The transformation of α-Fe nanoparticles into multi-domain FeNi-M₃O₄ (M = Fe, Ni) heterostructures by galvanic exchange

In this work we describe a novel method to prepare multi-domain metal-metal oxide heterostructured nanoparticles (NPs). We investigated the ability of Ni(acac)₂ to undergo galvanic exchange with pre-synthesized metallic α-Fe NPs. Findings indicate that an asymmetric heterostructure emerges from the exchange, which is followed by rapid oxidation to form a NiFe-M₃O₄ (M = Fe, Ni) alloy-oxide microstructure. Nucleation and growth was monitored using UV-vis and TEM, and crystal evolution, composition change, and oxidation states were measured by XRD and XPS respectively. Galvanic exchange and growth was studied by varying Ni:Fe molar feed ratios during synthesis. The findings indicate that at low Ni:Fe ratios, the NP forms multiple domains of oxides, whereas at higher ratios form regions with novel Ni-NiFe-M₃O₄ interfaces. These new heterostructures were highly magnetic, and the extent of magnetization was proportional to composition and morphology, where NPs prepared at high Ni:Fe feed ratios resulted in decreased saturation magnetization and increased magnetic hysteresis. The nickel deposition and NP growth mechanism was considered as a combination of both galvanic exchange and reduction, and the observed rapid oxidation of the remaining α-Fe core was considered in light of electron density change at the heterostructures interfaces. Nanomaterials like these may find use as components in magnetic storage and spintronic devices, probes in biomedicine, additives in corrosion resistant coatings, and even as 3D printing inks.