Single-walled carbon nanotube fibers, films and balls

Solid State Communications 141 (2007) 459–463 www.elsevier.com/locate/ssc

Single-walled carbon nanotube fibers, films and balls Lunhui Guan, Huanjun Li, Zujin Shi ∗ , Zhennan Gu ∗ Beijing National Laboratory for Molecular Sciences, State Key Lab of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, PR China Received 31 May 2005; received in revised form 24 November 2006; accepted 25 November 2006 by E.G. Wang Available online 18 December 2006

Abstract Well-defined fibers and films of single-walled carbon nanotubes (SWNTs) with high purity and narrow diameter distributions were obtained from the strand-like raw soot produced by a dc arc-discharge method. These architectures made up of SWNTs have very uniform smooth surfaces. When the strand-like product was placed on a silicon substrate, dipped into water, treated ultrasonically, and then dried in air, another interesting architecture, an SWNT ball, was obtained. This ball-like structure could also be found on the surface of purified SWNTs. We propose that the surface tension of water and the interaction between SWNTs and silica sphere played the key role in the ball (SWNTs outside and silica sphere inside) formation process. c 2006 Elsevier Ltd. All rights reserved.

PACS: 39.10.+j; 61.48.+c; 81.05.TP; 81.70.Jb Keywords: A. Carbon nanotube; B. Scanning electron microscope

1. Introduction Owing to their unique mechanical and electrical properties, single-walled carbon nanotubes (SWNTs) are desired for a broad range of applications in many fields [1], such as electrochemical devices [2], field emission devices [3], and sensors [4]. For some applications, their novel properties can only be actualized when a well-defined architecture can be obtained. So far, there has been much research toward producing well-defined SWNTs architectures, a key to building nanotube devices, or assembling remarkably flexible SWNTs into designed architectures. In 1998, Cheng et al. [5] synthesized long and wide ropes of SWNT bundles by catalytic decomposition of hydrocarbons. Subsequently, Vigolo et al. [6] obtained macroscopic SWNT fibers and ribbons by a postprocessing technique. Wu et al. [7] reported the formation of a 20 cm-long nanotube strand after the pyrolysis of hexane, ferrocene, and thiophene. Recently, Hata et al. [8] reported massive growth of superdense and vertically aligned SWNT forests. And in our recent work [9], we also succeeded in

synthesizing SWNT fibers with high purity by an arc-discharge method. SWNTs can also be induced to organize themselves into rings or coils which could be stabilized by van der Walls forces alone [10] or covalent bonds [11]. All these attempts are towards the goal of getting SWNTs with fine properties in one or more directions. Herein, we report the direct synthesis of fibers and films of well-aligned SWNTs by an arc discharge method. And motivated by the recent results in which multi-walled carbon nanotube (MWNT) 3-dimensional architecture was fabricated by using capillarity force [12], we succeeded in fabricating a ball structure with SWNTs outside and a silica sphere inside by a simple technique. Furthermore, this fascinating structure could be found on the surface of purified SWNT bulky paper. We propose that the surface tension of water and silicon dioxide (as the core of the ball) play the key role in the formation of the SWNT ball. Our findings may have an implication for utilization of the surface tension and capillarity force of water to manipulate SWNTs and construct SWNT microstructures. 2. Experiment section

∗ Corresponding author.

