For coating applications a uniform and stable dispersion of particulate matter plays an im‐ portant role. This requirement is especially critical when submicron or nano-sized particles are involved. CNTs tend to cohere in aqueous dispersion due to their high surface energy and lack of chemical affinity with the dispersing medium [Park et. al., 2002]. Surfactant ad‐ sorption on nanotube surfaces and chemical functionalization of nanotube sidewalls are two of the most widely-used methods for solubilization of nanotubes. Non-covalent surface treat‐ ment by surfactants or polymers has been used in the preparation of both aqueous and or‐ ganic solutions to obtain high weight fraction of individually dispersed nanotubes [Barraza et. al., 2002; Jiang et. al., 2003; Yurekli et. al., 2004]. When surfactants are employed in CNT dispersions, surfactant molecules work by adsorption at the interface and self-accumula‐ tion into supra-molecular structures, which help their dispersion retain a stable colloidal state. However, surfactants adsorbed on nanotubes create a physical barrier between the nanotubes and the environment [Hobbie et. al., 2006; Lee et. al., 2007]. Chemical methods use surface functionalization of CNT to improve their chemical compatibility with the tar‐ get medium that is to enhance wetting and reduce their tendency to agglomerate [Vais‐ man et. al., 2006]. Systematic investigation of the effects of nanotube length and functionalization for MWNT has revealed that the introduction of carboxylic or thiol groups on the surface of shortened nanotubes increases the stability of MWNT dispersions up to 0.24 mg/ml. Octadecyl-amide functionalized MWNT were reported to exhibit good solubil‐ ity in polar solvents [Qin et. al., 2003]. These long chain alkylamide-functionalized nano‐ tubes were obtained where surface-bound COOH groups are converted into thionyl chloride groups and subsequently reacted with amine. The introduction of surface charge on MWNT also has contrasting effects on stabilising their dispersions. MWNTs are modified with car‐ boxylic anion groups; the dispersion stability in water was significantly enhanced due to the combination of polar–polar affinity and electrostatic repulsion [Lee et. al., 2007]. MWNT uni‐ formly dispersed in water can be utilized in environmental friendly aqueous coating, di‐ rect conductive coating, further sol–sol process and aqueous nanocomposite system.

**Figure 16.** SEM image of the sodium silicate (a) and sodium silicate/functionalized MWNT composites prepared from aqueous coating system (b).

#### **3.8. Electromagnetic interference (EMI) shielding materials**

As electromagnetic radiation, particularly that at high frequencies tend to interfere with electronics, EMI shielding of both electronics and radiation source is needed and is increas‐ ingly required by governments around the world [Chung, 2001]. The radiation may be ei‐ ther electromagnetic in nature, such as X-rays and gamma rays, or charged particles, such as beta particles and electrons. The lifetime and efficiency of them can be increased by the ef‐ fective shielding. Generally, highly electroconductive materials such like metals are used for shielding application. However, metals have their own shortcomings like heavy weight, sus‐ ceptibility to corrosion, wear, and physical rigidity [Wu et al., 2006]. Many researches have been conducted to improve the EMI shielding of polymer materials by coating an electro‐ conductive layer on the surface, incorporating electroconductive fillers, or utilizing electro‐ conductive polymers. Among various electroconductive fillers that have been utilized, covalently functionalized MWNT is one of the most promising candidates, not only because of its good electrical conductivity but also because of its ability to improve mechanical prop‐ erties. Recently, the mass production of MWNTs causes price reduction. They are more af‐ fordable for application in nanocomposites [Wu et. al., 2006].

#### **4. Conclusion**

Recently, Neelgund and Oki synthesized the nanohybrids composed of silver nanoparti‐ cles and aromatic polyamide functionalized MWNTs. Prior to deposition of silver nanopar‐ ticles, acid treated MWNTs were successively reacted with p-phenylenediamine and methylmethacrylate to form series of NH2-terminated aromatic polyamide dendrimers on the surface of MWNTs through Michael addition and amidation. The antimicrobial activity of MWNT-Ar-NH2/Ag nanohybrids were measured against *E. coli*, *P. aeruginosa* and *S. aur‐ eu* and compared with MWNT-COOH and MWNT-Ar-NH2. The results showed that func‐ tionalization of MWNTs with aromatic polyamide dendrimers and successive deposition of Ag nanoparticles could play an important role in the enhancement of antimicrobial activity

For coating applications a uniform and stable dispersion of particulate matter plays an im‐ portant role. This requirement is especially critical when submicron or nano-sized particles are involved. CNTs tend to cohere in aqueous dispersion due to their high surface energy and lack of chemical affinity with the dispersing medium [Park et. al., 2002]. Surfactant ad‐ sorption on nanotube surfaces and chemical functionalization of nanotube sidewalls are two of the most widely-used methods for solubilization of nanotubes. Non-covalent surface treat‐ ment by surfactants or polymers has been used in the preparation of both aqueous and or‐ ganic solutions to obtain high weight fraction of individually dispersed nanotubes [Barraza et. al., 2002; Jiang et. al., 2003; Yurekli et. al., 2004]. When surfactants are employed in CNT dispersions, surfactant molecules work by adsorption at the interface and self-accumula‐ tion into supra-molecular structures, which help their dispersion retain a stable colloidal state. However, surfactants adsorbed on nanotubes create a physical barrier between the nanotubes and the environment [Hobbie et. al., 2006; Lee et. al., 2007]. Chemical methods use surface functionalization of CNT to improve their chemical compatibility with the tar‐ get medium that is to enhance wetting and reduce their tendency to agglomerate [Vais‐ man et. al., 2006]. Systematic investigation of the effects of nanotube length and functionalization for MWNT has revealed that the introduction of carboxylic or thiol groups on the surface of shortened nanotubes increases the stability of MWNT dispersions up to 0.24 mg/ml. Octadecyl-amide functionalized MWNT were reported to exhibit good solubil‐ ity in polar solvents [Qin et. al., 2003]. These long chain alkylamide-functionalized nano‐ tubes were obtained where surface-bound COOH groups are converted into thionyl chloride groups and subsequently reacted with amine. The introduction of surface charge on MWNT also has contrasting effects on stabilising their dispersions. MWNTs are modified with car‐ boxylic anion groups; the dispersion stability in water was significantly enhanced due to the combination of polar–polar affinity and electrostatic repulsion [Lee et. al., 2007]. MWNT uni‐ formly dispersed in water can be utilized in environmental friendly aqueous coating, di‐

rect conductive coating, further sol–sol process and aqueous nanocomposite system.

[Neelgund and Oki, 2011].

244 Physical and Chemical Properties of Carbon Nanotubes

**3.7. Environmental friendly aqueous coating system**

Current interest in CNTs has been generated and maintained because nanotubes exhibit unique properties include high modulus, high aspect ratios, excellent thermal and electrical conductivities, and magnetic properties not achievable with traditional filler. In this chapter, MWNTs were subjected to EB irradiation at various doses to determine the incidence of sur‐ face modification and, resultantly, deformation or destruction to the otherwise pristine graphitic structure. FTIR spectra obtained from EB-MWNT samples provide insight into the level of surface modification. The introduced carboxyl groups represent useful sites for fur‐ ther modifications, as they enable the covalent coupling of molecules through the creation of ester or amide bonds. Consequently, EB-MWNT was covalently functionalized with isocya‐ nate, epoxy, and hydroxyl compounds and their applicability was investigated. As has been shown in this study, the possible applications of them range widely, from nanocomposite materials to antibacterial agents. Functional groups such like isocyanate group on MWNT surface can interact with -OH group in polymers by urethane bonding and result in a better dispersion of MWNT in polyurethane matrix. Afterward we carry out extensive studies to investigate the properties and applicability for urethane link polydimethylsiloxane/MWNT nanocomposites using various isocyanate functionalized MWNT.

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#### **Acknowledgements**

We are grateful to the Small and Medium Enterprises (SMEs) Technology Innovation Pro‐ gram, Republic of Korea, for financial support of this experimental work.

#### **Author details**

Eun-Soo Park1\*

Address all correspondence to<EMAIL_ADDRESS>
1 Youngchang Silicone Co., Ltd., Korea

#### **References**

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We are grateful to the Small and Medium Enterprises (SMEs) Technology Innovation Pro‐

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Address all correspondence to<EMAIL_ADDRESS>
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**Chapter 10**

**Dispersion and Property Manipulation**

**of Amphiphilic Molecules**

http://dx.doi.org/10.5772/51967

**1. Introduction**

tubes are semiconducting.

Xia Xin , Guiying Xu and Hongguang Li

Additional information is available at the end of the chapter

**of Carbon Nanotubes by Self-Assemibles**

Over the past two decades, carbon nanotubes (CNTs) have grown as a novel type of nano‐ material and attracted great attention from scientists in different research fields [1-3]. Based on their unique one-dimensional nanostructure, CNTs exhibit excellent mechanical, optical and electronic properties as well as high chemical stability. As claimed by the Nobel lau‐ reate Richard Smalley [4], CNTs would be cheap, environmentally friendly, and do wonders for humankind. CNTs were discovered along with the research on fullerenes. In 1991, Iijima from Japan, who at that time was observing fullerenes produced by arc discharge method under high-resolution TEM, observed some tubular structures formed by coaxial cylinders of graphite layers [5]. These structures are nowadays well-known multi-walled carbon nanotubes (MWNTs) with diameters ranging from 2 to several hundred nanometers and lengths of microns. Two years later, single-walled carbon nanotubes (SWNTs) which contain only one cylinder of graphite were also observed and MWNTs can be equally regarded as a group of coaxial SWNTs with different diameters [6]. Theoretically, SWNTs can be regarded as the equivalent by rolling up a single layer of graphite (graphene). The way of the rolling will dominate the diameter and chirality of SWNTs and hence their electronic properties. Taking *a1* and *a2* as the basic vector of a graphite layer as shown in Figure 1, the tubes ob‐ tained by rolling up this layer along *a1* are called zigzag tubes, while those along *a2* are called armchair tubes. The tubes obtained along the vector *a* will have chirality (*a* = n*a1* + m*a2*). If n m = 3*q* (*q* is an integer), the tubes are metallic and can be conductive. When n - m ≠ 3*q*, the

