Sequential formation of calcium carbonate superstructure: From solid/hollow spheres to sponge-like/solid films

Organisms produce complex nanostructured biomineral materials that are supported by acidic proteins and have excellent mechanical properties. The strategy of mimicking these natural processes opens new avenues for material design in both scientific and industrial fields. Interestingly, amorphous minerals, which are less stable and more soluble than crystalline ones, have an important basic function in many biomineral formation processes as transient precursor phase of crystalline ones. Here we introduce a robust demonstration of the role of an amorphous precursor phase in biomineralization, and the sequential formation of an exquisite CaCO3 structures via a one-pot solution route, in which amorphous microparticles in the presence of a small amount of synthetic acidic macromolecules are sequentially transformed into hollow vaterite microparticles, sponge-like vaterite films and solid vaterite films by the CO2 diffusion method. We propose a mechanism for the formation of hollow spheres, which may have potential applications as host capsule materials, namely, their formation by crystallization on the particle surface, and the subsequent dissolution of their core. Our findings provide a new strategy for the sequential formation of biominerals in the presence of small amount of organic additives.