HF, CS2 and COF2 are three kind key partial discharge decomposition gas products of SF6 electrical insulation medium, which could be used for monitoring the type and degree of defect in gas insulated equipment. In this paper, the adsorption property including adsorption entropy change, adsorption distance, density of states, and frontier molecular orbitals, of HF, CS2 and COF2 on Au-doped anatase TiO2 (101) surfaces were simulated and analysis based on density function theory. The results demonstrated that Au-TiO2 incentive upon HF, CS2 and COF2 due to the little conductivity change and low adsorption entropy change, and this material could be not suitable to be used as a gas sensor for three decomposition gas detection in the application of condition monitoring and defect diagnosis in SF6 gas-insulated equipment based on DCA.
Research Article
A first principle simulation of adsorption behavior of HF, CS2 and COF2 on Au-doped anatase TiO2 (101)
https://doi.org/10.21203/rs.3.rs-228316/v1
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SF6
Au doping
Adsorption
Density function theory
Due to the excellent arc extinction properties SF6 has been widely used in gas insulated equipment as an insulating medium, [1–4]. However, SF6 would be decomposed to HF, SOF2, H2S, CS2, COF2 and so on with trace H2O, trace O2, metal vapor, and organic solid insulation material in the gas insulated equipment [5–7] under partial discharge or local overheating fault, which would threaten the operation safety of power grid. The method of condition monitoring and defect diagnosis in SF6 gas-insulated equipment based on SF6 decomposition components analysis is an advantageous technique owing to its high sensitivity and capability to identify defect type, without being affected by electromagnetic interference [8–10].
Currently, due to the excellent functions and various modification methods of the metal oxide semiconductor system, objective achievements have been made in sensor applications. As a remarkably high catalytic property [11–13], the adsorption ability of the intrinsic and noble metal (such as Au, Pt, Pd et.al) modified anatase TiO2 (101) nanotubes for certain SF6 decomposition gases has been studied to estimate the possibility of such material as the chemical sensor in the application of condition monitoring and defect diagnosis in SF6 gas-insulated equipment [14–18]. The noble metal modified TiO2 system is proven to have the potential to be an excellent chemical sensor for SO2 F2, SOF2, and SO2 [19]. However, there are few studies about the detection of some key SF6 decomposition
components, namely HF, CS2 and COF2, which are related to solid insulator and can more effectively represent the type and degree of defect in gas insulated equipment [19–23].
In the paper, the adsorption of Au doped anatase TiO2 (101) on HF, CS2 and COF2 are investigated by first principle simulation based on density function theory (DFT). The parameters of adsorption property including adsorption entropy change, charge transfer, density of states, the distributions of highest occupied molecule orbital (HOMO) and lowest unoccupied molecule (LUMO) were simulated and analyzed. All this work provides the fundamental theoretical information of TiO2 nanotubes adsorbed HF, CS2 and COF2.
All calculations were implemented in the Dmol3 model based on the density functional theory (DFT) [24]. The electron interaction effect was treated via the Perdew-Burke-Ernzerho (PBE) function of generalized gradient approximation (GGA), and the atomic orbital basis set was the double numerical plus polarization (DNP) [25]. Maximum force, energy tolerance accuracy, and maximum atom displacement were 0.002 Ha/Å, 1.0×10-5 Ha, and 5×10-3 Å, respectively [23]. The Brillouin zone was sampled as 2×3×1 k-point grid through the Monkhorst–Pack method for the model [15]. The Tkatchenko and Scheffler’s (TS) method was utilized for dispersion correction [26,27]. The self-consistent field convergence accuracy was 1×10−6 Ha. In addition, the global orbital cut-off radius of 5.2 Å and the smearing of 0.005 Ha were employed to make sure the accurate results of total energy.
The adsorption entropy change of Ed for HF, CS2 and COF2. gas molecules adsorption system was defined as shown in formula (1) [28]:
where, Egas+sur, Egas, and Esur represent the total system energy after gas molecules adsorption on Au-TiO2 surface, the energy of individual gas molecules, and the energy of insolated Au atom doped TiO2 surface, respectively. If the Ed <0, the energy is released in the process of gas adsorb on the Au-TiO2 surface, and If the Ed >0, the system will adsorb energy from the outside in the process.
