J. Phys. Chem. C 2007, 111, 18127-18134
18127
Infrared and Computational Studies of the Adsorption of Methanol and Ethanol on Single-Walled Carbon Nanotubes Mark D. Ellison,*,† Steven T. Morris,† Matthew R. Sender,† Jennifer Brigham,‡ and Nicholas E. Padgett‡ Department of Chemistry, Ursinus College, CollegeVille, PennsylVania 19426, and Department of Chemistry, Wittenberg UniVersity, Springfield, Ohio 45501 ReceiVed: August 7, 2007; In Final Form: September 18, 2007
The adsorption of methanol and ethanol on room-temperature single-walled carbon nanotubes (SWCNTs) has been investigated using Fourier transform infrared (FTIR) spectroscopy and two-level ONIOM calculations. Whereas methanol does not adsorb onto SWCNTs at room temperature, ethanol does adsorb molecularly under these conditions. The IR data show that the adsorbed ethanol is in an environment quite similar to that of liquid ethanol. Comparison to studies of clusters of ethanol indicates that adsorbed ethanol molecules likely form cyclic clusters of four or more molecules, and ONIOM calculations suggest that these clusters can form inside the largest-diameter nanotubes in the sample. The nature of ethanol adsorption has implications for previously reported measurements of changes in the electrical properties of SWCNTs upon ethanol adsorption.
Introduction The discovery of carbon nanotubes in 19911 unleashed a flurry of activity to investigate their physical and chemical properties. Of particular interest have been the electrical properties of pure nanotubes,2-7 functionalized nanotubes,8-10 and nanotubes to which gases are adsorbed.11-15 Because of their small size and their ability to conduct electricity, singlewalled carbon nanotubes (SWCNTs) show great promise for use in submicroscopic electronic devices. Although the electrical properties of individual SWCNTs are fairly well understood, much remains to be learned about the nature of the interactions of gas molecules with SWCNTs, particularly the means by which adsorbed molecules can alter the electrical properties of the nanotubes. The purpose of this study is 2-fold: to increase the basic understanding of the gas-nanotube interactions that lead to adsorption and to elucidate the aspect of these gasnanotube interactions that results in changes in the electrical properties of the nanotubes. A large number of systems involving the adsorption of gases onto SWCNTs have already been studied. Some gases, such as Xe,16-18 Kr,19 CH4,19,20 CF4,21 NO,22 and CO,23 interact weakly with SWCNTs and have not been studied in terms of altering the electrical properties of nanotubes. Turning to more strongly interacting gases, NH3 and NO2 were shown to affect the conductivity of a single SWCNT at room temperature.12 Infrared studies of the adsorption of each of these molecules on bundles of SWCNTs suggested that they adsorb preferentially in groove or interstitial sites of nanotube bundles.24 However, more recent studies suggest that NH3 adsorption is strongly dependent on the functional groups present on the nanotube walls.25 The adsorption of CO2 on SWCNTs has been well-studied and found to display fascinating behavior. Molecules of CO2 can become physically trapped in nanotube bundles26-28 and can also oxidize * Corresponding author. E-mail:
[email protected]. † Ursinus College. ‡ Wittenberg University.
