A new approach for design and synthesis of Co-II and Co-II,Co-III hydroxide materials

A synthetic investigation on the formation of cobalt hydroxides has been carried out under low pH and atmosphere-controlled conditions. It was found that various phases and compositions of cobalt hydroxides can be tailor-prepared via a careful control of preparative parameters such as the initial concentration ratio of [OH-]/[Co2+], preparative atmosphere and aging time. Through elemental and structural determinations, the formation sequence of a major cobalt hydroxide in air has been shown as: turbostratic Co-II hydroxide, Co-II hydrotalcite-like phase, Co-II,Co-III hydrotalcite-like phase and Co-II brucite-like phase. It is indicated that the oxidation of divalent cobalt in Co-II brucite-like phase becomes more difficult when the hydroxide is synthesized with a higher [OH-]/[Co2+] ratio. Defective crystallites and large interlayer spacing are attributed to be the two major factors contributing to the oxidation of divalent cobalt. In contrast to the samples prepared in air, the Co-II brucite-like phase is readily formed under a nitrogen atmosphere even at a low [OH-]/[Co2+] ratio. Thermal decomposition of prepared hydroxides has also been conducted, which confirms the findings revealed by the synthetic experiments and compositional and structural analyses. (C) 2000 Elsevier Science Ltd. All rights reserved