Composites of Titanate Nanotube and Carbon Nanotube as

ARTICLE pubs.acs.org/JPCC

Composites of Titanate Nanotube and Carbon Nanotube as Photocatalyst with High Mineralization Ratio for Gas-Phase Degradation of Volatile Aromatic Pollutant Zi-Rong Tang,*,† Fan Li,† Yanhui Zhang,†,‡ Xianzhi Fu,†,‡ and Yi-Jun Xu†,‡ † ‡

College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou 350108, People's Republic of China Research Institute of Photocatalysis, State Key Laboratory Breeding Base of Photocatalysis & College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou 350002, People's Republic of China ABSTRACT: The nanocomposites of one-dimensional titanate nanotubes and carbon nanotubes (TNT-CNT) have been synthesized by controlling the preparation conditions carefully during the hydrothermal treatment of TiO2 nanoparticles and carbon nanotube (CNT) in a concentrated alkali solution and the subsequent post-treatment. Using the gas-phase degradation of benzene, a volatile aromatic pollutant commonly present in urban atmosphere, as a testing reaction we for the first time have investigated the photocatalytic performance of TNTs and TNT-CNT nanocomposites together. The results show that one-dimensional tubular TNT exhibits enhanced photocatalytic performance toward the gas-phase degradation of benzene as compared to the reference photocatalyst of bare P25-TiO2 nanoparticles. Doping a certain amount of CNT into the matrix of TNT affects the conversion ratio of benzene only slightly; however, the mineralization ratio for degradation of benzene is remarkably increased to about 90%. This observation is particularly interesting because it is markedly different from that over the nanocomposites of TiO2 nanoparticles and CNT. The possible reasons have been put forward based on the results of photocatalytic activity and analysis of detailed characterization results including the transmission electron microscopy and electron spin resonance spectra. We ascribe the high mineralization ratio for benzene degradation over TNT-CNT to the following factors, that is, the unique one-dimensional nanotubular morphology associated with TNT-CNT, enhanced adsorptivity of benzene due to the doping of CNT, and enhanced light absorption intensity. In particular, the former factor of nanotubular morphology plays a more important role on enhancement of the mineralization ratio for degradation of benzene because the latter two factors are also found over the composites of TiO2 nanoparticles and CNT. Moreover, in view of the facile availability of tunable optical properties of TNT via substitution of sodium ions and proton ions in TNT with transition metal ions, there would be a wide scope to optimize the photocatalytic performance of TNT-based one-dimensional materials and their nanocomposites with CNT, which could be an interesting research topic with regard to TiO2-carbon composites as photocatalyst for the environmental remediation.

1. INTRODUCTION One-dimensional semiconducting nanostructures, such as nanorods, nanowires, and nanotubes, lie at the heart of current research on nanometer scale science and technology.1
4 Recent years have seen their extensive potential and applications in a diversity of fields, for example, photodetectors, light waveguides, solar energy conversion, lithium batteries, gas sensing, and photocatalysis.1
10 Titanium dioxide (TiO2) is a well-known and mostly studied wide band gap semiconductor material, which is able to be used as an effective photocatalyst for degradation of pollutants in air or water.11
16 In addition, TiO2-based materials have also found extensive applications in other fields, such as H2 generation, membrane solar cells, and catalysis.17
20 It has been well documented that the microscopic structure including morphology, crystalline phase, particle size, and crystallity affects the photocatalytic performance of TiO2 remarkably.17,20
22 Nanotubular TiO2 is a class of one-dimensional material with novel chemical structure and properties.23 In particular, as r 2011 American Chemical Society

compared to the commonly used nanoparticles or bulk materials, TiO2 in the form of nanotubes can provide unique benefits as photocatalysts in view of the following features.23
31 First, the one-dimensional geometry faciliates fast and long-distance electron transport. Second, nanotubular TiO2 is expected to have larger specific surface area and pore volume as compared with TiO2 nanoparticles. Third, the light absorption and scattering can be markedly enhanced owing to the high length-to-diameter ratio for the nanotubular TiO2. Therefore, it should be of significant interest to investigate the potential applications of such nanotubular TiO2 based materials in the field of heterogeneous photocatalysis. On the other hand, carbon nanotubes (CNT) represents another type of non-metal-oxide one-dimensional material. It is well established that the electron-accepting Received: December 6, 2010 Revised: February 4, 2011 Published: March 31, 2011 7880

