Abstract：The usage of semiconductor photocatalysts for removal of contaminants is one of the greenest and most effective methods under sunlight, whose core is obtaining high-efficient photocatalysts. The most widely studied photocatalysts are TiO2, ZnO, etc., but they cannot fully utilize sunlight due to their large band gapenergy, thus limiting their practical use. In addition to modifying TiO2 to improve its visible light catalytic activity, the development of other materials as photocatalysts is also an important solution. Bismuth based compound semiconductors have become important research objects for their abundant raw materials, various types, good solar response and excellent photocatalytic activity. Bismuth oxyhalide compounds [BiOX, X=Cl, Br, I] exhibit excellent photocatalytic activity owning to layered structure, but they still have low photocatalytic efficiency when used alone. However, their photocatalytic degradation efficiency could be improved by preparing solid solutions (a mixture of solids that are molecularly dispersed with each other). In this work, a low-temperature wet chemical method can be used to obtain solid solutions BiOCl1-xIx and BiOBr1-xIx with sheet like structures, which is prepared by the reaction of a certain proportion of KI/KBr or KI/KCl aqueous solution with Bi2O3/HAc solution for half an hour at room temperature. X-ray diffraction (XRD) patterns showed that the synthesized BiOCl1-xIx and BiOBr1-xIx samples have good crystallinity and can form a solid solution in the range of x=0～1. The prepared solid solution was found to have an irregular sheetlike shape by a transmission electron microscope (TEM). X-ray photoelectron spectroscopy (XPS) tests further demonstrate their surface element composition and chemical state. Ultraviolet-visible diffuse reflectance spectroscopy (DRS) analysis indicates the red-shifted absorption edge of the solid solution and decreased band gap energy as the iodine content increased, so the visible light absorption capacity is enhanced and the number of generated carriers is agumented. The photocatalytic tests of MO degradation under visible light excitation manifest that BiOCl0.25I0.75 and BiOBr0.25I0.75 exhibit the highest photocatalytic activity. Cyclic experiments show that BiOCl0.25I0.75 and BiOBr0.25I0.75 have high stability. Photocatalytic mechanism studies show that the active species in the photocatalytic degradation of MO in these bismuth oxyhalide samples were holes and superoxide ion radicals. Combined with their energy band structures, it is believed that the formation of solid solution not only increases the visible light absorption capacity, but also modulates its energy band structure. Compared with BiOI, the formation of solid solution lowers the valence band position and raises the conduction band position. Therefore, the reducing ability of the photogenerated electrons and the oxidizing ability of the holes are enhanced, so that the catalytic performance is improved. The novelty of this work is low-temperature solid solution preparation, which avoids hydrothermal method or the addition of surfactants. Furthermore, the prepared BiOCl1-xIx and BiOBr1-xIx solid solutions, especially BiOCl0.25I0.75 and BiOBr0.25I0.75, have excellent photocatalytic degradation ability for MO under visible light excitation. Moreover, the catalysts have good stability, so it is expected to be applied in environmental management.
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