Study On Controllable Growth, Microstructure And Properties Of Perovskite Ferroelectric Oxide Nanostructures

Low-dimensional perovskite-structure ferroelectric oxides have attracted extensive attention because of their fascinating physical and chemical properties, based on which promising applications in high density information storage, high sensitive sensor and catalysis become availiable. Therefore, studying on controllable growth, microstructure and properties of the low-dimensional perovskite oxides is highly important to explor the new phenomenon and application.In this dissertation, crystal structure and the current status of studies on low-dimensional perovskite ferroelectric oxides were reviewed at first. Then the progress and main problems on preparation and properties of low-dimensional perovskite ferroelectric oxides have been summarized in detail. In order to resolve the problems of0D ferroelectric oxides in large-scale preparation and controllable growth,0D PT nanocrystals on Si substrates were synthesized by modified sol-gel method. And0D polyhedral BT nanoparticles were synthesized by hydrothermal process. Compared to0D perovskite ferroelectric oxides, the preparation of2D ferroelectric oxides by wet-chemical method remains a great challenge. The free-standing PT nanoplates have been synthesized for the first time by a facile hydrothermal method. Crystal structure and microstructure of these nanomaterials have been investigated by different approaches, such as X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and atomic force microscope (AFM). And ferroelectricity, piezoelectricity and catalytic properties of these nanostructures were explored.The main contents are described as follows:(1) PT nanostructure on Si (100) substrates was synthesized by heat treatment of the amorphous powder precursor ethanol suspension. The precursor was obtained by the sol-gel and the subsequent freeze-drying process. XRD results displayed that the PT nanostructures show an oriented growth of the PT nanocrystals along [001]. AFM results revealed that an "aggregation-self-assembly-coarsen" process could be responsible for the formation of PT nanocrystals. During the crystal growth process, a novel intermediate, which was formed by self-assembly of eight oxide nanocrystals, were observed firstly. On the basis of above formation mechanism, the acac molecules were induced to sol of tetrabutyl titanate to controll the self-organization of the amorphous particles and therefore mediated crystal growth of PT nanocrystals on the substrates. Furthermore, piezoelectric force microscopy (PFM) investigations showed that the PT nanocrystals have obvious ferroelectricity and piezoelectricity. (2) Free-standing single-crystal and single domain ferroelectric PT nanoplates can be synthesized by a facile hydrothermal method for the first time. XRD and HRTEM results demonstrate that these PT nanoplates can be indexed to tetragonal-phase perovskite PT. The {001} crystal planes are prevailent in the PT sample. The PT nanoplates have a side length of600～1100nm, a height of～150nm, and an aspect ratio of about6:1. High-angle annular dark-field scanning transmission electron microscopy (HAADF STEM) and DC-EFM results showed that the "self-templated" mechanism was the main mechanism for the formation of PT nanoplates. Lead oxides (Pb3O4) are produced immediately in the alkaline solution and form the backbone of the nanoplate. TiO2particles are then absorbed and embedded within the lead oxide templates, and thin nanoplates are formed. Thin nanoplates then attach and stack along the vertical direction to the thin nanoplate, a process probably induced by electrostatic forces. Finally, perovskite-phase PT nanoplates are formed by the surface reconstruction and Ostwald repening process of thin nanoplates.(3) The effect of experimental condition, such as the concentration of mineralizer, the reaction time and temperature and the concentration of materials, have been studied systematicly on the formation of PT nanoplates. Using P25TiO2as titanium source,6M-8M KOH concentration and1.25:1～1.5:1of Pb/Ti molar ratio were determined to be desired condition to prepare PT nanoplates when the reaction temperature is below220℃.(4) DC-EFM results demonstrated that dark and bright regions images that correspond to the positively and negatively polarized domains were obtained in the amplitude images, which indicates that the PT nanoplates are single-domain ferroelectric nanoplates. Domain reveral depends on the magnitude and duration of the applied tip bias Vdc.The scan rate is0.5Hz. The voltage which made the domain reversal was10V and the voltage of the read of the all domains was1V. And the read and writte scale was500nm×500nm.(5) Pt/PT nanoparticles were prepared by impregnation-reduction method. PT nanoplates and H2PtC16.6H2O were used as the starting materials and NaBH4was used as the reductant. The catalytic activity of ferroelectric PT nanoplates and Pt/PT nanoplates for the oxidation of CO were evaluated. For the pure PT nanoplates, the CO conversion into CO2starts around110℃and increases slowly up to25℃. At least80%conversion is observed at250℃. For Pt/PT nanoplates, the CO conversion into CO2starts around60℃and increases significantly from70℃to100℃, with100%conversion occurring at100℃.(6) Cubic-phase-dodecahedral and tetragonal-phase-decaoctahedral BT nanocrystals were synthesized by a facile hydrothermal method. TEM and HRTEM results displayed that the dodecahedral nanocrystal with exposed{110} facets shows a hexagonal projection and the decaoctahedron nanocrystal with exposed{110} and{100} facets shows an octagonal projection. Based on the study of hydrothermal reaction time, BT polyhedral nanocrystals were suggested to grow via an "Ostwald ripening"(OR) mechanism. And the products consist of cubic-phase and tetragonal-phase BT nanocrystals and it is difficult to obtain100%pure tetragonal-phase BT nanocrystals. The high concentration OH" ions could serve as mineralizer and surfactant to promote the formation of BT polyhedral nanoparticles.(7) The shape and size of perovskite PT, BT et.al nanocrystals can be tailored during hydrothermal synthesis by using K2Ti6O13nanowires as the precursor. By changing alkaline concentration, reaction temperature and time, the tetragonal phase PT nanoparticles experienced an aggregation of plate-like particles (～200nm), irregular plate (～900nm), regular rectangular plate (～600nm) and then trapezoidal platform particles (～600nm). The perovskite BT nanoparticles developed from tetragonal-phase BT dendrites (～600nm), cubic-phase BT cubic particles composed of aggregated of small particles (～350nm), cubic-phase hexapod (6-pods) shape particles composed by aggregation of small particles (～350nm) to tetragonal-phase dodecahedral particles (～200nm). Simultaneously, the monoclinic structure of K2Ti6O13nanowires was synthesized by using the amorphous precipitation of titanium hydroxide from a highly disordered raw material. The results reveal that the formation mechanism for titanate nanostructures is dissolution/recrystallization model.