Microstructures Synthesized from Graphite

Chem. B , 2006, 110 (22), pp 10714–10719. DOI: 10.1021/jp0618274. Publication Date (Web): May 18, 2006. Copyright © 2006 American Chemical Society...
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J. Phys. Chem. B 2006, 110, 10714-10719

Tubular Carbon Nano-/Microstructures Synthesized from Graphite Powders by an in Situ Template Process Guozhen Shen,* Yoshio Bando, Chunyi Zhi, and Dmitri Golberg AdVanced Materials Laboratory, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 304-0044, Japan ReceiVed: March 23, 2006; In Final Form: April 21, 2006

Through the use of commercial graphite powders as the carbon sources, a variety of interesting tubular carbon nano- and microstructures, such as networked carbon nanotubes, aligned carbon microtubes with hexagonal cross-sections, aligned tapered carbon tubes, and hollow carbon microhorns, have been successfully synthesized. As-grown tubular carbon structures were characterized using scanning electron microscopy, transmission electron microscopy, and X-ray energy-dispersive spectroscopy. An in situ template mechanism was proposed to explain the possible growth process. The vibrational properties of the synthesized tubular carbon structures were also studied by Raman spectroscopy.

Introduction Since the discovery of carbon nanotubes in 1991, considerable efforts have been made to synthesize and investigate the physical properties and applications of tubular carbon nano- and microstructures.1-7 The myriad structural manifestations and their material properties have made tubular carbon structures very interesting not only for potential applications but also for understanding carbon at the nano-/microscale. Until now, several different tubular structures of carbon have been investigated intensely. A few of these include single and multiwalled nanotubes,8 helical nanotubes,9 multibranched carbon tubes,10 tubular graphite cones,11 horns,12 conical crystals,13 tapered carbon nanotubes,14 and nanopipets.15 Controlling the geometric structure and morphologies of tubular carbon nano-/microstructures is obviously necessary for applications in various types of functional devices with modified electronic characteristics, such as electrical conductivity or electron emission properties, etc. Now, it is still a challenge to fulfill these purposes. In the present work, using commercial graphite powders as the carbon sources, we report the synthesis of a variety of tubular carbon nano-/microstructures, including networked carbon nanotubes, aligned carbon microtubes with hexagonal cross-sections, aligned tapered carbon tubes, and carbon microhorns by a simple in situ template process. Our process offers a simple, efficient, economical method to produce tubular carbon nano-/microstructures, and the carbon structures obtained in this process may have some unique properties. For example, networked carbon nanotubes can find uses in integrated circuits or other devices such as rectifiers or switches.16 Carbon tubes with hexagonal cross-sections could be expected to have unique properties because of the sharp corners of the graphite walls, which may act as efficient electron emitting sites.17 Tapered carbon tubes and horns with wide hollow interiors here may be suitable for large-diameter particle encapsulation, nano-/ microfluidic, drug delivery, and nanoelectronic applications.15,18-21 * Author to whom correspondence should be addressed. Fax: +81-29851-6280. E-mail: [email protected].

Figure 1. Schematic diagram of the (a) vertical induction furnace and (b) graphite crucible utilized for product deposition.

Experimental Section Tubular carbon nano-/microstructures were synthesized in a vertical induction furnace, which consisted of a fused-quartz tube and an induction-heated cylinder made of high-purity graphite coated with a carbon-fiber thermo-insulating layer as shown in Figure 1a. The furnace has one inlet and one outlet on its base. A graphite crucible, containing a mixture of ZnS (1.2 g), SnS (0.3 g), and graphite (0.3 g) powders was placed at the center cylinder zone. Another graphite crucible (45 mm × 45 mm) is designed for the efficient deposition of different tubular carbon structures at different sites (shown in Figures 1b and 1c). There are many holes on the bottom of the crucible to provide efficient gas concentration changes at different sites during the experiments. After evacuation of the quartz tube to ∼20 Pa, a pure N2 flow was introduced and maintained through the inlet at a flow rate of 50 sccm at ambient pressure in the furnace, and the crucible was rapidly heated and kept at ∼1350 °C for 1 h. After the reaction was terminated and the furnace was cooled to room temperature, black powders were found on

10.1021/jp0618274 CCC: $33.50 © 2006 American Chemical Society Published on Web 05/18/2006

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Figure 2. XRD patterns of the (a) as-deposited product and (b) product after acid treatment.

