Theoretical and experimental study on competitive adsorption of SF6 decomposed components on Au-modified anatase (101) surface

Applied Surface Science 387 (2016) 437–445

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Theoretical and experimental study on competitive adsorption of SF6 decomposed components on Au-modified anatase (101) surface Xiaoxing Zhang a,b,∗ , Xingchen Dong a , Yingang Gui a a b

State Key Laboratory of Power Transmission Equipment & System Security and New Technology, Chongqing University, Chongqing 400044, China School of Electrical Engineering, Wuhan University, Wuhan 430072, China

a r t i c l e

i n f o

Article history: Received 17 April 2016 Received in revised form 15 May 2016 Accepted 17 May 2016 Available online 4 June 2016 Keywords: SF6 decomposed components TiO2 nanotubes (101) Surface Au doping Gas-sensing mechanism

a b s t r a c t Partial discharge inside gas insulated switchgear in electric systems will lead to the decomposition of SF6 gas, the insulating medium, producing several kinds of characteristic components. Detecting the species and concentrations of decomposed components of SF6 is considered a feasible way of early-warning to avoid occurrence of sudden fault. As a research hotspot in gas-sensing field, TiO2 nanotubes possess wide application prospect in online monitoring of fault gases in gas insulated switchgear. In this paper, adsorption parameters of SO2 , SOF2 , and SO2 F2 , characteristic products of SF6 decomposition, on Au-doped anatase TiO2 (101) surface were calculated using software Materials Studio. The adsorption processes of gas molecules on Au-doped anatase TiO2 (101) surface were theoretically analyzed, which can be used to explain the gas-sensing mechanism of TiO2 nanotubes sensor. Besides, adsorption parameters of Audoped anatase TiO2 (101) surface were compared with those of intrinsic anatase TiO2 (101) surface. As can be concluded, Au doping changes the sensitivity and selectivity of TiO2 nanotubes to the above three kinds of gases. Furthermore, gas-sensing experiment of intrinsic and Au-doped TiO2 nanotubes to SO2 , SOF2 , and SO2 F2 was carried out, of which the results were consistent with simulation analysis. Research of this paper illustrates sensitive and selective changes of TiO2 nanotubes gas sensor after Au doping, which lays foundation for preparation of gas sensors applied for detection of partial discharge inside gas insulated switchgear. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Operating experience shows that, the occurrence of sudden fault in electric gas insulated switchgear (GIS) is due to the potential insulation defects inside GIS. Partial discharge (PD) can be considered the result of potential insulation defects in early stage and will lead to decomposition of SF6 , with characteristic components such as SO2 , SOF2 , and SO2 F2 produced [1–4]. Detecting species and concentrations of these gases is essential to effectively find the potential insulation defects inside GIS in time, so that operating condition and insulating level can be evaluated to avoid occurrence of sudden faults [5–7]. With the advent of nanotechnology, nanomaterials are gaining more and more research focus as gas-sensing materials [8–10]. Doping and modification of active catalysts or functional groups on

∗ Corresponding author at: State Key Laboratory of Power Transmission Equipment & System Security and New Technology, Chongqing University, Chongqing 400044, China. E-mail address: [email protected] (X. Zhang). http://dx.doi.org/10.1016/j.apsusc.2016.05.087 0169-4332/© 2016 Elsevier B.V. All rights reserved.

surface of materials can greatly change gas-sensing performance of nanomaterials [11,12], so as to prepare sensor arrays made of mutisensor with specific sensitivity and selectivity to each component. What’s more, constituent identification and concentration quantification can be completed even in the environment of a variety of gases [13,14]. Therefore, gas sensor method is a sort of promising online monitoring approach. TiO2 nanotubes have been the research hotspot of nano-gassensing field owing to excellent properties. Refs. [15–18] studies show that, band gap of TiO2 nanotubes can be reduced by doping noble metal, so that sensitive and selective properties can be improved to specific gas. Enrico prepared a kind of TiO2 -NiO gas-sensing film with Au nanoparticles doped and investigated gassensing response of this film to H2 , CO, propane and H2 S [19]. Relevant researches have shown that, TiO2 nanomaterials with metal doped exhibit favorable application potential when detecting O2 , H2 , NO2 and NO [20–22]. Furthermore, simulations of interactions between gas molecules and TiO2 surface based on density functional theory have been reported in succession [23–25]. This research team have been devoted to the application of TiO2 nanotubes sensors to online monitoring fault gases of GIS

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Fig. 1. Ball-and-stick models of three gas molecules and anatase TiO2 (101) surface.

equipment. Anodic oxidation method was adopted to produce TiO2 nanotubes sensors and noble metal was doped to modify the surface of sensors. Newly-prepared gas sensors were employed to detect decomposed components of SF6 , with a lot of experimental data accumulated [26–29]. So far, however, research on mechanism of TiO2 nanotubes gas sensor from microcosmic aspect still needs strengthening. As a result, it is necessary to investigate gassensing mechanism of TiO2 nanotubes gas sensors for their wide application in the future.

