Vertically Aligned WO3 Nanowire Arrays Grown Directly on

Nov 29, 2010 - Grimes , C. A. ; Varghese , O. K. ; Ranjan , S. Light, Water, Hydrogen: The Solar Generation of Hydrogen by Water Photoelectrolysis; Sp...
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Vertically Aligned WO3 Nanowire Arrays Grown Directly on Transparent Conducting Oxide Coated Glass: Synthesis and Photoelectrochemical Properties Jinzhan Su,†,‡ Xinjian Feng,‡ Jennifer D. Sloppy,‡ Liejin Guo,† and Craig A. Grimes*,‡,§ †

State Key Laboratory of Multiphase Flow in Power Engineering, School of Energy and Power Engineering, Xi’an Jiaotong University, Shaanxi 710049, People’s Republic of China, and ‡ Department of Electrical Engineering, The Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ABSTRACT Photocorrosion stable WO3 nanowire arrays are synthesized by a solvothermal technique on fluorine-doped tin oxide coated glass. WO3 morphologies of hexagonal and monoclinic structure, ranging from nanowire to nanoflake arrays, are tailored by adjusting solution composition with growth along the (001) direction. Photoelectrochemical measurements of illustrative films show incident photon-to-current conversion efficiencies higher than 60% at 400 nm with a photocurrent of 1.43 mA/cm2 under AM 1.5G illumination. Our solvothermal film growth technique offers an exciting opportunity for growth of one-dimensional metal oxide nanostructures with practical application in photoelectrochemical energy conversion. KEYWORDS WO3, nanowire, tungsten trioxide, photoelectrochemical.

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properties.20 1-D semiconductor nanoarchitectures have been synthesized by a number of chemical and physical techniques, including vapor-liquid-solid,21 dielectrophoresis,22 Langmuir-Blodgett (LB),23,24 anodized aluminum oxide template (AAO),25 hydrothermal,26 lithographically patterned nanowire electrodeposition (LPNE),27 molecular beam epitaxy,28 etc. WO3 is recognized as one of the few n-type semiconductors resistant to photocorrosion in aqueous solutions, and significant incident photon-to-current conversion efficiencies (IPCEs) for oxidation of water have been reported for WO3 films.29 1-D-structured WO3 may prove a promising material with which to achieve efficient water photoelectrolysis. 1-D WO3 nanostructures have been synthesized by chemical vapor deposition,30 thermal vapor deposition,31 heating metal tungsten filaments/wires in vacuum or Ar atmosphere,32-35 and anodization of W foil.36 Hydrothermal/solvothermal techniques have been used to synthesize WO3 nanorods, nanowires, and nanobelts;37-39 however these structures are randomly oriented rather than vertically aligned from the substrate. There is a recent report on growth of WO3 nanoflake arrays synthesized by a solvothermal technique in ethanol.40 In this work, we report a facile way to deposit ordered nanowire, as well as nanoflake, WO3 arrays upon FTO coated glass. A WO3 seed layer is used to initiate growth, with the geometries tailored by adjusting the hydrothermal precursor composition; by adjustment of the amount of water and oxalic acid in the precursor, nanowire arrays can be selectively deposited. Film Synthesis. Before solvothermal growth, a 200 nm thick seed layer was deposited on a FTO coated glass substrate by spin coating a solution, made by dissolving

ydrogen production by water photoelectrolysis has been of considerable interest since Fujishima and Honda’s report of water splitting on a TiO2 surface under UV illumination in 1972.1 Since then there have been numerous reports on efforts to achieve a stable water photoelectrolysis system using materials responsive to solar spectrum energy.2-4 For example, significant efforts have focused on finding new materials with band edge alignments suitable for driving the necessary photoelectrochemical reactions,3 including semiconductor doping to achieve a lower band gap more suitable for visible light utilization and/ or superior electrical properties,5,6 formation of hybrid heterojunction structures,7 multiple band gap structures8 and p/n junctions,9 engineering of crystalline structures10 and modification of semiconductor surfaces by chemical and/or physical processes.11 It is now widely recognized that nanostructured semiconductors, in comparison to bulk materials, offer potential advantages in photoelectrochemical cell (PEC) application due to their large surface area and sizedependent properties, such as increased photon absorption, enhanced charge separation and migration, and surface reactions.12-15 One dimensional (1-D) semiconductor structures are currently of great interest,16-19 as they can offer photogenerated charges direct electrical pathways, with reduced grain boundaries, resulting in superior charge transport * To whom correspondence should be addressed, [email protected]. § Current address: Photonic Fuels, Innovation Park, State College, PA. 16803. Received for review: 09/30/2010 Published on Web: 11/29/2010

