Ti₃C₂T_x as a Sensor for SF₆/N₂ Nitrogen-Containing Fault Decomposition Characteristic Products: A Theoretical Study

Abstract

The SF₆/N₂ gas mixture is an alternative gas to SF₆. SF₆/N₂ will decompose and generate nitrogenous characteristic gases, such as NO, NO₂, N₂O, and NF₃, when exposed to long-term partial discharge. The adsorption models of Ti₃C₂T_x (T=O, F, OH) and NO, NO₂, N₂O, NF₃ were constructed, and the most stable adsorption structure was selected in this paper. The electron density and density of states of the adsorption system were further analyzed to study the adsorption behavior, and the sensing performance was evaluated in the end. The results are as follows: four gases could be spontaneously adsorbed on Ti₃C₂T_x, and strong adsorption occurred when surface terminal groups were OH, forming hydrogen or chemical bonds with significant charge transfer. Results show that Ti₃C₂(OH)₂ had a stronger sensing ability than Ti₃C₂F₂ and Ti₃C₂O₂. The conductivity of the Ti₃C₂T_x with different terminal groups was improved after the adsorption of NO and NO₂, showing Ti₃C₂T_x had a good sensing ability for NO and NO₂. It was difficult for the four gases to desorb from the Ti₃C₂(OH)₂ surface, but the adsorption on the Ti₃C₂F₂, Ti₃C₂O₂ surface had a short recovery time at room temperature.

1. Introduction

SF₆ has been widely used in high-voltage electrical equipment because of its excellent insulation and arc extinguishing properties. However, SF₆ is a greenhouse gas, and its global warming potential is 23,900 times that of CO₂, and it can exist in the atmosphere for more than 3200 years. It is one of the six greenhouse gases prohibited by the Kyoto Protocol [1,2,3]. In order to relieve the environmental pressure brought about by SF₆, the power industry has used the SF₆/N₂ gas mixture as an insulation medium. SF₆/N₂ not only has good insulation performance but also solves the problem of high SF₆ liquefaction temperature. It can effectively reduce the use and emission of SF₆ and relieve the greenhouse effect of SF₆ [4,5,6].

In the long-term running of power equipment, various insulation defects will unavoidably occur, and insulation faults, such as partial discharge or overheating, will be generated under the combined action of voltage and current. SF₆/N₂ will gradually decompose under the continuous action of these faults, generating not only sulfur-containing characteristic gases, such as SO₂, SOF₂, SO₂F₂, and H₂S [7], but also nitrogen-containing characteristic gases, such as NO, NO₂, N₂O, and NF₃ [8,9,10]. Since the SF₆/N₂ fault decomposition process is directly related to the fault properties, the internal insulation fault of the equipment can be detected in time by monitoring fault decomposition characteristic products, which is essential for the safe operation of gas-insulated equipment [11,12].

Two-dimensional layered nanomaterials have excellent electrical, mechanical, and optical properties due to their unique structure, and their high specific surface area is conducive to gas adsorption and sensing [13,14,15,16,17]. Two-dimensional transition metal carbides, carbon and nitrogen compounds (collectively called MXene) have received a lot of attention and research from researchers since their discovery in 2011. MXene has the chemical formula M_n+1X_nT_x (n = 1–3), where M represents the transition metal (such as Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, etc.), X is carbon or nitrogen, and T_x stands for terminal group, such as fluorine, oxygen, or hydroxyl. Ti₃C₂T_x was the first MXene synthesized and has been used in many sensing-related studies.

Eunji Lee et al. [18] prepared a sensor of Ti₃C₂T_x and successfully detected all the tested volatile organic compound gases at room temperature. Then, they proposed a possible sensing mechanism for the sensor by analyzing the interaction between the gas and the majority carrier of the material. Chen et al. [19] chose Ti₃C₂T_x and WSe₂ as materials for hybridization and prepared the Ti₃C₂T_x/WSe₂ hybrid sensor with low noise level and ultra-fast response and recovery time. It has high sensitivity and selectivity for detecting oxygenated volatile organic compounds. Wu et al. [20] realized the detection of NH₃ by Ti₃C₂T_x at room temperature and investigated its highly selective adsorption behavior using the density functional theory (DFT) calculations. Kong et al. [21] adsorbed SF₆ decomposition gas on Ti₃C₂T_x using Quantum Espresso software and modified the surface of Ti₃C₂T_x by adding atomic vacancies. The results showed that Ti₃C₂T_x with dot vacancies can detect SF₆ decomposition products with high sensitivity and low electronic noise, and the sensitive detection ability for SO₂ is especially obvious.

