2D/3D ZnS/SnO2 photocatalyst with Z-scheme heterojunction for efficient degradation of organic pollutants

Abstract

A series of 2D/3D ZnS/SnO₂-T (T = 400 °C, 500 °C, 600 °C, 700 °C) samples were synthesized, which were further applied as photocatalysts to degrade the organic pollutants. The crystal structures, particle morphologies, interfacial and photoelectric properties of the ZnS/SnO₂ semiconductor photocatalysts were characterized by XRD, SEM, FT-IR, XPS, UV–Vis, CV and electrochemical impedance spectra (EIS). The catalytic performance of the ZnS/SnO₂ photocatalysts was performed using methylene blue (MB) and rhodamine B (RhB) as the target pollutants, of which the ZnS/SnO₂-500 exhibited the highest photocatalytic degradation performance, capable of degrading 95.02 % of MB and 59.80 % of RhB, respectively. The degradation of MB was consistent with first-order kinetics, with a degradation rate constant of 0.587 h⁻¹ under optimal conditions (20 mg/L of MB, pH = 5, 40 mg of catalyst, 30 °C). This was attributed to the higher photoresponsive ability and lower electrochemical impedance as well as the enhanced visible light absorption capability of ZnS/SnO₂-500 sample. Importantly, the ZnS/SnO₂-500 photocatalyst can also be used to degrade various colorless pollutants including indole, tetracycline, norfloxacin, and ciprofloxacin, with degradation efficiencies of 46.65 %, 75.23 %, 53.98 %, and 59.19 %, respectively. Moreover, the presence of oxygen vacancies in the ZnS/SnO₂ solid was confirmed according to the results of XPS and ESR analyses, which played a key role in the photocatalytic process and promoted the generation of superoxide radicals and hydroxyl radicals. Finally, the high separation and distribution of electron-hole pairs of DFT calculations suggested that the electron was transmitted from SnO₂ to ZnS via the newly-formed Sn-S-Zn bond between the interfacial structures, further confirmed the proposed Z-scheme mechanism.