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1.25 g of H2WO4 and 0.5 g of poly(vinyl alcohol) (PVA) in 10 mL of 50 wt % H2O2, followed by 500 °C anneal for 2 h in air. A H2WO4 solution for solvothermal use was prepared by dissolving 1.25 g of H2WO4 into 30 mL of H2O by adding 10 mL of 50 wt % H2O2 while heating at 95 °C on a hot plate with stirring. The resulting clear solution was diluted using deionized water to 100 mL with a molar concentration of 0.05 M. Nanowire array growth was achieved using a 3 mL portion of H2WO4 (0.05 M) solution, with 0.5 mL of HCl (6 M) and 2.5 mL of deionized water added to 10 mL of acetonitrile. This solution was placed within a 23 mL Teflonlined stainless steel autoclave, holding a vertically oriented FTO-glass substrate (with a WO3 seed layer), which was then sealed and maintained at 180 °C for 6 h. The substrate was then rinsed with deionized water and dried in a nitrogen stream. We note that using the same general synthesis technique two distinct types of nanoflake array films were synthesized by modification of the nanowire array solvothermal conditions. For the first type, 3 mL of H2WO4 (0.05 M) solution, 0.02 g of oxalic acid, 0.02 g of urea, and 0.5 mL of HCl (6 M) were added into 12.5 mL of acetonitrile, and the reaction was kept at 180 °C for 2 h. For the second type, 3 mL of H2WO4 (0.25 M) solution, 0.2 g of oxalic acid, 0.5 mL of HCl (6 M), and 2.5 mL of deionized water were added into 10 mL of acetonitrile, and the reaction was kept at 180 °C for 2 h. The resulting films, of both types, were annealed in air at 500 °C for 1 h. Characterization. Film morphology was investigated by use of a field emission scanning electron microscope (FESEM, JEOL JSM 4700F) operated at 3 kV. Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns were obtained using a JEOL 2010 with a LaB6 emitter operated at 200 kV. X-ray diffraction (XRD) patterns were taken using a Scintag X2 diffractometer (Cu KR radiation). UV-vis absorption spectra measurements were performed using a Perkin-Elmer Lambda 950 UV-vis-NIR spectrophotometer with integrating sphere. Linear sweep voltammetry was obtained at a scan rate of 50 mV/s using a potentiostat (CH Instruments, model CHI 600C). A Spectra Physics simulator with an illumination intensity of 1 sun (AM 1.5, 100 mW/cm2) with a filter to remove light of wavelength below 400 nm was used as the light source; a PHIR CE power meter was used to calibrate input power. IPCE values were determined using a system comprising a monochromator (Cornerstone 130), a 300 W xenon arc lamp, a calibrated silicon photodetector, and a power meter. Intensity modulated photocurrent spectrum (IMPS) data were obtained using a custom built system: a UV emitting diode (NICHIA NCSU033A, λ ) 365 nm) was used as a light source whose dc illumination was adjusted to 2.53 mW/cm2. Light intensity modulation was conducted by current modulation with a depth of 5%. A lock-in amplifier (Stanford Research Systems SR 830) was used to record the photocurrent response as a function of frequency. © 2011 American Chemical Society

FIGURE 1. FESEM images of unannealed WO3: (a) nanowire, (b) NF1, and (c) NF2 arrays. Insets show film cross section.

Results and Discussion. Figure 1 presents FESEM images of an illustrative as-prepared WO3 nanowire array film, and the two types of nanoflake arrays; there was no discernible change in film morphology after annealing. Both the nanowire and nanoflake films grow perpendicular to the substrate. Nanowire length varies from 500 to 1500 nm, tapering in width from base (100 nm) to tip (30 nm). The thickness of the first type of nanoflake, NF1, is 20-30 nm, with a height of 1-2 µm. The second type of flake, NF2, has a 20-30 nm thickness and height of 5-6 µm. Figure 2 is a digital photograph of the different as-prepared and annealed films. Figure 3 shows the XRD patterns of the three film morphologies as-synthesized, and after a 500 °C 1 h anneal in air. The unannealed and annealed wires both exhibit hexagonal structure with, respectively, an oriented plane of 204

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FIGURE 2. Digital photograph of WO3 films as-prepared and after anneal.