Based on the previous research on gas-sensitive sensing of Ti₃C₂T_x for sulfur-containing gases [22], this paper performed first-principle calculations on the adsorption behavior of SF₆/N₂ nitrogen-containing fault decomposition characteristic components on the surface of Ti₃C₂T_x with different terminal groups, and adsorption morphology, adsorption distance, adsorption energy, charge transfer, electron density, and density of states were analyzed to explore the gas sensing ability of Ti₃C₂T_x with different terminal groups for NO, NO₂, N₂O, and NF₃.

2. Computational Methods

This paper is mainly based on the DFT. The simulation calculations regarding the adsorption system of gas molecules with Ti₃C₂T_x are carried out in the DMol³ module of Material Studio [23]. In this paper, a 3 × 3 × 1 periodic supercell of Ti₃C₂T_x is established. Firstly, NO, NO₂, N₂O, and NF₃ gases are placed on the surface of Ti₃C₂T_x, and multiple directions and different possible sites of gas molecules adsorption are considered. Then, the most stable adsorption site of gas molecules is finally selected for further analysis according to the energy of the system. The K-point settings of the Brillouin zone for structure optimization and electronic property calculation are 4 × 4 × 1 and 8 × 8 × 1, respectively, and the optimization of the structure, energy, and related properties of the gas–solid interface system are calculated by the GGA-PBE method [24,25]. The DNP basis group is chosen for the expansion of the electronic wave function [26], and the Grimme method in the DFT-D dispersion correction is used to describe the van der Waals interaction forces [27,28]. For the paramagnetic molecules NO, NO₂, and N₂O, the computational setup takes into account the spin polarization [29]. The all-electron model is used for the core treatment of the gas molecules, and the DFT Semi-core pseudopotential (DSPP for short) is used for the solid surface. The energy convergence threshold, the maximum force threshold, and the maximum displacement threshold for geometric optimization are set to 2.0 × 10⁻⁵ Ha, 0.004 Ha/Å, and 0.005 Å, respectively, and the convergence accuracy of SCF is 1.0 × 10⁻⁵. The direct inversion DIIS value in the SCF iterative subspace is set to 6, and the smearing value of the thermal tailing effect is set to 0.005 Ha. In addition, to eliminate the interaction of adjacent layers, a 25 Å vacuum layer is set in the z-direction. The strength of the interaction between the gas molecules and the sensing material is expressed by the adsorption energy (E_ads) as follows:

E_ads = E_total − E_gas − E_substrate

where $E_{total}$, $E_{gas}$, and $E_{substrate}$ represent the total energy of the gas/Ti₃C₂T_x adsorption system, isolated gas molecule, and pristine Ti₃C₂T_x, respectively.

Q_t represents the charge transfer between the gas molecule and Ti₃C₂T_x during the adsorption as follows:

Q_t = Q₁ − Q₂

where Q₁ and Q₂ represent the total charges of the adsorbed and the isolated gas molecules. A negative value of Q_t means the electrons’ transfer from the substrate to gas molecules.

3. Results and Discussion

3.1. Gas Adsorption on Ti₃C₂F₂

When gas molecules make contact with Ti₃C₂F₂ surface, adsorption may occur at different sites. The most stable adsorption sites of gas molecules and the orientation of adsorption on Ti₃C₂F₂ were selected according to the value of adsorption energy, as shown in Figure 1, where Figure 1a–h show the top and side views of the gas after adsorption. The structure after adsorption shows that no significant changes occur in the gases as well as the substrate material during the adsorption process. The adsorption distances of gas molecules with Ti₃C₂F₂ are all between 2.8 Å and 3.0 Å, as shown in

Table 1. Adsorption distance and bond length change of gas molecules on Ti₃C₂T_x.

