Enhanced Visible Light Activity of Single-Crystalline WO3 Microplates

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Enhanced Visible Light Activity of Single-Crystalline WO Microplates for Photoelectrochemical Water Oxidation Mira Park, Jong Hyeok Seo, Hyunjoon Song, and Ki Min Nam

J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b00389 • Publication Date (Web): 12 Apr 2016 Downloaded from http://pubs.acs.org on April 15, 2016

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The Journal of Physical Chemistry

Enhanced Visible Light Activity of SingleCrystalline WO3 Microplates for Photoelectrochemical Water Oxidation Mira Park,† Jong Hyeok Seo,‡ Hyunjoon Song†,*, and Ki Min Nam‡,* † Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), and Center for Nanomaterials and Chemical Reactions, Institute for Basic Science (IBS), 291 Daehak-ro, Yuseong-gu, Daejeon 34341, South Korea ‡ Department of Chemistry, Mokpo National University, 1666 Yeongsan-ro, Cheonggye-myeon, Muan-gun, Jeonnam 58554, South Korea

ABSTRACT

The preparation of metal oxides on a conductive substrate has been an important issue for improving photoelectrochemical water splitting efficiency. In this work, a facile synthetic process is reported for single-crystalline WO3 microplates with variable thicknesses grown directly on a fluorine-doped tin oxide substrate followed by an annealing procedure. The WO3

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microplate electrode showed an improved photocurrent under one sun irradiation (1.9 mA/cm2 at 0.6 V vs. Ag/AgCl, 100 mW/cm2) for water oxidation, including a significant enhancement in the visible light region compared to either a nanoparticle or a bulk film electrode. The enhanced water oxidation activity originates from both the single crystallinity with an optimum thickness and the oxygen-deficient characteristic of the WO3 microplates. To improve the photochemical stability, FeOOH electrocatalysts were deposited on the surfaces of the WO3 microplates. The resulting WO3/FeOOH composite electrode showed enhanced stability for water oxidation reactions.

1. Introduction Photoelectrochemical (PEC) water oxidation using semiconductor photoelectrodes is a promising approach for directly converting sunlight into chemical fuel.1-4 To realize efficient PEC water splitting, the electrode materials need to fulfill several requirements, including a suitable band position, an appropriate band gap, and long-term durability in an aqueous environment.5.6 While numerous semiconductors have been investigated for their potential use in the PEC water splitting reactions,7 the most critical issues remain unresolved for high efficiency in the visible light region. The intensity of solar light abruptly falls off below 400 nm, imposing an upper limit of 3.1 eV on the band gap; therefore, the optimum band gap should be within the range of 1.9 - 3.1 eV, corresponding to the visible region of the solar spectrum.8 Tungsten trioxide (WO3) is one of the most promising photoanode materials (n-type semiconductor) for PEC water oxidation due to its outstanding stability in acidic conditions and fast carrier mobility under solar irradiation.9 Moreover, WO3 can be responsive to the blue part


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of visible light given its band gap of 2.5-2.8 eV and the large potential of its valence band (3.0 eV vs. NHE), making it suitable for water oxidation reactions.10 Although these advantages of WO3 are suitable for photoanodes, the theoretical conversion efficiency (approximately 10 %) based on its band gap (2.5-2.8 eV) is very difficult to achieve, as the transfer of photogenerated holes to the electrolyte is too slow to compete against the rapid electron-hole recombination processes in the materials. The recombination generally occurs at the surface as well as in the bulk state of WO3, leading to reduced conversion efficiency for water oxidation. In order to suppress this type of recombination processes, the WO3 structure requires an average particle radius which is shorter than the hole diffusion length (150 - 500 nm) to reduce bulk recombinations,11 as well as capable co-catalysts to minimize surface recombinations. Nanoscale structural tailoring can enhance the kinetics parameters of the water oxidation reactions through the reduction of bulk recombinations; thus, intensive efforts have been made to realize the synthesis of WO3 nanostructures such as nanorods, nanoflakes, and nanocubes.12 However, nanosized structures are also associated with significant disadvantages, such as an increased number of surface recombinations and a reduced space-charge region.6 Typically, the relatively low activity in the visible light region owing to small grain sizes, stemming from quantum confinement, contributes to the poor overall efficiency of WO3 nanostructures.13-18 In the present study, we prepared single-crystalline WO3 microplates directly grown on a fluorine-doped tin oxide (FTO) substrate followed by annealing at 500 ºC for 3 h in air. The WO3 microplates had an average length of 2.4 µm with a thickness of 470 nm, and were perpendicularly oriented to the FTO substrate. A photoelectrode consisting of WO3 microplates showed an improved photocurrent (1.9 mA/cm2 at 0.6 V vs. Ag/AgCl) under one sun irradiation (100 mW/cm2), including a significant enhancement in the visible light region (0.9 mA/cm2 at



