| Citation: |
Peng LI, Ye YUAN, Rong ZHOU, Lin LI, Hao CUI, Ziheng PU, Xuelin YANG. Discharge fault detection of dry air switchgear based on ZnO-MoS2 gas sensor[J]. Plasma Science and Technology. DOI: 10.1088/2058-6272/ad8f0d
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When discharge faults occur in dry air switchgear, the air decomposes to produce diverse gases, with NO2 reaching the highest levels. Detecting the NO2 level can reflect the operation status of the equipment. This paper proposes to combine ZnO cluster with MoS2 to improve the gas-sensitive properties of the monolayer. Based on the Density Functional Theory (DFT), the effect of (ZnO)n size on the behavior of MoS2 is considered. Key parameters such as adsorption energy and band gap of (ZnO)n-MoS2/NO2 system were calculated. The ZnO-MoS2 heterojunction was successfully synthesized by a hydrothermal method. The gas sensor exhibits a remarkable response and a fast response-recovery time to 100 ppm NO2. In addition, it demonstrates excellent selectivity, long-term stability and a low detection limit. This work confirms the potential of the ZnO-MoS2 composite structure as a highly effective gas sensor for NO2 detection, which provides valuable theoretical and experimental insights for fault detection in dry air switchgear.
Dry air has been widely used in electrical equipment such as switchgear and ring network cabinets due to its excellent cost-effectiveness, environmental friendliness and insulating properties [1, 2]. However, in the long-term operation of the device, insulation defects will occur because of burrs, air gaps, overheating or bolt loosening, which can cause partial discharges and even arc faults, seriously threatening the safe and stable operation of the switch equipment [3–5]. Studies have shown that when a fault happens, the air will decompose to produce various components, such as NO2 and CO, with the highest levels of NO2 [6, 7]. Moreover, the NO2 concentration magnifies with the increase in discharge intensity and time. Therefore, detecting and monitoring the NO2 concentration hold great significance for effectively assessing and maintaining the operational condition of dry air switchgear.
The detection of insulation gases in switchgear commonly involves the use of detector tube methods, gas chromatography, mass spectrometry, spectroscopy and other techniques [8, 9]. However, the first three methods are constrained by the detection conditions, posing challenges for online monitoring. The spectral method is susceptible to interference from on-site noise. Resistive gas sensors made of semiconductor materials possesses the advantages of fast response time, high sensitivity and low cost [10, 11], which has been widely utilized in numerous aspects such as medical treatment, atmospheric monitoring and fault detection of electrical equipment [12, 13]. Two-dimensional transition metal dichalcogenides (TMDs) have garnered considerable interest in gas detection due to their large specific surface area, high carrier mobility and excellent semiconductor properties [14, 15]. Molybdenum disulfide (MoS2), a prominent material within TMDs, has been demonstrated to be a highly promising component for designing novel nano-sensors [16, 17]. Le et al [18] synthesized Ti3C2-MoS2 composites using 2D MXenes and MoS2, and the gas sensor can not only detect NO2 at room temperature but also effectively test harmful gases such as NH3 and CH4. Hu et al [19] reported an Au@MoS2 nano-heterostructure sensor that exhibits more than 8-fold enhanced response in detecting NO2 (1 ppm) under indoor white light activation. However, pure MoS2-based gas sensors are still sluggish in response and recovery rates, and sometimes even difficult to recover. For this reason, many studies have proved that combing MoS2 with metal oxides can greatly strengthen its gas-sensitive properties [20, 21]. Among them, zinc oxide (ZnO) has attracted widespread attention owing to its simple synthesis, low cost and excellent gas sensitivity [22]. In addition, ZnO is an n-type semiconductor and MoS2 is a p-type semiconductor material. To form a p-n heterojunction, its synergistic effect can be used to enhance the gas sensing performance of the MoS2 monolayer. Tang et al [23] fabricated ZnO nanorods/MoS2 nanoflower heterojunctions by hydrothermal and physical mixing methods, which presented efficient detection of SO2 in both air and SF6 environments. Ikram et al [24] successfully built a MoS2@ZnO heterojunction using a hydrothermal method, and the gas sensor has a response speed of more than 30 times that of pure MoS2. Although there are substantial studies on combining ZnO to promote the chemical activity and sensitivity of MoS2 gas sensors, there is still a lack of theoretical analysis on the mechanism of improving its detection performance and the influence of ZnO cluster size on the gas sensing characteristic.
