J. Phys. Chem. C 2010, 114, 20173–20177
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Adsorption Characteristics of Linear Alkanes Adsorbed on Purified HiPco Single-Walled Carbon Nanotubes D. S. Rawat, T. Furuhashi, and A. D. Migone* Department of Physics, Southern Illinois UniVersity, Carbondale, Illinois 62901, United States ReceiVed: August 30, 2010; ReVised Manuscript ReceiVed: October 19, 2010
We present the results of adsorption isotherm measurements for propane films on single-walled carbon nanotubes, for coverages limited to the first layer. Isotherms were measured at six different temperatures between 190 and 280 K. The results obtained are compared to our previous findings for ethane and butane on the same substrate. This comparison allows us to explore the evolution of the characteristics of adsorbed films as a function of alkane chain length. We found that the same groups of binding sites are available for all three molecules. We also found that there is a gradual smearing of the high-pressure substep in the monolayer data that occurs as the isotherm temperature is increased. This smearing becomes more pronounced as the length of the adsorbed molecule increases. In addition, for each value of the fractional coverage, we observed an increase in the value of the isosteric heat of adsorption as the length of the adsorbate increases. I. Introduction The unique structural and geometric features of carbon nanotubes have led to numerous experimental and theoretical studies of systems formed by gas adsorption on these materials.1 From a fundamental perspective, these systems offer the possibility of experimentally realizing matter in one dimension (1D).2-13 From a practical viewpoint, applications have been suggested in gas separation and storage. The majority of adsorption studies on carbon nanotubes has been conducted with spherical adsorbates. Adsorption studies of linear molecules and, in particular, of linear hydrocarbons are more limited,14-21 despite their importance in the petrochemical industry. One attractive feature of conducting adsorption studies of linear hydrocarbons on nanotube substrates is that these systems allow us to explore the evolution of the adsorption characteristics as a function of the length of the carbon chain. Recent simulation studies have indicated several interesting features of linear molecules when adsorbed on single-walled carbon nanotubes.14,15 Jiang et al.14 compared adsorption and gas mixture separation of linear and branched alkanes on single-walled carbon nanotubes using configurational-bias Monte Carlo simulation. For pure linear alkanes, near the zero coverage limits, longer alkanes were found to adsorb preferentially. On the other hand, as the coverage increases close to saturation, the amount of smaller molecules increases relative to the longer ones. This is due to the size entropy effect, in which small molecules can fit into partially filled pores more easily than larger ones. In addition, the pure linear isomer was found to adsorb to a greater extent than its pure branched counterpart. Cruz et al.15 described the thermodynamics of adsorption of light alkanes and alkenes on single-walled carbon nanotube bundles using configurationalbias grand canonical Monte Carlo simulation. Funk et al.16 studied the adsorption dynamics of n-/isobutene on closed-end and open-end single-walled carbon nanotubes using molecular beam scattering adsorption probability measurements. In another study,17 gravimetric techniques were employed to determine the * To whom correspondence should be addressed. E-mail: aldo@physics. siu.edu.
