High Energetic Polymeric Nitrogen Stabilized in the Confinement of

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High Energetic Polymeric Nitrogen Stabilized in the Confinement of Boron Nitride Nanotube at Ambient Conditions Shijie Liu, Mingguang Yao, Fengxian Ma, Bo Liu, Zhen Yao, Ran Liu, Tian Cui, and Bingbing Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04374 • Publication Date (Web): 12 Jul 2016 Downloaded from http://pubs.acs.org on July 17, 2016

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The Journal of Physical Chemistry

High Energetic Polymeric Nitrogen Stabilized in the Confinement of Boron Nitride Nanotube at Ambient Conditions

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Authors: Shijie Liu , Mingguang Yao , Fengxian Ma , Bo Liu , Zhen Yao , Ran †

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Liu , Tian Cui and Bingbing Liu*

†

Affiliations: †

State Key Lab of Superhard Materials, Jilin University, Changchun 130012, China

*To whom correspondence may be addressed. E-mail: [email protected] Tel/Fax: +86-431-85168256

Abstract Polymeric nitrogen, as a potential high-energy-density material (HEDM), has the promising applications for energy storage, propellants, and explosives. The searching for an effective method to recover polymeric nitrogen to ambient conditions is of great interest. Here, we study the confinement of polymeric nitrogen chain in boron nitride nanotube (BNNT) with different diameters, and find the polymeric nitrogen chain can be stable in BNNT (5, 5) at ambient conditions, while it can be also stabilized in other diameters of BNNTs with the application of little pressure. The polymeric nitrogen chains encapsulated in the BNNT dissociate into N2 molecules and release tremendous energy at above 1400K. The hybrid structure is favored by a charge transfer from BNNT to N chain. The hybrid material has a smaller charge transfer between the N chains and the hosting BNNT compared with other hosting materials. More importantly, this smaller charge transfer not only stabilizes the polymeric nitrogen chain at ambient conditions but also favors the energy release at a more gentle condition, which can be taken as an obvious advantage of BNNT when used as hosting material. Our findings offer a highly promising route for harvesting nitrogen-based HEDMs under ambient or near ambient conditions.

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Introduction Polymeric nitrogen has been attracting increasing attention due to its potential applications as a HEDM for energy storage system, propellants and explosive and they decompose into pure inert gas N2 molecules which are environmentally non-polluted. The polymeric nitrogen can release huge energy once it decomposes into N2 molecule, due to a uniquely large amount of the energy difference between single N-N or double N=N and triple N≡N bonds that is higher than most powerful energetic materials known today.1–4 Owing to the intriguing properties of the polymeric nitrogen, plenty of structures have been theoretically suggested under high pressure5–25. Among them, the so-called cubic gauche (CG)1 and layered polymeric nitrogen forms

26,27

have been proved experimentally and successfully synthesized at

extreme conditions. However, these forms of polymeric nitrogen can’t be recovered at ambient conditions. Therefore, searching for an effective method to stabilize polymeric nitrogen at ambient conditions has been a topic of interest and challenge. Recently, Abou-Rachid and coauthors theoretically predicted that a polymeric nitrogen chain becomes stable at ambient conditions when confined in carbon nanotube (CNT) and graphene matrix28,29. In hybrid materials, the configuration of the polymeric nitrogen chain N8 was stemmed from the previously predicted three-dimensional polymeric nitrogen existed in high pressure8. It is suggested that the Coulombic interaction due to charge transfer between polymeric nitrogen and carbon host material makes the hybrid structure stable at ambient conditions. Later on, an experimental group reported some traces that indicate the existence of polymeric nitrogen molecules in carbon nanotube bundles by electrochemical method30, suggesting that nano-confinement is a promising method to obtain energetic polymeric nitrogen at ambient conditions. This strategy was also studied by using silicon carbide nanotubes as host templates.31 In fact, as a HEDM, besides the high energy storage, chemical inertness and controllable release of energy are also very important requirements, it is thus highly desired to seek for other potential host material to confine polymeric nitrogen and develop the confinement method. The host material is expected to possess superb thermal and oxidation stability and chemical 2 / 18


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inertness which otherwise would be not stable in air, and/or easily contaminated at ambient conditions. Therefore, it is necessary to find a new hosting material which can stabilize the polymeric nitrogen chains at ambient condition and allow them to release the energy under a gentle condition and in a controllable way. BNNTs are analogous to CNTs, which have hollow interior space and could act as hosting material, such as applications in composites, hydrogen storage media.32–37 Unlike the nonpolar C-C bonds in CNTs, the polar B-N bonds with ionic character in BNNTs can induce dipole moment within the tube structure and result in buckled tubular structure.38–40 In addition, BNNTs are wide gap semiconductors with a band gap of 5.5eV

