Preparation Of Nitrogen And Phosphorus Doped Sulfur Quantum Dots And Their Visible Photocatalytic Properties

Various water contaminants are ubiquitous in the natural environment,and emerging and some classic contaminants,such as antibiotics,pharmaceuticals,pesticides,dyes and heavy metals from human activities,now pose a serious threat to the environment and human health.Many water pollutants are difficult to degrade and can only be partially degraded using traditional drinking water treatment and wastewater treatment processes.In recent decades,visible light activated photocatalysts have been widely used as a non-traditional method for the removal of organic pollutants in industrial wastewater and drinking water treatment.Elemental sulfur,one of the most abundant substances on earth,has been widely used for centuries.In recent years,sulfur quantum dots（SQDs）have emerged as a new class in the family of nanosulfides,consisting of discrete,quasi-spherical nanoscale particles less than 10 nm in size.In this paper,nitrogen-phosphorus-doped SQDs were prepared by hydrothermal method,loaded onto graphite-phase carbon nitride（g-C₃N₄）to obtain composite photocatalysts,and their visible photocatalytic properties were investigated.The main studies are as follows:（1）SQDs were synthesized by the water bath method,and phosphorus-doped sulfur quantum dots（P-SQDs）were prepared by the hydrothermal method using disodium hydrogen phosphate dodecahydrate as the nitrogen source,and loaded onto g-C₃N₄ to prepare phosphorus-doped sulfur quantum dots/graphite phase carbon nitride（P-SQDs/g-C₃N₄）composite photocatalytic materials.The results showed that the degradation rate of P-SQDs/g-C₃N₄ to methyl orange（MO）solution was as high as98.72%after 120 min of visible light irradiation,which was much higher than that of pure g-C₃N₄（63.36%）.This may be attributed to the lower direct recombination efficiency of the photogenerated electron-hole pairs of the composites and the corresponding enhancement of the photocurrent promoting the effective transfer of photogenerated carriers,thus increasing the degradation rate of the pollutants.（2）Nitrogen-doped sulfur quantum dots（N-SQDs）were synthesized by a hydrothermal method using urea and SQDs in a certain ratio and loaded onto g-C₃N₄ to obtain nitrogen-doped sulfur quantum dots/graphite phase carbon nitride（N-SQDs/g-C₃N₄）.The results showed that the photocatalytic degradation of MO by N-SQDs/g-C₃N₄ was only 58.0%after 120 min of xenon lamp irradiation,which was slightly lower than that of pure g-C₃N₄（63.36%）.This was attributed to the loading of N-SQDs on g-C₃N₄ which was not conducive to the effective separation of electron-hole pairs,thus reducing the photocatalytic efficiency.（3）Nitrogen-phosphorus doped sulfur quantum dots（NP-SQDs）were prepared by mixing ammonium dihydrogen phosphate and SQDs by hydrothermal method and loaded with g-C₃N₄ to obtain nitrogen-phosphorus doped sulfur quantum dots/graphite phase carbon nitride（NP-SQDs/g-C₃N₄）.The results showed that the degradation rate of MO by NP-SQDs/g-C₃N₄ reached 96.2%after 120 min of visible light irradiation,which was much higher than that of pure g-C₃N₄.This may be due to the introduction of NP-SQDs,which promotes the excitation,formation,transport and separation of carriers by visible light.In addition,a possible mechanism of photogenerated electron transfer in the photocatalytic process is proposed.