Fe K-Edge X-ray Absorption Spectroscopy Study of Nanosized Nominal Magnetite

Abstract

When studying nominal magnetite nanoparticles, it is mandatory to obtain a precise structural characterization to get an accurate relationship with their physiochemical properties. The great deal of information accumulated to date on the characterization of nominal magnetite and maghemite NPs does not clarify if the synthesized materials are single o multiphase systems involving bulk-like stoichiometric oxides (Fe 3 O 4 , γ-Fe 2 O 3 , α-Fe 2 O 3 , ...), or single or multiphase entities formed by nonstoichiometric oxides. In this work we propose a new approach to determine the structure of Fe oxide NPs by using the Fe K-edge X-ray absorption near edge spectroscopy. We report here an X-ray absorption near edge spectroscopy study at the Fe K-and L 2,3 -edges, on nominal magnetite nanoparticles synthesized by different methods. In addition, X-ray magnetic circular dichroism was recorded at the Fe L 2,3 -edges, in selected samples. We have found that the experimental spectra are not well reproduced by any linear combination of the absorption spectra of Fe 3 O 4 and γ-Fe 2 O 3 bulk references, even taking into account other oxides as goethite or ferrihydrite. The analysis of the Fe K-edge XANES spectra shows that it is the size, and not the synthesis method, which determines the structure of the NPs. Our experimental results indicate that, irrespective of the synthesis method, the nominal magnetite NPs are, actually, a single phase non stoichiometric Fe 3−δ O 4 oxide. At the origin of this phase are the cation vacancies, which lead to the modification of the structural arrangements at the Fe sites with respect to those found in bulk-like iron oxides.

Low toxic maghemite nanoparticles for theranostic applications

“…The spectra obtained for the nanoparticles under study are rather similar to the previously measured spectra of the nanoparticles synthesized without microwave assistance. 20,21 XPS Fe2p spectrum of our sample is shown in Figure 5. As one can see, the Fe2p spectrum consists of two main peaks A and C, corresponding to 2p 3/2 and 2p 1/2 states of Fe, arising from the spin-doublet splitting of 2p level.…”

Striped domains of coarse-grained magnetite observed by X-ray photoemission electron microscopy as a source of the high remanence of granites in the Vredefort dome

“…The corresponding X-ray absorption spectra of the host minerals are shown in Figure 2C, and those of the lamellae are shown in Figure 2D. Figure 2B shows the reference spectra of the Fe L 3,2 absorption edge of the iron oxides obtained from previous works (e.g., Kuiper et al, 1993;Pearce et al, 2006;Signorini et al, 2006;Kuzmin and Chaboy, 2014;Piquer et al, 2014). By comparing the spectral shapes of the data obtained from the present study with the references, the materials in areas 1-3 were determined to be magnetite, and those in areas 4-7 were determined to be hematite.…”

“…The cause of the little differences between the positive tetrahedral Fe 3+ and negative octahedral Fe 2+ is unknown. According to Piquer et al (2014), the shift of the energies of the absorption peaks in the magnetite XAS spectra is due to a single-phase non-stoichiometric Fe 3−δ O 4 oxide, and this modifies the structural arrangements at the Fe sites.…”

Determination of transition metal ion distribution in cubic spinel Co1.5Fe1.5O4 using anomalous x-ray diffraction

“…The edge energies of standard samples are in good agreement with literature. 22 From the figure, it is observed that the main edge of Cobalt Ferrite sample is blue shifted by about 0.6 eV, compared to that of the standard sample Fe 3 O 4 . As discussed above in the case of Co K-edge XANES spectra, the average oxidation state of Fe in the sample is more than 2.67.…”

“…Many authors have suggested a linear dependence of the average oxidation state on the energy positions of the absorption edge. 16,17,21,22 In Fig. 5 Fig.…”