The main goal of this work is to search for new stable porous carbon-based materials, which have the ability to accommodate and store hydrogen gas. Theoretical and experimental studies suggest a close relation between the nano-scale structure of the material and its storage capacity. In order to design materials with a high storage capacity, a compromise between the size and the shape of the nanopores must be considered. Therefore, a number of different carbon-based materials have been investigated: carbon foams, dislocated graphite, graphite intercalated by C60 molecules, and metal-organic frameworks. The structures of interest include experimentally well-known as well as hypothetical systems. The studies were focused on the determination of important properties and special features, which may result in high storage capacities. Although the variety of possible pure carbon structures and metal-organic frameworks is almost infinite, the materials described in this work possess the main structural characteristics, which are important for gas storage.

京都大学 / 新制・課程博士 / 博士(農学) / 甲第23243号 / 農博第2450号 / 新制||農||1084(附属図書館) / 学位論文||R3||N5333(農学部図書室) / 京都大学大学院農学研究科応用生命科学専攻 / (主査)教授 白井 理, 教授 三芳 秀人, 教授 森 直樹 / 学位規則第4条第1項該当 / Doctor of Agricultural Science / Kyoto University / DFAM

Solar thermal energy is emerging as an important source of renewable energy for meeting the ever-increasing energy requirements of the world. Solar selective coatings are known to enhance the efficiency of the photo thermal energy conversion. An ideal solar selective coating has zero reflectance in the solar spectrum region (i.e., 0.3-2.5 µm) and 100% reflectance in the infrared (IR) region (i.e. 2.5-50 µm). In this thesis, novel carbon nanotubes (CNT) and graphene based absorbers have been developed for photo thermal applications. Carbon nanotubes have good optical properties (i.e., α and ε close to 1), high aspect ratios (> 150), high surface area (470 m2/g) and high thermal conductivity (> 3000 W/mK), which enable rapid heat transfer from the CNTs to the substrates. Similarly, graphene also exhibits high transmittance (97%), low reflectance, high thermal conductivity (5000 W/mK) and high oxidation resistance behaviour. The major drawback of using CNTs for photothermal applications is that it exhibits poor spectral selectivity (i.e., α/ε = 1). In other words, it acts as a blackbody absorber. On the other hand, graphene exhibits poor intrinsic absorption behaviour (α - 2.3%) in a broad wavelength range (UV-Near IR). The main objective of the present study is to develop CNT and graphene based absorbers for photothermal conversion applications. The growth of CNT and graphene was carried out using chemical vapour deposition and sputtering techniques. An absorber-reflector tandem concept was used to develop the CNT based tandem absorber (Ti/Al2O3/Co/CNT). The transition from blackbody absorber to solar selective absorber was achieved by varying the CNT thicknesses and by using a suitable underlying absorber (Ti/Al2O3). A simple multilayer heat mirror concept was used to develop the graphene based multilayer absorber (SiO2/graphene/Cu/graphene). The transition from high transmitance to high absorptance was achieved by varying the Cu thickness. The refractive indices and the extinction coefficients of Ti/Al2O3, AlTiO and graphene samples were determined by the phase-modulated spectroscopic ellipsometric technique. Finally, the optical properties (i.e., absorptance and the emittance) of the CNT and graphene based absorbers were investigated. Chapter 1 gives a brief introduction about solar thermal energy, spectrally selective coating and photothermal conversion. The different types of absorbers used to achieve the spectral selectivity have also been discussed shortly. A brief description about the carbon-based materials/allotropes and their properties are outlined. The properties of carbon nanotubes and graphene which are the 1-D and 2-D allotropes of carbon, respectively are tabulated. A detailed literature survey was carried out in order to identify the potential candidates for the photothermal conversion applications. The objectives and the scope of the thesis are also discussed in this chapter. Chapter 2 discusses the deposition and characterization techniques used for the growth and the study of 1-D and 2-D carbon nanostructures. Atmospheric pressure chemical vapour deposition (CVD) and hot filament CVD techniques were used to grow CNT and graphene, respectively. The magnetron sputtering technique was used for the growth of ‘Ti’, ‘Al2O3’ and Co layers which were needed to grow the CNT based tandem absorber on stainless steel (SS) substrates. The important characterization techniques used to examine various properties of the 1-D and 2-D carbon nanostructures include: X-ray diffraction, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), phase modulated ellipsometry, UV-VIS-NIR spectrophotometer, Fourier-infrared spectroscopy (FTIR), micro-Raman spectroscopy and solar spectrum reflectometer and emissometer. Chapter 3 describes the design and development of Ti/Al2O3 coating for the growth of CNT-based tandem absorber on SS substrates. The power densities of the aluminum and titanium targets and the oxygen flow rates were optimized to deposit the Ti/Al2O3 coatings. The optimized Ti/Al2O3 coating with a Co catalyst on top was used as an underlying substrate to grow the CNT-based tandem absorber at 800°C in Ar+H2 atmosphere (i.e., CNT/Co/Al2O3/Ti/SS). The formation of aluminum titanium oxide (AlTiO) was observed during the CNT growth process and this layer enhances the optical properties of the CNT based tandem absorber. The optical constants of Ti, Al2O3 and AlTiO coatings were measured using phase modulated spectroscopic ellipsometry in the wavelength range of 300-900 nm. The experimentally measured ellipsometric parameters have been fitted with the simulated spectra using the Tauc-Lorentz model for generating the dispersion of the optical constants of the Al2O3 and the AlTiO layers. The Ti and Al2O3 layer thicknesses play a major role in the design of the CNT based tandem absorber with good optical properties. Chapter 4 describes the synthesis and characterization of the CNT based tandem absorber (Ti/AlTiO/CoO/CNTs) deposited on SS substrates. CNTs at different thicknesses were grown on Ti/AlTiO/CoO coated SS substrates using atmospheric CVD at various growth durations. The transition from blackbody absorber to solar selective absorber was achieved by varying the thicknesses of the CNTs and by suitably designing the bottom tandem absorber. At thicknesses > 10 µm, the CNT forest acts as near-perfect blackbody absorber, whereas, at thicknesses ≤ 0.36 µm, the IR reflectance of the coating increases (i.e., ε = 0.20) with slight decrease in the absorptance (i.e., α = 0.95). A spectral selectivity (α/ε) of 4.75 has been achieved for the 0.36 µm-thick CNTs grown on SS/Ti/AlTiO/CoO tandem absorber. Chapter 5 discusses the growth of graphene on polycrystalline copper (Cu) foils (1 cm × 1 cm) using hot filament CVD. The roles of the process parameters such as gas flow rates (methane and hydrogen), growth temperatures (filament and substrate) and durations on the growth of graphene were studied. The process parameters were also optimized to grow monolayer, bilayer and multilayer graphene in a controlled manner and the growth mechanism was deduced from the experimental results. The presence of graphene on Cu foils was confirmed using XPS, micro-Raman spectroscopy, FESEM and TEM techniques. The FESEM data clearly confirmed that graphene starts nucleating as hexagonal islands which later evolves into dendritic lobe shaped islands with an increase in the supersaturation. The TEM data substantiated further the growth of monolayer, bilayer and multilayer graphene. The intensity of 2D and G peak ratio (i.e., I2D/IG = 2) confirmed the presence of the monolayer graphene and the absence of the ‘D’ peak in the Raman spectrum indicated the high purity of graphene grown on Cu foils. The results show that the polycrystalline morphology of the copper foil has negligible effect on the growth of monolayer graphene. In Chapter 6, the design and development of graphene/Cu/graphene multilayer absorber and the study of its optical properties are discussed. The multilayer graphene grown on Cu foils has been transferred on quartz and SiO2 substrates in order to fabricate the graphene/Cu/graphene multilayer absorber. The sputtering technique was used to deposit copper on top of graphene/quartz substrates. The uniformity of the transferred multilayer graphene films was confirmed using Raman mapping. A simple multilayer heat mirror concept was used to develop the graphene/Cu/graphene absorber on quartz substrates and the transition from high transmittance to high absorptance was achieved. In order to further enhance the absorption, the graphene/Cu/graphene multilayer coating was fabricated on SiO2 substrates. The thickness of the Cu layer plays a major role in creating destructive interference, which results in high absorptance and low emittance. A high specular absorptance of 0.91 and emittance of 0.22 was achieved for the SiO2 graphene/Cu/graphene multilayer absorber. The specular reflectance of the multilayer absorber coatings was measured using the universal reflectance accessory of the UV-VIS-NIR spectrophotometer. Chapter 7 summarizes the major findings of the present investigation and also suggests future aspects for experimentation and analysis. The results obtained from the present work clearly indicate that both CNT and graphene based absorbers can be used as potential candidates for photothermal applications. In particular, the CNT based tandem absorber can be used for high temperature solar thermal applications and the graphene based multilayer absorber finds applications in the area of photodetectors and optical broadband modulators.

