Structural and photoelectrochemical investigation of boron-modified nanostructured tungsten trioxide films

Electrochimica Acta 104 (2013) 282–288

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Structural and photoelectrochemical investigation of boron-modified nanostructured tungsten trioxide films Piotr J. Barczuk a , Agata Krolikowska b , Adam Lewera b , Krzysztof Miecznikowski b , Renata Solarska b,c , Jan Augustynski a,c,∗ a

Centre for New Technologies, University of Warsaw, Zwirki i Wigury 93, 02-089 Warsaw, Poland Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland c Institute of Electronic Materials Technology, Wolczynska 133, 01-919 Warsaw, Poland b

a r t i c l e

i n f o

Article history: Received 21 January 2013 Received in revised form 28 March 2013 Accepted 17 April 2013 Available online 29 April 2013 Keywords: Tungsten trioxide Photoanode Water splitting Boron doping Photoelectrochemistry

a b s t r a c t We report a modification of nanostructured WO3 films by doping with boron. The films were obtained by a direct one-step sol–gel route involving tungstic acid/polyethelene glycol precursor. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) showed that the incorporation of boron results in the retention of a substantial amount of water and/or hydroxyl groups in the WO3 lattice and at the surface of nanoparticles occurring despite high temperature (550 ◦ C) annealing of the films. Another consequence of boron doping is the largely increased roughness factor revealed by atomic force microscopy (AFM) imaging. Both kinds of films are highly porous and consist of partly sintered particles with sizes in the range of tens of nanometers. The photoelectrochemical (PEC) studies performed under simulated solar AM 1.5 illumination showed significantly enhanced water oxidation photocurrents for B-WO3 photoanodes, by about 25% higher than those for the undoped WO3 films of similar thickness. The low extent of recombination of photogenerated charges was confirmed by incident photon-to-current conversion efficiencies (IPCEs) reaching 70% in the region of visible wavelengths at 420 nm. The improved PEC properties were attributed to the increased surface hydroxylation of B-WO3 nanoparticles favoring water photo-oxidation reaction and to the larger surface area of the film exposed to the electrolyte. © 2013 Elsevier Ltd. All rights reserved.

1. Introduction Photoelectrochemistry provides a means to store solar energy through converting water to oxygen and hydrogen [1]. Most of the recent research on H2 production via photoelectrochemical (PEC) water splitting has focused on the use of n-type semiconductor metal oxide materials employed as photoanodes [1,2]. Following the first report by Fujishima and Honda of photo-oxidation of water on a rutile titanium dioxide (TiO2 ) photoanode illuminated with ultraviolet (UV) light [3], initiated an extensive search of photoelectrode materials that are able to capture a substantial part of the visible spectrum. Early investigations involving a variety of inorganic semiconductors showed that the only materials able actually to split water whilst avoiding photocorrosion are metal oxides [4]. Besides investigations of TiO2 doped with a variety of elements [2], the recent work centered on hematite (␣-Fe2 O3 ) [5] and tungsten trioxide (WO3 ) [6] thin-film photoanodes with band gaps of 2.1 eV, respectively, 2.5 eV. Since any among these

∗ Corresponding author at: Centre for New Technologies, University of Warsaw, Zwirki i Wigury 93, 02-089 Warsaw, Poland. E-mail address: [email protected] (J. Augustynski). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.04.107

materials having photoaction spectra covering visible wavelengths can perform unassisted water splitting, due to the position of the conduction band edges more positive than the H2 evolution potential, continuing efforts are devoted to minimize the bias voltage required to perform visible light-driven photo-oxidation of water. Recently, unassisted solar production of H2 has been demonstrated in a dual-absorber tandem device combining a semitransparent WO3 photoanode and the latest version of the dye-sensitized solar cell (DSSC) providing a ca 1 V bias voltage to the photoelectrolyzer [7]. In such a tandem device, the WO3 photoanode, deposited on the conductive glass substrate, absorbs the near-UV and blue-green portions of the solar spectrum and the DSS photovoltaic cell, placed behind the photoelectrolyzer, captures longer wavelengths transmitted by the WO3 film. Efforts toward implementation of the solar water splitting include also the search of the low-cost, effective in terms of the energy payback, preparation methods of photoelectrode materials. This is illustrated by a recent use of solution-based colloidal synthesis approach, instead of chemical vapour deposition method, which offers better PEC performance, to form thin layer ␣-Fe2 O3 photoanodes [8]. In this paper, we report a modification with boron of nanostructured WO3 films formed using simple, one-step, sol–gel

