Shell Structure

May 13, 2009 - Kudo, Akihiko; Miseki, Yugo. Chemical Society Reviews (2009), 38 (1), 253-278CODEN: CSRVBR ; ISSN:0306-0012. (Royal Society of ...
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J. Phys. Chem. C 2009, 113, 10151–10157

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Role and Function of Noble-Metal/Cr-Layer Core/Shell Structure Cocatalysts for Photocatalytic Overall Water Splitting Studied by Model Electrodes Masaaki Yoshida,† Kazuhiro Takanabe,† Kazuhiko Maeda,†,| Akio Ishikawa,† Jun Kubota,† Yoshihisa Sakata,‡ Yasunari Ikezawa,§ and Kazunari Domen*,† Department of Chemical System Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, Graduate School of Science and Engineering, Yamaguchi UniVersity, 2-16-1 Tokiwadai, Ube-shi, Yamaguchi 755-8611, Japan, and Department of Chemistry, Rikkyo UniVersity, 3-34-1 Nishi-ikebukuro, Toyoshima-ku, Tokyo 171-8501, Japan ReceiVed: February 16, 2009; ReVised Manuscript ReceiVed: April 14, 2009

The mechanism of hydrogen evolution by a core/shell noble-metal/Cr2O3 particulate as a highly efficient cocatalyst for overall water splitting under visible light using the photocatalyst (Ga1-xZnx)(N1-xOx) is investigated by electrochemical and in situ spectroscopic measurements of model electrodes. The electrodes are prepared by electrochemical deposition of 1.8-3.5 nm thick Cr2O3 films on Rh and Pt plates and are evaluated as model systems of Rh/Cr2O3 and Pt/Cr2O3 core/shell particulates, which have previously been applied effectively as cocatalysts for hydrogen evolution in this system. Proton adsorption/desorption and H2 evolution currents are observed for both the Cr2O3-coated and the bare electrodes, and the infrared absorption band due to Pt-H stretching (2039 cm-1) is apparent for both the coated and the bare electrodes. These observations indicate that the Cr2O3 layer does not interfere with proton reduction or hydrogen evolution and that proton reduction takes place at the Cr2O3/Pt interface. However, the reduction of oxygen to water is suppressed only in the Cr2O3-coated samples. The Cr2O3 layer is thus permeable to protons and the evolved hydrogen molecules, but not to oxygen. Cocatalyst modification by thin films with this type of functionality thus appears to be a useful strategy for improving the efficiency of photocatalytic overall water splitting. 1. Introduction Photocatalytic overall water splitting, whereby water (H2O) is split into the gaseous products H2 and O2 by irradiation of an aqueous solution containing a suitable photocatalyst powders, is a promising technology for the sustainable production of hydrogen gas as an energy carrier. Substantial efforts have been made in recent years to develop visible-light-driven photocatalysts for this reaction, which would allow the utilization of visible solar radiation for storable energy production.1 Our group has recently demonstrated that the solid solution (Ga1-xZnx)(N1-xOx) evolves both H2 and O2 from H2O at ambient temperature under irradiation at visible wavelengths (>400 nm) when loaded with a suitable cocatalyst.2,3 The cocatalyst is an essential component of this photocatalytic system, providing hydrogen evolution sites to the base catalyst.2,3 To improve the catalytic efficiency of this system, it is therefore necessary to refine both the base catalyst and the cocatalyst. Particles of noble metals (e.g., Pt, Rh, Au, Ag)4 or transitionmetal oxides (e.g., NiOx, RuO2, Rh2-yCryO3)2,3c-f,j,5-7 have been investigated as cocatalysts providing hydrogen evolution sites for photocatalytic overall water splitting. To achieve overall water splitting, the base catalyst must be loaded with a suitable cocatalyst that is active for hydrogen evolution (providing photogenerated electrons with small potential loss), without being active for the reduction of evolved oxygen back to water. * To whom correspondence should be addressed. Phone: +81-3-58411148. Fax: +81-3-5841-0236. E-mail: [email protected]. † The University of Tokyo. ‡ Yamaguchi University. § Rikkyo University. | Research Fellow of the Japan Society for the Promotion of Science (JSPS).

