Oxidation Kinetics of Magnetite Nanoparticles: Blocking Effect of Surface Ligands and Implications for the Design of Magnetic Nanoheaters

Magnetite (Fe3O4) nanoparticles (NPs) are nowadays extensively used in biomedical, environmental, and catalytic applications. However, magnetite is known to oxidize to maghemite (γ-Fe2O3), leading to changes in the physical properties of the NPs. The factors that modulate such transformation and, particularly, the role of surface capping are often overlooked. In this work, we have studied monodisperse Fe3O4 NPs synthesized by organic phase methods with sizes between 9 and 28 nm and we report on the oxidation kinetics of stable NP colloids in organic and aqueous media. The fraction of Fe3O4 in the as-prepared NPs was found to depend on their size but, in contrast to usual assumptions, monochromated electron energy loss spectroscopy results reveal that the Fe2+ concentration is homogeneous across nonstoichiometric nanocrystals, without evidence of a core/shell structure with a γ-Fe2O3 outer layer. Additionally, we show that typical ligand-exchange procedures employed to remove oleate capping from the surface lead to partially oxidized NPs, indicating that surface ligands play a key role in hindering the oxidation reaction. To elucidate the effect of the capping agent in the redox transformation, we demonstrate that the oxidation process is notably slowed for NPs with increasing oleate coverages. Then, we interpreted these findings quantitatively by considering the coupling between the surface reactivity and diffusion of cations within the oxide. Finally, we demonstrate the remarkable impact of the oxidation process on the magnetic properties of the NPs and on their heating efficiencies under radio frequency magnetic fields. Overall, our results shed light on the importance of the design of iron oxide-based nanomaterials with increased chemical stability and greater control of their physical properties, which are key aspects for their successful application.