Facile Preparation of Platelike Tungsten Oxide Thin Film Electrodes

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Facile Preparation of Platelike Tungsten Oxide Thin Film Electrodes with High Photoelectrode Activity Fumiaki Amano,*,† Min Tian,‡ Guosheng Wu,‡ Bunsho Ohtani,† and Aicheng Chen‡ † ‡

Catalysis Research Center, Hokkaido University, Sapporo 001-0021, Japan Department of Chemistry, Lakehead University, Thunder Bay, Ontario P7B 5E1, Canada

bS Supporting Information ABSTRACT: Tungsten trioxide (WO3) thin film electrodes with platelike structures were prepared by a facile hydrothermal reaction of tungsten sheets in a dilute nitric acid solution at 100 180 °C and subsequent calcination at 450 °C. The calcination step facilitated the transformation of the crystal structure from tungsten oxide hydrates (WO3 3 H2O or WO3 3 2H2O) to monoclinic WO3 without significant modification to the platelike structures. The photoelectrochemical performance of the thin film electrodes for water splitting that took place in a dilute sulfuric acid was strongly dependent on both temperature and the time used for the hydrothermal reaction. This suggests that the thickness of the film influences the process of photoexcited electron transport. The time required for the hydrothermal reaction under higher temperatures was reduced in the generation of thin film electrodes with high photoelectrode activity, because the crystal growth is accelerated at high temperatures and the electron transport is restricted by a relatively thick compact layer that is comprised of WO3 nanoparticulates. The electrode exhibited sensitivity to the violet portion of the visible light spectrum due to the bandgap of 2.8 eV and high photoelectrode efficiency, as well as an incident photon-to-current conversion efficiency (IPCE) of 66.2%, for the photoelectrochemical oxidation of water. KEYWORDS: hydrothermal reaction, semiconductor electrode, photoelectrochemical water splitting, visible-light-responsive photocatalyst

’ INTRODUCTION Photoelectrochemical water splitting using n-type metal-oxide semiconductor electrodes has attracted the interest of many researchers, and stems from the pioneering work of Fujishima and Honda on titanium(IV) oxide (TiO2) single crystal electrodes.1 The key concept is that TiO2 has the capacity for inducing the four-electron oxidation of water to evolve oxygen without anodic photocorrosion.2 However, TiO2 is not a suitable material from the point view of the band gap (3.0 eV for rutile and 3.2 eV for anatase). Assuming that photons with energies higher than 3.0 eV are used up entirely in water splitting without the recombination of photoexcited carriers, the conversion efficiency of solar energy in hydrogen production is calculated to be only 2.2% utilizing the Gibbs free energy of 1.23 V. Tungsten trioxide (WO3), which exhibits a band gap of ca. 2.6 eV, is known as a robust electrode for the photoelectrochemical oxidation of water.3 6 The theoretical maximum conversion efficiency is 6.3% for the absorption of photons with energies higher than 2.6 eV. This indicates that WO3 holds more promise for practical utility than other metal oxide electrodes with smaller band gap energies. However, the flat-band potential of WO3 is too positive to induce water splitting without bias voltage.7 Fortunately, the association of a n-type semiconductor electrode with a p-type semiconductor electrode or a photovoltaic cell, is possible toward the realization of a water splitting system under solar illumination.7 11 r 2011 American Chemical Society

Despite the promising advantages of WO3,12 the number of reports that investigate the structural control of WO3 electrodes has been somewhat limited in comparison to the number of reports on TiO2 electrodes. We have reported a preparation method of WO3 thin films comprised of rectangular plates that are vertically aligned on a conductive glass substrate.13,14 Recently, WO3 thin films with comparable vertically aligned nanostructures have been reported by several groups who employed a similar procedure, in which a WO3 seed layer constituted an essential prerequisite for the initiation of crystal growth.15,16 These films exhibited relatively high incident photon-to-current conversion efficiencies (IPCEs) for photoelectrochemical water oxidation, suggesting that structural control is a critical requirement for enabling the fabrication of WO3 thin film electrodes with high efficiency.13 18 Therefore, the establishment of a facile preparation method for the structural control of WO3 thin films, as well as an elucidation of the relationship between distinct nanostructures and their photoelectrochemical properties comprise vital topics in the field of photoelectrochemisty. Widenkvist et al. have reported that WO3 3 H2O thin films consisting of platelike nanostructures were grown directly on a tungsten substrate by boiling in a dilute nitric acid solution at 50 95 °C.19 However, they have not reported on the photoelectrochemical Received: July 7, 2011 Accepted: September 15, 2011 Published: September 15, 2011 4047

