Ppb-Level Hydrogen Sulfide Gas Sensor Based on the Nanocomposite of MoS2 Octahedron/ZnO-Zn2SnO4 Nanoparticles

Hydrogen sulfide (H2S) detection is extremely necessary due to its hazardous nature. Thus, the design of novel sensors to detect H2S gas at low temperatures is highly desirable. In this study, a series of nanocomposites based on MoS2 octahedrons and ZnO-Zn2SnO4 nanoparticles were synthesized through the hydrothermal method. Various characterizations such as X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectrum (XPS) have been used to verify the crystal phase, morphology and composition of synthesized nanocomposites. Three gas sensors based on the nanocomposites of pure ZnO-Zn2SnO4 (MS-ZNO-0), 5 wt% MoS2-ZnO-Zn2SnO4 (MS-ZNO-5) and 10 wt% MoS2-ZnO-Zn2SnO4 (MS-ZNO-10) were fabricated to check the gas sensing properties of various volatile organic compounds (VOCs). It showed that the gas sensor of (MS-ZNO-5) displayed the highest response of 4 to 2 ppm H2S and fewer responses to all other tested gases at 30 °C. The sensor of MS-ZNO-5 also displayed humble selectivity (1.6), good stability (35 days), promising reproducibility (5 cycles), rapid response/recovery times (10 s/6 s), a limit of detection (LOD) of 0.05 ppm H2S (Ra/Rg = 1.8) and an almost linear relationship between H2S concentration and response. Several elements such as the structure of MoS2, higher BET-specific surface area, n-n junction and improvement in oxygen species corresponded to improving response.


Introduction
H 2 S gas, which has a rotten egg smell, could be considered one of the hazardous gases [1] which has an atrocious influence on human health. The long-term exposure to H 2 S gas at low concentrations (25-50 ppm) causes various diseases such as headaches, dizziness, nausea, vomiting and irritation in the eyes, etc. Moreover, high concentrations of H 2 S (more than 120 ppm) exposure may result in acute poisoning, paralysis and even sometimes death [2,3]. Thus, the veracious and real-time detection of H 2 S gas at low temperatures is very decisive, which is enabled by various semiconductor metal oxide (SMO)-based gas sensors.
Several gas sensors based on SMOs such as zinc oxide (ZnO), zinc gallate (ZnGa 2 O 4 ), tin oxide (SnO 2 ), nickel cobaltite (NiCo 2 O 4 ), zinc stannate (Zn 2 SnO 4 ), copper oxide (CuO), etc., possess some good properties such as precision, low cost, small dimensions, longterm stability and environmental friendliness; because of these properties, SMOs could be considered the primary candidates for the detection of toxic gases as well in photo-catalysis, etc. [4][5][6][7][8][9][10]. Plenty of research has shown that some complex metal oxides have been widely used as gas sensors in the last decades. Among these SMOs, an n-type SMOs Zn 2 SnO 4 is an imperative ternary metal oxide with some properties such as high chemical stability and electron mobility, high conductivity, low visible adsorption, etc., have been studied widely in various fields such as photo-catalysis, solar cells and gas sensors [11][12][13]. Furthermore, ZnO, an n-type semiconductor material, has also been studied in various fields. The study described by An et al. showed that the sensor based on Zn 2 SnO 4 detected the