E-mail addresses: [email protected] (Z. Shi), [email protected] (Z. Gu). c 2006 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2006.11.035

A direct current arc apparatus was used in this study. SWNTs were produced by a dc arc-discharge method similar

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to our earlier work [9]. Briefly, the cathode was a φ8 × 200 mm graphite rod. The anode was a φ6 × 150 mm spectrally pure graphite rod drilled with a φ4 × 100 mm hole and filled with graphite powder, Y–Ni alloy (YNi2 ), and a small quantity of FeS with weight ratio of 80:20:5. The arc discharge was created by a current of 80 A in helium atmosphere at a pressure of 760 Torr. The arc was maintained by continuously translating the cathode to keep a constant distance (∼3 mm) between two electrodes. Typically, it took 15 min to consume a 150 mm graphite anode. The products for investigation were obtained on the top of the cathode and in the inner wall of the furnace. They showed strand-like and clothlike appearance, respectively. The as-prepared SWNTs were characterized by a scanning electron microscope (SEM, JEOL, JSM-6700F), and Raman spectroscopy (Renishaw system 2000 UK). Composition analysis was performed in a focused ion beam apparatus (DB-235 FIB) equipped with an energy dispersive X-ray spectrometer (EDX) analyzer. The SWNT ball was obtained as follows. First, the strandlike product was placed on the surface of a silicon sheet (typically on the surface of silicon sheet there is a very thin silicon dioxide film). Secondly, the silicon sheet with strand-like product on its surface was totally dipped into water and treated ultrasonically (40 W) for 30 s, and then the silicon with strandlike raw soot was moved from the water, and dried naturally in air. The so-obtained material was characterized with an SEM. The SWNT raw soot was purified by a method similar to that in our previous report [13]. Briefly, the raw soot was first oxidized in an air flow of 100 sccm at 320 ◦ C for 1 h. The product was immersed in 2.6 M HNO3 in a glass flask. The glass flask containing the raw SWNTs and HNO3 was heated in a microwave oven for 2 h and refluxed for another 16 h. The obtained suspension was filtrated and dried in air to form a thin film (so-called SWNT bulky paper). 3. Results and discussion The Raman scattering technique has been shown to be a powerful tool to characterize SWNTs [14]. The Raman spectra were collected by using a 514.5 nm line of an Ar+ laser for excitation at room temperature and ambient pressure. The spectrometer was calibrated by the F1g mode of Si at 520.2 cm−1 . Fig. 1 shows typical Raman spectra of the clothlike and strand-like products, respectively. The spectra confirm that the soot contains large quantity of SWNTs, which is evidenced by the characteristic SWNT peaks, i.e. the shoulder peak around 1564 cm−1 and the radical breathing mode (RBM) from 100 to 220 cm−1 . The peak around 1590 cm−1 (G-band) originates from the SWNTs; on the other hand, the peak around 1350 cm−1 (D-band) is the Raman active mode of the residual ill-organized graphite, and the G/D value is a good index for the SWNT abundance [15,16]. The intensity of the D-band in Fig. 1(b) (G/D = 19.4) is lower than that in Fig. 1(a) (G/D = 5.7), which indicates that SWNTs are of higher purity in the strand-like products than their cloth-like counterparts. In the low-frequency range, the RBM mode is very sensitive to the diameter of tubes and can be used to probe the size of

Fig. 1. Raman spectra of: (a) the cloth-like raw soot; (b) the strand-like raw soot. The insets show the low-frequency RBM spectra.

the SWNTs in the bundles based on the following relationship between the diameter (nm) and the RBM frequency ω (cm−1 ): d = 224/(ω − 14) [17]. Although different diameter tubes with different mode frequencies couple with different efficiencies to the laser excitation frequency [18], the diameter distributions are expected to be similar within any given sample [19]. During the arc experiments, the wool-like soot can be seen to eject from the center of the arc and then to adhere to the chamber wall to form the cloth-like raw soot, while the SWNT strands are found in the localized region around the cathode. It is expected that the SWNTs in the strand-like products would show more homogeneity than that in the clothlike raw soot. The high-resolution RBM spectrum (Fig. 1 inset) shows that the SWNTs in the strand-like products share a narrower diameter distribution than that in the cloth-like products. Since the electronic and optical properties of SWNTs depend sensitively on the tube diameter, the SWNTs in the strand-like products with narrow diameter distribution would have greater superiority in application than their counterparts in the cloth-like raw soot. For SEM measurements, the products were cut and placed on the sample stage. Fig. 2(a) shows a high-magnification SEM image of the cloth-like SWNT raw soot. The SWNT bundles, together with a large number of catalyst particles, could be clearly seen in the products. The purity (volume) is estimated to be ∼20%. While taking the strand-like products into account, the products contain some well-aligned SWNTs