> © 2013 Xin et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

## **Dispersion and Property Manipulation of Carbon Nanotubes by Self-Assemibles of Amphiphilic Molecules**

Xia Xin , Guiying Xu and Hongguang Li

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/51967

#### **1. Introduction**

Over the past two decades, carbon nanotubes (CNTs) have grown as a novel type of nano‐ material and attracted great attention from scientists in different research fields [1-3]. Based on their unique one-dimensional nanostructure, CNTs exhibit excellent mechanical, optical and electronic properties as well as high chemical stability. As claimed by the Nobel lau‐ reate Richard Smalley [4], CNTs would be cheap, environmentally friendly, and do wonders for humankind. CNTs were discovered along with the research on fullerenes. In 1991, Iijima from Japan, who at that time was observing fullerenes produced by arc discharge method under high-resolution TEM, observed some tubular structures formed by coaxial cylinders of graphite layers [5]. These structures are nowadays well-known multi-walled carbon nanotubes (MWNTs) with diameters ranging from 2 to several hundred nanometers and lengths of microns. Two years later, single-walled carbon nanotubes (SWNTs) which contain only one cylinder of graphite were also observed and MWNTs can be equally regarded as a group of coaxial SWNTs with different diameters [6]. Theoretically, SWNTs can be regarded as the equivalent by rolling up a single layer of graphite (graphene). The way of the rolling will dominate the diameter and chirality of SWNTs and hence their electronic properties. Taking *a1* and *a2* as the basic vector of a graphite layer as shown in Figure 1, the tubes ob‐ tained by rolling up this layer along *a1* are called zigzag tubes, while those along *a2* are called armchair tubes. The tubes obtained along the vector *a* will have chirality (*a* = n*a1* + m*a2*). If n m = 3*q* (*q* is an integer), the tubes are metallic and can be conductive. When n - m ≠ 3*q*, the tubes are semiconducting.

© 2013 Xin et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

below.

5. Please make Figure 7 clearer as below.

1. We add something on Figure 1(A). Thus, please change Figure 1 to a new one as

to disperse CNTs in aqueous solutions has received much attention [10, 11]. In this chap‐ ter, we will first give a brief introduction and overview of the noncovalent method, with an emphasis on SWNTs and recent advances in this field. Then we will focus on the property manipulation of the dispersed tubes in the self-assemblies formed by amphiphil‐ ic molecules in water. Typical applications of the dispersed tubes in other research field will also be presented. These include the preparation of functional materials, fabrication of nano-devices and applications in life science. The work related to the dispersion of CNTs in organic solvents using water-insoluble conjugated polymers, which can be also included in the noncovalent method, is out of the scope of this chapter and hence will

Dispersion and Property Manipulation of Carbon Nanotubes by Self-Assemibles of Amphiphilic Molecules

http://dx.doi.org/10.5772/51967

257

Amphiphilic molecule possesses both hydrophobic and hydrophilic parts in the same mole‐ cule [12]. The hydrophobic part is typically the alkyl chain while the hydrophilic part can be an ionized functional group, an ethylene oxide group or the combination of them. Amphi‐ philic molecule has a special name in colloid and interface science, i.e., surfactant. The sur‐ face of CNTs is intrisically hydrophobic and thus has affinity with the hydrophobic part of surfactant. In this case, the hydrophilic part of surfactant stays in water and impedes tube aggregation. This forms the base of using surfactant to disperse and stabilize CNTs. Gener‐ ally, SWNTs are much more difficult to be dispersed than MWNTs due to the much stronger intertube attractions and thus attracted more attention of researchers in this field. In the fol‐

To facilitate the surfactant adsorption onto the tube surface, sonication is needed although example without sonication has also been reported [13]. In a typical dispersion procedure, SWNTs and surfactant are added into water and the mixture is sonicated. Recent report shows that the outermost tubes in a SWNTs bundle are treated more than the innermost tubes and the tubes tend to exfoliate from the bundle ends. Therefore, mechanical exfolia‐ tion of the bundles prior to surface treatment must occur in order to obtain individual car‐ bon nanotubes. A mechanism of nanotube isolation from a bundle (Figure 2 i), with the combined assistance of ultrasonication and surfactant adsorption, was proposed [14]. The role of ultrasonic treatment is likely to provide high local shear, particularly to the nanotube bundle end (Figure 2 ii). Once spaces or gaps at the bundle ends are formed, they are propa‐ gated by surfactant adsorption (Figure 2 iii), ultimatelym separating the individual nano‐ tubes from the bundle (Figure 2 iv). Since high-power sonication can potentially destroy the tubes, the power of sonication is usually low (< 10 W). On the other hand, to get a good dis‐ persion the sonication time should be sufficiently long (several to tens of hours). After this treatment a SWNTs dispersion with single tubes as well as tube bundles can be obtained. Large tube bundles can be removed from the dispersion by simply gravity sedimentation, while small tube bundles can be stable for weeks. To remove them, ultracentrifugation is recommended. After the sonication-ultracentrifugation circle, SWNTs dispersion up to sin‐

not be mentioned.

**2. Dispersing CNTs by amphiphilic molecules**

lowing, discussions will mainly be made on SWNTs.

gle tube level can be obtained.

**Figure 1.** Physical and electronic structures of semiconducting SWNTs. (**A**) Graphene sheet segment showing indexed lattice points. Nanotubes designated (*n,m*) are obtained by rolling the sheet from (0,0) to (*n*,*m*) along a roll-up vector. The chiral angle α (from 0 to 30°) is measured between that vector and the zigzag axis; the tube circumference is the vector's length. (**B**) The extent of electron transfer is dependent on the density of states in that electron density near *E*<sup>F</sup> leads to higher initial activity for metallic and semimetallic nanotubes [6, 9].

2. Figure 2 seems too big and not so clear, can we make it smaller as below?

**3. There is a line on the top of Figure 6, so we change it as below.**  This unique geometry at nanometer length scale imparts CNTs many intriguing properties. However, it also leads to a problem which must be solved before many practical applica‐ tions. That is, the as-produced CNTs, whatever synthetic method is used, are always a mix‐ ture of tubes with varying diameter, chirality and length. Besides the continuous effort to directly obtain the desired type of CNTs during synthesis [7], the post-sorting forms an al‐ ternative to solve this problem. In the latter case, the CNTs must be well dispersed in a me‐ dium. Moreover, the dispersion of CNTs is also a precondition of many fundamental research and practical applications because the aggregation of tubes can significantly lower the promising properties proposed to the single tube of CNTs and bring difficulties to han‐ dle these interesting nanomaterials. Unfortunately, in most cases the as-produced CNTs stay as aggregated bundles or ropes instead of single tubes driven by the van der Waals forces and π-π interactions between adjacent tubes. This aggregation trend becomes more pro‐ nounced in the case of SWNTs where the tube has a very high length to width ratio with the order of 100-1000 [8].

4. Figure 7 is not clear, please change it as below. From a chemistry viewpoint, an idea to disperse CNTs immediately comes into mind is to covalently link suitable functional groups to the sidewalls or end-caps of the tubes to render the CNTs desired solubility or dispersability in a given solvent. Among various routes to covalently functionalize the CNTs, oxidation using mixed acids [9] has received much interest due to its simplicity and effectiveness. After oxidation, carboxylic groups can be introduced, which opens a route for further functionalizations. This covalent meth‐ od, however, is proved to disturbing the π-electrons of the tubes and hence inevitably in‐ fluencing the intrinsic properties of single tubes. As an alternative and improved solution, in recent years, the so-called noncovalent method using amphiphilic molecules to disperse CNTs in aqueous solutions has received much attention [10, 11]. In this chap‐ ter, we will first give a brief introduction and overview of the noncovalent method, with an emphasis on SWNTs and recent advances in this field. Then we will focus on the property manipulation of the dispersed tubes in the self-assemblies formed by amphiphil‐ ic molecules in water. Typical applications of the dispersed tubes in other research field will also be presented. These include the preparation of functional materials, fabrication of nano-devices and applications in life science. The work related to the dispersion of CNTs in organic solvents using water-insoluble conjugated polymers, which can be also included in the noncovalent method, is out of the scope of this chapter and hence will not be mentioned.

#### **2. Dispersing CNTs by amphiphilic molecules**

1. We add something on Figure 1(A). Thus, please change Figure 1 to a new one as

**Figure 1.** Physical and electronic structures of semiconducting SWNTs. (**A**) Graphene sheet segment showing indexed lattice points. Nanotubes designated (*n,m*) are obtained by rolling the sheet from (0,0) to (*n*,*m*) along a roll-up vector. The chiral angle α (from 0 to 30°) is measured between that vector and the zigzag axis; the tube circumference is the vector's length. (**B**) The extent of electron transfer is dependent on the density of states in that electron density near *E*<sup>F</sup>

This unique geometry at nanometer length scale imparts CNTs many intriguing properties. However, it also leads to a problem which must be solved before many practical applica‐ tions. That is, the as-produced CNTs, whatever synthetic method is used, are always a mix‐ ture of tubes with varying diameter, chirality and length. Besides the continuous effort to directly obtain the desired type of CNTs during synthesis [7], the post-sorting forms an al‐ ternative to solve this problem. In the latter case, the CNTs must be well dispersed in a me‐ dium. Moreover, the dispersion of CNTs is also a precondition of many fundamental research and practical applications because the aggregation of tubes can significantly lower the promising properties proposed to the single tube of CNTs and bring difficulties to han‐ dle these interesting nanomaterials. Unfortunately, in most cases the as-produced CNTs stay as aggregated bundles or ropes instead of single tubes driven by the van der Waals forces and π-π interactions between adjacent tubes. This aggregation trend becomes more pro‐ nounced in the case of SWNTs where the tube has a very high length to width ratio with the

From a chemistry viewpoint, an idea to disperse CNTs immediately comes into mind is to covalently link suitable functional groups to the sidewalls or end-caps of the tubes to render the CNTs desired solubility or dispersability in a given solvent. Among various routes to covalently functionalize the CNTs, oxidation using mixed acids [9] has received much interest due to its simplicity and effectiveness. After oxidation, carboxylic groups can be introduced, which opens a route for further functionalizations. This covalent meth‐ od, however, is proved to disturbing the π-electrons of the tubes and hence inevitably in‐ fluencing the intrinsic properties of single tubes. As an alternative and improved solution, in recent years, the so-called noncovalent method using amphiphilic molecules

**DOS DOS**

2. Figure 2 seems too big and not so clear, can we make it smaller as below?

leads to higher initial activity for metallic and semimetallic nanotubes [6, 9].

