Investigation Of The Two-dimensional MX₂/ZnO Van Der Waals Heterostructures By First-principles Calculation

In recent years,two-dimensional(2D)materials are intensively studied.2D materials have a lot of novel and unique characteristics,largely broadening their application prospects.However,they are not spotless in practical applications,such as:photo-generated electron-hole pairs in monolayer MoS2 are extremely easy to recombine;the new 2D graphene-like structure of ZnO can only absorb light in the ultraviolet region,lowing the material utilization efficiency of solar energy.The van der Waals(vdW)heterostructure formed by the composite of 2D materials with different structures can not only retain the excellent performance of each layer of 2D materials,but also complement each other to obtain some unique structures and properties.In this paper,the first-principles calculation is used to systematically study the electronic and optical properties of monolayer MX2(M=Mo,W,X=S)transition metal sulfide(TMDs),monolayer ZnO and their composite vdW heterostructure,and explore their applications in photocatalysis.Firstly,by building MoS2,WS2 and ZnO monolayer models,and calculating their electronic properties,it is revealed that the monolayer MoS2,monolayer ZnO and monolayer WS2 have band gap widths of 2.25 eV,3.28 eV,and 2.30 eV,respectively,and they all have the properties of a direct bandgap semiconductor.Secondly,the first-principles calculations are performed on the 2D MoS2/ZnO and WS2/ZnO vdW heterostructure models,and the results show that:(1)The binding energies of the 2D MoS2/ZnO vdW heterostructure in different stacking models are all negative,indicating that the heterostructure is energetically stable.The 2D MoS2/ZnO vdW heterostructure is a typical type-? heterostructure with an indirect bandgap of 1.45 eV.Its conduction band minimum(CBM)is slightly higher than the reduction potential of water splitting,and its valence band maximum(VBM)is slightly lower than the oxidation potential of water splitting.Thus,the heterostructure satisfies the conditions required for the catalytic application in water splitting.(2)The binding energies of the 2D WS2/ZnO vdW heterostructure in different stacking models are also all negative,indicating that the heterostructures are energetically stable.Regardless of the 2D WS2/ZnO vdW heterostructures of the stacking models,their CBM are lower than the reduction potential of water splitting,while their VBM are slightly lower than the oxidation potential of water splitting.This shows that the 2D WS2/ZnO vdW heterostructure can be used as a good oxidant in the process of water splitting,but cannot fulfill the requirement for water reduction.When a biaxial stress is applied,the VBM of the heterostructure can be adjusted,thereby making the heterostructure into a possible good reduction agent in water splitting.(3)There is a built-in electric field at the interface of the 2D MoS2/ZnO and WS2/ZnO vdW heterostructures,which leads to the easy separation of photogenerated electron-hole pairs easily and reduces the recombination rate of photogenerated carriers.The heterostructure can not only respond to the light in the near-violet region,but also absorb the light in the visible region,broadening the light absorption range and enhancing the utilization efficiency of solar energy.It can be used as a candidate material for photovoltaic,photocatalysis,solar cell,water splitting,etc.