dx.doi.org/10.1021/jp1115838 | J. Phys. Chem. C 2011, 115, 7880–7886

The Journal of Physical Chemistry C

ARTICLE

Scheme 1. Flowchart of Preparation of Nanocomposites of TNT and CNT by a Hydrothermal Process in a Concentrated NaOH Solution Followed by the Wash Process with the Control of pH = 8 and Post-Treatment of Calcination in Air

and -transporting properties of CNT provide a convenient way to direct the flow of photogenerated charge carriers, thereby increasing the lifetime of electron
hole pairs generated by semiconductors under light irradiation.32
37 Thus, the integration of nanotubular TiO2 and CNT in a proper fashion could combine the respective advantages of these two nanotubular onedimensional materials, which could be utilized as photocatalyst with enhanced photocatalytic performance in comparison to the bare TiO2 bulk material or nanoparticles. Many approaches have been reported to prepare nanotubular TiO2, for example, deposition with templates,38
42 anodic oxidation of titanium,43,44 and hydrothermal treatment with concentrated alkali solutions.45
49 Among them, hydrothermal treatment of TiO2 particles with alkali solutions leading to nanotubular TiO2 is of particular interest because of easy scaleup synthesis at low temperature and high yield associated with this simple method.45
49 However, it should be noted that, during the post-treatment of TiO2 nanotubes prepared by a hydrothermal treatment of TiO2 nanoparticles with alkali solutions, many parameters must be paid to careful attention in order to keep the nanotubular structure intact. Thanks to the systematic work reported by Li et al.,50 they have disclosed the key factors affecting the nanotubular structure of the as-formed TiO2 nanotubes by a hydrothermal process, including the effect of the washing process, the important role of residual sodium ions in stabilization of the tubular framework structure, and the effect of calcination temperature in air. Also, they revealed that the nanotubes prepared by a hydrothermal process are actually titanate nanotubes (TNTs) rather than the so-called TiO2 nanotubes.50 The best characterization technique, judging whether the tubular structure of as-prepared TNTs is retained or not after the posttreatment, is X-ray powder diffraction (XRD), a general strategy to characterize the crystal structural features.50 Although Fugetsu and co-workers reported the first use of CNT/nanotubular-TiO2 composites for photodegradation of humic substances in the liquid phase, some problems are still open.51 For example, they used the method of hydrothermal treatment of TiO2 nanoparticles with concentrated NaOH solution to prepare the so-called nanotubular-TiO2, which in fact is TNTs as stated above.50 However, the product was washed with 0.1 M aqueous HCl solution and deionized water until its pH value reached to about 7; following that, the product was dried and calcined at 350 o C for 8 h in air. According to Li et al.’s work, washing with acid has a remarkable effect on the thermal stability of TNTs. If the pH value of 7 is reached by the washing process with acid and water, the thermal stability of TNTs will

become very poor; calcination process in air would transform TNTs into anatase titania by which the framework of tubular structure of TNTs is significantly destroyed.50 Our experiments in the current research work also support this viewpoint. This means that the as-obtained composites are actually TiO2 particles doped with CNT instead of nanotubular-TiO2/CNT as claimed by Fugetsu et al.51 In other words, at least some TNTs are collapsed, and the composite material is not totally comprised of TNTs and CNTs. Herein, we report the synthesis of composites of titanate nanotube and carbon nanotube (denoted as TNT-CNT) by controlling the preparation conditions carefully. By use of the degradation of benzene in the gas phase as a testing reaction, we for the first time have investigated the photocatalytic performance of TNT and TNT-CNT nanocomposites together. The results have demonstrated that one-dimensional TNT exhibits enhanced photocatalytic performance toward the gas-phase degradation of benzene as compared to the reference photocatalyst of bare P25-TiO2 nanoparticles. Doping a certain amount of CNT into the matrix of TNT affects the conversion ratio of benzene only slightly; however, the mineralization ratio is significantly increased. This observation is particularly interesting because it is markedly different from that over the nanocomposites of TiO2 nanoparticles and CNT. The possible reasons accounting for this phenomenon have been proposed in terms of the analysis of detailed characterization results including the transmission electron microscopy (TEM) and electron spin resonance spectra (ESR). Furthermore, in view of the facile availability of substitution of transition-metal ions for sodium ions and proton ions in TNT, there would be a wide scope to optimize the photocatalytic performance of TNT-based onedimensional materials and their nanocomposites with CNT, which could be an interesting research topic regarding TiO2-carbon composites as photocatalyst for environmental remediation.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Composites of TNT and CNTs. Multiwalled CNTs are supplied from Shenzhen Nanotech Port Co., Ltd., China. Before use, the CNT was first purified by refluxing in concentrated nitric acid for 8 h. Then, the samples were filtered, washed with distilled water, and finally dried at 100 °C in an oven. This purification step introduces oxygenated functionalities onto the nanotube surface and helps CNT disperse into the aqueous solution well.52 In a typical synthesis as roughly shown in Scheme 1, anatase titania powder (0.5 g, 10 nm in size, supplied from Alfa Aesar), a certain amount of CNT (feedstock ratio of 7881

