J. MATER. CHEM., 1991, 1(4), 621-627 621 Sol-Gel Synthesis of WO, Thin Films Patrick Judeinstein and Jaques Livage Laboratoire de Chimie de la Matiere Condensee, Universite Pierre et Marie Curie-4, Place Jussieu, 75252 Paris, Cedex 05, France Versatile molecular precursors for the sol-gel synthesis of tungsten oxide thin films can be obtained via the reaction of tungsten oxychloride WOCI, with alcohols. Oligomeric species [WOCl,-,(OR),], are formed. Their molecular structure is analysed by infrared spectroscopy, nuclear magnetic resonance (NMR)spectroscopy (lH, 13C, lE3W), X-ray absorption spectroscopy (EXAFS) and small-angle X-ray scattering (SAXS). Hydrolysis of these precursors leads to the formation of tungsten oxide colloidal solutions. They can be easily deposited by dip- coating.Thin films ca. 3000A thick are obtained. Their morphology depends on the nature of the alcohol. Homogeneous films are obtained with bulky alkyl groups such as Pr'OH. These amorphous tungsten oxide layers exhibit electrochromic properties and could be used for display devices or smart windows. They can also be easily transformed, at room temperature, into crystalline hydrates WO,*nH,O (n= 1 or 2) when left in a humid atmosphere. Keywords: Sol-gel processing; Tungsten chloroalkoxide; Electrochromism; Thin film 1. Introduction Electrochromic layers based on amorphous W03 have been extensively studied during the last decade. Optical switching from white to blue can be reversibly performed in electro- chemical cells and used for display devices or smart windows.' Amorphous W03 thin films are usually made by vacuum evaporation,2 ~puttering,~ spray deposition4 or anodic oxi- dati~n.~Electrochromic layers deposited from colloidal W03 solutions have been reported recently.These solutions are obtained via the sol-gel route from tungsten alkoxides W(OR)p8 tungsten oxyalkoxides WO(OR)4, and W02(OR)2,9 or tungstic acid aqueous solutions HxW03."-12 However, these precursors are not stable toward hydrolysis or condensation. They cannot be handled easily and have to be stabilized in order to avoid the rapid precipitation of hydrous tungsten oxides. This paper describes the synthesis and characterization of tungsten oxide (WO,) thin films obtained from chloroalkox- ides WOCl, -,.(OR),precursors.These molecular species appear to be versatile precursors for the sol-gel deposition of electrochromic W03 thin films by dip-coating. The mor- phology of these films depend on the nature of the alkoxy groups. Best results were obtained by dissolving W0Cl4 into isopropyl alcohol Pr'OH. 2. Results and Discussion 2.1 Spectroscopic Techniques The different chemical species formed during the sol-gel synthesis of tungsten oxide thin films were characterized using the techniques outlined in the following. Infrared spectra were recorded in the 4000-300 cm-' range with a 783 Perkin-Elmer spectrometer using KRSS discs. 'H and 13C NMR spectra were recorded on a Bruker AM250 spectrometer using standard procedures.Modified molecular precursors were first dried under vacuum in order to remove alcohol in excess. A brown powder is obtained which is then dissolved into CDC1, in order to avoid exchange reactions with the solvent. 183WNMR spectra were recorded on an MSL4OO Bruker spectrometer (9.6 T) at a frequency of 16.65 MHz. The length of the pulse was 10 ps (t9*=40 p) and the recycle time 2.0 s. About 10 h (104 scans) were required in order to obtain a good signal-to-noise ratio with 0.1 mol dm-3 solutions in CD2C12. Room-temperature X-ray absorption spectra at the tungsten LIII edge (EXAFS) were recorded at LURE, the French synchrotron radiation facility, using the EXAFS I spec-trometer. Operating conditions in the storage DCI were the following: positions at energies of 1.85 GeV and intensities of ca.150 mA. The two-crystal Si (31 l} monochromator was fixed at its maximum flux position. The photon flux was measured by two ionization chambers. The entrance slit was 0.5 mm wide. A 1 vm tungsten metallic foil was used for energy calibration. Energy was scanned by 2 eV steps over a 10 050-1 1 050 eV energy range. Accumulation time was 1.6 s per point. Solutions (0.1 mol dm-3) were sealed into the cells in order to avoid further hydrolysis. Cells were 2 mm thick with windows made of X-ray transparent Kapton. EXAFS modulations were analysed with standard method^.'