Pt Cluster Modified h-BN for Gas Sensing and Adsorption of Dissolved Gases in Transformer Oil: A Density Functional Theory Study

Hexagonal-Boron nitride nanotubes (h-BN) decorated with transition metals have been widely studied due to their enhanced physicochemical properties. In this paper, Pt cluster-modified h-BN is proposed as a sensitive material for a novel gas sensor for the online malfunction monitoring of oil-immersed transformers. The inner oil is ultimately decomposed to various gases during the long-term use of oil-immersed transformers. Exposure to excessively high temperatures produces the alkanes CH4 and C2H6, whereas different degrees of discharge generate H2 and C2H2. Therefore, the identification of H2, CH4, and C2H2 gas efficiently measures the quality of transformers. Based on the density functional theory, the most stable h-BN doped with 1–4 Pt atoms is employed to simulate its adsorption performance and response behavior to these typical gases. The adsorption energy, charge transfer, total density of states, projected density of states, and orbital theory of these adsorption systems are analyzed and the results show high consistency. The adsorption ability for these decomposition components are ordered as follows: C2H2 > H2 > CH4. Pt cluster-modified h-BN shows good sensitivity to C2H2, H2, with decreasing conductivity in each system, but is insensitive to CH4 due to its weak physical sorption. The conductivity change of Ptn-h-BN is considerably larger upon H2 than that upon C2H2, but is negligible upon CH4. Our calculations suggest that Pt cluster modified h-BN can be employed in transformers to estimate their operation status.


Introduction
Oil-immersed transformers are extensively employed in modern power systems. Oil-immersed transformers are recognized as one of the most essential electric equipment because of their outstanding power conversion function and universal application. However, inevitable malfunctions associated with the long-term use of transformers, such as local overheating, partial discharge, and equipment wear out, are likely to cause catastrophic damage to the entire power system. Thus, effective methods for the online monitoring of transformers have been developed by the power sector. Along with different types of faults, various impurity gases, including H 2 , CH 4 , C 2 H 6 , and C 2 H 2 , are generated by transformers [1][2][3]. Exposure to excessive temperature produces the alkanes CH 4 and C 2 H 6 , whereas different degrees of discharge generate H 2 and C 2 H 2 . Given that H 2 , CH 4 , and C 2 H 2 are the three typical gases in transformer faults, their identification effectively measures the quality of transformers. The condition of the transformer can be speculated by analyzing the concentration of these three typical gases and the variation of conductivity in the oil. Given the good sensitivity and versatility of where Qt indicates the charge transfer amount from the gas molecules to the surface of Pt cluster modified h-BN though Mulliken population analysis while Q1 and Q2 represent the total net charge after and before gas adsorption, respectively. In Equation (3), Eads represents the total energy of the gas molecules before adsorption. A negative Eads indicates that the adsorption process is exothermic and spontaneous. The adsorption mechanism can greatly affect the conductivity and gas concentration of the entire system, which is beneficial to fabricate a gas sensor with selectivity and sensitivity. The electrical conductivity is related to energy gap and the change of energy gap will affect conductivity exponentially [28]. To further obtain the conductivity of the entire system, we determined the energy gap (Eg) of the molecular orbital between the highest occupied molecular orbital (HOMO) and the lowest occupied molecular orbital (LUMO), as defined in Equation (4) [29].
Total density of states (TDOS) and projected density of states (PDOS) in the adsorption are analyzed in detail for comprehension of the mechanism of the adsorption process.

Optimized Structure of Gas Molecules and Intrinsic h-BN
As shown in Figure 1, the optimized structure of gas molecules and intrinsic h-BN are presented. Both H2 and C2H2 are linear gas molecules, whereas CH4 is a three-dimensional tetrahedral structure, the H-H bond length in the H2 molecule is 0.75 Å, given the weak atomic force and short atomic radius of the H atom. C-H bond lengths are 1.10 and 1.07 Å in CH4 and C2H2 molecules. Different orbital hybridization causes slight difference in the C-H bond lengths. In the CH4 molecule, the H-C-H bond angle is 103.39°. The symmetrical structure and four C-H bonds of the CH4 molecule imparts its chemical stability. An h-BN with a perfect crystal structure is shown in Figure 1b. The mesh-like hexagonal structure is beneficial to the adsorption performance. The B-N bond length of 1.44 Å is appropriate for structural stability, both axial and circumferential distances are around this value. When all aspects are considered, h-BN is a suitable material for gas adsorption.  Figure 2 shows the different doping structures with 1-4 Pt atoms. The adsorption distance, charge transfer, and adsorption energy are calculated to find the most stable structures with different doping Pt atoms.  Figure 2 shows the different doping structures with 1-4 Pt atoms. The adsorption distance, charge transfer, and adsorption energy are calculated to find the most stable structures with different doping Pt atoms.  Figure 2a shows the most stable structure doped with one Pt atom, which is also the only possible structure at this situation. The atomic arrangement of h-BN exhibits central symmetry and requires the location of the Pt atom to be centered. We considered different situations in the experiments. However, only the bridge site for Pt atom doping can be optimized successfully. The Pt-N bond length is 2.00 Å, which is slightly shorter than the Pt-B bond length (2.19 Å). This phenomenon illustrates the strong force between Pt and N atoms. The adsorption energy (Ed) of −1.98 eV demonstrates fine stability from the perspective of energy.

