Switching heterojunction system from Type-II to S-scheme for efficient photocatalytic degradation of ciprofloxacin

In this study, oxygen-doped g-C3N4/ZnIn2S4 nanoflower heterojunction (denoted as OxgCN/ZISy, x = mass percentage ratio of O doping, y = loading molar ratio of ZnIn2S4 to g-C3N4) was synthesized for Ciprofloxacin (CIP) photocatalytic degradation. The optimal photocatalyst O20gCN/ZIS20 possesses excellent photocatalytic performance, and 98.9 % of CIP can be degraded within 60 min. The corresponding reaction rate constant is 0.045 min−1, which is 15.3 and 2.7 times higher than that of g-C3N4 and gCN/ZIS20, respectively. Compared with other heterojunction photocatalysts, the degradation rate of O20gCN/ZIS20 has more advantages. Additionally, O20gCN/ZIS20 exhibits favorable reusability and chemical stability, and the removal rate still exceeds 90 % after six continuous cycles. The free radical identification experiments show that O2•− and h+ play a key role in the photocatalytic degradation process, with the contribution rates of 92.0 % and 83.1 %, respectively. The enhancement mechanism of O20gCN/ZIS20 is that oxygen doping switches the heterojunction model from Type-II to S-scheme by converting the direction of band bending and built-in electric field, which is proved by in-situ X-ray photoelectron spectroscopy analysis. S-scheme O20gCN/ZIS20 accelerates the separation of the photogenerated carriers and provides more active species to attack CIP. Furthermore, the impact of environmental factors including solution pH, inorganic salts, organic matter, and water matrices is investigated. Combined with DFT theoretical calculation, liquid chromatography-mass spectrometry, and toxicity assessment, the photocatalytic degradation behavior of CIP is predicted. This study innovatively transforms the heterojunction model from Type-II to S-scheme by element doping, which provides a new idea for designing S-scheme heterojunction.