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Probing the effects of redox conditions and dissolved Fe2+ on nanomagnetite stoichiometry by wet chemistry, XRD, XAS and XMCD

Abstract : Magnetite nanoparticles, commonly found in subsurface environments, are extensively used in various applications such as environmental remediation, catalysis, electronics and medicine. However, the oxidative transformation of magnetite (mixed-valent Fe-oxide) into maghemite (Fe(III)-oxide) that drastically affects magnetic, catalytic and redox properties of the mineral, is still poorly understood. In the present study, a thorough characterization of both particle core and surface of magnetite was performed to accurately assess the relationship between mineral composition and reactivity within the magnetite/maghemite core-shell structure. Previous work showed that X-ray absorption spectra (XAS) and X-ray magnetic circular dichroism (XMCD) can provide key insights into magnetite stoichiometry (R = Fe(II)/Fe(III)) of 10 nm sized particles, as compared to wet chemistry and X-ray diffraction (XRD). In the present study, XMCD signals have been used to further characterize the complex reactions involved in the magnetite/maghemite system upon oxidation and recharge processes, e.g. decreasing R from 0.5 to 0.1 using H2O2 or increasing from 0.1 to 0.5 through dissolved Fe2+ amendment. Indeed, surface recrystallization processes, induced by oxidation as well as Fe2+ diffusion into the solid phase and/or redistribution of electron equivalents between the aqueous solution and the magnetite bulk, led to decreased spin canting effects, altering XMCD signals. This study provides a fundamental understanding of the processes occurring in the magnetite/maghemite system upon the alteration of the redox conditions and offers a more accurate method for the determination of magnetite stoichiometry by XMCD.
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https://hal-insu.archives-ouvertes.fr/insu-03263142
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Submitted on : Thursday, June 17, 2021 - 8:25:31 AM
Last modification on : Tuesday, October 19, 2021 - 10:53:10 PM
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Phoomipat Jungcharoen, Mathieu Pédrot, Fadi Choueikani, Mathieu Pasturel, Khalil Hanna, et al.. Probing the effects of redox conditions and dissolved Fe2+ on nanomagnetite stoichiometry by wet chemistry, XRD, XAS and XMCD. Environmental science‎.Nano, Royal Society of Chemistry, 2021, pp.2098-2107. ⟨10.1039/D1EN00219H⟩. ⟨insu-03263142⟩

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