Chapter 2: Surface treatment of carbon nanotubes via plasma technology – Polymer-Carbon Nanotube Composites

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Surface treatment of carbon nanotubes via plasma technology

B. Ruelle, C. Bittencourt and P. Dubois,     University of Mons Materia Nova Research Centre, Belgium

Abstract:

The chapter begins by discussing carbon nanotube (CNT) surface chemistry and solution-based functionalization. It then reviews promising alternative ways to modify the CNT surface through plasma processes. Plasma treatments have the advantage of being non-polluting and provide a wide range of grafted functional groups, depending on the plasma parameters such as power, gases used, duration of treatment and gas pressure. However, the interaction of high energy particles with the CNT surface can induce damage in the CNT structure. A plasma post-discharge treatment, which can prevent the degradation of CNTs, is presented.

Key words

carbon nanotubes

functionalization

plasma

post-discharge treatment

2.1 Introduction

Since their observation by Iijima (1991), carbon nanotubes (CNTs) have attracted the attention of many researchers due to their outstanding properties (Thostenson et al., 2001). Their unique physical properties result from their structure, which at atomic scale can be thought of as a hexagonal sheet of carbon atoms rolled into a seamless one-dimensional cylindrical shape. Besides their extremely small (nanometric) size, their excellent electrical and thermal conducting performances, combined with their strong toughness and transverse flexibility make them promising for use in a wide variety of applications such as electronic components, chemical and biological sensors, chemical and genetic probes, field emission tips or mechanical memories (Collins and Avouris, 2000). Carbon nanotubes are thus expected to expand their unique properties in multidisciplinary fields and should play a key role in nanotechnology.

Characterized by both high aspect ratio (length-to-diameter) and low density, this allotropic variety of carbon is an ideal candidate to be used as filler materials in composites. In fact, CNTs can provide a three-dimensional conductive network through the polymer matrix with exceedingly low percolation thresholds (Kilbride et al., 2002). Furthermore, it has been suggested that their high thermal conductivity can be exploited to make thermally conductive composites (Bagchi and Nomura, 2006). In addition, the mechanical enhancement of polymer materials using CNTs as reinforcing nanofillers will be exploited in the very near future (Wang et al., 2008).

A key parameter for producing high quality nanocomposite materials with improved physical properties is the homogeneous dispersion of the individual CNTs. Unfortunately, the CNTs tend to form large bundles thermodynamically stabilized by van der Waals forces and physical entanglements between the tubes, which occur during their synthesis. The presence of these aggregates in addition to the CNT’s low solubility in water and organic solvents represent a drawback for the engineering of CNT in polymer nanocomposites. It has been reported that the homogeneous dispersion of CNTs is relatively difficult to achieve in the large majority of polymers (Andrews and Weisenberger, 2004).

The chemical functionalization of the CNT sidewalls, i.e. the CNT surface-anchoring of functional (reactive) groups, represents a solution for the tuning of interactions between CNTs and the host polymer matrix, improving their dispersion ability (Hirsch and Vostrowsky, 2005). This surface modification can be divided into two main approaches.

First is the non-covalent functionalization, used in different techniques such as the addition of surfactants (Hirsch, 2002), polymer wrapping (Liu, 2005) or polymerization-filling technique (PFT) (Bonduel et al., 2005), which allows the unaltered CNTs to preserve their physical properties. However, the interaction between the wrapping molecules and the nanotubes remains generally weak, limiting the efficiency of the property transfer between the nanotubes and the host polymer, in particular, there may be a low load transfer for mechanical properties.

The other approach relies upon the covalent grafting of functional chemical groups along the CNT sidewalls. These chemical functions can be used as anchoring sites for polymer chains (Hirsch and Vostrowsky, 2005) and to improve the interaction with the polymer matrix (Thostenson et al., 2005). The covalent bonding between CNTs and the polymer chains ensures an optimal interfacial strength and thus a better load transfer. Nevertheless, the covalent functionalization can influence the CNTs’ properties, depending on the nature and density of the functional sites. Indeed, the functionalization can take place on already existing CNT structural defects, or active sites can be created by grafting chemical functions. In the latter case, functionalization can degrade the CNTs’ physical properties if the density of the active site is significant, leading to damaged CNT structure (Fu et al,. 2001). The majority of developed covalent sidewall functionalizations are carried out in organic solvent. Nevertheless, as the CNT surface is largely inert, rather harsh conditions are needed for wet chemical functionalization of CNTs, which rarely results in a controlled covalent functionalization process (Hirsch and Vostrowsky, 2005).

To overcome these drawbacks, ‘dry’ processes are being developed. For example, the ball-milling of CNTs in a reactive atmosphere was shown to functionalize nanotubes with different chemical groups, such as amides, amines, carbonyls and thiols, for instance, depending on the reactant gas (Konya et al., 2002). Using this method, CNTs are broken, and dangling bonds are formed on their newly created extremities that can then react with a selected gas. A relatively long period of time, a minimum of 24 hours, is usually necessary to graft a significant amount of functional groups onto CNTs. However, the average length of the CNTs decreases as the treatment time increases and some amorphous carbon was also observed after the breaking process (Ma et al., 2008). Another drawback of this technique is the presence of functional groups exclusively at the ends of cup-stacked carbon nanotubes.

Alternative ‘dry’ approaches to chemical modification of CNTs using plasma discharges have been also proposed and developed. Considered the fourth state of matter, plasma is an ionized gas that constitutes a highly unusual and highly reactive chemical environment. Plasma treatments have evolved into a valuable technique to modify material surface properties without altering the bulk properties, allowing their use in various applications such as plasma cleaning, plasma sterilization or biomedical applications (D’Agostino et al., 2005). Recently, plasmas were also applied to modify the surface properties of CNTs. Characterized by a relatively low reaction time, surface plasma treatment is an environmentally friendly process without any use of organic solvents. The plasma technique allows the grafting of a wide range of different functional groups on the CNT surface depending on plasma parameters such as power applied to the discharge, nature of the gas used, duration of treatment and gas pressure.

