Kinetics of carbon nanotube growth with applications in hydrogen storage [electronic resource]
Keywords: Materials Science and Engineering.

Carbon nanotubes (CNTs) have unique transport and elastic properties due to their high aspect ratios. Hence there is considerable interest in using these tubes as field emitter cathodes, composite materials with enhanced electrical and mechanical properties, electronic components and recently for hydrogen storage applications taking advantage of the high specific surface areas. In this thesis three different aspects of carbon nanotubes were studied: (1) Controlled growth of single walled nanotubes (SWNTs), (2) Electric field directed Chemical Vapor Deposition (CVD) of multi walled (MWNTs) and (3) "Spillover" Mechanism of hydrogen storage in Pt-SWNT composites. The kinetics of carbon nanotube growth was studied in the context of the CVD process. A generic model for growth of 1-d nano structures via the Vapor-Liquid-Solid (VLS) mechanism is applied to the nanotube growth. This model considers the energetics of individual mass transfer steps through each phase and at the phase interfaces. The flux is then written in terms of the change in chemical potential. Laser interferometry was applied in a cold-wall thermal CVD reactor to measure the growth of the MWNT films in-situ. Temperature dependent studies in the steady-state regime were used to obtain activation energies which are consistent with the interfacial transport step. Consideration of the catalyst activation/de-activation process in the non-steady regimes requires the rate limiting step to be in the vapor-liquid transition. Application of an electric field during the MWNT growth was found to enhance both the growth rate and alignment of the MWNTs. Temperature dependent studies in the presence and absence of the electric field show that there are actually two activated processes involved, with rate-limiting step being independent of applied field at high temperature. At higher temperatures, the rate-limiting step is the carbon dissolution into the catalyst particle, while at lower temperatures it is the carbon dissociation at the catalyst-vapor interface that limits the growth. Application of an electric field enhances the decomposition of the C precursor in the vapor phase, thus circumventing this low temperature activation barrier. The enhanced alignment of the MWNTs with the electric field is explained by tensile stretching overcoming the defect-induced kinking of the MWNTs. Calculations show that this benefit is obtained at a minimum field level, with no benefit arising from further increase in field strength. The catalyst particle size is one of the key parameters that determine the morphology of the 1D carbon nanostructures in both processes studied. The thermodynamics of the nano-particle formation and carbon dissolution are studied and applied to these processes. While the diameter serves to template the CNT diameter, the Gibb's Thompson effect predicts a size dependent suppression of melting point which determines the nature of CNT formed. In the CVD process, higher pressures were found to form larger particle sizes which led to nanofiber growth. At these diameters, the melting point suppression puts the Fe-C particle in a dual solid-liquid phase. Carbon flux accumulates in the dual phase during growth until the dual phase becomes energetically unfavorable. At this point, the particle reverts to a single solid phase regime by discarding excess carbon, resulting in a discontinuous graphitic structure characteristic of Carbon nanofibers. For smaller particles, the phase is entirely liquid and leads to steady state carbon flux and CNT growth. Controlling the iron bearing precursor concentration of the solution fed into the floating catalyst reactor was found to control particle size, and hence SWNT diameter, within this regime. For similar catalyst particle size distributions, increasing the temperature increased the range of SWNT diameters and chiralities obtained. The thermodynamic energy barrier for SWNT formation at the different diameters was calculated and shown to be consistent with the observed variation. Finally, the mechanism of hydrogen uptake in transition metal-doped SWNT was studied. Molecular hydrogen, dissociated by metal catalyst nanoparticles, diffuses to the nanotube surface forming stronger bonds. In-situ 4-probe conductivity tests were performed on mats of Pt doped SWNT during hydrogen uptake. On hydrogen charging the resistivity of the Pt doped SWNT mat increased. This is due to the formation of C-H bonds, which breaks the symmetry of the CNT electronic structure resulting in formation of localized defects, thereby increasing the resistivity. Initial studies of the temporal dependence of hydrogen uptake suggest a diffusion-limited process. XPS was employed to measure the extent of sp3 C-H bonding.