FIGURE 3. XRD patterns of unannealed and 500 °C 1 h air-annealed samples.

(002) (PDF 97-008-0634; a ) 7.324 Å, c ) 7.663 Å) and (001) (PDF 00-033-1387; a ) 7.298 Å, c ) 3.899 Å). Different from the wires. The unannealed and annealed nanoflake arrays of the first-type were monoclinic (PDF 97-001-7003; a ) 7.3 Å, b ) 7.53 Å, c ) 7.68 Å, β ) 90.9°). For nanoflake arrays of the second type, the unannealed and annealed samples show, respectively, monoclinic structure referred to (PDF 00005-0393) and (PDF 97-001-7003). Peak broadening is pronounced for all samples. No hydrated tungsten oxide was found, presumably due to our use of the aprotic solvent acetonitrile. Figure 4 presents the TEM images and SAED patterns of annealed nanowire and nanoflakes. The clear SAED patterns reveal that the nanowire and nanoflakes are crystalline. The growth direction of hexagonal nanowires was indexed along [001], which gave the strongest peak intensity in the XRD pattern. The monoclinic nanoflakes were found to grow along [020] and [200] (zone axis ) [002]). The peak intensity of [002] for NF2 films was significantly enhanced after © 2011 American Chemical Society

FIGURE 4. TEM images of 500 °C 1 h annealed samples of (a) nanowire, (b) NF1, and (c) NF2. Inset is the selected area electron diffraction (SAED) pattern for each sample.

annealing, a behavior attributed to recrystallization of the interface between adjacent flakes; see Figure 1c. Figure 5 shows the UV-vis absorption spectra of the three sample types, annealed and unannealed. The band gap, EG, was determined using the equation41

αhν ) A(hν - EG)n

where h is Planck’s constant, ν is the frequency of light, A is a constant, and n is equal to 2 for an allowed indirect transition or 1/2 for an allowed direct transition. For WO3 205

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temperature to 120 °C resulted in a sparse sea urchin-like growth upon the seed layer. When the temperature was elevated to 160 °C, a particle film was grown. At 170 and 180 °C nanoflake array films were grown. Elevation of the temperature to 200 °C and above resulted in a dense mat of flakes seemingly comprised of particles. From the baseline nanowire growing conditions, nanowire arrays of the same morphology were grown with 0, 0.02, or 0.04 g of oxalic acid added. When the oxalic acid content was increased to 0.1 g, a mixture of nanowires as well as nanoflakes were grown. With 0.2 g of oxalic acid added to the solution, NF2 films were grown. The nanowire structure disappeared when the amount of urea was higher than 0.02 g. For the same growth condition as the NF1 films, when no oxalic acid was added to the precursor solution, the result was a compact layer, and when no urea was added, the result was a film comprised of particles mixed with sea urchin-like wires. Little variation in NF1 morphology was found when the amount of oxalic acid was varied from 0.01 to 0.08 g (0.02 g of urea added). XRD analyses showed that the hexagonal nanowires grow along [001] and monoclinic nanoflakes along [020]; similar results were reported for 1D WO3 nanostructures.37-39 The nanocrystal shapes are determined by the surface energies associated with facets of the crystal. One can control the final shape of a crystal by introducing appropriate surfactants/ capping reagents to change the free energies of the various crystallographic surfaces, thus altering their growth rates.44 Sulfate ions have been employed as capping agents to grow WO3 nanowire/nanorods in aqueous solution by hydrothermal deposition.37 In our experiments, Cl- appears to be the growth-directing ion as nanowire arrays were grown only with addition of HCl to the water and acetonitrile solution, while oxalic acid plays a key role in formation of the nanoflake films. A change from wire to ribbon morphology was observed by Gu45 with increasing K2SO4 in the hydrothermal reaction, which was explained as oriented aggregation of the nanowires induced by high sulfate concentrations. It was reported that with addition of oxalic acid, the hydrothermal products can change from irregularly aggregated WO3 nanorods to WO3 nanowire bundles.38 Figure 6 shows nanoflakes synthesized with addition of 0.1 g of oxalic acid; it is clearly observable that the flakes are assembled with nanowires. Evolution of WO3 from nanowires to nanosheets by thermal annealing was reported by Ko,46 who proposed that formation and recrystallization of an amorphous interface layer between two neighboring nanowires changes the nanowires to nanosheets. Urea was found essential for growing NF1 films. Urea can act as both a hydrogen-bond donor through its two NH protons or a hydrogen-bond acceptor through the CdO group47 and was used as a directing agent in an ethanol/WCl6 system for the synthesis of inorganic tungsten oxide nanotubes.48 Without addition of urea, more than 0.1 g of oxalic acid was needed to grow NF2 films, while with addition of urea (0.02 g), 0.01 g of

