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J. Phys. Chem. C 2010, 114, 2669–2676

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New Insight for Enhanced Photocatalytic Activity of TiO2 by Doping Carbon Nanotubes: A Case Study on Degradation of Benzene and Methyl Orange Yi-Jun Xu,* Yangbin Zhuang, and Xianzhi Fu Research Institute of Photocatalysis, State Key Laboratory Breeding Base of Photocatalysis, College of Chemistry and Chemical Engineering, Fuzhou UniVersity, Fuzhou, 350002, P.R. China ReceiVed: October 14, 2009; ReVised Manuscript ReceiVed: January 7, 2010

A carbon nanotubes (CNT)/TiO2 nanocomposite photocatalyst has been prepared by a simple impregnation method, which is used, for the first time, for gas-phase degradation of benzene. It is found that the asprepared CNT/TiO2 nanocomposite exhibits an enhanced photocatalytic activity for benzene degradation, as compared with that over commerical titania (Degussa P25). A similar phenomenon has also been found for liquid-phase degradation of methyl orange. The characterization of photocatalysts by a series of joint techniques, including X-ray diffraction, transmission electron microscopy, ultraviolet/visible (UV/vis) diffuse reflectance spectra, and photoluminescence spectra, discloses that CNT has two kinds of crucial roles in enhancement of photocatalytic activity of TiO2. One is to act as an electron reservoir, which helps to trap electrons emitted from TiO2 particles due to irradiation by UV light, therefore hindering electron-hole pairs recombination. The other is to act as a dispersing template or support to control the morphology of TiO2 particles in the CNT/TiO2 nanocomposite, and this important role was neglected in previous studies. Accordingly, a reasonable model is proposed to expain the role of CNT in CNT/TiO2 composites as a photocatalyst for degradation of organic pollutants. 1. Introduction Heterogeneous photocatalysis by semiconductor materials has been gaining increasing interest because, as a green technology, it can be widely applied to environmental purification (nonselective process) and selective organic transformations to fine chemicals in both gas and liquid phases.1,2 Among the abundant semiconductor materials, titanium oxide (TiO2)-based materials are generally believed to be the most reliable materials for photocatalytic reactions due to their nontoxicity, low cost, physical and chemical stability, availability, and unique electronic and optical properties.1,2 However, the high rate of electron-hole recombination in TiO2 particles results in low quantum efficiency of photocatalytic reactions. Therefore, increasing the photocatalytic activity of TiO2 is of great interest for the practical application of TiO2-based materials. To date, a variety of strategies have been utilized to improve the photocatalytic performance of TiO2, which can be generally classfied into two categories. One is to use chemical methods, such as coupling with secondary semiconductors, photosensitization of dye, and doping with transition metals (Au, Pd, Pt, Rh, etc.) or nonmetal elements (N, S, I, F, etc.).3,4 The other is to use physical approaches; for example, introducing microwave or ultrasonic irradiation into TiO2 photoreaction systems.5 In recent years, significant interest has been devoted to designing semiconductor-CNT composite materials, aiming at a synergetic combination of their intrinsic outstanding properties and, thus, enhanced performance to meet new requirements imposed by specific applications, such as solar energy utilization, optoelectronic quantum dot arrays, and heterogeneous photocatalysis.6 Indeed, some studies have proven that the electronaccepting and -transport properties of CNT provide a convenient way to direct the flow of photogenerated charge carriers, which * To whom correspondence should be addressed. E-mail Address: [email protected].

