Chem. Mater. 1996, 8, 2109-2113
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Preparation of Ultrafine Carbon Tubes in Nanochannels of an Anodic Aluminum Oxide Film Takashi Kyotani,* Li-fu Tsai, and Akira Tomita Institute for Chemical Reaction Science, Tohoku University, Katahira, Sendai 980-77, Japan Received January 30, 1996. Revised Manuscript Received March 21, 1996X
A template carbonization technique was applied to prepare carbon nanotubes and submicron-tubes in one dimensional channels. An anodic aluminum oxide film that has uniform and straight channels with a diameter at the nanometer level was used as a onedimensional template. The pyrolytic carbon deposition on the channel wall was carried out by exposing the anodic oxide film to propylene at 800 °C. The carbon was then liberated from the anodic oxide film by HF washing. It was found that the resultant carbon is comprised only of uniform hollow tubes with open ends. The infiltration of poly(furfuryl alcohol) into the channels of the film followed by heat treatment led to the formation of bamboolike carbon tubes. For all the carbon tubes obtained here, the outer diameter (30 or 230 nm) and the length (60 or 75 µm) precisely reflect the channel diameter and the thickness of the template used, respectively. Furthermore, in the case of the carbon tubes from propylene, the wall thickness of the tubes was easily controllable by changing the carbon deposition period.
Introduction We developed a template carbonization technique for synthesizing novel carbon materials. This technique requires using the opening or the pores whose size and shape can be controlled at the nanometer level. First, we succeeded in preparing a new type of thin graphite film from the carbonization of organic polymer in the two-dimensional opening between the lamellae of a layered clay such as montmorillonite and taeniolite.1-5 Then, we applied this template technique to the preparation of new types of porous carbon by using the threedimensionally controlled pores of a zeolite as a space for carbonization of poly(furfuryl alcohol).6 Our next approach in the template carbonization technique is the preparation of one-dimensional carbon such as carbon tubes by using the one-dimensional channel. Carbon tubes of nanometer level are now being expected as a new carbon material. Such tubes have been prepared thus far mainly using an arcdischarge evaporation technique7,8 or by thermal decomposition of benzene vapor.9,10 We have attempted to prepare carbon nanotubes and submicron-tubes by using an anodic aluminum oxide film as a one-dimensional template. This template was originally used by Martin et al. to synthesize nanocylinders or tubes of polymers, metals, and semiconductors.11-14 Very recently, they have attempted to prepare carbon tubes Abstract published in Advance ACS Abstracts, July 15, 1996. (1) Kyotani, T.; Sonobe, N.; Tomita, A. Nature 1988, 331, 331. (2) Sonobe, N.; Kyotani, T.; Hishiyama, Y.; Shiraishi, M.; Tomita, A. J. Phys. Chem. 1988, 92, 7029. (3) Sonobe, N.; Kyotani, T.; Tomita, A. Carbon 1991, 29, 61. (4) Kyotani, T.; Yamada, H.; Sonobe, N.; Tomita, A. Carbon 1994, 32, 627. (5) Kyotani, T.; Mori, T.; Tomita, A. Chem. Mater. 1994, 6, 2138. (6) Kyotani, T.; Nagai, T.; Tomita, A. Extended Abstracts of Carbon ’92; Essen, 1992; 437. (7) Iijima, S. Nature 1991, 354, 56. (8) Ebbesen, T. W.; Ajayan, P. M. Nature 1992, 358, 220. (9) Endo, M. Chemtech 1988, 18, 568. (10) Endo, M.; Takeuchi, K.; Igarashi, S.; Kobori, K.; Shiraishi, M.; Kroto, H. W. J. Phys. Chem. Solids 1993, 54, 1841. X
