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Feb 18, 2012 - (11, 21-23) Among various topological defects, Stone–Wales defect (SWD) is ... In order to provide theoretical data comparable to exp...
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Comprehensive Study on the Dissociative Chemisorption of NH3 on the Sidewalls of Stone−Wales Defective Armchair (5,5) Single-Walled Carbon Nanotubes M. A. Turabekova,†,‡ Tandabany C. Dinadayalane,*,† Danuta Leszczynska,†,‡ and Jerzy Leszczynski*,† †

Interdisciplinary Center for Nanotoxicity, Department of Chemistry and Biochemistry, and ‡Department of Civil and Environmental Engineering, Jackson State University, 1400 J. R. Lynch Street, Jackson, Mississippi, 39217 United States S Supporting Information *

ABSTRACT: Dissociative chemisorption of NH3 into H and NH2 on the sidewalls of (5,5) armchair Stone−Wales defective single-walled carbon nanotubes (SWCNTs) has been investigated using B3LYP/631G(d) and M06-2X/6-31G(d) levels of theory. In particular, H• and NH2• have been attached to the adjacent carbon atom sites on and around the Stone−Wales defect (SWD). Here, we consider SWD with two different orientations in finite H-terminated (5,5) SWCNTs. The reaction energy data suggest that the dissociative chemisorption of NH3 on the surface of the Stone−Wales defective SWCNTs is mostly endothermic except the adsorption at very few defect sites, wherein the chemisorption process is exothermic at the M06-2X/6-31G(d) level. Our systematic study reveals that the SWD generated by 90° rotation of nearly axial C−C bond in (5,5) SWCNT is more favorable for the chemisorption of ammonia than the nanotube containing SWD with another orientation. The HOMO−LUMO energy gap values of defect-free and Stone−Wales defective SWCNTs are slightly altered by the chemisorption of ammonia. We have reported characteristic vibrational frequencies associated with H and NH2 chemisorbed in the lowest energy products. Expectedly, the more preferred carbon atom sites for chemisorption of NH3 are located in the SW defect region. We identified low-energy adsorption sites in the Stone−Wales defective (5,5) SWCNTs.

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the binding of chemical vapors.21 Robinson et al. found that the chemical sensitivity of SWCNTs could be significantly increased by controllably introducing a low density of defects along the sidewalls of tubes.21 Scientists have tried to understand the role of structural defects in carbon nanotube based gas sensors.11,21−23 Among various topological defects, Stone−Wales defect (SWD) is very important in SWCNTs. SWD tubes have two adjacent pentagon−heptagon pairs in the hexagonal network.24,25 In 2007, the first direct-image of the pentagon-heptagon pair defect (Stone−Wales defect) in SWCNT was reported using high-resolution transmission electron microscopy (HRTEM).26 Very recently, pulsed neutron diffraction study has confirmed the presence of different topological defects including SWD in single-walled carbon nanohorns, which consist of SWCNTs ended with cone caps.27 Fujimori et al. have applied surface-enhanced Raman scattering technique to detect the Stone−Wales defect in SWCNTs.28 The interaction of NH 3 with SWCNTs stimulated considerable interest since it is somewhat controversial. Romero et al. have reported the experimental study of NH3

INTRODUCTION Single-walled carbon nanotubes (SWCNTs) have great potential for chemical, optical, and biomolecular sensing applications.1−4 The unique characteristics of carbon nanotubes (CNTs) such as high tensile strength, ultrahigh stiffness, high current carrying capacity, and thermal conducting ability make them very attractive materials in different areas of science and engineering, including chemistry, materials science, biomedicine, etc.3−10 Considering environmental applications, carbon nanotubes are expected to be a lightweight, miniature gas sensors for reliable detection of a wide variety of gases and chemical vapors such as NO2, O2, NH3, N2, CO2, H2, CH4, CO, and H2O.11 Chemical sensing application of SWCNTs for NO2 and NH3 gases was first reported by Kong et al.12 Other studies revealed that semiconducting SWCNTs could detect small concentrations of NH3 and NO2 with high sensitivity at room temperature.13,14 Researchers investigated binding of small molecules with the sidewalls of SWCNTs.15−19 Apart from pristine structure, in reality, the SWCNTs may contain defects that are introduced either by design or due to the stress applied. Significant charge transfer and desorption energies of ∼1 eV/molecule were reported in case of NH3 interaction with SWCNTs. The high values were attributed to the topological defects along the SWCNTs.20 Defect sites in SWCNTs play an important role in the electrical response for © 2012 American Chemical Society

Received: October 13, 2011 Revised: February 14, 2012 Published: February 18, 2012 6012

