Dissolved Gas Analysis in Transformer Oil Using Ni Catalyst Decorated PtSe₂ Monolayer: A DFT Study

Abstract

In this paper, the first-principles theory is used to explore the adsorption behavior of Ni catalyst decorated PtSe₂ (Ni-PtSe₂) monolayer toward the dissolved gas in transformer oil, namely CO and C₂H₂. Some Ni atoms from the catalyst are trapped in the Se vacancy on the pure PtSe₂ surface. The geometry configurations of Ni-PtSe₂ monolayer before and after gas adsorption, the electronic property of Ni-PtSe₂ monolayer upon gas adsorption, and the sensibility and recovery property of Ni-PtSe₂ monolayer are explored in this theoretical work. Through the simulation, the ${\mathrm{E}}_{\mathrm{ad}}$ of CO and C₂H₂ gas adsorption systems are calculated as −1.583 eV and −1.319 eV, respectively, both identified as chemisorption and implying the stronger performance of the Ni-PtSe₂ monolayer on CO molecule, which is further supported by the DOS and BS analysis. According to the formula, the sensitivity of Ni-PtSe₂ monolayer towards CO and C₂H₂ detection can reach up to 96.74% and 99.91% at room temperature (298 K), respectively, which manifests the favorable sensing property of these gases as a chemical resistance-type sensor. Recovery behavior indicates that the Ni-PtSe₂ monolayer is a satisfied gas scavenger upon the noxious gas dissolved in transformer oil, but its recovery time at room temperature is not satisfactory. To sum up, we monitor the status of the transformer to guarantee the stable operation of the power system through the Ni-PtSe₂ monolayer upon the detection of CO and C₂H₂, which may realize related applications, and provide the basis and reference to cutting-edge research in the field of electricity in the future.

1. Introduction

Transformers, which are static induction appliances using the principle of electromagnetic induction to change alternating current voltage and transmit alternating current power, are widely applied in modern industrial enterprises. Transformers can be classified as dry-type transformers, oil-immersed transformers, and gas-filled transformers. The oil-immersed transformer, an indispensable piece of electrical equipment in the modern era, accounts for more than 90% of all types of transformers and is of great significance in practical applications [1,2,3]. Although transformer oil is a petroleum-based liquid that has the possibility of burning and has disadvantages in environmental protection, most power transformers still use it as insulation and a cooling medium, for it possesses the characteristics of excellent insulation performance, low viscosity, admirable heat transfer performance, reduces the aging of insulation materials, and is of low price [4,5,6].

However, after long-term running, several transformer faults, such as the overheating of insulation oil, partial discharge, arc discharge, spark discharge, water ingress and dampness, natural aging, and so on, will give rise to great influence on the power system and may cause immeasurable economic losses [7,8,9]. These phenomena will bring on the breakage of certain C-C and C-H bonds in the transformer oil in the environment of electricity and heat, with a small number of active hydrogen atoms and unstable hydrocarbon free radicals emerging, which are further reorganized into hydrogen (H₂) and low molecular hydrocarbons, such as methane (CH₄), ethane (C₂H₆), ethylene (C₂H₄), acetylene (C₂H₂), and CO, CO_2, and other gases [10,11,12]. Among these by-products, the content of CO and C₂H₂ accounts for most of all the gases [13]. Therefore, it is quite essential to monitor online whether the transformer is operated normally by means of detecting the main components of decomposition gases of transformer oil, namely CO and C₂H₂.

