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carbon nanotube composites using supercritical ethanol and their photocatalytic activity for phenol degradation under visible light irradiation
Carbon 45 (2007) 1795–1801 www.elsevier.com/locate/carbon
Preparation of titania/carbon nanotube composites using supercritical ethanol and their photocatalytic activity for phenol degradation under visible light irradiation Guimin An, Wanhong Ma, Zhenyu Sun, Zhimin Liu *, Buxing Han, Shiding Miao, Zhenjiang Miao, Kunlun Ding Beijing National Laboratory for Molecular Sciences (BNLMS), Center for Molecular Science, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100080, China Received 14 February 2007; accepted 30 April 2007 Available online 13 May 2007
Abstract Titanium dioxide (anatase, TiO2) nanoparticles have been successfully deposited onto multi-walled carbon nanotubes (MWCNTs) via hydrolysis of titanium isopropoxide in supercritical ethanol. The as-prepared composites were characterized by X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy. It was demonstrated that the MWCNTs were decorated with welldispersed anatase nanoparticles less than 7 nm in diameter. The size and loading content of the nanoparticles on MWCNTs could be tuned by manipulating the ratio of precursor to MWCNTs, and the formation mechanism of the composites was also discussed. The absorbance spectrum of the resultant TiO2/MWCNT composites extended to the whole UV–visible region due to the decoration of TiO2 on MWCNTs. The TiO2/MWCNT composites were used as photocatalyst for phenol degradation under irradiation of visible light, which showed higher efficiency compared to a mixture of TiO2 and MWCNTs. 2007 Elsevier Ltd. All rights reserved.
1. Introduction Carbon nanotube (CNT)-based composites have attracted much attention due to their unique properties and promising applications [1–3]. Up to now, metal oxides [4–6], precious metal nanoparticles [7–9], polymers and functional organic molecules [10–12], etc., have been successfully decorated on CNTs via different methods including capillary action [13], supercritical fluid deposition [4–6], chemical vapor deposition [14,15], and so on [16–18]. These composites not only exhibit their intrinsic properties, such as electronic, mechanical, adsorption and thermal properties, but also display cooperative or synergetic effects. Titanium dioxide (TiO2) is an important semiconductor material, which has been applied as white pigment, cosmetic, catalyst and carrier owing to its excellent physical *
Corresponding author. Fax: +86 10 62562821. E-mail address: [email protected] (Z. Liu).
0008-6223/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2007.04.034
and chemical properties [19–23]. One of its most important applications is to act as photocatalyst for some chemical reactions, especially for the decontamination of water polluted with organic pollutants [24–28]. TiO2/UV catalytic system has been widely investigated in the heterogeneous photocatalytic process. There is a general agreement about the photocatalytic mechanism, i.e., when being irradiated and getting enough energy TiO2 can generate hole–electron couples, which move to the surface and react with OH group and O2 absorbed on the surface of the catalyst to yield hydroxyl radical and superoxide radical ion, respectively. Those species have a high activity and can oxidate organic compounds [29–32]. However, the major disadvantage in the practical application of TiO2 as photocatalyst is that only approximately 4% of the solar radiation is effective due to the large band gap energy (3.2 eV) for anatase. Therefore, it is desirable to modify and/or dope TiO2 and enable it to be excited by visible light [33–37]. As a good support for nanomaterials, multi-walled
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carbon nanotubes (MWCNTs) have been widely investigated, and TiO2/MWCNT composites have also obtained via different routes. Jitianu and his co-workers coated MWCNTs with anatase by a sol–gel method using classical alkoxides as precursors [38]; Wang et al. prepared TiO2/ MWCNT composites with MWCNTs embedded in TiO2 nanoparticles by a modified sol–gel method, and investigated their activity in photodegradation of phenol under irradiation of visible light [39]. However, the conventional preparation techniques usually suffer from their inherent disadvantages. For example, the CNTs need to be treated with strong acids to introduce active function groups on their surface; some organic stabilizers are introduced in order to prevent nanoparticles from agglomerating. Therefore, it is necessary to explore simple and effective ways to synthesize CNT-based composites. Herein we report a simple and effective route to deposit TiO2 nanoparticles on MWCNTs in supercritical ethanol. In this method, titanium isopropoxide (TIP) as precursor was hydrolyzed to form TiO2 and deposit on MWCNTs with the aid of supercritical ethanol, resulting in TiO2/ MWCNT composites. The used MWCNTs did not require tedious surface modification, and were readily decorated with metal oxide nanoparticles via controlling the reaction parameters. The resultant TiO2/MWCNT composites were characterized by different techniques including transmission electron microscopy (TEM), high-resolution TEM (HRTEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The photocatalytic activity of the as-prepared TiO2/MWCNT composites for phenol degradation under the visible light irradiation was also investigated. 2. Experimental 2.1. Synthesis of TiO2–MWCNT composites MWCNTs (purity, >95%; diameter, 40–60 nm; length, 5–15 lm; specific surface area, 40–300 m2/g) prepared by catalytic decomposition of CH4 using La2NiO4 as catalyst precursor, were provided by Shenzhen Nanoport Co. Ltd. (NPT). TIP was purchased from Acros Organics. Ethanol was A. R. grade, produced by Beijing Chemical Reagent Plant. In a typical experiment to prepare composites, suitable amount of TIP and MWCNTs were dissolved and ultrasonically dispersed in 8 mL of ethanol, respectively. Then the suspension was loaded in a stainless autoclave of 20 mL. The autoclave was sealed and moved to an oven of 270 C and maintained at this temperature for 2 h, and then cooled to room temperature naturally. Subsequently, the dark precipitate was separated from the solution by centrifugation and washed with absolute ethanol and distilled water repeatedly. The produced sample was vacuum-dried at 60 C for 6 h. By changing the TIP/MWCNT mass ratios, different samples were obtained. The samples prepared with the TIP/MWCNT mass ratios of 5:1 and 1:1 were named as sample A and sample B, respectively.
