Origin of hydrogen evolution activity on MS2 (M = Mo or Nb) monolayers

Catalytic activity often stems from surface atoms with dangling bonds. However, recent experiments show that the perfect surfaces of two-dimensional transition-metal disulfide (TMD) nanosheets are highly active for hydrogen generation. On the basis of first-principles calculations, we find that the surface activity of MS2 (M = Mo or Nb) monolayers largely originates from their bulk energy stabilization induced by electron injection in the Volmer reaction. A static computational model is proposed to refine the reaction Gibbs free energy of H adsorption into adiabatic electron and proton affinities for electronic-level thermodynamic description. It is found that the large adiabatic electron affinities of 1H-NbS2, 1T-MoS2 and 1T-NbS2 monolayers lead to their strong H adsorption at low surface H coverage. In contrast, the negative adiabatic electron affinity of 1H-MoS2 explains its catalytic inertness. For 1H-NbS2, the large adiabatic electron affinity comes from the quenching of d-band exchange splitting after H adsorption and the stabilization of bonding frontier states. For 1T-MoS2 and 1T-NbS2, their large adiabatic electron affinities are ascribed to the stabilization effect of charge density waves. It is suggested that tensile strains or modifications that could increase the adiabatic electron affinity can enhance H adsorption on the basal-planes of TMDs.