Characterization and Magnetic Properties of Helical Carbon

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Characterization and Magnetic Properties of Helical Carbon Nanotubes and Carbon Nanobelts Synthesized in Acetylene Decomposition over Fe-Cu Nanoparticles at 450 °C Xiaosi Qi,† Wei Zhong,*,† Yu Deng,† Chaktong Au,‡ and Youwei Du† Nanjing National Laboratory of Microstructures and Jiangsu ProVincial Laboratory for NanoTechnology, Nanjing UniVersity, Nanjing 210093, People’s Republic of China; and Chemistry Department, Hong Kong Baptist UniVersity, Hong Kong, People’s Republic of China ReceiVed: June 8, 2009; ReVised Manuscript ReceiVed: July 23, 2009

Over Fe-Cu nanoparticles derived from sol-gel synthesis followed by hydrogen reduction at 400 °C, helical carbon nanotubes (HCNTs) and carbon nanobelts (CNBs) were synthesized in large quantities in acetylene decomposition at 450 °C. The two carbon species could be separated easily as they deposited on different locations of the ceramic plate in which the Fe-Cu nanoparticles were placed. The advantages of this synthetic method are that the approach is simple and environmentally friendly and there is no need of using dilute gas such as argon or nitrogen. Field-emission and transmission electron microscopic investigations reveal that the selectivity to HCNTs (ca. 85%) or to CNBs (ca. 90%) is high. The corresponding yield of carbon species and CNBs is about 27 307% and 9867%, respectively, higher than any of those reported in the literature. It was found that the variation of reduction temperature (from 400 to 450 or 500 °C) of Fe2O3/CuO catalyst precursor and the temperature gradient during acetylene decomposition have profound influence on the morphology and yield of HCNTs and CNBs. The magnetic properties and the possible reaction mechanism of HCNTs and CNBs were also investigated in this study. Introduction The existence of helical carbon nanotubes (HCNTs) was predicted in the 1990s1-3 and was confirmed experimentally in 1994.4,5 Because of their unique physical and chemical properties, helical carbon materials have been studied extensively for various applications. They can be used as micromagnetic sensors, electromagnetic wave absorption materials, hydrogen storage materials, field emission materials, and so forth.6-9 In addition, HCNTs have been demonstrated to have novel electrical, magnetic, and mechanical properties that could be utilized in nanoengineering.10-16 Over the past years, various methods have been used for the synthesis of HCNTs under different experimental conditions.17-23 Usually, HCNTs were generated in pyrolysis of hydrocarbons such as acetylene, toluene, and pyridine (with the addition of a dilute gas such as H2, Ar, or N2) over metallic catalysts at reaction temperatures >650 °C. However, because of the formation of byproduct, the yield is low. Despite high selectivity to HCNTs being observed on various occasions,7,21,24 reports on high-yield generation of HCNTs are rare. Moreover, carbon nanobelts (CNBs), the quasi-2D carbon nanomaterial, are known to show excellent field emission characteristics, and with strip morphology, they have the potential of being used as electron-transport carriers.25,26 The synthesis of quasi-2D carbon nanomaterials is widely studied but that of CNBs is rarely reported. Kang et al. reported the synthesis of carbon nanotubes (CNTs) and CNBs using a polyoxometalate-assisted hydrothermal method.27 Qian and coworkers synthesized CNBs by means of a medial-reduction and the hydrothermal method.28,29 Through a chemical metath* To whom correspondence should be addressed. Phone: +86-2583621200. Fax: +86-25-83595535. E-mail: [email protected]. † Nanjing University. ‡ Hong Kong Baptist University.

