Magnetic Nanomaterials: Morphology-controlled Synthesis And Encapsulated In Carbon By Solution Route

Magnetic nanomaterials,owing to their novel magnetic and chemical properties, have wide potential applications in many fields such as high density magnetic recording apparatus,magnetic fluids,magnetic resonance imaging,carriers for targeted drugs, catalyst,and bio-separation.Morphology-controlled synthesis and property study of magnetic nanomaterials are the frontiers in material science and are of consequence to their applications.This thesis is focused on realizing morphology-and size-controlled synthesis of magnetic Fe-based alloys by modifying experiment parameters such as the composition of surfactants in aqueous solution with solution phase reduction technique; establishing novel solution phase routes to encapsulating magnetic nanomaterials in carbon shell by carbonizing carbohydrates with hydrothermal or sulfuric acid carbonization route,for the purpose of preventing their oxidation and agglomeration, averting their being influenced by environment,enhancing their biological compatibility, etc.; investigating the principal applications of carbon-coated magnetic nanomaterials in catalyst,medicine,and other fields; and exploring the relationships among the synthesis conditions,structures,and properties of magnetic nanomaterials.(1)A large-scale,mild(＜100℃,30 min)aqueous phase route was established to prepare FeCo nanocubes and microcages.The products were characterized by transmission electron microscopy(TEM),high-resolution TEM(HR-TEM),scanning electron microscopy(SEM),selected area electron diffraction(SAED),and X-ray powder diffraction(XRD).The influences of reaction conditions such as the concentration of polyethylene glycol(PEG)and cyclohexane,reaction time,the ratio of Fe²⁺to Co²⁺on the morphologies and sizes of FeCo alloys have been studied,which indicated that FeCo alloys with morphologies of nanoplatelets,nanocubes,cubic microcages,and microflowers,and in sizes of～68±6 nm,～117±20 nm,～500 nm, and～270 nm can be controllable synthesized.The formation process of FeCo nanocube was also investigated,which followed a mechanism of "first kinetics and then thermodynamics controlling".On the base of the results of our experiments and that of in literatures,a possible growth model to cubic microcages was proposed.(2)A mild solution phase reduction system was developed and morphology-and size-controlled synthesis of Fe-based magnetic alloy was achieved.(a)Spheric FeNi nanoparticles with average diameter of about 70 nm were obtained in the system of cetyltrimethyl ammonium bromide(CTAB)/H₂O,while most of the products were nanofilms/rods with the addition of cyclohexane; nearly monodisperse FeNi nanoparticles with average diameter of about 25 nm were prepared in the system of PEG/cyclohexane/H₂O.(b)With the assistance of ultrasonic,a no-surfactant method to 8.5 nm FeCo nanocrystals with high Ms(＞200.5 emu/g)has been developed with economical FeCl₃·6H₂O as iron sources.(c)FeCo nanowires/nanorods were also obtained by reducing Co²⁺and Fe²⁺in a mixture of water and n-octane in the presence of surfactant CTAB and n-octanol at 90℃,the diameter of FeCo nanowires/nanorods increased with prolonging the reaction time.(3)An new two-step route,preparation of magnetic particles then hydrothermal carbonization with glucose,was established to encapsulate magnetic nanomaterials in carbon shell,and core@shell nanostructures such as magnetic metals@C(Co@C, Ni@C),magnetic alloys@C(FeNi@C),magnetic oxides@C(Fe₃O₄@C,NiFe₂O₄@C, Zn_1.6Fe_1.6O₄@C)were obtained.The products were characterized by TEM,HR-TEM, SEM,SAED,XRD,IR,Raman,and X-ray photoelectron spectroscopy(XPS).There are many functional groups such as -OH on the outer carbon shell,and functional materials can be loaded on the surface.For example,(a)Ibuprofen(IBU)can be loaded on Co@C and Fe₃O₄@C,the corresponding products(such as Fe₃O₄@C@IBU)show sustained release properties in both acidic(pH=1.5)and neutral(pH=7)aqueous solution.(b) The loading of noble metal catalysts such as Ag,Pd can also be achieved by in-situ reduction route under aqueous refluxing and ultrasonic radiation,the obtained products such as Fe₃O₄@C@Pd and Zn_1.6Fe_1.6O₄@C@Pd show excellent catalytic properties for Suzuki cross-coupling.After the catalyst being recycled for five times,the yield is also up to＞90%.This route to carbon coated nanomaterials is an economical,clean, effective,facile way without carbides formed; the core materials did not change the morphology and crystal phase after carbonization; and the thickness of the carbon shell can be controlled by controlling the ratio of core materials to glucose.(4)Using classical carbonization reaction between sulfuric acid and sucrose,a novel room temperature,convenient pressure route to encapsulating magnetic Ni-based nanostructures(Ni,FeNi)in carbon shell was firstly established,which involves adding sulfuric acid to the mixture of Ni-based core materials and sucrose aqueous solution. This route may be related to the catalytic property of nickel,since encapsulating Ag, FeCo,and Fe₃O₄ in carbon shell are not effective with the similar conditions.The obtained Ni-based core@carbon shell shows excellent magnetic properties such as high saturation magnetization and coercivity,the outer carbon shell can also be loaded with functional molecules such as tyrosine.