2.2. Characterization techniques The obtained products were characterized by XRD on a D/MAX-RC diffractometer operated at 30 kV and 100 mA with Cu Ka radiation. The morphology and microstructure of the products were examined by means of TEM on a JEOL 2010 transmission electron microscope equipped with an EDS using an accelerating voltage of 200 kV. The XPS measurement of
the composites was performed on an ESCALab220i-XL spectrometer operated at 15 kV and 20 mA at a pressure of about 3 · 109 mbar using Al Ka as the exciting source (hm = 1486.6 eV).
2.3. Photocatalytic reaction The photocatalytic activity of TiO2/MWCNT composites was tested using phenol degradation in aqueous solution under visible light irradiation. The experiments were carried out in an open wide glass photochemical reactor charged with 40 mL of suspension and a magnetic stirrer. A 500 W high-pressure Hg lamp with a UV-filter that can eliminate effect of UV light was placed at about 20 cm from the reactor, and its emission light was in the range of 450–900 nm. A circulating water jacket was employed to cool the irradiation source and thus keep the system temperature constant. In a typical experiment, 40 mL of phenol aqueous solution with concentration ðC 00 Þ of 50 mg/L was mixed with TiO2/MWCNT composites (1 g/L), and continuously stirred with a magnetic stirrer in dark for 10 h in order to establish an adsorption–desorption equilibrium. Prior to turning on the lamp, the concentration of the solution was determined, which was considered as initial concentration (C0) of the phenol solution. Then the solutions were analyzed regularly by UV–vis spectroscopy (Tu-2100) at scheduled irradiation time.
3. Results and discussion 3.1. Morphology The morphology and microstructure of the products prepared with different TIP/MWCNT mass ratios were first examined by TEM, and it was indicated that MWCNT-based composites were obtained under our experimental conditions. Fig. 1a shows a representative TEM image of the sample prepared with the TIP/MWCNT mass ratio of 5:1 in supercritical ethanol at 270 C (named as sample A), which clearly illustrates that the MWCNTs were homogeneously decorated with well-dispersed nanoparticles less than 7 nm. Selected area electron diffraction (SAED) pattern taken from the square in Fig. 1a is displayed as a right inset, which reveals the crystalline feature of the composites. HRTEM may offer further insight into the morphology and microstructure of TiO2/MWCNT composites, as shown in a left inset of Fig. 1a. It can be seen that the well-resolved aligned lattice fringes of anatase with adjacent fringe spacing of about 0.33 nm, which also illustrates the crystalline nature of the composites [40]. The TIP/MWCNT mass ratio is an important factor to influence the final morphology of the composites. In the TIP/MWCNT mass ratio range from 1:1 to 5:1, almost all nanoparticles deposited on the MWCNTs, and almost every MWCNT was decorated with nanoparticles. Moreover, the amount of decorated nanoparticles increased with increasing the TIP/MWCNT ratio. For all the as-prepared composites, the nanoparticles firmly deposited on the surface of MWCNTs, and they could not be removed even if the composites were treated with ultrasonic for a long time. When the TIP/MWCNT mass ratio increased over 5:1, besides the decorated MWCNTs, some nanoparticle aggregates free of the MWCNTs could be observed from the TEM view, and their amount increased with increasing the TIP/MWCNT ratio. Fig. 1c is a TEM image of the
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Fig. 1. TEM images of the TiO2/MWCNT composites prepared in ethanol at 270 C with different TIP/MWCNT mass ratios and reaction times: (a) 5:1, 2 h, the left inset is the HRTEM image of the composite and the right one is an electron diffraction of the denoted rectangular area; (b) 1:1, 2 h; (c) 10:1, 2 h; and (d) 20:1, 0.5 h.
