Highly dispersed noble-metal/chromia (core/shell) nanoparticles as efficient hydrogen evolution promoters for photocatalytic overall water splitting under visible light

Abstract

Highly dispersed rhodium nanoparticles (1.7 ± 0.3 nm) prepared by a liquid-phase reduction method were loaded on a solid solution of GaN and ZnO without forming aggregates, achieving improved activity for visible-light-driven overall water splitting when the nanoparticles are coated with a chromia shell.

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The preparation of nanometre-sized particles has attracted remarkable research interest due to the wide range of potential applications of such materials in various fields.^1–3Nanoparticles of noble metals such as gold, platinum, rhodium, and palladium have been studied extensively, and effective preparation methods have been established.^4,5 In the field of photocatalysis, noble-metal nanoparticles, which are called ‘cocatalysts’, dispersed on a photocatalyst semiconductor improve charge separation and thereby increase photocatalytic activity.⁶ As the activity enhancement achieved is generally dependent on the uniformity of cocatalyst dispersion,⁷ methods for improving the dispersion of noble-metal nanoparticles are considered important to improving photocatalytic activity.

Photocatalytic water splitting under visible light has received much attention as a means for supplying clean and renewable hydrogen.^8–14 For application of a nanoparticle–semiconductor composite in the reaction, a number of metal oxides with d⁰ or d¹⁰ electronic configuration have been studied as semiconductor photocatalysts. Highly uniform dispersions of cocatalysts, which function as H₂ evolution promoters, on metal oxides have also been achieved in some cases, resulting in excellent photocatalytic activity.^12–14 The application of metal oxides for visible-light-driven overall water splitting is, however, complicated by the deep valence band positions (O 2p orbitals) of these materials, resulting in a band gap that is too large to absorb visible light.¹⁵ Since the N 2p orbital has a higher potential energy than the O 2p orbital, recent studies have focused on the introduction of a mid-gap level or a new valence band formed by N 2p or S 3p orbitals above the O 2p valence band to allow for both visible-light absorption and a conduction band potential negative enough for H₂ evolution.^8d,16

Our group has reported (oxy)nitrides (e.g., TaON and LaTiO₂N) as potential candidates for overall water splitting under visible light. Although (oxy)nitrides in general exhibit high photocatalytic activity for wateroxidation in the presence of an appropriate electron acceptor under visible light, the activity for water reduction is approximately one order of magnitude lower than that for wateroxidation.^8d To improve the overall efficiency of these oxynitrides, an effective modification method to promote the water reduction (i.e., H₂ evolution) is required. The promotion of H₂ evolution will be possible if nano-sized cocatalysts are highly dispersed on a photocatalyst. However, a modification method to allow for highly uniform dispersion of cocatalysts on (oxy)nitrides has yet to be reported, and thus remains a challenge.

The present authors have reported that an oxynitride, (Ga_1–xZn_x)(N_1–xO_x) solid solution,¹⁷ is capable of splitting water under visible light, when suitable cocatalysts are loaded.^9–11 Noble-metal-core/Cr₂O₃-shell nanoparticles, which are readily prepared by photodeposition, are a new type of H₂ evolution promoter.¹⁰ In this system, the backward reaction over the noble metal (water formation from H₂ and O₂) is prevented by the Cr₂O₃ shell;¹⁰ specifically, the amorphous Cr₂O₃ shell layer is permeable to protons and the evolved H₂ molecules, but not to O₂.¹⁸ Among the noble metals examined as the nanoparticle core, rhodium has been found to be the most effective for enhancing activity. However, in the original preparation method, the rhodium nanoparticles tended to aggregate on the catalyst.¹⁰ As the activity of a catalytic system is dependent on the surface area available for reaction,⁷ the activity of this system can be expected to be improved further by preventing the aggregation of cocatalyst nanoparticles. In the present work, a new method for loading noble-metal nanoparticles onto an (oxy)nitride photocatalyst is proposed using a catalytic system consisting of noble-metal/Cr₂O₃ (core/shell) nanoparticles and GaN:ZnO.¹⁰ The new preparation method achieves more uniform dispersion of nanoparticles and hence is expected to provide an increase in photocatalytic activity.

Scheme 1 Procedural flow of proposed LPR-based method: (A) Rh nanoparticles are stabilized by organic ligand molecules before cation exchange. (B) Stabilized Rh nanoparticles after cation exchange. (C) Electrostatic adsorption on GaN:ZnOcatalyst. (D) Removal of organic ligand.

