Abstract
MoS2 has been considered a potential novel material in various fields due to its large specific surface area, high carrier mobility, and tunable electronic properties. However, with the increasing demand for sensor substrates, different strategies have been made to achieve its high performance, usually by adopting the method of controlling the microstructure. SF6 gas-insulated electrical equipment has gained considerable attention in electric systems with the advantages of small volume, high security, strong breaking performance, and high-pressure fracture tolerance. Nevertheless, in the process of equipment operation, the SF6 gas will occur inevitably decompose due to partial discharge, resulting in the deterioration of the insulation performance of the equipment. Therefore, detecting SF6 decomposition products is significant for the safe and stable operation of power systems. In this mini review, we start from the synthesis of various MoS2 morphological structures. Then, the beneficial characteristics of the unique synthesized nanostructures at present are analyzed. Besides, we focus on the gas-sensing mechanisms and applications of MoS2-based sensors for detecting SF6 decomposition products. Finally, the future development in this field is proposed.
Introduction
Sulfur hexafluoride (SF6) is a non-color, tasteless, and non-flammable insulating gas, which is widely known for applications in gas insulated switchgear (GIS) (; ). Although GIS has advantage of high stability, insulation faults such as partial discharge, breakdown discharge, and spark discharge inevitably occur during a long running process (Zeng et al., 2015; ), which will lead to the decomposition of the SF6 gas into various sulfur fluorides, including H2S, SO2, SO2F2, and SOF2 etc. (Wei et al., 2020). Previous research has indicated that these characteristic gases can accelerate the corrosion rate of facilities and increase the probability of system paralysis (Zhang X. X. et al., 2017a; Zhou et al., 2018b). Therefore, it is of great necessity to evaluate the operational status of GIS equipment by effectively detecting these typically decomposed products of SF6 (Zhang X. X. et al., 2017b; Zhou et al., 2018d). In this respect, bidimensional nanomaterials have captured widespread attention for its multiple physical and chemical properties in the detection of SF6 decomposed gas (Zhou et al., 2018a; ).
Given this, various 2-D nanomaterials such as carbon nanotube (CNT), graphene, and molybdenum disulfide (MoS2) have been synthesized by different methods (Zhou et al., 2018e; ). As an n-type semiconductor material with wide band gap, MoS2 receives the most interest because of its high surface activity and chemical stability (; Zhang et al., 2019). Up to now, a large number of studies have been carried out on the various nanostructures of MoS2, including nanoflakes (), nanotubes (Zhong et al., 2020), nanospheres () and other complex hierarchical nanostructures (Zhang et al., 2018b; ), to realize more effective methods of detecting SF6 decomposed products. Besides, the evident correlation has been confirmed between unique structures and performances (; ). Therefore, the morphology synthesis and analysis of MoS2 nanostructures are of great significant to discuss. This mini review summarizes the morphological features and sensing applications of MoS2, especially for detecting SF6 decomposed products.
Morphology and analysis of MoS2
MoS2 is deemed as a viable and effective material owing to its stable semiconducting property and high thermal stability (; ). Until now, the diverse morphology of MoS2 nanostructures have been designed by investigators through various methods to achieve excellent sensing properties in the aspects of chemical, optical, and gas sensors (Zhang et al., 2018a; ). Kang et al. demonstrated vertical MoS2 nanoflakes (shown in Figure 1A) fabricated on SiO2/Si substrates by the deposition and thermal evaporation. They found the vertical flakes supplied an effective surface area and sufficient oxygen vacancies for the adsorption of NO2 gas. The minimum concentration of the NO2 detection was 0.15 ppm at room temperature (). Tu et al. synthesized hierarchical MoS2 spheres with flower-like structures (shown in Figure 1B) presented efficient aluminum storage properties. These flower-like microstructures for aluminum storage possessed open and well-defined hierarchical structures, leading to a higher specific capacity and prominent cycling stability. The as-prepared MoS2 electrodes delivered reversible capacities of 112.2 mAh g−1 at 153.6 mAh g−1 after 100 cycles ().
FIGURE 1
Sun et al. developed MoS2 nanorods (shown in Figure 1C) by the hydrothermal and ammonia annealing approach and found that the fabricated nanorods exhibited excellent catalytic performances due to a more specific surface area and a sufficient ion transportation channel (
MoS2 sensor for SF6 decomposing products
Theoretical calculations about MoS2-based sensors
In order to analyze and investigate the adsorption process between SF6 decomposed gases and MoS2-based material, corresponding adsorbing factors containing total adsorption energies (), the value of charge transfer (), and projected or total density of state can be attained (
The adsorption procedures between gas sensing materials and SF6 decomposing products were studied based on the density functional theory (DFT) (
In another instance, Qian et al. researched the adsorption property of H2S and SO2 on the Pt-decorated MoS2 and found that Pt-MoS2 showed a strong interaction with gas molecules due to the strong chemical activity of with Pt atom. The adsorption energy of H2S and SO2 molecules adsorbed on Pt-MoS2 was −1.465 and −1.584 eV, respectively, suggesting the occurrence of strong chemisorption in the contact surfaces. The charge transfer amount between H2S and Pt-MoS2 was up to 0.302 e, which is much higher than that of SO2 (0.036 e). The results indicated that Pt-MoS2 exhibited an excellent adsorption property for H2S gas (
In the above, the adsorption of H2S and SO2 molecules in different systems have been discussed in detail. The simulation results of H2S and SO2 molecules adsorption are in Table 1. These values may be obtained by different DFT functions. Compared with the adsorption construction of a pristine MoS2 system, the introduction of novel elements promotes the surface chemical activity of the MoS2 system. In addition, the addition of dopants enhances the affect of orbital hybridization between the MoS2 monolayer and gas molecules, and facilitates the electronic transfer. The adsorption capacity of the modified materials to the goal gas will be further improved, when the dopant is suitable (Zhang et al., 2017a).
