Estudo de microscopia eletrónica e espectroscopia de nanofios e nanotubos de titanato modificado

Dissertação de mestrado em PhysicsOne dimensional titanate nanostructures, such as titanate nanowires (TNWs) and nanotubes (TNTs), are promising alternatives to TiO2 in various applications including photocatalysis. They combine the properties of TiO2 nanoparticles with the properties of layered titanates such as ion exchange ability and higher speci c surface area, due to their open mesoporous morphology. In this thesis, investigations on cobalt doped and intercalated titanate nanowires and nanotubes using various characterization techniques are presented. Aberration corrected electron microscopy along with energy dispersive x-ray spectroscopy (EDX), is employed to ascertain the morphology, structure and chemical composition of the titanate nanostructures. In order to minimize electron beam induced damage, electron microscopy was carried out at 80 kV. EDX mapping revealed the distribution of the various elements including the dopant intercalant cobalt in both nanotubes and nanowires. L3/ L2 white line ratio in energy loss spectroscopy (EELS) is used to determine the oxidation state of cobalt in the modi ed titanate nanostructures, from single nanowire/nanotube. This analysis indicates the presence of cobalt in mixed oxidation state (2+ and 3+) in doped samples and 2+ oxidation state in intercalated samples. These findings were further confirmed by X-ray photoelectron spectroscopy (XPS) measurements. Raman analysis showed notable changes in the characteristic Raman peaks in the case of cobalt doped and intercalated titanate nanowires and nanotubes in comparison to the reference titanate nanowires and nanotubes. Cobalt incorporation was better achieved in the intercalated TNT and TNW samples in comparison to that of doped TNTs and TNWs. The findings in this work are very relevant because they can contribute to better understand, and even anticipate, the performance of such metal modified nanostructures, since they make possible to relate the performance with the amount, position and oxidation state of the metal in the nanostructure.As nanoestruturas unidimensionais de titanato, ou seja, nanofios (TNWs) e nanotubos (TNTs), apresentam-se como alternativas muito promissoras ao TiO2 em várias aplicações, em particular como fotocatalisadores. Estas estruturas combinam as propriedades e aplicações, das nanopartículas de TiO2, como por exemplo atividade (foto)catalítica com as propriedades dos titanatos lamelares, como a capacidade de troca iónica e elevada área superficial, decorrente da morfologia mesoporosa que apresentam. Nesta tese, são apresentados estudos de caracterização, de nanofios e nanotubos de titanato dopados e intercalados com cobalto, usando diferentes técnicas instrumentais. Neste ^âmbito, a microscopia eletrónica de aberração corrigida juntamente com a espectroscopia de energia dispersiva de raios X(EDX), foram utilizadas para averiguar a morfologia, estrutura e composição química das nanoestruturas de titanato. Nos estudos de microscopia eletrónica, e de forma a minimizar os danos criados pelo feixe de eletrões, foram aplicadas tensões na ordem dos 80 kV. O mapeamento de EDX revelou a distribuição dos vários elementos, incluindo o cobalto utilizado como dopante/intercalante nos nanotubos e nanofios. A razão L3/ L2 da linha branca na espectroscopia de perda de energia (EELS) foi usada para determinar o estado de oxidação do cobalto nas nanoestruturas elongadas de titanato modificado. As análises efetuadas em partículas isoladas de TNTs e TNWs modificados indicam a presença de cobalto com estado de oxidação misto (2+ e 3+) nas amostras dopadas e com estado de oxidação (2+) nas amostras intercaladas. Estes resultados foram confirmados por medidas de espetroscopia fotoeletrónica de raios X (XPS). Por espectrosocopia Raman verificaram-se mudanças significativas nos picos característicos para os casos dos nanofios e nanotubos de titanato dopados ou intercalados com cobalto, quando comparados com os obtidos para as estruturas de referência, TNTs e TNWs. A incorporação de cobalto foi melhor conseguida nas amostras intercaladas de TNTs e TNWs quando comparadas com os TNTs e TNWs dopados. Os resultados obtidos no decorrer deste trabalho são muito relevantes pois permitem compreender melhor, e mesmo antecipar, o desempenho destas estruturas modificadas, pois torna possível relacionar o desempenho de um determinado material, modificado por incorporação de um metal, com a quantidade, a posição e o estado de oxidação desse mesmo metal na estrutura

