SF6/N2 gas mixture decomposition components can reflect the operation status inside GIS, and be used for fault diagnosis and monitoring inside GIS. NF3 and N2O are the characteristic decomposition components of SF6/N2 mixed gas. In order to find a potential gas sensitivity material for the detection of NF3 and N2O. This paper investigated the adsorption properties of NF3 and N2O on Al- and Ga- doped graphene monolayers based on density functional theory. Through the analysis of adsorption distance, charge transfer, adsorption energy, energy band structure, etc., the results indicated that the adsorption effect of Al- and Ga-doped graphene to NF3 and N2O are probably good, and these nanomaterials are potential to apply for the monitoring of GIS internal faults.

1. Introduction

SF6 has been extensively adopted for high-voltage gas insulation devices due to its superb arc extinguishing and insulation abilities. However, it has a strong greenhouse effect [13]. Concurrently, the addition of N2 can greatly reduce the use of SF6 gas without significantly affecting the insulation performance, which is of great importance in achieving energy conservation and emission reduction [47]. However, SF6/N2 mixed gas under partial discharge (PD) or partial overheating conditions will produce gases such as NF3, SO2, CO2, and N2O [8]. It is a feasible technical method to comprehend the fault diagnosis of SF6/N2 mixed gas insulation equipment by monitoring this characteristic component decomposition information.

As the research hotspot of gas sensing materials in the sensor field, graphene has become an irreplaceable material with its ultrahigh electron mobility and specific surface area (SSA), along with superb mechanical characteristics [912]. Compared to other sensing materials, the following documents are available. According to He et al. C2H2 gas was analyzed for its sensing properties as well as electronic characteristics on diverse boron nitride nanotubes-modified transition metal oxide (Fe2O3, TiO2, and NiO) nanoparticles [13]. It was found that the conductivity of C2H2 gas on the three transition metal oxides and modified boron nitride nanotubes was different, especially on Fe2O3 and TiO2. In his studies of the gas sensing properties of single-walled carbon nanotubes doped with gold atoms for SO2 and H2S. Chen et al. found that SO2 and H2S have good adsorption properties on gold doped single-walled carbon nanotubes [14]. Syaahiran et al. implemented the DFT method to analyze CO, H2S, and H2 for their gas sensing performances on (WO3) () doped Cr [15]. The results indicate that when compared with undoped clusters (WO3) (), the energy gap of chromium-doped tungsten oxide clusters decreases () (), the reactivity increases and the stability is improved. Syaahiran et al. studied the interaction of CO gas on chromium doped-tungsten oxide/graphene composites [16]. From the energy gap, surface activity, and binding energy, it is found that chromium-doped tungsten oxide/graphene composite has a strong adsorption effect on CO.

Many scholars have studied the gas sensors of atom doped graphene, such as Mn, Pd, and Pt, to better study the interaction between graphene materials and gas molecules and to explore the gas-sensing characteristics of gases onto doped and intrinsic graphene surfaces. Gui et al. through DFT calculation, studied adsorption properties for typical oil soluble gases (C2H2, CH4, and CO) in transformers by doping Mn atoms at graphene bridge sites [17]. The gas sensing mechanism was analyzed by using the density of states (DOS) and molecular orbital theory. As a result, manganese-doped graphene was the potential gas-sensing substance in the detection of CO and C2H2. According to the literature, gas adsorption is more evident due to the doping of transition metals [18]. It is confirmed that doped graphene shows better gas sensing performance than intrinsic graphene, and metal doping remarkably enhances graphene chemical activity and adsorption performance.

According to above-mentioned, this paper studies the gas sensitivity of Al-doped Gra and Ga-doped Gra to NF3 and N2O gas molecules based on DFT. This work will guide the manufacturing of gas sensors and provide basic gas sensitivity information, as well as aluminum-doped graphene or gallium-doped graphene as a potential candidate for resistance chemical sensors for GIS internal fault diagnosis.

2. Computational Details

The present work conducted first-principle calculation by Dmol3 quantum chemistry module from Materials Studio [1921]. Perdew Burke ernzerho (PBE) function of the generalized gradient approximation (GGA) is used for managing the electron exchange relation [22]. Besides, double numerical plus polarization (DNP) is selected to be the atomic orbital basis group. The maximum atomic displacement, energy convergence accuracy, orbital tailing, and maximum force are set to 5 × 10-3 Å, 1.0 × 10-5 Ha, 0.005Ha, and 0.05 eV/Å, respectively [23, 24]. Also, to ensure precision in calculating total energy, this work established global orbital cut-off radius and self-consistent field error at 4.5 Å and 1.0x10-6Ha, respectively, Additively, a 2 × 2 × 1 Brillouin -point grid space was set [25, 26]. Dispersion force was controlled by using DFT-D (Grimme) approach, and the charge transfer amount () in the adsorption process was determined based on the Hirshfield approach [16, 27]. The total charge number , represents the transfer of electrons to the doped graphene surface from gas molecules, and the negative value stands for the opposite electron transfer path. Further, the definition of adsorption energy () is shown in [28]:

, , represent energy of gas adsorption on metal doped graphene, the energy of an Al-doped graphene and Ga-doped graphene surface, and Energy of gas, respectively. Generally, > 0 means that the adsorption process does not occur automatically, while < 0 means that the adsorption process is automatic [29].