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adsorption/desorption on graphene surfaces; small charge transfer has been observed from NH3 to graphene.29 Also in the case of graphene, the strength and character of the adsorption (chemisorption vs physisorption) and the degree of charge transfer between the analyte and graphene are still under debate. In order to understand the mechanisms of molecular sensing, it is important to explore both chemisorption and physisorption processes for the interaction between NH3 and SWCNTs. Computational study supported the experimental observation that ammonia interacts more strongly with the oxidized SWCNTs than with pristine or vacancy defective tubes.23 Only very few studies were reported for the chemisorption of NH3 with the defective SWCNTs. To date, the chemisorption of NH3 on model SWCNTs with and without SW defect was shown to be marginally exothermic and endothermic, respectively.11 However, only the carbon atom sites of the C−C bond shared by two heptagons was considered for the dissociative chemisorption of NH3 particularly with zigzag nanotubes.11,22 Therefore, we have performed a systematic investigation of chemisorptions of NH3 with the (5,5) armchair SW defect tube in this paper. Experimental and computational chemists have showed vast interest in understanding the influence of the topological defects on the reactivity of different carbon sites and other properties of the Stone−Wales defective SWCNTs.11,21−23,30,31 Dinadayalane and Leszczynski calculated the formation energy of a single Stone−Wales defect of two different orientations in (5,5) SWCNTs of different lengths.24 Robinson et al. pointed out that the defect sites form low-energy adsorption sites.21 Therefore, we believe that it is important to explore the dissociative chemisorption of NH3 with SWCNTs containing Stone−Wales defect in two different orientations. Also, we intend to identify the low-energy sorption sites for NH3 chemisorption in the defect region. In order to provide theoretical data comparable to experiments, which may be performed on imperfect materials, we present the vibrational frequency results for the NH3 chemisorbed Stone−Wales defective (5,5) SWCNTs (for the low energy products).

Chart 1. Addition Sites for Dissociative Chemisorption of NH3 Molecule within the SWD-I (a) and SWD-II (b) SingleWalled Carbon Nanotubes

attached simultaneously at the C1 and C2 positions for 1_2, whereas they were bonded correspondingly at the C2 and C1 sites for 2_1. All calculations were carried out using the Gaussian 09 suite of programs.35 Reaction energies for the dissociative NH3 chemisorption (Er) on the external surface of the SWCNTs (both defect-free and Stone−Wales defect tube) were calculated using the following equation: Er = ESWCNT + NH2 + H − ESWCNT − E NH3

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where ESWCNT+NH2+H denotes the total energy of ammonia chemisorbed (pristine or defect) tube and ESWCNT and ENH3 correspond to the energies of the SWCNT (pristine or defect tube) and the NH3 molecule. The chemisorption process is exothermic if the reaction energy Er is negative.

COMPUTATIONAL DETAILS As reported in earlier studies,11,22 the chemisorption of NH3 generally involves dissociation into H and NH2 fragments that form covalent bonds with carbon atoms of SWCNTs. Density functional theory (DFT) calculations were performed for the chemisorptions involving the attachment of H and NH2 to carbon atoms of the external surface of (5,5) armchair SWCNTs containing 11 carbon atom layers. The edges of SWCNTs were terminated with hydrogen atoms. The initial structures of (5,5) armchair SWCNTs with first and second type oriented Stone−Wales defect (with molecular formula C110H20) together with chemisorbed NH3 were fully optimized at the B3LYP/3-21G level followed by the calculations at B3LYP/6-31G(d) and M06−2X/6-31G(d) levels. Recently developed M06-2X functional has shown to produce accurate results for similar type of reactions.32−34 We have also performed harmonic vibrational frequency calculations at the M06-2X/6-31G(d) level for the lowest energy structures of NH3 chemisorbed Stone−Wales defective tubes and the functionalized defect-free SWCNT. It should be mentioned that NH2 and H were attached at adjacent carbon atom sites (Chart 1) on and around the defect area. In Chart 1, the atom numbers given in the structures indicate the positions where H and NH2 were attached. For example, H and NH2 were

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RESULTS AND DISCUSSIONS We have examined the dissociative chemisorption of NH3 on the external surface of SWCNTs with Stone−Wales defects of two possible orientations: SWD-I and SWD-II (Chart 1) generated by 90° rotation of the circumferential and nearly axial C−C bonds, respectively. In this systematic study, we have considered 31 and 66 structures of NH3 chemisorbed tubes correspondingly for SWD-I and SWD-II. Representative structures of dissociative NH3 chemisorbed defective SWCNTs are shown in Figure 1. The designations 1_2, 2_1, 10_1, etc. are used to indicate adjacent carbon atoms to which H (first number) and NH2 (second number) are attached (see Chart 1 and Figure 1). Importantly, obtained results for H and NH2 attached at adjacent carbon atoms Cx and Cy are comparable to those of H and NH2 attached at Cy and Cx (see Tables and Figures); where x and y are the numbers of carbon atoms (see Chart 1). This is applicable to SWD-I, SWD-II, and defect-free tubes at both B3LYP and M06-2X functionals. 6013

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Figure 1. Representative structures of NH3 chemisorbed SW defect nanotubes (a) SWD-I and (b) SWD-II (5,5) armchair SWCNTs.

Table 1. Reaction Energies (ΔE, in kcal/mol) Obtained for the Dissociative Chemisorption of NH3 at Specific Sites of SWD-Ia M06-2X/6-31G(d) X_Yb (Y_X)b 3_8 (8_3) 2_3 (3_2) 1_2 (2_1) B_C (C_B) 3_4 (4_3) 7_11 (11_7) 6_13 (13_6) 7_12 (12_7) 8_9 (9_8) 8_10 (10_8) A_B (B_A) 2_7 (7_2) 12_13 (13_12) 10_11 (11_10) 1_6 (6_1) 4_5d

ΔE (kcal/mol) −8.3 5.6 9.4 9.9 13.0 14.1 16.4 16.9 18.1 18.4 19.4 20.6 25.8 32.8 33.5 42.9

