Enhanced visible-light photocatalytic activity of BiOBr/BiOCl heterojunctions: A hybrid density functional investigation on the key roles of crystal facet and I-doping

In this work, (001) and (010) facets of BiOBr and BiOCl are selected to construct BiOBr(001)/BiOCl(001), BiOBr(010)/BiOCl(010), BiOBr(001)/BiOCl(010) and BiOBr(010)/BiOCl(001) heterostructure models for investigating the effect of crystal facet engineering on the photocatalytic activity of BiOBr/BiOCl through hybrid functional calculations. We found that the crystal facet is critical to the photocatalytic performance of BiOBr/BiOCl. The BiOBr(001)/BiOCl(001) and BiOBr(010)/BiOCl(010) with the same facet combinations in BiOBr/BiOCl present the unexpected type I band alignments and the poor photocatalytic activities. In contrast, the BiOBr(001)/BiOCl(010) and BiOBr(010)/BiOCl(001) with different facet combinations present the type II and direct Z-scheme characters, respectively, so that they can effectively separate the photo-excited electron-hole pairs in space. Especially, the direct Z-scheme BiOBr(010)/BiOCl(001) heterostructure can exhibit the stronger redox ability and better visible light absorption, further presenting excellent photocatalytic performance, in contrast to the type II BiOBr(001)/BiOCl(010). The investigations also show that Iodine (I) element doping can make type I BiOBr(001)/BiOCl(001) change to a direct Z-scheme band alignment and dramatically enhance its photocatalytic activity. This helps to interpret the related experimental results. Notably, I doping can significantly improve the visible light absorption of BiOBr(010)/BiOCl(001) heterostructure, enhance the solar-to-hydrogen (STH) efficiency, and still maintain its superior direct Z-scheme character. Therefore, these theoretical studies can provide valuable guidance in developing novel BiOBr/BiOCl heterostructures with high photocatalytic activities.