Noble metals provide efficient H2 evolution sites, yet also allow the back-reaction. Further research on cocatalysts is therefore essential in the development of an efficient photocatalytic system for overall water splitting under visible light. Our group has reported that noble-metal particles (Rh, Pt, Pd, Ir) coated with a thin layer of Cr2O3 to form a core/shell structure are excellent cocatalysts for the (Ga1-xZnx)(N1-xOx) system.3g,h Rh/Cr2O3 and Pt/Cr2O3 cocatalysts have been shown to substantially improve the photocatalytic activity of the (Ga1-xZnx)(N1-xOx) system for overall water splitting, while bare Pt and Rh metal particles do not work as efficient cocatalysts (Figure S1). The Cr2O3 cocatalyst on the (Ga1-xZnx)(N1-xOx) photocatalyst without Rh particulate is not active for overall water splitting,3h suggesting that the character of Cr2O3 semiconductor is not related to the photocatalytic activity for hydrogen evolution reaction. The Rh particulate fully covered with Cr2O3 shell was introduced as a means of suppressing the back-reaction of water formation from evolved H2 and O2 during the water splitting reaction.3h When Rh is partly covered with Cr2O3 shell, the rates of H2 and O2 evolutions are drastically decreased because of the back-reaction.3h It was tentatively suggested that the suppression was attributed to a tunneling mechanism that allows photogenerated electrons in the photocatalyst to migrate first to the metal core and then to the outer oxide surface prior to hydrogen evolution.3h In this Article, the characteristics of this core/shell noblemetal/Cr2O3 cocatalyst are examined in further detail by conducting a series of experiments using noble-metal electrodes coated with Cr2O3 as models of the core/shell system. Electrochemical measurements and in situ infrared reflection absorption spectroscopy (IRAS) reveal that the Cr2O3 layer is permeable to protons and hydrogen molecules. The function of the Cr2O3

10.1021/jp901418u CCC: $40.75  2009 American Chemical Society Published on Web 05/13/2009

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layer in the core/shell noble-metal/Cr2O3 cocatalyst is clarified, and a strategy for improvement of the cocatalyst is discussed. 2. Experimental Section Electrochemical Measurements. Two electrodes were prepared: a rhodium plate (10 × 10 × 0.5 mm) spot-welded to a rhodium wire and sealed in a Pyrex tube, and a platinum disk (10 mm diameter, 1 mm thick) mounted on the end face of a polytetrafluoroethylene (PTFE) rod. The electrodes were polished using 1 µm alumina slurry. Milli-Q water (total organic carbon < 5 ppb, resistivity > 18 MΩ cm) was used in all experiments. The electrochemical properties of the electrodes were measured in an aqueous solution of 0.5 M Na2SO4 (Wako Pure Chemicals, 99.9%) adjusted to pH 3.6 with H2SO4 (Aldrich Chemical, 99.999%) with continuous Ar, O2, or H2 bubbling. The potential with respect to an Ag/AgCl reference electrode was adjusted by a potentiostat (SDPS-501C, SYRINX) with a platinum counter electrode. The potential was corrected to that with respect to the reversible hydrogen electrode (RHE) by measurement of the pH of the electrolyte in this Article. The Cr2O3 layer was deposited electrochemically on the electrode in an aqueous solution of 0.5 M K2CrO4 (Kanto Chemicals, 99.9%) under continuous Ar bubbling. As the Cr2O3 dissolved at potentials above +0.8 V (Figure S2), the potential sweep for the Cr2O3-coated electrodes was limited within this range. X-ray Photoelectron Spectroscopy. The surface chemical composition and thickness of the Cr2O3 layer on the Rh electrode were estimated by angle-resolved X-ray photoelectron spectroscopy (AR-XPS) using a hemispherical analyzer (JPS-9000, JEOL). The samples were placed in a sample holder with adjustable takeoff angle (φ, between the surface normal and direction of electron energy analyzer). The measured binding energies were calibrated using the C1s band (284.6 eV). In Situ IRAS Measurements. IRAS measurements were performed by the subtractively normalized interfacial Fourier transform infrared reflection spectroscopy (SNIFTIRS) method using bare and Cr2O3-coated Pt electrodes.8 A three-electrode IRAS cell was used equipped with an R/β mixed-phase Pd/H reference electrode and gold wire counter electrode. A BaF2 IR prism designed for an incident angle of 65° was mounted less than 10 µm from the sample electrode separated only by the electrolyte. The electrode potential was corrected to that against RHE accounting for the potential difference due to the resistance of the thin electrolyte layer. The potential was controlled using a potentiostat (H-501, Hokuto Denko) interfaced with a Fourier transform infrared (FTIR) spectrometer (JIR 5500, JEOL). Two electrolytes were tested in this configuration: a 0.5 M Na2SO4 (Wako Pure Chemicals, 99.9%) aqueous solution adjusted to pH 3.6 with H2SO4 (Aldrich Chemical Co., 99.999%), and a 1.0 M H2SO4 aqueous solution. The electrolyte was bubbled with N2 before and during measurements. The electrode surfaces were cleaned prior to measurements by oxidation and reduction cycle between -0.1 and +1.5 V (bare Pt) or between -0.1 and +0.65 V (Cr2O3-coated Pt) until a stable constant voltammogram was obtained. The FTIR spectrometer was equipped with a liquid-N2-cooled mercury cadmium telluride (MCT) detector (Judson) and a wire-grid polarizer. The interferograms for 500 scans (2 scan s-1, 4 cm-1 resolution) were alternatively recorded at the sample and reference potentials, which were switched every 20 scans. 3. Results and Discussion 3.1. Thicknesses of Cr2O3 Films. The chemical composition and thickness of the thin Cr2O3 film deposited on the Rh

