Carbon-doped boron nitride nanosheets as highly sensitive materials for detection of toxic NO and NO2 gases: A DFT study

Vacuum 166 (2019) 127–134

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Carbon-doped boron nitride nanosheets as highly sensitive materials for detection of toxic NO and NO2 gases: A DFT study

T

Mehdi D. Esrafilia,∗, Farzad Arjomandi Radb a b

Department of Chemistry, Faculty of Basic Sciences, University of Maragheh, P.O. Box 55136-553, Maragheh, Iran Department of Chemistry, Bonab Branch, Islamic Azad University, Bonab, Iran

A R T I C LE I N FO

A B S T R A C T

Keywords: BN sheet C-doping Adsorption DFT Band gap

Using density functional theory calculations, we investigate sensing mechanism of C-doped hexagonal boron nitride (h-BN) nanosheets toward NO and NO2 molecules. The results indicate that C-doping induces a large spin density and electron density redistribution in h-BN, which leads to improved adsorption energy of NO and NO2. The adsorption of NO and NO2 is able to alter significantly electronic structure of C-doped h-BN nanosheets as evidenced by relatively large variation in the band gap values. Moreover, the application of an external electric field can avoid the strong adsorption of NO and NO2 molecules over C-doped h-BN sheets. Compared to NO and NO2, the adsorption of CO, CO2, H2O or NH3 on C-doped h-BN sheets is remarkably weaker, suggesting that the sensing of NO and NO2 can be performed selectively in the presence of these molecules. Therefore, C-doped h-BN nanosheets can be viewed as promising and sensitive room-temperature sensors for NO and NO2 molecules.

1. Introduction Nitrogen oxides (NOx; x = 1,2) are main air pollutants emitted by combustion of fossil fuels in vehicles, power plants and various industrial activities. Exposure to these toxic gases can cause a variety of health problems such as eye or throat irritation and decrease lung function [1]. NOx gases also contribute in the formation of acid rain as well as ground-level ozone, both of which are connected with unpleasant environmental effects [2]. According to the European Commission air quality standards [3], the annual average of NO2 concentration must be lower than the limit of 40 g/m3. Hence, development of any sensitive approach to detect low concentrations of NOx gases is highly desirable. In this context, carbon-based nanostructures [4–10] like graphene, carbon clusters and carbon nanotubes are viewed as promising sensing materials due to their outstanding properties such as high surface-to-volume ratio, short response times and unique thermal and electronic properties. For instance, previous studies [11–14] have indicated that graphene has high sensitivity to detect NO2 molecule, although it fails to detect NO due to its small adsorption energy and little charge transfer. Over the past decade, layered materials such as hexagonal boron nitride (h-BN) nanosheet have attracted considerable attention owing to their potential applications in gas sensors [15–19], batteries [20,21], supercapitors [22,23], emitters [24] and catalysis [25,26]. The

∗

combination of many exceptional properties such as high surface-tovolume ratio, high mechanical and chemical stability and limited crystal defects benefit h-BN as an ideal material for application in solidstate gas sensors [27,28]. Due to similarity between the graphene and hBN, it is expected that the charge carrier concentration induced by gas molecule adsorption affect the electrical resistivity of the h-BN in a similar way. However, h-BN is almost chemically inert, due to the presence of in-plane strong B–N bonds. Regardless, numerous experimental [29–31] and theoretical [32–35] studies indicate the surface reactivity of h-BN can be greatly improved through heteroatom chemical doping. For instance, theoretical studies have found that C-doped h-BN nanosheets exhibit superior catalytic activity for CO oxidation [36], N2O reduction [26] and oxygen reduction reaction [37]. The experimental study of Huang and coworkers [38] has also suggested that the incorporation of C atoms into h-BN enhances the chemical reactivity of system toward the reduction of CO2 under visible light illumination. Recently, C-doped h-BN nanosheets were considered as highly active catalysts for oxidation of SO2 in the gas phase and an aqueous solution [39]. The results indicated that the C-doping tends to activate h-BN toward the oxidation of SO2 mainly due to the localization of a high spin density over the C atom. Moreover, it was found that the activation energy for the oxidation of SO2 is significantly decreased in the aqueous solution. Despite all these research efforts, there is still a lack of comprehensive study about the sensing properties of C-doped h-BN toward

Corresponding author. E-mail address: esrafi[email protected] (M.D. Esrafili).

https://doi.org/10.1016/j.vacuum.2019.04.065 Received 23 January 2019; Received in revised form 29 April 2019; Accepted 30 April 2019 Available online 01 May 2019 0042-207X/ © 2019 Elsevier Ltd. All rights reserved.

