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.