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J. Phys. Chem. C 2007, 111, 12980-12986
Equilibration Time: Kinetics of Gas Adsorption on Closed- and Open-Ended Single-Walled Carbon Nanotubes Dinesh S. Rawat, M. Mercedes Calbi, and Aldo D. Migone* Department of Physics, Southern Illinois UniVersity, Carbondale, Illinois 62901 ReceiVed: April 10, 2007; In Final Form: June 15, 2007
We have explored the adsorption kinetics of argon and methane on chemically opened and on as-produced carbon nanotubes. We monitored the evolution of the adsorbate pressure after each gas dose was added to the cell during the performance of adsorption isotherms. We found significant differences between the equilibration times measured on these two groups of nanotubes. We propose an explanation for our experimental observations that makes use of recent computer simulations conducted for similar adsorbed systems.
I. Introduction The study of gas adsorption on carbon nanotubes has attracted a great deal of attention in recent years. The possibility of experimentally realizing matter in 1-D, through the adsorption of gases on the surface and inside carbon nanotubes, has resulted in numerous theoretical and experimental investigations of these systems.1 Interest in adsorption on this material also stems from potential practical applications, since nanotubes present characteristics that are favorable for their use in gas separation and in storage. Several methods have been employed to modify the surface characteristics of nanotubes2-7 with the aim of enhancing their sorptive capacity. In order to use adsorption on carbon nanotubes in practical applications involving storage or separation, however, in addition to controlling the sorptive capacity it is important to thoroughly investigate, understand, and control the kinetics of the adsorption/desorption processes. The experimental exploration of adsorption kinetics on nanotubes is the focus of the present work. Four possible binding sites have been identified in nanotube bundles: (i) internal sites (only accessible if the nanotube caps are removed, and the ends of the tubes are unblocked); (ii) interstitial channels (ICs); (iii) “grooves”, on the outer surface of the bundles; and (iv) the outer surface of those individual nanotubes lying at the external surface of the bundles (which we call here “outer surface” sites). Considerable effort has been devoted to determining which among these groups of sites are occupied by adsorbate molecules.1,8-11 A consensus is emerging that regards adsorption on closed-ended nanotubes as taking place on the grooves, the outer surface sites, and on large-diameter stacking-defectinduced ICs. The more numerous, smaller diameter, non-defect ICs, on the other hand, are inaccessible for adsorption. The interior sites of as-produced nanotubes are generally viewed as inaccessible for adsorption3 (there are, however, differing views on this point).12 Purified (i.e., chemically processed) nanotubes have at least a fraction of their ends uncapped. However, access to the interior space of these tubes is generally blocked by functional groups that get attached to the nanotube surfaces and ends as a result * To whom correspondence should be addressed. Tel.: (618) 453-1053. Fax: (618) 453-1056. E-mail:
[email protected].
of the purification process. These chemical groups have to be removed in order to make the interior space of the nanotubes accessible for adsorption.3 By contrast to what occurs for close-ended nanotubes, to date there is little agreement between the reported values for various experimental quantities for systems formed by gases adsorbed on treated, uncapped nanotubes. Some studies report isosteric heats for adsorption at the interior of uncapped nanotubes that are smaller than those found on the grooves of close-ended bundles, for the same adsorbate.13,14 Other reports find isosteric heat values at the interior of open-ended nanotubes which are higher than the highest values measured on closed nanotubes.15,16 Finally, there are reports in which the isosteric heat values corresponding to grooves and interior sites are found to be roughly comparable.17 We propose below a possible explanation for the wide variety of results for these systems that is based on the very different adsorption time scales that are present in bundles of closed-ended nanotubes and those present in uncapped and unblocked nanotubes. The rate at which the different sites are accessed and occupied provides important information regarding the behavior and growth of adsorbed systems on the nanotube bundles, and it can also provide information on which sites are being occupied. Adsorption experiments provide us a mean to probe adsorption kinetics by allowing us to monitor the time needed to reach equilibrium after a dose of gas is added to the cell containing the nanotube sample. These measurements also determine how the equilibration time changes as a function of monolayer fractional coverage. Here we present the results of adsorption and equilibration time measurements on as-prepared SWNTs and on chemically processed SWNTs. The effect of chemically uncapping the SWNTs was observed in two ways: (1) through the difference in the sizes of the substeps present in the monolayer adsorption data, and (2) through the very significant differences in the equilibration times measured in these two cases. Theoretical results for adsorption kinetics on nanotube bundles obtained very recently allow us to analyze and explain the kinetic behavior observed in the experiments.18 II. Experimental Methods The as-produced single-walled nanotubes (SWNTs) used in the argon and methane adsorption measurements were purchased
