Stabilization of Size-Controlled BaTiO₃ Nanocubes via Precise Solvothermal Crystal Growth and Their Anomalous Surface...

Crystal growth of barium titanate (BaTiO₃) using a wet chemical reaction was investigated at various temperatures. BaTiO₃ nanoparticles were obtained at an energy-efficient temperature of 80 °C. However, BaTiO₃ nanocubes with a preferred size and shape could be synthesized using a solvothermal method at 200 °C via a reaction involving titanium tetraisopropoxide [(CH₃)₂CHO]₄Ti for nucleation and fine titanium oxide (TiO₂) nanoparticles for crystal growth. The BaTiO₃ nanocubes showed a high degree of dispersion without the use of dispersants or surfactants. The morphology of BaTiO₃ was found to depend on the reaction medium. The size of the BaTiO₃ particles obtained using water as the reaction medium was the largest among the particles synthesized using various reaction media. In the case of alcohol reaction media, the BaTiO₃ particle size increased in the order methanol, ethanol, 1-propanol, 1-butanol, and 1-pentanol. Furthermore, BaTiO₃ powder obtained using alcohol reaction media resulted in cubic shapes as opposed to the round shapes obtained when water was used as the medium. We found that the optimal condition for the synthesis of BaTiO₃ nanocubes involved the use of 1-butanol as the reaction medium, resulting in an average particle size of 52 nm, which is the average distance of the cubes measured diagonally from corner to corner, and gives an average side length of 37 nm, and a tetragonal crystal system as evidenced by the powder X-ray diffraction pattern obtained using high-energy synchrotron X-rays. The origin of the spontaneous polarization of the BaTiO₃ tetragonal crystal structure was clarified by a pair distribution function analysis. In addition, surface reconstruction of BaTiO₃ nanocubes led to an outermost surface comprising two layers of Ti columns.