First-principles Study On Electronic And Photocatalytic Properties Of 2D MoS₂ Van Der Waals Heterojunction

Two-dimensional MoS₂ is a kind of two-dimensional transition metal dichalcogenide.As a high efficiency photocatalyst,the van der Waals heterojunctions of MoS₂ can effectively prevent the recombination of photogenerated electrons and holes,and they are hopeful materials to achieve a breakthrough in the efficiency of photocatalysis.In this thesis,the properties of van der Waals heterojunctions formed between MoS₂ and four new two-dimensional materials Si C,Zn O,Ga S and Ga₂S₃ are theoretically calculated and studied by the first-principles method,and their electronic and photocatalytic properties are also discussed.The main research contents and achievements of this thesis are as follows:（1）Through the Materials Visualizer in the Material Studio software platform,this thesis first established the structure model of MoS₂ heterojunction and because of the asymmetry of the geometry of Ga₂S₃,two kinds of MoS₂/Ga₂S₃ heterojunctions were established for the first time according to the contact surface of the heterojunctions.Using the structure optimization function of the CASTEP software module,the structure optimization of different stacking methods is carried out and the forming energy is calculated,determined the most stable stacking method,and obtained the optimized structure model of each heterojunction.（2）Based on the most stable heterojunction model after optimization,this thesis calculates the electrical properties such as energy band structure and electron density of states of each heterojunction.The calculated results show that the energy band structure of each two-dimensional material will partially change after the formation of heterojunction.The band gap values of MoS₂/ZnO,MoS₂/SiC,MoS₂/GaS and MoS₂/Ga₂S₃ heterojunctions are1.267e V,1.356e V,2.24e V,2.054e V and 1.547e V respectively calculated by HSE06 hybrid functional.The band gap value of each heterojunction was greater than 1.23e V,which satisfied the photocatalytic energy condition of the PN type photocatalytic heterojunction.From the energy bands contributed by two-dimensional materials in the heterojunction,it can be concluded that MoS₂/Ga S belongs to Type-I heterojunction,while MoS₂/ZnO,MoS₂/SiC and MoS₂/Ga₂S₃ heterojunction belong to Type-II heterojunction which is more favorable for photocatalysis.（3）Based on the first-principles method and using CASTEP software module,the photocatalytic characteristics of MoS₂ heterojunction were studied in this thesis.According to the analysis of the absorption coefficient in the optical properties of MoS₂ heterojunction,it is found that the absorption coefficient of MoS₂/Zn O,MoS₂/Si C,MoS₂/Ga S and MoS₂/Ga₂S₃ heterojunction in the visible light range is enhanced compared with that of the monolayer materials that compose the heterojunction,which is helpful to improve the photocatalytic efficiency of sunlight as the photocatalytic source.（4）Based on the first-principles method of density functional theory and using the HSE06 hybrid functional,the band edge position of each heterojunction relative to the vacuum level is calculated.The results show that the four heterojunctions studied in this thesis can meet the requirements of the band position of photocatalysis at a certain p H.According to the position of the band edge and the value of the band gap,the p H range of the two structures of MoS₂/ZnO,MoS₂/SiC,MoS₂/GaS and MoS₂/Ga₂S₃ is 0～0.4,1～2.5,0～4.6,0～4.1 and 0～14,respectively.（5）The biaxial strain of±5% was applied to each heterojunction,and the relationship between photocatalytic performance and applied strain was studied.The results show that the tensile strain can improve the light absorption capacity of each heterojunction in the visible light range to some extent,but the band gap value will be reduced and the photocatalytic oxidation energy or reduction energy will be reduced.The compressive strain can improve the light absorption capacity of each heterojunction in the ultraviolet range,but the absorption coefficient in the visible range is reduced except for the structure I heterojunction of MoS₂/Ga₂S₃.