SWCNTs to some extent.29 To our knowledge, however, no studies have been published that expressly investigate the effect of CO2 adsorption on the conductivity of nanotubes. More recently, the adsorption onto SWCNTs of substances that are liquids at room temperature has been investigated, both theoretically and experimentally. Agnihotri et al. measured and simulated the adsorption isotherms of hexane on SWCNTs.30 Bittner et al. investigated the adsorption kinetics of several organic compounds, including ethanol, on SWCNTs at room temperature.31 Also, Yang et al. determined the adsorption isotherms of methanol and ethanol on SWCNTs at room temperature.32 These studies establish that some volatile organic compounds adsorb on SWCNTs at room temperature. This fact was exploited in the construction of SWCNT field-effect transistors (SWCNT-FETs) whose drain current was found to be sensitive to the presence of a variety of alcohol vapors.15 In that study, methanol and ethanol vapors caused a change in the drain current of the SWCNT-FETs of 50% or greater, which led its authors to hypothesize that the alcohol vapors could be electrochemically adsorbing on or reacting with the SWCNT. Because these behaviors would show a distinct IR spectrum of the adsorbed molecules, we undertook the study of the IR spectra of methanol and ethanol adsorbed on SWCNTs at room temperature. Experimental Section Experiments were conducted in a fashion similar to previous studies24,33 that we have carried out on the adsorption of NH3, NO2, or H2O on SWCNTs, with a few important differences. HiPco SWCNTs were purchased from Carbon Nanotechnologies, Inc. Some were used as received, which is approximately 80% by mass carbon, with the remainder consisting of iron nanoparticles from the catalyst used in their production. For other studies, the SWCNTs were purified by being heated to 225 °C under a moist air stream for 5 h, sonicated in 6 M HCl, and then filtered and washed to remove the iron catalyst. This process has been shown to reduce the residual iron content to
10.1021/jp0763432 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/10/2007
18128 J. Phys. Chem. C, Vol. 111, No. 49, 2007 5% by mass or lower.34 Furthermore, infrared studies have demonstrated that this purification process is effective in removing the iron catalyst without adding functional groups to the SWCNTs.28 Spectroscopic analysis for iron in the acid filtrate indicated that this procedure removed about 15% of the prepurification mass as iron, which would leave about 5% iron by mass remaining in the samples. A UV/vis spectrum of the SWCNTs dispersed in DMF was collected after each purification (see Supporting Information). These spectra showed peaks arising from the van Hove singularities characteristic of individual nanotubes and agreed well with spectra obtained in the first reports of this purification method.34,35 Raman spectra collected after purification (see Supporting Information) are also consistent with those of purified SWCNTs. After purification, the SWCNTs were dispersed in toluene, and drops of the dispersion were applied to a tungsten screen (Buckbee-Mears, >85% optical transmission). Evaporation of the toluene left behind a fairly uniform mat of SWCNTs that adhered to the tungsten screen. The mat had a thickness of ∼1 mm and displayed an optical density of 0.75 that was essentially constant in the range of 1000-3700 cm-1. The screen was mounted in a stainless steel vacuum cube (base pressure of 4 × 10-7 Torr) with CaF2 windows. The SWCNTs were heated to about 800 K for 12 h to desorb gases and carboxylic functional groups from their surface.18 After being heated, the SWCNTs were allowed to cool to room temperature before being exposed to methanol or ethanol vapor. Neat methanol or ethanol (each Aldrich anhydrous, >99.5%) was placed in an oven-dried, sealed vial with a valve so that the vial could be connected to and disconnected from the system without exposing it or the system to ambient conditions. Before use, the methanol and ethanol were each subjected to several freeze-pump-thaw cycles. Because of the tubing between the alcohol vial and the screen on which the nanotubes were mounted, it was difficult to expose the nanotubes to a wellknown amount of alcohol vapor. The most reliable exposure method was found to be as follows: Alcohol in the vial was heated to increase its vapor pressure. The valve was opened, and the alcohol was continuously heated until small droplets were observed in the cube. At this point, the heating was stopped, and the valve was closed. Methanol or ethanol vapor was assumed to be in equilibrium with the droplets in the sample cube. The cube temperature was maintained at 298 K during the exposure, so the vapor pressures around the nanotubes were 127 Torr for methanol and 58 Torr for ethanol. The nanotubes were then exposed to that pressure of alcohol vapor for a fixed amount of time, after which the cube was purged with dry nitrogen