First-Principles Insight Into Au-Doped MoS2 for Sensing C2H6 and C2H4

C2H6 and C2H4 gases are two typical decompositions produced by partial discharge of transformer oil. To fully evaluate the feasibility of MoS2-based materials for the detection of C2H6 and C2H4 gases, the adsorption of C2H6 and C2H4 molecules on intrinsic and Au-doped MoS2 monolayer have been studied in this paper by the First-principle of Density Functional Theory (DFT). The adsorption mechanism of MoS2-based monolayer were investigated carefully in terms of adsorption energy, adsorption distance, bandgap structure, charge transfer and density of states (DOS). The calculated results show that the adsorption structures of the C2H6 and C2H4 molecules on Au-doped MoS2 monolayer with larger adsorption energies were stable, and have shorter adsorption distance, higher charge transfer, and stronger orbital hybridization compared with the corresponding MoS2 monolayer adsorption structures. It is concluded that the doped-Au atom affects the electronic structure of MoS2 monolayer to enhance the adsorption capacity. From this aspect, the present research offers a theoretical guidance to the application of Au-doped MoS2 materials as the sensing material for C2H6 and C2H4 gases.


INTRODUCTION
The transformers are considered to be the most important equipment in the power system, which affects the running state of power transmission (Chen W. G. et al., 2013;Uddin et al., 2016). After a long period of operation, the security and reliable operation ability of power transformers will be weakened due to inevitable defects, which will bring about power accidents and cause great inconvenience to the citizens and the society (Ma G. M. et al., 2015;Nobrega et al., 2019). Currently, failures within an oil-immersed transformer will cause the oil to decompose into several characteristic gases dissolved in it, including H 2 , CO, CO 2 , C 2 H 2 , CH 4 , C 2 H 6 , and C 2 H 4 . Some minor faults in the transformers may lead to the spark discharge in the transformer oil, then cause the oil decompose and produces C 2 H 6 , C 2 H 4 gases (Benounis et al., 2008;Yang et al., 2011). Therefore, dissolved gas analysis (DGA) to monitor dissolved gas in transformer oil is regarded to be the most direct method to guarantee the stable running of power transformer or to predict some fault trends (Chatterjee et al., 2013;Suryavanshi et al., 2017;Cun et al., 2019;de Lima et al., 2019).
In recent decades, metal inorganic compound semiconductor sensors have been widely used in the field of gas detection (Zhang et al., 2017c;Zhou et al., 2018cZhou et al., , 2019Chao et al., 2019;Kim et al., 2019Kim et al., , 2020Wang et al., 2019cWang et al., , 2020. Among them, nanowires or fiber materials sensors have attracted extensive attention due to their high sensitivity, fast response and recovery, low power consumption, and other excellent properties. The high sensitivity of nanowires or fiber materials sensors can be attributed to the large number of adsorption sites provided by high specific surface area Qin and Ye, 2016;Wang et al., 2018). Molybdenum disulfide (MoS 2 ) belongs to hexagonal crystal system and is a typical transition metal sulfide with a lamellar structure similar to graphene, which has the characteristics of surface effect, small size effect and quantum effect, and large specific surface area (Zhang et al., 2017b;Chen et al., 2019a). In addition, some studies have proved that nanometer MoS 2 , with excellent performances in adsorption capacity and other aspects, which is a special source of gas storage materials or sensing materials (Wen et al., 2016;Zhang et al., 2017a;Zhang D. Z. et al., 2018;Zhang Y. J. et al., 2018;Zhou et al., 2018b).
At present, the sensitivity or selectivity of most sensors are limited to some extent. In order to promote the sensing performance of gas sensors, dopant is usually used to enhance electron mobility and chemical reactivity of sensing materials. For instance, Zhou at al. verified that Pt nanoparticles doped SnO 2 nanoneedles can be an effective gas sensing material for the detection of CO, which exhibits excellent response and recovery characteristic and remarkable selectivity (Zhou et al., 2018d). Zhang et al. studied the behavior of a MoS 2 /Co 3 O 4 -based ammonia gas sensor. They concluded that the MoS 2 /Co 3 O 4 gas sensor exhibited excellent sensitivity performance to lowconcentration ammonia, which is better than the pure Co 3 O 4 gas sensor (Zhang et al., 2017a). Zhou at al. synthesized pure and Ni doped SnO 2 nanomaterials and found the Ni doped SnO 2 gas sensor shows lower optimum operating temperature and superior sensitivity properties to CO compared with the pure one (Zhou et al., 2018a).
Density functional theory (DFT) is a method to calculate the atomic or electronic structure and characteristic information of a material system by first-principles (Zhao and Wu, 2018;Wang et al., 2019c). Moreover, it has been widely employed to effectively predict the sensing properties of materials or interpret sensing mechanisms. Wang et al. analyzed the influence of Ni atom on the adsorption properties of ZnO (100) surface by using DFT calculations and found the doped Ni atom, as the active site, significantly enhance the adsorbability of SO 2 , SOF 2 , SO 2 F 2 gases (Wang et al., 2019a). Zahra et al. investigated the adsorption of H 2 S on transition element (such as Ni, Cu, and Zn) atomdoped graphene based on DFT and concluded that the adsorption configuration of the H 2 S molecule near the dopant on the doped graphene surface is most stable (Zahra, 2018). Esrafili et al. used DFT to analyze the sensing mechanism of C-doped h-BN nanosheets, and the results indicated that the sensing of NO and NO 2 can be selectively conducted in the presence of CO, H 2 O, CO 2 , and NH 3 gases (Esrafili and Rad, 2019). Many studies have reported the performance and mechanism of MoS 2 -based gas sensors. However, MoS 2 based fiber-materials as the gas sensor to detect transformer oil dissolved-gases has hardly been studied theoretically and experimentally.
In this study, Au was employed as a dopant to change the structural characteristics and electronic properties of MoS 2 fiber-materials, and the adsorption performance of Au-doped MoS 2 to C 2 H 6 and C 2 H 4 gases was investigated through DFT study. The stable adsorption structures of C 2 H 6 and C 2 H 4 molecules on intrinsic or Au-doped MoS 2 monolayer were built. Furthermore, the adsorption energies, charge transfer, adsorption distance, density of states, and bandgap have been calculated to comprehensively investigate the adsorption abilities of Au-doped MoS 2 monolayer to C 2 H 6 and C 2 H 4 gases. The results of this research have important guiding significance for the design of high efficiency MoS 2 fiber based sensors to detect fault characteristic gas in transformer oil (C 2 H 6 and C 2 H 4 gases).

