Periodic DFT study of adsorption of nitroamine molecule on α-Al2O3(0 0 1) surface

Applied Surface Science 258 (2012) 7334–7342

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Periodic DFT study of adsorption of nitroamine molecule on ␣-Al2 O3 (0 0 1) surface Su-Qin Zhou a,b , Xue-Hai Ju a,∗ , Feng-Qi Zhao c , Si-Yu Xu c a

Key Laboratory of Soft Chemistry and Functional Materials of MOE, School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, PR China Key Laboratory for Attapulgite Science and Applied Technology of Jiangsu Province, College of Life Science and Chemical Engineering, Huaiyin Institute of Technology, Huaian 223003, PR China c Science and Technology on Combustion and Explosion Laboratory, Xi’an Modern Chemistry Research Institute, Xi’an 710065, PR China b

a r t i c l e

i n f o

Article history: Received 13 January 2012 Received in revised form 31 March 2012 Accepted 31 March 2012 Available online 20 April 2012 Keywords: NH2 NO2 Al2 O3 (0 0 1) Adsorption Density functional theory

a b s t r a c t The adsorption of NH2 NO2 molecule on the Al2 O3 (0 0 1) surface were investigated by the generalized gradient approximation (GGA) of density functional theory (DFT). The calculations employ a supercell model represented with 2 × 2 of periodic boundary conditions. The strong attractive forces between NH2 NO2 molecule and Al2 O3 induce obvious change of the NH2 NO2 and Al2 O3 structure. Although the NH2 NO2 molecule does not decompose, Al O bonds partially decompose and some O O bonds form, whose bond lengths are intervenient between O O double bond length and O O single bond length. The largest adsorption energy is −453.8 kcal mol−1 . By the adsorption energy and the change with structure of NH2 NO2 and Al2 O3 , it can be concluded that aluminized explosive of NH2 NO2 keeps high reactivity even if the aluminum is oxidized to form a film of the alumina. This finding can make clear the activated aluminum as a stored energy source for propellants and the good performance of aluminized explosives. The energies of DOS for N and O atoms of the NH2 NO2 molecule match with those of Al atoms, and Al O or Al N bond forms easily at the corresponding energies range. The DOS projections on the N, O and Al atoms occur with obvious shift of peaks, which infers energy bands become broad and the interactions of chemical bonds are strengthened. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Metallic aluminum is a common energetic fuel in many traditional and nano-energetic material applications, including pyrotechnics, propellants, and thermobaric explosives, and as a result, the preparation of aluminum nanoparticles has become a mature technology [1–6]. Aluminum nanoparticles are commercially available in a variety of sizes ranging from 10 to 100 nm in diameter. Aluminum is a very electropositive metal, and the reason that it can be handled in air is that a thin and tightly adherent coating of alumina (Al2 O3 ) forms on the surface that passivates the metal. The thickness of the Al2 O3 layer is relatively constant for different particle sizes, which means that in the case of aluminum nanoparticles, the passivating Al2 O3 layer represents a significant fraction of the particles, which in turn, greatly decreases their reactivity [7–11]. Since the presence of the unreactive Al2 O3 passivation layer is unavoidable in air, this solicitation seeks proposals that aim to investigate the interaction between the energetic material molecules and Al2 O3 layer to make the aluminum nanoparticles

∗ Corresponding author. Tel.: +86 25 84315947 801; fax: +86 25 84431622. E-mail address: [email protected] (X.-H. Ju). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.03.187

more reactive. Despite the experimental interest in understanding the properties of nitro-containing compounds of energetic materials on alumina surface, to the best of our knowledge, no theoretical studies of the NH2 NO2 molecule on alumina surface are available to date. The experiment indicated that the decomposition of nitromethane on –alumina can take place above 373 K [12]. However, based on density functional theory calculations, the results investigated by Sorescu et al. show that nitromethane and FOX7 molecules on ␣-alumina surface do not decompose [13]. In this paper, density functional theory calculations were used to study the adsorption properties of NH2 NO2 on the ␣-Al2 O3 surface. The NH2 NO2 molecule contains only a nitro group and an amino group, which is a straightforward and representative high energetic material. To clarify some of the fundamental issues related to the interaction of energetic materials, especially nitro compounds, with the Al2 O3 surface, our work focuses on the atomic-level description of the interaction between the energetic compound of nitroamine and the ␣-Al2 O3 surface. The adsorption of NH2 NO2 molecule on Al2 O3 (0 0 1) surface can be extended to investigate the systems of other high energetic materials on the Al2 O3 surface. This study represents an extension of our recent density functional theory results of the adsorption and decomposition of the nitroamine molecule on Al(1 1 1) [14]. In that case, we

S.-Q. Zhou et al. / Applied Surface Science 258 (2012) 7334–7342

determined that oxidation of the aluminum surface readily occurs by partial or complete dissociation of the oxygen atoms from the NO2 groups in NH2 NO2 . In the case of dissociative chemisorption, abstraction of one or both O-atoms of a nitro group by Al surface atoms was seen to be the dominant mechanism. The dissociated oxygen atoms form strong Al O bonds with the neighboring Al sites around the dissociation sites. Additionally, the radical species obtained as a result of oxygen atom elimination remain bonded to the surface. In the current study, we analyze similar issues related to the adsorption of NH2 NO2 on the alumina surface. This paper is organized as follows: in Section 2, we describe the computational methods used to the adsorption calculations. The results and discussion are presented in Section 3, followed by a summary of main conclusions in Section 4.