the whole graphite, on both the bottom and the side walls (sites A-D in Figure 1). The collected products were characterized using X-ray diffraction (XRD, RINT 2200HF), scanning electron microscopy (SEM, JSM-6700F), and transmission electron microscopy (HRTEM, JEM-3000F) with energy-dispersive X-ray spectrometry (EDS). Raman scattering was measured by a laser Raman spectrophotometer (T64000, France) at room temperature. Results and Discussion Figure 2a shows the XRD pattern of a typical product deposited on the crucible. The sharp diffraction peaks with relatively high intensity in this figure can be indexed as crystalline body-centered tetragonal Sn phase (JCPDS 89-2761), while the peak marked with the star can be indexed to the hexagonal graphite (JCPDS 75-1621). After being treated with dilute hydrochloric acid, Sn was removed, and only carbon materials are left, which can be proved by the XRD pattern shown in Figure 2b. Networked Carbon Nanotubes (NCNTs). Figure 3a shows the SEM image of the deposited product on site A shown in Figure 1. It reveals that a large-area one-dimensional nanostructure network was successfully fabricated. The highmagnification image shown in Figure 3b shows the threedimensional features. The units for the builep of the network are uniform nanotubes with diameters of 100-300 nm and lengths of up to several tens of micrometers. The hollow tubular interior of the units for the buildup is clearly shown in the inset of Figure 3b. The structure and phase composition of the as-prepared network was further characterized with TEM and EDS. Figure 4a shows a typical TEM image of the product, revealing the network structures consistent with the SEM results. The highmagnification TEM image in Figure 4b clearly indicates that the composed nanotubes are highly branched and curved nanotubes with diameters of ∼200 nm and very thin wall thicknesses. The EDS spectrum shown in Figure 4c reveals that the nanotubes are made of carbon, indicating the formation of networked carbon nanotubes. A weak oxygen peak was also observed in the spectrum, which probably originates from unavoidable surface adsorption of oxygen onto the nanotubes arising from exposure to air during sample processing. No obvious signals of the elements Sn, Zn, or S were observed in the spectrum. Figure 4d shows that the carbon nanotubes have open ends. The selected area electron diffraction (SAED) pattern shown in Figure 4e is characteristic of a carbon nanotube with a hexagonal graphite crystalline structure. The rings in the pattern correspond to (002), (004), and (006) planes. A thin

Figure 3. SEM images of the synthesized networked carbon nanotubes.

Figure 4. (a) TEM image of the synthesized networked carbon nanotubes. (b) TEM image showing that each network is composed of multibranched carbon nanotubes. (c) EDS spectra. (d) TEM image showing the open tips. (e) Corresponding SAED pattern. (f) Highmagnification TEM image showing the thin wall thickness of ∼10 nm. (g-i) HRTEM images taken from the nanotube in part d.

wall thickness of ca. 10 nm was observed in Figure 4f for the synthesized carbon nanotubes. High-resolution TEM (HRTEM)

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Figure 6. SEM images of the synthesized aligned carbon tubes showing hexagonal cross-sections. Scale bars for parts c-f are 500 nm.

Figure 5. (a) SEM and (b) TEM images of the remaining networked Sn nanowires in the product.

images taken from the side wall, tip, and curved corner of the carbon nanotube are shown in Figures 4g-i. All these images reveal that the interlayer spacing of the carbon nanotube is about 0.34 nm, consistent with the (002) plane lattice parameter of graphite carbon. Besides the networked carbon nanotubes, it was found that some networked nanowires are also formed in the products. The SEM image and TEM image of these networked nanowires are shown in Figures 5a and 5b, respectively. It can be seen that these nanowires have diameters of about 150-300 nm, in accordance with the networked carbon nanotubes. The EDS analysis indicates that these nanowires are networked Sn nanowires (Supporting Information). Since the networked Sn nanowires and networked carbon nanotubes have similar dimensions and morphologies, it was thought that the Sn nanowires may act as a template formed in situ for the confined growth of networked carbon nanotubes. Aligned Carbon Microtubes with Hexagonal Cross-Sections (HCTs). Figures 6a and 6b show the SEM images with different magnifications of the sample deposited on site B of the graphite crucible. They reveal that the sample consists of one-dimensional well-aligned microtubes with diameters ranging from several hundred nanometers to about 2 µm and a length of several tens of micrometers (the tubular structures will be discussed below). Interestingly, it was found that all of the microtubes have well faceted hexagonal cross-sections as revealed in a series of SEM images in Figures 6c-f. Although

Figure 7. (a) TEM image of a single carbon tube. (b-d) TEM images of carbon tubes partially filled with Sn.

