Advances in VO2 Thermochromic Films

Journal ofMaterials Chemistry A

FEATURE ARTICLE

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Advances in therm

Mhf(hVEBtiU2r

chapters. His research interests incelectric eld assisted chemical vapthin lms and energy efficient buil

aDepartment of Chemistry, University C

Laboratories, 20 Gordon Street, London WCbUCL Energy Institute, Central House, 14 Up

UK

Cite this: J. Mater. Chem. A, 2014, 2,3275

Received 14th October 2013Accepted 28th November 2013

DOI: 10.1039/c3ta14124a

www.rsc.org/MaterialsA

This journal is © The Royal Society of C

ochromic vanadium dioxide films

Michael E. A. Warwickab and Russell Binions*c

Vanadium dioxide is a thermochromic material that undergoes a semiconductor to metal transitions at a

critical temperature of 68 �C. This phase change from a low temperature monoclinic structure to a

higher temperature rutile structure is accompanied by a marked change in infrared reflectivity and

change in resistivity. This review presents the fundamental chemical principles that describe the

electronic structure and properties of solids, and the chronological developments in the theory behind

the thermochromic transitions such as, the effects of electron–electron interactions and structural phase

changes due to lattice distortions. An extensive discussion and observations on the current

understanding of the nature of the semiconductor-to-metal transition exhibited by vanadium dioxide is

detailed. The possibility of manipulating the transition temperature by introducing various dopants,

additional layers or by size effects into the vanadium dioxide lattice are examined. Thermochromic

vanadium dioxide materials may be exploited in areas such as microelectronics, data storage, or

intelligent architectural glazing, thus are required to be synthesised as thin films for use in such

applications. The numerous synthetic techniques (physical vapour deposition, sol–gel method, pulsed

laser deposition, chemical vapour deposition), for making metal oxide thermochromic thin films are

described in reference to the production of vanadium dioxide with a particular focus on recent results.

r Michael Warwick completedis rst degree in Chemistryrom University College LondonUCL). He is currently nishingis Ph.D. titled “New Chemicalapour Deposition Methods fornergy Efficient Glazing” in theinions Group at UCL. He willake up a postdoctoral positionn the Barreca group at theniversity of Padua in early014. He is the co-author of 15esearch papers and 2 booklude nanocomposite thin lms,our deposition, thermochromicding simulation.

Dr Russell Binions completed hisrst degree (in Chemistry) fromthe College of St Hild and StBede, the University of Durhamin 2001. He subsequentlycompleted a Ph.D. in 2005 atUniversity College London(UCL). He spent a year workingin industry for the NorvilleOptical Group before returningto UCL in autumn 2005 tocomplete postdoctoral positionsworking on thermochromic thin

lms and zeolite modied gas sensors. He was awarded a RoyalSociety Dorothy Hodgkin Fellowship in 2008. Dr Binions iscurrently a Lecturer in Functional in the School of Engineering andMaterials at Queen Mary, University of London and an HonorarySenior Research Associate at UCL. He is the author of over 70 peerreviewed journal papers, 6 book chapters and 1 book. His researchinterests encompass new chemical vapour deposition techniques,metal oxide semiconductor materials, gas sensors, photocatalysis,chromogenic materials, nanocomposite lms and energy efficientbuilding materials. In his spare time he enjoys all aspects of music.

ollege London, Christopher Ingold

1H 0AJ, UK

per Woburn Place, London, WC1H 0NN,

cSchool of Engineering and Materials Science, Queen Mary University of London,

Mile End Road, London E1 4NS, UK

hemistry 2014 J. Mater. Chem. A, 2014, 2, 3275–3292 | 3275

http://dx.doi.org/10.1039/c3ta14124a

http://pubs.rsc.org/en/journals/journal/TA

http://pubs.rsc.org/en/journals/journal/TA?issueid=TA002010

Journal of Materials Chemistry A Feature Article

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Introduction

“Chromogenic”materials are those that exhibit changes in theiroptical properties due to some external stimulus. The mostcommon of these are photochromic, thermochromic and elec-trochromic materials, where the stimuli are irradiation by light(photons), change in temperature, and an applied electric eld,respectively. Thus, a thermochromic material changes colourupon reaching a characteristic ‘transition temperature’. Broadlyspeaking, thermochromism is the temperature-dependantchanges in the optical properties of a material. Typically, thethermochromic effect occurs over a range of temperatures,which is observed as a gradual colour change, i.e. continuousthermochromism. Discontinuous thermochromism involves astructural phase change at a specic transition temperature.This phase change can be rst- or second-order in nature, andmay be reversible or irreversible, as governed by the thermo-dynamics of the system.1 Thermochromism offers potential fortechnological applications, for example, in thermometers (feverindicators, gadgets, design applications, etc.), temperaturesensors for safety, laser marking, or warning signals. As well asinorganic oxides, many different compounds, for instance,liquid crystals,2 conjugated oligomers,3 leuco dyes,4 are allcommonly known to exhibit the ability to reversibly changecolour with temperature. However, thermochromic dyes areusually based on organic compounds, which show irreversiblecolour changes on heating.

Windows

The world's energy consumption is continuously increasing andthis creates a heavy demand for renewable energy sources to bedeveloped. The emission of carbon dioxide and other pollutantgases are posing a problem not only to the environment but tohuman health as well.5 This coupled with the increase of thegeneral standard of living has shown an increase in demand ofenergy. Buildings are said to be responsible for about 40% ofthe worlds total annual energy consumption due to the exces-sive use of lighting, air-conditioning and heating.6 A way inwhich this can be reduced is to use thin lm coatings onbuilding glazing in order to limit the amount of solar radiationentering or blackbody radiation leaving a building. Transparent

Fig. 1 Schematic of thermochromic behaviour.

3276 | J. Mater. Chem. A, 2014, 2, 3275–3292

conducting oxides (TCOs) and thin metallic coatings are usefulin solar applications because they are transparent in the specicrange of 400–700 nm.5,7 As a consequence of their metallicproperties they reect in the infrared but absorb in the ultra-violet. If the reectance occurs in the interval of 3000 < l <50 000 nm i.e. the range for thermal radiation at ambienttemperature, the emission of heat is hindered. If reectanceoccurs at 700 < l < 3000 nm then visible transmission iscombined with solar energy as well as low thermal emittance.Another important class of materials for solar control applica-tions is those that show chromogenic properties. By consider-ation of ambient temperature solar materials can be designedin order to achieve the desired properties7. These includematerials with high electrical conductivity and the combinationof high solar absorbance or transmittance with low thermalemittance; these properties are useful for low-emittancewindows coatings. Materials with high transmittance of ultra-violet radiation but the complete blockage of infrared solarradiation can be used for ‘solar control’ windows. Solar controland low-emittance are only practical in climates that do not varymuch from season to season. However, in parts of the worldwhere there are changeable climates such as the United States,Japan and Northern Europe, the use of chromogenic materials,such as thermochromic vanadium dioxide, can be used for‘smart windows’ as their properties allow them to adapt to thechanging environment and provide an energy benet all yearround.5 This has lead to signicant recent work on ‘chromo-genic’ materials.8,9 Thin lms which exhibit properties such asPhotochromism (TiO2 and MoO3 based lms10), thermo-chromism (VO2, V2O3 (ref. 11 and 12)) and electrochromism(NiO, WO3 (ref. 13 and 14)) has been in the forefront of thiswork. These oxide-based thin lms have now been producedthrough various experimental techniques and used as glasscoatings. We discuss the use of thermochromic vanadiumdioxide further in this review.

This can be visualised in Fig. 1, where the behaviour of athermochromic window is schematically shown. For tempera-tures below the transition, the material has a low infraredreectance; consequently, the solar radiation and associatedheat will reach the interior of the building, as it will not bereected. For T > Tc, the opposite is true, part of the energy fromthe sun will be reected, due to the higher material's infrared

This journal is © The Royal Society of Chemistry 2014


Fig. 2 The change in activation energy (3) versus reciprocal latticeconstant (1/b). a shows a continuous change, where as b shows adiscontinuous change.

