Nanoparticles (such as, carbon nanotubes, carbon black, clay etc.) have one or more dimensions of the order of 100 nm or less. Owing to very high van der Waals force of attraction, these nanoparticles exist in a highly aggregated state. It is often required to break these aggregates to truly experience the “nanosize” effect for any required end use. There are several strategies proposed for dispersing/exfoliating nanoparticles but limited progress has been made towards controlling their dispersion state. The ability to tailor nanoparticle dispersion state in liquid and solid media can ultimately provide a powerful method for tailoring the properties of solution processed nanoparticle-filled polymer composites. This dissertation reports the use of a variety of stimuli-responsive polymers to control the dispersion state of single-walled carbon nanotubes. Stimuli-responsive polymers exhibit conformational transitions as a function of applied stimulus (like pH, temp, chemical etc.). These variations in conformations of the polymer can be used tailor nanotube dispersion state in water and solid composites.The use of pH and temperature responsive polymers to stabilize/disperse single walled carbon nanotubes (SWNTs) in water is presented. Non-covalent functionalization of SWNTs using pH and temperature responsive polymer show tailored dispersion state as a function of pH and temperature, respectively. Carbon nanotube microstructure in these aqueous suspensions was characterized using several techniques (cryo-TEM, viscosity measurements, uv-vis spectroscopy, zeta potential measurements and settling behavior). Furthermore, nanotube dispersion state in aqueous suspensions is preserved to a large extent in the composites formed by drying these suspensions as evidenced by SEM images and electrical conductivity measurements. Based on the results obtained a mechanism is proposed to explain the tailored dispersion of SWNTs as a functions of applied external stimulus (i.e., pH, temperature). Such stimuli-controlled dispersion of carbon nanotubes could have a variety of applications in nanoelectronics, sensing, and drug and gene delivery systems. Furthermore, this dissertation also contains a published study focused on controlling the dispersion state of carbon black (CB) in epoxy composites using clay.

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Thesis (Ph. D.)--Rutgers University, 2009. / "Graduate Program in Materials Science and Engineering." Includes bibliographical references (p. 134-150).

Thesis (Ph.D)--Mechanical Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Graham, Samuel; Committee Member: Garimella, Srinivas; Committee Member: Kippelen, Bernard; Committee Member: Melkote, Shreyes; Committee Member: Ready, Jud. Part of the SMARTech Electronic Thesis and Dissertation Collection.

The production of cost-effective catalysts for the production of hydrogen by electrolysis of water is important for clean energy production. In this work we report on a study of molybdenum disulfide (MoS2) as catalyst for the hydrogen evolution reaction (HER). Nitrogen doped carbon nanotubes (NCNTs) directly synthesized onto carbon paper have been decorated with MoS2. The electrodes utilize the improved conductivity of the NCNTs and the carbon paper for electron transport, combined with the high catalytic activity of MoS2. The NCNTs were successfully decorated with co-axial nano-flakes of MoS2 by a single step solvothermal process using Dimethylformamide (DMF) and ammonium tetrathiomolybdate. MoS2 was also prepared with alternative methods for comparison. The effects of supporting MoS2 on NCNTs were studied by simulations with density functional theory (DFT). The most active adsorption sites for hydrogen on MoS2 were identified and were on the edges. The catalyst showed competitive activity with other earth-abun- dant catalysts with an onset potential of 170 mV and a small Tafel slope of 40 mV/dec. The improved catalytic activity of HER by having NCNTs as support was confirmed by DFT and experimental results.