for 10 min and then pumped out using an oil-free hand pump (Nalgene) capable of achieving a pressure of ∼10 Torr. The cube was then placed in the nitrogen-purged sample compartment of a Nicolet Research Series 6700 FTIR spectrometer. This exposure process was determined to give integrated IR peak areas within 5% of each other for the same exposure time. Typically, 1000 scans were acquired for each background and sample at a resolution of 4 cm-1. For each experiment, the background spectrum of the nanotube film was recorded before exposure to the alcohol vapor. The spectrometer computes the ratio between the sample and background spectra, effectively dividing out the absorbance of the carbon nanotube material. Thus, all peaks observed indicate species that are present in greater amounts in the sample scan (i.e., after exposure to methanol or ethanol) than in the background scan. Control experiments performed under identical conditions but without the SWCNTs showed no absorption, indicating that the alcohol
Ellison et al. was adsorbing on the SWCNTs and not on the tungsten screen or CaF2 windows. To help interpret the IR data, calculations were performed using the ONIOM multilayer method in Gaussian 03.36 Because ONIOM calculates two or three different regions at different levels of computation, it is possible to model a large system by applying a high-level quantum mechanical theory to the immediate region of interest and a lower level of theory, such as semiempirical or molecular mechanics, to the rest of the system. Such an approach has clear advantages for modeling nanoscale systems: It can avoid the computationally prohibitive step of applying high-level theory to a system with many atoms while still yielding accurate results. 37 Indeed, ONIOM calculations have been shown to successfully model both reactions and noncovalent interactions involving nanotubes.38-41 For our system, the SWCNT was simulated using a 11.26 Å long segment of a tube with H atoms to terminate the ends. Three different zigzag tubes were studied, namely, (10,0), (26,0), and (31,0), which have diameters of 8.53, 11.75, and 14.08 Å, respectively. Trials with different numbers of ethanol molecules, ranging from a single molecule to a cluster of four molecules, were carried out. Ethanol molecules were drawn approximately centrally inside the tube to minimize effects of interactions with the tube ends. The B3LYP density functional with either a 6-31G(d,p) or 6-31++G(3d,2p) basis set was applied to the ethanol molecules inside of the nanotube, whereas universal force field (UFF) molecular mechanics was applied to the nanotube. UFF is based on hybridization-dependent atomic radii, hybridization angles, van der Waals parameters, and effective nuclear charges.42 The combination of density functional theory (DFT) with UFF molecular mechanics for the SWCNT has been found to give good results for both reactions and noncovalent interactions.38,39 Indeed, UFF was found to give results superior to those using semiempirical methods.38 Therefore, we chose DFT/UFF ONIOM calculations for our system. Using Gaussian 03, the geometry that minimized the system’s energy was found, and then the vibrational frequencies were calculated. For comparison, the vibrational frequencies of an isolated ethanol molecule were also calculated using the B3LYP/6-31(d,p) and B3LYP/6-31++G(3d,2p) DFT methods. Results Infrared spectra of raw (unpurified) SWCNTs at room temperature exposed to methanol and ethanol for 5 min are shown in the Supporting Information and Figure 1, trace a, respectively. The methanol data show no peaks above the noise level, suggesting that methanol does not adsorb at room temperature above the detection level of about 0.006 mmol/g of SWCNTs. The ethanol data exhibit several peaks, indicating that ethanol does adsorb at room temperature. For comparison and to help identify the environment of the adsorbed ethanol molecules, spectra of gas-phase and liquid-phase ethanol are shown in Figure 1, traces b and c, respectively. Adsorbed ethanol clearly exhibits vibrational peaks that are much more similar to those of liquid-phase ethanol than to those of gas-phase ethanol. Although the C-H peaks near 2900 cm-1 and the other peaks in the “fingerprint region” (1000-1600 cm-1) are observed at essentially the same frequencies, the O-H peaks depend strongly on the phase. The similarity of the O-H stretch of adsorbed ethanol to that of liquid ethanol, as shown in Table 1, suggests that the adsorbed phase of ethanol is quite similar to the liquid phase. Another important point is that the purging process does indeed remove the gas-phase ethanol, to at least below our detection limit.
Methanol and Ethanol Adsorption on SWCNTs
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Figure 1. IR spectra of ethanol (a) adsorbed on raw SWCNTs, (b) in the liquid phase, and (c) in the gas phase.
Figure 2. FTIR spectra of ethanol adsorbed on (a) raw SWCNTs and (b) purified SWCNTs.