highest response to ethanol when compared with H 2 [14]. Additionally, during the study on th sensing properties of ZnO nanosheets and nanorods, it was revealed that the sensor of ZnO nanosheets detected 100 ppm ethanol; by making comparison, it was noted that the response of nanosheets was 4.7-fold that of nanorods [15]. Some other sensors, such as Zn 2 SnO 4 -/ZnO-loaded Pd-based sensors, enhanced H 2 sensing properties; the sensors based on the ZnO-SnO 2 -Zn 2 SnO 4 hetero-junction, Pt-Zn 2 SnO 4 hollow octahedron and Zn 2 SnO 4 /ZnO, revealed better gas sensing performances towards ethanol, acetone and formaldehyde, respectively [16][17][18]. However, pure metal oxides still face some demerits such as low response, high operating temperature, poor selectivity, etc. Therefore, their coupling with 2D materials is essential to increase the gas sensing properties of SMO-based gas sensors.
In the last few years, another research approach based on the emergence of 2D materials into SMOs has received great attention in various fields. Due to some rare properties such as narrow band gap, low density and thermal constancy, these have gained significant attention in the field of photo-catalysis, gas sensing, etc. [19][20][21][22][23][24]. Typical among transition metal dichalcogenides (TMDs), MoS 2 has received gear attention as a fascinating candidate. This is not only as a gas sensor, but MoS2 can also be used for potential applications in the fields of photodetectors, solar cells, etc. [25][26][27]. Consequently, the synthesis of 2D material MoS 2 is essential, which would accelerate the adsorption of oxygen molecules as well as increase gas sensing properties. For instance, various 2D materials and metal oxide-based gas sensors such as MoS 2 -reduced graphene oxide nanohybrid, MoS 2 @MoO 3 magnetic hetero-structure, wool-based carbon fiber/MoS 2 composite and ZnO-MoS 2 nanocomposites were used to detect various types of VOCs, accompanied by high response, good selectivity, rapid response/recovery times, etc. [28][29][30][31].
The purpose of the current study was the detection of hazardous gases. In this regard, a series of gas sensors based on pure ZnO-Zn 2 SnO 4 nanoparticles and octahedron MoS 2 were synthesized via a simple hydrothermal method. To date, no literature has been reported on low-temperature gas sensors, such as 30 • C H 2 S gas sensors based on a MoS 2 -ZnO-Zn 2 SnO 4 nanocomposite . Three gas sensors based on various nanocomposites (MS-ZNO-0, MS-ZNO-5, MS-ZNO-10) were tested to detect different hazardous gases, and our results studied that the highest response of 4 to 2 ppm H 2 S was received by the gas sensor of MS-ZNO-5. Furthermore, it revealed humble selectivity, good stability, rapid response/recovery times and LOD, promising reproducibility and an almost linear relationship between H 2 S concentration and response, suggesting its potential applicability in the field of gas sensors. Hence, the decoration of ZnO-Zn 2 SnO 4 nanoparticles with octahedron MoS 2 is expected to enable the generation of a novel sensor for H 2 S sensing.   [32] in MS-ZNO-0, MS-ZNO-5 and MS-ZNO-10, respectively . The addition of MoS 2 was reasoned to augment the crystallite sizes in both nanocomposites.

