A novel series of water splitting photocatalysts NiM2O6 (M=Nb,Ta) active under visible light

A novel series of water splitting solid photocatalysts NiM2O6 (M=Nb,Ta) were synthesized by the solid-state reaction method. The NiNb2O6 photocatalyst crystallizes in the Columbite-type structure, orthorhombic with space group pbcn, while NiTa2O6 belongs to the tri-Rutil-type structure, tetragonal system with space group P42/mnm. Both photocatalysts showed high activity to evolve H2 from an aqueous methanol solution under ultra-violet light irradiation. The new photocatalysts can also split water to generate H2 from pure water under visible light irradiation (λ>420 nm) without any co-catalyst. The band gaps of NiNb2O6 and NiTa2O6 were estimated to be 2.2 and 2.3 eV, respectively. The difference in the band gaps of the photocatalysts is supposed to come from their different conduction band levels formed by Nb 4d in NbO6 and Ta 5d in TaO6.