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Materials Letters 62 (2008) 4175–4176

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Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t

Low temperature synthesis of multi-walled carbon nanotubes via a sonochemical/hydrothermal method S. Manafi ⁎, H. Nadali, H.R. Irani Islamic Azad University-Shahrood Branch, P.O. Box 36155-163 Shahrood, Iran

A R T I C L E

I N F O

Article history: Received 24 April 2007 Accepted 23 May 2008 Available online 8 June 2008 Keywords: Carbon nanotube Reinforcement Retrofit Hydrothermal Sonochemistry

A B S T R A C T In this investigation, multi-walled carbon nanotubes (MWCNTs) have been prepared by a facile sonochemical/hydrothermal method. MWCNTs have been hydrothermally fabricated with using dichloromethane, cobalt chloride and metallic lithium as starting materials in 5 mol/lit NaOH aqueous solution. Ultrasonic pre-treatment of the solution mixture had an important step prior to the hydrothermal condition, which could generate a considerable amount of multi-walled carbon nanotubes for the subsequent hydrothermal growth. Finally, high pure MWCNTs with lengths of 2–5 μm and diameters of 60 ± 20 nm could be synthesized at as low temperature as 160 °C. As a matter of fact, the method of sonochemical/ hydrothermal guarantees the production of multi-walled carbon nanotubes (MWCNTs) for different applications, especially reinforcement materials. © 2008 Published by Elsevier B.V.

1. Introduction The discovery of carbon nanotubes in the arc-discharge apparatus was first published by Iijima in 1991, [1] which stimulated a worldwide research effort to improving their synthesis, determining their structure [2–7], calculating and measuring their physical properties [8–12]. In the following years, other synthesis methods of carbon nanotubes have also been reported: laser evaporation of a metal graphite composite target [13], carbon monoxide disproportionation on a metal catalyst [14], hydrocarbon pyrolysis using a metal catalyst [15] and hydrothermal synthesis [16–21]. Hydrothermal synthesis has been an interesting technique to prepare materials with different nanoarchitectures such as nanowires, nanorods, nanobelts, nanourchins, and so forth [22,23]. Herein, we report a direct sonochemical/hydrothermal synthesis of MWCNTs with aqueous C2H2Cl2 at a low temperature of about 160 °C. This hydrothermal process has many advantages in comparison with previous methods: (1) it is not necessary to prepare high temperature and all starting materials are easy to obtain and are stable in ambient condition; (2) it is not necessary to prepare a dangerous gas, for example, H2 as carrier gas in the other methods. This strategy may offer an opportunity for the further investigation of some fundamental properties of peculiar materials and may also serve as a general method for the synthesis of 1D nanostructures.

NaOH solution (5 mol/lit) under an ultrasonic bath (Power Sonic 405, 40 kHz and 350 W) for 0.5 h in ambient temperature. Then, this sonochemically produced aqueous solution was transferred to a Teflon-lined stainless steel autoclave. The hydrothermal synthesis was conducted at 150–180 °C for 24 h in an electric oven. After the reactions, black paste products were sequentially washed with ethanol, dilute acid, and distilled water to remove residual impurities, such as amorphous carbon and catalysts, and then dried at 110 °C for 24 h. The obtained powders were characterized with a scanning electron microscope (SEM, Philips XL30), and a transmission electron microscope (FEG-STEM, Philips CM200). Thermo-gravimetric analysis (TGA) was also conducted on the catalyst precursors to understand

2. Experimental In a typical experiment, 20 ml dichloromethane (C2H2Cl2), 0.5 g lithium (Li) and 0.5 g cobalt chloride (CoCl2) were dissolved into 15 ml ⁎ Corresponding author. Tel.: +98 273 3334530; fax: +98 273 3334537. E-mail address: manafi@iau.shahrood.ac.ir (S. Manafi). 0167-577X/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.matlet.2008.05.072

Fig. 1. SEM image of MWCNTs grown by hydrothermal treatment at 160 °C for 24 h.

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S. Manafi et al. / Materials Letters 62 (2008) 4175–4176

Fig. 3. Raman spectrum of the products containing the MWCNTs.

