Crystal Structure of Cobalt Ferrite Nanoparticles Dispersed in Ink: a Synchrotron X-Ray Diffraction Rietveld and Pair Distribution Function Analysis

Superparamagnetic cobalt ferrite nanoparticles are being intensively investigated for their potential use in magnetic devices, medical applications and ceramic decoration. Besides this increasing interest, the actual crystal structure of ferrite nanoparticles is poorly known, as their very small sizes (<10 nm) prevent a reliable determination with conventional approaches (e.g. X-ray diffraction). All crystal structural data available in the literature were obtained after annealing the Co-ferrite at temperatures over 300 °C, that brings about a remarkable crystallite size growth and may have significant effects on the as-synthesized characteristics of nanoparticles. The crystal structure of CoFe2O4 nanoparticles has been for the first time determined by synchrotron X-ray diffraction (ESRF, Grenoble) on both as dried (200 °C) powders and directly on diluted ink (12% wt solid load) at the ID31 and GILDA lines respectively, taking the microcrystalline cobalt ferrite as reference. The diffraction date were analysed using both Rietveld and pair distribution function (PDF) approach. The nanocrystalline cobalt ferrite exibits a well defined local atomic ordering that can be described with the spinel-type structure. The structural coherence length of the nanoparticles determined by the PDF analysis follows the dimension of 5-6 nm as observed by TEM measurements. The unit-cell parameter of cobalt ferrite is only slightly affected by the size reduction, being around 0.838 nm in microcrystals as well as in as dried nanoparticles.