Characterization of radiolytically synthesized feroxyhyte and oxidized magnetite nanoparticles

Abstract

Iron(III) chloride deaerated alkaline aqueous colloidal solutions in the presence of 2-propanol and DEAE-dextran (diethylaminoethyl-dextran hydrochloride) were γ-irradiated with doses of 36 and 130 kGy. The dose rates were ~7 and ~31 kGy h−1. The extremely stable aqueous suspension of substoichiometric Fe2.83O4 magnetite nanoparticles ~4.7 nm in size were formed at a dose rate of ~7 kGy h−1 and at a dose of 36 kGy. At a dose of 130 kGy the anisotropic δ-FeOOH magnetic discs with a diameter of ~256 nm and thickness of ~40 nm were formed. Each disc has a substructure consisting of approximately eighteen laterally stacked thin nanodiscs of about 2.2 nm in thickness. At a higher dose rate of ~31 kGy h−1and at a dose of 36 kGy, XRD revealed the presence of highly substoichiometric Fe2.75O4 magnetite, whereas TEM analysis revealed the presence of ~4.3 nm magnetite nanoparticles along with rod-shaped γ-Fe2O3 nanoparticles. The values of isomer shift in Mössbauer spectra of about 0.33 mm s−1 showed that all Fe(II) in substoichiometric magnetites oxidized to Fe(III) one month after synthesis. At high dose and dose rate, the rolled rod-like δ-FeOOH magnetic nanoparticles were formed having the diffraction patterns with streaks, which are characteristic of tubular structures. The magnetic measurements showed exceptional intrinsic room-temperature magnetic properties of both δ-FeOOH nanostructures with the Curie temperature above 300 K. In order to ascertain the formation of Fe(II) intermediate products prior to their oxidation, γ-irradiated samples were isolated by admixing glycerol. The amounts of Fe(II) in glycerol-isolated solid samples were determined using Mössbauer spectroscopy. Carbonate Green Rust I [GR(CO32−)] and Fe(OH)2 intermediate phases were confirmed by XRD and Mössbauer spectroscopy. The amounts of Fe2+ in the acidified solutions containing dissolved γ-irradiation ferrous products were determined using the potassium permanganate titration. The amounts of Fe2+ were 68.9% and 96.1% at doses of 36 and 130 kGy, respectively. Thus, at a dose of 130 kGy the radiolytically generated Fe(OH)2, after coming in contact with oxygen and CO2 from the air, topotactically oxidized to GR(CO32−) and further to δ-FeOOH without any structural changes. The lower dose rate (~7 kGy h−1) favored the δ-FeOOH nanodisc morphology, whereas the higher dose rate (~31 kGy h−1) favored the formation of tubular rod-like δ-FeOOH nanoparticles.