Feature Article Journal of Materials Chemistry A

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reectance. In this way, the heat gain from the solar radiationwill be high in winter and much lower in summer reducing theneed for cooling and heating respectively.

For the thermochromic glazing to be effective in energysaving, the thermochromic material should have some speciccharacteristics. The thermochromic coating should ideally betransparent in the visible region, to allow all the light from thesun to reach the interior of the building. In this way, there willno (or very little) reduction in the visibility, with no necessity foradditional articial lighting. Moreover, no signicant change inthe visibility should be associated with the phase change.

The change of the optical properties in the infrared region,on the contrary, should be as high as possible. With a muchhigher reectance for T > Tc, for instance, less solar energy willreach the inside of the building; however, if the increase in thereectance is lower, the process will be less effective.

The value of the transition temperature is also anotherimportant parameter; indeed, for the material to be used on aglassy window as solar controller, the change in the opticalproperties should take place for temperature values close toroom temperature, ideally between 20 and 25 �C in order tomaximise the time the coating spends in the higher tempera-ture, reective state.

A brief history of semiconductor-to-metal transitions

The non-interacting, free-electron model of the electronicstructure of solids by Wilson and Fowler in 1931 (ref. 15 and 16)successfully describes the distinction between metals and non-metals at absolute zero. It was acknowledged that insulatorswith small energy gaps between the highest lled band andlowest empty band would be semiconductors upon thermalexcitation of the electrons.

However, the ndings of transition-metal oxides existing asinsulators by de Boer and Verwey (1937)17 despite exhibitingpartially lled 3d bands, conrms the inadequacy of simpleband theory due to the neglect of electron repulsion. Followingthis nding, Peierls (1937)18 then pointed out the importance ofthe electron–electron correlation, stating that strong Coulombicrepulsion between the electrons could be the origin of theinsulating behaviour, where at low temperatures, the electronsare in their “proper” positions. Thermal excitation is required tocross the potential barrier for the movement of electrons intothe metallic, conducting state. Wigner in 1938 (ref. 19) intro-duced electron–electron interactions and suggested that a freeelectron gas should ‘crystallize’ in a non-conducting state. Mottconrmed this in 1949 (ref. 20) and suggested that electronsbecome localized by the Coulombic repulsion between two 3delectrons of opposite spin on the same ion. He described asemiconductor-to-metal transition by imagining a crystallinearray of one-electron atoms with a variable lattice constant, b. Atlarge values of b the material would be insulating, and at smallvalues it would be metallic. Thus, b has a critical value, b0, atwhich a transition occurs. If b is larger than b0, an activationenergy is required to form a pair of carriers. As b becomessmaller, the activation energy decreases. This drop in activationenergy is a discontinuous transition (Fig. 2), because an


electron and positive hole can form a pair owing to theirCoulombic attraction and mutual potential energy. In 1961,Mott proposed further that the transition between an insulatingground state and the conducting ground state, using a bandapproach, occurs sharply at b0 for each material. This is knownas the “Mott transition”.21

Mott's hypothesis has come to be known as the short-range,one band model. Hubbard22 developed the theory in 1963,where inter-ionic, long-range Coulombic interactions are alsoneglected and are only important when the electrons are on thesame atom. Hence, by quantitatively treating the Mott transi-tion, he found that at a critical ratio of the band-width to theintra-ionic Coulomb energy, the energy gap due to electroniccorrelations in the partially lled narrow bands had reduced tozero and thus an insulator-to-metal transition occurred.

A different approach using a two band model was proposedby Slater in 1951.23 He found that the insulating properties ofthe ground state in antiferromagnetic transition-metalcompounds with large values of b can be explained bysupposing the d-band splits at the Neel temperature, asobserved in NiS, which allows all bands to be full or empty.24

Therefore, it is antiferromagnetic ordering that leads to theinsulating nature of the ground state, but it is not clear why theinsulating property does not disappear above the Neel temper-ature. When the two bands are present (lower and upper Hub-bard bands), there is a band-crossing transition fromantiferromagnetic metal to antiferromagnetic insulator. Arelated transition from the antiferromagnetic metal to the‘normal’ metal was rst observed in vanadium oxides byBrinkman and Rice in 1970.25

Transition-metal oxides such as Ti2O3, V2O3, VO2, and VO areall semi-conducting at low temperatures and show a transitioninto a metallic state at the Neel temperature. The electricalproperties of these oxides were thoroughly studied usingthermo-conductive studies by Morin in 1959.26 One of thecomplications in studying these materials is the difficulty ingrowing pure, stoichiometric single crystals. He found that alllower oxides of titanium and vanadium exhibit this behaviour(Fig. 3) except for TiO, which is metallic over the entiretemperature range.

Morin attempted to explain the discontinuities in conduc-tivities of the oxides by adapting Slater's two-band model. The

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Fig. 3 Taken from26 shows a plot of conductivity versus reciprocaltemperature for the lower oxides of vanadium and titanium.


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transition was originally named a metal-to-insulator transitiondue to the changes in conductive properties of the materials,but other properties of these materials soon became apparentand it was therefore re-labelled as a metal-to-semiconductortransition.27,28 For this reason, the semiconductor-to-metaltransition due to the 3d band splitting (arising from antiferro-magnetism) would be expected at the Neel temperature. TheSlater–Morin theory has never been quantitatively applied to theoxides of vanadium and titanium, which is only one of thetheory's limitations. Another problem with the theory is thatantiferromagnetism had only been conrmed to exist in Ti2O3,29

which was found to be extremely small. Additionally, thestructure of the degenerate 3d bands and the antiferromagneticsplitting in attempt to explain how TiO, or VO could be insu-lating at 0 K, could not be modelled.

Not all semiconductor-to-metal transitions are the result ofelectron–electron interactions. The nature of the transition isnot entirely inexplicable in the theory of non-interacting elec-trons, since changes in crystal structure may also lead to theformation of a band gap. Their optical properties also showedlarge decreases in transmittance and increases in reectance onpassing through the transition temperature. In 1967–8, Adlerand Brooks30,31 found that transition-metal oxides can be insu-lators, semiconductors, metals or undergo metal to non-metaltransitions. They also discovered that band theory can beadapted to explain most of the materials that exhibit metal to

3278 | J. Mater. Chem. A, 2014, 2, 3275–3292

non-metal transitions. It was found that a lattice distortionoccurs at the transition point, causing an energy gap betweenoccupied and empty states. As the temperature is raised, theenergy gap between the valence and conduction bandsdecreases linearly with the number of electrons excited acrossthe gap. This results in rapid disappearance of the distortion,and hence the band gap. The material is then metallic. Thephase transition can be rst- or second-order in nature, whichdepends on themagnitude of the relative change in the gap withthe number of excited carriers.30 The ‘order’ of a phase transi-tion is classied by considering the thermodynamic potential,(for example the Gibbs free energy surface, G) and its derivativesat the transition. During a phase transition, the free energy ofthe solid remains continuous, but thermodynamic quantities,such as entropy, volume and heat capacity exhibit discontin-uous change. If the rst derivative of G, with respect to a ther-modynamic variable, is discontinuous the transition is called‘rst-order’. If the rst derivatives are continuous, but secondderivatives (at least one) exhibit discontinuities, the transitionis classed at ‘second-order’, for instance, a critical point on aphase diagram. In a rst-order transition where the G(p,T)surfaces of the initial state and nal state intersect sharply, theentropy and the volume show singular behaviour, thus there is alatent heat. Alternatively, in second-order transitions, the heatcapacity, compressibility, or thermal expansivity shows singularbehaviour, and so do not involve a latent heat, since the changeis entropy is continuous.32 Landau introduced the concept of anorder parameter, x, which is a measure of the order that resultsfrom a phase transition. In a rst-order transition, the change inx is discontinuous, but in a second-order transition, i.e., thechange of state is continuous, therefore the change in x is alsocontinuous. Landau postulated that in a second-order (orstructural) phase transition, G is not only a function of p and Tbut also of x. He consequently expanded G as a series of powersof x around the transition point, where the order parameter isseen to disappear at the critical temperature, Tc. Furthermore,Landau regarded the simultaneous symmetry changes from aphase of high symmetry to low symmetry during phase transi-tions, have an associated order parameter.

