Images of a single particle before (on the left) and after (on the right) annealing, showing dark Fe cores surrounded by lighter MgO shells.
Evolution of Fe 2p X-ray absorption spectra of as-grown Fe/MgO core/shell nanoparticles and after annealing: (a) Complete spectral range. Vertical lines indicate the peak positions in energy of metallic iron (Fe0), Fe2+, octahedral Fe3+(Oh) and tetrahedral Fe3+(Td). (b) Closer inspection of the Fe L3 region after annealing following different protocols: namely, rapid thermal annealing at 350 °C for 5 min, 450 °C for 10 min, and 450 °C for 5 h. Spectra were corrected for incomplete circular polarization and normalized, setting the absorption above the L3 edge equal to unity. (c) Fe L-edge XMCD spectra as a function of the different annealing protocols. As XMCD we understand the difference that appears in the absorption spectrum when reversing the incoming beam helicity. Asterisk (∗) denotes a small split (see text).
Enlarged view of the O K-edge pre-edge features of the XAS spectra of an as-grown Fe/MgO core/shell nanoparticles sample and after annealing at different temperature. The inset shows the complete XAS spectra scaled to the main peak S. The three peaks at 530.5, 531.7, and 532.7 eV have been interpreted as being due to transitions into eg↑, t2g↓ and eg↓ states (↑ and ↓ describing majority and minority spin), respectively. Other oxygen specie located at about 534 eV (indicated with asterisk) was also observed for the as-grown sample, which was assigned to −OH. We interpret this finding as the structure of the adsorbed water, 56 most probably forming brucite Mg(OH)2 during the passivation process in air.
Thermogravimetric curves of samples in an oxidizing (heavy line) and reducing atmosphere at heating rates of 5 °C/min. The dashed lines are guide to the eyes (see text for details).
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