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 α- and β-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 α-Co(OH)2 to crystalline β-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 α-Co(OH)2. X-ray diffraction and electronic microscopy investigations confirm the aforementioned Ostwald ripening process during the phase transformation whereby almost-amorphous α-Co(OH)2 dissolves to form crystalline β-Co(OH)2 5 μm in length. The UV–vis reflectance spectra reveal that the origin of the blue/green color in the α-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 α-polymorph to form octahedral Co(II) in the β-polymorph is shown to take place in seconds without induction time. α-Co(OH)2 was found to be mesoporous while the β-polymorph is microporous with low nitrogen adsorption capacities. Due to dipole–dipole broadening, no EPR spectrum was obtained for the β-polymorphs even at low temperature. In contrast, the obtained EPR spectrum of the α-polymorph was consistent with that of Co(II) in various materials.