**3. There is a line on the top of Figure 6, so we change it as below.**

**A B**

256 Physical and Chemical Properties of Carbon Nanotubes

4. Figure 7 is not clear, please change it as below.

order of 100-1000 [8].

5. Please make Figure 7 clearer as below.

below.

Amphiphilic molecule possesses both hydrophobic and hydrophilic parts in the same mole‐ cule [12]. The hydrophobic part is typically the alkyl chain while the hydrophilic part can be an ionized functional group, an ethylene oxide group or the combination of them. Amphi‐ philic molecule has a special name in colloid and interface science, i.e., surfactant. The sur‐ face of CNTs is intrisically hydrophobic and thus has affinity with the hydrophobic part of surfactant. In this case, the hydrophilic part of surfactant stays in water and impedes tube aggregation. This forms the base of using surfactant to disperse and stabilize CNTs. Gener‐ ally, SWNTs are much more difficult to be dispersed than MWNTs due to the much stronger intertube attractions and thus attracted more attention of researchers in this field. In the fol‐ lowing, discussions will mainly be made on SWNTs.

To facilitate the surfactant adsorption onto the tube surface, sonication is needed although example without sonication has also been reported [13]. In a typical dispersion procedure, SWNTs and surfactant are added into water and the mixture is sonicated. Recent report shows that the outermost tubes in a SWNTs bundle are treated more than the innermost tubes and the tubes tend to exfoliate from the bundle ends. Therefore, mechanical exfolia‐ tion of the bundles prior to surface treatment must occur in order to obtain individual car‐ bon nanotubes. A mechanism of nanotube isolation from a bundle (Figure 2 i), with the combined assistance of ultrasonication and surfactant adsorption, was proposed [14]. The role of ultrasonic treatment is likely to provide high local shear, particularly to the nanotube bundle end (Figure 2 ii). Once spaces or gaps at the bundle ends are formed, they are propa‐ gated by surfactant adsorption (Figure 2 iii), ultimatelym separating the individual nano‐ tubes from the bundle (Figure 2 iv). Since high-power sonication can potentially destroy the tubes, the power of sonication is usually low (< 10 W). On the other hand, to get a good dis‐ persion the sonication time should be sufficiently long (several to tens of hours). After this treatment a SWNTs dispersion with single tubes as well as tube bundles can be obtained. Large tube bundles can be removed from the dispersion by simply gravity sedimentation, while small tube bundles can be stable for weeks. To remove them, ultracentrifugation is recommended. After the sonication-ultracentrifugation circle, SWNTs dispersion up to sin‐ gle tube level can be obtained.

of tubes into water. This is quite important in applications where the amount of the dispersed tubes is the main concern. Consistent with the larger molecular size, the micelles formed by am‐ phiphilic macromolecules are usually larger compared to those formed by low molecular weight surfactants, which may induce a more pronounced depletion attraction if free micelles are present in the dispersion. Temperature is also an important influencing factor. An increase in temperature can induce desorption of the macromolecule from the tube surface due to a con‐

Dispersion and Property Manipulation of Carbon Nanotubes by Self-Assemibles of Amphiphilic Molecules

http://dx.doi.org/10.5772/51967

259

**Figure 3.** Cross-section model of (A) an individual and (B) a seven-tube bundle embedded in a cylindrical SDS micelle. C) A molecular dynamics simulations of water and the SDS micelle around an individual tube. D) The number density

Recent report reveals that the molecular architecture is also an important concern governing the quality of SWNTs dispersion. When the linear PEO-PPO-PEO is branched, an improved capability of SWNTs dispersion and stabilization is observed both experimentally and theo‐ retically. For example, in our laboratory, we have used a starlike amphiphilic block copoly‐ mer with PPO-PEO segments (AP432) to disperse CNTs in aqueous solutions. For comparison, two commercially available linear amphiphilic block copolymers, Pluronics L64 and F127, were also selected. It was found that AP432 and F127 can get good CNT disper‐ sions, while L64 was proved to be unable to disperse CNTs. AP432 with five branches could disperse CNTs efficiently at much lower concentrations compared with the linear F127, al‐ though it has a smaller molecular weight and shorter terminal EO groups. This indicated

profiles for SDS carbon atoms, sulfate head group atoms, water molecules, and sodium ions [15].

tinuous dehydration of the PPO and/or PEO segments.

**Figure 2.** Mechanism of nanotube isolation from bundle obtained by ultrasonication and surfactant stabilization [14].

#### **3. Influence of surfactant type**

Up to now, various types of surfactants, both common and uncommon, have been tested to disperse SWNTs in water. Sodium dodecyl sulfate (SDS), which is an anionic surfactant, is among the earliest and most common choices [6, 9, 15], as shown in Figure 3. Later on, in‐ vestigations have been expanded to cationic, nonionic and zwitterionic surfactants [16-23]. Although both ionic surfactants and nonionic ones can successfully disperse SWNTs in wa‐ ter, the stabilization mechanism for these two categories has subtle difference. While ionic surfactants stabilize the dispersed tubes mainly by electrostatic repulsion, the stabilization mechanism of nonionic surfactant-coated tubes is mainly achieved by steric repulsion. If surfactant molecules are in excess, they can form free micelles. When the size of micelles is above the average distance between the adjacent dispersed tubes, the micelles become diffi‐ cult to arrange themselves between the tubes and, driven by the depletion attraction, phase separation occurs. Besides the surfactant concentration, the quality of an aqueous dispersion of SWNTs can be also influenced by a variety of other experimental parameters including temperature, pH and ionic strength.

Amphiphilic macromolecules such as poly (ethylene oxide)-poly (propylene oxide)-poly (eth‐ ylene oxide) (PEO-PPO-PEO) tri-block copolymers are also efficient dispersing agents for SWNTs [20, 23-25]. The unique molecular structure and property of these amphiphilic macro‐ molecules lead to some differences in SWNTs dispersion compared to traditional low molecu‐ lar weight surfactants. For example, the hydrophobic part (PPO segment) is usually longer than that of low molecular weight surfactant and can thus wrap not only on single tubes but also on tube bundles (Figure 4). This will lead to a decreased fraction of single tubes in the dispersion, which seems a disadvantage in cases where single tubes are desired. However, this feature, to‐ gether with the improved steric repulsion created by the longer hydrophilic part (PEO seg‐ ment), also enables these amphiphilic macromolecules to disperse and stabilize a large amount of tubes into water. This is quite important in applications where the amount of the dispersed tubes is the main concern. Consistent with the larger molecular size, the micelles formed by am‐ phiphilic macromolecules are usually larger compared to those formed by low molecular weight surfactants, which may induce a more pronounced depletion attraction if free micelles are present in the dispersion. Temperature is also an important influencing factor. An increase in temperature can induce desorption of the macromolecule from the tube surface due to a con‐ tinuous dehydration of the PPO and/or PEO segments.

**Figure 2.** Mechanism of nanotube isolation from bundle obtained by ultrasonication and surfactant stabilization [14].

Up to now, various types of surfactants, both common and uncommon, have been tested to disperse SWNTs in water. Sodium dodecyl sulfate (SDS), which is an anionic surfactant, is among the earliest and most common choices [6, 9, 15], as shown in Figure 3. Later on, in‐ vestigations have been expanded to cationic, nonionic and zwitterionic surfactants [16-23]. Although both ionic surfactants and nonionic ones can successfully disperse SWNTs in wa‐ ter, the stabilization mechanism for these two categories has subtle difference. While ionic surfactants stabilize the dispersed tubes mainly by electrostatic repulsion, the stabilization mechanism of nonionic surfactant-coated tubes is mainly achieved by steric repulsion. If surfactant molecules are in excess, they can form free micelles. When the size of micelles is above the average distance between the adjacent dispersed tubes, the micelles become diffi‐ cult to arrange themselves between the tubes and, driven by the depletion attraction, phase separation occurs. Besides the surfactant concentration, the quality of an aqueous dispersion of SWNTs can be also influenced by a variety of other experimental parameters including

Amphiphilic macromolecules such as poly (ethylene oxide)-poly (propylene oxide)-poly (eth‐ ylene oxide) (PEO-PPO-PEO) tri-block copolymers are also efficient dispersing agents for SWNTs [20, 23-25]. The unique molecular structure and property of these amphiphilic macro‐ molecules lead to some differences in SWNTs dispersion compared to traditional low molecu‐ lar weight surfactants. For example, the hydrophobic part (PPO segment) is usually longer than that of low molecular weight surfactant and can thus wrap not only on single tubes but also on tube bundles (Figure 4). This will lead to a decreased fraction of single tubes in the dispersion, which seems a disadvantage in cases where single tubes are desired. However, this feature, to‐ gether with the improved steric repulsion created by the longer hydrophilic part (PEO seg‐ ment), also enables these amphiphilic macromolecules to disperse and stabilize a large amount

**3. Influence of surfactant type**

258 Physical and Chemical Properties of Carbon Nanotubes

temperature, pH and ionic strength.

**Figure 3.** Cross-section model of (A) an individual and (B) a seven-tube bundle embedded in a cylindrical SDS micelle. C) A molecular dynamics simulations of water and the SDS micelle around an individual tube. D) The number density profiles for SDS carbon atoms, sulfate head group atoms, water molecules, and sodium ions [15].

Recent report reveals that the molecular architecture is also an important concern governing the quality of SWNTs dispersion. When the linear PEO-PPO-PEO is branched, an improved capability of SWNTs dispersion and stabilization is observed both experimentally and theo‐ retically. For example, in our laboratory, we have used a starlike amphiphilic block copoly‐ mer with PPO-PEO segments (AP432) to disperse CNTs in aqueous solutions. For comparison, two commercially available linear amphiphilic block copolymers, Pluronics L64 and F127, were also selected. It was found that AP432 and F127 can get good CNT disper‐ sions, while L64 was proved to be unable to disperse CNTs. AP432 with five branches could disperse CNTs efficiently at much lower concentrations compared with the linear F127, al‐ though it has a smaller molecular weight and shorter terminal EO groups. This indicated clearly that, once branched, copolymers would get a much better ability to disperse CNTs [26, 27]. The detailed information is shown in Figure 5.

