Nonstoichiometric properties of nanoporous iron oxide films

Porous iron oxide films were prepared by the polymer precursor method with poly(vinyl alcohol) (PVA). Pore size distribution of the films was determined by the N-2 adsorption isotherm at 77K with the aid of the quartz crystal microbalance method. The crystal and pore structures of the iron oxide film varied with the amount of PVA in a coating solution; the crystal structure changed from alpha-Fe2O3 to gamma-Fe2O3 with an increase of PVA. XPS valence band spectra showed the presence of the layer of Fe3O4 in a mixed valence state on the surface of the microporous film. Also the temperature dependence of the electrical resistance of the film showed the presence of the Fe3O4 like layer on the film surface. The kinetics of the reduction of the porous iron oxide films was examined, and the activation energy of the reduction was determined. The Gibbs free energy of the film reduction of alpha-Fe2O3 to Fe3O4 decreased. Hence, it was shown that the surface energy of the porous films controls the crystal and surface structures of the film.