Studies of platinum electrodeposition

Further studies of an electroplating solution consisting of [Pt(NH3)4](HPO4) in aqueous 30 mM Na2HPO4 at pH 10-10.4, known commercially as Q bath, have been carried out. Measurements of the steady state currents as a function of potential for a copper disc electrode in this solution confirm the conclusion from voltammetry at microelectrodes that the rate of reduction of [Pt(NH3)4]2+ to Pt depends strongly on temperature and is not mass transport controlled even at 368 K. Indeed, a temperature above 350 K is essential to see Pt plating at a significant rate. These observations are interpreted in terms of a mechanism involving ligand substitution prior to electron transfer and it is suggested that the electroactive species is [Pt(NH3)4-x(H2O)x]2+. The same experiments show that the rate of Pt deposition passes through a sharp maximum as the potential is made more negative. The decline negative to the peak is attributed to the adsorption of hydrogen atoms onto the Pt surface.Potential step experiments at 368 K show that the deposition of Pt occurs via progressive nucleation and three dimensional growth under electron transfer control. This is also consistent with the surface morphology observed by scanning electron microscopy. In general, several types of growth are identified by SEM. The electroplated layers may appear featureless, show large hemispherical centres, cauliflower growth or a deposit made up of small angular crystallites. Whether plated at constant current or constant potential, the morphology of the deposit is mainly determined by the potential for electrodeposition. The featureless deposits (formed at low constant current) are also highly reflecting but they are always highly stressed and readily crack on cooling from the plating temperature, particularly when thick. The deposit made up of angular crystallites were matt and sometimes black, never cracked and formed at potentials where hydrogen adsorption occurs. The cauliflower deposits were formed at intermediate potentials where the highest rates of deposition were observed. Constant potential deposition had several advantages over constant current deposition; satisfactory deposits could be formed with a fivefold higher rate and thick deposits were much less stressed. The morphology of the deposits is also influenced by the pre-treatment of the copper electrode surfaces.It has been reported that adherent and bright, thick deposits can be formed with high current efficiency only when the bath is operated at > 363 K, an inconveniently high temperature. In fact, although the aret of deposition is lower than at 368 K, it is possible to obtain reasonable electroplates with a good current efficiency at 358 K but it is almost essential to use constant potential deposition in order to obtain a significant rate of deposition. The improved rate of deposition at constant potential arises because of the peaked current-potential characteristic; with constant current deposition, the cathode always takes up a stable potential well to one side of the peak.