Interaction of Oxidized Single-Walled Carbon Nanotubes with

Nov 28, 2001 - Centro de Instrumentos, UniVersidad Nacional Auto´noma de Me´xico, Apdo. Postal 70-186,. Me´xico, D.F. 04510, Mexico, Instituto de C...
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J. Phys. Chem. B 2002, 106, 1588-1597

Interaction of Oxidized Single-Walled Carbon Nanotubes with Vaporous Aliphatic Amines Elena V. Basiuk,*,† Vladimir A. Basiuk,‡ Jose´ -Guadalupe Ban˜ uelos,† Jose´ -Manuel Saniger-Blesa,† Valeriy A. Pokrovskiy,§ Taras. Yu. Gromovoy,§ Aleksandr V. Mischanchuk,§ and Boris G. Mischanchuk§ Centro de Instrumentos, UniVersidad Nacional Auto´ noma de Me´ xico, Apdo. Postal 70-186, Me´ xico, D.F. 04510, Mexico, Instituto de Ciencias Nucleares, UniVersidad Nacional Auto´ noma de Me´ xico, Apdo. Postal 70-543, Me´ xico, D.F. 04510, Mexico, and Institute of Surface Chemistry, National Academy of Sciences of the Ukraine, Prospekt Nauki 31, UA-03680 KieV, Ukraine ReceiVed: May 24, 2001; In Final Form: NoVember 28, 2001

The gas-phase derivatization procedure was employed for direct (i.e., without chemical activation of terminal carboxylic groups) amidization of oxidized single-walled carbon nanotubes (SWNTs) with simple aliphatic amines. The procedure includes treatment of SWNTs with amine vapors under reduced pressure and a temperature of 160-170 °C. Applicability of infrared (IR) spectroscopy and temperature-programmed desorption mass spectrometry (TPD-MS) for chemical characterization of the derivatized SWNTs was analyzed. It was concluded that IR spectra of oxidized SWNTs treated with amines under different conditions (described here and elsewhere) cannot correspond to amide derivatives on SWNT tips because of the very low concentration of the terminal groups relative to the whole sample mass, which implies a negligible contribution to the IR spectra. The bands detectable in the case of long-chain amines correspond to amine molecules physisorbed because of strong hydrophobic interactions of their hydrocarbon chains with SWNT walls. Energetically preferable adsorption sites are the channels inside SWNTs, according to MM+ molecularmechanics modeling. TPD-MS provided additional information on the chemical state of the amines. Heating of the amine-treated SWNTs at >200 °C causes cleavage of alkenes from the amine residues: nonene and pentene form in the case of nonylamine and dipentylamine, respectively. For the short-chain amine (dipentylamine), only one chemical form was detected, whereas two forms (amide and physisorbed amine) can be distinguished for the SWNTs treated with nonylamine. The content of physisorbed nonylamine is about 1 order of magnitude higher than the amide content. According to the results of two-level ONIOM quantum-chemistry-molecular-mechanics calculations, the direct formation of amides on armchair SWNT tips is more energetically favorable than that on the zigzag tips, although the activation barriers are of approximately equal height.

Introduction In the studies of the interaction of carbon nanotubes (CNTs) with organic compounds, amines have attracted special attention.1-9 Aspects of particular interest are the covalent functionalization of CNT probe tips for chemical force microscopy,1,2 increasing the solubility of single-walled CNTs (SWNTs),3-6 their self-assembly on gold substrates,7 plasma activation of CNTs for chemical modification,8 and chemical gating of individual semiconducting and metallic SWNTs.9 Among these, the most extensively explored is the formation of amide derivatives between carboxylic groups on oxidized CNT tips and long-chain amines.1-5 The reaction is currently performed through chemical activation of the carboxylic groups with thionyl chloride or carbodiimides in an organic solvent medium.1-5,7 * To whom correspondence should be addressed. E-mail: elenagd@ servidor.unam.mx. † Centro de Instrumentos, Universidad Nacional Auto ´ noma de Me´xico. ‡ Instituto de Ciencias Nucleares, Universidad Nacional Auto ´ noma de Me´xico. § National Academy of Sciences of the Ukraine.

