Interaction of Al, Ti, and Cu Atoms with Boron Nitride Nanotubes: A

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The Interaction of Al, Ti and Cu Atoms With Boron Nitride Nanotubes (BNNTs): A Computational Investigation Christoph Rohmann, Qiao Sun, and Debra J Searles J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10698 • Publication Date (Web): 14 Jan 2016 Downloaded from http://pubs.acs.org on January 30, 2016

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The Journal of Physical Chemistry

The Interaction of Al, Ti and Cu Atoms with Boron Nitride Nanotubes (BNNTs): A Computational Investigation Christoph Rohmann,† Qiao Sun‡ and Debra J. Searles†♯* †

Centre for Theoretical and Computational Molecular Science, Australian Institute for

Bioengineering and Nanotechnology, The University of Queensland, Queensland 4072, Australia ‡

Institute of Quantitative Biology and Medicine, Collaborative Innovation Center of Radiation

Medicine of Jiangsu Higher Education Institutions, School of Radiation Medicine and Protection, Medical College of Soochow University, Soochow University, Suzhou 215123, China ♯

School of Chemistry and Molecular Biosciences, The University of Queensland, Queensland

4072, Australia *

Email: [email protected] Phone: +61 (0)7 3346 3939

ABSTRACT

Quantum chemical calculations have been carried out to study the interactions of boron nitride nanotubes (BNNTs) with Al, Ti and Cu atoms. The interaction of these metals with pristine BNNTs, BNNTs with B or N vacancy defects, and BNNTs with C substitution have been investigated. Our results indicate that Ti exhibits the strongest binding to the BNNTs investigated, with the exception of the BNNTs where an N is substituted for a C atom. In this instance Al is found to bind equally strong. The adsorption of the metals onto B and N vacancies

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in BNNTs and C sites on BNNTs is significantly stronger than to pristine BNNTs, with the binding energies for a BNNT with a B vacancy being even stronger than that of carbon nanotubes with a vacancy. We find that in most cases the binding energy is little affected by changes in the nanotube radius and chirality.

1.

Introduction

Since the first theoretical prediction in 19941-2 and the synthesis of the pure material shortly afterwards,3-4 boron nitride nanotubes (BNNTs) have received ever-growing attention. BNNTs are low density materials (1.37 g/cm3)5 that have a high elastic modulus, thermal stability and resistance to oxidation.6-8 It has been shown that BNNTs are stable up to 700°C in air with oxidation occurring at ~800°C, whereas carbon nanotubes (CNTs) are found to be oxidized at 400°C.9 Furthermore, BNNTs are found to be stable in a metal matrix, while CNTs corrode, which is due to the redox potentials of CNTs and the insulating properties of BNNTs.10,11 Their low weight, stability and resistance to oxidation are the properties that have made BNNTs be considered as a reinforcing material in metal matrix composites (MMC). For this application, Al and Ti are of interest because they are low density materials, which is of extreme importance in aviation and space applications. Moreover Al has a melting point of 660°C,12 below the temperature at which BNNTs have been shown to be unstable.9 The reduction in density attained through formation of Al and/or Ti MMCs and the simultaneous improvement in mechanical performance have the potential to enable space missions and the design of novel aircraft that are not possible with current materials and structures technology.


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Although there is little experimental data on BNNT/Ti matrix composites available,13 the interactions of BNNTs within an Al matrix have recently been studied to a greater extent.14-15 In some instances strong BNNT-Al interactions and the formation of AlN and AlB2 regions are reported,14 while in other studies no such formations are found.15 The formation of AlN and AlB2 was attributed to the existence of bamboo-like structures15 which are clearly abundant on the BNNTs produced in that case. Incorporating BNNT into an Al matrix by high-pressure torsion (HPT), the tensile strength is reported to increase by roughly 50%, from 200 MPa for a BNNT free HPT-Al compact to 300 MPa for a sample containing 3 wt% BNNTs.15 Although this is a significant improvement over pure Al, Al alloys such as Al-2024 (wt. %: 3.8-4.9 Cu, 1.2-1.8 Mg, 0.3-0.9 Mn, others ≤ 0.5)16 exhibit a tensile strength of 505 MPa,16-17 far exceeding the value of 300 MPa. In addition this alloy exhibits a melting point of 502-638°C, which is below that of pure Al. Another interesting material is T-6Al-4V (wt. %: 6 Al, 4 V, others ≤ 0.25)18 a Ti alloy with a tensile strength of 950 MPa.18-19 While the Al alloy contains a significant amount of Cu, the Ti alloy contains significant amounts of Al. Incorporating BNNTs into these alloys to further improve their properties could make BNNT MMCs the material of choice for future applications. Fundamental to the functionality of BNNTs in a MMC is the interaction between the metal and the BNNT. This interaction should be strong enough to provide binding, but not so strong that the integrity of the nanotube is compromised. This happens when a strong covalent bond between the metal and atoms of the NT forms that destroys the sp2 network of the nanotube, which is mainly responsible for the excellent structural integrity of the nanotube. In extreme cases B or N could be completely removed from the nanotube as is the case for C atoms from CNTs which have been shown to form aluminium or silicon carbides (Al4C3 or SiC) on interaction with pure Al,20-21 Al-202422 or Al-23 wt. %Si.23 Previously the formation of Al4C3 in


