Edson Nunes Costa Pauraa,
Wiliam F. da Cunhaa,
Luiz Fernando Roncarattia,
João B. L. Martinsb,
Geraldo M. e Silvaa and
Ricardo Gargano*ac
aInstitute of Physics, University of Brasilia, CP 4455, 70919-970, Brazil. E-mail: gargano@unb.br
bInstitute of Chemistry, University of Brasilia, CP 4478, 70904-970, Brazil
cDepartments of Chemistry and Physics, Quantum Theory Project, University of Florida, Gainesville, Florida 32611, USA
First published on 10th March 2015
The adsorption of a CO2 molecule on the vacancy defect type of armchair (5,5) and zigzag (10,0) single-walled boron nitride nanotubes was studied based on Density Functional Theory (DFT). Vacancy defects were studied and the geometrical modifications implemented on the original hexagonal lattice yielded a considerable level of changes in the electronic properties. These changes are reflected in a greater level of CO2 reactivity in relation to the adsorption over a pristine structure. For all types of studied CO2 molecule interaction, we have found a chemical adsorption process based on binding energy. Furthermore, the CO2 adsorption takes place on the top of the vacancy region. A decomposition state was observed when the CO2 molecule interacted with the armchair nanotube with a vacancy on the nitrogen site. By comparing the values of the adsorption energies with those from other defect approaches present in the literature, we conclude that the proposed protocol presents a possible tool to develop stable and sensible carbon dioxide sensors.
All the aforementioned advantages are known to be valid for ideal nanotubes, i.e., structures which are defect free. In the actual process of fabrication, however, defects that are either spontaneously or artificially produced may alter the physical and electronic properties of BNNTs. These changes in properties might lead to effects that can alternatively limit or enhance the applicability of these structures. In this sense, one of the early works in this field has shown that mechanically-induced defects lead to the formation and dislocation of dipoles on BNNT walls, provided certain conditions of temperature and pressure are considered.10 Similarly, another work, which considered transition electronic microscopy (TEM) together with theoretical modeling, suggested that defects created through electronic irradiation are mainly of a divacancy nature. This study has also shown that a set of several vacancies induces the creation of extended defects, which locally alter the nanotube diameter and chirality, thus affecting their electronic and optical properties.11 TEM is an appropriate experimental method for imaging defective structures and also for inducing in situ the appearance of defects. By using energetic electrons (100 keV), this technique is capable of removing single atoms from the lattice through direct knock-on collisions.11 An interesting signatures of point defects (such as single vacancies) can be seen in the high-resolution electron microscopy images of a bundle of BNNTs taken after being irradiated by an energetic electron beam.11 Activated boron nitride, as an effective adsorbent for metal ions and organic pollutants, has been also studied in the literature.12
Some relevant theoretical works also propose that the presence of vacancies raises the concentration of electrons or holes depending on if the defect is produced in the boron or in the nitrogen atom sites.13 Likewise, a recent work by Li et al.14 shows that a charge redistribution takes place near the defect site on the wall of an armchair BNNT, such that this effect might increase the chemical reactivity of BNNTs regarding the capture of molecular species. Moreover, there is an urgent need for new carbon dioxide capture strategies. Therefore, a systematic investigation on the role played by vacancy-type defects on favoring the capture of CO2 is highly desired. The importance of this particular molecule lies in fact that CO2 is one of the main anthropic greenhouse gases, thus dangerously inducing severe climate changes.15
Recently, a theoretical study based on Density Functional Theory (DFT) calculations has shown that a single graphene-like hexagonal BN sheet endowed with a vacancy on the boron site could efficiently capture a carbon dioxide molecule.16 It was reported that in one of the steps of the adsorption process, the CO2 was broken down into an oxygen molecule and a carbon atom captured by the lattice. This step is observed to take place through an intermediate chemisorption state on the defective BN sheet. Therefore, we make use of single-walled armchair (henceforward referred to as BNNT(5,5)) and zigzag (BNNT(10,0)) boron nitride nanotubes to investigate the influence of nitrogen or boron sites on the reactivity of a CO2 molecule. Specifically, the main question is regarding the chiral angle effects as well as the influence on the CO2 adsorption process due to the defect site.
The remainder of this work is organized as follows: Section II presents the computational settings, followed by our results and their discussions in Section III; we summarize the main features of this work in Section IV.
