Hollow Structure Formation Mechanism of Calcium Carbonate Particles Synthesized by the CO2 Bubbling Method

The hollow structure formation mechanism of calcium carbonate particles synthesized by the CO2 bubbling method has been investigated. Samples were sequentially taken during the bubbling process to be structurally evaluated by X-ray diffraction and scanning electron microscopy. As a result, it has been elucidated that there are three development stages (A, B, and C) in the formation of the hollow structure. In stage A, amorphous CaCO3 precipitates as primary particles with the decreasing pH by CO2 bubbling. The primary particles then aggregate around pH 9.1 to form secondary particles, and turbidity ensues. Furthermore, because of the decreasing pH, the primary particles on the surface of the secondary particles begin to transform into the vaterite phase and form a shell. During stage B, the surface potential decreases along with a steep decrease in the pH, interparticle attraction of the primary particles on the surface of the secondary particles becomes stronger, and a highly compacted shell is formed. At the same time as vaterite precipitation on the surface, dissolution of the amorphous phase on the inside rapidly proceeds to precipitate vaterite particles by adhesion to the inside of the outer shell. At this time, contraction due to the release of water occurs and the hollow structure is formed. Moreover, it has been elucidated that, at pH <= 7, redissolution of the particulates inside the secondary particles occurs and definite shell structures are formed.