Double-Deck Inverse Opal Photoanodes - American Chemical Society

Sep 15, 2014 - Department of Chemical and Biomolecular Engineering, Sogang University, 1 Shinsu-dong, Seoul 121-742, Republic of Korea. •S Supportin...
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Double-Deck Inverse Opal Photoanodes: Efficient Light Absorption and Charge Separation in Heterojunction Ming Ma,†,⊥ Jung Kyu Kim,†,⊥ Kan Zhang,† Xinjian Shi,† Sung June Kim,† Jun Hyuk Moon,‡ and Jong Hyeok Park*,† †

SKKU Advanced Institute of Nanotechnology (SAINT) and School of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea ‡ Department of Chemical and Biomolecular Engineering, Sogang University, 1 Shinsu-dong, Seoul 121-742, Republic of Korea S Supporting Information *

ABSTRACT: For the first time, double-deck WO3/BiVO4 inverse opal photoanodes (DDIO-WO3/BiVO4) were prepared by swelling− shrinking mediated polystyrene template synthetic routes, and the use of the photoanodes in photoelectrochemical cells under simulated solar light was investigated. The double-deck photoanodes represented the compact interface between WO3 and BiVO4, inheriting the periodically ordered macroporous nanostructure. More significantly, the DDIOWO3/BiVO4 inverse opal photoanodes prepared from the optimized fabrication condition demonstrated a photocurrent that was ∼40 times higher than that of the pure inverse opal WO3 photoanodes at a bias of 1.23 V vs RHE. Even without an added catalyst, they produce an outstanding photocurrent density of ∼3.3 mA/cm2 at a bias of 1.23 V vs RHE, which profits from improving the poor charge carrier mobility of BiVO4 by combining it with a WO3 skeleton and a shrouded bilayer inverse opal structure with a large surface area and good contact with the electrolyte.

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INTRODUCTION Since photoelectrochemical (PEC) cells gained recognition through Fujishima and Honda’s report of titanium dioxide (TiO2) photoanodes for solar water splitting in 1972, this system has garnered much more attention for sustainable energy utilization and renewable fuel storage.1−5 However, TiO2 based PEC cells are limited by their low light absorption ability due to the relatively wide band-gap energy (∼3.2 eV), leading to only utilization of the UV region of solar light.6 For this technology to be commercially viable in the future, exploring new materials with broader solar light absorption or constructing a heterojunction configuration with the dual bandgap composite are two promising strategies for improving the solar-to-hydrogen (STH) conversion efficiency.3,7 Until now, many n-type metal oxide materials, such as TiO2, WO3, αFe2O3, and BiVO4, have been investigated for use in the PEC system because of their high photoelectrochemical stability, but they still suffer from low STH efficiency without using oxygen evolution catalysts.6,8−11 The dual band-gap heterojunction system with WO3 and BiVO4 is fascinating because it counterbalances the shortcomings of the individual components. The WO3 core has better electron transporting properties than the BiVO4 shell, © 2014 American Chemical Society

which can remedy the poor charge collection efficiency of BiVO4. Moreover, BiVO4 with a small band-gap energy can match better with the solar spectrum, and at the same time, excited electrons in the BiVO4 can efficiently transfer to the WO3, which can reduce the charge recombination in BiVO4. Because the roles of each component are varying, different nanostructures have to be designed for each individual component. For example, 1D nanostructures are more favorable for WO3 components in order to increase the electron transport ability.12 In addition, the vertically aligned nanostructure can increase the contact surface area between the photoanode and the electrolyte, thereby improving the water oxidation ability. In addition to the control of the nanostructure of the WO3 component, the control of the morphology and nanostructure of BiVO4 is another crucial factor for the design of efficient heterojunction photoelectrodes for PEC applications.13,14 Although BiVO4 has an excellent band-gap energy that is suitable for efficient solar light harvesting, the poor charge Received: June 6, 2014 Revised: September 11, 2014 Published: September 15, 2014 5592

