<p>Over the past decade, the study of zinc oxide (ZnO) II-VI semiconducting nanostructures has been a burgeoning research area because of this material's unique electrical and optical properties. Despite the promise of its characteristics for numerous applications, usage of ZnO in the fabrication of nanoscale devices on a commercial scale remains a challenge because of our lack of knowledge of the underlying physics and chemistry of nanostructures. Sustainable progress in nanowire manufacturing techniques requires that we first undertake basic studies to address these poorly understood underlying concepts before we embark on applied engineering. If these fundamental studies prove successful, then characterization, fabrication, and large-scale integration of nanostructures that use ZnO could be applied to a range of engineering fields. This doctoral dissertation is primarily concerned with the synthesis and doping required for the creation of novel ZnO nanostructures and the growth mechanisms of such structures. Numerous studies have been made of various kinds of ZnO nanostructures. However, no studies have been reported of systematic theoretical modeling that uses both density functional theory and as-synthesized nanostructures to explain the growth mechanisms involved in these devices. First, sulfur-doped ZnO nanostars, synthesized through a hydrothermal method, will be discussed. This section uses ab initio simulations in discussing the synthesis of novel ZnO nanostructures and their proposed growth mechanisms. Moreover, this discussion also addresses the optical properties of ZnO structures that cause sulfur doping to enhance their emission of green light. The next section introduces a novel synthetic methodology to reliably produce well-aligned vertical ZnO nanowire arrays on amorphous substrates. Vertical alignment of nanowires significantly improves the performance of devices like LEDs and solar cells. Because these vertically aligned arrays have historically been made using sapphire substrates that hinder their commercialization, substantial effort has been invested in using ZnO nanocrystal seeds to grow vertically aligned ZnO nanowires on silicon substrates. Well-known synthetic methods, such as zinc acetate dissolved in methanol or zinc acetate combined with sodium hydroxide (or potassium hydroxide), have typically been used in pursuit of this goal without a detailed understanding of the mechanisms of seed creation. The consequence of this lack of knowledge has been inconsistent reproducibility in growing vertically aligned nanowires on silicon substrates. This discussion includes the details of mechanisms that explain the why and how of creation of vertical/misoriented ZnO nanocrystal seeds on silicon substrates. In addition, a preferential c-axis-oriented ZnO nanocrystal seed has been successfully synthesized using a solution composed of ammonium hydroxide (NH4OH) and zinc acetate (Zn(O2CCH3)2). Lastly, the synthesis of sea urchin-like microstructures known as ZnO sea urchins will be introduced. Among the various kinds ZnO structures, the ZnO sea urchin is a integrated structure composed of a 3-D microsphere and 1-D nanowires. Dye-sensitized solar cells (DSSCs) made of ZnO sea urchins have shown a higher power conversion efficiency than planar nanowires. This is because ZnO sea urchins have a higher surface area per unit of volume than planar nanowire arrays. This larger surface area allows larger amounts of dye to access the semiconducting nanowires. We have synthesized the sea urchin structures composed of ZnOxPy microspheres, a mixed of zinc phosphide (Zn3P2) and ZnO phase, encapsulated in an array of ZnO nanowires. Synthesis of these interesting structures was achieved without resorting to the prefabricated 3-D microsphere templates that other groups used in previous studies. This new approach to the synthesis of ZnO sea urchin structures was accomplished by simply adding Zn3P2 powder to the C (graphite) and ZnO source powders in a chemical vapor transport method. The ZnO sea urchin's material properties and growth mechanism will be characterized and discussed in detail.</p> / Dissertation

