First Principles Study On Photocatalytic Hydrogen Production Of Two-Dimensional MoS₂-based Heterojunction

In the current society,non-renewable energy sources are increasingly depleted,and environmental pollution is serious.As a non-polluting,renewable green energy,hydrogen energy can effectively solve the problem.The water content of the earth is high,and the solar energy resources are abundant,so the photocatalytic splitting of water for hydrogen production can be realized by using solar energy.The two-dimensional transition metal sulfide MoS₂ has excellent electronic structure and optical properties,and its van der Waals heterojunction can effectively inhibit the recombination of photogenerated electrons and holes as a catalytic material for efficient photocatalytic water splitting for hydrogen production,and has great potential to improve photocatalysis Hydrogen production efficiency.In this paper,the first-principles methods are used to systematically study two-dimensional MoS₂,Hf Se₂ and their van der Waals heterojunctions,and discuss their photocatalytic properties.The main research contents and results of the paper are as follows:Firstly,based on first-principles calculations of density functional theory,the electronic structure of 2D monolayer MoS₂ were studied and compared with experimental values.The electronic structure were calculated by PBE functional and HSE06 hybrid functional respectively.The results show that the two-dimensional single-layer MoS₂ is a direct band gap semiconductor with the band gap of 1.67 e V and 2.14 e V under the two methods.Under the condition that the other calculation parameters remain unchanged,by adjusting the HSE06 hybrid functional parameter AEXX,at AEXX=0.12,the band gap of the two-dimensional monolayer MoS₂ is 1.91 e V,which is very close to the experimental value.The conduction band bottom potential is basically consistent with the experimental value.The valence band top potential is is 0.11 e V higher than the experimental value,and the calculated results meet the requirements of photocatalytic hydrogen production.Therefore,for the subsequent revised HSE06 hybrid functional calculation,the parameter AEXX are all taken as 0.12.On this basis,the MoS₂/Hf Se₂ vertical heterojunction with a lattice mismatch rate of only 1%was constructed to study its electronic structure and optical properties.It is calculated that the MoS₂/Hf Se₂ vertical heterojunction has an indirect band gap of 0.69 e V,which indicates that the band gap of the vertical heterojunction does not meet the requirements of photocatalytic hydrogen production.The density of states map of the vertical heterojunction shows that the top of the valence band is mainly provided by MoS₂,and the bottom of the conduction band is provided by Hf Se₂,indicating that the focus of energy band regulation should be Hf Se₂.The calculated optical absorption spectrum shows that the MoS₂/Hf Se₂ vertical heterojunction has a strong absorption capacity for part of the visible light,and also has a good response to the ultraviolet light.Secondly,the MoS₂/Hf Se₂ heterojunctions were doped with transition metal Ti atoms and non-metal O atoms,and the heterojunctions of Mo₄S₈/Hf₂Ti Se₆,Mo₄S₈/Hf₃Se₅O and Mo₄S₈/Hf₃Se₄O₂ were constructed（the doping concentrations of O atoms were respectively4.76%and 9.52%）,and their electronic structure were studied.At AEXX takes 0.12,the Mo₄S₈/Hf₂Ti Se₆ heterojunction has an indirect band gap of 0.38 e V.At the band gap of the MoS₂/Hf Se₂ heterojunction is reduced by 0.31 e V,it does not meet the expected requirements.Mo₄S₈/Hf₃Se₅O heterojunction has an indirect band gap of 1.43 e V.Compared with MoS₂/Hf Se₂ heterojunction,the band gap is increased by 0.74 e V.The increase in the band gap is due to the fact that the doping of non-metallic O makes the conduction band bottom and valence band top of the MoS₂/Hf Se₂ heterojunction move up and down,（the bottom potential of the conduction band is moved up by 0.47 e V,and the band top potential was moved down by 0.28 e V）,and its band gap and valence band top potential meet the requirements of photocatalytic hydrogen production.The Mo₄S₈/Hf₃Se₄O₂ heterojunction has an indirect band gap of 1.72 e V.Compared with the Mo₄S₈/Hf₃Se₅O heterojunction,the band gap is increased by 0.29 e V,the bottom potential of the conduction band is moved up by 0.42 e V,and the top potential of the valence band is moved up by 0.12 e V,its band gap,bottom potential of conduction band and top potential of valence band all meet the requirements of photocatalytic hydrogen production.It can be seen that the increase of the doping concentration of O atoms can effectively adjust the electronic structure of the MoS₂/Hf Se₂heterojunction.Finally,the optical properties of doped heterojunctions are calculated using PBE functional and compared with the MoS₂/Hf Se₂ heterojunctions.The calculation results show that the doping of transition metal Ti atom and non-metal O atom can improve the ultraviolet light absorption ability of the two-dimensional MoS₂-based heterojunction.The O atom doping can make the two-dimensional MoS₂-based heterojunction pair.Compared with the absorption edge of two-dimensional monolayer MoS₂,the wavelengths are red-shifted by118 nm and 72 nm.In conclusion,the Mo₄S₈/Hf₃Se₄O₂ vertical heterojunction has great potential to be applied in the field of photocatalytic water splitting for hydrogen production.