10.1021/jp072786u CCC: $37.00 © 2007 American Chemical Society Published on Web 08/15/2007
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TABLE 1: Types of Single-Walled Nanotube Samples and Gases Used in the Isotherm Measurements sample
treatment
type
wt (g)
gas
temp (K)
SWNTs SWNTs SWNTs SWNTs
as prepared acid treated as prepared as prepared
electric arc electric arc electric arc HiPco
0.131 0.051 0.130 0.132
argon argon methane methane
77 77 77 77
from Carbon Solutions Inc. and Carbon Nanotechnologies Inc. Detailed information on the different SWNT samples is provided in Table 1. SWNTs, produced by the electric arc method, were chemically treated by us by sonicating the nanotubes in a mixture of H2SO4 + HNO3 (3:1) for 24 h. This is a well-documented method to cut open the nanotubes and to uncap their ends.4 After the chemical treatment, the nanotubes were heated at 600 °C, under a vacuum of 10-6 Torr, for 6 h. This process is effective in the removal of the functional groups that get attached to the surface and to the uncapped ends of the nanotubes as a result of the chemical treatment.3 Transmission electron microscopy (TEM) (Hitachi, H-7100FA, 100 kV) was used to examine the wall structure of the SWNTs before and after subjecting the material to chemical treatment. The samples used in the microscopy study were sonicated in methanol for about 1 h. After sonication, droplets containing the nanotubes were deposited on a holey carbon grid, and the alcohol was allowed to evaporate. For the adsorption measurements, the nanotube samples were placed inside a copper cell and evacuated to a pressure below 10-6 Torr for a period of at least 48 h at room temperature prior to the performance of the adsorption measurements. The in-house built, automated, volumetric apparatus used for the adsorption measurements has been described previously.19 All the pressures were measured using 1, 10, and 1000 Torr fullscale capacitance pressure gauges. III. Results and Discussion In parts a and b of Figure 1 we present the results of the volumetric adsorption isotherm measurements of argon (77 K) on as-produced and on chemically treated SWNTs, respectively. The amount of Ar adsorbed in cm3 Torr (1 cm3 Torr ) 58.8 nmol) is presented as a function of natural logarithm of pressure (in Torr). At a fixed temperature, in equilibrium, the logarithm of the pressure of the 3-D adsorbate vapor is directly proportional to the chemical potential of the adsorbed film. As a result, the occupation of groups of sites with different binding energies (and correspondingly different chemical potentials) manifests itself in an adsorption isotherm through the presence of (rounded) substeps. In Figure 1a, the lower-coverage substep, indicated by an arrow, corresponds to adsorption occurring in grooves and in any of the largest diameter defect-induced ICs that may be present in the bundles; the higher-coverage substep, also indicated by an arrow, corresponds to the adsorption occurring on the external surface of the SWNTs, i.e., the outer surface sites. These results are consistent with previous experimental reports and simulations for argon adsorption on closeended SWNTs.14,20 According to computer simulations, at this temperature adsorption on closed-ended tubes starts at the lowest pressures with the formation of a single line of atoms on the grooves (binding energy ∼ 1500 K), this is followed by the completion of a monolayer consisting of five additional lines symmetrically located on the sides of the grooves, on the outer
surface sites.20 The binding energy on these outer surface sites is, roughly, 50% smaller than that on the grooves. Figure 1b displays adsorption results for argon on chemically treated SWNTs. Here again we observe the presence of two distinct substeps in the monolayer data. Interestingly, though, in this case there is a marked difference in the size of the lower substep: it is almost twice as large as the one observed for the untreated SWNTs (compare Figure 1b with Figure 1a). More importantly, this lower-pressure (high-binding-energy) step is now larger than the higher-pressure step. This is a reversal of what occurs for closed-ended tubes. Most of the adsorption in the first-layer portion of the isotherm for the chemically treated nanotubes is no longer taking place on the outside surface of individual nanotubes in the periphery of the bundle (as was the case for as-produced tubes), but it is occurring on higherbinding-energy sites. This observation is consistent with theoretical predictions for the binding energy inside the nanotubes that find a value of about 1400 K for this quantity; this is somewhat