DFT Insight to Ag₂O Modified InN as SF₆-N₂ Mixture Decomposition Components Detector

Abstract

In gas-insulated switchgear (GIS), partial discharge (PD) can be monitored by detecting sulfur hexafluoride-nitrogen (SF₆-N₂) decomposition components. In this paper, silver oxide (Ag₂O) modification was introduced to improve the gas-sensing properties of graphene-like indium nitride (InN). The adsorption process of NO₂, SO₂F₂, SOF₂ and SOF₄ on Ag₂O-InN was simulated based on the first principles calculation and density functional theory (DFT). The gas sensing mechanism was revealed by density of states theory and molecular orbital theory. It is found that Ag₂O doping greatly improves the adsorption properties of InN to NO₂ and SOF₂ molecules. The adsorption capacity of Ag₂O-InN to the four gas molecules is: NO₂ > SOF₂ > SOF₄ > SO₂F₂. All adsorptions can proceed spontaneously, and the gas molecules are electron donors and Ag₂O-InN is an electron acceptor. Through the analysis of recovery time, it is found that NO₂ is difficult to desorb from the substrate due to the significant adsorption energy of −2.201 eV, while SOF₄, SOF₂ and SO₂F₂ have a moderate adsorption energy of −0.185 eV, −0.754 eV and 0.173 eV and extremely short desorption time. The conductivity of the whole system changed after these four gases were adsorbed on the Ag₂O-InN monolayer. In summary, Ag₂O-InN can be used as NO₂ adsorbent and gas sensors to detect SOF₄, SOF₂ and SO₂F₂. This paper provides a method for on-line monitoring of partial discharge in GIS.

1. Introduction

Ensuring the safety and stability of gas-insulated switchgear (GIS) operation is the focus of the power system [1]. In GIS, SF₆ has become the most popular insulating gas due to its superior insulation performance and arc extinguishing performance [2,3,4,5,6]. However, with the development of society, the use of SF₆ increasingly violates the requirements of production and environmental friendliness [7,8,9]. For example, SF₆ has high liquefaction temperature, and under a nonuniform electric field, its dielectric strength will drop sharply [10,11]. As a kind of gas that causes the greenhouse effect, it is harmful to the environment [12]. Therefore, in the future, the use of SF₆ shows a limited trend. How to reduce the use of SF₆ in the power system is an urgent problem to be solved [13,14]. SF₆-gas mixtures instead of pure SF₆ can improve technical and economic performance [15,16,17,18]. SF₆-N₂ gas mixture is a great alternative, which can reduce the use of SF₆ while ensuring insulation performance. It was found that the mixture containing 30% SF₆ can be applied to GIS bus [19]. Partial discharge (PD) can cause insulation degradation and endanger the life of GIS [20]. Under PD, the SF₆-N₂ mixture decomposes mainly as SOF₂, SO₂, SO₂F₂, SOF₄, CF₄, CO₂, NO, NO₂ and NF₃ [21]. PD failure can be judged by detecting characteristic gases of NO₂, SO₂F₂, SOF₂ and SOF₄. SOF₂ and SO₂F₂ are the most common, and the sensitivity of detecting the two gases by gas chromatography can reach 1 ppmv [22].

Undoped InN is an n-type wide band gap semiconductor, a graphene-like material with excellent performance, which makes up for the defect of the zero band gap of graphene and becomes a research hotspot. According to the X-ray diffractometer (XRD) analysis, Zhang et al. [23] reported the influence of substrate temperature (Ts) on the growth of InN thin films on indium tin oxide (ITO) substrates, and clarified that gradually increasing the temperature above 200 °C is beneficial to the growth of InN (101) plane. Due to the graphene-like properties of InN, a new-type modified InN gas sensor can be developed to detect the SF₆-N₂ decomposition components. Studies showed the effect of transition metal (Cr, Fe, Ni, Mn and Co) doping on InN [24,25,26]. Metal oxide doping can be used to improve the gas sensing properties of materials in previous studies. G. Korotcenkov showed that the selectivity and sensitivity of SnO₂ and In₂O₃ thin films to gases can be improved by structural engineering [27]. Xu Pan et al. revealed that ZnO and CuO doped graphene has good adsorption and sensing properties for SO₂, SOF₂, and SO₂F₂ [28]. Hong Liu et al. showed that Ag₂O-doped MoSe is a potential SF₆ decomposition products sensor [29]. However, there is little study on the gas sensitivity studies of Ag₂O-modified InN monolayers to the of SF₆-N₂ decomposition components.

