Hydrothermal preparation and characterization of Cd-doped ZnS photocatalyst for efficient dye wastewater degradation

Cadmium (Cd)-doped ZnS photocatalysts were synthesized via a facile hydrothermal strategy to enhance visible-light responsiveness and photocatalytic degradation efficiency toward organic dyes. An optimal Cd/Zn molar ratio of 7.5% (denoted as 7.5-CZS) markedly improved photocatalytic activity, achieving a Rhodamine B (RhB) degradation efficiency of 97% under visible-light irradiation, compared with only 44% for pristine ZnS. Similarly, 7.5-CZS exhibited excellent degradation performance toward methylene blue (MB) and malachite green (MG), with removal efficiencies of 98% and 97%, respectively. Structural and morphological analyses revealed that Cd incorporation suppressed particle agglomeration, induced a porous surface architecture, reduced crystallite size, and significantly increased the specific surface area. Optical characterization demonstrated a pronounced bandgap narrowing from 3.19 eV (ZnS) to 2.56 eV (7.5-CZS), accompanied by substantially quenched photoluminescence intensity, indicating inhibited recombination of photogenerated charge carriers. Moreover, 7.5-CZS maintained 84% RhB degradation efficiency after four successive cycles, confirming good photocatalytic stability. Reactive species trapping experiments identified superoxide radicals (·O2⁻) and hydroxyl radicals (·OH) as the dominant active species responsible for dye degradation. Overall, this study demonstrates that Cd doping is an effective approach to activating ZnS under visible light, and the optimized Cd-doped ZnS photocatalyst shows strong potential for efficient degradation of refractory organic pollutants in wastewater treatment applications.