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A357, an Al alloy, reinforced with carbon fibres was attributed to the reduction in strength and premature failure.24 Furthermore, Lahiri and co workers14,25 showed that prolonged exposure of BNNTs to Al metal at 650°C results in the formation of AlN and AlB2 phases. Computational studies can provide important information on the interactions of Al, Ti and Cu at an atomic level, which can be used to guide selection of materials for synthesis. Properties of the nanotubes such as their diameter, chirality and presence of vacancies will also influence their interaction with metals, and these can be systematically studied using computational methods. Employing density functional theory (DFT) calculations, the interaction of a single atom with BNNTs has been successfully investigated in a wide variety of cases.26-29 These studies have been motivated by potential use of metal-doped BNNTs and BN nanosheets in applications such as catalysis,30 nano-electronics and spintronics,31-35 gas sensing36 and hydrogen storage.37 In this work we focus on the two main metals of interest in formation of low density MMCs, Al and Ti. In addition we consider Cu since it is present in Al alloys that look promising for use in MMCs. As well as being metals of relevance to MMC materials, these three metals have quite distinctive properties: Al is a main group metal; Ti a transition metal with unoccupied 3dorbitals, and Cu a transition metal with fully-occupied d-orbitals. It has previously been proposed that coupling between un-occupied 3d-orbitals on transition metals and 2p orbitals of C enhance the binding with C; and a similar effect is expected for the interaction with B and N of BNNTs.38-39 We carry out a thorough investigation of the binding of these metals with pristine BNNTs, those with a B or N vacancy and C substituted BNNTs. In each case, different chiralities and diameters are considered with an aim of determining how the properties of the nanotube influence the bonding. Nanotubes of diameter 6.4-13.9Å are considered which


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represent the diameter of recently synthesis single walled BNNTs,40 defects are formed by B and N vacancies and the substitution with carbon atoms. The defects we consider are sufficiently separated that they do not influence each other. Therefore this is in the limit of a reasonably small, but experimentally achievable, concentration of defects.41 In addition, armchair (n,n) zigzag (n,0) and chiral (n,x, x ≠ n,) BNNTs are considered. In section 2 we describe the computational methodology used in this work, in section 3 we present the results, which are discussed in detail in section 4. Some conclusions that can be obtained from this work are presented in section 5. 2.

Method

Periodic BNNTs or M@BNNTs (metal adsorbed on a BNNT) were studied with the supercell oriented so that the axis of the BNNT lies along the z-axis. Spin polarized DFT calculations were performed as implemented in the Vienna Ab initio Simulation Package (VASP)42 for all calculations. The Perdew-Burke-Ernzerhof (PBE) functional within the generalise gradient approximation approach (GGA) was employed to model the exchange correlations.43 The ionelectron interaction is described by ultra-soft pseudopotentials.44 All structures were fully relaxed until the total energy converged to 1.0 x 10-4 eV within the self-consistent loop and forces of less than 0.01 eV/Å were reached. The k-space was sampled with a 1 x 1 x 1 Monkhorst Pack mesh45 and an energy cut-off of 400 eV was used for all calculations. Van der Waals interactions were accounted for by means of the Grimme scheme.46 Density of state (DOS) calculations were carried out with a 1 x 1 x 12 Monkhorst Pack mesh. Initial test calculations were conducted to determine the appropriate energy cut-off and number of k-points, with results showing less than 0.01 eV changes in the binding energy upon an


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increase to 450 eV or a k-point mesh of 1 x 1 x 6. Prior to creating the supercells, the dimensions of each NT (nanotube) was optimized with respect to the c lattice constant (the axis of the NT), while vacuum layer of at least 15 Å was added in the a and b direction. To model the clean and adsorbate covered structures 1x 1 x 5 supercells were considered for the armchair NTs, and 1 x 1 x 3 supercells for the zigzag BNNTs. A single unit cell was sufficient for the (10,5) and (9,6) NTs due to their length. Minimum values of 11.5 Å for c (15 Å for a and b) were used to ensure interaction of the adsorbed species with its periodic image is small. A summary of the optimized lattice constants can be found in Table S1 of the supplementary material. The reported binding energies (BE) are defined as: ‫ܧ = ܧܤ‬ெ@ே் − ‫ܧ‬ே் − ‫ܧ‬ெ

(1)

where ‫ܧ‬ெ@ே் is the energy ‫ ܧ‬of a nanotube (which may be pristine, contain a vacancy or have a C atom substituting an N or B) which has a metal atom adsorbed on it; ‫ܧ‬ே் is the energy of the isolated, geometry optimized nanotube and ‫ܧ‬ெ is the energy of the isolated atom. For comparison, binding to the h-BN sheet was also considered. In this case a 15.3 Å x 15.3 Å hBN sheet consisting of 36 B and 36 N atoms was employed with a 10 Å vacuum layer.

3. 3.1

RESULTS

Adsorption of Single Al, Ti and Cu Atoms onto Pristine BNNTs

To determine the most stable adsorption site in each of the investigated systems, the ad-atom was initially placed in several different positions: above an N atom, B atom, the B-N bonds and in


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addition it was placed in the middle of a hexagon for the (5,5), (7,7), (10,10), for both zigzag and chiral BNNTs. The structures were then optimised to find the preferred binding site. During this procedure it was noted that the binding site is sensitive to the position of the bond within the BNNT (i.e. its orientation with respect to the axis of the tube). This provided two different environments for all armchair and zigzag BNNTs and three for the chiral (10,5) and (9,6) BNNTs, see Figure 1 and S1 for more detail. The chiral (9,6) and (10,5) and the (12,0) zigzag BNNTs were chosen because they have a similar radius to the (7,7) armchair BNNTs, although different orientations of the B-N bonds with respect to the tube axis. The adsorption geometries, binding energies and bond distances are presented in Figures 2, 3 and 4, Table 1 and Table S2 in the supplementary material.

Figure 1:

Top view of the (a) armchair (5,5), (b) zigzag (8,0) (c) chiral (10,5) and (d) chiral (9,6) BNNTs including the different B-N bond positions depicted in red, orange and yellow (N=blue, B=beige).