adsorption capacities for organic vapors of purified electric arcproduced nanotubes and HiPco-produced single-walled carbon nanotubes. A single-walled carbon nanotube can be viewed as a single graphene sheet rolled over itself, closed seamlessly, and capped with hemispherical ends.22,23 Individual nanotubes assemble into bundles. Four possible binding sites for adsorption have been identified on these bundles: (i) internal sites of nanotubes, which are available for the only case of uncapped and unblocked ends of nanotubes (ii) the space between the individual nanotubes at the interior of the bundle, that is, the interstitial channels (ICs); (iii) the convex “valley” formed in the space where two neighboring tubes, on the periphery of the bundle, get closest together, that is, the “grooves”; and (iv) the cylindrical outer surface of the individual nanotubes that lie at the external surface of the bundles or outer surface sites. As-produced SWNTs have capped ends; consequently, their interior sites are not accessible for adsorption.24,25 Opening the carbon nanotubes requires subjecting them to extreme treatments (for example, using harsh chemical treatment).26 A consensus view has emerged that, for closed-ended nanotubes, the grooves, outer surface sites, and a small number of large-diameter, defectinduced ICs are the groups of sites that are available for adsorption on the SWNT bundles.2-13 As each group of binding sites is characterized by a distinct value of binding energy, these different energies give rise to substeps in the adsorption isotherm. Here, we present the results of an adsorption study of propane on purified HiPco SWNTs. The measurements were conducted as part of a broader investigation devoted to exploring how the characteristics of adsorbed linear hydrocarbon films evolve as a function of carbon chain length. Specifically, we studied how the different adsorption substeps change as the length of the adsorbed molecule increases. For the current study, we compared the results of propane adsorption on purified HiPco SWNTs to those that were obtained for methane, ethane, and butane on the same sample.19-21
10.1021/jp108242x 2010 American Chemical Society Published on Web 11/09/2010
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Figure 1. Isotherms for propane adsorption on single-walled nanotubes at 190, 200, 220, 240, 260, and 280 K (isotherm temperatures increase from left to right). The coverage, on the Y axis in cm3 Torr (1 cm3 Torr at 273 K is equal to 3.54 × 1016 molecules), is presented as a function of the natural logarithm of pressure in Torr (X axis).
Rawat et al.
Figure 2. Derivative of the amount adsorbed as a function of pressure for data measured at four temperatures (this quantity is proportional to the isothermal compressibility of the film). The broad peaks, shown by the arrows, indicate the presence of two distinct groups of binding energy sites. The lines encompass the region where the higher-pressure substep is present.
II. Experimental Section The purified single-walled carbon nanotube sample used in these measurements was purchased from CNI (Carbon Nanotechnology Incorporated). The nanotubes were synthesized using the HiPco process and subsequently purified by the manufacturer. The reported purity of the sample is 91%. Because these nanotubes were not subjected to any harsh acid cutting treatments, we expect that the ends of the nanotubes are either capped or blocked. Therefore, for current study, the internal sites of the nanotubes are not accessible for adsorption. The mass of the nanotubes used was 0.325 g. The sample was placed inside a copper cell and was maintained under a pressure below 10-6 Torr for at least 48 h, at room temperature, prior to the performance of the adsorption measurements. The in-house built, automated, volumetric apparatus used for these adsorption measurements has been described previously.27 All the pressures were measured either by 1, 10, or 1000 Torr fullscale capacitance pressure gauges. Complete monolayer adsorption isotherms for propane were conducted at 190, 200, and 220 K. Three additional lowcoverage isotherm measurements, at 240, 260, and 280 K, were performed to obtain an estimate for the binding energy. III. Results and Discussion Adsorption Features. Full monolayer isotherms were performed to identify the features of the monolayer films of propane adsorbed on SWNTs and, in particular, the different sites on the nanotube bundles that are available for adsorption to these molecules. In Figure 1, we present adsorption isotherms of propane taken at six different temperatures between 190 and 280 K. The amount adsorbed (in units of cm3 Torr, with 1 cm3 Torr at 273 K ) 3.54 × 1016 molecules) is plotted as a function of the natural logarithm of pressure (in Torr). The presence of two rounded substeps in Figure 1, marked by the arrows, indicates the presence of at least two different sets of binding energy sites in the nanotube bundles. Because each group of binding energy sites has a significantly different energy, at low temperatures, the formation of a film on these sites takes place at a nearly constant value of the chemical potential (or, equivalently, of the pressure). Consequently, each substep in the monolayer isotherm corresponds to adsorption occurring on a distinct group of binding energy sites on the
