Front propagation, kinetics and mechanism of ionic intercalation/de-intercalation during the polymorphic transition of ╬▒- to ╬▓-cobalt hydroxide / by Janane Farid Rahbani.

Abstract

We study the spatiotemporal dynamics of the solid-state polymorphic formation and conversion of the hydrotalcite-like ╬▒-cobalt hydroxide to the brucite-like ╬▓-cobalt hydroxide in a gel matrix at the microscopic and macroscopic levels. At the macroscopic level, the system exhibits a blue front propagation consisting of ╬▒-cobalt hydroxide behind which another front of pink ╬▓-cobalt hydroxide forms. The dynamics of the fronts and their interaction are studied by analyzing the temporal evolution of the distance traveled by the fronts, their widths and mass contents. In that regard, the effect of temperature, nature of anions, cobalt salt concentration, gel concentration, and other parameters are studied. On the microscopic level, this transition is accompanied, in addition to breaking and forming of bonds, with intercalation-de-intercalation of ions between the hydroxide layers as revealed by XRD and FTIR. The kinetics of such a conversion are studied using ionic dyes, such as Rhodamine 6G, and fluorescence microscopy from which activation energies are calculated. Concomitant to this transition are morphological changes, which are captured by SEM and reveal an accompanying space-dependent Ostwald ripening mechanism. The results reflect the complexity of the system at different scales and constitute an attempt to relate the observed dynamics at the macroscopic level to those at various microscopic scales.