CALCULATION DETAILS
All the First-principles calculations in this study were processed in Dmol 3 package of Materials Studio software based on DFT. General Gradient Approximate (GGA) of Perdew-Burke-Ernzerhof (PBE) were selected to optimize the geometric and calculate the exchange-correlation energy (Qian et al., 2019). The double numerical plus polarization (DNP) and the density functional semi-core pseudopotential (DSPP) were taken to processed the core electron. In addition, the k-point grid was generated according to the Monkhorst-Pack scheme and the sampling k point of Brillouin zone was set to 5 × 5 × 1. The self-consistent field (SCF) convergence precision was set to 1 × 10 −6 Ha and the DIIS was set to 6 to shorten the convergence time of SCF. The convergence tolerance, maximum force and maximum displacement were set to 1 × 10 −5 Ha, 2 × 10 −3 Ha/Å, and 5 × 10 −3 Ha in the geometry optimization processes. All simulations results in present study were based on a 4 × 4 × 1 MoS 2 supercell with 48 atoms (including 32 S atoms and 16 Mo  atoms). With regard to the periodic boundary conditions (PBC), a vacuum layer in z-direction of 20 Å, which is perpendicular to the xy-plane, was employed to prevent the interaction between the adjacent layers. The adsorption energy (E ads ) of gases (C 2 H 6 , C 2 H 4 , and C 2 H 2 ) on the pristine or Au-doped MoS 2 monolayer is defined in Equation (1) (Chen et al., 2019b).
where E total , E monolayer , and E gas are the energy of pristine or Au-doped MoS 2 monolayer adsorbed system, corresponding monolayer and gas molecule, respectively. A negative adsorption energy represents spontaneous adsorption. The amount of charge transfer (Q t ) can be obtained based on the Mulliken analysis, and Q t is calculated by Equation (2) (Chen et al., 2015).
where Q a and Q b are the carried charge of the adsorbed gas and the isolated gas. A positive Q t suggests the gas accept charges from monolayer during the adsorption.

