Vertically Aligned WO3Nanowire ArraysGrown Directly on Transparent ConductingOxide Coated Glass: Synthesis andPhotoelectrochemical Properties

Vertically Aligned WO 3 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 WO 3 nanowire arrays are synthesized by a solvothermal technique on fluorine-doped tin oxide coated glass. WO 3 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/cm 2 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 WO 3 , nanowire, tungsten trioxide, photoelectrochemical. H ydrogen production by water photoelectrolysis has been of considerable interest since Fujishima and Honda’s report of water splitting on a TiO 2 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 structures 8 and p/n junctions, 9 engineering of crystalline structures 10 and modification of semiconductor surfaces by chemical and/or physical processes. 11 It is now widely recognized that nanostructured semiconductors, in comparison to bulk ma- terials, offer potential advantages in photoelectrochemical cell (PEC) application due to their large surface area and size- dependent 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 photoge- nerated charges direct electrical pathways, with reduced grain boundaries, resulting in superior charge transport properties. 20 1-D semiconductor nanoarchitectures have been synthesized by a number of chemical and physical techniques, including vapor-liquid-solid, 21 dielectrophore- sis, 22 Langmuir-Blodgett (LB), 23,24 anodized aluminum ox- ide template (AAO), 25 hydrothermal, 26 lithographically pat- terned nanowire electrodeposition (LPNE), 27 molecular beam epitaxy, 28 etc. WO 3 is recognized as one of the few n-type semiconductors resistant to photocorrosion in aqueous solu- tions, and significant incident photon-to-current conversion efficiencies (IPCEs) for oxidation of water have been re- ported for WO 3 films. 29 1-D-structured WO 3 may prove a promising material with which to achieve efficient water photoelectrolysis. 1-D WO 3 nanostructures have been syn- thesized 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 WO 3 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 WO 3 nanoflake arrays synthesized by a solvo- thermal technique in ethanol. 40 In this work, we report a facile way to deposit ordered nanowire, as well as nanoflake, WO 3 arrays upon FTO coated glass. A WO 3 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 * To whom correspondence should be addressed, craig.grimes40@gmail.com. § Current address: Photonic Fuels, Innovation Park, State College, PA. 16803. Received for review: 09/30/2010 Published on Web: 11/29/2010 pubs.acs.org/NanoLett © 2011 American Chemical Society 203 DOI: 10.1021/nl1034573 | Nano Lett. 2011, 11, 203–208 1.25gofH 2 WO 4 and 0.5 g of poly(vinyl alcohol) (PVA) in 10 mL of 50 wt % H 2 O 2 , followed by 500 °C anneal for2hin air. A H 2 WO 4 solution for solvothermal use was prepared by dissolving 1.25 g of H 2 WO 4 into 30 mL of H 2 O by adding 10 mL of 50 wt % H 2 O 2 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 usinga3mL portion of H 2 WO 4 (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 Teflon- lined stainless steel autoclave, holding a vertically oriented FTO-glass substrate (with a WO 3 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 condi- tions. For the first type, 3 mL of H 2 WO 4 (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 H 2 WO 4 (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 (FES- EM, JEOL JSM 4700F) operated at 3 kV. Transmission electron microscopy (TEM) images and selected area elec- tron diffraction (SAED) patterns were obtained using a JEOL 2010 with a LaB 6 emitter operated at 200 kV. X-ray diffrac- tion (XRD) patterns were taken using a Scintag X2 diffrac- tometer (Cu KR radiation). UV-vis absorption spectra mea- surements 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/cm 2 ) 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/cm 2 . Light intensity modulation was conducted by current modulation with a depth of 5%. A lock-in ampli- fier (Stanford Research Systems SR 830) was used to record the photocurrent response as a function of frequency. Results and Discussion. Figure 1 presents FESEM images of an illustrative as-prepared WO 3 nanowire array film, and the two types of nanoflake arrays; there was no discernible change in film morphology after annealing. Both the nano- wire and nanoflake films grow perpendicular to the sub- strate. 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 a20-30 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 FIGURE 1. FESEM images of unannealed WO 3 : (a) nanowire, (b) NF1, and (c) NF2 arrays. Insets show film cross section. © 2011 American Chemical Society 204 DOI: 10.1021/nl1034573 | Nano Lett. 2011, 11, 203-–208 (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 00- 005-0393) 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 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, E G , was determined using the equation 41 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 WO 3 FIGURE 2. Digital photograph of WO 3 films as-prepared and after anneal. FIGURE 3. XRD patterns of unannealed and 500 °C 1 h air-annealed samples. 