Study On Preparation, Microstructure And Applications Of Perovskite Oxide Ferroelectric Nanocomposites

Owing to their fascinating chemical and physical properties, ferroelectic nanomaterials have promising applications in various fields, such as memory devices, nanogenerators, optoelectric devices and photocatalysts. Thus, exploring the design, fabrication and relation between their microstructure and properties of ferroelectrics nanocomposites are highly important for further development of novel functional materials.In this dissertation, we firstly reviewed the crystal structure, preparation, properties and applications of ferroelectrics. In particular, we summriezed the present status and development prospects of ferroelectric based nanocomposites. However, the interfacial effect between ferroelectric and other materials （semiconductors, graphene, etc.） has been rarely investigated in the previous work. The systematic study focused on the interfacial effect modulated properties of nanocomposites remains a challenge. In this perspective, several typical ferroelectrics had been used to design and fabricate ferroelectric based nanocomposites. Based on microstructure and properties characterization, we focused on investigating the relations between ferroelectric interfaces and the properties of the nanocomposites. Firstly, we used single-crystal perovskite PbTiO₃ nanofibers and ZnO nanowires as pristine materials to design PbTiO₃/ZnO nanocomposites via hydrothermal process. And we used single-crystal perovskite PbTiO₃ nanofibers and CdS nanoparticles as pristine materials to design PbTiO₃/CdS nanocomposites. Then, we used graphene as two-dimentional matrix to fabricate BaTiO₃/Graphene and SrTiO₃/Graphene nanocomposites via solvothermal process. By using serial analytic approaches, comprehensive and systematic investigations on interfacial microstructure, ferroelectric polarization effect, piezo-optonic coupling and visible-light-driven photocatalysis of the as-prepared composites have been performed. The main contents and results are listed as follows:（1） PbTiO₃/ZnO nanocomposites were firstly prepared via a hydrothermal process. PbTiO₃/ZnO nanocomposites consist of ZnO branches with a diameter of 50nm and the length of 1μm grown on perovskite PbTiO₃ nanofibers. Based on HAADF-STEM and TEM characteriazation, the interfacial relationship between the{1000} plane of ZnO and the{101} plane of PbTiO₃ with a low lattice mismatch of 1.06% was revealed. And single-crystal ZnO grew epitaxial along the c axis on the surface of PbTiO₃,giving rise to an effective ferroelectric/semiconductor interface.（2） By using a PET flexible substrate, flexible PbTiO₃/ZnO nanocomposites were firstly designed and prepared. According to PL measuremnt, the samples display a direct band emission in UV region （λ=384nm） and a defect emission in visible region （λ=623nm）. In order to evaluate the ferroelectric effect on the PL properties of the as-synthesized composite, the sample was subjected to a bending force and excitation simultaneously. Interestingly, when a bending stress was loaded on the as-grown sample, the PL intensity of UV emission in the composite was enhanced with the increase of the bending curvature. When bending central angle was increased to a=190°, the intensity ratio of the UV to visible emission （Iuv/IVIS） raised to 8 times as that of the sample without bending. This intriguing phenomenon can be understood by the bending induced polarization field from the PbTiO₃ nanofibers, which could reduce the bending degree of the energy band of the ZnO nanowires through the interface. When a bending stress was loaded on the sample, a polarization field pointed to the side of the ZnO nanowires could be established along the axial direction of the PbTiO₃ nanofibers. Meanwhile, the positive bound charge at the ferroelectric/semiconductor interface will attract more negative charge on the surface of ZnO nanowires, which can lower the bending degree of the energy band and the width of the depletion region （Wd）. As a result, under UV laser illumination, the recombination of photo-generated electron-hole pairs in a bent PbTiO₃/ZnO nanocomposites could be promoted and hence lead to an enhanced intensity of UV emission. （3） Single-crystal heterostructured PbTiO₃/CdS nanocomposites were fabricated by a hydrothermal process. In the composites, well-crystalline CdS nanoparticles with diameters of 20-80nm grew on the surface of tetragonal perovskite PbTiO₃ nanofibers. Through the HAADF-STEM and TEM characterizations, there were large-scale sharp interfaces between PbTiO₃ and CdS in the composite. It was revealed that the as-synthesized heterostructured PbTiO₃/CdS nanorods exhibited higher first reaction rate in degradation of methylene blue（MB） under visible light irradiation （λ≥400nm） over pristine PbTiO₃ nanorods （1.7 times） and P25 （4.25 times）. The enhanced performance is associated with the proper band alignment and promoted separation of photogenerated carriers by forming shaip interfaces between PbTiO₃ and CdS in the heterostructure.（4） SrTiO₃/Graphene nanocomposite was synthesized by one-pot solvothermal process. In the SrTiO₃/Graphene nanocomposites, single-crystal SrTiO₃ nanoparticles with average size of 50nm were loaded on the micro graphene sheet matrix and formed large scale of continuous interfaces with graphene. It was found that SrTiO₃/Graphene nanocomposites demonstrated 5.5 times higher first reaction rate over pristine SrTiO₃ nanoparticles in degradation of various organic pollutants under visible light irradiation. The high charge transport properties and typical two-dimenetional structure of graphene is crucial for facilitating the separation and transport of photogenerated charge carriers, and hence inducing strongimprovement for the photocatalytic properties of the nanocomposites.（5） A high efficient visible-light-driven BaTiO₃/Graphene nanocomposite was firstly synthesized by one-pot solvothermal process. In the BaTiO₃/Graphene nanocomposites, single-crystal BaTiO₃ nanoparticles with average size of 20nm were anchored on the micro graphene sheet. Based on HAADF-STEM, TEM and second harmonic generation （SGH） measurements, it was revealed that the BaTiO₃ nanoparticles in the composites adopt a single ferroelectic domain and had large scale of high-quality interfaces with graphene. Furthermore, the XPS and Raman spectra of the samples confirmed the formation of chemical bonds between BaTiO₃ nanoparticles and graphene; about 73% of the carboxyl groups in the graphene oxides had been eliminated, implying the reduction of graphene and possible formation of Ti-O-C bond during solvothermal process.（6） BaTiO₃/Graphene nanocomposites demonstrated a dramatic degradation performance of various organic dye pollutants under visible light irradiation. Such composites demonstrated a dramatic degradation performance of various organic dye pollutants under visible light irradiation, which could degrade more than 80% dye molecules （MB, MO and Rh B） after 20 min irradiation of visible light. Compared to those of pure BaTiO₃ nanoparticles and SrTiO₃/Graphene nanocomposite, BaTiO₃/Graphene nanocomposite has a 9-fold and 2.5-fold enhancement for the first reaction rate of dye pollutants under visible light irradiation. The synergic effect of polarization effect in BaTiO₃ and high charge transport properties in graphene is crucial for facilitating the separation and transport of photogenerated charge carriers, and hence inducing significant enhancement in the photocatalytic properties of the nanocomposites.