Development of Two-Step Water Splitting Systems Under Visible Light Using Sulfide Photocatalysts

Photocatalytic water splitting using semiconductor materials has been attracting much attention due to the potential for the clean production of H₂ from water using solar energy. Most of metal sulfide semiconductors, such as CdS, possess appropriate band levels for water reduction and oxidation as well as a narrow band gap allowing visible light absorption because the valence bands of them are mainly formed by S 3p orbital. However, the most of sulfides have no capability of O₂ evolution from water due to the occurrence of self-oxidative deactivation by photogenerated holes. Even for the sacrificial H₂ evolution, they require particular electron donor such as S^2– and SO₃^2– to generate H₂ stably. In the present study, we attempted to combine some sulfides such as CdS with various reversible electron donors in order to develop two-step water splitting systems based on sulfide photocatalysts. We found that Pt/CdS can oxidize [Fe(CN)₆]^4– and generate H₂ stably, and combining it with an O₂ evolution system (TaON photoanode¹⁾) enabled us to split water into H₂ and O₂ under visible light irradiation (Fig. 1).

1) Higashi, M.; Domen, K.; Abe, R. J. Am. Chem. Soc. 2012, 134, 6968–6971.