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J. Phys. Chem. B 2001, 105, 936-940
Photoelectrochemical Properties of Nanostructured Tungsten Trioxide Films Clara Santato, Martine Ulmann,* and Jan Augustynski Department of Chemistry, UniVersity of GeneVa, CH-1211 GeneVa 4, Switzerland ReceiVed: June 21, 2000; In Final Form: October 12, 2000
The photoelectrochemical characteristics of highly transparent nanoporous WO3 films are described. The photocurrent versus excitation wavelength spectra of these photoelectrodes exhibit a maximum close to 400 nm and a significant photoresponse to the blue part of the visible spectrum. The observed conversion efficiencies attain 75% for the photogeneration of oxygen from 1 M aq HClO4 and reach 190% in the presence of methanol in the solution, denoting in the latter case the occurrence of a perfect photocurrent doubling. Experiments conducted under simulated solar AM 1.5 illumination resulted in steady-state anodic photocurrents of the order of several mA/cm2.
Introduction Tungsten trioxide is one of few n-type semiconductors resistant against photocorrosion in aqueous solutions. However, in contrast with widely investigated titanium dioxide, known to be stable toward photooxidation over a large range of pH (except for solutions of some acids), WO3 can be used as a photoanode in acidic (below ca. pH 4), including strongly acidic solutions. Although the photoelectrochemical characteristics of bulk polycrystalline WO3 films1 and those of a single crystal2 have been described by a number of authors, there have been only few studies regarding colloidal (nanostructured) WO3 films. Bedja et al.3 investigated relatively thick (ca. 10 µm) transparent films formed from quantum size WO3 nanoparticles. These films exhibited poor photocurrents for the photooxidation of I- ions with the incident-photon-to-current conversion efficiency (IPCE) not exceeding 0.42%, which have been assigned to an efficient electron trapping within the network of interconnected colloidal particles. To explain the observed decrease of the photocurrent as a function of time, the latter authors3 evoked also poor rectification properties of those semiconductor films. Recently, Shiyanovskaya and Hepel4 described the photoelectrochemical behavior of bicomponent WO3/TiO2 films consisting of a nanoporous amorphous WO3 matrix and of an overlayer of TiO2 nanocrystals. The authors point at the improved photoresponse of the composite films assigned to a more efficient charge separation than in single component (either TiO2 or WO3) films. Here we present photoelectrochemical data obtained using highly transparent WO3 films, consisting of a network of preferentially oriented nanocrystals, that demonstrate the ability of this semiconductor to operate as both efficient and stable solar-driven photoanode for oxidation of water and various organic compounds.
50 WX2, 100-200 mesh).5 A viscous solution, containing 0.4 mol dm-3 of tungstic acid and 25% w/w of poly(ethylene glycol), was applied to the substrate and then annealed in flowing oxygen at 550 °C for 30 min.6 The thickness of individual layers (as determined with a Tencor Alpha Step 200 profilometer) was close to 0.4 µm. The WO3 films used for most of the experiments described in this paper were ca. 2 µm thick and were formed by six consecutive applications of the solution, each followed by heat treatment. The photoelectrochemical measurements were carried out in a two-compartment Teflon cell equipped with a quartz window, by illuminating the WO3 electrode (except where otherwise stated) from the side of the film/solution interface. A platinum counter-electrode (large area Pt grid) was separated from the WO3 film electrode by a Nafion membrane. The potential of the WO3 electrode was monitored versus a mercurous sulfate-mercury reference electrode and is quoted versus reversible hydrogen electrode (RHE) in the same solution. The wavelength photoresponse (i.e., incident-photon-to-current conversion efficiency vs excitation wavelength) of the WO3 electrodes was determined using a 500 W xenon lamp (Ushio UXL-502HSO) set in an Oriel model 66021 housing and 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 from Optronic Lab. In various series of experiments, the WO3 electrodes were illuminated with the simulated solar AM 1.5 light obtained using a 150-W xenon lamp equipped with a Schott 113 filter and neutral density filters. All solutions were made from analytical grade reagents and twice distilled water. The photoelectrochemical measurements were carried out at ca. 25 °C under potential-controlled conditions. Results and Discussion
Experimental Procedures The nanoparticulate WO3 films used in this study were obtained by depositing on conducting glass substrates, layer by layer, a colloidal solution of tungstic acid. Conducting glass plates (Libbey Owens Ford, 12 Ω/square) comprised a 0.5-µm thick overlayer of F-doped SnO2. The precursor (tungstic acid) was obtained by passing an aqueous solution of sodium tungstate through a column filled with a proton exchange resin (Dowex
The above-described deposition method produces transparent (pale yellow), anisotropic WO3 films. Figure 1a shows a SEM image of a ca. 2-µm-thick film consisting of a network of platelike WO3 particles with diameters ranging from 20 to 50 nm. The diffraction pattern of such a film, obtained using λ ) 1.294°A synchrotron radiation, shown in Figure 1b, is consistent with strong preferential orientation of (200), (020), and (002) faces of WO3 crystallites parallel to the substrate. As confirmed
10.1021/jp002232q CCC: $20.00 © 2001 American Chemical Society Published on Web 01/11/2001
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J. Phys. Chem. B, Vol. 105, No. 5, 2001 937
a
b Figure 2. Photocurrent action spectrum for a 2-µm-thick WO3 film, recorded in 1 M aq HClO4 at 1 V vs RHE.