Pt Cluster Modified h-BN
Two Pt-atom doping is also considered in this work. Different systems have been computed, as presented in Figure 2b1,b2. The two structures are the adjacent and opposite sides of the hexagon where Pt atom doping occurs. The Pt-Pt bond lengths are 2.51 and 2.47 Å, meeting the bond length between heavy metals. The Ed values of the adjacent and opposite sides are −4.83 and −4.84 eV, as shown in Figure 2b1. Dispersed Pt atoms may cause the two-and single-atom-doped structures to be similar.
As shown in Figure 2c1-c5, five possible models were generated. The clustering of the three metal atoms has been widely employed in the field of materials. Thus, the Pt3 cluster is treated emphatically in this work to enhance gas sensitivity response. The surface structure of the intrinsic h-BN is greatly altered after Pt3 modification. The atom tends to protrude outward. Ed values from (c1) to (c5) are −9.02, −8.16, −8.31, −8.30, and −8.30 eV, respectively. A high Ed proves that the structure shown in Figure 2c1 exhibits stronger stability compared with the other structures.
As shown in Figure 2d1-d5, the Pt4 cluster is employed for analysis. Many possible structures are also obtained. We finally selected the most stable structure according to the Ed and geometry  Figure 2a shows the most stable structure doped with one Pt atom, which is also the only possible structure at this situation. The atomic arrangement of h-BN exhibits central symmetry and requires the location of the Pt atom to be centered. We considered different situations in the experiments. However, only the bridge site for Pt atom doping can be optimized successfully. The Pt-N bond length is 2.00 Å, which is slightly shorter than the Pt-B bond length (2.19 Å). This phenomenon illustrates the strong force between Pt and N atoms. The adsorption energy (E d ) of −1.98 eV demonstrates fine stability from the perspective of energy.
Two Pt-atom doping is also considered in this work. Different systems have been computed, as presented in Figure 2b1,b2. The two structures are the adjacent and opposite sides of the hexagon where Pt atom doping occurs. The Pt-Pt bond lengths are 2.51 and 2.47 Å, meeting the bond length between heavy metals. The E d values of the adjacent and opposite sides are −4.83 and −4.84 eV, as shown in Figure 2b1. Dispersed Pt atoms may cause the two-and single-atom-doped structures to be similar.
As shown in Figure 2c1-c5, five possible models were generated. The clustering of the three metal atoms has been widely employed in the field of materials. Thus, the Pt 3 cluster is treated emphatically in this work to enhance gas sensitivity response. The surface structure of the intrinsic h-BN is greatly  Figure 2c1 exhibits stronger stability compared with the other structures.
As shown in Figure 2d1-d4, the Pt 4 cluster is employed for analysis. Many possible structures are also obtained. We finally selected the most stable structure according to the E d and geometry structure, as shown in Figure 2d1. The adsorption energy of this structure is −12.59 eV, which is higher than that of any other structure. The three-dimensional feature also imparts stability to this structure.
To gain insight into the diversification in conductivity, we studied the band structure of systems in Figure 2a,b2,c1,d1. This analysis allows for the advanced understanding of the gas adsorption mechanism. The band structure of the systems is drawn as follows: As shown in Figure 3, the band gaps from Figure 3a-e are 3.73, 1.50, 0.30, 0.0, and 0.38 eV, respectively. The intrinsic h-BN exhibits a huge band gap of 3.73 eV, which explains the difficulty for electrons to jump from the top of the valence band to the bottom of the conduction band. This result is also consistent with h-BN being an insulator. After Pt cluster modification, the band gap in each case is greatly decreased, enhancing electrical conductivity throughout the system. After Pt 3 doping, the entire band structure is almost continuous, which may be largely due to the doping position. Nanomaterials 2019, 9, 1746 5 of 16 higher than that of any other structure. The three-dimensional feature also imparts stability to this structure.
To gain insight into the diversification in conductivity, we studied the band structure of systems in Figure 2a,b2,c1,d1. This analysis allows for the advanced understanding of the gas adsorption mechanism. The band structure of the systems is drawn as follows: As shown in Figure 3, the band gaps from Figure 3a-e are 3.73, 1.50, 0.30, 0.0, and 0.38 eV, respectively. The intrinsic h-BN exhibits a huge band gap of 3.73 eV, which explains the difficulty for electrons to jump from the top of the valence band to the bottom of the conduction band. This result is also consistent with h-BN being an insulator. After Pt cluster modification, the band gap in each case is greatly decreased, enhancing electrical conductivity throughout the system. After Pt3 doping, the entire band structure is almost continuous, which may be largely due to the doping position.