Vacuum 166 (2019) 127–134

M.D. Esrafili and F. Arjomandi Rad

energy (ZPE) obtained from the frequency calculations. Similarly, the change of enthalpy (ΔHads) and Gibbs free energy (ΔGads) due to the adsorption of NO or NO2 molecules were calculated (at 298 K and 1 atm) by following equations:

the toxic gases. Inspired by the above reports, the aim of this study is to investigate the sensing properties of C-BNNSs toward the toxic NOx molecules (x = 1 and 2). By performing detailed density functional theory (DFT) calculations, we explore the adsorption behavior and electronic structure of C-doped h-BN nanosheets (C-BNNSs) to detect toxic NO and NO2 gases. The C-BNNSs are chosen, because they have already been successfully fabricated experimentally [29,38]. Our earlier study has also indicated that these chemically modified h-BN nanosheets are thermally stable even at a reasonably elevated temperature [26,36]. Moreover, due to the accumulation of a large spin density over the C atom [37], it is natural to expect that C-BNNSs exhibit high affinity to interact with NO and NO2 radicals. In terms of these unique properties, following questions then rise: (i) can C-BNNSs serve as a potential sensing material for detecting NO and NO2? (ii) if can, what is the mechanism for sensing of these molecules?, and finally (iii) is the detection of NO and NO2 molecules selective in the presence of other molecules like CO and CO2? In order to answer these questions, the most stable adsorption configurations and the corresponding electronic structure of NO and NO2 over C-BNNSs are examined carefully. The results of the present study could be very helpful to design efficient and highly sensitive h-BN based sensors to detect toxic NO and NO2 molecules.

ΔGads = Gtot- Gadsorbate - Gsheet

(4)

3.1. Geometries and electronic structure of C-BNNSs To study the sensing properties of C-BNNSs toward NO and NO2 molecules, the geometry and electronic structure of these substrates should be considered first. Fig. 1 represents the optimized structure of two possible C-BNNSs, namely CB and CN, in which a carbon atom is substituted at the site of a boron (CB) or nitrogen (CN) vacancy of h-BN sheet. One can see that the C-doping induces a negligible structural distortion and bond rearrangement around the doping site, suggesting that the carbon atom can be introduced into in h-BN at a relatively high C/B or C/N atomic ratio. The optimized C–N and C–B bond distances in CB and CN are calculated to be 1.40 and 1.51 Å, respectively, which are almost similar to previous theoretical reports [26,37,50]. The formation energy (Ef) of CB and CN is 3.97 and 4.08 eV, respectively. This finding matches with other studies [37,51], and indicates that a carbon atom can be more easily incorporated into a B-vacancy site of h-BN than Nvacancy. According to the Hirshfeld analysis, the C atom in CB loses 0.10 |e| to its neighboring N atoms, while 0.16 |e| are transferred from the nearest boron atoms to the C in CN. On the other hand, the number of unpaired electrons located over the C atom of CB and CN is 0.42 and 0.45, which clearly indicates the potential of C atom to interact with radical species (Fig. 1). The electronic structure of C-BNNSs is also studied by the analysis of total density of states (TDOS). Fig. 1 depicts the TDOS plots of CB and CN. The band gap (Eg) value of pristine h-BN nanosheet is calculated to be 5.03 eV, indicating that it behaves as a semi-conductor material. This value is also in good agreement with the experimental value (≈5.5–5.8 eV) [52] and those of other theoretical studies [37,53], which verifies the reliability of our model and density functional used in the present study. Because the C atom has four valence electrons, hence it is expected that C-doping introduces some mid-gap states in the band gap, leading also to a shift in the Fermi level of h-BN. This finding is consistent with the previous theoretical study of Zhao and Chen [37]. These midgap states which are similar to those of doped phosphorene [54,55], are originated from the 2p states of doped C atom which can play a role as a bridge to transfer photo generated electrons from valence to conduction band. Moreover, the existence of such states helps to modify the energy gap of h-BN, the band gap of CB and CN is 0.99 and 4.29 eV, respectively (Fig. 1). For CB, it is expected that the C atom donates its outermost electron and thus the Fermi level of h-BN shifts from middle of energy gap toward conduction band. This shifting is large enough to convert the intrinsic h-BN into n-type semiconductor and therefore the midgap states introduced by the C atom can be counted as localized states. On the other hand, C-doping in CN leads to a shift of the Fermi level from middle of the band gap toward the valence band, hence converting the h-BN from intrinsic to p-type semiconductor. Since a small Eg implies low kinetic stability and high chemical reactivity, C-doping of h-BN sheet could make the electrons to be more easily excited from valence band to conduction band and hence enhancing its chemical reactivity. This finding is consistent with the molecular orbital analysis of CB and CN in Fig. S1 of Supporting Information, where the singly-occupied molecular orbital (SOMO) of CB and CN is mainly localized over the C atom. Hence, it is concluded that C-doping is a suitable approach to improve surface reactivity and electronic structure properties of h-BN nanosheet.

