| BiFeO3 is one of several rare single-phase multiferroic materials that are both ferroelectric and weakly ferromagnetic at room temperature. Recent studies demonstrated the BiFeO3 films have spontaneous polarization enhancement, switchable ferroelectric diode effects, photovoltaic effects, piezoelectric and THz radiation properties, which indicate potential applications in next-generation, lead-free, non-destructive memories, spin valve devices, actuators, and ultra-high speed telecommunication devices.Compared to thin films and bulk materials, one-dimensional nanostructured materials have special physical properties. This is due to their anisotropic property and a unique size effect. Theoretical calculations by Glinchuk has predicted that ferroic nanorods or nanowires will show giant magnetoelectric effects as their radii decrease, and then induce abnormal increase in the dielectric tenability and dramatical phase transition. In the experiment, nanowires, nanotubes, and arrays of BiFeO3 have been prepared by using the anodized aluminum oxide (AAO) template method, high voltage electrospinning techniques, surfactant and polymer (polyethylene pyrrole PVP) auxiliary hydrothermal techniques, and so on. All of the products prepared by these methods are polycrystalline structures, which severely affect their properties and applications. With such concerns in mind regarding basic physical properties, the high crystallinity of one-dimensional BiFeO3 nanomaterials is a fundamental characteristic that guarantees their magnetoelectric coupling. To the best of our knowledge, the synthesis of one-dimensional single-crystalline BiFeO3 nanowires has not yet been reported. Among the most likely reasons are that Bi components are volatile at high temperatures, and that bismuth nitrate hydrolyzes strongly in aqueous solutions, forming insoluble bismuth subnitrate or bismuthyl nitrate (Bi2O3·N2O5·2H2O). This latter setback can seriously influences the formation of single-phase BiFeO3, while easily forming the non-stoichiometric ratio components such as Bi2Fe4O9 and Bi12Fe0.63O18.945. Similarly, due to the peculiar structure of BiFeO3, it is still very difficult to prepare single-phase BiFeO3 while simultaneously controlling its components, morphology, and size using typical VS, VLS, and hydrothermal methods. Therefore, it is still a challenge to synthesize one-dimensional, single-phase, single crystal BiFeO3 nanomaterials in large amounts.Different morphologies and microstructures show different properties and unique application prospects, it greatly stimulated research interestings of tunable synthesis of bismuth ferrites with various morphologies. Bismuth ferrites basically have two kinds of structure BiFeO3 and Bi2Fe4O9, another important functional materials, Bi2Fe4O9 have attracted much attention because of its catalytic potential for oxidizing ammonia to NO and for semiconductor gas sensors. Therefore, tuning the morphology and microstructure of bismuth ferrite has important significance for study of ferroelectric, ferromagnetic and magnetoelectricity coupling effect on the fundamental theories and application.In the present doctoral dissertation, the precursors were dissolved in acetone, ethylene glycol, dilute nitric acid and distilled water, respectively.By adjusting the NaOH concentration, we obtained a series of special shape and structure characteristics of single crystalline bismuth ferrites, and studied magnetic properties at room temperature and low temperature. The temperature dependence of the zero-field-cooled (ZFC) and field-cooled (FC) susceptibility shows that single crystal BiFeO3 nanowires, sheets have experienced spin glass transition at low temperature. Due to spin glass transition, the magnetic properties significantly increase at low temperature.The research work is composed of the following parts:1. Tunable synthesis and characterization of bismuth ferrites with different morphologies in distilled water Firstly, the starting materials, [Bi(NO3)3·5H2O] and [Fe(NO3)3·9H2O] in a stoichiometric ratio (1:1 in molar ratios), were dissolved in distilled water, adjusted the pH value between 9 and 11 with ammonia, the sediment was washed to neutral, and then added different concentrations NaOH, the bismuth ferrites with special morphologies have been obtained by using a hydrothermal synthesis. It was found that, with the NaOH concentration increased, the morphology of reaction products evolve from irregular shape of the nanoparticles nano-disk, nanowires, micro triangle, square and hexagonal flakes mixture BiFeO3, the final a thin sheet BiFeO3 andBi2Fe4O9 mixture, and the microstructure transformed from pseudo-cubic BiFeO3 to orthorhombic Bi2Fe4O9. The results showed that the concentration of NaOH during the hydrothermal synthesis of bismuth ferrite can control the morphology and structure of nano-materials.2. Tunable synthesis and characterization of bismuth ferrites with different morphologies in dilute nitric acid solutionIn the first part of the work, based on the introduction of dilute nitric acid in the system, Bi(NO3)3·5H2O was completely dissolved, and it promoted hydrothermal reaction. We also investigated the influence of concentration of NaOH on the morphology and structure of the reaction products. It was found that, with the NaOH concentration increased, the evolution of the morphologies reaction products have gone from spindle-shaped BiFeO3 nanoparticles, BiFeO3 nanorods, hexagonal BiFeO3, thin flake BiFeO3 to Bi2Fe4O9, and the structure also changed from pseudo-cubic phase BiFeO3 to orthorhombic Bi2Fe4O9. The results showed that