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J. Phys. Chem. 1995, 99, 2042-2047
Nanocarbons Formed under ac Arc Discharge N. Koprinarov,***M. Marinov,* G. Pchelarov; M. Konstantinova,: and R. Stefanov' Bulgarian Academy of Sciences, Central Laboratory for Solar Energy & New Energy Sources, 72 Tzarigradsko shose, I784 Sofia, Bulgaria, and Bulgarian Academy of Sciences, Institute of Physical Chemistry, Academic Bonchev Street, Bl. I I , 1040 Sofia, Bulgaria Received: July 18, 1994; I n Final Form: November 4, 1994@
Large quantities of a wide range of carbon net forms have been synthesized and observed by a TEM in a deposit obtained by ac arc discharge in an argon ambient between close proximity electrodes. The forms differ in size and arrangement; these include closed or open end tubes, spiral tube cores, boomerang, single and double cone, arch, spherical with a large hollow core, spherical ordered structures, spherical amorphous, and a variety of polyhedral forms. The step by step growth process is discussed. Initially a two-dimensional carbon net grows spatially. Successive concentric layers grow temporally externally and internally to the initial surface. Due to the weak interaction between the atoms from the different layers, quite often the upper layers do not copy the initial form of the carbon net. The defects that arise define the nature of the deviations from the initial carbon net, the preferred spatial C-C bond angle, and the mechanism of growth characteristic for the structures.
Introduction Initially the idea that it is possible to build up different symmetric structures with the help of a carbon net undoubtedly belongs to the scientists who unveiled C6o.I The idea caught on so fast that almost all possible spheroidal forms that can be built up with the help of different six or five atom ring net arrangements were discussed very soon after that. The maximum number of atoms included in a closed net is theoretically unlimited, while the sizes of some of the structures obtained are very large. To the already renown spheroidal fullerenes, new forms were added with the discovery of carbon nanoWhat is more, in practice these were found to be easily obtainable in large quantities, while the structures synthesized are so large that they can be observed by an electron microscope of average magnification. Thus, the logical question arose, what other possible forms of carbon net structures can be built by existing carbon atom bonding. The answer to this question was sought by two routes, and by direct observation of carbon specimens produced in CHO The results presented here are a study of the material deposited on the cooler of the close proximity electrodes in an ac arc discharge."
A B
C Figure 1. Mechanism of growth of cylindrical nucleation sites: (A) atom attachment; (B) migration; (C) incorporation within the nucleation site structure.
Figure 2. Nanotubes (pointed out by arrows) dispersed within the extracted material.
Results and Discussion Although in our case the synthesis conditions are close to those described in ref 12, there are two peculiarities which have to be singled out: 1. Argon is an inert gas whose properties are different from those of helium. The destructive power of Ar is greatest when it bombards the material growing on the electrodes during the process of deposition. In this case it destroys and eliminates mainly the structures that have internal stress or defects. 2. The distance between the electrodes is maintained small. Due to ac power supply utilization, the accumulation of material takes place under the conditions of a cyclic work mode change.
* To whom correspondence should be addressed. ' Bulgarian Academy of Sciences. Central Laboratory for Solar Energy & New Energy Sources. @
Bulgarian Academy of Sciences. Institute of Physical Chemistry. Abstract published in Advmce ACS Ahsrrcrcts, January 1. 1995.
0022-365419512099-2042$09.00/0
Figure 3. Mechanism of concentric nanotube growth.
The high pressure and close proximity of the electrodes are prerequisites for a much higher density of the carbon vapor in and around the arc region. The periodic change in supply voltage polarity changes the conditions of growth on each electrode. At each half turn one of the electrodes is the source of carbon vapor which deposits as soot, fullerenes, and growth on the opposite electrode. During the next half turn, the opposite electrode itself supplies carbon vapor to the reaction space. Due to the peculiarities pointed out, we were able to obtain at ac discharge in an Ar ambient a percentage deposition of buckytubes that is, as our calculations show, higher than that 0 1995 American Chemical Society
Nanocarbons Formed under ac Arc Discharge
J. Phys. Chem., Vol. 99, No. 7,1995 2043
A
c
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. -.