Characterizations of Materials
Besides, the presence of MoS 2 in composites was confirmed by other characteristics as well, such as SEM, TEM, EDS, XPS, etc. In order to check the BET-specific surface areas of three samples, N 2 adsorption-desorption isotherms were studied in Figure 1b. The BET surface areas of MS-ZNO-0, MS-ZNO-5 and MS-ZNO-10 were 3.46, 20.15 and 13.16 m 2 /g, respectively. In addition, Figure 1c showed that the average pore sizes of MS-ZNO-0, MZ-ZNO-5 and MS-ZNO-10 were 11.1 nm, 8.1 nm and 11.3 nm, respectively.  As shown in Figure 2a-f, the morphology of the synthesized nanocomposites was observed from the SEM graphs. In Figure 2a,b, the SEM graphs of MS-ZNO-0 have been described. The ZnO-Zn2SnO4 nanoparticles, with an average particle size of 200-250 nm, were evaluated from the SEM images, while their modification with octahedron MoS2 was also confirmed from the SEM graphs of MS-ZNO-5 and MS-ZNO-10 in Figure 2c,d and Figure 2e,f, respectively. The SEM and TEM results showed that the relative particle sizes of ZnO-Zn2SnO4 nanoparticles and octahedron MoS2 were increased by the addition of As shown in Figure 2a-f, the morphology of the synthesized nanocomposites was observed from the SEM graphs. In Figure 2a,b, the SEM graphs of MS-ZNO-0 have been described. The ZnO-Zn 2 SnO 4 nanoparticles, with an average particle size of 200-250 nm, were evaluated from the SEM images, while their modification with octahedron MoS 2 was also confirmed from the SEM graphs of MS-ZNO-5 and MS-ZNO-10 in Figure 2c,d and Figure 2e,f, respectively. The SEM and TEM results showed that the relative particle sizes of ZnO-Zn 2 SnO 4 nanoparticles and octahedron MoS 2 were increased by the addition of MoS 2 contents in the nanocomposite of MS-ZNO-5 but decreased a little in the nanocomposite of MS-ZNO-10, which also corresponds to the XRD results.
MoS2 contents in the nanocomposite of MS-ZNO-5 but decreased a little in the nanocomposite of MS-ZNO-10, which also corresponds to the XRD results.  The XPS data were fitted by XPSPEAK41 software. The X-ray photo-electron spectroscopy (XPS) results were revealed in Figure 4. The full XPS spectrum of the MS-ZNO-5 composite was disclosed in Figure 4a, which revealed the presence of all the elements such as Zn, Sn, O, Mo and S. In the XPS spectrum of Zn 2p (Figure 4b), two peaks, positioned at 1020.7 and 1043.8 eV, corresponded to Zn 2p 3/2 and Zn 2p 1/2 [33]. These results pointed out that Zn ions in the composite have a valence state of "2+". In Figure 4c, the Sn 3d spectrum showed two peaks appearing at 485.6 and 496.1 eV corresponding to Sn 3d 5/2 and Sn 3d 3/2 , respectively [34]. Additionally, one satellite peak at the value of 497.1 eV was cited. The O 1s spectrum of MS-ZNO-5 demonstrated more oxygen adsorption sites (O V ), which was one of the factors required to enhance the gas sensing properties [8]. The peaks at the values of 529.5 and 530.8 eV in Figure 4d were matched to a typical metal-oxygen bond and defect sites in the XPS spectrum of MS-ZNO-5; on the contrary, in the XPS spectrum of MS-ZNO-0, the two peaks were cited at the values of 529.4 and 530.8 [35,36]. In the high-resolution spectra of Mo 3d, four peaks were cited. The peaks at the values of 225.1, 227.9, 231.1, 234.4 and 283.8 eV, shown in Figure 4e, were related to S 2s, Mo 4+ 3d 5/2 , Mo 4+ 3d 3/2 , and Mo 6+ 3d 5/2 , respectively [37]. In Figure 4f, the peaks at the values of 160.8 eV and 162.0 eV were related to S 2p 3/2 and S 2p 1/2 , respectively, in MoS 2 [37].  The XPS data were fitted by XPSPEAK41 software. The X-ray photo-electron spectroscopy (XPS) results were revealed in Figure 4. The full XPS spectrum of the MS-ZNO-5 composite was disclosed in Figure 4a, which revealed the presence of all the elements such as Zn, Sn, O, Mo and S. In the XPS spectrum of Zn 2p (Figure 4b), two peaks, positioned at 1020.7 and 1043.8 eV, corresponded to Zn 2p3/2 and Zn 2p1/2 [33]. These results pointed out that Zn ions in the composite have a valence state of "2+". In Figure 4c, the Sn 3d spectrum showed two peaks appearing at 485.6 and 496.1 eV corresponding to Sn 3d5/2 and Sn 3d3/2, respectively [34]. Additionally, one satellite peak at the value of 497.1 eV was cited. The O 1s spectrum of MS-ZNO-5 demonstrated more oxygen adsorption sites (OV), which was one of the factors required to enhance the gas sensing properties [8]. The peaks at the values of 529.5 and 530.8 eV in Figure 4d were matched to a typical metal-oxygen bond and defect sites in the XPS spectrum of MS-ZNO-5; on the contrary, in the XPS spectrum of MS-ZNO-0, the two peaks were cited at the values of 529.4 and 530.8 [35,36]. In the high-resolution spectra of Mo 3d, four peaks were cited. The peaks at the values of 225.1, 227.9, 231.1, 234.4 and 283.8 eV, shown in Figure 4e, were related to S 2s, Mo 4+ 3d5/2, Mo 4+ 3d3/2, and Mo 6+ 3d5/2, respectively [37]. In Figure 4f, the peaks at the values of 160.8 eV and 162.0 eV were related to S 2p3/2 and S 2p1/2, respectively, in MoS2 [37].