3.1.1. Pristine Armchair BNNTs Adsorption Geometry The adsorption of Al and Cu atoms were found to occur in a similar way on all the armchair BNNTs. We find the preferred adsorption site to be above a B-N bond that is perpendicular to


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the axis of the tube, with the Cu and Al situated slightly closer to the N (see Figure 2(a) and (c) as well as Table S2). In contrast, Ti is found to be situated above a ring between two neighbouring N atoms and slightly further from the B atom that is between them (see Figure 2(b)). Similarly to Cu and Al, the Ti adsorption geometry is found to be sensitive to the directions of the bonds relative to the tube axis. In all cases investigated we find Ti forms an NTi-N structure that is approximately parallel to the axis of the BNNT (see Figure 2 (b) and Table S2).

Figure 2:

Side view of (a) Al, (b) Ti and (c) Cu atoms adsorbed on a (5,5) BNNT. Only part of the supercell is shown for a clear display of the adsorption geometry (N=blue, B=beige, Al=purple, Ti=grey, Cu=light brown).

Upon an increase in tube diameter we note that the relative position of the Al remains unchanged. When Ti is considered, it results in a slight increase in the Ti-N distances, whereas the Ti-B distance remains approximately constant. The overall geometry, however, remains the same. Cu, in contrast to Al and Ti, changes its position upon an increase in tube diameter, moving towards a position above the N, which can be observed by the lengthening of the Cu-B distance while the Cu-N distance remains largely unchanged.


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Binding Energy For all nanotube diameters considered, the binding energy was found to be greatest for the Ti atom (-1.22 eV for a (5,5) BNNT), followed by the Al atom (-0.74 eV for a (5,5) BNNT) and smallest for the Cu atom (-0.55 eV for a (5,5) BNNT), with the binding energy decreasing with tube diameter and approaching the h-BN binding energies in the large diameter limit (see Figure 3 and Table 1).

Figure 3:

Variation on the binding energy of Ti, Al and Cu to pristine BNNTs as a function of the inverse diameter of the nanotube. Results for armchair (circles), zigzag (+) and chiral nanotubes (x) are shown. The lines are a guide to the eye.

A steady decrease in the binding energy with increase in diameter of the BNNT, coupled with a lengthening of the Al-N and B-N distance is observed for Al (see Figure 3, Table 1 and Table S2). Therefore it is assumed that the decrease in binding energy is due to the curvature of the BNNT. The Ti and Cu binding energies are less sensitive to changes in the tube diameter. Only for the smallest BNNT investigated, the (5,5), is a significant difference in binding energy observed as the diameter of the tube is increased. From the (6,6) BNNT onwards, the changes in the case of Ti are very small and for Cu the values remain largely constant. We note that with h-


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BN sheets (denoted with ∞ sign in Table 1) the binding energies of Al and Cu are very similar, 0.52 eV and -0.48 eV respectively, whereas Ti still exhibits the strongest binding of -1.15 eV. Furthermore, the binding strength of Ti and Cu to the (10,10) BNNT is almost identical to that with the h-BN sheet and for Al the binding to the (10,10) BNNT is within 0.1 eV of the h-BN sheet.

3.1.2 Pristine Zigzag and Chiral BNNTs There are some differences in the preferred binding sites for the zigzag and chiral BNNTs compared to the armchair nanotubes, due to the orientation of the bonds (See Figure 4 and Table S1; note that the binding geometry for the (8,0) BNNT is similar to that of the (12,0) BNNT and is therefore not shown in Figure 4). In all cases considered the Al and Cu atoms are situated above a B-N bond and closer to the N atom. Al is found to be most stable above bonds whose orientation is close to being perpendicular to the axis of the tube, as in the armchair BNNT. Cu is in this position only for the (9,6) BNNT whereas on the zigzag and chiral (10,5) BNNT it is on a bond that is closest to being parallel to the axis of the BNNT (see Figure 4). Ti is still situated above the ring and is found to give a similar structure to that observed for the armchair nanotubes when the two chiral tubes are considered. However, in the case of the zigzag BNNTs, the Ti-N bond distances are not of equal distance, in contrast to the chiral and armchair BNNTs. In addition, the (12,0) BNNT also shows a Ti-B bond length being shorter than the Ti-N running close to perpendicular to the axis of the BNNT (see Table S2). The obtained binding energies for all zigzag and chiral BNNTs investigated are consistent with those of the armchair BNNTs, see Figure 3 and Table 1 for more detail.


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Figure 4:

The adsorption sites for Al, Ti and Cu on zigzag (8,0) and chiral (10,5) and (9,6) BNNTs. For clarity, atoms of only a section of each supercell that includes the metal atom (Al, Ti and Cu) is displayed (N=blue, B=beige, Al=purple, Ti=grey, Cu=light brown). We note that the adsorption geometry of the (8,0) and (12,0) BNNT was the same.

3.1.3. Comparison with CNTs Adsorption studies of Al, Ti and Cu to the (7,7) and (12,0) CNTs were conducted to compare these results with those of the corresponding BNNTs. The adsorption of the three metals onto CNTs shows a significant difference in binding strength between armchair and zigzag CNTs (see, Table 2) while for BNNTs no significant difference in binding strength was noted. The


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different binding strength for the armchair and zigzag CNTs might be related to their different conductivity. The zigzag and armchair arrangement of the CNTs is found to show a semiconducting and metallic behaviour respectively,47 whereas the BNNTs are wide-gap semiconducting irrespective of the chirality.1 Our results further show that the binding of the metals to CNTs in both cases is significantly stronger than to BNNTs.

3.2 Adsorption of single Al, Ti and Cu atoms onto BNNTs with a B or N vacancy The interaction between each investigated metal and a BNNT was studied for cases where an N or B atom of a pristine BNNT was absent and therefore able to be replaced by an Al, Ti or Cu atom. For some initial configurations it was observed that the metal atom moved inside the nanotube on relaxation. In these cases the metal atom was placed only slightly above the vacant site before structure optimisation. However, in all cases investigated, the most stable configuration were those that showed an outward relaxation of the metal atom from the vacant site. In the following, only the properties of the most stable systems are discussed. The adsorption geometries of Al, Ti and Cu on N and B vacancies of the BNNT are shown in Figure 5 with distances given in the supporting information (Table S3 and S4). The binding energies are given in Table 3 and 4 and are presented in Figures 6 and 7. In all cases the binding energies are larger than the binding energies of the metals with pristine BNNTs.