Fig. 2. High resolution TEM image of MWCNTs.

their decomposition pathway (Shimadzu DTA-50 air atmosphere, dynamic heating). Raman spectra were taken at room temperature under ambient condition using an Almega Raman spectrometer with Ar+ at an excitation wavelength of 514.5 nm. The specific surface area of the synthesized material was determined by analyzing the N2 adsorption isotherm obtained at 77 K using Micromeritics Gemini 2375 equipment. The powders were degassed at 200 °C and 0.8– 1.3 kPa for 2 h prior to the measurement. The surface area of the powder was calculated with the Brunauer–Emmett–Teller (BET) equation [24]. 3. Results and discussion Fig. 1 shows a SEM image of a carbon nanotubes and indicates the large quantity that was achieved using the sonochemical/hydrothermal approach. The carbon nanotubes have diameters ranging from 60 ± 20 nm and lengths from 2–5 μm. The likelihood of finding Co particles in tube tips was in direct proportion to tube diameter, with most small tubes free of Co catalyst. The catalyst was particles with diameter ranging from 100 ± 20 nm, and consists of uniformly distributed within the aqueous solution as confirmed by EDAX analysis. The morphology of the products was further examined with high resolution TEM. As shown in Fig. 2, all powder samples dispersed on the TEM grids show nanotube morphology with diameter 60 ± 20 nm and lengths from 2–5 μm. So, Fig. 2 shows CNTs with closed tips and a ring shaped structure. Further observation of the prepared samples (by TEM) reveals the yield of nanotubes is about 70% from the original reagents, and the other contents of the obtained product are amorphous carbon and some carbon nanoparticles (CNPs). By reason of the high yield nanotube, we could easily find to get individual nanotube under electron microscope observation by dispersing the product in ethanol with an ultrasonic bath, which may be useful to utilize and operate carbon nanotube. In order to investigate the specific surface area of the synthesized CNTs, we used from Brunauer–Emmett–Teller (BET) technique. It shows that synthesized CNTs are without voids in the structure and as-synthesized CNTs have high specific area with average content 125.5 m2/g the quantity above represents nanostructure synthesized powder. It is necessary to measure the efficiency of the hydrothermal process. TG analysis of the sample washed with distilled water showed burn of the carbon material at 500 °C, which suggests that the ratio of the carbon prepared to the catalyst is approximately 20:1, which indicates an efficiency of about 95%. The Raman spectrum of the resulting product indicates two peaks at 1586 and 1340 cm− 1 (Fig. 3), corresponding to the vibration of sp2-bonded carbon atoms in a 2D hexagonal lattice and the vibrations of carbon atoms with dangling bonds in the plane terminations of turbostratic and poorly ordered carbon, respectively. The ratio of these peaks, in the range of G/D = 65/35, indicates that the carbon nanotubes exhibit a rather defective structure. This is quite consistent with what can be guessed from the TEM images, despite their low resolution. Regarding the growth process, it is believed that the ultrasonic pre-treatment of the solution mixture is an important step prior to the hydrothermal reactions at 160 °C, which could generate a considerable amount of multi-walled carbon nanotubes for the subsequent hydrothermal growth. Without this pre-treatment, the observed uniform

nanotube morphology simply cannot be attained. For example, much larger and shorter carbon nanotubes and nanoparticles had been generated from the untreated precursor solution after only 0.5 h reactions at 160 °C. Furthermore, the usage of a high basic condition and an alcoholic environment are the two crucial keys in ensuring the formation of MWCNTs under the hydrothermal condition. However, until now the role of NaOH for the formation of MWCNTs is not clear. This low temperature synthetic route, based on simple reactions without no participation of catalysts and catalyst supports and requiring no expensive and precise equipment, will ensure higher purity in the products and greatly reduce the production cost, and thus offer great opportunity for scale-up preparation of one dimensional nanostructure materials.

4. Conclusion In summary, a novel sonochemical/hydrothermal route to MWCNTs using reduction of dichloromethane by metallic lithium in the presence of Co catalyzer at 160 °C has been developed. The synthesis temperature is the lowest to our knowledge. The MWCNTs obtained from our experiment are well graphited. The catalytic metal particles may play an important role in the nucleation of nanotubes. So, it has been shown that ultrasonic pre-treatment of the solution mixture before hydrothermal condition culminated in the formation of MWCNTs. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]

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