Thesis (Ph.D.)--University of Delaware, 2006. / Principal faculty advisor: Stanley Sandler, Dept. of Chemical Engineering. Includes bibliographical references.

The ability to manipulate material response to dynamical processes depends on the extent of understanding of transport properties and their variation with chemical and structural features in materials. In this perspective, current work focuses on the thermal and electronic transport behavior of technologically important bulk and nanomaterials. Strontium titanate is a potential thermoelectric material due to its large Seebeck coefficient. Here, first principles electronic band structure and Boltzmann transport calculations are employed in studying the thermoelectric properties of this material in doped and deformed states. The calculations verified that excessive carrier concentrations are needed for this material to be used in thermoelectric applications. Carbon- and boron nitride-based nanomaterials also offer new opportunities in many applications from thermoelectrics to fast heat removers. For these materials, molecular dynamics calculations are used to evaluate lattice thermal transport. To do this, first, an energy moment term is reformulated for periodic boundary conditions and tested to calculate thermal conductivity from Einstein relation in various systems. The influences of the structural details (size, dimensionality) and defects (vacancies, Stone-Wales defects, edge roughness, isotopic disorder) on the thermal conductivity of C and BN nanostructures are explored. It is observed that single vacancies scatter phonons stronger than other type of defects due to unsatisfied bonds in their structure. In pristine states, BN nanostructures have 4-6 times lower thermal conductivity compared to C counterparts. The reason of this observation is investigated on the basis of phonon group velocities, life times and heat capacities. The calculations show that both phonon group velocities and life times are smaller in BN systems. Quantum corrections are also discussed for these classical simulations. The chemical and structural diversity that could be attained by mixing hexagonal boron nitride and graphene provide further avenues for tuning thermal and electronic properties. In this work, the thermal conductivity of hybrid graphene/hexagonal-BN structures: stripe superlattices and BN (graphene) dots embedded in graphene (BN) are studied. The largest reduction in thermal conductivity is observed at 50% chemical mixture in dot superlattices. The dot radius appears to have little effect on the magnitude of reduction around large concentrations while smaller dots are more influential at dilute systems.

Novel carbon materials such as carbon onions, nanotubes and amorphous carbon (a-C) are technologically important due to their useful properties. Normally synthesised using plasmas, their growth mechanisms are not yet fully understood. For example, the growth mechanism of the high density phase of a-C, tetrahedral amorphous carbon (ta-C), has been a subject of debate ever since its discovery. The growth mechanism of carbon nanostructures such as carbon onions and nanotubes is also not well known. The aim of this thesis is two-fold. Firstly, to provide insight into the growth of carbon films, in particular, the driving force behind the formation of diamond-like bonding in a-C which leads to ta-C. Secondly, to investigate the growth of carbon onions and other sp2 bonded carbon nanostructures such as nanotubes. To achieve the first aim, carbon thin films were deposited using cathodic arc deposition at a range of ion energies, substrate temperatures and Ar background gas pressures. These films were characterised using electron microscopy techniques to examine their microstructure, density and sp3 content. It was found that the formation of the ta-C is due to a stress-induced transition whereby a critical stress of 6.5±1.5 GPa is needed to change the phase of the film from highly sp2 to highly sp3. Within this region, a preferentially oriented phase with graphitic sheets aligned perpendicular to the substrate surface was found. By investigating the role of elevated temperatures, the ion energy-temperature

