First-principles Study On Photocatalytic Performance Of MoS₂/CdS And MoS₂/α-In₂Se₃

One of the most promising technologies to solve is to use solar energy to hydrolyze water to produce hydrogen to convert solar energy into chemical energy.And,it could directly provide raw materials to basic chemical industry,with high-efficiency photocatalyst being the key to the technology.Among them,molybdenum disulfide（MoS₂）is a p-type semiconductor with excellent optoelectronic properties.It stands out with its suitable energy band position,visible light response,and abundant storage.And it shows strong activity in degradation of organic compounds,nitrogen fixation,CO₂conversion and so on.However,the application of pure MoS₂is often limited by the shortcoming of limited light response range,easy recombination of electrons and holes,and bad stability.Although many researchers have used a series of methods to modify it,such as:doping,constructing defects,changing crystal phases,and building heterojunctions,etc.While the researches only focus on experimental phenomena and results generally,there is a lack of in-depth discussion on the internal mechanism of the photocatalytic performance of MoS₂and its heterojunction.Based on the above issues,this dessertation started from the First Principle,combining with the electronic structure,the band structure,the effective mass of photo-generated carriers,light absorption,and so on.We concentrated on explaining the microscopic catalysis mechanism of existed MoS₂-based heterojunction and tried to design a new type of heterojunction further.Besides,experimental methods was simulated to clarify its effects on catalytic performance of the heterojunction,such as changing the contacted surface and applying stress.The purpose was to provide the theoretical guidance for understanding how the photocatalysts work and designing catalysts in experiments.The major results and innovations of this research are as below:1.According to the previous experimental results,A MoS₂/CdS van der Waals heterojunction was constructed firstly in theory.The effects of different contact surfaces（（100）and（001））of CdS on the performance of MoS₂/CdS photocatalytic hydrogen production was studied,the aim was to find the best contacted interface.The calculation results revealed that the binding energy of MoS₂/CdS（100）was more negative than that of MoS₂/CdS（001）,which mean that the stability of the material was enhanced.And the band gap of MoS₂/CdS（100）was significantly smaller than that of MoS₂、CdS and MoS₂/CdS（001）.This was due to the exposure of more S and Cd atoms on the surface of MoS₂/CdS（100）,making S 3p orbit produce an additional energy level that caused the valence band position moving up,and further increased the light response range and had strong absorption of visible light.Moreover,MoS₂/CdS（100）was a type-II heterojunction,the relative offset calculated from the position of the valence band and the conduction band showed that the material generated a certain built-in electric field during the process of light excitation,the electrons was transferred from CdS to MoS₂,which promoted the separation of photogenerated carriers greatly.The capacity of photocatalytic hydrogen production was significantly enhanced.Based on the above results,the CdS（100）was the best contacted surface,which was consistent with the existed experimental results.2.A new photocatalyst was designed in theory,named MoS₂/α-In₂Se₃.The effects of different stress conditions on the photocatalytic oxygen generation capacity of heterostructure were explored.The electronic structure and optical characteristics of the heterojunction were analyzed by using two calculation methods,Perdew-Burke-Ernzerhof（PBE）and Heyd-Scuseria-Ernzerhof（HSE）,in order to obtain the optimal stress level at the highest catalytic efficiency.Studies presented that the stress changed from-4%,-2%,0%,2%,and 4%in order,the valence band of the composite catalyst shifted downward accordingly.Finally,the band gap decreased gradually,the light absorption range expanded,from the original infrared light to the near infrared light,and the optical intensity increased gradually.The oxidation ability of the material was correspondingly improved,and the application field of catalysis was expanded.When the applied normal stress was 4%,the photooxidation ability of MoS₂/α-In₂Se₃could be improved greatly.