Synthesis of highly spherical calcium carbonate nanoparticles using air nanobubbles as additives

Ultrasamll nanoparticles that simultaneously exhibit narrow size distribution and high circularity are key to enabling multifunctional applications in optoelectronic systems, integrated circuits and robots. Calcium carbonate (CaCO₃) nanoparticles with various shapes and sizes have found applications in both science and technology, while controlled synthetic approaches toward nanoparticles with high circularity remain challenging. Herein, we report the synthesis of spherical CaCO₃ nanoparticles using air nanobubbles as additives in reactive precipitation. Air nanobubbles (≈100 nm, stable > 5 days) were generated via a rotating packed bed. During precipitation, these nanobubbles acted as immobile impurities, directing crystal growth. This yielded spherical CaCO₃ nanoparticles (6.89 ± 0.75 nm in average) with calcite structure and exceptional circularity (0.97 in maximum). In situ liquid-phase TEM revealed the nanobubbles’ role in inducing anomalous growth, providing deep insight into spherical nanoparticle synthesis for diverse applications.