Wet-chemical synthesis of monodispersed barium titanate particles - hydrothermal conversion of TiO2 microspheres to nanocrystalline BaTiO3

A low-temperature hydrothermal reaction scheme has been developed to produc e pure, ultrafine, uniform-sized, nanocrystalline barium titanate (BaTiO3) microspheres from two inorganic precursors: synthesized titania microsphere s and barium hydroxide solutions. The size and morphology of titania (TiO2) microspheres were controlled using isopropanol to fine-tune the dielectric constant of the isopropanol-water mixed solvent system. Monodispersed tita nia microspheres approximately 0.1-1 mu m in diameter were successfully syn thesized for the further conversion to barium titanate. Barium titanate and titania microspheres were characterized by scanning electron microscopy (S EM) and room-temperature X-ray diffraction (RTXRD). High-temperature XRD (H TXRD) was also utilized for in situ study of the phase transformations and changes of crystallite size with calcination temperatures. The titania micr ospheres were predominant in the anatase (plus some brookite) phase at room temperature and were converted to the rutile phase when the calcination te mperature was increased from 650 degrees C to 900 degrees C. Monodispersed barium titanate microspheres were successfully synthesized from optimized t itania via a hydrothermal reaction (less than or equal to 100 degrees C) in barium hydroxide solutions. The size and morphology of the barium titanate particles remained the same as the precursor titania particles, indicating a "shrinking-core" diffusion-reaction mechanism. Barium carbonate in the f orm of witherite was also found along with the formation of barium titanate , especially under conditions with higher Ba/Ti ratios, but a formic acid w ashing procedure effectively removed this impurity phase from the barium ti tanate samples. The as-prepared barium titanate was in the cubic nanocrysta lline form and did not change when the temperature was increased from room temperature to as high as 750 degrees C. The cubic phase was also stable at high temperatures for over 5 h. (C) 2000 Elsevier Science S.A. All rights reserved.