composites prepared with the TIP/MWCNT mass ratio of 10:1, which indicates that nanoparticle aggregates appeared in the product as shown by the arrow. With the TIP/MWCNT mass ratio of 20:1, a large amount of TiO2 nanoparticle aggregates were observed during the TEM observation, and the MWCNTs were embedded into these aggregates. Careful TEM examination exhibited that almost every MWCNT was enwrapped with a TiO2 layer, as illustrated in Fig. 1d. The above results indicate that changing the TIP/MWCNT ratio can tune the composition and morphology of the resulting composites. Based on the assumption that the precursor was completely converted to TiO2 and all the formed nanoparticles deposited on the MWCNTs under the experimental conditions, the amounts of TiO2 in sample A and sample B are 54.8 wt% and 19.4 wt%, respectively. 3.2. XRD analysis The XRD pattern of the sample prepared with the TIP/ MWCNT mass ratio of 1:1 is displayed in Fig. 2, which demonstrates the highly crystalline nature of the composites. The diffraction peak at 2h = 26.5 can be well indexed as the 002 reflection of graphite, while the other diffraction peaks in the range of 15 < 2h < 80 correspond to the 101, 004, 100, 200, 105, 211, 204, 116, 220 and 215 reflections of anatase [19], respectively, which suggests that the nanopar-
Fig. 2. XRD pattern of sample B.
ticles decorated on the MWCNTs were anatase. This indicates that the precursor, titanium (IV) isopropoxide, was converted to anatase, which deposited on the MWCNTs under the experimental conditions. 3.3. XPS analysis To explore the formation mechanism of the TiO2/ MWCNT composites, sample B together with the pristine MWCNTs, and the mixture of TiO2 and MWCNTs with
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the same TiO2 content as that of sample B was analyzed by XPS. Fig. 3a displays the XPS spectrum of Ti 2p for sample B, which shows that Ti 2p3/2 and Ti 2p1/2 spin-orbital splitting photoelectrons were located at binding energies of 459.2 and 464.9 eV, respectively, slightly shifting toward higher binding energy compared to those of the pure bulk anatase [41]. This implies that the Ti in TiO2 nanoparticles decorated on MWCNTs was in a different environment from that of pure anatase, and also suggests that the strong interaction existed between TiO2 nanoparticles and MWCNTs. Fig. 3b exhibits the Ti 2p spectrum of the TiO2 and MWCNTs mixture, which shows a characteristic peak of pure anatase at binding energy of 458.9 eV. This indicates that the TiO2 in the mixture was in the same chemical environment as pure TiO2. In other word, the property of TiO2 in sample B is different from that of TiO2 in the mixture, which may generate some new functions for photocatalysis. Fig. 3c shows the C 1s XPS spectra of sample B (the bottom spectrum) and the pristine MWCNTs (the upper spectrum). Each C 1s spectrum could be deconvoluted into four peaks. The most resolved peak was assigned to CAC bond from MWCNTs with peak binding energy of 284.8 eV, while the peaks at 285.4, 286.7 and 290.0 eV, respectively, attributed to CAO, C@O and COO bonds [39]. The existence of these polar groups confirms that the surface of MWCNTs was oxidized to some extent, which is favorable to the adsorption of the precursor molecules and/or to the
nucleation of TiO2 on the surface of the MWCNTs. The O 1s XPS spectrum of sample B is illustrated in Fig. 3d, which can be deconvoluted as three peaks. The peaks at 532.8 and 530.4 eV ascribe to O 1s of H2O and TiO2, respectively, while the peak at 531.6 eV is assigned to CAO, C@O and COO bonds. This is consistent with the result obtained from C 1s spectrum. 3.4. Possible formation mechanism of TiO2/MWCNT composites The exact formation mechanism of the TiO2/MWCNT composites in supercritical ethanol is relatively complicated. In our experiment, though the used MWCNTs were not pre-treated by further surface modification, the surfaces of MWCNTs contained some polar groups such as CAO or C@O, confirmed by the XPS analysis, which was favorable to adsorption of the precursor molecules on MWCNTs. In addition, the unique properties of supercritical ethanol (such as low viscosity, high diffusion, near zero surface tension, etc.) also facilitate the precursor molecules to be adsorbed on MWCNTs. It has been reported that ethanol could condense to generate water to hydrolyze TIP under the supercritical state [42]. Under our experimental conditions, ethanol condensed to generate water, which was confirmed by the fact that ether was detectable in the reaction solution for preparing TiO2/MWCNT composites. Therefore, it can be deduced that the adsorbed TIP
Ti2p 3/2
Ti2p3/2 458.9 eV
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Intencity (a.u.)
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b
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Fig. 3. XPS spectra: (a,b) Ti 2p spectra of sample B, and the TiO2 and MWCNT mixture; (c) C 1s spectra: the upper for the MWCNTs and the bottom for sample B; and (d) O 1s spectrum for sample B.
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molecules could be hydrolyzed by the water generated from the ethanol condensation, forming TiO2 on the surface of MWCNTs. Meanwhile the supercritical ethanol could facilitate the transfer of reactants onto MWCNT surface and consequently promote a high uniform nanoparticles deposition on them. 3.5. Photocatalytic activity for phenol degradation Fig. 4 shows the photocatalytic conversion of phenol over different catalysts under the visible light irradiation. It is obvious that TiO2 powders showed little photocatalytic activity, and the mixture of TiO2 and MWCNTs exhibited some activity compared to the TiO2 powders. However, the TiO2/MWCNT composites prepared in this work showed much higher activity than the mixture. This suggests that the combining way between MWCNTs and TiO2 should be taken into account to explain the photocatalytic activity for phenol degradation. In order to support this, we examined the diffuse reflectance UV–vis spectra of the above composites. Fig. 5 shows the diffuse reflectance UV–vis spectra of pure TiO2 powders with size of about 7 nm, the TiO2/MWCNT composites (sample A and sam-
Conversion (%)
100 sample B mixture of TiO2 and MWNTs pure TiO2
80 60 40 20 0 0
100
200
300
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Time (minute) Fig. 4. Conversion of phenol photodegradation over pure TiO2 powders, the mixture of TiO2 and MWCNTs, and sample B.