Die vorliegende Promotion beinhaltet die Untersuchung von irregulären Kohlenstoff-Nanostrukturen, die mittels Hochenergie-Kugelmahlen hergestellt wurden. Die wissenschaftliche Herausforderung besteht darin, die strukturelle Vielfalt dieser Nanostrukturen experimentell zu erfassen, zu klassifizieren und bezüglich ausgewählter Eigenschaften zu bewerten, sowie mit den Herstellungsparametern in Zusammenhang zu bringen. Die Pulver konnten nach den Mahlungen hinsichtlich ihrer Struktur in zwei grundsätzliche Typen eingeteilt werden: (I) ein Nanopulver, das aus graphitischen Stapelpaketen besteht, welche in eine amorphe Matrix eingebettet sind, (II) ein vollständig amorphisiertes Pulver. Die Strukturanalyse in Bezug auf die Mahlbedingungen (Mahlatmosphäre, Mahltemperatur) zeigt, dass die Dauer der Nanostrukturierung sowie die Anzahl und Größe von graphitischen Stapelpaketen gezielt beeinflusst werden kann. Außerdem konnten Hinweise gefunden werden, die darauf hindeuten, dass Mahlen bei tiefen Temperaturen oder unter Wasserstoffatmosphäre die Agglomeration der Nanopartikel verringern kann. Das Kugelmahlen ermöglicht es ebenfalls, die spezifische Oberfläche des Graphitpulvers von 5,5 m2/g auf 725 m2/g innerhalb von fünf Mahlstunden zu erhöhen. Der Anteil der Verunreinigungen (Fe) liegt dabei nicht höher als 0,05 wt%. Es ist jedoch zu beachten, dass sämtliche Eigenschaften stark von den verschiedenen Mahlparametern (Mahltemperatur, Mahlmaterial) abhängen. Die für Adsorptionsuntersuchungen optimalen Eigenschaften (große spezifische Oberfläche, erhöhte Reaktivität, geringe Verunreinigungen) werden schon nach kurzer Mahldauer erreicht. Wiederholungsmahlungen und Wiederholungsmessungen verschiedener Eigenschaften (spezifische Oberfläche, Verbrennungstemperatur) machen deutlich, dass die Ergebnisse reproduzierbar sind, und dass keine Alterungserscheinungen während der Lagerung unter Argonatmosphäre im Zeitraum von einem Jahr auftreten. Die Wasserstoffspeicherung an nanostrukturierten Kohlenstoffpulvern konnte nachgewiesen werden. Die maximalen Speicherkapazitäten für Temperaturen nahe 77 K lagen bei 1,5 wt%. Für niedrigere Temperaturen Tist = 35 K zeigten sich höhere Speicherkapazitäten von bis zu 5 wt%. Die Korrelation der ermittelten Speicherkapazitäten mit den theoretisch erreichbaren Werten in Bezug auf die Oberfläche der Proben zeigt, dass im Experiment deutlich höhere Werte erhalten werden. Dies lässt den Schluss zu, dass neben der Speicherung an der Oberfläche der Pulver ein weiterer Speichermechanismus innerhalb der Mikroporen der Proben stattfindet.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Nanostructured carbon materials production can constitute an alternative for a sustainable management of residues originated from petrochemical waste and agriculture activities, toward the development of multifunctional ―green‖ materials. The coconut processing industry generate a significant amount of waste (45% of mass). The shell, fibers and coconut coir dust have been studied extensively to produce conventional carbon materials. The goal of this work was to produce carbon-clay nanocomposites and carbon nanostructures