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 degreesC, beta-Co(OH)(2) and Co-II(OH)(2-x)(NO3)(x).nH(2)O solid phases are gradually oxidized to a mixture of (Co1-xCoxIII)-Co-II(OH)(2)(NO3)(x).nH(2)O 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 O-2 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.