A scalable spray processing technique was used to fabricate carbon nanotube (CNT)-based film electrodes and solid-state supercapacitors. The sprayed CNT-based electrodes comprised a randomly interconnected meso-porous network with a high electrical conductivity. Layer-by-layer (LbL) deposition of functionalised and oppositely charged single-wall carbon nanotubes (SWNTs) increased the electrode density and improved charging and discharging kinetics when compared with carboxylic functionalised only SWNT electrodes. The capacitance was further increased to 151 F g-1 at 2 mV s-1 and 120 F g-1 at 100 mV s-1 after vacuum and H2 heat treatments that removed the functional groups, and resulted in a hybrid microstructure of SWNTs and multi-layer graphene sheets from unzipped SWNTs. Flexible solid-state supercapacitors were fabricated by directly spraying multi-wall carbon nanotube (MWNT)-based aqueous suspensions onto both sides of a Nafion membrane and dried. A single cell with MWNT-only electrodes had a capacitance of 57 F g-1 per electrode at 2 mV s-1 and 44 F g-1 at 150 mV s-1. Cells with MWNT/ionomer electrodes showed a higher H+ mobility and a lower charge transfer resistance, and the capacitance increased to 145 F g-1 at 2 mV s-1 and 91 F g-1 at 150 mV s-1. Finally, MWNT/TiO2 nanoparticle/ionomer hybrid electrodes were used in the same solid-state supercapacitor configuration and provided a capacitance of 484 F g-1 per electrode at 5 mV s-1 and 322 F g-1 at 100 mV s-1. A qualitative model of the charge storage mechanism was developed, where TiO2 promoted H+ ions via redox reactions that fed protons into the proton-conducting ionomer coating over the MWNTs (in which the TiO2 was embedded), while electrons were readily conducted through the MWNT scaffold. This solid-state supercapacitor provided both attractive energy (31.8 Wh kg-1) and power (14.9 kW kg-1) densities, where such high energy density is difficult to achieve for MWNTs alone and such high power density is difficult for metal oxides alone, especially in the solid state.

The desire for pure diatomic hydrogen gas, H2(g), has been on the rise since the concept of the hydrogen economy system was proposed back in 1970. The production of hydrogen has been extensively examined over 40 + years as the need to replace current fuel sources, hydrocarbons, has become more prevalent. Currently there are only two practical and renewable production methods of hydrogen; landfill gas and power to gas. This study focuses on the later method; using various renewable energy sources, such as photovoltaics, to provide off-peak energy to perform water electrolysis. Efficient electrolysis takes place in electrochemical cells which maximize performance efficiency with the use of noble metal electrocatalyst. Optimizing these electrocatalyst to be less material dependent, highly durable, and more efficient will support the implementation of power to gas electrolysis into the energy infrastructure. The main focus of this study is to explore RuO2 nanorods as a possible electrocatalyst for Proton Exchange Membrane (PEM) water electrolysis. A PEM electrolyzer cell has been constructed and fitted with a RuO2 nanorod decorated, mixed metal oxide (MMO) ribbon mesh anode catalyst structure. The current density-voltage characteristics were measured for the RuO2 nanorod electrocatalyst while under water feed operation. The electrocatalytic behavior was compared to that of ribbon mesh anode catalyst structures not decorated with RuO2 nanorods; one coated with a Ir/Ta MMO catalyst, the other was stripped of the MMO coating resulting in a Ti ribbon mesh anode. The results of these experiments show increased activity with the RuO2 nanorod electrocatalyst corresponding to a decrease in electrochemical overpotential. Through the collection of experimental data from various electrolyzer cell configurations, these overpotenials were able to be identified, resulting in categorical attributions of the enhanced catalytic behavior examined.

