Composition Dependence of the Photocatalytic Activities of BiOCl1-xBrx Solid Solutions under Visible Light

We prepared BiOCl1-xBrx (x=0-1) solid solutions and characterized their structures, morphologies, and photocatalytic properties by X-ray diffraction, diffuse reflectance spectroscopy, scanning electron microscopy, Raman spectroscopy, photocurrent and photocatalytic activity measurements and also by density functional theory calculations for BiOCl, BiOBr, BiOCl0.5Br0.5. Under visible-light irradiation BiOCl1-xBrx exhibits a stronger photocatalytic activity than do BiOCl and BiOBr, with the activity reaching the maximum at x=0.5 and decreasing gradually as x is increased toward 1 or decreased toward 0. This trend is closely mimicked by the photogenerated current of BiOCl1-xBrx, indicating that the enhanced photocatalytic activity of BiOCl1-xBrx with respect to those of BiOCl and BiOBr originates from the trapping of photogenerated carriers. Our electronic structure calculations for BiOCl0.5Br0.5 with the anion (O2-, Cl, Br) and cation (Bi3+) vacancies suggest that the trapping of photogenerated carriers is caused most likely by Bi3+ cation vacancies, which generate hole states above the conduction band maximum.