Synthesis and Characterization of TiO2 Coated Multiwalled Carbon

Jul 30, 2008 - Imperial College London, Prince Consort Road, London SW7 2BP, U.K.. A considerable ... reaction was kept under this condition for 120 m...
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Ind. Eng. Chem. Res. 2008, 47, 6598–6606

MATERIALS AND INTERFACES Synthesis and Characterization of TiO2 Coated Multiwalled Carbon Nanotubes Using a Sol Gel Method Sharif Hussein Sharif Zein*,† and Aldo R. Boccaccini*,‡ School of Chemical Engineering, Engineering Campus, UniVersiti Sains Malaysia, Seri Ampangan 14300 Nibong Tebal, Seberang Perai Selatan, Pulau Pinang, Malaysia, and Department of Materials, Imperial College London, Prince Consort Road, London SW7 2BP, U.K.

A considerable improvement in the quality of TiO2 coatings on multiwalled carbon nanotubes (MWNTs) was obtained by introducing a three-step purification and functionalization process which combines oxidation in air followed by sulfuric acid refluxes and reoxidation in air before synthesis of the TiO2 coating by the sol-gel method. A range of techniques, including transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive X-rays (EDX), X-ray diffraction (XRD), Raman spectroscopy, and thermal gravimetric analysis (TGA) were applied to characterize TiO2 coated MWNTs obtained by sol-gel. TEM results showed that the MWNTs were fully and homogeneously coated with TiO2 while SEM coupled with EDX confirmed the presence of a thick layer of TiO2 coating the MWNTs. XRD and Raman spectroscopy results revealed that the crystalline structure of TiO2 on the surface of MWNTs was anatase. Introduction Carbon nanotubes (CNTs) are promising new materials for electrodes of electrochemical energy storage and conversion devices, owing to their unique internal structure, high surface area, low mass density, remarkable chemical stability and electronic conductivity.1–6 Moreover, CNTs coated with metal oxides are expected to exhibit different physical properties than those of neat nanotubes, and they may be proven to be key components in the next generation of nano-optical and electronic devices such as quantum memory elements, high density magnetic storage media, semiconducting devices, field electron emitters,magneticfieldsensorsandscanningprobemicroscopes.7–16 Among the metal oxides investigated, TiO2 has recently been the focus of much research due to its photocatalytic activity. However, the photocatalytic activity of pure TiO2 is not high enough for industrial applications.17 Among the possible materials proposed for modifying TiO2, CNTs are highly promising due to their unique structure and properties.1–4 Many researchers have investigated the preparation of TiO2/ CNTs nanocomposites using different methods.8–26 The sol-gel technique is a well-known processing route for fabricating optical quality films, surface coatings on glass and for forming planar optical wave guides.27 This method permits the realization of homogeneous doping profiles of nanoparticles over molecular scales. However, the inhomogeneity of TiO2 coating on the surface of CNTs, the damage of the surface structure of CNTs after coating and the low quality of the TiO2 layer deposited on CNTs in terms of thermal stability have been reported. This might be due to the incomplete purification or lack of adequate functionalization of MWNTs before coating. Thus, there is further need to improve methods for the reliable fabrication of TiO2/CNT structures of high structural and chemical homogeneity. * Corresponding authors. S.H.S.Z. Phone: +604 5996442. E-mail: [email protected]. A.R.B. Phone: +44 (0)207 5946731. E-mail: [email protected]. † Universiti Sains Malaysia. ‡ Imperial College London.

