**1. Introduction**

The chemical, physical and mechanical properties of carbon nanotubes (CNTs) have stimu‐ lated extensive investigation since their discovery in the early 1990s (Iijima, 1991). CNTs, which are considered quasi-one dimensional nanostructures, are graphite sheets rolled up into cylinders with diameters of the order of a few nanometers and up to some millimeters in length. Types of nanotubes are the single-walled nanotubes (SWCNTs), double-walled nanotubes (DWCNTs) and the multi-walled nanotubes (MWCNTs). The MWCNTs consist of multiple layers of graphite arranged in concentric cylinders.

During the early stage, the primary research interests include the synthesis or growth of CNTs to prepare enough amounts of CNTs with desired dimension and purity. Several methods like arc discharge, laser ablation of graphite, the more productive chemical vapor deposition (CVD) and plasma enhanced CVD method, have been used to prepare high puri‐ ty CNTs with controllable wall-thickness and length and acceptable price (Meyyappan, 2004). CNTs attract considerable attention due to their special structure and high mechanical strength which makes them to be good candidates for advanced composites. They can be ei‐ ther semiconducting, semimetallic or metallic, depending on the helicity and the diameter of the tube (Ebbesen et al., 1996; Yang et al., 2003). Based on the structure and shape, CNTs conduct electricity due to delocalization of the pi bond electrons. On the other side, re‐ searchers found that CNTs are efficient adsorbents due to their large specific surface area, hollow and layered structures and the presence of pi bond electrons on the surface. Besides that, more active sites can be created on the nanotubes. Thus, CNTs can be used as a promis‐ ing material in environmental cleaning.

Photocatalytic oxidation using a semiconductor such as TiO2, ZnO and WO3 as photocatalyst is one of the advanced oxidation processes used for degradation of various pollutants in in‐

© 2013 Saleh; 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 Saleh; 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.

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 photogenerated holes, and reduction from photogenerated electrons.

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

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

pulverized in a ball-mill.

mission electron microscopy (TEM).

hydroxylic groups on the nanotube surface.

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

The Role of Carbon Nanotubes in Enhancement of Photocatalysis

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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

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‐

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

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

step is the attachment of the metal oxides on the active surface of the nanotubes.

The holes react with water molecules or hydroxide ions (OH¯ ) producing hydroxyl radicals (•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 hand, the excited electrons (e- ) to the conduction band (CB) can generate hydroxyl radical (•OH) and can also react with O2 and trigger the formation of very reactive superoxide radi‐ cal ion (O2¯ •) that can oxidize the target.

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

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 methods have been investigated. This includes:


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‐ ity of TiO2, ZnO and WO3 will be discussed.
