ARTICLE pubs.acs.org/JPCC
Energetic Molecules Encapsulated Inside Carbon Nanotubes and between Graphene Layers: DFT Calculations Manuel Smeu,*,† Ferdows Zahid,†,‡ Wei Ji,§,† Hong Guo,† Mounir Jaidann,|| and Hakima Abou-Rachid|| †
Centre for the Physics of Materials and Department of Physics, McGill University, Montreal, Quebec, Canada, Department of Physics, The University of Hong Kong, Hong Kong S.A.R., People's Republic of China, § Department of Physics, Renmin University of China, Beijing, People's Republic of China, and Defence R&D Canada, Valcartier, Quebec, Canada
)
‡
bS Supporting Information ABSTRACT: Insensitive energetic materials are desirable for propellants because of the reduced risks involved with their use. The ability to control the decomposition pathways for such materials is also of interest since it leads to optimal performance and controlled energy release. With these goals in mind, molecular structure and total energy calculations are used to investigate the confinement of energetic molecules inside carbon nanostructures. The molecules considered were FOX-7 (1,1-diamino-2,2-dinitroethylene), RDX (hexahydro-1,3,5-trinitro-striazine), HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine), DHT (3,6-di(hydrazino)-1,2,4,5-tetrazine), DiAT (3,6-diazido-1,2,4,5-tetrazine), DAAT (3,30 -azo-bis(6-amino-1,2,4,5-tetrazine)), and five different N-oxides of DAAT (DAATOn, with n = 15). Each of the eleven molecules is encapsulated inside a carbon nanotube (CNT) in order to determine if it is stabilized from such confinement. The calculations predict that each molecule could be stabilized by 3253 kcal/mol if a CNT of appropriate size is used. FOX-7, RDX, and HMX were also confined between graphene layers, resulting in these molecules being stabilized by 2840 kcal/mol. The stabilization stems from dispersion interactions between the molecules and carbon nanostructures, Coulombic interactions due to charge transfer, and intermolecular H-bonding in some cases. Overall, each molecule can be stabilized when encapsulated in a carbon nanostructure of appropriate size, thereby reducing its sensitivity.
’ INTRODUCTION The use of insensitive energetic materials for propellants is of interest because of the reduced shock wave sensitivity, resulting in lowered risks associated with their manufacture, transport and storage.13 However, for conventional high energy formulations, the insensitivity is accompanied by significant degradation in performance. In this regard, the relatively new insensitive highperformance material, molecular crystal FOX-7 (1,1-diamino2,2-dinitroethylene), has attracted considerable attention.4,5 FOX-7 has low sensitivity but a performance that is only a few percent lower than the energetic materials commonly used today. Other important insensitive materials include cyclic nitramines such as HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) and RDX (hexahydro-1,3,5-trinitro-striazine). The thermal decomposition processes of these compounds have been subjected to intensive studies in literature both experimentally and theoretically.615 For instance, several parallel pathways in the thermal decomposition of RDX and HMX in solid and liquid phases have been identified.811,1416 Placing molecules inside carbon nanostructures of reduced dimensionality such as one-dimensional carbon nanotubes (CNT) or between (two-dimensional) graphene layers is believed to offer unique opportunities to achieve unprecedented r 2011 American Chemical Society
properties. In fact, studies have shown that confinement of certain molecules to CNTs can improve stability, result in unique intermolecular arrangements, and lead to lowering of activation barriers for certain reactions.17 If the insensitive energetic materials mentioned above are confined in CNTs or between graphene layers, then the thermal decomposition pathways are expected to be markedly affected. We believe this confinement offers a unique opportunity to select desirable decomposition pathways that give optimal material performance while also providing insensitivity. It may also lead to better controlled energy release from such materials, which is a critical property for energetic materials. This novel idea has been proposed for stabilizing highly labile polymeric nitrogen inside CNTs,18 and between layers of graphene.19 In this work, the investigation is extended to a broader set of energetic molecules. Because of the experimental challenges that are present in fabricating such systems, the time and money costs associated with such efforts, and the large number of possible systems to be studied, a theoretical investigation is most suitable at this early stage. Such an investigation Received: February 22, 2011 Revised: April 19, 2011 Published: May 17, 2011 10985
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Figure 1. Energetic molecules.