Gas Sensing Properties
In this portion, the gas sensing properties of various sensors were studied. Their deep explanation was as follows: the response/recovery time curves for MS-ZNO-0, MS-ZNO-5 and MS-ZNO-10 were depicted in Figure 5a, which showed that response/recovery times for 2 ppm H2S were 10 s/6 s, (Figure 5b). Importantly, the minimum response of 1.8

Gas Sensing Properties
In this portion, the gas sensing properties of various sensors were studied. Their deep explanation was as follows: the response/recovery time curves for MS-ZNO-0, MS-ZNO-5 and MS-ZNO-10 were depicted in Figure 5a, which showed that response/recovery times for 2 ppm H 2 S were 10 s/6 s, (Figure 5b). Importantly, the minimum response of 1.8 to 0.05 ppm H 2 S was detected. The modification of octahedron MoS 2 with ZnO-Zn 2 SnO 4 nanoparticles not only enhances the BET surface area but also intensifies the adsorption of H 2 S molecules on the surface of the material, and it may also facilitate the adsorption of the oxygen species. These can be some factors which increase the gas sensing properties of nanocomposite (MS-ZNO-5). In Table 1, the gas sensing properties of the current sensor and some previous sensors were studied, which stated that the sensor based on MS-ZNO-5 received lower temperature gas sensing, better response, humble selectivity, LOD and rapid response/recovery times. Figure 5c was the temperature-resistance diagram of MS-ZNO-0, MS-ZNO-5 and MS-ZNO-10 at the operating temperature of 30 • C in air. When the operating temperature increased, the resistance showed a downward trend; this is because, when heating, the O 2− on the surface of both materials adsorbed oxygen and O 2− was converted into O − , which produces a large number of free electrons. At the same time, a large number of electrons reduced the concentration of negative ions on the surface of all the materials; in this way the potential barrier was twisted and the resistance was finally reduced [38,39]. As the temperature continued to enhance, the resistance of materials continued to decrease as the electrons moved faster. Figure 5d described the response vs. temperature curves, which specified that the response was improving with the increase in H 2 S concentrations. Thus, due to the linearity of the current sensor, it could be considered a promising material because of the linear relationship between H 2 S ppm and response. In Figure 5e, three sensors based on synthesized nanocomposites were tested to 2 ppm H 2 S at different operating temperatures. The maximum response detected by the sensor of MS-ZNO-5 was 4 to 2 ppm H 2 S at 30 • C. The decrease in response at higher temperatures could be explained as follows: the chemical activity is low at lower operating temperature, and vice versa. As a result, more gas molecules adsorbed onto the material surface quickly at higher temperatures, resulting in a decrease in response [40]. Figure 6a stated that the highest response to 2 ppm H 2 S was 4 and the second highest response to 2 ppm TMA was 2.5 at the operating temperature of 30 • C; in this way, the humble selectivity (S 2 ppm H2S /S 2 ppm TMA = 1.6) was detected. The results about the reproducibility in Figure 6b demonstrated that the sensor based on MS-ZNO-5 showed promising reproducibility; it was checked around five times at the operating temperature of 30 • C and a similar response was noted. While other sensors detected good reproducibility, our main concern was the gas sensing properties of MS-ZNO-5. For this reason, the promising reproducibility was studied in correspondence with the sensor of MS-ZNO-5. The improvement in gas sensing response corresponded to some aspects such as the octahedron structure of MoS 2 ; this enhanced BET surface area, allowing more oxygen species to adsorb onto the surface of the material and make the reaction faster to increase the gas sensing response [17].
The stability evaluation in Figure 7a explained that the sensors of MS-ZNO-0, MS-ZNO-5 and MS-ZNO-10 were almost stable for approximately 35 days. The resistance of all the sensors decreased slightly with time. To date, the majority of the gas sensors based on semiconductor gas sensors were not satisfied, which demerits their applications in the field of gas sensors. However, in the present case we may see that the long-term stability was quite stable for almost a month. Different responses (3.2, 4, 3.4, and 3.4) to 2 ppm H 2 S were detected by the sensor of MS-ZNO-5 at various relative humidities (RH) of 20, 40, 60, and 80, respectively; the detail was revealed in Figure 7b. The highest response was detected at the 40% RH value. The results explained that the response was decreased after 40% RH due to the presence of the higher amount of water molecules with increased relative humidity; these can be adsorbed onto the surface of the material, and accordingly, the resistance decreases [41]. The results proved that the sensor based on MS-ZNO-5 was noteworthy in all aspects such as high response, humble selectivity, rapid response-recovery times, LOD, good stability, an almost linear relation between H 2 S gas concentration and response, etc.
Molecules 2023, 28, x FOR PEER REVIEW 7 of 14 of MS-ZNO-5 was 4 to 2 ppm H2S at 30 °C. The decrease in response at higher temperatures could be explained as follows: the chemical activity is low at lower operating temperature, and vice versa. As a result, more gas molecules adsorbed onto the material surface quickly at higher temperatures, resulting in a decrease in response [40]. Figure 6a stated that the highest response to 2 ppm H2S was 4 and the second highest response to 2 ppm TMA was 2.5 at the operating temperature of 30 °C; in this way, the humble selectivity (S2 ppm H2S/S2 ppm TMA = 1.6) was detected. The results about the reproducibility in Figure 6b demonstrated that the sensor based on MS-ZNO-5 showed promising reproducibility; it was checked around five times at the operating temperature of 30 °C and a similar response was noted. While other sensors detected good reproducibility, our main concern was the gas sensing properties of MS-ZNO-5. For this reason, the promising reproducibility was studied in correspondence with the sensor of MS-ZNO-5. The improvement in gas sensing response corresponded to some aspects such as the octahedron structure of MoS2; this enhanced BET surface area, allowing more oxygen species to adsorb onto the surface of the material and make the reaction faster to increase the gas sensing response [17]. The stability evaluation in Figure 7a explained that the sensors of MS-ZNO-0, MS-ZNO-5 and MS-ZNO-10 were almost stable for approximately 35 days. The resistance of all the sensors decreased slightly with time. To date, the majority of the gas sensors based on semiconductor gas sensors were not satisfied, which demerits their applications in the field of gas sensors. However, in the present case we may see that the long-term stability

