**2. Synthesis of carbon nanotube/catalyst composites**

There are two main steps for the synthesis of CNT/catalyst nanocomposites. The first step is the grafting of oxygen-containing groups on the surface of the nanotubes and the second step is the attachment of the metal oxides on the active surface of the nanotubes.

#### **2.1. Grafting of oxygen-containing groups on CNTs**

dustrial wastewaters. As the semiconductor is illuminated with photons having energy con‐ tent equal to or higher than the band gap, the photons excite valence band (VB) electrons across the band gap into the conduction band(CB), leaving holes behind in the valence band.Thus, there must be at least two reactions occurring simultaneously: oxidation from

(•OH). The generation of such radicals depends on the pH of the media. Targeted pollutants which are adsorbed on the surface of the catalyst will then be oxidized by •OH. On the other

(•OH) and can also react with O2 and trigger the formation of very reactive superoxide radi‐

The band gap is characteristic for the electronic structure of a semiconductor and is defined as the energy interval (ΔEg) between the VB and CB (Koci et al.,2011). VB is defined as the highest energy band in which all energy levels are occupied by electrons, whereas CB is the lowest energy band without electrons. The rate of a photo catalytic reaction depends on sev‐ eral parameters. First and most important is the type of the photo catalytic semiconductor. The second factor is the light radiation used or the stream of photons, as over supply of light accelerates electron–hole recombination (Koci et al.,2008). Third factor is pH of the medium with which the semiconductor surface is in contact with the targeted molecules. Fourth fac‐ tor is the concentration of the substrate influencing the reaction kinetics. Fifth parameter is the temperature of the media where higher temperatures cause frequent collision between

The degradation rate can be enhanced by reducing the electron-hole recombination rate; preventing the particles agglomeration; and increasing the adsorption capacity, as it is a key process in the photocatalysis. In order to improve the photocatalytic efficiency, several

**2.** generation of defect structures to induce space-charge separation and thus reduce the

**4.** adding a co-sorbent such as silica, alumina, zeolite or clay (Yu et al. 2002; Rusu and

CNTs based composites have attracted considerable attentions due to the intrinsic proper‐ ties that have been created owing to the addition of CNTs into the composite. Functionaliza‐ tion of CNTs, or attachment of individual atoms, molecules or their aggregates to CNTs, further extend the field of application of these nanosystems in different fields like in photo‐ catalysis process (Dresselhaus & Dresselhaus, 2001; Burghard, 2005; Saleh, 2011). CNT/Metal oxide composites have been recently reported to be used for the treatment of contaminated water. In this chapter, therefore, the application of CNTs to enhance the photocatalytic activ‐

**1.** increasing the surface area of the metal oxide by synthesizing nano-size materials;

**3.** modification of the semiconductors with metal or other semiconductor; and

) to the conduction band (CB) can generate hydroxyl radical

) producing hydroxyl radicals

photogenerated holes, and reduction from photogenerated electrons.

The holes react with water molecules or hydroxide ions (OH¯

•) that can oxidize the target.

480 Syntheses and Applications of Carbon Nanotubes and Their Composites

the semiconductor and the substrate (Koci et al.,2010).

methods have been investigated. This includes:

ity of TiO2, ZnO and WO3 will be discussed.

recombination;

Yates, 1997)

hand, the excited electrons (e-

cal ion (O2¯

Grafting of oxygen-containing groups on the surface of the nanotubes or activation of CNTs can be achieved by oxidation treatment. It can be performed using oxidizing agents such as nitric acid, sulfuric acid, or a mixture of both. For example, oxygen-containing groups can be grafted on the surface of the nanotubes by the following procedure. Initially, CNTs are dis‐ persed by sonication in concentrated acidic media. Then, the mixture is treated by reflux while stirring vigorously at temperature of 100-120°C. After refluxing process, the mixture is allowed to cool at room temperature. The oxidized CNTs are purified by extraction from the residual acids by repeated cycles of dilution with distilled water, centrifugation and decant‐ ing the solutions until the pH is approximately 5-6. After the purification process, the oxi‐ dized CNTs are dried overnight in an oven at 100°C. After that, the dry oxidized CNTs are pulverized in a ball-mill.

