A novel ternary MQDs/NCDs/TiO2 nanocomposite that collaborates with activated persulfate for efficient RhB degradation under visible light irradiation

Novel TiO2 nanosheets co-modified with MoS2 quantum dots (MQDs) and N-doped carbon dots (NCDs), designated MQDs/NCDs/TiO2, were synthesized through a simple and mild process. The MQDs/NCDs/TiO2 nanocomposites exhibited significantly improved efficiency in the photocatalytic degradation of RhB compared to pure TiO2, mainly attributed to the synergy of excellent photoconversion by NCDs and efficient separation and transfer of photogenerated electron-hole pairs induced by the two heterojunctions formed in MQDs/NCDs/TiO2. More interestingly, the degradation rate was further promoted by 2.15 times after adding 2 mmol persulfate to the MQDs/NCDs/TiO2 photocatalytic system. This result is due to the excellent collaboration between the photocatalysis and persulfate activation, in which the persulfate was effectively activated by photogenerated electrons to generate abundant sulfate radicals (SO4-) and hydroxyl radicals (OH); meanwhile, the electron consumption caused by the persulfate effectually restrained surface electron-hole recombination, inducing more holes (h(+)) to participate in pollutant degradation. Radical quenching experiments and electron spin resonance spectra show that SO4-, OH, O-2(-) and h(+) were all in charge of RhB elimination in the MQDs/NCDs/TiO2-PS system. This work may provide guidance for employing dual quantum dots as co-catalysts, cooperating with activated persulfate, to enhance photocatalytic performance in contaminant degradation.