A very low solubility of carbon nanotubes (CNTs) hampers many of their potential applications. Especially this is true for multi-walled carbon nanotubes (MWNTs). One of the most efficient ways to increase CNT solubility and dispersibility is their covalent derivatisation (Bahr and Tour, 2002; Basiuk and Basiuk, 2004; Hirsch, 2002; Sun et al. 2002). All known types of chemical reactions employed for this purpose up to now can be divided into two main groups: (a) defect-group derivatisation; and (b) covalent sidewall derivatisation. The first group of reactions involve oxygenated functionalities (mainly carboxylic groups) formed on CNT tips, as well as on their sidewalls to some degree, as a result of oxidative treatment with strong mineral acids (Bahr and Tour 2002; Basiuk and Basiuk 2004; Chen et al. 1998; Hirsch 2002; Jia et al. 1999; Liu et al. 1998; Sun et al. 2002). Among these, the most extensively explored is the formation of amide derivatives between carboxylic groups on oxidised CNT tips and long-chain amines (Ausman et al. 2000; Hamon et al. 1999; Wong et al. 1998a,b). Traditionally, the reaction is performed through chemical activation of the carboxylic groups with thionyl chloride or carbodiimides in an organic solvent medium. For example, in the case of thionyl chloride and commonly used octadecylamine, the reaction can be schematically represented as follows:
While this reaction by itself is efficient and fast, such inevitable auxiliary operations as CNT filtering and centrifuging are not. And in addition, the use of organic solvents produces harmful wastes and cannot be particularly welcomed. From common considerations, for the functionalisation of CNTs one can use the same chemical approaches that have been developed for other poorly soluble inorganic materials, for example silica materials. A decade ago we performed systematic studies on the use of the gas-phase chemical derivatisation for the synthesis of chemically modified silicas, mainly for liquid-chromatographic applications (Basiuk and Chuiko 1990, 1993; Basiuk and Khil'chevskaya 1991; Basyuk 1991; Basyuk and Chuiko 1990). Among the derivatising reagents tested were polyazamacrocyclic ligands (Basyuk 1991; Basyuk and Chuiko 1990), pyrimidine bases (Basiuk and Chuiko 1993; Basyuk 1991), and solid carboxylic acids (Basiuk and Chuiko, 1990; Basiuk and Khil'chevskaya, 1991). Most of those compounds are poorly volatile under ambient temperature and pressure. Nevertheless, decreasing the pressure to a moderate vacuum and, on the other hand, increasing the temperature to >150°C provided efficient formation of the chemically bonded surface derivatives. In particular, the reaction between silica-bonded aminoalkyl groups and vaporous carboxylic acids to form surface amides proceeded smoothly at 150-180°C without chemical activation of the carboxylic groups, it was relatively fast (0.5-1 h), and provided high yields of the amide derivatives (>50% based on the starting surface concentration of aminoalkyl groups). Excess derivatising reagent was spontaneously removed from the reaction zone. In addition, there was no need to use an (organic) solvent medium: this feature is attractive not only from an ecological point of view, but also in that it helps to avoid undesirable particle aggregation of the material derivatised.
Bearing the above advantages of the gas-phase derivatisation in mind, we systematically worked on the development of a similar procedure for chemical derivatisation of single-walled CNTs (SWNTs) and MWNTs with amines.
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