by hydrothermal route. By using coconut fiber residue as carbonaceous precursor along with lamellar (montmorillonite and kaolinite) and fibrous clays (sepiolite and attapulgite).The obtained materials were characterized by X-ray diffraction, Raman and Infrared spectroscopy, thermogravimetry, scanning and transmission electron microscopy and area and porosity measurements by BET. Carbon phase formation was indicated by infrared results with bands at ~ 1444 cm-1 and ~ 1512 cm-1 assigned to C=C of aromatic groups. Raman spectroscopy results showed presence of carbonaceous species by the appearance of D and G bands assigned to disordered and graphitic crystallites, respectively. The estimated particle size based on Raman bands was found between 8-33 nm. SEM results showed that the morphology of coconut coir dust was preserved and all materials showed overlapping sheets and plates formation. In transmission electron microscopy (TEM) images it was possible to observe three types of carbon nanostructures: sheets, fibers and nanoparticles. It was observed the formation of very thin amorphous sheets, as well as the presence of partially ordered graphitic domains and disperse carbon nanoparticles. / A produção de materiais de carbono nanoestruturados pode constituir uma alternativa para a reutilização de resíduos provenientes da indústria petroquímica e atividades agrícolas, abrindo um caminho para o desenvolvimento de materiais ―verdes‖ multifuncionais. Da indústria do processamento do coco, origina-se uma quantidade significativa de resíduos (45% do fruto). A casca, fibras e pó de coco são estudados extensivamente para a produção de materiais de carbono convencionais. A proposta deste trabalho foi produzir nanocompósitos de carbono-argila e nanoestruturas de carbono, via rota hidrotérmica. Para tanto foi utilizado o pó de coco in natura como precursor carbonáceo e argilas lamelares (montmorillonita e caulinita) e fibrosas (atapulgita e sepiolita). Os materiais obtidos foram caracterizados por difração de Raios-X, espectroscopia Raman e no Infravermelho, Termogravimetria, Microscopia eletrônica de Varredura (MEV) e Transmissão (MET), e medidas de área superficial e porosidade por BET. A formação de carbono foi indicada pelos resultados de infravermelho com bandas em ~1444 cm-1 e ~1512 cm-1 atribuídas a C=C de grupos aromáticos. Os resultados de espectroscopia Raman evidenciaram a presença de espécies carbonáceas pelo aparecimento das bandas D e G atribuídas, respectivamente, a presença de desordem e cristalitos de grafite. A faixa de tamanho de partícula estimada a partir das bandas Raman está entre 8-33 nm. Os resultados de MEV mostraram que a morfologia do pó de coco foi preservada e todos os materiais obtidos apresentaram a formação de folhas sobrepostas e placas. Nas imagens de microscopia eletrônica de transmissão (MET) foi possível observar a formação de três tipos de nanoestruturas de carbono: folhas, fibras e nanopartículas. Observou-se a formação de folhas muito finas, de caráter predominantemente amorfo, bem como a presença de domínios grafiticos parcialmente ordenados, e nanopartículas de carbono dispersas.

