First-principles Study On Photocatalytic Properties Of ZnO Monolayer And Its Heterostructure

Photocatalytic water splitting into hydrogen is one of the most active research fields in recent years.Due to ultraviolet light accounts for 4.00% in the solar spectrum,and visible light accounts for 43.00% in the solar spectrum,improving the absorption in the visible light area has been the focus of photocatalytic water splitting.The key problem of photocatalytic water splitting is the choice of the photocatalysts.Compared with the corresponding bulk material,two-dimensional materials can shorten the distance necessary for the transport of the photo-generated carrier to the surface and have large surface areas,furnishing photocatalytic reaction sites.ZnO is one of the most widely studied semiconductor photocatalysts.Moreover,combined with the advantages of two-dimensional materials,ZnO monolayer has greater prospects in the field of photocatalytic decomposition of water.Unfortunately,the ZnO monolayer semiconductor has a wide band gap which limits its absorption in the visible light region.In view of this problem,this thesis puts forward the strategy of tuning the band gap to enhance the photocatalytic performance of the ZnO monolayer,using three methods of tuning the band gap by applying biaxial strain,applying an electric field,and forming a heterojunction with other single-layer materials.The calculation results show that the band gap of the intrinsic ZnO monolayer is 3.30 e V,which lead to a relatively low absorption in the visible light region.While the band gap of the ZnO monolayer at 10% biaxial tensile strain is reduced to 2.70 e V,at the same time the oxidizing capacity is improved,the light absorption is enhanced in the visible region and the band edge position still meets the redox potential of water.In addition,we found that the applied electric field had no significant effect on improving photocatalytic performance.Our theoretical work shows that biaxial strain can effectively enhance the performance of the ZnO monolayer for photocatalytic water splitting.The lattice constant of GaN monolayer is matched with the ZnO monolayer,which makes them be the ideal material for forming the heterojunction.However,its photocatalytic ability to decompose water is limited by excessively wide band gaps and indirect band gaps.The study found that when the doping content of In atom was 11.1 at.%,the band gap of GaN monolayer decreased from 3.30 e V to 2.90 e V,but it was still an indirect band gap.The GaN monolayer has a band gap of 2.42 e V under 7.5% biaxial compressive strain,and changes from an indirect bandgap semiconductor to a direct bandgap semiconductor.At the same time,it solves two problems with its intrinsic materials.The calculation results of the ZnO / GaN monolayer heterojunction show that the structure is stable,the band gap is 2.73 e V,and the band edge position is appropriate.These results show that biaxial strain and forming a heterojunction with other single-layer materials can effectively enhance the performance of the ZnO monolayer for photocatalytic water splitting,and it provides a reference for improving the photocatalytic performance of two-dimensional materials.