Dimensional Control of Cobalt-hydroxide-carbonate Nanorods and Their Thermal Conversion to One-dimensional Arrays of Co₃O₄ Nanoparticles

We report a detailed investigation on the formation of cobalt-basic-carbonate compounds [Co(OH)_x (CO₃) _0.5(2-x)·nH₂O] with dimensional and morphological controls. Two complementary precipitation methods developed in this work produce cobalt-basic-carbonate in the form of nanorods across a wide diameter span from 2 to 200 nm and a length up to 5 μm. Various controlling parameters were examined, such as anions in starting reagents, reaction temperature, aging time, and chemical compositions between the solid products and starting reagents. The resultant solids were characterized using XRD, FTIR, CHN, and TGA/DrTGA methods with respect to their formation process and thermal decomposition. More importantly, morphological aspects of the Co(OH) _x(CO₃)_0.5(2-x)·nH₂O] and their heat-treated product Co₃O₄ were investigated with TEMED methods. It has been found that the Co(OH)_x(CO ₃)_0.5(2-x)·nH₂O nanorods are grown along the [010] direction (orthorhombic). Upon thermal decomposition at ≥300 °C, the above nanorods self-assemble into one-dimensional arrays of Co ₃O₄ nanoparticles along the [111] axis (cubic) of the spinel. Co₃O₄ nanorods can be prepared with this method in a small diameter regime of 20-40 nm.