Engineering Stepped Edge Surface Structures of MoS2 Sheet Stacks to Accelerate the Hydrogen Evolution Reaction

Abstract

Two-dimensional molybdenum sulfide is an attractive noble-metal-free electrocatalyst for the hydrogen evolution reaction (HER). Significant efforts have been spent on increasing the number of exposed edge sites [1]. However, little attention has been paid to devising edge surface structures of MoS2 sheet stacks to promote the HER kinetics [2]. Herein we report the first demonstration of significantly enhanced HER kinetics by tailoring a stepped edge surface structure of MoS2 multilayers with respect to a flat edge surface. We provide an unambiguous interpretation on the HER performance difference in terms of the different H bonding on the stepped edge surface (se-MoS2) and on the flat edge surface (fe-MoS2). Vertical arrays of MoS2 sheets terminated with such a stepped surface structure have proved to be an outstanding HER electrocatalyst with overpotential of 104 mV at 10 mA/cm2, exchange current density of 0.2 mA/cm2 and high stability (Figure 1). DFT calculations suggest a more optimal ΔG H of the active se-MoS2 edge surface than that of the fe-MoS2 edge surface, and thereby a faster HER kinetics (Figure 1a). A perfectly designed stepped edge surface terminated MoS2 sheet array is schematically depicted in Figure 1d, in which the unique vertically terminated, stepped surface structure ensures an optimal hydrogen adsorption energy (ΔG H is ~0.02 eV); the vertical array would permit ultrafast electron transport and promote HER performance [3]. This approach presented here provides a new insight that we tailor the edge active sites to modulate the performance of HER and should be applicable to generalized transition-metal-dichalcogenide catalysts, by engineering their surface structures. we expect that our stepped-edge engineering strategy will prove more generally effective for creating catalysts from abundant noble-metal-free layered materials for hydrogen evolution.