Ultrasonically activated hydrothermal synthesis of fine TiO2 and ZrO2 powders

Fine TiO2 and ZrO2 powders were prepared by hydrothermal synthesis at 150 and 250 degreesC (10 min,1 h, 3 h) from aqueous titanyl and zirconyl salts and amorphous titanyl and zirconyl hydroxide gels with and without ultrasonic activation. The synthesized materials were characterized by x-ray diffraction, scanning and transmission electron-microscopic techniques, and nitrogen capillary adsorption measurements. The results demonstrate that ultrasonic activation of the hydrothermal process markedly accelerates the crystallization rates of the amorphous titanyl and zirconyl hydroxides and raises the content of thermodynamically stable phases in the reaction products. It is also shown that hydrothermal treatment of amorphous cobalt(II) hydroxide leads to its crystallization, whereas ultrasonic-hydrothermal treatment results in partial Co(II)-to-Co(III) oxidation, yielding a mixture of Co(OH)(2) and Co3O4.