Magnetic nanoparticles of the 3d transition metal oxides have gained enormous interest for applications in various fields such as data storage devices, catalysis, drug-delivery, and biomedical imaging. One major requirement for these applications is a narrow size distribution of the particles. We have studied the nucleation and growth mechanism for the formation of MnO nanoparticles synthesized by decomposition of a manganese oleate complex in high boiling nonpolar solvents using TEM, FT-IR, and AAS analysis. The exceptionally narrow size distribution indicates that nucleation and growth are clearly separated. This leads to a uniform growth with a very narrow size distribution on the existing nuclei. The particle size can be controlled by adjusting the reaction time, reaction temperature, solvent, and heating rate. The particle size increases with temperature, reaction time, and the chain length (boiling point) of the solvent. FT-IR and NMR spectra revealed that the oleate capping agent binds to the surface in a bidentate manner. In addition, XPS measurements indicate that MnO nanocrystals are air-stable. No significant oxidation of Mn2+ to Mn3+ occurred even after several days.
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