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method. The structure, surface composition and photoelectrochemical properties of WO3 and B(III)-WO3 films were studied. The B(III)-modified WO3 photoanodes exhibit enhanced by about 25% water oxidation photocurrents which attain 2.15 mA/cm2 at 1.23 V versus reversible hydrogen electrode (RHE), under standard solar conditions (AM 1.5). The incorporation of boron is found to increase roughness factor of the films and to induce retention of a significant amount of water and/or hydroxyl− groups within the WO3 structure, which are both proposed as contributing to the enhanced PEC performance. The increased hydroxylation, which is combined with excellent crystallinity of the B-WO3 films, offers also attractive prospects for the application of this material in nanocomposite electrocatalysts for oxidation of alcohols [9]. 2. Experimental Nanostructured WO3 films having thickness of approximately 1.2 ␮m were formed using a sol–gel method described in detail elsewhere [10,11]. Briefly, the employed precursor consisted of (poly)tungstic acid, freshly prepared by elution of a solution of Na2 WO4 through the cation exchange column, and of an organic complexing agent (acting also as porogen), low molecular weight polyethylene glycol (PEG) 300. The WO3 /PEG ratio was 0.5 w/w. Boric acid (H3 BO3 ) was a source of boron added to the precursor to obtain 10 at.%, respectively, 20 at.% B/W ratios. Fluorine-doped SnO2 (FTO) conductive glass (Pilkington Glass, resistance 8–10 /square) was used as a substrate. The FTO samples were first cleaned with acetone and then sonicated in deionized water for several hours. Some drops of the colloidal precursor were spread on the FTO substrate using doctor-blade technique, dried in air at 100 ◦ C and, subsequently, samples were annealed in flowing oxygen at 550 ◦ C for 30 min. The thickness of single WO3 layers was 0.35–0.45 ␮m; thicker films were formed by consecutive applications of the precursor, each followed by the annealing. Raman spectra were acquired in the backscattering configuration with a Labram HR800 (Horiba Jobin Yvon) confocal microscope system equipped with a Peltier-cooled CCD detector (1024 × 256 pixel), using a 150 mW diode pumped, frequency doubled Nd:YAG laser (532 nm). The confocal pinhole size was set to 200 ␮m and the holographic grating with 600 grooves/mm was used. The instrument was calibrated using a 520 cm−1 Raman signal of a silicon wafer. All Raman spectra were obtained using a 100× magnification Olympus objective. Deconvolution of the overlapping signals was conducted using the LabSpec5 software, assuming Gaussian shape of the Raman bands. The X-ray photoelectron spectra (XPS) were taken on a PHI 5700 Physical Electronics spectrometer. Monochromatized X-ray Al (K␣) radiation of 1486 eV was used. The spectra were recorded with 23.50 eV pass energy, 0.100 eV step and 100 ms dwell time. The total energy resolution was about 0.35 eV. The measurements were performed in a vacuum of 10−10 Torr. Depth profiling was realized by sputtering with Ar+ ions of 2 keV energy. The focused ion beam was scanned over the area 2.5 mm × 2.5 mm, whereas the analysis area had the diameter of 0.8 mm and was situated in the center of the created crater. The sputter rate was in the 1–2 nm/min rate. The X PS measurements were performed with the ion beam switched off. The X-ray diffraction (XRD) patterns of deposited films and powder samples were measured on a Philips X’Pert diffractometer using CuK␣1 radiation passed through a curved graphite monochromator. The patterns were recorded in the interval 15–140◦ , with the step of 0.02◦ . The crystal lattice, space group, cell parameters and site occupancies of the analyzed samples were evaluated from the corresponding XRD patterns, fitted and refined with a DBWS 9707 program. A Veeco Nanoscope V atomic force microscope equipped with a (di) MultiMode V scanner was used for topography imaging.