FIGURE 5. UV-vis absorption of unannealed and 500 °C 1 h airannealed samples of different film types.

the transition is indirect, and therefore (Rhν)1/2 is plotted as a function of hν from which the band gap energy is obtained. We find a band gap value for the unannealed nanowire samples of 3.14 eV, and 2.92 eV when annealed. For NF1 films we find a band gap value of 2.82 eV for unannealed and 2.61 eV for annealed. For NF2 films we find 2.54 eV for the unannealed samples and 2.51 eV when annealed. de Wijs and de Groot reported that for WO3 a larger band gap is obtained with inferior crystallization,42 hence the 0.2 eV band gap decrease with annealing for the nanowire and NF1 samples. Further, the electronic band gap increases with distortion of the octahedra that are building blocks of the various crystal structures;43 hence the monoclinic WO3 nanoflakes give a lower band gap than the hexagonal WO3 nanowires. The hydrothermal precursor composition plays a dominant role in controlling growth of the tungsten trioxide nanostructures. Nanowire or nanoflake arrays are selectively deposited by adjusting the amount of water added to the precursor. The total amount of water in the precursor included both the water added plus the 3.43 g of water in the 3 mL H2WO4 and 0.5 mL HCl (6 M) solutions. When the amount of water added to the precursor was varied, the amount of acetonitrile was adjusted to keep precursor volume at 16 mL. When more than 1 mL of H2O was added to the precursor solution, nanowire array films were grown. When no water was added to the precursor NF1 films were grown. Acidic conditions were necessary to grow the nanostructured WO3 films. In the growth of NF1 films, adding 0.1 g of NaCl instead of 0.5 mL of HCl (6 M) to the solution resulted in growth of a compact WO3 layer. To confirm that it is not Na+ that prevents growth of the nanostructured film, rather the acidic conditions, we added 0.05 g of NaCl and 0.144 mL of HCl (6 M) (keeping Cl- concentration constant) to the precursor and obtained nanoflake films. Nanostructured growth was achieved only within a narrow temperature window. For NF1 films, reducing the © 2011 American Chemical Society

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FIGURE 8. IPCE of three samples. The photocurrents were taken using a CHI600C potentiostat with a bias of 0.5 V in a two electrode setup with Pt foil as counter electrode. FIGURE 6. FESEM image of WO3 flakes synthesized with addition of 0.1 g of oxalic acid, indicating that the flakes are comprised of nanowires.

FIGURE 9. Complex plane plot of the IMPS response at a base light intensity of 2.53 mW/cm2, incident photon flux 0.465 × 1016 cm2s-1, using an UV LED (λ ) 365 nm). FIGURE 7. Current-potential plots for annealed nanowire, and two flake samples, under chopped visible light in an aqueous solution of 0.1 mol/L sodium sulfate (Na2SO4).

to-current-conversion efficiency (IPCE) measurements as a means of studying the photoactive wavelength regime for the nanostructured WO3 films (Figure 8). IPCE can be expressed as49

oxalic acid was enough to grow NF1 films. Urea together with oxalic acid promotes the translation from nanowires to nanosheets. Photoelectrochemical Properties. Photocurrent measurements of the nanostructured WO3 films were conducted in a 0.1 M Na2SO4 solution using a two electrode setup with aPtcounterelectrode.Figure7showschoppedcurrent-potential (I-V) curves of the three film morphologies. NF2 films give the highest saturation photocurrent value of 1.43 mA/cm2. As an indirect band gap semiconductor, WO3 has a relatively low absorption coefficient. The NF1 films have a thickness comparable to that of the nanowire array films but give about 3 times higher photocurrent, a behavior attributable to the lower band gap, and light scattering in the flake array structure (see Figure 2). The unannealed samples show very low, less than 1 µA/cm2, photocurrent values due to the poor crystallization. In order to make a quantitative correlation between nanowires and nanoflakes, we performed incident-photon© 2011 American Chemical Society