Gas Sensing Mechanism
N-type gas sensing behavior was discussed based on the sensor of MS-ZNO-5 described in Figure 7c. Usually, all SMOs involve three steps, such as (1) adsorption; (2) oxidation; and (3) desorption. When the sensor of MS-ZNO-5 was tested in the air ambiance, the process was similar to our previous work [10]; first, oxygen molecules were adsorbed onto the surface of the material. There, they converted the captured electrons from the conduction band of ZnO-Zn2SnO4 into oxygen ions such as O − , O 2− , and O2 − , while forming the electron depletion layer and enhancing the sensor resistance, which was described in Equations (1)-(4). The adsorbed oxygen species depends on the operating temperatures, as shown in the following equations [42]. When the sensor of MS-ZNO-5 was tested in gas ambiance, the molecules of H2S reacted with oxygen ions and converted them into H2O and SO2, as mentioned in Equation (5). After that, the captured electrons were discharged back, due to the decrease in the resistance and space charge layer [10]. Thus, the sensor of MS-ZNO-5 showed a low resistance in gas atmosphere [43,44]. Further detail about the sensing mechanism has been given in the Equations below.

Gas Sensing Mechanism
N-type gas sensing behavior was discussed based on the sensor of MS-ZNO-5 described in Figure 7c. Usually, all SMOs involve three steps, such as (1) adsorption; (2) oxidation; and (3) desorption. When the sensor of MS-ZNO-5 was tested in the air ambiance, the process was similar to our previous work [10]; first, oxygen molecules were adsorbed onto the surface of the material. There, they converted the captured electrons from the conduction band of ZnO-Zn 2 SnO 4 into oxygen ions such as O − , O 2− , and O 2 − , while forming the electron depletion layer and enhancing the sensor resistance, which was described in Equations (1)-(4). The adsorbed oxygen species depends on the operating temperatures, as shown in the following equations [42]. When the sensor of MS-ZNO-5 was tested in gas ambiance, the molecules of H 2 S reacted with oxygen ions and converted them into H 2 O and SO 2 , as mentioned in Equation (5). After that, the captured electrons were discharged back, due to the decrease in the resistance and space charge layer [10]. Thus, the sensor of MS-ZNO-5 showed a low resistance in gas atmosphere [43,44]. Further detail about the sensing mechanism has been given in the Equations below.
Enhancement in gas sensing response of MS-ZNO-5 may correspond to some parameters due to the attachment of octahedron MoS 2 . Firstly, the formation of the n-n junction can also be one of the factors used to enhance gas sensing properties; the octahedron structure of MoS 2 was checked by SEM and TEM [45]; the octahedron structure developed the exposure of active edge sites of MoS 2 , and improved the efficiency of gas transportation, reaction and carrier exchange, etc. This resulted in the enhanced H 2 S gas sensing properties and increased BET-specific surface area of MS-ZNO-5 (N 2 adsorption-desorption isotherms); this factor can be very helpful to the adsorption and diffusion of H 2 S molecules. Moreover, the attachment with octahedron MoS 2 increased the conductivity of the noteworthy composite [45] and enlarged the number of oxygen species (XPS), etc. This was also one of the parameters used to enhance their performance.