Adler and Brooks postulated two models for the likelymechanism of the semiconductor-to-metal transition. Onesuggesting that the band gap results from the splitting of therst Brillouin zone33 by an antiferromagnetic exchange inter-action, giving a second-order transition. In the Linear Combi-nation of Atomic Orbitals (LCAO) approach to band theory, therst Brillouin zone gives the range of wave vectors, k, that arenecessary in order to generate all possible, distinguishableBloch34 sums of atomic orbitals. Periodic arrangements ofatoms must satisfy the Bloch wave function eqn (1), constructedfrom the overlapping of atomic orbitals.

J(x) ¼ exp(ikx)u(x) (1)

where u(x) is any function that is periodic and must not bealtered when moving from one atom to another in a lattice. Theexp(ikx) term adjusts the amplitude of the wave. All functionshave a wave-like form, thus a crystal orbital has a wavelength




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determined by quantum number, k, also known as the wavenumber (or vector), where wavelength, l ¼ 2p/k. Since proper-ties such as conductivity depend on the motion of electron incrystals, the Bloch function equation is also applied in the free-electron theory, where k is proportional to the momentum of anelectron.

The other model suggests the band gap results from a crys-talline-structure distortion, in terms of the pairing ions in a one-dimensional crystal, to lower symmetry. This gives a rst-ordertransition. The band gap formed must decrease with thermalexcitation, and at a given temperature a transition to a metallicstate occurs. Generally, either rst- or second-order transitionsare possible for antiferromagnets and distorted crystals,although the former is more likely to undergo a second-ordertransition than a distorted crystal. The ndings of Alder andBrooks' study helped to explain the results of Morin's study onthe conductivities of titanium and vanadium oxides bydescribing the following: when the free energy of ametallic statefalls below the local minimum for the semiconducting state, arst-order transition will result. However, the continual exis-tence of the local minimum up to the second-order transitiontemperature could lead to a metastable state. Experimentally,this local minimum of free energy would appear as a hysteresiswhen the material is heated in the semiconducting state. Thehysteresis would not occur when the transition is second-order.Hence, the order of the transition can clearly be determineddirectly from Morin's electrical conductivity data (Fig. 3), indi-cating that V2O3, VO and VO2 undergo rst-order transitions,while Ti2O3 exhibits a second-order transition.

Fig. 4 The rutile structure of VO2, when T > Tc. The large red circlesrepresent V4+ ions, and the small blue circles are O2� ions.44

Vanadium dioxide

Transition-metal oxides are the most studied solid-state ther-mochromic materials, since the discovery of the phase transi-tions that occur at a critical temperature, Tc, where an abruptchange in optical and electronic properties is observed, makingthem ideal subjects for an investigation of non-metallic andmetallic states. Adler31 initially found that the main obstaclewhen studying these materials was the difficulty in growingpure, stoichiometric single crystals, and therefore found thatthe original electrical measurements obtained were not linkedto the intrinsic properties of the materials but due to the largeconcentrations of lattice defects and impurities. Owing to theimprovements and developments of crystal growth techniquesand equipment at the time of investigation, pure stoichiometriccrystalline transition metal oxides (and suldes) were classiedas metals or semiconductors and insulators that can undergosemiconductor-to-metal transitions. Accordingly, metals arecharacterised by a low resistivity (10�2 to 10�6 U cm) at roomtemperature, and as the temperature increases, the resistivityincreases linearly. Semiconductors and insulators have higherand large ranging resitivities (103–1017 U cm) at room temper-ature, which decreases exponentially with rising temperature.31

Several vanadium oxides display thermochromic properties.These materials are exploited in several technological applica-tions, such as electrical and optical switching devices along withseveral others.35,36 The solid-state physics of vanadium oxides is


focused around metal–insulator transitions and phase transi-tions as a function of temperature. These thermochromicmaterials display extraordinary electronic, structural, andmagnetic behaviour and are still a matter of some debate withrespect to theoretical remarks, particularly VO2 and V2O3.37–41

Vanadium(IV) oxide is by far the most studied solid-statethermochromic material. It shows great promise for use inapplications, such as “intelligent” architectural glazing. A singlepure crystal of VO2 has a semiconductor-to-metal transitiontemperature of 341 K (68 �C).42 There is a correspondingstructural phase change upon passing Tc, from the lowtemperature monoclinic crystal structure to the high tempera-ture rutile, tetragonal-type lattice.42,43

The high temperature rutile structure of metallic VO2 isbased on a simple tetragonal lattice (space group P42/mnm)(Fig. 4). The vanadium atoms are located at the equidistantWyckoff positions (4f), (0, 0, 0) and (1/2, 1/2, 1/2) and each Vatom is surrounded by an edge-sharing octahedron of oxygenatoms, VO6, which occupy the positions at�(u, u, 0) and�(1/2 +u, 1/2 � u, 1/2).

The low temperature semiconducting phase of VO2 belongsto the monoclinic crystal system (space group P21/c) (Fig. 5). At25 �C the lattice has unit cell parameters; a ¼ 5.75 A, b ¼ 4.52 A,c ¼ 5.38 A and b ¼ 122.60�. The lattice is the result of thedistortion and doubling in size of the high temperature metallictetragonal phase. The structure involves V4+–V4+ pairing withalternate shorter (0.265 nm) and longer (0.312 nm) V4+–V4+

distances along themonoclinic a axis, and tilting with respect tothe rutile c-axis. The pure VO2 phase is referred to as M1, sincedoping of vanadium(IV) oxide results in another monoclinicarrangement, M2 (space group C2/m).

The nature of the semiconductor-to-metal transition invanadium(IV) oxide has been investigated via computational,experimental and theoretical studies. The prime mechanism ofthe transition remains a mysterious phenomenon, since thethree phases of VO2 exhibit diverse lattice structures, buthave analogous electronic properties, such as, the existence of

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Fig. 5 The M1 monoclinic structure of semiconducting VO2 when T <Tc. Two types of oxygen ions can be distinguished.44


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the semiconductor-to-metal transition, similar activation ener-gies and conductivities; the Mott–Hubbard model of correlatingelectrons were initially postulated.45

Goodenough42 proposed a useful explanation of the twophases of vanadium(IV) oxide, based onmolecular orbitals and aband structure diagram (Fig. 6). An antiferroelectric transitionwas considered as the potential driving force for the semi-conductor-to-metal transition in vanadium(IV) oxide. It wassuggested that V4+–V4+ pairing in the tetragonal phase (Fig. 6a)becomes energetically stable aer cooling, following rear-rangement of the band structure to give the monoclinic phase(Fig. 6b). Subsequently, the onset of antiferroelectric and crys-tallographic distortion was found to occur at two differenttemperatures, but happen to be synchronized for vanadium(IV)oxide. Goodenough concluded that the anti-ferroelectriccomponent of the monoclinic low temperature phase in VO2 isthe driving force of the distortion. Furthermore, the transitiontemperature, Tc is not controlled by thermal excitation of

Fig. 6 Schematic band structure diagram of VO2.46 The hybridisationarrangement in the crystal lattice.

3280 | J. Mater. Chem. A, 2014, 2, 3275–3292

electrons into the anti-bonding bands, but by the entropy of thelattice vibrational modes.

Wentzcovitch et al.47 reported convincing evidence from theresults of LDA calculations, of the band-like character (Peierlsinsulator) of VO2 to account for the low temperature monoclinicstate, M1. These results were incorporated into Eyert's study thatalso used LDA calculations and a band theoretical approach,which elucidated the M2 monoclinic phase of semiconductingVO2.44 Cavalleri et al.43 carried out experimental studies toendorse the band-like character using femtosecond laser exci-tation to initiate the transition and establish a time domainhierarchy. Subsequent measurements of the characteristicX-rays, optical signatures, transient conductivity and coherentphonons associated with between the structural and electroniceffects in VO2 thin lms. The results indicated that the disap-pearance of the band gap was due to optical phonons thatbrought about the structural phase change. Thus, the atomicarrangement of the high-temperature rutile lattice was deemedto be essential for the occurrence of the metallic phase of VO2.

