Enhanced Photocatalytic Performance Depending on Morphology of

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Enhanced Photocatalytic Performance Depending on Morphology of Bismuth Vanadate Thin Film Synthesized by Pulsed Laser Deposition Sang Yun Jeong,† Kyoung Soon Choi,‡ Hye-Min Shin,† Taemin Ludvic Kim,§ Jaesun Song,† Sejun Yoon,† Ho Won Jang,§ Myung-Han Yoon,† Cheolho Jeon,‡ Jouhahn Lee,‡ and Sanghan Lee*,† †

School of Materials Science and Engineering, Gwangju Institute of Science and Technology, 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61005, Republic of Korea ‡ The Advanced Nano Surface Research Group, Korea Basic Science Institute, 169-148 Gwahak-ro, Yuseong-gu, Daejeon 34133, Republic of Korea § Department of Materials Science and Engineering, Research Institute of Advanced Materials, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea S Supporting Information *

ABSTRACT: We have fabricated high quality bismuth vanadate (BiVO4) polycrystalline thin films as photoanodes by pulsed laser deposition (PLD) without a postannealing process. The structure of the grown films is the photocatalytically active phase of scheelite-monoclinic BiVO4 which was obtained by X-ray diffraction (XRD) analysis. The change of surface morphology for the BIVO4 thin films depending on growth temperature during synthesis has been observed by scanning electron microscopy (SEM), and its influence on water splitting performance was investigated. The current density of the BiVO4 film grown on a glass substrate covered with fluorine-doped tin oxide (FTO) at 230 °C was as high as 3.0 mA/cm2 at 1.23 V versus the potential of the reversible hydrogen electrode (VRHE) under AM 1.5G illumination, which is the highest value so far in previously reported BiVO4 films grown by physical vapor deposition (PVD) methods. We expect that doping of transition metal or decoration of oxygen evolution catalyst (OEC) in our BiVO4 film might further enhance the performance. KEYWORDS: bismuth vanadate, oxide photoanode, pulsed laser deposition, physical vapor deposition, water-splitting solar cell

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phase was formed.6,7 BiVO4 thin films have been synthesized by numerous methods such as spray pyrolysis,8,9 metal−organic decomposition,3,10 electrodeposition,11−13 chemical vapor deposition,14 and physical vapor deposition including evaporation,15 molecular beam epitaxy,16 reactive sputtering,17 and pulsed laser deposition (PLD).18,19 PLD is a well-known physical vapor deposition technique which has advantages in deposition of pure and high crystalline film since an organic-free ceramic target is used. Besides, the use of the BiVO4 target which can vary the ratio of Bi and V elements makes compositional control of film by PLD easier than by other physical vapor deposition (PVD) methods.15−17 In recent years, Rettie and his co-workers first reported on the growth of BiVO4 film by PLD,18 and very recently, Chien et al. also demonstrated the pulsed laser deposition of the Au-BiVO4 heterostructure.19 However, despite the advantages of PLD growth mentioned above, PEC performances of their devices were far below that of BiVO4 fabricated by other methods.8−17

INTRODUCTION The photoelectrochemical (PEC) water splitting cell is considered as a promising energy-harvesting technology because of its unique characteristic that solar energy can be directly converted into chemical fuels. Considering that the water photoelectrolysis is conducted in the aqueous solution containing organic additives, the oxide semiconductor is a promising candidate for active materials owing to its resistance to most chemicals.1 However, the conversion efficiency of the oxide-based PEC cell is poor since most oxides have large band gap, and thus light absorption is limited to the UV region. Furthermore, the high electrical resistance of oxide causes carrier recombination in the bulk region of materials and at the interface between the electrode and electrolyte.1 To overcome forementioned issues, various oxides have been studied. For example, n-type semiconductors of bismuth vanadate (BiVO4),2,3 hematite (α-Fe2O3),4 and tungsten oxide (WO3)5 are used as photoanodes in the PEC cell because of their small band gap to absorb sunlight in the range of the visible region. Among those candidates, BiVO4 attracted much attention owing to its band gap of 2.4 eV and proper band edge position with respect to water redox potential when the monoclinic © XXXX American Chemical Society

