ARTICLE pubs.acs.org/JPCC
Improving the Chemical Reactivity of Single-Wall Carbon Nanotubes with Lithium Doping Pablo A. Denis* Computational Nanotechnology, DETEMA, Facultad de Química, and Centro Interdisciplinario en Nanotecnología y Química Física de Materiales, Espacios Intersiciplinarios, UDELAR, CC 1157, 11800 Montevideo, Uruguay ABSTRACT: The effect of lithium doping on the reactivity of single-wall carbon nanotubes (SWCNTs) was studied by means of first principle calculations. Two prototype reactions were considered, the 1,3 dipolar cycloaddition and the attachment of free radicals. The results obtained employing the PBE, M06L, and M06-2X functionals indicated that lithium significantly increases the reaction energies. Although this effect is slightly increased upon diameter enlargement, it decreases when the distance between the functional group and lithium is augmented. We observed a stronger enhancement of reactivity for semiconducting SWCNTs as compared with the metallic ones. Regarding the endohedral and exohedral lithium dopings, both increase the binding energies, although for some functional groups the latter doping is far less effective because exohedral lithium can remove the fluorine atoms or hydroxyl or thiol groups from the tube walls. Therefore, endohedral lithium is expected to be more useful to enhance the reactions performed onto SWCNTs, whereas in some cases exohedral lithium may have a negative effect depending on the affinity that lithium has with the functional groups attached to the SWCNTs. If the lithium functional group interaction is stronger than the binding energy between the nanotube and the functional group, the perfect sp2 framework of the nanotube will be restored. All of the functionals employed gave the same results from a qualitative standpoint, but important quantitative differences were observed in the magnitude of the reactivity enhancement.
1. INTRODUCTION In 1976, Oberlin et al.1 reported the “Filamentous growth of carbon through benzene decomposition”. Yet, it was not until 1991, when Iijima2 observed “helical microtubules of graphitic carbon”, that the carbon nanotube research woke up after 15 years of sleep. In contrast with fullerenes, the limited solubility of SWCNT hampered the progress of nanotube chemistry. The relentless work of chemists around the world solved this problem and in turn several methods to solubilize SWCNT were reported. Some of these procedures involve the use of surfactants,3 polymers,4 and covalent5 and noncovalent6 functionalization, just mention a few examples. Some reagents can improve the both the solubility and reactivity of nanotubes; such is the case of lithium. In effect, Liang et al.7 dissolved SWCNTs, lithium, and alkyl halides in liquid ammonia, yielding sidewall-functionalized nanotubes. The lithium doping plays a specific role in the reaction, debundling the SWCNTs by electrostatic interactions. A similar method was employed by Chattopadhyay et al.8 to show that SWCNTs salts prepared by treatment with lithium in liquid ammonia react with aryl and alkyl sulfides via single electron transfer to yield radical anions that dissociate into carbon-centered free radicals and mercaptide. The free radicals add to the SWCNT or recombine with the SWCNT radical anions. Lithium doping is not only capable of expanding the covalent chemistry of SWCNTs but also the noncovalent one. Buttner et al.9 proved that the adsorption energies of nonpolar molecules, like n-heptane, onto SWCNTs is increased by alkali metals. The most recent works about the effect of lithium on r 2011 American Chemical Society
the covalent chemistry of nanotubes were the joint theoretical and experimental work by Mandeltort et al.10 and the theoretical one by Choudhury and Johnson.11 Both investigations analyzed the interaction between lithium-doped nanotubes and chloromethane. The experimental and theoretical evidence indicated that lithium not only increases the adsorption of CH3Cl but also catalyzes the C Cl bond rupture in chloromethane. The CH3 radical is stabilized by a lithium atom and then it becomes chemically bound to defect sites of SWCNTs, yielding alkylated nanotubes. Covalent modification of the structure of carbon-based materials is an important tool that enables the synthesis of materials with remarkable properties.12 21 For this reason it is important to find new methods that can expand the chemistry of nanotubes and graphene. In this line, a recent publication22 studied the effect of lithium doping on the reactivity of graphene. Our theoretical calculations showed that a significant enhancement of reactivity can be expected because of the charge that lithium donates to graphene. For example, the reaction energy for the attachment of the hydroxyl radical to graphene is 12.2 kcal/mol, whereas that of lithium-doped graphene is 35.5 kcal/mol, almost 3 times larger. These findings are supported by the experimental work by Fan et al.,23 who studied the effect of charge puddles and ripples on the chemical reactivity of monolayer graphene. The latter results Received: August 5, 2011 Revised: September 6, 2011 Published: September 07, 2011 20282