The Synthesis of ZnO-Zn 2 SnO 4 Nanoparticles and MoS 2 -ZnO-Zn 2 SnO 4 Nanocomposite
Hydrothermal was used to synthesize ZnO-Zn 2 SnO 4 nanoparticles and MoS 2 -ZnO-Zn 2 SnO 4 nanocomposites. Concisely, Zn(NO 3 ) 2 . 6H 2 O (8 mmol, 1.477 g), and SnCl 4 .5H 2 O (4 mmol, 0.876 g) were mixed into three different beakers (1, 2 and 3) with 50 mL deionized water while stirring and then different contents of MoS 2 , such as 0 wt% MoS 2 (MS-ZNO-0), 5 wt% MoS 2 (MS-ZNO-5) and 10 wt% MoS 2 (MS-ZNO-10), were added into beaker numbers 1, 2 and 3, respectively. After half an hour, 2M NaOH solution was gradually added to all the beakers to adjust the pH to about 12. The stirring process was carried out for 24 h to thoroughly mix all the ingredients and to receive the milky solutions. Furthermore, there was an autoclave process in which the samples were transferred into 100 mL stainless steel autoclaves and placed into an oven, and the time (20 h) and temperature (190 • C) were adjusted. After the autoclave process, the powder materials were separated by centrifugation process (washing with ethanol and DI water, 8000 rpm) and drying process (heating 60 • C, 20 h). Finally, the white products were calcined at 300 • C, 3 h and 5 • C/min. After that, the fabrication of gas sensors for gas sensing performances and other characteristics was carried out to check crystallite size, morphology and other properties of all the samples. The dried samples after calcination were ground in a mortar for fabricating the sensors and also for other characterizations such as XRD, SEM, TEM, etc.

Characterizations of the Nanocomposites
Numerous characterizations have been used to identify the crystal size, BET-specific surface area, morphology and surface properties of synthesized products. Their deep explanation has been studied as follows: X-ray diffraction (

Fabrication of Gas Sensor
The gas sensor diagram and the electrical circuit were displayed in Figure 8. The fabrication process of the gas sensor was studied as follows: firstly, the paste was made with 0.2 g nanocomposite and 2-3 drops of terpineol and then the mixture was coated onto the outer surface of the alumina tube. After that, the alumina tube was heated in an oven for 10 h at 80 • C to remove the contents of terpineol. Then, a Ni-Cr alloy wire was placed in the alumina tube to control the operating temperature in the range of 30-400 • C. All the hazardous gases detected in the present work were bought from the Dalian Haide Technology Company Limited (Dalian China). The gas sensor based on MoS 2 -ZnO-Zn 2 SnO 4 nanocomposites showed n-type gas sensor behavior and the response was calculated as S = R a /R g , where R a was the resistance in air and R g was the resistance in the gas. The selectivity of the sensor was calculated in this study, which may be defined as the ratio of the highest response and second highest response; in the present case, it was 'S 10ppm H2S /S 10ppm TMA = 1.6', and likewise response/recovery times were stated as the time taken to reach 90% value of the final signal. The sensors were stable for 35 days and reproducible for five cycles.