Upon passing through the transition temperature, the elec-trical conductivity increases signicantly. This is accompaniedby a dramatic increase in infrared reectivity, with virtually nochange in the visible region. Above Tc, the material reectsinfrared radiation. Yet, below Tc, it is transparent, which iscrucial in its application as a thin lm coating for “intelligent”architectural glazing. An obvious problem for such applicationis the critical temperature of VO2 being much too high at 68 �C(in comparison to room temperature), which can be rectied byusing a dopant that will lower the critical temperature. This isdiscussed later in this review.

Synthetic techniquesSol gel & solution

Thin lms production via the sol–gel method has been widelyemployed for depositing thermochromic VO2 lms.48,49 In 1983,Greenberg50 introduced the ‘gelation-hydrolysis’ method formaking crystalline VO2, which involves partially hydrolysing the

of the V 3d and O 2p levels reflects the symmetries of the atomic




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initial coating and subsequently annealing it in a reducingatmosphere. The most commonly used precursors for the sol–gel preparation of VO2 lms are vanadyl tri(iso-propoxide) andvanadyl tri(tert-amyloxide)51. Additional metal alkoxides or saltsmay be included in the precursor solution, in the requiredproportions, in order to readily introduce dopants into thenetwork. The entire rst row and many of the second and thirdrow d-block elements have been doped into vanadium(IV) oxideusing the sol–gel method.

In the 1990s, Livage et al.52 found that vanadium dioxide andvanadium pentoxide gels can be synthesized via the acidica-tion of aqueous solutions of vanadates, such as NaVO3, orthrough the hydrolysis of vanadium oxo-alkoxides, for instance,VO(OR)3.53–57 Yin et al.58 demonstrated that V2O5 may be used asthe precursor, instead of a vanadium alkoxide, which they claimoffers advantages, such as the V2O5 precursor is cheap and easyto obtain, and the sol made by the quenching method theyemployed is stable.

The use of polyvanadate sols containing tungsten andmolybdenum reported by Takahashi et al.,59,60 who dissolvedmetallic V, W and Mo powder in 30% hydrogen peroxide solu-tion, which was then heated to form a hydrosol and spin coatedonto a suitable substrate. The doped VO2 lm was then reducedin a hydrogen atmosphere.

More recently Wu et al. have used a organic vanadic solprecursor based on V2O5 powder, isobutyl alcohol and benzylalcohol.61 The authors report good thermochromic propertiesand that the annealing temperature signicantly effects themicrostructure of the lms. However, no reduction in thethermochromic transition temperature was observed. Liuet al.62 have focused on preparing sols directly frommolten V2O5

powder and distilled water followed by vacuum annealing. Thelms were found to have transition temperatures around 60 �C.This synthetic approach has been further utilised by Pergamentet al. who described the electrical properties of their lms interms of “small polaron hopping conduction theory”.63 Zhanget al. investigated the use of vanadyl acetylacetonate andmethanol to produce VO2 thin lms. They found that under theappropriate annealing conditions of 440 �C the thermochromictransition temperature was reduced to 42 �C, although thechange in transmittance was also reduced to around 25%.64

Low temperature non-organic solvent routes to VO2 lmshave also received great attention in recent years. Kang et al.investigated the preparation of VO2 lms using a solution basedpolymer assisted deposition approach.65 They found that thelm grain size could be controlled by adjusting the ratio ofinorganic precursor to polymer template, and that this in turnhad a signicant effect on the thermochromic hysteresis of thedeposited lms. In particular the hysteresis width could bedramatically reduced from 50 �C to 10 �C. The same groupinvestigated the use of TiO2 buffer layer as an anti oxidationcoating using a similar aqueous process.66 The lms were foundto be signicantly more resistant to oxidation in heat cyclingtest. Kang et al. have donemore extensive work investigating thedecomposition of the polymer templates used in this process.They found that the order of polymer decomposition was crit-ical to the thermochromic performance of the lms.67 It was


also found that tungsten doping could be easily incorporatedinto this deposition method. Further work saw the lmsdeposited onto low emissivity substrates.68 The crystallinity ofthe lms was improved and the lms deposition temperaturecould be reduced. The multilayer lms were found to haveexcellent low emissivity properties, although this lead to adeterioration of the solar modulation.

Pulsed laser deposition

Thermochromic thin lms have also, more recently, beendeposited using laser ablation. Pulsed laser deposition (PLD) isanother physical vapour deposition technique, initially devel-oped for the deposition of oxide superconductors in the late1980s, and is ideal for metal oxide lm growth and was rstused for VO2 deposition by Borek et al.69 in 1993.

Borek et al. ablated a metallic vanadium target with a KrFpulsed excimer laser (248 nm), in a deposition chamber withargon and 10% oxygen atmosphere of 100–200 mTorr, whichwas the oxygen partial pressure that favoured the deposition ofpure VO2. Hence the oxygen to argon ratio was found to be acritical experimental parameter, since variations of partial O2

pressures stabilize many different vanadium–oxygen phasesthat can subsist, for example V2O5, V3O7 etc. The substratetemperature was maintained between 500 �C and 525 �C.Following the deposition, the samples were then annealed forapproximately 1 h at the same temperature and pressure toobtain VO2 lms. In 1994 Kim et al.70 reported that, using a KrFpulsed excimer laser (193 nm) on a target made of 99% purepressed V2O3 powder, VO2 thin lms were grown on sapphiresubstrates, whilst maintaining the temperature at 630 �C, aerwhich the samples were cooled to room temperature withoutpost-annealing, keeping the oxygen partial pressure constant.Room-temperature deposition of VO2 by PLD without post-annealing has also been investigated, by Maaza et al.,71 and theas-deposited lms exhibited sharp phase transitions atapproximately 70 �C. However, room-temperature PLD of VO2

thin lms had not yet been achieved.More recently several groups have investigated the use of

pulsed laser deposition to examine the impact of microstruc-tural strain on the thermochromic transition. It was noticedthat the lms that had been treated as so to minimise inter-lmstress produced the sharpest transitions72 whereas other workshowed that lattice mismatch could lead to a reduced transitiontemperature.73 Lysenko et al. demonstrated that a reducedparticle size led to a reduced transition temperature, typically to37 �C with a particle radius of 21 nm.74

Pulsed laser deposition has also been used to investigate theformation of composite lms. Orlianges et al. examined theproduction of VO2 and gold nanoparticle composites75. Theyfound that the incorporation of gold nanoparticles had no effecton the thermochromic properties of the lms relative to a plainVO2 lm. Though the additional surface plasmon resonanceeffects meant that the lms have potential use in optoelectronicdevices. Kaushal et al., examined the production of WO3–VO2

composites76. It was found that the composite lms had lowertransition temperatures, suggesting that W6+ ions were doping

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into the VO2 lm. Li et al. have observed anomalous opticalswitching and hysteresis behaviours from thin lms of VO2

deposited by PLD77 showing a clockwise hysteresis in the farinfrared but a counter clockwise hysteresis in the near infrared.

