Advancements in Polyol Synthesis: Expanding Chemical Horizons and Néel Temperature Tuning of CoO Nanoparticles

Abstract

Abstract The polyol synthesis of CoO nanoparticles (NPs) is typically conducted in diethylene glycol (DEG) by dissolving and heating tetrahydrate cobalt acetate and water. This process yields aggregates of approximately 100 nm made of partially aligned primary crystals. However, the synthesis requires careful temperature regulation to allow the nucleation of CoO and at the same time avoid its reduction due to DEG’s activity, impeding any freedom in tuning the synthesis conditions to obtain particles with different morpho-structural properties, which have a direct influence on chemical and physical properties. In this context, the growth of CoO NPs in polyol was studied focusing on the effect of the polyol chain length, the water/cations ratio, and the synthesis temperature, finding that longer polyol chains allow much higher temperatures to be reached. By this way, the aggregate size (20–150 nm), shape (octahedral spherical) and the crystalline length (8–35 nm) were successfully tuned. Our primary focus revolved around investigating the magnetic properties inherent in the synthesized products. Notably, a compelling correlation was observed between the crystallite size of the nanoparticles and their Néel temperature (TN) of CoO—an observation of substantial applicative significance across the spectrum of magnetic applications for this material. In the course of our investigation, two pivotal findings emerged. First, we the identified small quantities of a layered hydroxide ferromagnetic intermediate acting as an interference in our measurements, exhibiting magnetic properties congruent with features observed in other publications on CoO synthesized in systems compatible with the intermediate formation; second, we observed for the first time a branching of the curve in the TN vs crystal size, providing compelling evidence for a novel degree of freedom instrumental in fine-tuning the magnetic properties of these nanoparticles. These two findings shed light on the understanding of CoO and antiferromagnetic materials fundamental properties. Furthermore, good synthetic conditions were found to avoid the formation of ferromagnetic layered hydroxide salt (LHS) impurities, resolving several ambiguities already present in literature about CoO low-temperature magnetic behavior. The Néel temperature (TN) was also recognized in the zero-field cooled (ZFC) thermal variation of the magnetization of the resulting pure CoO particles, and a regular relation with their crystallite size was found, allowing us to regulate TN over ~ 80 K. Samples with spheroidal morphologies were consistently observed to have lower TN respect to the octahedral ones, providing a possible new degree of freedom with which the particles’ magnetic properties can be tuned.