Formation and Annealing of BaTiO_3 and SrTiO_3 Nanoparticles in KOH Solution

Barium titanate (BaTiO₃) and strontium titanate (SrTiO₃) nanoparticles were synthesized separately through hydrothermal reaction of crystalline TiO₂ particles and corresponding alkaline earth hydroxides, Ba(OH)₂ and Sr(OH)2 respectively, in 50 mol·dm^-3 KOH solution at 150°C. Each structural evolution of BaTiO₃ and SrTiO₃ during the hydrothermal treatment was investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), field emission transmission electron microscopy (FE-TEM) and thermogravimetry- differential analysis (TG-DTA). In the BaTiO₃ system, round particles with cubic perovskite-type structure were obtained within 1 h. However, these particles were gradually dissolved and then were re-precipitated in the form of cube-shape BaTiO₃ particles with a smaller lattice constant ac than that of the former phase. After the BaTiO₃ particles formed firstly have disappeared completely, or the two phases coexistence stage with different lattice constant ac passed, lattice constant ac of BaTiO₃ phase re-precipitated continuously decreased with annealing time. In contrast, once SrTiO₃ particles are formed, the lattice constant ac decreased continuously throughout the reaction. The result indicates that SrTiO₃ particles were annealed without dissolution and precipitation process under the present condition.