Oxidation of nanometer-sized iron particles

The evolution of the oxidation of ultrafine (5 nm diameter) α-iron particles in ambient air has been studied using Mössbauer spectroscopy and electron microscopy. A 1-2 nm thick oxide layer was found to appear almost immediately, whereafter the oxidation proceeded rather slowly. The rate of oxidation can be understood from the Caberra-Mott model of oxidation of metal surfaces. The oxide formed consists of a mixture of Fe₃O₄ and γ-Fe₂O₃, but with the magnetic properties significantly modified due to the finite size of the oxide crystallites, e.g. the magnetic hyperfine fields are somewhat smaller than for the bulk Fe₃O₄ and γ-Fe₂O₃, and a very strong spin-canting was revealed. A Verwey transition was found to occur between 12 and 80 K. The Debye temperature of the oxide layer was found to be about 185 K for the thinnest observed oxide layer, increasing to about 215 Kafter exposure of the α-iron particles to air for one week.