Dissolved Gas Analysis in Transformer Oil Using Ni Catalyst Decorated PtSe 2 Monolayer: A DFT Study

: In this paper, the ﬁrst-principles theory is used to explore the adsorption behavior of Ni catalyst decorated PtSe 2 (Ni-PtSe 2 ) monolayer toward the dissolved gas in transformer oil, namely CO and C 2 H 2 . Some Ni atoms from the catalyst are trapped in the Se vacancy on the pure PtSe 2 surface. The geometry conﬁgurations of Ni-PtSe 2 monolayer before and after gas adsorption, the electronic property of Ni-PtSe 2 monolayer upon gas adsorption, and the sensibility and recovery property of Ni-PtSe 2 monolayer are explored in this theoretical work. Through the simulation, the E ad of CO and C 2 H 2 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-PtSe 2 monolayer on CO molecule, which is further supported by the DOS and BS analysis. According to the formula, the sensitivity of Ni-PtSe 2 monolayer towards CO and C 2 H 2 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-PtSe 2 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-PtSe 2 monolayer upon the detection of CO and C 2 H 2 , which may realize related applications, and provide the basis and reference to cutting-edge research in the ﬁeld of electricity in the future.


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

Computational Details
In this work, the total geometric and electronic calculations on the basis of firstprinciples theory were carried out in the DMol 3 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 PtSe 2 monolayer supercell to perform the whole firstprinciples 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 PtSe 2 monolayer Chemosensors 2022, 10, 292 3 of 12 was obtained as 3.71 Å, in accordance with data from previous research reports [36]. The adsorption energy (E ad ) was calculated by E ad = E Ni−PtSe 2 /gas − E Ni−PtSe 2 − E gas (1) where in E Ni−PtSe 2 /gas , E Ni−PtSe 2 and E gas were the energies of the gas adsorbed system, isolated Ni-PtSe 2 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.

Analysis of Ni-PtSe 2 Monolayer and Gas Species
As shown in Figure 1, we optimize CO and C 2 H 2 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 C 2 H 2 , the C-O bond in CO (1.142 Å) is longer than the C-H bonds in C 2 H 2 (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 C 2 H 2 (1.211 Å) is longer than the C-O bond in CO. 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 (QT) 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.

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 Ead 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 Ead 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 After theoretical calculations, we found that the E ad of pristine PtSe 2 monolayer adsorbing CO and C 2 H 2 are 0.26 and 0.21 eV, respectively. The magnitude of the interaction between the pristine PtSe 2 monolayer and the gas molecules during the adsorption process is quite weak. Therefore, the pristine PtSe 2 monolayer is not suitable as a sensing material for detecting CO and C 2 H 2 .
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 PtSe 2 monolayer is usually easier to be formed than the Pt vacancy [38,39]. The lattice constant and interatomic bond length of the PtSe 2 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 PtSe 2 monolayer to the gas molecules can be achieved by atomic doping. Figure 2a exhibits the structurally optimized Se-vacancy from PtSe 2 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 PtSe 2 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-PtSe 2 system. This finding shows that the geometry of the PtSe 2 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 MoS 2 system [17]. In order to determine the chemical stability of the Ni-PtSe 2 monolayer, the value of E ad between Ni atom and S vacancy PtSe 2 monolayer was calculated. The result shows that the binding strength of the Ni-PtSe 2 monolayer (−4.50 eV) is much higher than that of the intrinsic PtSe 2 monolayer, indicating that its chemical stability is greatly improved compared to that of the intrinsic PtSe 2 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 PtSe 2 monolayer caused by the Ni dopant.
Chemosensors 2022, 10, x FOR PEER REVIEW 4 of 12 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. 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 PtSe 2 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 PtSe 2 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 PtSe 2 monolayer, Figure 3 exhibits the band structure (BS) and density of state (DOS) of the Ni-PtSe 2 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 PtSe 2 monolayer which is an indirect semiconductor is about 1.2 eV [23]. While the calculated bandgap for the Ni-PtSe 2 system shrinks down to 0.849 eV compared to the intrinsic PtSe 2 monolayer, this indicates that doping of the Ni atom can improve the electrical conductivity of the pristine PtSe 2 monolayer and Ni-PtSe 2 monolayer takes on the metallic properties. One can see from Figure 3b where the DOS of the PtSe 2 monolayer with Se-vacancy and Ni-PtSe 2 systems are both exhibited, several novel peaks emerge from the DOS curve of the PtSe 2 monolayer with Se-vacancy attributed to the doping of Ni dopant, bringing on the remarkably increased states of Ni-PtSe 2 monolayer at the Fermi level, from which the conclusion can be drawn that the conductivity of Ni-PtSe 2 has been enhanced. Besides, the DOS curve of the Ni-PtSe 2 monolayer is shifted to the right and a lower region compared with the PtSe 2 monolayer with Se-vacancy, since the electron-contributing behavior of the PtSe 2 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 PtSe 2 monolayer, which will lead to a higher conductivity.

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 E 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 E ad is between 0.1 and 0.6 eV, and chemical adsorption when the E ad is greater than 0.8 eV. The E 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.