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0.6 V vs. Ag/AgCl) compared to nanoparticle or bulk film electrodes. These enhanced performances mainly originated from the single crystallinity with an optimum thickness and an optimized oxygen-deficiency of the WO3 microplates. Furthermore, the deposition of FeOOH onto the surfaces of WO3 microplates as an oxygen-evolving electrocatalyst enhanced the stability for water oxidation reactions.

2. Experimental Section Materials. A fluorine-doped tin oxide (FTO, TEC 15, WY-GMS) coated glass was used as a substrate for thin film electrodes. (NH4)6H2W12O40·xH2O (≥ 99.0%, Sigma-Aldrich), ammonium metatungstate hydrate (99.99%, Sigma-Aldrich), and tungstic acid (Sigma-Aldrich, 99%), used as received, were used as metal precursor salts. D-(+)-Glucose (99.5%, SigmaAldrich), benzyl alcohol (99.0%, Junsei), HEPES (2-4-(2-hydroxyethyl)-1-piperazinyl)-ethanesulfonic acid, Sigma-Aldrich), Na2SO4, Na2HPO4, NaH2PO4, ethylene glycol (≥ 99%, SigmaAldrich), and oxalic acid dehydrate (≥ 99%, Sigma-Aldrich) were also used as received. Deionized water was used as a solvent in all of the electrochemical experiments.

Preparation of the WO3 Seed Layer. FTO substrates were cleaned by deionized water and ethanol and then sonicated in ethanol for 1 h. A drop-casting technique was used to fabricate thin film electrodes. The precursor solution ((NH4)6H2W12O40·xH2O, 10 mM, 200 µL) in ethylene glycol was dropped onto the FTO substrate (1.5 cm × 2 cm) followed by a drying step of 120 ºC in air. The film was annealed at 550 ºC for 3 h (with a 3 h ramp time) in air to form a WO3 seed layer.


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Preparation of WO3 Microplates (MP1 and MP2) and Nanoparticles (NP). WO3 microplates were grown on FTO substrates using a seed-mediated hydrothermal method. The seeded FTO substrate was vertically placed into a teflon-lined stainless steel autoclave with the volume of 50 mL. A reaction mixture was prepared by adding H2WO4 and oxalic acid into deionized water (30 mL). When the reaction mixture was dissolved in water, it was transferred into an autoclave. The sealed autoclave was heated in an electric oven at 170 ºC for 6 h and then cooled to room temperature. After being washed with acetone and water, the film was annealed in air at 500 ºC for 3 h. The corresponding amounts of H2WO4 and oxalic acid (1.6 mmol and 22 mmol, respectively) were added for microplates with a thickness of 470 nm (MP1), for microplates (0.40 mmol and 5.5 mmol, respectively) with a thickness of 330 nm (MP2), for microplates with mixed nanoparticles (0.20 mmol and 2.78 mmol, respectively) with a thickness of 143 nm, and for microplates (3.2 mmol and 44 mmol, respectively) with a thickness of 660 nm. For WO3 nanoparticles (NP), the seeded FTO substrate was horizontally placed into a teflon-lined stainless steel autoclave and H2WO4 (0.2 mmol) and oxalic acid (2.78 mmol) were added to the reaction mixture. Preparation of the WO3 Bulk Film. A drop-casting technique was used to create the bulk film electrodes. For the 500 nm film, precursor solution (200 µL, 20 mM) in ethylene glycol containing a tungsten precursor was dropped onto the FTO substrate (~ 1.5 cm × 2 cm) with a drying step at 120 ºC in air. To fabricate the ~ 2 µm film, multiple coatings (five times) of the precursor solution were carried out by drop-casting onto the FTO substrate followed by a drying step of 120 ºC in air between the coatings. The resulting films were annealed at 550 ºC for 3 h (with a 3 h ramp time) in air to form a continuous bulk film.