京都大学 / 0048 / 新制・課程博士 / 博士(薬科学) / 甲第22397号 / 薬科博第119号 / 新制||薬科||13(附属図書館) / 京都大学大学院薬学研究科薬科学専攻 / (主査)教授 髙倉 喜信, 教授 山下 富義, 教授 小野 正博 / 学位規則第4条第1項該当 / Doctor of Pharmaceutical Sciences / Kyoto University / DFAM

Thesis advisor: Zhifeng Ren / Solid-state cooling and power generation based on thermoelectric effects are attractive for a wide range of applications in power generation, waste heat recovery, air-conditioning, and refrigeration. There have been persistent efforts on improving the figure of merit (ZT) since the 1950's; only incremental gains were achieved in increasing ZT, with the (Bi1-xSbx)2(Se1-yTey)3 alloy family remaining the best commercial material with ZT ~ 1. To improve ZT to a higher value, we have been pursuing an approach based on random nanostructures and the idea that the thermal conductivity reduction that is responsible for ZT enhancement in superlattices structures can be realized in such nanostructures. The synthesis and characterization of various nanopowders prepared by wet chemical as well as high energy ball milling methods will be discussed in this dissertation. The solid dense samples from nanopowders were prepared by direct current induced hot press (DC hot press) technique. The thermoelectric properties of the hot pressed samples have been studied in detail. By ball milling ingots of bulk alloy crystals and hot pressing the nanopowders, we had demonstrated a high figure-of-merit in nanostructured bulk bismuth antimony telluride. In this dissertation, we use the same ball milling and hot press technique, but start with elemental chunks of bismuth, antimony, and tellurium to avoid the ingot formation step. We show that a peak ZT of about 1.3 can be achieved. Our material also exhibits a ZT of 0.7 at 250 °C, close to the value reached when ingot was used. This process is more economical and environmentally friendly than starting from bulk alloy crystals. The ZT improvement is caused mostly by the low thermal conductivity, similar to the case using ingot. Transmission electron microscopy observations of the microstructures suggest that the lower thermal conductivity is mainly due to the increased phonon scattering from the high density grain boundaries and defects. The performance of thermoelectric materials is determined by its dimensionless figure-of-merit (ZT) which needs to be optimized within a specific temperature range for a desired device performance. Hence, we show that by varying the Bi/Sb ratio, the peak ZT can be shifted to a higher or lower temperature for power generation applications or a cooling mode operation. A peak ZT of about 1.3 is achieved from a Bi0.4Sb1.6Te3 composition which is highest among the different compositions. These nanostructured bulk samples have a significantly low lattice thermal conductivity compared to the bulk samples due to the increased phonon scattering in the grain boundaries and defects. This study shows that Bi0.5Sb1.5Te3 may potentially perform better for cooling devices, while Bi0.3Sb1.7Te3 should be able to show better power generation efficiency. Several issues related to accurate measurement of thermoelectric properties were identified and many of them were solved during my studies and these are discussed in this thesis. With the data we obtained, it is clear that nanopowder-based thermoelectric materials hold significant promise. Therefore, a review of synthesis of nanostructured materials by solution-based methods, including a hydrothermal process for the Bi2Te3, Bi2Se3, and Bi2Te2.25Se0.75 nanoparticles, a solvothermal route for Sb2Te3 nanostructures, and a polyol process for the preparation of Bi nanostructures is presented in this dissertation. These new nanostructures may find applications in enhancing the thermoelectric performance. Although small sized and well dispersed nanopowders of various thermoelectric materials could be prepared by a solution method in large scale, contamination and partial oxidation are always big challenges in a chemical approach. Hence, a high energy ball milling technique to prepare thermoelectric nanopowders in large scale and without major contamination is still found to be more efficient and preferred. / Thesis (PhD) — Boston College, 2009. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Les matériaux à base de dioxyde de titane (TiO2) sont des candidats prometteurs pour remplacer le graphite utilisé actuellement dans les électrodes négatives des batteries lithium-ion (LIB), du fait de leur sécurité élevée, de leur capacité volumétrique supérieure et de leurs excellentes performances à hautepuissance.Dans cette thèse, différentes approches de synthèse à bas coût sont évaluées pour préparer du TiO2 nanostructuré avec différentes compositions de phase et des morphologies variées. L'influence de ces paramètres sur la capacité de TiO2 à insérer réversiblement le lithium est étudiée par des mesures électrochimiques. À cet égard nous avons également étudié l'effet du dopage aliovalent et de la morphologie poreuse sur les propriétés d'insertion du TiO2, révélant des résultats encourageants avec notamment un transfert de charge amélioré, principale limitation des matériaux à base d'oxyde de titane. Afin de comprendre le processus de stockage du lithium des deux phases de TiO2 synthétisées, des méthodes de diffraction et de caractérisation spectroscopique ont été utilisées dans des conditions opérando.Nous montrons qu'indépendamment de leur similitude de composition chimique, les deux phases révèlent des mécanismes