rg LPG

reacts with surface oxygen ions of the film Oxidation of film decreases the

number of free carriers Therefore resistance of the film increases with gas The

result of reaction of LPG with polycrystalline CeO2 is adsorbed CO2 and H2O on

the surface of the oxide The Ce harrLPG interactions on CeO2 are strong and Ce

sites probably get oxidized

Chapter ndash V

174

Fig 514 Variation of gas responses with operating temperature for LPG gas

5393 Effect of Gas Concentration (active region)

The variation of sensor response of CeO2 thick film sample with LPG gas

concentration at operating temperature 150degC is represented in Fig 515

Fig 515 Variation in sensor response with LPG gas concentration

Chapter ndash V

175

It is clear from the figure that the gas response goes on increasing with gas

concentration upto 600 ppm The rate of increase in response was relatively large

upto 1000 ppm and saturated beyond 600 ppm The monolayer of the gas

molecules formed on the surface covers the whole surface of the film The gas

molecules from that layer would reach the surface active sites of the film The

excess gas molecules remain idle and would not reach the surface active sites of

the sensor Thus the active region of the sensor would be up to 600 ppm

5394 Response and Recovery Time

The response of CeO2 thick film sensor was found to be quick (~ 28 s) to

600 ppm of while the recovery was fast (~ 10 s) (Fig 516) The fast response

may be attributed to faster oxidation of the gas The negligible quantity of the

surface reaction product and its high volatility explains its fast response and quick

recovery to its initial chemical status

Fig 516 Response and recovery time

Chapter ndash V

176

5395 Oxygen Adsorption-Desorption Mechanism

O2 (air) + 2e- 2O

-(film surface) --- 56

At higher temperature the atmospheric oxygen O2 adsorbs on the surface of the

thick film It captures the electrons (Fig517) from conduction band It would

result in decreasing conductivity of the film When LPG reacts with the adsorbed

oxygen on the surface of the film it gets oxidized to CO2 and H2O by following

series of intermediate stages This liberates free electrons in the conduction band

The final reaction takes place as

C4H10(gas) + 13Ominus

(filmsurface) rarr 4CO2(gas) + 5H2O(gas) + 13eminus

(condband) --- 57

Fig 517 Sensing mechanism on the surface of the film

This shows n-type conduction mechanism Thus generated electrons

contribute to a sudden increase in conductance of the thick film The mass of

Ce and O in sample was not as per the stoichiometric proportion and sample was

Chapter ndash V

177

observed to be the oxygen deficient This deficiency gets reduced due to

adsorption of atmosphericmolecular oxygen This helps in decreasing electronic

conductivity of the film Upon exposure LPG molecules got oxidized with the

adsorbed oxygen ions by following the series of intermediate stages producing

CO2 and H2O This results in evolving oxygen as electrically neutral atoms

trapping behind the negative charges (electrons) Upon exposure the energy

released in decomposition of LPG molecules would be sufficient for trapped

electrons to jump into the conduction band of activated CeO2 resulting in increase

in the conductivity of the film The drastic increase in conductivity of the sensor

could be attributed to the adsorptionndashdesorption mechanism resulted from the

electronic defects [26]

5310 Catalytic Activity

The utility of this catalytic system deprotection of several oximes were

studied (Table 52) The catalyst was also tested for reusability and it was found

that it could be reused without loss of initial activity for at least four cycles [19]

TLC analysis indicated the formation of only one product of corresponding

carbonyls FT-IR and NMR analysis showed that the oximes were converted into

carbonyls For checking the reusability (after each run) the catalyst was recovered

by simple filtration and washed several times with acetone and then dried at

373 K The catalyst thus obtained was used for the subsequent runs without any

further modification both activated and deactivated oximes were oxidized in better

yield more than ZrO2 catalyst as shown in Table 52

Chapter ndash V

178

Table 52 Oxidative Deoximation of Carbonyls on CeO2

Entry

Substrate

Product

Time

(h)

Yield

()

1

NOH

O

15

62

2

NOH

Cl

O

Cl

15

58

3

NOH

O

25

73

4

NOHCl

OCl

40

57

5

NOH

Cl

O

Cl

30

59

Solvent- Ethyl acetate Catalyst- ZrO2 Temperature- 353 K

Chapter ndash V

179

5311 Adsorption of Rhodamine B (Rh B)

The removal of dye pollutants are also of great importance for wastewater

treatment Rhodamine B a common cationic dye used in textile industry is one of

the most notorious contaminants in aquatic environments because of it huge

amounts slow biodegradation and toxicity In this study synthesized CeO2

nanocrystals are further used to remove Rh B The absorption spectrum of Rh B

solution was characterized by its characteristic absorption at 555 nm which was

attributed to the chromophore containing azo linkage of the dye molecules [22]

Fig 518 Absorption spectra of aqueous solution of Rh B in the presence of CeO2

nanocrystals at different times

UV-Visible absorption spectra represented in Fig518 confirms that over 93 of

Rh B in 100 mL of 20 mg L-1

Rh B aqueous solution could be removed with 100

mg of the as-prepared CeO2 nanocrystalline within 60 min The reasons may

account for the efficient removal of Rh B with CeO2 nanocrystals The first reason

Chapter ndash V

180

should be their to facilitate the adsorption of Rh B The second one is attributed to

the oxygen vacancies on the surface of CeO2 nanocrystals These abundant surface

oxygen vacancies could produce strong electrostatic attraction with the cationic

groups of Rh B as well as hydrogen bonding with the nitrogen atoms of Rh B

Chapter ndash V

181

Conclusions

Microwave synthesis is very beneficial to find a fast simple and energy

efficient approach to produce fine CeO2 nanoparticles It is relatively new method

to produce inorganic compounds for materials processing to enhance the material

properties as well as economic advantages through energy saving and acceleration

of product development Cerium oxide has outstanding physical and chemical

properties therefore it is used as LPG sensor In the area of catalysis nanoparticles

of ceria have been studied Also it is concluded as

1] The CeO2 nanoparticles were prepared by simple microwave assisted sol-gel

method

2] The well spherical and narrow size distribution with 10 nm size particles were

obtained by this method

3] It is very simple time as well as energy saving technique

4] The XRD pattern shows the CeO2 nanoparticles exists in face centered cubic

5] Scanning electron micrographs indicates that grains are uniformaly distributed

6] Transmission electron micrographs indicates that the mean diameter of the

particles as measured is about 10 nm

7] The conductivity of nano CeO2 thick film is represents the almost linear

relationship was found in the measured temperature range of 200-5000C It is

clear that the conduction in crystalline ceria is essentially an n-type electronic

process

8] The CeO2 nanoparticles in the thick film form prepared by screen printing

technique showed better response to LPG gas at 250degC temperature at 600 ppm

concentration of LPG gas

9] Catalyst can be prepared from inexpensive precursors using the microwave

assisted sol-gel technique is promising for the oxidative regeneration of ketones

10] This study reveals that these CeO2 nanocrystals are novel adsorbents for

wastewater treatment

Chapter ndash V

182

References

[1] A Ramesh H Hasegawa T Maki KUeda Sep Purif Tech

56 (2007) 90

[2] C Lettmann K Hildenbrand H Kisch W Macyk W F Maier

Appl Catal B 32 (2001) 215

[3] X Zhang Z H Ai F L Jia L Z Zhang J Phys Chem C 112 (2008) 747

[4] N Du H Zhang BD Chen X Y Ma D RYang J Phys ChemC

111 (2007) 12677

[5] F Zhang S Yang H Chen X Yu Cera Intern 30 (2004) 997

[6] F Gu SF Wang MK Lu GJ Zhou D Xu DR Langmuir J SciIR Iran

20 (2004) 3528

[7] H Xiao Z Ai L Zhang JPhys Chem 113 (2009) 16625

[8] L Combemale G Caboche D Stuerga D Chaumont MicroRese

Bull 40 (2005) 529

[9] M Zawadzki Jof Alloys and Comp 454 (2008) 347

[10] A V Patil C G Dighavkar S K Sonawane S J Patil R Y Borse

Sens and Transducers 108 (2009) 189

[11] AV Patil CG Dighavkar RY Borse Sens and Transducers

101 (2009) 96

[12] AV Patil CG Dighavkar SK Sonawane S J Patil RY Borse Sensand

Transducers 9 (2010) 11

[13] DR Patil LA Patil Talanta 77 (2009)1409

[14] R S Blackburn Environ Sci Tech 38 (2004) 4905

[15] GNaja CMustin JBerthelin BVolesky J Colloide Inter Sci

292 (2005) 537

[16] VKGupta IAli Environ Sci Tech 42 (2008) 766

[17] A Mittal VK Gupta A Malviya and JMittal J Hazard Mater

151 (2008) 821

[18] VKGupta IAli VKSaini J Colloid Interface Sci 315(2007) 87

Chapter ndash V

183

[19] S S Deshpande S U Sonavane R V Jayaram Cat Com 9 (2008) 639

[20] KM Garadkar BS Shirke YB Patil D R Patil Sens and Transducers

110 (2009) 17

[21] VKGupta AMittal VGajbe JMittal J Colloid Inter Sci

319 (2008) 30

[22] T A Khan I Ali V V Singh S Sharma J Envir Protec Sci 3 (2009)11

[23] A Hosseinnia M Keyanpour-Rad M Pazouki World Applied Sci J

8 (2010) 1327

[24] MI Baraton L Merhari J Am Ceram Soc 75 (2004) 1587

[25] CWang B Xu XWang J Zhao J Solid State Chem 178 (2005) 3500

[26] H Xiao Z Ai L Zhang J Phys Chem C doi 10 1021 JP9050269 (2009)

[27] A M Thompson Oxides of the Rare Earths Wiley New York (1978)

[28] H Noumlremberg G A D Briggs Phys Rev Lett 79 (1997) 4222

[29] A Hosseinnia M Keyanpour-Rad M Pazouki World Appl Sci J

8 (2010) 1327

[30] BD Cullity SR Stock Elements of X-Ray Diffraction 3rd edn

(Prentice Hall Upper Saddle River NJ) (2001) 07458

[31] G Ranga Rao B Gopal Mishra Bul Cat Soc of Ind 2 (2003)122

[32] A K Bhattacharyal T K Naiya S N Mondal S K Das Chem Eng J

137 (2008) 529

[33] G Liu J A Rodriguez Z Chang J Hrbek C H F Peden

J Phys Chem B 108 (2004) 2931

[34] P F Ji J L Zhang F Chen M Anpo J Phys Chem C 112 (2008) 17809

[35] A Trovarelli Catal Rev SciEng 38 (1996) 439

[36] B Li D M Pan J S Zheng Y J Cheng X Y Ma F Huang L Zhang

Langmuir 24 (2008) 9630

[37] M Fernaacutendez-Garcıacutea A A B Martıacutenez-Arias A Hungrıacutea Iglesias-Juez

J C Conesa J Soria J Phys Chem Chem Phys 4 (2002) 2473

[38] H M Chen J H He J Phys Chem C 112 (2008) 17540

Chapter ndash V

184

[39] F Zhang S Chan W Spanier J E Apak E Jin Q Robinson

R D Herman I P Appl Phys Lett 80 (2002) 127

[40] G Adachi TMasui In Catalysis by Ceria and Related Materials

ATrovarelli Ed Imperial College Press London (2002)

[41] J A Rodriguez X Wang J C Hanson G Liu A Iglesias-Juez

MFernaacutendez-Garcıacutea J Chem Phys 119 (2003) 5659

[42] S Eck C Castellarin-Cudia S Surnev M G Ramsey F PNetzer

Surf Sci 520 (2002) 173

[43] R B Yu L Yan P Zheng JChen X X R J Phys ChemC

112 (2008) 19896

[44] Y Nagai T Yamamoto T Tanaka SYoshida T Nonaka T Okamoto

A Suda M Sugiura Catal Today 74 (2002) 225

[45] Y M Chiang E B Lavik I Kosacki H L Tuller JY Ying

J Electroceram 1 (1997) 7

[46] D R Patil L A Patil G H Jain M S Wagh S A Patil

Sens and Transducers 74 (2006) 874

[47] K Sohlberg S K Pantelides S J Pennycook J Am Chem Soc

123 (2001) 6609

[48] L S Zhong J S Hu A M Cao Q Liu W G Song LWan

J Chem Mater 19 (2007) 1648

[49] J B Fei Y Cui X H Yan Y Yang K W Wang QHe J B Li

AdV Mater 20 (2008) 452

[50] P V Kamat J Phys Chem B 106 (2002) 7729

[51] R D Robinson J E Spanier F Zhang SW Chan I P Herman J Appl

Phys 92 (2002) 1936

[52] S W Cao Y J Zhu J Phys Chem C 112 (2008) 6253

[53] R Georgekutty M K Seery S C Pillai J Phys Chem C

112 (2008) 13563

[54] J S Hu L S Zhong W G Song L J Wan Adv Mater 20 (2008)1

Chapter ndash V

185

[55] HXu F L Jia Z H Ai L Z Zhang Cryst Growth Des (2007)

[56] H M Jia W J Xiao L Z Zhang Z F Zheng J PhysChem C

112 (2008) 1379

[57] Z H Ai H Y Xiao T Mei J Liu L Z Zhang K J Deng J R Qiu

J PhysChem C 112 (2008) 11929

[58] L M Ma W X Zhang EnViron Sci Technol 42 (2008) 5384

[59] Z H Ai Y Chen L Z Zhang J R Qiu EnViron Sci Technol

42 (2008) 69550

[60] Z Y Zhong J Ho J Teo S C Shen AGedanken ChemMater

19 (2007) 4776

[61] S C Kuiry S D Patil S Deshpande S Seal J Phys ChemB

109 (2005) 6936

[62] D R Patil L A Patil Sens IEEE 7 (2007) 434

[63] P Jasinski Anderson H U Sens Actuators B 95 (2003) 73

[64] J P Nair E Wachtel I Lubomirsky J Fleig JMaier AdVMater

15 (2003) 2077

[65] R J Qi Y J Zhu G F Cheng Y H Huang Nanotechnology

16 (2005) 2502

Chapter ndash V

166

The unit cell of ceria is shown in Fig56 In the face centered cubic (FCC)

structure of ceria Ce4+

ions form a cubic close packing arrangement and all the

tetrahedral sites are occupied by the oxide ions whereas the octahedral sites remain

vacant The unit cell of ceria can be considered as a simple cube in which the face

center positions and corners are occupied by Ce4+

ions The tetrahedral sites can be

visualized by dividing the cube into eight smaller cubes The body center positions

of all the small cubes are occupied by oxide ions and the alternate corners are

occupied by Ce4+

ions [31]

Fig57 βcos θ λ vs sin θ λ for CeO2 nanocrystals

The particle size of CeO2 nanoparticles was calculated by using Scherrerrsquos

formula The lattice strain and crystalline size was calculated from the following

equation [18]

β cos θ λ = (1 ε) + (τ sin θ λ) --- 53

Chapter ndash V

167

where β is the measured FWHM (Full Width at Half Maximum) in radian θ is the

Bragg angle of the peak λ is wavelength of X-ray (Cr Kα line 228 Aring) ε is the

effective particle size and τ is the effective strain The average particle size can be

estimated from the extrapolation of the Williamson-Hall plot shown in Fig 57

and the particle size was obtained 9 nm based on the intercept inverse

ie 1 ε = 0011 X 108

which yields ε = 909 X 10-9

m or 9 nm

534 Energy Dispersive Analysis of X-Rays (EDAX)

The cerium oxide nanoparticles were characterized by energy dispersive

X- ray analysis (EDAX) which is attachment of SEM The sample for EDAX was

prepared by depositing a very thin layer of cerium oxide nanoparticles on glass

slide The dispersive X ndash ray analysis was conducted by focusing an electron beam

on several different sectioned regions of copper nanoparticles

Fig58 EDAX spectrum of CeO2 nanoparticles

Chapter ndash V

168

The EDAX spectrum of cerium oxide nano particles synthesized by

microwave assisted sol-gel method is shown in Fig58 the composition of sample

shows the presense of cerium and oxygen elements The spectrum peak reveals the

presence of Ce and O at 4837 and 0525 keV respectively The atomic of Ce

and O is 3597 and 6403 respectively which confirms the presence of Ce and O in

the powder The present composition of Ce and O reveals that the formation of

non-stoichometric CeO2 which is superior for adsorption of toxic ions from

wastewater

535 Scanning Electron Microscopic (SEM) Analysis

The model JEOL JSM-6360 was used for the determination of morphology

of nanoparticle The cerium oxide nanoparticles were dispersed in isopropyl

alcohol and sonicated for frac12 hour One or two drop of sonicated solution was kept

on 1x1 cm glass plate dried naturally and coated the platinum The SEM images

are formed by scanning narrow beam of electrons across the surface of specimen

Fig59 (a) SEM micrograph of CeO2 nanoparticles by microwave

(without annealed)

Chapter ndash V

169

Collecting and processing emitting electron and displaying them on cathode ray

tubecomputer monitor Using a visual raster which is synchronized with the

beams scanned The dried thin films samples of cerium oxide nanoparticles were

scanned by SEM The SEM images of cerium oxide synthesized by microwave

(without annealed) and annealed at 300degC are shown in Figs59 (a) and (b) From

SEM images revealed that there were various sizes of particles in the as-prepared

sample The large particles are composed of small crystallites and show particle

aggregates of irregular shapes The particles are spherical in shapes which are

good agreement with XRD results of particles size These observations were

consistent with similar observations reported in the literature [42ndash44]

Fig59 (b) SEM micrograph of CeO2 nanoparticles annealed at 300degC

536 Transmission Electron Microscopic (TEM) Analysis

In case of cerium oxide the transmission electron micrograph was obtained

by employing Philips microscopy IIT Madras TEM is analogous to the optical

microscope It provides very high resolution which can reach approximately

Chapter ndash V

170

01 nm in the case of lattice images Consequently very high magnification can be

obtained TEM is used to examine very thin section through materials of the

surface A transmission electron microscope Philips 120 kV is equipped with CCD

camera This instrument has the resolution of 036 nm with 42 to 120 kV operating

voltage and magnifies object upto 6 lakhs times in high resolution mode Samples

for TEM examinations were prepared by placing a drop of sample suspension on a

copper grade coated with carbon film and were allowed to dry in air A typical

TEM image of CeO2 powders synthesized by cerium nitrate salts by microwave

assisted sol-gel method (without calcination) is presented in Fig 510 The mean

diameter of the particles as measured is about 10 nm These results are in good

agreement with that of estimated by X-ray line broadening Also the synthesized

Fig510 TEM image of the CeO2 nanoparticles

nanopowder is well crystallized without defects non-agglomerated with narrow

size distributions The selected area electron diffraction (SAED) pattern shown in

the Fig511 it also supports the d values obtained from XRD with rings which

can be assigned [111] [200] [220] and [311] diffractions of CeO2 structure

Chapter ndash V

171

Fig511 SAED image of the CeO2 nanoparticles

537 Thermo-Gravimetric and Differential Thermal Analysis

Fig512 shows the TG-DTA curves for the dried ceria precursor prepared

by the Ce(IV) methods The dried samples may consist of three portions of

materials (A) crystalline ceria CeO2 (B) crystalline ceria with structural water

CeO22H2O and (C) amorphous cerium hydroxide Ce(OH)4 The endothermic

peaks in 64degC could be due to crystallization of the amorphous portion in the

sample Ce(OH)4 to crystalline CeO2 The larger endothermic peak for the sample

indicates that the former contains more amorphous phase than the latter The total

weight loss up to 800 degC for the sample is about 1685 (or 194 for n which is

the number of water molecules per mole of cerium in the dried sample) This is

consistent with the change of [Ce (OH)4 + CeO22H2O] to [CeO2] The two

endothermic peaks at about 313 and 436degC in figure can be attributed to the loss of

the structural water molecules The less obvious endothermic peaks for the

structural water loss for this sample also agree with the weight loss data These

results suggest that the Ce(IV) derived sample contains primarily Ce(OH)4 and

CeO22H2O

Chapter ndash V

172

Fig 512 Weight loss of CeO2 nanoparticles as a function of temperature

538 Electrical Conductivity of Semiconductors CeO2

Semiconductors find wide appilication particularly in electronics Their

application is based on certain properties that will be an electrical conductivity of

semiconductors When the temperature is not very low the semiconductor will

conduct electricity due to the movement of the electrons in the conduction band

and that of holes in the valence band The conductivity of nano CeO2 thick film is

represented in Fig513 From the figure the almost linear relationship was found

in the measured temperature range of 200-500oC If it assumed that conduction in

crystalline ceria is essentially an n-type electronic process then using a simple

model we can express the conductivity-temperature relation by the equation

σ = σo e-Ea kT

where σo is a constant Ea is thermal activation energy T is the

Kelvin temperature and k is Boltzmanns constant Thus an Arrhenius plot of

conductivity and temperature should be linear over certain temperature range

Although the results in Fig513 are curved a linear section can be drawn above

280oC which gives activation energy of 236 eV

Chapter ndash V

173

Fig513 Electrical conductivity of CeO2 nanoparticles as a function of temperature

539 Gas Sensing Performance

5391 LPG Gas Sensing

Fig514 shows the variation of response of CeO2 fired at various

temperature The gas response increases with temperature from 50 to 150degC and

then decreases with a further increase in temperature In present work every time

prior to exposing the CeO2 film to LPG it was allowed to stabilize at an operating

temperature for 10 min and the stabilized resistance was taken as Ra After

exposing the film to the LPG gas the changed resistance was taken as Rg LPG

reacts with surface oxygen ions of the film Oxidation of film decreases the

number of free carriers Therefore resistance of the film increases with gas The

result of reaction of LPG with polycrystalline CeO2 is adsorbed CO2 and H2O on

the surface of the oxide The Ce harrLPG interactions on CeO2 are strong and Ce

sites probably get oxidized

Chapter ndash V

174

Fig 514 Variation of gas responses with operating temperature for LPG gas

5393 Effect of Gas Concentration (active region)

The variation of sensor response of CeO2 thick film sample with LPG gas

concentration at operating temperature 150degC is represented in Fig 515

Fig 515 Variation in sensor response with LPG gas concentration

Chapter ndash V

175

It is clear from the figure that the gas response goes on increasing with gas

concentration upto 600 ppm The rate of increase in response was relatively large

upto 1000 ppm and saturated beyond 600 ppm The monolayer of the gas

molecules formed on the surface covers the whole surface of the film The gas

molecules from that layer would reach the surface active sites of the film The

excess gas molecules remain idle and would not reach the surface active sites of

the sensor Thus the active region of the sensor would be up to 600 ppm

5394 Response and Recovery Time

The response of CeO2 thick film sensor was found to be quick (~ 28 s) to

600 ppm of while the recovery was fast (~ 10 s) (Fig 516) The fast response

may be attributed to faster oxidation of the gas The negligible quantity of the

surface reaction product and its high volatility explains its fast response and quick

recovery to its initial chemical status

Fig 516 Response and recovery time

Chapter ndash V

176

5395 Oxygen Adsorption-Desorption Mechanism

O2 (air) + 2e- 2O

-(film surface) --- 56

At higher temperature the atmospheric oxygen O2 adsorbs on the surface of the

thick film It captures the electrons (Fig517) from conduction band It would

result in decreasing conductivity of the film When LPG reacts with the adsorbed

oxygen on the surface of the film it gets oxidized to CO2 and H2O by following

series of intermediate stages This liberates free electrons in the conduction band

The final reaction takes place as

C4H10(gas) + 13Ominus

(filmsurface) rarr 4CO2(gas) + 5H2O(gas) + 13eminus

(condband) --- 57

Fig 517 Sensing mechanism on the surface of the film

This shows n-type conduction mechanism Thus generated electrons

contribute to a sudden increase in conductance of the thick film The mass of

Ce and O in sample was not as per the stoichiometric proportion and sample was

Chapter ndash V

177

observed to be the oxygen deficient This deficiency gets reduced due to

adsorption of atmosphericmolecular oxygen This helps in decreasing electronic

conductivity of the film Upon exposure LPG molecules got oxidized with the

adsorbed oxygen ions by following the series of intermediate stages producing

CO2 and H2O This results in evolving oxygen as electrically neutral atoms

trapping behind the negative charges (electrons) Upon exposure the energy

released in decomposition of LPG molecules would be sufficient for trapped

electrons to jump into the conduction band of activated CeO2 resulting in increase

in the conductivity of the film The drastic increase in conductivity of the sensor

could be attributed to the adsorptionndashdesorption mechanism resulted from the

electronic defects [26]

5310 Catalytic Activity

The utility of this catalytic system deprotection of several oximes were

studied (Table 52) The catalyst was also tested for reusability and it was found

that it could be reused without loss of initial activity for at least four cycles [19]

TLC analysis indicated the formation of only one product of corresponding

carbonyls FT-IR and NMR analysis showed that the oximes were converted into

carbonyls For checking the reusability (after each run) the catalyst was recovered

by simple filtration and washed several times with acetone and then dried at

373 K The catalyst thus obtained was used for the subsequent runs without any

further modification both activated and deactivated oximes were oxidized in better

yield more than ZrO2 catalyst as shown in Table 52

Chapter ndash V

178

Table 52 Oxidative Deoximation of Carbonyls on CeO2

Entry

Substrate

Product

Time

(h)

Yield

()

1

NOH

O

15

62

2

NOH

Cl

O

Cl

15

58

3

NOH

O

25

73

4

NOHCl

OCl

40

57

5

NOH

Cl

O

Cl

30

59

Solvent- Ethyl acetate Catalyst- ZrO2 Temperature- 353 K

Chapter ndash V

179

5311 Adsorption of Rhodamine B (Rh B)

The removal of dye pollutants are also of great importance for wastewater

treatment Rhodamine B a common cationic dye used in textile industry is one of

the most notorious contaminants in aquatic environments because of it huge

amounts slow biodegradation and toxicity In this study synthesized CeO2

nanocrystals are further used to remove Rh B The absorption spectrum of Rh B

solution was characterized by its characteristic absorption at 555 nm which was

attributed to the chromophore containing azo linkage of the dye molecules [22]

Fig 518 Absorption spectra of aqueous solution of Rh B in the presence of CeO2

nanocrystals at different times

UV-Visible absorption spectra represented in Fig518 confirms that over 93 of

Rh B in 100 mL of 20 mg L-1

Rh B aqueous solution could be removed with 100

mg of the as-prepared CeO2 nanocrystalline within 60 min The reasons may

account for the efficient removal of Rh B with CeO2 nanocrystals The first reason

Chapter ndash V

180

should be their to facilitate the adsorption of Rh B The second one is attributed to

the oxygen vacancies on the surface of CeO2 nanocrystals These abundant surface

oxygen vacancies could produce strong electrostatic attraction with the cationic

groups of Rh B as well as hydrogen bonding with the nitrogen atoms of Rh B

Chapter ndash V

181

Conclusions

Microwave synthesis is very beneficial to find a fast simple and energy

efficient approach to produce fine CeO2 nanoparticles It is relatively new method

to produce inorganic compounds for materials processing to enhance the material

properties as well as economic advantages through energy saving and acceleration

of product development Cerium oxide has outstanding physical and chemical

properties therefore it is used as LPG sensor In the area of catalysis nanoparticles

of ceria have been studied Also it is concluded as

1] The CeO2 nanoparticles were prepared by simple microwave assisted sol-gel

method

2] The well spherical and narrow size distribution with 10 nm size particles were

obtained by this method

3] It is very simple time as well as energy saving technique

4] The XRD pattern shows the CeO2 nanoparticles exists in face centered cubic

5] Scanning electron micrographs indicates that grains are uniformaly distributed

6] Transmission electron micrographs indicates that the mean diameter of the

particles as measured is about 10 nm

7] The conductivity of nano CeO2 thick film is represents the almost linear

relationship was found in the measured temperature range of 200-5000C It is

clear that the conduction in crystalline ceria is essentially an n-type electronic

process

8] The CeO2 nanoparticles in the thick film form prepared by screen printing

technique showed better response to LPG gas at 250degC temperature at 600 ppm

concentration of LPG gas

9] Catalyst can be prepared from inexpensive precursors using the microwave

assisted sol-gel technique is promising for the oxidative regeneration of ketones

10] This study reveals that these CeO2 nanocrystals are novel adsorbents for

wastewater treatment

Chapter ndash V

182

References

[1] A Ramesh H Hasegawa T Maki KUeda Sep Purif Tech

56 (2007) 90

[2] C Lettmann K Hildenbrand H Kisch W Macyk W F Maier

Appl Catal B 32 (2001) 215

[3] X Zhang Z H Ai F L Jia L Z Zhang J Phys Chem C 112 (2008) 747

[4] N Du H Zhang BD Chen X Y Ma D RYang J Phys ChemC

111 (2007) 12677

[5] F Zhang S Yang H Chen X Yu Cera Intern 30 (2004) 997

[6] F Gu SF Wang MK Lu GJ Zhou D Xu DR Langmuir J SciIR Iran

20 (2004) 3528

[7] H Xiao Z Ai L Zhang JPhys Chem 113 (2009) 16625

[8] L Combemale G Caboche D Stuerga D Chaumont MicroRese

Bull 40 (2005) 529

[9] M Zawadzki Jof Alloys and Comp 454 (2008) 347

[10] A V Patil C G Dighavkar S K Sonawane S J Patil R Y Borse

Sens and Transducers 108 (2009) 189

[11] AV Patil CG Dighavkar RY Borse Sens and Transducers

101 (2009) 96

[12] AV Patil CG Dighavkar SK Sonawane S J Patil RY Borse Sensand

Transducers 9 (2010) 11

[13] DR Patil LA Patil Talanta 77 (2009)1409

[14] R S Blackburn Environ Sci Tech 38 (2004) 4905

[15] GNaja CMustin JBerthelin BVolesky J Colloide Inter Sci

292 (2005) 537

[16] VKGupta IAli Environ Sci Tech 42 (2008) 766

[17] A Mittal VK Gupta A Malviya and JMittal J Hazard Mater

151 (2008) 821

[18] VKGupta IAli VKSaini J Colloid Interface Sci 315(2007) 87

Chapter ndash V

183

[19] S S Deshpande S U Sonavane R V Jayaram Cat Com 9 (2008) 639

[20] KM Garadkar BS Shirke YB Patil D R Patil Sens and Transducers

110 (2009) 17

[21] VKGupta AMittal VGajbe JMittal J Colloid Inter Sci

319 (2008) 30

[22] T A Khan I Ali V V Singh S Sharma J Envir Protec Sci 3 (2009)11

[23] A Hosseinnia M Keyanpour-Rad M Pazouki World Applied Sci J

8 (2010) 1327

[24] MI Baraton L Merhari J Am Ceram Soc 75 (2004) 1587

[25] CWang B Xu XWang J Zhao J Solid State Chem 178 (2005) 3500

[26] H Xiao Z Ai L Zhang J Phys Chem C doi 10 1021 JP9050269 (2009)

[27] A M Thompson Oxides of the Rare Earths Wiley New York (1978)

[28] H Noumlremberg G A D Briggs Phys Rev Lett 79 (1997) 4222

[29] A Hosseinnia M Keyanpour-Rad M Pazouki World Appl Sci J

8 (2010) 1327

[30] BD Cullity SR Stock Elements of X-Ray Diffraction 3rd edn

(Prentice Hall Upper Saddle River NJ) (2001) 07458

[31] G Ranga Rao B Gopal Mishra Bul Cat Soc of Ind 2 (2003)122

[32] A K Bhattacharyal T K Naiya S N Mondal S K Das Chem Eng J

137 (2008) 529

[33] G Liu J A Rodriguez Z Chang J Hrbek C H F Peden

J Phys Chem B 108 (2004) 2931

[34] P F Ji J L Zhang F Chen M Anpo J Phys Chem C 112 (2008) 17809

[35] A Trovarelli Catal Rev SciEng 38 (1996) 439

[36] B Li D M Pan J S Zheng Y J Cheng X Y Ma F Huang L Zhang

Langmuir 24 (2008) 9630

[37] M Fernaacutendez-Garcıacutea A A B Martıacutenez-Arias A Hungrıacutea Iglesias-Juez

J C Conesa J Soria J Phys Chem Chem Phys 4 (2002) 2473

[38] H M Chen J H He J Phys Chem C 112 (2008) 17540

Chapter ndash V

184

[39] F Zhang S Chan W Spanier J E Apak E Jin Q Robinson

R D Herman I P Appl Phys Lett 80 (2002) 127

[40] G Adachi TMasui In Catalysis by Ceria and Related Materials

ATrovarelli Ed Imperial College Press London (2002)