The positions of all the atoms were fully relaxed until the following convergence criteria were met: 0.001 Ha Å−1 for the force constant and 0.003 Å for displacement. The self-consistent field computations criterion was chosen to be 10−6 Ha. The electronic wave functions were expanded in a 4.4 version double numerical plus polarization basis set (DNP) truncated at a real space cut-off of 4.1 Å. The DNP basis set was found to lead to a small basis set superposition error (BSSE) (about 1.62 kJ mol−1).22 Therefore, we have used BSSE correction. Due to the presence of boron and nitrogen atoms in the model, all calculations were spin unrestricted. A 0.005 Ha smearing23 and 6 Pulay direct inversion of the iterative subspace (DIIS)24 was applied to the system to facilitate convergence of the electronic structures.
It must be noted that BNNTs used in nanotechnology applications are known to be of very large length. However, the explicit consideration of such a large system tends to be computationally expensive. In order to surpass this difficulty, while treating the system with a suitable approach, the 3D periodic boundary conditions were applied to the whole system, accomplishing simulation of the infinitely large BNNTs. The size of the vacuum space in-between two neighbor image tubes was set to be 20 Å to prevent the interaction between the atoms of each unit cell. The Brillouin zone for a single cell was sampled by 1 × 1 × 11 special k-points.
In order to compute the adsorption energy,25 we make use of the total energy of the isolated BNNT (EBNNT) and the CO2 molecule energy (ECO2), as well as of the energy of the complex system composed of the gas molecule interacting with the nanotube (EBNNT+CO2). The equation below employs these terms in the calculation of the adsorption energy:
Eads = EBNNT+CO2 − EBNNT − ECO2. |
It is important to remark that negative and positive values of Eads denote exothermic and endothermic adsorption, respectively. The Eads values and other properties are calculated based only on the lowest-energy structural configuration for the CO2–BNNT systems. In this work, the charge distribution on the system was analyzed using the Mulliken population method,26 despite its known limitations, since our interest was to analyze the trend of charge distribution across the same models.
Our work concerns the influence of the vacancy defects of BNNTs and of their chiralities during the CO2 adsorption process. In other words, we compare the adsorption of the gas molecule with and without structural defects for two different symmetries.
As a next step, we placed the CO2 molecule in the neighborhood of pristine BNNTs, and carried out a geometry optimization. Fig. 1 presents the frontal and side views of the equilibrium configuration for the two different nanotube chiralities considered. One can observe that, regardless of the chirality, both the BNNTs and the CO2 molecule preserve their original geometrical characteristics. For the case of BNNT(5,5), the CO2 molecule configuration was parallel to the tube axis and located outside the BNNT wall, with its carbon atom directly in front of the nitrogen atom of the BNNT, and its oxygen atoms close to the boron atoms of the BNNT.
This behavior can be understood in terms of the polar character of the B–N bonding, which creates a higher charge concentration over the nitrogen atoms, due to the greater electronegativity of nitrogen when compared to boron. Thus, it is expected that the CO2 atoms are organized by means of electronic affinity: the carbon atom of the carbon dioxide is positively polarized and interacts with a nitrogen atom whereas the oxygen atoms are attracted by the boron of the nanotubes.
As for BNNT(10,0), it was calculated that the CO2 molecule was positioned above the center of a nanotube hexagon. This configuration was achieved after the molecule performed a 38° rotation in relation to its original position around its axis, with its oxygen atoms directly above the nanotube boron atoms. In this case, as in previous reported works,27 the zigzag bonding pattern of the B–N plays an important role in the molecular orientation related to the tube axis.
Table 1 presents the adsorption energies (Eads), equilibrium distances (D), charge transfers (QT) and energy gaps (Eg) for the BNNT–CO2 for both considered chiralities. As it is shown, the optimized configurations present small values of adsorption energies and large equilibrium distances, which indicate that the interaction between the carbon dioxide molecule and the pristine boron nitride nanotube is of a physical adsorption nature. In this case, the interaction is known to be mediated by van der Waals type interactions. A comparison of the values obtained for Eads between the (5,5) and (10,0) tubes highlights a small difference of 0.02 eV. This fact suggests a slight tendency of the molecule to interact better with the zigzag rather than with the armchair pristine nanotube.