Figure 4. Interlayer variations in closing of concentric nanotubes: (A) growth of the layers; (B) closing of the first layer and induction of changes in the second layer; (C) symmetric closing; (D) consecutive delayed closure without changing the diameter of the tubes; (E) consecutive delayed closure with symmetric change of the diameter up to the diameter of closure; (F) consecutive delayed closure with the change in diameter and direction; (G) consecutive delayed closure with the nonsymmetric change in diameter.
obtained by dc discharge,12given the resublimation following the change in polarity. The material studied was prepared by mechanically grinding a piece of the deposit which grows onto the thick electrode, and then this was destroyed following a 20 h interaction with ultrasound in a xylene bath. After boiling in xylene for 4 h the nonfiltered material was dried and deposited onto the susceptor for observation with a TEM. Part of the observations were carried out on a piece of material broken off directly from within the deposit crater (see ref 11). The results obtained are similar to those attained by the method of extraction. The large quantity of diverse forms were observed in all the samples investigated. Together with all forms known from the literature so far, we observed not only variations of these in size and extent of closure but also forms which had not been observed previously. Judging from the exceedingly large sizes of the structures and their high symmetry, it was not possible to assume that these arise spontaneously in accordance with the mechanisms described in refs 13 and 14. One possibility is to assume that microstructures are built up spontaneously, whereas macrostructures are built up step by step, moving from the stage of creation after initial nucleation through intense growth, reaching the stage of end formation, which is sometimes realized when the flow of carbon vapor is stopped. The idea that buckytubes grow at their open ends, the open-ended growth model, was first proposed by Iijima et a1.I0 In our case the whole process of growth takes place within a single supply voltage half-cycle.
Considering the complexity and size of the structures formed during this half-cycle, it can be concluded that the process of structure growth is very rapid. Most of the structures observed by us appear to support the above hypothesis, accepting that the initial nucleation site can be a carbon ringI5 or sphere. The growing carbon surface edge and region around it are places which are especially favorable for intensive nucleation, when considering a single-wall buckytube whose growing end faces the arc, where the carbon vapor density and the energy emitted at the arc have the highest possible values. The carbon vapor atoms and the mobile atoms standing in close proximity to the growing deposit migrate toward the sites of intense growth, after they come into contact with the growing plane. In our view, intense growth is due not only to all the atoms that fall directly within the region of buckytube growth but also to the migrating atoms, as shown in Figure 1. The attraction of the migrating atoms to the cylindrical nucleation surface is due to weak van der Waals forces. In accordance with our model the forces of attachment between the plane already built and the atoms that lay close to the plane play a crucial role in the process of growth not only when considering a single-layer buckytube but also for multiple-layer buckytubes. After impact the carbon atoms begin to migrate, Figure lB, until they reach either end of the nucleation surface, Figure lC, where they attach permanently, forming CT bonds. The speed of nucleation surface growth, in accordance with the above mechanism, is directly dependent on the density of the carbon atom influx. Figure 2 shows cylindrical nanotubes surrounded by other carbon formations.
Koprinarov et al.
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The utilization of an ac arc discharge increases the gas stream fluctuations, the arrival of groups of already bound carbon atoms, and the unification of atoms into groups moving along the nucleation surface. Due to the high energy, a number of additional concentric simultaneously growing rings can arise from these groups of atoms, as shown in Figure 3. The first cylindrical surface is employed as the mother matrix which the growing follow-up planes strictly copy. However, the tendency of carbon to migrate along the surface of the nanotube and to build up its plane in accordance with the mechanism shown in Figure 1 is much greater than the tendency to create new parallel planes. That is why buckytubes are very long and relatively thin. This coincides with the studies carried out in carbon layer growth from the gas phase at lower temperaturesIh where a graphite plane speed of growth is observed which is several orders higher longitudinally in the plane with respect to growth perpendicular to the plane. The mechanism of growth as shown in Figure 3 lead us to expect that if for some reason the nucleation surface bottom layer experiences a change, it has to also induce respective changes in the manner of layer growth mimicking its structure. The reason for this is that the change of the angle of the carbon atom o bond orientation at warping leads also to a change of the n bond,I7 which acts on the atoms from the next layer. In accordance