Metal-free N-GQDs/P-g-C3N4 photocatalyst with broad-spectrum response: Enhanced exciton dissociation and charge migration for promoting H2 evolution and tetracycline degradation

Abstract

Graphitic carbon nitride (g-C3N4) is a promising candidate for solving energy and environmental problems. However, narrow photoresponse range, difficult exciton dissociation, and high charge recombination rate severely restrict its activity. Herein, a novel ultrathin porous N-GQDs/P-g-C3N4 (PG-CNS) nanocomposite was fabricated by combining P-element doping, N-doped graphene quantum dots (N-GQDs) coupling, and morphology regulation strategies. The optimal sample (PG-CNS-0.2) was found to extend the photoresponse range over 780 nm due to the combination of midgap state induced by P-doping and excellent upconversion performance of N-GQDs, achieving a fuller utilization of solar energy. Photoluminescence and electrochemical tests confirmed that both exciton dissociation and charge transfer within PG-CNS-0.2 were dramatically enhanced, as reflected by a 3.0-fold increase in carrier density. This enhancement was attributed to reduced exciton binding energy resulting from P-doping, superior electron conduction ability of N-GQDs, as well as larger specific surface area and shorter migration paths provided by the 2D porous structure. Besides, the trapping experimental result indicated that targeted regulation of reactive oxygen species was achieved by changing the addition amount of N-GQDs, thereby enhancing the controllability of photocatalytic reactions. Compared with g-C3N4 nanosheets (CNS), the H2 production rate and tetracycline degradation rate of PG-CNS-0.2 under visible light were improved by 15.7 and 6.9 times, respectively. Meanwhile, the molecular oxygen activation ability of PG-CNS-0.2 was also significantly increased, as reflected by the generation of O2– and H2O2 at concentrations 2.5 and 3.5 times higher than that of CNS, respectively. This work not only comprehensively reveals the exciton and carrier behaviors in g-C3N4-based reaction systems, but also provides a new strategy for designing low-cost, environmentally friendly, and efficient broad-spectrum responsive photocatalysts.