Polyol Synthesis of Size-Controlled Rh Nanoparticles and Their

Jul 31, 2012 - This article is part of the Nanostructured-Enhanced Photoenergy ... it was demonstrated that smaller Rh cores gave higher activity than...
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Polyol Synthesis of Size-Controlled Rh Nanoparticles and Their Application to Photocatalytic Overall Water Splitting under Visible Light Takahiro Ikeda,†,‡ Anke Xiong,§ Taizo Yoshinaga,†,‡ Kazuhiko Maeda,§,∥ Kazunari Domen,§ and Toshiharu Teranishi*,‡ †

Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571, Japan Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan § Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ∥ Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan ‡

S Supporting Information *

ABSTRACT: A gradual size control of poly(N-vinyl-2-pyrrolidone)-protected Rh nanoparticles (PVP-Rh NPs) was successfully achieved in the range of 1.7−7.7 nm by tuning the pH value and the reaction temperature of the ethylene glycol (EG) solution; smaller NPs were formed at higher pH and at higher temperature. This trend can be interpreted by the change in the nucleation rate caused by tuning the pH value and/or the temperature of the solution. When the size tuned PVP-Rh NPs were applied for use as cocatalysts in a photocatalyst (solid solution of GaN and ZnO (Ga1−xZnx)(N1−xOx)) for overall water splitting under visible light, it was demonstrated that smaller Rh cores gave higher activity than the larger ones.

on (Ga1−xZnx)(N1−xOx) improves its photocatalytic activity,12 and we have found that the effect of high dispersion of the Rh core is more important than its oxidation state in increasing photocatalytic activity.13 As it is difficult to tune the size of the Rh core by current methods utilizing Rh NPs synthesized in the liquid phase,12 the size effect of Rh NP cocatalysts on the photocatalytic activity of (Ga1−xZnx)(N1−xOx) modified with Rh/Cr2O3 NPs has not been investigated yet. By investigating the relationship between the size of the Rh core and photocatalytic activity, we should be able to shed light on the nanoscale effect of cocatalysts on photocatalytic water splitting. The size14,15 and shape16,17 of noble metal NPs has been widely studied in the investigation of morphology dependent properties of NPs, such as catalysis,18,19 plasmonic properties,20,21 and Coulomb charging effects.22,23 For an application to catalysis, the size of noble metal NPs should be precisely tuned in the range of 1−10 nm, in which noticeable size dependency frequently appears.24−26 As mentioned above, Rh is one of the important elements as a cocatalyst for overall water splitting, and there have been several reports on the

1. INTRODUCTION Hydrogen production from water using particulate photocatalysts and sunlight can hold significant advantages and benefits. Recently, it has been demonstrated that a solid solution of GaN and ZnO, (Ga1−xZnx)(N1−xOx),1−3 is capable of splitting water into H2 and O2 under visible light when modified with a suitable cocatalyst,4−11 which functions as a hydrogen evolution site. Because the photocatalytic performance of (Ga1−xZnx)(N1−xOx) for water splitting is strongly dependent on the loaded cocatalyst, it is important to develop an efficient cocatalyst material.4−11 Among the cocatalysts we have investigated so far, Rh/Cr2O3 (core/shell) nanoparticles (NPs)9−11 are effective and structurally interesting materials for (Ga1−xZnx)(N1−xOx) to promote overall water splitting. In this system, undesirable backward reactions such as H2−O2 recombination and the photoreduction of O2 are prevented by the Cr2O3 shell. This is due to the amorphous Cr2O3 shell layer being permeable to protons and the evolved H2 molecules but not to O2.11 However, the Rh cores play a role in extracting photogenerated electrons from the conduction band of (Ga1−xZnx)(N1−xOx) and of providing catalytically active sites for H2 evolution. Therefore, the physicochemical properties of the Rh core such as morphology and oxidation state are important. To date, we have demonstrated that highly dispersive deposition of Rh NPs © 2012 American Chemical Society

