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Thin Solid Films 516 (2

The structure and composition of oxidized andreduced tungsten oxide thin films

Simon Penner ⁎, Xianjie Liu, Bernhard Klötzer, Frederik Klauser,Bernd Jenewein, Erminald Bertel

Institute of Physical Chemistry, University of Innsbruck, Innrain 52a, A-6020 Innsbruck, Austria

Received 5 December 2006; received in revised form 16 April 2007; accepted 17 May 2007Available online 29 May 2007

Abstract

The structure, morphology and composition of pure WO3 thin films deposited onto vacuum-cleaved NaCl(001) single crystals have beenstudied at different substrate temperatures up to 580 K and under different oxidative and reductive treatments in the temperature range 373–873 Kby Transmission Electron Microscopy, Selected-Area Electron Diffraction and X-ray Photoelectron Spectroscopy (XPS). A transition from anamorphous structure obtained after deposition at 298 K to a more porous structure with small crystallites at the highest substrate temperatures hasbeen observed. XPS spectra reveal the presence of W6+ irrespective of the preparation procedure. Significant changes in the film structure wereonly observed after an oxidative treatment in 1 bar O2 at 673 K, which induces crystallization of a monoclinic WO3 structure. After raising theoxidation temperature to 773 K, the film shows additional reconstruction and a hexagonal WO3 structure becomes predominant. This hexagonalstructure persists at least up to 873 K oxidation temperature. However, these structural transformations observed upon oxidation were almostcompletely suppressed by mixing the WO3 thin film with a second oxide, e.g. Ga2O3. Reduction of the WO3 films in 1 bar H2 at 723–773 Keventually induced the formation of the β-W metal structure, as evidenced by electron diffraction and XPS.© 2007 Elsevier B.V. All rights reserved.

Keywords: Electron microscopy; Selected area electron diffraction; X-ray photoelectron spectroscopy; Tungsten oxide; Oxidation; Reduction; β-W

1. Introduction

Like other transition metal oxides, WO3 is an interestingmaterial with respect to a wide range of different physico-chemical applications [1–5]. Among those to be mentionedare its use as electrochromic devices [1] and its gas sensingproperties [2–4], especially for nitrogen [2] — and sulphur-containing compounds [3]. WO3 is also catalytically relevant ina wide range of reactions including propene oxidation [6], NOx

reduction [7] or skeletal rearrangements of hydrocarbons [8]. Asthese properties are crucially influenced by the structure andmorphology and in turn by the preparation conditions of thematerial, WO3 thin film systems are particularly well-studied[5,9–12]. Thin films have been prepared by thermal evaporationof WO3 powder [9,13], by Radio-Frequency (RF)-sputtering

⁎ Corresponding author. Tel./fax: +43 5125075056.E-mail address: simon.penner@uibk.ac.at (S. Penner).

0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2007.05.041

from metallic W [14] or WO3 targets [15] in Ar/O2 atmosphere,by sol–gel deposition [16] and by chemical vapor deposition[17]. Depending on the preparation conditions, crystalline oramorphous samples are obtained [9,10,12,17]. Al Mohammedet al. [9] observed a monoclinic WO3 phase after thermal evap-oration onto α-Al2O3(0001) at substrate temperatures around573 K. The same was observed by Wang et al. [10] for RF-sputtered WO3 films. In addition, amorphous samples wereobtained after deposition at 373 K substrate temperature. Pal andJacob [17] discussed the influence of the substrate temperaturefor chemical–vapor deposited films and concluded that thecritical temperature for crystallization is around 600 K, belowwhich only amorphous samples are produced. The influence ofoxygen on the grain size was addressed by Manno et al. [12] andgenerally it was observed for RF-sputtered films that increasingthe O2/Ar ratio leads to an increase in WO3 grain size.

