Nucleation and Growth of Cobalt Hydroxide Crystallites in Organized Polymeric Multilayers

In this paper, we report the hydrolysis of Co2+ ions absorbed in organized, multilayered polymer films to form cobalt hydroxide nanocrystals. Polymer films were prepared using the layer-by-layer deposition technique that consists of adsorbing polycations and polyanions alternately on a quartz substrate. By monitoring the UV−vis absorbance of the polymer films as a function of the number of absorption−hydrolysis cycles, it was shown that the crystals continued to grow with additional cycles. It is also found in this study that using more polymer layer pairs results in more crystal growth. Hydrolysis of the Co2+ ions in a nitrogen-rich environment gives rise to mainly needlelike crystallites of α-Co(OH)2 that are initially about 100 nm in length and then increase in size with the number of absorption−hydrolysis cycles. However, in an oxygen-rich microenvironment, hexagonal crystallites were found to be predominant. X-ray diffraction and transmission electron microscopic studies revealed that these hexagonal crystallites mainly consist of β-Co(OH)2 with a contribution of CoOOH. Our studies suggest that by varying the nitrogen-to-oxygen ratio the formation of α-Co(OH)2 and β-Co(OH)2 can be controlled.