Metallic tungsten disulfide (WS2) monolayers have been demonstrated as promising electrocatalysts for hydrogen evolution reaction (HER) induced by the high intrinsic conductivity, however, the key challenges to maximize the catalytic activity are achieving the metallic WS2 with high concentration and increasing the density of the active sites. In this work, single-atom-V catalysts (V SACs) substitutions in 1T-WS2 monolayers (91% phase purity) are fabricated to significantly enhance the HER performance via a one-step chemical vapor deposition strategy. Atomic-resolution scanning transmission electron microscopy (STEM) imaging together with Raman spectroscopy confirm the atomic dispersion of V species on the 1T-WS2 monolayers instead of energetically favorable 2H-WS2 monolayers. The growth mechanism of V SACs@1T-WS2 monolayers is experimentally and theoretically demonstrated. Density functional theory (DFT) calculations demonstrate that the activated V-atom sites play vital important role in enhancing the HER activity. In this work, it opens a novel path to directly synthesize atomically dispersed single-metal catalysts on metastable materials as efficient and robust electrocatalysts.

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There exists a strong demand to replace expensive noble metal catalysts with efficient and earth-abundant catalysts for hydrogen evolution reaction (HER). Recently the Co-and Mobased sulfides such as CoS 2 , Co 9 S 8 , and MoS x have been considered as several promising HER candidates. Here, a highly active and stable hybrid electrocatalyst 3D flower-like hierarchical Co 9 S 8 nanosheets incorporated with MoS x has been developed via a one-step sulfurization method. Since the amounts of Co 9 S 8 and MoS x are easily adjustable, we verify that small amounts of MoS x promotes the HER activity of Co 9 S 8 , and vise versa. In other words, we validate that symmetric synergy for HER in the Co-and Mo-based sulfide hybrid catalysts, a long-standing question requiring clear experimental proofs. Meanwhile, the best electrocatalyst Co 9 S 8-30@MoS x /CC in this study exhibits excellent HER performance with an overpotential of-98 mV at-10 mA/cm 2 , a small Tafel slope of 64.8 mV/dec, and prominent electrochemical stability.

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