A Comprehensive Review on Separation Methods and Techniques for Single-Walled Carbon Nanotubes

Structural control of single-walled carbon nanotubes (SWNTs) is attracting enormous interest in view of their applications to nanoelectronics and nanooptics. Actually, more than 200 papers regarding separation of SWNTs have been published since 1998. In this review, they are classified into the following five sections according to the separation methods; electrophoresis, centrifugation, chromatography, selective solubilization and selective reaction. In each method, all literature is summarized in tables showing the separated objects (metallic/semiconducting (M/S), length, diameter, (n, m) structure and/or handedness), the production process of the used SWNTs (CoMoCAT, HiPco, arc discharge and/or laser vaporization) and the employed chemicals, such as detergents and polymers. Changes in annual number of publications related to this subject are also discussed.


Introduction
Structural control of single-walled carbon nanotubes (SWNTs) is attracting enormous interest because the physical properties are closely correlated to their structures and the application frequently requires homogeneous properties. In this context, extensive investigations have been made in pursuit of selective synthesis and separation of SWNTs with specific structural properties. As for the OPEN ACCESS synthesis, limited (n, m) structures of SWNTs were synthesized using a silica-supported Co-Mo catalyst [1,2] and the catalyst system of Co-incorporated mobile composition of matter 41 (Co-MCM-41) [3,4]. Great progress has been made quite recently on the synthesis using non-metal catalysts [5][6][7][8][9], and selective synthesis of metallic and semiconducting SWNTs [10,11].
On the other hand, separation of SWNTs is an alternative way to obtain SWNTs with specific electrical and structural properties, and a wide variety of methods have been devised so far for the separation according to metallic and semiconducting (M/S) property, diameter, length, roll-up index ((n, m) structure) and handedness (M or P structure defined in Figures 2 and 3). Herein, papers related with CNT separation are categorized and tabulated according to the five methods based on electrophoresis, centrifugation, chromatography, selective solubilization and selective reaction. As for solubilization of carbon nanotubes (CNTs), there have been a number of reviews [12][13][14][15][16][17]. Several excellent reviews have also been published on separation of CNTs [18][19][20][21][22][23][24]. Therefore, the authors mainly focus on the table summary of the papers reported so far and make a brief overview in each section. On the basis of the data summarized in the tables, we discuss the changes in the annual number of publications in the last part of this review.

Terminology of SWNTs
Before going into the main subject of this review, the terminology of SWNTs is briefly introduced in this section. The properties of SWNTs are determined by the alignment of the hexagons as shown in Figure 1. For their electronic properties, armchair SWNTs have metallic property, and zigzag and chiral ones are either metallic or semiconducting, depending on the roll-up index (see Figure 2). While armchair and zigzag are achiral, a chiral SWNT has a pair of helical isomers as shown in Figures 1 and 2. Optically active SWNTs have been found by enrichment of either left-or right-handed structures [25][26][27][28][29][30].  Since a SWNT can be prepared by rolling up a graphene into a seamless cylinder, the structure is defined by a roll-up vector C h defined by two unit vectors a 1 and a 2 ; C h = na 1 + ma 2 , where n and m are integers and designated as the roll-up index (n, m) as shown in Figure 2 [31,32]. The (n, m) and C h have been referred to as -chiral‖ index (or simply -chirality‖) and -chiral‖ vector, respectively. However, the meaning of the -chiral‖ is not consistent with the original meaning defined by the International Union of Pure and Applied Chemistry (IUPAC); that is, -the geometric property of a rigid object of being non-superposable on its mirror image‖ [33]. Since this terminology is confusing, as Strano pointed out in the article [34], an explicit nomenclature is required to define the structures of CNTs. In this review, the terms of -chiral‖ and -chirality‖ are only used according to the definition of IUPAC mentioned above, and the term of (n, m) is used for describing the roll-up index of SWNTs.
The handedness of chiral SWNTs is defined as M and P as follows. Every SWNT has three zigzag lines (Z lines) as indicated as solid arrows in Figure 3. These Z lines cannot be superposed on their mirror-image in the case of chiral SWNTs, while they can be superposed in zigzag and armchair types. When two of the three Z lines are rotated to the left and the third Z line to the right, the chiral SWNT is designated as M as shown in Figure 3a. Similarly, the chiral SWNT with two Z lines rotated to the right is P as shown in Figure 3b. This terminology is based on ZR and ZL in the literature [31]. In this review, we propose M and P stereodescriptors according to the IUPAC nomenclature.