The proposed loading method involves the preparation of nanoparticles by liquid-phase reduction (LPR) of rhodium(III) chloride (RhCl₃) in the presence of sodium 3-mercapto-1-propanesulfonate to afford Rh nanoparticles stabilized by organic ligands. LPR is a commonly used method for the preparation of noble-metal nanoparticles.^4,5 The preparation procedure is summarized in Scheme 1. An aqueous solution containing equal molar amounts (15–90 µmol, corresponding to 0.5–3.0 wt% with respect to GaN:ZnO powder) of RhCl₃ and sodium 3-mercapto-1-propanesulfonate was prepared in a conical flask and reduced by sodium borohydride under air. Sodium mercapto-1-propanesulfonate was chosen as a short protective ligand because it can serve to form monodisperse small Rh nanoparticles and be easily removed after immobilization onto GaN:ZnO. The amount of sodium borohydride (NaBH₄) added to the solution was 10 times the amount of RhCl₃ employed. The solution exhibited a broad tailing absorption in the ultraviolet–visible region after the addition of NaBH₄, indicating that the Rh(III) species were reduced to form metallic Rh (see Fig. S1 of the ESI† ). The as-prepared Rh nanoparticles were 1.7 ± 0.3 nm in size (Fig. 1). After purification, H-type Amberlyst was added to the solution to convert the terminal sulfonate groups to sulfonic acid groups, because sulfonate-terminated Rh nanoparticles do not adsorb to GaN:ZnO. After removal of the Amberlyst by filtration, the nanoparticles were loaded onto the GaN:ZnOcatalyst by adding 0.300 g of the powdered catalyst to the solution. During the loading process, both hydrogen bonding and acid–base interactions between sulfonic acid groups on Rh nanoparticles and surface hydroxyl and amino groups on GaN:ZnO might play an important role. After letting the solution stand for 30 min, the organic ligands were eliminated by calcination under vacuum at 673 K for 30 min. Elemental analysis by X-ray fluorescence spectroscopy confirmed that sulfur species derived from organic ligands were completely eliminated by calcination. Transmission electron microscopy (TEM) images of the Rh-loaded catalyst (Figs. 2a and b) indicate that the Rh nanoparticles are present as individual particles and are rarely aggregated. The particle size distribution histograms for 200 nanoparticles on the catalyst (Fig. S2 of the ESI† ) indicate an average particle diameter of 1.9 ± 0.6 nm, 4 times smaller than the particles prepared previously by photodeposition.^10a No appreciable change in particle size was observed upon annealing. The present preparation method is thus effective for achieving highly uniform dispersion of Rh nanoparticles on GaN:ZnO, which is difficult to realize by the photodeposition method.

Fig. 1 TEM image of Rh nanoparticles stabilized by sodium 3-mercapto-1-propanesulfonate.

Fig. 2 (a, b) TEM images of Rh nanoparticles loaded on GaN:ZnO. (c, d) The same specimens after coating with Cr₂O₃. Nanoparticles were prepared using 75 µmol of Rh.

To prepare this catalytic system for use in overall water splitting, the loaded Rh nanoparticles were subsequently coated with a Cr₂O₃ shell by photodeposition according to the reported method.¹⁰ Our previous study by means of X-ray photoelectron spectroscopy and X-ray absorption spectroscopy has revealed that Cr species introduced according to this manner on Rh/GaN:ZnO are trivalent chromium oxide (Cr₂O₃).^10bTEM images of the Cr₂O₃/Rh/GaN:ZnO system thus prepared (Figs. 2c and d) show that a Cr₂O₃ layer of ca. 2 nm in thickness was uniformly deposited on the nanoparticles. This Cr₂O₃ thickness is the same as that achieved in previous syntheses,¹⁰ suggesting that the shell thickness is independent of the loading method employed for Rh nanoparticles. We have also attempted to control the thickness of the Cr₂O₃ shell on Rh/GaN:ZnO.^10b Although the precise control of the shell thickness still remains a challenge, the result showed that water-splitting rate tends to increase with increasing the shell thickness.

Overall water splitting reactions were conducted using this catalytic system under visible irradiation (λ > 400 nm) according to the same conditions as reported previously.¹⁰ As shown in Fig. 3, the activity was found to increase with the amount of Rh added, with the maximum activity obtained with Rh addition of 45 µmol (1.5 wt%) or higher. The experimental error in determination of activity is considered to be approximately 10%. The saturation behavior observed with Rh additions higher than 45 µmol (1.5 wt%) suggests that adsorption–desorption equilibrium is achieved in the preparation procedure. In fact, it was confirmed by inductively-coupled plasma atomic emission spectroscopy (ICP-AES) analysis that the actual loading amount of Rh remains unchanged (0.3–0.4 wt%) at addition levels higher than 45 µmol (1.5 wt%).