All the quantum chemical calculations in the present study were performed within the spin-polarized DFT using the DMol3 [40,41]. The Perdew-Burke-Ernzerhof (PBE) [28] formulation of the generalized gradient approximation (GGA) was used to treat electron exchange and correlation effects, with a long-range dispersion correction by Grimme's scheme [42,43]. PBE is a computationally-less demanding method, which is widely used to study geometric and electronic structure of periodic systems [44,45]. According to the earlier studies, this method can provide reliable bond lengths, vibrational frequencies and surface properties, particularly when is corrected to the long-range dispersion effects [46–48]. Double numerical plus polarization (DNP) was taken as the basis set, which has been shown to give comparable results to those of Pople's 6–31G** basis set [49]. To improve the results, a Fermi smearing of 0.005 Ha and a real-space cutoff of 4.6 Å was employed in the calculations. In the geometry optimization, a convergence tolerance of 10−5 Ha, 0.001 Ha/Å and 0.005 Å was used for the energy, force, and displacement, respectively. To simulate C-BNNSs, a 4 × 4 supercell of pristine h-BN sheet was first constructed including 16 B and 16 N atoms. A vacuum layer of 20 Å perpendicular to the h-BN sheet was adopted to avoid the artificial interactions between the layers. By replacing a B or N atom of pristine h-BN, two distinct C-doped h-BN monolayers were obtained (see Fig. 1). The Brillouin zone integration was sampled by 6 × 6 × 1 (for the geometry optimization) and 10 × 10 × 1 k-points (for the density of states analysis) using the Monkhorst–Pack scheme. To evaluate the thermodynamic stability of C-BNNSs, formation energy was calculated using the following equitation: (1)

Here EC-BNNS and EBNNS are the total energy of the C-doped and pristine h-BN sheets, respectively. μC is the chemical potential of one single carbon atom in the pristine graphene, μN is the chemical potential of the nitrogen atom defined as half of N2 molecule energy, and μB is the energy of a single B atom in the α-rhombohedral B crystal. The adsorption energy (Eads) for a given adsorbate was obtained as Eads = Etot- Eadsorbate - Esheet

(3)

3. Results and discussion

2. Computational details

Eform = EC-BNNS - EBNNS - μC + μB (or μN)

ΔHads = Htot- Hadsorbate - Hsheet

(2)

where Etot, Eadsorbate and Esheet are the total energy of relaxed adsorbate/nanosheet complex, adsorbate molecule and nanosheet, respectively. All calculated Eads values were corrected for the zero-point 128

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Fig. 1. The optimized structure (left), spin density isosurface (isovalue = 0.05 au, middle) TDOS/PDOS plots (right) of CB and CN. The dashed line in the DOS plots refers to the Fermi level, which set to be zero.

Table 1, the NO adsorption over CB and CN is accompanied with a charge-transfer from the surface to NO, which occupies the half-filed NO-2π* orbital and resulting in the elongation of the N–O bond distance compared to the isolated state (1.16 Å). It is expected that the adsorption of NO alters electronic structure of CB and CN, which could be reflected by the change in the corresponding TDOS plots. Fig. 2a and b indicate the TDOS plot of NO adsorbed configuration over CB and CN, respectively. It is found that NO adsorption leads to a substantial downshift in the Fermi energy of CB. As a result, the band gap of CB increases by 0.24 eV upon the adsorption of NO (Table 1). As also evident, the states around the Fermi energy are mainly distributed over NO, indicating that CB can respond to NO sensitively. Notably, comparing the PDOS plots reveals that there is an additional state above the Fermi level for the NO adsorbed over CN, which should be responsible for the larger adsorption energy of this molecule on this surface. Moreover, NO molecule contributes a localized band above the Fermi level of CN and so decreases the band gap value of this system significantly. These results clearly indicate a different response of CB and CN toward NO molecule. Hence, according to the following equation [57], a change in the electric conductivity (σ) of CB and CN is expected due to the adsorption of NO molecule:

