Covalent Functionalization of Single-Walled Carbon Nanotubes

Language
en
Document Type
Doctoral Thesis
Issue Date
2021-09-13
Issue Year
2021
Authors
Schirowski, Milan
Editor
Abstract

Single-Walled Carbon Nanotubes (SWCNTs) are an amazing material, which will eventually find their way into everyday life due to their outstanding material properties. Until ready for mass production, a lot of fundamental research needs to be conducted first. In this work, I succeeded to promote our understanding of nanotube chemistry, underlying reaction mechanisms and selectivities, and establish inimitably precise analytic techniques. The reaction of SWCNTs and diazonium compounds was explicitly investigated. Using the interplay between Raman-, TG-MS and TG-GC-MS analysis, the resulting product could be analyzed with unprecedented precision. Not only could the desired covalent sidewall-bonded moiety be distinguished from other compounds, but the degree of functionalization could be determined with unique accuracy. Differentiation was achieved by temperature-dependent Raman spectroscopy and cleavage temperature in TGA experiments, where residual solvent and non-covalently bonded aromates cleave at <250 °C, covalent sidewall-bonds at 200 400 °C, and lattice decomposition products at ~450 °C. The combination of these analytical methods allows for a controlled tuning of the degree of functionalization up to 1 %. Furthermore, the reaction mechanism of the underlying reaction has been studied in detail. After an initial well-known single electron transfer (SET) from nanotube to diazonium cation, the coupling occurs and a positively charged, functionalized nanotubium cation is formed. This intermediate does not react with nucleophiles, as could be expected, but undergoes a subsequent SET, resulting in the functionalized, neutral nanotube and acidification of the reaction mixture. In a joint experimental and theoretical study, the selectivity of the reaction between negatively charged carbon nanotubides and diazonium compounds was elucidated. In-depth Raman analysis was performed to investigate the phenyl-functionalized product, revealing that metallic nanotubes bear more sidewall-bonded aryl moieties than semiconducting ones. This is in good accordance with the performed DFT calculations, revealing that the initial and selectivity-determining step of a π-complex between diazonium cation and SWCNT is energetically favored for metallic tubes. However, this metallicity-depending selectivity is superimposed by a diameter-dependent selectivity favoring thinner nanotubes due to a higher ring strain. Furthermore, in order to establish new reaction pathways for the functionalization of SWCNTs, I conducted reactions with phenyl cations and with tert-butyloxycarbonyl-(Boc-)protected diazenes. The first reaction, initiated by photoinduced heterolysis of 4-fluoroaromates, did not succeed due to the light absorbing properties of nanotubes. The second reaction using Boc-protected compounds, however, worked well. It does not only represent a quick and cheap method for SWCNT functionalization, but also proceeds in a cleaner fashion and higher degrees of functionalization compared to the commonly used diazonium route. The recently developed route of nanotube halogenation was enhanced as well. The resulting brominated nanotubes could be substituted with various strong nucleophiles, such as amines, alcoholates, and thiolates. This underlines the crucial role of this halogenated nanomaterial as a potential universal precursor for a variety of functionalization reactions. Moreover, successful chlorination with ICl could be established. The reaction could be even monitored by in situ Raman reactions under strictly inert conditions. Cross-Linked SWCNTs were also synthesized in the scope of this thesis due to their importance in possible applications due to improved properties such as porosity, conductivity, and material strength. The reaction using p-diiodobenzene and biphenyl gave better results compared to the corresponding bisdiazonium compounds. Bi-linked tethers were verified next to mono-linked moieties. Microscopy with atomic resolution enabled to visually observe a biphenyl group and its cleavage upon continuous irradiation was seen for the first time. Cross-linking with longer, polyvalent, or aliphatic tethers could not be proven. Lastly, the reaction conditions for covalent reductive functionalization on graphene have been investigated. Among many existing reaction protocols, the best pathway has been developed, using KC8 and iodoalkanes and iodoarenes. Analytic novelties were found, revealing that aliphatic moieties dimerize during TGA experiments, which is not observed for aromatic groups. Like for nanotubes, at high temperatures >400 °C, lattice decomposition is observed accompanied by small aromatic fragments like toluene and xylene. I am deeply convinced that these scientific novelties will support future works by improving our understanding of carbon allotropes. Hopefully, the synthetic and analytic progress demonstrated in this work will pave the way for the entrance of manifold nanotube and graphene applications supporting and facilitating our everyday life.

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