First-principles Study On Adsorption And Photocatalytic Activity Of Defective G-C₃N₄

Two-dimensional materials have good controllability,which can reduce the scattering phenomenon caused by illumination,and also improve the refraction times of light when it is transmitted inside the material.Moreover,the two-dimensional material itself is very thin,and the specific surface area will become relatively large,therefore it will reduce the migration time of photogenerated carriers in the two-dimensional material,which is the best choice of photocatalyst.Although the two-dimensional g-C₃N₄ material has small hardness and relatively stable properties,the defects of its wide band gap and low utilization rate of visible light absorption still affect its photocatalytic performance.Element doping is an important means to adjust the band gap and improve the light absorption rate.For this reason,various theoretical calculations are used to simulate element doping and instrict defects for modification.It can not only reduce the waste of resources,but also provide theoretical verification and guidance for experimental research.In this paper,the electronic and optical properties of g-C₃N₄ were studied by first-principles calculations,and the influence of defects on the electronic and optical properties of g-C₃N₄ materials was systematically studied.The interaction between H₂O molecule and the surface structure of g-C₃N₄ with C vacancy carbon defects was studied,and the microscopic mechanism of the effect of C vacancy defects on the surface and interior of the material was discussed.The result of the calculation is as follows:(1)The structure and energy of g-C₃N₄ containing C vacancy and N vacancy defects were constructed and studied by means of first-principles using hybrid functionals,and the electronic and optical properties were further calculated under different C vacancy defect concentrations.The results show that the defect formed at the C2 position is the most stable configuration with the lowest formation energy.It was found that with the increase of C vacancy defect concentration,the band gap of the defect g-C₃N₄ also showed an increasing trend,and the visible light response range will also be increased.It can be found that there is a spatially separated charge distribution phenomenon on the conduction and valence band edge states,which may help to suppress the recombination of photogenerated electron-hole pairs.By comparing with the standard potential of photolyzed water,it was found that g-C₃N₄ containing C2 vacancy defect has suitable band gap and band edge potential.These results indicate that changing the concentration of C vacancy defects is a feasible method to improve the photocatalytic water splitting ability,and provide ideas for the next step to synthesize novel and efficient photocatalysts.(2)The adsorption behavior of H₂O on the surface of V_C2-g-C₃N₄ containing C vacancy defect was investigated by the first-principles method,and the effect of the initial adsorption distance of H₂O molecules on the adsorption properties was analyzed.By comparing the adsorption energy,the most stable adsorption site of H₂O molecule on the substrate surface was determined,and the electronic properties such as density of state and energy band at the most stable adsorption site were studied.The calculational results show that the most stable adsorption site is located at the boundary of the surface unit.The adsorption of H₂O molecules will distort the planar structure of V_C2-g-C₃N₄ and through the density of states it can be concluded that there is a strong interaction between H₂O and the substrate.The band gap of the system increases after adsorption,its VBM and CBM can meet the redox potential of photolyzed water and defect energy levels is generated inside the structure,which is beneficial to improve the photocatalytic ability.