(−6.9) (5.0) (11.1) (9.8) (15.0) (15.4) (15.7) (16.7) (18.1) (19.4) (19.4) (20.2) (23.8) (32.3) (35.0)

rC−C (Å) 1.559 1.593 1.585 1.561 1.570 1.586 1.562 1.609 1.574 1.528 1.608 1.597 1.563 1.631 1.623 1.490

B3LYP/6-31G(d) HL gap (eV)c

(1.565) (1.592) (1.585) (1.561) (1.566) (1.595) (1.562) (1.609) (1.574) (1.532) (1.608) (1.606) (1.557) (1.632) (1.622)

2.67 2.79 3.14 2.89 2.77 2.73 2.96 2.92 2.82 2.64 2.83 2.34 2.50 2.78 2.33 2.47

(2.67) (2.80) (3.11) (2.93) (2.73) (2.72) (2.91) (2.93) (2.82) (2.67) (2.83) (2.33) (2.52) (2.76) (2.33)

ΔE (kcal/mol) 3.1 15.8 20.9 20.1 25.3 24.0 26.2 26.0 26.7 27.9 28.4 28.0 34.3 38.4 37.7 51.0

(5.0) (15.0) (22.6) (20.0) (27.1) (25.5) (25.9) (25.8) (26.7) (28.8) (28.4) (28.0) (32.1) (37.8) (38.9)

rC−C (Å) 1.566 1.609 1.599 1.571 1.582 1.599 1.572 1.622 1.584 1.535 1.624 1.611 1.572 1.651 1.637 1.495

(1.573) (1.607) (1.599) (1.571) (1.580) (1.609) (1.572) (1.623) (1.584) (1.540) (1.623) (1.622) (1.566) (1.648) (1.638)

HL gap (eV)c 1.28 1.44 1.69 1.53 1.35 1.37 1.58 1.59 1.40 1.26 1.49 1.05 1.12 1.45 1.18 1.08

(1.28) (1.46) (1.65) (1.57) (1.29) (1.36) (1.53) (1.60) (1.40) (1.29) (1.49) (1.04) (1.15) (1.43) (1.14)

a

The HOMO-LUMO energy gap (HL gap, in eV) obtained for the chemisorbed products and the distance of C−C bond (rC−C, in Å) wherein NH3 chemisorbed. The values were obtained at the M06-2X and B3LYP functionals using 6-31G(d) basis set. bH and NH2 were attached correspondingly at X and Y adjacent carbon atoms. The data given in parentheses correspond to the reverse case. cHOMO−LUMO gap. For SWD-I, HL gap is 2.75 and 1.49 at the M06-2X/6-31G(d) and B3LYP/6-31G(d), respectively. dThe results of 5_4 are not given since they are same as 4_5 due to symmetry.

reported earlier,33,34 the results obtained at the M06-2X/631G(d) level are taken for discussion unless otherwise stated. The reaction energy data suggest that the dissociative chemisorption of NH3 on the surface of SWD-I is endothermic for all considered cases. The only exception is the chemisorption at adjacent carbon sites of C3 and C8. Using the M06-2X functional, the reaction energy for 3_8 is −8.3 kcal/mol and 8_3 is −6.9 kcal/mol. Thus, the most favorable sites for the dissociative chemisorption of NH3 are C3 and C8 sites, which are part of five- and six-membered rings. Interestingly, the dissociative addition of NH3 at C4 and C5, which share one five- and two seven-membered rings, is the least preferred in case of SWD-I. Similar situation has been recently reported in case of H and F addition in Stone−Wales defect tube of this type.30 Moreover, the bond between C4 and C5 is the shortest (1.351 Å). The reaction energy obtained for the addition at C4 and C5 sites is approximately 2 or 4 times

Chemisorption of NH3 with First Type Stone−Wales Defective SWCNT (SWD-I). The values of reaction energies, important C−C bond distances and HOMO−LUMO energy gap obtained at the M06-2X/6-31G(d) and B3LYP/6-31G(d) levels for the dissociative chemisorption of NH3 with SWD-I are listed in Table 1. M06-2X and B3LYP functionals exhibit similar trend of reaction energies. In general, the values obtained using the M06-2X functional are lower than the B3LYP results but still positive except dissociative chemisorption of NH3 at C3 and C8. The observed discrepancy in reaction energy values can be explained by the fact that M06-2X functional is known to be better parametrized to cover London dispersion forces.36 Moreover, M06-2X functional was showed to outperform B3LYP in predicting the bond lengths in large molecules, e.g. cyclophanes.36 Owing also to the good performance of M06-2X functional for these types of reactions 6014

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Figure 2. M06-2X/6-31G(d) level C−C bond length (Å) after dissociative chemisorption of NH3 at carbon atoms of various C−C bonds in SWD-I. The corresponding C−C bond distances before NH3 chemisorption were plotted for comparison. X and Y indicate carbon atom numbering (see Chart 1).

