Experimental And Theoretical Study On Photocatalytic Properties Of Graphite Phase Carbon Nitride And Its Nanocomposites

With the development of global industrialization,the problems of environmental pollution and energy crisis have become increasingly prominent,which seriously threaten people’s health and normal life.Semiconductor photocatalysis technology is one of the important approaches and research hotspots to solve the above problems.It is a major challenge to develop an efficient,economic,environment friendly and solar response photocatalyst.Graphite-phase carbon nitride（g-C₃N₄）,as a non-metallic photocatalytic material,has attracted wide attention due to its visible light response,suitable edge potential,good acid-base stability,environmental friendliness and wide source of raw materials.Photocatalytic hydrogen production,photocatalytic degradation of organic pollutants,photocatalytic carbon dioxide reduction,photocatalytic synthesis,photocatalytic disinfection and sterilization have been widely studied.However,its further application in the field of photocatalysis is limited by its small specific surface area and easy recombination of photogenerated carriers.In order to improve this situation,a closed calcination experimental method was adopted in this paper to prepare porous structure g-C₃N₄ nanosheets,which improved the crystallinity,specific surface area and thermal stability,and improved the photocatalytic performance.In order to further reduce the photogenerated carrier recombination and improve the photocatalytic performance,MXene titanium carbide/g-C₃N₄/Zn O ternary nanocomposite was designed and its photocatalytic carbon dioxide reduction performance was investigated.In addition,g-C₃N₄/N-rTiO₂ nanocomposites were designed to reduce photogenerated carrier recombination and improve its photocatalytic performance by constructing Z-scheme heterojunction.Specific research content is as follows:（1）A porous structure g-C₃N₄ nanosheet with few layers was successfully prepared by closed calcination method with urea as precursor and no other excipients added.The average thickness of g-C₃N₄ nanosheets is 4.8 nm,and the specific surface area is 95.10m² g^-1,which is twice higher than that of the bulk g-C₃N₄.The band gap is 2.96 e V,which can respond to visible light,and the crystallinity and thermal stability are also significantly improved.By the efficient separation of photogenerated carrier and lower charge transfer resistance,g-C₃N₄ nanosheets with porous,few layers and high crystallization has shown better photocatalytic activity than bulk g-C₃N₄,its photocatalytic hydrogen production performance achieved 8116μmol g^-1 h^-1,6 times higher than bulk g-C₃N₄.The photocatalytic degradation rate of Rh B reached 0.035 min^-1,which was nearly 4 times higher than that of bulk g-C₃N₄.It is further verified by first-principles calculation that fewer layers of graphite phase carbon nitride extend the band gap,reduce the effective mass of carriers,and have smaller Gibbs free energy of hydrogen adsorption,so it has good photocatalytic hydrogen production performance.（2）A novel ternary structure g-C₃N₄/Zn O/Ti₃C₂ photocatalyst was synthesized by co-precipitation method by adding titanium carbide（Ti₃C₂）,a two-dimensional MXene material with metal conductivity characteristics.To the g-C₃N₄/Zn O nanocomposites,the photocatalytic carbon dioxide reduction performance of the composites were studied.The results show that the addition of MXene Ti₃C₂ can further promote the migration and separation of photogenerated carriers,and the photocatalytic carbon dioxide reduction efficiency is improved by 8 times and 12 times compared with g-C₃N₄ and Zn O.The theoretical calculation shows that the addition of MXene Ti₃C₂ forms an internal electric field on interface of g-C₃N₄ and Zn O,promotes the further migration of photogenerated electrons to MXene Ti₃C₂,effectively separates the photogenerated carriers,and further improves the performance of photocatalytic carbon dioxide.（3）Binary g-C₃N₄/N-rTiO₂ nanocomposites were prepared by co-sintering method,and direct Z-type heterojunction was constructed,which solved the problem that nitrogen doped rutile titanium dioxide（N-rTiO₂）did not meet the thermodynamic conditions of photocatalytic hydrogen production and could not be used as a catalyst for photocatalytic hydrogen production.The designed and synthesized g-C₃N₄/N-rTiO₂nanocomposites promoted the separation and migration of charge carriers.The photocatalytic hydrogen production efficiency increased to 2.57 times.The relaxation structure,band structure,state density,work function,differential charge density and Bader charge of the materials were analyzed by VASP,revealing that the formation of g-C₃N₄ and N-rTiO₂ is not a traditional type II heterojunction,but a direct Z-scheme structure.The photocatalytic hydrogen production performance of the nanocomposites was improved.