Synthesis of aligned carbon nanotube arrays and fabrication of carbon nanotube structures for gas and water separation

The purpose of this project is to optimise the synthesis of aligned carbon nanotube (CNT) arrays and explore the application of porous CNT structures in the gas and water separation. Aligned CNT arrays and CNT buckypapers are the main two types of porous CNT structures used as separation membranes. In this project, aligned multi-walled CNTs were synthesised by supported-catalyst and floating-catalyst chemical vapour deposition (CVD). In the supported-catalyst CVD, H2 pre-treatment was found to be a critical factor affecting the CNT growth. CNT array height and alignment was strongly dependent on the duration of H2 pre-treatment, with optimal height and alignment achieved using 10-15 minutes pre-treatment. Small angle X-ray scattering was used to quantify the alignment, distribution, and size of the CNTs in arrays produced from varying pre-treatment times and the results correlated with microscopy measurements. Compared to the supported-catalyst CVD, the floating-ferrocene CVD is promising due to its low cost and facile fabrication process, as well as its scalability. A two-step floating-ferocene CVD, was developed to prepare aligned CNT arrays. The effect of ferrocene loading on the size and distribution of Fe catalysts, as well as the morphology, diameter, and height of the CNT arrays, was investigated. CO2, as a weak oxidant, was introduced into the floating-ferrocene CVD to improve the growth of aligned CNT arrays. The CO2-assisted floating-ferrocene CVD method facilitates the well-controlled growth of aligned CNT arrays, where the quality of the aligned CNT arrays was significantly improved in the presence of CO2, as demonstrated by improved height, alignment and purity. A novel vacuum extraction process was employed to release the as-prepared CNT array from the Si wafer after water etching at 750 °C. The resulting free-standing CNT arrays were applied as membranes for gas permeation test (H2, N2, and CO2), showing much faster gas transport than that calculated theoretically. Alignment of the CNT arrays plays an important role in the gas permeation through the aligned CNT arrays. Compared to the theoretical calculation, the gas diffusion across the well-aligned CNT arrays was enhanced by a factor of ~ 45, which is significantly more than that across the poorly­aligned CNT arrays, with an enhancement factor of~ 8. Multi-walled CNT buckypaper was fabricated by a facile, surfactant-free assembly process. The permeation of different gases through the as-prepared buckypaper was investigated. The gas permeance is 2~3 times higher than the theoretical permeance, with ideal H2 selectivities to Ar, C2H4, and CO2, of 4.20, 3.62, and 4.59, respectively. The buckypaper was also used in water treatment as a filter membrane for removal of natural organic matters (NOM). Humic acid (HA) was used as a model NOM. Purification of the CNTs introduced carboxylic and hydroxylic functional groups that improved the rejection of HA through the buckypaper. The buckypaper prepared from purified CNTs exhibited excellent rejection of HA(> 93%). Overall, in the supported-catalyst CVD, the synthesis of aligned CNT arrays was optimised by H2 pre-treatment. In the floating-ferrocene CVD, the height, alignment, diameter and other properties of the aligned CNT arrays were firstly controlled by the addition of CO2. New methods were employed to prepare free-standing CNT arrays and buckypapers as membranes. The effect of alignment was firstly investigated by using free-standing arrays with different alignment in gas separation experiments. Buckypaper was used in a new field, NOM removal, and showed excellent performance