Engineering active edge sites of fractal-shaped single-layer MoS2 catalysts for high-efficiency hydrogen evolution

The hydrogen evolution reaction (HER) is a crucial step in electrochemical water splitting that demands an efficient, cheap, and stable catalyst to succeed in practical applications. Two-dimensional (2D) layered molybdenum disulfide (MoS2), known to contain active edge sites and a chemically inert basal plane, has provided great promise as a non-precious alternative to platinum-based catalysts for electrochemical hydrogen production from water. Here, we directly synthesize fractal-shaped single-layer MoS2 with large tensile strain on fused silica. The as-synthesized MoS2 with a large amount of exposed edge sites is superior to the triangle-shaped MoS2 grown on SiO2 for catalyzing HER. By controlling the MoS2 grain size and coverage, etc., we identify the active sites of the MoS2. Electrocatalytic activity of the MoS2 for the HER correlates linearly with the number of edge sites with an enhanced activity of similar to 2.74 x 10(-7) mu A mu m(-1), due to the increased supplying of electrons to the active edge sites caused by the large tensile strain. The optimal HER electrocatalyst of the fractal-shaped single-layer MoS2, which has an edge-to-substrate ratio of about 0.33 mu m(-1), exhibits superior HER catalytic activities such as a low overpotential of 185 mV at a current density of 10 mA cm(-2), a Tafel slope of 45 mV/dec, an exchange current density of 50.9 mu A cm(-2), and long-term stability. The present study provides new ways to design 2D HER electrocatalysts, including controlling the geometry, strain, and modulating the electrical conductivity.