Figure 1. (a) Scanning electron micrograph, showing surface view of a ca. 2-µm-thick nanostructured WO3 film after sintering at 550 °C, obtained using a Hitachi S-900 microscope. Magnification 230000×. (b) Diffraction pattern of the same film (λ ) 1.294 Å, synchrotron radiation, θ/2θ scan). Sharp diffraction peaks correspond, from left to right, to 002, 020, and 200 faces of WO3 crystallites.
by the experiments employing decreasing glancing angles of the λ ) 1.294°A synchrotron radiation, this preferential orientation of WO3 nanocrystals is preserved across the whole film. The pattern of WO3 film detached from the substrate, measured in transmission mode, could be identified as the monoclinic phase.7 Figure 2 displays photocurrent action spectrum for a ca. 2-µmthick WO3 film illuminated in 1 M aq. HClO4 solution and subjected to an anodic potential of 1 V vs NHE. The photoresponse extends into the blue part of the spectrum, up to 500 nm, corresponding to a band-gap energy of ca. 2.5 eV. The latter value is identical with that reported for a bulk WO3 film formed by oxidation of the metal at 700-750 °C and consisting of a triclinic phase.1i The maximum photocurrent efficiency of ca. 75% for the photogeneration of oxygen from 1 M HClO4 solution observed in Figure 2 is much higher than the corresponding values reported until now for nanocrystalline films of other oxide semiconductors. It is to be recalled, in this connection, that nanoparticulate TiO2 (anatase) films, which promote quite efficient photooxidation of a large variety of organic molecules, exhibit in contrast poor IPCE for the splitting
of water from both acidic and alkaline solutions.8 The latter behavior has been assigned8b to the easy occurrence of the back charge-transfer reactions involving intermediates of the photooxidation of water and molecular oxygen itself. In fact, all these species can already be reduced at potentials positive with respect to the flat-band potential of TiO2.9 In the case of an anodically biased nanoparticulate TiO2 electrode, unable to develop a regular depletion layer, the conduction band electrons may therefore not only recombine with photogenerated holes but may also react with the intermediates of the photooxidation of H2O. On the other hand, in the case of WO3, having markedly (ca 0.5V) more positive flat-band potential than anatase TiO2, the reduction of oxygen by conduction band electrons is apparently a slow process,10 too slow to affect the actual photocurrent efficiency to any significant extent. It is apparently also the slow down of the back charge-transfer reactions that is the main reason for the improved efficiency of photooxidation of water observed for composite TiO2-SnO2 nanocrystalline films.11 In the case of films composed of ca. 30 nm in diameter TiO2 and smaller (d ∼ 5 nm) SnO2 particles in the w/w ratio of 1:1, the photon-to-current efficiency determined in a 0.02 M NaOH solution reached a maximum of 25% at 325 nm. The improved charge separation in the composite films was explained by the transfer of photogenerated electrons to the SnO2 nanoparticles having lower lying (by ca. 0.5 eV) conduction band edge.11 Using a recently developed version of nanocrystalline Fe2O3 electrodes, Khan and Akikusa12 reported a ca. 20% IPCE (at 400 nm) for the photogeneration of oxygen from a 1 M NaOH solution. It is to be noted, however, that the latter result has been achieved at a high anodic potential, 1.67 V vs RHE (reversible hydrogen electrode in the same solution), i.e., 0.44 V above the equilibrium potential for oxygen evolution in the dark. The photocurrents observed at 1.23 V vs RHE were already three times smaller. Consequently, despite the fact that their absorption spectrum extends to ca. 600 nm, the photoresponse of the Fe2O3 film electrodes toward the solar light remains for the moment lower than that of the WO3 counterparts.
