Water splitting over new niobate photocatalysts with tungsten-bronze-type structure and effect of transition metal-doping

Photophysical properties and photocatalytic activities for water splitting over KM₂Nb₅O₁₅ (M=Sr and Ba) and K ₂LnNb₅O₁₅ (Ln=La, Pr, Nd, and Sm) with tungsten bronze-type structure were investigated. Single phases of KM₂Nb ₅O₁₅ and K₂LnNb₅O₁₅ were successfully prepared by solid-state reaction (SSR) method and polymerizable complex (PC) method. The band gaps of these niobates were estimated to be 3.1-3.5a eV. These metal oxides loaded with an activated NiO_x cocatalyst showed photocatalytic activities for water splitting into H ₂ and O₂ under UV irradiation. When K₂LaNb ₅O₁₅ was doped by Rh ions, a new visible-light absorption band was observed around 400-500a nm in addition to the band gap absorption band of the K₂LaNb₅O₁₅ host. K₂LaNb ₅O₁₅:Rh showed photocatalytic activities for H₂ or O₂ evolution from an aqueous solution containing a sacrificial reagent under visible-light irradiation.