2.2 Carbon nanotube surface chemistry and solution-based functionalization

As mentioned above, even though chemical functionalization displays some drawbacks, it still represents a very important method investigated by several research groups. Hereafter are discussed some important results recently reported in the literature.

Actually, the chemical functionalization of CNTs via covalent grafting reaction can be divided into: (a) end-caps and ‘defect groups’ chemistry; and (b) sidewall functionalization.

2.2.1 End-caps and defect-sites chemistry

Functionalization via ‘end-caps and defect-site’ chemistry consists of grafting functional groups directly on the already existing defects in the structure of CNTs using wet chemistry. Carbon nanotubes are in general described as perfect graphite sheets rolled into nanocylinders. In reality, CNTs present structural defects: typically, ca. 1–3% of the carbon atoms of a nanotube are located at a defect site (Hu et al., 2001).

The end-caps of nanotubes are composed of highly curved fullerene-like hemispheres, which are much more reactive when compared to the sidewalls (Sinnott, 2002). The sidewalls themselves contain defect-sites such as pentagonheptagon pairs called Stone-Wales defects, sp3-hybridized defects or holes in the carbon sheet (Charlier, 2002). The most frequently encountered type of defect is the Stone-Wales (or 7–5-5–7) defect, which leads to a local deformation of the nanotube curvature. Grafting reactions are most favored at the carbon–carbon double bonds in these positions. The different defects commonly observed in CNTs are shown in Fig. 2.1 (Hirsch, 2002).

2.1 Typical defects along a single-wall CNT: (A) five- and seven-membered rings forming a bend in the tube; (B) dangling bonds; (C) hole in the carbon framework; and (D) open end of CNT terminated with –COOH groups (Hirsch, 2002).

Frequently, the techniques applied for the purification of the raw material, such as acid oxidation, induce the opening of the tube caps as well as the formation of holes in the sidewalls. These vacancies and tube extremities are therefore functionalized with oxygenated functional groups such as carboxylic acid, ketone, alcohol and ester groups (Chen et al., 1998). The introduced carboxyl groups can present useful sites for further modifications in organic solvents such as the coupling of molecules through the creation of amide or ester bonds. Figure 2.2 represents common functionalization routes on single-wall CNT ends and defect-sites through solution-based chemistry (Banerjee et al., 2005).

2.2 Some representative functionalization reactions at CNT ends and defect-sites (Banerjee et al., 2005). SWNTs, single-walled nanotubes.

Using solution-based chemistry, nanotubes can be grafted with a wide range of functional moieties via amidation or esterification reactions, for which bifunctional molecules are often utilized as linkers/coupling agents. This method was used to graft amine moieties onto carbon nanotubes via the reaction with diamines such as triethylenetetramine (Gojny et al., 2003), ethylenediamine (Meng et al., 2008) or 1,6-hexamethylenediamine (Li et al., 2007). Another approach involves the reduction of the carboxyl groups to hydroxyls, followed by transformation into amino groups via phthalimide coupling and hydrolysis (Ramanathan et al., 2005). Gromov et al. (2005) have developed two other approaches, using amino-decarboxylation substitution, to make primary amino groups directly attach onto the carbon nanotube surface. The carboxylic groups, the majority grafted at the open ends of CNTs, are replaced with amino groups via Hofmann rearrangement of carboxylic amide or via Curtius reaction of carboxylic acid chloride with sodium azide.

One advantage of the defect-site chemistry is that functionalized carbon nanotubes mostly retain their pristine electronic and mechanical properties (Zhang et al., 2003). However, as mentioned earlier, the spatial distribution of grafted functions is not homogeneous. In fact, the majority of carboxylic groups, used as anchoring sites, are localized at the extremities of the carbon nanotubes.

2.2.2 Sidewall functionalization

The other approach for covalent grafting reactions is the ‘ sidewall functionalization’, which consists of grafting chemical groups through covalent reactions onto the π-conjugated skeleton of CNTs (Hirsch, 2002). Unlike defect-site chemistry, which takes advantage of defects already present in the CNT structure, the direct covalent sidewall functionalization is associated with a change in hybridization from sp2 to sp3. This simultaneous loss of conjugation of CNTs influences their physical properties, and more particularly their electrical conductivity, depending on the density of functionalization.

First, covalent sidewall functionalization was carried out on the basis of well-developed grafting chemistry on fullerenes whose reactivity depends strongly on the curvature of the carbon framework. However, the sidewall reaction chemistry of CNTs differs from that of fullerenes as the chemical reactivity in carbon systems arises from two factors that induce local strain: the pyramidalization at the carbon atoms and the π-orbital misalignment between adjacent pairs of conjugated carbon atoms (Chen et al., 2003). The fullerene structures and the CNT end-caps present a pronounced pyramidalization of the carbon atoms further improving chemical reactions. In the CNT sidewalls, the pyramidalization strain is not as acute and, thus, π-orbital misalignment has a greater influence on sidewall chemical reactivity (Hamon et al., 2001). In Fig. 2.3, the reactivity of the C–C bond in CNT structure is presented in function of its angle to the tube circumference. Because the deformation energy of sp2 bond is inversely proportional to the diameter of the CNT, nanotubes with a smaller diameter have higher reactivity (Hirsch, 2002).

