Electrochemical Deposition of Gold in Citrate Solution Containing Thallium

Electrochemical deposition of gold from citrate solution, with and without thallium, has been studied using a rotating disc electrode (RDE) and linear sweep voltammetry. RDE was made from a mono-crystal silicon wafer, coated with a thin film of gold, deposited by a vapor deposition process (VDP), serving as a substrate for the electrochemical deposition of gold. Limiting current density is achieved neither in citrate nor in thallium free electrolyte. The rotation speed significantly influences the process rate only at lower temperatures. At elevated temperatures gold deposition occurs in a kinetic domain. Tafel slopes confirmed that the electrochemical reaction occurs in two steps, depending on both the electrode potential and the presence of thallium ions in the electrolyte. The reaction kinetics changes in the presence of thallium ions as well. The mechanism of gold deposition is predominantly dependent on the electrolyte composition. In the citrate, thallium free solution, the adsorption of Au(CN)– and CN– on the electrode surface is presumed, resulting in the formation of AuCNad species, which are afterwards being reduced to the metallic gold. Thallium presence in the citrate solution leads to the adsorption of Tl+ ion on the surface at a potential of –750 mV/SCE, including it’s catalytic effect as an adatom. AES and SEM analyses did not detect thallium presence in the deposited gold layer. It was found that the presence of thallium in the citrate solution shifts the deposition potential towards more positive values.