Titanium Oxide Multilayer ... - ACS Publications

J. Phys. Chem. C 2007, 111, 18479-18492

18479

Sol-Gel Tungsten Oxide/Titanium Oxide Multilayer Nanoheterostructured Thin Films: Structural and Photoelectrochemical Properties Vittorio Luca,*,† Mark G. Blackford,† Kim S. Finnie,† Peter J. Evans,† Michael James,† Matthew J. Lindsay,† Maria Skyllas-Kazacos,‡ and Piers R. F. Barnes§,⊥ Institute of Materials and Engineering Sciences PMB 1, Australian Nuclear Science and Technology Organisation, Menai, NSW 2234, Australia, School of Chemical Sciences and Engineering, The UniVersity of New South Wales, Sydney, New South Wales 2052, Australia, and CSIRO Industrial Physics, P.O. Box 218, West Linfield, New South Wales 2070, Australia ReceiVed: April 13, 2007; In Final Form: September 20, 2007

Multilayer structures of alternating thin titanium and tungsten oxide layers having dimensions of ∼20 nm have been fabricated from titanium alkoxide and various tungstate precursor solutions using the dip coating technique. Single, double, and triple layer titanate and tungstate thin films were deposited on silicon substrates, and these films were initially annealed at 400 °C. Structural and microstructural aspects of the films were investigated using a variety of techniques, including X-ray reflectometry, grazing incidence X-ray absorption spectroscopy (GIXAS), cross-sectional transmission electron microscopy (TEM), and secondary ion mass spectrometry. The dimensions of the films and the character of the interfaces were principally gauged by cross-sectional TEM and X-ray reflectometery. All films were continuous on a local scale and had relatively low surface roughness. At the treatment temperature of 400 °C, only the tungsten oxide component showed appreciable crystallinity. The multilayer films had relatively diffuse interfaces, even after annealing in air at this temperature. At these temperatures, easily measurable diffusion of tungsten into the titanium oxide component was observed, whereas the diffusion of titanium into the tungsten oxide component occurred to a lesser degree. At higher temperatures, interdiffusion of components was found to be significant. TEM, X-ray diffraction, and Ti K-edge GIXAS measurements indicated that annealing at 400 °C generated films in which the titanate component remained amorphous while the tungstate component crystallized in the tetragonal modification of WO3, which is normally stable only at high temperatures. Grazing incidence X-ray absorption spectroscopy allowed the degree of distortion of the tungsten oxygen polyhedra to be monitored as a function of depth into the film. The photoelectrochemical activity of the multilayer film electrodes was investigated, and the activity for water photo-oxidation was assessed. The photoelectrochemical response was greatest when crystalline WO3 was bounded on both sides by amorphous TiO2 layers. In this bounded state, WO3 had unique structural characteristics.

Introduction Electrodes based on nanocrystalline titanium oxide (anatase, with band gap Eg ) 3.2 eV) and tungsten oxide (Eg ) 2.7 eV) thin films have attracted considerable interest in the past decade because of the enormous potential for application. For instance, nanocrystalline titania electrodes have featured prominently in a new generation of efficient dye-sensitized solar cells,1,2 as humidity sensors,3,4 in electrochromic coatings,5,6 in lithium ion batteries,7-9 and as photocatalytic films and smart windows.10 Additionally, titania has also been widely investigated for its water-splitting properties after generation of hydrogen and oxygen from water using sunlight was first demonstrated in 1972.11 Although the efficiency of energy conversion is limited by its wide band gap, TiO2 has the significant advantage of photolytic stability.12 Tungsten trioxide is another oxide that has been intensively studied because of its interesting electronic and optoelectronic * Corresponding author. E-mail: [email protected]. † Australian Nuclear Science and Technology Organisation. ‡ The University of New South Wales. § CSIRO Industrial Physics. ⊥ Current address: Department of Chemistry, Imperial College, London, UK.

properties and applications as a chromogenic element,13-15 including smart windows,10 gasochromic,16,17 and gas-sensing devices and even as optical storage media.18 The electrochromism of tungsten oxide films, which has been particularly intensively studied, has been thoroughly reviewed.5,6,19 It has also recently been shown that UV irradiation of tungsten trioxide thin films can induce a change in film wetting.20 Digiulio et al.21 demonstrated the usefulness of tungsten oxide as a sensor element for NO2, and Ferroni et al.22 have studied titanium tungstate mixed oxide films and found them to have enhanced NO2-sensing properties.23,24 A combination of titanium and tungsten oxides are involved in commercial catalysts for the selective catalytic reduction of NOx.25 Most recently, tungsten oxide has shown impressive performance for the photoelectrolysis of water to produce hydrogen.26-28 In electronic applications, the exact electrical response exhibited by a metal oxide film depends strongly on composition29 and on microstructural characteristics, such as particle size, shape, crystallinity, and defect structure.30 It is also wellknown that intimate contacts between one nanoscale semiconducting oxide and another with different electronic properties can result in hybrid properties, including electronic rectifica-

10.1021/jp0729112 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/29/2007

18480 J. Phys. Chem. C, Vol. 111, No. 50, 2007

Figure 1. Schematics of different types of nano heterojunction structures.

tion.31 An additional advantage that heterostructures may have over single-component materials is the ability to shield a photochemically unstable material from the surrounding electrolyte with a corrosion-resistant outer layer,32 allowing some narrower band gap materials to be used. There have been many reports of the improved photoelectrochemical activity that can be achieved using such coupled semiconductor systems.33-39 In some of these studies, one semiconductor acts as a sensitizer for another, whereas in others, it is the modification of surface properties that appears to play a greater role in the enhancement of photoelectrochemical activity. The present study focuses on the preparation and properties of nanoheterostructured thin films assembled from the two semiconductors discussed above. Gao et al.40 have recently exemplified the particular case of the photooxidation of dichloroacetic acid using TiO2 in which a monolayer or more of WO3 was dispersed over the titania particle surfaces. Shinguu recently reported on the formation of WO3-TiO2 multilayer thin films using pulsed laser deposition and showed an enhanced photocatalytic response by this type of coupling.41 Shiyanovskayaa and Hepel42 have also studied bicomponent films of TiO2 on amorphous WO3 and have also demonstrated improved photocurrent responses over either individual component. In none of these studies, however, has a comprehensive investigation of the structural and electronic properties of the resulting composite heterostructured systems been undertaken. There are many ways of achieving nanoscale contacts, and some possibilities are shown in Figure 1 in order of structural complexity. Moreover, when a particular dimension of one or both components of such heterostructures falls below the exciton wavelength, then various properties, such as melting point and optical band gap, are strongly modulated.43,44 Thus, to control the properties of thin film assemblies, it is important to be able to control the desired crystallographic phases formed in each layer and the dimensions and shape of the particles and then to be able to place each component in intimate contact with a second component having distinct electronic properties exerting at the same time some control over the interfaces. To some extent, the multilayer film heterostructure (Figure 1a) case can be considered a model system for the study of the structurally more complex heterostructures, and these multilayer films represent the focus of the present contribution. We have chosen titanium and tungsten oxides as the multilayer components because, in addition to the interest being shown in them