TABLE 1: O-H Stretching Frequencies (cm-1) for Ethanol Adsorbed on SWCNTs and for Various Ethanol Clusters
the amount adsorbed per unit mass of sample was estimated. From the data, an average value of 0.043 mmol of adsorbed ethanol per gram of sample was calculated. This value is in very good agreement with the value of 0.05 mmol/g obtained by Bittner et al. for similarly treated nanotubes.31 Then, using a coverage area of 22.0 Å2 per molecule for ethanol,43 the coverage per unit mass was calculated to be about 10 m2/g. ONIOM calculations indicate that adsorption of ethanol molecules is energetically favorable inside (10,0), (26,0), and (31,0) nanotubes. A single ethanol molecule fits easily into each of the nanotubes modeled. However, the largest cluster that can fit in a (10,0) nanotube is a linear cluster of two molecules, whose optimized geometry is shown in Figure 3a,b. For the (26,0) and (31,0) nanotubes, larger clusters can fit inside of the tube, including linear clusters of two to four molecules, and cyclic clusters of four molecules. The (26,0) tube is the smallest tube in which a cyclic cluster of four molecules can fit, a fact whose importance will be discussed in the next section. When performing DFT calculations of vibrational frequencies, it is common to find a scale factor that brings the calculated frequencies into agreement with the experimental frequencies, and two approaches to do so were taken here. First, the C-H and O-H stretch frequencies calculated for an isolated ethanol molecule were compared to those of gas-phase ethanol. From these comparisons, an average scale factor of 0.960 was determined, which is quite similar to the scale factor obtained in a previous calculation of vibrational frequencies of ethanol clusters.44 The second approach was to assume that the C-H stretch frequencies would not significantly shift for ethanol molecules adsorbed inside an SWCNT. Indeed, this was the case, as the calculated C-H stretch frequencies for molecules inside an SWCNT were only very slightly different from those calculated for an isolated ethanol molecule. The ratio of the calculated C-H stretch frequencies to those of gas-phase ethanol resulted in a scale factor of 0.959. The high degree of similarity between the two scale factors determined by different methods is a strong indication of their reliability. Furthermore, the scaling factors bring the O-H stretching frequencies of isolated ethanol molecules into very good agreement with the experimental gasphase value (cf. Tables 1 and 2). The scaled calculated O-H stretching frequencies of a single ethanol molecule inside a (10,0) nanotube are reported in Table 2. These results suggest that a single ethanol molecule adsorbed
adsorbed on SWCNTs
liquid phase
gas phase 3675
3355
3345
ethanol in hexane ref 44
nozzle expansion clusters ref 49
3645 3535 3360 3280 3280
3450 3270 3200 (“larger clusters”) 3200 (“larger clusters”)
cluster type monomer linear dimer cyclic trimer cyclic tetramer cyclic pentamer cyclic hexamer
Figure 2 compares the high-frequency region of the spectra of ethanol adsorbed on raw SWCNTs and on purified SWCNTs in traces a and b, respectively. The C-H peaks are essentially identical, but the O-H peaks do show some slight differences. For ethanol adsorbed on the raw SWCNTs, the O-H peak is about 50 cm-1 lower in frequency than the O-H peak of ethanol adsorbed on purified SWCNTs. In multiple trials, we noticed that the O-H peak could shift by 10-15 cm-1 from trial to trial, possibly because of slight differences in the SWCNT sample after heating. Thus, we cannot confidently claim that the difference between peak frequencies is meaningful. Exposure-time experiments (see Supporting Information) indicate that no additional ethanol adsorption occurred after 5 min of exposure. That is, the nanotubes were saturated after 5 min of exposure to ethanol vapor. We note that, during the postexposure purge process, it took at least 2-3 min to remove any visible alcohol droplets from the inside of the sample chamber. Thus, it is possible that saturation coverage was reached before the 5 min exposure time. However, we could not achieve better time resolution with our experimental setup. Integrated peak areas were used to estimate the coverage of ethanol molecules on the nanotubes. Because the C-H peaks for adsorbed ethanol were essentially at the same frequency and had the same shape as those for liquid-phase and gas-phase ethanol, those peaks were used for this analysis. Assuming that adsorption does not change the C-H absorption strength, the integrated C-H peak areas for known amounts of liquid ethanol were used to extrapolate a number density (molecules/cm3) of ethanol adsorbed on the SWCNTs. Then, using the known IR beam geometry, the measured thickness of the nanotube layer on the screen, and the known mass of nanotubes on the screen,
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Ellison et al.