Special Issue: Nanostructured-Enhanced Photoenergy Conversion Received: June 17, 2012 Revised: July 30, 2012 Published: July 31, 2012 2467

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polyol synthesis of size-controlled Rh NPs.26−28 While the polyol process is one of the most common methods for synthesizing metal NPs with controlled shapes and sizes, due to its wide applicability and ease of use, the precise control of Rh NPs in the size range of 1−10 nm has been rarely achieved. In the present work, the size of the poly(N-vinyl-2pyrrolidone)-protected Rh NPs (PVP-Rh NPs) was precisely controlled in the range of 1.7−7.7 nm by changing the nucleation rate of the polyol synthesis. The resulting PVP-Rh NPs were then deposited on (Ga1−xZnx)(N1−xOx) without any sintering by chemical adsorption, followed by calcinations. After the photodeposition of Cr2O3 on the deposited Rh NPs, the photocatalytic activity for overall water splitting under visible light was investigated with respect to the size of Rh NPs.

The deposition of Rh NPs on GaN:ZnO was carried out as follows. An ethanol solution (50 mL) of the desired amount of size tuned PVP-Rh NPs (1.6 ± 0.3, 2.7 ± 0.3, and 5.1 ± 0.5 nm) was first prepared. GaN:ZnO (0.26 g) was then added into the solution (1.3−1.7 wt % Rh vs GaN:ZnO), followed by stirring for 24 h under ambient conditions, to reach adsorption/ desorption equilibrium. GaN:ZnO modified with PVP-Rh NPs was collected by filtration, thoroughly washed with ethanol, and calcined under air at 673 K for 3 h to remove PVP on Rh NPs. UV−visible spectroscopy of an as-prepared PVP-Rh NP solution and a supernatant after adding GaN:ZnO showed that the amount of adsorbed Rh NPs on GaN:ZnO was 0.1− 0.3 wt %. The resulting powder suspended in 0.16 mM K2CrO4 aq. solution (400 mL, Cr 1.5 wt % vs GaN:ZnO) was exposed to visible light irradiation (λ > 400 nm) for 4 h to deposit Cr2O3 on the surface of Rh NPs. 2.5. Characterization. TEM observations were carried out using a JEM-1011 Transmission Electron Microscope (JEOL) at an accelerating voltage of 100 kV. The diameters and standard deviations of the obtained NPs were calculated by counting a few hundreds of NPs. XRD patterns were taken on X’Pert Pro MPD (PANalytical) with CuKα radiation (λ = 1.542 Å) at 45 kV and 40 mA. The UV−visible absorption spectroscopy was conducted using a U-3310 spectrophotometer (HITACHI). X-ray absorption near-edge structure (XANES) analysis was conducted at the BL01B1 Beamline of the SPring-8 synchrotron facility (Hyogo, Japan) using a ring energy of 8 GeV and a stored current of 100 mA in top-up mode (proposal number 2011A1601). The spectra were measured in transmission mode at room temperature using a Si(111) two-crystal monochromator, and the photon energies in the XANES spectra were corrected with reference to Cu foil (8980.3 eV). 2.6. Photocatalytic Water Splitting Reactions. The photocatalytic water splitting reactions were carried out at room temperature using the same experimental setup as used for the photodeposition of Rh/Cr2O3 (core/shell) NPs on GaN:ZnO. The reactant solution consisting of the catalyst (0.15 g) and H2SO4 aq. solution (pH 4.5, 400 mL) was evacuated several times prior to the reaction to ensure that no air remained in the reaction vessel. The evolved gases were analyzed by gas chromatography (Shimadzu, GC-8A with TCD detector and MS-5A column, argon carrier gas).

