Harnessing Composition of Iron Oxide Nanoparticle: Impact of Solvent-Mediated Ligand–Ligand Interaction and Competition between Oxidation and Growth Kinetics

The composition of metal oxide nanoparticles is of great importance for their applications because defects and/or deviation from stoichiometry strongly affect their physical properties. We report here on the crucial role of synthesis parameters such as solvent, ligand, and iron precursors on the composition of spinel iron oxide nanoparticles synthesized by the thermal decomposition method. At first, the investigation of the thermal decomposition of iron stearates bearing either two or three stearate chains by thermogravimetric analysis, infrared spectroscopy, and Mössbauer spectrometry as a function of temperature and syntheses with only oleic acid and iron stearate confirmed that the composition of the first nuclei is wüstite Fe1–xO. The synthesis of nanoparticles with high sizes requires the use of very high boiling point solvents to ensure an effective growth step. We observed that when the grain growth and oxidation kinetics are similar, nanoparticles with a spinel composition and no defects are produced. An oxidation rate slower than the nuclei growth rate favors the formation of core–shell Fe1–xO@Fe3–xO4 NPs. The oxidation kinetics is shown to be influenced by surfactant and solvent natures. Indeed, surfactants such as oleic acid form a dense monolayer at the nuclei surface, and oxidation kinetics will depend on this monolayer permeability. Temperature, solvents with high surfactant affinity, deprotonated surfactants, or decomposition products of solvents affect the monolayer stability and thus the nanoparticle composition. The solvents’ nature and solvent-mediated ligand–ligand interactions are thus evidenced to be important parameters to control the formation of defect-free and stoichiometric oxide nanoparticles.