substrate. The number of quasi-vertical substeps in the monolayer identifies the number of distinct binding energy sites present on the substrate. The results shown in Figure 1 for propane are similar to our previous findings for ethane and butane adsorption on the same sample. In our previous studies, the lower substep was assigned to adsorption occurring on the grooves and in the small number of large-diameter, defect-induced, ICs. The higher-pressure substep was attributed to adsorption occurring on the external surface of the nanotube bundles. The same interpretation holds for the current data for propane adsorbed on nanotube bundles. Although all three alkane molecules exhibit similar characteristics in the adsorption isotherms, there are some important distinguishing features present in each individual case. We will discuss them in the rest of this paper. Change in the High-Pressure Substep with Alkane Molecular Size. An interesting feature present in the data is the gradual smearing of the high-pressure substep with increasing temperature. As is evident from the isotherms displayed in Figure 1, the high-pressure substep, which is clear for 190 K, becomes impossible to discern in the isotherm measured at 240 K. Figure 2 shows a plot of the isothermal compressibility of propane films adsorbed on SWNTs at four different temperatures. The two arrows, corresponding to broad peaks in the compressibility, again indicate the presence of two different groups of binding energy sites in the nanotube bundles. The gradual smearing of the high-pressure substep in the isotherms appears in this graph as a gradual disappearance of the compressibility peak corresponding to it. We also studied regions corresponding to this higher-pressure substep in our previous exploration of ethane and butane adsorption on SWNTs. For ethane, this higher-pressure substep was visible over a wide range of temperatures (between 103 and 170 K). By contrast, in the case of butane, the same higherpressure substep was clearly identifiable only at the two lowest temperatures we studied (180 and 190 K). This result suggests that it would be of interest to compare the evolution of this feature for all the alkanes we have studied, in order to determine and understand the dependence of the higher-pressure substep on the size of the adsorbed molecule. The data obtained on the various alkanes had to be measured at different absolute temperatures. This was necessary because
Linear Alkanes Adsorbed on Purified HiPco SWCNTs
Figure 3. Monolayer isotherms, for the same scaled temperature, for methane (99 K), ethane (160 K), propane (190 K), and butane (220 K) adsorbed on single-walled carbon nanotubes. The scaled temperatures were determined by dividing the isotherm temperature by the bulk critical temperature (the data presented in the figure correspond to Tisotherm/Tcritical ∼ 0.5). The fractional coverage (Y axis) is presented as a function of the natural logarithm of the pressure in Torr (X axis). The bottom arrows (A) indicate the high binding energy substeps, whereas the top arrows (B) indicate the low binding energy substeps in the monolayer isotherms. (Note that, for the sake of visual clarity, the values of the pressure for the methane isotherm have been shifted to the left from the original data.)
the saturated vapor pressures of different alkanes are vastly different at the same temperature. Similarly, the pressures measured at the same temperature and fractional coverage are vastly different for the different alkanes used. As a result, pressures corresponding to an isotherm feature, which are easily measurable for a given alkane at one temperature, lie beyond the measuring capacity of our experimental setup at the same temperature for a different alkane. To study the evolution of an isotherm feature as a function of alkane chain length, we need to scale the isotherm temperatures. In this study, we scaled the temperatures by dividing the isotherm temperatures by the respective bulk critical temperatures for the alkanes studied. The approach used here is similar to that used in arriving at the law of corresponding states. We, thus, compared adsorption isotherms for alkanes taken at a nearly constant