<p>Energy storage systems are critically important for many areas in modern society including consumer electronics, transportation and renewable energy production. This dissertation summarizes our efforts on improving the performance metrics of energy storage and conversion devices through rational design and fabrication of hybrid nanomaterial systems. </p><p>This dissertation is divided into five sections. The first section (chapter 2) describes comparison of graphene and carbon nanotubes (CNTs) on improving the specific capacitance of MnO2. We show that CNTs provided better performance when used as ultrathin electrodes but they both show similar performance with rapid MnO2 specific capacitance decrease as electrodes become thicker. We further designed ternary composite electrodes consisting of CNTs, graphene and MnO2 to improve thick electrode performance (chapter 3). We demonstrate that these electrodes were flexible and mechanically strong, had high electrical conductivity and delivered much higher capacity than electrodes made without CNTs. </p><p>Chapter 4 describes assembly of flexible asymmetric supercapacitors using a graphene/MnO2/CNTs flexible film as the positive electrode and an activated carbon/CNTs flexible film as the negative electrode. The devices were assembled using roll-up approach and can operate safely with 2 V in aqueous electrolytes. The major advantage of these devices is that they can deliver much higher energy under high power conditions compared with those designed by previous studies, reaching a specific energy of 24 Wh/kg at a power density of 7.8 kW/kg. </p><p>Chapter 5 describes our approach to improve the energy and power densities of nickel hydroxides for supercapacitors. This was done by assembling CNTs with Co-Ni hydroxides/graphene nanohybrids as freestanding electrodes. The assembled electrodes have dramatically improved performance metrics under practically relevant mass loading densities (~6 mg/cm2), reaching a specific capacitance of 2360 F/g at 0.5 A/g and 2030 F/g even at 20 A/g (~86% retention). </p><p>Finally, we discuss our efforts on designing highly active electrocatalysts based on winged nanotubes for oxygen reduction reactions (ORR). The winged nanotubes were prepared through controlled oxidization and exfoliation of stacked-cup nanotubes. When doped with nitrogen, they exhibited strong activity toward catalyzing ORR through the four-electron pathway with excellent stability and methanol/carbon monoxide tolerance owning to their unique carbon structure.</p> / Dissertation

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No description available.

