Ozone Oxidation of Single Walled Carbon Nanotubes from Density

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J. Phys. Chem. C 2009, 113, 17636–17642

Ozone Oxidation of Single Walled Carbon Nanotubes from Density Functional Theory Wai-Leung Yim†,‡,| and J. Karl Johnson*,†,§ Department of Chemical Engineering, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15261, Department of Chemistry, Surface Science Center, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260, and National Energy Technology Laboratory, United States Department of Energy, Pittsburgh, PennsylVania 15236 ReceiVed: August 21, 2009

Ozone is known to react with single-walled nanotubes (SWNTs) to form oxide species on the nanotubes and, upon annealing, to etch the SWNTs. However, the mechanism of ozone attack is not known. We use gradientcorrected density functional theory to compute the potential energy surfaces for O3 dissociation on the sidewall of a pristine (8,8) SWNT. Two decomposition pathways were considered; the first involves the formation of a Criegee intermediate, with a barrier of 17 kcal/mol, followed by transformations leading to lactone, quinone, and carbonyl functional groups. The activation barriers for these transformations are below 23 kcal/mol. The cleavage of the lactone group, evolving CO and CO2, have barrier heights of 39.4 and 49.3 kcal/mol, respectively. This agrees well with experimental findings that the evolution of CO2 and CO occur at 600 K. The second decomposition pathway involves the direct cleavage of the ozonide, forming a singlet O2 and an ether or epoxide group on the SWNT. This pathway competes with the Criegee mechanism; the barrier for forming singlet O2 is 7.9 kcal/mol, which is 9.1 kcal/mol lower than the barrier to formation of the Criegee intermediate, indicating that formation of ether or epoxide groups is kinetically favored. However, formation of ester and carbonyl groups could proceed by subsequent addition of O3 on newly generated defect sites. Vibrational frequency calculations were carried out on cluster models in order to predict infrared absorption signals of local structures. The calculated results for CdO stretching frequencies agree well with experiments. Analysis of the calculated frequencies indicates that the unassigned experimental band at 1380 cm-1 is due to ester and ether groups, while the unassigned band at 925 cm-1 is due to epoxide groups. The vibrational frequency of the O+-O- stretch in the Criegee intermediate is in the range 1055-1096 cm-1. Introduction Single walled carbon nanotubes (SWNTs) provide excellent sites for the physisorption of molecules due to the deep potential wells in the interior of the nanotubes and in the groove sites, which exist on the outside of nanotube bundles.1-8 Most asproduced SWNTs are closed, having end-caps and few defects in the nanotube walls. However, there is some evidence that SWNTs produced by the HiPco process have a fraction of nanotubes that are opened.9,10 Most nanotubes must therefore be opened in order for the interior sites to become available for physical adsorption.2,11 Opening of a SWNT is typically accomplished by oxidation of the nanotubes.1-3 The oxidation may be accomplished by the use of acid oxidiants, such as H2SO4:H2O2,12 by oxidation at high temperatures in oxygen gas,13,14 or by high-temperature oxidation in CO2.15,16 Recently, the high oxidative capability of O3(g) at room temperature has been shown to lead to the opening of nanotubes when the ozonized product is heated to about 1000 K to remove the oxidized groups as CO and CO2.2,17-20 The oxidation of SWNTs by O3 has been experimentally studied by several groups.17,19,21-26 Mawhinney et al. used transmission IR spectroscopy to observe the functional groups produced by O3 treatment at 300 K.17 Vibrational modes * To whom correspondence should be addressed. E-mail: [email protected]. † Department of Chemical Engineering, University of Pittsburgh. ‡ Department of Chemistry, Surface Science Center, University of Pittsburgh. § United States Department of Energy. | Present address: Institute of High Performance Computing, 1 Fusionopolis Way, #16-16 Connexis, Singapore 138632.

