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

Abstract

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.