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. 2021 Sep 7;6(37):23776–23781. doi: 10.1021/acsomega.1c02562

First-Principles Calculations for Adsorption of HF, COF2, and CS2 on Pt-Doped Single-Walled Carbon Nanotubes

Zhengqin CAO †, Xiaoyu WU †, Gang WEI †,*, Gang HU †, Qiang YAO ‡, Haiyan ZHANG †
PMCID: PMC8459352  PMID: 34568657

Abstract

graphic file with name ao1c02562_0013.jpg

HF, CS2, and COF2 are three important decomposition components of the SF6 gas insulation medium. In this paper, the gas sensitivity of Pt doped on (8, 0) single-walled carbon nanotube (SWCNT) to HF, CS2, and COF2 is investigated based on density functional theory. The binding energy, charge transfer, density of states, and frontier molecular orbital theory are discussed. It is found that all processes of HF, CS2, and COF2 adsorbed on Pt-SWCNT are exothermic. Pt-SWCNT donated 0.182 electrons to CS2 molecules during the interaction process but acts as an electron acceptor during adsorption of HF and COF2 on it. After comprehensive consideration of binding energy and charge transfer, the response of Pt- SWCNT to CS2 may be the best, and those to HF and COF2 are almost the same. In addition, after the adsorption of the three kinds of gases on Pt-SWCNT, the order of the conductivity of the Pt-SWCNT material is CS2 > COF2 ≈ HF via frontier molecular orbital theory analysis. The Pt-SWCNT material is probably more suitable as a gas sensor for the detection of CS2 in the application of gas-insulated equipment.

1. Introduction

SF6 has been widely used as insulation medium for the operation safety of gas-insulated equipment due to its insulation and excellent arc extinguishing characteristics.1−4 However, in long-running gas-insulated equipment, SF6 would decompose in partial discharge to F atoms and other low fluorinated sulfur species, which further react with trace H2O, trace O2, and organic solid insulation material to generate HF, COF2, CS2, SO2, SOF2, SO2F2, S2F10, etc.5−9 It has been proved that detecting SF6 characteristic decomposed products by chemical sensors is a feasible and effective method to online monitoring and insulation defect diagnosis of gas-insulated equipment.10−13

In recent years, due to the strong responsivity, small size, and high sensitivity of a single-walled carbon nanotube (SWCNT),14−16 it has been a research hotspot as a sensor material. Moreover, the gas sensitivity of the SWCNT will enhance after decorating its surface with transition metals17−21 and realize its application to detect some gases.22 Cui et al. found that the SWCNT doped with Pt, Pd, and Rh has good gas sensitivity to SOF2, CO, CH4, and H2S, due to strong orbital interactions and chemisorption between the adsorbent and the target gas10,23,24 by simulations. Yoosefian studied the adsorption properties of SO2 on Pt- and Au-doped SWCNTs (5, 5) and found that the energy gap of SO2 adsorbed on Pt-SWCNT changes more significantly than that on Au-SWCNT.25

There are a few studies about the gas-sensitive materials for HF, CS2, and COF2, which can more effectively represent the degree of solid insulator defects in SF6 gas-insulated equipment, and Pt doped on the SWCNT surface is a promising material for gas sensor development. The adsorption properties of HF, CS2, and COF2 adsorbing on Pt doped on (8, 0) SWCNT were calculated and analyzed based on density functional theory (DFT). The work provides fundamental adsorption information of Pt doped on (8, 0) SWCNT as a possible candidate for a chemical sensor applied in condition monitoring and defect diagnosis in SF6 gas-insulated equipment.

2. Computation Details

All of the calculations in the work were performed by the Dmol3 module based on DFT.26,27 The atomic orbital basis sets adopted double numerical plus polarization (DNP).28 The Perdew–Burke–Ernzerhof (PBE) generalized gradient approximation (GGA) was used to deal with electron exchange and correlation. The Brillouin zone k-point was sampled as 1 × 1 × 8 from the Monkhorst–Pack mesh10,29 with semicore pseudopots being adopted as the core treatment.26 Maximum force, energy tolerance accuracy, and maximum atom displacement were selected as 0.002 Ha/Å, 1.0 × 10–5 Ha, and 5 × 10–3 Å, respectively.16,30 The charge density convergence accuracy of the self-consistent field was 1.0 × 10–6 Ha.

