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.

References

[1] Jaramillo TF, Jorgensen KP, Bonde J, Nielsen JH, Horch S, Chorkendorff I. Identification of active edge sites for electrochemical H₂ evolution from MoS₂ nanocatalysts. Science. 2007;317:100-2.

[2] Hu J, Zhang C, Meng X, Lin H, Hu C, Long X, et al. Hydrogen evolution electrocatalysis with binary-nonmetal transition metal compounds. J Mater Chem A. 2017;5:5995-6012.

[3] Hu J, Huang B, Zhang C, Wang Z, An Y, Zhou D, et al. Engineering stepped edge surface structures of MoS₂ sheet stacks to accelerate the hydrogen evolution reaction. Energy Environ Sci. 2017;10:593-603.

Figure caption

(a) Free energy diagram for hydrogen adsorption at the stepped and flat MoS2 edges with one-quarter hydrogen coverage on one of the MoS2 layers. The molecular structures depicted at the top and bottom of (a) show H adsorption on the fe-MoS2 and se-MoS2 edges, respectively. The yellow, dark cyan and violet spheres represent S, Mo and H atoms, respectively. (b) HRTEM image and schematic illustration of the se-MoS2 layers, which clearly shows that crystal fringes of the S-Mo-S layers along the edge are stepped. (c) Polarization curves of the commercial MoS2, r-MoS2, fe-MoS2 and se-MoS2, and commercial Pt/C catalysts in 0.5M H2SO4 solution. (d) Schematic illustration of the designed stepped edge surface terminated MoS2 sheet array, in which the unique vertically terminated, stepped surface structure ensures optimal hydrogen adsorption energy (ΔGH is ~0.02 eV) and ultrafast electron transport to the stepped MoS2 edge surface.