2. Computational method The calculations described in this paper have been performed using the CASTEP package [15] with Vanderbilt-type ultrasoft pseudopotentials [16] and a plane-wave expansion of the wave functions. Exchange and correlation were treated with the generalized gradient approximation, using the functional form of Perdew, Burke, and Enzerh of PBE [17]. The electronic wave functions were obtained by a density-mixing scheme [18] and the structures were relaxed using the Broyden, Fletcher, Goldfarb, and Shannon (BFGS) method [19]. The cutoff energy of plane waves was set to 300.0 eV. Brillouin zone sampling was performed using the Monkhost–Pack scheme. The values of the kinetic energy cutoff and the k-point grid were determined to ensure the convergence of total energies. Several tests have been performed to verify the accuracy of the method when applied to bulk Al2 O3 , the Al2 O3 (0 0 1) surface and to the isolated NH2 NO2 molecules, such as the optimum cutoff energy for calculations. For bulk alumina, the 10-atom rhombohedral primitive cell of ␣-Al2 O3 has been optimized using a cutoff energy of 300 eV. We have tested for convergence, using the kpoint sampling density and the kinetic energy cutoff. When a cutoff energy is 300 eV, a Monkhorst–Pack scheme with mesh parameters of 5 × 5 × 5 has been used, leading to 19 k–points in the irreducible Brillouin zone. Based on this calculation, the optimized rhombo˚ ˛calc = 55.29◦ and the hedral unit parameters are acalc = 5.128 A, volume of the unit cell (Vcalc ) is 84.93 A˚ 3 . When a cutoff energy is 380 eV, a Monkhorst–Pack scheme with mesh parameters of 6 × 6 × 6 has been used, leading to 28 k-points in the irreducible Brillouin zone. On the calculation of 380 eV, the obtained rhombohedral unit parameters (i.e. acalc , ˛calc and Vcalc ) are same with those of 300 eV. It can be concluded that, at Ecut = 300 eV, the bulk structure is well converged, with respect to the cutoff energy. The calculated lattice constant (acalc ) of 5.128 A˚ is very close to ˚ and ˛calc = 55.29◦ is almost the experimental value (aexp = 5.136 A), identical to the experiment value (˛exp = 55.30◦ ). The relative deviation of Vcalc (84.93 A˚ 3 ) to Vexp (85.11 A˚ 3 ) is 0.21%. [20] These results indicate that the present set of pseudo-potentials is able to provide a very good representation of the structural properties of bulk aluminum. The surface model selected in this study corresponds to the (0 0 1) basal plane of ␣-Al2 O3 . Based on the optimized unit cell values of ␣-Al2 O3 , a supercell model was represented with 2 × 2 of periodic boundary conditions, containing three layers of O atoms (36 ions) and three layers of Al atoms (20 ions). The surface slab is separated by a vacuum layer of 12 A˚ from the top slab to the surface along c axes direction. The cell size with rhombic box ˚ In addition, we tested the of a × b × c is 9.52 A˚ × 9.52 A˚ × 17.17 A. adsorption energy of 4-layer slab at the Al-hollow site and found that the adsorption energies of 3-layer and 4-layer slab are −420.3 and −394.1 kcal mol−1 , respectively. The relative deviation of

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adsorption energies for the 4-layer slab to that of the 3-layer slab is 6.2%. A vacuum layer of 18 A˚ from the top slab to the surface along c axes direction was tested. The deviation of single-point energy for a vacuum layer of 18 A˚ to 12 A˚ is 5.4 kcal mol−1 . However, the computation of 4-layer slab with a vacuum layer of 18 A˚ is much more time-consuming than that of 3-layer slab with a vacuum ˚ Hence, in balance of the computational precision layer of 12 A. and source, 3-layer slab model with a vacuum layer of 12 A˚ were selected to study the adsorption of the NH2 NO2 molecule on the Al2 O3 (0 0 1) surface. Initial surface optimizations have been performed by relaxing all ions in the systems. The surface energy (Esurf ) is calculated by equation, Esurf =

Eslab − nEbulk 2A

(1)

here Eslab is total energy of the selected slab supercell, Ebulk is the primitive cell energy of the bulk, n is the multiple of the atoms for the selected supercell in comparison with those of the primitive cell, A is the area of the slab. Our calculated Al2 O3 (0 0 1) surface energy is 2.31 J m−2 , which is close to the experiment value of 2.6 J m−2 [21] and also close to the previous calculated value of 2.0 J m−2 [22]. As a result of surface optimization, significant inward relaxations of about 0.4 A˚ take place for the surface Al atoms, while about 0.5 A˚ for the surface O atoms. The separation between the top plane Al atoms and the immediate adjacent O ˚ For the surface top layer, Al O bonds decrease from atoms is 0.2 A. ˚ These values are close to those the bulk value of 1.856–1.675 A. reported by Sorescu et al. [13].An equally good representation has been observed for the geometric parameters of the isolated NH2 NO2 molecules. For example, on the basis of optimizations of the isolated NH2 NO2 molecule in a rhombic box with dimensions ˚ we obtained the following: equilibof 9.52 A˚ × 9.52 A˚ × 17.17 A, ˚ r (N O) = 1.252 A, ˚ and rium bond lengths of r (N N) = 1.400 A, ˚ and bond angles of  (H N H) = 116.8◦ and r (N H) = 1.025 A,  (O N O) = 127.8◦ at Ecut = 300 eV. The increase of the cutoff ˚ energy to 380 eV leads to the following values: r (N N) = 1.398 A, ˚ r (N H) = 1.026 A, ˚  (H N H) = 117.8◦ , and r (N O) = 1.250 A,  (O N O) = 127.4◦ . We noticed that there are no significant differences between the values obtained at the two cutoff energies, indicating convergence of the results even at Ecut = 300 eV. These values are also very similar to those calculated at the ˚ N O: 1.215 A, ˚ CCSD/cc–pVDZ theoretical level (N N: 1.405 A, ˚ H N H: 112.4◦ and O N O: 127.8◦ ). In addition, N H: 1.021 A, the calculated values are also similar to the experimental data ˚ N O: 1.206 A, ˚ N H: for gaseous NH2 NO2 (N N: 1.427 ± 0.002 A, ˚ H N H: 115◦ 11 ± 2◦ and O N O: 130◦ 8 ± 15 ). 1.005 ± 0.01 A, [23] Our largest deviation of 0.046 A˚ is observed for N O bonds. The good agreement between our calculated properties of alumina bulk, the Al2 O3 (0 0 1) surface and the isolated NH2 NO2 molecule with the experiment and CCSD/cc–pVDZ theoretical predictions made us confident to proceed to the next step: the investigation of molecular adsorption on the Al2 O3 (0 0 1) surface. This also suggests that the performed computational method is proper to the adsorption system of NH2 NO2 molecule on the Al2 O3 (0 0 1) surface. In calculations of molecular adsorption on the surface, we have relaxed all atomic positions of the molecule, as well as the Al and O atoms of the slab. For the case of adsorption configurations, the corresponding adsorption energy (Eads ) was calculated according to the expression. Eads = E(adsorbate+slab) − E(molecule+slab)