Received: November 23, 2016 Accepted: December 14, 2016 Published: December 14, 2016 A

DOI: 10.1021/acsami.6b15034 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) X-ray diffraction θ−2θ scans of the polycrystalline monoclinic BiVO4 thin films on FTO glass. Asterisk signs (*) and triangle indicate signals from the FTO-coated glass substrate and vanadate phase, respectively. Vertical line is pattern for monoclinic BiVO4 (PDF# 83-1699). (b) UV−vis transmission spectra and (c) diffuse reflectance UV−vis absorption spectra of BiVO4 thin films on FTO glass. Inset: Tauc plot analysis of the data shown in (c). Temperatures given in the figure are process temperatures of each sample. sintering processes. The whole heating process was conducted in a box furnace under air ambient. After sintering, the target was cooled to room temperature naturally. In our experiment, 5% of bismuth was additionally added to the mixture in order to compensate for loss of the bismuth element during the sintering and calcination of the BiVO4 target as well as the deposition process in the PLD chamber. For the preparation of the films, KrF (248 nm) excimer laser source generated from Coherent Compex Pro 205F was used. F-doped tin oxide coated glasses (FTO glass, Pilkington, TEC 15) were used as substrates. FTO was sonicated successively in deionized water, acetone, and isopropyl alcohol followed by attaching to the sample holder using silver paste. The size of the substrates was typically 1 × 1.5 cm2. For PEC measurement of the sample, 1 × 0.5 cm2 of the FTO conductive layer was masked by a silicon wafer. After the FTOattached sample holder was loaded in the chamber, a single BiVO4 target was ablated and rotated during deposition in oxygen ambient. The oxygen partial pressure before deposition was 300 mTorr and was maintained during deposition. Energy density of the laser beam was 2 J/cm2, and the pulse repetition rate was 3 Hz. Based on a deposition rate of BiVO4 film at the given growth condition (∼0.23 Å/pulse), the thickness of the films was controlled by total number of pulses and was 140 nm, unless otherwise indicated. The substrate temperature was calibrated by a K-type thermocouple prior to deciding the process temperature. In order to control the morphology of the film, the process temperature was decreased from 530 to 130 °C, while other process variables were fixed. Finally, the sample was cooled to room temperature for 10 min under the oxygen partial pressure of 300 mTorr and then detached from the sample holder. Crystalline phase analysis was performed by an X-ray diffraction technique using Cu Kα radiation (λ = 1.5405 Å) with a Rigaku D/ MAX-2500 X-ray diffractometer. The diffractometer was operated at 40 kV and 100 mA for the measurement of polycrystalline BiVO4 film on FTO glass. The theta−2theta (θ−2θ) scan was measured between 10° and 55° with scan speed of 0.1°/s. Diffuse reflectance UV−vis absorption spectra were measured by a Varian Cary 500 Scan spectrometer. The surface morphology of the sample was confirmed by field-emission scanning electron microscopy (FESEM, Hitachi S4700).

Strategies for enhanced photocatalytic performance frequently include lowering the electrical resistance of BiVO4 by doping of a foreign element10,20 or modification of surface morphology for increased surface area.12,15,17 Considering the thermodynamics of film growth which depend on surface energy between the film and the substrate, a growth condition of PLD at low temperature is expected to satisfy a threedimensional island growth mode (the Volmer−Weber growth).21,22 Thus, we were motivated by an idea that a porous structure of BiVO4 thin films can be formed during the PLD process as a result of suppressed diffusion of synthesized BiVO4 nuclei at low temperature followed by grain growth at the nuclei surface. Hence, enhanced performance of the device is expected to be exhibited from the film with increased surface area as reported in many researches.12,15,23,24 In this study, we report on the deposition of high quality stoichiometric BiVO4 thin films by PLD at process temperature of as low as 230 °C. Also, the capabilities of facile morphology to control the film by simply controlling the process temperature during deposition are demonstrated. Moreover, compositional analysis is thoroughly investigated which is hardly found among previous reports on BiVO4. As a consequence of the morphology control of the highly crystallized polycrystalline BiVO4 film, enhancement of the device performances affected by the surface area is discussed.