Luca et al. individually and in combination,23,24,30,41,45-47 the pairing of low and high atomic number elements results in high X-ray contrast, which then facilitates study by synchrotron-based X-ray absorption and scattering techniques. In addition, the coupling of a wide band gap semiconductor, such as TiO2 (Eg ) 3.2 eV), and a narrower band gap semiconductor, such as WO3 (Eg ) 2.7 eV), resembles to some extent a conventional tandem photoelectrochemical water-splitting device.48 Many methods have been developed for the deposition of thin titanium and tungsten oxide films, including (1) spray pyrolysis,49 (2) cathodic deposition,50 (3) oxidation of metallic films deposited by r.f. sputtering (RFS) and vacuum thermal evaporation (VTE) techniques,47,51 (4) chemical vapor deposition,52,53 and (5) the sol-gel method.23,54,55 We have selected the sol-gel method for the preparation of the films being considered in this work because this method boasts the significant advantages of process simplicity, straightforward scale-up, and low temperature. In the sol-gel method, metalorganic precursors, such as metal alkoxides (MORx), are allowed to hydrolyze to form M-OH bonds. Condensation of hydroxyl groups then gives M-OH-M and M-O-M bonds. The solgel technique in principle allows for control over the growth of the metal oxide network by regulating various synthesis parameters, including precursor type, water content, temperature, solvent, pH, and ancillary chemicals. Although there are numerous applications of the sol-gel technique for the formation of thin metal oxide films, in general, the films that have been studied have had dimensions in excess of several hundred angstroms. Where thinner films are required, researchers have typically relied on methods in which small concentrations of reactants are introduced from the vapor phase, such as chemical vapor deposition and reactive sputtering. The present program of work has been undertaken in the belief that the fabrication of sophisticated “intelligent” devices with novel properties will require combining nanocrystalline semiconductors in sometimes complex geometries. As a start down this path, the stacking of simple thin films will be an important and basic technology underpinning the construction of micromechanical and microelectronic systems. One of the specific objectives of the present study was to better understand the interfacial structure between titanium and tungsten oxide components of nanoscale multilayer thin films deposited through sol-gel techniques. Issues such as the degree of intermixing, the nature of the crystallographic phases formed, and the local environment of the atoms making up the film components were the focus. Another specific objective of this work was to explore and understand the photoelectrochemical activity of these multilayer films as photoanodes for photoelectrochemical watersplitting. Experimental Preparation of Films. The titanium oxide component of the multilayer films was applied by dip-coating from a solution of 4 wt % (0.6 mol/L) tetraethoxy titanate (TET) in dry ethanol onto silicon wafers or titanium metal substrates. To allow a full range of characterization techniques to be applied to the films and for electrochemical studies to be undertaken, films were prepared on silicon wafers as well as titanium metal substrates. In addition, for these reasons, the tungstate layer was applied by different methods. In type 1 films, the tungstate layer was deposited by dip-coating onto silicon wafers from a tungstic acid sol produced by the method of Chemsiddine et al.56 Our procedure differed slightly in that an excess of thoroughly washed resin (Dowex 50WX4-400) required to completely

Sol-Gel WO3/TiO2 Multilayer Thin Films exchange the Na+ for H+ was stirred for 5-10 min with 50 mL of 0.5 M sodium tungstate solution. The resin was then separated from the solution by vacuum filtration, leaving a pale yellow, transparent colloidal solution. Twenty milliliters of additional water was added to give a total tungsten concentration of 0.37 mol/L. Fresh colloid was prepared for each film that was deposited, and it was used within 1 h of preparation. Several films were prepared as part of this study, including T-Si, W-Si, T-W-Si, W-T-Si, and T-W-T-Si, where T stands for titanium and W for tungsten oxide-based layers. Type 2 films were, in general, used for X-ray absorption spectroscopy (XAS) studies. The tungstate component on these films was applied to Si substrates from W5+-ethoxide solution (Gelest AKT890, FW 409.15) of concentration 0.48 mol/L in ethanol. This deposition method was chosen for the XAS experiments because these films had visually a somewhat superior surface finish compared with type 1 films and because this technique was expected to be particularly sensitive to small variations in thickness over large areas. Type 3 films were prepared on titanium metal substrates for the purpose of performing cyclic voltammetry and water photoelectrolysis measurements where a conducting substrate and a high level of purity was required. These films were made via the peroxopolytungstate route.35 In this method of film preparation, 100 mL of 30% H2O2 was added to 12.0 g of H2WO4. The tungstic acid dissolved on warming, and excess peroxide was destroyed using Pt wire. The titania film was deposited as per the other films from a solution of 0.6 mol/L TET. It will be shown in what follows that the different film deposition methods result in films with similar microstructural properties. All films were applied to the substrates by the dip-coating procedure carried out in ambient air and employing a withdrawal rate of 5 cm/min for all of the precursor solutions. Each deposited layer was annealed in air at 400 °C for 10 min prior to deposition of a subsequent layer. X-ray Absorption Spectroscopy. Grazing incidence X-ray absorption spectra (GIXAS) and X-ray absorption near edge spectra (XANES) were recorded on beam line 20B of the Australian National Beam Line Facility (ANBF) at The Photon Factory, Tsukuba, Japan, using a Si(111) double monochromator. To reject harmonics, the monochromator was detuned to between 50 and 70%. Fluorescence yield data were acquired using a 30-element, solid-state germanium detector. A measure of the relative resolution could be made from the peak-to-peak first-derivative line width of the first pre-edge feature in the titanium metal foil reference spectrum. This value was 1.42 eV for the ANBF data. Normalization of X-ray absorption data was performed using the program WINXAS,57 which fits a zero-order spline to a specified region after the edge step. Analysis of the extended X-ray absorption fine structure (EXAFS) was performed with the programs XFIT58 and FEFFIT.59 To fit the EXAFS, typical values of E0 (the energy zero) and S02 (the amplitude reduction factor) were obtained by fitting the EXAFS of a crystalline reference samples with N (coordination number), R (interatomic distance) and interatomic distances fixed at their expected crystallographic values and allowing E0 and S02 to float simultaneously. The obtained values of E0 and S02 were then used as initial values for the modeling of the EXAFS of the film samples. X-ray Diffraction. X-ray diffraction patterns were acquired on a Scintag X1 instrument using Cu KR radiation and the θ-θ