Adsorption of H2, CH4, C2H2 on the Surface of Intrinsic h-BN
The optimization results for the adsorption of H2, CH4, and C2H2 on intrinsic h-BN are presented in Figure 4. The geometric optimization results were calculated as the lowest energy structure according to the DFT, that is, relatively thermodynamic stability, so as to determine the adsorption energy and adsorption distance. According to the final analysis results, intrinsic h-BN exhibits low gas sensitivity response to these typical gases. The H2, C2H2, and CH4 molecular structures remain unchanged during the adsorption process, and the long adsorption distance and weak adsorption energy suggest a weak physical mechanism between gas molecules and the intrinsic h-BN. The specific parameters in the adsorption process are provided in Table 1 to further explain the adsorption performance.

Adsorption of H 2 , CH 4 , C 2 H 2 on the Surface of Intrinsic h-BN
The optimization results for the adsorption of H 2 , CH 4 , and C 2 H 2 on intrinsic h-BN are presented in Figure 4. The geometric optimization results were calculated as the lowest energy structure according to the DFT, that is, relatively thermodynamic stability, so as to determine the adsorption energy and adsorption distance. According to the final analysis results, intrinsic h-BN exhibits low gas sensitivity response to these typical gases. The H 2 , C 2 H 2 , and CH 4 molecular structures remain unchanged during the adsorption process, and the long adsorption distance and weak adsorption energy suggest a weak physical mechanism between gas molecules and the intrinsic h-BN. The specific parameters in the adsorption process are provided in Table 1 to further explain the adsorption performance.
The distance of the gas molecules to h-BN is very far, ranging from 3.23 Å to 3.63 Å, explaining the weak interaction between the molecules. The adsorption energies of H2, CH4, and C2H2 are −0.08, −0.06, and −0.10 eV, respectively. Such small adsorption energy values also correspond to the long adsorption distance. The final simulation data show that the intrinsic h-BN response to these three gases is ordered as follows: C2H2 > H2 > CH4. This order seems to be related to the characteristics of the gas molecules.

Adsorption of Gas Molecules on Pt, Pt2 Doped h-BN
Figure 5a-f depicts the gas adsorption system on Pt-and the Pt2-doped h-BN. The calculation parameters are shown in Table 2. Based on the optimized adsorption structure, Pt doping can improve the gas sensitivity of the entire system, especially for H2 and C2H2, where strong chemical adsorption process occurs after Pt and Pt2 doping. However, the adsorption capacity for CH4 is insufficient, with merely weak physical and catalytic effects between the molecules. The adsorption capacity based on the simulation results is as follows: C2H2 > H2 > CH4, and the effects in all occasions are strengthened.  1.97 Å −3.40 eV 0.14 e   The distance of the gas molecules to h-BN is very far, ranging from 3.23 Å to 3.63 Å, explaining the weak interaction between the molecules. The adsorption energies of H 2 , CH 4 , and C 2 H 2 are −0.08, −0.06, and −0.10 eV, respectively. Such small adsorption energy values also correspond to the long adsorption distance. The final simulation data show that the intrinsic h-BN response to these three gases is ordered as follows: C 2 H 2 > H 2 > CH 4 . This order seems to be related to the characteristics of the gas molecules.  Table 2. Based on the optimized adsorption structure, Pt doping can improve the gas sensitivity of the entire system, especially for H 2 and C 2 H 2 , where strong chemical adsorption process occurs after Pt and Pt 2 doping. However, the adsorption capacity for CH 4 is insufficient, with merely weak physical and catalytic effects between the molecules. The adsorption capacity based on the simulation results is as follows: C 2 H 2 > H 2 > CH 4 , and the effects in all occasions are strengthened. Figure 5a-d shows that the H-H bond is broken during the adsorption process. Two H atoms are bonded to the Pt atom at a bond length of 1.55 Å, and the adsorption energy in both cases are 1.94 and 1.97 eV. No difference is found between single-and two-atom doping due to the far distance between the H 2 molecule and the second Pt atom. After Pt atom doping, E ads is increased from −0.08 eV to −1.94 eV. The rapid increase of E ads with a large Q t (0.26 e) indicates that the Pt atom can enhance the adsorption activity of the system, thereby producing a strong chemical action with the H 2 molecule.