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from polyacrylonitrile using the template.15 Our attempt for the carbon tube preparation was briefly reported in a short communication.16 In the present work, we report the more details of the method and the characteristics of the resultant carbon tubes. Furthermore, we would like to emphasize here how effectively uniform carbon tubes can be prepared and how easily their size can be controlled by using this method. Experimental Section Preparation of Anodic Aluminum Oxide Films. The following two types of circular anodic oxide films were used as a template. One was a commercially available membrane filter of 25 mm o.d. wide and 60 µm thick (Whatman Ltd., Anodisc 25), whose porosity consisted of an array of parallel and straight channels with a diameter of about 230 nm. Another one was prepared by anodic oxidation of aluminum plate (purity 99.99%) at a cell voltage of 20 V in 20 wt % sulfuric acid at 0 °C for 2 h. Following the electrooxidation, an anodic oxide film was separated from the aluminum substrate by reversing the polarity of the cell voltage. Then an impervious layer (referred to as the barrier layer) was etched by immersing the film in 20 wt % sulfuric acid for 1 h. The diameter and the thickness of this film were 15 mm and 75 µm, respectively, and the diameter of its straight channels was about 30 nm. Carbonization of Poly(furfuryl alcohol). The commercial anodic films were dried at 150 °C for 3 h under vacuum. After cooling to room temperature, the films (5 or 6 sheets) were impregnated under vacuum with a mixture of furfuryl alcohol (100 cm3) and oxalic acid (0.25 g) as an acid catalyst. The mixture was stirred with the films for 3 days. The polymerization of furfuryl alcohol and its subsequent (11) Liang, W.; Martin, C. R. J. Am. Chem. Soc. 1990, 112, 9666. (12) Brumlik, C. J.; Martin, C. R. J. Am. Chem. Soc. 1991, 113, 3174. (13) Klein, J. D.; Herrick, II, R. D.; Palmer, D.; Sailor, M. J.; Brumlik, C. J.; Martin, C. R. Chem. Mater. 1993, 5, 902. (14) Martin, C. R. Science 1994, 266, 1961. (15) Parthasarathy, R. V.; Phani, K. L. N.; Martin, C. R., Adv. Mater. 1995, 7, 896. (16) Kyotani, T.; Tsai, L.; Tomita, A. Chem. Mater. 1995, 7, 1427.
© 1996 American Chemical Society
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Figure 1. Schematic drawing of the formation process of carbon tubes. carbonization in the channels of the films were carried out by heating the impregnated films under N2 flow up to 900 °C at a rate of 5 °C/min and holding it for 3 h. The resultant carbon-anodic oxide composite films were washed with an excess amount of 46% HF solution at room temperature to dissolve the anodic aluminum oxide template. As a result, carbon was obtained as an insoluble fraction. Pyrolytic Carbon Deposition from Propylene. In addition to the above impregnation method, we attempted to deposit pyrolytic carbon on the inside of the straight channels of anodic oxide film in the following way. Each anodic oxide film was cut into two semicircular films. About four pieces of the semicircular films were placed on quartz wool in a vertical quartz reactor (i.d. 20 mm). The reactor temperature was increased to 800 °C under N2 flow and then propylene gas (2.5% in N2) was passed through the reactor at a total flow rate of 200 cm3 (STP)/min. The thermal decomposition of propylene in the uniform straight channels of the anodic oxide films results in carbon deposition on the channel walls. After the desired period of time, the reactor was cooled to room temperature and the films were removed. Then they were washed with HF as described above, and only carbon was left as an insoluble fraction. The formation process of carbon tubes is illustrated in Figure 1. Some of the carbon samples from the two methods were heat-treated at 2800 °C for 1 h under Ar flow, using an electric furnace with a graphite heater. Characterization. The microscopic features of the samples were observed with a scanning electron microscope (SEM, Hitachi, S900) and transmission electron microscopes (TEM; JEOL, JEM-2000EXII, and JEM-ARM1250). Electron diffraction patterns for selected areas (SAD) were also taken. The interplanar spacing, d002, and the average crystallite size, Lc, of the carbon samples were determined with an X-ray diffractometer (XRD, Shimadzu, XD-D1 with Cu KR radiation) by referring to a silicon standard. The surface area and the pore structure were investigated with an automatic volumetric sorption analyzer (Quantachrome, Autosorb-1) using N2 as adsorbent at -196 °C. The surface area and the pore size distribution were determined using the BET equation and the BJH method, respectively.
