This study demonstrates the control of crystal phase of zirconium oxide (ZrO2) nanoparticles synthesized from zirconium basic carbonate via supercritical hydrothermal method. The pH of the slurries of starting materials was adjusted using acetic acid. The lower pH reduces the amount of tetragonal phase of ZrO2 nanoparticles, which are stabilized by CO2 adsorbed on their surfaces. Single-phase monoclinic ZrO2 nanoparticles are achieved at a pH of 3.0. This is because the pH of the slurries of the starting materials affects the states of CO2-H2O system. These results will lead to large-scale industrial production of phase-controlled ZrO2 nanoparticles.