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Figure 5:

Top view of (a) Al, (b) Ti and (c) Cu atom adsorbed onto a (5,5) BNNT with an N vacancy and (d) Al, (e) Ti and (f) Cu adsorbed onto a (5,5) BNNT with a B vacancy (N=blue, B=beige, Al=purple, Ti=grey, Cu=light brown).

Figure 6:

Variation on the binding energy of Al, Ti, and Cu to BNNTs with an N vacancy as a function of the inverse diameter of the nanotube. Results for armchair (filled circles), zigzag (+) and chiral nanotubes (x) are shown. The result for the smallest diameter nanotube (8,0) is not shown for Ti as this binding energy is much larger than the others shown in this figure (-6.10 eV). The lines are a guide to the eye.


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Figure 7:

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Variation on the binding energy of Ti, Al and Cu to BNNTs with a B vacancy defect as a function of the inverse diameter of the nanotube. Results for armchair (filled circles), zigzag (+) and chiral nanotubes (x) are shown. The lines are a guide to the eye.

3.2.1. Armchair BNNTs with an N vacancy Adsorption Geometry Armchair BNNTs with an N vacancy result in Al and Ti adsorption that takes place with the metal occupying a position slightly above the site vacated by the N atom (see Figure 5 (a), (b), (d) and (e)). The binding of Al results in one short and two long Al-B bonds, whereas Ti (see Figure 5 (b) and (e) shows the opposite behaviour having one long and two short Ti-B bonds. Cu occupies a position between two B atoms forming a B-Cu-B structure roughly parallel to the axis of the BNNT, see Figures 5 (c) and (f). Upon an increase in tube diameter we notice a very small shortening of the short Al-B bond in the case of Al, whereas the two longer Al-B bonds remain constant. For Ti, the long Ti-B bond is found to slightly decrease while the other two remain constant. In contrast, Cu shows a


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significant decrease of the longest Cu-B bond, with the other Cu-B bonds remain constant upon an increase in tube diameter; see Table S3 for details. The adsorption onto an h-BN sheet shows an unchanged geometry for Al, Ti however exhibits three equidistant Ti-B bonds, whereas the geometry for Cu is reversed forming one short and two long Cu-B bonds.

Binding Energy All three metals exhibit a significant increase in binding strength (~3-6 times stronger) compared to the interactions with pristine BNNTs (compare Figures 3 and 6; and Tables 1 and 3). Interestingly, the ordering of the strongest to weakest adsorbate changes from Ti> Al > Cu on pristine BNNTs to Ti> Cu > Al for BNNTs with an N vacancy. Al showed a significant decrease in binding energy with an increase in diameter when adsorbed to a pristine tube. Although the absolute value of the change is similar in this case, the binding energy is much greater thus the relative change is small (compare Figures 3 and 6). As in the case of the pristine BNNT, we also note that the binding energies of the three metals to the (10,10) BNNT are very close to those of the h-BN sheet.

3.2.2. Zigzag and chiral BNNTs with an N vacancy The observed adsorption geometries for Al, Ti and Cu are similar to those of the armchair BNNTs with Al and Ti most stable above the vacancy while Cu prefers to from a B-Cu-B bond.


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Some differences in the binding energy are observed comparing the armchair, zigzag and chiral BNNTs of similar diameter. We note a slightly weaker binding of Al, Ti and Cu to zigzag and chiral BNNTs of a similar diameter to their armchair counterparts (comparing the (12,0), (10,5) and (9,6) to the (7,7) and (8,8) BNNTs). Major differences are seen comparing the binding energy of the (8,0) to the (5,5) BNNTs, with Al showing a slightly stronger binding what could be expected due to tube diameter, Cu shows a less strong binding and Ti an over 2 eV stronger binding compared to the armchair BNNT of similar diameter.

3.2.3. Armchair BNNTs with a B vacancy Adsorption Geometry Al, Ti and Cu are found to be most stable in positions just slightly above the vacant site (see Figure 5 (d)-(f)). All three metals exhibit similar asymmetric bonding geometries on top a B vacancy with one long and two short bonds of equal distance (see Table S4 for more detail). These geometries remain largely unchanged upon an increase in tube diameter. The adsorption to an h-BN sheet results in Al and Ti to be equidistant from the three N atoms with Cu maintaining the above mentioned symmetry. While Al and Cu are located at the site vacated by the B atom, Ti is slightly above that site.

Binding Energy The binding of Al, Ti and Cu onto BNNTs with a B vacancy is even stronger than onto those with an N vacancy (see Figure 7 and Table 4 for more detail). In fact, the binding energy of Ti is


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~ -12 eV which is approaching the binding energy of a B in a pristine BNNT (-16 eV). Furthermore, we note that the binding strength for the different metals is in the order Ti> Al > Cu, consistent with the trend for the adsorption on pristine BNNTs. Upon an increase in tube diameter an interesting, although small, trend in the binding energies is observed. Up to a certain tube diameter the binding energy increases; this diameter corresponds to the (9,9) BNNT for Al and the (7,7) BNNT for Ti and Cu. From that point onward the energy remains constant for Ti and Cu until reaching the diameter of the (9,9) BNNT before the value starts to decrease again. This trend is further confirmed by the adsorption onto an h-BN sheet with a B vacancy, which shows an even lower binding energy for Al, Ti and Cu.

