Study On Preparation And Properties Of G-C₃N₄ With Nanoporous Structure Modification

Graphitic carbon nitride（g-C₃N₄）is a new kind of carbon material in recent years,which has high stability,non-toxicity,high adsorption capacity and good optical properties.Besides,the production process of g-C₃N₄ is simple and low-cost.However,traditional blocky g-C₃N₄ is prone to agglomeration,resulting in low specific surface area,few active sites and poor electrical conductivity,which is the main obstacle to its application in the fields of electrochemistry and photocatalysis.The above problems can be effectively solved by structural modification,defect control and atomic doping.In this thesis,the modification of nanoporous structure and the regulation of nitrogen defects about g-C₃N₄ were studied.The g-C₃N₄ was modified by controlling the calcination temperature in argon atmosphere,acid etching time and etching object,so as to obtain g-C₃N₄ with rich nitrogen defects and nanoporous structure.The influence of the defects and nanoporous structure on its electrochemical and photocatalytic performance was further studied.Nanoporous sheet g-C₃N₄（S-g-C₃N₄）was prepared by thermocondensation method,and then the S-g-C₃N₄ was calcined under argon atmosphere at 650℃,750℃and 850℃.Through morphology and structure analysis,nanoporous S-g-C₃N₄-750 with nitrogen defects was obtained when the calcination temperature was 750℃.Abundant defects cannot be introduced when the temperature was too low（650℃）.The porous structure was destroyed when the temperature is too high（850℃）.The hollow tubular g-C₃N₄（T-g-C₃N₄）was prepared with melamine as precursor by thermocondensation method,and then etched with acetic acid for 12 h,24 h and 48 h.When T-g-C₃N₄ was treated with acetic acid for 24h,a large number of vertical porous nanotubes appeared on the outer wall of the product（T-g-C₃N₄-2）,and abundant two-dimensional mesoporous pores appeared on the inner wall.If the acetic acid treatment time is too short（12 h）,only a small amount of nanopore structure can be formed,while if the acetic acid treatment time was too long（48 h）,the porous structure will collapse.At the same time,acetic acid was used to etch melamine in the same time and then thermal polycondensation reaction was carried out.Only when the etching time of acetic acid was 24 h,some nanopore was formed on the surface of the product（T-g-C₃N₄-5）.If the time is too short（12 h）,no nanopore structure is generated,and if the time is too long（24 h）,the tubular structure is destroyed.The lithium-sulfur battery based on S-g-C₃N₄-750/CNT-modified separators showed excellent electrochemical performance,and the specific discharge capacity of the first cycle was up to 1128 m Ah g^-1 at 0.2 C,and the capacity retention rate was 69%after 100 cycles.The lithium-sulfur battery based on T-g-C₃N₄-2/S electrode achieves an excellent area capacity of 4.72 m Ah cm^-2 at 0.1 C under the high sulfur loading of 7 mg cm^-2.Under visible light irradiation,the degradation rate of methyl orange on S-g-C₃N₄-750photocatalyst reached 80%after irradiation for 4 h,which was more than twice of the photocatalytic efficiency of S-g-C₃N₄.The degradation rate of methyl orange by T-g-C₃N₄-2 was also as high as 80%,which was much higher than 47%of T-g-C₃N₄ and 59%of T-g-C₃N₄-5.