smaller than the binding on the grooves, but considerably larger than that on the outer surface sites.20,21 In a previous study of argon adsorption on open-ended SWNTs, the first large substep was identified as simultaneous adsorption of argon occurring in grooves, internal sites, and large ICs present in the SWNTs.17 Our adsorption results (as well as the results for equilibration time measurements that we describe below) agree with this scenario. The higher-pressure substep, which is almost universally attributed to adsorption on the outer surface of the nanotubes, is now not as pronounced as the corresponding feature for the as-produced nanotubes. Chemical treatments are known to damage the wall structure of the SWNTs.22 The TEM studies we conducted on the chemically treated SWNTs confirm this effect. Close inspection of Figure 2b shows that, due to the chemical treatment, the external surface is more amorphous. By contrast, for the inner wall, the structural integrity is maintained even after the chemical treatment. This change in the structure of the external wall can explain the broadening observed for the isotherm featured when compared to the corresponding one in as-produced nanotubes. In Figure 1c,d we show linear plots for argon adsorption on as-produced and on chemically treated SWNTs. In order to get an estimate of the monolayer capacity for these two cases, we used the so-called “point B” method.23 In this method (Figure 1c), a straight line is plotted along the slanted plateau region of isotherms (i.e., the region of relatively slow increase in coverage present at intermediate pressures in the isotherm following the very steep increase in coverage at low pressure). The lowest point where the linear segment separates from the isotherm (the “point B”) provides a rough estimate for monolayer capacity. For the as-produced SWNTs, we found a monolayer capacity of 48 000 cm3 Torr/g, while for the chemically treated SWNTs, the monolayer capacity was found to be 55 000 cm3 Torr/g. This corresponds to an increase of 15% in specific surface area for the chemically treated SWNTs. The relatively small increase in the specific surface area suggests that only a fraction of SWNTs got their ends opened with the combined result of chemical and vacuum heat treatments. We also note that for the chemically treated SWNTs (Figure 1d), the slope of the slanted plateau in the isotherm is much smaller than the slope of the corresponding region in the untreated tubes (compare the slopes of the linear portions of parts c and d of Figure 1). A small slope in the isotherm for this region is characteristic of adsorption on a microporous substrate. Isotherms like the one in Figure 1d are classified as
12982 J. Phys. Chem. C, Vol. 111, No. 35, 2007
Rawat et al.
Figure 1. (a) Full monolayer isotherm for Ar on as-produced SWNTs at 77 K. The coverage (amount adsorbed) is presented as a function of the natural logarithm of pressure (in Torr, X axis). (b) Full monolayer isotherm for Ar on chemically treated SWNTs at 77 K. The choice of axes is the same as for part a. (c) Linear plot of the isotherm shown in part a. The coverage is presented as a function of the pressure (in Torr, X axis). The arrow (corresponding to “point B”) indicates monolayer completion for as-produced SWNTs. (d) Linear plot of the isotherm shown in part b. The choice of axes is the same as for c. The arrow indicates monolayer completion for the chemically treated SWNTs.
Figure 2. (a) TEM image of as-produced SWNTs (5 nm). (b) TEM image of acid- treated SWNTs (50 nm). Note that individual tubes are readily resolvable near the surface of the bundles in part a. This is not the case in part b, probably due to the formation of an amorphous region as a result of the chemical treatment.
type I (IUPAC). The change in isotherm shape to type I for the chemically opened nanotubes correlates well with the observation made before that a smaller fraction of the adsorption is now occurring at the outer surface of the individual tubes and that the relative amount adsorbed on the higher-energy sites is greater on the treated tubes. (The sharp coverage increase seen in both isotherms at higher pressures, near 200 Torr, corresponds to the system reaching the saturated vapor pressure.) In order to study the kinetics of adsorption for as-produced and chemically treated SWNTs, we monitored the equilibration times, i.e., the time required for the pressure to equilibrate after a dose of gas is introduced into the sample cell. These
measurements were performed for each coverage value along an adsorption isotherm. Figure 3a shows the variation of pressure with time for argon on as-produced SWNTs for different monolayer fractional coverages (the coverage increases from bottom to top in this figure). Inspection of the different pressure evolution curves reveals that the time for the pressure to equilibrate becomes progressively smaller with increasing coverage. For fractional coverages near 0.1 layer, the minimum equilibration time is approximately 60 min. This equilibration time decreases to less than 10 min when the monolayer fractional coverage increases to 0.75, as shown in Figure 3b.