From common considerations, for the functionalization of CNT tips, one can use the same chemical approaches that have been developed for other poorly soluble inorganic materials, for example silica materials. A decade ago we performed systematic studies on the use of the gas-phase chemical derivatization for the synthesis of chemically modified silicas, mainly for liquid-chromatographic applications.10-14 Among the derivatizing reagents tested were compounds that are not volatile under ambient temperature and pressure, such as polyazamacrocyclic ligands,10,12 pyrimidine bases,12,14 and solid carboxylic acids.11,13 However, decreasing the pressure to a moderate vacuum and, on the other hand, increasing the temperature to >150 °C provided efficient formation of the chemically bonded surface derivatives. In particular, the reaction between silica-bonded aminoalkyl groups and vaporous carboxylic acids to form surface amides proceeds smoothly at 150180 °C without chemical activation of the carboxylic groups, it is relatively fast (0.5-1 h), and it provides high yields of the amide derivatives (>50% yield based on the starting surface concentration of aminoalkyl groups). Excess derivatizing reagent is spontaneously removed from the reaction zone. In addition, there is no need to use a (organic) solvent medium; this feature

10.1021/jp0120110 CCC: $22.00 © 2002 American Chemical Society Published on Web 01/24/2002

Oxidized Single-Walled Carbon Nanotubes

Figure 1. Vacuum manifold used for the gas-phase derivatization of SWNTs: (1-5) Teflon valves; (6-9) 10-mm i.d. O-ring joints; (1012) 41.4-mm i.d. O-ring joints; (13) milligram-scale reactor; (14) gramscale reactor.

is attractive not only from an ecological point of view, but also in that it helps to avoid undesirable particle aggregation of the material derivatized. With the above advantages of the gas-phase derivatization in mind, in the present study, we attempted to apply this procedure to oxidized SWNTs containing carboxylic groups on their tips, in other words, to verify whether the amide derivatives can be synthesized directly according to the following general scheme:

SWNT-COOH + HNR1R2 f SWNT-CO-NR1R2 + H2O (1) where HNR1R2 is an aliphatic amine. Nonylamine, dipentylamine, ethylenediamine, and propylenediamine were used as test compounds. Applicability of infrared (IR) spectroscopy and temperature-programmed desorption mass spectrometry (TPDMS) for chemical characterization of the derivatized SWNTs was analyzed. Molecular-mechanics and quantum-chemistrymolecular-mechanics calculations were employed to gain insight into the mechanisms of SWNT-amine interactions. Experimental Section Materials. Nonylamine, dipentylamine, anhydrous ethylenediamine, and propylenediamine from Aldrich were used as received, without further purification. SWNTs from MER Corporation were purified/oxidized, following the reported procedures.15,16 Gas-Phase Derivatization. The procedure was performed using the custom-made Pyrex glass vacuum manifold presented in Figure 1. In a typical experiment, 100 mg of oxidized SWNTs was placed into the bigger reactor 14. To remove volatile contaminants from the SWNTs, the reactor was pumped out to a vacuum of ca. 10-2 Torr (valves 1 and 4 open; 2, 3, and 5 closed), and its bottom was heated for 0.5 h at 100-120 °C by means of a heating mantle. Then, the reactor was cooled and opened and ca. 50 mg of amine was dropped to the bottom containing the SWNTs. After pumping the reactor out to ca. 1 Torr at room temperature, its valve 4 was closed, and the bottom was heated at 160-170 °C for 1 h. During this procedure, amine evaporated and reacted with SWNTs, and its excess condensed

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Figure 2. Custom-made system used for temperature-programmed desorption mass spectrometric measurements.

