Various photocatalyst materials aiming at water splitting are introduced [1]. Alkali and alkaline earth tantalates have arisen as a new group of photocatalyst materials for water splitting into H₂ and O₂ under ultraviolet light irradiation. Among the tantalates, a NiO (0.2 wt %)/NaTaO₃:La (2%) photocatalyst with a 4.1-eV band gap showed high activity for water splitting into H₂ and O₂ with an apparent quantum yield of 56% at 270 nm. The NiO/NaTaO₃:La photocatalyst have demonstrated highly efficient water splitting using a powdered photocatalyst system. Many visible-light-driven photocatalysts have also been developed through band engineering by doping of metal cations, new valence formation, and by making solid solution. BiVO₄, AgNbO₃, and TiO₂ co-doped with Rh and Sb photocatalysts showed high activities for O₂ evolution in the presence of sacrificial reagent (Ag⁺) under visible light irradiation. Pt/SrTiO₃ doped with Rh showed high activity for H₂evolution from aqueous solutions containing a reducing reagent. Solar water splitting under visible light irradiation has been achieved by construction of a Z-scheme photocatalysis system employing the visible-light-driven photocatalysts, Ru/SrTiO₃:Rh for H₂evolution and BiVO₄ for O₂ evolution, and an Fe³⁺/ Fe²⁺ redox couple as an electron relay. On the other hand, AgInS₂-CuInS₂-ZnS solid solution photocatalysts showed activity for solar H₂ productoin from aqueous solutions including sulfur compounds as electron donor.

[1] A. Kudo, Int. J. Hydrogen Energy, 32, 2673-2678 (2007), Pure Appl. Chem., 79 [11], 1917-1927 (2007).