In this study, the effect of (ZnO)n scale on the structural and electronic properties of MoS2 was initially considered, and simulations of (ZnO)n-MoS2 with n = 1, 2, 3 and 4 were conducted respectively based on the Density Functional Theory (DFT). Finally, the ZnO-MoS2 heterojunction consisting of (ZnO)4 and MoS2 was focused. To investigate its adsorption performance on NO2, some key parameters of the ZnO-MoS2/NO2 system were calculated. Subsequently, the ZnO-MoS2 material was synthesized by a hydrothermal method and characterized by various means. The adsorption behaviors of the ZnO-MoS2 sensor on NO2 gas were explored. This study can lay the foundational theory for improving the gas sensing performance of MoS2 materials, and provide a reference for further development of discharge fault detection sensors for switchgear.
All the calculation results are obtained in the DMol3 module of Materials Studio software. The generalized gradient approximation (GGA) and the Perdew-Burke-Ernzerhof (PBE) functions are used to handle the electron exchanges and related terms. The introduction of relativity by the DFT semi-core pseudopods aims to decrease the computing cost [25]. In addition, a 10×10×1 Brillouin zone k-point grid is defined for geometric optimization and electronic structure calculation of super-cell. The energy tolerance accuracy, maximum force and displacement are set as 10−5 Ha, 2×10−3 Ha/Å and 5×10−3 Å, respectively. A 4×4×1 MoS2 super-cell consisting of 16 Mo and 32 S atoms is constructed. After geometry optimization, the lattice constant of the MoS2 monolayer is 3.19 Å, which is close to the previous results [3].
ZnO is prone to form clusters during preparation, whose size and shape usually affect their structures and properties. Despite the numerous studies on various ZnO clusters, limited attention was given to the gas sensing mechanism of intrinsic MoS2 monolayer doped with different (ZnO)n. So, this study simulates four MoS2 monolayers doped with different (ZnO)n clusters. Figure 1 displays the geometrically optimized structures of the MoS2, (ZnO)n clusters and NO2 molecule.
The binding energy (Eb) and adsorption energy (Ead) are applied to scale the adsorption intensity of each system. The formulas are set as:
| Eb=E(ZnO)n-MoS2−EMoS2−E(ZnO)n, | (1) |
| Ead=E(ZnO)n-MoS2/NO2−E(ZnO)n-MoS2−ENO2, | (2) |
where E(ZnO)n-MoS2,EMoS2,E(ZnO)n, E(ZnO)n-MoS2/NO2 and ENO2 are the energies of (ZnO)n-MoS2, MoS2, (ZnO)n clusters, (ZnO)n-MoS2/NO2 system and individual NO2 molecule, respectively. When E > 0, the adsorption process is endothermic and cannot proceed spontaneously. When E < 0, the adsorption process is exothermic and spontaneous. The smaller E is, the easier the process is to occur and the more stable the structure is. Moreover, by the Hirshfeld method to analyze the charge transfer (QT) of each system, the charge distribution and electronic properties can be derived. The value can reflect the interaction strength between the monolayer and the molecule in each system.
The required gas-sensitive materials were synthesized by a hydrothermal method. Initially, 4.13 mmol Na2MoO4•2H2O and 15.76 mmol CH4N2S were dissolved to 80 mL deionized water and stirred at room temperature for 60 min to form a clear solution. Then 0.6 g HC2O4 was taken into the solution and agitated continuously for 30 min. The obtained solution was transferred to a 100 mL polytetrafluoroethylene high-pressure reactor and reacted at 200 °C for 24 h. After cooling to room temperature, the solution underwent repeated centrifugation with ultra-pure water and dried in a vacuum oven at 80 °C for 24 h. After that, the ground and dried samples were annealed in a tube furnace at 700 °C for 2 h in a pure nitrogen environment to acquire MoS2 powder. The composite material was formed by dispersing some MoS2 powder and 0.5 g ZnO in 30 mL anhydrous ethanol, ultrasonically treating for 2 h, and drying at 80 °C for 5 h. It was also annealed in a tube furnace for 2 h to get the final ZnO-MoS2 sample. The specific synthesis process is visually depicted in figure 2, and the reagents are all analytically pure.