[41] J A Rodriguez X Wang J C Hanson G Liu A Iglesias-Juez

MFernaacutendez-Garcıacutea J Chem Phys 119 (2003) 5659

[42] S Eck C Castellarin-Cudia S Surnev M G Ramsey F PNetzer

Surf Sci 520 (2002) 173

[43] R B Yu L Yan P Zheng JChen X X R J Phys ChemC

112 (2008) 19896

[44] Y Nagai T Yamamoto T Tanaka SYoshida T Nonaka T Okamoto

A Suda M Sugiura Catal Today 74 (2002) 225

[45] Y M Chiang E B Lavik I Kosacki H L Tuller JY Ying

J Electroceram 1 (1997) 7

[46] D R Patil L A Patil G H Jain M S Wagh S A Patil

Sens and Transducers 74 (2006) 874

[47] K Sohlberg S K Pantelides S J Pennycook J Am Chem Soc

123 (2001) 6609

[48] L S Zhong J S Hu A M Cao Q Liu W G Song LWan

J Chem Mater 19 (2007) 1648

[49] J B Fei Y Cui X H Yan Y Yang K W Wang QHe J B Li

AdV Mater 20 (2008) 452

[50] P V Kamat J Phys Chem B 106 (2002) 7729

[51] R D Robinson J E Spanier F Zhang SW Chan I P Herman J Appl

Phys 92 (2002) 1936

[52] S W Cao Y J Zhu J Phys Chem C 112 (2008) 6253

[53] R Georgekutty M K Seery S C Pillai J Phys Chem C

112 (2008) 13563

[54] J S Hu L S Zhong W G Song L J Wan Adv Mater 20 (2008)1

Chapter ndash V

185

[55] HXu F L Jia Z H Ai L Z Zhang Cryst Growth Des (2007)

[56] H M Jia W J Xiao L Z Zhang Z F Zheng J PhysChem C

112 (2008) 1379

[57] Z H Ai H Y Xiao T Mei J Liu L Z Zhang K J Deng J R Qiu

J PhysChem C 112 (2008) 11929

[58] L M Ma W X Zhang EnViron Sci Technol 42 (2008) 5384

[59] Z H Ai Y Chen L Z Zhang J R Qiu EnViron Sci Technol

42 (2008) 69550

[60] Z Y Zhong J Ho J Teo S C Shen AGedanken ChemMater

19 (2007) 4776

[61] S C Kuiry S D Patil S Deshpande S Seal J Phys ChemB

109 (2005) 6936

[62] D R Patil L A Patil Sens IEEE 7 (2007) 434

[63] P Jasinski Anderson H U Sens Actuators B 95 (2003) 73

[64] J P Nair E Wachtel I Lubomirsky J Fleig JMaier AdVMater

15 (2003) 2077

[65] R J Qi Y J Zhu G F Cheng Y H Huang Nanotechnology

16 (2005) 2502

Chapter ndash V

167

where β is the measured FWHM (Full Width at Half Maximum) in radian θ is the

Bragg angle of the peak λ is wavelength of X-ray (Cr Kα line 228 Aring) ε is the

effective particle size and τ is the effective strain The average particle size can be

estimated from the extrapolation of the Williamson-Hall plot shown in Fig 57

and the particle size was obtained 9 nm based on the intercept inverse

ie 1 ε = 0011 X 108

which yields ε = 909 X 10-9

m or 9 nm

534 Energy Dispersive Analysis of X-Rays (EDAX)

The cerium oxide nanoparticles were characterized by energy dispersive

X- ray analysis (EDAX) which is attachment of SEM The sample for EDAX was

prepared by depositing a very thin layer of cerium oxide nanoparticles on glass

slide The dispersive X ndash ray analysis was conducted by focusing an electron beam

on several different sectioned regions of copper nanoparticles

Fig58 EDAX spectrum of CeO2 nanoparticles

Chapter ndash V

168

The EDAX spectrum of cerium oxide nano particles synthesized by

microwave assisted sol-gel method is shown in Fig58 the composition of sample

shows the presense of cerium and oxygen elements The spectrum peak reveals the

presence of Ce and O at 4837 and 0525 keV respectively The atomic of Ce

and O is 3597 and 6403 respectively which confirms the presence of Ce and O in

the powder The present composition of Ce and O reveals that the formation of

non-stoichometric CeO2 which is superior for adsorption of toxic ions from

wastewater

535 Scanning Electron Microscopic (SEM) Analysis

The model JEOL JSM-6360 was used for the determination of morphology

of nanoparticle The cerium oxide nanoparticles were dispersed in isopropyl

alcohol and sonicated for frac12 hour One or two drop of sonicated solution was kept

on 1x1 cm glass plate dried naturally and coated the platinum The SEM images

are formed by scanning narrow beam of electrons across the surface of specimen

Fig59 (a) SEM micrograph of CeO2 nanoparticles by microwave

(without annealed)

Chapter ndash V

169

Collecting and processing emitting electron and displaying them on cathode ray

tubecomputer monitor Using a visual raster which is synchronized with the

beams scanned The dried thin films samples of cerium oxide nanoparticles were

scanned by SEM The SEM images of cerium oxide synthesized by microwave

(without annealed) and annealed at 300degC are shown in Figs59 (a) and (b) From

SEM images revealed that there were various sizes of particles in the as-prepared

sample The large particles are composed of small crystallites and show particle

aggregates of irregular shapes The particles are spherical in shapes which are

good agreement with XRD results of particles size These observations were

consistent with similar observations reported in the literature [42ndash44]

Fig59 (b) SEM micrograph of CeO2 nanoparticles annealed at 300degC

536 Transmission Electron Microscopic (TEM) Analysis

In case of cerium oxide the transmission electron micrograph was obtained

by employing Philips microscopy IIT Madras TEM is analogous to the optical

microscope It provides very high resolution which can reach approximately

Chapter ndash V

170

01 nm in the case of lattice images Consequently very high magnification can be

obtained TEM is used to examine very thin section through materials of the

surface A transmission electron microscope Philips 120 kV is equipped with CCD

camera This instrument has the resolution of 036 nm with 42 to 120 kV operating

voltage and magnifies object upto 6 lakhs times in high resolution mode Samples

for TEM examinations were prepared by placing a drop of sample suspension on a

copper grade coated with carbon film and were allowed to dry in air A typical

TEM image of CeO2 powders synthesized by cerium nitrate salts by microwave

assisted sol-gel method (without calcination) is presented in Fig 510 The mean

diameter of the particles as measured is about 10 nm These results are in good

agreement with that of estimated by X-ray line broadening Also the synthesized

Fig510 TEM image of the CeO2 nanoparticles

nanopowder is well crystallized without defects non-agglomerated with narrow

size distributions The selected area electron diffraction (SAED) pattern shown in

the Fig511 it also supports the d values obtained from XRD with rings which

can be assigned [111] [200] [220] and [311] diffractions of CeO2 structure

Chapter ndash V

171

Fig511 SAED image of the CeO2 nanoparticles

537 Thermo-Gravimetric and Differential Thermal Analysis

Fig512 shows the TG-DTA curves for the dried ceria precursor prepared

by the Ce(IV) methods The dried samples may consist of three portions of

materials (A) crystalline ceria CeO2 (B) crystalline ceria with structural water

CeO22H2O and (C) amorphous cerium hydroxide Ce(OH)4 The endothermic

peaks in 64degC could be due to crystallization of the amorphous portion in the

sample Ce(OH)4 to crystalline CeO2 The larger endothermic peak for the sample

indicates that the former contains more amorphous phase than the latter The total

weight loss up to 800 degC for the sample is about 1685 (or 194 for n which is

the number of water molecules per mole of cerium in the dried sample) This is

consistent with the change of [Ce (OH)4 + CeO22H2O] to [CeO2] The two

endothermic peaks at about 313 and 436degC in figure can be attributed to the loss of

the structural water molecules The less obvious endothermic peaks for the

structural water loss for this sample also agree with the weight loss data These

results suggest that the Ce(IV) derived sample contains primarily Ce(OH)4 and

CeO22H2O

Chapter ndash V

172

Fig 512 Weight loss of CeO2 nanoparticles as a function of temperature

538 Electrical Conductivity of Semiconductors CeO2

Semiconductors find wide appilication particularly in electronics Their

application is based on certain properties that will be an electrical conductivity of

semiconductors When the temperature is not very low the semiconductor will

conduct electricity due to the movement of the electrons in the conduction band

and that of holes in the valence band The conductivity of nano CeO2 thick film is

represented in Fig513 From the figure the almost linear relationship was found

in the measured temperature range of 200-500oC If it assumed that conduction in

crystalline ceria is essentially an n-type electronic process then using a simple

model we can express the conductivity-temperature relation by the equation

σ = σo e-Ea kT

where σo is a constant Ea is thermal activation energy T is the

Kelvin temperature and k is Boltzmanns constant Thus an Arrhenius plot of

conductivity and temperature should be linear over certain temperature range

Although the results in Fig513 are curved a linear section can be drawn above

280oC which gives activation energy of 236 eV

Chapter ndash V

173

Fig513 Electrical conductivity of CeO2 nanoparticles as a function of temperature

539 Gas Sensing Performance

5391 LPG Gas Sensing

Fig514 shows the variation of response of CeO2 fired at various

temperature The gas response increases with temperature from 50 to 150degC and

then decreases with a further increase in temperature In present work every time

prior to exposing the CeO2 film to LPG it was allowed to stabilize at an operating

temperature for 10 min and the stabilized resistance was taken as Ra After

exposing the film to the LPG gas the changed resistance was taken as Rg LPG

reacts with surface oxygen ions of the film Oxidation of film decreases the

number of free carriers Therefore resistance of the film increases with gas The

result of reaction of LPG with polycrystalline CeO2 is adsorbed CO2 and H2O on

the surface of the oxide The Ce harrLPG interactions on CeO2 are strong and Ce

sites probably get oxidized

Chapter ndash V

174

Fig 514 Variation of gas responses with operating temperature for LPG gas

5393 Effect of Gas Concentration (active region)

The variation of sensor response of CeO2 thick film sample with LPG gas

concentration at operating temperature 150degC is represented in Fig 515

Fig 515 Variation in sensor response with LPG gas concentration

Chapter ndash V

175

It is clear from the figure that the gas response goes on increasing with gas

concentration upto 600 ppm The rate of increase in response was relatively large

upto 1000 ppm and saturated beyond 600 ppm The monolayer of the gas

molecules formed on the surface covers the whole surface of the film The gas

molecules from that layer would reach the surface active sites of the film The

excess gas molecules remain idle and would not reach the surface active sites of

the sensor Thus the active region of the sensor would be up to 600 ppm

5394 Response and Recovery Time

The response of CeO2 thick film sensor was found to be quick (~ 28 s) to

600 ppm of while the recovery was fast (~ 10 s) (Fig 516) The fast response

may be attributed to faster oxidation of the gas The negligible quantity of the

surface reaction product and its high volatility explains its fast response and quick

recovery to its initial chemical status

Fig 516 Response and recovery time

Chapter ndash V

176

5395 Oxygen Adsorption-Desorption Mechanism

O2 (air) + 2e- 2O

-(film surface) --- 56

At higher temperature the atmospheric oxygen O2 adsorbs on the surface of the

thick film It captures the electrons (Fig517) from conduction band It would

result in decreasing conductivity of the film When LPG reacts with the adsorbed

oxygen on the surface of the film it gets oxidized to CO2 and H2O by following

series of intermediate stages This liberates free electrons in the conduction band

The final reaction takes place as

C4H10(gas) + 13Ominus

(filmsurface) rarr 4CO2(gas) + 5H2O(gas) + 13eminus

(condband) --- 57

Fig 517 Sensing mechanism on the surface of the film

This shows n-type conduction mechanism Thus generated electrons

contribute to a sudden increase in conductance of the thick film The mass of

Ce and O in sample was not as per the stoichiometric proportion and sample was

Chapter ndash V

177

observed to be the oxygen deficient This deficiency gets reduced due to

adsorption of atmosphericmolecular oxygen This helps in decreasing electronic

conductivity of the film Upon exposure LPG molecules got oxidized with the

adsorbed oxygen ions by following the series of intermediate stages producing

CO2 and H2O This results in evolving oxygen as electrically neutral atoms

trapping behind the negative charges (electrons) Upon exposure the energy

released in decomposition of LPG molecules would be sufficient for trapped

electrons to jump into the conduction band of activated CeO2 resulting in increase

in the conductivity of the film The drastic increase in conductivity of the sensor

could be attributed to the adsorptionndashdesorption mechanism resulted from the

electronic defects [26]

5310 Catalytic Activity

The utility of this catalytic system deprotection of several oximes were

studied (Table 52) The catalyst was also tested for reusability and it was found

that it could be reused without loss of initial activity for at least four cycles [19]

TLC analysis indicated the formation of only one product of corresponding

carbonyls FT-IR and NMR analysis showed that the oximes were converted into

carbonyls For checking the reusability (after each run) the catalyst was recovered

by simple filtration and washed several times with acetone and then dried at

373 K The catalyst thus obtained was used for the subsequent runs without any

further modification both activated and deactivated oximes were oxidized in better

yield more than ZrO2 catalyst as shown in Table 52

Chapter ndash V

178

Table 52 Oxidative Deoximation of Carbonyls on CeO2

Entry

Substrate

Product

Time

(h)

Yield

()

1

NOH

O

15

62

2

NOH

Cl

O

Cl

15

58

3

NOH

O

25

73

4

NOHCl

OCl

40

57

5

NOH

Cl

O

Cl

30

59

Solvent- Ethyl acetate Catalyst- ZrO2 Temperature- 353 K

Chapter ndash V

179

5311 Adsorption of Rhodamine B (Rh B)

The removal of dye pollutants are also of great importance for wastewater

treatment Rhodamine B a common cationic dye used in textile industry is one of

the most notorious contaminants in aquatic environments because of it huge

amounts slow biodegradation and toxicity In this study synthesized CeO2

nanocrystals are further used to remove Rh B The absorption spectrum of Rh B

solution was characterized by its characteristic absorption at 555 nm which was

attributed to the chromophore containing azo linkage of the dye molecules [22]

Fig 518 Absorption spectra of aqueous solution of Rh B in the presence of CeO2

nanocrystals at different times

UV-Visible absorption spectra represented in Fig518 confirms that over 93 of

Rh B in 100 mL of 20 mg L-1

Rh B aqueous solution could be removed with 100

mg of the as-prepared CeO2 nanocrystalline within 60 min The reasons may

account for the efficient removal of Rh B with CeO2 nanocrystals The first reason

Chapter ndash V

180

should be their to facilitate the adsorption of Rh B The second one is attributed to

the oxygen vacancies on the surface of CeO2 nanocrystals These abundant surface

oxygen vacancies could produce strong electrostatic attraction with the cationic

groups of Rh B as well as hydrogen bonding with the nitrogen atoms of Rh B

Chapter ndash V

181

Conclusions

Microwave synthesis is very beneficial to find a fast simple and energy

efficient approach to produce fine CeO2 nanoparticles It is relatively new method

to produce inorganic compounds for materials processing to enhance the material

properties as well as economic advantages through energy saving and acceleration

of product development Cerium oxide has outstanding physical and chemical

properties therefore it is used as LPG sensor In the area of catalysis nanoparticles

of ceria have been studied Also it is concluded as

1] The CeO2 nanoparticles were prepared by simple microwave assisted sol-gel

method

2] The well spherical and narrow size distribution with 10 nm size particles were

obtained by this method

3] It is very simple time as well as energy saving technique

4] The XRD pattern shows the CeO2 nanoparticles exists in face centered cubic

5] Scanning electron micrographs indicates that grains are uniformaly distributed

6] Transmission electron micrographs indicates that the mean diameter of the

particles as measured is about 10 nm

7] The conductivity of nano CeO2 thick film is represents the almost linear

relationship was found in the measured temperature range of 200-5000C It is

clear that the conduction in crystalline ceria is essentially an n-type electronic

process

8] The CeO2 nanoparticles in the thick film form prepared by screen printing

technique showed better response to LPG gas at 250degC temperature at 600 ppm

concentration of LPG gas

9] Catalyst can be prepared from inexpensive precursors using the microwave

assisted sol-gel technique is promising for the oxidative regeneration of ketones

10] This study reveals that these CeO2 nanocrystals are novel adsorbents for

wastewater treatment

Chapter ndash V

182

References

[1] A Ramesh H Hasegawa T Maki KUeda Sep Purif Tech

56 (2007) 90

[2] C Lettmann K Hildenbrand H Kisch W Macyk W F Maier

Appl Catal B 32 (2001) 215

[3] X Zhang Z H Ai F L Jia L Z Zhang J Phys Chem C 112 (2008) 747

[4] N Du H Zhang BD Chen X Y Ma D RYang J Phys ChemC

111 (2007) 12677

[5] F Zhang S Yang H Chen X Yu Cera Intern 30 (2004) 997

[6] F Gu SF Wang MK Lu GJ Zhou D Xu DR Langmuir J SciIR Iran

20 (2004) 3528

[7] H Xiao Z Ai L Zhang JPhys Chem 113 (2009) 16625

[8] L Combemale G Caboche D Stuerga D Chaumont MicroRese

Bull 40 (2005) 529

[9] M Zawadzki Jof Alloys and Comp 454 (2008) 347

[10] A V Patil C G Dighavkar S K Sonawane S J Patil R Y Borse

Sens and Transducers 108 (2009) 189

[11] AV Patil CG Dighavkar RY Borse Sens and Transducers

101 (2009) 96

[12] AV Patil CG Dighavkar SK Sonawane S J Patil RY Borse Sensand

Transducers 9 (2010) 11

[13] DR Patil LA Patil Talanta 77 (2009)1409

[14] R S Blackburn Environ Sci Tech 38 (2004) 4905

[15] GNaja CMustin JBerthelin BVolesky J Colloide Inter Sci

292 (2005) 537

[16] VKGupta IAli Environ Sci Tech 42 (2008) 766

[17] A Mittal VK Gupta A Malviya and JMittal J Hazard Mater

151 (2008) 821

[18] VKGupta IAli VKSaini J Colloid Interface Sci 315(2007) 87

Chapter ndash V

183

[19] S S Deshpande S U Sonavane R V Jayaram Cat Com 9 (2008) 639

[20] KM Garadkar BS Shirke YB Patil D R Patil Sens and Transducers

110 (2009) 17

[21] VKGupta AMittal VGajbe JMittal J Colloid Inter Sci

319 (2008) 30

[22] T A Khan I Ali V V Singh S Sharma J Envir Protec Sci 3 (2009)11

[23] A Hosseinnia M Keyanpour-Rad M Pazouki World Applied Sci J

8 (2010) 1327

[24] MI Baraton L Merhari J Am Ceram Soc 75 (2004) 1587

[25] CWang B Xu XWang J Zhao J Solid State Chem 178 (2005) 3500

[26] H Xiao Z Ai L Zhang J Phys Chem C doi 10 1021 JP9050269 (2009)

[27] A M Thompson Oxides of the Rare Earths Wiley New York (1978)

[28] H Noumlremberg G A D Briggs Phys Rev Lett 79 (1997) 4222

[29] A Hosseinnia M Keyanpour-Rad M Pazouki World Appl Sci J

8 (2010) 1327

[30] BD Cullity SR Stock Elements of X-Ray Diffraction 3rd edn

(Prentice Hall Upper Saddle River NJ) (2001) 07458

[31] G Ranga Rao B Gopal Mishra Bul Cat Soc of Ind 2 (2003)122

[32] A K Bhattacharyal T K Naiya S N Mondal S K Das Chem Eng J

137 (2008) 529

[33] G Liu J A Rodriguez Z Chang J Hrbek C H F Peden

J Phys Chem B 108 (2004) 2931

[34] P F Ji J L Zhang F Chen M Anpo J Phys Chem C 112 (2008) 17809

[35] A Trovarelli Catal Rev SciEng 38 (1996) 439

[36] B Li D M Pan J S Zheng Y J Cheng X Y Ma F Huang L Zhang

Langmuir 24 (2008) 9630

[37] M Fernaacutendez-Garcıacutea A A B Martıacutenez-Arias A Hungrıacutea Iglesias-Juez

J C Conesa J Soria J Phys Chem Chem Phys 4 (2002) 2473

[38] H M Chen J H He J Phys Chem C 112 (2008) 17540

Chapter ndash V

184

[39] F Zhang S Chan W Spanier J E Apak E Jin Q Robinson

R D Herman I P Appl Phys Lett 80 (2002) 127

[40] G Adachi TMasui In Catalysis by Ceria and Related Materials

ATrovarelli Ed Imperial College Press London (2002)

[41] J A Rodriguez X Wang J C Hanson G Liu A Iglesias-Juez

MFernaacutendez-Garcıacutea J Chem Phys 119 (2003) 5659

[42] S Eck C Castellarin-Cudia S Surnev M G Ramsey F PNetzer

Surf Sci 520 (2002) 173

[43] R B Yu L Yan P Zheng JChen X X R J Phys ChemC

112 (2008) 19896

[44] Y Nagai T Yamamoto T Tanaka SYoshida T Nonaka T Okamoto

A Suda M Sugiura Catal Today 74 (2002) 225

[45] Y M Chiang E B Lavik I Kosacki H L Tuller JY Ying

J Electroceram 1 (1997) 7

[46] D R Patil L A Patil G H Jain M S Wagh S A Patil

Sens and Transducers 74 (2006) 874

[47] K Sohlberg S K Pantelides S J Pennycook J Am Chem Soc

123 (2001) 6609

[48] L S Zhong J S Hu A M Cao Q Liu W G Song LWan

J Chem Mater 19 (2007) 1648

[49] J B Fei Y Cui X H Yan Y Yang K W Wang QHe J B Li

AdV Mater 20 (2008) 452

[50] P V Kamat J Phys Chem B 106 (2002) 7729

[51] R D Robinson J E Spanier F Zhang SW Chan I P Herman J Appl

Phys 92 (2002) 1936

[52] S W Cao Y J Zhu J Phys Chem C 112 (2008) 6253

[53] R Georgekutty M K Seery S C Pillai J Phys Chem C

112 (2008) 13563

[54] J S Hu L S Zhong W G Song L J Wan Adv Mater 20 (2008)1

Chapter ndash V

185

[55] HXu F L Jia Z H Ai L Z Zhang Cryst Growth Des (2007)

[56] H M Jia W J Xiao L Z Zhang Z F Zheng J PhysChem C

112 (2008) 1379

[57] Z H Ai H Y Xiao T Mei J Liu L Z Zhang K J Deng J R Qiu

J PhysChem C 112 (2008) 11929

[58] L M Ma W X Zhang EnViron Sci Technol 42 (2008) 5384

[59] Z H Ai Y Chen L Z Zhang J R Qiu EnViron Sci Technol

42 (2008) 69550

[60] Z Y Zhong J Ho J Teo S C Shen AGedanken ChemMater

19 (2007) 4776

[61] S C Kuiry S D Patil S Deshpande S Seal J Phys ChemB

109 (2005) 6936

[62] D R Patil L A Patil Sens IEEE 7 (2007) 434

[63] P Jasinski Anderson H U Sens Actuators B 95 (2003) 73

[64] J P Nair E Wachtel I Lubomirsky J Fleig JMaier AdVMater

15 (2003) 2077

[65] R J Qi Y J Zhu G F Cheng Y H Huang Nanotechnology

16 (2005) 2502

Chapter ndash V

168

The EDAX spectrum of cerium oxide nano particles synthesized by

microwave assisted sol-gel method is shown in Fig58 the composition of sample

shows the presense of cerium and oxygen elements The spectrum peak reveals the

presence of Ce and O at 4837 and 0525 keV respectively The atomic of Ce

and O is 3597 and 6403 respectively which confirms the presence of Ce and O in

the powder The present composition of Ce and O reveals that the formation of

non-stoichometric CeO2 which is superior for adsorption of toxic ions from

wastewater

535 Scanning Electron Microscopic (SEM) Analysis

The model JEOL JSM-6360 was used for the determination of morphology

of nanoparticle The cerium oxide nanoparticles were dispersed in isopropyl

alcohol and sonicated for frac12 hour One or two drop of sonicated solution was kept

on 1x1 cm glass plate dried naturally and coated the platinum The SEM images

are formed by scanning narrow beam of electrons across the surface of specimen

Fig59 (a) SEM micrograph of CeO2 nanoparticles by microwave

(without annealed)

Chapter ndash V

169

Collecting and processing emitting electron and displaying them on cathode ray

tubecomputer monitor Using a visual raster which is synchronized with the

beams scanned The dried thin films samples of cerium oxide nanoparticles were

scanned by SEM The SEM images of cerium oxide synthesized by microwave

(without annealed) and annealed at 300degC are shown in Figs59 (a) and (b) From

SEM images revealed that there were various sizes of particles in the as-prepared

sample The large particles are composed of small crystallites and show particle

aggregates of irregular shapes The particles are spherical in shapes which are

good agreement with XRD results of particles size These observations were

consistent with similar observations reported in the literature [42ndash44]

Fig59 (b) SEM micrograph of CeO2 nanoparticles annealed at 300degC

536 Transmission Electron Microscopic (TEM) Analysis

In case of cerium oxide the transmission electron micrograph was obtained

by employing Philips microscopy IIT Madras TEM is analogous to the optical

microscope It provides very high resolution which can reach approximately

Chapter ndash V

170

01 nm in the case of lattice images Consequently very high magnification can be

obtained TEM is used to examine very thin section through materials of the

surface A transmission electron microscope Philips 120 kV is equipped with CCD

camera This instrument has the resolution of 036 nm with 42 to 120 kV operating

voltage and magnifies object upto 6 lakhs times in high resolution mode Samples

for TEM examinations were prepared by placing a drop of sample suspension on a

copper grade coated with carbon film and were allowed to dry in air A typical

TEM image of CeO2 powders synthesized by cerium nitrate salts by microwave

assisted sol-gel method (without calcination) is presented in Fig 510 The mean

diameter of the particles as measured is about 10 nm These results are in good

agreement with that of estimated by X-ray line broadening Also the synthesized

Fig510 TEM image of the CeO2 nanoparticles

nanopowder is well crystallized without defects non-agglomerated with narrow

size distributions The selected area electron diffraction (SAED) pattern shown in

the Fig511 it also supports the d values obtained from XRD with rings which

can be assigned [111] [200] [220] and [311] diffractions of CeO2 structure

Chapter ndash V

171

Fig511 SAED image of the CeO2 nanoparticles

537 Thermo-Gravimetric and Differential Thermal Analysis

Fig512 shows the TG-DTA curves for the dried ceria precursor prepared

by the Ce(IV) methods The dried samples may consist of three portions of

materials (A) crystalline ceria CeO2 (B) crystalline ceria with structural water

CeO22H2O and (C) amorphous cerium hydroxide Ce(OH)4 The endothermic

peaks in 64degC could be due to crystallization of the amorphous portion in the

sample Ce(OH)4 to crystalline CeO2 The larger endothermic peak for the sample

indicates that the former contains more amorphous phase than the latter The total

weight loss up to 800 degC for the sample is about 1685 (or 194 for n which is

the number of water molecules per mole of cerium in the dried sample) This is

consistent with the change of [Ce (OH)4 + CeO22H2O] to [CeO2] The two

endothermic peaks at about 313 and 436degC in figure can be attributed to the loss of

the structural water molecules The less obvious endothermic peaks for the

structural water loss for this sample also agree with the weight loss data These

results suggest that the Ce(IV) derived sample contains primarily Ce(OH)4 and

CeO22H2O

Chapter ndash V

172

Fig 512 Weight loss of CeO2 nanoparticles as a function of temperature

538 Electrical Conductivity of Semiconductors CeO2

Semiconductors find wide appilication particularly in electronics Their

application is based on certain properties that will be an electrical conductivity of

semiconductors When the temperature is not very low the semiconductor will

conduct electricity due to the movement of the electrons in the conduction band

and that of holes in the valence band The conductivity of nano CeO2 thick film is

represented in Fig513 From the figure the almost linear relationship was found

in the measured temperature range of 200-500oC If it assumed that conduction in

crystalline ceria is essentially an n-type electronic process then using a simple

model we can express the conductivity-temperature relation by the equation

σ = σo e-Ea kT

where σo is a constant Ea is thermal activation energy T is the

Kelvin temperature and k is Boltzmanns constant Thus an Arrhenius plot of

conductivity and temperature should be linear over certain temperature range

Although the results in Fig513 are curved a linear section can be drawn above

280oC which gives activation energy of 236 eV

Chapter ndash V

173

Fig513 Electrical conductivity of CeO2 nanoparticles as a function of temperature

539 Gas Sensing Performance

5391 LPG Gas Sensing

Fig514 shows the variation of response of CeO2 fired at various

temperature The gas response increases with temperature from 50 to 150degC and

then decreases with a further increase in temperature In present work every time

prior to exposing the CeO2 film to LPG it was allowed to stabilize at an operating

temperature for 10 min and the stabilized resistance was taken as Ra After

exposing the film to the LPG gas the changed resistance was taken as Rg LPG

reacts with surface oxygen ions of the film Oxidation of film decreases the

number of free carriers Therefore resistance of the film increases with gas The

result of reaction of LPG with polycrystalline CeO2 is adsorbed CO2 and H2O on

the surface of the oxide The Ce harrLPG interactions on CeO2 are strong and Ce

sites probably get oxidized

Chapter ndash V

174

Fig 514 Variation of gas responses with operating temperature for LPG gas

5393 Effect of Gas Concentration (active region)

The variation of sensor response of CeO2 thick film sample with LPG gas

concentration at operating temperature 150degC is represented in Fig 515

Fig 515 Variation in sensor response with LPG gas concentration

Chapter ndash V

175

It is clear from the figure that the gas response goes on increasing with gas

concentration upto 600 ppm The rate of increase in response was relatively large

upto 1000 ppm and saturated beyond 600 ppm The monolayer of the gas

molecules formed on the surface covers the whole surface of the film The gas

molecules from that layer would reach the surface active sites of the film The

excess gas molecules remain idle and would not reach the surface active sites of

the sensor Thus the active region of the sensor would be up to 600 ppm

5394 Response and Recovery Time

The response of CeO2 thick film sensor was found to be quick (~ 28 s) to

600 ppm of while the recovery was fast (~ 10 s) (Fig 516) The fast response

may be attributed to faster oxidation of the gas The negligible quantity of the

surface reaction product and its high volatility explains its fast response and quick

recovery to its initial chemical status

Fig 516 Response and recovery time

Chapter ndash V

176

5395 Oxygen Adsorption-Desorption Mechanism

O2 (air) + 2e- 2O

-(film surface) --- 56

At higher temperature the atmospheric oxygen O2 adsorbs on the surface of the

thick film It captures the electrons (Fig517) from conduction band It would

result in decreasing conductivity of the film When LPG reacts with the adsorbed

oxygen on the surface of the film it gets oxidized to CO2 and H2O by following

series of intermediate stages This liberates free electrons in the conduction band

The final reaction takes place as

C4H10(gas) + 13Ominus

(filmsurface) rarr 4CO2(gas) + 5H2O(gas) + 13eminus

(condband) --- 57

Fig 517 Sensing mechanism on the surface of the film

This shows n-type conduction mechanism Thus generated electrons

contribute to a sudden increase in conductance of the thick film The mass of

Ce and O in sample was not as per the stoichiometric proportion and sample was

Chapter ndash V

177

observed to be the oxygen deficient This deficiency gets reduced due to

adsorption of atmosphericmolecular oxygen This helps in decreasing electronic

conductivity of the film Upon exposure LPG molecules got oxidized with the

adsorbed oxygen ions by following the series of intermediate stages producing

CO2 and H2O This results in evolving oxygen as electrically neutral atoms

trapping behind the negative charges (electrons) Upon exposure the energy

released in decomposition of LPG molecules would be sufficient for trapped

electrons to jump into the conduction band of activated CeO2 resulting in increase

in the conductivity of the film The drastic increase in conductivity of the sensor

could be attributed to the adsorptionndashdesorption mechanism resulted from the

electronic defects [26]

5310 Catalytic Activity

The utility of this catalytic system deprotection of several oximes were

studied (Table 52) The catalyst was also tested for reusability and it was found

that it could be reused without loss of initial activity for at least four cycles [19]

TLC analysis indicated the formation of only one product of corresponding

carbonyls FT-IR and NMR analysis showed that the oximes were converted into

carbonyls For checking the reusability (after each run) the catalyst was recovered

by simple filtration and washed several times with acetone and then dried at

373 K The catalyst thus obtained was used for the subsequent runs without any

further modification both activated and deactivated oximes were oxidized in better

yield more than ZrO2 catalyst as shown in Table 52

Chapter ndash V

178

Table 52 Oxidative Deoximation of Carbonyls on CeO2

Entry

Substrate

Product

Time

(h)

Yield

()

1

NOH

O

15

62

2

NOH

Cl

O

Cl

15

58

3

NOH

O

25

73

4

NOHCl

OCl

40

57

5

NOH

Cl

O

Cl

30

59

Solvent- Ethyl acetate Catalyst- ZrO2 Temperature- 353 K

Chapter ndash V

179

5311 Adsorption of Rhodamine B (Rh B)

The removal of dye pollutants are also of great importance for wastewater

treatment Rhodamine B a common cationic dye used in textile industry is one of

the most notorious contaminants in aquatic environments because of it huge

amounts slow biodegradation and toxicity In this study synthesized CeO2

nanocrystals are further used to remove Rh B The absorption spectrum of Rh B

solution was characterized by its characteristic absorption at 555 nm which was

attributed to the chromophore containing azo linkage of the dye molecules [22]

