- Email: [email protected]
Theoretical study on the interaction between SF6 molecule and BaTiO3(0 0 1) surface: A DFT study
Applied Surface Science 483 (2019) 409–416
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full length article
Theoretical study on the interaction between SF6 molecule and BaTiO3(0 0 1) surface: A DFT study
T
⁎
Zhaolun Cuia, Xiaoxing Zhanga,b, , Dachang Chena, Yuan Tiana a
School of Electrical Engineering, Wuhan University, Wuhan 430072, China State Key Laboratory of Power Transmission Equipment & System Security and New Technology, College of Electrical Engineering, Chongqing University, Chongqing 400030, China
b
A R T I C LE I N FO
A B S T R A C T
Keywords: SF6 BaTiO3(0 0 1) Adsorption DFT study
Based on the first principle, the adsorption and decomposition process of SF6 gas molecules on the surface of BaTiO3(0 0 1) was calculated. The results show that there is a strong interaction between SF6 molecule and BaTiO3 surface, and the adsorption energy reached 4.453–4.963 eV, which is likely to be a chemical adsorption process. According to Mulliken analysis, about 1.6 e of electrons transferred from BaTiO3 surface to the SF6 gas molecule, the SF6 molecule behaved as an electron acceptor. According to density of states calculation results, there were obvious electron orbital interactions between SF6 and BaTiO3 surface atoms, especially F and O atoms, and the charge transfer process was also confirmed by the differential charge density results. The SF6 molecules interacted strongly with the BaTiO3 surface, which led to the elongation of SeF bonds and made SF6 tend to undergo a decomposition process. The results indicate that BaTiO3 may have a catalytic potential for SF6 abatement and provide a simulation support for the experimental study of efficient and harmless treatment of SF6 gas.
1. Introduction Barium titanate (BaTiO3) as a perovskite type crystal, has excellent piezoelectricity, pyroelectricity, high dielectric permittivity and high voltage tunability [1]. As a catalyst or packing material, it is widely used in the field of volatile organic compounds (VOC) treatment often combined with nonthermal plasma (NTP) discharge, and has been proved to have good catalytic effects [2–6]. In Ref [2], Ogata et al. reported the experimental results of three VOC gases treatment in a packed bed dielectric barrier discharge (DBD) reactor with 5% BaTiO3 participation, indicating the catalyst effect of BaTiO3. In Ref [3], Liang et al. reported that the BaTiO3 packed bed reactor has the highest toluene removal efficiency when compared to a NaNO2 packed reactor. In the Ref [4–6], scholars have proposed different sizes of BaTiO3 pellets affect the degradation effects of benzene and CCl4 in DBD discharge. All of the above experiments have proved the effect of BaTiO3 on the discharge enhancement and VOC gases abatement. This is because the addition of BaTiO3 can change the dielectric constant of the reactor gap, promote the discharge process and increase the micro-discharge on the surface of the packing materials. On the other hand, gas molecules such as VOCs may be adsorbed on the surface of BaTiO3 to promote the combination of gas molecules and plasma, so that the abatement effect
⁎
of VOCs and other waste gases is significantly improved [7]. Sulfur hexafluoride (SF6), as an industrial gas, is widely used in power industry and semiconductor processing. Due to its excellent insulation performance and arc-extinguishing performance, it has been increasingly used in the power industry in China, India and other countries in recent years [8]. However, SF6 is one of the six major greenhouse gases proposed by the Kyoto Protocol in 1997, and its global warming potential (GWP) is 23,500 times that of the same volume fraction of CO2 [9]. Therefore, how to effectively deal with the exhaust SF6 gas is a problem and has attracted much attention. At present, NTP has been proved to be a reliable method for the treatment of SF6 and other industrial waste gases. High-energy particles generated by NTP can effectively decompose SF6 molecules in the discharge region [10]. Among the discharge types of NTP, microwave discharge, radio frequency discharge, dielectric barrier discharge (DBD) and corona discharge have applied to SF6 degradation process [11–16]. The device scales of microwave and radio frequency (RF) discharge are relatively large and the energy efficiency is relatively low [11,12]. Besides, the processing rate of DBD and corona discharge is relatively small [13–16]. Therefore, some scholars have promoted the decomposition process of SF6 by adding packing materials such as Al2O3 in the DBD discharge reactor, and showed a promotion effect [17,18].
Corresponding author at: School of Electrical Engineering, Wuhan University, Wuhan 430072, China. E-mail address: [email protected] (X. Zhang).
https://doi.org/10.1016/j.apsusc.2019.03.102 Received 26 November 2018; Received in revised form 6 March 2019; Accepted 11 March 2019 Available online 16 March 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 483 (2019) 409–416
Z. Cui, et al.
theoretical support for SF6 abatement process by BaTiO3 catalyzed discharge. 2. Computational details In this study, all the calculations were based on DFT using Dmol3 package in Material Studio 8.0 [25]. The generalized gradient approximation (GGA) was selected to describe the exchange–correlation interaction combined with the Perdew–Burke–Ernzerhof (PBE) functions [26]. The structures were built based on materials visualize model and the BaTiO3 (0 0 1) slab contains 58 atoms. The periodic region of BaTiO3 was (x = 8.020 Å, y = 8.020 Å, z = 21.015 Å) along XYZ direction with an optimized vacuum space of 15 Å. The Monkhorst-Pack k-point was set at 4 × 4 × 1 for geometric optimization and other properties [27]. The DFT Semi-core Pseudopots (DSSP) and the double numerical atomic orbital augmented by d-polarization (DNP) were applied [28]. TS for DFT + D applied to correct the van der Waals forces [29]. The energy convergence tolerance and maximum force were set at 1.0 × 10−5 Ha and 0.002 Ha/Å. The maximum displacement chose as 0.005 Å and the global orbital cutoff was 5.0 Å [30–32]. In BaTiO3, atoms in the bottom layer were fixed and upper three layers were free in the calculations. In addition, there were no constraints applied for the geometry optimization of the SF6 molecule. The adsorption energy (Ead) of SF6 molecules on BaTiO3 surface was calculated by the following equation:
Fig. 1. Molecular structure of SF6.
As BaTiO3 has been mentioned as a very common and effective DBD discharge packing material, many scholars have studied the adsorption process of different materials on the surface of BaTiO3 (0 0 1), which proves BaTiO3 (0 0 1) surface has certain adsorption and catalytic properties [1,19–21]. Ref [21, 22] have proved that BaO-terminated surface has a good stability which appears to be the most prone to occur in nature and has a stronger surface adsorption process with H2O or ethanol molecule. Hence, we chose the BaO terminated slab to study in this paper. Based on the density functional theory (DFT) calculation, the physical properties of solid materials and gas molecules can be calculated and the adsorption process of SF6 gas molecules on BaTiO3 (0 0 1) surface was analyzed [22–24]. It was found that SF6 gas molecules may undergo chemical adsorption on BaTiO3 surface, resulting in SeF bonds breaking. In addition, the interaction process between SF6 molecule and BaTiO3 surface was analyzed by means of electron transfer calculation, density of state (DOS) analysis and differential charge density analysis. The results show that BaTiO3 material may produce certain catalytic effect on SF6 molecule. The results may provide
Ead = EBaTiO3 + ESF6 − ESF6/BaTiO3
(1)
where ESF6/BaTiO3 is the total energy of SF6 gas molecule absorbed on the BaTiO3 (0 0 1) surface, EBaTiO3 and ESF6 is the energy of the BaTiO3 slab and SF6 gas molecule, respectively. Following this equation, a positive value of Ead indicates that adsorption process is exothermic. To further understand the interaction process, we else calculated the charge transfer Qt and the electron density difference between the SF6 gas molecule and BaTiO3 (0 0 1) surface by Mulliken analysis. The negative Mulliken charge (e) values mean the atom is positively charged and the positive Mulliken charge (e) values mean the atom is negatively charged. DOS analysis helped to obtain the electronic structure and the properties of the relaxed structure before and after
Fig. 2. Simulation structure of BaTiO3 (0 0 1): (a) main view (b) top view. 410
Applied Surface Science 483 (2019) 409–416
Z. Cui, et al.
Fig. 3. Initial adsorption configurations of SF6 on BaTiO3 (0 0 1) surface.
