In Situ Scanning Tunneling Microscopy Imaging Self-Assembled Monolayers of Mercaptoacetic Acid and Cupric Ion on Au(111) Electrode

We employed voltammetry and scanning tunneling microscopy (STM) to study the coadsorption of mercaptoacetic acid (MAA) and cupric ions (Cu²⁺) on Au(111) electrode from 0.5 M sulfuric acid + 0.8 M CuSO₄ + 0.03 mM MAA. Irreversibly adsorbed Cu²⁺ ions were reduced at 0.27 V first then at -0.2 V (vs. Ag/AgCl), ascribed to the underpotential and overpotential deposition processes, respectively. Counting the amount of charge involved in the stripping peak of the Cu deposit rendered quantification of Cu²⁺ ions codeposited with MAA on the Au(111) electrode, which yielded roughly 7.7 x 10¹⁴ Cu²⁺ ions/cm². MAA alone and MAA + Cu²⁺ were adsorbed in Au(111) - (3 x r13) and (p x r3) structures, respectively. Cu²⁺ could be imaged by the STM, but their high mobility made it difficult to obtain high – quality STM results needed to quantify the amount of adsorbed Cu²⁺. Spatial structures of MAA molecules segregated onto the Cu deposit were found to vary with potential. They returned to the Au(111) electrode once the Cu deposit was oxidized at E > 0.4 V, yielding MAA + Cu²⁺ structures different from those seen at the pristine sample.