The morphology and structure of the prepared ZnO-MoS2 were observed by JSM-7500F scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (TEM). The composition and crystal structure were tested by X-ray diffractometer (XRD) of the Japanese Rigaku SmartLab (9 kW). The chemical valence was analyzed by X-ray photoelectron spectroscopy (XPS) of Shimadzu AXIS Supra.
A little deionized water and 0.5 g ZnO-MoS2 powder were meticulously blended in a mortar until a smooth, homogeneous slurry was achieved. The slurry was then uniformly coated on the surface of the ceramic tube with a brush, which was dried at room temperature overnight. Then, four test ends of the tube were welded to the hexagonal base. The heating resistance wire was passed through the tube, with two leads exposed and welded at the heating ends. The details are shown in figure 3.
The experimental platform mainly consisted of the gas tank, DGD-IV dynamic configuration system and CGS-8 gas sensitive analysis system, as presented in figure 3. The dynamic gas distribution system effectively dilutes target gas with dry air, achieving precise ppm-level assignment and simulating the generation of dry air decomposition components under discharge fault. Due to the insufficient closure of the gas chamber of the analyzing system, it was decided to conduct the test in a separate chamber. Using an extension wire to connect the test chamber to the system, the heating currents of each channel can be controlled individually and real-time data acquisition can be realized, and the closed chamber can also prevent humidity from affecting the test results.
To measure the effectiveness of the sensor in detecting gas, the response is defined as Re:
| Re=|Rg−RaRa|×100%, | (3) |
where Rg and Ra are the stabilized resistance values of the gas-sensitive material in the target gas and air, respectively. Also, the response time and the recovery time is the time required for the resistance to change by 90% after filling or stopping charging the target gas.
When n = 1, two possible composite structures are considered: (ZnO)1 is perpendicular to the monolayer, and the Zn atom is close to an S atom; or the O atom is close to an Mo atom. After optimization, it is found that the Zn–S bond is formed in the upper layer. The specific parameters are summarized in table 1. The Eb of both is −1.364 eV, which suggests that the processes both are spontaneous. The adsorption distance is 2.255 Å and 2.256 Å, respectively. The QT is 0.1407e and 0.1413e, respectively. Moreover, the band gap (BG) of both is 0.756 eV, which is 0.96 eV less compared to the value of pure MoS2. The smaller the BG, the easier it is for electrons to be excited from the valence band to the conduction band, the higher the intrinsic carrier concentration, and the greater the conductivity of the structure.
| Structure before adsorption | Structure after adsorption | Eb (eV) | Adsorption distance (Å) | QT/e | BG (eV) | |
| (ZnO)1 |  |  | −1.364 | 2.255 | 0.1407 | 0.756 |
 |  | −1.364 | 2.256 | 0.1413 | 0.756 | |
| (ZnO)2 |  |  | −1.283 | 2.302 | 0.1273 | 1.026 |
 |  | −1.232 | 2.302 | 0.151 | 0.925 | |
 |  | −1.259 | 2.314 | 0.1494 | 0.975 | |
| (ZnO)3 |  |  | −0.899 | 2.415 | 0.1897 | 1.505 |
 |  | −1.222 | 2.637 | 0.1807 | 1.493 | |
| (ZnO)4 |  |  | −1.35 | 2.715 | 0.1866 | 1.566 |
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When n < 5, (ZnO)n are more stable in the ring structure [26]. Therefore, ring structures are considered when n = 2, 3 and 4. As MoS2 adsorbs (ZnO)2, in addition to considering the same two cases as (ZnO)1, (ZnO)2 is also placed horizontally above the monolayer. Then, calculations show that (ZnO)2 in all structures forms a Zn–S bond in the upper layer. From table 1, the absolute value of Eb in the first structure is the largest, but its QT is small. The adsorption distance of the third structure is the farthest, indicating that when (ZnO)2 is adsorbed, the tendency of (ZnO)2 to be tightly bound to the monolayer is relatively weak. So, the second structure, with the greatest BG change, is selected as the optimal structure for MoS2 adsorbing (ZnO)2.
When n = 3, two structures are considered: (ZnO)3 is parallel or perpendicular to the MoS2 monolayer. The Eb values of the two structures are −0.899 eV and −1.222 eV, respectively. There is a new Zn–S bond formed in the first one, and the adsorption distances are measured to be 2.415 Å and 2.637 Å. (ZnO)3 obtains electrons as acceptors, and the QT in the first structure is slightly larger than that in the second structure. In summary, the first structure is more stable than the second. Hence, the adsorption properties of (ZnO)3-MoS2 on NO2 will be evaluated using the first structure. Besides, the Eb of the ring (ZnO)4 is −1.35 eV. The cluster is parallel to the MoS2 monolayer, and the nearest distance between them is 2.715 Å. The BG of the system decreases to 1.566 eV, which is consistent with the changes in the above three structures.