3.2. Adsorption of NO over C-BNNSs To find out the most favorable adsorption configurations, a single NO molecule was initially placed at different positions above the CB and CN surface. Several distinct starting structures were used, including those when O or N atom of NO is close to the C atom of surface. Fig. 2a and b shows the most energetically favored configurations of NO molecule over CB and CN, respectively, along with the corresponding total and partial density of states (TDOS/PDOS) plots. Table 1 collects the corresponding ZPE-corrected adsorption energies, change of enthalpy (ΔHads) and Gibbs free energy (ΔGads) and net Hirshfeld charge-transfer (qCT) values. To understand the effect of C-doping, NO adsorption on the pristine h-BN nanosheet is also considered (Fig. S2). The inspection of the results indicates that NO is physisorbed over the pristine h-BN with a binding distance of about 3 Å. The calculated ΔHads and ΔGads value of NO is 0.02 and 0.03 eV, respectively, which clearly indicate that NO cannot be stably adsorbed over h-BN nanosheet at room temperature. Hence, from a theoretical point of view, the surface of pristine h-BN is not active enough to absorb NO. For C-BNNSs studied here, our results indicate that NO adsorption via its N-site is energetically more stable than through its O-end, which can be attributed to the relatively larger spin density over the nitrogen atom (0.71 au) compared to O (0.29 au) of NO molecule (see Fig. 2a and b). The binding distance between the nitrogen atom of NO and carbon atom of CB and CN is 1.66 and 1.65 Å, respectively. These short binding distances clearly indicate that NO molecule is stably adsorbed over these surfaces due to the C-doping. Meanwhile, the adsorption of NO is found to induce a locally structural deformation around the C atom, as evidenced by elongated C–N (C–B) bond distances in CB (CN). The strong interaction between NO and the C atom is also confirmed by the calculated large negative adsorption energies of this molecule over CB (−1.37 eV) and CN (−1.77 eV). Note that these adsorption energies are in good agreement with those of previous studies [50], suggesting that CN has a quite larger tendency to interact with NO molecule. This can be partly related to the larger number of unpaired electrons over C atom of the latter surface. Moreover, the negative values of ΔHads and ΔGads indicate that the adsorption of NO is an exothermic and spontaneous process at normal condition. Besides, the adsorption energy of NO over CB is smaller than that of over Si-doped h-BN sheets (−1.67 eV) [56], which can be mainly related to the larger positive charge located on the dopant of the latter system. As also indicated in

σ ∝ exp (-Eg/2 kT)

(5)

where.k.is the Boltzman's constant and T is the temperature. Since adsorption of NO leads to a large variation in the Eg value, and hence, electrical conductivity of CB and CN nanosheets is significantly changed in the presence of this molecule. Consequently, it is expected that Cdoped surfaces exhibit high sensitivity toward NO molecule. 3.3. Adsorption of NO2 over C-BNNSs Fig. 2c and d also indicate the relaxed adsorption configurations of NO2 over CB and CN, respectively. Like NO, NO2 molecule is found to be adsorbed over these surfaces via its N atom. The N atom of NO2 attaches to the C atom and the N–O bonds are oriented upwards, which is consistent with those of other related studies [58–62]. The binding distance between the NO2 and C atom is 1.79 and 1.54 Å over CB and CN, respectively. The corresponding adsorption energy is −2.17 and −2.49 eV, demonstrating that NO2 has a stronger interaction with the CN surface than CB. Note that the relatively stronger interaction between NO2 and CN can be related to the greater number of unpaired 129

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Fig. 2. Optimized geometry and corresponding TDOS/PDOS plot of (a) NO/CB, (b) NO/CN, (c) NO2/CB and (c) NO2/CN systems. All bond distances are in Å. In the TDOS and PDOS plots, the dashed line indicates the Fermi level, which set to be zero.

