Controlling Cation Distribution and Morphology in Colloidal Zinc Ferrite Nanocrystals

This paper describes the first synthetic method to achieve
independent
control over both the cation distribution (quantified by the inversion
parameter x) and size of colloidal ZnFe2O4 nanocrystals. Use of a heterobimetallic triangular
complex of formula ZnFe2(μ3-O)­(μ2-O2CCF3)6(H2O)3 as a single-source precursor, solvothermal reaction conditions,
absence of hydroxyl groups from the reaction solvent, and the presence
of oleylamine are required to achieve well-defined, crystalline, and
monodisperse ZnFe2O4 nanoparticles. The size
of the ZnFe2O4 nanocrystals increases as the
ratio of oleic acid and oleylamine ligands to precursor increases.
The inversion parameter increases with increasing solubility of the
precursor in the reaction solvent, with the presence of oleic acid
in the reaction mixture, and with decreasing reaction temperature.
These results are consistent with a mechanism in which ligand exchange
between oleic acid and carboxylate ligands bound to the precursor
complex influences the degree to which the reaction produces a kinetically
trapped or thermodynamically stable cation distribution. Importantly,
these results indicate that preservation of the triangular Zn–O–Fe2 core structure of the precursor in the reactive monomer species
is crucial to the production of a phase-pure ZnFe2O4 product and to the ability to tune the cation distribution.
Overall, these results demonstrate the advantages of using a single-source
precursor and solvothermal reaction conditions to achieve synthetic
control over the structure of ternary spinel ferrite nanocrystals.