A New Model for the Synthesis of Hollow Particles via the Bubble Templating Method

Hollow CaCO3 particles were synthesized by passing CO2/N-2 bubbles into a CaCl2 solution. The formation of such hollow particles was investigated via the nucleation mechanism of calcium carbonate. It was found that homogeneous and heterogeneous nucleation both occurred via a three phase gas-liquid-solid reaction. However. only the latter played a role in the formation of the described hollow particles. Nuclei were initially formed at the bubble surface and subsequently aggregated together to form the initial shell. Growth of the initial shell structure proceeded along the bubble surface by the subsequent aggregation of nuclei. Concurrently it was found that the combination of reaction ions, Ca2+ and CO32-, adsorbed on the outer surface of the growing shell resulted in the increase of shell thickness in a direction perpendicular to the bubble template surface. Both aggregation of nuclei and crystal growth contributed to the formation of the shell structure, with distinguishable inner and outer layers present. A new model was proposed based on these investigations, in which crystal growth was for the first time highlighted as a key component in the formation of hollow particles by the bubble template route.