Preparation Of GC₃N₄-based Composite Photocatalyst Driven By Visible Light And Study On Degradation Of Tetracyclin

In recent decades,the widespread use of antibiotics has reaped huge benefits for human society.However,the misuse of antibiotics and their difficult degradability pose a challenge to the ecosystem and human health.Traditional methods of treating wastewater,including physical adsorption and biodegradation,do not fully meet the needs of human beings due to their low degradation efficiency and long degradation cycles.Semiconductor photocatalysis technology has rapidly attracted the attention of many scholars at home and abroad due to its advantages of green,high degradation efficiency and fast reaction rate,and is considered a promising new technology for wastewater treatment.Graphitic phase carbon nitride（g-C₃N₄）,as a new type of non-metallic photocatalyst,has been widely used in the fields of sensors,conversion of CO₂,degradation of organic matter and decomposition of aquatic hydrogen due to its simple preparation,low cost and non-toxic and non-hazardous advantages.However,the application of g-C3N4 in photocatalytic degradation of wastewater is severely limited by its small specific surface area,small visible light response range and high photogenerated electron-hole complexation rate.In this paper,three composite photocatalysts were prepared by loading semiconductor materials into g-C₃N₄and constructing heterojunctions.The morphology,structure and optical properties of the prepared catalysts were investigated by several characterisation techniques including X-ray diffraction（XRD）,transmission electron microscopy（TEM）,Fourier infrared spectroscopy（FT-IR）,solid-state ultraviolet（UV-vis DRS）and fluorescence spectroscopy（PL）.The optimum reaction conditions for several catalysts were explored using photocatalytic degradation experiments.The stability of the catalysts was tested using cycling experiments.Finally,the reaction mechanisms of several catalysts were speculated using free radical trapping experiments.The specific studies are as follows.（1）CuSnO₃/g-C₃N₄photocatalysts were prepared by a simple chemical co-precipitation and calcination method.The catalysts were characterized using XRD,SEM,TEM,FT-IR,UV-vis DRS and PL to investigate the morphological structure and optical properties of the catalysts.The results demonstrate the successful preparation of the composite photocatalyst and effectively improve the utilization of visible light by g-C₃N₄and reduce the complexation rate of photogenerated electron-hole pairs.The enhanced photocatalytic activity and stability of the composite photocatalyst were demonstrated by degradation experiments.The radical trapping experiments illustrate that it is the superoxide radical that plays a major role in the degradation process,and thus the Z-scheme degradation mechanism was proposed.（2）Zn₂In₂S₅/g-C₃N₄composite photocatalysts were prepared by a one-pot hydrothermal method.The successful preparation of the composite photocatalyst was demonstrated using several characterization tools and effectively broadened the light-absorbing edge of the catalyst and accelerated the migration rate of photogenerated carriers.The results of degradation experiments show that the composite photocatalyst has better photodegradation effect.The radical capture experiments demonstrated that the superoxide radical is the main active factor in the reaction.Based on the energy band structure of the semiconductor and the radical trapping results,it is speculated that the degradation of tetracycline by the Zn₂In₂S₅/g-C₃N₄composite photocatalyst may follow a type II heterojunction reaction mechanism in conjunction with literature reports.（3）The CdS/Zn₂In₂S₅/g-C₃N₄composite photocatalyst was prepared by loading CdS into the Zn₂In₂S₅/g-C₃N₄binary photocatalyst using a chemical co-precipitation method.The characterization results showed that the introduction of CdS further enhanced the absorption ability of the photocatalyst for visible light,broadened the absorption edge of the catalyst and enhanced the transfer and separation efficiency of photogenerated carriers.The results of photocatalytic degradation experiments showed that the tetracycline could be degraded by the ternary photocatalyst up to about 94.74%after 60 min of visible light irradiation under the optimal reaction conditions,successfully improving the photocatalytic activity of the catalyst.The results of the capture experiments indicate that superoxide radicals play a major role in the degradation process.The results of the cycling experiments demonstrate that the ternary composite photocatalyst has good stability.The double type II heterojunction reaction mechanism was proposed based on the results of the capture experiments and the characterization results.