thus increases the lifetime of electron-hole pairs generated by semiconductors upon light irradiation.7,8 Consequently, the photocatalytic activity of semiconductors will be improved for target reactions. For example, most recently, Yao et al. have demonstrated that a hydration/dehydration process can create TiO2/CNT composites that enhance the photocatalytic activity for degradation of phenol in the liquid phase in comparison to commercial titania (P25).7a This preparation procedure requires only a hydration/dehydration process, which is much simpler than the sol-gel, chemical vapor deposition (CVD) and physical vapor deposition (PVD) methods.7c,d,9 However, it should be stressed that the multiwalled carbon nanotube (MWCNT)/TiO2 composite prepared by this way is not as effective as single-walled carbon nanotube (SWCNT)/TiO2 composites for degradation of phenol in the liquid phase. That is, the MWCNT/TiO2 composite shows lower photocatalytic activity for degradation of phenol than that over commercial P25.7a In view of the much lower price of MWCNT than SWCNT, it would be more meaningful to synthesize effective MWCNT/ TiO2 composites with improved photocatalytic performance. Yu and co-workers reported that a simple mechanical mixture of TiO2 (P25) and MWCNTs improved the photocatalytic activity of TiO2 for color removal in an aqueous solution of azo-dyes under UV light.8a However, the researchers did not fully explain how a simple mixture of TiO2 and MWCNTs would interact to accelerate the decay of the dye;7a moreover, it still remains unknown if the MWCNT/TiO2 prepared by such a simple approach is effective to boost the performance for gas-phase degradation of acetone.8b In addition, previous studies take only one type of reaction to compare the photocatalytic performance between MWCNT (or SWCNT)/TiO2 and bulk TiO2. Therefore, it would be of great significance to synthesize a MWCNT/TiO2 nanocomposite via a simple process, and importantly, this as-

10.1021/jp909855p  2010 American Chemical Society Published on Web 01/22/2010

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SCHEME 1: The Overall Sketch for Preparation of CNT/TiO2 Nanocomposite Photocatalyst Using a Wet Impregnation Method Followed by Calcination in Air

synthesized material exhibits enhanced photocatalytic activity for both liquid- and gas-phase degradation of organic pollutants. The purpose of this paper is two-fold. The first purpose aims to create the effective TiO2/MWCNT nanocomposite by a facile wet impregnation method (Scheme 1), which is able to show improved photocatalytic performance for the application to both liquid- and gas-phase environmetal cleanup. By comparison to TiO2/MWCNT prepared by a simple mechanical mixing process in conjuction with microscopic characterizations, we attempt to establish the relationship between stucture and photocatalytic activity. Accordingly, a reasonable model is proposed to illustrate the key roles of MWCNT in the nanocomposite of TiO2/MWCNT. Noteworthily, there has been no report regarding using a nanocomposite of MWCNT/TiO2 for gas-phase photocatalytic degradation of benzene, which is one of the most abundant volatile aromatic hydrocarbons found in polluted urban atmospheres and has severe toxic effects on the human body.10,11 Therefore, the second purpose is, for the first time, to investigate whether the as-prepared MWCNT/TiO2 nanocomposite has the enhanced photocatalytic activity toward gas-phase degradation of benzene. 2. Experimental Section 2.1. Catalyst Preparation. The preparation of TiO2/ MWCNT nanocomposite photocatalyst is outlined in Scheme 1. MWCNTs (supplied from Shenzhen Nanotech Port Co., Ltd., China) were first purified by refluxing in concentrated nitric acid for 8 h. Then, the MWCNTs were filtered; washed with distilled water; and finally, dried at 100 °C in an oven. This purification step also introduced oxygenated functionalities onto the nanotube surface.12 To synthesize TiO2/MWCNT nanocomposites, a simple wet impregation method was used. MWCNTs were ultrasoniated in 20 mL of anhydrous ethanol solution to disperse them well, then a given amount of titanium tetrasiopropoxide was added to the above MWCNT solution to prepare 95, 80, and 60 wt % TiO2/MWCNT nanocomposite catalysts. This solution was aged with stirring for 12 h and then dried at 80 °C in an oven for 12 h. The final TiO2/MWCNT photocatalysts were obtained by calcination at 400 °C in air for 2 h. Similarly, a blank experiment in the absence of MWCNT was also performed to understand the role of MWCNT during the process of prepartion of TiO2/MWCNT nanocomposites. The as-obtained material is denoted as blank-TiO2. For comparison, three other photocatalystssnamely, 95, 80, and 60 wt % TiO2/ MWCNT nanocompositesswere also prepared by a simple mechanical mixing method. That is, commercial titania (P25) and MWCNT were dispersed into 20 mL of anhydrous ethanol solution and ultrasonicated to make them mingle well. After that, this mixture was fully dried at 80 °C in an oven for 12 h. To discriminate the composites of TiO2/MWCNT prepared by a mechanical mixing method from those prepared by an impregnation method, we denote the composites of TiO2/

