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

alpha-AgVO3, beta-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 alpha-NaVO3 with that of alpha-AgVO3 with a diopside-type structure revealed that a band gap (2.5 eV) of alpha-AgVO3 was 0.6 eV smaller than that (3.1 eV) of alpha-NaVO3. The electronic band structure study using a plane-wave based density functional method indicated that the decrease in the band gap of alpha-AgVO3 was due to Ag 4d orbitals which formed a valence band at a more negative level than O 2p orbitals. Among alpha-AgVO3, beta-AgVO3, Ag4V2O7 and Ag3VO4, only Ag3VO4 showed a photocatalytic activity for O-2 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 O-2.