2.3 Pyramidalization angles (ΘP) and the π-orbital misalignment angles (Φ) along C1–C4 in the CNT framework and its capping fullerene C60 (Hirsch and Vostrowsky, 2005).

However, whereas the nanotube end-caps are quite reactive due to their fullerene-like structure, even taking into account the folding of the graphene sheet, the reactivity of CNT sidewalls remains low and sidewall-functionalization is only successful if a highly reactive reagent is used. As mentioned, an additional constraint on sidewall functionalization is the tendency of CNTs to form bundles, which limits the available nanotube surface for the grafting of chemical reagents.

Typically, the covalent sidewall functionalization is carried out in organic solvent, which allows the utilization of the sonication process to improve the dispersion of CNTs and, thus, to increase the available surface of carbon nanotubes. However, precipitation immediately occurs when this process is interrupted (Tasis et al., 2006). The required reactive species such as carbenes, nitrenes or radicals are in general made available through thermally activated reactions (Balasubramanian and Burghard, 2005). Normally, the grafting reaction can be initiated exclusively on the intact sidewall or in parallel at defect-sites. The most common sidewall functionalizations using organic solvents, such as carbene (Lee et al., 2001) or nitrene (Holzinger et al., 2004) [2+1] in cycloaddition reactions or radical additions via diazonium salts (Bahr et al., 2001), are shown in Fig. 2.4.

2.4 Representative covalent sidewall functionalization reactions of CNTs as carried out in organic solvents (Banerjee et al., 2005).

The first sidewall functionalization studied was the fluorination of CNTs. Carbon nanotubes were fluorinated using fluorine in the range between room temperature and 600 °C (Mickelson et al., 1998). This reaction is very useful because further nucleophilic substitutions can easily be accomplished and make possible a flexible approach to provide the CNT sidewalls with various types of functional groups (Khabashesku et al., 2002).

Among these reactions, several diamines are reported to react with ‘fluoronanotubes’ via nucleophilic substitution reactions (Stevens et al., 2003), leading to the formation of amino-functionalized CNTs. However, if bifunctional reagents such as diamines with sufficiently long carbon chains are used, the nanotubes can be covalently cross-linked with each other. Modification with amino-containing substituents was developed using photolysis of acetonitrile (Nakamura et al., 2008). The 1,3-dipolar cycloaddition reaction can be also used to graft linkers, with amino groups at their ends, uniformly distributed along the sidewalls (Pantarotto et al., 2003).

As discussed before, the carbon nanotube covalent functionalizations carried out in organic solvent present some drawbacks. The need to agitate carbon nanotubes by sonication to improve their dispersion in organic solvents can trigger severe damage to the tube walls and the average length of CNTs can be reduced (Monthioux et al., 2001). Moreover, certain highly reactive chemical products also involve harsh conditions causing detrimental damage to the tips and sidewalls of CNTs, which decreases their stability and can cut them into short pieces (Liu et al., 1998). In addition, the solvent functionalization process generally requires time-consuming multistep reactions. Finally, the use of organic solvent pollutes and is difficult to upgrade for industrial processes.

In the next section, the use of plasmas as an optimal alternative for grafting functional groups at the CNT surface will be presented and discussed.

2.3 Plasma treatment of carbon nanotubes

2.3.1 Introduction to plasma: the fourth state of matter

Plasma is physically defined as an ionized gas with an equal number of positively and negatively charged particles. It consists of free electrons, ions, radicals, UV radiation and various highly excited neutral and charged species (Chapman, 1980). The entire plasma is electrically neutral; however, the displacement of the species is controlled by electric and magnetic fields. According to the gas temperature, the plasma can be classified into thermal plasma, which is characterized by being fully ionized (gas temperature Tg = electron temperature Te), or non-thermal (or cold) plasma where the gas is only partially ionized (Tg is much lower than Te).

Thermal plasma implies that the temperature of all active species (electrons, ions and neutral species) is the same. This is true for stars, for example, as well as for fusion plasma. High temperatures are required to form these equilibrium plasmas, typically ranging from 4000 K to 20000 K (Bogaerts et al., 2002).

In the non-thermal or cold plasmas, the temperature of neutral and positively charged species is low, with the electrons having a much higher temperature than the other particles because they are light and easily accelerated by the applied electromagnetic field. As a result, the plasma is in the non-equilibrium state and the reactions may proceed at low temperature. Indeed, the electrons can reach temperatures of 104–105 K (1–9 eV) while the temperature of the gas can be as low as room temperature (Grill, 1994). Figure 2.5 shows the image of a cold plasma discharge.

2.5 Radio frequency cathodic magnetron sputtering of titanium target in an argon plasma.

2.3.2 Characteristics and principal applications of plasma

To reach the plasma state of atoms and molecules, energy for the ionization must be input into the atoms and the molecules from an external energy source. The ionization occurs when an atom or a molecule gains enough energy from the excitation source or via collisions with other particles. There are many kinds of plasma sources differing greatly from one another. The electrically induced discharge in gas is the most common method for plasma ignition because of handling convenience. Direct current (DC) discharges, pulsed DC discharges, radio frequency (RF) discharges (13.56 MHz) and microwave (μ-wave) discharges (2.45 GHz) represent the plasma categorization based on electric apparatuses (Denes and Manolache, 2004). These are the common standard frequencies used for RF, and μ-wave plasmas are authorized frequencies for industrial, scientific and medical applications worldwide in order to avoid interfering with telecommunication. The basic feature of a variety of electrical discharges is that they produce plasmas in which the majority of the electrical energy primarily goes into the production of energetic electrons, instead of heating the entire gas stream (Fridman et al., 2005). These energetic electrons induce ionization, excitation and molecular fragmentation of the background gas molecules to produce excited species that create a ‘chemically-rich’ environment. Due to their essential role, the electrons are therefore considered to be the primary agents in the plasma.