Orientadores: Rubens Maciel Filho, André Luiz Jardini Munhoz / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Química / Made available in DSpace on 2018-08-17T01:50:40Z (GMT). No. of bitstreams: 1 MaciasMacias_Carolina_M.pdf: 33756712 bytes, checksum: 756466b4de64af3709bb1a7410020b1a (MD5) Previous issue date: 2010 / Resumo: Após a descoberta dos nanotubos de Carbono, o estudo de novos materiais de Carbono surgiu como um fascinante assunto de pesquisa para a comunidade científica. As propriedades excepcionais destes materiais os fazem potenciais aditivos para lubrificantes, suportes para catalisadores, materiais para estocagem de energia, e materiais para dispositivos nanoeletrônicos. Técnicas de síntese muito versáteis como o método Laser-Forno e o método de Deposição Química de Vapor são usadas para produzir vários tipos de materiais de Carbono como nanotubos, esferas e fibras a partir de hidrocarbonetos líquidos e gasosos e de outras matérias-primas. Por outro lado, o processamento do petróleo gera resíduos que vêm sendo tratados por meio de processos inovadores como a Destilação Molecular visando à obtenção de produtos finais de alto valor agregado. Porém, esta técnica gera resíduos ainda mais pesados (ultrapesados) de petróleo de composição rica em hidrocarbonetos, resinas, asfaltenos, aromáticos e heteroátomos, característica que lhes confere potencial como precursores de estruturas carbonosas, mas paralelamente impõe desafios tecnológicos em função da sua complexidade. Neste contexto, o resíduo ultrapesado da destilação molecular aplicada ao resíduo de vácuo foi avaliado como matéria prima para a síntese de estruturas de carbono. Em razão disso, foi construído um reator bifuncional para a síntese de materiais carbonosos por meio do método Laser- Forno e do método de Deposição Química de Vapor. Com o projeto adequado do reator foram desenvolvidas políticas e condições operacionais que possibilitaram a síntese de esferas de Carbono. Estas foram caracterizadas e foi possível demonstrar a potencialidade do resíduo da destilação molecular como material precursor de nanoestruturas de Carbono, abrindo um novo caminho de pesquisa que envolve o processamento de resíduos com a obtenção de produtos de alto valor agregado. / Abstract: Since the discovery of carbon nanotubes and related carbon materials, an intense scientific activity around these new structures has been stimulated. Because of the exceptional properties showed by these materials, they have been investigated as lubricant additives, catalyst supports, energy storage materials and nanoelectronic devices. Various methods, including Chemical Vapor Deposition and Laser-Furnace technique, have been developed for the production of carbon nanomaterials (nanotubes, spheres and fibers) from liquid and gaseous hydrocarbons, and other carbon precursors. Additionally, petroleum processing generates oil wastes that have been treated by various innovative processes as Molecular Distillation, with the intention of producing high value final products as gasoline and lubricant oils. Nevertheless, this technique generates oil wastes even heavier (ultraheavy oil), that exhibit a complex chemical composition based on saturates, resins, asphalthenes, aromatics and heteroatoms compounds. This feature suggests that Molecular Distillation Residua could be used as a carbon precursor for carbon nanostructures, but additionally imposes process challenges. In this context, the ultraheavy oil residue from Molecular Distillation of vacuum residue was assessed as raw material for the synthesis of carbon nanostructures. The performance evaluation experiments were carried out in a reactor for carbon materials production by Chemical Vapor Deposition and Laser-Furnace technique, built for that purpose. There were developed policies and operational conditions that made possible the Carbon Spheres production. The as-produced Carbon Spheres were characterized by various techniques, and the potential of Molecular Distillation Residue, as carbon nanostructures precursor, was demonstrated. Finally, a new branch of research involving oil wastes processing and carbon nanomaterials synthesis was successfully initiated. / Mestrado / Desenvolvimento de Processos Químicos / Doutor em Engenharia Química

