Application of optical properties and photocatalytic degradation behavior of N, P-CQDs in organic dyes degradation.

In this study, nitrogen and phosphorus co-doped carbon quantum dots (N, P-CQDs) were synthesized via a one-step hydrothermal synthesis method, and citric acid was used as the carbon precursor and ammonium dihydrogen phosphate as the simultaneous nitrogen and phosphorus source. The morphology, chemical composition, and crystal structure of the N, P-CQDs were systematically characterized by TEM, XPS, FT-IR, and XRD. Their optical properties and photocatalytic degradation mechanisms were further revealed by combining PL, UV-Vis, and ESR spectroscopy with photocatalytic experiments. The N, P-CQDs solution was used to catalyze the degradation of rhodamine B, methylene blue, and malachite green. Among these dyes, rhodamine B exhibited the highest catalytic degradation efficiency, followed by malachite green, whereas methylene blue showed the lowest efficiency. We propose a possible photocatalytic degradation mechanism for N, P-CQDs in which under light irradiation the material relies on the generated reactive oxygen species (ROS), with the main active species being •O₂ and •OH radicals, and its VB potential of 2.80 eV and CB potential of -1.25 eV can meet the thermodynamic requirements for the generation of •O₂ and •OH. Subsequently, using TOC analysis results, we further confirmed that N, P-CQDs can convert part of the dye molecules into inorganic products such as CO₂, with other degradation intermediates also present. Additionally, the effect of catalyst dosage was investigated by varying the volume of N, P-CQD solution from 1 to 4 mL. After 120 min of photocatalysis, the degradation efficiencies using 3 mL and 4 mL of the N, P-CQDs solution reached approximately 90 %, whereas only about 74 % efficiency was obtained with 1 mL and 2 mL of the N, P-CQDs solution. This work further expands the research on the fluorescence properties and catalytic degradation applications of carbon quantum dots.