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EXPERIMENTAL SECTION

The BiVO4 target was synthesized by a conventional solid-state method. Bismuth oxide (Bi2O3, High Purity Chemicals, Japan) and vanadium oxide (V2O5, High Purity Chemicals, Japan) powders were first dissolved in ethanol without further purification. The mixture was ball milled homogeneously and dried at 90 °C for 24 h. After, calcination was conducted at 760 °C for 4 h. Finally, the powder was pressed into a round pallet without any binder followed by sintering at 900 °C for 5 h. Heating rates were 2 °C/min for calcination and B


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Figure 2. SEM surface and cross-sectional images of BiVO4 films synthesized at (a, b) 230 °C, (c, d) 330 °C, and (e, f) 430 °C which show the microstructural changes of the BiVO4 grains. Cross-sectional SEM images of the BiVO4 films were tilted and measured at 30°. The underlying substrate was FTO-coated glass. photographs of each sample. Linear sweep voltammetry measurements were conducted using an Autolab PGSTAT302N potentiostat with a scan rate of 10 mV/s. For incident photon-to-current efficiency (IPCE) measurement, monochromatic light was obtained using a monochromator and light source. The IPCE spectra were measured at 1.23 VRHE (Ivium Technologies, Nstat).

Oxidation states of each element and chemical composition of the films were obtained using X-ray photoelectron spectroscopy (XPS). XPS was carried out of an AXIS Ultra DLD model (KRATOS, U.K.) operating at a base pressure of 5 × 10−10 mbar at 300 K with a monochromatic Al Kα line at 1486.69 eV. High-resolution spectra were measured within the spectral range of interest (ca. ±20 eV around the core level emission peaks) with 0.02 eV steps and 100 ms dwell time per data point. Survey and narrow XPS spectra scans were obtained with analyzer pass energies of 160 and 40 eV, respectively. Corrections due to charging effects were performed by the use of C 1s at 284.5 eV as an internal reference and the Fermi edge of a gold sample. Photoelectrochemical measurements were performed under an illuminated condition of 1 Sun (100 mW/cm2) using a 300 W Xe lamp (Newport) with an AM 1.5G filter (Newport). The light was illuminated on the front side (BiVO4) of the sample in a threeelectrode glass cell through a quartz window. In our configuration, the BiVO4 was used as a working electrode, while Pt and Ag/AgCl (saturated KCl) were employed as a counter electrode and a reference electrode, respectively. The potential applied on the BiVO4 electrode was measured by the Ag/AgCl (saturated KCl) and calculated to the potential versus the reversible hydrogen electrode (VRHE) through the following equation VRHE = VAg/AgCl + V 0 Ag/AgCl + 0.059 × pH

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RESULTS AND DISCUSSION The crystalline phase of BiVO4 thin films was determined by Xray diffraction patterns of the films synthesized at different temperatures. As shown in Figure 1a, polycrystalline monoclinic BiVO4 film was synthesized even at very low temperature of 230 °C without noticeable peaks of unwanted vanadate phase such as V2O5.11 The intensity increment of peaks implies that better crystallinity was obtained for the film synthesized at 430 °C by comparison with the case of 230 or 330 °C, and clearly, the XRD patterns exhibit that pure and highly crystallized BiVO4 films were allowed to form during the PLD process. At higher temperature of 530 °C, however, a secondary vanadate phase was formed (peak at 2θ of 12.1°, red triangle), while the film was amorphous at lower process temperature of 130 °C. Optoelectronic behavior of the BiVO4 thin film including visible-light absorption was measured by the diffuse reflectance UV−vis spectroscopy (Figure 1b and 1c). Note that, as shown in Figure 1c, the films synthesized at 230 and 330 °C clearly show the visible-light absorption corresponding to its indirect gap energy of 2.4 eV which is further confirmed by Tauc plot analysis (inset in Figure 1c). The results well match with the

(1)

where V Ag/AgCl is 0.197 V at 25 °C. Aqueous solutions of 0.5 M sodium sulfate (Na2SO4, ≥99%, Aldrich) with and without 0.5 M sodium sulfite (Na2SO3, ≥98%, Aldrich) were introduced as electrolyte. The samples were sealed with insulating epoxy and soaked in the aqueous electrolyte. The contact area of the BiVO4 with the electrolyte was measured from ruler-scaled 0