In this paper, the coating of MWNTs with TiO2 by the sol-gel method is investigated. The novelty of this work lies in the development of a three-step purification and functionalization process combining oxidation in air followed by sulfuric acid refluxes and reoxidation in air. With this additional threestep process, a high purity of the MWNTs was obtained and the surface of MWNTs was conveniently modified, in order to significantly influence surface interactions during the coating procedure. As a comparison, MWNTs without the three-step purification/functionalization process resulted in poor and nonhomogenous coating of TiO2 on the surface of MWNTs. The TiO2-MWNTs nanocomposites obtained were examined by a range of techniques including TEM, SEM, EDX, XRD, TGA, and Raman spectroscopy. Experimental Procedure The MWNTs were produced by catalytic decomposition of methane in a stainless-steel fixed-bed reactor. The catalyst used was 5 mol % MnOx/20 mol % NiO/TiO2 and was prepared by impregnation method. The catalyst was distributed in the middle part of the reactor. Subsequently, the reactor was heated to 725 °C in argon flow (99.999% purity, Sitt Tatt Industrial Gases Sdn. Bhd.). High purity methane (99.999% supplied by Malaysian Oxygen Bhd.) was mixed with argon in the volume ratio of 1:1 and both gases were introduced into the reactor with a total flow rate of 80 mL/min at the reaction temperature. The reaction was kept under this condition for 120 min. After the reaction, the reactor was cooled down to room temperature. The CNTs obtained were MWNTs containing mixed impurities, including catalyst particles, and with average outer diameter of around 30-50 nm. The synthesized MWNTs were then subjected to a three-step purification/functionalization process involving oxidation in air followed by sulfuric acid refluxes and reoxidation in air. A complete description of the MWNTs synthesis and purification have been explained elsewhere.28–32 The coating process was performed using a conventional sol-gel preparation method employing titanium tetraisopro-

10.1021/ie701770q CCC: $40.75  2008 American Chemical Society Published on Web 07/30/2008

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poxide solution [Ti[OCH(CH3)2]4, Aldrich]. Once the hydrolysis and condensation reactions have been performed, the solution was then stirred until it progressively became more viscous. The next step in the procedure is the introduction of the purified/ functionalilzed MWNTs in the sol-gel solution. As a comparison, MWNTs with and without having been subjected to the three-step purification/functionalization process were also tested. The concentration of MWNTs introduced in the sol-gel solution was varied according to the properties desired in the final TiO2/ MWNTs material. It was observed that the MWNTs dispersability in the solution increased with time. A good dispersion of MWNTs is important in order to prevent their precipitation in the sol-gel solution and hence to allow the production of homogeneous coatings. At a given point, the particles coalesced to form an elastic gel. The resulting gel was air-dried in a preheated, well ventilated oven and spread thinly (10 mm deep) onto a silica tray before being air-calcined in a preheated furnace at 400 °C for 5 h. In order to study the effect of the concentration of TiO2 in the coating process, TiO2 concentrations of 1.25, 2.5, 5, and 7.5 wt % were used under the same conditions. The TiO2-coated MWNTs were characterized using a transmission electron microscope (TEM) (Philips CM12). Field emission scanning electron microscopy (FESEM) and energy dispersive X-ray (EDX) analysis (ZEISS SUPRA 35 VP) were also used. X-ray diffraction (XRD) patterns of the coated samples were measured using a Siemens D-5000 diffractometer, using Cu KR radiation and a graphite secondary beam monochromator. The intensity was measured by step scanning in the 2θ range 10°-80° with a step of 0.02° and a measuring time of 2 s per point. The thermal stability of the coated MWNTs nanocomposites was determined from weight loss measurements in air at a heating rate of 10 °C/min by using thermogravimetric analysis (TGA) (Perkin-Elmer TGA7 thermogravimetric analyzer). A Raman and photoluminescence spectroscopy system (Model Jobin-Yvon HR800 UV) with a wavelength of 200-1100 nm was used to detect the defect and crystalline structure of the TiO2-coated MWNTs. Raman spectra were collected using 514.5 nm radiation from an argon ion laser in the backscattering geometry and a monochromator equipped with a peltier cooling CCD detector. Results and Discussion It is well-known that as-synthesized CNTs contain impurities such as catalyst particles, amorphous carbon, and fullerenes. These impurities are closely entangled with the CNTs and thereby influence their properties and possible applications. The three-step purification/functionalization process resulted in very high efficiency in purifying the as-synthesized MWNTs. TGA is used to detect the percentage of MWNTs, metal catalysts, and other impurities according to the combustion temperature difference between these materials. Figure 1a shows the TGA analysis of the MWNTs after the three-step purification and functionalization process. It shows purity of 99.9 wt % of the total dry original mass. There was no mass loss between the temperature of 300 and 400 °C, which indicates that the purified MWNTs are free of amorphous carbon. The MWNTs started burning at 525 °C. As reported by Shi et al.,33 the combustion of amorphous carbon occurs between 300 and 400 °C, whereas the burning temperature of CNTs is between 400 and 700 °C. The residue at 845 °C amounted to 0.01 wt % of the catalyst used in synthesizing the MWNTs. Figure 1b shows the TEM image of the purified MWNTs with a diameter of 30-50 nm while Figure 1c is an SEM image of the purified MWNTs showing no bright spots, which also confirms that the purified MWNTs are free of metal catalysts.

