Puzzling Mechanism behind a Simple Synthesis of Cobalt and Cobalt Oxide Nanoparticles: In Situ Synchrotron X-ray Absorption and Diffraction Studies

Here, we show a simple approach to synthesize cobalt and cobalt oxide nanoparticles in an organic solvent. We find that the cubic Co3O4 nanoparticles can be easily obtained, even at temperatures as low as 80 °C. Moreover, exactly the same reaction at 180 °C leads to metallic Co nanoparticles. Thus, in addition to the synthetic efforts, we study the mechanism of occurrence of oxidation and reduction of a Co2+ precursor in benzyl alcohol. Remarkably, the in situ X-ray absorption and diffraction measurements of the synthesis at 140 °C reveal that oxidation of Co2+ to Co3+/2+ and reduction of Co2+ to Co0 reactions take place simultaneously. It is followed by a rapid formation of Co3O4 nanoparticles and its consecutive solid-state reduction to CoO. In parallel, metallic Co nanoparticles begin to grow. In addition, Multicomponent Curve Resolution–Alternating Least Squares (MCR-ALS) analysis of X-ray absorption spectroscopy (XAS) data efficiently reveals the nontrivial interdependence between four different reactions. Our strategy to control reduction and oxidation of Co-based nanoparticles as they grow opens up an elegant pathway for the one-pot-synthesis of the hybrid materials for energy-related applications.