Figure 3. Optimized geometry of a linear cluster of two ethanol molecules inside a (10,0) SWCNT segment: (a) side view, (b) oblique view.
TABLE 2: Scaled O-H Stretching Frequencies (cm-1) for Isolated Ethanol and Ethanol Adsorbed in a (10,0) SWCNT Calculated Using the ONIOM DFT/UFF Method O-H stretch frequency (cm-1)
theoretical level/model
isolated ethanol molecule
ethanol inside nanotube
difference
B3LYP/6-31(d,p):UFF B3LYP/6-31(3d,2p):UFF
3664 3673
3663 3670
1 3
TABLE 3: Scaled O-H Stretching Frequencies (cm-1) for Ethanol Adsorbed in a (10,0), (26,0), or (31,0) SWCNT Calculated Using the ONIOM B3LYP/6-31(d,p):UFF Method
cluster size and type
nanotube type
2, linear
(10,0)
2, linear
(26,0)
3, linear
(26,0)
4, linear
(26,0)
4, cyclic
(26,0)
2, linear
(31,0)
3, linear
(31,0)
4, cyclic
(31,0)
scaled O-H stretch frequencies (cm-1) 3687 3509 3700 3578 3679 3603 3547 3655 3442 3406 3345 3422 3353 3289 3187 3695 3577 3681 3604 3571 3311 3266 3257 3166
inside an SWCNT does not exhibit a meaningful shift in its O-H stretching frequency compared to the gas phase. This is the case for both basis sets, indicating that increasing the number of polarized functions beyond B3LYP/6-31G(d,p) in the basis set makes little difference for these calculations. This finding is in agreement with a study on the vibrational frequencies of amidines.45 Consequently, for the frequency calculations of ethanol molecules inside of SWCNTs, ONIOM calculations using the B3LYP/6-31(d,p) level of theory for the ethanol molecules and the UFF level of theory for the nanotube segment were used. Finally, Table 3 summarizes the scaled O-H vibrational frequencies from the ONIOM calculations for clusters and
nanotubes of varying sizes. Several trends are immediately clear. First, clusters of two and three ethanol molecules exhibit O-H vibrational frequencies that are similar to those of ethanol in the gas phase and only somewhat shifted to lower frequencies. Second, linear clusters of any size must necessarily have one H atom that is not hydrogen bonded to another ethanol molecule and therefore have an O-H stretching frequency quite similar to that of gas-phase ethanol. Third, only cyclic clusters of ethanol molecules have O-H stretching frequencies that are similar to those observed for adsorbed ethanol. These results are quite helpful in understanding the FTIR data, as discussed below. Discussion The IR results show that, under our experimental conditions, methanol does not remain adsorbed on SWCNTs at room temperature without the vapor present. This observation seemingly contradicts previous results that measured the adsorption isotherms of methanol on SWCNTs32 and also the findings that exposure to methanol vapor alters the conductivity of SWCNTs15 and their thermoelectric power (TEP) response.43 These dissimilarities are attributed to different experimental conditions. In this work, the methanol vapor was purged from the sample chamber, whereas in refs 15, 32, and 43 this was not the case. In fact, in refs 15 and 43, the methanol vapor was maintained at a constant pressure around the SWCNTs as the conductivity or TEP was measured. In those experiments, the conductivity and TEP returned to their pre-exposure values after the methanol vapor was removed, suggesting that, without methanol vapor present, methanol does not remain adsorbed on SWCNTs. We hypothesize that methanol’s high volatility allows it to evaporate readily from the SWCNTs, which would imply that the forces between the nanotube(s) and the methanol molecules are quite weak. Adsorption energies supporting this hypothesis are presented later in this section. In contrast to methanol, ethanol does remain adsorbed on SWCNTs, even after the ethanol vapor has been purged from the sample chamber. The IR data in Figure 1 suggest that the adsorbed ethanol is in an environment quite similar to that of liquid ethanol, namely, one in which the ethanol molecules are involved in a high degree of hydrogen bonding. The significant red shift of the O-H peak is indicative of hydrogen bonding. The liquid phase of ethanol is known to consist of hydrogenbonded clusters of ethanol molecules, with the clusters taking both cyclic and linear forms.46 The similarity between the IR spectra of adsorbed ethanol and liquid-phase ethanol strongly suggests similar environments, specifically hydrogen-bonded clusters. However, for adsorbed ethanol, the fingerprint region between 1000 and 1400 cm-1 is similar to that of gas-phase ethanol. Several distinct vibrations contribute to the peak at 1380 cm-1,47
Methanol and Ethanol Adsorption on SWCNTs
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Figure 4. Optimized geometry of a cyclic cluster of four ethanol molecules inside a (26,0) SWCNT segment: (a) side view, (b) oblique view.