will help in directing future experiments toward the most promising systems. The molecules studied in this work are shown in Figure 1. These include the insensitive high-performance materials FOX7, RDX, and HMX, which are related by having the same stoichiometric form: CnH2nN2nO2n, with n = 2,3,4 for FOX-7, RDX and HMX, respectively. A set of eight other molecules were also considered, including DHT (3,6-di(hydrazino)-1,2,4,5-tetrazine), DiAT (3,6-diazido-1,2,4,5-tetrazine), DAAT (3,30 -azobis(6-amino-1,2,4,5-tetrazine)) and various N-oxides of DAAT, as shown in Figure 1. The stabilizing effect of confining each of these molecules in a one-dimensional nanostructure is investigated by modeling them encapsulated inside CNTs. Additionally, FOX-7, RDX, and HMX are also modeled between graphene layers to investigate two-dimensional confinement. To this end, molecular structure and total energy calculations were carried out with the Gaussian20 and VASP21 electronic structure packages.
’ THEORETICAL METHODS The geometry of each molecule from Figure 1 was optimized with the Gaussian20 program using the B3LYP/6-31G(d) method.22 In most cases, several starting conformations were considered and the minimum energy structure was used thereafter. Only one conformation was considered for the DAATOn set, in which the structure of the related molecule with one fewer O atom was used as a starting point. For example, the optimized DAAT structure was used as the starting point for DAATO1, which was relaxed and then used as the starting point for DAATO2; and so on. The other exception was for the HMX molecule, which is large and has a vast conformational space, so a structure from the literature23 was used as the starting point in the relaxation. The minimum energy HMX structure was then compared and found to agree (bond lengths within 0.02 Å and angles within 1°) with other calculations.24 After the molecules are relaxed using the Gaussian program, they are confined inside carbon nanostructures (CNTs) and graphene, and further relaxation is carried out by the Vienna ab initio simulation package (VASP)21 since for periodic structures
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the planewave method is most naturally applicable. To verify that VASP provides reliable geometries for the molecules studied, each isolated molecule was also independently relaxed with VASP. The VASP relaxed molecular structures were nearly identical to those relaxed by Gaussian,25 therefore we concluded that VASP is adequate for structure relaxation of these energetic molecules, and we used it for all other calculations. The VASP calculations were carried out using the PerdewBurke-Ernzerhof generalized gradient approximation (PBEGGA) for the exchange correlation energy26 with a semiempirical van der Waals (vdW) correction to account for dispersion interactions.27,28 A projector augmented wave method was used for the ionic potentials,29 with a kinetic energy cutoff for the plane wave basis of 400 eV for structure relaxations and 800 eV for total energy calculations. During the structure relaxations, all atoms were fully relaxed until the net force on every atom was less than 0.005 eV/Å. For the CNT systems, the unit cell length along the CNT was optimized; while for the graphene systems, all three lattice constants were optimized. All the VASP calculations were carried out using a supercell approach. For the isolated molecules, at least 15 Å of vacuum space separated each molecule from its image in a neighboring unit cell. For the CNT systems, there was a minimum of 8 Å of vacuum space between the CNT wall and its image. For all periodic systems, the Brillouin zone was sampled with sufficient k points so that the energy was converged to less than 1 meV/ atom. From the total energy calculations, the stabilization energy was obtained as follows: Estab ¼ Emolþns ðEmol þ Ens Þ
ð1Þ
where Emolþns is the energy of the system with the molecule inside the carbon nanostructure, Emol is the energy of the isolated molecule, and Ens is the energy of the empty carbon nanostructure. In other words, the energy difference between the molecule being inside and outside the nanostructure is of our interest in this work.