a few centimeters above the heating mantle. After the procedure was finished, valve 4 was opened again for ca. 15 min to remove volatiles. Then, the heating mantle was removed, and the reactor was cooled and disconnected from the manifold. Before unloading the SWNTs, the upper reactor part with the condensed excess amine was wiped with cotton wool wet with ethanol. For milligram-scale syntheses, the narrow reactor 13 can be used in a similar way. Infrared Spectra. IR spectra of SWNTs were recorded in KBr pellets (4 mg of SWNTs per 130 mg of KBr) on a Nicolet 5SX FTIR spectrometer. Atomic Force Microscopy. Atomic force microscopy (AFM) measurements were performed using an AutoProbe CP instrument from Park Scientific Instruments (tapping mode; typical force constant of 2.1 N/m; typical resonance frequency of 80 kHz). SWNTs were dissolved in tetrahydrofurane with sonication and deposited onto a freshly cleaved mica surface. Temperature-Programmed Desorption Mass Spectrometry. TPD-MS measurements were performed using a custommade system, the general scheme of which is shown in Figure 2. The system was based on a MX-7304A mass spectrometer (Sumy, Ukraine) with a mass range of 1-400 Da and a sensitivity of 10-8 g. The temperature ramp was variable in a range of 0.05-30 °C min-1. SWNT samples (0.1-1 mg) were placed into the quartz-molybdenum tube and the tube was evacuated to 10-1 Pa and then attached to the mass spectrometer inlet. The reactor-to-mass spectrometer interface included a highvacuum valve with a 5-mm orifice and a 20-cm long tube, which was kept at 150 °C. The sample tube was opened in the ionsource direction, and under the heating rate used (about 0.1 °C s-1), the observed intensity of the ion current is expected to be proportional to the desorption rate so that diffusion inhibition may be neglected. We assumed quasi-stationary conditions when the shape and position of desorption peaks do not depend on the temperature of the spectrometer interface, sample mass, and its geometric characteristics. An average experiment duration was 1 h. More details on TPD-MS methodology and data interpretation were described in our previous papers.17-19 Theoretical Section Molecular Mechanics. For purely molecular mechanics modeling, the MM+ method was used, implemented in the HyperChem version 5.1 package (by HyperCube Inc., Canada). In all calculations, full geometry optimization was performed

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with the Polak-Ribiere conjugate gradient algorithm and a rms gradient of 0.001 kcal Å-1 mol-1. For some bigger molecular systems (SWNTs larger than 15-Å diameter), the latter convergence criteria were difficult to meet because of a large number of shallow local minima; in such cases, the calculations were stopped when the total energy did not change anymore after >500 cycles. Quantum Chemistry/Molecular Mechanics. Reaction mechanisms were studied using a two-level ONIOM approach20,21 implemented in the Gaussian 98W suite of programs.21,22 The universal force field23 (UFF) was used for the low-level treatment, and the Becke’s three-parameter hybrid method24 with the exchange functional of Lee, Yang, and Parr25 (B3LYP) was used for the high-level description, in conjunction with the 4-31G basis set by Pople et al.26-28 The usefulness of this theoretical approach to study chemical reactions of SWNTs was demonstrated recently.29 A search for transition states was carried out using the QST2 procedure. The stationary point geometries were fully optimized and characterized as minima (0 imaginary frequencies) or first-order saddle points (1 imaginary frequency) by calculations of vibration frequencies. All of the optimizations met the default convergence criteria set in Gaussian 98W. Results and Discussion IR spectra of SWNTs treated with organic amines have been measured by different research groups.3,4,6,7 Unfortunately, the data reported so far do not allow an unambiguous interpretation of the spectral changes observed because they turn out to be quite contradictory. In particular, after transforming oxidized SWNTs into their octadecylamide derivatives, Chen et al.3 found two νCdO (amide I) bands at 1663 and 1642 cm-1 of almost equal intensity and not one band as one normally expects. Afterward, the same group4 performed a similar SWNT derivatization with 4-dodecylaniline; now, they found two bands at 1655 and 1598 cm-1 and assigned them to νCdO (amide I) and δNH (amide II) vibrations. This is a reasonable assignment for the waVenumbers. However, the band intensities must equally be taken into account: in the IR spectrum of SWNT 4-dodecylanilide,4 the intensity of the δNH band is several times higher than that of the νCdO band, whereas in amides the carbonyl band is always more intense (usually by about a factor of 2) than the amide II band. In addition, in both cases, the intensities of the bands appearing after amide formation turn out to be much higher than those of the bands typical for oxidized SWNTs (at 1200, 1590, and 1720 cm-1), thus giving the impression that in the synthesized samples amine content is higher than SWNT content. Finally, according to Liu et al.,7 SWNTs derivatized with cysteamine exhibit a band around 1600 cm-1, which was assigned to amide I vibrations: this interpretation can hardly agree with the preceding data. Bearing the above observations in mind, we were not very optimistic about the possibility of providing a straightforward IR-spectral characterization for our samples. Still, the IR spectra obtained appeared even worse than we expected. Of four amines (nonylamine, dipentylamine, ethylenediamine, and propylenediamine) used for the gas-phase treatment of oxidized SWNTs, only nonylamine treatment caused obvious changes in the IR spectra (Figure 3a,b). In the other three cases, the changes detected were at the noise level. If we suppose that all of the amines studied form amide bonds with carboxylic groups at SWNT tips (there are no logical reasons to expect that this is not the case), such spectral behavior is hard to explain: for smaller amine molecules, the efficiency of amide formation