41,42

. Due to their unexpected thermal and oxidation stability and

chemical inertness43,44, BNNTs has been widely used as protective capsules for encapsulating many kinds of materials. For example, Li et al. investigated fullerene coalescence into metallic hetero structures in boron nitride nanotubes;32 Mickelson et al. recently found that the oxidation degradation of CNTs can be reduced by coating them with BNNTs45. However, the potential of BNNTs as a host material to encapsulate polymeric nitrogen is still an open question. Here, we have systematically studied the BN nanotubes with various diameters as host materials to confine polymeric nitrogen chain N8 by ab initio simulations. Finite temperature simulations demonstrate that the polymeric nitrogen chain can be stable in BNNT (5, 5) at ambient conditions, while it can be also stabilized in other diameter BNNT with the application of little pressure. We further reveal the mechanism of nitrogen chain stabilized by electronic structure analysis. More importantly, the MD simulations show that the polymeric N chains encapsulated in the BNNT dissociate into N2 molecules and release tremendous energy at a gentle condition. We thus propose a feasible and practical template to recover polymeric nitrogen to ambient or near ambient conditions and extend the application of the confinement method.

Theoretical methods All calculations performed in this paper are based on the first-principles plane-wave pseudopotential density functional theory (DFT) as implemented in the 3 / 18



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CASTEP code.46 An ultrasoft pseudopotential is used. The exchange and correlation effects are described by the generalized gradient approximation (GGA) PBE (Perdew–Burke–Ernzerhof).47 A convergence test gives an energy cutoff of 400 eV with 1×1×5 Monkhors-Pack grid spacing for the one-dimension Brillouin zone (BZ) integration of the hybrid materials. Calculations are carried out in a supercell approach, keeping a vacuum distance of 10 Å between the wall of the nanotube and its image in the next unit cell. All parts under study are fully relaxed both with respect to internal atomic positions and to cell size. The geometry relaxation is stopped when the maximum atomic force in the system is less than 0.005 eV /atom and the energy difference of two iterations is less than 10-6 eV/atom. To verify the accuracy of electronic properties, the calculations are also performed using the VASP code.48 Molecular dynamic (MD) simulations are performed in constant volume and constant temperature for 2ps time period. During the simulation the temperature is set to 300 5000 K with the step 200 K by using Nose thermostat. Calculations of the band structure require a very high degree of precision with 1×1×9 Monkhorst - Pack grid spacing. The armchair chain N8 is used as the starting polymeric nitrogen chain, which stems from the previously predicted three–dimensional polymeric nitrogen trans-cis chain under high pressure8, same as the polymeric nitrogen chain in N8@CNT28. To test the validity of the method, we performed atomic relaxation of armchair periodic nitrogen chains. For the armchair chain containing four nitrogen atoms per unit cell, the bond lengths are calculated to be 1.28 and 1.36 Å, which are in good agreement with previously reported theoretical values (1.27, 1.37 Å).28 For the host nanotubes, we consider the different radiuses of the BNNT to host the N8 chains, including the (4, 4), (5, 5), (6, 6), (7, 7) and (8, 8). The unit cells of the hybridization structures with various diameters of BNNTs contain 2 elementary unit cells of nitrogen chain (8 atoms) and 3 elementary rings of the BNNT. These unit cells are used to calculate band structures and density of states, and to perform ab initio molecular dynamics (MD) calculations.

Results and Discussion 4 / 18


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Figure 1. The maps of cross section (a) and longitudinal section (b) for double cells of N8@BNNT (5, 5) structure; (c) The isosurface of electron density difference for N8@BNNT (5, 5) system. Red and yellow denote the effective positive and negative charge, respectively. (d) The electron density distribution of N8@BNNT (5, 5) along the N8 chain.

We insert the N8 chains into a series of BNNTs with different diameters, including the (4, 4), (5, 5), (6, 6), (7, 7) and (8, 8). The tests of MD simulations at ambient conditions and geometry relaxations at zero temperature are performed for these hybrid systems. Our simulations show that N8 chains can be stable only in the (5, 5) BNNT at ambient conditions with the N-N bond lengths of 1.29 Å and 1.34 Å, which are similar to the bond lengths of N chain encapsulated in CNTs28. The optimized structures of N8@BNNT (5, 5) are shown in Figure 1(a, b). For the other diameter BNNTs, the polymeric nitrogen chains also can be stable but the application of a slight pressure is required, as shown in Table 1. Herein, we focus on the N8@ BNNT (5, 5) hybrid structure. 5 / 18



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Table 1. The pressures required to stabilize the system and the amount of charge transfer (e/N atom) between the N8 chain and the BNNTs with different diameters. BNNTs

Diameters

Pressure (GPa) to

Charge transfer by

Charge transfer by

of BNNTs

stabilize the

the integrated DOS

Bader analysis

(Å)

system

and Mulliken

method

(4, 4)

