Ó Springer 2007
Journal of Nanoparticle Research (2007) 9:1087–1096 DOI 10.1007/s11051-006-9199-x
Synthesis of anatase titania-carbon nanotubes nanocomposites with enhanced photocatalytic activity through a nanocoating-hydrothermal process Qun Wang, Dong Yang, Daimei Chen, Yabo Wang and Zhongyi Jiang* Key Laboratory for Green Chemical Technology, School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300072, China; *Author for correspondence (Tel.: +86-22-2789-2143; Fax: +86-222789-2143; E-mail:
[email protected]) Received 21 August 2006; accepted in revised form 1 December 2006
Key words: carbon nanotubes, MWNT, titanium dioxide, nanocomposite, nanocoating-hydrothermal process, photocatalysis
Abstract Anatase TiO2 nanoparticles were covalently anchored onto acid-treated multi-walled carbon nanotubes (MWNTs) through a nanocoating-hydrothermal process to obtain TiO2-MWNTs nanocomposites. The composition and structural properties of the nanocomposites were characterized by XRD, BET, TG, TEM, HRTEM, EDX, XPS, and FTIR, and the formation of ester-bond linkage between TiO2 nanoparticles and MWNTs was demonstrated. The enhanced photocatalytic activity of TiO2-MWNTs nanocomposites was probed by photodegradation reaction of methylene blue under visible-light irradiation.
Introduction Titanium dioxide (TiO2) has been extensively employed as heterogeneous photocatalysts for the degradation of many kinds of contaminants in air and water, because of its strong oxidizing power, low cost, chemical inertness and long-term photostability (Fujishima et al., 2000; Hoffmann et al., 1995; Linsebigler et al., 1995). In order to enhance their photocalatytic activity, TiO2 nanoparticles with higher surface area and lower spacecharge recombination are usually employed. However, the size decrease of photocatalysts will induce some practical problems such as serious aggregation of TiO2 nanoparticles, sticky separation of catalyst from the reaction system, and unsuitability to the continuous flow systems, etc (Kedem et al., 2005). To date, several strategies have been proposed to solve these problems, such
as immobilizing TiO2 nanoparticles on solid carriers (Silva et al., 2003; Takeda et al., 1995; Tryba et al., 2003), or using the porous TiO2 materials with hierarchically ordered structure (Schattka et al., 2002; Shchukin et al., 2003a, b; Taguchi et al., 2005). Recently, the composites of TiO2 nanoparticles with carbon nanotubes (CNTs) (Huang et al., 2003; Jitiannu et al., 2004; Lee et al., 2003; Li et al., 2003; Sun et al., 2004; Wang et al., 2005; Yu et al., 2005a, b) have attracted much attention since CNTs have high capability to conduct electrons and adsorb hydrophobic substances, hardly adsorbed by TiO2 nanoparticles themselves. For example, Lee et al. (Lee et al., 2003) deposited anatase TiO2 nanoparticles on CNTs by the controlled hydrolysis and condensation of titanium bis-ammonium lactate dihydroxide in CNT-containing aqueous media. Gao and his
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coworkers (Huang et al., 2003; Sun et al., 2004) immobilized TiO2 nanoparticles on the sidewall of CNTs by chemical adsorption or using polyethylemine as the linker. Yu et al. (Yu et al., 2005a, b) found that the physical mixture of CNTs and nanocrystalline titania showed high photodegradation activity toward azo dye and acetone. However, TiO2 nanoparticles were usually linked with CNTs by weak interaction forces such as Van der Waals force, hydrogen bond or electrostatic force in most of above-mentioned TiO2-CNTs nanocomposites. In order to increase the stability of these nanocomposites and facilitate the electron transfer from TiO2 nanoparticles to CNTs, it is necessary to find a facile method obtaining TiO2-CNTs nanocomposites through covalent linkage. The nanocoating is an effective approach to prepare the structured materials (Caruso et al., 2001.), and the hydrothermal process can conveniently transform metal oxide from amorphous form to crystal form under mild conditions (Nagaveni et al., 2004). It can be thus naturally conjectured that, nanocoating-hydrothermal coupling process may be an ideal choice to prepare nanocrystal materials with desired structures. In this study, we attempted to use this process to prepare the nanocomposites of TiO2 nanoparticles with multi-walled carbon nanotubes (MWNTs). It is found that the TiO2 nanoparticles can be firmly anchored on the MWNTs through ester bonds, and the resulting nanocomposites showed higher photocatalytic activity in degrading methylene blue (MB) than pure TiO2 nanoparticles and Degussa P25. To the best of our knowledge, it is the first time to obtain covalent TiO2-MWNTs nanocomposites by the nanocoating-hydrothemal process.
