A First-principles Study Of Van Der Waals Heterojunctions Based On GaS And Its Janus Structure

The symmetry of a 2D material is significant for its electronic properties,and the introduction of asymmetry in different planes can bring novel properties to the material.The Janus group-III monochalcogenides with intrinsic dipole has extensive application prospects in the field of optoelectronics.The van der Waals heterojunctions based on the group-Ⅲ monochalcogenides GaS and its Janus structure are established in this work,the electronic transport properties,optical properties and photocatalytic performance of heterojunctions are investigated systematically by first-principles calculations.First,the stability,electron properties,charge transfer,band alignment and optical absorption of novel g-C6N6/GaS heterojunctions are investigated systematically.The interlayer distance indicates that the g-C6N6/GaS heterojunction is the typical van der Waals heterojunction,and the negative formation energy means that heterojunction is energetically favorable.After contacting,the redistribution of charge leads to a built-in electric field at the interface.The g-C6N6/GaS heterojunction presents a type-II alignment,photo-generated electrons and photo-generated holes can be effectively separated.The band edge of type-II g-C6N6/GaS heterojunction straddles the redox potential of water,which can spontaneously undergo hydrogen and oxygen evolution reactions to photocatalytic water splitting.In addition,the g-C6N6/GaS heterojunction exhibits a strong absorption in the visible light region,which can capture and convert solar energy efficiently.The electronic characteristics and charge distribution of the heterojunction are tuned by applying external electric field to change the coupling between monolayers.The results indicate that the type-II g-C6N6/GaS heterojunction is a promising candidate in photocatalytic water splitting and solar energy conversion.Then,we build the Janus Ga2SSe by breaking the mirror symmetry of GaS,and construct the g-C6N6/Janus Ga2SSe heterojunction with g-C6N6 monolayer.Considering the influence of different contact surfaces,two kinds of structures g-C6N6/SGa2Se and gC6N6/SeGa2S are designed.The stability,electronic structure,optical properties of the heterojunction have been investigated systematically.The interlayer distance indicates that the g-C6N6/Janus Ga2SSe heterojunction is the typical van der Waals heterojunctions,and the negative formation energy means that g-C6N6/Janus Ga2SSe is energetically favorable.The redistribution of charge leads to the built-in electric field at the interface.Both g-C6N6/SGa2Se and g-C6N6/SeGa2S present type-II alignment.The band edge of type-II g-C6N6/SGa2Se heterojunction straddles the redox potential of water,which can spontaneously undergo hydrogen and oxygen evolution reactions.In addition,the gC6N6/SeGa2S heterojunction exhibits a stronger absorption than g-C6N6/GaS in the visible light region,the external electric field can further improve its optical response.The results indicate that the type-II g-C6N6/Janus Ga2SSe heterojunction is a promising candidate in photocatalytic water splitting and solar energy conversion.Finally,we build Janus Ga2STe and construct GaSe/Janus Ga2STe heterojunction.Considering the influence of different contact surfaces,g-C6N6/SGa2Se and gC6N6/SeGa2S are designed.The interlayer distance indicates that the g-C6N6/Janus Ga2STe heterojunctions is the typical van der Waals heterojunction,and the negative formation energy means the heterojunction is energetically favorable.The redistribution of charge leads to the built-in electric field at the interface.The order of S atoms and Te atoms relative to GaSe affects the properties of the heterojunction.GaSe/SGa2Te presents a type-I alignment with the band edge straddling the reduction and oxidation potential of water,and band alignment turn to type-Ⅱ under the external electric field,which significantly enhanced photocatalytic performance.GaSe/TeGa2S exhibits a type-II alignment with a high oxidation activity.Both heterojunctions exhibit excellent visible light response.The results show that the GaSe/Janus Ga2STe heterojunction is a promising photocatalyst candidate material.