Size-dependent oxidation in iron/iron oxide core-shell nanoparticles

Signorini, Luca; Pasquini, Luca; Savini, L.; Carboni, R.; Boscherini, Federico; Bonetti, E.; Giglia, Angelo; Pedio, M.; Mahne, Nicola; Nannarone, Stefano

doi:10.1103/physrevb.68.195423

Cited by 188 publications

(187 citation statements)

References 27 publications

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“…21 At the top of Fig. 4 the experimental Fe K-edge XANES spectrum of Fe-AC sample is shown together with those of ␣-Fe, ␥-Fe, ␥-Fe 2 O 3 , and Fe 3 O 4 .…”

Section: X-ray Absorption Near-edge Structure (Xanes)mentioning

confidence: 99%

“…Consequently, the NPs present core-shell morphology. [17][18][19][20][21][22][23][24][25] In this situation, strong exchange magnetic coupling between the iron particle core and the iron oxide shell gives rise to modifications in the coercivity, the magnetic anisotropy, and to the appearance of an exchange-bias ͑EB͒ effect. [26][27][28][29][30][31][32][33][34][35] This phenomenology is mostly observed in thin films composed of alternating ferromagnetic/antiferromagnetic ͑FM/AFM͒ layers when the Néel temperature ͑T N ͒ of the AFM layer is smaller than the Curie temperature ͑T C ͒ of the FM one.…”

Section: Introductionmentioning

confidence: 99%

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Microstructure and magnetism of nanoparticles withγ-Fecore surrounded byα-Feand iron oxide shells

et al. 2010

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Iron-carbon nanocomposites have been elaborated by means of a simple chemical procedure based on in situ synthesis of iron nanoparticles within the nanopores of an activated carbon. The Fe nanoparticles present a broad particle-size distribution ͑5-40 nm͒. A combined structural and magnetic study seems to suggest that most of the nanoparticles of mean size ϳ15 nm have exotic "onionlike" core-shell morphology of ␥-Fe nucleus surrounded by a concentric double shell of ␣-Fe and maghemitelike oxide. The true nature of Fe-oxide was successfully evidenced through room temperature x-ray absorption spectroscopy. The whole system does not reach a fully superparamagnetic regime even at 750 K, probably due to higher blocking temperatures for the largest nanoparticles. Mössbauer spectrometry indicates that low temperature para-to-antiferromagnetic transition for the ␥-Fe phase cannot be discarded. In addition, the external Fe-oxide shell exhibits spin-glass behavior giving rise to the freezing of its magnetic moments at low temperatures. Hence, we propose a competing double magnetic coupling: ͑i͒ the oxide shell/␣-Fe interaction and ͑ii͒ the possible antiferromagnetic coupling between ␥-Fe nucleus and ␣-Fe layer; as being both responsible for the observed exchange bias effect at T =5 K ͑H ex Ϸ 150 Oe͒.

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“…21 At the top of Fig. 4 the experimental Fe K-edge XANES spectrum of Fe-AC sample is shown together with those of ␣-Fe, ␥-Fe, ␥-Fe 2 O 3 , and Fe 3 O 4 .…”

Section: X-ray Absorption Near-edge Structure (Xanes)mentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Microstructure and magnetism of nanoparticles withγ-Fecore surrounded byα-Feand iron oxide shells

et al. 2010

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“…Oxidized nanoparticles show no band at the pre-edge corresponding to Fe 2+ and a shift of the absorption spectrum to higher energies than that obtained for magnetite. 13 Both features allow us to identify the iron oxide phase of the completely oxidized particles as maghemite. Further, maghemite is a semiconductor with a 2.03 eV band gap, while magnetite is a semimetal with a 0.14 eV band gap.…”

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confidence: 99%

Vacancy Coalescence during Oxidation of Iron Nanoparticles

et al. 2007

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Supporting Information Available: Detailed synthesis and experimental procedures, UV-vis, XRD, and XAS of the iron/iron oxide nanoparticles, SQUID of the final iron oxide shells, histograms of the particle sizes, and analysis of the electron beam influence during TEM imaging and of the high-temperature oxidation of the particles in solution. This material is available free of charge via the Internet at

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“…In addition, magnetite oxidation is relevant to environmental applications of zerovalent iron (ZVI) nanoparticles, which can be effective for the reductive transformation of environmental contaminants such as chlorinated hydrocarbons and arsenic 10,11 but which are rapidly oxidized upon exposure to air. Magnetite is the first oxidation product, forming either an exterior shell 12 or hollow oxide nanocrystals due to the nanoscale Kirkendall effect. 13,14 Further oxidation leads to the formation of a ferric oxide surface layer that reduces the effective reactivity of the nanomaterials.…”

Section: Introductionmentioning

confidence: 99%

Soft X-ray Spectroscopy Study of the Electronic Structure of Oxidized and Partially Oxidized Magnetite Nanoparticles

et al. 2010

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The crystal structure of magnetite nanoparticles may be transformed to maghemite by complete oxidation, but under many relevant conditions the oxidation is partial, creating a mixed valence material with structural and electronic properties that are poorly characterized. We used X-ray diffraction, Fe K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy and soft X-ray absorption and emission spectroscopy to characterize the products of oxidizing uncoated and oleic-acidcoated magnetite nanoparticles in air. The oxidization of uncoated magnetite nanoparticles creates a material that is structurally and electronically indistinguishable from maghemite. By contrast, while oxidized oleic-acid-coated nanoparticles are also structurally indistinguishable from maghemite, Fe Ledge spectroscopy revealed the presence of interior reduced iron sites even after a 2-year period. We used X-ray emission spectroscopy at the O K-edge to study the valence bands (VB) of the iron oxide nanoparticles, using resonant excitation to remove the contributions from oxygen atoms in the ligands and from low-energy excitations that obscured the VB edge. The bonding in all nanoparticles was typical of maghemite, with no detectable VB states introduced by the long-lived, reduced-iron sites in the oleic-acid-coated sample. However, O K-edge absorption spectroscopy observed a ~0.2 eV shift in the position of the lowest unoccupied states in the coated sample, indicating an increase in the semiconductor band gap relative to bulk stoichiometric maghemite that was also observed by optical absorption spectroscopy. The results show that the ferrous iron sites within ferric iron oxide nanoparticles coated by an organic ligand can persist under ambient conditions with no evidence of a distinct interior phase, and exert an effect on the global electronic and optical properties of the material.This phenomenon resembles the band gap enlargement caused by electron accumulation in the conduction band of TiO 2 .

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Size-dependent oxidation in iron/iron oxide core-shell nanoparticles

Cited by 188 publications

References 27 publications

Microstructure and magnetism of nanoparticles withγ-Fecore surrounded byα-Feand iron oxide shells

Microstructure and magnetism of nanoparticles withγ-Fecore surrounded byα-Feand iron oxide shells

Vacancy Coalescence during Oxidation of Iron Nanoparticles

Soft X-ray Spectroscopy Study of the Electronic Structure of Oxidized and Partially Oxidized Magnetite Nanoparticles

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