Role of Ag⁺ in the Band Structures and Photocatalytic Properties of AgMO₃ (M:  Ta and Nb) with the Perovskite Structure

Photophysical and photocatalytic properties of perovskite-type materials AgMO3 (M: Ta and Nb) consisting of Ag+ and d0 ions were investigated. The band gaps of AgTaO3 and AgNbO3 were 3.4 and 2.8 eV, respectively, being 0.6 eV smaller than the band gaps of NaTaO3 (4.0 eV) and NaNbO3, even if the crystal structures of AgMO3 were similar to those of NaMO3. It was found from the electronic band structure study, using the plane-wave-based density functional method, that a hybrid orbital of Ag 4d and O 2p formed a valence band at a more negative level than O 2p orbitals. AgTaO3 showed photocatalytic activity for pure water splitting into H2 and O2 under UV-light irradiation. AgNbO3 has arisen as a new visible-light-driven photocatalyst possessing the ability to evolve H2 or O2 from water in the presence of sacrificial reagents.