value of the relative temperature, that is, Tisotherm/Tcritical, where Tisotherm is the temperature at which the isotherm was measured and Tcritical is the bulk critical point for the alkane. We used a value of 0.5 for this ratio for the various alkanes studied. Figure 3 shows semilogarithmic plots of isotherms for methane (99 K), ethane (160 K), propane (190 K), and butane (220 K). As can be seen, the lowest-pressure substep (A) is clearly distinguishable for all four alkanes. The higher-pressure substep (B), however, is barely resolvable for the case of the butane isotherm, and it is also quite difficult to resolve for the propane isotherm. We believe that this smearing of the higherpressure substep with increasing carbon chain length has the same origin as the size entropy effect. Experimentally, the size entropy effect is realized when a 3D pore, a 2D surface, or a 1D line is filled with linear molecules. Linear molecules are not able to pack into these spaces as efficiently as spherical molecules are. As a consequence, there are unoccupied sites left. This effect becomes more pronounced and important as the total space available decreases, that is, as more molecules occupy the volume, surface, or line. In a previous study of comparative adsorption of alkanes on zeolites and on silicalite,28,29 the role of three different entropy
J. Phys. Chem. C, Vol. 114, No. 47, 2010 20175 effects was explored to estimate the saturation capacities of linear and branched alkanes. The selectivity in the saturation coverage values was attributed to the difference in the sizes of the adsorbed molecules; that is, near saturation loadings, the vacant sites in the substrate are more easily filled up with the smaller molecules than with the larger ones. Similarly, Jiang et al.14 compared the sorptive capacities of linear alkane mixtures limited to the interior sites of the nanotube bundles. At low pressures, considerable adsorption for a long molecule, and almost no adsorption for a short molecule, was found. The difference was attributed to the larger number of interaction sites for the longer molecule. However, as the pressure was increased, the short alkane molecule was found to adsorb more. This change in the adsorption behavior was explained by taking the size entropy effect into account; size entropy manifests itself at higher coverage (or, equivalently, at larger pressure values). For studies involving single-component alkane adsorption, a similar conclusion can be drawn by comparing the sizes of the adsorption substeps in the monolayer isotherm (i.e., the saturation coverages corresponding to each group of sites) and, also, by comparing the characteristics of corresponding adsorption features. As was discussed in our previous work on butane adsorption on purified HiPco single-walled carbon nanotubes,20 the smearing of the higher-pressure substep is related to the changing contribution from the size entropy to the free energy (F ) U - TS) of the adsorbed film as the coverage increases. Longer molecules are not able to pack on the surface of the bundles as efficiently as shorter or spherical molecules (this result is clearly visible in Figure 3 when we compare the corresponding higher-pressure substeps for methane and butane isotherms). Consequently, as the size of the alkane molecule increases, an increasing fraction of the surface of the bundles remains uncovered. There will be a change in the entropy of the film; this change is related to the size entropy effect. In Figures 1 and 3, the horizontal axis is proportional to the chemical potential of the film. If the entropy per particle remains fairly constant as the coverage increases, the chemical potential will remain fairly constant, as well, when a group of sites of similar binding energies is being filled. This, in turn, will result in the formation of a well-defined substep, as is the case for methane and ethane. But, if the entropy per particle does not remain essentially constant, then the chemical potential will not be essentially constant either, and no substep will result as the similar binding energy sites get filled. Experimentally, this will appear as a smearing of the isotherm substep. This type of smearing (that has the same origin as the size entropy effect) will be more prominent for weaker groups of binding energy sites (that is, for the higher-pressure substeps, in the present case). The more homogeneous term in the free energy (i.e., the U in F ) U - TS) that is due to the interaction energy between the adsorbate and the substrate will make a smaller contribution to the free energy over a group of weaker sites. Thus, changes in the entropy will have a greater impact on the free energy. Consequently, the smearing effect will be more noticeable for these groups of sites than for sites with stronger binding energy. A similar reasoning can also be applied to explain the relative sharpness (compare, for example, the slopes of methane and butane substeps) observed in the case of the lower-pressure (higher binding energy) substep (see Figure 3) as the size of the alkane molecules increases. With the increase in the size of the adsorbed molecule, the strength of interaction between the adsorbate and the substrate increases. So, in this case, the free energy of the adsorbed film, for the coverage region in the lower-