(2)

where E(adsorbate+slab) is the total energy of the adsorbate/slab system after the NH2 NO2 molecule being absorbed by Al2 O3 slab and E(molecule+slab) is the single-point energy of the NH2 NO2 /slab system as a whole but without interactions between NH2 NO2 molecule and

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S.-Q. Zhou et al. / Applied Surface Science 258 (2012) 7334–7342

Fig. 1. (a) Top view of the surface, surface sites were depicted in panel; (b) NH2 NO2 molecule on the Al2 O3 (0 0 1) surface with no interaction.

the Al2 O3 slab (NH2 NO2 is as far as 5.3 A˚ away from the top-Al atom of Al2 O3 surface). When the distance of the NH2 NO2 molecule to ˚ the interaction decreases with the Al2 O3 surface is less than 5 A, the distance increasing. However, their distance increases from 5 A˚ ˚ the energy with the system of the NH2 NO2 molecule and to 8 A, Al2 O3 surface decreases less than 1.2 kcal mol−1 . Therefore, when ˚ non-bonding the distance of the molecule from the surface is 5 A, action of the NH2 NO2 /slab system can be neglected and the singlepoint energy at this distance may be used as the reference energy without any adsorption interactions. The E(adsorbate+slab) and E(molecule+slab) were calculated with the same periodic boundary conditions and the same Brillouin-zone sampling. A negative Eads value corresponds to a stable adsorbate/slab system. Fig. 1 shows the pictorial view of the Al2 O3 (0 0 1) surface model, the absorbed surface sites and the configuration of NH2 NO2 molecule on the Al2 O3 (0 0 1) surface with no interactions of adsorbate-Al2 O3 .

3. Results and discussion The adsorption of NH2 NO2 molecule on the Al2 O3 (0 0 1) results in formation of Al O and Al N bonds due to strong interacting of Al atom with O and N atoms of NH2 NO2 molecule. There are three cases as follows:

(1) There is not chemical bond between NH2 NO2 molecule and Al2 O3 , for example, V-D2, V-F2 in Fig. 2 and P-C2 in Fig. 3. (2) An Al O bond forms between the Al ion of Al2 O3 and the O atom of NO2 group. (V-A2, V-B2, V-C2 in Fig. 2, P-A2, P-D2, P-E2, and P-F2 in Fig. 3). (3) An Al N bond forms between the Al ion of Al2 O3 and the N atom of NH2 group. (V-E2 in Fig. 2 and P-B2 in Fig. 3).

According to the orientation of N N bond relative to the Al2 O3 (0 0 1) surface, V and P denote vertical and parallel adsorptions of NH2 NO2 , respectively, as shown in Fig. 2 and Fig. 3. For each configuration in Figs. 2 and 3, two views are given. The first one, denoted with index 1, represents the initial configuration at the start of optimization process. The second ones, denoted with index 2, represent lateral view of the optimized adsorption configuration after full relaxation of the atomic positions.

3.1. Geometries Figs. 2 and 3 presented some geometrical parameters of NH2 NO2 molecule for adsorption configurations. Some geometrical parameters of Al2 O3 for adsorption configurations were listed in Table 1. As can be seen in Fig. 2, the adsorption configurations of NH2 NO2 were presented when the N N bond was initially vertical to the Al2 O3 surface, with the nitro or amino groups pointing down toward the Al2 O3 surface. Configurations V-A2 to V-C2 illustrate that the adsorption of NH2 NO2 occur at an Al-top, an Al-hollow and an O-top sites with the nitro groups pointing down toward the Al2 O3 surface, respectively. At these sites, the adsorptions lead to formation of an Al O bond and obvious change of the bond length of Al2 O3 . However, NH2 NO2 molecule does not dissociate. As can be seen in Fig. 2, for V-A2 to ˚ V-C2, the N H bond lengths stretch from 1.016 A˚ to 1.023–1.025 A; ˚ the N N bond lengths shrink from 1.390 A˚ to 1.345–1.348 A; ˚ whereas one N O bond stretches from 1.224 A˚ to 1.232–1.235 A, another one stretches to 1.299–1.305 A˚ due to formation of a new Al O bond. Thus it can be seen, except that the change of lengths of a N O bond is obvious due to the Al O bond formation, the change of other lengths for NH2 NO2 molecule is small. The newly ˚ In addition, built Al O bonds are in the lengths of 1.882–1.914 A. the Al O bonds in Al2 O3 become 1.667–3.000 A˚ (Table 1), compared to the initial values of 1.856 and 1.969 A˚ in Fig. 1b (i.e. the NH2 NO2 molecule is not interacting with the surface Al atoms); ˚ the O O distance becomes to 1.362–2.999 A˚ from 2.522 to 2.867 A, and the Al Al distance becomes to 2.527–2.630 A˚ from 2.650 to ˚ This shows that the change of Al O bonds and the O O 2.790 A. distance is large, especially, the minimum O O distance for V-A2 to V-C2 is 1.362–1.429 A˚ and these values are intermediate between O O double bond value of 1.12 A˚ and O O single bond value of ˚ which implies that O atoms of Al2 O3 (0 0 1) surface become 1.48 A, easily reactive due to the adsorption of NH2 NO2 molecule on the Al2 O3 (0 0 1) surface. V-D2 to V-F2 of Fig. 2 illustrate the adsorption at an Al-top, an Al-hollow and an O-top sites with the amino groups of NH2 NO2 initially oriented toward the Al2 O3 surface, respectively. In these cases, the adsorption at an Al-hollow site, i.e. configuration V-E2, ˚ As results in a Al N bond forming with bond length value of 2.032 A. can be seem from Fig. 2, the N H bond lengths for NH2 NO2 stretch ˚ the N N bond length from from 1.016 A˚ to 1.040 and 1.070 A; ˚ the N O bond lengths from 1.224 A˚ to 1.237 1.390 A˚ to 1.494 A; ˚ For V-D2 and V-F2, the N H bond lengths stretch to and 1.238 A.

