Interaction of Water with Single-Walled Carbon Nanotubes: Reaction

Mark D. Ellison, Steven T. Morris, Matthew R. Sender, Jennifer Brigham, and Nicholas E. Padgett. The Journal of Physical Chemistry C 2007 111 (49), 18...
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J. Phys. Chem. B 2005, 109, 10640-10646

Interaction of Water with Single-Walled Carbon Nanotubes: Reaction and Adsorption Mark D. Ellison,* Adam P. Good, Carrie S. Kinnaman, and Nicholas E. Padgett Department of Chemistry, Wittenberg UniVersity, Springfield, Ohio 45501 ReceiVed: December 7, 2004; In Final Form: April 4, 2005

The interaction of water vapor with carbon nanotubes at room temperature has been investigated using Fourier transform (FT) IR spectroscopy and density functional theory (DFT) calculations. FTIR data indicate that water molecules adsorb on single-walled carbon nanotubes at room temperature. Comparison to previous studies suggests that the water forms hydrogen-bonded structures inside the nanotubes. Analysis of the FTIR data demonstrates that a small number of water molecules react with the nanotubes, forming C-O bonds, whereas a majority of the water molecules adsorb intact. The DFT calculations show that cleavage of an O-H bond upon adsorption to form adsorbed -H and -OH groups is energetically favorable at defect sites on nanotubes.

Introduction The discovery of carbon nanotubes1 in 1991 has prompted an ever-increasing number of investigations of their properties, with branches of study continually emerging. Nanotubes have shown promise for applications ranging from biotechnology to microelectronic devices. If nanotubes are to be utilized as elements in these applications, they must first be purified and functionalized. Such processes involve exposing the nanotubes to a variety of environments. Because microelectronic devices will likely depend on the reliable performance of a single nanotube, thorough understanding and control of the environment to which the nanotube has been subjected will be necessary. This means that a topic of importance is the interaction of nanotubes with a variety of gas molecules. Activity in this field began in earnest when it was discovered that exposure to NH3, NO2, or O2 dramatically affects the electrical conductivity of the nanotubes.2,3 Although computational studies on the adsorption of gases have been numerous,4-16 the systems studied experimentally have been few in number. The need for more experimental data on the interaction of gases with carbon nanotubes is clearly evident for both the basic understanding of the forces that govern adsorption and the reporting of results so that computational models can be refined. Previous experimental studies have investigated gas adsorption on both the inside and outside of single-walled carbon nanotubes. Because the inside of a nanotube represents a large volume for gas storage, Yates et al. have studied the adsorption of Xe,17-19 CF4,20 and NO21 inside carbon nanotubes. These systems involve physisorption, so the gases are not bound to the nanotubes at room temperature. Theoretical studies indicate that chemisorption on the outside of nanotubes is more energetically favorable in groove or interstitial sites between nanotubes in a bundle than on the surface of an individual nanotube.6,10 Studies of the chemisorption of molecules such as CO2, NH3, and NO2 to the outside of nanotubes has bolstered this conclusion. NH3 and NO2 were found to adsorb in the interstitial grooves of nanotube bundles, interacting via both ends of the molecules.22 CO2 displays an interesting behavior * Author to whom correspondence should be addressed. E-mail: [email protected].

in that some molecules become trapped in the interstitial grooves in a nanotube bundle and do not escape, even when the nanotubes are heated above 1000 K.23-25 These findings notwithstanding, a great deal of fundamental knowledge about gas-nanotube chemistry remains to be discovered. Nearly any ambient environment in which nanotubes exist will entail exposure to water vapor. Water-saturated air has been shown to be effective in an oxidative purification scheme.26 Recently, the interaction of water molecules with single-walled nanotubes (SWNTs) was investigated experimentally using neutron scattering27,28 and X-ray diffraction.29 These studies found that water formed stable hydrogen-bonded structures inside the nanotubes at temperatures from 9 to above 300 K. Several computational studies have also been carried out to probe the interaction of water with single-walled carbon nanotubes. In a density functional theory (DFT) calculation, Peng and Cho found that a water molecule with the oxygen atom pointed toward a nanotube experienced only repulsive forces and therefore would not adsorb.5 However, another DFT calculation by Pati et al. determined that a water molecule could adsorb by interacting via one of its hydrogen atoms.9 The calculated interaction energy in the latter case was rather low, 0.035 eV. These theoretical results suggest that water can adsorb on single-walled carbon nanotubes and that the orientation of the molecule is a critical factor. Finally, Marti et al. studied the adsorption of water in and on SWNTs using molecular dynamics simulations. Their results indicate that water molecules adsorb on nanotubes at room temperature and exhibit two phases of differing density.8,11,30 To test these theoretical predictions and to further the understanding of this important system, we have conducted experiments on the adsorption of water on SWNTs at room temperature. Experimental Section The experimental details have been reported in a previous publication.22 Briefly, SWNTs produced by the HiPco process were obtained from Carbon Nanotechnologies, Inc. Most published purification techniques26,31-34 involve strongly oxidizing conditions that have been shown to remove the end caps and oxidize the walls of the nanotubes.17 Because we wished to avoid opening large pores and adding additional functional

