Hydrothermal/microwave and hydrothermal/ultrasonic synthesis of nanocrystalline titania, zirconia, and hafnia

We compare the physical-chemical properties (X-ray diffraction (XRD), powder X-ray diffraction, TGA, TEM, and BET) of titania, zirconia, and hafnia nanopowders (d = 10–15 nm) synthesized from amorphous titanyl hydroxide TiO2 · nH2O, zirconyl hydroxide ZrO(OH)2 · nH2O, and hafnyl hydroxide HfO(OH)2 · nH2O using hydrothermal (HT), hydrothermal/microwave (HT-MW), and hydrothermal/ultrasonic (HT-US) methods at 150, 180, and 250°C with treatment lasting 0.5–24 h. Titania, zirconia, and hafnia crystallization from amorphous hydroxides is substantially enhanced, and the percentage of the thermally stable zirconia phase (m-ZrO2) in the HT-MW and HT-US processes increases compared to conventional HT synthesis. The observed similar effects have completely different causes. A common factor in both cases is likely the uniformity of heating of treated suspensions. Local overheating in the reaction mixture, which appears during both ultrasonication and microwave treatment, can also play a role in accelerating the hydrothermal processes.