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Electrochemical Characterization of WO3 Electrodes. Electrochemical characterization was performed in a specially designed cell in a three-electrode configuration with the thin film as the working electrode, a Pt wire counter electrode, and an Ag/AgCl reference electrode. The working electrode with the actual geometric area of 0.28 cm2 was exposed to electrolyte solution. A 150 W xenon lamp (ABET technologies) was used as the light source in the PEC characterization step, and light illumination area was 0.28 cm2. Chopped light linear sweep voltammetry (LSV) was utilized to obtain the photocurrent responses using a DY2321 potentiostat (Digi-Ivy). The PEC measurements were taken in aqueous solutions of Na2SO4 (0.1 M) with a phosphate buffer (pH 7) for water oxidation. In all tests, the intensity of the lamp on the sample was measured and found to be 100 mW/cm2 using a Si solar cell (AIST). A 425 nm long-pass filter was used to cut the UV portion of the spectrum and to provide only visible light illumination. A monochromator (ORIEL) was used to obtain the action spectra of photo-response as a function of the wavelength. Material Characterization of the WO3 Electrodes. The thin film electrodes were characterized by scanning electron microscopy (SEM, Magellan 400 operated at 2 kV). Transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) were conducted using a Tecnai TF30 ST instrument at 300 kV. A focused ion beam (FIB, Helios Nanolab 450 F1) was used to prepare the samples for the SAED measurement. The X-ray diffraction (XRD) spectra were measured using Cu K radiation at 40 kV and 300 mA (Rigaku, D/MAX-2500). X-ray photoelectron spectroscopy (XPS) measurements were taken using a Kalpha spectrometer with an X-ray source of Al K and at a pass energy level of 40 eV. The UVVis absorption spectra were acquired from the photoelectrode film with a UV-3600 UV-VIS-


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NIR spectrophotometer using a solid sample holder in the wavelengths ranging from 300 to 800 nm.

3. Results and Discussion

Preparation of Single-Crystalline WO3 Microplates with Controlled Thicknesses Single-crystalline WO3 microplates were synthesized on a FTO substrate using a seedmediated hydrothermal method. The resulting WO3·H2O microplates showed full coverage of the FTO substrate. The as-prepared WO3·H2O microplates on FTO were annealed at 500 ºC for 3 h in air to provide WO3 microplates. The scanning electron microscopy (SEM) image showed high uniformity of the WO3 microplates (MP1) in terms of the morphology, with an orientation perpendicular to the substrate (Figure 1a). Each plate had an average length of 2.4 µm and an average thickness of 470 nm (Figure S1, Supporting Information). The transmission electron microscopy (TEM) image and the selected-area electron diffraction (SAED) pattern reveal the single-crystalline nature of the individual WO3 microplates (Figure S2, Supporting Information). The lattice spacing in the high-resolution TEM (HRTEM) image was measured and found to be 3.86 Å, corresponding to the distance between the (002) planes of a monoclinic WO3 phase. The concentration change in both H2WO4 and oxalic acid with a fixed ratio provided precise control of the WO3 microplates in terms of the length and thickness while full coverage on the FTO substrate was maintained. Figure 1b shows an SEM image of WO3 microplates with an average length of 1.8 µm and an average thickness of 330 nm (MP2) using only one quarter of the reactant in each case. A further decrease in the amount of the reactants yielded WO3 nanoparticles with an average diameter of 145 ± 40 nm (NP, Figure 1c). The X-ray diffraction (XRD) patterns of the MP1, MP2, and NP electrodes (Figure 2) are in good agreement with that



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of monoclinic WO3 (JCPDS No. 01-089-4476). In the MP1 and MP2 electrodes, the relative intensity of the (002) reflection is large compared to the reference peak, indicating the coexistence of an oxygen-deficient structure (monoclinic WO2.9).19,20 The thermal stability of the WO3 microplates before annealing was investigated using thermogravimetric analysis (TGA), showing a steady weight loss until 450 ºC with the total weight loss of about 2% (Figure S3, Supporting Information). The oxygen deficiency was presumably induced through the removal of water from the WO3·H2O lattice21 and also through the oxidative decomposition of oxalic acid adsorbed on the surface during the annealing process.22,23 The UV-visible spectra of the electrodes with the distinct WO3 microplates and thin films were measured (Figure S4, the Supporting Information). In the visible region ( > 400 nm), the light absorption proportionally increased with the thickness of the WO3 microplates on the electrodes.

