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Electronic and magnetic properties of isolated atoms on surfaces
Sandra Gardonio, C. Grazioli, 2013, objavljeni povzetek znanstvenega prispevka na konferenci

Najdeno v: ključnih besedah
Ključne besede: electron states in nanoscale materials, magnetic properties of nanostructures, photoemission, photoelectron spectra
Objavljeno: 06.05.2014; Ogledov: 2024; Prenosov: 5
URL Polno besedilo (0,00 KB)

Metka Benčina, 2015, doktorska disertacija

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.
Najdeno v: ključnih besedah
Ključne besede: nanoparticles, pyrochlore structure, photocatalysis, visible light
Objavljeno: 26.06.2015; Ogledov: 3268; Prenosov: 74
.pdf Polno besedilo (7,29 MB)

Development of Advanced TiO2/SiO2 Photocatalyst for Indoor Air Cleaning
Andraž Šuligoj, 2015, doktorska disertacija

Opis: TiO2 - SiO2 composites were synthesized by low-temperature sol-gel impregnation method, using four different titania sources (P-25 from Degussa, PC500 from Millennium, CCA 100 AS and CCA 100 BS from Cinkarna, later denoted as AS and BS, respectively) and deposited on aluminium and glass carriers. Ordered and disordered mesoporous silicas were impregnated with ce{TiO2} in powder or suspension form in the Ti : Si molar ratio 1 : 1. Structure, size, band gap, chemical composition and specific surface area of nanoparticles were determined by X-ray diffraction (XRD), scanning (SEM) and transmission electron microscopy (TEM), UV-vis diffuse reflectance spectroscopy (DR-UV-vis), Fourier transform infrared spectroscopy (FT-IR) and ce{N2} physisorption. Additionally, quantity of surface hydroxyl groups, surface acidity and mechanical stability of the coatings were determined by temperature programmed Fourier transform infrared spectroscopy (TPD-IR) and Wolff-Wilborn method, respectively. The photocatalytic activity of TiO2 and TiO2 - SiO2 composites was evaluated in the photodegradation of toluene and formaldehyde, as model VOCs, under UVA light irradiation in lab-made photoreactor system with two different regimes; batch and plug-flow mode. These two VOCs are being considered as examples of two of the six major classes of indoor air contaminants. Adsorption properties of the samples with toluene, have shown that the addition of mesoporous silica was beneficial. The increase of the adsorption of the bare AS TiO2 (9.5 %) was higher in the case of ordered silica, SBA-15 (2.8 times for AS/SBA15 to 26.8 %) than disordered SiO2 KIL-2 (2.4 times for AS/KIL2 to 22.7 %) although it was significant in both types of mesoporous silica supports (over 20 %). Adsorption was found to be dependent mostly on the quantity of surface Si-OH groups. Regarding the photocatalytic activities towards toluene degradation, the results with pure TiO2 showed the fastest kinetics in case of sample AS followed by PC500 and P25. The observed behaviour was ascribed to smaller particle size, and consequently higher specific surface area. Grafting titania onto silica showed the importance of structural parameters. Most importantly, if the pore structure of bare TiO2 collapsed, this resulted in decreased activity. On the other hand, retainment of the pore structure improved the distribution of nanoparticles, crystallinity and optical properties, which resulted in improved photocatalytic activity. In the degradation of formaldehyde, it was found that adsorption and degradation abilities of the materials were much more dependent on the structural properties of the samples, i.e., the pore structure. This was explained as a consequence of the different degradation mechanisms of both pollutants. Toluene degradation is governed by the oxidation through photogenerated holes - direct oxidation, whereas in the degradation of formaldehyde, the major oxidative species are OH radicals. In addition, the same as in toluene oxidation, the activity was dependent mainly on the number of crystal defects and the band gap values, that is the oxidation and reduction potentials of the catalysts. The degradation efficiency was increased from 88 % in pure AS TiO2 to 97 % when this titania was grafted onto colloidal silica (7C). An important highlight of the thesis is synthesis of a novel photocatalyst, labelled AS7C, which comprises an acidic colloidal suspension AS as TiO2 source and colloidal silica, using a low-temperature sol-gel impregnation method. This sample used all the above mentioned properties that improve photocatalytic activity towards both pollutants. Mechanical stability of the samples was also tested. The tests showed that binder, in the form of colloidal SiO2 (in size of 25 nm) in combination with colloidal titania of appropriate size - 6 nm, produced the highest mechanical stability of the coatings, which also showed excellent photocatalytic activity. Stability of the coatings, using AS as titania source, was greatly improved. The Wolff-Wilborn test on AS coating showed no mechanical resistance, while with the optimal (in terms of photocatalytic performance) addition of 7C SiO2 binder stability was excellent (F, which is in the middle of 6B-6H hardness scale). This sample (AS7C) was also tested for release of aerosols, during operation of the reactor, which could be harmful for human health at longer exposure times. It was found that aerosols are formed, probably as a consequence of detachment of nanoparticles in the first period of photodegradation test. However, their formation in consecutive tests was greatly reduced. Last but not least, a pilot plug-flow reactor was constructed to test the photocatalyst's efficiency in one-pass degradation of toluene. Sample AS7C was able to degrade toluene at conditions applied (v= 400 mL/min, m(catalyst) = 1049 mg, C(0) = 1 ppmv), which means that the out-flow from reactor was clean of the pollutant and any possible intermediates, comprising only of humid air and CO2. Deactivation of the catalyst was found at higher air flow and higher initial concentration of the pollutant. However, the concentrations of pollutants in living conditions are few orders of magnitude lower, hence this is a promising result.
Najdeno v: ključnih besedah
Ključne besede: Air remediation, TiO2 photocatalysis, Immobilization, Thin layers, TiO2/SiO2 composites
Objavljeno: 31.08.2015; Ogledov: 3475; Prenosov: 66
.pdf Polno besedilo (33,47 MB)