Plasma processes provide a cost-effective and environmentally friendly alternative to many important industrial processes because this reliable method produces no waste products and in most cases exposes operators to no significant hazards. Plasmas thus find well-established use in industrial applications, however, they are also gaining more interest in the field of life sciences, related to environmental issues (Hammer, 1999) and biomedical applications (Frauchiger et al., 2004). Recently, the production of CNTs via plasma-enhanced chemical vapor deposition has been reported as well (Boskovic et al., 2005).

Gas discharge plasmas are used in a large variety of applications requiring surface modification. Plasma processing is generally used for film deposition and may also be used for resistant materials development (McOmber and Nair, 1991). Surface modification by plasmas also plays a crucial role in the microelectronics industry, in the microfabrication of integrated circuits and in materials technology (Hino and Akiba, 2000). Actually, the versatility of plasma technologies stems from its many advantages: different sizes, shapes, geometries and type of materials can be treated, surface topography and bulk properties are usually not affected and they exhibit high reproducibility. For surface modification, plasma approaches can be classified into different categories: plasma etching (Inoue and Kajikawa, 2003), plasma polymerization and plasma functionalization (Chu et al., 2002) based on the outcomes of plasma interactions. Etching, deposition or grafting of chemical groups can dominate in modifications of the material’s surface, depending on various factors such as the gas used, the nature of the surface and the plasma parameters.

Plasma etching occurs when the surface material removal effect, due to ions and active neutral species as well as vacuum ultraviolet radiation, is prominent in the modification effect. During this process, the plasma generates volatile etched products from the chemical reactions between the elements present on the material surface and the plasma species.

In the plasma polymerization process, the plasma interacts with organic molecules (monomers) and involves their fragmentation and subsequent deposition. The plasma polymer formed thus is deposited in the form of a thin film. Notwithstanding the use of the word polymer, ‘plasma polymer’ refers to a new class of material that has little in common with the conventional polymer. In the case of the plasma polymer, the chains are short and randomly branched and terminate with a high degree of cross-linking, and are not constituted by regularly repeating units.

Plasma treatment using gases such as O2, N2, NH3 or CF4 allows reactive chemical functionalities to be inserted onto the material’s surface. Compared with other chemical modification methods, plasma-induced functionalization presents interesting characteristics such as being a solvent-free and time-efficient technique. Moreover, this treatment allows the grafting of a wide range of different functional groups depending on plasma parameters such as power, nature of the gas used and its pressure, duration of treatment, etc. This method also provides the possibility of scaling up to produce large quantities necessary for commercial use. Plasma treatment is widely used for surface activation of various materials, ranging from organic polymers to ceramics and metals. In this field, we can cite the polymer surface functionalization in pulsed and continuous nitrogen microwave plasma, under admixture of hydrogen (Meyer-Plath et al., 2003). This process makes it possible to graft nitrogen functional groups onto polystyrene with a high selectivity in primary amine groups.

2.3.3 Plasma-based treatment of carbon nanotubes

The surface modification of carbon nanotubes can be carried out through a wide range of plasma processes. In addition to the already cited advantages of plasma treatments, the number of functional groups grafted on the CNT surface can also be tailored. This is an important characteristic of this type of surface treatment since having saturation of these groups on the surface can alter the electronic conductivity of nanotube materials.

To facilitate the discussion, we will classify the plasma treatment of CNTs into: (1) CNT surface etching, which is generally carried out via an argon (Ar) plasma treatment; (2) coating of CNTs through plasma polymerization; and (3) plasma functionalization, which allows the grafting of reactive functional groups on to CNT surfaces and opens the way to a wide variety of subsequent chemical reactions.

Carbon nanotube surface etching

The plasma surface modification of CNTs is a non-reactive treatment when Ar gas is used. The effects of argon RF plasma treatment on the surface of vertically grown CNTs have been investigated. The inert Ar plasma produces an efficient etching and cleaning process of CNT films, causing structural changes in the CNTs and leading to an increase in their field emission ability (Ahn et al., 2003). An observed improvement in gas ionization in comparison with that of untreated aligned CNTs was also observed after Ar RF plasma treatment (Yan et al., 2005). The Ar plasma was also used to activate the CNT surface, allowing the subsequent grafting of maleic anhydride (Tseng et al., 2007) or 1-vinylimidazole (Yan et al., 2007).

Nevertheless, the destruction of carbon nanotube sidewalls was observed after treatment with an Ar-non reactive μ-wave plasma (Qin and Hu, 2008). Figure 2.6 shows high resolution transmission electronic microscopy (HRTEM) images of multi-wall CNTs recorded before and after Ar μ-wave plasma treatment. We can observe that ion bombardment and irradiation in Ar plasma deform the graphite walls and destroy the CNT layered structure; a reduction in the power supplied to the plasma or in the treatment duration can reduce the destruction of sidewalls.

2.6 HRTEM images of MWNTs (a) before and (b) after Ar microwave plasma treatment for 3 minutes (Qin and Hu, 2008).

Hydrogenation of CNT films through H2 RF plasma processes improved the CNT field emission properties via creation of defects along the CNT structure (Yu et al., 2004; Feng et al., 2007). The enhancement of emission properties was attributed to apparition of nodular CNTs due to bending of graphene sheets along the CNT wall (Fig. 2.7 (a)) (Zhang et al., 2004). During the H2 RF plasma treatment, it was suggested that hydrogen ions bombarding the tube sidewall remove carbon atoms from the CNT surface under –CHx radical forms. Meanwhile, a small fraction of –CHx redeposited on the remaining CNT surface induces the formation of nanoscale particles with an onion-like structure distributed along the tubes (Fig. 2.7 (b)). The improvement of the emission characteristics was attributed to the change in the electronic (formation of sp3 defects in sp2 graphite network) and/or geometrical CNT structure (Zhang et al., 2004).

