Influence of Ligand Exchange and Heteroatom Substitution on Surface Chemistry of Cobalt Ferrite Nanoparticles

Abstract

The nanoparticles (NPs) of cobalt ferrite (CoFe2O4, CFO) and Zn-/Ga-substituted (Co0.5Zn0.5Fe2O4, CFO_Zn and CoFe1.5Ga0.5O4, CFO_Ga) were synthesized via a solvothermal method using oleic acid (OA) and dihydrocaffeic acid (DHCA) as surface ligands to investigate the effects of ligand exchange and heteroatom on surface chemistry. The NPs were also subjected to thermal treatment at 450 °C to investigate the stability of surface functional groups and overall surface chemistry. X-ray diffraction (XRD) confirmed the formation of a cubic spinel structure, while transmission electron microscopy (TEM) revealed uniform, spherical NPs with an average size of 5 ± 1 nm. Thermal treatment at 450 °C led to agglomeration of NPs due to partial removal of the surfactant. To understand the changes in surface chemistry, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD) measurements were performed. FTIR indicated successful attachment of OA and DHCA on the NPs surface through carboxyl and catechol groups. The FTIR analysis reveals that thermal treatment at 450 °C leads to almost complete removal of OA from the nanoparticle surface, while DHCA is partially retained in a modified form due to surface group recombination. XPS and TPD analyses revealed a significant reduction in surface carbon and oxygen content after annealing, along with a redistribution of functional groups. The obtained results highlight how ligand exchange and heteroatom substitution, combined with thermal treatment, greatly influence surface composition and properties of CFO nanomaterials. These findings suggest that such materials hold potential for applications in supercapacitors, where establishing a clear correlation between surface properties and electrochemical performance is essential.