Mechanistic Investigation on Salt-Mediated Formation of Free-Standing Co₃O₄ Nanocubes at 95 °C

The transformation process of various cobalt hydroxide and hydroxide nitrate into final free-standing Co3O4 nanocubes with a uniform size of ca. 47 nm has been investigated with X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and high-resolution transmission electron microscopy (HRTEM) methods. It is found that during their sequential oxidation under air bubbling at 95 °C, β-Co(OH)2 and CoII(OH)2-x(NO3)x·nH2O solid phases are gradually oxidized to a mixture of CoII1-xCoIIIx(OH)2(NO3)x·nH2O and Co3O4 in high concentration nitrate salt (e.g., NaNO3) solution, through which the single-phase Co3O4 nanocubes have been formed with a prolonged oxidation. With the mediation of NaNO3 salt in synthesis, formation of perfectly faceted Co3O4 nanocubes can be attributed to a lowering in O2 solubility and creation of salt−(solvent)n diffusion boundary on the surfaces, which retards the cobalt oxidation and alters the normal interfacial growth under nonsalted conditions.