Experimental Preparation of TiO2-MWNTs nanocomposites In a typical experiment, raw MWNTs (purity>95%, diameter 30–40 nm, length 1–2 lm, CVD method, purchased from Shenzhen Nanotech Port Co., Shenzhen, China) were firstly converted into acid form (designated as MWNTsCOOH) by ultrasonicating in the concentrated sulfuric acid/nitric acid mixture reported in the
literature (Sun et al., 2004). Subsequently, 200 mg MWNTs-COOH was dispersed in 50 g isopropanol with ultrasonication, and then, the resulting suspension was added into 40 wt% isopropanol solution of tetra-n-butyl titanium (TBT), which was used as a TiO2 precursor. The mixture of MWNTs-COOH and TBT was magnetically stirred for 30 min, and kept for another 12 h at room temperature. After centrifuging and drying at room temperature for 48 h, a black complex of TBT-MWNTs was obtained. The TBT-MWNTs complex was grinded into fine powders and transferred into a 100 ml Teflon-lined stainless steel autoclave, along with a certain amount of deionized water. Finally, the autoclave was maintained at high temperature for a certain time to make TiO2 crystallize, and then, cooled to room temperature naturally. After drying at 80°C, a series of TiO2-MWNTs nanocomposites were obtained corresponding to different temperature and time in hydrothermal process. The TiO2MWNTs samples were denoted as TM-X-Y, where X and Y represent the temperature and time of hydrothermal process, respectively. For comparison, pure TiO2 without MWNTs was also prepared with the same procedure.
Characterization of TiO2-MWNTs nanocomposites The X-ray powder diffraction (XRD) patterns were obtained on a Philips XÕPertpro diffractometer using Co Ka radiation to determine the crystallite size and identity of TiO2 samples. The Brunaure-Emmett-Teller (BET) surface area was determined by using a ThermoFinnigan Sorptomatic 1990 nitrogen adsorption apparatus. The thermal behavior of the TiO2-MWNTs nanocomposites was analyzed with a thermogravimetry (TG, Pyris, Perkin-Elmer, USA) at a heating rate of 10° C/min under an air flow rate of 70 ml/min. The IR spectra were recorded on a NICOLET560 Fourier transform infrared (FTIR) spectrometer. X-ray photoelectron spectroscopy (XPS) was obtained by a Perkin-Elmer PHI 1600 ESCA system with a monochromatic Mg Ka source and a charge neutralizer. Transmission electron microscopy (TEM, JEM-100CXa) and high resolution TEM (HRTEM, Tecnai G2 F20) were employed to observe the microstructure of the nanocomposites.
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Results and Discussion
ing, the ester bonds and a few C-O-Ti bonds formed, meanwhile, amorphous TiO2 nanoparticles also converted into the crystal form. Moreover, a number of hydroxyl groups on TiO2 surface can be generated in the hydrothermal processing, which are desirable for the enhancement of TiO2 photoactivity. TiO2 nanoparticles attached on the MWNTs did not fall off even after vigorous washing and sonication for a long time. It is noted that the resulting nanocomposites preserve the structure of MWNTs very well after the nanocoating-hydrothemal processing, meanwhile, the size of TiO2 crystal particles still remains at nanoscale. Therefore, this methodology may be widely effective for the nanocomposites preparation of MWNTs with metal or semiconductor nanocrystals. In order to ensure the adsorption equilibrium, nanocoating process time was fixed as long as 12 h. Thus, the hydrothermal conditions including reaction temperature and time became the main factors affecting the morphology and structure of the resulting nanocomposites. In the current work, nine TiO2-MWNTs samples were prepared at different reaction temperature and time as listed in Table 1.
