Carbon Nanotubes Deposited by Hot Wire Plasma CVD and water assisted CVD for Energetic and Environmental Applications

Abstract

Nanoscience and Nanotechnology have experienced a tremendous growth in few years. Nanotechnologies are the design, characterization, production and application of structures, devices and systems by controlling shape and size at nanometer scale. Carbon exists in several forms, depending on how the carbon atoms are arranged, their properties vary. One of the carbon forms is carbon nanotubes, which are capped at each end by half of a fullerene, and have aroused great interest in the research community because of their exotic electrical, thermal and mechanical properties. MWCNTs and SWCNTs were discovered in 1991 and 1993, respectively, by Ijima. A single-wall carbon nanotube can be described as a graphene sheet rolled into a cylindrical shape so that the structure has one-dimensional axial symmetry. Carbon nanotubes (CNTs) have unique characteristics that allow them to act as electrodes in charge storage devices, sensors and traps for pollutants, among others. On the one hand, for applications that require a certain amount of energy in pulse form, the traditional capacitors used in electronic circuits are not suitable because they cannot store enough energy in the volume and weight available. However, given the characteristics of CNTs that have a narrow size distribution, large specific surface area, low resistivity and high stability, CNTs have been regarded as a suitable material for electrodes in supercapacitors. On the other hand, the development of new systems, based in CNTs, which could overcome some of the current limitations in the capture of emerging pollutants in fluids, such as nanometric particles (being, moreover, difficult to detect) and organic pollutants at very low concentrations, is an additional objective of the present thesis. Water plasma and nitrogen plasma treatments were performed to remove amorphous carbon and to functionalize the surface of CNTs with different oxygen or nitrogen groups. Conditions of plasma treatments were optimized by adopting a Box-Wilson experimental design. Various microscopic and spectroscopic techniques were used to characterize the morphology, structure and elemental compositions before and after the plasma treatments. Electrochemical measurements show that water plasma treatment significantly increases the active surface area of CNTs, and nitrogen plasma is more effective to improve the charge transfer. Both nitrogen and water plasma raise the capacitance of CNTs notably in comparison to untreated CNTs. Manganese dioxide was deposited by galvanostatic method on untreated CNTs and plasma treated nanotubes. The MnO2 structure changes from nanoflower (as deposited) to needle like or to a layer coating on the surface of CNTs depending on the voltage applied during the cycling measurements. CNTs treated with 75 W plasma power and 10 Pa nitrogen pressure, and further functionalized with MnO2, exhibit the highest specific capacitance obtained in this thesis; 955 Fg-1 at 10 mVs-1. This value is almost 87% of the theoretical value for MnO2. The structural evolution of CNTs during water assisted growth has also been studied. The obtained length of CNTs was ~ 800 µm on silicon wafer. Transfer of ultralong CNTs on conductive adhesive aluminum tape was carried out using a novel methodology that lowers the series resistance of the electrode. The specific capacitance of CNTs/Al increases from 87 to 148 Fg-1 for untreated and water plasma treated CNTs/Al, respectively. In addition, we found that for a successful and faster growth of CNTs on copper substrate, strong adhesion of the buffer layer (Al2O3) is essential. A multilayered setup (Cu/Ni/Ti/Al2O3) prior to catalyst deposition boosts the growth rate and quality of CNTs. Vertically-aligned CNTs were synthesized on quartz fiber filters for environmental applications. Three chlorinated VOCs; trichloroethylene, chloroform and 1,2-dichlorobenzene were used to study the adsorption/desorption properties of CNTs/QF. The ability to detect or remove organic pollutants increases after the water plasma treatment, which functionalizes the CNTs surface and removes the catalyst from the top of CNTs (inner cavities are available for use). We found that molecules with aromatic rings present stronger interactions with CNTs (Phi-stacking).

La nanociencia y la nanotecnología han experimentado un enorme crecimiento en pocos años. Una de las formas del carbono son los nanotubos de carbono, que están limitados en cada extremo por medio fulereno, y que han despertado un gran interés en la comunidad científica debido a sus exóticas propiedades eléctricas, térmicas y mecánicas. Un nanotubo de carbono de pared simple puede ser descrito como una hoja de grafeno enrollada en forma cilíndrica de modo que la estructura tiene una simetría axial. Los nanotubos de carbono (CNTs) tienen características únicas que les permiten actuar como electrodos en dispositivos de almacenamiento de carga, sensores y adsorción de contaminantes, entre otros.

Los principales objetivos de esta tesis son la síntesis de CNTs sobre los diversos sustratos para aplicaciones de almacenamiento de carga (supercondensadores) y ambientales.

Tratamientos de plasma de agua y nitrógeno se realizaron para eliminar el carbono amorfo y funcionalizar la superficie de los CNTs con diferentes grupos de oxígeno o nitrógeno. Las condiciones de los tratamientos de plasma fueron optimizados mediante la utilización de un diseño experimental de Box-Wilson. Las medidas electroquímicas muestran que el tratamiento con plasma de agua aumenta significativamente el área superficial activa de los CNTs, y el plasma de nitrógeno es más eficaz para mejorar la transferencia de carga. Tanto el plasma de nitrógeno como el de agua aumentan la capacidad de los nanotubos de carbono en comparación con los CNTs no tratados. El dióxido de manganeso se depositó electroquímicamente mediante el método galvanostático sobre los nanotubos de carbono sin tratar y tratados con plasma. La estructura de MnO2 cambia de una estructura de “nanoflor” (inicialmente) a una en forma tipo aguja o de capa superficial en función del voltaje aplicado durante los experimentos de ciclado. Los CNTs tratados con 75 W de potencia de plasma y 10 Pa de presión de nitrógeno, y posteriormente funcionalizados con MnO2, exhiben la capacitancia específica más alta obtenida en esta tesis; 955 Fg-1 a 10 mVs-1. Este valor es aproximadamente el 87% del valor teórico para MnO2. La evolución estructural de los nanotubos de carbono durante su crecimiento asistido por agua ha sido estudiada. La longitud obtenida de los CNTs es de ~ 800 micras sobre una oblea de silicio. La transferencia de CNTs ultralargos a una cinta adhesiva de aluminio conductor se llevó a cabo utilizando una metodología novedosa que reduce la resistencia en serie del electrodo. La capacidad específica de los CNTs / Al aumenta de 87 a 148 Fg-1 para los CNTs / Al sin y con tratamiento de plasma de agua, respectivamente. Una configuración de múltiples capas (Cu/Ni/Ti/Al2O3) antes de depositar el catalizador aumenta la velocidad de crecimiento y la calidad de los CNTs. CNTs verticalmente alineados se sintetizaron sobre filtros de fibra de cuarzo para aplicaciones ambientales. Tres compuestos orgánicos volátiles clorados; tricloroetileno, cloroformo y 1,2 diclorobenceno se utilizaron para estudiar las propiedades de adsorción / desorción de CNTs / QF. Se vio que las moléculas con anillos aromáticos presentan interacciones más fuertes con los nanotubos de carbono (apilamiento de tipo pi).