Syntheses and characterization of wurtzite CoO, rocksalt CoO, and spinel Co₃O₄ nanocrystals: Their interconversion and tuning of phase and morphology

Pure rocksalt (c-CoO) and wurtzite (h-CoO) phases of cobaltous oxide (CoO) have been selectively prepared through thermodynamically and kinetically controlled reactions of a single molecular precursor Co(acac)₃ (acac: acetylacetonate), respectively. Changing thermal decomposition conditions of the precursor produces different phases and distinct morphologies of the cobaltous oxides. Hexagonal pyramidal shaped h-CoO nanocrystals have been formed by a flash heating of the reaction mixture (185 °C for 2 h, kinetic control condition), whereas cube shaped c-CoO nanocrystals have been produced by a prolonged heating at a relatively low temperature (130 °C for 12 h, thermodynamic control condition). Addition of o-Dichlorobenzene (o-DCB) to the reaction mixture alters the reaction condition to the thermodynamic control regime by slowing down the decomposition rate of the precursor. Further increase of the concentration of o-DCB in the reaction mixture changes the morphology of product from h-CoO hexagonal pyramids to h-CoO nanorods with various aspect ratios and finally to c-CoO nanocrystals. Air oxidation at 240 °C for 5 h of either h-CoO or c-CoO nanocrystals yields spinel Co₃O₄ nanocrystals with retention of the original crystal morphology. During the oxidation process, the h-CoO phase has been converted into Co₃O ₄ via formation of the c-CoO phase, but the c-CoO phase has been directly oxidized to Co₃O₄. The electrochemical properties of the h-CoO, c-CoO, and spinel Co₃O₄ nanocrystals toward lithium exhibit characteristic features reflecting their Gibbs free energies. This work allows understanding of the detailed mechanism and energetics of selective formation, phase transformation, morphology control, and electrochemical properties in the closely related nanostructured cobalt oxides.