In this work, pristine and Ag₂O doped InN monolayers are analyzed using density functional theory (DFT), focusing on the spin unrestricted electronic structure behavior. The adsorption capacity and sensing behavior of NO₂, SOF₄, SOF₂, and SO₂F₂ gas molecule on the adsorbent surface are the key research targets. Therefore, this paper simulated the adsorption process of gas molecules on the InN monolayer surface, and systematically calculated the adsorption energy, band structure, density of states (DOS), partial density of states (PDOS), and charge transfer, and thus provided a theoretical basis for the experimental development of new-type gas sensor.

2. Materials and Methods

Based on the first principle, the adsorption and gas-sensing properties of SF₆-N₂ mixture decomposition components (NO₂, SOF₄, SOF₂, SO₂F₂) on pristine and Ag₂O-doped InN are investigated in this paper. The exchange-correlation function is described by a generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional [30,31]. By constructing a 4 × 4 × 1 InN supercell, the adsorption properties and doping were investigated. The Monkhorst–Pack (MP) method was used to describe the Brillouin zone, and 7 × 7 × 1 k-point was set for all simulation calculations [32,33]. The energy convergence accuracy, maximum stress, and max displacement were set as 1 × 10⁻⁵ Ha, 2 × 10⁻³ Ha/Å, 5 × 10⁻³ Å, respectively. A 15 Å vacuum slab was adopted in order to prevent interactions between the adjacent layers.

The formation energy (E_form) describes the stability of the doped structure, and the calculation formula is as follows:

E_form = E_Ag2O-InN − E_Ag2O − E_InN

(1)

where E_Ag2O-InN, E_Ag2O, and E_InN are the total energy of Ag₂O-InN, isolated Ag₂O atom and pristine InN monolayer.

The adsorption capacity of the InN monolayer towards gas molecules is described by adsorption energy (E_ads). The E_ads is defined as:

E_ads = E_gas/suf − E_gas − E_suf

(2)

where E_gas/suf is the total energy of absorbed pristine or Ag₂O-doped InN system, and E_suf, E_gas represents the total energy of pristine or doped InN and isolated gas molecule, respectively.

In addition, Mulliken analysis was considered to analyze the mechanism of charge transfer (Q_t) throughout the work. Q_t of each gas adsorption process can be calculated by the following equation:

Q_t = Q_gas/suf − Q_gas

(3)

Q_gas/suf and Q_gas denotes the charge transfer amount after adsorption carried by the gas and isolated gas, respectively. If Q_t is negative, that the substrate material loses electrons.

E_g = |E_LUMO − E_HOMO|

(4)

To further analyze the energy gap (E_g) of molecular orbital, the highest occupied molecular orbit energy (E_HOMO) and the lowest occupied molecular orbit energy (E_LUMO) is investigated. E_g is defined as Equation (4).

3. Results

3.1. InN Monolayer and Gas Molecules

The optimized configurations of Ag₂O-InN monolayers are shown in Figure 1a–d, and four possible dopings are considered. The formation energies of the four kinds of Ag₂O-doped substrate material are given in

Table 1. Formation energies of three Ag₂O substrate material.

Doping Site	(a)	(b)	(c)	(d)
Eform (eV)	−5.909	−6.459	6.663	−7.038

, the formation energy of (d) is the largest, reaching −7.038 eV. The top view and side view of the doping mode (d) are shown in Figure 1. It is evident that the lengths of three Ag-N bonds are calculated to be 2.199 Å, 2.296 Å, and 2.227 Å, respectively. Both the triangular structure of Ag₂O molecule and the large formation energy indicate the stability of the doping mode (d), and all following studies focus on the (d) configuration.

After structure optimization, the structure of NO₂, SOF₄, SOF₂, and SO₂F₂ molecules are demonstrated in Figure 2a–d. The N–O bond of NO₂ molecule is 1.210 Å with a planar V-shaped structure, and O-N-O bond angle is 133.489° (Figure 2a). As shown in Figure 2b, bond lengths of S=O, S-F1, and S-F2 in distorted triangular bipyramid SOF₄ are 1.444 Å, 1.655 Å and 1.610 Å while the bond angle of O=S-F1 and F1-S-F2 is 97.897° and 85.558°, respectively. Obviously, in the planar triangular SOF₂, the S-O bond length (1.461 Å) is shorter than the S-F bond length (1.670 Å), and the bond angles of O-S-F and F-S-F are 107.164° and 93.050°, as shown in Figure 2c. Besides, SO₂F₂ molecule exhibits a regular tetrahedron, with O-S bond length of 1.443 Å and O-F bond length of 1.612 Å. The angles of O-S-O bond, O-S-F and F-S-F in SO₂F₂ are 102.703°, 107.775° and 90.409°, respectively. It is worth noting that in SOF₄, SOF₂, and SO₂F₂, the length of the S-F bond is negatively correlated with the electronegativity of F atom.

