Sensing properties of Ni-doped boron nitride nanotube to SF₆ decomposed components: A DFT study Open Access

SF₆ is a widely used insulation media in the high-voltage devices of electrical engineering due to its excellent arc-extinguishing performance.^1,2 However, in a long-running device, there may appear various degrees of partial discharge (PD) due to inevitably inherent defects or some new insulation problems. The energy engaged by PD could make SF6 decompose into several low-fluorine sulfides such as SF₄, SF₃, and SF₂.^3,4 These by-products of SF₆ will further react with trace water and oxygen in the insulation devices, forming compounds such as H₂S, SO₂F₂, SOF₂ and SO₂. Previous reports revealed that qualitatively and quantificationally detecting these decomposed components is a workable manner to evaluate the operation status of SF₆ insulation devices, guaranteeing the safe running of the power system.^5,6 Therefore, scholar in the electrical engineering always attempt to explore novel sensing materials for the detection of SF₆ decomposed components in order to realize their research significance and application value. For example, the germanene nanosheet has been theoretical proposed to realize the sensitive detection of H₂S and SO₂,⁷ which gives the motivation to explore novel nanosensors for this purpose largely.

Being predicted theoretically in 1994, boron nitride nanotubes (BNNT) were successfully synthesized for the first time in 1995 by Chopra et al^8,9 Due to its unique mechanical properties, chemical stability and thermal stability, more and more researches are devoted to the development of BNNT for hydrogen storage and environmental monitoring devices.^10,11 For example, recent studies have demonstrated the splendid performance of BNNT upon gas adsorption.^10,12–14 Moreover, surface-doping on the BNNT sidewall can further improve its adsorption property largely, which is similar with those systems of graphene and carbon nanotubes^15–18 in which scholars use transition metals (TM) as the dopants on their surfaces in order to increase their sensitivity to gas molecules. Dopants such as V, Cr, and Mn significantly modulates the electronic behavior and thus enhances the conductivity of BNNT,¹⁹ which can attribute to the introduced impurity state within the bandgap of BNNT after TM-doping improving its electron transport capability accordingly.^20,21 The doping of Ni atom enhances the chemical reactivity of BNNT and thus improves the adsorption capacity for CO molecule.²² Al-doping on BNNT surface greatly increases the adsorption behavior of CO₂ molecules as reported in Ref. 23. In this regard, we assume that TM-doped BNNT may have potential application for detection of SF₆ decomposed gases given its superior adsorption behavior upon gaseous molecules that can lead to notable change in electrical conductivity.

To the best of our knowledge, few experimental or theoretical investigation were reported on the adsorption of SF₆ decomposed gases on the TM-doped BNNT, which stimulates us to given a detailed study in this area to exploit novel sensing material based on density functional theory (DFT). Ni is one of the TM that has strong catalytic behavior for gas interaction and is frequently employed as the impurity atom to enhance the adsorption and sensing behaviors of certain surfaces for gas molecules.^24–29 In this paper, the adsorption behavior of three SF₆ decomposed gases, namely SO₂, SOF₂ and SO₂F₂, on Ni-doped BNNT (Ni-BNNT) have been investigated theoretically, in order to systemically comprehend the adsorbing mechanism of gas molecules on TM-doped BNNT. Understanding of the interactions between TM atom and gas molecules is important since this determines the enhanced mechanism for gas adsorption of BNNT. Our research aims at suggesting novel gas sensor to be employed in the electrical engineering.

The whole calculations were performed by DFT in Dmol³ package.³⁰ The Perdew-Burke-Ernzerhof (PBE) formulation of generalized gradient approximation (GGA) function was chosen to address the electron-electron exchange correlation interaction.³¹ The double numerical plus polarization (DNP) was applied as the basis set, while density functional semi-core pseudopotential (DSSP) method was selected to deal with the relativistic effect of TM atom.³² The Grimme method was chosen to get a better understanding of van der Waals interaction.³³ The energy convergence tolerance, the maximum force and the maximum displacement of geometry optimizations were set to 1.0×10⁻⁵ Ha, 0.002 Ha/Å and 0.005 Å.³⁴ In order to avoid the interaction between adjacent cells, the crystal was built in size of 20 Å × 20 Å × 8.5 Å.³⁵ The global orbital cutoff radius was set at 0.005 Å. The Brillouin zone integration is performed using the Monkhorst-Pack k-point grid with 1 × 1 × 3 and k=3 was set along the tube axis.³⁶ 

The adsorption energy (E_ad) of the combined systems is defined as:

where the E_Ni-BNNT/gas, the E_Ni-BNNT, the E_gas are the energies of the gas/Ni-BNNT hybrid structure, isolated Ni-BNNT and free gas molecule, respectively.

