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Resumo: Dióxido de estanho, 'SNO IND.2', é um material semicondutor com transição de banda de energia (bandgap) larga, sendo que para o cristal (bulk), a energia pode variar de 3,6 até 4,2 eV, dependendo do método utilizado no preparo do material e também do teórico utilizado para o cálculo do bandgap. Suas propriedades ópticas, elétricas e estruturais são responsáveis pela grande quantidade de aplicações tecnológicas, tais como sensores para deteccção de gases, dispositivos óptico-eletrônicos, varistores e mostradores (displays) de cristal liquido entre outras. A natureza de transição de bandas de energia de 'SNO IND.2' tem sido motivo de controvérsia entre vários trabalhos já publicados, tanto teóricos quanto experimentais. Neste trabalho, apresentamos uma revisão de trabalhos teóricos e experimentais, selecionados da literatura, a respeito da natureza da transição direta ou indireta do bandgap do material. Apresentamos também um estudo das propriedades ópticas e estruturais de filmes finos de 'SNO IND.2', depositados sobre substrato de vidro e principalmente sobre substrato de quartzo, pela técnica de dip-coating via sol-gel. A influência dos dopantes foi analisada através de caracterização estrutural por difração de raios-X e também de medidas de absorção óptica. Além disso, também foram realizados cálculos da estrutura de bandas de cristal (bulk), superfície (110) e superfície (101), usando métodos de Primeiros-Princípios. Estes resultados são comparados ao bandgap obtido experimentalmente através de dados de absorção óptica e fotocondutividade em função do comprimento de onda da luz. / Abstract: Tin dioxide, 'SNO IND.2', is a wide bandagap semiconductor. For the bulk, the bandgap energy may vary in the range 3.6 to 4.2 eV, depending on the method used for its preparation or on the theoretical method employed for the bandgap calculation. The combination of its optical, electrical and structural properties is responsible by the large number of technological applications, such as gas sensors, opto-electronic devices, varistors and liquid crystal display, among others. The bandgap nature has been the focus of controversy among many published papers, either experimental as well as theoretical. In this work, we present a review of published theoretical and experimental papers, selected from the literature, concerning the driect or indirect bandgap nature. A study of optical and structural properties od 'SNO IND.2' thin films, deposited by sol-gel-dip-coating technique, on glass and mainly quartz substrates, is also shown. The influence of doping is also analyzed by the structural characterization through X-ray diffraction data as well as optical absorption measurements. Besides, band structure calculation is also shown, performed on bulk, besides (110) and (101) surfaces, using the First Principles method. These results are compared to experimentally obtained bandgap, through experimental data of optical absorption and photoconductivity as function of light wavelength. / Orientador: Luís Vicente de Andrade Scalvi / Coorientador: Júlio Ricardo Sambrano / Banca: Margarida Juri Saeki / Banca: Sérgio Ricardo de Lázaro / O Programa de Pós-Graduação em Ciência e Tecnologia de Materiais, PosMat, tem caráter institucional e integra as atividades de pesquisa em materiais de diversos campi da Unesp / Mestre

Orientador: Luis Vicente de Andrade Scalvi / Banca: Margarida Juri Saeki / Banca: Tomaz Catunda / O Programa de Pós-Graduação em Ciência e Tecnologia de Materiais, PosMat, tem caráter institucional e integra as atividades de pesquisa em materiais de diversos campi da Unesp / Resumo: A proposta deste trabalho é a investigação das propriedades elétricas e ópticas de filmes finos de dióxido de estanho (Sn'O IND. 2') obtidos via sol-gel e por solução alcoólica depositados via dip-coating, e, filmes de alumina ('Al IND. 2''O IND. 3') obtidos por deposição de filmes de alumínio (Al) via evaporação resistiva e tratamento térmico em diferentes ambientes, para promover a oxidação de Al. A investigação individual quanto às propriedades ópticas e elétricas desses materiais conhecer seu comportamento na forma de filmes, e estudar a região interfacial de Sn'O IND. 2' e 'Al IND. 2''O IND. 3'. As caracterizações estruturais dos filmes foram feitas por difração de raios-X (DRX), e, no caso dos filmes de alumina, utilizou-se também microscopia