Morphological control of CaCO3 superstructures in seawater: Insights into Ca-source anion influence and formation mechanism

This work provides crucial insights into the crystallization and morphogenesis of CaCO3 polymorphs in seawater at elevated temperatures. Using Ca source anions as a controlling factor, we modify the crystallization of CaCO3 and demonstrate that stable and intricate particle morphologies of CaCO3, such as hexagonal bipyramidal, disks, and twin structures, can be generated in seawater without surfactants. The synthesis of CaCO3 was accomplished through a direct hydrothermal method utilizing water-soluble Ca salts (calcium chloride, calcium nitrate, and calcium acetate) as Ca source and urea as a carbonate source. Depending on the specific source anions used, various morphological changes were observed. The CaCO3 prepared with the nitrate source yields vaterite as the major polymorph with hexagonal bipyramidal morphology. Pseudohexagonal prisms of aragonite were the primary product obtained with calcium acetate as the source. In contrast, chloride ions yielded vaterite and aragonite depending on the reaction temperature and time. The purity of CaCO3 prepared with seawater was > 94 % regardless of the Ca source. We observe that the phase formation follows the order of gypsum-vaterite-aragonite with an increase in hydrothermal reaction time. Further, we propose the formation mechanism of vaterite hexagonal bipyramidal and aragonite pseudohexagonal prism superstructures where directional aggregation of particles occurs in the presence of ammonium ions and seawater solvent. These findings, obtained through the exploration of seawater and different Ca sources, offer a comprehensive understanding of how source anions influence the selective formation of CaCO3 polymorph and its morphological tuning. This information can be applied to tailor the synthesis of CaCO3 for specific applications.