According to experimental reports, whether from this team [30] or other researchers [31–35], anatase TiO2 (101) surface appeared frequently when surfaces of TiO2 nanotubes were investigated using X-ray diffraction (XRD) method. Therefore, this paper concentrates on anatase TiO2 (101) surface with Au doped and simulates the adsorption of gas molecules on this kind of surface, with adsorption mechanism analyzed in detail. The adsorption experiments were also conducted to study gas-sensing performances of intrinsic and Au-doped TiO2 nanotubes sensors. Furthermore, this paper

Fig. 2. Views of intrinsic and Au-doped anatase TiO2 (101) surface models from different angles.

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focuses on two comparisons including simulation results comparison of Au-doped anatase TiO2 (101) surface with intrinsic anatase TiO2 (101) surface, as well as comparison of experimental results with simulation results. Simulation results were coincided with experimental results, which perfected adsorption mechanism of Au-doped anatase TiO2 nanotubes sensors to SF6 decomposed components under partial discharge.

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2. Parameters and calculation method

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Structure models in this paper were established in Materials Visualizer module of Materials Studio, while calculation and analysis were completed in Materials Dmol3 module [36,37]. Dmol3 module of Materials Studio includes algorithm based on density functional theory (DFT) of quantum mechanics. Total functional energy of the system is represented by numerical wave function of atoms’ orbits. Wave functions and energies of molecules were calculated by continuous iteration so that physicochemical property of the system can be mastered from view of atom. Anatase TiO2 (101) surface as well as individual SO2 , SOF2 , SO2 F2 molecules were constructed in Visualizer module, shown in Fig. 1. To avoid interactions between surfaces caused by periodic boundary condition, surface models in this paper were set as periodic boundary models. Anatase TiO2 (101) surface as well as individual SO2 , SOF2 , SO2 F2 molecules were optimized, respectively. After golden atom was added at the bridge site constructed by two 2-coordinated oxygen atoms, Au-doped anatase TiO2 (101) surface was optimized again to be closer to physicochemical reality. Adsorption processes between individual gas molecule and Au-doped TiO2 (101) surface were simulated and optimized until stable adsorption structures were formed. Characteristic physical and chemical parameters were calculated and analyzed then. There are many atoms in anatase TiO2 (101) surface model, and calculation included d electronic orbits of Ti atom as well as band gap. Therefore, general gradient approximate (GGA) of higher degree of calculation accuracy was adopted rather than local density approximation (LDA). PBE function was used to deal with exchange and correlation interaction effect among electrons [38]. In order to obtain more accurate results, double numerical basis set plus polarization functions (DNP) was utilized to achieve approximation of d and p orbitals’ polarization function. Convergence accuracy of energy was set as 1.0e−5 Ha, while the energy gradient and atom displacement were set as 0.002 Ha/Å and 0.005 Å, respectively. Convergence accuracy of charge density of self-consistent field was 1.0e−6 Ha and Brillouin k point was 1 × 1 × 1. In addition, direct inversion of iterative subspace (DIIS) was chosen to accelerate convergence speed of charge density of self-consistent field to reduce calculation time and enhance efficiency. 3. Results and discussions 3.1. Au-doped anatase TiO2 (101) surface Intrinsic anatase TiO2 (101) surface model was optimized in Dmol3 module shown in Fig. 2(a) (b) while Au-doped anatase TiO2 (101) surface model after optimization was exhibited in Fig. 2(c) (d). In Fig. 3, density of states (DOS) of intrinsic and Au-doped anatase (101) surfaces as well as the doped Au atom was compared. As can be concluded, Au-doping could reduce band gap of TiO2 , to some extent. To obtain more precise values, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) were calculated and band gap of Au-doped anatase (101) surface was 1.906 eV, less than1.932 eV, band gap of intrinsic anatase (101) surface. The decrease of band gap made it easier for

