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.