To analyze the change in the electrical properties of the MoS2 after adsorbing (ZnO)n, figure 4 exhibits the total density of states (TDOS) and the partial density of states (PDOS) of the four structures. According to the TDOS, when the MoS2 monolayer is composited with (ZnO)n clusters, the overall curves are discernibly shifted to the left to a certain extent. This phenomenon suggests a favorable interaction between them, resulting in a stabilization of the system at a lower energy level. Meanwhile, there are obvious variations in the DOS of four structures at the Fermi level. The BG of each structure shows different degrees of reduction, which makes the electrons inside the material more prone to transition. As can be observed from the PDOS plotted in figures 4(b), (d), (f) and (h), orbital hybridization occurs between the areas of −7.5 to −5 eV, signifying some intriguing interactions between the clusters and the substrates.
Reference [27] mentioned that when using organometallic precursors, such as (MeZnOtBu)4 to synthesize ZnO, its molecular structure is pre-organized as (ZnO)4 molecules and is hexahedral. Therefore, subsequent analytical calculations will be performed on MoS2 monolayer adsorbing cubic (ZnO)4. As described in figure 1(e), the Zn–O bond length (2.017 Å) approximates the previous calculation report (2.00 Å) [27]. After geometric optimization, (ZnO)4 is always captured by MoS2 through two S atoms to form chemical bonds, with the most stable configuration (MSC) displayed in figure 5(a). It can be realized that the structure of (ZnO)4 has changed compared to its original state. Each Zn atom forms new bonds with the other three Zn atoms on the basis of having three Zn–O bonds. The Eb is −1.63 eV, and the adsorption distance is 2.44 Å. The BG is 1.147 eV, which is reduced by 0.569 eV from the intrinsic MoS2. Figure 5(b) is the deformation charge density (DCD) of (ZnO)4-MoS2. It can be viewed that there is obvious charge aggregation at two Zn–S bonds while charge dissipation on the monolayer, reflecting a strong binding force between them. According to the Hirshfeld method, (ZnO)4 received 0.1662e from the MoS2, which verified the electronic properties of the (ZnO)4 cluster.
To delve deeper into the effect of (ZnO)4 on the electronic properties of MoS2, the TDOS, PDOS and band structure (BS) of (ZnO)4-MoS2 are calculated. The results, drawn in figure 6, reveal intriguing findings. From figure 6(a), it can be observed that when MoS2 adsorbs (ZnO)4, its TDOS moves to the left, accompanied by the emergence of a new peak between −17 and −15 eV. The peak intensity at −5 eV increases significantly, indicating the strong interaction between them. It can also be noted from figure 6(b) that the Zn 3d and S 2p orbitals have evident overlapping peaks in the range of −6 to −4 eV, which suggests a significant hybridization between them. Moreover, the BS of (ZnO)4-MoS2 is presented in figure 6(c). The BG experiences a reduction of 0.569 eV after adsorbing (ZnO)4, which makes it easier for electrons to transfer between the valence and conduction bands. Consequently, this leads to reduced resistance and heightened conductivity in the monolayer.
In the preceding analysis, the adsorption of different (ZnO)n structures on the MoS2 monolayer was fully investigated, and the respective optimal configurations were obtained. In this section, the effects of various (ZnO)n on the gas-sensing properties of materials are explored.
Similar to the last section, for each material adsorbing NO2 molecule, three different adsorption structures were considered: the NO2 is parallel to the monolayer; or the NO2 is adsorbed by an N atom; or the NO2 is adsorbed by two O atoms. By comparing the Ead, QT and BG values of each structure, each MSC was finally obtained and the parameters are shown in table 2. It is found that each structure shows a pronounced adsorption effect on the NO2 molecule, and NO2 always acts as an electron acceptor to acquire electrons from the substrate material. Additionally, the DOS for each system is illustrated in figure 7. When n = 1 or 2, the BG of the material after adsorbing NO2 increases, as seen in figures 7(a) and (c). Whereas, when n = 3 or 4, the BG reduces as depicted in figures 7(e) and (g). This reveals an interesting phenomenon: as the size of the (ZnO)n cluster gets bigger, the detection characteristics of the sensor also change.
