Nanopolygons of Monolayer MS₂: Best Morphology and Size for HER Catalysis

With first-principles calculations, we find a new strategy for developing high-performance catalysts for hydrogen evolution reaction (HER) via controlling the morphology and size of nanopolygons of monolayer transition-metal dichalcogenides (npm-MS₂, with M = Mo, W, or V). Particularly, through devising a quantitative method to measure HER-active sites per unit mass and using such HER site density to comparatively gauge npm-MS₂ performance, we identify three keys in making npm-MS₂ with optimal HER performance: (a) npm-MS₂ should be triangular with each edge being M-terminated and each edge-M atom passivated by one S atom; (b) each edge of npm-MoS₂ and WS₂ should have 5-6 metal atoms as HER site density drops below/above these sizes optimal both for HER and practical npm growth; and (c) npm-VS₂ is immune to this overly fastidious size dependence. Known experimental data on npm-MoS₂ indeed support the plausibility of practicing these design rules. We expect that raising the nucleation density and controlling the growth time to favor the production of our proposed ultrasmall npm-MS₂ are critical but practical. Research on npm-VS₂ would bear the highest impact because of its size-forgiving HER performance and relatively high abundance and low cost.