Enhanced Photocatalytic Water Splitting on Very Thin WO3 Films

Oct 8, 2018 - Further advancement in sunlight-driven splitting of water as a means of producing hydrogen and oxygen is mainly hampered by the availabi...
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Letter

Enhanced Photocatalytic Water Splitting on Very Thin WO Films Activated by High Temperature Annealing. 3

Aldona Jelinska, Krzysztof Bienkowski, Michal Jadwiszczak, Marcin Pisarek, Marcin Strawski, Dominik Kurzyd#owski, Renata Solarska, and Jan Augustynski ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03497 • Publication Date (Web): 08 Oct 2018 Downloaded from http://pubs.acs.org on October 8, 2018

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Enhanced Photocatalytic Water Splitting on Very Thin WO3 Films Activated by High Temperature Annealing Aldona Jelinska*†, Krzysztof Bienkowski†, Michal Jadwiszczak†, Marcin Pisarek‡, Marcin Strawski§, Dominik Kurzydlowski¤, Renata Solarska*†, Jan Augustynski*† †Centre

of New Technologies, University of Warsaw, 02-097 Warsaw, Poland

‡Institute

of Physical Chemistry Polish Academy of Science, 02-668 Warsaw, Poland

§Faculty

of Chemistry, University of Warsaw, 02-097 Warsaw, Poland

¤Faculty

of Mathematics and Natural Sciences, Cardinal Stefan Wyszynski University, 01-815 Warsaw, Poland

ABSTRACT: Further advancement in sunlight-driven splitting of water as a means of producing hydrogen and oxygen is mainly hampered by the availability of easy-to-prepare, inexpensive n-type semiconductor materials able to operate as stable and efficient photoanodes in a water photoelectrolysis cell. Here, we demonstrate that photocatalytic water oxidation currents on thin-layer semitransparent WO3 electrodes, deposited by one-step sol-gel method on conductive oxide F-SnO2 substrates, are dramatically improved following additional higher-temperature (ca 700°C) annealing. Largely reduced recombination of charge carriers photogenerated in activated WO3 associated with enhanced light absorption yields at 1.23 V vs RHE, under simulated solar AM 1.5G irradiation (100 mW cm-2), water photo-oxidation currents close to 4.2 mA cm-2 on a 1.2 µm-thick photoanode - about 2 times larger than on the electrodes of the same thickness only annealed at 550°C. The relative enhancement of the photocurrent induced by the further annealing at 700°C scaled up with decreasing the film thickness with a threefold increase observed for the thinnest tested, 0.25 µm-thick WO3 electrode that reaches 2.75 mA cm-2. We obtained such high photocatalytic water splitting performance without depositing any additional water oxidation catalyst.

tungsten

trioxide,

tin

oxide,

water

photo-oxidation,

photoanode,

photoelectrochemistry,

photocatalysis

Photoelectrochemical (PEC) devices employing semiconductor electrodes provide a potentially viable means for converting solar energy into storable fuels such as hydrogen.1-5 In addition to effective absorption of solar light, the semiconductors used to drive kinetically difficult 4electron photo-oxidation of water should guarantee stability under highly oxidizing conditions. Numerous earlier investigations identified several n-type metal oxides able to operate as stable photoanodes in water splitting PEC cells.6-9 Nevertheless, only few of them, hematite ( α -Fe2O3) with band gap EG = 2.1-2.2 eV10-14, bismuth vanadate (BiVO4) EG = 2.4 eV15-18 and tungsten trioxide (WO3) EG = 2.5 eV19-23 absorb enough photons in the visible spectral range to enable them to deliver significant anodic photocurrents.

microns.25 Although nanostructuring of the semiconductor films allowed to partly circumvent the mismatch existing between the photon penetration depth and the distance over which the photogenerated charge carriers can be separated and collected, other unavoidable consequences are substantial Ohmic drops and uneven current distribution within the pores of thicker nanostructured photoanodes.26 Nevertheless, a particularly important advantage of the WO3 as water splitting photoanode is a relatively long hole diffusion length (LD) of about 150 nm20,27,23 that in the case of nanostructured WO3 films allows to reduce bulk recombination of photogenerated charge carriers, resulting in high incident photon-to-current conversion efficiencies (IPCEs).22

