The Journal of
Physical Chemistry
0 Copyright 1994 by the American Chemical Society
VOLUME 98, NUMBER 49, DECEMBER 8,1994
LETTERS Nanoclusters Produced in Flames H. M. Duan* and J. T. McKinnon Department of Chemical Engineering, Colorado School of Mines, Golden, Colorado 80401 Received: July 22, 1994; In Final Form: October 25, 1994@
Nanotubes have been synthesized by low-pressure fuel-rich benzene-oxygen flames. They were usually solitary and seldom appeared in a bundle. Both closed-end and open-end nanotubes were observed. A new type of nanocluster was also synthesized. We suggest that these nanoclusters are not hollow and are composed of a series of concentric tubes of 6.2 A radius increments.
Carbon atoms in hot carbon vapor can coagulate and form fullerenes, nanotubes, and graphite onions. 1-5 Very fuel-rich low-pressure flames can also produce f~llerenes.~,’ Nanotubes and graphite onions exhibit interlayer spacing close to the 3.4 8, spacing of graphite. To date, nanotubes have been made in an atmosphere where carbon is the only reactive species. Here we report the synthesis of nanotubes and other nanoclusters in flames. Both closed-end and open-end nanotubes have been observed. We have also synthesized a new type of nanocluster that, we suggest, is composed of a series of concentric tubes of 6.2 8, radius increments. It appears that these new clusters are not hollow. The bumer is a larger size version of the bumer used in ref 8. A premixed flame of benzene vapor, oxygen, and argon is stabilized approximately 5 mm above a water-cooled bumer surface. Large quantities of fullerene-rich soot are produced in this flame front. The pressures range from 30 to 70 Torr, and the C:O ratio is 0.9. We used both JOEL FX2000 and JOEL JM- 1000 transmission electronic microscopes (TEM) to study the samples. Before TEM observation, soot was dispersed in water or methanol by sonication, and small droplets were placed on carbon substrates or holey substrates. Nanotubes are randomly distributed in the soot. Soot contains nanotubes and rich fullerenes. There is no significantdifference in fullerene concentration in the soot collected from different @Abstractpublished in Advance ACS Abstracts, November 15, 1994.
0022-365419412098-12815$04.50/0
parts of the bumer chamber surfaces. The abundance of the nanotube on a TEM microgrid is different from microgrid to microgrid. However, the difference does not occur as the result of nonuniform distribution of nanotubes in the soot; it is rather the result of sample preparation. A sample can contain numerous nanotubes and no soot particles, while another sample from the same soot can contain only a few nanotubes and a large number of soot particles. (These soot particles are the products of coagulation of aromatic hydrocarbons attracted to each other by dispersion forces.) The abundance is therefore not quantitatively established. Although the number of the nanotubes on a microgrid varies significantly, in most samples nanotubes are easy to find. Previously, some single-walled nanotubes were found on the wall of the synthesis ~ h a m b e r . ~These J ~ single-walled nanotubes are believed to be formed in hot carbon vapor and pushed to the chamber wall in the thermal gradient between the hot arc and the chamber wall. Multiwalled nanotubes were usually found in bundles attached to some kind of substrate. By attaching to a substrate, a multiwalled nanotube is properly oriented during the full course of growth. In electric arc methods, tubes are found on the tip of an ele~trode.~ In the vapor growth method, nanotubes were found on a graphite sheet.4 All the nanotubes we have seen in the flame-generated soot are multiwalled. The number of walls on each side of the hollow center is usually higher than ten. For some tubes the walls can be as thick as 0.1 pm. In the flame method, the tubes 0 1994 American Chemical Society
Letters
12816 J. Phys. Chem., Vol. 98, No. 49, 1994 t
Figure 1. Nanotubes in a flame can be both closed, A, and open, B and C,at the ends. The bar represents 50 nm.
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Figure 2. Fringes of a nanotube. The spacing between the fringes is 3.4 A. The bar represents 3.6 nm.
are free, and growing space is unlimited. While the temperature gradient in the mass growth reaction zone in the electric arc method is high, that in the flame method is minimal. These highly anisotropic nanotubes are synthesized in an atmosphere in which there is no significant thermal and composition gradient to make a specific orientation favorable. Nanotubes from flames are usually solitary, seldom showing up in bundles of two and never in bundles of three or more. Their lengths range from
0.3 to 2 pm. Unlike the capped nanotubes grown in a carbononly atmosphere, nanotubes from flames can be both capped and open. Figure 1 shows the types of nanotubes. In nanotube A, the uniform thickness region is followed by gradually shrinking segment and ends in a long needlelike area. Tubes B and C open at their ends and they bundle together. Figure 2 shows the frin4es of a tube. The spacing between the adjunct fringes is 3.4 A. The number of fringes is the same on both
J. Phys. Chem., Vol. 98, No. 49, 1994 12817
Letters
Figure 3. Image of a new type nanostructure. The spacing between the fringes is 6.2 A. The bar represents 4 nm. The orientations of segment A and segment B are about 6" apart.
