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. 2020 Aug 18;5(34):21662–21668. doi: 10.1021/acsomega.0c02499

Adsorption Property of CS2 and COF2 on Nitrogen-Doped Anatase TiO2(101) Surfaces: A DFT Study

Zhengqin Cao †, Gang Wei †,*, Haiyan Zhang †, Dingwen Hu ‡, Qiang Yao ‡
PMCID: PMC7469397  PMID: 32905343

Abstract

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SF6 has been utilized widely as an electrical insulation medium because of its excellent arc extinguishing performance and insulation characteristics. In this paper, the adsorption property of two kinds of key SF6 carbon-containing decomposition components (CS2 and COF2) on nitrogen-doped anatase TiO2(101) (N–TiO2) surfaces was simulated and analyzed based on density functional theory. The results demonstrated that N–TiO2 shows good gas sensitivity toward CS2 with the increase of conductivity but is insensitive toward COF2. In addition, the gas-sensing property of CS2 on N–TiO2 is stronger than that of COF2. This work provides the theoretical information on such a gas-sensitive material for key SF6 carbon-containing decomposition components, supporting its utilization as a chemical sensor applied in condition monitoring and defect diagnosis in SF6 gas-insulated equipment based on decomposition component analysis.

1. Introduction

SF6 has been utilized widely in electrical equipment1−6 because of its excellent arc extinguishing performance and insulation characteristics.7−9 However, harmful insulation defects are inevitably generated in the production, transportation, installation, and operation via vibration, mechanical friction, collision, and other factors. Moreover, such insulation defects would cause partial discharge (PD), which is also the mark of insulation defects. However, the withstand voltage of the equipment would considerably decrease without the defects not handled in time.10,11

Under the action of PD, the SF6 gas molecule would decompose to the primary decomposition products, namely, SF5, SF4, SF3, SF2, and F, which will react chemically with trace H2O, trace O2, metal vapor, and organic solid insulation materials in the gas chamber of gas-insulated equipment to generate stable products including SOF4, H2S, SOF2, SO2F2, CS2, S2F10, CF4, HF, COF2, and so on.12−14 The method of condition monitoring and defect diagnosis in SF6 gas-insulated equipment based on decomposition component analysis (DCA) is an advantageous technique owing to its high sensitivity and capability to identify the fault type, without being affected by electromagnetic interference.15−17

With a remarkably high catalytic property, TiO2(101) nanotubes with good surface thermodynamic stability3 have attracted considerable interest as gas sensors.18,19 Noble metal doping of TiO2 can improve the gas-sensing ability for specific gases.20−22 Currently, the nitrogen-doped anatase TiO2 (N–TiO2) nanotube for photocatalytic activity has been studied.23−26 The literature27 reported the detection of SF6 sulfur-containing decomposition components, namely, SOF2, SO2F2, and SO2, by N-doped anatase TiO2. However, there are few studies about the adsorption of SF6 carbon-containing decomposition components on the N–TiO2 nanotube material, especially some key SF6 decomposition components, namely, CS2 and COF2. The amounts of CS2 and COF2 could reach 12 and 80 μL/L, respectively, during the operation of gas electrical equipment. They are related to solid insulators and can more effectively represent the type and degree of solid insulator defect in gas-insulated equipment.28−31 Consequently, this paper focuses on the gas sensitivity of N–TiO2 on CS2 and COF2. Some parameters of the adsorption property were simulated and analyzed. This work provides the fundamental gas sensitivity information on TiO2 nanotubes as the chemical sensors applied in condition monitoring and defect diagnosis in SF6 gas-insulated equipment based on DCA.