In this study, we selected the (2 × 2) BaTiO3 (0 0 1) surface to study the adsorption of SF6 molecules. In Ref [33], (2 × 2) BaTiO3 surface has proved to suffice for carrying out the adsorption of one molecule onto its surface and meet the calculation accuracy. There are three adsorption sites on the surface of BaTiO3 (0 0 1), which are above the Ba atom, above the labelled ‘2’ oxygen atom (O1) and above the labelled ‘3’ oxygen atom (O2), as shown in Fig. 2. Other sites like ‘bridge’ and ‘hollow’ sites are not discussed here as they are not representative and the geometric optimization results of these two sites are similar to the O2 site. The relative analysis provides in Supplementary material file. Fig. 3 shows the six configurations of SF6 molecules in BaTiO3. In M1 configurations, the straight line of the two SeF bonds of SF6 was perpendicular to the BaTiO3 (0 0 1) surface. In M2 configurations, two bottom F atoms were close to the adsorption sites at the same distance. The six adsorption configurations are defined as M1-Ba, M1-O1, M1-O2, M2-Ba, M2-O1, M2-O2, respectively. In both configurations, the
adsorption. Qt represents the total charge transfer. If the total charge transfer Qt < 0, it represents that electrons transfer from the BaTiO3 (0 0 1) surface to SF6 gas molecule and the gas molecule has positive charge after interactions. If the total charge transfer Qt > 0, it represents that electrons transfer from SF6 gas molecule to BaTiO3 (0 0 1) surface. 3. Result and discussion 3.1. Surface models Fig. 1 is the molecular structure of SF6. The SF6 gas molecule has a regular octahedral structure with an SeF bond length of 1.613 Å. The angle between the two adjacent F atoms and the S atom is 90°, and the Mulliken charges of the S atom and F are 1.865e and −0.311e, respectively. 411
Applied Surface Science 483 (2019) 409–416
Z. Cui, et al.
Table 1 Adsorption energy (Eads) and total charge transfer of SF6 adsorption on BaTiO3 (0 0 1). Adsorption site
Adsorption energy Ead (eV)
Total charge transfer of SF6 Qt (e)
M1-Ba
4.741
−1.578
M1-O1
4.455
−1.583
M1-O2
4.952
−1.567
M2-Ba
4.453
−1.587
M2-O1
4.951
−1.569
M2-O2
4.963
−1.567
Table 2 Valence electron occupation of atoms in M2-O2 adsorption structure. Atom species
Bond-length (Å) F1-S: F2-S: F3-S: F4-S: F1-S: F2-S: F3-S: F1-S: F2-S: F3-S: F1-S: F2-S: F3-S: F1-S: F2-S: F3-S: F1-S: F2-S: F3-S:
S F1 F2 F3 O1 O2 Ba
4.561 1.955 1.971 1.671 2.216 2.002 2.781 2.282 2.126 2.274 2.431 2.003 2.425 2.283 2.124 2.286 2.283 2.114 2.285
Before adsorption
After adsorption
s
p
d
s
p
d
2.557 1.980 1.980 1.980 1.935 1.951 1.176
4.020 2.660 2.660 2.660 2.537 2.459 3.080
0.491
2.919 1.983 1.983 1.985 1.945 1.943 1.164
4.094 2.853 2.853 2.827 2.488 2.473 3.043
0.253
O1 is the marked ‘2’ O atom in Fig. 2, O2 is the marked ‘3’ O atom in Fig. 2.
distance of the bottom F atom from the BaTiO3 plane was set to 2.000 Å. 3.2. SF6 molecule adsorbed on BaTiO3 (0 0 1) surface The adsorption energy and total charge transfer of the six adsorption configurations of SF6 gas on BaTiO3(0 0 1) surface are shown in Table 1, where Fx-S is expressed as the bond length of the labelled ‘Fx’ atom and S atom. Fig. 4 shows the geometric optimization results for the six adsorption configurations. In Table 1, under six configurations, SF6 molecule had relative large adsorption energies and charge transfers on the surface of BaTiO3. The adsorption energies in three M1configurations were 4.741, 4.455 and
Fig. 4. Adsorption results of BaTiO3 and SF6 gas molecules after geometric optimization. 412
Applied Surface Science 483 (2019) 409–416
Z. Cui, et al.
bond lengths of the three F atoms near the surface of BaTiO3 changed greatly. In the M1-O1 adsorption configuration, the bond lengths of the three bottom F atoms were stretched from 1.613 Å to 2.216, 2.002 and 2.781 Å. In the M1-O2 configuration, the three SeF bonds were elongated to 2.282, 2.126, and 2.274 Å, respectively. In the M2-Ba configuration, there were also three significant changes in the SeF bonds, and the bond length extended to 2.431, 2.003, and 2.425 Å. The SF6 molecule drifted from above the Ba atom to the direction of the O1 and O2 atom during the adsorption process. The above results show that under the six configurations, the SF6 molecule had a strong adsorption process on the surface of BaTiO3, which caused the elongation of some SeF bonds in SF6, and the SF6 molecule has a certain displacement. Besides, we also investigated the influence of atomic distances between SF6 and BaeO surface. We chose the M1-Ba configuration and the initial distances were set at 1.5 Å, 2.0 Å, 2.5 Å and 3.0 Å. The adsorption results shows in Fig. S1 (Supplementary material file). Compared with the calculated initial structure of M1-Ba (initial adsorption distance is 2.0 Å), the geometric optimization results of the three newly calculated structures were very similar to the previous one. The adsorption energies of the three new configurations obtained after calculation were 4.742, 4.750 and 4.749 eV, which were very close to the previously calculated 4.741 eV. In Fig. S1, SF6 shifted to the O2 site during the adsorption process under each adsorption structure, indicating that SF6 has the tendency towards the O2 site after adsorption. The influence of atomic distances in the initial adsorption is not obvious. Beside, we believe that the initial adsorption distance of 2.0 Å is representative.
Table 3 Charge transfer for characteristic atoms of SF6 and BaTiO3 (0 0 1) surface. Adsorption configuration
Characteristic atom
Charge transfer (e−)
M1-Ba
F1 F2 F3 F4 S Ba O1 O2 F1 F2 F3 S Ba O1 O2 F1 F2 F3 S Ba O1 O2
−0.454 −0.271 −0.275 −0.099 −0.389 0.057 0.012 0.072 −0.356 −0.298 −0.437 −0.422 0.080 0.002 0.062 −0.369 −0.330 −0.367 −0.398 0.067 0.011 0.073
M1-O1
M1-O2
3.3. Electronic structure of SF6 on BaTiO3 (0 0 1) surface According to the valence electron occupation of atoms in Table 2, after the adsorption, the valence electrons in different orbits of the SF6 and BaTiO3 atoms underwent certain changes. After adsorption of SF6 gas molecules, the valence electrons in the s orbital and p orbitals of S atoms increased from 2.557 and 4.020 to 2.919 and 4.094, respectively, and the valence electrons in d orbit decreased from 0.491 to 0.253. In addition to the electron transfer of the SF6 molecule and the BaTiO3 surface, the electrons of the S atom's d orbital may partially transferred to the s orbital and p orbitals. The valence electron changes of the F1 and F2 atoms were the same, the s orbital and p orbital increased from 1.980 and 2.660 to 1.983 and 2.853, respectively, and the s orbital and p orbital of the F3 atom increased from 1.980 and 2.660 to 1.985 and 2.827, respectively. The s orbital and p orbital of the O1 atom changed from 1.935 and 2.537 to 1.945 and 2.488, respectively. The s orbital and p orbital of the O2 atom changed from 1.951 and 2.459 to 1.943 and 2.473, respectively. The s orbital and p orbital may occur electronic transfer in each O atom. The valence electron of s and p orbital of the Ba atom were reduced from 1.176 and 3.080 to 1.164 and 3.043, respectively. Changes in BaTiO3 were relatively smaller than that in SF6. The main orbitals of Ba atoms lost electrons and may transferred to SF6 molecules. According to Mulliken analysis, we obtained the charge transfer results of characteristic atoms in three M1 configurations, as shown in Table 3. In the M1-Ba configuration, the four F atoms obtained 0.454e, 0.271e, 0.275e and 0.099e electrons, respectively, while the S atom obtained 0.389e. Ba lost 0.057e of electrons, O1 and O2 atom lost 0.012e and 0.072e, respectively. At this time, SF6 appeared as an electron acceptor, and BaTiO3 appeared as an electron donor. In the M1-O1 system, three F atoms and S atoms obtained electrons of 0.356e, 0.298e, 0.437e and 0.422e, respectively, while Ba and two O atoms lost 0.080e, 0.002e and 0.062e, respectively. In the M1-O2 system, three F atoms and S atoms respectively obtained electrons of 0.369e, 0.330e, 0.367e and 0.398e, while Ba and two O atoms lost 0.067e, 0.011e and 0.073e. The charge transfer under the M2 adsorption systems was very similar to that in M1. The SF6 molecule was also an electron acceptor and the BaTiO3 surface was an electron donor. According to the results
Fig. 5. DOS result of BaTiO3 system before and after adsorption of SF6 molecules in M1-Ba configuration.