Figure 3. Variation of HOMO−LUMO energy gap at the M06-2X/6-31G(d) and B3LYP/6-31G(d) levels for a series of products of NH3 chemisorbed Stone−Wales defective tube SWD-I. The lines in red and blue color symbolize the HOMO−LUMO energy gap of SWD-I at the M062X/6-31G(d) and B3LYP/6-31G(d) levels, respectively. X and Y indicate carbon atom numbering (see Chart 1).

more endothermic than that of addition at A and B or B and C, which are defect-free sites located on the exactly opposite side of the SW defect region, in SWD-I. It is important to note that C4 and C5 sites have been commonly considered in the earlier studies for predicting the reactivity of the SW defect tube since they are fully located within the SW defect region and the bond between them is the shortest.11,37−39

An examination of the bond lengths of the nanotubes in the vicinity of NH2 and H additions reveals notable changes in the bond distances associated with the addition sites as the consequence of changing hybridization of carbon atoms from sp2 to sp3. An analysis of bond length data given in Table 1 indicates that the M06-2X functional produced slightly smaller values of bond distances, compared to the B3LYP functional. However, the data obtained using both functionals exhibit 6015

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Table 2. Reaction Energies (ΔE, in kcal/mol) Obtained for the Dissociative Chemisorption of NH3 at Specific Sites of SWD-IIa M06-2X/6-31G(d) X_Yb (Y_X)b

ΔE (kcal/mol)

12_13 (13_12) 5_6 (6_5) 1_2 (2_1) 10_11 (11_10) 3_4 (4_3) 1_10 (10_1) i_8 (8_i) 3_16 (16_3) 11_12 (12_11) d_5 (5_d) e_f (f_e) 8_9 (9_8) 6_7 (7_6) 4_a (a_4) B_C (C_B) 9_10 (10_9) 1_13 (13_1) b_c (c_b) 2_3 (3_2) d_e (e_d) A_B (B_A) 4_5 (5_4) 2_6 (6_2) g_h (h_g) 14_15 (15_14) 7_8 (8_7) 13_14 (14_13) f_7 (7_f) 15_16 (16_15) f_g (g_f) c_d (d_c) a_b (b_a) h_i (i_h)

−10.8 −6.4 −5.9 0.3 6.4 10.5 9.2 9.7 13.0 13.2 13.3 13.4 14.9 15.4 16.5 17.8 18.3 18.4 18.5 19.3 20.4 20.5 21.0 21.9 26.1 26.1 26.6 27.7 30.8 33.3 34.9 37.9 40.8

(−11.7) (−6.4) (−5.1) (0.1) (7.0) (6.8) (10.0) (10.6) (13.6) (14.0) (13.2) (15.7) (15.4) (15.7) (16.0) (17.5) (13.9) (17.5) (16.7) (17.2) (20.1) (20.6) (16.9) (22.2) (26.5) (28.1) (27.1) (27.2) (33.7) (34.7) (33.8) (37.3) (41.1)

rC−C (Å) 1.571 1.564 1.530 1.573 1.576 1.598 1.574 1.570 1.547 1.571 1.576 1.629 1.570 1.529 1.564 1.580 1.612 1.580 1.595 1.558 1.609 1.533 1.607 1.541 1.560 1.570 1.565 1.649 1.707 1.545 1.586 1.631 1.648

B3LYP/6-31G(d) HL gap (eV)c

(1.561) (1.563) (1.531) (1.577) (1.575) (1.612) (1.569) (1.580) (1.541) (1.570) (1.577) (1.628) (1.565) (1.535) (1.566) (1.576) (1.620) (1.572) (1.613) (1.552) (1.609) (1.534) (1.617) (1.544) (1.570) (1.581) (1.560) (1.648) (1.700) (1.551) (1.578) (1.633) (1.647)

3.08 2.77 2.87 3.20 2.85 1.98 2.73 2.76 2.24 2.72 2.83 2.74 2.70 2.81 2.76 2.24 2.42 2.64 1.82 2.62 2.87 2.09 2.33 2.81 2.77 2.58 2.23 2.63 2.48 2.00 2.16 2.54 2.54

(3.07) (2.80) (2.82) (3.22) (2.87) (2.06) (2.71) (2.81) (2.23) (2.67) (2.81) (2.76) (2.73) (2.87) (2.76) (2.24) (2.50) (2.62) (2.19) (2.61) (2.87) (2.04) (2.36) (2.78) (2.73) (2.56) (2.27) (2.72) (2.50) (2.02) (2.18) (2.52) (2.53)

ΔE (kcal/mol) 0.6 4.0 4.6 10.3 15.1 15.2 19.7 18.7 18.9 20.2 22.3 21.9 23.9 24.1 25.3 22.1 24.2 26.4 22.0 28.4 28.3 22.3 26.6 31.9 34.4 34.4 32.0 35.4 33.3 36.5 38.8 42.1 44.5

(−1.0) (3.2) (5.3) (10.0) (15.5) (12.0) (20.2) (19.2) (19.9) (20.9) (21.5) (24.0) (24.1) (24.6) (25.0) (22.8) (19.5) (25.7) (21.6) (26.0) (28.6) (23.1) (22.3) (32.1) (34.6) (34.6) (32.3) (35.1) (36.1) (38.3) (37.9) (41.2) (44.8)

rC−C (Å) 1.580 1.572 1.540 1.585 1.586 1.608 1.587 1.582 1.554 1.582 1.587 1.649 1.578 1.537 1.574 1.593 1.631 1.594 1.605 1.567 1.625 1.541 1.623 1.549 1.572 1.581 1.573 1.668 1.724 1.553 1.597 1.656 1.675

