Facile synthesis of N-doped carbon dots/g-C3N4 photocatalyst with enhanced visible-light photocatalytic activity for the degradation of indomethacin

In this study, a novel visible-light-driven N-doped carbon dot (NCDs)/g-C3N4 composite was successfully synthesized by loading NCDs nanoparticles onto the interlayers and surface of g-C3N4 via a facile polymerized method. The photocatalytic activity of the NCDs/g-C3N4 was remarkably higher than that of g-C3N4 and CDs/g-C3N4 toward the degradation of indomethacin (IDM) under visible light irradiation. With increasing NCDs loading volumes, the photocatalytic activity of NCDs/g-C3N4 initially increased, and then decreased. A very low NCDs content of 1.0 wt% resulted in a 13.6 fold higher reaction rate than that of pristine g-C3N4. This enhanced photocatalytic activity might have been ascribed to the unique up-converted PL properties, efficient charge separation, as well as bandgap narrowing of the NCDs. Reactive species (RS) scavenging experiments revealed that superoxide radical anions (O2−) and photogenerated holes (h+) played key roles during the photocatalytic degradation of IDM. The quantification of O2− showed that NCDs/g-C3N4 formed a larger amount of O2− than that of pristine g-C3N4. Potential photocatalytic pathways of IDM were proposed through the identification of intermediates using HPLC–MS/MS and the prediction of reaction sites based on Frontier Electron Densities (FEDs) calculations, which involved decarboxylation, hydroxylation, as well as the addition and cleavage of indole rings. Toxicity and mineralization evaluations revealed that NCDs/g-C3N4 provided a very desirable performance for the toxicity reduction and mineralization of IDM under longer exposures of visible light irradiation.