Photocatalytic activities of layered titanates and niobates ion-exchanged with Sn²⁺ under visible light irradiation

Energy gaps (EG) of Sn²⁺-exchanged layered metal oxides, Sn ²⁺/KTiNbO₅, Sn²⁺/K₄Nb ₆O₁₇, Sn²⁺/CsTi₂NbO₇, Sn²⁺/K₂Ti₄O₉, Sn²⁺/K ₂Ti₂O₅, and Sn₂₊/Cs ₂Ti₆O₁₃, were 0.7-1.2 eV narrower than those of the original layered metal oxides. The Sn²⁺-exchanged layered metal oxides had visible-light absorption bands that were due to the electronic transition from an electron donor level (a valence band maximum) consisting of Sn5s orbitals to conduction bands consisting of Ti3d and Nb4d orbitals. The potential of the electron donor level negatively shifted as the amount of ion-exchanged Sn²⁺ increased. They showed photocatalytic activities for H₂ evolution from an aqueous methanol solution under visible light irradiation. Sacrificial oxidation of Sn²⁺ also proceeded partly accompanied with the H₂ evolution. Sn²⁺/K ₄Nb₆O₁₇ also showed an activity for O ₂ evolution from an aqueous silver nitrate solution under visible light irradiation. The stable Sn²⁺ in Sn²⁺/K ₄Nb₆O₁₇ to redox reactions contributed to the photocatalytic H₂ and O₂ evolution reactions in the presence of sacrificial reagents.