Photophysical properties and photocatalytic activities under visible light irradiation of silver vanadates

Abstract

α-AgVO₃, β-AgVO₃, Ag₄V₂O₇ and Ag₃VO₄ prepared by precipitation and solid-state reactions showed intense absorption bands in the visible light region due to band gap transitions. Comparison of a diffuse reflectance spectrum of α-NaVO₃ with that of α-AgVO₃ with a diopside-type structure revealed that a band gap (2.5 eV) of α-AgVO₃ was 0.6 eV smaller than that (3.1 eV) of α-NaVO₃. The electronic band structure study using a plane-wave based density functional method indicated that the decrease in the band gap of α-AgVO₃ was due to Ag 4d orbitals which formed a valence band at a more negative level than O 2p orbitals. Among α-AgVO₃, β-AgVO₃, Ag₄V₂O₇ and Ag₃VO₄, only Ag₃VO₄ showed a photocatalytic activity for O₂ evolution from an aqueous silver nitrate solution under visible light irradiation. Holes photogenerated in Ag₃VO₄ can migrate to the reaction sites on the surface more easily than those of other silver vanadates, because the content of silver forming a valence band is large. It resulted in that holes photogenerated in Ag₃VO₄ are able to oxidize H₂O to form O₂.