Pyrolytic Synthesis of Conical Carbon Fibers with Carbon Nanotube

Apr 7, 2009 - The pyrolytic synthesis of conical carbon fibers (CCFs) with tips of carbon nanotubes (CNTs) that run through the CCFs as their cores ha...
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J. Phys. Chem. C 2009, 113, 7629–7632

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Pyrolytic Synthesis of Conical Carbon Fibers with Carbon Nanotube Tips on Carbon Substrates Wei Zhang,* Zhonghe Xi, Xin Bai,† and Gengmin Zhang Key Laboratory for the Physics and Chemistry of NanodeVices and Department of Electronics, Peking UniVersity, Beijing 100871, People’s Republic of China, and Institute of Electronics, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ReceiVed: January 24, 2009; ReVised Manuscript ReceiVed: March 11, 2009

The pyrolytic synthesis of conical carbon fibers (CCFs) with tips of carbon nanotubes (CNTs) that run through the CCFs as their cores has been investigated under different experimental conditions. The core construction of the carbon composite was found to be mainly determined by the presence of a nickel catalyst and the use of a high reaction temperature of 1800 °C. The graphon (a graphitized carbon black) and pyrolytic carbon film substrate make a greater contribution to the nucleation of the CCF and CNT composites than the basal surface of highly oriented pyrolytic graphite with a deposit of nickel. Raman spectra show that the two former substrates have many more sp2 defects than the latter. The size of the CCFs was determined by the concentration of carbon atoms during growth, which could be controlled by adjusting the temperature and etching hydrogen flow. 1. Introduction CNTs with perfect structure show attractive prospects in view of the fact that carbon fibers (CFs) have found wide industrial application in both the military and civilian sectors. These two carbon materials may also benefit each other in applications. CNTs have been deposited on the surfaces of CFs to form composite materials, which enhances the performance of the CFs in mechanical application by improving the interfacial load transfer.1,2 Recently, carbon nanofibers based on CNTs have been constructed as macroscale fibers, which combine the merits of CNTs and CFs.3,4 On the other hand, during the synthesis of vapor-grown carbon fibers (VGCFs), CNTs are also found in the center of pyrolytic CFs with high crystallinity or even as single crystals.5-7 These composite materials composed of CNTs and CFs could yield individual, fresh, clean, and straight CNTs with high crystallinity after peeling off the outer CF layers, and these might be used as electron emitters or for investigating the telescopic mechanical behavior of CNTs.8,9 However, there have been few literature reports concerning the synthesis of composites of CFs and CNTs from the viewpoint of the CNTs. We have reported the rapid synthesis of CCFs with CNT tips by chemical vapor deposition (CVD), and the super field emission ability of the products thus obtained.10 In the present work, we have further studied the reaction by varying the experimental conditions, such as the catalyst and temperature, in order to gain a better understanding of the formation of these CCF and CNT composites. 2. Experimental Section Fast-heating CVD experiments were performed in an in-house constructed stainless steel chamber with a vacuum system (Figure 1). The setup is similar to arc-discharge equipment without performing discharging. Carbonaceous gas and protective gas were introduced into the reaction chamber to a pressure * To whom correspondence should be addressed at Peking University. E-mail: [email protected]. † Chinese Academy of Sciences.

Figure 1. Schematic of the setup for the growth of CNTs and CCFs. The red parts in the center are the substrate and electrode heated by Joule heat.

of about 104 Pa, after a transition metal catalyst had been coated on the graphon (a graphitized carbon black) substrate by thermal evaporation. The graphon substrate was heated by Joule heating upon application of a 50-100 A alternating current to the circuit including the substrate at a low voltage (24 V). Details of the experiments were provided in our previous article.10 3. Results and Discussion A typical product is shown in Figure 2a. Generally speaking, CFs are present in three shapes: cylinders with hemispherical ends, hemispheres, and cones with sharp CNT tips (Figure 2b). The most interesting structures are the conical fibers with sharp CNTs protruding at their tips, which exhibit remarkable multiple telescopicextensibilityandoutstandingfieldemissioncharacteristics.9,10 From the point of view of the growth mechanism, the two former shapes can be regarded as early stages of the conical one, for which growth has terminated prior to forming the final conical shape with a protruding CNT. Figure 2b shows two typical CCFs with inner CNTs progressively exposed. By further peeling with the tungsten probe of the micromanipulator system in the scanning electron microscope (SEM, Tecnai XL30), the cross

10.1021/jp900732m CCC: $40.75  2009 American Chemical Society Published on Web 04/07/2009