’ RESULTS AND DISCUSSION The energetic molecules are confined in carbon nanotubes and several are also confined between graphene layers. The results are presented in this section. Confinement Inside CNTs. Selecting the appropriate CNT to encapsulate a given molecule is not a trivial task because of the large number of possible CNTs and molecular combinations. For this work, armchair (n,n)-CNTs of different sizes have been considered. This choice was motivated by a previous work on polymeric nitrogen inside CNTs,18 where it was found that armchair CNTs are better suited for stabilizing such systems. Nevertheless, it may also be interesting to compare encapsulation of the energetic molecules into other confinement templates such as the zigzag CNT and CNTs with different chiralities. The main idea is that placing molecules inside the CNT may stabilize them due to the interactions with the CNT wall. Therefore, the CNT needs to be large enough so that the molecule would fit inside but small enough for there to be some interaction. Additionally, the molecule and CNT must not be substantially deformed since this would likely have an overall destabilizing effect on the system. Because the molecules are of different shapes and sizes, the appropriate CNT had to be determined in each case. 10986
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Table 1. Optimal CNT Size and Estab charge transferred ideal CNT size
Estab (kcal/mol)
to molecule (e)
FOX-7a
(7,7)
34.0
0.06
RDX
(7,7)
31.8
0.08
HMX
(8,8)
35.5
0.03
DHTa
(6,6)
34.8
0.01
DiAT
(6,6)
33.8
0.23
DAAT
(6,6)
52.6
0.19
DAATO1 DAATO2
(6,6) (7,7)
48.1 44.6
0.29 0.21
DAATO3
(7,7)
45.8
0.13
DAATO4
(7,7)
42.4
0.18
DAATO5
(8,8)
39.8
0.18
molecule
a
Intermolecular H-bonding present.
Figure 3. DAAT(On) set in their respective CNTs.
Figure 2. DHT (top) and FOX-7 (bottom) inside CNTs.
The stabilization energy Estab of eq 1 was calculated for each molecule inside CNTs of different sizes. In each case, several molecular conformations inside the CNT were considered including different molecular orientations relative to the CNT and different intermolecular separations. Out of the set, the lowest energy structure for each moleculeCNT pair was used afterward. For every molecule, there were several CNT sizes that could stabilize the molecule (i.e., give negative Estab). Table 1 lists the ideal armchair CNT size for stabilizing each molecule, along with the calculated Estab, which range from 32 to 53 kcal/mol. Since they are all negative, it means that each molecule would be more stable inside a CNT than in free space. Out of the stoichiometrically related set (FOX-7, RDX, HMX), all are stabilized by about 32 to 36 kcal/mol when placed inside CNTs. DHT and DiAT also have comparable Estab values. Interestingly, the systems containing FOX-7 and DHT have intermolecular H-bonding in a head-to-tail arrangement, resulting in one-dimensional molecular chains inside the CNTs, as shown in Figure 2. Note that the minimum energy structure for FOX-7 is a repeating wave-like pattern with two molecules per unit cell, analogous to the two-dimensional wave-like pattern observed in crystalline FOX-7.5,30 In order to investigate the amount of stabilization gained from such interactions, for the DHT system, the Estab value was computed with H-bonding and also in a system where the molecules are farther apart. There was a difference of 2 kcal/mol between the two geometries, which can
be attributed to the H-bonding interaction. This is lower than expected for a typical H-bond (ca. 3 kcal/mol for NH 3 3 3 N), likely due to the fact that the molecules are confined and the appropriate groups cannot orient themselves for optimal interaction. The molecules in the DAAT(On) set are the most stabilized by CNTs (shown in Figure 3), with DAAT having the most negative Estab value, at 52.6 kcal/mol. This is likely due to their overall long and narrow shape, which makes for a good geometrical fit maximizing the interaction between the molecule and the CNT. As oxygen atoms are added (going down Table 1) the amount of stabilization tends to decrease.31 At first glance, this may seem to be a reasonable trend since O atoms are sterically bulky and will make for a tighter fit inside the CNT. However, the nanotube size was chosen on a per molecule basis, so the CNTs listed are still the optimal ones for each molecule. For example, the ideal CNT size to encapsulate DAATO1 is (6,6), while for DAATO2 it is (7,7). Therefore, the reason for the trend is not based on steric repulsion, but appears to be related to the number of O atoms, and their effect on the dispersion interactions. This is an intriguing phenomenon which may be investigated in future studies. In previous work on polymeric nitrogen encapsulated inside a CNT,18 the stabilizing interaction was attributed to charge transfer between the lone pairs on N atoms and the inside of the CNT wall. For the present work, Bader charge analysis32 was carried out to determine the amount of charge transferred between the CNT and the molecules. The results, listed in Table 1, show that for each molecule negative charge is transferred from the CNT to the molecule, in agreement with the polymeric nitrogen case. Interestingly, there is no direct relationship between the amount of charge transferred and Estab, suggesting that the stabilization for these systems is dominated by dispersion interactions and the charge transfer plays a smaller 10987
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Table 2. Optimal Graphene Layerlayer Separation and Estab molecule
a
graphene layerlayer
Estab
charge transferred
separation (Å)
(kcal/mol)
to molecule (e)
FOX-7a
6.5
39.8
0.04
RDX
7.6
28.2
0.08
HMX
8.1
33.2
0.10
Intermolecular H-bonding.
