Theoretical Studies Of Metal-induced 2D Transition Metal Dichalcogenide Phase Transition And Electrocatalytic Applications

Since graphene was successfully prepared by micromechanical exfoliation two-dimensional materials have attracted widespread interest.Hexagonal boron nitride（h-BN）,silicene,black phosphorus（BP）,transition metal sulfides,borophene and other two-dimensional materials have been experimentally synthesized.Recently,two-dimensional transition metal sulfides（2D TMDs）have received much attention due to their unique atomic structures and band structures.Two-dimensional（2D）transition metal dichalcogenides exhibit many phases（2H,1T and 1T’）.Metallic TMDs can be used to prepare supercapacitors and low contact resistance field-emission transistors,and they also has applications in electrocatalytic and photocatalytic water splitting.So phase engineering of 2D TMDs is very important.A large number of studies have shown that the edges of 2H TMDs are highly active for hydrogen evolution reaction（HER）,and the basal plane of 2H TMDs is chemically inert.We need to find new ways to enhance the HER activity of 2H TMDs.In addition,electrocatalytic CO₂reduction is an effective approach to realize the efficient storage of electrical energy,the cyclic utilization of carbon resource and the reduction of CO₂emission.So far,TMDs for electrocatalytic CO₂ reduction has not been systematically explored.Therefore,we will investigate metal substrate induced phase transition by density functional theory（DFT）computations.Electrocatalytic applications（HER and CO₂RR）of TMDs tuned by substrate introduction,the vacancy and doping are also studied.The main conclusions of the research are as follows:Two-dimensional（2D）transition metal dichalcogenides MX₂（M=Mo;X=Te,Se）exhibit three crystalline phases:the semiconducting 2H as well as the metallic 1T and 1T’.The 2H phase is normally more stable than both the 1T and 1T’phases.We found that1T’-Mo Te₂becomes more stable than 2H-Mo Te₂when Mo Te₂is adsorbed on Cu（111）,Ni（111）,and Ti（0001）surfaces by comprehensive first-principles calculations.A Mo Se₂monolayer on Ti（0001）substrate prefers the 1T’phase.Electronic accumulation on Mo atoms of MX₂monolayers with the introduction of the metal substrates is crucial in the2H-to-1T’phase transition.These findings offer new ways to stabilize metastable phase of transition metal dichalcogenides.We performed density functional theory calculations to study the influence of substrate introduction,the vacancy and transition metal doping on electrocatalytic hydrogen evolution performances of 2H/1T’-Mo Te₂and 2H/1T’-Mo Se₂.The introduction of Te vacancies improves electrocatalytic hydrogen evolution performances of1T’-Mo Te₂/Cu（111）,1T’-Mo Te₂/Ni（111）and 1T’-Mo Te₂/Ti（0001）,and the Gibbs free energies of H adsorbed on these catalysts are 0.07 e V,0.09 e V and 0.02 e V,respectively.Likewise,the introduction of Se vacancies promotes electrocatalytic hydrogen evolution performances of 2H-Mo Se₂/Ti and 1T’-Mo Se₂/Ti,and the Gibbs free energies of H adsorbed on the two catalysts are 0.01 e V and-0.14 e V respectively.Furthermore,we investigate the influence of Ti substrate introduction and the vacancy on electrocatalytic CO₂reduction performances of 2H/1T’-Mo Se₂.For the CO₂electroreduction reaction of2H/1T’-Mo Se₂,2H/1T’-Mo Se₂＿V_Seand 2H/1T’-Mo Se₂/Ti（0001）,the speed-determining step is the formation of*COOH intermediates,and 1T’-Mo Se₂has good electrocatalytic CO₂reduction performance.The research will provide a theoretical basis for the applications of TMDs on electrocatalytic hydrogen evolution and CO₂ reduction.