First-principles Research On Silicon-based Composite Heterojunction Photocatalysts

Converting solar energy into hydrogen energy by photocatalytic water cracking is one of the effective ways to solve the increasingly severe energy and environment crisis.Among various photocatalytic materials,two-dimensional（2D）materials have attracted much attention due to their unique structural and electronic properties.However,the inherent defects of two-dimensional materials result in higher recombination rates of carrier and hole.Among them,two-dimensional silicon-based materials have poor light absorption ability,which limits their application in the field of photocatalysis.In this paper,based on first principles,two dimensional photocatalytic composites based on silicon are designed and their photocatalytic properties are studied.The main research contents are as follows:（1）As a two-dimensional silicon based material with zero band gap,the application of silicene in the field of photocatalysis is limited due to the low light absorption capacity of visible light.The electronic structure and optical properties of two dimensional silicene materials were studied by density functional theory（DFT）calculation.It is found that the light absorption capacity of silicene structure can be effectively adjusted by constructing heterojunction engineering with g-C₃N₄,and the electronic properties of silicon-based heterojunction can be adjusted by changing the number of stacking layers.On this basis,the method to improve the photocatalytic performance of silicene/g-C₃N₄ material was explored.It is found that the stacking mode of multimolecular layers can affect the light absorption and electronic properties of heterojunction.Moreover,the low light absorption performance of silicene is improved by constructing heterojunction.Finally,a heterojunction structure with the best light absorption performance was determined.This provides a way to adjust the optical properties of silicene in the field of photocatalytic hydrogen production.（2）The application of two-dimensional SiC materials with wide band gap in the field of photocatalytic decomposition of water to produce hydrogen is also limited because of their poor light absorption performance in the visible part.In this study,the application prospect of two-dimensional SiC materials in the field of photocatalysis was investigated using the first principles method.The band width and optical absorption of SiC materials were improved by constructing heterojunction of sic nanosheets and hexagonal boron nitride（h BN）.On this basis,the band gap of h BN/SiC heterojunction is improved from 3.17 e V to 2.47 e V by impurity engineering doped with double carbon atoms.In addition,impurity engineering also optimized the type-I h BN/SiC heterojunction into the Z-scheme h BNC/SiC heterojunction which is more suitable for photocatalyst.Subsequently,by analyzing the hydrogen evolution reaction process on the surface of hb NC/SiC 2D material,it is demonstrated that the h BNC/SiC structure doped with double C atoms can not only meet the requirements as photocatalyst,but also has a faster reaction rate for hydrogen production from decomposed water than the original structure.This provides theoretical support for the application of SiC in the field of photocatalysis.