Studies of annealing, oxidation and reduction of WO3 arefurther complicated by the vast number of different WO3 phases

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[9]. Although the most stable one at room temperature has amonoclinic structure, hexagonal and orthorhombic structuresare also present depending on the preparation and annealingconditions [9]. Moreover, reduction of WO3 leads to a varietyof under stoichiometric WO3−x structures along with more re-duced phases like WO2 and even metallic W [18]. Electron-microscopy has proven to be a powerful tool especially forstudying not only the structures of these substoichiometrictungsten oxides [19] but also for investigation of the phasetransformation in WO3 thin films upon annealing [9,10,12].

The present contribution aims at a better understanding ofthe processes and structures occurring during annealing, oxi-dation and reduction treatments. Based on previous studiesof structural changes of WO3 and on recent experiments on theco-deposition of WO3/Ga2O3 systems we especially focussedon the different properties of our pure and “Ga2O3-doped”WO3 thin film in comparison to previously described films. Weexpect the differences to alter both the annealing and reductionbehaviour of WO3. Transmission Electron Microscopy (TEM),Selected Area Electron Diffraction (SAED) and X-ray Photo-electron Spectroscopy (XPS) were chosen as suitable experi-mental techniques to monitor these changes.

2. Experimental details

A high-vacuum chamber (base pressure 10−4 Pa) was usedto prepare the tungsten oxide films. WO3 (Alfa Aesar-99.998%)was thermally evaporated from a tungsten crucible onto vacuum-cleaved NaCl (001) surfaces at varying substrate temperatures(298 K–580 K) in 10−2 Pa O2. Ga2O3 (Alfa Aesar-99.99%)was evaporated from a tantalum crucible at 580 K substratetemperature, also in 10−2 Pa O2. (Co-) deposition of the re-spective oxides was carried out at comparable evaporation rates(10–14 Å/s). For co-deposition, the deposition rates of the in-dividual oxides were set up at first to result in a superposeddeposition rate required for the desired film stoichiometry. De-position rates and the nominal film thickness (usually 25 nm)were measured by a quartz crystal microbalance. The resultingfilmswere floated and rinsedwith distilledwater, dried and finallymounted on gold grids for electronmicroscopy. Subsequently, thethin films were subjected to oxidative (up to 873 K, 1 bar O2, 1 heach) and reductive treatments (up to 773 K, 1 bar H2, 1 h each)and the corresponding structural and morphological changesmonitored by TEM and SAED with a ZEISS EM 10C micro-scope. The electron diffraction patterns were externally cali-brated with respect to the reflections of an as-deposited, untreatedPt/SiO2 catalyst.

XPS was used to control the purity of the sample and itsoxidation state and were performed in a Thermo MultiLab 2000spectrometer equipped with a Mg/Al standard twin anode X-raysource. Usually, spectra were collected using Al Kα radiation.Due to the small-size sample on the TEM grid, small area XPSdetection with a nominal spatial resolution of 600 μm waschosen in the experiments. All XPS measurements were carriedout at room temperature in ultra-high vacuum with a basepressure of about 3×10−8 Pa. A non-linear least squares curvefitting program (XPSPEAK Version 4.1 software) with a Voigt

line shape function and Shirley background subtraction wasused to deconvolute the XPS spectra.