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Fig. 3. DOS of intrinsic and Au-doped anatase TiO2 (101) surface.

electrons to transfer from valence band to conduction band. In addition, Au-doping enhanced the DOS below Fermi level and increased density of electrons, providing more electrons which were possible to transfer to conduction band. 3.2. Adsorption of three gas molecules on doped Au atom Gas molecules approach Au-doped anatase TiO2 (101) surface by different atoms. Considering structure features of SO2 F2 that S atom is inside the tetrahedron made of O and F atoms, two conditions that SO2 F2 approaches Au-doped anatase TiO2 (101) surface by O and F atoms were taken into consideration. Adsorption energy Ea is used to represent total energy variation before and after gas molecules are adsorbed on Au-doped anatase TiO2 (101) surface, reflecting abilities of gas molecules adsorbed on surface. Ea is calculated by the following formula. Ea = Esur+gas − Egas − Esur where Egas is the energy of individual gas molecule, Esur is the energy of crystal surface without gas molecules adsorption, Esur+gas is total energy of adsorbed system after adsorption of gas molecule. Ea < 0 represents that adsorption process is exothermic and spontaneous. Furthermore, the larger the value of Ea is, the larger the released or adsorbed energy in the adsorption process is, and the stronger the interaction is, with more stable adsorption structure constructed. Mulliken charge population of gas molecules and crystal surface were calculated, respectively, so that the change of charge population can be obtained in the adsorption process. Charge transfer Q was defined as charge variation before and after adsorption of gas molecules. If Q > 0, electrons transfer from gas molecules to crystal surface. Table 1 shows adsorption parameters of gases adsorbed on Audoped anatase TiO2 (101) surface. As can be found that, when SO2 molecule approaches Au atom by O or S atoms, adsorption parameters are close to each other, while similar condition occurs when SOF2 approaches Au atom. When SO2 F2 approaches Au atom by O atom, the adsorption energy and charge transfer are obviously smaller than those of the other condition that SO2 F2 approaches Au atom by F atom, which indicates that it is easier for SO2 F2 to adsorb on Au atom by F atom. By comparison of adsorption energy, it can be concluded that, whatever figure gas molecules approach adsorption surface by, adsorption energy of SO2 is the largest, about 2–3 times that of SOF2 and SO2 F2 . It can also be found from the view of charge transfer that, electrons transfer from adsorption surface to SO2 molecule, the opposite direction of electron transfer in SOF2 and SO2 F2

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Fig. 4. Adsorption structures of three gas molecules on Au-doped anatase TiO2 (101) surface.

adsorption processes. The phenomenon can be explained by the strong oxidizing ability of SO2 . In addition, SO2 adsorption possesses larger absolute value of charge transfer in adsorption process than SOF2 and SO2 F2 adsorption processes (Fig. 4). Figs. 5, 6 and 7 showed the total density of states (TDOS) of adsorption system and partial density of states (PDOS) of adsorbed gas molecules, SO2 , SOF2 , and SO2 F2 , respectively. “SO2 -O-TiO2 ” represents that SO2 approaches anatase TiO2 (101) surface by O atom, and the like.

It should be noted that, in reference [39], SO2 and SOF2 molecules were adsorbed on intrinsic anatase TiO2 (101) surface by O atom, while SO2 F2 approached adsorption surface by F atom. Therefore, the comparisons of adsorption parameters are mainly based on calculation results when gas molecules approached sur-

Table 1 Adsorption parameters of gases adsorbed on Au-doped anatase TiO2 (101) surface. Gas molecules Adsorption structure

3.3. Comparison with simulation results of gas molecules adsorbed on intrinsic anatase TiO2 (101) surface In order to investigate the effect of Au-doping on gas-sensing properties of TiO2 (101) surface, simulation results were compared with those of gas molecules adsorbed on intrinsic TiO2 (101) surface. Detailed calculation and results of adsorption on intrinsic TiO2 (101) surface were included in the published reference [39].

SO2 SOF2

SO2 F2

−S −O −S −O −F −O −F

Adsorption energy Ea /eV

Charge transfer Q/e

Adsorption distance d/Å

−0.657 −0.571 −0.238 −0.363 −0.593 −0.071 −0.200

−0.156 −0.182 0.067 0.042 −0.006 0.009 0.039

2.601 2.419 2.657 2.636 2.656 2.667 2.924

“−S”, “−O” and “−F” represent that gas molecules approach adsorption surface by S, O and F atoms, respectively.

