Article pubs.acs.org/cm
Observation of Substrate Orientation-Dependent Oxygen Defect Filling in Thin WO3−δ/TiO2 Pulsed Laser-Deposited Films with in Situ XPS at High Oxygen Pressure and Temperature Artur Braun,*,†,‡ Funda Aksoy Akgul,§,⊥ Qianli Chen,†,¶ Selma Erat,†,∥,▽ Tzu-Wen Huang,† Naila Jabeen,§,# Zhi Liu,§ Bongjin S. Mun,×,+ Samuel S. Mao,▼,◆ and Xiaojun Zhang▼,◆ †
Laboratory for High Performance Ceramics, Empa. Swiss Federal Laboratories for Materials Science and Technology, CH-8600 Dübendorf, Switzerland ‡ Hawaii Natural Energy Institute, University of Hawaii at Manoa, Honolulu, Hawaii 96822, United States § Advanced Light Source (ALS), Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ⊥ Physics Department, Nigde University, TR-51240 Nigde, Turkey ¶ Physics Department, ETH Zürich, Swiss Federal Institute of Technology, CH-8093 Zürich, Switzerland ∥ Department of Materials, ETH Zürich, Swiss Federal Institute of Technology, CH-8093 Zürich, Switzerland ▽ Faculty of Engineering, Electrical-Electronics Department, Toros University, TR-33140 Yenisehir, Mersin, Turkey # Nanosciences & Catalysis Division, National Centre for Physics, Islamabad 44000, Pakistan × Department of Applied Physics, Hanyang University, ERICA, Ansan, Korea + Ertl Center for Electrochemistry and Catalysis, Gwangju Institute of Science and Technology, Gwangju, Korea ▼ Department of Mechanical Engineering, University of California at Berkeley, Berkeley, California 94720, United States ◆ Environmental Energy Technologies Division, Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States S Supporting Information *
ABSTRACT: Substoichiometric tungsten oxide films of approximately 10 nm thickness deposited with pulsed laser ablation on single-crystal TiO2 substrates with (001) and (110) orientation show defect states near the Fermi energy in the valence-band X-ray photoelectron spectroscopy (XPS) spectra. The spectral weight of the defect states is particularly strong for the film grown on the (001) surface. In situ XPS under an oxygen pressure of 100 mTorr shows that the spectral weight of the defect states decreases significantly at 500 K for the film on the (110) substrate, whereas that of the film grown on the (001) substrate remains the same at a temperature up to 673 K. Furthermore, diffusion of titanium from the substrate to the film surface is observed on the (110) substrate, as is evidenced by the sudden appearance of the Ti 2p core level signature above 623 K and below 673 K. The film grown on the (001) surface does not show such an interdiffusion effect, which suggests that the orientation of the substrate can have a significant influence on the high-temperature integrity of the tungsten oxide films. Quantitative analysis of the O 1s core level XPS spectra shows that chemisorbed water from sample storage under ambient conditions is desorbed during heating under oxygen exposure. KEYWORDS: tungsten oxide, oxygen defect, oxygen vacancy, valence band, photoemission spectroscopy, XPS, defect filling, ambient-pressure XPS, photoanode, solar water splitting, thin film, WO3, TiO2, photoelectrochemistry, interdiffusion, diffusion barrier
■
INTRODUCTION
materials such as electrodes, current collectors, and solid electrolytes or barrier layers. We demonstrate in this work how defect states in the electronic structure of tungsten oxide films grown on TiO2 single crystals with two different crystallographic orientations behave upon oxidative thermal treatment. TiO2 is an ultraviolet-absorbing wide-band-gap semiconductor
The function of modern electric devices depends more and more on heterostructured architectures of materials and components.1 For example, in multijunction solar cell electrodes, various layers of semiconductors are stacked in order to enhance their efficiency.2 On top of the general layout of such heterostructures, the processing steps can have a significant, sometimes even decisive, influence on the functionality of the components and devices.3 It has been highly regarded that the electronic structure is the key to relevant properties for © 2012 American Chemical Society
Received: June 12, 2012 Revised: August 1, 2012 Published: August 1, 2012 3473
dx.doi.org/10.1021/cm301829y | Chem. Mater. 2012, 24, 3473−3480
Chemistry of Materials
Article