dx.doi.org/10.1021/jp1115838 |J. Phys. Chem. C 2011, 115, 7880–7886

The Journal of Physical Chemistry C CNT/TiO2 is 0, 5, 10, 20, and 30 wt %, respectively), and an aqueous solution of concentrated NaOH (10 M, 40 mL) were mixed and kept stirring to form a homogeneous suspension. This suspension was transferred into a Teflon-lined autoclave and hydrothermally treated at 140 o C for 12 h in an oven. After the reaction, the precipitate was separated by filtration and washed with deionized water until a pH value near 8 was reached. The precipitate was then dispersed in anhydrous ethanol with the assistance of ultrasonication. After a second filtration and washing step by ethanol, the sample was dried in a vacuum oven at 80 o C overnight. The as-obtained sample is finally calcined at 400 °C for 6 h in air, thus giving rise to the TNT-CNT nanocomposites. To confirm the key effect of washing process on the tubular framework structure of TNT, we also synthesized the samples for which the washing step is controlled with pH value = 7 while the other treatment conditions are the same as those as mentioned above. 2.2. Characterization. The phase composition of the samples was determined by XRD on a Bruker D8 Advance X-ray diffractometer at 40 kV and 40 mA with Ni-filtered Cu KR radiation. The optical properties were analyzed by the UV
vis diffuse reflectance spectroscopy (DRS) using a UV
vis spectrophotometer (Cary-500, Varian Co.), in which BaSO4 was used as the internal standard. The nitrogen adsorption
desorption isotherm was carried out using a Micromeritic-ASAP2020 equipment. TEM images were obtained using a JEOL model JEM 2010 EX instrument at the accelerating voltage of 200 kV. ESR spectra of the radicals trapped by 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) were measured using a Bruker A300 EPR electron paramagnetic resonance spectrometer equipped with a quantaRay Nd: YAG laser system as their radiation light source (λ = 266 nm). The settings were the center field at 3480.00 G, microwave frequency at 9.83 GHz, and power at 6.35 mW. 2.3. Photocatalytic Activity Test. The photocatalytic degradation of benzene in the gas phase was performed in a tubular vessel microreactor operating in a continuous flow mode.32 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 (FID) and a thermal conductivity detector (TCD). Conversion of benzene and mineralization of benzene were defined as the following % conversion ¼ ½ðC0
CÞ=C0   100 % mineralization ¼ ½ðCO2 Þproduced =6ðBenzeneÞconverted   100 where C0 is the initial concentration of benzene and C is the concentration of benzene after photocatalytic reaction.

3. RESULTS AND DISCUSSION Figure 1A shows the XRD patterns of the as-prepared samples including TNT and nanocomposites of TNT-CNT. It is clear to find that the post-treatment of washing process with control pH = 8 and calcination at 400 °C for 6 h in air does not affect the

ARTICLE

Figure 1. XRD patterns of TNT and composites of TNT-CNT. (A) Samples with the control of pH = 8 during the wash process using diluted HCl acid followed by calcination at 400 o C for 6 h in air; (B) samples with the control of pH = 7 during the wash process using diluted HCl acid and deionized water followed by calcination at 400 o C for 6 h in air.

tubular framework structure at all since all of the characteristic crystalline phase peaks of TNT are retained.50 In contrast, for the samples with control pH = 7 during the washing process, the post-treatment of calcination in air will significantly destroy the tubular structure of TNT as can be clearly reflected by the disappearance of characteristic peaks of TNT (Figure 1B); in this case, all the newly appeared peaks are attributed to the anatase phase of TiO2 particles. The nice tubular framework structure of the as-prepared TNT can also be seen from the TEM images in top panels of Figure 2. It is clear that the aggregation-free nanotubes are evenly distributed in the sample of TNT and the average inner diameter of TNT is ca. 5 nm. This is also evidenced by the pore-size distribution curve obtained from the Barrett
Joyner
Halenda (BJH) pore size distribution as shown in the inset in Figure 3. The specific Brunauer
Emmett
Teller (BET) surface area and pore volume of the as-synthesized TNT 7882

dx.doi.org/10.1021/jp1115838 |J. Phys. Chem. C 2011, 115, 7880–7886

The Journal of Physical Chemistry C

ARTICLE

Figure 4. UV
vis DRS of samples of TNT, CNT, and composites of TNT-CNT.