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Figure 3. Adsorption kinetics of Ar on as-produced SWNTs at 77 K. (a) Pressure as a function of time as different values of the equilibrium coverage are approached. The fractional equilibrium coverages (relative to monolayer coverage obtained from Figure 1) corresponding to each curve are shown on the left; they increase from the bottom curve to the top one. (b) Equilibration time, derived from each curve in part a, as a function of fractional coverage.
Figure 4. Adsorption kinetics of Ar on chemically treated SWNTs at 77 K. (a) Time dependence of the pressure as equilibrium is approached. The fractional coverages increase from bottom to top. The inset shows the corresponding curves for the smallest coverages. (b) Equilibration time plotted as a function of fractional coverage from the curves shown in part a.
We performed similar equilibration time measurements for the chemically treated tubes. Figure 4a shows plots of the variation of the pressure with time for argon adsorption on chemically treated SWNTs. There are important differences between the results obtained for as-produced and for chemically treated nanotubes. Perhaps the most striking one among them is the difference between the magnitudes of the times required for the pressure to reach equilibrium for the two sets of nanotubes. For the chemically treated SWNTs the equilibration times were at least 2 orders of magnitude larger than the ones found for the as-prepared SWNTs. The second important difference is the presence of an apparent non-monotonic evolution of the equilibration times for the chemically treated nanotubes. There is an apparent increase in the equilibration time with increasing fractional coverages between 0 and 0.6 (see Figure 4b). This trend is opposite to the one observed for argon on as-prepared SWNTs. For the chemically opened tubes the equilibration times decrease with increasing fractional
coverage for coverages beyond n ) 0.6, a behavior that is consistent with the results observed for as-prepared SWNT (Figure 4a). We note that an analogous apparent increase in equilibration times was reported for Ar on open-ended nanotubes,13 which was attributed to an increase in dose size. Next, we discuss our results in light of a recent theoretical study of adsorption kinetics on nanotube bundles.18 In that work, the adsorption kinetics on various regions of the bundle was explored by analyzing the time evolution of the coverage on one-dimensional chains of sites. Each chain is characterized by specific binding energies () and subjected to different adsorption dynamics. The system is allowed to evolve according to a kinetic Monte Carlo algorithm that was tested for simple cases with exact analytical models. Two kinds of dynamics were simulated to represent the adsorption on external and internal sites in a bundle: (1) In the case of adsorption on the exterior of the bundle, the chain is considered to be directly exposed to the gas, so that adsorption/desorption processes can take place
12984 J. Phys. Chem. C, Vol. 111, No. 35, 2007 at every site of the chain. That adsorption behavior is meant to reproduce the adsorption kinetics on grooves and outer surface sites. (2) In order to model the kinetics of the pore-like phases that could form either inside the tubes or in the ICs, adsorption/ desorption processes were allowed to occur only at the end sites of the chain, and occupation of the internal sites was achieved only by particle displacements along the chain. For given values of the ratio /kBT and of the pressure of the external gas, the equilibration times are obtained from the time evolution of the coverage as a function of the amount of gas eventually adsorbed at equilibrium.18 The behavior for adsorption on as-produced nanotubes observed in the experiments readily matches that seen in the simulations for adsorption occurring solely on external sites. In both cases one observes a decrease in the equilibration time with increasing coverage that approaches zero as monolayer completion is reached. As shown in ref 18, this decreasing trend is due to the fact that the adsorption rate is proportional to the external pressure, and higher pressures are needed to achieve larger equilibrium coverages. In the case of this simple type of external adsorption, the occupation of sites does not depend on diffusion processes; it is mainly a function of the value of the external pressure and the number of empty sites on the lattice. Under these circumstances, the equilibration time is expected to drop to very small values as the lattice becomes completely filled. The very significant difference in equilibration times observed in the experiments between the as-produced and chemically treated nanotubes is similar to the one seen in the simulations when contrasting adsorption exclusively on the external surface of the nanotube bundles (grooves and outer surface sites) and adsorption in the pore-like spaces (inside open-ended tubes and in the ICs).18 The origin of the time-scale difference in the simulations is the fact that in the first case adsorption can occur at any point along the nanotube bundle surface, while adsorption on the inside of a pore-like space can only occur at the ends of the pores (the adsorbed species can then migrate to the pore’s interior). This difference in adsorption mechanisms explains quite well our experimental data. In the case of the as-produced nanotubes, the high-binding-energy sites are the grooves (and some of the largest, defect-induced ICs which may be present; given the relatively small