Polymorphs Co hydroxides formed between hydrazine and Co2+ as Liesegang bands in semisolid agar gel

Co hydroxide polymorphs (green α-Co(OH) 2 , red β-Co(OH) 2 and brown β-Co(OH) 3 ) formed at separate positions in a test-tube. Diffusion of hydrazine hydrate downward in agar gel facilitated the reaction between hydrazine and Co 2+ leading a dilatational wave of reactant concentration. The resulting periodic Liesegang bands of α-Co(OH) 2 were observed due to the diffusion hindrance of reactants. It can be deduced that the high concentration of Co 2+ causes formation of α-Co(OH) 2 while the low concentrations produce β-Co(OH) 2 . β-Co(OH) 2 is oxidized readily to form brown β-Co(OH) 3 at the upper area of agar gel with high alkali condition and O 2 distribution. This study is helpful to learn the formation of more than one product and prepare multiple materials in single chemical reaction system. In addition, the obtained α-Co(OH) 2 has been stabilized in the gel medium for at least 3 months. It may facilitate its application in many areas especially in electrochemical area since α-Co(OH) 2 typically exhibits higher conductivity and electrocatalysis than β-Co(OH) 2 while in the past its application often is limited by thermodynamically instability.