Bifunctional CdS QDs@ZnS/ZnO composites with double S-scheme heterojunctions and oxygen-rich vacancies for enhanced photocatalysis

Abstract

Oxygen vacancies (OVs) are recognised as active sites in photocatalytic reactions, but their key role in heterojunction composites remains to be elucidated. A double S-scheme CdS QDs@ZnS/ZnO heterojunction composite with OVs was successfully prepared by modifying ZnO-based semiconductor materials via an in situ chemical deposition and microwave-assisted hydrothermal method. Further characterization results confirmed that the prepared CdS QDs@ZnS/ZnO composites had a 3D rod-like flower cluster structure with a double S-scheme heterojunction structure. The constructed heterojunction composites improved the separation efficiency of electron–hole pairs through a synergistic effect of the inner-boundary electric field and OVs, narrowing the bandgap and broadening the light absorption range of ZnO, which can effectively degrade RhB under multi-mode photocatalytic conditions. The optimized CdS QDs@ZnS/ZnO heterojunction composite achieved an RhB photodegradation efficiency of 91.3% under simulated sunlight irradiation for 120 min, which was 2.3-times that of ZnO. Its hydrogen production reached 11.78 mmol g⁻¹ over 8 h, which was 127.6-times that of ZnO. The composite exhibited high cycling stability and reusability. The possible photocatalytic mechanism of the composite double S-scheme heterojunction structure was hypothesised based on the results of trapping experiments and density functional theory calculations. Our study provides an effective strategy for heterojunction photocatalysts for photocatalytic environmental remediation in practical applications for wastewater treatment.