Silver Anchored Polyaniline@Molybdenum Disulfide Nanocomposite (Ag/Pani@MoS2) for Highly Efficient Ammonia and Methanol Sensing under Ambient Conditions: A Mechanistic Approach

We report the synthesis of silver anchored and para toluene sulfonic acid (pTSA) doped polyaniline/molybdenum disulfide nanocomposite (pTSA/Ag-Pani@MoS2) for highly reproducible room temperature detection of ammonia and methanol. Pani@MoS2 was synthesized by in situ polymerization of aniline in the presence of MoS2 nanosheets. The chemical reduction of AgNO3 in the presence of Pani@MoS2 led to the anchoring of Ag to Pani@MoS2 and finally doping with pTSA produced highly conductive pTSA/Ag-Pani@MoS2. Morphological analysis showed Pani-coated MoS2 along with the observation of Ag spheres and tubes well anchored to the surface. Structural characterization by X-ray diffraction and X-ray photon spectroscopy showed peaks corresponding to Pani, MoS2, and Ag. The DC electrical conductivity of annealed Pani was 11.2 and it increased to 14.4 in Pani@MoS2 and finally to 16.1 S/cm with the loading of Ag. The high conductivity of ternary pTSA/Ag-Pani@MoS2 is due to Pani and MoS2 π–π* interactions, conductive Ag, as well as the anionic dopant. The pTSA/Ag-Pani@MoS2 also showed better cyclic and isothermal electrical conductivity retention than Pani and Pani@MoS2, owing to the higher conductivity and stability of its constituents. The ammonia and methanol sensing response of pTSA/Ag-Pani@MoS2 showed better sensitivity and reproducibility than Pani@MoS2 owing to the higher conductivity and surface area of the former. Finally, a sensing mechanism involving chemisorption/desorption and electrical compensation is proposed.


Introduction
Nanomaterials that undergo a change in physical or chemical characteristics, such as color, optical properties, electrical conductivity, etc., on exposure to various gases, can be effectively used to monitor different types of volatile organic compounds (VOCs), toxic gases, environmental humidity, etc. [1,2]. Time and conductivity indicators can reliability track the nature of emitted gases and their toxicity, which is a vital function of sensors. In food samples, especially fish and meat, the humidity and biogenic amines present can also be related to food freshness or spoilage [3]. For example, the concentration of amines has been reported to increase from 130 to 330 ppm with the aging of fish samples and thus levels of gases and amines are a direct indicator of food freshness [4]. Apart from these, the environmental pollution caused by the release of toxic gases, such as NO x , SO x , Cox, etc., from vehicle fuel combustion and gas leakage from industrial plants and laboratories, is not only a threat to human health but also to animals and plants [5]. Thus large-scale, portable sensing devices are much needed to selectively monitor VOCs and gas leakages, as well as to predict the freshness of food items. microscopy (SEM; JSM7600F, JEOL, Tokyo, Japan) and transmission electron microscopy (JEOL ARM-200F, HRTEM, Tokyo, Japan).

DC Electrical Conductivity Retention and Ammonia Sensing Studies
DC electrical conductivity was measured by a 4-in-line probe device connected to a temperature controller (PID-200). The calculations were performed by using the following equations: G7 (W/S) = ln2 (2S/W) Here, G7(W/S)-the correction factor-depends on sample thickness and probe spacing; I is current (A), V is the voltage (V), W is sample thickness (cm), and S is probe spacing (cm). The ρ is the resistivity, ρo is the specific resistivity (Ω-cm), and σ is the conductivity (S/cm) [18].
The thermal stability of the electrical conductivities of pTSA/Pani, pTSA/Pani@MoS 2, and pTSA/Ag-Pani@MoS 2 was analyzed using cyclic and isothermal heating experiments and subsequent measurements of DC electrical conductivity. For isothermal studies, the pellet of the nanocomposite was heated to temperatures of 50, 70, 90, 110, and 130 • C for 40 min, and conductivity values were recorded at intervals of 10 min. In the cyclic studies, the nanocomposite pellet was heated from 40-150 • C five times at an interval of 1 h [19]. The details of the gas sensing set-up and studies can be seen elsewhere [20]. Prior to the cyclic, isothermal and gas sensing measurements the samples were annealed at 150 • C for 2 h.

Fabrication of pTSA/Ag-Pani@MoS 2
For the synthesis of pTSA/Ag-Pani@MoS 2 , firstly, MoS 2 nanosheets were prepared, then coated with Pani to create Pani@MoS 2, and finally, Ag was anchored onto Pani@MoS 2 to produce pTSA/Ag-Pani@MoS 2 . For the synthesis of MoS 2 nanosheets, 0.23 g of MoO 3 and 0.53 g of thiourea were stirred in 70 mL of water for 1 h, and then the whole reaction mixture was charged into a 100 mL Teflon-lined hydrothermal reactor, and subsequently heated at 200 • C for 20 h. The resulting black MoS 2 precipitate was separated by centrifugation, washed with solvents (water and ethanol), and subsequently dried at 80 • C for 12 h. The Pani@MoS 2 was synthesized by in situ oxidative polymerization of aniline in the presence of MoS 2 . In a typical process, 0.5 g of MoS 2 was vigorously stirred in 100 mL of 1M HCl solution with the addition of 0.25 g of CTAB and 5 mL of aniline. The whole reaction mixture was constantly stirred for 1 hr for the proper adsorption of aniline over MoS 2 . Subsequently, the oxidant solution (7.4 g of PPs in 100 mL 1M HCl) was added to the above mixture to initiate the polymerization process. The whole reaction mixture was constantly stirred for 24 h, followed by centrifugation to collect MoS 2 @Pani and subsequent washing with solvents (water and ethanol). Thus, prepared wet Pani@MoS 2 was further doped with pTSA by stirring in pTSA solution (1 g pTSA in 100 mL water) for 30 min, followed by separation with centrifugation and subsequent drying at 80 • C for 12 h. For the Ag-MoS 2 @Pani, the wet MoS 2 @Pani was stirred in 100 mL water, with the subsequent addition of 0.30 g of ascorbic acid, followed by the addition of 0.5 g of AgNO 3 , leading to the precipitation of Ag over MoS 2 @Pani. The prepared Ag-Pani@MoS 2 was washed with solvents and subsequently doped with pTSA as per the procedure described earlier.