^,'^ The continuous absorption background was estimated by fitting the spectrum before the edge by a Victoreen function.The main absorption, beyond the edge p(E), was fitted with an iterative procedure. Normalization of the EXAFS signal was achieved by using z(E)=[po(E)-p(E)]/[p(E)]. Structural information was extracted on the basis of the single-scattering theory. Tabulated values of Teo and Lee" were checked with reference compounds such as Na2W04, (BUqN)2W6019 and W03. The electron mean-free path A(k)was approximated by ;I=k/T,where r is a fitting parameter. Small-angle X-ray scattering (SAXS) spectra were recorded with the synchrotron radiation of the DCI storage ring (LURE) in order to take advantage of the intense X-ray beam associated with point collimation and the D22 bench device which allows very small angles to be reached. Collected data cover q values from 0.04 to 0.8 A-1 and from 0.003 to 0.1 A-', depending on the sample-detector distance. The scattering- vector amplitude q =2 sin 8/A,where 28 is the scattering angle and A the selected wavelength, is equal to 1.5 A.The analysed Bragg zone ranges between 5 and lo3 A. Samples were sealed into cells 1 mm thick. As a result of the narrow beam, the only correction was to subtract scattering arising from the solvent. Usual SAXS data analysis procedure was used in order to obtain the Guinier radius of gyration and the mass of the smallest particles.16 For larger particles, a log-log plot of the experimental scattering functions led to curves exhibit- ing one or two linear parts. The cross-over distance, B, is a characteristic length of ~oherence.'~ The slope of the linear parts give information on the compacity of aggregates. They can be analysed as more or less dense particles18 or fractal aggregates.'' 2.2 Synthesis and Characterization of Molecular Precursors Tungsten alkoxides W(OR)6 can be synthesized by reacting WCl, with an alcohol ROH as follows:2o WCl6 +ROH WCl6 -,(OR), +XHCl (1) This reaction does not go to completion unless a base is added in order to remove HC1." Moreover, Wv' is reduced into Wv species such as WCl,(OR)3, and the solution rapidly turns blue." Spontaneous oxidation then occurs when the solution is left in dry air and the colour turns yellow within a few days.Reduction can be avoided by using tungsten oxychloride instead of WC16.WOCl, (5 g) was dissolved in 100 cm3 of pure alcohol (isopropyl alcohol distilled on sodium) under a dry atmosphere. A violent exothermic reaction occurs while gaseous HCl evolves but the solution remains yellow. Accord- ing to similar experiments performed on MoOC~,,~~the overall reaction could be described as follows: WOCl, +xROH eWOCl,-,(OR), +xHCl (2) Alcohol in excess is removed upon heating the solution under vacuum at ca. 60 "C. A brown powder is obtained which can be dissolved in the parent alcohol or even in a neutral solvent such as CCl,. Concentrated solutions and powders are very sensitive towards moisture and therefore rather difficult to handle. Dilute solutions are much less reactive. They can be kept in a closed vessel and remain stable for months.Infrared spectra of concentrated solutions of the chloro- alkoxide in CC14 are shown in Fig. 1. All the bands typical of alkyl groups can be seen above 1OOOcm-'. The intensity of the v(C0) stretching vibration at 1130 cm-' increases significantly compared with pure isopropyl alcohol suggesting that isopropoxy groups are bonded to tungsten via the oxygen atom. Vibrations involving tungsten atoms are on the low- energy side. The sharp band close to 970 cm-' can be assigned to W=O double bonds.', Broad bands between 800 and 730 cm-' suggest the formation of W-0-W bridges" similar to those observed in polyanions. W-Cl vibrations can be seen at ca. 350 cm-'.26 'H and 13C NMR spectra of oxychloroalkoxide precursors were recorded in CDC13 in order to avoid exchange reactions between alkoxy ligands and the solvent.The 'H NMR spectrum exhibits several sets of broad peaks typical of isopropoxy groups. However, the corresponding chemical shifts are shifted towards low field C1.07 (CH3)2CHOH, 3.88 (CH3)2CHOH for isopropyl alcohol, 1.2-1.5 -n A7-'CH J. MATER. CHEM., 1991, VOL. 1 (CH3)2CHO-WW, 4.3 and 5.8 pprn (broad peaks) (CH3)2CNO-W]. Such unshielding effects could be due to the presence of tungsten atoms. Similar chemical shifts have already been observed in W(OPri)6.27 The 13C NMR spectrum of the same solution exhibits two groups of peaks as for pure Pr'OH [Fig. 2(a)]. The corresponding chemical shifts also suggest a strong unshielding due to the presence of heavy tungsten atoms.Three broad signals corresponding to different W-0-C bonds are observed at ca. 83,84 and 87 ppm. The large chemical shifts and the multiplicity observed for both 'H and 13C NMR peaks suggest that bridging and terminal alkoxy groups are simultaneously present in the molecular precursor. The rather broad linewidth could be due to some chemical exchange between these alkoxy groups. The 183W NMR spectrum was recorded with a solution of tungsten oxychloroalkoxide (0.2 mol dm-3) dissolved in a Pr'OH-CD2C12 mixture. It exhibits three signals [Fig. 2(b)] at ca. 36.2, -37.0 and -122.8 ppm. The 6 =O ppm reference corresponds to Na,W04 in D20 (pH 9). This suggests three different chemical environments for tungsten.Two processes could account for the observed chemical shifts which are small compared to WC16(2181 ppm) and W(OR)6 (ca. -480 ppm).28 (i) The substitution of OR groups by heavier and more electronegative C1 atoms changes the shielding effect. Similar features have been reported for WF6-,(OR),29 and for "V in the NMR spectra of VO(C1)3~,(OR),.30 In both cases chemical shifts larger than 200ppm are observed when x increases one unit. (ii) Axial distortion of local symmetry from a regular octahedron (0,) to a pyramidal C4"symmetry due to the W=O double bond also leads to an unshielding effect as was already reported in polyanion~.~~ Both processes could 100 80 60 40 20 I IIIIIIIIIIIIIIIIIIIIII 80 ' 50 20 -10 -40 -70 -100 -130 'w-0 6 (PPm) Ill I I I1; 111 I 1 I 4000 3000 2000 1000 500 Fig.2 (a) 13C-('H) decoupled NMR spectra of the WOC1,-Pr'OH v/cm -precursor in CDC1, (+, chemical shifts for Pr'OH; *, CDCl, signal); (b) lS3W NMR spectra of the WOC1,-Pr'OH precursor in Fig. 1 Infrared spectra of the WOC1,-Pr'OH precursor in CCl, CD2C12-Pr'OH J. MATER. CHEM., 1991, VOL. I actually occur simultaneously leading to chemical shifts in opposite directions. It is therefore difficult to claim which one is predominant. All resonance peaks are rather broad. This could be due to short relaxation items arising from the formation of oligomeric species and exchange reactions between different sites. Therefore a definite conclusion is not possible.NMR spectra could be assigned to a single oligomeric molecule as well as a mixture of different chemical species. EXAFS spectra were recorded with tungsten oxychloro- alkoxide solutions in isopropyl alcohol (0.2 mol dm-3). The radial distribution function of the EXAFS signal and the k-space filtered EXAFS spectrum are shown in Fig. 3. The EXAFS curve was fitted with the conventional single-scat- tering formalism. It was not possible to get a good fit unless both oxygen and chlorine atoms were included in the first- neighbour shell. The best fit then leads to the values reported in Table 1. The 0:C1 first neighbours ratio is close to 3 :2, suggesting the following chemical formula WO(OR)2C1,. A short tungsten-oxygen distance is observed at ca.1.76 A. It could correspond to the W=O double-bond contribution. However, the Debye-Waller factors associated with all W-0 distances are rather large, suggesting a broad distribution of these distances arising for instance from strongly distorted octahedra. W-C1 distances at ca. 2.36A are rather long compared with the mean W-Cl distances found in WC1, (2.24A, monomer), W0Cl4 (2.29 A) or W02C12 (2.31 A, poly-meric structure with W-0 bridging bonds).31 They could therefore be assigned to longer bridging C1 atoms rather than terminal ones. This would suggest the presence of oligomeric species. However W ... W correlations are not observed. This could be due to the small size or a linear structure of Y, Y 3 4 5 6 7 a 910 0123456 k/A-RIA h h Y 3 4 5 6 7 8 9 10 0123456 klk' RIA Fig.3 EXAFS spectrum of the WOC1,-Pr'OH precursor in isopropyl alcohol (a), (b)and the corresponding hydrolysed sample (h=20) (c), (d).