Figure 2. Typical TEM images of the samples of TNT (top panels) and composites of TNT-30 wt % CNT (bottom panels).

Figure 3. BET adsorption isotherm of TNT; inset is the pore size distribution.

are 379 m2/g and 0.97 cm3/g, respectively, which is in agreement with the previous report; namely, the tubular structured TNT has a large surface area and pore volume.50 We have also performed the TEM analysis of nanocomposite of TNT-30 wt % CNT. As shown in bottom panels of Figure 2, a clear distinction between CNT and TNT can be easily found in the TNT-CNT nanocomposite. The UV
vis diffuse reflectance spectra (DRS) of all the samples are displayed in Figure 4. As is seen clearly, both TNT and composites of TNT-CNT exhibit good light absorption capability in the UV region. In particular, the composites of TNT-CNT show much higher light absorption capability than the sample of bare TNT, therefore indicating that TNT-CNT material might have better photocatalytic performance toward a specific reaction. The presence of CNT regarding TNT-CNT composites leads to the continuous absorption band in the range of 400
800 nm, which is in agreement with the black color of the

samples.32 A higher amount of CNT doping into the matrix of TNT causes a stronger background absorption in the range of 400
800 nm as compared to that for lower amount of CNT doping. Figure 5 summarizes the time-online photocatalytic results for the gas-phase degradation of benzene over the samples of TNT and TNT-CNT nanocomposites. For the TNT sample, the average conversion ratio of benzene is ca. 5% along with the produced CO2 of ca. 32 ppm during the reaction of 20 h. This represents an average mineralization ratio of 42.7%. For the nanocomposite of TNT-CNT, it is clear to see that a certain amount of CNT doping into the matrix of TNT only affects the conversion ratio of benzene slightly. However, the produced amount of CO2 is higher over all of the TNT-CNT samples than the bare TNT. This means that, for the TNT-CNT nanocomposites, the presence of CNT is able to improve the mineralization ratio for photocatalytic degradation of benzene. In particular, TNT-30 wt % CNT shows the best photocatalytic performance for degradation of benzene based on the mineralization ratio of benzene. The average produced amount of CO2 over TNT-30 wt % CNT is ca. 50 ppm during the steady state of reaction along with an average conversion ratio of benzene ca. 4%. This represents a much higher mineralization ratio of benzene, ca. 90%, which is two times higher than that over the bare TNT. Notably, the mineralization ratio for degradation of benzene over the TNT-30 wt % CNT is also higher than the best photocatalyst of TiO2 nanoparticles doped with 5 wt % CNT (TiO2-5 wt % CNT) as reported recently,32 for which the average mineralization ratio of benzene is about 64.6% with an average conversion ratio of 6.4%. The observation of strikingly high mineralization ratio for photocatalytic degradation of benzene over the TNT-30 wt % CNT is interesting because it is a crucial factor affecting the sustainability of a photocatalyst. The high mineralization ratio means the fewer amount generation of intermediates. This is particularly important for the gas-phase degradation of benzene because the photocatalytic active sites of the surface will be blocked if the intermediates are deposited on the photocatalyst surface during the reaction.32,53
55 For example, over the commerical P25-TiO2, the photocatalytic activity for gas-phase degradation of benzene is quite unstable, as displayed in Figure 6. This deactivation is attributed to the blockage of active sites by the stable intermediates deposited 7883

dx.doi.org/10.1021/jp1115838 |J. Phys. Chem. C 2011, 115, 7880–7886

The Journal of Physical Chemistry C

ARTICLE

Figure 6. Time-online data for gas-phase photocatalytic degradation of benzene over the reference sample of commercial P25-TiO2.