2. Computation Details

The simulation was implemented in Materials Studio with the DMol3 module based on the density functional theory (DFT).32 The supersize of the N–TiO2(101) surface is 10.88 1011.33 1.19.35 Å with a 12 Å vacuum layer. The composition of the N dopant is 0.833%. The Perdew–Burke–Ernzerhof functional generalized gradient approximation was utilized to deal with the electron exchange and correlation.33 The maximum force, energy tolerance accuracy, and maximum atom displacement were selected as 0.002 Ha/Å, 1.0 110–5 Ha, and 5 a10–3 Å, respectively.34 The k-point of the Monkhorst–Pack grid was sampled to 2 × 2 × 1 of the Brillouin zone.27 The charge density convergence accuracy of the self-consistent field was 1.0 Ch10–6 Ha. The Grimme method (DFT-D) was utilized for a better considering of the dispersion forces or van der Waals interactions.35 A smearing of 0.005 Ha was employed to ensure the accurate results of total energy. In addition, the atomic orbital basis set was the double numerical plus polarization (DNP).

The adsorption energy Ed for a gas molecule adsorption system was as shown in formula 1(36,37)

2. 1

where Egas+sur, Egas, and Esur represent the total system energy after gas molecule adsorption, the energy of individual gas molecules, and the energy of the insolated TiO2 surface, respectively. If Ed > 0, extra energy is required for the process of gas adsorption on the TiO2 surface, which could not happen spontaneously and vice versa.38−40

The electron population in the process of adsorption was calculated via Mulliken charge population analysis. If the Mulliken charge population Qd > 0, it denotes that the gas molecules donated electrons to the TiO2 surface during the process.41,42

3. Results and Discussion

3.1. Model Analysis of N–TiO2 and Gas Molecules

Many research studies show that the doping of TiO2 can improve the gas-sensing ability significantly.43−45 The geometric structures of CS2, COF2, and N–TiO2 are optimized before the simulation of the adsorption process and shown in Figure 1, where the parameters of bond length and bond angle are in Å and °, respectively. Figure 1a,b shows the optimized structures of the CS2 and COF2 gas molecules, where the bond length of C–S in CS2 is 1.571 Å. The bond lengths of C–O and C–F in COF2 are 1.185 and 1.337 Å, respectively. The doping position of the N atom is based on the previous studies,46,47 that is, one oxygen atom on the surface is replaced with a nitrogen atom. The new nitrogen atom and titanium atom form the bond Ti–N.

Figure 1.

Figure 1

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Views of (a) CS2, (b) COF2, and (c) N-doped TiO2(101) surface and (d) DOS distribution of native TiO2 and nitrogen-doped TiO2.

The comparison of the density of states (DOS) distribution between native TiO2 and nitrogen-doped TiO2 is shown in Figure 1d. One can observe that the energy gap decreases, to a large scale, after doping nitrogen on anatase TiO2(101) surfaces. This means that the electrons in the valence band would be easier to move to the conduction band and enhance the gas-sensing ability.

3.2. Adsorption Property of CS2 and COF2 on N–TiO2

The optimized model and parameters (adsorption energy, adsorption distance, and Mulliken population analysis) of CS2 and COF2 on the N–TiO2 surface are shown in Figures 2 and 3 and Table 1. As for CS2, there are two adsorption modes which were considered. The electrons of 0.005 e transfer from CS2 molecules to the N–TiO2 surface in the S atom-oriented system, that is, CS2 approaches N–TiO2 by the S atom. Moreover, the bond angle changes from 180 to 150.065° and the bond length changes to 1.635 and 1.583 Å. However, in the C atom-oriented system, N–TiO2 acts as the electron donor. Moreover, the bond angle changes from 180 to 78.634° and the bond length changes to 1.779 and 1.749 Å. For the two adsorption modes of CS2, the adsorption distances are 1.253 and 1.696 Å, respectively, and the adsorption energies are −1.188 and −2.394 eV, respectively, implying that the energy is released in the adsorption process. Therefore, considering the above comparison, it could be easier for CS2 to adsorb on the N–TiO2 surface by the S atom.

Figure 2.

Figure 2

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Adsorption configuration of CS2 on the N-doped TiO2(101) surface. (a) S atom-oriented system and (b) C atom-oriented system.

Figure 3.

Figure 3

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Adsorption configuration of COF2 on the N-doped TiO2(101) surface. (a) C atom-oriented system, (b) F atom-oriented system, and (c) O atom-oriented system.