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Figure 2. SEM images of a typical CCF and CNT composite. (a) Overall view of the substrate with carbon hemispheres, CFs, and CCFs. (b) CFs of two shapes, the inner CNTs of which are unexposed and just exposed, respectively. (c) A CCF with a CNT tip protruding.

sections of all three types of CFs could be observed. The same inner construction, with one CNT being located in the core of the CF, wrapped by turbostratic carbon layers, was found. Moreover, all of the CNTs in the CFs were found to be straight, clean, and highly crystalline.9,10 3.1. Effect of the Catalyst on the Formation of the CNT Core. Nickel was chosen as the catalyst for use in the CVD experiments, which was evaporated and deposited onto the graphon substrate prior to the synthesis. Transition metals are widely used as catalysts and have proved to be effective in numerous experiments on the synthesis of CFs and nanotubes. The nickel catalyst used in these experiments served to produce a great amount of CFs and CNTs, and especially aided the core construction. As reported in some previous studies on the synthesis of CNTs, self-catalytic growth was also observed.11 Thus, when the experiments were performed in the absence of a catalyst, large numbers of carbon hemispheres and fibers still appeared on the substrates. However, the production was significantly less than in the synthesis with the catalyst. Irrespective of the other conditions, such as the temperature, there were still a few CFs but none of the conical fibers with CNT tips. No CNTs were found inside the turbostratic layers of the CFs synthesized without catalyst after peeling off the outer layers by means of the micromanipulators. The general theoretical view of CNT growth in catalyzed CVD suggests that the CNTs initiate from eutectic particles of nickel and carbon, and then grow with excess carbon atoms isolated from the saturated eutectic particles. Here, the presence of nickel is conducive to the formation of inner CNTs, and then a large number of carbon atoms wrap the fresh CNT to construct the outer CF. Without the nickel catalyst, CFs were also formed by wrapping of the so-called filament instead of the CNT, as is also the case with conventional VGCFs. 3.2. The Influence of Temperature. The common synthesis method of VGCFs comprises two independent stages. In the first stage, the filaments of the VGCFs initiate and lengthen in a low hydrocarbon concentration. In the second stage, the filaments are thickened by carbon deposition and the CCFs form in a higher hydrocarbon concentration. In our CVD experiments, the substrates were directly heated to 1800 °C by Joule heating at a high speed of 10*2 deg/s. Observation by transmission electron microscopy (TEM, Tecnai G20) indicated that the CNT had high crystallinity and that the outer layers were composed of graphene stacks (region G) and amorphous carbon (region A) (Figure 3). Though none of the stages were intentionally meant to emulate the synthesis of VGCFs, the construction of the composite is similar to that of typical VGCFs, which indicates that the formation may also be interpreted in terms of growth of the CNT filament and thickening of the outer layers. A rapid increase in temperature produces a considerable amount of carbon from hydrocarbons

Figure 3. TEM images of (a) a CF with an embedded CNT, (b) the CNT in the CF, and (c) the layers of the CF (A and G denote amorphous carbon and graphene stacks, respectively).

in a short time, and only a small quantity of this carbon needs to dissolve in the metal particles to form the CNT cores. To investigate the influence of temperature, a cone-shaped graphon rod was used as the substrate, the tip of which was placed against another graphon electrode (Figure 1) to form the contact resistance in the heating circuit. Since the contact resistance was located at the tip of the electrode (substrate), the temperature there was the highest and decreased to room temperature along the axis of the electrode. The CCFs were distributed within a small area (a circular ring) at the front of the graphon electrode. In the middle of this area, most of the CCFs aggregated, and their production obviously reduced along the axis away from this area. Considering that the density of the nickel catalyst was the same over the whole surface of the substrate, these differences could be attributed to the temperature distribution. The temperature immediately in front of the substrate was so high that the nickel catalyst diffused or was readily evaporated before forming eutectic particles. On the contrary, the temperature far from the tip was too low to activate the catalyst. Furthermore, we also found that the volume of the CCFs decreases incrementally from the front to the rear in this area. The largest CCFs at the front have root diameters of around 40 µm and lengths of 100 µm, while the smallest ones have diameters and lengths down to 1 and 2 µm, respectively. Most of CCFs have diameters and lengths of about 6 and 15 µm, respectively. Observing cross-sectional SEM images, we found that the diameters of the inner CNTs had almost the same size in the synthesis, irrespective of the size of their outer CFs.9 This was because all of the CNTs grew from the particles deposited in the same deposition, the diameters of which were approximately uniform. When the temperature of the substrate was rapidly increased, there were a great number of carbon atoms around the hot substrate surface that had been pyrolyzed from the carbonaceous gas. A small amount of this carbon dissolved in the nickel particles and formed the CNT filaments, while the large amount of excess carbon wrapped the nascent CNTs. At the front of the substrate, the higher temperature produced more carbon atoms, and so the CCFs wrapping the inner CNTs were of larger diameter than those at the rear of the electrode. Raman spectra (Figure 4) were measured with a Renishaw System 1000 spectrometer (He-Ne laser, 632.8 nm, 25 mW) along the axis from the rear (low-temperature area) to the front (high-temperature area). It was found that the G peak (1582 cm-1), corresponding to the E2g mode, clearly intensifies from the rear to the front. This change in the Raman spectrum indicates that the graphitization of the products along the graphon substrate gets better, which was also manifested in increased order of pyrolytic carbon in terms of the surface morphology observed by SEM.