4.952 eV, respectively. At this time, the adsorption effect was most obvious under the configuration of M1-O2. The adsorption energies in three M2 configurations were 4.453, 4.951 and 4.963 eV, respectively. The adsorption effect was most obvious under the configuration of M2O2. The O2 adsorption site may have the greatest adsorption capacity and catalytic ability. According to Mulliken analysis, we obtained total charge transfer results under six adsorption systems. In Table 1, the charge transfer results in the six adsorption configurations were very similar, and the charges were all transferred from BaTiO3 (0 0 1) surface to the SF6 molecule. In the M1 configuration, the SF6 molecule obtained 1.578, 1.583 and 1.567e electrons in the three adsorption structures, respectively. In the M2 configuration, the SF6 molecules obtained 1.587, 1.569, and 1.567e electrons in the three adsorption structures, respectively. It can be seen from Fig. 4 that SF6 structure had obvious changes after adsorption in the six configurations. We calculated the bond length results of the SeF bond under each adsorption structure, as shown in Table 1. The unshown SeF bonds did not change obviously after adsorption. According to Table 1, in M1-Ba configuration, the SeF bonds of four F atoms were significantly elongated, and the bond lengths of the F1 to F4 atoms were stretched from 1.613 Å to 4.561, 1.955, 1.971 and 1.671 Å respectively, the SF6 molecule also drifted a bit, moving from above the Ba atom to above the O2 atom. The SeF bonds changes of the other five configurations were similar, and the 413
Applied Surface Science 483 (2019) 409–416
Z. Cui, et al.
Fig. 6. PDOS results of BaTiO3 system before and after SF6 adsorption in six adsorption structures.
the characteristic atoms in the six adsorption configurations, as shown in Fig. 6. Fig. 6(a) is the M1-Ba PDOS result, the 3p orbital of the S atom in the SF6 molecule overlapped with the 2p orbital of the O2 atom is near −2 eV. In addition, the 2p orbital of the F1 atom overlapped with the 2p orbital of O2 at −1 eV, and the 2p orbital of the F2 and F3 atoms overlapped with the 2p orbital of O2 at −2 eV. The Ba 6s orbital slightly overlapped with F2 and F3 2p orbital. The above results indicate that there is a certain orbital interaction process between SF6 molecule and the Ba and O atoms of BaTiO3 surface. A similar result occurred in the M1-O1 configuration of Fig. 6(b). The 2p orbital of the O1 atom overlapped with the 2p orbital of the three F atoms at −2 eV, and the 6s orbital of the Ba atom had a cross with the 2p of the F1 and F2 atoms near −3 .5eV. In Fig. 6(c), the 2p orbital of O2 overlapped with the 3p orbital of the S atom and the 2p
of Mulliken analysis, in the adsorption process, there was a strong charge transfer process on the BaTiO3 surface, and the electrons on the surface of BaTiO3 transferred to SF6 gas molecules. This may indicate that the SF6 molecule underwent a strong chemical adsorption process on the surface of BaTiO3. Fig. 5 is the total density of states (TDOS) of SF6 molecules before and after adsorption on the surface of BaTiO3 in the M1-Ba configuration. After the adsorption of SF6 molecules, the DOS of the BaTiO3 system changed in some energy ranges, and there were significant increases in the vicinity of −13 eV, −9 eV, −6 eV and −4 eV and −1 eV, which was due to the adsorption of SF6 molecule. Considering that there may be strong interaction and electron exchange between the SF6 molecule and the BaTiO3 surface during the adsorption process, we calculated the partial density of states (PDOS) of 414
Applied Surface Science 483 (2019) 409–416
Z. Cui, et al.
configurations. It shows a strong electrical interaction between the SF6 molecule and the BaTiO3 surface. Charge density increased near the F atoms, and decreased near the SeF bonds, which indicates that there, may have a charge transfer process inside the SF6 molecule during the adsorption process. Surface O atoms did not show obvious changes in charge density as the charge transfer of these O atoms are not large in Table 3. At the same time, the BaTiO3 surface layer exhibited a weak increase of electron densities, while the layer below the surface exhibited a decrease of electron density, which may indicate that part of the electrons in the BaTiO3 structure were transferred from the interior of the structure to the surface and then transferred to the SF6 molecule. Although some electrons inside SF6 molecule transferred from S to F, the SF6 totally acts as an electron acceptor as many electrons transferred to it from BaeO surface. In summary, in the above six adsorption configurations, SF6 molecules interacted strongly with BaTiO3, and it has speculated that a chemical adsorption process may occur. During the process, there were obvious charge transfer processes on BaTiO3 surface and SF6. At the same time, there were certain orbital overlaps between them and the F atoms have strong interaction with the O atoms with charge transfer. After the adsorption, the structure of SF6 molecules changed obviously, the SeF bonds were elongated, and the SF6 molecule drifted towards to the O2 atom on the surface of BaTiO3, which made the SF6 molecule more likely to be decomposed. We believe that the O2 site is the most stable adsorption site and the interaction main occurs between the surface O atoms and F atoms. The results showed the catalytic potential of BaTiO3 surface for SF6 molecules.
Fig. 7. The supplemented PDOS results of BaTiO3 with SF6 adsorption in M1-Ba structure.
orbital of the F atom at −1 eV, overlapped with the 3p orbital of the S atom near −3 eV. Under the three M2 configurations, there were similar PDOS calculation results. The above results indicate that there may be some interactions between SF6 molecules and BaTiO3 surface atoms in the electron orbitals during the adsorption of SF6 and BaTiO3. Through PDOS analysis, we found that in the surface adsorption system of SF6 and BaTiO3, the interaction between the F atom and the O atom in the 2p orbital was the main electron orbital interaction, and the interaction between the F and Ba as well as S and O were not obvious. Therefore, we supplemented the PDOS distribution of the 2p orbitals of the other O atoms below the SF6 molecule in the PDOS of the M1-Ba system, as shown in Fig. 7. In Fig. 7, in addition to the O2 atom, the 2p orbital of O1 overlapped with the 2p orbital of the F1 atom near −1 eV, and the 2p orbital of several nearby O atoms overlapped with the 2p orbital of the F2 and F3 atoms near −4 eV, with the F1 atom at −1 eV. This indicates that there exhibits a strong interaction between F and O atoms in 2p orbital. The SF6 molecule also moved from above Ba to above the O2 atom and the interaction between F and O may lead to the chemical adsorption process. Fig. 8 shows the differential charge calculation results for two M1
4. Conclusion Based on the first-principle calculation, we theoretically analyzed the interaction mechanism of the insulating gas SF6 on the BaTiO3 (0 0 1) surface with six initial configurations. The results show that the SF6 gas molecules on the surface of BaTiO3 had strong adsorption processes and it has presumed to be a chemical adsorption process. In addition, according to DOS calculation results, S atoms and F atoms in SF6 molecules had obvious electron orbital interactions with Ba atoms and O atoms on BaTiO3 surface. According to Mulliken analysis, about 1.6 e electrons transferred from the surface of BaTiO3 to SF6 molecules. SF6 Fig. 8. Differential charge density distribution of SF6 molecules and BaTiO3 surface after adsorption in three configurations (the purple part indicates an increase in the density of the electron cloud and the yellow part indicates a decrease in the density of the electron cloud). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
415
Applied Surface Science 483 (2019) 409–416
Z. Cui, et al.
molecule behaved as electron acceptors during charge transfer, and similar phenomena existed in the results of differential charge density analysis. The atomic distance did not show an obvious influence on adsorption process. The molecular structure of SF6 changed significantly during the adsorption process, and the SeF bonds were elongated, making the SF6 molecule more likely to be decomposed. The results provide a theoretical support for the catalytic decomposition research of SF6 by BaTiO3 catalyst.