2. EXPERIMENTAL SECTION 2.1. Chemicals. A 38 mM aqueous (aq.) solution of rhodium(III) chloride hydrate (RhCl3·xH2O, Rh 38−40%, Aldrich) (calculated as x = 3) was prepared by dissolving RhCl3·xH2O (1.0 g, 3.8 mmol) in deionized water (100 mL). A 173 mM aq. solution of hydrogen hexachloroplatinate(IV) hexahydrate (H2PtCl6·6H2O, 98.5%, Kishida Chemicals) was prepared by dissolving H2PtCl6·6H2O (2.24 g, 4.33 mmol) in deionized water (25 mL). A 60 mM aq. solution of palladium(II) chloride (PdCl2, High Purity Chemicals) was prepared by dissolving PdCl2 (106 mg, 0.600 mmol) in 0.12 M HCl (10 mL). A 15.6 M aq. solution of sodium hydroxide (NaOH, 96%, Wako Chemicals) was prepared by dissolving NaOH (6.24 g, 156 mmol) in deionized water (8.33 mL). Zinc oxide (ZnO, 99%, Kanto Chemicals), gallium nitride (GaN, 99.9%, High Purity Chemicals), trisodium hexachlororhodate(III) hydrate (Na3[RhCl6]·12H2O, Rh 99.9%, Mitsuwa Chemicals), potassium chromate (K2CrO4, 99%, Kanto Chemicals), poly(N-vinyl-2-pyrrolidone) (PVP, Mw: 10000 or 40000, Tokyo Chemical Industry), ethylene glycol (EG, 99%, Wako Chemicals), and nitric acid (HNO3, 15.6 M, Kanto Chemicals) were used without further purification. 2.2. Synthesis of PVP-Rh NPs. A 38 mM RhCl3 aq. solution (1.04 mL, 40 μmol) was evaporated to remove water in a 100 mL three-necked flask. PVP (53 mg, 0.48 mmol as a monomeric unit), EG (10 mL), and either HNO3 or NaOH aq. (1−15 μL) were then added into the flask. After stirring for several minutes, the solution was refluxed for 1 h under nitrogen atmosphere. The solution was then purified with a combination of good/poor solvents, 2-propanol/hexane, and redispersed in ethanol (50 mL). The reaction temperature and the reaction time were changed in the 376−453 K and 1−48 h ranges to precisely tune the Rh NP size. The ethanol solution of the Rh NPs was used for next adsorption process. 2.3. Synthesis of PVP-Pt and PVP-Pd NPs. PVP-Pt and PVP-Pd NPs were synthesized with a similar procedure to the PVP-Rh NPs. 2.4. Preparation of Photocatalysts. (Ga1−xZnx)(N1−xOx) solid solution was prepared by a method reported in the literature.1,3 Briefly, a powdered mixture of ZnO (0.94 g) and Ga2O3 (1.08 g) was heated at 1123 K for 15 h under NH3 flow (250 mL·min−1). After nitridation, the sample was cooled to room temperature under NH3 flow. The band gap of the as obtained (Ga1−xZnx)(N1−xOx) is ca. 2.66 eV, as estimated from the onset of the diffuse reflectance spectrum. The specific surface area determined by nitrogen adsorption at 77 K was ca. 9.6 m2·g−1. In this article, the as-prepared (Ga1−xZnx)(N1−xOx) is referred to as GaN:ZnO for simplicity.

3. RESULTS AND DISCUSSION 3.1. Size Control of PVP-Rh NPs. PVP-Rh NPs were synthesized by the polyol reduction method and were prepared by refluxing an EG solution of RhCl3 and PVP under N2 atmosphere. The obtained PVP-Rh NPs were 4.7 ± 0.6 nm in size as shown in Figure 1a. It has been previously reported that the reducing ability of EG can be tuned by adjusting the pH value,29 which would in turn affect the nucleation process of the Rh NPs. The pH value of EG was changed to 1.8 (calculated value; see Supporting Information) with 15.6 M HNO3 and the resulting size of the Rh NPs increased to 6.7 ± 0.8 nm (Figure 1b). Correspondingly, when the pH was raised to 14 by the addition of 15.6 M NaOH aq. solution, the size of the Rh NPs decreased to 1.7 ± 0.3 (Figure 1c). Figure 1d shows the XRD peaks of these three samples. The PVP-Rh NPs resulting from no additives and the addition of HNO3 showed the XRD patterns assigned to face-centered cubic (fcc) Rh, whereas the other product did not show any characteristic peaks because of its small size. According to the literature, such small Rh NPs 2468

