Original Research Paper
1T/2H-MoS2 engineered by in-situ ethylene glycol intercalation for improved toluene sensing response at room temperature

https://doi.org/10.1016/j.apt.2020.02.022Get rights and content

Highlights

  • Molybdenum sulfide was successfully synthesized by solvothermal treatment process.

  • The phase composition could be engineered by intercalation of ethylene glycol.

  • The MoS2 intercalated with EG possessed higher 1T conductive phase.

  • The relative humidity could modify the toluene sensing performance of MoS2.

Abstract

Molybdenum sulfide (MoS2) was engineered by intercalation of ethylene glycol to form 2H (semiconductor) and 1T (metallic) phases. MoS2 was successfully synthesized under solvothermal conditions with different solvents. Water, ethylene glycol (EG), and a mixture of water and EG were used as the solvent. The obtained samples were denoted as MoS2 (W), MoS2 (EG), and MoS2 (EG:W), respectively. The use of ethylene glycol as a solvent expands the (0 0 2) lattice spacing which indicated the expansion of interlayer spacing by intercalation of EG through solvothermal reaction. The MoS2 intercalated with EG possessed higher 1T phase compared to MoS2 without any intercalation. The obtained MoS2 was applied for room temperature toluene sensing with different relative humidity (RH, 20, 40, 60, 80%). The increase of relative humidity could continuously increase the base resistance and also the sensing response of MoS2 (W) and MoS2 (EG:W). For MoS2 (EG), 60% relative humidity showed the optimum condition for sensing applications. The all MoS2 (W), MoS2 (EG), and MoS2 (EG:W) had good sensing performance at 60% RH with sensing ability (ΔR/Rair%) in 100 ppm toluene around 12.50, 14.53, 16.29%, respectively. 1T-MoS2 is taking a major contribution on the sensing properties of MoS2.

Introduction

Recently, transition metal dichalcogenides (TMDs) have attracted huge attention in gas sensing applications because of their low working temperature (room temperature) [1], [2], [3]. The conventional sensing materials such as ZnO, SnO2, and NiO usually work at high temperatures (>100 °C) [4], [5], [6], which can cause additional problems including high energy cost operations, high probability of gas explosion and combustion, and short lifetime of sensors [7]. Recently, morphological controllable nitrides such as GaN, etc. were also utilized a gas sensing applications [8], [9]. Therefore, room temperature sensing is highly demanded, since the environmental-friendly green chemical process is very important for future applications [10].

Among TMDs materials, molybdenum disulfide (MoS2) has been widely investigated due to its abundance in nature as molybdenite and works as a sensor material at comparatively low temperatures. MoS2 possesses several unique characteristics such as tunable band gap (1.2–1.8 eV), good carrier mobility (~700 cm2V-1s−1), high current on/off ratio (107–108), large optical absorption (~107 m−1 in the visible range) [11]. MoS2 can be classified into 2 phases based on different coordination of molybdenum atoms. These different phases lead to different electronic and also optical properties [12], [13], [14]. 2H-MoS2 with trigonal prismatic coordination possesses semiconducting characteristics, whereas 1T-MoS2 with octahedral coordination possesses metals or semimetals characteristics [15]. Compare to pure 2H-MoS2, 1T or mix phase of 1T/2H-MoS2 is more promising for sensing applications due to its metallic properties which pose higher conductivity (107 times higher than 2H), high charge carrier mobility [16], and small contact resistance for FETs (200–300 Ω·μm at zero gate bias for 1T-MoS2 and 0.7–10 kΩ·μm for 2H-MoS2) [17]. Moreover, abundant active sites on the basal plane of 1T-MoS2 compare to 2H-MoS2 [18] is beneficial for enhancing gas sensor performance. Therefore, the phase manipulation between 2H and 1T structure of MoS2 is important for the improvement of MoS2 gas sensing properties.

Recently the using of MoS2 as a gas sensor is limited to 2H phase only [19], [20], [21]. Until now based on our knowledge, only Yang et al. [22], reported the mix-phase 1T/2H-Mox-1WxS2 for acetone sensing response under room temperature. The formation of 1T/2H (metal/semiconductor) heterojunctions, the alloyed TMD nanosheets with an optimized 1T concentration exhibited much-enhanced gas sensing capability compared to the highly metallic nanosheets or the annealed semiconducting nanosheets with the same chemical composition.