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Tapping mode AFM (TM AFM) was applied as less destructive toward the sample surface. Cantilevers with a spring constant of 42 N m−1 and resonance frequency range 311–380 kHz (manufacturer data), tuned prior to further measurement, were employed. The Nanoscope 7.30 software was used for topography visualization and cross-sectional profiling. The roughness parameter was calculated with aid of the NanoScope II software version 7.20. It is calculated as the difference between the image’s three-dimensional surface area (this value is the sum of the area of all of the triangles formed by three adjacent data points) and two-dimensional projected surface area. The analysis was performed over the entire image. A Teflon electrochemical cell with a quartz window, platinum foil counter electrode (separated from the photoanode by a Nafion membrane) and an Ag/AgCl/Cl− reference electrode was used for all photoelectrochemical studies. The photoanode (with the exposed surface area of 0.28 cm2 ) was illuminated from the side of the WO3 film/solution interface. Photocurrent action spectra (i.e., incident photon-to-current conversion efficiencies plotted against excitation wavelength) were obtained by illuminating the sample with the light of a 500 W xenon lamp (Ushio UXL-502HSO), set in an Oriel model 66021 housing, passing through a Multispec 257 monochromator (Oriel) with a bandwidth of 4 nm. The absolute intensity of the incident light from the monochromator was measured with a model 730 A radiometer/photometer (Optronic Laboratories). The photocurrent–voltage characteristics were recorded by scanning the potential of the WO3 photoanode at 10 mV s−1 under simulated AM 1.5 solar irradiation provided by an Oriel 150 W solar simulator fitted with a Schott 113 filter and a neutral density filter. The applied electrode potential versus Ag/AgCl/Cl− was converted to be reported versus the RHE at pH 2. 3. Results and discussion The preparation of nanostructured WO3 films relies on the sol–gel method involving a mixture of tungstic acid and polyethylene glycol (PEG). Deposition of the precursor on the FTO substrate followed by annealing in flowing oxygen at ca 550 ◦ C produces well crystallized WO3 with monoclinic structure (m-WO3 ). In fact, Raman spectra taken for both the boron (III)-modified and undoped films exhibit a series of features typical of m-WO3 (Fig. 1). In particular, the strong bands lying around 715 and 805 cm−1 and weaker close to 270 and 325 cm−1 , corresponding, respectively, to the

Fig. 1. Raman spectra, illustrating changes in the structure of tungsten trioxide induced by the addition of boron. The spectra, taken at exc = 532 nm, correspond to B-modified (at 10 and 20 at.%) and undoped ca 1.2 ␮m thick WO3 films annealed at 550 ◦ C. The amount of boron added to the precursor and the assignments of different Raman modes are indicated in the legend. (For interpretation of the references to spectra in figure legend, the reader is referred to the web version of the article.)

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O W O stretching and bending modes are entirely consistent with the monoclinic crystalline structure for both types of studied WO3 films [10,12]. To compare the spectra collected from different locations on the surface of the electrodes, Raman point mapping (here not shown) was performed on selected areas (around 50 ␮m × 50 ␮m) of the boron modified samples. The mapping revealed that the films are largely homogeneous concerning their crystalline phase and structural features. Hence, particular spectra presented in Fig. 1 can be considered as representative of the entire samples. It is also worth mentioning that, according to the observed differences in relative intensity scale, the boron-modified samples seem to exhibit higher Raman scattering efficiency than the undoped WO3 film. Good crystallinity of the examined films was further confirmed by the presence in the Raman spectra of lattice modes and other bands which positions (marked, respectively, with italics and underlined in Fig. 1) match closely those observed for the crystalline solid WO3 . Interestingly, the intensities of those bands, visible on the low wavenumber side of the spectra, are apparently even increased by the presence of boron in the films (cf. signals at 304, 202 and 180 cm−1 ) and this effect is still more pronounced for the samples fabricated with a larger amount of the boron additive. It is to be noted that the spectra displayed in Fig. 1 do not provide any indication of boron incorporation into and/or modifying the WO3 lattice. However, Raman spectroscopy might be somewhat misleading, because of the high Raman scattering cross-section of m-WO3 as determined by Chan et al. [13] who demonstrated that the detectable limit of m-WO3 in WO3 /Al2 O3 catalysts is below 0.1 wt%. As shown later by the analysis of XRD patterns, the boron incorporation into the m-WO3 induces, in fact, detectable changes in the lattice parameters and site occupancies. The major difference revealed by the comparison of the Raman spectra in Fig. 1 is a large increase in the extent of hydration (hydroxylation) for B(III)-modified WO3 films. This is consistent with the presence of two bands around 640 and 680 cm−1 , strongly overlapping with the O W O stretching vibration at 715 cm−1 . Since the resolution of our Raman instrument does not allow for direct observation of the separate bands, the Gaussian fit of these two extra components was necessary. The results of fitting, performed on the spectrum collected for the WO3 film modified with 20 at.% of boron, are shown in Fig. 1S. The two bands clearly visible on the lower wavenumber side of the spectrum can be attributed to the ␯(O W O) vibrations of the tungsten trioxide hydrates: WO3 × H2 O (the band around 640 cm−1 ) and WO3 × 1/3H2 O (the band around 680 cm−1 ) [12]. This latter hydrate is considered to be particularly sensitive to the action of atmospheric water. The less intense band lying at 377 cm−1 (cf. Fig. 1) can be assigned either to ␯(W OH2 ) of WO3 × H2 O or to water libration characteristic of WO3 × 2H2 O. All these results indicate that the addition of boron causes retention of a significant amount of water in the WO3 lattice. Importantly, this occurs despite the high temperature (550 ◦ C) at which all samples have been annealed, which is otherwise sufficient to totally withdraw water and/or OH groups from the film prepared without boron. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta. 2013.04.107. It is to be noted, in this connection, that WO3 films are known to exhibit hydrophilic properties both in the presence and even in the absence of UV illumination [14]. However, the extent of hydrophilicity has been shown to reach a maximum for the WO3 films annealed at 400 ◦ C and then to decrease with a further increase of the annealing temperature [15]. The fact that the B(III)modified WO3 films retain water even after annealing at 550 ◦ C strongly suggests that they remain hydrophilic despite such hightemperature heat treatment.

