A Physical Model for Understanding the Activation of MoS Basal-Plane Sulfur Atoms for the Hydrogen Evolution Reaction.

Weak binding of hydrogen atoms to the 2H-MoS basal plane renders MoS inert as an electrocatalyst for the hydrogen evolution reaction. Transition-metal doping can activate neighboring sulfur atoms in the MoS basal plane to bind hydrogen more strongly. Our theoretical studies show strong variation in the degree of activation by dopants across the 3d transition-metal series. To understand the trends in activation, we propose a model based on the electronic promotion energy required to partially open the full valence shell of a local S atom and therefore enable it to bond with a H atom. In general, the promotion is achieved through an electron transfer from the S to neighboring metal-atom sites. Furthermore, we demonstrate a specific, electronic-structure-based descriptor for the hydrogen-binding strength: Δ , the local interband energy separation between the lowest empty d-states on the dopant metal atoms and occupied p-states on S. This model can be used to provide guidelines for chalcogen activation in future catalyst design based on doped transition-metal dichalcogenides.