Study On The Controlled Synthesis Of Iron Oxides And Iron Particles With Hierarchical Nanostructures

Materials with hierarchical nanostructures have extensive applications in photoelectronic devices, drug delivery, active material encapsulation, ionic intercalation, surface functionalization, catalyst carriers, and sizeselective reactions, etc., because their unique structures may generate novel properties. Hierarchical nanostructures can be prepared via traditional templating methods and newly developed template-free methods based on Ostwald ripening, oriented attachment, and Kirkendall effect etc. In this dissertation, hierarchical nanostructures of iron oxides and iron materials are prepared via soft templating method, self-assembly of nanoparticles, Ostwald ripening and oriented attachment. The structures of the obtained materials are characterized and some of their properties are studied.Single-crystal star-like arrayed particles ofα-Fe2O3 are prepared by a precipitation-oxidation process with FeSO4, urea and air in the pressense of dispersant CTAB. Reaction temperature and amount of precipitant urea are found to strongly affect their morphology and phase composition of the products. The formation mechanism of the star-like arrayed particles is a combination of oriented attachment and epitaxial overgrowth. The typical single-crystal star-like arrayed particles of a-Fe2O3 show a weak ferromagnetic behavior with a remanent magnetization of 0.569 emu/g and a coercivity of 156.08 Oe at room temperature.Mesoporousβ-FeOOH andα-Fe2O3 particles are synthesized using cetyl trimethylammonium bromide (CTAB) as the soft template and [FeF6]3-, [Fe(C2O4)3]3-as the iron sources, respectively. Nitrogen adsorption/desorption analysis reveals that they both have peak pore sizes at 3-4 nm and 20 nm, and their specific areas are 164.4 and 102.0 m2/g, respectively.Mesoporous iron submicro-spheres assembled with primary nanoparticles are synthesized by reduction of Fe2+ in aqueous solution with NaBH4 in the presence of ethylenediamine (EDA).The highly concentrated Fe2+ ions and excessive NaBH4 tend to cause a very fast reaction, but EDA can slow down the reaction owing to its alkalinity and chelating effect. The morphological structure of the product can be adjusted kinetically by the reaction rate, as the consequence of varying the amount of EDA. The typical submicro-spheres have a diameter of ca.300-1000 nm and the primary nanoparitcles are ca.9 nm in size. The specific, surface area and peak pore size of the typical product are 11.4 m2·g-1 and 4.2 nm, respectively. Magnetic measurement reveals a superparamagnetic behavior. The simulated microwave reflection loss curve obtained from the measured electromagnetic parameters shows a peak reflection loss of-11.79 dB at 11.84 GHz and a range of about 7 GHz that the reflection loss is less than-8dB. When the typical mesoporous iron submicro-spheres are used as the iron reagent in a sono-Fenton process at neutral pH,77% of rhodamine B (RhB) is degraded within 60 min.Highly concentrated Fe2+ aqueous solution, excessive NaBH4 and highly concentrated EDA are reacted in a closed autoclave at 50℃and hollow spheres with a shell of arrayed dendritic nanofibers are obtained. Because of the alkalinity and strong chelating effect of EDA, the reduction rate is slow and micrometer iron spheres are densely assembled with primary iron nanoparticles firstly. During the followed EDA-assisted Ostwald ripening process, iron is transported from the central part of the spheres to the surface by dissolution and reprecipitation of iron through the coordination equilibrium of EDA and Fe2+, therefore, hollow spheres with a shell of arrayed dendritic nanofibers are formed. When the typical sample is used as the iron reagent in a sono-Fenton process at neutral pH,55% of RhB is degraded within 60 min.[Fe(EDA)3]2+, formed with Fe2+ and EDA, are reacted with excessive NaBH4 in the cosolvent of H2O/ethanol. The undissolved NaBH4 solid acts as the matrix of heterogeneous nucleation and assembly. When the matrix NaBH4 is washed off with water, micrometer hemispheres densely assembled with primary iron nanoparticles are obtained. In the case of high EDA/FeⅡmolar ratio in addition to undissolved NaBH4, the heterogeneous nucleation combined with the EDA-assisted Ostwald ripening results in hemispheres with radially arrayed dendritic nanofibers. In the case of complete dissolution of NaBH4 and high EDA/FeⅡmolar ratio, microparticles with interior structure of dendritic nanofibers are formed. These samples are used as the iron reagents in a sono-Fenton process at neutral pH, and the hemispheres with radially arrayed dendritic nanofibers show the highest RhB-degrading activity, degrading 82% of RhB within 60 min.