Figure 1. X-ray reflectograms of the nominal 10 nm thin films grown on TiO2 with orientations (100), (110), and (001) before (blue color) and after (yellow color) thermal treatment in oxygen. Parallel, nominally 100-nm-thick films were prepared under the same conditions, the study of which was reported in ref 14. The deposition time for the 10 nm thin films studied here was 90 s. The asdeposited films had a bluish color. The films were subject to X-ray diffraction (XRD) and X-ray reflectometry (Philips X’pert, Cu Kα). Xray photoelectron spectroscopy (XPS) spectra were recorded at an ambient-pressure XPS end station15 at Beamline 9.3.2 of the ALS, Lawrence Berkeley National Laboratory. The energy resolution of the beamline was set to E/ΔE = 3000.16 This end station allows for XPS studies while the sample can be heated up to 1273 K and be exposed to up to 1 Torr of oxygen (or other gases) pressure during XPS data acquisition. After recording conventional XPS spectra at an ultrahigh vacuum (UHV) of 10−10 Torr, we supplied approximately 100 mTorr of oxygen to the chamber. We then heated the sample stepwise while recording XPS spectra and also constantly monitoring the sample temperature, chamber base pressure, and oxygen partial pressure. It has been reported that films deposited at 100 mTorr of oxygen pressure at 846 K on glass substrates were near-stoichiometric monoclinic WO3.17
with an energy band gap of 3.1 eV and an excellent photocatalyst.4 WO3 is a semiconductor with 2.7-eV band-gap energy and absorbing in the yellow visible-wavelength range.5 Tungsten oxide is inherently substoichiometric WO3−δ and typically requires an oxidative thermal after-treatment to be converted to WO3 with yellow color. WO3−δ is blue and has metal-type conductivity (dρ/dT > 0); the latter does not develop its photoelectrochemical function unless oxidized to yellow WO3. An assumed proportionality between the density of color centers and the density of states contributing to hopping conductivity has been experimentally supported.6 WO3/TiO2 is an interesting combination of materials in which UV-irradiated TiO2 ejects photoelectrons into WO3 and thus turns the tungsten oxide blue. In practical applications, this can be used for charge storage,7 anticorrosive coatings,8 and improved photocatalysts.9 Thin TiO2/WO3 multilayers have higher photocatalytic activity in visible light than TiO2 alone.10 Epitaxial WO3 films can have conductivity and gas-sensing response superior to those of polycrystalline films11 and can be deposited up to 700 nm on SrTiO3(001).12 Films postannealed in oxygen have a better electric-fieldinduced resistive switching behavior than as-deposited films. This switching improvement was attributed to the decrease in the density of states at the surface.13 The gas-phase oxidation of WO3−δ is a complex process by itself. We have recently shown how WO3 deposited on TiO2 single crystals can have a remnant substoichiometric, oxygen-deficient layer at the top surface and the interface with TiO2,14 which potentially could affect their function as photoanodes in photoelectrochemical cells. Oxidative treatment at elevated temperatures is generally a necessary process step to convert WO3−δ into WO3 or to make metal oxides stoichiometric in general. Such high-temperature treatment may activate unwanted diffusion and chemical processes that can destroy the architecture of film assemblies.
■
■
RESULTS AND DISCUSSION X-ray Reflectivity and XRD of the Films. The asdeposited films had a bluish color, which is typical for a substoichiometric tungsten oxide with oxygen vacancies.7 After thermal oxidation at 673 K, the films assumed a yellow color. The X-ray reflectograms of the films in Figure 1 show the influence of substrate orientation and thermal oxidative aftertreatment on the growth behavior. The film as-deposited on the (100) surface shows Kiessig oscillations corresponding to 12.5 ± 0.2 nm total thickness including both a surface roughness of 2 ± 0.3 nm and an interface roughness of 0.5 ± 0.1 nm from the refinement results.14 The thermal after-treatment in oxygen causes a slight shift of the reflectrogram toward larger angles, revealing a slight shrinking of the film thickness. An alternative possibility after thermal treatment is that the roughness of the film could be increasing. However, given that in our investigation the surface roughness is about the same value as the error bar before and after heat treatment, it appears that the film thickness is rather shrinking. This is likely a consequence of oxygen vacancies becoming filled, which results in an increase of the W6+ ion concentration. With the ionic radius of W5+ being larger than that of W6+,18 the unit cell and ultimately the film volume is expected to decrease, which is in agreement with the reflectograms for the film grown on the (100) substrate. On the basis of the angle of total reflection, we find that the film density is around 95% of the bulk density of WO3, i.e., approximately 6.8 g/cm3.
