Hydroxide-Source-Dependent Polymorphism and Phase Stability of Cobalt(II) Hydroxides in Diffusion-Driven Systems

Hydroxides of cobalt(II) exist predominantly in two polymorphic forms, namely, the blue-green α-form [α-Co(OH)₂] and reddish β-form [β-Co(OH)₂]. These hydroxides have a layered structure with interlayer galleries of around 7 and 4 Å, respectively, for α- and β-Co(OH)₂. In most of the previous studies, both the polymorphs were synthesized separately, and a few of them showed that the α-form gets converted to a thermodynamically more stable β-form via physical processes. In the present work, we have optimized the conditions for the simultaneous synthesis of both polymorphs under identical conditions in the same reactor using the 1D reaction-diffusion framework by employing different outer electrolytes. We found that the polymorph chemistry of Co(OH)₂ depends on the source and concentration of OH^- rather than other reaction conditions or later physical transformation. The products are characterized to confirm their morphology, structure, and chemical environment. We observed that the use of NaOH and NH₄OH as the OH^- precursor leads to α-Co(OH)₂ only; however, with NaOH, a continuous precipitate is formed, and with NH₄OH, periodic precipitation is formed. On the other hand, with hydrazine (HYZ) as the OH^- source, Liesegang bands of α-Co(OH)₂ and β-Co(OH)₂ as granules are formed throughout the diffusion reactor. Another intriguing observation on the HYZ system is that at its high concentration, the bands of α-Co(OH)₂ get converted to β-Co(OH)₂. We articulate the reasons and mechanism of those observations.