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WO3 precursor solution in the swelled PS template. Then, the samples were stored at 100 °C for pyrolyzing the WO3 precursor to an amorphous WO3 frame and shrinking the PS spheres to their original size, which will induce voids between the WO3 skeleton and the PS spheres. The remaining voids were further filled with the BiVO4 precursor solution, and after thermal annealing, a BiVO4 crystal layer was uniformly coated onto the WO3 crystal skeleton with DDIO-WO3/BiVO4, and the PS spheres were removed (Figure S1, Supporting Information).

separation and charge transport ability are intrinsic shortcomings of BiVO4. Therefore, the optimization and manipulation of BiVO4 on WO3 in the heterojunction photoanodes is crucial for improving the STH conversion efficiency. However, in most BiVO4/WO3 heterojunction photoanodes, BiVO4 films were randomly coated on a WO3 host film by using sol−gel spin-coating or the drop-casting method,6,12 which induce irregular particle growth and aggregation, thereby diminishing the positive effects of the heterojunction photoanode system. As far as we know, there is no report on systematic approaches to control the BiVO4 growth on a WO3 host for the uniform film thickness. Here, we demonstrate the uniform growth of BiVO4 on WO3 host material by using a newly designed synthetic scheme. In this study, we adapted periodically ordered macroporous nanostructures, which have attracted a lot of attention as the architecture of photoelectrodes in solar or PEC cells,14−20 because of the photonic-crystal-based optical coupling effects and highly porous nanostructures to increase light harvesting.16,17 Recently, numerous semiconducting materials, such as TiO2,14 ZnO,18 WO3,19 Fe2O3,9 and BiVO4,20 with periodically ordered macroporous nanostructures have been tried in an attempt to maximize the efficiency of both light harvesting and STH conversion. Unfortunately, to the best of our knowledge, it has still remained a challenge to prepare a fully integrated double or multiple component heterojunction with periodically ordered macroporous nanostructures. Herein, a succinct crack permeated method was successfully employed to fabricate the double-deck WO3/BiVO4 inverse opal (DDIO-WO3/BiVO4) photoanodes for PEC cells using two steps, as illustrated in Figure 1. The double-deck inverse opal nanostructure, which leads to full integration with a large area of the interface between the BiVO4 and WO3 layers, can optimize the charge transfer efficiency across their interface.21,22 As shown in Figure 1, the monodisperse polystyrene (PS) opalline scaffold film was swelled via a methanol solvent, followed by the infiltration of a

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RESULTS AND DISCUSSION The top-view scanning electron microscopy (SEM) image of the PS templates showed the compactly arranged PS spheres with diameters of ∼500 nm (Figure 2A). The swelled PS template was obtained by immersing it into methanol for 1 h, and then a WO3 precursor solution was infiltrated into the swelled PS template. As shown in Figure 2B,C, because of the different shrinkage ratios of PS nanobeads and the infiltrated WO3 during the first drying step at 100 °C, the WO3/PS

Figure 2. FE-SEM images of (A) PS templates, (B) WO3/PS templates (top-view), (C) WO3/PS templates (cross-sectional view), (D) IO-WO3 (top-view), (E) DDIO-WO3/BiVO4 (top-view), (F) conventional IO-WO3/BiVO4 from drop-casting of BiVO4 precursor solution on IO-WO3, (G) DDIO-WO3/BiVO4 (cross-sectional view), and (H) conventional IO-WO3/BiVO4 (cross-sectional view). In these figures, DDIO-WO 3 /BiVO 4 is the DDIO-WO 3 /40 μL-BiVO 4 electrode.