Hazard. Mater. 294 (2015) 41. [12] C.H. Tsai, J.M. Shao, Formation of fluorine for abating sulfur hexafluoride in an atmospheric-pressure plasma environment[J], J. Hazard. Mater. 157 (1) (2008) 201–206. [13] X. Zhang, H. Xiao, X. Hu, et al., Effects of background gas on sulfur hexafluoride removal by atmospheric dielectric barrier discharge plasma[J], AIP Adv. 6 (11) (2016) (495202-3855). [14] Q. Zhuang, B. Clements, A. Mcfarlan, et al., Decomposition of the most potent greenhouse gas (GHG) sulphur hexafluoride (SF6) using a dielectric barrier discharge (DBD) plasma[J], Can. J. Chem. Eng. 92 (1) (2014) 32–35. [15] Zhang Renxi, Wang Jingting, Cao Xu, et al., Decomposition of potent greenhouse gases SF6, CF4 and SF5CF3 by dielectric barrier discharge[J], Plasma Sci. Technol. 18 (4) (2016) 388–393. [16] H.L. Chen, H.M. Lee, L.C. Cheng, et al., Influence of nonthermal plasma reactor type on CF4 and SF6 abatements[J], IEEE Trans. Plasma Sci. 36 (2) (2008) 509–515. [17] R.J. Van Brunt, J.T. Herron, Fundamental processes of SF6, decomposition and oxidation in glow and corona discharges[J], IEEE Trans. Electr. Insul. 25 (1) (1990) 75–94. [18] M. Young Sun, K. Donghong, Decomposition of sulfur hexafluoride by using a nonthermal plasma-assisted catalytic process[J], J. Korean Phys. Soc. 59 (61) (2011) 3437. [19] X. Li, Y. Bai, B.C. Wang, et al., Water adsorption induced in-plane domain switching on BaTiO3 surface[J], J. Appl. Phys. 118 (9) (2015) 15. [20] M. Salazar-Villanueva, A.B. Hernandez, E.C. Anota, et al., Theoretical study on small clusters of BaTiO3 using DFT calculations[J], Mol. Simul. 39 (7) (2013) 5. [21] F. Maldonado, R. Rivera, L. Villamagua, et al., DFT modelling of ethanol on BaTiO3(001) surface[J], Appl. Surf. Sci. 456 (2018). [22] Z. Yuhong, D. Shijie, L. Hu, et al., First-principle investigation of pressure and temperature influence on structural, mechanical and thermodynamic properties of Ti3AC2 (A = Al and Si) [J], Comput. Mater. Sci. 154 (2018) 365–370. [23] Y. Zhao, L. Qi, Y. Jin, et al., The structural, elastic, electronic properties and Debye temperature of D022-Ni3V under pressure from first-principles[J], J. Alloys Compd. 647 (2015) 1104–1110. [24] Z. Jinbo, W. Lili, Z. Chuanxing, et al., Overview of polymer nanocomposites: computer simulation understanding of physical properties[J], Polymer 133 (2017) 272–287. [25] B. Delley, An all-electron numerical method for solving the local density functional for polyatomic molecules[J], J. Chem. Phys. 92 (1) (1990) 508–517. [26] J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple[J], Phys. Rev. Lett. 77 (18) (1996) 3865. [27] M. Ramos, C. Díaz, A.E. Martínez, et al., Dissociative and non-dissociative adsorption of O2 on Cu (111) and Cu/Ru (0001) surfaces: adiabaticity takes over[J], Phys. Chem. Chem. Phys. 19 (16) (2017) 10217–10221. [28] B. Delley, Hardness conserving semilocal pseudopotentials[J], Phys. Rev. B 66 (15) (2002) 155125. [29] S. Grimme, J. Antony, S. Ehrlich, et al., A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu[J], J. Chem. Phys. 132 (15) (2010) (154104-0). [30] M. Sokolov, R.I. Eglitis, S. Piskunov, et al., Ab initio hybrid DFT calculations of BaTiO3 bulk and BaO-terminated (001) surface F-centers[J], Int. J. Mod. Phys. B 31 (31) (2017) 1. [31] S. Piskunov, E.A. Kotomin, E. Heifets, The electronic and atomic structure of SrTiO3, BaTiO3, and PbTiO3(001) surfaces: ab initio DFT/HF hybrid calculations [J], Microelectron. Eng. 81 (2) (2005) 472–477. [32] R.A. Evarestov, A.V. Bandura, First-principles calculations on the four phases of BaTiO3[J], J. Comput. Chem. 33 (11) (2012) 1123–1130. [33] F. Yang, S. Lin, L. Yang, et al., First-principles investigation of metal-doped cubic BaTiO3[J], Mater. Res. Bull. 96 (2017) 372–378, https://doi.org/10.1016/j. materresbull.2017.03.023.
Acknowledgement This study is funded by National Natural Science Foundation of China (NSFC, funding number is 51777144) and China State Grid Corporation Science and Technology Project (SGHB0000KXJS 1800554). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.03.102. References [1] F. Maldonado, A. Stashans, DFT study of Ag and La codoped BaTiO3[J], J. Phys. Chem. Solids 102 (2017) 136–141. [2] A. Ogata, D. Ito, K. Mizuno, et al., Effect of coexisting components on aromatic decomposition in a packed-bed plasma reactor, Appl. Catal. A Gen. 236 (2002) 9–15. [3] W. Liang, J. Li, J. Li, et al., Abatement of toluene from gas streams via ferro-electric packed bed dielectric barrier discharge plasma, J. Hazard. Mater. 170 (2009) 633–638. [4] T. Yamamoto, K. Mizuao, I. Tamori, A. Ogata, M. Nifuku, M. Michalska, G. Prieto, Catalysis-assisted plasma technology for carbon tetrachloride destruction, IEEE Trans. Ind. Appl. 32 (1996) 100–105. [5] A. Ogata, N. Shintani, K. Mizuno, et al., Decomposition of benzene using a nonthermal plasma reactor packed with ferroelectric pellets, IEEE Trans. Ind. Appl. 35 (1999) 753–759. [6] A. Ogata, H. Einaga, H. Kabashima, S. Futamura, et al., Effective combination of nonthermal plasma and catalysts for decomposition of benzene in air, Appl. Catal. B Environ. 46 (2003) 87–95. [7] T.D. Butterworth, R. Elder, R.W.K. Allen, Effects of particle size on CO2 reduction and discharge characteristics in a packed bed plasma reactor[J], Chem. Eng. J. 293 (2016) 55–67. [8] M. Rabie, Franck C.M. An, Assessment of eco-friendly gases for electrical insulation to replace the most potent industrial greenhouse gas SF6[J], Environ. Sci. Technol. 52 (2) (2017) 369–380. [9] J. Reilly, R. Prinn, J. Harnisch, et al., Multi-gas assessment of the Kyoto Protocol[J], Nature 401 (6753) (1999) 549–555. [10] H. Xiao, X. Zhang, X. Hu, et al., Experimental and simulation analysis on by-products of treatment of SF6 using dielectric barrier discharge[J], IEEE Trans. Dielectr. Electr. Insul. 24 (3) (2017) 1617–1624. [11] J.H. Kim, C.H. Cho, D.H. Shin, et al., Abatement of fluorinated compounds using a 2.45 GHz microwave plasma torch with a reverse vortex plasma reactor[J], J.
