Preparation and characterization of CNTs–TiO 2 composites Li Chen * , Bai-Lan Zhang, Mei-Zhen Qu, Zuo-Long Yu * Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, Graduate School of Chinese Academy of Sciences, China Received 7 October 2004; received in revised form 28 February 2005; accepted 15 April 2005 Available online 26 May 2005 Abstract Carbon nanotubes-based TiO 2 composites were fabricated by hydrolysis, and the transmission electron microscopy(TEM) results showed that carbon nanotubes were partly coated with TiO 2 . X-ray photoelectron spectroscopy (XPS) results of purified carbon nanotubes indicated that there were some polar oxygenated groups such as C–O, C=O and O – C=O which might stimulate formation of the composites, and enhance the interfacial combination of TiO 2 with carbon nanotubes. The formation of TiO 2 and its compounding with CNTs happened almost simultaneously in this process. The method is a convenient route to fabricate CNTs-based TiO 2 composites with different ratios. D 2005 Elsevier B.V. All rights reserved. Keywords: Carbon nanotubes; TiO 2 ; Composites; Transmission electron microscopy 1. Introduction Since the discovery of carbon nanotubes(CNTs), poly- mer-based composites including carbon nanotubes have attracted considerable attention in the research and industrial communities, due to their good electrical conductivity, high stiffness and high strength at relatively low CNTs content [1–6]. Currently, three methods are commonly used to introduce CNTs into polymers: (1) solution mixing or film casting of suspensions of CNTs in dissolved polymer [7], (2) in situ polymerization of CNT–polymer monomer mixture [8], and (3)mechanically melt mixing of CNTs with polymers [9]. Besides that, CNTs can also form composites with many inorganic materials. For examples, Ma et al. [10] have prepared CNT–SiC composites by hot-pressing the mixture of large multi-wall carbon nanotubes(MWNTs: 30–40 nm in diameter) and SiC powder. Novel composite powders such as CNT – Fe/Co– MgAl 2 O 4 and CNT–Co–MgO have been synthesized [11–14]. Peigney, A. et al. have produced the CNT–Fe–Al 2 O 3 powders [15] in which the CNTs are very homogeneously dispersed between the metal oxide grain s. These CNT–metal oxide composites are electrical conduc- tors owing to the percolation of the carbon nanotubes. SiO x coated CNTs [16] have also been fabricated through a sol–gel technique at room temperature. Multi-walled carbon nano- tube (MWNT)-based metal oxide composites [17] were prepared by an impregnation method using organometallic compounds as precursor. Chen et al. [18] obtained SnO– CNT composites by a sol–gel method as anode active material for lithium-ion batteries. Han and Zettl [19] have coated single-walled carbon nanotubes with a thin SnO 2 layer (about 4 nm) by a chemical-solution route. The MWNTs– SnO 2 composites fabricated by a new and simple one-step wet chemical method [20] have also been reported recently. Here we produced CNTs – TiO 2 composites by hydrolysis in which the formation of TiO 2 and its compounding with CNTs happened almost simultaneously. The as produced composites were examined by transmission electron micro- scopy(TEM). Results showed that purified CNTs were well coated with TiO 2 . The formation mechanism of the compo- sites was deduced from XPS results of the purified CNTs. 2. Experimental CNTs were synthesized from methane via catalytically chemical vapor deposition (CCVD) and were purified with acid. The as purified MWNTs existed as agglomerates and 0032-5910/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2005.04.028 * Corresponding authors. Fax: +86 288 522 3978. E-mail address: chenli720516@hotmail.com (L. Chen). Powder Technology 154 (2005) 70 – 72 www.elsevier.com/locate/powtec curved intertwined entanglements with an average diameter of 20 nm. The fabrication of CNTs–TiO 2 composites was carried out as follows: firstly, certain amount of CNTs was dispersed into acetone solution of Ti (OBu) 4 by ultrasonication; Then, hydrolysis was initiated by adding some deionized water into the mixture; Final ly, CNTs – TiO 2 composites could be obtained by filtering