Defect Control via Compositional Engineering of Zn-Cu-In-S Alloyed QDs for Photocatalytic H₂O₂ Generation and...

Photocatalytic H₂O₂ production and recalcitrant pollutant degradation are regarded as promising clean technology toward achieving sustainable solar-to-chemical energy conversion. Herein, nonstoichiometric Zn-Cu-In-S (ZCIS) quaternary alloyed quantum dots (QDs) are rationally fabricated via a reflux method toward H₂O₂ generation and ciprofloxacin degradation under visible light irradiation. The optimum catalyst (ZCIS-2) exhibits a notable H₂O₂ production of 1685.2 μmole h^-1 g^-1 (solar-to-chemical conversion efficiency (SCC), 0.19%), which is 5.3 times higher than that of CuInS₂ (CIS), and a ciprofloxacin (CIP) degradation efficiency of 96% in 2 h. The observed improvement in activity corresponds to optimized exciton separation/transfer, broad photon absorption, tunable band alignment, and effective adsorption/activation. In addition, oxygen reduction goes through both direct two-electron single-step reduction and single-electron two-step superoxide radical pathways, whereas CIP degradation proceeds via direct ^•O₂^- and indirect ^•OH radical pathways, as confirmed by scavenger experiments. An appropriate amount of defects improves the adsorption/activation of O₂ toward H₂O₂ and active oxygen species generation that facilitates CIP degradation. The effect of operational parameters, such as pH, surrounding environment, presence of ions, sacrificial agent, etc., on both H₂O₂ formation and CIP removal is vividly studied. Hence, the current study will provide an in-depth insight into O₂ photoreduction and micropollutant removal, which encourages further advancement of potent alloyed quantum dot-oriented photocatalytic systems.