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Theoretical study of sarin adsorption on (12,0) boron nitride nanotube doped with silicon atoms
Journal Pre-proofs Research paper Theoretical study of sarin adsorption on (12,0) boron nitride nanotube doped with silicon atoms Jeziel Rodrigues dos Santos, Elson Longo da Silva, Osmair Vital de Oliveira, José Divino dos Santos PII: DOI: Reference:
S0009-2614(19)30797-3 https://doi.org/10.1016/j.cplett.2019.136816 CPLETT 136816
To appear in:
Chemical Physics Letters
Received Date: Revised Date: Accepted Date:
14 August 2019 13 September 2019 1 October 2019
Please cite this article as: J. Rodrigues dos Santos, E. Longo da Silva, O. Vital de Oliveira, J. Divino dos Santos, Theoretical study of sarin adsorption on (12,0) boron nitride nanotube doped with silicon atoms, Chemical Physics Letters (2019), doi: https://doi.org/10.1016/j.cplett.2019.136816
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© 2019 Published by Elsevier B.V.
Theoretical study of sarin adsorption on (12,0) boron nitride nanotube doped with silicon atoms
Jeziel Rodrigues dos Santosa*, Elson Longo da Silvab, Osmair Vital de Oliveirac* and José Divino dos Santosa
a
Universidade Estadual de Goiás, Campus Anápolis, CEP: 75.132-903, GO, Brazil
b
INCTMN, LIEC, Departamento de Química da Universidade Federal de São Carlos, CEP: 13.565-
905, São Carlos, SP, Brazil c
Instituto Federal de Educação, Ciência e Tecnologia de São Paulo, Campus Catanduva, CEP:
15.808-305, Catanduva, SP, Brazil
Corresponding authors: Jeziel Rodrigues dos Santos ([email protected]); Osmair Vital de Oliveira ([email protected])
Telephone number: +55 (17) 3524-9718
Abstract Sarin gas is one of the most lethal nerve agent used in chemical warfare, which its detection is import to prevent a chemical attack and to identify a contamination area. Herein, density functional theory was used to investigate the (12,0) boron nitride nanotube (BNNT) and Si–doped BNNT as possible candidates to sarin detection. The Si-atoms doped improve the electronic properties of nanotubes by altering the electrostatic potential, HOMO and LUMO energies. Based in the adsorption energies and the conductivity increased to ~33 and 350%, respectively, for Si- and 2SiBNNT imply that they can be used for sarin detection.
Keywords: Nerve agent sarin, Gas sensor, Boron nitride nanotube, DFT
1
1. Introduction Neurotoxic chemical agents are a class of substances which direct and indirectly perturb the Human and animal nervous system by acting in the neural cell or in the metabolic process of this system [1]. These substances are chemically classified into the organophosphorus group and they are organic compounds degradable. Moreover, these compounds are very harmful and/or lethal for Human, consequently they has been used as high-impact military artifice in called chemical warfare. These agents inhibit the acetylcholinesterase irreversibly leading the loss of the control of central nervous system. Among the neurotoxic agents, the most used in the chemical warfare are belong the G-series as tabun, sarin and soman gases, which these two last were developed during the World War II [2]. In this series, they are absorbed through the lungs causing seizures, loss of body control, paralyses muscles including heart and diaphragm. Sarin gas is the most neurotoxic agent known and largely used in chemical attack. It was used in the World War II, Iraq/Iran War (1981-1989), in the Tokyo subway attack (1995), in the Syrian Civil War (2012), and recently in the Syrian attack in 2017. Therefore, the detection of sarin can be useful for military and civil defense to prevent chemical attack using this neurotoxic agent and others. In this way, different methodologies have been used to detect chemical agents like infrared spectroscopy [3], mobility spectroscopy [4], calorimetric [5], surface acoustic waver sensors [6], electrochemical detectors [7], carbon nanotube (CNT) chemical sensor [8]. Among them, chemical sensors based in CNT have an advantage to produce portable sensing method and they can be coupled with electrical devices as conductometric, electrochemical, and others [8]. However, CNTs offer non-specific sensing responses for nerve agent and their mimics. Another disadvantage, the electronic properties of CNTs are high dependent of their chirality [9]. So, the boron nitride nanotube (BNNT) appears as an excellent candidate to substitute the CNT because their electronic properties are independent of their diameter and chirality [10-12]. For instance, the band gap of the BNNT is 5-6 eV [13, 14] independently of these properties. Therefore, the BNNT is an excellent insulator, contrary to CNTs which are semimetallic and semiconductor material [15]. Moreover, BNNTs are stable in oxidation resistance and it has high thermal resistance above 900 °C. Experimentally, BNNT was synthesized by first time in 1995 thought arc discharge [16], and actually others methods like chemical vapor deposition [17], laser ablation [18] and thermal plasma jet [19] are used to their synthesis. BNNTs have been applied for polymer composite reinforcement [20], for piezo actuators [21], drug carrier [22], in field emission technology [23, 24], sensing [25, 26], etc. Recently, the possible use of BNNTs as gas sensor has attracted the attention of many researchers. In the manner that, theoretical methods were used to study the adsorption of oxazole and isoxazole [27], hydrogen halides [28], hydrogen cyanide [29], carbon monoxide [30], cyanogen chloride [31] and ammonia [32] in BNNTs. Regarding the chemical agents, the adsorption of soman and chlorosoman by (8,0) BNNT 2
[33], and a sarin derivative by (6,0) BNNT [34] were studied at theoretical level. In both studies, the authors find that these nerve agents interact weakly with pristine BNNT. Therefore, to improve the BNNT electronic sensitivity, herein a large zigzag (12,0) BNNT and it doped with silicon atoms were studied at density functional theory (DFT) with intention to enhance the sarin-BNNT interactions. For instance, Si-doped BNNT was forecast by Guerini [35] from theoretical methods, and posterior it was synthesized in 2009 by Cho [36] using thermal chemical vapor deposition. In both studies, it was confirmed the improvement of BNNT reactivity up replacing B by the Si atom.
