Electrochemical and Structural Aspects of Gold Electrodeposition from Dilute Solutions by Direct Current

Abstract

The electrochemical and structural aspects of gold deposition from dilute cyanide, citrate, and phosphate‐buffered plating solutions were studied using rotating disk electrodes. Current‐potential curves were recorded galvano‐statically for 0.005M Au solutions at 60°C, and the morphology of deposits approximately 1µ thick, which corresponded to different regions of the i–V curves, was determined by scanning electron microscopy. Transmission electron microscopy was used to elucidate the early stages of growth (∼0.1µ). The cyanide system, which has an exchange current density of 0.82 mA/cm2 and a transfer coefficient of 0.7, showed the greatest tendency to form an out‐ward growth morphology consisting of fine features termed spikes. These features are responsible for the characteristic brown appearance of deposits plated under certain conditions. Increasing current density and decreasing rotation speed, in general, both favor the formation of spikes which occurs over a given range of potential and not at a particular concentration of gold at the interface. The spikes, which are typically 0.1– 1.0µ in size and >108/cm2 in density, were also observed for deposits from the citrate system and to a much lesser extent in the phosphate system. At low overpotentials, a smooth, layer type of growth is formed. The formation of the various morphological features is governed by the electrode potential below the limiting current density and by mass transport near or above limiting current.