938 J. Phys. Chem. B, Vol. 105, No. 5, 2001
Figure 3. Photocurrent efficiency (IPCE) vs wavelength plot for a 2 µm-thick WO3 film, determined at 1 V vs RHE in 1 M aq. HClO4/ 0.01M CH3OH and/or in 1 M aq HClO4/0.1 M CH3OH (both plots were practically identical).
As shown in Figure 3, addition of methanol to a HClO4 solution caused a large increase of the photoresponse of the WO3 electrode. In particular, the maximum in the action spectrum, arising again at ca. 400 nm, amounts to almost 190% corresponding to the occurrence of an almost perfect photocurrent doubling.13 This indicates that the photooxidation of methanol is the dominant process with respect to the photogeneration of oxygen and that it is also able to compete efficiently with the surface hole-electron recombination, even under moderate anodic bias. Consistent with the high IPCEs exhibited by the nanostructured WO3 electrodes in the blue part of the visible spectrum are photocurrent-voltage curves, recorded under simulated solar AM 1.5 illumination, shown in Figure 4. Closely similar iph-E curves were obtained when illuminating the WO3 films from the backside (i.e., through the conducting glass substrate). The observed onset potential of the photocurrent, at 0.45 V in 1 M HClO4 and at 0.40 V in the solution containing methanol, was quite close to the flat band potential determined by Nenadovic et al.10 in the case of a colloidal suspension containing WO3 particles of a similar size. Under simulated solar illumination, the saturation photocurrent associated with the photogeneration of oxygen reaches 2.4 mA/cm2 and twice as much for the photooxidation of methanol. The steep rise of the photocurrent as well as the negative shift of the onset potential observed in the latter case are typical of a number of small organic molecules including, besides aliphatic alcohols, formic acid and formaldehyde. Comparison of the iph-E curves in Figure 4 shows that, under low positive bias, the multiplication factor of the photocurrent by far exceeds 2 expected from the simple occurrence of the photocurrent doubling. The photooxidation of methanol results first in the formation of ‚ CH2OH radical with large negative redox potential, E° (‚CH2OH/CH2O) ) -0.97 V,14 able to transfer an electron to the conduction band of WO3. Because of the strongly (ca. 1.4 eV) downhill character of the latter reaction, the possibility of the
Santato et al.
Figure 4. Photocurrent-potential plots for a 2-µm-thick WO3 electrode illuminated with simulated AM 1.5 solar light. Curve A recorded in 1 M aq. HClO4, curve B obtained after addition of 0.1 mol dm-3 of methanol.
back electron transfer (i.e., the re-reduction of the ‚CH2OH species) is negligible. The situation is essentially the same for other two reactants, CH2O and HCOOH, involved in the total 6 e- oxidation of CH3OH to CO2. The observed ability of methanol to compete efficiently with water molecules for positive holes raises the question of the mechanism (direct or indirect) of charge transfer at illuminated WO3 electrodes. Le´austic et al.15 obtained clear evidence for the formation of ‚OH radicals at illuminated suspensions of different WO3 powders in water by means of spin-trapping ESR experiments. In fact, the involvement of ‚OH radicals in the photooxidation of water at nanocrystalline WO3 electrodes is consistent with the presence in HClO4 solution, after a prolonged photoelectrolysis conducted under simulated solar AM 1.5 illumination, of a significant amount of H2O2 corresponding to ca. 5% faradaic yield.16 The situation appears more complex for the solutions containing alcohols. The ESR measurements performed upon irradiation of WO3 powders suspended in a 10% v/v ethanol solution in water showed, in fact, the presence of hydroxyalkyl radicals.15 Depending on the nature of the WO3 powder (i.e., its surface area and degree of crystallinity) used for these experiments, the ESR signal due to ‚CH3CHOH radicals was accompanied or not by the signal assigned to ‚OH radicals. In the experiments carried out with the best crystallized (and, at the same time, the most photoreactive) samples, the only species detected in the ethanol solution were hydroxyalkyl radicals.15b The above results suggest that, at least for relatively concentrated solutions, the photooxidation of alcohols at WO3 may actually occur via direct hole transfer
CH3OH (ads) + h+ f ‚CH2OH (ads) + H+ (aq)
(1)
followed by the electron injection to the conduction band of
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J. Phys. Chem. B, Vol. 105, No. 5, 2001 939
Figure 6. CO2 production (expressed as faradaic yield) during photoelectrolyses of 0.01 M solutions of formic acid and methanol in 1 M Na2SO4 at a 0.8 cm2 WO3 electrode, plotted against the amount of electric charge. The photoelectrolyses were conducted for 50, respectively 100 h with photocurrents ranging from ca. 2 mA cm-2 at the beginning to less than 0.5 mA cm-2 at the end of the photoelectrolysis run. Simulated solar AM 1.5 illumination; E ) 1 V vs RHE; solution volume 30 cm3. Formation of CO2 was followed by GC (using Hewlett-Packard 5890 series II gas chromatograph equipped with a TCD detector and a molecular sieves 5 Å column).