| Structure | Ead (eV) | QT/e | BG (eV) | |
| 1 | (ZnO)1-MoS2/NO2 | −2.42 | 0.12 | 1.32 |
| 2 | (ZnO)2-MoS2/NO2 | −1.28 | 0.13 | 1.03 |
| 3 | (ZnO)3-MoS2/NO2 | −0.68 | 0.22 | 1.39 |
| 4 | (ZnO)4-MoS2/NO2 | −0.997 | 0.21 | 0.929 |
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From the above analysis, it can be known that no matter which cluster is doped on the MoS2, the cluster always acquires electrons from the monolayer. At the same time, the composite formed by combining the most stabilized (ZnO)4 with MoS2, which is abbreviated as ZnO-MoS2, exhibits a reduced band gap and increased conductivity after adsorbing NO2. The interaction of NO2 with the ZnO-MoS2 surface creates a more conductive path for electron transport. The notable decreased resistance can be taken as the primary sensing mechanism for detecting the target gases. Consequently, the resistive gas sensor made of ZnO-MoS2 material can be utilized to realize the effective detection of NO2.
Figures 8(a) and (b) exhibit the SEM images of the pure MoS2 and the ZnO-MoS2 sample. It can be seen that the MoS2 is in the form of nanoflowers with curled edges, while ZnO is a nanosheet. This composite structure can be further viewed by the TEM result in the figure 8(c). The lattice fringes in figure 8(d) correspond to the (002) crystal plane of MoS2 and the (100) crystal plane of ZnO, with lattice spacings of 0.63 nm and 0.28 nm, respectively. Moreover, the region in figure 8(c) is scanned using energy dispersive spectroscopy (EDS) to observe the elemental distribution of the sample at a microscopic level, and the results are displayed in figures 8(e)–(h). It can be clearly found that the Mo, S, Zn and O elements are more uniformly distributed in the sample, demonstrating the successful preparation of the ZnO-MoS2 heterojunction.
Figure 9(a) draws the XRD spectra of ZnO, MoS2 and ZnO-MoS2 samples. The diffraction peaks labeled “▲” and “●” are the characteristic peaks of the crystalline phases of ZnO and MoS2, respectively. There are eight distinct characteristic peaks located at 31.74°, 34.40°, 36.33°, 47.50°, 56.54°, 62.82°, 67.94° and 69.02° for pure ZnO, belonging to the (100), (002), (101), (102), (110), (103), (112) and (004) crystal planes of ZnO, respectively. The results are in good agreement with its standard diffraction pattern (JCPDS No. 36-1451). As can be noted from the figure 9(a), the diffraction peaks of the composite ZnO-MoS2 appear at 14.20°, 33.28°, 39.50° and 58.70°, which pertain to the (002), (100), (103) and (105) crystal planes of MoS2, consistent with the MoS2 standard card (JCPDS No. 37-1492). Apart from these, no other obvious peaks occur in the graph, meaning that the synthesized sample is pure and free from other heterogeneous phases.
The elemental composition and chemical valence of the sample are further analyzed by XPS. The full spectrum in figure 9(b) proves the presence of the Zn, O, Mo and S elements. From figure 9(c), the precise binding energies of two fitted peaks, Zn 2p1/2 and Zn 2p3/2, are 1044.85 eV and 1021.74 eV, respectively, indicating that the Zn element in the sample exists in the form of Zn2+. Figure 9(d) shows the fitting results of O 1s, consisting of three spectral peaks: a high energy of 532.57 eV in the form of defect vacancy oxygen (OV); a medium energy of 531.33 eV, corresponding to lattice oxygen (OL) in ZnO; and a low energy of 528.73 eV, which is probably adsorbed oxygen (OS) from the surface of the structure. From the Mo 3d pattern in figure 9(e), it is obvious that the Mo 3d3/2 and Mo 3d5/2 peaks are at 232.78 eV and 229.65 eV, respectively, which are the characteristic peaks for Mo4+. The weak peak at 234.30 eV belongs to Mo6+, implying that a small amount of MoO3 may exist in the sample. Moreover, an additional small peak located at 226.87 eV can be found, which can be attributed to S2− in MoS2. The spectrum of S 2p is illustrated in figure 9(f), where two distinct characteristic peaks can be viewed at 163.66 eV and 162.45 eV, being part of the S 2p1/2 and S 2p3/2 electronic states, respectively. There is also a minor peak at 160.81 eV, signifying the presence of S–Zn bonds and formation of ZnO-MoS2 heterojunction. The XPS results further evinced the successful fabrication of the nanocomposite.