Fig. 7 Transmission hysteresis loops of varying thickness VO2 thinfilms at 2000 nm.101

Physical vapour deposition

The physical vapour deposition (PVD) technique involves foursteps including evaporation, transportation, reactions, anddeposition. Thematerial to be deposited, known as the target, isusually a solid-state metal precursor, which is bombarded by ahigh-energy source (such as a beam of electrons or ions) underreduced pressure. Thus, the atoms on the surface of the metaltarget become displaced and vaporised. There have been severalsystems employed to energetically remove atoms from a metaltarget, and most have been used to prepare vanadium(IV) oxidethin lms. Reactive sputtering is one of the most common PVDtechniques, and thermochromic thin lms have been syn-thesised using RF magnetron sputtering,78–80 DC magnetronsputtering81–83 and ion beam sputtering.84 RF magnetron sput-tering uses non-conducting targets, whereas DC magnetronsputtering deposits thin lms from metallic i.e. conductingtargets. Reactive sputtering of a metal in an inert gas, with asmall amount of an active gas present, has been used exten-sively as a technique for forming metallic compounds, forexample, transition metal compounds of V, Nb, Ta, Mo and W,which are otherwise difficult to achieve. VO2 thin lms were rstgrown by reactive sputtering in Fuls et al.85 who synthesised thelms by reactive ion-beam sputtering of a vanadium target in anargon–oxygen atmosphere.

Recently the use of PVD systems to deposit thermochromicvanadium dioxide lms has garnered great interest.86–95 The useof ion implantation as a method of doping96 deposition on tosteel substrates97 and preparation via thermal oxidation ofsputtered vanadium lms98 have all been explored. Saitzek et al.deposited VO2 lms using a reactive sputtering technique.99

They were able to demonstrate a correlation between lmthickness and change in transmittance on undergoing thethermochromic transition. Gentle et al. used DC magnetronsputtering and observed a number of anomalies in the switch-ing behaviour of their lms; they explored the gradual nature ofthe thermochromic transition and explained this as a functionof grain size and the relative activation energy towards switch-ing.100 Ma et al. demonstrated that at a thickness of 50 nmoptimal thermochromic properties were observed for lmsdeposited by PVD.101 They showed that for lms with a lowerthickness that the transition became less sharp, broader andless extensive (Fig. 7).

The impact of oxygen rich and oxygen poor lms on ther-mochromic properties has also been evaluated.102 It was foundthat oxygen rich lms gave a higher transmission in the opticalportion of the spectrum, but that the extent of the thermo-chromic transition was less in the infrared portion relative tooxygen poor lms. Chen et al. demonstrated that substratetemperature is also important in the sputtering process; theydemonstrated an ability to control the transition hysteresisgradient.103

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Atomic layer deposition

Atomic layer deposition (ALD) is a well-known method for thedeposition of ultra thin lms. It is known for it's high repeat-ability, uniformity and conformity as a result of the fact it is aunique self-limiting process.104 Whilst the relatively slow growthrate of ALD excludes it from signicant use in the glazingindustry the previously mentioned properties make it an idealtechnique for the coating of multi-dimensional and hierarchicalnanostructures; in the case of vanadium dioxide principally forapplications in energy storage and resistance switching.104

The rst report for ALD was examining the use of vanadiumoxide thin lms in electrochemical batteries105. It was found thatan ALD regime could be maintained below substrate tempera-tures of 180 �C. Subsequent reports examined the use of vanadylacetylacetonate and oxygen as precursors with a substratetemperature in the range of 400–475 �C.106 The lms were foundto contain multiple phases, which had a deleterious effect on thethermochromic transition magnitude. Typically the transitiontemperature was in the range of 60–66 �C limiting the potentialuse in smart window type technology, although the lms didshow very thin transition hysteresis widths (2–4 �C).

Work on the use of VO2 in photonic systems via ALD hasbeen reported by Povey et al.107 VOCl3 and water were used asprecursors at a deposition temperature of 490 �C, VOCl3 wasfavoured due to its higher volatility and reactivity than thepreviously used VO(acac)2. The purity of the lms were found tobe poor with many other vanadium oxide phases present,although a thermochromic transition was observed. Furtherdevelopments in VO2 ALD occurred in 2011 and 2012,108,109

when deposition using an ozone process to oxidize vanadiummetal was developed. The lms were intended for use in




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resistance switching devices and it was found that an abrupttransition of greater than two orders of magnitude could beachieved.108 Ozone has also been used as a precursor alongsidetetrakis(ethylmethylamido)vanadium (TMEAV) for the deposi-tion of VO2 thin lms via ALD.110 The use of this precursorsystem allowed for signicantly lower substrate temperatures tobe used, typically in the region of 100–150 �C, with the lmsshowing excellent uniformity although post annealing wasrequired to produce mono-phase lms. The lms showedsimilar resistance changes of two orders of magnitude aspreviously reported108 with a switching temperature of 75 �C.TMEAV has also been used with water as an oxygen precursor.111

It was found that this enhanced the growth rate relative to theozone system (0.8 A per cycle compared to 0.34 A per cycle) withno loss of lm quality.

Fig. 8 Thin films of VO2 deposited from the APCVD reaction of VOCl3and H2O at 550 �C.121

Chemical vapour deposition

The CVD process used for the production of vanadium(IV) oxidethin lms is mainly based on the use of organometallicprecursors, hence is frequently entitled metal–organic chemicalvapour deposition (MOCVD) or organometallic chemical vapourdeposition (OMCVD). Historically vanadium(V) oxide lms areusually deposited and subsequent reduction results in theproduction of vanadium(IV) oxide thin lms.

The rst reported use of CVD for the deposition of VO2 thinlms was by Koide and Takei in 1966.112 They initially grew bulksingle crystals of VO2 by CVD. The following year they depositedthin lms of VO2, using the same method with vanadium oxy-chloride (VOCl3) precursors and N2 carrier gas, which hydro-lyzed onto the substrates producing epitaxial VO2 lms.113

The next development occurred in the following year byMacChesney et al.,114 who exchanged the nitrogen carrier gas,with carbon dioxide, to transport the VOCl3 precursor. Vana-dium pentoxide, V2O5 formed on single-crystal sapphiresubstrates, which was then reduced to vanadium dioxide, VO2

by annealing in a controlled atmosphere. This consisted of amixture of CO and CO2 gases at low oxygen partial pressures,between 500 �C and 550 �C.

Greenberg50 was the rst to successfully attempt CVD usingvanadyl tri-isopropoxide, VO(OC3H7)3 as a single sourceprecursor in open atmosphere, with and without post-anneal-ing. This resulted in pure VO2 thin lms coated onto glasssubstrates. Takahashi et al.115 used vanadyl tri(isobutoxide)VO(O-i-Bu)3 as a single source precursor for depositing VO2 thinlms by dip-coating, as well as MOCVD at low pressure ontoglass substrates, which resulted in discontinuous thin lmswith ne needle-like VO2 crystals. Low-pressure CVD (LPCVD)has also been used to deposit thin lms of VO2 onto glasssubstrates.116,117 This work demonstrated that variations in thecharacteristics of the phase transition were a function of lmmicrostructure. The lms were very dense at 475 �C and showeda large change in resistance at 66 �C, displaying a smalltemperature hysteresis in the transition. However, deposition at520 �C led to lms that exhibited a higher transition tempera-ture of 72 �C, while the change in resistance was smaller with alarger hysteresis width.


In addition to organometallic precursors, vanadium halideprecursors may be used, for example, VCl4 or VOCl3, with wateror ethanol as a source of oxygen,118,11 and the resultant lms canthen be reduced to vanadium(IV) oxide in an appropriateatmosphere. Furthermore Barreca et al.119 used the CVDmethodwith vanadyl precursors of general formula VO(L)2(H), where Lis a b-diketonate ligand, at around 380 �C, in various atmo-spheres, such as O2, N2 and a mixture of N2 and H2O, whichresulted in VO2 and V2O5 thin lms. The precursors used inmost of these studies are generally expensive and require post-deposition reduction to form the desired vanadium(IV) oxidethin lms.