All of the sizes of supercells were 20 Å × 20 Å × 8.50 Å with angles of α = 90°, β = 90°, and γ = 120°. The initial HF adsorption orientation includes two adsorption modes, namely, the H and F atoms approaching the Pt site of the Pt-SWCNT surface (as shown in Figure S1). For CS2, the initial adsorption orientation includes two modes as well, namely, the C and S atoms approaching the Pt site of the Pt-SWCNT surface (as shown in Figure S2). Three modes, namely, the C, O, and F atoms approaching the Pt site of the Pt-SWCNT surface, were considered for the initial COF2 adsorption orientation (as shown in Figure S3). Only the detailed analysis of the adsorption modes with the largest binding energy was discussed in this paper, including charge transfer, density of states (DOS), morphology, and frontier molecular orbital.

The binding energy Ead represents the change of the gas molecule adsorption system and is given in formula (1)(31,32)

2. 1

where Emolecule/Pt-SWCNT represents the energy of the total system after adsorption of one gas molecule. Emolecule and EPt-SWCNT represent the energies of one gas molecule and the intrinsic Pt-SWCNT surface, respectively.

The charge transfer Qt between Pt-SWCNT and gas molecules was estimated via Mulliken population analysis. If Qt < 0, the electrons transfer from the Pt-SWCNT surface to gas molecules during the adsorption process, and the electrons transfer from gas molecules to the Pt-SWCNT surface when Qt > 0.

3. Results and Discussion

Before the adsorption calculation, the structures of HF, CS2, COF2, and Pt-SWCNT were optimized. SWCNT (8, 0) is selected as the carrier, and the Pt atom is connected to two adjacent C atoms on the SWCNT, which forms a bridge site outside the SWCNT,33 as shown in Figure 1. The bond lengths of both Pt and its adjacent two C atoms are 2.264 Å with a C–Pt–C angle of 36.996°. In addition, there is no deformation of the SWCNT after Pt doping.

Figure 1.

Figure 1

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Optimized structure of Pt-SWCNT: (a) front view and (b) top view.

The optimized structures and their parameters of HF, CS2, and COF2 molecules are shown in Figure S4.

3.1. Adsorption Configurations of HF, CS2, and COF2 on Pt-SWCNT

For all adsorption configurations of HF, CS2, and COF2 on the Pt-SWCNT surface, we only discuss the ones with the largest binding energy. The parameters of the adsorption configurations of the three gas molecules are listed in Table 1.

Table 1. Parameters of the Adsorption Configurations of HF, CS2, and COF2 on Pt-SWCNT.

gas molecule binding energy (eV) charges transfer (e) distance (Å)
HF –0.325 0.045 2.268
CS2 –1.037 –0.182 2.211
COF2 –0.464 0.029 2.454
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For HF, when the HF molecule is close to the Pt-SWCNT surface by the H atom, the largest binding energy is −0.325 eV in the two adsorption orientation modes. The adsorption configuration of HF is shown in Figure 2. The adsorption distance (between Pt and F) is 2.268 Å, and the bond lengths of Pt and its adjacent two C atoms change to 2.239 and 2.247 Å after adsorption, respectively. However, the structures of the SWCNT and the HF molecule have not changed during the adsorption process. In total, 0.045 electrons transfer from HF to the Pt-SWCNT surface, which indicates that HF acts as an electron donator.

Figure 2.

Figure 2

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Adsorption configuration of HF on the Pt-SWCNT surface: (a) front view and (b) top view.