Results and Discussion Structure of Anodic Aluminum Oxide Films. Figure 2 shows the SEM photographs of the surface and the cross section for the two types of the anodic oxide films. Many openings with an uniform diameter (about 230 and 30 nm for photographs a and c, respectively) are observed on the surface. The cross-section views (b and d) indicate the presence of parallel and straight channels perpendicular to the film surface. The BET surface area for the anodic oxide film with the channels of 30 nm in diameter was indicated in Table 1, where the area calculated by the geometrical dimension esti-
Figure 2. SEM photographs of the aluminum anodic oxide film with 230 nm channels (surface a, cross section b) and the film with 30 nm channels (surface c, cross section d).
Figure 3. Pore distribution curves for the anodic oxide films with 30 nm channels and the carbon tubes prepared by carbon deposition of propylene for 1 h on the above film. Table 1. BET Surface Area and the Area Calculated by the Geometrical Dimension from the SEM Observation surface area (m2/g) sample
BET
calc
anodic oxide film with 30 nm channels carbon tubes from the above film
23 225
19 270
mated from the SEM observation was put in the third column. The BET surface area is in fair agreement with the calculated value. The pore distribution curve of this film (Figure 3) has a sharp peak at about 30 nm of pore diameter, which is consistent with the channel diameter from the SEM observation (Figure 2c). These findings imply the size and shape of the channels to be very uniform and non porous nature of the aluminum oxide substrate. Carbon from Poly(furfuryl alcohol). Figure 4a,b shows the SEM photographs of the resultant carbon and its 2800 °C treated sample. Figure 4a indicates the formation of tubular carbon, whose diameter is almost equal to the channel diameter (230 nm) of the corresponding anodic oxide film as a template. Each tube
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Figure 4. SEM and TEM photographs of the original carbon prepared from PFA (SEM image a, TEM image c) and the sample following heat treatment at 2800 °C (SEM image b, TEM image d). In the TEM images a microscope grid is seen under the sample.
Figure 5. SEM photographs of the carbon tubes prepared by carbon deposition of propylene: (a and b) a carbon deposition period of 1 h on the anodic oxide film with 30 nm channels; (c and d) a period of 12 h on the film with 230 nm channels.
ramifies into several thin tubes near the end. This branchlike structure originates from a similar branching in pore structure of the commercial anodic oxide film near its surface. The heat treatment at 2800 °C changed the microscopic feature of the tubes, although the tubular structure still remained. The surface of the walls became rough, and there are several holes on some parts on the surface (Figure 4b). The following TEM photographs (Figure 4c,d) give a clearer view of the structure of these carbon samples. The photograph of the carbon without the heat treatment shows voids and knots in the tubes (Figure 4c). This is just like bamboo. After the heat treatment, some carbon tubes kept the bamboolike structure, but the others show the structure as in Figure 4d, where they look as if many bubbles appeared in the tubes. Although we do not have now a clear explanation for the formation of such structure as bamboo and bubble, it was confirmed that the carbonization in the one-dimensional channels gives the formation of one-dimensional carbon. Formation of Carbon Tubes from Propylene. After the pyrolytic carbon deposition from propylene, the anodic films were treated with HF to obtain carbon as a residue. Figure 5 shows the SEM photographs of the carbon samples from the two types of the films (the film with 30 nm channels was subjected to the carbon deposition for 1 h and the film with 230 nm channels for 12 h). These figures reveal that in both cases the samples consist of only cylindrical tubes and their outer diameter is almost the same as the channel diameter of the corresponding anodic oxide film. No other form of carbon was found in the SEM and TEM observation. In the SEM photographs with low magnification (Figure 5c), many bundles of the tubes can be observed, and the length of a whole tube in a bundle was almost the same as the thickness of the corresponding template film. The presence of so many bundles implies that most of the tubes are connected at both ends of each tube, because