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Figure 4. SEM image of a whole substrate and resonance Raman spectra corresponding to low- and high-temperature areas of the substrate.

Figure 5. SEM and AFM images of the HOPG substrate. (a) AFM image of the HOPG substrate with nickel particles deposited (particles are concentrated at the edges). (b) SEM image of the nucleation of CFs on the HOPG substrate, including the basal graphite and pyrolytic carbon surfaces.

3.3. Reactions on Different Carbon Substrates. Research has been carried out on the influence of substrates on materials synthesis, which has led to the conclusion that defects in graphite and its topography significantly affect the pyrolysis of hydrocarbons and the deposition of catalyst particles.12-14 To study the influence of the substrate on the growth of the CNT and CF composite, we chose graphon with different degrees of roughness and highly oriented pyrolytic graphite (HOPG) as the substrates. The graphon substrates were obtained from Beijing City Sanye Carbon Co., Ltd., and had been compressed from high-purity carbon black, while the HOPG was obtained from NT-MDT Co., Russia. The nickel catalyst was evaporated and deposited on these substrates before the pyrolysis reactions, and the particle size was measured by atomic force microscopy (AFM, SPM Solver P47, NT-MDT Co., Russia). Since the surface of the graphon substrate was too rough for AFM scanning, the particle sizes were estimated by observing those deposited on a small silicon slice beside the graphon during the evaporation and deposition. AFM images showed that the nickel particles were evenly distributed on the silicon and HOPG surfaces, except that there were obvious concentrations at the step edges on the cleaved HOPG surface (Figure 5a). It is believed that these concentrations may be attributed to zigzag and armchair defects on the edges, which are more active in absorbing reactive metal atoms than the perfect basal graphite surface.15 Experimental results have shown that there is no obvious difference in the production of CCFs and their morphology on the graphon substrates with different degrees

Figure 6. Raman spectra of the graphon substrate and the pyrolytic carbon film.

of roughness. The CF and CNT composites were also synthesized on the HOPG substrates, but the success rate and the level of production were lower than those on the graphon. Besides, the catalyst particles chemisorbed at the edges did not form an obvious concentration of CFs corresponding to the nickel particles, which means that the chemisorption is not significantly superior to the physical absorption on the basal graphite surface under the high-temperature conditions. An interesting result is that the surface coated with the pyrolytic carbon film exhibited greater catalytic activity than the HOPG substrate in promoting the growth of the CNT and CF composites. This phenomenon was tested through three steps: (1) coating a pyrolytic carbon film on the HOPG surface by heating the substrate in a methane/hydrogen environment, (2) cleaving sections of the graphene layers with pyrolytic carbon film on them, and (3) performing CF and CNT composite growth experiments on these substrates with the two kinds of surfaces, namely pyrolytic carbon film and HOPG basal surface, including nickel evaporation and deposition and methane decomposition. The results show that pyrolytic carbon has an obvious advantage in terms of nucleation compared to the HOPG basal surface (Figure 5b). The similar nucleation abilities of the graphon substrate and the pyrolytic carbon film may be rationalized by reference to the Raman spectra (Figure 6). Both of the Raman spectra show strong D and G peaks, the intensity ratios of which are also similar. Here, the D peaks are shifted to 1333 cm-1 from 1360 cm-1, which is believed to be caused

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Zhang et al. advance, but that the size of individual CFs may be adjusted by controlling the hydrogen flow in tandem with heating. 4. Conclusions The CVD synthesis of CCF and CNT composite structures has been investigated in detail. Although CFs could be produced without any catalyst, nickel has been proved to play an important role in forming the CNT core. A high temperature attained by fast heating produced a lot of carbon atoms from methane in a short time, some of which formed highly crystalline CNTs inside the CFs, and then the excess carbon immediately wrapped the CNTs with many turbostratic layers. Analysis of TEM micrographs has revealed that the temperature also determines the graphitization of the outer layers of the CFs. The nucleation and growth of the CNT and CF composite proceeded more favorably on the graphon and pyrolytic carbon film substrate surfaces, as opposed to the basal surface of HOPG. Raman spectra indicated that both the graphon and pyrolytic film were covered by lattice defects, which have been proved to be active sites for growth in other studies. The shape of the fibers was mainly determined by etching by hydrogen introduced into the reactor before the synthesis, but could be further adjusted by controlling the hydrogen flow during heating.