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Fig. 2. SEM images of (a) the cloth-like SWNT raw soot obtained from the inner wall of the furnace; (b) the SWNT bundles obtained from the cathode; (c) the SWNT film obtained from the cathode; (d) higher magnification of the film.

Fig. 3. SEM images of (a) a typical SWNT ball; (b) higher magnification in region 1; (c) higher magnification in region 2; (d) higher magnification in region 3.

fibers (Fig. 2(b)), which is quite in accord with our previous results [9]. Besides the SWNT fibers, another interesting product, SWNTs films, could be easily found in the products. A typical SEM image of an SWNT film is shown in Fig. 2(c). The film, with width of nearly 200 µm, has a very uniformly smooth surface, and it can be folded without breakage. The edges of the SWNT film are continuous and smooth, with a few SWNT bundles protruding out. A higher magnification of this film (Fig. 2(d)) shows the high purity of the SWNTs. The film is composed of abundant snarled SWNT bundles. Only a few impurities could be found adhering to the SWNT bundles. It is a real two-dimensional architecture which is made up of SWNTs. In the synthesis of well-aligned SWNTs, we found that using FeS as promoter was pivotal. In our previous work [9], when we added a little Fes (about 1.5 weight %) as promoter, we obtained high-purity SWNT fibers. When we increased the relative amount of FeS in the weight ratio graphite:YNi2 :FeS from 80:20:1.5 to 80:20:5, we obtained SWNTs films on the cathode surface, together with SWNT fibers. Without FeS, the quantity and quality of SWNT fibers decreased dramatically:, we could only obtain some cloth-like and cotton-like products. However, when we continued to increase the amount of FeS until it accounted for 10% of the total materials encapsulated in the graphite rod, very little SWNTs could be obtained in the arc apparatus. We could hardly obtain SWNTs films and fibers. We can only get very little cloth-like SWNT raw soot. The actual role played by FeS in the formation of SWNT films is still open to question; however, it is known that the presence of a little amount of S assists the graphitization of the nanotube and promotes the catalytic action, especially when associated with iron [20]. When large amounts of FeS were used, we estimated that S will cover the metal catalyst’s surface and

poison the catalyst then prevent the carbon atom from adsorbing on the metal catalyst. As a result, very little SWNTs could be synthesized. When the strand-like product was placed on a silicon substrate, dipped into deionized water, treated ultrasonically, and dried in air, some interesting objects, SWNT balls, were found. Fig. 3 shows this fascinating structure. The SWNT ball has a rough surface, and its diameter is about 30 µm (Fig. 3(a)). A higher-magnification image of region 1 (Fig. 3(b)) indicates that the surface of the ball is composed of tangled SWNT bundles, together with some catalyst particles. In the border of the ball (Fig. 3(c)), we can find that a relatively smooth surface, and some SWNT bundles stick tightly to the surface. Fig. 3(c) is of relatively different appearance to Fig. 3(b), which implies that the different regions have different composition. The inner structure and composition of the balls (mainly silicon dioxide) were identified and will be discussed later. Wei et al. [21] have reported that when well-aligned nanotube fibers were ultrasonically dispersed in ethanol solution for several tens of minutes, the strands got tangled and formed macroscopical balllike structures. As regards our microscopical SWNT ball, the synthesis mechanism needs to be established. As published recently, aligned MWNT film [22] can selfassemble to large-scale micropatterns through the capillary force of water on quartz substrates [12]. This finding may have implications for the formation of some architectures based on SWNTs. We repeated the experiment and got the same result. Furthermore, we accidentally discovered some ball structures, which occupy about 3% of the area on the surface of the bulky paper. Fig. 4 shows detailed SEM images of the purified SWNT bulky paper. As shown in Fig. 4(a), the ball-like structures are clearly visible on the surface of the SWNT bulky paper. Higher-

