First-Principles Study of Au-Doped InN Monolayer as Adsorbent and Gas Sensing Material for SF6 Decomposed Species

As an insulating medium, sulfur hexafluoride (SF6) is extensively applied to electrical insulation equipment to ensure its normal operation. However, both partial discharge and overheating may cause SF6 to decompose, and then the insulation strength of electrical equipment will be reduced. The adsorption properties and sensing mechanisms of four SF6 decomposed components (HF, SO2, SOF2 and SO2F2) upon an Au-modified InN (Au-InN) monolayer were studied in this work based on first-principles theory. Meanwhile, the adsorption energy (Ead), charge transfer (QT), deformation charge density (DCD), density of states (DOS), frontier molecular orbital and recovery property were calculated. It can be observed that the structures of the SO2, SOF2 and SO2F2 molecules changed significantly after being adsorbed. Meanwhile, the Ead and QT of these three adsorption systems are relatively large, while that of the HF adsorption system is the opposite. These phenomena indicate that Au-InN monolayer has strong adsorption capacity for SO2, SOF2 and SO2F2, and the adsorption can be identified as chemisorption. In addition, through the analysis of frontier molecular orbital, it is found that the conductivity of Au-InN changed significantly after adsorbing SO2, SOF2 and SO2F2. Combined with the analysis of the recovery properties, since the recovery time of SO2 and SO2F2 removal from Au-InN monolayer is still very long at 418 K, Au-InN is more suitable as a scavenger for these two gases rather than as a gas sensor. Since the recovery time of the SOF2 adsorption system is short at 418 K, and the conductivity of the system before and after adsorption changes significantly, Au-InN is an ideal SOF2 gas-sensing material. These results show that Au-InN has broad application prospects as an SO2, SOF2 and SO2F2 scavenger and as a resistive SOF2 sensor, which is of extraordinary meaning to ensure the safe operation of power systems. Our calculations can offer a theoretical basis for further exploration of gas adsorbent and resistive sensors prepared by Au-InN.


Introduction
Nowadays, SF 6 is extensively applied to gas-insulated switchgears (GIS) because of its excellent insulation and arc extinguishing properties [1][2][3][4]. In cases where the high-voltage insulation equipment has been working for a long time, partial overheating and discharge is very likely to occur, leading to the decomposition of SF 6 . After the decomposition of SF 6 , gases such as HF, SO 2 , SOF 2 and SO 2 F 2 will be generated [5][6][7], which will cause aging and even damage to high-voltage insulation equipment [8][9][10]. Previous reports have shown that it is possible to judge whether electrical equipment is malfunctioning through detecting the type and content of the SF 6 decomposed products in GIS. Thus, finding an excellent gas sensor and adsorbent for monitoring and adsorbing the SF 6 decomposed products in GIS is of great significance to ensure the normal operation of power systems. Two-dimensional (2D) materials have been broadly applied in electronics, fieldeffect devices, supercapacitors, sensing materials, energy storage and other fields [11][12][13][14][15]. Graphene is a typical representative of 2D materials, but in some cases, the zero band gap property of it will restrict its application [16,17]. Thus, researchers are exploring other materials with graphenelike structures and appropriate band gaps. Among these novel graphenelike materials, AlN and InN nanosheets, which belong to group III-V nitrides, have drawn extensive attention due to their excellent semiconducting property [18][19][20][21]. In particular, the high carrier mobility and large specific surface area of InN mean that it has great potential to be applied in the field of gas adsorption and sensing [22][23][24]. A large number of reports have demonstrated that transition metal (TM) modified semiconductor materials have superior sensing performance and strong gas adsorption ability [25][26][27]. Guo [28] et al. found that TM (Pd, Pt, Ag, Au) modified InN was a promising candidate material for detecting NO 2 gas. Wang [29] et al. found that TM (Ag, Au) doped MoS 2 was considered to be a promising H 2 O molecule adsorbent. Therefore, the doping of Au atom may enhance the adsorption capacity of InN to SF 6 decomposed products. Au is a precious metal, but so far few studies have analyzed the effects of precious metal doping on the adsorption ability of InN toward SF 6 decomposed species.
The adsorption and electronic behaviors of Au modified InN monolayer toward four SF 6 decomposed products have been studied based on first-principles theory in this work. The E ad (adsorption energy), Q T (charge transfer), DCD (deformation charge density), DOS (density of states), frontier molecular orbital and recovery property were calculated so as to obtain the adsorption properties and sensing mechanism of the Au-InN monolayer for four SF 6 decomposed products. The results suggest that Au-InN has strong adsorption capacity for three SF 6 decomposed species except HF. Combining the frontier molecular orbital and recovery property analysis can lead to the conclusion that the gas adsorbent and resistive sensor prepared by Au-InN has great potential for adsorbing SO 2 , SO 2 F 2 and sensing SOF 2 .