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Figure 2. pH-dependence of PVP-Rh NP size in the pH ranges of (a) 13.0−14.3 and (b) 1.7−2.8. Solid circles and the error bars represent the mean diameters and the standard deviations. Figure 1. TEM images of synthesized PVP-Rh NPs (a) at pH 7.9 without any additives (4.7 ± 0.6 nm), (b) at pH 1.6 with HNO3 (6.7 ± 0.8 nm), and (c) at pH 14 with NaOH aq. solution (1.7 ± 0.3 nm). (d) XRD patterns of PVP-Rh NPs in panels a−c.

(ca. 2 nm) are partially oxidized under ambient conditions.26 Our 1.7 ± 0.3 nm PVP-Rh NPs were also found to be partially oxidized, as revealed by the XANES measurement. Since we found that the size of PVP-Rh NPs was drastically changed by the EG pH value, we then systematically changed the pH value between 1.6 and 14 by adding HNO3 or NaOH aq. solution to the EG solution in order to finely control the particle size. Figure 2 shows the relationship between the pH value of the EG solution and the size of the resulting PVP-Rh NPs. The overall relationship across the entire pH range is shown in Figure S1, together with corresponding TEM images in Figure S2A,B (Supporting Information). A clear relationship between pH value and NP size is observed. As pH decreases from pH 2, the size of PVP-Rh NPs gradually increases, and conversely, as the pH increases from pH 13, the size decreases. PVP-Rh NPs size remains constant in the 2−13 pH range. Consequently, we can precisely control the particle size from 1.7 to 6.7 nm by tuning the EG pH value. As mentioned previously, there are several reports on the pH-dependent size change of noble metal NPs,29,30 which showed the similar results to ours; larger NPs at lower pH and smaller NPs at higher pH. This pH-dependent size change might be caused by the change of the nucleation rate of Rh NPs. To elucidate the mechanism of this phenomenon, a spectroscopic approach was adopted during the reaction. The color of the EG solution began to turn brown faster in the order of pH = 14 > 7.9 > 1.6. As shown in Figure 3, which presents the change in absorbance against the reaction temperature, the absorbance at λ = 700 nm assigned to intra-

Figure 3. Change in absorbance of the EG solutions against the reaction temperature at various pH. The absorbance is normalized by the value at the end of the 1 h reaction time.

and interband transitions of Rh NPs (see Figure S3, Supporting Information; UV−visible absorption spectra of the EG solutions before and after reaction) started to increase at higher reaction temperatures in the order of pH = 14 > 7.9 > 1.6. These results strongly suggest that the reduction of Rh(III) ions is facilitated at higher pH and suppressed at lower pH. Goia et al., reported that, in certain pH regions, the difference between the redox potentials of reductant and metal precursor (ΔE0) induces the change in the nucleation rate of metal precursor.30 By calculation of the redox potential shifts for reductant and metal precursor, they concluded that the higher pH value induces larger ΔE0, leading to a faster nucleation rate. It is well-known that a change of nucleation rate can drastically affect the final size of the NPs; a faster rate provides smaller NPs, and a slower rate, larger NPs.31,32 Therefore, the pHdependent size control mechanism can be interpreted by the 2469

