Synthesis and characterization of high-performance ZnO/graphene quantum dot composites for photocatalytic degradation of metronidazole

Background: Industrial and medical wastewater treatment has been the focus of significant research and development in recent years. Especially untreated antibiotics commonly found within the wastewater generated by the hospitals or/and manufacturers have caused serious environmental concerns. Methods: In this work, we proposed a heterogeneous graphene quantum dot (GQD)/ZnO composite as a photocatalyst for degrading antibiotics by using co-precipitation and thermal carbonization processes. Significant findings: The introduction of N-GQDs efficiently facilitates the photocatalytic activity and reaction rate. Indeed, the N-functionalized GQDs, including pyrrolic/pyridinic N and graphitic N, can accelerate the charge transfer rate in the heterostructures. Optimizing the composition of GQD/ZnO catalyst, ultra-high removal ratio (∼100%) and significantly improved rate constant (i.e., 1.74 times higher compared to pristine ZnO) was achieved. In the binary catalyst, the presence of N-GQDs enhances the absorption in visible-light region and increases the photo-induced charge carrier, resulting in an elevated photocatalytic activity. This finding reveals that the GQD/ZnO composite structure demonstrated in this work can be employed as an in-expensive photocatalyst for degrading metronidazole (MNZ) in liquid phase under the UV-light irradiation.