Synthetic approach
Characterization of TiO2-MWNTs nanocomposites
Scheme 1 is the illustration of TiO2-MWNTs nanocomposites preparation via a nanocoatinghydrothemal process. At first, raw MWNTs were treated by mixed acid, which can not only shorten carbon nanotubes, but also generate large amount of hydroxyl ()OH), carboxyl ()COOH), and carbonyl ()C = O) groups for further modification. These functional groups ensure the considerable adsorption of the chemical substance on the surface of MWNTs through hydrogen bond and Van der Waals attraction. Thus, when MWNTsCOOH was mixed with TBT, TBT molecules were promptly adsorbed on the sidewalls and end caps of MWNTs-COOH through noncovalent bonds, named as a nanocoating process. After the TBTMWNTs complex was exposed to air, amorphous TiO2 nanoparticles and some Ti (IV) complex ion [TiOHn(OH2)m]4)n gradually formed on the sidewalls of MWNTs due to the water existed in air. At this time, the noncovalent essence of linkage between MWNTs and titanium complexes did not change. In the subsequent hydrothermal process-
XRD pattern of TiO2 and TiO2-MWNTs nanocomposites prepared in different hydrothermal conditions is shown in Figure 1. From this pattern, it can be observed that the anatase constitutes the major crystal form in the pure TiO2 sample, with minor amount of brookite. Due to the presence of MWNTs, a new peak at about 50 degree appears, which corresponds to the (100) reflection of MWNTs. However, it is hard to elicit other main characteristic peaks of MWNTs because of the distinct crystallization of anatase TO2 and their thorough coating on the MWNTs. Due to the overlap of the highest intensity peak for anatase (101) and brookite (210) reflection, the phase composition of TiO2 in these samples was calculated from the integrated intensities of anatase (101) and brookite (211) peaks according to Zhang et al. (Zhang et al., 2000), and the average size of TiO2 particles in all samples is estimated by the Scherrer formula. All the results are listed in Table 1, from which it can be seen that the content of brookite in TiO2-MWNTs nanocomposites is
Determination of photocatalytic activity The photocatalytic activity of TiO2-MWNTs nanocomposites was evaluated by the photodegradation of methylene blue (MB) under visiblelight irradiation at room temperature. A total of 80 mg TiO2-MWNTs powder was dispersed in 80 ml of 30 mg/l MB aqueous solution, and the suspension was stirred not less than 8 h to reach adsorption equilibrium in darkness. Then, the suspension was irradiated with a 150 W highpressure xenon lamp (k > 420 nm) with continuous stirring, atmosphere bubbling (about 0.5 dm3/ min), and cooling by cold water. At a defined time interval, the concentration of MB in the suspension was analyzed using a UV-Vis spectrophotometer (U-2800, Hitachi) at 664 nm. The photocatalytic activity of TiO2-MWNTs samples was calculated from the decrease amount of MB. For comparison, the MB photodegradation experiment was conducted under the same conditions using pure TiO2 nanoparticles and Degussa P25 powder as photocatalysts.
1090 O
O
C-OH acid treatment
OH
C-OH coating
OH OH
C-OH O
MWNTs
MWNTs-COOH hydrolyzation
O
O C
O
O
C
C
O
hydrothermal processing
O
TiO2-MWNTs
amorphous TiO2;
anatase TiO2
Scheme 1. Schematic illustration of the preparation of anatase TiO2-MWNTs composites via covalent linkage.