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properties. In this study, we investigated the photoelectrochemical properties of WO3 thin films prepared by a facile hydrothermal method at 100 180 °C, and by subsequent calcination at 450 °C. It was found that the increase in hydrothermal reaction temperature significantly affected the photocurrent for water oxidation. The influence of temperature and time as relates to the hydrothermal reactions on the nanostructures and the resulting photoelectrochemical properties are discussed.

’ EXPERIMENTAL SECTION Preparation of Films. A method that was previously reported by Widenkvist et al. was expanded in this study via the application of higher temperatures using a Teflon-lined autoclave.19,20 Tungsten sheets (99.95% purity, ca. 10  11 mm, 0.25 mm in thickness) were heated in 8 mL of dilute nitric acid (1.5 mol L 1 HNO3) at 100 180 °C for 0.5 12 h. The hydrothermal reaction resulted in the production of yellowish-green films on the surfaces of the tungsten sheets. The films thus obtained were rinsed with pure water, dried at room temperature (ca. 20 °C), and then calcined at 450 °C for 3 h. Prior to calcination, the samples were designated as Wx-yh, in which x and y indicated the hydrothermal reaction temperatures in the unit of °C, and reaction times in the unit of hours, respectively. The calcined samples were denoted as Wx-yh-C. The samples were characterized by scanning electron microscope (SEM, JEOL JSM-7400F), transmission electron microscope (TEM, JEOL JEM-2100F), X-ray diffraction (XRD, PANalytical X’Pert PRO Diffractometer) with Cu Kα radiation, and diffuse reflectance UV vis spectroscopy (Varian Cary 5E spectrometer). Photoelectrochemical Study. The photoelectrochemical splitting of water was carried out in a glass cell using a three electrode configuration which was controlled by a potentiostat (Radiometer analytical VolataLab 40). Sulfuric acid (H2SO4) (20 mL; 0.1 mol L 1) was used as an electrolyte solution, which was stirred continuously via a small magnetic bar. The thin films prepared in this study were welded to a titanium wire and used as the working electrode. A platinum coil was used as a counter electrode, and a silver/silver chloride (Ag/AgCl) electrode in an aqueous solution of 1 mol L 1 potassium chloride (+0.222 V vs a standard hydrogen electrode) was employed as a reference electrode. The applied potential was swept to the anodic direction by a scanning rate of 50 mV s 1. A 50 W metal halide lamp (ADAC Systems Cure Spot 50 lamp) was used for photoirradiation. The primary range of the light was UV-A and blue-visible light (300 450 nm). The light was introduced to the cell using a fiber cable, which was placed above the electrode. The distance between the fiber tip and the electrode surface was 1.5 cm. The intensity at a ∼ 312 nm (half bandwidth 12 nm) wavelength was measured to be ca. 0.6 mW cm 2. A 300-W xenon arc lamp (LOT Oriel LSB530) with integrated long-pass glass filters (Edmund Optics) was used for photoirradiation.

Figure 1. Top-view SEM images of the samples before calcination: (a) W100-3 h, (b) W140-2 h, and (c) W180-1 h. The insets show a top-view photograph of the sample in real color.

’ RESULTS AND DISCUSSION Morphology and Characterization. Figure 1 depicts photographs of the thin films prior to calcination, W100-3 h, W140-2 h, and W180-3 h. The formation of films by the hydrothermal reaction could be visually confirmed owing to the change of surface color of the tungsten sheet to a yellowish-green color. SEM images (top-views) show the presence of micrometer scale, platelike rectangular geometries on the surfaces of the thin films. A similar morphology has been reported for a WO3 3 H2O thin film, which was formed when a tungsten substrate was immersed in nitric acid at 50 °C and then annealed at 100 °C.19,20 The average dimensions of the platelets were estimated via measurements of more than fifty particles. It was found that the lateral dimension and thickness of the

Figure 2. XRD patterns of the samples before calcination: (a) W100-3 h, (b) W140-2 h, (c) W180-1 h, (d) WO3 3 2H2O (ICDD #18 1420), and (e) WO3 3 H2O (ICDD #43 0679).

platelets of the W100-3 h were 540 ( 200 nm and 66 ( 23 nm, respectively. For the platelets prepared at 140 °C, the lateral dimension was 2100 ( 550 nm and the thickness was 220 ( 54 nm. For the platelets prepared at 180 °C, the lateral dimension was 1500 ( 340 nm and the thickness was 210 ( 66 nm. Widenkvist et al. reported that the size and shape of the platelets 4048

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Figure 4. XRD patterns of the calcined samples: (a) W100-3 h-C, (b) W140-2 h-C, (c) W180-1 h-C, and (d) monoclinic WO3 (ICDD #43 1035).

