Carbon Nanotubes Composites: Engineering, Properties and Applications

Abstract

Carbon nanotubes (CNTs) an unique 1-dimensional sp² carbon structure have attracted significant scientific interest in the last decades due to their outstanding and unique transport, electrical, mechanical and thermal properties. These nanostructures have been envisaged for a broad range of applications. Nevertheless, their applicability for potential applications are strongly dependent on their physical and chemical properties. Therefore, new nanofabrication approaches that allow the production of CNTs with precisely engineered properties (i.e. dimension, surface chemistry, chemical composition, etc.) could enable and spread the applicability of these carbon-based nanostructures across more relevant applications. In this context, this thesis aims at developing advanced fabrication methodologies to produce CNTs-based composites with precisely engineered dimensions and properties for addressing gap in knowledge about molecular transport inside CNTs structures and specific problems related to molecular separation and advanced photocatalysts for environmental and energy applications. CNTs composites were produced by growing vertically aligned multi-walled carbon nanotubes (MWCNTs) inside nanoporous anodic alumina membranes (NAAMs) and titania nanotubes (TNTs), produced by anodisation of aluminium and titanium substrates, respectively, through a catalyst-free chemical vapour deposition (CVD) approach. It was demonstrated that the dimensional features of CNTs-NAAMs composite membranes can be precisely engineered by the anodisation conditions and electrolyte used to produce the host templates and the deposition time during the CVD process. Thermal annealing and wet and dry oxidation processes were explored as means of controlling the surface chemistry of the inner walls of CNTs. Both dimensional features and surface chemistry were observed to have significant impacts on the molecular transport and selectivity properties of CNTs-NAAMs composite membranes assessed by analysing the transport of two models molecules with different hydrophilic-hydrophobic and charge properties. Electrochemical impedance spectroscopy (EIS) investigations on CNTs-NAAMs composite membranes featuring different chemistry (i.e. as-produced and modified with charged oxygen species) revealed that their electrochemical properties can be readily tuned by controlling their surface chemistry, which is critical to understand the properties of these composite membranes for desalination applications. Furthermore, a new strategy aiming to modify the inner wall surfaces of CNTs-NAAMs composite membranes by doping their structure with nitrogen was developed. The crucial role of the precursor source C/N ratio in the formation of N-doped CNTs has been revealed. Characterisations of the structure of N-doped CNTs composite membranes and liberated tubes indicate that this nanofabrication method enables the chemical modification of CNTs without obstruction or morphological changes. The chemical composition analysis showed a dependency of nitrogen doping levels on the precursor source. The tunability and enhancement in the transport performance and chemical selectivity properties of CNTs-NAAMs after nitrogen doping have been established for the first time. Finally, a composite platform for photocatalytic applications based on CNTs-TNTs has been developed. Investigations on their photocatalytic properties evaluated via the degradation of an organic model molecule revealed that CNTs induces a synergistic effect on the photocatalytic activity of TNTs, enhancing it up to one order of magnitude as compared to bare TNTs. The results and investigations presented in this thesis advance the knowledge in designing CNTs and CNTs-based composite membranes with different physical and chemical properties in a precise manner for transport and photocatalysis applications. These results are envisaged valuable contribution towards advancing the existing knowledge in the field of CNTs composite membranes and carbon materials in general and boosting the applicability of these systems across multiple disciplines and industrial applications.