Properties of Gas Molecules and MoS 2 Monolayer
The optimized structures with corresponding parameters of C 2 H 6 and C 2 H 4 are shown in Figure 1. The bond length of C-C, C-H in C 2 H 6 are 1.527 and 1.100 Å, and the bond angle of H-C-H is 107.4 • . For C 2 H 4 molecule, the structure is symmetric around the C-C bond (the length is 1.335 Å), the H-C-H angle is 116.6 • and the length of C-H bond is similar to C 2 H 6 , which is 1.092 Å. Mulliken atomic charge analysis ( Table 1) exhibits that the C atom of C 2 H 6 molecule has negative charges of −0.868e, while the carried charges of H atoms is 0.047e. The carried charges of C and H atoms in C 2 H 4 are −0.101e and 0.050e. It can be concluded that the electrons of C atoms are transferred from H atoms, which indicates the interaction between C atoms and H atoms. The optimized structure of MoS 2 monolayer model with the lattice parameters of 12.424, 12.584, and 18.152 Å in the a, b, and c directions, as shown in Figure 2A. Besides, the optimized bond angles of S-Mo-S and Mo-S-Mo are achieved as 81.5 • , which are comparable to 81.5 • in previous study (Wang et al., 2019b). The electronic structural characteristics of MoS 2 monolayer were investigated, the relevant band gap structure and DOSs are It is easily to distinguish the valence band maximum (VBM) and conduction band minimum (CBM), and the Fermi level (represented by the line of zero energy) is located in the energy bandgap. As a result, the considered MoS 2 monolayer with band gap of 2.058 eV is observed to be a direct band semiconductor. The total density of state in Figure 2C is composed of the conduction band, upper valence band, and lower valence band with energy values of 1.9∼2.5 eV, −5.8∼0 eV, and −14∼−11.8 eV, respectively. According to the PDOS in Figure 2D, the conduction band is mainly dominated by 4dorbital of Mo atom, while the 4p, 4d-orbitals of Mo atom and 3s, 3p-orbitals of S atom contribute to the valence band.

Adsorption Properties of Molecules on MoS 2 Monolayer
The C 2 H 6 , C 2 H 4 , and C 2 H 2 molecules were placed above the MoS 2 monolayer, the optimized adsorption structures and DOSs are shown in Figure 3. Table 2 reports the corresponding adsorption energy (E ads ), charge transfer (Q t ), adsorption distance (d), and band gap (E g ). As exhibited in Figures 3C,D, the density of state of two adsorption systems change only slightly. In addition, the E ads of C 2 H 6 , C 2 H 4 , and C 2 H 2 adsorbed MoS 2 monolayer are only −0.082, −0.106, and −0.101 eV, the corresponding Q t and the change of band gap are very small, which clearly suggests that the adsorptions belong to physical adsorption with Van der Waals force. In conclusion, the C 2 H 6 , C 2 H 4 , and C 2 H 2 gases are difficult to be stably adsorbed on pristine MoS 2 monolayer at room temperature, because the material is theoretically not active enough to adsorb them.

Properties of Au-Doped MoS 2 Monolayer
According to the previous research, it is the stable doped position that putting Au atom on the top of the Mo atom. As shown in Figure 4A, three Au-S bonds with the same length of 2.809 Å have been formed and the distance (3.749 Å) between Au and Mo atoms is too far to form bond due to the weak interaction. Compared with the undoped structure, the bond angle of Mo-S-Mo (80.8 • ) and S-Mo-S (81.3 • ) in Au doped MoS 2 monolayer have slightly decreases. The carried electrons of the Au atom to MoS 2 monolayer was calculated to 0.043e, suggesting the interaction between Au atom and the surface of MoS 2 monolayer leads to a steady doping structure. For the PDOS (Figure 4D) of doped system, it can be found there is large overlap area from −3.7 to 0 eV of S-3p orbital and Au-5d orbital, verifying that strong hybridization occurs between S-3p and Au-5d orbitals. It can be inferred that chemical interaction exists between the doped Au atom and MoS 2 to promote the formation of the final stable doping structure. Figure 4C shows the waveform of DOS has barely changed except for the obvious left shift after the dope of Au atom, which not only indicates an increase in the amount of system electrons, but means a promotion in the metallicity. The Au-doping introduces some impurity energy levels from the Au-6s and 5d orbitals into the band gap, leading to a narrowing of the bandgap (0.266 eV), which is corresponding with Figure 4B. The smaller band gap implies the lower kinetic stability and the higher chemical activity. Therefore, Au-doping makes the transfer from valence band to conduction band of electrons become easier, thus the conductivity and reactivity are improved.