Fig 518 Absorption spectra of aqueous solution of Rh B in the presence of CeO2

nanocrystals at different times

UV-Visible absorption spectra represented in Fig518 confirms that over 93 of

Rh B in 100 mL of 20 mg L-1

Rh B aqueous solution could be removed with 100

mg of the as-prepared CeO2 nanocrystalline within 60 min The reasons may

account for the efficient removal of Rh B with CeO2 nanocrystals The first reason

Chapter ndash V

180

should be their to facilitate the adsorption of Rh B The second one is attributed to

the oxygen vacancies on the surface of CeO2 nanocrystals These abundant surface

oxygen vacancies could produce strong electrostatic attraction with the cationic

groups of Rh B as well as hydrogen bonding with the nitrogen atoms of Rh B

Chapter ndash V

181

Conclusions

Microwave synthesis is very beneficial to find a fast simple and energy

efficient approach to produce fine CeO2 nanoparticles It is relatively new method

to produce inorganic compounds for materials processing to enhance the material

properties as well as economic advantages through energy saving and acceleration

of product development Cerium oxide has outstanding physical and chemical

properties therefore it is used as LPG sensor In the area of catalysis nanoparticles

of ceria have been studied Also it is concluded as

1] The CeO2 nanoparticles were prepared by simple microwave assisted sol-gel

method

2] The well spherical and narrow size distribution with 10 nm size particles were

obtained by this method

3] It is very simple time as well as energy saving technique

4] The XRD pattern shows the CeO2 nanoparticles exists in face centered cubic

5] Scanning electron micrographs indicates that grains are uniformaly distributed

6] Transmission electron micrographs indicates that the mean diameter of the

particles as measured is about 10 nm

7] The conductivity of nano CeO2 thick film is represents the almost linear

relationship was found in the measured temperature range of 200-5000C It is

clear that the conduction in crystalline ceria is essentially an n-type electronic

process

8] The CeO2 nanoparticles in the thick film form prepared by screen printing

technique showed better response to LPG gas at 250degC temperature at 600 ppm

concentration of LPG gas

9] Catalyst can be prepared from inexpensive precursors using the microwave

assisted sol-gel technique is promising for the oxidative regeneration of ketones

10] This study reveals that these CeO2 nanocrystals are novel adsorbents for

wastewater treatment

Chapter ndash V

182

References

[1] A Ramesh H Hasegawa T Maki KUeda Sep Purif Tech

56 (2007) 90

[2] C Lettmann K Hildenbrand H Kisch W Macyk W F Maier

Appl Catal B 32 (2001) 215

[3] X Zhang Z H Ai F L Jia L Z Zhang J Phys Chem C 112 (2008) 747

[4] N Du H Zhang BD Chen X Y Ma D RYang J Phys ChemC

111 (2007) 12677

[5] F Zhang S Yang H Chen X Yu Cera Intern 30 (2004) 997

[6] F Gu SF Wang MK Lu GJ Zhou D Xu DR Langmuir J SciIR Iran

20 (2004) 3528

[7] H Xiao Z Ai L Zhang JPhys Chem 113 (2009) 16625

[8] L Combemale G Caboche D Stuerga D Chaumont MicroRese

Bull 40 (2005) 529

[9] M Zawadzki Jof Alloys and Comp 454 (2008) 347

[10] A V Patil C G Dighavkar S K Sonawane S J Patil R Y Borse

Sens and Transducers 108 (2009) 189

[11] AV Patil CG Dighavkar RY Borse Sens and Transducers

101 (2009) 96

[12] AV Patil CG Dighavkar SK Sonawane S J Patil RY Borse Sensand

Transducers 9 (2010) 11

[13] DR Patil LA Patil Talanta 77 (2009)1409

[14] R S Blackburn Environ Sci Tech 38 (2004) 4905

[15] GNaja CMustin JBerthelin BVolesky J Colloide Inter Sci

292 (2005) 537

[16] VKGupta IAli Environ Sci Tech 42 (2008) 766

[17] A Mittal VK Gupta A Malviya and JMittal J Hazard Mater

151 (2008) 821

[18] VKGupta IAli VKSaini J Colloid Interface Sci 315(2007) 87

Chapter ndash V

183

[19] S S Deshpande S U Sonavane R V Jayaram Cat Com 9 (2008) 639

[20] KM Garadkar BS Shirke YB Patil D R Patil Sens and Transducers

110 (2009) 17

[21] VKGupta AMittal VGajbe JMittal J Colloid Inter Sci

319 (2008) 30

[22] T A Khan I Ali V V Singh S Sharma J Envir Protec Sci 3 (2009)11

[23] A Hosseinnia M Keyanpour-Rad M Pazouki World Applied Sci J

8 (2010) 1327

[24] MI Baraton L Merhari J Am Ceram Soc 75 (2004) 1587

[25] CWang B Xu XWang J Zhao J Solid State Chem 178 (2005) 3500

[26] H Xiao Z Ai L Zhang J Phys Chem C doi 10 1021 JP9050269 (2009)

[27] A M Thompson Oxides of the Rare Earths Wiley New York (1978)

[28] H Noumlremberg G A D Briggs Phys Rev Lett 79 (1997) 4222

[29] A Hosseinnia M Keyanpour-Rad M Pazouki World Appl Sci J

8 (2010) 1327

[30] BD Cullity SR Stock Elements of X-Ray Diffraction 3rd edn

(Prentice Hall Upper Saddle River NJ) (2001) 07458

[31] G Ranga Rao B Gopal Mishra Bul Cat Soc of Ind 2 (2003)122

[32] A K Bhattacharyal T K Naiya S N Mondal S K Das Chem Eng J

137 (2008) 529

[33] G Liu J A Rodriguez Z Chang J Hrbek C H F Peden

J Phys Chem B 108 (2004) 2931

[34] P F Ji J L Zhang F Chen M Anpo J Phys Chem C 112 (2008) 17809

[35] A Trovarelli Catal Rev SciEng 38 (1996) 439

[36] B Li D M Pan J S Zheng Y J Cheng X Y Ma F Huang L Zhang

Langmuir 24 (2008) 9630

[37] M Fernaacutendez-Garcıacutea A A B Martıacutenez-Arias A Hungrıacutea Iglesias-Juez

J C Conesa J Soria J Phys Chem Chem Phys 4 (2002) 2473

[38] H M Chen J H He J Phys Chem C 112 (2008) 17540

Chapter ndash V

184

[39] F Zhang S Chan W Spanier J E Apak E Jin Q Robinson

R D Herman I P Appl Phys Lett 80 (2002) 127

[40] G Adachi TMasui In Catalysis by Ceria and Related Materials

ATrovarelli Ed Imperial College Press London (2002)

[41] J A Rodriguez X Wang J C Hanson G Liu A Iglesias-Juez

MFernaacutendez-Garcıacutea J Chem Phys 119 (2003) 5659

[42] S Eck C Castellarin-Cudia S Surnev M G Ramsey F PNetzer

Surf Sci 520 (2002) 173

[43] R B Yu L Yan P Zheng JChen X X R J Phys ChemC

112 (2008) 19896

[44] Y Nagai T Yamamoto T Tanaka SYoshida T Nonaka T Okamoto

A Suda M Sugiura Catal Today 74 (2002) 225

[45] Y M Chiang E B Lavik I Kosacki H L Tuller JY Ying

J Electroceram 1 (1997) 7

[46] D R Patil L A Patil G H Jain M S Wagh S A Patil

Sens and Transducers 74 (2006) 874

[47] K Sohlberg S K Pantelides S J Pennycook J Am Chem Soc

123 (2001) 6609

[48] L S Zhong J S Hu A M Cao Q Liu W G Song LWan

J Chem Mater 19 (2007) 1648

[49] J B Fei Y Cui X H Yan Y Yang K W Wang QHe J B Li

AdV Mater 20 (2008) 452

[50] P V Kamat J Phys Chem B 106 (2002) 7729

[51] R D Robinson J E Spanier F Zhang SW Chan I P Herman J Appl

Phys 92 (2002) 1936

[52] S W Cao Y J Zhu J Phys Chem C 112 (2008) 6253

[53] R Georgekutty M K Seery S C Pillai J Phys Chem C

112 (2008) 13563

[54] J S Hu L S Zhong W G Song L J Wan Adv Mater 20 (2008)1

Chapter ndash V

185

[55] HXu F L Jia Z H Ai L Z Zhang Cryst Growth Des (2007)

[56] H M Jia W J Xiao L Z Zhang Z F Zheng J PhysChem C

112 (2008) 1379

[57] Z H Ai H Y Xiao T Mei J Liu L Z Zhang K J Deng J R Qiu

J PhysChem C 112 (2008) 11929

[58] L M Ma W X Zhang EnViron Sci Technol 42 (2008) 5384

[59] Z H Ai Y Chen L Z Zhang J R Qiu EnViron Sci Technol

42 (2008) 69550

[60] Z Y Zhong J Ho J Teo S C Shen AGedanken ChemMater

19 (2007) 4776

[61] S C Kuiry S D Patil S Deshpande S Seal J Phys ChemB

109 (2005) 6936

[62] D R Patil L A Patil Sens IEEE 7 (2007) 434

[63] P Jasinski Anderson H U Sens Actuators B 95 (2003) 73

[64] J P Nair E Wachtel I Lubomirsky J Fleig JMaier AdVMater

15 (2003) 2077

[65] R J Qi Y J Zhu G F Cheng Y H Huang Nanotechnology

16 (2005) 2502

Chapter ndash V

169

Collecting and processing emitting electron and displaying them on cathode ray

tubecomputer monitor Using a visual raster which is synchronized with the

beams scanned The dried thin films samples of cerium oxide nanoparticles were

scanned by SEM The SEM images of cerium oxide synthesized by microwave

(without annealed) and annealed at 300degC are shown in Figs59 (a) and (b) From

SEM images revealed that there were various sizes of particles in the as-prepared

sample The large particles are composed of small crystallites and show particle

aggregates of irregular shapes The particles are spherical in shapes which are

good agreement with XRD results of particles size These observations were

consistent with similar observations reported in the literature [42ndash44]

Fig59 (b) SEM micrograph of CeO2 nanoparticles annealed at 300degC

536 Transmission Electron Microscopic (TEM) Analysis

In case of cerium oxide the transmission electron micrograph was obtained

by employing Philips microscopy IIT Madras TEM is analogous to the optical

microscope It provides very high resolution which can reach approximately

Chapter ndash V

170

01 nm in the case of lattice images Consequently very high magnification can be

obtained TEM is used to examine very thin section through materials of the

surface A transmission electron microscope Philips 120 kV is equipped with CCD

camera This instrument has the resolution of 036 nm with 42 to 120 kV operating

voltage and magnifies object upto 6 lakhs times in high resolution mode Samples

for TEM examinations were prepared by placing a drop of sample suspension on a

copper grade coated with carbon film and were allowed to dry in air A typical

TEM image of CeO2 powders synthesized by cerium nitrate salts by microwave

assisted sol-gel method (without calcination) is presented in Fig 510 The mean

diameter of the particles as measured is about 10 nm These results are in good

agreement with that of estimated by X-ray line broadening Also the synthesized

Fig510 TEM image of the CeO2 nanoparticles

nanopowder is well crystallized without defects non-agglomerated with narrow

size distributions The selected area electron diffraction (SAED) pattern shown in

the Fig511 it also supports the d values obtained from XRD with rings which

can be assigned [111] [200] [220] and [311] diffractions of CeO2 structure

Chapter ndash V

171

Fig511 SAED image of the CeO2 nanoparticles

537 Thermo-Gravimetric and Differential Thermal Analysis

Fig512 shows the TG-DTA curves for the dried ceria precursor prepared

by the Ce(IV) methods The dried samples may consist of three portions of

materials (A) crystalline ceria CeO2 (B) crystalline ceria with structural water

CeO22H2O and (C) amorphous cerium hydroxide Ce(OH)4 The endothermic

peaks in 64degC could be due to crystallization of the amorphous portion in the

sample Ce(OH)4 to crystalline CeO2 The larger endothermic peak for the sample

indicates that the former contains more amorphous phase than the latter The total

weight loss up to 800 degC for the sample is about 1685 (or 194 for n which is

the number of water molecules per mole of cerium in the dried sample) This is

consistent with the change of [Ce (OH)4 + CeO22H2O] to [CeO2] The two

endothermic peaks at about 313 and 436degC in figure can be attributed to the loss of

the structural water molecules The less obvious endothermic peaks for the

structural water loss for this sample also agree with the weight loss data These

results suggest that the Ce(IV) derived sample contains primarily Ce(OH)4 and

CeO22H2O

Chapter ndash V

172

Fig 512 Weight loss of CeO2 nanoparticles as a function of temperature

538 Electrical Conductivity of Semiconductors CeO2

Semiconductors find wide appilication particularly in electronics Their

application is based on certain properties that will be an electrical conductivity of

semiconductors When the temperature is not very low the semiconductor will

conduct electricity due to the movement of the electrons in the conduction band

and that of holes in the valence band The conductivity of nano CeO2 thick film is

represented in Fig513 From the figure the almost linear relationship was found

in the measured temperature range of 200-500oC If it assumed that conduction in

crystalline ceria is essentially an n-type electronic process then using a simple

model we can express the conductivity-temperature relation by the equation

σ = σo e-Ea kT

where σo is a constant Ea is thermal activation energy T is the

Kelvin temperature and k is Boltzmanns constant Thus an Arrhenius plot of

conductivity and temperature should be linear over certain temperature range

Although the results in Fig513 are curved a linear section can be drawn above

280oC which gives activation energy of 236 eV

Chapter ndash V

173

Fig513 Electrical conductivity of CeO2 nanoparticles as a function of temperature

539 Gas Sensing Performance

5391 LPG Gas Sensing

Fig514 shows the variation of response of CeO2 fired at various

temperature The gas response increases with temperature from 50 to 150degC and

then decreases with a further increase in temperature In present work every time

prior to exposing the CeO2 film to LPG it was allowed to stabilize at an operating

temperature for 10 min and the stabilized resistance was taken as Ra After

exposing the film to the LPG gas the changed resistance was taken as Rg LPG

reacts with surface oxygen ions of the film Oxidation of film decreases the

number of free carriers Therefore resistance of the film increases with gas The

result of reaction of LPG with polycrystalline CeO2 is adsorbed CO2 and H2O on

the surface of the oxide The Ce harrLPG interactions on CeO2 are strong and Ce

sites probably get oxidized

Chapter ndash V

174

Fig 514 Variation of gas responses with operating temperature for LPG gas

5393 Effect of Gas Concentration (active region)

The variation of sensor response of CeO2 thick film sample with LPG gas

concentration at operating temperature 150degC is represented in Fig 515

Fig 515 Variation in sensor response with LPG gas concentration

Chapter ndash V

175

It is clear from the figure that the gas response goes on increasing with gas

concentration upto 600 ppm The rate of increase in response was relatively large

upto 1000 ppm and saturated beyond 600 ppm The monolayer of the gas

molecules formed on the surface covers the whole surface of the film The gas

molecules from that layer would reach the surface active sites of the film The

excess gas molecules remain idle and would not reach the surface active sites of

the sensor Thus the active region of the sensor would be up to 600 ppm

5394 Response and Recovery Time

The response of CeO2 thick film sensor was found to be quick (~ 28 s) to

600 ppm of while the recovery was fast (~ 10 s) (Fig 516) The fast response

may be attributed to faster oxidation of the gas The negligible quantity of the

surface reaction product and its high volatility explains its fast response and quick

recovery to its initial chemical status

Fig 516 Response and recovery time

Chapter ndash V

176

5395 Oxygen Adsorption-Desorption Mechanism

O2 (air) + 2e- 2O

-(film surface) --- 56

At higher temperature the atmospheric oxygen O2 adsorbs on the surface of the

thick film It captures the electrons (Fig517) from conduction band It would

result in decreasing conductivity of the film When LPG reacts with the adsorbed

oxygen on the surface of the film it gets oxidized to CO2 and H2O by following

series of intermediate stages This liberates free electrons in the conduction band

The final reaction takes place as

C4H10(gas) + 13Ominus

(filmsurface) rarr 4CO2(gas) + 5H2O(gas) + 13eminus

(condband) --- 57

Fig 517 Sensing mechanism on the surface of the film

This shows n-type conduction mechanism Thus generated electrons

contribute to a sudden increase in conductance of the thick film The mass of

Ce and O in sample was not as per the stoichiometric proportion and sample was

Chapter ndash V

177

observed to be the oxygen deficient This deficiency gets reduced due to

adsorption of atmosphericmolecular oxygen This helps in decreasing electronic

conductivity of the film Upon exposure LPG molecules got oxidized with the

adsorbed oxygen ions by following the series of intermediate stages producing

CO2 and H2O This results in evolving oxygen as electrically neutral atoms

trapping behind the negative charges (electrons) Upon exposure the energy

released in decomposition of LPG molecules would be sufficient for trapped

electrons to jump into the conduction band of activated CeO2 resulting in increase

in the conductivity of the film The drastic increase in conductivity of the sensor

could be attributed to the adsorptionndashdesorption mechanism resulted from the

electronic defects [26]

5310 Catalytic Activity

The utility of this catalytic system deprotection of several oximes were

studied (Table 52) The catalyst was also tested for reusability and it was found

that it could be reused without loss of initial activity for at least four cycles [19]

TLC analysis indicated the formation of only one product of corresponding

carbonyls FT-IR and NMR analysis showed that the oximes were converted into

carbonyls For checking the reusability (after each run) the catalyst was recovered

by simple filtration and washed several times with acetone and then dried at

373 K The catalyst thus obtained was used for the subsequent runs without any

further modification both activated and deactivated oximes were oxidized in better

yield more than ZrO2 catalyst as shown in Table 52

Chapter ndash V

178

Table 52 Oxidative Deoximation of Carbonyls on CeO2

Entry

Substrate

Product

Time

(h)

Yield

()

1

NOH

O

15

62

2

NOH

Cl

O

Cl

15

58

3

NOH

O

25

73

4

NOHCl

OCl

40

57

5

NOH

Cl

O

Cl

30

59

Solvent- Ethyl acetate Catalyst- ZrO2 Temperature- 353 K

Chapter ndash V

179

5311 Adsorption of Rhodamine B (Rh B)

The removal of dye pollutants are also of great importance for wastewater

treatment Rhodamine B a common cationic dye used in textile industry is one of

the most notorious contaminants in aquatic environments because of it huge

amounts slow biodegradation and toxicity In this study synthesized CeO2

nanocrystals are further used to remove Rh B The absorption spectrum of Rh B

solution was characterized by its characteristic absorption at 555 nm which was

attributed to the chromophore containing azo linkage of the dye molecules [22]

Fig 518 Absorption spectra of aqueous solution of Rh B in the presence of CeO2

nanocrystals at different times

UV-Visible absorption spectra represented in Fig518 confirms that over 93 of

Rh B in 100 mL of 20 mg L-1

Rh B aqueous solution could be removed with 100

mg of the as-prepared CeO2 nanocrystalline within 60 min The reasons may

account for the efficient removal of Rh B with CeO2 nanocrystals The first reason

Chapter ndash V

180

should be their to facilitate the adsorption of Rh B The second one is attributed to

the oxygen vacancies on the surface of CeO2 nanocrystals These abundant surface

oxygen vacancies could produce strong electrostatic attraction with the cationic

groups of Rh B as well as hydrogen bonding with the nitrogen atoms of Rh B

Chapter ndash V

181

Conclusions

Microwave synthesis is very beneficial to find a fast simple and energy

efficient approach to produce fine CeO2 nanoparticles It is relatively new method

to produce inorganic compounds for materials processing to enhance the material

properties as well as economic advantages through energy saving and acceleration

of product development Cerium oxide has outstanding physical and chemical

properties therefore it is used as LPG sensor In the area of catalysis nanoparticles

of ceria have been studied Also it is concluded as

1] The CeO2 nanoparticles were prepared by simple microwave assisted sol-gel

method

2] The well spherical and narrow size distribution with 10 nm size particles were

obtained by this method

3] It is very simple time as well as energy saving technique

4] The XRD pattern shows the CeO2 nanoparticles exists in face centered cubic

5] Scanning electron micrographs indicates that grains are uniformaly distributed

6] Transmission electron micrographs indicates that the mean diameter of the

particles as measured is about 10 nm

7] The conductivity of nano CeO2 thick film is represents the almost linear

relationship was found in the measured temperature range of 200-5000C It is

clear that the conduction in crystalline ceria is essentially an n-type electronic

process

8] The CeO2 nanoparticles in the thick film form prepared by screen printing

technique showed better response to LPG gas at 250degC temperature at 600 ppm

concentration of LPG gas

9] Catalyst can be prepared from inexpensive precursors using the microwave

assisted sol-gel technique is promising for the oxidative regeneration of ketones

10] This study reveals that these CeO2 nanocrystals are novel adsorbents for

wastewater treatment

Chapter ndash V

182

References

[1] A Ramesh H Hasegawa T Maki KUeda Sep Purif Tech

56 (2007) 90

[2] C Lettmann K Hildenbrand H Kisch W Macyk W F Maier

Appl Catal B 32 (2001) 215

[3] X Zhang Z H Ai F L Jia L Z Zhang J Phys Chem C 112 (2008) 747

[4] N Du H Zhang BD Chen X Y Ma D RYang J Phys ChemC

111 (2007) 12677

[5] F Zhang S Yang H Chen X Yu Cera Intern 30 (2004) 997

[6] F Gu SF Wang MK Lu GJ Zhou D Xu DR Langmuir J SciIR Iran

20 (2004) 3528

[7] H Xiao Z Ai L Zhang JPhys Chem 113 (2009) 16625

[8] L Combemale G Caboche D Stuerga D Chaumont MicroRese

Bull 40 (2005) 529

[9] M Zawadzki Jof Alloys and Comp 454 (2008) 347

[10] A V Patil C G Dighavkar S K Sonawane S J Patil R Y Borse

Sens and Transducers 108 (2009) 189

[11] AV Patil CG Dighavkar RY Borse Sens and Transducers

101 (2009) 96

[12] AV Patil CG Dighavkar SK Sonawane S J Patil RY Borse Sensand

Transducers 9 (2010) 11

[13] DR Patil LA Patil Talanta 77 (2009)1409

[14] R S Blackburn Environ Sci Tech 38 (2004) 4905

[15] GNaja CMustin JBerthelin BVolesky J Colloide Inter Sci

292 (2005) 537

[16] VKGupta IAli Environ Sci Tech 42 (2008) 766

[17] A Mittal VK Gupta A Malviya and JMittal J Hazard Mater

151 (2008) 821

[18] VKGupta IAli VKSaini J Colloid Interface Sci 315(2007) 87

Chapter ndash V

183

[19] S S Deshpande S U Sonavane R V Jayaram Cat Com 9 (2008) 639

[20] KM Garadkar BS Shirke YB Patil D R Patil Sens and Transducers

110 (2009) 17

[21] VKGupta AMittal VGajbe JMittal J Colloid Inter Sci

319 (2008) 30

[22] T A Khan I Ali V V Singh S Sharma J Envir Protec Sci 3 (2009)11

[23] A Hosseinnia M Keyanpour-Rad M Pazouki World Applied Sci J

8 (2010) 1327

[24] MI Baraton L Merhari J Am Ceram Soc 75 (2004) 1587

[25] CWang B Xu XWang J Zhao J Solid State Chem 178 (2005) 3500

[26] H Xiao Z Ai L Zhang J Phys Chem C doi 10 1021 JP9050269 (2009)

[27] A M Thompson Oxides of the Rare Earths Wiley New York (1978)

[28] H Noumlremberg G A D Briggs Phys Rev Lett 79 (1997) 4222

[29] A Hosseinnia M Keyanpour-Rad M Pazouki World Appl Sci J

8 (2010) 1327

[30] BD Cullity SR Stock Elements of X-Ray Diffraction 3rd edn

(Prentice Hall Upper Saddle River NJ) (2001) 07458

[31] G Ranga Rao B Gopal Mishra Bul Cat Soc of Ind 2 (2003)122

[32] A K Bhattacharyal T K Naiya S N Mondal S K Das Chem Eng J

137 (2008) 529

[33] G Liu J A Rodriguez Z Chang J Hrbek C H F Peden

J Phys Chem B 108 (2004) 2931

[34] P F Ji J L Zhang F Chen M Anpo J Phys Chem C 112 (2008) 17809

[35] A Trovarelli Catal Rev SciEng 38 (1996) 439

[36] B Li D M Pan J S Zheng Y J Cheng X Y Ma F Huang L Zhang

Langmuir 24 (2008) 9630

[37] M Fernaacutendez-Garcıacutea A A B Martıacutenez-Arias A Hungrıacutea Iglesias-Juez

J C Conesa J Soria J Phys Chem Chem Phys 4 (2002) 2473

[38] H M Chen J H He J Phys Chem C 112 (2008) 17540

Chapter ndash V

184

[39] F Zhang S Chan W Spanier J E Apak E Jin Q Robinson

R D Herman I P Appl Phys Lett 80 (2002) 127

[40] G Adachi TMasui In Catalysis by Ceria and Related Materials

ATrovarelli Ed Imperial College Press London (2002)

[41] J A Rodriguez X Wang J C Hanson G Liu A Iglesias-Juez

MFernaacutendez-Garcıacutea J Chem Phys 119 (2003) 5659

[42] S Eck C Castellarin-Cudia S Surnev M G Ramsey F PNetzer

Surf Sci 520 (2002) 173

[43] R B Yu L Yan P Zheng JChen X X R J Phys ChemC

112 (2008) 19896

[44] Y Nagai T Yamamoto T Tanaka SYoshida T Nonaka T Okamoto

A Suda M Sugiura Catal Today 74 (2002) 225

[45] Y M Chiang E B Lavik I Kosacki H L Tuller JY Ying

J Electroceram 1 (1997) 7

[46] D R Patil L A Patil G H Jain M S Wagh S A Patil

Sens and Transducers 74 (2006) 874

[47] K Sohlberg S K Pantelides S J Pennycook J Am Chem Soc

123 (2001) 6609

[48] L S Zhong J S Hu A M Cao Q Liu W G Song LWan

J Chem Mater 19 (2007) 1648

[49] J B Fei Y Cui X H Yan Y Yang K W Wang QHe J B Li

AdV Mater 20 (2008) 452

[50] P V Kamat J Phys Chem B 106 (2002) 7729

[51] R D Robinson J E Spanier F Zhang SW Chan I P Herman J Appl

Phys 92 (2002) 1936

[52] S W Cao Y J Zhu J Phys Chem C 112 (2008) 6253

[53] R Georgekutty M K Seery S C Pillai J Phys Chem C

112 (2008) 13563

[54] J S Hu L S Zhong W G Song L J Wan Adv Mater 20 (2008)1

Chapter ndash V

185

[55] HXu F L Jia Z H Ai L Z Zhang Cryst Growth Des (2007)

[56] H M Jia W J Xiao L Z Zhang Z F Zheng J PhysChem C

112 (2008) 1379

[57] Z H Ai H Y Xiao T Mei J Liu L Z Zhang K J Deng J R Qiu

J PhysChem C 112 (2008) 11929

[58] L M Ma W X Zhang EnViron Sci Technol 42 (2008) 5384

[59] Z H Ai Y Chen L Z Zhang J R Qiu EnViron Sci Technol

42 (2008) 69550

[60] Z Y Zhong J Ho J Teo S C Shen AGedanken ChemMater

19 (2007) 4776

[61] S C Kuiry S D Patil S Deshpande S Seal J Phys ChemB

109 (2005) 6936

[62] D R Patil L A Patil Sens IEEE 7 (2007) 434

[63] P Jasinski Anderson H U Sens Actuators B 95 (2003) 73

[64] J P Nair E Wachtel I Lubomirsky J Fleig JMaier AdVMater

15 (2003) 2077

[65] R J Qi Y J Zhu G F Cheng Y H Huang Nanotechnology

16 (2005) 2502

Chapter ndash V

170

01 nm in the case of lattice images Consequently very high magnification can be

obtained TEM is used to examine very thin section through materials of the

surface A transmission electron microscope Philips 120 kV is equipped with CCD

camera This instrument has the resolution of 036 nm with 42 to 120 kV operating

voltage and magnifies object upto 6 lakhs times in high resolution mode Samples

for TEM examinations were prepared by placing a drop of sample suspension on a

copper grade coated with carbon film and were allowed to dry in air A typical

TEM image of CeO2 powders synthesized by cerium nitrate salts by microwave

assisted sol-gel method (without calcination) is presented in Fig 510 The mean

diameter of the particles as measured is about 10 nm These results are in good

agreement with that of estimated by X-ray line broadening Also the synthesized

Fig510 TEM image of the CeO2 nanoparticles

nanopowder is well crystallized without defects non-agglomerated with narrow

size distributions The selected area electron diffraction (SAED) pattern shown in

the Fig511 it also supports the d values obtained from XRD with rings which

can be assigned [111] [200] [220] and [311] diffractions of CeO2 structure

Chapter ndash V

171

Fig511 SAED image of the CeO2 nanoparticles

537 Thermo-Gravimetric and Differential Thermal Analysis

Fig512 shows the TG-DTA curves for the dried ceria precursor prepared

by the Ce(IV) methods The dried samples may consist of three portions of

materials (A) crystalline ceria CeO2 (B) crystalline ceria with structural water

CeO22H2O and (C) amorphous cerium hydroxide Ce(OH)4 The endothermic

peaks in 64degC could be due to crystallization of the amorphous portion in the

sample Ce(OH)4 to crystalline CeO2 The larger endothermic peak for the sample

indicates that the former contains more amorphous phase than the latter The total

weight loss up to 800 degC for the sample is about 1685 (or 194 for n which is

the number of water molecules per mole of cerium in the dried sample) This is

consistent with the change of [Ce (OH)4 + CeO22H2O] to [CeO2] The two

endothermic peaks at about 313 and 436degC in figure can be attributed to the loss of

the structural water molecules The less obvious endothermic peaks for the

structural water loss for this sample also agree with the weight loss data These

results suggest that the Ce(IV) derived sample contains primarily Ce(OH)4 and

CeO22H2O

Chapter ndash V

172

Fig 512 Weight loss of CeO2 nanoparticles as a function of temperature

538 Electrical Conductivity of Semiconductors CeO2

Semiconductors find wide appilication particularly in electronics Their

application is based on certain properties that will be an electrical conductivity of

semiconductors When the temperature is not very low the semiconductor will

conduct electricity due to the movement of the electrons in the conduction band

and that of holes in the valence band The conductivity of nano CeO2 thick film is

represented in Fig513 From the figure the almost linear relationship was found

in the measured temperature range of 200-500oC If it assumed that conduction in

crystalline ceria is essentially an n-type electronic process then using a simple

model we can express the conductivity-temperature relation by the equation

σ = σo e-Ea kT

where σo is a constant Ea is thermal activation energy T is the

Kelvin temperature and k is Boltzmanns constant Thus an Arrhenius plot of

conductivity and temperature should be linear over certain temperature range

Although the results in Fig513 are curved a linear section can be drawn above

280oC which gives activation energy of 236 eV

Chapter ndash V

173

Fig513 Electrical conductivity of CeO2 nanoparticles as a function of temperature

539 Gas Sensing Performance

5391 LPG Gas Sensing

Fig514 shows the variation of response of CeO2 fired at various

temperature The gas response increases with temperature from 50 to 150degC and

then decreases with a further increase in temperature In present work every time

prior to exposing the CeO2 film to LPG it was allowed to stabilize at an operating

temperature for 10 min and the stabilized resistance was taken as Ra After

exposing the film to the LPG gas the changed resistance was taken as Rg LPG

reacts with surface oxygen ions of the film Oxidation of film decreases the

number of free carriers Therefore resistance of the film increases with gas The

result of reaction of LPG with polycrystalline CeO2 is adsorbed CO2 and H2O on

the surface of the oxide The Ce harrLPG interactions on CeO2 are strong and Ce

sites probably get oxidized

Chapter ndash V

174

Fig 514 Variation of gas responses with operating temperature for LPG gas

5393 Effect of Gas Concentration (active region)

The variation of sensor response of CeO2 thick film sample with LPG gas

concentration at operating temperature 150degC is represented in Fig 515

Fig 515 Variation in sensor response with LPG gas concentration

Chapter ndash V

175

It is clear from the figure that the gas response goes on increasing with gas

concentration upto 600 ppm The rate of increase in response was relatively large

upto 1000 ppm and saturated beyond 600 ppm The monolayer of the gas

molecules formed on the surface covers the whole surface of the film The gas

molecules from that layer would reach the surface active sites of the film The

excess gas molecules remain idle and would not reach the surface active sites of

the sensor Thus the active region of the sensor would be up to 600 ppm

5394 Response and Recovery Time

The response of CeO2 thick film sensor was found to be quick (~ 28 s) to

600 ppm of while the recovery was fast (~ 10 s) (Fig 516) The fast response

may be attributed to faster oxidation of the gas The negligible quantity of the

surface reaction product and its high volatility explains its fast response and quick

recovery to its initial chemical status

Fig 516 Response and recovery time

Chapter ndash V

176

5395 Oxygen Adsorption-Desorption Mechanism

O2 (air) + 2e- 2O

-(film surface) --- 56

At higher temperature the atmospheric oxygen O2 adsorbs on the surface of the

thick film It captures the electrons (Fig517) from conduction band It would

result in decreasing conductivity of the film When LPG reacts with the adsorbed

oxygen on the surface of the film it gets oxidized to CO2 and H2O by following

series of intermediate stages This liberates free electrons in the conduction band