2.7 HRTEM images of a formed nodular CNT and an untreated CNT in the insert (a) and schematic diagram of the formation of nodular CNTs (b) (Zhang et al., 2004).

Vertically aligned CNTs were hydrogenated in a H2 + Ar pulsed dc plasma, and correlations between the plasma characteristics and the CNT surface chemistry were discussed (Jones et al., 2008). Actually, scanning electron microscope (SEM) morphological analysis of CNTs treated in this way showed the etching of the tangled nanotubes and the ‘welding’ of nanoparticles. The extent of the etching effect was correlated with the quantity of plasma-excited hydrogen H* interacting with CNTs.

Plasma polymerization

The plasma polymerization process has been used to coat CNTs with plasma polymer films. Several examples are reported in the literature. A plasma polymerization method spraying a mixture of aniline and CNTs was developed to deposit plasma polyaniline onto CNTs leading to the formation of composite films with improved electrical properties in comparison to neat plasma (Nastase et al., 2006). Chen et al. (2001) developed an approach based on RF plasma activation, followed by chemical reactions between derivatized dextran- and plasma-generated functional groups such as aldehyde groups from acetaldehyde and amino groups from ethylenediamine. The resulting polysaccharide-grafted CNTs proved highly hydrophilic.

Multi-walled CNTs were also modified using plasma polymerization with ethylene glycol (EG) as the monomer (Avila-Orta et al., 2009). The plasma-polymerized EG-coated CNTs showed very stable dispersion with water, methanol and ethylene glycol, confirming the hydrophilic behavior of the treated CNTs. This plasma polymerization technique was successfully used to produce a plasma poly(methyl methacrylate) (PMMA) coating on CNTs (Gorga et al., 2006). Excellent suspensions of CNTs were achieved in toluene after plasma treatment, and plasma PMMA coating was observed using SEM (Fig. 2.8). The coated CNTs were incorporated into the PMMA matrix via melt mixing and the mechanical properties of the nanocomposites formed in this way were determined with tensile measurements. The CNT coating slightly improved the load transfer from the PMMA matrix to the nanotubes, over the uncoated CNTs, but it did not significantly influence the dispersion of CNTs. The mechanical properties were not dramatically improved.

2.8 SEM images of multi-walled CNTs before treatment (a) and after MMA plasma polymerization treatment (b) (Gorga et al., 2006).

Styrene plasma was also reported to deposit polystyrene (PS) plasma polymer coating onto multi-walled CNTs (Felten et al., 2007). It was shown that polystyrene nanocomposite filled with plasma PS-coated CNTs presented better mechanical properties than the ones filled with unmodified CNTs (Shi and He, 2004).

Plasma chemical functionalization

CNT sidewalls can be fluorinated via CF4 plasma treatment (Plank et al., 2003). Fluorination is one of the most effective modifications of the CNT surface and makes possible a wide range of subsequent derivatization reactions (Lee et al., 2003). It has been observed that bundles of pristine CNTs can be transformed into unroped fluorinated CNTs without structural deformation (Valentini et al., 2005). Moreover, unlike other fluorination procedures, the CF4 plasma treatment was demonstrated to enhance the reactivity of fluoronanotubes with aliphatic amines at room temperature. Two minutes of CF4 plasma treatment was reported to increase the field emission current (Zhu et al., 2005). Fluorine atoms were grafted on the CNT sidewalls when these were exposed to a CF4 plasma that had been generated in a surface wave microwave plasma reactor (Kalita et al., 2008). The amount of grafted fluorine (up to 24 atomic %) was determined by X-ray Photoelectron Spectroscopy (XPS) and Raman studies revealed a structure change in the treated CNTs. HRTEM showed modification of the outer wall with fluorine incorporation while the inside walls remained unaffected. Super-hydrophobic CNTs were obtained by NF3 glow-discharge plasma supplied by alternating current (AC) power (Hong et al., 2006). The treatment time to obtain hydrophobic CNT powders was shorter than 1 minute. Figure 2.9 illustrates liquid droplets on CNT cushions of about 1 mm thickness. Figure 2.9 (a) was taken after a water droplet fell onto untreated CNTs. The other photographs (Figs 2.9 (b)–(d)) show respectively the droplets of polyethylene glycol (PEG), glycerol and water placed on CNTs that had been exposed for 10 minutes to NF3 plasma treatment. These images reveal a significant enhancement of the hydrophobicity.

2.9 Photographic images of liquid droplets. Untreated CNT powder (a) completely absorbed liquid droplets and (b)–(d) the droplets of PEG, glycerol and distilled water on 10-min plasma treated CNTs, respectively (Hong et al., 2006).

The grafting of oxygen-containing groups using plasma processes has been extensively reported. Hydroxyl, carbonyl and carboxyl groups were grafted onto CNTs via O2 and CO2 RF plasma in order to form polar groups that could improve the overall adhesion of CNTs to a polyamid matrix (Bubert et al., 2003). Oxygen concentration of up to 14% was determined by XPS, which was considered by the authors as equivalent to a complete saturation of the outer CNT surface. The efficiency of oxygen grafting in function of plasma frequency (RF or μ-wave) was determined through contact angle measurement, showing that μ-wave plasma treatment leads to better wettability (Chirila et al., 2005). Other plasma processes were proposed to readily functionalize CNTs with oxygenated groups such as air-atmospheric pressure dielectric barrier discharge (Okpalugo et al., 2005) or air μ-wave oven-generated plasma (Hojati-Talemi et al., 2009).