Besides generally too positive values of the photocurrent onset potentials, another major limitation of a number of metal oxide semiconductors used as photoanodes is the indirect nature of optical transition resulting in relatively low absorption coefficients, especially close to the semiconductor band edge.24 Such is also the case of WO3 films that in order to absorb significant portion of blue visible light have to attain thicknesses of the order of several

In the work presented here, we describe thin-layer nanostructured WO3 film electrodes activated by higher temperature post-annealing, formed by a simple and rapid one-step, solution-based sol-gel method, with dramatically improved PEC and optical properties. As detailed in SI, the WO3 films are prepared by doctor blading on conductive fluorine-doped tin oxide (FTO) substrates a colloidal precursor solution of tungstic acid including polyethylene

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glycol (PEG) acting as structure-directing and binding agent. Obtaining by this procedure a crystalline nanostructured WO3 film and removing residual carbon requires annealing in oxygen at temperatures of 500-550°C. However, synthesis of adherent and coherent films thicker than 0.4-0.5 µm necessitates sequential layer-by-layer deposition of the precursor and annealing. The here described activation procedure allows to drastically restrict the WO3 film thickness (and consequently the amount of the photomaterial) required to effectively absorb blue light and supply substantial anodic photocurrents. Figure 1a shows the water oxidation photocurrentpotential (j-E) plots for 1.2 µm - thick WO3 electrodes recorded in a 1 M methane-sulfonic acid supporting electrolyte26 under simulated AM 1.5G (100 mW cm-2) sunlight.

Figure 1. PEC water oxidation currents plotted against applied potential for WO3 electrodes annealed at 550°C (black curves) and after further annealing for 36 min. at 670°C (red curves) for (a) a 1.2 µm-thick and (b) for a 0.25 µm-thick film. Measurements were performed in a 1 M CH3SO3H supporting electrolyte under simulated AM 1.5G (100 mW cm-2) sunlight. The additional film annealing at 670°C (this temperature was measured with a thermocouple placed in the tube of the furnace programmed at 700°C at the same position of the samples) for up to 1 h produced a very large enhancement of the photocurrent that measured at 1.23 V vs RHE reaches 4.2

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mA cm-2, almost twice as large as for the WO3 electrode only annealed at 550°C. The effect of the higher temperature annealing is even more apparent for a very thin, ca. 0.25 µmthick, WO3 film for which the further heat treatment at 670°C resulted in about threefold increase of the photocurrent to attain at 1.23 VRHE 2.75 mA cm-2(Figure 1b). The j-E plots recorded under chopped light (Figure S1) do not indicate at 1.23 VRHE any perceptible dark current that appears only at potentials of 1.6-1.8 V. Another consequence of the 670°C post-annealing of the WO3 electrodes, visible in Figure 1, is a more marked Ohmic character of the j-E plots suggesting a possible drop of conductivity of the F-SnO2 substrate. Following further tests, we avoided annealing the WO3 films at temperatures higher than 700°C where softening of the glass in the FTO substrate was already observed. In the present work, we used as substrates 7 /square FTO samples originated from different batches provided by Sigma-Aldrich bearing an approximately 0.9 µmthick conductive F-SnO2 layer. Although the extent of degradation in conductivity of the FTO substrates caused by heating to high temperatures may in part depend on the manufacturer of the conductive glass28, the FTO coatings are generally considered to be stable over short heating times up to around 800°C.29,3o After a systematic screening, based on the photocurrents measured at 1.23 VRHE, we found the best PEC performance for the samples annealed at 670°C for 3040 min. We also note that experiments performed using another acidic electrolyte 1 M H2SO4 gave results quite similar to those displayed in Figure 1. However, as demonstrated in earlier work 31,32,26,23 the photo-oxidation of acidic sulfate ions at the WO3 electrode leads to the formation of persulfates rather than oxygen evolution. We obtained direct evidence of the interaction taking place between the WO3 film and the F-SnO2 substrate occurring during annealing at 670°C, that suggests migration of tin species into the WO3 film, from X-ray photoelectron scattering (XPS) spectra (Figure S2). Since we did not detect tin on the surface of 1.2 µm-thick film, systematic XPS measurements focused on thinner films. The amount of tin observed on the surface of about 0.25 µm-thick WO3 film, reaches about 0.6 at. % (2 at. % based on metal) and it substantially increases over argon-ion film sputtering to attain after 3600 s of etching about 3.5 at. % (7.7 at. % based on metal) as indicated by the etching-time profile in Figure 2a. We obtained quite similar results from XPS measurements taken on a 0.4 µm-thick WO3 film. The binding energy of the Sn 3d5/2 signal observed on the surface of the 0.25 µm-thick WO3 film prior to argon-ion etching was 487.7 eV (vs BE of C 1s at 285.o eV) consistent with the presence of SnO2. However, it is important to note that the XPS spectra taken from different places on such WO3 sample surface showed quite uneven tin distribution, with the detected amounts ranging from the aforementioned upper limit to a virtual zero (i.e., below the detection limit of our spectrometer). On the other hand, we did not detect any tin on the surface of a WO3 sample of similar thickness only annealed at 550°C. Consistently with the results of XPS analyses indicating uneven distribution of the tin species within the WO3 films on FTO annealed at 670°C, similar conclusion can be taken from energy selective backscattered image in Figure 2b obtained using the ESB detector of the scanning electron microscope (SEM). The red dots on the ESB image that represent the tin species suggest their

