Physical properties of multiwalled carbon nanotubes

Physical properties of multiwalled carbon nanotubes

International Journal of Inorganic Materials 1 (1999) 77–82 Physical properties of multiwalled carbon nanotubes Y. Ando a , *, X. Zhao a , H. Shimoya...

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International Journal of Inorganic Materials 1 (1999) 77–82

Physical properties of multiwalled carbon nanotubes Y. Ando a , *, X. Zhao a , H. Shimoyama b , G. Sakai c , K. Kaneto c b

a Department of Physics, Meijo University, Tempaku-ku, Nagoya 468 -8502, Japan Department of Electrical and Electronic Engineering, Meijo University, Tempaku-ku, Nagoya 408 -8502, Japan c CSE, Kyushu Institute of Technology, Iizuka, Fukuoka 820 -8502, Japan

Abstract Multiwalled carbon nanotubes (MWNTs), which were prepared by hydrogen arc discharge, were purified by using an infrared radiation heating system. The morphology, structure, vibrational modes and crystalline perfection of purified MWNTs were investigated by using scanning electron microscopy, high-resolution transmission electron microscopy, an X-ray diffractometer and a Raman spectrometer. Moreover, the electrical conductivity of individual purified MWNTs was measured using a two-probe method using a micro manipulator system. It turned out that the MWNTs had a high degree of graphitization, an electrical conductivity of about 1.85310 3 S cm 21 along the long axis, and an enormous current density of more than 10 7 A cm 22 .  1999 Elsevier Science Ltd. All rights reserved. Keywords: A. fullerenes; C. electron microscopy; C. X-ray diffraction; C. Raman spectroscopy; D. electron properties

1. Introduction Carbon nanotubes [1] have a unique crystal structure, and their theoretical calculations indicated that they would be either metallic or semiconducting depending on the tube diameter and chirality [2–6]. Recently, Raman spectra studies [7] and electrical transport experiments [8] on single-wall carbon nanotubes (SWNTs) [9,10] showed that the SWNTs were a new class of quantum wire. Multiwalled carbon nanotubes (MWNTs) [1] are made of a few to a few tens of cylindrical graphene sheets. The observation of the Young’s modules for individual MWNTs showed that MWNTs were significantly stiffer than currently available carbon fibers [11]. MWNTs usually can be prepared on the cathode by DC arc discharge of graphite electrodes in inert gas [12–14] and CH 4 gas [15,16]. Ebbesen et al. [17] measured the electrical conductivity of individual MWNTs, which were prepared by graphite evaporation using DC arc discharge in He gas and then further annealed at 28508C for 15 min under Ar gas for eliminating the defects of MWNTs. We replaced the atmospheric gas by H 2 gas [18], and

*Corresponding author. Tel.: 181-52-832-1151 (ext. 5280); fax: 18152-832-1170. E-mail address: [email protected] (Y. Ando)

obtained high-quality MWNTs co-existing with fewer carbon nanoparticles. Moreover, the MWNTs could be easily purified from co-existing carbon nanoparticles by infrared radiation [19]. In this paper, the results of the characterization of purified MWNTs by SEM, TEM and XRD (CuKa), and measurements of the Raman spectra and the electrical conductivity of individual purified MWNTs are reported.

2. Experimental As-grown MWNTs were prepared by DC arc discharge between two graphite electrodes, which were installed vertically in H 2 gas [18,19]. After introduction of H 2 gas at 60 Torr in the evaporation chamber, a DC arc current of 50 A was applied through the lower anode of 6 mm diameter. In order to stabilize the DC arc discharge, the electrodes were not moved through graphite evaporation for 30|60 s. A thin disk-like deposit containing as-grown MWNTs was obtained on the upper cathode of 10 mm diameter. Generally, carbon nanoparticles co-exist with as-grown MWNTs prepared by a DC arc discharge of graphite [12–16,18,19]. In order to purify MWNTs by removing carbon nanoparticles, the disk-like cathode deposit was placed on a quartz specimen holder in air, and was

1466-6049 / 99 / $ – see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S1463-0176( 99 )00012-5

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irradiated using an infrared radiation heating system from the top surface [20] of the deposit for 30 min at 5008C. As a result, a spongy bulk of purified MWNTs with a surface area of approximately 10 mm 2 and thickness of 0.1 mm was obtained [19]. As-grown MWNTs and purified MWNTs were observed by scanning electron microscopy (SEM; Topcon ABT150F) and high-resolution transmission electron microscopy (HRTEM; Topcon EM-002B). To make the specimens for X-ray diffraction (XRD; Shimadzu XRD-6000), we handled the spongy bulk of purified MWNTs with a pair of tweezers, and glued them to a non-diffraction silicon plate with a paste. The Raman spectra were recorded using a Raman spectrometer (Jobin Yvon RAMANOR T64000) with a laser beam of 514.5 nm wavelength and 100 mW power. A 1003 objective lens was used, giving an illuminated spot size of 1 mm on the top surface of the spongy bulk of purified MWNTs. The electrical conductivity of individual purified MWNTs was measured using a two-probe method using a micro manipulator system.