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and the aforementioned experimental and theoretical ones about the reactivity of lithium-doped carbon nanotubes clearly indicate that lithium increases the reactivity. However, there are plenty of unanswered questions. For example, (a) we do not know if the exohedral and endohedral lithium dopings make any difference, (b) if the increase in reactivity is similar for metallic and semiconducting nanotubes, (c) how the effect of lithium is influenced by the curvature of nanotubes, or (d) if exohedral lithium is capable of removing the functional groups attached to the nanotubes, as observed for lithium-doped graphene. (e) Can one expect a larger or smaller effect as compared with that determined for lithiumdoped graphene? (f) Which are the electronic properties of the functionalized lithium-doped nanotubes? The answers to these questions is not intuitive and thus motivated me to perform first principle calculations to investigate the 1,3 dipolar cycloaddition of azomethine ylides and the addition of free radicals, namely, OH, SH, F, CH3, and H. It is my hope that the present work can answer the above-mentioned questions and elicit new experimental and theoretical investigations about Li-doped nanotubes.
2. METHODS We performed density functional theory calculations employing two different implementations: SIESTA24,25 and Gaussian 2009.26 The calculations carried out with SIESTA are similar Table 1. Electronic Binding Energies (kcal/mol) Determined for the System Li@(5,5) method
system
model
binding energy
PBE/DZP
Li@(5,5)
∞ (three unit cells)
30.7
PBE/6-31G
Li@(5,5)
∞ (three unit cells)
26.1
PBE/6-31G*
Li@(5,5)
∞ (three unit cells)
30.3
M06-L/6-31G
Li@(5,5)
C80H20
33.5
M06-2X/6-31G B3LYP/6-31G
Li@(5,5) Li@(5,5)
C80H20 C60H20
29.4 29.8
PBE/DZP
Li-(5,5)
∞ (three unit cells)
23.8
PBE/6-31G*
Li-(5,5)
∞ (three unit cells)
23.5
to the ones applied to successfully study the covalent chemistry of graphene22,27 33 and nanotubes.34 37 PBE38 periodic calculations were performed in conjunction with the double-ζ basis set with polarization functions and fixed the orbital confining cutoff to 0.01 Ry. The split norm used was 0.15. The DFT implementation in SIESTA can be prone to significant basis set superposition error (BSSE), even with a relatively low degree of radial confinement.29 To avoid this problem, the counterpoise correction suggested by Boys and Bernardi were used.39 In all cases, relaxed structures were employed to estimate the BSSE corrected binding energies, and monomer deformation energies were taken into account. The interaction between ionic cores and valence electrons was described by the Troullier Martins norm conserving pseudopotentials.40 The convergence of the mesh cutoff was checked; using a value of 200 Ry, converged binding energies (within 0.02 eV) were obtained. Geometry optimizations were performed using the conjugate gradient algorithm until all residual forces were smaller than 0.01 eV/Å. For SWCNTs, periodic boundary calculations were undertaken for tubes that have similar diameter to those produced by the HiPco method.41 The (5,5) and (10,0) SWCNTs were selected and three unit cells were used to model the metallic tube and two for the semiconducting one. The lattice parameter along the c direction was fixed to 7.38 and 8.52 Å, for the (5,5) and (10,0) SWCNT, respectively. For the a and b directions, they were fixed to 20 Å, a value large enough to prevent the interaction between adjacent images. Bearing in mind the symmetry of the SWCNTs, a Monkhorst Pack sampling of 1 1 300 was used. In the case of the Gaussian-based calculations, the B3LYP,42,43 M06-2X,44 M06L,44 and PBE39 functionals were our choice. The B3LYP and M06-2X methods are more accurate than the PBE and M06L ones. Yet, the problem that arises is that periodic calculations with functionals that include exact HF exchange become complex. For this reason, cluster models were used to investigate the effect of lithium doping on the reactivity of SWCNTs at the B3LYP and M06-2X levels. C60H20 and C80H20 models were constructed to mimic the infinite (5,5) tube. As the metallic properties of this nanotube cannot be reproduced by a finite model, smaller reactivity is
Figure 1. Optimized structures for the lithium-doped (5,5) SWCNT with and without functional groups attached to the sidewalls. Corresponding structures for (10,0) SWCNT are similar. 20283
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Table 2. Electronic Binding Energies (kcal/mol) for the Addition of Radicals and Azomethine to Bare and Lithium-Doped (5,5) and (6,6) SWCNTs, Determined at Different Levels PBE DZP
PBE 6-31G*
extra bind. bind. energy
energy
dist C Xa
bind.