2. Methodology The initial structure of the zigzag (12,0) BNNT formed by 96 boron and 69 nitrogen atoms was built using a script written in-house. The end atoms were saturated by 24 hydrogen atoms to avoid the boundary effects, forming the B96N96H24 compound. This nanotube model has 9.4 and 15.4 Å of diameter and length, respectively. For the complexes, the initial configurations were built by positioning sarin (C4H10FO2P) molecule in different regions around the BNNT and Si atoms–doped BNNT surfaces. The sarin, the pristine BNNT and it doped with Si atom (B95N96SiH24 in doublet state and B94N96Si2H24 in singlet state), and the complexes (sarin–BNNT, sarin–B95N96SiH24 and sarin–B94N96Si2H24) were optimized initially with the semiempirical Hamiltonian PM7 [37] using MOPAC2016 program [38]. Posteriorly, the structures with minimum energies were re-optimized with DFT calculations considering the B3LYP hybrid functional [39] with 6-31G(d) basis set. This procedure was chosen to balance the computational cost and quality of the results. Moreover, some works reported in the literature show that this theory level is sufficiently to describe pristine BNNT [28, 34, 40]. The stationary points were characterized as a point of minimum energy using the harmonic vibrational states, which it was not observed negative frequencies. All DFT calculations were carried out in vacuum using the Gaussian 09 package [41]. The natural bond orbital (NBO) method [42] was used to compute the atomic charges. The adsorption energy (Ead) was calculated using the equation, Ead = Ecomplex – (Enanotube + Esarin)
(I)
where Ecomplex is the total energy of complexes (sarin–BNNT, sarin–B95N96SiH24 and sarin– B94N96Si2H24), Enanotube is the total energy of BNNT, B95N96SiH24 or B94N96Si2H24, and Esarin is the total energy of the sarin.
3
3. Results and discussion 3.1 Structural analysis In the present work, DFT method was used to study the pristine BNNT and it doped with Si atoms with intention to obtain a new gas sensor to identify the presence of the nerve agent sarin. Figure 1 shows the optimized structures with minimum energy obtained at DFT method.
Figure 1. Structures of the compounds optimized with DFT//B3LYP/6-31G(d) method. Nitrogen in blue, oxygen in red, silicon in yellow, carbon in gray, hydrogen in white, phosphorous in orange, fluorine in green and boron in salmon colors.
Overall, as can see in the Figure 1, it was not observed significant structural change in the BNNT doped with Si atoms. The main difference noted is attributed to the Si atoms that are outside of 0.09 nm from the B95N96SiH24 and B94N96Si2H24 surface. This is due the Van der Waals radius (0.210 nm) of the Si atom to be slight high than the boron atom (0.192 nm). Wang [43] obtained this same geometrical distortion in Si-doped BNNT using periodic DFT calculations. Regarding the structural change of nanotubes, the same pattern was observed for the complexes, implying that the sarin adsorbed preserve the BNNT, B95N96SiH24 and B94N96Si2H24 structures. These observations are in concordance with the lowest root mean square deviation (RMSD) values (< 0.01 nm) calculated from the superposition between Si–doped BNNT and pristine BNNT structure. For sarin–BNNT, it was observed a shortest distance between the boron and oxygen atom (sarin) with distance of 0.294 4
nm. Whereas, for the sarin–B95N96SiH24 and sarin–B94N96Si2H24, this distance was, respectively, 0.165 and 0.178 nm. From the NBO analysis, it was confirmed that the oxygen bound covalently with Si atom of the B95N96SiH24 and partially with B94N96Si2H24 compounds with bond order of 1.11 and 0.63, respectively. Contrary, there is not formation of covalent bond between BNNT and sarin, keeping bond order of 0.04. Therefore, the O–Si bond obtained in our calculations is in good agreement with experimental data (0.161 nm). For all nanotubes, the boron-nitrogen bond is 0.144– 0.145 nm in excellent agreement with the hexagonal crystal structure of the boron nitride (0.145 nm). In the next section, the electronic structures for pure compounds and for complexes are presented and discussed to understand the energetic process involved.
3.2 Electronic structure The electronic properties were used to clarify the chemisorption process of the sarin by BNNT and Si atoms–doped BNNT. Initially, it is important evaluate the Si–doped BNNT stability, which this can be addressed by the formation energy (Eformation) using the equation, Eformation = (Edoped-BNNT + nEB) – (EBNNT + nESi)
(II)
where Edoped-BNNT is the total energy of one or two silicon doped with BNNT, n is the number of Si or B atoms substituted, nEB is the total energy B atom, EBNNT is the total energy of the pristine BNNT and nESi is the total energy of Si atom. The Eformation values for B95N96SiH24 and B94N96Si2H24 are 3.46 and 8.30 eV, respectively. For 1Si–doped BNNT, the Eformation has low value compared to the reported in the literature (4.06 eV) using DFT//B3LYP/ECP method [44], but these values are in good agreement considering the specificity and limitation of each basis set used. Table 1 summarizes some important electronic properties obtained and discussed here.
