Carbon quantum dots and reduced graphene oxide modified self-assembled S@C3N4/B@C3N4 metal-free nano-photocatalyst for high performance degradation of chloramphenicol

Graphitic carbon nitride based photocatalytic materials have proved to be one of most promising candidates for hazardous pollutant removal and clean energy production. Structural modification and engineering of various materials based on g-C3N4 have been attempted to achieve solar energy harvesting, slow charge transfer and efficiency. Here we report synthesis of all metal free self-assembled carbon quantum dots and reduced graphene oxide layers modified S@g-C3N4/B@g-C3N4 (CRSB) photocatalyst for visible and solar degradation of chloramphenicol (CMP) (10 mg L-1). Differently doped g-C3N4 with different band structure forms a junction which grows into an effective Z-scheme photocatalyst. Extended visible absorption is achieved with optimized modification by reduced graphene oxide (RGO) and carbon quantum dots. The optical response, band structure and charge separation efficiency were tested by optical absorption, electrochemical impedance, Mott-Schottky plots and photo-luminescence. CQDs convert a typical type-II mechanism into effective Z-scheme transfer. For the best sample CRSB 99.1% CMP was photo-degraded in 90 min exposure to visible light and retention of high activity in natural solar light (92.4% in 120 min). A stable heterojunction has been attempted to design using boron doped g-C3N4 and sulphur doped g-C3N4 with different band positions via ratio optimization with enhanced charge transfer. The degradation rate for CRSB (0.0810 min(-1)) is 10 times than bare g-C3N4 (0.00802 min(-1)). The optimized combination of carbon quantum dots and RGO with intimate interfacial contact for charge flow & reduced recombination, better adsorption of pollutant, high visible absorption and production of reactive oxygen species. The effect of pH, electrolytes and water matrix was studied for CMP degradation. The mechanism of degradation and mineralization was confirmed by liquid chromatography-mass spectrometry (LC-MS) analysis, scavenging experiments and electron spin resonance. It was observed that the chances of O-center dot(2)- and (OH)-O-center dot radicals increased on the formation of the junction as compared to individual doped and bare carbon nitride. The mineralization was ensured by high total organic removal and cyto-toxicity studies of treated water sample onto human peripheral blood (PBL) cells with high cell viability equivalent to untreated cells. This experiment supports the competence of optimally designed metal free and environmentally friendly cost-effective catalysts with higher photocatalytic efficiency for removal of hazardous pollutants. (C) 2019 Elsevier B.V. All rights reserved.