extra binding
PBE 6-31G:
M06L 6-31G:
M06-2X 6-31G:
energy
energy
extra binding energy
extra binding energy
extra binding energy
model
∞
∞
∞
∞
∞
∞
∞
C80H20
(5,5)+SH Li@(5,5)+SH
7.8 21.3
13.5
1.917 1.913
10.2 29.1
18.9
19.4
15.1
25.8
Li-(5,5)+SH-close
19.6
11.8
1.936
Li-(5,5)+SH-far
12.2
4.4
(5,5)+OH
31.6
Li@(5,5)+OH
46.8
15.2
1.449
58.6
22.3
23.1
18.5
27.7
Li-(5,5)+OH-close
45.9
14.3
1.467
57.8
21.5
(5,5)+F
52.4
1.457
57.2
Li@(5,5)+F (5,5)+H
65.8 37.5
13.4
1.451 1.127
80.7 36.9
23.5
23.7
19.6
31.7
Li@(5,5)+H
47.9
10.4
17.1
18.0
12.8
23.0
(5,5)+CH3
26.1
Li@(5,5)+CH3
37.4
18.9
19.4
(5,5)+azomethine
23.1
Li@(5,5)+azomethine
24.6
1.942 1.461
11.3
36.3
1.126
54.0
1.564
27.8
1.554
46.7
1.586
20.3
1.558 1.5
1.610
Li@(6,6)+OH
43.8
16.3
1.582 1.450
Li@(6,6)+F
62.3
16.1
1.454
25.9
5.6
12.5
a
X = OH, SH, F, H, azomethine. For the (5,5) SWCNT, the addition of the azomethine ylide is to a CC bond slanted with respect to the tube axis, whereas for the (10,0), it corresponds to the addition onto a CC bond parallel to the tube axis.
expected as compared with that corresponding to the infinite model.45,46 However, this effect is not expected to be a problem because interest lies in the enhanced reactivity associated with lithium doping and not in determining highly accurate bond dissociation energies. The basis set that accompanied the hybrid functionals was the 6-31G.47 In the case of M06L, periodic calculations were performed using the 6-31G and 6-31G* basis sets.47 The ultrafine grid was employed for the M06-2X and M06L calculations. The periodic PBE calculations conducted by means of Gaussian 2009 are intended to test those performed with SIESTA. These results are expected to be more accurate than those determined with SIESTA, because they do not rely in the use of pseudopotentials and use basis sets of better quality, like the 6-31G* one.
3. RESULTS AND DISCUSSION 3.1. Lithium Encapsulation Energies. In Table 1 is listed the encapsulation energies for a single lithium atom and the (5,5) SWCNT using different functionals. The calculations employing four different functionals gave similar interaction energies. Indeed, at the PBE/DZP level the complexation energy is 30.7 kcal/mol, whereas at the M06-2X/6-31G level it is 29.4 kcal/mol. We expect that the agreement would be better if periodic calculations are performed with the M06-2X, because cluster models tend to underestimate the reactivity of metallic nanotubes.45,46 This comparison is important because it validates the use of PBE for this type of interaction, which is mostly ionic. In line with the results obtained by Baker and Head-Gordon,48 the charge transfer observed is very sensitive to the basis set and method used.
