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Surface structure controlling Nanoparticle behavior: Magnetism of Ferrihydrite, Magnetite, and Maghemite

Abstract

Iron oxide nanoparticles are omnipresent in nature and of great importance for environmental sciences and technology. The size-dependent magnetic behavior of ferrihydrite (Fh), magnetite (Fe3O4), and maghemite (γ-Fe2O3) has been studied in relation to the surface structure. The selected minerals have in common the presence of tetrahedral Fe. This Fe polyhedron is unstable at the surface when forming singly coordinated ligand(s). It leads to a size-dependency of the polyhedral composition, which is for Fh in excellent agreement with the relative contributions of edge and corner sharing measured with high-energy total X-ray scattering. For Fh, superparamagnetic behavior scales with the particle volume according to theory, but has additionally an inter-particle contribution. Magnetic saturation at low temperature scales with size and is predominantly due to polyhedral surface depletion. The mineral core of Fh may behave ferrimagnetic as well as antiferromagnetic. Both have an opposite particle size dependency, for which a surface structural model has been developed. The relative stability of ferrimagnetic and antiferromagnetic Fh is related to a slight difference in surface Gibbs free energy (∼0.03 J m-2). At the same surface structure, the predicted crossing-over point is at ∼4 nm, above which the core of Fh shifts from antiferromagnetic to ferrimagnetic. For magnetite and maghemite, the size dependency of the ferrimagnetic behavior can be described with the same model as developed for Fh by only adjusting the maximum magnetic saturation of the ideal bulk material to its theoretical value. As discussed and quantified, structural defects of superparamagnetic Fe-oxide nanoparticles (SPION) will lower the magnetic saturation at a given size.

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Publication details

The article was received on 10 Nov 2017, accepted on 07 Jan 2018 and first published on 08 Jan 2018


Article type: Paper
DOI: 10.1039/C7EN01060E
Citation: Environ. Sci.: Nano, 2018, Accepted Manuscript
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    Surface structure controlling Nanoparticle behavior: Magnetism of Ferrihydrite, Magnetite, and Maghemite

    T. Hiemstra, Environ. Sci.: Nano, 2018, Accepted Manuscript , DOI: 10.1039/C7EN01060E

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