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Fig. 5. TDOS of adsorption systems and PDOS of adsorbed SO2.

face by the same atom. In this paper, more approaching figures of gas molecules adsorbed on TiO2 (101) surface are involved. As can be concluded from the adsorption parameters, adsorption energy and charge transfer increased obviously after SO2 molecule was adsorbed on Au-doped anatase TiO2 (101) surface than intrinsic anatase TiO2 (101) surface, while adsorption distance changed slightly. More importantly, Au-doping changed the direction of charge transfer, causing the resistivity of adsorption system changing from decrease to increase. As for SOF2 , adsorption energy increased by one third while charge transfer and adsorption distance kept almost unchanged. It can be judged that, adsorption ability of SOF2 on Au-doped surface slightly increased compared with that of SOF2 on intrinsic surface. After SO2 F2 was adsorbed on Au-doped anatase TiO2 (101) surface, adsorption distance became smaller while adsorption energy and charge transfer became larger compared with the adsorption on intrinsic anatase TiO2 (101) surface. It can also be found that, absolute values of adsorption energy and charge transfer became larger when three molecules were adsorbed on Au-doped TiO2 (101) surface with adsorption distance almost unchanged compared with the adsorption on intrinsic surface. That is to say, Au-doping improved adsorption performance of intrinsic anatase TiO2 (101) surface to SO2 , SOF2 , and SO2 F2 . When SO2 molecule was adsorbed on Au-doped adsorption surface by S atom, adsorption energy was 0.657eV > 0.6 eV, so the process belonged to chemical adsorption, while adsorption energy was 0.571 eV when SO2 molecule was adsorbed by O atom, slightly smaller than 0.6 eV, which can be considered close to chemical adsorption. Adsorption energies of SOF2 and SO2 F2 molecules adsorbed on Au-doped adsorption surface were much smaller than 0.6 eV, belonging to physical adsorption. TDOS of adsorption systems and gas molecules were compared, too. Ref. [39] shows TDOS of adsorption systems and PDOS of gas

molecules adsorbed on intrinsic anatase TiO2 (101) surface. By comparing with adsorption on intrinsic anatase TiO2 (101) surface, an obvious TDOS peak did not appear above Fermi level when SO2 was adsorbed on Au-doped anatase TiO2 (101) surface. As can be concluded, SO2 molecule contributed less electrons to conduction band, leading to increase of adsorption system’s resistivity, which is consistent with the above analysis based on adsorption parameters. Au-doping changed variation tendency of adsorption system’s resistivity when SO2 was adsorbed on anatase TiO2 (101) surface. Compared with adsorption of SOF2 on intrinsic anatase TiO2 (101) surface, SOF2 made obvious contribution to DOS near Fermi level when it was adsorbed on Au-doped anatase TiO2 (101) surface, which increased the amount of electrons in conduction band. From macroscopic view, resistivity of adsorption system dropped more severely and resistance declined more wildly as well in the adsorption process. That is to say, Au-doping improved the sensitivity of anatase TiO2 (101) surface to SOF2 , to some extent. As far as SO2 F2 , adsorption of SO2 F2 molecule did not contribute much to the DOS near Fermi level when SO2 F2 was adsorbed on intrinsic anatase TiO2 (101) surface, that is, SO2 F2 provided almost no electrons for conduction band of adsorption system. When SO2 F2 was adsorbed on Au-doped anatase TiO2 (101) surface, however, obvious DOS peaks appeared above Fermi level, and SO2 F2 supplied more electrons to conduction band of adsorption system. Macroscopically, sensitivity of Au-doped anatase TiO2 (101) surface to SO2 F2 increased obviously compared with intrinsic anatase TiO2 (101) surface. Based on the above analysis of adsorption parameters and density of states, it can be concluded that, resistance’s variation tendency of Au-doped anatase TiO2 (101) surface after the adsorption of SO2 is decreasing, different from intrinsic anatase TiO2 (101) surface. Resistance of Au-doped anatase TiO2 (101) surface

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Fig. 6. TDOS of adsorption systems and PDOS of adsorbed SOF2.

dropped more, and the sensitivity increased slightly after SOF2 adsorption, while resistance of Au-doped anatase TiO2 (101) surface dropped severely, and the sensitivity increased obviously after SO2 F2 adsorption.

tested. Gas-sensing responses of sensors to aimed gases were represented by R%, relative change of resistance, formulated by the following equation. R% =