Fig. 3 Dependence of the rates of H₂ and O₂ evolution over Cr₂O₃/Rh/GaN:ZnO on the amount of Rh added in the preparation. Reaction conditions: catalyst, 0.15 g; distilled water, 400 mL; light source, high-pressure mercury lamp (450 W) via aqueous NaNO₂ solution filter to cut ultraviolet light; reaction vessel, Pyrex inner-irradiation type.

The time course of overall water splitting is shown in Fig. 4 with the result achieved by the previous photodeposition method. Both catalysts contain almost the same amount of Rh (0.3–0.4 wt%). Compared to previous method, the average rate of gas evolution from the present system is 3 times higher.¹⁹ The rate of gas evolution declines slightly at the beginning of the reaction, but the evolution of H₂ and O₂ generally proceeds steadily and stoichiometrically. The total H₂ and O₂ evolution over 5 h of reaction (3.2 mmol) in the present system is substantially larger than the amount of catalyst employed (0.15 g; 1.85 mmol), confirming the catalytic cycle. This improvement in activity is attributed to the highly uniform dispersion of nanoparticles on the catalyst surface as efficient H₂ evolution sites. It has been confirmed that Pd- and Pt-based nanoparticles can also be loaded onto GaN:ZnO by the present method, and the resultant activities are also higher than those achieved previously by the photodeposition method (see Fig. S3 of the ESI† ). This new loading method is therefore superior to the previous photodeposition method in terms of the suppression of nanoparticle aggregation, leading to higher activities.

Fig. 4 Time course of overall water splitting over Cr₂O₃/Rh/GaN:ZnO prepared by the present method, and the previous photodeposition method. Reaction conditions: catalyst, 0.15 g; distilled water, 400 mL; light source, high-pressure mercury lamp (450 W) via aqueous NaNO₂ solution filter to cut ultraviolet light; reaction vessel, Pyrex inner-irradiation type. Almost the same amount of Rh (0.3–0.4 wt%) is loaded on each catalyst.

It should be noted that the activity of the present catalytic system reaches saturation at the Rh loading of 0.3–0.4 wt%, while the optimal loading amount Rh in the previous photodeposition method is around 0.75 wt%. Although the quantum efficiency of the present catalyst remains lower than that of a similar analogue modified with a Rh–Cr mixed oxide,¹¹ it is expected that the performance of the present system will be improved by refining the preparation to allow for more loading of Rh on GaN:ZnO and/or by employing an appropriate post-treatment. Using the present method in conjunction with other methods to control the size of noble-metal nanoparticles,²⁰ it may be also possible to achieve precise control of cocatalyst particle size. These possibilities are currently under investigation.

In conclusion, a new method for the preparation and loading of a nanoparticle cocatalyst for H₂ evolution in overall water splitting was proposed using GaN:ZnO as a photocatalyst. It was demonstrated that the proposed method achieves highly uniform dispersion of nanoparticles on the catalyst surface, with a corresponding increase in catalytic activity for overall water splitting.

Acknowledgements

The authors thank the staff of Mitsubishi Chemicals Co. for elemental analyses. This work was supported by the 21st Century Center of Excellence (COE) and the Research and Development in a New Interdisciplinary Field Based on Nanotechnology and Materials Science programs of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. One of the authors (K.M.) gratefully acknowledges the support of a Japan Society for the Promotion of Science (JSPS) Fellowship.

Notes and references

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19. Even at the optimal Rh loading condition for each case, the present catalyst recorded much better performance (426 µmol·h⁻¹ H₂, 213 µmol·h⁻¹ O₂) than the catalyst prepared by photodeposition (169 µmol·h⁻¹ H₂, 86 µmol·h⁻¹ O₂).
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Footnotes

† Electronic supplementary information (ESI) available: Experimental details, UV–vis spectra for the aqueous preparation solution before and after reduction by sodium borohydride, particle size distribution histograms for Rh nanoparticles on GaN:ZnO, and photocatalytic activity of Cr₂O₃/M/GaN:ZnO (M = Pd, Pt) for overall water splitting under visible irradiation (λ > 400 nm). See DOI: 10.1039/b9nr00186g

‡ Present address: Pioneering Research Unit for Next Generation, Kyoto University.

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