The final reaction takes place as

C4H10(gas) + 13Ominus

(filmsurface) rarr 4CO2(gas) + 5H2O(gas) + 13eminus

(condband) --- 57

Fig 517 Sensing mechanism on the surface of the film

This shows n-type conduction mechanism Thus generated electrons

contribute to a sudden increase in conductance of the thick film The mass of

Ce and O in sample was not as per the stoichiometric proportion and sample was

Chapter ndash V

177

observed to be the oxygen deficient This deficiency gets reduced due to

adsorption of atmosphericmolecular oxygen This helps in decreasing electronic

conductivity of the film Upon exposure LPG molecules got oxidized with the

adsorbed oxygen ions by following the series of intermediate stages producing

CO2 and H2O This results in evolving oxygen as electrically neutral atoms

trapping behind the negative charges (electrons) Upon exposure the energy

released in decomposition of LPG molecules would be sufficient for trapped

electrons to jump into the conduction band of activated CeO2 resulting in increase

in the conductivity of the film The drastic increase in conductivity of the sensor

could be attributed to the adsorptionndashdesorption mechanism resulted from the

electronic defects [26]

5310 Catalytic Activity

The utility of this catalytic system deprotection of several oximes were

studied (Table 52) The catalyst was also tested for reusability and it was found

that it could be reused without loss of initial activity for at least four cycles [19]

TLC analysis indicated the formation of only one product of corresponding

carbonyls FT-IR and NMR analysis showed that the oximes were converted into

carbonyls For checking the reusability (after each run) the catalyst was recovered

by simple filtration and washed several times with acetone and then dried at

373 K The catalyst thus obtained was used for the subsequent runs without any

further modification both activated and deactivated oximes were oxidized in better

yield more than ZrO2 catalyst as shown in Table 52

Chapter ndash V

178

Table 52 Oxidative Deoximation of Carbonyls on CeO2

Entry

Substrate

Product

Time

(h)

Yield

()

1

NOH

O

15

62

2

NOH

Cl

O

Cl

15

58

3

NOH

O

25

73

4

NOHCl

OCl

40

57

5

NOH

Cl

O

Cl

30

59

Solvent- Ethyl acetate Catalyst- ZrO2 Temperature- 353 K

Chapter ndash V

179

5311 Adsorption of Rhodamine B (Rh B)

The removal of dye pollutants are also of great importance for wastewater

treatment Rhodamine B a common cationic dye used in textile industry is one of

the most notorious contaminants in aquatic environments because of it huge

amounts slow biodegradation and toxicity In this study synthesized CeO2

nanocrystals are further used to remove Rh B The absorption spectrum of Rh B

solution was characterized by its characteristic absorption at 555 nm which was

attributed to the chromophore containing azo linkage of the dye molecules [22]

Fig 518 Absorption spectra of aqueous solution of Rh B in the presence of CeO2

nanocrystals at different times

UV-Visible absorption spectra represented in Fig518 confirms that over 93 of

Rh B in 100 mL of 20 mg L-1

Rh B aqueous solution could be removed with 100

mg of the as-prepared CeO2 nanocrystalline within 60 min The reasons may

account for the efficient removal of Rh B with CeO2 nanocrystals The first reason

Chapter ndash V

180

should be their to facilitate the adsorption of Rh B The second one is attributed to

the oxygen vacancies on the surface of CeO2 nanocrystals These abundant surface

oxygen vacancies could produce strong electrostatic attraction with the cationic

groups of Rh B as well as hydrogen bonding with the nitrogen atoms of Rh B

Chapter ndash V

181

Conclusions

Microwave synthesis is very beneficial to find a fast simple and energy

efficient approach to produce fine CeO2 nanoparticles It is relatively new method

to produce inorganic compounds for materials processing to enhance the material

properties as well as economic advantages through energy saving and acceleration

of product development Cerium oxide has outstanding physical and chemical

properties therefore it is used as LPG sensor In the area of catalysis nanoparticles

of ceria have been studied Also it is concluded as

1] The CeO2 nanoparticles were prepared by simple microwave assisted sol-gel

method

2] The well spherical and narrow size distribution with 10 nm size particles were

obtained by this method

3] It is very simple time as well as energy saving technique

4] The XRD pattern shows the CeO2 nanoparticles exists in face centered cubic

5] Scanning electron micrographs indicates that grains are uniformaly distributed

6] Transmission electron micrographs indicates that the mean diameter of the

particles as measured is about 10 nm

7] The conductivity of nano CeO2 thick film is represents the almost linear

relationship was found in the measured temperature range of 200-5000C It is

clear that the conduction in crystalline ceria is essentially an n-type electronic

process

8] The CeO2 nanoparticles in the thick film form prepared by screen printing

technique showed better response to LPG gas at 250degC temperature at 600 ppm

concentration of LPG gas

9] Catalyst can be prepared from inexpensive precursors using the microwave

assisted sol-gel technique is promising for the oxidative regeneration of ketones

10] This study reveals that these CeO2 nanocrystals are novel adsorbents for

wastewater treatment

Chapter ndash V

182

References

[1] A Ramesh H Hasegawa T Maki KUeda Sep Purif Tech

56 (2007) 90

[2] C Lettmann K Hildenbrand H Kisch W Macyk W F Maier

Appl Catal B 32 (2001) 215

[3] X Zhang Z H Ai F L Jia L Z Zhang J Phys Chem C 112 (2008) 747

[4] N Du H Zhang BD Chen X Y Ma D RYang J Phys ChemC

111 (2007) 12677

[5] F Zhang S Yang H Chen X Yu Cera Intern 30 (2004) 997

[6] F Gu SF Wang MK Lu GJ Zhou D Xu DR Langmuir J SciIR Iran

20 (2004) 3528

[7] H Xiao Z Ai L Zhang JPhys Chem 113 (2009) 16625

[8] L Combemale G Caboche D Stuerga D Chaumont MicroRese

Bull 40 (2005) 529

[9] M Zawadzki Jof Alloys and Comp 454 (2008) 347

[10] A V Patil C G Dighavkar S K Sonawane S J Patil R Y Borse

Sens and Transducers 108 (2009) 189

[11] AV Patil CG Dighavkar RY Borse Sens and Transducers

101 (2009) 96

[12] AV Patil CG Dighavkar SK Sonawane S J Patil RY Borse Sensand

Transducers 9 (2010) 11

[13] DR Patil LA Patil Talanta 77 (2009)1409

[14] R S Blackburn Environ Sci Tech 38 (2004) 4905

[15] GNaja CMustin JBerthelin BVolesky J Colloide Inter Sci

292 (2005) 537

[16] VKGupta IAli Environ Sci Tech 42 (2008) 766

[17] A Mittal VK Gupta A Malviya and JMittal J Hazard Mater

151 (2008) 821

[18] VKGupta IAli VKSaini J Colloid Interface Sci 315(2007) 87

Chapter ndash V

183

[19] S S Deshpande S U Sonavane R V Jayaram Cat Com 9 (2008) 639

[20] KM Garadkar BS Shirke YB Patil D R Patil Sens and Transducers

110 (2009) 17

[21] VKGupta AMittal VGajbe JMittal J Colloid Inter Sci

319 (2008) 30

[22] T A Khan I Ali V V Singh S Sharma J Envir Protec Sci 3 (2009)11

[23] A Hosseinnia M Keyanpour-Rad M Pazouki World Applied Sci J

8 (2010) 1327

[24] MI Baraton L Merhari J Am Ceram Soc 75 (2004) 1587

[25] CWang B Xu XWang J Zhao J Solid State Chem 178 (2005) 3500

[26] H Xiao Z Ai L Zhang J Phys Chem C doi 10 1021 JP9050269 (2009)

[27] A M Thompson Oxides of the Rare Earths Wiley New York (1978)

[28] H Noumlremberg G A D Briggs Phys Rev Lett 79 (1997) 4222

[29] A Hosseinnia M Keyanpour-Rad M Pazouki World Appl Sci J

8 (2010) 1327

[30] BD Cullity SR Stock Elements of X-Ray Diffraction 3rd edn

(Prentice Hall Upper Saddle River NJ) (2001) 07458

[31] G Ranga Rao B Gopal Mishra Bul Cat Soc of Ind 2 (2003)122

[32] A K Bhattacharyal T K Naiya S N Mondal S K Das Chem Eng J

137 (2008) 529

[33] G Liu J A Rodriguez Z Chang J Hrbek C H F Peden

J Phys Chem B 108 (2004) 2931

[34] P F Ji J L Zhang F Chen M Anpo J Phys Chem C 112 (2008) 17809

[35] A Trovarelli Catal Rev SciEng 38 (1996) 439

[36] B Li D M Pan J S Zheng Y J Cheng X Y Ma F Huang L Zhang

Langmuir 24 (2008) 9630

[37] M Fernaacutendez-Garcıacutea A A B Martıacutenez-Arias A Hungrıacutea Iglesias-Juez

J C Conesa J Soria J Phys Chem Chem Phys 4 (2002) 2473

[38] H M Chen J H He J Phys Chem C 112 (2008) 17540

Chapter ndash V

184

[39] F Zhang S Chan W Spanier J E Apak E Jin Q Robinson

R D Herman I P Appl Phys Lett 80 (2002) 127

[40] G Adachi TMasui In Catalysis by Ceria and Related Materials

ATrovarelli Ed Imperial College Press London (2002)

[41] J A Rodriguez X Wang J C Hanson G Liu A Iglesias-Juez

MFernaacutendez-Garcıacutea J Chem Phys 119 (2003) 5659

[42] S Eck C Castellarin-Cudia S Surnev M G Ramsey F PNetzer

Surf Sci 520 (2002) 173

[43] R B Yu L Yan P Zheng JChen X X R J Phys ChemC

112 (2008) 19896

[44] Y Nagai T Yamamoto T Tanaka SYoshida T Nonaka T Okamoto

A Suda M Sugiura Catal Today 74 (2002) 225

[45] Y M Chiang E B Lavik I Kosacki H L Tuller JY Ying

J Electroceram 1 (1997) 7

[46] D R Patil L A Patil G H Jain M S Wagh S A Patil

Sens and Transducers 74 (2006) 874

[47] K Sohlberg S K Pantelides S J Pennycook J Am Chem Soc

123 (2001) 6609

[48] L S Zhong J S Hu A M Cao Q Liu W G Song LWan

J Chem Mater 19 (2007) 1648

[49] J B Fei Y Cui X H Yan Y Yang K W Wang QHe J B Li

AdV Mater 20 (2008) 452

[50] P V Kamat J Phys Chem B 106 (2002) 7729

[51] R D Robinson J E Spanier F Zhang SW Chan I P Herman J Appl

Phys 92 (2002) 1936

[52] S W Cao Y J Zhu J Phys Chem C 112 (2008) 6253

[53] R Georgekutty M K Seery S C Pillai J Phys Chem C

112 (2008) 13563

[54] J S Hu L S Zhong W G Song L J Wan Adv Mater 20 (2008)1

Chapter ndash V

185

[55] HXu F L Jia Z H Ai L Z Zhang Cryst Growth Des (2007)

[56] H M Jia W J Xiao L Z Zhang Z F Zheng J PhysChem C

112 (2008) 1379

[57] Z H Ai H Y Xiao T Mei J Liu L Z Zhang K J Deng J R Qiu

J PhysChem C 112 (2008) 11929

[58] L M Ma W X Zhang EnViron Sci Technol 42 (2008) 5384

[59] Z H Ai Y Chen L Z Zhang J R Qiu EnViron Sci Technol

42 (2008) 69550

[60] Z Y Zhong J Ho J Teo S C Shen AGedanken ChemMater

19 (2007) 4776

[61] S C Kuiry S D Patil S Deshpande S Seal J Phys ChemB

109 (2005) 6936

[62] D R Patil L A Patil Sens IEEE 7 (2007) 434

[63] P Jasinski Anderson H U Sens Actuators B 95 (2003) 73

[64] J P Nair E Wachtel I Lubomirsky J Fleig JMaier AdVMater

15 (2003) 2077

[65] R J Qi Y J Zhu G F Cheng Y H Huang Nanotechnology

16 (2005) 2502

Chapter ndash V

171

Fig511 SAED image of the CeO2 nanoparticles

537 Thermo-Gravimetric and Differential Thermal Analysis

Fig512 shows the TG-DTA curves for the dried ceria precursor prepared

by the Ce(IV) methods The dried samples may consist of three portions of

materials (A) crystalline ceria CeO2 (B) crystalline ceria with structural water

CeO22H2O and (C) amorphous cerium hydroxide Ce(OH)4 The endothermic

peaks in 64degC could be due to crystallization of the amorphous portion in the

sample Ce(OH)4 to crystalline CeO2 The larger endothermic peak for the sample

indicates that the former contains more amorphous phase than the latter The total

weight loss up to 800 degC for the sample is about 1685 (or 194 for n which is

the number of water molecules per mole of cerium in the dried sample) This is

consistent with the change of [Ce (OH)4 + CeO22H2O] to [CeO2] The two

endothermic peaks at about 313 and 436degC in figure can be attributed to the loss of

the structural water molecules The less obvious endothermic peaks for the

structural water loss for this sample also agree with the weight loss data These

results suggest that the Ce(IV) derived sample contains primarily Ce(OH)4 and

CeO22H2O

Chapter ndash V

172

Fig 512 Weight loss of CeO2 nanoparticles as a function of temperature

538 Electrical Conductivity of Semiconductors CeO2

Semiconductors find wide appilication particularly in electronics Their

application is based on certain properties that will be an electrical conductivity of

semiconductors When the temperature is not very low the semiconductor will

conduct electricity due to the movement of the electrons in the conduction band

and that of holes in the valence band The conductivity of nano CeO2 thick film is

represented in Fig513 From the figure the almost linear relationship was found

in the measured temperature range of 200-500oC If it assumed that conduction in

crystalline ceria is essentially an n-type electronic process then using a simple

model we can express the conductivity-temperature relation by the equation

σ = σo e-Ea kT

where σo is a constant Ea is thermal activation energy T is the

Kelvin temperature and k is Boltzmanns constant Thus an Arrhenius plot of

conductivity and temperature should be linear over certain temperature range

Although the results in Fig513 are curved a linear section can be drawn above

280oC which gives activation energy of 236 eV

Chapter ndash V

173

Fig513 Electrical conductivity of CeO2 nanoparticles as a function of temperature

539 Gas Sensing Performance

5391 LPG Gas Sensing

Fig514 shows the variation of response of CeO2 fired at various

temperature The gas response increases with temperature from 50 to 150degC and

then decreases with a further increase in temperature In present work every time

prior to exposing the CeO2 film to LPG it was allowed to stabilize at an operating

temperature for 10 min and the stabilized resistance was taken as Ra After

exposing the film to the LPG gas the changed resistance was taken as Rg LPG

reacts with surface oxygen ions of the film Oxidation of film decreases the

number of free carriers Therefore resistance of the film increases with gas The

result of reaction of LPG with polycrystalline CeO2 is adsorbed CO2 and H2O on

the surface of the oxide The Ce harrLPG interactions on CeO2 are strong and Ce

sites probably get oxidized

Chapter ndash V

174

Fig 514 Variation of gas responses with operating temperature for LPG gas

5393 Effect of Gas Concentration (active region)

The variation of sensor response of CeO2 thick film sample with LPG gas

concentration at operating temperature 150degC is represented in Fig 515

Fig 515 Variation in sensor response with LPG gas concentration

Chapter ndash V

175

It is clear from the figure that the gas response goes on increasing with gas

concentration upto 600 ppm The rate of increase in response was relatively large

upto 1000 ppm and saturated beyond 600 ppm The monolayer of the gas

molecules formed on the surface covers the whole surface of the film The gas

molecules from that layer would reach the surface active sites of the film The

excess gas molecules remain idle and would not reach the surface active sites of

the sensor Thus the active region of the sensor would be up to 600 ppm

5394 Response and Recovery Time

The response of CeO2 thick film sensor was found to be quick (~ 28 s) to

600 ppm of while the recovery was fast (~ 10 s) (Fig 516) The fast response

may be attributed to faster oxidation of the gas The negligible quantity of the

surface reaction product and its high volatility explains its fast response and quick

recovery to its initial chemical status

Fig 516 Response and recovery time

Chapter ndash V

176

5395 Oxygen Adsorption-Desorption Mechanism

O2 (air) + 2e- 2O

-(film surface) --- 56

At higher temperature the atmospheric oxygen O2 adsorbs on the surface of the

thick film It captures the electrons (Fig517) from conduction band It would

result in decreasing conductivity of the film When LPG reacts with the adsorbed

oxygen on the surface of the film it gets oxidized to CO2 and H2O by following

series of intermediate stages This liberates free electrons in the conduction band

The final reaction takes place as

C4H10(gas) + 13Ominus

(filmsurface) rarr 4CO2(gas) + 5H2O(gas) + 13eminus

(condband) --- 57

Fig 517 Sensing mechanism on the surface of the film

This shows n-type conduction mechanism Thus generated electrons

contribute to a sudden increase in conductance of the thick film The mass of

Ce and O in sample was not as per the stoichiometric proportion and sample was

Chapter ndash V

177

observed to be the oxygen deficient This deficiency gets reduced due to

adsorption of atmosphericmolecular oxygen This helps in decreasing electronic

conductivity of the film Upon exposure LPG molecules got oxidized with the

adsorbed oxygen ions by following the series of intermediate stages producing

CO2 and H2O This results in evolving oxygen as electrically neutral atoms

trapping behind the negative charges (electrons) Upon exposure the energy

released in decomposition of LPG molecules would be sufficient for trapped

electrons to jump into the conduction band of activated CeO2 resulting in increase

in the conductivity of the film The drastic increase in conductivity of the sensor

could be attributed to the adsorptionndashdesorption mechanism resulted from the

electronic defects [26]

5310 Catalytic Activity

The utility of this catalytic system deprotection of several oximes were

studied (Table 52) The catalyst was also tested for reusability and it was found

that it could be reused without loss of initial activity for at least four cycles [19]

TLC analysis indicated the formation of only one product of corresponding

carbonyls FT-IR and NMR analysis showed that the oximes were converted into

carbonyls For checking the reusability (after each run) the catalyst was recovered

by simple filtration and washed several times with acetone and then dried at

373 K The catalyst thus obtained was used for the subsequent runs without any

further modification both activated and deactivated oximes were oxidized in better

yield more than ZrO2 catalyst as shown in Table 52

Chapter ndash V

178

Table 52 Oxidative Deoximation of Carbonyls on CeO2

Entry

Substrate

Product

Time

(h)

Yield

()

1

NOH

O

15

62

2

NOH

Cl

O

Cl

15

58

3

NOH

O

25

73

4

NOHCl

OCl

40

57

5

NOH

Cl

O

Cl

30

59

Solvent- Ethyl acetate Catalyst- ZrO2 Temperature- 353 K

Chapter ndash V

179

5311 Adsorption of Rhodamine B (Rh B)

The removal of dye pollutants are also of great importance for wastewater

treatment Rhodamine B a common cationic dye used in textile industry is one of

the most notorious contaminants in aquatic environments because of it huge

amounts slow biodegradation and toxicity In this study synthesized CeO2

nanocrystals are further used to remove Rh B The absorption spectrum of Rh B

solution was characterized by its characteristic absorption at 555 nm which was

attributed to the chromophore containing azo linkage of the dye molecules [22]

Fig 518 Absorption spectra of aqueous solution of Rh B in the presence of CeO2

nanocrystals at different times

UV-Visible absorption spectra represented in Fig518 confirms that over 93 of

Rh B in 100 mL of 20 mg L-1

Rh B aqueous solution could be removed with 100

mg of the as-prepared CeO2 nanocrystalline within 60 min The reasons may

account for the efficient removal of Rh B with CeO2 nanocrystals The first reason

Chapter ndash V

180

should be their to facilitate the adsorption of Rh B The second one is attributed to

the oxygen vacancies on the surface of CeO2 nanocrystals These abundant surface

oxygen vacancies could produce strong electrostatic attraction with the cationic

groups of Rh B as well as hydrogen bonding with the nitrogen atoms of Rh B

Chapter ndash V

181

Conclusions

Microwave synthesis is very beneficial to find a fast simple and energy

efficient approach to produce fine CeO2 nanoparticles It is relatively new method

to produce inorganic compounds for materials processing to enhance the material

properties as well as economic advantages through energy saving and acceleration

of product development Cerium oxide has outstanding physical and chemical

properties therefore it is used as LPG sensor In the area of catalysis nanoparticles

of ceria have been studied Also it is concluded as

1] The CeO2 nanoparticles were prepared by simple microwave assisted sol-gel

method

2] The well spherical and narrow size distribution with 10 nm size particles were

obtained by this method

3] It is very simple time as well as energy saving technique

4] The XRD pattern shows the CeO2 nanoparticles exists in face centered cubic

5] Scanning electron micrographs indicates that grains are uniformaly distributed

6] Transmission electron micrographs indicates that the mean diameter of the

particles as measured is about 10 nm

7] The conductivity of nano CeO2 thick film is represents the almost linear

relationship was found in the measured temperature range of 200-5000C It is

clear that the conduction in crystalline ceria is essentially an n-type electronic

process

8] The CeO2 nanoparticles in the thick film form prepared by screen printing

technique showed better response to LPG gas at 250degC temperature at 600 ppm

concentration of LPG gas

9] Catalyst can be prepared from inexpensive precursors using the microwave

assisted sol-gel technique is promising for the oxidative regeneration of ketones

10] This study reveals that these CeO2 nanocrystals are novel adsorbents for

wastewater treatment

Chapter ndash V

182

References

[1] A Ramesh H Hasegawa T Maki KUeda Sep Purif Tech

56 (2007) 90

[2] C Lettmann K Hildenbrand H Kisch W Macyk W F Maier

Appl Catal B 32 (2001) 215

[3] X Zhang Z H Ai F L Jia L Z Zhang J Phys Chem C 112 (2008) 747

[4] N Du H Zhang BD Chen X Y Ma D RYang J Phys ChemC

111 (2007) 12677

[5] F Zhang S Yang H Chen X Yu Cera Intern 30 (2004) 997

[6] F Gu SF Wang MK Lu GJ Zhou D Xu DR Langmuir J SciIR Iran

20 (2004) 3528

[7] H Xiao Z Ai L Zhang JPhys Chem 113 (2009) 16625

[8] L Combemale G Caboche D Stuerga D Chaumont MicroRese

Bull 40 (2005) 529

[9] M Zawadzki Jof Alloys and Comp 454 (2008) 347

[10] A V Patil C G Dighavkar S K Sonawane S J Patil R Y Borse

Sens and Transducers 108 (2009) 189

[11] AV Patil CG Dighavkar RY Borse Sens and Transducers

101 (2009) 96

[12] AV Patil CG Dighavkar SK Sonawane S J Patil RY Borse Sensand

Transducers 9 (2010) 11

[13] DR Patil LA Patil Talanta 77 (2009)1409

[14] R S Blackburn Environ Sci Tech 38 (2004) 4905

[15] GNaja CMustin JBerthelin BVolesky J Colloide Inter Sci

292 (2005) 537

[16] VKGupta IAli Environ Sci Tech 42 (2008) 766

[17] A Mittal VK Gupta A Malviya and JMittal J Hazard Mater

151 (2008) 821

[18] VKGupta IAli VKSaini J Colloid Interface Sci 315(2007) 87

Chapter ndash V

183

[19] S S Deshpande S U Sonavane R V Jayaram Cat Com 9 (2008) 639

[20] KM Garadkar BS Shirke YB Patil D R Patil Sens and Transducers

110 (2009) 17

[21] VKGupta AMittal VGajbe JMittal J Colloid Inter Sci

319 (2008) 30

[22] T A Khan I Ali V V Singh S Sharma J Envir Protec Sci 3 (2009)11

[23] A Hosseinnia M Keyanpour-Rad M Pazouki World Applied Sci J

8 (2010) 1327

[24] MI Baraton L Merhari J Am Ceram Soc 75 (2004) 1587

[25] CWang B Xu XWang J Zhao J Solid State Chem 178 (2005) 3500

[26] H Xiao Z Ai L Zhang J Phys Chem C doi 10 1021 JP9050269 (2009)

[27] A M Thompson Oxides of the Rare Earths Wiley New York (1978)

[28] H Noumlremberg G A D Briggs Phys Rev Lett 79 (1997) 4222

[29] A Hosseinnia M Keyanpour-Rad M Pazouki World Appl Sci J

8 (2010) 1327

[30] BD Cullity SR Stock Elements of X-Ray Diffraction 3rd edn

(Prentice Hall Upper Saddle River NJ) (2001) 07458

[31] G Ranga Rao B Gopal Mishra Bul Cat Soc of Ind 2 (2003)122

[32] A K Bhattacharyal T K Naiya S N Mondal S K Das Chem Eng J

137 (2008) 529

[33] G Liu J A Rodriguez Z Chang J Hrbek C H F Peden

J Phys Chem B 108 (2004) 2931

[34] P F Ji J L Zhang F Chen M Anpo J Phys Chem C 112 (2008) 17809

[35] A Trovarelli Catal Rev SciEng 38 (1996) 439

[36] B Li D M Pan J S Zheng Y J Cheng X Y Ma F Huang L Zhang

Langmuir 24 (2008) 9630

[37] M Fernaacutendez-Garcıacutea A A B Martıacutenez-Arias A Hungrıacutea Iglesias-Juez

J C Conesa J Soria J Phys Chem Chem Phys 4 (2002) 2473

[38] H M Chen J H He J Phys Chem C 112 (2008) 17540

Chapter ndash V

184

[39] F Zhang S Chan W Spanier J E Apak E Jin Q Robinson

R D Herman I P Appl Phys Lett 80 (2002) 127

[40] G Adachi TMasui In Catalysis by Ceria and Related Materials

ATrovarelli Ed Imperial College Press London (2002)

[41] J A Rodriguez X Wang J C Hanson G Liu A Iglesias-Juez

MFernaacutendez-Garcıacutea J Chem Phys 119 (2003) 5659

[42] S Eck C Castellarin-Cudia S Surnev M G Ramsey F PNetzer

Surf Sci 520 (2002) 173

[43] R B Yu L Yan P Zheng JChen X X R J Phys ChemC

112 (2008) 19896

[44] Y Nagai T Yamamoto T Tanaka SYoshida T Nonaka T Okamoto

A Suda M Sugiura Catal Today 74 (2002) 225

[45] Y M Chiang E B Lavik I Kosacki H L Tuller JY Ying

J Electroceram 1 (1997) 7

[46] D R Patil L A Patil G H Jain M S Wagh S A Patil

Sens and Transducers 74 (2006) 874

[47] K Sohlberg S K Pantelides S J Pennycook J Am Chem Soc

123 (2001) 6609

[48] L S Zhong J S Hu A M Cao Q Liu W G Song LWan

J Chem Mater 19 (2007) 1648

[49] J B Fei Y Cui X H Yan Y Yang K W Wang QHe J B Li

AdV Mater 20 (2008) 452

[50] P V Kamat J Phys Chem B 106 (2002) 7729

[51] R D Robinson J E Spanier F Zhang SW Chan I P Herman J Appl

Phys 92 (2002) 1936

[52] S W Cao Y J Zhu J Phys Chem C 112 (2008) 6253

[53] R Georgekutty M K Seery S C Pillai J Phys Chem C

112 (2008) 13563

[54] J S Hu L S Zhong W G Song L J Wan Adv Mater 20 (2008)1

Chapter ndash V

185

[55] HXu F L Jia Z H Ai L Z Zhang Cryst Growth Des (2007)

[56] H M Jia W J Xiao L Z Zhang Z F Zheng J PhysChem C

112 (2008) 1379

[57] Z H Ai H Y Xiao T Mei J Liu L Z Zhang K J Deng J R Qiu

J PhysChem C 112 (2008) 11929

[58] L M Ma W X Zhang EnViron Sci Technol 42 (2008) 5384

[59] Z H Ai Y Chen L Z Zhang J R Qiu EnViron Sci Technol

42 (2008) 69550

[60] Z Y Zhong J Ho J Teo S C Shen AGedanken ChemMater

19 (2007) 4776

[61] S C Kuiry S D Patil S Deshpande S Seal J Phys ChemB

109 (2005) 6936

[62] D R Patil L A Patil Sens IEEE 7 (2007) 434

[63] P Jasinski Anderson H U Sens Actuators B 95 (2003) 73

[64] J P Nair E Wachtel I Lubomirsky J Fleig JMaier AdVMater

15 (2003) 2077

[65] R J Qi Y J Zhu G F Cheng Y H Huang Nanotechnology

16 (2005) 2502

Chapter ndash V

172

Fig 512 Weight loss of CeO2 nanoparticles as a function of temperature

538 Electrical Conductivity of Semiconductors CeO2

Semiconductors find wide appilication particularly in electronics Their

application is based on certain properties that will be an electrical conductivity of

semiconductors When the temperature is not very low the semiconductor will

conduct electricity due to the movement of the electrons in the conduction band

and that of holes in the valence band The conductivity of nano CeO2 thick film is

represented in Fig513 From the figure the almost linear relationship was found

in the measured temperature range of 200-500oC If it assumed that conduction in

crystalline ceria is essentially an n-type electronic process then using a simple

model we can express the conductivity-temperature relation by the equation

σ = σo e-Ea kT

where σo is a constant Ea is thermal activation energy T is the

Kelvin temperature and k is Boltzmanns constant Thus an Arrhenius plot of

conductivity and temperature should be linear over certain temperature range

Although the results in Fig513 are curved a linear section can be drawn above

280oC which gives activation energy of 236 eV

Chapter ndash V

173

Fig513 Electrical conductivity of CeO2 nanoparticles as a function of temperature

539 Gas Sensing Performance

5391 LPG Gas Sensing

Fig514 shows the variation of response of CeO2 fired at various

temperature The gas response increases with temperature from 50 to 150degC and

then decreases with a further increase in temperature In present work every time

prior to exposing the CeO2 film to LPG it was allowed to stabilize at an operating

temperature for 10 min and the stabilized resistance was taken as Ra After

exposing the film to the LPG gas the changed resistance was taken as Rg LPG

reacts with surface oxygen ions of the film Oxidation of film decreases the

number of free carriers Therefore resistance of the film increases with gas The

result of reaction of LPG with polycrystalline CeO2 is adsorbed CO2 and H2O on

the surface of the oxide The Ce harrLPG interactions on CeO2 are strong and Ce

sites probably get oxidized

Chapter ndash V

174

Fig 514 Variation of gas responses with operating temperature for LPG gas

5393 Effect of Gas Concentration (active region)

The variation of sensor response of CeO2 thick film sample with LPG gas

concentration at operating temperature 150degC is represented in Fig 515

Fig 515 Variation in sensor response with LPG gas concentration

Chapter ndash V

175

It is clear from the figure that the gas response goes on increasing with gas

concentration upto 600 ppm The rate of increase in response was relatively large

upto 1000 ppm and saturated beyond 600 ppm The monolayer of the gas

molecules formed on the surface covers the whole surface of the film The gas

molecules from that layer would reach the surface active sites of the film The

excess gas molecules remain idle and would not reach the surface active sites of

the sensor Thus the active region of the sensor would be up to 600 ppm

5394 Response and Recovery Time

The response of CeO2 thick film sensor was found to be quick (~ 28 s) to

600 ppm of while the recovery was fast (~ 10 s) (Fig 516) The fast response

may be attributed to faster oxidation of the gas The negligible quantity of the

surface reaction product and its high volatility explains its fast response and quick

recovery to its initial chemical status

Fig 516 Response and recovery time

Chapter ndash V

176

5395 Oxygen Adsorption-Desorption Mechanism

O2 (air) + 2e- 2O

-(film surface) --- 56

At higher temperature the atmospheric oxygen O2 adsorbs on the surface of the

thick film It captures the electrons (Fig517) from conduction band It would

result in decreasing conductivity of the film When LPG reacts with the adsorbed

oxygen on the surface of the film it gets oxidized to CO2 and H2O by following

series of intermediate stages This liberates free electrons in the conduction band