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ple B), and the mixture of TiO2 and MWCNTs with the same content and size of TiO2 as sample B. As expected, the neat TiO2 powders showed its characteristic absorption edge at 400 nm. Interestingly, the TiO2/MWCNT composites (sample A and sample B) displayed unusual UV–vis spectra, which almost covered the whole UV–vis region and the absorption increased with increasing the TiO2 content in the composites, as shown in Curves c and d of Fig. 5. This result suggests that the TiO2/MWCNT composites combined the features of MWCNTs and TiO2 together, and generated new properties, which would be favorable to widening the absorption from UV light region to visible light region. For comparison, the diffuse reflectance UV–vis spectrum of the MWCNTs and TiO2 mixture was also examined, as shown in Curve b of Fig. 5, which just shifted slightly toward visible light region. This indicates that the TiO2/MWCNT composites prepared in this work are greatly different from the mixture of TiO2 and MWCNTs. The characters of the TiO2 and MWCNTs have been changed due to the indubitable interaction of TiO2 nanoparticles with the MWCNTs, which may modify the process of the electron/hole pair formation under visible light irradiation [43]. So it may be the unique structure of the TiO2/MWCNT composites that led to the high catalytic activity for phenol degradation. It was reported that the photodegradation of phenol in aqueous suspension containing MWCNT/TiO2 composites under visible light irradiation followed pseudo-first-order kinetics [41]. We fitted our results of photocatalytic reaction over TiO2/MWCNT composites (sample B) and the mixture of TiO2 and MWCNTs according to the apparent first-order equation ln(C0/C) = f(t). As shown in Fig. 6, the slope of Curve a (for sample B) is almost two times as that of Curve b (for the TiO2 and MWCNTs mixture), which means that the photodegradation rate of phenol using sample B as catalyst is two times as that using the mixture of TiO2 and MWCNTs as catalyst. The reaction over the mixture of TiO2 and MWCNTs well accorded the pseudo-firstorder kinetics, however, it had a discrepancy when sample 2.5
d
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Wavelength (nm) Fig. 5. Diffuse reflectance UV–vis spectra of (a) pure TiO2, (b) mixture of TiO2 and MWCNTs, (c) sample B, and (d) sample A.
Fig. 6. Apparent first-order linear transform ln(C0/C) = f(t) of phenol degradation kinetic plots for sample B and the mixture of MWCNTs and TiO2 with the same TiO2 content.
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B was used as catalyst. This may attribute to the different structures of the catalysts. Kamat and co-workers investigated the photochemical behavior of C60 and C70 adsorbed on TiO2 particles under the visible light irradiation [44]. Because the bandgap of C60 is smaller than that of TiO2, it can be excited under irradiation of visible light, and the photo-induced electrons transferred between TiO2 and C60. Since MWCNTs have similar structure and characters to C60, we can deduce the possible role of MWCNTs in the photocatalytic process as follows. MWCNTs were induced to generate electron (e) by visible light irradiation, and the photo-induced electrons transferred to conduction band of TiO2 and the electrons of valence band (VB) of TiO2 transferred to MWCNTs simultaneously. In other words, the positive charged holes (h+) formed while the induced electrons migrated from MWCNTs to TiO2. The injecting electrons from MWCNTs to TiO2 could catch the O2 adsorbed on the surface of TiO2 to yield very active radicals, superoxide radical ions, and the positive charged hole could catch OH to yield hydroxyl radicals. Both superoxide radical ions and hydroxyl radicals were responsible for the degradation of phenol. Therefore, the connection between MWCNTs and TiO2 particles may play a key important role in the electron transfer. For sample B, the TiO2 nanoparticles firmly combined with the surface of MWCNTs as described above, while the TiO2 nanoparticles freely dispersed and no particles decorated on MWCNTs in the mixture of TiO2 and MWCNTs, confirmed by TEM observation. It is the interphase interactions between TiO2 nanoparticles and MWCNTs in the as-prepared TiO2/MWCNT composites rather than the simple contact as the case in mechanical mixture that improved the activity of catalyst.
4. Conclusion Nanometer-sized crystalline anatase was successfully anchored on MWCNTs using a simple supercritical fluid deposition method. The loading content of the TiO2 nanoparticles on MWCNTs could be conveniently tuned by changing the precursor/MWCNT ratios. The photodegradation of phenol under visible light irradiation demonstrated that the modified MWCNTs with TiO2 nanoparticles exhibited a significant increment of photoactivity in comparison to the pure TiO2 and the mechanical mixture of TiO2 and MWCNTs. The as-prepared composites may have promising applications in photodegradation of organic compounds in aqueous solution under the irradiation of visible light.