Figure 1. XPS results in the (a) Rh3d and (b) Cr2p bands with respect to takeoff angle.

Figure 2. Change in ratio of Rh3d/Cr2p XPS peak area with takeoff angle for various Cr2O3-coated Rh electrodes prepared at electrodeposition potentials of -0.69 V (O), -0.79 V (0), and -0.89 V (4) (vs RHE). Solid lines denote calculations by eq 1.

electrode were investigated by XPS over a range of takeoff angles, as shown in Figure 1. The binding energies of Rh3d5/2 and Cr2P1/2 are identical to those reported for metallic rhodium (ref 306.4 eV)9 and chromium(III) oxide (ref 5768 eV).10 The Rh3d band intensities decreased with increasing takeoff angle, whereas the Cr2p band intensities remained constant, indicating that the thickness of the Cr2O3 layer coating the surface of the electrode was within the range of the mean-free path of the photoelectrons. Figure 2 plots the ratio of XPS peak area (Rh3d/ Cr2p) as a function of takeoff angle for samples prepared at different electro-deposition potentials. The Rh3d/Cr2p ratio can be seen to increase with takeoff angle in all samples. The

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Figure 3. Cyclic voltammograms for (a) bare and (b) Cr2O3-coated Rh electrodes in 0.5 M Na2SO4 aqueous solution adjusted to pH 3.6 with H2SO4 under Ar bubbling (scan rate, 30 mV s-1).

Figure 4. Cyclic voltammograms for (a) bare and (b) Cr2O3-coated Pt electrodes in 0.5 M Na2SO4 aqueous solution adjusted to pH 3.6 with H2SO4 under Ar bubbling (scan rate, 50 mV s-1).

reported mean-free path of photoelectrons (λ) for Rh is 13.5 Å with a kinetic energy of hV ) 948 eV,11 while that for Cr2O3 is in the range 12.0-34.5 Å depending on the reference papers.10 The λ values for Rh and Cr2O3 were both assumed to be 13.5 Å for simplicity, corresponding to potential error of -11% to +156% for Cr2O3. The thickness of the thin film (d) as a uniform layer based on the AR-XPS results is then obtained by

[

d ) λCr2O3 cos φ ln

]

σRhFRhMCr2O3λRh ICr2O3 +1 σCr2O3FCr2O3MRhλCr2O3 IRh

(1)

where σ is the ionization cross-section, F is the bulk density, M is the molecular weight, and I is the peak area after applying Shirley background subtraction.12 The fitting curves using eq 1 are shown in Figure 2. The thicknesses of the Cr2O3 films deposited at -0.69, -0.79, and -0.89 V are thus estimated to be ca. 1.8, 2.7, and 3.5 nm, respectively. These thicknesses are close to those determined for the particulate core/shell Rh/Cr2O3 cocatalysts (ca. 2 nm) by transmission electron microscopy (TEM).3g,h The Cr2O3-coated Rh electrode prepared at -0.79 V (2.7 nm Cr2O3 thickness) was used for all subsequent measurements. 3.2. Electrochemical Measurements. Cyclic voltammograms of the bare and Cr2O3-coated Rh electrodes obtained with continuous Ar bubbling are shown in Figure 3. The currents indicating proton adsorption/desorption and hydrogen evolution were observed at 0.1 and 0.0 V, respectively, on both the bare and the Cr2O3-coated Rh electrodes. The similarity of potentials suggests that protons are reduced at the interface between Rh and Cr2O3 in the Cr2O3-coated Rh electrode. The corresponding cyclic voltammograms of the Pt electrodes are shown in Figure 4. The currents of proton adsorption/desorption and hydrogen evolution in this case were observed at 0.05-0.40 and 0.0 V, respectively. The calculated electric charges for proton reduction on the bare and Cr2O3-coated Pt electrodes are 1.7 and 1.3 mC cm-2 (1.1 cm2 geometrical area), respectively. This result suggests that the density of hydrogen (proton) adsorption sites

