Solubilisation, purification and processing of single walled carbon nanotubes using organic molecules
Document Type Article
The aims of this research are seven fold; • To enhance the solubility of SWNTs in organic solvents. • To purify an untreated sample of SWNTs that contains many side products. • To gain a greater understanding of the interaction between the polycyclic aromatic hydrocarbons and SWNTs. • To debundle and disperse SWNTs. • To select SWNTs of a specific structure based on the structures of the polycyclic aromatic hydrocarbons used. • To observe and compare the interaction of polycyclic aromatic hydrocarbons with SWNTs produced by different methods. • To elucidate factors which contribute to different degrees of interaction between different SWNT samples and the hydrocarbon molecules. In this research, the solubilisation, purification and debundling of single walled carbon nanotubes (SWNT) on interaction with the polycyclic aromatic hydrocarbons (PAHs), anthracene and p-terphenyl are explored. The investigations were performed in toluene due to its poor affinity for the retention of SWNTs. Thus this solvent can act as a good indicator for improvements in the solubility of the SWNTs due to the presence of the PAHs. The hydrocarbons anthracene and p-terphenyl were chosen because of their high degree of solubility in toluene and their simple structure that is proposed in this research to exhibit molecular recognition of SWNTs with a similar backbone structure to that of the hydrocarbon. The study begins with the exploration of the interaction between the SWNTs in an untreated sample and the PAHs, and results reveal an enhanced solubility of the SWNTs in toluene and subsequently the purification of the SWNT sample. Interaction between the PAHs and the SWNTs draws the SWNTs into solution allowing all side products to precipitate out. The composite solution is a stable suspension of finely dispersed SWNTs with no evidence of SWNT precipitation observed in over a five year period. This observation indicates an enhanced stability of the SWNTs in solutions due to an interaction with the PAH molecules and this observation is supported by Raman and fluorescence spectroscopic studies which are presented in this thesis. Both anthracene and p-terphenyl map via a π - π interaction to the SWNT backbone with strong indications of structural selectivity between the SWNT surfaces and PAHs. The literature is scarce with regard to the interaction between organic molecules and SWNTs. In this research an investigation into the interaction that occurs between PAHs and SWNTs is conducted. The interactions and debundling of SWNTs over a broad concentration range are probed through spectroscopic methods including fluorescence and Raman. Fluorescence concentration dependence studies define the concentration range where aggregated PAHs and isolated PAHs exist. A fluorescence model based on the fraction of free hydrocarbons is used to indicate the concentration range where debundled and bundled SWNTs and PAHs are interacting with atomic force microscopic (AFM) studies supporting the results. AFM is used to obtain a surface profile of the SWNTs at a range of concentrations to image the debundling of SWNTs with decreasing concentration. The fluorescence model also allows the calculation of a binding energy between the SWNTs and PAHs at low concentrations. The binding energy determined is further supported by alternative theoretical models and the results obtained correlate well. Raman studies support fluorescence and AFM studies with regard to the interaction and debundling of SWNTs and the Raman radial breathing modes (RBMs) are probed to investigate selective interaction of the RAH molecules at low concentrations and results reveal a degree of selective interaction. To conclude this work, a study of SWNTs produced by different methods is performed to investigate any changes which may occur to the SWNT/PAH/solvent system as a result of replacing one tube type for another. The study elucidates factors that are favourable for interaction with the hydrocarbon in the given solvent toluene. Spectroscopic methods such as fluorescence and microscopic studies such as AFM are used as the investigative tools. The studies reveal that large bundles with larger diameter SWNTs are most favourable for interaction with p-terphenyl and debundling in toluene. Such a study is of interest for the development of a standardised purification and processing protocol.