assigned to CdO moieties in carboxylic groups and quinone groups were observed, as well as C-O moieties characteristic of ether and ester groups. Heating the ozonized SWNTs led first to the loss of the CdO groups followed by loss of the C-O groups at higher temperatures, for SWNTs oxidized by acidic treatment18 or oxidized by O3.17 Oxidation has been assumed to occur mainly at the nanotube end-caps, which are assumed to be more reactive toward oxidation because of the larger strain and the presence of fivemembered rings on the end caps.17,27,28 Mawhinney and coworkers estimated the percentage of carbon atoms directly located at a defect site in oxidized SWNTs to be about 5%.18 If the estimate of 5% of the carbon atoms being at defect sites is accurate, then oxidation must be occurring on the sidewalls of the nanotubes in addition to the end-caps; a 5% density of defects located only at the ends of the nanotubes requires an average nanotube length of about 5 nm, which is much less than the measured average length.18 Moreover, Simmons et al. have demonstrated that UV-generated ozone readily oxidizes the side walls and diminishes the aromatic character of the nanotubes.29 They speculate that the carbonyl and epoxide groups they observed could have resulted from the decomposition of short-lived Criegee ozonides.29 It is therefore reasonable to assume that O3 is capable of oxidizing the side walls of SWNTs with or without defects. In addition, UV/ozone treatments were used to tailor carbon nanotubes and change their intrinsic properties, such as solubility,30-32 catalytic activity,33 optical and electrical properties.29,34,35 Ozone has also been used to oxidize multiwalled nanotubes, producing the same types of oxide groups as on SWNTs.31,32,36

10.1021/jp908089c CCC: $40.75  2009 American Chemical Society Published on Web 09/17/2009

Ozone Oxidation of SWNTs from DFT

Figure 1. Schematic diagram of the Criegee mechanism.

Several theoretical studies of the reaction of ozone with perfect or defective sidewalls of SWNTs have been performed over the past few years.37-42 These previous studies have only examined the chemisorption and dissociation of the primary ozonide. To the best of our knowledge, the question of how ozone etches the sidewalls of SWNTs has not been explored. In this paper we use first-principles density functional theory (DFT) to examine the reaction of O3 with a (8,8) SWNT to form a primary ozonide (POZ) and the subsequent decomposition of the ozonide along different reaction channels. We have calculated the energetics and identified the transition states for five different etching reactions resulting from a single initial POZ. The etching is accomplished by evolution of CO and CO2. We considered two main competitive channels for the conversion of the POZ at the SWNT wall to other oxygen-containing functionalities: (1) the decomposition pathway via Criegee intermediates that produce carbonyl oxide (-CdO-O-) and carbonyl groups (-CdO), and (2) the direct cleavage of a POZ to form singlet O2 and an ether or epoxide on the SWNT. Reaction channel (2) has been studied theoretically on a (5,5) SWNT37 but evolution of CO and CO2 was not observed when considering only this channel. We therefore have studied the potential energy surfaces for Reaction channel (1) and have found that the formation of CO and CO2 is observed from reactions branching off this channel. The Criegee mechanism43,44 (Figure 1) was proposed to explain the stereochemistry of reactions between ozone and liquid alkenes.43-47 The primary and secondary ozonides have been isolated and completely characterized. However, the intermediate between themsthe so-called Criegee intermediateshas never been observed.48 It was speculated to consist of a pair of molecules, a carbonyl oxide and a carbonyl compound. The most likely reason for the failure to observe Creigee intermediates in gas-phase reactions is that the carbonyl oxide possess too much excess energy after ring rupture of the POZ,49,50 leading to rapid degradation of the unstable species. A study of the ozone reaction with C60 by Shang et al. showed that the formation of the POZ and Criegee intermediates were highly exothermic, around 40 kcal/mol,51 which was similar to ozone reactions with simple alkenes. On the other hand, the energetics of the reactions between ozone and the sidewalls of SWNTs would be much different because the Criegee intermediates on SWNTs can destabilize the tube surface and make the reaction less exothermic or perhaps even endothermic. We have found that direct cleavage of the POZ, resulting in the formation of singlet O2 and an epoxide or an ether, is kinetically more favorable than the Criegee channel on the SWNT surface. We show that chemisorption of O3 at low