Adsorption of Gas Molecules on Pt, Pt 2 Doped h-BN
The optimization results for CH 4 adsorption are shown in Figure 5b-e. Compared with intrinsic h-BN, the Pt-and Pt 2 -doped h-BN can considerably improve the adsorption capacity, as proven by the large E ads and short adsorption process distance. The bond lengths of the H atom in the CH 4 molecules to the Pt atom are 1.76 and 2.53 Å, and the molecular structure of CH 4 remains substantially unchanged. We speculate that the role between the CH 4 and Pt is biased toward physical adsorption.
The structure in Figure 5e shows a weaker effect, and the stability of the doped structure causes the lower adsorption of diatomic doping compared with that of monoatomic doping. Nanomaterials 2019, 9,   Figure 5a-d shows that the H-H bond is broken during the adsorption process. Two H atoms are bonded to the Pt atom at a bond length of 1.55 Å, and the adsorption energy in both cases are 1.94 and 1.97 eV. No difference is found between single-and two-atom doping due to the far distance between the H2 molecule and the second Pt atom. After Pt atom doping, Eads is increased from −0.08 eV to −1.94 eV. The rapid increase of Eads with a large Qt (0.26 e) indicates that the Pt atom can enhance the adsorption activity of the system, thereby producing a strong chemical action with the H2 molecule.
The optimization results for CH4 adsorption are shown in Figure 5b-e. Compared with intrinsic h-BN, the Pt-and Pt2-doped h-BN can considerably improve the adsorption capacity, as proven by the large Eads and short adsorption process distance. The bond lengths of the H atom in the CH4 molecules to the Pt atom are 1.76 and 2.53 Å, and the molecular structure of CH4 remains substantially unchanged. We speculate that the role between the CH4 and Pt is biased toward physical adsorption.
The structure in Figure 5e shows a weaker effect, and the stability of the doped structure causes the lower adsorption of diatomic doping compared with that of monoatomic doping. Figure 5c-e demonstrates the large influence between C2H2 molecule and doped h-BN. In the single Pt atom-doped system, the Pt-B bond is fractured because of the great force, and the structure of C2H2 is changed from a straight line to a distorted planar structure. In the two Pt atom-doped system, the degree of distortion is evident. Eads with magnitudes of 2.60 and 3.4 eV at different situations suggest a complicated process that occurs, and the adsorption effect is improved by a leap.    In the single Pt atom-doped system, the Pt-B bond is fractured because of the great force, and the structure of C 2 H 2 is changed from a straight line to a distorted planar structure. In the two Pt atom-doped system, the degree of distortion is evident. E ads with magnitudes of 2.60 and 3.4 eV at different situations suggest a complicated process that occurs, and the adsorption effect is improved by a leap.
For improved understanding of the adsorption for manufacturing suitable gas sensors, the total density of states (TDOS), partial density of states (PDOS), and molecular orbitals for different gas adsorption scenarios were analyzed. An accurate sensor applied to the online monitoring of transformers can be implemented by identifying different parameters, including conductivity and E g .
As shown in Figure 6a-f, the TDOS is changed after gas adsorption, and Figure 6a1-f1 presents the PDOS of corresponding situations. The adsorption of different gases shows varying effects on the TDOS. The conductivity of the entire system can be precisely modified through TDOS analysis. Finally, a sensitive gas sensor can be fabricated by analyzing the resistance value of the system. PDOS was calculated to explore advanced adsorption mechanisms, particularly the mechanism of intermolecular chemical bonding. A combination analysis can provide accurate simulation data applied to gas sensor manufacturing. Nanomaterials 2019, 9, 1746 8 of 16 For improved understanding of the adsorption for manufacturing suitable gas sensors, the total density of states (TDOS), partial density of states (PDOS), and molecular orbitals for different gas adsorption scenarios were analyzed. An accurate sensor applied to the online monitoring of transformers can be implemented by identifying different parameters, including conductivity and Eg.
As shown in Figure 6a-f, the TDOS is changed after gas adsorption, and Figure 6a1-f1 presents the PDOS of corresponding situations. The adsorption of different gases shows varying effects on the TDOS. The conductivity of the entire system can be precisely modified through TDOS analysis. Finally, a sensitive gas sensor can be fabricated by analyzing the resistance value of the system. PDOS was calculated to explore advanced adsorption mechanisms, particularly the mechanism of intermolecular chemical bonding. A combination analysis can provide accurate simulation data applied to gas sensor manufacturing.  In TDOS, both H 2 and C 2 H 2 show remarkable changes in TDOS, but the change in CH 4 is not evident. In a single-atom-doped system, TDOS is approximately equal to 0 eV between the conduction band and the valence band, reflecting the decline in conductivity. However, the occurrence of this Nanomaterials 2019, 9, 1746 9 of 16 phenomenon is minimal in the two-atom-doped system, because two-Pt-atoms doping increases the conductivity of the system. As shown in Figure 1a, TDOS moves to the right as a whole and then drops slightly at the right side of the Fermi level. This result indicates the reduced number of electronic fillings of the conduction band. Furthermore, the conductive property of the structure is decreased, the reason for which can be analyzed from the distribution of PDOS. Considerable orbital hybridization exists in the 5d orbital of Pt and the 1s orbital of H, which is related to the formation of Pt-H bond in the adsorption structure. Strong chemical action decreases conductivity, but this effect is minimal in the two Pt atoms doping system.
