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.