Fe K-edge X-ray absorption spectroscopy study of nanosized nominal magnetite

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 (Fe₃O₄, γ-Fe₂O ₃, α-Fe₂O₃,...), 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 Fe₃O₄ and γ-Fe₂O₃ 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-δO₄ 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.