3.2.4. Zigzag and Chiral BNNTs with a B vacancy Also consistent with the results of the armchair BNNTs, the most stable position for Al, Ti and Cu is found to be on top of the B vacancy. The bond length of each metal to its N neighbours varies slightly compared to the armchair BNNTs, see Table S4 for more details. We observe a notable difference in the binding energy of the metals to the (10,5) BNNT which is found to be approximately 0.3 eV weaker than the armchair BNNTs for Al, Ti and Cu. A drastic difference is noted for binding of Al, Ti and Cu to the (8,0) zigzag BNNT resulting in a binding energy that is approximately 2 eV weaker than the (5,5) BNNT which is of similar diameter.

3.2.5. Comparison to Armchair and Zigzag CNTs with a C vacancy


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The most stable adsorption site of Al, Ti and Cu to the (7,7) and (12,0) CNTs is found to be directly above the C vacancy. While the (12,0) zigzag CNT has one long and two short M-C bonds the (7,7) armchair CNT has a short and two slightly longer M-C bonds. The binding to the CNTs containing a C vacancy is stronger for the all CNTs. It is important to point out that the binding energies obtained for both zigzag and for armchair CNTs are higher than the binding energies of the metals to pristine BNNTs, but significantly smaller than those on BNNTs with a B vacancy (compare results in Table 2 with those in Tables 1 and 3 -4).

3.3 Adsorption onto C substituted N and B vacancies of zigzag, armchair and chiral BNNTs 3.3.1. Adsorption Geometry The adsorption of Al, Ti and Cu onto armchair BNNTs containing C impurities is modelled employing a (7,7) BNNT where either one B or N is being substituted by a C. To simplify the calculations the most stable geometries of Al, Ti and Cu interacting with a (7,7) BNNT were employed as the initial structures with one C atom replacing an N or B atom. The C doped (12,0) zigzag BNNT, (9,6) and (10,5) chiral nanotubes were treated in the same way. The adsorption geometries of the armchair BNNTs and binding energies of the zigzag, armchair and chiral BNNTs are summarized in Table 5 and Table S6, the optimised geometries are shown in Figure 8. Our results show that all three metals are situated close to the C impurity regardless of whether it replaces the N or B atom, or of the chirality of the BNNTs. Whereas Al and Cu are situated


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almost directly above the C site, Ti is most stable above the ring, like in the pristine system. When a B atom is substitute by a C atom, the Ti-C bond is shorter than the two Ti-N bonds which are of equal length, which is the opposite behaviour to that observed for the pristine systems. An exception occurs in the case of the zigzag BNNT where C substitutes an N. Here, Ti is found above the C-N bond, closer to the C atom.

Figure 8:

The adsorption sites for Al, Ti and Cu on (7,7) armchair BNNTs doped with a C atom at an N or B site. For clarity, only a section of each supercell that includes the metal atom (Al, Ti and Cu) is displayed (N=blue, B=beige, Al=purple, Ti=grey, Cu=light brown).


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3.3.2. Binding Energy Calculations of the binding of Al, Ti and Cu to the BNNTs containing C impurities suggest that the binding strength of Al, Ti and Cu on nanotubes of the same diameter is independent of the chirality of the nanotubes. The exception being the (12,0) zigzag BNNT where Ti adsorbs onto a C that is substituting an N, and shows a slight decrease in binding strength. Furthermore Al is found to exhibit a binding energy equal or stronger to Ti, while Cu binds least strongly onto a C substituting an N, regardless of tube chirality. In addition, the binding strength of Al to the C substituting an N is stronger than that of an N vacancy while those of Ti and Cu are close to that of an N vacancy. In general we observe a significant increase in binding energy compared to the pristine BNNT. When C substitutes an N atom, a stronger binding is observed than when a C substitutes a B atom. We find the binding strength to decrease from Al ~ Ti > Cu when C substitutes N and Ti > Al > Cu when C substitutes B, with the latter trend agreeing with the findings for the pristine system.

4.

Discussion

The observed adsorption geometries of Al, Ti and Cu for the investigated systems show that the coordination number of the metals is ordered of Ti > Al > Cu, which in most cases corresponds to order of binding strength. The adsorption on the pristine BNNTs shows Ti in a configuration interacting with one B and two N, while Al and Cu are situated between B and N. With increasing tube diameter, Cu moves towards the N thus reducing coordination further. In


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addition, the lower coordination of Cu is also seen when it fills an N vacancy, where Cu forms a B-Cu-B bond in contrast to Al and Ti. This may be partly explained simply by the relative atomic sizes of the metals, however contributions due to electronegativity and strong interaction between the unfilled 3d-orbitals of Ti and the 2p-orbitals of the B and N will also contribute.38-39 To show that the metals actually are interacting with their neighbouring atoms in the manner described above we perform a Bader charge analysis48 on the pristine (7,7) BNNT. The charges on Ti, Al and Cu were found to be +0.61, +0.54 and +0.03 respectively, which can be explained by the fact that Ti and Al have roughly the same electronegativity (EN) of 1.5, with Cu being the most electronegative, EN = 1.9.49 The configuration described above (Figure 2(b)) has Ti interacting with two electron withdrawing N (EN = 3.0)49 atoms that have a higher charge (~0.15e) than the other N atoms in the structure and the B atom close to the Ti also has a significantly higher charge (~0.1e) than the other B atoms that are further from the adsorption site). The N and B atoms interacting with the Al and Cu also have a 0.1-0.2e higher charge than all other B and N atoms in the structure. Furthermore an increase in tube diameter form a (5,5) to (10,10) BNNT where Cu moves to the top of the neighbouring N is coupled with a steady decrease in charge on the B atom (0.89e in case of the (5,5) to 0.84 in case of the (10,10) BNNT), thus representing the lowered coordination of Cu upon an increase in tube diameter. The trends in adsorption energy on the pristine BNNTs are consistent with those obtained in a previous study employing a pristine (8,0) CNT: the binding of Ti being strongest followed by Al, then Cu.50 Furthermore, that study reports that the binding energy of Ti decreases when being adsorbed onto a larger (6,6) CNT, which agrees with our finding for the BNNTs. The adsorption of Al and/or Cu onto a (6,6) BNNT was not considered in that investigation. It has also been