3. Results and Discussion

3.1. The Optimization of Al-Doped Graphene, Ga-Graphene, NF3, and N2O

Firstly, the adsorption characteristics of the aluminum and gallium atoms onto the graphene surface are discussed through the formation energy (). Aluminum and gallium atoms moving onto the graphene surface is defined in [30]:

, energy after doping graphene into aluminum and gallium; and are the initial energies of aluminum, gallium, and graphene substrates, respectively. We considered the doping configurations of the top, bridge, heart, and replacement sites. By comparing the formation energies of different doping structures in Table 1, we found that the configuration of the replacement site is the most stable.

Figure 1 presents optimal geometry for aluminum-doped graphene, gallium-doped graphene, NF3, and N2O, with bond angle and bond length being expressed as and , respectively. According to Figure 1(a), the aluminum-doped graphene surface is composed of (4 × 4 × 1) supercells with a 20 Å() vacuum layer for reducing the interaction of neighboring clusters and for preventing interactions between planes resulting from periodic boundary conditions [31]. The optimal top view for aluminum-doped graphene exhibit in Figure 1(b). The bond length between the Al and the surrounding three carbon atoms is 1.746 Å, which is increased by 0.321 Å compared with the carbon-carbon before doping, since aluminum has a larger atomic orbital radius than carbon. The ∠C1-Al-C2 is 120°, which remained unchanged from that before doping. The optimal top view for gallium-doped graphene is shown in Figure 1(c). Optimal N2O and NF3 gas molecules structures are shown in Figures 1(d) and 1(e).

Figure 2 shows the TDOS for graphene, Al- and Ga-doped graphene. The structural properties of doped-graphene are further analyzed through TDOS. Contrast undoped graphene, it is known that the charge distribution of TDOS increases remarkably near the Fermi level after doping aluminum and gallium relative to intrinsic graphene, suggesting that aluminum and gallium doping enhances graphene structure conductivity.

The band structure of intrinsic graphene is shown in Figure 3(a). It can be seen from the figure that the valence band and conduction band are almost tangent at the Fermi energy level, and the band gap is 0.005 eV, approaching 0 eV. However, after doping aluminum atoms, as shown in Figure 3(b), the valence band moves up and intersects with the . After doping, the band gap of graphene has obviously increased, which is 0.227 eV. At the same time, a new energy level has been introduced near the , indicating that the electrical and physical properties of graphene have changed significantly due to the doping of aluminum atoms as shown in Figure 3(c). Doped gallium atoms have similar properties. The influence of aluminum-doped graphene and gallium-doped graphene on gas adsorption characteristics needs to be further studied.

3.2. The Adsorption Properties of NF3 on Aluminum-Doped Graphene

Different original approach sites for NF3 on aluminum doped sites were calculated for obtaining adsorption structure with the highest stability to further analyze the adsorption characteristics of aluminum-doped graphene on target gas. A characteristic adsorption structure is obtained following optimization. Figure 4 shows its top view and side view. Table 2 displays the charge transfer and adsorption energy, together with structural parameters.

As observed from the adsorption structure in (Figures 4(a) and 4(b)). The bond length formed by A1-F1 is 1.689 Å, and the number of electrons transferred from aluminum-doped graphene surface onto NF3 reaches 0.291e. Noteworthily, the NF3 structure alters the following adsorption, among them, the F1-N bond length increases to 3.072 Å, and the angle of F1-N-F2 becomes 75.591°. The of NF3 onto aluminum-doped graphene surface reaches -1.476 eV. From the perspective of electron transfer and , the reaction of NF3 gas is strongest when F atom is close to the aluminum-doped graphene surface.

Figure 5(a) shows the TDOS for NF3 on aluminum-doped graphene surface. The TDOS showed significant changes at -14 eV, -9.2 eV, -7.3 eV, -5.4 eV, -3.4 eV, and other positions when compared with the nonadsorbed gas. Due to the outermost electrons of atoms contributing the most during adsorption, just PDOS for Al-3p, F-2p, and N-2p is discussed. From the PDOS in Figure 5(b), we can see that for the above orbitals, their overlapped peaks can be seen at approximately, -9.3 eV, -7.3 eV, -3.9 eV, Fermi level, and 5.2 eV. PDOS and TDOS analyses implicit that there are great interaction between NF3 and aluminum-doped graphene.

