Optical properties and visible light-induced photocatalytic activity of bismuth sillenites (Bi12XO20, X = Si, Ge, Ti)

Three bismuth sillenite compounds with the general composition, Bi12XO20 (X = Si, Ge, Ti), were prepared by a conventional solid state synthesis route and their local crystal structure, light absorption and visible-light induced photocatalytic activities were systematically investigated. The prepared powder samples show comparable particle size distribution (~ 0.5-5 µm) and BET surface area (~ 0.8 m2/g). The optical band gap energies were estimated from diffuse reflectance spectra to be in the range 2.6-2.9 eV. The local crystal structure variation and its influence on the electronic band structure was elucidated in the three sillenite compounds by Raman analysis and density functional theory (DFT). The visible-light induced photocatalytic activity was evaluated by monitoring the degradation of rhodamine B dye solution under visible-light irradiation (> 420 nm). We found that the Bi12TiO20 powder shows the highest photocatalytic activity, which is attributed to the higher visible light absorption and the smaller band gap resulting from its distinct local crystal structure (longer Bi-O distance).

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