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Gas-Catalyst Dynamics in Vertically Aligned Carbon Nanotube Synthesis: Advanced Materials for Environmental Engineering.

Record Type:	Electronic resources : Monograph/item
Title/Author:	Gas-Catalyst Dynamics in Vertically Aligned Carbon Nanotube Synthesis: Advanced Materials for Environmental Engineering./
Author:	Shi, Wenbo.
Published:	Ann Arbor : ProQuest Dissertations & Theses, : 2018,
Description:	330 p.
Notes:	Source: Dissertations Abstracts International, Volume: 80-02, Section: B.
Contained By:	Dissertations Abstracts International80-02B.
Subject:	Nanotechnology. -
Online resource:	http://pqdd.sinica.edu.tw/twdaoapp/servlet/advanced?query=10927909
ISBN:	9780438194793

Gas-Catalyst Dynamics in Vertically Aligned Carbon Nanotube Synthesis: Advanced Materials for Environmental Engineering.
Shi, Wenbo.

Gas-Catalyst Dynamics in Vertically Aligned Carbon Nanotube Synthesis: Advanced Materials for Environmental Engineering. - Ann Arbor : ProQuest Dissertations & Theses, 2018 - 330 p.

Source: Dissertations Abstracts International, Volume: 80-02, Section: B.

Thesis (Ph.D.)--Yale University, 2018.

This item must not be added to any third party search indexes.

Although both materials and environmental scientists recognize the promise of advanced nanomaterials to address environmental issues and to promote societal sustainability, converting the intrinsic nanoscale properties of those materials to macroscopic practical applications remains challenging. In addition, nanomaterial production and utilization might bear unintended environmental concerns. Thus, the convergence of materials science advances and environmental objective optimization is in demand to address those intertwined challenges for an environmentally sustainable future. This dissertation considers vertically aligned carbon nanotubes (VACNTs) as an example of advanced materials to illustrate the efforts towards concurrently (1) reducing the resource and energy consumption associated with production and (2) promoting material applications to ultimately provide energy and water benefits. In the first part of the dissertation, fundamental understanding of the gas phase composition (i.e., O2, H2O, CO2, H2, and different hydrocarbon sources) involved in CNT synthesis via chemical vapor deposition (CVD) was investigated with the ultimate motivation of advancing the controllability, sustainability, and scalability of VACNT manufacturing. In an engineered Fe/Al2O 3 thin film catalyst system, trace amount (hundreds ppm) of molecular O 2, rather than the traditional suspect oxidant, H2O, was shown to exert substantial influences on Fe thin film dewetting in the Hz-dominant annealing gas environment, implying the importance of controlling O2 in atmospheric-pressure CVD to guarantee reproducible and controlled VACNT production. Following film dewetting, carbon introduction typically initiates CNT formation, and the growth performance of commonly-used hydrocarbon precursors (i.e., CH4, C2H4, and C2H2) was compared through data mining of over 200 published articles and complimentary experimental testing. With simultaneous evaluation of product quality and environmental metrics, C2H 2 exhibited advantages over the other two precursors with respect to a lower temperature requirement, higher atom efficiency, and accordingly fewer hazardous byproducts emission. Note that this thermal dependence on precursor identity is consistent with polymerization schema to explain CNT formation. Lastly, aiming to further improve feedstock efficiency, the role of CO 2 was investigated as a promoter for C2H2 to form CNT structures. Based on experimental evidence and complimentary theoretical simulation, a CO2-assisted dehydrogenation reaction mechanism ( i.e., wherein CO2 removes H atoms in the growing CNT structure following C2H2 incorporation) was proposed, explaining previous reports demonstrating CNT synthesis at low temperature in the presence of CO2. Taken together, these synthesis studies provide a holistic understanding of the critical roles of gas species at each individual stage of CNT manufacturing, enriching CNT growth mechanism knowledge, informing production metrics, and providing inroads to more environmentally and economically sustainable nanomaterials synthesis. In the second part of the dissertation, the most desirable applications of CNTs for environmental engineering are reviewed thoroughly. Several of these promising applications rely on capacitance, but have yet to be realized. Manganese oxide (MnOx) has high theoretical specific capacitance, and is a low cost, low toxicity, earth-abundant material. However, its poor conductivity and low accessible surface area limit its applications as electrode materials. In response. I designed and fabricated vertically aligned arrays of CNT-MnOx core-shell via atomic layer deposition of MnO x onto VACNTs. The hierarchical and anisotropic morphology of VACNT arrays effectively extended the intrinsic extraordinary nanoscale properties (mechanical, electrical, and thermal) of individual nanotubes to the macroscale, which positions them well for device integration (i.e., macroscopic applications). Providing high electrolyte accessibility via well-defined intertube spacing and exceptionally conductive support, VACNTs were found to compensate for the poor electrical conductivity and low surface area shortcomings of MnOx resulting in a hybrid material that exhibited exceptional capacitance. This outstanding capacitance enables potential applications in energy storage as supercapacitors and water desalination via capacitive deionization. Concurrently advancing sustainable production and applications for sustainability, this dissertation functions at the interface of environmental and material engineering and uses VACNTs as a case study to explore the convergence of material and environmental insights for employing advanced materials to address global sustainability challenges.