In the TDOS and PDOS of CH 4 adsorption, no considerable change is found, as shown in Figure 7b-e. Therefore, the electrical conductivity of the entire system remains unchanged. The 2p orbital of C is mainly distributed between −7.5 and −5 eV. Therefore, the effect on the entire energy band structure is minimal, as demonstrated by the weak physical function and small E ads (0.97 and 0.1 eV). Such feature can be used to study new types of sensors with selectivity. evident. In a single-atom-doped system, TDOS is approximately equal to 0 eV between the conduction band and the valence band, reflecting the decline in conductivity. However, the occurrence of this phenomenon is minimal in the two-atom-doped system, because two-Pt-atoms doping increases the conductivity of the system. As shown in Figure 1a, TDOS moves to the right as a whole and then drops slightly at the right side of the Fermi level. This result indicates the reduced number of electronic fillings of the conduction band. Furthermore, the conductive property of the structure is decreased, the reason for which can be analyzed from the distribution of PDOS. Considerable orbital hybridization exists in the 5d orbital of Pt and the 1s orbital of H, which is related to the formation of Pt-H bond in the adsorption structure. Strong chemical action decreases conductivity, but this effect is minimal in the two Pt atoms doping system.
In the TDOS and PDOS of CH4 adsorption, no considerable change is found, as shown in Figure  7b-e. Therefore, the electrical conductivity of the entire system remains unchanged. The 2p orbital of C is mainly distributed between −7.5 and −5 eV. Therefore, the effect on the entire energy band structure is minimal, as demonstrated by the weak physical function and small Eads (0.97 and 0.1 eV). Such feature can be used to study new types of sensors with selectivity.
The same analytical method was used to analyze the adsorption of C2H2. The Pt 5d and C 2p orbitals show simultaneous peaks at the same energy level. The orbital hybridization illustrates the strong interaction between the Pt and C atoms, and the high effect between these atoms also reduces the distribution of TDOS around the Fermi level. Thus, the conductivity of the entire system will also decrease. Frontier molecular orbital theory was analyzed for different systems, and the effect can be obtained based on the electronic behavior of Pt-and Pt2-doped h-BN in the presence of gas molecules. Determining the features that can be practically modified will be helpful for the exploitation of gas sensors. Based on molecular orbital theory, we calculated the HOMO and LUMO distributions of the adsorption system, as shown in Figure 7. We also calculated Eg to evaluate conductivity changes, as presented in Table 3. HOMO and LUMO are mostly located near the Pt atom doping site, which is associated with the good conductivity and insulation properties of h-BN. After gas adsorption, the LUMO position is drastically changed, whereas the HOMO is slightly altered. As shown in Figure  7a-d, the 1.51 eV Eg of Pt-h-BN reflects good conductivity. When the gas molecules are close to h-BN, Eg is rapidly increased to >2.8 eV. As gas molecules transfer charge to the h-BN system, many LUMOs are on the surface of the gas molecules. In the H2 molecule model, Eg can be remarkably improved, and a strong chemical adsorption process occurs, greatly reducing the conductivity of the entire The same analytical method was used to analyze the adsorption of C 2 H 2 . The Pt 5d and C 2p orbitals show simultaneous peaks at the same energy level. The orbital hybridization illustrates the strong interaction between the Pt and C atoms, and the high effect between these atoms also reduces the distribution of TDOS around the Fermi level. Thus, the conductivity of the entire system will also decrease.
Frontier molecular orbital theory was analyzed for different systems, and the effect can be obtained based on the electronic behavior of Pt-and Pt 2 -doped h-BN in the presence of gas molecules. Determining the features that can be practically modified will be helpful for the exploitation of gas sensors. Based on molecular orbital theory, we calculated the HOMO and LUMO distributions of the adsorption system, as shown in Figure 7. We also calculated E g to evaluate conductivity changes, as presented in Table 3. HOMO and LUMO are mostly located near the Pt atom doping site, which is associated with the good conductivity and insulation properties of h-BN. After gas adsorption, the LUMO position is drastically changed, whereas the HOMO is slightly altered. As shown in Figure 7a-d, the 1.51 eV E g of Pt-h-BN reflects good conductivity. When the gas molecules are close to h-BN, E g is rapidly increased to >2.8 eV. As gas molecules transfer charge to the h-BN system, many LUMOs are on the surface of the gas molecules. In the H 2 molecule model, E g can be remarkably improved, and a strong chemical adsorption process occurs, greatly reducing the conductivity of the entire system. Based on the comparison of different situations, almost no HOMO and LUMO distribution occurs on the CH 4 molecules, and weak physical effects cannot largely change the conductivity. The contribution of C 2 H 2 to the conductivity is slightly lesser than that of H 2 . Thus, the conductivity of the entire system is slightly higher than that of the H 2 adsorption system. Figure 7e-h show that the HOMO and LUMO distribution of the gas molecule/Pt 2 -h-BN system presents a similar situation. CH 4 exhibits a minor role, and E g remains unchanged. In general, the molecular adsorption of H 2 shows the largest influence on conductivity, whereas that of CH 4 exhibits nearly no effect on conductivity. Table 3. Molecular orbitals and energy gaps of individual molecules before and after adsorption.