Computation Methods
All the theoretical calculations based on density functional theory (DFT) were carried out in Dmol 3 package in this study [30]. To handle the electron exchange-correlation terms, the Perdew-Burke-Ernzerhof (PBE) function within generalized gradient approximation (GGA) method was chosen [31]. The DFT-D method was employed for the further understanding of van der Waals forces and long-range interactions [32,33]. Besides, double numerical polarization (DNP) was adopted for calculation while the DFT semicore pseudopotential (DSSP) method was applied to handle core electron relativity effects [34]. In terms of the setup of Monkhorst-Pack k-point mesh, 5 × 5 × 1 was set for geometric optimization and 10 × 10 × 1 for the calculation of static electronic structure [35]. The energy tolerance accuracy, maximum force and displacement were severally set as 10 −5 Ha, 0.002 Ha/Å, and 0.005 Å [36,37].
A 4 × 4 InN supercell with 16 In atoms and 16 N atoms was established. In order to avoid adjacent supercell interaction, the vacuum region of InN supercell was set as 15 Å [38]. The lattice constant of the fully optimized InN monolayer is calculated to be 3.62 Å, which is consistent with a previous report (3.63 Å [39]).
Through calculating the value of E ad , the interaction strength between substrate material and gas molecules can be roughly obtained. Thus, the most stable adsorption configuration can be found by comparing the value of E ad . The calculation formula of E ad is as follows [40]: In the above formula, E Au−InN/gas denotes the energy of the gas adsorption system, while E Au−InN and E gas denote the energies of Au-InN monolayer and isolated gas molecule, respectively. Besides, the charge transfer (Q T ) during the doping and adsorption process is analyzed through the Hirshfeld method [41]. A positive value of Q T implies that the analyte acts as an electron donator, and conversely implies that the analyte acts as an electron acceptor [42].

Results and Discussion
3.1. Isolated HF, SO 2 , SOF 2 , SO 2 F 2 Molecules and Au-InN Monolayer The geometrical configurations of four optimized SF 6 decomposed products-HF, SO 2 , SOF 2 and SO 2 F 2 -are shown in Figure 1. Meanwhile, the geometrical parameters of four optimized gas configurations are displayed in Table 1, which are basically consistent with previous reports [36,43,44]. Table 2 lists the single atom charges of gas molecules in the gas phase.
In order to obtain the most stable doping configuration for subsequent analysis, four possible doping sites of Au atom are considered [28]. The Au atom could be doped not only directly above the N atom (T N ) or In atom (T In ) in the InN monolayer but also right above the In-N bond (T B ) or at the hollow center of the six-membered ring of the InN monolayer (T H ). Afterwards, so as to measure the stability of each optimized doping system, the binding energy (E b ) was calculated. The E b of each doping system is calculated as follows: In the above formula, E Au−InN represents the energy of the Au-InN monolayer, while E InN and E Au represent the energies of the pure InN monolayer and Au atom, respectively. The negative E b values of the four doping systems in this study indicate that they all emit heat during their establishment. The doping system is the most stable when the Au atom is doped to the T N site, as displayed in Figure 2b, because of the absolute value of its E b is the largest (E b = −1.61 eV). In this doping system, the length of the In-N bond increases from 2.091 Å to 2.172 Å, which shows the strong interaction between the InN monolayer and Au dopant.
From the DCD shown in Figure 2b, the area where the charge density increases is displayed in red, otherwise it is displayed in blue. In the doping system, the positive charge of the Au atom (0.116 e) implies that Au dopant provides electrons to the InN monolayer. It can be found from DCD that the electron accumulation region is principally confined to In atoms while the consumption region is principally confined to N atoms. The relatively large charge transfer and structural deformation suggest the strong interaction between Au dopant and InN monolayer; in other words, the doping structure is very stable. Meanwhile, DOS was considered so as to have a better understanding about the electronic behavior of the doping system. As can be observed from Figure 3a, the total DOS (TDOS) of the doping system shifts to the left compared with that of the pure InN monolayer. In addition, the spin up and spin down curves of TDOS in InN are highly symmetrical while those of Au-InN are asymmetric, which suggests that Au doping makes the InN monolayer change from non-magnetic to magnetic [45]. Meanwhile, the TDOS of the entire system shifts to the left, and a new peak appears near −2.5 eV, which means that several impurity states introduced by the doping of Au have changed the electronic behavior of the entire system. In atomic DOS (PDOS) (Figure 3b), it can be observed that N 2p orbital and Au 6s, 5p, 5d orbitals have a considerable overlap near −5.2, −3.5 and 0 eV in the spin up and −5.1, −3.0 and 1.0 eV in the spin down. Besides, the In 5p orbital and Au 6s orbitals overlap near −2.5 eV in the spin up and −2.2 eV in the spin down. Meanwhile, the In 5p orbital and Au 5p orbitals have an obvious overlap near 2.7 and 3.5 eV. These phenomena suggest that the orbital hybridization between the Au atom and the In, N atom is very strong, and a stable Au-N bond is formed. In particular, since the orbital hybridization between Au and N atom is in the vicinity of the Fermi level, the electronic behavior of the entire system will undergo greater changes [46]. In summary, the electronic behavior of the InN monolayer will be significantly changed by Au doping.