Ti3C2Tx	Gas	D(Å) a	L(Å) b
F	NO	2.978	−0.012
	NO2	2.807	0.011
	N2O	2.979	−0.002
	NF3	2.849	0
O	NO	2.769	−0.028
	NO2	2.806	0.005
	N2O	3.021	−0.001
	NF3	2.842	−0.002
OH	NO	1.809	0.08
	NO2	1.666	0.073
	N2O	-- c	--
	NF3	--	--

^a The adsorption distance D refers to the shortest distance between the atom of gas and atom of substrate. ^b The bond length change L, L = L₂−L₁, L₁ and L₂ represents the chemical bond of gas before and after adsorption. A negative value indicates that the molecular bond length becomes shorter. ^c The -- symbol indicates that the chemical bond of the gas is broken and is not included in the measurement.

. After measurement and comparison, the bond length of the adsorbed gas molecules does not change significantly, and the specific values of the bond length change are shown in Table 1 (The original bond lengths of the four gas molecules are shown in Figure S1).

Table 2. Adsorption energy and charge transfer of gas molecules on Ti₃C₂T_x.

Ti3C2Tx	Gas	Eads(eV)	Qt€ a
F	NO	−0.216	0.111
	NO2	−0.213	−0.129
	N2O	−0.273	0.008
	NF3	−0.323	0.014
O	NO	−0.507	0.284
	NO2	−0.115	0.077
	N2O	−0.240	−0.008
	NF3	−0.386	0.036
OH	NO	−1.709	−0.607
	NO2	−3.806	−0.753
	N2O	−5.461	−0.707
	NF3	−9.065	−1.428

^a A positive Q_t means that the gas molecule loses electrons, and a negative Qt means that it gains electrons.

gives the adsorption energies of the four gas molecules on the best adsorption sites of Ti₃C₂F₂, and the adsorption energies of NO, NO₂, N₂O, and NF₃ are −0.216 eV, −0.213 eV, −0.273 eV, and −0.323 eV, respectively. The negative adsorption energies indicate that the adsorption of gases does not require external energy.

Electron density represents the probability of finding electrons at specific locations around atoms or molecules. The deformation charge density is the difference between the electron density after bonding and the atomic charge density at the corresponding point. By calculating and analyzing the deformation charge density, one can understand the bonding of the atoms in the system and the charge transfer during the bonding process. Figure 2 lists the deformation charge density diagrams for the NO, NO₂, N₂O, and NF₃ adsorption models (red represents electron gain, and blue represents electron loss). There is not much charge aggregation between the gas molecules and the atoms on the material surface, as shown in the four pictures below, indicating a weak bonding behavior between them. In addition, the charge transfer values in the adsorption structure are given in Table 2. NO, N₂O, and NF₃ lose 0.111 e, 0.008 e, and 0.014 e, respectively, while NO₂ gains 0.129 e from the surface of Ti₃C₂F₂, and NO and NO₂ have a large charge transfer in the process of adsorption.

To further clarify the electronic structure characteristics of the adsorption systems, the total density of states (TDOS) and partial density of states (PDOS) of all adsorption systems are analyzed. The TDOS plots of Ti₃C₂F₂ after gas adsorption are shown in Figure 3. The density of states of the whole system after the adsorption of NO and NO₂ experiences a significant increase near the Fermi energy level, indicating that the adsorption of these two gases enhances the conductivity of the material. This phenomenon does not occur after the adsorption of N₂O and NF₃. As seen from the figure, the effects of N₂O and NF₃ on the whole adsorption system are below −3 eV, and the density of states of the whole system does not change greatly after adsorption. So, the adsorption of these two gases does not significantly enhance the conductivity of the material. These findings are consistent with the value of the charge transfer reflected in Table 2.

In the PDOS of Figure 4, only a small part of the orbital overlaps between the atoms of NO, NO₂, and N₂O and the atoms of the material surface, indicating that the interactions between the atoms are not strong. The F 2p orbital of NF₃ has a partial overlap with the F 2p orbital of the surface around −7 eV, indicating that there are some interactions between them but not the formation of a strong chemical bond. Taking all the analyses together, the adsorption of the gases on the Ti₃C₂F₂ surface is a physical adsorption, and there is no new chemical bond formed between the atoms composing the gas molecules and the atoms on the material surface.