Figure 1. Schematic diagram of the fabrication procedures for the DDIO-WO3/BiVO4 photoanodes and PEC cells with the DDIOWO3/BiVO4 photoanodes. 5593

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BiVO4 amounts were analyzed by X-ray diffraction(XRD) using Cu Kα radiation and were compared with the pure WO3 inverse opal anode (Figure 4A). For comparison purposes, the

templates can have enough additional voids to be infiltrated further with the second BiVO4 precursors, which results in a DDIO-WO3/BiVO4 photoelectrode consisting of a core/shell morphology. After the infiltration of the second BiVO4 precursor solution, DDIO-WO3/BiVO4 photoanodes were obtained by removing the PS colloid spheres at 500 °C for 6 h, as shown in Figure 2E. For comparison, conventional WO3 inverse opal (IO-WO3) photoanodes were obtained by directly annealing a WO3/PS template at 500 °C for 6 h, as shown in Figure 2D. The topview SEM images of WO3 and DDIO-WO3/BiVO4 demonstrated that these macropores in the top layer connect to the corresponding macropores in the sublayer through the necks. On the contrary, when the BiVO4 precursor was directly dropped into the WO3 inverse opal template, which is the conventional method that has been used until now, the macroporous structures were destroyed due to the irregular growth of BiVO4 crystals inside the WO3 skeletons (Figure 2F). The regular and periodically ordered macroporous structures of DDIO-WO3/BiVO4 were further confirmed by their crosssectional SEM images, as shown in Figure 2G,H. Note that the well-organized double-deck inverse opal structures could be retained with varying amounts of the BiVO4 precursor, as shown in Figure S2 (Supporting Information). For comparison, the cross-sectional images of plain BiVO4 and plain WO3/ BiVO4 films by drop-casting with different BiVO4 amounts are shown in Figures S3 and S4 (Supporting Information). Obviously, the infiltration of the precursor solution of BiVO4 inside the voids with pore sizes less than 100 nm generated by the swelling−shrinking process of the WO3/PS template can result in the uniform coating of BiVO4 crystals on WO3 skeletons. The morphologies of DDIO-WO3/BiVO4 were investigated further using transmission electron microscopy (TEM) (Figure 3A,B), which delivered a more detailed view of the BiVO4/

Figure 4. (A) XRD patterns of IO-WO3 and various DDIO-WO3/ BiVO4. (B) Photocurrent−time (I−t) curves of IO-WO3, IO-WO3/40 μL-BiVO4, and DDIO-WO3/40 μL-BiVO4 at 1.23 V (vs RHE) under chopped AM 1.5G light illumination. (C) Chopped photocurrent densities as a function of applying potential (vs RHE) under chopped AM 1.5G light illumination. (D) IPCE spectra of the IO-WO3, IOWO3/40 μL-BiVO4, and DDIO-WO3/40 μL-BiVO4 at 1.23 V (vs RHE).

XRD pattern of the pure BiVO4 film obtained by dropping the BiVO4 precursor onto the FTO glass is shown in Figure S5 (Supporting Information). The diffraction peaks of the inverse opal WO3 skeleton were verified to correspond to the monoclinic phase (JCPDS No. 43-1035), and those of the pure BiVO4 film were indexed to a uniform scheelitemonoclinic structure (JCPDS 14-0688). From the XRD patterns, the typical 002, 020, 200, and 220 planes of monoclinic WO3 were obtained at around 23.1°, 23.6°, 24.0°, and 33.4°, respectively; the typical 121 and 040 planes of scheelite-monoclinic BiVO4 were observed at around 29.0° and 30.5°, respectively. These indicated that the DDIO-WO3/ BiVO4 photoanodes consist of monoclinic WO3 and scheelitemonoclinic BiVO4, and the peak intensities of 29.0° and 30.5° increased with increasing amounts of BiVO4. PEC experiments were carried out to assess the double-deck effect on the photocurrent generation of the WO3/BiVO4 composite electrodes, which were conducted using a threeelectrode system in a 0.5 M Na2SO4 electrolyte. The typical photocurrent−time (I−t) curves at 1.23 V (vs RHE) under chopped AM 1.5G light illumination are shown in Figure 4B and Figure S6B (Supporting Information), and the linear sweep voltammograms (LSVs) were measured for all of the photoanodes under chopped AM 1.5G light illumination conditions (Figure 4C and Figure S6A). The photocurrent density of the DDIO-WO3/40 μL-BiVO4 electrode as the optimized condition (see the Experimental Section) was significantly improved by ∼40 times compared to that of the pure WO3 inverse opal, which reached ∼3.3 mA/cm2 at 1.23 V vs RHE, while that of the pure WO3 inverse opal photoanode was about 0.08 mA/cm2. On the contrary, the maximum photocurrent density of IO-WO3/BiVO4, which was preapred from the conventional method, was ∼1.5 mA/cm2 at 1.23 V vs