However, the spin density is nearly the same in all cases and it indicates a complete charge transfer from lithium to the nanotube, again in agreement with Baker and Head-Gordon, who proved that charge transfer between lithium and PAH larger than benzene is expected.48 3.2. Reactivity Enhancement Due to Lithium Doping. The structures optimized for the functionalized lithium-doped (5,5) SWCNT are presented in Figure 1 [those corresponding to the (10,0) SWCNT are similar; thus, they are not presented]. Electronic reaction energies are listed in Tables 2 and 3 for the metallic (5,5) and semiconducting (10,0) nanotubes, respectively. We define the increased reactivity due to lithium as the difference between the binding energy determined for the bare and lithium-doped nanotubes. For example, at the PBE/DZP level the binding energy for the attachment of OH to the (5,5) SWCNT is 31.6 kcal/mol, whereas for Li@(5,5) it is 46.8 kcal/mol. Therefore, the extra binding energy due to lithium doping for the addition of OH is 15.2 kcal/mol. The three functionals selected indicated that lithium doping significantly increases the reactivity of carbon nanotubes. Yet, the extra binding energies computed with different methodologies showed some quantitative differences. The values obtained at the PBE/6-31G* level as implemented in Gaussian are systematically larger than those determined with PBE/DZP (SIESTA). Inspection of the results presented in Tables 2 and 3 shows that for metallic nanotubes the extra binding energies at the PBE/6-31G* level are between 4 and 10 kcal/mol larger, and those for the semiconducting nanotubes are larger by 3 4 kcal/mol. The extra binding energies computed with M06L and using infinite models are bracketed by the PBE/DZP and PBE/6-31G* results. In the case of the M06-2X/6-31G* methodology, when 20284
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Table 3. Electronic Binding Energies (kcal/mol) for the Addition of Radicals and Azomethine to Bare and Lithium-Doped (10,0) and (14,0) SWCNTs, Determined at the PBE/DZP Level PBE DZP total bind. extra bind. energy Li@(10,0)
26.3
Li-(10,0)
21.5
(10,0)+SH
energy
PBE 6-31G* dist C Xa
total bind. extra binding energy
energy
29.1 1.949
5.6
Li@(10,0)+SH
21.4
3.9 17.5
1.922
26.8
Li-(10,0)+SH-
10.1
6.2
1.942
far (10,0)+OH
26.1
1.465
32.1
Li@(10,0)+OH
46.5
20.4
1.453
55.6
Li-(10,0)+OH-
45.9
19.8
1.478 1.463
52.1
21.8
1.454
77.8
1.130
32.9
21.2
23.5
close (10,0)+F
45.4
Li@(10,0)+F
67.2
(10,0)+H
33.7
Li@(10,0)+H (10,0)
48.6 19.1
14.9
1.127 1.573
50.7 25.2
17.8
30.7
11.6
1.569
33.1
7.9
Li@(14,0)+OH
47.9
25.1
1.461
Li@(14,0)+F
63.1
22.2
1.457
25.7
+azomethine Li@(10,0) +azomethine
a
X = OH, SH, F, H, azomethine. For the (5,5) SWCNT, the addition of the azomethine ylide is to a CC bond slanted with respect to the tube axis, whereas for the (10,0), it corresponds to the addition onto a CC bond parallel to the tube axis.