(a),(c)k-space filtered EXAFS spectrum; (b),(d) radial distribution function oligomeric species leading to a small number of tungsten neighbours. Moreover a rather broad distribution of W-W distances together with the large extent of 5d tungsten orbitals could reduce significantly the intensity of such contributions. SAXS experiments were performed with tungsten oxychlor- ide solutions in isopropyl alcohol. Absolute intensity measure- ments suggest the presence of anisotropic molecular species with a Guinier radius close to 5 A. Scattering curves, expressed as In I =f(Q2) and In (IQ)=f(Q2)plots, can be assigned to oligomeric species ca.3-4 A in diameter and 10 A long. Molecular precursors are rather sensitive toward reduction. They become blue upon UV irradiation. Spontaneous reduction is even observed when WCI, is dissolved into Pr'OH. The electron paramagnetic resonance (EPR) spectra of such reduced solutions are similar. They are characteristic of Ws+ ions in an axial ligand field (Fig. 4).The frozen solution spectrum recorded at low temperature is well resolved. Satellite lines can be seen on each side of the spectrum. They are due to the hyperfine interaction of the electron spin (S= 1/2) with the nuclear spin of the lg3W tungsten nucleus (I = 1/2, natural abundance 14.8%).EPR parameters are gll= 1.81, g,= 1.78, All=.125 G and Al= 150G. They are close to those found in (WOC1s)2- or (WOC1,,H20)3-where W5 + is surrounded by oxygen and chlorine atoms.32 g, is smaller than gI1suggesting that the unpaired electron is in a d,, orbital. An EPR signal is still observed at room temperature. This is quite unusual in most tungsten-oxygen compounds in which electrons are so delo-calized that EPR signals can no longer be seen.33 This suggests that unpaired electrons remain rather localized even at room temperature. Such an electron localization could be due to the small size of the oligomeric species and the nature of W-OR-W and W-Cl-W bridges between W atoms. 2.3 Hydrolysis and Condensation Hydrolysis of precursor solutions in Pr'OH (0.2mol dm -3, was performed at room temperature by adding a mixture H20-Pr'OH (10% H20 in weight).The hydrolysis ratio is 3200 3450 3700 3950 4200 magnetic field/G Fig. 4 EPR spectrum of the WOC1,-Pr'OH precursor in Pr'OH reduced upon UV irradiation (recorded at 77 K) Table 1 EXAFS parameters of precursor compound (WOC1,-Pr'OH) and colloids obtained after hydrolysis (h=20) nature of neighbour RIA N Deb ye- Waller coeff. (6) AEIeV fitting coefficient WOC1,-Pr'OH 0 0c1 1.76 1.88 2.36 0.9 1.8 2.4 0.04 0.08 0.05 10.4 10.4 -0.6 r =0.7 R = 3% C 3.7 5.5 0.12 0.1 WOCI,-Pr'OH--20H20 0 0 1.76 1.92 1.1 3.5 0.06 0.06 10.4 10.4 r =0.7 W 3.50 5.0 0.10 15.2 R = 5% 0 3.10 6.8 0.15 10.4 624 given by h= H20:W.Transparent yellow colloidal solutions are obtained up to h=5, while white gelatinous precipitates are obtained beyond h= 10 (Table 2). Infrared spectra are progressively modified as hydrolysis proceeds (Fig. 5). Absorption bands at ca. 1100 cm-' disap-pear as well as the W-C1 bands at ca. 350cm-'. The IR spectrum between 500 and 1000 cm-' becomes more and more poorly resolved. A broad absorption with two maxima rising from the formation of an oxide network is seen when h=50 [Fig. 5(d)] with a small shoulder at ca. 980 cm-' corresponding to W =0 bonds. The Fourier transform of the EXAFS spectrum of hydro- lysed samples (h=20) exhibits two peaks corresponding to W-0 and W-W distances (Fig. 3). A careful analysis of the first-neighbours shell shows that all Cl atoms have been removed from the co-ordination sphere of tungsten, whereas W-0 distances are not significantly modified.The best fit (Table 2) leads to two W-0 distances (1.76 and 1.92 A) corresponding to a distorted octahedron with one short W=O bond. Tungsten neighbours can be seen in the second shell at 3.50A. Such a distance lies between those reported for edge sharing (W-W =3.3 A) and corner sharing (W-W =3.7 A) W06 ~ctahedra.~, It could therefore corre- spond to a random distribution of both species. Small-angle scattering curves of samples hdyrolysed with h= 10 recorded at different times after water has been added vco / vw-o Ill I I I Ill I I III 4000 3000 2000 1000 500 vfcm-Fig.5 Infrared spectra of the hydrolysis product of WOC1,-Pr'OH solutions with different hydrolysis ratio h=H20:W (recorded after I h): (a) 1, (b) 5, (c) 10, (d) 50 J. MATER. CHEM., 1991, VOL. 1 are shown in Fig. 6. For the first 2 h scattering curves In I= Aln Q) exhibit a single linear variation in the Porod region, with a slope of -1.4 suggesting the anisotropic growth of rod-like particles. A maximum can be seen at low Q values (Q=0.002 k').It does not vary with dilution and can be attributed to particles ca. 400 A long. Colloidal particles keep on growing as hydrolysis and condensation proceed further. Two linear regimes are observed on the scattering curve. The first one at high Q with a smaller slope close to -1.1 and the other one at low Q with a larger slope of ca.