5.52

0.5

0.08

0.09748

(5, 5)

6.90

0

0.04

0.04392

(6, 6)

8.28

1

0.01

0.01734

(7, 7)

9.67

3.5

0.005

0.00921

(8, 8)

11.05

2

0.0021

0.00310

In order to give deep insight on the stability of the N8@BNNT (5, 5), we calculated the electron density difference for the N8@BNNT (5, 5) by subtracting the electron of N8 chain and BNNT from the density of N8@BNNT (5, 5). The result demonstrates how the electron distributes due to charge transfer between the N chain and the BNNT as shown in Figure 1(c). The electron density is increased around the N chain and decreased near the wall of BNNT. Thus, BNNT transfers small amount of electrons to the polymeric nitrogen chain, in which the charge transfer mainly occurs between the nitrogen atom in BNNT and N chain due to the polar B-N bonds with most of electrons located near nitrogen atoms. The electron transfer from BNNT to polymeric N chains favors the stabilization of the hybrid system because the Coulombic interactions between the inner wall of the nanotube (positive charged) and the nitrogen chain (negative charged) stabilize the hybrid system. Similar phenomenon has also been reported in the case of polymeric nitrogen chains confined in CNT28, graphene29, and SiCNT31. The electron distribution on N chain is clearly shown in Figure 1(d), which is similar to that of armchair N chain in the bulk8.

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Figure 2. Band structures for (a) the hybrid material N8@BNNT (5, 5), (b) the empty boron nitride nanotube and (c) the free N-chain.

We further study the band structure for the N8@BNNT (5, 5) system along the one-dimensional Brillouin zone. To highlight the effects of the interaction between the N chain and BNNT, we also calculated the band structures of a free N chain and a BNNT using the same atomic coordinates as in the N8@BNNT (5, 5) system. Through comparing these bands structures, we find that bottom of the conduction bands of N8@BNNT (5, 5) is from the N chain; the first two bands in the top of the valence band of the system is from the BNNT, and the later two bands are from N chain (see Figure 2(a-c)). The results show that no coupling occurs between the occupied orbits of the BNNT and N chain. However, we find that the conduction bands of the N chain shifts downwards and the first valence bands of N chain are modified to a nearly flat band due to the interaction between BNNT and N chain.

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Figure 3. The density of states for empty BNNT (5, 5) and BNNT in the N8@BNNT (5, 5) system (a), and density of states for the stand-alone N-chain and N-chain in the N8@BNNT (5, 5) system (b).

In addition, the hybridization between the N-chain and the BNNT is clearly seen from the total projected density of states (PDOS) of N8@BNNT. PDOS for the N8@BNNT (5, 5) system, projected on the orbits of the nitrogen chain atoms, as well as on the orbits of the nanotube are shown in Figure 3. Density of states for the free N-chain and BNNT (5, 5) are also shown in the figure for comparison. From Figure 3, the PDOS of BNNT (5, 5) in N8@BNNT (5, 5) preserves its pristine curve similar to that of empty BNNT (5, 5). For polymeric nitrogen chains in N8@BNNT (5, 5), the curve of density of state (DOS) also does not change but the energies of the corresponding DOS peaks downshift compared with the free N chain. These changes of DOS in BNNT and polymeric nitrogen chain in N8@BNNT (5, 5) compared with those of the isolated ones originate from the hybridizations between BNNT and N chains, leading to the charge transfer from BNNT to N chain and stabilization of the hybrid system. In addition, using the integrated DOS and the Mulliken charge calculations, we quantitatively analyze the charge transfer between N chain and BNNTs in the hybrid materials at their stable configurations. The results indicate that the amount of transfer charge gradually decreases with the increase of the diameters of the hosting BNNTs, as shown in Table 1. More accurately, we also performed a 8 / 18


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Bader charge transfer analysis. As summarized in Table 1, the results and resulting conclusion are consistent with each other.

Figure 4. The N–N bond lengths as a function of time in our MD simulations at 300K and 1300K. The red and black lines show the bond lengths of N–N which parallel and nonparallel to the tube axis, respectively.

To study the stable temperature range of the N8@BNNT (5, 5), we performed the MD simulations in the temperature range from 300 up to 5000K. At 300K, the two bond lengths in N8-chain evolve slightly in an alternative way. As the temperature increases, the bond lengths evolve more significantly but still keep continuous up to 1300K. The evolution of bond lengths is given in Figure 4. In addition, no chemical bonding between N chains and the BNNTs wall and dissociation of N chains have been observed. The results show that the N8@BNNT (5, 5) is stable up to 1300K.