Figure 3. High-magnification (A) and low-magnification (B) TEM images, selected-area HR-TEM image (C), and element mapping (D) of the DDIO-WO3/BiVO4. In these figures, DDIO-WO3/BiVO4 is the DDIO-WO3/40 μL-BiVO4 electrode.

WO3 opal surface, and clearly showed the macroporous structures. The high-resolution TEM image in Figure 3C indicated crystalline WO3 and BiVO4 with the (002) crystalline plane of monoclinic WO3 and the crystallographic planes of (121) of monoclinic BiVO4, respectively, suggesting doubledeck WO3 and BiVO4 layers with a compact interface. This result can be further confirmed by the elemental mapping images (Figure 3D), where the homogeneous distribution of W, V, Bi, and O elements in the entire range was observed. Crystalline structures of the DDIO-WO3/BiVO4 with different 5594

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RHE (Figure S6C,D). As a reference, the photocurrent densities of the plain BiVO4 and plain WO3/BiVO4 photoanodes with different BiVO4 amounts also are shown in Figure S7 (Supporting Information). Note that the optimized sample has the highest photocurrent density ever achieved from a WO3/BiVO4 heterojunction system. Interestingly, this value was achieved without using any additional oxygen evolution catalyst, such as Pt or Co-Pi. The incident photon-to-current efficiency (IPCE) analyses (Figure 4D) were conducted for the pure WO3, IO-WO3/40 μL-BiVO4, and DDIO-WO3/40 μLBiVO4 photoanodes at 1.23 V vs RHE. The enhanced IPCE values and expanded functional wavelength region for the DDIO-WO3/40 μL-BiVO4 electrode indicated that the formation of the ordered inverse opal heterojunction with a solid interface and the optimal component ratio effectively improve light absorption and charge separation in WO3/BiVO4 photoanodes, resulting in excellent solar energy conversion efficiency. First, well-ordered spherical voids can act as light scattering centers in the photoanodes, which were realized with PS particles, thus enhancing the light harvesting. We can observe that the DDIO-WO3/40 μL-BiVO4 electrode has excellent IPCE values between 400 and 500 nm. One of the powerful advantages of inverse opal nanostructures is their outstanding light scattering ability. In the case of photoanodes where the light scattering system consists of large particles or pores, which have the sizes comparable to the wavelength of the incident light, Mie scattering theory is an applicable theoretical description. To investigate how the inverse opal skeleton can help the light absorption of BiVO4 shells, the scattering power versus scattering angle was simulated, based on the Mie theory.23,24 The cross section of Mie scattering is given by eq 1, and the Mie scattering efficiency, Qscat,Mie, can be simplified as shown in eq 2 δscat =

Q scat =

λ2 2π

Figure 5. (A) Simulated incident light scattering intensity for WO3/ BiVO4 anodes under 400 and 500 nm of wavelength. Diffuse reflectance curves (B), transmittance spectra (C), and UV−vis absorption spectra (D) of DDIO-WO3/40 μL-BiVO4 and plainWO3/40 μL-BiVO4.