controlling the geometric structure of carbon tubes is necessary for applications in various types of functional devices with modified electronic characteristics, reports seldom could be found on the fabrication of carbon tubes with controlled crosssectional shapes.22 So our work presented here provides an efficient way for the synthesis of carbon tubes with crosssections different from the conventional circles. Figure 7a shows the TEM image of an as-synthesized typical carbon microtube, which has a uniform diameter along its whole length. The hollow interior is clearly shown in this image. The carbon tube also has a very thin wall thickness of about 30 nm as indicated in Figure 7b compared with its hollow cavity. Besides the hollow carbon tubes, it is also found that some of the carbon tubes are partially filled with Sn rods (Figures 7c and 7d). In some cases, only Sn particles are left within the carbon tubes. The compositions of the inner filled Sn rods are confirmed using EDS analyses. Figure 8 shows the EDS

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Figure 8. EDS spectrum taken from the filled Sn.

Figure 10. (a) SEM image of the obtained carbon horns. (b) Highmagnification SEM image of a typical carbon horn. (c) Highmagnification SEM image of the tips of the carbon horns. (d and e) TEM image and its SAED pattern of the carbon horns. (f) HRTEM image of the carbon horn.

Figure 9. (a and b) SEM image of the synthesized tapered carbon tubes. (c) TEM image of several tapered carbon tubes. (d) Highmagnification TEM image showing the thin wall thickness of ∼17 nm and (e) its corresponding SAED pattern. (f and g) HRTEM images showing highly crystallized graphite layers.

spectrum taken from the inner rods. It shows the signal of the element tin, indicating that they are Sn rods. Aligned Tapered Carbon Tubes (TCTs). On site C of the crucible, another interesting carbon product was deposited, and its SEM image is shown in Figure 9a, which depicts the formation of highly aligned tapered carbon tubes with diameters ranging from about 1 µm at the root to ∼200 nm at the tip. The high-magnification SEM image in Figure 9b shows that some of the tapered carbon tubes have closed tips while some have open tips. The TEM image in Figure 9c gives more clear information about the formation of tapered carbon tubes instead of the convenient carbon tubes with uniform diameters along their whole lengths. The wall thickness of the tapered carbon tubes is in the range of 10-20 nm as shown in Figure 9d. Its corresponding SAED pattern is shown in Figure 9e, which also confirms the formation of crystallized carbon tubes. Our present

results show that the diameters of the tapered carbon tubes gradually decrease from the roots to the tips, which is quite different from the tapered carbon nanotubes reported by Hu et al., in which tapered carbon nanotubes with diameters increasing from the roots to the tips are formed by a VLS process,23 possibly indicating a different present growth mechanism versus the previous report, which is also confirmed by the randomly distributed wall thicknesses compared with the previous report. The HRTEM images shown in Figures 9f and 9g reveal that the wall is composed of regularly ordered graphitic layers with an interlayer spacing of 0.34 nm, also consistent with the (002) plane lattice parameter of graphite carbon. Hollow Carbon Microhorns (HCMHs). Figure 10a shows the SEM image of the product deposited on site D. It can be seen that the product contains uniform conical structures, resembling microhorns, on a large scale. Figure 10b shows the SEM image of a typical as-obtained carbon microhorn. The SEM images indicate that the diameters of the carbon microhorns increase from 200-300 nm at their bottom parts to 700-1000 nm at the top parts; the length of the carbon microhorns is uniform at about 3 µm. Figure 10c shows that some of the carbon microhorns have broken tips, indicating that the carbon microhorns have hollow interiors; namely, hollow carbon microhorns are formed at site D. The TEM image of several HCMHs is shown in Figure 10d, consistent with the SEM results. The SAED pattern shown in Figure 10e clearly depicts the (002) and (004) diffraction rings, confirming the formation of crystallized carbon tubes. A HRTEM image, Figure 10f, confirmed the structural uniformity of the HCMHs; the lattice fringes of the (002) plane with a d spacing of 0.34 nm are visible.

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Figure 12. Raman spectra taken from the synthesized (a) networked carbon nanotubes, (b) aligned carbon tubes with hexagonal crosssections, (c) aligned tapered carbon tubes, and (d) carbon horns. Figure 11. Schematic illustrations for the formation of carbon nano-/ microstructures.

