Photophysical properties and pbotocatalytic 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 O₂ evolution from an aqueous silver nitrate solution under visible light irradiation, among various bismuth molybdates (Bi₂MoO₆, Bi₂Mo₂O ₉, and Bi₂Mo₃O₁₂) synthesized by solid-state and reflux reactions. Bi₂Mo₃O₁₂ (BG: 2.88 eV) also showed photocatalytic activity for O₂ evolution under full-arc irradiation of a Xe lamp (λ > 300 nm). The photocatalytic activity of the Aurivillius structure Bi₂MoO ₆ 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.