Synergistic Effect of Ion Doping and Type-II Heterojunction Construction and Ciprofloxacin Degradation by MIL-68(In,Bi)-NH2@BiOBr under Visible Light

Abstract

Heterojunction structure and ion doping techniques are viable tactics in facilitating the generation and separation of photogenerated electrons and holes in photocatalysis. In the current study, a novel Bi ion-doped MIL-68(In,Bi)-NH₂@BiOBr (MIBN@BOB) type-II heterojunction was first synthesized in a one-step solvothermal reaction. Doping of Bi ions not only broadened the light-sensing range but also provided reliable anchor sites for the in situ growth of BiOBr. Meanwhile, the heterostructure supplied new channels for photogenerated carriers, accelerating the transfer and inhibiting the recombination of photogenerated electron–hole. The obtained MIBN@BOB exhibited enhanced photocatalytic performance (91.1%) than MIL-68(In)-NH₂ (40.8%) and BiOBr (57.5%) in ciprofloxacin (CIP) degradation under visible light, with excellent reusability. Photocatalysts were characterized in detail, and a series of photoelectrochemical tests were utilized to analyze the photoelectric properties. MIBN@BOB were deduced to conform the electron conduction mechanism of conventional type-II heterojunctions. More importantly, based on the above experiments and density functional theory (DFT) calculation, BiOBr-Bi in MIBN@BOB can serve as the major active sites of CIP enrichment, and •O₂^– and ¹O₂ generated at the BiOBr interface can react with the adsorbed CIP directly. Lastly, the possible degradation products and pathways of CIP were analyzed by liquid chromatography-tandem mass spectrometry (LC/MS/MS). This study provides a reference for the construction of ion-doping-modified metal–organic framework (MOF)-based heterojunction photocatalysts and their application in antibiotic removal.