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irregular distribution in the subsurface region of the WO3 film annealed at 670°C. We also note that we did not see any tin on a backscattered image of a similar WO3 film annealed only at 550°C.

Figure 2. (a) Example of the evolution of tin concentration in an about 0.25 µm-thick WO3 film after annealing at 670°C determined by XPS over in-depth argon-ion etching at the rate of 0.032 nm s-1. (b) Energy selective backscattered image of a similar 0.25 µm-thick WO3 film after post-annealing at 670°C. (c) SIMS profiles of W and Sn observed across the latter WO3 film before and after further annealing at 670°C. In an attempt to determine the part of the WO3 film across which the migration of tin from the FTO substrate during the higher-temperature annealing takes place, we also performed secondary-ion mass spectroscopy (SIMS) analysis on a 1.2 µm-thick film formed by successive deposition of three layers of the precursor. The presence of progressively increasing, down to the FTO substrate, amounts of tin is observed across the large part of such film annealed at 670°C (Figure S3). Like in the case of the XPS analyses, we did not find in the SIMS measurements any evidence of tin incorporation into the WO3 sample only annealed under standard (550°C) conditions. Another consequence of the WO3 film annealing at 670°C was a marked sintering of the oxide nanoparticles (NPs) with mean individual particle sizes increasing from 30-50 nm, for the film only annealed at 550°C, to 60-80 nm. As shown by the top-view SEM image in Figure 3b, the film annealing at 670°C results also in the formation of a number of much larger (some of them exceeding 200 nm), sintered features, consisting of agglomerates of smaller WO3 NPs, accompanied by several large apertures in the film. The transmission electron microscopic (TEM) image of WO3 NPs removed from the film annealed at 670°C, displayed in Figure 3c, shows large single crystalline domains and a particle size of 40-60 nm. It is interesting to note that a similar effect has been reported for a α-Fe2O3 hematite film consisting of nanorods grown on the FTO substrate that acquired single crystalline character following annealing at even higher temperature of 800°C.33

Figure 3. Comparison of SEM images of mesoporous WO3 films (a) annealed only at 550°C and (b) after further annealing for 36 min. at 670°C. (c) TEM image of NPs removed from the WO3 film annealed at 670°C. The annealing of the WO3 films at 670°C had also a major effect upon UV-Vis spectra (Figure S4) that show an important increase of the absorptivity over the entire 320600 nm range due in part (particularly in the sub-bandgap region) to light scattering by larger NPs. These changes in optical properties are one of the reasons of the observed enhancement in the IPCE spectra (Figure 4) that we discuss later in context with the incorporation of tin species into the WO3 films. We note however, in this connection, that the values of IPCEs shown in Figure 4 are affected by the low intensity of light passing through the narrow bandwidth (4 nm) of the monochromator that generates the photocurrents too weak to allow filling of all electron traps present within the film that continued to act as recombination centers.

Figure 4. IPCE photoaction spectra determined for (a) a 1.2 µm thick WO3 film and (b) for a ca. 0.25 µm thick film annealed at 550°C (black curves) and further annealed at 670°C (red curves).