doped h-BN is larger than that for the B-site one. The adsorption of NO2 molecule also induces a locally structural deformation at the adsorption site, where the adsorbing C atom is slightly pulled out of the surface. The latter indicates some changes in the hybridization of the C atom due to the formation of C–N bond. Moreover, the Eads values of NO2 over CB and CN are much larger than that of NO2 over pristine h-BN (−0.02 eV), which clearly indicates that C-doping improves surface reactivity of h-BN toward NO2 (Fig. S2). The calculated negative values of ΔHads and ΔGads also suggest that the adsorption of NO2 over both surfaces is an exothermic reaction and the resulted complexes should be thermodynamically stable in the gas phase at normal temperate and pressure (298 K, 1 atm). The effect of the NO2 adsorption on the electronic structure and TDOS plots of CB and CN is also studied. It is found that the adsorption of NO2 is able to substantially change the Eg value of these systems due to the appearance of some local states around the Fermi level. This indicates that similar to NO, NO2 adsorption is able to effectively change the electronic structure of CB and CN. Hence, according to equation (5), CB and CN can be viewed as highly sensitive materials to sense toxic NO2 molecule.

Table 1 Calculated ZPE-corrected adsorption energy (Eads, eV), change of enthalpy (ΔHads, eV) and Gibbs free energy (ΔGads, eV), net Hirshfeld charge-transfer (qCT, e), band gap (Eg, eV) and change in the band gap (ΔEg, eV) due to the adsorption of NO and NO2 over CB and CN nanosheetsa,b. surface

molecule

Eads

ΔHads

ΔGads

qCT

Eg

ΔEg

CB

NO NO2 NO NO2

−1.37 −2.17 −1.77 −2.49

−1.24 −2.02 −1.58 −2.37

−1.22 −1.99 −1.53 −2.32

−0.08 −0.20 −0.01 −0.12

1.23 3.18 1.51 3.34

0.24 2.19 −2.78 −0.95

CN

Calculated ΔHads and ΔGads values refer to 298 K and 1 atm. The negative qCT value indicates the charge-transfer from CB or CN surface to the adsorbate, while positive qCT corresponds to the charge-transfer from the adsorbate to CB or CN. a

b

electrons located on the C atom of CN (Fig. 1). Moreover, the Hirshfeld analysis reveals that for the adsorption of NO2 over CN, there is a quite larger charge separation between the N atom of NO2 and C atom, which leads to a larger electrostatic attraction between NO2 and the CN surface. This finding is also in good agreement with other theoretical studies [63], in which the adsorption energy of NO2 for the N-site 130

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Table 2 Calculated ZPE-corrected adsorption energy (Eads, eV), net Hirshfeld chargetransfer (qCT, e), band gap (Eg, eV) and change in the band gap (ΔEg, eV) due to the adsorption of CO, CO2, H2O and NH3 over CB and CN nanosheetsa. surface

molecule

Eads

qCT

Eg

ΔEg

CB

CO CO2 H2O NH3 CO CO2 H2O NH3

−0.34 −0.25 −0.23 −0.17 −0.16 −0.18 −0.25 −0.23

0.40 0.38 −0.03 −0.02 −0.96 0.03 0.10 −0.33

1.19 1.17 0.96 0.97 3.73 4.32 4.39 3.96

0.20 0.18 −0.03 −0.02 −0.56 0.03 0.10 −0.33

CN

Table 3 Calculated ZPE-corrected adsorption energy (Eads, eV) of NO and NO2 in the presence of different electric fields. surface

CB CN

molecule

NO NO2 NO NO2

Electric filed (au) +0.01

+0.02

+0.03

+0.04

+0.05

−1.23 −1.92 −1.21 −1.71

−1.10 −1.73 −1.07 −1.52

−1.03 −1.60 −0.96 −1.34

−0.35 −0.82 −0.60 −0.78

−0.12 −0.35 −0.16 −0.37

BNNSs are also given in Table 2. As seen, the Eads values of these molecules are between −0.17 and −0.34 eV, which are much smaller than the corresponding values for the adsorption of NO and NO2 molecules over these surfaces. The adsorption of CO, CO2, H2O and NH3 is also characterized by a negligible qCT value, which is regarded as another proof for the physisorption of these molecules over the title surfaces. Moreover, the small variation in the Eg values suggests that none of these molecules can affect the electronic structure of CB or CN. Consequently, these molecules cannot be detected by CB and CN.

a

The negative qCT value indicates the charge-transfer from CB or CN surface to the adsorbate, while positive qCT corresponds to the charge-transfer from the adsorbate to CB or CN.

