| Fe2O3nanoparticles (NPs) possessed many research values including semi-conductor, magnetic separation and bio-sensor technologies. In particular, the modification by SiO2coatings and surface functionlization woud further expand their applications. However, the monodisperse Fe2O3NPs with the large sizes would face to many challenges. The main challenges were as below:(i) the absence of thermodynamic stability could cause the worse productivity and monodispersity;(ii) the self-aggragation phenomena of magnetic NPs were obviously presented in modification process, which was hard to be uniformly coated;(iii) the increase of their own weights would restrict the self-assemblying process.In this project, we prepared the monodisperse α-Fe2O3NPs with different shapes by the hydrothermal synthesis. After researching systhsis processes, the best reaction conditions were determined by the time for12h and the temperature at120℃. The as-prepared α-Fe2O3NPs displayed a narrower distribution width and aspect ratio of3.2. Moreover, the monodispersity and aspect ratio could be adjusted by some factors, i. e. raction time, temperature, concentration of participate and surfactant. Utilizing the ultra-voilet and visible (UV-Vis) spectroscopy, the analytical results of optical gap band demonstrated that the values of optical gap band were enhanced by the increase of aspect ratio, which was2.08eV for spindle α-Fe2O3NPs. The magnetic properties measured by VSM showed that the spindle α-Fe2O3NPs had higher magnetic parameters. Here, the coercivity (Hc) and residual magnetization (Mr) attained1645Oe and0.0951emu/g, respectively. Analyzed by HRTEM images, the the spindle α-Fe2O3NPs presented more growth orientation in long-axis direction and grain boundries, which could reduce the optical gap band and enhance the coercivity.The surfaces of the spindle α-Fe2O3NPs were modified by the surfactant of PVP. We researched the influences of PVP, temperature, concentration and initiator on the morphologies and structures of the as-prepared α-Fe2O3@SiO2NPs. The shell thicknesses between15nm and125nm were controllable and adjustable by these factors. Furthermore, we observed these core shell structures by TEM. Their structures were measured by XRD and UV-Vis spectra. It was confirmed that using PVP as modified agent and ammonia as initiator were easily controlling the coating effects of SiO2shells, to avoid producing the individual SiO2morsphere. The results of optical and magnetic properties showed that the value of optical gap band was increased and coercivity was decreased with the SiO2shell thickened. Thermal stabilities were increased with the increase of shell thickness. For65nm of SiO2shell thickness, the Fe2O3@SiO2displayed the ourstanding sharp stability, which was no obvious deformation treated in the temperature of800℃for1h.Thermal analysis methods could supply more informations of structural transformation of Fe2O3. In the reduced atmosphere of H2flow, the phase transfomation from α-Fe2O3to Fe3O4at292℃, and in oxidized atmosphere of O2flow, the phase transfomation from Fe3O4to γ-Fe2O3was realizeed at302℃by TG-DTA curves. By the XRD, Raman and FTIR spectra in reduction-oxidation process, these phase transformations were high effectively complished, according to the crystalline structure of typical products. In particular, Mossbaur spectra display that the transformation yeilds are exceed99.99%after treating for1h. According to the transformed mechanisms, the coated α-Fe2O3@SiO2NPs was treated by reduction and oxidation process. We obtained the high-quality ellipsoid-typed γ-Fe2O3@SiO2NPs. The results of XRD patterns and FTIR spectra demonstrated that the products are typical core shell structures. The analysis in magnetic properties showed that the coevitiy reached up to149.9Oe for the ellipsoid NPs with110nm shell thicknes.We researched the self-assembly processes. When some factors, such as temperature, concentration, magnetic fields, were controlled strictly, we could easily obtain the highly ordered ellipsoid γ-Fe2O3@SiO2films by the air-liquid interfacial self-assembly. For the reflective spectra of these ellipsoidal templates, the photonic band gap was centralized at585nm, and the relative width was12.6%. Using the forced absorption method, Au nanocrytals uniformly coated on the sufaces of SiO2shells. By TEM images, the different contents of Au nanocrystal were uniformly coated on the ellipsoid templates. Using thiosalicylic acid as the probe molecule, the SERS effects were obviously presented in the ordered ellipsoid templates, the limited concentration were below10-7M. The fluorescence spectra displayed that three dimentional ordered ellipsoid templates exhibited the obvious fluorescence enhancement. |
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