Title

Synthesis and functionalization of coiled carbon filaments

Date of Award

2014

Degree Name

Ph.D. in Materials Engineering

Department

Department of Chemical and Materials Engineering

Advisor/Chair

Advisor: Khalid Lafdi

Abstract

Coiled carbon filaments have one of the most attractive three-dimensional forms in carbon materials due to their helical morphologies. Because of their shape and carbon structure, they exhibit excellent mechanical and electrical properties such as superelasticity, low Young's modulus, relatively high electrical conductivity, and good electromagnetic (EM) wave absorption. Therefore, they are good candidates as fillers in composite materials for tactile sensor and electromagnetic interference shielding. In medical areas of interests, coiled carbon filaments can be used as micro and nano heaters or trigger for thermotherapy and biosensors using EM wave exposure because absorbed EM waves by coiled carbon filaments are converted into heat. Although various shapes of coiled carbon filaments have been discovered, optimum synthesis conditions and growth mechanisms of coiled carbon filaments are poorly understood. The study of growth kinetics is significant not only to analyze catalyst activity but also to establish the growth mechanisms of coiled carbon filaments. The establishment of growth mechanisms would be useful for determining optimum synthesis conditions and maximizing the quantity of carbon filaments synthesized for a given application. In the first study, tip grown single helical carbon filaments or carbon nanocoils (CNCs) were synthesized by a chemical vapor deposition method using tin-iron-oxide (Sn-Fe-O) xerogel film catalyst. The Sn-Fe-O catalyst was prepared by a low-cost sol-gel method using stannous acetate and ferric acetate as precursors. The growth kinetics of CNCs were monitored by a thermogravimetric analyzer, and the experimental result was correlated using a one-dimensional kinetic model, corresponding to one-dimensional tip growth. In the second study, bidirectionally grown double helical filaments or carbon microcoils (CMCs) were synthesized using a chemical vapor deposition method. CMCs obtained at two reaction temperatures were compared. CMCs grown at the higher temperature had smaller fiber size and coil diameter, and longer lifetime of catalyst. For in-depth analysis, growth kinetics of CMCs were studied using an exponential decay model for catalyst poisoning. In the third study, CMCs were functionalized for improvement of water dispersion, optical properties and self-assembly. As-grown CMCs were hydrophobic. To improve water dispersion for biological applications, the surface of as-grown CMCs were oxidized by concentrated nitric acid at room temperature. After the oxidation, the acid-treated CMCs were well dispersed in water. For optical property, CMCs were functionalized with octadecylamine (ODA). Upon photoexcitation, the functionalized CMCs exhibited photoluminescence in the visible region. Similar to carbon based nanoparticles, the photoluminescence of CMCs was attributed to electron-hole radiative recombination after surface passivation. The results suggest that these functionalized CMCs might be used as a new class of optical agents for biological applications. As a primary experiment to study Au-S bonding, aminoethanethiol (HSCH2CH2NH2) was attached to the surface of gold-coated CMCs. Energy dispersive X-ray (EDX) mapping shows gold, sulfur and nitrogen on the surface of CMCs. Then, a thiol-modified ssDNA attachement experiment was performed using a similar functionalization procedure as aminoethanethiol. The existence of phosphorus, nitrogen, oxygen and sulfur on surface of Au-coated CMCs immersed in thiol-modified ssDNA solution was confirmed by the EDX spectrum. The result indicates that ssDNA was fixed on their surface. In the fourth study, the effect of oxidized CMCs on mouse embryonic stem (MES) cells was examined to determine their toxicity. Mouse embryonic stem cells represent a unique cell population with the ability to undergo both self-renewal and differentiation. Results indicate that oxidized CMCs had very little toxicity on stem cell viability. There was no observed loss of alkaline phosphatase (AP) as a stem cell marker and mitochondrial membrane potential (MMP) of oxidized CMC-treated MES cells was unaffected. Oxidized CMCs did not show toxicity to MES cells at the cellular level.

Keywords

Carbon fibers Testing, Materials Science, coiled carbon filament, carbon microcoil, carbon nanocoil, CMC, CNC, chemical vapor deposition, toxicity study, mouse embryonic stem cell, MES cell

Rights Statement

Copyright 2014, author

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