applied to hydrogen-terminated (5,5) SWCNT sections, the largest enhancement of reactivity was obtained. Although these results are not directly comparable with those obtained with infinite models, they support the significant enhancement of reactivity. The reason for not being directly comparable is that for metallic nanotubes, cluster models are less reactive.47,48 Yet, some error cancellation may occur, and the results computed with the M062X functional may be close to the true values. Although the evaluation of the extra binding energies depends on the basis set (6-31G* vs DZP, for example) and functional selected, all DFT methods support the strong effect of lithium doping in the reactivity of nanotubes. The important deviations observed between the selected functionals is rather surprising because, as discussed above, the encapsulation energies determined for the Li@(5,5) system with different functionals show smaller deviations than the extra binding energies. Finally, it is worth mentioning that in recent work about lithium-doped graphene22 it was observed that Moller Plesset second-order perturbation theory (MP2/cc-pVQZ) calculations support the enhanced reactivity due to lithium. 3.3. Metallic versus Semiconducting Nanotubes. For the addition of free radicals, the effect of Li doping is higher for the (10,0) tube, even though again differences between the results obtained with Gaussian and SIESTA were found. In effect, at the PBE/DZP level (SIESTA) it is predicted that the metallic nanotubes are less reactive by 4 8 kcal/mol, while Gaussian suggests that this difference is only 1 2 kcal/mol. In the case of the 1,3 dipolar
cycloaddition considered, the extra binding energy is 1.5 kcal/mol at the PBE/DZP level of theory. However, for the semiconducting tube it is larger by 10.1 kcal/mol. Yet, at the PBE/6-31G* level the latter difference decreases to 2.3 kcal/mol. Despite the quantitative differences in the extra binding energies computed with SIESTA and Gaussian, for both types of reactions, semiconducting nanotubes display a larger enhancement of reactivity due to lithium doping. This may be related to the change in the electronic properties caused by lithium. In Figure 2 the band structure for the (5,5) and Li@(5,5) is presented, whereas those of the (10,0), and Li@(10,0) SWCNTs are shown in Figure 3. In both cases one can appreciate that lithium increases the energy of the Fermi level, and thus, the semiconducting tube becomes metallic and more states are available near this level. For the (5,5) tube the Fermi level is also shifted upward, but the changes at the Fermi level are smaller as compared with the (10,0) tube. The band gaps determined at the PBE/DZP level are presented in Table 4. The analysis of the band structure and density of states of the functionalized Li@(5,5) and Li@(10,0) SWCNTs indicated that the metallic properties of lithium-doped nanotubes disappear when the tubes are functionalized with free radicals but are maintained when the azomethine group is attached. 3.4. Effect of Curvature: Diameter Enlargement and Comparison with Graphene. Two nanotubes with larger diameter were investigated, namely, the metallic (6,6) and semiconducting (14,0) SWCNT. In particular, the addition of the F and OH radicals to the metallic (6,6) SWCNT whose diameter is 8.3 Å, only 0.3 Å longer than the one corresponding to the (10,0) SWCNT, was studied. The results are grouped in Table 1 and confirmed that the reactivity of the semiconducting nanotube was more affected by lithium doping as compared with that of the (6,6) one, in agreement with their electronic properties. On the other hand, the extra binding energies computed for the (6,6) SWCNT are slightly larger than those observed for the (5,5) SWCNT, but smaller than those computed for the (10,0) one. Therefore, the enhanced reactivity due to lithium doping was not decreased upon diameter enlargement. To test this hypothesis further, the addition of F and OH to the semiconducting (14,0) SWCNT, whose diameter is 11.2 Å, about 3.2 Å longer that that of the (10,0) SWCNT, was investigated. The results presented in Table 3 indicated that the extra binding energies computed for the (14,0) SWCNT are slightly larger than those computed for the (10,0) SWCNT, probably because the gap of the (14,0) tube is 0.05 eV larger. It is interesting to compare the effect of lithium doping on nanotubes and graphene. To that end, the reader is referred to Table 3 of our previous work.22 The comparison is performed by considering the PBE/ DZP results shown in Tables 2 and 3, because this was the methodology employed for graphene. For the SH, OH, and F radicals, lithium has a slightly stronger effect on the reactivity of graphene as compared with the (10,0) SWCNT, whereas for hydrogen and the 1,3 dipolar cycloaddition, the opposite behavior is observed. Yet, when the comparison is performed with the (14,0) SWCNT and fluorine, lithium has a stronger effect on the nanotube. This comparison indicates that the curvature of the nanotube does not induce important modifications on the reactivity enhancement due to lithium. Yet, it must be considered that for some functional groups only endohedral doping is effective to enhance the reactivity of nanotubes. 3.5. Endohedral versus Exohedral Doping. Inspection of the results obtained for all tubes reveals that the effect of endohedral and exohedral dopings are quite similar, even though the former is always larger. Indeed, for the addition of the SH radical to the (5,5) tube, the binding energy is increased by 13.5 20285
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Figure 2. Band structure determined for the (5,5) and Li@(5,5) SWCNTs. The Fermi level is at 0 eV. (For the lithium-doped tube only the spin up band is shown, and the spin down is very similar.)