5B,C. In general, the light absorption can be expressed as 1 − A(reflectivity) − R(transmittance). It can be clearly seen that inverse opal electrodes represented low light reflectance and transmittance values compared to the plain sample, which can confirm the superior light absorption ability of the macroporous nanostructures (Figure 5D). The enhanced IPCE values for the double-deck WO3/40 μLBiVO4 inverse opal electrode indicated that the formation of the heterojunction with a solid interface and the optimal component ratio maintaining the ordered nanostructure effectively improve the photoinduced charge carrier separation, resulting in excellent solar energy conversion efficiency. Presumably, the high photocurrent density of the doubledeck WO3/BiVO4 inverse opal originates from the uniform film thickness of BiVO4 and the high contact surface area between BiVO4 and WO3. The DDIO-WO3/40 μL-BiVO4 electrode exhibited superior photocurrent compared to either conventional IO-WO3/BiVO4 electrodes or other double-deck inverse opal electrodes, which is the optimum condition for obtaining the maximum synergistic effects of WO3 and BiVO4. In morphological terms as mentioned above, the ordered inverse opal structure could be maintained until 40 μL of BiVO4 has been deposited. However, if 40 μL is exceeded, the morphology of the double-deck WO3/BiVO4 inverse opal electrode starts to be disordered slightly due to the excess coating of BiVO4, resulting in a larger possibility of charge recombination due to the incompetent interface not only between WO3 and BiVO4 but also between a photoanode and an electrolyte. The electrochemical impedance spectroscopy (EIS) was also carried out (Figure 6A, B) under illumination at a bias potential of 1.23 V vs RHE, which is further evidence for the efficient charge transfer in the DDIO-WO3/40 μL-BiVO4 photoanode. As the arc diameter in the Nyquist plot is related to the interface charge transfer,28 the smaller arc diameter in the Nyquist plot of various DDIO-WO3/BiVO4 photoanodes indicated the promoted generation of electrons and superior charge transfer at the heterojunction interface compared to the conventional BiVO4 coated WO3 inverse opal photoanode. In addition, the surface area factor for our samples could influence the Nyquist plot of impedance analysis. Therefore, the surface

∞

∑ (2n + 1)(|an|2 − |bn|2 ) n=0

(1)

δscat πr 2

(2)

where the parameters |an| and |bn| are defined by the Riccati− Bessel functions Ψ and ξ.24 Here, one single cavity of water in the IO-WO3 matrix was considered, where the radius of the cavity was set to 230 nm and the refractive index of the DDIOWO3/BiVO4 matrix was set to 2.2 (average value of WO3 and BiVO4). The intensity was obtained upon exposure of this cavity to the incident light (unpolarized) with the intensity of 1 W/m2. The scattering powers of nonporous films were also calculated for the comparison. The scattered wave from each cavity may induce interferences. However, the scattering by multi cavities or particles have been predicted by simple superposition of each scatter qualitatively.25−27 As can be seen in Figure 5A, much stronger light scattering within the DDIOWO3/40 μL-BiVO4 anode compared to that of nonporous anodes was observed, which can enhance the solar light capture inside the PEC device. Solar light reflected this way will bounce back and forth as it travels down through the photoanodes (named increased optical film thickness), which can increase light absorption of the photoadnode. To further confirm this enhancing effect, both UV−vis diffuse reflection and transmittance spectra of plain- and DDIO-WO3/40 μL-BiVO4 were measured, as shown in Figure 5595