The charge transfer in the process of adsorption was calculated via Mulliken charge population analysis [29]. If the Mulliken charge population Qd <0, it denotes the electrons transfers from Au-TiO2 surface to gas molecules during the process; If Qd >0, it denotes the electrons transfers from gas molecules to Au-TiO2 surface during the process.
The total density of states (TDOS), the partial density of states (PDOS), HOMO and LUMO were also analyzed to investigated the adsorption mechanism of HF, CS2 and COF2. gas molecules on Au-TiO2 surface.
3.1. Adsorption distance, adsorption entropy change, and charge transfer of HF, CS2 and COF2 on Au-TiO2
The optimized model and parameters (adsorption distance, adsorption entropy change, and charge transfer) of HF, CS2, and COF2 on Au-TiO2 surface are shown in Fig. 1–3 and Table 1.
Figure 2 shows the adsorption of CS2 molecules on Au-TiO2 surface. There is also two adsorption modes were considered, that is CS2 approaches to Au-TiO2 by C atom and S atom respectively. In C atom oriented system, CS2 molecule donates 0.093 e to Au-TiO2 surface with the − 0.378 eV adsorption entropy energy and 2.636 Å adsorption distance. For S atom oriented system, the CS2 gas molecule is also as the electrons donor and donate 0.116 e in the adsorption process. The adsorption entropy change and adsorption distance are − 0.885 eV and 2.669 Å.
As for COF2, three adsorption modes were considered, namely C atom, F atom, and O atom oriented system. In C atom oriented system, COF2 gas molecule donates 0.053 e electrons. The adsorption entropy change and adsorption distance are − 4.331 eV and 3.633 Å. In O atom oriented system, the charge transfer and adsorption entropy change are the same as those in C atom oriented system, however, the adsorption distance of 2.618 Å is smaller than that in C atom oriented system. In F atom oriented system, the adsorption entropy change and adsorption distance are − 2.228 eV and 3.996 Å, and the gas molecule donated a few of electrons of 0.004 e.
| Gas molecules | Calculation system | Adsorption entropy change (eV) | Adsorption distance (Å) | Charge transfer (e) |
|---|---|---|---|---|
| HF | -H -F | -0.368 -0.368 | 3.266 2.636 | 0.044 0.043 |
| CS2 | -C -S | -0.796 -0.885 | 3.523 2.669 | 0.116 0.053 |
| COF2 | -C -O -F | -0.431 -0.431 -0.228 | 3.633 2.618 3.996 | 0.053 0.043 0.004 |
3.2. The density of states of HF, CS2 and COF2 on Au-TiO2
The DOS distribution of HF absorbed on Au-TiO2 is shown in Fig. 4. Both TDOS distribution in H atom oriented system and F atom oriented system are similar to that of isolated Au-TiO2 surface,it indicates that the number of surface electron transitions in this system is fewer. The only difference is a novel peak appears around − 11 eV in the two oriented systems. As for the PDOS distribution, it could be confirmed that the 2p orbital of F atom mainly contributes to the new peak. In addition, the hybridization between 5d orbital of Au atom and 2p orbital of F atom is weak, and the overlapping area is also very small near the Fermi level. This further indicates that the interaction between Au-TiO2 and HF may be weak.
The DOS distribution of CS2 on Au doped TiO2 surface is exhibited in Fig. 5. one can observe the weak interaction between CS2 and Au-TiO2 surface by the comparing TDOS distribution of CS2 adsorption system with that of isolated Au-TiO2, where two TDOS distributions are basically similar to each other at the area near the Fermi level and the range among − 20 ~ -17 eV. But two novel peaks appear in -15.5 eV and − 9 eV of CS2 adsorption system. In addition, it can find that the 2p orbital of C atom is the main contributor of the new peak at -15.5 eV, and the 3P orbital of S atom is the main contributor of the new peak at -9 eV. The 2p orbital of C atom and the 3P orbital of S atom overlap evidently the 5d orbital of Au among − 7.5 ~ -5 eV. Moreover, the overlapping peak between the 2p orbital of C atom and 5d orbital of Au is little small than that between the 3P orbital of S atom and 5d orbital of Au, which shows that the interaction between CS2 and Au-TiO2 by S oriented system is more obvious.