much lower than that of pure TiO2. Moreover, with the increase of temperature, the content of brookite decreases gradually and the content of anatase increases correspondingly. These results indicate that MWNTs can suppress the formation of brookite or catalyze the transformation from brookite to anatase like B3+ and F) (Chen et al., 2006; Yu et al., 2002). On the other hand, the average size of all samples changes little, which is almost around 10 nm, suggesting that the key
factors affecting the size of TiO2 nanoparticles should come from the nanocoating process. The BET surface area (SBET) of TiO2-MWNTs nanocomposites was also influenced by hydrothermal conditions. The surface areas of MWNTsCOOH and neat TiO2 are 75.0 and 97.0 m2/g, respectively, while most of TiO2-MWNTs composites show high SBET except TM-180-48. It should be particularly mentioned that TM-180-10 has much higher SBET than others due to the
Table 1. Summary of physicochemical properties of TiO2, MWNTs and TiO2-MWNTs composites Sample
TiO2 TM-120-10 TM-120-24 TM-120-48 TM-150-10 TM-150-24 TM-150-48 TM-180-10 TM-180-24 TM-180-48 MWNTs
Hydrothermal conditions
Anatase
Temp (°C)
Time (h)
Crystalline size (nm)
Content (%)
180 120 120 120 150 150 150 180 180 180
10 10 24 48 10 24 48 10 24 48
11.2 9.6 10.5 10.6 9.8 10.2 10.8 10.5 11.0 11.5
84.5 96.4 92.4 95.2 96.6 90.7 94.2 97.0 96.1 99.5
Brookite content (%)
SBET (m2/g)
15.5 3.6 7.6 4.8 3.4 9.3 5.8 3.0 3.9 0.5
97.0 100 123 96.0 100 104 97.0 156 102 81.0 75.0
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A(105)
A(200)
C(100)
40 2 Theta (degree)
f e d c b a 60
Figure 1. XRD spectra of pure TiO2 (a) and TiO2-MWNTs nanocomposites prepared under different hydrothermal conditions: (b) TM-120-10, (c) TM-150-10, (d) TM-180-10, (e) TM-180-24, (f) TM-180-48.
effective suppression of TiO2 nanoparticles conglomeration by the presence of MWNTs. Moreover, it can be found that the surface area value is rather high and comparable to TiO2-MWNTs composites obtained by a sol-gel method (Wang et al., 2005) and a physically mixed method (Yu et al., 2005b). The synergistic combination of high surface area and nanostructure of these composites is expected to enhance the wide and evergrowing application of titania. In addition, with the increase of hydrothermal time at 120 °C or 150 °C, SBET of nanocomposites firstly increases and then decreases; however, the changing trend is monotonic at 180 °C. The tentative analysis is presented as follows: the hydrothermal treatment process can be further divided into two basic steps: conversion of TiO2 from amorphous form to crystal phase and growing up of crystal TiO2. In the former step, the size of TiO2 particles changes little while TiO2 evolutes from disordered form to ordered form substantially; in the later step, the morphology of TiO2 nanoparticles changes little while the size of TiO2 nanoparticles increases significantly. Weighing the factors including surface area, crystalline size, and phase composition, TM180-10 may have the highest photocatalytic activity among all samples, which is thus chosen as the main concern in the following investigations. The thermal gravimetric (TG) analysis of TiO2MWNTs composites was performed to assess their stability and the content of TiO2 loaded on the
nanotubes. Figure 2 shows the typical TG curves of TM-180-10 before and after hydrothermal processing. There are three separate weight-loss steps in the TG curve of TM-180-10 after hydrothermal treatment (Figure 2b). The first one appearing in the range of room temperature )500 °C can be assigned to the residual TiO2 precursor and organic solvent. Following that, a steep decrease in mass centered at 630 °C occured, corresponding to thermo-decomposition of MWNTs and crystallization of amorphous TiO2. This result hints that the crystallization at high temperature is not appropriate for the preparation of nanocomposites like anatase TiO2-MWNTs (Wang et al., 2005); the hydrothermal approach may be a good choice instead. Finally, there is a very slow weight-loss from 670 to 800 °C, and TiO2 becomes the major component in the product. Accordingly, the weight percentage of TiO2 in the TM-180-10 was estimated to be about 30 wt%. Compared with Figure 2a, the TG curve of TM180-10 after hydrothermal treatment (Figure 2b) shows two main differences: one is that the weightloss (5 wt%) during the first step becomes less, owing to the complete hydrolyzation of TBT; the other is that the decomposition temperature of MWNTs becomes higher, indicating the stability enhancement of MWNTs after modified with TiO2 nanoparticles. The morphologies of TiO2-MWNTs composites were revealed by TEM investigation and the typical TEM images of TM-180-10 are showed in Figure 3. From Figure 3a, a low magnification TEM micrograph of TM-180-10, MWNTs coated
0
Mass Change (%)
20
A: Anatase B: Brookite C: MWNTs
A(004)
B(211)
Intensity (a.u.)