but they involve primarily C-H bending motions and are therefore not expected to be affected much by hydrogen bonding of the molecules. The broad peak centered at 1240 cm-1 is attributed to a C-O-H bend. These vibrations are often obscured by CH3 bend features but can be red shifted by hydrogen bonding.47 The presence of this peak in the spectrum of the adsorbed ethanol indicates hydrogen bonding between molecules, but not exactly the same as in the liquid, because this peak is not apparent in the spectrum of the liquid. Lastly, the intense peak at 1265 cm-1 in the gas-phase spectrum, which is split in the liquid-phase spectrum, is attributed to a C-O stretch.47 This peak is present but in much-diminished intensity in the spectrum of adsorbed ethanol. Although all of these peaks are not exactly similar to the liquid-phase features, they are consistent with ethanol molecules involved in hydrogen bonding. Finally, the data in Figure 2 showing that the adsorption is essentially the same on raw and purified nanotubes strongly suggest that the adsorption takes place on the SWCNTs themselves and not on impurities. The nature of the adsorption of ethanol on SWCNTs involves two potentially interrelated factors: the nature of the interaction between ethanol molecules (cluster size) and the location of the ethanol molecules relative to the SWCNTs. An ethanol molecule adsorbed in the hydrophobic confines of the endohedral site of a nanotube might be expected to show a shift in vibrational frequency relative to the gas phase. Indeed, CO2 adsorbed in endohedral sites of SWCNTs shows a slight shift from its gas-phase vibrational frequency.27 A small shift for ethanol might similarly be possible. However, the ONIOM calculations suggest that single ethanol molecules adsorbed inside of SWCNTs probably do not have O-H vibrational frequencies that are significantly different from those in the gas phase. The observed O-H stretch for adsorbed ethanol is quite different from that of the gas phase, indicating that, at most, a few single ethanol molecules adsorb in endohedral sites. Furthermore, the (10,0) tube in this study is at the small end of the range of nanotube sizes in a sample of HiPco SWCNTs.48 Ethanol molecules inside larger tubes would be expected to have even weaker interactions with the walls. These factors effectively rule out the likelihood that adsorption of single ethanol molecules in endohedral sites is responsible for the observed large red shift of the O-H stretch frequency. As noted above, the IR data strongly suggest that the adsorbed ethanol is in an environment similar to that of liquid-phase ethanol, which is known to consist of small clusters of hydrogenbonded molecules. To help further determine the type and size of the adsorbed clusters, we compared our results to those of ethanol clusters in ethanol-hexane solutions44 and in jet-cooled ethanol clusters.49 Both of these studies used infrared spectroscopy and ab initio calculations to assign O-H stretching frequencies to specific ethanol clusters. For the ethanol-hexane
Figure 5. Comparison of the experimental O-H stretch peak with that obtained from the ONIOM calculation of a cyclic tetramer ethanol cluster inside a (26,0) SWCNT.