3. Results

3.1. SEM and HRTEM An example of an SEM micrograph of as-grown MWNTs prepared by a DC arc discharge of 50 A in H 2 gas of 60 Torr is shown in Fig. 1. A large number of long and fine MWNTs are observed, and the average aspect ratio is larger than 1000. Although the quantity of coexisting carbon nanoparticles is very little, still they can be seen. Fig. 2 shows an example of a HRTEM micrograph of as-grown MWNT. The inner central channel is very narrow, about 1.0 nm in diameter, which is much thinner than that of MWNTs prepared in He gas and nearly the same as that of SWNTs [9,10]. The outer diameter is near 11.2 nm. A little bit of amorphous carbon covering the surface of the as-grown MWNT can be found, and several carbon nanoparticles can be observed in the left-bottom of the HRTEM micrograph. When the as-grown MWNTs were heat-irradiated in air, the morphology of the MWNTs changed markedly. An example of an SEM micrograph of the MWNTs heat irradiated at 5008C for 30 min is shown in Fig. 3. Only the tangled MWNTs with random orientation can be observed, and the co-existing carbon nanoparticles were entirely removed. This means that the MWNTs were successfully purified. By this method, a spongy bulk of purified MWNTs with a volume of about 1 mm 3 can be obtained. The spongy bulk of purified MWNTs was fairly large and could be easily handled by a pair of tweezers. The observation of HRTEM on the purified MWNTs showed

Fig. 1. A typical micrograph of SEM of the as-grown MWNTs.

that the oxidation not only burned out the carbon nanoparticles co-existing with MWNTs but also damaged the outer layers and the tips of the MWNTs.

3.2. XRD Measurement of XRD for purified MWNTs stuck on non-diffraction silicon plate was carried out. For comparison, the XRD patterns of the outer shell of a cathode deposit, graphene sheets [21] and a raw graphite rod were also recorded. In the XRD patterns of purified MWNTs, the (002) and (004) peaks were found to shift to a lower angle as compared to those from a raw graphite rod, indicating the wide d 002 spacings of the MWNTs [22,23]. The XRD measurement also indicated that the XRD patterns of graphene sheets were similar to those of purified MWNTs and those of the outer shell and the raw graphite rod bore a resemblance to each other.

3.3. Raman spectra The micro-Raman spectra of the purified MWNTs excited by an Ar-laser (514.5 nm) were taken in the frequency range 12–4800 cm 21 . The Raman spectra of

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Fig. 2. An example of HRTEM of an as-grown MWNT. The inner diameter of the MWNT is approximately 1.0 nm, while the outer diameter is near 11.2 nm.

four carbon allotropes, i.e. graphene sheets, an outer shell, purified MWNTs and a raw graphite rod, in the frequency region 12–2000 cm 21 were shown in Fig. 4(a) and (b). In the typical first-order Raman spectrum of the graphite rod (frequency range 1200–2000 cm 21 ), a sharp strong peak appears at 1579 cm 21 , and two weak peaks appear at 1350

Fig. 3. An example of an SEM micrograph of the MWNTs after heat irradiation for 30 min at 5008C.

cm 21 and 1622 cm 21 . The strong peak at 1579 cm 21 can be assigned to the Raman-allowed phonon mode E 2g [24], and the peaks at 1350 cm 21 and 1622 cm 21 correspond to

Fig. 4. Raman spectra in the frequency range 12–2000 cm 21 of graphene sheets, outer shell, purified MWNTs (indicated as MWNT) and graphite rod. (a) The Raman spectra in the range 12–1200 cm 21 ; and (b) 21 1200–2000 cm .

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the disorder-induced phonon mode due to the finite size of crystals and defects [25]. In comparison to the first-order Raman spectrum of the graphite rod, the following features can be found in the Raman spectrum of purified MWNTs in Fig. 4(b). (1) The Raman-allowed phonon mode E 2g downshifts to 1567 cm 21 and the half-width more than doubles. (2) The relative intensity of the peak at 1350 cm 21 decreases and the peak shifts to a lower wave number (1338 cm 21 ). (3) The peak at 1622 cm 21 cannot be observed, and (4) a new peak appears at 1837 cm 21 , although its appearance strongly depends on the specimen. In Fig. 4(a), several new peaks can be observed in the Raman spectrum of purified MWNTs, only in the frequency range 180–500 cm 21 , which strongly depend on the position of the specimen. In the very low frequency range, a lot of peaks labeled * can be seen at the same positions for all carbon allotropes although their intensities vary. The sharp peak at 1125 cm 21 was certain to be the fluorescent lamp line, and the three pairs of peaks at 837 cm 21 , 894 cm 21 and 912 cm 21 labeled . are probably not intrinsic to carbon but to the equipment [23].

