Passivation of alpha-Fe nanoparticle by epitaxial gamma-Fe2O3 shell

Nanoparticles of iron prepared by inert gas condensation of plasma evaporat ed vapour exhibit remarkable resistance to oxidation. They remain rust free in air and in water for years. We have found by transmission electron micr oscopy and X-ray photoelectron spectroscopy, that all the passivated nanopa rticles of iron are covered by an epitaxial shell of gamma-Fe2O3 about 4 nm thick. The epitaxial relationship between the gamma-Fe2O3 shell and the ir on core is (001)gamma-Fe2O3//(001)(alpha-Fe), and [110]gamma-Fe2O3//[100](a lpha-Fe), [(1) over bar 10]gamma-Fe2O3//[010](alpha-Fe). The passivation of the nanoparticles of iron by an epitaxial oxide can be accounted for by th e 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 oxid e layer thickens to 4 nm in a few hours, growth virtually stops. The 4-nm e pitaxial oxide shell protects the iron core from further oxidation at room temperature. (C) 2000 Elsevier Science S.A. All rights reserved.