Adsorption Properties of Molecules on Au-Doped MoS 2 Monolayer
To investigate the adsorption properties of Au-doped MoS 2 monolayer to C 2 H 6 molecule, a C 2 H 6 was placed at different positions to close the Au atom, and geometric optimization was conducted. The most stable structure is shown in Figure 5A, the corresponding adsorption energy, charge transfer, adsorption distance and band gap of this adsorption configurations are listed in Table 3  The C 2 H 6 molecule was trapped at the Au doping site and the distance between Au atom and C atom is 2.757 Å, which is significantly smaller than that in MoS 2 monolayer adsorption system (3.274 Å). Besides, it can be found that the structure of C 2 H 6 molecule changes little after being adsorbed by Audoped MoS 2 monolayer, while the change of the Au-S bonds is clearly observed. According to Table 3, the adsorption energy is −0.463 eV, which is greater than the intrinsic adsorption structure. Moreover, the value of transferred electrons that calculated by Mulliken analysis is 0.118e, indicating the Au-MoS 2 monolayer acts as an acceptor for the electrons transferred from the C 2 H 6 molecule. It is observed that the band gap of Au-doped MoS 2 monolayer system reduce from 0.266 to 0.206 eV. Therefore, the conclusion that the C 2 H 6 molecule was adsorbed on the Au-MoS 2 monolayer by chemisorption is obtained.
To further explore the adsorption mechanism between the Au-doped MoS 2 monolayer and C 2 H 6 molecule, the total density of states (TDOS) and the projected density of states (PDOS) of the adsorption system are analyzed in detail. Adsorption is mainly affected by the outer atomic orbitals, hence the Mo-5s, Mo-4d, S-3s, S-3p, Au-5d, C-2s, and H-1s orbitals were investigated, as shown in Figure 5D. Distinctly, some changes of TDOS after C 2 H 6 adsorption can be observed in Figure 5B, such as the increases around the Fermi level, which may be consistent with the reduction of the bandgap. It can be deduced that this phenomenon is due to the introduction of impurity energy level caused by a large amount of electrons transfer in the adsorption. In addition, a narrower bandgap suggests the adsorption structure conducts electricity better and the electrons are more easily to transfer. The TDOS has a left shift and some new peaks at −10 and −2.4 eV appear. By the comparison with Figure 5C, it can be found that the changes of TDOS are influenced by the C 2 H 6 molecule and the slight peak near the Fermi level in the DOS of C 2 H 6 contributes to the decrease of the bandgap, particularly. Moreover, it can be implied that some orbitals hybridization occur to the C 2 H 6 adsorption Au-doped MoS 2 monolayer system. Figure 5D concludes that the C-2p, Mo-4d and Au-5d orbitals dominate the changes of TDOS. The H-1s and Au-5d overlap around −3 eV, while the obvious overlapping region of C-2p, Mo-4d, and Au-5d orbitals at −5.5 and −2.5 eV. The PDOS overlap of the orbitals indicates hybridization happen between them. Therefore, it can be concluded by the contribution of the C-2p orbitals that during the adsorption process, the Au-doped MoS 2 monolayer mainly adsorbs C 2 H 6 molecule by capturing C atom.
Above all, the Au-doped MoS 2 monolayer has a large adsorption energy, more transferred electrons, short adsorption distance and more obvious orbitals hybridization than the corresponding intrinsic adsorption system. In conclusion, the adsorption performance of MoS 2 monolayer has a significant improvement after Au doped and the doped structure can effectively absord the C 2 H 6 molecule. For C 2 H 4 molecule adsorption on Au-doped MoS 2 monolayer, the most stable adsorption structure is presented in Figure 6A, where the molecule was adsorbed by the direction of parallel to MoS 2 monolayer. The adsorption properties parameters are classified at Table 3.
The adsorption of C 2 H 4 molecule is characterized by a large negative adsorption energy (−0.952 eV), suggesting the chemical adsorption happen to the C 2 H 4 molecule and Audoped MoS 2 monolayer. By comparing with the adsorption on pristine MoS 2 monolayer, the higher adsorption energy implies that the interaction is stronger. Moreover, it can be noticed that the adsorption distance between doped Au atom and C atom in C 2 H 4 molecule is 2.170 Å, illustrating the formation of Au-C covalent bond. Additionally, the amount of transferred charges in this adsorption system is 0.309e. The reorganization of charges indicates that the Au-doped MoS 2 monolayer acquires charges from C 2 H 4 molecule. It is observed the charge transfer is much larger than the corresponding pristine MoS 2 monolayer adsorption structure, which indicates the dope of Au atom make the adsorption reaction between monolayer and C 2 H 4 stronger. The band gap is decreased to 0.122 eV compared with that of Audoped MoS 2 monolayer (0.266 eV), and the change represents the C 2 H 4 molecule has the influence on the electronic structure of monolayer. In a conclusion, the C 2 H 4 molecule could be detected effectively by the Au-doped MoS 2 monolayer.
The DOS and PDOS of C 2 H 4 molecule adsorbed Au-doped MoS 2 monolayer were conducted to understand the effect of the C 2 H 4 on the electronic properties of Au-doped MoS 2 monolayer. Adsorption is mainly affected by the outer atomic orbitals, hence the Mo-5s, Mo-4d, S-3s, S-3p, Au-5d, C-2s, and H-1s orbitals were investigated, as shown in Figure 6D. From Figure 6A, it is observed that the DOS of adsorption system has a certain displacement compared with the Au-doped MoS 2 monolayer, in which there is the apparent transition near the Fermi level. Furthermore, the bandgap decreased after the adsorption of C 2 H 4 molecule, indicating the transfer of electrons between the conduction band and the valance band is easier. The DOS shifts to a lower region, which is consistent well with the charges accepting behavior of Au-doped MoS 2 monolayer. To be specific, the electron density in the conduction band is improve due to the electron-gaining and right shift of the Fermi level is happened, which in accordance well with the left shift of DOS. In conclusion, the electrical conductivity of Audoped MoS 2 monolayer is greatly improved after adsorbing C 2 H 4 molecule. Combined with the observation of Figures 6B,C, it can be found that the contributions of C 2 H 4 to the DOS of system are at the energies of −17, −12, −9, −4, and 0.5 eV. The obvious overlapping peak Au-5d, C-2p, and H-1s orbitals at −6 eV displayed in Figure 6D reveals the strong hybridization among them, which elucidate the strong attraction of Au-doped MoS 2 monolayer to C 2 H 4 molecule.
The large adsorption energy, larger transferred electrons, shorter adsorption distance and stronger orbitals hybridization perhaps reveal that the stronger interaction in Au-doped adsorption system than in MoS 2 monolayer adsorption structure. In consequence, the Au-doped MoS 2 monolayer is sensitive enough to detect C 2 H 4 gas and it is able to be a potential sensing material for C 2 H 4 sensors.