MWCNT prepared by a mechanical mixing process as TiO2/ MWCNT-mixing. 2.2. Catalyst Characterization. The phase composition of the samples was determined by X-ray diffraction (XRD) on a Bruker D8 Advance X-ray diffractometer at 40 kV and 40 mA with Ni-filtered Cu KR radiation. The optical properties of the samples were analyzed by UV-vis diffuse reflectance spectroscopy using a UV-vis spectrophotometer (Cary-500, Varian Co.), in which BaSO4 was used as the internal standard. The photoluminescence spectra were measured using an Edinburgh Analytical Instrument FL/FSTCSPC920 spectrophotometer. Transmission electron microscopy (TEM) images were obtained using a JEOL model JEM 2010 EX instrument at an accelerating voltage of 200 kV. 2.3. Catalyst Activity. The photocatalytic degradation of benzene in the gas phase was conducted in a tubular vessel microreactor operating in a continuous flow mode. All of the photocatalysts were sieved to obtain particles of 0.21-0.25 mm size. Four 4 W UV lamps with a wavelength centered at 254 nm (Philips, TUV 4W/G4 T5) were used as the light source. A bubbler that contained benzene was immersed in an ice-water bath, and benzene (about 250 ppm) bubbled with oxygen from the bubbler was fed to 0.3 g catalyst at a total flow rate of 20 mL/min. The reaction temperature was controlled at 30 ( 1 °C by an air-cooling system. Simultaneous determination of benzene and CO2 concentrations was performed with an online gas chromatograph (HP6890) equipped with a flame ionization detector and a thermal conductivity detector. Conversion of benzene and mineralization of benzene were defined as the following:

% conversion ) [(C0 - C) /C0 × 100] % mineralization ) [(CO2)produced /6(benzene)converted] × 100 where C0 is the initial concentration of benzene and C is the concentration of benzene after photocatalytic reaction. The liquid phase degradation of dye, that is, methyl orange, was carried out in a quartz tube under UV irradiation. A 0.08 g portion of catalyst was suspended in 80 mL of 10 ppm methyl organge solution. Four 4 W UV lamps with a wavelength centered at 365 nm (Philips, TUV 4W/G4 T5) were used as the illuminating source. Before UV irradiation, the suspensions were stirred in the dark for 2 h to ensure the establishment of adsorption-desorption equilibrium. A 3 mL sample solution was taken at a certain time interval during the experiment and centrifuged to remove the catalyst completely. After that, this solution was analyzed on a Varian UV-vis spectrophotometer (Cary-50, Varian Co.). The percentage of degradation is reported as C/C0. Here, C is the absorption of methyl organge solution at each irradiated time interval of the main peak of the adsorption

Photocatalytic Activity of TiO2

Figure 1. XRD patterns of the samples of CNT and TiO2/MWCNT nanocomposites with different weight ratios prepared by an impregnation method.

Figure 2. UV-vis diffuse reflectance spectra of the samples of TiO2/ MWCNT nanocomposites with different weight ratios: CNT, blankTiO2, and P25.

spectrum, and C0 is the absorption of the initial concentration when the adsorption-desorption equilibrium is reached. 3. Results and Discussion Figure 1 shows the XRD patterns of the as-prepared MWCNT/ TiO2 composites with different weight ratios. Clearly, the particles of TiO2 in MWCNT/TiO2 have formed the anatase phase, since characteristic diffraction peaks at 25.3°, 37.8°, 48.0°, 53.9°, 55.1°, and 62.7° are observed, which can be attributed to the (101), (004), (200), (105), (211), and (204) faces of anatase TiO2. Notably, in these MWCNT/TiO2 composites, there is no apparent peak at the position of 26.0°, which is the characteristic peak for CNTs.12 The reason could be due to the fact that the main peak of CNTs at 26.0° might be shielded by the main peak of anatase TiO2 at 25.4°.7e The optical property of the as-prepared TiO2/MWCNT composites, together with CNT, P25 and blank TiO2, has been measured by UV-vis diffuse reflectance spectra, as displayed in Figure 2. All of these samples show the typical absorption with an intense transition in the UV region of the spectra, which is due to electron promotion of TiO2 from the valence band to the conduction band.1,2

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Figure 3. Time-online photocatalytic degradation of benzene in the gas phase over P25 and 95 wt % TiO2/MWCNT prepared by an impregnation method.