Acknowledgement This work is financially supported by National Natural Science Foundation of China (No. 50472096).
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Preparation of titania/carbon nanotube composites using supercritical ethanol and their photocatalytic activity for phenol degradation under visible light irradiation Guimin An, Wanhong Ma, Zhenyu Sun, Zhimin Liu *, Buxing Han, Shiding Miao, Zhenjiang Miao, Kunlun Ding Beijing National Laboratory for Molecular Sciences (BNLMS), Center for Molecular Science, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100080, China Received 14 February 2007; accepted 30 April 2007 Available online 13 May 2007
Abstract Titanium dioxide (anatase, TiO2) nanoparticles have been successfully deposited onto multi-walled carbon nanotubes (MWCNTs) via hydrolysis of titanium isopropoxide in supercritical ethanol. The as-prepared composites were characterized by X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy. It was demonstrated that the MWCNTs were decorated with welldispersed anatase nanoparticles less than 7 nm in diameter. The size and loading content of the nanoparticles on MWCNTs could be tuned by manipulating the ratio of precursor to MWCNTs, and the formation mechanism of the composites was also discussed. The absorbance spectrum of the resultant TiO2/MWCNT composites extended to the whole UV–visible region due to the decoration of TiO2 on MWCNTs. The TiO2/MWCNT composites were used as photocatalyst for phenol degradation under irradiation of visible light, which showed higher efficiency compared to a mixture of TiO2 and MWCNTs. 2007 Elsevier Ltd. All rights reserved.
1. Introduction Carbon nanotube (CNT)-based composites have attracted much attention due to their unique properties and promising applications [1–3]. Up to now, metal oxides [4–6], precious metal nanoparticles [7–9], polymers and functional organic molecules [10–12], etc., have been successfully decorated on CNTs via different methods including capillary action [13], supercritical fluid deposition [4–6], chemical vapor deposition [14,15], and so on [16–18]. These composites not only exhibit their intrinsic properties, such as electronic, mechanical, adsorption and thermal properties, but also display cooperative or synergetic effects. Titanium dioxide (TiO2) is an important semiconductor material, which has been applied as white pigment, cosmetic, catalyst and carrier owing to its excellent physical *
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0008-6223/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2007.04.034
and chemical properties [19–23]. One of its most important applications is to act as photocatalyst for some chemical reactions, especially for the decontamination of water polluted with organic pollutants [24–28]. TiO2/UV catalytic system has been widely investigated in the heterogeneous photocatalytic process. There is a general agreement about the photocatalytic mechanism, i.e., when being irradiated and getting enough energy TiO2 can generate hole–electron couples, which move to the surface and react with OH group and O2 absorbed on the surface of the catalyst to yield hydroxyl radical and superoxide radical ion, respectively. Those species have a high activity and can oxidate organic compounds [29–32]. However, the major disadvantage in the practical application of TiO2 as photocatalyst is that only approximately 4% of the solar radiation is effective due to the large band gap energy (3.2 eV) for anatase. Therefore, it is desirable to modify and/or dope TiO2 and enable it to be excited by visible light [33–37]. As a good support for nanomaterials, multi-walled
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carbon nanotubes (MWCNTs) have been widely investigated, and TiO2/MWCNT composites have also obtained via different routes. Jitianu and his co-workers coated MWCNTs with anatase by a sol–gel method using classical alkoxides as precursors [38]; Wang et al. prepared TiO2/ MWCNT composites with MWCNTs embedded in TiO2 nanoparticles by a modified sol–gel method, and investigated their activity in photodegradation of phenol under irradiation of visible light [39]. However, the conventional preparation techniques usually suffer from their inherent disadvantages. For example, the CNTs need to be treated with strong acids to introduce active function groups on their surface; some organic stabilizers are introduced in order to prevent nanoparticles from agglomerating. Therefore, it is necessary to explore simple and effective ways to synthesize CNT-based composites. Herein we report a simple and effective route to deposit TiO2 nanoparticles on MWCNTs in supercritical ethanol. In this method, titanium isopropoxide (TIP) as precursor was hydrolyzed to form TiO2 and deposit on MWCNTs with the aid of supercritical ethanol, resulting in TiO2/ MWCNT composites. The used MWCNTs did not require tedious surface modification, and were readily decorated with metal oxide nanoparticles via controlling the reaction parameters. The resultant TiO2/MWCNT composites were characterized by different techniques including transmission electron microscopy (TEM), high-resolution TEM (HRTEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The photocatalytic activity of the as-prepared TiO2/MWCNT composites for phenol degradation under the visible light irradiation was also investigated. 2. Experimental 2.1. Synthesis of TiO2–MWCNT composites MWCNTs (purity, >95%; diameter, 40–60 nm; length, 5–15 lm; specific surface area, 40–300 m2/g) prepared by catalytic decomposition of CH4 using La2NiO4 as catalyst precursor, were provided by Shenzhen Nanoport Co. Ltd. (NPT). TIP was purchased from Acros Organics. Ethanol was A. R. grade, produced by Beijing Chemical Reagent Plant. In a typical experiment to prepare composites, suitable amount of TIP and MWCNTs were dissolved and ultrasonically dispersed in 8 mL of ethanol, respectively. Then the suspension was loaded in a stainless autoclave of 20 mL. The autoclave was sealed and moved to an oven of 270 C and maintained at this temperature for 2 h, and then cooled to room temperature naturally. Subsequently, the dark precipitate was separated from the solution by centrifugation and washed with absolute ethanol and distilled water repeatedly. The produced sample was vacuum-dried at 60 C for 6 h. By changing the TIP/MWCNT mass ratios, different samples were obtained. The samples prepared with the TIP/MWCNT mass ratios of 5:1 and 1:1 were named as sample A and sample B, respectively.