-2.5. These curves suggest some complex growth mechanism involving two different steps. The first regime could be assigned to an aggregation process arising from the diffusion of rod-like particles while the second one could correspond to the forma- tion of branched polymers arising from the cross-linking of small rods. The crossing point between these two curves gives an order of magnitude of the coherence length, B, of these rods. This coherence length decreases as the hydrolysis ratio, h, increases, from B =600 A for h= 10 to B =200 A for h= 20. It does not vary with time for a given hydrolysis ratio. More nuclei are formed but they do not grow as much. The growth process seems to stop when the hydrolysis ratio becomes too small (h~5).2.4 Thin-film deposition W03 thin films can be deposited easily from molecular precursor solutions via the dip-coating pr~cedure.~' A glass plate covered with a conducting transparent layer (antimony- doped tin oxide) is dipped into an alcoholic solution of WOCl, (0.lmol-0.3 mol dm-3). It is then pulled out at a controlled speed of ca. 1 cm s-'. The plate is left in air so that hydrolysis and condensation occur spontaneously in the ambient moisture. Evaporation of the solvent (alcohol) occurs simultaneously and a dry coating ca. 800 A thick is obtained O1 -2 OO-'I OUo0 -6 I I I I I I I 1 -6 -5.5 -5 -4.5 -4 -3.5 -3 -2.5 -2 In Q Fig. 6 SAXS curves of the hydrolysis product of WOC1,-Pr'OH + 10H20.Delay after hydrolysis, tfh: (+) 0.2, (+) 2, (0)10 Table 2 Hydrolysis of 0.1 mol dm-3 isopropoxy oxochloroalkoxide solutions in isopropyl alcohol hydrolysis ratio h=HzO:W final appearance of the compounds stability 0 1 2 5 10 20 50 yellow solution oligomers [WOC14-,(OR),], n I3 yellow solution yellow solution rod-like particles 25 A long white opaque solution polymeric particles diameter >600 A white gelatinous precipitate precipitate yellow scattering solution rod-like particles 100 long months months weeks weeks 5 days J. MATER. CHEM., 1991, VOL. 1 within 30 min. The process can be repeated several times in order to have thicker films. Up to six layers have been superimposed by successive coatings leading to films ca.5000 A thick. Three dips only were required in order to obtain films exhibiting good electrochromic behaviour. Rather uniform coatings are obtained as shown by scanning electron microscopy (Fig. 7). Their morphology mainly depends on the nature of the alcohol in which WOCl, is dissolved. Spherical particles embedded into a continuous film are obtained with methanol [Fig. 7(a)]. More uniform films are obtained with alcohols such as isopropyl alcohol or butanol [Fig. 7(b)].The surface of these films appears to be smoother when the size of the alcohol molecule increases. This is probably related to the slower hydrolysis rate of tungsten precursors and the slower evaporation rate of the solvent.Both of them are known to decrease when the steric hindrance of the alkoxy group increases. Fig. 7 Scanning Electron Microscopy of sol-gel W03 thin films: (a)dip coating in WOC1,-MeOH solution; (b)dip coating in WOCI,- BuOH solution X-Ray diffraction patterns of W03 layers are typical of amorphous samples. The infrared absorption spectrum of a thin film deposited onto a KRS5 plate is quite similar to that of WOC1,-PriOH+50H20 [Fig. 5(d)]. It exhibits a strong absorption below 1000 cm- due to the formation of an oxide network. These bands are very broad, suggesting a large distribution of W-0 distances and W-0-W angles in the amorphous oxide. The shoulder at 980 cm -could be assigned to a short W=O bond. Absorption bands arising from water molecules can be seen on the high-energy side of the spectrum at ca.3500 and 1600 cm-'. No band corresponding to alcohol molecules or alkoxy ligands can be seen showing that all organics have been removed upon hydrolysis and drying. DTA experiments show two phenomena: an endothermic process between 50 and 170 "C corresponding to the departure of water molecules, and an exothermic process at 350 "C corresponding to the crystallization of orthorhombic W03. When dried under ambient conditions, hydrated tungsten- oxide films correspond to amorphous W03 * 1 .8H20. They can be easily transformed into other tungsten oxide phases as follows. (i) Crystallization occurs upon heating at 350 "C leading to orthorhombic