Metal oxide catalysts for carbon nanotubes growth : the growth mechanism using NiO and doped ZnO

摘要

The thesis describes the preparation and characterisation of novel oxide catalysts (NiO and doped ZnO) and catalyst free growth of carbon nanotubes (CNTs). Additionally a new growth mechanism has been proposed for these oxide catalysts, based on intra-granular charge transfer and lattice strain. The intra-granular charge transfer process is believed to help dissociate the hydrocarbon molecules and the lattice strain is believed to be responsible for the diffusion of carbon atoms through the catalyst nanoparticles. These two processes collectively give rise to CNTs growth. The materials were prepared through solid state reaction with a range of temperatures. The pellets were then thermally evaporated on to the substrates in order to use these catalyst nanoparticles for CNTs growth. The chemical state and the chemical environment of the dopants (Ni, Cu, Sm, Tb and Ho) in the ZnO host material were identified through x-ray photoelectron spectroscopy (XPS). It was observed through XPS that the mixed oxidation state of the dopants in the ZnO gives rise to the intra-granular charge transfer process. The crystal structure and the lattice strain produced by the incorporation of dopant ions into the ZnO matrix were observed through x-ray diffraction (XRD). It was observed that dopant ions produce lattice strain and the extent of it, depends upon the ionic radii of the dopants. The higher the lattice strain the higher will be the diffusion of carbon atoms through the catalyst nanoparticles. The size and shape of the catalyst nanoparticles were obtained using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Energy dispersive x-ray spectrometry (EDS) in the TEM and SEM was applied to see the elemental composition of individual nanoparticles and the pellets respectively. After growth the CNTs were characterised through SEM and TEM to see the morphologies and EDS for the elemental composition of the particle at the tip of CNTs. The study of these catalysts regarding the growth of CNTs showed that those catalyst nanoparticles with a strong intra-granular charge transfer mechanism and higher lattice strain gives rise to CNTs growth. The catalyst nanoparticles with dopants in their minimum or maximum oxidation states do not give rise to intra-granular charge transfer and hence no CNTs growth.