Regulation of built-in electric field in heterojunction for improved photocatalytic degradation of Ciprofloxacin: Role of element doping

For obtaining superior photocatalytic activity, further research is required to ascertain the nature of element doping on heterojunction photocatalysts. In this work, g-C3N4/CuS (Type-II heterojunction) and BiOBr/CdS (S-scheme heterojunction) were used as examples to explore how element doping affects heterojunction photocatalysts. The low-electronegativity B and Zn were doped in g-C3N4 and BiOBr, respectively. Then, a series of BαgCN/CSβ and ZnγBOB/CSδ heterojunction photocatalysts (α and γ were the molar ratio of doping elements, β and δ were the mass ratio of heterojunction components) were prepared and characterized. Among them, B10gCN/CS50 and Zn5BOB/CS10 showed the optimal photocatalytic degradation activity of Ciprofloxacin (CIP), and the kinetic constants were 0.0283 and 0.0461 min−1, which were 1.7 and 3.9 times than that of gCN/CS50 and BOB/CS10, respectively. Further reactive species identification experiments manifested that e− and O2•− played a key role in the B10gCN/CS50 photocatalytic system, and •OH and h+ are the primary active species involved in the Zn5BOB/CS10 system. The charge transfer path of the heterojunction was determined by X-ray photoelectron spectroscopy (XPS) and in-situ irradiated XPS, and the results revealed that element doping changed the Fermi level of the photocatalyst and enhanced the intensity of the built-in electric field (IEF). Through experimental data, characterization measures, and theoretical calculation, the feasibility of regulating the IEF at the heterojunction interface by element doping was confirmed, and a simple but effective approach to designing a heterojunction that boasts excellent photocatalytic performance was offered.