Iron fatty acid complexes (IFACs) are prepared via the dissolution of porous hematite powder in hot unsaturated fatty acid. The IFACs are then decomposed in five different organic solvents under reflux conditions in the presence of the respective fatty acid. The XRD analysis results indicate that the resulting NPs comprise a mixture of wustite, magnetite, and maghemite phases. The solvents with a higher boiling point prompt the formation of larger NPs containing wustite as the major component, while those with a lower boiling point produce smaller NPs with maghemite as the major component. In addition, it is shown that unstable NPs with a mixed wustite–magnetite composition can be oxidized to pure maghemite by extending the reaction time or using an oxidizing agent.

This study identifies submicrometer hematite (R-Fe 2 O 3 ) powder as a cost-effective and efficient starting material for the synthesis of monodisperse magnetic iron oxide nanoparticles (MMIONPs). The feature of this process is the slow dissolution of hematite powder in a mixture of unsaturated fatty acids and organic solvents. The hematite particles are dissolved by the complexation of their iron with the unsaturated fatty acid ligands to give iron acid salt as a soluble compound. This method presents an interesting example for the production of monodisperse oxide NPs from bulk oxide powder with a moderate solubility in fatty acids. Conceptually, it can be simply viewed as a controlled dissolution-recrystallization process. The relatively low solubility of magnetic iron oxides (maghemite and magnetite) in the reaction condition determines their appearance in the recrystallization process. Moreover, the monodispersity of the formed NPs is sustained during the whole process, which results from the slow supply and the fast consumption of growing units by the slow dissolution of hematite and the fast enough growth rate of magnetic iron oxide NPs. The compositions of the MMIONPs were determined by Mössbauer spectroscopy. It appears that γ-Fe 2 O 3 is preferentially precipitated out at lower ligand concentrations, while the Fe 3 O 4 appears along with γ-Fe 2 O 3 at higher ligand concentrations.

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