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.
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