The structure, magnetic characterization, and oxidation of colloidal iron dispersions

Abstract

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.