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

Abstract

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 Co2+ 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 Co2+ 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 O2 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.