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With regard to the increased absorptivity of the 670°C annealed WO3 films, we wish to recall that similar changes in optical properties were earlier observed by Sivula et al.34 for mesoporous α -Fe2O3 films deposited onto F-SnO2 coated substrates as a result of sintering the films at a higher temperature of 800°C. Even if our work was not directly inspired by the latter results, we consider important to discuss shortly the main similarities and differences between our WO3 samples post-annealed at 670°C and the hightemperature (generally 800°C) processed α -Fe2O3 films on FTO investigated by many authors.30,33-42Based on the results of XPS analyses, Sivula et al. assigned the increased absorptivity of the α-Fe2O3 films to the structural distortion of the hematite lattice caused by the incorporation of Sn4+ ions from the FTO substrate.34 However, despite large improvement in visible light absorption, the final water splitting currents delivered by those α -Fe2O3 photoanodes annealed at 800°C remained modest due the important sintering of the film nanostructure34 making sizes of the individual features by far larger than the hole diffusion length (LD of about 2-4 nm) in hematite.11,13 Comparison of the values of LD for α -Fe2O3 with that much longer (ca. 150 nm) for WO320,27 gives a direct explanation why, in the case of our samples, the post-annealing at 670°C may still cause an important enhancement of the anodic photocurrents. In fact, such large values of LD provide an effective means of hole extraction in the case of porous, like our WO3 (Figure 3b), semiconductor films even when they consist of a network of relatively large NPs permeated by the electrolyte. The observations reported in ref. 34, were followed by a series of further investigations that lead to a number of complementary interpretations of the effect of high temperature annealing of iron oxide films deposited on FTO and the consecutive tin doping.30,33-42 We note in this connection that, unlike our WO3 electrodes, the hematite films used in those experiments were practically in all cases photoelectrochemically inactive prior to high temperature (750-800°C) annealing. Quite generally, when the film composition was probed by XPS, the incorporation of tin species from the FTO substrate into the α -Fe2O3 film was indicated although it is unclear whether the reported tin concentrations were the mean or the maximum observed values. Increased the majority charge carrier density35, reduced width of a space charge layer36, larger surface roughness and better wettability of the films39,42 have been evoked among possible reasons of the improved PEC behavior of the high–temperature annealed hematite electrodes. When comparing the n-type WO3 and α -Fe2O3 semiconductors, it is important to recall very low electrical mobility (in the range of 0.01-0.1 cm2 V-1 s-1) in hematite43,11 that is the reason for which to become conductive the hematite is frequently doped with group IV elements.44,11,9 This makes an essential difference with the WO3 that, due to its two orders of magnitude larger electron mobility of about 12 cm2 V-1 s-1,45 does not require extrinsic n-doping to exhibit good conductivity.22,23 To clarify the effect of tin doping upon the PEC activity of WO3, we also prepared films from the precursor with added (as SnCl4) 2 at. % of Sn4+. Interestingly, such intentionally doped Sn(IV):WO3 electrode exhibits pronounced drop of the photocurrents in comparison to an (extrinsically) undoped film prepared in the same way (Figure S5), the observation consistent with the results of an earlier study of