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. αhν ) A(hν - E G ) n © 2011 American Chemical Society 205 DOI: 10.1021/nl1034573 | Nano Lett. 2011, 11, 203-–208 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 WO 3 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 WO 3 nanoflakes give a lower band gap than the hexagonal WO 3 nanowires. The hydrothermal precursor composition plays a domi- nant 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 the3mLH 2 WO 4 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 H 2 O 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 nanostruc- tured WO 3 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 WO 3 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 nar- row temperature window. For NF1 films, reducing the 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, nano- wire 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 WO 3 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 WO 3 nanowire/nanorods in aqueous solution by hydrother- mal 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 Gu 45 with increasing K 2 SO 4 in the hydro- thermal reaction, which was explained as oriented aggrega- tion of the nanowires induced by high sulfate concentrations. It was reported that with addition of oxalic acid, the hydro- thermal products can change from irregularly aggregated WO 3 nanorods to WO 3 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 WO 3 from nanowires to nanosheets by thermal annealing was reported by Ko, 46 who proposed that formation and recrystallization of an amorphous inter- face 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 group 47 and was used as a directing agent in an ethanol/WCl 6 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 air- annealed samples of different film types. © 2011 American Chemical Society 206 DOI: 10.1021/nl1034573 | Nano Lett. 2011, 11, 203-–208 oxalic acid was enough to grow NF1 films. Urea together with oxalic acid promotes the translation from nanowires to nanosheets. Photoelectrochemical Properties. Photocurrent mea- surements of the nanostructured WO 3 films were conducted ina0.1MNa 2 SO 4 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/cm 2 . As an indirect band gap semiconductor, WO 3 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/cm 2 , photocurrent values due to the poor crystallization. In order to make a quantitative correlation between nanowires and nanoflakes, we performed incident-photon- to-current-conversion efficiency (IPCE) measurements as a means of studying the photoactive wavelength regime for the nanostructured WO 3 films (Figure 8). IPCE can be expressed as 49 where I is the photocurrent density, λ the incident light wavelength, and J light 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 photo- currents 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 (τ n ) can be determined from the frequency at the imaginary maximum, given by 50 FIGURE 6. FESEM image of WO 3 flakes synthesized with addition of 0.1 g of oxalic acid, indicating that the flakes are comprised of nanowires. 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 (Na 2 SO 4 ). 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 9. Complex plane plot of the IMPS response at a base light intensity of 2.53 mW/cm 2 , incident photon flux 0.465 × 10 16 cm 2 s -1 , using an UV LED (λ ) 365 nm). IPCE ) (1240I)/(λJ light ) © 2011 American Chemical Society 207 DOI: 10.1021/nl1034573 | Nano Lett. 2011, 11, 203-–208 The electron transport times calculated for nanowires, NF1, and NF2 films are 2.89, 3.35, and 26.99 ms, respec- tively. 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 compa - rable in thickness, and gave similar electron transport times. Comparing the electron transport in TiO 2 nanotube and nanoparticle films, 20 in which a value of 5-7mswas reported for a film thickness of 4.3 µm under similar incident photon flux (4.65 × 10 15 cm 2 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 WO 3 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 photoelectroly- sis. 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/cm 2 under AM 1.5G illumination. Acknowledgment. Jinzhan Su was supported by a schol- arship grant from the China Scholarship Council. Partial support of this work through the Department of Energy, GrantNumberDE-FG36-08GO18074,isgratefullyacknowledged. REFERENCES AND NOTES (1) Fujishima, A.; Honda, K. Nature 1972, 238, 37–38. (2) Khaselev, O.; Turner, J. A. Science 1998, 280, 425–427. (3) Bak, T.; Nowotny, J.; Rekas, M.; Sorrell, C. C. Int. J. Hydrogen Energy 2002, 27, 991–1022. (4) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nano Lett. 2005, 5, 191–195. (5) Grimes, C. A.; Varghese, O. K.; Ranjan, S. 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