Fabrication of morphology and crystal structure controlled nanorod and nanosheet cobalt hydroxide based on the difference of oxygen-solubility between water and methanol, and conversion into Co3O4

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Abstract
Films of brucite-type cobalt hydroxide with nanorod morphology and hydrotalcite-type cobalt hydroxide with nanosheet morphology films were fabricated by heterogeneous nucleation in a chemical bath using water and a mixed solution of water–methanol as solvents, respectively. Since oxygen is around 25 times more soluble in methanol than in water, a methanol solution was used to convert a part of divalent cobalt ions into trivalent cobalt ions through oxidation, due to the amount of dissolved oxygen. The resultant cobalt hydroxides were of the hydrotalcite type, with a sheet-like morphology, and di- and trivalent cobalt ions. On the other hand, brucite-type hydroxides with a rod morphology, constructed using only divalent cobalt ions, were fabricated due to the scarcity of dissolved oxygen in a water-only solvents. Both the brucite and hydrotalcite types of cobalt hydroxide films were transformed into Co3O4 through pyrolysis without nanostructural deformation. The Co3O4 films were porous structures with a large surface area because both rod and sheet were constructed through nanoparticles and nanopores once the self-template was removed.

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