Over recent years, two-dimensional (2D) layered materials such as graphene and transition metal dichalcogenides (TMDs), have attracted the attention of researchers around the world on account of their strong chemical reactivity to various gas species. With the discovery of graphene, a growing amount of related research has appeared in the fields of digital information, field-effect devices, biomedicine, and sensing materials for its high thermal conductivity, admirable charge mobility, and large specific surface area [9,14,15]. Likewise, TMDs such as MoS₂ which is widely studied, are also favorable 2D materials and possess outstanding performance in gas sensing [3,16,17,18,19,20,21]. Among all those TMDs, platinum diselenide (PtSe₂) monolayer, as another member of 2D-layered materials, exhibits eminent semiconductor property since the band gap of its monolayer is about 1.2 eV [22,23]. Besides, several research reports indicate that the PtSe₂ monolayer possesses excellent performance in detecting gas molecules on the basis of the first-principles theory [22,24]. Even so, we need to make use of transition metal (TM) doping to improve the electron redistribution of the whole system and further enhance the sensitivity and responsiveness in gas interaction, since the binding force between the gas molecule and the PtSe₂ monolayer is not enough to bring on electron transfer between the two [22]. Moreover, it is propitious for the filling of the doped atoms that the lattice constant and the bond length between atoms in PtSe₂ monolayer are larger than that in MoS₂ monolayer [25,26,27,28]. It is reported that the Ni atom, as a dopant, possesses favorable catalytic property and electron mobility in gas interactions, imposing a significant promotion for gas detection. Therefore, it is reasonable for us to assume a Ni-doped PtSe₂ (Ni-PtSe₂) monolayer as a promising novel sensing material to explore its sensing potential toward the main parts of dissolved gas in transformer oil based on first-principles theory. This theoretical work will be quite significant for the stable operation of the power system and provide a critical reference to future scientific studies.

2. Computational Details

In this work, the total geometric and electronic calculations on the basis of first-principles theory were carried out in the DMol³ package [29]. The Perdew-Burke-Ernzerhof (PBE) function within the generalized gradient approximation (GGA) was taken to conduct the exchange correlation interaction [30]. DFT-D2 method proposed by Grimme was chosen to correct dispersion to consider the Van der Waals force and long-range interactions [31]. The Monkhorst-Pack k-point mesh of 10 × 10 × 1 was adopted for the Brillouin zone integration [32]. We selected the energy tolerance accuracy, maximum force, and displacement to be 10⁻⁵ Ha, 2 × 10⁻³ Ha/Å, and 5 × 10⁻³ Å [33], respectively. Self-consistent loop energy of 10⁻⁶ Ha, global orbital cut-off radius of 5.0 Å, and smearing of 0.005 Ha were applied to ensure the accuracy of the total energy [34].

We establish a 4 × 4 × 1 PtSe₂ monolayer supercell to perform the whole first-principles calculations, which include 16 Pt atoms and 32 Se atoms with a vacuum region of 15 Å to prevent the possible interactions between the adjacent units [35] since previous studies have shown that a 4 × 4 × 1 supercell would be large enough to conduct the gas adsorption process [10]. The lattice constant of the optimized pristine PtSe₂ monolayer was obtained as 3.71 Å, in accordance with data from previous research reports [36]. The adsorption energy (E_ad) was calculated by

$${\mathrm{E}}_{\mathrm{ad}}={\mathrm{E}}_{\mathrm{Ni}\text{-}\mathrm{P}\mathrm{t}\mathrm{S}{\mathrm{e}}_2/\mathrm{gas}}-{\mathrm{E}}_{\mathrm{Ni}\text{-}\mathrm{P}\mathrm{t}\mathrm{S}{\mathrm{e}}_2}-{\mathrm{E}}_{\mathrm{gas}}$$

(1)

where in ${\mathrm{E}}_{\mathrm{Ni}\text{-}\mathrm{P}\mathrm{t}\mathrm{S}{\mathrm{e}}_2/\mathrm{gas}}$, ${\mathrm{E}}_{\mathrm{Ni}\text{-}\mathrm{P}\mathrm{t}\mathrm{S}{\mathrm{e}}_2}$ and ${\mathrm{E}}_{\mathrm{gas}}$ were the energies of the gas adsorbed system, isolated Ni-PtSe₂ monolayer and gas molecule, respectively. The Hirshfeld method was adopted to consider the charge-transfer (Q_T) between the target molecule and the adsorbent surface, which is proposed to describe carried electron value by gas molecule after adsorption. Only the most stable configurations (MSC) for gas adsorption would be plotted and discussed below.