Besides the hydrophobic/hydrophobic interaction, π–π stacking could also play a role in SWNTs dispersion (Figure 6). A typical example is the anionic surfactant sodium dodecyl‐ benzene sulfonate (SDBS) which can greatly enhance SWNTs dispersion in water due to the existance of a benzene ring in the alkyl chain [20, 28]. Other examples include the amphi‐ philic pyrene [29], fluorescein [30], perylene [31] and fullerene [32, 33]. In some cases the af‐ finity between SWNTs and the dispersant can be complicated and not so obvious. There are reports on SWNTs dispersion using polyelectrolytes such as poly (acrylic acid) (PAA) (Fig‐ ure 7) [34] and naturally occurring macromolecules or biomolecules such as Gum Arabic [35], Hyaluronic Acid [36], starch [37], protein [38] and DNA [39, 40]. The surface activity of these molecules is not as high as traditional surfactants, but satisfactory SWNTs dispersion

Dispersion and Property Manipulation of Carbon Nanotubes by Self-Assemibles of Amphiphilic Molecules

http://dx.doi.org/10.5772/51967

261

**Figure 6.** Schematic representation of how surfactants may adsorb onto the nanotube surface. NaDDBS and TX100 were believed to disperse the tubes better than SDS because of their benzene rings. NaDDBS disperses better than TX100 because of its headgroup and slightly longer alkyl chain. The spacing between the benzene rings on the surfac‐

**Figure 7. (A)** Schematic showing the change in nanotube microstructure that occurs as the pH of poly(acrylic acid) is changed. At low pH, the polymer is uncharged and highly coiled with extensive intrachain hydrogen bonding. At high pH, the carboxylic acid groups are deprotonated and the polymer is more extended as the negatively charged side groups repel one another. **(B)** Viscosity as a function of shear rate for aqueous suspensions containing 1 wt % PAA-

charged groups [28].

can be also obtained by optimizing the experimental parameters.

tants and the tube surface is large enough to accommodate the SO3

SWNT (SWNT is 10 wt % of the total solids) as pH is progressively increased [34].

**Figure 4.** Block copolymers (designated A-B or A-B-A). They are comprised of covalently linked incompatible moieties and disperse SWNT in selective solvents that act as a "good solvent" for one of the blocks (i.e., A), while simultaneously acting as a "poor solvent" for the other block (B). Under these conditions polymer chains may adsorb via physical at‐ tachment of the B-block while the A blocks dangle into the solution repelling other polymer-decorated CNT and form‐ ing a long-lived dispersion. Nonselective solvents fail to disperse the SWNT [24, 25].

**Figure 5.** (A) Molecular details of linear (F127 and L64) and star-like amphiphilic block copolymers (AP432). The value of m and n for AP432 is the average value calculated according to the molecular weight and EO content. It is evident that L64 has no ability to disperse CNTs. (B) Schematic Representation of the Possible Mechanism for Nanotube Dis‐ persion by AP432 and F127 [27].

Besides the hydrophobic/hydrophobic interaction, π–π stacking could also play a role in SWNTs dispersion (Figure 6). A typical example is the anionic surfactant sodium dodecyl‐ benzene sulfonate (SDBS) which can greatly enhance SWNTs dispersion in water due to the existance of a benzene ring in the alkyl chain [20, 28]. Other examples include the amphi‐ philic pyrene [29], fluorescein [30], perylene [31] and fullerene [32, 33]. In some cases the af‐ finity between SWNTs and the dispersant can be complicated and not so obvious. There are reports on SWNTs dispersion using polyelectrolytes such as poly (acrylic acid) (PAA) (Fig‐ ure 7) [34] and naturally occurring macromolecules or biomolecules such as Gum Arabic [35], Hyaluronic Acid [36], starch [37], protein [38] and DNA [39, 40]. The surface activity of these molecules is not as high as traditional surfactants, but satisfactory SWNTs dispersion can be also obtained by optimizing the experimental parameters.

clearly that, once branched, copolymers would get a much better ability to disperse CNTs

**Figure 4.** Block copolymers (designated A-B or A-B-A). They are comprised of covalently linked incompatible moieties and disperse SWNT in selective solvents that act as a "good solvent" for one of the blocks (i.e., A), while simultaneously acting as a "poor solvent" for the other block (B). Under these conditions polymer chains may adsorb via physical at‐ tachment of the B-block while the A blocks dangle into the solution repelling other polymer-decorated CNT and form‐

[26, 27]. The detailed information is shown in Figure 5.

260 Physical and Chemical Properties of Carbon Nanotubes

ing a long-lived dispersion. Nonselective solvents fail to disperse the SWNT [24, 25].

+

*n*

**B**

*n*

persion by AP432 and F127 [27].

Low AP432 concentration

Low F127 concentration

**Figure 5.** (A) Molecular details of linear (F127 and L64) and star-like amphiphilic block copolymers (AP432). The value of m and n for AP432 is the average value calculated according to the molecular weight and EO content. It is evident that L64 has no ability to disperse CNTs. (B) Schematic Representation of the Possible Mechanism for Nanotube Dis‐

High AP432 concentration

> High F127 concentration

+

**A**

**Figure 6.** Schematic representation of how surfactants may adsorb onto the nanotube surface. NaDDBS and TX100 were believed to disperse the tubes better than SDS because of their benzene rings. NaDDBS disperses better than TX100 because of its headgroup and slightly longer alkyl chain. The spacing between the benzene rings on the surfac‐ tants and the tube surface is large enough to accommodate the SO3 charged groups [28].

**Figure 7. (A)** Schematic showing the change in nanotube microstructure that occurs as the pH of poly(acrylic acid) is changed. At low pH, the polymer is uncharged and highly coiled with extensive intrachain hydrogen bonding. At high pH, the carboxylic acid groups are deprotonated and the polymer is more extended as the negatively charged side groups repel one another. **(B)** Viscosity as a function of shear rate for aqueous suspensions containing 1 wt % PAA-SWNT (SWNT is 10 wt % of the total solids) as pH is progressively increased [34].

#### **4. Evaluation of the SWNTs dispersion**

The quality of a SWNTs dispersion is evaluated mainly based on the requirement of the spe‐ cific application. However, there are also some common criterions. The first one is the amount of SWNTs that can be dispersed by surfactants in water. This can be calculated by substracting the undispersed tubes from the total tubes or by simply visual inspections since the dispersion with more tubes usually has a heavier black color. The second and more im‐ portant criterion is the dispersing extent of the tubes, i.e., bundles or single tubes. This can be checked either directly by imaging methods such as high resolution transmission electron microscopy (HRTEM) or atomic force microscopy observations, or indirectly by spectro‐ scopic characterizations including UV-vis-NIR absorption, Raman spectroscopy and photo‐ luminescence. In the latter case, researchers have shown that once SWNTs are dispersed up to a single tube level, fine structures in both absorption and emission spectra can be ob‐ served [6, 9]. While in a tube bundle, the fine spectroscopic characteristics will be signifi‐ cantly suppressed, highlighting the advantage of SWNTs dispersion by surfactants. The third criterion is the stability of the dispersion. The SWNTs dispersed in water is a kinetical‐ ly rather than thermodynamically stable system. Thus aggregation and sedimentation can occur with time. If the dispersion is only an intermediated stage in a work and the dispersed tubes will be immediately used for the next step, short period of stability may be sufficient. In some applications, however, long term stability up to weeks or months may be required.

[28] and random adsorption model [41]. Figure 8 shows the schematic representations of the mechanism by which surfactants help to disperse SWNT. Although each of them has gained some experimental evidence, the real picture in the microscopic length scale is still to be clarified. In this context computer simulation is frequently adopted and various interaction modes between surfactant molecules and tube surfaces have been proposed [42-44]. Now it is generally accepted that for cylindrical micelle forming surfactant, SWNTs may be encap‐ sulated in the micelles while for spherical micelle forming surfactant, random adsorption

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In some cases, after the dispersion of SWNTs bundles the tubes are needed to be effectively aligned. SWNTs are anisotropic particles with diameters on the order of nanometers but lengths ranging from micrometers to centimeters. They display most of their expected prop‐ erties along the tube axis. If the tubes are randomly oriented, the properties will be averaged which should be avoided in some applications such as nanodevice fabrication. The uniform alignment is therefore a crucial condition in these cases. Surfactant can form various self-as‐ semblies in water above the critical micellar concentration (cmc). At medium-to-high con‐ centrations, these self-assemblies can further organize into long range ordered phases called lyotropic liquid crystals (LLC) [45, 46]. An idea immediately comes into mind is that wheth‐ er these LLC can be utilized to align SWNTs (Figure 9). A typical strategy is that SWNTs are dispersed first in a dilute surfactant aqueous solution before introduced to an LLC matrix since directly disperse SWNTs in a viscous LLC phase could be difficult. The surfactant used for SWNTs dispersion and LLC construction can be the same type or different. The incorpo‐ ration of the tubes is found not to destroy the LLC matrix. This indirectly proves that the tubes are aligned along the director of the LLC phase since otherwise the system will be en‐

An innovative development in this research field is the fabrication of SWNTs/LLC hybrid by a spontaneous phase separation induced by hydrophilic polymer. In this method, dispersed SWNTs, surfactant and hydrophilic polymer are added into water and homogenized. Initial‐ ly the concentration of surfactant is well below the critical point for LLC formation and only randomly oriented micelles exist in the solution. If the radius of gyration of the hydrophilic polymer in water is above the average distance between adjacent micelles, the polymer mol‐ ecules will be driven out and phase separation will occur. The surfactant micelles together with the dispersed SWNTs will be compressed to form a new phase and the hydrophilic pol‐ ymer forms another. Since the volume shrinks during phase separation, the surfactant con‐ centration in the newly formed phase will be increased to exceed the critical point of LLC formation. This method has been successfully utilized in several surfactant and polymer combinations including nonionic surfactant/nonionic polymer system (Figure 10) [47] and ionic surfactant/polyelectrolyte system where the surfactant and polyelectrolyte have the same sign of charges [48]. The most striking advantage of this method is that the dispersed

**5. SWNTs alighment in ordered surfactant self-assemblies**

may also exist.

ergetically unfavorable.

