Local crystal structure of iron oxide nanoparticles synthesized from Ferritin

We have investigated the size dependence of the local crystal structure of nanosized iron oxide by extended x-ray absorption fine structure (EXAFS) at the iron K-edge. Hematite ($\alpha $-Fe$_{2}$O$_{3})$ nanoparticles of different diameters were produced by thermal treatment of horse spleen ferritin molecules and remineralized apo-ferritin molecules, respectively. The structure of these particles was compared to $\alpha $-Fe$_{2}$O$_{3}$ and $\gamma $-Fe$_{2}$O$_{3}$ nanopowder references. The Fourier transformed EXAFS spectra of the nanoparticles differ significantly from hematite and maghemite reference spectra and change systematically as a function of particle diameter, signalling a corresponding evolution of the structure. We show that the Fe--O bond length decreases with decreasing diameter of the particles and with decreasing particle density. This is explained by a core-shell model, in which the fraction of a $\gamma $-Fe$_{2}$O$_{3}$ like particle shell increases while the hematite core decreases with decreasing particle size.