Edge-Vacancy Synergy in Cracked Janus WSeS Monolayers for Efficient Hydrogen Evolution

The catalytic performance of two-dimensional (2D) transition metal dichalcogenides (TMDs) for the hydrogen evolution reaction (HER) is severely limited by the scarcity of active edge sites and intrinsically inert basal planes. Although Janus TMDs mitigates this issue by activating the basal plane through an inherent vertical asymmetry structure and synthesis-induced chalcogen vacancies, achieving a high density of edge sites and exploring their interplay with Janus dipole remains a critical challenge. Herein, we demonstrate a strain-driven crack engineering strategy for monolayer Janus WSeS. The lattice strain inherent in the room-temperature sulfurization conversion from WSe₂ to Janus WSeS spontaneously induces a dense distribution of nanoscale cracks, creating abundant active edges. In addition, these crack edges synergistically interact with sulfur vacancies to modulate the electronic states near E_F and optimize the hydrogen adsorption/desorption kinetics for enhanced HER activity. The resulting material achieves outstanding HER performance in acidic electrolyte, with a low overpotential of 214 mV at 10 mA cm^-2 and a Tafel slope of 68 mV dec^-1. This work provides an approach to precisely tailor edge morphology in 2D materials, bridging the structure-performance relationship for advanced electrocatalysis.