Figure 5. Linear-sweep voltammograms for (a) bare and (b) Cr2O3coated Rh electrodes in 0.5 M Na2SO4 aqueous solution adjusted to pH 3.6 with H2SO4 under Ar (dashed line) and O2 bubbling (solid line) (scan rate, 5 mV s-1).

on the Cr2O3-coated Pt electrode is 76% of that for the bare Pt electrode. The Cr2O3-coated Cr(metal) electrode exhibits no activity for H2 evolution (see Figure S2), even at -0.6 V, even though the electrode allows electron transfer to the interface between the electrode and electrolyte, confirming that the Cr2O3 shell alone does not evolve H2 in photocatalytic overall water splitting. The activity of the bare and Cr2O3-coated electrodes for oxygen reduction, corresponding to the back-reaction (water formation) of overall water splitting, was determined by acquiring voltammograms while bubbling with O2, as shown in Figure 5. Cathodic current due to oxygen reduction on the bare Rh electrode was observed at +0.85 V, in agreement with previous reports.13 However, negligible oxygen reduction current

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Figure 7. Tafel plots of hydrogen evolution on bare and Cr2O3-coated Rh electrodes with different thicknesses in 0.5 M Na2SO4 aqueous solution adjusted to pH 3.6 with H2SO4 under Ar bubbling (scan rate, 5 mV s-1). The “O” denote bare Rh electrode, and solid symbols denote Rh electrodes coated with a Cr2O3 layer of ca. 1.8 nm (b), 2.7 nm (9), and 3.5 nm (2) in thickness. Solid lines denote fitting curves obtained by eq 2. Figure 6. Linear-sweep voltammograms for (a) bare and (b) Cr2O3coated Pt electrodes in 0.5 M Na2SO4 aqueous solution adjusted to pH 3.6 with H2SO4 under Ar bubbling (dashed line) and O2 bubbling (solid line) (scan rate, 50 mV s-1).

TABLE 1: Exchange Current Densities of Hydrogen Evolution for Bare and Cr2O3-Coated Rh Electrodes with Different Thicknesses and Selected Noble-Metal Electrodes in Acidic Solutions

a

electrodes

(log j0)/j0 in A cm-2

bare Rha Cr2O3/Rh (2.2 nm)a Cr2O3/Rh (3.3 nm)a Cr2O3/Rh (4.3 nm)a Rhb Ptb Pdb Irb

-3.2 -3.7 -3.9 -3.8 -1.9 to -3.8 -2.6 to -3.7 -1.2 to -3.7 ca. -3.8

This work. b Reference data.15

was observed for the Cr2O3-coated Rh electrode, demonstrating that the Cr2O3 layer suppresses oxygen reduction. The Pt electrodes also showed oxygen reduction current, and the presence of Cr2O3 suppresses the oxygen reduction as shown in Figure 6.14 This result also indicates that the Rh or Pt electrode was completely coated with Cr2O3, preventing interaction between the bare noble metal and O2 in the reactant solution. The exchange current density (j0) for hydrogen evolution, which might be affected by the thickness of the Cr2O3 layer, can be determined from the Tafel equation as follows.

E ) a + b log i

(2)

Here, a and b are characteristic constants, and b is known as the Tafel slope. The values of a and b are listed in Table 1, as obtained from Tafel plots (Figure 7) of the hydrogen evolution reaction for bare and Cr2O3-coated Rh electrodes with various thicknesses of Cr2O3 (ca. 1.8, 2.7, and 3.5 nm). The hydrogen evolution current for the Cr2O3-coated Rh electrode was smaller than that for the bare Rh electrode,15 attributable to the reduction in the density of hydrogen reaction sites. The activity of the Cr2O3-coated Rh electrodes for hydrogen evolution is largely invariant with respect to the thickness of the Cr2O3 layer in the