with the above, when the growing cylindrical surface, Figure 4A, closes at the bottom end due to an energetic fluctuation, stimulated by the utilization of an ac power supply, the growth of the innermost layer halts, as shown in Figure 4B. In further growth the follow-up cylindrical structures will be stimulated by the mother matrix to copy warping. An integral repetition of the initial carbon net can be witnessed in Figure 4C. Closure induced by the mother matrix without changing the diameters of the concentric nanotubes can be seen in Figure 4D. Such arrangements have been reported in refs 8, 9, and 18. In the case of Figure 4E the cylindrical layers acquire a conical shape and close when a set diameter is reached. A large number of analogous structures have been reported previously.3 This is possibly the diameter at which the closure of the nanotube becomes energetically favorable. Characteristic of the last two cases is that the process of growth continues after the mother matrix ceases to grow. Each subsequent layer acts as a mother matrix until it closes. Often the deformations of the mother matrix don't induce equivalent deformations of the copy layers. The changes are expressed in terms of warping of the cylindrical layers with a radius of curvature equivalent to the angle allowed for the C-C bond. This changes the whole subsequent growth of the formation, as seen in Figure 4F. When a defect which does not have a circular symmetry within the carbon matrix arises with respect to the axis of the growing buckytube, then growth and cylindrical layer closure are not symmetric, Figure 4G. A similar structure is shown in ref 3. In the same figure we can also see the effect of every closing cylindrical layer on the next layer, including the warping which it causes while closing. It was observed that the process of closure does not proceed identically for every concentric layer, as it should have if every layer grown was in semblance to the mother matrix. Obviously, factors other than copying the mother matrix, factors that change during growth, have come into play here; these yield different end closures of the type in Figure 4G. At an ac power supply, the periodic stoppages and the changes in the percentage of growth are common phenomena. In accordance with the scheme in Figure 3, if the stream of C atoms toward a growing structure of this nature is halted, the later will remain with unopened cylindrical layers, as observed in
Figure 5. Halt in the growth of a concentric nanotube at the termination of the supply of C atoms.
Figure 6. Halt in growth due to warping of the mother matrix.
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g . . .
Figure 7. Carbon vapor introduction within the insides of the nanotubes.
Figure 5 (the spot marked by an arrow). The mother matrix, if not closed, can sometimes close after the stream of vapor halts, fed by migrating atoms, moved by the necessity to reduce the number of unsaturated C bonds, and aided by the high temperature. The upper layers, which are incomplete, lack the favorable conditions to close in time. A large diversity of incomplete concentric structures have been reported in ref 10, regardless of the fact that the later researchers employed a dc discharge, with which the induced halt in the growth process is not such a common occurrence. Their conclusion also is that the growth of these structures halts because the growth conditions do not allow the newly arriving C atoms to continue to build the nonotubes any further. After considering Figure 6 we are prepared to accept that the termination in growth can also be caused by a large defect in the mother matrix, since the conditions allowing for successive layer growth at the spot of warping are highly aggravated. Due to the high C vapor density and a highly disordered movement of C atoms, it is quite likely that C atoms can infiltrate into the ringlike cylindrical mother matrices, schematically shown in Figure 7. Formations created on the external surface copy the C net mother matrix and, due to its smaller diameter, will be more prone to close than the mother matrix. This mechanism explains the structures seen in Figure 8. Such structures have been observed previously by other authors, but obtained under different condition^.^.'^ The conditions suited to nanotube synthesis stimulate the buildup of a perfect cylindrical surface as a mother matrix. However, the larger the fluctuations at deposit preparation, the larger the mother matrix curving. A boomerang form is shown in Figure 9. These have been described previously? Sometimes warping is not characteristic for one point alone, but can encompass a segment of the growing structure such as the arc seen in Figure 10.
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Nanocarbons Formed under ac Arc Discharge
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Figure 8. Closed structures within the insidcs of the nanotubes. .
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Figure 12. Spiral curving of the nanotuhe core.
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Figure 13. (A) Single-cone structures. (B) Ilouble-cone structures.
Figure 9. Boomerang type fullerene.
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Figure 10. Arc curving fullerene.
1 Figure 14. Spherical carbon formations of different density.
Figure 11. Symmetric growth of concentric nanotubes.