Zschoerper et al. (2009) have characterized Ar/O2 and Ar/H2O RF plasma-treated CNTs by XPS. To overcome the limitations of XPS analysis in distinguishing between functional groups with similar binding energies, alcohol, keto/aldehyde and carboxyl groups were tagged using derivatization techniques with fluorine-containing reagents. Trifluoroacetic anhydride (TFAA) was used for the derivatization of alcohol groups, (trifluoromethyl)phenylhydrazine (TFMPH) for keto/aldehyde groups and trifluoroethanol for carboxyl groups through reactions carried out in the saturated vapor phase. Despite the fact that the total oxygen content is almost identical, variations in different functional group concentration are observed in the function of treatment parameters such as pressure or treatment time. Based on this work, CNT bucky papers were modified using Ar/O2 plasma and thereafter melt-mixed into polycarbonate (Pötschke et al., 2009). Carboxylic acid and ester groups were formed on the CNT surface, allowing better macrodispersion and better phase adhesion to the matrix, as showed by morphological investigations.

A flexible amperometric biosensor based on O2 RF plasma-functionalized CNTs films on polydimethylsiloxane substrates has been developed (Lee et al., 2009). The plasma-treated samples presented better glucose response than non-treated ones, showing their potential application as biosensors. The O2 RF plasma treatment was also found to improve the sensing potential of CNTs to detect NO2 (Ionescu et al., 2006) or to clean CNT surfaces by removing amorphous carbon (Rawat et al., 2006). In fact, it has been observed that purification of carbon nanotube powder can be attained after CNT treatment in glow discharges (RF or μ-wave). Amorphous carbon domains are eliminated and the impurities are removed by ion bombardment and irradiation in an O2 RF plasma (Xu et al., 2007). However, it was also reported that the average diameter of multi-walled CNTs decreases with treatment duration. Figure 2.10 shows a three-step model proposed to explain this decrease. First, ion bombardment causes the creation of vacancies and interstitials in CNTs; the superficial structure, including the amorphous carbon that was produced, is thus lost. In a second step, the ion beams continue to react with amorphous carbon until they are peeled totally away from the CNTs. Finally, the oxidation of amorphous carbon in CO2 occurs, inducing the decrease of the CNT’s average diameter.

2.10 Proposed model for the transformation process of the multi-walled CNTs under O2 plasma treatment. (a) Oxygen functional groups are grafted by O2 plasma; (b) defects and carbon nanowires are created by ion bombardment, losing the superficial structure; (c) ions continue to react with amorphous carbon which are peeled totally from the carbon nanotubes; (d) amorphous carbon is oxidized under O2 plasma which forms carbon dioxide, the diameter of the multi-walled carbon nanotubes decreases due to the disappearance of the outer wall (Xu et al., 2007).

Plasma-oxidized CNTs were reported to present improved dispersion in epoxy resin in comparison with non-treated CNTs (Kim et al., 2006). The epoxy nanocomposites filled with plasma-modified CNTs exhibited higher storage and loss moduli than CNT/epoxy nanocomposites as well as improved tensile strength and elongation at break due to better dispersion and stronger interaction between the CNTs and the polymer matrix.

Felten et al. showed that cluster dispersion on thermal evaporation coating of CNTs with various metals can be tuned by increasing the cluster nucleation density through the formation of interaction sites when CNTs are treated in an O2 RF plasma (Felten et al., 2007). The same effect was also observed when the CNTs were exposed to an NH3 RF plasma before the metal coating.

The nitrogen-containing groups can be grafted through plasma treatment if gases, such as N2, N2/H2 or NH3, are used. The treatment of multi-walled CNTs in an N2/Ar RF plasma was shown to be an effective procedure to enhance the field emission characteristics of CNTs due to the doping of the CNT structure with N atoms (Gohel et al., 2005). The N1s core level peak was fitted with components at 398.2, 398.6, 399.7 and 400.8 eV, which corresponds to C-N, C≡N (sp3 bonding), C=N (sp2 bonding) and NO. The field emission properties of screen-printed CNT films were also improved after NH3 or N2 RF plasma treatment (Feng et al., 2007). For the CNT films treated by N2 plasma, the emission current density and fluorescent photos at 6.4 V/μm clearly demonstrated the influence of the treatment time on field emission properties (Fig. 2.11).

2.11 The emission current density and fluorescent photos of the CNT films at 6.4 V/μm after various N2 plasma treatment times (Feng et al., 2007).

The improvement of the field emission properties was attributed to the change in CNT morphology after plasma treatment with the formation of nano-protuberances, i.e. ‘multi-tip’ CNTs. The dependence of the emission current with treatment duration showed an optimum after 20 minutes of treatment. Prolonged treatment was shown to reduce the field emission properties that were associated with the destruction of CNTs and nano-protuberances.

The N2 RF plasma treatment was also used to improve the Pt catalyst deposition at the CNT surface, resulting in an enhancement of the electrochemical activity (Kim et al., 2008).

Bystrzejewski et al. reported the treatment of single-walled CNTs in a DC hollow cathode glow discharge (HCGD) (Bystrzejewski et al., 2009). Various gases (N2, H2O, N2 + H2O and NH3 + H2O) were used to functionalize CNTs. The N2 and/or H2O plasma treatment resulted in the presence of amorphous carbon species at the CNT surface while the use of NH3 + H2O yielded a very clean product consisting of functionalized CNTs. In-situ optical emission studies showed that the functionalization occurs via radical addition channels with the initial assistance of H2+ radical ions. The H2+ bombardment breaks the C–C bonds on the CNT surface, after which other chemical radicals are subsequently added and quenched (Fig. 2.12).