IPCE ) (1240I)/(λJlight)

where I is the photocurrent density, λ the incident light wavelength, and Jlight is the measured irradiance. As shown in Figure 8, the IPCEs measured for the three film types were consistent with the I-V curves, with the NF2 films giving the highest efficiency. Below 400 nm, the NF2 films gave IPCE values higher than 60%. The onset wavelengths of photocurrents were 430, 468, and 480 nm for nanowire, NF1, and NF2 films, respectively, which track results of the UV-vis absorption spectra. IMPS was employed to investigate electron transport. Figure 9 shows the complex plane plot of the IMPS response. The electron transport time (τn) can be determined from the frequency at the imaginary maximum, given by50 207

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τn ) (2πf(IMPS))-1

(13) Zhu, J.; Za¨ch, M. Curr. Opin. Colloid Interface Sci. 2009, 14, 260– 269. (14) Li, Y.; Zhang, J. Z. Laser Photonics Rev. 2010, 4, 517–528. (15) van de Krol, R.; Liang, Y.; Schoonman, J. J. Mater. Chem. 2008, 18, 2311–2320. (16) Mor, G. K.; Varghese, O. K.; Paulose, M.; Shankar, K.; Grimes, C. A. Sol. Energy Mater. Sol. Cells 2006, 90, 2011–2075. (17) Gudiksen, M. S.; Lauhon, L. J.; Wang, J.; Smith, D. C.; Lieber, C. M. Nature 2002, 415, 617–620. (18) Su, J.; Guo, L.; Yoriya, S.; Grimes, C. A. Cryst. Growth Des. 2010, 10, 856–861. (19) Yang, X.; Wolcott, A.; Wang, G.; Sobo, A.; Fitzmorris, R. C.; Qian, F.; Zhang, J. Z.; Li, Y. Nano Lett. 2009, 9, 2331–2336. (20) Zhu, K.; Neale, N. R.; Miedaner, A.; Frank, A. J. Nano Lett. 2007, 7, 69–74. (21) Wang, D.; Qian, F.; Yang, C.; Zhong, Z.; Lieber, C. M. Nano Lett. 2004, 4, 871–874. (22) Freer, E. M.; Grachev, O.; Duan, X.; Martin, S.; Stumbo, D. P. Nat. Nanotechnol. 2010, 5, 525–530. (23) Whang, D.; Jin, S.; Wu, Y.; Lieber, C. M. Nano Lett. 2003, 3, 1255– 1259. (24) Mai, L.; Gu, Y.; Han, C.; Hu, B.; Chen, W.; Zhang, P.; Xu, L.; Guo, W.; Dai, Y. Nano Lett. 2009, 9, 826–830. (25) Shankar, K. S.; Raychaudhuri, A. K. Nanotechnology 2004, 15, 1312–1316. (26) Feng, X.; Shankar, K.; Varghese, O. K.; Paulose, M.; Latempa, T. J.; Grimes, C. A. Nano Lett. 2008, 8, 3781–3786. (27) Yang, Y.; Kung, S. C.; Taggart, D. K.; Xiang, C.; Yang, F.; Brown, M. A.; Guell, A. G.; Kruse, T. J.; Hemminger, J. C.; Penner, R. M. Nano Lett. 2008, 8, 2447–2451. (28) Tchernycheva, M.; Cirlin, G. E.; Patriarche, G.; Travers, L.; Zwiller, V.; Perinetti, U.; Harmand, J. C. Nano Lett. 2007, 7, 1500–1504. (29) Santato, C.; Ulmann, M.; Augustynski, J. J. Phys. Chem. B 2001, 105, 936–940. (30) Huang, R.; Zhu, J.; Yu, R. Chin. Phys. B 2009, 18, 3024–3030. (31) Hong, K.; Xie, M.; Wu, H. Nanotechnology 2006, 17, 4830–4833. (32) Chi, L.; Xu, N.; Deng, S.; Chen, J.; She, J. Nanotechnology 2006, 17, 5590–5595. (33) Gu, G.; Zheng, B.; Han, W.; Roth, S.; Liu, J. Nano Lett. 2002, 2, 849–851. (34) Qi, H.; Wang, B.; Liu, J. Adv. Mater. 2003, 15, 411–414. (35) Wang, H.; Quan, X.; Zhang, Y.; Chen, S. Nanotechnology 2008, 19, 065704-9. (36) Mukherjee, N.; Paulose, M.; Varghese, O. K.; Mor, G. K.; Grimes., C. A. J. Mater. Res. 2003, 18, 2296–2299. (37) Lou, X.; Zeng, H. Inorg. Chem. 2003, 42, 6169–6171. (38) Gu, Z.; Ma, Y.; Yang, W.; Zhang, G.; Yao, J. Chem. Commun. 2005, 41, 3597–3599. (39) Zhao, Y.; Hu, W.; Xia, Y.; Smith, E.; Zhu, Y.; Dunnill, C.; Gregory, D. J. Mater. Chem. 2007, 17, 4436–4440. (40) Amano, F.; Li, D.; Ohtani, D. Chem. Commun. 2010, 46, 2769– 2771. (41) Tauc, J.; Grigorovici, R.; Vancu, A. Phys. Status Solidi 1966, 15, 627–637. (42) Wijs, G.; Groot, R. Phys. Rev. B 1999, 60, 16463–16474. (43) Wijs, G.; Boer, P.; Groot, R. Phys. Rev. B 1999, 59, 2684–2693. (44) Zou, G.; Li, H.; Zhang, Y.; Xiong, K.; Qian, Y. Nanotechnology 2006, 17, S313–S320. (45) Gu, Z.; Zhai, T.; Gao, B.; Sheng, X.; Wang, Y.; Fu, H.; Ma, Y.; Yao, J. J. Phys. Chem. B 2006, 110, 23829–23836. (46) Ko, R.; Wang, S.; Tsai, W.; Lioub, B.; Lin, Y. CrystEngComm 2009, 11, 1529–1531. (47) Custelcean, R. Chem. Commun. 2008, 21, 295–307. (48) Zhao, Z.; Miyauchi, M. Angew. Chem., Int. Ed. 2008, 47, 7051– 7055. (49) Varghese, O. K.; Grimes, C. A. Sol. Energy Mater. Sol. Cells 2008, 92, 374–384. (50) Kruger, J.; Plass, R.; Gratzel, M.; Cameron, P. J.; Peter, L. M. J. Phys. Chem. B 2003, 107, 7536–7539. (51) Hagfeld, A.; Gratzel, M. Chem. Rev. 1995, 95, 49–68.