Table 1. Adsorption Parameters of CS2 and COF2 on the N–TiO2 Nanotube Surface.

gas calculation system adsorption energy (eV) adsorption distance (Å) charge transfer (e)
CS2 –S –2.394 1.696 0.005
  –C –1.188 1.253 –0.003
COF2 –F –0.116 3.096 0.002
  –O –0.616 3.211 –0.002
  –C –0.273 2.772 0.006
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As for COF2, there are three adsorption modes which were considered, namely, the C atom-, F atom-, and O atom-oriented systems. All the bond angles and lengths of COF2 have small changes after adsorption in the three adsorption modes. In the C atom-oriented system, the COF2 gas molecule donates electrons of 0.006 e, and the adsorption distance and energy are 2.772 Å and −0.273 eV, respectively. In the F atom-oriented system, the calculation adsorption distance and energy are 3.096 Å and −0.116 eV, respectively, and the gas molecule donates a few of electrons of 0.002 e. The final adsorption structure of COF2 approaching the N–TiO2 surface by the O atom is shown in Figure 3c. The calculation adsorption distance and energy in the O atom-oriented system are the largest in the three kinds of adsorption modes. Meanwhile, the gas molecule acts as the charge recipient. The comparison of the three adsorption modes shows that it could be easier for COF2 to adsorb on the N–TiO2 surface by the O atom. In addition, all the adsorption energies of COF2 on N–TiO2 are markedly smaller than those of CS2 on N–TiO2, which suggests that the adsorption effect of CS2 on N–TiO2 is quite stronger and there is a more stable adsorption structure.

The DOS of CS2 on the N-doped TiO2 surface is shown in Figure 4. Compared with the DOS of the isolated N–TiO2 surface, both the DOS of the S atom-oriented system and the DOS of the C atom-oriented system are shifted to the right significantly. There appear two novel peaks in the DOS distribution of the S atom-oriented system around −16 and −8 eV in comparison with that of isolated N–TiO2 and one novel peak in the DOS distribution of the C atom-oriented system around −8 eV in comparison to that of isolated N–TiO2. Considering that the pseudogap of the S atom-oriented system is wider than that of the C atom-oriented system, the interaction between the S atom and N-doped TiO2 surface is stronger, which confirms the previous conclusion. In addition, the p orbital of the CS2 gas molecule contributes the largest to the DOS of CS2 gas molecules, which is also shown in Figure 4b,d, and one can find that the p orbital DOS of the S atom-oriented system and the C atom-oriented system is similar to their total DOS, respectively.

Figure 4.

Figure 4

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DOS of CS2 on the N-doped TiO2(101) surface. (a) DOS of the S atom-oriented system, (b) p orbital DOS of the S atom-oriented system, (c) DOS of the C atom-oriented system, and (d) p orbital DOS of the C atom-oriented system.

The DOS distribution of COF2 on the N-doped TiO2 surface is shown in Figure 5. It could confirm that the weak interaction between COF2 and the N–TiO2 surface by the comparison of the C atom-, O atom-, and F atom-oriented systems with isolated N–TiO2 in Figure 5a,c,e, where three DOS distributions are basically overlapped at the area near the Fermi level and the range between −20 and −15 eV.47−49 Moreover, the only change between the DOS distribution of the COF2 system and that of isolated N–TiO2 are four new peaks in the F atom- and C atom-oriented systems and two new peaks in the O atom-oriented system between −11 and −7 eV. The pseudogaps of the C atom-, O atom-, and F atom-oriented systems are similar. In addition, the DOS and p orbital of COF2 gas molecule distributions are also shown in Figure 5b,d,f.

Figure 5.

Figure 5

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DOS of COF2 on the N-doped TiO2(101) surface. (a) DOS of the F atom-oriented system, (b) p orbital DOS of the F atom-oriented system, (c) DOS of the O atom-oriented system, (d) p orbital DOS of the O atom-oriented system, (e) DOS of the C atom-oriented system, and (f) p orbital DOS of the C atom-oriented system.