Fig. 2. Comparison of the O 1s XPS signals measured for a 10 at.% B-modified (a) and an undoped (b) WO3 films.

The XPS spectrum displayed in Fig. 2 shows that the modification of WO3 with boron leads to an increase in intensity of the O 1s signal at the higher binding energy (BE) side, at 532–534 eV, consistent with a partial coverage of the film surface with the hydroxyl groups. The observed increase in the intensity can be quantified as an OH− coverage of ca 15% of a monolayer. Oxygen, tungsten, boron and carbon (originating from adventitious impurities) were identified in the XP spectra. The appearance in the spectrum of the B 1s signal centered at ca 193 eV confirms that boron is present as B(III) [16,17]. The results of Raman spectroscopic and XPS analyses of the B(III)-modified WO3 samples point at a rather unusual combination of the excellent crystallinity and the hydrophilicity of the films. Note that a good crystallinity of the semiconductor material is quite generally considered as a prerequisite to obtain high activity in photoinduced oxidation reactions [10,18]. On the other hand, increased hydrophilicity might be particularly important in the case of the nanoparticulate films where it should favor electrolyte accessibility and, therefore, the transport of the reacting species within the mesoporous structure. The results of XRD analysis of an undoped WO3 film and 10 at.% boron (III)-modified sample refined by the Rietveld method [19] are shown in Fig. 3 and are summarized in Table 1 together with the data for a 20 at.% B (III)-WO3 sample. The monoclinic crystal system of WO3 phase, with space group P21/n, was identified in both diffraction patterns however the ˇ angle was more disordered in the presence of boron. Consistent with the earlier observations [10], the preferred orientation of crystallites was (0 0 2), (0 2 0), (0 0 2). The refined values of lattice parameters (cf. Table 1) show that the presence of boron causes a slight decrease in size of the unit cell, by introducing local lattice strains (<a/a>). The restrains on the composition might be imposed by the boron substitution in the tungsten or oxygen sites, which is illustrated by changes in site occupancies and the occupancy parameters. The fact that the observed disorder in site occupancies does not exceed a few percent suggests that B(III) is also present, at least in part, in interstitial positions in the m-WO3 lattice. Consequently, the changes in the values of refined lattice parameters might be the result of attractive and repulsive interactions of B3+ with O2− , respectively, W6+ ions. No impurity peaks, indicative of the presence of either boron oxide or boric acid were observed in the patterns of B(III)modified WO3 samples. As also shown in Table 1, the incorporation of boron is accompanied by a decrease in size of the crystallites. This is consistent with the hypothesis that B3+, with its crystallographic radius more than two times smaller than that of W6+ (0.23 A˚ vs. ˚ [20], occupies partially tungsten sites. 0.54 A)

