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Oxygen Adsorption Characteristics on Hybrid Carbon and Boron-Nitride Nanotubes Haining Liu and C. Heath Turner* In this work, first-principles density functional theory (DFT) is used to predict oxygen adsorption on two types of hybrid carbon and boron-nitride nanotubes (CBNNTs), zigzag (8,0), and armchair (6,6). Although the chemisorption of O2 on CBNNT(6,6) is calculated to be a thermodynamically unfavorable process, the binding of O2 on CBNNT(8,0) is found to be an exothermic process and can form both chemisorbed and physisorbed complexes. The CBNNT(8,0) has very different O2 adsorption properties compared with pristine carbon nanotubes (CNTs) and boron-nitride nanotube (BNNTs). For

example, O2 chemisorption is significantly enhanced on CBNNTs, and O2 physisorption complexes also show stronger binding, as compared to pristine CNTs or BNNTs. Furthermore, it is found that the O2 adsorption is able to increase the conductivity of CBNNTs. Overall, these properties suggest that the CBNNT hybrid nanotubes may be useful as a gas sensor or as C 2014 Wiley a catalyst for the oxygen reduction reaction. V Periodicals, Inc.

Introduction

shown[20] that by changing the relative ratio of CNT to BNNT and/or the nanotube chirality, one can systematically tune the electronic properties of the CBNNT composite, (such as incrementally converting a CBNNT from semiconducting to metallic). As a result, synthesizing hybrid CBNNTs from CNT and BNNT segments may lead to new nanomaterials with customizable electronic properties. Although previous experimental and computational studies have helped to identify some of the fundamental properties of CBNNTs, it is important to understand the ability of such materials to adsorb small gas molecules, such as O2, to further shed light on potential catalytic and sensing applications. In this work, we use first-principles density functional theory (DFT) to investigate the ability of CBNNT (formed by 1:1 stoichiometric combination of CNT and BNNT) to adsorb O2. We consider both the zigzag (8,0) and armchair (6,6) CBNNT structures, and we find that CBNNT(8,0) demonstrates much stronger O2 adsorption versus pristine CNT and BNNT, and the CBNNT band gaps show distinct changes upon O2 adsorption.

Since their discovery in 1991,[1] great efforts have been made to investigate the properties of carbon nanotubes (CNTs). A remarkable feature of CNTs is their tunable electronic properties, which depends in part on their chirality. For example, the armchair (n,n) CNT is metallic while zigzag (n,0) CNT can be metallic (when n 5 3m) or semiconducting (when n 6¼ 3m).[2] In addition, the conductivity of CNTs has been found to be sensitive to small molecule adsorbates. For example, it is known that a small amount of O2 is able to convert a semiconducting CNT to a conductor.[3] Thus, CNTs are promising materials for gas sensors and nanoelectronic devices.[4–7] Other analogous nanotube structures, which have also attracted great interest, are boron-nitride nanotubes (BNNTs), the III–V analogs of CNTs. It is known that BNNTs have several unique properties, as compared with CNTs. For example, BNNTs have a large band gap (resulting in insulating behavior).[8] In addition, H2 binds with BNNTs much stronger than with CNTs, due to the existence of the polar BAN bonds.[9,10] Furthermore, BNNTs have higher thermal and chemical stability than CNTs.[11,12] These properties make BNNTs promising materials for hydrogen storage applications and as high-temperature ceramics.[11] Considering the dramatically different properties of CNTs and BNNTs, we are particularly interested in the characteristics of composite nanotube structures, such as the hybrid carbon and boron-nitride nanotubes (CBNNT) structure shown in Figure 1. Several experimental studies have previously reported on the synthesis of such hybrid nanotubes.[13–16] Various synthetic methods, such as chemical vapor deposition and laser vaporization, have been recently reviewed.[17] In addition, computational methods have been used to study the geometries, formation energies, and electronic properties of hybrid CBNNTs.[18–23] It has been found that CBNNTs have stabilities comparable to CNTs and BNNTs.[18,20] Furthermore, we have 1058

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DOI: 10.1002/jcc.23589

Computational Methods All calculations were performed using the Vienna ab initio simulation package.[24–27] The generalized gradient approximation (GGA) functional of Perdew, Burke, and Ernzerhof,[28] combined with the projector-augmented wave[29,30] method was used for geometry optimizations. The energy cutoff for the plane-wave basis set was set to be 400 eV. The dimension of the unit cell Haining Liu, C. Heath Turner Department of Chemical and Biological Engineering, The University of Alabama, Tuscaloosa, Alabama 35487-0203 E-mail: [email protected] Contract grant sponsor: The University of Alabama Research Stimulation Fund and the National Science Foundation; Contract grant number: CBET0747690 C 2014 Wiley Periodicals, Inc. V

