Microwave-Assisted Preparation of Carbon Nanotubes with Versatile Functionality

Since the accidental discovery of carbon nanotubes (CNTs) during the synthesis of fullerene by Iijima (Iijima 1991), tremendous research has been done on CNTs. CNTs are under intense investigation owing to their spectacular mechanical and electrical properties (Vazquez, Georgakilas, and Prato 2002). Many ways are currently available for the production of CNTs, which include arc-discharge (Journet et al. 1997), pulsed laser vaporization (Kokai et al. 2000), chemical vapor deposition (Campos-Delgado et al. 2010). However, commercial applications of CNTs have been inhibited by the lack of large-scale production of purified CNTs. Moreover, the intrinsic Vander Waals attraction of CNTs towards each other leads easily to entangle agglomerates, which results in their insolubility in most of organic and aqueous solvents. One of the most powerful approaches to improve CNT solubility is the covalent functionalization of their side walls and tips (Tasis et al. 2006). A wide variety of reactions have been described, most of them employing the conventional chemical techniques, such as refluxing and/or sonication in organic solvents or mineral acids, often making use of high temperature and/or pressure (Hu et al. 2003), long reaction time (Holzinger et al. 2003), or highly reactive species (Brunetti et al. 2007). With the aim of optimizing a multiple funcitonalization approach, many research groups have developed new synthetic strategies to produce multifunctional CNTs avoiding the use of long reaction time, toxic solvent, and extreme condition. Vazquez and coworkers described that 1,3-dipolar cycloaddition of azomethineylides can readily occur on CNTs (Brunetti et al. 2007). However, a severe limitation of this process is the great amount of the DMF and the lengthy time (five days) needed to complete this reaction. Recently, a microwave-assisted synthesis is enabling technology that has been extensively used in organic synthesis (Dallinger and Kappe 2007; Brunetti et al. 2007; Nuchter et al. 2004). Microwave-assisted modification of CNTs is non-invasive, simple, fast, environmentally friendly, and clean method as compared to traditional methods. Usually, the use of the microwave facilitates and accelerates reactions, often improving relative yields. In case of microwave-assisted functionalization of CNTs, microwave irradiation of CNT reduces the reaction time and gives rise to products with higher degrees of functionalization than those obtained by the conventional thermal methods (Vazquez and Prato 2009). Interestingly, Liu and coworkers (Liu, Zubiri et al. 2007) suggest a competitive effect of microwave irradiation that both promotes functionalization and removes some