coverage interval spanned by the lower-pressure step, these latter can only constitute a small fraction of the highbinding-energy sites). Hence, for the as-produced nanotubes, most of the high-energy binding sites (grooves) are accessible throughout the surface of the bundles. This is not the case with the chemically treated nanotubes. For them, as we have seen, the high-energy binding sites are more numerous than the surface sites. These high-energy sites (which, in this case, are mostly on the interior of the open tubes) can only be accessed from the open ends of the tubes and not from everywhere along the side of the bundle. Our results provide clear experimental evidence of the existence of two very different time scales for adsorption processes in these materials depending on whether access to the tube interior is possible or not. The simulations found that for both the pore-like sites and the external sites the equilibration times varied inversely with coverage. Thus, according to the simulations, the slowest sites to fill up are the pore-like ones at low coverages. The computer simulations found that when adsorption is considered for a system in which there are a comparable number of adsorption sites with substantially different equilibration times (i.e., simultaneous adsorption on surface-like and pore-like sites)
Rawat et al. it is harder to extract a single equilibration time. Apparent “equilibrium time” values can be reached in this case that, in actual fact, do not correspond to equilibrium. Such situations occur when the equilibrium coverage on the shorter equilibration time sites is reached, while that in the longer ones is not. This is true especially at lower coverages, so little adsorption takes place in the pore-like sites during the time scale of equilibration on the external sites, such that the total coverage in this time scale appears not to change any more and equilibrium appears to have been reached when, in fact, it has not. We believe that the initial increase in the equilibration wait time with increasing fractional coverage seen for chemically treated SWNTs (Figure 4b) is an artifact explained by the apparent equilibration phenomenon we have just described. In the experiment, at low pressure, the highest-binding-energy sitessgrooves, large-diameter defect-induced ICs, and internal sitessget filled simultaneously, but at very different rates. At low coverages, the rate of filling of internal sites is very small, and the attainment of equilibrium cannot be assured under practical experimental conditions. Specifically, in these lowcoverage regions the amount of gas adsorbed in the pore-like sites is very small, and, owing to the exceedingly long equilibration times and low pressures, the adsorption setup is working under severe limitations: (1) the pressure gauges are working close to their resolution limit, and (2) any minute amount of outgassing from the gas-handling system eventually ends up affecting the background pressure. As the pressure increases, atoms start to access more internal sites, in addition to the exterior sites, within the experimental times. Hence the “effective” equilibration time appears to increase with coverage. Eventually, all the high-energy external sites are filled and we measure only the equilibration times corresponding to the highenergy internal sites. This corresponds to the apparent maximum in the experimental data. As already has been stated, the simulations find that the pore-like sites have equilibration times that decrease with increasing coverage (just as the external sites do). The experiments start measuring this decrease in equilibration times beyond this point. We have noted before the wide range of values for the binding energies and isosteric heats of adsorption that have been reported for measurements on open-ended nanotubes in the literature. Our results for equilibration times suggest a possible explanation for this. Given that a careful examination of the times needed to reach equilibrium pressure had not been conducted prior to this study, and, given that there are two very different time scales for adsorption present which can easily mimic equilibrium, we propose that the variation in the reported values for the isosteric heats and binding energies on open tubes is the result of misjudging the attainment of equilibrium. In order to explore whether the adsorption kinetics depend on how the nanotubes were produced, we monitored the equilibration times for methane adsorbed on as-produced HiPco and on as-produced electric arc discharge nanotubes. Since neither of these samples were subjected to any treatment, the ends of the nanotubes are intact and the internal sites are not available for adsorption. Figure 5a shows a plot of the variation of the equilibration time as a function of fractional coverage for methane adsorbed on electric arc discharge produced SWNTs. Figure 5b shows a similar plot for measurements on HiPco nanotubes; both isotherms were measured at the same temperature, 77 K. We compare the waiting times needed for equilibration for two fractional monolayer coverages 0.2 and 0.8. For electric arc discharge SWNTs, the equilibration times were 97 min and 42
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Figure 5. Adsorption kinetics of methane (at 77 K) on two different samples of as-produced SWNTs. (a) Equilibration time as a function of fractional coverage on as-produced SWNTs, synthesized by the electic arc discharge method. (b) Equilibration time as a function of fractional coverage on as-produced SWNTs, synthesized by the HiPco method.