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Figure 1. The optimized structure of our CBNNT(8,0) model. Gray 5 carbon, blue 5 nitrogen, pink 5 boron. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

˚ 3 19.57 A ˚ 3 8.69 A ˚ with a 5 b 5 90 and c 5 120 . is 19.37 A The a and b parameters are set to be large enough to prevent the interaction between periodic images, while the c parameter was optimized to minimize the energy of the nanotube. A one dimensional periodic boundary condition was used along the tube (c) axis with the Brillouin-zone integration sampled by 1 3 1 3 3 k-points using the Monhorst-Pack scheme,[31] as performed in our previous study.[20] To obtain the densities of states (DOS), a finer 1 3 1 3 11 k-points grid was used. The binding energy (BE) discussed in this study is defined as: BE 5EðO2 =TubeÞ2EðO2 Þ2EðTube Þ

(1)

where E(O2), E(Tube), and E(O2/Tube) are the energies of an isolated O2 molecule, an isolated nanotube (CBNNT, CNT, or BNNT), and the O2/nanotube complex, respectively. Thus, a negative binding energy indicates a thermodynamically favored exothermic adsorption process. Also, because it is possible for O2 to exist in two different spin states (singlet and triplet), we have considered both spin states for all the complexes using spin-polarized DFT. The energy of isolated triplet O2, which is the ground state of O2, was used in the above equation to calculate the binding energies. For selected structures, to compare with the binding energies reported in the literature, the free singlet O2 was also used as the reference. It was previously shown that spin-polarized DFT significantly lowers the energy of singlet O2 due to spin contamination.[32] As a result, for those structures, we used spin-restricted energies of isolated singlet O2 to obtain the binding energies of the complexes. It is known that GGA usually underestimates the binding energy due to its inability to treat long-range dispersion interactions appropriately. In a

previous study[33] on the interaction of O2 with (5,5) BNNT, it was estimated that the dispersion contributes an additional 1 kcal mol21 to the total binding energies, which is also expected for the adsorption complexes obtained in this study. To further shed light on the charge distribution within the O2/ nanotube complexes, deformation charge density plots have been generated for selected structures (for consistency, the isovalue for all the surfaces was chosen to be 0.004). Partial charges were calculated by the Bader charge analysis.[34–36]

Results and Discussion Geometries and binding energies We first considered the adsorption of O2 on the CBNNT(8,0) model. In our study, both chemisorbed and physisorbed O2/ CBNNT complexes were found, including three different possible chemisorbed complexes. Their optimized structures are shown in Figure 2, and the binding energies are listed in Table 1 (along with the key binding distances). It is predicted that the lowest energy spin state for O2/CBNNT(8,0)-1 and O2/ CBNNT(8,0)-3 is the triplet state, whereas O2/CBNNT(8,0)-2 has a singlet ground state. The O2/CBNNT(8,0)-1 complex was found to be the most stable, with a calculated binding energy of 27.16 kcal mol21, which is 1.17 and 1.71 kcal mol21 lower in energy than those of O2/CBNNT(8,0)-2 and O2/CBNNT(8,0)-3, respectively. In all of these complexes, O2 interacts either with only the boron atom [O2/CBNNT(8,0)-1 and O2/CBNNT(8,0)-3], or with both the boron and carbon atoms [O2/CBNNT(8,0)-2]. A chemisorbed complex with an ON interaction could not be obtained. In both O2/CBNNT(8,0)-1 and O2/CBNNT(8,0)-3, O2 sits in a tilted configuration with one of the oxygen atoms interacting directly with a boron atom. The OAO bond lengths in O2/CBNNT(8,0)-1 and O2/CBNNT(8,0)-3 have been slightly ˚ , respectively, compared with lengthened to 1.337 and 1.333 A that in free O2 which is calculated to be 1.234 A˚ using the same computational method. In contrast, in O2/CBNNT(8,0)-2, the O2 stays parallel to the nanotube, with the OB and OC ˚ , respectively. interaction distances equal to 1.509 and 1.510 A Different from O2/CBNNT(8,0)-1 and O2/CBNNT(8,0)-3, when both oxygen atoms interact with the nanotube, the ground state of the resulting complex is in the singlet spin state. The OAO bond in O2/CBNNT(8,0)-2 has been significantly Table 1. The calculated interaction distances (d) associated with the O2 ˚. complexes in units of A Spin state O2/CBNNT(8,0)-1 O2/CBNNT(8,0)-2