d'insertion du lithium très différents, menant à des propriétés électrochimiques de charge/décharge très différentes.Nous avons également amélioré les performances électrochimiques en travaillant sur la formulation d'électrodes à base de TiO2 nanostructuré, en optimisant le choix des composants (additif carboné, liant, électrolyte) et le processus de préparation. De nombreuses réactions parasites électrode-électrolyte ont été mises en évidence à travers cette étude, phénomènes très peu décrits dans la littérature à ce jour. / Titania based electrode materials are promising candidates to replace widely used graphite as negative electrode material in lithium ion batteries (LIB), due to their increased safety, volumetric capacity, and high rate performance.In this thesis different low-cost synthesis approaches are evaluated to prepare nanostructured TiO2 with various phase composition and morphology. The influence of these parameters on its ability to reversibly insert lithium are studied in electrochemical measurements. In this regard we also investigated the effect of aliovalent doping and porous structures on the insertion properties of two main polymorphs of TiO2, Anatase and TiO2(b), revealing encouraging results in overcoming the low charge transfer, which is the main drawback of titanium oxide based materials.In order to understand the mechanism of lithium storage process of the two synthesized TiO2 phases, diffraction and spectroscopic characterization methods were carried out under operando conditions. We show that, regardless of their chemical similarity, both phases reveal very different lithium insertion processes, leading to distinct electrochemical cycling properties.Another field of interest is the adaptation of electrode components to the nanostructured TiO2 active insertion material. The choice of binder, carbon additive, and electrolyte components can have significant impacts on the performance. Especially the origin and prevention of parasitic side reactions were in the focus of our work, as these pose an under estimated hindrance in the application of titania based electrode materials in LIB.

Neste trabalho, foram desenvolvidos filmes finos de composição SrTi1-xFexO3 (0,00&le; x &le;0,150) nanoestruturados visando sua aplicação como sensor de gás ozônio. Os filmes finos foram depositados através da técnica de deposição por feixe de elétrons (EBD) cujo alvo utilizado foram pastilhas obtidas a partir do pó cristalino SrTi1-xFexO3 (0,00&le; x &le;0,150) sintetizadas através do método dos precursores poliméricos. Foi observado a partir das analises termogravimétrica e térmica diferencial que a incorporação de ferro no sistema diminui a temperatura de queima do pó precursor, sugerindo que o ferro atua como catalisador na cadeia polimérica. Os filmes depositados por DFE apresentam-se no estado amorfo sendo necessário um tratamento térmico ex-situ para que a fase cristalina desejada seja obtida. Todos os filmes apresentaram uma boa aderência aos diferentes tipos de substratos utilizados. Após o processo de cristalização, os filmes depositados sobre diferentes substratos foram caracterizados através das técnicas de difração de raios-X, espectroscopia UV-Vis e microscopia de força atômica (MFA). Foi observado que um aumento na temperatura de tratamento térmico dos filmes leva a um aumento no grau de cristalização e a uma diminuição no valor da energia de gap, calculada a partir dos espectros UV-VIS. A analise por MFA mostrou que a mudança do tipo de substrato utilizado não influencia as propriedades estruturais e microestruturais dos filmes. Através da realização de medidas de resistência elétrica, observou-se que os filmes cristalinos submetidos a um tratamento térmico ex-situ a 500oC por 4 horas apresentaram uma boa sensibilidade ao gás ozônio sendo possível detectar a presença de até 75 ppb de ozônio. / In this study, nanostructured thin films of SrTi1-xFexO3 (0.00 &le; x &le; 0.150) compositions were prepared looking their application as ozone gas sensor. The thin films were deposited using the technique of electron beam deposition (EBD) whose targets were obtained from polycrystalline SrTi1-xFexO3 (0.00 &le; x &le; 0.150) powders synthesized by the polymeric precursor method. It was observed from the thermogravimetric and differential thermal analysis that the incorporation of iron in the system decreases the calcination temperature of the precursor powders, suggesting that the iron acts as a catalyst in the polymer chain. The as obtained films deposited by EBD present an amorphous state being necessary a ex-situ heat treatment to obtain the desired crystalline phase. All films showed good adhesion to different substrates. After the crystallization process, the films deposited on different substrates were characterized through X-ray diffraction, UV-Vis spectroscopy and atomic force microscopy (AFM) trechniques. It was observed that an increase in the annealing temperature of the film results in an increase in the degree of crystallization and a decrease in the value of band gap energy, which was calculated from the UV-VIS spectra. The AFM analysis showed that changing the type of substrate does not influence the structural properties and microstructure of the films. By carrying out measurements of electrical resistance, it was observed that the crystalline films subjected to a ex-situ heat treatment at 500oC for 4 hours showed a good sensitivity to the ozone gas being possible to detect the presence of up to 75 ppb ozone.

