  The catalytic sites of MoS$_{2}$ 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 defective areas on the basal
plane of Mo$S_{2}$ monolayers by most of defect-engineering routes. In this
work, the fabrication of etched arrays on ultra-large supported and
free-standing MoS$_{2}$ 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 spread 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. High-resolution transmission electron microscopy images obtained on FIB
patterned freestanding MoS$_{2}$ monolayers reveal the presence of amorphous
regions and X-ray photoelectron spectroscopy indicates sulfur excess 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 MoS$_{2}$ monolayers.