Table 1. Electronic properties obtained from DFT//B3LYP/6-31G(d) calculations.
a
Compounds
HOMO (eV)
LUMO (eV)
Ega
Ead (kcal/mol)
sarin
–7.93
1.47
6.46
–
BNNT
–6.43
–0.38
6.05
–
B95N96SiH24
–5.28
–0.37
4.91
–
B94N96Si2H24
–4.06
–3.82
0.24
–
sarin–BNNT
–6.34
–0.28
6.06
–3.79
sarin–B95N96SiH24
–4.77
–0.12
4.65
–13.44
sarin–B94N96Si2H24
–2.96
–1.23
1.74
–39.43
|HOMO – LUMO| in eV
5
The reactivity parameters were based in the highest occupied molecular orbital (HOMO, HOMO) and lowest unoccupied molecular orbital (LUMO, LUMO) energies. For instance, according to the Koopmans’ theorem [45] the ionization potential (IP) is described by the negative of the HOMO energy, and the electronic affinity (AE) can be approximated by the LUMO energy with opposite sign. Energy gap (Eg) is another important property obtained from these orbital energies by the difference between HOMO and LUMO energies in module. The large and small Eg values imply a high stability and reactivity, respectively, of a compound in chemical reactions. Therefore, the sarin is the most stable compound studied here with Eg value of 6.46 eV with highest IP and with lowest AE. The lowest reactivity of the sarin predicted here is in agreement with a recent work reported by our research group using DFT//B3LYP/6-31(d,p) [46]. In the same way, the large energy gap (6.05 eV) obtained here show that BNNT is an insulator in agreement with literature [13, 14], which they reported a band gap of 5-6 eV. Moreover, the energy gap obtained for others authors [28, 32-33, 40, 44] using different theoretical methods and different size of BNNT are in good accordance with our predicted Eg value. Therefore, these experimental and theoretical studies validate our BNNT model, and likewise the theory level used is accurate enough to describe the electronic properties of the present pristine BNNT. For Si–doped BNNT, it was observed an increase in the reactivity with Eg values of 4.91 and 0.24 eV for B95N96SiH24 and B94N96Si2H24, respectively. This improvement of the reactivity caused by the Si impurity on BNNT was also obtained in others theoretical [35, 43, 44] and experimental [36] studies. Moreover, the Si doped with a small (6,0) BNNT enhance it reactivity in one way that it can be used in catalysis for CO oxidation [47], N2O reduction by SO2 [48] and CO oxidation by N2O [49]. According to the Wang [43], the Si-doped BNNT reactivity is relationship with its structural change in the doping process. The lowest Eg of B94N96Si2H24 indicates that it is a conductor nanotube, whereas the B95N96SiH24 is a semiconductor. A Si atom doped in BNNT, does not alter significant the LUMO energy of the BNNT, which they have a similar value of ~ –0.38 eV. Contrary, the LUMO energy (–3.82 eV) of the 2Si–doped BNNT decrease drastically in ten times compared with pristine BNNT (–0.38 eV), which become the B94N96Si2H24 an excellent electron acceptor. The doped atoms increase the HOMO energy in 1.15 and 2.37 eV for B95N96SiH24 and B94N96Si2H24, respectively, compared to the pure BNNT. For best visualization of the electronic density, in the Figure 2 is presented the HOMO and LUMO energies, and the molecular electrostatic potential (MEP) obtained from NBO atomic charges at DFT//B3LYP/6-31G(d) level.
6
Figure 2. Representation of HOMO and LUMO energies, and molecular electrostatic potential (MEP, in eV) of the sarin, BNNT and Si–atoms doped BNNT. For MEP, the negative and positive charges range from red to blue colors, respectively.
Figure 2 shows lowest density of the HOMO energies for all nanotubes, which the highest density is observed on Si atoms (Si–atoms doped BNNT) and, for pure BNNT, there small density energy on the N atoms. On the other hand, the BNNT and B95N96SiH24 present high density in LUMO energy centered in the boron-nitrogen bonds, while for B94N96Si2H24, the LUMO energy is condensed only in the Si atoms. For nerve agent sarin, the LUMO is distributed along the molecule and the HOMO energy is centered in the oxygen atom. Therefore, the highest reactivity of Si atoms-doped BNNT can be attributed the generation of specific active sites, which they are suitable to react with others chemical species. From MEP analysis, it is interesting to point that the 2Si–doped BNNT does not change the electrostatic surface of the BNNT as can see in Figure 2. Contrary, it was observed a significant change caused by one Si impurity in the BNNT due the increasing of the atomic charges of B and N atoms. For instance, the average of NBO atomic charges obtained for B and N are, respectively, 1.18 electron (e) and –1.18e for pristine BNNT. Whereas, for B95N96SiH24 are 0.59 (B) and –0.59e 7
(N), and for B95N96SiH24 are 1.18 (B) and –1.18e (N). Except the edge regions, the distribution of MEP is homogeneous for pristine BNNT implying that there are not specific sites for nucleophilic (donates an electron pair) and/or electrophilic (an electron pair acceptor) attack. Therefore, the lowest reactivity of BNNT (high Eg value of 6.05 eV, Table 1) is relationship with its electrostatic potential distribution in agreement with its low density of HOMO energy (Figure 2). Otherwise, in both Si–doped BNNT structures, there is the presence of electrophilic sites (blue region in the middle nanotube structure) in the Si atom and, it may explain their high reactivity as confirmed by the low Eg values (Table 1) and the high density of HOMO energies in the Si (Figure 2). Therefore, from the HOMO, LUMO and MEP analysis, we can infer that the sarin acts as