System | Eads (eV) | D (Å) | Eg (eV) | QT (e) |
---|---|---|---|---|
BNNT(5,5)–CO2 | −0.16 | 3.00 | 4.35 | −0.003 |
BNNT(10,0)–CO2 | −0.18 | 2.95 | 4.03 | −0.005 |
The charge analysis, carried out through the Mulliken method, suggests that about 0.61e and 0.54e are transferred from the boron atom to the adjacent nitrogen atom in the (5,5) and (10,0) BNNTs, respectively. This fact endorses the partially ionic character of the B–N bond in BNNTs. By using a Density of States (DOS) analysis, our results also indicate the presence of band gaps of 4.35 eV and 4.03 eV for the (5,5) and (10,0) nanotubes, respectively. These results are in agreement with previous data present in the literature.13,28 From Table 1, we also note that no appreciable energy gap difference is observed for the BN nanotubes. This is an indication that the physical adsorption does not affect the electronic properties of pristine BNNTs. It is important to remark that these results are consistent with the fact that pure BNNTs are quasi-inert structures. Indeed, the low values observed for the interaction energies are a measure of the high stability of these materials without defects.2
We present, in Fig. 2, the optimized geometries for the vacancy defect BNNTs. The geometry deformation due to the vacant atom site is shown for all structures. Fig. 2a suggests a stretching in the radial direction for the BNNT(5,5)VB in the region of the removed boron atom. One can note that the relaxation induces the displacement of a nitrogen atom (N21) toward the tube and of two other nitrogen atoms, N23 and N4, outside the tube. In this case, we observed the formation of a 12-fold ring, without new bonding types involving nitrogen close to the vacancy due to the dangling bond. The equilibrium distances between the nitrogen atoms were found to be 2.67 Å for N23–N4, 2.38 Å for N23–N21 and 2.38 Å for N21–N4. These values indicate that, indeed, no new bonding was established between the nitrogen atoms.
Fig. 2b concerns the effect of a nitrogen vacancy instead of the boron vacancy of Fig. 2a. In this case, one can see that deformations appeared on a smaller scale than in the previous case. The two-coordinated boron atom B20 relaxes outside the tube whereas the opposite happens for both B22 and B39. As can be seen, the formation of new bonding between atoms B22 and B39 takes place, thus altering the original hexagonal lattice around the vacancy site. The distance between the aforementioned boron atoms (B39–B22) is 1.82 Å, which corresponds to the formation of a five-fold and nine-fold adjacent rings arrangement. It is important to remark that these geometrical results for the vacancy defect of BNNT(5,5) are in agreement with other previous results from the literature.14
Switching to the zigzag chirality, Fig. 2c represents the BNNT(10,0)VB case. A pronounced radial deformation is observed around the vacancy site. Atom N39 is displaced outside the nanotube, whereas atoms N77 and N41 move inside the nanotube. Also, a local rebuilding of the bonding between nitrogen atoms N77 and N41 takes place, thus promoting changes in the original hexagonal lattice. The bond distance between nitrogen atoms N77–N41 is 1.46 Å, thus being responsible for the formation of a five-fold and nine-fold adjacent rings arrangement.
A smaller level of geometrical deformation was observed to take place for the BNNT(10,0)VN, as can be seen in Fig. 2d. A weak bonding is observed between boron atoms B2 and B38. The bonding length was 1.78 Å, which also results in the formation of five-fold and nine-fold adjacent rings. It can be seen that both the binding boron atoms are dislocated inside the nanotube, whereas the two-coordinated boron atom B79 is pushed outward from the nanotube. We must, again, stress that these results obtained for the vacancy defect of BNNT(10,0) are in agreement with previously published work.29 We thus conclude, by analyzing Fig. 2, that for both BNNT(5,5) and BNNT(10,0), the vacancy formation of boron atom causes greater structural deformation than the nitrogen vacancy.
These geometrical distortions result in important modifications of the electronic properties of the nanotubes. Fig. 3 represents the DOS due to the vacancy defect type. We emphasize that this kind of defect induces impurity states near to the Fermi level in all cases.
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Fig. 3 DOS of BNNT(5,5) on the left and BNNT(10,0) on the right, for pristine and vacancy defect BNNTs with boron and nitrogen vacancies. |
For BNNT(5,5)VB, a final state in the valence band reduced the original band gap to 2.85 eV. For BNNT(5,5)VN, the appearance of an isolated state between the end of the valence band and the beginning of the conduction band due to the vacancy formation creates a state of 1.37 eV near the valance band. The zigzag BNNT(10,0) presents the same trend in the electronic behavior changes to its armchair counterpart when vacancy defects are introduced. A final state inside the original gap of BNNT(10,0)VB at 2.11 eV occurs near the valence band. BNNT(10,0)VN, on the other hand, presents an isolated state of 0.97 eV between the end of the valence band and the beginning of the conduction band, close to the valence band.
