Langmuir 1997, 13, 7197-7201
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Adsorption Studies of Methane Films on Catalytic Carbon Nanotubes and on Carbon Filaments E. B. Mackie,† R. A. Wolfson,† L. M. Arnold,† K. Lafdi,‡ and A. D. Migone*,† Department of Physics and Center for Advanced Friction Studies, Southern Illinois University, Carbondale, Illinois 62901 Received July 21, 1997. In Final Form: October 10, 1997X Adsorption measurements of CH4 on catalytically produced carbon nanotubes were used to determine the wetting behavior of the films, the presence of capillary condensation, and the specific surface area of the tubes. Two groups of carbon tubes were produced. The set of larger tubes had inner diameters on the order of 1 µm, while the set of narrower tubes had inner diameters on the order of 10-100 nm. The narrower carbon tubes were either oxidized in nitric acid, or subjected to high-temperature treatment at 2400 °C in vacuum, or left in as-produced condition. The activation processes were used to open the ends of the tubes. Surface area determinations for untreated and treated tubes established that the specific surface area increased as a result of activation. Isothermal adsorption-desorption cycles were measured on treated and untreated tubes. Hysteresis loops, indicative of the formation of a capillary condensate on the substrate, were present only for the open-ended treated tubes; no hysteresis loops were present on the untreated tubes. The wetting behavior of methane films above and below the bulk triple point was determined. Solid films incompletely wet the tubes. This behavior contrasts with the complete wetting exhibited by CH4 on exfoliated graphite, in spite of the fact that both the tubes and the exfoliate have essentially the same composition. Liquid methane completely wets the nanotubes.
I. Introduction Carbon nanotubes can be thought of as one (or more) planar sheets of graphite rolled into a cylinder (or several concentric cylinders), closed seamlessly. Nanotubes are capped at the ends by carbons of a different symmetry than those constituting the walls of the tube. The center of the carbon nanotube is hollow. Typically, its inner diameter is on the order of a few nanometers, while its length is on the order of micrometers resulting in aspect ratios of 100 or greater. Carbon nanotubes, produced by arc discharge from carbon electrodes, were discovered by Ijima in the early 1990s.1 Many of the properties of carbon nanotubes have been reviewed recently.2-4 It has been suggested that a material filling the center of a nanotube will behave, to a substantial degree, as if it were one-dimensional (1-D). From a fundamental point of view, the possibility of the experimental realization of 1-D matter provides a powerful stimulus for studying nanotubes and their filling. There is also a strong practical impetus to study the problem of filling nanotubes: this process is closely linked with several of the possible uses suggested for these materials (catalysis, differential adsorption, storage technology, nanowire production, etc.).5,6 Catalytically-produced carbon tubes, similar to arcdischarge nanotubes, have been known for decades. They were first prepared by thermal decomposition of benzene between 1150 and 1300 °C.7-9 The reproducible synthesis †
Department of Physics. Center for Advanced Friction Studies. X Abstract published in Advance ACS Abstracts, December 1, 1997. ‡
(1) Ijima, S. Nature 1991, 354, 56. (2) Dresselhaus, M. S.; Dresselhaus, G. D.; Eklund, P.C. Science of Fullerenes and Carbon Nanotubes; Academic Press: San Diego, CA, 1996. (3) Ebbesen, T.W. Annu. Rev. Mater. Sci. 1994, 24, 235. (4) Ebbesen, T.W. Phys. Today 1996, 33 (June), 26. (5) Tsang, S. C.; Chen, Y. K.; Harris, P. J. F.; Green, M. L. H. Nature 1994, 372, 159. (6) Ajayan, P. M.; Ebbesen, T. W.; Ichihashi, T.; Ijima, S.; Tanigaki, K.; Hiura, H. Nature 1993, 362, 522. (7) Koyama, T.; Onuma, Y. Ohyo Butsuri 1963, 11, 857. (8) Koyama, T. Carbon 1972, 10, 757.