nucleophile and Si– atoms doped BNNT acts as an electrophile. For instance, the NBO charges of the silicon are 1.28 and 1.73e for 1Si and 2Si–doped BNNT, respectively, and for oxygen atom (sarin) is -1.05e. Consequently, as expected, it was observed strong interactions between sarin and Si-doped BNNT, confirmed by the covalent bond formation, as presented in the Figure 1. The Ead values (Table 1) follower the order: –39.43 < –13.44 < –3.79 kcal/mol for B94N96Si2H24, B95N96SiH24 and pristine BNNT, respectively. The highest Ead value for BNNT indicates a weakly physical adsorption between sarin and BNNT due the Van der Waals interactions. This finding is in agreement with others studies reporting the nerve agents adsorbed on pristine BNNT [33, 44]. Indeed, the energy gap (Table 1) is very similar of BNNT and sarin–BNNT complex indicating that the adsorption of sarin does not alters significantly the electronic properties of the BNNT. Consequently, this result suggests that the BNNT is not suitable to be used as gas sensor for the nerve agent sarin detection using electric conductivity. For instance, the conductivity can be estimated from Eg using the equation below [50],
(III) where σ is the electric conductivity, Eg is the energy gap, kb is the Boltzmann’s constant and T is the temperature. Therefore, lowest Eg value implies a higher electric conductivity. As expected, the BNNT doped with 1 and 2 Si atoms have the lowest Ead due their high reactivity as confirmed by their lowest Eg. Among them, the lowest Ead value (–39.43 kcal/mol) of B94N96Si2H24 imply that the sarin molecule interact strongly with B94N96Si2H24 due electrostatic interactions, because in this nanotube the large charges of B and N atoms of BNNT (1.18 for B and –1.18e for N atom) are preserved. On the other hand, for B95N96SiH24 the B and N charges were decreased and increased, respectively, by two times compared with pristine BNNT. The Ead is very favorable for sarin-B95N96SiH24 interaction, and the conductivity is increased in ~33% (using equation III) implying that this nanotube may be promissory as sensor gas for neurotoxic sarin. 8
Likewise, the B94N96Si2H24 appear to be an excellent sensor to detect the presence of sarin due its lowest Ead and by the decreasing of ~350% (from equation III) of the conductivity along the adsorption process of sarin. Although all nanotubes interact favorably with sarin, its presence does not affect significantly the density of HOMO and LUMO energies of them as can see in the Figures 2 and 3.
Figure 3. Representation of HOMO and LUMO energies, and molecular electrostatic potential (MEP) for all complexes. For MEP, the negative and positive charges range from red to blue colors, respectively. In details, the sarin adsorbed on BNNT and B95N96SiH24 decrease slight their density of LUMO energy; whereas for B94N96Si2H24 the LUMO is located only in the sarin molecule (see Figure 2 and 3). For HOMO energy, the 1Si and 2Si-doped BNNT use their electronic density centered in the Si atom to bind with oxygen from sarin molecule. An interesting find is that in sarin–B94N96Si2H24 complex, there is another density of HOMO energy and a partial positive charge in the Si atom unbound with sarin, suggesting that a second sarin can bind in this site. A good sensor chemical must be recovered after it worked. Thus, the sarin desorption from nanotubes surface can be obtained using physical (e.g. temperature) and/or chemical process, because the HOMO and LUMO energies are located only in the sarin molecule in B95N96SiH24 and B94N96Si2H24. The MEP analyses show that the sarin adsorbed on nanotubes does not alters significantly their electrostatic surfaces. The main changes observed are due the charge transfer from sarin to nanotubes (–0.02, –0.21 and – 0.24e to pristine BNNT, B95N96SiH24 and B94N96Si2H24, respectively.) Finally, our results show the 9
improvement of the electronic properties of BNNT by substitution the B by Si atom, which become the Si-doped BNNT as a promissory candidate to be used for sarin gas detection.
4. Conclusions In this work, DFT method was used to study the adsorption of nerve agent sarin in BNNT and Si– doped BNNT with intention to obtain a new gas sensor for this lethal neurotoxic agent. The BNNT structure is preserved along the doping process by Si atom. However, the electronic properties of the BNNT with Si impurity atoms were improved increasing their reactivity by altering the HOMO and LUMO energies, and the electrostatic potential. The adsorption energy values indicate that the interaction between the nanotubes and sarin is a favorable process, which 2Si-doped BNNT has the lowest Ead with value of –39.43 kcal/mol. The large Ead and energy gap imply that the pristine BNNT cannot be used as gas sensor based in the conductivity. On the other hand, for B95N96SiH24 and B94N96Si2H24 it was observed a variation of ~33 and 350% of the conductivity along the adsorption process. Therefore, among than, the B94N96Si2H24 is more suitable to be a new gas sensor for sarin molecule. Overall, the theoretical results herein can be used to support future experimental and theoretical studies concerning the use of BNNT doped with atoms to be used as sensor gas to detect nerve agents.
Acknowledgments This research was carried out with the support of the High Performance Computing Center at the Universidade Estadual de Goiás. Also, Jeziel Rodrigues dos Santos acknowledge the Fundação de Amparo à Pesquisa do Estado de Goiás (FAPEG) for the award of a scholarship.