The presence of MWCNT leads to the continuous absorption band in the range of 420-800 nm, which is in agreement with the black color of the samples. Interestingly, for the composite of 95 wt % TiO2/MWCNT, it exhibits a stronger absorption intensity in the UV region than commercial P25. However, regarding the composites of 80 and 60 wt % MWCNT/TiO2, the higher amount of MWCNTs shield the UV light for the absorption by TiO2. As a result, it is reasonable that the composite of 95 wt % TiO2/MWCNT has a better absorption capability of UV light than 80 and 60 wt % TiO2/MWCNT, suggesting that it may have the higher photocatalytic activity for target reactions. These also indicate clearly that controlling the composition ratio in TiO2/MWCNT is crucial to obtain an optimal synergistic effect between MWCNT and TiO2. On the other hand, it should be noted that all of the composites of TiO2/MWCNT-mixing prepared by a simple mechanical mixing process show lower absorption intensity of UV light than P25 and TiO2/MWCNT prepared by an impregnation method. This suggests that the preparation method has an important effect on the structure of the as-prepared composite materials, thereby affecting the photocatalytic properties significantly. In the following, we will demonstrate that the asprepared TiO2/MWCNT composites via an impregnation approach exhibit enhanced photocatalytic performance for both gas-phase degradation of benzene and liquid-phase degradation of methyl organge. First, let us discuss the gas-phase photocatalytic degradation of benzene over the as-prepared TiO2/MWCNT composites. Figure 3 shows the conversion of benzene and the amount of produced CO2 over the 95 wt % TiO2/MWCNT photocatalyst prepared by an impregnation method. As can be seen, after 1 h of reaction, an average conversion ratio of 6.4% is reached. Before 10 h of reaction, the amount of produced CO2 declines gradually. After that, the average amount of produced CO2 is about 62 ppm, which corresponds to an average mineralization ratio of 64.6%. During this reaction range, no significant deactivation is observed over such a 95 wt % TiO2/MWCNT photocatalyst based on benzene conversion and the produced amount of CO2. However, with regard to degradation of benzene over commercial P25, the photocatalytic activity is quite unstable. After 22 h of reaction, the conversion ratio of benzene remarkably decreases to 1.2% and the amount of produced CO2 is only 12 ppm. Namely, the percentage decrease of activity over P25 is about 76.9%. This can be ascribed to the blockage of photocatalytic active sites by stable intermediates on the

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Figure 4. Time-online photocatalytic degradation of benzene in the gas phase over P25 and 95 wt % TiO2/MWCNT-mixing prepared by a mechanical mixing process.

surface of TiO2 during the reaction, thereby leading to the deactivation of the photocatalyst.13 For comparison, the 95 wt % TiO2/MWCNT composite, prepared by simple mechanical mixing of P25 and MWCNTs, was also tested for gas-phase degradation of benzene. The data are displayed in Figure 4. Obviously, a simple mechanical mixing process is not able to produce an effective TiO2/