S.-Q. Zhou et al. / Applied Surface Science 258 (2012) 7334–7342

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˚ for nitroamine of equilibrium Fig. 2. Adsorption configurations of nitroamine on the Al2 O3 (0 0 1) surface obtained from initial vertical configurations and bond lengths (A) adsorption. Initial configurations are depicted in panels V-A1 to V-F1, whereas the final configurations are shown in panels V-A2 to V-F2.

˚ the N N bond lengths shrink to 1.365 and 1.388 A; ˚ 1.027–1.031 A; ˚ For Al2 O3 of V-D2 the N O bond lengths stretch to 1.253–1.238 A. to V-F2, the minimum O O distances are appreciably longer than those of V-A2 to V-C2 and are intermediate between O O double ˚ while bond value of 1.12 A˚ and O O single bond value of 1.48 A, the Al O bond lengths and the Al Al distances are close to those of V-A2 to V-C2. These results show that Al2 O3 becomes active when

the NH2 NO2 molecule adsorbs on the Al2 O3 surface even if NH2 NO2 molecule does not dissociate. We have also studied the cases in which the NH2 NO2 molecule was initially parallel to the Al2 O3 surface. Six cases have been investigated: the N atoms of nitro and amino groups at an Al-top and an Al-hollow sites. Fig. 3 gives six adsorption configurations, i.e. P-A2 to P-F2. Our study indicated that the NH2 NO2 molecule rotates to

˚ for nitroamine of equilibrium Fig. 3. Adsorption configurations of nitroamine on the Al2 O3 (0 0 1) surface obtained from initial parallel configurations and bond lengths (A) adsorption. Initial configurations are depicted in panels P-A1 to P-F1, whereas the final configurations are shown in panels P-A2 to P-F2.

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Table 1 Some geometrical parameters of Al2 O3 for adsorption equilibrium configurations.a bb

V-A2

V-B2

V-C2

V-D2

V-E2

V-F2

P-A2

P-B2

P-C2

P-D2

P-E2

P-F2

L(Al O)min L(Al O)max

1.856 1.969

1.677 2.989

1.667 3.000

1.682 2.986

1.667 3.000

1.684 2.989

1.678 2.998

1.678 2.983

1.682 2.995

1.680 2.989

1.667 2.987

1.665 2.999

1.668 2.997

L(O O)min L(O O)max

2.522 2.867

1.367 2.983

1.362 2.995

1.429 2.999

1.470 2.986

1.376 2.999

1.392 2.995

1.406 2.990

1.350 2.998

1.323 2.996

1.379 2.991

1.305 2.978

1.376 2.994

L(Al Al)min L(Al–Al)max

2.650 2.790

2.588 2.987

2.527 2.987

2.630 2.988

2.636 2.996

2.615 2.982

2.623 2.989

2.568 2.952

2.546 2.992

2.449 2.994

2.612 2.982

2.583 2.990

2.612 2.993

a b

˚ Units are in A. NH2 NO2 molecule on the Al2 O3 (0 0 1) surface with no interaction.

Table 2 Adsorption energies (Eads ) and adsorption sites of NH2 NO2 on the Al2 O3 (0 0 1) surface. Configurations

Relation of the N N bond with Al2 O3 (0 0 1) surface

Orientation of N O and N H bonds to Al2 O3 (0 0 1) surface

Adsorption sites

Eads (kcal mol−1 )

V-A2 V-B2 V-C2 V-D2 V-E2 V-F2

Vertical

N O bonds orient toward the Al2 O3 surface

Al-top Al-hollow O-top Al-top Al-hollow O-top

−405.0 −420.3 −384.3 −382.3 −415.1 −424.9

P-A2 P-B2 P-C2

Parallel

Al-top Al-top Al-top

−422.6 −428.9 −359.4

Al-hollow Al-hollow Al-hollow

−350.2 −453.8 −403.2

N H bonds orient toward the Al2 O3 surface N H bonds orient toward the vacuum of Al2 O3 surface N H bonds orient toward the Al2 O3 surface N H bonds orient toward the vacuum of Al2 O3 surface N H bonds orient toward the Al2 O3 surface

