P-band center theory guided activation of MoS₂ basal S sites for pH-universal hydrogen evolution

The edge S sites of thermodynamically stable 2H MoS₂ are active for hydrogen evolution reaction (HER) but the active sites are scarce. Despite the dominance of the basal S sites, they are generally inert to HER because of the low p-band center. Herein, we reported a synergistic combination of phase engineering and NH₄⁺ intercalation to promote the HER performance of MoS₂. The rational combination of 1T and 2H phases raises the p-band center of the basal S sites while the intercalated NH₄⁺ ions further optimize and stabilize the electronic band of these sites. The S sites with regulated band structures afford moderate hydrogen adsorption, thus contributing to excellent HER performance over a wide pH range. In an acid medium, this catalyst exhibits a low overpotential of 169 mV at 10 mA·cm⁻² and Tafel slope of 39 mV·dec⁻¹ with robust stability, superior to most of recently reported MoS₂-based non-noble catalysts. The combined use of in/ex-situ characterizations ravels that the appearance of more unpaired electrons at the Mo 4d-orbital reduces the d-band center which upshifts the p-band center of the adjacent S for essentially improved HER performance. This work provides guidelines for the future development of layered transition-metal-dichalcogenide catalysts.