Catalytic Activities of Sulfur Atoms in Amorphous Molybdenum Sulfide for the Electrochemical Hydrogen Evolution Reaction

In-situ Raman spectroscopy during cyclic voltammetry was used for the first time to investigate the hydrogen evolution reaction (HER) occurring on electrodeposited amorphous MoSx 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 MoSx 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 S2- and/or bridging S2 2-. Quantum chemical simulations using density functional theory rule out the apical S2- 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.