Passivation of α-Fe nanoparticle by epitaxial γ-Fe2O3 shell

Nanoparticles of iron prepared by inert gas condensation of plasma evaporated vapour exhibit remarkable resistance to oxidation. They remain rust free in air and in water for years. We have found by transmission electron microscopy and X-ray photoelectron spectroscopy, that all the passivated nanoparticles of iron are covered by an epitaxial shell of γ-Fe2O3 about 4 nm thick. The epitaxial relationship between the γ-Fe2O3 shell and the iron core is (001)γ-Fe2O3//(001)α-Fe, and [110]γ-Fe2O3//[100]α-Fe, [1̄10]γ-Fe2O3//[010]α-Fe. The passivation of the nanoparticles of iron by an epitaxial oxide can be accounted for by the Caberra–Mott theory of oxidation of metal. The oxide layer grows rapidly at 420 K but slows down dramatically when the layer thickens. When the oxide layer thickens to 4 nm in a few hours, growth virtually stops. The 4-nm epitaxial oxide shell protects the iron core from further oxidation at room temperature.