The frontier molecular orbital theory is employed to explore the conductivity of the N-doped TiO2(101) surface via gas adsorption.9 The distributions of the highest occupied molecular orbital (HOMO) and their energies, lowest unoccupied molecular orbital (LUMO) and their energies, and the energy gap Eo (Eo = ELUMO – EHOMO) are shown in Figure 6.

Figure 6.

Figure 6

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HOMO and LUMO distributions and relative energies of (a) isolated N–TiO2, (b) CS2, and (c) COF2 for different adsorption systems (the iso value is 0.03 e/A3).

One can observe that the energies of the HOMO and LUMO of isolated N–TiO2 are −7.451 and −5.723 eV, respectively, with an Eo of 1.728 eV. As for the CS2 adsorption, the HOMO and LUMO are redistributed evidently. The energies of the HOMO and LUMO of the S atom-oriented system are −4.951 and −4.918 eV, respectively, in Figure 6b1, which are lower than those of isolated N–TiO2. Moreover, the energy gap (0.033 eV) of the S atom-oriented system decreases remarkably in comparison to that of isolated N–TiO2.

The molecular orbital distribution of the C atom-oriented system is shown in Figure 6b2, where the energies of the HOMO and LUMO are −4.674 and −4.653 eV, respectively, and the energy gap Eo surprisingly decreases to 0.021 eV. Consequently, both the energy gaps of the S atom- and C atom-oriented systems when CS2 adsorbs on N–TiO2 are much lower than that of isolated N–TiO2, leading to electrons in the valence band to easily jump to the conduction band. Therefore, it can be assumed that to a large scale, the conductivity of N–TiO2 would be increased after adsorbing CS2.

In Figure 6c, the orbital distributions of the HOMO and LUMO for COF2 adsorption are exhibited. One can observe that COF2 contributes less to the orbital distributions in comparison to N–TiO2. In addition, the energies of the HOMO in the F atom-, O atom-, and C atom-oriented systems are −7.452, −7.373, and −7.478 eV, respectively, and the energies of the LUMO in the F atom-, O atom-, and C atom-oriented systems are −5.718, −5.662, and −5.750 eV, respectively. Consequently, all the energy gaps for COF2 adsorption are similar to that of isolated N–TiO2, denoting that the conductivity of N–TiO2 would be slightly changed after adsorbing COF2.

In summary, it can be concluded that N-doped TiO2 is a desirable material to detect CS2 gas based on the increased conductivity and stable adsorption structure. However, because of the small conductivity change and low adsorption energies, this material is probably not suitable for detecting the presence of COF2 precisely. Therefore, it could be hypothesized that the material can function as a sensor in the application of condition monitoring and defect diagnosis in SF6 gas-insulated equipment based on DCA.

4. Conclusions

In this paper, the parameters of adsorption property, namely, adsorption energy, distance, DOS, and frontier molecular orbital theory, were simulated and analyzed for comprehensively investigating the gas sensitivity of N-doped anatase TiO2(101) on two kinds of SF6 carbon-containing decomposition components, CS2 and COF2, based on DFT. The results manifest the following:

  • (1)

    The main adsorption positions for CS2 on the N–TiO2 surface are probably by the S atom. However, the N–TiO2 surface adsorbing COF2 is probably by the O atom. The gas-sensing property of CS2 on N–TiO2 is quite stronger than that of COF2. In addition, there is less charge transfer in both the CS2 and COF2 adsorption processes.

  • (2)

    The simulation and analysis results of DOS and frontier molecular orbital theory consistently indicate that the N–TiO2 surface has good sensitivity to the CS2 gas molecule because of the increase of conductivity but exhibits insensitivity toward the COF2 gas molecule.

This work provides theoretical information on such a gas-sensitive material for SF6 carbon-containing decomposition components, which could function as a sensor in the application of condition monitoring and defect diagnosis in SF6 gas-insulated equipment based on DCA.

Acknowledgments

The authors acknowledge the support of the Chongqing Technological Innovation and Application Development Project (Cstc2019jscx-msxm0182) and the Research Foundation of Chongqing University of Science and Technology (Ckrc2019043).

The authors declare no competing financial interest.

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