Dependence of Photoelectrochemical Activity on Morphology of WO3 Microplates and Nanoparticles The PEC performance capabilities of the WO3 microplate and nanoparticle electrodes were investigated using linear sweep voltammetry (LSV) in a phosphate buffer (pH 7) with Na2SO4. The LSV was conducted from -0.2 V to +0.8 V vs. Ag/AgCl at a scan rate of 20 mV/s with chopped light under UV-visible irradiation (Figure 3a). All electrode samples successfully generated anodic photocurrents, which confirmed the n-type characteristics of the WO3 electrodes. The photocurrent of the MP1 electrode was measured and found to be 1.9 mA/cm2 at 0.6 V vs. Ag/AgCl, while the MP2 electrode generated 1.3 mA/cm2 at the same potential. The NP electrode only generated 0.5 mA/cm2 under the present condition, indicating that the MP1 electrode is highly effective for photocurrent generation. In order to investigate the photocurrent


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dependence on the domain thickness in detail, we have synthesized WO3 microplates with a small average diameter of 143 nm and a large diameter of 660 nm as uniform films covering entire electrode surfaces (Figure S5, Supporting Information). The photocurrents of the electrodes with these microplates were measured to be 0.9 and 1.8 mA/cm2 at 0.6 V vs. Ag/AgCl, respectively, indicating that the MP1 electrode had an optimal structure for the maximum photocurrent (Figures S6 and S7, Supporting Information). Note that the photocurrent of the MP1 electrode under visible light irradiation ( > 425 nm) is 0.9 mA/cm2 at 0.6 V vs. Ag/AgCl, thus showing a much higher value compared to those of the MP2 and NP electrodes (Figure 3b). These WO3 microplates, particularly MP1 and MP2, exhibited either superior or comparable photochemical performances to the other reported results using WO3 nanostructured materials (Table S1, Supporting Information). For comparison, the highest photocurrent values reported thus far were 1.5 mA/cm2 using vertically oriented WO3 nanosheets36 and 2.0 mA/cm2 using WO3 nanorods directly grown on a substrate at 0.6 V vs. Ag/AgCl.21 The action spectra of the WO3 electrodes show typical photocurrents depending on the wavelength with a 10 nm interval (Figure 3c). The photocurrents were generated in all samples by light with a wavelength longer than 350 nm, and the MP1 electrode generated much higher photocurrents than those of the MP2 and NP electrodes over all wavelength ranges. The action spectrum of the MP1 electrode shows a band gap of approximately 2.5 eV, and its profile is in good agreement with the UV-visible absorption spectra (Figure S4a, Supporting Information). The MP1 electrode showed a wide absorption range from 300 nm to 470 nm, but the NP electrode showed a narrow light absorption feature. Consequently, the MP1 electrode exhibited high photocurrent generation with a wide range of absorption, indicating that the optimal thickness of the WO3 microplates is ~500 nm. This value corresponds to the maximum hole



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diffusion length of WO3, which is known to range from 150 to 500 nm.11 As the thickness of the active material increases, the light absorption increases proportionally, allowing more photocurrent to be generated. However, when the thickness is larger than the average hole diffusion length, the holes generated in the middle areas of the materials cannot reach the semiconductor/electrolyte interface, impacting the PEC water oxidation reaction. In the present condition, the thickness of MP1 is an optimal value to maximize the effective charge separation and conduction processes (Figure S7, Supporting Information). Effective photocurrent generation in these electrodes is mainly attributed to the singlecrystalline nature of the WO3 microplates. To investigate the role of the single-crystalline domains regarding the performance of the WO3 electrodes, polycrystalline WO3 bulk films were fabricated with average thicknesses of 500 nm and 2 µm and with small grain boundaries (~30 nm) on the FTO substrates via a solid-state method (Figures S8 and S9, Supporting Information).24 The photocurrents of the WO3 bulk films were measured to be 0.10 and 0.15 mA/cm2 at 0.6 V vs. Ag/AgCl under UV-visible irradiation, respectively, with negligible visible light activities. These values are far smaller than those of the electrodes with single-crystalline materials, i.e., the MP1, MP2, and NP electrodes. These findings indicate that the singlecrystalline structure has fewer grain boundaries, which can offer direct conduction paths for photogenerated electrons and yield superior PEC performances.25,26