(1.570) (1.572) (1.541) (1.590) (1.586) (1.626) (1.580) (1.592) (1.547) (1.580) (1.588) (1.647) (1.574) (1.543) (1.575) (1.590) (1.643) (1.584) (1.625) (1.560) (1.625) (1.541) (1.637) (1.553) (1.583) (1.584) (1.569) (1.668) (1.702) (1.560) (1.589) (1.658) (1.674)

HL gap (eV)c 1.61 1.32 1.46 1.75 1.44 0.79 1.29 1.35 0.87 1.33 1.42 1.30 1.31 1.34 1.31 0.97 1.09 1.24 0.72 1.15 1.39 0.88 0.95 1.32 1.33 1.13 0.99 1.18 1.23 0.80 0.80 1.14 1.17

(1.59) (1.36) (1.41) (1.77) (1.46) (0.88) (1.27) (1.40) (0.83) (1.29) (1.41) (1.31) (1.33) (1.42) (1.31) (0.97) (1.20) (1.22) (0.73) (1.13) (1.38) (0.93) (1.01) (1.30) (1.31) (1.13) (0.99) (1.24) (1.23) (0.82) (0.82) (1.14) (1.17)

a

The HOMO-LUMO energy gap (HL gap, in eV) obtained for the chemisorbed products and the distance of C−C bond (rC−C, in Å) wherein NH3 chemisorbed. The values were obtained at the M06-2X and B3LYP functionals using 6-31G(d) basis set. bH and NH2 were attached correspondingly at X and Y adjacent carbon atoms. The data given in parentheses correspond to the reverse case. cHOMO−LUMO gap. For SWD-II, HL gap is 2.55 and 1.17 at the M06-2X/6-31G(d) and B3LYP/6-31G(d) levels, respectively.

chiral angle.40,41 The HOMO−LUMO energy gap values for the finite length SWCNTs and how they are altered by physisorption or chemisorption of substrates have been studied earlier by several groups.18−22,30,31,37−40 Figure 3 depicts the variation of HOMO−LUMO energy gap values before and after NH3 chemisorption with SWD-I. The values of HOMO− LUMO energy gap at the M06-2X functional are approximately 1.5−2 times larger than the results obtained using the B3LYP functional. However, a similar trend is obtained with these two DFT functionals. The dissociative chemisorptions of NH3 at adjacent carbon sites of SWD-I decrease (at least slightly) the HOMO−LUMO energy gap in many cases compared to the nonfunctionalized Stone−Wales defective SWCNT. The trend of HOMO−LUMO energy gap values does not follow the results of reaction energy. Chemisorption of NH3 with Second Type Stone− Wales Defective SWCNT (SWD-II). We have performed the calculations of the dissociative chemisorption of NH3 not only in the SW defect region but also for the carbon atoms of the three six-membered rings adjacent to the defect (Chart 1b). Unlike SWD-I, there are many possibilities for addition of NH3

similar trend (see the Supporting Information, Figure S1). Figure 2 depicts the bond distances between two adjacent carbon atoms, to which H and NH2 were bonded, before and after chemisorption. Overall, majority of bond distances are above 1.54 Å, which represents a typical sp3−sp3 C−C single bond, indicating weakening of C−C bonds by NH 3 chemisorption. The shortest distance of 1.495 Å was obtained for 4_5 that has somewhat a unique bond in terms of being shared by two heptagons. Some of the C−C bond lengths are ≥1.60 Å. The longest bond lengths are observed for the 10_11 (11_10) and 1_6 (6_1) chemisorbed tubes; the values at M062X are 1.631 Å (1.632 Å) and 1.623 Å (1.622 Å), respectively. These elongated circumferential bond lengths might be attributed to the release of existing local strain upon addition of H and NH2 at the carbon sites. Figure 2 clearly shows that C7−C12, CA−CB, C10−C11, and C1−C6 bonds, which are of circumferential type bond shared by two hexagons (see Chart 1), in SWD-I are elongated to a large extent (up to 0.21 Å) upon addition of H and NH2. It is known that the infinite length SWCNTs can be either semiconducting or metallic depending on their diameter and 6016

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Figure 4. M06-2X/6-31G(d) level C−C bond length (Å) after dissociative chemisorption of NH3 at carbon atoms of various C−C bonds in SWDII. The corresponding C−C bond distances before NH3 chemisorption were plotted for comparison. X and Y indicate carbon atom numbering (see Chart 1).

Figure 5. Variation of HOMO−LUMO energy gap at the M06-2X/6-31G(d) and B3LYP/6-31G(d) levels for a series of products of NH3 chemisorbed Stone−Wales defective tube SWD-II. The lines in red and blue color symbolize the HOMO−LUMO energy gap of SWD-II at the M06-2X/6-31G(d) and B3LYP/6-31G(d) levels, respectively. X and Y indicate carbon atom numbering (see Chart 1).

surface of SWD-II are highly endothermic except few cases. We have identified three possible sites as moderately favorable for chemisorption of NH3. The reaction energies for 12_13 (13_12), 5_6 (6_5), and 1_2 (2_1) sites are −10.8 (−11.7), −6.4 (−6.4), and −5.9 (−5.1) kcal/mol, respectively. Our results are similar to the data reported by Govind et al.11 who also observed the reaction of NH3 chemisorption to be marginally exothermic (−3.46 kcal/mol) but the calculations were performed for the zigzag type tubes containing SW defect. In our present study, the reactions at the sites of 10_11

in case of SWD-II due to no symmetry in the structure. Reaction energies, length of C−C bond where chemisorption took place, HOMO−LUMO energy gap obtained at the M062X and B3LYP functionals using 6-31G(d) basis set are given in Table 2. As mentioned earlier in case of SWD-I, the M06-2X/ 6-31G(d) level predicts lower values of reaction energies than B3LYP/6-31G(d) level. The data obtained in the former level is taken for discussion. Reaction energy data indicate that likewise the first set of calculations, the reactions of H and NH2 chemisorptions on the 6017