The charge density analysis also reveals important information related to vacant BNNTs. Fig. 4a shows that in the case of BNNT(5,5)VB, the charge is concentrated over the aromatic ring, particularly above the nitrogens with dangling bonds (N23, N4 and N21). For BNNT(5,5)VN (Fig. 4b), one can observe a higher charge concentration over the B39 and B22 bonds.
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Fig. 4 Charge density of vacancy defect BNNTs. (a) BNNT(5,5)VB, (b) BNNT(5,5)VN, (c) BNNT(10,0)VB, (d) BNNT(10,0)VN. |
For the BNNT(10,0)VB (Fig. 4c), the charge is also mainly concentrated over the two-coordinated N39 nitrogen atom. It is important to note that, despite the presence of the bonding between atoms N41 and N77, Fig. 4c does not reveal a significant charge concentration between these nitrogen species that would favor strong interaction in bound systems. BNNT(10,0)VN (Fig. 4d), similarly to BNNT(5,5)VN, presents a high concentration of charge over the B38–B2 bond. These results show that for both chiralities, the variation in charge distribution occurs on the vacancy region. It also demonstrates that the vacancy of a nitrogen atom induces a region with an excess of electrons, whereas that of boron causes a deficiency of electrons. Therefore, the electrostatic behavior of these defects can be useful for the capture of molecular species.
Table 2 presents the comparison between Eads, D, Eg and QT for the different vacancy defect BNNTs interacting with the CO2 molecule. Considering BNNT(5,5)VB–CO2 (Fig. 5a), CO2 is interacting with the nanotube through a C–N bond, whose length is 1.40 Å. In this case, the CO2 molecule presents an inner angle of 118.4°, with the C–O distances being altered to 1.20 Å and 1.36 Å. The binding energy between the carbon dioxide molecule and BNNT(5,5)VB is −4.42 eV (Table 2), suggesting that the interaction is of a chemical adsorption nature. The adsorption energies obtained for CO2 in (8,0) BNNT with a boron antisite (BN) using PAW-PBE30 are smaller than that obtained with vacancy formation. The charge transfer is 0.6e and occurs from the carbon to the nitrogen atom due to electronegativity differences. In the interesting case of BNNT(5,5)VN–CO2, we observe that a complete dissociation of the CO2 molecule takes place. The final product is an oxygen atom and a carbon monoxide (CO) molecule. In this case, the binding energy was not included due to the carbon dioxide dissociation. Additionally, these investigations of CO2–BNNT interactions may provide insight into the mechanism and surface characterization on nanotube planes, for the application of CO2 storage and the screening and design of materials for CO2 capture, and how the dissociation may affect the partial charge distribution of the vacancy sites.31–33
System | Eads (eV) | D (Å) | Eg (eV) | QT (e) |
---|---|---|---|---|
BNNT(5,5)VB–CO2 | −4.42 | 1.40 | 3.41 | −0.50 |
BNNT(5,5)VN–CO2 | — | — | 2.65 | — |
BNNT(10,0)VB–CO2 | −4.26 | 1.40 | 2.14 | −0.45 |
BNNT(10,0)VN–CO2 | −6.53 | 1.30 | 1.31 | 0.82 |
As can be seen in Fig. 5b, the dissociated oxygen atom ends up binding to the three boron atoms in the vacancy region, thus yieling an average B–O bond length of approximately 1.50 Å. The substitutional energy of an oxygen atom in the (5,5) BN nanotube was calculated from the chemical potentials (Esub = EBNNT+O − EBNNT − νO + νN); the covalent bond between the oxygen and the boron atoms is of about −13.46 eV, thus being of the same order of magnitude as that of the B–N bond in the nanotube. Interestingly, the CO molecule, rather than being separated from the nanotube lattice, turns out to interact with BNNT(5,5)VN through a relatively weak bond estimated to be of about 0.8 eV, which characterizes a physisorption process. An evidence of the weak interaction between the CO molecule and the BN nanotube is that the C–O distance preserves the same 1.15 Å value of the free species. It is important to remark that experimental studies present in the literature16 have dealt with a similar situation regarding the dissociation, albeit in the scope of a graphene-type sheet with a boron vacancy interacting with a CO2 molecule. It was reported that the CO2 molecule suffered decomposition in such a way that the carbon atom was bound to the nitrogen of the sheet, whereas a formed oxygen molecule maintained a weak interaction with the BN sheet.16 Although in our work this process took place for a nitrogen vacancy system, these theoretical results confirm that dissociation phenomena can indeed be observed experimentally.