Adsorption Behavior of HF, SO 2 , SOF 2 and SO 2 F 2 on Au-InN Monolayer
In this section, the adsorption behaviors of Au-InN to HF, SO 2 , SOF 2 and SO 2 F 2 are analyzed. The gas molecules are placed on the surface of the Au-InN monolayer in different directions so as to find the most stable adsorption structure for subsequent analysis. Figure 4 displays the steadiest structure of each gas adsorption system, the adsorption characteristic parameters of which are listed in Table 3.
As can be seen from Figure 4a, the HF molecule prefers to be adsorbed at a position vertical to the InN plane with a small slope and the H-F bond elongates from 0.932 Å to 0.957 Å. The slight structural deformation implies the weak interaction between HF and Au-InN. Meanwhile, the small absolute values of Q T (0.05 e) and E ad (−0.31 eV) also prove that the adsorption of HF upon Au-InN is not stable. Thus, Au-InN is unsuitable for detecting and removing HF. In the SO 2 adsorption system, the most stable configuration bears a resemblance to the HF absorption configuration. After the adsorption of SO 2 , the length of S-O bond increases from 1.481 Å to 1.603 Å. The significant structure deformation indicates that SO 2 molecule is activated during the interaction with Au-InN [47]. From the molecular point of view, SO 2 has a negative charge of 0.24 e, indicating the electronreceiving property of SO 2 . Meanwhile, according to the DCD in Figure 4b, the electron accumulation region is principally confined to the S atom. The absolute value of E ad (−1.38 eV) in Au-InN/SO 2 system is higher than 0.8 eV, hence this adsorption process can be regarded as chemisorption [48]. SOF 2 and SO 2 F 2 molecules tend to be adsorbed on the side of the Au dopant in the Au-InN monolayer rather than on the top. In SOF 2 and SO 2 F 2 adsorption systems, the S-F bond of SOF 2 elongates from 1.671 Å to 2.698 Å, while that of SO 2 F 2 increases from 1.612 Å to 4.776 Å. The more significant deformation of SO 2 F 2 molecule is related to the larger absolute values of E ad (−2.48 eV) and Q T (−1.12 e) in the SO 2 F 2 adsorption system. As can be observed from DCD in Figure 4c,d, the electron density around the Au dopant decreases while that around the F atom increases. As electron acceptors, the SOF 2 and SO 2 F 2 molecules obtain 0.57 e and 1.12 e from the Au-InN monolayer, respectively. Besides, the relatively large absolute values of Q T in the SO 2 , SOF 2 and SO 2 F 2 adsorption systems imply that after the adsorption of SO 2 , SOF 2 and SO 2 F 2 , the redistribution of the electrons in the entire system causes the electronic behavior of the Au-InN monolayer to be changed. In conclusion, the Au-InN monolayer has large adsorption capacity toward three SF 6 decomposed species except HF, and these adsorption processes can be regarded as chemisorption. The relatively large absolute values of Q T and E ad in SO 2 , SOF 2 and SO 2 F 2 adsorption systems not only reflects the strong orbital hybridization between atoms in excited gas molecules and Au atoms but also shows the good stability of the adsorption structure [49,50].   Table 4 lists the Q T and E ad of Cu-InN/gas and Au-InN/gas systems (the Q T and E ad of Cu-InN/gas system are obtained from previous report [14]). By comparison, it can be found that the absolute value of E ad in Au-InN/gas system is significantly larger than that in Cu-InN/gas system, except for the SO 2 adsorption system. This result shows that Au atom doping can improve the adsorption performance of InN to HF, SOF 2 and SO 2 F 2 more significantly than Cu atom doping. In summary, the Au-InN monolayer has good potential as an efficient SF 6 decomposed product adsorbent. Therefore, in order to further study the electron behavior of four adsorption systems, the TDOS and PDOS of the Au-InN/gas systems are investigated. As shown in Figure 5, the TDOS of each adsorption system has varying degrees of deformation in comparison with that of Au-InN monolayer. In the TDOS of HF and SO 2 F 2 adsorption systems, the gap at the bottom of the guide band in the spin up curve narrowed slightly. This phenomenon implies that the conductivity of the Au-InN monolayer may be changed after adsorbing HF and SO 2 F 2 . The TDOS of the SO 2 adsorption system has an obvious left shift compared with that of the Au-InN monolayer, and the degree of TDOS deformation is larger than HF, SOF 2 and SO 2 F 2 adsorption systems. Besides, since gas molecules are activated during the interaction with the Au-InN monolayer, some novel states appear in TDOS. The spin up and spin down curves of TDOS in HF, SO 2 and SOF 2 adsorption systems are asymmetric; in other words, the magnetic property of the Au-InN monolayer does not change after the adsorption of HF, SO 2 and SOF 2 . However, the TDOS spin up and down of the SO 2 F 2 adsorption system is highly symmetrical, which implies that the adsorption of SO 2 F 2 makes the Au-InN monolayer change from magnetic to non-magnetic.
In the PDOS of the HF adsorption system, the hybridization between F 2p orbital and Au 5d, 6s orbitals is in the vicinity of −3.5 eV. In SO 2 adsorption system, the S 2p and O 2p orbitals of activated SO 2 have certain hybridization with Au 5d orbital at −6.2, −3.5 eV in the spin up and at −6.0, −3.2 eV in the spin down. As can be seen from the PDOS of SOF 2 adsorption system, the large overlap area between the F 2p orbital and Au 6s, 5p and 5d orbitals near −5.1, −3.5, −2.5 and −1.0eV indicates the strong interaction between F and Au atom. It can be observed from the PDOS of the SO 2 F 2 system that the F 2p orbital overlaps with the Au 5d orbital near −5.0, −2.3, −0.5 eV, and the overlap area is bigger than that of the HF, SO 2 and SOF 2 adsorption systems. These phenomena show that the interaction between SO 2 F 2 and Au-InN is the strongest, which supports the large absolute values of E ad and Q T in SO 2 F 2 adsorption system. Besides, the strong orbital hybridization between the atoms in excited SO 2 , SOF 2 , SO 2 F 2 and Au atoms verifies the previous conclusion that three SF 6 decomposed species (except HF) can be stably adsorbed by Au-InN.