EXPERIMENTAL SECTION
Three compact films of approximately 10 nm thickness were deposited on three different single-crystal TiO2 substrates (rutile, tetragonal symmetry, lattice constants a = b = 0.459 nm and c = 0.296 nm) with (100), (001), and (110) orientations (CRYSTEC, Berlin) by pulsedlaser ablation from a WO3 target (99.9% purity, American Elements) with 1126 K substrate temperature and 10 mTorr oxygen partial pressure. The excimer laser (Lambda Physik LPX 210i, 248 nm wavelength) was run at 160 mJ pulse energy and 10 Hz repetition rate. The laser spot size was 1 mm × 4 mm (rectangular). Because of the energy loss of mirrors and windows (50% loss), the actual energy fluence was 2.0 J/cm2. 3474
dx.doi.org/10.1021/cm301829y | Chem. Mater. 2012, 24, 3473−3480
Chemistry of Materials
Article
The reflectogram of the film deposited on the (110) surface shows in the reflection angle range from 1° to 1.5° a convolution of two oscillations, indicative of two layers. Modeling of the reflectogram with least-squares fitting suggests that we have a layer of 6.5 ± 0.2 nm and also a layer of 6 ± 0.2 nm thickness. We are not able to discriminate which of those is the bottom layer and which is the top layer. After annealing in oxygen to 673 K, the two-layer film reconstructs toward a more homogeneous single layer, but a remaining slight shoulder at 1.4° suggests that the film is not entirely relaxed into a completely homogeneous phase. The film deposited on the TiO2(001) surface has a reflectogram similar to that grown on the (100) surface but with far better developed intensity maxima. Upon oxidation, the reflectogram shifts toward smaller angles, suggesting an increase of the film thickness. XRD analyses of our as-deposited films (shown in Figure S1 in the Supporting Information) show that the film grows with a {022} preferred orientation on the (001) substrate. The diffractogram shows the monoclinic WO (pseudocubic) structure and, for comparison, also the diffractogram of a 100 nm film grown under the same conditions, which also grows in a {022} orientation. The Bragg reflections around 32.8−34.5° belong to a combination of broadened Bragg (022), (−202), (202), and (220) peaks, the widths of which are determined by the low thickness of the films and by crystal size effects. The same holds also for the 10 and 100 nm14 films grown on the (100) and (110) substrates (Figures S2 and S3 in the Supporting Information). The XPS survey scans of the as-deposited blue films on the (110) and (001) substrates in Figure 2 were recorded in UHV
at ambient temperature before annealing in oxygen. Both spectra look virtually identical. Within the resolution limits of XPS with 700 eV photon energy, we identify no traces of titanium at the expected energy positions of the titanium core levels (for example, between 455 and 465 eV) prior to the thermal oxidative after-treatment. WO3/TiO2(110). The as-deposited blue films were subject to XPS measurements during exposure to 100 mTorr of oxygen, while the films were simultaneously heated stepwise from ambient temperature to 678 K. Emphasis was put on the valence-band (VB) characteristic because defect states arising from oxygen vacancies show up in the VB spectra of substoichiometric tungsten oxide as a series of four to six peaks.14,19,20 Figure 3 shows magnified the evolution of the VB region of the film deposited on the (110) surface while being heated during exposure to 100 mTorr of oxygen (left panel), together with the evolution of the temperature with time (right panel). The VB XPS spectrum of WO3−δ has been studied previously; see, for example, ref 20. The as-deposited film shows near the Fermi energy (EF) a slight W 5d defect state structure at ambient temperature from oxygen vacancies, and up to 524 K, beyond which it becomes barely noticeable. The O 2p bonding peak at 4 eV is followed by hybridized O 2p−W 5d resonances around 7−8 eV. In our recent ex situ study on 100-nm-thick WO3 films, we deconvoluted the defect region near EF into four resonances.14 Here, we performed a quantitative analysis on all recorded spectra shown in Figure 4; we subtracted a linear background so as to be able to give an absolute height for the spectral weight of the defect-state region. This is exemplary exercised for the spectra recorded at 296, 477, 524, and 678 K in Figure 4. The spectra recorded at 296 and 477 K show a pronounced defect peak intensity around 0 eV, whereas this energy range is virtually flat for the spectra recorded above a critical temperature of 523 K, i.e., for the two spectra recorded at 524 and 678 K. The relative peak height, i.e., spectral weight of this cumulated W 5d defect peak, is plotted versus the temperature in Figure 5. At ambient temperature and above, this peak height is 0.2 units or slightly above. At 523 K, the peak height decreases abruptly to below 0.1 units and remains at about this value for 673 K, the highest temperature to which the film was exposed. The gray solid line in Figure 5 is a least-squares fit of an arctan function in order to determine the critical temperature Tcrit = 500 K, at which the defect states
Figure 2. XPS survey scans of the 10 nm film as-deposited on the (110) substrate, bottom spectrum, and on the (001) substrate, top spectrum, with Eph = 700 eV.
Figure 3. VB XPS spectra of the 10 nm WO3−δ/TiO2(110) film recorded in situ while the film was heated from 296 K (bottom spectrum) to 678 K (top spectrum) at 100 mTorr of oxygen pressure with Eph = 700 eV. Spectra are shifted on the intensity axis for comparison. The temperature and gas concentration profiles during the experiment are shown on the right. 3475
dx.doi.org/10.1021/cm301829y | Chem. Mater. 2012, 24, 3473−3480
Chemistry of Materials
Article
adjustment of the oxygen octahedral using shear mechanisms, without formation of oxygen vacancies,19 as was already earlier suggested by Matthias’ observation of the softness of WO3 single crystals.24 WO6 octahedra are the building blocks of tungsten bronzes, which provide open structures for the insertion of cations.25 However, the 5% titanium-substituted WO3 suffers from structural instabilities; i.e., titanium is a crystallization inhibitor,26,27 which may be an additional potential origin for titanium diffusion. Figure 6 shows the oxygen core level (O 1s) spectra of the film grown on the (110) substrate. We show the spectra aligned
Figure 4. Range of the VB XPS spectra of 10 nm WO3−δ/TiO2(110) showing the W 5d defect peaks near the Fermi level, recorded in situ while the film was heated from 296 to 678 K at 100 mTorr of oxygen pressure with Eph = 700 eV. The spectra are shifted on the intensity axis for easier comparison. The horizontal black lines mark the level of zero intensity.