Figure 3. Band structure determined for the (10,0) and Li@(10,0) SWCNTs. The Fermi level is at 0 eV. (For the lithium-doped tube only the spin up band is shown, and the spin down is very similar.)
and 11.8 kcal/mol for the endohedral and exohedral doping at the PBE/DZP level, respectively. In the case of hydroxyl radical, for the same nanotube, the difference between the extra binding
energy computed for both types of doping is 0.9 kcal/mol. When this analysis is performed for the (10,0) SWCNT, one finds that for OH the effect of endohedral doping is higher by 0.6 kcal/mol. 20286
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Table 4. Band Gaps Determined at the PBE/DZP Level for the Li@SWCNT and Functionalized Li@SWCNT Systems PBE DZP gap
PBE DZP gap model
model
(5,5) (5,5)+Li
metal metal
(5,5)+SH
metal
(10,0)+SH
metal/0.30
Li@(5,5)+SH
0.24
Li@(10,0)+SH
0.31
(5,5)+OH
metal
(10,0)+OH
metal
Li@(5,5)+OH
0.20
Li@(10,0)+OH
0.36/0.30
(5,5)+F
metal
(10,0)+F
metal
Li@(5,5)+F
0.10
Li@(10,0)+F
0.20
(5,5)+H Li@(5,5)+H
metal (10,0)+H semimetal Li@(10,0)+H
(5,5)+CH3
0.26/0.20
(10,0)+CH3
Li@(5,5)+CH3
0.20
Li@(10,0)+CH3
(5,5)+azomethine
metal
(10,0)+azomethine
Li@(5,5)+azomethine metal
(10,0) (10,0)+Li
0.42 metal
0.33/0.22 0.20
0.30
Li@(10,0)+azomethine metal
However, for SH the problem that lithium removes SH from the nanotube was faced. The generation of the unwanted side product makes the exohedral doping much less efficient than the endohedral one in enhancing the reactivity of the nanotube. This effect shows the strong reductive capabilities of lithium. Indeed, when for OH the optimization is performed with extremely tight thresholds, the C OH is also broken by the presence of lithium. However, it was found that for the methyl radical and the hydrogen atom, lithium does not break the C CH3 bond or the C H one. For the sake of completeness, the same analysis was performed using Gaussian and the PBE/6-31G* methodology. The results were in line with those obtained with SIESTA, as the rupture of the C OH by lithium is more favorable by 17.0 kcal/mol. When constrained optimizations are performed, the extra binding energy at the PBE/6-31G* level for addition of OH to the Li@(5,5) SWCNT is 2 kcal/mol larger than that determined for the exohedral lithium, in good agreement with the values obtained with SIESTA (0.9 kcal/mol see above) In the same line, for the Li@(10,0) SWCNT it is higher by 1.0 kcal/mol (0.6 kcal/mol with SIESTA). 3.6. Effect of Lithium Proximity to the Attachment Site. the binding energies computed for the addition of SH to the (5,5) and (10,0) tubes suggests that the effect of Li doping is distancedependent. Indeed, when lithium is located in the diametrically opposite site in relation to the functional group, the extra binding energy at the PBE/DZP level is 7.4 kcal/mol smaller than the one computed when lithium is close to the addition site. This may be explained by the charge transfer observed between lithium and the tube, which is mostly a local effect. Our calculations at the PBE/DZP level indicated that 0.5 e is transferred from exohedral lithium to the (10,0) tube. This charge is not equally distributed along the tube but concentrated on the hexagon, which is below lithium, which holds 0.35 e, equivalent to 70% of the total charge accepted by the tube. We have analyzed the charge transfer for the remaining three systems, Li@(5,5), Li-(5,5), and Li@(10,0), and the whole picture is similar: 0.5 e is transferred from lithium to the nanostructure and around 70% of the charge is located on the hexagon, which is close to lithium. The higher effect of endohedral lithium doping can be attributed to proximity. When lithium is inside the tube, the charge can be more easily donated
Figure 4. Charge distribution for the Li@(5,5)+F (left) and (5,5)+F (right) systems, at the PBE/6-31G* level. Only the carbon atoms close to the addition site are shown.