min, respectively; while for HiPco SWNTs, the equilibration times were 86 min and 35 min. The comparable values obtained indicate that adsorption kinetics does not depend on the method used to produce the nanotubes. The measurements for methane on these two different samples of closed-ended tubes exhibit the same trend as the ones for argon (Figure 3b). The longer equilibration times observed for methane at low coverages are due to its stronger binding to the external sites compared to argon. The simulations show that, for a given fractional coverage and temperature, the equilibration times increase with increasing binding energy because adsorption occurs at lower pressures for stronger binding. The lower pressures needed, in turn, decrease the adsorption rate. IV. Conclusions In conclusion, we have investigated the kinetics of gas adsorption on closed (as-produced) and open-ended (chemically treated) single-walled carbon nanotubes by monitoring the evolution of the equilibration time as a function of fractional coverage on both types of substrates. The equilibration time was found to depend on the geometrical accessibility of the adsorption sites. For chemically opened nanotubes (for which the interior sites are the most numerous ones) the waiting times were found to be at least 2 orders of magnitude larger than the ones for as-produced nanotubes (for which adsorption occurs almost exclusively on external sites). When both pore-like and external sites are available for adsorption, this large difference between equilibration times can easily lead to underestimating the total coverage through the misjudgment of the attainment of equilibrium. Conversely, carefully monitoring equilibration times provides clear evidence of the occurrence of internal adsorption. Our measurements are consistent with recent computer simulation results for physisorption kinetics on nanotube bundles. In agreement with prior studies on uncapped nanotubes, we found that the lower-pressure substep (corresponding to higher-binding-energy sites) was larger than that of the corresponding one for as-produced nanotubes, and larger, as well, than the substep corresponding to adsorption on the lower-binding-energy sites. This provides additional support to the proposed adsorption scenario in which sites with binding energies similar to those of the grooves become available as a result of the chemical treatment. In addition, for the chemically treated nanotubes, the higher-pressure (lower-binding-energy) substep was found to be not as pronounced as the one obtained for as-produced nanotubes. TEM analysis indicated that structural changes occur on the surface of the chemically treated nanotubes, thus providing an explanation for the observed
changes in the characteristics of the isotherms. Finally, we found that, for untreated nanotubes, the equilibration times are independent of the method used to produce the nanotubes. Acknowledgment. A.D.M. acknowledges support provided for this study by the National Science Foundation through Grant DMR-0089713 and by the Materials Technology Center of Southern Illinois University. Acknowledgment is also made to the donors of the American Chemical Society Petroleum Research Fund for partial support of this research (M.M.C.). References and Notes (1) For a