Figure 3. SEM images (top views and cross-sectional side views) of the calcined samples: (a) W100-3 h-C, (b) W140-2 h-C, and (c) W180-1 h-C. The insets show a top-view photograph of the sample in real color.

are dependent on the temperature (50 95 °C), acid concentration, and immersion time.19 In the present study, aggregates of large and small platelets were generated on the surfaces of thin films that were prepared at 100 °C after a prolonged reaction. A protracted hydrothermal reaction at 140 and 180 °C resulted in lateral growth that added to the thickness of the platelets.

Figure 2 shows XRD patterns of the thin films before calcination. The peak at 2θ of 40.3° is assignable to the diffraction from the tungsten substrate. W100-3 h exhibited patterns assignable to tungsten oxide dihydrate (hydrotungstite, WO3 3 2H2O). For W140-2 h, the films contained crystalline segments of WO3 3 2H2O and tungsten oxide hydrate (tungstite, WO3 3 H2O). W180-1 h comprised WO3 3 H2O; however, there is a slight portion of WO3 3 2H2O. These results suggest that higher temperatures applied in the hydrothermal reaction resulted in the enhanced formation of WO3 3 H2O crystallites. Since it is known that WO3 3 2H2O can be converted to WO3 3 H2O by mild dehydration, (e.g., drying at 120 °C),21,22 we dried the prepared samples at room temperature after the hydrothermal reaction. WO3 3 H2O (orthorhombic) and WO3 3 2H2O (monoclinic) exhibited a layered crystalline structure comprised of cornersharing WO5(OH2) octahedral sheets; the resident tungsten atom is coordinated by five oxygen atoms and a water molecule.23,24 The stacking of the WO5(OH2) octahedral sheets by hydrogen bonding forms a layered structure of WO3 3 H2O.23 For WO3 3 2H2O, a second water molecule is placed within the interlayer as a structural element.24 Rectangular, platelike structures are generated by the crystal-plane selective growth of layered WO3 3 H2O and WO3 3 2H2O with a two-dimensional anisotropic crystal structure.13,14 The relatively low intensities of (020) planes are related to the anisotropic crystal growth. Figure 3 shows photos of the thin films calcined at 450 °C. The yellowish-green color was changed to dark blue via calcination. SEM images reveal that the morphology of the platelets on the surface was scarcely altered by calcination. The cross-sectional side view shows that the thin films are comprised of layers of rectangular platelets, nanoparticles, and a compact layer. The compact layer was located between those of the platelets and nanoparticles and the surface of the tungsten substrate. It was evident that platelet growth was in the vertical direction, perpendicular to the substrate. The thicknesses of the three layers were ca. 1.8, 2.0, and 1.6 μm for W100-3 h-C, W140-2 h-C, and W180-1 h-C, respectively. Figure 4 shows XRD patterns of W100-3 h-C, W140-2 h-C, and W180-1 h-C. The patterns were assignable to monoclinic WO3. The complete conversion of WO3 3 2H2O and WO3 3 H2O to monoclinic WO3 was confirmed by the absence of the diffraction peaks due to layered crystallites. 4049

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Figure 5. (A) UV vis diffuse reflection spectra of the samples before calcination: (a) W100-3 h, (b) W140-2 h, and (c) W180-1 h. (B) Spectra of the calcined samples: (d) W100-3 h-C, (e) W140-2 h-C, and (f) W180-1 h-C. Corresponding SEM images can be found in Figure 1 and Figure 3, respectively.

