12464
J. Phys. Chem. B 2001, 105, 12464-12468
Growth and Microstructure of Catalytically Produced Coiled Carbon Nanotubes K. Hernadi† UniVersity of Szeged, Applied and EnVironmental Chemistry Department, H-6720 Szeged, Rerrich B. ter 1., Hungary
L. Thieˆ n-Nga* and L. Forro´ ‡ DP, Ecole Polytechnique Fe´ de´ rale de Lausanne, CH-1015 Lausanne, Switzerland ReceiVed: March 29, 2001; In Final Form: July 23, 2001
We investigated the influence of catalyst preparation on the growth of coil-shaped nanotubes by varying the pH of the solution during catalyst impregnation, to elucidate their formation process. The spirals were observed by transmission electron microscopy, and we determined the dimensions of these coils for a large number of them, to compare them to straight nanotubes, and also to establish correlations between these dimensions and catalyst preparation. We also looked for the nature of curling at the nanometer scale. A growth mechanism for these spirals is finally suggested.
I. Introduction The CVD route for nanotube production has become a popular method to make large amounts of multiwall carbon nanotubes. It is indeed the best way of producing large quantities for materials science applications, such as the fabrication of nanotube-reinforced composites. Occurrence of coil-shaped fibers in the jungle of straight catalytically grown nanotubes has been noticed from the early days of studying their catalytic synthesis.1 These coiled tubes do not appear when an arc-discharge process is used, nor any other process. It is clear that such nanospirals would on one hand have a toughness resembling the toughness of nanotubes more than of carbon fibers, and that, on the other hand, if used in composites, they would be better anchored in their embedding matrix than straight nanotubes. Their shape would favor a better load transfer to the matrix than in the case of ordinary tubules, and possibly easier infiltration. It would be interesting therefore to find ways of increasing their yield. From a nanomechanics point of view, with the recent advances in the study of individual nanotubes,2-5 nanocoils appear as a naturally attractive object for NEMS (nanoelectromechanical systems). Previous studies made us think that preparation of the catalyst with a slightly basic solution is favorable to spiral carbon nanotube formation.6 One might indeed hope that these coils would have the extraordinary stiffness displayed by straight nanotubes. They should be superior, in terms of the mechanical properties, to coiled carbon fibers which are likely to fracture. Attempts to measure the mechanical properties of these springs with the atomic force microscope have been made,7 but it is still not known what the spring constant of such a helix is. The coiled nanotubes have a strikingly regular structure; with a pitch of 50 nm, typically, the helix is regularly coiled like a miniature telephone cord (Figure 1). The coils can be very long: the longest coil we actually observed was as long as 10 * To whom correspondence should be addressed. Fax: 41-21-693-4470. E-mail:
[email protected]. † Fax: 36-62-544-619. E-mail:
[email protected]. ‡ Fax: 41-21-693-4470. E-mail:
[email protected].
Figure 1. A typical coil, with its regular pitch.
µm. Keeping these facts in mind, we have investigated the coil microstructures in more detail, both by measuring the dimensions of these coils and observing the structure of the nanofiber carbon planes at the atomic scale. We also tried to see if there is a correlation between the way the catalyst is prepared and the density of these nanocoils; this would help both to produce coils in larger quantities and to understand how and why they are formed, and to suggest growth mechanisms for these tubules. II. Experimental Section II.1. Catalyst Preparation and Synthesis. Catalytic nanotubes were grown by acetylene decomposition at 720 °C for 30 min with a gas feed of 70 L/h of nitrogen and 10 mL/min of acetylene. The catalyst used for this purpose was a cobalt catalyst supported on silicagel H (5-40 µm, Fluka). It was
10.1021/jp011208p CCC: $20.00 © 2001 American Chemical Society Published on Web 11/21/2001
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J. Phys. Chem. B, Vol. 105, No. 50, 2001 12465
TABLE 1: Carbon Yield as a Function of pH catalyst 1 2 3 4 5 a
Co2+ a mmol 2.51 2.51 2.51 2.51 2.51
NH3a mmol 1.447 2.894 5.787 7.234 11.575
pH measured 7.5 8.0 8.5 9.0 10.0
yieldb%
free Co2+ mol dm-3
free Co2+ a mmol
50.33 45.01 38.89 31.67 22.33
2.5 × 2.5 × 10-4 2.5 × 10-5 2.5 × 10-6 2.5 × 10-8
0.125 1.25 × 10-2 1.25 × 10-3 1.25 × 10-4 1.25 × 10-6
10-3
In each 50 mL of solution for 1 g of support. b Deposited carbon compared to the amount of initial catalyst.