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EXPERIMENTAL SECTION

Preparation of PS Colloidal Films on FTO. At first, we prepared polystyrene (PS) colloids by using the emulsifier-free emulsion polymerization method. The 500 nm sized PS colloidal particle was controlled by setting the synthesis temperature, the composition of the reaction mixture, and the type of solvent used.31,32 In this PS colloid fabrication process, a styrene (Showa) monomer, potassium persulfate (Aldrich) initiator, potassium bicarbonate (Sigma) buffer, and divinylbenzene (Aldrich) comonomer were used as the starting materials.33−35 After centrifugation and cleaning with deionized (DI) water, the PS colloidal particles were dispersed in DI water at a certain concentration. In addition, the 1.5 cm × 1.5 cm sized fluorine-doped tin oxide (FTO, TEC-8, Pilkilton) glass substrates were cleaned by immersion in an ethanol and acetone mixture solution (1:1 by vol %) with sonication for 10 min and were then put into a miscible liquor of sulfuric acid and hydrogen peroxide (7:3 by vol %) for 10 min to make their surfaces hydrophilic. The surface-treated FTO glasses were immersed into 20 mL of a 0.15 wt % PS colloidal suspension by leaning them on the walls of Petri dishes, which were placed in an oven at 70 °C for 2 days to get the PS films. Preparation of WO3 and BiVO4 Precursor Solutions. The precursor solution used for the deposition of WO3 was composed of peroxy-tungstic acid and 2-propanol (IPA, Aldrich) as an organic stabilizer. The peroxy-tungstate precursor was obtained by dissolving 0.9 g of tungsten powder (Acros) in 5 mL of hydrogen peroxide (50% in water, Aldrich), which was added into 20 mL of IPA to form a complex of tungsten oxoanions.10 The BiVO4 precursor solution was prepared from stoichiometric bismuth nitrate hexahydrate (0.173 g, BiN3O9·6H2O, Aldrich) and vanadylacetylacetonate (0.095 g, Aldrich) in a 5 mL mixed solvent of 1:0.12 acetylacetone (Fluka) and acetic acid (Aldrich).36 After 10 min of sonication, the dark green solution was obtained without sedimentation. Fabrication of WO3/BiVO4 Photoanodes. At first, 15 μL of the WO3 precursor was dropped onto the PS template, which was immersed into the methanol solvent for 1 h to induce the swelling of the PS template. The samples were then heated at 100 °C in order to pyrolyze the WO3 precursor into a WO3 monoclinic crystal and to shrink the PS spheres to a smaller size. The WO3 skeleton and PS particles were interspaced in a WO3/PS template after the swelling− shrinking process. The BiVO4 precursor solution was slowly dropped onto the prepared WO3/PS templates and then maintained for 30 min to allow the solution to penetrate into the cracks between the WO3 skeleton and the PS spheres. Note that we used three different amounts of the BiVO4 precursor solutions: 20, 40, and 60 μL. After the BiVO4 precursor solution penetrated into the cracks absolutely, the double-deck WO3/BiVO4 inverse opal (DDIO-WO3/BiVO4) photoanodes were obtained by removing the PS colloid spheres at 500 °C for 6 h. The WO3 inverse opal (IO-WO3) photoanodes were obtained by directly annealing the WO3/PS template at 500 °C for 6 h. After drop-casting of 20, 40, and 60 μL of BiVO4 precursor onto the IOWO3, the conventional IO-WO3/BiVO4 photoanodes could be obtained through annealing at 500 °C for 6 h. As doing the same thermal treatment one by one, plain WO3 and plain WO3/BiVO4 photoanodes can be prepared by using 15 μL of WO3 precursor and 20, 40, and 60 μL of BiVO4 precursor with the drop-casting method. Characterization. The morphologies of the samples were observed by field emission scanning electron microscopy (FESEM, JSM-7000F, Japan). The thermogravimetric analysis (TGA) was conducted by using a Seiko Exstar 6000 in an air atmosphere and at a heating rate of 10 °C min−1. The XRD measurements were carried out with a Siemens diffractometer D500/5000 in a Bragg−Brentano geometry under Cu Kα radiation. The transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) observations were performed on a JEOL JEM-2100F (Japan) electron microscope. Photoelectrochemical Measurements. PEC measurements were performed in a conventional three-electrode system with the photoanodes as the working electrode, a Ag/AgCl (3 M NaCl) reference electrode, and a Pt foil as the counter electrode by using a