Figure 3. IR spectra of oxidized SWNTs (a), SWNTs after gas-phase treatment with nonylamine (b), and SWNTs after liquid-phase impregnation with nonylamine from ethanol solution with an SWNTnonylamine mass ratio of 3:1 (c) and 1:1 (d). Samples used were 3% SWNT samples in KBr; spectra are presented without baseline correction.

should be higher than for longer-chain molecules (such as nonylamine) because of better accessibility to the reacting carboxylic groups of the SWNTs. The IR spectrum of nonylamine-treated SWNTs (Figure 3b) contains several new bands due to nonylamine at 1358 (C-N stretch.), 1462 (C-H def.),30 2856 (sym. C-H stretch.), 2927 (asym. C-H stretch.), and 1586 cm-1. Assignment of the last band is the most important to characterize a chemical state of nonylamine, but obviously its frequency is too low to attribute this band to νCdO (amide I) vibrations; it should be assigned to δNH vibrations of the NH2 group. Along with the above bands, there is one more at 1715 cm-1, having a shoulder at ∼1750 cm-1. However, before trying to suggest a further explanation, it is worthwhile to analyze in more detail what kind of information on the terminal organic groups can be expected from IR spectra of typical SWNTs. The first thing to remember is that in some respect NTs are similar to chemically derivatized inorganic adsorbents (silica,

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Figure 4. Schematic representation of a (20,0) zigzag SWNT of ∼1.55-nm diameter and 10-nm length, containing 10 carboxylic groups at each tip.

Figure 5. AFM micrograph of SWNTs gas-phase treated with nonylamine.

alumina, amorphous carbon, graphite, etc.). For the latter, the contribution of surface organic groups to the whole IR spectrum is relatively low because of an overwhelming mass fraction of the adsorbent itself; for CNTs, the same effect should be expected because of their typically large aspect ratios. Even SWNTs called “short”, prepared by oxidation and purification in strong acids, are on average 100-300 nm in length, at a diameter of ∼1.4 nm.3,4,6,7,15,31,32 To demonstrate the implications of these parameters, we performed the following experiment. To evaluate the SWNT-nonylamine mass ratio in the gasphase-treated sample, we impregnated oxidized SWNTs with the amine from ethanol solution taking two different proportions, 3:1 and 1:1 (w/w). The samples were dried in a vacuum, and IR spectra were measured for them under the same conditions (3% SWNTs in KBr) as for oxidized SWNTs and the gas-phasetreated sample (Figure 3c,d). The resulting spectra are qualitatively similar to each other, as well as to the spectrum of the gas-phase-treated sample (Figure 3b). Comparing the transmittance scale and amine-band intensities, one can conclude that the latter sample has a closer resemblance to the impregnated sample of the 3:1 SWNT-nonylamine mass ratio. Implications of the above observations can be understood in terms of SWNT aspect ratios. For illustrative purpose, one can take a (20,0) zigzag SWNT backbone, containing 1920 carbon atoms (Figure 4) of ∼1.55-nm diameter and 10-nm length. Such a SWNT can have about 10 carboxylic groups at each tip (a higher density of COOH groups at the tip is unlikely for steric reasons). As a result, it can form amide bonds with 20 nonylamine molecules, and this stoichiometry would correspond to a SWNT-nonylamine mass ratio of 8.4:1. Thus, even if we suppose that oxidized SWNTs are on average 10 nm in length, to obtain the IR-band intensities shown in Figure 3b,c, one would have to have a nonylamine content roughly triple that of the stoichiometric one. The sample would have to contain >60% nonylamine in a physically adsorbed form, and correspondingly >60% of the IR absorption would be due to the chemically unbound fraction. It should be obvious that for 100-300-nmlong SWNTs, the amide contribution to the IR absorption will be even lower, by 10-30 times, and such bands are almost

Figure 6. TPD-mass spectrum (a) of volatile products evolved at 325 °C from SWNTs gas-phase treated with nonylamine and experimental thermograms (b) for selected hydrocarbon peaks at m/z ) 42, 43, 55, 57, and 69.