Figure 6 displays the difference in electron density of NF3 adsorbed onto aluminum-doped graphene surfaces from different sides, in which the red and blue areas indicate elevated and declined electron density, respectively. The post-gas adsorption charge distribution is analyzed intuitively based on the difference in electron density. From Figure 6, it shows that the red area around the F atom shows that electrons are received, while the blue area around the N atom, Al atom, and C atom indicates that electrons are lost due to the reduction in electron density. Gas molecules are suggested to play the role of electron acceptors, while Al-Graphene is the electron donors. Therefore, NF3 molecules bring drastic changes in electron density to the surface of aluminum-doped graphene.

Collectively, based on the structural parameters of DOS, adsorption energy, and charge transfer, together with the difference in NF3 electron density adsorbed on aluminum-doped graphene, it is obvious that the interaction between NF3 and aluminum-doped graphene is very severe.

The band structure of aluminum-doped graphene without adsorbed gas exhibit in Figure 7(a). Figure 7(b) exhibit the band structure of NF3 molecules adsorbed on aluminum-doped graphene surface. From above, it is known that the band structure of aluminum-doped graphene has an obvious band gap near the , and a new energy level is introduced at the . Moreover, a lot of new energy levels have been added in the valence band. It can be determined that the generation of the new energy level is due to the adsorption of gas on the surface.

3.3. The Adsorption Properties of N2O on Aluminum-Doped Graphene

Gas molecules approach the aluminum-doped graphene surface with different atoms for the adsorption of N2O. 3 characteristic adsorption structures are acquired following geometric optimization, according to Figure 8, the parameters of the above configurations are exhibited in Table 3. Figures 8(a) and 8(b) exhibits top and side view for M1 configuration. N2O is close to the doped surface with an N1 atom, and the is 2.035 Å. The N1-N2 bond length of N2O adsorbed on the surface reaches 1.142 Å, with the 1.180 Å N2-O bond length. The bond angle does not change, which is not different from the free N2O molecule. Therefore, the N2O structure shows low alterations in adsorption. In M1 configuration, adsorption energy is -1.376 eV, with the transfer of 0.238 e onto aluminum-doped graphene surface from the gas molecule, indicating the strong interaction between N2O and aluminum-doped graphene surface.

Figures 8(c) and 8(d) exhibits side and top view for M2 configuration. N2O is close to the doped surface with an N2 atom, and the is 2.080 Å. Based on structural parameters observed in Table 3, the N2O structure has low alterations before and after adsorption. M2 configuration has adsorption energy of -1.407 eV, which indicates that the M2 configuration is more stable than M1. In addition, the transfer of 0.212e onto aluminum doped graphene surface from gas molecules is detected in the M2 configuration, with the charges transferred from N1, N2, and O atom being 0.247 e, -0.004 e, and 0.031 e, respectively.

The side and top views for the M3 configuration are displayed in Figures 8(e) and 8(f). N2O is close to the doped surface with the O atom, and the is 2.157 Å. The N1-N2 bond length of N2O adsorbed onto aluminum-doped graphene surface reaches 1.214 Å, which is slightly longer than the N1-N2 bond (1.141 Å) of free N2O molecules. The charge transfer in the M3 configuration is 0.204 e, which significantly increases relative to M1/M2 configuration. However, according to parameters observed in Table 3, the bond angle and bond length changed a little after adsorption, and the adsorption energy was -1.240 eV, which was slightly decreased when compared with the M2 configurations. In conclusion, this was based on great adsorption energy between N2O and M2 configurations, and therefore, the M2 system may exhibit the highest stability. For better verification, this work also analyzed the difference in electron density and DOS.

TDOS for M2 configuration is exhibited in Figure 9(a). When N2O molecules are adsorbed onto an aluminum-doped graphene surface, there are obvious changes around -12.07 eV, -11.01 eV, -5.71 eV, and 0.89 eV. Due to the outermost electrons of atoms contributing the most to adsorption, this work only analyzes PDOS for Al-3p, N-2p, and O-2p. From PDOS observed in Figure 9(b), N-2p together with O-2p orbital peaks coincide around -12.15 eV, -10.97 eV, -10.25 eV, -5.62 eV, and 0.86 eV. According to PDOS, potent chemisorption of N2O with aluminum-doped graphene was seen. At the same time, considering that the N-2p orbital contributes the most to the adsorption process, the N2O adsorption structure shows extremely high stability within the M2 configuration.

The difference in electron density of M2 configuration is exhibited in Figure 10. Red and blue areas stand for elevated and declined electron densities, respectively. O and N1 receive charge in the adsorption process, and the charge close to the N2 atom declines. There is also an increase in electron density on the aluminum-doped graphene surface. According to the distribution on electron density, N2O gets electrons.