Neste trabalho utilizamos métodos computacionais ab initio, baseados na Teoria do Funcional da Densidade (DFT), para simular em nível atomístico propriedades estruturais, eletrônicas e magnéticas de nanoestruturas de grafeno. Estudamos nanoflocos de grafeno (GNFs) em estado pristino e GNFs com defeitos intrínsecos (monovacância, divacância e Stone-Wales). Escolhemos GNFs com diferentes terminações e formas, e estudamos também empilhamentos duplos - biflocos - em diferentes composições. Empregamos dois enfoques diferentes de DFT, a aproximação de gradiente generalizado simples no nível teórico de Perdew-Burke-Ernzerhof (PBE), e PBE híbrida (PBEh), incorporando uma fração de troca de Hartree-Fock. Todos os cálculos foram realizados através do código all-electron AIMS, incluindo correções de van der Waals. Nossos GNFs foram escolhidos com simetrias específicas: D2h, D3h e D6h, e com diferentes bordas, armchair (AC), zigue-zague (ZZ) e misturas das duas. Os flocos hexagonais D6h apresentam um gap de energia e nao apresentam spin, enquanto flocos perfeitos com bordas zigue-zague e mistas apresentam spin intrínseco. Esse spin não nulo é devido à diferença no numero de átomos entre uma e outra subrede do grafeno (Liebs imbalance). Defeitos em materiais de carbono sao frequentes, e tem sido estudados experimental e teoricamente. Aqui, estudamos a monovaçancia, através de modelos de cluster e supercélulas, e obtemos para esse defeito o momento magnético de = 2B, (B ´e o magneton de Bohr). Mostramos que as diferenças entre resultados anteriores são oriundas do erro de auto-interação presente na DFT simples, amenizado através do uso de PBEh. Através da mesma metodologia estudamos a interação de nanoestruturas de grafeno com moléculas de água, focalizando em propriedades estruturais. A grafite é um material hidrofóbico, mas a nanoestrutura poderia favorecer a interação com a água. Obtemos que pequenos agregados de água são adsorvidos na superfície de GNFs e biflocos, entretanto a inclusão desses agregados na região interna dos biflocos é altamente desfavorável. Assim podemos esperar que essas nanoestruturas empilhadas sejam também hidrofóbicas. / In this work, computational ab initio methods based on density functional theory (DFT) are used to simulate on an atomistic level the structural, electronic and magnetic properties of graphene nanostructures. We study pristine graphene nanoflakes (GNFs), and GNFs with intrinsic defects (monovacancy, divacancy, Stone-Wales). We design GNFs with different terminations and shapes and also studied stacked forms -biflakes- in different compositions. We employed two DFT approaches, plain generalized gradient approximation in the Perdew-Burke-Ernzerhof (PBE) level of the theory, and hybrid PBE (PBEh) incorporating a fraction of Hartree-Fock exchange. All calculations were performed with the all-electron code AIMS, including van der Waals corrections. Our GNFs were chosen from three symmetry groups: D2h, D3h and D6h, and with different edges, armchair (AC), zigzag (ZZ) and a mixture of both. Our chosen D6h- hexagonal flakes present an energy gap and no spin, while perfect trigonal zigzag and mixed edges GNFs have an intrinsic spin. This non-zero spin is due to the graphene sublattice imbalance (Liebs imbalance). Defects are common in carbon materials, and have been experimentally and theoretically studied in graphene. Here, the single vacancy in graphene was studied, by cluster and supercell approaches, finding that the vacancy induces a magnetic moment = 2B (Bohr magneton). We show that conflicting results for the magnetic moment coming from theoretical studies come from the self-interaction error present in plain PBE, cured through the use of PBEh. Using the same methodology we studied the interaction of carbon nanostructures with water molecules, focusing on structural properties. Graphite is a hydrophobic material but nanostructuring could favor the interaction with water. We obtained that small water groups are adsorbed on the surface of GNFs and biflakes, however the inclusion of these groups in the internal region of biflakes is highly unfavorable, thus we can expect these stacked nanostructures to be also hydrophobic.