10.1021/jp0444417 CCC: $30.25 © 2005 American Chemical Society Published on Web 05/11/2005

Interaction of Water with SWNTs groups to the tube walls, we used the SWNTs as received (∼80% carbon by mass). The major impurity in HiPco SWNTs is graphite-covered iron catalyst particles. It is possible that water molecules could adsorb onto these particles. However, water adsorption on graphite occurs only at cryogenic temperatures,35 so this is unlikely to be a complicating factor in our roomtemperature experiments. Furthermore, as will be presented later, the similarity of some of our IR data to results from previously published studies on nanotubes is strong evidence that the water is interacting primarily with the SWNTs. Dry SWNTs were placed on a tungsten screen (BuckbeeMears, >85% optical transmission), wetted with toluene, and pressed into the screen. The resulting nanotube layer was uniform and had a thickness of ∼1 mm. This layer was found to have an optical density of about 0.75 that was roughly constant over 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 SWNTs were heated to about 800 K for 12 h to desorb gases and carboxylic functional groups from the SWNTs.17 After being heated, the SWNTs were allowed to cool to room temperature before being exposed to water vapor. Nanopure water or D2O (Aldrich, 99.9% atom purity) was placed in a 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 water was subjected to several freeze-pump-thaw cycles. Because of the tubing between the water vial and the screen on which the nanotubes were mounted, it was difficult to expose the nanotubes to a well-known amount of water vapor. The most reliable exposure method was found to be as follows: Water in the vial was heated to increase its vapor pressure. The valve was opened, and the water was continuously heated until small droplets of water were observed in the cube. At this point, the heating was stopped, and the valve was closed. Water 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 pressure of water around the nanotubes was 23 Torr. The nanotubes were then exposed to that pressure of water 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 a nitrogen-purged sample compartment of a Mattson Research Series Fourier transform (FT) IR 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 each were acquired for the background and sample at a resolution of 1 cm-1. For each experiment, the background spectrum was taken of the nanotube film before exposure to water. The spectrometer computes the ratio of the sample to the background spectra, effectively dividing out the absorbance of the carbon material. Thus, all peaks observed indicate species that are present in greater amounts in the sample scan (i.e., after exposure to H2O) than in the background scan. Control experiments showed no absorption, indicating that the water is adsorbing on the nanotubes and not the screen or CaF2 windows. Results Figure 1 shows FTIR spectra of SWNTs after exposure to water for a range of times. A large, broad feature is observed from 3000 to 3500 cm-1. This feature is nearly resolved into two peaks at 3255 and 3390 cm-1. A curve-fitting procedure using Igor (Wavemetrics) shows that this feature is much better

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Figure 1. IR spectra of SWNTs exposed to water for 8.5, 30, and 120 min.

fit by two Gaussians at 3255 and 3390 cm-1 with nearly equal intensities than by a single Gaussian at 3300 cm-1. These observed values are similar to those of liquid water,36 3280 and 3490 cm-1, and are therefore assigned to the stretching motions (symmetric and asymmetric) of an O-H bond of intact, adsorbed water molecules. Although these observed peaks are somewhat red-shifted with respect to those of liquid water, their similarity indicates that adsorbed water molecules are in an environment similar to that of bulk liquid water, i.e., one involving extensive hydrogen bonding. Additionally, several peaks are observed in the range of 1000-1700 cm-1. In accord with other FTIR studies of SWNTs,17,25 we assign the peak at 1170 cm-1 to a C-O stretching motion. The peak at 1360 cm-1 has previously been termed unassigned,17,24,25,37 but because of good agreement with an observed C-H bend motion on hydrogenated SWNTs,38 we assign it as a C-H bending motion. A peak at 1640 cm-1 has two possible assignments: a CdO stretch in a quinone-like group or a H-O-H bend in an intact water molecule. In liquid water, the H-O-H bend has a frequency of 1645 cm-1.36 Small narrow peaks around 1600 and 3700 cm-1 are due to incomplete subtraction of water vapor in the spectrometer during background and sample scans. Finally, we note that the IR data can be reproducibly obtained after exposure to water by heating the SWNTs as described above and reexposing the nanotubes. This suggests that the adsorption process is entirely reversible. To aid with the assignment of the peaks in the FTIR data, we exposed the SWNTs to D2O for 30 min. The resulting data are shown in Figure 2. The broad peak from 2000 to 2500 cm-1 is assigned to the O-D stretching motions. In comparison to the H2O data, the peaks at 1360 and 1640 cm-1 are no longer present, indicating that they are due to vibrational motions directly involving H atoms. Two peaks are apparent at 1140 and 1240 cm-1 and are assigned to a C-O stretch and D-O-D bend, respectively. On the basis of observed C-D bend motions for deuterated SWNTs,38 a C-D bend is believed to be a

