Structural change of molybdenum sulfide facilitates the electrocatalytic hydrogen evolution reaction at neutral pH as revealed by in situ Raman spectroscopy

Abstract

Molybdenum sulfides are promising electrocatalysts for the hydrogen evolution reaction (HER). S- and Mo-related species have been proposed as the active site for forming adsorbed hydrogen to initiate the HER; however, the nature of the interaction between Mo centers and S ligands is unclear. Further, the development of cost-effective water-splitting systems using neutral water as a proton source for H2 evolution is highly desirable, whereas the mechanism of the HER at neutral pH is rarely discussed. Here, the structural change in the Mo−Mo and S−S species in a synthesized molybdenum sulfide was monitored at neutral pH using in situ electrochemical Raman spectroscopy. Analysis of the potential dependent Raman spectra revealed that the band assigned to a terminal S−S species emerged along with synchronized changes in the frequency of the Mo−Mo, Mo3−μ3S, and Mo−S vibrational bands. This indicates that Mo−Mo bonds and terminal S−S ligands play synergistic roles in facilitating hydrogen evolution, likely via the internal reorganization of trinuclear Mo3−thio species. The nature and role of metal-ligand interactions in the HER revealed in this study demonstrated a mechanism that is distinct from those reported previously in which the S or Mo sites function independently.