The DOS distribution of COF2 on Au doped TiO2 surface is shown in Fig. 6. The TDOS distribution after Au-TiO2 surface absorbing COF2 gas molecule also has a little change, especially among the Fermi level. Comparing with the TDOS distribution of isolated Au-TiO2, four novel peaks appeared near − 14.5 eV, -11.5 eV, -10.5 eV, and − 8.5 eV in those of C atom and O atom oriented system. And four novel peaks appeared near − 13 eV, -11 eV, -9.5 eV, and − 8.5 eV in that of F atom oriented system. According to the PDOS distribution, the overlapping area between the 2p orbital of O atom and 5d orbital of Au is largest, followed by the one between the 2p orbitals of C atoms and the 5d orbitals of Au, the one of between the 2p orbitals of F atoms and the 5d orbitals of Au is smallest.
3.3. The HOMO and LUMO of HF, CS2 and COF2 on Au-TiO2
Based on frontier molecular orbital theory, the energies of HOMO, LUMO, and the energy gap Eo (Eo=ELUMO-EHOMO) were obtained and exhibited in Table 2. It can observe that the HOMO and LUMO of isolated Au-TiO2 are − 4.4987 and − 4.4826 eV respectively. The energy gap is 0.0161 eV.
As for HF, the energies of HOMO and LUMO in F and H atom oriented systems are larger than those of isolated Au-TiO2. And both the energy gaps in F and H atom oriented systems increase slightly in contrast to that of isolated Au-TiO2. As for CS2, the energies of HOMO in C and S atom oriented system are − 4.4806 and − 4.4725 eV respectively. And the energies of LUMO in C and S atom oriented are − 4.4623 and − 4.4488 eV respectively. Consequently, both energy gaps for CS2 adsorption are a little larger than that of isolated Au-TiO2. As for COF2, one can observe that the energies of HOMO in C, O, and F atom oriented system are − 4.4228, -4.4275, and --4.4737 eV respectively. And the energies of LUMO in F, O, and C atom oriented system are − 4.4035, -4.4094, and − 4.4563 eV respectively.
If the energy gap becomes larger, the conductivity of the system would decrease, while if the energy gap becomes smaller, the conductivity of the system becomes stronger [30]. So it could hypothesize that, to a large scale, the conductivity of Au-TiO2 would be decreased after adsorbing HF, CS2, and COF2. However, considering the little conductivity change and low adsorption energies, it could assume that this material is probably not suitable to detect the presence of HF, CS2, and COF2 precisely.
| Calculation system | Adsorption system | HOMO/eV | LUMO/eV | ∣HOMO―LUMO∣/eV |
|---|---|---|---|---|
| Au-TiO2 | / | -4.4987 | -4.4826 | 0.0161 |
| CS2 | C atom S atom | -4.4806 -4.4725 | -4.4623 -4.4488 | 0.0184 0.0237 |
| COF2 | C atom O atom F atom | -4.4228 -4.4275 -4.4737 | -4.4035 -4.4094 -4.4563 | 0.0194 0.0182 0.0174 |
| HF | H atom F atom | -4.4424 -4.4402 | -4.4218 -4.4221 | 0.0206 0.0182 |
In this paper, the adsorption parameters, namely adsorption entropy change, adsorption distance, DOS, and frontier molecular orbitals, of HF, CS2, and COF2 gas molecules adsorption on Au doped anatase TiO2 (101) surface were simulated and analyzed to comprehensively investigating the gas sensitivity based on DFT. We found that the energies are released in the interaction of HF, CS2, and COF2 absorbing on Au-TiO2, and all the TDOS distribution of HF, CS2, and COF2 are similar to that of isolated Au-TiO2 surface. In addition, considering the little conductivity change and low adsorption entropy change, this material could be not suitable to be used as a gas sensor for the detection in the application of condition monitoring and defect diagnosis in SF6 gas-insulated equipment based on DCA.
Acknowledgements
The authors appreciate the supported of science and Technology Research Program of Chongqing Municipal Education Commission (Grant No. KJQN202001524, Grant No. KJZD-K202001505), Chongqing Natural Science Foundation (cstc2020jcyj-msxmX0267)., Chongqing Research Program of Technology Innovation and Application Development (No. cstc2020jscx-msxmX0188) and Chongqing Market Supervision Administration Research Program (No. CQSJKJ2020008)
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No competing interests reported.