J. Phys. Chem. C, Vol. 113, No. 41, 2009 17637 temperature, followed by thermal annealing is expected to form more ether functional groups than carbonyl groups; this prediction is in accord with findings from experiments.17 The Criegee mechanism, which forms a lactone moiety, competes with the direct cleavage channel at higher temperatures. It should be noted that relative importance of the reaction channels can be estimated by comparing reaction barrier heights. On the other hand, it is very challenging to predict the actual yields of intermediates and products. Computational Details. The electronic structure of O3 can be expressed as two O-O bonds and two singlet-coupled π electrons, one on each terminal oxygen atom.52 Higher-order excitations are required to describe the diradical character of O3 and improve the accuracy for geometry optimization and vibrational frequency calculation.52-56 However, Murray and coworkers have shown that DFT calculations with nonlocal functionals can predict the properties of molecules having multireference character with reasonable accuracy, including O3.57 We have ignored the spin contamination of O3 in our calculations. This may shift the reference energy of the reactants, but the barrier heights should not be affected. One may model a nanotube as being of infinite length by using periodic boundary conditions.6,58-63 Alternatively, one may use a short segment of a nanotube to represent a much longer nanotube.37,64-72 Cluster calculations on nanotubes usually involve a multilevel method, such as ONIOM, which treats a small section of the cluster with a high-level method (e.g., DFT) and the surrounding atoms with a lower level of theory (e.g., AM1). The reason for using a multilevel method is that hundreds of atoms are required to model a nanotube that is sufficiently long to be free from end effects. It has been shown that the calculated energies are sensitive to the size of the high-level core in the ONIOM method.39 Use of a small high-level core was found to overestimate the binding energies for chemisorption.67 Further discussion of using the ONIOM method can be found in the literature.73 We have used planewave periodic DFT as implemented within VASP74-76 to compute geometries, energetics, and reaction pathways in this work. The effect of nanotube diameter on reaction energetics was assessed by performing calculations for selected reactions on (8,8) and (10,10) SWNTs and comparing these results with previously published results for the (5,5) and (6,6) SWNTs (see Supporting Information, Table S1). We have found that the reaction energies are similar for the (8,8) and (10,10) nanotubes, whereas reaction energetics for the smaller nanotubes were usually qualitatively different from results on the larger tubes. We have performed extensive calculations for reactions on the (8,8) SWNT because of the lower computational cost. We used three primitive unit cells of the (8,8) SWNT in a hexagonal supercell (Figure 2) in our calculations, where the separation between the nanotubes in the adjacent cells was 10 Å and the supercell lattice parameter in c-direction was 7.4 Å. We used the generalized gradient approximation (GGA) with the PW91 exchange and correlation functionals with Vanderbilt ultrasoft pseudopotentials.77 The energy cutoff was set to 396 eV, and the augmentation charge cutoff was set to 928 eV. The Monkhorst-Pack scheme78 was used to generate 1 × 1 × 3 meshes for k-space integration. The electronic energies were converged to 10-4 eV. The geometries of stationary points were optimized by conjugate gradient minimization, and the ionic energies were converged to 10-3 eV. The nudged elastic band (NEB) method and the CLIMBING method developed by Jo´nnson et al.79,80 were used to locate transition state structures.

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Yim and Johnson

Figure 2. Schematic diagrams of the simulation models for periodic supercell calculations (left-hand side) and for cluster models (righthand side).