Figure 7. Logarithm of desorption rate k for three values of reaction order (0sfirst order, )ssecond order, Osthird order) as a function of inverse temperature τ ) (KT)-1, calculated from eq 6 in ref 19. Temperature dependence is linear for first-order reaction; values of activation energy E and preexponential factor k0 are E ) 79.6 kJ mol-1 and k0 ) 105 s-1.

impossible to distinguish (especially given the generally poor quality of the CNT spectra). SWNTs gas-phase treated with nonylamine have dimensions of the same order of magnitude, as can be seen from the AFM microphotograph in Figure 5. Consequently, the data presented here and elsewhere3,4,6,7 reflect mainly amines strongly physisorbed because of hydrophobic

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Figure 8. Experimental thermogram (solid line) for the peak at m/z ) 43 for SWNTs gas-phase treated with nonylamine as a superposition of two first-order effects: (a) E1 ) 81; (b) E2 ) 93.5, k0 ) 105.8 s-1.

Figure 10. Mutual orientation (a) of a zigzag (n,0) SWNT-COOH ((20,0), as an example) and the nonylamine molecule inside it in MM+ molecular mechanics simulation (carboxylic O-atoms and amine N-atom are highlighted) and effect of n on the difference between total energies (b) of nonylamine “adsorption” on the outside and inside SWNT wall.

Figure 9. A (12,0) zigzag SWNT-COOH (a) as an example of (n,0) SWNT models used to simulate adsorption of nonylamine on the inside and outside walls (carboxylic O-atoms are highlighted) and effect of n on total SWNT energy (b) calculated with the MM+ force field.

interactions between their long-chain hydrocarbon radicals and CNT side walls. As we already mentioned, in the IR spectrum of the gasphase-treated sample (Figure 3b), there is one more band at 1715 cm-1 with a shoulder at ∼1750 cm-1. This band (observed in the 1700-1735-cm-1 range by different authors) is believed to correspond to the terminal COOH groups.3,4,6,7,32,33 A higherfrequency absorption was reported as well; in particular, for

SWNTs after oxidation with gaseous ozone (i.e., no liquid-phase treatment with strong acids), the band at 1739 cm-1 was assigned to ester νCdO vibrations.34 A band at 1750 cm-1 was found also for CNTs after boiling in concentrated HNO3;35 in this case, however, the existence of ester groups must be excluded: in strong acid media, they are hydrolyzed into COOH groups. In our case, the shoulder at 1750 cm-1 cannot be due to ester absorption, for the same reason. One might suggest the formation of anhydrides through thermal condensation of neighboring carboxylic groups; however, carboxy anhydrides are extremely reactive, especially with amines. Besides that, we did not detect this absorption in IR spectra of oxidized SWNTs heated to 150-180 °C in a vacuum in the absence of amines. One more observation that is difficult to explain is that in the liquid-phase-impregnated samples (Figure 3c,d) the band at 1720 cm-1 significantly increased as compared to that in the

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Figure 11. TPD mass spectrum (a) of volatile products evolved at 252 °C from SWNTs gas-phase treated with dipentylamine; experimental thermograms (b) for selected hydrocarbon peaks at m/z ) 42, 55, 56, 57, and 70.