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Figure 3. X-ray photoelectron spectroscopy data of BiVO4 films. (a) Core level spectra and curve fitting results of Bi 4f, V 2p3/2, and O 1s. Based on fitting results, binding energies of individual peaks are presented in different colors as indicated in the figure. Temperatures given in (a) are process temperatures of each sample. (b) Variation of Bi (triangle), V (circle), and O (square) composition depending on the process temperatures obtained from the XPS analysis. (c) Trend of changes in the atomic portion of V2p5+ originated from the BiVO4 (circle) and V2O5 (square) within total number of the V versus the process temperatures.

voids are clearly distinguishable for the film synthesized at 230 °C, which shows the largest surface area. At the process temperature of 330 °C, the voids were tightened since diffusion of the synthesized BiVO4 was promoted, so that the film had larger grains and rather smooth surface morphology as shown in Figure 2c and 2d. Coalescence of grains significantly occurred at higher temperature of 430 °C and resulted in the formation of huge grain and exposure of the underlying FTO layer between them (Figure 2e and 2f). Obviously, the thickness of the film was increased to ∼400 nm as a result of the coalescence of the BiVO4 grains at 430 °C although total number of laser pulses was identical for all samples (see Figure S1 in Supporting Information). The oxidation states of Bi, V, and O elements and its compositional changes were thoroughly investigated using XPS analysis. Preservation of the Bi during synthesis is an important issue for Bi-containing materials such as BiVO4 since point defects of Bi vacancies can act as a carrier recombination

indirect gap energy of polycrystalline film reported by Chen et al.17 (∼2.4 eV) and indicate again that the monoclinic BiVO4 phase was synthesized as discussed in XRD results. After synthesis, the cooled films appear as yellow green color to the eye except the film synthesized at 130 °C which was amorphous as shown in Figure 1a. The light absorption of the film synthesized at 130 °C can be regarded as the absorption of high energy visible light (near UV) by amorphous film. In the case of the film synthesized at 430 °C, the film is still yellow green but is opaque which makes it hard to measure the absorption spectrum as shown in transmittance spectra of the samples (Figure 1b). Microstructural changes induced by the control of process temperature were analyzed by scanning electron microscopy. As shown in Figure 2a−2f, the grain size was increased, and concomitantly, the surface area was decreased with increasing the process temperature from 230 to 430 °C. In Figure 2a and 2b, vertically arranged individual BiVO4 grains separated by D


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Figure 4. Photoelectrochemical measurement data for the BiVO4 films synthesized at different process temperatures (a) with and (b) without Na2SO3 which were introduced as a hole scavenger. Inset: illustration of the surface morphology of the samples. (c) IPCE spectra of the samples measured in aqueous solution of 0.5 M Na2SO4 with 0.5 M Na2SO3. Inset: Calculated IPCE enhancement factor with respect to the IPCE spectrum for the film synthesized at 430 °C. The absorption edges of the monoclinic BiVO4 (∼517 nm) are expressed as a dashed line in (c). (d) Photoelectrochemical measurement data for the films with different thicknesses and deposition temperatures. The curves of the same color (red, blue, and olive) in (d) represent the performance of the films which were grown by the same number of laser pulses. Temperatures given in the figure are process temperatures of each sample. The electrolytes were 0.5 M Na2SO4 aqueous solution with and without 0.5 M Na2SO3. Analysis was conducted under AM 1.5G illumination (100 mW/cm2).