A(101)+B(210, 111)
b a
-40
-80 200
400
600
800
Temperature (°C) Figure 2. TG analysis of TM-180-10 before (a) and after (b) hydrothermal treatment.
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Figure 3. TEM (a and b) and HRTEM (c) images of TM180-10.
with TiO2 nanoparticles are identified and no apparent agglomeration of MWNTs can be observed. An enlarged image of a segment of
nanotube (Figure 3b) clearly shows that the TiO2 nanoparticles with about 10 nm in size are attached on the sidewall of MWNTs, which is consistent with XRD results. The corresponding energy dispersive X-ray (EDX) pattern (Figure 4) exhibits distinct peaks of Ti and O, and the atomic ratio of O to Ti is close to 2.0, confirming that the nanoparticles on the MWNTs is actually composed of TiO2. However, TiO2 nanoparticles did not cover all the surface of MWNTs, since the active sites produced by the acid-treatment are relatively low (Li et al., 2003). A typical HRTEM image of the tube walls (Figure 3c) demonstrates the presence of some nanocrystals showing clear (101) lattice fringes of anatase TiO2 and the interface region between MWNTs and TiO2 nanoparticles, indicating that TiO2 nanocrystals are well attached on the outer shell of MWNTs through strong covalent interaction. Figure 5 presents the FTIR spectra of MWNTsCOOH (Figure 5a) and TM-180-10 (Figure 5b). Both of the spectra show a peak at 1588 cm)1, which corresponds to the active carbon stretching mode of MWNTs (Konstantinou et al., 2004). The appearance of two peaks at 1714 and 1626 cm)1 in the IR spectrum of MWNTs-COOH clearly indicates the formation of carboxyl groups and carbonyl groups after ultrasonicating in the mixed acid. It is noteworthy that when TiO2 is immobilized on the sidewalls of MWNTs, the characteristic peak of carboxyl groups of MWNTs shifts from 1714 to 1742 cm)1, meaning the formation of covalent bonding between oxidized MWNTs and TiO2. It is deduced that the possible type of covalent bonds is O = C-O-Ti due to the esterification between the )COOH of MWNTs-COOH and the -OH of TiO2. Additionally, a low-frequency band in the range of 900–500 cm)1 appears in the IR spectrum of TM-180-10, which corresponds to the mTiOTi of the mineral network, confirming the presence of TiO2 structure (Soler-Illia et al., 2002). X-ray photoelectron spectroscopy (XPS) measurements were performed to further determine the interaction of MWNTs with TiO2 nanoparticles. Figure 6 shows the typical high-resolution and curve fitting of C1s and O 1s XPS of TM-180-10 sample. It is observed that the C1s spectrum (Figure 6a) could be fitted to three peaks, located at 284.8, 285.8 and 286.7 eV, which are attributed to C-C, C-O and C = O bonds, respectively.
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Figure 4. EDX spectrum of TM-180-10.
a
C-C: 284.5eV, 41.0%(Area) C-O: 285.8eV, 42.5%(Area) C=O: 287.0eV, 16.5%(Area)
Intensity
Moreover, the contents of C-O (42.5%) and C = O (16.5%) are relative high, suggesting the covalent binding between these two components. On the other hand, the O1s spectrum (Figure 6b) is also composed of three peaks: Ti-O bonds at 531.2 eV, C-O bonds at 532.6 eV and H-O bonds at 534.1eV. The Ti 2p spectrum (data not shown) was recorded and only one peak at 458.9eV, the characteristic peak of the Ti in the TiO2 lattice, was identified. These results further confirm the essence of covalent linkage in TiO2-MWNTs nanocomposites.