esis route, Qi et al. synthesized CNBs,30 and adopting a template method, Lin et al. obtained CNBs.26 To the best of our knowledge, the selective generation of relatively regular CNBs in large quantity has never been reported. In previous publications, we reported a low-cost route to synthesize double helical carbon nanofibers (HCNFs) and HCNTs selectively by means of decomposing acetylene at 450 °C over Fe nanoparticles generated by a combined sol-gel/ reduction method.7,31 Recently, we reported the mass production of carbon nanorods (CNRs) and nanotubes (CNTs) selectively through benzene decomposition over Ni nanoparticles fabricated using a sol-gel/hydrogen reduction method.32 In the present paper, we report that, during acetylene decomposition at 450 °C over Fe-Cu nanoparticles fabricated through similar approach, HCNTs and CNBs were synthesized in large quantities. The two species deposited on different locations of the ceramic plate in which the Fe-Cu catalyst was placed. We also report the effects of reduction temperature of catalyst precursor (Fe2O3/ CuO) on the yield and nature of HCNTs and CNBs, as well as on the magnetic properties of the obtained samples. Moreover, on the basis of the models proposed in the literature,33-35 we conduct discussion on the formation mechanism of HCNTs and CNBs. Experimental Section Preparation of Catalyst Precursor. The approach is similar to that reported elsewhere.31,32 Briefly speaking, 0.025 mol of FeCl2 · 4H2O, 0.005 mol of CuCl2 · 2H2O, and 0.045 mol of citric acid monohydrate were well mixed with 200 mL of absolute ethanol (with stirring at 60 °C for 6 h). The as-obtained mixture was heated at 80 °C for several hours and then heated at 150 °C for the generation of a xerogel. Finally, the xerogel was heated in air at 500 °C for 4 h for the generation of the catalyst precursor (i.e., Fe2O3/CuO powder).

10.1021/jp905387v CCC: $40.75  2009 American Chemical Society Published on Web 08/12/2009

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Figure 1. Schematic for the synthesis of HCNTs (black) and CNBs (brown). Figure 3. XRD pattern of as-fabricated catalyst precursor.

Results and Discussion

Figure 2. Operating conditions for the generation of HCNTs and CNBs.

Synthesis of HCNTs and CNBs. The Fe2O3/CuO powder (0.05 g; giving 0.037 g of iron and copper after reduction) was dispersed on each of the two ceramic plates that were symmetrically placed on each side of the thermocouple (located at the center of the furnace) in a head-to-tail manner inside a quartz reaction tube (6 cm in inner diameter and 80 cm in length, equipped with temperature and gas-flow controls) (Figure 1). Subsequently, the catalyst precursor was in situ reduced in H2 at (A) 400 °C, (B) 450 °C, and (C) 500 °C for 4 h (as shown in Figure 2). After reduction of catalyst precursor, acetylene was introduced into the reaction tube, and decomposition of acetylene was carried out at 450 °C for 6 h under atmospheric pressure. After cooling to room temperature (RT), a small amount of liquid oligomers was detected at the end of the quartz reaction tube, and 9.504, 3.860, and 2.342 g of products were collected in each ceramic plate in cases (A), (B), and (C), respectively. In each case, the obtained products can be divided into two parts: black and brown. The products can be separated easily according to colors (as shown in Figure 1). As depicted in Figure 2, the black and the brown samples collected in cases (A), (B), and (C) are denoted as X400 and Y400, X450 and Y450, and X500 and Y500 hereinafter, respectively. The samples were examined at RT on an X-ray powder diffractometer (XRD) for phase identification using Cu KR radiation (model D/Max-RA, Rigaku, Japan). Thermoanalysis was carried out on a thermal analysis system (Perkin-Elmer TGA7 series) with approximately 5.0 mg of sample heated in air at a rate of 10 °C/min. The morphologies of various samples were examined over a transmission electron microscope (TEM) (model JEM-2000EX) operated at an accelerating voltage of 80 kV and a field emission scanning electron microscope (FESEM, model FEI Sirion 200) operating at accelerating voltages of 5 kV. The magnetic properties of the samples were measured at 300 K using a Quantum Design MPMS superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS-XL) equipped with a superconducting magnet capable of producing fields of up to 50 kOe. Fourier transform infrared (FTIR) spectroscopic studies of samples (in KBr pellets) were conducted over a Nicolet 510P spectrometer.

