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.