**4. Purification of fabricated CNTs**

CNTs usually contain a large amount of impurities such as metal particles, amorphous car‐ bon, and multi shell. There are different steps in purification of CNTs. Purification of CNTs is a process that separates nanotubes from non-nanotube impurities included in the raw products, or from nanotubes with undesired numbers of walls. Purification has been an im‐ portant synthetic effort since the discovery of carbon nanotubes, and there are many publi‐ cations discussing different aspects of the purification process. Good review articles on the purification of CNTs are available in the recent literature [64, 65].

The current industrial methods applied oxidation and acid-refluxing techniques that affect the structure of tubes. Purification difficulties are great because of insolubility of CNT and the limitation of liquid chromatography.

CNT purification step (depending on the type of the purification) removes amorphous car‐ bon from CNTs, improves surface area, decomposes functional groups blocking the en‐ trance of the pores or induces additional functional groups.

Most of these techniques are combined with each other to improve the purification and to remove different impurities at the same time. These techniques are as follow:

#### **4.1. Oxidation**

Oxidation is a way to remove CNTs impurities. In this way CNTs and impurities are oxi‐ dized. The damage to CNTs is less than the damage to the impurities. This technique is more preferable with regard to the impurities that are commonly metal catalysts which act as oxidizing catalysts [66, 67].

Altogether, the efficiency and yield of the procedure are highly depending on a lot of factors, such as metal content, oxidation time, environment, oxidizing agent and tempera‐ ture [67].

#### **4.2. Acid treatment**

Refluxing the sample in acid is effective in reducing the amount of metal particles and amor‐ phous carbon. Different used acids are hydrochloric acid (HCl), nitric acid (HNO3) and sul‐ phuric acid (H2SO4), while HCl is identified to be the ideal refluxing acid. When a treatment in HNO3 had been used the acid had an effect on the metal catalyst only, and no effects was observed on the CNTs and the other carbon particles. [66-69]. Figure 14 shows the SEM im‐ ages of CNTs after and before purification stage with HCl [51].

**Figure 14.** The SEM images of CNTs (A) after (B) before purification stages with HCl [51].

High temperature has effect on the productions and paralizes the graphitic carbon and the short fullerenes. When high temperature is used, the metal will be melted and can also be

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**4.3. Annealing and thermal treatment**

removed [69].

If a treatment in HCl is used, the acid has also a little effect on the CNTs and other carbon particles [69, 70]. A review of literature demonstrates the effects that key variables like acid types and concentration & temperature have on the acid treatment [69, 70].

Fabrication, Purification and Characterization of Carbon Nanotubes: Arc-Discharge in Liquid Media (ADLM) http://dx.doi.org/10.5772/51116 67

**Figure 14.** The SEM images of CNTs (A) after (B) before purification stages with HCl [51].

#### **4.3. Annealing and thermal treatment**

**4. Purification of fabricated CNTs**

66 Syntheses and Applications of Carbon Nanotubes and Their Composites

the limitation of liquid chromatography.

**4.1. Oxidation**

ture [67].

**4.2. Acid treatment**

as oxidizing catalysts [66, 67].

purification of CNTs are available in the recent literature [64, 65].

trance of the pores or induces additional functional groups.

ages of CNTs after and before purification stage with HCl [51].

types and concentration & temperature have on the acid treatment [69, 70].

CNTs usually contain a large amount of impurities such as metal particles, amorphous car‐ bon, and multi shell. There are different steps in purification of CNTs. Purification of CNTs is a process that separates nanotubes from non-nanotube impurities included in the raw products, or from nanotubes with undesired numbers of walls. Purification has been an im‐ portant synthetic effort since the discovery of carbon nanotubes, and there are many publi‐ cations discussing different aspects of the purification process. Good review articles on the

The current industrial methods applied oxidation and acid-refluxing techniques that affect the structure of tubes. Purification difficulties are great because of insolubility of CNT and

CNT purification step (depending on the type of the purification) removes amorphous car‐ bon from CNTs, improves surface area, decomposes functional groups blocking the en‐

