Patterning Edge-Like Defects and Tuning Defect Propagation on the Basal Plane of Ultra-Large MoS₂ Monolayers Toward Hydrogen Evolution Reaction

The catalytic sites of MoS₂ monolayers towards hydrogen evolution are well known to be vacancies and edge-like defects. However, it is still very challenging to control the position, size, and propagation of defects on the basal plane of MoS₂ monolayers by most of defect-engineering routes. In this work, the fabrication of arrays of etched windows on ultra-large supported [1] and free-standing MoS₂ monolayers using focused ion beam (FIB) is reported for the first time. By tuning the Ga⁺ ion dose it is possible to confine defects near the etched edges or propagate them over ultra-large areas on the basal plane. The electrocatalytic activity of the arrays toward hydrogen evolution reaction (HER) was measured by fabricating microelectrodes using a new method that preserves the catalytic sites. We demonstrate that the overpotential can be decreased up to 290 mV by assessing electrochemical activity only at the basal plane. HRTEM images obtained on FIB patterned freestanding MoS₂ monolayers reveal the presence of amorphous regions and X-ray photoelectron spectroscopy indicate excess of sulfur in these regions. Density-functional theory calculations provide identification of catalytic defect sites. Our results demonstrate a new rational control of amorphous-crystalline surface boundaries and future insight for defect optimization in MoS₂ monolayers.<br/>[1] <i>Nanoscale</i>, 2022,14, 6811-6821.<br/>Acknowledgements: Serrapilheira institute (1912-31228) and FAPESP (2022/00955-0)