3.2. Gas Adsorption on Ti₃C₂O₂

Figure 1i–p are top and side views of the four gas molecules after adsorption on the Ti₃C₂O₂ surface. The pictures show that there is no significant structural change in the gas and the substrate material after adsorption occurs, and the bond length changes of the gas molecules are measured, as shown in Table 1, which further confirms that there is only a slight change in the molecular structure after adsorption. Table 1 also shows that the gas adsorption distances are all within the range of 2.7 Å–3.1 Å. From the slight structural changes and large adsorption distances, it can be assumed that the adsorption of gases on Ti₃C₂O₂ is a physical adsorption. The adsorption energies of the four gases on the Ti₃C₂O₂ surface are given in Table 2 (−0.507 eV, −0.115 eV, −0.240 eV, and −0.386 eV for NO, NO₂, N₂O, and NF₃, respectively). All have negative adsorption energies, as in the case of adsorption on the Ti₃C₂F₂ surface, indicating that the adsorption of the gases does not require energy from outside.

The deformation charge density plots of the four adsorption systems are shown in Figure 5, which shows that the aggregation of electrons between the atoms of the four gases and the material surface is difficult due to the long adsorption distance, indicating that they have difficulty in forming chemical bonds with strong interactions. The charge transfer given in Table 2 shows that 0.284e is transferred from NO to Ti₃C₂O₂ surface, 0.077 e from NO₂ to Ti₃C₂O₂ surface, 0.008 e from Ti₃C₂O₂ surface to N₂O, and 0.036 e from NF₃ to Ti₃C₂O₂ surface. Compared with adsorption on Ti₃C₂F₂ surface, the changes of NO and NO₂ charge transfer are more obvious, with enhanced adsorption of NO and more charge transfer, and weakened adsorption of NO₂ and less charge transfer.

Figure 6 shows the TDOS of the whole adsorption system of Ti₃C₂O₂ after gas adsorption. Similar to Ti₃C₂F₂, after the adsorption of NO and NO₂, the density of states of the system experiences a significant increase near the Fermi energy level, which indicates that the adsorption of these two gases enhances the electrical conductivity of the material. Meanwhile, the adsorption of N₂O and NF₃ only makes the density of electronic states at some lower energy levels increase a little, and the effect on the density of states of the system is not great, which also reflects that NO and NO₂ have a larger charge transfer, and N₂O and NF₃ have a lower charge transfer. In the PDOS of Figure 7, only a small part of the orbital overlap between the atoms of NO, NO₂, and N₂O and the atoms of the material surface occurs at some energy levels, indicating a weak interaction between the atoms, while the F 2p orbital of NF₃ has a partial overlap with the O 2p orbital of the surface at around −4.5 eV, indicating some interaction between them. In general, the adsorption of all the four gases on the Ti₃C₂O₂ surface is a physical adsorption.

3.3. Gas Adsorption on Ti₃C₂(OH)₂

The top and side views of the gases after adsorption on the best adsorption site on Ti₃C₂(OH)₂ are shown in Figure 1q–x. Unlike the adsorption on Ti₃C₂F₂ and Ti₃C₂O₂, it can be seen that after the adsorption of NO and NO₂, some H atoms around the gas molecules significantly deflect due to the attraction of the gas molecules. Other H atoms of the surface have different degrees of movement. The N atom of NO is attracted to the H on the surface in Figure 1r, and the O of NO₂ is attracted to the H on the surface in Figure 1t, and the bond angle has an obvious change. So, the adsorption distances of these two gases with the Ti₃C₂(OH)₂ surface shown in Table 1 are significantly smaller than those of Ti₃C₂F₂ and Ti₃C₂O₂. In addition, the bond length changes of the gases in Table 1 are also greater due to the stronger interactions between the gas and the surface. This strong adsorption is even more significant after the adsorption of N₂O and NF₃. Figure 1u,w show that the H atoms on the Ti₃C₂(OH)₂ surface are stripped from the surface and form new chemical bonds with the atoms of gases. The N₂O reacts with the H atoms of the surface to form H₂O and N₂. After NF₃ adsorbs on the surface, two HF molecules form. All these figures indicate a stronger chemisorption occurring among them. The adsorption energies in Table 2 also reflect the phenomena above. It shows NF₃ having the strongest interaction with the surface, with an adsorption energy of −9.065 eV, followed by N₂O with an adsorption energy of −5.461 eV, and finally NO₂ and NO with adsorption energies of −3.806 eV and −1.709 eV, respectively. Compared to the previous two materials with different terminal groups, the adsorption between four gases and Ti₃C₂(OH)₂ is enhanced in different degrees.

