Enhancing the visible light absorption of MOF-808 by surface decoration with sulfur and nitrogen co-doped carbon quantum dots: Highly efficient photocatalyst for improved photocatalytic water remediation

Metal-organic frameworks (MOFs) have aroused growing attention in the domain of photocatalysis. However, the photocatalytic performance of MOFs has been severely restricted by their fast recombination of photo-generated electrons and low utilization of solar energy. Combining MOFs with narrow-gap semiconductors can help to overcome their functional shortcomings. In this work, MOF-808 and sulfur and nitrogen co-doped carbon quantum dots (S,N-CQDs) were synthesized using a simple solvothermal technique. Then, novel MOF-808/S,N-CQDs(x) (MS-x) composites were fabricated through a simple solvent-deposition technique to decorate various amounts of S,N-CQDs on the surface of MOF-808. The morphology of MOF-808 and MS-0.5 composite, along with the lattice spacing, and size distribution of decorated S,N-CQDs on MOF-808 surface were investigated by SEM and HR-TEM analyses. According to the UV–Vis DRS and Tauc plots findings, MOF-808 was not photo-active in visible light region because of its large band gap energy (3.85 eV). In contrast, the photocatalytic performance of the prepared composites in Rhodamine B (RhB) degradation was significantly enhanced compared with MOF-808 under visible light irradiation. The results confirmed that S,N-CQDs play a key role in enhancing the visible light absorption efficiency. MS-0.5 composite exhibited the highest photocatalytic activity among the other composites and demonstrated high RhB photodegradation up to 89 % under visible light and 95 % under sunlight irradiation in optimal conditions. According to the kinetic studies, MS-0.5 composite had the highest rate constant, 29.67 and 6.93 times greater than S,N-CQDs and MOF-808, respectively. The scavenger tests revealed that h+ and OH radicals were the main active intermediates in the photocatalytic reaction. In addition, MS-0.5 composite exhibited excellent reusability because only a minor loss of photoactivity was observed after four reaction cycles.