C-C C-O C=O
292
290
288
286
284
282
Binding energy (eV)
1742 1626 1588 1714
Intensity
1626 1588
Absorbance
b
b
Ti-O: 531.2eV, 62.1%(Area) C-O: 532.6eV, 24.4%(Area) H-O: 534.1eV, 13.5%(Area)
Ti-O C-O
a
H-O
4000
3000
2000
Wavenumber
1000
(cm-1)
Figure 5. FTIR spectra of (a) MWNTs-COOH and (b) TM-180-10.
538
536
534
532
530
528
Binding energy (eV)
Figure 6. C 1s (a) and O 1s (b) XPS spectra of TM-180-10.
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1.0
d C/C0
0.8
c 0.6
a
ln(C0/C)
1.2
0.4
0.4 0.0
0.2
0
b
0.8
0
4 Reaction time (h)
2
b a
8
4
6
8
Reaction time (h) Figure 7. Kinetics of methylene blue degradation under visible-light irradiation without any solid (d) and in the presence of different solids: (a) TM-180-10; (b) P25; (c) MWNTs. The inset shows the semilogarithmic plots of the results (a) and (b).
Photocatalytic Activity of TiO2-MWNTs nanocomposites To examine the photocatalytic efficiency (E) of TiO2-MWNTs nanocomposites, the photodegradation of MB under visible-light irradiation has been introduced. Figure 7 illustrates the changes of MB concentration with reaction time photocatalyzed by different samples and the inset is a plot of ln(C0/C) versus irradiation time for MB degradation with TM-180-10 and Degussa P25. It is found that the direct visible-light illumination without any solid and the presence of MWNTs can lead to 13 and 25% decomposition within 7 h, which are higher than the data of phenol conversion within 4 h UV irradiation (Wang et al., 2005). This can be explained by the high-absorption capability of MWNTs for visible light, which can accelerate the direct photolysis of MB. After 7 h, the photodegradation efficiency of TM-180-10 can reach 70.7%, only six percent higher than P25. When the reaction time exceeds 7 h, the MB concentration in the reaction system involving TiO2MWNTs composites enhances again due to the desorption of MB molecules from the surface of MWNTs. Thus, to keep the direct comparison, the reaction time discussed in this article is no more than 7 h. On the other hand, because there is only 30 wt% TiO2 component in TM-180-10, a parameter of relative photocatalytic efficiency (RE) is introduced, which is equal to the quotient
of E and quality of photocatalyst. Accordingly, negligible with the photoactivity of MWNTs, the RE value of TM-180-10 is calculated to be 2.95 wt% per milligram TiO2, which is almost 2.5 times higher than that of P25. The large enhancement of photocatalytic activity can be attributed to the kinetically synergetic effect between MWNTs and TiO2 nanoparticles. Two plots of ln(C0/C) versus irradiation time in the presence of TM-180-10 and P25 both give straight line, if the first data point at 0 h is excluded (small insert in Figure 7). From the slopes, apparent first-order rate constants are fitted to be 0.170 and 0.145 h)1, respectively, indicating that this reaction is of pseudofirst-order (Lettmann et al., 2001). The photocatalytic oxidation mechanism of MB under visible-light irradiation by TiO2 (k > 420 nm) is different from the pathway implicated under UV irradiation (Konstantinou et al., 2004). In the former case, the adsorbed dye rather TiO2 was excited by visible light to appropriate singlet or triplet states, followed by electron injection from the excited dye molecules into the conduction band of the TiO2 particles, whereas the dye is converted to the cationic dye radicals that undergo degradation (Chen et al., 2002). Carbon nanotubes act an important role in the improvement of photocatalytic efficiency of the TiO2-MWNTs nanocomposites. On the one hand, MWNTs can prevent TiO2 from agglomerating as the immobilized support, and suppress the formation of brookite TiO2 without any photocatalytic activity, thus providing a high active surface area of the resulting composite catalyst. On the other hand, MWNTs have a high adsorption capacity for MB molecules, causing the concentration of MB in the vicinity of MWNTs is higher than other places in the reaction system. When some MB molecules on the surface of TiO2 nanocrystals are degraded, the other MB molecules adsorbed on the carbon nanotubes can transfer to the residual vacancies through slip-induced surface diffusion, which may be a faster process than the free diffusion in solution (Wang et al., 2005).