The presence of oxygen containing groups on the surface of the oxidized nanotubes are characterized by the means of Fourier transform infrared spectroscopy (FT-IR), X-ray pow‐ der diffraction (XRD), field emission scanning electron microscopy (FESEM ) and the trans‐ mission electron microscopy (TEM).

As an example, IR spectra, in the range of 400-4000 cm-1, were recorded in KBr pellets using a Thermo Nicolet FT-IR spectrophotometer at room temperature. Samples were prepared by gently mixing 10 mg of each sample with 300 mg of KBr powder and compressed into discs at a force of 17 kN for 5 min using a manual tablet presser. Figure 1 depicts IR spectrum of oxidized MWCNTs. In the spectrum, a characteristic peak at 1580 cm-1 can be assigned to C=C bond in MWCNTs. The band at about 1160cm−1 is assigned to C–C bonds. Also, the spectrum shows the carbonyl characteristic peak at 1650 cm-1, which is assigned to the car‐ bonyl group from quinine or ring structure. More characteristic peak to the carboxylic group is the peak at 1720 cm-1 (Ros et al., 2002; Yang et al., 2005; Xia et al., 2007). The observation of IR spectra corresponding to the oxidized MWCNTs suggests the presence of carboxylic and hydroxylic groups on the nanotube surface.

Figure 2 depicts the typical XRD pattern of the oxidized MWCNTs. The strongest diffraction peak at the angle (2θ) of 25.5° can be indexed as the C(002) reflection of the hexagonal graphite structure (Rosca et al., 2005; Saleh et al., 2011; Lu et al., 2008). The sharpness of the peak at the angle (2θ) of 25.5° indicates that the graphite structure of the MWCNTs was acid-oxidized without significant damage since any decrease in the order of crystallinity in MWCNTs will make the XRD peaks broader and shift the peak diffraction towards lower angles. The other characteristic diffraction peaks of graphite at 2θ of about 43°, 53° and 77° are associated with C(100), C(004) and C(110) diffractions of graphite, respectively.

Energy dispersive X-ray spectroscope (EDX) measurement is also used as a quantitative analysis for the presence of the oxygen containing groups on the surface of the nanotubes. Figure 4 represents the results of the oxidized MWCNTs. The results shows the presence of oxygen in the sample in addition to carbon element. SEM and TEM are used to characterize the morphology of the nanotube and to ensure that the structure of the nanotube has not been destroyed by the acid treatment. As an example, SEM image and the inset TEM image in Figure 3 confirm that there is no damage effect on the nanotubes using mixtures of nitric acid sulfuric acid for the treatment of the nanotubes.

**Figure 3.** Field emission scanning electron microscopy (FESEM ) image of the MWCNTs oxidized with H2SO4/HNO3

The Role of Carbon Nanotubes in Enhancement of Photocatalysis

http://dx.doi.org/10.5772/51050

483

**Figure 4.** EDX spectrum of the MWCNTs oxidized with H2SO4/HNO3 mixture for 6h at 100°C; inset is the table showing

CNT/metal oxide nanocomposites can be synthesized by different methods which fall into two basic classes. The first class involves the prior synthesis of nanoparticles that subse‐ quently connected to surface functionalized CNTs by either covalent or noncovalent interac‐ tions (Eder, 2010; Peng et al., 2010; Hu et al., 2010). The second class is the one step method which involves direct deposition of nanoparticles onto MWCNT surface, in situ formation of nanoparticles through redox reactions or electrochemical deposition on CNTs (Chen et al., 2006; Gavalas et al., 2001; Yang et al., 2010; Sahoo et al., 2001; Lee et al., 2008). The second class has the advantages where uniform nanomaterials can be prepared due to the presence

As an example, CNT/ZnO nanocomposites are prepared by the following procedure (Saleh et al., 2010). Zinc precursor like Zn(NO3)2.6H2O, is dissolved in doubly deionized water. Then,

mixture for 6h at 100°C; Inset is the transmission electron microscopy (TEM) image of the same.

the percentage of each component in the nanotubes.

of active sites on oxidized CNT surfaces.

