Synthesis of Alumina Nanoparticles Using Plasma-Induced Microbubbles

This study investigates the selective synthesis of α- and γ-alumina nanoparticles using plasma-induced microbubbles. Although plasma-induced bubbles provide an effective reaction environment for the synthesis of nanomaterials, precise phase control remains challenging. Herein, we demonstrate that the modulation of the pulse off time regulates the thermal environment within the bubbles. Optical emission spectroscopy revealed that a shorter off time maintains a high electron temperature, indicating substantial heat accumulation. This high-energy state promotes the atomization of the precursor mist and the subsequent growth of molten droplets, providing sufficient activation energy for the formation of the thermodynamically stable α-phase. In contrast, a longer off time leads to the formation of a metastable γ-phase because of insufficient heating and rapid quenching. These findings prove that alumina nanoparticles with desired crystal phase and size can be synthesized by controlling the thermal energy inside the plasma-induced microbubbles.