significantly decrease. This temperature is indicated in the temperature profile in Figure 3 (right panel) with a horizontal line. It is noteworthy that the relative height of the defect peak does not completely vanish above Tcrit, suggesting that the film contains some remaining defect states despite the oxidative treatment at high temperature. The right panel in Figure 5 shows the Ti 2p core level region recorded under 100 mTorr at 623 K and also at 673 K. At 673 K, we identify the Ti 2p peaks as a clear signature from titanium. A comparison with the literature spectra21 reveals that the peak position shown in the right part of Figure 5 is Ti4+. At 623 K, we cannot identify any Ti 2p signature yet; see Figure 5, right panel. We believe that diffusion channels open up in the tungsten oxide film between 623 and 673 K, which allows interdiffusion of titanium from the TiO2(110) substrate to the WO3−δ film surface, notwithstanding that in return also tungsten might have diffused into the substrate, as was reported to occur at 673 K.22 It remains open whether there are structural defects such as dislocations that allow for titanium diffusion22 or whether there is bulk diffusion of titanium similar to proton conductivity in perovskites.23 WO3 belongs to a class of materials that can tolerate deviations from stoichiometry by
Figure 6. Evolution of the O 1s spectra of the film grown on the (110) substrate during heating recorded at a photon energy of 700 eV with p(O2) = 111 mTorr. In the middle are two deconvoluted spectra from 300 K (bottom with three Gaussians) and 673 K (top with one Gaussian for Ox only).
on the energy axis. At first, under UHV at 300 K, the spectral weights of adsorbed water [H2O(v)] at 533 eV and hydroxyl groups (OH−) at 531 eV are in about the same intensity range as that from structural oxygen Ox (WO double bonds) at 530 eV. Water adsorbed from the ambient environment can dissociate on the WO3 film and form adsorbed component radicals28 according to the reaction H 2O(g) ↔ H 2O(a) → •H(a) + •OH(a)
On iron sulfides, for example, this reaction is the dominant surface reaction causing adsorbed hydroxyl on the surface.29 The water partial pressure on our WO3−δ films under ambient conditions is high enough to allow for the above reaction to take place and thus be valid as well. Upon exposure to oxygen and then heating, the spectral weight of the water peak decreases homogeneously to below
Figure 5. (Left) Relative peak height of the W 5d defect state range near EF in the VB XPS spectra of 10 nm WO3−δ/TiO2(110) recorded in situ with Eph = 700 eV while the film was heated from ambient temperature to 678 K, keeping 100 mTorr of oxygen partial pressure. (Right) Ti 2p spectra recorded at 623 K (open symbols, no signature from Ti) and 673 K (filled symbols), the latter of which shows clearly the Ti 2p peaks. 3476
dx.doi.org/10.1021/cm301829y | Chem. Mater. 2012, 24, 3473−3480
Chemistry of Materials
Article
Above 328 K, it is obvious that the relative portion of structural oxygen is steadily increasing with the cost of the hydroxyl and water, which are decreasing. At around 500 K, the relative portions of structural oxygen and hydroxyl assume a plateau level or saddle point, from which they depart again at above 550 K. The trend of the structural oxygen and hydroxyl groups is indeed mirrored, as is obvious from Figure 7. The water content is at 500 K not significantly changing and very close to zero intensity within the error bars. A comparison of the temperature dependence of the W 5d defect peak in the VB spectra (Figure 5) and the O 1s spectra (Figure 7) shows immediately the correlation of the diminishing of the W5 d defect peak at 500 K and the saddle point of the structural oxygen, as is evident from the O 1s spectra. Considering both trends, i.e., that of defect peak depletion and oxygen vacancy filling, in Figure 8, it is interesting to note
the detection limit of XPS at 473 K, suggesting thermal desorption of the water. The spectral weight of the hydroxyl at 531 eV increases and that from structural oxygen at 530 eV decreases slightly up to around 373 K. Then, the peak for structural oxygen increases and that of the hydroxyl decreases steadily until the temperature reaches 473 K. Shpak et al. used four molecular species for deconvolution of their O 1s XPS spectra, i.e., O2−, OH−, O−, and H2O,30 whereas Azimirad et al.31 did not include the O− species. Inclusion of the O− species in our deconvolution was not significant and thus was not applied in our further analysis. The decrease of the spectral weight for structural oxygen at 300 K does not imply that the concentration of the structural oxygen in the film decreases; it rather shows that the relative spectral weight grows in favor of the hydroxyl groups. With still increasing temperature, during oxygen exposure, the intensity at 530 eV from the structural oxygen is substantially increasing, more so than the decrease of the hydroxyl and absorbed water signature could compensate for when considering the absolute intensity in the original XPS spectra in Figure 6. This suggests that the WO3−δ film is consuming and integrating the oxygen constantly supplied into the UHV recipient. At about 573 K, the absolute intensity of the Ox peak increases drastically and the peak from water disappears completely to below the resolution limit of XPS. Additionally, the O 1s spectrum shifts toward lower binding energy. This finding appears to be in contrast to a report saying that a temperature as high as 673 K is necessary for reduced tungsten oxide in order to uptake oxygen into the bulk.32 Hence, in the 12 nm thin pulsed laserdeposited film grown on TiO2(110), a temperature of 573 K is sufficient to trigger the incorporation of oxygen into tungsten oxide. We have deconvoluted the spectra in the range from 527 to 536 eV with three Gaussians representative to the components Ox, OH−, and H2O(v), plus a Shirley function for the background. The relative spectral weight of the three chemical components (given in percent) is plotted versus the temperature in Figure 7. The open squares right before 300 K denote the spectral weight at ambient temperature in UHV conditions, while the open circles denote the spectra recorded under exposure of 100 mTorr of oxygen. The dotted lines are cubic splines connecting the data points from spectra of the film deposited on the (110) substrate.