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generally occurred for circumferential bond type. The longer C−C bond distances (>1.6 Å) in the functionalized SWD-II may be attributed to the release of local strain by the dissociative NH3 chemisorption. Similar to SWD-I, the C1−C2 bond sharing two heptagons is the shortest (1.36 Å) in SWD-II and it is elongated considerably by the chemisorption of NH3. Similarly to the data for the NH3 chemisorbed SWD-I, the HOMO−LUMO energy gap values obtained by M06-2X and B3LYP differ from each other for the functionalized SWD-II (Figure 5). However, the qualitative trend in the energy gap of HOMO−LUMO is the same for the two functionals. Figure 5 shows that in most cases, dissociative NH3 chemisorption leads to increase of HOMO−LUMO energy gap of SWD-II. The largest increase of HOMO−LUMO energy gap is observed for 11_10 (10_11). Chemisorption of NH3 at most favored sites results in increase of HOMO−LUMO energy gap of SWD-II. Our data will be useful for experimentalists to target controlled functionalization of defect tube with desired HOMO−LUMO gap. Comparison with the Defect-Free SWCNT. Finally, we have also examined the chemisorption of NH3 on the sidewalls of pristine (5,5) armchair SWCNT to understand the role of Stone−Wales defect in the adsorption of NH3. We have attached H and NH2 to adjacent carbon atoms A, B and B, C as depicted in Chart 2. The values of reaction energies, C−C bond distances after chemisorption, HOMO−LUMO energy gap are provided in Table 3. Our results clearly indicate that the dissociative chemisorption of ammonia on the surface of pristine SWCNT is an endothermic process, which is in line with the previous study.11 Similar trends are observed for each data of A−B−C adsorption sites on pristine, SWD-I and SWDII SWCNTs. In particular, in all three tubes, the dissociative chemisorption of NH3 to the atoms of A−B requires more energy than that of B−C (Tables 1−3). This may be explained in terms of difference in two bonds’ nature. Thus, the A−B and B−C bonds are perpendicular and diagonal to the direction of tube axis, respectively. The orientation of the SW defect has significantly larger influence on the chemisorption of NH3 to the atoms of B−C than the A−B bond. In all three tubes pristine, SWD-I, and SWD-IIthe bond A−B elongates longer than the bond B−C upon the dissociative addition of NH3. Furthermore, the B−C and A−B distances in the products of defect tube are comparable to the values of the functionalized pristine tube. The orientation of the defect has an influence on the HOMO−LUMO energy gap. The SWD-II has noticeably lower HOMO−LUMO gap than the defect-free tube. The NH3 chemisorption to pristine tube results in slight increase of its HOMO−LUMO energy gap. Vibrational Frequency Analysis of Selected Functionalized SWD-I, SWD-II and Pristine Tubes. Infrared (IR)

Chart 2. Addition Sites for the Dissociative Chemisorption of NH3 within Pristine (5,5) Armchair Single-Walled Carbon Nanotube

(11_10) and 3_4 (4_3) are predicted to be endothermic despite C10−C11 and C3−C4 bonds are located in the five membered rings of SW defect like C12−C13 and C5−C6 bonds. In contrast to SWD-I, the dissociative addition of NH3 to the carbon sites of C1−C2, which is shared by two seven membered rings, is an exothermic process. These carbon atom sites are the third most preferred in SWD-II. It should be noted that those sites in SWD-I showed the least preference for the NH3 addition. Unlike the SWD-I, the least favored sites for the addition of NH3 with SWD-II are not located in the defect region. The data provided in Table 2 indicate that the adjacent h,i and a,b carbon atoms, which are part of six-membered rings as well as the C−C bonds of these atoms are of circumferential type, are correspondingly the first and second least favorable sites. Thus, the orientation of the SW defect has significant influence on the dissociative chemisorption of NH3. A critical analysis of the lengths of C−C bonds, wherein the carbon atoms are bonded with H and NH2, showed a very little discrepancy in values obtained between B3LYP and M06-2X functionals, consistently for all functionalized tubes (Table 2). Again, the varying pattern of bond distances is alike for both functionals. Expectedly, the lengths of C−C bonds of SWD-II are elongated by the dissociative chemisorption of NH3 at the carbon sites of these bonds (Figure 4). The bond distances show that the change in hybridization (from sp2 to sp3) takes place for the carbon atoms attached by H and NH2. About sixteen NH3 functionalized SWD-II exhibit bond lengths ≥1.60 Å, indicating cleavage of C−C bonds by chemisorption. The longest bond distance is observed for 15_16 (16_15). The bond length of about 1.70 Å indicates rupture of the C15−C16 bond that is a circumferential type bond shared by a six- and a seven-membered ring. This bond is the most elongated one in SWD-II by NH3 chemisorption and it is followed by Cf−C7 bond. It should be noted that the bond distances of >1.6 Å have

