Unveiling Active Sites for the Hydrogen Evolution Reaction on Monolayer MoS2

Here, the hydrogen evolution reaction (HER) activities at the edge and basal-plane sites of monolayer molybdenum disulfide (MoS2) synthesized by chemical vapor deposition (CVD) are studied using a local probe method enabled by selected-area lithography. Reaction windows are opened by e-beam lithography at sites of interest on poly(methyl methacrylate) (PMMA)-covered monolayer MoS2 triangles. The HER properties of MoS2 edge sites are obtained by subtraction of the activity of the basal-plane sites from results containing both basal-plane and edge sites. The catalytic performances in terms of turnover frequencies (TOFs) are calculated based on the estimated number of active sites on the selected areas. The TOFs follow a descending order of 3.8 +/- 1.6, 1.6 +/- 1.2, 0.008 +/- 0.002, and 1.9 +/- 0.8 x 10(-4) s(-1), found for 1T'-, 2H-MoS2 edges, and 1T'-, 2H-MoS2 basal planes, respectively. Edge sites of both 2H- and 1T'-MoS2 are proved to have comparable activities to platinum (approximate to 1-10 s(-1)). When fitted into the HER volcano plot, the MoS2 active sites follow a trend distinct from conventional metals, implying a possible difference in the reaction mechanism between transition-metal dichalcogenides (TMDs) and metal catalysts.