Dependence of the PEC Activity on the Oxygen Deficiency of WO3 Microplates The WO3 microplates are oxygen-deficient materials, as indicated by the large (002) intensity in the XRD spectra (Figure 2). It was reported that an annealing process can induce surface disorder or oxygen deficiencies of metal oxides.27 Recently, enhanced PEC performance


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capabilities were observed via the introduction of a moderate degree of oxygen deficiency to form reduced tungsten oxide (WO3-X).28-30 Because the oxygen deficiency of the active materials directly influences the PEC activity, precise adjustment of the degree of oxygen deficiency and its quantitative analysis may provide critical information for the fabrication of highly efficient PEC electrodes. In the present study, we employed various reduction conditions for the asprepared WO3·H2O electrode to determine the relationship between the degree of oxygen deficiency and the resulting PEC activity on the water oxidation reactions. As discussed above, the annealing procedure could effectively remove the water and capping agents adsorbed on the surface while also inducing oxygen deficiencies in the WO3 microplates. To determine the chemical state of the WO3 microplates, X-ray photoelectron spectroscopy (XPS) was performed (Figure 4). In the W 4f region, the spectra have two peaks assigned as characteristic W 4f5/2 and W 4f7/2 peaks at 37.7 and 35.5 eV for W6+ and at 36.5 and 34.3 eV for W5+, respectively.20,30-32 By adjusting the annealing conditions, the intensities of the W6+ and W5+ signals change distinctively. The amount of W5+ was markedly increased by the prolonged annealing process at 500 ºC for 3 h in air (Air_3 h, Figure 4b) relative to that of the sample treated only for 10 min under identical conditions (Air_10 min, Figure 4a). In contrast, the population of the W5+ peaks were decreased by annealing with an O2 flow under the present reaction conditions (O2_3 h, Figure 4c) due to the supply of sufficient O2 atoms on the surface. With the treatment of the H2 flow for 30 min (H2_30 min, Figure 4d), the W5+ peaks were significantly increased and exceeded the peak intensities of the Air_3 h sample. The normalized XPS spectra of H2, Air, and O2 were shown in the Figure S10 in the Supporting Information. The 4f peak positions of the W5+ were similarly located at 36.5 eV and 34.3 eV, and the population of the W5+ peaks apparently increased by annealing the samples under air and H2 environments.



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The XRD patterns of the microplate electrodes depending on the various annealing conditions are shown in Figure 5. The diffraction peaks of the as-prepared WO3·H2O electrode were precisely indexed to tungsten trioxide hydrates (WO3·H2O). When the as-prepared MP1 electrode was annealed at 500 ºC in air for 10 min (Air_10 min), a phase transition from orthorhombic WO3·H2O to monoclinic WO3 was observed. With additional annealing in air, the (002) and (200) peaks showed high intensity levels, whereas the (020) peak decreased continuously until an annealing time of 3 h (Air_3 h). The peak patterns in Air_3 h were similar to those in the H2_30 min case, indicating the existence of monoclinic WO2.9. However, prolonged annealing in air for up to 9 h (Air_9 h) led to the growth of both the (020) and (200) peaks due to re-oxidation into monoclinic WO3 by the O2 present in the air. Annealing with O2 for 3 h (O2_3 h) also showed peak patterns assignable to monoclinic WO3, similar to those of the Air_9 h sample. The photocurrents were measured for all samples treated under different annealing conditions, from -0.2 V to + 0.8 V vs. Ag/AgCl at a scan rate of 20 mV/s with chopped light under UVvisible irradiation in a phosphate buffer (pH 7) with Na2SO4. The photocurrent gradually increased as the annealing time increased from the as-prepared (0.1 mA/cm2 at 0.6 V vs. Ag/AgCl) to the Air_3 h samples (1.8 mA/cm2), as shown in Figure 6. These photocurrents correlate well with the intensity of the W5+ peaks in the XPS spectrum. On the other hand, the O2_3 h sample showed a smaller photocurrent (1.4 mA/cm2) than that of the Air_3 h sample, also indicating that the oxygen deficiency played a pivotal role in enhancing the efficiency during the PEC water oxidation reactions. In a comparison between the Air_3 h and O2_3 h samples, the Air_3 h sample always generated higher photocurrents over the entire potential range as compared to the O2_3 h sample (Figure S11, Supporting Information). The action


