Study On The Preparation And Photocatalytic Properties Of Nonmentallic Elements Doped Graphitic Carbon Nitride

Graphitic carbon nitride（g-C₃N₄）is a metal free polymer semiconductor photocatalyst,which has the advantages of good chemical stability,simple preparation process,cheap raw materials,non-toxic and harmless.However,g-C₃N₄ still cannot be put in to large-scale industrial application due to the serious recombination of photogenerated electron-hole pairs and narrow visible light response range.Therefore,the researchers used element doping,heterojunction construction,micro morphology control and other means to improve its photocatalytic activity.In this paper,supramolecular precursor was synthesized by self-assembly technology,non-metallic elements doped g-C₃N₄ were prepared by thermal condensation polymerization or high-energy microwave irradiation.Rhodamine B（Rh B）solution or tetracycline（TC）solution was degraded under visible light to investigate the photocatalytic activity of the products.The photocatalytic mechanism was explored by active species capture experiment.The main research contents and conclusions are as follows:（1）Using melamine and trithiocyanuric acid as raw materials,the rod like supramolecular precursor was synthesized by water bath heating.The S doped g-C₃N₄（SCN）was obtained by thermal condensation polymerization at different temperatures.The experimental results showed that SCN began to decompose from the center of precursor and changed from rod structure to tubular structure with the increase of polycondensation temperature.When the polycondensation temperature increased,the band gap of SCN increased from 2.66 e V of g-C₃N₄ to 2.91 e V of SCN-600.In addition,the photogenerated electron-hole pairs migration of SCN was effectively improved.The results of photocatalytic experiments showed that the photocatalytic performance of SCN was optimized with the increase of thermal polycondensation temperature.When the thermal polycondensation temperature was 600℃,the photocatalytic performance of SCN was the best.The degradation rate of Rh B was 99.19%after 9 min irradiation,and its pseudo-first order kinetic rate constant was about 15.6 times of bulk g-C₃N₄（BCN）.The main active species in the process of photocatalytic reaction were h⁺and·O₂^-.（2）Using melamine-trithiocyanuric acid supramolecular precursor as raw material,carbon fiber as microwave absorbent,respectively.SCN porous microtubules were efficiently and rapidly synthesized by high energy microwave irradiation.The results showed that S doping can effectively change the electronic structure of g-C₃N₄and reduce the photogenerated electron-hole pairs recombination rate of it.The obtained SCN-m had excellent degradation effect on TC solution under visible light.The degradation rate of TC reached 95.72%after 40 min irradiation,and its pseudo-first order kinetic rate constant was 7.94 times of pure g-C₃N₄.（3）Using melamine and oxalic acid as raw materials,melamine-oxalic acid supramolecular precursor was synthesized by hydrothermal method.Using carbon fiber as microwave absorbent,O-doped g-C₃N₄（OCN）was synthesized by high energy microwave irradiation.The results showed that with the increase of oxalic acid concentration,the band gap of OCN decreased gradually and the optical absorption range extended.In addition,the migration of photogenerated carriers of OCN was effectively improved.Under visible light,the degradation rate of TC solution by OCN series samples was effectively improved.Among them,the photocatalytic performance of 0.1-OCN was the best.The degradation rate of TC reached 96.49%after 60 min irradiation,and its pseudo-first order kinetic rate constant was about 6.01 times that of g-C₃N₄.In the process of photocatalytic reaction,active species h⁺,·OH and·O₂^-all played an important role.