The aim of this thesis is the development of solid contact ion selective electrodes, ISEs, where the transducer layer is made of a network of carbon nanotubes.Potentiometric classical ion selective electrodes (ISEs) have been used for analytical applications since the beginning of 1900's. Determination of pH by a glass membrane ion selective electrode emerged at the beginning, being the first ISEs developed. pH glass electrode is still one the most useful and robust sensors for routine measurements both in laboratories and industries.Throughout the years, new technologies, ideas and designs have been developed and incorporated successfully in the potentiometric fields so as to provide answers to the new society's needs. Therefore, the ion selective electrodes developed in this thesis are a step further in the progress of ISEs and must be considered as products of the scientific envisioning, growth, and interdisciplinary cooperation of many research teams over many years of continuous efforts. The sensing part can be regarded nowadays as well developed, although it has been during only the last few years when considerable improvements have taken place in the development of new polymeric membranes, ionophores and lipophilic ions. Moreover, the understanding of the theoretical sensing mechanism has been a powerful solid backbone in the rise of ISEs. Miniaturization of classical ISEs requires making all solid contact electrodes to avoid the intrinsic drawbacks of the inner solution. In this manner, the transduction layer has been the focus of attention for the two last decades. New solid contact transducers having the capacity to convert an ionic current into an electronic current have been emerging. Within them, conducting polymers have played an important role in the transduction of the potentiometric signal, being the most used in solid contact ion selective electrodes (SC-ISEs) up to now. However, the behaviour of conducting polymers can be further improved. For instance, their sensitivity to light one of main operational issues yet to be solved.In the present context of searching for new materials able to transduce potentiometric signals we selected and tested carbon nanotubes (CNTs). CNTs, which were rediscovered by Ijima in 1991, display excellent electronic properties in terms of signal transduction. In addition, due to their chemical reactivity CNTs can be easily functionalized with receptors or other functional groups. In fact, depending on the type of functionalization the macroscopic and microscopic properties of CNTs can be drastically changed. This nanostructured material had not been used previously as a solid contact material in ISEs.The main aim of this thesis is to demonstrate that CNTs can act as a clean and efficient transducer in SC-ISEs overcoming the drawbacks displayed by the previously assayed solid contact materials. The developed electrodes were used in different conditions to determine several ions in different sample types, demonstrating the capabilities of this nanostructured material.The thesis has been structured in different chapters, each one containing the following information:· Chapter 1 provides a short historical overview of potentiometric ISEs. The evolution from the "classical ISEs" to the SC-ISEs is briefly illustrated. Once the motivation for thesis is described, the general and specific objectives of the thesis are reported.· Chapter 2 reports the scientific foundations of the developed electrodes. All components of the ISE, sensing layer, transducers and detection systems are introduced. Analytical performance characteristics of ISEs are also described.· Chapter 3 corresponds to the experimental part. Reagents, protocols, procedures and instruments used in the thesis are reported.· Chapter 4 provides the demonstration that CNTs can act as a transducer layer in SC-ISEs. The first SC-ISEs based on CNTs are characterized by electrochemical and optical techniques.· Chapter 5 contains the experimental results that lead to the elucidation of the possible transduction mechanism of CNTs in SC-ISEs. Electrochemical impedance spectroscopy (EIS) is employed as the main characterization technique. · Chapter 6 is composed of four sections reporting different analytical applications. In the first section, the common pH electrode is developed using a solid contact technology based on CNTs. In the second section, the development of SC-ISEs based on a new synthetic ionophore selective to choline, and CNTs as transducers is shown. In the third section, watertight and pressure-resistant SC-ISEs based on CNTs are developed and tested in aquatic research to obtain information about the gradient profiles along the depth of the lakes. In the fourth section, SC-ISEs based on CNTs are adapted for the on-line control of a denitrification catalytic process.· Chapter 7 reports the possibilities of miniaturization of the SC-ISEs based on CNTs to reach a nanometric electrode. Potentiometric and optical characterizations are described in this section. Moreover, a discussion about the limitations of the real miniaturization in potentiometry is undertaken.· Chapter 8 points out the conclusions of the thesis. In addition, future prospects are suggested.· Finally, several appendices are added to complete the doctoral thesis. / El principal objetivo de esta tesis es el desarrollo de electrodos selectivos de iones de contacto sólido, ESIs-CS, utilizando como capa transductora una red compuesta de nanotubos de carbono.Los electrodos potenciométricos selectivos de iones han sido utilizados en aplicaciones analíticas desde comienzos de 1900. La determinación de pH mediante electrodos de vidrio selectivo de iones fue el primer ESI desarrollado. Hoy en día, el electrodo de vidrio para la determinación de pH es todavía uno de los más útiles y robustos sensores utilizados en mediciones rutinarias tanto en laboratorios como en industrias.A lo largo de los años, nuevas tecnologías, ideas y diseños han sido desarrollados e incorporados satisfactoriamente en el campo potenciométrico proporcionando soluciones a las necesidades en continua evolución de la sociedad. De esta manera, los electrodos selectivos de iones desarrollados en esta tesis son un paso más en el progreso de los ESIs y deben ser considerados como el producto de una sólida base científica, del crecimiento y de la cooperación interdisciplinaria de diversos grupos de investigación durante varios años.La parte del sensor donde tiene lugar el reconocimiento químico y donde se genera el potencial dependiente de la muestra en estudio en los ESIs se puede considerar, en estos días, ampliamente desarrollada, aunque considerables mejoras han tenido lugar durante los últimos años, especialmente en el desarrollo de nuevas membranas poliméricas, ionóforos e iones lipofílicos. Sobretodo, el estudio y la comprensión del mecanismo teórico del sensor ha sido muy importante en el crecimiento y desarrollo de los ESIs.El concepto de electrodos selectivos de iones de estado sólido surge como requisito vital para evitar las intrínsecas desventajas de la solución interna, en el proceso de miniaturización de los ESIs clásicos. De esta forma, la capa transductora ha sido el principal punto de atención durante dos décadas. Así, nuevos transductores de contacto sólido con la capacidad de convertir una corriente iónica en una corriente electrónica han sido desarrollados. Entre ellos, los polímeros conductores han jugado un importante papel en la transducción de la señal potenciométrica, siendo éstos los más empleados en los electrodos selectivos de iones de contacto sólido (ESIs-CS). Sin embargo el comportamiento de los polímeros conductores puede ser mejorado. Por ejemplo, la sensibilidad hacia la luz de estos materiales es un inconveniente todavía no resuelto. En este contexto de investigación de nuevos materiales capaces de actuar como transductor de una señal potenciométrica, se han escogido y estudiado los nanotubos de carbono (NTCs) como transductores. Los NTCs fueros redescubiertos por Ijima en 1991, y muestran excelentes propiedades electrónicas en términos de traducción de señal. Además, debido a su reactividad química, los NTCs pueden ser fácilmente funcionalizados con receptores u otros grupos funcionales. De hecho, sus propiedades macroscópicas y microscópicas pueden ser afectadas drásticamente dependiendo del tipo y grado de funcionalización. Este material nanoestructurado no había sido previamente utilizado como transductor en ISEs.El principal propósito de esta tesis es demostrar que los nanotubos de carbono pueden actuar de forma eficiente como transductor en electrodos selectivos de iones de estado sólido logrando vencer las desventajas de los transductores previamente mencionados. Los electrodos desarrollados fueron usados en diferentes condiciones para determinar distintos iones en diversos tipos de sistemas, demostrando las extraordinarias capacidades de este material nanoestructurado. Esta tesis ha sido estructurada en capítulos que contienen la siguiente información:· El Capítulo 1 proporciona una breve visión histórica de lo electrodos potenciométricos selectivos de iones. Se ilustra la evolución desde los "clásicos ESIs" hasta los actuales "ESIs-CS". Además se señalan en esta sección los objetivos generales y específicos. · El Capitulo 2 contiene las bases científicas de los electrodos desarrollados. Se introducen todos los componentes que integran un ESI, tales como: capa reconocedora, capa transductora y sistema de detección. A continuación se describen los parámetros analíticos de calidad de los ESIs. · El Capitulo 3 describe la parte experimental. Se recogen los reactivos, protocolos, procedimientos e instrumentos usados a lo largo de la tesis.· El Capitulo 4 provee de la demostración de que los NTCs pueden actuar eficientemente como capa transductora en SC-ISEs. Se caracteriza el primer ESI-CS integrado por NTCs mediante técnicas ópticas y electroquímicas.· El Capitulo 5 contiene los resultados experimentales que permiten la posible elucidación del mecanismo de transducción de los NTCs en los ESIs-CS. La Espectroscopia de Impedancia Electroquímica (ESI) es utilizada como la principal técnica de caracterización. · El Capitulo 6 está integrado por cuatro secciones con diferentes aplicaciones analíticas. En la primera sección, se desarrolla un electrodo de pH que usa NTCs como nueva tecnología transductora en ESIs-CS. En la segunda sección se muestra el desarrollo de un ESI-CS integrado por un ionóforo sintético selectivo a colina, y NTCs como transductores. En la tercera sección, ESIs-CS basados en NTCs, resistentes a altas presiones y totalmente herméticos, se desarrollan y prueban en investigaciones acuáticas con la finalidad de obtener información sobre los gradientes de concentración de iones en función de la profundidad de un lago. En la cuarta sección ESIs-CS basados en NTCs se adaptan para el control on-line de un proceso catalítico de desnitrificación.· El Capitulo 7 presenta la posibilidad de la miniaturización de los ESIs-CS basados en NTCs logrando obtener un electrodo nanométrico. Se muestran en esta sección la caracterización óptica y potentiométrica. Además, se discuten las limitaciones de la miniaturización real de los ESIs en potenciometría.· El Capitulo 8 contiene las conclusiones de la tesis. Adicionalmente, se sugieren las perspectivas futuras del trabajo presentado.· Finalmente, se añaden algunos apéndices como complemento de la tesis doctoral.