Figure 2. The optimized structures of the chemisorbed O2/CBNNT(8,0) complexes in their lowest energy spin state. Key distances are shown in angstroms. (Color scheme: gray 5 carbon, blue 5 nitrogen, pink 5 boron, red 5 oxygen.) [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

O2/CBNNT(8,0)-3

Singlet Triplet Singlet Triplet Singlet Triplet

d(OAB) 1.537 1.544 1.509 1.500 1.533 1.556

d(OAC)

d(OAO)

Binding energy

1.510 1.563

1.338 1.337 1.510 1.509 1.343 1.333

3.87 27.16 25.99 11.10 3.53 25.45

The binding energies (kcal mol21) of the chemisorbed O2/CBNNT(8,0) complexes are reported in both singlet and triplet spin states.

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three complexes. In addition, the Bader charge analysis shows that O2 in O2/CBNNT(8,0)-1, O2/CBNNT(8,0)-2, and O2/ CBNNT(8,0)-3 carries a partial charge of 20.70, 21.05, and 20.66 e2, respectively, which also demonstrates the charge transfer from the nanotube to O2. The finding that O2 carries a partial negative charge may also explain why the OB (instead of the ON) interaction is observed in the chemisorbed O2/ CBNNT(8,0) complexes, as boron is more electron-poor than nitrogen. This facilitates a favorable electrostatic interaction Figure 3. 3D-deformation charge density isosurface of the chemisorbed O2/ between boron and the partial negative charge of O2. CBNNT(8,0) complexes. The magenta and cyan colors indicate electron rich Four physisorbed O2/CBNNT(8,0) complexes were obtained in and electron poor regions, respectively. Atom colors are consistent with Figure 2. our study. Their optimized structures are shown in Figure 4, and the binding energies and key interaction distances are listed in Table 2. In these complexes, O2 stays in a tilted [O2/CBNNT(8,0)-4 and O2/CBNNT(8,0)-6] or close-to-vertical [O2/CBNNT(8,0)-5 and O2/CBNNT(8,0)-7] position, relative to the nanotube axis. In all cases, the triplet spin state lies lower in energy than the singlet spin state, and the O2/CBNNT(8,0)-4 complex has the largest calculated binding energy at 20.57 kcal mol21. However, this complex has a BE significantly lower than those of the chemisorbed complexes, suggesting that O2 is only weakly bound in these physisorbed configurations. This conclusion is also supported by ˚ , Table 2) in the the much shorter OAO bond lengths (1.24 A physisorbed complexes. In addition, these distances are close to the OAO bond length in free O2, further indicating that O2 does not strongly interact with the nanotube. As a result, the distances between O2 and the nanotube are also much larger in the physisorbed complexes than in the chemisorbed complexes (see Table 2). Due to such weak interactions, no significant charge transfer between O2 and the nanotube is observed in the physiFigure 4. The optimized structures of physisorbed O2/CBNNT(8,0) comsorbed complexes, as can be seen from the three-dimension plexes at their lowest energy spin state. Key distances are shown in ang(3D) deformation charge density isosurfaces shown in Supportstroms. Atom colors are consistent with Figure 2. ing Information Figure S1. To compare the O2 adsorption properties of CBNNT with a pristine CNT and BNNT, we performed additional calculations ˚ lengthened to 1.510 A. Notably, this is even longer than the of O /CNT(8,0) and O2/BNNT(8,0) complexes. The adsorption of 2 ˚ typical OAO single bond in H2O2 (calculated to be 1.47 A at O on various chiral CNTs and BNNTs has been well docu2 the same level of theory), which indicates that the OAO bond mented in several previous studies.[32,37–42] For simplicity, we in O2/CBNNT(8,0)-2 has a significant single bond character. only discuss the most stable O2/CNT(8,0) and O2/BNNT(8,0) The negative binding energies (thermodynamically favored complexes obtained in this work. process) of O2/CBNNT(8,0)-1, O2/CBNNT(8,0)-2, and O2/ For both CNT and BNNT, the most stable O2/CNT(8,0) and CBNNT(8,0)-3 can be attributed in part to the significant O /BNNT(8,0) complexes are in the physisorbed state. The opti2 charge transfer from the nanotube to O2, which can be clearly mized structures are shown in Figure 5. In all of these comseen in Figure 3. The adsorption results in significant electron plexes, [O2/CNT(8,0)-1 and O2/BNNT(8,0)-2], O2 is only weakly density being transferred to O2, which reduces its double bound to the nanotube, and the triplet spin state is calculated bond character and results in the lengthened OAO bond in all to be the ground state. The distances between O2 and the ˚ ), the bond length (A ˚ ) of O2, and binding energies (kcal Table 2. The calculated interaction distance (A nanotube are 3.5–3.8 A˚ and mol21) of the physisorbed O2/CBNNT(8,0) complexes in both singlet and triplet spin states. the OAO bond length is close to that of free O2. The calcuSpin state d(OAO) d(OAN) d(OAB) d(OAC) Binding energy lated binding energies of O2/ O2/CBNNT(8,0)-4[a] Triplet 1.245 3.293 3.496 20.57 CNT(8,0) and O2/BNNT(8,0) are [a] O2/CBNNT(8,0)-5 Triplet 1.243 3.549 3.683 20.50 20.20 and 20.23 kcal mol21, Singlet 1.260 3.691 3.022 20.91 O2/CBNNT(8,0)-6 Triplet 1.241 3.872 3.632 20.45 respectively, which are much O2/CBNNT(8,0)-7[a] Triplet 1.241 3.705 4.184 20.27 weaker than the binding energy [a] Optimization of the singlet spin state O2/CBNNT(8,0)-4, O2/CBNNT(8,0)-5, and O2/CBNNT(8,0)-7 leads to a of the most stable chemisorbed chemisorbed complex. O2/CBNNT(8,0) complex (27.16 1060