the introduction of nitric acid solution effectively solves the Bi (NO3) 3·5H2O dissolved the problem of incomplete and greatly changed the morphology and structure of reaction products.3. Tunable synthesis and magnetic properties of bismuth ferrites with different morphologies in ethylene glycolAlthough the introduction of nitric acid solution effectively solves the problems of Bi (NO3)3·5H2O solubility, the product appeared BiFeO3 nanorod, but still did not get a good nanowires, the reason may be that the decreased pH values of the precursor solution caused by the introduction of nitric acid. In the fourth chapter, ethylene glycol, which commonly used as organic solvents in preparing nanoparticles by sol-gel technology, will be introduced into hydrothermal synthesis BiFeO3, without affecting the pH value of precursor solution under the premise of improving the Bi (NO3)3·5H2O solubility . The results show that, the concentration of NaOH has an important role in regulating morphology and structure of bismuth ferrite, a series of different morphologies and structural of the BiFeO3 were obtained. On this basis, the magnetic properties of the circular sheet, flakes and cubic BiFeO3 were studied. For circular sheet, flakes and cubic BiFeO3 obtained at 3 M and 5 M concentration of NaOH, the ZFC and FC curves show that they presence spin glass transition at the low temperature, hysteresis loops show that ferromagnetism of the circular sheet, flakes and cubic BiFeO3 at the low temperature was significantly enhanced than room temperature. For BiFeO3 sample obtained at 8 M concentration of NaOH , the ZFC and FC curves show that it does not exist spin-glass transition, hysteresis loops show paramagnetic properties at both 5 K and 300 K , Bi2Fe4O9 phase appears in BiFeO3 may be the main reason.4. Tunable synthesis and magnetic properties of bismuth ferrites with different morphologies in acetoneThe design and introduction of ethylene glycol system without changing the system pH value of the premise, we achieved the Bi (NO3)3·5H2O completely dissolved, and obtained a series of special structural features of the BiFeO3 nanomaterials. The analysis showed that the initial morphology and structure of the precursor has an important effect on that of the reaction products, and solvent system significantly changed the initial appearance of the precursor. For example, in the aqueous phase, Bi (NO3) 3·5H2O and Fe (NO3)3·9H2O precursor particles were irregular shape of the form, after the introduction of nitric acid, the precursor form of rods and particles. In the ethylene glycol system, the initial appearance of the Fe (OH) and Bi (NO3)3 precursor were rods and particles. Therefore, the solvent would effectively change the initial appearance, and ultimately affect the morphology and structure of the reaction products. In the fifth chapter, we would replace the fourth chapter of the ethylene glycol with acetone, and we investigated the influence of the concentration of NaOH on morphology and structure of the reaction products in acetone system. The results showed that the acetone effectively regulated of the initial appearance of the precursor, single crystalline BiFeO3 nanorods and nanowires were obtained. As NaOH concentration increased, the structure of products gradually transformed from the rhombohedral distortion BiFeO3 into the orthorhombic Bi2Fe4O9. On this basis, the magnetic properties of BiFeO3 nanowires and Bi2Fe4O9 sheets were studied. For BiFeO3 nanowires obtained at 3 M and 5 M concentration of NaOH, the ZFC and FC curves show that they presence spin glass transition at the low temperature, hysteresis loops show that ferromagnetism of BiFeO3 nanowires at the low temperature was significantly enhanced than room temperature. For Bi2Fe4O9 sheets obtained at 8 M concentration of NaOH , the ZFC and FC curves show that Bi2Fe4O9 sheets does not exist spin-glass transition, hysteresis loops of Bi2Fe4O9 sheets show paramagnetic properties at both 5 K and 300 K.5. Hydrothermal synthesis and magnetic properties of single-crystalline BiFeO3 nanowiresOn the basis of obtained pure phase single crystalline BiFeO3 nanowires with the diameter of 40-200 nm in fifth Chapter, we investigate the structure and magnetic properties of the BiFeO3 nanowires in the sixth chapter. Zero-field cooling (ZFC) and field cooling (FC) magnetic temperature (MT) curves (DC) show peak Tf (freezing temperature or blocking temperature), a series of experiments confirmed that single-crystalline BiFeO3 nanowires occurs the spin glass transition below freezing temperature Tf. Hysteresis loop (M-H curve) of single-crystalline BiFeO3 nanowires show that ferromagnetism significantly enhanced at the 5 K than room temperature. The M-H curve of single-crystal nanowires BiFeO3 shows a remarkable hysteresis loop characteristics at the 5 K, saturated magnetization, residual magnetization, coercive force larger than observed results previously reported in the literature in the polycrystalline nanowires. The synthesized single crystalline BiFeO3 nanowires provide important material foundation for the study of light, electric and magnetic fields from low temperature to room temperature under the excitation of the longitudinal (along the nanowire length direction) and horizontal (perpendicular to the length of the nanowires direction) electrical transport properties, polarization properties, variable temperature magnetic properties and the properties of the magneto-electric coupling, giant magneto-electric coupling properties and phase transition mutations.
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