3.2. SF₆-N₂ Decomposition Components Adsorption on InN Monolayer

Figure 3 and

Table 2. Adsorption parameters of NO₂, SOF₄, SOF₂ and SO₂F₂ on undoped InN monolayer.

Adsorption Structures	Figure	Adsorption Distance	Eads	Atom	Mulliken Charge	Qt
NO2/InN	Figure 3a	3.132 Å	−0.839 eV	N	0.217 e	−0.400 e
				O	−0.369 e
				O	−0.248 e
SOF4/InN	Figure 3b	2.986 Å	−2.242 eV	S	0.794 e	−1.188 e
				O	−0.329 e
				F	−0.275 e
				F	−0.549 e
				F	−0.293 e
				F	−0.536 e
SOF2/InN	Figure 3c	3.077 Å	−0.483 eV	S	0.750 e	−0.022 e
				O	−0.231 e
				F	−0.269 e
				F	−0.272 e
SO2F2/InN	Figure 3d	3.297 Å	−0.380 eV	S	0.896 e	−0.007 e
				O	−0.218 e
				O	−0.244 e
				F	−0.222 e
				F	−0.219 e

exhibit the most stable adsorption configurations of SF₆-N₂ decomposition components on undoped InN. It can be seen that adsorption distances of NO₂, SOF₄, SOF₂, and SO₂F₂ on the InN monolayer are 3.132 Å, 2.986 Å, 3.077 Å and 3.297 Å, respectively. SOF₄ tends to be adsorbed on the InN monolayer by F atom (seen in Figure 3b), and the absolute value of the adsorption energy (2.242 eV) is much greater than 0.8 eV, indicating that it is strong chemisorption. Apart from that, the adsorption energy is −0.839 eV in NO₂ system, −0.483 eV in SOF₂ system, and −0.380 eV in SO₂F₂ system. Tiny adsorption energy means weak adsorption. Additionally, for the NO₂, SOF₄, SOF₂ and SO₂F₂ adsorption, the charge transfer direction is from the InN monolayer to the gas molecule, as further verified by the −0.400 e, −1.188 e, −0.022 e and −0.007 e of these systems. The negative charge transfers of O atoms and F atoms indicate that they are electron acceptors. For SOF₂ and SO₂F₂ adsorption, the tiny charge transfer mainly occurs in the gas molecules themselves, further confirming the weak interaction, which is consistent with the adsorption energy analysis.

In brief, the negative adsorption energy indicates that the InN monolayer adsorbs NO₂, SOF₄, SOF₂, and SO₂F₂ molecule spontaneously. InN monolayer substrate is electron donor, and gas molecules are electron acceptors. The adsorption capacity of InN monolayer to SF₆-N₂ decomposition components is in the following order: SOF₄ > NO₂ > SOF₂ > SO₂F₂.

3.3. SF₆-N₂ Decomposition Components Adsorption on Ag₂O-InN Monolayer

To analyze the adsorption behaviors of Ag₂O-InN to NO₂, SOF₄, SOF₂, SO₂F₂ molecules, the gas molecules are approached to the Ag₂O-InN monolayer, and the most stable adsorption structure and adsorption parameters are shown in Figure 4 and

Table 3. Adsorption parameters of NO₂, SOF₄, SOF₂ and SO₂F₂ on Ag₂O-InN monolayer.

Gas Molecules	Figure	Adsorption Distance (Å)	Eads (eV)	Charge Transfer (Qt)
NO2	Figure 4a	1.383	−2.021	−0.174 e
SOF4	Figure 4b	2.880	−0.185	−0.028 e
SOF2	Figure 4c	2.143	−0.754	−0.279 e
SO2F2	Figure 4d	3.110	−0.173	−0.015 e

.