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Table 1 Values of refined by Rietveld method lattice parameters for the undoped and boron-modified WO3 ; the crystallites sizes (D) and the strains of the crystal lattice (<a/a>). WO3 and B-WO3 films deposited onto conductive glass FTO substrates

ICDD 0%B 10%B 20%B

a0 [nm]

b0 [nm]

c0 [nm]

ˇ [◦ ]

D [nm]

<a/a> [%]

0.73060 0.73054(3) 0.72996(3) 0.72981(3)

0.75400 0.75001(3) 0.74963(3) 0.74818(3)

0.76920 0.76799(3) 0.76490(3) 0.76363(3)

90,881 90,878(2) 90,905(2) 90,978(2)

– 46.3(8) 40.2(8) 34.8(8)

– 0.66(3) 0.75(4) 0.82(4)

We observed that the addition of boron affected also the morphology of the tungsten trioxide films. Fig. 4 displays the tapping mode atomic force microscopy (TM AFM) images collected for an undoped (a) and a 10 at.% boron-modified (b) ca. 1.2 ␮m thick WO3 films. The combustion of PEG from the precursor occurring during annealing of the samples at 550 ◦ C results in a largely porous structure of the films. As can be seen from cross-sectional profiles, shown at the bottom of Fig. 4, nanoparticles with a diameter of about 30–40 nm prevail in both cases. Most of the particles are partly sintered and there are also some substantially larger particles. The feature that clearly differentiates the B-modified from the undoped WO3 samples is the film roughness factor indicated in the top right corner of the AFM images in Fig. 4. The latter parameter is defined as a difference in the actual image surface area and the projected image surface area, given in percent. Based on the AFM images shown in Fig. 4, the estimated roughness factor is

substantially larger for the B(III)-modified film (27%) than for the undoped WO3 film (10.2%) The increased roughness factor translates into a larger surface contact area with the electrolyte for the B(III)-modified WO3 photoanode. The influence of the surface area and geometrical orientation of the crystallites that form a nanostructured film upon its optical absorption and photoelectrochemical properties is illustrated by a recent work describing a WO3 photoanode with “flake-wall structure” [21]. Although the presence of crystalline WO3 flakes oriented perpendicularly to the substrate apparently improves the light capture, it also induces relatively large ohmic drops, which affect adversely the actual photocurrent-potential performance of the film. Fig. 5 shows the photocurrent density, I, plotted as a function of the applied potential, E, for both the 10 at.% B(III)-modified and the undoped WO3 photoanodes in contact with a 0.5 M sodium chloride (pH 2) solution, recorded under simulated solar AM 1.5 (100 mW/cm2 ) illumination. The choice of the latter electrolyte – with salinity close to that of sea water – was dictated by the remarkable stability of the photocurrents exhibited by the WO3 photoanodes during long-lasting polarization experiments. Under such conditions oxygen remains the main product of the photoelectrolysis: H2 O + 2h+ → ½O2 + 2Haq +

(1)

with chlorine formation accounting for less than 25% of the current efficiency: 2Cl− + 2h+ → Cl2

Fig. 3. The results of Rietveld refinement for (a) WO3 and (b) WO3 modified with 10 at.% of boron, showing the observed (dashed) and calculated (solid line) X-ray diffraction patterns. The vertical bars indicate the calculated characteristic Bragg peaks for two refined phases, WO3 and SnO2 -originating from the FTO substrate. The line at the bottom shows the difference between the observed and calculated profiles.

(2)

The obtained results open the prospect to use the sea water, the widely spread natural electrolyte, for sustainable production of hydrogen. It is to be noted, in this connection, that for the WO3 photoanode in a number of aqueous electrolytes, O2 is not the only product of the photo-oxidation reaction [22,23], the persulphates and hydrogen peroxide being formed simultaneously in H2 SO4 [22], respectively, in HClO4 [23] solution. This affects the long-term stability of the photocurrent through the formation of surface peroxo species [23]. Recently, this question has been discussed by Seabold and Choi in connection with water splitting experiments on the WO3 photoanode performed in KH2 PO4 (pH 7) electrolyte [24]. Interestingly, they observed that electrodeposition of the oxygen evolution catalyst Co–Pi at WO3 , oriented photoanodic reaction toward formation of O2 instead of that of peroxo species occurring on bare WO3 (without electrocatalyst). However, the presence of a relatively thick Co-Pi over-layer caused finally some decrease of the photocurrent clearly due to a reduced absorption of the incident light by the WO3 . An alternative way to avoid formation at the WO3 surface of the peroxo species is the addition to acidic electrolytes of even small amount of Cl− or Br− ions [23]. Such an effect may be related to the known reactivity of the halides toward peroxo complexes of tungsten(VI) in solution [25]. There have been earlier attempts to produce hydrogen from the sea water in photocatalytic systems (i.e., employing suspensions of powder semiconductors) [26] and, recently, in a PEC