Crystalline Structures of Fe2O3/CuO Precursor. The XRD pattern of the as-fabricated catalyst precursor is shown in Figure 3. The peaks can be indexed to a rhomb-centered hexagonal phase of Fe2O3 (JCPDS card file no. 84-0306; lattice parameters a ) 5.034 Å and c ) 13.74 Å) and an end-centered monoclinic phase of CuO (JCPDS card file no. 80-1916; lattice parameters a ) 4.692 Å, b ) 3.428 Å, and c ) 5.137 Å). Since no peaks other than those of Fe2O3 and CuO are observed, we deduce that the catalyst precursor is of high purity. Microstructures of X400 and Y400. In case (A), 9.504 g of X400 and Y400 was collected. As indicated by arrows in Figure 4(a), black and brown products are produced in different locations of the ceramic plate (as depicted in Figure 1). The XRD pattern of X400 and Y400 is shown in Figure 4(b) and (c), respectively. The peaks can be attributed to (002) and (101) reflections of hexagonal graphite (JCPDS card file no. 41-1487; lattice parameters a ) 2.470 Å and c ) 6.756 Å), denoting the formation of graphitic species. Since the peaks of Figure 4(b) are stronger and narrower than those of Figure 4(c), we deduce that the crystallinity of X400 is better than that of Y400. There is no XRD signal assignable to Fe and Cu particles; the result suggests that the contents of Fe and Cu in the obtained samples are low. It is understandable because the weight percentage of iron and copper in the collected 9.504 g of X400 and Y400 is only 0.39%. The yield of carbon species as defined by the equation

yield )

mtotal - mcatalyst × 100% mcatalyst

is extremely high, up to approximately 27 307%. In the literature, high yields of CNTs (ca. 3050% at 900 °C over FeMoMgO),36 CNRs (ca. 4579% at 460 °C over Ni nanoparticles),32 and carbon nanocoils (CNCs) (ca. 21 270% at 425 °C over Ni nanoparticles)37 were reported. To the best of our knowledge, a yield of nanocarbon species as high as approximately 27 307% is unprecedented. Since the metal content is 0.37 wt %, the carbon purity of the sample is up to 99.63 wt %, much higher than that of HCNTs obtained over Fe at 450 °C.7 The result of thermogravimetric (TG) analysis shows a weight loss of 99.46% in the 500-700 °C range (Figure 4(d)), and the 0.54 wt % leftover can be considered as Fe2O3 and CuO. The weight loss hence confirms the high purity of the collected sample. The results indicate that the Fe-Cu catalyst fabricated using the combined sol-gel/hydrogen reduction method is suitable for large-scale production of carbon species of high purity.

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Qi et al.

Figure 4. (a) Photo of X400 and Y400 deposited in a ceramic plate; (b, c) XRD pattern of X400 and Y400, respectively; (d) TG curve of as-obtained carbon sample.

Figure 5. (a, b) FE-SEM images and (c, d) TEM images of X400 at different magnifications.

Figure 5 shows the FE-SEM and TEM images of X400. One can see that the content (ca. 85%) of HCNTs is high. The HCNTs are with nozzles that are indicated by arrows in Figure 5(a) and (b). Compared to the case of generating double-HCNTs over Fe at 450 °C,7 the selectivity to double-HCNTs in case (A) is low. Instead of double-HCNTs, plaitlike HCNTs are synthesized as observed in the FE-SEM and TEM images (Figure 5). The HCNTs are tens of micrometers in length and 30 to 100 nm in diameter. Moreover, CNTs could also be observed. As shown in Figure 5(c) and (d), the HCNTs are coiled in a regular and tight fashion, and the coil pitch is short. The walls of the HCNTs are thick, and the hollow part of the tubes cannot be observed in TEM examination. Similar phenomena were reported before.7,23 It is noted that carbon species with uncoiled morphologies can be detected occasionally (Figure 5(a)-(c)). The FE-SEM and TEM images of Y400 are shown in Figure 6. From Figure 6(a) and (b), one can see that CNBs are generated in large majority (ca. 90%). The width of the nanobelts