Figure 5. Time-online data for gas-phase photocatalytic degradation of benzene over the samples of TNT, and composites of TNT-CNT.

on the photocatalyst surface during the time-online reaction.32,53
55 On the contrary, no obvious deactivation trend is observed on the TNT-CNT composites, as reflected in Figure 5. Analogous photocatalytic stability was also found over the nanocomposite of TiO2 nanoparticles doped with CNT.32 Also, it should be pointed out that, similar to the nanocomposite of TiO2 nanoparticles doped with CNT, it is crucial to control the composition ratio in TNT-CNT to obtain an optimal synergistic effect between TNT and CNT toward the photocatalytic degradation of a given organic pollutant. Thus, it can be concluded that the presence of appropriate amount of CNT in the matrix of either TNT or TiO2 nanoparticles will strengthen the photocatalyst stability toward the gasphase degradation of benzene. Different from the TiO2-5 wt % CNT nanocomposite as reported earlier,32 the introduction of CNT into the matrix of TNT will remarkably improve the mineralization ratio for degradation of benzene; however, the conversion ratio of benzene is nearly identical or even decreased as compared to the bare TNT. As is well-known, CNT has the excellent properties of transporting and shuttling electrons.32
37 The large electron storage capacity can be evidenced by the research work reported by Kamat’s group in which they found that one electron can be stored by 32 carbon atoms.56 If so, the

lifetime of photogenerated charge carriers will be significantly boosted, and in turn, the amount of photoactive radical species with strong oxidation power, such as hydroxyl radical (•OH) and superoxide radical (O2•
) species, should be increased. This would, in principle, improve the conversion ratio of benzene over the TNT-CNT nanocomposites as compared to the bare TNT. However, we did not observe the higher conversion ratio of benzene over the TNT-CNT nanocomposites than the bare TNT during our experiments regarding the photocatalytic degradation of benzene. To explore the possible reason that accounts for this interesting phenomenon, we have done the ESR analysis of the TNT30 wt % CNT nanocomposite and the bare TNT. On the basis of the ESR data in Figure 7, it is interesting to see that the intensity of hydroxyl radical and superoxide radical species as detected over the bare TNT sample is higher than that over the TNT-30 wt % CNT nanocomposite, suggesting the more amount of radical species with strong oxidation power generated over the bare TNT than TNT-30 wt % CNT nanocomposite. This explains why the higher conversion ratio of benzene is not observed over the TNT-CNT nanocomposites than the bare TNT. On the contrary, the TNT-CNT nanocomposites show an averagely decreased conversion ratio of benzene than the bare TNT. However, because of the much higher mineralizatio ratio of benzene over TNT-CNT than the bare TNT, no obvious catalyst deactivation is found over the TNT-CNT nanocomposite. For the bare TNT, a deactivation trend is clearly observed after 16 h of reaction although it still exhibits much higher photocatalytic performance than P25 nanoparticles, which could be attributed to the one-dimensional tubular structure of TNT with high surface area. The TEM images, as shown in parts c and d of Figure 2, can also provide the evidence to illustrate indirectly why the intensity of hydroxyl radical and superoxide radical species as detected over the TNT-CNT nanocomposite is not enhanced as compared to the bare TNT. As seen from the TEM images, there is no spatially intimate interface contact between TNT and CNT in the composite of TNT-CNT. Thus, the transfer of photogenerated electrons from TNT under the irradiation of UV light will be not as effective as the composite materials of TiO2 nanoparticles doped with CNT, for which there is a close contact between the TiO2 nanoparticles and CNT.32 This in turn explains reasonably why the detected 7884

dx.doi.org/10.1021/jp1115838 |J. Phys. Chem. C 2011, 115, 7880–7886

The Journal of Physical Chemistry C

ARTICLE

Figure 8. Bar plot showing the remaining fraction of benzene in the dark over the TNT-CNT nanocomposites after reaching the adsorption equilibrium for gas-phase degradation of benzene.

Figure 7. ESR spectra of radical adducts trapped by DMPO. (A) DMPO
•OH radical species of detected for the samples dispersion in water; (B) DMPO-O2•
radical species detected for the samples dispersion in methanol.

