Pristine basal- and edge-plane-oriented molybdenite MoS2 exhibiting highly anisotropic properties

Abstract

The layered structure of molybdenum disulfide (MoS2) is structurally similar to that of graphite, with individual sheets strongly covalently bonded within but held together through weak van der Waals interactions. This results in two distinct surfaces of MoS2: basal and edge planes. The edge plane was theoretically predicted to be more electroactive than the basal plane, but evidence from direct experimental comparison is elusive. Herein, the first study comparing the two surfaces of MoS2 by using macroscopic crystals is presented. A careful investigation of the electrochemical properties of macroscopic MoS2 pristine crystals with precise control over the exposure of one plane surface, that is, basal plane or edge plane, was performed. These crystals were characterized thoroughly by AFM, Raman spectroscopy, X-ray photoelectron spectroscopy, voltammetry, digital simulation, and DFT calculations. In the Raman spectra, the basal and edge planes show anisotropy in the preferred excitation of E2g and A1g phonon modes, respectively. The edge plane exhibits a much larger heterogeneous electron transfer rate constant k0 of 4.96×10-5 and 1.1×10-3 cm s-1 for [Fe(CN)6]3-/4- and [Ru(NH3)6]3+/2+ redox probes, respectively, compared to the basal plane, which yielded k0 tending towards zero for [Fe(CN)6]3-/4- and about 9.3×10-4 cm s-1 for [Ru(NH3)6]3+/2+. The industrially important hydrogen evolution reaction follows the trend observed for [Fe(CN)6]3-/4- in that the basal plane is basically inactive. The experimental comparison of the edge and basal planes of MoS2 crystals is supported by DFT calculations.