Synthesis and characterization of multilayer graphene nanostructures

Abstract

The goal of the present investigation is to examine the processing-structure-property relationships of multilayer graphene nanowall materials. Various plasma enhanced chemical vapor deposition (PECVD) processing parameters were altered to control the structure and morphology of the material. Growth parameters and substrate material were the major structural features studied, and these were characterized by spectroscopic techniques. The direct synthesis of graphene without catalysis on dielectric substrates, compatible with the complementary metal oxide semiconductor technology, is a stimulating but complex task. The goal of the thesis has different tasks consisting of: a) The design and construction of a new inductively coupled plasma remote chemical vapor deposition reactor in the PECVD-FEMAN laboratory of the Universitat de Barcelona. b) Fabrication of vertical graphene nanostructures at low temperature on different conductive and nonconductive substrates. c) Characterization of the vertical graphene obtained through different synthesis parameters in order to optimize their physical and surface properties; such as structural and morphological studies by Raman spectroscopy, SEM and TEM. d) Functionalization of MLGNWs by MnO nanoparticles for hybrid supercapacitor systems. The thesis consists of the following main parts: In the first part of the thesis provides a brief introduction of carbon materials, graphene, graphene nanowalls and their history, discovery, outstanding properties and all the technologies that prompted their development during these years until the first application. Moreover, in this section the methods for the synthesis of carbon nanostructures and brief explanation of the fundamentals of each technique explains. In the second part the concepts and technologies of plasma, plasma enhanced chemical vapor deposition (PECVD) and PECVD related techniques are exposed. In addition, in this section a deposition reactor designed by us are described, where all experiments carried out during this thesis took place. Also, the basics and work principles of different characterization techniques are briefly described. Furthermore, this part discusses the growth mechanism of MLGNWs synthesized by PECVD. In the third part, which is the main results part, discusses the study of growing material along the entire length of the tube and the importance of sample location inside a tubular quartz reactor. The influence of the substrate material, growth time and growth temperature on the MLGNWs growth process have been examined by field emission scanning electron microscopy (FESEM),

transmission electron microscopy (TEM) and high resolution TEM (HRTEM) techniques. The chemical characteristics of as grown structures were studied by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectrometer. The hydrogen and carbon contents in grown samples were determined by elemental analysis (EA). To study the photoluminescent properties of the carbon structure grown in the whole length of the tubular reactor, the room temperature PL spectra were conducted. In addition, the chemical reactions inside the tube under plasma deposition were controlled by optical emission spectroscopy (OES). Also, in the part of results synthesis and characterization of MLGNWs/CNTs hybrid structure are discussed. The goal of MLGNWs/CNTs hybrid structure is increased chemical activity of CNTs. The morphological and structural characterization was carried out using SEM, High resolution TEM and Raman scattering analysis. Electrochemical properties of transferred MLGNWs/CNTs were studied by CV and charge/discharge measurements. The last section of the part of the results are exposed information about application of MLGNWs in supercapacitors, in particular, supercapacitive performance of manganese dioxide/ graphene nanowalls electrodes deposited on stainless steel current collectors and annealing temperature effect are discussed. Composite electrodes MLGNWs/MnO2 were characterized by FESEM and Raman shift spectroscopy. The electrochemical properties of the MLGNWs/MnO2 for supercapacitor applications were investigated by cyclic voltammetry (CV), charge/discharge and electrochemical impedance spectroscopy (EIS). The influence of annealing temperature on the electrochemical performance has been studied as well.

El grafeno, como material basado en el carbono, es un logro del desarrollo y los avances de la Nanotecnología. La síntesis directa de grafeno sin catálisis sobre sustratos dieléctricos, compatible con la tecnología de los semiconductores complementarios de óxido metálico, es una tarea estimulante pero compleja. La técnica PECVD, permite la síntesis directa de nanoestructuras de carbono a temperaturas más bajas y es el método principal utilizado en esta tesis. El objetivo de esta tesis es la síntesis y optimización de nanoparedes verticales de grafeno y su posible extensión a aplicaciones en sistemas que requieran superficies macroscópicas. Para ello, se han realizado diferentes tareas: a) Se ha diseñado y construido un reactor prototipo con plasma remoto en el laboratorio PECVD-FEMAN de la Facultad de Física (Universidad de Barcelona) con el fin último de crecer grafeno en forma de paredes/tabiques verticales nanométricos mediante la técnica PECVD. b) Se ha desarrollado un proceso PECVD modificado con el fin de mejorar los resultados actuales en términos de: 1) el tiempo de crecimiento, 2) la temperatura, 3) la naturaleza del substrato, 4) la presión, y 5) la cantidad de gas precursor para crecer grafeno vertical. Las muestras obtenidas fueron caracterizadas mediante microscopía TEM, SEM, XPS, XRD y mayormente mediante espectroscopia Raman, con el objetivo de optimizar el proceso y las propiedades físico-químicas y del grafeno vertical. c) Se ha desarrollado una estructura híbrida con nanoparedes y nanotubos de carbono. Para ello, se utilizaron tres equipos: el reactor “PEDRO” para la preparación del substrato, el reactor “CNTs” para el crecimiento de nanotubos de carbono y el reactor ICP-CVD para el crecimiento de nanoparedes de grafeno. En esta tesis se investigaron las caracterizaciones morfológicas y electroquímicas, pero aún se necesitan más estudios para confirmar posibles futuras aplicaciones. d) Para mejorar las propiedades de los supercapacitores basados en los electrodos desarrolladas con nanoparedes de grafeno y acero inoxidable, se ha realizado el crecimiento de capas delgadas de MnO2 mediante el método de electrodeposición. El efecto de la temperatura de recocido (annealing) en las propiedades electroquímicas de las muestras se ha estudiado en el rango de 70° C a 650° C.