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COVALENT ORGANIC FUNCTIONALIZATION AND CHARACTERIZATION OF CARBON NANOTUBES

Cécilia Ménard-Moyon

Laboratoire d'Immunopathologie et Chimie Thérapeutique, CNRS - Institut de Biologie Moléculaire et Cellulaire, Strasbourg, France

1.1 INTRODUCTION

More than 20 years ago, Iijima reported the structural morphology of carbon nanotubes (CNTs) by use of high-resolution transmission electron microscopy (HRTEM) and electron diffraction [1]. A CNT can be defined as a graphene sheet rolled up to form a cylinder (Fig. 1.1a). CNTs can be classified into different types: single-wall CNTs (SWCNTs), double-wall CNTs (DWCNTs), and multi-wall CNTs (MWCNTs) depending on the number of layers. SWCNTs have diameters ranging from 0.7 to 2 nm and lengths up to several micrometers, while MWCNTs have diameters from a few to tens of nanometers and lengths up to a few micrometers. Hence, the structure of CNTs is characterized by a high aspect ratio (i.e., ratio between length and diameter). Approximately two-thirds of as-produced SWCNTs are semiconducting, whereas one-third is metallic. CNTs contain defects in their structure, such as vacancies, and five- or seven-membered rings that induce curvature, as illustrated in the transmission electron microscopy (TEM) image in Figure 1.1b.

Figure 1.1 Schematic representation of a SWCNT (a) and TEM image of MWCNTs (b).

The breadth and range of research involving CNTs has expanded greatly over the past years. Indeed, CNTs possess unique electronic, mechanical, and thermal properties that can be exploited for potential applications in a variety of fields from materials science [2], molecular electronics [3], photovoltaic devices [4] to nanomedicine [5]. However, CNTs have poor solubility in all solvents due to strong intermolecular cohesive forces among the nanotubes that form bundles, thus hampering full exploitation of their properties and presenting obstacles to their practical applications. Therefore, functionalization is required for manipulating and processing CNTs by inducing exfoliation, increasing dispersibility, and giving the possibility to associate molecules with specific properties to nanotubes.

Functionalization can be classified into two categories: covalent and noncovalent derivatization, the latter relying on hydrophobic, p-p, and/or electrostatic interactions [6-8]. Covalent functionalization can be achieved by oxidation of defect sites of CNTs and subsequent derivatization of the generated carboxylic acid groups. Other methods are based on halogenation, cycloaddition reactions, or direct additions of highly reactive species on the nanotube sidewall. Grafting functional groups on the nanotube surface in a covalent manner allows to obtain stable conjugates with desired properties by tailoring the physicochemical properties of the CNTs. Depending on the level of functionalization, the electrical conductivity of the CNTs can be significantly altered. It is of high importance to rigorously characterize functionalized CNTs. For this purpose, different spectroscopic, microscopic, and thermal techniques can be used for morphological, structural, and elemental analysis of functionalized CNTs.

This chapter is focused on covalent methodologies for nanotube functionalization. Section 1.2 is dedicated to the different chemical strategies used for covalent functionalization of CNTs, while Section 1.3 describes the analytical techniques for characterization of functionalized CNTs. Finally, Section 1.4 contains some concluding remarks.

1.2 COVALENT FUNCTIONALIZATION OF CARBON NANOTUBES WITH ORGANIC MOLECULES

1.2.1 Defect-Site Chemistry

Among various surface functionalization techniques, amidation or esterification of oxidized CNTs is probably the most extensively used to prepare soluble materials either in organic solvents or in water and for linking a wide range of molecules [9]. Generally, oxidation of CNTs is performed by treatment with strong acids such as nitric acid [10,11], sulfuric/nitric acid mixture [10], or with other strong oxidizing agents (H2SO4/KMnO4 [12] or OsO4 [13]). The oxidative treatment, in particular when assisted by sonication, usually induces shortening of the CNTs [10], but also frequently causes nanotube damage, limiting their use as mechanical and electrical reinforcements. Among treatments using strong acids, low-power sonication of MWCNTs in nitric acid followed by treatment with hydrogen peroxide was found to minimize nanotube damage [14]. Many research groups have studied the chemical nature of the oxygenated moieties (e.g., carboxylic acids, carbonyls, hydroxyls) [15] introduced on the nanotube surface by different techniques such as infrared (IR) spectroscopy [16] and thermogravimetry [15]. Oxidized CNTs are mainly decorated with carboxylic groups, as suggested by the pioneering work of the group of Smalley who derivatized the carboxyl functions with thiolalkylamines by amidation [10]. The CNTs bearing thiol moieties were labeled with gold nanoparticles and visualized by atomic force microscopy (AFM). Gold nanoparticles were found mainly at the nanotube ends. By using scanning tunneling microscopy (STM), Prato and coworkers visualized alkyl chains introduced by amidation of carboxylic acid functions, confirming that oxidation of CNTs occurs mainly at the nanotube tips [17].

