Study On Hydrothermal Synthesis, Characterization And Properties Of Perovskite Oxide Nanostructures

Perovskite oxides have been the focus of many researchers because of theirfruitful physical properties, such as ferromagnetism, ferroelectricity, piezoelectricity,colossal magnetoresistances, and so on. As we all knew that nanomaterials exhibit awide range of physical and chemical properties that depend sensitively on both sizeand shape, and are of both fundamental and technological interest.In this thesis, the structure and synthesis method of the perovskite oxides werefirst introduced, then the progress of the dendrites and multiferroic materials weresummarized detailedly. In order to resolve the problems, such as the weakferromagnetism of BiFeO₃, the sparsity of single phase multiferroic materials innature and the controllable growth of the perovskite oxides, we made a detail researchon the perovskite oxides, such as BiFeO₃, Na_0.5Bi_0.5TiO₃, BaTiO₃, SrTiO₃, andinvestigated detailedly the hydrothermal condition, factors of controllable growth andthe magnetic properties by doping.The main results include:1. Bismuth nitrate and iron nitrate were used as the starting materials and potassiumhydroxide (KOH) was used as mineralizer. The influence factors were investigatedduring the hydrothermal synthesis of BiFeO₃. It was found that the obtained productswere determined by KOH concentration. When KOH concentration was 7M and 12M,BiFeO₃ particles with a diameter 100-300nm were prepared. As KOH concentrationwas low, the as-prepared products were perovskite BiFeO₃ and Bi₂Fe₄O₉ with anorthorhombic structure. When KNO₃ was introduced during the hydrothermal process,BiFeO₃ nanoparticles were synthesized due to the adsorption of NO₃^- ions on thenucleus. In addition, BiFeO₃ nanoflakes were obtained as the reaction time wasprolonged, which was well assistant with the growth rule of Ostward2. The ferromagnetism of BiFeO₃ was noticeably improved by the doping oftransition metal ions (Co²⁺ ions or Ni²⁺ ions). BiFeO₃ exhibited a strongestferromagnetism when Co and Ni doping concentration was 5% and 0.5%, respectively, and the saturation ferromagnetism was 2emu/g and 0.8emu/g, respectively. Themechanism was also explained, which was that the G-type anti-ferromagnetismstructure in BiFeO₃ was destroyed by the doping, and net ferromagnetism wasobtained. In addition, the ferromagnetism was not increased as the dopingconcentration because of the weakness of Co and Ni atomic magnetization comparedto Fe atomics.3. Controllable growth of Na_0.5Bi_0.5TiO₃ could be realized by the choice of differentprecipitator. Na_0.5Bi_0.5TiO₃ crystallites with a spheric morphology were preparedwhen NaOH was used as precipitator. However, Na_0.5Bi_0.5TiO₃ microcubes wereprepared when Na₂CO₃ was used as precipitator, and its morphology was similar tothe cell morphology, which may be due to the average surface growth speed ofNa_0.5Bi_0.5TiO₃ crystallites.4.Based on the research of diluted magnetic semiconductors (DMSs).Na_0.5Bi_0.5TiO₃ crystals doping transition metal ions (Fe or Co) in the perovskitestructure showed weak ferromagnetism. As transition metal ions doping contentincreased, the magnetism of Na_0.5Bi_0.5TiO₃ nanocrystals developed fromdiamagnetism to ferromagnetism and paramagnetism. It is suggested that theferromagnetic component was not growing as Fe or Co doping increasing. This couldbe attributed to the competition between ferromagnetic and paramagneticcontributions to the magnetism of the sample. The mechanism of the origin offerromagnetism in this material was explained. Ferromagnetism dominated whendoping concentration was low, while paramagnetism dominated in the case of highdoping concentration.5. Single-crystal BaTiO₃ dendrites were controllably synthesized by adjusting KOHconcentration. When KOH concentration was low, BaTiO₃ dendrites were obtainedbecause the growth of BaTiO₃ nucleus was in the state far away from equilibriumstate. As KOH concentration was increased gradually, the morphology of BaTiO₃varied from dendrite, hexagon to sphericity.6. Single-crystal SrTiO₃ dendrites were controllably synthesized by adjustingKOH concentration. When KOH concentration was low, SrTiO₃ dendrites were obtained. As KOH concentration was increased gradually, the morphology of SrTiO₃varied from dendrite, quadrangle to sphericity. The model of the growth units of anioncoordination polyhedra can explain the growth mechanism of SrTiO₃ dendrites.During dendrite growth, the anion coordination polyhedra as growth units areconnected with the structure units on the crystal surface along the direction of stablecombination. The anion coordination polyhedra (Ti-O₆) as the growth units of SrTiO₃perovskite structure connected along the direction of corner. The coordinationpolyhedra has two corners to connect with other coordination polyhedra along [110],while it has only one corner along [100], therefore, the SrTiO₃ dendrites grew along[110] because of the more stable character along [110] compared to that along [100].