3. Results and Discussion

3.1. Analysis of Ni-PtSe₂ Monolayer and Gas Species

As shown in Figure 1, we optimize CO and C₂H₂ molecules to their most stable geometry structure, where we mark the bond length for the sake of better comparing the changes before and after gas absorption. One can see from Figure 1a,b, that with sp orbital hybridization in CO and C₂H₂, the C-O bond in CO (1.142 Å) is longer than the C-H bonds in C₂H₂ (1.071 Å) for the reason of the smaller radius of H atom than O atom. Likewise, since the larger radius of the C atom than the O atom, the C-C bond in C₂H₂ (1.211 Å) is longer than the C-O bond in CO.

After theoretical calculations, we found that the E_ad of pristine PtSe₂ monolayer adsorbing CO and C₂H₂ are 0.26 and 0.21 eV, respectively. The magnitude of the interaction between the pristine PtSe₂ monolayer and the gas molecules during the adsorption process is quite weak. Therefore, the pristine PtSe₂ monolayer is not suitable as a sensing material for detecting CO and C₂H₂.

It is a known fact that mono-sulfur vacancy is the most occurred point defect structure in the TMDs [18,37]. Recently, it is proven that the Se vacancy in PtSe₂ monolayer is usually easier to be formed than the Pt vacancy [38,39]. The lattice constant and interatomic bond length of the PtSe₂ monolayer is relatively large, which is conducive to replacing the filling of atoms. Therefore, it is reasonable to speculate that the enhanced response of the PtSe₂ monolayer to the gas molecules can be achieved by atomic doping. Figure 2a exhibits the structurally optimized Se-vacancy from PtSe₂ monolayer. It can be seen that the geometry around the Se vacancy is slightly deformed compared with the intrinsic structure that the Se-Pt bond length around the Se vacancy is shortened by 2%, due to the Se vacancy in the PtSe₂ monolayer. Besides, as shown in Figure 2b, the MSC is formed by Ni atom doping in the Se vacancy after structural optimization. The Se-Pt bond length is restored to a large extent in the Ni-PtSe₂ system. This finding shows that the geometry of the PtSe₂ monolayer, which can be restored by filling the vacancy with other atoms is deformed by the presence of Se vacancy. This finding is consistent with that demonstrated in the MoS₂ system [17]. In order to determine the chemical stability of the Ni-PtSe₂ monolayer, the value of E_ad between Ni atom and S vacancy PtSe₂ monolayer was calculated. The result shows that the binding strength of the Ni-PtSe₂ monolayer (−4.50 eV) is much higher than that of the intrinsic PtSe₂ monolayer, indicating that its chemical stability is greatly improved compared to that of the intrinsic PtSe₂ monolayer [22]. It can be seen from the front view and top view in Figure 2b, that the Ni dopant forms three new chemical bonds with three neighboring Pt atoms, namely Ni-Pt bonds, which are all shorter than the pristine Se-Pt bond length. The position of the Ni atom is also slightly lower than the Se atomic plane from the top view on account of the smaller radius of the Se atom than the Ni atom. This also indicates the deformation of the geometric structure upon the PtSe₂ monolayer caused by the Ni dopant.

Based on the Hirshfeld analysis, it can be found that the carried charge of the Ni dopant is −0.0823 e, which manifests that the Ni atom behaves as an electron recipient, charging 0.0823 e from the PtSe₂ monolayer with Se-vacancy. Se atoms serve as electron acceptors while Pt atoms as electron contributors in the doping process. As we can see from Figure 2b, where electron density difference (EDD) of PtSe₂ monolayer is exhibited, the green electron accumulation is mainly concentrated around the Ni dopant and the Se atoms, while the rosy electron depletion around the Pt atoms. These phenomena exhibit the electron hybridization between the Ni dopant and the adjacent atoms during the doping process, and meanwhile also are in accordance with the electron transfer property in the Hirshfeld analysis.

