| Due to the widely applications in high-density magnetic data storage media, magnetic responsive photonic crystals, and magnetic separation, an important research effort has been directed towards the study on the synthesis of nanoscale magnetic materials. In addition, magnetic nanomaterials with ordered functional structure have some brand-new properties resulted from the spatial orientation and arrangement of the building blocks and can be viewed as the foundation for the formation of novel miniaturized nanodevices. Therefore, the research about the synthesis, assembly and application of nanoscale magnetic materials is very important from the point of view of science and applications.The main objective of this dissertation is to develop new avenue for the solution-based synthesis of nanoscale magnetic materials, and investigate the relationship between their magnetic parameters and their structure (size and/or shape), and demonstrate some simple and effective technologies for assembling the magnetic block buildings into highly ordered functional structures. Furthermore, the pratical application of functional magnetic nanomaterials will be discussed. The main parts of the as-obtained results are summarized below:1. Magnetite nanoclusters with average size about 120 nm have been prepared and allowed to self-assemble into one-dimensional (1D) nanochain structure with the average length about 2μm by a simple magnetic-field-induced (MFI) assembly approach (0.20 T). The constituent, phase, and morphology of these 1D nanochains have been characterized by X-ray diffraction and Transmission electron microscopy. Magnetic measurement reveals that these 1D nanochains are weakly ferromagnetic at room temperature. In this paper, we discuss the influence of magnetization time and strength of external magnetic field on the formation of 1D nanochains. We also shows that by changing the amount of hydrogen peroxide in starting materials, 1D nanochains with different interparticle spacing can be obtained. This 1D nanochain structure with different interparticle spacing would be an ideal system for the further study of magnetization properties of 1D ordered magnetic nanostructures.2. Necklace-like magnetite and maghemite nanorings, composed of magnetic nanoparticles (NPs) with average size about 40 nm, have been prepared via a solvothermal process in a colloidal solution by a self-assembly process. The composition, phase, and morphology of these nanorings have been characterized by X-ray diffraction, X-ray absorption, and transmission electron microscopy. In this paper, we discuss the influence of reaction conditions on the formation of nanorings structure including the amount of PVP in starting materials, reaction time and temperature. On the basis of experimental observation, we supposed that magnetite NPs may first assemble into chains by magnetic dipole-dipole interactions. These dipolar chains, which are metastable structures relative to necklace-like nanorings, then produced the rings. So, the stability of chains may determine the yield, size, and morphologies of necklace-like nanorings.3. The core/shell structure of magnetite/carbon colloidal nanoparticles (CNPs) with average size about 190 nm has been prepared via a one-step solvothermal process using ferrocene as a single reactant. The composition, phase, and morphology of the nanostructure have been characterized by X-ray diffraction, and transmission electron microscopy. Magnetic measurements reveal the superparamagnetic nature of the material with a magnetization saturation 40.2 emu/g at room temperature. Under the induction of an external magnetic field, strong diffraction in the visible light spectrum can be observed in a suspension of CNPs in ethanol and the diffraction wavelength varies with the strength of the external magnetic field. After being stored for eight months in an ethanol solution, these CNPs can still diffract visible lights when a magnetic field was applied, which is attributed to carbon coating and creating carboxyl groups on the surface of carbon shells introducing both a steric hindrance and electrostatic repulsions between magnetite nanoparticles.Size and solvent-dependent of magnetic responsive photonic crystals were investigated using carbon-encapsulated superparamagnetic magnetite nanoparticles as building blocks. The wavelength dependence of optical diffraction on particle size was demonstrated using colloidal nanoparticles spherical in shape and having an average diameter of 100 nm,150 nm,200 nm,250 nm, respectively. A colloidal suspension with a particle size of 250 nm can only diffract red light, while the suspension with particle size 100 nm can only diffract blue light no matter how the magnetic induction changed in strength. Solvent-dependent diffraction of photonic crystals under an external magnetic field were carried out by dispersing the colloidal nanoparticles in solvents, including acetone, ethanol, DMSO and water. It is shown that the reflection intensity was reduced as the polarity and reflactive index of solvent increased.4. Superparamagnetic colloidal nanochains (CNCs) coated and linked by carbon were prepared by solvothermal decomposition of ferrocene at a temperature of 180-200℃with a 0.20 T magnetic-field applied. It is demonstrated that CNCs with different intra-particle spaces (216 nm,186 nm, and 173 nm) can be obtained by changing the reaction temperature. Under the induction of an external magnetic field (e.g.0.10 T), the ethanol suspension of the three samples can diffract different wavelengths of visible light, displaying red, green and blue color, respectively. The three samples did not show the dependence of the diffraction wavelength on the strength of induction magnetic field, which indicates that the superparamagnetic CNCs can be used for magnetic responsive Bragg Reflector (BR).In addition, we have also prepared magnetic responsive Bragg reflector with purple visible light and the diffraction wavelength of 404 nm. The BR composed of monodisperse magnetic colloidal nanochains keep fixed diffraction wavelength under different strength of magnetic field (0.10 T-0.20 T) because of their immobile inter-particle spacing (155 nm). The synthetic process of monodisperse magnetic colloidal nanochains are carried out at room temperature by inducing the self-assembly of colloidal nanoparticles when a magnetic field is applied. Therefore, the demonstrated method is simple, convenient and harmless for the magnet used in the synthetic process of magnetic responsive Bragg reflector.5. Superparamagnetic magnetite nanoparticles coated with a thick (11 nm) and porous carbon layer have been prepared, which can serve as an efficient adsorbent for removing heavy metal ions (Pb2+) from an acidic aqueous solution. The nanoparticles show high removal capacities at lower pH values such as 3 and treat high pollution of wastewater (100 mg/L), which are aroused from their porous carbon layer with a carboxyl surface. The material demonstrated in this work also allows the possibility to separate other toxic metal ions (Hg2+, Cd2+) from an aqueous solution via tailoring their surface with special functional groups.
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