The final reaction takes place as

C4H10(gas) + 13Ominus

(filmsurface) rarr 4CO2(gas) + 5H2O(gas) + 13eminus

(condband) --- 57

Fig 517 Sensing mechanism on the surface of the film

This shows n-type conduction mechanism Thus generated electrons

contribute to a sudden increase in conductance of the thick film The mass of

Ce and O in sample was not as per the stoichiometric proportion and sample was

Chapter ndash V

177

observed to be the oxygen deficient This deficiency gets reduced due to

adsorption of atmosphericmolecular oxygen This helps in decreasing electronic

conductivity of the film Upon exposure LPG molecules got oxidized with the

adsorbed oxygen ions by following the series of intermediate stages producing

CO2 and H2O This results in evolving oxygen as electrically neutral atoms

trapping behind the negative charges (electrons) Upon exposure the energy

released in decomposition of LPG molecules would be sufficient for trapped

electrons to jump into the conduction band of activated CeO2 resulting in increase

in the conductivity of the film The drastic increase in conductivity of the sensor

could be attributed to the adsorptionndashdesorption mechanism resulted from the

electronic defects [26]

5310 Catalytic Activity

The utility of this catalytic system deprotection of several oximes were

studied (Table 52) The catalyst was also tested for reusability and it was found

that it could be reused without loss of initial activity for at least four cycles [19]

TLC analysis indicated the formation of only one product of corresponding

carbonyls FT-IR and NMR analysis showed that the oximes were converted into

carbonyls For checking the reusability (after each run) the catalyst was recovered

by simple filtration and washed several times with acetone and then dried at

373 K The catalyst thus obtained was used for the subsequent runs without any

further modification both activated and deactivated oximes were oxidized in better

yield more than ZrO2 catalyst as shown in Table 52

Chapter ndash V

178

Table 52 Oxidative Deoximation of Carbonyls on CeO2

Entry

Substrate

Product

Time

(h)

Yield

()

1

NOH

O

15

62

2

NOH

Cl

O

Cl

15

58

3

NOH

O

25

73

4

NOHCl

OCl

40

57

5

NOH

Cl

O

Cl

30

59

Solvent- Ethyl acetate Catalyst- ZrO2 Temperature- 353 K

Chapter ndash V

179

5311 Adsorption of Rhodamine B (Rh B)

The removal of dye pollutants are also of great importance for wastewater

treatment Rhodamine B a common cationic dye used in textile industry is one of

the most notorious contaminants in aquatic environments because of it huge

amounts slow biodegradation and toxicity In this study synthesized CeO2

nanocrystals are further used to remove Rh B The absorption spectrum of Rh B

solution was characterized by its characteristic absorption at 555 nm which was

attributed to the chromophore containing azo linkage of the dye molecules [22]

Fig 518 Absorption spectra of aqueous solution of Rh B in the presence of CeO2

nanocrystals at different times

UV-Visible absorption spectra represented in Fig518 confirms that over 93 of

Rh B in 100 mL of 20 mg L-1

Rh B aqueous solution could be removed with 100

mg of the as-prepared CeO2 nanocrystalline within 60 min The reasons may

account for the efficient removal of Rh B with CeO2 nanocrystals The first reason

Chapter ndash V

180

should be their to facilitate the adsorption of Rh B The second one is attributed to

the oxygen vacancies on the surface of CeO2 nanocrystals These abundant surface

oxygen vacancies could produce strong electrostatic attraction with the cationic

groups of Rh B as well as hydrogen bonding with the nitrogen atoms of Rh B

Chapter ndash V

181

Conclusions

Microwave synthesis is very beneficial to find a fast simple and energy

efficient approach to produce fine CeO2 nanoparticles It is relatively new method

to produce inorganic compounds for materials processing to enhance the material

properties as well as economic advantages through energy saving and acceleration

of product development Cerium oxide has outstanding physical and chemical

properties therefore it is used as LPG sensor In the area of catalysis nanoparticles

of ceria have been studied Also it is concluded as

1] The CeO2 nanoparticles were prepared by simple microwave assisted sol-gel

method

2] The well spherical and narrow size distribution with 10 nm size particles were

obtained by this method

3] It is very simple time as well as energy saving technique

4] The XRD pattern shows the CeO2 nanoparticles exists in face centered cubic

5] Scanning electron micrographs indicates that grains are uniformaly distributed

6] Transmission electron micrographs indicates that the mean diameter of the

particles as measured is about 10 nm

7] The conductivity of nano CeO2 thick film is represents the almost linear

relationship was found in the measured temperature range of 200-5000C It is

clear that the conduction in crystalline ceria is essentially an n-type electronic

process

8] The CeO2 nanoparticles in the thick film form prepared by screen printing

technique showed better response to LPG gas at 250degC temperature at 600 ppm

concentration of LPG gas

9] Catalyst can be prepared from inexpensive precursors using the microwave

assisted sol-gel technique is promising for the oxidative regeneration of ketones

10] This study reveals that these CeO2 nanocrystals are novel adsorbents for

wastewater treatment

Chapter ndash V

182

References

[1] A Ramesh H Hasegawa T Maki KUeda Sep Purif Tech

56 (2007) 90

[2] C Lettmann K Hildenbrand H Kisch W Macyk W F Maier

Appl Catal B 32 (2001) 215

[3] X Zhang Z H Ai F L Jia L Z Zhang J Phys Chem C 112 (2008) 747

[4] N Du H Zhang BD Chen X Y Ma D RYang J Phys ChemC

111 (2007) 12677

[5] F Zhang S Yang H Chen X Yu Cera Intern 30 (2004) 997

[6] F Gu SF Wang MK Lu GJ Zhou D Xu DR Langmuir J SciIR Iran

20 (2004) 3528

[7] H Xiao Z Ai L Zhang JPhys Chem 113 (2009) 16625

[8] L Combemale G Caboche D Stuerga D Chaumont MicroRese

Bull 40 (2005) 529

[9] M Zawadzki Jof Alloys and Comp 454 (2008) 347

[10] A V Patil C G Dighavkar S K Sonawane S J Patil R Y Borse

Sens and Transducers 108 (2009) 189

[11] AV Patil CG Dighavkar RY Borse Sens and Transducers

101 (2009) 96

[12] AV Patil CG Dighavkar SK Sonawane S J Patil RY Borse Sensand

Transducers 9 (2010) 11

[13] DR Patil LA Patil Talanta 77 (2009)1409

[14] R S Blackburn Environ Sci Tech 38 (2004) 4905

[15] GNaja CMustin JBerthelin BVolesky J Colloide Inter Sci

292 (2005) 537

[16] VKGupta IAli Environ Sci Tech 42 (2008) 766

[17] A Mittal VK Gupta A Malviya and JMittal J Hazard Mater

151 (2008) 821

[18] VKGupta IAli VKSaini J Colloid Interface Sci 315(2007) 87

Chapter ndash V

183

[19] S S Deshpande S U Sonavane R V Jayaram Cat Com 9 (2008) 639

[20] KM Garadkar BS Shirke YB Patil D R Patil Sens and Transducers

110 (2009) 17

[21] VKGupta AMittal VGajbe JMittal J Colloid Inter Sci

319 (2008) 30

[22] T A Khan I Ali V V Singh S Sharma J Envir Protec Sci 3 (2009)11

[23] A Hosseinnia M Keyanpour-Rad M Pazouki World Applied Sci J

8 (2010) 1327

[24] MI Baraton L Merhari J Am Ceram Soc 75 (2004) 1587

[25] CWang B Xu XWang J Zhao J Solid State Chem 178 (2005) 3500

[26] H Xiao Z Ai L Zhang J Phys Chem C doi 10 1021 JP9050269 (2009)

[27] A M Thompson Oxides of the Rare Earths Wiley New York (1978)

[28] H Noumlremberg G A D Briggs Phys Rev Lett 79 (1997) 4222

[29] A Hosseinnia M Keyanpour-Rad M Pazouki World Appl Sci J

8 (2010) 1327

[30] BD Cullity SR Stock Elements of X-Ray Diffraction 3rd edn

(Prentice Hall Upper Saddle River NJ) (2001) 07458

[31] G Ranga Rao B Gopal Mishra Bul Cat Soc of Ind 2 (2003)122

[32] A K Bhattacharyal T K Naiya S N Mondal S K Das Chem Eng J

137 (2008) 529

[33] G Liu J A Rodriguez Z Chang J Hrbek C H F Peden

J Phys Chem B 108 (2004) 2931

[34] P F Ji J L Zhang F Chen M Anpo J Phys Chem C 112 (2008) 17809

[35] A Trovarelli Catal Rev SciEng 38 (1996) 439

[36] B Li D M Pan J S Zheng Y J Cheng X Y Ma F Huang L Zhang

Langmuir 24 (2008) 9630

[37] M Fernaacutendez-Garcıacutea A A B Martıacutenez-Arias A Hungrıacutea Iglesias-Juez

J C Conesa J Soria J Phys Chem Chem Phys 4 (2002) 2473

[38] H M Chen J H He J Phys Chem C 112 (2008) 17540

Chapter ndash V

184

[39] F Zhang S Chan W Spanier J E Apak E Jin Q Robinson

R D Herman I P Appl Phys Lett 80 (2002) 127

[40] G Adachi TMasui In Catalysis by Ceria and Related Materials

ATrovarelli Ed Imperial College Press London (2002)

[41] J A Rodriguez X Wang J C Hanson G Liu A Iglesias-Juez

MFernaacutendez-Garcıacutea J Chem Phys 119 (2003) 5659

[42] S Eck C Castellarin-Cudia S Surnev M G Ramsey F PNetzer

Surf Sci 520 (2002) 173

[43] R B Yu L Yan P Zheng JChen X X R J Phys ChemC

112 (2008) 19896

[44] Y Nagai T Yamamoto T Tanaka SYoshida T Nonaka T Okamoto

A Suda M Sugiura Catal Today 74 (2002) 225

[45] Y M Chiang E B Lavik I Kosacki H L Tuller JY Ying

J Electroceram 1 (1997) 7

[46] D R Patil L A Patil G H Jain M S Wagh S A Patil

Sens and Transducers 74 (2006) 874

[47] K Sohlberg S K Pantelides S J Pennycook J Am Chem Soc

123 (2001) 6609

[48] L S Zhong J S Hu A M Cao Q Liu W G Song LWan

J Chem Mater 19 (2007) 1648

[49] J B Fei Y Cui X H Yan Y Yang K W Wang QHe J B Li

AdV Mater 20 (2008) 452

[50] P V Kamat J Phys Chem B 106 (2002) 7729

[51] R D Robinson J E Spanier F Zhang SW Chan I P Herman J Appl

Phys 92 (2002) 1936

[52] S W Cao Y J Zhu J Phys Chem C 112 (2008) 6253

[53] R Georgekutty M K Seery S C Pillai J Phys Chem C

112 (2008) 13563

[54] J S Hu L S Zhong W G Song L J Wan Adv Mater 20 (2008)1

Chapter ndash V

185

[55] HXu F L Jia Z H Ai L Z Zhang Cryst Growth Des (2007)

[56] H M Jia W J Xiao L Z Zhang Z F Zheng J PhysChem C

112 (2008) 1379

[57] Z H Ai H Y Xiao T Mei J Liu L Z Zhang K J Deng J R Qiu

J PhysChem C 112 (2008) 11929

[58] L M Ma W X Zhang EnViron Sci Technol 42 (2008) 5384

[59] Z H Ai Y Chen L Z Zhang J R Qiu EnViron Sci Technol

42 (2008) 69550

[60] Z Y Zhong J Ho J Teo S C Shen AGedanken ChemMater

19 (2007) 4776

[61] S C Kuiry S D Patil S Deshpande S Seal J Phys ChemB

109 (2005) 6936

[62] D R Patil L A Patil Sens IEEE 7 (2007) 434

[63] P Jasinski Anderson H U Sens Actuators B 95 (2003) 73

[64] J P Nair E Wachtel I Lubomirsky J Fleig JMaier AdVMater

15 (2003) 2077

[65] R J Qi Y J Zhu G F Cheng Y H Huang Nanotechnology

16 (2005) 2502

Chapter ndash V

173

Fig513 Electrical conductivity of CeO2 nanoparticles as a function of temperature

539 Gas Sensing Performance

5391 LPG Gas Sensing

Fig514 shows the variation of response of CeO2 fired at various

temperature The gas response increases with temperature from 50 to 150degC and

then decreases with a further increase in temperature In present work every time

prior to exposing the CeO2 film to LPG it was allowed to stabilize at an operating

temperature for 10 min and the stabilized resistance was taken as Ra After

exposing the film to the LPG gas the changed resistance was taken as Rg LPG

reacts with surface oxygen ions of the film Oxidation of film decreases the

number of free carriers Therefore resistance of the film increases with gas The

result of reaction of LPG with polycrystalline CeO2 is adsorbed CO2 and H2O on

the surface of the oxide The Ce harrLPG interactions on CeO2 are strong and Ce

sites probably get oxidized

Chapter ndash V

174

Fig 514 Variation of gas responses with operating temperature for LPG gas

5393 Effect of Gas Concentration (active region)

The variation of sensor response of CeO2 thick film sample with LPG gas

concentration at operating temperature 150degC is represented in Fig 515

Fig 515 Variation in sensor response with LPG gas concentration

Chapter ndash V

175

It is clear from the figure that the gas response goes on increasing with gas

concentration upto 600 ppm The rate of increase in response was relatively large

upto 1000 ppm and saturated beyond 600 ppm The monolayer of the gas

molecules formed on the surface covers the whole surface of the film The gas

molecules from that layer would reach the surface active sites of the film The

excess gas molecules remain idle and would not reach the surface active sites of

the sensor Thus the active region of the sensor would be up to 600 ppm

5394 Response and Recovery Time

The response of CeO2 thick film sensor was found to be quick (~ 28 s) to

600 ppm of while the recovery was fast (~ 10 s) (Fig 516) The fast response

may be attributed to faster oxidation of the gas The negligible quantity of the

surface reaction product and its high volatility explains its fast response and quick

recovery to its initial chemical status

Fig 516 Response and recovery time

Chapter ndash V

176

5395 Oxygen Adsorption-Desorption Mechanism

O2 (air) + 2e- 2O

-(film surface) --- 56

At higher temperature the atmospheric oxygen O2 adsorbs on the surface of the

thick film It captures the electrons (Fig517) from conduction band It would

result in decreasing conductivity of the film When LPG reacts with the adsorbed

oxygen on the surface of the film it gets oxidized to CO2 and H2O by following

series of intermediate stages This liberates free electrons in the conduction band

The final reaction takes place as

C4H10(gas) + 13Ominus

(filmsurface) rarr 4CO2(gas) + 5H2O(gas) + 13eminus

(condband) --- 57

Fig 517 Sensing mechanism on the surface of the film

This shows n-type conduction mechanism Thus generated electrons

contribute to a sudden increase in conductance of the thick film The mass of

Ce and O in sample was not as per the stoichiometric proportion and sample was

Chapter ndash V

177

observed to be the oxygen deficient This deficiency gets reduced due to

adsorption of atmosphericmolecular oxygen This helps in decreasing electronic

conductivity of the film Upon exposure LPG molecules got oxidized with the

adsorbed oxygen ions by following the series of intermediate stages producing

CO2 and H2O This results in evolving oxygen as electrically neutral atoms

trapping behind the negative charges (electrons) Upon exposure the energy

released in decomposition of LPG molecules would be sufficient for trapped

electrons to jump into the conduction band of activated CeO2 resulting in increase

in the conductivity of the film The drastic increase in conductivity of the sensor

could be attributed to the adsorptionndashdesorption mechanism resulted from the

electronic defects [26]

5310 Catalytic Activity

The utility of this catalytic system deprotection of several oximes were

studied (Table 52) The catalyst was also tested for reusability and it was found

that it could be reused without loss of initial activity for at least four cycles [19]

TLC analysis indicated the formation of only one product of corresponding

carbonyls FT-IR and NMR analysis showed that the oximes were converted into

carbonyls For checking the reusability (after each run) the catalyst was recovered

by simple filtration and washed several times with acetone and then dried at

373 K The catalyst thus obtained was used for the subsequent runs without any

further modification both activated and deactivated oximes were oxidized in better

yield more than ZrO2 catalyst as shown in Table 52

Chapter ndash V

178

Table 52 Oxidative Deoximation of Carbonyls on CeO2

Entry

Substrate

Product

Time

(h)

Yield

()

1

NOH

O

15

62

2

NOH

Cl

O

Cl

15

58

3

NOH

O

25

73

4

NOHCl

OCl

40

57

5

NOH

Cl

O

Cl

30

59

Solvent- Ethyl acetate Catalyst- ZrO2 Temperature- 353 K

Chapter ndash V

179

5311 Adsorption of Rhodamine B (Rh B)

The removal of dye pollutants are also of great importance for wastewater

treatment Rhodamine B a common cationic dye used in textile industry is one of

the most notorious contaminants in aquatic environments because of it huge

amounts slow biodegradation and toxicity In this study synthesized CeO2

nanocrystals are further used to remove Rh B The absorption spectrum of Rh B

solution was characterized by its characteristic absorption at 555 nm which was

attributed to the chromophore containing azo linkage of the dye molecules [22]

Fig 518 Absorption spectra of aqueous solution of Rh B in the presence of CeO2

nanocrystals at different times

UV-Visible absorption spectra represented in Fig518 confirms that over 93 of

Rh B in 100 mL of 20 mg L-1

Rh B aqueous solution could be removed with 100

mg of the as-prepared CeO2 nanocrystalline within 60 min The reasons may

account for the efficient removal of Rh B with CeO2 nanocrystals The first reason

Chapter ndash V

180

should be their to facilitate the adsorption of Rh B The second one is attributed to

the oxygen vacancies on the surface of CeO2 nanocrystals These abundant surface

oxygen vacancies could produce strong electrostatic attraction with the cationic

groups of Rh B as well as hydrogen bonding with the nitrogen atoms of Rh B

Chapter ndash V

181

Conclusions

Microwave synthesis is very beneficial to find a fast simple and energy

efficient approach to produce fine CeO2 nanoparticles It is relatively new method

to produce inorganic compounds for materials processing to enhance the material

properties as well as economic advantages through energy saving and acceleration

of product development Cerium oxide has outstanding physical and chemical

properties therefore it is used as LPG sensor In the area of catalysis nanoparticles

of ceria have been studied Also it is concluded as

1] The CeO2 nanoparticles were prepared by simple microwave assisted sol-gel

method

2] The well spherical and narrow size distribution with 10 nm size particles were

obtained by this method

3] It is very simple time as well as energy saving technique

4] The XRD pattern shows the CeO2 nanoparticles exists in face centered cubic

5] Scanning electron micrographs indicates that grains are uniformaly distributed

6] Transmission electron micrographs indicates that the mean diameter of the

particles as measured is about 10 nm

7] The conductivity of nano CeO2 thick film is represents the almost linear

relationship was found in the measured temperature range of 200-5000C It is

clear that the conduction in crystalline ceria is essentially an n-type electronic

process

8] The CeO2 nanoparticles in the thick film form prepared by screen printing

technique showed better response to LPG gas at 250degC temperature at 600 ppm

concentration of LPG gas

9] Catalyst can be prepared from inexpensive precursors using the microwave

assisted sol-gel technique is promising for the oxidative regeneration of ketones

10] This study reveals that these CeO2 nanocrystals are novel adsorbents for

wastewater treatment

Chapter ndash V

182

References

[1] A Ramesh H Hasegawa T Maki KUeda Sep Purif Tech

56 (2007) 90

[2] C Lettmann K Hildenbrand H Kisch W Macyk W F Maier

Appl Catal B 32 (2001) 215

[3] X Zhang Z H Ai F L Jia L Z Zhang J Phys Chem C 112 (2008) 747

[4] N Du H Zhang BD Chen X Y Ma D RYang J Phys ChemC

111 (2007) 12677

[5] F Zhang S Yang H Chen X Yu Cera Intern 30 (2004) 997

[6] F Gu SF Wang MK Lu GJ Zhou D Xu DR Langmuir J SciIR Iran

20 (2004) 3528

[7] H Xiao Z Ai L Zhang JPhys Chem 113 (2009) 16625

[8] L Combemale G Caboche D Stuerga D Chaumont MicroRese

Bull 40 (2005) 529

[9] M Zawadzki Jof Alloys and Comp 454 (2008) 347

[10] A V Patil C G Dighavkar S K Sonawane S J Patil R Y Borse

Sens and Transducers 108 (2009) 189

[11] AV Patil CG Dighavkar RY Borse Sens and Transducers

101 (2009) 96

[12] AV Patil CG Dighavkar SK Sonawane S J Patil RY Borse Sensand

Transducers 9 (2010) 11

[13] DR Patil LA Patil Talanta 77 (2009)1409

[14] R S Blackburn Environ Sci Tech 38 (2004) 4905

[15] GNaja CMustin JBerthelin BVolesky J Colloide Inter Sci

292 (2005) 537

[16] VKGupta IAli Environ Sci Tech 42 (2008) 766

[17] A Mittal VK Gupta A Malviya and JMittal J Hazard Mater

151 (2008) 821

[18] VKGupta IAli VKSaini J Colloid Interface Sci 315(2007) 87

Chapter ndash V

183

[19] S S Deshpande S U Sonavane R V Jayaram Cat Com 9 (2008) 639

[20] KM Garadkar BS Shirke YB Patil D R Patil Sens and Transducers

110 (2009) 17

[21] VKGupta AMittal VGajbe JMittal J Colloid Inter Sci

319 (2008) 30

[22] T A Khan I Ali V V Singh S Sharma J Envir Protec Sci 3 (2009)11

[23] A Hosseinnia M Keyanpour-Rad M Pazouki World Applied Sci J

8 (2010) 1327

[24] MI Baraton L Merhari J Am Ceram Soc 75 (2004) 1587

[25] CWang B Xu XWang J Zhao J Solid State Chem 178 (2005) 3500

[26] H Xiao Z Ai L Zhang J Phys Chem C doi 10 1021 JP9050269 (2009)

[27] A M Thompson Oxides of the Rare Earths Wiley New York (1978)

[28] H Noumlremberg G A D Briggs Phys Rev Lett 79 (1997) 4222

[29] A Hosseinnia M Keyanpour-Rad M Pazouki World Appl Sci J

8 (2010) 1327

[30] BD Cullity SR Stock Elements of X-Ray Diffraction 3rd edn

(Prentice Hall Upper Saddle River NJ) (2001) 07458

[31] G Ranga Rao B Gopal Mishra Bul Cat Soc of Ind 2 (2003)122

[32] A K Bhattacharyal T K Naiya S N Mondal S K Das Chem Eng J

137 (2008) 529

[33] G Liu J A Rodriguez Z Chang J Hrbek C H F Peden

J Phys Chem B 108 (2004) 2931

[34] P F Ji J L Zhang F Chen M Anpo J Phys Chem C 112 (2008) 17809

[35] A Trovarelli Catal Rev SciEng 38 (1996) 439

[36] B Li D M Pan J S Zheng Y J Cheng X Y Ma F Huang L Zhang

Langmuir 24 (2008) 9630

[37] M Fernaacutendez-Garcıacutea A A B Martıacutenez-Arias A Hungrıacutea Iglesias-Juez

J C Conesa J Soria J Phys Chem Chem Phys 4 (2002) 2473

[38] H M Chen J H He J Phys Chem C 112 (2008) 17540

Chapter ndash V

184

[39] F Zhang S Chan W Spanier J E Apak E Jin Q Robinson

R D Herman I P Appl Phys Lett 80 (2002) 127

[40] G Adachi TMasui In Catalysis by Ceria and Related Materials

ATrovarelli Ed Imperial College Press London (2002)

[41] J A Rodriguez X Wang J C Hanson G Liu A Iglesias-Juez

MFernaacutendez-Garcıacutea J Chem Phys 119 (2003) 5659

[42] S Eck C Castellarin-Cudia S Surnev M G Ramsey F PNetzer

Surf Sci 520 (2002) 173

[43] R B Yu L Yan P Zheng JChen X X R J Phys ChemC

112 (2008) 19896

[44] Y Nagai T Yamamoto T Tanaka SYoshida T Nonaka T Okamoto

A Suda M Sugiura Catal Today 74 (2002) 225

[45] Y M Chiang E B Lavik I Kosacki H L Tuller JY Ying

J Electroceram 1 (1997) 7

[46] D R Patil L A Patil G H Jain M S Wagh S A Patil

Sens and Transducers 74 (2006) 874

[47] K Sohlberg S K Pantelides S J Pennycook J Am Chem Soc

123 (2001) 6609

[48] L S Zhong J S Hu A M Cao Q Liu W G Song LWan

J Chem Mater 19 (2007) 1648

[49] J B Fei Y Cui X H Yan Y Yang K W Wang QHe J B Li

AdV Mater 20 (2008) 452

[50] P V Kamat J Phys Chem B 106 (2002) 7729

[51] R D Robinson J E Spanier F Zhang SW Chan I P Herman J Appl

Phys 92 (2002) 1936

[52] S W Cao Y J Zhu J Phys Chem C 112 (2008) 6253

[53] R Georgekutty M K Seery S C Pillai J Phys Chem C

112 (2008) 13563

[54] J S Hu L S Zhong W G Song L J Wan Adv Mater 20 (2008)1

Chapter ndash V

185

[55] HXu F L Jia Z H Ai L Z Zhang Cryst Growth Des (2007)

[56] H M Jia W J Xiao L Z Zhang Z F Zheng J PhysChem C

112 (2008) 1379

[57] Z H Ai H Y Xiao T Mei J Liu L Z Zhang K J Deng J R Qiu

J PhysChem C 112 (2008) 11929

[58] L M Ma W X Zhang EnViron Sci Technol 42 (2008) 5384

[59] Z H Ai Y Chen L Z Zhang J R Qiu EnViron Sci Technol

42 (2008) 69550

[60] Z Y Zhong J Ho J Teo S C Shen AGedanken ChemMater

19 (2007) 4776

[61] S C Kuiry S D Patil S Deshpande S Seal J Phys ChemB

109 (2005) 6936

[62] D R Patil L A Patil Sens IEEE 7 (2007) 434

[63] P Jasinski Anderson H U Sens Actuators B 95 (2003) 73

[64] J P Nair E Wachtel I Lubomirsky J Fleig JMaier AdVMater

15 (2003) 2077

[65] R J Qi Y J Zhu G F Cheng Y H Huang Nanotechnology

16 (2005) 2502

Chapter ndash V

174

Fig 514 Variation of gas responses with operating temperature for LPG gas

5393 Effect of Gas Concentration (active region)

The variation of sensor response of CeO2 thick film sample with LPG gas

concentration at operating temperature 150degC is represented in Fig 515

Fig 515 Variation in sensor response with LPG gas concentration

Chapter ndash V

175

It is clear from the figure that the gas response goes on increasing with gas

concentration upto 600 ppm The rate of increase in response was relatively large

upto 1000 ppm and saturated beyond 600 ppm The monolayer of the gas

molecules formed on the surface covers the whole surface of the film The gas

molecules from that layer would reach the surface active sites of the film The

excess gas molecules remain idle and would not reach the surface active sites of

the sensor Thus the active region of the sensor would be up to 600 ppm

5394 Response and Recovery Time

The response of CeO2 thick film sensor was found to be quick (~ 28 s) to

600 ppm of while the recovery was fast (~ 10 s) (Fig 516) The fast response

may be attributed to faster oxidation of the gas The negligible quantity of the

surface reaction product and its high volatility explains its fast response and quick

recovery to its initial chemical status

Fig 516 Response and recovery time

Chapter ndash V

176

5395 Oxygen Adsorption-Desorption Mechanism

O2 (air) + 2e- 2O

-(film surface) --- 56

At higher temperature the atmospheric oxygen O2 adsorbs on the surface of the

thick film It captures the electrons (Fig517) from conduction band It would

result in decreasing conductivity of the film When LPG reacts with the adsorbed

oxygen on the surface of the film it gets oxidized to CO2 and H2O by following

series of intermediate stages This liberates free electrons in the conduction band

The final reaction takes place as

C4H10(gas) + 13Ominus

(filmsurface) rarr 4CO2(gas) + 5H2O(gas) + 13eminus

(condband) --- 57

Fig 517 Sensing mechanism on the surface of the film

This shows n-type conduction mechanism Thus generated electrons

contribute to a sudden increase in conductance of the thick film The mass of

Ce and O in sample was not as per the stoichiometric proportion and sample was

Chapter ndash V

177

observed to be the oxygen deficient This deficiency gets reduced due to

adsorption of atmosphericmolecular oxygen This helps in decreasing electronic

conductivity of the film Upon exposure LPG molecules got oxidized with the

adsorbed oxygen ions by following the series of intermediate stages producing

CO2 and H2O This results in evolving oxygen as electrically neutral atoms

trapping behind the negative charges (electrons) Upon exposure the energy

released in decomposition of LPG molecules would be sufficient for trapped

electrons to jump into the conduction band of activated CeO2 resulting in increase

in the conductivity of the film The drastic increase in conductivity of the sensor

could be attributed to the adsorptionndashdesorption mechanism resulted from the

electronic defects [26]

5310 Catalytic Activity

The utility of this catalytic system deprotection of several oximes were

studied (Table 52) The catalyst was also tested for reusability and it was found

that it could be reused without loss of initial activity for at least four cycles [19]

TLC analysis indicated the formation of only one product of corresponding

carbonyls FT-IR and NMR analysis showed that the oximes were converted into

carbonyls For checking the reusability (after each run) the catalyst was recovered

by simple filtration and washed several times with acetone and then dried at

373 K The catalyst thus obtained was used for the subsequent runs without any

further modification both activated and deactivated oximes were oxidized in better

yield more than ZrO2 catalyst as shown in Table 52

Chapter ndash V

178

Table 52 Oxidative Deoximation of Carbonyls on CeO2

Entry

Substrate

Product

Time

(h)

Yield

()

1

NOH

O

15

62

2

NOH

Cl

O

Cl

15

58

3

NOH

O

25

73

4

NOHCl

OCl

40

57

5

NOH

Cl

O

Cl

30

59

Solvent- Ethyl acetate Catalyst- ZrO2 Temperature- 353 K

Chapter ndash V

179

5311 Adsorption of Rhodamine B (Rh B)

The removal of dye pollutants are also of great importance for wastewater

treatment Rhodamine B a common cationic dye used in textile industry is one of

the most notorious contaminants in aquatic environments because of it huge

amounts slow biodegradation and toxicity In this study synthesized CeO2

nanocrystals are further used to remove Rh B The absorption spectrum of Rh B

solution was characterized by its characteristic absorption at 555 nm which was

attributed to the chromophore containing azo linkage of the dye molecules [22]

Fig 518 Absorption spectra of aqueous solution of Rh B in the presence of CeO2

nanocrystals at different times

UV-Visible absorption spectra represented in Fig518 confirms that over 93 of

Rh B in 100 mL of 20 mg L-1

Rh B aqueous solution could be removed with 100

mg of the as-prepared CeO2 nanocrystalline within 60 min The reasons may

account for the efficient removal of Rh B with CeO2 nanocrystals The first reason

Chapter ndash V

180

should be their to facilitate the adsorption of Rh B The second one is attributed to

the oxygen vacancies on the surface of CeO2 nanocrystals These abundant surface

oxygen vacancies could produce strong electrostatic attraction with the cationic

groups of Rh B as well as hydrogen bonding with the nitrogen atoms of Rh B

Chapter ndash V

181

Conclusions

Microwave synthesis is very beneficial to find a fast simple and energy

efficient approach to produce fine CeO2 nanoparticles It is relatively new method

to produce inorganic compounds for materials processing to enhance the material

properties as well as economic advantages through energy saving and acceleration

of product development Cerium oxide has outstanding physical and chemical

properties therefore it is used as LPG sensor In the area of catalysis nanoparticles

of ceria have been studied Also it is concluded as

1] The CeO2 nanoparticles were prepared by simple microwave assisted sol-gel

method

2] The well spherical and narrow size distribution with 10 nm size particles were

obtained by this method

3] It is very simple time as well as energy saving technique

4] The XRD pattern shows the CeO2 nanoparticles exists in face centered cubic

5] Scanning electron micrographs indicates that grains are uniformaly distributed

6] Transmission electron micrographs indicates that the mean diameter of the

particles as measured is about 10 nm

7] The conductivity of nano CeO2 thick film is represents the almost linear

relationship was found in the measured temperature range of 200-5000C It is

clear that the conduction in crystalline ceria is essentially an n-type electronic

process

8] The CeO2 nanoparticles in the thick film form prepared by screen printing

technique showed better response to LPG gas at 250degC temperature at 600 ppm

concentration of LPG gas

9] Catalyst can be prepared from inexpensive precursors using the microwave

assisted sol-gel technique is promising for the oxidative regeneration of ketones

