Nitrogen-Plasma-Treated Continuous Monolayer MoS₂ for Improving Hydrogen Evolution Reaction

Abstract

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.