J. Phys. Chem. C, Vol. 111, No. 50, 2007 18481 geometry. This instrument was operated with a 270 mm goniometer radius and was fitted with a Peltier-cooled detector. X-ray Reflectivity. X-ray reflectivity (XRR) measurements were performed on a conventional Siemens D5000 laboratory X-ray diffractometer. This instrument employed a long fine focus line Cu X-ray tube, which was set up with a projected source width of 0.04 mm. A near parallel incident beam was achieved using narrow 0.05 mm divergence slits at 110 mm from the source. Soler slits with apertures of 2.3° were also included in both the incident and reflected beams to limit any axial divergence. The reflected beam was detected through 0.05 mm receiving slits in front of a curved graphite monochromator and scintillation counter. The samples were monitored on a stage fitted with three adjustable pins allowing height adjustments to be made within 0.005 mm and tilt adjustments of ∼0.01° to be made perpendicular to the beam. In this paper, φ is defined as the angle of the incident beam onto the sample, and 2θ is the angle of the detector relative to the incident beam. Before each sample was measured, various alignment procedures were carried out, including the 2θ zero of the detector, incident beam direction, zero φ angle, specimen height, and specimen tilt perpendicular to the diffractometer axis. For some of the thin films, X-ray reflectivity measurements were also performed on a purpose-built facility at the Australian National University designed for the study of liquid surfaces. Reflectivity data was modeled using the software package Parratt32.60,61 Simulations of multilayer reflectivities and electric field penetration were also performed using the IMD software package, which calculates the reflectivity of a multilayer stack, taking into account interface imperfections through modified Fresnel coefficients.62 Electron Microscopy. Cross-sectional transmission electron microscope (TEM) specimens of type 1 and type 2 films were prepared by gluing a coated wafer onto a glass slide using highstrength epoxy resin. A core, 2.3 mm in diameter, was then ultrasonically machined so that the coated surface ran through the center of the core. The core was then glued into a brass tube of 3 mm external diameter. A wafer was produced that was then ground and ion-milled to produce electron-transparent, cross-sectional specimens. From a knowledge of the silicon wafer normal and by appropriate tilting of the silicon specimen in the TEM, it was possible to ensure that the film was viewed edge-on. The type 3 film cross-sectional TEM specimen was prepared by focused ion beam (FIB) milling on a FEI Nova Nanolab 200 plus (FEI Company, Netherlands). Prior to FIB milling, the film was coated with a 100 nm gold layer for protection during the initial stages of subsequent platinum deposition and FIB milling.63 When FIB milling was completed, the TEM foil was extracted from the bulk sample using ex situ micromanipulators and placed onto a carbon support film ready for TEM analysis. Orientation of the film in the TEM was achieved by tilting the specimen until the layer thicknesses were minimized and the interfaces were as distinct as possible. TEM was performed using a JEOL 2010F (JEOL, Japan) equipped with a field emission gun electron source operated at 200 kV. The TEM was equipped with a LINK energy dispersive X-ray spectrometer (EDS) (Oxford Instruments, UK) and ESVision microanalysis system for qualitative X-ray analysis. Energy-filtered TEM was performed using a GIF 2001 electron energy filter (Gatan) attached to the JEOL 2010F. TEM images were recorded using the 1k × 1k CCD camera in the GIF 2001. The JEOL 2010F was operated in scanning transmission electron


Figure 2. Schematic of the photoelectrochemical cell and potentiostat showing the metal oxide working electrode (photoanode), the saturated calomel reference electrode (SCE), and the platinum wire counter electrode (cathode), Vbias ) VW-C. The working and counter electrodes are separated by a porous glass frit, and the electrolyte in the vicinity of the counter electrode is purged with N2 (reproduced from Murphy et al.12).

microscopy mode to record EDS datasets from which elemental maps and line profiles were generated. Secondary Ion Mass Spectrometry. Depth profiling of the multilayer film was performed on a dynamic secondary ion mass spectrometry (SIMS) instrument (Cameca ims 5f). The primary Cs+ ion beam was rastered over an area of 250 µm × 250 µm, and secondary cesium cluster ions (MCs+) from a ∼55 µm diameter central region of the sputtered crater were detected. The depths denoted in Figure 8 were determined by terminating the SIMS depth profiles at a point corresponding to the interface between the individual layers. Profilometry of the series of craters formed in this way yielded the specified depths. Photoelectrochemical Measurements. The photoelectrochemical measurements were made in a standard three-electrode cell with a saturated calomel reference electrode and a platinum wire counter electrode and the oxide electrode as the photoanode (Figure 2). The measurement procedure has been described in detail elsewhere.12 The cell was filled with a 1.0 M H2SO4 electrolyte solution and was fitted with a quartz window to allow the illumination of the photoanode. Several sets of measurements were performed on different film preparations. The bias potential was either manually adjusted and the steady-state photocurrent measured or it was scanned at a rate of 1 or 20 mV/s and the photocurrent automatically recorded. The potential difference between the working and counter electrodes (Vw-c) was recorded to determine the applied bias. The cathode region was purged using a flow of nitrogen gas to prevent oxygen reduction from occurring instead of hydrogen evolution. The photo anode was illuminated using an Oriel 6271 ozonefree xenon lamp with a fitted Oriel 61945 water filter. The spectral irradiance of the xenon lamp was measured by the Australian National Measurement Institute. Intensity measurements were made using an Oriel Instruments miniature thermopile detector (model 71751) with a sapphire window. Potentiometry. Preliminary electrochemical characterization of the multilayer type 3 films with metallic titanium substrates as the working electrodes were performed on an Elchema model PS-605 potentiostat using a standard three-electrode configuration. The counter electrode was platinum wire, and a Ag/AgCl reference electrode was used. The electrolyte was a 0.5 M

Luca et al.

Figure 3. X-ray powder diffraction patterns of selected films (a) silicon (100) substrate, (b) W-Si-1, (c) W-T-Si-1, and (d) T-W-T-Si-1 and (e) simulated powder pattern of tetragonal tungsten oxide including miller indices. Lines not attributable to the substrate are consistent with a tetragonal tungsten bronze. Values above certain reflections are the d spacings in angstroms.

sulfuric acid solution. A scan rate of 20 mV/s was employed for these measurements. Results X-ray Diffraction. X-ray diffraction patterns of type 1 films are shown in Figure 3. The silicon substrate peaks were easily observed in all of the films (Figure 3a). No peaks that were not due to the substrate were observed in the XRD pattern of the single layer T-Si-1 film (not shown) or in the XRD pattern of the W-Si-1 film (Figure 3b). Only in the XRD patterns of W-T-Si-1 (Figure 3c) and T-W-T-Si-1 films (Figure 3d) were relatively intense reflections observed that could be attributed to the deposited films. These reflections occurred at 23.3, 34.4, and 49.9° 2θ and do not correspond to any known polymorph of TiO2. Although the films were thin, the XRD suggested that for type 1 films, the titanate layers did not possess any significant long-range order after annealing at 400 °C. The observed reflections agreed reasonably well with the hightemperature tetragonal form of WO3 or the related H0.1WO3 bronze (JCPDS 23-1448). A simulated X-ray powder pattern for this phase is shown in Figure 3e, and the structure of tetragonal WO3 is shown in Figure 4. Apart from the (001) reflection that has lower relative intensity than in the simulated pattern, the other reflections featured in the experimental pattern had (hk0) indices, indicating that the tungstate crystallites have quite a high degree of preferred orientation. Cross-Sectional TEM. Bright-field cross-sectional TEM images at two magnifications were taken of the three type 1 films: T-Si-1, W-Si-1, and W-T-Si-1 (Figure 5). The layers in these films were of constant thickness in large areas of the sample, with only occasional areas having large thickness variations. The native SiO2 layer of ∼1.0 nm was clearly evident in all the cross sections. The thickness of the titanium oxide layer on the T-Si-1 film (Figure 5a) was consistently on the order of 17.5 nm, whereas for the single layer W-Si-1 film (Figure 5b), there was greater variation in thickness, ranging from 20.0 nm in large regions down to 5.0 nm in isolated regions. It is one of these thin regions that is shown in Figure

Sol-Gel WO3/TiO2 Multilayer Thin Films

J. Phys. Chem. C, Vol. 111, No. 50, 2007 18483

Figure 6. Cross-sectional TEM image of the T-W-T-Si-1 triple layer film (a) and associate EDS line scan taken at right angles to the substrate (b).