Figure 6. (a, b) FE-SEM images and (c, d) TEM images of Y400.

ranges from 200 nm to 1.0 µm; the thickness is about 5-10 nm, and the length ranges from tens of micrometers to several millimeters. The width of the nanobelt pointed by the arrow is approximately 500 nm (Figure 6(c) and (d)). On the basis of the TEM and FE-SEM images, we estimate that the width-tothickness ratio of the nanobelts varies from 10 to 200. The amount of Y400 collected was 3.468 g, slightly higher than onethird of the total mass. In other words, the yield of CNBs was approximately 9867%. Again, such a high yield of CNBs has never been reported. Effects of Reduction Temperaure of Catalyst Precursor. In case (B), the amount of X450 and Y450 collected was 3.860 g. The yield and purity of carbon species is estimated to be around 10 994% and 99.10 wt %, respectively. Compared to Y400 of case (A), Y450 is deeper in color and higher in content. The IR spectrum of X450 and Y450 is shown in Figure 7(b) and (c), respectively. As shown in Figure 7(b), no IR signals attributable to -CHdCH-, -CH2-, and -CH3 entities are detected over X450, indicating complete decomposition of acetylene. Over Y450, a peak at 3044.0 cm-1 ascribable to the stretching vibration of

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Figure 7. (a) Photo of X450 and Y450; (b) and (c) IR spectrum of X450 and Y450, respectively; (d) and (e) typical magnetization curve of X450 and Y450, respectively (inset: enlarged part close to origin).

unsaturated -CHdCH- is observed. The peaks at 2925.8 and 2867.7 cm-1 can be ascribed to the stretching vibration of -C-H while that at 1600.5 cm-1 can be ascribed to the skeleton vibration of graphene. The other peaks can be ascribed to the deformation vibration of -C-H (Figure 7(c)). The results suggest that Y450 is a result of incomplete decomposition of acetylene. It is apparent that there was a temperature gradient in the quartz reaction tube. At the middle of the quartz tube where the thermocouple was located, the temperature was 450 °C, and black X450 was generated. Slightly away from the middle, the temperature was slightly lower, and brown Y450 was produced (as shown in Figure 2). Figure 7(d) and (e) shows the magnetization-coercivity (M-H) curves of X450 and Y450 (measured at 300 K). The saturation magnetization (MS) and the coercivity (HC) of X450 are 0.98 emu/g and 625 Oe, respectively. We deduce that the magnetizm is due to the catalyst

nanoparticles that distribute throughout the samples. Compared with X450, Y450 shows a MS (0.21 emu/g) and a HC (466 Oe) much lower in value. The results suggest that the distribution of catalyst nanoparticles in X450 is different from that in Y450 and the content of Fe-Cu nanoparticles in the latter is much lower than that in the former. Figure 8 shows the FE-SEM and TEM images of X450. One can see that HCNTs and wormlike CNTs are produced in large scales (Figure 8(a) and (b)). The nozzles of the nanotubes can be clearly seen as indicated by arrows in Figure 8(b). Compared to case (A), case (B) shows poorer selectivity to HCNTs but clear generation of wormlike CNTs (Figure 8(a) and (c)). The FE-SEM and TEM images indicate that the majority of wormlike CNTs are with diameters in the 150-250 nm range. A closer examination (Figure 8(c) and (d)) reveals that the wormlike CNTs are attached to a catalyst nanoparticle of

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Figure 8. (a, b) FE-SEM images and (c, d) TEM images of X450.

