Photocatalytic Activities of Layered Titanates and Niobates Ion-Exchanged with Sn2+ under Visible Light Irradiation

Energy gaps (EG) of Sn2+-exchanged layered metal oxides, Sn2+/KTiNbO5, Sn2+/K4Nb6O17, Sn2+/CsTi2NbO7, Sn2+/K2Ti4O9, Sn2+/K2Ti2O5, and Sn2+/CS2Ti6O13, 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 H-2 evolution from an aqueous methanol solution under visible light irradiation. Sacrificial oxidation of Sn2+ also proceeded partly accompanied with the H-2 evolution. Sn2+/K4Nb6O17 also showed an activity for O-2 evolution from an aqueous silver nitrate solution under visible light irradiation. The stable Sn2+ in Sn2+/K4Nb6O17 to redox reactions contributed to the photocatalytic H-2 and O-2 evolution reactions in the presence of sacrificial reagents.