Study On The Surface, Interface And Properties Of Single-crystal Nanostructures Of Perovskite Ferroelectric Oxide

Single-crystal perovskite ferroelectric oxide nanostructures have attracted extensive attention because of their unique physical properties and fascinating ferroelectric surface chemistry, based on which promising applications in high-density information storage, energy conversion and catalysis are predicted. Therefore, studies on controllable synthesis, surface/interfaces and properties of single-crystal perovskite ferroelectric oxide nanostructures are highly important for of both theoretical and practical interest.In this dissertation, the crystal structure of perovskite oxides was reviewed firstly. Then the microstructure, spontaneous polarization and the screening-related ferroelectric surface chemistry of perovskite ferroelectric oxides, especially the stability mechanism of single-domain perovskite PbTiO3 (PTO) nanostructures and their interface chemistry have been emphasized and summarized in detail. Moreover, the synthesis and up-to-date research status on ferroelectric oxide nanostructures have been discussed. Especially, the faceted perovskite ferroelectric nanocrystals, chemistry of ferroelectric surface and the effect of ferroelectric polarization on the molecular adsorption, the growth of noble metals on the ferroelectric surface as well as their catalytic properties have been documented and analyzed comprehensively. Accordingly, faceted perovskite PTO nanocrystals and STO/PTO nanocomposites have been synthesized by solid-state-reaction (SSR) and hydrothermal method in our research. Subsequently, the microstructure, chemical state and stability of the exposed surface and the photocatalytic property of the faceted perovsktie ferroelectric oxide nanostructures have been explored systematically. Furthermore, the effect of the polarization of the PTO surfaces on the growth of noble metal nanostructures, their CO catalytic oxidation property and the room-temperature ferromagnetism at STO/PTO interface have been investigated and discussed. The mechanism generating the ferromagnetism in the STO/PTO nanocomposite that both components are non magnetic has been proposed on the basis of ferroelectric polarization and interface defects. The main contents are generalized as following.(1) Octahedral-shaped single-crystal perovsktie PTO nanostructure (PT OCT) have been synthesized by an inorganic metal oxide salt-assisted hydrothermal method for the first time. PT OCT nanocrystals have a size of 50-100 nm with exposed facets of {111} and a Curie temperature at 485.56 ℃. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and electron energy loss spectroscopy (STEM-EELS) results showed there exists a lithium concentrated layer with ～2 nm in thickness closed to the PT OCT surface, while no lithium signal can be detected in the inner side of the PT OCT nanocrystals. Li-O bonding formed on the surface has been determined to be crucial for the formation and stability of the single-crystal PT OCT.(2) The crystal growth process of the single-crystal perovskite PT OCT nanostructure can be ascribed to be an oriented attachment (OA) mechanism. Firstly, tetragonal perovskite PTO nanoparticles with size of 2-4 nm were formed at an initial stage of the hydrothermal reaction. Then, the aggregations of these primary PTO nanoparticles were formed under the co-effect of electrostatic force of the particle surface, the presence of Li+ions and Van der Waals force, where the PTO particles gradually adjusted the orientation to achieve the lowest surface energy and formed the final octahedral shaped PTO nanocrystals. During the formation of basic OCT morphology, Li+was diffused from inner PT OCT nanostructure towards the surface and concentrated on the surface. The concentration of Li+ on the surface of PT OCT was determined to play a key role in stabilizing {111} plane of PT OCT.(3) PT OCT nanostructure has been demonstrated to exhibit an excellent photocatalytic property under visible-light irradiation. Blue MB aqueous solution of 10-5 M can be decomposed into colorless solution within 60 min by 0.1 g PT OCT and the first-order reaction constant K was estimated to be 0.042 min-1, an order of improvement compared to those of other perovskite oxide nanostructures under the similar experimental condition. UV-VIS absorption spectrum revealed an obvious absorption in the visible-light range of 500-700 nm for the PT OCT, and the corresponding band gap of the PT OCT was determined to be 2.58 eV, much lower than that of the bulk PTO (2.8～3.0 eV). In addition, electronic spinning resonance(ESR) measurement suggested that a localized state of Ti3+ defects may have been formed in the band structure of the PT OCT nanostructure, leading to a decrease in the band gap width, enhanced absorption within visible light range of 500-700 nm and thus a high visible-light photocatalytic performance.