Figure 4. Polyhedral representation of the structure of tetragonal hydrogen tungsten bronze. Tungsten atoms occupy the centers of the octahedra with oxygen atoms at the corners.

Figure 5. Cross-sectional TEM images of (a) T-Si-1, (b) W-Si-1, (c) W-T-Si-1 and (d) the line scan of the film in (c) taken at right angles to the substrate.

5b. For the W-T-Si-1 film (Figure 5c), the layer thickness was reasonably consistent and on the order of the dimensions observed in the single layer films. The tungsten oxide component had a thickness of about 10-20 nm, and the titania component had a thickness of ∼10.0 nm. The interface between the two components in these images does not appear abrupt but, rather, was somewhat smeared over a range of ∼2.0 nm. An energydispersive X-ray spectroscopy line scan was taken across the profile of the W-T-Si-1 film (Figure 5d), and this indicated that tungsten could be found across almost the entire width of

the buried titania component, decreasing roughly exponentially from the region of highest concentration. Also to be noted was the migration of some silicon from the substrate into the titanium oxide layer. The EDS line scan analysis has the limitation that the small beam used for analysis has finite dimensions on the order of 2.0 nm. Even accepting this limitation, it seems that the graded nature of the interface agrees with the contrast variations observed in the high-resolution images. In addition to the detection of tungsten in the titania component, some titanium was also observed in the tungsten trioxide layer but not across its entire cross section, as occurred in the titanium oxide layer. Importantly, the surface regions of the top titania layer remained essentially free of tungsten. From the EDS analysis profile, it is possible to calculate values of the diffusion coefficients of one metal species into the neighboring phase at an interface. Typically, values between 0.5 × 10-20 and 3 × 10-20 m2 s-1 were obtained for both W and Ti in films heated at 400 °C for approximately 10 min. For the triple layer T-W-T-Si-1 film (Figure 6), some variation in thickness of each individual component was also observed, but on average, all components had a similar thickness of between 15 and 20 nm. A 2 nm thick dark layer appeared between the silicon substrate and the native SiO2 layer. This layer was most likely due to polishing damage caused by preparation of the substrate surface. In addition, there was a layer of light contrast between the first titania and tungsten oxide layers. This layer showed evidence of S and P by EDS that was probably due to the ion-exchange resin used in the preparation. The S- and P-containing layer was not apparent in all regions of the triple layer film and was not observed in any of the other type 1 films. The EDS line profile of the T-W-T-Si-1 triple layer is shown in Figure 6b. Twenty EDS spectra were acquired across the film profile, each analysis being separated by a distance of ∼3.5 nm. Drift correction was applied after each spectrum was acquired. Again, it was estimated that the spatial resolution of the EDS data was on the order of 2.0 nm. Therefore, although the shapes of the EDS profiles are broadened somewhat by the limited spatial resolution, the line scan shows that the bottom titanate layer contained significant concentrations of tungsten and some Si even within the limitations of the analysis. Titanium also appears to have diffused as far as the center of the tungstate layer. This seems to have occurred in both directions to give a parabolic distribution. Diffusion of W seems to have occurred through the entire cross section of the bottom titanium oxide layer and reached almost the middle of the top titanium oxide layer. This could be understood to be a consequence of the fact that the film was annealed at 400 °C after application of each layer so that the each subsequent layer that was applied spent


Luca et al.

Figure 8. SIMS analysis of the T-W-T-Si-1 triple layer film calcined at 400 °C. Values above vertical lines are the depth calibration markings in nanometers.

Figure 7. Low (a) and high (b) magnification cross-sectional TEM images of T-W-Si-2 double layer film calcined at 400 °C and low (c) and high (d) magnification images of the same film as in a and b after calcination at 800 °C.

TABLE 1: Layer Dimensions (in nanometers) for Type 2 Films Calcined at 400 °C Determined Using Spectroscopic Ellipsometrya T1 T-Si W-Si W-T-Si T-W-Si T-W-T-Si a

22.5 ( 0.1 20.6 ( 0.5 15.6 ( 0.6

W 23.7 ( 0.1 12.4 ( 0.4 21.5 ( 0.5 25 ( 1

T2

Si

27.7 ( 0.3 19 ( 1

T1 is the top titania film, and T2 is the bottom titania film.

correspondingly less time at temperature. Once again, no W appeared to have reached the surface of the top titania layer, and once again, the diffusion of some Si from the silicon substrate into the overlying titanium oxide layer was also observed. Two examples of the TEM images of the type 2 films are shown in Figure 7. For the T-W-Si-2 film calcined at 400 °C (Figure 7a), the individual component thicknesses were clearly visible and were of dimensions similar to the type 1 films. One set of type 2 films was heated to 800 °C (Figure 7c,d), and these showed dramatic microstructural differences as compared to those heated at 400 °C. The interface region in these films was significantly smeared. This implies significant diffusion of W in the direction of the titania. In addition, both the EDS of the film profile and XPS of the film top surface indicated significant concentrations of silicon that apparently migrated from the substrate. Spectroscopic ellipsometry gave dimensions for these films similar to those observed by TEM (Table 1). Namely, each component had a thickness of between 15.0 and 20.0 nm. The film annealed at 800 °C will not be considered further. TEM results on type 1 and 2 films indicated that the different precursors used resulted in very similar films having comparable dimensions and microstructure. For device application, conducting substrates and high purity were, however, deemed necessary, and therefore, to check that similar results could be expected on such substrates, a focused-ion-beam-milled specimen of the

T-W-Ti-3 film annealed at 400 °C was prepared. TEM examination of the cross section of this T-W-Ti-3 film (see the Supporting Information) indicated that the thicknesses of each layer of the film was of dimensions similar to the type 1 and 2 films prepared on silicon substrates annealed at a similar temperature. However, the TEM measurements and EDS elemental mapping also indicated growth of a native TiO2 layer with a thickness of 10 - 20 nm under the deposited films on the Ti substrate. The native oxide layer on the titanium metal substrates formed after annealing at 800 °C is expected to be ∼1 µm thick. Hence, for the purposes of the present study, it was only considered meaningful to examine the photoelectrochemical response of the type 3 films annealed at 400 °C. It is also worth recalling that cross-sectional TEM images of these type 3 films indicated both the native and deposited titanium oxide to be relatively amorphous, whereas the tungsten oxide component appeared to be crystalline. These results are therefore entirely consistent with what was observed for type 1 and 2 films. In the case of the underlying native oxide, amorphization was somewhat unexpected but could easily have been induced by migration of tungsten atoms to this interface. SIMS Analysis. The triple layer type 1 film was also analyzed by secondary ion mass spectrometry (SIMS) in conjunction with sputter depth profiling to provide elemental compositions for the multilayer structure as a function of depth. The results of the SIMS analysis are shown in Figure 8. Substantial variations in the relative amounts of elements in the different layers are clearly seen in this figure. This is in agreement with the TEM EDS analyses wherein Ti was found to be the principal metallic element present in the top titania layer. In addition, however, SIMS shows that Ti and O do not exhibit a constant ratio at any point in this layer. Oxygen was in large excess at the surface, as were carbon, hydrogen, and silicon, which suggests surface contamination. The more highly oxidized surface region relative to deeper in the film is consistent with observations of thicker thermally oxidized films of TiO2.64 Beneath the top titania layer surface, at a depth of ∼10 nm, the ratio of O to Ti increased again. Although this behavior may be partly due to the thinness of the three layers, the relatively large variations in the Ti and O signals on either side of the central region of this layer suggests changing stoichiometry, possibly suggesting oxygen is segregated at the layer boundaries or interfaces during annealing. The SIMS analysis accorded with the EDS analysis of the cross section of this film in that the tungstate layer appeared not to be compositionally uniform. The trends in the W and oxygen signals were similar as a function of time, both trending up toward the center of this layer. The Ti, C, and Si profiles were similar in the tungstate layer and passed through a