The adsorption configuration of CS2 is shown in Figure 3. The binding energy is −1.037 eV and the adsorption distance is 2.211 Å. Both C–S bond lengths of CS2 increase from 1.567 to 1.593 Å and 1.636 Å, while the bond angle of C–S–C decrease from 180 to 156.845° after the adsorption. In addition, the bond lengths of Pt and its adjacent two C atoms decrease to 2.237 and 2.232 Å, respectively. The Pt-SWCNT surface donated 0.182 electrons to the CS2 molecule.

Figure 3.

Figure 3

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Adsorption configuration of CS2 on the Pt-SWCNT surface: (a) front view and (b) top view.

Figure 4 exhibits the adsorption configuration of COF2. The adsorption distance is 2.454 Å with a binding energy of −0.464 eV. The structure of COF2 has changed slightly after adsorption. The bond lengths of Pt and its adjacent two C atoms decrease to 2.238 and 2.249 Å, respectively. In addition, the Pt-SWCNT surface obtains 0.029 electrons from the COF2 molecule during the adsorption process.

Figure 4.

Figure 4

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Adsorption configuration of COF2 on the Pt-SWCNT surface: (a) front view and (b) top view.

Larger adsorption energy means a larger amount of gas could adsorb on the surface of a gas-sensing material.34 Meanwhile, more charge transfer could give rise to higher sensitivity in the same adsorption condition.35 Therefore, due to larger binding energy and charge transfer, Pt- SWCNT may have the best response to CS2 among the three kinds of gas molecules. However, due to the larger binding energy and the smaller charge transfer of the COF2 system than those of the HF system, the response of Pt-SWCNT to COF2 may be nearly the same as that to HF.

3.2. Density of States Analysis for HF, CS2, and COF2 Adsorbed on Pt-SWCNT

To further estimate the interaction mechanism between HF, CS2, and COF2 and Pt-SWCNT, the density of states (DOS) distributions of the three gas molecules on the Pt-SWCNT surface are discussed; 0 eV refers to the Fermi level. Figure 5 shows the total density of states (TDOS) and local density of states (LDOS) distribution of HF adsorbed on Pt-SWCNT. The TDOS increase little at the Fermi level but increase obviously at −9.8, −6.5, and −5.5 eV after HF adsorption. According to LDOS, the overlap between the 1s orbital of the H atom, the 2p orbital of the F atom, and the 5d orbital of the Pt atom is not significant, which indicates that the interaction between HF and the Pt-SWCNT surface is weak. In addition, the band gap decreases 0.010 eV after HF adsorption

Figure 5.

Figure 5

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DOS distribution of HF adsorbed on the Pt-SWCNT surface.

Figure 6 shows the DOS distribution of CS2 adsorbed on the Pt-SWCNT surface. The TDOS increases at −15.8, −13, −7.5, −5, −4.2, −1.8, and 2.5 eV after CS2 adsorption. Compared with LDOS distribution, the increased areas of the TDOS derive from the CS2 molecule. In addition, the 3p orbital of the S atom and the 2p orbital of the C atom in CS2 overlaps with the 5d orbital of Pt at −5.2 to 0 and 1.2–3.6 eV, respectively, indicating that the interaction between the CS2 molecule and Pt-SWCNT is strong. The overlap illustrates the hybridization between atomic orbitals.

Figure 6.

Figure 6

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DOS distribution of CS2 adsorbed on the Pt-SWCNT surface.

The DOS distribution of COF2 adsorbed on Pt-SWCNT is exhibited in Figure 7. It can be found that the TDOS has changed significantly after COF2 adsorption and decrease obviously at the Fermi level. However, the hybridization between 2p orbitals of C, O, and F atoms in COF2 and the 5d orbital of the Pt atom is slight according to the LDOS results.

Figure 7.

Figure 7

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DOS distribution of COF2 adsorbed on the Pt-SWCNT surface.