Adsorption system
Structure Pt-h-BN Figure 7a  In summary, for different adsorption scenarios, conductivity decreases at varying degrees. H 2 molecular adsorption exhibits the greatest influence on conductivity, followed by C 2 H 2 adsorption. CH 4 molecule adsorption is very weak, minimally contributing to conductivity. The estimated final conductivity is arranged as follows: CH 4 adsorption system >C 2 H 2 adsorption system >H 2 adsorption system. Figure 8a-f. Table 4 shows very similar results between Pt-and Pt 2 -doped h-BN systems. Doping structures with more modified Pt atoms are more responsive to H 2 and C 2 H 2 than to CH 4 , and the adsorption process with the CH 4 molecule is very weak. Drastic changes in the charge transfer amount occur at different situations. Different doping structures were compared, and the results are presented as follows.

The gas adsorption system on Pt 3 -and Pt 4 -doped h-BN is depicted in
In the H 2 adsorption system, a strong chemical adsorption process exists between Pt 3 -or Pt 4 -doped structure and the H 2 molecule, the adsorption energy slightly decreasing from −1.97 eV to −1.67 eV. Different degrees of adsorption process mechanism cause large changes in Q t . In the H 2 /Pt 3 -h-BN system, such a large Q t indicates that the H atom acquires numerous electrons. Therefore, great changes in conductivity exist.
In the CH 4 adsorption model, the adsorption distance of approximately 2.30 Å and adsorption energy of <1 eV greatly reflect the weak physical effects, which is consistent with previous conclusions. In the system of C 2 H 2 /Pt 3 -and Pt 4 -doped h-BN, E ads of 2.16 and 2.88 eV are approaching that of the C 2 H 2 /Ptand Pt 2 -doped h-BN systems. This result illustrates that E ads remains unchanged, and only slight differences in conductivity occurs.
The TDOS and PDOS of gas molecule adsorption on Pt 3 -and Pt 4 -doped h-BN are presented in Figure 9. When three Pt atoms are doping, TDOS is decreased rapidly at the Fermi level, and the TDOS to the right of the Fermi level is increased, indicating increased electronic filling conduction band. The band gap of Pt 3 -h-BN is 0.01 eV, which reflects very good electrical conductivity. Additional impurity bands and chemical action worsens the conductivity. Figure 9b,c shows a spike at −5 eV, which is caused by the hybridization of the C-2p orbit and Pt-5d orbit from the PDOS. In C 2 H 2 adsorption, the C-2p and Pt-5d orbits show strong hybridization at many energy levels. Strong molecular forces exhibit a great effect between the two atoms. Thus, the C 2 H 2 molecule undergoes a great bend.  The TDOS and PDOS of gas molecule adsorption on Pt3-and Pt4-doped h-BN are presented in Figure 9. When three Pt atoms are doping, TDOS is decreased rapidly at the Fermi level, and the TDOS to the right of the Fermi level is increased, indicating increased electronic filling conduction band. The band gap of Pt3-h-BN is 0.01 eV, which reflects very good electrical conductivity. Additional impurity bands and chemical action worsens the conductivity. Figure 9b,c shows a spike at −5 eV, which is caused by the hybridization of the C-2p orbit and Pt-5d orbit from the PDOS. In C2H2 adsorption, the C-2p and Pt-5d orbits show strong hybridization at many energy levels. Strong molecular forces exhibit a great effect between the two atoms. Thus, the C2H2 molecule undergoes a great bend.
The overall change of TDOS in the Pt4-h-BN adsorption system is relatively weak, as shown in Figure 9d-f. Only the adsorption of the H2 gas molecule changes the TDOS at the Fermi level, and a low TDOS decreases the conductivity in this situation. As shown in Figure 9e, almost no change occurs in TDOS around the Fermi level, indicating that the CH4 molecule contributes slightly to conductivity. The PDOS from −2.5 eV to 2.5 eV in this case is also a good illustration of this phenomenon. Almost no electronic filling is expected in the Pt atom. Only the Pt doping changes the band structure. Therefore, the conductivity does not change substantially after CH4 adsorption. The results in Figure 9f-f1 are almost identical to the previous analysis of C2H2. Extraordinary orbit hybridizations occur between the C-2p and Pt-5d orbits, but the change in conductivity may be small due to the weak change around the Fermi level.  The overall change of TDOS in the Pt 4 -h-BN adsorption system is relatively weak, as shown in Figure 9d-f. Only the adsorption of the H 2 gas molecule changes the TDOS at the Fermi level, and a low TDOS decreases the conductivity in this situation. As shown in Figure 9e, almost no change occurs in TDOS around the Fermi level, indicating that the CH 4 molecule contributes slightly to conductivity. The PDOS from −2.5 eV to 2.5 eV in this case is also a good illustration of this phenomenon. Almost no electronic filling is expected in the Pt atom. Only the Pt doping changes the band structure. Therefore, the conductivity does not change substantially after CH 4 adsorption. The results in Figure 9f-f1 are almost identical to the previous analysis of C 2 H 2 . Extraordinary orbit hybridizations occur between the C-2p and Pt-5d orbits, but the change in conductivity may be small due to the weak change around the Fermi level. Based on the molecular orbital theory, the HOMO and LUMO distributions of the adsorbed structures were determined using the same formula. Figure 10 shows an intuitive graphical distribution, and Table 5 lists the arranged data. Based on the molecular orbital theory, the HOMO and LUMO distributions of the adsorbed structures were determined using the same formula. Figure 10 shows an intuitive graphical distribution, and Table 5 lists the arranged data.        As shown in Figure 10, Eg is decreased between HOMO and LUMO, indicating that increased metal atom doping can reduce Eg, thereby increasing the conductivity. In other respects, the contribution of gas molecules to electrical conductivity is not as remarkable as previously observed, but the degree of discrimination is still quite valuable. In general, gas adsorption can decrease the conductivity of the system, and conductivity caused by Pt3 doping is more intense than that by Pt4 doping.
In the Pt3 doping system, the Eg of 0.1 eV reflects good electrical conductivity of the system. When accompanied by the adsorption of different gases, the conductivity is decreased, especially when H2 is close to the doped structure. Eg is changed from 0.1 eV to 1.15 eV, indicating that strong adsorption with chemical bond formation decreases electrical conductivity. The role of CH4 is nonnegligible under this model, and the relatively large Eg (0.63 eV) also remarkably changes the conductivity of the system. The same effect is observed in the C2H2 adsorption model. Finally, we analyzed the gas/Pt4-h-BN. As shown in Figure 10e-h, the effect of gas molecules on conductivity is slightly decreased, and no considerable changes occur after gas adsorption.
In summary. The adsorption of different molecules decreases conductivity at varying degrees. The adsorption of H2 and CH4 shows the greatest and weakest influence on conductivity, respectively. The final conductivity is ordered as follows: CH4 adsorption system >C2H2 adsorption system >H2 adsorption system, which is the same as that in Pt-and Pt2-doped h-BN systems.  Table 5. Molecular orbitals and energy gaps of individual molecules before and after adsorption.

