Underpotential deposition of copper on gold and effects of thiourea studied by AC impedance

This paper reports ac impedance cyclic voltammetry studies of underpotential deposition (UPD) of Cu on Au from an electrolyte composed of CuSO4 and H2SO4. Formation of metal monolayers on foreign substrates by UPD offers the electrochemist an opportunity to study adsorption precisely since coverage can be controlled with potential. This characteristic of UPD makes ac impedance an ideal tool for studying the kinetic mechanisms involved. A theoretical model was derived which fits experimental impedance spectra over frequencies ranging from 0.001 to 20,000 Hz. The coadsorption of thiourea and Cu on Au was also studied by ac impedance since thiourea significantly affects the morphological properties of Cu electrodeposits and is one of the most important organic additives for commercial CuSO4 plating baths