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 ~ 2.74 × 10⁻⁷ μA μm⁻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 µm⁻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 μA 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.