Abstract

Crystals of Co2CO3(OH)2 have been synthesized under ambient conditions, in contrast to hydrothermal methods reported previously. We have developed a simple but efficient methodology to obtain an initial amorphous phase that evolves to a crystalline cobalt hydroxide carbonate after one week of maturation. X-ray diffraction analysis indicates that this phase crystallizes in the space group P21/a (a = 12.886(6), b = 9.346(3), c = 3.156(1) Å, β = 110.358(6)°). The platelet morphology of Co2CO3(OH)2 agrees with its lamellar crystal structure. High-resolution transmission electron microscopy (HRTEM) reveals that each individual platelet is comprised of nanodomains disoriented with respect to their neighbours. The kinetics and the activation energy (Ea = 6.26 kJ mol–1) of the transformation process have been estimated using the rate constant method. The precipitation of solids leads to a decrease in the cobalt concentration in the solution (∼88%) reaching values of ∼150 ppm, which can be considered a successful reduction from the perspective of water quality. The calcination in air of the synthetized platelets produced exclusively Co3O4. The thermo-X-ray difraction results confirm that Co2CO3(OH)2 is transformed over a small range of temperatures (225 – 235°C) into pure Co3O4. HRTEM images show that the lamellar nanomorphology is preserved in the Co3O4 phase. Therefore, understanding the crystallization behaviour of Co2CO3(OH)2 can help to minimize environmental problems caused by cobalt pollution and may facilitate the management of methods to obtain phases with specific nanomorphologies used widely in material sciences.

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