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Graphene is a two-dimensional carbon structure that exhibits remarkable structure-property relations. Consequently, there has been immense effort undertaken towards developing methods for graphene synthesis. Chemical vapor deposition (CVD) and chemical exfoliation from colloidal suspensions are two common methods used for obtaining graphene films. However, the underlying experimental conditions have to be carefully optimized in order to obtain graphene films of controllable thickness and morphology. In this context, a significant part of this dissertation was devoted towards developing and improving current CVD-based and chemical exfoliation based methods for synthesizing high quality graphene films. Specifically, in the CVD based procedure for growing graphene on copper, the effect of surface pretreatment of copper was investigated and the quality of graphene grown using two different pretreatment procedures was compared and analyzed. In particular, graphene grown on electropolished copper (EP-Cu) was analyzed with respect to its surface morphology, surface roughness and thickness, and compared with graphene grown on as cold-rolled acetic acid cleaned copper (AA-Cu). It was shown that electropolishing of the Cu substrates prior to graphene growth greatly enhanced the ability to obtain flat, uniform, predominantly single layer graphene surface coverage on copper. The reported surface roughness of the graphene on EP-Cu was found to be much lower than for previously reported systems, suggesting that the electropolishing procedure adopted in this work has great promise as a pretreatment step for Cu substrates used in CVD growth of graphene. Obtaining graphene from colloidal suspensions of graphitic systems was also examined. In this work, an acid (H₂SO₄ + HNO₃) treatment process for intercalating natural graphite flakes was examined and the ability to reversibly intercalate and deintercalate acid ions within graphitic galleries was investigated. More importantly, a rapid-thermal expansion (RTP) processing was developed to thermally expand the acid-treated graphite, followed by exfoliation of predominantly bilayer graphene as well as few layer graphene flakes in an organic solvent (N, N-Dimethylformamide - DMF). The developed method was shown to provide bilayer and few layer graphene flakes in a reliable fashion. Fullerene is another carbon nanostructure that has garnered attention due to unique structure and chemical properties. Recently, there has been increased focus towards harnessing the properties of fullerenes by synthesizing fullerene self-assemblies in the form of extended rods, tubes and more complex shapes. Current methods to synthesize these self-assemblies are either cumbersome, time consuming or expensive. In this context, an alternate, straightforward dip-coating procedure technique to self-assemble equal-sized, faceted, polymerized fullerene nanorods on graphene-based substrates in a rapid fashion was developed. By suitably modifying the kinetics of self-assembly, the ability to reliably control the spatial distribution, size, shape, morphology and chemistry of fullerene nanorods was achieved.

The thesis consists of four parts of which part 1 presents a brief overview of nanomaterials. Parts 2, 3 and 4 contain results of investigations of graphene, nanofilms of noble metal nanoparticles and ZnO nanostructures respectively. Investigations of graphene are described in Part 2 which consists of six chapters. In Chapter 2.1, changes in the electronic structure and properties of graphene induced by molecular charge-transfer have been discussed. Chapter 2.2 deals with the results of a study of the interaction of metal and metal oxide nanoparticles with graphene. Electrical and dielectric properties of graphene-polymer composites are presented in Chapter 2.3. Chapter 2.4 presents photo-thermal effects observed in laser-induced chemical transformations in graphene and other nanocarbons system. Chapter 2.5 describes the mechanical properties of polymer matrix composites reinforced by fewlayer graphene investigated by nano-indentation. The extraordinary synergy found in the mechanical properties of polymer matrix composites reinforced with two nanocarbons of different dimensionalities constitute the subject matter of Chapter 2.6. Investigations of noble metal nanoparticles have been described in Part 3. In Chapter 3.1, ferromagnetism exhibited by nanoparticles of noble metals is discussed in detail while Chapter 3.2 deals with surface-enhanced Raman scattering (SERS) of molecules adsorbed on nanocrystalline Au and Ag films formed at the organic–aqueous interface. Factors affecting laser-excited photoluminescence from ZnO nanostructures are examined in great detail in Part 4.