Single Atomic Vacancy Catalysis.

Single atom catalysts provide exceptional activity. However, measuring the intrinsic catalytic activity of a single atom in real electrochemical environments is challenging. Here, we report the activity of a single sulfur vacancy for electrocatalytically evolving hydrogen in two dimensional (2D) MoS2. Surprisingly, we find that the catalytic activity per vacancy is not constant but increases with their concentration - reaching a sudden peak in activity at 5.7 x 1014 cm-2 where the intrinsic turn over frequency (TOF) and Tafel slope of a single atomic vacancy was found to be ~ 5 s-1 and 44 mV/dec, respectively. At this vacancy concentration, we also find local strain of ~ 3% and a semiconductor to metal transition in 2D MoS2. Our results suggest that, along with increasing the number of active sites, engineering the local strain and electrical conductivity of catalysts is essential in increasing their activity.