Size-dependent electrocatalytic hydrogen evolution activity of arrays of edge-like defects in MoS2 crystals patterned by focused ion beam

Abstract

Introducing surface defects on molybdenum disulfide (MoS₂) crystals plays a crucial role in enhancing the electrocatalytic activity toward the hydrogen evolution reaction (HER). Despite the remarkable progress in this area, the precise control of the size, distance, and defective areas with minimal surface contamination remains unachieved. Conventional microfabrication methods often leave organic residues on the surface that impair electrocatalytic activity and hinder interface elucidation by advanced characterization tools. In this study, we pattern for the first-time arrays of pillars in nanometer-thick MoS₂ flakes using a focused ion beam to enhance HER activity. We observe size-dependent stability of the edge-like defects under HER conditions with defective areas. The pillars undergo chemical and structural changes post-patterning due to an amorphization process, unambiguously confirmed by atomic force microscopy, Raman spectroscopy, and synchrotron X-ray photoelectron spectroscopy. Furthermore, the amorphization process is more pronounced in specific in-plane directions, as confirmed by transmission electron microscopy. The electrocatalytic activity of the pillars was measured using a recently reported fabrication method, allowing us to unequivocally correlate defects with HER activity. Microelectrodes with less pillars, i.e. less catalytic sites, demonstrate a lower overpotential of 349 mV at 10 mA cm⁻² showcasing a groundbreaking advancement in their fabrication.