Size-controlled synthesis of quasi-monodisperse transition-metal ferrite nanocrystals in fatty alcohol solutions

Quasi-monodisperse hydrophobic transition-metal ferrite MFe ₂O₄ (M = Mn, Fe, Co, Ni, Zn) nanocrystals were synthesized by the thermolysis of transition-metal acetates in oleyl alcohol solutions under aerobic conditions. The proposed reaction protocol is simple, rapid, and highly versatile because it takes advantage of the multiple roles of the oleyl alcohol molecules, namely, solvent for the precursors, reaction medium, and capping ligand for the metal oxide nanoparticles. A systematic FT-IR spectroscopy study has indicated that the transition-metal ferrite nanoparticles are precipitated in oleyl alcohol solutions via a thermal decomposition process with no evidence about a potential esterification reaction, involving the long-chain alcohol and the metal acetate salt. A detailed characterization of the oleyl alcohol capped ferrite nanoparticles was performed by XRD, TEM, SAED, EXAFS, FT-IR, and SQUID measurements. The as-prepared transition-metal ferrite nanocrystals are spherically shaped, and their average diameter can be conveniently tuned between 4 and 15 nm by increasing the heating rate of the solution. The surface composition of the nanoparticles can be modified via ligand-exchange reactions through which the nanocrystals can be rendered soluble in polar solvents without altering their morphology. The oleyl alcohol capped ferrite nanocrystals typically exhibit a superparamagnetic behavior with blocking temperatures depending on both the nature of the transition metal and the size of the nanocrystals.