Reaction-diffusion based co-synthesis of stable α- and β-cobalt hydroxide in bio-organic gels

We report the preparation, dynamics of formation and extensive characterization of a stable two-phase system of crystalline alpha- and beta-Co(OH)(2). The method is based on the reaction and diffusion of hydroxide ions into a biopolymer gel (agar, gelatin) containing Co(II). The spatio-temporal dynamics leading to the formation and coexistence of two polymorphs exhibit a complicated and rich pattern whereby the system proceeds as a propagating Ostwald ripening front that continuously transforms blue/green alpha-Co(OH)(2) to crystalline beta-Co(OH)(2). Depending on the nature of the gel, the system might further exhibit fascinating Liesegang bands. The coexisting polymorphs were characterized using XRD, FTIR, UV-vis, TGA, SEM and TEM, and EPR. The FTIR spectra reveal the intercalation of water molecules and chloride ions between the hydroxyl layers in the case of alpha-Co(OH)(2). X-ray diffraction and electronic microscopy investigations confirm the aforementioned Ostwald ripening process during the phase transformation whereby almost-amorphous alpha-Co(OH)(2) dissolves to form crystalline beta-Co(OH)(2) 5 mu m in length. The UV-vis reflectance spectra reveal that the origin of the blue/green color in the alpha-polymorph is due to the tetrahedrally coordinated Co(II) ions existing within the octahedral Co(II) layers. The reorganization of these tetrahedral Co(II) ions in the alpha-polymorph to form octahedral Co(II) in the beta-polymorph is shown to take place in seconds without induction time. alpha-Co(OH)(2) was found to be mesoporous while the beta-polymorph is microporous with low nitrogen adsorption capacities. Due to dipole-dipole broadening, no EPR spectrum was obtained for the beta-polymorphs even at low temperature. In contrast, the obtained EPR spectrum of the alpha-polymorph was consistent with that of Co(II) in various materials. (C) 2009 Elsevier B.V. All rights reserved.