Polymorph Control of Calcium Carbonate on the Surface of Mesoporous Silica

The influence of mesoporous silica on the polymorph selectivity in CaCO3 has been investigated. Vaterite is selectively formed from an aqueous solution containing CaCl2 and Na2CO3 by precipitation under the presence of KIT-6-type mesoporous silica. Crystallization time of vaterite from amorphous calcium carbonate (ACC) is much longer on the addition of KIT-6 than that reported previously, indicating the remarkable stabilization of ACC on the surface of mesoporous silica. Other types of silica affect the polymorph selectivity; the addition of amorphous silica gel or assembled silica nanoparticles 12 nm in particle size induced the formation of vaterite as a main phase whereas the presence of assembled silica nanoparticles 30 nm in particle size resulted in the formation of calcite as a main phase with a minor component of vaterite. Therefore, the porous nature of the surfaces of silica greatly influences the polymorph, and a sort of surface confinement should play a major role in the selectivity of polymorph.