3.3. Frontier Molecular Orbital Theory Analysis for HF, CS2, and COF2 Adsorbed on Pt-SWCNT

Frontier molecular orbital theory is investigated to better understand the Pt-SWCNT response to HF, CS2, and COF2. The distributions of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) accompanied by related energies are obtained and shown in Figures 8–11. Subsequently, the energy difference Eg = LUMO–HOMO, which could be a feasible parameter to measure the conductivity of materials.34 The values of HOMO and LUMO are shown in Table 2. It can be found that both the HOMO and LUMO of isolated Pt-SWCNT are mainly around the tube and Pt has an Eg of 0.747 eV. For HF, CS2, and COF2 adsorption systems, all of the three gas molecules have little contribution to the HOMO and LUMO according to Figure 9–1011. However, the HOMO and LUMO areas around the Pt atom are obviously reduced after CS2 adsorbed on Pt-SWCNT. In addition, Eg values of the three gas molecule adsorption systems have narrowed compared with that of isolated Pt-SWCNT, namely, 0.711 eV for the HF system, 0.693 eV for the CS2 system, and 0.710 eV for the COF2 system. This would lead to an increase in the conductivity of the Pt-SWCNT material interacting with the three kinds of gases in the order CS2 > COF2 ≈ HF. The result is consistent with the order of the response of the Pt-SWCNT surface to the three kinds of gases. This provides theoretical evidence for the detection of SF6 decomposition components, HF, CS2, and COF2, by a Pt-SWCNT-based sensor through the change in electrical conductivity.

Figure 8.

Figure 8

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Configurations of the HOMO and LUMO of Pt-SWCNT with front and top view: (a) HOMO and (b) LUMO.

Figure 11.

Figure 11

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Configurations of the HOMO and LUMO of COF2 adsorbed on the Pt-SWCNT surface: (a) HOMO and (b) LUMO.

Table 2. Values of HOMO and LUMO.

calculation system LUMO (eV) HOMO (eV) Eg (eV)
Pt-SWCNT –4.377 –5.084 0.747
HF/Pt-SWCNT –4.288 –4.998 0.711
CS2/Pt-SWCNT –4.612 –5.307 0.695
COF2/Pt-SWCNT –4.283 –4.993 0.710
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Figure 9.

Figure 9

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Configurations of the HOMO and LUMO of HF adsorbed on the Pt-SWCNT surface: (a) HOMO and (b) LUMO.

Figure 10.

Figure 10

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Configurations of the HOMO and LUMO of CS2 adsorbed on the Pt-SWCNT surface: (a) HOMO and (b) LUMO.

4. Conclusions

In the work, the adsorption properties of HF, CS2, and COF2 on Pt-SWCNT have been studied based on DFT, including adsorption structure, binding energy, charge transfer, DOS, and frontier molecular orbital theory. The conclusions are summarized as follows.

  • (1)

    All processes of HF, CS2, and COF2 adsorbed on Pt-SWCNT are exothermic. After comprehensive consideration of binding energy and charge transfer, the response of Pt- SWCNT to CS2 may be the best, and those to HF and COF2 are almost the same.

  • (2)

    After the adsorption of the three kinds of gases on Pt-SWCNT, the order of the conductivity of the Pt-SWCNT material is CS2 > COF2 ≈ HF via frontier molecular orbitals theory analysis.

  • (3)

    The Pt-SWCNT material is probably more suitable as a gas sensor for the detection of CS2 in the application of gas-insulated equipment.

Acknowledgments

The authors gratefully appreciate the support of the Science and Technology Research Program of Chongqing Municipal Education Commission (Grant Nos. KJQN202001524 and KJZD-K202001505), the Chongqing Natural Science Foundation (cstc2020jcyj-msxmX0267), the Chongqing Research Program of Technology Innovation and Application Development (No. cstc2020jscx-msxmX0188), and the Chongqing Market Supervision Administration Research Program (No. CQSJKJ2020008).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c02562.

  • All initial adsorption orientation configurations of HF, CS2, and COF2 adsorbing on Pt-SWCNT; adsorption parameters after optimization; and optimal molecular structures of HF, CS2, and COF2 (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao1c02562_si_001.pdf (512.2KB, pdf)

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Supplementary Materials

ao1c02562_si_001.pdf (512.2KB, pdf)

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