3. Results and discussion

3.1. Influence of the substrate temperature on the structure ofthe WO3 thin film

Fig. 1 shows the evolution of the WO3 film structure as afunction of increasing substrate temperature. Starting with auniform amorphous layer after deposition at around roomtemperature (298 K, Fig. 1a), beginning structural changes werenoticed after deposition at around 430 K. A porous filmstructure with increased contrast starts to develop (Fig. 1b) andgets even more pronounced as the deposition temperature isincreased to ∼580 K. At this temperature, an array of lengthy,interconnected, irregularly-shaped grains is observed. We notethat, irrespective of the preparation conditions, the electrondiffraction patterns only show very fine-crystalline, almostamorphous samples. To further clarify the chemical state of thetungsten oxide films grown at different substrate temperatures,we performed XPS studies on representative samples. Fig. 2shows a set of W 4f spectra taken from the films grown at298 K, 430 K and 580 K (second, third and fourth spectrumfrom the bottom) in comparison with an oxidized sample,prepared by annealing the 298 K-deposited sample in 1 barO2 at 773 K for 1 h. This oxidized sample (discussed in detailbelow) unambiguously shows an SAED pattern mainly attri-butable to hexagonal WO3. The W 4f5/2 and W 4f7/2 peaksmeasured at binding energies of 38.3 eV and 36.2 eV, re-spectively, coincide with literature-reported W 4f binding ener-gies measured on similar WO3 thin films [8,10,20], and thusare used as the binding energy reference point for the XPSexperiments discussed below. Obviously, none of the spectraexhibit a considerable binding energy shift as compared tothe oxidized sample, which provides clear evidence that thestoichiometry of the tungsten oxide is close to WO3 irrespectiveof the preparation conditions. Studies of Pal and Jacob [17] onthe influence of the substrate temperature on the oxide filmgrowth in a modified hot filament chemical vapor depositionsystem showed a continuous change of ex-situ collected W 4fspectra from sub-stoichiometric WOx to complete WO3 as thesubstrate temperature increased from 298 K to 700 K. Hence, weexclude a possible reoxidation of the samples upon transforma-tion into the electron microscope and conclude that thestoichiometry is close to the ideal WO3 already after depositionat 298 K.

Although several studies on the influence of the substratetemperature on the structure and morphology of WO3 thin films(prepared by different methods) are available [10,17], we notesome differences to the WO3 films discussed here. Reportshave been presented that the crystallinity of the resulting filmsstrongly depends on the substrate temperature, ranging fromalmost amorphous films at very low substrate temperatures(i.e. 298–373 K) to crystalline samples at high temperatures(523–573 K) [10]. Nevertheless, it was also reported for chem-ical vapor-deposited films of about similar nominal thickness

Fig. 1. TEM images of the WO3 thin films deposited at different substratetemperatures. (a) 298 K, (b) 430 K and (c) 580 K. Two representative SAEDimages are shown as insets in (a) and (c).

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that the formation of a crystalline structure only started ataround 600 K, below which the deposited material was amor-phous [17]. For films sputtered from a WO3 target in Ar/O2

mixtures it was also observed, that the degree of crystallinity isstrongly influenced by the O2 partial pressure [12]. For low O2

partial pressures, almost amorphous structures were obtained,whereas increasing the O2 pressure led to crystalline WO3 filmsat the same substrate temperature. However, the film discussedin the present contribution is amorphous irrespective of thesubstrate temperature used. We attribute this result to our highdeposition rate, which suppresses the ordering and hence, thecrystallization of WO3.

3.2. Oxidation of the WO3 thin film

WO3 is known to undergo considerable structural trans-formations upon annealing in the temperature range between473 and 773 K [9]. Al Mohammed et al. conducted a detailedstudy on the phase transformation in WO3 thin films (depositedat ∼600 K) during annealing in the temperature range 300 K–723 K. Starting with a monoclinic structure after deposition theyobserved the transformation into a hydrated structure whichthen becomes converted into a hexagonal structure at around423–473 K. This structure was found to be stable up to around673 K and finally re-transformed into the monoclinic structure[9]. As our films are clearly amorphous after deposition even atthe highest substrate temperatures, it was therefore interesting tosee whether there are substantial differences in the annealing/oxidation behaviour upon starting from an amorphous WO3 thinfilm.

First notable differences in the film structure with respect toearlier work can already be detected in the temperature range

Fig. 2. W 4f XPS spectra of WO3 film deposited at different substratetemperatures (first three spectra from the top) and aWO3 film deposited at 298 Kand subsequently oxidized at 773 K in 1 bar O2 for 1 h.

