Carbon Nanotubes grown by chemical vapour deposition: a catalyst activation study

Nowadays the main challenge to fully exploit carbon nanotubes (CNTs) for potential applications consists of achieving complete control over their synthesis. Synthesis control means to be able to selectively obtain isolated CNTs as well as bundles, different types of CNTs (SWNT/MWNT/CNF), different chiralities and diameters, their location and orientation. Other aims are decreasing defect and impurity concentrations, and increasing yields. These goals can be reached only by a complete understanding of the role of the catalyst during its interaction with the environment (substrate and feed stock). In this thesis, we have investigated the effects of the substrate-catalyst interaction on the growth and the chemical decomposition of the carbon precursor gas on the catalyst clusters and the consequent formation of carbon structures during the growth process itself. We have mainly concentrated on studying the growth by surface bound chemical vapour deposition method using Fe and/or Ni as catalysts, Al2O3 and SiO2 as the support substrate, and C2H2 as the precursor gas. We have identified how different catalyst-substrate interactions between Fe-Al2O3 and Fe-SiO2, determine the difference in density, direction and type of carbon nanotubes obtained by using the same pretreatment and growth conditions. Different experimental conditions and apparatus were employed to study the catalyst-substrate interactions effects. We monitored the chemical state of the catalyst and the substrate in situ by X-ray photoemission spectroscopy and, in parallel, the morphology of the surface at each intermediate state of the catalyst preparation (by ex situ atomic force microscopy (AFM)). Further we confirmed the results by post-growth characterization by transmission electron microscope (TEM). Studying the catalyst-hydrocarbon interaction in situ via both environmental TEM (ETEM) and XPS techniques has allowed us to make progress towards an atomistic model of CNT growth. By in situ time-resolved ETEM we have found that structural selectivity is determined by the dynamic interplay between carbon network formation and catalyst crystalline particle deformation. Our in situ time-resolved XPS study shows the selective acetylene chemisorption on metallic Fe catalyst, which is rapidly followed by the formation of a carbon-rich phase (iron carbide), to finally the formation of a sp2 carbon network characteristic of graphite. Carbidic carbon has also been detected, even if gradually attenuated from the graphitic peak, up to the intensity saturation of the sp2 C peak. Summarizing, we have observed selective acetylene chemisorption at the nucleation stage and we have demonstrated that the formation of a carbon-rich (sub)surface layer on crystalline transition metal nanoparticles is an integral part of catalyst dynamics during CNT growth.