Gas Adsorption Configurations on Ni-Ptse 2 Monolayer
Various gas adsorption configurations were established for the sake of exploring whether the Ni-PtSe 2 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-PtSe 2 surface is depicted, the CO molecule lies diagonally above the Ni atom, with the plane of Ni-PtSe 2 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-PtSe 2 system. The C-O bond length elongates from 1.142 Å to 1.158 Å. These phenomena manifest interaction between the Ni-PtSe 2 monolayer and CO molecule, giving rise to apparent deformation toward the geometric structure of the Ni-PtSe 2 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 E 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 E ad is between 0.1 and 0.6 eV, and chemical adsorption when the E ad is greater than 0.8 eV. The E 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-PtSe 2 monolayer. The Ni dopant as an electron embracer, carries 0.133 e from the Ni-PtSe 2 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. nsors 2022, 10, x FOR PEER REVIEW 6 of 12 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 E 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.  For the C 2 H 2 adsorption system, the different views of MSC, as well as EDD, are exhibited in Figure 5. It can be found that the C 2 H 2 molecule traps in the Ni-PtSe 2 surface, with the parallel position and a small slope to the Ni-PtSe 2 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 C 2 H 2 molecule is absorbed into the Ni dopant, the linear structure of the C 2 H 2 molecule, namely the C≡C bond, gets stretched, and elongated from 1.211 Å to 1.262 Å, which shows that the C 2 H 2 molecule is highly activated during gas adsorption. Moreover, chemisorption can be confirmed in the C 2 H 2 adsorption system, on account of the simulation result that the calculated E ad is −1.319 eV, larger than the critical value of 0.8 eV. Based on the Hirshfeld analysis, the C 2 H 2 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 C 2 H 2 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 C 2 H 2 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 C 2 H 2 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-PtSe 2 monolayer conducts strong chemisorption on CO and C 2 H 2 , with its adsorption strength upon two gases in the order CO > C 2 H 2 , judging by the value of E ad . 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.

Electronic Property of Ni-Ptse 2 Monolayer upon Gas Adsorption
The BS and DOS of CO and C 2 H 2 systems are exhibited in Figure 6 to further analyze the electronic behavior of the Ni-PtSe 2 monolayer upon gas adsorption. We can clearly find that the bandgap of the Ni-PtSe 2 monolayer is calculated as 0.673 eV and 0.489 eV, respectively, after adsorption of CO and C 2 H 2 molecules, less than that of 0.849 eV in the pristine Ni-PtSe 2 system, which implies that the electrical conductivity of the Ni-PtSe 2 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 C 2 H 2 > CO, contrary to the order of E ad : CO > C 2 H 2 . 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 E ad . Therefore, a conclusion can be drawn that the CO adsorption system matched with a larger bandgap possesses superior sensing response properties than the C 2 H 2 system.

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 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-PtSe 2 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 C 2 H 2 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-PtSe 2 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 C 2 H 2 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 C 2 H 2 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-PtSe 2 monolayer can be effectively explored in the dissolved gas of transformer oil on account of the change of electrical conductivity after gas adsorption.

Sensing Response and Recovery Property of Ni-PtSe 2 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-PtSe 2 monolayer. Furthermore, the variational electrical conductivity can result in the altered sensitivity of the Ni-PtSe 2 monolayer. We list the following two formulas for electrical conductivity (σ) and sensitivity (S) of the Ni-PtSe 2 surface to accurately capture the small change in the electronic behavior [10,22].
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-PtSe 2 monolayer, respectively. Based on formula (2), the decreasing bandgap of the CO and C 2 H 2 systems can give rise to the increase in electrical conductivity of the Ni-PtSe 2 monolayer, which provides the basic mechanism to explore the electrical conductivity of various adsorption systems for the Ni-PtSe 2 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-PtSe 2 monolayer towards CO and C 2 H 2 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-PtSe 2 monolayer with high sensitivity to detect CO and C 2 H 2 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]: where A is the attempt frequency (10 12 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 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-PtSe 2 monolayer based on the impact of temperature on τ.
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.

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: 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 C 2 H 2 from the Ni-PtSe 2 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 C 2 H 2 desorption at room temperature also confirm their strong chemical adsorption strength. We can find that the recovery times in CO and C 2 H 2 systems dive to 21.34 s and 0.128 s at 598 K, respectively, which implies that the Ni-PtSe 2 monolayer as a gas sensing material can be reusable with a temperature of 598 K for the detection of CO and C 2 H 2 . Beyond that, the recovery time with various temperatures in the C 2 H 2 system is lower than that in the CO system as a result of the weaker binding force with the Ni-PtSe 2 monolayer. In the meantime, the longer desorption time for CO just illustrates the stronger chemisorption compared with C 2 H 2 . 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 C 2 H 2 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-PtSe 2 monolayer is a satisfied gas scavenger upon the noxious gas dissolved in transformer oil, but its recovery time at room temperature is not satisfactory.

Conclusions
In this work, the theoretical analysis for the sensing performance of Ni-PtSe 2 monolayer towards two dissolved gases in transformer oil, namely CO and C 2 H 2 , 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 PtSe 2 surface to form the Ni-PtSe 2 supercell, and then simulate the adsorption of CO and C 2 H 2 on the Ni-PtSe 2 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-PtSe 2 monolayer toward the dissolved gas in transformer oil, namely CO and C 2 H 2 , is sorted in ascending order as CO > C 2 H 2 , where the E 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 C 2 H 2 detection possesses high sensitivity, and the decreasing bandgap of the CO and C 2 H 2 systems can bring on the increase in electrical conductivity of the Ni-PtSe 2 monolayer. (4) Ni-PtSe 2 monolayer is a reusable gas scavenger for detecting CO and C 2 H 2 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-PtSe 2 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.