Effect of solvothermal synthesis parameters on crystallite size and atomic structure of cobalt iron oxide nanoparticles

We investigate how synthesis method affect the crystallite size and atomic structure of cobalt iron oxide nanoparticles. By using a simple solvothermal method, we first synthesize cobalt ferrite nanoparticles of ca. 2 and 7 nm, characterized by Transmission Electron Microscopy (TEM), Small Angle X-ray scattering (SAXS), X-ray and neutron total scattering. The smallest particle size corresponds to only a few spinel unit cells, nevertheless, Pair Distribution Function (PDF) analysis of X-ray and neutron total scattering data show that the atomic structure in even the smallest nanoparticles is well described by the spinel structure, however with significant disorder and a contraction of the unit cell parameter. These effects can be explained by surface oxidation of the small nanoparticles, which is confirmed by X-ray near edge absorption spectroscopy (XANES). Neutron total scattering data and PDF analysis reveal a larger degree of inversion of the spinel of the smallest nanoparticles. Neutron total scattering data furthermore allows magnetic PDF (mPDF) analysis, which show that the ferrimagnetic domains correspond to ca. 80% of the crystallite size in the larger particles. A similar but less well-defined magnetic ordering was observed for the smallest nanoparticles. Finally, we use a co-precipitation synthesis method at room temperature to synthesize ferrite nanoparticles of similar size as the smallest crystallites synthesized by the solvothermal method. Structural analysis with PDF demonstrates that the ferrite nanoparticles synthesized via this method exhibit a significantly more defective structure compared to those synthesized via a solvothermal method.