Environment-Friendly Carbon Quantum Dots/ZnFe2O4 Photocatalysts: Characterization, Biocompatibility, and Mechanisms for NO Removal

A highly efficient and environmentally-friendly oxidation process is always desirable for air purification. This study reported a novel carbon quantum dots (CQDs)/ZnFe₂O₄ composite photocatalyst for the first time through a facile hydrothermal process. The CQDs/ZnFe₂O₄ (15 vol %) composite demonstrates stronger transient photocurrent response, approximately 8 times higher than that of ZnFe₂O₄, indicating superior transfer efficiency of photogenerated electrons and separation efficiency of photogenerated electron-hole pairs. Compared with pristine ZnFe₂O₄ nanoparticles, CQDs/ZnFe₂O₄ displayed enhanced photocatalytic activities on gaseous NO_x removal and high selectivity for nitrate formation under visible light (λ > 420 nm) irradiation. Electron spin resonance analysis and a series of radical-trapping experiments showed that the reactive species contributing to NO elimination were ·O₂^- and ·OH radicals. The possible mechanisms were proposed regarding how CQDs improve the photocatalytic performance of ZnFe₂O₄. The CQDs are believed to act as an electron reservoir and transporter as well as a powerful energy-transfer component during the photocatalysis processes over CQDs/ZnFe₂O₄ samples. Furthermore, the toxicity assessment authenticated good biocompatibility and low cytotoxity of CQDs/ZnFe₂O₄. The results of this study indicate that CQDs/ZnFe₂O₄ is a promising photocatalyst for air purification.