Centrifugation
This process is very powerful and versatile to separate SWNTs according to their electrical and structural properties such as M/S, diameter, length, (n, m) and handedness [63]. In 2005, Arnold and Hersam reported the first example of density gradient ultracentrifugation (DGU) for separation of the diameter of SWNTs [64]. They successfully applied the process to the separation of M/S electrical property, (n, m) structures and even handedness [25,65]. The methodology was extended to (n, m) separation and enantiomer sorting of HiPco SWNTs [30]. The papers reported so far are summarized in Table 2. SDS and/or SC have been commonly used as detergents to dissolve SWNTs. In addition, iodixanol is required as a gradient-induced agent to the aqueous medium. Organic system was employed in only one example in the presence of polymer and heavily halogenated benzene in organic solvent [66]. Quite recently, facile M/S separation has been realized by use of SDS and agarose gel [53,67]. Table 2. Separation of SWNTs by density gradient ultracentrifugation (DGU).

Chromatography
Chromatography was first employed for separation of CNTs by Duesberg and coworkers in 1998. In addition to removal of metallic impurities and amorphous carbon, length separation of SWNTs as well as MWNTs was accomplished by size exclusion chromatography (SEC) after dissolvation of CNTs in the presence of SDS [91][92][93]. In 2003, Zheng and coworkers separated DNA-wrapped SWNTs by ion-exchange chromatography (IEC) to realize M/S, length and diameter separations [94,95]. Quite recently, the IEC separation was extended to strict (n, m) separation by applying appropriate sequences of DNA, providing many kinds of pure (n, m) of SWNTs [96]. The separations of length and M/S by field flow fractionation (FFF) and optical trapping, respectively, are incorporated in this section, as shown in Table 3 [97][98][99][100][101][102][103]. Table 3. Separation of SWNTs by chromatography.

Concluding Remarks
Separation of SWNTs has been described in this review. The related papers reported so far are classified into the following five methods; electrophoresis (Chapter 3.1), centrifugation (3.2), chromatography (4.1), selective solubilization (4.2) and selective reaction (4.3). All the data, summarized in Tables 1-5, will be analyzed on the basis of  Changes in the annual number of publications from 1998 to 2009 are shown in Figure 4. The first few reports, published in 1998-1999, dealt with length separation of CNTs by chromatography and the related technique. CNTs were solubilized by use of sonication in the presence of surfactant in 1997 [225,226], making the SEC [91][92][93] and FFF [97] separations possible. The first papers on selective solubilization by polymer wrapping [134] and electrophoresis of polymer-wrapped SWNTs [45] were published in 2000 and 2001, respectively. The annual number of publications is less than 10 until 2002. During this period, about 60% of the papers focus on length separation, while investigations started on diameter and M/S separations ( Figure 5). Noteworthy is that the first M/S separation was accomplished by electrical breakdown in 2001 [209,210]. In 2003, the number of papers suddenly leapt to more than 15 papers (Figure 4). This is probably because individualization of SWNTs in aqueous solution and the spectroscopic analysis were accomplished in 2002 by Smalley and Weisman [122,227]. The findings greatly facilitate more precise evaluation of the distribution of diameters and (n, m) structures of the SWNTs before and after separation. Novel methods of selective reaction of carbenes and diazonium salts with metallic SWNTs were devised in 2003 [188][189][190]194], also increasing the number of publications in 2003 ( Figure 4). Especially, publications related to M/S and diameter separations increased remarkably in 2003, as shown in Figure 5. In 2005 and 2006, the first papers of DGU were published by Hersam and coworkers [64,65], causing a sudden increase of the number of papers to more than 25 in 2007 ( Figure 4). Fluorene-based polymers also found to extract the specific structure in 2007 [132]. These novel methodologies realized precise (n, m) separation and enhanced the number of publications related to the (n, m) separation shown in Figure 5. In the same year, optically active SWNTs were obtained for the first time by separating the handedness of chiral nanotubes [29]. Conclusively, the changes in annual number of publications shown in    . Rate in targeted object of SWNTs to be separated in the following five methods; electrophoresis (3-1), centrifugation (3-2), chromatography (4-1), selective solubilization (4-2) and selective reaction (4-3). Figure 6 summarizes the rates of the separated objects in SWNTs in each of the five separation methods; electrophoresis, centrifugation, chromatography, selective solubilization and selective reaction. The rate is different among these methods as shown in the figure; for example, electrophoresis and selective reaction have been employed mainly for M/S separation, while length separation has been carried out mostly by chromatography. Centrifugation has been used for separating SWNTs according to a variety of structural and electrical features including (n, m) and even handedness. However, the method is not considered to be suitable for large-scale separation. For M/S separation, scalable methods have been reported recently by Kataura and coworkers [140]. A more facile method to obtain specific (n, m) in large quantities is awaited in view of electrical and optical applications of SWNTs.