Table 1. Band Gap Energy (Eg) Estimated from Diffuse Reflection Spectra of the Prepared Films sample

Eg (eV)

sample

Eg (eV)

W100- 3 h

2.65

W100- 3 h-C

2.89

W140- 2 h

2.47

W140- 2 h-C

2.79

W180- 1 h

2.48

W180- 1 h-C

2.82

On the basis of the results of SEM observations, the crystal transformation does not initiate a drastic change in the morphology of the material, i.e. plate-like structures.25 TEM images and selected area electron diffraction (SAED) patterns of WO3 platelets that were dislodged from calcined films indicate that a number of segments of the platelet exhibit a single crystalline nature (see Figure S1 in the Supporting Information). The platelets were not composited of finer plates and particles. Figure 5 presents the UV vis diffuse reflection spectra of the films. As mentioned above, the color of films was transitioned from yellowish-green to dark blue via calcination. The absorption edges of the thin films before calcination were located at ∼500 nm. On the other hand, the calcined thin films can absorb light only at a wavelength that is smaller than ca. 450 nm. Calcination also caused an increase of the baseline; in another words, it decreased the reflectance. Table 1 summarizes the band gaps that were estimated from the spectra using a Tauc plot (Figure S2 in the Supporting Information). The band gaps were ca. 2.65 and 2.5 eV for the thin films of WO3 3 2H2O and WO3 3 H2O, respectively. The band gaps of the WO3 thin films were 2.8 2.9 eV, which are larger than the typical band gap energy of WO3. The reasons for this difference are not yet understood; hence further study is required in order to elucidate the cause. Photoelectrochemical Efficiency. Figure 6 depicts a series of linear sweep voltammograms of the calcined thin film electrodes in 0.1 mol L 1 H2SO4 (pH 1), in the absence of light and under photoirradiation. The films exhibited a dark anodic current at ∼ +0.1 V vs Ag/AgCl (VAg/AgCl). The current is attributable to electrochemical oxidation of partially reduced WO3. Electrochromism of WO3 is known to be induced by redox reactions.

Figure 6. Linear sweep voltammograms of the calcined films in 0.1 mol L 1 H2SO4 in the dark (dashed curves) and under photoirradiation using a 50-W metal halide lamp (solid curves): (A) W100-yh-C, (B) W140-yh-C, and (C) W180-yh-C.

Figure 7. Effect of hydrothermal reaction time on the photocurrent density at +1.2 VAg/AgCl: (a) W100-yh-C, (b) W140-yh-C, and (c) W180-yh-C. Experimental conditions are identical to that of Figure 6.

It was observed that films prepared for longer durations exhibited larger dark current. Photoirradiation increased the anodic current in the milliampere range. The photocurrent was due to the oxidation of water by photogenerated holes in WO3 platelets under anodic polarization. The onset potential was found to be ca. +0.2 VAg/AgCl, which is consistent with the potential of +0.48 V versus a reversible hydrogen electrode (RHE). In comparison with previous reports, the present onset potential value is reasonable for a WO3 electrode.3,4,26 29 The photocurrent density of samples was contingent both on the temperature and the duration of the hydrothermal reaction. Figure 7 shows the photocurrent density plots at +1.2 VAg/AgCl relative to the hydrothermal reaction time for thin film preparation. For thin films prepared at 100 °C, the photocurrent density 4050

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Figure 8. XRD patterns of (a) W100-1 h-C, (b) W100-12 h-C, (c) W140-2 h-C, (d) W140-12 h-C, (e) W180-1 h-C, and (f) W180-3 h-C.

