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Abstract: 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 ∼22kGy/h. Upon 1h of γ-irradiation the XRD pattern of the precipitate showed goethite and unidentified low-intensity peaks. Upon 6h 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 , 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 18h of γ-irradiation the precipitate exhibited dual behaviour, it was a more oxidised product than the precipitate obtained after 6h 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 18h of γ-irradiation the pH of the medium shifted from the alkaline to the acidic range. The high dose rate of ∼22kGy/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 25h at a lower dose rate of 16kGy/h produced pure substoichiometric nanosize magnetite particles of about 25nm in size and with the stoichiometry of Fe2.83O4. [Copyright &y& Elsevier]

Abstract: The commercial magnetite powder was characterised as substoichiometric magnetite (Fe2.93O4) with a small hematite fraction (∼7wt%). It consisted of micrometer-sized particles of regular (octahedral) and irregular morphologies. Reference sample was subjected to reduction by hydrogen gas and reoxidation at a temperature of up to 600°C. The oxidation experiments were performed in an oxygen stream. Reference sample oxidised to a maghemite–hematite mixture, whereas the fraction of each phase was very sensitive to the oxidation temperature. XRD and FT-IR spectroscopy indicated the superstructure character of obtained maghemite samples with the ordering of cation vacancies in the maghemite crystal lattice. Mössbauer spectroscopy was very sensitive to magnetite stoichiometry, however, it could not detect less than 10wt% of hematite in the maghemite–hematite mixture. In reduction experiments, conditions for the reduction of reference sample to stoichiometric magnetite (Fe3.00O4) were found. The reduction was performed by hydrogen gas under static conditions (375°C, 150min). Under this static condition the relatively high amount of water condensed inside the evacuated quartz tube. The formation of water is due to the chemisorption of hydrogen molecules on the iron oxide, which then dissociate from iron oxide generating an intermediate hydroxyl group according to the general equations: where (s) signifies a solid (hematite or substoichiometric magnetite), (g) a gas phase and an anionic vacancy formed in hematite or substoichiometric magnetite. Thus, in order to ensure a good reduction condition, water vapour was removed from the quartz tube and the tube was refilled with hydrogen gas every 30min. [Copyright &y& Elsevier]

1,10-phenanthroline spectrophotometric method was used in order to systematically measure the quantity of Fe2+ ions that were generated upon γ-irradiation of alkaline Fe(III) aqueous solutions in the presence of diethylaminoethyl (DEAE)-dextran. γ-irradiation was performed in a range of doses from 5 to 130 kGy and the dose rate was ~26 kGy h-1. The results showed that γ-irradiation reduces Fe3+ to Fe2+; the reduction was initially very fast, but quickly slowed down and then reached a plateau of 100% reduction. The quantity of Fe2+ in γ-irradiated suspensions and isolated solid products roughly overlap up to 45% of Fe2+, because in this range the inverse spinel structure of substoichiometric magnetite nanoparticles was able to capture 30.1% of Fe2+. The stoichiometries of the formed magnetite nanoparticles were very similar, which indicated that the absorbed dose did not have a significant influence on the magnetite stoichiometry, even though Fe2+ molar fraction increased from 22 to 45% as a function of absorbed dose. When γ-irradiation generated 69% or more of Fe2+ the powder samples consisted exclusively of Fe(III), i.e. of δ-FeOOH nanodiscs and poorly crystallized α-FeOOH nanoparticles about 4 nm in size. The volume-averaged domain sizes and crystal aspect ratio of the δ-FeOOH nanodiscs increased from 16 nm to 25 nm and from 1.6 to 2.1 with the increase of absorbed dose, respectively. The use of DEAE-dextran in the γ-irradiation synthesis enabled the generation of up to 100% of Fe2+ and synthesis of extremely stable aqueous suspensions of superparamagnetic magnetite nanoparticles as well as the synthesis of δ-FeOOH nanodiscs with high aspect ratios. • γ-irradiation generated Fe2+ was quantitatively measured using 1,10-phenanthroline. • Fe2+ generated initially very quickly and then slowed down to reach a value of 100%. • Magnetite formed when γ-irradiation generated between 22 to 69% of Fe2+ in suspension. • DEAE dextran and at least 69% of Fe2+ are crucial for synthesis of δ-FeOOH nanodiscs. [ABSTRACT FROM AUTHOR]