The electron transport times calculated for nanowires, NF1, and NF2 films are 2.89, 3.35, and 26.99 ms, respectively. Electron transport in the small feature size films, ≈20-30 nm, is dominated by diffusion due to the lack of band bending.51 The nanowire and NF1 films are comparable in thickness, and gave similar electron transport times. Comparing the electron transport in TiO2 nanotube and nanoparticle films,20 in which a value of 5-7 ms was reported for a film thickness of 4.3 µm under similar incident photon flux (4.65 × 1015 cm2 s-1), the transport time of 26.99 ms for the NF2 films, 5.6 µm thickness, is relatively long. A longer transport time can decrease the IPCE because of carrier recombination. However the NF2 films showed high IPCE values indicating efficient electron transport. Conclusions. In summary, ordered WO3 nanowire and nanoflake films with, respectively, hexagonal and monoclinic structure were synthesized on FTO coated glass substrates by solvothermal deposition with morphologies controlled through solution composition. The amounts of water, oxalic acid, and urea in the precursor play important roles in determining film morphology. Structural and photoelectrochemical properties were investigated to demonstrate their utility in photoelectrolysis. Annealing decreased the band gap and improved the photocurrent significantly, with the nanoflakes showing lower band gap values than the nanowires. The NF2 films, 5.6 µm thick, gave the highest saturation photocurrent of 1.43 mA/cm2 under AM 1.5G illumination. Acknowledgment. Jinzhan Su was supported by a scholarship grant from the China Scholarship Council. Partial support of this work through the Department of Energy, GrantNumberDE-FG36-08GO18074,isgratefullyacknowledged. REFERENCES AND NOTES (1) (2) (3)

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