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Opis: Bi2O3-based pyrochlore nanomaterials and their photocatalytic, optoelectronic and crystallographic properties are the research topics of this doctoral dissertation. We synthesized these materials at nanoscale, since they are expected to possess photoactivity in the visible-light spectrum, which is an important target for the high-performance photocatalysts. To prepare nanoparticles that are not agglomerated, different synthesis methods (coprecipitation, coprecipitation followed by the digestion, reverse micelle) and post-treatments (furnace or hydrothermal reactor) were examined. Micrometer size materials were prepared with the solid-state reaction and annealing at high temperature. Bi2Ti2O7 nanoparticles were synthesized with the coprecipitation reaction, followed by annealing at 570 °C in the furnace or in the hydrothermal reactor at 230 °C for 18h in NH4OH medium. UV-Vis diffuse reflectance spectra of the Bi2Ti2O7 samples showed that they exhibit the maximum absorption edge at ~420-440 nm. The UV-Vis diffuse reflectance measurements of Bi2Ti2O7 loaded with 10 wt. % of Ag as a co-catalyst showed an enhanced absorbance in the visible region, presumably due to a contribution of Local Surface Plasmon Resonance. The Bi2Ti2O7 nanoparticles are not photocatalytically active under the visible light irradiation without assistance of a sacrificial reagent. The photocatalytic activity does not increase even with Ag as a co-catalyst under visible light irradiation. All samples are active under UV light irradiation, especially the sample with Ag that is able to decolourize 7 mg/L of metyl orange in just 3 min of exposure to UV light. However, H2O2 as a sacrificial reagent increases the photocatalytic activity of Bi2Ti2O7 under visible light irradiation. Phase pure pyrochlore Bi1.647Nb1.118Fe1.157O7 (BNF) nanoparticles were synthesized with coprecipitation reaction and further annealing in the furnace at 570 °C for 7h or 15h and in the hydrothermal reactor at 160 °C for 18h in 3 M or 6 M NaOH medium. The BNF materials exhibit shift toward longer wavelengths in absorbance of visible light (up to ~550-650 nm). This result confirmed that incorporation of Fe in the pyrochlore structure decreases the band gap. The BNF nanoparticles annealed for 7h showed intense photoactivity under visible light irradiation in the presence of a sacrificial reagent. The improved photoactivity, much better than with BNF, was shown for the composite nanoparticles BiOCl/BNF. They decolourize 100 % of the 7 mg/L methyl orange within only 2h of the visible light irradiation in the presence of H2O2. Phase pure pyrochlore Bi1.9Te0.58Fe1.52O6.87 (BTF) nanoparticles were synthesised by coprecipitation reaction and further annealing in the furnace at 570 °C for 7h. Although the BTF nanoparticles include higher amount of Fe than BNF nanoparticles and exhibit narrower band gap, the photoactivity of these nanoparticles is a bit lower than that of BNF nanoparticles. We assume that Fe at some critical concentration causes formation of mid-band states, which act as recombination centers. The same as BiOCl/BNF, the BiOCl/BTF composite is able to decolourize 100 % of the 7 mg/L MO within 2h of the visible light irradiation in the presence of H2O2. Based on the band gap determination and estimation of valence and conduction band levels, the proposed photodecolourization mechanisms are discussed. The conduction band levels of all synthesized materials lie more positively than O2/O2● redox potential (vs. negative hydrogen electrode potential), but are more close to H2O2/●OH redox potential. Thus, we assume that the photogenerated electrons form ●OH radicals, the species that induce dye decolourization under visible light irradiation. We believe that better performance of composite materials (BiOCl/BNF, BiOCl/BTF) is due to the formation of heterojunction, which facilitates the electron transfer between semiconductors, and O2 vacancy states formation in the BiOCl material.
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Povzetek najdenega: ...that incorporation of Fe in the pyrochlore structure decreases the band gap. The BNF nanoparticles...
