Preparation and Characterization of CaO-Bi2O3 Complex Oxide

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The single phase of Ca4Bi6O13 crystal was obtained at lower calcination temperature in a citric acid complex method in comparison with a solid-state reaction method. It was confirmed that the photocatalytic activity of Ca4Bi6O13 for the degradation of methylene blue (MB) was higher in a citric acid complex method than in a solid-state reaction under visible-light irradiation (λ ? 420 nm). When Sr was added to Ca4Bi6O13, the optical band gap of Ca4Bi6O13 decreases slightly and the photocatalytic activity was improved. Further, we performed first-principles band calculation of Ca4Bi6O13 to determine the type of the optical transition, the characters of valence and conduction bands, and the band-gap energy. Ca4Bi6O13 is a direct-gap oxide and the energy states near the valence band maximum (VBM) and conduction band minimum (CBM) show wide dispersion, indicating a higher mobility of holes and electrons generated by band gap excitation. In addition, Ca4Bi6O13 has a distorted local structure consisting of [BiO6] polyhedron and a consequent dipole moment. Thus, a high photocatalytic ability of Ca4Bi6O13 is attributable to both the wide band dispersion near the valence band maximum and the conduction band minimum and its distorted local crystal structure.