W03. (ii) Crystallization is also observed at room temperature when the film is left in a humid atmosphere.This leads to the formation of the well known dihydrate W03 *2H20.This hydrous oxide exhibits a layered structure. Water molecules are intercalated between the W03 layers and the basal distance corresponds to d=6.91 However, no preferential orientation of these layers is observed while anisotropic layers were deposited from aqueous solu- tions of colloidal tungstic acid.36 (iii) Dehydration of the crystalline dihydrate occurs when heated at 120 "C during 20 h. It leads to the monohydrate phase W03- 1H,O which also exhibits a layered structure with a basal distance d= 5.34 Scanning electron microscopy of these tungsten oxide layers shows that the coverage and adhesion of the oxide coating onto the substrate are not affected by crystallization when the amorphous film is heated or left in a humid atmosphere.However, electrochromic properties strongly depend on the water content and ~rystallinity.~~ 3. Conclusions This work shows that electrochromic W03 thin films can be synthesized easily via the hydrolysis and condensation of cheap and stable molecular precursors. The reaction of alcohol with WCl, or better WOCl, leads to the formation of WOC1,-,(OR), solutions, which can be kept without major transformation for more than 6 months in a closed vessel. Moreover these solutions remain reactive enough to give uniform coatings when deposited onto a glass substrate in the presence of a humid atmosphere.WOCl, -,(0Pri), precursors have been prepared by dissolv- ing tungsten oxychloride in isopropyl alcohol. The molecular structure of these chloride alkoxides is not easy to determine. No crystal can be extracted from the solution and an amorph- ous powder is obtained upon evaporation. Even the chemical composition is not obvious because it is almost impossible to remove all alcohol and HCl in excess. Therefore chemical titrations were not successful. Infrared spectra show that both W-C1 and W-OR bonds are present in the molecular precursor as well as short W =0 double bonds. These results agree with EXAFS data suggest- ing that tungsten is surrounded by both oxygen and chlorine atoms with a ratio 0:C1 close to 3 :2 suggesting the following composition WOC12(0Pri)2.Infrared bands in the low-fre- quency range and SAXS measurements suggest the formation of oligomeric species. However, W ... W correlations cannot J. MATER. CHEM., 1991, VOL. 1 be seen in the EXAFS spectrum. This does not preclude the presence of oligomers. It could be due to the large extension of 5d atomic orbitals and the small number of tungsten neighbours. 183W NMR spectra suggest the presence of three different tungsten species. They might be found in a single oligomeric molecule. It seems, however, more reasonable to suggest that several molecular species such as [WOCl, -x(OPri)x],, with nI 3 are in equilibrium. Their rela- tive abundance should depend mainly on tungsten concen- tration and the steric hindrance of the alkyl chain.Similar analyses (IR, EXAFS, SAXS) have been performed on WC1,-PriOH solutions. They give the same features as for W OC1,-Pr'OH precursors, i.e. an oligomeric structure with W-C1, W-0 and W=O bonds. It shows that alcoholysis of WCl, leads to a change of co-ordination around tungsten from an Oh(WX,) to a C4"(WOX,) symmetry. Such changes are energetically favoured by the formation of an 0x0 group bonded to tungsten.34 Such bonds involve a large overlap of the d,-p, orbitals in order to produce a strong double bond W=O. Moreover, they are reinforced by the proximity of more ionic W-C1 bonds. This could explain the differences observed between chlororalkoxides and oxyal- koxides of the WO(OR), series.39 According to literature, double W=O bonds are not observed in these solid com- pounds, except for large tert-butoxy derivatives.The corre- sponding vibration band at ca. 950 cm-' cannot be seen on infrared spectra. One could also suggest a modifica-tion of these structures from solid-state [poly-meric-W(OR),-0-W(OR),-] structure to solution in parent alcohol [O=W(OR),], as observed with WOCl,, by IR spectro~copy.