Nickel and either tantalum or niobium-codoped TiO2 and SrTiO3 photocatalysts with visible-light response for H2 or O2 evolution from aqueous solutions

TiO2 (band gap = 3.0 eV) and SrTiO3 (band gap = 3.2 eV) codoped with nickel and either tantalum or niobium ions showed photocatalytic activities for O2 evolution from an aqueous silver nitrate solution and H2 evolution from an aqueous methanol solution under visible light irradiation (λ > 420 nm). The visible-light responses were due to the charge-transfer transition from the electron donor levels formed by the 3d orbitals of doped Ni2+ to the conduction bands of the host materials. Codoping of a tantalum or niobium ion made the absorption intensity in the visible light region strong for TiO2 doped with nickel ions, resulting in the enhancement of the photocatalytic activity for O2 evolution under visible light irradiation. SrTiO3 codoped with nickel and tantalum showed higher activity for H2 evolution than SrTiO3 doped with only nickel. This is due to the fact that codoped Ta5+ ions played a role in charge compensation, resulting in the suppression of the formation of Ni3+ ions, which are expected to trap photogenerated electrons.