eletrônica de varredura (MEV) e microscopia óptica. Nas caracterizações ópticas foram utilizadas técnicas de espectroscopia na região do ultravioleta e no infravermelho próximo (UV-Vis-Nir). Tanto os filmes obtidos por meio alcoólico como obtidos via SGDC foram caracterizados como sendo de Sn'O IND. 2' de estrutura tetragonal do tipo rutilo, sendo que os filmes obtidos via processo alcoólico apresentaram condutividade elétrica maior do que os filmes obtidos via SGDC. Os resultados referentes aos filmes finos de alumínio indicam que independentemente da quantidade de camadas de alumínio depositadas e da atmosfera de tratamento térmico, tem-se a oxidação do alumínio à alumina ('Al IND. 2''O IND. 3'), sendo que a estrutura dominante depende da atmosfera de tratamento. A sua utilização como camada isolante no gate em dispositivo metal-óxido-semicondutor é viável, pois a corrente fonte-dreno apresenta valores significativamente maiores do que a corrente fonte-gate. / Abstract: The main goal of this work is the investigation of properties of tin dioxide (Sn'O IND. 2') and alumina ('Al IND. 2''O IND. 3) thin films. The first one was obtained through the sol-gel process as well as alcoholic solution, via dip-coating. The alumina thin films were obtained by resistive evaporation of aluminum (Al) followed by thermal annealing in distinct atmospheres, to promote the Al oxidation. The individual investigation of optical and electrical properties of these materials aims the knowledge of their behavior as thin films, which allows studying the interface layer of the heterojunction Sn'O IND. 2' e 'Al IND. 2''O IND. 3'. Structural characterization of films was carried out by X-ray diffraction (XRD) technique and particularly on the alumina films, scanning electron microscopy (SEM) and optical microscopy were done. For the optical characterization, wide spectra were obtained, with spectroscopy from ultraviolet to near infrared (UV-Vis-Nir). Either the films obtained in the alcoholic solution as well as via SGDC, where characterized as Sn'O IND. 2' of tetragonal structure of rutile type, and the films obtained through alcoholic process present electrical conductivity higher than the films obtained via SGDC. Results on aluminum thin films indicate that independent on the amount of deposited aluminum and thermal annealing atmosphere, the oxidation of aluminum to alumina ('Al IND. 2''O IND. 3) takes place, but the dominant alumina structure depends on the thermal annealing atmosphere. Besides, its utilization as insulating layer at the gate of a metal-oxide semicondutor device is achievable, because the source-drain current is significantly higher than the source-gate current. / Mestre

abstract: The thesis studies new methods to fabricate optoelectronic Ge1-ySny/Si(100) alloys and investigate their photoluminescence (PL) properties for possible applications in Si-based photonics including IR lasers. The work initially investigated the origin of the difference between the PL spectrum of bulk Ge, dominated by indirect gap emission, and the PL spectrum of Ge-on-Si films, dominated by direct gap emission. It was found that the difference is due to the supression of self-absorption effects in Ge films, combined with a deviation from quasi-equilibrium conditions in the conduction band of undoped films. The latter is confirmed by a model suggesting that the deviation is caused by the shorter recombination lifetime in the films relative to bulk Ge. The knowledge acquired from this work was then utilized to study the PL properties of n-type Ge1-ySny/Si (y=0.004-0.04) samples grown via chemical vapor deposition of Ge2H6/SnD4/P(GeH3)3. It was found that the emission intensity (I) of these samples is at least 10x stronger than observed in un-doped counterparts and that the Idir/Iind ratio of direct over indirect gap emission increases for high-Sn contents due to the reduced gamma-L valley separation, as expected. Next the PL investigation was expanded to samples with y=0.05-0.09 grown via a new method using the more reactive Ge3H8 in place of Ge2H6. Optical quality, 1-um thick Ge1-ySny/Si(100) layers were produced using Ge3H10/SnD4 and found to exhibit strong, tunable PL near the threshold of the direct-indirect bandgap crossover. A byproduct of this study was the development of an enhanced process to produce Ge3H8, Ge4H10, and Ge5H12 analogs for application in ultra-low temperature deposition of