Surfactant-free hydrothermal fabrication of vaterite CaCO3 with hexagonal bipyramidal morphologies using seawater

We describe a simple hydrothermal process for synthesizing vaterite-type CaCO 3 microparticles with a hexagonal bipyramidal shape using seawater. Vaterite with a spherical morphology is synthesized at room temperature relatively frequently, and a few high-temperature synthetic methods have been reported. However, the majority of these procedures require specific conditions, such as the use of detrimental surfactants, ultrasonication, and microwave irradiation. In this study, we successfully synthesized vaterite in Mg-free seawater using Ca(NO 3 ) 2 and urea, which do not require surfactants/organic additives or complex experimental conditions. The obtained vaterite hexagonal bipyramids had sharp edges and facets and a purity of 96%. We investigated the effects of the solvent type, reaction temperature, reaction time, and the ratio of Ca 2+ and CO 3 2− , on the crystallization and morphology of the CaCO 3 polymorph. The phase and morphology of the vaterite were primarily determined by the synergistic effects of seawater and the ammonium ions from urea. • Vaterite CaCO 3 was synthesized by Mg-free seawater aided hydrothermal method. • Unique hexagonal bipyramidal morphology was observed for the vaterite particles. • No organic additive/solvent is used for hexagonal bipyramidal vaterite formation. • Temperature (T) and time (t) play major roles in the vaterite formation. • CaCO 3 polymorphs follow same phase transition route irrespective of T and t.