**Figure 8. (A)** Schematic representations of the mechanism by which surfactants help to disperse SWNT. (a) SWNT en‐ capsulated in a cylindrical surfactant micelle (both cross section and side-view); (b) hemimicellar adsorption of surfac‐ tant molecules on a SWNT; (c) random adsorption of surfactant molecules on a SWNT [11]. **(B)** Coherent SANS intensities calculated for cylindrical (with an embedded nanotube) and spherical core-shell micelles at a concentration of 0.25 wt % SDS in D2O. The combined cylindrical and spherical micelle prediction shown is calculated on the basis of 48% of the SDS molecules participating in cylindrical micelles [41].

A concern in SWNTs dispersion is the way of surfactant adsorption on the tube surface. Un‐ derstanding this can not only help to gain the insight of the interaction mechanism between surfactant molecules and SWNTs, but also provide guidelines to select or design specific surfactants to further improve the quality of the SWNTs dispersions. To fully address this issue experimentally, however, is not an easy task. Up to now, three different adsorption models have been proposed including the cylindrical micelle model [6], semi-sphere model [28] and random adsorption model [41]. Figure 8 shows the schematic representations of the mechanism by which surfactants help to disperse SWNT. Although each of them has gained some experimental evidence, the real picture in the microscopic length scale is still to be clarified. In this context computer simulation is frequently adopted and various interaction modes between surfactant molecules and tube surfaces have been proposed [42-44]. Now it is generally accepted that for cylindrical micelle forming surfactant, SWNTs may be encap‐ sulated in the micelles while for spherical micelle forming surfactant, random adsorption may also exist.

#### **5. SWNTs alighment in ordered surfactant self-assemblies**

**4. Evaluation of the SWNTs dispersion**

262 Physical and Chemical Properties of Carbon Nanotubes

The quality of a SWNTs dispersion is evaluated mainly based on the requirement of the spe‐ cific application. However, there are also some common criterions. The first one is the amount of SWNTs that can be dispersed by surfactants in water. This can be calculated by substracting the undispersed tubes from the total tubes or by simply visual inspections since the dispersion with more tubes usually has a heavier black color. The second and more im‐ portant criterion is the dispersing extent of the tubes, i.e., bundles or single tubes. This can be checked either directly by imaging methods such as high resolution transmission electron microscopy (HRTEM) or atomic force microscopy observations, or indirectly by spectro‐ scopic characterizations including UV-vis-NIR absorption, Raman spectroscopy and photo‐ luminescence. In the latter case, researchers have shown that once SWNTs are dispersed up to a single tube level, fine structures in both absorption and emission spectra can be ob‐ served [6, 9]. While in a tube bundle, the fine spectroscopic characteristics will be signifi‐ cantly suppressed, highlighting the advantage of SWNTs dispersion by surfactants. The third criterion is the stability of the dispersion. The SWNTs dispersed in water is a kinetical‐ ly rather than thermodynamically stable system. Thus aggregation and sedimentation can occur with time. If the dispersion is only an intermediated stage in a work and the dispersed tubes will be immediately used for the next step, short period of stability may be sufficient. In some applications, however, long term stability up to weeks or months may be required.

**Figure 8. (A)** Schematic representations of the mechanism by which surfactants help to disperse SWNT. (a) SWNT en‐ capsulated in a cylindrical surfactant micelle (both cross section and side-view); (b) hemimicellar adsorption of surfac‐ tant molecules on a SWNT; (c) random adsorption of surfactant molecules on a SWNT [11]. **(B)** Coherent SANS intensities calculated for cylindrical (with an embedded nanotube) and spherical core-shell micelles at a concentration of 0.25 wt % SDS in D2O. The combined cylindrical and spherical micelle prediction shown is calculated on the basis of

A concern in SWNTs dispersion is the way of surfactant adsorption on the tube surface. Un‐ derstanding this can not only help to gain the insight of the interaction mechanism between surfactant molecules and SWNTs, but also provide guidelines to select or design specific surfactants to further improve the quality of the SWNTs dispersions. To fully address this issue experimentally, however, is not an easy task. Up to now, three different adsorption models have been proposed including the cylindrical micelle model [6], semi-sphere model

48% of the SDS molecules participating in cylindrical micelles [41].

In some cases, after the dispersion of SWNTs bundles the tubes are needed to be effectively aligned. SWNTs are anisotropic particles with diameters on the order of nanometers but lengths ranging from micrometers to centimeters. They display most of their expected prop‐ erties along the tube axis. If the tubes are randomly oriented, the properties will be averaged which should be avoided in some applications such as nanodevice fabrication. The uniform alignment is therefore a crucial condition in these cases. Surfactant can form various self-as‐ semblies in water above the critical micellar concentration (cmc). At medium-to-high con‐ centrations, these self-assemblies can further organize into long range ordered phases called lyotropic liquid crystals (LLC) [45, 46]. An idea immediately comes into mind is that wheth‐ er these LLC can be utilized to align SWNTs (Figure 9). A typical strategy is that SWNTs are dispersed first in a dilute surfactant aqueous solution before introduced to an LLC matrix since directly disperse SWNTs in a viscous LLC phase could be difficult. The surfactant used for SWNTs dispersion and LLC construction can be the same type or different. The incorpo‐ ration of the tubes is found not to destroy the LLC matrix. This indirectly proves that the tubes are aligned along the director of the LLC phase since otherwise the system will be en‐ ergetically unfavorable.

An innovative development in this research field is the fabrication of SWNTs/LLC hybrid by a spontaneous phase separation induced by hydrophilic polymer. In this method, dispersed SWNTs, surfactant and hydrophilic polymer are added into water and homogenized. Initial‐ ly the concentration of surfactant is well below the critical point for LLC formation and only randomly oriented micelles exist in the solution. If the radius of gyration of the hydrophilic polymer in water is above the average distance between adjacent micelles, the polymer mol‐ ecules will be driven out and phase separation will occur. The surfactant micelles together with the dispersed SWNTs will be compressed to form a new phase and the hydrophilic pol‐ ymer forms another. Since the volume shrinks during phase separation, the surfactant con‐ centration in the newly formed phase will be increased to exceed the critical point of LLC formation. This method has been successfully utilized in several surfactant and polymer combinations including nonionic surfactant/nonionic polymer system (Figure 10) [47] and ionic surfactant/polyelectrolyte system where the surfactant and polyelectrolyte have the same sign of charges [48]. The most striking advantage of this method is that the dispersed SWNTs take part in the process of LLC formation instead of post-introduction, which is ben‐ eficial for tube alignment.

The dispersed SWNTs could also be aligned in randomly oriented surfactant micelles. For example, Tannenbaum's group probed the effects of shear flow on the alignment of dis‐ persed single-walled carbon nanotubes in polymer solutions. Two different systems were compared: Single-walled carbon nanotubes dispersed using an anionic surfactant (sodium dodecyl benzene sulfonate, NaDDBS) and single-walled carbon nanotubes dispersed using an anionic surfactant and a weakly binding polymer (carboxyl methylcellulose, CMC). In this case, an additional force, typically shear, will be needed. It was determined that the ad‐ dition of a weakly binding polymer serves two purposes: constituting the polymer matrix in which the SWNT will be dispersed and aligned and providing a secondary mechanism for the promotion of carbon nanotube dispersion [49]. The results showed that the tubes are found to be preferentially align along the direction of shear flow, as shown in Figure 11. Some reports also revealed that the presence of SWNTs can have a pronounced effect on the ordering of micelles [50]. This disadvantage of such method is, however, once the shear flow

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**Figure 11. (A)** Schematic representation of the stabilization mechanism of carbon nanotubes: Interaction between SWNT with NaDDBS, followed by the addition of CMC. It is important to note that the molecules were not drawn to scale, and the dimension of the polymer molecules is about 2 orders of magnitude larger that the dimension of the surfactant molecules (*R*g ≈1000 Å for CMC as compared to ≈20 Å for NaDDBS); **(B)** Summary of the calculated shear stresses of the various SWNT-containing solutions: Plots of shear stresses as a function of angular velocity [49].

The formation of well-dispersed SWNTs in surfactant aqueous solutions opens the door for further investigation and practical applications of these interesting nanomaterials, as shown in Figure 12 [51]. Here we just briefly give some typical examples in different research field to elucidate the significance of the dispersion of SWNTs by surfactants. The first example comes from the preparation of advanced functional materials using SDS-coated SWNTs. When subjected to shear flow in a polymer solution, the tubes will be recondensed and

is stopped, the tubes and micelles tend to be disordered again.

**6. Applications of the surfactant-coated SWNTs**

**Figure 9. (A)** Organizing CNTs with LC solvents. (a) A droplet of liquid crystal (ellipsoids) with suspended carbon nano‐ tubes (rods) is applied to a porous membrane substrate. (b) The bulk LC is then aligned using a grooved surface or external field, which in turn orders the nanotubes. (c) The LC is drained through the porous membrane leaving behind an ordered nanotube film [45]. **(B)** Schematic illustrations of the mechanism of CNT alignment in a lyotropic nematic LC host, for the cases of rod- and disk-micelle-type adsorption, (a) and (b) respectively, of surfactant molecules on the CNT surface. The CNT is drawn as a vertical black rod in the center of each picture, only partially covered by surfactant molecules for clarity [46]. **(C)** AFM image of oriented CNT film of MWCNTs from 5CB (50×50 μm) [45].

**Figure 10.** Schematic illustration of the phase separation process. (A) SWNTs dispersion in 0.1 wt%C12E6 aqueous solu‐ tion; (B) mixture of SWNTs dispersion, C12E6 (10 wt %) and PEG(20 wt %) after being homogenized; (C) intermediate stage during phase separation; (D) final state after equilibrium. (E) Schematic Representation of the Phase Separation Process in the Four Component Mixture of the Surfactant C12E6, PEG 20000, SWNTs, and Water [47].

