Graphitic Carbon Nitride Based Single Atom Catalysts For Enhanced Photocatalytic Hydrogen Evolution

On entering the 21st century,human faces two significant issues of energy shortages and environmental pollution with the rapid development of science and technology.Therefore,the search for new renewable and clean energy sources has become a hot topic of concern for all mankind.Semiconductor photocatalysis is a promising technology for hydrogen production via water splitting,achieving effective solar-to-hydrogen energy conversion and degrade organic pollutants,and alleviating energy scarcity and promoting environmental pollution prevention.The core issue in semiconductor photocatalytic hydrogen production is the development of highly efficient photocatalysts.Graphitic carbon nitride（g-C₃N₄）has attracted the extensive attention,due to its excellent photoelectric performance,good stability,simple synthesis method,low cost,and green and pollution-free characteristics.However,its low specific surface area,and the rapid recombination of photogenerated electron-hole pairs limit its practical application.Herein,elemental doped g-C₃N₄ based photocatalysts were synthesized to enhance its photocatalytic activity for photocatalytic hydrogen production.The main research content of this thesis can be summarized as follows:In the first part,co-doped S and Sr g-C₃N₄ photocatalyst with a microwire morphology was synthesized by calcining urea,2-thiobarbituric acid,and strontium chloride.After doping modification,the sample exhibited excellent photocatalytic performance.Under visible light irradiation withλ>420 nm,the optimal hydrogen production rate of 6Sr-SCN reached 13.9μmol/h,which is approximately 6.6 times that of pure g-C₃N₄.Experimental results showed that S and Sr doping narrows the bandgap,forming an in-plane heterojunction,promoting charge transfer and increasing molecular buckling and specific surface area as well.These factors were synergistically interwoven to enhance the visible light capture of g-C₃N₄ and improve the hydrogen production efficiency.This experiment provides certain theoretical support for the co-doping of g-C₃N₄ with metal and non-metal atoms.In the second part,Mo-doped g-C₃N₄catalysts were prepared by simply calcining melamine,cyanuric acid and sodium molybdate.Experimental results implied that Mo atoms are located in the triangular vacancies of g-C₃N₄ and form covalent bonds with N atoms.The Mo atoms act as an electron transfer pathway between the layers of g-C₃N₄,which accelerates the transfer of photogenerated carriers.Under visible light withλ>420 nm,the hydrogen production rate of 3Mo-CN increased to 16.66μmol/h,about 7.9times that of pure CN.Theoretical simulations show that Mo doping reduces the bandgap of g-C₃N₄,tunes energy band structure,and builts-in electric fields,increases the specific surface area.Thus improving the capture of visible light,and enhancing the separation and transfer of photogenerated carriers.This experiment provides new insights into the study of transition metal ion doping modification of g-C₃N₄.In the third part,W element-doped g-C₃N₄ were synthesized by thermally polymerizing urea,trithiocyanuric acid and ammonium metatungstate.Under visible light withλ>420 nm,the optimal photocatalytic hydrogen production rate of 10W-SCN reached 19.59μmol/h,approximately 10.2 times that of pure CN（1.93μmol/h）.Compared to pure g-C₃N₄,10W-SCN has a larger specific surface area,providing more active sites in photocatalytic reactions.Electrochemical tests show that the co-doped W and S have more excellent electrochemical performance.The experimental results indicate that W⁶⁺occupies the triangular vacancy center of g-C₃N₄,while S atoms replace N atoms in the skeleton of g-C₃N₄.S and W act as dual channels for photogenerated electron transfer between the layers of g-C₃N₄.Doping S and W induced the dislocalized HOMO and LUMO of g-C₃N₄ from an alternating distribution,effectively inhibiting the recombination of photogenerated electrons and holes.The doping of W and S endows g-C₃N₄ with excellent photocatalytic performance,which provides certain reference for synthesizing highly efficient g-C₃N₄ photocatalysts.