role. For example, the DHT molecule has a comparable Estab value to the other molecules, while receiving essentially no charge from the CNT. Confinement between Graphene Layers. FOX-7, RDX, and HMX were also modeled as confined in two-dimensions, i.e., sandwiched between graphene layers. Those are really threedimensional periodic structures, consisting of alternating layers of molecules and graphene in the third dimension. For simplicity, the atoms in the graphene layers are sitting directly on top of each other, which is a reasonable approximation since they are separated by a layer of molecules and are thus relatively far from each other. The graphene interlayer distance was varied to find the minimum energy separation, as summarized in Table 2. The relaxed systems are shown in Figure 4. Note that the empty nanostructure energy, Ens for eq 1 is obtained for empty graphene where the interlayer separation is kept at the value as if the molecule were there (i.e., the values listed in Table 2). Of course, this is not a minimum energy structure for real graphene, but this configuration gives a better representation of the stabilizing effect of graphene on the energetic molecules. For FOX-7, there are four molecules in each unit cell, to allow for intermolecular H-bonding extending in the plane parallel to the graphene (see Figure 4a). Note that, although the relaxation was started as a wave-like geometry resembling the crystal structure for FOX-7, the lowest energy structure is mostly planner with the exception of some nitro groups. Just as for confinement in CNTs, all molecules studied were stabilized when confined between graphene layers. Some charge transfer is predicted in each system, similar to the case with polymeric nitrogen between graphene layers.19 Out of the set, FOX-7 is stabilized by the highest amount, with Estab = 39.8 kcal/mol. Note that when confined inside CNTs, HMX was the most stabilized from this set of molecules. It is interesting that FOX-7 is more stabilized in graphene than in a CNT, while the reverse is true for RDX and HMX. This can be attributed to two factors. First, the FOX-7 molecule has a flat shape, which is ideal for maximizing interactions with a two-dimensional structure such as graphene; whereas RDX and HMX are nonplanar structures. Second, in a CNT, FOX-7 forms intermolecular H-bonds along a one-dimensional chain, but in graphene, it forms H-bonds with more partners in a two-dimensional network; while RDX and HMX do not form H-bonds inside either nanostructure. For the confinement of energetic molecules, it might also be possible to use multiwalled CNTs or multiple layers of graphene separating the molecular layers. For the multiwalled CNTs, we expect the molecules to be stabilized by a comparable amount by the inner CNT, but an added benefit may be that the properties of the outer CNTs would remain unaffected by the presence of the encapsulated molecules. This would be an advantage if such CNTs are to be packaged into bundles. In the case of multiple graphene layers between the layers of molecules, we expect even more negative stabilization energies since more charge can transfer to each molecule from the graphene. Therefore, the
Figure 4. Top and side views of (a) FOX-7, (b) RDX, and (c) HMX confined between graphene layers. The dashed region in panel (a) outlines a repeating unit cell containing four FOX-7 molecules.
