Adsorption and Dissociation of H<sub>2</sub>O on Monolayered MoS<sub>2</sub> Edges: Energetics and Mechanism from <i>ab Initio</i> Simulations

The dissociation of water on 2D monolayer molybdenum disulfide (MoS<sub>2</sub>) edges was studied with density functional theory. The catalytically active sites for H<sub>2</sub>O, H, and OH adsorption on MoS<sub>2</sub> edges with 0% (Mo-edge), 50% (S50-edge), and 100% (S100-edge) sulfur coverage were determined, and the Mo-edge was found to be the most favorable for adsorption of all species. The water dissociation reaction was then simulated on all edges using the climbing image nudged elastic band (CI-NEB) technique. The reaction was found to be endothermic on the S100-edge and exothermic for the S50- and Mo-edges, with the Mo-edge having the lowest activation energy barrier. Water dissociation was then explored on the Mo-edge using metadynamics biased <i>ab initio</i> molecular dynamics (AIMD) methods to explore the reaction mechanism at finite temperature. These simulations revealed that water dissociation can proceed by two mechanisms: the first by splitting into adsorbed OH and H species produced a particularly small activation free energy barrier of 0.06 eV (5.89 kJ/mol), and the second by formation of desorbed H<sub>2</sub> and adsorbed O atom had a higher activation barrier of 0.36 eV (34.74 kJ/mol) which was nevertheless relatively small. These activation barrier results, along with reaction rate calculations, suggest that water dissociation will occur spontaneously at room temperature on the Mo-edge.