Adsorption System
Structure       As shown in Figure 10, E g is decreased between HOMO and LUMO, indicating that increased metal atom doping can reduce E g , thereby increasing the conductivity. In other respects, the contribution of gas molecules to electrical conductivity is not as remarkable as previously observed, but the degree of discrimination is still quite valuable. In general, gas adsorption can decrease the conductivity of the system, and conductivity caused by Pt 3 doping is more intense than that by Pt 4 doping.
In the Pt 3 doping system, the E g of 0.1 eV reflects good electrical conductivity of the system. When accompanied by the adsorption of different gases, the conductivity is decreased, especially when H 2 is close to the doped structure. E g is changed from 0.1 eV to 1.15 eV, indicating that strong adsorption with chemical bond formation decreases electrical conductivity. The role of CH 4 is non-negligible under this model, and the relatively large E g (0.63 eV) also remarkably changes the conductivity of the system. The same effect is observed in the C 2 H 2 adsorption model. Finally, we analyzed the gas/Pt 4 -h-BN. As shown in Figure 10e-h, the effect of gas molecules on conductivity is slightly decreased, and no considerable changes occur after gas adsorption.
In summary. The adsorption of different molecules decreases conductivity at varying degrees. The adsorption of H 2 and CH 4 shows the greatest and weakest influence on conductivity, respectively. The final conductivity is ordered as follows: CH 4 adsorption system >C 2 H 2 adsorption system >H 2 adsorption system, which is the same as that in Pt-and Pt 2 -doped h-BN systems.

