First-principles Studies On Photocatalytic Properties Of Type-Ⅱ G-GaN/C₂N,In₂STe/C₂N And Ga₂SSe/g-C₆N₆ Van Der Waals Heterojunctions

Photocatalytic water splitting for hydrogen production has been a hot research topic in recent years.Two-dimensional materials are widely used in photocatalysis because of their unique properties such as large specific surface area,abundant active sites and tunable band gap,but single two-dimensional materials often exhibit problems of too wide band gap and high carrier complexation rate,which limit their further improvement of photocatalytic efficiency.To address these problems,researchers have proposed the strategy of constructing"van der Waals heterojunctions"to further improve the photocatalytic performance of two-dimensional materials.Van der Waals heterojunctions have many excellent properties,such as high carrier mobility,tunable band gap,and broad absorption spectrum.These properties make the heterojunctions exhibit excellent photocatalytic performance in photocatalytic water splitting.In this thesis,we have constructed g-GaN/C₂N,In₂STe/C₂N and Ga₂SSe/g-C₆N₆ van der Waals heterojunctions.First-principles calculations are carried out to investigated systematically the electronic band structure,projected density of states,band edge positions,charge transfer modes and optical properties of the heterostructures,and their application prospects in the field of photocatalytic water splitting for hydrogen production were discussed.The main research contents of this thesis are as follows:（1）We have investigated the electronic and optical properties of the g-GaN/C₂N heterostructure and discussed around its application in photocatalytic water splitting.The calculated results show that the g-GaN/C₂N heterostructure possesses a staggered band alignment,which is a typical type-Ⅱ heterostructure.This unique band arrangement makes the photogenerated carriers effectively separated in space,and the heterojunction is an indirect bandgap semiconductor with a bandgap value of 1.92 e V,and has a wide absorption spectrum from visible to ultraviolet light,and the absorption coefficient can reach the magnitude order of 10⁵ cm^-1.Meanwhile,its band edges straddle water redox potentials,satisfying the requirements of photocatalytic water splitting.In addition,it is found the application of strain can effectively modulate the electronic and optical properties of heterostructure,especially,A transition from indirect to direct bandgap occurs at the-4%compressive strain,resulting in great increase of optical absorption in the visible light These theoretical predictions suggest that g-GaN/C₂N van der Waals heterostructure is promising for photocatalytic water splitting.（2）Since the g-GaN/C₂N heterojunction is indirect bandgap,we consider to find a direct bandgap structure with moderate bandgap to expect better optical absorption and improved photocatalytic efficiency.We have performed first-principles calculations to investigate the electronic and optical properties of In₂STe/C₂N van der Waals heterostructure.The calculated results show that the In₂STe/C₂N heterostructure is direct bandgap with a bandgap value of 1.58 e V and its conduction band minimum and valence band minimum are contributed by different materials,thus it is a type Ⅱ heterojunction.Further differential charge density maps show the formation of a built-in electric field at the heterojunction interface pointing from the C₂N layer to the In₂STe layer,which can effectively suppress the compounding of photogenerated carriers and contribute to the improvement of photocatalytic efficiency.Meanwhile,the position of its band edge straddles the redox potential,indicating that it can spontaneously carry out redox reactions.Compared with the independent monolayer material,the light absorption coefficient of the heterojunction is significantly enhanced in the visible and UV regions.The above results indicate that the In₂STe/C₂N heterojunction is an efficient photocatalyst.（3）Employing First-principles calculation,we have investigated the electronic and optical properties of the Ga₂SSe/g-C₆N₆ heterostructure.The calculated results show that the Ga₂SSe/g-C₆N₆heterojunction is a 2.23 e V indirect bandgap semiconductor and forms a staggered energy band arrangement,which is a type Ⅱ heterostructure.Moreover,the Ga₂SSe/g-C₆N₆ heterojunction is able to cross the redox potential of water and exhibits good light absorption properties in the visible to UV region.Therefore,Ga₂SSe/g-C₆N₆ heterojunction can be an excellent photocatalytic candidate material.