Table 3. Reaction Energies (ΔE, in kcal/mol) Obtained for the Dissociative Chemisorption of NH3 at Specific Sites of DefectFree (5,5) SWCNTa M06-2X/6-31G(d)

B3LYP/6-31G(d)

X_Yb (Y_X)b

ΔE (kcal/mol)

rC−C (Å)

HL gap (eV)c

ΔE (kcal/mol)

rC−C (Å)

HL gap (eV)c

B_C (C_B) A_B (B_A)

13.8 (13.9) 21.0 (21.0)

1.563 (1.564) 1.609 (1.609)

2.95 (2.97) 3.24 (3.24)

23.5 (23.6) 29.3 (29.3)

1.573 (1.574) 1.624 (1.626)

1.52 (1.54) 1.81 (1.81)

a

The HOMO-LUMO energy gap (HL gap, in eV) obtained for the chemisorbed products and the distance of C−C bond (rC−C, in Å) wherein NH3 chemisorbed. The values were obtained at the M06-2X and B3LYP functionals using 6-31G(d) basis set. bH and NH2 were attached correspondingly at X and Y adjacent carbon atoms. The data given in parentheses correspond to the reverse cases. cHOMO−LUMO gap. For the defect-free (5,5) tube, HL gap is 2.85 and 1.51 at the M06-2X/6-31G(d) and B3LYP/6-31G(d), respectively. 6018

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6019

(4) (7) (5) (28) (44) (56) (9)

1833 3028 3498 3596

(14) (29) (1) (3)

1299 (56) 1679 (50) 1688 (15)c

1271 (16) 1281 (10)

1160 1179 1192 1235 1256 1299 1330

1137 (4)

(4) (4) (6) (6) (6) (25) (45) (14)

8_3 300 305 306 873 882 886 890 900

13_12

1728 3047 3571 3695

(28) (6) (12) (10)

1656 (25) 1661 (41)

1686 (42) 1687 (10)c (28) (6) (1) (1)

b

b

1732 3069 3478 3569

1205 (6) 1243 (8) 1258 (36)

1076 (2) 1278 (8)

1150 (6) 1152 (5) 1139 (21)

449 (38) 467 (50) 479 (46)

255 (7) 262 (4)

1262 (25) 1294 (9)

1313 (7) 1337 (6) 1352 (7)

1192 (3)

1122 (15)

(4) (3) (8) (85) (36) (35) (31)

12_13 309 319 321 914 931 965 969

5_6

(31) (18) (57) (38) (48) (30)

(39) (25) (13) (11)

1727 3061 3484 3573

(92) (13) (2) (2)

1313 (6) 1691 (22) 1694 (12)

1234 (17)

1264 1270 1294 1311

1122 (13)

1074 (13)

891 894 926 931 936 956

324 (12)

SWD-II 6_5

1306 1686 1689 1695 1728 3044 3491 3587

1148 1152 1278 1232 1239 1264 1270 1289 1182 1260

(13) (22) (25) (13) (90) (4) (1) (2)

(6) (6) (9) (16) (6) (20) (34) (44) (5) (23)

827 (52) 872 (24) 889 (44) 906 (27) 914 (47) 937 (22) 950 (14) 1083 (3)

288 (16)

2_1

(8) (9) (65) (84) (13)

e

3014 (5) 3491 (2) 3584 (2)

(39) (4) (6) (5) (9) (38) (7)

3012 (3) 3490 (2) 3584 (2)

1272 1340 1346 1350 1320 1693 1694

1351 (4) 1365 (3)

1041 (8) 1098 (6)

1041 (8)

786 871 915 927 955

358 (3) 363 (14)

e

1695 (44)

b

1340 (3) 1341 (16)

1325 (5) 1341 (16)

1098 (10)

1125 (37)

(4) (5) (7) (20) (19) (50) (25) (18) (21)

1_2 349 358 362 848 873 916 937 945 955 (22) (19) (13) (13) (70)

3038 (0.1) 3465 (1) 3559 (2)

f

1298 (28) 1693 (44)

1256 (28) 1314 (13)

1243 (8) 1251 (20)

1007 (15) 1018 (14) 1116 (14)

886 926 932 944 966

342 (11)

A_B

(19) (11) (83) (66) (16)

3022 (8) 3481 (1) 3576 (2)

f

1338 (17) 1692 (40)

1270 (6) 1294 (15) 1343 (15)

1240 (10) 1278 (20)

1112 (14)

1081 (7)

876 881 924 934 954

326 (17)

B_C

defect-free tube

The values are given only for the products of the highly feasible reactions of the dissociative NH3 chemisorption with the SWD-I and SWD-II. The relevant data of NH3 chemisorbed defect-free tubes are given for comparison. The values given in parentheses correspond to the IR intensity (in km/mol). bEither the frequency corresponding to this mode does not exist or the intensity is very low. Hence, the value is not given. cThis vibrational mode also relevant to CC stretching of the five-membered ring in which NH2 and H are attached. dThis stretching mode is for the CC bond shared by two sevenmembered rings in the Stone−Wales defect tubes. eThe double bond is transformed to single bond by functionalization, thus the corresponding stretching frequency is absent. fNo result since this is defect-free tube.

a

1295 1684 1685 1695 1841 3081 3480 3571

NH2 rocking NH2 scissoring

CC stretchingd C−H stretching of chemisorbed H N−H stretching

1267 (26) 1287 (7)