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Fig. 5. EDX spectrum of the SWNT ball. Fig. 4. SEM images of purified SWNT bulky paper: (a) low magnification; (b) and (c) higher magnification, in which SWNT balls are clearly visible; (d) surface of the SWNT ball.

magnification images (Fig. 4(b), (c)) show that the SWNTs are of relatively high purity, which was in accord with our previous results [13]. The tangled SWNT bundles stick tightly to both the surface and border of the ball. A detailed SEM image (Fig. 4(d)) of the ball indicates that the surface of the ball is composed solely of SWNTs; no visible impurity could be found. There are still some problems that remain unclear; for instance, what is the inner structure of the ball? To uncover the inner structure and composition of the SWNT balls on the bulky paper, we performed energy dispersive Xray spectrometry (EDX) directly on the SWNT balls. With the EDX analyzer installed in the FIB, the compositions of SWNT balls on the bulky paper were determined to be carbon, silicon, oxygen and a small amount of sodium (Fig. 5). The Al signals were due to the aluminum sample holder. The residual contaminants in purified SWNTs, mainly chemical residues, are from the purification, dispersion and filtration processes. Since the purification process involves refluxing the raw soot in a glass flask, it is likely that a small amount of silicate and/or silicon dioxide would dissolve in the acid (HCl or HNO3 ) solvent. Goldoni et al. [23] reported that these residues, especially sodium-containing species, affect the transport and electronic properties of SWNTs upon exposure to gases such as O2 , CO and N2 . We also checked the composition of the ball on the strand-like raw soot; it had nearly the same chemical composition as the ball on bulky paper. The silicon dioxide in the ball on strand-like raw soot might come from the silicon wafer or come from the strand-like raw soot as impurity. Although the source of the silica spheres remains unclear, it is obvious that the silicon spheres play an important role in the formation of the SWNT balls. Jeong et al. [24] reported a sonochemical route to synthesize SWNTs in which the presence of silica powder is quite essential. The result implies that there

is an interaction between SWNTs and silica surface, especially accompanied with ultrasonic treatment. Zhou et al. [25] directly grew SWNTs on silica spheres by chemical vapor deposition and found that SWNTs tend to wrap silica spheres to form uniform SWNT nanoclaws. In their results, it was calculated that the van der Waals contribution to the energy decrease of tube–silica system per unit nanotube length is 1.59 eV nm−1 . As to our experiment, we estimated that during the SWNT dispersion and filtration processes, the SWNT bundles would deposit on the silicon wafer. When the water evaporated in air, the surface tension and the capillary action of water would draw the SWNT bundles tightly on the surface of silica spheres and finally form the ball structure composed of SWNTs outside and a silica sphere inside. In summary, fibers and films of SWNTs with high purity and narrow diameter distributions were directly synthesized by a dc arc discharge method under the existence of YNi2 catalyst and a small amount of FeS as promoter. The twodimensional film with width of nearly 200 µm has a very uniform smooth surface. A highly ordered SWNT architecture, SWNT balls, could be obtained on a silicon sheet through the surface tension and capillary action of water, van der Waals interaction between SWNTs and silicon dioxide as well. Our result may provide valuable information on SWNT properties and provide an implication for the utilization of the interaction between SWNTs and silicon dioxide, together with the surface tension and capillarity of water, to manipulate SWNTs and construct SWNT microstructure. Acknowledgements This work was supported by the National Natural Science Foundation of China, No. 20151002, 90206048 and 20371004. References [1] R.H. Baughman, A.A. Zakhidov, W.A. de Heer, Science 397 (2002) 787.

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