Figure 5. The transformation pathway of N8@BNNT (5, 5) to N2@BNNT (5, 5) at high temperature, (a) the initial state N8@BNNT (5, 5) at room temperature; (b) the intermediate state N6+N2@BNNT (5, 5) and (c) the final state 4N2@BNNT (5, 5) at above 1400K. 9 / 18



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With the temperature increasing, interestingly, according to our MD simulations, a phase transformation from N8@BNNT (5, 5) to 4N2@BNNT (5, 5) takes place at above 1400K, as shown in Figure 5. By the reaction pathway simulations, we found that the N chain firstly dissociated into a N2 and a N6 inside the BNNT and then transformed into four N2 molecules. Note that the dissociation temperature of the polymeric nitrogen chains confined in CNT is higher than 5000K which makes the energy release difficult.49 The BNNT starts to disintegrate when the temperature is increased up to 4300K, which agrees with the disintegration temperature of 4100-4600K for armchair BN nanotubes reported previously50. Compared with the disintegration temperature (4100-4300K) of similar unfilled BNNTs, we suggest that the N chain encapsulation does not change the thermal stability of BNNTs. These results provide us a comprehensive understanding of energy storage and release in N8@BNNT (5, 5) as a HEDM. In addition, from the above results, it is clear that the band structure overlap and the hybridization of DOS between N chain and BNNT (5, 5) are smaller than that in N8@CNT28. And the decomposition temperature of the nitrogen chain in the N8@BNNT (5, 5) system is lower than that of the N chain in the N8@CNT (5, 5)49. To understand these observed differences, we compared N8@BNNT (5, 5) with N8@CNT and other hybrid materials. The results show that the charge transfer in N8@BNNT (5, 5) is smaller than that in N8@CNT (0.05e/N atom). The smaller charge transfer between N chain and BNT indicates that the electron interaction between N chain and tube wall should be weaker, which results in the smaller overlap in band structure and hybridization in DOS. On the contrary, more charge transfer leads to a stronger interaction between polymeric nitrogen chains and the hosting materials which increases the dissociation temperatures and makes it difficult to release energy. That’s the reason why the N8@BNNT (5, 5) can release energy at a mild temperature of about 1400K, while N8@CNT dissociates at temperature higher than 5000K49. Furthermore, we compared the amount of charge transfer in N8@BNNT (5, 5) with previously reported systems, such as the N8@graphene29, and SiCNT31. The results show that N8@BNNT has the suitable amount of charge transfer 10 / 18


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and thus mild energy release conditions compared with previous systems. Therefore, N8@BNNT (5, 5) is a very promising candidate for HEDMs. To analyze the formation of N8@BNNT (5, 5), we calculated the energy difference between N8@BNNT (5, 5) and N2 molecules filled BNNTs (4N2@BNNT (5, 5)). The results suggest an energy difference of 0.49eV/N2 between these two phases. Compared with the corresponding value of 2.2eV/N2 in N8@graphene system29 and 0.9eV/N2 in N8@CNT system

28,49

, the N8 should be easier to be formed in BNNT

than those in graphene and CNT. Experimentally, a possible way to overcome the energy barrier can be the application of external pressure and temperature on the hybrid N2-BNNT system. Therefore, N8@BNNT (5, 5) shows remarkable advantages in the formation and energy release than the N8@CNT and N8@graphene, and should be an excellent candidate for HEDM. Conclusions In summary, we have performed ab initio DFT calculations on the electronic and structural properties of polymeric nitrogen chains encapsulated in BNNTs with different diameters. The results demonstrate that the polymeric nitrogen chain can be stable in BNNT (5, 5) at ambient pressure, which is favored by a charge transfer from BNNT to N chain. For different diameters of BNNTs, slight pressure is required to stabilize the N8 chains. Finite-temperature molecular dynamics simulations show that the polymeric N chains encapsulated in the BNNT dissociate into N2 molecules and release energy at above 1400 K, while the BNNT show high stability even up to 4300 K. More importantly, N8@BNNT has a smaller charge transfer between the N chains and the hosting BNNT compared with other hosting materials. Such a smaller amount of charge transfer not only stabilizes the polymeric nitrogen chain at ambient conditions but also favors the energy release at a more gentle condition, which can be taken as a remarkable advantage of BNNT when used as hosting material. Our findings provide a promising way for truly harvesting nitrogen-based HEDMs under ambient or near ambient conditions.

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Acknowledgments

The authors sincerely thank Dr. Ketao Yin, Hanyu Liu and Pengyue Gao for helpful discussions. This work was supported financially by the National Natural Science Foundation of China (51320105007), the Cheung Kong Scholars Programme of China, and the Program of Changjiang Scholars and Innovative Research Team in University (IRT1132). We acknowledge the use of computing facilities at the High Performance Computing Center of Jilin University.

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High Energetic Polymeric Nitrogen Stabilized in the Confinement of

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