Figure 9. Experimental (markers) and simulated (solid line) XRR data of type 1 films (a) T-Si-1, (b) W-Si-1 and (c) W-T-Si-1. Note that data have been scaled vertically.

minimum close to the midpoint of the tungstate layer. Although the tungsten oxide layer appeared somewhat more abrupt than indicated by EDS, diffusion of components was nonetheless apparent, and the penetration of Ti into tungsten oxide layer was very obvious. The Ti/O ratio observed in the bottom titanate layer increased from the titanate-tungstate interface to the titanate-silica interface. As with the top titanate layer, this is indicative of changing stoichiometry. Comparable quantities of C and H were also observed in this layer, but the amount of Si was much less. In agreement with the EDS line scan data taken from the TEM cross sections, there was considerable diffusion of W into this bottom titanate layer. Similarly, there was evidence of the diffusion of Ti into the silica layer. X-ray Reflectommetry. To check if the dimensions observed by TEM were representative of the average film, XRR patterns were measured on single, double, and triple layer type 1 films. The XRR measurements on the simpler single and double layer films were performed to gain preliminary information on which to base analyses of the more complex systems. The experimental and simulated XRR of a selection of films are shown in Figure 9. For the T-Si-1 film (Figure 9a), the data could be wellsimulated using a film thickness, z, of 16.3 nm; a vacuumtitania root-mean-square interface roughness, σ, of 0.6 nm; and a density slightly lower than that of bulk anatase or rutile. This lower density and low air-TiO2 interface roughness was consistent with a titanate layer that was poorly crystalline or amorphous after annealing at 400 °C. The XRR data of the single layer W-Si-1 film is shown in Figure 9b. This film could be well-modeled using a film thickness of ∼16.9 nm and a vacuum-tungstate root-meansquare interface roughness of 7.0 nm. For the more complex W-T-Si-1 film (Figure 9c), the dimensions of the W and T components were found to be about 21.7 and 16.3 nm, respectively. These values are in reasonable agreement with the values obtained from the single layer films. Similar simulations for the triple layer T-W-T-Si film gave values of ∼20.0 nm for each of the components. This again agrees well with the values obtained for the single components and is in line with the images obtained by TEM. Thus, TEM analysis of small areas of these films is capable of providing

Figure 10. X-ray reflectivity, R, and corresponding XANES data as a function of grazing angle, φ, for (a) W-Si-2, (b) W-T-Si-2 (c) T-W-Si-2, and (d) T-W-T-Si-2 annealed at 400 °C.

dimensions that are representative of the average film thickness. Overall, the XRR-determined dimensions were in general in agreement with both TEM and spectroscopic ellipsometry. W LIII Edge XAS. Having obtained basic dimensional and microstructural information on sol-gel films of types 1 and 2, we chose to examine the atomic level structure of the type 2 films as a function of depth using X-ray absorption spectroscopy. The W LIII edge XANES of these films is shown in Figure 10, together with the corresponding X-ray reflectivity traces measured at 10.3 keV. XANES data were acquired at a range of grazing angles from just below the critical angle to just above so as to adjust the penetration depth from a few tens of nanometers to the entire film profile. This was born out in simulations of all of the films, taking appropriate values of film thickness and interface roughness (see Supporting Information). The strong edge feature centered at ∼10.20 keV is known as a white line (WL) and is attributed to the transition of the photoelectron between the 2p3/2 level and a final state in the continuum with 5d atomic character.65 The W LIII white-line structure and intensity gives an indication of the degree of departure from regular octahedral symmetry. In regular octahedral symmetry, a weak WL is observed, whereas in more distorted coordination, the WL intensity can reach three times the edge step. Indeed, Pauporte et al.66 have observed a linear trend in WL intensity as a function of the condensation of the tungsten oxide polyhedra. XANES data recorded at increasing incidence angles can therefore be taken to reflect coordination environment or degree of condensation as a function of depth into the film. For the W-Si-2 film (Figure 10a), the intensity of the WL decreased with increasing grazing angle and, therefore, penetration depth. This implies that the distortion of the bonding about

18486 J. Phys. Chem. C, Vol. 111, No. 50, 2007 the tungsten center decreases as the penetration depth increases and the WO3-SiO2 interface is approached. This seems entirely reasonable given that the degree of coordinative unsaturation is expected to be greatest at the interface with air. When titania underlies tungsten oxide, such as in the W-T-Si-2 film (Figure 10b), a similar WL intensity profile was observed again, indicating that the predominant influence on the degree of distortion is a result of incomplete coordinative unsaturation at the surface air-WO3 interface rather than as a result of upward migration of Ti. For the T-W-Si-2 film (Figure 10c), the reflectivity profile was steeper than when tungsten oxide is the top layer. The critical edge occurred at a lower angle as compared with the previous films with WO3 as the top layer, and this agrees with simulations. The white line remained intense until well after the critical edge. This suggests that the coordination environment of W atoms that migrated up through the TiO2 layer is rather distorted and that the level of distortion is significantly reduced only well away from the TiO2-WO3 interface. It is to be emphasized that the intensity of the W LIII edge also gives an indication of the concentration of W in the film as a function of penetration depth. Thus, tungsten is detectable in this film even in the first 5.0 nm of the titania layer, indicating that 400 °C is sufficient to induce migration of tungsten, in accordance with the SIMS and TEM. Grazing-angle XANES data recorded for a T-W-Si-2 film annealed at 800 °C (see Supporting Information) differed from the data taken at the lower temperature in that the WL remained intense at all grazing angles. The XANES of the triple layer T-W-T-Si-2 film (Figure 10d) was quite distinct to all the previous films in that the WL remained intense at all grazing angles, indicating that the level of distortion was constant throughout the film profile. Clearly, in the case of these triple layer film structures, only a very small proportion of the W atoms occur near the exposed interface. As shown by the EDS analyses of a T-W-T-Si-1 film (Figure 6), Ti is observed throughout the tungsten oxide layer with a well-shaped profile. This means that all regions of the tungsten oxide layer were to some degree doped by Ti4+. Given the results presented so far, it is clear that significant migration of both Si from the underlying substrate and Ti from the titanium oxide top layer occurs even at annealing temperature as low as 400 °C. This significant interdiffusion is clearly observed in cross-sectional TEM images of the T-W-Si-2 film heated at 800 °C (Figure 7c and d). In fact, XPS measurements have evidenced significant concentrations of Si at the surface of this film that were not observed in films annealed at lower temperatures. This proves that migration of Si is very significant at these higher temperatures. The simulated variation in electric field intensity as a function of grazing angle and depth into a hypothetical T-W-T-Si triple layer film with characteristics (dimensions and surface roughness) similar to the actual film and at an incident beam energy of 10.3 KeV is shown in (Figure 11). At such high energies, the electric field intensity is tightly confined at angles close to the critical angle (∼0.2°). The critical angle is much lower than at the Ti K-edge, and the reflectivity drops precipitously at angles above the critical angle. Simulations indicated that at a grazing angle of ∼0.15°, there was negligible penetration of X-rays into the tungstate layer, but at a 0.2° grazing angle, almost all of the electric field intensity is restricted to the first 10.0 nm below the film surface. That is, most of the electric field intensity is confined to the titanate component of the double or triple layer structures with titanium oxide as the