Atmospheric pressure chemical vapour depositon

There have been numerous studies of the growth of VO2 thinlms on glass or silica substrates. Maruyama and Ikuta120 in1993 used APCVD followed by post-deposition annealing, usingvanadium(III) acetylacetonate, V(acac)3, as a single-sourceprecursor to deposit polycrystalline, pure VO2 lms on asubstrate of fused quartz and sapphire single crystals. Manninget al. have extensively investigated APCVD of vanadium oxidethin lms, and have successfully demonstrated doping of VO2

with W, Ti, Mo, and Nb. They initially carried out APCVD usingVCl4 and water, to form thin lms of V2O5, VO2, VOx (x z 2.00–2.50) and V6O13 on glass substrates at 400–550 �C.11

Close control the deposition temperature of the reactionallowed the various vanadium oxide phases to be isolated.Higher temperatures encouraged the formation of oxygen poorphases. Also, the concentrations of the gaseous precursors inthe reactions were also found to effect the phases deposited,where higher concentrations favoured the formation of oxygenrich vanadium oxides. VO2 thin lms were synthesised withoutrequiring post-treatment reduction. In the following years,VOCl3 and H2O (with an excess of water over VOCl3) were usedas dual source precursors for APCVD of thermochromic VO2

thin lms onto glass substrates, at reactor temperatures greaterthan 600 �C (Fig. 8). A mixed phase of V2O5 and V6O13 were alsoproduced in some small patches, which were reduced bycontrolling the ow rates through the reactor. V2O5 thin lmswere also prepared when the reactor temperatures were lowedbelow 600 �C, or where an excess of VOCl3 over H2O occurred.122

Tungsten-doped lms were also prepared using APCVD of

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vanadium(IV) chloride, tungsten(VI) ethoxide, and water at 500–600 �C, producing V0.99W0.01O2 thin lms on glass substrates.121

The lms displayed signicantly reduced thermochromicswitching temperatures, from 68 �C in bulk VO2 to 42 �C,showing great promise for commercial use as a window coatingin “intelligent” glazing. Further work was conducted on thissystem to rene the lm properties further for use in architec-tural glazing.123 The same group has reported the growth ofcomposite VO2–TiO2 thin lms using the precursor combina-tions of VOCl3, titanium(IV) chloride (TiCl4) and water,124 andalso with the combination of VOCl3, titanium isopropoxide andwater. These composite lms exhibited photo-induced hydro-philicity with low contact angles, photocatalysis, and a reducedthermochromic switching temperature of 54 �C.

More recently, APCVD of vanadyl acetylacetonate, tungstenhexachloride in a 2% oxygen and 98% nitrogen atmosphere byBinions et al.125 led to the production of tungsten doped or un-doped vanadium(IV) oxide lms on glass substrates (Fig. 9).Tungsten doping was found to decrease the transition temper-ature by 20 �C per 1% tungsten incorporated.126 The propertiesof the thermochromic transition were notably affected by the

Fig. 9 XRD patterns (left) and thermochromic hysteresis behaviours (righVO(acac)2 and 2% O2 at 525 �C; (a) 10 nm min�1, (b) 50 nm min�1 and (

3284 | J. Mater. Chem. A, 2014, 2, 3275–3292

crystallographic orientation of the lm, which is inuenced byvariations of growth rate as has subsequently been conrmed.127

Along with lm morphology, the lm thickness was found torule the extent of the transition and visible light transmittancethrough the lm, but not the temperature at which the transi-tion occurred. An evaluation of the literature available revealsthat widely varying conditions have been investigated in termsof reactor types, ow rates, temperature ranges, and precursorconcentrations.

Aerosol assisted chemical vapour deposition

Aerosol Assisted Chemical Vapour Deposition (AACVD) is wherethe precursor is dissolved into an appropriate solvent (i.e. itpossesses the correct physical and chemical properties to allowformation of the aerosol), followed by the generation of anaerosol where the precursors are atomised into nely dividedsub-micrometer liquid droplets. The aerosol droplets are thentransported to the heated active deposition site via a ow ofinert gas, generally N2, and the solvent is evaporated or com-busted to produce a solid thin lm. One advantage of AACVD is

t) of films produced at different growth rates from the APCVD reactionc) 120 nm min�1.125




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that the precursor is not required to be volatile (as in APCVD)but must be soluble or dispersible in the solvent. Hence, the useof unconventional precursors that were not previously func-tional with other CVD techniques may be used, such as poly-oxometallates or ionic powders.128,129 Other advantages includethe lower cost of the process, since the vapour precursorgeneration and delivery method is simplied, compared tomoral usual CVD methods that use a bubbler/vaporiser, and itcan take place in an open environment without the requirementof a vacuum system or CVD chamber, when depositing oxidelms. Additionally, AACVD oen uses single-source precursorsand it is oen easier to control the precursor proportions in thesolution than in the gas phase, hence multi-component mate-rials are synthesised with relative ease and well-controlledstoichiometry. Furthermore, rapid deposition can occur atmoderately low temperatures owing to the small diffusiondistances between reactant and intermediates.130 AACVD of thinlms suffers from disadvantages such as adhesion of the lm tothe substrate may being at times weak, and gas phase reactionscan take place, leading to lm defects (e.g. pin holes due to thedeposition of large particles), which can result in the produc-tion of powdery lms.

Sahana et al.131 used AACVD of vanadium(III) acetylacetonate[V(acac)3] in a spray pyrolysis system with a controlled atmo-sphere to develop VO2, V2O3 and V2O5 lms, and vanadyl(IV)acetylacetonate, [VO(acac)2], has been used to prepare the meta-stable rutile VO2. This phase can be converted to the tetragonalstructure by annealing in argon at 500 �C.

Piccirillo et al.132 deposited thin lms of vanadium oxides onglass substrates using AACVD from V(acac)3 and VO(acac)2. Thephase of vanadium oxide formed (V2O3, VO2, or V2O5) was foundto be determined by the varying experimental parameters, suchas, the vanadium precursor, solvent, and carrier gas ow rate.Modication of the conditions allowed single-phase V2O3, VO2,and V2O5 to be formed across whole substrates, and this studywas the rst to report the CVD synthesis of V2O3 thin lms. Theresultant lms displayed interesting functional properties,including hydrophobicity (VO2), heat mirror properties for solarcontrol applications (V2O3), and hydrophilicity (V2O5). Theythen synthesised tungsten-doped VO2 thin lms via the sameconditions as the previous used, i.e. ethanol and VO(acac)2, andW(OC2H5)5 was employed as the tungsten precursor. Various

Fig. 10 Figure showing the correlation between tungsten content in prectungsten incorporation (right) and thermochromic transition temperaturAACVD reaction of VO(acac)2, W(OEt)6 and ethanol at 550 �C.133


depositions were carried out using different tungsten concen-trations to investigate the effect of the tungsten concentrationon the thermochromic transition temperature, Tc (Fig. 10).133

It was found that monoclinic VxW1�xO2 was the only phasepresent with a tungsten content of up to �2% atom, wheremixed phases, such as W–O or W–V did not form. There is alinear relationship between the amount of tungsten in thesolution and the amount incorporated in the lms, demon-strating the potential of the AACVD methodology for thedeposition of doped thin lms. The undoped VO2 sampledeposited in the initial study using this methodology showed atransition temperature at roughly 58 �C, which is 10 �C lowerthan the value normally observed for VO2 (Tc ¼ 68 �C). Thedecrease is thought to be caused by a strain in the lm, which isusually observed for lms thinner than 300 nm.120 They reportthat an amount of dopant of �1% reduces the transitiontemperature of VO2 by nearly 22 �C. Picirrillo et al. alsoproduced niobium-doped vanadium dioxide (VxNb1�xO2, x¼ 0–0.037) thin lms were prepared by AACVD of vanadyl(IV) aceto-nate and niobium(V) ethoxide in ethanol.134 The data indicated agood linear correlation between the value of Tc and niobiumcontent in the lm and that 2% niobium in the lm lowers thetransition temperature by 15 �C. Although niobium is a lesseffective dopant than tungsten, to decrease the value of Tc,signicant changes in the transmittance and reectance prop-erties of the lms were observed; hence it is still considered asuitable material for application in intelligent architecturalglazing.