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pressure substep, is controlled by the strength of the interaction between the adsorbate and the substrate. As the size of the adsorbed molecules increases from methane to butane, this interaction increases, and hence, increased sharpness in the lower-pressure substep is observed. In addition to a gradual smearing of the higher-pressure substep, another manifestation related to the size entropy effect is also observed in single-component isotherms, when the specific surface area of the same substrate is determined using adsorbates of progressively increasing sizes. In a previous study of the determination of specific surface areas of purified HiPco SWNTs with alkanes, we used the “Point B” method to calculate this quantity using methane, ethane, propane, and butane isotherms.21 We found a progressively decreasing value for the specific surface area of the same substrate as the alkane size increased from methane (690 m2/g) to butane (580 m2/g).21 The decrease in the specific surface area of the substrate was attributed to the increasing fraction of substrate left uncovered by adsorption of larger molecules and by the inability of traditional methods, such as the Point B and the BET equation, to account for such an effect. By contrast, near constant values of the specific surface areas were obtained when a modified equation was employed to calculate this quantity. The modified equation specifically takes into account the multisite occupancy, which gives rise to this phenomenon, related to the size entropy effect.21 Increase in Binding Energy with Increasing Alkane Length. At least two previous theoretical studies of linear hydrocarbon adsorption on SWNTs have predicted an increase in the binding energy with increasing carbon chain length.14,15 Both simulation studies employed single-walled carbon nanotubes with open ends, which are different from the closed-ended SWNTs used for the current study. However, the basic argument behind this observation, namely, that the longer alkane molecule is in contact with more adsorption sites and this results in an increase in the value of the isosteric heat as the size of the adsorbed molecule increases, holds true for both simulations as well as for our experiments. One of the simulation studies14 considered only the effect of adsorption at the interior of the nanotubes and neglected surface adsorption contributions (in essence, modeling the bundle as infinite). The other simulation study considered adsorption on both surface sites (grooves and external surface), as well as on interstitials and interior sites. This second study can be more directly compared to our experiments, which almost exclusively probe adsorption behavior on the surface sites. The isosteric heat of adsorption, qst, is the amount of heat released when a molecule gets adsorbed on a substrate. Experimentally, this quantity can be calculated from adsorption isotherms by using the following expression:30
( ∂ ∂Tln P )
qst ) kBT2
N
(1)
In eq 1, kB is the Boltzmann’s constant, N is the coverage of the adsorbed gas, ln P is the natural logarithm of pressure of the coexisting 3D gas, and T is the isotherm temperature. In practice, the partial differentials are approximated by differences between the logarithms of the pressures divided by the difference between isotherm temperatures, evaluated at the same fractional coverage. As was discussed in greater detail elsewhere,31 the relation between the low-coverage value of the isosteric heat of adsorption and the binding energy for an adsorbed film is given by
qst ) E + γkBT
(2)
