X-Ray Absorption Spectroscopy Study of the Local Structure of Heavy Metal Ions Incorporated into Electrodeposited Nickel Oxide Films

The incorporation of heavy metal ions into simulated corrosion films has been investigated using spectroscopic and electrochemical techniques. The films were formed by electrodeposition of the appropriate oxide (hydroxide) onto a graphite substrate. Synchrotron X-ray absorption spectroscopy (XAS) was used to determine the structure and composition of the host oxide film, as well as the local structure of the impurity ion. Results on the incorporation of Ce and Sr into surface films of Ni(OH)₂ and NiOOH are reported. Cathodically deposited Ni(OH)₂ was found to be mainly in the alpha form while anodically prepared NiOOH showed the presence of Ni⁺² and Ni⁺⁴. Cerium incorporated into Ni(OH)₂ exists as mixed Ce⁺³ and Ce⁺⁴ phases; a Ce⁺⁴ species was found when Ce was codeposited with NiOOH. The structure of the Ce⁺⁴ phase in anodic films appears similar to a Ce(OH)₄ standard. However, XAS, X-ray diffraction, and laser Raman measurements indicate that the latter chemical formulation is probably incorrect and that the material is really a disordered form of hydrous cerium oxide. The local structure of this material is similar to CeO₂ but has much higher strucutral disorder. The significance of this finding on the question of the structure of Ce-based corrosion inhibitors in aluminum oxide films is pointed out. Moreover, we found it possible to form pure Ce oxide (hydroxide) films on graphite by both cathodic and anodic electrodeposition; their structures have also been elucidated. Strontium incorporated into nickel oxide films consists of Sr⁺² which is coordinated to oxygen atoms and is likely to exist as small domains of coprecipitated material.