It can be seen from Figure 4 that the four gas molecules all undergo slight deformation on Ag₂O-InN monolayer. The adsorption energies of NO₂, SOF₄, SOF₂, and SO₂F₂ molecules on Ag₂O-InN are −2.021 eV, −0.185 eV, −0.754 eV and −0.173 eV, respectively. Obviously, after Ag₂O doping, the adsorption energy of NO₂ and SOF₂ increase, on the contrary, the adsorption energies of SOF₄ and SO₂F₂ decrease. Meanwhile, the charge transfer of NO₂, SOF₄, SOF₂, and SO₂F₂ molecules on Ag₂O-InN are −0.174 e, −0.028 e, −0.279 e and −0.015 e, respectively. The negative charge transfer indicates that the gas molecules are the electron acceptors and Ag₂O-InN is the electron donor. Among the four gases, only SOF₂ and SO₂F₂ adsorption resulted in an increase in the charge transfer quantity compared undoped InN. In NO₂ adsorption, the NO₂ molecule is activated and absorbed by O atom, forming a N-O bond (1.383 Å) with Ag₂O-InN. None of the other three gases formed bonds with Ag₂O-InN. From the large adsorption energy, small adsorption distance, and large charge transfer, it can be inferred that the NO₂ adsorption is chemisorption. As for SOF₂ adsorption, the adsorption distance is 2.143 Å, which is smaller than that on pristine InN. It is worth noting that the absolute value of adsorption energy of SOF₂ is larger than 0.7 eV, which also belongs to chemisorption. For SOF₄ and SO₂F₂ adsorption, it is inferred that they belong to physisorption by the tiny adsorption energy, large adsorption distance and small charge transfer. In conclusion, NO₂, SOF₄, SOF₂, SO₂F₂ molecules tend to adsorb on Ag₂O molecules in the Ag₂O-InN monolayer. The adsorption capacity of the four gas molecules is: NO₂ > SOF₂ > SOF₄ > SO₂F₂.

3.4. Analysis of Density of States of Gas Adsorption on Ag₂O-InN

Figure 5 presents the DOS (density of states) and PDOS (partial density of states) before and after NO₂, SOF₄, SOF₂, SO₂F₂ are absorbed on Ag₂O-InN monolayer. The outermost electrons of atoms have the greatest influence on the adsorption process, so their PDOS was analyzed and compared. For NO₂ adsorption, the redistribution of TDOS is dramatic. The clear hybridization of N-2p and O-2p at −12 eV, −9 eV, 2 eV is consistent with the phenomenon of bonding between N atom and O atom. Similarly, SOF₂ undergoes redistribution of TDOS and strong orbital hybridization. The strong orbital hybridization indicates that the adsorption reaction of NO₂ and SOF₂ is strong, which is consistent with the conclusion that they are chemisorption. As for the SOF₄ and SO₂F₂ adsorption, a slight redistribution of TDOS occurred, and PDOS did not reflect the bonding information, which is consistent with Figure 4. The new peaks between −5~−12.5 eV mainly caused by F-2p, S-2p and O_gas-2p, while the contribution of Ag-4d is mainly located at the Fermi level. However, the adsorption of these two molecules on the substrate was strong physisorption. In short, after gas adsorption, the redistribution of TDOS proves the existence of charge transfer. The adsorption behavior will change the conductivity of adsorption system, which provide the feasibility of preparing Ag₂O modified InN gas sensor.

3.5. Molecular Orbital Analysis and Recovery Time of Gas Adsorption on Ag₂O-InN

According to the molecular orbital theory, the HOMO and LUMO of Ag₂O-InN and the four gas adsorption systems were analyzed, as shown in Figure 6. The distribution of HOMO and LUMO reflects the electronic behavior during the adsorption process. The energy gap between HOMO and LUMO reveals the ability of electron transition from the valence band to conduction band, and is an important parameter to measure the conductivity of adsorption system. Before gas adsorption, HOMO and LUMO are mainly located on Ag₂O dopant, Ag₂O is the active site on the InN monolayer. After adsorption, the LUMO of NO₂ adsorption system is mainly distributed on Ag₂O dopant, and the HOMO of SOF₄, SOF₂ and SO₂F₂ adsorption systems is mainly located on Ag₂O dopant. In addition, the gas adsorption leads to a small reduction in energy gaps compared to the unadsorbed Ag₂O-InN energy gap (1.768 eV). Energy gaps of the NO₂, SOF₄, SOF₂ and SO₂F₂ system drop to 1.669 eV, 0.952 eV, 1.659 eV and 1.742 eV, respectively. All gas adsorptions lead to enhancement in system conductivity. Especially, the energy gap of SOF₄ system is 0.816 eV narrower than that of non-adsorbed Ag₂O-InN, indicating a much better conductivity. The conductivity of the four adsorption systems is arranged as: SOF₄ > SOF₂ > NO₂ > SO₂F₂. The change of conductivity after gas adsorption provides a theoretical basis for the preparation of resistive gas sensors.