and drying the aforesaid mixture in a vaccum oven at 150 -C for at least 8 h. The surface state analysis of the CNTs was performed by XPS employing a Kratos XSAM 800 spectrometer with AL Ka´ (1.48keV) radiation. The system pressure was normally maintained at 6.7 Â 10 À 7 Pa. The spectrometer was run in fixed retarding ratio (FAT) mode at a pass energy 12 kV and 15 mA. All binding energies were referred to the C1s (BE = 284.6 eV) peak for sample charging. The as fabricated CNTs–TiO 2 composites were charac- terized by TEM using a JEM-100cx instrument with an accelerating voltage of 20 kV. The samples were ultra- sonicated in ethanol for 2 min and then a drop of dispersion was deposited on a Cu/Rh grid covered with formvar, a vinyl polymer, and the grid was dried overnight under vacuum. 3. Results and discussion It is well-known that most as produced CNTs contain some impurities such as amorphous carbon, fullerenes and catalyst particles, which are a serious impediment for CNTs to be directly used as functional filler in composites. Fig. 1 shows the TEM image of purified MWNTs in which the long interwined CNTs with a diameter of about 20 nm were very clean and almost all imp urities had been removed. Thus the purification for MWNTs was effective. Fig. 2 shows the XPS spectrum of the purified CNTs. According to the XPS studies about carbon nanotubes [21], the broader C 1s peak region at 284.6 eV could be fitted to four line shapes with binding energies at 285.000 eV, 286.400 eV, 288.000 eV, 290.600 eV. These different binding energy peaks were assigned to C–C at 285.000 eV, C–O at 286.400 eV, C=O at 288.000 eV and O – C=O at 290.600 eV [22], respectively. So, in addition to C–C groups, the surface of as purified CNTs stil l c on tained other carbon-base d polar oxygenated groups which were beneficial to its combination with TiO 2 then. From the above results, the total area of the C 1s peak region of the MWNTs sample consists of 68.37% C– C, 22.25% C–O, 6.80% C=O and 2.58% O–C=O. These surface polar functional groups were supposed to come from the purification process of CNTs. Fig. 3 shows TEM image of CNTs–TiO 2 composites in which the black materials of different size absorbed onto Fig. 1. TEM image of as purified CNTs. Fig. 2. XPS spectrum of purified CNTs. (a) ( × 20k) (b) ( × 50k) Fig. 3. TEM image of as produced TiO 2 – CNTs composites at two different magnitudes. L. Chen et al. / Powder Technology 154 (2005) 70 – 72 71 walls of as used CNTs were believed to be TiO 2 particles formed in the course of hydrolysis of Ti (OBu) 4 . Due to the –OH group on TiO 2 [23] and the C –O, C=O and O– C=O groups on purified CNTs as aforesaid, TiO 2 –CNTs composites formed naturally through some physicochem- ical actions such as van der Waals force, H bonding and other bondings. For example, the –OH group on TiO 2 may possibly react with the –OH and –COOH groups on CNTs in removing H 2 O contai ned in wet fresh composites, thus the bonding C – O – Ti or O=C–O–Ti might form through the dehydration reaction happened among the groups on the two materials. However, the high specific surface area of CNTs was also the basis for formati on of the composites. Though the density of TiO 2 is larger than that of CNTs, the TiO 2 – CNTs composites could form a homogeneous dispersion in ethanol and no TiO 2 was observed to drop from CNTs after 1 h of sonication. So, the interaction between the molecules of these two materials was supposed to be very strong. 4. Conclu sion CNTs-based TiO 2 composites were fabricated success- fully by means of hydrolysis, and CNTs were partly coated with TiO 2 . There were some polar oxygenated groups such as C–O, C=O and O–C=O w hich might stimulat e formation of the composites, and enhance the interfacial combination of TiO 2 with carbon nanotubes. The formation of TiO 2 and its compounding with CNTs happened almost simultaneously in this process. The method is a convenient route to fabricate CNTs-based TiO 2 composites with differ- ent ratios, which could be used as new functional fillers with more effects than the one component fillers. 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