Microwave-assisted synthesis of CQDs/ZnO hollow microspheres for complete NO_x oxidation under visible light

The development of ZnO-based composites with high charge separation and effective inhibition of toxic by-products is admirable for effective photocatalysis of nitrogen oxides (NO_x) oxidation. In this study, carbon quantum dots (CQDs)/ZnO hollow microspheres, synthesized through a rapid microwave-assisted method, achieved over a 30-fold higher NO_x removal efficiency compared to ZnO, with complete inhibition of NO₂ by-products and good durability. The enhanced photocatalytic activity was ascribed to the unique role of CQDs, as revealed by in-situ photoelectric techniques. Results demonstrated that the electron directional migration from ZnO to CQDs at the composite interface accounts for the enhanced charge separation. Active free radicals for NO_x oxidation were identified, and in-situ diffuse reflectance infrared Fourier transform spectroscopy analysis elucidated the conversion pathways of NO_x oxidation under visible light irradiation. This work sheds light on the mechanisms of electron transfer and charge separation at the composite interface, offering guidance for designing superior ZnO-based photocatalysts for complete NO_x removal.