Unraveling the Defects Introduced on the Basal Plane of MoS ₂ Monolayers by H ₂ O ₂ for the Hydrogen Evolution Reaction

The introduction of defects is crucial for tuning the electrocatalytic activity of MoS2 monolayers on the basal plane toward the hydrogen evolution reaction (HER). Hydrogen peroxide is a mild, metal-free, and carbon-free oxidant that can activate the basal plane to enhance the HER activity. Depending on the experimental conditions, H2O2 can lead to the formation of edge-like defects and vacancies on the basal plane. In this work, we demonstrate for the first time that exposure to H2O2 solution induces both local strain and sulfur vacancy on MoS2 monolayers. Using high-resolution transmission electron microscopy, we tracked the same sample to understand the significant effects of H2O2 on free-standing MoS2 monolayers at both nanoscale and atomic resolutions. We measured the electrocatalytic activity of the monolayers without introducing unintentional defects associated with the microfabrication process. Our findings revealed an anodic shift in the overpotential required to drive the HER. Thus, our work provides a fundamental basis for understanding the chemical transformations of MoS2 monolayers following the defect engineering step.