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Fig. 4. TM AFM images and the corresponding cross visible-sectional profiles (bottom) obtained for ca 1.2 ␮m thick WO3 films deposited on FTO using (a) an undoped and (b) a 10 at.% boron-modified tungstic acid/PEG precursor. The samples were annealed at 550 ◦ C. For comparison image collected for uncoated FTO is also presented (c). The roughness factors are given in the top right corners of the AFM images (see the text for the details). The Z scale bar is shown on the right of each image. Image size: 500 nm × 500 nm.

cell using a photoanode consisting of TiO2 nanotubes [27]. In the latter case, a UV–vis illumination source and a large bias voltage of ca 3 V were applied. In contrast, as visible in Fig. 5, the newly developed B(III)-modified WO3 photoanodes offer sea water splitting under sunlight irradiation supplying photocurrents of 2 mA/cm2 already at an imposed potential of 1 V vs. RHE and reaching 2.15 mA/cm2 at 1.23 V. As shown recently [7], the

observed steep rise of the photocurrent from the onset potential at ca 0.5 V (vs. RHE) to attain the plateau within 0.5 V of additional bias is essential in view of the effective use of the WO3 -based photoanodes to split water in a tandem device. In fact, the most recent version of the dye-sensitized solar cell can provide substantial currents starting from an operating voltage of 1 V [28].

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Fig. 5. Photocurrent versus electrode potential plots for 10 at.% B-modified (a) and undoped (b) WO3 photoanodes measured in 0.5 M NaCl (pH 2) under simulated solar AM 1.5 illumination (100 mW cm−2 ).

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to their porous structure, largely permeated by the electrolyte, with the mean size of individual nanoparticles (cf. Fig. 4) smaller than the hole diffusion length in WO3 (0.15 ␮m based on the measurements of Butler [29] performed on a single crystal WO3 photoelectrode). Although this can prevent to a large extent the e− –h+ recombination inside the WO3 particles, further improvement, associated with the presence of boron, is the result of reduced charge recombination at the semiconductor/electrolyte interface. The small size of semiconductor particles that form the film favors diffusion of the minority charge carriers (holes) toward the junction with the electrolyte, however, another consequence is the increase in the number of nanoparticle contacts which may hamper transport of the electrons to the FTO substrate. The particle contacts are produced by high temperature sintering occurring simultaneously with the combustion of carbonaceous residues from the film. The steep rise of the photocurrent vs. potential curves in Fig. 5 is indicative of a good electronic connectivity of the nanoparticles that form the films, which may be attributed to the crystalline nature of the grain boundaries [10]. 4. Conclusions

The reported improvement of the PEC performance of the WO3 photoanode induced by the modification with boron is assigned both to an increase of the surface area exposed to the electrolyte (illustrated by the large roughness factor in Fig. 4b) and to the increased surface hydroxylation of B(III)-WO3 films expected to favor water photo-oxidation reaction. The reduced recombination of the photogenerated electrons and holes (e− –h+ ) is consistent with the large incident photon-tocurrent efficiencies (IPCEs) displayed in Fig. 6 and reaching ca 70% at 400–420 nm, for the 10 at.% B(III)-WO3 photoanode which corresponds to an improvement of about 20% relative to the undoped WO3 film. Increase of the amount of added boron to 20 at.% did not produce any improvement of the IPCEs. These spectral photoresponse measurements were performed using low intensity monochromatic light corresponding to a relatively narrow exit illumination bandwidth of 4 nm. The IPCEs were obtained by dividing the recorded photocurrent (after subtraction of the, usually very weak, dark current) by the photon flux. Measurable photocurrents for both the undoped and B(III)-modified films were observed for wavelengths up to 500 nm, consistent with the band-gap energy of monoclinic, m-WO3 which is about 2.5 eV [11]. The relatively large photocurrent efficiencies observed for both kinds of films are due

