Physical and chemical properties of functionalized titanate nanostructures

Nanostrukturni titanati (nanocjevčice, nanokristali, nanožice) pokazuju jedinstvenu kombinaciju fizikalnih i kemijskih svojstava, kao što je velika specifična površina, veliki omjer dužine i promjera, mogućnost interkalacije i ionske izmjene, protonska vodljivost, te fotokatalitička aktivnost. Cilj disertacije je bio istražiti svojstva titanata i funkcionalizirati ih za primjenu kod fotokatalize, kao ojačavala u kompozitnim polimerima, te za dobivanje feroelektričnih nanostruktura. Kako bi se postigla željena svojstva nanostruktura od ključne važnosti je bilo odrediti strukturu, temperaturnu i mehanokemijsku stabilnost i prilagoditi uvjete sinteze. Prilikom funkcionalizacije titanata za primjenu u fotokatalizi titanatne nanocjevčice su dekorirane nanočesticama srebra i grijane u vodikovoj atmosferi. Za primjenu u kompozitnim polimerima titanatne nanocjevčice su silanizirane i istražena je njihova temperaturna stabilnost i čvrstoća. Prilikom dobivanja feroelektričnog nanostrukturnog BaTiO3, titanatne nanostrukture su korištene kao početni materijal za sintezu. Rezultati su pokazali koji su granični uvjeti mehanokemijske stabilnosti titanatnih nanocjevčica. Odredio se utjecaj funkcionalizacije i modifikacije titanatnih nanostruktura na njihova strukturna, kemijska i fizikalna svojstva kao što je apsorpcija Sunčevog zračenja i feroelektričnost barijevih titanatnih nanostruktura. Određivanje i poboljšanje navedenih svojstava od posebne je važnosti zbog moguće primjene navedenih nanostruktura u razgradnji otpadnih voda, solarnim ćelijama, povećanju čvrstoće i temperaturne otpornosti polimera, te moguće primjene dobivenih fotovodljivih BTO-TiO2 heterostruktura za holografsku memoriju.Titanate nanostructure (nanotubes, nanocrystals, nanowires) show a unique combination of physical and chemical properties, such as high specific surface area, a large aspect ratio, possibility of intercalation and ion exchange, proton conductivity and photocatalytic activity. The aim of the dissertation is to investigate the properties of titanate nanostructures and functionalize them for potential applications in photocatalysis, as strengthen in the composite polymers, and to obtain ferroelectric nanostructures. To achieve desired properties of nanostructures, it is crucial to determine the structure, temperature and mechanochemical stability and optimize the synthesis. For the application in photocatalysis, titanate nanotubes surface were decorated by silver nanoparticles and annealed in hydrogen atmosphere. The temperature stability and the strength of silanized titanate nanotubes were studied to improve their application in polymer composites. For the synthesis of nanostructured BaTiO3, titanate nanostructures were used as a precursor. The obtained results showed limitation of mechanochemical stability for titanate nanostructures, the influence of functionalization and modification of titanate nanostructures to their structural, chemical and physical properties. Properties as the Sun light absorption and ferroelectricity of barium titanate was studied. Determination and improvement of mentioned properties is of main importance for possible application of nanostructures for degradation of waste water, in solar cells, for increase of strength and thermal resistivity of polymers and possible application of photoconductive BTO-TiO2 heterostructures for holographic memories

Influence of RF excitation during pulsed laser deposition in oxygen atmosphere on the structural properties and luminescence of nanocrystalline ZnO:Al thin films

Thin ZnO:Al layers were deposited by pulsed laser deposition in vacuum and in oxygen atmosphere at gas pressures between 10 and 70 Pa and by applying radio-frequency (RF) plasma. Grazing incidence small angle x-ray scattering and grazing incidence x-ray diffraction (GIXRD) data showed that an increase in the oxygen pressure leads to an increase in the roughness, a decrease in the sample density, and changes in the size distribution of nanovoids. The nanocrystal sizes estimated from GIXRD were around 20 nm, while the sizes of the nanovoids increased from 1 to 2 nm with the oxygen pressure. The RF plasma mainly influenced the nanostructural properties and point defects dynamics. The photoluminescence consisted of three contributions, ultraviolet (UV), blue emission due to Zn vacancies, and red emission, which are related to an excess of oxygen. The RF excitation lowered the defect level related to blue emission and narrowed the UV luminescence peak, which indicates an improvement of the structural ordering. The observed influence of the deposition conditions on the film properties is discussed as a consequence of two main effects: the variation of the energy transfer from the laser plume to the growing film and changes in the growth chemistry