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Tb673 K. The WO3 film obtained after an oxidative treatmentat 473 K is shown in Fig. 3a as a representative example.Compared to the as-grown state, the film shows minor signsof sintering indicated by the annealing of grain boundaries.However, no other considerable changes in the film structure aswell as in the SAED patterns (inset), which show only diffuserings and halos, are noticeable. This result is already in strongcontrast to previous studies on crystalline WO3 films, where –at comparable temperatures – the film was already transformedinto the hexagonal WO3 structure. Noticeable changes toward acrystallization of the film can only be detected after oxidationat 673 K (Fig. 3b). It now consists of a porous low-contrastbackground structure with large, irregularly-shaped darkercrystals with a characteristic SAED pattern (inset). The SAEDpatterns of these patches are typically very complex and seem toarise from the simultaneous presence of various WO3 structuresin different orientations. Nevertheless, it is sometimes possibleto detect islands which appear to consist of a single phase.The SAED pattern in Fig. 3b (inset) reveals a quasi tetragonalstructure with pronounced reflections at (among others) ∼5.25,3.62, 3.10, 2.69, 2.32, 1.94 and 1.80 Å. Indeed, the reflectionscan be assigned to a tetragonal WO3 phase (a=7.39 Å, c=3.88 Å, space group P-421 m) [21]. However, the reflectionscan also be attributed to a (almost tetragonal) monoclinic WO3

structure (a=7.29 Å, b=7.53 Å, c=7.68 Å, β=90.91°) [22]previously reported after deposition of WO3 at high substratetemperatures [9]. We do also not fully exclude the simultaneouspresence of two different phases, but in this case a pronouncedcrystallographic relationship and a high degree of interfacialordering between these WO3 phases would be required to obtainan SAED pattern like the one shown in Fig. 3b.

Raising the oxidation temperature to 773 K induces evenmore changes to the film structure (Fig. 4). Although the large,dark islands are still present, most of the film seems to consist ofrather extended platelet-like grains with varying contrast. In theupper left corner, two plates with pronounced contrast areclearly visible. On some places of the film with lower contrast

Fig. 3. TEM images of WO3 films oxidized at 473 K (a) and 673 K (b) in 1

(denoted “A” in Fig. 4a), extended lattice fringes of about 6.3 Ådistance are visible (Fig. 4b). These spacings can be assignedto the (100) lattice spacings of one of the hexagonal WO3

structures (a=7.32 Å, c=7.66 Å; dtheor(100)=6.34 Å [23];a=7.29 Å, c=3.89 Å, dtheor(100) = 6.32 Å [24]). Thecorresponding SAED pattern of this area shows a single-crystalline, hexagonal structure, but the lattice spacings of bothhexagonal structures are again very similar, and therefore a clearassignment is difficult. We, however, note that the hexagonalstructure discussed in the work of Al Mohammed et al. is theone with lattice spacings of 7.32 Å and 7.66 Å, respectively [9].The hexagonal structure discussed here was exclusivelyobserved on low-contrast regions on the film, but never onthe darker patches or the platelets. The interpretation of thedarker patches is not as straightforward. Taking the SAEDpatterns into account, which usually exhibit (quasi) tetragonalstructures in various orientations, it appears that these areasare dominated by tetragonal/monoclinic WO3 structures. Theplatelets usually show a fringe contrast typical for ordered planedefects occurring in reduced WO3 crystals [9]. Hexagonal WO3

persists and is the dominant structure even at higher oxidationtemperatures (873 K).

Summarizing the oxidation/annealing behaviour, it appearsthat an initially present amorphous structure inhibits thecrystallization and reconstruction of the entire WO3 film. Thecrystallization sets in at around 673 K, accompanied by theformation of a tetragonal/monoclinic WO3 structure. Afteroxidation at 773 K, the film structure is dominated by hexagonalWO3 phases, but still other WO3 modifications (most probablya tetragonal/monoclinic structure and defect WO3−x structures)are present. In contrast to the work of Al Mohammed et al., whoreported the monoclinic structure(s) as the most stable onesat the highest oxidation temperatures (700 K), a hexagonalstructure is in our case the dominant one at comparable oxida-tion temperatures (773–873 K).

Also co-deposited Ga2O3 as a “dopant” can induce differentstructural properties of WO3 films and change the oxidation and

bar O2 for 1 h. The corresponding SAED patterns are shown as insets.

Fig. 4. Overview TEM image of the WO3 thin film after oxidation at 773 K in 1 bar O2 for 1 h (a), a high-resolution detail with WO3 (100) lattice fringes (b) and thecorresponding SAED pattern (c). Both the high-resolution and the SAED pattern are taken from the area denoted as “A” in the overview TEM image.

