Dispersion And Assembly Of Single-Walled Carbon Nanotubes In Surfactant Association System

Since the discovery of carbon nanotubes (CNTs) in 1991 by Iijima, they have attracted tremendous attention due to their unique mechanical, electronic, optical and magnetic properties. CNT materials and related composites are expected to be ideal candidates for various applications including nanoelectronics, field emission devices, nanocompositions, sensors, detectors, etc. However, the as-prepared carbon nanotubes form aggregated bundles due to the stong intertube van der Walls interaction, which is the main barrier for realizing the practical application of carbon nanotubes. So far, various carbon nanotubes dispersing techniques have been developed, which fall into two main categories:covalent and noncovalent functionalization. The covalent functionalization would inevitably disrupt the electronic structure of carbon nanotubes and alter the intrinsic properties of CNTs. The noncovalent approach preserves the intrinsic properties and performance of CNTs, and hence is considered to be more advantageous. Amphiphilic surfactants and polymers are the common noncovalent modifiers of carbon nanotubes according to references in the the literature. By using these CNT dispersing systems, nice dispersion efficiency have been achieved, while some challenges still exist, which actually restrict the performance of the CNTs in applications. For instance, the as-prepared SWNTs are not only aggregated bundles but also a mixture of metallic and semiconducting tubes. However, the property of SWNTs was solely depended on their electric structure. The mechanical mixing of SWNTs with various electronic types prevents their widespread application as high-performance materials. Effective dispersion of SWNTs with different diameters and further separation of metallic and semiconducting SWNTs are still the challenges for their practical application. Besides, there were few reports about using the biocompatible and environmentally friendly surfactants as CNT dispersants.In this paper, the dispersion of SWNTs was realized by using biocompatible amino-acid based surfactants and alkanolamide; and not only the intensive dispersion and synchronous assembly of SWNTs but also the selective dispersion of semiconducting SWNTs were achieved by adjusting the transformation of the micro association structures of surfactants.The main contents and innovations of this paper are summarized as follows:(1) The reversible dispersion of SWNTs by amino-acid based surfactant with biocompatibility. Considering the bio-applications of SWNTs, the essentially dispersing agent needs to be low-toxic, biocompatible as well as biodegradable. Sodium N-Cocoyl Sarcosinate and sodium N-Cocoyl-Glycinate are amino-acid based zwitterionic surfactants. It was found that the hydrophobic tails of amino-acid based surfactants adsorbed on the surface of SWNTs by hydrophobic interaction under mild sonicating; the static repulsion between the adsorbed amino-acid based surfactants overcame the inter-tubes van der Waals interaction; and a well-dispersed and stable SWNTs suspension was achieved after mild centrifugation. The reversible dispersion of SWNTs was realized by adjusting pH of the zwitterionic surfactant (sodium N-Cocoyl Sarcosinate and sodium N-Cocoyl-Glycinate) aqueous solution.(2) N,N-bis(2-hydroxyethyl)dodecanamide (DDA) is a kind of amphiphilic nonionic surfactant with low toxicity and biodegradability. The microstructures of self-assembled DDA in water and oil were investigated by combining experimental and theoretical methods. It was found that DDA can self-assemble to form bicontinuous network structures in both water and oil phase. Liquid crystal formed by self-assembled DDA in water displayed high electric conductivity, which have potential application in high-performance materials. The excellent biocompatibility and processability of DDA liquid crystals might expand their application scope in the biosensors and biomedicine field in the future.(3) The intensive dispersion and synchronous assembly of SWNTs were realized by the DDA association system. We used the mixture of surfactant and oil insteading of surfactant aqueous solution as the initial medium of dispersing SWNTs. According to the self-assembly properties of DDA in water and oil, the intensive dispersion and synchronous assembly of SWNTs were finally realized by gradually adjusting the transformation of the micro association structure of DDA-oil-water system. SWNTs suspension dispersed by the surfactant-oil-water association system was demonstrated to be a prospective candidate of saturable absorber in passively Q-switched laser. The discussion about the mechanism revealed a novel approach for thorough dispersing and synchronous assembling SWNTs, which might be helpful to break through the bottleneck in some application areas of SWNTs.(4) The as-synthesized SWCNTs are always a mixture of metallic and semiconducting tubes. The effective and selective seperation of semiconducting SWNTs with narrow chirality distributions was achieved by the difference of the interactions between DDA and the metallic and semiconducting SWNTs through a unique experimental arrangement. The electronic and optical properties of SWNTs strongly depend on their diameter and chiralities, the effective and selective dispersion of semiconducting SWNTs might clear the major obstacle in SWNTs applications.