Theoretically, the edges of a MoS2 flake and S-vacancy within the lattice have nearly zero Gibbs free energy for hydrogen adsorption, which is essentially correlated to the exchange currents in hydrogen evolution reaction (HER). However, MoS2 possesses insufficient active sites (edges and S-vacancies) in pristine form. Interestingly, active sites can be effectively engineered within the continuous MoS2 sheets by treating it with plasma in a controlled manner. Here, we employed N2 plasma on a large-area continuous-monolayer MoS2 synthesized via metal-organic chemical vapor deposition to acquire maximum active sites that are indeed required for an efficient HER performance. The MoS2 samples with maximum active sites were acquired by optimizing the plasma exposure time. The newly induced edges and S-vacancies were directly verified by high-resolution transmission electron microscopy. The 20 min treated MoS2 sample showed maximum active sites and thereby maximum HER activity, onset overpotential of ∼-210 mV vs reversible hydrogen electrode (RHE), and Tafel slope of ∼89 mV/dec. Clearly, the above results show that this approach can be employed for improving the HER efficiency of large-scale MoS2-based electrocatalysts.
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