In eq 2, E is the binding energy and γ is a constant that depends on the dimensionality of the adsorbed film (γ ) 2 in the case of one-dimensional adsorption). Repeated application of eq 1 helps evaluate the variation of the isosteric heat with fractional coverage. In the case of SWNTs, the value of the isosteric heat at low coverage also determines the value of binding energy for the most attractive site present on the nanotube bundles, through the application of eq 2. Our previous studies of ethane19 and butane20 adsorption on the same sample had yielded binding energy values on the grooves of 308 and 345 meV, respectively. These values were determined by applying eqs 1 and 2 to the data for these gases at the lowest experimental fractional coverages measured for them. If we use the same procedure for the present data for propane, we obtain a binding energy of 318 meV on the grooves for this gas. The experimental data for the isosteric heat for propane, ethane, and butane does not start from the same low value of the fractional coverage (see Figure 4 for propane and the corresponding figures in refs 19 and 20). To take this into account, we have used a polynomial fit of degree 2 to obtain isosteric heat values corresponding to near-zero fractional coverage for all three gases. Following this approach for propane, we find a value of 332 ( 21 meV for the binding energy on the groove sites of nanotube bundles (332 meV is the average between the zero coverage predicted by the polynomial fit and the lowest coverage measured in our experiments for this gas). Similarly, using this approach on the results obtained in our previous studies of ethane and butane, we find values of 309 ( 3 and 377 ( 32 meV, respectively, for the binding energies on the grooves. (These binding energy values are slightly higher than those we have reported previously for these gases19,20 because our previous results were obtained without any extrapolation to zero coverage.) There is previous indirect experimental evidence that the binding energy values of alkanes on the grooves increase with increasing alkane chain length. In a temperature-programmed desorption (TPD) study of pentane, hexane, heptane, octane, and nonane, the peak corresponding to desorption from the grooves was observed to move to higher temperatures as a function of increasing alkane chain length.32 Our experimental results can be directly compared to computer simulations for ethane and propane on the grooves of closed bundles.15 Computer simulation studies15 performed for adsorption on the outside of a bundle have found the following binding energies on the grooves: 188.4 and 238.2 meV for ethane and propane, respectively. Although these theoretical values are not in good quantitative agreement with our experimental values, they follow the same qualitative trend of increasing binding energy with increasing molecular length. By contrast, an experimental determination of the binding energy of butane on the groove sites (using thermally programmed desorption spectroscopy)16 obtained a value of 349.4 meV for this quantity, in good agreement with our thermodynamically determined value of 377 ( 32 meV. A direct comparison cannot be made with other computer simulation studies for alkanes14 because those calculations were made for essentially an infinitely large bundle in which contributions from adsorption at the surface were negligible. This is, evidently, not the case in our experiments. In addition to comparing our experimental results with those of computer simulations and other experiments, we can also compare
Linear Alkanes Adsorbed on Purified HiPco SWCNTs
J. Phys. Chem. C, Vol. 114, No. 47, 2010 20177 (a) There is a decrease in the relative sharpness of the higherpressure substep as the length of the alkane molecule increases. This effect (which, we believe, has the same origin as the size entropy effect for these systems) becomes increasingly dominant as the size of the adsorbed molecules increases. (b) For comparable coverages, the isosteric heat was found to increase as the chain length of the adsorbed alkane molecule increased. It was found to be highest for the case of butane and lowest for the case of ethane. With increasing chain length, the adsorbed molecules have access to more interaction sites with the substrate, resulting in the higher isosteric heat values. Our results were compared to those available from computer simulations and experiments for these systems. Acknowledgment. A.D.M. would like to acknowledge support provided for this study by the National Science Foundation through Grant No. DMR-0705077. References and Notes
Figure 4. Fractional coverage dependence of the isosteric heat of adsorption for ethane, propane, and butane (bottom to top). The isosteric heat in meV (Y axis) is presented as a function of monolayer fractional coverage, in layers (X axis).