Figure 10(a) portrays the response-recovery properties of the ZnO-MoS2 sensor to 100 ppm NO2 at room temperature. The Re is 43.51%, with a response time of 176 s. It can be recognized that the sensor can still show a remarkable detection effect on NO2 without heating, but the recovery process is always sluggish. With the temperature rising, the Re undergoes a distinct trend of initial growth followed by a decline, reaching the maximum at 90 °C. The response values at 50 °C, 70 °C, 90 °C, 110 °C, 130 °C, 150 °C, 170 °C and 190 °C are 44%, 49.9%, 54.25%, 39.61%, 27.01%, 25.5%, 18.18% and 15.76%, respectively.
Figure 10(b) depicts the response-recovery behavior of the sensor to NO2 at different temperatures. The response time is within 200 s, with the fastest response at 130 °C, indicating that elevating the working temperature can accelerate the response speed. However, the sensor still has the problem of slow recovery velocity. It also struggles to revert to its original resistance value. This issue is also partially mitigated at 130 °C. At this time, the sensor exhibits a more rapid recovery speed and achieves a final resistance that is only 5.9% different from its preliminary Ra.
Therefore, only the maximum response value cannot fully explain the best detection effect at a certain temperature. It is necessary to further analyze the response-recovery properties of the sensor, rather than blindly pursuing the maximum value. When the temperature increases to 150 °C, the sensor can completely recover to the original value in a limited time, and the response-recovery times (56 s/157 s) are both short. Despite the Re measuring slightly lower at 25.5% compared to the low-temperature section, it can still show that the sensor has an obvious detection effect on NO2. To recapitulate, it can be judged that the best working temperature of the ZnO-MoS2 sensor is 150 °C, and in the earlier experiment using the pure MoS2 sensor to detect NO2, the sensor had a Re of 30.29% at room temperature and peaked at 120 °C. In addition, when the temperature is lower than 240 °C, it is difficult for the Re of the MoS2 sensor to recover to the initial value within a limited time, which cannot meet the practical working requirements. Thus, it can be concluded that combining ZnO with MoS2 can improve the response intensity of the sensor to NO2, speed up the response recovery and reduce the operating temperature.
The response value of the gas sensor is closely related to the gas concentration. Exploring this relationship is of great significance for accurately evaluating the test precision of the sensor, defining its concentration detection range and determining the detection limit [28]. It has been reported that when corona or partial discharge occurs in the switchgear, the internal NO2 concentration is detected to be about 0‒60 ppm; when an arc fault happens, the NO2 concentration can reach several hundred ppm [29]. So, the sensing characteristics of the ZnO-MoS2 sensor to 5‒60 ppm NO2 are tested at 150 °C, and the Re values of the ZnO-MoS2 sensor for 5, 10, 20, 30, 40, 50 and 60 ppm NO2 are 5.95%, 9.79%, 11.82%, 15.17%, 17.99%, 19.37% and 20.02% respectively. It can be seen that the sensor has a certain response to each concentration of NO2, and the higher the concentration is, the greater the Re is. Meanwhile, it is realized that there is a good linear relationship between them. Through curve fitting, the numerical relationship between the Re(y) and the NO2 concentration (x) is derived as follows:
| y=0.259x+6.241. | (4) |
The linear correlation coefficient R2 of the curve is 0.9289, showing strong linearity. The fitting function is presented in figure 11(a). According to this, the minimum detection limit of the ZnO-MoS2 sensor can reach the level of hundreds of ppb.
In practical gas detection, sensors exhibit diverse response behaviors to different gases. To investigate its selectivity, the responses of the ZnO-MoS2 sensor to different gases are tested. As shown in figure 11(b), the response values of the ZnO-MoS2 sensor to 100 ppm NO2, 200 ppm SO2, 300 ppm CO and C2H5OH are analyzed. The corresponding values are 25.5%, 2.56%, 2.1% and 1.9%, respectively, which manifests the strong selectivity of ZnO-MoS2 sensor towards NO2.