Vibrational frequency calculations were performed on selected functional groups using the ONIOM cluster method within the Gaussian 03 package.81 The infrared absorption property of oxygen-containing groups is highly localized in nature and can therefore be predicted with reasonable accuracy using a cluster approach. Ether, epoxide, and Criegee intermediates on the SWNT wall were optimized at the ONIOM(B3LYP/6-31G(d): AM1) level for the vibrational frequency calculations. The (8,8) SWNT cluster contained 208 carbon atoms, terminated with 32 hydrogen atoms (Figure 2). The core at the high level of theory contained 32 carbon atoms and the chemisorbed oxygen atoms. The high-level cores were used for vibrational frequency calculations. The edge carbon atoms were saturated with hydrogen atoms, and the coordinates of the hydrogen atoms were further optimized. The vibrational frequencies were calculated at the B3LYP/6-31G(d) level of theory, and the scaling factor used for the frequencies at this level of theory was 0.9613.82 Results and Discussions Chemisorption of O3 on the (8,8) SWNT. Previous calculations for chemisorption of O3 on a (5,5) SWNT revealed that the reaction was very facile, with an activation barrier of only 0.2 kcal/mol.39 The binding energy of O3 on the (5,5) SWNT was found to be -11.7 kcal/mol.39 Chemisorption of O3 on the (8,8) SWNT should be less exothermic and have a higher activation energy than on the (5,5) SWNT because the larger nanotube has a smaller curvature and therefore less sp3 character. Hence, it will require more energy to buckle the carbon atoms out to form C-O bonds. We calculate the O3/(8,8) SWNT reaction to be exothermic by 1.2 kcal/mol relative to the reactants, which is 10.5 kcal/mol less exothermic than the binding energy reported on the (5,5) nanotube.39 Our calculated activation energy is 7.2 kcal/mol, which is 7 kcal/mol larger than for the O3/(5,5) SWNT system computed by periodic DFTGGA.39 The ONIOM(B3LYP/6-31G(d):AM1) method was used to predict the addition barrier for O3 on an (8,8) SWNT of 6.2 kcal/mol,66 which is similar to our DFT-GGA result. However, the ONIOM method gave a binding energy for O3 chemisorption of -21.2 kcal/mol,66 which is larger in magnitude than our result by a factor of about 18. The deficiency of cluster models for describing SWNT chemistry was reported in the literature.39 These energetics suggest that the chemisorption of O3 can occur at or below room temperature. This result is in agreement with experiments, where SWNT samples are observed to react with O3 at room temperature.17,29 Decomposition through the Criegee Intermediate. A decomposition energy landscape involving the Criegee intermedi-

Figure 3. Reaction energy profile for decomposition of the POZ via a Criegee intermediate. Energies in kcal/mol.

ate is shown in Figure 3. The starting point is gas-phase O3 and an isolated SWNT (zero of energy), followed by the formation of the physisorbed state, to the POZ (structure 1), and from there to etching reactions involving the formation of CO and CO2. Ozone physisorbs onto the SWNT before it chemisorbs, as shown by the first minimum in the energy landscape of Figure 3. The chemisorption barrier for O3 relative to the physisorbed state of O3/(8,8)SWNT is 14.3 kcal/mol, which is comparable to the barrier from the POZ to the Criegee intermediate (structure 2) of 17.0 kcal/mol. These barriers should be surmountable at room temperature; assuming a typical prefactor of 1013 s-1 gives a turn over frequency of 4 s-1 for a process with a barrier of 17 kcal/mol. The ring-opening of the POZ forming the Criegee intermediate (from 1 to 2) is endothermic by 12.8 kcal/mol, relative to the POZ. The Criegee intermediate is less stable than the POZ because the SWNT has to be deformed to cleave one C-C bond on the nanotube. In contrast, ab initio calculations for the gas phase reaction between ethene and O3 indicate that the reaction is highly exothermic with an energy change of -49.2 kcal/mol.49,50 Kinetic experiments by Herron et al.48 failed to observe the Criegee intermediate, probably because the internal energy of the fragment is so high that the carbonyl oxide dissociates almost instantaneously to form other products. The Criegee intermediate (structure 2) can either revert to the POZ (2 to 1) with an energy barrier of 4.2 kcal/mol or react to form an epoxide (2 to 3) through a higher energy barrier of 10.6 kcal/mol. Given these reaction barriers, the Criegee intermediate may be observable if the reaction is quenched to low temperature. However, the presence of other functional groups may make the observation of the Criegee intermediate by IR spectroscopy difficult. See the discussion below on calculated vibrational frequencies. The epoxide is formed as the carbonyl oxide adds its oxygen to the neighboring C-C bond. The barrier from 2 to 3 is much higher than from 2 to 1 because the neighboring carbon atoms must be buckled out to form the C-O bond. The transition state geometry (TS3) is highly strained, having a four-member-ring structure. Once the epoxide is formed (structure 3 in Figure 3), there are two initial pathways for rearrangement. The epoxide oxygen can migrate to bind to the carbonyl oxygen and a neighboring carbon, forming a lactone between carbons C1 and C3 and a