Xu et al. MWCNT composite photocatalyst, which thus shows lower activity for benzene degradation than bulk TiO2 alone. In fact, this observation is in good agreement with a recent work regarding phenol degradation using TiO2/MWCNT composite materials reported by Gray’s group.7a Namely, similar to a simple hydration/dehydration process, a mechanical mixing process is not able to create good interfacial contact between TiO2 and MWCNTs. A larger amount of MWCNTs in TiO2/ MWCNT composites will hinder the absorption of UV light by TiO2 particles. As a result, the photocatalytic activity would be decreased significantly. This is true, since the composites of 80 wt % TiO2/MWCNT and 60 wt % TiO2/MWCNT, prepared by either an impregnation method or a simple mechanical mixing process, show much lower or trace activity for benzene degradation as compared to commerical P25 alone (not shown here). Interestingly, we have also tested the photocatalytic activity of benzene degradation over blank-TiO2, which is prepared by a similar process (in the absence of MWCNT) to preparation of TiO2/MWCNT composites via an impregnation method. It is found that blank-TiO2 exhibits only trace photocatalytic activity for benzene degradation. This observation leads us to speculate that the presence of MWCNT, during the prepration of TiO2/MWCNT composites by an impregnation method, play an important role in controlling the morphology of TiO2 particles. This speculation is confirmed by the transmission

Figure 5. TEM images of 95 wt % TiO2/MWCNT prepared by an impregnation method.

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Figure 6. TEM images of blank-TiO2 prepared by an impregnation method in the absence of MWCNTs.

electron microscopy analysis. Figures 5 and 6 display the TEM images of 95 wt % TiO2/MWCNT and blank-TiO2, respectively. It can be clearly seen from the top panel in Figure 5 that, regarding the composite of 95 wt % TiO2/MWCNT, there is a good dispersion of nanosized TiO2 particles (5-25 nm). Higher-resolution TEM images, as displayed in the bottom panels in Figure 5, show that some nanosized TiO2 particles have formed along the side wall of the CNT. For this segment of TiO2 particles, there is good contact between the MWCNT and the surface of the TiO2 particles. A remaining large number of nanosized TiO2 particles in the composite of 95 wt % TiO2/ MWCNT are also observed, which do not have a close contact with the surface of the MWCNTs. However, with regard to blank-TiO2, the morphology of TiO2 particles is quite different from those in the nanocomposite of 95 wt % TiO2/MWCNT. Clearly, in the absence of MWCNTs, the as-formed TiO2 particles are inclined to agglomerate (see the top panel in Figure 6), although a small number of nanosized TiO2 particles are still observed (the bottom panels in Figure 6). This suggests that MWCNTs act as a “dispersing template or support” to control the morphology of TiO2 particles in the TiO2/MWCNT composite. In other words, the presence of MWCNTs is able to prevent the agglomeration of as-formed TiO2 particles, thus downsizing TiO2 particles in a small nanometer dimension (5-25 nm). It is well-known that the photocatalytic activity of functional materials strongly depends on their morphological structure.14 The small-sized TiO2 particles in the nanocomposite

of 95 wt % TiO2/MWCNT are beneficial for improving its photocatalytic activity, as compared to agglomerated TiO2 particles in the bulk material of blank-TiO2, because of the large specific area of small-sized TiO2 particles, which ensures efficient UV light absorption and provides more photoreactive sites for degradation reactions. In addition, the other important role of MWCNTs is to act as an electron reservoir to trap electrons emitted from TiO2 particles due to irradiation by UV light, thus hindering electron-hole pair recombination and improving the photocatalytic activity of TiO2. This can be verifid by the photoluminescence (PL) spectra analysis. PL sepctra are often employed to study surface processes involving the electron-hole fate of TiO2.7a,15 With electron-hole pair recombination after a photocatalyst is irradiated, photons are emitted, resulting in photoluminescence. This behavior is due to the reverse radiative deactivation from the excited state of the Ti species. Clearly, as shown in Figure 7, the 95 wt % TiO2/MWCNT shows significantly diminished PL intensity as compared to bulk TiO2 (blank-TiO2 or P25) alone, indicating reduced charge recombination. Most recently, Kamat’s goup demonstrated that CNTs have a large electron-storage capacity (one electron per 32 carbon atoms).16 Thus, as proposed by previous studies,7a the photonexcited electrons from TiO2 particles can be shuttled freely in the conducting network of MWCNTs, as displayed in Figure 8. A similar mechanism has also been found in the C60-TiO2

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Figure 7. Photoluminescence spectra of 95 wt % TiO2/MWCNT, P25, and MWCNT.