Most of these techniques are combined with each other to improve the purification and to

Oxidation is a way to remove CNTs impurities. In this way CNTs and impurities are oxi‐ dized. The damage to CNTs is less than the damage to the impurities. This technique is more preferable with regard to the impurities that are commonly metal catalysts which act

Altogether, the efficiency and yield of the procedure are highly depending on a lot of factors, such as metal content, oxidation time, environment, oxidizing agent and tempera‐

Refluxing the sample in acid is effective in reducing the amount of metal particles and amor‐ phous carbon. Different used acids are hydrochloric acid (HCl), nitric acid (HNO3) and sul‐ phuric acid (H2SO4), while HCl is identified to be the ideal refluxing acid. When a treatment in HNO3 had been used the acid had an effect on the metal catalyst only, and no effects was observed on the CNTs and the other carbon particles. [66-69]. Figure 14 shows the SEM im‐

If a treatment in HCl is used, the acid has also a little effect on the CNTs and other carbon particles [69, 70]. A review of literature demonstrates the effects that key variables like acid

remove different impurities at the same time. These techniques are as follow:

High temperature has effect on the productions and paralizes the graphitic carbon and the short fullerenes. When high temperature is used, the metal will be melted and can also be removed [69].

#### **4.4. Ultrasonication**

This technique is based on the separation of particles due to ultrasonic vibrations and also agglomerates of different nanoparticles will be more dispersed by this method. The separa‐ tion of the particles is highly dependable on the surfactant, solvent and reagents which are used [67-70].

When an acid is used, the purity of the CNTs depends on the sonication time. During the tubes vibration to the acid for a short time, only the metal is solvated, but in a more extend‐ ed period, the CNTs are also chemically cut. [69].

**Figure 15.** Electron micrographs of CNT (A) SEM of the CNT. (B) TEM of the CNT [57].

bands are modified according to the carbon forms [80].

This technique is used to obtain some information on the interlayer spacing, the structural strain and the impurities. However, in comparing CNTs with x-ray incident beam, CNTs

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Raman spectroscopy is one of the most powerful tools for characterization of CNTs. Without sample preparation, a fast and nondestructive analysis is possible. All allotropic forms of carbon are active in Raman spectroscopy [79]. The position, width, and relative intensity of

A Raman spectrum of a purified sample (after applying the purification procedure) is shown in figure 16. The peaks at 1380 cm–1 and 1572 cm–1 correspond to disorder (D-band) and graphite (G-band) bands, respectively. The former is an indication of the presence of de‐

**1.** Low-frequency peak <200 cm-1 characteristic of the SWNT, whose frequency is depend‐

**2.** D line mode (disorder line), which is a large structure assign of residual ill-organized

**3.** High-frequency bunch that is called G band and is a characteristic of CNTs. This bunch

Raman spectroscopy is considered an extremely powerful tool for characterizing CNT, which gives qualitative and quantitative information on its diameter, electronic struc‐ ture, purity and crystalline, and distinguishes metallic and semiconducting material as

has the ability to be superimposed with the G-line of residual graphite [81].

fective material and the latter one refers to the well-ordered graphite [62].

ed on the diameter of the tube mainly (RBM: radical breathing mode).

The most characteristic features are summarized as following:

have multiple orientations. This leads to a statistical characterization of CNTs [78].

**5.2. X-ray diffraction (XRD)**

**5.3. Raman spectroscopy**

graphite.

well as chirality.

#### **4.5. Micro-filtration**

Micro-filtration is based on particle size. Usually CNTs and a small amount of carbon nano‐ particles are trapped in a filter. The other nanoparticles (catalyst metal, fullerenes and car‐ bon nanoparticles) are passing through the filter [65, 69, 70, 72].

A special form of filtration is cross flow filtration. Through a bore of fiber, the filtrate is pumped down at head pressure from a reservoir and the major fraction of the fast flowing solution is reverted to the same reservoir in order to be cycled through the fiber again. A fast hydrodynamic flow down the fiber bore sweeps the membrane surface and prevents build‐ ing up of a filter cake [67].
