Synthesis And Application Of Ferromagnetic Metal, Ferrite And Their Composites

Magnetic nanoparticles are of great interest for researchers from a widerange of disciplines, including magnetic fluids, catalysis,biotechnology/biomedicine, magnetic resonance imaging, data storage, andenvironmental remediation. A number of literatures have been reported on thethe synthesis, protection, functionalization, and application of magneticnanoparticles, as well as the magnetic properties of nanostructured systems.However, these nano-sized materials are easy to gather together due to theirsmall size, and they are unstable because of the active surface, and they havewide size distributions. Hence it is necessary to obtain magnetic nanoparticleswith smaller and narrower size distribution than the ones manipulated withexternal magnetic fields and with dispersion stability. Another potential application of these nanoparticles is their use as tertiary treatment of residualwaters acting as powerful reducer agents of organic and inorganic material,with the advantage that it could be possible to recycle and separate themagnetite particles by an external magnetic field. In this paper, synthesis andapplication of ferromagnetic metal, ferrite and their composites the conditionsof synthesis of magneitc nanomaterials are studied.The main results obtainedin the thesis are divided into seven parts as following:Firstly, we compare the adsorption capacity of different MFe₂O₄（M=Mn,Fe, Co, Ni） ferrite nanocrystals synthesized a by hydrothermal method forCongo red （CR）. It is the first time to give a comprehensive comparison andanalysis of the adsorption capacity of ferrite nanocrystals with spinel structurefor CR. Research indicates that the cations distribution of MFe₂O₄ferrites isthe most important factor to decide their adsorption capacity. Electrostaticabsorption was conceived as the main adsorption mechanism. Meanwhile, the MFe₂O₄nanoparticles exhibited a clearly ferromagnetic behavior under appliedmagnetic field, which allowed their high-efficient magnetic separation fromwastewater. Furthermore, acetone is an effective desorption agent fordesorption of MFe₂O₄nanoparticles loaded by CR. All of the spinel ferritenanocrystals possess good soft-magnetism, especially, CoFe₂O₄nanocrystalsexhibit a higher saturation magnetization of86.1emu·g^-1as well as theoutstanding adsorption capacity for CR. By the calculation of Langmuirisotherm model, the maximum adsorption capacity of CoFe₂O₄for CR is244.5mg·g^-1.Secondly, this investigation was to increase the adsorption capacity ofmagnetite for CR by adulterating a small quantity of La³⁺ions into it. Theadsorption capability of nanocrystalline Fe_3-xLa_xO₄（x=0,0.01,0.05,0.10）ferrite to remove CR from aqueous solution was evaluated carefully.Compared with undoped magnetite, the adsorption values were increased from 37.4to79.1mg·g^-1. The experimental results prove that it is effectual toincrease the adsorption capacity of magnetite by doped La³⁺ions. Among theLa³⁺-doped magnetite, Fe2.95La0.05O4nanoparticles exhibit the highestsaturation magnetization and the maximum adsorption capability. Thedesorption ability of La³⁺-doped magnetite nanoparticles loaded by CR canreach92%after the treatment of acetone. Furthermore, the Fe_3-xLa_xO₄nanoparticles exhibited a clearly ferromagnetic behavior under appliedmagnetic field, which allowed their high-efficient magnetic separation fromwastewater. It is found that high magnetism facilitates to improve theiradsorption capacity for the similar products.Thirdly, a facile low-temperature hydrothermal process to synthesizeα-Fe/Fe₃O₄nanocomposite is reported. TEM and HRTEM revealed that theα-Fe/Fe₃O₄nanocomposite was composed of catenulate α-Fe and lamellarstructured Fe₃O₄. The weight ratio of α-Fe in the α-Fe/Fe₃O₄nanocomposite is 35.6%. The α-Fe/Fe₃O₄nanocomposite demonstrates an extremely high Congored （CR） removal efficiency from the wastewater almost complete removalwithin3minutes. For100mg·L1of CR aqueous solution, the maximum of CRremoval can reach1297.06mg·g^-1. The large saturation magnetization (80.5emu·g^-1) of the nanocomposite allows fast separation of α-Fe/Fe₃O₄nanopartilces loaded with CR from the liquid suspension. The synergisticeffect of the nanocomposite may contribute to the enhanced CR removal ability,because the CR can be removed by reduction reaction and adsorption at thesame time. Based on the degradation products identified by UV–Vis spectra,XRD and FTIR spectra, a possible degradation mechanism of CR on theα-Fe/Fe₃O₄composite was proposed. The significantly reduced treatment timerequired to remove the CR and the simple, low-cost and pollution-freepreparation method make α-Fe/Fe₃O₄nanocomposite promising for the highly Fourthly，cobalt crystals with hcp and fcc mixture structure and hcpstructure were prepared by a solvothermal process based on the dosages ofN2H4·H2O, in the meantime effect of crystal structure on their magneticproperties and Congo red （CR） removal abilities were evaluated. To our bestknowledge, it is the first time to report the CR removal by micron andsub-micron sizes of Co crystals, and the best CR removal ability can reach694.4mg·g^-1. For the hcp and fcc mixture structure of Co crystal, the degree ofmixing can be clearly observed from the HRTEM images. Furthermore, thesaturation magnetization （Ms） of Co crystal is increased with the increase ofhcp phase, while its coercivity （Hc） is decreased with the increase of hcp phase.The Co crystal with the most mixture structure has the highest CR removalability. And the micron and sub-micron sizes of Co crystals will be good formagnetic separation after CR removal.Fifth, we synthesized water-soluble Fe₃O₄nanoparticles （NPs） with sufficiently high solubility （28mg·mL1） and stability （at least one month）through hydrothermal approach, also found that they exhibited excellentremoval ability for heavy-metal ions from waste water. It is noteworthy thatthe adsorption ability of the water-soluble Fe₃O₄NPs to Pb2+and Cr6+isstronger than water-insoluble Fe₃O₄NPs. Furthermore, the water-soluble Fe₃O₄NPs exhibited relatively high saturation magnetization (83.4emu·g^-1), whichallowed their high-efficient magnetic separation from wastewater. The mostimportant thing is that the water-soluble magnetite as an adsorbent can directlydissolve in water without the help of mechanical stir or any extraneous forces,which may solve a key problem for practical application of magnetic powdersin the field of sewage purification. Moreover, the water-soluble Fe₃O₄NPsshow high-efficient adsorption capacity for10ppm of Pb2+ions solution whichcan reach90%within2minutes. Sixth, we report a facile approach to prepare cobalt hybrid/graphene（Co/G） nanocomposite via a general one-pot hydrothermal synthesis. NaBH4isused as the reducing agent. Co/G nanocomposite possesses narrowsize-distribution and good dispersion, enabling their tremendous potential forenergy and environment applications. As a proof of concept, we demonstratethe use of Co/G nanocomposite in a lithium-ion battery and an adsorbent forCongo red （CR）, respectively. More importantly, more than97%of capacityretention (605mAh·g^-1) is retained after50cycles, indicative of highcharge/discharge reversibility of the Co/G nanocomposite electrode. And theCR removal ability of Co/G nanocomposite can reach934.9mg·g^-1.Lastly, we report a facile approach to synthesize water-dispersiblenanocomposite with Fe₃O₄nanoparticles attached to graphene, which combinesthe growth of Fe₃O₄nanoparticles and the reduction of graphene oxide （GO） in one single step. The Fe₃O₄/G nanocomposite showed high T2relaxivityindicating its potential as an ultrasensitive T2contrast agent.