In order to describe the change in electron transport performance of BNNT, charge transfer (Q_t) from gas molecules to Ni-BNNT was calculated using Mulliken charge population analysis. Q_t is defined as the charge change of gas molecules adsorbed on Ni-BNNT. If Q_t<0, it means that electrons transfer from Ni-BNNT to gas molecule and when Q_t>0, the electrons transfer from gas molecule to Ni-BNNT. In addition, the density of states (DOS) was calculated to analyze the interaction mechanism between gas molecules and the Ni-BNNT. A zigzag (8, 0) BNNT is chosen here, and the supercell contains two unicells with 64 atoms (B₃₂N₃₂). Only the most favorable configurations for Ni-doping on BNNT and gas adsorption on Ni-BNNT will be analyzed in the following parts.

The geometric structures of three gas molecules (SO₂, SOF₂ and SO₂F₂) and Ni-BNNT were optimized to their most stable configurations prior to the adsorption processes. Figure 1 shows the structure of SO₂, SOF₂ and SO₂F₂ molecules. In SO₂, the S-O bond has the length of 1.464 Å, and the bond angle equals to 119.646°, showing as a bent molecule. The central S atom in SOF₂ molecule has sp² hybridization and bonds with F atoms and O atom. The S-O bond and S-F bond is 1.447 Å, 1.650 Å, respectively. The bond angle of F-S-O and F-S-F equals to 106.931 Å and 1.650 Å, respectively. The central S atom in SO₂F₂ molecule has sp³ hybridization and bonds with F atoms and O atoms. The S-O bond and S-F bond is 1.596 Å, 1.431 Å, respectively. These results are in good agreement with the previous report.³⁷ 

In order to obtain the most stable structure of Ni atom doped on (8, 0) BNNT, we consider the cases that a Ni atom replaces a B atom (Ni_B-BNNT) and an N atom (Ni_N-BNNT) of BNNT in a super-lattice, respectively. The simulation results show that the Ni_B-BNNT structure has a lower binding energy, in agreement with the previous report.³⁸ Therefore, we chose the Ni_B-BNNT structure for the study of this paper. Figure 2 shows the optimal structures of the intrinsic BNNT and Ni-BNNT. The Ni dopant protrudes from the sidewall of the BNNT along the Z axis after doping. The calculated bond lengths between Ni atom and its three neighboring N atoms are all 1.824 Å, which is increased compared with the original 1.430 Å (B-N).

To further analyze the effect of Ni-doping on the electronic properties of BNNT, the total density of states (TDOS) and projected density of states (PDOS) of intrinsic BNNT and Ni-BNNT have been calculated as seen in Figure 3. It can be seen that in the Ni-BNNT system there appear several novel peaks within the bandgap of pure BNNT, indicating that the conductivity of the system is supposed to be enhanced. In addition, the Ni 3d is strong hybrid with the N 2p near the Fermi level in the PDOS, suggesting the strong binding force between Ni and N atoms. Moreover, according to the band structure, we found that the bandgap of Ni-BNNT is reduced to 0.601 eV after doping from 3.712 eV of intrinsic BNNT. It is worth noting that the PBE function somewhat underestimates the bandgap, and our results here is slightly smaller than the reported one of 4.29 eV.³⁹ Thus, we assume that our results are in good accordance with the previous ones and are reliable. The reduction in bandgap facilitates the transfer of electrons between the valence band and the conduction band, which therefore would enhance the electrical conductivity for the Ni-doped system. And Ni dopant could be regarded as the active center for interaction with the target gas molecules since its strong electron mobility.

