Magnetic Properties And Application In Photocatalysis Of Nano-structured Magnetic Particles

Nano-scaled magnetic composite exhibits the different properites and applications from the single magnet in the composite.Hard/soft magnetic composite is the one that has been widely attractive.The hard magnet possesses the high coercivity while the soft magnet has the high saturation magnetization.It is predicted theoretically that the magnetic product of the composite would be greatly larger than the single magnet due to the exchange-coupling between hard and soft magnets,and such the magnetic composite likely becomes the forth-generation permanent magnet.However,lots of experimental researches reported the magnetic product far below the theoretically predicted value(1 MJ/m3),and the fundamental reasons need the further investigation.In chapter 1 of this thesis,the magnetic properites of hard CoFe2O4/soft CoFe2 composites were studied.The results indicate that the content of soft magnet plays a crucial role in the magnetization reversal and hence the macroscopic magnetic properties.Magnetic metal@oxide core@shell structure is another type of magnetic composite.From the viewpoint of practical applications,the metal is prone to oxidation and corrosion.To solve this problem,the oxide shell is synthesized to build magnetic metal@oxide core@shell structure.Moreover,such the oxide shell is biocompatible and prone to functionalization.Therefore these core-shell nanoparticles are expected to be promising materials for photocatalytic and bio-sensing applications.For the oxide shell synthesized through the solid stae reaction,there exist disadvantages such as agglomeration and difficulty in controlling the shell thichness.In chapter 2 of this thesis,the Fe@Fe2O3 core-shell structure was synthesized through the electrochemically anodization method developed by ourselves;Then,it was functionalized by TiO2 to obtain Fe@Fe2O3@TiO2.The stability of magnetization and photocatalytic performance are investigated.In chapter 3 of this thesis,we studied the effects of reduction in the H2 atmosphere on the magnetic and photocatalytic properties of NiFe2O4@NiFe2@TiO2.With increasing the reduction temperature,the ratio of NiFe2 in NiFe2O4@NiFe2 magnetic composite increases,enhancing the magnetization of composite and hence the magnetic separability of catalyst.The main results of this thesis are as follows:1.Preparation and magnetic properties of CoFe2O4@CoFe2 core-shell particlesThe temperature dependent field cooling(FC)and zero-field cooling(ZFC)magnetizations,i.e.MFC and MZFC,under different magnetic fields from 500 Oe to 20 kOe have been investigated on two exchange-spring CoFe2O4/CoFe2 composites with different relative content of CoFe2.Two samples exhibit different magnetization reversal behaviors.With decreasing temperature,a progressive freezing of the moments in two composites occurs at a field-dependent irreversible temperature Tirr.For the sample with low content of CoFe2,the curves of-d(MFC-MZFC)/dT versus temperature T exhibit a broad relaxing peak at an intermediate temperature T2 below Tirr,and the moments are suggested not to fully freeze till to the lowest measuring temperature 10 K.However for the sample with high content of CoFe2,it exhibits the reentrant spin-glass state around 50K;In d(MFC-MZFC)/dT curves,besides a broad relaxing peat at T2,a rapid rise around the low temperature 1?15 K is observed,below which the moments are suggested to fully freeze.Increase of magnetic field from 2 kOe leads to the shift of T2 and Tirr towards a lower temperature and the shift of T2 is attributable to the moment reversal of CoFe2O4.2.Preparation of Fe@Fe2O3 core-shell particles and TiO2 functionalizationThe Fe@y-Fe2O3 core/shell nanoparticles were synthesized by an electrochemical anodization method in a potentiostatic way followed by the crystallization in the N2 flow at 500?.The anodization was performed in an electrolytic cell with 1.0×1.5 cm2 Pt foil as cathode and Pt filament as anode;The Pt filament anode was penetrated into a thin glass tube in which contains Fe nanoparticles;the glass tube was submerged in the NH4F electrolytic solutin.The Fe/y-Fe2O3 core/shell structure was clearly shown by the HRTEM observation and the thickness of y-Fe2O3 shell can be well controlled by the anodization potential and time.Compared with the naked Fe nanoparticles,the anodized Fe/y-Fe2O3 core/shell nanoparticles show the better magnetization stability;furthermore,they can be more easily functionalized by TiO2.The synthesized Fe/y-Fe2O3 core/shell nanoparticles have the advantages of large magnetization with good stability and easy functionalization;Therefore they are promissing materials in applications of not only photocatalysis but also targeted drug delivery and biosensing.3.Chemical state and photocatalytic activity of NiFe2O4@NiFe2@TiO2The NiFe2O4@NiFe2@TiO2 magnetic photocatalyst was prepared by hydrothermal synthesis method and then reduced in the H2/N2 mixed atmosphere at 400 ?,600 ?,800 ? and 1000 ? to change the chemical state of the element and obtain strong magnetic core which is beneficial for magnetic separation.Unlike previous reports,the reduction reaction did not introduce oxygen vacancies Ov and TiOx chemical states in the TiO2 lattice.The catalyst prepared at 800 ? has the best performance,its degradation efficiency De and reaction rate constant kapp reach 99.9%and 3×10-2 min-1,respectively.The reason can not be interpreted as visible light and the appropriate amount of Ov and TiOx chemistry state.It shall be explained by synergetic effect of proper defect types in the TiO2 crystal lattice,and between TiO2 and magnetic particles,such as TiOH inside of TiO2 lattice,or Ti-O-Fe(Ni)at interface,and the defect concentration.