Search for energy dependent patterns in the arrival direction of cosmic rays at the Pierre Auger Observatory
Marko Zavrtanik, Danilo Zavrtanik, Lili Yang, Serguei Vorobiov, Darko Veberič, Marta Trini, Samo Stanič, Ahmed Saleh, Gašper Kukec Mezek, Andrej Filipčič, Tobias Winchen, 2015, objavljeni znanstveni prispevek na konferenci

Opis: Energy-dependent patterns in the arrival directions of cosmic rays are expected from deflections in galactic and extragalactic magnetic fields. We report on searches for such patterns in the data of the surface detector of the Pierre Auger Observatory at energies above E = 5 EeV in regions within approximately 15◦ around events with energy E > 60 EeV. No significant patterns are found with this analysis which can be used to constrain parameters in propagation scenarios.
Najdeno v: ključnih besedah
Ključne besede: ultra-high energy cosmic rays galactic and extragalactic magnetic fields magnetic deflection patterns Pierre Auger Observatory
Objavljeno: 02.03.2016; Ogledov: 1288; Prenosov: 95
.pdf Polno besedilo (1019,52 KB)

Search for Ultra-relativistic Magnetic Monopoles with the Pierre Auger Observatory
Marko Zavrtanik, Danilo Zavrtanik, Lili Yang, Serguei Vorobiov, Darko Veberič, Marta Trini, Samo Stanič, Ahmed Saleh, Gašper Kukec Mezek, Andrej Filipčič, Toshihiro Fujii, 2015, objavljeni znanstveni prispevek na konferenci

Opis: Ultra-relativistic magnetic monopoles, possibly a relic of phase transitions in the early universe, would deposit an amount of energy comparable to UHECRs in their passage through the atmosphere, producing highly distinctive air shower profiles. We have performed a search for ultra-relativistic magnetic monopoles in the sample of air showers with profiles measured by the fluorescence detector of the Pierre Auger Observatory. No candidate was found to satisfy our selection criteria and we establish upper limits on the flux of ultra-relativistic magnetic monopoles - the first from an UHECR detector - improving over previous results by up to an order of magnitude.
Najdeno v: ključnih besedah
Ključne besede: ultra-relativistic magnetic monopoles, extensive air showers, Pierre Auger Observatory
Objavljeno: 03.03.2016; Ogledov: 1211; Prenosov: 65
.pdf Polno besedilo (188,24 KB)

A meshless model of electromagnetic braking for the continuous casting of steel
Božidar Šarler, Robert Vertnik, Katarina Mramor, 2015, izvirni znanstveni članek

Najdeno v: ključnih besedah
Ključne besede: continuous casting of steel, turbulent flow, magnetic field, macro-segregation
Objavljeno: 17.05.2016; Ogledov: 1169; Prenosov: 24
URL Polno besedilo (0,00 KB)

Whiskerisation of polycrystalline SiC fibres during synthesis
Gleb N. Yushin, Z. G. Cambaz, Yury G. Gogotsi, Kateryna L. Vyshnyakova, Lyudmila N. Pereselentseva, 2004, izvirni znanstveni članek

Najdeno v: ključnih besedah
Ključne besede: carbothermal synthesis, whiskers, composites, spectroscopy
Objavljeno: 09.05.2017; Ogledov: 874; Prenosov: 22
URL Polno besedilo (0,00 KB)

Packing patterns of silica nanoparticles on surfaces of armored polystyrene latex particles
Sara Fortuna, Catheline A. L. Colard, Stefan A. F. Bon, Alessandro Troisi, 2009, izvirni znanstveni članek

Opis: Fascinating packing patterns of identical spherical and discotic objects on curved surfaces occur readily in nature and science. Examples include C60 fullerenes,(1, 2)13-atom cuboctahedral metal clusters,(3) and S-layer proteins on outer cell membranes.(4) Numerous situations with surface-arranged objects of variable size also exist, such as the lenses on insect eyes, biomineralized shells on coccolithophorids,(5) and solid-stabilized emulsion droplets(6) and bubbles.(7) The influence of size variations on these packing patterns, however, is studied sparsely. Here we investigate the packing of nanosized silica particles on the surface of polystyrene latex particles fabricated by Pickering miniemulsion polymerization of submicrometer-sized armored monomer droplets. We are able to rationalize the experimental morphology and the nearest-neighbor distribution with the help of Monte Carlo simulations. We show that broadening of the nanoparticle size distribution has pronounced effects on the self-assembled equilibrium packing structures, with original 12-point dislocations or grain-boundary scars gradually fading out.
Najdeno v: ključnih besedah
Ključne besede: packing patterns, silica, simulation, Monte Carlo, order, disorder, transition, armored particles, nanoparticles, Pickering emulsion
Objavljeno: 10.10.2016; Ogledov: 1110; Prenosov: 0
.pdf Polno besedilo (3,85 MB)

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