Ozone Oxidation of SWNTs from DFT

Figure 4. Reaction energy profile for decomposition of the POZ forming singlet O2 and epoxide/ether moieties. Energies in kcal/mol.

carbonyl group on C2 (3 to 4 in Figure 3). The barrier for this reaction, TS4, is 24.8 kcal/mol. The other reaction pathway leads to a very similar product, but the lactone group is between carbons C1 and C2, while the carbonyl is located at C3 (3 to 5 in Figure 3). The reaction barrier at TS5 is 16.7 kcal/mol, somewhat lower than the barrier for 3 to 4. These two products have slightly different energies because of the curvature of the nanotube. On a planar graphene sheet, structures 4 and 5 would be identical. Structure 4 can decompose to produce either CO or CO2 upon heating. The decomposition 4 f TS6 f 6 leaves behind a quinone group and produces CO2 with a barrier of 49.3 kcal/ mol. The pathway 4 f TS7 f 7 produces CO and leaves a carbonyl and an ether group; the barrier is 39.4 kcal/mol, indicating that more CO than CO2 should be produced from this decomposition pathway. The energy barriers predicted for decomposition are sufficiently high to preclude significant etching at room temperature. This is in good agreement with experiments, which indicate that the rate of production of CO and CO2 becomes appreciable only at temperatures near 600 K.2 We note that structure 5 will have decomposition pathways that are very similar to those for structure 4. We do not expect the energetics to be significantly different for structure 5, so we have not computed those pathways. Formation of Ether/Epoxide and Singlet O2. The POZ on the nanotube can decompose to form an epoxide group on the SWNT and singlet O2, as shown in Figure 4, 1 f TS8 f 8. The calculated barrier for this reaction is 7.9 kcal/mol. This low activation barrier makes the reaction channel more favorable than the Criegee mechanism and very facile at room temperature. The activation barrier for adding singlet O2 to a (8,0) SWNT was predicted to be 14.1 kcal/mol.61 The addition barrier for singlet O2 on an (8,8) SWNT will be larger than this value because the (8,8) SWNT has larger diameter (10.8 Å) than the (8,0) SWNT (6.3 Å) and will therefore be less reactive. Thus, the functionalization of SWNTs by singlet oxygen is expected to be less effective than by ozone. The epoxide group may migrate to a neighboring C-C bond, as seen in Figure 4, 8 f TS10 f 8. The epoxide migration has an energy barrier of 16.2 kcal/mol, indicating that the diffusion of the epoxide group on the surface of a nanotube at room temperature may proceed at a reasonable rate. However, the epoxide may also transform to an ether through a transition state, TS9, with a barrier of 19 kcal/molsslightly higher than the diffusion barrier. The ether product, 8 f 9, is 15 kcal/mol more stable than the epoxide (see Figure 4). The reaction barrier from

