Adsorption of thiourea on polycrystalline platinum: Influence on electrodeposition of copper

Although previous works have shown the marked influence of thiourea as an organic additive in electrolyte bulk on copper electrodeposition, a variety of mechanisms could be responsible. The present paper concerns the effect of a thiourea layer adsorbed prior to copper electrodeposition. First evidence of a monolayer on platinum substrate was revealed by X-ray photoelectron spectroscopy. Two different times (15 h and 1 min) for platinum immersion were tested, and it was observed that, even for short times, thiourea is chemisorbed on the surface. Then, effects of thiourea adsorption on copper underpotential deposition (UPD) and overpotential deposition (OPD) processes on polycrystalline platinum were investigated by polarisation curves, atomic force microscopy and X-ray photoelectrons. Thiourea monolayer increases the copper UPD process: in fact the current density of UPD cathodic peak is higher in the presence of thiourea on the platinum surface. However no modifications were observed on the AFM images and on the XPS spectra. For the copper OPD process, the presence of thiourea monolayer inhibits copper deposition: the current density of OPD cathodic peak is lower whatever the immersion time of the platinum substrate in the thiourea solution. Moreover, XPS spectra show that the presence of thiourea modified the oxidation state of the copper coating, as an oxide film is formed more quickly on the surface of the coating.