intensity of radical species over TNT-CNT, as shown in Figure 7, is lower than that over the bare TNT. From the TEM images in Figure 2, it can also be concluded that the presence of CNT does not provide the solid template for attaching the TNT onto the sidewall of CNT during the hydrothermal treatment of the suspension of TiO2 nanoparticles and CNT, meaning that the CNT and TNT are simply interwined each other. On the basis of the above discussions, we can primarily ascribe the high mineralization ratio for gas-phase photocatalytic degradation of benzene over the TNT-CNT nanocomposites to the following factors, including enhanced adsorptivity, light intensity absorption, and the special one-dimensional tubular nanostructure. It has been well recognized that adsorption of the substrate is an important factor that is able to control the reaction mechanism and the formation of products.12
16,57,58 In turn, adsorption is influenced by many factors, for example, reaction medium and type of photocatalysts.57,58 The increased adsorptivity of TNT-CNT can be reflected from Figure 8. It can be seen that, with the increased doping amount of CNT, the TNT-CNT nanocomposites will have the improved adsorptivity for benzene in the dark. The second factor of increased light intensity absorption is evidenced by the UV
vis DRS spectra as shown in Figure 4. It should be noted that these two factors associated with TNT-CNT nanocomposites have also been observed with regard to the composite of TiO2 nanoparticles doped with CNT as reported earlier.32 However, interestingly, the remarkably high mineralization ratio for gas-phase degradation of benzene is only observed over the TNT-CNT nanocomposite. For the composite of TiO2 nanoparticles doped with CNT as reported previously, the average optimum mineralization ratio is about 64.6% over the nanocomposite of TiO2 nanoparticles doped with 5 wt % CNT along with a 6.4% conversion ratio of benzene,32 which is much lower than the mineralization ratio of ca. 90% over the tubular nanocomposite of TNT doped with 30 wt % CNT. We

primarily ascribe this intriguing difference of photocatalytic performance to the specific morphology distinction between TNT and TiO2 nanoparticles. In other words, the nanotububular morphology endows TNT with the specific advantages as photocatalyst.23
31 First, the one-dimensional geometry of TNT faciliates fast and long-distance electron transport. Second, nanotubular TiO2 has larger specific surface area and pore volume as compared with that of TiO2 nanoparticles. Third, the light absorption and scattering can be markedly enhanced owing to the high length-todiameter ratio for the nanotubular TNT. It is hoped that our study regarding gas-phase degradation of benzene over bare TNT and TNT-CNT would enrich the chemistry of the TiO2-carbon composite materials as photocatalyst for the environmet remediation and stimulate further research interest on this intriguing topic. Furthermore, considering the tunable optical property of TNT by facile substitution of transition-metal ions, such as Co2þ, Ni2þ, and Cu2þ, via ion-exchange with sodium ions and proton ions in TNT,50 there would be a wide scope or potential to further optimize the photocatalytic performance of TNT-based onedimensional materials and their nanocomposites with CNT.

4. CONCLUDING REMARKS In summary, we have reported the synthesis of nanocomposites of TNT-CNT by controlling the preparation conditions carefully. For the first time, using the gas-phase degradation of benzene as a testing reaction, we have investigated the photocatalytic performance of TNT and TNT-CNT nanocomposites together. The results have demonstrated that one-dimensional TNT exhibits enhanced photocatalytic performance toward the gas-phase degradation of benzene as compared to the reference photocatalyst of bare P25-TiO2 nanoparticles, hence indicating the advantageous feature of TNT as photocatalyst for the degradation of pollutants. Further doping a certain amount of CNT into the matrix of TNT will increase the mineralization ratio significantly; however, the conversion ratio of benzene is only affected slightly. The possible reasons accounting for this interesting phenomenon can be primarily attributed to the specific nanotubular morphology associated with TNT-CNT nanocomposites, because such a behavior has not been observed for the gas-phase degradation of benzene over the nanocomposites of TiO2 nanoparticles and CNT. 7885

dx.doi.org/10.1021/jp1115838 |J. Phys. Chem. C 2011, 115, 7880–7886

The Journal of Physical Chemistry C In addition, in view of the easy availability of tunable optical property by substitution of sodium ions and proton ions in TNT with transition-metal ions, there would be a wide scope to optimize the photocatalytic performance of TNT-based onedimensional materials and their nanocomposites with CNT, which, as we believe, would enrich the chemistry of TiO2-carbon composites as photocatalyst for the environmental cleanup. We hope that the current work would stimulate further research interest on this intriguing research topic.

’ AUTHOR INFORMATION Corresponding Author

*Phone/Fax: þ86 591 22866126. E-mail: [email protected].

