Direct water splitting into H_2 and O_2 under visible light irradiation with a new series of mixed oxide semiconductor photocatalysts

摘要

The research on a new series of solid photocatalysts with different crystal structures was reviewed. The first system is A_2B_2O_7 pyrochlore-crystal type: Bi_2MNbO_7 (M = Al, Ga, In and Y, rare earth, and Fe),which is cubic system and space group Fd3m. The second system is ABO_4 stibotantlite-crystal type: BiMO_4 (M = Nb~(5+), Ta~(5+)) in which both the triclinic system with space group P1 in the csae 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: InMO_4 (M = Nb~(5+), Ta~(5+)), which is monoclinic system and space group P2/a. Although these photocatalysis crystallize in the different crystal structure, they contain the same octahedral TaO_6 and/or NbO_6 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 2.4eV. Metal doped InTaO_4 photocatalysts were also investigated. Under visible light (λ > 420 nm) or ultra-violet irradiation, the H_2 and/or O_2 evolutions were observed from pure water as well as aqueous CH_3OH/H_2O and AgNO_3 solutions. The photocatalytic activity increase significantly by loading co-catalysts such as Pt, RuO_2 and NiO_x 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 NiO_x (partly oxidized nickel) promoted in_(0.9)Ni_(0.1)TaO_4 photocatalyst.