Energy level modulation of MoS2 monolayers by halide doping for an enhanced hydrogen evolution reaction

MoS2 monolayers are promising materials for electrochemical water-splitting catalysts due to their cost-effectiveness and high catalytic activities. However, as its most stable 2H-phase that corresponds to semiconducting nature, the conventional MoS2 possesses less conductance than metallic catalysts, resulting in a sluggish catalytic reaction. Here, we propose a method of engineering the energy levels of MoS2 monolayers via halide doping that can greatly contribute to the acceleration of charge transfer and enhancing catalytic activities. We found that the halide atom doped MoS2 monolayer modifies potential barriers at both electrode-catalyst and catalyst-electrolyte interfaces. Especially, electron-rich Cl doped MoS2 exhibited superior catalytic performance with low overpotential and Tafel slopes due to the increase of the Fermi level that forms favorable potential barriers for efficient charge transfer. Furthermore, by selectively fabricating an exposed MoS2 surface for doping, we confirmed the potential barrier effects at both the catalyst interfaces to obtain the optimized catalytic activities of MoS2. Our findings provide new insights into designing 2D semiconductor electrocatalysts by modulating the energy level of catalysts in an effective way.