Conclusions In summary, a simple nanocoating-hydrothermal method was successfully developed for the
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preparation of TiO2-MWNTs composites, and their characterization of TEM, HRTEM, FTIR and XPS confirmed that TiO2 nanoparticles were immobilized on the nanotubes with the covalent bonds. The presence of MWNTs can efficiently inhibit the agglomeration of the TiO2 nanoparticles and the formation of brookite TiO2, so that the resulting composite possesses more photocatalytically active sites than the pure titania using the same method. Moreover, the difference of MB concentration between MWNTs and TiO2, owing to high adsorption capacity of MWNTs, caused the surface diffusion of MB molecules from MWNTs to TiO2. These resulted in higher photocatalytic activity of TiO2-MWNTs composites than that of pure TiO2 nanomaterials. The nanocoating-hydrothermal process may open a new and versatile way to prepare the nanocomposites like TiO2-MWNTs under mild conditions.
Acknowledgements This work is supported by the Program for the Cross-Century Talent Raising Program of Ministry of Education of China and Changjiang Scholars and Innovative Research Teams in University (PCSIRT). We would like to thank Professor Fei He for their kind assistance in XPS measurements.
References Caruso R.A. & M. Antonietti, 2001. Sol-gel nanocoating: an approach to the preparation of structured materials. Chem. Mater. 13, 3272–3282. Chen C., X. Li, W. Ma & J. Zhao, 2002. Effect of transition metal ions on the TiO2-assisted photodegradation of dyes under visible irradiation: a probe for the interfacial electron transfer process and reaction mechanism. J. Phys. Chem. B 106, 318–324. Chen D., D. Yang, Q. Wang & Z. Jiang, 2006. Effects of boron doping on photocatalytic activity and microstructure of Titanium dioxide nanoparticles. Ind. Eng. Chem. Res. 45, 4110–4116. Fujishima A., T.N. Rao & D.A. Tryk, 2000. Titanium dioxide photocatalysis. J. Photochem. Photobiol. C: Photochem. Rev. 1, 1–21. Hoffmann M.R., S.T. Martin, W. Choi & D.W. Bahnemann, 1995. Environmental applications of semiconductor photocatalysis. Chem. Rev. 95, 69–96.
Huang Q. & L. Gao, 2003. Immobilization of rutile TiO2 on multiwalled carbon nanotubes. J. Mater. Chem. 13, 1517– 1519. Jitiannu A., T. Cacciaguerra, R. Benoit, S. Delpeux, F. Be´guin & S. Bonnamy, 2004. Synthesis and characterization of carbon nanotubes-TiO2 nanocomposites. Carbon 42, 1147– 1151. Kedem S., J. Schmidt, Y. Paz & Y. Cohen, 2005. Composite polymer nanofibers with carbon nanotubes and Titanium dioxide particles. Langmuir 21, 5600–5604. Konstantinou I.K. & T.A. Albanis, 2004. TiO2-assisted photocatalytic degradation of azo dyes in aqueous solution: kinetic and mechanistic investigations: A review. Appl. Catal. B: Environ. 49, 1–14. Lee S.-W., S.W. Sigmund, 2003. Formation of anatase TiO2 nanoparticles on carbon nanotubes. Chem.Commun. 6, 780– 781. Lettmann C., K. Hildenbrand, H. Kisch, W. Macyk & W.F. Maier, 2001. Visible light photodegradation of 4-chlorophenol with a coke-containing Titanium dioxide photocatalyst. Appl. Catal. B:Environ. 