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Controllable Preparation, Structure And Properties Of The Complex Metal Oxides

Posted on:2009-02-24Degree:DoctorType:Dissertation
Country:ChinaCandidate:Z J GuFull Text:PDF
GTID:1101360278980192Subject:Applied Chemistry
Abstract/Summary:PDF Full Text Request
Complex metal oxides are an class of inorganie functional materials, which are widely applied in many important fields such as information, energy, electronics, metallurgy, aerospace, chemical industry, biology and medical science, due to their gas-sensing, magnetism, giant magnetoresistance, conductivity and catalytic activity. Among these materials, spinel and perovskite attract excessive interests for their versatility basing on the composition and structure. Recently, with the research about the relationship of function-structure-preparation developing, the study on rational design and controllable preparation of complex metal oxides is extremely noticed by researchers of all over the world. In this thesis we proposed two new methods for the preparation of complex metal oxides: the layered double hydroxide (LDHs) precursor approach to perovskite-like materials and the reduction-oxidation approach to the spinels nanomaterials. The structure, properties and catalytic activities of these products were systematically studied. The main results were as follows:(1) A series of LDHs precursors were prepared by codeposition method. Because the rates of bi- to trivalence metal ions is 2-4 in LDHs, reductive bivalence metal ions had to be introduced into layers of LDHs to overcome the deficiency of trivalent ions, for bivalence metal iosn in air giving additional trivalence ions by calcination.(2) Within the LHDs, the cations are uniformly distributed on an atomic level without segregation of 'lakes' of separate cations. Therefore, calcination of LDHs precursors at a lower temperature and shorter time can give perovskite-like complex metal oxides with uniform composition and structure. The composition and structure of calcined products are mainly affected by the species and rates of metal ions in the layers of LDHs. For example: pure perovskite-like oxides were obtain by calciantion of CaCo2+Fe3+-LDHs precursors, and as the amount of Co increasing, the crystal structure of perovskite-like oxides changed from Pnma to P212121 space group. However, calcination of CaMn2+Fe3+-LDHs and CaFe2+Cr3+-LDHs precursors gave mixtures of perovsktie and oxides except for sample with Ca2+/Mn2+/Fe3+ rate of 2/0.8/1.2. XPS analysis showed that a large amount of absorbed oxygen and oxygen vacancy in the surface of perovskite-like oxides had important effect on their catalytic properties.(3) The high catalytic activity of perovskite-like oxides in the reaction of oxidation of active carbon was owned to the absorbed oxygen in the surface of catalyst. The catalyst with more absorbed oxygen showed higher catalytic activity in the oxidation of active carbon. The kinetic study showed that the process of oxidation of active carbon was suitable for 3-Dimensional sphere diffusion mechanism.(4) A series of spinel nanomaterials were prepared by reduction-oxidation method. Ultilizing the high dispersing ability of the colloid mill, the metal ions were rapidly mixed with sodium borohydride and were reduced to metal nucleus. Then, the resulting metal nucleus were oxidated by the dissolved oxygen to give monodispersed spinel nanoparticels. The particle sizes of obtained spinels varied with the reaction condition, such as the crystallization temperature, the aging time, the concentration of reactant and the species, amount of surfactant. et al.(5) The structure and properties of spinels were studied detailedly, including crystal structure, micro-morphology, magnetic and optical properties, properties of the surface ions, and so on. The magnetic property was related to the particle sizes of the sphere-like spinel nanoparticles. As particle sizes decreasing, the saturation magnetization, residual magnetism and the coercive force also decreased, which resulted from the surface spin canting, surface disorder, as well as the presence of a magnetic dead or antiferromagnetic layer on the surface of nanoparticles. Furthermore, the 9 nm CoFe2O4, 14 and 22 nm NiFe2O4 were supermagnetism at room-temperature. UV-vis diffuse reflectance spectra analysis showed that the band gap of ZnFe2O4 increased with the decreasing particle sizes, which was the result of quantum confinement effects arising from the small size regime. The introduction of doping ions had a major impact to the magnetic and optical properties of spinels.(6) The spinels obtained by reduction-oxidation method showed great catalytic active in the growth of carbon nanotubes film. The smaller particels were facile to the growth of the more rough-diameter carbon nanotubes which formed films with higher porosity leading to the stronger hydrophobicity. During the photo-degradation of dyes, the smaller particles showed higher catalytic activity which was because of the quanta effect, specific surface effect and more photogenerated holes. Although the decrease of band gap of spinel by introduction of Co2+ was conducive to the generation of holes, it aslo decreased the oxidation ability of the holes, and eventually decreased the activity of catalyst.
Keywords/Search Tags:complex metal oxides, perovskite-like, spinels, reduction-oxidation method, LDHs precursor method, catalyst
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