Luca et al.

Figure 11. IMD simulation of electric field intensity of the T-WT-Si multilayer as a function of grazing angle, φ, and depth, z, into the film. Thickness of each component used in the simulation was 16 nm.

first component (i.e., T-W-Si and T-W-T-Si). It would therefore be expected that most Ti atoms would not experience a significant contribution from W in the next-nearest-neighbor coordination sphere, since SIMS and cross-sectional TEM suggest that the concentration of W in this layer is not large. For angles at or about the critical angle (0.4°), the electric field intensity is distributed relatively evenly throughout the film, and therefore, this is where the greatest contribution from W atoms to the next-nearest-neighbor coordination of Ti should occur. This gives an opportunity to derive genuine depthsensitive structural information on the tungsten oxide component by careful examination of W LIII-edge EXAFS. The W LIII-edge EXAFS of the W-Si-2, T-W-Si-2, and W-T-Si-2 were recorded at grazing angles above and below the critical edge. Data for only the W-Si-2 and T-W-Si-2 film are presented in Figure. 12. The fitted EXAFS for the W-T-Si-2 film is presented in the Supporting Information. The EXAFS data generally have a poor signal-to-noise ratio due to the extremely small dimensions of these films. Nevertheless, bond length data obtained from simulation of the spectra is expected to be reliable, and this is presented in Table 2. All of the data required two oxygen shells for an adequate fit of the first Fourier transform (FT) peak. In the case of the W-Si-2 film, the first shell consisted of a short W-O bond distance around 0.18 nm and a longer W-O bond at 0.22 nm. Similar values were found for data taken above and below the critical angle. In the case of the T-W-Si-2 film, similarly short first shell W-O values were obtained at both 0.2° and 0.3°. However, below the critical edge at 0.15°, the first shell W-O bond length appeared to be somewhat larger at 0.194 nm. Given that the poor signal-to-noise ratio of the EXAFS allowed fitting only up to k ) 10 Å-1, at this very small grazing angle, little confidence can really be placed in this value for this particular film. For the W-T-Si-2 film, bond lengths around 0.18 nm were also obtained irrespective of grazing angle. Such short and long W-O bond length values are generally typical for WO3 polymorphs that exist at temperatures between 0 and 1000 °C. For instance, the room-temperature WO3 polymorph in P21/n has W-O bond lengths ranging from 0.1790 to 0.2129 nm. Therefore, the W LIII-edge EXAFS data support the notion that there is no significant variation in the short-range W bonding environment throughout the film profile. Since the edge position does not vary measurably as a function of incidence angle, it seems reasonable to also conclude that there is no change in the tungsten oxidation state with depth. Ti K-Edge XAS. The amorphous nature of the titanate component of the type 1 films suggested by the XRD and TEM was clearly borne out in the Ti K-edge XANES of the T-W-


Figure 12. Room-temperature W LIII-edge EXAFS of W-Si-2 at (a) 0.1° and (b) 0.20° and T-W-Si-2 at (c) 0.15°, (d) 0.20°, and (e) 0.30°.

T-Si-1 film (Figure 13) at a range of incidence angles. The XANES of the film at all values of φ resembles that of the amorphous TiO2 film rather than crystalline TiO2 (anatase or rutile), clearly indicating the poor crystallinity of the TiO2 component. Therefore, the diffusion of W into the TiO2 component suppresses crystallization, but the diffusion of Ti into the WO3 does not. Indeed, it is well-known that dopants in anatase can suppress crystallization of rutile, and the specific case for tungsten dopants has been demonstrated.67 The present results therefore clearly indicate that the crystallization of anatase from the initially amorphous state is also strongly suppressed in these very thin films. When the Ti K-edge EXAFS data of the T-W-T-Si-1 film was splined and transformed into k space, it was apparent that there was no useable data beyond k ∼ 11 (see Supporting Information). Nevertheless, a reasonable Fourier transform could be performed and the data modeled in k and R space. Comparison of the three data sets obtained at each of the three grazing angles (0.3°, 0.4°, and 0.6°) indicated that there was a slight increase in the intensity of the FT peaks due to nextnearest-neighbor correlations for data obtained at grazing angles of 0.4° and 0.6°, as compared to the data obtained at 0.3°. The data obtained at 0.3° (see Supporting Information) was typical of poorly crystalline titania containing significant numbers of dangling Ti-OH bonds.68,69 These data in general showed lower values for the Ti-O bond lengths than typically observed in crystalline titania. The reduced coordination of Ti is reflected in the XANES and also the EXAFS, since the Ti-O bond length should be shorter than that for crystalline bulk anatase or rutile. The amorphous nature of the titanate component of the type 1 films was further supported by the low value of the air-TiO2 interface roughness value deduced from the XRR results for films with titanium oxide as the top component, which is not