Hybrid CVD systems, the mixing of aerosol and atmosphericprecursor ows, have been developed, boasting lm character-istics similar to those produced by APCVD (i.e. good lmadhesion, uniformity, and coverage) with the added versatilityof the AACVD technique. For example, enhanced surfacecoverage was observed and an assortment of lms of varyingthickness and dopant levels were produced with reasonableease. Binions et al.135 produced thin lms of gold nanoparticle-doped monoclinic vanadium(IV) oxide thin lms via hybridaerosol-assisted (AA) and atmospheric pressure (AP) CVD ofvanadyl acetylacetonate and auric acid in methanol. Theyincorporated gold nanoparticles with strongly absorbingsurface plasmon resonance (SPR) into VO2 thin lms. SPR isreversible and is stimulated thermally within the temperature

ursor solution and tungsten content in deposited films (left) and in filme for tungsten doped vanadium dioxide thin films produced from the

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Fig. 12 Example microstructures of vanadium dioxide thin filmsproduced using the electric field assisted CVDmethod from VO(acac)2in ethanol at 525 �C.141


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range of 25–120 �C. The frequency of SPR (548 nm to 600 nm) ishighly dependent on the dielectric properties of the host matrix,i.e. VO2, and the size of the gold nanoparticles.71 This researchwas implemented for the reason that, although tungsten hascontinually proved to be the most successful dopant fordecreasing the transition temperature of VO2, the major limita-tion on its commercial viability for exploitation in architecturalglazing, is that tungsten-doped vanadium(IV) oxide thin lmspossess an unpleasant brown/yellow colour, which is retainedupon thermochromic switching. Signicant changes in thecolour of the lm, from the brown colour to a variety of greensand blues, can therefore be tuned by incorporation gold nano-particles of the appropriate size and concentration (Fig. 11).

The use of auric acid as a single-source precursor for AACVDto produce gold nanoparticle lms led to a wide nanoparticlesize distribution.137 Therefore, Binions et al.138,139 incorporatedthe use of a coordinating surfactant into the hybrid CVDsynthesis for Au-doped VO2 thin lms, which controlled the sizeand shape of the deposited gold nanoparticles. The method wasa one-step process that used a solution of preformed nano-particles, VO(acac)2 and tetraoctylammonium bromide (TOAB)in methanol. The TOAB was found not only to template thegrowth of gold nanoparticles doped into the lm, but also thegrowth of the lm itself. Thus, additional strain was establishedin the lms, which caused a reduction in the thermochromictransition temperature.136 It has also been demonstrated thatpreformed nanoparticles can be incorporated into the depos-ited lms using this hybrid technique.140 In this work pre-formed titania nanoparticles were incorporated into thegrowing lm and both thermochromic and photocatalyticproperties were demonstrated.

The most recent development has been the use of electric eldassisted chemical vapour deposition (EACVD).141–144 In this tech-nique electricelds are added into a conventional AACVD reaction.Electric elds have been demonstrated to have a profound effecton the deposited lm morphology, with various nanostructuredand dendritic type growths being reported (Fig. 12).141–144

Electric elds have also been shown to effect crystallographicorientation, reduce particle size and reduce the thermochromictransition temperature.143,144 This is thought to occur through acombination of gas phase and surface effects.142

Nanoparticles

In the last few years there has been increasing interest in theproduction of vanadium dioxide nanoparticles as this provides

Fig. 11 Examples of glass with gold and vanadium dioxide nanocomposdoped VO2), (B) 0.09, (C) 0.15, (D) 0.30, (E) 0.36 and (F) (gold nanopartic

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another way to incorporate thermochromic materials intowindows and devices. He et al. prepared mica and VO2 particlesvia a sol gel route,145 the particles were processed into a trans-parent resin composite and their optical properties examined;the composites were found to have a narrow hysteresis widthbut the transition temperature was high at 71 �C. Colloidalsuspensions of vanadium oxide particles coated in poly-(ethylene glycol) diacrylate (PEGDA) were created by Lu et al.146

The PEGDA coating enable a liquid suspension to be formedand prevented the VO2 nanoparticles from being oxidised,however, the thermochromic properties were sub optimalhaving both high transition temperatures (71–74 �C) and widehysteresis widths (20 �C).

Gao and co-workers have been particularly active in this area.They have synthesised core shell structures where VO2 nano-particles are coated in SiO2 outer layers.147 The nanoparticles

ite films. The films had an Au/V ratio determined by EDAX of: (A) 0 (W-le film).136




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were incorporated into exible and transparent polyurethanesubstrates. The nanoparticles had reduced transition tempera-tures (�55 �C) and narrow hysteresis widths (10 �C), suggestingpossible application in energy efficient glazing. Gao's group havefurther reported on antimony doping of VO2 nanoparticles.148

The materials were incorporated into polyethylene lms andtheir optical properties determined. Excellent near infraredmodulation was observed (up to 41%), however the transitiontemperature of 61 �C and hysteresis width of 20 �C are likely tohigh for practical application. Gao et al. have also combinedtheir VO2 nanoparticles with low emissivity antimony doped tinoxide nanoparticles in a polyurethane foil.149 The foils wereutilised in a simple physical box model that were heated exter-nally and the temperature measured internally. The modiedfoil performed signicantly better than a plain oat glass pane.

Fig. 13 Comparison of the luminous transmittance of fluorine dopedvanadium dioxide and undoped vanadium dioxide thin films at a varietyof thicknesses. The films were made using a sputtering technique.165

Dopants

Amongst all of the thermochromic transition metal oxides dis-cussed, vanadium(IV) oxide is the closest to room temperature,with its critical temperature, Tc at 68 �C. This is too high to beeffective in many applications. For example the ideal transitiontemperature for “intelligent” glazing is in the region of 18–25 �C.150 Dopants can be incorporated into VO2 to increase ordecrease the thermochromic switching temperature, in order tomake the VO2 more commercially viable.151

The way in which the introduction of dopants into the VO2

lattice affects the temperature of the phase transition is, in fact,less understood than the nature of the semiconductor-to-metaltransition itself. A comprehensive and elaborate discussionusing analysis via X-ray diffraction is given by Goodenough,42

who considered the presence of another semiconducting phaseat high temperatures above Tc, existing between the monoclinicand tetragonal phase. This phase has an orthorhombic crystalstructure for low-valence dopant ions and forms the rutileconguration for high-valence dopant ions.

Even though tungsten(VI) has been shown to reduce thethermochromic switching temperature of VO2 by the greatestextent per atom%, there are a number of other dopants that canbe incorporated into vanadium(IV) oxide. Dopant ion size andcharge, and electron carrier density are factors that have beendetermined to affect Tc of vanadium(IV) oxide. Dopants with anatomic radii that is larger than the V4+ ion, or that create V5+

defects in the lattice, cause a decrease in Tc to around 25 �C, forexample, the high valence metal ions tungsten(VI), niobium(V)and titanium(IV).53 Whereas, those with smaller ionic radiiincrease Tc such as, the low valence metal ions aluminium(III)and chromium(III).53 Although, changes in the critical temper-ature are only evident when large concentrations of the dopantsare incorporated into the crystal structure.152 A 2 atom% loadingof tungsten proves to be the most effective dopant for reducingthe critical temperature to approximately 25 �C, when lms areprepared by physical vapour deposition153 and sol–gel coating.54

A charge-transfer mechanism takes place, since a tungsten(VI)ion replaces a vanadium(IV) ion. The mechanism is postulatedto be either via insertion of extra electrons into the vanadiumd-band,154 as well as the larger ionic radius of tungsten over


vanadium. Tungsten doping of vanadium dioxide has beenextensively using CVD techniques11,123,125,133,155,156

Sol-gel synthesis of VO2 thin lms can easily accommodatedopants and demonstrates a wide-ranging selection of dopantions, since most of the rst row transition metals having beenemployed.157 Other dopants that have also been integrated intothe VO2 lattice by sol–gel methods, include gold158 and molyb-denum.159 Only a small amount of gold (0.25 atom%) is requiredto decrease the transition temperature, however as theconcentration of Au increases, the high temperature rutilephase of the doped VO2 becomes less far infrared reecting.Molybdenum has been found to lower Tc to 24 �C at a 7 atom%loading. Co-doping of molybdenum and tungsten, or tungstenand titanium into the VO2 lattice works to afford very lowthermochromic transition temperatures.59 There have morerecently been several reports into titanium doping.160,161 Tita-nium doping of VO2 nanoparticles was shown to signicantlyreduced the hysteresis width of the thermochromic transition.This could be manipulated from between 30 �C (no titanium)and 5 �C (for V0.91Ti0.09O2 particles).160

PVD techniques for producing doped-VO2 thin lms have notbeen as extensively studied and the range of metal ions thathave been incorporated into the VO2 lattice is not as ample asthose investigated with sol–gel. Additionally, those that areknown to induce the largest decrease in the semiconductor-to-metal transition temperature are the most studied. i.e.tungsten153,154 and molybdenum.162,163 Indeed co-sputtering ofvanadium and tungsten in magnetron sputtering has shown tobe a facile way to produce tungsten doped vanadium dioxidethin lms.164 Fluorine has also been shown to reduce the tran-sition temperature to 20 �C, whereby the uorine atomsreplacing oxygen atoms during doping, with 1.2 atom% uo-rine165,166 (Fig. 13).