~~ Hydrolysis of WOC1,-Pr'OH solutions leads to the forma- tion of yellow colloidal solutions or white gelatinous precipi- tates depending on tungsten concentration and the hydrolysis ratio. Infrared and EXAFS data show that both chlorine and alkoxy ligands are removed upon hydrolysis, leading to the formation of condensed species evidenced by W ...W corre- lations in the EXAFS spectrum. A mean distance W-W= 3.5 A is found suggesting that condensation occurs via both edge and corners sharing [WO,] octahedra. Two linear variations of In I=f(ln Q) are seen on the small-angle X-ray scattering curves. They show that condensation leads to the formation of large branched aggregates (diameter d >600 A) with a complex structure. It could be assumed that rod-like particles are first formed which then aggregate to give dense ramified polymers. The anisotropic growth of the colloidal particles should be related to the functionality of the molecular precursor. In the case of tungstic acid prepared upon acidification of tungstate aqueous solutions it was shown that large condensed species were formed from neutral precursors such as [H2 W04].40*41 Co-ordination expansion occurs via the nucleophilic addition of two water molecules leading to the formation of sixfold co-ordinated precursors with one short W=O bond and a long W-OH2 bond along the z axis.Four equivalent W-OH bonds are formed along x and y axis so that neutral precursors have a functionality of four in the xy plane. This leads to the anisotropic growth of platelet- like particles as observed by electron micro~copy.~~ Scattering curves, performed by SAXS, with such colloids give a single linear In I=Aln Q) plot.43 The corresponding slope of -2 agrees with an anisotropic two-dimensional growth process leading to the formation of platelets.Chloride alkoxides WOCl,-,(OR), should behave in a different way from the previous inorganic precursors. W=O bonds are conserved during the hydrolysis process. Both chlorine and alkoxy groups are removed. However, these ligands do not exhibit the same reactivity toward hydrolysis. W-C1 bonds are more ionic than W-OR bonds. They should be hydrolysed first. The first apparent functionality therefore decreases around 2 leading to the anisotropic growth of rod-like particles. These rods then condense together. Moreover, branched polymeric networks are also formed during the hydrolysis of WO(OEt), solution^.^ Depending on the hydrolysis ratio, h, alkoxide precursors lead to the forma- tion of more or less branched polymers with a small Porod exponent, -1.8 to -2 measured by SAXS.The functionality of these alkoxide precursors is rather high, and the reactivity of all hydrolysable functions are quite similar. This leads to species of complex geometry described in terms of fractals. Thin films of large area can be deposited easily by dip- coating. The morphology of these films mainly depends on the nature of the alcohol used for the sol-gel synthesis. It appears that more uniform films are obtained with bulky alcohols such as Pr'OH. This can be attributed to the lower reactivity of the corresponding alkoxy groups toward hydroly- sis and the slow evaporation rate of the solvent. Such morpho- logical variations reflect the differences observed on the growing mechanism of different WOC14-PrOHi precursors, especially the value of the coherence length of the particles.These differences appear to play an important role on the microstructure of tungsten oxide coatings. Films deposited from organic solutions [Fig. 7(a),(b)] are made of spherical particles embedded into a continuous film. They are similar to those obtained from polymeric gels. Films deposited from aqueous solutions of tungstic acid exhibit a layered structure made of the stacking of flat colloidal particles.,' Moreover, amorphous W03 1.8H20 thin films deposited via the sol-gel process appear to be very versatile precursors in order to obtain other W03 *nH20 coatings. The hydration state can be controlled by the relative humidity of the ambient atmos- phere and the temperature. Crystalline layers are formed upon heating at 350°C (orthorhombic W03) or even at room temperature in the presence of water vapour (W03.2H20 and W03 *H20).The sol-gel process therefore leads to a wide range of tungsten oxide thin films which could be used for making display devices or smart windows. The electrochromic proper- ties of these oxides can be tailored via the chemical control of parameters such as the nature of the alcohol, the OR :C1 ratio in the precursor, the hydrolysis ratio h= H20: W, the amount of water in the layer or even the crystalline state. 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