P-D2 P-E2 P-F2

maximize the interaction with the Al2 O3 surface during the optimization. As a result, an O atom of the NH2 NO2 molecule with an Al atom form an Al O bond, for example P-A2, P-D2, P-E2 and PF2; the N atom of the amino group with an Al atom form an Al N bond, for example P-B2. In addition, for the adsorption with amino group lying down toward the Al2 O3 surface at an Al-top site, i.e. P-C2, no bond forms between the NH2 NO2 molecule and the Al2 O3 surface. From Fig. 3, the forming Al O bonds are in the lengths ˚ The N H bond lengths stretch from 1.016 A˚ to of 1.882–1.942 A. 1.026–1.101 A˚ for P-A2 to P-F2; the N N bond lengths shrink from 1.390 A˚ to 1.335–1.354 A˚ for P-A2, P-D2, P-E2 and P-F2, while the N N bond length of P-B2 stretches to 1.501 A˚ due to forming an ˚ lengths of a Al N bond and the N N bond length of P-C2 is 1.390 A; ˚ whereas lengths N O bond stretch from 1.224 A˚ to 1.232–1.239 A, of the others stretch to 1.293–1.320 A˚ due to formation of Al O bond for P-A2, P-D2, P-E2 and P-F2; N O bond lengths of P-B2 become to 1.231 and 1.239 A˚ and those of P-C2 become to 1.253 ˚ From Table 1 for Al2 O3 of P-A2 to P-F2, the difference and 1.255 A. of the distance for Al Al, Al O and O O is small. Their changes are similar to those of V-A2 to V-F2. However, compared to the some significant change of Al O, O O and Al Al distances on the top Al/O layer, the largest changes of Al O, O O and Al Al distances in the second and the third Al/O ˚ respectively. The geomelayers of the slab are 0.17, 0.1 and 0.01 A, try change of Al2 O3 is small in the second and the third Al/O layers of the slab. As can be seen from Table 1, Figs. 2 and 3, when Al atom and an O atom of nitro group form an Al O bond, the corresponding N O bond length is longer than that of another N O bond, for example

V-A2, V-B2, V-C2, P-A2, P-D2, P-E2 and P-F2, whereas their N N bonds shrink. When Al atom and the amino N atom form an Al N bond, for example V-E2 and P-B2, all the corresponding N N, N H and N O bonds increase, and the non-adsorpted N O bond stretches less than the adsorpted N O bond. But the N H bond stretches less than that of configurations with newly built Al O bond. When the NH2 NO2 molecule and the Al2 O3 do not form any chemical bond, the change of bond length for NH2 NO2 molecule is small. For all adsorption configurations, the change of Al O bond length of Al2 O3 is large. Based on the maximum distance of Al O of Al2 O3 and the minimum O O distance of Al2 O3 , it can concluded that the Al O bonds partially decompose and some O O bonds form, whose bond lengths are intermediate between O O double bond value of 1.12 A˚ and O O single bond value of ˚ This implies O ions of Al2 O3 become easily reactive due to 1.48 A. the adsorption of the NH2 NO2 molecule on the Al2 O3 surface. 3.2. Adsorption energies Table 2 gives the adsorption energies (Eads ) of the NH2 NO2 molecule on the Al2 O3 (0 0 1) surface. As can be seen in Table 2, the adsorption energies of all adsorption configurations are in the range of −350.2 to −453.8 kcal mol−1 , and these values are large. This is mainly induced by the interacting of the NH2 NO2 molecule and Al2 O3 resulting in the distortion of the structures of NH2 NO2 and Al2 O3 . When the N N bond of the NH2 NO2 molecule is placed parallelly to the Al2 O3 (0 0 1) surface, there exist a smallest adsorption energy and a largest one, which are −350.2 kcal mol−1 for P-E2 and −453.8 kcal mol−1 for P-D2, respectively. Their adsorption

Table 3 Coverage degree () of NH2 NO2 on the Al2 O3 (0 0 1) surface.



V-A2

V-B2

V-C2

V-D2

V-E2

V-F2

P-A2

P-B2

P-C2

P-D2

P-E2

P-F2

0.091

0.134

0.143

0.143

0.135

0.115

0.159

0.183

0.163

0.139

0.160

0.091

0.03

c

NH2 NO2 molecule on the Al2 O3 (0 0 1) surface with no interaction. Total charges for all Al atoms of Al2 O3 surface. Total charges for all O atoms of Al2 O3 surface. a

b

−0.05 0.02

−0.02 0.03

−0.05 −0.04

0.04 −0.04

0.06 0.08

−0.09

−0.02

0 −0.02

0.01 0.13

−0.12 0.01

0.12 0.1

−0.1

0.09

−0.11

0.02

NH2 NO2 -total charge Al2 O3 -total charge

O

−0.09

−34.78 −34.74 −34.82 −34.5 −34.64 −34.71 −34.65 −34.65 −34.87 −34.65 −34.62 −33.35

−34.66

34.81 34.72 34.85 34.54 34.6 34.62 34.65 34.63 34.75 34.56 34.56 34.51

1.6–2.03 1.55–1.76

33.36

c

1.62–1.93 1.57–1.92 1.61–1.92 1.6–1.94 1.63–1.93 1.6–1.93 1.62–1.92 1.6–1.91 1.63–1.95 1.61–1.94 1.6–1.94

−0.35 to −1.22 −0.06 to −1.19 −0.33 to −1.22 −0.25 to −1.18 −0.3 to −1.19 −0.33 to −1.19 −0.39 to −1.21 −0.33 to −1.2 −0.47 to −1.2 −0.18 to −1.15 −0.31 to −1.18 −0.65 to −1.08