review of recent literature on gas adsorption on carbon nanotubes, see: Migone, A. D.; Talapatra, S. In Encyclopedia of Nanoscience and Nanotechnology; Nalwa, H. S., Ed.; American Scientific Publishers: Los Angeles, CA, 2004; Vol. 4, pp 749-767. (2) Lee, H.; Kang, Y. S.; Kim, S. H.; Lee, J. Y. Appl. Phys. Lett. 2002, 80, 577. (3) (a) Kuznetsova, A.; Yates, J. T., Jr.; Liu, J.; Smalley, R. E. J. Chem. Phys. B 2000, 112, 9590. (b) Kuznetsova, A.; Mawhinney, D. B.; Naumenko, V.; Yates, J. T., Jr.; Liu, J.; Smalley, R. E. Chem. Phys. Lett. 2000, 321, 292. (4) Rinzler, A. G.; Liu, J.; Dai, H.; Nikolaev, P.; Huffman, C. B.; Rodriguez-Macias, F. J.; Boul, P. J.; Lu, A. H.; Heymann, D.; Colbert, D. T.; Lee, R. S.; Fischer, J. E.; Rao, A. M.; Eklund, P. C.; Smalley, R. E. Appl. Phys. A 1998, 67, 29. (5) Hirscher, M.; Becher, M.; Haluska, M.; Dettlaff-Weglikowska, U.; Quintel, A.; Duesberg, G. S.; Choi, Y. M.; Downes, P.; Hulman, M.; Roth, S.; Stepanek, I.; Bernier, P. Appl. Phys. A 2001, 72, 129. (6) Byl, O.; Liu, J.; Yates, J. T., Jr. Langmuir 2005, 21, 4200. (7) Stepanek, I.; Maurin, G.; Bernier, P.; Gavillet, J.; Loiseau, A.; Edwards, R.; Jaschinski, O. Chem. Phys. Lett. 2000, 331, 125. (8) Talapatra, S.; Zambano, A. Z.; Weber, S. E.; Migone, A. D. Phys. ReV. Lett. 2000, 85, 138. (9) Muris, M.; Dufau, N.; Bienfait, M.; Dupont-Pavlovsky, N.; Grillet, Y.; Palmari, J. P. Langmuir 2000, 16, 7019. (10) Muris, M.; Dupont-Pavlovsky, N.; Bienfait, M.; Zeppenfeld, P. Surf. Sci. 2001, 492, 67. (11) Fujiwara, A.; Ishii, K.; Suematsu, H.; Kataura, H.; Maniwa, Y.; Suzuki, S.; Achiba, Y. Chem. Phys. Lett. 2001, 336, 205. (12) Matranga, C.; Bockrath, B. J. Phys. Chem. B 2005, 109, 4853. (13) (a) Jakubek, Z. J.; Simard, B. Langmuir 2004, 20, 5940-5945. (b) Yoo, D. H.; Rue, G. H.; Seo, J. Y.; Hwang, Y. H.; Chan, M. H. W.; Kim, H. K. J. Phys. Chem. B 2002, 106, 9000. (14) (a) Krungleviciute, V.; Heroux, L.; Migone, A. D.; Kingston, C. T.; Simard, B. J. Phys. Chem. B 2005, 109, 9317. (b) Talapatra, S.; Rawat, D. S.; Migone, A. D. J. Nanosci. Nanotechnol. 2002, 2, 467. (15) Babaa, M. R.; Stepanek, I.; Masenelli-Varlot. K.; Dupont-Pavlovsky, N.; McRae, E.; Bernier, P. Surf. Sci. 2003, 531, 86-92. Note: while this article does not report on isoteric heats at the interior of the nanotubes, the isotherms shown correspond to stronger binding occuring on sites on the inside the open nanotubes. (16) Yoo, D. H.; Rue, G. H.; Chan, M. H. W.; Hwang, Y. H.; Kim, H. K. J. Phys. Chem. B 2003, 107, 1540-1542. (17) Rols, S.; Johnson, M. R.; Zeppenfeld, P.; Bienfait, M.; Vilches, O. E.; Schneble, J. Phys. ReV. B. 2005, 71, 155411.
12986 J. Phys. Chem. C, Vol. 111, No. 35, 2007 (18) Burde, J. T.; Calbi, M. M. J. Phys. Chem. C 2007, 111, 5057. (19) Shrestha, P.; Alkhafaji, M. T.; Lukowitz, M. M.; Yang, G.; Migone, A. D. Langmuir 1995, 10, 3244. Wolfson, R. A.; Arnold, L. M.; Shrestha, P.; Migone, A. D. Langmuir 1996, 12, 2868. (20) Gatica, S. M.; Bojan, M. J.; Stan, G.; Cole, M. W. J. Chem. Phys. 2001, 114, 3765.
Rawat et al. (21) Stan, G.; Bojan, M. J.; Curtarolo, S.; Gatica, S. M.; Cole, M. W. Phys. ReV. B 2000, 62, 2173. (22) Monthioux, M.; Smith, B. W.; Burteaux, B.; Claye, A.; Fischer, J. E.; Luzzi, D. E. Carbon 2001, 39, 1251. (23) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1967; pp 54-56.