Figure 1. Purified MWNTs using oxidation in air followed by sulfuric acid refluxes and then reoxidation in air: (a) TGA, (b) TEM image, and (b) SEM image.

TEM images of the TiO2-coated MWNT with and without three-step purification/functionalization pretreatment are presented in Figures 2 and 3, respectively. Figure 2a and b show that TiO2 is not uniformly coated on the MWNTs. It is also noted that the thickness of the TiO2 layer is not homogeneous. However, when the MWNTs are pretreated by the three-step purification and functionalization process, the primary goal of MWNTs coating has been achieved, as shown by the TEM images in Figure 3a and b.

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Figure 2. TEM images of the TiO2-coated MWNTs without introducing three-step purification/functionilization process at two different magnifications: (a) low magnification and (b) higher magnification.

Figure 3a shows a TiO2 layer uniformly deposited on the MWNTs. It is also noted that the TiO2 layer with homogeneous thickness was wrapped around both the top and the side of the MWNTs. Figure 3b shows a magnified TEM image of the TiO2coated MWNTs. The hollow structure of the nanotubes is invisible and the external diameter was found to be about 80-130 nm. Since the diameter of the MWNTs was originally of about 30-50 nm, the image indicates that a relatively thick TiO2 layer (about 30–80 nm) was deposited on the MWNTs. The driving force for the surface coverage might be related to the density

of the defect sites that existed on the CNTs.6 Besides that, the defects after oxidation during purification also provide the pathway and the adsorption sites of TiO2.34 The same phenomena have been observed during hydrogen adsorption.35 It is suggested that this behavior is due to the fact that CNTs have lattice defects and these lattice defects might contribute to the adsorption of TiO2 on their surface. In order to study the effect of the concentration of TiO2 on the coating process, the presence of 1.25%, 5%, and 7.5% TiO2 was studied under the same conditions and the result is shown in Figure 4a-c, respectively. Figure 4a shows that the coating

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Figure 3. TEM images of the TiO2-coated MWNTs with introducing three-step purification/functionilization process at two different magnifications: (a) low magnification and (b) higher magnification.

was as uniform as that achieved with TiO2 concentration of 2.5% (shown previously in Figure 3) and that there was no significant difference in the thicknesses of the coatings. Moreover, separated TiO2 aggregates were observed at concentration of 5% in the TEM image of Figure 4b, and this aggregation increased as the percentage of TiO2 was increased to 7.5%, as shown Figure 4c. Thus, it was found that the driving force for the surface coverage is determined by how much the defect sites existed on the initial MWNTs.6 Raman spectra of the MWNTs shown later in Figure 8a reports that the two characteristic peaks (1354 and 1581 cm-1), corresponding to the defects of MWNTs that

contributed to the adsorption of TiO2 on the surface of MWNTs are no longer available after coating with TiO2. This explains that the defect sites might be already fully coated with TiO2 particles and thus why the thicknesses of the TiO2 coated MWNTs was the same in all the cases. Figure 5 shows the TGA and the differential thermogravimetric analysis (DTA) curves of the TiO2-coated MWNTs. The solid lines and dotted lines correspond to the TGA and DTA curves, respectively. On the basis of the TGA curve, the combustion temperatures range from 0 to 100 °C was assumed to be water vapor. There was a small peak in the DTA curve at

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Figure 4. TEM images of the TiO2-coated MWNTs with introducing three-step purification/functionilization process at different concentarions of TiO2: (a) 1.25%, (b) 5%, and (c) 7.5%.

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Figure 5. TGA graph of the TiO2-coated MWNTs.