416
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full length article
Theoretical study on the interaction between SF6 molecule and BaTiO3(0 0 1) surface: A DFT study
T
⁎
Zhaolun Cuia, Xiaoxing Zhanga,b, , Dachang Chena, Yuan Tiana a
School of Electrical Engineering, Wuhan University, Wuhan 430072, China State Key Laboratory of Power Transmission Equipment & System Security and New Technology, College of Electrical Engineering, Chongqing University, Chongqing 400030, China
b
A R T I C LE I N FO
A B S T R A C T
Keywords: SF6 BaTiO3(0 0 1) Adsorption DFT study
Based on the first principle, the adsorption and decomposition process of SF6 gas molecules on the surface of BaTiO3(0 0 1) was calculated. The results show that there is a strong interaction between SF6 molecule and BaTiO3 surface, and the adsorption energy reached 4.453–4.963 eV, which is likely to be a chemical adsorption process. According to Mulliken analysis, about 1.6 e of electrons transferred from BaTiO3 surface to the SF6 gas molecule, the SF6 molecule behaved as an electron acceptor. According to density of states calculation results, there were obvious electron orbital interactions between SF6 and BaTiO3 surface atoms, especially F and O atoms, and the charge transfer process was also confirmed by the differential charge density results. The SF6 molecules interacted strongly with the BaTiO3 surface, which led to the elongation of SeF bonds and made SF6 tend to undergo a decomposition process. The results indicate that BaTiO3 may have a catalytic potential for SF6 abatement and provide a simulation support for the experimental study of efficient and harmless treatment of SF6 gas.
1. Introduction Barium titanate (BaTiO3) as a perovskite type crystal, has excellent piezoelectricity, pyroelectricity, high dielectric permittivity and high voltage tunability [1]. As a catalyst or packing material, it is widely used in the field of volatile organic compounds (VOC) treatment often combined with nonthermal plasma (NTP) discharge, and has been proved to have good catalytic effects [2–6]. In Ref [2], Ogata et al. reported the experimental results of three VOC gases treatment in a packed bed dielectric barrier discharge (DBD) reactor with 5% BaTiO3 participation, indicating the catalyst effect of BaTiO3. In Ref [3], Liang et al. reported that the BaTiO3 packed bed reactor has the highest toluene removal efficiency when compared to a NaNO2 packed reactor. In the Ref [4–6], scholars have proposed different sizes of BaTiO3 pellets affect the degradation effects of benzene and CCl4 in DBD discharge. All of the above experiments have proved the effect of BaTiO3 on the discharge enhancement and VOC gases abatement. This is because the addition of BaTiO3 can change the dielectric constant of the reactor gap, promote the discharge process and increase the micro-discharge on the surface of the packing materials. On the other hand, gas molecules such as VOCs may be adsorbed on the surface of BaTiO3 to promote the combination of gas molecules and plasma, so that the abatement effect
⁎
of VOCs and other waste gases is significantly improved [7]. Sulfur hexafluoride (SF6), as an industrial gas, is widely used in power industry and semiconductor processing. Due to its excellent insulation performance and arc-extinguishing performance, it has been increasingly used in the power industry in China, India and other countries in recent years [8]. However, SF6 is one of the six major greenhouse gases proposed by the Kyoto Protocol in 1997, and its global warming potential (GWP) is 23,500 times that of the same volume fraction of CO2 [9]. Therefore, how to effectively deal with the exhaust SF6 gas is a problem and has attracted much attention. At present, NTP has been proved to be a reliable method for the treatment of SF6 and other industrial waste gases. High-energy particles generated by NTP can effectively decompose SF6 molecules in the discharge region [10]. Among the discharge types of NTP, microwave discharge, radio frequency discharge, dielectric barrier discharge (DBD) and corona discharge have applied to SF6 degradation process [11–16]. The device scales of microwave and radio frequency (RF) discharge are relatively large and the energy efficiency is relatively low [11,12]. Besides, the processing rate of DBD and corona discharge is relatively small [13–16]. Therefore, some scholars have promoted the decomposition process of SF6 by adding packing materials such as Al2O3 in the DBD discharge reactor, and showed a promotion effect [17,18].
Corresponding author at: School of Electrical Engineering, Wuhan University, Wuhan 430072, China. E-mail address: [email protected] (X. Zhang).
https://doi.org/10.1016/j.apsusc.2019.03.102 Received 26 November 2018; Received in revised form 6 March 2019; Accepted 11 March 2019 Available online 16 March 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 483 (2019) 409–416
Z. Cui, et al.
theoretical support for SF6 abatement process by BaTiO3 catalyzed discharge. 2. Computational details In this study, all the calculations were based on DFT using Dmol3 package in Material Studio 8.0 [25]. The generalized gradient approximation (GGA) was selected to describe the exchange–correlation interaction combined with the Perdew–Burke–Ernzerhof (PBE) functions [26]. The structures were built based on materials visualize model and the BaTiO3 (0 0 1) slab contains 58 atoms. The periodic region of BaTiO3 was (x = 8.020 Å, y = 8.020 Å, z = 21.015 Å) along XYZ direction with an optimized vacuum space of 15 Å. The Monkhorst-Pack k-point was set at 4 × 4 × 1 for geometric optimization and other properties [27]. The DFT Semi-core Pseudopots (DSSP) and the double numerical atomic orbital augmented by d-polarization (DNP) were applied [28]. TS for DFT + D applied to correct the van der Waals forces [29]. The energy convergence tolerance and maximum force were set at 1.0 × 10−5 Ha and 0.002 Ha/Å. The maximum displacement chose as 0.005 Å and the global orbital cutoff was 5.0 Å [30–32]. In BaTiO3, atoms in the bottom layer were fixed and upper three layers were free in the calculations. In addition, there were no constraints applied for the geometry optimization of the SF6 molecule. The adsorption energy (Ead) of SF6 molecules on BaTiO3 surface was calculated by the following equation:
Fig. 1. Molecular structure of SF6.
As BaTiO3 has been mentioned as a very common and effective DBD discharge packing material, many scholars have studied the adsorption process of different materials on the surface of BaTiO3 (0 0 1), which proves BaTiO3 (0 0 1) surface has certain adsorption and catalytic properties [1,19–21]. Ref [21, 22] have proved that BaO-terminated surface has a good stability which appears to be the most prone to occur in nature and has a stronger surface adsorption process with H2O or ethanol molecule. Hence, we chose the BaO terminated slab to study in this paper. Based on the density functional theory (DFT) calculation, the physical properties of solid materials and gas molecules can be calculated and the adsorption process of SF6 gas molecules on BaTiO3 (0 0 1) surface was analyzed [22–24]. It was found that SF6 gas molecules may undergo chemical adsorption on BaTiO3 surface, resulting in SeF bonds breaking. In addition, the interaction process between SF6 molecule and BaTiO3 surface was analyzed by means of electron transfer calculation, density of state (DOS) analysis and differential charge density analysis. The results show that BaTiO3 material may produce certain catalytic effect on SF6 molecule. The results may provide
Ead = EBaTiO3 + ESF6 − ESF6/BaTiO3
(1)
where ESF6/BaTiO3 is the total energy of SF6 gas molecule absorbed on the BaTiO3 (0 0 1) surface, EBaTiO3 and ESF6 is the energy of the BaTiO3 slab and SF6 gas molecule, respectively. Following this equation, a positive value of Ead indicates that adsorption process is exothermic. To further understand the interaction process, we else calculated the charge transfer Qt and the electron density difference between the SF6 gas molecule and BaTiO3 (0 0 1) surface by Mulliken analysis. The negative Mulliken charge (e) values mean the atom is positively charged and the positive Mulliken charge (e) values mean the atom is negatively charged. DOS analysis helped to obtain the electronic structure and the properties of the relaxed structure before and after
Fig. 2. Simulation structure of BaTiO3 (0 0 1): (a) main view (b) top view. 410
Applied Surface Science 483 (2019) 409–416
Z. Cui, et al.
Fig. 3. Initial adsorption configurations of SF6 on BaTiO3 (0 0 1) surface.