use of multiwalled CNTs and multilayered graphene would be very interesting for future consideration. It should be pointed out that the quantity Estab is calculated relative to each molecule in free space. However, the energetic materials involving these molecules are generally present in a condensed state, which also stabilizes them. For example, the enthalpy of sublimation for FOX-7 is reported to be 26 kcal/ mol,33 for RDX it is 2731 kcal/mol,34 for HMX it is 4466 kcal/mol,16 while for DHT, DiAT, and DAAT, it is reported as 25, 26, and 40 kcal/mol, respectively.35 However, even though some of these molecules might be as stable or more stable in a pure condensed phase than inside a nanostructure,36 the latter may still be advantageous if they imply moderated decomposition pathways which ultimately lead to controlled energy release from these materials. Confinement to a nanostructure may also provide the opportunity to select desirable decomposition pathways to yield optimal material performance while also providing insensitivity.
’ CONCLUSIONS The confinement of energetic molecules inside CNTs and between graphene layers has been investigated with total energy 10988
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The Journal of Physical Chemistry C calculations. A set of eleven molecules was encapsulated in CNTs. The stabilization energies were calculated for each molecule inside several different CNTs to determine the optimal CNT size. For these ideal CNTs, the calculated stabilization energies were all negative, indicating that the each molecule has the potential to be stabilized when confined to a CNT of an appropriate size. H-bonding is also possible in some cases, where the molecules formed one-dimensional chains inside the CNT, thereby providing another stabilizing interaction for those systems. FOX-7, RDX, and HMX were also placed between layers of graphene. FOX-7, which is a flat molecule, was more stabilized in graphene than in CNTs, while RDX and HMX were more stable in CNTs due to their nonplanar shapes. Also, each FOX-7 molecule can participate in more H-bonds with neighboring molecules in graphene than in CNTs. The calculations predict charge transfer to occur between the carbon nanostructure and the confined molecule. This charge transfer contributes in part to the molecule’s stabilization. However, other important factors include the chemical groups and overall shape of the molecule, as well as possible H-bonding. In conclusion, each of the energetic molecules studied in this work has the potential to be stabilized if confined inside an appropriate carbon nanostructure.
’ ASSOCIATED CONTENT
bS
Supporting Information. Complete ref 20. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (M.S.); hakima.abou-rachid@ drdc-rddc.gc.ca(H.A-.R.).
’ ACKNOWLEDGMENT We gratefully acknowledge financial support from the Technology Innovation Fund from the Government of Canada and the Natural Sciences and Engineering Research Council of Canada (NSERC). The calculations were performed at the computation facilities of the Reseau Quebecois de Calcul de Haute Performance (RQCHP), and Consortium Laval, Universite du Quebec, McGill and Eastern Quebec (CLUMEQ). ’ REFERENCES (1) Liu, H.; Zhang, Y.; Li, R.; Abou-Rachid, H.; Lussier, L.-S.; Sun, X. Int. J. Energetic Mater. Chem. Prop. 2010, 9, 55–69. (2) Liu, H.; Zhang, Y.; Li, R.; Sun, X.; Desilets, S.; Abou-Rachid, H.; Jaidann, M.; Lussier, L.-S. Carbon 2010, 48, 1498–1507. (3) Zhong, Y.; Jaidann, M.; Zhang, Y.; Zhang, G.; Liu, H.; Ionescu, M. I.; Li, R.; Sun, X.; Abou-Rachid, H.; Lussier, L.-S. J. Phys. Chem. Solids 2010, 71, 134–139. (4) Lochert, I. J. FOX-7—A New Insensitive Explosive; DSTO-TR1238; DSTO Aeronautical and Maritime Research Laboratory: Fishermans Bend, Victoria, Australia, 2001. (5) Bellamy, A. J. Struct. Bonding (Berlin) 2007, 125, 1–33. (6) Hu, A.; Larade, B.; Abou-Rachid, H.; Lussier, L.-S.; Guo, H. Propellants, Explos., Pyrotech. 2006, 31, 355–360. (7) Burnham, A.; Weese, R.; Wang, R.; Kwok, Q.; Jones, D. SolidSolid Phase Transition Kinetics of FOX-7; 2005 NATAS Annual Conference, Universal City, CA, September 1821, 2005; UCRL-CONF213671.
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