S0743-7463(97)00817-2 CCC: $14.00
of long fibers by controlled catalytic processes led to a number of property and structural studies of these materials, as well as to developments of technological interest.10,11 Although catalytic tubes can be made with inner diameters of only a few nanometers, these materials have generally larger diameters than arc-discharge nanotubes. Catalytic nanotubes have a lower degree of graphitization, and hence less uniformity, than nanotubes produced by arc discharge; however, their full graphitization can be readily achieved by further high-temperature treatment of the tubes. A third form of tubular carbon, carbon fibers, has been known and well studied for some time.2 Carbon fibers have inner diameters on the order of micrometers and lengths on the order of centimeters. They provide close micrometer-diametered analogs to carbon nanotubes.2 One aspect of nanotube research which has received only limited attention is the study of adsorption on these materials. This is somewhat surprising, since adsorption measurements on open-ended tubes afford the opportunity to study the filling of nanotubes under equilibrium conditions. Adsorption measurements also permit the determination of the specific surface area of the tubes, the strength of the substrate potential, and the wetting behavior of films on these materials. In this paper, we report the results of adsorption measurements on different groups of catalytically produced carbon tubes. The advantage offered by catalytically produced nanotubes is that they can be manufactured with a wider range of diameters than arc nanotubes. This allows for the study of the evolution of the properties of the film as a function of tube diameter. The adsorbate used in all the measurements was CH4. The diameters of the tubes ranged from 10 nm to 2 µm. We focused on three aspects: the surface area of closed and open nanotubes; the presence or absence of a capillary condensate in the tubes; and the wetting behavior of the adsorbed films. We complemented our adsorption measurements (9) Koyama, T.; Endo, M.; Onuma, Y. Jpn. J. Appl. Phys. 1972, 13, 1933. (10) Endo, M.; Koyama, T.; Hishiyama, Y. Jpn. J. Appl. Phys. 1976, 15, 2073. (11) Koyama, T.; Endo, M. Kogyo Zairyo 1982, 30, 109.
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with scanning electron microscopy studies of all the groups of tubes used in the thermodynamic experiments. Previous studies of adsorption on carbon nanotubes include a small number of experimental measurements12,13 as well as theoretical14 and computer simulation15 work. The surface area of carbon nanotubes before and after treatment with CO212 was determined from adsorption measurements (an increase in the specific area from 21 to 31.7 m2/g was found). Temperature-programmed spectroscopy was used to study hydrogen adsorption in carbon nanotubes.13 This study found very high hydrogen uptake at the interior of the tubes.13 Monte Carlo simulations15 of the adsorption and desorption of Ar and N2 on carbon nanotubes of two different diameters (1.02 and 4.78 nm) have been reported. No phase transitions were observed in the simulations for the smaller diameter nanotube; by contrast, capillary condensation was found in the larger diameter tubes.15 Stan and Cole14 have recently investigated the adsorption potential of several rare gases on carbon nanotubes; they calculated the well depth of the potentials, as well as the ratio of the adsorption capacity on and inside the tube. Interestingly, this study found a strong dependence of the calculated properties on the diameter of the tubes. The nature of the adsorbate inside the tube evolved from 1-D to 2-D to 3-D as a function of the tube diameter.14 II. Experimental Section (A) Samples. We review, next, some general aspects of the production of catalytic carbon nanotubes and carbon filaments. Long carbon fibers result from the controlled catalytic pyrolysis of benzene, or other hydrocarbons, on a heated substrate. The catalysts used in this process are nanometer-diametered metallic particles. The hollow tubes form by the attachment of carbon