Growth Mechanism. Above, we have shown that networked Sn nanowires were found in coexistence with the networked carbon nanotubes. They have similar dimensions and morphologies. Partial Sn-nanorod-filled carbon tubes with novel hexagonal cross-sections were also formed in the experiments. All of these results indicate that Sn nanostructures formed in situ during the experiments may act as templates to confine the growth of final carbon nano-/microstructures.24 So we deduce the possible growth mechanism as an in situ template process as follows. During the experiments, SnS will thermal decompose (boiling point of Sn is 1210 °C) to generate Sn and S vapors at a temperature higher than 1210 °C.25 The newly generated Sn vapors are transferred to low-temperature regions, where they deposit on the graphite crucible (Figure 11I). It is well-known that the gas concentration changes with the distance between the source material and the deposit site.26 Due to the differences of Sn concentrations at different sites and the anisotropic nature of Sn, the Sn vapors transferred to different sites will form different morphologies, networked Sn nanowires, Sn rods with hexagonal cross-sections, tapered Sn rods, and Sn horns in the present cases (Figures 11II and 11III). Meanwhile, a mixture of ZnS and graphite powders undergo the reaction 2ZnS + C T 2Zn + CS2 to produce CS2 gas at a high temperature. The CS2 gas is also transferred to the low-temperature regions and absorbs on the Sn nano-/microstructures, where it decomposes into gaseous C and S2 and forms carbon-coated Sn nano-/ microstructures (Figure 11IV). Annealing for a long period, Sn is evaporated and forms the tubular carbon nano-/microstructures (Figure 11V). If the Sn core is partially evaporated, then Snfilled carbon nano-/microstructures are formed. Raman Analyses. Raman spectroscopy has been used to investigate the vibrational properties of the synthesized tubular carbon nano-/microstructures, which also allows us to draw further conclusions about the crystallography or morphology. The typical Raman spectra were shown in Figure 12. All these spectra show two strong peaks at 1346 and 1576 cm-1. The peak at 1576 cm-1 (G-band) is closely related to the vibration in all sp2-bonded carbon atoms in a two-dimensional hexagonal lattice, such as in the graphite layer.27 The peak at 1346 cm-1 (D-band) could be assigned to the vibrations of C atoms having dangling bonds due to the in-plane terminations of disordered graphite.28 Besides, another weak peak located at 2650 cm-1 (2 × D) was also found in the spectra, which results from the

local symmetry breakdown.29 From these spectra, for the networked carbon nanotubes, carbon tubes with hexagonal crosssections, and tapered carbon tubes, it was also found that they have similar values for the relative intensities (R) of the D-band at 1346 cm-1 to the G-band at 1576 cm-1 (R ) 0.95, 0.99, and 1.01), while the hollow carbon horns have the highest R value (R ) 1.8), suggesting that the hollow carbon horns have the lowest degree of long-range order. Conclusion In conclusion, various tubular carbon nano-/microstructures, including networked carbon nanotubes, carbon tubes with hexagonal cross-sections, tapered carbon tubes, and hollow carbon microhorns, have been successfully synthesized from commercial graphite powders by a simple in situ template process. The present method provides a simple, commercial, and efficient method to produce tubular carbon nano-/microstructures. Due to the special morphologies of the carbon nano-/ microstructures, they may have unique properties and applications in integrated circuits, large-diameter particle encapsulation, nano-/microfluidics, drug delivery, and nanoelectronics. Supporting Information Available: Photograph of the carbon product deposited on the crucible, low-magnification SEM image of the networked carbon nanotubes, EDS spectrum taken from the networked Sn nanowires, HRTEM image taken from the corner part of the carbon tube with the hexagonal crosssection, TEM image of a single carbon microhorn, highmagnification TEM of the carbon microhorn wall, TEM image of a hollow carbon microhorn partially filled with Sn. This material is available for free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Iijima, S. Nature 1991, 354, 56. (2) (a) Ren, Z. F.; Huang, Z. P.; Xu, J. W.; Wang, J. H.; Bush, P. Science 1998, 282, 1105. (b) Liu, C.; Fan, Y. Y.; Liu, M.; Cong, H. T.; Cheng, H. M.; Dresselhaus, M. S. Science 1999, 286, 1127. (3) (a) Fan, S. S.; Chapline, M. G.; Frank, N. R.; Tommbler, T. W.; Cassell, A. M.; Dai, H. J. Science 1999, 283, 512. (b) Ebbesen, T. W.; Ajayan, P. M. Nature 1992, 358, 220. (4) (a) Lee, R. S.; Kim, H. J.; Fischer, J. E.; Thess, A.; Smalley, R. E. Science 1997, 288, 255. (b) Zhu, H. W.; Xu, C. L.; Wu, D. H.; Wei, B. Q.; Vajtai, R.; Ajayan, P. M. Science 2002, 296, 884. (5) (a) Baughman, R. H.; Cui, X. C.; Zakhidov, A. A.; Iqbal, Z.; Barisci, J. N.; Spinks, G. M.; Wallace, G. G.; Mazzoldi, A.; De Rossi, D.; Rinzler,

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