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spectra at the potential of 0.5 V vs. Ag/AgCl showed that the Air_3 h sample generated particularly high photocurrents in the long wavelength region (> 410 nm) compared to the O2_3 h sample, indicating that the optimal oxygen deficiency widened the light absorption range of the original materials and thus improved the PEC activity. Although the oxygen deficiency significantly increased the photocurrent, an additional increase over a certain level had a negative effect. For the H2_30 min case, the W5+ signal intensities were higher than those of the Air_3 h case (Figure 4d), but the resulting electrode could not exhibit any PEC activity. This result implied that a high degree of oxygen deficiency increased the defect levels of the WO3 microplates, which could behave as recombination sites of photogenerated holes and electrons. Clearly, the degree of oxygen deficiency reached its optimum level for high efficiency during the PEC water oxidation reactions, which was readily achieved by annealing at 500 ºC for 3 h in air.

Enhancement of Electrode Stability by the Photodeposition of FeOOH Photocorrosion has commonly been observed on WO3 electrodes during photocurrent measurements in neutral conditions. This phenomenon indicates that the water oxidation reaction is not the only reaction occurring on the WO3 surface.14 To improve the stability, FeOOH electrocatalysts were deposited on the surface of MP1 (Air_3 h). The photodeposition of FeOOH was carried out in a 0.1 M FeSO4 aqueous solution using a three-electrode cell under an external bias of 0.3 V vs. Ag/AgCl for 20 min.33 Light was illuminated through the FTO side (back side) at an intensity level of 100 mW/cm2. Through this process, FeOOH layers were uniformly formed on the electrode surface while maintaining the original morphology of MP1 (Figure S12, Supporting Information). After the deposition step, the resulting MP1/FeOOH



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electrode showed a photocurrent nearly identical to that of the original MP1 electrode (Figure S13, Supporting Information). To assess the stability of the MP1/FeOOH electrodes, chronoamperometry was carried out at 0.3 V vs. Ag/AgCl in a 0.2 M phosphate buffer solution (pH 7) with 0.1 M Na2SO4 under UV-visible irradiation (Figure 7). After an initial drop, the photocurrent was stabilized at a steady-state value of 0.4 mA/cm2 at 0.3 V vs. Ag/AgCl. However, the photocurrent of the original MP1 electrode decayed rapidly under these conditions due to the intrinsic instability of WO3 during the measurement (Figures 7 and S14a). LSV was conducted after the stability test. The photocurrent of the MP1/FeOOH electrode was essentially identical before and after the stability test (Figure 8b), whereas the photocurrent of the original MP1 electrode (Air_3 h) decreased by 60 % at 3 h (Figures 8a and S14a). This instability of WO3 arises from the direct reaction between the electrolyte anions, including phosphates or sulfates, and the WO3 electrode surface under UV-visible irradiation. When a phosphate or sulfate solution was used, the evolution of oxygen and the formation of peroxo species were two major photo-oxidation reactions occurring on the WO3 surface due to its low valence band edge (3.0 eV vs. NHE).14,34 Accumulation of the peroxo species (S2O82-) on the WO3 surface is known to cause gradual photocurrent decay, and lead to the suppression of water oxidation.34 This happened on the MP1 electrode where the large decay of the photocurrent was observed (Figure S14a). To prevent the surface peroxo species, electrocatalysts such as Co-Pi were deposited on the WO3 electrode, which exhibited improved stability.14 Similarly in our experiment, the presence of the FeOOH electrocatalyst may suppress the formation of surface bound peroxo species and prevent the photochemical deactivation of the WO3 electrode.