Frontier Molecular Orbital Analysis
From frontier molecular orbital (FMO) theory, we know that the energy between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) is called the energy gap (E g ). It is a feasible method to measure the conductivity of a material through calculating E g . In this section, the effect of gas adsorption on the conductivity of the whole system is investigated. Through analyzing the frontier molecular orbital, we can explore the feasibility of Au-InN as a gas resistive sensor for detecting SF 6 decomposed species. As can be seen from Figure 6, HOMO and LUMO are mainly located near to an Au atom before gas adsorption, indicating that the Au dopant has strong electron mobility. In the Au-InN doping system, the energy of HOMO is −5.106 eV while that of LUMO is −4.441 eV, and the E g is 0.665 eV. Besides, the E g of the HF, SO 2 , SOF 2 and SO 2 F 2 adsorption systems are 0.634 eV, 0.252 eV, 0.994 eV and 1.805 eV, respectively. The obvious change of E g after adsorbing SO 2 , SOF 2 and SO 2 F 2 suggesting the adsorption of these three gases has a significant effect on the conductivity of Au-InN. However, in the HF adsorption system, the E g is basically the same as that of the Au-InN monolayer, which shows that the adsorption of HF basically does not affect the conductivity of Au-InN. Therefore, Au-InN may be used as a resistive gas sensor for detecting SO 2 , SOF 2 and SO 2 F 2 , which is of extraordinary importance to ensure the normal working state of high-voltage insulation equipment. The gas sensor must pay attention to the problem of recycling, that is to say, the desorption process of gas molecules from the sensor surface must be considered. Only after a comprehensive analysis of the gas adsorption and desorption processes can the application feasibility of the Au-InN monolayer as adsorbent and gas sensor be further explored. Recovery time is an important parameter for evaluating the desorption capacity of gas-sensitive materials, and its calculation formula is as follows [51]: In the above formula, A represents the attempt frequency (10 12 s −1 [52]), while T and K B are the tested temperature and Boltzmann constant (8.62 × 10 −5 eV/K), respectively. E a represents the potential barrier of the desorption process. Since adsorption and desorption are inverse processes of each other, the value of E a can be equal to that of E ad . In order to fully understand the desorption performance of the four SF 6 decomposition products on the Au-InN monolayer, we used three temperatures of 298 K (ambient temperature), 348 K, and 418 K as test temperatures. The recovery time of four adsorption systems at various temperatures was calculated, as shown in Figure 7. As can be seen from Figure 7, the instantaneous recovery time at 298 K (ambient temperature) indicates that HF is easily desorbed from the surface of the Au-InN monolayer. This phenomenon also confirms the previous conclusion that the Au-InN monolayer has a weak adsorption capacity for HF. In summary, Au-InN is not suitable as an adsorbent and sensor for HF gas. At the same time, it is very difficult to desorb SO 2 and SO 2 F 2 from Au-InN at 298 K (ambient temperature), which also confirms the strong interaction between the Au-InN monolayer and these two gases. Although the recovery time is significantly shortened with the increase in temperature, the recovery time of the two adsorption systems is still very long even at 418 K, especially in the SO 2 F 2 adsorption system. Therefore, Au-InN is very suitable as a scavenger for SO 2 and SO 2 F 2 gas in GIS, and thus has great application prospects in the field of ensuring the safe operation of power systems. In addition, SOF 2 is difficult to desorb from the surface of the Au-InN monolayer at ambient temperature, but the recovery time at 418 K is significantly shorter, about 72.6 s. This result shows that Au-InN has a strong adsorption performance for SOF 2 at ambient temperature, and can be recycled due to its short recovery time at high temperature, indicating that Au-InN is an ideal SOF 2 gas sensing material.