3.3. Explanation of phenomenon in gas-sensing experiment In order to raise scientific reliability of theoretical analysis, in this part, simulation results and practical phenomenon were compared through gas-sensing experiment. Anatase TiO2 nanotubes were prepared by anodic oxidation method and Au was doped to modify the surface of TiO2 nanotubes sensors. Scanning electron microscope (SEM) images of intrinsic and Au-doped TiO2 nanotubes are shown in Fig. 8. Gas-sensing properties of intrinsic and Au-doped TiO2 nanotubes sensors to SO2 , SOF2 and SO2 F2 were

(R − R0 ) × 100% R0

where R is the resistance of gas sensors in the atmosphere of detected gases, and R0 is the resistance of gas sensor exposed to initial carrier gases. Fig. 9 shows the gas-sensing responses of intrinsic and Au-doped TiO2 nanotubes to different SF6 decomposed components. In comparison with intrinsic TiO2 nanotubes, Au-doped TiO2 nanotubes responded a little better to SOF2 and much better to SO2 F2 , respectively. Those experimental consequences were coincided with the above gas-sensing analysis based on simulation results. As far as

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gas-sensing response of Au-doped TiO2 nanotubes to SO2 , the value decreased from −74.6% to −8.73%, that is, the adsorption of SO2 made resistance of nanotubes sensor decline less, even to the trend of increase, which is not consistent with the above analysis that SO2 adsorption enhanced resistance of Au-doped TiO2 nanotubes. Inconformity here is not contradictory to the gas-sensing mechanism analysis, actually. It should be noted that, there are both strong adsorption sites and weak adsorption sites on the surface of Audoped TiO2 nanotubes. According to the simulation results, doped Au atoms are strong adsorption sites while the un-doped surface is the weak adsorption sites to SO2 . When SO2 molecules came close to TiO2 nanotubes, it was easier for them to approach strong adsorption sites, that is, the Au atoms, causing the resistance to increase

while the adsorption on weak adsorption sites will lead to decrease of the resistance of TiO2 nanotubes. It should be noted that, the doped Au atoms cover only part of the adsorption surface, that is, the number of strong adsorption sites is much smaller than those of weak adsorption sites. Considering strong adsorption effect which decreased the resistance and weak adsorption effect which functioned oppositely, synthesized consequence of experiment was a slight decline of resistance of the sensor. It can be inferred from the above analysis that, if the coverage of doped Au atoms can be changed within a proper range, the sensitivity and selectivity of Au-doped TiO2 nanotubes to SO2 can be altered, so that TiO2 nanotubes sensors of specific gas-sensing performances to SO2 can be prepared.

Fig. 8. SEM images of intrinsic and Au doped TiO2 nanotubes.

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intrinsic Au doped

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4. Conclusions

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In this paper, models of Au-doped anatase TiO2 (101) surface as well as SF6 decomposed components SO2 , SOF2 , SO2 F2 were built and adsorption parameters such as adsorption energy, charge transfer and adsorption distance of three gases on built surface were calculated. DOS of adsorption structures was also analyzed for gas-sensing mechanism. The conclusions are as follows.

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because violent thermal motions of gas molecules makes it not easy for them to adsorb on the surface of nanotubes.

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Fig. 9. Gas-sensing responses of intrinsic and Au-doped TiO2 nanotubes to different SF6 decomposed components.

Fig. 10 shows sensitivity of Au-doped TiO2 nanotubes sensor on different working temperatures, which can be explained based on the mechanism analyzed previously. When temperature is below 60 ◦ C, gas molecules tend to adsorb on Au-doped nanotubes sensor physically, with small gas-sensing response no more than 2% in general. It is difficult to precisely distinguish SO2 , SO2 F2 and SOF2 . When temperature ranges from 60 ◦ C to 100 ◦ C, gas-sensing responses increase gradually, which indicates that activities of both Au-doped surface and un-doped surface improve, so that gas molecules are easier to adsorb on nanotubes surface. With temperature changing from 100 ◦ C to 120 ◦ C, sensitivity of doped Au nanoparticles rises obviously, making the adsorption of SOF2 and SO2 F2 much easier. Gas-sensing response to SO2 F2 was enhanced greatly while the response to SOF2 increased a little. In comparison, gas-sensing response of the sensor to SO2 becomes weak because adsorption on Au nanoparticles which increased resistance counteracts adsorption on un-doped surface which decreased the resistance. Gas-sensing responses of Au-doped TiO2 nanotubes sensors to three gases are SO2 F2 > SOF2 ≈ SO2 . When temperature rises more than 120 ◦ C, gas-sensing responses do not change significantly, even to very small values,

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Fig. 10. Sensitivity of Au-doped TiO2 nanotubes sensor at different working temperatures.