Characterizations of the Nanocomposites
Numerous characterizations have been used to identify the crystal size, BET-specific surface area, morphology and surface properties of synthesized products. Their deep explanation has been studied as follows: X-ray diffraction (XRD, D/MAX-Ultima, Cu Kα source, 2°/min scanning rate, scanning angle from 10°-80° as well as power of 40 kV and 40 mA, Rigaku, Tokyo, Japan), BET method (ASAP2010C instrument, Norcross, GA, USA), scanning electron microscopy (SEM, Suppa 55 Sapphire, Carl Zeiss AG, Jena, Germany), transmission electron microscopy (TEM, JEM-3200FS, JEOL, Tokyo, Japan), energy-dispersive X-ray spectroscopy (EDS, Sapphire 55 Supra, Zeiss, Jena, Germany) and X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250 XI, Ther-moFisher Scientific, Waltham, MA, USA) were used to check the crystal size, BET-specific surface area, morphology and surface properties of synthesized products. These

Fabrication of Gas Sensor
The gas sensor diagram and the electrical circuit were displayed in Figure 8. The fabrication process of the gas sensor was studied as follows: firstly, the paste was made with 0.2 g nanocomposite and 2-3 drops of terpineol and then the mixture was coated onto the outer surface of the alumina tube. After that, the alumina tube was heated in an oven for 10 h at 80 °C to remove the contents of terpineol. Then, a Ni-Cr alloy wire was placed in the alumina tube to control the operating temperature in the range of 30-400 °C. All the hazardous gases detected in the present work were bought from the Dalian Haide Technology Company Limited (Dalian China). The gas sensor based on MoS2-ZnO-Zn2SnO4 nanocomposites showed n-type gas sensor behavior and the response was calculated as S = Ra/Rg, where Ra was the resistance in air and Rg was the resistance in the gas. The selectivity of the sensor was calculated in this study, which may be defined as the ratio of the highest response and second highest response; in the present case, it was 'S10ppm H2S/S10ppm TMA = 1.6 , and likewise response/recovery times were stated as the time taken to reach 90% value of the final signal. The sensors were stable for 35 days and reproducible for five cycles.

Conclusions
The pure ZnO-Zn2SnO4 nanoparticles and nanocomposites were synthesized via a hydrothermal method. The synthesized materials were characterized using XRD, BET

Conclusions
The pure ZnO-Zn 2 SnO 4 nanoparticles and nanocomposites were synthesized via a hydrothermal method. The synthesized materials were characterized using XRD, BET method, SEM, TEM, EDS and XPS, respectively. The XRD and SEM results may relate to each other, and we studied to see why the crystallite size and particle were increased when ZnO-Zn 2 SnO 4 nanoparticles were attached with octahedron MoS 2 , respectively. From BET and XPS results, it was concluded that the MS-ZNO-5 nanocomposite revealed higher BET-specific surface area and more adsorption of oxygen species than MS-ZNO-0, which could be the main factors enhancing the gas sensing properties. The gas sensing properties of three gas sensors based on MS-ZNO-0, MS-ZNO-5 and MS-ZNO-10 were studied. The gas sensor based on an MS-ZNO-5 nanocomposite detected the highest response to 2 ppm H 2 S, and humble selectivity, rapid response/recovery time, good stability, promising reproducibility and LOD (0.05 ppm) were noticed. Furthermore, the sensor of MS-ZNO-0 and MS-ZNO-10 detected far fewer responses towards all gases. The enhancement in gas sensing response of MS-ZNO-5 corresponded with some parameters such as layered structure, n-n junction, higher BET surface area, more adsorption of oxygen species, etc. An almost linear relation between response and concentration of H 2 S (0.05-2 ppm) could allow the current sensor to be considered a potential candidate for gas sensing applications in the detection of and warning about leakage of hazardous VOCs.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The data can be provided on the responsible request to the corresponding author.