Orientador: Oswaldo Luiz Alves / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Química / Made available in DSpace on 2018-08-26T03:04:22Z (GMT). No. of bitstreams: 1 Simoes_MateusBatista_M.pdf: 4986679 bytes, checksum: 2b115e73f65ae444a8545b2ab4b05173 (MD5) Previous issue date: 2014 / Resumo: As diferenças nas propriedades observadas considerando um material no seu estado bulk e na sua escala nanométrica são, possivelmente, a característica mais marcante e fascinante da nanotecnologia. Os carbon dots são nanomateriais baseados em carbono que apresentam fluorescência quando menores do que 10 nm, mas que podem fluorescer após tratamento da sua superfície, quando em partículas da ordem de até 100 nm. É interessante notar que o comprimento de onda no qual ocorrerá a fluorescência é dependente do tamanho das partículas. Assim, é possível modular a fluorescência controlando o tamanho dos carbon dots, os quais apresentam grande potencial para aplicação em fotocatálise, bioimagem, sensores e optoeletrônica, sendo possível funcionalizar estes materiais, objetivando uma aplicação in vivo, a fim de aumentar sua biocompatibilidade. As sínteses hidrotérmicas vêm despertando interesse para a obtenção dos carbon dots, por ser uma técnica simples, econômico e eficiente. Além disto, é possível obter materiais com grande homogeneidade e com controle de morfologia e tamanho, fatores estes que irão influenciar a fluorescência. Desta forma, o presente trabalho teve como objetivo estudar a influência das condições de síntese hidrotérmica na fluorescência dos carbon dots, realizar a funcionalização deste material e avaliar a capacidade de utilização in vivo do material por meio de ensaios de hemólise. Carbon dots foram obtidos por meio da carbonização hidrotérmica de glicose e as condições de síntese foram otimizadas utilizando-se um planejamento fatorial. Observou-se que temperatura e tempos de síntese elevados e uma menor concentração inicial da fonte de carbono leva a nanopartículas com maior rendimento quântico (variando entre 3,3 e 5,8%). Os carbon dots foram caracterizados por espectroscopia na região do infravermelho, espectroscopia na região do ultravioleta-visível, microscopia eletrônica de transmissão, além de ter seu perfil de fluorescência estudado, sendo que o máximo de excitação ocorre na região do ultravioleta e o máximo de emissão na região do azul. Testes hemolíticos foram realizados com as nanopartículas que apresentaram maior rendimento quântico e mostraram que não há indução de hemólise, demonstrando que o material tem elevado potencial para aplicação in vivo. Por fim, utilizando-se as condições ótimas de síntese, carbon dots também foram obtidos por meio da carbonização hidrotérmica de pectina, demonstrando que o método de síntese é robusto e válido para fontes de carbono alternativas. Os carbon dots obtidos de pectina apresentam um rendimento quântico de 3,6% e foram caracterizados pelas mesmas técnicas utilizadas para os carbon dots de glicose / Abstract: The differences in observed properties considering a material in its bulk state and its nanoscale are possibly the most striking and fascinating feature of nanotechnology. Carbon dots are carbon-based nanomaterials that present fluorescence when smaller than 10 nm, but may fluoresce after treatment of its surface considering particles of the order of until 100 nm. Interestingly, the wavelength at which the fluorescence occurs is dependent on the particle size. Thus, it is possible to modulate the fluorescence controlling the size of the carbon dots, which have great potential for application in photocatalysis, bioimage, optoelectronics and sensors, being possible to functionalize these materials, aiming an application in vivo, in order to increase its biocompatibility. The hydrothermal syntheses have attracted interest for obtaining the carbon dots, being a simple, cheap and efficient technique. Moreover, it is possible to obtain materials with high homogeneity and controlled morphology and size, factors that will influence the fluorescence. Thus, the present work aimed to study the influence of the conditions of hydrothermal synthesis in the fluorescence of carbon dots, perform the functionalization of this material and evaluate the ability to in vivo use of the material by hemolytic trials. Carbon dots were obtained by hydrothermal carbonization of glucose and the synthesis parameters were optimized by a factorial design of experiments. It was observed that higher temperature and time of synthesis and a lower initial concentration of the carbon source leads to nanoparticles with a higher quantum yield (varying between 3.3 and 5.8%). The carbon dots were characterized by infrared spectroscopy, ultraviolet-visible spectroscopy, transmission electron microscopy, beyond to have its fluorescence profile studied, and it was observed that the maximum excitation occurs at the ultraviolet range and the maximum emission at the blue range of the spectrum. Hemolytic trials were performed with the nanoparticles of highest quantum yield, and the results showed that no hemolysis was provoked, demonstrating that he material have a raised potential to in vivo applications. Lastly, with the optimized synthesis parameters, carbon dots were also obtained by hydrothermal carbonization of pectin, evidencing that the synthesis protocol is robust and effectual to alternatives carbon sources. The carbon dots of pectin presented a quantum yield of 3.6% and were characterized by the same techniques utilized to the carbon dots of glucose / Mestrado / Quimica Inorganica / Mestre em Química

Microbial fuel cells (MiFCs) have been suggested as a means to harness energy that is otherwise unutilized during the wastewater treatment process. MiFCs have the unique ability to treat influent waste streams while simultaneously generating power which can offset energy associated with the biological treatment of wastewater. During the oxidation of organic and inorganic wastes, microorganisms known as exoelectrogens have the ability to move electrons extracellularly. MiFCs generate electricity by facilitating the microbial transfer of these electrons from soluble electron donors in feedstocks to a solid-state anode. While MiFCs are a promising renewable energy technology, current systems suffer from low power densities which hinder their practical applicability. In this study, a novel anode design using flame-deposited carbon nanostructures (CNSs) on stainless steel mesh is developed to improve the electron transfer efficiency of electrons from microorganisms to the anode and thus the power densities achievable by MiFCs. These new anodes appear to allow for increased biomass accumulation on the anode and may aid in the direct transfer of electrons to the anode in mediatorless MiFC systems. Experiments were conducted using anaerobic biomass in single-chamber MiFCs with CNS-enhanced and untreated stainless steel anodes. Fuel cells utilizing CNS-enhanced anodes generated currents up to two orders of magnitude greater than cells with untreated metal anodes, with the highest power density achieved being 510 mW m-2. / Master of Science