J. Phys. Chem. C, Vol. 113, No. 41, 2009 17639 ethoxide to epoxide (9f TS9f 8) is 34 kcal/mol, which means that once the epoxide transforms to an ethoxide the oxygen atom is kinetically trapped and diffusion will effectively stop for moderate temperatures. For example, the hopping rate at room temperature is on the order of 10-12 s-1, assuming a prefactor of 1013 s-1. Note that the two epoxides shown in Figure 4 (the two structures labeled 8) lie on C-C bonds that are oriented at 30° to the SWNT axis. We have not been able to find epoxides that are formed across C-C bonds perpendicular to the tube axis (parallel to the tube circumference). Our calculations indicate that only ethers are formed on C-C bonds that lie parallel to the circumference of the SWNT (Figure 4, structure 9). This is probably due to the strain on the C-C bond due to curvature of the nanotube when the bond lies parallel to the tube circumference, which would favor rupture of the C-C epoxide bond to form an ether moiety. The epoxide-ether transformation observed in Figure 4, 8 f 9, may affect the formation of the lactone structures 4 and 5 in Figure 3. We have found that the transformation of the carbonyl oxide in structure 2 to the lactone does not proceed by a direct pathway but instead involves the formation of the epoxide, as seen from Figure 3, 2 f 3 f 4. This situation is similar to the gas phase reaction investigated by Alpincourt and Ruiz-Lo´pez.83 After the carbonyl oxide decomposes to a carbonyl group and an epoxide at the β-carbon, the epoxy oxygen may move away from the carbonyl group instead of recombining to form the ester (lactone). When the epoxide in 3 migrates to form an ethoxide on C4-C5, then the formation of the lactone will be inhibited. Thereafter, the quinone group remains intact on the tube wall; the stability of the quinone group is consistent with previous experiments.3 Experiments indicate that the ethoxy C-O-C group is thermally very stable on the SWNT surface. The C-O-C stretching frequency was observed to disappear only at the very high temperature of 1073 K.3 Cai et al. also indicate that high temperatures (873-1073 K) are needed to remove surface functional groups from ozonized SWNTs.22 Our calculations show why this is the case. The ethoxy oxygen is fairly immobile and so cannot easily recombine with other oxygen-containing groups to escape. The barrier for direct decomposition of C-O-C is computed to be very high, at 127.8 kcal/mol (8 f 10 in Figure 4). Hence, etching of the tube by decomposition of the ethoxy group is predicted to occur only at very high temperatures, precisely as observed in experiments.3,22 Comparison of the barrier heights for competing reaction channels provides some insight into the relative rate of reactions but not a complete description of the reaction network. Assuming similar prefactors, a simple analysis indicates that formation of 8 is favored over the formation of 2 by a factor of about 4 × 106. This would indicate that ether and epoxide groups would be virtually the only observable products. There are at least two issues that must be considered in light of this analysis. First, the energy released by exothermic reactions will raise the local temperature and will mitigate the differences in the energy barriers to some extent. A second factor is that the epoxides and ethers resulting from the reaction channels shown in Figure 4 may react with additional O3 molecules from the gas phase to form carbonyl, lactone, and quinone groups. It is beyond the scope of this work to investigate such secondary reaction pathways, but Li et al. have shown that chemisorption of multiple O3 species can etch graphene,84 indicating that similar pathways may exist for nanotubes.

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TABLE 1: Scaled Vibrational Frequencies of Selected Functional Groupsa

a

The scaling factor is 0.9613.82

Vibrational Frequencies of Functional Groups. Previous experimental work revealed that ozonolysis produced carbonyl and ether groups on SWNTs, as observed by IR spectroscopy.3 It is well known that vibrational frequencies of functional groups may be shifted through coupling between other proximate functional groups and nanotube carbons. We have therefore computed the vibrational frequencies of the POZ (1), the Criegee intermediate (2), an ester (4), an epoxide (8), and an ether (9) on a SWNT. We used the ONIOM method to model the functionalized SWNTs, with B3LYP/6-31G(d) used for the highlevel section shown in Figure 2. The computed vibrational frequencies are reported in Table 1. The simulated intensities are given in Figure S3 of the Supporting Information for selected species. To the best of our knowledge, the Criegee intermediate has not previously been observed experimentally. As noted above, we predict that the Criegee intermediate contains less internal energy on a SWNT than on small molecules in the gas phase. Our calculations suggest that the weakly bound Criegee intermediate should be observable if the ozonolysis process is carefully controlled. The vibrational frequency of the O+-Ostretch in 2 is predicted to lie in the range 1055-1096 cm-1, as shown in Table 1. However, the calculated C1-O1-C2 asymmetric stretch of the ethoxy group for structure 9 (Figure 4) is also active in this frequency range (Table 1), which may make the identification of the Criegee intermediate from IR

