From iron(III) precursor to magnetite and vice versa

• Published in: Materials research bulletin Vol. 44; no. 10; pp. 2014 - 2021
• Main Authors: Gotić, M., Jurkin, T., Musić, S.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Ltd 01.10.2009
• Subjects: A. Oxides; B. Chemical synthesis; C. Electron microscopy; C. Mössbauer spectroscopy; D. Microstructure; DOSE RATES; ELECTRON MICROSCOPY; GAMMA RADIATION; GOETHITE; IRON IONS; IRRADIATION; MAGNETITE; MATERIALS SCIENCE; MICROEMULSIONS; MOESSBAUER EFFECT; NANOSTRUCTURES; OXIDATION; PARTICLES; RADIOLYSIS; SYNTHESIS; X-RAY DIFFRACTION; B. Chemical synthesis; D. Microstructure; C. Electron microscopy; C. Mössbauer spectroscopy; A. Oxides
• ISSN: 0025-5408; 1873-4227
• DOI: 10.1016/j.materresbull.2009.06.002

The syntheses of nanosize magnetite particles by wet-chemical oxidation of Fe 2+ 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 (∼Fe 2.71O 4) particles with insignificant amount of goethite particles found in the precipitate. Hydrated electrons ( e a q − ) , organic radicals and hydrogen gas as radiolytic products were responsible for the reductive dissolution of iron oxide in the microemulsion and the reduction Fe 3+ → Fe 2+. 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 (∼Fe 2.93O 4). 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 Fe 3+ → Fe 2+ resulted in the formation of more stoichiometric magnetite particles, whereas the oxidation conditions in the acidic medium permitted the oxidation Fe 2+ → Fe 3+. The Fe 3+ 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 Fe 2.83O 4.