Figure 6. EDX spectrum of the TiO2-coated MWNTs.

a temperature of 300 °C, which could indicate the presence of a small amount of amorphous carbon in the MWNTs. The MWNTs started burning at 418 °C and this was completed at 650 °C. In this temperature range, the weight percent of the sample dropped from 95.87 to 88.40 wt %. This result indicates that the sample contains only approximately 7.47 wt % of MWNTs. EDX analysis carried out on the TiO2-coated MWNTs confirmed the presence of the TiO2 coating as shown in Figure 6. The EDX analysis of the original MWNTs showed only the presence of carbon. After they had been coated with TiO2, Ti, C, and O peaks were detected by EDX (Ti, 41.95; O, 31.23, and C, 26.82 wt %). These percentages are different to the one obtained from TGA analysis (Figure 5), which might be possibly due to inaccuracies of values given by the EDX system. SEM images of the TiO2-coated MWNTs are presented in Figure 7a and b. The purified MWNTs shown previously in Figure 1c have very smooth surfaces, and they are about 30-50 nm in diameter and several micrometers in length. However,

the TiO2-coated MWNTs shown in Figure 7a are rod-like. Figure 7b shows a magnified image of a single TiO2-coated MWNT, and it is observed that the external diameter is approximately 135 nm. This observation demonstrates that a relatively thick layer of TiO2 nanoparticles was deposted on the MWNTs which is consistent with the TEM observation in Figure 3. The structural features of the TiO2-coated MWNTs have also been thoroughly identified using Raman spectroscopy and XRD technique. Figure 8a and b show the Raman spectra before and after TiO2 coating, respectively. The Raman spectra have been taken in the 100-2000 cm-1 spectral region. Figure 8a shows that the Raman spectra of the uncoated MWNTs consists of two characteristic peaks for the MWNTs. The broad peak at 1354 cm-1 which is called D band can be attributed to disordered carbon atoms, while the feature at 1581 cm-1 which is called G band originates from MWNTs.36 However, after coating with TiO2, these two distinct peaks were not observed, as shown in Figure 8b. This indicates that most of the MWNTs are fully coated with TiO2. Thus, these results show the lattice

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Figure 7. SEM images of the TiO2-coated MWNTs at two different magnifications: (a) low magnification and (b) higher magnification.

defects contributed to the coating of TiO2. We thus consider that the purification and functionalization process applied in this process is effective in enhancing TiO2 coating. Besides that, defects after oxidation during the purification also provide the pathway and the adsorption sites of TiO2.34 The analysis of the spectra shown in Figure 8b also confirms a strong peak at 143 cm-1 and weaker signals at 401, 521, and 640 cm-1, which are all typical of the TiO2-anatase phase.37,38 No other features corresponding to different TiO2 phases were detected. The XRD investigation of the TiO2-coated MWNTs also confirms the presence of TiO2-anatase phase as shown in Figure 9, which shows also the XRD spectrum of MWNTs. The two peaks at

25°, 9°, and 44.1° in Figure 9a correspond to the (0 0 2) and (1 0 0) reflections of MWNTs, respectively.18,25,39,40 The peaks in the diffraction pattern of the TiO2-coated MWNTs in Figure 9b correspond to the anatase phase of TiO2. The peak at 2θ 25.9° of MWNTs overlaps with the anatase TiO2 peak (1 0 1) at the reflection plane of 2θ 25.3°. The formation of rutile phase was not observed, as noted by the absence of the most intense rutile peak (2θ ) 27.5°).39 Conclusions TiO2-coated MWNTs were fabricated successfully by sol-gel. MWNTs were fully and homogenously coated with TiO2, which

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Figure 8. Raman spectrum of (a) uncoated MWNTs two characteristic peaks for the MWNTs at 1354 and 1581 cm-1 and (b) TiO2-coated MWNTs, showing characteristic anatase phase peaks at 143, 401, 521, and 640 cm-1.

Figure 9. XRD patterns of (a) MWNTs and (b) TiO2-coated MWNTs.

was confirmed by a range of techniques including TEM, SEM, EDX, XRD, TGA, and Raman spectroscopy. Considering the developed sol-gel technique and taking into consideration the semiconducting character of TiO2, the TiO2-coated MWNTs nanocomposites are candidate materials for application in the photocatalytic decomposition of aromatic pollutants in aqueous medium under UV irradiation. Acknowledgment The financial support provided by the Ministry of Science, Technology and Innovation through ScienceFund (Project No. 03-01-05-SF0126) is gratefully acknowledged. Literature Cited (1) Treacy, M. M. J.; Ebbesen, T. W.; Gibson, J. M. Exceptionally High Young’s Modulus Observed for Individual Carbon Nanotubes. Nature 1996, 381, 678.

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ReceiVed for reView December 26, 2007 ReVised manuscript receiVed May 20, 2008 Accepted May 25, 2008 IE701770Q