Cu underpotential deposition on Au(111) in methyl sulfonic acid: Analysis of Frumkin interactions on nucleation and growth

Methylsulfonic acid (MSA) is widely utilized in commercial electroplating. However, fundamental investigations into its influence on metal deposition remain scarce. Most studies of MSA adsorption on metal single crystals have focused on bare electrode surfaces. This work investigates the influence of MSA on the Cu underpotential deposition (UPD) on Au(111), one of the most thoroughly characterized model systems in electrochemistry. In MSA, the Cu UPD process exhibits a distinct potential-dependent behavior. The onset of Cu UPD is identical to that in perchloric acid, a classic non-specifically adsorbing electrolyte. Kinetic analysis suggests Cu island formation driven by an attractive interaction between copper adatoms. In this potential regime, shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS) reveals no evidence of specific methylsulfonate adsorption. At lower potentials, however, the interfacial behavior changes significantly. Sharp voltammetric features and the SHINER spectra demonstrate a transition to specific methylsulfonate adsorption on the Cu-covered Au(111) surface. Furthermore, Cu bulk deposition is kinetically hindered, similar to the Cu UPD process on Au(111) in the presence of acetic acid. This inhibition is discussed with regard to the hydrophobic moiety shared by both acetate and methyl sulfonate.