to the carbon atom holding the functional group, but when lithium is outside, it will always be closer to other atoms rather than that carbon bonded to the radical. Indeed, for Li@SWCNT, in all cases the lithium atom moves from the below hexagon position to that below the sp3 carbon atom, upon functionalization, facilitating the charge transfer toward the tube. The bond angle formed by the lithium atom, the functionalized carbon atom, and the functional group is 178° for all radicals. The results obtained at the PBE/6-31G* level support those obtained with SIESTA; i.e., half e is transferred to the tube, even though the spin density indicates that one electron is donated to the tube and most of it is located on the carbon atoms near to lithium, in agreement with the charge distribution discussed above. The analysis of the charge distribution in the Li@(5,5)+F and (5,5)+F systems can be very instructive. It can be performed with the aid of Figure 4. The charge distribution indicated that the fluorine atom bonded to Li@(5,5) has the same negative charge as in the system (5,5)+F. However, those three carbon atoms bonded to the C atom where fluorine is attached experience significant changes. In effect, they concentrate almost all the electron density donated by lithium in the Li@(5,5)+F system. Therefore, in contrast with the results expected on the basis of the electronegativity of fluorine, it was found that the charge fluorine is not affected by the presence of lithium and it helps to concentrate the negative charge in one small region of the nanotube, favoring electrostatic interactions with lithium that increase the binding energy for the attachment of fluorine.
4. CONCLUSIONS The PBE, M06L, and M06-2X density functionals have been used to study the effect of lithium doping on the reactivity of carbon nanotubes. All of the methodologies selected indicated that lithium increases the reaction energies. Although this effect is slightly augmented upon diameter enlargement, it decreases when the distance between the functional group and lithium is increased. We observed a weaker enhancement of reactivity for metallic SWCNTs as compared with the semiconducting ones. Regarding the endohedral and exohedral lithium dopings, both increase the binding energies, although for some functional groups the latter is far less effective because exohedral lithium can remove 20287
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’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Tel: 0059899714280. Fax: 00589229241906.
’ ACKNOWLEDGMENT The author thanks the PEDECIBA Quimica for financial support. ’ REFERENCES (1) Oberlin, A.; Endo, M.; Koyama, T. J. Cryst. Growth 1976, 32, 335. (2) Iijima, S. Nature 1991, 354, 56. (3) Patel, N.; Egorov, S. A. J. Am. Chem. Soc. 2005, 127, 14124. (4) Usrey, M. L.; Chaffe, A.; Jeng, E. S.; Strano, M. S. J. Phys. Chem. C 2009, 113, 9532. (5) Georgakilas, V.; Bourlinos, A.; Gournis, D.; Tsoufis, T.; Trapalis, C.; Mateo-Alonso, A.; Maurizio, P. J. Am. Chem. Soc. 2008, 130, 8733. (6) Jain, D.; Saha, A.; Marti, A. A. Chem. Commun. 