However, the hysteresis width of the thermochromic transi-tion became substantially wider; hence uorine doping is lessappropriate for use in applications such as thermochromic

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window coatings. A further more recent report of uorine dopedVO2 via aerosol assisted chemical vapour deposition suggeststhat uorine doping improves lm luminosity whilst reducingthe transition temperature.167 Magnesium doping has alsobeen shown as a useful route for improving the luminosity ofVO2 lms.168

The co-doping of tungsten and uorine (in the range of up to3 atom% for both F and W) into VO2 thin lms by PVD is foundto improve visible transmittance greater than tungsten-dopedvanadium(IV) oxide, whilst reducing thermochromic transitiontemperature to around 27 �C.169 Co-doping has receivedparticular interest more recently.170,171 Specically a sample ofcomposition V0.25Mo0.5W0.25O2 had a transition temperature of36 �C and a hysteresis width of 15 �C.171 Although in neitherpaper are the optical properties in the visible portion of thespectrum reported.

Multi-layers

A number of recent articles have reported on the use of multi-layer systems as a way to improve the optical and thermochro-mic properties of VO2 thin lms. Granqvist et al. reported that acarefully tailored TiO2/VO2/TiO2 optical stack could increasevisible transition by some 30%with only a minor degradation inthe thermochromic properties of the lms (Fig. 14).172

Heinilehto et al. examined the use of a ITO/VO2/ITO stack asan infrared optical shutter,173 they found that the stack system

Fig. 14 Comparison of the thermochromic properties of a VO2 thinfilm and a TiO2/VO2/TiO2 stack.172

3288 | J. Mater. Chem. A, 2014, 2, 3275–3292

had superior cycling performance than a non stack system. Luoet al. reported on the use of a VOx/W/VOx sandwich structure.174

Post annealing saw the formation of a tungsten doped vana-dium oxide phase. Annealing time gave some control over theextent of tungsten doping and a corresponding drop in transi-tion temperature. Viswanath et al. investigated the effect ofHfO2 under and over layers as a way to achieve full integration ofvanadium dioxide lms onto MEMS substrates. It was foundthat the HfO2 over layer signicantly reduced the transititontemperature (to 46 �C) and decreased the lm stress by up totwo thirds.175

Kang et al. examined the effects of VO2 thin lms coveredwith a thin layer of platinum deposited by sputtering.176 Theydemonstrated a reduction in emissivity with increased plat-inum coating thickness, however the thermochromic propertiesof the lms were not signicantly improved in any of thesamples investigated. The same group has investigated the useof ZnO:Al/VO2 systems,177 the ZnO:Al layer led to a reduction intransmission in the infrared and a signicant improvement inlow emissivity performance. The overlayer also dramaticallydecreased the hysteresis width of the transition from 30 �C toless than 5 �C. This group have also proposed an all solutionmethod to prepare double layer lms containing VO2 and SiO2/TiO2 materials, the lms show good solar modulation in thenear infrared but no signicant reduction in transitiontemperature (65 �C).178

Energy modelling

Recently the ability to model the energy demand reductioncharacteristics in buildings of thermochromic fenestration hasgained signicant interest. This is an important development astraditionally the only way to evaluate a glazing systems energyperformance was to install it in a test bank for a year and makethousands of temperature measurements, not only is this timeconsuming, but also costly. Improved modelling systems helpto identify potential candidates for new and improved glazingsystems. The rst report of its kind was published by Saeli et al.and demonstrated that in warm climates real thermochromiclms could have an enhanced energy reduction benet, anadditional 10%, compared to a number of current standards(Fig. 15).150

The study also included calculations for a hypothetical best-case thermochromic system that was demonstrated to poten-tially reduce energy demand by 20% when modelled forPalermo, the Sicilian capital. Subsequent work by the samegroup demonstrated that gold nanoparticle vanadium dioxidecomposite thermochromic lms were able to give a similarenergy demand benet, although with signicantly improvedcolouration and aesthetic properties136. Ye et al. have publishedlooking at the use of thermochromic thin lms in variousChinese climates,179 they report that no signicant energybenet is obtained, though it is worth noting that the benchmark they compare to is a theoretical idealised glazing systemand not a current standard. Ye et al. have also compared thepotential energy benets of thermochromic systems with otherchromogenic systems such as photochromic, electrochromic



Fig. 15 Total annual energy consumption (top) and percentageimprovement relative to a clear–clear glazing system (bottom) for aglazing wall model in a variety of climates; Cairo, Palermo, Rome,Milan, Paris, London, Moscow, Helsinki.150


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and gasochromic systems.180 Kim et al. have also reviewed currentwork in this area.181Most recentlyWarwick et al. have investigatedthe effect that thermochromic transition hysteresis width plays inthe energy demand reduction characteristics of VO2 thin lms.182

Their results show that a low transition temperature is thedominating force in dening energy demand reduction behav-iour, however, hysteresis width was found to be important, ahysteresis that was thinner by 1 �C led to an energy demandperformance that was 0.5% better, with a much more signicantimprovement seen for hysteresis widths less than 5 �C.

Conclusion

Vanadium(IV) oxide is the most studied thermochromic mate-rial due to its transition temperature being the closest to roomtemperature, and as such exhibits the greatest potential forapplication in thermochromic devices and “intelligent” glazingsystems. However, signicant challenges must be overcome inorder to manufacture a commercial window coating includefactors such as colour, scale-up, and thermal cycling.

A transition temperature of 68 �C is still too high, applica-tions such as “intelligent” thermochromic glass requireswitching temperatures between 18 and 25 �C. The preciseswitching temperature can be tuned, by doping the material.Tungsten(VI) has been found to be the most promising reducingthe transition temperature by 22 �C per atomic% incorporated.

Incorporating gold nanoparticles into the lms via a hybridaerosol-assisted and atmospheric pressure CVD technique


shows potential to improve the colour of the VO2 lms,compared to the tungsten doped lms, however, the cost of goldis likely to be an issue. Flourine or magnesium doping or theuse of optical stacks may well prove to be the optimal wayforward.

The energy-saving performance of the lms is predominantlycontrolled by the thermochromic switching temperature andnumber of hours the lm spends in the hot state. Simulationresults show that, in warmer climates, a lower transitiontemperature leads to energy savings, thus reduction of the TC toaround room temperature will lead to further energy-savingbehaviour.

A possible drawback to this new technology is that avail-ability of the resources needed in order to produce these coat-ings. One must not forget that this will be a large scaleproduction of new technology and issues such as accessibility ofthe raw materials needed should be considered. Nevertheless,the current energy situation and future issues, such as pop-ulation growth and increasing standards of living, will inevi-tably demand a safe and affordable solution to the shortage offossil fuels and environmental effects such as global warming.We believe that thermochromic technology and thin lmmaterials can play an important role in nding the appropriatesolution.

Acknowledgements

RB would like to acknowledge the signicant contribution ofProf. Ivan P. Parkin to the subject of vanadium dioxide thinlms and for sparking the author's interest in thermochromicmaterials.

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