In this dissertation, we determine how the dispersion, network formation and alignment of carbon nanotubes in polymer nanocomposites affect the electrical properties of two different polymer composite systems: high temperature bismaleimide (BMI) and polyethylene. The knowledge gained from this study will facilitate optimization of the above mentioned parameters, which would further enhance the electrical properties of polymer nanocomposites. BMI carbon fiber composites filled with nickel-coated single walled carbon nanotubes (Ni-SWNTs) were processed using high temperature vacuum assisted resin transfer molding (VARTM) to study the effect of lightning strike mitigation. Coating the SWNTs with nickel resulted in enhanced dispersions confirmed by atomic force microscopy (AFM) and dynamic light scattering (DLS). An improved interface between the carbon fiber and Ni-SWNTs resulted in better surface coverage on the carbon plies. These hybrid composites were tested for Zone 2A lightning strike mitigation. The electrical resistivity of the composite system was reduced by ten orders of magnitude with the addition of 4 weight percent Ni-SWNTs (calculated with respect to the weight of a single carbon ply). The Ni-SWNTs - filled composites showed a reduced amount of damage to simulated lightning strike compared to their unfilled counterparts indicated by the minimal carbon fiber pull out. Methods to reduce the electrical resistivity of 10 weight percent SWNTs -- medium density polyethylene (MDPE) composites were studied. The composites processed by hot coagulation method were subjected to low DC electric fields (10 V) at polymer melt temperatures to study the effect of viscosity, nanotube welding, dispersion and, resultant changes in electrical resistivity. The electrical resistivity of the composites was reduced by two orders of magnitude compared to 10 wt% CNT-MDPE baseline. For effective alignment of SWNTs, a new process called Electric field Vacuum Spray was devised to overcome viscosity within the dispersed nanotube polymer system, and produce conductive MDPE-SWNT thin films. Polarized Raman spectroscopy and scanning electron microscopy (SEM) analysis on the samples showed an improvement in SWNT -- SWNT contacts and alignment in the polymer matrix. The resistivity of the samples processed by this new method was two order magnitudes lower than the samples processed by hot coagulation method subjected to electric field.