Colloidal iron dispersions have been prepared by the thermolysis of Fe(CO)5 in solutions of functional polymers. The structure of the particles was very disordered at sizes ≲100 Å but changed to become single crystal with a disordered core as the size increased to 100–200 Å. Particles ≲100 Å were superparamagnetic, and particles in the 100–200-Å range had a time-dependent hysteresis. On exposure to the atmosphere an ∼30-Å-thick γ-Fe2O3 oxide film was produced on the surface of the particles. This is the ’’passive oxide film’’ detected previously by a number of techniques but never before imaged directly in situ. As water was absorbed from the atmosphere the chlorinated solvent-based dispersions reacted further to give β-FeOOH. This reaction was promoted by chloride-ion impurity. The magnetic moment decayed with oxidation roughly in proportion to the quantities of Fe, γ-Fe2O3, and β-FeOOH present. Disorder in the structure of ≲100-Å particles formed in nonchlorinated solvent-based dispersions produced an initial oxide film with magnetic characteristics distinct from those of γ-Fe2O3.
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Griffiths et al. (1979) studied this question.