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

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.

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

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

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

presence of zinc oxide particles on the surface of the nanotubes.

484 Syntheses and Applications of Carbon Nanotubes and Their Composites

ponent in the nanotubes.

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 from metal oxides, thereby hindering charge recombination.

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

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

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 gap of TiO2.

The composite has been reported for photoinactivation of E. coli in visible light irradiation (Akhavan et al., 2009). The efficiency of the nanocomposite was high toward photocatalytic hydrogen generation and for the reduction of CO2 with H2O (Dai et al., 2009; Xia et al., 2007).

charge vacancy or holes (h+

+

), in the VB. Some of the charges quickly recombine without creat‐

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

487

ing efficient photodecomposition of the pollutant. In the case where the composite is ap‐ plied, the strong interaction between the nanotube and the metal oxide results in a close contact to form a barrier junction which offers an effective route of reducing electron-hole re‐ combination by improving the injection of electrons into the nanotube. Therefore, CNTs acts as a photo-generated electron acceptor to promote interfacial electron transfer process since CNTs are relatively good electron acceptor while the semiconductor is an electron donor un‐ der irradiation (Saleh and Gupta, 2011; Riggs et al., 2000; Subramanian et al., 2004; Geng et al., 2008). The adsorbed oxygen molecules on the nanotubes react with the electrons forming very reactive superoxide radical ion (O2•-) which oxidize the target. On the other side, the hole (h

) oxidize hydroxyl groups to form hydroxyl radical (•OH) which can decompose the target.

**Figure 7.** Schematic diagram of the proposed mechanism of photodegradation over CNT/MO composite.

**•** Stronger adsorption on photocatalyst for the targeted moleculs of pollutant is achieved by the incorporation of the nanotubes, due to their large specific surface area and high quali‐

**•** The nanotubes can act as effective electron transfer unitbecause of their high electrical

**•** The nanotubes manifest higher capture electron ability and can prompt electron transfer from the conduction band of the metal oxide or semiconductor nanoparticles (NP) to‐

**•** Schottky barrier forms at the interface between the CNTs and the semiconductor. The photo-generated electrons may move freely towards the CNT surface, thus the left holes

wards the nanotube surface due to their lower Fermi level (Cong et al., 2011).

may move to the valence band (Woan et al., 2009; Chen et al., 2005).

Some important points of such process can be highlighted as:

conductivity and high electron storage capacity.

ty active sites.

Zinc oxide, a direct wide band gap (3.37 eV) semiconductor with a large excitation binding energy (60 meV), has been investigated as a potential non-toxic photocatalyst used to suc‐ cessfully degrade organic pollutants. Recently, ZnO nanoparticles have received much at‐ tention due to its high photoactivity in several photochemical, UV light response, photoelectron-chemical processes and its low cost production possibility (Wu et al., 2008; Neudeck et al., 2011; Gondal et al., 2010; Drmosh et al., 2010).

Experimental results proved that CNT/ ZnO nanocomposites display relatively higher pho‐ tocatalytic activity than ZnO nanoparticles for the degradation of some dyes like rhodamine B, azo-dyes, methylene blue, methylene orange (Dai et al., 2012; Zhu et al., 2009). The com‐ plete removal of azo-dyes such as acid orange, acid bright red, acid light yellow, after selec‐ tion of optimum operation parameters such as the illumination intensity, catalyst amount, initial dye concentration and the different structures of the dye on the photocatalytic proc‐ ess, can be achieved in relatively short time by using CNT/ZnO composites.

The MWCNT/ZnO nanocomposites exhibits excellent photocatalytic activity toward other pollutants such as acetaldehyde and cyanide in model solutions (Saleh et al., 2011; Saleh et al., 2010). CNTs act as a photogenerated electron acceptor and retard the recombination of photoinduced electron and hole. The adsorption and photocatalytic activity tests indicate that the CNTs serve as both an adsorbent and a visible light photocatalyst. The experimental results show that the photocatalytic activity of the ZnO/MWCNTs nanocomposites strongly depends on the synthetic route, which is probably due to the difference of surface states re‐ sulted from the different preparation processes (Zhang, 2006; Kim and Sigmund, 2002; Jiang and Gao, 2005;Agnihotri et al., 2006).

CNT/WO3 nanocomposites have been synthesized via different routs (Pietruszka et al., 2005; Wang et al., 2008; Saleh and Gupta, 2011). The utilization of carbon nanotubes to enhance photocatalytic activity of tungsten trioxide has also been investigated. The photocatalytic ac‐ tivities are greatly improved when CNT/WO3 nanocomposite has been used for the degra‐ dation of pollutants such as rhodamine B under ultraviolet lamp or under sunlight. The results showed that photocatalytic activity of the MWCNT/WO3 composites prepared by chemical process is higher than that prepared by mechanical mixing. The photocatalytic ac‐ tivity is enhanced when WO3 nanoparticles are loaded on the surface of CNTs. The en‐ hanced photocatalytic activity may be ascribed to the effective electron transfer between the nanotubes and the metal nanoparticles.