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Figure 2. IR spectrum of SWNTs exposed to D2O for 30 min.

component of the peak at 1140 cm-1. A summary of the observed peaks and their assignments and a comparison to liquid and gaseous water is presented in Table 1. Figure 1 shows several scans taken after different exposure times. These are representative of the data, and many such scans were collected. For each scan, the areas under the peaks were determined, and Figure 3a presents the normalized peak area for each peak (normalized to the peak of maximum area) for the O-H stretches. The line is intended to guide the eye. The integrated area initially rises steadily and then levels off around 90-min exposure time. This implies a very slow adsorption process and also suggests that water initially adsorbs and then reaches saturation coverage after some time. For comparison, the normalized areas of the C-O and C-H peaks as a function of exposure time are displayed in Figures 3b and 3c, respectively. These data indicate essentially no change in the areas of these peaks with increasing exposure, strongly suggesting that these peaks are caused by an initial process that does not continue with further exposure to water. Finally, we note the very long exposure time needed to saturate the SWNTs, which implies a very slow adsorption process. Finally, Figure 4 shows IR data collected at specific time intervals after exposing the SWNTs to water for 30 min. The first spectrum was taken immediately after the exposure process described above. Subsequent scans were acquired after 1 and 4 days. During that time, the SWNTs remained under vacuum in the cell. The figure demonstrates that little change occurs in the adsorbates over the course of several days. The peaks at 1170 and 1400 cm-1 decrease in intensity somewhat, the peak at 1400 cm-1 shifts to a slightly higher frequency, and the peak at 1170 cm-1 shifts to a slightly lower frequency. This suggests that the absorbed species are stable in a vacuum for a time period of several days.

Figure 3. Normalized integrated peak areas for water adsorbed on SWNTs.

Discussion The FTIR data confirm that water adsorbs intact on SWNTs at room temperature. The vibrational peaks observed at 1640, 3255, and 3390 cm-1 are consistent with water molecules adsorbed in a state that is similar to liquid water, and the large intensity of these peaks indicates that most of the water molecules adsorb into this state. Integration of the peak areas of the O-H stretches demonstrates that water molecules steadily adsorb into this state, saturating after about 120 min of exposure. The IR data for the adsorbed water molecules indicate that they are in a state that is similar to that of bulk water, with extensive hydrogen bonding. In contrast to the molecular

TABLE 1: Vibrational Frequencies for Water Adsorbed on SWNTs and Liquid and Gaseous Water vibrational motion

SWNTs/H2O (cm-1)

liquid H2O36 (cm-1)

gaseous H2O42 (cm-1)

SWNTs/H2O28 (cm-1)

SWNTs/D2O (cm-1)

liquid D2O36 (cm-1)

gaseous D2O42 (cm-1)

ν1 (symmetric stretch) ν3 (asymmetric stretch) ν2 (bend) ν(C-O) δ(C-H)

3255 3390 1640 1170 1360

3280 3490 1645

3657 3756 1595

3403 3403 1653

2230 2490 1240 1140 1140

2394 2590 1215

2671 2788 1178

Interaction of Water with SWNTs

Figure 4. IR spectra of SWNTs exposed to H2O for 30 min. Scans were acquired for the same sample immediately after exposure, 1 day after exposure, and 4 days after exposure.

