The Synthesis, Structure And Properties Of Perovskite And Pre-Perovskite Oxide Nanomaterials

Perovskite type oxides have been the focus of many researches because of their fruitful physical properties,and the notable examples are ferroelectrics such as PbTiO₃ and PZT,the superconducting copper oxide(e.g.(La,Ba)₂CuO₄)and manganite perovskites having colossal magnetoresistances(e.g.(La,Ca)MnO₃).Perovskite oxide functional materials are not only the essential content for condensed-matter physics study,but also afford wide practical applications,significantly leading to the advancement of many industrial techniques.With the minimization of devices,there is growing interest in preparing perovskite oxide nanostructures and exploring their properties,which is of great importance for both of fundamental research and technical applications.In this dissertation,the current status of studies on typical perovskite ferroelectric oxide PbTiO₃ and PZT was reviewed briefly at first.The research progress and main problems about preparation and properties of perovskite ferroelectric oxide nanostructures have been summarized,and the methods used to synthesize perovskite oxide nanomaterials have also been summarized and analyzed.Based on this analysis,a novel polymer-assisted hydrothermal route is proposed for the first time to prepare one-dimensional nanosized perovskite oxides.By this method,various perovskite and pre-perovskite oxide nanostructures have been successfully synthesized,including nanodots,single-crystalline nanorods and nanowires of PZT,single-crystalline nanotubes of pre-perovskite PbTiO₃ and self-assembled superstructures of PbTiO₃ nanocrystals doped by transition metal elements.Crystal structure and microstructure of these nanomaterials have been investigated by different analysis methods such as neutron powder diffraction,combined with computer simulations.And many properties of these nanostructures have also been intensively explored,such as mechanics, electromechanical coupling,electron transport,ferroelectricity,dielectricity and magnetism. The main contents of the dissertation are described as follows:(1)Single-crystalline nanorods and nanowires of PZT have been prepared for the first time by a polymer-assisted hydrothermal method using PVA or PVA/PAA as surfactant.The mechanism for the formation of one-dimensional PZT nanostructures has been proposed.The polymer molecules adsorbed on the surfaces of PZT nanocrystals by weak hydrogen bonding or strong chemical interaction,which modified the growth rate of different facets and thus led to orientation growth of PZT nanocrystals.Such process resulted in the formation of PZT nanorods or nanowires as well as shape control of PZT nanocrystals. (2)Extending a polymer-assisted hydrothermal method to PbTiO₃ system,a new structured single-crystalline PbTiO₃ nanotube has been synthesized for the first time.Such new structure is referred to a "pre-perovskite" because its transformation to perovskite PbTiO₃ under certain condition.Different analysis methods including neutron powder diffraction have been employed to refine crystal structure of pre-perovskite PbTiO₃,and a tetragonal unit cell with a=12.37(?),c=3.81(?)and space group I4/m(87)was found to be the most promising indexing scheme.According to crystal structure parameters,a large unit cell of the pre-perovskite PbTiO₃ containing 40 atoms with Pb:Ti:O=1:1:3 can be readily constructed,in which each pair of TiO₆ sharing an edge is stacked upwards along c axis in an interlaced manner with next pair to form 1D TiO₆ octahedron columns.The TiO₆ octahedron columns are linked respectively by Pb-O covalence.Prep-perovskite PbTiO₃ nanotube grows along c axis of pre-perovskite structure and its length can be up to several hundred micrometers. Prep-perovskite PbTiO₃ nanotube presents cubic facets with outer diameter in range of 100nm～300nm and inner one of 10nm～50nm,and the exposed facets of the nanotube is {110} crystalline plane.(3)Controllable bending behavior of pre-perovskite PbTiO₃ nanotubes has been observed by TEM.One-end-suspended pre-perovskite PbTiO₃ nanotube can be bent up to 180°reversibly and repeatedly when tuning the electron beam intensity in TEM and the bent angle of the nanotube is determined by the strength of the electron beam intensity.Especially,the bending occurs on a certain direction,{110} crystal planes of the nanotubes.Such bending behavior indicates that pre-perovskite PbTiO₃ nanotubes are remarkably flexible and resilient. The driving force for the bending behavior is expected to be an electromechanical coupling between electric dipoles and an electric field.This electromechanical coupling of pre-perovskite PbTiO₃ nanotubes is further illustrated by in situ direct-current(DC)electric field-driven deflection using a probe in TEM and DC electric field-driven rotation experiment in ethanol solution,which also support the bending mechanism.(4)Typical polarization hysteresis loops have been observed in single pre-perovskite PbTiO₃ nanotube and nanotubes/PVP composited films,indicating that pre-perovskite PbTiO₃ nanotubes are good ferroelectrics.But ferroelectricity of pre-perovskite PbTiO₃ nanotube conflicts with its centralsymmetry crystal structure.Ferroelectricity of pre-perovskite PbTiO₃ nanotube could arise from a local crystal structure breaking induced nanoscale effect,internal stress and domain structure.Dielectric property of pre-perovskite PbTiO₃ nanotube as a function of temperature at different measurement frequency has been studied.The peak near 520℃can be well fitted by Curie-Weiss law with T_c=793K,and this peak should correspond to a ferroelectric phase transformation.(5)In-situ XRD study indicates that in air pre-perovskite PbTiO₃ firstly transforms into cubic perovskite PbTiO₃ when temperature increases,and then cubic perovskite PbTiO₃ transforms into tetragonal perovskite PbTiO₃ as temperature decreases to room temperature. During this process,1D column structure of TiO₆ octahedron of pre-perovskite PbTiO₃ develops into 3D network structure of TiO₆ octahedron of perovskite PbTiO₃.After annealed in air,single-crystalline pre-perovskite PbTiO₃ nanotube can transform into single-crystalline perovskite PbTiO₃ nanotube,and the 1D morphology of the nanotube can be basically retained when the phase transformation is completed.Pre-perovskite PbTiO₃ nanotube grows along[001]direction,while perovskite PbTiO₃ nanotube grows along[010]direction of tetragonal perovskite structure.(6)Fe-doped PbTiO₃ nanocrystals have been prepared by PEG-assisted hydrothermal method.As Fe doping content increases,the magnetism of PbTiO₃ nanocrystals develops from diamagnetism to ferromagnetism and paramagnetism.Due to Fe doping,a fine structure of Fe³⁺-Vo^2--Fe³⁺forms in PbTiO₃,and an F-center exchange mechanism based on such fine structure could be the origin of room-temperature in Fe-doped PbTiO₃ nanocrystals.In addition,the presence of PEG as a surfactant has promoted the self-assembly of Fe-doped PbTiO₃ nanocrystals,which significantly enhances room-temperature ferromagnetism Fe-doped PbTiO₃ samples.The morphology of PbTiO₃ nanocrystals develops from irregular aggregation to regular 1D ring-like and planar-like assembled superstructures on nominal Fe doping content increasing.These assembled structures demonstrate single-crystal-like behavior due to oriented-attachment growth between Fe-doped PbTiO₃ nanocrystals that imperfectly share the same crystallographcial plane.