This work investigates the compositional and nanostructural variability of celluloses isolated from plants and the impact of the variability in the production of cellulose nanocrystals. A variable set of cellulose isolated from plants were generated starting with a range of feedstocks (coconut fiber, sisal fiber, eucalyptus sawdust, pine sawdust, sugarcane rind and sugarcane pith), applying a range of cellulose isolation processes (acetossolv, liquid hot water, alkaline, and liquid hot water + alkaline) and adding commercial cellulose (eucalyptus kraft pulp, dissolving pulp, and microcrystalline cellulose) as reference materials. The nanostructural characteristics were evaluated by calorimetric thermoporometry, X-ray diffraction, and moisture sorption isotherms. Composition was evaluated by standard wet chemical analysis and insights on functional groups were obtained by infrared spectroscopy. The cellulose nanocrystals were produced by acid hydrolysis with sulfuric acid and characterized by atomic force microscopy and X-ray diffraction. The measured parameters of the isolated celluloses were spread, showing we could achieve a highly diverse set of substrates. Significant correlations between measured variables across the sample set, indicating possible unforeseen multivariate relations among cellulose features. For example, we could show that cellulose monolayer hydration is determined by both hemicelluloses content (compositional parameter) as well as cellulose crystal width (structural parameter). Cellulose nanocrystals were successfully produced, although in some cases such as for the acetossolv pulps the acid conditions were too aggressive and oxidized the substrates. Finally, some quantitative correlations were seen between the parameters of cellulose substrates and the resulting cellulose nanocrystals. These results supply the first hints about how the nanostructural variability of isolated cellulose can influence the cellulose nanocrystals produced from them. / Este trabalho investiga a variabilidade composicional e nanoestrutural de celuloses isoladas de plantas e o seu impacto na variabilidade na produção de nanocristais de celulose. Um conjunto variável de celuloses isoladas de plantas foi gerado a partir de uma série de matérias-primas (fibra de coco, sisal, serragem de eucalipto, serragem de pinheiro, casca de cana e miolo de cana), aplicando uma série de processos de isolamento de celulose (hidrotérmico, alcalino, hidrotérmico + alcalino e acetosolve) e adicionando celuloses comerciais (polpa kraft de eucalipto, polpa para dissolução e celulose microcristalina) como materiais de referência. As características nanoestruturais foram avaliadas por termoporometria calorimétrica, difração de raios X e isotermas de sorção de umidade. A composição foi avaliada por análise química húmida padrão e os conhecimentos sobre grupos funcionais foram obtidos por espectroscopia de infravermelhos. Os nanocristais de celulose foram produzidos por hidrólise ácida com ácido sulfúrico e caracterizados por microscopia de força atômica e difração de raios-X. Os parâmetros medidos das celuloses isoladas foram distribuídos, demonstrando que poderíamos alcançar um conjunto altamente diversificado de substratos. Correlações significativas entre as variáveis medidas foram observadas em todo o conjunto amostral, indicando possíveis relações multivariadas imprevistas entre as características da celulose. Por exemplo, poderíamos demonstrar que a monocamada de hidratação de celulose é determinada tanto pelo conteúdo de hemiceluloses (parâmetro de composição) quanto pela largura do cristal de celulose (parâmetro estrutural). Os nanocristais de celulose foram produzidos com sucesso, embora em alguns casos, como nas polpas acetosolve, as condições ácidas fossem muito agressivas e oxidassem os substratos. Finalmente, algumas correlações quantitativas foram observadas entre os parâmetros dos substratos de celulose e os nanocristais de celulose resultantes. Estes resultados fornecem as primeiras dicas sobre como a variabilidade nanoestrutural da celulose isolada pode influenciar os nanocristais de celulose produzidos a partir deles.

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