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ΔE0 shift, and in our case, this pH effect works well at pH < 2 and pH > 13. This size-tuning technique can also be applied to other noble metals such as Pt and Pd. We additionally carried out the syntheses of PVP-Pt and PVP-Pd NPs (Figure S4, Supporting Information), and although, the resulting Pt and Pd NPs are polydisperse, the trend followed the results of the PVP-Rh NPs indicating that this strategy can be widely used to control the particle size of noble metal NPs. Although larger PVP-Rh NPs were obtained at lower pH conditions, they were relatively polydisperse (σ = 11−13%), likely due to the heterogeneous growth of the Rh nuclei. To synthesize monodisperse PVP-Rh NPs larger than 5 nm, the reaction conditions were changed in order to encourage a homogeneous slow growth of the Rh nuclei. When the reaction temperature was decreased by 20−80 K from the reflux point of EG (approximately 453 K in this system) and the reaction time was prolonged to complete the reaction (confirmed by terminating the size evolution; see Figure S5, Supporting Information), PVP-Rh NPs with sizes ranging from 5.7 to 7.7 nm (σ = 8.2−9.1%) were obtained (Figure 4). It should be Figure 5. TEM images of synthesized PVP-Rh NPs (a) at 379 K for 12 h, pH 13.7 (4.0 ± 0.4 nm), (b) at 389 K for 5 h, pH 13.7 (3.8 ± 0.4 nm), (c) at 376 K for 14 h, pH 13.9 (2.6 ± 0.2 nm), and (d) at 393 K for 18 h, pH 13.9 (2.3 ± 0.2 nm).

protected Rh NPs.12 The PVP-Rh NPs are likely to be chemically adsorbed on GaN:ZnO because they did not desorb even after thorough washing with ethanol, as confirmed by TEM observations and UV−visible spectroscopy. The adsorption mechanism may be ascribed to the multisite hydrogen bond between the surface OH and/or NH2 groups of GaN:ZnO (confirmed by XPS analysis)3 and oxygen atoms in γ-lactam rings of PVP, which do not participate in coordination to the surface of Rh NPs. Three kinds of PVPRh NPs with sizes of 1.6 ± 0.3, 2.7 ± 0.3, and 5.1 ± 0.5 nm were adsorbed on GaN:ZnO by mixing the NP solution with GaN:ZnO powder. PVP was then removed by calcination under air at 673 K for 3 h33 to make a heterointerface between the Rh NPs and GaN:ZnO. TEM images of these samples during the loading process are shown in Figure 6. Although a slight thermal aggregation occurred upon calcination, the loaded Rh NPs were fairly monodispersed compared to that prepared by photodeposition. Therefore, the size effect of Rh cores on the photocatalytic activity of GaN:ZnO modified with Rh/Cr2O3 (core/shell) NPs was investigated by using these samples. To evaluate the oxidation state of the loaded Rh NPs, XANES analysis was carried on the as synthesized samples. We first conducted the XANES analysis for samples after the adsorption of PVP-Rh NPs without calcination (Figure 7a), which indicated that, before calcination, the valence state of Rh species loaded on GaN:ZnO became close to metallic with increasing size. Figure 7b shows the Rh K-edge XANES spectra of different-sized Rh NPs loaded on GaN:ZnO by calcination. Regardless of the size, the spectral shape of the three samples is similar to each other, indicating similar valence states of Rh, although the 1.5 nm sample exhibited less oxidized features. The absorption-edge positions of these samples differ from that of the Rh foil, and are located at slightly lower photon energy than the Rh2O3 reference. The Rh K-edge spectra were not

Figure 4. TEM images of synthesized PVP-Rh NPs (a) at 409 K for 12 h (5.7 ± 0.5 nm), (b) at 396 K for 24 h (6.1 ± 0.5 nm), (c) at 390 K for 24 h (7.2 ± 0.6 nm), and (d) at 376 K for 48 h (7.7 ± 0.7 nm).

noted that this method can provide still larger NPs with narrow size distributions and demonstrates that the homogeneous slow growth of nuclei is of great importance in generating high quality large NPs. Furthermore, we found that the combination of low temperature reaction with high pH conditions was also effective in the formation of more monodisperse small Rh NPs (σ = 9−11%). Figure 5 shows the TEM images of the PVP-Rh NPs synthesized at temperatures lower than the reflux point of EG at high pH conditions. The monodispersity of the assynthesized PVP-Rh NPs indicates the versatility of this method. 3.2. Loading Rh NPs on GaN:ZnO. The PVP-Rh NPs are capable of being adsorbed onto the surface of GaN:ZnO by mixing an ethanol solution of NPs with GaN:ZnO, similar to the previously reported 3-mercapto-1-propane sulfonate2470