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DFT method was used to study the adsorption of nerve agent sarin by BNNT. Electronic properties of pristine BNNT are improved by Si impurity atoms. The adsorption of sarin by Si-doped BNNT is highest favorable than the pure BNNT. Si-doped BNNT can be a new gas sensor for sarin gas detection and its derivatives.
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Declaration of interests (X) The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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S0009-2614(19)30797-3 https://doi.org/10.1016/j.cplett.2019.136816 CPLETT 136816
To appear in:
Chemical Physics Letters
Received Date: Revised Date: Accepted Date:
14 August 2019 13 September 2019 1 October 2019
Please cite this article as: J. Rodrigues dos Santos, E. Longo da Silva, O. Vital de Oliveira, J. Divino dos Santos, Theoretical study of sarin adsorption on (12,0) boron nitride nanotube doped with silicon atoms, Chemical Physics Letters (2019), doi: https://doi.org/10.1016/j.cplett.2019.136816
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© 2019 Published by Elsevier B.V.
Theoretical study of sarin adsorption on (12,0) boron nitride nanotube doped with silicon atoms
Jeziel Rodrigues dos Santosa*, Elson Longo da Silvab, Osmair Vital de Oliveirac* and José Divino dos Santosa
a
Universidade Estadual de Goiás, Campus Anápolis, CEP: 75.132-903, GO, Brazil
b
INCTMN, LIEC, Departamento de Química da Universidade Federal de São Carlos, CEP: 13.565-
905, São Carlos, SP, Brazil c
Instituto Federal de Educação, Ciência e Tecnologia de São Paulo, Campus Catanduva, CEP:
15.808-305, Catanduva, SP, Brazil
Corresponding authors: Jeziel Rodrigues dos Santos ([email protected]); Osmair Vital de Oliveira ([email protected])
Telephone number: +55 (17) 3524-9718
Abstract Sarin gas is one of the most lethal nerve agent used in chemical warfare, which its detection is import to prevent a chemical attack and to identify a contamination area. Herein, density functional theory was used to investigate the (12,0) boron nitride nanotube (BNNT) and Si–doped BNNT as possible candidates to sarin detection. The Si-atoms doped improve the electronic properties of nanotubes by altering the electrostatic potential, HOMO and LUMO energies. Based in the adsorption energies and the conductivity increased to ~33 and 350%, respectively, for Si- and 2SiBNNT imply that they can be used for sarin detection.
Keywords: Nerve agent sarin, Gas sensor, Boron nitride nanotube, DFT
1
1. Introduction Neurotoxic chemical agents are a class of substances which direct and indirectly perturb the Human and animal nervous system by acting in the neural cell or in the metabolic process of this system [1]. These substances are chemically classified into the organophosphorus group and they are organic compounds degradable. Moreover, these compounds are very harmful and/or lethal for Human, consequently they has been used as high-impact military artifice in called chemical warfare. These agents inhibit the acetylcholinesterase irreversibly leading the loss of the control of central nervous system. Among the neurotoxic agents, the most used in the chemical warfare are belong the G-series as tabun, sarin and soman gases, which these two last were developed during the World War II [2]. In this series, they are absorbed through the lungs causing seizures, loss of body control, paralyses muscles including heart and diaphragm. Sarin gas is the most neurotoxic agent known and largely used in chemical attack. It was used in the World War II, Iraq/Iran War (1981-1989), in the Tokyo subway attack (1995), in the Syrian Civil War (2012), and recently in the Syrian attack in 2017. Therefore, the detection of sarin can be useful for military and civil defense to prevent chemical attack using this neurotoxic agent and others. In this way, different methodologies have been used to detect chemical agents like infrared spectroscopy [3], mobility spectroscopy [4], calorimetric [5], surface acoustic waver sensors [6], electrochemical detectors [7], carbon nanotube (CNT) chemical sensor [8]. Among them, chemical sensors based in CNT have an advantage to produce portable sensing method and they can be coupled with electrical devices as conductometric, electrochemical, and others [8]. However, CNTs offer non-specific sensing responses for nerve agent and their mimics. Another disadvantage, the electronic properties of CNTs are high dependent of their chirality [9]. So, the boron nitride nanotube (BNNT) appears as an excellent candidate to substitute the CNT because their electronic properties are independent of their diameter and chirality [10-12]. For instance, the band gap of the BNNT is 5-6 eV [13, 14] independently of these properties. Therefore, the BNNT is an excellent insulator, contrary to CNTs which are semimetallic and semiconductor material [15]. Moreover, BNNTs are stable in oxidation resistance and it has high thermal resistance above 900 °C. Experimentally, BNNT was synthesized by first time in 1995 thought arc discharge [16], and actually others methods like chemical vapor deposition [17], laser ablation [18] and thermal plasma jet [19] are used to their synthesis. BNNTs have been applied for polymer composite reinforcement [20], for piezo actuators [21], drug carrier [22], in field emission technology [23, 24], sensing [25, 26], etc. Recently, the possible use of BNNTs as gas sensor has attracted the attention of many researchers. In the manner that, theoretical methods were used to study the adsorption of oxazole and isoxazole [27], hydrogen halides [28], hydrogen cyanide [29], carbon monoxide [30], cyanogen chloride [31] and ammonia [32] in BNNTs. Regarding the chemical agents, the adsorption of soman and chlorosoman by (8,0) BNNT 2
[33], and a sarin derivative by (6,0) BNNT [34] were studied at theoretical level. In both studies, the authors find that these nerve agents interact weakly with pristine BNNT. Therefore, to improve the BNNT electronic sensitivity, herein a large zigzag (12,0) BNNT and it doped with silicon atoms were studied at density functional theory (DFT) with intention to enhance the sarin-BNNT interactions. For instance, Si-doped BNNT was forecast by Guerini [35] from theoretical methods, and posterior it was synthesized in 2009 by Cho [36] using thermal chemical vapor deposition. In both studies, it was confirmed the improvement of BNNT reactivity up replacing B by the Si atom.