For BNNT(10,0)VB–CO2, Fig. 5c shows that the interaction with CO2 results in the breaking of the weak bond between N41 and N77. In this case, the CO2 molecule was linked to the tube by means of a bond between the carbon atom and N41 as well as by another bond between the oxygen and N39. The C–N41 presents a bond length of 1.40 Å whereas that of O–N39 is about 1.38 Å. The inner angle of CO2 in this configuration was found to be 116.4°, and the distances are 1.20 Å and 1.36 Å. The obtained binding energy between the CO2 molecule and BNNT(10,0)VB was −4.26 eV, thus suggesting that this interaction is also of a chemisorption nature. For BNNT(10,0)VN–CO2 (Fig. 5d), a bond breaking between sites B2–B38 and the formation of two bonds between the CO2 molecule and BNNT(10,0)VN were observed. An interaction between the carbon atom and B38, and another between the oxygen atom and B79, were observed. In this configuration, the C–B38 distance was found to be 1.64 Å and that of O–B79 was 1.37 Å. The inner angle of the molecule changed to 120.2° and the C–O distances were altered to 1.20 Å and 1.35 Å. In this case, it is possible to note that the oxygen binding to the boron atom in the nanotube is closer than that of the carbon. The binding energy was observed to be −6.53 eV, which also suggests a chemisorption interaction between the molecule and BNNT(10,0)VN.
These results suggest that BNNT(5,5)VB is more suitable for capture of CO2 than BNNT(5,5)VN, since while the former interacts with the molecule by means of a chemical adsorption, the interaction of the latter yields the decomposition of the molecule to a carbon monoxide molecule, which is highly toxic. Regarding the chirality effects, we note that both zigzag nanotubes (BNNT(10,0)VB and BNNT(10,0)VN) capture the CO2 molecule through a chemisorption process. In this case the adsorption energy of the BNNT(10,0)VN–CO2 complex is observed to be 2.27 eV larger than that of BNNT(10,0)VB–CO2. Thus, a system derived from the boron vacancy is shown to be more efficient as far as carbon dioxide adsorption is concerned.
By comparing the binding energies of the zigzag and armchair nanotubes, one concludes that the kind of defect plays an important role in the adsorption energy. Although BNNT(5,5)–CO2 presents only a slight difference of 0.16 eV from BNNT(10,0)–CO2 in the complex binding energy, when the structures contain a vacancy defect this difference greatly increases. Specifically, the adsorption energy is 2.11 eV greater for BNNT(5,5)VB–CO2 than for BNNT(10,0)VN–CO2.
Table 2 also shows the charge transfer between the tubes and the CO2 molecule. We can note that in the case of the vacancy on the boron site, the nanotube presents a charge deficient region, in such a way that the carbon dioxide molecule behaves like an n-type dopant. On the other hand, when the vacancy occurs on a nitrogen site, the BNNT presents an excess of charge, in which case the carbon dioxide molecule behaves like a p-type dopant.
In order to exploit the adsorption effects of the CO2 molecule on the electronic properties of vacancy defect BNNTs, we plotted the density of states due to the molecular adsorption in Fig. 6. It can be observed that, in the case of BNNT(5,5)VB, the absence of an electronic state at the end of the valence band occurs after the adsorption of the CO2 molecule. Also, the associated gap is observed to be 3.41 eV. In the case of BNNT(5,5)VN, despite the molecular decomposition, a state near the conduction band remains present, which yields an associated gap of 2.65 eV.
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Fig. 6 DOS of BNNT(5,5) on the left and BNNT(10,0) on the right, for pristine and vacancy defect BNNTs with an adsorbed CO2 molecule. |
For both BNNT(10,0)VB–CO2 and BNNT(5,5)VB–CO2, the vanishing of a state at the end of the valence band occurs, which yields a gap of 2.14 eV. A state between the valence and conduction bands of BNNT(10,0)VN suffers a small shift of 1.31 eV. Table 2 presents an expected gap for all the analyzed structures. Fig. 7 plots the charge density in order to show the charge concentration in the interaction between BNNTs and the CO2 molecule. We observe that CO2 was bound exactly on the vacancy region with those atoms that present dangling bonds.
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Fig. 7 Charge density of vacancy defect BNNTs with an adsorbed CO2 molecule. (a) BNNT(5,5)VB, (b) BNNT(5,5)VN, (c) BNNT(10,0)VB, (d) BNNT(10,0)VN. |
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