The Preparation And Application Of Carbon Nitride Based Heterostructure Photocatalysts

Due to the widespread use of personal pharmaceuticals and veterinary drugs,the detection of ecotoxic antibiotics（a class of persistent pollutants）in natural waters has become more and more frequent,posing a nonnegligible threat to the ecological environment and human health.Photocatalytic technology has become an effective way to solve the problem of antibiotics pollution because of its feasible process and low consumption.Polymeric carbon nitride（C₃N₄）has shown to be a promising metal-free photocatalyst with merits of low cost,visible light activity and environmental friendliness,but it however suffers from narrow light absorption range,strong charge recombination and poor conductivity.Therefore,the optimization of C₃N₄ material to prepare photocatalyst with high photocatalytic degradation activity is of key significance to realize the application of cost-effective photocatalytic technology in water pollution treatment.In this article,carbon nitride materials were modified with heterostructure in order to prepare photocatalysts with high degradation performance of tetracycline hydrochloride under visible light.The details are as follows:（1）MXenes have shown to be perfect co-catalysts for the photocatalytic process but show poor stability.In this study,we successfully constructed a robust heterostructure photocatalyst in which few-layer Ti₃C₂T_x was embedded into alkalized C₃N₄ without being oxidized.The photocatalyst showed stable and effective photocatalytic performance for the removal of tetracycline hydrochloride and other organic compounds under visible light irradiation,and the kinetic constant of tetracycline hydrochloride degradation reached to0.307 min^-1.Different characterization methods were used to elucidate the morphology and structure of the as-prepared photocatalyst.The robust heterostructure and the intimate interaction between the two constituents of the composite were verified.Based on the van der Waals heterostructure,Ti₃C₂T_x acts as the electron acceptor and helps to form Schottky junction,preventing charge recombination of the photocatalyst.And in the meantime,the electrons from C₃N₄ protect Ti₃C₂T_x from oxidation.XRD and SEM results demonstrated that the Ti₃C₂T_x structure remains unchanged after calcination and after photodegradation experiments.Furthermore,a possible mechanism for photocatalytic tetracycline hydrochloride degradation was proposed based on the results of radical scavenging experiments.This work provides a strategy to strengthen heterostructure between 2D materials,and shows that carbon nitride and Mxenes could be promising materials for photocatalytic wastewater pre-treatment applications.（Chapter 2）.（2）Due to the merits of non-toxic,facile synthesis process and excellent electron transferring,carbon quantum dots（CQD）have been developed for varies applications.Through simply mechanical mixing,there are some limitations in improving the photocatalytic performance of C₃N₄（CN）materials by constructing nonplanar 0D/2D heterostructure.In this chapter,we hydrothermally synthesized sulfur and nitrogen co-doped CQD using thiourea and citric acid.A series of CQD-CN materials were prepared by calcining the mixture of different loading of obtained CQD and urea precursor.The morphology and photochemical properties of the photocatalyst were analyzed by different characterization methods.A small amount of load of S and N co-doped CQD reduces the surface energy of C₃N₄ and prevents stacking.Moreover,the admixture of CQD increases the conjugate components of C₃N₄ unit structure and improves the light absorption capacity of the material.Meanwhile,in the formed planar heterostructure,the excellent electron transfer capacity of CQD and the electron capture site caused by S and N doping make up for the serious carrier recombination of C₃N₄.The photocatalyst showed an efficient and stable degradation performance to tetracycline hydrochloride under visible light irradiation,and the kinetic constant of degradation reached to 0.0473 min^-1 within 40 min.The mechanism of degradation was proposed according to radical quenching experiments.（Chapter 3）.