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Figure 6. TEM images of three kinds of PVP-Rh NPs (a−c) as-synthesized, (d−f) adsorbed on GaN:ZnO, and (g−i) loaded on GaN:ZnO after calcination.

more efficient charge separation.35 Similarly, the smaller Rh NPs on GaN:ZnO might have some positive effects not only on the surface reaction (H2 evolution) but on charge separation, leading to improved performance. Furthermore, decreasing the size of the cocatalyst would induce additional positive effects as the decrease in particle size increases the number of coordinative unsaturated atoms.36 Because H2 evolution reaction is known to be a structure-sensitive reaction, it is strongly conceivable that the increase in the number of the unsaturated atoms and/or the facets with high surface energy accelerates the rate of H2 evolution over smaller cocatalyst NPs.37−39 There are also drawbacks that can affect the photocatalytic activity. For example, as the particle size of Rh decreases while maintaining the total amount of Rh, the number of the loaded Rh NPs should increase. Our recent spectroscopic study has suggested that the deposited metal species such as Rh on a semiconductor material work not only as cocatalysts to promote the forward reactions (water photoredox reactions) but also as electron−hole recombination centers.40 Moreover, an increase of the number of cocatalysts is likely to avoid the photon absorption by photocatalysts as well as to cover even over the active sites on the surface of photocatalysts. If we assume that the loading amount of Rh is 0.3 wt % for all the samples, the coverage of Rh NPs on GaN:ZnO can be roughly estimated to be 1, 2, and 5% for 6.6, 3.8, and 1.5 nm Rh NPloaded samples, respectively. Therefore, this negative effect might affect the catalytic activities somewhat. Thus, increasing the number of Rh NPs on GaN:ZnO may reduce the efficiency of the forward reaction. While we cannot discuss the effect of these factors individually, the result of photocatalytic reactions

completely consistent with that of the Rh2O3, which means that Rh in these calcined samples had valence states close to that of Rh2O3, but slightly reduced. This result is almost identical to our previous XANES analysis for an analogue prepared using 3mercapto-1-propane sulfonate-protected Rh NPs.13 Thus, the change in the oxidation state of the Rh NPs is concluded to be caused by the calcination step in the loading process. 3.3. Photocatalytic Activity. Before conducting the water splitting reaction, GaN:ZnO modified with Rh NPs was treated by K2CrO4 under visible light to deposit Cr2O3 on the Rh NPs according to the reported method.9,10 The photocatalytic activities of the as-obtained samples were measured under visible light irradiation (λ > 400 nm).9,10 Without loading a cocatalyst (only GaN:ZnO), no reaction took place. However, modification of GaN:ZnO with Rh NPs and subsequent Cr2O3 deposition resulted in stoichiometric H2 and O2 evolution regardless of the particle size of Rh NPs. Figure 8a shows the time course of overall water splitting using GaN:ZnO modified with 1.5 nm Rh NPs and Cr2O3 under visible light. The amount of evolved H2 and O2 (3.45 mmol) is larger than that of the used catalyst (1.80 mmol), confirming the catalytic cycle of this reaction. These results indicate that the Rh NPs loaded on GaN:ZnO, further modified with Cr2O3, achieve the functionality as cocatalysts to promote overall water splitting by GaN:ZnO. Importantly, the water splitting rate increased with decreasing the size of Rh NPs (Figure 8b), consistent with a general trend in heterogeneous (photo) catalysis that highly dispersed catalytic species lead to improved performance.12,24,34 The increased activity can be ascribed to not only an increase of surface area of active sites but also other factors.24,34 Kamat et al. have reported that smaller Au NPs loaded on TiO2 induces 2471