range 1.8-3.5 nm, indicating that the reduction reaction does not proceed via a tunneling mechanism. If the electrochemical reactions on the outer surface were driven by the tunneling transfer of electrons through the Cr2O3 layer, the current density would be expected to decrease exponentially with increasing thickness of the Cr2O3 layer.16 This result therefore suggests that H2 evolution proceeds via proton transfer through the Cr2O3 layer to the Rh surface. Such a mechanism is less sensitive to the thickness of the Cr2O3 layer. For the production of H2 by this core/shell cocatalyst, the H2 molecules at the core/shell interface must be able to escape through the shell layer. However, as the H2 molecules are much larger than protons, the ability of H2 molecules to permeate out through the shell layer should be considered. As bare Rh and Pt electrodes are known to be active for hydrogen oxidation reaction (HOR) above 0.0 V,17 the penetration of hydrogen molecules through the Cr2O3 layer was investigated by performing voltammetric measurements while bubbling H2 through the electrolyte. Figure 8a shows voltammograms for Cr2O3-coated Rh electrodes in 0.5 M Na2SO4 aqueous solution adjusted to pH 3.6 with H2SO4 under Ar, O2, or H2 bubbling. The current due to HOR was observed above 0.0 V under H2 bubbling for the Cr2O3-coated electrode, confirming that H2 was oxidized at the Rh electrode surface through the Cr2O3 layer. Similar results were obtained for the Cr2O3-coated Pt electrode (Figure 8b). The constant currents of HOR at the potentials above 0.0 V were considered to be diffusion-limited currents. These currents for the Cr2O3-coated Rh electrode (0.046 mA cm-2, average 0.40-0.50 V at a scan rate of 30 mV s-1, hydrogen pressure of 152 Torr) were much smaller than those for the bare Rh electrode (0.28 mA cm-2, average 0.40-0.50 V at a scan rate of 30 mV s-1, hydrogen pressure of 152 Torr). These results suggest that the diffusion-limited currents for the Cr2O3-coated Rh electrode are most likely to depend on the diffusion through the Cr2O3 layer. If the diffusion of hydrogen through the Cr2O3 layer is driven in the form of dihydrogen molecules or hydrogen atoms, the currents would increase with kinetic orders in hydrogen pressure of 0-1 or 0-0.5, respectively. Figure 9 shows the relationship between the current density of HOR and the partial pressure of hydrogen for the Cr2O3-coated Rh electrode. The observed kinetic order for the partial pressure of

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Figure 8. Linear-sweep voltammograms for Cr2O3-coated Rh (a) and Pt (b) electrodes in 0.5 M aqueous Na2SO4 solution adjusted to pH 3.6 with H2SO4 under Ar, O2, and H2 bubbling (scan rate, 5 mV s-1). Figure 10. Potential-dependent SNIFTIR spectra of adsorbed hydrogen atoms on bare Pt electrode in 1.0 M H2SO4 aqueous solution and 0.5 M Na2SO4 aqueous solution adjusted to pH 3.6 with H2SO4 under N2 bubbling (reference potential, +0.95 V vs RHE). Baselines are corrected and spectra are offset for clarity.

Figure 9. Current density of hydrogen oxidation for Cr2O3-coated Rh electrode (average 0.40-0.50 V vs RHE at a scan rate of 30 mV s-1) relative to partial pressure of added hydrogen (0-760 Torr partial pressure, 760 Torr total pressure, balance Ar) in logarithmic scales. The solid line denotes linear fitting.

H2 was 1.1 ( 0.1, suggesting that the diffusion of hydrogen proceeds in the form of molecular dihydrogen through the Cr2O3 layer. 3.3. In Situ IRAS Measurements. The property of hydrogen atoms adsorbed on the bare and Cr2O3-coated Pt electrodes was investigated by in situ IRAS measurements using the SNIFTIRS method. The reference potential was set at +0.95 V for the bare Pt electrode. The dependence of the SNIFTIR spectra on potential is shown in Figure 10 for the bare Pt electrode. A peak at 2089 cm-1 due to the Pt-H stretching of terminal H atoms can be observed at +0.05 V in 1.0 M H2SO4 aqueous solution, consistent with previous reports (2080-2100 cm-1).18 In 0.5 M Na2SO4 aqueous solution adjusted to pH 3.6, the peak due to the Pt-H stretching mode shifts to a lower frequency of around 2063 cm-1 at +0.05 V due to the effect of pH. As the potential is scanned negatively from +0.05 to -0.20 V, the peak position shifts from 2063 to 2052 cm-1, which is explainable by the change in the Pt-H bond strength (electrochemical Stark effect).19 The peak intensities increase with decreasing potential due to an increase in the amount of hydrogen adsorbed on the Pt electrode, consistent with the