The band structure of aluminum-doped graphene without adsorbed gas exhibit in Figure 11(a). Figure 11(b) shows the band structure of N2O molecules adsorbed on aluminum-doped graphene surface. It can be seen that after adsorbing N2O, the energy gap of aluminum doped graphene decreases, the conduction band near the Fermi level becomes more smooth, and introduced new energy levels. Accordingly, the density of states near the increases greatly.

3.4. Prediction of Desorption of Aluminum-Doped Graphene

The decomposed components of SF6/N2 mixed gas can be desorbed from the sensing material surface under heating conditions. The and distance of NF3 and N2O on aluminum-doped graphene surface are exhibited in Tables 2 and 3, 375 K, 575 K, and 757 K are the temperature gradient of the desorption time of the sensing material. The desorption time is related to the adsorption energy and temperature, as shown in [32, 33]:

In which, is the trial frequency of the system, generally 1012 s-1 [34], is the adsorption energy of NF3 and N2O on aluminum-doped graphene surface, : a constant, eV/K, is the temperature (unit: K).

The desorption time of NF3 and N2O gas molecules at 375 K, 575 K, and 775 K is shown in Figure 12. The optimal desorption time of NF3 molecule at 775 K is , and the desorption time of the whole system increases with the decrease of temperature, and the maximum recovery time is . The desorption time of N2O molecule in the system is shorter than that of NF3 molecule. At 375 K and 775 K, the recovery time is and , respectively.

3.5. Adsorption of NF3 and N2O Gases on Gallium-Doped Graphene

Since the number of outermost electrons of Ga atom is the same as that of Al atom, the doping of Ga atom is also considered. Its adsorption mode is the same as that of Al doped graphene, so only consider that NF3 molecules are close to Ga-doped graphene as F atoms, and N2O molecules are close to Ga-doped graphene as N atoms. As shown in Figures 13(a) and 13(b), NF3 molecules are adsorbed on Ga-doped graphene surface, the is 1.801 Å, and the electron transformation is 0.287 e. As shown in Figures 13(c) and 13(d), N2O is adsorbed on Ga-doped graphene surface, the is 2.285 Å, and the electron transformation is 0.227 e. It can be seen from Table 4 that the adsorption energy of both gases is less than that on Al-doped graphene surface. The adsorption parameters show that there is a good reaction between NF3 and N2O molecules on Ga-doped graphene surface.

As shown in Figures 14(a)14(d), it is the TDOS and PDOS of NF3 and N2O on Ga-doped graphene surface. For NF3 molecule, a new peak appears at the of PDOS, which is contributed by NF3 molecule, where the 2p orbital of N atom, the 2p orbital of F atom, and the 3p orbital of Ga atom are coupled. For N2O molecule, it can be seen from the PDOS diagram that there is no new peak at the , and a fresh peak occur in the conduction band region, and it hybridizes with the 3p orbital of Ga atom, indicating that N2O molecule has a strong interaction with Ga-doped graphene.

Figures 15(a) and 15(b) shows the charge difference density of NF3 and N2O on Ga-doped graphene, respectively. It can be seen that after the adsorption of NF3 and N2O, the color near F atom turns red, indicating that the charge increases, and the color near N atom turns blue, indicating that the charge decreases. The charge near N1 atom and O atom in N2O molecule increases, and the electron concentration near N2 atom and graphene surface decreases, indicating that the gas obtains charge.

4. Conclusions

The adsorption characteristics of NF3 and N2O molecules on Al- and Ga-doped graphene surfaces were studied in this paper, for the sake of finding the resistance chemical sensor material for GIS internal fault diagnosis. Among them, the most likely adsorption mode of NF3 molecule is that F atom is close to Al-doped graphene. They have large charge transfer and , short , and strong interaction. The most likely adsorption mode of N2O molecules on Al-doped graphene is M2 mode, which is close to the N atom. From the adsorption parameters obtained, it can be seen that there is also a very strong interaction between N2O molecules and Al-doped graphene. As Ga atom and Al atom belongs to the same family of elements, the simulation of Ga atom is also carried out. The results show that the adsorption parameters of Ga-doped graphene and Al-doped graphene are similar, and Ga-doped graphene may also be a sensing material.

The present work sheds more light on the association of characteristic decomposition components of SF6/N2 gas mixture and Al- and Ga-doped graphene, and provides the theoretical foundation to adsorb characteristic decomposition components in SF6/N2 gas mixture onto Al- and Ga-doped graphene.

Conflicts of Interest

The authors declare that they have no conflicts of interest.


The authors would like to acknowledge the support of Science and Technology Research Program of Chongqing Municipal Education Commission (Grant No. KJZD-K202001505, KJQN202001503) and the Graduate Science and Technology Innovation Project of Chongqing University of Science and Technology (Grant No. YKJCX2120404).