Figure 9. (a, b) FE-SEM images and (c, d) TEM images of Y450 at low and high magnification.

approximately 80 nm diameter. This can be considered as a clear evidence for the existence of magnetic Fe-Cu in X450. Figure 9 shows the FE-SEM and TEM images of Y450. The FE-SEM images (Figure 9(a) and (b)) indicates that the selectivity to CNBs is high, up to approximately 90%. The CNBs are wider than those observed in case (A). The dimensions of the obtained CNBs are estimated to be tens of micrometers to several millimeters in length, 40 ( 30 nm in thickness, and 200 nm to 2.0 µm in width. As shown in Figure 9(c) and (d), the displayed nanobelts are uniform in width and thickness, and it is uncommon to have catalyst nanoparticles attached to them. After separation of X450 and Y450, 1.831 g of the latter was collected (almost half of the total mass). The yield of CNBs is hence estimated to be approximately 5163%. In general, we find that the reduction temperature of catalyst precursor has profound effects on the yield and

Qi et al.

Figure 11. (a, b) FE-SEM images and (c, d) TEM images of Y500.

morphology of products. In order to confirm such a view, the Fe2O3/CuO powder was reduced at 500 °C as well. In case (C), 2.342 g of the product was collected (Figure 10(a)), giving a yield of approximately 6631%. In contrast to X400 (Figure 5) and X450 (Figure 8), there are CNTs of low helicity (L-HCNTs) in X500. The nozzles of HCNTS and L-HCNTs can be seen clearly as indicated in Figure 10(b) and (c). In Figure 10(d) and (e), one can see two L-HCNTs stemmed out from a catalyst nanoparticle. The diameters of the nanotubes are in the 80-100 nm range, and the grain size of the catalyst nanoparticle is 50 ( 10 nm. Similar to the cases of X400 (Figure 5(c) and (d)) and X450 (Figure 8(c) and (d)), the hollow part of the multiwalled L-HCNTs cannot be observed clearly in TEM analysis (Figure 10(d) and (e)). Figure 11 shows Y500 at different magnifications. The FESEM images (Figure 11(a) and (b)) show that the content of CNBs in Y500 is at least up to approximately 90%. Compared to that of Y400 (Figure 4(a)) and Y450 (Figure 7(a)), the content of Y500 (2.342 g) is high. After separation, 1.263 g (exceeding half of the total mass) of Y500 is obtained, and the yield of CNBs is up to 3 530%. As shown in Figure 11(a) and (b), crooked CNBs are generated in large quantity. The CNBs are tens of micrometers to several millimeters in length, 5-100 nm in thickness, and 100 nm to 2.0 µm in width. Almost no trace of catalyst nanoparticles can be observed in the TEM images (Figure 11(c) and (d)), indicating that the distribution of the catalyst is uneven throughout the entire sample. It is without doubt that the temperature for the reduction of catalyst precursor has profound effects on the morphology and nature of carbon products. It is observed that a rise in reduction temperature from 400 to 500 °C favors the generation of CNBs but lowers the total yield of carbon product. Since the decomposition of

Figure 10. (a) Photo of X500 and Y500; (b, c) FE-SEM images and (d, e) TEM images of X500.

Helical Carbon Nanotubes and Carbon Nanobelts

J. Phys. Chem. C, Vol. 113, No. 36, 2009 15939 products. In general, we find that the Fe-Cu nanoparticles derived from sol-gel synthesis followed by hydrogen reduction are suitable for large-scale production of CNBs and the formation of CNBs is subject to subtle changes of reaction conditions. A clear understanding of the growth mechanism of CNBs requires further investigation. Possible Reaction Mechanism. The results of the comparison experiments indicate that the Fe-Cu catalyst and the temperature gradient influence the kinds of products being formed. It is proposed that the main reaction processes of acetylene are as follow ((a) stands for adsorbed):