32, 215–227. Li X., J. Niu, J. Zhang, H. Li & Z. Liu, 2003. Labeling the defects of single-walled carbon nanotubes using Titanium dioxide nanoparticles. J. Phys. Chem. B 107, 2453–2458. Linsebigler A.L., G. Lu & J.T. Yates, 1995. Photocatalysis on TiO2 surfaces: principles, mechanisms, and selected results. Chem. Rev. 95, 735–758. Nagaveni K., M.S. Hegde, N. Ravishankar, G.N. Subbanna & M. Giridhar, 2004. Synthesis and structure of nanocrystalline TiO2 with lower band gap showing high photocatalytic activity. Langmuir 20, 2900–2907. Schattka J.H., D.G. Shchukin, J. Jia, M. Antonietti & R.A. Caruso, 2002. Photocatalytic activities of porous Titania and Titania/Zirconia structures formed by using a polymer gel templating technique. Chem. Mater. 14, 5103–5108. Shchukin D.G., J.H. Schattka, M. Antonietti & R.A. Caruso, 2003. Photocatalytic properties of porous metal oxide networks formed by nanoparticle infiltration in a polymer gel template. J. Phys. Chem. B 107, 952–957. Shchukin D.G. & R.A. Caruso, 2003. Inorganic macroporous films from preformed nanoparticles and membrane templates: synthesis and investigation of photocatalytic and photoelectrochemical properties. Adv. Funct. Mater. 13, 789–794. Silva C.G. & J.L. Faria, 2003. Photochemical and photocatalytic degradation of an azo dye in aqueous solution by UV irradiation. J. Photochem. Photobiol. A: Chem. 155, 133–143. Soler-Illia G.J. de A.A., A. Louis & C. Sanchez, 2002. Synthesis and characterization of mesostructured Titania-based materials through evaporation-induced self-assembly. Chem. Mater. 14, 750–759. Sun J., M. Iwasa, L. Gao & Q.H. Zhang, 2004. Single-walled carbon nanotubes coated with titania nanoparticles. Carbon 42, 885–901. Taguchi A. & F. Schu¨th, 2005. Ordered mesoporous materials in catalysis. Micropor. Mesopor. Mater. 77, 1–45.
1096 Takeda N., T. Torimoto, S. Sampath, S. Kuwabata & H. Yoneyama, 1995. Effect of inert supports for Titanium dioxide loading on enhancement of photodecomposition rate of gaseous propionaldehyde. J. Phys. Chem. 99, 9986–9991. Tryba B., A.W. Morawske & M. Inagaki, 2003. Application of TiO2-mounted activated carbon to the removal of phenol from water. Appl. Catal. B: Environ. 41, 427–433. Wang W.D., P. Serp, P. Kalck & J.L. Faria, 2005. Photocatalytic degradation of phenol on MWNT and titania composite catalysts prepared by a modified sol–gel method. Appl. Catal. B: Environ. 56, 305–312. Yu J.C., J. Yu, W. Ho, Z. Jiang & L. Zhang, 2002. Effects of Fdoping on the photocatalytic activity and micro-structures of nanocrystalline TiO2 Powders. Chem. Mater. 14, 3808–3816.
Yu Y., J.C. Yu, C.-Y. Chan, Y.-K. Che, J.-C. Zhao, L. Ding, W.-K. Ge & P.-K. Wong, 2005. Enhancement of adsorption and photocatalytic activity of TiO2 by using carbon nanotubes for the treatment of azo dye. Appl. Catal. B: Environ. 61, 1–11. Yu Y., J.C. Yu, J.-G. Yu, Y.-C. Kwok, Y.-K. Che, J.-C. Zhao, L. Ding, W.-K. Ge & P.-K. Wong, 2005. Enhancement of photocatalytic activity of mesoporous TiO2 by using carbon nanotubes. Appl. Catal. A: Gen. 289, 186–196. Zhang H. & J.F. Banfield, 2000. Understanding polymorphic phase transformation behavior during growth of nanocrystalline aggregates: insights from TiO2. J. Phys. Chem. 104, 3481–3487.