voltammograms shown in Figure 4a. The peaks in the 1967-2008 cm-1 region can be ascribed to the presence of linear-bonded CO. This impurity could not be eliminated even with careful cleaning of the electrodes and the use of high-quality reagents. However, the peak intensity of the linear CO band (ca. 5 × 10-5 absorbance) is 1000 times smaller than that for saturated CO coverage (5 × 10-2 absorbance), indicating that the CO impurity comprises less than 0.1% of a monolayer roughly assuming a linear relationship between peak intensity and coverage.18d,20 The SNIFTIR spectra for the Cr2O3-coated Pt electrode in 0.5 M Na2SO4 aqueous solution adjusted to pH 3.6 with H2SO4 are shown in Figure 11. The peak at 2039 cm-1 is due to Pt-H stretching, as observed for the bare Pt electrode. The peak due to terminal H adsorbed on Cr2O3, which has been reported to appear at 1581 cm-1 under an H2 atmosphere,21 is not observed for this sample, indicating that the hydrogen species are adsorbed on Pt and not Cr2O3. The position of the Pt-H peak for the Cr2O3-coated electrode is slightly lower than that for but the bare Pt electrode (2063-2052 cm-1), tentatively attributed to weakening of the Pt-H bond by H-Cr2O3 or Pt-Cr2O3 interaction. The intensity of the Pt-H peak increased with decreasing potential, consistent with the increase in the progress of hydrogen adsorption as shown in Figure 4b. The observation of Pt-H vibration directly suggests that hydrogen is reversibly oxidized and reduced at the Cr2O3/Pt interface and that H+ penetrates through the Cr2O3 layer to the Cr2O3/Pt interface. The intensity ratio of Pt-H peak area between the Cr2O3coated Pt electrode and the bare Pt electrode is estimated to be ca. 0.87 at -0.2 V. The ratio (87%) on the coated electrode is similar to that estimated by electrochemical measurements (76%). These results thus indicate that 80-90% of surface sites remain available for proton reduction when the surface is coated

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Figure 12. Schematic model of H2 evolution reaction on core/shell noble-metal/Cr2O3 particulate system as a cocatalyst for photocatalytic overall water splitting. Figure 11. Potential-dependent SNIFTIR spectra of adsorbed hydrogen atoms on Cr2O3-coated Pt electrode in 0.5 M Na2SO4 aqueous solution adjusted to pH 3.6 with H2SO4 under N2 bubbling (reference potential, +0.65 V vs RHE). Baselines are corrected and spectra are offset for clarity.

with Cr2O3. The shell layer also weakens the PtCO stretching peak associated with CO impurities, indicating that the shell layer inhibits the penetration of organic impurities in the electrolyte. The Rh-H vibrational peak due to hydrogen atoms adsorbed on the Rh electrode unfortunately could not be observed in the present IRAS measurements. The results shown in Figures 3 and 4 indicate that much less hydrogen adsorbed on the Rh electrode than on the Pt electrode, and the hydrogen adsorption/ desorption peaks in the voltammograms are broader for the Rh electrode. These factors, in addition to the small absorption coefficient for H-metal stretching, render it difficult to observe the Rh-H stretching on the Rh electrode. 3.4. Function of Core/Shell Noble-Metal/Cr2O3 Cocatalysts. The noble-metal core should have high exchange current density and low overvoltage for hydrogen evolution, an appropriate work function allowing contact between the photocatalyst and the noble metal with a negligible Schottky barrier, and the ability to be deposited as small particles with high surface area. We consider that Rh satisfies all of these requirements and achieves the highest activity of the noble metals tested with the (Ga1-xZnx)(N1-xOx) system. The Cr2O3 shell material has been determined previously by X-ray near-edge structure (XANES) and XPS to have a corundum Cr2O3 structure.3g,h Such a structure is too rigid to allow the permeation of protons or molecules. However, many studies using a variety of techniques suggested that Cr2O3 substances develop a passivated CrO(1.5-m)(OH)2m · xH2O (m ) 0, Cr2O3 · xH2O; m ) 0.5, CrOOH · xH2O; m ) 1.5, Cr(OH)3 · xH2O) thin layer at the interface with an aqueous solution.22-26 Lindbergh et al. suggested that the Cr(OH)3 · xH2O layer is permeable to protons but not to O2, by the electrochemical measurements on the Pt electrode in a chromate solution, because of smaller hydrogen and/or hydroxide ions than O2.27 In aqueous solution, Cr2O3 layer on the noble-metal cocatalyst is thus considered to form CrO(1.5-m)(OH)2m · xH2O, which would possess this proton permeability. A proposed model of proton reduction on the core/shell noble-metal/Cr2O3 cocatalyst is illustrated in Figure 12. The functional enhancement by the deposition of this Cr-based layer on particulate noble-metal cocatalysts for photocatalytic overall water splitting was dem-

