Size‐Dependent Enhancement of Electrocatalytic Oxygen‐Reduction and Hydrogen‐Evolution Performance of MoS 2 Particles

MoS 2 particles with different size distributions were prepared by simple ultrasonication of bulk MoS 2 followed by gradient centrifugation. Relative to the inert microscale MoS 2 , nanoscale MoS 2 showed significantly improved catalytic activity toward the oxygen‐reduction reaction (ORR) and hydrogen‐evolution reaction (HER). The decrease in particle size was accompanied by an increase in catalytic activity. Particles with a size of around 2 nm exhibited the best dual ORR and HER performance with a four‐electron ORR process and an HER onset potential of −0.16 V versus the standard hydrogen electrode (SHE). This is the first investigation on the size‐dependent effect of the ORR activity of MoS 2 , and a four‐electron transfer route was found. The exposed abundant Mo edges of the MoS 2 nanoparticles were proven to be responsible for the high ORR catalytic activity, whereas the origin of the improved HER activity of the nanoparticles was attributed to the plentiful exposed S edges. This newly discovered process provides a simple protocol to produce inexpensive highly active MoS 2 catalysts that could easily be scaled up. Hence, it opens up possibilities for wide applications of MoS 2 nanoparticles in the fields of energy conversion and storage.