Yim and Johnson spectroscopy difficult. We note that the O2-O3 stretching (2 in Figure 3) is blue-shifted by 122.9 cm-1 compared to the calculated stretching frequency of O+-O- in H2CdO+-O-. Moreover, the C2-O2 stretching is softened at the nanotube surface when compared to the stretching of -CdO+- in H2CdO+-O-. Our computed results are in reasonable agreement with previous IR absorption studies.17 The C2-O2 stretching frequency for structure 4 in Figure 3 is 1772 cm-1, which is comparable to the observed IR peak at 1739 cm-1 for CdO (ester) stretching.17 The C1-O1 stretching frequency is 1677 cm-1, which is comparable to the IR peak at 1650 cm-1 for CdO (quinone) stretching.17 The C-O-C (ester) stretching appears in the range from about 1126 to 1350 cm-1 (4 in Table 1), which is consistent with the experimental findings of Mawhinney et al.17 However, they assigned the IR band centered at 1040 cm-1 to symmetric stretching of the C-O-C (ester) mode and the IR band centered at 1200 cm-1 was assigned to the asymmetric C-O-C (ester) mode. It is commonly believed that the asymmetric stretching has a higher frequency than the symmetric stretching; however, the situation for groups on the nanotube surface may differ because the functional groups are constrained by the surrounding carbon network. For example, the C13-C2-O3-C3 group in 4 has a higher frequency for the symmetric stretching than for the asymmetric stretching. So it is not always the case that C-O-C asymmetric stretching has a higher frequency than symmetric stretching, as previously assumed.17 The C-O-C (ester) stretching frequency is spread over about 200 cm-1 because the vibrational motions are coupled with the nanotube surface vibrations, which involve C-C-C bending motions parallel and normal to the nanotube surface. Our calculations indicate that the experimentally observed band17 at 1200 cm-1 could be due to ester groups (structure 4 in Table 1) or due to epoxide C-O-C breathing modes (structure 8 in Table 1). The mode observed experimentally at 1040 cm-1 has been attributed to an ester vibration.17 Our calculations indicate that this mode could be an epoxide, an ether (structure 9), or an O-O stretching mode of a Criegee intermediate (Structure 2). The unassigned IR absorption peak at 1380 cm-1 observed by Mawhinney et al.17 may be the C-O-C symmetric stretching of the nanotube ether group (structure 9), or the C13-C2-O3-C3 symmetric stretching computed for structure 4. The unassigned peak at 925 cm-1 may be due to the C-O-C deformation mode of the epoxide (structure 8) having a frequency of 876 cm-1. In general, our calculated frequencies are red-shifted compared with the experimental values, so the 49 cm-1 difference between experimental and computed IR bands is reasonable. Symmetric stretching of the POZ (1) is centered at about 900 cm-1. However, the calculated energetics suggest that the POZ is subject to ring-opening and is not thermally stable at high temperature. Experimental data indicate that the unassigned peak at 925 cm-1 survives thermal annealing at 823 K.17 Therefore, it is unlikely that this unassigned peak is due to the POZ moiety. Conclusions We have performed DFT-GGA calculations to identify the potential energy surface for O3 dissociation on a (8,8) SWNT surface. Two decomposition pathways were considered. The first decomposition pathway involved the formation of a Criegee intermediate, followed by transformation leading to ester functional groups. The activation barriers for these transformations were below about 23 kcal/mol, which should be accessible