dynamics simulations,11,30 we do not observe any peaks around 3700 cm-1 attributable to water molecules with a single hydrogen-bonded hydrogen. Rather, our data suggest that essentially all adsorbed water molecules involve both hydrogen atoms in hydrogen bonds. The previously reported molecular dynamics simulations also predicted two distinct phases of adsorbed water molecules with different densities. Because our IR data do not exhibit the same features as predicted by the simulations, we do not find any evidence for the existence of different adsorption phases at room temperature. Table 1 shows a comparison of the results of this work to the vibrational motions of water molecules inside the SWNTs as determined by neutron scattering.28 The strong similarity suggests that the adsorbed water that we observe is in an environment similar to that probed in the previous study, namely, inside the nanotubes. The water molecules likely enter the nanotubes through pores and vacancies produced by decarboxylation during the heating prior to exposure to water. This process is expected to be kinetically slow, which would explain the observed long time needed to saturate the nanotubes with adsorbed water. We note that water is known to adsorb on graphite(0001) at 85 K, with a weak water-graphite interaction and a strong water-water interaction.35 In that experiment, electron energy loss spectroscopy results indicated that the O-H stretching frequency was 3380 cm-1 and the H-O-H bend frequency was 1610 cm-1. These values are quite similar to those for H2O/ SWNTs, suggesting that the interaction of H2O with the SWNTs is roughly as weak as that of H2O with graphite. Water is also known to adsorb on rough carbon surfaces, such as glassy carbon and the edge surface of graphite, at room temperature.39 In that study, the adsorption was attributed to the formation of hydroxyl groups on the reactive areas of the surfaces. Finally, we note that the vibrational frequencies observed here are similar to those of water adsorbed on a wide variety of metal surfaces.40 In those systems, the hydrogen bonding between water molecules is stronger than the water-surface interaction. Indeed, the substrates must be cooled to less than 100 K for water to adsorb in most of those systems. The similarity in vibrational frequencies between these systems and H2O/SWNTs suggests that, in water adsorbed to SWNTs, intermolecular hydrogen bonding is the

J. Phys. Chem. B, Vol. 109, No. 21, 2005 10643 dominant force. It also suggests that the vibrational data do not provide conclusive evidence for the location of the water molecules when they are adsorbed on SWNTs. However, the fact that water adsorbs to SWNTs at room temperature and remains adsorbed for at least several days indicates that the force holding water to the SWNTs is stronger than the forces holding water to graphite or metal substrates. This force seemingly does not exert a strong influence on the vibrations of the water molecules. A possible explanation is that this “force” is no more than the physical trapping of the water molecules inside of the SWNTs. The FTIR data also show evidence that water molecules react with the SWNTs. The peak observed at 1170 cm-1 for H2O/ SWNTs is consistent with a C-O stretch observed in previous studies of the functional groups on the walls of SWNTs.25,41 That this peak appears after exposure of the SWNTs to water strongly suggests that some water molecules are reacting with the nanotubes to form C-O bonds. Such a reaction is likely responsible for the peak that also appears at 1360 cm-1. A peak area analysis (Figures 3b and 3c) shows that both of these peaks reach their maximum intensity after a very short exposure, suggesting that these peaks share a common cause. The peak at 1360 cm-1 is not present in the D2O/SWNTs data, indicating that this vibration directly involves the motion of a hydrogen atom. Furthermore, Khare et al. observed a C-H bend at 1370 cm-1 for nanotubes that were hydrogenated by exposure to H atoms.38 However, in their work, they also observed a peak at 1460 cm-1 that they attributed to a C-H bend. Such a peak is not observed in our data. Because a C-H bend can have only one distinct frequency, it is possible that the two peaks observed by Khare et al. are due to hydrogen at two different binding sites on the nanotubes. Our observation of a single peak suggests that the H atoms adsorb in roughly equivalent sites. We believe then that the C-H bend indicates that some water molecules react with SWNTs to form an adsorbed H atom. The D2O/SWNTs data also lend circumstantial credence to the adsorption of H atoms. Khare et al. observed C-D bends at 1095, 1189, and 1251 cm-1.38 Our data do not show peaks at any of these frequencies. However, the peak in our data at 1140 cm-1 does tail off slowly to the low-frequency side, so it is possible that the C-D bend is a slight shoulder on that peak. This would be reasonable, because for H2O reacting with SWNTs, our data display only the lowest-frequency C-H bend of those observed by Khare et al. Consequently, we would expect to observe only the lowest-frequency C-D bend. Finally, the C-D bend observed by Khare et al. is very low in intensity, which could explain why it would be a low-intensity shoulder in our data. These observations of C-O and C-H vibrations imply that some molecules adsorb dissociatively, forming C-OH and C-H bonds on the outside of the nanotubes. Yet, the observation of a C-H bend would be expected to be accompanied by the presence of a C-H stretch around 2940 cm-1. Such a peak is not apparent in the data that we collected. However, the observed O-H stretches are so broad that they cover this frequency, making it possible that a C-H stretch peak is obscured by the O-H stretch peaks. Several possible reaction pathways are possible for this system. We rule out any reaction products that produce CdO bonds, such as carboxylic acid or quinone groups, because the IR data show no evidence for a CdO vibration. The FTIR data for H2O/SWNTs do show a peak at 1645 cm-1. However, this peak is not present for SWNTs exposed to D2O. If water reacts with SWNTs to form a CdO group, then it should do so