Oxidized CNTs have been widely used as precursors for further covalent derivatization via amidation or esterification reactions, with amine or alcohol derivatives, respectively (Fig. 1.2). The carboxylic acid functions have to be pre-activated via the formation of acyl chlorides using oxalyl or thionyl chloride, followed by the addition of the appropriate amine or alcohol. Alternatively, the amidation can be performed by using the carbodiimide coupling chemistry. In this case, the carboxyl groups are treated with N-hydroxy succinimide (NHS) or 1-hydroxybenzotriazole (HOBt) in the presence of a carbodiimide, usually N,N-dicyclohexylcarbodiimide (DCC) or 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC). The corresponding esters are then displaced by amine or hydroxyl functions to form the amide or ester bonds, respectively.

Figure 1.2 Oxidation of CNTs (a) and, amidation (b) and esterification of oxidized CNTs (c). For clarity, a SWCNT segment is shown with only a single added functional group.

A range of molecules with various properties have been linked on the CNTs by this method, including several organic molecules [18,19], chromophores with optoelectronic properties [20], bioactive molecules [21-23], or polymers [24,25]. To provide evidence of the ester bond formation in soluble CNTs functionalized with lipophilic or hydrophilic chains, defunctionalization was performed by acid- or base-catalyzed hydrolysis, leading to recovery of the starting insoluble CNTs [26]. In another study, esterification between oxygenated functions at the tips of single oxidized SWCNTs has been exploited to form rings of nanotubes with a narrow size distribution according to AFM [27].

1.2.2 Halogenation

1.2.2.1 Fluorination.

Fluorination has been one of the first chemical methods developed to functionalize CNTs [28]. Most strategies involve elemental fluorine at high temperatures (up to 600°C) (Fig. 1.3a) [29-33]. The best temperature conditions are between 150°C and 400°C. The highest degree of functionalization was found to be one fluorine atom for every two carbon atoms according to elemental analysis [34].

Figure 1.3 Fluorination of CNTs (a). Substitution with Grignard reagents or alkyllithium derivatives (b). Substitution with amino compounds (c) or diols (d).

Alternative conditions implying CF4 plasma treatment have also been developed [35,36]. Due to rehybridation of a high number of sp2 carbon atoms to sp3, the resulting fluoronanotubes are insulating.

Fluorination drastically enhances the reactivity of the nanotube sidewalls. Therefore, derivatization of fluoronanotubes by nucleophilic substitution reactions is possible (Figs. 1.3b-1.3d) [37]. Indeed, a variety of nucleophilic reagents has been used such as alkyl magnesium bromides (Grignard reagents) [38] and alkyllithium derivatives [39]. Fluoronanotubes have also been reacted with several amines [40], diamines [41], diols [42], or amino alcohols [42].

1.2.2.2 Bromination.

A few methods for bromination of CNTs have been recently reported using various conditions. DWCNTs have been brominated by elemental bromine using microwaves, leading to a mild alteration of the p-conjugated sidewall of the nanotubes according to Raman spectroscopy (5-8 wt% of Br) (Fig. 1.4a) [43]. Alternatively, bromination of DWCNTs using Br2 vapor at room temperature results in 5-6 at% bromine concentration [44]. Plasma using gaseous bromine has been applied for the functionalization of SWCNTs. The treatment is very efficient as one bromine atom per two carbon atoms is introduced on the nanotube surface in these conditions [45]. The bromo-functionalized nanotubes have been further derivatized by nucleophilic substitution with amine derivatives. Elemental bromine in the presence of a Lewis acid or dibenzoylperoxide as radical initiator allows to functionalize MWCNTs with...