C2H2(g) f C2H2(a) Liquid oligomers: lC2H2(a) f -(-CH)CH-)l- (l > 4)

(1) Figure 12. (a, b) FE-SEM images of the sample obtained over Fe nanoparticles; (c, d) TEM images of the sample collected over Cu nanoparticles.

acetylene was conducted at equal temperature (450 °C) in cases (A), (B), and (C), any discrepancy in yield of carbon species should be related to the reduction temperature of catalyst precursor. It is of significance to investigate the influence of the reduction temperature of Fe2O3/CuO on the Fe-Cu catalyst. However, because of the high affinity to oxygen, the Fe-Cu nanoparticles ignite spontaneously in air even at room temperature. Such an intrinsic chemical property makes the characterization of the catalyst extremely difficult. To investigate the factors that influence the formation of CNBs, a series of comparison experiments were designed and conducted: (I) Only Fe nanoparticles fabricated according to the described method were used as catalyst while the other experimental conditions were kept as those of case (B). In such a case, all the obtained samples (0.915 g) were black in color (the same as previously reported31), and TEM observations showed no trace of CNBs. The FE-SEM images of the obtained samples indicate that the yield selectivity (85%) of HCNTs is high (Figure 12(a) and (b); the arrows indicate the nozzle of HCNTs). We hence deduce that the presence of Cu in the catalyst is crucial for the formation of CNBs. (II) Only Cu nanoparticles fabricated according to the described method were used as catalyst while the other conditions were kept as those of case (B); 1.205 g of products were collected on the two ceramic plates. In contrast to the results of Qin and coworkers,38-42 the yield of HCNTs is low, and thin carbon nanofibers (CNFs) are produced in large quantity as revealed in TEM investigations. A closer examination shows that the morphology of some sections of the CNFs are beltlike (as indicated by the arrows in Figure 12(d)). The results further confirm that Cu is an essential component in the catalyst for the formation of CNBs. (III) Only one ceramic plate (rather than two) with 0.05 g of Fe2O3/CuO powder was placed at the middle of the quartz reaction tube. After cooling to RT, 7.447 g of product was collected, and the majority of it was black and only a minute amount brown. The results of TEM investigation of the brown material indicate CNBs of high purity. It is apparent that a temperature gradient during pyrolysis has a great impact on the production of CNBs. (IV) The other experimental conditions were kept as those of case (A), and the pyrolysis temperature was raised to 600 °C. In such a case, the obtained product (11.296 g) in each ceramic plate was black in color. It is understandable because, if the pyrolysis temperature is high enough, acetylene decomposes completely and H-free carbon species (black) would be the

Solid polymers: mC2H2(a) f -(-CH)CH-)m- (l > m)

(2) Carbon:

nC2H2(a) f 2nC + 2nH

(3)

Because of the existence of the temperature gradient across the Fe-Cu nanoparticles during the pyrolysis process, solid polymers are generated at the region of lower temperature, and the obtained brown sample is a mixture of carbon entities and the solid polymers. Moreover, according to the model of Gamaly and Ebbesen,35 the distribution velocity of carbon atoms on the surface of catalysts is a major factor that governs the kinds of carbon nanomaterials being formed. It is known that the mobility of carbon atoms on a catalyst surface can be affected by factors such as catalyst category and temperature as well as concentration of surface carbon atoms. Under the conditions adopted in our experiments, some adsorbed acetylene molecules couple to form liquid oligomers (eq 1) that subsequently vaporize and finally condense at the end of the quartz reaction tube. Meanwhile, a large number of acetylene molecules polymerize to form polymers (eq 2). It is only in eq 3 that acetylene undergoes complete decomposition to generate carbon atoms. Under the influence of the Fe-Cu catalyst and subject to temperature, the generated carbon atoms that differ in mobility assemble into hexagonal, pentagonal, and heptagonal carbon rings, forming the different carbon products at different regions. Because of the complexity of the reaction processes, the exact formation mechanism of the obtained carbon nanomaterials still needs further research. Conclusions In this article, we report a simple and environmentally friendly approach of chemical vapor deposition for the simultaneous synthesis of HCNTs and CNBs in large quantities. The HCNTs and CNBs species could be separated easily because they deposited on distinct locations of the ceramic plate in which the Fe-Cu nanoparticles were placed. The yield and selectivity of HCNTs and CNBs can be regulated by controlling the reduction temperature of the catalyst precursor (Fe2O3/CuO). The maximum yield of carbon species and CNBs was 27 307% and 9867%, respectively. Such high yields of carbon species and CNBs are unprecedented. Moreover, the obtained HCNTs (X450) and CNBs (Y450) show distinct magnetic properties. The results of a series of comparison experiments designed to investigate the influence of synthesis conditions indicate that