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shown in previous work50-51 that the strength of binding to CNTs varies with the number of d electrons for transition metals, with Ti being particularly strongly bound. Beheshtian et al. reported the binding energy of Al onto a (5,0) zigzag BNNT terminated by hydrogen atoms to be -0.61 eV.52 Wu and Zeng26 reported the binding energy of Ti and Cu on a (8,0) BNNT to be -1.19 eV and -0.46eV respectively, similar to the values obtained in this work (-1.29 eV and -0.56 eV, respectively). The adsorption of a single Cu atom onto h-BN sheets35,53 and the interaction of a Cu(111) surface with an h-BN sheets54 have previously been investigated. The single atom studies conducted by Zhou et al.35 and Liu et al.53 report a Cu binding energy of -0.79 eV and -0.22 eV and a Cu-N bond length of 2.22 Å and 2.18 Å respectively. While the bond distances are in excellent agreement with our results, our reported binding energy of -0.48 eV lies between the two earlier results. This difference might be attributed to consideration of dispersion forces in our case and the different methods employed. Our results further show that the binding of Al, Ti and Cu to pristine CNTs is significantly stronger than to pristine BNNTs considered in our study, see Table 1 and 2 for comparison. This is in good agreement with literature results for a (8,0) CNT reporting a -2.2 eV binding for Ti and -0.7 eV with Cu.50 This study further presents data showing a stronger binding of the investigated metals to zigzag compared to an armchair CNT. The value for Ti was given as -2.2 eV for the (8,0) zigzag CNTs compared to -1.8 eV of the (6,6) armchair CNT that also confirms our results. In contrast, overall we note rather small differences in binding energy of Al, Ti and Cu to armchair, zigzag and chiral BNNTs of similar diameter, whether they are pristine, have an N or B vacancy or are C doped BNNTs, with the exception of the smallest diameter BNNT


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investigated above, i.e. the (8,0) BNNT. With decreasing NT diameter it is expected that more internal stress and orbital deformation will develop due to the increasing curvature. This makes the NT more unstable, deformable and reactive, and the formation of sp3 bonding more likely. The different behaviour of the (8,0) BNNT is also noted from results of preliminary density of States (DOS) calculations. These reveal a significant difference in the electronic structure compared to other BNNTs of larger diameter. While we calculate a band gap of 3.70 eV for the (8,0), values of roughly 4.50 eV are obtained for all BNNTs exhibiting larger diameter (DOSs are displayed in Figure S3 and band gaps in Table S8). Decreasing the diameter even further, as done in case of the (6,0) BNNT, causes the band gap to decrease to 2.78 eV (see Table S8 for more detail). These results indicate that the decrease in diameter of the nanotube is changing the bonding in these very small NTs, whereas for all others consider the bonding is similar. We would like to point out that using our current methodology, the band gaps are well know to be underestimated (the experimentally measured value of is 6.0 eV for large diameter nanotubes ~ 60 nm55), however the trends are expected to be the same if a higher level of theory is used. As noted above, in the case of pristine BNNTs we find Ti to bind most strongly, followed by Al and Cu. We observe that only very small changes in binding energy occur upon a changing diameter for Ti and Cu, whereas Al shows a greater dependence on tube diameter. For the cases investigated, we generally observe a decrease in binding energy upon an increase in tube diameter. The adsorption geometry may offer an explanation for the different behaviour of Al. While Ti forms roughly an N-Ti-N bond running parallel to the axis, any effects from an increase in diameter are marginal, since it affects mainly regions running perpendicular to the axis of the tubes. Cu moves towards the neighbouring N thus reducing its coordination to one, which also minimises the effect. Al, on the other hand, remains its adsorption geometry at a site


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perpendicular to the axis of the BNNT, thus most affected by an increase in diameter. Comparing the binding energy of Al, Ti and Cu for the largest BNNTs considered with those of an h-BN sheet, we find almost identical values for Ti and Cu, and the values of Al are still within 0.1 eV. Therefore our results suggest that large diameter pristine BNNT can be represented by h-BN sheets in the case of Al, Ti and Cu. BNNTs with an N or B vacancy also show small differences in binding strength when different chiralities are considered with the exception of the (8,0), the smallest BNNTs investigated. Removing a single N atom from an (8,0) BNNT to create an N vacancy results in significant structural deformations and an overall increase in energy by roughly 17 eV (see Figure S2 and Table S7 in the supplementary material for more detail). All other BNNTs investigated, show less deformation and an increase in energy of only approximately 15 eV. Therefore one needs to take the nanotube structure into consideration when discussing the binding energy in this case. It might be expected that the 2 eV that is associated with structural deformation would be added to the binding energy if adsorption of the metals “healed” this deformation. This change in energy and a ‘healing’ of the deformation is observed for Ti. In contrast, the deformation remains and a significantly weaker binding energy is observed for Al and Cu. Examining those structures (represented in Figure S2) it can be seen that the structural deformation is still present in the case of Al and Cu, while the adsorption of Ti onto an N vacancy reverses the deformation. This structural deformation also causes Al and Ti to bind roughly 0.3 eV more weakly to an N vacancy for the (12,0) BNNT compared to the armchair BNNTs of similar diameter and chirality. Cu on the other hand shows binding energies for these BNNTs that are more consistent to those of other BNNTs considered. This however seems to be caused by the binding geometry:


