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

High Resolution Image Download MS PowerPoint Slide The introduction of defects is crucial for tuning the electrocatalytic activity of MoS 2 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, H 2 O 2 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 H 2 O 2 solution induces both local strain and sulfur vacancy on MoS 2 monolayers. Using high-resolution transmission electron microscopy, we tracked the same sample to understand the significant effects of H 2 O 2 on free-standing MoS 2 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 MoS 2 monolayers following the defect engineering step.