The porous nanostructured WO3 films synthesized by aqueous sol–gel method were modified with boron. The presence of B(III) species in the monoclinic WO3 lattice is accompanied by an increased extent of hydroxylation of the films observed despite the high temperature final annealing. An additional effect of the boron incorporation is a modification of the film morphology with a pronounced increase of the roughness factor. The changed film morphology resulting in larger surface area exposed to the aqueous solution and the increased surface hydroxylation are both expected to contribute to the enhanced PEC performance of the B(III)-modified WO3 photoanodes by improving the electrolyte accessibility within the film and affecting directly the rate of water photo-oxidation reaction. Further improvement of the PEC performance of WO3 photoanodes is conceivable through incorporation of plasmonic metal nanostructures to enhance the light capture [30] and by the use of electrocatalysts. Acknowledgements This work was supported by a grant from Switzerland through the Swiss Contribution to the enlarged European Union. XPS measurements were performed at the Department of Physics, Silesia University, Katowice, Poland. We gratefully acknowledge J. Szade, A. Winiarski, M. Kulpa and M. Pilch for the XPS measurements and helpful discussions. We also acknowledge G. Dercz, from Silesia University, for having carried out the XRD spectra. References

Fig. 6. Incident photon-to-current efficiency (IPCE) plots for a 10 at.% B-modified (a) and an undoped (b) ca 1.2 ␮m thick WO3 photoanodes determined in 0.5 M. NaCl (pH 2) at an applied potential of 1.2 V versus RHE.

[1] F.E. Osterloh, Inorganic nanostructures for photoelectrochemical and photocatalytic water splitting, Chemical Society Reviews (2013), http://dx.doi.org/10.1039/c2cs35266d. [2] J. Augustynski, B.D. Alexander, R. Solarska, Metal oxide photoanodes for water splitting, Topics in Current Chemistry 303 (2011) 1. [3] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37. [4] K. Rajeshwar, Hydrogen generation at irradiated oxide semiconductor–solution interfaces, Journal of Applied Electrochemistry 37 (2007) 765. [5] K. Sivula, F. Le Formal, M. Grätzel, Solar water splitting: progress using hematite (␣-Fe2 O3 ) photoelectrodes, ChemSusChem 4 (2011) 432. [6] C. Janáky, K. Rajeshwar, N.R. De Tacconi, W. Chanmanee, M.N. Huda, Tungstenbased oxide semiconductors for solar hydrogen generation, Catalysis Today 199 (2013) 53. [7] J. Brillet, J.-H. Yum, M. Cornuz, T. Hisatomi, R. Solarska, J. Augustynski, M. Grätzel, K. Sivula, Highly efficient water splitting by a dual-absorber tandem cell, Nature Photonics 6 (2012) 824.