onstrated to be due to a selective permeation mechanism. It seems that there are micropores in the CrO(1.5-m)(OH)2m · xH2O layer, which allow the diffusion of H2 and inhibit that of O2, but, in the view of van der Waals radii of H2 and O2 molecules of 1.20 and 1.52 Å, respectively, we lack any techniques that can detect such small micropores.28 The mechanism responsible for this selective gas permeability at the shell layer remains unclear, but this mechanism must be related to the nonrigid structure mixed with many different varieties of CrO(1.5-m)(OH)2m · xH2O. The functionality that inhibits oxygen reduction reaction was also reported for the electrodes based on Fe,29 Ce,30 and Ni,31 in the functionality similar to that of Cr.27,30,32 Such thin films can thus be considered additional candidates for the modification of noble metals to inhibit the back-reaction in overall water splitting. Organic passivation layers such as ion-exchanged polymers may also provide a selective permeation characteristic suitable for the present photocatalytic system, although preparing such materials on the noble-metal core and ensuring durability for photocatalytic reactions remain significant challenges. There appears to be considerable scope for refinement of the cocatalyst modifier, which may potentially lead to further increases in photocatalytic activity for overall water splitting. 4. Conclusion Electrochemical and in situ IRAS analyses of Cr2O3-coated noble-metal electrodes performed in the aqueous solution revealed the distinct properties of the Cr-based layer (presumably forming CrO(1.5-m)(OH)2m · xH2O composites) that selectively permeate protons and hydrogen, but not oxygen atoms and molecules. The fact that the current was observed due to proton reduction reactions and that IRAS measurements showed the Pt-H stretching peaks for both the coated and the bare Pt electrodes indicates that the protons permeate through the CrO(1.5-m)(OH)2m · xH2O layer and reach the noble-metal/ CrO(1.5-m)(OH)2m · xH2O interface, and hydrogen molecules and atoms produced at the interface then permeate out through the CrO(1.5-m)(OH)2m · xH2O layer. In contrast to high ORR current on the bare noble-metal electrodes, no ORR current was observed on Cr-coated noble-metal electrode, indicating that the back-reaction of O2 reduction to water, which preferentially occurs on the bare electrodes, was strongly suppressed for the CrO(1.5-m)(OH)2m · xH2O-coated samples. The presence of a passivation layer explains the high activity and functionality of the core/shell noble-metal/CrO(1.5-m)(OH)2m · xH2O as an effective cocatalyst for photocatalytic overall water splitting.