In the past two decades carbon nanotubes and graphene have attracted a lot of research attention due to their exceptional electronic properties. The research focus on improving the synthesising techniques will eventually lead to their applications in terahertz wave, millimetre wave and microwave frequencies. In this thesis, a modelling technique based on the transmission line theory is proposed to calculate the 2-port S-parameters of vertically aligned CNT arrays with finite sizes and arbitrary cross sections. The process takes into account all the coupling in the array and gives the analytical solution of S-parameters. The simulation results from the proposed technique are compared with results obtained by effective single conductor model and shows a good matching for small arrays and an increasing difference with the increase of array sizes. From the S-parameters, the fundamental properties of CNT arrays such as input impedance and absorption are obtained and compared with measurement results in microwave frequencies. The dependence of these properties on ambient temperature and host medium are also presented to explore the tunability of CNT arrays. From the Fabry-Perot the wave propagating velocity is also calculated for arrays with different sizes and fitted with a power function. The S-parameters allows the extraction of the complex permittivity, permeability and conductivity of the CNT array. The extracted permittivity and absorption are compared with measurement results. The graphene nanoribbons are simulated in the same manner. The graphene sheet on top of a microstrip gap is simulated using transmission line model at microwave frequencies to show the impact of parasitics and contact resistances. Finally, a graphene based microwave absorber is proposed and modelled under both electric and magnetic bias. The absorber shows good broadband absorption rate and a potential for turning transparent and opaque to microwaves under both electric and magnetic bias.

The dissociative and non-dissociative hydrogen uptake in carbon nanostructures and metallic films were investigated by measurements and analysis of solubility isotherms. The total, non-dissociative, uptake for multi-walled nano-barrels and amorphous nanoporous carbon was determined to be 6.2 and 4.2 wt. % respectively at 77 K and the adsorption energies (at lowest coverage) -7.2 and -4.2 kJ/mol. At 298 K the H-uptake was negligible. At low concentrations the H-uptake of Nb-films is strongly affected by the film thickness. For thicknesses less then about 31 nm, the absorption energy was found to be temperature dependent. Such changes have not been observed in Nb films before. The presence of multiple absorption energies was shown to limit the possibility to obtain relevant absorption and interaction energies by traditional Sievert's and van 't Hoff analysis. The Mg1-xNix system (0<0.43) was investigated with respect to the hydrogen uptake. For Mg2Ni the hydrogen uptake, at an external hydrogen pressure of 1 bar, is close to 1.33 H/M (Mg2NiH4). The enthalpy of formation is smaller in the film as compared to bulk material. The changes in the absorption energy are caused by the adhesion to the substrate as well as the nanocrystallinity of the absorbing layers. The optical band gap of Mg2NiH4 was determined to be 2.4 eV. In Mg1-xYx (0<0.17) it was found that the Y-concentration limits the hydrogen uptake at 1 bar. However, the kinetics of the uptake improves substantially with a minimum of 7 at.% of Y. For Mg-Y the optical band gap (3.6 eV) is independent of Y concentration within the concentration range investigated, while the transmittance decreases with increasing Y content.