For the adsorption of SO₂ molecule on the surface of Ni-BNNT, we designed two different initial adsorption sites around the Ni dopant (as shown in Figure S1) in order to obtain the most stable adsorption structure. The adsorption energies of SO₂ system with different modes are given in Table I. We can see that the initial adsorption structure of M2 has the largest adsorption energy (-0.864 eV), and the negative value indicates that the adsorption process is an exothermic reaction. Under the same conditions, the gas adsorption of this structure has the highest probability of occurrence. It is worth noting that the interaction between SO₂ and the Ni-BNNT is a chemisorption process as its larger adsorption energy more than 0.8 eV.^40,41 In fact, for possible application as a resistance-type gas sensor, chemisorption would be important to yield admirable interaction between sensing nanomaterial and the gas molecules, so that leads to detectable electrical response for gas sensing. Thus, the adsorption energy larger than 0.8 eV should be necessary for exploration of potential gas sensors.⁴² 

Therefore, the most stable structure M2 adsorbed on the surface of Ni-BNNT is shown in Figure 4. We can find that SO₂ is captured by Ni as a dopant atom, wherein the closest distance from the surface to the molecule is Ni-S, which equals to 2.177 Å. The bond length of Ni-N changed from 1.824 Å to 1.910 Å after adsorption. Compared to isolated SO₂ molecule, the angle of O1-S-O2 was increased from 119.646° to 120.704°, indicating that the SO₂ molecule is slightly activated by Ni atom after adsorption. In addition, 0.105 e transfers from the SO₂ molecule to Ni-BNNT according to the Mulliken charge analysis, indicating that the SO₂ molecule acts as an electron donor during the adsorption reaction.

The total electron state density (TDOS), partial electron density state (PDOS) and electron density difference before and after adsorption were calculated to further explore the interaction mechanism between SO₂ and Ni-BNNT surface. Figure 5 shows the TDOS and PDOS of SO₂ molecule absorbed on Ni-BNNT with M2 initial structure. The TDOS at -3.5, -7.5, and -9 eV increased significantly, and a new peak appeared near -11 and -21 eV after the SO₂ adsorption. Since the main contribution in the adsorption process is the outer orbitals of the interacting atom, only the PDOS of S-3p and Ni-3d is discussed. As one can see from the PDOS results, the 3p orbitals of the S atom and the 3d orbitals of the Ni atom overlapped at -11, -7.5 and 1.5 eV, which can illustrate the strong interaction between the SO₂ molecule and Ni-BNNT surface.^43,44

Figure 6 shows the electron density difference of SO₂ adsorbed on Ni-BNNT. The increase and decrease in electron density are indicated by the red and blue areas in the figure. It can be seen that the electron density in the vicinity of S atom in the SO₂ molecule is remarkably lowered, and the electron density in the vicinity of the Ni atom increased, which further confirms that the gas molecule acts as an electron donor and the Ni-BNNT acts as an electron acceptor.

For the adsorption of SOF₂ molecule, three different initial adsorption sites (as shown in Figure S2) were calculated to obtain the most stable adsorption structure. Table I shows the adsorption energy of SOF₂ adsorbed on the Ni-BNNT with different initial position. We can see that the interaction between the SOF₂ molecule and Ni-BNNT surface is weaker than that of SO₂. The SOF₂ adsorbed on the surface of Ni-BNNT with the initial structure of M3 has the highest adsorption energy (-0.522 eV).

Figure 7 shows the optimized structure of SOF₂ molecule absorbed on Ni-BNNT with M3 initial structure. The SOF₂ molecule was captured by Ni-BNNT system with an O atom closest to the surface, and the distance between Ni atom and O atom is 2.07 Å. The bond length of S-O is slightly stretched from 1.447 Å to 1.478 Å after SOF₂ adsorption, demonstrating the slightly activation of SOF₂ molecule. The Ni-N and S-F bonds experience small changes to varying degrees after adsorption. All of these are well matched to the small adsorption energy of -0.522 eV in this system.

