Growth Orientation of Transition Metal Sulfides Confined in Carbon Nanospace and Their Impacts on the Hydrogen Evolution Reaction

Abstract

Two-dimensional layered transition metal dichalcogenides (2D TMDs) have been of tremendous recent interests as active electrocatalysts for the hydrogen evolution reaction (HER). Since TMD catalysts are commonly grown on conductive carbon supports, revealing their growth behavior on carbon surface can guide rational design concept for the TMD-based HER catalysts. With an aim to investigate the growth behavior on carbon surface, we synthesized MS2 (M = W or Mo) nanoplates (NPs) within porous carbon nanorod arrays by limiting their growth space at the nanoscale. A combined experimental and computational study revealed that the horizontal growth is preferred in WS2 giving rise to monolayer NPs. In contrast, MoS2 NPs favor to adhere to the carbon surface in a vertical, edge-on bonding mode, which promotes subsequential stacking to generate multilayer NPs. In the space-confinement growth within carbon nanostructure, the monolayer WS2 grown along the curved surface of carbon nanorods may have highly strained surface sites affording a high density of active sites on basal surfaces. A series of monolayer WS2 NPs with different lateral sizes were prepared on carbon nanorods, and showed efficient HER activity with a high turnover frequency. A linear correlation between mass activities and ratios of S2-/S22- species is established, suggesting that active sites primarily originate from unsaturated S2- species in highly strained basal surfaces in monolayer WS2 NPs.