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the co-authors.46 Since the drop of PEC activity for the WO3 films formed using the precursor containing Sn4+ was earlier correlated46 with the disruption of crystallinity of WO3, indicated by the increased breadth of the Raman 715 and 805 cm−1 peaks, we performed Raman analyses of the bare WO3 samples, either annealed only at 550°C or further at 670°C, as well as for the film formed from the precursor with added 2 at. % of Sn4+. While annealing the bare WO3 at 670°C results in increase in intensity of the Raman peaks – consistent with improvement in crystallinity - the introduction of Sn4+ into the film precursor has clearly an opposite effect causing important decrease of intensity and widening of the 715 and 805 cm−1 peaks (cf. Figure S6). Note that an earlier work47 demonstrated direct correlation between the high crystallinity and the PEC performance of mesoporous WO3 films. The excellent crystallinity of the WO3 films annealed at 670°C is further confirmed by XRD patterns (Figure S7) that exhibit three main sharp peaks characteristic of the preferential orientation of (200), (020) and (002) faces of monoclinic WO3 crystallites parallel to the substrate.47 To be noted is the change observed in the relative intensity of the peaks, with the 200 reflection becoming the most intense for the sample post-annealed at 670°C. According to DFT/thermochemistry calculations the photo-oxidation of water on the 200 WO3 surface requires the lowest overpotential.48 The contrasting PEC behavior of the intentionally tindoped WO3 (with Sn4+ added to the precursor), as compared to the 670°C annealed bare WO3 films - having interacted with the FTO substrate, points at an important difference with the α -Fe2O3 films where Sn(IV) acts as n-type dopant improving the conductivity and photoactivity of the electrode.49,11 This is probably the reason for which the drastic improvement of the PEC performance resulting from the annealing of hematite films on FTO at 800°C has been frequently assigned to incorporation of Sn4+ from the substrate into the α-Fe2O3.30,34-37,39 It is also to be noted that the 800°C annealing temperature coincides with an abrupt increase of the diffusion coefficient of cations in hematite.34,50 A different insight into the nature of the interactions between an α -Fe2O3 film and the F-SnO2 substrate occurring during the high temperature processing has been subsequently brought about by a soft X-ray absorption spectroscopic film study of Kronawitter et al.38 The authors measured changes in the electronic structure of the interface between a very thin hematite film and the FTO substrate induced by short annealing at 800°C. Of broader significance is the observation that the large increase in the anodic photocurrents was not accompanied by the incorporation of the Sn4+ into the hematite lattice in substitution for Fe3+ ions. The fact that the Sn M4,5–edge absorption spectrum, measured for the 800°C annealed sample, showed the chemical environment around Sn quite similar to that of SnO2, lead the authors38 to propose that tin is incorporated into α -Fe2O3 in the form of microstructural regions of the defect clusters of rutile SnO2.51 The latter model suggests a possible interpretation of the results of our investigation of higher-temperature annealed WO3 films on FTO. In fact, quite uneven distribution of the Sn species within the WO3 film after annealing at 670°C, showed by aforementioned series of XPS measurements and the ESB image (Figure 2b) together with lack of evidence that such species are incorporated into the WO3 lattice (cf. Raman

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spectra in Figure S6) suggest that tin from the FTO substrate may diffuse along the WO3 crystallites to form defective SnO2 clusters. Such mechanism should be facilitated by significant initial porosity of the WO3 films (Figure 3a) in which the migration of tin species from FTO may occur during the 670°C annealing in parallel with sintering of the WO3 NPs (Figure 3b) forming an inter-diffused junction. An indication in that sense is provided by the results of scanning transmission electron microscopic-energy dispersive X-ray spectroscopic (STEM-EDX) analyses of the cross section of the WO3 film on FTO annealed at 670°C represented in Figure 5. Panel (a) displaying the SEM image of the analyzed cross section, shows an uneven coverage of the FTO substrate by the WO3 NPs implying that during the photoelectrolysis a part of the substrate may be in direct contact with the electrolyte penetrating through the pores in the film. Panel (b) displays results of EDX line scans for W, Sn and O performed following the red line within the delineated fragment in the STEM image showed as an inset. These scans suggest presence of significant amount of tin across the major part of the analyzed fragment of the WO3/FTO film; we also note a not zero background for Sn in the outer part of the film. In an attempt to evaluate the resistance of the semiconductor film, we performed AC impedance measurements of the WO3 films shown on the example of Figure S8 a, b. However, as shown in Table S8b, due to large apertures in the film leading to the direct contact of the FTO substrate with the electrolyte, the impedance Rs values are clearly consistent with the FTO resistance and not the resistance of WO3 in series with the film capacitance. Inspection of the photoaction spectra in Figure 4 reveals a very large (up to three times) relative increase in the IPCEs for the samples annealed at 670°C, occurring in the range of visible wavelengths that penetrate in-depth the WO3 film. We assign that increase of IPCEs to the formation of interdiffused nano-heterojunction involving WO3 crystallites and defective microcrystalline SnO2 clusters, consecutive to the diffusion of tin species from the FTO substrate into the film.

Figure 5. STEM-EDX analyses of the cross section of approximately 0.25 µm thick WO3 film after annealing at 670°C. Panel (a) shows a SEM image of the analyzed cross section, panel (b) displays EDX line scans for W, Sn and O corresponding to the red line within the delineated fragment in the STEM image showed as an inset. Included are also in panels (c) and (d) maps for elemental Sn and W.

