Structural Design, Characterization, And Pho-tocatalytic Property Investigation Of Nanostruc-tured Iron Oxides

Global environmental pollution and energy shortage are considered as two greatest challenges that human society is facing now. Semiconductor photocatalysis is expected as a highly prom-ising strategy for both harvesting solar energy and decomposing toxic organics in water and air by solar light irradiation. The main problem of TiO2, the most widely studied photocatalyst material by now, is that its band gap is as wide as3.1eV, making it absorb only the UV part of the incident solar irradiation. Iron oxide has a proper band gap of about2.1eV which lies in the visible region of solar spectrum and allows utilization of～45%of the solar radiation. Due to the virtue of environmental compatibility and low cost, iron oxide is considered as a good candidate for visible light responsive photocatalyst. For the purpose of exploiting iron oxide as an effective photocatalyst, this dissertation focuses on structure design, synthesis and preparation, photocatalytic activity, as well as property improvement of iron oxide nanostruc-tures. The main conclusions drawn from the work are as follows.1. Nanostructured iron oxide and oxyhydroxide nanoparticles with various morphologies were designed and synthesized via hydrothermal route. Stabilizer molecules with particular capping groups were applied in the reaction system to control the morphology of the final products. A terrace-step-kink growth mechanism was thus proposed based on the experimen-tal observations. The morphology variation was attributed to the interaction between capping groups of stabilizer molecules and iron oxide surfaces. When small stabilizer molecules were used, the anisotropic adsorption of the molecules on the surface of iron oxide crystals was predominant.3-aminopropanol and urea chelate with surface Fe atoms via-N and-O, result-ing in hexagonal and platelet particles with high-indexed planes exposed. PEG molecules with long chain structures are inclined to interact with each other to form’cages templates’instead of to adsorb on the surface of iron oxides, confining the growth iron oxide. Research work on growth of goethite indicated that the growth of1D structure is due to the high anisotropy of the crystal structure.2. To elucidate the growth mechanism of iron oxide particles, a TEM based trace method was proposed to characterize the crystalline planes and directions of a faceted nanoparticle, and therefore its shape. The Miller indices of surface planes can be determined through coor-dinate transformation after the determination of the edge vectors in the TEM screen coordi-nate system. The core of the method is to determine the coordinates of the edges vectors in the TEM screen coordinate system. The orientation of the particle with respect to the sample holder is determined by acquiring and indexing the corresponding diffraction pattern. The Miller indices of the surface planes and edges of the particle are calculated through coordinate transformation from the screen coordinate system to the crystal coordinate system. To illu-strate the feasibility of the proposed method, the hydrothermally synthesized hematite nano-particles were characterized by applying the technique.3. The photocatalytic performances of hydrothermally synthesized iron oxides were in-vestigated. Methyl orange, a representative azo dye pollutant in textile industry, was chosen as the model contaminant molecule. A systematic study, including the influence of the reac-tion conditions, the kinetics and the route of the oxidation of the methyl orange molecules, was carried out to explore the mechanism of photocatalytic degradation of organic molecules over iron oxide under visible light irradiation. It was found that the photodegradation of me-thyl orange over iron oxide was more like a semiconductor photocatalysis process, rather than a surface reaction process of ligands to metal charge transfer. Moreover, the visible light pho-tocatalytic activities of iron oxide can be tuned by controlling the morphologies, especially the crystallographic facet of the iron oxide nanoparticles. The highest photocatalytic activity of iron oxide powders synthesized with3-aminopropanol and urea can be obtained under the condition of hydrothermal treatment at160℃for5hours and140℃for5hours separately.4. Based on the understanding of photocatalysis mechanism of iron oxide, post-treatment techniques were adopted to enhance their abilities to photo-decompose organic pollutant mo-lecules. The iron oxide nanopowders were modified by loading noble metals, such as Ag, Au and Pt nanoparticles. Photocatalytic evaluation showed that the abilities of loaded metal par-ticles to inhibit the electron-hole pair recombination depend on the relative positions between their work functions and conductive band position of the iron oxide.5. A simple method was suggested to fabricate iron oxide thin films on substrates, and thus to facilitate the applications of these photocatalyst integrated as devices. It was found that ordered arrays of hematite were easily grown on textured sapphire substrate rather than on glass substrate. The growth of iron oxide nanorods array can grow on glass substrate with a seed layer prepared by thermal decomposition of ferric salts. Using this method, problems caused by the short free distance of charge carriers inside iron oxide can be avoided.