Structural and compositional properties of sol-gel formed Ni, Co and Ni-Co oxide films

Abstract

Sol-gel (SG) formed oxide films (between 60 and 800 nm in thickness) were deposited on Pt foil substrates by dip-coating from Ni, Co and mixed Ni-Co sols. After withdrawal at a constant rate, the films were dried at temperatures between 100 and 400°C for various periods of time (15 min to 1 hour). Electron diffraction and transmission electron microscopy studies have indicated that the as-formed SG films are nanocrystalline oxide materials, consisting of cubic units of NiO and/or CoO (and Co3O4, in pure Co SG films) in crystallites which are 1–4 nm in diameter, the size depending on the oxide composition and the drying conditions employed. Oxide films formed at 200°C are highly hydrous in nature, and drying at higher temperatures than this results in the loss of water, the formation of more compact films and an increase in the crystallite size. Based on the observed charge efficiencies (ca. 75% for pure Ni oxide and ca. 60% for 50:50 Ni:Co oxide films), it is suggested that only metal sites on the outer surfaces of the crystallites, with easy access to the solution ions and water, participate in the Ni-Co oxide redox reaction in alkaline solutions.