Figure 5. Saturation photocurrents for a 2-µm-thick WO3 electrode, measured in a 1 M aq. HClO4/0.1 M CH3OH solution, plotted against incident light intensity expressed as a multiple of AM 1.5 solar light (i.e., 1 stands for 1 sun AM 1.5).
WO3 by the adsorbed ‚CH2OH radical
‚CH2OH (ads) f CH2O + H+ (aq) + e-cb
(2)
The direct transfer of mobile WO3 holes to adsorbed methanol, expected to compete very efficiently with hole-electron recombination, offers in fact a plausible explanation for the steep rise of the iph-E curve (curve B) in Figure 4 and for the strong dependence of the photocurrent on methanol concentration (below 0.1 M). The observed drastic decrease of the photocurrent when lowering the CH3OH concentration from 0.1 to 10-2 M may reflect the rapidly increasing contribution of the indirect pathway of methanol oxidation (mediated by the precursors of H2O2, i.e., surface bound or free ‚OH radicals)
Despite the expected predominance of an indirect photooxidation pathway in diluted solutions of organic compounds under simulated solar light illumination, the nanocrystalline WO3 electrodes maintain excellent selectivity toward oxidation of the latter species down to very low concentrations. In various longterm photoelectrolysis experiments at the WO3 electrodes, we monitored the mineralization of different organic substances by analyzing the amount of formed CO2 as a function of the electric charge passed through the electrode. Such kind of data, collected for the photodegradation of 0.01 M solutions of formic acid and methanol, are represented in Figure 6 in terms of faradaic yield of CO2. High current efficiencies were maintained for a major part of photoelectrolysis runs, at the end of which the total organic carbon (TOC) amount decreased below 3 ppm. Conclusions Despite their absorption spectrum limited to the blue portion of the solar light, the nanostructured WO3 photoelectrodes are shown to be able to deliver significant photocurrents under AM 1.5 illumination, especially in the case of oxidation of various organic molecules. Prolonged photoelectrolysis experiments also confirmed their ability to act as both efficient and stable photoanodes for the degradation of a variety of organic effluents. The 0.8-1 V bias required for the operation of such a photoelectrolysis cell can eventually be provided by a suitable photovoltaic cell.
H2O + h+ f ‚OH + H+ (aq)
(3)
CH3OH + ‚OH f ‚CH2OH + H2O
(4)
‚CH2OH + ‚OH f CH2O + H2O
(5)
Acknowledgment. This work was supported by the Swiss Federal Office of Energy and the Swiss National Science Foundation.
‚CH2OH + h+ f CH2O + H+ (aq)
(6)
References and Notes
followed by
or
In fact, such a pathway involving CH3OH diffusing to the WO3 electrode, instead of the adsorbed species, is expected to decrease markedly the extent of the photocurrent doubling (reaction 2). The competition between the direct hole transfer and the ‚OH mediated photooxidation pathway is also perceptible in the case of a 0.1 M CH3OH solution from the relationship between the maximum photocurrent and the light intensity that progressively deviates from linearity (cf. Figure 5).