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Figure 5. The optimized most stable O2/CNT(8,0) (O2/CNT(8,0)-1) and O2/ BNNT(8,0) (O2/BNNT(8,0)-1) complexes, and a chemisorbed O2/CNT(8,0) complex (O2/CNT(8,0)-2) obtained in this study. Key distances are shown in angstroms, and colors are consistent with Figure 2. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

kcal mol21). Furthermore, the BE values are even slightly weaker than the most physisorbed O2/CBNNT(8,0) complex (20.57 kcal mol21), indicating that the hybrid CBNNT has much stronger O2 adsorption characteristics. We also attempted to obtain chemisorbed O2/CNT(8,0) and O2/BNNT(8,0) complexes. As for the former, one chemisorbed complex was found (in the singlet spin state), which is similar to a previous study.[40] The optimized structure is shown in

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Figure 5 (O2/CNT(8,0)-2). However, the binding energy of this complex is calculated to be 18.80 kcal mol21, which indicates an endothermic (thermodynamically unfavorable) process, consistent with previous studies.[32,40] A previous study[32] obtained an exothermic binding energy of 211.07 kcal mol21 using the isolated singlet O2 as the reference. In that study, to avoid spin contamination, the energy of singlet O2 was calculated using the spin-restricted DFT. If the same approach is used for O2/CNT(8,0)-2, the binding energy is calculated to be 27.35 kcal mol21, in reasonable agreement with the literature.[32] As for the O2/BNNT(8,0) complex, no chemisorbed binding could be obtained, in agreement with previous computational studies on (5,5) and (10,0) BNNTs.[33,43] Thus, CBNNT shows dramatically different O2 adsorption properties, as compared with its individual CNT and BNNT segments. Specifically, the CNT and BNNT segments have limited or no ability to chemisorb triplet O2. In contrast, the composite material shows significantly enhanced O2 adsorption, which implies that CBNNT may be useful in sensing, catalytic, or electrochemical applications (such as the oxygen reduction reaction). We also considered the O2 adsorption on CBNNT(6,6). In contrast to CBNNT(8,0), the binding energies of the three