center.13 Moreover, the Bi vacancies induce the formation of the VxOy phase where x and y are determined by the oxidation state of the V. One of the problems caused by the formation of such a phase is reduction of the visible-light absorption of the device and thus its performance. In this point of view, XPS measurement was conducted so that impacts of the temperature control on chemical composition of BiVO4 were demonstrated. In order to conduct the componential and surface atomic analysis of fabricated samples, we have measured high-resolution XPS spectra with intervals of 0.02 eV. For the curve fitting of core level spectra, the chemical bonding states were simulated with Casa XPS software with a Shirley background and Gaussian−Lorentzian (70−30) line shapes. The results are given in Figure 3 and Figure S2. The signal of Sn in XPS result of the film synthesized at 430 °C (Figure S2a) was attributed to the Sn element in the FTO electrode which was exposed between the BiVO4 grains, as shown in Figure 2e. Figure 3a shows the core level spectra and the fitted curves of Bi 4f, V 2p3/2, and O 1s. In the XPS spectrum of the Bi 4f peak, there are two peaks which are corresponding to the Bi 4f7/2 and 4f5/2 with 5.3 eV of binding energy differences. The area ratio of Bi 4f7/2 to 4f5/2 is 4 to 3, indicating that only Bi3+ existed in all samples.17,25 The V 2p core levels were observed at binding energy range of 514−520 eV, and the curve fitting was conducted for the V 2p3/2 peaks. The V mainly existed as the oxidation state of V5+, as shown in the fitted curves. The V 2p5+ could be separated into two different peaks which are corresponding to V 2p5+ at V2O5 and the BiVO4 phase since the binding energy of V 2p5+ at the BiVO4 phase is lower than that of the V2O5 phase.26,27 Based on the fitted curves, we have calculated the atomic composition of fabricated samples. Even

though there is no change in atomic composition of V 2p, the atomic composition of Bi 4f drastically decreases with respect to the annealing temperature, as shown in Figure 3b. Moreover, as annealing temperature increases, the component of V 2p5+ at the BiVO4 phase decreased, and that of V 2p5+ at the V2O5 phase increased (Figure 3c). It is well consistent with the reduction of atomic composition of the Bi element. Figure S2b shows the difference of vanadium oxidation states and error range of each core line (V5+ 2p3/2 at V2O5: 517.7 ± 0.1 eV, V5+ 2p3/2 at BiVO4: 516.9 ± 0.1 eV, V4+ 2p3/2: 515.84 ± 0.2 eV).26,27 Interestingly, unlike the fitting results of XPS spectra, only the monoclinic BiVO4 phase was distinguished in the XRD patterns for the film synthesized at 430 °C (Figure 1a). Considering that photoelectrons are generated from the finite region of the film surface during the XPS analysis (a few nanometers in depth), this discrepancy can be regarded as follows: the secondary phase of V2O5 presented in very small amounts and was confined to the surface region of the film.11 Photocatalytic water splitting reactions of the samples were investigated by the PEC measurement (Figure 4). Onset potential, which is defined as the potential where the current density reaches 0.3 mA/cm2, and the current density of the samples were listed in Table S1. As the process temperature was decreased, the onset potentials showed negative shift from 0.6 VRHE to 0.38 VRHE for the film synthesized at 430 and 230 °C, respectively. Meanwhile, the current density was remarkably enhanced from 0.72 mA/cm2 to 3.0 mA/cm2 at 1.23 VRHE. However, as can be expected from the XRD results (Figure 1a), poor performances of devices were exhibited from the films synthesized at 530 and 130 °C (Figure 4a). The value is decently comparable to the best performance of the BiVO4 film E


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Figure 5. Illustration of effects of process temperature on (a−c) microstructural changes of synthesized film and (d−f) its charge collection abilities. Photoexcited electrons and holes in the vicinity of interfaces between BiVO4 and electrolyte are indicated as red and blue spheres, respectively. Schematic representation of carrier recombination is expressed as a yellow dotted circle.