2011, 47, 2246. (7) Liang, F.; Sadana, A. K.; Chattopadhyay, J.; Gu, Z.; Hauge, R. H.; Billups, W. E. Nano Lett. 2004, 4, 1257. (8) Chattopadhyay, J.; Chakraborty, S.; Mukherjee, A.; Wang, R.; Engel, P. S.; Billups, W. E. J. Phys. Chem. C 2007, 111, 17928. (9) Buttner, M.; Xiao, L.; Mandeltort, L.; Edington, S.; Johnson, J. K.; Yates, J. T. J. Phys. Chem. C 2009, 113, 4829. (10) Mandeltort, L.; Buttner, M.; Yates, J. T.; Choudhury, P.; Xiao, L.; Johnson, J. K. J. Phys. Chem. C 2010, 114, 17148. (11) Choudhury, P.; Johnson, J. K. J. Phys. Chem. C 2011, 115, 11694. (12) Ghaderi, N.; Peressi, M. J. Phys. Chem. C 2010, 114, 21625. (13) Chandrachud, P.; Pujari, B. S.; Haldar, S.; Sanyal, B.; Kanhere, D. K. J. Phys.: Condens. Matter 2010, 22, 465502. (14) Al-Aqtash, N.; Vasiliev, I. J. Phys. Chem. C 2009, 113, 12970. (15) Dai, J.; Yuan, J.; Giannozzi, P. Appl. Phys. Lett. 2009, 95, 232105. (16) Dai, J.; Yuan, J. J. Phys.: Condens. Matter 2010, 22, 225501. (17) Suggs, K.; Reuven, D.; Wang, X. -Q. J. Phys. Chem. C 2011, 115, 3313. (18) Yu, Y.; Yuan, S.; Wu, S.; Russ. J. Phys. Chem. A 2011, 85, 2031. (19) Hussain, C. M.; Mitra, S. Anal. Bioanal. Chem. 2011, 399, 75. (20) Abergel, D. S. L.; Apalkov, V.; Berashevich, J.; Ziegler, K.; Chakraborty, T. Adv. Phys. 2010, 59, 261. (21) Paraknowitsch, J. P.; Thomas, A.; Schmidt, J. Chem. Commun 2011, 47, 8280. (22) Denis, P. A. J. Phys. Chem. C 2011, 115, 11694. (23) Fan, X.; Nouchi, R.; Tanigaki, K. J. Phys. Chem. C 2011, 115, 12960. (24) Soler, J. M.; Artacho, E.; Gale, J. D.; Garcia, A.; Junquera, J.; Ordejon, P.; Sanchez-Portal, D. J. Phys.: Condens. Matter 2002, 14, 2745. (25) Ordejon, P.; Artacho, E.; Soler, J. M. Phys. Rev. B 1996, 53, R10441. (26) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, Jr., J. A.; Peralta, J. E.; Ogliaro, F.;
ARTICLE
Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, € Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, O.; Revision A.1; Gaussian, Inc.: Wallingford CT, 2009. (27) Denis, P. A.; Iribarne, F. J. Mol. Struct. THEOCHEM 2009, 907, 93. (28) Denis, P. A.; Faccio, R.; Mombru, A. W. ChemPhysChem 2009, 10, 715. (29) Denis, P. A. J. Phys. Chem. C 2009, 113, 5612. (30) Denis, P. A. Chem. Phys. Lett. 2010, 492, 251. (31) Denis, P. A. Chem. Phys. Lett. 2011, 508, 95. (32) Denis, P. A.; Iribarne, F. J. Phys. Chem. C 2011, 115, 195. (33) Denis, P. A.; Iribarne, F. Int. J. Quantum Chem. 2010, 110, 1764. (34) Denis, P. A.; Faccio, R. Chem. Phys. Lett. 2008, 460, 486. (35) Denis, P. A. Int. J. Quantum Chem. 2009, 109, 772. (36) Denis, P. A.; Gancheff, J. S. J. Mater. Sci. 2010, 45, 1039. (37) Denis, P. A.; Iribarne, F.; Faccio, R. J. Chem. Phys. 2009, 130, 194704. (38) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865. (39) Boys, F. S.; F. Bernardi, F. Mol. Phys. 1970, 19, 553. (40) Troullier, N.; Martins, J. L. Phys. Rev. B 1991, 43, 1993. (41) Nikolaev, P.; Bronikowski, M. J.; Bradley, R. K.; Rohmund, F.; Colbert, D. T.; Smith, K. A.; Smalley, R. E. Chem. Phys. Lett. 1999, 313, 91. (42) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (43) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (44) Zhao, Y.; Truhlar, D. G. J. Chem. Phys. 2006, 125, 194101. (45) Denis, P. A.; Iribarne, F. J. Comput. Chem. 2011, 32, 2397. (46) Bettinger, H. F. Org. Lett. 2004, 6, 731. (47) Ditchfeld, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54, 724. (48) Baker, T. A.; Head-Gordon, M. J. Phys. Chem. A 2010, 114, 10326.
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