Morphological Transformation of Co(OH)2 Microspheres from Solid to Flowerlike Hollow Core-Shell Structures

We report, for the first time, a detailed investigation into the formation of highly uniform, 3D, flowerlike, hollow, spherical architectures of cobalt hydroxide through a facile solvothermal process. Various controlling parameters were examined, such as water content in starting materials, reaction time, cobalt(II) precursor concentration, and reaction temperature. On the basis of the experimental results, the formation mechanism of these flowerlike cobalt hydroxide hollow spheres involves aggregation of cobalt hydroxide building clusters into solid spheres and hollowing effect through subsequent dissolution, diffusion, and re-deposition of the smaller crystallites under the surface layer driven by an Ostwald ripening process. Metallic cobalt hollow spheres have also been obtained by thermal decomposition of cobalt hydroxide flowers in a mixed gas of Ar+4% H-2 at 400 degrees C. The morphology and composition of the products were characterized by Xray diffraction, field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, and (high resolution) transmission electron microscopy.