Figure 7. SEM images and size distribution histograms of the CCFs synthesized (a) with a H2 flow and (b) without a H2 flow and (c) the distribution of the CCFs in terms of diameter and length.

by a change in the laser wavelength to 643 nm.16 According to the different results obtained for the three types of substrates, we believe that the sp2 defects represented by the D peaks contribute greatly to the nucleation in the growth of the CFs. 3.4. Hydrogen Etching. Etching by hydrogen during the reaction is believed to be the key reason for the conical shape of the CFs, as we have proposed in earlier work.10 Here, we investigated the influence of hydrogen etching by carrying out contrast experiments. Before the reactions, the experiments were prepared with the same conditions as those performed previously, including introducing methane and hydrogen into the chamber to the desired partial pressure. The difference in these contrast experiments was whether or not a hydrogen flow was introduced during the heating process. The results are shown in Figure 7, and point to two aspects of hydrogen etching during these reactions. First, the percentage of the conical shape in all of the CFs is close between the two contrast experiments (Figure 7, panels a and b). Second, the CCFs synthesized without hydrogen flow are obviously larger than those synthesized with a hydrogen flow (Figure 7, panels a, b, and c). The average length and root diameter of the CCFs synthesized with the H2 etching flow are 3.49 and 1.63 µm, respectively. The corresponding data without H2 flow are 5.40 and 2.01 µm, respectively. The two results suggest that the conical shape of the CFs is mainly determined by the hydrogen introduced in

ACKNOWLEDGMENT. . This work was supported by the National Natural Science Foundation of China (No. 60771004) and the MOST of China (No. 2006CB932402). References and Notes (1) Otsuka, K.; Abe, Y.; Kanai, N.; Kobayashi, Y.; Takenaka, S.; Tanabe, E. Carbon 2004, 42, 727. (2) Thostenson, E. T.; Li, W. Z.; Wang, D. Z.; Ren, Z. F.; Chou, T. W. J. Appl. Phys. 2002, 91, 6034. (3) Dror, Y.; Salalha, W.; Khalfin, R. L.; Choen, Y.; Yarin, A. L.; Zussman, E. Langmuir 2003, 19, 7012. (4) Liu, L. Q.; Tasis, D.; Prato, M.; Wagner, H. D. AdV. Mater. 2007, 19, 1228. (5) Endo, M.; Takeuchi, K.; Kobori, K.; Takahashi, K.; Kroto, H. W.; Sarkar, A. Carbon 1995, 33, 873. (6) Qin, L. C.; Iijima, S. Mater. Lett. 1997, 30, 311. (7) Shang, N. G.; Milne, W. I.; Jiang, X. J. Am. Chem. Soc. 2007, 129, 8907. (8) Zhao, G. P.; Zhang, J.; Zhang, Q.; Zhang, H.; Zhou, O.; Qin, L. C. Appl. Phys. Lett. 2006, 89, 193113. (9) Zhang, W.; Xi, Z. H.; Zhang, G. M.; Li, C. Y.; Guo, D. Z. J. Phys. Chem. C 2008, 112, 14714. (10) Zhang, W.; Xi, Z. H.; Zhang, G. M.; Wang, S.; Wang, M. S.; Wang, J. Y.; Xue, Z. Q. Appl. Phys. A: Mater. Sci. Process. 2007, 86, 171. (11) Zhu, Z. P.; Lu, Y.; Qiao, D. H.; Bai, S. L.; Hu, T. P.; Li, L.; Zheng, J. F. J. Am. Chem. Soc. 2005, 127, 15698. (12) Hoffman, W. P.; Vastola, F. J.; Walker, P. L., Jr. Carbon 1985, 23, 151. (13) Hoffman, W. P.; Vastola, F. J.; Walker, P. L., Jr. Carbon 1988, 26, 485. (14) Miranda-Hernandez, M.; Gonzalez, I.; Batina, N. J. Phys. Chem. B 2001, 105, 4214. (15) Stabel, A.; Eichhorst-Gerner, K.; Rabe, J. P.; Gonzalez-Elipe, A. R. Langmuir 1998, 14, 7324. (16) Vidano, R. P.; Fischbach, D. B.; Willis, L. J.; Loehr, T. M. Solid State Commun. 1981, 39, 341.

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