2.12 Radical addition yielding functional groups (-CH3, -CN, -OH, -NH2′ …) covalently attached to CNT sidewall (Bystrzejewski et al., 2009).

An NH3 μ-wave plasma treatment has been also used to enhance the solubility of CNTs (Wu et al., 2007). The introduction of polar functional groups increases the hydrophilicity of CNTs, making the application of CNTs in the immobilization of biomolecules and construction of biosensors more convenient.

The studies cited above demonstrate the wide range of plasma processes that can be used to modify the CNT surface. The excited species such as electrons, ions and radicals within plasma interact with the surface of CNTs and break the C=C bonds, which promotes the creation of active sites to bind functional groups as well as some physical modification of the CNT surface. Moreover, it was also reported that UV photons interact with CNTs and create active sites on their sidewalls, however, they can at the same time promote the defunctionalization of moieties grafted onto CNTs (Khare et al., 2002). Due to the interaction of the different plasma reactive species with CNTs, the grafting of functional groups can therefore occur through different simultaneous reactions. This wide range of interactions involves a lack of control on the grafting of wanted chemical function and makes it difficult to determine the weight of each functional mechanism. Moreover, prolonged plasma treatments promote damage of CNTs, resulting in diminution of their diameter with destruction of the outer wall, the forming of onion-like structures, or even their complete destruction.

Plasma post-discharge treatment of carbon nanotubes

To prevent the unwanted effects of plasma functionalization, a solution was proposed by Ruelle et al. (2007). These authors proposed avoiding the functionalization of CNTs directly inside the plasma where the density of high energy ions is very high, and instead, placing the CNTs outside the discharge production zone to reduce the detrimental effects associated with ion bombardment and irradiation. In this approach, radicals are the most important reactive species to graft functional groups onto the CNT surface. In this case, the application of microwave discharge sources for the production of intense beams of atomic, radical and metastable species is well established. For example, carbon nanotubes have been hydrogenated by atomic hydrogen generated in a H2 microwave plasma (Wu et al., 2007). In this type of functionalization, the exact location of the sample and its distance from the plasma discharge zone become a key factor in determining the density of functionalization. Khare et al. studied the exposition of SWNTs to microwave-generated N2 plasma, by placing CNTs at different distances from discharge (Khare et al., 2005). At the shortest distance (1 cm), they observed the highest concentration in nitrogen groups but also a loss of integrity of SWNTs due to highly reactive species, like H2+. At intermediate distances, the incorporation of nitrogen, but also a high quantity of oxygen, onto the CNTs is obtained while functionalization was not observed for the maximal distance of 7 cm due to total recombination of atomic nitrogen before arriving at the CNT surface. It is thus important to place CNTs outside the glow discharge zone at an optimal distance to avoid interaction with highly reactive ions and UV photons. In this treatment configuration, radicals become the most important reactive species for the grafting of functional groups onto carbon nanotubes. The efficiency of this approach depends on the density of the radicals produced.

In this context, a surface wave discharge set-up was developed (Godfroid et al., 2003). The microwave-induced plasmas present a higher density of electrons in comparison with other type of plasmas, such as RF plasmas, for instance, because electrons are more easily created with microwaves (at the same power).

The high electron density of the μ-wave-induced plasmas is the key parameter in the creation of atomic nitrogen in Ar + N2 microwave plasma (Fig. 2.13). Godfroid et al. have demonstrated that production of atomic nitrogen in Ar + N2 μ-wave discharge is achieved through two electronic mechanisms (Godfroid et al., 2005). The first is dissociation by direct electronic impact, which is achieved by collision between a nitrogen molecule and an electron:

2.13 An Ar + N2 microwave discharge sustained by a surface wave launched on the quartz tube. A high quantity of atomic nitrogen is produced in this plasma discharge (Ruelle, 2009).

[2.1]

This reaction leads to the production of atomic nitrogen in the fundamental state N(4S).

The second is called dissociative recombination and is achieved by recombination between an electron and an ionized nitrogen molecule:

[2.2]

Finally, the set-up and parameters of the microwave discharge were studied to find the higher nitrogen dissociation rate (Godfroid et al., 2003). The most influential parameters proved to be the power supply pulse, the dilution of N2 in Ar and the total gas flow, which makes it possible to achieve a high homolytic dissociation up to 40% of the molecular N2.

The CNTs were thus treated in the post-discharge of an Ar + N2 microwave plasma sustained by a surface wave launched in a quartz tube via a surfaguide supplied by a 2.45 GHz microwave generator, which can be pulsed (Ruelle et al., 2007). In this plasma-induced functionalization, the plasma was not used to directly interact with the surface of CNTs but as a source of atomic nitrogen N• reactive flow. It is important to note that the discharge tube and the post-discharge chamber are separated by a diaphragm that consists of an aluminium ring with a circular aperture whose diameter measures a few millimetres. This diaphragm plays two roles in the post-discharge treatment. First, the separation between the plasma set-up and the post-discharge protects the samples placed in the postdischarge from irradiation of high-energy particles from μ-wave plasma. Moreover, the average distance of 40 cm between the end of the discharge tube and the sample holder promotes the interaction of CNTs with atomic nitrogen species that have enough mean lifetime to reach the sample holder, in contrast to other reactive species.

High resolution photoelectron spectroscopy analysis showed that by exposing multi-walled CNTs to atomic nitrogen N• generated in the μ-wave plasma, nitrogen chemical groups were grafted onto the CNT surface, altering the density of electronic states (Ruelle et al., 2008). High-resolution TEM and SEM images revealed that the atomic nitrogen exposition did not damage the surface of the CNTs (Fig. 2.14). The absence of damage in the structure of carbon nanotubes after the plasma treatment also indicated that nitrogen group grafting resulted only from chemical reaction between CNT surfaces and atomic nitrogen species, without the side-effects of irradiation or high energy particle interaction at the CNT surface.