Comparison of Charge Transfer, Adsorption Energy, Energy Gap at Different Systems Through Table and Histogram
A comparative analysis of adsorption parameters (Q t , E ads , E g ) are carried out to present more intuitive comparison as shown in Table 6 and Figure 11. After gas molecules adsorption, Q t is ranging from −0.05-0.28 e. It is obvious that the charge transfer amount of CH 4 is smaller than H 2 and C 2 H 2 . By comparing the Q t and E ads of each adsorption systems, chemical effect occurs in in H 2 , C 2 H 2 , while physical effect occurs in CH 4 during the adsorption process. The adsorption ability for these decomposition components are ordered as follows: C 2 H 2 > H 2 > CH 4 . As shown in Figure 11c, E g has different degrees of reduction after different Pt atoms modification, meaning that the conductivity of the entire system increases. When gas molecules are close to the Pt n -h-BN, the rising E g indicates that the conductivity of the system decrease with different degrees, H 2 and C 2 H 2 molecules drastically decrease the conductivity, while CH 4 nearly does not affect electronic distribution. The final conductivity is ordered as follows: CH 4 adsorption system >C 2 H 2 adsorption system >H 2 adsorption system. This work can be applied to develop new gas sensors based on the Pt cluster modified h-BN using online monitoring of the dissolved gases in transformer oil. A comparative analysis of adsorption parameters (Qt, Eads, Eg) are carried out to present more intuitive comparison as shown in Table 6 and Figure 11. After gas molecules adsorption, Qt is ranging from −0.05-0.28 e. It is obvious that the charge transfer amount of CH4 is smaller than H2 and C2H2. By comparing the Qt and Eads of each adsorption systems, chemical effect occurs in in H2, C2H2, while physical effect occurs in CH4 during the adsorption process. The adsorption ability for these decomposition components are ordered as follows: C2H2 > H2 > CH4. As shown in Figure 11c, Eg has different degrees of reduction after different Pt atoms modification, meaning that the conductivity of the entire system increases. When gas molecules are close to the Ptn-h-BN, the rising Eg indicates that the conductivity of the system decrease with different degrees, H2 and C2H2 molecules drastically decrease the conductivity, while CH4 nearly does not affect electronic distribution. The final conductivity is ordered as follows: CH4 adsorption system >C2H2 adsorption system >H2 adsorption system. This work can be applied to develop new gas sensors based on the Pt cluster modified h-BN using online monitoring of the dissolved gases in transformer oil.

Conclusions
In this study, the most stable structure of h-BN under various doping strategies is analyzed, and the Pt cluster-modified h-BN is employed to investigate its adsorbing performance on oildecomposed components. DOS analysis and frontier molecular orbital theory are considered to comprehensively understand the interactions between gas molecules and Pt cluster-modified h-BN. A new gas sensor with selectivity and sensitivity can be developed by analyzing the responsiveness to typical gases. The specific response mechanism and conclusions are as follows:

Conclusions
In this study, the most stable structure of h-BN under various doping strategies is analyzed, and the Pt cluster-modified h-BN is employed to investigate its adsorbing performance on oil-decomposed components. DOS analysis and frontier molecular orbital theory are considered to comprehensively understand the interactions between gas molecules and Pt cluster-modified h-BN. A new gas sensor with selectivity and sensitivity can be developed by analyzing the responsiveness to typical gases. The specific response mechanism and conclusions are as follows: 1.
Pt cluster-modified h-BN exhibits good sensitivity to C 2 H 2 and H 2 due to chemical adsorption process but is insensitive to CH 4 , with only weak physical adsorption process between them.
The adsorption ability for these decomposition components occurs in the following order: C 2 H 2 > H 2 > CH 4 .

2.
In all doping systems, the adsorption of each gas molecule decreases the conductivity of the entire system. H 2 and C 2 H 2 molecules can drastically change the conductivity, and the decreased conductivity values for C 2 H 2 and H 2 are ordered as follows: H 2 > C 2 H 2 . CH 4 does not affect electronic distribution. 3.
In doping by different Pt atoms, the adsorption process mechanisms and adsorption are slightly different, but the good sensitivity to H 2 and C 2 H 2 is consistent. On the basis of the large difference in characteristics after adsorption, Pt cluster-modified h-BN is a suitable gas sensor.
Therefore, our calculations suggest that Pt cluster-modified h-BN prepared sensors provide a facile way to practically detect transformer oil-dissolved components and can effectively estimate the operation status of transformers. Pt cluster-modified h-BN is a promising material as a gas sensor and for employment in power systems.