The dispersed SWNTs could also be aligned in randomly oriented surfactant micelles. For example, Tannenbaum's group probed the effects of shear flow on the alignment of dis‐ persed single-walled carbon nanotubes in polymer solutions. Two different systems were compared: Single-walled carbon nanotubes dispersed using an anionic surfactant (sodium dodecyl benzene sulfonate, NaDDBS) and single-walled carbon nanotubes dispersed using an anionic surfactant and a weakly binding polymer (carboxyl methylcellulose, CMC). In this case, an additional force, typically shear, will be needed. It was determined that the ad‐ dition of a weakly binding polymer serves two purposes: constituting the polymer matrix in which the SWNT will be dispersed and aligned and providing a secondary mechanism for the promotion of carbon nanotube dispersion [49]. The results showed that the tubes are found to be preferentially align along the direction of shear flow, as shown in Figure 11. Some reports also revealed that the presence of SWNTs can have a pronounced effect on the ordering of micelles [50]. This disadvantage of such method is, however, once the shear flow is stopped, the tubes and micelles tend to be disordered again.

SWNTs take part in the process of LLC formation instead of post-introduction, which is ben‐

**Figure 9. (A)** Organizing CNTs with LC solvents. (a) A droplet of liquid crystal (ellipsoids) with suspended carbon nano‐ tubes (rods) is applied to a porous membrane substrate. (b) The bulk LC is then aligned using a grooved surface or external field, which in turn orders the nanotubes. (c) The LC is drained through the porous membrane leaving behind an ordered nanotube film [45]. **(B)** Schematic illustrations of the mechanism of CNT alignment in a lyotropic nematic LC host, for the cases of rod- and disk-micelle-type adsorption, (a) and (b) respectively, of surfactant molecules on the CNT surface. The CNT is drawn as a vertical black rod in the center of each picture, only partially covered by surfactant

**Figure 10.** Schematic illustration of the phase separation process. (A) SWNTs dispersion in 0.1 wt%C12E6 aqueous solu‐ tion; (B) mixture of SWNTs dispersion, C12E6 (10 wt %) and PEG(20 wt %) after being homogenized; (C) intermediate stage during phase separation; (D) final state after equilibrium. (E) Schematic Representation of the Phase Separation

Process in the Four Component Mixture of the Surfactant C12E6, PEG 20000, SWNTs, and Water [47].

molecules for clarity [46]. **(C)** AFM image of oriented CNT film of MWCNTs from 5CB (50×50 μm) [45].

**E**

eficial for tube alignment.

264 Physical and Chemical Properties of Carbon Nanotubes

**Figure 11. (A)** Schematic representation of the stabilization mechanism of carbon nanotubes: Interaction between SWNT with NaDDBS, followed by the addition of CMC. It is important to note that the molecules were not drawn to scale, and the dimension of the polymer molecules is about 2 orders of magnitude larger that the dimension of the surfactant molecules (*R*g ≈1000 Å for CMC as compared to ≈20 Å for NaDDBS); **(B)** Summary of the calculated shear stresses of the various SWNT-containing solutions: Plots of shear stresses as a function of angular velocity [49].

#### **6. Applications of the surfactant-coated SWNTs**

The formation of well-dispersed SWNTs in surfactant aqueous solutions opens the door for further investigation and practical applications of these interesting nanomaterials, as shown in Figure 12 [51]. Here we just briefly give some typical examples in different research field to elucidate the significance of the dispersion of SWNTs by surfactants. The first example comes from the preparation of advanced functional materials using SDS-coated SWNTs. When subjected to shear flow in a polymer solution, the tubes will be recondensed and aligned and finally a nanotube fiber can be obtained which has a high elastic modulus [16]. The surfactant-coated SWNTs can be also used as starting materials for SWNTs sorting i.e., separating semiconducting tubes from metallic ones. In recent years, great progress has been made in this direction by density-gradient ultracentrifugation and tubes with defined diameter and chirality can be obtained (Figure 13) [52, 53]. The sorting of SWNTs is necessa‐ ry and important especially in the fabrication of high performance nanodevices such as field effect transistor arrays [54].

**Figure 13.** Sorting of SWNTs by diameter, bandgap and electronic type using density gradient ultracentrifugation. **a,** Schematic of surfactant encapsulation and sorting, where r is density. **B-g,** Photographs and optical absorbance (1 cm path length) spectra after separation using density gradient ultracentrifugation. A rich structure–density relationship is observed for SC-encapsulated SWNTs, enabling their separation by diameter, bandgap and electronic type. In con‐ trast, no separation is observed for SDBS-encapsulated SWNTs. **b,c,** SC encapsulated, CoMoCAT-grown SWNTs (7–11 A° ). **d, e,** SDBS-encapsulated CoMoCAT-grown SWNTs (7-11 A°). **f, g,** SC-encapsulated, laser-ablation-grown SWNTs

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**Figure 14.** Carbon nanotubes with high NIR absorbance solubilized in water. (*a*) Schematic of a Cy3-DNA-functional‐ ized SWNT. The drawing is only a graphic presentation and does not represent the precise way DNA binds on SWNTs. (*b*) UV-visible spectra of solutions of individual SWNTs functionalized noncovalently by 15-mer Cy3 labeled-DNA at various nanotube concentrations. (*c*) Absorbance at 808nmvs.SWNTconcentration (optical path = 1 cm). Solid line is Beer's law fit to obtain molar extinction coefficient of SWNT≈7.9×10<sup>6</sup> M-1•cm-1. (*Inset*) A photo of a DNA functional‐ ized SWNT solution. (*d*) AFM image of DNA-functionalized individual SWNTs (height of 1–10 nm) deposited on a SiO2

substrate (Scale bar: 200 nm.) [55].

(11-16 A°). pH = 7 for all parts. SWNTs before sorting are depicted as a dashed grey line in **c** and **g** [53].

**Figure 12.** The aligned SWNT/polymer films can be used to orient the molecules of the liquid crystal. a) Schematic diagrams for arrays of LCs on a glass substrate showing random directional local phases. b) On a shear-aligned com‐ posite thin film (1a–SWNTarc-discharge 1:1, 2mg mL-1 in DCB) showing uniformly aligned LC domains. c) Transmission cross-polarized microscope image of a drop of LC 5CB on a bare glass substrate showing many small randomly aligned domains. d) A drop of LC 5CB on a glass substrate with sheared composite thin filmwith the shearing direction parallel to one of the polarizers. e) Shearing direction 45° to one of the polarizers. Arrows indicate the shearing direction [51].

Due to the unique geometry, ideal size and low cytotoxicity, SWNTs have great potential applications in life science. For this purpose the surfactant used to disperse SWNTs should be biocompatible and nonimmunogenity. One good choice is lipid derivative with a PEO segment as the hydrophilic part [55, 56]. SWNTs functionalized with single stranded-DNA can find application in gene therapy since the tubes can transport the cell membranes [55]. SWNTs are also known to absorb the near infrared light which will subsequent induce a lo‐ cal temperature rise, as shown in Figure 14. The surfactant-coated SWNTs can thus be used in photothermal treatments in some diseases such as cancer [55]. Besides, other functionali‐ ties including fluorescent dyes, cell receptors and drugs can be also integrated onto the sur‐ factant-coated SWNTs to construct multifunctional materials.

Dispersion and Property Manipulation of Carbon Nanotubes by Self-Assemibles of Amphiphilic Molecules http://dx.doi.org/10.5772/51967 267

aligned and finally a nanotube fiber can be obtained which has a high elastic modulus [16]. The surfactant-coated SWNTs can be also used as starting materials for SWNTs sorting i.e., separating semiconducting tubes from metallic ones. In recent years, great progress has been made in this direction by density-gradient ultracentrifugation and tubes with defined diameter and chirality can be obtained (Figure 13) [52, 53]. The sorting of SWNTs is necessa‐ ry and important especially in the fabrication of high performance nanodevices such as field

**Figure 12.** The aligned SWNT/polymer films can be used to orient the molecules of the liquid crystal. a) Schematic diagrams for arrays of LCs on a glass substrate showing random directional local phases. b) On a shear-aligned com‐ posite thin film (1a–SWNTarc-discharge 1:1, 2mg mL-1 in DCB) showing uniformly aligned LC domains. c) Transmission cross-polarized microscope image of a drop of LC 5CB on a bare glass substrate showing many small randomly aligned domains. d) A drop of LC 5CB on a glass substrate with sheared composite thin filmwith the shearing direction parallel to one of the polarizers. e) Shearing direction 45° to one of the polarizers. Arrows indicate the shearing direction [51].

Due to the unique geometry, ideal size and low cytotoxicity, SWNTs have great potential applications in life science. For this purpose the surfactant used to disperse SWNTs should be biocompatible and nonimmunogenity. One good choice is lipid derivative with a PEO segment as the hydrophilic part [55, 56]. SWNTs functionalized with single stranded-DNA can find application in gene therapy since the tubes can transport the cell membranes [55]. SWNTs are also known to absorb the near infrared light which will subsequent induce a lo‐ cal temperature rise, as shown in Figure 14. The surfactant-coated SWNTs can thus be used in photothermal treatments in some diseases such as cancer [55]. Besides, other functionali‐ ties including fluorescent dyes, cell receptors and drugs can be also integrated onto the sur‐

factant-coated SWNTs to construct multifunctional materials.

effect transistor arrays [54].

266 Physical and Chemical Properties of Carbon Nanotubes

**Figure 13.** Sorting of SWNTs by diameter, bandgap and electronic type using density gradient ultracentrifugation. **a,** Schematic of surfactant encapsulation and sorting, where r is density. **B-g,** Photographs and optical absorbance (1 cm path length) spectra after separation using density gradient ultracentrifugation. A rich structure–density relationship is observed for SC-encapsulated SWNTs, enabling their separation by diameter, bandgap and electronic type. In con‐ trast, no separation is observed for SDBS-encapsulated SWNTs. **b,c,** SC encapsulated, CoMoCAT-grown SWNTs (7–11 A° ). **d, e,** SDBS-encapsulated CoMoCAT-grown SWNTs (7-11 A°). **f, g,** SC-encapsulated, laser-ablation-grown SWNTs (11-16 A°). pH = 7 for all parts. SWNTs before sorting are depicted as a dashed grey line in **c** and **g** [53].

