Due to their unique and extraordinary mechanical, thermal and electrical properties, carbon nanotubes (CNTs) have long been considered a leading material candidate for next-generation technologies in fields such as catalysis, electronics and energy storage. However, wide adoption is still hindered by the lack of economic and efficient synthesis methods. The properties of bulk CNT materials are dictated by the individual CNTs, their nanoscale properties (e.g. diameter, length, chirality) and their structural organisation, which are often determined by the synthesis processes. Chemical vapour depositions (CVD) on substrates and floating catalysts (FCCVD) are considered the most promising routes for the scalable production of high-quality CNTs.

Aerosol dynamics play a uniquely important role in CNT synthesis and aerosol processes have emerged as a versatile approach for the scalable synthesis of highly functional nanomaterials. Motivated by this prospect, this dissertation aims to understand, apply and develop gas-phase processes for the production of CNTs. It studies the synthesis of catalyst nanoparticles with controlled size and defined composition, their effects on the structure-controlled synthesis of single-walled CNTs (SWCNTs) and the gas-phase reorientation dynamics of CNTs, which is a critical step preceding aerogel formation in the FCCVD process.

This dissertation develops a novel aerosol synthesis route induced by corona discharge at ambient conditions to harness the effect of unipolar charge in suppressing coagulation and producing nanoparticles with a more monodisperse particle size distribution (PSD). The produced aerosol has a tunable size within 3 - 10 nm and the PSD was found to be distinctively narrower (geometric standard deviation GSD = 1.15−1.38) than the default self-preserving PSDs (GSD= 1.46−1.48). A formation mechanism driven by positive ions was proposed and investigated with an aerosol dynamics model, which revealed that a unipolar charge fraction as small as 0.1% could significantly narrow the PSD. The mechanism was confirmed by measurements of the mobility spectra in the range of 0.8 - 5 nm at the early stage of particle formation. This is also the first known experimental measurement of cluster formation from ferrocene which leads to aerosol generation, providing new insights into the relationship between process parameters and aerosol growth dynamics.

This dissertation then applied three aerosol synthesis processes to make two monometallic and four bimetallic catalyst nanoparticles with a controllable size range of 6 - 15 nm. Size-controlled nanoparticles were used for the synthesis of SWCNTs via substrate chemical vapour deposition and their effects on the nanotube diameter (dt) and electrical type of the SWCNTs were examined. SWCNTs with a narrow dt distribution (1.4±0.2 nm) could be synthesized from Fe or Co nanoparticles under optimal synthesis conditions. However, for SWCNTs smaller than 1.8 nm in diameter, the dependency of dt on catalyst size weakens. To further facilitate the structure-controlled synthesis of SWCNTs by tuning catalyst composition, four Fe-containing bimetallic NPs were synthesized and tested. Fe/W was found to be a promising candidate for catalysing the growth of ultra-small SWCNTs (dt <1.2 nm); Fe/Re for better thermal stability and the growth of semi-conducting SWCNTs (80%); and Fe/Cu for the growth of metallic SWCNTs (87%) with a narrow dt distribution (1.24±0.1 nm). This study also discusses the advantages and challenges of synthesizing catalysts via aerosol processes and proposes future research opportunities accordingly.

When producing bulk CNT materials from an FCCVD reactor, the high CNT number concentration in the gas phase leads to collision and agglomeration, giving rise to a CNT aerogel. To understand this unique phenomenon, a physics-based semi-analytical model was developed to study the gas-phase reorientation dynamics of high-aspect-ratio CNTs, which is a critical step preceding aerogel formation. The model can predict the reorientation behaviour at ±10% accuracy compared with mesoscale molecular dynamics simulations but at <0.1% the computational cost. It is therefore possible to understand the effects of various physical parameters on reorientation time, such as CNT length, type, and CNT bundle size, which then allows the reorientation timescale to be compared against other gas-phase dynamics in an FCCVD reactor. The reorientation time was found to span from 10 ns to 0.1 s, which is still shorter than the collision timescale until the emergence of ultra-long (>50 μm) CNTs/bundles. This study not only explained the gas-phase reorientation behaviour of CNTs, but also opened research avenues into studying the statistical variability in aerogel formation. The methods developed can also be adapted to study other 1D aerosol systems beyond carbon.

This dissertation presents new understandings and progress on the aerosol synthesis of catalyst nanoparticles, the structure-controlled synthesis of SWCNTs, and the gas-phase reorientation dynamics of CNTs. Novel modelling and experimental techniques were developed that not only provide the foundation for future research on the scalable synthesis of high-quality CNTs but also help advance progress in the broad field of the gas-phase synthesis of functional nanomaterials.