Photocatalytic hydrogen and oxygen formation over SiO2-supported RuS2 in the presence of sacrificial donor and acceptor
Introduction
Sulfide semiconductors such as CdS which are able to absorb visible light have been studied extensively as attractive material for the photoelectrochemical conversion of light energy into electrical and chemical energy. These compounds, however, are generally unstable in aqueous solution under irradiation due to self-oxidation by photogenerated holes (photocorrosion). On the other hand, it is known that ruthenium disulfide (RuS2) is highly stable against photocorrosion. Band gap (Eg) values of RuS2, 1.85 eV [1], 1.3 eV [2], [3] for single-crystal, 1.48 eV [4] for a film obtained by electrochemical deposition were reported, suggesting potentially good efficiency in photoelectrochemical energy conversion.
Tributsch and co-workers have been studying the photoelectrochemical property of RuS2 extensively and have discussed its stability [1], [2], [5], [6], [7]. They reported that single-crystal RuS2 electrodes (n-type) with an Eg of 1.85 eV and flat band potential (Efb) of −0.48 V versus NHE (pH = 0) were stable photoanodes in water oxidation in aqueous electrolyte [1]. They explained the stability of RuS2 in terms of d-states in the valence band [2], [6]. The valence band of RuS2 has only d-character, different from ZrS2, MoS2, and WS2. The oxidation reaction with photogenerated holes takes place on the Ru-based surface states, leading to kinetic inhibition of corrosion reaction, i.e. oxidation of S22− with holes. Sakata and co-workers studied the photoelectrochemical behavior of the TiO2 electrodes coated with colloidal RuS2 particles as a semiconductor sensitization electrode [8], [9], [10]. They concluded from the photoelectrochemical investigations that the Eg of RuS2 colloids was 2.8 eV and that the Efb was –0.6 V versus NHE (pH = 7) [10]. They supposed that such a large Eg of colloidal RuS2 compared to the single-crystal result was due to a size-effect.
While electrochemical and photoelectrochemical studies of RuS2 have been reported previously, heterogeneous photocatalytic reactions over an RuS2 catalyst have not been investigated so far. It is expected from the value of the Eg and the Efb of RuS2 colloids [10], and from their high stability, that hydrogen and oxygen formation by photocatalytic decomposition of water can proceed energetically over an RuS2 catalyst. Hydrogen formation over sulfide and supported-sulfide semiconductor photocatalysts, CdS [11], [12], [13], CdS/SiO2 [14], CdS–ZnS/SiO2 [15], and WS2/SiO2 [16], [17] in sulfide ion and methanol aqueous solutions have been reported. On the other hand, a sulfide semiconductor photocatalyst finds it hard to produce oxygen photocatalytically from water due to competing self-oxidized photocorrosion, while those energy levels of the valence band can bring about water oxidation energetically. Oxygen formation over RuO2/CdS and Rh2O3/CdS photocatalysts under visible light irradiation in PtCl62− aqueous solution was only reported by Grätzel and co-workers [18].
We have reported for the first time that hydrogen and oxygen were produced, respectively, over RuS2 and SiO2-supported RuS2 photocatalysts under UV light (>300 nm) irradiation in the presence of a sacrificial electron donor and acceptor [19]. In this study, in order to clarify the photocatalytic activity of RuS2 semiconductor photocatalysts for water decomposition, detailed study of RuS2 photocatalytic system including characterization of the catalysts was conducted. The results of the effects of the amount of supported RuS2, loading of a Pt catalyst, support on TiO2 in hydrogen and oxygen formation over RuS2 photocatalysts under UV and visible light irradiation are shown and discussed in this publication.
Section snippets
Preparation of photocatalysts and characterization
Non-supported RuS2 catalyst was prepared by mixing a 0.15 mmol dm−3 RuCl3 nH2O (Soekawa Chemicals) acetonitrile solution containing a small amount of water and a 1.3 mol dm−3 Na2S (Wako Pure Chemical Industries, Ltd.) aqueous solution. The mixture was stirred continuously for 15 h at room temperature to form an RuS2 black precipitate. Dried RuS2 precipitate was heat-treated at 400–500°C under Ar atmosphere for 2 h. Supported RuS2 was prepared by the following procedure: RuCl3 was supported on SiO2
Characterization of the catalysts
Fig. 1 shows the XRD spectrum of a RuS2 black powder prepared in acetonitrile solution and then treated at 400°C under an Ar atmosphere for 2 h. The peaks of this spectrum indicate the formation of RuS2. The ratio of peak intensity is in agreement with that reported previously [9], while it is different from that of RuS2 thin film synthesized electrochemically [4]. XRF measurements of prepared RuS2 catalysts also supported RuS2 formation (as shown in Table 2 later).
UV–VIS absorption spectra of
Photocatalytic activity of the RuS2/SiO2 catalyst
The Eg of SiO2-supported RuS2 estimated from UV–VIS absorption spectrum was 2.3 eV, which is smaller than that of colloidal RuS2, 2.8 eV, reported previously [10]. This value, however, is larger than that of single-crystal: 1.3–1.85 eV [1], [2], [3]. An increase in the Eg compared to that of single-crystal would be due to the size effect. The result of the TEM analysis of 1 wt.% RuS2/SiO2 indicated that the particle size of supported RuS2 was 3–5 nm, as shown in Fig. 3a. From the results showing
Conclusions
Hydrogen and oxygen were produced, respectively, over an SiO2-supported RuS2 photocatalyst whose particle size is 3–5 nm and band gap is 2.3 eV, under UV light (>300 nm) and visible light (>400 nm) irradiation in the presence of sacrificial electron donor and acceptor. Non-supported RuS2 showed quite a low activity for hydrogen formation under UV irradiation. These results suggests that RuS2 small particles supported on SiO2 have high photocatalytic activity for water decomposition, due to large
Acknowledgements
This work was assisted by Japan Science and Technology Corporation.
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