The phase engineered of 1T and 2H-MoS2 could be done by several techniques such as doping on MoS2 layers and intercalation into interlayers [23], [24], [25]. Recently, some researchers have proved that intercalations of MoS2 with some species including alkali metal intercalations (Li+, K+) [26], oxygen intercalations [27], amine intercalations [28], and ammonia intercalations [29], tailored the phase of MoS2. The intercalation could change the phase structure of MoS2 from 2H to 1T due to the ejection of a large number of the electron by the intercalated materials [17], [30], [31].

However, the above intercalation process is quite complex due to the two-step process and also required additive reagent as intercalated materials. Therefore, a new simple and effective technique is required to solve this issue. The solvent intercalation of MoS2 is never been reported to produce 1T-MoS2 or 1T/2H-MoS2. We hypothesize that the reductive solvent is suitable to produce 1T-MoS2 or 1T/2H-MoS2 due to its ability to produce additional electrons to maintain 1T-MoS2 or 1T/2H-MoS2. Among reductive solvents, ethylene glycol is one of the best choices due to its more safety compare to other reductive solvents such as DMF and hydrazine [32]. The MoS2 synthesis using ethylene glycol as a solvent has been yet reported by Wang and Zhou [33] by one step solvothermal reactions. However, the detail explanation about the intercalations, phase engineering, and gas sensing properties has not been reported yet.

The toluene gas was chosen as a gas model to examine the gas sensing ability of MoS2 with different humidity. Toluene (C7H8) is one of the most hazardous pollutants among the volatile organic compounds (VOCs), which is found to be associated with asthma, nasopharyngeal cancer and multiple severe damages on human health because of its wide applications in chemical production and interior decoration [34]. The humidity is expected to influence the properties of MoS2. Several reports have been suggested that the humidity can accept the electron from the MoS2 [35], [36], [37]. The enhancement of the sensing properties by introducing humidity has been done by Su et al. [38], who investigated the toluene sensing of reduce graphene oxide-polyethylene oxide. Therefore, we proposed the new strategy to enhance the sensing properties by adjusting the humidity on the toluene sensing process at room temperature. The influence of ethylene glycol intercalation on the electronic properties, surface properties, and gas sensing properties of MoS2 were also analyzed systematically.

Section snippets

Synthesis MoS2 powder

In the typical process, 0.36 g of Na2MoO4·2H2O and 0.14 g sulphur powder were dissolved in 30 mL of a solvent (distilled water, ethylene glycol, and a mixture of 15 mL water and 15 mL ethylene glycol solvent). After that, the mixture was transferred into a Teflon-sealed autoclave and heated at 180 °C for 24 h. The precipitate was filtered and washed with distilled water and ethanol. The final product was obtained after drying at 60 °C for 6 h. The MoS2 with water, ethylene glycol, and mix water

Structural analysis

Fig. 1(a) shows the XRD patterns of MoS2 obtained in different solvents. The XRD peaks of MoS2 (W) were located at 2θ value of 14.22°, 32.94°, 39.37°, and 58.46°. These peak positions were similar to that of typical MoS2, indicating the formation of MoS2 phase. On the other hand, the use of EG during the synthesis (MoS2 (EG) and MoS2 (EG:W)) shift the peak related to layer stacking ((0 0 2) plane in MoS2) to the lower 2θ angle (9.85°). The shifting of the (0 0 2) diffraction of MoS2 indicates

Conclusions

In this study, the 1T/2H-MoS2 phase has been successfully engineered by using ethylene glycol in the solvothermal synthesis process. The obtained MoS2 possessed ethylene glycol in the interlayers, indicating the “in situ” intercalation occurred during the synthesis. Intercalation led to the increment of the 1T properties of MoS2 due to the change of MoS2 coordination by transferring electron through the reduction process. The ethylene glycol intercalation also leads to increase of the specific

Acknowledgements

This research was partly supported by Japan Society for the Promotion of Science KAKENHI (Grant Number JP16H06439, Grant-in-Aid for Scientific Research on Innovative Areas), the Dynamic Alliance for Open Innovations Bridging Human, Environmental and Materials the Cooperative Research Program of Network Joint Research Center for Materials and Devices and the Hosokawa Powder Technology Foundation.

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