gas-phase-treated sample. This band is apparently due to SWNTs and not to nonylamine because it does not increase in intensity to the same degree as the bands corresponding to nonylamine do. To summarize, we believe that some of the existing interpretations of IR spectra of CNTs are premature and need further extensive studies. Looking for other techniques that could be more informative for chemical characterization of SWNT-amine systems, we tested temperature-programmed desorption mass spectrometry (TPD-MS), which is widely used to study organic groups and molecules on inorganic adsorbent surfaces,17-19 including carbon materials.36-39 A first feature found in the mass spectra of SWNTs gas-phase treated with nonylamine is a series of hydrocarbon peaks appearing at temperatures of ∼200 °C (Figure 6a) at m/z ) 27 (C2H3), 29 (C2H5), 41 (C3H5), 42 (C3H6), 43 (C3H7), 55 (C4H7), 56 (C4H8), 57 (C4H9), 67 (C5H7), 69 (C5H9), 70 (C5H10), 71 (C5H11), 79 (C6H7), 81 (C6H9), 83 (C6H11), 84 (C6H12), 85 (C6H13), 97 (C7H13), 98 (C7H14), and 99 (C7H15). Thermograms (i.e., plots of peak intensities vs temperature) for all of these peaks have a similar shape; they pass through a maximum at ∼325 °C (as an example, Figure 6b shows thermograms for selected peaks at m/z ) 42, 43, 55, 57, and 69), and total disappearance of the hydrocarbon fragments is observed at ∼400 °C. (No hydrocarbon peaks were found in TPD mass spectra of SWNTs without amine treatment, recorded for comparison under the same conditions, in the same temperature interval.) Taking into account the consistency of their behavior, one can infer a common origin of the hydrocarbon fragments. Moreover, by searching for similar mass spectra in the Wiley 138 K Mass Spectral Library40 (including about 138 000 spectra), we found that both the mass numbers

and the peak intensity distribution correspond to nonene. The smooth curve of decreasing intensities (Figure 6a) is characteristic of straight-chain hydrocarbons and not of branched ones. At the same time, location of the double bond is difficult, as usual for acyclic olefins, because of its facile migration in the fragments.41 We suggest it to be at position 1 and explain the formation of 1-nonene by pyrolysis of nonylamide terminal groups in SWNTs according to the following scheme:

SWNT-CO-NH-(CH2)8-CH3 f SWNT-CO-NH2 + H2CdCH-(CH2)6-CH3 (2) Apparently, nonylamine molecules strongly adsorbed in SWNTs decompose in a similar way, forming nonene and ammonia. The presence of two forms of nonylamine can be confirmed by the following results. For a detailed analysis of the thermal evolution of the hydrocarbon fragments, we selected the peak at m/z ) 43 as one of the most abundant and illustrative peaks. According to the analytical procedure described in ref 19, for this peak, we plotted the logarithm of the desorption rate k as a function of inverse temperature, τ ) (KT)-1, supposing first, second, and third reaction orders (Figure 7). The dependence appears to be linear for the first-order reaction. The calculated values of activation energy and preexponential factor (E ) 79.6 kJ mol-1 and k0 ) 105 s-1, respectively) are very low, suggesting the existence of an activation-energy distribution for the process of destruction of the nonylamine species. An attempt to fit the observed dependence with a rectangular distribution of activation energies18 gave unsatisfactory results. The reason becomes clear if we consider the shape of the thermodesorption curve (for m/z

