Deciphering the multifaceted role of oleic acid in synthesizing multibranched cobalt ferrite nanoparticles with enhanced magnetic properties

Abstract

Multibranched cobalt ferrite octapod nanoparticles (NPs), with their distinctive eight-branched structure, offer a promising platform for harnessing their shape anisotropy to enhance magnetic anisotropy and magnetic stray fields. These characteristics make them suitable for applications in medicine, permanent magnets and data storage. However, reproducible synthesis of octapods with controlled size and arm length remains challenging, owing to limited mechanistic understanding of surfactant roles, particularly oleic acid. Here, we present a reproducible strategy for fabricating multibranched cobalt ferrite NPs and elucidating how modulation in thermodynamic and kinetic factors governs NP growth, dissolution and stability. Time-dependent studies unveil a dynamic shape progression from spherical and cubic NPs to multibranched octapod NPs driven by oleic acid-mediated slow thermolysis and facet-specific growth. Prolonged reaction time leads to octapod arm regression and reversion to cubic and polyhedral shapes. Notably, we discover that this shape reversion from octapods to polyhedra is predominantly driven by oleic acid inducing a selective dissolution of high energy crystal facets, rather than surface atomic diffusion, highlighting the dynamic role of oleic acid. Our findings also challenge the prevailing assumption that multibranched octapods are transient kinetic intermediates by demonstrating their inherent stability, even at elevated temperature. We establish a detailed framework for understanding and controlling the anisotropic growth of nanoparticles with high energy facets and their stability. Additionally, post-synthesis oxidation confirms the stability of a multibranched CFO structure and enhances its magnetic properties by eliminating residual phase impurities. This work provides critical insights into the rational design of magnetic nanomaterials with tailored properties, with implications for applications in catalysis, permanent magnets, data storage, and medicine.