Engineering stepped edge surface structures of MoS₂ sheet stacks to accelerate the hydrogen evolution reaction

Two-dimensional molybdenum sulfide is an attractive noble-metal-free electrocatalyst for the hydrogen evolution reaction (HER). Significant efforts have been made to increase the number of exposed edge sites. However, little attention has been paid to devising edge surface structures of MoS₂ sheet stacks to promote the HER kinetics. Herein we report the first demonstration of significantly enhanced HER kinetics by controllably fabricating a stepped MoS₂ surface structure. Vertical arrays of MoS₂ sheets terminated with such a stepped surface structure have proved to be an outstanding HER electrocatalyst with an overpotential of 104 mV at 10 mA cm^-2, an exchange current density of 0.2 mA cm^-2 and high stability. Experimental and theoretical results indicate that the enhanced electrocatalytic activity of the vertical MoS₂ arrays is associated with the unique vertically terminated, highly exposed, stepped surface structure with a nearly thermoneutral H-adsorption energy. This work opens a new avenue to designing and developing layered materials for electrochemical energy applications.