2.2. Characterization techniques The obtained products were characterized by XRD on a D/MAX-RC diffractometer operated at 30 kV and 100 mA with Cu Ka radiation. The morphology and microstructure of the products were examined by means of TEM on a JEOL 2010 transmission electron microscope equipped with an EDS using an accelerating voltage of 200 kV. The XPS measurement of
the composites was performed on an ESCALab220i-XL spectrometer operated at 15 kV and 20 mA at a pressure of about 3 · 109 mbar using Al Ka as the exciting source (hm = 1486.6 eV).
2.3. Photocatalytic reaction The photocatalytic activity of TiO2/MWCNT composites was tested using phenol degradation in aqueous solution under visible light irradiation. The experiments were carried out in an open wide glass photochemical reactor charged with 40 mL of suspension and a magnetic stirrer. A 500 W high-pressure Hg lamp with a UV-filter that can eliminate effect of UV light was placed at about 20 cm from the reactor, and its emission light was in the range of 450–900 nm. A circulating water jacket was employed to cool the irradiation source and thus keep the system temperature constant. In a typical experiment, 40 mL of phenol aqueous solution with concentration ðC 00 Þ of 50 mg/L was mixed with TiO2/MWCNT composites (1 g/L), and continuously stirred with a magnetic stirrer in dark for 10 h in order to establish an adsorption–desorption equilibrium. Prior to turning on the lamp, the concentration of the solution was determined, which was considered as initial concentration (C0) of the phenol solution. Then the solutions were analyzed regularly by UV–vis spectroscopy (Tu-2100) at scheduled irradiation time.
3. Results and discussion 3.1. Morphology The morphology and microstructure of the products prepared with different TIP/MWCNT mass ratios were first examined by TEM, and it was indicated that MWCNT-based composites were obtained under our experimental conditions. Fig. 1a shows a representative TEM image of the sample prepared with the TIP/MWCNT mass ratio of 5:1 in supercritical ethanol at 270 C (named as sample A), which clearly illustrates that the MWCNTs were homogeneously decorated with well-dispersed nanoparticles less than 7 nm. Selected area electron diffraction (SAED) pattern taken from the square in Fig. 1a is displayed as a right inset, which reveals the crystalline feature of the composites. HRTEM may offer further insight into the morphology and microstructure of TiO2/MWCNT composites, as shown in a left inset of Fig. 1a. It can be seen that the well-resolved aligned lattice fringes of anatase with adjacent fringe spacing of about 0.33 nm, which also illustrates the crystalline nature of the composites [40]. The TIP/MWCNT mass ratio is an important factor to influence the final morphology of the composites. In the TIP/MWCNT mass ratio range from 1:1 to 5:1, almost all nanoparticles deposited on the MWCNTs, and almost every MWCNT was decorated with nanoparticles. Moreover, the amount of decorated nanoparticles increased with increasing the TIP/MWCNT ratio. For all the as-prepared composites, the nanoparticles firmly deposited on the surface of MWCNTs, and they could not be removed even if the composites were treated with ultrasonic for a long time. When the TIP/MWCNT mass ratio increased over 5:1, besides the decorated MWCNTs, some nanoparticle aggregates free of the MWCNTs could be observed from the TEM view, and their amount increased with increasing the TIP/MWCNT ratio. Fig. 1c is a TEM image of the
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Fig. 1. TEM images of the TiO2/MWCNT composites prepared in ethanol at 270 C with different TIP/MWCNT mass ratios and reaction times: (a) 5:1, 2 h, the left inset is the HRTEM image of the composite and the right one is an electron diffraction of the denoted rectangular area; (b) 1:1, 2 h; (c) 10:1, 2 h; and (d) 20:1, 0.5 h.
composites prepared with the TIP/MWCNT mass ratio of 10:1, which indicates that nanoparticle aggregates appeared in the product as shown by the arrow. With the TIP/MWCNT mass ratio of 20:1, a large amount of TiO2 nanoparticle aggregates were observed during the TEM observation, and the MWCNTs were embedded into these aggregates. Careful TEM examination exhibited that almost every MWCNT was enwrapped with a TiO2 layer, as illustrated in Fig. 1d. The above results indicate that changing the TIP/MWCNT ratio can tune the composition and morphology of the resulting composites. Based on the assumption that the precursor was completely converted to TiO2 and all the formed nanoparticles deposited on the MWCNTs under the experimental conditions, the amounts of TiO2 in sample A and sample B are 54.8 wt% and 19.4 wt%, respectively. 3.2. XRD analysis The XRD pattern of the sample prepared with the TIP/ MWCNT mass ratio of 1:1 is displayed in Fig. 2, which demonstrates the highly crystalline nature of the composites. The diffraction peak at 2h = 26.5 can be well indexed as the 002 reflection of graphite, while the other diffraction peaks in the range of 15 < 2h < 80 correspond to the 101, 004, 100, 200, 105, 211, 204, 116, 220 and 215 reflections of anatase [19], respectively, which suggests that the nanopar-
Fig. 2. XRD pattern of sample B.