**Figure 14.** Carbon nanotubes with high NIR absorbance solubilized in water. (*a*) Schematic of a Cy3-DNA-functional‐ ized SWNT. The drawing is only a graphic presentation and does not represent the precise way DNA binds on SWNTs. (*b*) UV-visible spectra of solutions of individual SWNTs functionalized noncovalently by 15-mer Cy3 labeled-DNA at various nanotube concentrations. (*c*) Absorbance at 808nmvs.SWNTconcentration (optical path = 1 cm). Solid line is Beer's law fit to obtain molar extinction coefficient of SWNT≈7.9×10<sup>6</sup> M-1•cm-1. (*Inset*) A photo of a DNA functional‐ ized SWNT solution. (*d*) AFM image of DNA-functionalized individual SWNTs (height of 1–10 nm) deposited on a SiO2 substrate (Scale bar: 200 nm.) [55].

#### **7. Summary and outlook**

The combined development of colloid and interface science and nanotechnology has paved an effective way to disperse CNTs, especially SWNTs, into water using surfactants. Up to now, almost all the common surfactants have been tested and quite a few newly synthesized surfactants with unique molecular structures have been tried. The effective dispersing meth‐ odology has been found and a general dispersing mechanism has been proposed. These ad‐ vances enable us to get SWNTs dispersions up to single tube level, which significantly facilitates the property manipulation of the tubes and leads to a variety of important appli‐ cations of SWNTs in physics, biology and life science.

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At the same time, one should also keep in mind that challenges still exist. Although good dispersions with large amount of tubes or single tubes can be obtained individual‐ ly, a combination of them, i.e., dispersions with large amount of single tubes, is still dif‐ ficult to get. Achievement at this point will rely on the appearance of novel surfactant with improved performance and/or further optimization of the dispersion methodology. In nanodevice fabrication, the insulating surfactant layer on the tube surface may be un‐ desirable and effective removal of them might be an issue. Despite these limitations, dis‐ persing SWNTs using surfactants has provide an elegant way to manipulate these interesting nanomaterials and we believe the challenges mentioned above will be con‐ quered in near future based on the continuous efforts made by scientists from related disciplines.

#### **Acknowledgments**

The authors gratefully acknowledge support of this work by the Natural Science Foundation of China (21203109) and (20873077) and the program of Hundreds of Talents of the Chinese Academy of Sciences.

#### **Author details**

Xia Xin 1 , Guiying Xu 1 and Hongguang Li 2

1 National Engineering Technology Research Center for Colloidal Materials, Shandong Uni‐ versity, Jinan, , P. R. China

2 Laboratory of Clean Energy Chemistry and Materials, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, , P. R. China

#### **References**

**7. Summary and outlook**

268 Physical and Chemical Properties of Carbon Nanotubes

disciplines.

**Acknowledgments**

Academy of Sciences.

**Author details**

, Guiying Xu 1

versity, Jinan, , P. R. China

Xia Xin 1

cations of SWNTs in physics, biology and life science.

The combined development of colloid and interface science and nanotechnology has paved an effective way to disperse CNTs, especially SWNTs, into water using surfactants. Up to now, almost all the common surfactants have been tested and quite a few newly synthesized surfactants with unique molecular structures have been tried. The effective dispersing meth‐ odology has been found and a general dispersing mechanism has been proposed. These ad‐ vances enable us to get SWNTs dispersions up to single tube level, which significantly facilitates the property manipulation of the tubes and leads to a variety of important appli‐

At the same time, one should also keep in mind that challenges still exist. Although good dispersions with large amount of tubes or single tubes can be obtained individual‐ ly, a combination of them, i.e., dispersions with large amount of single tubes, is still dif‐ ficult to get. Achievement at this point will rely on the appearance of novel surfactant with improved performance and/or further optimization of the dispersion methodology. In nanodevice fabrication, the insulating surfactant layer on the tube surface may be un‐ desirable and effective removal of them might be an issue. Despite these limitations, dis‐ persing SWNTs using surfactants has provide an elegant way to manipulate these interesting nanomaterials and we believe the challenges mentioned above will be con‐ quered in near future based on the continuous efforts made by scientists from related

The authors gratefully acknowledge support of this work by the Natural Science Foundation of China (21203109) and (20873077) and the program of Hundreds of Talents of the Chinese

1 National Engineering Technology Research Center for Colloidal Materials, Shandong Uni‐

2 Laboratory of Clean Energy Chemistry and Materials, Lanzhou Institute of Chemical

and Hongguang Li 2

Physics, Chinese Academy of Sciences, Lanzhou, , P. R. China

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**Chapter 11**

**Aqueous Solution Surface Chemistry of Carbon**

Since the rediscovery of carbon nanotubes (CNTs) by Iijima in 1991, a plethora of applica‐ tions have been developed in the fields of biomolecular science, catalysis, environmental chemistry and medicine. Relevant to the development of these new technologies, it is impor‐ tant to effectively characterize and tune the chemical and electronic structures of these mate‐ rials for desired properties. Within the last 15 years, an array of surface characterization methods have been developed to assay the surface structures of single- (SWNTs) and multiwalled (MWNTs) carbon nanotubes, in particular as organic moieties and catalytically active metal nanoparticles are tethered to them. Distinctive physical, chemical, electrical and high thermal properties of CNTs make these materials suitable for widespread applications, such as fuel cells, semiconducting materials in electronics, atomic force microscopy probes, mi‐ croelectrodes, adsorbents to remove pollutants from waste water, electrochemical sensing and drug carriers. Aqueous surface chemistry plays a vital role in determining the fate and transport of CNTs. A large fraction of the atoms in CNTs reside at or near the surface (Sayes et al., 2006; Bottini et al., 2006). Pristine carbon nanotubes are barely soluble in liquids. To introduce nanotubes in more easily dispersible forms, they require functionalization. These processes entail attaching various organic moities to the sidewalls, which can be used to tether catalytically reactive nanoparticles. Biomolecules require electron mediators to pro‐ mote electron transfer needed for effective biosensing (Sampath et al., 1998). Electrochemical metal ion sensors require certain functional groups which show potential affinity towards particular metal ions (Mojica et al., 2007). Surface electrostatic interactions in solution also influence the sorption properties of these materials to entrain environmental contaminants

> © 2013 Deb and Chusuei; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

> © 2013 Deb and Chusuei; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Nanotubes**

**1. Introduction**

Anup K. Deb and Charles C. Chusuei

on the CNT sidewalls (Tavallai et al., 2012).

http://dx.doi.org/10.5772/51869

Additional information is available at the end of the chapter

### **Aqueous Solution Surface Chemistry of Carbon Nanotubes**

Anup K. Deb and Charles C. Chusuei

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/51869

#### **1. Introduction**

Since the rediscovery of carbon nanotubes (CNTs) by Iijima in 1991, a plethora of applica‐ tions have been developed in the fields of biomolecular science, catalysis, environmental chemistry and medicine. Relevant to the development of these new technologies, it is impor‐ tant to effectively characterize and tune the chemical and electronic structures of these mate‐ rials for desired properties. Within the last 15 years, an array of surface characterization methods have been developed to assay the surface structures of single- (SWNTs) and multiwalled (MWNTs) carbon nanotubes, in particular as organic moieties and catalytically active metal nanoparticles are tethered to them. Distinctive physical, chemical, electrical and high thermal properties of CNTs make these materials suitable for widespread applications, such as fuel cells, semiconducting materials in electronics, atomic force microscopy probes, mi‐ croelectrodes, adsorbents to remove pollutants from waste water, electrochemical sensing and drug carriers. Aqueous surface chemistry plays a vital role in determining the fate and transport of CNTs. A large fraction of the atoms in CNTs reside at or near the surface (Sayes et al., 2006; Bottini et al., 2006). Pristine carbon nanotubes are barely soluble in liquids. To introduce nanotubes in more easily dispersible forms, they require functionalization. These processes entail attaching various organic moities to the sidewalls, which can be used to tether catalytically reactive nanoparticles. Biomolecules require electron mediators to pro‐ mote electron transfer needed for effective biosensing (Sampath et al., 1998). Electrochemical metal ion sensors require certain functional groups which show potential affinity towards particular metal ions (Mojica et al., 2007). Surface electrostatic interactions in solution also influence the sorption properties of these materials to entrain environmental contaminants on the CNT sidewalls (Tavallai et al., 2012).

© 2013 Deb and Chusuei; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Deb and Chusuei; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Historically, the synthesis, fabrication and characterization of carbon nanomaterials have been carried out in vacuum environments. As these materials proliferate in use, knowledge pertaining to their environmental impact (i.e., involving fate and transport in aqueous sys‐ tems) becomes increasingly important (Cho et al., 2008). Furthermore, preparation and syn‐ thesis of these materials in non-vacuum conditions makes these processes more amenable for industrial scale up. Recent attention has focused on modifying SWNTs and MWNTs in solution media.

surface groups is available using this approach. Boehm titrations can be used to quantify the number of proton-containing functional groups (carboxylic acids, hydroxyl groups, lactones, etc.) on the CNT sidewall surface (Boehm et al., 1964). The titrant typically involves various bases ideal for each protic group, e.g., Na2CO3, NaOH, NaHCO3, etc.), the acidity constants (pKa) of which differ by orders of magnitude, rendering the analysis selective to the func‐ tional group of interest. But, this technique is ineffective for characterizing CNTs functional‐

ATR-IR, AFM, QCM and Raman spectroscopy can be performed in ambient environments (i.e., not requiring vacuum conditions for analysis). ATR-IR is useful for qualitative identifi‐ cation of CNT surface moieties; however, quantitation is not available and some modes are too small to be observed relative to background (Brundle et al., 1992). AFM offers the capa‐ bility of probing changes in CNT surface morphology, sidewall surface coverage and CNT lengths. However, the technique is not amenable to subnanometric scales as thermal noise becomes a major interference at this lengthscale (Magonov et al., 1996). QCM provides a means of monitoring mass changes during the assembly process as CNTs undergo function‐ alization, but accurate mass measurements are readily hampered by changes in temperature or cavitation (i.e., during ultrasonication) (Brown and Gallagher, 2007). The "diamond" D and G band shifts observed in Raman spectroscopy at ~1300 and ~1600 cm−1, respectively, is a useful tool for assessing the degree of sidewall surface damage encountered in some func‐ tionalization methods (i.e., ultrasonication) as well as CNT purity and composition. D and G

sheets, respectively, and are commonly used markers for elucidating covalent bond forma‐ tion (Dresselhaus et al., 2001). However, spectral interpretion, involving relative D and G