Figure 6. Nyquist plots of (A) IO-WO3/BiVO4 and (B) DDIO-WO3/ BiVO4 at 1.23 V (vs RHE).

area and charge transport resistance values were quantified from the impedance results by using Z-Viewer software. We plotted the calculated lines with the experimental results and made a table for these values (Table S1, Supporting Information). As a result, the surface area of DDIO-WO3/ BiOV4 was slightly larger than that of IO-WO3/BiVO4 when it came to the same amount of BiVO4 (40 μL); however, the charge transport resistance of DDIO-WO3/BiOV4 (371.1 Ω) was much lower than that of IO-WO3/BiVO4 (816.8 Ω). The CPE-T values, strongly related to the surface area, were 1.64 × 10−4 F for DDIO-WO3/BiOV4 and 1.97 × 10−7 F for IO-WO3/ BiVO4.29,30 The long-term stability of DDIO-WO3/BiVO4 photoanodes was tested by obtaining a J−t curve. A photocurrent density of ∼3 mA/cm2, obtained by applying 1.23 V vs RHE, was maintained for 2 h without showing any sign of decay, proving its favorable stability (Figure S8A, Supporting Information). For testing the mechanical stability of our optimized sample, the morphology after using more than 10 times was measured through SEM (Figure S8B). In this study, H2 and O2 production at the Co-Pi catalyst modified photoanode and Pt counter electrode was also detected with gas chromatography at 1.2 V vs Pt (Figure S9A, Supporting Information).14 As a comparison, the current density of the optimized anode with the Co-Pi catalyst is shown in Figure S9B. Because the optimization of the catalyst in photoanodes is out of scope of this study, further improvement of the cell efficiency is expected when various strategies of tuning compositions of oxygen evolution catalysts for DDIO-WO3/BiVO4 photoanodes are utilized.

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CONCLUSION In summary, compact double-deck heterojunction (WO3/ BiVO4) photoanodes with an inverse opal nanostructure were successfully fabricated by controlling the PS template using a swelling−shrinking process. The critical advance in this work is the use of controlled growth of BiVO4 inside a confined geometry, which was generated by the swelling−shrinking process of the PS soft template. The controlled growth of BiVO4 from the sol−gel method, resulting in the uniform distribution of BiVO4 on a WO3 skeleton, could overcome the intrinsically poor charge transport properties of BiVO4 without compromising the light absorption. The optimum amount of BiVO4 coated on the WO3 skeleton leads to efficient charge generation and separation, thereby improving the photocurrent density remarkably, which is ∼40 times that of the pure WO3 inverse opal anodes. We demonstrate a photoanode that achieves the highest reported photocurrent density (3.3 mA/ cm2 @ 1.23 V vs RHE) among WO3/BiVO4 heterojunction photoanodes without using any additional oxygen evolution catalysts and doping atoms. 5596

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potentiostat (CH Instruments, CHI 660) under 1 sun illumination. The photoanodes were painted with silver paste on the top in order to increase the conductivity, and the contact area with the electrolyte was determined by masking an aperture in order to evaluate the photo- and dark current precisely.33,34 The photocurrent characteristics of the samples were measured in 0.5 M Na2SO4 (pH = 6.8) from the illumination under AM 1.5G simulated sunlight with 100 mW cm−2 using a solar simulator for the irradiation (PEC-L01, PECCELL) calibrated with a standard Si solar cell. The electrochemical impedance spectra (EIS) were measured in potentiostatic mode with an ac voltage amplitude of 5 mV and a frequency range of 0.1−100 kHz under AM 1.5G illumination. A silicon reference cell (Fraunhofer ISE, Certificate No. C-ISE269) was used to calibrate the light intensity. The incident photon-to-current efficiency (IPCE) data were obtained using a monochromator (Polaronix K3100 IPCE Measurement System, McScience) with a 300 W xenon light source.