1594 J. Phys. Chem. B, Vol. 106, No. 7, 2002 ) 43, as well as for other hydrocarbon peaks); it is noticeably asymmetric. Good agreement with the experimental curve was obtained for two activation energies, admitting the existence of two types of nonylamine species in the SWNTs, with E1 ) 81 and E2 ) 93.5 kJ mol-1 (Figure 8). This agrees well with the IR spectral results if one assumes that one of them is the terminal nonylamide derivative (due to the first thermodesorption maximum at ∼250 °C and lower abundance of hydrocarbon decomposition products) and that the other, more abundant, is nonylamine physisorbed on SWNTs because of strong hydrophobic interactions (due to the second thermodesorption maximum at ∼320 °C and higher abundance of hydrocarbon decomposition products). Their abundance ratio estimated from the maxima of curves a and b is ∼1:5. Where exactly in the SWNTs are nonylamine molecules most likely to physisorb? Evidently, there should be a well-manifested energetic difference between adsorption outside and inside CNTs because of a cooperative wall effect in the latter case. We attempted to estimate until what SWNT size this difference can be observed, using MM+ molecular mechanics. As SWNT models, zigzag species were usedsvariable in diameter (starting with n ) 12) but of the same length (Figure 9a), sufficient to accommodate one nonylamine molecule oriented along the SWNT axis. For narrower SWNTs (n ) 12 and 13), which are more strained because of a stronger distortion from the plain graphite sheet, the calculated total energies were positive; the values become negative starting with the (14,0) SWNT, as can be seen from Figure 9b. Because of the high hydrophobicity of nonylamine, it sticks strongly at any site on the SWNT walls, and depending on that, the total calculated energies can vary within up to 2 kcal mol-1. To obtain more uniform and comparable results, in all simulations, we chose the same starting geometry, in which the following three criteria are met: (1) the amino group of nonylamine is placed close to the SWNT carboxylic group, (2) the two molecules are parallel to each other, and (3) the C-chain plane is parallel to the closest SWNT wall. For narrower SWNTs, in the calculated “inside” complexes, the molecules tended to remain parallel, but the nonylamine gradually rotated with increasing SWNT diameter, as can be seen, for example, for a (20,0) SWNT (Figure 10a). In the “outside” complexes, this effect was already observed for n ) 12, for which the angle between the SWNT and the nonylamine axes was ∼10°. For a (30,0) SWNT (the biggest considered in the present study), the angles for the “inside” and the “outside” nonylamine orientation both reached 30°. For even bigger n (e.g., 40 and 60), the angles increase further, although we have not been able to finish the geometry optimization because of convergence problems. Nevertheless, this trend is evident, and one can conclude that hydrophobic interactions contribute more significantly to long-chain amine adsorption on a SWNT than interaction between the polar NH2 and COOH groups does. As regards the energetic differences between adsorption outside and inside a SWNT (plotted in Figure 10b), it was biggest (∼28 kcal mol-1) for a (12,0) SWNT because of the greater effect of other walls. Then, ∆Eout/in drops rapidly with increasing n up to ∼20, and its further decrease becomes very slow: even for a (30,0) SWNT (of which the diameter is 2.3 nm for the MM+ geometry), ∆Eout/in is almost 5 kcal mol-1. Thus, the energetic difference discussed should be significant for a wide size range of CNTs (both single- and multiwalled), and the preferential sites for adsorption of long-chain amines (and most likely of other organic molecules) are inside SWNTs. As was mentioned before, we found no IR-spectral manifestations of interaction of gaseous dipentylamine with oxidized

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Figure 12. Monocarboxyl-substituted fragments of (10,0) zigzag (a) and (5,5) armchair (b) SWNTs used for two-level ONIOM calculations of the model gas-phase reaction with methylamine. The highlighted (dark) atoms and those constituting the methylamine molecule were treated at the B3LYP/4-31G level of theory; the remaining SWNT atoms were treated with UFF molecular mechanics.

SWNTs. However, the TPD-MS method was able to provide useful information on the reaction products. In some respect, its behavior is similar to that of the nonylamine sample. The mass spectra of the desorbed species (Figure 11a, for 252 °C, as an example) contain a series of peaks due to hydrocarbon fragments, but the highest mass number detected was 70. This peak, as well as the peaks at lower m/z, corresponds to pentene (molecular weight 70); this identification was made by searching in the Wiley 138 K Mass Spectral Library.40 The formation of pentene can be explained by thermal decomposition of the dipentylamide groups on the SWNT tips, by analogy with the previous case [ reaction 2]:

SWNT-CO-N((CH2)4CH3)2 f SWNT-CO-NH2 + 2H2CdCH-(CH2)2-CH3 (3) Thermodesorption curves (Figure 11b) for different hydrocarbon peaks have similar profiles. However, a big difference between the two samples is that the curves are almost symmetric for the dipentylamide derivative. Their maximum was found at ∼250 °C; this approximately coincides with the first maximum for the nonylamine sample. Evolution of the hydrocarbon species ceased at 200 °C causes cleavage of alkenes from the amine residues: nonene and pentene form in the case of nonylamine and dipentylamine, respectively. For the short-chain amine (dipentylamine), only one chemical form was detected, whereas two forms (amide and physisorbed amine) can be distinguished for the SWNTs treated with nonylamine. The content of physisorbed nonylamine is about 1 order of magnitude higher than the amide content. (3) According to the results of two-level ONIOM quantumchemistry-molecular-mechanics calculations, the direct formation of amides on armchair SWNT tips is energetically preferable to that on the zigzag tips, although the activation barriers are of approximately equal height.

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