reported by Kim et al.12 For clear comparison, the PEC performances of several BiVO4-based photoanodes and their synthesis conditions are summarized in Table S2. Note that the listed current densities were exhibited from a single layer of pristine BiVO4 thin films. The effect of hole scavenger (sulfite, Na2SO3) on PEC performance of the samples can be confirmed by comparing the data in Figure 4a and 4b. In the case of the film synthesized at 230 °C, the current density was improved from 0.75 mA/cm2 to 3.0 mA/cm2 at 1.23 VRHE without and with the scavenger, respectively. Since the photoexcited holes in the BiVO4 are more easily absorbed by the sulfite than the sulfate,12 it is obvious that the improvement is originated from the reduced carrier recombination at the interface between the BiVO4 and the electrolyte. The improvement of the performance is clearly represented in the IPCE spectra of the samples (Figure 4c and Figure S3a). At the wavelength of 400 nm, the IPCE is approaching 35% for the film synthesized at 230 °C which is 7 times higher than that of 430 °C (inset in Figure 4c). It is noteworthy that the efficiency rapidly rises below the absorption edge of the monoclinic BiVO4 (∼517 nm), and it is very consistent with the absorption spectra (Figure 1c). Overall, the results indicate that the impact of increased surface area, induced by the morphological change, on the photocatalytic activity is significant by comparing the PEC performances of the films and its morphologies (Figure 4a and Figure 2). Influences of other factors such as film thickness, grain size, and shape were also demonstrated so that we could verify the significance of the morphological change among them. The results are given in Figure 4d. It can be observed that the films synthesized at 230 °C exhibit the highest current density regardless of its thickness. Note that the grain shapes for the films synthesized at 230 °C are almost the same (Figure S1a and d). Under operation of photocatalytic measurement, the photoexited electrons generated in the BiVO4 grain flow directly to the bottom electrode due to the presence of the electric field. Therefore, the grain growth in the vertical direction perpendicular to the FTO electrode, i.e., the increase of film thickness, is responsible for the difference in the performance. As shown in Figure 4d and Figure S3, the thickness dependence of the J−V curve for each temperature implies that there is optimal thickness which is around 140 nm. The results can be inferred as an offset between the light

absorption and the carrier recombination at a certain thickness; the thicker film has longer penetration length of light than the thinner one so that more photons are absorbed, but at the same time, the carrier recombination rates are increased owing to the elongated pathway for carriers. The latter will be reinforced when the film becomes thicker than 70 nm which is the estimated minority carrier diffusion length of BiVO4.9 For clarifying the origin of the enhanced PEC performance, schematic representations of the film morphologies synthesized by the PLD at different process temperatures are given in Figure 5. The microstructural changes induced by the control of process temperature and its influence on charge collection abilities can be explained as follows. The film synthesized at the low process temperature consists of the BiVO4 grains separated by the voids owing to the suppressed diffusion of synthesized BiVO4 (Figure 5a). Since the voids can be formed near the FTO electrode, the photoelectrolysis of water, in other words, consumption of photoexcited holes for the photoanode can take place in the vicinity of the electrode (Figure 5d). It will be resulted in increased lifetime of photoexcited electrons and thus rise of the current density as shown in Figure 4a. When the process temperature is increased, the voids are disappeared by the intergrain diffusion (Figure 5b), and eventually the coalescence of the grains is significantly occurred (Figure 5c). Therefore, the performances of the films are reduced because of the decrease in surface area (Figure 5e and 5f) and, additionally, elongated diffusion length for the electrons from the surface of grains to the FTO electrode (Figure 5f). Moreover, in addition to the influence of the surface area, the significant loss of Bi at the surface of the grains (Figure 3b) should increase the carrier recombination rate as indicated in Figure 5f. Consequentially, the BiVO4 film synthesized at 230 °C exhibits the best performance, although the film synthesized at 430 °C shows the best crystalline quality. These results suggest that the increase in the interfacial area between the BiVO4 and the electrolyte governs the overall performance of the device.

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CONCLUSIONS The purpose of this study was to elucidate the relationship between the performance of PEC cell and its surface morphology. To accomplish the purpose, we have synthesized the pure and high quality polycrystalline monoclinic BiVO4 film F