Role and Function of Noble-Metal/Cr-Layer Cocatalysts Acknowledgment. This work was supported by the Research and Development in a New Interdisciplinary Field Based on Nanotechnology and Materials Science program of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. Acknowledgement is also extended to the Global Center of Excellence (GCOE) Program for Chemistry Innovation. We wish to thank Tokyo Metropolitan Collaboration of Regional Entities for the Advancement of Technological Excellence, Japan Science and Technology Agency (JST), for their partial financial support. K.M. gratefully acknowledges the support of a fellowship from the Japan Society for the Promotion of Science (JSPS). Supporting Information Available: Photocatalytic activities of (Ga1-xZnx)(N1-xOx) photocatalysts modified with Rh, Pt, Pt/ Cr2O3, or Rh/Cr2O3 cocatalysts of the Cr2O3/Rh/(Ga1-xZnx)(N1-xOx) photocatalysts and cyclic voltammograms on the Cr2O3-coated Cr electrode. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Kudo, A.; Miseki, Y. Chem. Soc. ReV. 2009, 38, 253. (b) Osterloh, F. E. Chem. Mater. 2008, 20, 35. (c) Maeda, K.; Domen, K. J. Phys. Chem. C 2007, 111, 7851. (d) Hoertz, P. G.; Mallouk, T. E. Inorg. Chem. 2005, 44, 6828. (e) Lee, J. S. Catal. SurV. Asia 2005, 9, 217. (2) Maeda, K.; Teramura, K.; Lu, D.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K. Nature 2006, 440, 295. (3) (a) Maeda, K.; Takata, T.; Hara, M.; Saito, N.; Inoue, N.; Kobayashi, H.; Domen, K. J. Am. Chem. Soc. 2005, 127, 8286. (b) Maeda, K.; Teramura, K.; Takata, T.; Hara, M.; Saito, N.; Toda, K.; Inoue, Y.; Kobayashi, H.; Domen, K. J. Phys. Chem. B 2005, 109, 20504. (c) Sun, X.; Maeda, K.; Le Faucheur, M.; Teramura, K.; Domen, K. Appl. Catal., A 2007, 327, 114. (d) Maeda, K.; Teramura, K.; Saito, N.; Inoue, Y.; Domen, K. J. Catal. 2006, 243, 303. (e) Maeda, K.; Teramura, K.; Masuda, H.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K. J. Phys. Chem. B 2006, 110, 13107. (f) Maeda, K.; Teramura, K.; Lu, D.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K. J. Phys. Chem. B 2006, 110, 13753. (g) Maeda, K.; Teramura, K.; Lu, D.; Saito, N.; Inoue, Y.; Domen, K. Angew. Chem., Int. Ed. 2006, 45, 7806. (h) Maeda, K.; Teramura, K.; Lu, D.; Saito, N.; Inoue, Y.; Domen, K. J. Phys. Chem. C 2007, 111, 7554. (i) Hirai, T.; Maeda, K.; Yoshida, M.; Kubota, J.; Ikeda, S.; Matsumura, M.; Domen, K. J. Phys. Chem. C 2007, 111, 18853. (j) Maeda, K.; Teramura, K.; Domen, K. J. Catal. 2008, 254, 198. (k) Jensen, L. L.; Muckerman, T.; Newton, D. J. Phys. Chem. C 2008, 112, 3439. (l) Maeda, K.; Hashiguchi, H.; Masuda, H.; Abe, R.; Domen, K. J. Phys. Chem. C 2008, 112, 3447. (4) (a) Kraeutler, B.; Bard, A. J. J. Am. Chem. Soc. 1978, 100, 4317. (b) Yamaguti, K.; Sato, S. J. Chem. Soc., Faraday Trans. 1 1985, 81, 1237. (c) Sayama, K.; Arakawa, H. J. Chem. Soc., Faraday Trans. 1997, 93, 1647. (d) Abe, R.; Sayama, K.; Arakawa, H. Chem. Phys. Lett. 2003, 371, 360. (e) Tsuji, I.; Kato, H.; Kobayashi, H.; Kudo, A. J. Am. Chem. Soc. 2004, 126, 13406. (f) Tsuji, I.; Kato, H.; Kobayashi, H.; Kudo, A. J. Phys. Chem. B 2005, 109, 7323. (g) Subramanian, V.; Wolf, E.; Kamat, P. V. J. Phys. Chem. B 2001, 105, 11439. (h) Tada, H.; Ishida, T.; Takao, A.; Ito, S. Langmuir 2004, 20, 7898. (5) (a) Domen, K.; Naito, S.; Soma, M.; Onishi, T.; Tamaru, K. J. Chem. Soc., Chem. Commun. 1980, 543. (b) Domen, K.; Kudo, A.; Onishi, T.; Kosugi, N.; Kuroda, H. J. Phys. Chem. 1986, 90, 292. (c) Domen, K.; Kudo, A.; Onishi, T. J. Catal. 1986, 102, 92. (d) Kato, H.; Kudo, A. Catal. Today 2003, 78, 561. (e) Kudo, A.; Nakagawa, S.; Kato, H. Chem. Lett. 1999, 28, 1197. (f) Kudo, A. Catal. SurV. Asia 2003, 7, 31. (g) Kudo, A.; Tanaka, A.; Domen, K.; Maruya, K.; Aika, K.; Onishi, T. J. Catal. 1988, 111, 67. (h) Kato, H.; Asakura, K.; Kudo, A. J. Am. Chem. Soc. 2003, 125, 3082. (i) Domen, K.; Naito, S.; Onishi, T.; Tamaru, K. J. Phys. Chem. 1982, 86, 3657.

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