The interaction of the gas molecules with the Ti₃C₂(OH)₂ surface is further analyzed by deformation charge density, and the deformation charge density diagram of the adsorption system is shown in Figure 8. The N atom of NO in Figure 8a is closer to the H atom on the surface, and the red charge aggregation shown between them indicates a stronger interaction. Combined with Table 2, 0.607 e is transferred from the Ti₃C₂(OH)₂ surface to NO. There is also more charge aggregation between the O atom of NO₂ and the H atom on the surface in Figure 8b, with 0.753 e transferred from the Ti₃C₂(OH)₂ surface to NO₂. In Figure 8c,d, it can be seen that a large amount of charge aggregates between the H atoms, which are removed from the surface, and the atoms of the gas, further demonstrating the formation of chemical bonds between them, accompanied by 0.707 e and 1.428 e transferring from Ti₃C₂(OH)₂ to N₂O and NF₃.

The TDOS of Ti₃C₂(OH)₂ after gas adsorption is shown in Figure 9. Apparently, the density of states of the whole system before and after the adsorption of NO, NO₂, N₂O, and NF₃ experiences a great change. It can be clearly seen that all the TDOS show a right shift, and the electronic states of the system move to higher energy levels after the adsorption of these gases, indicating that the adsorption of the gases has a relatively large effect on the adsorption system, and the densities of states all increase around the Fermi energy level, meaning that the conductivity of the material is enhanced. All these changes are consistent with the large charge transfer between the gases and Ti₃C₂(OH)₂. In addition, after the adsorption of NF₃, a new peak at −4.5 eV clearly appears, which changes the whole system the most, which also reflects the strongest interaction of NF₃ with Ti₃C₂(OH)₂.

Figure 10 shows the PDOS for some gas atoms and surface atoms. The N 2p orbital of NO and the H 1s orbital from the surface have the same density of state peaks at 0 eV, −9 eV in Figure 10a, which indicates a resonance of the electrons in the orbital, a manifestation of a stronger interaction. The O 2p orbital of NO₂ and the H 1s orbital at 0 eV, −2 eV, −8 eV, −9 eV have an overlap in Figure 10b, which likewise indicates a stronger interaction between them, and the O 2p orbital of N₂O and the H 1s orbital of the surface have the same density of state peaks at −6.5 eV, −9 eV in Figure 10c, demonstrating that O–H bonds are indeed formed between O and H. The N 2p and F 2p orbitals of NF₃ and the H1 1s and H2 1s of the surface likewise show multiple overlaps in Figure 10d, representing the formation of N–H bonds and F–H bonds.

In summary, the adsorption that occurs on the Ti₃C₂(OH)₂ surface is significantly different from that of Ti₃C₂F₂ and Ti₃C₂O₂, which has greater adsorption energy, more charge transfer, and stronger interactions. The reason is that the hydroxyl groups on the surface are more chemically active than oxygen and fluorine atoms, and the gas molecules interact more strongly with the OH surface. When NO and NO₂ make contact with the Ti₃C₂(OH)₂ surface, N and O are attracted by OH and form O–H···N and O–H···O hydrogen bonds, respectively. When N₂O and NF₃ make contact with the Ti₃C₂(OH)₂ surface, O atoms, N atoms, and F atoms are attracted by OH, forming O–H···O, O–H···N, and O–H···F hydrogen bonds and approaching to the surface, which eventually leads to H atom stripping from surface and forming new chemical bonds.

3.4. Ti₃C₂T_x Gas Sensing Performance Evaluation

An important parameter reflecting the sensing performance is the recovery time τ, which represents the time required to remove the adsorbed gas molecules from the material surface, defined as [30]

$$\tau =v_0^{-1}{e}^{\left(-\frac{E_{ads}}{kT}\right)}$$

$v_0$ indicates the attempt frequency, assuming that all gases have the same order of magnitude as that of NO₂ (1.0 × 10¹² s⁻¹) [31]; E_ads is the energy barrier for desorption, which is set equal to the adsorption energy; k is the Boltzmann constant (8.62 × 10⁻⁵ eV·K⁻¹); and T is the Kelvin temperature. It can be seen from the equation that the larger the adsorption energy is for a gas molecule, the more difficult its desorption will become accordingly. The recovery time of each gas on Ti₃C₂T_x at room temperature is given in

Table 3. Recovery time at room temperature (25 °C).