Figure 8. Correlation of the filling of structural oxygen (top panel) and the depletion of oxygen defects (bottom panel), as shown by the correspondence of variation of O 1s spectra and W 5d defects in the VB spectra. The solid black line in the top panel is the TGA (recorded in air) from ref 23.
that thermogravimetric analysis (TGA), shown as the solid black line, reflects our reasoning surprisingly well. The top panel in Figure 8 shows the TGA curve from Figure 4 in ref23, which we have digitized from their published data. Li et al.23 interpret this TGA curve in the framework of dehydration of two different types of water in their WO3·2H2O powders. Metal oxide surfaces in contact with ambient air are typically hydrated, i.e., contain water molecules and hydroxyl groups.6,30 There, each tungsten atom has one terminal oxygen atom, one coordinated water oxygen atom, and four bridging oxygen atoms connecting the WO5(H2O) octahedra to form a charge-neutral WO 3 [H 2 O] layer. 23 The other half of the water is accommodated between these layers as interlayer crystal water.23 The first step of mass loss at 343 K is assigned to the loss of interlayer crystal water. This temperature coincides with the temperature at which the hydroxyl group signature decreases and the structural oxygen signature increases. In parallel, the signature from water is steadily decreasing. The TGA step at 463 K is assigned to the loss of coordinated water. This temperature coincides with the temperature at which the
Figure 7. Evolution of the spectral weight of structural oxygen Ox (top dashed line), hydroxyl groups OH− (middle dashed line), and water vapor H2O(v) (bottom dashed line) in the film grown on TiO2(110) during heating from 300 to 673 K under 100 mTorr of oxygen pressure. Open squares denote the data from spectra recorded at 300 K in UHV. They are shifted for better distinction to lower temperature. 3477
dx.doi.org/10.1021/cm301829y | Chem. Mater. 2012, 24, 3473−3480
Chemistry of Materials
Article
Figure 9. Structural evolution of the film grown on (001) during heating in 100 mTorr of oxygen. The figure on the right shows the temperature profile over time.
Figure 10. (Left) Comparison of the spectral intensity of the oxygen defect structure near the Fermi level of the (001) deposited film during heating in 100 mTorr of oxygen (bottom) and variation of the relative spectral weight of Ox, OH−, and H2O(v). (Right) Corresponding oxygen spectra aligned on the energy axis for a facile comparison of the spectral shape.
The WO3−δ lattice, decorated with crystal water and adsorbed OH− groups, interacts strongly with irradiation by ions, X-rays, UV light, and electrons, influencing the spectral signatures of the samples (radiation damage) and thus potentially causing misinterpretation of the spectra.33 The harsh thermal oxidative treatment in our in situ experiment exceeds the radiation damages, which become negligible. Between 623 and 673 K, titanium is activated to diffuse through the WO3 film to the surface. Given that the thermal treatment causes the two-layer structure, which we know from the X-ray reflectivity measurements, to convert into a one-layer structure, it is not all surprising that such severe structural transformation is paralleled by the diffusion of titanium to the film surface. It would need further structural investigations in order to describe the actual crystallographic and microstructure scenario that takes place during annealing. This is beyond the scope of this work, but we are working toward addressing this issue as well in the near future. WO3/TiO2(001). The situation is somewhat different for the film grown on the (001) substrate. The spectral weight of the defect signature in WO3−δ/ TiO2(001), as shown in Figure 9, is significantly larger than that
decrease of the hydroxyl signature becomes slower, where the increase of the structural oxygen signature becomes slower, and assumes a plateau. At about this temperature, the XPS water signature increases slightly again before it disappears at around 623 K. For the film grown on the (110) substrate, we arrive, hence, at the following interpretation. The as-deposited film has W 5d defects, the spectral weight of which decreases upon heating in 100 mTorr of oxygen at above 500 K to around 50% of its original intensity. The as-deposited film contains a strong O 1s signal from residual water, assigned to interlayer crystal water,23 and hydroxyl groups, assigned to the coordinated water, with reference to ref 23, which all decrease significantly during heating in oxygen. At the same time, the spectral signature from structural oxygen in tungsten oxide is increasing, revealing that the supplied oxygen is filling the oxygen vacancies that were created during film growth. This is corroborated by the observation that the spectral weight from structural oxygen is approaching a plateau at T > 500 K and in line with the rationale proposed in ref 30, where the OH− concentration is proportional to the oxygen deficiency δ in WO3−δ. 3478
dx.doi.org/10.1021/cm301829y | Chem. Mater. 2012, 24, 3473−3480
Chemistry of Materials
Article