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(16) Chow, E.; Lin, S. Y.; Johnson, S. G.; Villeneuve, P. R.; Joannopoulos, J. D.; Wendt, J. R.; Vawter, G. A.; Zubrzycki, W.; Hou, H.; Alleman, A. Nature 2000, 407, 983. (17) Chen, X.; Ye, J.; Ouyang, S.; Kako, T.; Li, Z.; Zou, Z. ACS Nano 2011, 5, 4310. (18) Juárez, B. H.; García, P. D.; Golmayo, D.; Blanco, A.; López, C. Adv. Mater. 2005, 17, 2761. (19) Kim, J. K.; Moon, J. H.; Lee, T. W.; Park, J. H. Chem. Commun. 2012, 48, 11939. (20) Zhou, M.; Wu, H. B.; Bao, J.; Liang, L.; Lou, X. W.; Xie, Y. Angew. Chem., Int. Ed. 2013, 52, 8579. (21) Shiyanovskaya, I.; Hepel, M. J. Electrochem. Soc. 1999, 146, 243. (22) Pilli, S. K.; Duetsch, T. G.; Furtak, T. E.; Brown, L. D.; Turner, J. A.; Herring, A. M. Phys. Chem. Chem. Phys. 2013, 15, 3273. (23) Ishimaru, A. Wave Propagation and Scattering in Random Media; Academic Press: New York, 1978. (24) Zhang, Q. F.; Myers, D.; Lan, J. L.; Jenekhe, S. A.; Cao, G. Z. Phys. Chem. Chem. Phys. 2012, 14, 14982. (25) Yang, Q.; Li, M.; Liu, J.; Shen, W.; Ye, C.; Shi, X.; Jiang, L.; Song, Y. J. Mater. Chem. A 2013, 1, 541. (26) Ha, S. J.; Moon, J. H. Sci. Rep. 2014, 4, 5375. (27) Durrant, J. R. Philos. Trans. R. Soc. A 2013, 371, 20120195. (28) Walter, G. W. Corros. Sci. 1986, 26, 681. (29) Hu, Y.; Yella, A.; Guldin, S.; Schreier, M.; Stellacci, F.; Gratzel, M.; Stefik, M. Adv. Energy Mater. 2014, 1400510. (30) Dao, V. D.; Kim, S. H.; Choi, H. S.; Kim, J. H.; Park, H. O.; Lee, J. K. J. Phys. Chem. C 2011, 115, 25529. (31) Morrison, S. R.; Freund, T. J. Chem. Phys. 1967, 47, 1543. (32) Goodwin, J. W.; Hearn, J.; Ho, C. C.; Ottewill, R. H. Colloid Polym. Sci. 1974, 252, 464. (33) Shi, X.; Zhang, K.; Park, J. H. Int. J. Hydrogen Energy 2013, 38, 12725. (34) Zhang, K.; Shi, X.; Kim, J. K.; Lee, J. S.; Park, J. H. Nanoscale 2013, 5, 1939. (35) Moon, J. H.; Kim, S.; Yi, G.-R.; Lee, Y.-H.; Yang, S.-M. Langmuir 2004, 20, 2033. (36) Zhong, D. K.; Choi, S.; Gamelin, D. R. J. Am. Chem. Soc. 2011, 133, 18370.

ASSOCIATED CONTENT

S Supporting Information *

Additional TGA, SEM, XRD, photocurrent and gas evolution data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.H.P.). Author Contributions ⊥

M.M. and J.K.K. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NRF of Korea Grant funded by the Ministry of Science, ICT, and Future Planning (NRF2013R1A2A1A09014038, 2009-0083540).

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dx.doi.org/10.1021/cm502073d | Chem. Mater. 2014, 26, 5592−5597