Ferroelectric Mesocrystalline BaTiO₃/SrTiO₃ Nanocomposites with Enhanced Dielectric and Piezoelectric Responses

A platelike mesocrystalline BaTiO₃/SrTiO₃ (BT/ST) nanocomposite is prepared via a clever two-step solvothermal soft chemical process. First a protonated titanate H_1.07Ti_1.73O₄·<i>n</i>H₂O (HT) crystal with a layered structure and platelike morphology is solvothermally treated in a Ba­(OH)₂ solution to generate a homogeneous platelike BaTiO₃/HT (BT/HT) nanocomposite. Second the generated BT/HT nanocomposite is solvothermally treated in a Sr­(OH)₂ solution to generate the mesocrystalline BT/ST nanocomposite with platelike particle morphology. The transformation reactions from the HT precursor to the mesocrystalline BT/ST nanocomposite are topochemical conversion reactions, and the formed BT/ST nanocomposite is constructed from well-aligned BT and ST nanocrystals in the same crystal-axis orientation. The BT/ST nanocomposite annealed at 900 °C shows a ferroelectric behavior and drastically enhanced piezoelectric and dielectric responses owing to the introduction of a lattice strain at a three-dimensional heteroepitaxial interface between the BT and ST nanocrystals in the mesocrystal. The nanostructure of the BT/ST mesocrystal is suitable for simultaneous application of the strain engineering and the orientation engineering to develop high performance piezoelectric and dielectric materials

Ferroelectric Mesocrystals of Bismuth Sodium Titanate: Formation Mechanism, Nanostructure, and Application to Piezoelectric Materials

Ferroelectric mesocrystals of Bi_0.5Na_0.5TiO₃ (BNT) with [100]-crystal-axis orientation were successfully prepared using a topotactic structural transformation process from a layered titanate H_1.07Ti_1.73O₄·<i>n</i>H₂O (HTO). The formation reactions of BNT mesocrystals in HTO–Bi₂O₃–Na₂CO₃ and HTO–TiO₂–Bi₂O₃–Na₂CO₃ reaction systems and their nanostructures were studied by XRD, FE-SEM, TEM, SAED, and EDS, and the reaction mechanisms were given. The BNT mesocrystals are formed by a topotactic structural transformation mechanism in the HTO–Bi₂O₃–Na₂CO₃ reaction system and by a combination mechanism of the topotactic structural transformation and epitaxial crystal growth in the HTO–TiO₂–Bi₂O₃–Na₂CO₃ reaction system, respectively. The BNT mesocrystals prepared by these methods are constructed from [100]-oriented BNT nanocrystals. Furthermore, these reaction systems were successfully applied to the fabrication of [100]-oriented BNT ferroelectric ceramic materials. A BNT ceramic material with a high degree of orientation, high relative density, and small grain size was achieved

Mesocrystalline Nanocomposites of TiO₂ Polymorphs: Topochemical Mesocrystal Conversion, Characterization, and Photocatalytic Response

Four kinds of platelike mesocrystalline nanocomposites of TiO₂ polymorphs were successfully synthesized for the first time based on a topochemical mesocrystal conversion mechanism. In this conversion process, a [010]-oriented titanate H_1.07Ti_1.73□_0.27O₄ (□: vacancy of Ti) single crystal with lepidocrocite-like structure and platelike morphology was successively transformed into [001]- and [102]-oriented TiO₂(B) phases including a {010}-faceted TiO₂(B) twinning, [010]-oriented anatase phase, and [110]-oriented rutile phase. The platelike particle morphology is retained in the topochemical conversion process. The platelike particles are constructed from nanocrystals which well-aligned in the same orientation for the same phase, resulting in the formations of HTO/TiO₂(B), HTO/TiO₂(B)/anatase, TiO₂(B)/anatase, and anatase/rutile mesocrystalline nanocomposites. The reaction mechanism and the crystallographic topological correspondences between the precursor, intermediates, and the final product were given on the basis of the nanostructural analysis results. The mesocrystalline nanocomposite of anatase/rutile polymorphs exhibits unexpectedly high surface photocatalytic activity, which can be explained by the superior electron–hole separation effect and the high activity of {010}-faceted anatase surface in the mesocrystalline nanocomposite. Such mesocrystalline anatase/rutile nanocomposite is an ideal photocatalytic system