Conclusions
In this study, the most stable doping structure and the adsorption properties andsensing mechanism of four SF 6 decomposed species (HF, SO 2 , SOF 2 and SO 2 F 2 ) on the Au-InN monolayer were analyzed based on first-principles theory. The main conclusions of this study are listed as below: 1.
When the Au atom is doped at the T N site in InN, the doping system is the most stable.

2.
The Au-InN monolayer has strong adsorption capacity toward three SF 6 decomposed species except HF, and the adsorption can be identified as chemisorption. These results indicate that Au-InN can be a promising scavenger for SO 2 , SOF 2 and SO 2 F 2 .

3.
Compared with Cu atom doping, Au atom doping can improve the adsorption capacity of InN to HF, SOF 2 and SO 2 F 2 more significantly.

4.
Combined with the analysis of DOS, the strong orbital hybridization between the atoms of excited SO 2 , SOF 2 , SO 2 F 2 and the Au atom can be observed, which not only makes the adsorption configuration more stable but also reflects the good electron mobility of the Au dopant.

5.
It can be obtained from the analysis of the frontier molecular orbital and recovery properties that Au-InN has broad application prospects as SO 2 , SOF 2 and SO 2 F 2 scavenger and resistive SOF 2 sensors, which is of extraordinary importance to ensure the safe operation of power systems.