Electrodeposition of Ni/ceria composites: an in situ visible reflectance investigation

Ni/cerium oxide coatings were electrodeposited from particle-free aqueous baths containing NiCl₂.6H₂O and CeCl₃.7H₂O. The mechanism of deposition was studied systematically by a combination of voltammetric, in situ spectroelectrochemical (visible reflectivity spectroscopy (VRS) and surface Raman spectroscopy), ex situ spectroscopic (spectroscopic ellipsometry) methods, as well as by scanning electron microscope imaging; yielding details on the steps of the composite formation process. Time- and potential-dependent electro VRS data were interpreted on the basis of an optical model, accounting for the formation of metal and ceramic phases and corresponding relative distribution and morphology. In the VRS curves measured with the pure Ni and Ce-containing solutions, the value of reflectivity drops sharply when the potential is lower than ca. ?0.9?V. The VRS curves measured in the Ce-containing solutions exhibit a second drop when the potential is lower than ca. ?1.1?V while, instead, for pure Ni solution an increase in reflectivity is observed. According to the proposed optical model, the drop found in the reflectivity transient can be explained with the nucleation of Ni on the Cu substrate, while the second one measured with Ce-containing solutions is due to secondary nucleation of Ni. The results showed that the deposition processes of Ni and Ni/cerium oxide can be divided into two and four stages, respectively. (1) In the case on Ni: nucleation and 3D growth, accompanied by roughening; (2) as far as Ni/cerium oxide is concerned: nucleation, formation of cerium oxide, secondary nucleation, and 3D growth and roughening.