In-situ Raman spectroscopy during cyclic voltammetry was used for the first time to investigate the hydrogen evolution reaction (HER) occurring on electrodeposited amorphous MoS_x films, which consist of Mo₃-based motiffs. A Raman peak at 2530 cm^-1 was observed on these films during HER in aqueous HClO₄ electrolyte. This peak was assigned as a MoS-H stretching vibration. This identification was corroborated by H/D isotopic exchange experiments. Hence, we could directly establish that sulfur atoms are the catalytic active sites in amorphous MoS_x for proton reduction.

We further distinguish the activities of the S atoms present in these amorphous films. Using X-ray photoelectron spectroscopy as a means to identity the types of S atoms present, we found that the turnover frequency for H₂ production increases linearly with the percentage of S atoms with higher electron binding energies. These S atoms could be apical S^2- and/or bridging S₂^2-. Quantum chemical simulations using density functional theory rule out the apical S^2- atoms as reaction sites for HER. In situ Raman spectroscopy of the model [Mo₃S₁₃]^2- cluster further demonstrate the higher catalytic reactivity of the bridging S₂^2- over terminal S₂^2- (which have lower electron binding energy) for proton reduction.