J. Phys. Chem. C, Vol. 111, No. 50, 2007 18487 what would be expected had crystallization occurred. Therefore, crystallization of the titanium oxide component cannot occur in these films at an annealing temperature of 400 °C. This notion is also consistent with retardation of crystallization on doping of titania with a hetero element, which in this case is tungsten. Finally, the amorphous nature of the titania component in all of the film types is also confirmed by the TEM cross sections, which showed no lattice fringes in the titania. Cyclic Voltammetry. Figure 14 shows CV curves for a T-W-Ti-3 film annealed at 400 and 800 °C. The notable feature of these CV curves for these type 3 films is that in the cathodic region, just before where hydrogen evolution occurs, numerous reductive shoulders could be observed in the film annealed at 400 °C. For this film, there were at least two cathodic shoulders, whereas for the film annealed at 800 °C, there were up to three such features. It is tempting to tentatively ascribe these shoulders to hydrogen insertion into the titanate and tungstate components of the multilayer structure. Such an assertion is supported by the literature on the electrochemistry of tungsten oxide films.70,71 This interpretation seems reasonable, given that the experiments being reported here indicate that distinct layers are to be observed after annealing to 400 °C. However, annealing at higher temperature has been observed to result in dissolution of one layer component into the other such that the distinction between the layers may be more blurred at higher annealing temperatures. Nonetheless, the reductive peaks for these films are more numerous and more pronounced than would be expected for a single film of a mixed tungsten titanium oxide. Additional experiments are clearly required to understand detailed aspects of the electrochemistry of these structurally complex films. Photoelectrochemical Measurements. Having thoroughly characterized the sol-gel-derived, multilayer heterostructures on silicon substrates, it is interesting to ascertain how such systems might perform in a practical device. In this section, we consider the photoelectrochemical properties of the films by undertaking the photoelectrolysis of water with the multilayered films as photoanodes in the setup described in the Experimental Section. It is well-established that in the electrodeelectrolyte system employed here, the photocurrent generated is proportional to the evolution of oxygen at the photoanode and the evolution of hydrogen at the platinum cathode. Six combinations of films on Ti substrates (type 3) annealed at 400 °C were tested, including T-Ti-3, W-Ti-3, T-W-Ti3, W-T-Ti-3, T-W-T-Ti-3, and W-T-W-Ti-3. The peroxopolytungstic acid route was used for the preparation of these films in order to totally exclude the possibility of interferences from the presence of adventitious dopants such as were identified in the type 1 films. Photocurrent measurements as a function of applied bias for the type 3 films are shown in Figure 15. The photoelectrochemical properties of films annealed at 800 °C on Ti metal were not considered in this study due to the formation of a significant native oxide layer on the Ti substrate. The data were measured against a saturated calomel reference electrode (SCE) with 80 mW cm-2 irradiance from a xenon lamp, which approximates sunlight but required correction for higher intensities in the ultraviolet region of the spectrum.12 The spectrum of the xenon lamp with a water filter in place and the AM1.5 spectrum together the optical constants (refractive indices and extinction coefficients) for the various cell components are provided in the Supporting Information. The magnitude of the photocurrent generated by the different devices in the 0.1-0.8 V versus SCE range was ranked in the


Luca et al.

TABLE 2: Room Temperature W LIII-Edge EXAFS Fits for Type 2 Multilayer Filmsa f

N

W-Si 400 °C 0.22 O 2.2 O 9.8 W 0.84 W-Si 400 °C 0.3 O 2.8 O 7.7 W 0.9 T-W-Si 400 °C 0.15 O 0.5 O 44

R (Å)

σ2 (Å2)

1.83 0.001 2.21 0.001 2.83 0.002 1.81 0.001 2.23 0.001 2.60 0.002 1.94 0.001 2.22 0.007

f

N

T-W-Si 400 °C 0.20 O 4.2 O 17 W 1.4 T-W-Si 400 °C 0.3 O 1.6 O 127 W-T-Si 400 °C 0.12 O 2.1 O 20 W 1.63

R (Å)

σ2 (Å2)

1.80 0.001 2.27 2.4 0.007 1.81 0.001 2.27 0.001

f

N

R (Å)

σ2 (Å2)

W-T-Si 400 °C 0.22 O 1.5 1.69 0.001 O 6.8 2.32 0.001 W 0.6 2.45 0.001 W-T-Si 400 °C 0.30 O 0.8 1.84 0.001 O 3.6 2.29 0.001 W 1.5 2.42 0.005

1.81 0.001 2.26 0.003 2.4 0.003

a φ is the grazing angle, N is the coordination number, R is the distance between absorbing atom W and the various atomic shells listed in the first column, and σ is the mean square relative displacement.

Figure 13. Ti K-edge XANES of the T-W-T-Si-1 at various incidence angles (a) 0.6°, (b) 0.4°, (c) 0.3° and of (d) amorphous TiO2 and (e) anatase.

Figure 14. First, second, and third CV scans for the T-W-Ti-3 film calcined at (a) 400 °C and (b) 800 °C.

following order (lowest first): T-Ti-3, W-T-W-Ti-3, W-TTi-3, W-Ti-3, T-W-Ti-3, then T-W-T-Ti-3. At potentials exceeding ∼0.8 V, the W-Ti layers showed higher photocur-

Figure 15. Photocurrents generated in type 3 multilayer structures as a function of cell potential (vs SCE) in 1 M H2SO4 under 80 mW cm-2 illumination from a xenon lamp: (a) T-Ti, (b) W-Ti, (c) W-T-Ti, (d) T-W-Ti, (e) T-W-T-Ti, (f) W-T-W-Ti, and (g) T-W-Ti. (a-e) Scan rate was 1 mV/s; data have been smoothed; (f, g) steadystate measurements.

rents. The most notable feature of these comparisons is that the strongest photocurrents were generated by structures wherein WO3 layers were capped by TiO2 layers. These heterostructures also showed relatively stronger photocurrents at lower applied biases, which could be attributed to higher conduction band energy positions of the TiO2 phase relative to WO3. It should be noted that data sets in Figure 15f and g were from pseudosteady-state measurements wherein 30-60 s was allowed for equilibration at each bias voltage. The similarity between data sets of Figure 15d and g of different preparations of the same films helps demonstrate that the reproducibility of both preparation and measurement is good (the measured range of photocurrents at 1 V versus SCE are shown in Table 3), although one preparation of W-Ti-3 showed substantially lower photocurrent than the data set of Figure 15b (see range of photocurrent values for W-Ti structure in Table 3). Photocurrent measurements made at faster scan rates of 20 mV/s (compared to 1 mV/s) were very similar to those reported in Figure 15. To make meaningful comparisons between the performance of these different heterostructures, it is necessary to consider the relative fraction of incident light absorbed into each device. Without correcting for this factor, any comparison is confused by the differing photon harvesting efficiency in each structure. To estimate the photon absorption profile in each structure, we assume each layer has a thickness of 20 nm and is optically smooth with abrupt layer interfaces (in contrast to our actual



TABLE 3: Calculated Flux of Incident Photons Absorbed in Each Layer of the Film Structures on Titanium Substrates (Type 3) and the Corresponding Estimated Proportion of the Total Absorbed Photons that are Converted To Photocurrent (Quantum Yield) at 1 V vs SCE (Illumination from a Xe lamp, 80 mW cm-2, Total Photon Flux 2.74 × 1021 m-2 s-1)a absorbed photons film structure

layer 1 (× 1019 m-2 s-1)

layer 2 (× 1019 m-2 s-1)

layer 3 (× 1019 m-2 s-1)

total (× 1019 m-2 s-1)

photocurrent at 1 V vs SCE (mA cm2)

quantum yield (%)

T-Ti W-Ti T-W-Ti W-T-Ti T-W-T-Ti W-T-W-Ti

2.3 0.9 2.3 0.9 2.3 0.9

0.3 2.8 0.2 2.4

0.3 0.2

2.3 0.9 2.6 3.7 2.9 3.5

0.02 0.04-0.14 0.08-0.11 0.03-0.05 0.19 0.03

4 28-96 19-27 5-8 42-44 5

a

The range of photocurrents presented indicate the spread of values observed on independent measurements and samples.

observations which showed diffuse interfaces) and only firstorder reflections are considered. With normal incidence, the fraction of light of wavelength λ reflected from an interface between phases a and b is given by