(1) (a) Hodes, G.; Cahen, D.; Manassen, J. Nature 1976, 26, 312. (b) Hardee, K. L.; Bard, A. J. J. Electrochem. Soc. 1977, 124, 215. (c) Gissler, W.; Memming, R. J. Electrochem. Soc. 1977, 124, 1710. (d) Derrington, C. E.; Godek, W. S.; Castro, C. A.; Wold, A. Inorg. Chem. 1978, 17, 977. (e) Gerrard, W. A. J. Electroanal. Chem. 1978, 86, 421. (f) Reichman, B.; Bard, A. J. J. Electrochem. Soc. 1979, 126, 2133. (g) Di Quarto, F.; Russo, G.; Sunseri, C.; Di Paola, A. J. Chem. Soc., Faraday Trans. 1 1982, 78, 3433. (h) Ulmann, M.; Augustynski, J. In Photoelectrochemistry: Fundamental Processes and Measurement Techniques edited by W. L. Wallace, A. J. Nozik, S. K. Deb, and R. H. Wilson (The Electrochemical Society, Pennington, NJ, 1982, pp 663-672). (i) Spichiger-Ulmann, M.; Augustynski, J. J. Appl. Phys. 1983, 54, 6061.
940 J. Phys. Chem. B, Vol. 105, No. 5, 2001 (2) (a) Butler, M. A.; Nasby, R. D.; Quinn, R. K. Solid State Commun. 1976, 19, 1011. (b) Butler, M. A. J. Appl. Phys. 1977, 48, 1914. (3) Bedja, I.; Hotchandani, S.; Carpantier, R.; Vinodgopal, K.; Kamat, P. V. Thin Solid Films 1994, 247, 195. (4) (a) Shiyanovskaya, I.; Hepel, M. J. Electrochem. Soc. 1998, 145, 3981. (b) Shiyanovskaya, I.; Hepel, M. J. Electrochem. Soc. 1999, 146, 243. (5) (a) Richardson, E. J. Inorg. Nucl. Chem. 1959, 12, 79. (b) Chemseddine, A.; Babonneau, F.; Livage, J. J. Non-Cryst. Solids 1987, 91, 271. (6) Augustynski, J.; Ulmann, M.; Santato, C. Patent pending WO9967181. (7) Loopstra, B. O.; Rietveld, H. M. Acta Crystallogr. 1969, B25, 1420. (8) (a) Wahl, A.; Ulmann, M.; Carroy, A.; Augustynski, J. J. Chem. Soc., Chem. Commun. 1994, 2277. (b) Wahl, A.; Ulmann, M.; Carroy, A.; Jermann, B.; Dolata, M.; Kedzierzawski, P.; Chatelain, C.; Monnier, A.; Augustynski, J. J. Electroanal. Chem. 1995, 396, 41. (9) Ulmann, M.; De Tacconi, N. R.; Augustynski, J. J. Phys. Chem. 1986, 90, 6523. (10) The effect of oxygen upon concentration of excess free electrons in WO3 colloids was followed by Nenadovic M. T., Rajh T., Micic O. I., Nozik A. J. (J. Phys. Chem. 1984, 88, 5827) through their absorption
Santato et al. spectrum at λ > 700 nm. Although the observed initial rate of oxygen reduction was significant in the presence of large concentrations of electrons injected into WO3, it became quite low for smaller e- concentrations. (11) Vinodgopal, K.; Bedja, I.; Kamat, P. V. Chem. Mater. 1996, 8, 2180. (12) Khan, S. U. M.; Akikusa, J. J. Phys. Chem. B 1999, 103, 7184. (13) (a) Morrison, S. R.; Freund, T. J. Chem. Phys. 1967, 47, 1543. (b) Freund, T.; Gomes, W. P. Catal. ReV. 1969, 3, 1. (14) Breitenkamp, M.; Henglein, A.; Lilie, J. Ber. Bunsen-Ges. Phys. Chem. 1976, 80, 973. (15) (a) Le´austic, A.; Babonneau, F.; Livage, J. J. Phys. Chem. 1986, 90, 4193. (b) Le´austic, A.; Babonneau, F.; Chemseddine, A.; Livage, J. New. J. Chem. 1989, 13, 111. (16) It is however to be noted, in this connection, that in the case of measurements performed by Le´austic et al. the photooxidation reactions were conducted at a negatively charged surface of the WO3 photocatalyst dispersed in (and equilibrated with) pure water. In contrast, the pH of the solutions used in our measurements involving the nanocrystalline films was close to the point of zero charge of WO3, pH ) 0.4; cf. El Wakkad, R. K. S.; Risk, H. A. J. Phys. Chem. 1957, 61, 494 and ref 15b.