to the catalytic particles. These particles are supersaturated with carbon from a hydrocarbon gas present in the reaction chamber at 900 °C.2 The carbon filament growth rate is very sensitive to the reaction conditions (e.g., partial pressure of hydrocarbon, purity, flow rate, residence time in the reaction chamber, furnace temperature, etc.). The fiber yield is an inverse function of the diameter of the metallic particles.16,17 In the fiber growth process, fiber nucleation and elongation is initiated at 1000 °C while fiber thickening occurs subsequently at somewhat higher temperatures by thermal decomposition of the hydrocarbons. The catalyst/ hydrocarbon ratio determines the fiber diameter and the aspect ratios of the catalytic carbon nanotubes and filaments. The metallic particle catalysts can be produced by a variety of procedures.16,17 These procedures generally involve two steps: the formation of a metallic film (usually Fe or Ni) on a substrate and its subsequent heating in a hydrogen atmosphere to convert the film into small spherules. The diameter of the metallic spherules can be roughly controlled by the preparation conditions. An efficient variation of fiber production integrates fiber seeding and fiber formation: the metallic spherules are formed in a spray, and fiber formation begins before the catalyst particles deposit on the substrate. With the metallic particles dispersed before aggregation, the number of nucleated fibers can be increased significantly. All the carbon tubes studied were produced catalytically at the Center for Advanced Friction Materials of Southern Illinois University utilizing the last method described in the previous paragraph. Fe was the catalyst used in a reaction chamber with flowing methane. Two groups of carbon tubes were produced. The inner diameter of the smaller tubes studied are in the range (12) Tsang, S.C.; Harris, P. J. F.; Green, H. L. Nature 1993, 362, 520. (13) Dillon, A.C.; Jones, K. M.; Bekkedahi, T. A.; Kiang, C. H.; Bethune, D. S.; Heben, M. J. Nature 1997, 386, 377. (14) Stan, G.; Cole, M. W. To be published. (15) Maddox, M.; Ulberg, D.; Gubbins, K. E. Fluid Phase Equilib. 1995, 104, 145. Maddox, M.; Gubbins, K. E. Langmuir 1995, 11, 3988. (16) Endo, M.; Ueno, H. MRS Symposium on Graphite and Intercalation Compounds (Pittsburgh, PA), 1984; p 177. (17) Endo, M.; Shikata, T. Ohyo Butsuri 1985, 54, 507.
Mackie et al. of 10-100 nm (with 70% in the range between 40 and 70 nm); the lengths of these tubes are on the order of 10 µm or larger. The larger tubes studied have hollow center diameters on the order of 1 to 2 µm (with 80% between 1.2 and 1.5 µm) and lengths on the order of centimeters. The group of narrower nanotubes was divided into three sets. Two of these sets were subjected to one of two methods of activation. One set of tubes was activated by soaking in nitric acid, followed by ambient drying. The second set was heated under vacuum to 2400 °C. Both of these treatments result in an opening of the capped ends of the tubes. The third set was studied without subjecting it to any treatment after production. (B) Apparatus. The automated apparatus in which the adsorption and desorption isotherms were performed has been previously discussed in detail.18 Electropneumatic valves were employed to dose a computer-stipulated amount of gas from a reservoir to the gas handling system and from the gas handling system to the cell. MKS Baratron gauges and an IBM PCcompatible computer were used to measure and record pressures. Equilibrium conditions were controlled and monitored by the computer. The low temperatures were achieved using a He closed-cycle refrigerator and a two-stage temperature control setup. The temperature of the sample cell remained within (0.005 K of the selected experimental value throughout the entire adsorption-desorption cycle. Scanning electron (SEM) micrographs were taken of all the groups of carbon tubes used in the adsorption measurements. The images were taken at the Center for Electron Microscopy of Southern Illinois University at Carbondale, using a Hitachi Model S570 electron microscope.