increased concurrently with an increase in the reaction duration to a threshold of 3 h, beyond which there was a gradual decrease. Similar phenomena were observed for thin films that were prepared at higher temperatures. However, the reaction times required to attain each photocurrent density maximum was dependent on the reaction temperature (e.g., 2 h for thin films that were prepared at 140 °C and 1 h for thin films that were prepared at 180 °C). The photocurrent density of thin films that were prepared at high temperatures was significantly diminished by prolonged reaction times that extended beyond the requisite optimum time for obtaining the maximum photocurrent density. Figure 8 shows the XRD patterns of W100-12 h-C, W140-12 h-C, and W180-3 h-C compared with W100-1 h-C, W140-2 h-C, and W180-1 h-C. For thin films prepared at 100 °C, the peak intensities were not radically changed by a prolonged reaction. However, extended reaction times resulted in the alteration of XRD patterns for thin films that were prepared at higher temperatures (e.g., the intensity of the peak at 2θ of 40.3° due to the tungsten substrate was decreased, whereas the intensities of the peaks that were assignable to WO3 increased). These results indicate that crystal growth at low temperatures is a sluggish process and that prolonged reaction times at high temperatures served to accelerate the growth of the hydrated WO3 layer, which is converted to WO3 subsequent to calcination. A gradual increase in the thin film thickness along with an increase in the hydrothermal reaction time was also confirmed by the SEM observation of a cross-sectional side view (see Figure S3 in the Supporting Information). The thickness of the compact layer as well as the platelike crystalline layer was increased over time. In this case, the compact layer comprised nanoparticles. As shown in Figure 7, higher temperature exposures required less time to obtain thin film electrodes that exhibited high photoelectrode activity. This is because crystal growth is accelerated at high temperatures and electron transport is constrained by the thick compact layer that is composed of WO3 nanoparticulates. Because crystal growth at 100 °C is slow, the thickness of the WO3 layer of W100-12 h-C is moderate. In fact, the photocurrent density of W100-12 h-C is shown to be much higher than that of W140-12 h-C, as shown in Figure 7. Figure 9 illustrates the pseudo action spectrum of W180-1 h-C and a thin film of TiO2 nanotubes, which was prepared by the anodization of a titanium sheet at 40 V in dimethyl sulfoxide (DMSO) with 2% hydrofluoric acid and subsequent calcination at 450 °C for 3 h (see Figure S4 in the Supporting Information).30 33 The photocurrent density at +1.2 VAg/AgCl in linear sweep voltammetry was plotted against the cutoff

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Figure 9. Dependence of photocurrent in 0.1 mol L 1 H2SO4 at +1.2 VAg/AgCl on the cutoff wavelength of long-pass glass filters: (a) W180-1 h-C and (b) TiO2 nanotube thin film. Inset shows an enlargement of the action spectra in the visible light region. A 300-W xenon arc lamp was used for photoirradiation.

wavelength of a long-pass glass filter in the “pseudo” action spectrum.34 The photocurrent density of W180-1 h-C was much higher than that of TiO2 nanotube electrode. The onset wavelength of W180-1 h-C was located at ∼450 nm. This wavelength is close to the photoabsorption edge that is observed in the diffuse reflection spectrum. On the other hand, the onset wavelength of TiO2 nanotubes was ∼380 nm. The light intensity dependence of photocurrent under 365-nm photoirradiation (see Figures S5 and S6 in the Supporting Information) shows a linear relationship with a slope value of 0.195 mA mW 1 between the irradiance and photocurrent density, suggesting that the IPCE is constant despite the intensity of light. The IPCE under 365-nm photoirradiation was calculated to be 66.2% for the photoelectrochemical oxidation of water at +1.2 VAg/AgCl. This value could be categorized as a relatively high level of IPCEs among WO3 electrodes.3,4,18,26 29,35,36 This high efficiency might be due to nanocrystalline platelet structures, which exhibit large surface areas and extensive crystallinity. Further investigation is necessary in order to elucidate the reasons behind this high photoelectrode activity.

’ CONCLUSION This study provides a facile and reproducible method for the preparation of WO3 photoelectrodes with high efficiency. Electrodes of monoclinic WO3 thin films containing platelike structures exhibited high efficiencies for the photoelectrochemical splitting of water. It was found that the increase in hydrothermal reaction temperature significantly affected the photoelectrochemical properties of films grown in a dilute nitric acid. The duration under exposure to higher temperatures enabled the more rapid synthesis of tungsten oxide photoelectrodes with enhanced performance, and the photoelectrode activity was significantly diminished by prolonged reaction times that extended beyond the requisite optimum time for obtaining the maximum photocurrent density. These are because crystal growth is concurrently accelerated at high temperatures along with increases in the thickness of the compact layer. Electron transport was impeded at a particular threshold by the thickness of a compact layer comprised of nanoparticulate WO3. This knowledge is of critical value in the development of highly efficient WO3 photoelectrodes. ’ ASSOCIATED CONTENT

bS

Supporting Information. TEM images of WO3 platelets, Tauc plot, SEM images of films prepared for prolonged hydrothermal

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ACS Applied Materials & Interfaces reaction and film of TiO2 nanotubes, IPCE measurement. This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the Institutional Program for Young Researcher Overseas Visits from the Japan Society for the Promotion of Science (JSPS) and a Grant-in-Aid for Young Scientists (A) (23686114) from JSPS.

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