ticles decorated on the MWCNTs were anatase. This indicates that the precursor, titanium (IV) isopropoxide, was converted to anatase, which deposited on the MWCNTs under the experimental conditions. 3.3. XPS analysis To explore the formation mechanism of the TiO2/ MWCNT composites, sample B together with the pristine MWCNTs, and the mixture of TiO2 and MWCNTs with
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the same TiO2 content as that of sample B was analyzed by XPS. Fig. 3a displays the XPS spectrum of Ti 2p for sample B, which shows that Ti 2p3/2 and Ti 2p1/2 spin-orbital splitting photoelectrons were located at binding energies of 459.2 and 464.9 eV, respectively, slightly shifting toward higher binding energy compared to those of the pure bulk anatase [41]. This implies that the Ti in TiO2 nanoparticles decorated on MWCNTs was in a different environment from that of pure anatase, and also suggests that the strong interaction existed between TiO2 nanoparticles and MWCNTs. Fig. 3b exhibits the Ti 2p spectrum of the TiO2 and MWCNTs mixture, which shows a characteristic peak of pure anatase at binding energy of 458.9 eV. This indicates that the TiO2 in the mixture was in the same chemical environment as pure TiO2. In other word, the property of TiO2 in sample B is different from that of TiO2 in the mixture, which may generate some new functions for photocatalysis. Fig. 3c shows the C 1s XPS spectra of sample B (the bottom spectrum) and the pristine MWCNTs (the upper spectrum). Each C 1s spectrum could be deconvoluted into four peaks. The most resolved peak was assigned to CAC bond from MWCNTs with peak binding energy of 284.8 eV, while the peaks at 285.4, 286.7 and 290.0 eV, respectively, attributed to CAO, C@O and COO bonds [39]. The existence of these polar groups confirms that the surface of MWCNTs was oxidized to some extent, which is favorable to the adsorption of the precursor molecules and/or to the
nucleation of TiO2 on the surface of the MWCNTs. The O 1s XPS spectrum of sample B is illustrated in Fig. 3d, which can be deconvoluted as three peaks. The peaks at 532.8 and 530.4 eV ascribe to O 1s of H2O and TiO2, respectively, while the peak at 531.6 eV is assigned to CAO, C@O and COO bonds. This is consistent with the result obtained from C 1s spectrum. 3.4. Possible formation mechanism of TiO2/MWCNT composites The exact formation mechanism of the TiO2/MWCNT composites in supercritical ethanol is relatively complicated. In our experiment, though the used MWCNTs were not pre-treated by further surface modification, the surfaces of MWCNTs contained some polar groups such as CAO or C@O, confirmed by the XPS analysis, which was favorable to adsorption of the precursor molecules on MWCNTs. In addition, the unique properties of supercritical ethanol (such as low viscosity, high diffusion, near zero surface tension, etc.) also facilitate the precursor molecules to be adsorbed on MWCNTs. It has been reported that ethanol could condense to generate water to hydrolyze TIP under the supercritical state [42]. Under our experimental conditions, ethanol condensed to generate water, which was confirmed by the fact that ether was detectable in the reaction solution for preparing TiO2/MWCNT composites. Therefore, it can be deduced that the adsorbed TIP
Ti2p 3/2
Ti2p3/2 458.9 eV
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Intencity (a.u.)
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Fig. 3. XPS spectra: (a,b) Ti 2p spectra of sample B, and the TiO2 and MWCNT mixture; (c) C 1s spectra: the upper for the MWCNTs and the bottom for sample B; and (d) O 1s spectrum for sample B.
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molecules could be hydrolyzed by the water generated from the ethanol condensation, forming TiO2 on the surface of MWCNTs. Meanwhile the supercritical ethanol could facilitate the transfer of reactants onto MWCNT surface and consequently promote a high uniform nanoparticles deposition on them. 3.5. Photocatalytic activity for phenol degradation Fig. 4 shows the photocatalytic conversion of phenol over different catalysts under the visible light irradiation. It is obvious that TiO2 powders showed little photocatalytic activity, and the mixture of TiO2 and MWCNTs exhibited some activity compared to the TiO2 powders. However, the TiO2/MWCNT composites prepared in this work showed much higher activity than the mixture. This suggests that the combining way between MWCNTs and TiO2 should be taken into account to explain the photocatalytic activity for phenol degradation. In order to support this, we examined the diffuse reflectance UV–vis spectra of the above composites. Fig. 5 shows the diffuse reflectance UV–vis spectra of pure TiO2 powders with size of about 7 nm, the TiO2/MWCNT composites (sample A and sam-
Conversion (%)
100 sample B mixture of TiO2 and MWNTs pure TiO2
80 60 40 20 0 0
100
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300
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Time (minute) Fig. 4. Conversion of phenol photodegradation over pure TiO2 powders, the mixture of TiO2 and MWCNTs, and sample B.