III. Results (A) Wetting. Wetting behavior describes the evolution of the thickness of the adsorbed film as the pressure approaches saturated vapor pressure. If a liquid or solid completely wets a substrate, the film thickness diverges as the saturated vapor pressure is approached. If only a finite film forms as the saturated vapor pressure is reached, the adsorbate exhibits incomplete wetting. We studied the wetting behavior on both treated and nontreated carbon tubes above and below the triple point of methane (90.7 K). Liquid methane completely wets all the tubes studied, whereas solid methane incompletely wets them. This behavior is shown in Figures 1and 2. The former displays a 92.5 K adsorption isotherm measured on short nanotubes subjected to nitric acid treatment, while the latter shows a 77 K isotherm measured on short, untreated tubes. By contrast, both liquid and solid methane completely wet exfoliated graphite.19 We note that this difference in solid wetting occurs despite the fact that both substrates essentially consist of hexagonal graphene sheets. The uniformity of the carbon tube surface is partly responsible for the adsorbate’s wetting behavior.20 On more highly graphitized carbon tubes, the thickness of the solid CH4 film at saturation is greater than that for films grown on less graphitized tubes. This can be seen by comparing Figures 2 and 3; the latter shows an isotherm measured at 75 K on the highly graphitized tubes (heated to 2400 °C), while the former displays an isotherm measured at 77 K on tubes which were not subjected to additional heat treatment after production. The number of layers present at saturation is greater on the heattreated tubes. The higher degree of graphitization of the heat-treated tubes is made apparent by the presence of steps in the adsorption isotherm (Figure 3). These steps (18) Shrestha, P.; Alkhafaji, M. T.; Lukowitz, M. M.; Yang, G.; Migone, A. D. Langmuir 1994, 10, 3244. (19) Hess, G. B. In Phase Transitions in Surface Films 2; Taub, H., Torzo, G., Lauter, H. J., Fain, S. C., Jr., Eds.; NATO ASI series B; Plenum Press: New York, 1991; Vol. 267. (20) Bruschi, L.; Torzo, G.; Chan, M. H. W. Europhys. Lett. 1988, 6, 541.
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Figure 1. Adsorption isotherm at 92.5 K for CH4 adsorbed on narrow nanotubes, treated with nitric acid. The film thickness increases asymptotically as the saturated vapor pressure is approached, indicating complete wetting for this liquid film. For Figures 1, 2, 4, and 5, the coverage corresponding to one layer was determined from the BET isotherm equation.
Figure 3. Adsorption isotherm of CH4 on narrow nanotubes subjected to in vacuo heat treatment at 2400 °C. The presence of steps in the multilayer adsorption isotherm is clear evidence of the high degree of graphitization of these tubes. The steps occur at the same relative pressures as they do for exfoliated graphite. At least four or five steps are present before reaching saturation. At the saturated vapor pressure, however, only a finite thickness film is present on the tubes. This indicates that in spite of the fact that there is a greater degree of wetting than in the data shown in Figure 2, the solid film still incompletely wets the substrate. The coverage corresponding to one layer was determined from the “knee” of the isotherm at the location of the first step.
Figure 2. Adsorption isotherm at 77 K for CH4 on narrow, untreated nanotubes. The sharp intercept present at saturation is an indication of the coexistence of a finite film with bulk at the saturated vapor pressure. The solid film, thus, incompletely wets the substrate.
occur at the same relative pressures as they do on exfoliated graphite.19 The incomplete wetting by solid films is also due in part to a geometric effect. The curved surfaces of the nanotubes stress the solid film, making it energetically favorable for the adsorbate to form bulk crystallites rather than to continue growing as film.21 (B) Capillary Condensation. Capillary condensation is the formation of a condensed bulk phase (solid or liquid)
at pressures below saturated vapor pressure which occurs when a film adsorbs on nonplanar regions of a substrate. This phenomenon manifests itself distinctly in isothermal adsorption-desorption cycles by the presence of a hysteresis loop.22 In Figure 4 we present adsorption-desorption data on long, untreated tubes measured at 85 K. No hysteresis loop is present in the data, within experimental resolution. Because these tubes were not subjected to activation they are closed at the ends; this was confirmed by electron microscopy. The absence of capillary condensation is the expected behavior for closed tubes. It also indicates that very little capillary condensation is occurring in cavities and crevices on the tubes’ outer surface. The same lack of hysteresis loops in adsorption-desorption cycles was found for isotherms measured on short, untreated nanotubes. The small-diametered tubes which were opened by soaking in nitric acid displayed very different behavior. After activation, a hysteresis loop was present in the adsorption-desorption cycle (see Figure 5). Electron micrographs taken of the treated tubes (see Figure 6) (21) The effect of strain on the wetting by a solid film has been investigated: Gittes, F. T.; Schick, M. Phys. Rev. B 1984, 30, 209. (22) Defay, R.; Prigongine, I. Surface Tension and Adsorption; John Wiley: New York, 1966. Adamson, A. W. Physical Chemistry at Surfaces, 4th ed.; John Wiley and Sons: New York, 1982. Lowell, S.; Shields, J. R. Powder Surface Area and Porosity, 2nd ed.; Chapman and Hall: London, 1984.
