Hydroxide-Source-Dependent\nPolymorphism and Phase\nStability of Cobalt(II) Hydroxides in Diffusion-Driven Systems

Hydroxides of cobalt(II) exist predominantly in two polymorphic\nforms, namely, the blue-green α-form [α-Co(OH)<sub>2</sub>] and reddish β-form [β-Co(OH)<sub>2</sub>]. These hydroxides\nhave a layered structure with interlayer galleries of around 7 and\n4 Å, respectively, for α- and β-Co(OH)<sub>2</sub>. In most of the previous studies, both the polymorphs were synthesized\nseparately, and a few of them showed that the α-form gets converted\nto a thermodynamically more stable β-form via physical processes.\nIn the present work, we have optimized the conditions for the simultaneous\nsynthesis of both polymorphs under identical conditions in the same\nreactor using the 1D reaction–diffusion framework by employing\ndifferent outer electrolytes. We found that the polymorph chemistry\nof Co(OH)<sub>2</sub> depends on the source and concentration of OH<sup>–</sup> rather than other reaction conditions or later physical\ntransformation. The products are characterized to confirm their morphology,\nstructure, and chemical environment. We observed that the use of NaOH\nand NH<sub>4</sub>OH as the OH<sup>–</sup> precursor leads\nto α-Co(OH)<sub>2</sub> only; however, with NaOH, a continuous\nprecipitate is formed, and with NH<sub>4</sub>OH, periodic precipitation\nis formed. On the other hand, with hydrazine (HYZ) as the OH<sup>–</sup> source, Liesegang bands of α-Co(OH)<sub>2</sub> and β-Co(OH)<sub>2</sub> as granules are formed throughout the diffusion reactor.\nAnother intriguing observation on the HYZ system is that at its high\nconcentration, the bands of α-Co(OH)<sub>2</sub> get converted\nto β-Co(OH)<sub>2</sub>. We articulate the reasons and mechanism\nof those observations.