Figure 3. Defects allowing curvature at a strong bend of a spiral. On the upper part, the stretching of the planes leads to a regular curvature. In the inner part of the spiral both plane buckling, and interrupted layers close to the core can be seen.
Figure 2. Variations in core diameter along a lightly coiled spiral. The zoomed micrograph shows the buckling of the planes which allows the tube to curve.
obtained by precipitation of a cobalt acetate (Co(CH3COO)2, × H2O, purum p.a., Fluka) solution at different pH values. [Coacetate solution is a precursor which is well-suited to obtain proper catalyst particles. Decomposition of either cobalt acetate or cobalt hydroxide results in pure CoO on the surface upon heating. Since the solubility of cobalt salts depends strongly on the pH of the solution, the size and the shape of the deposited catalyst particles vary a lot with the pH of solution used during impregnation.] Amounts of Co2+ ions and NH3 loaded are summarized in Table 1. During catalyst preparation the pH was set by ammonia addition so that the starting pH values could be measured for the different preparations: 7.5, 8, 8.5, 9, and 10. To each portion 1 g of catalyst support was added, then the preparation was filtered after 4 h. The final product was then purified: the metallic particles were removed by immersion into a 30% HNO3 solution,
followed by filtration. The catalyst support was separated by repeated sonication in an organic solvent mixture (n-heptane: acetone:2-propanol ) 1:1:1, found to be the best in our previous work8) and decantation. The detailed purification procedure has been described elsewhere.8 II.2. Coil Dimensions and Microstructures. The observations were then performed using a Philips CM-20 conventional transmission electron microscope and a Philips CM-300 FEG high resolution one. Usually the spirals are real nanotubes, fairly well graphitized, with a core. Graphitization of the walls can be seen on the HREM micrographs (Figures 2-4). The less well-graphitized fibers are the thin ones with a pitch in the order of the diameter (tightly coiled helices), or the very large ones. The coils can have a great variety of diameters and pitches, appearing sometimes as slightly twisted tubes, at other times as tightly coiled spirals. We have measured the dimensions of the projections of the helices on a large number of tubes to get their distribution for different pH values. This was facilitated by the fact that the measurements were done on TEM holey carbon grid samples, where the spirals lie flat. The geometrical parameters of the coils are represented in Figure 5, where the pitch and tube diameter of a typical coil are shown. According to Figures 6 and 7, which shows samples of the size distributions at pH ) 7.5 and pH ) 10, the diameter of these helical tubes is generally between 10 and 100 nm. The size distribution is wider than for the surrounding straight nanotubes, which is also clear from our statistics (Figure 8). The average diameter is larger as well (25 nm for the spirals on average, 15 nm for the straight tubes).
12466 J. Phys. Chem. B, Vol. 105, No. 50, 2001
Hernadi et al.
Figure 4. Intricate tube structure at a bend, with internal closures.
Figure 5. Coiled tube with its projection (left) showing d ) helix diameter and p ) coil pitch.
The pitch of the helix varies between 10 and 300 nm. From the statistics shown in these figures the number of thicker tubes is larger on the catalyst prepared at higher pH. This fact is probably due to the great variety of precipitated catalyst particles. The probability of deposition of larger and asymmetric particles increases with increasing pH. Nevertheless, the correlation between the tube diameter and pitch shows large scatter in the data (third plot in Figures 6 and 7): this confirms the fact that there can be tightly or weakly coiled tubes at any diameter. A linear fit shows that roughly speaking the ratio of the pitch to the diameter in twice larger at pH ) 7.5 compared to pH ) 10. This means that on average the spirals would be “stretched” more at low pH than at high pH. To achieve the coiling there are different types of bends. Some parts of the tubes seem to be bent with a constant curvature. Some parts clearly display local buckling of the planes, especially in the case of slightly coiled tubes with a pitch in the 0.3 µm range, where there can be a separation of as much as 0.1 µm between the bends (Figure 2a,b). Other parts exhibit angles, the general curvature of the tube being achieved by a succession of straight segments. This polygonization has been observed previously by other authors.9 As pointed out in this reference, the polygonal character of these tubes can also be seen on the spotty diffraction patterns, which show that the distribution of angles is not continuous through the bends. Some regions can show mixed characters as can be seen in Figure 3. While inner layers are buckling, the outer graphene sheets have constant curvature. The diameter of the core is often irregular too, with frequent constrictions (Figures 3 and 4). At a finer scale, these variations appear to be due to buckling of the graphite planes or defect structures, the planes being interrupted. Sometimes (Figure 4) a complicated structure of imbricated tubes also appears.
Figure 6. Distribution of diameters, helix pitch, and correlation between pitch and diameter at pH ) 7.5.
