Environmental friendly nano-star CdS coupled ZnS on bi-polymer matrix: Unravelling defects-rich nanoplatform for ultrahigh white light active direct S-scheme photodegradation of organic pollutants

Photocatalysis is a reliable technology to treat polluted wastewater. Herein, we report S-scheme photocatalyst namely ZnS/CdS nanocomposites (NCs) with S-vacancies and interstitial defect states, immobilised on the polymer matrix of cellulose and chitosan. It showed significant performance in photocatalytic degradation of Rhodamine B (RhB) dye. Defect sites present in ZnS facilitates charge separation and two-photon excitation concomitantly with a bandgap shift in ZnS/CdS NCs to 2.65 eV and promotes visible light harvesting. XRD analysis demonstrated the highly crystalline nature of the NCs. TEM and SEM were used to observe the crystal phases, particle morphology and defect sites. XPS and EDAX ensured the composition and purity of the synthesized NCs. FTIR displayed the peaks representing the bonds present in metal sulphides and chitosan/cellulose matrix. The photodegradation of RhB by ZnS/CdS NCs reached a maximum of 99.18% in 120 min which displayed 6.8 and 10.25 times higher rate than pure ZnS and CdS respectively. Further, ZnS/CdS NCs was able to achieve the TOC reduction of 86% after the photodegradation of RhB. Defect engineering played a vital role in enhancing the photocatalytic activity of NCs. PL spectra revealed the prolonged life time of photogenerated charge carriers and EIS analysis was used to investigate the charge mobility of the fabricated materials. Superoxide radical was found to be the major radical involved in photodegradation of RhB by NCs. The degradation pathway of RhB was elucidated based on GC-MS/MS analysis. The recyclability of NCs was tested for six consecutive cycles and remarkable stability was observed. The results of in vitro cytotoxicity assay, and cellular reactive oxygen species generation suggest that ZnS/CdS NCs are safety photocatalysts to environment and human being.