Vacuum-based characterization tools (the most cost-prohibitive class of these analytical methods), include electron spectroscopy, microscopy and mass analysis. XPS is an excellent tool for monitoring analyte surface oxidation states, and useful for elemental quantification and qualitative identification of surface functional groups. However, large amounts of sam‐ ple (~ 5 mg) are needed for analysis and peakfitted interpretation can be complex. TEM of‐ fers powerful imaging capabilities of the CNT sidewalls to allow for observation of surface roughening that can result from either functionalization or the creation of surface defects. Material length, diameter and dispersion state can also be readily determined by TEM. In addition, spatial elemental analysis is available via energy dispersive X-ray spectroscopy (EDX), as it is often an available technique built into many TEM instruments. However, CNTs are susceptible to beam damage from TEM electrons. Another caveat is that variation in technique involving dispersing samples onto TEM grids and subsequent drying can skew observed results. TGA and TPD can be used to quantify the concentration of moieties teth‐ ered to the CNT sidewalls; but, limiting case assumptions, e.g., all of the mass lost (TGA) and bonding modes remain unchanged (TPD), need to be made for assessments, which may not be accurate if the CNT surface chemistry is complex. In addition, large amounts of sam‐

Aqueous Solution Surface Chemistry of Carbon Nanotubes

http://dx.doi.org/10.5772/51869

277

ized with aprotic moieties.

bands emanate from disordered and ordered sp2

ple (> 10 mg) are required for TPD and TGA analysis.

band intensity determinations can be complex (Brundle et al., 1992).

A review of recent advancements to modify CNT surfaces in aqueous media is described in this chapter. Changes in the material properties are often observed concomitant to altera‐ tions in surface structure, such as colloidal dispersion and electrocatalytic activity. In the in‐ troductory section, the strengths and weaknesses of various traditional CNT surface chemistry probes are presented. Following this, nanotubes that have been chemically modi‐ fied via chemical oxidation and organic derivatization are discussed. The technique of elec‐ trochemical functionalization using carbon nanotubes as the working electrode surface is presented. The next section describes the applications of derivatized carbon nanotubes as it applies to catalysis (involving noble metal nanoparticles), sensing, and selective cancer cell destruction, in which the nanotube sidewall structure plays a key role. The use of transmis‐ sion electron microscopy (TEM) in conjunction with point-of-zero charge (PZC) measure‐ ments for exploring structure-property relationships is shown. The final sections present the effects of CNT functionalization on properties pertaining to colloidal stability and isoelectric points relevant for applications in environmental chemistry, catalyst synthesis, and design‐ ing materials for the remediation of contaminated ground water.

#### **2. Overview of analytical techniques: strengths and weaknesses**

Traditional analysis methods of carbon nanotubes include Boehm titrations, settling speed measurements, atomic force microscopy (AFM), and quartz crystal microbalance (QCM) measurements, X-ray photoelectron spectroscopy (XPS), attenuated total reflection infrared spectroscopy (ATR-IR), transmission electron microscopy (TEM), Raman spectroscopy, ther‐ mogravimetric analysis (TGA) and temperature programmed desorption (TPD). Each of these techniques has its own advantages in the chemical/structural information that they can provide as well as drawbacks.

Wet chemical characterization methods provide a rapid means of characterizing the CNT surface structure. Settling speed measurements is a crude, but rapid technique for measur‐ ing the extent of CNT sidewall oxidation containing protic groups with which the solvent can undergo hydrogen bonding (Xing et al., 2005). In this simple experimental setup, the rate at which CNTs fall in a buret (by gravity) is measured and correlated with the extent of surface functionalization. However, no qualitative information regarding the identity of the surface groups is available using this approach. Boehm titrations can be used to quantify the number of proton-containing functional groups (carboxylic acids, hydroxyl groups, lactones, etc.) on the CNT sidewall surface (Boehm et al., 1964). The titrant typically involves various bases ideal for each protic group, e.g., Na2CO3, NaOH, NaHCO3, etc.), the acidity constants (pKa) of which differ by orders of magnitude, rendering the analysis selective to the func‐ tional group of interest. But, this technique is ineffective for characterizing CNTs functional‐ ized with aprotic moieties.

Historically, the synthesis, fabrication and characterization of carbon nanomaterials have been carried out in vacuum environments. As these materials proliferate in use, knowledge pertaining to their environmental impact (i.e., involving fate and transport in aqueous sys‐ tems) becomes increasingly important (Cho et al., 2008). Furthermore, preparation and syn‐ thesis of these materials in non-vacuum conditions makes these processes more amenable for industrial scale up. Recent attention has focused on modifying SWNTs and MWNTs in

A review of recent advancements to modify CNT surfaces in aqueous media is described in this chapter. Changes in the material properties are often observed concomitant to altera‐ tions in surface structure, such as colloidal dispersion and electrocatalytic activity. In the in‐ troductory section, the strengths and weaknesses of various traditional CNT surface chemistry probes are presented. Following this, nanotubes that have been chemically modi‐ fied via chemical oxidation and organic derivatization are discussed. The technique of elec‐ trochemical functionalization using carbon nanotubes as the working electrode surface is presented. The next section describes the applications of derivatized carbon nanotubes as it applies to catalysis (involving noble metal nanoparticles), sensing, and selective cancer cell destruction, in which the nanotube sidewall structure plays a key role. The use of transmis‐ sion electron microscopy (TEM) in conjunction with point-of-zero charge (PZC) measure‐ ments for exploring structure-property relationships is shown. The final sections present the effects of CNT functionalization on properties pertaining to colloidal stability and isoelectric points relevant for applications in environmental chemistry, catalyst synthesis, and design‐

ing materials for the remediation of contaminated ground water.

**2. Overview of analytical techniques: strengths and weaknesses**

Traditional analysis methods of carbon nanotubes include Boehm titrations, settling speed measurements, atomic force microscopy (AFM), and quartz crystal microbalance (QCM) measurements, X-ray photoelectron spectroscopy (XPS), attenuated total reflection infrared spectroscopy (ATR-IR), transmission electron microscopy (TEM), Raman spectroscopy, ther‐ mogravimetric analysis (TGA) and temperature programmed desorption (TPD). Each of these techniques has its own advantages in the chemical/structural information that they can

Wet chemical characterization methods provide a rapid means of characterizing the CNT surface structure. Settling speed measurements is a crude, but rapid technique for measur‐ ing the extent of CNT sidewall oxidation containing protic groups with which the solvent can undergo hydrogen bonding (Xing et al., 2005). In this simple experimental setup, the rate at which CNTs fall in a buret (by gravity) is measured and correlated with the extent of surface functionalization. However, no qualitative information regarding the identity of the

solution media.

276 Physical and Chemical Properties of Carbon Nanotubes

provide as well as drawbacks.

ATR-IR, AFM, QCM and Raman spectroscopy can be performed in ambient environments (i.e., not requiring vacuum conditions for analysis). ATR-IR is useful for qualitative identifi‐ cation of CNT surface moieties; however, quantitation is not available and some modes are too small to be observed relative to background (Brundle et al., 1992). AFM offers the capa‐ bility of probing changes in CNT surface morphology, sidewall surface coverage and CNT lengths. However, the technique is not amenable to subnanometric scales as thermal noise becomes a major interference at this lengthscale (Magonov et al., 1996). QCM provides a means of monitoring mass changes during the assembly process as CNTs undergo function‐ alization, but accurate mass measurements are readily hampered by changes in temperature or cavitation (i.e., during ultrasonication) (Brown and Gallagher, 2007). The "diamond" D and G band shifts observed in Raman spectroscopy at ~1300 and ~1600 cm−1, respectively, is a useful tool for assessing the degree of sidewall surface damage encountered in some func‐ tionalization methods (i.e., ultrasonication) as well as CNT purity and composition. D and G bands emanate from disordered and ordered sp2 -hybridized carbon from the graphene sheets, respectively, and are commonly used markers for elucidating covalent bond forma‐ tion (Dresselhaus et al., 2001). However, spectral interpretion, involving relative D and G band intensity determinations can be complex (Brundle et al., 1992).

Vacuum-based characterization tools (the most cost-prohibitive class of these analytical methods), include electron spectroscopy, microscopy and mass analysis. XPS is an excellent tool for monitoring analyte surface oxidation states, and useful for elemental quantification and qualitative identification of surface functional groups. However, large amounts of sam‐ ple (~ 5 mg) are needed for analysis and peakfitted interpretation can be complex. TEM of‐ fers powerful imaging capabilities of the CNT sidewalls to allow for observation of surface roughening that can result from either functionalization or the creation of surface defects. Material length, diameter and dispersion state can also be readily determined by TEM. In addition, spatial elemental analysis is available via energy dispersive X-ray spectroscopy (EDX), as it is often an available technique built into many TEM instruments. However, CNTs are susceptible to beam damage from TEM electrons. Another caveat is that variation in technique involving dispersing samples onto TEM grids and subsequent drying can skew observed results. TGA and TPD can be used to quantify the concentration of moieties teth‐ ered to the CNT sidewalls; but, limiting case assumptions, e.g., all of the mass lost (TGA) and bonding modes remain unchanged (TPD), need to be made for assessments, which may not be accurate if the CNT surface chemistry is complex. In addition, large amounts of sam‐ ple (> 10 mg) are required for TPD and TGA analysis.

#### **3. Functionalizing carbon nanotubes**

Both single- and multiwalled carbon nanotubes have a tendency to aggregate into bundles very efficiently via van der Waals interactions in solution. These bundles can be exfoliated by using ultrasonication in combination with suitable surfactants. Typically, the outer walls of pristine carbon nanotubes are chemically inactive. Two major functionalization routes are used to activate CNT sidewalls: (i) endohedral and (ii) exohedral functionalization.

2005). Noncovalent exohedral functionalization, on the other hand, is achieved by wrapping nanotubes using polymers or surfactants (Hirsch, 2002). They leave the CNT carbon frame‐