Figure 8. Model picture demonstrating CNT as an electron reservoir to trap electrons emitted from TiO2 particles due to UV irradiation and active species during the photocatalysis process.

(or ZnO) composite photocatalysts in which C60 is a good electron acceptor.17 The longer-lived electron-hole pairs are beneficial for generating a larger amount of photoreactive species, such as O2- and · OH, with very strong oxidation capability, which account for the higher photocatalytic activity of the 95 wt % TiO2/MWCNT composite than bulk TiO2 alone. Thus far, we can conclude that there are two important roles of MWCNTs in the nanocomposite of TiO2/MWCNT. One is to act as an electron reservoir to trap electrons emitted from TiO2 particles due to irradiation by UV light, thus hindering electron-hole pairs recombination. The other is to act as a dispersing template or support to control the morphology of TiO2 particles in a small nanosized dimension. These two aspects together can explain why the TiO2/MWCNT material with an appropriate composition exhibits enhanced photocatalytic activity for gas-phase degradation of benzene as compared to bulk TiO2 alone. To further test if the TiO2/MWCNT nanocomposite prepared by an impregnation method has improved photocatalytic activity for degradation of pollutants in the liquid phase, we have also investigated the performance of the as-prepared TiO2/MWCNT nanocomposites for degradation of methyl orange, which is a

Figure 9. Photocatalytic degradation of methyl orange in the liquid phase over P25, blank-TiO2, and TiO2/MWCNT composites with different weight ratios prepared by an impregnation method.

well-known organic dye pollutant in wastewater produced from textile and other industrial processes. Figure 9 shows the data for liquid-phase degradation of methyl orange over P25, blankTiO2, and TiO2/MWCNT composites with different weight ratios. It is clearly demonstrated that the 95 wt % TiO2/MWCNT composite shows the best performance because its lowering rate in C/C0 is the fastest among the five samples (Figure 9a). On the basis of previous studies,18 the degradation of dyes can be ascribed to a pseudo-first-order reaction with a simplified Langmuir-Hinshelwood model when C0 is very small: ln(C0/ C) ) kat, where ka is the apparent first-order rate constant, as displayed in Figure 9b. Obviously, on the basis of the kinetic plot, the 95 wt % TiO2/MWCNT composite has the highest rate constant as compared to the other samples. In contrast, as shown in Figure 10, the samples of TiO2/MWCNT composite prepared by a simple mechanical mixing of P25 and MWCNTs are not able to exhibit the enhanced photocatalytic activity toward methyl orange degradation as compared to P25, which is similar to gas-phase degradation of benzene as discussed above. Therefore, a simple mechanical mixing process cannot create more photoreactive TiO2/MWCNT composites with good interfacial contact between TiO2 and MWCNT.

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J. Phys. Chem. C, Vol. 114, No. 6, 2010 2675 electron reservoir to trap electrons generated from TiO2 particles due to UV irradiation and prolong the lifetime of electron-hole pairs. These two important roles of MWCNTs together lead to the enhanced photocatalytic activity. 4. Concluding Remarks

Figure 10. Photocatalytic degradation of methyl orange in the liquid phase over P25 and TiO2/MWCNT composites with different weight ratios prepared by a simple mechanical mixing process.

This is in good agreement with a recent study regarding liquid-phase degradation of phenol using TiO2/CNT composites as reported by Gray’s research group.7a They suggest that a similar hydration/dehydration process is not able to combine P25 and MWCNT effectively due to less individual interfacial contact between MWCNT and TiO2, thus leading to a lower photocatalytic performance for degradation of phenol in the liquid phase. In line with the work from Gray’s group,7a we do not observe the enhancement of photocatalytic activity for degradation of organic pollutants in the liquid phase, which is different from the experimental observation as reported by Yu et al.8a In comparison to previous studies regarding preparation of TiO2/MWCNT photocatalysts using the sol-gel, chemical vapor deposition and physical vapor deposition methods,7c,d,9 we show for the first time that a much simpler impregnation method can be directly used to prepare the nanocomposites of TiO2/ MWCNT, which have the enhanced photocatalytic activity for both gas-phase degradation benzene and liquid-phase degradation of methyl orange. The presence of a small number of MWCNTs during the preparation of nanocomposites of TiO2/ MWCNT is able to control the as-formed TiO2 particles in a small nanosized dimension and, on the other hand, to act as an

