Single Source Precursor Approach for the Sol−Gel Synthesis of Nanocrystalline ZnFe₂O₄ and Zinc−Iron Oxide Composites

Nanostructured ZnFe2O4 and zinc oxide−iron oxide composites were obtained by sol−gel processing of a hetero-bimetallic alkoxide. For this purpose the novel molecular precursors Fe2Zn(OR)8 (R = tBu, iPr), fully characterized by spectroscopic and single-crystal diffraction techniques, have been synthesized and their applications in the sol−gel procedure were investigated. The obtained xerogels were annealed at very low temperatures (down to 200 °C) yielding crystalline phases with very small crystallite grain sizes. The phase compositions of the material strongly depend on the polarity of the solvents used in the hydrolysis reaction. When the hydrolysis was performed in the polar propan-2-ol, the spinel ZnFe2O4 (franklinite) formed at annealing temperature of 200 °C with a mean crystallite grain size Dvol of 2.24 nm. However, when using the less polar toluene as solvent in the sol−gel process, iron oxide and zinc oxide are formed together with the franklinite phase. This mixture of phases transforms to the spinel as unique crystalline phase with increasing sintering temperature.