solutions, it was determined that the ethanol molecules primarily form cyclic tetramer, pentamer, and hexamer clusters.44 The nozzle-expansion clusters exhibited primarily ethanol dimers, trimers, and tetramers.49 A summary of these results and the results of the current work is presented in Table 1. The results of refs 44 and 49 show a clear trend of decreasing O-H stretch frequency with increasing cluster size. The very close agreement between the O-H vibrational frequency of adsorbed ethanol molecules and that of cyclic tetramers of ethanol in hexane strongly suggests that adsorbed ethanol forms cyclic clusters of four molecules. This point is further supported by the results of the ONIOM calculations. As shown in Table 3, only adsorbed cyclic tetramer ethanol clusters exhibit O-H vibrational frequencies that are similar to those observed for adsorbed ethanol. In fact, the calculated frequencies for a cyclic tetramer in an endohedral site in a (26,0) nanotube agree quite well with the experimental data. The optimized geometry of this system is shown in Figure 4a,b. To emphasize the agreement between the experimental data and the ONIOM calculations of a cyclic tetramer cluster inside an SWCNT, Figure 5 presents the scaled calculated frequencies and the FTIR results for adsorption on purified SWCNTs. ONIOM calculations report vibrational frequencies and their corresponding intensities. Although the absolute intensities are not accurate, relative intensities within a particular calculation can be compared. Using the calculated frequencies and intensities and a Gaussian line shape with ∼100 cm-1 FWHM, which is consistent with the broadening of the O-H stretch determined by transient spectroscopy,50 the calculated O-H band was determined. For the purpose of Figure 5, the experimental data were scaled to have the same height as the calculated band. The agreement is quite good, with the exception of an overestimate of the intensity of the calculated low-frequency
18132 J. Phys. Chem. C, Vol. 111, No. 49, 2007 O-H stretch. This good agreement suggests that cyclic tetramer ethanol clusters can explain much of the observed experimental FTIR data. Therefore, on the basis of a comparison of this work to the ethanol cluster studies and the ONIOM calculations, we conclude that the adsorbed ethanol molecules primarily form cyclic tetramer clusters inside the nanotubes. However, the width of the O-H stretch peak in this work indicates that the formation of other clusters, such as cyclic pentamers and possibly cyclic trimers and cyclic hexamers, is also possible. Although the ONIOM calculations and FTIR data suggest that adsorption is dominated by cyclic tetramer clusters in endohedral sites, it is important to consider other possibilities. A bundle of SWCNTs presents several possible adsorption sites for molecules, including interstitial sites between multiple tubes; endohedral sites, or sites inside a tube; groove sites between two or three tubes; double-groove sites with two nanotubes adjacent to an underlying nanotube; and sites on the outside wall of a tube. Additionally, nanotubes have functional groups on the outside and at the tube ends to which the molecules could adsorb. Each one of these other possibilities, however, is inconsistent with the data, which will be briefly discussed. Raw and purified SWCNTs are known to contain functional groups28 such as CdO groups and carboxylic acid moieties that could be involved in hydrogen bonds with ethanol. The method by which the experiments in this study were performed is sensitive to changes in the nanotubes after adsorption, and no changes were observed. However, if the ethanol molecules adsorb by forming hydrogen bonds to carboxylic acid groups on the SWCNTs, small changes in the O-H stretching frequencies of the carboxylic acid groups would be obscured by the O-H peak of the ethanol. Indeed, hydrogen bonding of CO molecules with functional groups on SWCNTs has been inferred by blue shifts of CtO stretches rather than by direct observation of changes in vibrational frequencies in the O-H region of the spectrum.23 To help provide information about the vibrational frequencies of the O-H stretches of ethanol molecules that are hydrogen bonded to CdO and carboxylic acid groups, the IR spectra of solutions of 1 × 10-3 mole fraction of ethanol in acetone and acetic acid were collected (see Supporting Information). For ethanol in acetone, the O-H stretch was observed at ∼3500 cm-1, and for ethanol in acetic acid, the O-H stretch was observed at ∼3400 cm-1. Both of these values are significantly higher