Carbon nanostructures are a wide field with many applications. The use of carbon nanostructures as support in heterogeneous catalysis is a key development that led together with the use of nanoparticles to a significant cost reduction of catalysts. Catalysts designed in this way are widely applied in fuel cell technologies. For portable devices especially low temperature fuel cells are desirable with low hazards for the user. One technology which fulfills these requirements is the direct formic acid fuel cell (DFAFC). DFAFC have many promising characteristics, such as high electromotive force and easy fuel handling. However, they still suffer from too low power output and lifetime for commercialization. This thesis focusses on two main aspects: the synthesis of carbon nanostructures by chemical vapor deposition (CVD) and their application as catalyst support. The materials are investigated by many different techniques ranging from transmission electron microscopy (TEM) to fuel cell tests. Different carbon nanostructures could be synthesized by catalytic CVD on palladium (Pd) nanoparticles. Multi-walled carbon nanotubes (MWCNTs), carbon nanofibers (CNFs) and helical carbon nanofibers (HCNFs) were grown, selectively, dependent on temperature, using acetylene as carbon precursor. Especially HCNF raised further interest due to their unique structure. A growth model for HCNFs was developed based on an anisotropic extrusion model. The synthesis conditions for HCNFs were optimized until an almost 100 % purity with very high efficiency was obtained. The unique helical but fiber-like structure made the material very interesting as support for heterogeneous catalysis. Several catalysts based on Pd nanoparticle decorated HCNFs were developed. The synthesis methods ranged from standard methods like the polyol method to phase-transfer methods. The catalysts showed very promising results for the electro-oxidation of methanol, ethanol and formic acid. This makes them highly attractive for fuel cell applications. The catalysts were tested in DFAFC. The superiority of HCNF-based catalysts is attributed to the good attachment of nanoparticles to the defect-rich and easy to functionalize surface of HCNFs in combination with adequate film forming properties during electrode preparation. / Nanostrukturerat kol är ett mycket brett fält med ett stort antal tillämpningar. Användning av kolnanostrukturer som support för heterogena katalysmaterial har tillsammans med utvecklingen av nanopartiklar lett till en avsevärd minskning av kostnaden för katalysatorer. Katalysatorer designade på detta sätt används frekvent i bränsleceller. För portabla tillämpningar är utvecklingen av säkra och miljövänliga lågtemperaturceller mycket viktig. En teknologi som uppfyller dessa kriterier är bränsleceller som drivs med myrsyra (DFAFC). Sådana bränsleceller har många önskvärda egenskaper, såsom en hög elektromotorisk kraft och en enkel hantering av bränslet. Trots dessa goda egenskaper har de också en del nackdelar som hindrar en full kommersialisering. De två mest problematiska är en för låg genererad effekt samt en för kort livslängd på katalysatorerna. Denna avhandling fokuserar på två huvudpunkter som adresserar dessa problem; tillverkning och karaktärisering av kolnanostrukturer producerade med CVD, och deras tillämpningar som support för katalysatorer. Materialen karaktäriseras med en rad olika tekniker, allt från transmission-elektronmikroskopi till bränslecellstester. Olika kolnanostrukturer har syntetiserats med katalytisk CVD på palladium (Pd) nanopartiklar. Produktionen av flerväggiga kolnanorör, kolfibrer och heliska kolnanofibrer har tillverkats med acetylen som kolkälla och genom att variera temperaturen kunde innehållet av olika typer av nanostrukturerat kol kontrolleras. Särskilt stort intresse har de heliska kolnanofibrerna rönt på grund av deras unika struktur. Vi beskriver en tillväxtmekanism baserad på en anisotrop diffusionsmodell. Genom att justera produktionsparametrarna visar vi att heliska kolnanofibrer kunde tillverkas med nära 100 %-ig renhet och hög effektivitet. Den unika heliska och fiberlika strukturen är mycket intressant for tillämpningar som support för heterogena katalysatorer. Ett flertal kompositer för katalytiska tillämpningar har utvecklats baserade på heliska kolnanofibrer, dekorerade med heterogena katalysatorer genom en rad olika kemiska/fysikaliska tekniker. De syntetiserade materialen visar mycket goda katalytiska egenskaper för att oxidera metanol, etanol och myrsyra. Därigenom blir de mycket attraktiva för användning i bränsleceller. Vi korrelerar de goda katalytiska egenskaperna med en bra vidhäftning av nanopartiklarna på de heliska kolnanofibrerna defekter, deras goda ledningsförmåga, bra egenskaper för att förbereda elektroder, samt deras stora yta i förhållande till deras volym och vikt.

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.

Two different interfaces were the focus of study: 1) the interface between disordered amorphous carbon and inorganic materials (metal nanostructures and silicon), and 2) the interface between partially ordered graphene (graphene oxide) and synthetic polymer matrix. Specifically, the uniqueness of this study can be summarized through the following novel findings, fabrication processes, and characterization techniques: • A simple and efficient process for faster, greener, less-expensive, and highly localized transformation of amorphous carbon nanostructures into graphitic nanostructures using low temperature heat and light treatments was developed for the fabrication of low-resistance interfaces between carbon nanomaterials and inorganic metal surfaces. • A new protocol for high resolution mapping the charge distribution and electronic properties of nanoscale chemically heterogeneous domains on non-homogeneous surfaces such as graphene oxide was established. • High strength laminated mechanical nanocomposites based on high interfacial stress transfer between polymer matrices and large area, flat, and non-wrinkled graphene oxide sheets were suggested and demonstrated. • Scanning Thermal Twist Microscopy – a thermal microscopy based technique was developed and demonstrated for characterizing the thermal properties of homogeneous and heterogeneous interfaces with nanoscale spatial resolution and high thermal sensitivity unachievable using traditional techniques.