In this study, we selected the (2 × 2) BaTiO3 (0 0 1) surface to study the adsorption of SF6 molecules. In Ref [33], (2 × 2) BaTiO3 surface has proved to suffice for carrying out the adsorption of one molecule onto its surface and meet the calculation accuracy. There are three adsorption sites on the surface of BaTiO3 (0 0 1), which are above the Ba atom, above the labelled ‘2’ oxygen atom (O1) and above the labelled ‘3’ oxygen atom (O2), as shown in Fig. 2. Other sites like ‘bridge’ and ‘hollow’ sites are not discussed here as they are not representative and the geometric optimization results of these two sites are similar to the O2 site. The relative analysis provides in Supplementary material file. Fig. 3 shows the six configurations of SF6 molecules in BaTiO3. In M1 configurations, the straight line of the two SeF bonds of SF6 was perpendicular to the BaTiO3 (0 0 1) surface. In M2 configurations, two bottom F atoms were close to the adsorption sites at the same distance. The six adsorption configurations are defined as M1-Ba, M1-O1, M1-O2, M2-Ba, M2-O1, M2-O2, respectively. In both configurations, the
adsorption. Qt represents the total charge transfer. If the total charge transfer Qt < 0, it represents that electrons transfer from the BaTiO3 (0 0 1) surface to SF6 gas molecule and the gas molecule has positive charge after interactions. If the total charge transfer Qt > 0, it represents that electrons transfer from SF6 gas molecule to BaTiO3 (0 0 1) surface. 3. Result and discussion 3.1. Surface models Fig. 1 is the molecular structure of SF6. The SF6 gas molecule has a regular octahedral structure with an SeF bond length of 1.613 Å. The angle between the two adjacent F atoms and the S atom is 90°, and the Mulliken charges of the S atom and F are 1.865e and −0.311e, respectively. 411
Applied Surface Science 483 (2019) 409–416
Z. Cui, et al.
Table 1 Adsorption energy (Eads) and total charge transfer of SF6 adsorption on BaTiO3 (0 0 1). Adsorption site
Adsorption energy Ead (eV)
Total charge transfer of SF6 Qt (e)
M1-Ba
4.741
−1.578
M1-O1
4.455
−1.583
M1-O2
4.952
−1.567
M2-Ba
4.453
−1.587
M2-O1
4.951
−1.569
M2-O2
4.963
−1.567
Table 2 Valence electron occupation of atoms in M2-O2 adsorption structure. Atom species
Bond-length (Å) F1-S: F2-S: F3-S: F4-S: F1-S: F2-S: F3-S: F1-S: F2-S: F3-S: F1-S: F2-S: F3-S: F1-S: F2-S: F3-S: F1-S: F2-S: F3-S:
S F1 F2 F3 O1 O2 Ba
4.561 1.955 1.971 1.671 2.216 2.002 2.781 2.282 2.126 2.274 2.431 2.003 2.425 2.283 2.124 2.286 2.283 2.114 2.285
Before adsorption
After adsorption
s
p
d
s
p
d
2.557 1.980 1.980 1.980 1.935 1.951 1.176
4.020 2.660 2.660 2.660 2.537 2.459 3.080
0.491
2.919 1.983 1.983 1.985 1.945 1.943 1.164
4.094 2.853 2.853 2.827 2.488 2.473 3.043
0.253
O1 is the marked ‘2’ O atom in Fig. 2, O2 is the marked ‘3’ O atom in Fig. 2.
distance of the bottom F atom from the BaTiO3 plane was set to 2.000 Å. 3.2. SF6 molecule adsorbed on BaTiO3 (0 0 1) surface The adsorption energy and total charge transfer of the six adsorption configurations of SF6 gas on BaTiO3(0 0 1) surface are shown in Table 1, where Fx-S is expressed as the bond length of the labelled ‘Fx’ atom and S atom. Fig. 4 shows the geometric optimization results for the six adsorption configurations. In Table 1, under six configurations, SF6 molecule had relative large adsorption energies and charge transfers on the surface of BaTiO3. The adsorption energies in three M1configurations were 4.741, 4.455 and
Fig. 4. Adsorption results of BaTiO3 and SF6 gas molecules after geometric optimization. 412
Applied Surface Science 483 (2019) 409–416
Z. Cui, et al.
bond lengths of the three F atoms near the surface of BaTiO3 changed greatly. In the M1-O1 adsorption configuration, the bond lengths of the three bottom F atoms were stretched from 1.613 Å to 2.216, 2.002 and 2.781 Å. In the M1-O2 configuration, the three SeF bonds were elongated to 2.282, 2.126, and 2.274 Å, respectively. In the M2-Ba configuration, there were also three significant changes in the SeF bonds, and the bond length extended to 2.431, 2.003, and 2.425 Å. The SF6 molecule drifted from above the Ba atom to the direction of the O1 and O2 atom during the adsorption process. The above results show that under the six configurations, the SF6 molecule had a strong adsorption process on the surface of BaTiO3, which caused the elongation of some SeF bonds in SF6, and the SF6 molecule has a certain displacement. Besides, we also investigated the influence of atomic distances between SF6 and BaeO surface. We chose the M1-Ba configuration and the initial distances were set at 1.5 Å, 2.0 Å, 2.5 Å and 3.0 Å. The adsorption results shows in Fig. S1 (Supplementary material file). Compared with the calculated initial structure of M1-Ba (initial adsorption distance is 2.0 Å), the geometric optimization results of the three newly calculated structures were very similar to the previous one. The adsorption energies of the three new configurations obtained after calculation were 4.742, 4.750 and 4.749 eV, which were very close to the previously calculated 4.741 eV. In Fig. S1, SF6 shifted to the O2 site during the adsorption process under each adsorption structure, indicating that SF6 has the tendency towards the O2 site after adsorption. The influence of atomic distances in the initial adsorption is not obvious. Beside, we believe that the initial adsorption distance of 2.0 Å is representative.
Table 3 Charge transfer for characteristic atoms of SF6 and BaTiO3 (0 0 1) surface. Adsorption configuration
Characteristic atom
Charge transfer (e−)
M1-Ba
F1 F2 F3 F4 S Ba O1 O2 F1 F2 F3 S Ba O1 O2 F1 F2 F3 S Ba O1 O2
−0.454 −0.271 −0.275 −0.099 −0.389 0.057 0.012 0.072 −0.356 −0.298 −0.437 −0.422 0.080 0.002 0.062 −0.369 −0.330 −0.367 −0.398 0.067 0.011 0.073
M1-O1
M1-O2
3.3. Electronic structure of SF6 on BaTiO3 (0 0 1) surface According to the valence electron occupation of atoms in Table 2, after the adsorption, the valence electrons in different orbits of the SF6 and BaTiO3 atoms underwent certain changes. After adsorption of SF6 gas molecules, the valence electrons in the s orbital and p orbitals of S atoms increased from 2.557 and 4.020 to 2.919 and 4.094, respectively, and the valence electrons in d orbit decreased from 0.491 to 0.253. In addition to the electron transfer of the SF6 molecule and the BaTiO3 surface, the electrons of the S atom's d orbital may partially transferred to the s orbital and p orbitals. The valence electron changes of the F1 and F2 atoms were the same, the s orbital and p orbital increased from 1.980 and 2.660 to 1.983 and 2.853, respectively, and the s orbital and p orbital of the F3 atom increased from 1.980 and 2.660 to 1.985 and 2.827, respectively. The s orbital and p orbital of the O1 atom changed from 1.935 and 2.537 to 1.945 and 2.488, respectively. The s orbital and p orbital of the O2 atom changed from 1.951 and 2.459 to 1.943 and 2.473, respectively. The s orbital and p orbital may occur electronic transfer in each O atom. The valence electron of s and p orbital of the Ba atom were reduced from 1.176 and 3.080 to 1.164 and 3.043, respectively. Changes in BaTiO3 were relatively smaller than that in SF6. The main orbitals of Ba atoms lost electrons and may transferred to SF6 molecules. According to Mulliken analysis, we obtained the charge transfer results of characteristic atoms in three M1 configurations, as shown in Table 3. In the M1-Ba configuration, the four F atoms obtained 0.454e, 0.271e, 0.275e and 0.099e electrons, respectively, while the S atom obtained 0.389e. Ba lost 0.057e of electrons, O1 and O2 atom lost 0.012e and 0.072e, respectively. At this time, SF6 appeared as an electron acceptor, and BaTiO3 appeared as an electron donor. In the M1-O1 system, three F atoms and S atoms obtained electrons of 0.356e, 0.298e, 0.437e and 0.422e, respectively, while Ba and two O atoms lost 0.080e, 0.002e and 0.062e, respectively. In the M1-O2 system, three F atoms and S atoms respectively obtained electrons of 0.369e, 0.330e, 0.367e and 0.398e, while Ba and two O atoms lost 0.067e, 0.011e and 0.073e. The charge transfer under the M2 adsorption systems was very similar to that in M1. The SF6 molecule was also an electron acceptor and the BaTiO3 surface was an electron donor. According to the results
Fig. 5. DOS result of BaTiO3 system before and after adsorption of SF6 molecules in M1-Ba configuration.