of the film grown on the (110) substrate, revealing that the tungsten oxide grows largely oxygen-deficient on (001), more so than on (110). Even annealing to 673 K with 100 mTorr of oxygen does not change the defect structure in the spectra, which is surprising when we compare with ref 32, within which it was found that a temperature of 673 K is necessary for the incorporation of oxygen ions into reduced tungsten oxide films. We also do not find any signal from titanium at the tungsten oxide film surface after heating in oxygen to 673 K, in contrast to the observations with the film grown on the (110) substrate. We have to note, however, that the film grown on (001) was subject to thermal oxidation only for about 2 h (see the temperature profile in Figure 9), whereas the film grown on (110) showed titanium diffusion after around 4 h. This difference in the treatment time was primarily caused by the circumstance that the kind of in situ experiment as presented here depends on instrumental complexity, combined with the limited available synchrotron beamtime. Hence, we cannot absolutely rule out that, after longer oxidation, the W 5d defect peak in the film grown on (001) would decrease and eventually vanish. In analogy to Figure 5 (left panel), we made a quantitative analysis of the height of the defect peak; see the bottom/left panel in Figure 10. The spectral weight of the defect peak remains constant up to at least 673 K. Above this constant trend, we plotted the relative spectral weight of the O 1s spectra from structural oxygen Ox, hydroxyl OH−, and water H2O(v) of the film grown on (001). Unlike with the film grown on the (110) substrate, there is no depletion of the W 5d defect states during heating in oxygen up to 673 K. The four O 1s spectra that we recorded for the film grown on (001) are shown on the right of Figure 10. The as-deposited film contains at 300 K in UHV a considerable spectral signature from water and hydroxyl groups, but to a lesser extent than the film grown on (110). The right panel in Figure 10 shows a direct comparison of the O 1s spectra recorded at 373 K during oxygen exposure. The film grown on the (110) substrate has a more pronounced shoulder from OH− groups and from adsorbed water than the one grown on the (001) substrate. The relative spectral weight for the structural oxygen decreases slightly from 300 to 423 K and then remains constant, whereas the water peak is constant from 300 to 423 K and then decreases slightly. In return, the hydroxyl peak is steadily increasing from 300 to 473 K. A comparison of the oxygen spectra in Figure 10 shows that the absolute intensity at around 530.5 eV, indicative of the structural oxygen, is increased for 423 and 473 K, corroborating that some oxygen is incorporated into the WO3 film lattice. At this point, it remains unclear why the oxygen defect formation appears to depend on the orientation of the substrate surfaces they were grown on. Our observation as such warrants extended studies, beginning with monitoring of the growth modes, for example, with reflection high- or medium-energy electron diffraction in order to see to what extent of layer-bylayer growth is possible. This should be complemented by a crystallographic analysis with glancing-incidence XRD or lowenergy electron diffraction in order to verify potential epitaxial growth. The observation that the tungsten oxide film grows as a two-layer system on (110) and then relaxes toward a one-layer film suggests that a strained phase may have been grown on (110), in contrast to (001). Probably the first 6 or 6.5 nm tungsten oxide grows in a strained phase on (110), and the other 6 nm grows relaxed. The substrate thus preconditions the
growth and possibly also the oxygen vacancies, even though the interaction of the film with the surrounding gas atmosphere takes place at the film surface and then may propagate into the film interior. A recent review35 gives account of the oxygen vacancies predominantly at surfaces of TiO2, ZrO2, CeO2, and V2O5 and the computational tools necessary to calculate the energies necessary to form oxygen defects. For hematite, the energies necessary to form oxygen vacancies at the surface and in the subsurface have been calculated in ref 36; there the defect formation energy shows an oscillatory behavior around 24 eV within the first five oxygen layers underneath the (0001) hematite surface. Hence, computational methods are promising for the further comprehension of defect formation in metal oxides, including tungsten oxide. Meanwhile, experimental progress has been made with oxygen isotope exchange and secondary-ion mass spectroscopy on single-crystal SrTiO3(100), where an oxygen defect concentration profile of several tens of nanometers has been obtained, which is attributed to an equilibrium space-charge layer depleted of oxygen vacancies, followed by a profile extending several micrometers into the solid, which is attributed to diffusion in a homogeneous bulk phase.37 In particular, it was found that the surface termination with oxygen significantly affects the surface exchange coefficients. This approach is, in principle, applicable to tungsten oxide films as well and warrants further studies.