In order to explore the effect on the electron behavior of the Ni atom doping on the PtSe₂ monolayer, Figure 3 exhibits the band structure (BS) and density of state (DOS) of the Ni-PtSe₂ system by means of simulation. As we all know, we can judge the electrical conductivity of certain materials by the band gap. The widening of the band gap corresponds to a decrease in conductivity, otherwise, the opposite is true [40,41,42]. According to the research, the bandgap of the pristine PtSe₂ monolayer which is an indirect semiconductor is about 1.2 eV [23]. While the calculated bandgap for the Ni-PtSe₂ system shrinks down to 0.849 eV compared to the intrinsic PtSe₂ monolayer, this indicates that doping of the Ni atom can improve the electrical conductivity of the pristine PtSe₂ monolayer and Ni-PtSe₂ monolayer takes on the metallic properties. One can see from Figure 3b where the DOS of the PtSe₂ monolayer with Se-vacancy and Ni-PtSe₂ systems are both exhibited, several novel peaks emerge from the DOS curve of the PtSe₂ monolayer with Se-vacancy attributed to the doping of Ni dopant, bringing on the remarkably increased states of Ni-PtSe₂ monolayer at the Fermi level, from which the conclusion can be drawn that the conductivity of Ni-PtSe₂ has been enhanced. Besides, the DOS curve of the Ni-PtSe₂ monolayer is shifted to the right and a lower region compared with the PtSe₂ monolayer with Se-vacancy, since the electron-contributing behavior of the PtSe₂ monolayer. From the partial DOS of Figure 3c, the Ni 3d orbital overlap constantly with the Se 3p orbital from—6.5 to 2 eV, demonstrating the strong electron hybridization between the Ni and Se atoms on account of the electron transfer, which further verifies the formation of new chemical bonds, namely Ni-Pt bonds. Above all, the Ni dopant has generated a significant impact on the geometric structure of the pristine PtSe₂ monolayer, which will lead to a higher conductivity.

3.2. Gas Adsorption Configurations on Ni-PtSe₂ Monolayer

Various gas adsorption configurations were established for the sake of exploring whether the Ni-PtSe₂ monolayer possesses eminent sensing and adsorbing performance toward the dissolved gas in transformer oil. From Figure 4a, where the most stable geometric structure for CO adsorption onto the Ni-PtSe₂ surface is depicted, the CO molecule lies diagonally above the Ni atom, with the plane of Ni-PtSe₂ monolayer into a slope. The C atom of CO is captured by the Ni dopant, generating the new chemical bond, namely the C-Ni bond, with a bond length of 1.769 Å. The average Ni-Pt bond length is obtained as 2.560 Å, longer than that in the isolated Ni-PtSe₂ system. The C-O bond length elongates from 1.142 Å to 1.158 Å. These phenomena manifest interaction between the Ni-PtSe₂ monolayer and CO molecule, giving rise to apparent deformation toward the geometric structure of the Ni-PtSe₂ monolayer in the process of gas absorption. It is pointed out that the C-Ni bond length (1.769 Å) is shorter than the covalent radii (1.92 Å) between the C atom and Ni atom, which is consistent with the conclusion that the CO adsorption system is classified as chemisorption, since ${\mathrm{E}}_{\mathrm{ad}}$ is calculated as −1.583 eV, larger than the reference value. According to the research [43], the interaction mechanism can be determined as physical adsorption when the ${\mathrm{E}}_{\mathrm{ad}}$ is between 0.1 and 0.6 eV, and chemical adsorption when the ${\mathrm{E}}_{\mathrm{ad}}$ is greater than 0.8 eV. The ${\mathrm{E}}_{\mathrm{ad}}$ is an important parameter to measure the strength of the adsorption effect during adsorption reaction based on the first-principles theory, which can directly indicate the magnitude of the interaction between the adsorbent and the adsorbed substrate during the adsorption process.

