Defect physics of intrinsic point defects in BiPO₄ photocatalysts: a hybrid functional study

In this work, the intrinsic point defect properties of bulk BiPO4 under different growth conditions are intensively investigated and explored using first-principles hybrid functional calculations. It is found that Bi vacancies and O vacancies are the primary native defects in BiPO4. Under O-poor conditions, BiPO4 acts as an intrinsic insulator because the O vacancy defects (donor) and the Bi vacancy defects (acceptor) compensate for each other. Under Bi-poor conditions, good p-type conductivity is observed in BiPO4, which affirms the observed p-type conductivity behavior in experiments. Bi vacancies in BiPO4 are very shallow, which make it an excellent acceptor and are mostly responsible for the p-type character. In addition, it is found that the primary Bi vacancy defects of BiPO4 hardly affect its electronic structure and optical absorption spectrum regardless of the charge states. In contrast, the neutral O vacancy defects in BiPO4 introduce an impurity energy level near the VBM and induce a new optical absorption peak at around 370 nm. Furthermore, the O vacancies should be favorable for enhancing the production and separation efficiencies of the photo-generated electrons and holes in BiPO4. While Bi vacancies easily provide p-type carriers, simultaneously, they could become the active sites for the photocatalytic reactions because of their dominant -3 charge state. Therefore, understanding the defect physics in BiPO4 photocatalysts is believed to be beneficial for more research in developing BiPO4 or BiPO4-based photocatalysts.