Figure 4. A structural anomaly in the new nanostructure. The widening of the fringe spacing may be caused by a reduction of the binding forces between the layers. The bar represents 6.5 nm.
sides of the hollow area. We therefore believe these nanotubes to be the same kind reported b e f ~ r e . ~The - ~ coexistence of the closed and open tubes may be a result of the complicated environment of flames. Oxygen and C02 in the flame can react with the carbon on the cap and open the nanotubes as demonstrated before.11J2 H atom (mole fraction -1% in a flame), OH, or 0 atom oxidation in flames may also open tubes. Alternatively, the flame free radicals may react with the growing tube edge and terminate the tube growth. The growth mechanism of nanotubes is not well understood. It has been suggested that the formation of fullerenes and graphite onions is energetically more favorable than that of n a n ~ t u b e s . ~The ~ . ~carbon ~ atoms in a flame should form fullerenes rather than tubes. This is consistent with the observation of high Cm and C ~ yield O in our soot. However, unlike the electric arc method, in which high gas pressure (hundreds of Torr) favors nanotube growth and low gas pressure (tens of Torr) favors fullerene formation, the flame method produces both abundant nanotubes and fullerenes under the same conditions. Though the weight percentage of the nanotubes in the soot is not as high as that of the fullerenes, the ease of locating nanotubes under TEM suggests the weight percentage of nanotubes is not negligible. The inclusion of hydrogen and oxygen atoms and the complicated reaction atmosphere and mechanisms in a flame may cause the differences between the soot produced by the electric arc method and that produced by the flame method. In the electric arc method, the extremely high electric field in the tube tip area is believed to overcome the energetically favored tip closing, keeping the tip open and making the tube
grow.l5 A flame, on the other hand, has no highly anisotropic driving force; the gradients of temperature and chemical composition are negligible on a scale of the nanotube length. The orientation of a nanotube changes constantly in a flame (where T 2000 K). Given that nanotube formation is not energetically favored, the rather surprising observation of abundant nanotubes in the soot is still unexplained. Figure 3 shows a new type of nanostructure, not observed in arc methods or other methods. The fringes are uniformly spaced 6.2 %, from each other, and there is no empty space at the center. This fringe spacing does not match any reported graphite lattice. We have repeatedly observed this kind of cluster; in some samples they are even more prevalent than the nanotubes. This type of cluster may be explained as a set of concentric tubes of 6.2 8, radius increments with the innermost tube having a diameter of 6.2 A. This value is of the same order as the smallest nanotube1° and of the innermost sphere in a paphite onion.5 Figure 4 shows a cluster which also has 6.2 A fringe spacing. The region marked as "A" shows a structural anomaly which may be due to a weakening of interltyer binding forces. We have attempted to interpret the 6.2 A fringe spacing in terms of nontubular structures or interference patterns. For example, we have considered that the structure might be a hydrocarbon crystal (the windings which appear in Figure 3 suggest it is not very rigid), and the 6.2 %, fringes are the interference pattern of a crystal under two different but coherent electron beams. However, adjustments to make the TEMs generate this kind of pattern, a difficult task, were never attempted. The pictures were taken under the conditions of conventional observation, just like the conditions for the
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12818 J. Phys. Chem., Vol. 98,No. 49, 1994 observation of the nanotubes. Given the fact that this kind of pattem has repeatedly been seen in our samples and has been seen with two different TEMS in two independent groups (one in Alfred University in New York and another in NREL in Colorado), it is very unlikely to be the interference pattem of a crystal under two coherent electron beams. This conclusion is further bolstered by the continuous but winding nature of the fringes. In Figure 3 the directions of segments A and B are about 6” apart. This would be equivalent to a 6” tilting in the plane and would result in the disappearance of fringes if the fringes were the interference pattem of a crystal. Also, if this long ropelike structure were a conventional crystal, we would expect such a thin and soft crystal to twist somewhat over the 0.3 pm sample length, making the fringes disappear or become nonuniform. The long, regular pattern, retained throughout the sample length, makes these interpretations unlikely. We also tried to interpret these fringes as MoriC interference patterns resulting from an electron beam passing through two overlying crystals. However, we have found 6.2 A fringe spacing in every one of the new nanostructures that we have studied. The only way that a Morie interference pattern could cause this would be for two crystals of the same size to overlay each other in same direction throughout every sample length, an impossibility. At present, we have no explanation for the 6.2 A interlayer spacing in this structure. If it is not a graphite structure, then possibly it could be an array of sp3-hybridized carbon atoms with hydrogen atoms filling the space between the layers. This could account for the “delamination” observed in region “A”
Letters of Figure 4. A structure like this would have much lower interlayer attractive forces than graphite.
Acknowledgment. We acknowledge the National Science Foundation for support through Grant CTS-9215795 and the National Young Investigator Program as well as support from NIH through Grant RR00592. We are also in debt to Divid Hoelzer, Bob McGrew, Jim Kremer, Kim Jones, and Steve Thomson for their assistance to our TEM observations and their helpful discussions. References and Notes (1) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. Nature 1985,318,162. (2) Kriitschmer, W. Lamb, L. D.; Fostiropoulos, K.; Huffamnn, D. R. Nature 1990,347,354. (3) Iijimia, S . Nature 1991,354,56. (4) Ge, M.; Sattler, K. Science 1993,260,515. (5) Ugarte, D. Nature 1992,359,707. (6) Gerhardt, P.; Loffler, S. H.; Homann, K. Chem. Phys. Lett. 1987, 137?306. (7) Howard, J. B.; McKinnon, J. T.; Makarovsky, Y.; Lafleur, A. L.;Johnson, M. E. Nature 1991,352, 139. (8) Howard, J. B.; McKinnon, J. T.; Johnson, M.E.; Makarovsky, Y.; Lafleur, A. L. J. Phys. Chem. 1992,96, 6657. (9) Bethune, D. S. Nature 1993,363,605. (10) Iijima, S.; Ichihashi, T. Nature 1993,363,603. (11) Tsang,S. C.; Harris, P. J. F.; Green, M. L. H. Nature 1993,362, 520.
(12) Ajayan, P.M. Nature 1993,363,522. (13) Adams, G. B.; Shankey, 0. F.; Page, J. B.; O’Keeffe, M.; Drabold, D. A. Science 1992,256, 1792. (14) Lamb, L. D. Science 1992,255, 1413. (15) Smalley, R. E. Mater. Sci. Eng. 1993,B19, 1.
