Stability And Electronic Properties Of Two-dimensional M₆B₆T_Z（M=Mo,W,Ta,Nb,T_Z=-O,-OH,-F） Monolayers

Two-dimensional（2D）materials,such as graphene,transition metal chalcogenides,hexagonal boron nitride,etc,exhibit novel electronic,mechanical,optical,magnetic and other properties that are different from the bulk due to the influence of quantum confinement effect,and thus attract wide attention.2D materials are generally bound by weak Van der waals forces in their bulk phases,allowing them to be exfoliate relatively easily into two-dimensional crystalline phases.Different from theseVan der waals force layered materials,some layered materials such as the layered hexagonal transition metal carbon,nitride MAX phase are bonded through strong metal,covalent,ionic bonds,resulting the difficulty in the corresponding two-dimensional crystal phase synthesis.Recently,a team from Sweden reported a 2D boride with ordered metal vacancy Mo_4/3B_2-x（Science373,801-805（2021））obtained by chemical stripping,opening a new stage in the production of large quantities of 2D transition metal borides by chemical stripping of layered compounds.In this work,the general expression for 2D transition metal borides is:Mo_4/3B_2-xT_z,where T_z represents the surface terminal-F,-O,or-OH.This kind of material has ordered metal vacancy,which is different from the previously reported 2D materials.This drives us to further investigate the effect of vacancy concentration on this type of 2D transition metal borides.In this paper,effects of different vacancy concentration on the stability of Mo_4/3B_2-xT_z have been investigated by first principles calculations.Several new 2D transition metal borides have been discovered,and the functional groups can effectively stabilize the structure of these materials.We further investigated their structures,electronic and mechanical properties,and hydrogen evolution reaction（HER）performance.The main contents of the article are as follows:1.According to the structure of Mo₄B₆,the structural property of Mo₄B₆ were analyzed,and the model structure of different vacancy concentration was constructed,and the influence of vacancy concentration on Mo₄B₆ was explored.2.It is found that the free vocancy structure Mo₆B₆ has a low energy.After surface functional group modification,it is found that the structure is the most stable when the functional group is in bridge position.Mo₆B₆O₆and Mo₆B₆O₆H₆ with high stability were obtained,and their structural and electronic properties were analyzed.3.On the basis of the above structures,the structure screening based on element replacement is carried out.We replace Mo atoms with Ta,Nb,and W,respectively.Ta₆B₆,Nb₆B₆ and W₆B₆ structures were obtained and passivated respectively.Among them,Ta₆B₆O₆,Ta₆B₆F₆,W₆B₆O₆ and Nb₆B₆F₆ are kinetically and thermodynamically stable,which is expected to be synthesized experimentally.W₆B₆O₆H₆ is kinetically stable.And energy band calculations show that they are all metallic.At the same time,Ta₆B₆O₆has a good HER characteristic,the value ofΔG_H^*is 0.06eV,which tends to the ideal hydrogen evolution reaction performance.The relationship between HER and stress action is explored.When strain=-2%,the value ofΔG_H^*is 0.003eV,and the result tends to 0,which is close to the ideal catalytic performance.The relationship between HER efficiency and hydrogen coverage was studied.It was found that HER catalytic performance decreased slightly with the increase of hydrogen coverage.This kind of material has a promising application in catalysis.Our systematic study of 2D transition metal borides provides a theoretical basis for the experimental preparation of such materials.