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Figure 4. Adsorption-desorption isothermal cycle measured at 85 K for CH4 on long, untreated tubes. The monolayer capacity for this sample is less than 100 cm3 Torr. Within the experimental resolution of our apparatus the adsorption and desorption curves coincide, indicating the absence of a capillary condensate under these conditions.
provide direct evidence that the ends of the tubes are open. The activation process not only attacks the ends of the carbon tubes but also attacks defects present on the outer walls of the tubes. Some of the capillary condensation is presumably occurring within the hollow center of the tube, while the remainder is taking place on the walls. Similar hysteretic behavior, in the adsorption-desorption cycles, was found for isotherms measured on tubes subjected to heating to 2400 °C under vacuum. It should be noted that for catalytically-produced tubes the heating process results in the expulsion of the metal catalyst and, hence, in the opening of the tube. Our SEM images confirmed that the heated nanotubes were open at the ends. By contrast, arc-evaporated nanotubes cannot be opened by heating under vacuum. (C) Specific Surface Area. In order to estimate how much area inside the tubes is available for adsorption, we compared the surface area of the unopened tubes with that of the opened tubes. Surface area was calculated using the BET equation. An increase from 17.3 to 23 m2/g was observed after the tubes were activated with nitric acid. Reported increases in specific surface area for CO2-activated carbon nanotubes are on the order of 50%.12 Two explanations were put forth for this increase. Tsang et al.12 argued that some of the tubes were opened by the activation process, resulting in newly accessible internal surfaces. Additionally, they suggested that the activation treatment besides opening the ends of the tubes also attacked the tubes, thinning them. The combined effect of these two mechanisms is to increase the specific surface area of the tubes. Both of these explanations probably apply to carbon tubes of different inner diameters and to different activation processes.
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Figure 5. Adsorption-desorption isothermal cycle at 92 K for CH4 on narrow nanotubes activated with a nitric acid soak. A hysteresis loop is clearly visible in the data. Careful inspection shows the presence of a step in the desorption data, near 25 Torr, which has no counterpart in the adsorption curve. This additional step is characteristic of the closing of the hysteresis loop in these cycles and provides a signature for capillary condensation.
Figure 6. Scanning electron micrograph of the end of a narrow nanotube which has been subjected to a nitric acid soak. The treatment was conducted in order to open the ends of the tubes. The tube, which appears open in the micrograph, has an inner diameter on the order of 10 nm.