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the Cu in the catalyst is crucial in CNBs fabrication and a temperature gradient during pyrolysis would affect the yield of CNBs. Acknowledgment. This work was supported by the National Natural Science Foundation of China (grant no. 10674059), the National High Technology Research and Development Program of China (grant no. 2007AA021805), and the National Key Project for Basic Research (grant nos. 2010CB923402 and 2005CB623605), People’s Republic of China. Supporting Information Available: Figure of helical carbon nanotubes and carbon nanobelts. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Itoh, S.; Ihara, S.; Kitakami, J. Phys. ReV. B 1993, 47, 1703. (2) Itoh, S.; Ihara, S.; Kitakami, J. Phys. ReV. B 1993, 48, 5643. (3) Itoh, S.; Ihara, S. Phys. ReV. B 1993, 48, 8323. (4) Ivanov, V.; Nagy, J. B.; Lambin, Ph.; Lucas, A. A.; Zhang, X. B.; Zhang, X. F.; Bernaerts, D.; Van Tendeloo, G.; Amelinckx, S.; Van Landuyt, J. Chem. Phys. Lett. 1994, 223, 329. (5) Zhang, X. B.; Zhang, X. F.; Bernaerts, D.; Van Tendeloo, G.; Amelinckx, S.; Van Landuyt, J.; Ivanov, V.; Nagy, J. B.; Lambin, Ph.; Lucas, A. A. Europhys. Lett. 1994, 27, 141. (6) Motojima, S.; Hoshiya, S.; Hishikawa, Y. Carbon 2003, 41, 2658. (7) Tang, N. J.; Zhong, W.; Au, C. T.; Gedanken, A.; Yang, Y.; Du, Y. W. AdV. Funct. Mater. 2007, 17, 1542. (8) Xie, G. W.; Wang, Z. B.; Cui, Z. L.; Shi, Y. L. Carbon 2005, 43, 3181. (9) Furuya, Y.; Hashishin, T.; Iwanaga, H.; Motojima, S.; Hishikawa, Y. Carbon 2004, 42, 331. (10) Ihara, S.; Itoh, S. Carbon 1995, 33, 931. (11) Akagi, K.; Tamura, R.; Tsukada, M.; Itoh, S.; Ihara, S. Phys. ReV. Lett. 1995, 74, 2307. (12) Iijima, S.; Toshinary, I.; Ando, Y. Nature 1992, 356, 776. (13) Amelinckx, S.; Zhang, X. B.; Bernaerts, D.; Zhang, X. F.; Ivanov, V.; Nagy, J. B. Science 1994, 265, 635. (14) Hsu, W. K.; Terrones, M.; Hare, J. P.; Terrones, H.; Kroto, H. W.; Walton, D. R. M. Chem. Phys. Lett. 1996, 262, 161. (15) Ajayan, P. M.; Nugent, J. M.; Siegel, R. W.; Wei, B.; KohlerRedlich, P. Nature 2000, 404, 243. (16) Kong, X. Y.; Wang, Z. L. Nano Lett. 2003, 3, 1625.

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