Photophysical properties and photocatalytic activities of bismuth molybdates under visible light irradiation

Aurivillius structure Bi2MoO6 (BG: 2.70 eV) that is a low-temperature phase showed an intense absorption band in the visible light region and photocatalytic activity for O-2 evolution from an aqueous silver nitrate solution under visible light irradiation, among various bismuth molybdates (Bi2MoO6, Bi2Mo2O9, and Bi2Mo3O12) synthesized by solid-state and reflux reactions. Bi2Mo3O12 (BG: 2.88 eV) also showed photocatalytic activity for O-2 evolution under full-arc irradiation of a Xe lamp (gimel > 300 nm). The photocatalytic activity of the Aurivillius structure Bi2MoO6 prepared by the reflux method was dependent on the annealing temperature after the preparation. The crystallinity was the important factor for the activity. Calculation by the density functional method indicated that the conduction band of Aurivillius structure Bi2MoO6 was made up of Mo 4d orbitals. It turned out that the visible-light absorption of this photocatalyst was due to the transition from the valence band consisting of O 2p orbitals to the conduction band. The corner-sharing structure of the MoO6 octahedra contributed to the visible light response and the photocatalytic performance because excitation energy and/or photogenerated electron and hole pairs began to migrate easily in the Aurivillius structure.