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Figure 8. (a) Time courses of overall water splitting using GaN:ZnO modified with Rh (1.5 nm)/Cr2O3 (core/shell) NPs under visible light irradiation (λ > 400 nm). (b) Initial rates of H2 and O2 evolution over GaN:ZnO modified with different-sized Rh/Cr2O3 (core/shell) NPs. Black and white symbols or bars indicate H2 and O2, respectively. Reaction conditions: catalyst, 0.15 g; H2SO4 aq. (pH 4.5), 400 mL; light source, high-pressure Hg lamp (450 W) through NaNO2 aq. filter to cut UV light; reaction vessel, Pyrex inner-irradiation type.

Figure 7. Rh K-edge XANES spectra of different-sized PVP-Rh NPs (a) after adsorption on GaN:ZnO and (b) after loading on GaN:ZnO by calcination (the spectra of Rh2O3 and Rh are also shown as references).

strongly suggests that smaller cocatalysts are highly effective in improving the activity. It should be also noted that the loading amount of Rh was estimated to be 0.1−0.3 wt % by the decrease in absorbance of the ethanol solution of the PVP-Rh NPs before and after the adsorption step. Similar to our previous study using 3mercapto-1-propane sulfonate-protected Rh NPs,12 the amount of Rh introduced onto the surface of GaN:ZnO should be limited to a certain level because the present loading method involves adsorption/desorption equilibrium. Another important point we have learned so far is that metallic Rh is superior as an electron collector from GaN:ZnO and as a H2 evolution site.41 The valence states of Rh loaded on GaN:ZnO are close to Rh(III) oxide (Figure 7), although they are reduced somewhat. The performance of this system will thus be further improved if one could introduce smaller Rh NPs in the metallic state. To adsorb more Rh having the valence state close to metallic, further refinement of preparation conditions is required and is now under investigation in our group.

temperature, based on the homogeneous growth mechanism of the nuclei. Size-tuned PVP-Rh NPs were successfully deposited on GaN:ZnO by calcination with slight thermal growth, leading to the oxidation of Rh NPs to the similar state. The smaller Rh cores gave higher activity than the larger ones when loaded by the same conditions. This is possibly because of (1) an increase of surface area of Rh core, (2) an improvement of charge separation, and (3) an increase of active sites for H2 evolution as the size of Rh NPs decreases. Thus, in the system of GaN:ZnO modified with Rh/Cr 2 O 3 (core/shell) NPs, decreasing the size of the Rh core is highly effective in improving the photocatalytic activity. Deposition of the Rh core in a more metallic state and increasing the loading of Rh to an optimal amount are essential to obtain higher photocatalytic activity for overall water splitting under visible light.

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ASSOCIATED CONTENT

S Supporting Information *

4. CONCLUSIONS We have developed a novel method to load size tuned Rh NPs on GaN:ZnO. The precise size control of Rh NPs is conducted in the liquid phase with PVP as a surfactant prior to the deposition, and sizes in the range of 1.7−6.7 nm are achieved by tuning the pH of the system. Furthermore, we obtained monodisperse PVP-Rh NPs in the size range of 2.3−7.7 nm (σ = 8−11%) by a combination of pH control and low reaction

Calculation method of EG pH, additional TEM images, UV− visible absorption spectra, and temporal change of size distribution. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: +81-774-38-3120. E-mail: [email protected]. 2472

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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 and the KAITEKI Institute, Inc. Acknowledgments are also extended to KAKENHI (23245028 to T.T.) from the Japanese Society for the Promotion of Science (JSPS) and a JSPS Research Fellowship for Young Scientists (23·1430 to T.I.). One of the authors (K.M.) thanks the Nippon Sheet Glass Foundation for Materials Science and Engineering for funding support.

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dx.doi.org/10.1021/jp305968u | J. Phys. Chem. C 2013, 117, 2467−2473