Ključne besede: nanoparticles, pyrochlore structure, photocatalysis, visible light
Objavljeno: 26.06.2015; Ogledov: 5869; Prenosov: 146
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Ključne besede: silicon carbide, fine powders, structure
Objavljeno: 09.05.2017; Ogledov: 2189; Prenosov: 15
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Ključne besede: Cherenkov Telescope Array, small-size telescope (SST) design for CTA, dual-mirror (2M) Schwarzschild-Couder optical design, Gamma Cherenkov Telescope (GCT): SST-2M telescope design, mechanical structure
Objavljeno: 21.11.2016; Ogledov: 2880; Prenosov: 188
Polno besedilo (422,88 KB)

Opis: This work presents a paradigm for high temperature stabilization of bulk amorphous aluminium oxide. The thermodynamic stabilization is achieved by preparing a nanocomposite, where polymethylsilane dendritic molecules are dispersed in an aluminium hydroxide gel. Upon heat-treatment the gel transforms to the amorphous aluminium oxide that is stable up to 900°C. The dispersion of the macromolecules and their covalent bonding to the alumina matrix induce homogeneously distributed strain fields that keep the alumina amorphous. The first part of the thesis focuses on the synthesis, characterization and solubility properties of the dendritic polymethylsilane. The polymethylsilane is synthetized by electrochemical polymerization from trichloromethylsilane monomer. The polymerization mechanism, involving a single polymerization pathway, is identified. The polymer growth proceeds through reduction of the monomers to the silyl anions and their addition to the growing polymer. The solubility of three chemically related but topologically different polysilanes (linear, dendritic and network) were studied by dynamic light scattering. At room temperature the agglomerates in a range from 500 to 1300 nm are present. They undergo de-agglomeration at slightly elevated temperatures of around 40°C. The de-agglomeration results in formation of stable solutions, where a hydrodynamic diameter of the individual polymer molecules was measured to be in a range from 20 to 40 nm. The obtained diameters of two dendritic polymethylsilane macromolecules, synthesized under different electrolysis conditions, are much larger than the theoretical size estimated for an ideal dendrimer. We determined by 29Si NMR that the reason for this is in a large number of branching irregularities (defects) contained in the molecular structure. Combining the experimental values obtained by DLS and density measurements with a structural model that considers the branching irregularities, it is shown that the inclusion of the defects allows the dendritic polymer to exceed the sterical limitations and form the hyperbranched dendritic structure. The final size depends on a relative amount of the branching defects. In the second part, the synthetized polymethylsilane molecules were successfully used for the nanocomposite formation. The aluminium hydroxide gel with the dispersed polymethylsilane molecules was prepared as a precursor. Upon heat-treatment it gives the amorphous aluminium oxide stable up to 900°C. The dispersed macromolecules induce homogeneously distributed strain fields that keep the aluminium oxide amorphous during the thermal treatment the dispersed macromolecules covalently bind to the matrix, inducing the interface strain. The amorphous state was confirmed by the presence of penta-coordinated aluminium detected by 27Al NMR and a low bandgap measured by UV-vis absorption spectroscopy.
Najdeno v: ključnih besedah
Povzetek najdenega: ...branching irregularities (defects) contained in the molecular structure. Combining the experimental values obtained by DLS...
Ključne besede: amorphous aluminium oxide, polymethylsilane, nanocomposite, electropolymerization, solubility, agglomeration, de-agglomeration, dendrimer, hyperbranched dendritic structure, dynamic light scattering, thermal analysis, transmission electron microscopy, scanning electron microscopy, X-ray diffraction, infrared spectroscopy, UV-Vis spectroscopy
Objavljeno: 19.07.2018; Ogledov: 3405; Prenosov: 145
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Ključne besede: Nanoparticles, Magnetic properties, Size effect, Structure properties relationship, Ferrite.
Objavljeno: 06.05.2019; Ogledov: 1554; Prenosov: 0
Polno besedilo (1,16 MB)