Nanocatalysts Unravel the Selective State of Ag

In the present work, we report on a comparative study of model catalysts during ethylene epoxidation reaction under industrially relevant conditions. The catalysts consist of Ag nanoparticles <6 nm and a reference sample ∼100 nm. Combining catalytic data with transmission electron microscopy, thermal desorption spectroscopy, and density functional theory allows us to show that catalytic performance is linked to the oxygen concentration in/on the Ag particles. Isotope experiments using 18O2 and C18O2 are conducted to gain insight into the nature and location of oxygen in/on the Ag nanoparticles. The oxygen species responsible for the CO2 formation and inhibition of the overall catalytic activity are identified, and the abundance of those species is shown to depend strongly on the pre‐treatment and reaction conditions, showing both are critical for effective oxygen management. By comparison with a conventional Ag/α‐Al2O3 catalyst, we demonstrate a low concentration of oxygen in/on Ag leads to the highest selectivity regardless of particle size. However, particle size dependent oxophilicity leads to significantly lower TOFs for the Ag nanoparticles. This study provides fundamental understanding of the performance of supported Ag particles in ethylene epoxidation and offers new strategies to improve performance under industrially relevant conditions

Surface Electron-Hole Rich Species Active in the Electrocatalytic Water Oxidation.

Iridium and ruthenium and their oxides/hydroxides are the best candidates for the oxygen evolution reaction under harsh acidic conditions owing to the low overpotentials observed for Ru- and Ir-based anodes and the high corrosion resistance of Ir-oxides. Herein, by means of cutting edge operando surface and bulk sensitive X-ray spectroscopy techniques, specifically designed electrode nanofabrication and ab initio DFT calculations, we were able to reveal the electronic structure of the active IrOx centers (i.e., oxidation state) during electrocatalytic oxidation of water in the surface and bulk of high-performance Ir-based catalysts. We found the oxygen evolution reaction is controlled by the formation of empty Ir 5d states in the surface ascribed to the formation of formally IrV species leading to the appearance of electron-deficient oxygen species bound to single iridium atoms (μ1-O and μ1-OH) that are responsible for water activation and oxidation. Oxygen bound to three iridium centers (μ3-O) remains the dominant species in the bulk but do not participate directly in the electrocatalytic reaction, suggesting bulk oxidation is limited. In addition a high coverage of a μ1-OO (peroxo) species during the OER is excluded. Moreover, we provide the first photoelectron spectroscopic evidence in bulk electrolyte that the higher surface-to-bulk ratio in thinner electrodes enhances the material usage involving the precipitation of a significant part of the electrode surface and near-surface active species

Thermal Stability and Utilization of 1D-Nanostructured Co3O4 Rods Derived by Simple Solvothermal Processing

For p-type semiconductor nanoparticles, such as the cobalt oxide spinel, enhancing the nanoparticle geometry can expose more of the surface and bring up the sensitivity and applicability, pointing to even more advantageous behaviour in comparison to n-type semiconductors which are known for a somewhat faster reactivity. Here, we present a strategy that relies on fostering a simple synthetic route that can deliver reasonably or comparably performing p-type-semiconducting partially 1D-Co3O4 material prepared under less technically and economically demanding conditions. Structurally monophasic Co3O4 nanoparticles with a spinel structure were indicated by powder X-ray diffraction, while the presence of traces of organic-phase residuals in otherwise chemically homogeneous material was observed by Fourier-transform infrared spectroscopy. Scanning electron microscopy further showed that the observed fine nanoparticle matter formed agglomerates with the possible presence of rod-like formations. Interestingly, using transmission electron microscopy, it was possible to reveal that the agglomerates of the fine nanoparticulated material were actually nanostructured, i.e., the presence of 1D-shaped Co3O4 rods embedded in fine nanoparticulated matrix was confirmed. In conjunction with the N2 adsorption–desorption isotherms, discussion about the orientation, exposure of nanostructured rod domains, and derivative geometry parameters was possible. The nanostructured Co3O4 material was shown to be stable up to 800 °C whereat the decomposition to CoO takes place. The specific surface area of the nanostructured sample was raised. For the purpose of testing the photoactivity of the prepared samples, simple sorption/photodegradation tests using methylene blue as the model pollutant were performed. The degradation performance of the prepared nanostructured Co3O4 was better described by a pseudo-second-order fit, suggesting that the prepared material is worth further development toward improved and stable immobilized photocatalysts.ISSN:2073-434