Role of sulfur vacancies and undercoordinated Mo regions in MoS₂ nanosheets toward the evolution of hydrogen

Low-dimensional materials have been examined as electrocatalysts for the hydrogen evolution reaction (HER). Among them, two-dimensional transition metal dichalcogenides (2D-TMDs) such as MoS₂ have been identified as potential candidates. However, the performance of TMDs toward HER in both acidic and basic media remains inferior to that of noble metals such as Pt and its alloys. This calls for investigating the influence of controlled defect engineering of 2D TMDs on their performance toward hydrogen production. Here, we explored the HER activity from defective multilayered MoS₂ over a large range of surface S vacancy concentrations up to 90%. Amorphous MoS₂ and 2H MoS₂ with ultrarich S vacancies demonstrated the highest HER performance in acid and basic electrolytes, respectively. We also report that the HER performance from multilayered MoS₂ can be divided into two domains corresponding to "point defects" at low concentrations of surface S vacancies (Stage 1) and large regions of undercoordinated Mo atoms for high concentrations of surface S vacancies (Stage 2). The highest performance is obtained for Stage 2 in the presence of undercoordinated Mo atoms with a TOF of ∼2 s^-1 at an overpotential of 160 mV in 0.1 M KOH which compares favorably to the best results in the literature. Overall, our work provides deeper insight on the HER mechanism from defected MoS₂ and provides guidance for the development of defect-engineered TMD-based electrocatalysts.