Furthermore, based on charge partitioning by the Hirshfeld method, the CO molecule performs as an electron acceptor, withdrawing 0.0079 e from the Ni-PtSe₂ monolayer. The Ni dopant as an electron embracer, carries 0.133 e from the Ni-PtSe₂ monolayer during gas absorption, accounting for its strong electron receptivity. As depicted in Figure 4b, the CO molecule and Ni atom are mainly surrounded by the green electron accumulation, manifesting their electron-accepting behavior, whereas, the rosy depletion around the Pt atom demonstrates its electron-loss property in the CO adsorption system. Massive green electron accumulation on the C-Ni bond verifies the electron hybridization between the C atom and Ni atom. These phenomena are in accordance with the above Hirshfeld analysis.

For the C₂H₂ adsorption system, the different views of MSC, as well as EDD, are exhibited in Figure 5. It can be found that the C₂H₂ molecule traps in the Ni-PtSe₂ surface, with the parallel position and a small slope to the Ni-PtSe₂ plane, forming two new Ni−C covalent bonds whose bond length are 1.948 Å and 1.915 Å, respectively, much larger than the length of covalent radii of Ni and C atoms (1.92 Å). Severe deformation occurs after the C₂H₂ molecule is absorbed into the Ni dopant, the linear structure of the C₂H₂ molecule, namely the C≡C bond, gets stretched, and elongated from 1.211 Å to 1.262 Å, which shows that the C₂H₂ molecule is highly activated during gas adsorption. Moreover, chemisorption can be confirmed in the C₂H₂ adsorption system, on account of the simulation result that the calculated ${\mathrm{E}}_{\mathrm{ad}}$ is −1.319 eV, larger than the critical value of 0.8 eV. Based on the Hirshfeld analysis, the C₂H₂ molecule transfers 0.0281 e to the Ni-PtSe2 surface, while the Ni dopant as a catalyst withdraws 0.1183 e as well as the Se atom receiving electrons during gas adsorption, which implies the electron-donating property of C₂H₂ molecules. In the EDD, it is obvious that the strong green electron accumulations are mainly situated at the newly formed Ni−C bonds and the Ni dopant, which demonstrates not only the formation of Ni−C bonds but also the electron-accepting behavior of the Ni dopant in C₂H₂ adsorption. The rosy electron depletions are mainly located on the Ni−Pt and C≡C bonds, which implies the weakness of the Ni−Pt and C≡C bonds, in line with the elongation of C≡C bonds and Ni−Pt bonds after the C₂H₂ is absorbed.

On all accounts, the Ni dopant plays a significant impact on electron redistribution toward the above two various gas absorption systems. We can infer that the Ni-PtSe₂ monolayer conducts strong chemisorption on CO and C₂H₂, with its adsorption strength upon two gases in the order CO > C₂H₂, judging by the value of ${\mathrm{E}}_{\mathrm{ad}}$.

3.3. Electronic Property of Ni-PtSe₂ Monolayer upon Gas Adsorption

The BS and DOS of CO and C₂H₂ systems are exhibited in Figure 6 to further analyze the electronic behavior of the Ni-PtSe₂ monolayer upon gas adsorption. We can clearly find that the bandgap of the Ni-PtSe₂ monolayer is calculated as 0.673 eV and 0.489 eV, respectively, after adsorption of CO and C₂H₂ molecules, less than that of 0.849 eV in the pristine Ni-PtSe₂ system, which implies that the electrical conductivity of the Ni-PtSe₂ monolayer has been enhanced with different amplitude in various adsorption systems. It is worth pointing out that the descending order of the bandgap in these systems is C₂H₂ > CO, contrary to the order of ${\mathrm{E}}_{\mathrm{ad}}$: CO > C₂H₂. Since the smaller molecule, which is more likely to be trapped in the Ni dopant, leads to the stronger binding force of the Ni−C bond, corresponding to the larger ${\mathrm{E}}_{\mathrm{ad}}$. Therefore, a conclusion can be drawn that the CO adsorption system matched with a larger bandgap possesses superior sensing response properties than the C₂H₂ system.

