A Theoretical Study Of Defective Molybdenum Disulfide/Graphene Composite Material For Hydrogen Evolution Reaction

Transition metal dichalcogenides molybdenum disulfide（MoS₂）has been a promising candidate for hydrogen evolution reaction（HER）because of it’s low cost,abundant reserve and alternative to Pt for electrochemical hydrogen production.But the catalytic performance of MoS₂ is limitied for it’s scarce intrinsic active site and low conductivity.In this article,we systematically explore the structure,electronic structure and hydrogen evolution catalytic activity of defective MoS₂/graphene composites by using density functional theory（DFT）,thus provides theoretical basis to design better MoS₂ based material for hydrogen evolution.The study of defective MoS₂/graphene composites shows that,the double layered composites are stabilized by the Van der Waals force between the graphene layer and MoS₂ layer。The electronic structure of MoS₂/graphene composites is different from that pure MoS₂ or graphene,and the graphene layer promotes the conductivity of the MoS₂ layer.The hydrogen evolution study on seven types of defective(V_S,V_S2,V_Mo,V_MoS3,V_MoS6,S2_Moo and Mo_S2)MoS₂/graphene composites indicates that H adsorption Gibbs free energy（ΔG_H）values of V_S-MoS₂/G,V_S2-MoS₂/G,V_MoS3-MoS₂/G and Mo_S2-MoS₂/G cases are close to Pt（0.09 eV）,showing prominent hydrogen evolution activity.The electronic structure research demonstrates that Mo-4d orbital appears near the Fermi level is beneficial for the overlap of H-1s and Mo-4d orbital.Additionally,the good linear relationship between hydrogen adsorption energy（ΔE_H）and d-band-center-|ε′_d|also shows that the d orbital electrons near the Fermi level is vital for hydrogen adsorption performance.A further study of the hydrogen evolution mechanism shows,V_S-MoS₂/G,V_S2-MoS₂/G and Mo_S2-MoS₂/G cases own low Tafel and Heyrovsky reaction barrier and exhibit excellent kinetic performance for hydrogen evolution.Water splitting reaction on V_S-MoS₂/G,V_S2-MoS₂/G and Mo_S2-MoS₂/G cases goes through Volmer-Tafel and Volmer-Heyrovsky reaction both,while on the case of V_MoS3-MoS₂/G,it happens in favor of Volmer-Heyrovsky reaction.Furthermore,our results also shows that the distance between the adsorbed H*is the core factor affecting the barrier of Tafel reaction.The study of hydrogen evolution property on perfect MoS₂/G and defective(V_S,V_S2,V_Mo,V_MoS3,V_MoS6,S2_Moo and Mo_S2)MoS₂/G composites under-5%to+5%biaxial strain condition demonstrates,that the strain effect influence perfect MoS₂/G case little,while the catalytic activity of defective(V_S,V_S2,V_MoS3 and Mo_S2)MoS₂/G cases can be optimized.Especially,ΔG_H of V_S-MoS₂/G,V_S2-MoS₂/G,V_MoS3-MoS₂/G and Mo_S2-MoS₂/G cases approach to 0 eV under+1%,+1%,+4%and+1%strain condition respectively.It is clear that strain effect is capable to tune the hydrogen evolution performance of defective MoS₂/G.The electronic structure analyze shows that strain is effective to change the d orbital state near the Fermi level,ΔE_H′and-|ε_d|of MoS₂/G composites correlate each other with good linear relationship.The deeper hydrogen evolution reaction mechanism study indicates,that compared with no strain condition circumstance,biaxial strain effectively reduces Heyrovsky reaction barrier of V_MoS3-MoS₂/G,boosting the kinetics process of hydrogen evolution thus promote the catalytic property of materials.In summary,the strain effect optimizes the thermodynamics process of hydrogen adsorption and also accelerates the kinetics process of hydrogen evolution.