Structures And Properties Of Iron Oxide Based Nanocomposites By Flame Spray Pyrolysis Route

Recently, advanced nanomaterials have attracted more attention owing to the unique physical and chemical properties. Especially, nanocomposites with versatile morphologies have an extensive application and are one of research hot topics of novel materials, which have different compositions, diverse function and unique interfacial interaction at nanoscale. Iron oxide based nanocomposites, as an important fuctional material, have a promising application such as magnetic recyclable catalysts, magnetic resonance imaging, lithium ion battery, solar light splitting water and so on. Therefore, it is still a challenge that how to design and synthesize multifunctional iron oxide based nanocomposites effectively. Flame spray pyrolysis technology has been demonstrated to be an effective, continuous, easily scalable approach for production of nanomaterials. Liquid-feeding route could offer more chocies for employing versatile precursor and make flame spray pyrolysis method possess more advantages in the construction of nanocomposites with multicomposition and diverse morphologies. In this thesis, combining flame reactor designing, optimum process control and the design of structured materials, Iron oxide based nanocomposites, such as oxides stabilized metallic Fe and Co3Fe7alloy core-shell nanoparticles, double faced γ-Fe2O3‖SiO2nanhybrids, γ-Fe2O3‖SiO2core-shell nanostructures and α-Fe2O3/SnO2nanoheterostructures, have been prepared by a facile flame spray pyrolysis approach. The flame process, materials structures and properties have been investigated in detail. The major contents have been summarized as follows.1. Air stable, metallic Fe and Co3Fe7nanoparticles have been synthesized via one-step flame spray pyrolysis of an organicmetal precursor solution under stronger reducing atmosphere. The as-synthesized nanoparticles with diameters of20-80nm showed a typical core shell structure and high stability for one month in air, which metallic Fe or Co3Fe7cores were protected against oxidation by a surface shell of about3-6nm oxides (Fe3O4and CoFe2O4). The ratio of metallic Fe/Co alloy nanoparticles was7:3. The alloy nanoparticles exhibited enhanced saturation magnetization (126.1emu/g), compared with flame sprayed iron nanoparticles with the same conditions. By investigating the relationship between the ratio of fuel to oxygen, the morphology and composition of obtained materials and magnetic properties, reducing flame synthesis process is created for the synthesis of novel nanomaterials. Moreover, the formation mechanism of metal and alloy nanoparticles with core-shell structure was investigated in detail, which included three stages:flame combustion, reducing and surface oxidation during flame process. It is reckoned that such a continuous production approach an effective way to produce the stable advanced nanomaterials with a reduced valence state.2. Double faced γ-Fe2O3‖SiO2nanohybrids (NHs) and their in-situ selective modification on silica faces with the3-Methacryloxypropyltrimethoxysilane molecules have been successfully prepared by a simple, rapid and scalable flame aerosol route. The double faced NHs perfectly integrates magnetic hematite hemispheres and non-magnetic silica parts into an almost intact nanoparticle as a result of phase segregation during the preparation process. The unique feature allows us to easily manipulate these particles into one-dimension chain-like nanostructures. By tailoring the mole ratio of Fe and Si in precursor, Janus nanoparticles with different volume of segregated SiO2hemisphere could be designed. The high temperature phase segregation mechanism based on thermal dynamic control is proposed to explain the formation of double faced γ-Fe2O3‖SiO2NHs in flame. Furthermore, the co-oxidation of different precursor (Ni, Mn, Co) and silica resource in flame eventually leads to the products with different composition and morphologies, indicating that the final morphology undergoing a high temperature aerosol process are independent of the bulk physiochemical features of the corresponding metal oxides. On the other hand, in situ selectively modificated double faced γ-Fe2O3‖SiO2NHs possess excellent interfacial activities, which can assemble into many interesting architectures, such as interfacial film, magnetic responsive capsules, novel magnetic liquid marbles and so forth. The modified NHs prefers to assemble at the interface of water/oil or oil/water systems. It is believed that the highly interfacial active NHs is not only beneficial for the development of interface reaction in a miniature reactor, but also very promising functional materials for other smart applications.3. Core-shell γ-Fe2O3‖SiO2nanohybrids were fabricated via a simple flame-assisted spray pyrolysis (FASP). The shell layer composed of SnO2nanocrystals (8-10nm) was in-situ grown on the pristine Fe2O3nanoparticles with a tailored thickness ranging from6-20nm. The construction of such intriguing structure thus provided a rational and efficient combination of two kinds of gas sensitive materials, resulting in a remarkably enhanced sensing sensitivity (Ra/Rg) of22.8to100ppm ethanol vapor at300℃with high selectivity, compared to the pure Fe2O3and SnO2materials synthesized by FASP, respectively. The enhancement can be mainly attributed to the synergistic effects, i.e. the change of heterojunction barrier height at the interface between them. In addition, the in-situ flame coating process provides a promising and versatile choice for the synthesis of core-shell nanostructures with multifunctional composition. 4. Novel core-shell α-Fe2O3/SnO2heterostructures (HSs) are successfully prepared by a one-step flame-assisted spray copyrolysis of iron and tin precursor. The effect of SnO2component is investigated for the evolution of phase composition and morphology in detail. For the first time, it is noted that SnO2as a dopant can effectively promote the phase transition of γ-Fe2O3to α-Fe2O3during flame synthesis. A phase-segregation induced growth mechanism is proposed to explain the formation of unique core-shell structure. Such core-shell HSs as LIB anode materials exhibits an enhanced lithium storage capacity in comparison to pure Fe2O3and SnO2. This enhancement could be ascribed to the synergetic effect of both single components as well as the unique core-shell HSs.