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

We report a detailed morphological and structural characterization of iron/iron oxide core-shell nanoparticles with x-ray diffraction and x-ray absorption spectroscopy performed at both the Fe and O $K$ edges. Core-shell nanoparticles with core size ranging from 7 to 21 nm were synthesized using the inert gas condensation technique followed by 12 h of controlled surface oxidation. Rietveld analysis of diffraction patterns shows the presence of \ensuremath{\alpha}-Fe nanoparticles surrounded by a 2--3 nm-thick oxide layer with a disordered cubic spinel structure. Magnetite $({\mathrm{Fe}}_{3}{\mathrm{O}}_{4})$ and maghemite $(\ensuremath{\gamma}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}),$ two different iron oxides, share this lattice structure, but x-ray diffraction was not able to distinguish between the two. An analysis of the Fe and O x-ray absorption spectra in both the near-edge and the extended energy regions is described. The analysis of the extended spectra was performed using the ab initio calculation of all significant contributions to the absorption cross section. We show that there are size-dependent changes in the local structure and oxidation state of the oxide shell, the relative fraction of maghemite increasing at the expense of magnetite as the core dimensions decrease. This size/structure correlation has been explained in terms of morphological and structural disorder arguments