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TABLE 2: DFT Calculation Results for Several Possible Reaction Products of Water with a Single SWNT nanotube part

location of -H and -OH

tube, Stone-Wales defect between seven-membered and six-membered rings tube, Stone-Wales defect between seven-membered and five-membered rings

∆E (kJ/mol) -42 -66

regardless of the hydrogen isotope involved. The absence of any peak attributable to a carbonyl stretch in the D2O/SWNTs IR data effectively rules out the formation of a carboxylic acid or a quinone-like CdO group. Another possibility is the formation of adsorbed -OH and -H. The presence of these reaction products is supported by the FTIR data. In the H2O/SWNTs FTIR spectra, the peak at 1360 cm-1 is most likely due to a C-H bend. Cleavage of an O-H bond upon adsorption would produce adsorbed -OH and -H groups. The fate of such H atoms has been discussed above. The presence of a C-O stretch at 1170 cm-1 in the FTIR data supports the hypothesis of an -OH group adsorbing to a nanotube. In the literature, this peak is referred to as a C-O stretch without explanation of what else is bonded to the O atom. A reasonable explanation is that this is the C-O stretch of an alcohol group. For example, the C-O stretch of phenol occurs at 1250 cm-1, and that of cyclohexanol occurs at 1230 cm-1.42 Additionally, for the D2O/SWNT data, the peak shifts to 1140 cm-1, as would be expected if a D atom were substituted for a H attached to the O atom. Therefore, the IR data provide strong evidence that some water molecules react with the nanotubes, forming -OH and -H groups. The heating prior to the experiments causes decarboxylation of the nanotubes, probably leaving behind “dangling bonds” that would be highly reactive toward adsorbing molecules. However, without knowing the exact nature of these dangling bonds, it is difficult to predict likely reaction products. A dangling bond with a single unfilled valence on a C atom cannot easily account for adsorption of both -H and -OH groups. A dangling bond with two unfilled valences could, however, attack a water molecule and cleave an O-H bond, resulting in the adsorption of both -OH and -H groups. Little is known about the exact nature of the species left behind after decarboxylation, so it is difficult to determine whether water molecules react with a dangling bond on the tube wall. Molecular dynamics calculations indicate that a vacancy defect causes reconstruction to saturate dangling bonds.43 Certainly, the reaction of water with dangling bonds is plausible and cannot be ruled out. Although nanotubes are generally unreactive, recent theoretical studies have shown that topological defects in the tube walls are significantly more reactive. To determine the likelihood of dissociative adsorption of water molecules, we carried out density functional theory (DFT) calculations to estimate the energetics of several possible reactions. The nanotube was modeled using a section of a (7,0) zigzag tube 12.7 Å long with ends terminated with H atoms. Using this model offered a compromise between being able to place the adsorbed -OH and -H groups away from the ends of the tube to avoid edge effects and keeping the number of atoms reasonable so that the computation time was not excessively long. Still, optimizing the geometry of a bare nanotube took over 100 h. Initially, calculations were run at the semiempirical PM3 level using Cache (Fujitsu Software) to narrow the candidates for DFT calculations. Products in these calculations included an alcohol, a CdO group, and a “diol” that consists of two -OH groups. For the diol, ortho, meta, and para positions were compared.

Figure 5. Optimized geometry for adsorbed -H and -OH groups for reaction between seven-membered and five-membered rings at a defect on the wall of a SWNT.