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(6) Iwase, A.; Kudo, A. Photoelectrochemical Water Splitting Using Visible-Light-Responsive BiVO4 Fine Particles Prepared in an Aqueous Acetic Acid Solution. J. Mater. Chem. 2010, 20, 7536−7542. (7) Zhao, Z.; Li, Z.; Zou, Z. Electronic Structure and Optical Properties of Monoclinic Clinobisvanite BiVO4. Phys. Chem. Chem. Phys. 2011, 13, 4746−4753. (8) Abdi, F. F.; van de Krol, R. Nature and Light Dependence of Bulk Recombination in Co-Pi-Catalyzed BiVO4 Photoanodes. J. Phys. Chem. C 2012, 116, 9398−9404. (9) Abdi, F. F.; Savenije, T. J.; May, M. M.; Dam, B.; van de Krol, R. The Origin of Slow Carrier Transport in BiVO4 Thin Film Photoanodes: A Time-Resolved Microwave Conductivity Study. J. Phys. Chem. Lett. 2013, 4, 2752−2757. (10) Zhong, D. K.; Choi, S.; Gamelin, D. R. Near-Complete Suppression of Surface Recombination in Solar Photoelectrolysis by “Co-Pi” Catalyst-Modified W: BiVO4. J. Am. Chem. Soc. 2011, 133, 18370−18377. (11) Seabold, J. A.; Choi, K.-S. Efficient and Stable Photo-Oxidation of Water by a Bismuth Vanadate Photoanode Coupled with an Iron Oxyhydroxide Oxygen Evolution Catalyst. J. Am. Chem. Soc. 2012, 134, 2186−2192. (12) Kim, T. W.; Choi, K.-S. Nanoporous BiVO4 Photoanodes with Dual-Layer Oxygen Evolution Catalysts for Solar Water Splitting. Science 2014, 343, 990−994. (13) Xiao, S.; Chen, H.; Yang, Z.; Long, X.; Wang, Z.; Zhu, Z.; Qu, Y.; Yang, S. Origin of the Different Photoelectrochemical Performance of Mesoporous BiVO4 Photoanodes between the BiVO4 and the FTO Side Illumination. J. Phys. Chem. C 2015, 119, 23350−23357. (14) Alarcón-Lladó, E.; Chen, L.; Hettick, M.; Mashouf, N.; Lin, Y.; Javey, A.; Ager, J. W. BiVO4 Thin Film Photoanodes Grown by Chemical Vapor Deposition. Phys. Chem. Chem. Phys. 2014, 16, 1651− 1657. (15) Berglund, S. P.; Flaherty, D. W.; Hahn, N. T.; Bard, A. J.; Mullins, C. B. Photoelectrochemical Oxidation of Water Using Nanostructured BiVO4 Films. J. Phys. Chem. C 2011, 115, 3794−3802. (16) Stoughton, S.; Showak, M.; Mao, Q.; Koirala, P.; Hillsberry, D. A.; Sallis, S.; Kourkoutis, L. F.; Nguyen, K.; Piper, L. F. J.; Tenne, D. A.; Podraza, N. J.; Muller, D. A.; Adamo, C.; Schlom, D. G. Adsorption-Controlled Growth of BiVO4 by Molecular-Beam Epitaxy. APL Mater. 2013, 1, 042112. (17) Chen, L.; Alarcón-Lladó, E.; Hettick, M.; Sharp, I. D.; Lin, Y.; Javey, A.; Ager, J. W. Reactive Sputtering of Bismuth Vanadate Photoanodes for Solar Water Splitting. J. Phys. Chem. C 2013, 117, 21635−21642. (18) Rettie, A. J.; Mozaffari, S.; McDaniel, M. D.; Pearson, K. N.; Ekerdt, J. G.; Markert, J. T.; Mullins, C. B. Pulsed Laser Deposition of Epitaxial and Polycrystalline Bismuth Vanadate Thin Films. J. Phys. Chem. C 2014, 118, 26543−26550. (19) Chien, N. V.; Chang, W. S.; Chen, J.-W.; Tsai, K.-A.; Tzeng, W.Y.; Lin, Y.-C.; Kuo, H.-H.; Liu, H.-J.; Chang, K.-D.; Chou, W.-C.; Wu, C.-L.; Chen, Y.-L.; Luo, C.-W.; Hsu, Y.-J.; Chu, Y.-H. Heteroepitaxial Approach to Explore Charge Dynamics Across Au/BiVO4 Interface for Photoactivity Enhancement. Nano Energy 2015, 15, 625−633. (20) Luo, W.; Yang, Z.; Li, Z.; Zhang, J.; Liu, J.; Zhao, Z.; Wang, Z.; Yan, S.; Yu, T.; Zou, Z. Solar Hydrogen Generation from Seawater with a Modified BiVO4 Photoanode. Energy Environ. Sci. 2011, 4, 4046−4051. (21) Chrisey, D. B.; Hubler, G. K. Pulsed Laser Deposition of Thin Films, 1st ed; Wiley: New York, 1994. (22) Ashfold, M. N.; Claeyssens, F.; Fuge, G. M.; Henley, S. J. Pulsed Laser Ablation and Deposition of Thin Films. Chem. Soc. Rev. 2004, 33, 23−31. (23) Zhou, Y.; Vuille, K.; Heel, A.; Probst, B.; Kontic, R.; Patzke, G. R. An Inorganic Hydrothermal Route to Photocatalytically Active Bismuth Vanadate. Appl. Catal., A 2010, 375, 140−148. (24) Zhou, M.; Zhang, S.; Sun, Y.; Wu, C.; Wang, M.; Xie, Y. Coriented and {010} Facets Exposed BiVO4 Nanowall Films: TemplateFree Fabrication and Their Enhanced Photoelectrochemical Properties. Chem. - Asian J. 2010, 5, 2515−2523.