In order to explore the desorption capacity of Ag₂O-InN, the recovery time of gas molecules was calculated by Equation (5).

τ = γ⁻¹ exp (−E_ads/K_BT)

(5)

γ, K_B and T represent the attempt frequency (10⁻¹²s⁻¹), Fahrenheit temperature (298 K, 348 K and 418 K) and Boltzmann constant (8.62 × 10⁻⁵ eV K⁻¹), respectively. The recovery times of the four gas molecules at 298 K, 348 K and 418 K are shown in

Table 4. Recovery time of NO₂, SOF₄, SOF₂ and SO₂F₂ on Ag₂O-InN monolayer.

Adsorption System	KB (K)	τ (s)
NO2/Ag2O-InN	298	1.474 × 1023
	348	1.817× 1018
	418	2.288 × 1013
SOF4/Ag2O-InN	298	1.342 × 10−8
	348	4.768 × 10−9
	418	1.698 × 10−9
SOF2/Ag2O-InN	298	55.94
	348	8.244 × 10−1
	418	1.225 × 10−2
SO2F2/Ag2O-InN	298	8.411 × 10−9
	348	3.196 × 10−9
	418	1.217 × 10−9

. Due to the large adsorption energy and strong chemisorption, the recovery time of NO₂ at 298K is 1.474 × 10²³ s, indicating that NO₂ cannot desorb form Ag₂O-InN. Therefore, Ag₂O-InN is suitable as a NO₂ scavenger, but not as a NO₂ sensor. However, the recovery time of SOF₄, SOF₂ and SO₂F₂ on Ag₂O-InN is extremely short, and Ag₂O-InN has good reusability as a sensor for detecting these three gases.

4. Discussion

Based on the first principles calculation, the adsorption process of SF₆-N₂ decomposition components (NO₂, SOF₄, SOF₂, SO₂F₂) on pristine and Ag₂O-doped InN was simulated. By analyzing the adsorption energy, charge transfer, density of states, molecular orbital theory and recovery time, the adsorption mechanism of gas molecules on Ag₂O-doped InN was further revealed. Pristine InN shows strong adsorption to SOF₄ molecules, but exhibits weak adsorption to NO₂, SOF₂, and SO₂F₂ molecules. In particular, the charge transfer quantity for SOF₂ and SO₂F₂ adsorption on pristine InN are only 0.022 e and 0.007 e. The gas sensing capacity of InN to SOF₂ and SO₂F₂ is poor. Compared with undoped InN, Ag₂O-InN has a significantly higher adsorption energy for NO₂, and a significantly larger adsorption energy and charge transfer quantity for SOF₂. The large adsorption energy makes it difficult for NO₂ to desorb form Ag₂O-InN, so Ag₂O-InN is difficult to be a NO₂ sensor. At the same time, the moderate adsorption energy, large charge transfer, and obvious changes in conductivity make Ag₂O-InN a good sensor for SOF₄, SOF₂, and SO₂F₂. In conclusion, this paper not only explores the gas sensing mechanism of Ag₂O-doped InN to SF₆-N₂ decomposed components, but also provides a theoretical basis for fabricating Ag₂O-InN sensors for online monitoring of partial discharge in GIS.

5. Conclusions

This paper reveals the gas-sensing mechanism of pristine and Ag₂O-doped InN to SF₆-N₂ decomposition components. Studies show that pristine InN exhibits poor sensing ability for SF₆-N₂ decomposition components. The Ag₂O dopant is the active site on the substrate material surface, and the four gas molecules tend to be adsorbed on Ag₂O. The order of adsorption capacity is: NO₂ > SOF₂ > SOF₄ > SO₂F₂. The doping of Ag₂O dramatically increases the absolute value of the adsorption energy of NO₂ to 2.021 eV, resulting in that NO₂ cannot be desorbed from Ag₂O-InN. In addition, moderate adsorption energy, large charge transfer, and obvious conductivity change make Ag₂O-InN a potential material for the fabrication of SOF₄, SOF₂, and SO₂F₂ sensors, and the extremely short desorption time reveals the reusability of Ag₂O-InN as a gas sensor.