Nitrogen Plasma-treated Continuous Monolayer MoS₂ Film for Improving Hydrogen Evolution Reaction

Large density of active sites and near zero Gibbs free energy renders two-dimensional (2D) MoS₂ is ideal platforms to investigate low-cost hydrogen evolution reaction (HER) compared to the noble metal in terms of cost. Recently reported metal-organic chemical vapor deposition (MOCVD) is a good synthetic method for continuous and conformal monolayer transition metal dichacogenide. [1,2] However, the intrinsic defects and the grain boundaries in MOCVD-grown TMDC are still limited to provide enough active sites for high efficient HER. In this work, we employed nitrogen plasma to enhance the active sites on the continuous MoS₂ surface for further improving the ratio of the edge to basal sites. Initially, the optical properties were investigated by using in-situ PL and Raman spectroscopy. Furthermore, the surface morphology was examined by using FE-SEM and in-situ AFM. The HER results shows that the sample treated for 20 minutes at 0.02 mA and 20 kV showed the best performance. The sample showed reduced onset overpotential of -0.25 mV compared to the as-grown MoS₂ (-0.48 mV). [1] K. Kang, et al., Nature 520, 656 (2015). [2] E. Kim, et al., Nanoscale 10, 18920 (2018).