’ ACKNOWLEDGMENT The support from the National Natural Science Foundation of China (20903022, 20903023), the Award Program for Minjiang Scholar Professorship, Program for Changjiang Scholars and Innovative Research Team in Universities (PCSIRT0818), National Basic Research Program of China (973 Program: 2007CB613306), the Science and Technology Development of Foundation of Fuzhou University (2009-XQ-10), the Open Fund of Photocatalysis of Fuzhou University (0380038004), and Program for Returned High-Level Overseas Chinese Scholars of Fujian province is gratefully acknowledged. ’ REFERENCES (1) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425. (2) Tian, B.; Zheng, X.; Kempa, T. J.; Fang, Y.; Yu, N.; Yu, G.; Huang, J.; Lieber, C. M. Nature 2007, 449, 885. (3) Hochbaum, A. I.; Chen, R.; Delgado, R. D.; Liang, W.; Garnett, E. C.; Najarian, M.; Majumdar, A.; Yang, P. Nature 2008, 451, 163. (4) Gautam, U. K.; Fang, X.; Bando, Y.; Zhan, J.; Golberg, D. ACS Nano 2008, 2, 1015. (5) Wang, J.; Gudiksen, M. S.; Duan, X.; Cui, Y.; Lieber, C. M. Science 2001, 293, 1455. (6) McPhililips, J.; Murphy, A.; Jonsson, M. P.; Hendren, W. R.; Atkinson, R.; Hook, F.; Zayats, A. V.; Pollard, R. J. ACS Nano 2010, 4, 2210. (7) Mor, G. K.; Varghese, O. K.; Paulose, M.; Shankar, K.; Grimes, C. A. Sol. Energy Mater. Sol. Cells 2006, 90, 2011. (8) Chen, X.; Mao, S. S. Chem. Rev. 2007, 107, 2891. (9) Wang, Y.; Jiang, X.; Xia, Y. J. Am. Chem. Soc. 2003, 125, 16176. (10) Wang, Z. L. ACS Nano 2008, 2, 1987. (11) Zhou, H.; Wong, S. S. ACS Nano 2008, 2, 944. (12) Hoffmann, M. R.; Marin, S. T.; Choi, W.; Bahnemannt, D. W. Chem. Rev. 1995, 95, 69. (13) Linsebigler, A. L.; Lu, G.; Yates, J. T., Jr. Chem. Rev. 1995, 95, 735. (14) Gaya, U. I.; Abdullah, A. H. J. Photochem. Photobiol., C: Photochem. Rev. 2008, 9, 1. (15) Fox, M. A.; Dulay, M. T. Chem. Rev. 1993, 93, 341. (16) Rajeshwar, K.; Osugi, M. E.; Chanmanee, W.; Chenthamarakshan, C. R.; Zanoni, M. V. B.; Kajitvichyanukul, P.; Krishnan-Ayer, R. J. Photochem. Photobiol., C: Photochem. Rev. 2008, 9, 171. (17) Stone, V. F.; Davis, R. J. Chem. Mater. 1998, 10, 1468. (18) Adachi, M.; Murata, Y.; Harada, M.; Yoshikawa, S. Chem. Lett. 2000, 942. (19) Park, N. G.; Lagemaat, J.; Frank, A. J. J. Phys. Chem. B 2000, 104, 8989. (20) Shen, G.; Zhang, X. H.; Ming, Y.; Zhang, L.; Zhang, Y.; Hu, J. J. Phys. Chem. C 2008, 112, 4029. (21) Zhang, Z.; Wang, C. C.; Zakaria, R.; Ying, J. Y. J. Phys. Chem. B 1998, 102, 10871.

