Effect of thiourea and substituted thioureas on copper underpotential deposition on gold

The formation of metal monolayers through underpotential deposition (UPD) depends on the interactions between the electrode surface and the depositing species. Other adsorbable species, if present in the solution, will influence the formation of UPD metal films through their adsorption on the electrode surface, taking place simultaneously. Copper UPD formation on polycrystalline gold surfaces has been investigated in the presence of small amounts of urea, thiourea (TU) and N-substituted thioureas using cyclic voltammetry. The results show that the UPD of copper on gold is significantly influenced by the presence of thiourea in solution. The charge corresponding to the stripping peak obtained in the presence of both Cu2+ and TU in solution is always larger than the sum of the charges resulting when either of them is present alone. Again, the charge observed during stripping in the presence of both Cu2+ and TU is believed to arise from (i) stripping of the UPD Cu, (ii) adsorption–desorption of TU and (iii) oxidation of Cu(TU)n+ complexes. The addition of N-substituted TUs is found to influence the UPD of Cu to different extents. This is tentatively explained in terms of the structure of the organic molecules.