Study On Photocatalytic Performance Of G-C₃N₄ Or MoS₂-based Heterojunctions Based On Density Functional Theory

g-C₃N₄ is a new type of two-dimensional semiconductor material with a layered structure,which has been extensively researched in the field of photocatalysis due to its abundant natural resources,low production cost and high stability.But,its high photogenerated carrier recombination rate and other unfavourable factors lead to poor photocatalytic performance,then its practical application also reduced accordingly.MoS₂ is a photocatalytic material that exhibits both metallic phase（which cannot exist stably）and semiconductor phase,and is highly sought after in photocatalytic research because of its good hydrogen production efficiency,fast carrier mobility and high thermal stability.However,factors such as its small number of active sites and poor electrical conductivity limit its application.The method of constructing a heterojunction has been shown in the past experimental results to be a convenient and effective way to improve the photocatalytic performance,but there is no reasonable explanation for the effect of charge transfer in the heterojunction on the photocatalytic performance.Therefore,the analysis of the photocatalytic mechanism may be inaccurate.Therefore,the band structure,the density of states,work function,interface charge transfer,electron wave function and photocatalytic mechanism of g-C₃N₄/（001）-BiOBr heterojunction are calculated quantitatively based on the HSE06 hybrid density functional method.The results indicate that g-C₃N₄/（001）-BiOBr is an S-type heterojunction,which is consistent with previous experimental observations.In addition,by comparing the charge transfer and built-in electric field intensity,the results showed that the S-type g-C₃N₄/（001）-BiOBr heterojunction photocatalyst has better hydrogen production activity and photocatalytic performance than the II-type g-C₃N₄/（001）-BiOCl heterojunction.To further discuss the effect of constructing heterojunctions of different materials on the photocatalytic performance.Therefore,asymmetric 1T-MoS₂/BiOCl Janus heterostructure and asymmetric 2H-MoS₂/BiOCl Janus heterostructure were constructed,respectively,and their geometric and electronic structure,partial（band decomposed）charge density,charge transfer,electron localization function and photocatalytic mechanism were systematically studied.For the asymmetric 1T-MoS₂/BiOCl Janus heterostructure,the calculations results showed that the hydrogen generation activity is weakened due to the decrease of the CBM,and its ability to oxidize pollutants is also weakened.For the asymmetric 2H-MoS₂/BiOCl Janus heterostructure,the calculations showed that there exist several newly formed weak Bi-S bonds with shorter bond lengths between BiOCl and 2H-MoS₂ which act as an electron transport bridge along the direction perpendicular to the heterojunction interface.This newly weak bonds lead to the formation of occupied shallow defect levels approximately 0.0-0.9 eV below the bottom of the conduction band.Electrons located at these defect levels can easily jump into the conduction band as a donor energy level under thermal fluctuations and simultaneously further promote the effective separation of photo-generated electron-hole pairs in the BiOCl.The photogenerated electrons located around Bi-atom layer in the conduction band of BiOCl transfer to the valence band of 2H-MoS₂ around the S-atom layer through the interface of the asymmetric 2H-MoS₂/BiOCl Janus heterostructure,which significantly reduce photo-generated holes in the 2H-MoS₂ and electrons in the BiOCl.The large numbers of photogenerated electrons from the 2H-MoS₂cannot recombine with holes owing to lack of sufficient holes.They will move to the surface and greatly improve the hydrogen production activity in the 2H-MoS₂.While the photogenerated holes from the BiOCl will significantly improve the ability of BiOCl to oxidize pollutant in the water owing to the absence of sufficient electrons.Therefore,the asymmetric 2H-MoS₂/BiOCl Janus heterostructure belongs to the I-type energy band arrangement of the S-type photocatalytic mechanism.This paper provides theoretical guidance for the preparation of S-type heterojunction materials and a new approach to design asymmetric Janus bilayer heterostructures with newly formed weak chemical bonds.