Long-term stability is also one of the important parameters for maintaining the detection ability of any practical gas sensor [24, 30]. To assess the stability of the ZnO-MoS2 sensor, 100 ppm NO2 gas was detected each time with 5 days as a cycle, and the average value of each test in the morning and evening was taken as the effective response value. The response curve of the sensor within 30 days is drawn in figure 11(c). It is found that the response value of the sensor remains around 25 within 30 days, and there is only slight fluctuation and no obvious downward trend. The overall results suggest that the sensor can guarantee a stable and reliable detection effect within 30 days, and has the reliability of long-term detection in practical application.
In this work, the effect of (ZnO)n cluster scale on MoS2 was considered, especially focusing on the structure of n = 4. The detection mechanism of the ZnO-MoS2 for NO2 was deeply analyzed. Then, the material was successfully synthesized by a hydrothermal method and the sensor was prepared. The detection performance of ZnO-MoS2 sensor on NO2 has been studied by gas sensing experiment. The main conclusions are as follows:
(1) No matter which (ZnO)n cluster is doped on the MoS2, the cluster always acquires electrons from the monolayer, and the BG of the composite decreases. It can be concluded that doping clusters can improve the conductivity of the sensor. In addition, the BG of the composite increases after adsorbing NO2 when n < 3, and decreases when n ⩾ 3. It can be seen that as the size of the cluster becomes larger, the electronic properties of the gas-sensitive material also change.
(2) The Ead of (ZnO)4-MoS2 on NO2 adsorption is −0.997 eV, which is much larger than that of the intrinsic MoS2 monolayer. In addition, the BG of this system is reduced by 19%, which means that the conductivity of the material increases a lot after adsorbing the NO2 gas molecule. This makes it possible to use the (ZnO)4-MoS2 material as a resistive gas sensor for NO2 detection.
(3) At 150 °C, the response-recovery time of the ZnO-MoS2 sensor is 56 s/157 s, and the maximum Re is 25.5%, showing an obvious response effect and rapid speed. The Re reflects a strong linear correlation with NO2 concentration, demonstrating the feasibility of quantitatively detecting NO2. Furthermore, the sensor has a detection limit in the hundreds of ppb, and the ZnO-MoS2 sensor exhibits excellent selectivity and long-term stability.
In conclusion, this study combines simulation and experiment to comprehensively analyze the gas sensing detection mechanism of the gas sensor. It is found that compounding the ZnO cluster with MoS2 can significantly enhance the adsorption performance of the monolayer on NO2, and the prepared ZnO-MoS2 gas sensor has multiple advantages. This paper provides strong theoretical and practical guidance for designing fault detection sensors for dry air switchgear. In the future, the sensor array can be further studied and developed for other characteristic gas components to further enhance the detection effect of the sensor.
The authors gratefully appreciate the experimental support provided by the Laboratory of New Energy Materials of China Three Gorges University. The authors also thank the financial support of National Natural Science Foundation of China (Nos. 52207175 and 52407178).
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Figures(11) / Tables(2)
Supported by: Beijing Renhe Information Technology Co., Ltd. 
| Structure before adsorption | Structure after adsorption | Eb (eV) | Adsorption distance (Å) | QT/e | BG (eV) | |
| (ZnO)1 | | | −1.364 | 2.255 | 0.1407 | 0.756 |
| | −1.364 | 2.256 | 0.1413 | 0.756 | |
| (ZnO)2 | | | −1.283 | 2.302 | 0.1273 | 1.026 |
| | −1.232 | 2.302 | 0.151 | 0.925 | |
| | −1.259 | 2.314 | 0.1494 | 0.975 | |
| (ZnO)3 | | | −0.899 | 2.415 | 0.1897 | 1.505 |
| | −1.222 | 2.637 | 0.1807 | 1.493 | |
| (ZnO)4 | | | −1.35 | 2.715 | 0.1866 | 1.566 |
DownLoad:
CSV
| Structure | Ead (eV) | QT/e | BG (eV) | |
| 1 | (ZnO)1-MoS2/NO2 | −2.42 | 0.12 | 1.32 |
| 2 | (ZnO)2-MoS2/NO2 | −1.28 | 0.13 | 1.03 |
| 3 | (ZnO)3-MoS2/NO2 | −0.68 | 0.22 | 1.39 |
| 4 | (ZnO)4-MoS2/NO2 | −0.997 | 0.21 | 0.929 |
DownLoad:
CSV