Far from equilibrium ultrafast high-temperature sintering of ZrO2-SiO2 nanocrystalline glass-ceramics

Ultrafast high-temperature sintering (UHS) is a novel sintering technique with ultrashort firing cycles (e.g., a few tens of seconds). The feasibility of UHS has been validated on several ceramics and metals; however, its potential in consolidating glass-ceramics has not yet been demonstrated. In this work, an optimized carbon-free UHS was utilized to prepare ZrO2-SiO2 nanocrystalline glass-ceramics (NCGCs). The phase composition, grain size, densification behavior, and microstructures of NCGCs prepared by UHS were investigated and compared with those of samples sintered by pressureless sintering. Results showed that NCGCs with a high relative density (similar to 95%) can be obtained within similar to 50 s discharge time by UHS. The UHS processing not only hindered the formation of ZrSiO4 and cristobalite but also enhanced the stabilization of t-ZrO2. Meanwhile, owing to the ultrashort firing cycles, the UHS technology allowed the NCGCs to be consolidated in a far from equilibrium state. The NCGCs showed a microstructure of spherical monocrystalline ZrO2 nanocrystallites embedded in an amorphous SiO2 matrix

Single scan STEM-EMCD in 3-beam orientation using a quadruple aperture

The need to acquire multiple angle-resolved electron energy loss spectra (EELS) is one of the several critical challenges associated with electron magnetic circular dichroism (EMCD) experiments. If the experiments are performed by scanning a nanometer to atomic-sized electron probe on a specific region of a sample, the precision of the local magnetic information extracted from such data highly depends on the accuracy of the spatial registration between multiple scans. For an EMCD experiment in a 3-beam orientation, this means that the same specimen area must be scanned four times while keeping all the experimental conditions same. This is a non-trivial task as there is a high chance of morphological and chemical modification as well as non-systematic local orientation variations of the crystal between the different scans due to beam damage, contamination and spatial drift. In this work, we employ a custom-made quadruple aperture to acquire the four EELS spectra needed for the EMCD analysis in a single electron beam scan, thus removing the above-mentioned complexities. We demonstrate a quantitative EMCD result for a beam convergence angle corresponding to sub-nm probe size and compare the EMCD results for different detector geometries

Electron microscopy and spectroscopy studies of modified titanate nanostructures

Titanate nanotubes (TNTs), such as that of Na2Ti3O7, are investigated as alternatives to TiO2 for photocatalysis, as they combine the properties of TiO2 nanoparticles with the properties of layered titanates such as cation exchangeability and open mesoporous morphology with higher specific surface area. Recently spectroscopic investigations using XPS and Raman spectroscopy on cobalt modified TNT samples showed that the presence of the Co cations and its concentration influences the optical and photocatalytic properties. The structure, composition and morphology of the material play a significant role in their catalytic activity. In this work, aberration corrected electron microscopy (AC-TEM/STEM) along with the associated spectroscopic techniques, electron energy loss spectroscopy (EELS) and energy dispersive x-ray spectroscopy (EDS) are employed to study TNTs/cobalt modified TNTs to provide an accurate and in-depth understanding of their structure, composition and morphology of the specimen in a single experiment, from the same region, with high spatial resolution

Silicon-Nanographite Aerogel-Based Anodes for High Performance Lithium Ion Batteries

To increase the energy storage density of lithium-ion batteries, silicon anodes have been explored due to their high capacity. One of the main challenges for silicon anodes are large volume variations during the lithiation processes. Recently, several high-performance schemes have been demonstrated with increased life cycles utilizing nanomaterials such as nanoparticles, nanowires, and thin films. However, a method that allows the large-scale production of silicon anodes remains to be demonstrated. Herein, we address this question by suggesting new scalable nanomaterial-based anodes. Si nanoparticles were grown on nanographite flakes by aerogel fabrication route from Si powder and nanographite mixture using polyvinyl alcohol (PVA). This silicon-nanographite aerogel electrode has stable specific capacity even at high current rates and exhibit good cyclic stability. The specific capacity is 455 mAh g−1 for 200th cycles with a coulombic efficiency of 97% at a current density 100 mA g−1