Construction of a BiOCl/Bi3O4Cl heterojunction and its photocatalytic oxidation performance toward emerging contaminants in water

Bismuth oxychloride (BiOCl) possesses strong oxidative ability but suffers from limited visible-light utilization and rapid recombination of photogenerated charge carriers. In this study, BiOCl/Bi 3 O 4 Cl heterojunctions were fabricated by an ultrasound-assisted hydrothermal method, with sonication duration used to regulate phase coupling, nanosheet morphology, oxygen-defect abundance, and carrier dynamics. Among the prepared catalysts, U-20 exhibited the optimal photocatalytic performance. Under optimized conditions (pH 7 and catalyst dosage of 0.8 g/L), U-20 achieved 98.8% removal of ciprofloxacin (CIP) within 210 min and 93.4% removal of tetrabromobisphenol A (TBBPA) within 180 min, with apparent rate constants of 0.0177 and 0.0134 min −1 , respectively. The catalyst also showed good reusability, maintaining 91.8% and 86.1% removal efficiencies for CIP and TBBPA after five consecutive cycles, respectively. Structural and photoelectrochemical analyses showed that 20 min of sonication produced intimate BiOCl/Bi 3 O 4 Cl interfacial contact, the largest specific surface area, and the most efficient charge separation and transfer. By contrast, prolonged sonication increased the defect-related signal but induced nanosheet deformation and aggregation. Scavenger tests and electron spin resonance (ESR) spectroscopy identified photogenerated holes as the dominant oxidizing species, whereas superoxide radicals contributed through an auxiliary pathway. These results demonstrate that photocatalytic performance is governed by the balance among morphology, defect density, and interfacial charge transport rather than by the maximum oxygen-vacancy abundance, providing a simple strategy for optimizing BiOCl-based photocatalysts for emerging-contaminant treatment.