Theoretical Study On Electronic Properties And CO₂ Reduction Performance Of Graphite Carbon Nitride Derivatives

Graphite phase carbon nitride?g-C₃N₄?has a unique electronic structure and chemical properties.It is a new-type metal-free functional material and has received extensive attention in the field of solar energy conversion clean energy.However,pristine g-C₃N₄ exhibits fast recombination of photo-generated holes and electrons,low carrier mobility,and low visible light response,which hinders its application to some extent.In order to improve its photocatalytic activity,many studies have focused on the modification of g-C₃N₄ to adjust the band gap,visible light absorption and solve the problem of easy recombination of electrons and holes.The common modification methods are doping modification and compositing with semiconductors.Metal and non-metal doping can effectively reduce the band gap and widen the visible light absorption range.g-C3N4-based heterostructure can not only reduce the band gap,but also solve the problem of fast recombination of photo-generated holes and electrons,enriching electrons and holes on two semiconductors,and achieving spatial separation.It is proved that the photocatalytic activity of the heterojunction is much higher than that of the single component semiconductor in experiments.At present,a great deal of research has focused on the synthesis and application of catalysts,and the mechanisms of catalytic reactions are limited.It is of great significance to design catalysts with excellent performance and explore the micro-reaction mechanism and the relationship between the structure and performance of catalysts.The first part of this thesis mainly introduces the modification methods of g-C₃N₄ and its application in the field of photocatalysis.The second part gives a brief introduction to the first principles and density functional theory.The third to fifth parts are the main research parts of this thesis,which are in the following:1.The study on Electronic and optical properties of?C₆N₇?_n,[C₆N₇?C₂?_1.5]_n,[C₆N₇?C₄?_1.5]_n,[C₆N₇?C₃N₃?]_n and[C₆N₇?N₂?_1.5]_n were investigated by first-principles calculations.The results show that the band gaps of the five carbon nitride derivatives are reduced compared to g-C3N4.The work function of the carbon nitride derivative is greater than that of g-C₃N₄.The strong absorption peak of g-C₃N₄ is around 350 nm,while the absorption spectra extents of five carbon nitride derivatives increase,the absorption wavelength ranges extend to the visible region For the five carbon nitride derivatives,compared with g-C3N4,the lowest unoccupied molecular orbitals?LUMOs?and the highest occupied molecular orbitals?HOMOs?have significant delocalization,which is beneficial to enhance the migration ability of carriers and enhance photocatalytic activity.2.The electronic structure and optical properties of S-doped g-C₃N₄ were studied by density functional theory method,and the optimal reaction path of S-doped g-C3N4as photocatalyst for CO₂RR was investigated.The electronic and optical properties indicate that doping S improves the catalytic performance of g-C3N4.From the thermodynamic calculation,the optimal path for S-doped g-C3N4 as a photocatalyst on CO₂RR is CO₂?COOH*?CO?HCO*?HCHO?H₃CO*?CH₃OH.In comparison with g-C₃N₄,doping S can alter the rate-determining step and reduce the Gibbs free energy from 1.43 to 1.15 eV.Therefore,the CO₂ reduction activity of S-doped g-C3N4 is better than that of g-C3N4,which is in good agreement with the experimental results.3.The band structure,absorption spectra and electron transfer mechanism of Z-scheme B-doped g-C₃N₄/SnS₂ were studied by density functional theory.The optimal reaction path and catalytic product of CO₂ reduction as photocatalyst were investigated.The results show that the band gaps of the g-C₃N₄/SnS₂ and B-doped g-C₃N₄/SnS₂ heterojunctions are reduced compared to the single-component g-C₃N₄,B-doped g-C₃N₄ and SnS₂.The simulated absorption spectra of the g-C₃N₄/SnS₂ and B-doped g-C₃N₄/SnS₂ heterojunctions show red shift and stronger absorption.The work function and charge density difference of g-C₃N₄/SnS₂ and B-doped g-C₃N₄/SnS₂ heterojunctions indicates that the charge transfer mechanism is a Z-scheme charge transfer mechanism.Under illumination,the built-in electric field acceleratesthe transfer of photoexcited electrons in the CB of SnS₂ into the VB of g-C₃N₄ or B-doped g-C₃N₄,and the light-excited electrons and holes are concentrated in different semiconductor surfaces,effectively extending the lifetime of the photo-generated carriers and improving the photocatalytic ability.In thermodynamic calculations,for Z-scheme g-C₃N₄/SnS₂,the rate-determining step is CO₂?COOH*,?G is 1.10 eV,and the main products are CH₃OH and CH₄.For B-doped g-C₃N₄/SnS₂,the optimal CO2RR path is CO2?COOH*?CO*?HCO*?CHOH*?CH*?CH₂*?CH₃*?CH₄,The?G of rate-determination step?CH₃*?CH₄?is 0.40 eV.The?G of rate-determining step of B-doped g-C₃N₄/SnS₂ is much lower than that of g-C₃N₄/SnS₂,indicating that the photocatalytic activity of B-doped g-C₃N₄/SnS₂ is better than that of g-C₃N₄/SnS₂.