The total DOS, orbital DOS, and molecular DOS of the gas before and after gas adsorption are displayed in Figure 6a2,a3,b2,b3. After the adsorption of various targeted molecules, one can see that the total DOS distributions of Ni-PtSe₂ monolayer, contain several deformations, where the curves are split into numerous novel small wave crests and left-shifted to a region, especially the CO adsorption system in Figure 6a2. The novel generated peaks are mainly located at −1.5, −6.25 and −9.0 eV in Figure 6a2 and 0.8, 0.5, −8.0 and −9.5 eV in Figure 6b2, respectively. Most of the BS curve in Figure 6b2, before and after C₂H₂ adsorption, almost overlaps, implying a stronger interaction for the CO system. These deformations may be put down to the orbital interaction between atoms bringing about the electrons on the gas molecules to activate and further alter the electronic behavior of the whole adsorption system. In other words, the novel generated peaks around the Fermi level turn out the strong chemical reactions between the gas molecules and Ni-PtSe₂ surface, which results in the formation of covalent bonds.

From the partial DOS in Figure 6a3,b3, where the orbital interactions occur between bonded atoms, namely the Ni dopant and the C atom of targeted molecules, the enormous overlaps are mainly located at −5.0, −1.2, and 1.2 eV in the CO system and at −5.1, −1.0, and 1.2 eV in the C₂H₂ system, indicating the strong orbital hybridization between Ni 3d orbital and C 2p orbital, and the strong binding force of Ni-C bonds. Otherwise, these overlaps of the two orbitals give rise to the deformations of the total DOS in large part, where several novel peaks emerge and occur at the offset of the curve position. Moreover, the scope of these overlaps between Ni 3d orbital and C 2p orbital in the CO adsorption system is larger than that in the C₂H₂ system, which verifies the stronger chemical interaction of the CO adsorption system.

On all accounts, stronger orbital interaction in the CO adsorption system can be confirmed by means of the above analysis, as consistent with the inference of gas adsorption configurations. The sensing mechanism of the Ni-PtSe₂ monolayer can be effectively explored in the dissolved gas of transformer oil on account of the change of electrical conductivity after gas adsorption.

3.4. Sensing Response and Recovery Property of Ni-PtSe₂ Monolayer

During the process of gas adsorption, the changes in the electronic behavior in various gas atmospheres correspond to respective conductivity responses, which are directly reflected in the value of the bandgap and provide the intuitive data to judge the sensing performance of the Ni-PtSe₂ monolayer. Furthermore, the variational electrical conductivity can result in the altered sensitivity of the Ni-PtSe₂ monolayer. We list the following two formulas for electrical conductivity (σ) and sensitivity (S) of the Ni-PtSe₂ surface to accurately capture the small change in the electronic behavior [10,22].

$$\sigma =A\ast e^{(-B_g/2kT)}$$

(2)

$$S=({{\sigma }^{-1}}_{gas}-{{\sigma }^{-1}}_{pure})/{{\sigma }^{-1}}_{pure}=({\sigma }_{pure}-{\sigma }_{gas})/{\sigma }_{gas}$$

(3)

where A is a constant, B_g is the bandgap, k is the Boltzmann constant and T is working temperature, while σ_gas and σ_pure mean the electrical conductivity of the gas absorption system and isolated Ni-PtSe₂ monolayer, respectively. Based on Formula (2), the decreasing bandgap of the CO and C₂H₂ systems can give rise to the increase in electrical conductivity of the Ni-PtSe₂ monolayer, which provides the basic mechanism to explore the electrical conductivity of various adsorption systems for the Ni-PtSe₂ monolayer. However, the measure of separating the mixed gas needs to be carried out to realize efficient detection, since a sensing material cannot select certain gas in the mixed gas environment. According to the above formula calculation, the sensitivity of the Ni-PtSe₂ monolayer towards CO and C₂H₂ detection can reach up to 96.74% and 99.91% at room temperature (298 K), respectively, which not only exhibits the detailed conductivity values but also manifests the favorable sensing property of these gases as a chemical resistance-type sensor. Taken together, it is a promising gas sensor for the Ni-PtSe₂ monolayer with high sensitivity to detect CO and C₂H₂ at room temperature.