A possible synergistic effect between the semiconductor nanoparticles and CNTs on the en‐ hancement of photocatalytic activity is proposed in Figure 7. The mechanism is based on the results of the structure characterizations and the enhancement in photocatalytic activity of the prepared composite.

When the catalyst is irradiated by photons, electrons (e- ) are excited from the valence band (VB) to the conduction band (CB) of catalysts or the metal oxide nanoparticles (NP) creating a charge vacancy or holes (h+ ), in the VB. Some of the charges quickly recombine without creat‐ ing efficient photodecomposition of the pollutant. In the case where the composite is ap‐ plied, the strong interaction between the nanotube and the metal oxide results in a close contact to form a barrier junction which offers an effective route of reducing electron-hole re‐ combination by improving the injection of electrons into the nanotube. Therefore, CNTs acts as a photo-generated electron acceptor to promote interfacial electron transfer process since CNTs are relatively good electron acceptor while the semiconductor is an electron donor un‐ der irradiation (Saleh and Gupta, 2011; Riggs et al., 2000; Subramanian et al., 2004; Geng et al., 2008). The adsorbed oxygen molecules on the nanotubes react with the electrons forming very reactive superoxide radical ion (O2•-) which oxidize the target. On the other side, the hole (h + ) oxidize hydroxyl groups to form hydroxyl radical (•OH) which can decompose the target.

**Figure 7.** Schematic diagram of the proposed mechanism of photodegradation over CNT/MO composite.

Some important points of such process can be highlighted as:

The composite has been reported for photoinactivation of E. coli in visible light irradiation (Akhavan et al., 2009). The efficiency of the nanocomposite was high toward photocatalytic hydrogen generation and for the reduction of CO2 with H2O (Dai et al., 2009; Xia et al., 2007). Zinc oxide, a direct wide band gap (3.37 eV) semiconductor with a large excitation binding energy (60 meV), has been investigated as a potential non-toxic photocatalyst used to suc‐ cessfully degrade organic pollutants. Recently, ZnO nanoparticles have received much at‐ tention due to its high photoactivity in several photochemical, UV light response, photoelectron-chemical processes and its low cost production possibility (Wu et al., 2008;

Experimental results proved that CNT/ ZnO nanocomposites display relatively higher pho‐ tocatalytic activity than ZnO nanoparticles for the degradation of some dyes like rhodamine B, azo-dyes, methylene blue, methylene orange (Dai et al., 2012; Zhu et al., 2009). The com‐ plete removal of azo-dyes such as acid orange, acid bright red, acid light yellow, after selec‐ tion of optimum operation parameters such as the illumination intensity, catalyst amount, initial dye concentration and the different structures of the dye on the photocatalytic proc‐

The MWCNT/ZnO nanocomposites exhibits excellent photocatalytic activity toward other pollutants such as acetaldehyde and cyanide in model solutions (Saleh et al., 2011; Saleh et al., 2010). CNTs act as a photogenerated electron acceptor and retard the recombination of photoinduced electron and hole. The adsorption and photocatalytic activity tests indicate that the CNTs serve as both an adsorbent and a visible light photocatalyst. The experimental results show that the photocatalytic activity of the ZnO/MWCNTs nanocomposites strongly depends on the synthetic route, which is probably due to the difference of surface states re‐ sulted from the different preparation processes (Zhang, 2006; Kim and Sigmund, 2002; Jiang

CNT/WO3 nanocomposites have been synthesized via different routs (Pietruszka et al., 2005; Wang et al., 2008; Saleh and Gupta, 2011). The utilization of carbon nanotubes to enhance photocatalytic activity of tungsten trioxide has also been investigated. The photocatalytic ac‐ tivities are greatly improved when CNT/WO3 nanocomposite has been used for the degra‐ dation of pollutants such as rhodamine B under ultraviolet lamp or under sunlight. The results showed that photocatalytic activity of the MWCNT/WO3 composites prepared by chemical process is higher than that prepared by mechanical mixing. The photocatalytic ac‐ tivity is enhanced when WO3 nanoparticles are loaded on the surface of CNTs. The en‐ hanced photocatalytic activity may be ascribed to the effective electron transfer between the

A possible synergistic effect between the semiconductor nanoparticles and CNTs on the en‐ hancement of photocatalytic activity is proposed in Figure 7. The mechanism is based on the results of the structure characterizations and the enhancement in photocatalytic activity of

(VB) to the conduction band (CB) of catalysts or the metal oxide nanoparticles (NP) creating a

) are excited from the valence band

Neudeck et al., 2011; Gondal et al., 2010; Drmosh et al., 2010).

486 Syntheses and Applications of Carbon Nanotubes and Their Composites

and Gao, 2005;Agnihotri et al., 2006).

nanotubes and the metal nanoparticles.

When the catalyst is irradiated by photons, electrons (e-

the prepared composite.

ess, can be achieved in relatively short time by using CNT/ZnO composites.


**•** The presence of the nanotubes in the composite can inhibit the recombination of photogenerated electrons and holes, thus, improving the photocatalytic activity.

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**•** The transmission stability of promoted electron between the nanotubes and the conduc‐ tion band is enhanced by the strong interaction and intimate contact between the nano‐ particles and the surface of the nanotubes.
