Polymorph design of CaCO3 from stable calcite to metastable vaterite and amorphous CaCO3 via altering reaction system step by step

Despite the pivotal roles of amorphous calcium carbonate (ACC) and vaterite in the biomineralization process, drug delivery and biomedical engineering, their thermodynamic instability poses substantial challenges in their preparation. Although many synthesis methods for ACC and vaterite have been developed, the polymorph discrimination of CaCO₃ in different reaction system remains unclear. In the present work, the polymorph of CaCO₃ is designed by altering reaction system step by step under the same or similar parameters, i.e. liquid–liquid reaction between CaCl₂ and Na₂CO₃/K₂CO₃ in aqueous solution, ethanol–water solution, ethanol/ethylene glycol solution, liquid–solid in ethylene glycol solvent and solid–solid reaction assisted by mechanical energy. The results indicate that the partial substitution of water by ethanol in aqueous reaction system has no obvious advantage on the formation and stability of vaterite, while the whole substitution of CaCl₂ aqueous solution by CaCl₂ ethanol solution has significant advantage on vaterite formation and its stability. Furthermore, the reaction between CaCl₂ and K₂CO₃ in ethylene glycol solution has huge advantage on the formation and stability of ACC. The rigorous operation is needed to obtain ACC in liquid–solid and solid–solid reaction systems. This work contributes to thorough understanding the preparation of ACC and vaterite in different reaction systems.