2. Methodology The initial structure of the zigzag (12,0) BNNT formed by 96 boron and 69 nitrogen atoms was built using a script written in-house. The end atoms were saturated by 24 hydrogen atoms to avoid the boundary effects, forming the B96N96H24 compound. This nanotube model has 9.4 and 15.4 Å of diameter and length, respectively. For the complexes, the initial configurations were built by positioning sarin (C4H10FO2P) molecule in different regions around the BNNT and Si atoms–doped BNNT surfaces. The sarin, the pristine BNNT and it doped with Si atom (B95N96SiH24 in doublet state and B94N96Si2H24 in singlet state), and the complexes (sarin–BNNT, sarin–B95N96SiH24 and sarin–B94N96Si2H24) were optimized initially with the semiempirical Hamiltonian PM7 [37] using MOPAC2016 program [38]. Posteriorly, the structures with minimum energies were re-optimized with DFT calculations considering the B3LYP hybrid functional [39] with 6-31G(d) basis set. This procedure was chosen to balance the computational cost and quality of the results. Moreover, some works reported in the literature show that this theory level is sufficiently to describe pristine BNNT [28, 34, 40]. The stationary points were characterized as a point of minimum energy using the harmonic vibrational states, which it was not observed negative frequencies. All DFT calculations were carried out in vacuum using the Gaussian 09 package [41]. The natural bond orbital (NBO) method [42] was used to compute the atomic charges. The adsorption energy (Ead) was calculated using the equation, Ead = Ecomplex – (Enanotube + Esarin)
(I)
where Ecomplex is the total energy of complexes (sarin–BNNT, sarin–B95N96SiH24 and sarin– B94N96Si2H24), Enanotube is the total energy of BNNT, B95N96SiH24 or B94N96Si2H24, and Esarin is the total energy of the sarin.
3
3. Results and discussion 3.1 Structural analysis In the present work, DFT method was used to study the pristine BNNT and it doped with Si atoms with intention to obtain a new gas sensor to identify the presence of the nerve agent sarin. Figure 1 shows the optimized structures with minimum energy obtained at DFT method.
Figure 1. Structures of the compounds optimized with DFT//B3LYP/6-31G(d) method. Nitrogen in blue, oxygen in red, silicon in yellow, carbon in gray, hydrogen in white, phosphorous in orange, fluorine in green and boron in salmon colors.
Overall, as can see in the Figure 1, it was not observed significant structural change in the BNNT doped with Si atoms. The main difference noted is attributed to the Si atoms that are outside of 0.09 nm from the B95N96SiH24 and B94N96Si2H24 surface. This is due the Van der Waals radius (0.210 nm) of the Si atom to be slight high than the boron atom (0.192 nm). Wang [43] obtained this same geometrical distortion in Si-doped BNNT using periodic DFT calculations. Regarding the structural change of nanotubes, the same pattern was observed for the complexes, implying that the sarin adsorbed preserve the BNNT, B95N96SiH24 and B94N96Si2H24 structures. These observations are in concordance with the lowest root mean square deviation (RMSD) values (< 0.01 nm) calculated from the superposition between Si–doped BNNT and pristine BNNT structure. For sarin–BNNT, it was observed a shortest distance between the boron and oxygen atom (sarin) with distance of 0.294 4
nm. Whereas, for the sarin–B95N96SiH24 and sarin–B94N96Si2H24, this distance was, respectively, 0.165 and 0.178 nm. From the NBO analysis, it was confirmed that the oxygen bound covalently with Si atom of the B95N96SiH24 and partially with B94N96Si2H24 compounds with bond order of 1.11 and 0.63, respectively. Contrary, there is not formation of covalent bond between BNNT and sarin, keeping bond order of 0.04. Therefore, the O–Si bond obtained in our calculations is in good agreement with experimental data (0.161 nm). For all nanotubes, the boron-nitrogen bond is 0.144– 0.145 nm in excellent agreement with the hexagonal crystal structure of the boron nitride (0.145 nm). In the next section, the electronic structures for pure compounds and for complexes are presented and discussed to understand the energetic process involved.
3.2 Electronic structure The electronic properties were used to clarify the chemisorption process of the sarin by BNNT and Si atoms–doped BNNT. Initially, it is important evaluate the Si–doped BNNT stability, which this can be addressed by the formation energy (Eformation) using the equation, Eformation = (Edoped-BNNT + nEB) – (EBNNT + nESi)
(II)
where Edoped-BNNT is the total energy of one or two silicon doped with BNNT, n is the number of Si or B atoms substituted, nEB is the total energy B atom, EBNNT is the total energy of the pristine BNNT and nESi is the total energy of Si atom. The Eformation values for B95N96SiH24 and B94N96Si2H24 are 3.46 and 8.30 eV, respectively. For 1Si–doped BNNT, the Eformation has low value compared to the reported in the literature (4.06 eV) using DFT//B3LYP/ECP method [44], but these values are in good agreement considering the specificity and limitation of each basis set used. Table 1 summarizes some important electronic properties obtained and discussed here.
