Hydrothermal Synthesis of N-Doped GQD/CuO and N-Doped GQD/ZnO Nanophotocatalysts for MB Dye Removal Under Visible Light Irradiation: Evaluation of a New Procedure to Produce N-Doped GQD/ZnO

During the last decade, dye pollution has been a major environmental concern, making it necessary to develop removal methods. In this study, three nanophotocatalysts, namely ZnO, CuO, and ZnO/CuO composite, were synthesized via a hydrothermal method and then examined for degradation of methylene blue (MB) dye solution under visible light irradiation. Pure ZnO and CuO showed rather poor photocatalytic activities due to their limited photo response ranges under visible light. Accordingly, in order to overcome this limitation and improve the photocatalytic activity of ZnO and CuO, nitrogen-doped graphene quantum dots (N-GQDs) were employed. Results proved that the N-GQDs can significantly enhance photocatalytic activity of the ZnO sample, while no significant effect was observed on CuO sample. Furthermore, effects of synthesis method and added content of N-GQDs on the activity of the N-GQDs/ZnO composite samples were studied. Firstly, pure ZnO was synthesized via a hydrothermal reaction and then added to the N-GQDs precursor solution to produce N-GQD/ZnO composite in a final hydrothermal reaction (method I). Alternatively, a hydrothermal method was used to prepare the N-GQDs firstly, and then zinc acetate was introduced into this solution for final hydrothermal process (method II). The results showed higher elimination performance of the second method, by which the MB dye could be removed completely. The best nanophotocatalyst (the sample containing 3 mmol of ascorbic acid in the primary solution, prepared via the second method) demonstrated maximum dye removal efficiency (70% in about 30 min, 100% < 60 min). Thus, nanophotocatalysts of similar characteristics are suggested for industrial effluent purification under visible light.