| Summary: | Monolayered, semiconducting MoS<sub>2</sub> and their transition metal dichalcogenides (TMDCs) families are promising and low-cost materials for hydrogen generation through electrolytes (HER, hydrogen evolution reaction) due to their high activities and electrochemical stability during the reaction. However, there is still a lack of understanding in identifying the underlying mechanism responsible for improving the electrocatalytic properties of theses monolayers. In this work, we investigated the significance of controlling carrier densities in a MoS<sub>2</sub> monolayer and in turn the corresponding electrocatalytic behaviors in relation to the energy band structure of MoS<sub>2</sub>. Surface functionalization was employed to achieve p-doping and n-doping in the MoS<sub>2</sub> monolayer that led to MoS<sub>2</sub> electrochemical devices with different catalytic performances. Specifically, the electron-rich MoS<sub>2</sub> surface showed lower overpotential and Tafel slope compared to the MoS<sub>2</sub> with surface functional groups that contributed to p-doping. We attributed such enhancement to the increase in the carrier density and the corresponding Fermi level that accelerated HER and charge transfer kinetics. These findings are of high importance in designing electrocatalysts based on two-dimensional TMDCs.
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