3.4. Electrical conductivity of individual purified MWNTs The electrical conductivity of individual purified MWNTs was measured using a two-probe method. Fig. 5

Fig. 6. The current–voltage curves of four individual purified MWNTs at high voltage. The length of four MWNTs was 4, 6, 9 and 10 mm, respectively.

shows the experimental data of resistance per unit length (1mm) along the long axis of purified MWNTs. In the experimental data, the biggest resistance per unit length was 28.1 kV mm 21 , the smallest one was only 1.95 kV mm 21 , and the average resistance was 7.2 kV mm 21 , respectively. In our HRTEM observations on the purified MWNTs, the biggest value of MWNT diameter was determined to be 31.1 nm and the average diameter of the inner central channel was 1.0 nm. If we use 31.1 nm as the outer diameter of the measured purified MWNTs and 1.0 nm as the inner diameter, we can easily calculate the electrical conductivity of the individual purified MWNTs, which is approximately 1.85310 3 S cm 21 . The current–voltage curves of four individual purified MWNTs, whose lengths were 4, 6, 9 and 10 mm, respectively, at a high applied voltage are shown in Fig. 6. When the applied voltage was approximately 4 V, the measured MWNTs were burned out. We can use the current value of 100 mA to calculate the current density of the individual MWNTs, because any one of the four individual MWNTs could conduct a current of 100 mA before they were burned out. The calculated current density is about 1.323 10 7 A cm 2 2 .

4. Discussion Fig. 5. The resistance per unit length (1 mm) of individual purified MWNTs. The average resistance is 7.2 kV mm 21 .

In the first-order Raman spectrum of purified MWNTs, which were shown in Fig. 4(b), the relative intensity of the

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peak at 1338 cm 21 with respect to the peak at 1567 cm 21 was very weak. It is well known that the relative intensity of the peak at 1338 cm 21 depends on the finite size of the crystals and defects [26]; therefore the very weak relative intensity shows the high degree of graphitization of purified MWNTs. This result is consistent with that from HRTEM and XRD analysis. The Raman-allowed phonon mode of purified MWNTs, E 2g , was found to downshift to more than 10 cm 21 compared with that of the raw graphite rod. Initially, it was thought that the very narrow central tubes of the purified MWNTs caused the significant phonon softening. However, when we decreased the power of the Ar-laser source for the measurement of Raman spectra, the downshift values were found to decrease. Obviously, the power of the laser source plays an important role in phonon softening, related to the specimen temperature. It is of interest to note that several new peaks appeared in the Raman spectrum of purified MWNTs in the frequency range 180–500 cm 21 , which were shown in Fig. 6(a). According to the observation of the HRTEM, the MWNTs prepared in H 2 gas possess very narrow inner diameters of approximately 1.0 nm, a value which is the same order as that of the SWNTs. The narrow central tubes of purified MWNTs perhaps give rise to the appearance of the breathing modes of the MWNTs, similar to those of the SWNTs [7]. In the experimental data of resistance per unit length (1 mm) along the long axis of purified MWNTs, the distribution of experimental data is dispersal. We think that the dispersion should come from the difference of diameters of purified MWNTs, whose electrical conductivities were measured. When a voltage of approximately 4 V was applied between individual purified MWNTs, whose length was a few mm, the measured MWNTs were burned out. This phenomenon could be explained by Joule’s heat. Another interesting phenomenon is that the current–voltage curves of four individual purified MWNTs were not straight lines at high applying voltage, i.e. Ohm’s law was not followed at a high applying voltage.

5. Conclusions Evaporation of graphite electrodes in H 2 gas by DC arc discharge was found to form fine and long MWNTs coexisting with fewer carbon nanoparticles. Infrared irradiation of as-grown MWNTs in air could easily purify MWNTs from co-existing carbon nanoparticles. The MWNTs prepared in H 2 gas had a very narrow inner channel, of about 1.0 nm, and a high degree of graphitization. The heat-oxidation burned co-existing carbon nanoparticles and damaged the outer layers and the tips of MWNTs.

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In Raman spectra of purified MWNTs, a strong new peak at 1837 cm 21 , and several weak peaks in the frequency range 180–500 cm 21 were observed. Perhaps the latter came from the breathing modes. The electrical conductivity measurements of individual purified MWNTs indicated that the purified MWNTs obtained using this method possessed an electrical conductivity of approximately 1.85310 3 S cm 21 , and could conduct an enormous current density of more than 10 7 A cm 2 2 . These suggest that this kind of MWNTs can be expected to be used as a conducting wire in nano-techniques in the future.

Acknowledgements The authors thank Drs. S. Iijima and T. Ichihashi of the R&D group of the NEC Corporation for the high-resolution transmission electron microscopy facility. We also thank Dr. M. Hiramatsu of the Department of Electrical and Electronic Engineering in Meijo University for allowing the use of the Raman spectrometer. This work was partially supported by three Grants-in Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture, Japan (Nos. 07455014, 08555157 and 09450013), and by a subsidy from the Iketani Science and Technology Foundation.

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