Selective and controlled synthesis of α- and β-cobalt hydroxides in highly developed hexagonal platelets

We report on the controlled synthesis of single-crystal platelets of α- and β-Co(OH)₂ via homogeneous precipitation using hexamethylenetetramine as a hydrolysis agent. The α- and β-Co(OH)₂ hexagonal platelets of several micrometers in width and about 15 nm in thickness were reproducibly yielded in rather dilute CoCl₂ solutions in the presence and absence of NaCl at 90 °C, respectively. The phase and size control of the products were achieved by varying both CoCl₂ and NaCl concentrations. Polarized optical microscope observations revealed clear liquid crystallinity of colloidal suspensions of these high aspect-ratio platelets. The as-prepared α-Co(OH)₂ containing interlayer chloride ions was intercalated with various inorganic or organic anions, keeping its high crystallinity and hexagonal platelike morphology.