<i>In Situ</i> X-ray Scattering Guided Polymorphism Control in ZrO ₂ Nanoparticle Synthesis

Zirconium dioxide (ZrO2) is widely used as a structural ceramic and in various technological applications, where performance often depends on stabilizing the high-temperature t-ZrO2 phase under ambient conditions. However, green synthesis routes using benign precursors typically yield phase mixtures dominated by the thermodynamically stable m-ZrO2 phase. Here, in situ X-ray scattering is used to investigate the solvothermal synthesis of ZrO2 nanoparticles across different solvents (methanol, ethanol, 2-propanol, ethylene glycol, and water) and temperatures (150–400 °C). The metastable t-ZrO2 phase forms initially as a kinetic phase in all syntheses except water before the transition into the m-ZrO2 phase in a solid-state transformation. The conditions required to isolate a phase-pure t-ZrO2 product are established, demonstrating the in situ solvothermal synthesis as an efficient screening tool. Ex situ continuous-flow solvothermal synthesis is then employed to reproduce the required conditions, enabling the isolation of phase-pure t-ZrO2. Continuous flow solvothermal synthesis is an efficient green method for production of nanoparticles, and the fast heating rate and flexible design provide versatility to reach the short reaction time required in this case.