Electrode Kinetic Parameters for Copper Deposition on Clean and Soiled Copper Cathodes

Abstract

A galvanostatic method for measuring the overpotentials and obtaining the charging curves for the deposition of copper was developed. The copper was plated from an acid copper sulfate bath, on flat plate copper cathodes. Rigorous standards of solution and electrode purity were maintained, and the results of runs using clean electrodes are in excellent agreement with those reported by other investigators for spherical electrodes of pure copper. Values of αc and io for the clean cathodes were 0.57 and 11.0 ma/cm2, respectively. The techniques developed are easily applied to study the effects of surface treatments on deposition kinetics. Hence some cleaned cathodes were soiled with stearic acid and with mineral oil by means of the evaporation technique, and charging curves were obtained for these cathodes. In addition, sequences of charging curves were obtained for the soiled electrodes as deposition proceeded. The soils when present in quantities equivalent to a monolayer greatly increase activation overpotential and decrease the value of αc, as determined from the slope of the Tafel line, to about 0.3. The exchange currents for the cathodes on which the equivalent of a monolayer of stearic acid is freshly deposited are higher than those for similar clean electrodes. An explanation is suggested for this observation based on the assumption that the freshly applied stearic acid is preferentially deposited at sites other than those active for electrodeposition. Cathodes soiled with paraffin oil and oriented stearic acid in quantities equal in thickness to a monolayer of stearic acid have exchange currents which are close to those for clean electrodes.The presence of these trace contaminants causes a reduction in the double layer capacity, and consequently the charging curves for the soiled electrodes are much steeper than those for cleaned ones. Sequential charging curves reveal that as plating proceeds the electrode contaminants are eventually covered by the freshly deposited copper, and, therefore, the charging curves tend to approach in shape and magnitude those of clean electrodes. Cathodes soiled with the equivalent of 0.5 monolayer of freshly applied stearic acid have charging curves which are similar to those of cleaned electrodes, and the values of αc and io as determined from the Tafel line are not appreciably different. The implication is made that organic contaminants similar to stearic acid present in quantities less than about 0.5 monolayer will not have an appreciable effect on the mechanism and hence the kinetic parameters of the electrodeposition.