Apart from that, the recovery time (τ) is also a critical parameter to judge the repeatability of the sensing material. The formula is shown below [10,22]:

$$\tau =A^{-1}{e}^{(-E_a/K_{B}T)}$$

(4)

where A is the attempt frequency (10¹² s⁻¹), T is temperature, and K_B is the Boltzmann constant (8.314 × 10⁻³ kJ/(mol·K)) [28]. $E_a$ is a potential barrier, which is equal to $E_{\mathit{ad}}$ in this work. In other words, the recovery time of a sensing material is the time required for desorption. As exhibited in Figure 7, we list the various temperatures of 298, 448, and 598 K to contrast and analyze the desorption property of the Ni-PtSe₂ monolayer based on the impact of temperature on τ.

From Formula (4), it is obvious that T and $E_a$ are the two significant parameters that affect the value of τ, where a lower $E_a$ and a higher temperature will give rise to a smaller τ in the case of other parameters remaining the same, respectively. As portrayed in Figure 7, one can see that the desorption of CO and C₂H₂ from the Ni-PtSe₂ monolayer are rather tough at room temperature, while the recovery time drops along with the boosting of temperature. On the flip side, the long recovery time for CO and C₂H₂ desorption at room temperature also confirm their strong chemical adsorption strength. We can find that the recovery times in CO and C₂H₂ systems dive to 21.34 s and 0.128 s at 598 K, respectively, which implies that the Ni-PtSe₂ monolayer as a gas sensing material can be reusable with a temperature of 598 K for the detection of CO and C₂H₂. Beyond that, the recovery time with various temperatures in the C₂H₂ system is lower than that in the CO system as a result of the weaker binding force with the Ni-PtSe₂ monolayer. In the meantime, the longer desorption time for CO just illustrates the stronger chemisorption compared with C₂H₂. However, we must consider the thermostability of this material and the high energy consumption at high temperatures. The typical highest operating temperature of a transformer reaches around 60 to 90 degrees, namely 333 K to 363 K, approximately. Though the recovery time of CO and C₂H₂ adsorption systems between 60 and 90 degrees is not short, it can be improved by other means, such as increasing the operating temperature of the sensor, which will be further studied in depth later. Hence, the Ni-PtSe₂ monolayer is a satisfied gas scavenger upon the noxious gas dissolved in transformer oil, but its recovery time at room temperature is not satisfactory.

4. Conclusions

In this work, the theoretical analysis for the sensing performance of Ni-PtSe₂ monolayer towards two dissolved gases in transformer oil, namely CO and C₂H₂, is conducted by the first-principles theory in order to explore the potential of this novel material for enhancing the operation status of the transformer in power systems. We use a Ni atom as the dopant doped on the pure PtSe₂ surface to form the Ni-PtSe₂ supercell, and then simulate the adsorption of CO and C₂H₂ on the Ni-PtSe₂ monolayer to explore its adsorption performance. The main conclusions from the whole paper can be summed up as followed:

(1)

The substituted Ni atom drops into the Se vacancy, which forms the optimized configuration.

(2)

The sensing property of Ni-PtSe₂ monolayer toward the dissolved gas in transformer oil, namely CO and C₂H₂, is sorted in ascending order as CO > C₂H₂, where the ${\mathrm{E}}_{\mathrm{ad}}$ of two gas systems are calculated as −1.583 eV and −1.319 eV, respectively, both classified as chemisorption, and the stronger interaction in the CO system is further verified by the BS and DOS analysis.

(3)

This novel gas sensing material upon CO and C₂H₂ detection possesses high sensitivity, and the decreasing bandgap of the CO and C₂H₂ systems can bring on the increase in electrical conductivity of the Ni-PtSe₂ monolayer.

(4)

Ni-PtSe₂ monolayer is a reusable gas scavenger for detecting CO and C₂H₂ removal from a transformer at 598 K, but its recoverability needs to be further explored and improved at room temperature.

In conclusion, the above findings in this theoretical work indicate that the Ni-PtSe₂ monolayer is a promising candidate for scavenging the noxious gas decomposed in transformer oil, which will be extremely meaningful for gas-sensing application in the electrical field and be of great reference significance for future researchers.