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 NOx 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. Copyright © 2017 American Chemical Society.