■
CONCLUSIONS The tungsten oxide films grown on single-crystal TiO2 surfaces with (110) and (001) orientations with their blue visual appearance were both oxygen-deficient. The XPS spectra with Eph = 700 eV sample information deep in the film and show that the films are substoichiometric in the bulk and interface regions with a high W5 d defect state density for the film grown on the (001) substrate. The oxidative treatment of the film grown on (110) removes the defect states at a critical temperature of 523 K by an extent that the spectral weight decreases to about 50% of its original intensity. Upon a further temperature increase, the defect density signature remains at that 50% level, but interdiffusion of titanium from the substrate to the film and film surface is activated between 623 and 673 K. Upon this treatment, the bilayer structure transforms into a single layer without significant improvement of X-ray reflectivity. The same thermal oxidative treatment on the film grown on (001) is not sufficient to remove the defect state signature in the VB XPS spectra up to at least 673 K. The relative spectral weight instead remains virtually constant at 0.3 units. The thermal oxidative treatment shifts the spectra toward 0.5 eV higher binding energy, revealing n-type doping upon oxidation. This holds, however, only for the surface. In contrast to the film grown on the (110) substrate, we notice no interdiffusion of titanium into the bulk of the film grown on the (001) substrate or its surface. The film thickness grows slightly with oxidation, and X-ray reflectivity improves. It is difficult to exactly model the X-ray reflectivity curves of the (001) and (110) grown films, likely because the films have a systematic yet unknown oxygen stoichiometry gradient. However, we observe that the (110) grown film has a twolayer structure that reconstructs upon thermal treatment to a one-layer film. This study shows that the electronic structure of thin tungsten oxide films can be different depending on the surface orientation of the substrate on which they were grown. 3479
dx.doi.org/10.1021/cm301829y | Chem. Mater. 2012, 24, 3473−3480
Chemistry of Materials
Article
(12) Garg, A.; Leake, J. A.; Barber, Z. H. Phys. D: Appl. Phys. 2000, 33, 1048−1053. (13) Shang, D. S.; Shi, L.; Sun, J. R.; Shen, B. G.; Zhuge, F.; Li, R. W.; Zhao, Y. G. Appl. Phys. Lett. 2010, 96, 072103. (14) Braun, A.; Erat, S.; Zhang, X.; Chen, Q.; Aksoy, F.; Löhnert, R.; Liu, Z.; Mao, S. S.; Graule, T. J. Phys. Chem. C 2011, 115 (33), 16411− 16417. (15) Grass, M. E.; Karlsson, P. G.; Aksoy, F.; Lundqvist, M.; Wannberg, B.; Mun, B. S.; Hussain, Z.; Liu, Z. Rev. Sci. Instrum. 2010, 81, 053106. (16) Hussain, Z.; Huff, W. R. A.; Kellar, S. A.; Moler, E. J.; Heimann, P. A.; McKinney, W.; Padmore, H. A.; Fadley, C. S.; Shirley, D. A. J. Electron Spectrosc. Relat. Phenom. 1996, 80, 401−404. (17) Ramana, C. V.; Utsunomiya, S.; Ewing, R C.; Julien, C. M.; Becker, U. J. Phys. Chem. B 2006, 110, 10430−10435. (18) Shannon, R. D. Acta Crystallogr. 1976, A32, 751−767. (19) Bringans, R. D.; Höchst, H.; Shanks, H. R. Phys. Rev. B 1981, 24, 3481−3489. (20) Bullett, D. W. J. Phys. C: Solid State Phys. 1983, 16, 2197−2207. (21) Nolan, M.; Elliott, S. D.; Mulley, J. S.; Bennett, R. A.; Basham, M.; Mulheran, P. Phys. Rev. B 2008, 77 (23), 235424. (22) Luca, V.; Blackford, M. G.; Finnie, K. S.; Evans, J.; James, M.; Lindsay, M. J.; Skyllas-Kazacos, M.; Barnes, P. R. F. J. Phys. Chem. C 2007, 111, 18479−18492. (23) Li, Y. M.; Hibino, M.; Miyayania, M.; Kudo, T. Solid State Ionics 2000, 134, 271−279. (24) Matthias, B. T. Phys. Rev. 1949, 76, 430−431. (25) Guo, J.-D.; Reis, K. P.; Whittingham, M. S. Solid State Ionics 1992, 53−56, 305−314. (26) Kalidindi, N. R.; Manciu, F. S.; Ramana, C. V. Appl. Mater. Interfaces 2011, 3 (3), 863−868. (27) Kalidindi, N. R.; Kamala Bharathi, K.; Ramana, C. V. Appl. Phys. Lett. 2010, 97, 142107. (28) Thiel, P. A.; Madey, T. E. Surf. Sci. 1987, 7, 211−385. (29) Knipe, S. W.; Mycroft, J. R.; Pratt, A. R.; Nesbitt, H. W.; Bancroff, G. M. Geochim. Cosmochim. Acta 1995, 59 (6), 1079−1090. (30) Shpak, A. P.; Korduban, A. M.; Medvedskij, M. M.; Kandyba, V. O. J. Electron Spectrosc. Relat. Phenom. 2007, 156−158, 172−175. (31) Azimirad, R.; Naseri, N.; Akhavan, O.; Moshfegh, A. Z. J. Phys. D: Appl. Phys. 2007, 40, 1134−1137. (32) Schiavello, M.; Pepe, F.; Cannizzaro, M.; de Rossi, S.; Tilley, R. J. D. Z. Phys. Chem. 1977, 106, 45−51. (33) Weinhardt, L.; Blum, M.; Baer, M.; Heske, C.; Cole, B.; Marsen, B.; Miller, E. L. J. Phys. Chem. C 2008, 112, 3078−3082. (34) Braun, A.; Sivula, K.; Bora, D. K.; Zhu, J.; Zhang, L.; Grätzel, M.; Guo, J.; Constable, E. C. J. Phys. Chem. C 2012, DOI: 10.1021/ jp304254k. (35) Ganduglia-Pirovano, M. V.; Hofmann, A.; Sauer, J. Surf. Sci. Rep. 2007, 62, 219−270. (36) Warschkow, O.; Ellis, D. E.; Hwang, J.; Mansourian-Hadavi, N.; Mason, T. O. J. Am. Ceram. Soc. 2002, 85, 213−220. (37) De Souza, R. A.; Metlenko, V.; Park, D.; Weirich, T. E. Phys. Rev. B 2012, 85, 174109.