All of the PM3 calculations indicated that these reactions would be energetically unfavorable on the (7,0) tube, with the formation of an alcohol being the least unfavorable. Because defects are expected to be more reactive, the PM3 calculations were repeated using a nanotube that had a defect on the tube wall. The defect considered was a Stone-Wales defect: two seven-membered rings and two five-membered rings in place of four six-membered rings.44 The formation of a Cd O group on the defect was still energetically unfavorable. However, the formation of an alcohol was energetically favorable at the defect on the nanotube. The Stone-Wales defect offers several distinct possible reaction sites. Formation of an alcohol between two sites connecting the two five-membered rings in the defect was found to be energetically most favorable. Therefore, DFT calculations were carried out for alcohol formation on a nanotube with the Stone-Wales defect described above. DFT calculations were performed with the Gaussian 03 program,45 the STO-3G basis set, and the B3LYP density functional. Attempts to perform calculations with a (7,0) nanotube using a larger basis set resulted in failure to converge. Geometry optimizations were carried out for a bare nanotube, a water molecule, and a nanotube with adsorbed -H and -OH groups. The optimized bare nanotube has C-C bond lengths of 1.43 Å, very close to the experimental value of 1.42 Å.46 The energies of the optimum geometries were used to calculate the energy change of the reaction of water with a nanotube to form adsorbed -H and -OH groups. Table 2 shows the results of the DFT calculations. The calculations indicate that the reaction of water with a defect on the tube wall would be energetically favorable. The optimized geometry of adsorbed -H and -OH groups on the (7,0) nanotube is shown in Figure 5. These results are in agreement with other DFT calculations on the adsorption of gases to Stone-Wales defects on SWNTs. For NO2, chemisorption was found to be slightly favored on a Stone-Wales defect compared to a defect-free tube wall.16 Also,

Interaction of Water with SWNTs the binding energy of O2 to a Stone-Wales defect is significantly higher than that for O2 binding to a defect-free tube wall.14 Finally, ozone adsorption to a defect-free tube wall is classified as weak physisorption, but adsorption to a Stone-Wales defect was calculated to be much stronger, justifying classification as chemisorption.15 These results support our findings that the reaction of H2O at a Stone-Wales defect could be energetically favorable. Reaction in the manner supported by the DFT calculations is consistent with our IR data. An adsorbed -OH group would exhibit a C-O stretch, which we observe at 1140 cm-1, and an O-H stretch. The latter vibration would fall in the region of 3100-3500 cm-1 and is probably obscured by the O-H stretches from the more numerous adsorbed intact water molecules. The adsorbed -H group would exhibit a C-H stretch, which is also obscured by the O-H stretches as discussed previously, and a C-H bend, which is our assignment for the peak at 1360 cm-1. On the basis of DFT calculations, we believe that some water molecules react with defects on the SWNT wall to form -H and -OH groups. The process of adsorption of water to SWNTs likely proceeds with the initial molecules reacting with the nanotubes at defects in the walls of the tubes. Once these sites have reacted, further reaction does not take place. This is consistent with the peak area data presented in Figures 3b and 3c. These peaks arise from products of the reaction, so the fact that the peak areas quickly rise and then remain at a given level indicates that reaction ceases after a brief exposure. A plausible explanation is that the reaction no longer proceeds once the accessible defects have reacted. Finally, the data in Figure 4 show that the adsorbates, both intact water molecules and -OH and -H groups, are stable in a vacuum over the course of several days. Whether the adsorbates would be stable during this time period in ambient conditions is an open question. However, under these controlled conditions, the adsorbed species do not appear to change, indicating that they are stable for an extended period of time. Thus, this could be the starting point of future studies on the stability of these adsorbates under ambient conditions. Conclusions We have studied the adsorption of water on single-walled carbon nanotubes at room temperature. Intact water molecules adsorb inside the nanotubes, in agreement with previous studies. The FTIR data and DFT calculations suggest that some water also reacts with the nanotubes at dangling bonds or defect sites on the tube wall to form adsorbed -H and -OH groups. These results indicate that any potential application of nanotubes should take into account the fact that nanotubes exposed to water in ambient conditions will have functional groups and adsorbed water present. This is likely to affect the performance of any device constructed with nanotubes. Acknowledgment. The authors thank Richard York for valuable laboratory assistance. We also gratefully acknowledge Erica Ellison for a critical proofreading and editing of the manuscript. We acknowledge the Ohio Supercomputer Center for a grant of computer time to perform the DFT calculations. This work was supported by a Cottrell College Science Award from the Research Corporation. References and Notes (1) Ijima, S. Nature 1991, 354, 56. (2) Kong, J.; Franklin, N. R.; Zhou, C.; Chapline, M. G.; Peng, S.; Cho, K.; Dai, H. Science 2000, 287, 622.

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