C matrix curving of the type shown in Figure 11 is encountered quite often and requires more attention due to the manner of its growth, which very well compares with our own growth model. Core growth changes direction twice, leading to chiral symmetry. C structures of such a form can be obtained only if growth along the length of the innermost layer starts from its midpoint first, proceeding toward both ends simultaneously. The growth material was comprised of weakly joined atoms that reach the growing ends by way of simultaneous antipode migration, a mechanism which very well coincides with the one shown in Figure 1, but migration is toward both ends of the nanotube. The cylindrical or globular mother matrix forms are well established. However, P. J. F. Harris et al. show'* that they have observed transverse cross sections of nanotubes, which differ from the proper circle. In the material extracted by us and shown in Figure 12 we observed a core structure that in our view can be interpreted as a core with an elliptical cross section. In our view the photograph shows an elliptic core
turned by 180"; that is why we observe the change in the si7e of the core cross section. As a distinction from J. D. Fitzgerald et al.," who observed fibrous formations with a pancake structure, and Iijima et al.,") all of whom do not report a cone, we observed cone formations subtending an angle of 20°, with a fully closed base. These are clear-cut cones as expressed in Figure 13A or double cones whose axes conclude an angle as seen in Figure 13B. The double-cone structures observed are most probably grown simultaneously from the midpoint toward both ends; this seems apparent when looking at the chiral symmetry structures grown. Many of the structures observed are prone to terminate as such cones, but the form of the body in distinction from that of ref 6 is cylindrical. Often the axis of the cylindrical formation and the cone conclude an obtuse angle. Our deposits contain a multitude of spherical formations with hundreds of nanometer and micrometer sizes that in order to be so large should consist of hundreds of thousands of C atoms. Regardless of their size, these can be recognized by a TEM as being of two types. One of the sphere types has a low density and high electron beam transmission (indicated with a circle and arrow in Figure 14), while the other type of sphere has a high density and low electron beam transmission (indicated with a point and arrow in Figure 14). In all probability the first type
2046 J. Phys. Chem., Vol. 99, No. 7, 1995
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Koprinarov et al. outermost layer can be the first because it is the first to fall within the hot region. After that, the internal layers restructure. Because of the fact that the growth process starts from the periphery and is aimed toward the periphery, the final structure is a hollow closed system. After closure, whether the hollow system takes up a spherical form or possesses a multitute of plane sections, as in Figure 15, depends on what is energetically more favorable for the system. Two of the hollow formations in Figure 15 are indicated with a circle and arrow, while two compact formations are indicated with a point and arrow. A large quantity of perfect spheres-Fullerenes-composed of different numbers of C atoms can exist for a protracted period of time between the electrodes. Thus, additional coats grow onto the C matrix. Such onion structures were observed also in refs 7, 19, and 20. These spheres do not transmit electrons when analyzed by E M . Nontransmissive spheres can also be obtained by multiple-layer dressing of hollow spheres. Which of the observed nontransmissive spheres are compact and which have a hollow core cannot be determined with the aid of a "EM. We consider that the probability of starting with a Cw molecule and building up such large spheres consisting of from 100 to 200 layers, equivalent to tens of millions of carbon atoms, is less than a sphere with a 'Ore* In our investigations we found formations that cannot be restructured into orderly systems. This is also true for part of the spherical formations. Information is obtained characterizing the degree of order of the respective systems by utilizing the bright field (BF), Figure 16A, or dark field (DF), Figure 16B, TEM work modes. Some of the spheres (those which remain dark) at electron beam irradiation do not have an ordered structure; hence, they are not restructured. Concentric polyhedral carbon coats are shown in Figure 17. Such structures, which F. Beguin et al. observed in large quantities,8 in our case account for only a small part of the material obtained. Due to the changes in the radius of curvature, the deformation and consequent rise in strain increase with subsequent coats. Eventually, the angles of layer curving in the plane are changed, and the sum total C net energy is reduced. Such changes in angle are shown in Figure 17 with arrows. These can be so large that the layer can detach from the internal surface, thus enclosing and capturing next-door structures.
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Figure 15. Formations of different density and partially planar surface regions.
is composed of one or several layers and is hence highly transparent, while the second is composed of many more layers and a hollow or compact core. We consider it unacceptable that such large formations can be obtained from a large multitude of successively grown and concentrically arranged spheres lying within one another. In our opinion the only viable alternative is to suppose that these formations are the result of a quick restructuring of formerly randomly arranged carbon clusters. Carbon soot restructuring at a lower temperature and the synthesis of perfect compact spherical structures have already been observed by Ugarte.I9 The conditions under which Ugarte conducted his experiments differ significantly from the conditions of ac arc discharge synthesis between close proximity electrodes. Also in our case we do not employ gas stream C vapor purging from the arc region and the quantity of C atoms obtained is very large, while their transport is slow. Spontaneously arising agglomerates of C atoms appear in a great variety of soot forms; these are ordered to different degrees. Some are of low density and lack a compact core, while others are packed. When employing close proximity electrodes, there is a large temperature gradient. The arc ignition and extinction as well as the constant shift in polarity change the position of the arc region between the electrodes. This increases the possibility for such conglomerates to fall within the arc and to initiate restructuring. In this case the I
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0
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Figure 16. Investigation of the extent of system order: (A) TEM (bright field); (B) TEM (dark field).