O atoms of Al2 O3 surface Al atoms Of Al2 O3 surface Alb

−0.32 to −1.19

−0.54 −0.27

−0.55 −0.54

−0.3 −0.36

−0.37 −0.29

−0.23 −0.27

−0.49 −0.37

−0.36 −0.28

−0.29 −0.32

−0.32 −0.23

−0.51

−0.52 −0.39

−0.26 −0.26 −0.4 O1 of NH2 NO2

−0.5

−0.3

7339

O2 of NH2 NO2

0.51 0.41 0.38 0.44 0.49 0.47 0.43 0.47 0.48 0.53 0.54 0.52 H2

0.53

0.38 0.52 0.51 0.42 0.48 0.52 0.46 0.42 0.46 0.54 0.53 0.51 H1

0.53

0.48 0.48 0.48 0.5 0.5 0.5 0.51 0.51 0.51 0.48 0.48 0.53 N2

0.48

P-F2 P-E2

−0.57 −0.58

P-D2 P-C2

−0.67 −0.89

P-B2 P-A2

−0.65 −0.69

V-F2 V-E2

−0.82 −0.68

V-D2 V-C2

−0.68 −0.75

V-B2 V-A2 ba

A useful tool to provide a quantitative measure of charge transfer is the Mulliken population analysis method [26], in which the electronic charge is distributed among the individual atoms. Table 4 gives atomic charges of equilibrium adsorption of the NH2 NO2 molecule on the Al2 O3 (0 0 1) surface. The obtained charges indicate that slight charge transfer occurs as a result of adsorption between the NH2 NO2 molecule and Al2 O3 surface. For the N atom of amino group, its charge of V-E2 and P-B2 decreases 0.07 and 0.14 e, respectively, due to forming an Al N bond. However charges of other adsorption configurations increase, with maximum charge increment of 0.18 e for P-E2. The charge of two H atoms for amino group change slightly, except that the charge changes of P-D2 and P-F2 are large because the amino H atoms and surface alumina O atoms are ˚ respectively. The charges neighboring, which are 1.541 and 1.510 A, of the nitro N atom decrease slightly for all adsorption configurations. As for two nitro O atoms, when the O and Al atoms form an Al O bond, charge at one O atom decreases while the charge of another O atom increases. Of all configurations, the change of nitro O atoms charge of V-C2 and P-F2 is the largest. To the O atoms of the Al2 O3 , the maximal decreasing charge is from −0.65 e to −1.22 e, as for the O atoms of P-D2 and P-F2 and

N1

3.4. Charge transfer

Table 4 Atomic charge (e) of equilibrium adsorption of NH2 NO2 on the Al2 O3 (0 0 1) surface.

To understand the interacting of NH2 NO2 molecule with Al2 O3 (0 0 1) surface, the surface coverage is investigated. The surface coverage is based on the ratio of the projective area of adsorbed molecule to the total area of the surface. Table 3 gives the surface coverage degree of all adsorption configurations. By counting the number of the grids for the projective area of adsorbed NH2 NO2 molecule and the total grid number of Al2 O3 surface, the coverage degree was obtained from the ratio of projective area to the total surface area. The surface coverage for configurations in Figs. 2 and 3 are between 9.13% and 18.27%. The smallest surface coverage degree of configuration V-A2 is 9.13%, and the largest surface coverage degree of P-B2 is 18.27%. As seen from the adsorption energy in Table 2 and the surface coverage in Table 3, the order of the surface coverage for the NH2 NO2 molecule on the Al2 O3 disagrees with that of the adsorption energy. Therefore, it can be concluded that the interaction between the NH2 NO2 molecule and Al2 O3 is hardly influenced by the surface coverage when the coverage is low.

−0.69

3.3. Coverage degree of surface

−0.68

sites are of N atoms for nitro and amino groups at an Al-hollow, respectively. The maximum difference of the adsorption energies for configurations with forming an Al O bond is 103.6 kcal mol−1 , i.e. P-E2 and P-D2; the difference of the adsorption energies for configurations with forming an Al N bond is 13.8 kcal mol−1 , i.e. V-E2 and P-B2; the maximum difference of the adsorption energies for configurations without forming bond is 65.5 kcal mol−1 , i.e. V-F2 and P-C2. Based on the adsorption energies, it can be concluded that interacting of the NH2 NO2 molecule and Al2 O3 is very strong. In a word, whether the NH2 NO2 molecule and Al2 O3 form chemical bond or not, interaction of the NH2 NO2 molecule and Al2 O3 is very strong. The reason is that Al2 O3 is ionic and the Al ion can attract O or N atom of NH2 NO2 . Making a comprehensive view of the adsorption energy and the change with structure of NH2 NO2 and Al2 O3 after the adsorption of the NH2 NO2 molecule on the Al2 O3 surface, aluminized explosive of NH2 NO2 keeps high reactivity, even if the aluminum is oxidized to form a film of the alumina. This finding may make clear the activated aluminum as a stored energy source for propellants and the good performance of aluminized explosives. [7,24,25]

−0.58

S.-Q. Zhou et al. / Applied Surface Science 258 (2012) 7334–7342

7340

S.-Q. Zhou et al. / Applied Surface Science 258 (2012) 7334–7342

3

2

3

b-N1

2

1

0

0

3

3

2

V-A2-N1

2

1

0

3

3

Density of States(electrons/eV)

