Selection of phase and control of orientation during physisorption on surfaces of homogeneously formed calcium carbonate nuclei

The physisorption of homogeneously formed nuclei was shown to result in the preferential formation of different phases of calcium carbonate on different surfaces and the orientation of the crystallites. Calcium carbonate polymorphs were grown on clean polystyrene and glass substrates by diffusion of ammonium carbonate vapors into calcium chloride solutions. The substrates were arranged both vertically and horizontally. Oriented crystallites of vaterite and calcite were formed on the two substrates, although the number and type of crystallites were strongly dependent on both the type of substrate and the arrangement of the substrate. Polystyrene strongly favored vaterite, whereas glass favored calcite. The observation of crystal orientation and the preference of each surface for a particular polymorph suggest a significant role for the surface, presumably via heterogeneous nucleation. However, comparison of the density of crystals on horizontal and vertical substrates indicates that sedimentation influences the distribution of the crystallites. To achieve an influence of sedimentation, the homogeneously formed nuclei must grow to a significant size (0.03-1.0 μm) prior to physisorption onto the substrates. Subsequently, these nuclei either undergo a phase transformation to the observed polymorph or two different polymorphs are formed spontaneously and physisorbed preferentially on the two types of substrate. Orientation of the crystallites was thought to occur during physisorption. This work demonstrates that heterogeneous nucleation is not required to control orientation and achieve phase selection.