Subsequent process steps like thermal after-treatment in an oxidative atmosphere can adjust or improve the electronic structure with respect to a particular application, such as, for example, for photoelectrochemical photoanodes. However, additional processes malign to the function may be triggered as well. We have demonstrated how in situ XPS analyses can aid to monitor the evolution of the electronic structure during processing. It is desirable to complement these structural studies also by operational measurements of the transport properties, in the ideal case under realistic photoelectrochemical device operation conditions, such as was recently shown operationally for hematite in an illuminated photoelectrochemical cell using near-edge X-ray absorption fine structure spectroscopy.34 The development of such instruments for XPS is currently in progress.
■
ASSOCIATED CONTENT
S Supporting Information *
Figures S1−S3. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The research leading to these results received funding from the European Community’s Sixth Framework Marie Curie International Reintegration Program (Grant 042095; HiTempEchem, X-ray and Electrochemical Studies on Solid Oxide Fuel Cells and Related Materials), Seventh Framework Program Novel Materials for Energy Applications (Grant 227179; NanoPEC, Nanostructured Photoelectrodes for Energy Conversion), Swiss NSF Grants 200021-116688 (to S.E.), 200021-132126 and IZK0Z2-133944 (to A.B.), and 20021-124812 (to Q.C.), and Swiss Federal Office of Energy Contracts 152316-101883, 153613-102809, and 153476-102691. The ALS is supported by the Director, Office of Science/BES, of the U.S. Department of Energy (Contract DE-AC02-05CH11231).
■
REFERENCES
(1) Zubko, P.; Gariglio, S.; Gabay, M.; Ghosez, P.; Triscone, J.-M. Annu. Rev. Condens. Matter Phys. 2011, 2, 141−165. (2) Yoon, H.; Granata, J. E.; Hebert, P.; King, R. R.; Fetzer, C. M.; Colter, P. C.; Edmondson, K. M.; Law, D.; Kinsey, G. S.; Krut, D. D.; Ermer, J. H.; Gillanders, M. S.; Karam, N. H. Prog. Photovolt. Res. Appl. 2005, 13, 133. (3) Auciello, O. J. Appl. Phys. 2006, 100, 051614. (4) Fujishima, A.; Zhang, X.; Tryk, D. A. Surf. Sci. Rep. 2008, 63 (515), 582. (5) Granqvist, C. G. Sol. Energy Mater. Sol. Cells 2000, 60, 201−262. (6) Heinz, B.; Merz, M.; Widmayer, P.; Ziemann, P. J. Appl. Phys. 2001, 90, 4007−4018. (7) Deb, S. K. Sol. Energy Mater. Sol. Cells 2008, 92, 245−258. (8) Tatsuma, T.; Saitoh, S.; Ohko, Y.; Fujishima, A. Chem. Mater. 2001, 13 (9), 2838−2842. (9) Akurati, K. K.; Vital, A.; Dellemann, J.-P.; Michalowa, K.; Graule, T.; Ferri, D.; Baiker, A. Appl. Catal. B: Environ. 2008, 79, 53−62. (10) Shinguu, H.; Bhuiyan, M. M. H.; Ikegami, T.; Ebihara, K. Thin Solid Films 2006, 506−507, 111−114. (11) LeGore, L. J.; Greenwood, O. D.; Pauls, J. W.; Frankel, D. J.; Lad, R. J. J. Vac. Sci. Technol. 1997, A 15, 1223−1227. 3480
dx.doi.org/10.1021/cm301829y | Chem. Mater. 2012, 24, 3473−3480
