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

Sol-gel (SG) formed oxide films (between 60 and 800 nm in thickness) were d eposited on Pt foil substrates by dip-coating from Ni, Co and mixed Ni-Co s ols. After withdrawal at a constant rate, the films were dried at temperatu res between 100 and 400 degreesC for various periods of time (15 min to 1 h our). Electron diffraction and transmission electron microscopy studies hav e 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 fil ms) in crystallites which are 1-4 nm in diameter, the size depending on the oxide composition and the drying conditions employed. Oxide films formed a t 200 degreesC are highly hydrous in nature, and drying at higher temperatu res than this results in the loss of water, the formation of more compact f ilms 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 f ilms), it is suggested that only metal sites on the outer surfaces of the c rystallites, with easy access to the solution ions and water, participate i n the Ni-Co oxide redox reaction in alkaline solutions. (C) 2001 Kluwer Aca demic Publishers.