C−H bending of chemisorbed H

(75) (81)c (11) (11)c (13) (6) (0.1) (2)

1251 (12) 1275 (10) 1312 (36)

NH2 rocking + C−H bending

C−N stretching + C−H bending

998 (3) 1144 (7) 1145 (7) 1155 (3)

C−N stretching

(6) (24) (63) (49) (15)

855 886 931 950 986

NH2 wagging

3_8 301 (5) 305 (5)

assignment of frequencies

NH2 twisting

SWD-I

Table 4. Selected Harmonic Vibrational Frequencies (in cm−1) Obtained at the M06-2X/6-31G(d) Levela

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the C−C bond shared by two heptagons in SWD-I has been generally considered in many studies but the dissociative chemisorption of NH3 to the atoms of unique C4−C5 bond appears to be the least favorable. However, the atoms of the C− C bond of same type in SWD-II exhibit the third most favorable positions for NH3 chemisorption. In both SWD-I and SWD-II, the C−C bonds of circumferential type undergo significant elongation by the dissociative chemisorption of NH3 to the carbon atoms of those bonds. Reported IR frequency data for the most favorable cases of NH3 chemisorbed defect tubes might help experimentalists in future to characterize the chemisorption of NH3 with the SW defect tubes.

spectroscopy was extensively used to identify functional groups added with the SWCNTs.42−44 Kar et al. have reported a computational study on vibrational frequency analysis of monoand tetra-functionalization of acid, amide, and ester groups at the tip of (5,5) and (10,0) SWCNTs. They have observed several differences in frequency and intensity of the characteristic CO bond between (5,5) and (10,0) SWCNTs, and this data were expected to help experimentalists to identify different tubes.45 Very recently, combined experimental and computational study has utilized IR spectroscopy in characterizing the adsorption of small molecules with SWCNTs.46 We have performed harmonic vibrational frequency calculations at the M06−2X/6-31G(d) level for 3_8 (8_3) in case of SWD-I; for 12_13 (13_12), 5_6 (6_5), and 1_2 (2_1) in case of SWD-II, and for chemisorbed defect-free SWCNTs. The structures considered in case of SWD-I and SWD-II are the most favorable chemisorbed products in each category. Selected IR frequencies and their intensities relevant to attached NH2 and H for the above-mentioned systems are given in Table 4. Computed IR spectra for all of these systems are given in Supporting Information. These vibrational frequency results might help experimentalists in characterization of the chemisorption of NH3 with the SW defect tubes. Large intensities are observed for the NH2 wagging, NH2 rocking coupled with C−H bending and NH2 scissoring vibrational modes that may aid experimentalists to confirm NH2 attached with SWCNTs. The strong stretching mode of CC bond sharing two seven membered ring is useful in characterizing the functionalization in the SW defect tube and the orientation of the defect. However, this mode disappears if the functionalization takes place at those carbon sites. The data in Table 4 reveal that vibrational frequencies are useful to distinguish functionalization in SW defect tube and defect-free tube.

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ASSOCIATED CONTENT

* Supporting Information S

Total energies, C−C bond distances before and after the dissociative addition of NH3 at different positions of the (5,5) armchair SWCNTs with the Stone−Wales defect, IR spectra for selected products, figures of bond length changes before and after chemisorption at two different levels. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.L); [email protected] (T.C.D). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Department of Defense (DoD) for the HPCDNM project (Contract No. W912HZ-09-C-0108) through the U.S. Army/Engineer Research and Development Center (Vicksburg, MS) and the National Science Foundation (NSF/CREST HRD-0833178 and NSF EPSCoR Award No.: 362492-190200-01\NSFEPS-0903787) for the financial support. HPCMP is thanked for computational facilities through ERDC. Mississippi Center for Supercomputing Research (MCSR) is acknowledged for generous computational facilities.

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CONCLUSIONS In this paper, by applying quantum chemical calculations, we have demonstrated how the Stone−Wales defect and its orientation in (5,5) armchair SWCNT alter the dissociative chemisorption of ammonia. The DFT calculations using M062X/6-31G(d) level predict lower values of reaction energies compared to B3LYP/6-31G(d) level. The former one produced slightly smaller values of bond distances compared to the later level. The values of HOMO−LUMO energy gap at the M062X functional are approximately 1.5 to 2 times larger than the results of B3LYP functional. However, we observed a similar trend of results with these two DFT functionals for reaction energies, bond distances and HOMO−LUMO energy gap. We have identified the most favored carbon atom sites in the SW defect tubes for the chemisorption of NH3. The present study reveals that the most and the least preferred sites for the NH3 chemisorption are observed in the SW defect region in SWD-I. Evidently, for the SWD-II, the most preferred sites are located in the SW defect region, while the least preferred sites for NH3 addition are the carbon atoms of the C−C bonds sharing sixmembered rings near to the SW defect. In agreement with the experimental study of Robinson et al.,21 low energy sorption sites are located in the defect region in (5,5) SWCNTs containing the Stone−Wales defect with two different orientations. Our data provide theoretical rationalization of experimental findings. Our study reveals that orientation of the Stone−Wales defect plays important role in dissociative chemisorption of NH3 in case of (5,5) armchair SWCNT. It is also worth to mention that

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REFERENCES

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