We tried to observe the spiral ends whenever possible. We never found metallic particles at the tube ends on as-grown material, and we believe that, as for the surrounding straight tubes, growth proceeds from the bottom, at a metallic particle dispersed on the substrate (Figure 9). Even this small region displays spirals of various helix and tube diameters, and pitches. The spirals are very regularly coiled in general. Only occasionely does one see changes in the coiling of the tube. The regularity in the coiling of the spirals on a micron scale does not correspond to a perfect periodic repetition of the atomic structures: looking closely at a number of periodic bends, it appears that the places where there is buckling to achieve the angle compatible with the coil periodicity is not perfect. In this respect the curvature of the spirals is not generated by a repeated translation of a nanometer wide cross-sectional ring during the growth process. The periodicity must be determined at a larger scale. Although it is not easy to assess the percentage of coiled nanotubes, there is a definite influence of the pH on their density. At higher pH the relative amount of spirals compared to straight tubes is larger. The amount of carbon deposited on the surface of the catalyst was weighed. Carbon yields, as summarized in Table 1, decrease with increasing pH. Although the number of efficient catalyst
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J. Phys. Chem. B, Vol. 105, No. 50, 2001 12467
Figure 8. Comparison of tube diameter distributions for straight nanotubes and spirals.
Figure 7. Distribution of diameters, helix pitch, and correlation between pitch and diameter at pH ) 10.
particles seems to increase with pH, a smaller number of catalytic centers could explain the lower value of deposited carbon at higher pH. III. Discussion From a macroscopic point of view, the formation of coils was shown to correspond to an energy-minimum shape as well as that of the straight tube10 if one describes the coil energy accounting for the van der Waals bonding between layers, the surface tensions, and the curvature elastic energy of the layers. At an atomistic scale, one can imagine than this curvature is achieved either by plane buckling, which we observed quite frequently, or by the formation of pentagon-heptagon pairs. The latter cannot be detected easily, but it is clear that they form the basis of additional curvature on the fullerene networks. However, buckling is infrequent in tubes having quite large inner diameter. At lower diameters, structure is less strained with the formation of buckling planes. In the formation of fiberlike thin tubes of disordered structure, some kind of buckling effect might also take part. Which of these different configurationssbuckling or defect formationscosts more energy is difficult to assess.10 Whatever the equilibrium shape one would like to know what growth mechanism can lead to such structures, coiled in such a
Figure 9. Growth from the particle up (the end of these tubes was found to be capped with no metallic particle).
regular way. The idea of our experiments was that, by treating the catalyst at different pH conditions, the shape of the metal particles is made more irregular; during exothermic reaction of acetylene on the particle surface, the energy deposited by the reaction does not flow homogeneously through the whole particle and there can be areas with different catalytic activities. Our assumption is that coil growth is rather similar to straight tube growth except for the fact that these different rates for the catalytic reaction at the surface of the particle probably induces different growth speeds around the catalyst particle. A higher carbon deposition rate at one side of the particle would generate the “outer” part of the spiral (following a spiral, a line of maximum length and a line of minimum length can be drawn). This is more likely to happen on bigger particles, which could also explain that the average diameter of the spirals is larger than that of the straight tubes. With this growth mechanism in mind, some special features can be interpreted. For example, it happened once that we could observe a tube, which, assuming growth from the root, first grew into a helix then continued growing straight, possibly due to changes in catalyst morphology.