in frequency than the O-H stretch peak observed for ethanol adsorbed on SWCNTs. The observed peak is broad enough to have a small but nonzero intensity at these higher frequencies, suggesting that it is possible that some adsorbed ethanol molecules are hydrogen bonded to CdO and/or carboxylic acid groups on the nanotubes. However, it is unlikely that the majority of the adsorbed molecules are in such a binding configuration. Using size considerations, it is highly unlikely that ethanol molecules adsorb in the interstitial channels. Previous studies have indicated that even small molecules such as He, H2, CH4, Ne, and Xe are too large to enter the insterstitial channels between nanotubes in a bundle.51 Additionally, using adsorption isotherms, Yang et al. estimated an average pore size of approximately 1-2 nm for purified SWCNT bundles,32 which is not large enough to readily allow entry of an ethanol molecule. It is known that purification processes open holes in the walls and end caps of nanotubes.18 However, this is complicated by the fact that the acid-treatment purification step also produces functional groups that can block the holes in the walls and ends of nanotubes.18 The annealing process removes some18 but not
Ellison et al. all28 of these functional groups. However, those openings not blocked by functional groups could become blocked by adsorbate molecules. Calbi and Riccardo showed that the ends of nanotubes are favorable binding sites and that adsorbate molecules could adsorb there and effectively block access to the interstitial sites.52 By analogy, it is reasonable to think that adsorbate molecules would also adsorb at high-energy sites where end caps had been etched away and block other molecules from entering the interstitial sites. Having ruled out the likelihood of hydrogen bonding to functional groups on the SWCNTs or adsorption in interstitial sites as the cause of the observed O-H red shift, we consider the formation of adsorbed clusters of ethanol molecules in endohedral sites to be the most likely explanation. Further evidence supporting this conclusion comes from the adsorption coverage.53 From our data, the estimated coverage of 10 m2/g could support endohedral adsorption in a select number of SWCNTs. Using geometric considerations, Williams and Eklund calculated that endohedral sites offer approximately 800 m2/ g.54 If ethanol were adsorbing in all endohedral sites, the estimated coverage should be much larger. However, the data sheet provided by Carbon Nanotechnologies, Inc., for the HiPco SWCNTs states that the tube diameters in the sample range from 8 to 12 Å. As noted earlier, the (26,0) SWCNT, with a diameter of 11.75 Å, used in the ONIOM calculations is therefore among the very largest diameter nanotubes in the sample. Because cyclic tetramer clusters of ethanol could not form in narrower nanotubes, it is likely that the ethanol adsorbs by forming such clusters in only the largest diameter SWCNTs in the sample. Assuming a Gaussian distribution of nanotube diameters with a most probable diameter of 9.5 Å and a width of 2 Å, the same assumptions as were made in simulating photoluminescence spectra of SWCNT samples,55 we calculate that approximately 1% of the sample consists of SWCNTs with diameters large enough to accommodate cyclic tetramer ethanol clusters. These SWCNTs would offer ∼8 m2/g adsorption area in their endohedral sites, in very good agreement with the observed coverage. The existence of adsorbed ethanol clusters in only 1% of the SWCNTs in the sample seems surprising, especially given recent work that estimates 50-55% open tubes in samples similar to the ones used here.56 However, under our experimental conditions, most (but not all) of the functional groups at the ends of the nanotubes have been removed, allowing ethanol molecules to adsorb at those sites. It is possible that those molecules block the entrance to the endohedral sites for all but the largest nanotubes, just as they block access to the interstitial sites, as discussed previously. That is, the initial adsorbing ethanol molecules could adsorb at the sites at the ends of the tubes, physically blocking later-arriving ethanol molecules from adsorbing inside the smaller-diameter tubes. With only the largest diameter tubes available for adsorption of clusters of ethanol molecules, this could account for the low fraction observed. The results of other studies suggest that adsorption does not occur on the outside of SWCNTs or in the groove sites. Migone et al. studied the adsorption of Xe and CF4 on SWCNTs at low temperatures (
