Theoretical Study On The Photocatalytic Mechanism Of Graphite Phase C₃N₄/2D Out-of-plane Polar Composites

The depletion of fossil fuels and continuous CO₂emissions have caused emerging global energy and environmental crises.Using solar energy to split water into efficient new energy source is considered as one of the effective ways to deal with the above situation,which is called photocatalytic technology.Therefore,many researchers have focused on developing excellent semiconductor photocatalysts.However,the high photogenerated carrier recombination rate of a single material does not meet the requirements for practical applications.Recently,polarization electric field engineering has been shown to be an effective strategy.Among them,the intrinsic electric penetration caused by spontaneous polarization can not only improve charge separation and transfer in bulk structures,but also couple polar materials with 2D materials to form composite materials（called heterojunction later）to facilitate the charge separation of photocatalysts.Therefore,this paper studiesα-In₂Se₃/g-C₃N₄heterojunctions of two configurations（A and B）are studied through first-principles calculations.The spontaneous out-of-plane electric polarization ofα-In₂Se₃points g-C₃N₄for configuration A,in which the layer spacing and formation energy are lower than those of configuration B.Configuration A has the characteristics of the I-type energy band alignment and the photocatalytic performance as an S-scheme heterojunction,which is different from the II-type band alignment characteristic of the previous traditional Z-scheme heterojunctions with a larger built-in electric field.The photogenerated electrons（e^-）inα-In₂Se₃migrate to the interface region ofα-In₂Se₃/g-C₃N₄in the effect of a stronger intrinsic electric field ofα-In₂Se₃,and then pass through the interface to recombine with holes（h⁺）from the VBM of g-C₃N₄under the field strength of build-in electric field,reducing the number of photogenerated holes in the VBM of g-C₃N₄for configuration A.The photogenerated electrons in the CBM of g-C₃N₄move toward the surface of g-C₃N₄due to the absence of holes and play a significant role in the hydrogen generation activity.Acoording to this the intrinsic mechanism of the experimentally enhanced photocatalytic performance can be well explained.To further study the role of 2D Janus transition metal dichalcogenides with intrinsically polar electric fields in heterojunctions,the photocatalytic mechanism of Janus Mo SX（X=Se,Te）/g-C₃N₄heterojunctions（defined as configurations C,D,E,and F,respectively）was investigated.Different from the band alignment characteristic of Z-scheme heterojunctions with type-Ⅱenergy band alignment,the configuration C and D heterojunctions belong to the S-type heterojunctions with the I-type energy band arrangement,and the F-configuration heterojunction belong to S-type heterojunctions with the II-type band arrangement.In addition,since the intrinsic electric field of Mo STe is greater than that of Mo SSe,it has a greater effect on the arrangement of the energy bands,resulting in forming different types of heterojunctions.Finally,in the C-configured heterojunction,the intrinsic electric field of Mo SX can promote more photogenerated electrons to participate in the photocatalytic reaction.Based on the above studies,it can be seen that the intrinsic polar electric field existing in the heterojunction can be used as a direct driving force to promote the rapid migration of e^-and h⁺to the two sides of the photocatalyst structure,respectively.In this situation the photogenerated carriers can effectively separate,which solve the problem of high composite rate of single material.In addition,the magnitude of the intrinsic electric field can determine the energy level arrangement of the heterojunction.In this paper,the I-type band arrangement S-type heterojunction with better photocatalytic performance is build.Yet a method to improve the photocatalytic efficiency of materials by introducing an intrinsically polarized electric field in the heterojunction is proposed.