Microwave-assisted synthesis of CQDs/ZnO hollow microspheres for complete NOx 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 (NOx) oxidation. In this study, carbon quantum dots (CQDs)/ZnO hollow microspheres, synthesized through a rapid microwave-assisted method, achieved over a 30-fold higher NOx 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 NOx oxidation were identified, and in-situ diffuse reflectance infrared Fourier transform spectroscopy analysis elucidated the conversion pathways of NOx 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 NOx removal.