From iron(III) precursor to magnetite and vice versa

The syntheses of nanosize magnetite particles by wet-chemical oxidation of Fe2+ have been extensively investigated. In the present investigation the nanosize magnetite particles were synthesised without using the Fe(II) precursor. This was achieved by γ -irradiation of water-in-oil microemulsion containing only the Fe(III) precursor. The corresponding phase transformations were monitored. Microemulsions (pH ∼ 12.5) were γ -irradiated at a relatively high dose rate of ∼ 22 kGy/h. Upon 1 h of γ -irradiation the XRD pattern of the precipitate showed goethite and unidentified low-intensity peaks. Upon 6 h of γ -irradiation, reductive conditions were achieved and substoichiometric magnetite (∼ Fe2.71O4) particles with insignificant amount of goethite particles found in the precipitate. Hydrated electrons (ea q-), organic radicals and hydrogen gas as radiolytic products were responsible for the reductive dissolution of iron oxide in the microemulsion and the reduction Fe3+ → Fe2+. Upon 18 h of γ -irradiation the precipitate exhibited dual behaviour, it was a more oxidised product than the precipitate obtained after 6 h of γ -irradiation, but it contained magnetite particles in a more reduced form (∼ Fe2.93O4). It was presumed that the reduction and oxidation processes existed as concurrent competitive processes in the microemulsion. After 18 h of γ -irradiation the pH of the medium shifted from the alkaline to the acidic range. The high dose rate of ∼ 22 kGy/h was directly responsible for this shift to the acidic range. At a slightly acidic pH a further reduction of Fe3+ → Fe2+ resulted in the formation of more stoichiometric magnetite particles, whereas the oxidation conditions in the acidic medium permitted the oxidation Fe2+ → Fe3+. The Fe3+ was much less soluble in the acidic medium and it hydrolysed and recrystallised as goethite. The γ -irradiation of the microemulsion for 25 h at a lower dose rate of 16 kGy/h produced pure substoichiometric nanosize magnetite particles of about 25 nm in size and with the stoichiometry of Fe2.83O4.