We expect the formation of such heterojunction to decrease the surface charge carrier recombination while improving light absorption through enhanced internal scattering within the mesoporous film structure52 that extends the travelling distance of photons across the WO3 film. The fact that the lower CB edge of bulk SnO2 is located below that of WO353 suggests that incorporation of SnO2 clusters might facilitate the charge separation within the 670°C-annealed photoanode. There are several recent reports54-57 indicating that in the case of nanostructured SnO2 photomaterials the optical band gap may be much lower than the EG = 3.6 eV of bulk SnO2, due principally to the shift of the upper edge of the VB to less positive potentials.57 Actually, only small negative changes in the onset potential of WO3 resulting from the post-annealing at 670°C have been measured. It is important to also note here that the decreased extent of charge recombination is not clearly apparent on the initial incline of the j-E plots (Figure 1) visibly hidden by the increased resistance of the FTO substrate after annealing at 670°C. To evaluate the effect of the annealing at 670°C upon the optical characteristic of the WO3 film, independently of the incorporation of the tin species, we also prepared and annealed in the same way the WO3 samples deposited on quartz substrates covered with a 20 nm-thick Cr adhesion layer. Although in the case of such about 0.25 µm-thick film the annealing at 670°C for 36 min. produces also marked sintering (see Figure S9 a, b) this is, however, accompanied only by a modest change in absorptivity (Figure S10a) and practically unchanged water photo-oxidation currents (Figure S10b), showing that incorporation of tin from the FTO substrate is crucial to the performance of the WO3 photoanode. We used WO3 films formed on FTO and post-annealed at 670°C in various series of water splitting experiments that gave reproducible j-E plots with changes in the saturation photocurrents that did not exceed 3%. Figure S11a shows the evolution of the photocurrent for the 1.2 µm-thick WO3 electrode recorded along prolonged photoelectrolysis conducted at an applied potential of 1.2 VRHE. After an initial drop we assign to filling with formed oxygen the pores within the mesoporous film, the electrode maintained a practically constant photocurrent over the whole test. After that experiment and a period of vigorous stirring of the solution, the electrode recovered the initial j-E plot (Figure S11b). The very thin (0.25 µm-thick) film WO3 electrodes provided also quite reproducible j-E plots (similar to that in Figure 1b) with no significant changes in the saturation photocurrents in series of subsequent CV experiments. However, attempts to perform continuous photoelectrolyses with these electrodes, applying a 1 V vs RHE potential, resulted in the decline of the photocurrents occurring more or less rapidly within a few hours. For this reason, further stability tests focused on 1.2 µm thick electrodes. Although the optimization of the PEC performance of the investigated WO3 electrodes was beyond the scope of the present study, a comparison with the literature data shows that, with the attained at 1.23 VRHE water oxidation photocurrents of 4.2 +-0.1 mA-cm-2, the 1.2 µm-thick film WO3 electrode activated by annealing at 670°C outperforms other recently described water splitting photoanodes. For example, a nanostructured WO3 electrode consisting of highly crystalline hyper-branched nano-trees

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reached the saturation photocurrent of 1.85 mA cm-2 already at 0.8 VRHE that offers a potential advantage for the application in a tandem cell.58 An indium and tin co-doped hematite photoanode yielded 2.5 mA cm-2 at 1.23 VRHE also under simulated AM 1.5G sunlight.41 In conclusion, we have demonstrated that an additional higher (670°C) temperature annealing of thin nanostructured WO3 films formed on transparent conductive oxide F-SnO2 substrates results in a very large enhancement of solar lightdriven water splitting photocurrents. In contrast with the wide number of similar studies regarding the hightemperature photoactivation of α -Fe2O3 films deposited on FTO, the improvement of the PEC performance of our WO3 electrodes occurs over annealing under milder conditions i.e., at an about 100°C lower temperature. Importantly, parallel experiments consisting in intentional doping the WO3 by introducing Sn4+ ions into the film precursor gave results opposite to those obtained with the 670°Ctemperature annealed bare films, namely the drastic drop in water photo-oxidation currents attributable to the disruption of WO3 crystallinity in such films. Although structural analyses of the 670°C processed WO3 samples evidence the migration of the Sn species from the F-SnO2 substrate into the film, they also reveal uneven tin distribution within the WO3 film. Consequently, we postulate that, instead of being included into the WO3 lattice as Sn4+ ions, the Sn species diffusing from the FTO substrate into the film during the treatment at 670°C form nano-heterostructures involving defective SnO2 clusters51 and the WO3 crystallites. We postulate that the formation of inter-diffused SnO2-WO3 heterojunction modifies electronic structure of the interface in contact with the electrolyte improving the efficiency of oxygen evolution photoreaction within significant portion of the film. The WO3 electrodes modified by the 670°C postannealing perform remarkably better than similar films only annealed at 550°C yielding twice as large photo-oxidation currents, i.e., 4.2+-0.1 mA cm-2 under standard conditions: at 1.23 VRHE and simulated AM 1.5G sunlight (100 mW cm-2). Supporting Information. The WO3 film preparation and characterization procedures, a j-E plot for a 0.25 µm-thick photoanode recorded under chopped light, the XPS survey spectrum of the post-annealed WO3 film, UV-Vis spectra of WO3 films, the j-E plot for a Sn(IV)-doped WO3 photoanode, Raman and XRD spectra of WO3 films, SEM images, UV-Vis spectra and j-E plots for WO3 films deposited on quartz/Cr, the photocurrent stability test for a post-annealed WO3 photoanode and subsequent j-E plot, AC impedance measurements of a WO3 film. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]; [email protected]