with different surface morphologies as confirmed by XRD, XPS, and SEM measurement. By comparing PEC measurement results of each cell, it is demonstrated that the entire performance of the device is dominated by the surface area of the film. As a result, the current density of the film synthesized at 230 °C exhibited significantly enhanced value of as high as 3.0 mA/cm2 at 1.23 VRHE. Further investigation on the doping of transition metal or the formation of heterojunction with OEC should be conducted to improve the performance of the device since the results of this study are obtained from a single layer of BiVO4. To realize those strategies, PLD will be a powerful methodology because the films of interest can be synthesized by means of using dopedtarget or multiple targets of BiVO4 and other catalytic materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b15034. SEM cross-sectional images, X-ray photoelectron spectroscopy spectra, and details of device performances (J− V curves and tables) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sanghan Lee: 0000-0002-5807-864X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program and Global Research Network Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, the Ministry of Science and ICT & Future Planning (NRF-2014R1A1A2053552, NRF2014S1A2A2028361), and by the GIST (Gwangju Institute of Science and Technology) Research Institute (GRI) Project through a grant provided by GIST in 2016. The authors acknowledge S.-M. Kim (Korea Photonics Technology Institute) for fruitful discussions and analyses on optoelectronic behavior of BiVO4 films.

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REFERENCES

(1) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446−6473. (2) Park, Y.; McDonald, K. J.; Choi, K.-S. Progress in Bismuth Vanadate Photoanodes for Use in Solar Water Oxidation. Chem. Soc. Rev. 2013, 42, 2321−2337. (3) Chen, Y.-S.; Manser, J. S.; Kamat, P. V. All Solution-Processed Lead Halide Perovskite-BiVO4 Tandem Assembly for Photolytic Solar Fuels Production. J. Am. Chem. Soc. 2015, 137, 974−981. (4) Zhong, D. K.; Cornuz, M.; Sivula, K.; Grätzel, M.; Gamelin, D. R. Photo-Assisted Electrodeposition of Cobalt−Phosphate (Co−Pi) Catalyst on Hematite Photoanodes for Solar Water Oxidation. Energy Environ. Sci. 2011, 4, 1759−1764. (5) Seabold, J. A.; Choi, K.-S. Effect of a Cobalt-Based Oxygen Evolution Catalyst on the Stability and the Selectivity of PhotoOxidation Reactions of a WO3 Photoanode. Chem. Mater. 2011, 23, 1105−1112. G


Research Article

ACS Applied Materials & Interfaces (25) National Institute of Standards and Technology, X-ray Photoelectron Spectroscopy Database. URL: http://srdata.nist.gov/ xps/ (accessed 3rd Dec, 2016). (26) Silversmit, G.; Depla, D.; Poelman, H.; Marin, G. B.; De Gryse, R. Determination of the V2p XPS Binding Energies for Different Vanadium Oxidation States (V5+ to V0+). J. Electron Spectrosc. Relat. Phenom. 2004, 135, 167−175. (27) Schuhl, Y.; Baussart, H.; Delobel, R.; Le Bras, M.; Leroy, J.-M.; Gengembre, L.; Grimblot, J. Study of Mixed-Oxide Catalysts Containing Bismuth, Vanadium and Antimony. Preparation, Phase Composition, Spectroscopic Characterization and Catalytic Oxidation of Propene. J. Chem. Soc., Faraday Trans. 1 1983, 79, 2055−2069.

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Enhanced Photocatalytic Performance Depending on Morphology of

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