CONCLUSION
In this paper, the adsorption mechanism of C 2 H 6 and C 2 H 4 gases on MoS 2 based sensing materials were analyzed by the First-principles with DFT calculations. Firstly, the model of intrinsic MoS 2 monolayer was optimized and its structural parameters and electronic characteristics were calculated. It is found that MoS 2 monolayer with band gap of 2.058 eV is observed to be a direct band semiconductor. Then the adsorption of C 2 H 6 and C 2 H 4 molecules on MoS 2 monolayer were investigated, the corresponding structural parameters, charge transfer, adsorption energies and density of states were calculated to obtain the adsorption capacity. The results show that the intrinsic MoS 2 monolayer cannot adsorb gas effectively. The Au atom was chose as the dopant to enhance the properties of MoS 2 monolayer and the bandgap of Au-doped MoS 2 monolayer is decreased to 0.266 eV, which indicates a promotion in the metallicity. In final, the adsorption structures of Audoped MoS 2 monolayer to C 2 H 6 and C 2 H 4 molecules were modeled and the electronic structure of adsorption systems was investigated in detail. The adsorption energies to C 2 H 6 and C 2 H 4 are −0.463 and −0.952 eV, and the transferred charges from molecules to Au-doped MoS 2 monolayer are 0.118e and 0.309e, respectively. The impressive evidences from PDOS verify the hybridization between the orbitals of molecules and Audoped MoS 2 monolayer, which implies the strong interaction between them. In addition, the increased conductivity after gases adsorption were indicated by the decreased band gap. Therefore, it is concluded that the Au doping promotes the adsorption capacity of MoS 2 monolayer to C 2 H 6 and C 2 H 4 molecules by influencing the electronic properties. In summary, the present study demonstrates the surface reactivity to C 2 H 6 and C 2 H 4 gases of MoS 2 based sensing materials, which is meaningful to develop the MoS 2 based gas sensors for the detection of C 2 H 6 and C 2 H 4 gases.

DATA AVAILABILITY STATEMENT
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation, to any qualified researcher.