**2.2. Synthesis of CNT/catalysts nanocomposites**

**Figure 1.** FTIR spectrum of MWCNT oxidized with H2SO4/HNO3 mixture for 6h at 100°C.

**Figure 2.** XRD patterns of MWCNT oxidized with H2SO4/HNO3 mixture for 6h at 100°C.

**Figure 3.** Field emission scanning electron microscopy (FESEM ) image of the MWCNTs oxidized with H2SO4/HNO3 mixture for 6h at 100°C; Inset is the transmission electron microscopy (TEM) image of the same.

**Figure 4.** EDX spectrum of the MWCNTs oxidized with H2SO4/HNO3 mixture for 6h at 100°C; inset is the table showing the percentage of each component in the nanotubes.

#### **2.2. Synthesis of CNT/catalysts nanocomposites**

MWCNTs will make the XRD peaks broader and shift the peak diffraction towards lower angles. The other characteristic diffraction peaks of graphite at 2θ of about 43°, 53° and 77°

Energy dispersive X-ray spectroscope (EDX) measurement is also used as a quantitative analysis for the presence of the oxygen containing groups on the surface of the nanotubes. Figure 4 represents the results of the oxidized MWCNTs. The results shows the presence of oxygen in the sample in addition to carbon element. SEM and TEM are used to characterize the morphology of the nanotube and to ensure that the structure of the nanotube has not been destroyed by the acid treatment. As an example, SEM image and the inset TEM image in Figure 3 confirm that there is no damage effect on the nanotubes using mixtures of nitric

are associated with C(100), C(004) and C(110) diffractions of graphite, respectively.

acid sulfuric acid for the treatment of the nanotubes.

482 Syntheses and Applications of Carbon Nanotubes and Their Composites

**Figure 1.** FTIR spectrum of MWCNT oxidized with H2SO4/HNO3 mixture for 6h at 100°C.

**Figure 2.** XRD patterns of MWCNT oxidized with H2SO4/HNO3 mixture for 6h at 100°C.

CNT/metal oxide nanocomposites can be synthesized by different methods which fall into two basic classes. The first class involves the prior synthesis of nanoparticles that subse‐ quently connected to surface functionalized CNTs by either covalent or noncovalent interac‐ tions (Eder, 2010; Peng et al., 2010; Hu et al., 2010). The second class is the one step method which involves direct deposition of nanoparticles onto MWCNT surface, in situ formation of nanoparticles through redox reactions or electrochemical deposition on CNTs (Chen et al., 2006; Gavalas et al., 2001; Yang et al., 2010; Sahoo et al., 2001; Lee et al., 2008). The second class has the advantages where uniform nanomaterials can be prepared due to the presence of active sites on oxidized CNT surfaces.

As an example, CNT/ZnO nanocomposites are prepared by the following procedure (Saleh et al., 2010). Zinc precursor like Zn(NO3)2.6H2O, is dissolved in doubly deionized water. Then, ammonia is added drop-wise under continuous stirring into the solution to form a clear sol‐ ution. The oxidized MWCNTs is added into the solution. The mixture is refluxed at 100°C. The composite are separated and dried at 80°C prior to the calcination in vacuum at 300°C.

**3. Applications of CNT/Catalyst nanocomposites**

from metal oxides, thereby hindering charge recombination.

composite (Yu et al., 2005a,b).

gap of TiO2.

