Abstract Tuning surface strain is a new strategy for boosting catalytic activity to achieve sustainable energy supplies; however, correlating the surface strain with catalytic performance is scarce because such mechanistic studies strongly require the capability of tailoring surface strain on catalysts as precisely as possible. Herein, a conceptual strategy of precisely tuning tensile surface strain on Co 9 S 8 /MoS 2 core/shell nanocrystals for boosting the hydrogen evolution reaction (HER) activity by controlling the MoS 2 shell numbers is demonstrated. It is found that the tensile surface strain of Co 9 S 8 /MoS 2 core/shell nanocrystals can be precisely tuned from 3.5% to 0% by changing the MoS 2 shell layer from 5L to 1L, in which the strained Co 9 S 8 /1L MoS 2 (3.5%) exhibits the best HER performance with an overpotential of only 97 mV (10 mA cm −2 ) and a Tafel slope of 71 mV dec −1 . The density functional theory calculation reveals that the Co 9 S 8 /1L MoS 2 core/shell nanostructure yields the lowest hydrogen adsorption energy (∆ E H ) of −1.03 eV and transition state energy barrier (∆ E 2H* ) of 0.29 eV (MoS 2 , ∆ E H = −0.86 eV and ∆ E 2H* = 0.49 eV), which are the key in boosting HER activity by stabilizing the HER intermediate, seizing H ions, and releasing H 2 gas.
