3.2. Pt Cluster Modified h-BN
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.
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 Pt
3 cluster is treated emphatically in this work to enhance gas sensitivity response. The surface structure of the intrinsic h-BN is greatly altered after Pt
3 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–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
Ed 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.
3.4. Adsorption of Gas Molecules on Pt, Pt2 Doped h-BN
Figure 5a–f depicts the gas adsorption system on Pt- and the Pt
2-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 H
2 and C
2H
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
2H
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,
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 H
2 molecule.
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
Eads 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.
Figure 5c–e demonstrates the large influence between C
2H
2 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 C
2H
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.
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.
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
2H
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 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
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 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
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 H
2 molecule model,
Eg 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
2H
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
Eg 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.
In summary, for different adsorption scenarios, conductivity decreases at varying degrees. H2 molecular adsorption exhibits the greatest influence on conductivity, followed by C2H2 adsorption. CH4 molecule adsorption is very weak, minimally contributing to conductivity. The estimated final conductivity is arranged as follows: CH4 adsorption system >C2H2 adsorption system >H2 adsorption system.
3.5. Adsorption of Gas Molecules on Pt3, Pt4 Doped h-BN
The gas adsorption system on Pt
3- and Pt
4-doped h-BN is depicted in
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
2H
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.
In the H2 adsorption system, a strong chemical adsorption process exists between Pt3- or Pt4-doped structure and the H2 molecule, the adsorption energy slightly decreasing from −1.97 eV to −1.67 eV. Different degrees of adsorption process mechanism cause large changes in Qt. In the H2/Pt3-h-BN system, such a large Qt indicates that the H atom acquires numerous electrons. Therefore, great changes in conductivity exist.
In the CH4 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 C2H2/Pt3- and Pt4-doped h-BN, Eads of 2.16 and 2.88 eV are approaching that of the C2H2/Pt- and Pt2-doped h-BN systems. This result illustrates that Eads 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
2H
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
2H
2 molecule undergoes a great bend.
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
2H
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.
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 Pt
3 doping is more intense than that by Pt
4 doping.
In the Pt
3 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 H
2 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 CH
4 is non-negligible 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 C
2H
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 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.