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ple B), and the mixture of TiO2 and MWCNTs with the same content and size of TiO2 as sample B. As expected, the neat TiO2 powders showed its characteristic absorption edge at 400 nm. Interestingly, the TiO2/MWCNT composites (sample A and sample B) displayed unusual UV–vis spectra, which almost covered the whole UV–vis region and the absorption increased with increasing the TiO2 content in the composites, as shown in Curves c and d of Fig. 5. This result suggests that the TiO2/MWCNT composites combined the features of MWCNTs and TiO2 together, and generated new properties, which would be favorable to widening the absorption from UV light region to visible light region. For comparison, the diffuse reflectance UV–vis spectrum of the MWCNTs and TiO2 mixture was also examined, as shown in Curve b of Fig. 5, which just shifted slightly toward visible light region. This indicates that the TiO2/MWCNT composites prepared in this work are greatly different from the mixture of TiO2 and MWCNTs. The characters of the TiO2 and MWCNTs have been changed due to the indubitable interaction of TiO2 nanoparticles with the MWCNTs, which may modify the process of the electron/hole pair formation under visible light irradiation [43]. So it may be the unique structure of the TiO2/MWCNT composites that led to the high catalytic activity for phenol degradation. It was reported that the photodegradation of phenol in aqueous suspension containing MWCNT/TiO2 composites under visible light irradiation followed pseudo-first-order kinetics [41]. We fitted our results of photocatalytic reaction over TiO2/MWCNT composites (sample B) and the mixture of TiO2 and MWCNTs according to the apparent first-order equation ln(C0/C) = f(t). As shown in Fig. 6, the slope of Curve a (for sample B) is almost two times as that of Curve b (for the TiO2 and MWCNTs mixture), which means that the photodegradation rate of phenol using sample B as catalyst is two times as that using the mixture of TiO2 and MWCNTs as catalyst. The reaction over the mixture of TiO2 and MWCNTs well accorded the pseudo-firstorder kinetics, however, it had a discrepancy when sample 2.5
d
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Wavelength (nm) Fig. 5. Diffuse reflectance UV–vis spectra of (a) pure TiO2, (b) mixture of TiO2 and MWCNTs, (c) sample B, and (d) sample A.
Fig. 6. Apparent first-order linear transform ln(C0/C) = f(t) of phenol degradation kinetic plots for sample B and the mixture of MWCNTs and TiO2 with the same TiO2 content.
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B was used as catalyst. This may attribute to the different structures of the catalysts. Kamat and co-workers investigated the photochemical behavior of C60 and C70 adsorbed on TiO2 particles under the visible light irradiation [44]. Because the bandgap of C60 is smaller than that of TiO2, it can be excited under irradiation of visible light, and the photo-induced electrons transferred between TiO2 and C60. Since MWCNTs have similar structure and characters to C60, we can deduce the possible role of MWCNTs in the photocatalytic process as follows. MWCNTs were induced to generate electron (e) by visible light irradiation, and the photo-induced electrons transferred to conduction band of TiO2 and the electrons of valence band (VB) of TiO2 transferred to MWCNTs simultaneously. In other words, the positive charged holes (h+) formed while the induced electrons migrated from MWCNTs to TiO2. The injecting electrons from MWCNTs to TiO2 could catch the O2 adsorbed on the surface of TiO2 to yield very active radicals, superoxide radical ions, and the positive charged hole could catch OH to yield hydroxyl radicals. Both superoxide radical ions and hydroxyl radicals were responsible for the degradation of phenol. Therefore, the connection between MWCNTs and TiO2 particles may play a key important role in the electron transfer. For sample B, the TiO2 nanoparticles firmly combined with the surface of MWCNTs as described above, while the TiO2 nanoparticles freely dispersed and no particles decorated on MWCNTs in the mixture of TiO2 and MWCNTs, confirmed by TEM observation. It is the interphase interactions between TiO2 nanoparticles and MWCNTs in the as-prepared TiO2/MWCNT composites rather than the simple contact as the case in mechanical mixture that improved the activity of catalyst.
4. Conclusion Nanometer-sized crystalline anatase was successfully anchored on MWCNTs using a simple supercritical fluid deposition method. The loading content of the TiO2 nanoparticles on MWCNTs could be conveniently tuned by changing the precursor/MWCNT ratios. The photodegradation of phenol under visible light irradiation demonstrated that the modified MWCNTs with TiO2 nanoparticles exhibited a significant increment of photoactivity in comparison to the pure TiO2 and the mechanical mixture of TiO2 and MWCNTs. The as-prepared composites may have promising applications in photodegradation of organic compounds in aqueous solution under the irradiation of visible light.
Acknowledgement This work is financially supported by National Natural Science Foundation of China (No. 50472096).
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