4.952 eV, respectively. At this time, the adsorption effect was most obvious under the configuration of M1-O2. The adsorption energies in three M2 configurations were 4.453, 4.951 and 4.963 eV, respectively. The adsorption effect was most obvious under the configuration of M2O2. The O2 adsorption site may have the greatest adsorption capacity and catalytic ability. According to Mulliken analysis, we obtained total charge transfer results under six adsorption systems. In Table 1, the charge transfer results in the six adsorption configurations were very similar, and the charges were all transferred from BaTiO3 (0 0 1) surface to the SF6 molecule. In the M1 configuration, the SF6 molecule obtained 1.578, 1.583 and 1.567e electrons in the three adsorption structures, respectively. In the M2 configuration, the SF6 molecules obtained 1.587, 1.569, and 1.567e electrons in the three adsorption structures, respectively. It can be seen from Fig. 4 that SF6 structure had obvious changes after adsorption in the six configurations. We calculated the bond length results of the SeF bond under each adsorption structure, as shown in Table 1. The unshown SeF bonds did not change obviously after adsorption. According to Table 1, in M1-Ba configuration, the SeF bonds of four F atoms were significantly elongated, and the bond lengths of the F1 to F4 atoms were stretched from 1.613 Å to 4.561, 1.955, 1.971 and 1.671 Å respectively, the SF6 molecule also drifted a bit, moving from above the Ba atom to above the O2 atom. The SeF bonds changes of the other five configurations were similar, and the 413
Applied Surface Science 483 (2019) 409–416
Z. Cui, et al.
Fig. 6. PDOS results of BaTiO3 system before and after SF6 adsorption in six adsorption structures.
the characteristic atoms in the six adsorption configurations, as shown in Fig. 6. Fig. 6(a) is the M1-Ba PDOS result, the 3p orbital of the S atom in the SF6 molecule overlapped with the 2p orbital of the O2 atom is near −2 eV. In addition, the 2p orbital of the F1 atom overlapped with the 2p orbital of O2 at −1 eV, and the 2p orbital of the F2 and F3 atoms overlapped with the 2p orbital of O2 at −2 eV. The Ba 6s orbital slightly overlapped with F2 and F3 2p orbital. The above results indicate that there is a certain orbital interaction process between SF6 molecule and the Ba and O atoms of BaTiO3 surface. A similar result occurred in the M1-O1 configuration of Fig. 6(b). The 2p orbital of the O1 atom overlapped with the 2p orbital of the three F atoms at −2 eV, and the 6s orbital of the Ba atom had a cross with the 2p of the F1 and F2 atoms near −3 .5eV. In Fig. 6(c), the 2p orbital of O2 overlapped with the 3p orbital of the S atom and the 2p
of Mulliken analysis, in the adsorption process, there was a strong charge transfer process on the BaTiO3 surface, and the electrons on the surface of BaTiO3 transferred to SF6 gas molecules. This may indicate that the SF6 molecule underwent a strong chemical adsorption process on the surface of BaTiO3. Fig. 5 is the total density of states (TDOS) of SF6 molecules before and after adsorption on the surface of BaTiO3 in the M1-Ba configuration. After the adsorption of SF6 molecules, the DOS of the BaTiO3 system changed in some energy ranges, and there were significant increases in the vicinity of −13 eV, −9 eV, −6 eV and −4 eV and −1 eV, which was due to the adsorption of SF6 molecule. Considering that there may be strong interaction and electron exchange between the SF6 molecule and the BaTiO3 surface during the adsorption process, we calculated the partial density of states (PDOS) of 414
Applied Surface Science 483 (2019) 409–416
Z. Cui, et al.
configurations. It shows a strong electrical interaction between the SF6 molecule and the BaTiO3 surface. Charge density increased near the F atoms, and decreased near the SeF bonds, which indicates that there, may have a charge transfer process inside the SF6 molecule during the adsorption process. Surface O atoms did not show obvious changes in charge density as the charge transfer of these O atoms are not large in Table 3. At the same time, the BaTiO3 surface layer exhibited a weak increase of electron densities, while the layer below the surface exhibited a decrease of electron density, which may indicate that part of the electrons in the BaTiO3 structure were transferred from the interior of the structure to the surface and then transferred to the SF6 molecule. Although some electrons inside SF6 molecule transferred from S to F, the SF6 totally acts as an electron acceptor as many electrons transferred to it from BaeO surface. In summary, in the above six adsorption configurations, SF6 molecules interacted strongly with BaTiO3, and it has speculated that a chemical adsorption process may occur. During the process, there were obvious charge transfer processes on BaTiO3 surface and SF6. At the same time, there were certain orbital overlaps between them and the F atoms have strong interaction with the O atoms with charge transfer. After the adsorption, the structure of SF6 molecules changed obviously, the SeF bonds were elongated, and the SF6 molecule drifted towards to the O2 atom on the surface of BaTiO3, which made the SF6 molecule more likely to be decomposed. We believe that the O2 site is the most stable adsorption site and the interaction main occurs between the surface O atoms and F atoms. The results showed the catalytic potential of BaTiO3 surface for SF6 molecules.
Fig. 7. The supplemented PDOS results of BaTiO3 with SF6 adsorption in M1-Ba structure.
orbital of the F atom at −1 eV, overlapped with the 3p orbital of the S atom near −3 eV. Under the three M2 configurations, there were similar PDOS calculation results. The above results indicate that there may be some interactions between SF6 molecules and BaTiO3 surface atoms in the electron orbitals during the adsorption of SF6 and BaTiO3. Through PDOS analysis, we found that in the surface adsorption system of SF6 and BaTiO3, the interaction between the F atom and the O atom in the 2p orbital was the main electron orbital interaction, and the interaction between the F and Ba as well as S and O were not obvious. Therefore, we supplemented the PDOS distribution of the 2p orbitals of the other O atoms below the SF6 molecule in the PDOS of the M1-Ba system, as shown in Fig. 7. In Fig. 7, in addition to the O2 atom, the 2p orbital of O1 overlapped with the 2p orbital of the F1 atom near −1 eV, and the 2p orbital of several nearby O atoms overlapped with the 2p orbital of the F2 and F3 atoms near −4 eV, with the F1 atom at −1 eV. This indicates that there exhibits a strong interaction between F and O atoms in 2p orbital. The SF6 molecule also moved from above Ba to above the O2 atom and the interaction between F and O may lead to the chemical adsorption process. Fig. 8 shows the differential charge calculation results for two M1
4. Conclusion Based on the first-principle calculation, we theoretically analyzed the interaction mechanism of the insulating gas SF6 on the BaTiO3 (0 0 1) surface with six initial configurations. The results show that the SF6 gas molecules on the surface of BaTiO3 had strong adsorption processes and it has presumed to be a chemical adsorption process. In addition, according to DOS calculation results, S atoms and F atoms in SF6 molecules had obvious electron orbital interactions with Ba atoms and O atoms on BaTiO3 surface. According to Mulliken analysis, about 1.6 e electrons transferred from the surface of BaTiO3 to SF6 molecules. SF6 Fig. 8. Differential charge density distribution of SF6 molecules and BaTiO3 surface after adsorption in three configurations (the purple part indicates an increase in the density of the electron cloud and the yellow part indicates a decrease in the density of the electron cloud). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
415
Applied Surface Science 483 (2019) 409–416
Z. Cui, et al.
molecule behaved as electron acceptors during charge transfer, and similar phenomena existed in the results of differential charge density analysis. The atomic distance did not show an obvious influence on adsorption process. The molecular structure of SF6 changed significantly during the adsorption process, and the SeF bonds were elongated, making the SF6 molecule more likely to be decomposed. The results provide a theoretical support for the catalytic decomposition research of SF6 by BaTiO3 catalyst.
