Perovskite nanoparticles and nanowires: Microwave-hydrothermal synthesis and structural characterization by high-resolution transmission electron microscopy

Perovskite pure BaTiO3 (BT) nanoparticles with spherical morphology and PX-phase PbTiO3 (PT) nanowires with acicular morphology, were synthesized by microwave-hydrothermal process, and their atomic-scale microstructures were characterized by electron microscopy. Both X-ray diffraction (XRD) and selected area electron diffraction (SAED) patterns demonstrated that the produced BT nanoparticles remained a cubic perovskite structure at room temperature. The BT nanoparticles exhibited a spherical morphology with a nearly uniform size of 50 nm. The PX-phase PT nanowires exhibited acicular morphology with diameter sizes of 40-60 nm and length up to several micrometers, and the aspect ratio was close to 90. They tended to grow into a regular structure with parallel arrangement along their long axis in the [001] direction. The extinction rule for the diffraction indexes (h, k, l ) in the XRD pattern is the sum of (h, k, l ) equal to odd number, similar to body-centered cubic crystal. In the SAED patterns of the PX-phase PT nanowires, both super electron diffraction spots with a threefold modulated periodicity along the [110] direction and the appearance of strong diffraction spot (008) but the forbidden (004) spot, were clearly observed, which indicated that the PX-phase PT has a threefold modulated periodicity along the [110] direction, and fourfold modulated periodicity in the [001] direction, respectively. This result was confirmed by high-resolution transmission electron microscopy images. The Raman bands centered near 146, 193, and 292 cm(-1) observed in the nanowires at room temperature, could be assigned to the E(1LO), E(2TO), and E+B-1 phonon modes of the tetragonal PT structure, respectively. However, no phonon modes of the tetragonal PT structure were responsible for the additional three Raman bands centered near 175, 548, and 832 cm(-1), which are presumed to be resulted from the additional Raman modes of the PX-phase PT with a large and complex unit cell.