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[8] K. Sivula, R. Zboril, F. Le Formal, R. Robert, A. Weidenkaff, J. Tucek, J. Frydrych, M. Grätzel, Photoelectrochemical water splitting with mesoporous hematite prepared by a solution based colloidal approach, Journal of the American Chemical Society 132 (2010) 7436. [9] K.W. Park, J.H. Choi, K.S. Ahn, Y.E. Sung, PtRu alloy and PtRu–WO3 nanocomposite electrodes for methanol electrooxidation fabricated by a sputtering deposition method, Journal of Physical Chemistry B 108 (2004) 5989. [10] C. Santato, M. Odziemkowski, M. Ulmann, J. Augustynski, Crystallographically oriented mesoporous WO3 films: synthesis, characterization, and applications, Journal of the American Chemical Society 123 (2001) 10639. [11] B.D. Alexander, P.J. Kulesza, I. Rutkowska, R. Solarska, J. Augustynski, Metal oxide photoanodes for solar hydrogen production, Journal of Materials Chemistry 18 (2008) 2298. [12] F. Daniel, B. Desbat, J.C. Lassegues, B. Gerand, M. Figlarz, Infrared and Raman study of WO3 tungsten trioxides and WO3 xH2 O tungsten trioxide hydrates, Journal of Solid State Chemistry 67 (1987) 235. [13] S.S. Chan, I.E. Wachs, L.L. Murrell, Relative Raman cross-sections of tungsten oxides: [WO3 , Al2 (WO4 )3 and WO3 /Al2 O3 ], Journal of Catalysis 90 (1984) 150. [14] M. Miyauchi, A. Nakajima, T. Watanabe, K. Hashimoto, Photocatalysis and photoinduced hydrophilicity of various metal oxide thin films, Chemistry of Materials 14 (2002) 2812. [15] R. Azimirad, N. Naseri, O. Akhavan, A.Z. Moshfegh, Hydrophilicity variation of WO3 thin films with annealing temperature, Journal of Physics D: Applied Physics 40 (2007) 1134. [16] C.D. Wagner, A.V. Naumkin, A. Kraut-Vass, J.W. Allison, C.J Powell, J.R. Rumble Jr, NIST Standard Reference Database 20, Version 3.4 (Web Version). [17] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy. A Reference Book of Standard Spectra for Identification and Interpretation of XPS Spectra, Physical Electronics, Inc., Eden Praire, Minnesota, USA, 1995. [18] J.K. Leland, A.J. Bard, Photochemistry of colloidal semiconducting iron oxide polymorphs, Journal of Physical Chemistry 91 (1987) 5076. [19] H.M. Rietveld, Line profiles of neutron powder-diffraction peaks for structure refinement, Acta Crystallographica 22 (1967) 151.

[20] B.V.R. Chowdari, P. Pramoda Kumari, Synthesis and characterization of silver borotellurite glasses, Solid State Ionics 86–88 (1996) 521. [21] F. Amano, D. Ding Li, B. Ohtani, Fabrication and photoelectrochemical property of tungsten(VI) oxide films with a flake-wall structure, Chemical Communications 46 (2010) 2769. [22] J. Desilvestro, M. Graetzel, Photoelectrochemistry of polycrystalline n-WO3 . Electrochemical characterization and photoassisted oxidation processes, Journal of Electroanalytical Chemistry 238 (1987) 129. [23] (a) B.D. Alexander, J. Augustynski, in: L. Vayssieres (Ed.), On Solar Hydrogen and Nanotechnology, Wiley, Singapore, 2009, p. 333; (b) J. Augustynski, R. Solarska, H. Hagemann, C. Santato, Nanostructured thinfilm WO3 photoanodes for solar water and sea-water splitting, Proceedings of the Society of Photo-Optical Instrumentation Engineers 6340 (2006) 63400J. [24] J.A. Seabold, K.S. Choi, Effect of a cobalt-based oxygen evolution catalyst on the stability and the selectivity of photo-oxidation reactions of a WO3 photoanode, Chemistry of Materials 23 (2011) 1105. [25] M.S. Reynolds, K.J. Babinski, M.C. Bouteneff, J.L. Brown, R.E. Campbell, M.A. Cowan, M.R. Durwin, T. Foss, P. O’Brien, H.R. Penn, Kinetics of bromide oxidation by peroxo complexes of molybdenum (VI) and tungsten (VI), Inorganica Chimica Acta 263 (1997) 225. [26] S. Ji, H. Jun, J. Jang, H. Son, P.H. Borse, J. Lee, Photocatalytic hydrogen production from natural seawater, Journal of Photochemistry and Photobiology A 189 (2007) 141. [27] W. Nam, S. Oha, H. Joo, S. Sarp, J. Cho, B.W. Nam, J. Yoon, Preparation of anodized TiO2 photoanode for photoelectrochemical hydrogen production using natural seawater, Solar Energy Materials and Solar Cells 94 (2010) 1809. [28] J.-H. Yum, E. Baranoff, F. Kessler, T. Moehl, S. Ahmad, T. Bessho, A. Marchioro, E. Ghadiri, J.-E. Moser, C. Yi, M.K. Nazeeruddin, M. Grätzel, A cobalt complex redox shuttle for dye-sensitized solar cells with high open-circuit potentials, Nature Communications 3 (2012) 631. [29] M.A. Butler, Photoelectrolysis and physical properties of the semiconducting electrode WO3 , Journal of Applied Physics 48 (1977) 1914. [30] R. Solarska, A. Krolikowska, J. Augustynski, Silver nanoparticle induced photocurrent enhancement at WO3 photoanodes, Angewandte Chemie International Edition 49 (2010) 7980.