3.4. Selectivity In order to verify the selectivity of CB and CN nanosheets toward NOx molecules, we also consider the adsorption of other potential gases (CO, CO2, H2O and NH3) over these surfaces. Fig. S3 of Supporting Information shows the optimized structure of the resulting adsorption configurations. The corresponding adsorption energy, net Hirshfeld atomic charge, band gap and its variation with respect to that of C-

3.5. The moisture effects on the NO and NO2 adsorption Although the C-BNNSs can selectively detect NOx species in the

Fig. 3. Optimized geometry and corresponding TDOS/PDOS plot of (a) NO/CB, (b) NO/CN, (c) NO2/CB and (c) NO2/CN systems in the presence of a water molecule. All bond distances are in Å. In the TDOS and PDOS plots, the dashed line indicates the Fermi level, which set to be zero. 131

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Fig. 4. Representation of the direction of the applied perpendicular electric field and the optimized geometry of (a) NO/CB, (b) NO/CN, (c) NO2/CB and (c) NO2/CN systems in the presence of an electric field of +0.03 au. All bond distances are in Å.

Fig. 5. Variation of C–N binding distance with the magnitude of applied electric field in (a) NO/CB, (b) NO/CN, (c) NO2/CB and (c) NO2/CN systems.

3.6. The effect of an external electric field

presence of H2O, the coadsorption of H2O may affect the adsorption and sensing properties of these molecules. Fig. 3 shows the coadsorbed structures of NO/NO2 and water molecules over CB and CN. Considering the adsorption energies of H2O and NOx molecules as discussed above, it is expected that the active site of CB and CN is dominantly covered by NOx. This is also evident from Fig. 3, where NO and NO2 directly interact with the C atom of surface, while H2O weakly interacts with these molecules. Compared to the optimized structures in Fig. 2, it is found that the addition of the water molecule tends to decrease the binding distances between the NO/NO2 and the C atom of surface. The calculated change in the Eg value of CB (CN) due to the coadsorption of NO/ H2O and NO2/H2O molecules is calculated to be 0.28 (−2.84) and 2.30 (−1.03) eV, respectively. These are larger than the corresponding values obtained in the absence of water molecule, which indicates that the coadsorption of H2O molecule improves the sensing properties of both CB and CN surface due to the formation of a O···H hydrogen-bonding. Such improved sensing properties can be understood in terms of a favorable electrostatic interaction between the negatively charged oxygen atom of NO or NO2 and hydrogen atom of the water molecule. This induces a stronger attraction between the surface and NO or NO2 molecule, and hence a greater change in the Eg value of CB and CN. Thus, it can be concluded that the sensing properties of CB and CN is improved in the presence of an atmospheric water molecule.

The recovery of a sensing material after adsorption of molecules is an important requested property, since strong adsorptions lead to the poisoning and hence deactivation of the active sites of the sensor [64]. Hence, for repetitive applications of a sensing material, such strong adsorptions should be avoided. According to the earlier studies [65–68], the adsorption behavior of h-BN is greatly modified in the presence of an external electric field due to the polarization of charge density over the adsorbed molecule as well as the surface. Hence, the adsorption of NO and NO2 molecules is also studied over CB and CN under different external electric fields using the ground state geometries. To this aim, a perpendicular electric field is applied in the z direction with values between 0.01 and 0.05 au. Table 3 shows the variation of adsorption energies of NO and NO2 molecules with respect to the electric field. The relaxed structures of the adsorption configurations in the presence of an electric field of 0.03 au are indicated in Fig. 4. The variation of C–N bond distance between the C atom of surface and NO or NO2 molecule is also depicted in Fig. 5. For both NO and NO2 molecules, one can see that application of a positive electric field leads to a decrease in the Eads value and hence reactivation of the surface (Table 3). For example, when an external electric field of 0.04 au is applied, the adsorption energy of NO over CB and CN decreases about 72 and 50%, respectively, with respect to the neutral field state. Meanwhile, the corresponding C–N binding distance between the

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surface and NO or NO2 molecule increases by 54 and 62%. To rationalize this finding, we have also analyzed the net charge-transfer values associated with the adsorption of NO and NO2 molecules under electric field. Considering the fact that the charge-transfer due to the adsorption of these molecules occurs in the C-BNNS → NOx direction, the application of a positive electric field tends to avoid the electron transfer from the surface to NOx, and therefore leads to a loss of electron density over this molecule. This decreases the adsorption energy of NOx. In particular, application of an electric field larger than 0.04 au is found to cause desorption of NOx molecules from the CB and CN surfaces. Therefore, application of a positive electric field can serve as a useful strategy to reduce recovery time of CB and CN, and thus to improve the sensing properties of these systems via the weakening of the orbital interaction between the surface and NOx.

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