In summary, we have first reported that the wet impregnation method, which is much simpler than the sol-gel, CVD, and PVD methods, can be directly used to prepare the photoreactive TiO2/MWCNT nanocomposite materials. The as-prepared TiO2/ MWCNT nanocomposite, for the first time, has been used for gas-phase degradation of benzene, a notorious volatile organic pollutant in urban atmospheres.10,11 It is found that the TiO2/ MWCNT nanocomposite shows an enhanced photocatalytic activity toward benzene degradation, as compared to commercial TiO2 (P25, Degussa). Such a TiO2/MWCNT nanocomposite also exhibits an improved photocatalytic activity for degradation of methyl orange in the liquid phase. For comparison, TiO2/ MWCNT nanocomposites have also been prepared by a simple mechanical mixing of P25 and MWCNTs. In contrast, TiO2/ MWCNT composites prepared by a simple mechanical mixing process are not able to exhibit improved photocatalytic activity for either gas-phase degradation of benzene or liquid-phase degradation of methyl orange. This is in good agreement with a recent study reported by Gray’s research group.7a They demonstrated that a simple hydration/dehydration approach is not able to produce efficient TiO2/MWCNT composite materials with enhanced photocatalytic activity for liquid-phase degradation of phenol as compared to commercial P25.7a The presence of a small number of MWCNTs in the composite of TiO2/MWCNT has two important roles. One is to act as a dispersing template or support to downsize the as-formed TiO2 particles in a small, nanosized dimension during the preparation of TiO2/MWCNT, thus increasing the UV absorption capability and more photoreactive sites for degradation reactions. The other role of MWCNTs is to act as an electron reservoir to trap electrons from TiO2 particles due to UV irradiation, hence, hindering the electron-hole pairs recombination, which is beneficial for improving the photocatalytic performance of TiO2. Accordingly, a reasonable model is proposed to explain the role of MWCNTs in TiO2/MWCNT composites as a photocatalyst for degradation of organic pollutants. Acknowledgment. Support by the Award Program for the Minjiang Scholar Professorship, the National Natural Science Foundation of China (20903023), Program for Changjiang Scholars and Innovative Research Team in Universities (PCSIRT0818), National Basic Research Program of China (973) Program (2007CB613306), and Program for Returned HighLevel Overseas Chinese Scholars of Fujian province is gratefully acknowledged. References and Notes (1) (a) Hoffmann, M. R.; Marin, S. T.; Choi, W.; Bahnemannt, D. W. Chem. ReV. 1995, 95, 69. (b) Mills, A.; Davies, R. H.; Worsley, D. Chem. Soc. ReV. 1993, 22, 417. (c) Gaya, U. I.; Abdullah, A. H. J. Photochem. Photobiol., C 2008, 9, 1. (d) Carp, O.; Huisman, C. L.; Reller, A. Prog. Solid State Chem. 2004, 32, 33. (e) Anpo, M. Pure Appl. Chem. 2000, 72, 1265. (f) Fujishima, A.; Zhang, X.; Tryk, D. A. Surf. Sci. Rep. 2008, 63, 615. (g) Rajeshwar, K.; Osugi, M. E.; Chanmanee, W.; Chenthamarakshan, C. R.; Zanoni, M. V. B.; Kajitvichyanukul, P.; Krishnan-Ayer, R. J. Photochem. Photobiol., C 2008, 9, 171. (2) (a) Fox, M. A.; Dulay, M. T. Chem. ReV. 1993, 93, 341. (b) Fox, M. A. Acc. Chem. Res. 1983, 16, 314. (c) Yoshida, H. Curr. Opion. Solid State Mater. Sci. 2003, 7, 435. (d) Shiraishi, Y.; Hirai, T. J. Photochem. Photobiol., C 2008, 9, 1571. (e) Palmisano, G.; Augugliaro, V.; Pagliaro,

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