Ti3C2Tx	τ(NO)/s	τ(NO2)/s	τ(N2O)/s	τ(NF3)/s
F	$4.4\times$10−9	$3.9\times$10−9	$4.1\times$10−8	$2.8\times$10−7
O	$3.7\times$10−4	$8.8\times$10−11	$1.1\times$10−8	$3.3\times$10−6
OH	$7.8\times$1016	$2.2\times$1052	$2.1\times$1080	$1.8\times$10141

. The adsorption energies on Ti₃C₂F₂ and Ti₃C₂O₂ are generally small, so they reflect a fast recovery time, and the slowest is 0.37 ms for NO adsorption on Ti₃C₂O₂, and the very short recovery time may not achieve effective detection in the actual sensing detection. In contrast, the adsorptions of four gases on Ti₃C₂(OH)₂ have a large adsorption energy and exhibit a long recovery time. It is difficult for gas molecules to desorb from the material surface at room temperature, which is not conducive to the reuse of the sensor but has potential value as a gas adsorbent. The desired sensor performance can be achieved by surface modification. There are many ways of surface modification, such as controlling the ratio of surface F, O and OH groups, doping with other metals and their compounds, adding atomic vacancies, adding other terminal groups, etc. [32,33,34,35,36].

4. Conclusions

In this paper, the adsorption process of gases on the surface of these materials was studied by constructing the adsorption models of NO, NO₂, N₂O, and NF₃ on Ti₃C₂T_x with three terminal groups of F, O, and OH, and the adsorption behavior of gases was analyzed according to adsorption energy, charge density, and density of states through DFT calculations:

(1)

The adsorption energies of NO on Ti₃C₂F₂, Ti₃C₂O₂, and Ti₃C₂(OH)₂ are −0.216eV, −0.507 eV, and −1.709 eV, respectively, and the charge transfers are 0.111 e, 0.284 e, and −0.607 e, respectively. After NO adsorption, the density of states of the adsorbed systems all increase near the Fermi energy level, and conductivity is enhanced.

(2)

The adsorption energies of NO₂ on Ti₃C₂F₂, Ti₃C₂O₂, and Ti₃C₂(OH)₂ are −0.213 eV, −0.115 eV, and −3.806 eV, respectively, and the charge transfer is −0.129 e, 0.077 e, and −0.753 e, respectively, and the adsorption of NO₂ leads to the enhancement of electrical conductivity of the materials.

(3)

The adsorption energies of N₂O on Ti₃C₂F₂, Ti₃C₂O₂, and Ti₃C₂(OH)₂ are −0.273 eV, −0.240 eV, and −5.461 eV, respectively, with charge transfer of 0.008 e, −0.008 e, and -0.707e. After the adsorption of N₂O, only Ti₃C₂(OH)₂ exhibits an increase in electrical conductivity, and it is the chemisorption that occurs with strong interactions.

(4)

The adsorption energies of NF₃ on Ti₃C₂F₂, Ti₃C₂O₂, and Ti₃C₂(OH)₂ are −0.323 eV, −0.386 eV, and −9.065 eV, respectively, and the charge transfers are 0.014 e, 0.036 e, and −1.428 e, respectively. NF₃ exhibits strong chemisorption only on Ti₃C₂T_x terminated with -OH, accompanied by a large amount of charge transfer.

(5)

The adsorption of all four gases on the surface of Ti₃C₂F₂ and Ti₃C₂O₂ exhibits short recovery times, while the adsorption on the surface of Ti₃C₂(OH)₂ makes it difficult to achieve desorption at room temperature due to the stronger adsorption effect.

Combining all the analyses, Ti₃C₂T_x with three terminal groups has good sensing ability for NO and NO₂, while Ti₃C₂T_x with -OH surface has great adsorption energy and significant charge transfer for NO, NO₂, N₂O, NF₃, which makes it easy to achieve gas detection, but it is also found to have the disadvantage of long recovery time, so the performance of the material needs to be improved by further methods.