Adsorption of 2-Mercapto-1-methylimidazole on the (√3 × 22) Reconstructed and (1 × 1) Phases of an Ordered Au(111) Electrode under Potential Control

The self-assembled monolayers (SAMs) of organosulfur compounds bearing an imidazole (Im) functional group can form hydrogen bonds with others and tailor specific interfacial structures used for CO₂ reduction, biosensing, and corrosion prevention. The current study employed cyclic voltammetry and in situ scanning tunneling microscopy (STM) to address the adsorption and structure of 2-mercapto-1-methylimidazole (MMI) on the reconstructed and unreconstructed Au(111) electrodes as a function of potential. The adsorbed MMI molecule assumed the unprotonated form, allowing its S- and N-ends to bind with the Au electrode at a positive potential. Molecular resolution STM images showed an ordered Au(111)-(√21 × √21)R10.9°-MMI structure. The Au-N bond could weaken and be broken at a negative potential, leading to a disordered MMI adlayer. This restructuring event was coupled with the protonation of the Im group. These processes resulted in a sharp peak, which shifted negatively by 60 mV with an increase of 1 pH unit. MMI admolecules were desorbed at a more negative potential to unveil a pitted Au surface, resulting from etching of MMI adsorption. This feature supports the S-Au-S motif for MMI adsorbed to the Au(111) electrode, irrespective of the initial Au surface structure. The effect of MMI as an additive to the deposition bath of Ni was also explored.