ACKNOWLEDGMENT We thank Dr Piotr Kedzierzawski from the Institute of Physical Chemistry PAS for the EIS measurements. This work was supported by the MAESTRO Grant UMO2013/10/A/ST5/00245 awarded to J.A. by the Polish National Science Centre. K.B. acknowledges also the support from the

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PRELUDIUM Grant 2013/09/N/ST5/02976 (National Science Centre). The SIMS measurements have been performed using equipment purchased by Biological and Chemical Research Centre, University of Warsaw, established within the project co-financed by the European Union from the European Regional Development Fund under the Operational Programme Innovative Economy, 2007–2013.

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(41) Kaouk, A.; Ruoko, T-P.; Pyeon, M.; Gönüllü, Y.; Kaunisto, K.; Lemmetyinen, H.; Mathur, S. High Water-Splitting Efficiency Through Intentional In and Sn Codoping in Hematite Photoanodes. J. Phys. Chem. C 2016, 120, 28345-28353. (42) Carvalho-Jr, W.M.; Souza, F.L. Thermal Enhancement of Water Affinity on the Surface of Undoped Hematite Photoelectrodes. Sol. Energy Mater. Sol. Cells 2016, 144, 395-404. (43) Morin, F.J. Electrical Properties of α-Fe2O3, Phys. Rev. 1954, 93, 1195-1199. (44) Sanchez, H.L.; Steinfink, H. Solid Solubility of Ge, Si, and Mg in Fe2O3 and Photoelectric Behavior. J. Solid State Chem. 1982, 41, 9096. (45) Berak, J.M.; Sienko, M.J. Effect of Oxygen-Deficiency on Electrical Transport Properties of Tungsten Trioxide Crystals, J. Solid State Chem. 1970, 2, 109-133. (46) Solarska, R.; Alexander, B.D.; Braun, A.; Jurczakowski, R.; Fortunato, G.; Stiefele, M.; Graule, T.; Augustynski, J. Tailoring the Morphology of WO3 Films with Substitutional Cation Doping: Effect on the Photoelectrochemical Properties. Electrochim. Acta 2010, 55, 7780-7787. (47) Santato, C.; Odziemkowski, M.; Ulmann, M.; Augustynski, J. Crystallographically Oriented Mesoporous WO3 Films:  Synthesis, Characterization, and Applications. J. Am. Chem. Soc. 2001, 123, 10639-10649. (48) Valdes, A.; Kroes, G.J. First Principles Study of the Photooxidation of Water on Tungsten Trioxide (WO3). J. Chem. Phys. 2009, 130, 114701 -1-9. (49) Glasscock, J.A.; Barnes,P.R.F.; Plumb, I.C.; Savvides, N.J. Enhancement of Photoelectrochemical Hydrogen Production from Hematite Thin Films by the Introduction of Ti and Si. J. Phys. Chem. C 2007, 111, 16477-16488. (50) Atkinson, A.; Taylor, R.I. Diffusion of 55Fe in Fe2O3 Single Crystals. J. Phys. Chem. Solids 1985, 46, 469-475. (51) Berry, F.J.; Greaves, C.; McManus, J.G.; Mortimer, M.; Oates, G.J. The Structural Characterization of Tin- and Titanium-Doped α-Fe2O3 Prepared by Hydrothermal Synthesis. J. Solid State Chem. 1997, 130, 272-276. (52) Qian, J.F.; Liu,P.; Xiao,Y.; Jiang,Y.; Cao,Y.L.; Ai, X.P.; Yang, H.X. TiO2-Coated Multilayered SnO2 Hollow Microspheres for Dye-Sensitized Solar Cells. Adv. Mater. 2009, 21, 3663-3667. (53) Grätzel, M. Photoelectrochemical Cells. Nature 2001, 414, 338344. (54) Wang, H.; Sun, F.; Zhang, Y.; Li, L.; Chen, H.; Wu, Q.; Yu, J.C. Photochemical Growth of Nanoporous SnO2 at the Air–Water Interface and Its High Photocatalytic Activity. J. Mater. Chem. 2010, 20, 5641-5645. (55) Liu, X.; Pan, L.; Chen, T.; Li, J.; Yu, K.; Sun, Z.; Sun, C. Visible Light Photocatalytic Degradation of Methylene Blue by SnO2 Quantum Dots Prepared via Microwave-Assisted Method. Catal. Sci. Technol. 2013, 3, 1805-1809. (56) Zhang,S.; Li,J.; Niu,H.; Xu, W.; Xu, J.; Hu, W.; Wang, X. Visible-Light Photocatalytic Degradation of Methylene Blue Using SnO2/ α -Fe2O3 Hierarchical Nanoheterostructures. ChemPlusChem. 2013, 78, 192-199. (57) Zulfigar, Y.; Khan, R.; Yuan, Y.; Iqbal, Z.; Yang, J.; Wang, W.; Ye, Z.; Lu, J. Variation of Structural, Optical, Dielectric and Magnetic Properties of SnO2 Nanoparticles. J. Mater. Sci. ; Mater electron., 2017, 28, 4625-4636. (58) Balandeh,M.; Mezzetti,A.; Tacca,A.; Leonardi, S.; Marra,G.; Divitini,G.; Ducati,C.; Meda,L.; Di Fonzo,F. Quasi-1D Hyperbranched WO3 Nanostructures for Low-Voltage Photoelectrochemical Water Splitting. J. Mater. Chem. A 2015, 3, 6110-6117.