10] This study reveals that these CeO2 nanocrystals are novel adsorbents for

wastewater treatment

Chapter ndash V

182

References

[1] A Ramesh H Hasegawa T Maki KUeda Sep Purif Tech

56 (2007) 90

[2] C Lettmann K Hildenbrand H Kisch W Macyk W F Maier

Appl Catal B 32 (2001) 215

[3] X Zhang Z H Ai F L Jia L Z Zhang J Phys Chem C 112 (2008) 747

[4] N Du H Zhang BD Chen X Y Ma D RYang J Phys ChemC

111 (2007) 12677

[5] F Zhang S Yang H Chen X Yu Cera Intern 30 (2004) 997

[6] F Gu SF Wang MK Lu GJ Zhou D Xu DR Langmuir J SciIR Iran

20 (2004) 3528

[7] H Xiao Z Ai L Zhang JPhys Chem 113 (2009) 16625

[8] L Combemale G Caboche D Stuerga D Chaumont MicroRese

Bull 40 (2005) 529

[9] M Zawadzki Jof Alloys and Comp 454 (2008) 347

[10] A V Patil C G Dighavkar S K Sonawane S J Patil R Y Borse

Sens and Transducers 108 (2009) 189

[11] AV Patil CG Dighavkar RY Borse Sens and Transducers

101 (2009) 96

[12] AV Patil CG Dighavkar SK Sonawane S J Patil RY Borse Sensand

Transducers 9 (2010) 11

[13] DR Patil LA Patil Talanta 77 (2009)1409

[14] R S Blackburn Environ Sci Tech 38 (2004) 4905

[15] GNaja CMustin JBerthelin BVolesky J Colloide Inter Sci

292 (2005) 537

[16] VKGupta IAli Environ Sci Tech 42 (2008) 766

[17] A Mittal VK Gupta A Malviya and JMittal J Hazard Mater

151 (2008) 821

[18] VKGupta IAli VKSaini J Colloid Interface Sci 315(2007) 87

Chapter ndash V

183

[19] S S Deshpande S U Sonavane R V Jayaram Cat Com 9 (2008) 639

[20] KM Garadkar BS Shirke YB Patil D R Patil Sens and Transducers

110 (2009) 17

[21] VKGupta AMittal VGajbe JMittal J Colloid Inter Sci

319 (2008) 30

[22] T A Khan I Ali V V Singh S Sharma J Envir Protec Sci 3 (2009)11

[23] A Hosseinnia M Keyanpour-Rad M Pazouki World Applied Sci J

8 (2010) 1327

[24] MI Baraton L Merhari J Am Ceram Soc 75 (2004) 1587

[25] CWang B Xu XWang J Zhao J Solid State Chem 178 (2005) 3500

[26] H Xiao Z Ai L Zhang J Phys Chem C doi 10 1021 JP9050269 (2009)

[27] A M Thompson Oxides of the Rare Earths Wiley New York (1978)

[28] H Noumlremberg G A D Briggs Phys Rev Lett 79 (1997) 4222

[29] A Hosseinnia M Keyanpour-Rad M Pazouki World Appl Sci J

8 (2010) 1327

[30] BD Cullity SR Stock Elements of X-Ray Diffraction 3rd edn

(Prentice Hall Upper Saddle River NJ) (2001) 07458

[31] G Ranga Rao B Gopal Mishra Bul Cat Soc of Ind 2 (2003)122

[32] A K Bhattacharyal T K Naiya S N Mondal S K Das Chem Eng J

137 (2008) 529

[33] G Liu J A Rodriguez Z Chang J Hrbek C H F Peden

J Phys Chem B 108 (2004) 2931

[34] P F Ji J L Zhang F Chen M Anpo J Phys Chem C 112 (2008) 17809

[35] A Trovarelli Catal Rev SciEng 38 (1996) 439

[36] B Li D M Pan J S Zheng Y J Cheng X Y Ma F Huang L Zhang

Langmuir 24 (2008) 9630

[37] M Fernaacutendez-Garcıacutea A A B Martıacutenez-Arias A Hungrıacutea Iglesias-Juez

J C Conesa J Soria J Phys Chem Chem Phys 4 (2002) 2473

[38] H M Chen J H He J Phys Chem C 112 (2008) 17540

Chapter ndash V

184

[39] F Zhang S Chan W Spanier J E Apak E Jin Q Robinson

R D Herman I P Appl Phys Lett 80 (2002) 127

[40] G Adachi TMasui In Catalysis by Ceria and Related Materials

ATrovarelli Ed Imperial College Press London (2002)

[41] J A Rodriguez X Wang J C Hanson G Liu A Iglesias-Juez

MFernaacutendez-Garcıacutea J Chem Phys 119 (2003) 5659

[42] S Eck C Castellarin-Cudia S Surnev M G Ramsey F PNetzer

Surf Sci 520 (2002) 173

[43] R B Yu L Yan P Zheng JChen X X R J Phys ChemC

112 (2008) 19896

[44] Y Nagai T Yamamoto T Tanaka SYoshida T Nonaka T Okamoto

A Suda M Sugiura Catal Today 74 (2002) 225

[45] Y M Chiang E B Lavik I Kosacki H L Tuller JY Ying

J Electroceram 1 (1997) 7

[46] D R Patil L A Patil G H Jain M S Wagh S A Patil

Sens and Transducers 74 (2006) 874

[47] K Sohlberg S K Pantelides S J Pennycook J Am Chem Soc

123 (2001) 6609

[48] L S Zhong J S Hu A M Cao Q Liu W G Song LWan

J Chem Mater 19 (2007) 1648

[49] J B Fei Y Cui X H Yan Y Yang K W Wang QHe J B Li

AdV Mater 20 (2008) 452

[50] P V Kamat J Phys Chem B 106 (2002) 7729

[51] R D Robinson J E Spanier F Zhang SW Chan I P Herman J Appl

Phys 92 (2002) 1936

[52] S W Cao Y J Zhu J Phys Chem C 112 (2008) 6253

[53] R Georgekutty M K Seery S C Pillai J Phys Chem C

112 (2008) 13563

[54] J S Hu L S Zhong W G Song L J Wan Adv Mater 20 (2008)1

Chapter ndash V

185

[55] HXu F L Jia Z H Ai L Z Zhang Cryst Growth Des (2007)

[56] H M Jia W J Xiao L Z Zhang Z F Zheng J PhysChem C

112 (2008) 1379

[57] Z H Ai H Y Xiao T Mei J Liu L Z Zhang K J Deng J R Qiu

J PhysChem C 112 (2008) 11929

[58] L M Ma W X Zhang EnViron Sci Technol 42 (2008) 5384

[59] Z H Ai Y Chen L Z Zhang J R Qiu EnViron Sci Technol

42 (2008) 69550

[60] Z Y Zhong J Ho J Teo S C Shen AGedanken ChemMater

19 (2007) 4776

[61] S C Kuiry S D Patil S Deshpande S Seal J Phys ChemB

109 (2005) 6936

[62] D R Patil L A Patil Sens IEEE 7 (2007) 434

[63] P Jasinski Anderson H U Sens Actuators B 95 (2003) 73

[64] J P Nair E Wachtel I Lubomirsky J Fleig JMaier AdVMater

15 (2003) 2077

[65] R J Qi Y J Zhu G F Cheng Y H Huang Nanotechnology

16 (2005) 2502

Chapter ndash V

175

It is clear from the figure that the gas response goes on increasing with gas

concentration upto 600 ppm The rate of increase in response was relatively large

upto 1000 ppm and saturated beyond 600 ppm The monolayer of the gas

molecules formed on the surface covers the whole surface of the film The gas

molecules from that layer would reach the surface active sites of the film The

excess gas molecules remain idle and would not reach the surface active sites of

the sensor Thus the active region of the sensor would be up to 600 ppm

5394 Response and Recovery Time

The response of CeO2 thick film sensor was found to be quick (~ 28 s) to

600 ppm of while the recovery was fast (~ 10 s) (Fig 516) The fast response

may be attributed to faster oxidation of the gas The negligible quantity of the

surface reaction product and its high volatility explains its fast response and quick

recovery to its initial chemical status

Fig 516 Response and recovery time

Chapter ndash V

176

5395 Oxygen Adsorption-Desorption Mechanism

O2 (air) + 2e- 2O

-(film surface) --- 56

At higher temperature the atmospheric oxygen O2 adsorbs on the surface of the

thick film It captures the electrons (Fig517) from conduction band It would

result in decreasing conductivity of the film When LPG reacts with the adsorbed

oxygen on the surface of the film it gets oxidized to CO2 and H2O by following

series of intermediate stages This liberates free electrons in the conduction band

The final reaction takes place as

C4H10(gas) + 13Ominus

(filmsurface) rarr 4CO2(gas) + 5H2O(gas) + 13eminus

(condband) --- 57

Fig 517 Sensing mechanism on the surface of the film

This shows n-type conduction mechanism Thus generated electrons

contribute to a sudden increase in conductance of the thick film The mass of

Ce and O in sample was not as per the stoichiometric proportion and sample was

Chapter ndash V

177

observed to be the oxygen deficient This deficiency gets reduced due to

adsorption of atmosphericmolecular oxygen This helps in decreasing electronic

conductivity of the film Upon exposure LPG molecules got oxidized with the

adsorbed oxygen ions by following the series of intermediate stages producing

CO2 and H2O This results in evolving oxygen as electrically neutral atoms

trapping behind the negative charges (electrons) Upon exposure the energy

released in decomposition of LPG molecules would be sufficient for trapped

electrons to jump into the conduction band of activated CeO2 resulting in increase

in the conductivity of the film The drastic increase in conductivity of the sensor

could be attributed to the adsorptionndashdesorption mechanism resulted from the

electronic defects [26]

5310 Catalytic Activity

The utility of this catalytic system deprotection of several oximes were

studied (Table 52) The catalyst was also tested for reusability and it was found

that it could be reused without loss of initial activity for at least four cycles [19]

TLC analysis indicated the formation of only one product of corresponding

carbonyls FT-IR and NMR analysis showed that the oximes were converted into

carbonyls For checking the reusability (after each run) the catalyst was recovered

by simple filtration and washed several times with acetone and then dried at

373 K The catalyst thus obtained was used for the subsequent runs without any

further modification both activated and deactivated oximes were oxidized in better

yield more than ZrO2 catalyst as shown in Table 52

Chapter ndash V

178

Table 52 Oxidative Deoximation of Carbonyls on CeO2

Entry

Substrate

Product

Time

(h)

Yield

()

1

NOH

O

15

62

2

NOH

Cl

O

Cl

15

58

3

NOH

O

25

73

4

NOHCl

OCl

40

57

5

NOH

Cl

O

Cl

30

59

Solvent- Ethyl acetate Catalyst- ZrO2 Temperature- 353 K

Chapter ndash V

179

5311 Adsorption of Rhodamine B (Rh B)

The removal of dye pollutants are also of great importance for wastewater

treatment Rhodamine B a common cationic dye used in textile industry is one of

the most notorious contaminants in aquatic environments because of it huge

amounts slow biodegradation and toxicity In this study synthesized CeO2

nanocrystals are further used to remove Rh B The absorption spectrum of Rh B

solution was characterized by its characteristic absorption at 555 nm which was

attributed to the chromophore containing azo linkage of the dye molecules [22]

Fig 518 Absorption spectra of aqueous solution of Rh B in the presence of CeO2

nanocrystals at different times

UV-Visible absorption spectra represented in Fig518 confirms that over 93 of

Rh B in 100 mL of 20 mg L-1

Rh B aqueous solution could be removed with 100

mg of the as-prepared CeO2 nanocrystalline within 60 min The reasons may

account for the efficient removal of Rh B with CeO2 nanocrystals The first reason

Chapter ndash V

180

should be their to facilitate the adsorption of Rh B The second one is attributed to

the oxygen vacancies on the surface of CeO2 nanocrystals These abundant surface

oxygen vacancies could produce strong electrostatic attraction with the cationic

groups of Rh B as well as hydrogen bonding with the nitrogen atoms of Rh B

Chapter ndash V

181

Conclusions

Microwave synthesis is very beneficial to find a fast simple and energy

efficient approach to produce fine CeO2 nanoparticles It is relatively new method

to produce inorganic compounds for materials processing to enhance the material

properties as well as economic advantages through energy saving and acceleration

of product development Cerium oxide has outstanding physical and chemical

properties therefore it is used as LPG sensor In the area of catalysis nanoparticles

of ceria have been studied Also it is concluded as

1] The CeO2 nanoparticles were prepared by simple microwave assisted sol-gel

method

2] The well spherical and narrow size distribution with 10 nm size particles were

obtained by this method

3] It is very simple time as well as energy saving technique

4] The XRD pattern shows the CeO2 nanoparticles exists in face centered cubic

5] Scanning electron micrographs indicates that grains are uniformaly distributed

6] Transmission electron micrographs indicates that the mean diameter of the

particles as measured is about 10 nm

7] The conductivity of nano CeO2 thick film is represents the almost linear

relationship was found in the measured temperature range of 200-5000C It is

clear that the conduction in crystalline ceria is essentially an n-type electronic

process

8] The CeO2 nanoparticles in the thick film form prepared by screen printing

technique showed better response to LPG gas at 250degC temperature at 600 ppm

concentration of LPG gas

9] Catalyst can be prepared from inexpensive precursors using the microwave

assisted sol-gel technique is promising for the oxidative regeneration of ketones

10] This study reveals that these CeO2 nanocrystals are novel adsorbents for

wastewater treatment

Chapter ndash V

182

References

[1] A Ramesh H Hasegawa T Maki KUeda Sep Purif Tech

56 (2007) 90

[2] C Lettmann K Hildenbrand H Kisch W Macyk W F Maier

Appl Catal B 32 (2001) 215

[3] X Zhang Z H Ai F L Jia L Z Zhang J Phys Chem C 112 (2008) 747

[4] N Du H Zhang BD Chen X Y Ma D RYang J Phys ChemC

111 (2007) 12677

[5] F Zhang S Yang H Chen X Yu Cera Intern 30 (2004) 997

[6] F Gu SF Wang MK Lu GJ Zhou D Xu DR Langmuir J SciIR Iran

20 (2004) 3528

[7] H Xiao Z Ai L Zhang JPhys Chem 113 (2009) 16625

[8] L Combemale G Caboche D Stuerga D Chaumont MicroRese

Bull 40 (2005) 529

[9] M Zawadzki Jof Alloys and Comp 454 (2008) 347

[10] A V Patil C G Dighavkar S K Sonawane S J Patil R Y Borse

Sens and Transducers 108 (2009) 189

[11] AV Patil CG Dighavkar RY Borse Sens and Transducers

101 (2009) 96

[12] AV Patil CG Dighavkar SK Sonawane S J Patil RY Borse Sensand

Transducers 9 (2010) 11

[13] DR Patil LA Patil Talanta 77 (2009)1409

[14] R S Blackburn Environ Sci Tech 38 (2004) 4905

[15] GNaja CMustin JBerthelin BVolesky J Colloide Inter Sci

292 (2005) 537

[16] VKGupta IAli Environ Sci Tech 42 (2008) 766

[17] A Mittal VK Gupta A Malviya and JMittal J Hazard Mater

151 (2008) 821

[18] VKGupta IAli VKSaini J Colloid Interface Sci 315(2007) 87

Chapter ndash V

183

[19] S S Deshpande S U Sonavane R V Jayaram Cat Com 9 (2008) 639

[20] KM Garadkar BS Shirke YB Patil D R Patil Sens and Transducers

110 (2009) 17

[21] VKGupta AMittal VGajbe JMittal J Colloid Inter Sci

319 (2008) 30

[22] T A Khan I Ali V V Singh S Sharma J Envir Protec Sci 3 (2009)11

[23] A Hosseinnia M Keyanpour-Rad M Pazouki World Applied Sci J

8 (2010) 1327

[24] MI Baraton L Merhari J Am Ceram Soc 75 (2004) 1587

[25] CWang B Xu XWang J Zhao J Solid State Chem 178 (2005) 3500

[26] H Xiao Z Ai L Zhang J Phys Chem C doi 10 1021 JP9050269 (2009)

[27] A M Thompson Oxides of the Rare Earths Wiley New York (1978)

[28] H Noumlremberg G A D Briggs Phys Rev Lett 79 (1997) 4222

[29] A Hosseinnia M Keyanpour-Rad M Pazouki World Appl Sci J

8 (2010) 1327

[30] BD Cullity SR Stock Elements of X-Ray Diffraction 3rd edn

(Prentice Hall Upper Saddle River NJ) (2001) 07458

[31] G Ranga Rao B Gopal Mishra Bul Cat Soc of Ind 2 (2003)122

[32] A K Bhattacharyal T K Naiya S N Mondal S K Das Chem Eng J

137 (2008) 529

[33] G Liu J A Rodriguez Z Chang J Hrbek C H F Peden

J Phys Chem B 108 (2004) 2931

[34] P F Ji J L Zhang F Chen M Anpo J Phys Chem C 112 (2008) 17809

[35] A Trovarelli Catal Rev SciEng 38 (1996) 439

[36] B Li D M Pan J S Zheng Y J Cheng X Y Ma F Huang L Zhang

Langmuir 24 (2008) 9630

[37] M Fernaacutendez-Garcıacutea A A B Martıacutenez-Arias A Hungrıacutea Iglesias-Juez

J C Conesa J Soria J Phys Chem Chem Phys 4 (2002) 2473

[38] H M Chen J H He J Phys Chem C 112 (2008) 17540

Chapter ndash V

184

[39] F Zhang S Chan W Spanier J E Apak E Jin Q Robinson

R D Herman I P Appl Phys Lett 80 (2002) 127

[40] G Adachi TMasui In Catalysis by Ceria and Related Materials

ATrovarelli Ed Imperial College Press London (2002)

[41] J A Rodriguez X Wang J C Hanson G Liu A Iglesias-Juez

MFernaacutendez-Garcıacutea J Chem Phys 119 (2003) 5659

[42] S Eck C Castellarin-Cudia S Surnev M G Ramsey F PNetzer

Surf Sci 520 (2002) 173

[43] R B Yu L Yan P Zheng JChen X X R J Phys ChemC

112 (2008) 19896

[44] Y Nagai T Yamamoto T Tanaka SYoshida T Nonaka T Okamoto

A Suda M Sugiura Catal Today 74 (2002) 225

[45] Y M Chiang E B Lavik I Kosacki H L Tuller JY Ying

J Electroceram 1 (1997) 7

[46] D R Patil L A Patil G H Jain M S Wagh S A Patil

Sens and Transducers 74 (2006) 874

[47] K Sohlberg S K Pantelides S J Pennycook J Am Chem Soc

123 (2001) 6609

[48] L S Zhong J S Hu A M Cao Q Liu W G Song LWan

J Chem Mater 19 (2007) 1648

[49] J B Fei Y Cui X H Yan Y Yang K W Wang QHe J B Li

AdV Mater 20 (2008) 452

[50] P V Kamat J Phys Chem B 106 (2002) 7729

[51] R D Robinson J E Spanier F Zhang SW Chan I P Herman J Appl

Phys 92 (2002) 1936

[52] S W Cao Y J Zhu J Phys Chem C 112 (2008) 6253

[53] R Georgekutty M K Seery S C Pillai J Phys Chem C

112 (2008) 13563

[54] J S Hu L S Zhong W G Song L J Wan Adv Mater 20 (2008)1

Chapter ndash V

185

[55] HXu F L Jia Z H Ai L Z Zhang Cryst Growth Des (2007)

[56] H M Jia W J Xiao L Z Zhang Z F Zheng J PhysChem C

112 (2008) 1379

[57] Z H Ai H Y Xiao T Mei J Liu L Z Zhang K J Deng J R Qiu

J PhysChem C 112 (2008) 11929

[58] L M Ma W X Zhang EnViron Sci Technol 42 (2008) 5384

[59] Z H Ai Y Chen L Z Zhang J R Qiu EnViron Sci Technol

42 (2008) 69550

[60] Z Y Zhong J Ho J Teo S C Shen AGedanken ChemMater

19 (2007) 4776

[61] S C Kuiry S D Patil S Deshpande S Seal J Phys ChemB

109 (2005) 6936

[62] D R Patil L A Patil Sens IEEE 7 (2007) 434

[63] P Jasinski Anderson H U Sens Actuators B 95 (2003) 73

[64] J P Nair E Wachtel I Lubomirsky J Fleig JMaier AdVMater

15 (2003) 2077

[65] R J Qi Y J Zhu G F Cheng Y H Huang Nanotechnology

16 (2005) 2502

Chapter ndash V

176

5395 Oxygen Adsorption-Desorption Mechanism

O2 (air) + 2e- 2O

-(film surface) --- 56

At higher temperature the atmospheric oxygen O2 adsorbs on the surface of the

thick film It captures the electrons (Fig517) from conduction band It would

result in decreasing conductivity of the film When LPG reacts with the adsorbed

oxygen on the surface of the film it gets oxidized to CO2 and H2O by following

series of intermediate stages This liberates free electrons in the conduction band

The final reaction takes place as

C4H10(gas) + 13Ominus

(filmsurface) rarr 4CO2(gas) + 5H2O(gas) + 13eminus

(condband) --- 57

Fig 517 Sensing mechanism on the surface of the film

This shows n-type conduction mechanism Thus generated electrons

contribute to a sudden increase in conductance of the thick film The mass of

Ce and O in sample was not as per the stoichiometric proportion and sample was

Chapter ndash V

177

observed to be the oxygen deficient This deficiency gets reduced due to

adsorption of atmosphericmolecular oxygen This helps in decreasing electronic

conductivity of the film Upon exposure LPG molecules got oxidized with the

adsorbed oxygen ions by following the series of intermediate stages producing

CO2 and H2O This results in evolving oxygen as electrically neutral atoms

trapping behind the negative charges (electrons) Upon exposure the energy

released in decomposition of LPG molecules would be sufficient for trapped

electrons to jump into the conduction band of activated CeO2 resulting in increase

in the conductivity of the film The drastic increase in conductivity of the sensor

could be attributed to the adsorptionndashdesorption mechanism resulted from the

electronic defects [26]

5310 Catalytic Activity

The utility of this catalytic system deprotection of several oximes were

studied (Table 52) The catalyst was also tested for reusability and it was found

that it could be reused without loss of initial activity for at least four cycles [19]

TLC analysis indicated the formation of only one product of corresponding

carbonyls FT-IR and NMR analysis showed that the oximes were converted into

carbonyls For checking the reusability (after each run) the catalyst was recovered

by simple filtration and washed several times with acetone and then dried at

373 K The catalyst thus obtained was used for the subsequent runs without any

further modification both activated and deactivated oximes were oxidized in better

yield more than ZrO2 catalyst as shown in Table 52

Chapter ndash V

178

Table 52 Oxidative Deoximation of Carbonyls on CeO2

Entry

Substrate

Product

Time

(h)

Yield

()

1

NOH

O

15

62

2

NOH

Cl

O

Cl

15

58

3

NOH

O

25

73

4

NOHCl

OCl

40

57

5

NOH

Cl

O

Cl

30

59

Solvent- Ethyl acetate Catalyst- ZrO2 Temperature- 353 K

Chapter ndash V

179

5311 Adsorption of Rhodamine B (Rh B)

The removal of dye pollutants are also of great importance for wastewater

treatment Rhodamine B a common cationic dye used in textile industry is one of

the most notorious contaminants in aquatic environments because of it huge

amounts slow biodegradation and toxicity In this study synthesized CeO2

nanocrystals are further used to remove Rh B The absorption spectrum of Rh B

solution was characterized by its characteristic absorption at 555 nm which was

attributed to the chromophore containing azo linkage of the dye molecules [22]

Fig 518 Absorption spectra of aqueous solution of Rh B in the presence of CeO2

nanocrystals at different times

UV-Visible absorption spectra represented in Fig518 confirms that over 93 of

Rh B in 100 mL of 20 mg L-1

Rh B aqueous solution could be removed with 100

mg of the as-prepared CeO2 nanocrystalline within 60 min The reasons may

account for the efficient removal of Rh B with CeO2 nanocrystals The first reason

Chapter ndash V

180

should be their to facilitate the adsorption of Rh B The second one is attributed to

the oxygen vacancies on the surface of CeO2 nanocrystals These abundant surface

oxygen vacancies could produce strong electrostatic attraction with the cationic

groups of Rh B as well as hydrogen bonding with the nitrogen atoms of Rh B

Chapter ndash V

181

Conclusions

Microwave synthesis is very beneficial to find a fast simple and energy

efficient approach to produce fine CeO2 nanoparticles It is relatively new method

to produce inorganic compounds for materials processing to enhance the material

properties as well as economic advantages through energy saving and acceleration

of product development Cerium oxide has outstanding physical and chemical

properties therefore it is used as LPG sensor In the area of catalysis nanoparticles

of ceria have been studied Also it is concluded as

1] The CeO2 nanoparticles were prepared by simple microwave assisted sol-gel

method

2] The well spherical and narrow size distribution with 10 nm size particles were

obtained by this method

3] It is very simple time as well as energy saving technique

4] The XRD pattern shows the CeO2 nanoparticles exists in face centered cubic

5] Scanning electron micrographs indicates that grains are uniformaly distributed

6] Transmission electron micrographs indicates that the mean diameter of the

particles as measured is about 10 nm

7] The conductivity of nano CeO2 thick film is represents the almost linear

relationship was found in the measured temperature range of 200-5000C It is

clear that the conduction in crystalline ceria is essentially an n-type electronic

process

8] The CeO2 nanoparticles in the thick film form prepared by screen printing

technique showed better response to LPG gas at 250degC temperature at 600 ppm

concentration of LPG gas

9] Catalyst can be prepared from inexpensive precursors using the microwave

assisted sol-gel technique is promising for the oxidative regeneration of ketones

10] This study reveals that these CeO2 nanocrystals are novel adsorbents for

wastewater treatment

Chapter ndash V

182

References

[1] A Ramesh H Hasegawa T Maki KUeda Sep Purif Tech

56 (2007) 90

[2] C Lettmann K Hildenbrand H Kisch W Macyk W F Maier

Appl Catal B 32 (2001) 215

[3] X Zhang Z H Ai F L Jia L Z Zhang J Phys Chem C 112 (2008) 747

[4] N Du H Zhang BD Chen X Y Ma D RYang J Phys ChemC

111 (2007) 12677

[5] F Zhang S Yang H Chen X Yu Cera Intern 30 (2004) 997

[6] F Gu SF Wang MK Lu GJ Zhou D Xu DR Langmuir J SciIR Iran

20 (2004) 3528

[7] H Xiao Z Ai L Zhang JPhys Chem 113 (2009) 16625

[8] L Combemale G Caboche D Stuerga D Chaumont MicroRese

Bull 40 (2005) 529

[9] M Zawadzki Jof Alloys and Comp 454 (2008) 347

[10] A V Patil C G Dighavkar S K Sonawane S J Patil R Y Borse

Sens and Transducers 108 (2009) 189

[11] AV Patil CG Dighavkar RY Borse Sens and Transducers

101 (2009) 96

[12] AV Patil CG Dighavkar SK Sonawane S J Patil RY Borse Sensand

Transducers 9 (2010) 11

[13] DR Patil LA Patil Talanta 77 (2009)1409

[14] R S Blackburn Environ Sci Tech 38 (2004) 4905

[15] GNaja CMustin JBerthelin BVolesky J Colloide Inter Sci

292 (2005) 537

[16] VKGupta IAli Environ Sci Tech 42 (2008) 766

[17] A Mittal VK Gupta A Malviya and JMittal J Hazard Mater

151 (2008) 821

[18] VKGupta IAli VKSaini J Colloid Interface Sci 315(2007) 87

Chapter ndash V

183

[19] S S Deshpande S U Sonavane R V Jayaram Cat Com 9 (2008) 639

[20] KM Garadkar BS Shirke YB Patil D R Patil Sens and Transducers

110 (2009) 17

[21] VKGupta AMittal VGajbe JMittal J Colloid Inter Sci

319 (2008) 30

[22] T A Khan I Ali V V Singh S Sharma J Envir Protec Sci 3 (2009)11

[23] A Hosseinnia M Keyanpour-Rad M Pazouki World Applied Sci J

8 (2010) 1327

[24] MI Baraton L Merhari J Am Ceram Soc 75 (2004) 1587

[25] CWang B Xu XWang J Zhao J Solid State Chem 178 (2005) 3500

[26] H Xiao Z Ai L Zhang J Phys Chem C doi 10 1021 JP9050269 (2009)

[27] A M Thompson Oxides of the Rare Earths Wiley New York (1978)

[28] H Noumlremberg G A D Briggs Phys Rev Lett 79 (1997) 4222

[29] A Hosseinnia M Keyanpour-Rad M Pazouki World Appl Sci J

8 (2010) 1327

[30] BD Cullity SR Stock Elements of X-Ray Diffraction 3rd edn

(Prentice Hall Upper Saddle River NJ) (2001) 07458

[31] G Ranga Rao B Gopal Mishra Bul Cat Soc of Ind 2 (2003)122

[32] A K Bhattacharyal T K Naiya S N Mondal S K Das Chem Eng J

137 (2008) 529

[33] G Liu J A Rodriguez Z Chang J Hrbek C H F Peden

J Phys Chem B 108 (2004) 2931

[34] P F Ji J L Zhang F Chen M Anpo J Phys Chem C 112 (2008) 17809

[35] A Trovarelli Catal Rev SciEng 38 (1996) 439

[36] B Li D M Pan J S Zheng Y J Cheng X Y Ma F Huang L Zhang

Langmuir 24 (2008) 9630

[37] M Fernaacutendez-Garcıacutea A A B Martıacutenez-Arias A Hungrıacutea Iglesias-Juez

J C Conesa J Soria J Phys Chem Chem Phys 4 (2002) 2473

[38] H M Chen J H He J Phys Chem C 112 (2008) 17540

Chapter ndash V

184

[39] F Zhang S Chan W Spanier J E Apak E Jin Q Robinson

R D Herman I P Appl Phys Lett 80 (2002) 127

[40] G Adachi TMasui In Catalysis by Ceria and Related Materials

ATrovarelli Ed Imperial College Press London (2002)

[41] J A Rodriguez X Wang J C Hanson G Liu A Iglesias-Juez

MFernaacutendez-Garcıacutea J Chem Phys 119 (2003) 5659

[42] S Eck C Castellarin-Cudia S Surnev M G Ramsey F PNetzer

Surf Sci 520 (2002) 173

[43] R B Yu L Yan P Zheng JChen X X R J Phys ChemC

112 (2008) 19896

[44] Y Nagai T Yamamoto T Tanaka SYoshida T Nonaka T Okamoto

A Suda M Sugiura Catal Today 74 (2002) 225

[45] Y M Chiang E B Lavik I Kosacki H L Tuller JY Ying

J Electroceram 1 (1997) 7

[46] D R Patil L A Patil G H Jain M S Wagh S A Patil

Sens and Transducers 74 (2006) 874

[47] K Sohlberg S K Pantelides S J Pennycook J Am Chem Soc

123 (2001) 6609

[48] L S Zhong J S Hu A M Cao Q Liu W G Song LWan

J Chem Mater 19 (2007) 1648

[49] J B Fei Y Cui X H Yan Y Yang K W Wang QHe J B Li

AdV Mater 20 (2008) 452

[50] P V Kamat J Phys Chem B 106 (2002) 7729

[51] R D Robinson J E Spanier F Zhang SW Chan I P Herman J Appl

Phys 92 (2002) 1936

[52] S W Cao Y J Zhu J Phys Chem C 112 (2008) 6253

[53] R Georgekutty M K Seery S C Pillai J Phys Chem C

112 (2008) 13563

[54] J S Hu L S Zhong W G Song L J Wan Adv Mater 20 (2008)1

Chapter ndash V

185

[55] HXu F L Jia Z H Ai L Z Zhang Cryst Growth Des (2007)

[56] H M Jia W J Xiao L Z Zhang Z F Zheng J PhysChem C

112 (2008) 1379

[57] Z H Ai H Y Xiao T Mei J Liu L Z Zhang K J Deng J R Qiu

J PhysChem C 112 (2008) 11929

[58] L M Ma W X Zhang EnViron Sci Technol 42 (2008) 5384

[59] Z H Ai Y Chen L Z Zhang J R Qiu EnViron Sci Technol

42 (2008) 69550

[60] Z Y Zhong J Ho J Teo S C Shen AGedanken ChemMater

19 (2007) 4776

[61] S C Kuiry S D Patil S Deshpande S Seal J Phys ChemB

109 (2005) 6936

[62] D R Patil L A Patil Sens IEEE 7 (2007) 434

[63] P Jasinski Anderson H U Sens Actuators B 95 (2003) 73

[64] J P Nair E Wachtel I Lubomirsky J Fleig JMaier AdVMater

15 (2003) 2077

[65] R J Qi Y J Zhu G F Cheng Y H Huang Nanotechnology

16 (2005) 2502

Chapter ndash V

177

observed to be the oxygen deficient This deficiency gets reduced due to

adsorption of atmosphericmolecular oxygen This helps in decreasing electronic

conductivity of the film Upon exposure LPG molecules got oxidized with the

adsorbed oxygen ions by following the series of intermediate stages producing

CO2 and H2O This results in evolving oxygen as electrically neutral atoms

trapping behind the negative charges (electrons) Upon exposure the energy

released in decomposition of LPG molecules would be sufficient for trapped

electrons to jump into the conduction band of activated CeO2 resulting in increase

in the conductivity of the film The drastic increase in conductivity of the sensor

could be attributed to the adsorptionndashdesorption mechanism resulted from the

electronic defects [26]