Hazard. Mater. 294 (2015) 41. [12] C.H. Tsai, J.M. Shao, Formation of fluorine for abating sulfur hexafluoride in an atmospheric-pressure plasma environment[J], J. Hazard. Mater. 157 (1) (2008) 201–206. [13] X. Zhang, H. Xiao, X. Hu, et al., Effects of background gas on sulfur hexafluoride removal by atmospheric dielectric barrier discharge plasma[J], AIP Adv. 6 (11) (2016) (495202-3855). [14] Q. Zhuang, B. Clements, A. Mcfarlan, et al., Decomposition of the most potent greenhouse gas (GHG) sulphur hexafluoride (SF6) using a dielectric barrier discharge (DBD) plasma[J], Can. J. Chem. Eng. 92 (1) (2014) 32–35. [15] Zhang Renxi, Wang Jingting, Cao Xu, et al., Decomposition of potent greenhouse gases SF6, CF4 and SF5CF3 by dielectric barrier discharge[J], Plasma Sci. Technol. 18 (4) (2016) 388–393. [16] H.L. Chen, H.M. Lee, L.C. Cheng, et al., Influence of nonthermal plasma reactor type on CF4 and SF6 abatements[J], IEEE Trans. Plasma Sci. 36 (2) (2008) 509–515. [17] R.J. Van Brunt, J.T. Herron, Fundamental processes of SF6, decomposition and oxidation in glow and corona discharges[J], IEEE Trans. Electr. Insul. 25 (1) (1990) 75–94. [18] M. Young Sun, K. Donghong, Decomposition of sulfur hexafluoride by using a nonthermal plasma-assisted catalytic process[J], J. Korean Phys. Soc. 59 (61) (2011) 3437. [19] X. Li, Y. Bai, B.C. Wang, et al., Water adsorption induced in-plane domain switching on BaTiO3 surface[J], J. Appl. Phys. 118 (9) (2015) 15. [20] M. Salazar-Villanueva, A.B. Hernandez, E.C. Anota, et al., Theoretical study on small clusters of BaTiO3 using DFT calculations[J], Mol. Simul. 39 (7) (2013) 5. [21] F. Maldonado, R. Rivera, L. Villamagua, et al., DFT modelling of ethanol on BaTiO3(001) surface[J], Appl. Surf. Sci. 456 (2018). [22] Z. Yuhong, D. Shijie, L. Hu, et al., First-principle investigation of pressure and temperature influence on structural, mechanical and thermodynamic properties of Ti3AC2 (A = Al and Si) [J], Comput. Mater. Sci. 154 (2018) 365–370. [23] Y. Zhao, L. Qi, Y. Jin, et al., The structural, elastic, electronic properties and Debye temperature of D022-Ni3V under pressure from first-principles[J], J. Alloys Compd. 647 (2015) 1104–1110. [24] Z. Jinbo, W. Lili, Z. Chuanxing, et al., Overview of polymer nanocomposites: computer simulation understanding of physical properties[J], Polymer 133 (2017) 272–287. [25] B. Delley, An all-electron numerical method for solving the local density functional for polyatomic molecules[J], J. Chem. Phys. 92 (1) (1990) 508–517. [26] J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple[J], Phys. Rev. Lett. 77 (18) (1996) 3865. [27] M. Ramos, C. Díaz, A.E. Martínez, et al., Dissociative and non-dissociative adsorption of O2 on Cu (111) and Cu/Ru (0001) surfaces: adiabaticity takes over[J], Phys. Chem. Chem. Phys. 19 (16) (2017) 10217–10221. [28] B. Delley, Hardness conserving semilocal pseudopotentials[J], Phys. Rev. B 66 (15) (2002) 155125. [29] S. Grimme, J. Antony, S. Ehrlich, et al., A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu[J], J. Chem. Phys. 132 (15) (2010) (154104-0). [30] M. Sokolov, R.I. Eglitis, S. Piskunov, et al., Ab initio hybrid DFT calculations of BaTiO3 bulk and BaO-terminated (001) surface F-centers[J], Int. J. Mod. Phys. B 31 (31) (2017) 1. [31] S. Piskunov, E.A. Kotomin, E. Heifets, The electronic and atomic structure of SrTiO3, BaTiO3, and PbTiO3(001) surfaces: ab initio DFT/HF hybrid calculations [J], Microelectron. Eng. 81 (2) (2005) 472–477. [32] R.A. Evarestov, A.V. Bandura, First-principles calculations on the four phases of BaTiO3[J], J. Comput. Chem. 33 (11) (2012) 1123–1130. [33] F. Yang, S. Lin, L. Yang, et al., First-principles investigation of metal-doped cubic BaTiO3[J], Mater. Res. Bull. 96 (2017) 372–378, https://doi.org/10.1016/j. materresbull.2017.03.023.
Acknowledgement This study is funded by National Natural Science Foundation of China (NSFC, funding number is 51777144) and China State Grid Corporation Science and Technology Project (SGHB0000KXJS 1800554). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.03.102. References [1] F. Maldonado, A. Stashans, DFT study of Ag and La codoped BaTiO3[J], J. Phys. Chem. Solids 102 (2017) 136–141. [2] A. Ogata, D. Ito, K. Mizuno, et al., Effect of coexisting components on aromatic decomposition in a packed-bed plasma reactor, Appl. Catal. A Gen. 236 (2002) 9–15. [3] W. Liang, J. Li, J. Li, et al., Abatement of toluene from gas streams via ferro-electric packed bed dielectric barrier discharge plasma, J. Hazard. Mater. 170 (2009) 633–638. [4] T. Yamamoto, K. Mizuao, I. Tamori, A. Ogata, M. Nifuku, M. Michalska, G. Prieto, Catalysis-assisted plasma technology for carbon tetrachloride destruction, IEEE Trans. Ind. Appl. 32 (1996) 100–105. [5] A. Ogata, N. Shintani, K. Mizuno, et al., Decomposition of benzene using a nonthermal plasma reactor packed with ferroelectric pellets, IEEE Trans. Ind. Appl. 35 (1999) 753–759. [6] A. Ogata, H. Einaga, H. Kabashima, S. Futamura, et al., Effective combination of nonthermal plasma and catalysts for decomposition of benzene in air, Appl. Catal. B Environ. 46 (2003) 87–95. [7] T.D. Butterworth, R. Elder, R.W.K. Allen, Effects of particle size on CO2 reduction and discharge characteristics in a packed bed plasma reactor[J], Chem. Eng. J. 293 (2016) 55–67. [8] M. Rabie, Franck C.M. An, Assessment of eco-friendly gases for electrical insulation to replace the most potent industrial greenhouse gas SF6[J], Environ. Sci. Technol. 52 (2) (2017) 369–380. [9] J. Reilly, R. Prinn, J. Harnisch, et al., Multi-gas assessment of the Kyoto Protocol[J], Nature 401 (6753) (1999) 549–555. [10] H. Xiao, X. Zhang, X. Hu, et al., Experimental and simulation analysis on by-products of treatment of SF6 using dielectric barrier discharge[J], IEEE Trans. Dielectr. Electr. Insul. 24 (3) (2017) 1617–1624. [11] J.H. Kim, C.H. Cho, D.H. Shin, et al., Abatement of fluorinated compounds using a 2.45 GHz microwave plasma torch with a reverse vortex plasma reactor[J], J.
416