The TDOS and PDOS of SOF₂ system in Figure 8 show the electronic behavior of Ni-BNNT upon SOF₂ adsorption clearly. The TDOS at -14, -9.5, -6.5, -5.5, and 2 eV increased after adsorption, and based on the PDOS results, one can see that the increase of the above-mentioned density peaks generated by the SOF₂ molecule. Similarly, since the adsorption process is mainly contributed by the outermost orbitals of the atoms, only the PDOS of Ni-3d and O-2p are discussed, as shown in Figure 8b. The overlap of electron density states means hybridization between atomic orbitals. The 2p orbitals of the S atom and the 3d orbitals of the Ni atom overlapped in the range from -7.5 eV to -3 eV, and the overlapped peaks of these two orbitals appear at around -14, -9.5 and -6 eV, suggesting the adsorption of SOF₂ gas molecule on the surface of Ni-BNNT. Figure 9 shows the electron density difference of SOF₂ adsorbed on Ni-BNNT. It can be found that the SOF₂ molecule acts as electron donor, transferring 0.078 e towards the Ni-BNNT. In addition, the Ni atom acts as an active center carrying 0.105 e after adsorption, revealing that it extracted electrons from both the BNNT and the gas molecule, which demonstrates the strong electron accepting property of TM.^45,46

In this part, three different initial adsorption sites (as shown in Figure S3) were calculated to obtain the most stable SO₂F₂ adsorption structure. The adsorption energy of SO₂F₂ adsorbed on the Ni-BNNT with different initial positions was shown in Table I. We can see that the interaction between the SO₂F₂ molecule and Ni-BNNT surface is the weakest among SO₂, SOF₂, and SO₂F₂. The adsorption energy of SO₂F₂ adsorbed on Ni-BNNT with M2 initial structure is the highest (-0.223 eV).

Figure 10 shows the optimized structure of the SO₂F₂ molecule absorbed on Ni-BNNT with M2 initial position. The closest distance between SO₂F₂ molecule and Ni-BNNT surface is Ni-O and the length is 2.307 Å. We also observed the bond length of the SO₂F₂ molecule, and the distance between Ni atom and N, B atoms experience little changes after adsorption. During the interaction, the SO₂F₂ molecule acts as an electron acceptor and Ni-BNNT acts as an electron donor. The number of electrons transferring from the Ni-BNNT to the SO₂F₂ molecule is 0.035 e according to the Mulliken analysis. Through comparison, we could find that the amount of charge transfer between the Ni-BNNT and gas molecules is in sequence as SO₂ > SOF₂ > SO₂F₂, which is in agreement with the adsorption energy, indicating the strongest adsorption performance of Ni-BNNT upon SO₂, followed by SOF₂ and the last one of SO₂F₂.

Figure 11 shows the TDOS and PDOS of SO₂F₂ molecule absorbed on Ni-BNNT with M2 initial structure. The TDOS of the system increased at -14, -12, -9, -6 and +2 eV after adsorption, and based on the PDOS results, we can see that the above-mentioned density peaks are derived from the SO₂F₂ molecule. The 2p orbital of the O atom only has a quite small overlap with the 3d orbital of Ni atom at around -6 eV, confirming the weak interaction between SO₂F₂ molecule and Ni-BNNT. In addition, we also studied the DOS of SO₂F₂ molecules before and after adsorption (as shown in Figure 11 (c–d)). One can see that the DOS of the SO₂F₂ molecule hardly changes after adsorption. Through the electron density difference shown in Figure 12, we can also find that the O atom nearest to the Ni atom has pretty weak interaction with the Ni-BNNT and there is no significant charge transfer between them during gas adsorption.

To explore the potential application of a gas sensor, the thorough investigation of the adsorption and desorption behavior of the Ni-BNNT are essential. From the above calculations, we can see that the closest distances between the gas molecules and the surface of Ni-BNNT are 2.177 Å (S-Ni), 2.07 Å (O-Ni), 2.307 Å (O-Ni), respectively. And they are larger than the sum of corresponding covalent radii (R), calculated to be 1.976 Å (S-Ni), 1.637 Å (O-Ni).⁴⁷ This shows that the Ni-BNNT does not have a strong binding force with the trapped atoms of the adsorbed molecules. It is therefore predictable that these gas molecules can be desorbed from the surface of our proposed material through heating process.^48,34