the results of our measurements on SWNTs to those obtained on planar graphite. The energy corresponding to the lower-pressure substep on the bundles should be greater than that for propane on graphite (because there are more C neighbors near each propane molecule in the grooves or in the very few large-diameter, defectinduced ICs than there are on planar graphite), and the energy corresponding to adsorption on the exterior surface of an individual nanotube on the periphery of a bundle should be intermediate between those for the first and second layers of propane on planar graphite. For the current study, both of these expectations are realized. The binding energy for propane on the grooves is 332 ( 21 meV, whereas the value for the binding energy on graphite is 312 meV.33 Additionally, adsorption on the exterior surface of an individual nanotube on the periphery of a bundle corresponds to an isosteric heat of about 254 meV, whereas the isosteric heat at the beginning of the second layer of propane on graphite is 239.2 meV.33 Finally, in Figure 4, we present the coverage dependence of the isosteric heat for ethane, propane, and butane adsorbed on purified single-walled carbon nanotube bundles as a function of fractional coverage for the first layer. As is the case with other adsorbates, the isosteric heats are generally decreasing functions of coverage. However, for all coverages, the value of the isosteric heat increases as the carbon chain length in the alkane increases. We note that the two main results of this study, that is, smearing of the higher-pressure substep and an increase in binding energy value with the increase in carbon chain length, suggest that it would be of interest to perform similar measurements using alkanes longer than butane and also with alkanes that are not just linear, but branched, as well. IV. Conclusions We have studied adsorption isotherms of propane on purified HiPco SWNTs. Comparison of the results of our current study with our previous measurements of ethane and butane adsorption, on the same sample, indicates a strong dependence of the adsorption characteristics on the length of the adsorbed alkane molecules. The two most important findings of the current study are the following:
(1) For a review of recent literature on gas adsorption on carbon nanotubes, see: Migone, A. D. In Adsorption by Carbons; Tascon, J. M., Ed.; Elsevier: Amsterdam, The Netherlands, 2008. (2) Stan, G.; Bojan, M. J.; Curtarolo, S.; Gatica, S. M.; Cole, M. W. Phys. ReV. B 2000, 62, 2173. (3) Calbi, M. M.; Cole, M. W. Phys. ReV. B 2002, 66, 115413. (4) Gatica, S. M.; Hernandez, E. S.; Szybisz, L. Phys. ReV. B 2003, 68, 144501. (5) Calbi, M. M.; Gatica, S. M.; Bojan, M. J.; Cole, M. W. J. Chem. Phys. 2001, 115, 9975. (6) Shi, W.; Johnson, J. K. Phys. ReV. Lett. 2003, 91, 015504. (7) Talapatra, S.; Migone, A. D. Phys. ReV. B 2002, 65, 045416. (8) Talapatra, S.; Rawat, D. S.; Migone, A. D. J. Nanosci. Nanotechnol. 2002, 2, 467. (9) Rawat, D. S.; Heroux, L.; Krungleviciute, V.; Migone, A. D. Langmuir 2006, 22, 234. (10) Bienfait, M.; Zeppenfeld, P.; Dupont- Pavlovsky, N.; Muris, M.; Johnson, M. R.; Wilson, T.; DePies, M.; Vilches, O. E. Phys. ReV. B 2004, 70, 035410. (11) Muris, M.; Dupont- Pavlovsky, N.; Bienfait, M.; Zeppenfeld, P. Surf. Sci. 2001, 492, 67. (12) LaBrosse, M. R.; Shi, W.; Johnson, J. K. Langmuir 2008, 24, 9430. (13) LaBrosse, M. R.; Johnson, J. K. J. Phys. Chem. C 2010, 114, 7602. (14) Jiang, J.; Sandler, S. I.; Schenk, M.; Smit, B. Phys. ReV. B 2005, 72, 045447. (15) Cruz, F. J. A. L.; Mota, J. P. B. Phys. ReV. B 2009, 79, 165426. (16) Funk, S.; Burghaus, U.; White, B.; O’Brien, S.; Turro, N. J. J. Phys. Chem. C 2007, 111, 8083. (17) Agnihotri, S.; Rood, M. J.; Rostam-Abadi, M. Carbon 2005, 43, 2379. (18) Esteves, I. A. A. C.; Cruz, F. J. A. L.; Muller, E. A.; Agnihotri, S.; Mota, J. P. B. Carbon 2009, 47, 948. (19) Rawat, D. S.; Migone, A. D. Phys. ReV. B 2007, 75, 195440. (20) Rawat, D. S.; Furuhashi, T.; Migone, A. D. Langmuir 2009, 25, 973. (21) Rawat, D. S.; Migone, A. D.; Riccardo, J. L.; Ramirez-Pastor, A. J.; Roma, F. J. Langmuir 2009, 25, 9227. (22) Ajayan, P. M.; Ebbesen, T. W. Rep. Prog. Phys. 1997, 60, 1025. (23) Saito, R.; Dresselhaus, G.; Dresselhaus, M. S. Physical Properties of Carbon Nanotubes; Imperial College Press: London, 1998. (24) Kuznetsova, A.; Yates, J. T., Jr.; Liu, J.; Smalley, R. E. J. Chem. Phys. 2000, 112, 9590. (25) Kuznetsova, A.; Mawhinney, D. B.; Naumenko, V.; Yates, J. T., Jr.; Liu, J.; Smalley, R. E. Chem. Phys. Lett. 2000, 321, 292. (26) Rawat, D. S.; Calbi, M. M.; Migone, A. D. J. Phys. Chem. C 2007, 111, 12980. (27) Shrestha, P.; Alkhafaji, M. T.; Lukowitz, M. M.; Yang, G.; Migone, A. D. Langmuir 1995, 10, 3244. (28) Schenk, M.; Vidal, S. L.; Vlugt, T. J. H.; Smit, B.; Krishna, R. Langmuir 2001, 17, 1558. (29) Krishna, R.; Smit, B.; Calero, S. Chem. Soc. ReV. 2002, 31, 185. (30) Dash, J. D. Films on Solid Surfaces; Academic Press: New York, 1975. (31) Wilson, T.; Tyburski, A.; DePies, M. R.; Vilches, O. E.; Becquet, D.; Beinfait, M. J. Low Temp. Phys. 2002, 126, 403. (32) Kondratyuk, P.; Wang, Y.; Johnson, J. K.; Yates, J. T., Jr. J. Phys. Chem. B 2005, 109, 20999. (33) Zhao, X.; Kwon, S.; Vidic, R. D.; Borguet, E.; Johnson, J. K. J. Chem. Phys. 2002, 117, 7719.
JP108242X