(4) Truncated octahedral shaped perovskite PTO nanocrystals have been prepared successfully with a size of 25-50 nm and well-dispersion by a SSR method. A homogeneous nucleation-crystal growth has been proposed to explain the formation mechanism of the perovskite PTO nanocrystals, where a quasi-liquid environment was attributed from a local melting of Pb3O4. HAADF-STEM and TEM tomography identified that the as-synthesized PTO nanocrystals have a truncated octahedral shape with{111} and {011} exposed mainly, accompanied by a low ratio of {100}.(5) Pt-PTO nanocomposite has been synthesized with truncated octahedral PTO nanocrystals as substrate by a facile wet-chemical route. Microstructure characterization of the Pt-PTO revealed that well dispersed single-crystal Pt nanoparticles of 2-5 nm were selectively deposited on the {111} surface of the truncated octahedral PTO nanocrystals. By using such nanocomposite as catalyst, the result of CO catalytic oxidation measurement showed that the starting temperature of CO oxidation reaction was as low as～30 ℃ and a 100% CO conversion was achieved at ～50 ℃. The conversion temperature implies that the Pt-PTO nanocomposite has excellent low-temperature reactivity towards CO oxidation.(6) Pt nanoparticles were successfully deposited on three kinds of perovskite PTO nanostructures, including truncated octahedral shaped nanoparticles, nanofibers and nanoplates, respectively, via a wet chemical reaction. The exposed facets of the three PTO nanostructures have previously been determined mainly to be {111}, {100}/{010} and {001} planes. Microstructure characterization revealed that the size of Pt increased from 3-5 nm,5-20 nm to～100 nm on the {111},{100}/{010} and {001} planes of the PTO nanostructures, respectively. Perovskite PTO nanoparticles, nanofibers and nanoplates without Pt loading were employed as catalysts for CO oxidation, in which the CO conversion rate was determined to be 60%,5% and 85% at 250 ℃, respectively. The perovskite PTO nanoplates exhibited the highest catalytic activity while the PTO nanofibers exhibited the lowest one for CO oxidation. The main mechanism leading to a difference in CO catalytic reactivity can be understood by a polarization effect of the PTO nanostructures. Considering PTO nanostructure as the CO oxidation reaction center, the reaction energy of the CO oxidation can be decreased on a highly polarized facet of PTO nanostructures, which is favorable for the catalytic reactivity.(7) The full conversion temperature from CO to CO2 was measured to be 50 ℃, 100 ℃ and 100 ℃, respectively for Pt-loaded PTO nanoparticles, nanofibers and nanoplates. And the apparent activation energy (Ea) was estimated to be 22.9(±0.4) kcal/mol,32.7(±2.9) kcal/mol and 26.5(±1.6) kcal/mol, respectively. When Pt-PTO nanostructures were used as catalysts, Pt was served as the catalytic reaction center, thus the microstructure and surface chemistry of Pt nanocrystals dominated the reaction-kinetics. Single-crystal Pt nanocrystals of 3-5 nm grown on truncated octahedral PTO nanoparticles were mono-dispersed. However, Pt nanocrystals on PTO nanofibers and nanoplates were observed to be seriously aggregation of tens of nanometers in size, resulting in less reactive sites on the surface of Pt nanocrystals.(8) Perovskite SrTiO3 (STO) epitaxially grown on single-crystal and single domain ferroelectric PTO nanoplates to form the STO/PTO nanocomposite by a hydrothermal method. TEM and Cs-TEM investigation showed that perovskite STO was selectively grown on the four non-polar lateral facets of PTO nanoplates and positive polar facet of {001} plane, forming a core-shell-like composite. An atomic resolution STO/PTO interface was observed without any mutual diffusion of Pb and Sr atoms across the interface. Microstructure characterization showed that STO with 15-20 nm in thickness was epitaxially grown on both the non-polar and the polar facets of PTO nanoplates in a topological-like manner. The thickness of STO/PTO interface was observed to be 1-2 unit cell (～1 nm) on the positive polar facets and about 1 unit cell (-0.4 nm) on non-polar facets.(9) Obvious room-temperature ferromagnetism of STO/PTO has been observed. The saturation magnetization Ms was estimated to be 2.5×10-3 emu/g at 300 K and increased to be 2.5×10-2 emu/g at 5 K with a corresponding coercive magnetic field enhanced from 138 Oe to 375 Oe. The typical ferromagnetism curve disappeared and resulted in a near-zero magnetization(M) transition when the magnetic field was beyond 5000 Oe at 300 K,150 K and 100 K. Further increasing magnetic field to 2500 Oe, a steep transition occurred and the near-zero M was jumped into a negative section, resulting in a diamagnetism. The ferromagnetism recovered when the magnetic field decreased, indicating that the magnetic transition is reversible. As the temperature decreased, the crucial magnetic field need for the magnetic transition are greatly increased. HAADF-STEM, STEM-EELS and the first-principle calculation revealed that the ferromagnetism for the STO/PTO nanocomposite is tightly related to the large-scale presence of Ti3+ on the interface of STO/PTO growing at the positive polar surface. The investigation on microstructure and ferromagnetism suggests that such STO/PTO composite could be a novel multi-ferroic oxide model to realize the coexistence of ferroelectric and ferromagnetism.