S-scheme MnS/ZnO heterojunction composites for photocatalytic degradation of tetracycline

Abstract

The practical applications of ZnO are somewhat limited due to its large wide band gap, requiring UV light for photocatalysis. To improve the visible-light performance of ZnO, a series of S-scheme MnS/ZnO heterojunction composites were fabricated by hydrothermal methods. Thorough characterization methods were utilized to investigate the microtopography, crystal structure, and photoelectric performance of the materials. The visible-light performance of MnS/ZnO was assessed with simulated antibiotic wastewater containing tetracycline hydrochloride (TC) as degradation object. The crystal structure of MnS/ZnO is mainly of ZnO hexagonal wurtzite structure and the overall morphology is short strip-like. MnS is tightly loaded on the ZnO surface and forms S-scheme heterojunctions at the contact sites. This structure can recombine ineffective carriers efficiently due to the existence of the internal electric field (IEF). Additionally, the presence of IEF can help retain effective carriers, leading to a significant promotion of the visible-light photocatalytic performance of MnS/ZnO. The trapping experiments indicate that hydroxyl radical (OH) and superoxide radical (O₂⁻) are the main active species in the photocatalytic reaction. MnS/ZnO composite photocatalysts exhibit better degradation efficiency of TC than pure MnS and ZnO, especially when the loading amount of MnS is 6 wt%. Under optimized experimental conditions, the MnS/ZnO composite catalyst (0.67 g/L) was employed to treat TC solution with an initial concentration of 20 mg/L. When the pH of the system was adjusted to approximately 7, a remarkable degradation efficiency of 97.8% was achieved after 100 min under visible light irradiation. Additionally, after four cycles of usage, MnS/ZnO can still degrade 90.3% of the TC, demonstrating the stability of this composite catalyst.