ISBN: 9780438194793Subjects--Topical Terms:

526235
Nanotechnology.

Gas-Catalyst Dynamics in Vertically Aligned Carbon Nanotube Synthesis: Advanced Materials for Environmental Engineering.
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520 $a Although both materials and environmental scientists recognize the promise of advanced nanomaterials to address environmental issues and to promote societal sustainability, converting the intrinsic nanoscale properties of those materials to macroscopic practical applications remains challenging. In addition, nanomaterial production and utilization might bear unintended environmental concerns. Thus, the convergence of materials science advances and environmental objective optimization is in demand to address those intertwined challenges for an environmentally sustainable future. This dissertation considers vertically aligned carbon nanotubes (VACNTs) as an example of advanced materials to illustrate the efforts towards concurrently (1) reducing the resource and energy consumption associated with production and (2) promoting material applications to ultimately provide energy and water benefits. In the first part of the dissertation, fundamental understanding of the gas phase composition (i.e., O2, H2O, CO2, H2, and different hydrocarbon sources) involved in CNT synthesis via chemical vapor deposition (CVD) was investigated with the ultimate motivation of advancing the controllability, sustainability, and scalability of VACNT manufacturing. In an engineered Fe/Al2O 3 thin film catalyst system, trace amount (hundreds ppm) of molecular O 2, rather than the traditional suspect oxidant, H2O, was shown to exert substantial influences on Fe thin film dewetting in the Hz-dominant annealing gas environment, implying the importance of controlling O2 in atmospheric-pressure CVD to guarantee reproducible and controlled VACNT production. Following film dewetting, carbon introduction typically initiates CNT formation, and the growth performance of commonly-used hydrocarbon precursors (i.e., CH4, C2H4, and C2H2) was compared through data mining of over 200 published articles and complimentary experimental testing. With simultaneous evaluation of product quality and environmental metrics, C2H 2 exhibited advantages over the other two precursors with respect to a lower temperature requirement, higher atom efficiency, and accordingly fewer hazardous byproducts emission. Note that this thermal dependence on precursor identity is consistent with polymerization schema to explain CNT formation. Lastly, aiming to further improve feedstock efficiency, the role of CO 2 was investigated as a promoter for C2H2 to form CNT structures. Based on experimental evidence and complimentary theoretical simulation, a CO2-assisted dehydrogenation reaction mechanism ( i.e., wherein CO2 removes H atoms in the growing CNT structure following C2H2 incorporation) was proposed, explaining previous reports demonstrating CNT synthesis at low temperature in the presence of CO2. Taken together, these synthesis studies provide a holistic understanding of the critical roles of gas species at each individual stage of CNT manufacturing, enriching CNT growth mechanism knowledge, informing production metrics, and providing inroads to more environmentally and economically sustainable nanomaterials synthesis. In the second part of the dissertation, the most desirable applications of CNTs for environmental engineering are reviewed thoroughly. Several of these promising applications rely on capacitance, but have yet to be realized. Manganese oxide (MnOx) has high theoretical specific capacitance, and is a low cost, low toxicity, earth-abundant material. However, its poor conductivity and low accessible surface area limit its applications as electrode materials. In response. I designed and fabricated vertically aligned arrays of CNT-MnOx core-shell via atomic layer deposition of MnO x onto VACNTs. The hierarchical and anisotropic morphology of VACNT arrays effectively extended the intrinsic extraordinary nanoscale properties (mechanical, electrical, and thermal) of individual nanotubes to the macroscale, which positions them well for device integration (i.e., macroscopic applications). Providing high electrolyte accessibility via well-defined intertube spacing and exceptionally conductive support, VACNTs were found to compensate for the poor electrical conductivity and low surface area shortcomings of MnOx resulting in a hybrid material that exhibited exceptional capacitance. This outstanding capacitance enables potential applications in energy storage as supercapacitors and water desalination via capacitive deionization. Concurrently advancing sustainable production and applications for sustainability, this dissertation functions at the interface of environmental and material engineering and uses VACNTs as a case study to explore the convergence of material and environmental insights for employing advanced materials to address global sustainability challenges.
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