IV. Discussion Our experimental results for the presence of capillary condensation can be compared to those from the Monte Carlo computer simulations for these systems.15 The inside diameter of the activated nanotubes used in our experiments are, at their smallest, on the order of 10 nm. The Monte Carlo simulations for nanotubes of two different
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diameters found this phenomenon present in 5 nm tubes but absent in 1 nm tubes. Our results are consistent with the computer simulation predictions for the larger diameter tubes. The evolution toward 1-D behavior with decreasing tube diameter is one of the most remarkable phenomena predicted for films adsorbed on carbon nanotubes.14 Thus, it is of great interest to determine how capillary condensation changes as a function of the inner diameter of the tubes. Our experimental results strongly suggest that 1-D behavior is not present in films formed on tubes having inner diameters of 10 nm, or greater. The availability of tubes of a wide range of diameters is an essential precondition for the experimental study of the evolution of the dimensionality of the adsorbed films. The catalytic process of carbon tube manufacturing permits the production of tubes of different diameters, which make this type of systematic study possible. It has been noted that filling the inside of a nanotube is intimately related to, and controlled by, the wetting characteristics of the adsorbate-nanotube system.23 Early theoretical calculations for dipolar molecules24 found that the nanotubes would act as “molecular straws”, sucking to their interior molecules from the vapor or fluid phases. Experimental results have shown that only those adsorbates which in their liquid phase wet the outside of the nanotube penetrate the inside of the tubes.4 Most studies of nanotube filling have not been conducted at equilibrium. Our adsorption isotherm measurements allow for the determination of the equilibrium wetting characteristics of liquid and solid films. Our results on the incomplete wetting of solid films strongly suggest that an additional important factor which needs to be taken into account when filling a carbon nanotube is the state of the adsorbate (that is, whether the adsorbate at the filling temperature is a solid or a liquid). V. Conclusions We have measured adsorption and desorption isotherms of CH4 on catalytically produced carbon nanotube substrates. The carbon tubes studied were prepared under two different sets of conditions. One set had inner diameters on the order of 10 nm, while the other set had inner diameters of up to 2 µm. Samples of the narrower tubes were subjected to two different activation treatments (nitric acid soaking and in vacuo heating to 2400 °C). The purpose of these treatments was to open the ends of the tubes; tube opening was confirmed by scanning electron microscopy. Three issues were investigated with our (23) Dujardin, E.; Ebbesen, T. W.; Hiura, H.; Tanigaki, K. Science 1994, 265, 1850. (24) Pederson, M. R.; Broughton, J. Q. Phys. Rev. Lett. 1992, 69, 2689.
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thermodynamic measurements: the presence (or absence) of a capillary condensate in the different sets of tubes; the wetting behavior of liquid and solid CH4 films on the different groups of tubes; the surface areas of the different sets of carbon tubes. Cycles of adsorption and desorption show no hysteresis loops in data measured on untreated carbon tubes, while hysteresis loops are present on data measured on the activated tubes. Hysteresis loops are an indication of capillary condensation occurring on the substrate; they signal the presence of a condensed phase on, and probably in, the narrow open tubes. The formation of a capillary condensate on activated tubes of 10 nm inner diameter, and greater, is in general qualitative agreement with the predictions of Monte Carlo simulations for N2 and Ar films on carbon nanotubes.15 Such simulations found adsorption hysteresis loops for adsorption-desorption cycles simulated on 5 nm tubes while no hysteresis was found in simulations for 1 nm diameter tubes. Our results strongly suggest that the 1-D behavior expected to be exhibited by matter inside the carbon nanotubes, which should be identifiable by the lack of a hysteresis loop on adsorptiondesorption isothermal cycles, is not present for tubes of diameters greater than or equal to 10 nm. Our adsorption measurements found that CH4 completely wets the tubes in the liquid phase, while it incompletely wets the tubes in the solid phase. A greater degree of wetting was found on the fully graphitized tubes (i.e., those which had been subjected to in vacuo heating to 2400 °C). We suggest that a combination of surface imperfections and geometric effects account for the incomplete wetting by the solid films. Our surface area determinations found an increase in the specific surface area upon activation/tube opening. This is in agreement with findings by other authors12 for arc-discharge carbon nanotubes. Specific surface area increases are probably due to a combination of the increase in area available for adsorption from the newly opened inner volume of the tubes and from increase in external lateral area resulting from the activation process. Experiments studying the adsorption of methane on the more uniform nanotubes produced by the arc vaporization method are currently in progress. Additionally, adsorption studies will also be performed on catalytically produced nanotubes of diameters less than 10 nm. The manufacture of these tubes is currently under way. Acknowledgment. We gratefully acknowledge Professor M. W. Cole for sharing with us his theoretical results prior to publication. Acknowledgment is made to the donors of The Petroleum Research Fund, administered by the ACS, for support of this research. LA970817E