Ozone Oxidation of SWNTs from DFT at room temperature. The cleavage of the ester group results in etching of the SWNT, evolving CO and CO2, has a barrier higher than 39 kcal/mol. This agrees well with the experimental findings that the evolution of CO2 and CO from ozonized SWNTs is observable only above 600 K.2 The second decomposition pathway involves the direct cleavage of the ozonide, forming singlet O2 and ether/epoxide groups. This pathway strongly competes with the Criegee mechanism because the barrier for forming singlet O2 is lower. Oxygen migration on (8,8) SWNT would result in trapping of oxygen in the ethoxy form. The immobility of the ether group inhibits the formation of ester groups. The larger activation barriers for removing ethoxy oxygens compared with that for removing the ester groups is consistent with pevious experimental findings.2 Vibrational frequency calculations were carried out on cluster models. The calculated results for CdO stretching modes agrees well with experiments. We suggest the unassigned band at 1380 cm-1 is due to ester and ether groups, while the unassigned band at 925 cm-1 may be due to epoxide groups. Acknowledgment. We gratefully acknowledge the Army Research Office for the support of this work. Computations were performed at the Center for Molecular and Material Simulations at the University of Pittsburgh and at the U.S. Army Research Laboratory Major Shared Resource Center through a Department of Defense High Performance Computing challenge grant. We acknowledge Professor K. D. Jordon for many helpful discussions. Supporting Information Available: Energetics of selected reactions on (n,n) SWNTs (n ) 5, 6, 8, 10), optimized structural parameters of the stationary and transition structures, and simulated infrared absorption spectra of 1-9. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kuznetsova, A.; Yates, J. T., Jr.; Liu, J.; Smalley, R. E. J. Chem. Phys. 2000, 112, 9590. (2) Kuznetsova, A.; Mawhinney, D. B.; Naumenko, V.; Yates, J. T., Jr.; Liu, J.; Smalley, R. E. Chem. Phys. Lett. 2000, 321, 292. (3) Kuznetsova, A.; Yates, J. T., Jr.; Simonyan, V. V.; Johnson, J. K.; Huffman, C. B.; Smalley, R. E. J. Chem. Phys. 2001, 115, 6691. (4) Byl, O.; Kondratyuk, P.; Forth, S. T.; FitzGerald, S. A.; Chen, L.; Johnson, J. K.; Yates, J. T., Jr. J. Am. Chem. Soc. 2003, 125, 5889. (5) Byl, O.; Kondratyuk, P.; Yates, J. T., Jr. J. Phys. Chem. B 2003, 107, 4277. (6) Yim, W.-L.; Byl, O.; Yates, J. T., Jr.; Johnson, J. K. J. Chem. Phys. 2004, 120, 5377. (7) Matranga, C.; Chen, L.; Smith, M.; Bittner, E.; Johnson, J. K.; Bockrath, B. J. Phys. Chem. B 2003, 107, 12930. (8) Ellison, M. D.; Crotty, M. J.; Koh, D.; Spray, R. L.; Tate, K. E. J. Phys. Chem. B 2004, 108, 7938. (9) Matranga, C.; Bockrath, B. J. Phys. Chem. B 2004, 108, 6170. (10) Agnihotri, S.; Mota, J. P. B.; Rostam-Abadi, M.; Rood, M. J. Langmuir 2005, 21, 896. (11) Hemraj-Benny, T.; Bandosz, T. J.; Wong, S. S. J. Colloid Interface Sci. 2008, 317, 375. (12) Liu, J.; Rinzler, A. G.; Dai, H.; Hafner, J. H.; Bradley, R. K.; Boul, P. J.; Lu, A.; Iverson, T.; Shelimov, K.; Huffman, C. B.; Rodriguez-Macias, F.; Shon, Y.-S.; Lee, T. R.; Colbert, D. T.; Smalley, R. E. Science 1998, 280, 1253. (13) Tsang, S. C.; Chen, Y. K.; Harris, P. J. F.; Green, M. L. H. Nature 1994, 372, 159. (14) Ajayan, P. M.; Ebbesen, T. W.; Ichihashi, T.; Iijima, S.; Tanigaki, K.; Hiura, H. Nature 1993, 362, 522. (15) Smith, M. R.; Hedges, S. W.; LaCount, R.; Kern, D.; Shah, N.; Huffman, G. P.; Bockrath, B. Carbon 2003, 41, 1221. (16) Smith, M. R.; Bittner, E. W.; Shi, W.; Johnson, J. K.; Bockrath, B. C. J. Phys. Chem. B 2003, 107, 3752. (17) Mawhinney, D. B.; Naumenko, V.; Kuznetsova, A.; Yates, J. T., Jr.; Liu, J.; Smalley, R. E. J. Am. Chem. Soc. 2000, 122, 2383.

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