Date of Award
Fall 2022
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Chemical and Environmental Engineering (ENAS)
First Advisor
Plata, Desirée
Abstract
Carbon nanotubes (CNTs) have exceptional electronic properties that give rise to promising use as sensors, interconnects, and semiconductor materials. However, functionalizing carbon nanotubes with covalent modifications to enable sensing applications, or selecting desirable semiconductive tubes via liquid treatment, both dominates the environmental impact of CNTs and suffers limitations in control and product quality. In this thesis, I explored alternative approaches to functionalization and chirality direction that leverage exotic gas-phase precursors or vapor-phase treatments. In the first part of this work, I leveraged oxygen-bearing, short-chained alkynes. Briefly, terminal alkyne precursors polymerize on a metal nanoparticle catalyst to form carbon nanotubes, and I hypothesized that alkyl or heteroatom substituents could become part of the resulting CNT structure. Two oxygen-functionalized C3 alkynes, propargyl alcohol and propiolic acid, were selected as potential carriers of hydroxyl and carboxyl functionality, respectively. Each successfully produced CNT forests with elemental oxygen content at or below 2% (by elemental analysis). These levels were not observable via X-ray photoelectron spectroscopy (XPS), but could be detected via attenuated total reflectance (ATR)-Fourier-transform infrared spectroscopy (FTIR). This suggested that oxygen-functionalized precursors imparted structural differences in the CNTs. Notably, the limited oxygen content was likely the result of a critical need to form CNTs from purely carbonaceous precursors, and an analysis of reactor effluent via gas chromatography with flame ionization detector (GC-FID) revealed possible thermal breakdown pathways of oxygen-bearing precursors. While acetylene-based growths ultimately yielded higher carbon conversion efficiencies, propargyl alcohol and propiolic acid present opportunities for targeted growth of minimally-functionalized structures. The second approach to CNT functionalization sought to explore the viability of vapor and liquid post-processing schemes in combination with variable CNT surface chemistries imparted by alkynes. CNTs were derived from three precursors (acetylene, methylacetylene, and vinylacetylene) to determine whether distinct precursors yielded materials that were more amenable to differing acid treatment modalities. In particular, vapor-phase acid treatment was explored as a means to minimize material utilization relative to liquid-phase treatment. Two acids were explored: hydrochloric acid (HCl), which has an implicitly lower environmental footprint due to lower greenhouse gas and toxic emissions during its life cycle production, and nitric acid (HNO3), a common reagent used for CNT oxygen addition via reflux. XPS analysis of treated samples demonstrated an affinity for vapor-based Cl addition via HCl, and liquid-based O addition via HNO3. Additionally, vinylacetylene-derived CNTs were prone to O addition via HCl, in an acid-catalyzed pathway (e.g., oxygen derived from water added to the CNT structure). From a material standpoint, life cycle impact assessment (LCIA) showed that vinylacetylene bears a 2.5-fold greater global warming potential, and a staggering 150-fold greater ecotoxicity, over acetylene. These differences were not remediated by switching to vapor- or HCl-based treatment strategies. While a switch from HNO3 to HCl reduces both global warming potential and ecotoxicity by over 90%, the primary burden across growth and treatment modalities was the energy required for liquid reflux, where proposed room temperature vapor treatments offer a 3.5 order of magnitude footprint reduction; this highlights the importance of vapor treatments, should Cl functionality be desirable. Finally, a series of terminal alkyne precursors (acetylene, methylacetylene, vinylacetylene, 1-butyne, (R)-3-butyn-2-ol, (S)-3-butyn-2-ol, and a racemic mixture of (R)- and (S)-3-butyn-2-ol) were used to grow CNTs on a range of metal catalysts (Fe, FeMo, and varied proportions of CoMo, known to restrict or randomly give rise to variable CNT chiral angles) to determine whether precursor structure could influence the chirality of resulting single-walled CNTs (SWCNTs). An extensive multipoint spectral analysis of Raman scattering across all precursor-catalyst combinations supports the known role of the catalyst in influencing chiral distribution. Though a uniform chiral shift was not detected across precursors, differences in the broadening and narrowing of the observed diameter distribution (especially with acetylene, where more sub-nanometer CNTs were detected) implies an influence of precursor structure on chiral distribution that cannot be achieved by catalyst choice alone. While a deeper understanding of catalyst-precursor synergy may yield new possibilities of chiral control, this work suggests that a narrow chiral distribution may rely more heavily upon CNT seed growth and cloning strategies. As a whole, this thesis seeks to delineate the limits of gas-based control over carbon nanotube structures, which is often overlooked in favor of catalyst-focused approaches. These strategies of gas-phase control, which leverage and further inform a fundamental understanding of CNT growth mechanisms, offer the potential for synthesis of tailored structures while reducing cost, waste, and environmental footprint – an attractive prospect for implementation in industrial-scale syntheses.
Recommended Citation
Johnson, Eric Phillip, "Gas-Phase Control of Heteroatom Placement and Structure in Carbon Nanotube Synthesis" (2022). Yale Graduate School of Arts and Sciences Dissertations. 854.
https://elischolar.library.yale.edu/gsas_dissertations/854