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Cu forms one short and two longer Cu-B bonds, thus mostly interacting with one B only and therefore being less sensitive to changes in the BNNT environment. As in the case of the adsorption onto N vacancies, a 0.3 eV weaker binding for Al, Ti and Cu to a B vacancy for the (10,5) BNNT can be attributed to structural deformations of the clean structure exhibiting a B vacancy (see Table S7 for more detail). The last case to consider is the strongly reduced binding energy (roughly 2 eV) of Al, Ti and Cu to a B vacancy for the (8,0) BNNT. We believe that this drastic reduction in binding strength is also related to a structural effect within the (8,0) BNNT. In contrast to the (8,0) with an N vacancy, our results show that the geometry of the (8,0) remains intact and similar to the structures of other BNNTs with a B vacancy except the (10,5) BNNT. However, the reduced binding strength seems associated with the distortion of the (8,0) BNNT upon adsorption of Al, Ti or Cu at the B vacancy. This distortion is evident from consideration of the bond distance of the metal to the second nearest neighbour N where we notice a significant elongation of two and the shortening of the third M-N bonds, see Table S5 for more detail. The adsorption of the metals to BNNT with one C substituted by an N or B atom resulted in an increase in binding strength compared to that on the pristine BNNTs, consistent with stronger binding on CNTs. The binding strength was found to be stronger in the case where a C substitutes an N atom than when it substitutes a B atom. An increase was expected since our adsorption studies of Al, Ti and Cu on the (7,7) and (12,0) CNTs showed a stronger binding compared to all BNNTs investigated. However, we find the binding of Al to the BNNT when a C substitutes an N to exceed that of an N vacancy, whereas the energies are similar when Ti and Cu are considered. In addition, the binding strength of Al was equal or stronger to that of Ti in those


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cases. This may suggest that it is possible that targeted C substitution can specifically enhance the binding strength of one metal over another. This may also be possible with targeted creation of vacancies, since the binding strength on an N vacancy was stronger for Cu adsorption than Al, whereas all other cases showed a stronger adsorption of Al.

5.

Conclusion

In this work we compared the adsorption behaviour of Al, Ti and Cu atoms to BNNTs and its dependence on BNNT diameter, chirality and the influence of B and N vacancies. We generally find a trend of Ti > Al > Cu in terms of the binding energy which can be attributed to the combined result of several factors including the size and coordination number of the atoms, their electronegativity and the interaction unfilled 3d-orbitals with the 2p-orbitals of B and N. However, there are two exceptions to the trend: (i) when adsorption takes place on a BNNT containing a C that is substituted for an N where Al binds at least as strong as Ti; (ii) the adsorption at an N vacancy of the BNNT where Cu is found to bind more strongly than Al. We show that vacancies (N or B) and C substitutions strongly enhance the binding of Al, Ti and Cu. In addition our results show that the adsorption-taking place at a B vacancy results in a significantly stronger binding of the metal to the BNNT than to a CNT (with or without a Cvacancy) of similar diameter. The binding of the metals to the CNT is sensitive to the chirality of the nanotube, unlike in the case of the BNNT, and is likely to be linked to the greater changes in the electronic structure that occur with change in chirality of the CNT; in contrast to the BNNT.


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Our results further show that the (8,0) BNNT exhibits significantly different binding energy and adsorption geometry in all cases investigated compared to larger diameter BNNTs considered here. Therefore, this suggests that once the diameter of the tube is less than a critical value, the behaviour will change. In contrast, our data shows that the interaction of Al, Ti and Cu with hBN sheets is comparable to that of the BNNTs with the largest diameter considered with the exception of those systems were the adsorption takes place at an N vacancy. Therefore we note that h-BN sheets can provide good models of BNNTs, provided they are not too small. In order for BNNT to be useful as reinforcement materials for MCCs, it is necessary that the interactions are strong enough that they bind, but not so strongly that the BNNTs are degraded in the presence of the metals. Our results suggest that the binding with pristine nanotubes is moderate and generally decreases with increasing tube diameter. This is consistent with the behaviour observed for CNTs and is caused by the increasing curvature resulting a change in the electronic structure of the smaller BNNTs and their higher reactivity. This highlights the need to produce small diameter BNNTs.41,56 There is also very strong binding of the metals with defective nanotubes – particularly when there is a B vacancy. Despite this, the nanotube structure appears to remain, with the metal atom forming part of the nanotube. Further work on the interaction of BNNTs with clusters of metal atoms and/or metal surfaces is required to validate the obtained results for the adsorption of single atoms.

Acknowledgement This research was undertaken with the assistance of resources from the National Computational Infrastructure (NCI), which is supported by the Australian Government and the Queensland


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Cyber Infrastructure Foundation, the Queensland University Research Computing Centre and computing facilities funding through a LIEF grant from the Australian Research Council. We are grateful for funding and support from the Asian Office for Aerospace Research and Development (AOARD) and the Air Force Office of Scientific Research (AFOSR). We would like to thank Dr Vesselin Yamakov and Dr Cheol Park for helpful discussions and valuable input.

Supporting Information. Tables list: cell parameters; metal-N/B/C bond distances of all systems investigated and those to the nearest neighbour N atoms in case of the (8,0), (5,5) and (12,0) BNNTs; energy changes upon removal of an N/B atom; and band gaps. Figures present: the tested adsorption sites; distortion of the (8,0) BNNT containing an N vacancy; and the total density of states of the (8,0) and (5,5) BNNTs


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Tables Table 1:

Binding energies of Al, Ti and Cu on pristine BNNTs of different diameters and chiralities. The ∞ sign denotes h-BN sheets, representing BNNTs with infinitely large diameters.

Diameter

Binding Energy Al

Binding Energy Ti

[eV]

[eV]

Binding Energy Cu [eV]

BNNT

[Å]

5,5

6.9

-0.74

-1.22

-0.55

6,6

8.3

-0.66

-1.18

-0.51

7,7

9.7

-0.63

-1.17

-0.51

8,8

11.1

-0.61

-1.16

-0.51

9,9

12.5

-0.61

-1.18

-0.52

10,10

13.9

-0.57

-1.15

-0.50

8,0

6.4

-0.79

-1.28

-0.56

12,0

10.6

-0.64

-1.15

-0.51

9,6

10.5

-0.61

-1.15

-0.49

10,5

10.6

-0.60

-1.14

-0.48

∞

∞

-0.48

-1.14

-0.48


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Table 2:

Page 30 of 45

Binding energies of Al, Ti and Cu on the (7,7) and (12,0) pristine CNTs and at a C vacancy.