Rab )

(

)

n˜ a - n˜ b n˜ a + n˜ b

2

where n˜ is the complex refractive index, n + iκ. The intensity of light transmitted through a layer varies according to

I ) I0 exp(-4πκx/λ) The fraction of light transmitted across an interface is given by 1 - Rab. The photon absorption rate is given by the intensity of light (the sum of intensities traveling in both the incident and reflected directions) at a depth x in the film by

A(x) ) I(x)

4πκ λ

Using these relationships along with the photon flux spectra and optical constants for the cell components given in the Supporting Information, we calculated the integrated photon absorption profiles (over all incident wavelengths) of the films investigated, and these are shown in Figure 16. Table 3 shows the calculated flux of incident photons absorbed in each layer of the film structures and the proportion of the total absorbed photons converted to photocurrent (quantum yield) at 1 V vs SCE. These calculations must be viewed as estimations due to the diffuse nature of the film interfaces and the variation in thickness observed. A more sophisticated approach would be required to account for these effects, which we do not pursue here.72 Nevertheless, these calculations make it clear that most photon absorption occurs in the TiO2 layers due to the relatively weak extinction coefficient of WO3 (despite its narrower band gap) in the ultraviolet part of the spectrum relative to TiO2. Because of the amorphous nature of the TiO2 layers, this enhanced photon absorption is unlikely to result in substantial electron-hole separation and therefore photocurrent (compare curves a and b in Figure 15). Discussion The present results have shown that titanium and tungsten oxide film structures with uniform dimensions (10-20 nm for each component) could be formed on both silicon and titanium substrates, regardless of the precursor chemistry used, provided the films produced were deposited at the same dipping speeds and were similarly annealed. A range of techniques, including XRR, XRD and TEM, also showed that regardless of the substrate or precursor chemistry, the titanium oxide component

Figure 16. Calculated photon absorption profiles integrated over all incident wavelengths in different multilayer structures, assuming optical constants presented in the Supporting Information, and illumination with 80 mW/cm-2 from a xenon arc lamp with spectra shown in the Supporting Information. The insets show schematic diagrams indicating the ideal relative energy levels of the conduction and valence bands in the neighboring phases of the multilayer structures (not to scale).82

was always amorphous, and the tungsten oxide was always crystalline after annealing at 400 °C. Although the complete range of available techniques could not be applied to films deposited on titanium metal substrates (type 3) owing to the need for perfectly flat substrates, cross-sectional TEM observations also supported similar conclusions regarding dimensions and crystallinity of these films. A consequence of multilayer structures comprising crystalline WO3 layers and amorphous or poorly crystalline TiO2 layers was that most of the photochemical activity derived from the former component. This was evidenced by the fact that the W-Ti-3 film showed a factor of up to seven times higher photocurrent at 1 V versus SCE (see Figure 15) as compared with the T-Ti-3 film, which showed very little activity. Indeed, the observed inactivity of the amorphous titania layer is consistent with previous observations that amorphous titania shows negligible photo activity.73

18490 J. Phys. Chem. C, Vol. 111, No. 50, 2007 The TEM and SIMS reported here have clearly shown that annealing at 400 °C results in some interdiffusion of components, especially of Ti into the WO3 layer, and to a lesser extent, W into the TiO2. At higher annealing temperatures, such as 800 °C, this diffusion was even more pronounced. It is therefore quite apparent that we are for the most part not dealing with layers of pure TiO2 or WO3, but instead, at least part of each layer is likely to comprise TixW1-xO3-n-type oxides having a compositional gradient, that is, x and n vary with depth. If we consider migration of Ti into WO3, then the replacement of W5+/6+ by Ti4+ is likely to result in the generation of a gradient of defect centers, which simplistically could be either oxygen interstitials or W5+/6+ vacancies. Indeed, some precedent exists for the suggestion that Ti-W mixed oxide can be formed. Bulk Ti-W mixed oxides have been prepared using microemulsion methods, and it has been suggested that for tungsten contents up to 20 atom %, it is possible to have a complete solid solution in which W6+ substitutes for Ti4+ in the anatase structure to give compounds with the formula WxTi1-xO2+x.74 These materials have previously been observed to give enhanced photo degradation of toluene relative to pure TiO2.75 Such improved photo activity was ascribed to a red shift in the band gap absorption due to W doping. Likewise, in recent studies of Ti4+-doped WO3 films, increased photoelectrochemical response has also been observed relative to undoped WO3 films, and this has been ascribed to a decreased rate of electron-hole recombination.76 More specifically, such a recombination rate reduction was attributed to the presence of acceptor levels in the otherwise little-altered band structure of WO3.77 It is also worth noting that the dissolution limit of Ti into WO3 in this study was determined to be on the order of only ∼0.35 atom %, which is obviously at variance with the present observations. To further explore the doping aspect in the present nanoscopic films and to obtain direct structural information on the very thin layers produced, grazing incidence W LIII EXAFS measurements were performed, and these showed variation in the tungsten coordination environment as a function of depth. Typically, the W environment in regions of the WO3 layer close to an interface with either air or a TiO2 layer was found to be highly distorted. This could be due to coordinative unsaturation at the air-WO3 interface or from migration of Ti4+ into the WO3 layer near the WO3-TiO2 interface. The most uniform distortion of tungsten sites occurred either when higher temperatures were involved or when a WO3 layer was sandwiched between two TiO2 layers where migration of Ti4+ occurred both from the top and from the bottom. Substitution of some W6+ sites by Ti4+ suggests that oxygen vacancies are likely to be the predominant defect in the interface region giving rise to coordinative unsaturation of the tungsten as suggested by X-ray absorption spectroscopy. Indeed, these defect states at the interfaces might represent the trap states tentatively identified in the CV data. It is becoming increasingly accepted that the performance of crystalline TiO2 photocatalysts is intimately related to defect structure.78,79 In similar fashion, it can be expected that the improved performance of the T-W-T-Ti-3 must also be strongly dependent on the defect structure of the Ti-doped WO3 layer in which the initial charge separation takes place. Given the model of the film structure arrived at here, the main result requiring explanation is the fact that, by far, the highest photocurrents (below 0.8 V bias) were observed when WO3 was in this sandwiched configuration (T-W-T-Ti-3). In fact, the photocurrent observed for this film was significantly

Luca et al. higher than W-Ti-3 and W-T-W-Ti-3, for which calculations indicated that a significantly higher fraction of the incident light was absorbed into the WO3 layer (see Table 3) when no amorphous TiO2 capping layer was involved. Although defect states are obviously important, as discussed above, we need also to consider the overall configuration of the multilayer stack itself. That is, we need to consider the influence of the amorphous top TiO2 layer (or more precisely, the graded W-doped TiO2) in which electron-hole separation is poor but which provides the actual interface for electron transfer to and from redox species in the electrolyte, despite the fact that initial charge separation must take place in the underlying Ti-doped WO3. Indeed, Figure 15 suggests that the enhanced performance of the T-W-Ti-3 and T-W-T-Ti-3 films relative to W-Ti-3 (

Titanium Oxide Multilayer ... - ACS Publications

Recommend Documents