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.

中文翻译：

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磁铁矿纳米颗粒的氧化动力学：表面配体的阻挡效应及其对磁性纳米加热器设计的影响


磁铁矿 （Fe₃O₄） 纳米颗粒 （NPs） 如今广泛用于生物医学、环境和催化应用。然而，已知磁铁矿会氧化成磁赤铁矿 （γ-Fe₂O₃），导致 NP 的物理性质发生变化。调节这种转变的因素，特别是表面封端的作用经常被忽视。在这项工作中，我们研究了通过有机相法合成的尺寸在 9 到 28 nm 之间的单分散 Fe₃O₄ NPs，并报告了稳定 NP 胶体在有机和水介质中的氧化动力学。发现所制备的 NPs 中 Fe₃O₄ 的分数取决于它们的大小，但与通常的假设相反，单色电子能量损失光谱结果表明，Fe²⁺ 浓度在非化学计量纳米晶体中是均匀的，没有证据表明具有 γ-Fe₂O₃ 的核/壳结构外层。此外，我们表明，用于从表面去除油酸帽的典型配体交换程序会导致部分氧化的 NPs，表明表面配体在阻碍氧化反应中起关键作用。为了阐明封端剂在氧化还原转化中的作用，我们证明随着油酸盐覆盖率的增加，NP 的氧化过程显着减慢。然后，我们通过考虑氧化物内阳离子的表面反应性和扩散之间的耦合来定量解释这些发现。最后，我们证明了氧化过程对 NPs 的磁性及其在射频磁场下的加热效率的显着影响。 总体而言，我们的结果阐明了设计基于氧化铁的纳米材料的重要性，这些材料具有更高的化学稳定性和更好的物理性质控制，这是其成功应用的关键方面。