5310 Catalytic Activity

The utility of this catalytic system deprotection of several oximes were

studied (Table 52) The catalyst was also tested for reusability and it was found

that it could be reused without loss of initial activity for at least four cycles [19]

TLC analysis indicated the formation of only one product of corresponding

carbonyls FT-IR and NMR analysis showed that the oximes were converted into

carbonyls For checking the reusability (after each run) the catalyst was recovered

by simple filtration and washed several times with acetone and then dried at

373 K The catalyst thus obtained was used for the subsequent runs without any

further modification both activated and deactivated oximes were oxidized in better

yield more than ZrO2 catalyst as shown in Table 52

Chapter ndash V

178

Table 52 Oxidative Deoximation of Carbonyls on CeO2

Entry

Substrate

Product

Time

(h)

Yield

()

1

NOH

O

15

62

2

NOH

Cl

O

Cl

15

58

3

NOH

O

25

73

4

NOHCl

OCl

40

57

5

NOH

Cl

O

Cl

30

59

Solvent- Ethyl acetate Catalyst- ZrO2 Temperature- 353 K

Chapter ndash V

179

5311 Adsorption of Rhodamine B (Rh B)

The removal of dye pollutants are also of great importance for wastewater

treatment Rhodamine B a common cationic dye used in textile industry is one of

the most notorious contaminants in aquatic environments because of it huge

amounts slow biodegradation and toxicity In this study synthesized CeO2

nanocrystals are further used to remove Rh B The absorption spectrum of Rh B

solution was characterized by its characteristic absorption at 555 nm which was

attributed to the chromophore containing azo linkage of the dye molecules [22]

Fig 518 Absorption spectra of aqueous solution of Rh B in the presence of CeO2

nanocrystals at different times

UV-Visible absorption spectra represented in Fig518 confirms that over 93 of

Rh B in 100 mL of 20 mg L-1

Rh B aqueous solution could be removed with 100

mg of the as-prepared CeO2 nanocrystalline within 60 min The reasons may

account for the efficient removal of Rh B with CeO2 nanocrystals The first reason

Chapter ndash V

180

should be their to facilitate the adsorption of Rh B The second one is attributed to

the oxygen vacancies on the surface of CeO2 nanocrystals These abundant surface

oxygen vacancies could produce strong electrostatic attraction with the cationic

groups of Rh B as well as hydrogen bonding with the nitrogen atoms of Rh B

Chapter ndash V

181

Conclusions

Microwave synthesis is very beneficial to find a fast simple and energy

efficient approach to produce fine CeO2 nanoparticles It is relatively new method

to produce inorganic compounds for materials processing to enhance the material

properties as well as economic advantages through energy saving and acceleration

of product development Cerium oxide has outstanding physical and chemical

properties therefore it is used as LPG sensor In the area of catalysis nanoparticles

of ceria have been studied Also it is concluded as

1] The CeO2 nanoparticles were prepared by simple microwave assisted sol-gel

method

2] The well spherical and narrow size distribution with 10 nm size particles were

obtained by this method

3] It is very simple time as well as energy saving technique

4] The XRD pattern shows the CeO2 nanoparticles exists in face centered cubic

5] Scanning electron micrographs indicates that grains are uniformaly distributed

6] Transmission electron micrographs indicates that the mean diameter of the

particles as measured is about 10 nm

7] The conductivity of nano CeO2 thick film is represents the almost linear

relationship was found in the measured temperature range of 200-5000C It is

clear that the conduction in crystalline ceria is essentially an n-type electronic

process

8] The CeO2 nanoparticles in the thick film form prepared by screen printing

technique showed better response to LPG gas at 250degC temperature at 600 ppm

concentration of LPG gas

9] Catalyst can be prepared from inexpensive precursors using the microwave

assisted sol-gel technique is promising for the oxidative regeneration of ketones

10] This study reveals that these CeO2 nanocrystals are novel adsorbents for

wastewater treatment

Chapter ndash V

182

References

[1] A Ramesh H Hasegawa T Maki KUeda Sep Purif Tech

56 (2007) 90

[2] C Lettmann K Hildenbrand H Kisch W Macyk W F Maier

Appl Catal B 32 (2001) 215

[3] X Zhang Z H Ai F L Jia L Z Zhang J Phys Chem C 112 (2008) 747

[4] N Du H Zhang BD Chen X Y Ma D RYang J Phys ChemC

111 (2007) 12677

[5] F Zhang S Yang H Chen X Yu Cera Intern 30 (2004) 997

[6] F Gu SF Wang MK Lu GJ Zhou D Xu DR Langmuir J SciIR Iran

20 (2004) 3528

[7] H Xiao Z Ai L Zhang JPhys Chem 113 (2009) 16625

[8] L Combemale G Caboche D Stuerga D Chaumont MicroRese

Bull 40 (2005) 529

[9] M Zawadzki Jof Alloys and Comp 454 (2008) 347

[10] A V Patil C G Dighavkar S K Sonawane S J Patil R Y Borse

Sens and Transducers 108 (2009) 189

[11] AV Patil CG Dighavkar RY Borse Sens and Transducers

101 (2009) 96

[12] AV Patil CG Dighavkar SK Sonawane S J Patil RY Borse Sensand

Transducers 9 (2010) 11

[13] DR Patil LA Patil Talanta 77 (2009)1409

[14] R S Blackburn Environ Sci Tech 38 (2004) 4905

[15] GNaja CMustin JBerthelin BVolesky J Colloide Inter Sci

292 (2005) 537

[16] VKGupta IAli Environ Sci Tech 42 (2008) 766

[17] A Mittal VK Gupta A Malviya and JMittal J Hazard Mater

151 (2008) 821

[18] VKGupta IAli VKSaini J Colloid Interface Sci 315(2007) 87

Chapter ndash V

183

[19] S S Deshpande S U Sonavane R V Jayaram Cat Com 9 (2008) 639

[20] KM Garadkar BS Shirke YB Patil D R Patil Sens and Transducers

110 (2009) 17

[21] VKGupta AMittal VGajbe JMittal J Colloid Inter Sci

319 (2008) 30

[22] T A Khan I Ali V V Singh S Sharma J Envir Protec Sci 3 (2009)11

[23] A Hosseinnia M Keyanpour-Rad M Pazouki World Applied Sci J

8 (2010) 1327

[24] MI Baraton L Merhari J Am Ceram Soc 75 (2004) 1587

[25] CWang B Xu XWang J Zhao J Solid State Chem 178 (2005) 3500

[26] H Xiao Z Ai L Zhang J Phys Chem C doi 10 1021 JP9050269 (2009)

[27] A M Thompson Oxides of the Rare Earths Wiley New York (1978)

[28] H Noumlremberg G A D Briggs Phys Rev Lett 79 (1997) 4222

[29] A Hosseinnia M Keyanpour-Rad M Pazouki World Appl Sci J

8 (2010) 1327

[30] BD Cullity SR Stock Elements of X-Ray Diffraction 3rd edn

(Prentice Hall Upper Saddle River NJ) (2001) 07458

[31] G Ranga Rao B Gopal Mishra Bul Cat Soc of Ind 2 (2003)122

[32] A K Bhattacharyal T K Naiya S N Mondal S K Das Chem Eng J

137 (2008) 529

[33] G Liu J A Rodriguez Z Chang J Hrbek C H F Peden

J Phys Chem B 108 (2004) 2931

[34] P F Ji J L Zhang F Chen M Anpo J Phys Chem C 112 (2008) 17809

[35] A Trovarelli Catal Rev SciEng 38 (1996) 439

[36] B Li D M Pan J S Zheng Y J Cheng X Y Ma F Huang L Zhang

Langmuir 24 (2008) 9630

[37] M Fernaacutendez-Garcıacutea A A B Martıacutenez-Arias A Hungrıacutea Iglesias-Juez

J C Conesa J Soria J Phys Chem Chem Phys 4 (2002) 2473

[38] H M Chen J H He J Phys Chem C 112 (2008) 17540

Chapter ndash V

184

[39] F Zhang S Chan W Spanier J E Apak E Jin Q Robinson

R D Herman I P Appl Phys Lett 80 (2002) 127

[40] G Adachi TMasui In Catalysis by Ceria and Related Materials

ATrovarelli Ed Imperial College Press London (2002)

[41] J A Rodriguez X Wang J C Hanson G Liu A Iglesias-Juez

MFernaacutendez-Garcıacutea J Chem Phys 119 (2003) 5659

[42] S Eck C Castellarin-Cudia S Surnev M G Ramsey F PNetzer

Surf Sci 520 (2002) 173

[43] R B Yu L Yan P Zheng JChen X X R J Phys ChemC

112 (2008) 19896

[44] Y Nagai T Yamamoto T Tanaka SYoshida T Nonaka T Okamoto

A Suda M Sugiura Catal Today 74 (2002) 225

[45] Y M Chiang E B Lavik I Kosacki H L Tuller JY Ying

J Electroceram 1 (1997) 7

[46] D R Patil L A Patil G H Jain M S Wagh S A Patil

Sens and Transducers 74 (2006) 874

[47] K Sohlberg S K Pantelides S J Pennycook J Am Chem Soc

123 (2001) 6609

[48] L S Zhong J S Hu A M Cao Q Liu W G Song LWan

J Chem Mater 19 (2007) 1648

[49] J B Fei Y Cui X H Yan Y Yang K W Wang QHe J B Li

AdV Mater 20 (2008) 452

[50] P V Kamat J Phys Chem B 106 (2002) 7729

[51] R D Robinson J E Spanier F Zhang SW Chan I P Herman J Appl

Phys 92 (2002) 1936

[52] S W Cao Y J Zhu J Phys Chem C 112 (2008) 6253

[53] R Georgekutty M K Seery S C Pillai J Phys Chem C

112 (2008) 13563

[54] J S Hu L S Zhong W G Song L J Wan Adv Mater 20 (2008)1

Chapter ndash V

185

[55] HXu F L Jia Z H Ai L Z Zhang Cryst Growth Des (2007)

[56] H M Jia W J Xiao L Z Zhang Z F Zheng J PhysChem C

112 (2008) 1379

[57] Z H Ai H Y Xiao T Mei J Liu L Z Zhang K J Deng J R Qiu

J PhysChem C 112 (2008) 11929

[58] L M Ma W X Zhang EnViron Sci Technol 42 (2008) 5384

[59] Z H Ai Y Chen L Z Zhang J R Qiu EnViron Sci Technol

42 (2008) 69550

[60] Z Y Zhong J Ho J Teo S C Shen AGedanken ChemMater

19 (2007) 4776

[61] S C Kuiry S D Patil S Deshpande S Seal J Phys ChemB

109 (2005) 6936

[62] D R Patil L A Patil Sens IEEE 7 (2007) 434

[63] P Jasinski Anderson H U Sens Actuators B 95 (2003) 73

[64] J P Nair E Wachtel I Lubomirsky J Fleig JMaier AdVMater

15 (2003) 2077

[65] R J Qi Y J Zhu G F Cheng Y H Huang Nanotechnology

16 (2005) 2502

Chapter ndash V

178

Table 52 Oxidative Deoximation of Carbonyls on CeO2

Entry

Substrate

Product

Time

(h)

Yield

()

1

NOH

O

15

62

2

NOH

Cl

O

Cl

15

58

3

NOH

O

25

73

4

NOHCl

OCl

40

57

5

NOH

Cl

O

Cl

30

59

Solvent- Ethyl acetate Catalyst- ZrO2 Temperature- 353 K

Chapter ndash V

179

5311 Adsorption of Rhodamine B (Rh B)

The removal of dye pollutants are also of great importance for wastewater

treatment Rhodamine B a common cationic dye used in textile industry is one of

the most notorious contaminants in aquatic environments because of it huge

amounts slow biodegradation and toxicity In this study synthesized CeO2

nanocrystals are further used to remove Rh B The absorption spectrum of Rh B

solution was characterized by its characteristic absorption at 555 nm which was

attributed to the chromophore containing azo linkage of the dye molecules [22]

Fig 518 Absorption spectra of aqueous solution of Rh B in the presence of CeO2

nanocrystals at different times

UV-Visible absorption spectra represented in Fig518 confirms that over 93 of

Rh B in 100 mL of 20 mg L-1

Rh B aqueous solution could be removed with 100

mg of the as-prepared CeO2 nanocrystalline within 60 min The reasons may

account for the efficient removal of Rh B with CeO2 nanocrystals The first reason

Chapter ndash V

180

should be their to facilitate the adsorption of Rh B The second one is attributed to

the oxygen vacancies on the surface of CeO2 nanocrystals These abundant surface

oxygen vacancies could produce strong electrostatic attraction with the cationic

groups of Rh B as well as hydrogen bonding with the nitrogen atoms of Rh B

Chapter ndash V

181

Conclusions

Microwave synthesis is very beneficial to find a fast simple and energy

efficient approach to produce fine CeO2 nanoparticles It is relatively new method

to produce inorganic compounds for materials processing to enhance the material

properties as well as economic advantages through energy saving and acceleration

of product development Cerium oxide has outstanding physical and chemical

properties therefore it is used as LPG sensor In the area of catalysis nanoparticles

of ceria have been studied Also it is concluded as

1] The CeO2 nanoparticles were prepared by simple microwave assisted sol-gel

method

2] The well spherical and narrow size distribution with 10 nm size particles were

obtained by this method

3] It is very simple time as well as energy saving technique

4] The XRD pattern shows the CeO2 nanoparticles exists in face centered cubic

5] Scanning electron micrographs indicates that grains are uniformaly distributed

6] Transmission electron micrographs indicates that the mean diameter of the

particles as measured is about 10 nm

7] The conductivity of nano CeO2 thick film is represents the almost linear

relationship was found in the measured temperature range of 200-5000C It is

clear that the conduction in crystalline ceria is essentially an n-type electronic

process

8] The CeO2 nanoparticles in the thick film form prepared by screen printing

technique showed better response to LPG gas at 250degC temperature at 600 ppm

concentration of LPG gas

9] Catalyst can be prepared from inexpensive precursors using the microwave

assisted sol-gel technique is promising for the oxidative regeneration of ketones

10] This study reveals that these CeO2 nanocrystals are novel adsorbents for

wastewater treatment

Chapter ndash V

182

References

[1] A Ramesh H Hasegawa T Maki KUeda Sep Purif Tech

56 (2007) 90

[2] C Lettmann K Hildenbrand H Kisch W Macyk W F Maier

Appl Catal B 32 (2001) 215

[3] X Zhang Z H Ai F L Jia L Z Zhang J Phys Chem C 112 (2008) 747

[4] N Du H Zhang BD Chen X Y Ma D RYang J Phys ChemC

111 (2007) 12677

[5] F Zhang S Yang H Chen X Yu Cera Intern 30 (2004) 997

[6] F Gu SF Wang MK Lu GJ Zhou D Xu DR Langmuir J SciIR Iran

20 (2004) 3528

[7] H Xiao Z Ai L Zhang JPhys Chem 113 (2009) 16625

[8] L Combemale G Caboche D Stuerga D Chaumont MicroRese

Bull 40 (2005) 529

[9] M Zawadzki Jof Alloys and Comp 454 (2008) 347

[10] A V Patil C G Dighavkar S K Sonawane S J Patil R Y Borse

Sens and Transducers 108 (2009) 189

[11] AV Patil CG Dighavkar RY Borse Sens and Transducers

101 (2009) 96

[12] AV Patil CG Dighavkar SK Sonawane S J Patil RY Borse Sensand

Transducers 9 (2010) 11

[13] DR Patil LA Patil Talanta 77 (2009)1409

[14] R S Blackburn Environ Sci Tech 38 (2004) 4905

[15] GNaja CMustin JBerthelin BVolesky J Colloide Inter Sci

292 (2005) 537

[16] VKGupta IAli Environ Sci Tech 42 (2008) 766

[17] A Mittal VK Gupta A Malviya and JMittal J Hazard Mater

151 (2008) 821

[18] VKGupta IAli VKSaini J Colloid Interface Sci 315(2007) 87

Chapter ndash V

183

[19] S S Deshpande S U Sonavane R V Jayaram Cat Com 9 (2008) 639

[20] KM Garadkar BS Shirke YB Patil D R Patil Sens and Transducers

110 (2009) 17

[21] VKGupta AMittal VGajbe JMittal J Colloid Inter Sci

319 (2008) 30

[22] T A Khan I Ali V V Singh S Sharma J Envir Protec Sci 3 (2009)11

[23] A Hosseinnia M Keyanpour-Rad M Pazouki World Applied Sci J

8 (2010) 1327

[24] MI Baraton L Merhari J Am Ceram Soc 75 (2004) 1587

[25] CWang B Xu XWang J Zhao J Solid State Chem 178 (2005) 3500

[26] H Xiao Z Ai L Zhang J Phys Chem C doi 10 1021 JP9050269 (2009)

[27] A M Thompson Oxides of the Rare Earths Wiley New York (1978)

[28] H Noumlremberg G A D Briggs Phys Rev Lett 79 (1997) 4222

[29] A Hosseinnia M Keyanpour-Rad M Pazouki World Appl Sci J

8 (2010) 1327

[30] BD Cullity SR Stock Elements of X-Ray Diffraction 3rd edn

(Prentice Hall Upper Saddle River NJ) (2001) 07458

[31] G Ranga Rao B Gopal Mishra Bul Cat Soc of Ind 2 (2003)122

[32] A K Bhattacharyal T K Naiya S N Mondal S K Das Chem Eng J

137 (2008) 529

[33] G Liu J A Rodriguez Z Chang J Hrbek C H F Peden

J Phys Chem B 108 (2004) 2931

[34] P F Ji J L Zhang F Chen M Anpo J Phys Chem C 112 (2008) 17809

[35] A Trovarelli Catal Rev SciEng 38 (1996) 439

[36] B Li D M Pan J S Zheng Y J Cheng X Y Ma F Huang L Zhang

Langmuir 24 (2008) 9630

[37] M Fernaacutendez-Garcıacutea A A B Martıacutenez-Arias A Hungrıacutea Iglesias-Juez

J C Conesa J Soria J Phys Chem Chem Phys 4 (2002) 2473

[38] H M Chen J H He J Phys Chem C 112 (2008) 17540

Chapter ndash V

184

[39] F Zhang S Chan W Spanier J E Apak E Jin Q Robinson

R D Herman I P Appl Phys Lett 80 (2002) 127

[40] G Adachi TMasui In Catalysis by Ceria and Related Materials

ATrovarelli Ed Imperial College Press London (2002)

[41] J A Rodriguez X Wang J C Hanson G Liu A Iglesias-Juez

MFernaacutendez-Garcıacutea J Chem Phys 119 (2003) 5659

[42] S Eck C Castellarin-Cudia S Surnev M G Ramsey F PNetzer

Surf Sci 520 (2002) 173

[43] R B Yu L Yan P Zheng JChen X X R J Phys ChemC

112 (2008) 19896

[44] Y Nagai T Yamamoto T Tanaka SYoshida T Nonaka T Okamoto

A Suda M Sugiura Catal Today 74 (2002) 225

[45] Y M Chiang E B Lavik I Kosacki H L Tuller JY Ying

J Electroceram 1 (1997) 7

[46] D R Patil L A Patil G H Jain M S Wagh S A Patil

Sens and Transducers 74 (2006) 874

[47] K Sohlberg S K Pantelides S J Pennycook J Am Chem Soc

123 (2001) 6609

[48] L S Zhong J S Hu A M Cao Q Liu W G Song LWan

J Chem Mater 19 (2007) 1648

[49] J B Fei Y Cui X H Yan Y Yang K W Wang QHe J B Li

AdV Mater 20 (2008) 452

[50] P V Kamat J Phys Chem B 106 (2002) 7729

[51] R D Robinson J E Spanier F Zhang SW Chan I P Herman J Appl

Phys 92 (2002) 1936

[52] S W Cao Y J Zhu J Phys Chem C 112 (2008) 6253

[53] R Georgekutty M K Seery S C Pillai J Phys Chem C

112 (2008) 13563

[54] J S Hu L S Zhong W G Song L J Wan Adv Mater 20 (2008)1

Chapter ndash V

185

[55] HXu F L Jia Z H Ai L Z Zhang Cryst Growth Des (2007)

[56] H M Jia W J Xiao L Z Zhang Z F Zheng J PhysChem C

112 (2008) 1379

[57] Z H Ai H Y Xiao T Mei J Liu L Z Zhang K J Deng J R Qiu

J PhysChem C 112 (2008) 11929

[58] L M Ma W X Zhang EnViron Sci Technol 42 (2008) 5384

[59] Z H Ai Y Chen L Z Zhang J R Qiu EnViron Sci Technol

42 (2008) 69550

[60] Z Y Zhong J Ho J Teo S C Shen AGedanken ChemMater

19 (2007) 4776

[61] S C Kuiry S D Patil S Deshpande S Seal J Phys ChemB

109 (2005) 6936

[62] D R Patil L A Patil Sens IEEE 7 (2007) 434

[63] P Jasinski Anderson H U Sens Actuators B 95 (2003) 73

[64] J P Nair E Wachtel I Lubomirsky J Fleig JMaier AdVMater

15 (2003) 2077

[65] R J Qi Y J Zhu G F Cheng Y H Huang Nanotechnology

16 (2005) 2502

Chapter ndash V

179

5311 Adsorption of Rhodamine B (Rh B)

The removal of dye pollutants are also of great importance for wastewater

treatment Rhodamine B a common cationic dye used in textile industry is one of

the most notorious contaminants in aquatic environments because of it huge

amounts slow biodegradation and toxicity In this study synthesized CeO2

nanocrystals are further used to remove Rh B The absorption spectrum of Rh B

solution was characterized by its characteristic absorption at 555 nm which was

attributed to the chromophore containing azo linkage of the dye molecules [22]

Fig 518 Absorption spectra of aqueous solution of Rh B in the presence of CeO2

nanocrystals at different times

UV-Visible absorption spectra represented in Fig518 confirms that over 93 of

Rh B in 100 mL of 20 mg L-1

Rh B aqueous solution could be removed with 100

mg of the as-prepared CeO2 nanocrystalline within 60 min The reasons may

account for the efficient removal of Rh B with CeO2 nanocrystals The first reason

Chapter ndash V

180

should be their to facilitate the adsorption of Rh B The second one is attributed to

the oxygen vacancies on the surface of CeO2 nanocrystals These abundant surface

oxygen vacancies could produce strong electrostatic attraction with the cationic

groups of Rh B as well as hydrogen bonding with the nitrogen atoms of Rh B

Chapter ndash V

181

Conclusions

Microwave synthesis is very beneficial to find a fast simple and energy

efficient approach to produce fine CeO2 nanoparticles It is relatively new method

to produce inorganic compounds for materials processing to enhance the material

properties as well as economic advantages through energy saving and acceleration

of product development Cerium oxide has outstanding physical and chemical

properties therefore it is used as LPG sensor In the area of catalysis nanoparticles

of ceria have been studied Also it is concluded as

1] The CeO2 nanoparticles were prepared by simple microwave assisted sol-gel

method

2] The well spherical and narrow size distribution with 10 nm size particles were

obtained by this method

3] It is very simple time as well as energy saving technique

4] The XRD pattern shows the CeO2 nanoparticles exists in face centered cubic

5] Scanning electron micrographs indicates that grains are uniformaly distributed

6] Transmission electron micrographs indicates that the mean diameter of the

particles as measured is about 10 nm

7] The conductivity of nano CeO2 thick film is represents the almost linear

relationship was found in the measured temperature range of 200-5000C It is

clear that the conduction in crystalline ceria is essentially an n-type electronic

process

8] The CeO2 nanoparticles in the thick film form prepared by screen printing

technique showed better response to LPG gas at 250degC temperature at 600 ppm

concentration of LPG gas

9] Catalyst can be prepared from inexpensive precursors using the microwave

assisted sol-gel technique is promising for the oxidative regeneration of ketones

10] This study reveals that these CeO2 nanocrystals are novel adsorbents for

wastewater treatment

Chapter ndash V

182

References

[1] A Ramesh H Hasegawa T Maki KUeda Sep Purif Tech

56 (2007) 90

[2] C Lettmann K Hildenbrand H Kisch W Macyk W F Maier

Appl Catal B 32 (2001) 215

[3] X Zhang Z H Ai F L Jia L Z Zhang J Phys Chem C 112 (2008) 747

[4] N Du H Zhang BD Chen X Y Ma D RYang J Phys ChemC

111 (2007) 12677

[5] F Zhang S Yang H Chen X Yu Cera Intern 30 (2004) 997

[6] F Gu SF Wang MK Lu GJ Zhou D Xu DR Langmuir J SciIR Iran

20 (2004) 3528

[7] H Xiao Z Ai L Zhang JPhys Chem 113 (2009) 16625

[8] L Combemale G Caboche D Stuerga D Chaumont MicroRese

Bull 40 (2005) 529

[9] M Zawadzki Jof Alloys and Comp 454 (2008) 347

[10] A V Patil C G Dighavkar S K Sonawane S J Patil R Y Borse

Sens and Transducers 108 (2009) 189

[11] AV Patil CG Dighavkar RY Borse Sens and Transducers

101 (2009) 96

[12] AV Patil CG Dighavkar SK Sonawane S J Patil RY Borse Sensand

Transducers 9 (2010) 11

[13] DR Patil LA Patil Talanta 77 (2009)1409

[14] R S Blackburn Environ Sci Tech 38 (2004) 4905

[15] GNaja CMustin JBerthelin BVolesky J Colloide Inter Sci

292 (2005) 537

[16] VKGupta IAli Environ Sci Tech 42 (2008) 766

[17] A Mittal VK Gupta A Malviya and JMittal J Hazard Mater

151 (2008) 821

[18] VKGupta IAli VKSaini J Colloid Interface Sci 315(2007) 87

Chapter ndash V

183

[19] S S Deshpande S U Sonavane R V Jayaram Cat Com 9 (2008) 639

[20] KM Garadkar BS Shirke YB Patil D R Patil Sens and Transducers

110 (2009) 17

[21] VKGupta AMittal VGajbe JMittal J Colloid Inter Sci

319 (2008) 30

[22] T A Khan I Ali V V Singh S Sharma J Envir Protec Sci 3 (2009)11

[23] A Hosseinnia M Keyanpour-Rad M Pazouki World Applied Sci J

8 (2010) 1327

[24] MI Baraton L Merhari J Am Ceram Soc 75 (2004) 1587

[25] CWang B Xu XWang J Zhao J Solid State Chem 178 (2005) 3500

[26] H Xiao Z Ai L Zhang J Phys Chem C doi 10 1021 JP9050269 (2009)

[27] A M Thompson Oxides of the Rare Earths Wiley New York (1978)

[28] H Noumlremberg G A D Briggs Phys Rev Lett 79 (1997) 4222

[29] A Hosseinnia M Keyanpour-Rad M Pazouki World Appl Sci J

8 (2010) 1327

[30] BD Cullity SR Stock Elements of X-Ray Diffraction 3rd edn

(Prentice Hall Upper Saddle River NJ) (2001) 07458

[31] G Ranga Rao B Gopal Mishra Bul Cat Soc of Ind 2 (2003)122

[32] A K Bhattacharyal T K Naiya S N Mondal S K Das Chem Eng J

137 (2008) 529

[33] G Liu J A Rodriguez Z Chang J Hrbek C H F Peden

J Phys Chem B 108 (2004) 2931

[34] P F Ji J L Zhang F Chen M Anpo J Phys Chem C 112 (2008) 17809

[35] A Trovarelli Catal Rev SciEng 38 (1996) 439

[36] B Li D M Pan J S Zheng Y J Cheng X Y Ma F Huang L Zhang

Langmuir 24 (2008) 9630

[37] M Fernaacutendez-Garcıacutea A A B Martıacutenez-Arias A Hungrıacutea Iglesias-Juez

J C Conesa J Soria J Phys Chem Chem Phys 4 (2002) 2473

[38] H M Chen J H He J Phys Chem C 112 (2008) 17540

Chapter ndash V

184

[39] F Zhang S Chan W Spanier J E Apak E Jin Q Robinson

R D Herman I P Appl Phys Lett 80 (2002) 127

[40] G Adachi TMasui In Catalysis by Ceria and Related Materials

ATrovarelli Ed Imperial College Press London (2002)

[41] J A Rodriguez X Wang J C Hanson G Liu A Iglesias-Juez

MFernaacutendez-Garcıacutea J Chem Phys 119 (2003) 5659

[42] S Eck C Castellarin-Cudia S Surnev M G Ramsey F PNetzer

Surf Sci 520 (2002) 173

[43] R B Yu L Yan P Zheng JChen X X R J Phys ChemC

112 (2008) 19896

[44] Y Nagai T Yamamoto T Tanaka SYoshida T Nonaka T Okamoto

A Suda M Sugiura Catal Today 74 (2002) 225

[45] Y M Chiang E B Lavik I Kosacki H L Tuller JY Ying

J Electroceram 1 (1997) 7

[46] D R Patil L A Patil G H Jain M S Wagh S A Patil

Sens and Transducers 74 (2006) 874

[47] K Sohlberg S K Pantelides S J Pennycook J Am Chem Soc

123 (2001) 6609

[48] L S Zhong J S Hu A M Cao Q Liu W G Song LWan

J Chem Mater 19 (2007) 1648

[49] J B Fei Y Cui X H Yan Y Yang K W Wang QHe J B Li

AdV Mater 20 (2008) 452

[50] P V Kamat J Phys Chem B 106 (2002) 7729

[51] R D Robinson J E Spanier F Zhang SW Chan I P Herman J Appl

Phys 92 (2002) 1936

[52] S W Cao Y J Zhu J Phys Chem C 112 (2008) 6253

[53] R Georgekutty M K Seery S C Pillai J Phys Chem C

112 (2008) 13563

[54] J S Hu L S Zhong W G Song L J Wan Adv Mater 20 (2008)1

Chapter ndash V

185

[55] HXu F L Jia Z H Ai L Z Zhang Cryst Growth Des (2007)

[56] H M Jia W J Xiao L Z Zhang Z F Zheng J PhysChem C

112 (2008) 1379

[57] Z H Ai H Y Xiao T Mei J Liu L Z Zhang K J Deng J R Qiu

J PhysChem C 112 (2008) 11929

[58] L M Ma W X Zhang EnViron Sci Technol 42 (2008) 5384

[59] Z H Ai Y Chen L Z Zhang J R Qiu EnViron Sci Technol

42 (2008) 69550

[60] Z Y Zhong J Ho J Teo S C Shen AGedanken ChemMater

19 (2007) 4776

[61] S C Kuiry S D Patil S Deshpande S Seal J Phys ChemB

109 (2005) 6936

[62] D R Patil L A Patil Sens IEEE 7 (2007) 434

[63] P Jasinski Anderson H U Sens Actuators B 95 (2003) 73

[64] J P Nair E Wachtel I Lubomirsky J Fleig JMaier AdVMater

15 (2003) 2077

[65] R J Qi Y J Zhu G F Cheng Y H Huang Nanotechnology

16 (2005) 2502

Chapter ndash V

180

should be their to facilitate the adsorption of Rh B The second one is attributed to

the oxygen vacancies on the surface of CeO2 nanocrystals These abundant surface

oxygen vacancies could produce strong electrostatic attraction with the cationic

groups of Rh B as well as hydrogen bonding with the nitrogen atoms of Rh B

Chapter ndash V

181

Conclusions

Microwave synthesis is very beneficial to find a fast simple and energy

efficient approach to produce fine CeO2 nanoparticles It is relatively new method

to produce inorganic compounds for materials processing to enhance the material

properties as well as economic advantages through energy saving and acceleration

of product development Cerium oxide has outstanding physical and chemical

properties therefore it is used as LPG sensor In the area of catalysis nanoparticles

of ceria have been studied Also it is concluded as

1] The CeO2 nanoparticles were prepared by simple microwave assisted sol-gel

method

2] The well spherical and narrow size distribution with 10 nm size particles were

obtained by this method

3] It is very simple time as well as energy saving technique

4] The XRD pattern shows the CeO2 nanoparticles exists in face centered cubic

5] Scanning electron micrographs indicates that grains are uniformaly distributed