ARTICLE

(22) Nag, M.; Guin, D.; Basak, P.; Manorama, S. V. Mater. Res. Bull. 2008, 43, 3270. (23) Shankar, K.; Basham, J. I.; Allam, N. K.; Varghese, O. K.; Mor, G. K.; Feng, X.; Paulose, M; Seabold, J. A.; Choi, K. S.; Grimes, C. A. J. Phys. Chem. C 2009, 113, 6327. (24) Martinson, A. B. F.; McGarrah, J. E.; Parpia, M. O. K.; Hupp, J. T. Phys. Chem. Chem. Phys. 2006, 8, 4655. (25) Ohsaki, Y.; Masaki, N.; Kitamura, T.; Wada, Y.; Okamoto, T.; Sekino, T.; Niihara, K.; Yanagida, S. Phys. Chem. Chem. Phys. 2005, 7, 4157. (26) Tachikawa, T.; Tojo, S.; Fujitsuka, M.; Sekino, T.; Majima, T. J. Phys.Chem. B 2006, 110, 14055. (27) Jennings, J. R.; Ghicov, A.; Peter, L. M.; Schmuki, P.; Walker, A. B. J. Am. Chem. Soc. 2008, 130, 13364. (28) Zhu, K.; Vinzant, T. B.; Neale, N. R.; Frank, A. J. Nano Lett. 2007, 7, 3739. (29) Jennings, J. R.; Ghicov, A.; Peter, L. M.; Schmuki, P.; Walker, A. B. J. Am. Chem. Soc. 2008, 130, 13364. (30) Zhu, K.; Neale, N. R.; Miedaner, A.; Frank, A. J. Nano Lett. 2007, 7, 69. (31) Tachikawa, T.; Majima, T. J. Am. Chem. Soc. 2009, 131, 8485. (32) Xu, Y. J.; Zhuang, Y.; Fu, X. J. Phys.Chem. C 2010, 114, 2669. (33) Yao, Y.; Li, G.; Ciston, S.; Lueptow, R. M.; Gray, K. A. Environ. Sci. Technol. 2008, 42, 4952. (34) Lee, S. H.; Pumprueg, S.; Moudgil, B.; Sigmund, W. Colloids Surf., B 2005, 40, 93. (35) Yu, H.; Zhao, H.; Quan, X.; Chen, S. Chin. Sci. Bull. 2006, 51, 2294. (36) Yu, Y.; Yu, J. C.; Chan, C. Y.; Che, Y. K.; Zhao, J. C.; Ding, L.; Ge, W. K.; Wong, P. K. Appl. Catal., B 2005, 61, 1. (37) Yu, Y.; Yu, J. C.; Yu, J. G.; Kwok, Y. C.; Che, Y. K.; Zhao, J. C.; Ding, L.; Ge, W. K.; Wong, P. K. Appl. Catal., A 2005, 289, 186. (38) Hoyer, P. Langmuir 1996, 12, 1411. (39) Lei, Y.; Zhang, L. D.; Meng, G. W.; Li, G. H.; Zhang, X. Y.; Liang, C. H.; Chen, W.; Wang, S. X. Appl. Phys. Lett. 2001, 78, 1125. (40) Imai, H.; Takei, Y.; Shimizu, K.; Matsuda, M.; Hirashima, H. J. Mater. Chem. 1999, 9, 2971. (41) Michailowski, A.; AlMawlawi, D.; Cheng, G.; Moskovits, M. Chem. Phys. Lett. 2001, 349, 1. (42) Liu, S. M.; Gan, L. M.; Liu, L. H.; Zhang, W. D.; Zeng, H. C. Chem. Mater. 2002, 14, 1391. (43) Yu, J.; Dai, G.; Cheng, P. J. Phys. Chem. C 2010, 114, 19378. (44) Varghese, O. K.; Gong, D.; Paulose, M.; Grimes, C. A.; Dickey, E. C. J. Mater. Res. 2003, 18, 156. (45) Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino, T.; Niihara, K. Langmuir 1998, 14, 3160. (46) Tsai, C. C.; Teng, H. Chem. Mater. 2004, 16, 4352. (47) Du, G. H.; Chen, Q.; Che, R. C.; Yuan, Z. Y.; Peng, L. M. Appl. Phys. Lett. 2001, 79, 3702. (48) Seo, D. S.; Lee, J. K.; Kim, H. J. Cryst. Growth 2001, 229, 428. (49) Tian, Z. R.; Voigt, J. A.; Liu, J.; McKenzie, B.; Xu, H. J. Am. Chem. Soc. 2003, 125, 12384. (50) Sun, X.; Li, Y. Chem.
Eur. J. 2003, 9, 2229. (51) Zhu, Z.; Zhou, Y.; Yu, H.; Nomura, T.; Fugetsu, B. Chem. Lett. 2006, 35, 890. (52) Hou, P. X.; Liu, C.; Chen, H. M. Carbon 2008, 46, 2003. (53) Hou, Y.; Wang, X.; Wu, L.; Ding, Z.; Fu, X. Environ. Sci. Technol. 2006, 40, 5799. (54) Mendez-Roman, R.; Cardona-Martinez, N. Catal. Today 1998, 40, 353. (55) Einaga, H.; Futamura, S.; Ibusuki, T. Environ. Sci. Technol. 2001, 35, 1880. (56) Kongkanand, A.; Kamat, P. V. ACS Nano 2007, 1, 13. (57) Shiraishi, Y.; Saito, N.; Hirai, T. J. Am. Chem. Soc. 2005, 127, 12820. (58) Maldotti, A.; Molinari, A.; Amadelli, R. Chem. Rev. 2002, 102, 3811.

7886

dx.doi.org/10.1021/jp1115838 |J. Phys. Chem. C 2011, 115, 7880–7886