The recovery time (τ) is a parameter to evaluate the time costed for a gas molecule desorption from the sensing material’s surface. In order to evaluate the recovery properties of the sensing material, we calculated the recovery time according to the transition state theory and Van’t-Hoff-Arrhenius expression:⁴⁹ 

where A represents the apparent frequency factor which we determine as 10¹² s^-1,⁵⁰ T means the working temperature, and B means Boltzmann constant. Since desorption is the inverse process of adsorption, the value of E_ad is assumed to be equal to the energy barrier (E_a) of the desorption. Three temperatures including 298 K, 348 K and 398 K were considered to get a full understanding of the desorption property of Ni-BNNT, with desorption situation plotted in Figure 13.

The large E_ad undoubtedly reflects the high energy barrier during desorption, so we can see that the SO₂ system has a long recovery time of 400 s at 298 K. In contrast, the other two gases are easily desorbed from the Ni-BNNT surface at 298 K, within 669 μs for SOF₂, 5.9 ns for SO₂F₂, respectively. With the increase of working temperature, the desorption time of the three gases on Ni-BNNT was significantly reduced, especially for the SO₂ desorption, with the desorption time of 3 s when the temperature was raised to 348 K. In most cause, the recovery time should be around several to several hundred seconds which would be acceptable for sensing application.^51,52 Accompanied with this, we believe that Ni-BNNT can be a satisfactory SO₂ gas sensing material given its stable adsorption of SO₂ molecules at ambient temperature and short recovery time at a relatively low heating temperature, which allows this material to be cyclically used quickly. However, the extremely short recovery time of the SO₂F₂ desorption system at 298 K demonstrates that Ni-BNNT is not suitable for the detecting of SO₂F₂.

Resistivity-type sensors reflect the gas-sensitive response of the sensing material by detecting changes in conductivity. Therefore, the electrical conductivity is also an important factor to consider in resistivity-type sensor applications. In the above analysis, it is found that the Ni-BNNT is not suitable for the detection of SO₂F₂ gas due to the fast desorption rate. Therefore, we only analyze the electrical conductivity of SO₂ and SOF₂ here. We evaluated the conductivity of the proposed sensing material Ni-BNNT before and after adsorbing gas molecules as follows:⁵³ 

where E_g, B and T is band gap of the proposed material, Boltzmann constant and working temperature, respectively. From the formula we can clearly see that the smaller the band gap, the higher the conductivity at a specific temperature.

The calculated E_g for Ni-BNNT and two gas adsorption systems are shown in Figure 14. The E_g trends to rise in both SO₂ and SOF₂ systems, which indicates that the conductivity of Ni-BNNT decreases after adsorption of corresponding gas molecules. Therefore, the adsorption of the gas molecules can be detected by the changes in conductivity. Unfortunately, the difference between the increase of E_g in the SO₂ and SOF₂ systems is small, making it impossible to guarantee their selectivity. In spite of this, the Ni-BNNT could evaluate the working operation of the SF₆ insulation devices through the sensitive detection of SO₂ considering the consistent conductivity trend caused by gas interaction.

In this work, the theoretical calculations were applied to investigate the adsorption performance of Ni-doped (8, 0) zigzag BNNT toward SF₆ decomposition gas: SO₂, SOF₂, SO₂F₂. The desorption property and response mechanism of Ni-BNNT upon the gases were also studied. The main conclusions we got are as follows:

1. When doping by substitution, Ni atom is likely to adsorb onto the BNNT surface with replacing the B atom than N atom.

2. Ni-BNNT can be used as a satisfactory material for SO₂ sensing given its good adsorption and desorption performance at specific condition. However, due to weak interaction and extremely short recovery time, it is not suitable for detecting SO₂F₂.

3. Ni-BNNT has great potential for evaluating the working operation of SF₆ insulation devices through the sensitive detection of SO₂ considering the consistent conductivity trend caused by gas interaction.

See supplementary material for the initial configurations for gas molecules adsorption on Ni-BNNT.

We gratefully acknowledge the financial support from National Natural Science Foundation of P. R. China (Project No. 51777144).

The authors declare no conflict of interest.