Group-IV semiconductors. The thesis also studies synthesis routes of an entirely new class of semiconductor compounds and alloys described by Si5-2y(III-V)y (III=Al, V= As, P) comprising of specifically designed diamond-like structures based on a Si parent lattice incorporating isolated III-V units. The common theme of the two thesis topics is the development of new mono-crystalline materials on ubiquitous silicon platforms with the objective of enhancing the optoelectronic performance of Si and Ge semiconductors, potentially leading to the design of next generation optical devices including lasers, detectors and solar cells. / Dissertation/Thesis / Ph.D. Chemistry 2013

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Neste trabalho foram investigados o efeito da adição de diferentes quantidades de IrO2 (entre 1 e 30% em mol) em eletrodos de SnO2 e suas atividades eletrocatalíticas para a reação de oxidação de etanol. A inovação deste trabalho está na composição do eletrodo onde a concentração de IrO2 varia de níveis da ordem de dopante até concentrações típicas de ADE. O método de preparação utilizado permitiu a obtenção de filmes homogêneos e com estequiometria controlada, comprovados pela técnica de Energia Dispersiva de Raios-X (EDX). A análise de Microscopia Eletrônica de Varredura (MEV) mostrou que os filmes possuem morfologia com trincas. Na análise por Difração de Raios-X (DRX) ficou evidente a formação de solução sólida. A caracterização por Voltametria Cíclica mostrou que a carga voltamétrica e a corrente relacionada a reação de desprendimento de oxigênio (RDO) aumentam com a concentração de IrO2 na composição do eletrodo. Adicionalmente, a ordem da estabilidade eletroquímica encontrada é a seguinte: 30 >> 1 @ 10 > 5% em mol de IrO2. Para avaliar a atividade eletrocatalítica destes eletrodos para a reação de oxidação de etanol foram utilizadas as técnicas de voltametria cíclica, eletrólise a corrente constante e Infravermelho com Transformada de Fourier (FTIR) in situ. Foram identificados como produtos acetaldeído, acido acético e CO2. Comprovouse que o ácido acético não é oxidado nas condições experimentais empregadas. Observou-se que os eletrodos contendo 5 e 10% em mol de IrO2 apresentaram as maiores velocidades de oxidação do etanol formando preferencialmente ácido acético. Os eletrodos contendo 1 e 30% em mol de IrO2 apresentaram maior seletividade para CO2 em relação as outras composições. A adição de 1% em mol de IrO2 no SnO2 foi suficiente para promover uma estabilidade satisfatória para a aplicação deste eletrodo. Outra observação importante foi que este eletrodo apresentou a menor seletividade para a formação de ácido acético permitindo uma maior formação de CO2. / In this work was investigated the effect of the IrO2 addition (1 30 mol% contents) on SnO2 electrodes and its electrocatalytic activities towards ethanol oxidation. The new approach of this work is related to the IrO2 contents on the electrodes compositions shifts between the dopant levels until Dimensionally Stable Anodes (DSA®) concentrations. The preparation method used allowed to obtain homogeneous films with controlled stoichiometry, that was confirmed by Energy Dispersive X-ray (EDX) analysis. The Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD) analysis showed a cracked morphology and the formation of solid solution, respectively. The voltammetric experiments showed an increase in the Oxygen Evolution Reaction (OER) currents and the voltammetric charges for the higher IrO2 contents. In addition, the stability order found is 30 >> 1 @ 10 > 5 mol% IrO2. In order to investigate the electrodes activities towards ethanol oxidation, the cyclic voltammetry, electrolysis and Fourier Transform Infrared (FTIR) in situ techniques were used. The presence of products as acetaldehyde, acetic acid and CO2 was detected. Acetic acid was not oxided in the experimental conditions employed. The 5 and 10 mol% IrO2 electrodes showed higher ethanol oxidation rates with the preferential acetic acid formation. The electrodes containing 1 and 30 mol% IrO2 are more selective to CO2 production than the others compositions. The addition of 1 mol% IrO2 was enough to enhance the electrode stability, that make it applicable. Another interesting observation is that the IrO2 1 mol% electrode presented the smaller selectivity for the acid acetic formation allowing the complete oxidation of ethanol possible.