Table 1. Electronic properties obtained from DFT//B3LYP/6-31G(d) calculations.
a
Compounds
HOMO (eV)
LUMO (eV)
Ega
Ead (kcal/mol)
sarin
–7.93
1.47
6.46
–
BNNT
–6.43
–0.38
6.05
–
B95N96SiH24
–5.28
–0.37
4.91
–
B94N96Si2H24
–4.06
–3.82
0.24
–
sarin–BNNT
–6.34
–0.28
6.06
–3.79
sarin–B95N96SiH24
–4.77
–0.12
4.65
–13.44
sarin–B94N96Si2H24
–2.96
–1.23
1.74
–39.43
|HOMO – LUMO| in eV
5
The reactivity parameters were based in the highest occupied molecular orbital (HOMO, HOMO) and lowest unoccupied molecular orbital (LUMO, LUMO) energies. For instance, according to the Koopmans’ theorem [45] the ionization potential (IP) is described by the negative of the HOMO energy, and the electronic affinity (AE) can be approximated by the LUMO energy with opposite sign. Energy gap (Eg) is another important property obtained from these orbital energies by the difference between HOMO and LUMO energies in module. The large and small Eg values imply a high stability and reactivity, respectively, of a compound in chemical reactions. Therefore, the sarin is the most stable compound studied here with Eg value of 6.46 eV with highest IP and with lowest AE. The lowest reactivity of the sarin predicted here is in agreement with a recent work reported by our research group using DFT//B3LYP/6-31(d,p) [46]. In the same way, the large energy gap (6.05 eV) obtained here show that BNNT is an insulator in agreement with literature [13, 14], which they reported a band gap of 5-6 eV. Moreover, the energy gap obtained for others authors [28, 32-33, 40, 44] using different theoretical methods and different size of BNNT are in good accordance with our predicted Eg value. Therefore, these experimental and theoretical studies validate our BNNT model, and likewise the theory level used is accurate enough to describe the electronic properties of the present pristine BNNT. For Si–doped BNNT, it was observed an increase in the reactivity with Eg values of 4.91 and 0.24 eV for B95N96SiH24 and B94N96Si2H24, respectively. This improvement of the reactivity caused by the Si impurity on BNNT was also obtained in others theoretical [35, 43, 44] and experimental [36] studies. Moreover, the Si doped with a small (6,0) BNNT enhance it reactivity in one way that it can be used in catalysis for CO oxidation [47], N2O reduction by SO2 [48] and CO oxidation by N2O [49]. According to the Wang [43], the Si-doped BNNT reactivity is relationship with its structural change in the doping process. The lowest Eg of B94N96Si2H24 indicates that it is a conductor nanotube, whereas the B95N96SiH24 is a semiconductor. A Si atom doped in BNNT, does not alter significant the LUMO energy of the BNNT, which they have a similar value of ~ –0.38 eV. Contrary, the LUMO energy (–3.82 eV) of the 2Si–doped BNNT decrease drastically in ten times compared with pristine BNNT (–0.38 eV), which become the B94N96Si2H24 an excellent electron acceptor. The doped atoms increase the HOMO energy in 1.15 and 2.37 eV for B95N96SiH24 and B94N96Si2H24, respectively, compared to the pure BNNT. For best visualization of the electronic density, in the Figure 2 is presented the HOMO and LUMO energies, and the molecular electrostatic potential (MEP) obtained from NBO atomic charges at DFT//B3LYP/6-31G(d) level.
6
Figure 2. Representation of HOMO and LUMO energies, and molecular electrostatic potential (MEP, in eV) of the sarin, BNNT and Si–atoms doped BNNT. For MEP, the negative and positive charges range from red to blue colors, respectively.
Figure 2 shows lowest density of the HOMO energies for all nanotubes, which the highest density is observed on Si atoms (Si–atoms doped BNNT) and, for pure BNNT, there small density energy on the N atoms. On the other hand, the BNNT and B95N96SiH24 present high density in LUMO energy centered in the boron-nitrogen bonds, while for B94N96Si2H24, the LUMO energy is condensed only in the Si atoms. For nerve agent sarin, the LUMO is distributed along the molecule and the HOMO energy is centered in the oxygen atom. Therefore, the highest reactivity of Si atoms-doped BNNT can be attributed the generation of specific active sites, which they are suitable to react with others chemical species. From MEP analysis, it is interesting to point that the 2Si–doped BNNT does not change the electrostatic surface of the BNNT as can see in Figure 2. Contrary, it was observed a significant change caused by one Si impurity in the BNNT due the increasing of the atomic charges of B and N atoms. For instance, the average of NBO atomic charges obtained for B and N are, respectively, 1.18 electron (e) and –1.18e for pristine BNNT. Whereas, for B95N96SiH24 are 0.59 (B) and –0.59e 7
(N), and for B95N96SiH24 are 1.18 (B) and –1.18e (N). Except the edge regions, the distribution of MEP is homogeneous for pristine BNNT implying that there are not specific sites for nucleophilic (donates an electron pair) and/or electrophilic (an electron pair acceptor) attack. Therefore, the lowest reactivity of BNNT (high Eg value of 6.05 eV, Table 1) is relationship with its electrostatic potential distribution in agreement with its low density of HOMO energy (Figure 2). Otherwise, in both Si–doped BNNT structures, there is the presence of electrophilic sites (blue region in the middle nanotube structure) in the Si atom and, it may explain their high reactivity as confirmed by the low Eg values (Table 1) and the high density of HOMO energies in the Si (Figure 2). Therefore, from the HOMO, LUMO and MEP analysis, we can infer that the sarin acts as nucleophile and Si– atoms doped BNNT acts as an electrophile. For instance, the NBO charges of the silicon are 1.28 and 1.73e for 1Si and 2Si–doped BNNT, respectively, and for oxygen atom (sarin) is -1.05e. Consequently, as expected, it was observed strong interactions between sarin