(1) Au-doping changes the variation tendency of gas-sensing response of anatase TiO2 (101) surface to SO2 gas. Adsorption of SO2 makes the resistance of anatase TiO2 (101) surface from decreasing to increasing. Au-doping does not change the variation tendency of gas-sensing response to SOF2 , with slight enhancement instead, while the gas-sensing response to SO2 F2 rises significantly and resistance of sensor declines more violently. (2) On the basis of simulation analysis, gas-sensing experiments of intrinsic and Au-doped anatase TiO2 nanotubes to SO2 , SOF2 and SO2 F2 were carried out to verify scientific correctness of mechanism theory. By comparison, it can be concluded that, consequences of gas-sensing response of Au-doped TiO2 nanotubes sensor to SO2 , SOF2 and SO2 F2 were consistent with those of simulation analysis. (3) According to the simulation analysis and gas-sensing experiments, sensitivity of Au-doped TiO2 nanotubes sensor to SO2 , SOF2 and SO2 F2 at different working temperatures was explained, which improved and perfected gas-sensing mechanism of Au-doped anatase TiO2 nanotubes sensors. Acknowledgment We gratefully acknowledge the financial support from National Natural Science Foundation of P.R. China (project No. 51277188). References [1] F. Zeng, J. Tang, Q. Fan, et al., Decomposition characteristics of SF6 under thermal fault for temperatures below 400 ◦ C, IEEE Trans. Dielectr. Electr. Insul. 21 (21) (2014) 995–1004. [2] J. Tang, F. Zeng, J. Pan, et al., Correlation analysis between formation process of SF6 decomposed components and partial discharge qualities, IEEE Trans. Dielectr. Electr. Insul. 20 (3) (2013) 864–875. [3] A. Casanovas, J. Casanovas, F. Lagarde, A. Belarbi, Study of the decomposition of SF6 under dc negative polarity corona discharges (point to plane geometry): influence of the metal constituting the plane electrode, J. Appl. Phys. 72 (8) (1992) 3344–3354. [4] A. Derdouri, J. Casanovas, R. Hergli, R. Grob, J. Mathieu, Study of the decomposition of wet SF6 : subjected to 50 Hz ac corona discharges, J. Appl. Phys. 65 (5) (1989) 1852–1857. [5] W. Ding, R. Hayashi, K. Ochi, J. Suehiro, K. Imasaka, M. Hara, N. Sano, E. Nagao, T. Minagawa, Analysis of PD generated SF6 decomposition gases adsorbed on carbon nanotubes, IEEE Trans. Dielectr. Electr. Insul. 13 (6) (2006) 1200–1207. [6] Zhang Xiaoxing, Yao Yao, Tang Ju, et al., Actuality and perspective of proximate analysis of SF6 decomposed products under partial discharge, High Voltage Eng. 34 (4) (2008) 664–669. [7] Zhongqi Zhang, Lian Hongsong, Using SO2 detection for failure checking of SF6 electricity equipment, Electric Power 34 (1) (2001) 77–80. [8] D. Yang, M.K. Fuadi, K. Kang, et al., Multiplexed gas sensor based on heterogeneous metal oxide nanomaterial array enabled by localized liquid-phase reaction, ACS Appl. Mater. Interfaces 7 (19) (2015) 10152–10161. [9] E. Sotter, E. Llobet, E.H. Espinosa, et al., New TiO2 and Carbon Nanotube Hybrid Microsensors for Detecting Traces of O2 in Beverage Grade CO2 , in: Solid-State Sensors, Actuators and Microsystems Conference, 2007, Transducers 2007 International. IEEE, 2007, pp. 1039–1042. [10] Xuan Tianmei, Yin Guilin, Ge Meiying, et al., Research progress on nano-ZnO gas sensors, Mater. Rev. 29 (1) (2015) 132–136. [11] C.L. Zhu, H.L. Yu, Z. Yue, et al., Fe2 O3 /TiO2 tube-like nanostructures: synthesis, structural transformation and the enhanced sensing properties, ACS Appl. Mater. Interfaces 4 (2) (2012) 665–671.

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