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第15389号 / 工博第3268号 / 新制||工||1492(附属図書館) / 27867 / 京都大学大学院工学研究科物質エネルギー化学専攻 / (主査)教授 江口 浩一, 教授 井上 正志, 教授 垣内 隆 / 学位規則第4条第1項該当

Philosophiae Doctor - PhD / Direct methanol fuel cells (DMFCs) are attractive power sources as they offer high conversion efficiencies with low or no pollution. One of the major advantages DMFCs has over PEMFCs is that methanol is a liquid and can be easily stored where in the case for PEMFCs storing hydrogen requires high pressures and low temperatures. However, several challenging factors especially the sluggish oxygen reduction reaction (ORR) and the high cost of Pt catalysts, prolong their commercialization. With the aim to search for more active, less expensive more active ORR catalysts and methanol tolerant catalysts than pure Pt, this dissertation focuses on the development of low loading Pt electrocatalyst and the understanding of their physical and electrochemical properties. Pt-Sn/C electrocatalsyts have been synthesized by a modified polyol reduction method. The effect of temperature, pH, water, sonication and addition of carbon form were studied before a standard polyol method was established. From XRD patterns, the Pt-Sn/C peaks shifted slightly to lower 2Ө angles when compared with commercial Pt/C catalyst, suggesting that Sn is alloying with Pt. Based on HRTEM data, the Pt-Sn/C nanoparticles showed small particle sizes well-dispersed onto the carbon support with a narrow particle distribution. The particle sizes of the different as-prepared catalysts were found to be between 2-5 nm. The Pt-Sn/C HA Slurry pH3 catalysts was found to be the best asprepared catalyst and was subjected to heat-treatment in a reducing atmosphere at 250-600 °C which led to agglomeration yielding nanoparticles of between 5-10 nm. The Methanol Oxidation Reaction (MOR) on the as-prepared Pt-Sn/C HA Slurry pH3 catalyst appeared at lower currents (+7.11 mA at 860 mV vs. NHE) compared to the commercial Pt/C (+8.25 mA at +860 mV vs. NHE) suggesting that the Pt-Sn/C catalyst has „methanol tolerance capabilities‟. Pt-Sn/C HA Slurry pH3 and Pt-Sn/C 250 °C catalysts showed better activity towards the ORR than commercial Pt/C with specific and mass activities higher than Pt/C at +0.85 V vs NHE. The Tafel slopes of Pt-Sn/C HA Slurry pH3 catalyst was -62 and -122 mV dec-1 for the low and high current regions respectively and suggests that the ORR mechanism is similar to that of commercial Pt/C indicating that the ORR kinetics was not negatively influenced by the addition of tin. It was found that the electrochemical oxidation reduction reaction follows first order kinetics of a multi-electronic (n=4ē) charge transfer process producing water. All the Pt-Sn/C catalysts showed resistance towards MOR and it was found for the heat-treated catalysts that an increase in temperature resulted in an increase in methanol tolerance. The synthesized Pt-Sn/C HA Slurry pH3 catalysts were also tested in a fuel cell environment. Electrodes were prepared by either spraying on Toray carbon paper with the Asymtek machine or by VI spraying directly on the membrane with a hand spray gun the catalysts coated membrane (CCM) technique. Polarization curves obtained in DMFC with CCM showed superior performance than electrode prepared by spraying on the carbon paper with the machine. In our study, the Pt-Sn/C catalyst appears to be a promising methanol tolerant catalyst with activity towards the ORR in the DMFC.