Self-Assembled Monolayers of Molybdenum Sulfide Clusters on Au Electrode as Hydrogen Evolution Catalyst for Solar Water Splitting

Hydrogen evolution reaction (HER) activities of self-assembled monolayers (SAMs) of [Mo3S7(S2CNMe2)3] and several other MoSx molecular clusters are presented on planer Au electrode. Our study suggests that such Mo-S clusters are unstable under HER reaction conditions of a strongly acidic electrolyte. The [Mo3S7(S2CNEt2)3]I monolayer prepared from DMF showed greater stability among all the studied precursors. The X-ray photoelectron spectroscopy (XPS) analysis on a monolayer of [Mo3S7(S2CNMe2)3]I in THF assembled on Au/ITO suggested sulfur-rich composition with S:Mo ratio of 2.278. The Mo-S monolayer clusters resulting from [Mo3S7(S2CNMe2)3]I in THF showed a Tafel slope of 75.74 mV dec&minus
1 and required a lower overpotential of 410 mV to reach a high HER catalytic current density of 100 mA cm&minus
2 compared to the other studied precursors. Surface coverage of the Mo-S clusters on the Au surface was confirmed by cyclic voltammetry (CV) curves from K3Fe(CN)6 and anodization of Au surface. Further, the rotating ring-disk electrode (RRDE) measurements were performed for the monolayer of [Mo3S7(S2CNMe2)3]I prepared in THF to study its reaction kinetics. The HER catalytic activity of such monolayer Mo-S clusters can further be improved by controlling the sulfur vacancy.