Figure 6. Calculated band structures and total DOS for CBNNT(8,0), and the most stable chemisorbed and physisorbed O2/CBNNT(8,0) complexes. The red dashed line indicates the Fermi level, which has been adjusted to zero. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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chemisorbed O2/CBNNT(6,6) complexes we obtained (Supporting Information Figure S2) are all positive, indicating that the chemisorption of O2 on CBNNT(6,6) is an endothermic process (thermodynamically unflavored). We note that a previous computational study[32] obtained an endothermic binding energy of O2 (both singlet and triplet) on pristine CNT(6,6) likely due to its larger diameter compared with (8,0) CNT. Similarly, the larger diameter of CBNNT(6,6) also prevents the adsorption of O2 on this nanotube. In addition to pristine CNT and BNNT, CBNNT also shows stronger O2 adsorption ability than certain doped nanotubes, such as phosphorus-nitrogen doped carbon nanotube (BE 5 21.41 kcal mol21).[44] However, it is weaker than the phosphorus doped carbon nanotube (BE 5 217.43 kcal mol21),[44] silicon carbide nanotube (BE 5 223.53 kcal mol21),[45] nitrogen doped carbon nanotube (BE 5 217.07 kcal mol21),[46] nitrogen doped short carbon nanotube (BE 5 259.27 kcal mol21),[47] and silicon doped carbon nanotube (BE 5 260.58 kcal mol21).[48] Nevertheless, there are likely possibilities for further tuning the O2 adsorption ability of the CBNNT materials. We recently showed[20] that the highest occupied crystal orbital/lowest unoccupied crystal orbital (HOCO–LUCO) gap of CBNNT can be tuned by changed the relative ratio of CNT and BNNT. In principle, the change of the HOCO–LUCO gap should affect the electron transfer ability of the CBNNT, which provides a route for further tuning the O2 adsorption ability of the CBNNT. Electronic structures To further elucidate the potential applications of hybrid CBNNT materials, we analyzed the band structures and total DOS (Fig. 6) of the most stable chemisorbed and physisorbed O2/CBNNT(8,0) complexes, which are compared with isolated CBNNT substrate. The CBNNT(8,0) is a semiconductor with a calculated band gap at 0.98 eV. When O2 is chemisorbed on CBNNT to form O2/ CBNNT(8,0)-1, the calculated band gap is significantly lowered to 0.15 eV. Hence, the nanotube shows significantly enhanced conductivity, as can be also seen from the change of the peaks across the Fermi level in the total DOS (Fig. 6). When O2 is physisorbed on CBNNT(8,0) to form O2/CBNNT(8,0)-4, the band gap also decreases compared to the isolated CBNNT(8,0), although to a lesser extent to 0.71 eV, likely due to the weak interaction between O2 and CBNNT. As a result, when O2 interacts with CBNNT(8,0), an increase of the conductivity of the nanotube is expected, similar to the pristine carbon nanotube[3] and other nanotubes, such as silicon carbide nanotube[45] and silicon doped carbon nanotube.[48] Considering the large binding energy in the chemisorbed O2/CBNNT(8,0) complexes and the increased conductivity upon such binding processes, the hybrid CBNNT may be useful in gas sensing applications.

Conclusions In this study, we used first-principles DFT to investigate the interaction of O2 with zigzag (8,0) and armchair (6,6) hybrid CBNNT. It is found that the CBNNT(8,0) shows dramatically different O2 adsorption properties compared with pristine CNT 1062

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and BNNT. For example, the chemisorption of triplet O2 by pristine CNT and BNNT is confirmed to be a thermodynamically unlikely process. In contrast, CBNNT is able to chemisorb triplet O2 with a large binding energy of 27.16 kcal mol21. Furthermore, the physisorbed O2/CBNNT complexes also have larger binding energies than those of O2/CNT and O2/BNNT complexes. The increased O2 binding properties of CBNNT can be attributed to the significant charge transfer between O2 and CBNNT, likely due to the formation of strong CAO and/or BAO bonds. In addition, the adsorption of O2 with CBNNT(8,0) is found to result an increase in the conductivity of the nanotube, as evidenced by the decreased band gap in the O2/ CBNNT(8,0) complexes. These properties suggest that hybrid CBNNT nanotube composites may be useful in future sensing devices, catalytic applications, and electrocatalytic technologies, further motivating experimental investigations of these materials.

Acknowledgment The authors appreciate the computational resources of the Alabama Supercomputer Authority. Keywords: carbon nanotubes  boron-nitride nanotubes  adsorption  oxygen  gas sensor  density functional theory

How to cite this article: H. Liu, C. H. Turner J. Comput. Chem. 2014, 35, 1058–1063. DOI: 10.1002/jcc.23589

]

Additional Supporting Information may be found in the online version of this article.

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Received: 18 December 2013 Revised: 17 February 2014 Accepted: 23 February 2014 Published online on 23 March 2014

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Oxygen adsorption characteristics on hybrid carbon and boron-nitride nanotubes.

In this work, first-principles density functional theory (DFT) is used to predict oxygen adsorption on two types of hybrid carbon and boron-nitride na...
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