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Figure 1. PEC water oxidation currents plotted against applied poten-tial for WO3 electrodes annealed at 550°C (black curves) and after further annealing for 36 min. at 670°C (red curves) for (a) a 1.2 µm-thick and (b) for a 0.25 µm-thick film. Measurements were performed in a 1 M CH3SO3H supporting electrolyte under simulated AM 1.5G (100 mW cm-2) sunlight. 230x344mm (150 x 150 DPI)

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Figure 2. (a) Example of the evolution of tin concentration in an about 0.25 µm-thick WO3 film after annealing at 670°C determined by XPS over in-depth argon-ion etching at the rate of 0.032 nm s-1. (b) Energy selective backscattered im-age of a similar 0.25 µm-thick WO3 film after post-annealing at 670°C. (c) SIMS profiles of W and Sn observed across the latter WO3 film before and after further annealing at 670°C. 397x370mm (150 x 150 DPI)

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Figure 3. Comparison of SEM images of mesoporous WO3 films (a) annealed only at 550°C and (b) after further annealing for 36 min. at 670°C. (c) TEM image of NPs removed from the WO3 film annealed at 670°C. 257x350mm (150 x 150 DPI)

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Figure 4. IPCE photoaction spectra determined for (a) a 1.2 µm thick WO3 film and (b) for a ca 0.25 µm thick film annealed at 550°C (black curves) and further annealed at 670°C (red curves). 404x171mm (150 x 150 DPI)

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Figure 5. STEM-EDX analyses of the cross section of ap-proximately 0.25 µm thick WO3 film after annealing at 670°C. Panel (a) shows a SEM image of the analyzed cross section, panel (b) displays EDX line scans for W, Sn and O corresponding to the red line within the delineated fragment in the STEM image showed as an inset. Included are also in panels (c) and (d) maps for elemental Sn and W. 407x285mm (150 x 150 DPI)

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TOC graphic 149x88mm (78 x 78 DPI)

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