the carbon deposition also took place on the outer surface of the anodic film. But some of the tubes were separated from the others during the stirring in the HF solution, as observed in the other pictures. It is noteworthy that the tubes from the anodic oxide film with the smaller channels (Figure 5b) look transparent under the SEM observation with an acceleration voltage of 15 kV, indicating that the wall thickness of these tubes is very thin. The BET surface area and the pore distribution curve of the tubes from the film with 30 nm channels are shown in Table 1 and Figure 3. As in the case of the anodic oxide film, the surface area of the carbon tubes was calculated from their geometrical dimension with assuming carbon density to be about 2 g/cm3. A good agreement between the BET and the calculated surface areas was obtained. On the other hand, their pore distribution curve is found to be broader especially to the large diameter side than that of the anodic oxide film used as a template. This can be explained from the fact that there is a long space among the carbon tubes in a bundle, and it was also counted as pore. Figure 6 shows the SEM and TEM photographs for the carbon tubes under different carbon deposition periods. The SEM photographs (Figure 6a-c), which were taken from the end of the tubes, show the openend structure of these carbon tubes, and the TEM photographs (Figure 6d-f) confirm that the shape of the carbon samples here are tubular. These were also the case for the carbon tubes from the anodic oxide film with the 30 nm channels (see Figure 7a). These images clearly demonstrate that the wall thickness of the tubes increases with an increase in the deposition period. The wall thickness can be roughly estimated from the TEM photographs, where the thicknesses are in the ranges 3-5, 40-45, and 60-80 nm for 1, 6, and 12 h deposition, respectively. Furthermore, the TEM observation exhibits that the morphology of the inside of the tubes looks rougher with increasing the deposition period.
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Figure 6. SEM and TEM photographs of the carbon tubes prepared in the anodic oxide film (channel diameter 230 nm) under different carbon deposition periods: (a-c) SEM images; (d-f) TEM images.
Figure 7. TEM images of the carbon tubes prepared by carbon deposition for 1 h: (a) bright-field image of the tubes prepared from the anodic oxide film with 30 nm channels; (b) SAD pattern from the area indicated by a circle in a; (c) brightfield image of the tubes from the film with 230 nm channels; (d and e) SAD patterns from the areas indicated by circles in c.
Structural Analysis for Carbon Tubes from Propylene. The structural parameters such as the interplanar spacing (d002) and the average crystallite size (Lc)
Kyotani et al.
Figure 8. High-resolution TEM image of the carbon tubes from the anodic oxide film with 30 nm channels under a carbon deposition period of 6 h.
were, respectively, determined from the position and the half-width of a broad (002) X-ray diffraction peak for the carbon tubes with a 30 nm-diameter (carbon deposition period 1 h) and the ones with a 230 nm diameter (carbon deposition period 3 h). The values of d002 and Lc were 0.352 and 2.0 nm for the former carbon tubes and 0.354 and 2.7 nm for the latter tubes, respectively. Their d002 values are much larger than that of ideal graphite (d002 ) 0.3354 nm) and Lc are very small in comparison with these of graphitized carbon materials, indicating low crystallinity of the carbon tubes obtained here. Figure 7 presents the TEM bright field images of the carbon tubes and their corresponding SAD patterns. The selected area for each SAD pattern is indicated by a circle in the images. The SAD patterns (Figure 7b,d) exhibit a pair of small but strong arcs for 002, together with weak 10 and 11 diffraction rings. The appearance of the 002 diffraction neither as a ring nor as clear spots, but as a pair of small arcs, indicates some orientation of the 002 planes in the carbon tubes and its poor crystallinity. The latter accords with the results from the XRD measurement. For the carbon tubes with a 230 nm diameter, the SAD pattern (Figure 7e) was taken from a different area. It is noted that, in the area d of Figure 7c, part of electron beam passed through the tube wall in parallel, but no beam paralleled the wall in the area e. The 002 arcs as observed in Figure 7d are not seen in the case of Figure 7e, where only 10 and 11 rings are observed. This finding suggests that the tube wall consists of cylindrically stacked 002 planes. This would also be the case for the tubes with a diameter of 30 nm, which are, however, too thin to take such a SAD pattern as in Figure 7d,e. Accordingly, the observation of 002 lattice image was attempted for these carbon tubes. The lattice image for the carbon tubes with a diameter of 30 nm is shown in Figure 8, where at least four tubes cross each other. The thickness of the walls is about 10 nm, and consequently the carbon has a hollow with a diameter as small as 10 nm. Many small lines, which correspond to 002 lattice planes, are observed in the cross section of the walls for each tube. This image demonstrates that the size of most 002 planes is less
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observed in the lattice image (Figure 8) would cause such high graphatizabilty. Conclusions
Figure 9. TEM images of the carbon tubes (diameter; 230 nm, carbon deposition period; 3 h) heat-treated at 2800 °C: (a) bright-field image; (b) SAD pattern from the area indicated by circle in a.
