Stud On Hydrothermal Synthesis, Characterization And Properties Of Perovskite Oxide Micro-and Nanostructures

Perovskite oxides have found wide applications in various fields because of the piezoelectric, ferroelectric and high-temperature superconductive properties. Therefore, exporing the controllable synthesis of perovskite oxides and the correlation between their microstructure and their property is of significant importance for providing theoretic guidance to practical applications, which is fundamental scientific values.In this dissertation, the crystal structure characteristics and the property of the perovskite oxides were reviewed in brief firstly. And then the characteristics of BiFeO3, BaTiO3and CaTiO3and their preparation methods have been summarized and commented in detail. However, the difficulty in obtaining pure phase BiFeO3remains a challenge in scientific research. Focusing on this problem, we report on the successfully synthesis of pure phased BiFeO3by a hydrothermal method. Moreover, the preparation of monodispersed anisotropic single-crystal perovskite ferroelectric materials remains a problem. Focusing on this problem, it was reported that for the first time, we successfully synthesized monodispersed2D single-crystal BiFeO3microplates and monodispersed hollow-structured single-crystal BaTiO3nanoparticles. Thirdly, the difficulty in the preparation of nanoceramics with high dielectric constant inspired us to successfully prepare nanoceramics with high dielectric constant by using spark plasma sintering method. By employing a variety of characterization approaches, such as XRD, SEM, HRTEM and FT-IR, comprehensive and in-depth investigations on the phase, microstructure, growing process and the formation mechanism of crystal growth has been performed. The main contents and results are listed as follows:(1) The optimal hydrothermal experimental parameters to synthesized pure phase BiFeO3have been achieved. Ca-doped BiFeO3has been successfully synthesized by the hydrothermal method. The resultant product is pure-phased BiFeO3polyhedrons with a size distribution in a range of150～180nm. The Ca doping is favorable for the improved photocatalytic and magnetic property of the product. In particular, the most efficient photocatalytic activity was discovered when the Ca doping concentration was20%, which100%degradation of RhB can be achieved after4.5h of illumination. Moreover, the product exhibited a remarkable ferromagnetic property at ambient temperature, which the remnant magnetization was determined to be Mr=3.8×10-3emu/g, and the coercive field was288Oe.(2) Monodispersed single-crystal BiFeO3microplates have been successfully synthesized by using C6H10BiNO8as both starting material and surfactant. The optimal condition was determined to be Bi/Fe molar ratio of1:1, the concentration of KOH of0.4M, the stirring time for precursor of12h, ultra sonic dispersion time of20min, the hydrothermal time and temperature of12h and200℃. The as-synthesized BiFeO3microplates adopt perovskite orthorhombic structure (JCPDS86-1518) with a regular rectangle shape. The exposed facet of the as-synthesized BiFeO3microplates was determined to be{012} with a lateral size of about8μm and a thickness of510-550nm. Investigations on time serial hydrothermal experiments reveal that the organic ligand in the starting material of CeH10BiNO8was selectively absorbed on the surface of Bi25FeO40nanoparticles as well as BiFeO3nanoplates, resulting in the2D oriented aggregation procedure and a subsequent self-assembly process.The BiFeO3nanoplates were firstly formed by the fast formation of Bi25FeO40nanoparticles and their2D oriented aggregation. Secondly, a self-assembly process occurred in the direction vertical to the BiFeO3thin plates, leading to the formation of the sphereical-shaped BiFeO3microplates with a coarse surface. Thus in the final process, BiFeO3microplates with smooth surface were obtained by a surface structure reconstruction and Ostawald ripening process of the above as-synthesized BiFeO3microplates with coarse surface.(3) The measurement on the optical adsorption reveals that the BiFeO3microplates have a band gap value of1.88eV, smaller than that of the reported value (2.2-2.8eV). The magnetic measurement shows that the as-synthesized BiFeO3microplates have weak ferromagnetism. Then, BiFeO3microplate/PVDF composited film (the mass load of BiFeO3microplates was set as30%) on ITO substrate was prepared by a spin-coating method. The as-prepared BiFeO3microplates/PVDF composite film exhibited a permittivity of～210in the frequency range of1000Hz～1MHz, which is far larger than that of pure phase BiFeO3(BiFeO3ceramics, εr<130; BiFeO3thin films, εr<1～10) in the previous reports.(4) Monodispersed BaTiO3nanoparticles have been synthesized via a sol-gel combined with a hydrothermal method. And then BaTiO3nanoceramics were prepared by using the above BaTiO3nanoparticles with a size of10～40nm as feedstock via a spark plasma sintering method. The as-sintered BaTiO3nanoceramics consists of both cubic and tetragonal perovskite BaTiO3(JCPDF31-0174and05-0626, respectively). The density of the BaTiO3nanoceramics is above90%and the particle size distribution is in the range of10～500nm. The results from the dielectric measurement reveal that the dielectric constant of BaTiO3nanoceramics in the low frequency range is up to105, and the relaxation frequency was above105Hz with a dielectric loss lower than0.06. Furthermore, the relaxation frequency and corresponding peak dielectric loss both shift towards high frequency as the density of the BaTiO3nanoceramics increased. The temperature-dependence of dielectric measurement reveals that the dielectric permittivity of the BaTiO3nanoceramics decreased as the frequency increased at the same testing temperature.(5) Monodispersed and hollow-structured single-crystal BaTiO3nanoparticles have been synthesized via a hydrothermal treatment at200℃for6h when the molar ratio of starting materials of TiO2and Ba(CH3COO)2was set as1:3and the concentration of KOH was4M. The particle size of the as-synthesized BaTiO3nanoparticles was in the range of50-90nm and are well dispersed. the BET surface area of the powder BaTiO3nanoparticles has been deteremined to be27.01m2g-1on the basis of N2adsorption-desorption measurement. Comprehensive investigations on the microstructure and growing process of the BaTiO3nanoparticles have been performed, indicating that the hollow-structured BaTiO3nanoparticles should experience a gradient crystallization process.(6) Mesoporous orthorhombic perovskite CaTiO3(JCPDS22-0153) has been successfully synthesized via a sol-gel method by the self-assembly of nanoparticles. The BET surface area was anlysized to be in the range of2-40m2/g with the average pore size of18～34nm. The formation mechanism of the as-synthesized CaTiO3was proposed. Firstly, the molecules of P123were coated on the surface of the nucleates that enabled a regular alignment of these nucleates; then the excess CaCO3reacted with HNO3solution to form Ca(NO3)2and CO2, thus leading to the pore formation within the previous sites occupied by CaCO3to form porous CaTiO3.