Direct water splitting into H2 and O2 under visible light irradiation with a new series of mixed oxide semiconductor photocatalysts

The research on a new series of solid photocatalysts with different crystal structures was reviewed. The first system is A(2)B(2)O(7) pyrochlore-crystal type: Bi2MNbO7 (M = Al, Ga, In and Y, rare earth, and Fe), which is cubic system and space group Fd3m. The second system is ABO(4) stibotantalite-crystal type: BiMO4 (M = Nb5+, Ta5+), in which both the triclinic system with space group PI in the case of M = Ta and the orthorhombic system with space group Pnna in the case of M = Nb. The third system is ABO(4) wolframite-crystal type: InMO4 (M = Nb5+, Th5+), which is monoclinic system and space group P2/a. Although these photocatalysts crystallize in the different crystal structure, they contain the same octahedral TaO6 and/or NbO6 in the different photocatalysts. The band structure of the photocatalysts is defined by Ta/Nb d-level for a conduction band and O 2p-level for a valence band. The band gaps of the photocatalysts were estimated to be between 2.7 and 2.4 eV. Metal doped InTaO4 photocatalysts were also investigated. Under visible light (lambda > 420 nm) or ultra-violet irradiation, the H-2 and/or O-2 evolutions were observed from pure water as well as aqueous CH3OH/H2O and AgNO3 solutions. The photocatalytic activity increases significantly by loading co-catalysts such as Pt, RuO2 and NiOx on the surface of the photocatalysts. Finally, direct water splitting into H-2 and O-2 under visible light irradiation was firstly established using newly synthesized NiOx (partly oxidized nickel) promoted In0.9Ni0.1TaO4 photocatalyst. (C) 2003 Elsevier Science B.V. All rights reserved.