and Si-doped BNNT, confirmed by the covalent bond formation, as presented in the Figure 1. The Ead values (Table 1) follower the order: –39.43 < –13.44 < –3.79 kcal/mol for B94N96Si2H24, B95N96SiH24 and pristine BNNT, respectively. The highest Ead value for BNNT indicates a weakly physical adsorption between sarin and BNNT due the Van der Waals interactions. This finding is in agreement with others studies reporting the nerve agents adsorbed on pristine BNNT [33, 44]. Indeed, the energy gap (Table 1) is very similar of BNNT and sarin–BNNT complex indicating that the adsorption of sarin does not alters significantly the electronic properties of the BNNT. Consequently, this result suggests that the BNNT is not suitable to be used as gas sensor for the nerve agent sarin detection using electric conductivity. For instance, the conductivity can be estimated from Eg using the equation below [50],
(III) where σ is the electric conductivity, Eg is the energy gap, kb is the Boltzmann’s constant and T is the temperature. Therefore, lowest Eg value implies a higher electric conductivity. As expected, the BNNT doped with 1 and 2 Si atoms have the lowest Ead due their high reactivity as confirmed by their lowest Eg. Among them, the lowest Ead value (–39.43 kcal/mol) of B94N96Si2H24 imply that the sarin molecule interact strongly with B94N96Si2H24 due electrostatic interactions, because in this nanotube the large charges of B and N atoms of BNNT (1.18 for B and –1.18e for N atom) are preserved. On the other hand, for B95N96SiH24 the B and N charges were decreased and increased, respectively, by two times compared with pristine BNNT. The Ead is very favorable for sarin-B95N96SiH24 interaction, and the conductivity is increased in ~33% (using equation III) implying that this nanotube may be promissory as sensor gas for neurotoxic sarin. 8
Likewise, the B94N96Si2H24 appear to be an excellent sensor to detect the presence of sarin due its lowest Ead and by the decreasing of ~350% (from equation III) of the conductivity along the adsorption process of sarin. Although all nanotubes interact favorably with sarin, its presence does not affect significantly the density of HOMO and LUMO energies of them as can see in the Figures 2 and 3.
Figure 3. Representation of HOMO and LUMO energies, and molecular electrostatic potential (MEP) for all complexes. For MEP, the negative and positive charges range from red to blue colors, respectively. In details, the sarin adsorbed on BNNT and B95N96SiH24 decrease slight their density of LUMO energy; whereas for B94N96Si2H24 the LUMO is located only in the sarin molecule (see Figure 2 and 3). For HOMO energy, the 1Si and 2Si-doped BNNT use their electronic density centered in the Si atom to bind with oxygen from sarin molecule. An interesting find is that in sarin–B94N96Si2H24 complex, there is another density of HOMO energy and a partial positive charge in the Si atom unbound with sarin, suggesting that a second sarin can bind in this site. A good sensor chemical must be recovered after it worked. Thus, the sarin desorption from nanotubes surface can be obtained using physical (e.g. temperature) and/or chemical process, because the HOMO and LUMO energies are located only in the sarin molecule in B95N96SiH24 and B94N96Si2H24. The MEP analyses show that the sarin adsorbed on nanotubes does not alters significantly their electrostatic surfaces. The main changes observed are due the charge transfer from sarin to nanotubes (–0.02, –0.21 and – 0.24e to pristine BNNT, B95N96SiH24 and B94N96Si2H24, respectively.) Finally, our results show the 9
improvement of the electronic properties of BNNT by substitution the B by Si atom, which become the Si-doped BNNT as a promissory candidate to be used for sarin gas detection.
4. Conclusions In this work, DFT method was used to study the adsorption of nerve agent sarin in BNNT and Si– doped BNNT with intention to obtain a new gas sensor for this lethal neurotoxic agent. The BNNT structure is preserved along the doping process by Si atom. However, the electronic properties of the BNNT with Si impurity atoms were improved increasing their reactivity by altering the HOMO and LUMO energies, and the electrostatic potential. The adsorption energy values indicate that the interaction between the nanotubes and sarin is a favorable process, which 2Si-doped BNNT has the lowest Ead with value of –39.43 kcal/mol. The large Ead and energy gap imply that the pristine BNNT cannot be used as gas sensor based in the conductivity. On the other hand, for B95N96SiH24 and B94N96Si2H24 it was observed a variation of ~33 and 350% of the conductivity along the adsorption process. Therefore, among than, the B94N96Si2H24 is more suitable to be a new gas sensor for sarin molecule. Overall, the theoretical results herein can be used to support future experimental and theoretical studies concerning the use of BNNT doped with atoms to be used as sensor gas to detect nerve agents.
Acknowledgments This research was carried out with the support of the High Performance Computing Center at the Universidade Estadual de Goiás. Also, Jeziel Rodrigues dos Santos acknowledge the Fundação de Amparo à Pesquisa do Estado de Goiás (FAPEG) for the award of a scholarship.
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DFT method was used to study the adsorption of nerve agent sarin by BNNT. Electronic properties of pristine BNNT are improved by Si impurity atoms. The adsorption of sarin by Si-doped BNNT is highest favorable than the pure BNNT. Si-doped BNNT can be a new gas sensor for sarin gas detection and its derivatives.
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Declaration of interests (X) The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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