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annealing behaviour, as will be outlined below. WO3–Ga2O3

films were prepared as outlined in the Experimental detailssection and subjected to similar oxidative treatments as the pureWO3 films. We performed these studies with two differentmixtures of WO3 with Ga2O3 with varying stoichiometry (W:Ga ∼80:20 and 50:50%, respectively). The results of anoxidation at 773 K in 1 bar O2 for 1 h are shown in Fig. 5a forthe 80:20 film and in Fig. 5b for the 50:50 sample. It is clearthat, irrespective of the Ga2O3 content, the reconstruction of theWO3 films is completely suppressed at comparable tempera-tures. Both films largely maintained the structure of the as-deposited films (not shown here) and the respective SAEDpatterns indicate amorphous samples.

Fig. 5. TEM images of co-deposited WO3/Ga2O3 thin films (deposition temperature∼ 80:20, (b) W/Ga ratio ∼50:50. The corresponding SAED patterns are shown as i

3.3. Reduction of the WO3 thin film

Mainly in view of earlier catalytic and structural studies onWO3-supported Pd and Pt catalysts used in hydrogenation andalkane isomerization reactions [8], we completed the study ofthe WO3 thin film properties by focussing on the reductivebehaviour of the WO3 structure. It was reported that manyreaction mechanisms occurring during the reduction in hydrogenatmosphere are crucially influenced by the presence of WO3−x,WO2 and evenWmetal [25]. A detailed study of the reduction ofWO3 as a function of temperature was performed by Schubertand indicated the stepwise reduction ofWO3 overWO3−x phasesinto WO2 and finally metallic W (β- and α-W) [8].

580 K) and subsequently oxidized in 1 bar O2 for 1 h at 773 K. (a) W/Ga rationsets.

Fig. 7. W 4f XPS spectra of WO3 film reduced in bar 1 h H2 for 1 h at differenttemperatures. From the bottom to the top: 573 K, 673 K, 723 K and 773 K. Thetop spectrum represents metallic bulk W.

Fig. 6. TEM images of WO3 films reduced at (a) 573 K, (b) 673 K and (c) 773 K.The corresponding SAED patterns are shown as insets.

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3.3.1. TEM analysisElectron micrographs of the WO3 film deposited at 298 K

and subsequently reduced at increasing temperatures in 1 bar H2

for 1 h, are shown in Fig. 6. Below 673 K no structural changeswere noticed. Fig. 5a shows the film after reduction at 573 Kand, by comparison with Fig. 1a, it is obvious that the filmstill exhibits a low-contrast structure represented by an SAEDpattern with diffuse rings. First changes in the structure areinduced by raising the reduction temperature to 673 K (Fig. 5b).Small irregularly-shaped dark grains start to form (size 5–25 nm). The corresponding SAED patterns confirm beginningcrystallization and show some broadened ring reflections. Thestructural changes become more pronounced if the reductiontemperature is further raised to 773 K. The film structure nowhas two very characteristic features: Rather large, darker patches(size up to 50 nm) and a grey, small-size grain structure (sizeabout 3–5 nm). The SAED pattern (inset) exhibits sharp ringreflections at ∼3.56, 2.51, 2.23, 2.03, 1.77, 1.44, 1.39 and1.33 Å, coinciding with the (110), (200), (210), (211), (220),(222), (320) and (321) reflections of the cubic β-W metal struc-ture [dtheor(110)=3.56 Å, dtheor(200)=2.52 Å, dtheor(210)=2.25 Å, dtheor(211)=2.05 Å, dtheor(220)=1.786 Å, dtheor(222)=1.45 Å, dtheor(320)=1.40 Å, dtheor(321)=1.34 Å] [26]. β-Wcrystallizes in a cubic A15 structure (a=5.04 Å, space groupPm-3n) and is usually prepared by hydrogen reduction of WO3

powder, among other methods [27,28]. We also note, that thestructure obtained by reduction at 773 K very much resemblesthe structure of β-W films prepared by RF-sputtering andimaged by AFM and TEM [29,30].