4. Conclusion


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Single-crystalline WO3 microplates were directly synthesized on FTO substrates via a hydrothermal method followed by an optimized annealing procedure. The thickness of the WO3 microplates (470 nm) was in good agreement with the maximum hole diffusion length, indicating an optimized morphology for effective photogenerated charge separation. The resulting WO3 microplate electrode showed an improved photocurrent (1.9 mA/cm2 at 0.6 V vs. Ag/AgCl) for water oxidation compared to either a nanoparticle or bulk film electrode, particularly with significantly enhanced activity in the visible region. These enhanced levels of activity mainly originated from two factors: 1) the morphology of the single-crystalline domains with an optimum thickness, and 2) the optimized oxygen deficiency of WO3 due to the use of an appropriate annealing procedure. The photodeposition of the FeOOH electrocatalyst on the WO3 microplates greatly improved the stability of the electrodes for PEC water oxidation under a neutral condition. These results demonstrate the promise of WO3 microplates as an efficient and stable PEC water oxidation electrode by the optimization of the thickness and oxygen deficiency of the active materials and with the assistance of an electrocatalyst on the surface.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. The TEM and SEM images, UV-visible spectra, LSV and chronoamperometry, and the action spectra of the WO3 microplates and the WO3 nanoparticles and their electrodes are shown.

AUTHOR INFORMATION



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Corresponding Author *E-mail: [email protected]. *E-mail: [email protected].

ACKNOWLEDGMENT This work was supported by the Saudi-Aramco-KAIST CO2 Management Center. This work was also supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF2015R1C1A1A02037373, NRF-2015R1A2A2A01004196). ABBREVIATIONS PEC, photoelectrochemical; FTO, fluorine-doped tin oxide; SEM, scanning electron microscopy; TEM, transmission electron microscopy; SAED, selected-area electron diffraction; HRTEM, high-resolution TEM; XRD, X-ray diffraction; XPS, X-ray photoelectron spectroscopy; LSV, linear sweep voltammetry; TGA, thermogravimetric analysis; MP1, microplates 1; MP2, microplates 2.

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Figure 1. SEM images of (a) WO3 microplates (MP1, 470 nm), (b) WO3 microplates (MP2, 330 nm), and (c) WO3 nanoparticles (NP) on a FTO substrate.


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Figure 2. XRD patterns of WO3 microplate (MP1, 470 nm, red line), WO3 microplate (MP2, 330 nm, blue line), and WO3 nanoparticle (NP, black line) electrodes annealed at 500 ºC for 3 h in air.



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Figure 3. LSVs of WO3 microplate (MP1, MP2) and nanoparticle (NP) electrodes after annealing at 500 ºC for 3 h in air (a) under UV-visible illumination, and (b) under visible illumination (> 425 nm) in a phosphate buffer (pH 7). (c) Action spectrum at an applied potential of 0.5 V versus Ag/AgCl in a phosphate buffer (pH 7). Scan rate: 20 mV/s. Light intensity: 100 mW/cm2.


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Figure 4. XPS spectra of the MP1 electrode annealed at 500 ºC (a) for 10 min in air (Air_10 min), (b) for 3 h in air (Air_3 h), (c) for 3 h in an oxygen atmosphere (O2_3 h), and (d) for 30 min in a hydrogen atmosphere (H2_30 min).



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Figure 5. XRD patterns of MP1 electrodes under various annealing conditions.


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Figure 6. Photocurrent density of MP1 electrodes in various annealing conditions under UVvisible illumination at an applied potential of 0.6 V vs. Ag/AgCl in a phosphate buffer (pH 7). Scan rate: 20 mV/s. Light intensity: 100 mW/cm2.



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Figure 7. Current-time response curve of the MP1/FeOOH composite electrode (red line) and MP1 electrode without FeOOH (black line) at an applied potential of 0.3 V vs. Ag/AgCl under UV-visible illumination in a phosphate buffer (pH 7). Light intensity: 100 mW/cm2.


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Figure 8. LSVs of the MP1 (Air_3 h) and MP1/FeOOH electrodes after the stability test under UV-visible illumination in a phosphate buffer (pH 7). Scan rate: 20 mV/s. Light intensity: 100 mW/cm2.



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Table of Contents Mira Park, Jong Hyeok Seo, Hyunjoon Song*, and Ki Min Nam* Enhanced

Visible

Light

Activity

of

Single-Crystalline

WO3

Microplates

for

Photoelectrochemical Water Oxidation


Enhanced Visible Light Activity of Single-Crystalline WO3 Microplates

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