V-B2-N1

2

1

0

3

3

2

V-C2-N1

2

1

0

3

3

V-D2-N1

2

1

0

3

3

V-E2-N1

2

1

0

3

3

V-F2-N1

2

1

-30 -25 -20 -15 -10

-5

0

5

V-F2-O of NH2NO2

0

0

10

60

60

50

V-B2-Al

40

10-30 -25 -20 -15 -10

-5

0

5

40 30

20

20

2

10

10

0

0

0

10

60

60

50

V-C2-Al

40

50

V-C2-O of Al2O3

40

30

30

20

20

2

10

10

0

0

0

10

60

60

50

V-D2-Al

40

50

V-D2-O of Al2O3

30

20

20

2

10

10

0

0

0

10

60

60

50

V-E2-Al

40

50

V-E2-O of Al2O3

40

30

30

20

20

2

10

10

0

0

0

10

60

60

V-F2-Al

50 40

50

V-F2-O of Al2O3

30

20

20

2

10

10

0

0 -5

0

5

10-30 -25 -20 -15 -10

V-B2-Al2O3

V-C2-Al2O3

V-D2-Al2O3

V-E2-Al2O3

V-F2-Al2O3

40

30

10-30 -25 -20 -15 -10

V-A2-Al2O3

40

30

4

0

50

V-B2-O of Al2O3

30

6

2

5

10

0

8

4

0

10

4

0

-5

20

2

6

2

40 30

8

V-E2-O of NH2NO2

50

V-A2-O of Al2O3

20

4

6

10-30 -25 -20 -15 -10

40

6

0

0

50

30

8

2

1

0

E

V-D2-O of NH2NO2

V-F2-N2

60

4

4

6

60

6

0

4

0

10

8

2

1

0

2

V-C2-O of NH2NO2 4

V-E2-N2

0

4

6

V-D2-N2

10

0

6

0

6

10

8

2

1

0

2

V-B2-O of NH2NO2 4

V-C2-N2

20

2

V-A2-Al

b-Al2O3

40 30

4

6

V-B2-N2

50

20

6

0

60

b-O of Al2O3

30

8

V-A2-O of NH2NO2

2

1

0

2

6 4

40

4

0

V-A2-N2

50

b-Al

6

2

1

0

8

4

1

0

2

b-O of NH2NO2

1

60

10

6

b-N2

0 -5

0

5

10-30 -25 -20 -15 -10

-5

0

5

10

Energy/eV Fig. 4. Partial DOS on the N, O, Al atoms and Al2 O3 for equilibrium configurations from initial vertical ones. Configuration b refers to Fig. 1b. The dashed energy is Fermi level.

the maximal increasing charge is from −0.65 e to −0.06 e, as for the O atoms of P-E2. The maximal change of the charge for Al atoms is from 1.56 e to 1.95 e. When the Al and O atoms of the NH2 NO2 molecule form an Al O bond, the Al atom charges increase largely, whereas the change of other Al atoms charges is small. In a word, although the charge transfer between the NH2 NO2 molecule and Al2 O3 surface is small, there exist some charge transfers among atoms in NH2 NO2 or Al2 O3 moiety. In addition, for all adsorption configurations, the charges of the surface Al atoms are between 1.55 and 2.03 e, whereas charges of the nitro O atoms are between −0.23 and −0.55 e, and those of amino N atoms are in the range of −0.57 and −0.89 e. Based on these data, it can be concluded that electrostatic attraction of Al atoms with O and N atoms is very strong, which contributes to the adsorption energy of NH2 NO2 on the Al2 O3 surface. This explains that the adsorption energy is large even if no chemical bond between NH2 NO2 and Al2 O3 is formed in the adsorption.

3.5. The density of states The electronic structure is intimately related to their fundamental physical and chemical properties. Moreover, the electronic structures and properties are related to the adsorptions for the adsorbates. The discussion above suggests that the adsorption of NH2 NO2 molecule on the Al2 O3 surface results in the formation of Al O and Al N bonds. Therefore the knowledge of their electronic and properties appears to be useful for further understanding the behaviors of NH2 NO2 molecule on the Al2 O3 surface. Fig. 4 displays the calculated partial density of states (DOS) on the N, O and Al atoms for the adsorption configurations from initial vertical ones. Clearly, for V-A2, V-B2 and V-C2, the density of states for their N1 (i.e. amino N atom) and N2 (i.e. nitro N atom) atoms are similar each other because the difference of their adsorption configurations is small. In comparison with b, their peaks shift slightly to the left. The peak values of DOS on N2 atoms are small. At the range of −15 to −5 eV, the peak number of DOS on N1 atoms becomes

S.-Q. Zhou et al. / Applied Surface Science 258 (2012) 7334–7342 3

3

2

2

1

1

0

0

3

3

2

P-B2-N 1

0

3

3

Density of States/(electrons/eV)

P-C2-N1 2

2

1

1

0

0

3

3

1

0

3

3

P-E2-N1

2

1

0

0

3

3

2

P-F2-N1

2

1

-30 -25 -20 -15 -10 -5

0

5

P-E2-O of NH2NO2

50

P-B2-Al

P-F2-O of NH2NO2

30

30

20

20

2

10

10

0

0

0

10

60

60

50

P-C2-Al

40

10-30 -25 -20 -15 -10 -5

0

5

40

30

30

20

20

10

10

0

0

0

10

60

60

50

P-D2-Al

P-D2-O of Al2O3

50

40

40

30

30

20

20

2

10

10

0

0

0

10

60

60

50

P-E2-Al

40

50

P-E2-O of Al2O3

40

30

30

20

20

2

10

10

0

0

0

10

60

60

50

P-F2-Al

P-F2-O of Al2O3

50

40

40

30

30

20

20

2

10

10

0

0

4

0

50

P-C2-O of Al2O3

2

6

2

50 40

8

4

P-B2-O of Al2O3

40

4

6

5

60

6

0

0

60

8

2

10-30 -25 -20 -15 -10 -5

0

10

4

4

0

0

6

6

P-F2-N2

10

0

8

0

1

0

P-D2-O of NH2NO2

2

1

10

4

4

P-E2-N2

20

2

6

0 6

30

20

8

2

1

0

2

P-C2-O of NH2NO2

2

30

4

4

50 40

6

6

60

P-A2-O of Al2O3

40

8

0

P-D2-N2

P-D2-N1

P-B2-O of NH2NO2

2

P-C2-N2

50

4

4

1

P-A2-Al

6

0

P-B2-N2

0

8

2

6

60

10

P-A2-O of NH2NO2

4

2

1

2

6

P-A2-N2

P-A2-N1

7341

10-30 -25 -20 -15 -10 -5

0

5

P-A2-Al2O3

P-B2-Al2O3

P-C2-Al2O3

P-D2-Al2O3

P-E2-Al2O3

P-F2-Al2O3

0 10-30 -25 -20 -15 -10 -5

0

5

10-30 -25 -20 -15 -10 -5

0

5

10

Energy/eV Fig. 5. Partial DOS on the N, O, Al atoms and Al2 O3 for equilibrium configurations from initial parallel ones. The dashed energy is Fermi level.