TABLE 1
| SF6 Decomposingproducts | System | (eV) | (e) | References |
|---|---|---|---|---|
| H2S | MoS2 | −0.22 | 0.02 | |
| Si-MoS2 | −0.68 | 0.16 | ||
| Ni-MoS2 | −1.319 | 0.254 | Wei et al., 2018 | |
| Pt-MoS2 | −1.465 | 0.302 | ||
| SO2 | MoS2 | −0.30 | −0.04 | |
| Ni-MoS2 | −1.382 | −0.016 | Wei et al., 2018 | |
| Pt-MoS2 | −1.584 | 0.036 | ||
| Al-MoS2 | −2.33 | −3.43 | Zhang et al., 2018 |
Comparison of adsorption parameters for H2S and SO2 in different systems.
Experimental analysis about MoS2-based sensors
At present, there are few experimental studies on the detection of SF6 decomposed byproducts, the few that do mainly focus on the H2S, SO2, and CO gases (Wang et al., 2019a; Zhou et al., 2019). Park et al. prepared a MoS2 gas sensor modified with Pt nanoparticles with a low detection limit and high sensitivity for H2S gas. The Pt-MoS2-based sensor reduced the minimum detection concentration of H2S gas to 5Â ppm, which was much lower than that of the pure MoS2 sensor (30Â ppm) (
Gas sensing mechanism
The gas sensing mechanism of the MoS2-based sensors to SF6 decomposed products can be explained by the change of conductivity caused by the interaction between oxyanions (mainly O-) on the material surface and target gas molecules. Firstly, the oxygen molecules in air will adsorb on the surface of semiconductor materials and capture electrons from the adsorption sites to form oxyanions. The formation of an electron depletion layer and the increase of resistance are caused by oxygen adsorption. Then, when sensors are placed in an environment of reductive gas, the test gas molecules react with oxyanions to release the electrons back to the surface of the material, resulting in the decrease of the electron depletion layer and the occurrence of conductivity change. The relevant reactions are depicted as follows (take H2S gas as an example):
Conclusion
In this paper, we focused on the morphological characteristics and application of MoS2 materials for the detection of SF6 decomposed products in GIS. Firstly, partial reports of sensing properties of MoS2 with various morphologies were concluded. Diverse MoS2 structures presented relatively different sensing properties at specific operating temperatures, so it was possible to promote sensing performances of MoS2 by simple surface morphology modification. With the increasing demand for sensor materials, simples and more convenient synthetic routes and more favorable morphology structures should be proposed. Next, it was shown that the selectivity and sensitivity of MoS2 gas sensors could also be enhanced by the enlargement of the active surface area and the introduction of metal or non-metal elements based on formerly theoretical and experimental investigations. Then, the gas-sensing mechanism of the MoS2 based sensors to SF6 decomposed products was described by comparing the characteristics of material substrate and gas molecules. Although some developments have been obtained in theoretical research into SF6 decomposed products, a large number of fundamental experiments are needed to further prepare MoS2 sensors for practical industrial applications.
Statements
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.
Author contributions
All authors listed have made a substantial, direct, and intellectual contribution to the work, and approved it for publication.
Conflict of interest
Authors GQ, QP, DZ, SW, and BY were employed by the Electric Power Science Research Institute of Yunnan Power Grid Co., Ltd.
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Summary
Keywords
molybdenum disulfide, morphological characteristics, SF6 decomposing products, sensing application, gas insulated switchgear
Citation
Qian G, Peng Q, Wang H, Wang S and Dai W (2020) Morphological Characteristics of Molybdenum Disulfide and Current Application on Detecting SF6 Decomposing Products. Front. Mater. 7:580245. doi: 10.3389/fmats.2020.580245
Received
05 July 2020
Accepted
02 September 2020
Published
06 November 2020
Volume
7 - 2020
Edited by
Wen Zeng, Chongqing University, China
Reviewed by
Yanqiong Li, Chongqing University of Arts and Sciences, China
Jianbo Yin, Northwestern Polytechnical University, China
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© 2020 Qian, Peng, Wang, Wang and Dai.
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*Correspondence: Guochao Qian, 164688847@qq.com
This article was submitted to Smart Materials, a section of the journal Frontiers in Materials
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