  Large scale implementation of electrochemical water splitting for hydrogen
evolution requires cheap and efficient catalysts to replace expensive platinum.
Molybdenum disulfide is one of the most promising alternative catalysts but its
intrinsic activity is still inferior to platinum. There is therefore a need to
explore new active site origins in molybdenum disulfide with ultrafast reaction
kinetics and to understand their mechanisms. Here, we report a universal cold
hydrogen plasma reduction method for synthesizing different single atoms
sitting on two-dimensional monolayers. In case of molybdenum disulfide, we
design and identify a new type of active site, i.e., unsaturated Mo single
atoms on cogenetic monolayer molybdenum disulfide. The catalyst shows
exceptional intrinsic activity with a Tafel slope of 35.1 mV dec-1 and a
turnover frequency of ~10^3 s-1 at 100 mV, based on single flake microcell
measurements. Theoretical studies indicate that coordinately unsaturated Mo
single atoms sitting on molybdenum disulfide increase the bond strength between
adsorbed hydrogen atoms and the substrates through hybridization, leading to
fast hydrogen adsorption/desorption kinetics and superior hydrogen evolution
activity. This work shines fresh light on preparing highly-efficient
electrocatalysts for water splitting and other electrochemical processes, as
well as provides a general method to synthesize single atoms on two-dimensional
monolayers.
