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Hydroxide-Source-Dependent Polymorphism and Phase Stability of Cobalt(II) Hydroxides in Diffusion-Driven Systems
Akshay K Dimble1, Nikhil D Bagul1, Prasad C Walimbe1
1Post Graduate and Research Center, Department of Chemistry, S. P. Mandali's, Sir Parashurambhau College, Tilak Road, Pune, Maharashtra 411 030, India.
Langmuir : the ACS Journal of Surfaces and Colloids
|August 20, 2024
Summary
Cobalt(II) hydroxide polymorphs (α- and β-Co(OH)2) can be synthesized simultaneously. The OH- source dictates the resulting polymorph, influencing structure and precipitation patterns, with hydrazine enabling unique Liesegang band formation.
Area of Science:
- Materials Science
- Inorganic Chemistry
- Crystallography
Background:
- Cobalt(II) hydroxide exists as two main polymorphs: blue-green α-Co(OH)2 and reddish β-Co(OH)2.
- These polymorphs possess distinct layered structures with different interlayer gallery spacings.
- Previous research often synthesized these forms separately, with α-Co(OH)2 sometimes converting to the more stable β-form.
Purpose of the Study:
- To achieve simultaneous synthesis of both α- and β-Co(OH)2 polymorphs under identical conditions.
- To investigate the influence of the hydroxide ion (OH-) source on polymorph formation.
- To explore the reaction-diffusion framework for controlled synthesis of cobalt hydroxide polymorphs.
Main Methods:
- Utilized a 1D reaction-diffusion framework for simultaneous polymorph synthesis.
- Employed various outer electrolytes, focusing on different sources and concentrations of OH- (NaOH, NH4OH, hydrazine).
- Characterized the synthesized products to confirm morphology, structure, and chemical environment.
Main Results:
- The polymorph chemistry of Co(OH)2 is critically dependent on the OH- source and concentration, not other reaction parameters.
- NaOH and NH4OH exclusively yield α-Co(OH)2, with distinct continuous and periodic precipitation, respectively.
- Hydrazine (HYZ) leads to Liesegang bands of both α- and β-Co(OH)2, with α-Co(OH)2 converting to β-Co(OH)2 at higher HYZ concentrations.
Conclusions:
- The OH- source is the primary determinant in the simultaneous synthesis of Co(OH)2 polymorphs.
- Reaction-diffusion with specific OH- sources offers a pathway to control polymorph formation and morphology.
- The observed conversion of α- to β-Co(OH)2 in the HYZ system provides insights into phase transformation mechanisms.
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