CNTs are considered to be good support material for catalysts, because they provide large surface area support and also stabilize charge separation by trapping electrons transferred

The Role of Carbon Nanotubes in Enhancement of Photocatalysis

http://dx.doi.org/10.5772/51050

485

A significant number of papers have been published on the application of CNTs in conjunc‐ tion with TiO2, reflecting the focus of recent research (Jitianu et al., 2004; Huang and Gao, 2003; Woan et al., 2009; Feng et al., 2005). One of the most important applications of such composite is to act as photocatalyst for some chemical reactions, especially for the decon‐ tamination of organic pollutants in waste waters. The photocatalytic activity of MWCNT/TiO2 composite toward the degradation of acetone under irradiation of UV light was investigated by the detection of the hydroxyl radical (•OH) signals using electron para‐ magnetic resonance. It has been reported that the agglomerated morphology and the parti‐ cle size of TiO2 in the composites change in the presence of CNTs, which provide a large surface area resulting in more hydroxyl group on the surface of the composite with no effect on the mesoporous nature of the TiO2. The composite have been reported to be of higher photocatalytic activity than commercial photocatalyst (P25) and TiO2/activated carbon (AC)

The photocatalytic activities of MWCNT/TiO2under visible light have also been reported us‐ ing the decolorization of dyes like methylene blue, methyl orange, azo dye and other dyes in model aqueous solutions (Cong et al., 2011; Gao et al., 2009; Hu et al., 2007; Saleh and Gupta, 2012; Yu et al., 2005; Kuo, 2009). TiO2 loading of 12% was found to result in the highest pho‐ toactivity in comparison with 6% and 15% loadings. Little TiO2 or excessive nanotubes addi‐ tion shielded the TiO2 and reduced the UV intensity, due to photon scattering by the nanotubes. However, a high TiO2 content was found to be ineffective in suppressing exciton recombination because of the large distance between the titania and the nanotubes (Li et al., 2012). Optimum ratio of titania and nanotubes provides a large surface area support and stabilize charge separation by trapping electrons transferred from TiO2, thereby hindering charge recombination with minimum photon scattering. The composite provides high sur‐ face area which is beneficial for photocatalytic activity, as it provides high concentration of

Also, the activity of MWCNT/TiO2 composites has been investigated in photodegradation of phenol and photocatalytic oxidation of methanol under irradiation of visible light (Wang et al., 2005; An et al., 2007; Yao et al., 2008; Dechakiatkrai et al., 2007). The catalysts exhibited enhanced photocatalytic activity for degradation of toluene in gas phase under both visible and simulated solar light irradiation compared with that of commercial Degussa P25 (Wu et al., 2009). It exhibited high activity for the photoreduction of Cr(VI) in water (Xu et al., 2008). Its efficiency was higher compared to a mechanical mixture of TiO2 and MWCNTs. A proba‐ ble synergistic effect of TiO2 and MWCNTs in a composite MWCNT/TiO2 on the enhance‐ ment of visible light performance, have been proposed where MWCNTs act as support, absorbent, photo-generated transfer station and carbon-doping source to narrow the band

target organic substances around sites activated by ultraviolet (UV) radiation.

Different techniques can be applied for the characterization of the nanocomposite. For exam‐ ple XRD is employed to determine crystalline phases and average crystalline size. FT-IR is used for qualitative analysis of the binding of the metal oxide into the nanotube surface. The morphology of the nanotubes and particle size are examined by the field emission scanning electron microscope (FESEM) and high resolution transmission electron microscopy (HRTEM). EDX measurement is also used as a quantitative analysis for the presence of the oxygen containing groups on the surface of the nanotubes. As an example, Figure 5 depicts the EDX data of the CNT/ZnO nanocomposite. The table shows the percentage of each com‐ ponent in the composite. Figure 6, SEM image and the inset HRSEM image, confirm the presence of zinc oxide particles on the surface of the nanotubes.

**Figure 5.** EDX spectrum of the MWCNT/ZnOnanocomposites; inset is the table showing the percentage of each com‐ ponent in the nanotubes.

**Figure 6.** Field emission scanning electron microscopy (FESEM) image of the MWCNT/ZnO; Inset is the HRSEM image.