more. For V-D2, V-E2 and V-F2, their peaks shift to the right but other changes are small, since their amino groups point toward the Al2 O3 surface and the interaction of the NH2 NO2 molecule and the Al2 O3 is weak, compared to V-A2, V-B2 and V-C2. To the O atoms of the NH2 NO2 molecule for V-A2, V-B2 and VC2, the highest peak values of DOS become smaller than those of b, whereas the number of DOS peak becomes more than those of b at the range of −15 to −5 eV. For V-D2, V-E2 and V-F2, their peaks transfer to the right but other changes are small. To the Al atoms for V-A2, V-B2, V-C2, V-D2, V-E2 and V-F2, the highest peak values of DOS decrease in comparison with those of b, and the DOS become smooth and broad at the range of −7.5 to 0 eV. To Al2 O3 and the O atoms of the Al2 O3 , the changes of the DOS for other adsorption configurations (except V-D2) are similar each other. Their highest peak values of DOS decrease in comparison with those of b, and the DOS become smooth and broad at the range of −22.5 to −12.5 eV. This maybe related with the N atom of amino group on an Al-top site and the amino group pointing toward the Al2 O3 surface for V-D2. Fig. 5 collects the calculated partial density of states (DOS) on the N, O and Al atoms for the adsorption configurations from initial parallel ones. As shown in Fig. 5, for the N1 and N2 atoms of P-A2, the peak values of DOS become smaller than those of b. For N1 atom of P-B2, the peak value at the energy of 10 eV becomes larger than

that of b due to the formation of an Al N bond between Al and N1 atoms, while the change of N2 atom DOS is small. To N1 , N2 and O atoms of the NH2 NO2 molecule for P-C2, the DOS shifts to the right because the N atom of the amino group is on an Al-top site and the amino group points toward the Al2 O3 surface. For N1 and N2 atoms of P-D2, P-E2 and P-F2, the changes of their DOS are similar to each other in comparison with that of b because their adsorption sites are all Al-hollow, and they all form a new Al O bond between surface Al atom and nitro O atoms. To the O atoms of the NH2 NO2 molecule for P-A2, P-D2, P-E2 and P-F2, the changes of their DOS are similar, since an Al O bond between Al and nitro O atoms comes into being. Their peak values become smaller than that of b. For O atoms of the NH2 NO2 molecule for P-B2, the change of DOS is small. To the O and Al atoms of the Al2 O3 , the changes of the DOS for adsorption configurations P-A2 to P-F2 are similar. Their highest peak values of DOS decrease in comparison with those of b, and the DOS become smooth and broad at the range of −22.5 to −12.5 eV. In a word, these changes are mainly induced by the strong interaction between the NH2 NO2 molecule and the Al2 O3 , and the formation an Al O or an Al N bond between the NH2 NO2 molecule and the Al2 O3 . In addition, for V-A2 to V-F2 and P-A2 to P-F2, their peak values of DOS for alumina O atoms is smaller than those of nitroamine O atoms, and the Al and O atoms band

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S.-Q. Zhou et al. / Applied Surface Science 258 (2012) 7334–7342

energies are broader than those of b (i.e. Fig. 1b). This shows that the strong interaction between NH2 NO2 and Al2 O3 results in overlapping of the electronic outer orbitals for Al and O or N atoms of NH2 NO2 . 4. Conclusion The adsorption of the NH2 NO2 molecule on the Al2 O3 (0 0 1) surface have been investigated on the basis of optimizations performed using the PBE exchange-correlation functional of the plane-wave density functional theory. The major findings can be summarized as follows. (1) For all adsorption configurations, the change of the structure with the NH2 NO2 molecule is small while the change of bond length with Al O bond of Al2 O3 is large. By the maximum distance of Al O of Al2 O3 and the minimum distance of O O of Al2 O3 , it can concluded that the Al O bonds partially decompose and some O O bonds form, whose bond lengths are intermediate between the O O double bond value of 1.12 A˚ and ˚ This implies O ions of Al2 O3 O O single bond value of 1.48 A. become easily reactive due to the adsorption of the NH2 NO2 molecule on the Al2 O3 surface. This result also explains why aluminized explosive keeps high reactivity even if the aluminum is oxidized to form a film of the alumina. (2) The smallest adsorption energy and the largest one are −350.2 kcal mol−1 for P-E2 and −453.8 kcal mol−1 for P-D2, respectively. By the adsorption energies, it can be concluded that interacting of the NH2 NO2 molecule and Al2 O3 is very strong. The smallest surface coverage of V-A2 is 9.13%, and the largest surface coverage degree of P-B2 is 18.27%. However the surface coverage of NH2 NO2 molecule on the Al2 O3 surface is not proportional to the adsorption energy. Therefore, the interaction between the NH2 NO2 molecule and Al2 O3 is hardly influenced by the surface coverage. (3) The charge transfer between the NH2 NO2 molecule and Al2 O3 surface is small, but there exist some charges transfer among atoms within NH2 NO2 or Al2 O3 moiety. (4) The energies of DOS for N and O atoms of the NH2 NO2 molecule match with those of Al atoms at the energy of −25 to 7.5 eV, and Al O or Al N bond forms easily at this energy level. In addition, the DOS projections on the N, O and Al atoms occur with obvious shift of peaks, which infers energy bands become broad and the interactions of chemical bonds are strengthened. Acknowledgments The authors gratefully acknowledge the foundation provided by the Science and Technology on Combustion and Explosion Laboratory (grant no. 9140C35010201) and Jiangsu Applied Chemistry and Materials Graduate Center for Innovation and Communication Foundation (grant no. 2010ACMC04) for this work. S.Q.Z. thanks the Innovation Foundation from the Graduate School of NJUST and the Innovation Project for Postgraduates in Universities of Jiangsu Province (grant no. CX09B093Z) for partial financial support.

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