Ellipsometric Study of the Initial Layer Formation of the Preferential Zn Deposition in Zn-Ni Electroplating

Abstract

The initial layer formation of Zn, Ni, and Zn-Ni deposition on Au electrode in sulfate bath has been investigated by three-parameter ellipsometry. The formation of deposited layer has been monitored by the in-situ optical method in the thickness range of nm with the complex refractive index of the layer.
The underpotential deposition (UPD) of Zn is observed from the change of ellipsometric parameters and the Zn deposition is found to start in the UPD potential region more positive than 0.5 V than the redox potential of Zn²⁺/Zn. The Ni deposition is strongly inhibited by the presence of Zn²⁺ ion. It is suggested that the thin layer of Zn initially formed in the UPD potential region inhibits the Ni deposition. The inhibition by the thin surface layer of Zn, which is preferentially formed, is thought to occur also on the growing Zn-Ni electrodeposited alloy. It is, therefore, speculated that the preferential deposition of Zn in the Zn-Ni codeposition process is caused by the thin Zn layer on the deposited Zn-Ni alloy.