The Origin of High Activity of Amorphous MoS(2) in the Hydrogen Evolution Reaction.

Molybdenum disulfide (MoS(2) ) and related transition metal chalcogenides can replace expensive precious metal catalysts such as Pt for the hydrogen evolution reaction (HER). The relations between the nanoscale properties and HER activity of well-controlled 2H and Li-promoted 1T phases of MoS(2) , as well as an amorphous MoS(2) phase, have been investigated and a detailed comparison is made on Mo-S and Mo-Mo bond analysis under operando HER conditions, which reveals a similar bond structure in 1T and amorphous MoS(2) phases as a key feature in explaining their increased HER activity. Whereas the distinct bond structure in 1T phase MoS(2) is caused by Li(+) intercalation and disappears under harsh HER conditions, amorphous MoS(2) maintains its intrinsic short Mo-Mo bond feature and, with that, its high HER activity. Quantum-chemical calculations indicate similar electronic structures of small MoS(2) clusters serving as models for amorphous MoS(2) and the 1T phase MoS(2) , showing similar Gibbs free energies for hydrogen adsorption (ΔG(H*) ) and metallic character.