12468 J. Phys. Chem. B, Vol. 105, No. 50, 2001 From the point of view of catalyst preparation, it is interesting to consider what the processes which take place in the solution actually are. To form a Co precipitate, Co2+ can be either dissolved or precipitated as Co(OH)2 in the aqueous phase, and can be deposited on the surface of silicagel. With increasing NH3 concentration, amino-complexes of Co2+ have to be also taken into account. The following equilibria have an influence on ion concentrations in the solution during catalyst preparation:
Co2+ + 2OH- h Co(OH)2 CH3COO- + HOH h CH3COOH + OHNH3 + HOH h NH4+ + OHCo(NH3)2+nNH3 h Co(NH3)2+n+1
n ) 0, 1, 2, ..., 5
Because the equilibrium constants for these steps are wellknown, the Co distribution and actual pH could be calculated. Since silicagel support has a significant effect on both Co2+ concentration and pH, these calculations would be irrelevant for further considerations. Some kind of estimation can be given for determining free Co2+ by using the measured pH values and solubility product of Co(OH)2 (2.5 × 10-16). From these data it can be seen that the overwhelming part of Co2+ is precipitated. These particles which are colloidal in nature are likely to have an irregular shape. They can adsorb on the surface of the silicagel support. To vary the shape and the size of the cobalt clusters that precipitate, we used solutions of various pH for catalyst preparation. With increasing pH, Co(OH)2 aggregates are larger in size, and the formation of asymmetric adsorbates has a higher probability. This fact explains both the increasing number of helices and the higher average tube diameter obtained with increasing pH. Decreasing the yield with increasing pH can be explained by two facts: the formation of an amino-complex increases the solubility of Co at higher pH on one hand, and bigger catalyst particles result in smaller amount of active sites on the other hand. It was already proved that, for straight tubes, the tube diameter depends on the average size of the catalyst particles.12 To account for the other parameters that determine the shape of helices (diameter of the helix, pitch), the following considerations can be given. In our system we found a strong interaction between the catalyst particles and the support. Indicating the importance of interaction, the activity of the catalyst strongly depends on the thickness of the CoO layer on the surface. If the catalyst particle deposited from a homogeneous solution is symmetric, the catalyst activity will be equal at the edge of this circular catalyst particle and acetylene decomposition will result
Hernadi et al. in the formation of a straight carbon nanotube. When asymmetry appears in the spherical particle, helical tubes start to grow as a result of differences in local acetylene decomposition rate. If there is a big difference between maximum and minimum activity within the same particle, the curvature of the growing helical nanotube will be quite large, thus, the diameter of the helix will be large. With a lower activity difference, the diameter of the growing helix is smaller. Irregularities in the deposited catalyst particle can be significant. Consequently, a difference in activity has a crucial effect on the final shape of the tube. Recently Chen et al.13 showed that it is possible to grow large amounts of carbon microcoils by pyrolysis of acetylene onto a Ni catalyst. In that case the carbon fibers are amorphous, and growth occurs at the bottom of the metallic particle, which is lifted up by carbon fiber formation. The authors suggested that curling occurs to accommodate the different carbon formation rates on the sides of the metal particle, often found to be faceted. Such a mechanism is indeed close to the explanation we offer for coiled-nanotubule growth. We hope that understanding this growth mechanism will help to control syntheses of carbon nanocoils by designing the features of the catalyst. Acknowledgment. The authors acknowledge the TMR network of the European Community, NANOCOMP, OFES, as well as the Swiss National Science Foundation for financial support. K. Hernadi thanks the Hungarian Ministry of Education (FKFP 0643/2000) for financial support. References and Notes (1) Ivanov, V.; Nagy J. B.; Lambin, P.; Lucas, A.; Zhang, X. B.; Zhang, X. F.; Bernaerts, D.; VanTendeloo, G.; Amelinckx, S.; Van Landuyt, J. Chem. Phys. Lett. 1994, 223, 329. (2) Yu, M.-F.; Yakobson, B. I.; Ruoff, R. S. J. Phys. Chem. B 2000, 104, 8764. (3) Yu, M.-F.; Files, B. S.; Arepalli, S.; Ruoff, R. S. Phys. ReV. Lett. 2000, 84, 5552. (4) Yu, M.-F.; Lourie, O.; Dyer, M. J.; Moloni, K.; Kelly, T. F.; Ruoff, R. S. Science 2000, 287, 637. (5) Cumings, J.; Zettl, A. Science 2000, 289, 602. (6) Hernadi, K.; Fonseca, A.; Piedigrosso, P.; Nagy, J. B.; Bernaerts, D.; Riga, J.; Lucas, A. Catal. Lett. 1997, 48, 229. (7) Volodin, A.; Ahlskog, M.; Seynaeve, E.; Van Haesendonck, C.; Fonseca, A.; Nagy, J. B. Phys. ReV. Lett. 2000, 84, 3342. (8) Hernadi, K.; Fonseca, A.; Nagy, J. B.; Bernaerts, D.; Riga, J.; Lucas, A. Synth. Met. 1996, 77, 31. (9) Zhang, X. B.; Zhang, X. F.; Bernaerts, D.; Van Tendeloo, G.; Amelinckx, S.; Van Landuyt, J.; Ivanov, V.; Nagy, J. B.; Lambin, P.; Lucas, A. Europhys. Lett. 1994, 27, 141. (10) Ou Yang Zhong Can; Zhao Bin Su; Chui Lin Wang Phys. ReV. Lett . 1997, 78, 4055. (11) Han, J. Chem. Phys. Lett. 1998, 282, 187. (12) Piedigrosso, P.; Bosquillon, A. G.; Fonseca, A.; Nagy, J. B. Molecular Nanostructures; Kuzmany, H., Ed.; World Scientific: New York, 1997; p 391. (13) Chen, X.; Saito, T.; Kusunoki, M.; Motojima, S. J. Mater. Res. 1999, 14, 4329.