2.14 HRTEM images of CNTs treated in the post-discharge chamber of Ar + N2 μ-wave plasma (Ruelle et al., 2008).

With the addition of H2 in the post-discharge, an efficient and selective grafting of primary amine groups was developed. Actually, two-thirds of grafted nitrogenated functional groups were primary amine groups, as determined by XPS after (trifluoromethyl)benzaldehyde derivative reaction (Ruelle, 2009). These grafted amine groups have been used as initiation sites for promoting the ring opening polymerization (ROP) of ε-caprolactone (ε-CL) yielding polyester-grafted CNT nanohybrids (Ruelle et al., 2007). The morphology of the recovered nanohybrids has been characterized by TEM, showing that PCL islets, and so the initiator primary amines were homogeneously dispersed along the CNT sidewalls (Fig. 2.15). Moreover, the nanohybrids displayed the best dispersion ability in chloroform, a good PCL solvent, in comparison to pristine and amino-CNTs.

2.15 TEM image of one CNT with PCL islets resembling a pearl-necklace structure (Ruelle, 2009).

The CNT-g-PCL nanohybrids, mixed with free PCL chains, presented a high degree of CNT pre-disaggregation and were thus used as nanofillers in PCL; pristine and amino-CNT-filled nanocomposites were also prepared as comparative materials. TEM images showed that slightly disrupted CNT bundles are present in the CNT-filled PCL nanocomposites while CNTs are homogeneously dispersed when CNT-g-PCL nanohybrids are used (Fig. 2.16). Electrical conductivity measurements of the PCL nanocomposites showed a similar percolation threshold (~ 0.35 wt%) for each PCL nanocomposite, corresponding to a theoretical value for optimal dispersion of CNTs, which is equal to the inverse of CNT aspect ratio.

2.16 TEM images of PCL nanocomposites filled with (a) 3 wt% CNTs and (b) 3 wt% CNT-g-PCL nanohybrids. CNTs remain agglomerated (black circles) in the CNTs-filled PCL nanocomposites while very small bundles are observed (circled) when PCL is filled with CNT-g-PCL nanohybrids (Ruelle, 2009).

The different nanofillers (CNTs, amino-CNTs and CNT-g-PCL nanohybrids) were also dispersed in a high-density polyethylene (HDPE) matrix. TEM analysis indicated that CNTs and amino-CNTs were poorly dispersed in HDPE while CNT-g-PCL were more homogeneously dispersed, even though PCL and HDPE are known to be non-miscible polymers. Electrical measurements confirmed these observations, showing an electrical percolation threshold of 2.5 wt% for CNTs and amino-CNT-filled HDPE nanocomposites and 1.1 wt% for samples filled with a CNT-g-PCL nanohybrid.

The grafting of primary amine groups on the CNT surface did not influence the dispersion ability of CNTs in PCL and HDPE matrices. However, the electrical measurements showed that the electrical conductivity of amino-CNTs was higher than that of pristine CNTs; functionalization can be used to improve the electrical conductivity. This last result confirmed that the CNT structure is preserved after CNT functionalization in the microwave plasma post-discharge.

2.4 Summary

Offering both a high aspect ratio (length-to-diameter) and a low density, CNTs show strong application potential as advanced filler materials to replace or complement the conventional nanofillers, such as nanoclays, in the fabrication of multifunctional polymer nanocomposites. In this field, the degree of dispersion of the individual CNTs is an essential parameter to produce nanocomposite materials with enhanced properties. In order to homogeneously disperse CNTs throughout polymer matrices, the CNT entanglements and bundles, stabilized by intermolecular van der Waals forces, must be disrupted. One of the most reported techniques to overcome these drawbacks is the surface modification of CNTs. In this context, solvent-based covalent functionalizations are well reported in the literature. Nevertheless, this type of functionalization presents different drawbacks, such as CNT insolubility, which can limit control over the grafting of the functional group onto the CNT surface.

Plasma processes constitute a promising alternative for surface modification. Plasma treatments have the advantage of being non-polluting and provide a wide range of different functional groups depending on plasma parameters such as power, type of gas used, treatment duration and gas pressure.

Generally, CNTs are placed inside the plasma, where their surface interacts with plasma-excited species such as radicals, electrons, ions and UV radiation which induces the breaking of C–C bonds and the creation of active sites for bonding of functional groups on the CNT surface.

Argon and hydrogen plasmas are reported to etch CNT surface and end-caps, improving their field emission properties. When organic monomers are introduced in the plasma, CNTs are coated with plasma polymer films. This results in the pre-disaggregation of CNT bundles, which can be well dispersed in polymer matrices. Fluorinated, oxygenated and nitrogenated gases were also used to generate functional groups at the CNT surface. This surface modification of CNTs opens the way for compatibilization with polymer matrices or for further attachment of other molecules in order to form polymer nanohybrids. Nevertheless, total control over the grafting of functional groups on the CNT surface is difficult to achieve when CNTs are treated directly in the plasma discharge chamber.

To prevent the degradation of the CNTs during plasma treatment, it was proposed to use plasma discharges as a source of reactive species, such as radicals, which can interact with the CNT surface outside the plasma discharge, avoiding structural damage and destruction due to high energy species bombardment. This process can be considered as a ‘dry’ chemical reaction at the CNT surface. The grafted functional groups can be used as initiator sites for polymerization, yielding CNT nanohybrids that can be introduced into polymer matrices, forming high-performance polymer nanocomposites and nanohybrids.

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