J. Phys. Chem., Vol. 99, No. 7,1995 2047
Nanocarbons Formed under ac Arc Discharge
( f
Figure 17. Polyhedral carbon formations.
Conclusion The set of examples of created C net formations thus far presented are only a small part of all possible C net forms. The physical and chemical properties of the C net atoms change when the net warps or a local deviation in the periodicity of the atomic arrangement arises. In this respect each new observed form manifests new properties and presents an interest for unraveling the mechanisms by which it is built. We can envisage that these forms will find unique practical applications.
Acknowledgment. The sponsorshipof the Bulgarian Science Foundation is much appreciated. References and Notes (1) Kroto, H. W. Science 1988, 242, 1139. (2) Iijima. S. Nature 1991, 354, 56. (3) Iijima, S.; Ichihashi, T.; Ando, Y. Nature 1992, 356, 776. (4) Fowler, P. W.; Manolopoulos, D. E.; Rayan, R. P. Carbon 1992, 30, 1235. (5) Vanderbilt, D.; Tersoff, J. Phys. Rev. Lett. 1992, 68, 51 1. (6) Pang, L. S. K.;Wilson, M. A.; Fitz Gerald, J. D.; Brunckhorst, L. Carbon 1993.31, 240. (7) Dravid, V. P.; Lin, X.; Wang, Y.; Wang, X. K.;Yee, A.; Ketterson, J. B.; Chang, R. P. H. Science 1993, 259, 1601.
(8) Beguin, F.; Chard, C.; Conard, J.; Rouzaud, J.-N. Extended Abstracts of the 21st Biennial Conference on Carbon, Buffalo, NY, June 13-18, 1993; American Carbon Society: 1993; p 223. (9) Endo, M.; Takeuchi, K.; Ikarashi, S.; Kobouri, K. Extended Abstracts of the 21st Biennial Conference on Carbon, Buffalo, NY, June 13- 18, 1993; American Carbon Society: 1993; p 23 1. (IO) Iijima, S.; Ajayan, P. M.; Ichihashi, T. Phys. Rev. Lett. 1992, 69, 3 100. (1 1) The authors utilize arc discharge between carbon electrodes of different cross sections. The thick electrode has a cross section of 8 x 8 mm, while the thin electrode has a cross section of 3 x 3 mm. The power supply is 75 A ac, 25 V. The pressure in the reaction space is 250 TOITin an Ar ambient. The distance between the electrodes is maintained at 0.1 mm. The electrodes are water cooled. The thicker electrode is cooled better; a round deposit resembling a bar grows across its face at a speed of 1 m d m i n . Its diameter is around 6 mm, while its weight is 63% of the weight of the carbon which evaporates from the thin electrode. The remaining 37% leaves the vicinity of the arc as soot and fullerenes. Cm is about 5%. The cross section of the growing deposit has a shallow crater which is surround by a thin and stiff cylindrical cover with a gray metallic sheen. The longitudinal bisection of the deposit shows that its inside is full of column formations equal in length to the length of the deposit. These are oriented in the direction of growth. (12) Ebbesen, T. W.; Ajayan, P. M. Nature 1992, 358, 220. (13) Curl, R. F.; Smalley, R. E. Science 1988, 242, 1017. (14) Wilson, M. A.; Pang, L. S. K.;Willett, G. D.; Fisher, K.J.; Dance, I. G . Carbon 1992,30, 675. (15) Robertson, D. H.; Brenner, D. W.; White, C. T. J. Phys. Chem. 1992, 96, 6133. (16) Fedosayev, D. V.; Deryagin, B. V.; Varasavskja, I. G. The Crystallization of Diamond; Nauka: Moscow, 1984; p 563. (17) Haddon, R. C. Science 1993, 261, 1545. (18) Harris, P. J. F.; Tsang, S. C.; Green, M. L. H. The First International Interdisciplinary Colloquium on the Science and Technology of the Fullerenes, Santa Barbara, CA, June 27-July 1, 1993; Elsevier Science Publishers: 1993; p 252. (19) Ugarte, D. Nature 1992, 359, 707. (20) Jose-Yacaman, M., Miki, M.; Reyes-Gasga, J. The First Intemational Interdisciplinary Colloquium on the Science and Technology of the Fullerenes, Santa Barbara, CA, June 27-July 1, 1993; p 248. (21) Taylor, R.; Walton, R. M. Nature 1993, 363, 685.
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