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3.3.2. XPS measurementsFurther information on the redox state of the differently

prepared WO3 films can be obtained by XPS experiments[8,17]. Fig. 7 shows a set of W 4f spectra from the WO3 thinfilm reduced at different temperatures (573–773 K) togetherwith a W 4f spectrum from bulk metallic W (top graph). Fig. 8represents all the spectra of the reduced samples decomposedinto six different components corresponding to three W 4fdoublets. The W 4f7/2 and W 4f5/2 peaks of the W

6+ componentwere assigned to the components at 36.2 eV and 38.3 eV,respectively. The spectrum obtained from the sample reduced at573 K is similar to the WO3 spectra shown in Fig. 2 and doesneither exhibit a considerable binding energy shift nor an ad-ditional component of reduced WO3. This observation supportsthe TEM measurements showing no structural changes andno significant change in the SAED pattern after reduction at573 K. In agreement with the TEM reduction experiments at673 K, the W 4f spectrum of this sample exhibits a pronouncedshoulder at ∼34.4 eV, assigned to the presence of a second,more reduced WO3−x phase. The decomposed spectrum conse-quently accounts for the presence of two additional components(W 4f components at 34.4 eVand 36.5 eV, respectively). This W4f doublet occurs at lower binding energies and coincides withthe W 4f peaks of W5+ [20]. By comparing the peak areas therelative ratio of W6+ to W5+ was determined to be ∼82:18%.Note that this is the temperature (i.e. 673 K) where first signsof crystallization/reconstruction of the films were evident inTEM. The spectrum of the sample reduced at 723 K shows twovery pronounced components at much lower binding ener-gies, but the W 4f doublet of W6+ still persists. Decompositionreveals the simultaneous presence of W6+, W5+ and the W 4f

Fig. 8. W 4f spectra of the WO3 film reduced at 573 K (bottom right), 673 K (bottom(solid line), W5+ (dashed line) and metallic β-W (dotted line).

doublet of two additional components at 32.0 eV and 34.2 eV.The latter doublet gets even more pronounced after reduction at773 K. Combining SAED patterns and the reference bulk metalW 4f spectrum, we conclude that these components correspondto metallic tungsten (β-W). The relative ratios of W6+:W5+:W0

are approximately 58:31:11% and 55:14:31% after reduction at723 K and 723 K, respectively. Note that the amount of W6+ hasdecreased from 82% to about 58% after reduction at 673 K and773 K, but stagnates at reduction temperatures above 723 K at55%. In contrast the amount of W5+ has increased from 18% to31% and decreases again to about 14%. Correspondingly, W0

increases from 11% to 31%, i.e. by almost the same amountas the W5+ component decreases. As the W6+ component staysconstant, this implies that the W0 component is formed byreduction of the W5+ component rather than by direct reductionof W6+. We also note, that a considerable amount of W6+

remains even after reduction at 773 K but the correspondingSAED patterns only showW0. This can be explained in terms ofthe amorphicity of WO3 in the as-grown state.

4. Conclusions

In conclusion the present results suggest a dominant roleof the initial film crystallinity and the presence of dopants onthe thermal annealing and oxidation behaviour of WO3. Incomparison to initially crystallized WO3 films the reconstruc-tion (and hence, crystallization) occurring during annealing/oxidation treatments is suppressed in amorphous WO3 films upto 673 K. By deliberate doping with Ga2O3, this reconstructionwas also found to be completely suppressed in co-depositedWO3/Ga2O3 films at least up to 773 K. As many catalytic

left), 723 K (top right) and 773 K (top left) plus their decomposition into W6+

2836 S. Penner et al. / Thin Solid Films 516 (2008) 2829–2836

reactions involving hydrogen occur in the presence of partiallyreduced WO3, the outlined results on the reduction of WO3,which was found to proceed stepwise from W6+ over W5+ toW0, may also help to gain more insight into the interaction ofWO3 with hydrogen.

Acknowledgements

Financial support by the Austrian Science Fund (FWF) andthe West Austrian Initiative for Nano Networking (WINN) isgreatly acknowledged.

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