6] Transmission electron micrographs indicates that the mean diameter of the

particles as measured is about 10 nm

7] The conductivity of nano CeO2 thick film is represents the almost linear

relationship was found in the measured temperature range of 200-5000C It is

clear that the conduction in crystalline ceria is essentially an n-type electronic

process

8] The CeO2 nanoparticles in the thick film form prepared by screen printing

technique showed better response to LPG gas at 250degC temperature at 600 ppm

concentration of LPG gas

9] Catalyst can be prepared from inexpensive precursors using the microwave

assisted sol-gel technique is promising for the oxidative regeneration of ketones

10] This study reveals that these CeO2 nanocrystals are novel adsorbents for

wastewater treatment

Chapter ndash V

182

References

[1] A Ramesh H Hasegawa T Maki KUeda Sep Purif Tech

56 (2007) 90

[2] C Lettmann K Hildenbrand H Kisch W Macyk W F Maier

Appl Catal B 32 (2001) 215

[3] X Zhang Z H Ai F L Jia L Z Zhang J Phys Chem C 112 (2008) 747

[4] N Du H Zhang BD Chen X Y Ma D RYang J Phys ChemC

111 (2007) 12677

[5] F Zhang S Yang H Chen X Yu Cera Intern 30 (2004) 997

[6] F Gu SF Wang MK Lu GJ Zhou D Xu DR Langmuir J SciIR Iran

20 (2004) 3528

[7] H Xiao Z Ai L Zhang JPhys Chem 113 (2009) 16625

[8] L Combemale G Caboche D Stuerga D Chaumont MicroRese

Bull 40 (2005) 529

[9] M Zawadzki Jof Alloys and Comp 454 (2008) 347

[10] A V Patil C G Dighavkar S K Sonawane S J Patil R Y Borse

Sens and Transducers 108 (2009) 189

[11] AV Patil CG Dighavkar RY Borse Sens and Transducers

101 (2009) 96

[12] AV Patil CG Dighavkar SK Sonawane S J Patil RY Borse Sensand

Transducers 9 (2010) 11

[13] DR Patil LA Patil Talanta 77 (2009)1409

[14] R S Blackburn Environ Sci Tech 38 (2004) 4905

[15] GNaja CMustin JBerthelin BVolesky J Colloide Inter Sci

292 (2005) 537

[16] VKGupta IAli Environ Sci Tech 42 (2008) 766

[17] A Mittal VK Gupta A Malviya and JMittal J Hazard Mater

151 (2008) 821

[18] VKGupta IAli VKSaini J Colloid Interface Sci 315(2007) 87

Chapter ndash V

183

[19] S S Deshpande S U Sonavane R V Jayaram Cat Com 9 (2008) 639

[20] KM Garadkar BS Shirke YB Patil D R Patil Sens and Transducers

110 (2009) 17

[21] VKGupta AMittal VGajbe JMittal J Colloid Inter Sci

319 (2008) 30

[22] T A Khan I Ali V V Singh S Sharma J Envir Protec Sci 3 (2009)11

[23] A Hosseinnia M Keyanpour-Rad M Pazouki World Applied Sci J

8 (2010) 1327

[24] MI Baraton L Merhari J Am Ceram Soc 75 (2004) 1587

[25] CWang B Xu XWang J Zhao J Solid State Chem 178 (2005) 3500

[26] H Xiao Z Ai L Zhang J Phys Chem C doi 10 1021 JP9050269 (2009)

[27] A M Thompson Oxides of the Rare Earths Wiley New York (1978)

[28] H Noumlremberg G A D Briggs Phys Rev Lett 79 (1997) 4222

[29] A Hosseinnia M Keyanpour-Rad M Pazouki World Appl Sci J

8 (2010) 1327

[30] BD Cullity SR Stock Elements of X-Ray Diffraction 3rd edn

(Prentice Hall Upper Saddle River NJ) (2001) 07458

[31] G Ranga Rao B Gopal Mishra Bul Cat Soc of Ind 2 (2003)122

[32] A K Bhattacharyal T K Naiya S N Mondal S K Das Chem Eng J

137 (2008) 529

[33] G Liu J A Rodriguez Z Chang J Hrbek C H F Peden

J Phys Chem B 108 (2004) 2931

[34] P F Ji J L Zhang F Chen M Anpo J Phys Chem C 112 (2008) 17809

[35] A Trovarelli Catal Rev SciEng 38 (1996) 439

[36] B Li D M Pan J S Zheng Y J Cheng X Y Ma F Huang L Zhang

Langmuir 24 (2008) 9630

[37] M Fernaacutendez-Garcıacutea A A B Martıacutenez-Arias A Hungrıacutea Iglesias-Juez

J C Conesa J Soria J Phys Chem Chem Phys 4 (2002) 2473

[38] H M Chen J H He J Phys Chem C 112 (2008) 17540

Chapter ndash V

184

[39] F Zhang S Chan W Spanier J E Apak E Jin Q Robinson

R D Herman I P Appl Phys Lett 80 (2002) 127

[40] G Adachi TMasui In Catalysis by Ceria and Related Materials

ATrovarelli Ed Imperial College Press London (2002)

[41] J A Rodriguez X Wang J C Hanson G Liu A Iglesias-Juez

MFernaacutendez-Garcıacutea J Chem Phys 119 (2003) 5659

[42] S Eck C Castellarin-Cudia S Surnev M G Ramsey F PNetzer

Surf Sci 520 (2002) 173

[43] R B Yu L Yan P Zheng JChen X X R J Phys ChemC

112 (2008) 19896

[44] Y Nagai T Yamamoto T Tanaka SYoshida T Nonaka T Okamoto

A Suda M Sugiura Catal Today 74 (2002) 225

[45] Y M Chiang E B Lavik I Kosacki H L Tuller JY Ying

J Electroceram 1 (1997) 7

[46] D R Patil L A Patil G H Jain M S Wagh S A Patil

Sens and Transducers 74 (2006) 874

[47] K Sohlberg S K Pantelides S J Pennycook J Am Chem Soc

123 (2001) 6609

[48] L S Zhong J S Hu A M Cao Q Liu W G Song LWan

J Chem Mater 19 (2007) 1648

[49] J B Fei Y Cui X H Yan Y Yang K W Wang QHe J B Li

AdV Mater 20 (2008) 452

[50] P V Kamat J Phys Chem B 106 (2002) 7729

[51] R D Robinson J E Spanier F Zhang SW Chan I P Herman J Appl

Phys 92 (2002) 1936

[52] S W Cao Y J Zhu J Phys Chem C 112 (2008) 6253

[53] R Georgekutty M K Seery S C Pillai J Phys Chem C

112 (2008) 13563

[54] J S Hu L S Zhong W G Song L J Wan Adv Mater 20 (2008)1

Chapter ndash V

185

[55] HXu F L Jia Z H Ai L Z Zhang Cryst Growth Des (2007)

[56] H M Jia W J Xiao L Z Zhang Z F Zheng J PhysChem C

112 (2008) 1379

[57] Z H Ai H Y Xiao T Mei J Liu L Z Zhang K J Deng J R Qiu

J PhysChem C 112 (2008) 11929

[58] L M Ma W X Zhang EnViron Sci Technol 42 (2008) 5384

[59] Z H Ai Y Chen L Z Zhang J R Qiu EnViron Sci Technol

42 (2008) 69550

[60] Z Y Zhong J Ho J Teo S C Shen AGedanken ChemMater

19 (2007) 4776

[61] S C Kuiry S D Patil S Deshpande S Seal J Phys ChemB

109 (2005) 6936

[62] D R Patil L A Patil Sens IEEE 7 (2007) 434

[63] P Jasinski Anderson H U Sens Actuators B 95 (2003) 73

[64] J P Nair E Wachtel I Lubomirsky J Fleig JMaier AdVMater

15 (2003) 2077

[65] R J Qi Y J Zhu G F Cheng Y H Huang Nanotechnology

16 (2005) 2502

Chapter ndash V

181

Conclusions

Microwave synthesis is very beneficial to find a fast simple and energy

efficient approach to produce fine CeO2 nanoparticles It is relatively new method

to produce inorganic compounds for materials processing to enhance the material

properties as well as economic advantages through energy saving and acceleration

of product development Cerium oxide has outstanding physical and chemical

properties therefore it is used as LPG sensor In the area of catalysis nanoparticles

of ceria have been studied Also it is concluded as

1] The CeO2 nanoparticles were prepared by simple microwave assisted sol-gel

method

2] The well spherical and narrow size distribution with 10 nm size particles were

obtained by this method

3] It is very simple time as well as energy saving technique

4] The XRD pattern shows the CeO2 nanoparticles exists in face centered cubic

5] Scanning electron micrographs indicates that grains are uniformaly distributed

6] Transmission electron micrographs indicates that the mean diameter of the

particles as measured is about 10 nm

7] The conductivity of nano CeO2 thick film is represents the almost linear

relationship was found in the measured temperature range of 200-5000C It is

clear that the conduction in crystalline ceria is essentially an n-type electronic

process

8] The CeO2 nanoparticles in the thick film form prepared by screen printing

technique showed better response to LPG gas at 250degC temperature at 600 ppm

concentration of LPG gas

9] Catalyst can be prepared from inexpensive precursors using the microwave

assisted sol-gel technique is promising for the oxidative regeneration of ketones

10] This study reveals that these CeO2 nanocrystals are novel adsorbents for

wastewater treatment

Chapter ndash V

182

References

[1] A Ramesh H Hasegawa T Maki KUeda Sep Purif Tech

56 (2007) 90

[2] C Lettmann K Hildenbrand H Kisch W Macyk W F Maier

Appl Catal B 32 (2001) 215

[3] X Zhang Z H Ai F L Jia L Z Zhang J Phys Chem C 112 (2008) 747

[4] N Du H Zhang BD Chen X Y Ma D RYang J Phys ChemC

111 (2007) 12677

[5] F Zhang S Yang H Chen X Yu Cera Intern 30 (2004) 997

[6] F Gu SF Wang MK Lu GJ Zhou D Xu DR Langmuir J SciIR Iran

20 (2004) 3528

[7] H Xiao Z Ai L Zhang JPhys Chem 113 (2009) 16625

[8] L Combemale G Caboche D Stuerga D Chaumont MicroRese

Bull 40 (2005) 529

[9] M Zawadzki Jof Alloys and Comp 454 (2008) 347

[10] A V Patil C G Dighavkar S K Sonawane S J Patil R Y Borse

Sens and Transducers 108 (2009) 189

[11] AV Patil CG Dighavkar RY Borse Sens and Transducers

101 (2009) 96

[12] AV Patil CG Dighavkar SK Sonawane S J Patil RY Borse Sensand

Transducers 9 (2010) 11

[13] DR Patil LA Patil Talanta 77 (2009)1409

[14] R S Blackburn Environ Sci Tech 38 (2004) 4905

[15] GNaja CMustin JBerthelin BVolesky J Colloide Inter Sci

292 (2005) 537

[16] VKGupta IAli Environ Sci Tech 42 (2008) 766

[17] A Mittal VK Gupta A Malviya and JMittal J Hazard Mater

151 (2008) 821

[18] VKGupta IAli VKSaini J Colloid Interface Sci 315(2007) 87

Chapter ndash V

183

[19] S S Deshpande S U Sonavane R V Jayaram Cat Com 9 (2008) 639

[20] KM Garadkar BS Shirke YB Patil D R Patil Sens and Transducers

110 (2009) 17

[21] VKGupta AMittal VGajbe JMittal J Colloid Inter Sci

319 (2008) 30

[22] T A Khan I Ali V V Singh S Sharma J Envir Protec Sci 3 (2009)11

[23] A Hosseinnia M Keyanpour-Rad M Pazouki World Applied Sci J

8 (2010) 1327

[24] MI Baraton L Merhari J Am Ceram Soc 75 (2004) 1587

[25] CWang B Xu XWang J Zhao J Solid State Chem 178 (2005) 3500

[26] H Xiao Z Ai L Zhang J Phys Chem C doi 10 1021 JP9050269 (2009)

[27] A M Thompson Oxides of the Rare Earths Wiley New York (1978)

[28] H Noumlremberg G A D Briggs Phys Rev Lett 79 (1997) 4222

[29] A Hosseinnia M Keyanpour-Rad M Pazouki World Appl Sci J

8 (2010) 1327

[30] BD Cullity SR Stock Elements of X-Ray Diffraction 3rd edn

(Prentice Hall Upper Saddle River NJ) (2001) 07458

[31] G Ranga Rao B Gopal Mishra Bul Cat Soc of Ind 2 (2003)122

[32] A K Bhattacharyal T K Naiya S N Mondal S K Das Chem Eng J

137 (2008) 529

[33] G Liu J A Rodriguez Z Chang J Hrbek C H F Peden

J Phys Chem B 108 (2004) 2931

[34] P F Ji J L Zhang F Chen M Anpo J Phys Chem C 112 (2008) 17809

[35] A Trovarelli Catal Rev SciEng 38 (1996) 439

[36] B Li D M Pan J S Zheng Y J Cheng X Y Ma F Huang L Zhang

Langmuir 24 (2008) 9630

[37] M Fernaacutendez-Garcıacutea A A B Martıacutenez-Arias A Hungrıacutea Iglesias-Juez

J C Conesa J Soria J Phys Chem Chem Phys 4 (2002) 2473

[38] H M Chen J H He J Phys Chem C 112 (2008) 17540

Chapter ndash V

184

[39] F Zhang S Chan W Spanier J E Apak E Jin Q Robinson

R D Herman I P Appl Phys Lett 80 (2002) 127

[40] G Adachi TMasui In Catalysis by Ceria and Related Materials

ATrovarelli Ed Imperial College Press London (2002)

[41] J A Rodriguez X Wang J C Hanson G Liu A Iglesias-Juez

MFernaacutendez-Garcıacutea J Chem Phys 119 (2003) 5659

[42] S Eck C Castellarin-Cudia S Surnev M G Ramsey F PNetzer

Surf Sci 520 (2002) 173

[43] R B Yu L Yan P Zheng JChen X X R J Phys ChemC

112 (2008) 19896

[44] Y Nagai T Yamamoto T Tanaka SYoshida T Nonaka T Okamoto

A Suda M Sugiura Catal Today 74 (2002) 225

[45] Y M Chiang E B Lavik I Kosacki H L Tuller JY Ying

J Electroceram 1 (1997) 7

[46] D R Patil L A Patil G H Jain M S Wagh S A Patil

Sens and Transducers 74 (2006) 874

[47] K Sohlberg S K Pantelides S J Pennycook J Am Chem Soc

123 (2001) 6609

[48] L S Zhong J S Hu A M Cao Q Liu W G Song LWan

J Chem Mater 19 (2007) 1648

[49] J B Fei Y Cui X H Yan Y Yang K W Wang QHe J B Li

AdV Mater 20 (2008) 452

[50] P V Kamat J Phys Chem B 106 (2002) 7729

[51] R D Robinson J E Spanier F Zhang SW Chan I P Herman J Appl

Phys 92 (2002) 1936

[52] S W Cao Y J Zhu J Phys Chem C 112 (2008) 6253

[53] R Georgekutty M K Seery S C Pillai J Phys Chem C

112 (2008) 13563

[54] J S Hu L S Zhong W G Song L J Wan Adv Mater 20 (2008)1

Chapter ndash V

185

[55] HXu F L Jia Z H Ai L Z Zhang Cryst Growth Des (2007)

[56] H M Jia W J Xiao L Z Zhang Z F Zheng J PhysChem C

112 (2008) 1379

[57] Z H Ai H Y Xiao T Mei J Liu L Z Zhang K J Deng J R Qiu

J PhysChem C 112 (2008) 11929

[58] L M Ma W X Zhang EnViron Sci Technol 42 (2008) 5384

[59] Z H Ai Y Chen L Z Zhang J R Qiu EnViron Sci Technol

42 (2008) 69550

[60] Z Y Zhong J Ho J Teo S C Shen AGedanken ChemMater

19 (2007) 4776

[61] S C Kuiry S D Patil S Deshpande S Seal J Phys ChemB

109 (2005) 6936

[62] D R Patil L A Patil Sens IEEE 7 (2007) 434

[63] P Jasinski Anderson H U Sens Actuators B 95 (2003) 73

[64] J P Nair E Wachtel I Lubomirsky J Fleig JMaier AdVMater

15 (2003) 2077

[65] R J Qi Y J Zhu G F Cheng Y H Huang Nanotechnology

16 (2005) 2502

Chapter ndash V

182

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56 (2007) 90

[2] C Lettmann K Hildenbrand H Kisch W Macyk W F Maier

Appl Catal B 32 (2001) 215

[3] X Zhang Z H Ai F L Jia L Z Zhang J Phys Chem C 112 (2008) 747

[4] N Du H Zhang BD Chen X Y Ma D RYang J Phys ChemC

111 (2007) 12677

[5] F Zhang S Yang H Chen X Yu Cera Intern 30 (2004) 997

[6] F Gu SF Wang MK Lu GJ Zhou D Xu DR Langmuir J SciIR Iran

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[7] H Xiao Z Ai L Zhang JPhys Chem 113 (2009) 16625

[8] L Combemale G Caboche D Stuerga D Chaumont MicroRese

Bull 40 (2005) 529

[9] M Zawadzki Jof Alloys and Comp 454 (2008) 347

[10] A V Patil C G Dighavkar S K Sonawane S J Patil R Y Borse

Sens and Transducers 108 (2009) 189

[11] AV Patil CG Dighavkar RY Borse Sens and Transducers

101 (2009) 96

[12] AV Patil CG Dighavkar SK Sonawane S J Patil RY Borse Sensand

Transducers 9 (2010) 11

[13] DR Patil LA Patil Talanta 77 (2009)1409

[14] R S Blackburn Environ Sci Tech 38 (2004) 4905

[15] GNaja CMustin JBerthelin BVolesky J Colloide Inter Sci

292 (2005) 537

[16] VKGupta IAli Environ Sci Tech 42 (2008) 766

[17] A Mittal VK Gupta A Malviya and JMittal J Hazard Mater

151 (2008) 821

[18] VKGupta IAli VKSaini J Colloid Interface Sci 315(2007) 87

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183

[19] S S Deshpande S U Sonavane R V Jayaram Cat Com 9 (2008) 639

[20] KM Garadkar BS Shirke YB Patil D R Patil Sens and Transducers

110 (2009) 17

[21] VKGupta AMittal VGajbe JMittal J Colloid Inter Sci

319 (2008) 30

[22] T A Khan I Ali V V Singh S Sharma J Envir Protec Sci 3 (2009)11

[23] A Hosseinnia M Keyanpour-Rad M Pazouki World Applied Sci J

8 (2010) 1327

[24] MI Baraton L Merhari J Am Ceram Soc 75 (2004) 1587

[25] CWang B Xu XWang J Zhao J Solid State Chem 178 (2005) 3500

[26] H Xiao Z Ai L Zhang J Phys Chem C doi 10 1021 JP9050269 (2009)

[27] A M Thompson Oxides of the Rare Earths Wiley New York (1978)

[28] H Noumlremberg G A D Briggs Phys Rev Lett 79 (1997) 4222

[29] A Hosseinnia M Keyanpour-Rad M Pazouki World Appl Sci J

8 (2010) 1327

[30] BD Cullity SR Stock Elements of X-Ray Diffraction 3rd edn

(Prentice Hall Upper Saddle River NJ) (2001) 07458

[31] G Ranga Rao B Gopal Mishra Bul Cat Soc of Ind 2 (2003)122

[32] A K Bhattacharyal T K Naiya S N Mondal S K Das Chem Eng J

137 (2008) 529

[33] G Liu J A Rodriguez Z Chang J Hrbek C H F Peden

J Phys Chem B 108 (2004) 2931

[34] P F Ji J L Zhang F Chen M Anpo J Phys Chem C 112 (2008) 17809

[35] A Trovarelli Catal Rev SciEng 38 (1996) 439

[36] B Li D M Pan J S Zheng Y J Cheng X Y Ma F Huang L Zhang

Langmuir 24 (2008) 9630

[37] M Fernaacutendez-Garcıacutea A A B Martıacutenez-Arias A Hungrıacutea Iglesias-Juez

J C Conesa J Soria J Phys Chem Chem Phys 4 (2002) 2473

[38] H M Chen J H He J Phys Chem C 112 (2008) 17540

Chapter ndash V

184

[39] F Zhang S Chan W Spanier J E Apak E Jin Q Robinson

R D Herman I P Appl Phys Lett 80 (2002) 127

[40] G Adachi TMasui In Catalysis by Ceria and Related Materials

ATrovarelli Ed Imperial College Press London (2002)

[41] J A Rodriguez X Wang J C Hanson G Liu A Iglesias-Juez

MFernaacutendez-Garcıacutea J Chem Phys 119 (2003) 5659

[42] S Eck C Castellarin-Cudia S Surnev M G Ramsey F PNetzer

Surf Sci 520 (2002) 173

[43] R B Yu L Yan P Zheng JChen X X R J Phys ChemC

112 (2008) 19896

[44] Y Nagai T Yamamoto T Tanaka SYoshida T Nonaka T Okamoto

A Suda M Sugiura Catal Today 74 (2002) 225

[45] Y M Chiang E B Lavik I Kosacki H L Tuller JY Ying

J Electroceram 1 (1997) 7

[46] D R Patil L A Patil G H Jain M S Wagh S A Patil

Sens and Transducers 74 (2006) 874

[47] K Sohlberg S K Pantelides S J Pennycook J Am Chem Soc

123 (2001) 6609

[48] L S Zhong J S Hu A M Cao Q Liu W G Song LWan

J Chem Mater 19 (2007) 1648

[49] J B Fei Y Cui X H Yan Y Yang K W Wang QHe J B Li

AdV Mater 20 (2008) 452

[50] P V Kamat J Phys Chem B 106 (2002) 7729

[51] R D Robinson J E Spanier F Zhang SW Chan I P Herman J Appl

Phys 92 (2002) 1936

[52] S W Cao Y J Zhu J Phys Chem C 112 (2008) 6253

[53] R Georgekutty M K Seery S C Pillai J Phys Chem C

112 (2008) 13563

[54] J S Hu L S Zhong W G Song L J Wan Adv Mater 20 (2008)1

Chapter ndash V

185

[55] HXu F L Jia Z H Ai L Z Zhang Cryst Growth Des (2007)

[56] H M Jia W J Xiao L Z Zhang Z F Zheng J PhysChem C

112 (2008) 1379

[57] Z H Ai H Y Xiao T Mei J Liu L Z Zhang K J Deng J R Qiu

J PhysChem C 112 (2008) 11929

[58] L M Ma W X Zhang EnViron Sci Technol 42 (2008) 5384

[59] Z H Ai Y Chen L Z Zhang J R Qiu EnViron Sci Technol

42 (2008) 69550

[60] Z Y Zhong J Ho J Teo S C Shen AGedanken ChemMater

19 (2007) 4776

[61] S C Kuiry S D Patil S Deshpande S Seal J Phys ChemB

109 (2005) 6936

[62] D R Patil L A Patil Sens IEEE 7 (2007) 434

[63] P Jasinski Anderson H U Sens Actuators B 95 (2003) 73

[64] J P Nair E Wachtel I Lubomirsky J Fleig JMaier AdVMater

15 (2003) 2077

[65] R J Qi Y J Zhu G F Cheng Y H Huang Nanotechnology

16 (2005) 2502

Chapter ndash V

183

[19] S S Deshpande S U Sonavane R V Jayaram Cat Com 9 (2008) 639

[20] KM Garadkar BS Shirke YB Patil D R Patil Sens and Transducers

110 (2009) 17

[21] VKGupta AMittal VGajbe JMittal J Colloid Inter Sci

319 (2008) 30

[22] T A Khan I Ali V V Singh S Sharma J Envir Protec Sci 3 (2009)11

[23] A Hosseinnia M Keyanpour-Rad M Pazouki World Applied Sci J

8 (2010) 1327

[24] MI Baraton L Merhari J Am Ceram Soc 75 (2004) 1587

[25] CWang B Xu XWang J Zhao J Solid State Chem 178 (2005) 3500

[26] H Xiao Z Ai L Zhang J Phys Chem C doi 10 1021 JP9050269 (2009)

[27] A M Thompson Oxides of the Rare Earths Wiley New York (1978)

[28] H Noumlremberg G A D Briggs Phys Rev Lett 79 (1997) 4222

[29] A Hosseinnia M Keyanpour-Rad M Pazouki World Appl Sci J

8 (2010) 1327

[30] BD Cullity SR Stock Elements of X-Ray Diffraction 3rd edn

(Prentice Hall Upper Saddle River NJ) (2001) 07458

[31] G Ranga Rao B Gopal Mishra Bul Cat Soc of Ind 2 (2003)122

[32] A K Bhattacharyal T K Naiya S N Mondal S K Das Chem Eng J

137 (2008) 529

[33] G Liu J A Rodriguez Z Chang J Hrbek C H F Peden

J Phys Chem B 108 (2004) 2931

[34] P F Ji J L Zhang F Chen M Anpo J Phys Chem C 112 (2008) 17809

[35] A Trovarelli Catal Rev SciEng 38 (1996) 439

[36] B Li D M Pan J S Zheng Y J Cheng X Y Ma F Huang L Zhang

Langmuir 24 (2008) 9630

[37] M Fernaacutendez-Garcıacutea A A B Martıacutenez-Arias A Hungrıacutea Iglesias-Juez

J C Conesa J Soria J Phys Chem Chem Phys 4 (2002) 2473

[38] H M Chen J H He J Phys Chem C 112 (2008) 17540

Chapter ndash V

184

[39] F Zhang S Chan W Spanier J E Apak E Jin Q Robinson

R D Herman I P Appl Phys Lett 80 (2002) 127

[40] G Adachi TMasui In Catalysis by Ceria and Related Materials

ATrovarelli Ed Imperial College Press London (2002)

[41] J A Rodriguez X Wang J C Hanson G Liu A Iglesias-Juez

MFernaacutendez-Garcıacutea J Chem Phys 119 (2003) 5659

[42] S Eck C Castellarin-Cudia S Surnev M G Ramsey F PNetzer

Surf Sci 520 (2002) 173

[43] R B Yu L Yan P Zheng JChen X X R J Phys ChemC

112 (2008) 19896

[44] Y Nagai T Yamamoto T Tanaka SYoshida T Nonaka T Okamoto

A Suda M Sugiura Catal Today 74 (2002) 225

[45] Y M Chiang E B Lavik I Kosacki H L Tuller JY Ying

J Electroceram 1 (1997) 7

[46] D R Patil L A Patil G H Jain M S Wagh S A Patil

Sens and Transducers 74 (2006) 874

[47] K Sohlberg S K Pantelides S J Pennycook J Am Chem Soc

123 (2001) 6609

[48] L S Zhong J S Hu A M Cao Q Liu W G Song LWan

J Chem Mater 19 (2007) 1648

[49] J B Fei Y Cui X H Yan Y Yang K W Wang QHe J B Li

AdV Mater 20 (2008) 452

[50] P V Kamat J Phys Chem B 106 (2002) 7729

[51] R D Robinson J E Spanier F Zhang SW Chan I P Herman J Appl

Phys 92 (2002) 1936

[52] S W Cao Y J Zhu J Phys Chem C 112 (2008) 6253

[53] R Georgekutty M K Seery S C Pillai J Phys Chem C

112 (2008) 13563

[54] J S Hu L S Zhong W G Song L J Wan Adv Mater 20 (2008)1

Chapter ndash V

185

[55] HXu F L Jia Z H Ai L Z Zhang Cryst Growth Des (2007)

[56] H M Jia W J Xiao L Z Zhang Z F Zheng J PhysChem C

112 (2008) 1379

[57] Z H Ai H Y Xiao T Mei J Liu L Z Zhang K J Deng J R Qiu

J PhysChem C 112 (2008) 11929

[58] L M Ma W X Zhang EnViron Sci Technol 42 (2008) 5384

[59] Z H Ai Y Chen L Z Zhang J R Qiu EnViron Sci Technol

42 (2008) 69550

[60] Z Y Zhong J Ho J Teo S C Shen AGedanken ChemMater

19 (2007) 4776

[61] S C Kuiry S D Patil S Deshpande S Seal J Phys ChemB

109 (2005) 6936

[62] D R Patil L A Patil Sens IEEE 7 (2007) 434

[63] P Jasinski Anderson H U Sens Actuators B 95 (2003) 73

[64] J P Nair E Wachtel I Lubomirsky J Fleig JMaier AdVMater

15 (2003) 2077

[65] R J Qi Y J Zhu G F Cheng Y H Huang Nanotechnology

16 (2005) 2502

Chapter ndash V

184

[39] F Zhang S Chan W Spanier J E Apak E Jin Q Robinson

R D Herman I P Appl Phys Lett 80 (2002) 127

[40] G Adachi TMasui In Catalysis by Ceria and Related Materials

ATrovarelli Ed Imperial College Press London (2002)

[41] J A Rodriguez X Wang J C Hanson G Liu A Iglesias-Juez

MFernaacutendez-Garcıacutea J Chem Phys 119 (2003) 5659

[42] S Eck C Castellarin-Cudia S Surnev M G Ramsey F PNetzer

Surf Sci 520 (2002) 173

[43] R B Yu L Yan P Zheng JChen X X R J Phys ChemC

112 (2008) 19896

[44] Y Nagai T Yamamoto T Tanaka SYoshida T Nonaka T Okamoto

A Suda M Sugiura Catal Today 74 (2002) 225

[45] Y M Chiang E B Lavik I Kosacki H L Tuller JY Ying

J Electroceram 1 (1997) 7

[46] D R Patil L A Patil G H Jain M S Wagh S A Patil

Sens and Transducers 74 (2006) 874

[47] K Sohlberg S K Pantelides S J Pennycook J Am Chem Soc

123 (2001) 6609

[48] L S Zhong J S Hu A M Cao Q Liu W G Song LWan

J Chem Mater 19 (2007) 1648

[49] J B Fei Y Cui X H Yan Y Yang K W Wang QHe J B Li

AdV Mater 20 (2008) 452

[50] P V Kamat J Phys Chem B 106 (2002) 7729

[51] R D Robinson J E Spanier F Zhang SW Chan I P Herman J Appl

Phys 92 (2002) 1936

[52] S W Cao Y J Zhu J Phys Chem C 112 (2008) 6253

[53] R Georgekutty M K Seery S C Pillai J Phys Chem C

112 (2008) 13563

[54] J S Hu L S Zhong W G Song L J Wan Adv Mater 20 (2008)1

Chapter ndash V

185

[55] HXu F L Jia Z H Ai L Z Zhang Cryst Growth Des (2007)

[56] H M Jia W J Xiao L Z Zhang Z F Zheng J PhysChem C

112 (2008) 1379

[57] Z H Ai H Y Xiao T Mei J Liu L Z Zhang K J Deng J R Qiu

J PhysChem C 112 (2008) 11929

[58] L M Ma W X Zhang EnViron Sci Technol 42 (2008) 5384

[59] Z H Ai Y Chen L Z Zhang J R Qiu EnViron Sci Technol

42 (2008) 69550

[60] Z Y Zhong J Ho J Teo S C Shen AGedanken ChemMater

19 (2007) 4776

[61] S C Kuiry S D Patil S Deshpande S Seal J Phys ChemB

109 (2005) 6936

[62] D R Patil L A Patil Sens IEEE 7 (2007) 434

[63] P Jasinski Anderson H U Sens Actuators B 95 (2003) 73

[64] J P Nair E Wachtel I Lubomirsky J Fleig JMaier AdVMater

15 (2003) 2077

[65] R J Qi Y J Zhu G F Cheng Y H Huang Nanotechnology

16 (2005) 2502

Chapter ndash V

185

[55] HXu F L Jia Z H Ai L Z Zhang Cryst Growth Des (2007)

[56] H M Jia W J Xiao L Z Zhang Z F Zheng J PhysChem C

112 (2008) 1379

[57] Z H Ai H Y Xiao T Mei J Liu L Z Zhang K J Deng J R Qiu

J PhysChem C 112 (2008) 11929

[58] L M Ma W X Zhang EnViron Sci Technol 42 (2008) 5384

[59] Z H Ai Y Chen L Z Zhang J R Qiu EnViron Sci Technol

42 (2008) 69550

[60] Z Y Zhong J Ho J Teo S C Shen AGedanken ChemMater

19 (2007) 4776

[61] S C Kuiry S D Patil S Deshpande S Seal J Phys ChemB

109 (2005) 6936

[62] D R Patil L A Patil Sens IEEE 7 (2007) 434

[63] P Jasinski Anderson H U Sens Actuators B 95 (2003) 73

[64] J P Nair E Wachtel I Lubomirsky J Fleig JMaier AdVMater

15 (2003) 2077

[65] R J Qi Y J Zhu G F Cheng Y H Huang Nanotechnology

16 (2005) 2502