Binding Energy Al [eV]

Binding Energy Ti [eV]


7,7

-0.94

-1.92

-0.60

12,0

-1.65

-2.81

-0.90

7,7

-3.31

-3.43

-3.16

12,0

-4.74

-7.06

-2.93

CNT Pristine

C Vacancy


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Binding energies of Al, Ti and Cu on BNNTs with an N vacancy defect, and different

Table 3:

diameters and chiralities. The ∞ sign denotes h-BN sheets, representing BNNTs with infinitely large diameters.

BNNT

Binding Energy [eV]

Binding Energy [eV]

Binding Energy [eV]

Al

Ti

Cu

-2.85

-3.84

-3.25

-2.77

-3.78

-3.23

-2.71

-3.71

-3.22

-2.67

-3.68

-3.21

-2.63

-3.65

-3.19

-2.63

-3.64

-3.19

8,0

-3.12

-6.10

-3.17

12,0

-2.37

-3.43

-3.17

9,6

-2.50

-3.59

-3.20

10,5

-2.58

-3.50

-3.17

-2.63

-3.69

-3.08

5,5 6,6 7,7 8,8 9,9 10,10

∞


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Binding energies of Al, Ti and Cu on BNNTs with a B vacancy defect, and different

Table 4:

diameters and chiralities. The ∞ sign denotes h-BN sheets, representing BNNTs with infinitely large diameters.

BNNT

Binding Energy

Binding Energy [eV]

Binding Energy [eV]

[eV] Al

Ti

Cu

-11.22

-12.23

-6.61

-11.29

-12.25

-6.63

-11.35

-12.28

-6.67

-11.37

-12.28

-6.67

-11.38

-12.28

-6.67

-11.25

-12.13

-6.52

8,0

-8.80

-10.20

-4.74

12,0

-11.29

-12.26

-6.60

9,6

-11.42

-12.33

-6.70

10,5

-11.08

-12.02

-6.40

-10.44

-11.96

-5.30

5,5 6,6 7,7 8,8 9,9 10,10

∞


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Table 5:

Binding energies of Al, Ti and Cu on various BNNTs doped with a C atom at a B or N site. “C sub N/B” represents the substitution of a C for an N/B.

Binding Energy Al [eV]

Binding Energy Ti [eV]


(7,7)

-3.66

-3.67

-2.96

(12,0)

-3.67

-3.52

-2.94

(9,6)

-3.67

-3.68

-2.95

(10,5)

-3.66

-3.61

-2.93

(7,7)

-2.21

-2.44

-2.02

(12,0)

-2.21

-2.47

-1.97

(9,6)

-2.18

-2.42

-2.00

(10,5)

-2.19

-2.42

-2.00

BNNT

C sub N

C sub B


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Figure 1: Top view of the (a) armchair (5,5), (b) zigzag (8,0) (c) chiral (10,5) and (d) chiral (9,6) BNNTs including the different B-N bond positions depicted in red, orange and yellow (N=blue, B=beige). 175x36mm (300 x 300 DPI)



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Figure 2: Side view of (a) Al, (b) Ti and (c) Cu atoms adsorbed on a (5,5) BNNT. Only part of the supercell is shown for a clear display of the adsorption geometry (N=blue, B=beige, Al=purple, Ti=grey, Cu=light brown). 82x27mm (300 x 300 DPI)


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Figure 3: Variation on the binding energy of Ti, Al and Cu to pristine BNNTs as a function of the inverse diameter of the nanotube. Results for armchair (circles), zigzag (+) and chiral nanotubes (x) are shown. The lines are a guide to the eye. 80x44mm (288 x 288 DPI)



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Figure 4: The adsorption sites for Al, Ti and Cu on zigzag (8,0) and chiral (10,5) and (9,6) BNNTs. For clarity, atoms of only a section of each supercell that includes the metal atom (Al, Ti and Cu) is displayed (N=blue, B=beige, Al=purple, Ti=grey, Cu=light brown). We note that the adsorption geometry of the (8,0) and (12,0) BNNT was the same. 82x94mm (300 x 300 DPI)


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Figure 5: Top view of (a) Al, (b) Ti and (c) Cu atom adsorbed onto a (5,5) BNNT with an N vacancy and (d) Al, (e) Ti and (f) Cu adsorbed onto a (5,5) BNNT with a B vacancy (N=blue, B=beige, Al=purple, Ti=grey, Cu=light brown). 82x45mm (300 x 300 DPI)



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Figure 6. Variation on the binding energy of Al, Ti, and Cu to BNNTs with an N vacancy as a function of the inverse diameter of the nanotube. Results for armchair (filled circles), zigzag (+) and chiral nanotubes (x) are shown. The result for the smallest diameter nanotube (8,0) is not shown for Ti as this binding energy is much larger than the others shown in this figure (-6.10 eV). The lines are a guide to the eye. 80x44mm (288 x 288 DPI)


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Variation on the binding energy of Ti, Al and Cu to BNNTs with a B vacancy defect as a function of the inverse diameter of the nanotube. Results for armchair (filled circles), zigzag (+) and chiral nanotubes (x) are shown. The lines are a guide to the eye. 80x45mm (288 x 288 DPI)



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Figure 8: The adsorption sites for Al, Ti and Cu on (7,7) armchair BNNTs doped with a C atom at an N or B site. For clarity, only a section of each supercell that includes the metal atom (Al, Ti and Cu) is displayed (N=blue, B=beige, Al=purple, Ti=grey, Cu=light brown). 82x98mm (300 x 300 DPI)


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For Table of Contents Only 85x46mm (300 x 300 DPI)


Interaction of Al, Ti, and Cu Atoms with Boron Nitride Nanotubes: A

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