than 10 nm, and they wrinkle to a great extent. This structure is far from a graphitic one. It should be noted that all 002 planes are orientated toward the direction of carbon tube axis. This lattice image confirms that the thinner tubes are also comprised of cylindrically stacked 002 planes as well as the case of the thick tubes. The carbon tubes prepared from the anodic oxide film with 230 nm channels were subjected to the heat treatment at 2800 °C. Figure 9 shows the TEM photograph and the SAD pattern for the heat-treated carbon tubes. Unlike the case of the carbon tubes from PFA, no bubble pattern appeared even after the heat treatment. Some of the tube walls look broken or collapsed. The SAD pattern was taken from one of the carbon tubes, which is indicated by a circle in the brightfield image. In the pattern, 002 diffraction was observed as a pair of spots, not as arcs. Furthermore, 100 (and probably 101) and 110 diffractions appeared as many spots along circle lines, not as continuous circles. These findings clearly indicate the significant increase in the degree of ordering of 002 planes with the heat treatment. The carbon tubes from the pyrolytic carbon deposition can be, thereby, considered as a graphitizable carbon. The orientated structure in 002 planes as
We can conclude that one-dimensional carbon was prepared by the template carbonization method using an anodic oxide film. The infiltration of furfuryl alcohol into the channels of the film followed by heat treatment led to the formation of bamboolike carbon tubes and the pyrolytic carbon deposition from propylene on the channels allowed us to synthesize uniform carbon nanotubes and submicron-tubes. The nanotubes prepared from the other techniques such as arc-discharge evaporation are much more graphitic and finer than the present carbon tubes, and their structure is close-ended. These techniques do not, however, allow the preparation of uniform carbon tubes and the accurate control of the tube length, diameter, and thickness. In addition, the arc-discharge evaporation technique produces a large amount of amorphous carbon as well as carbon nanotubes. On the other hand, one can prepare monodisperse carbon tubes by the present method. Since the length and the inner diameter of the channels in an anodic oxide film can easily be controlled by changing the anodic oxidation period and the current density during the oxidation, it is possible to control the length and the diameter of the carbon tubes. Furthermore, by changing the carbon deposition period, the wall thickness of the carbon tubes is controllable. The present method makes it possible to produce no other form of carbon, but only carbon tubes that are not capped at both ends. Thus, the encapsulation of other material into the present tubes would be very easy. This opens up the possibility for the formation of a novel one-dimensional composite with carbon. Acknowledgment. We thank the High Voltage Electron Microscope Laboratory of Tohoku University for microscopical analysis. This study was partly supported by Special Coordination Funds for Promising Science and Technology from Science and Technology Agency, Japan, the Ministry of Education, Culture and Science of Japan (07650777) and the Mitsubishi Foundation. CM960063+