Results and Discussion
A hydrothermal methodology was employed for the MoS 2 sheets and its composite with Pani was obtained by in situ oxidative polymerization of aniline in its presence. Finally, Ag nanoparticles were anchored onto the Pani@MoS 2 by the reduction of AgNO 3 . This synthesized pTSA/Ag-Pani@MoS 2 nanocomposite is expected to show a strong electrical response and gas sensing characteristics owing to the additional effect of the conductive Pani, MoS 2, and Ag. Figure 1 presents a schematic diagram of the synthesis of pTSA/Ag-Pani@MoS 2 nanocomposites. 30 min, followed by separation with centrifugation and subsequent drying at 80 °C for 12 h. For the Ag-MoS2@Pani, the wet MoS2@Pani was stirred in 100 mL water, with the subsequent addition of 0.30 g of ascorbic acid, followed by the addition of 0.5 g of AgNO3, leading to the precipitation of Ag over MoS2@Pani. The prepared Ag-Pani@MoS2 was washed with solvents and subsequently doped with pTSA as per the procedure described earlier.

Results and Discussion
A hydrothermal methodology was employed for the MoS2 sheets and its composite with Pani was obtained by in situ oxidative polymerization of aniline in its presence. Finally, Ag nanoparticles were anchored onto the Pani@MoS2 by the reduction of AgNO3. This synthesized pTSA/Ag-Pani@MoS2 nanocomposite is expected to show a strong electrical response and gas sensing characteristics owing to the additional effect of the conductive Pani, MoS2, and Ag. Figure 1 presents a schematic diagram of the synthesis of pTSA/Ag-Pani@MoS2 nanocomposites.

Morphological Analysis
The SEM analysis of pTSA/Pani, pTSA/Pani@MoS2, and pTSA/Ag-Pani@MoS2 is presented in Figure 2. The pTSA/Pani shows mostly interconnected fibrous structures with the presence of other structures, such as stacked globules and sheets. As interpreted by the naked eye, the Pani fibers seem to be of two types: either smooth and thin, or thick and rough. It can be interpreted that during the rapid mixing technique, some fibers adsorbed more aniline or oxidant during their genesis and grew more along the radial axis, becoming thicker, or generating other structures. Apart from this, some porosity can also be seen between different agglomerates. In the case of pTSA/Pani@MoS2, large floating sheets of MoS2, Pani tubes, and Pani coating over MoS2 covering large areas, as well as small agglomerates of Pani, can be seen. In the case of ternary pTSA/Ag-Pani@MoS2, Panicoated MoS2 is visible, along with Ag spheres and tubes well anchored to the surface. The SEM images of pure MoS2 sheets is presented in Figure S1.
The TEM analysis showed well dispersed Pani and Ag inside/on the MoS2 sheets ( Figure 3). It can be clearly seen that all the constituents are well linked, which is anticipated to create an interconnected network for charge transfer. The dimensions of the MoS2 platform are in the range of micrometers and Pani tubes of diameter ~100 nm. Ag nanoparticles are in the range of 20-30 nm in diameter, except for a few larger particles in the 60-100 nm range. The EDAX analysis showed the presence of C, O, N, Mo, S, and Ag ( Figure 4) while the elemental mapping showed their uniform mixing, thereby suggesting the efficacy of the synthesis methodology.

Morphological Analysis
The SEM analysis of pTSA/Pani, pTSA/Pani@MoS 2, and pTSA/Ag-Pani@MoS 2 is presented in Figure 2. The pTSA/Pani shows mostly interconnected fibrous structures with the presence of other structures, such as stacked globules and sheets. As interpreted by the naked eye, the Pani fibers seem to be of two types: either smooth and thin, or thick and rough. It can be interpreted that during the rapid mixing technique, some fibers adsorbed more aniline or oxidant during their genesis and grew more along the radial axis, becoming thicker, or generating other structures. Apart from this, some porosity can also be seen between different agglomerates. In the case of pTSA/Pani@MoS 2 , large floating sheets of MoS 2 , Pani tubes, and Pani coating over MoS 2 covering large areas, as well as small agglomerates of Pani, can be seen. In the case of ternary pTSA/Ag-Pani@MoS 2 , Pani-coated MoS 2 is visible, along with Ag spheres and tubes well anchored to the surface. The SEM images of pure MoS 2 sheets is presented in Figure S1.
The TEM analysis showed well dispersed Pani and Ag inside/on the MoS 2 sheets ( Figure 3). It can be clearly seen that all the constituents are well linked, which is anticipated to create an interconnected network for charge transfer. The dimensions of the MoS 2 platform are in the range of micrometers and Pani tubes of diameter~100 nm. Ag nanoparticles are in the range of 20-30 nm in diameter, except for a few larger particles in the 60-100 nm range. The EDAX analysis showed the presence of C, O, N, Mo, S, and Ag ( Figure 4) while the elemental mapping showed their uniform mixing, thereby suggesting the efficacy of the synthesis methodology.

XRD
The XRD of pTSA/Pani, pTSA/Pani@MoS2, and pTSA/Ag-Pani@MoS2 is presented in Figure 5. Pure Pani shows semi-crystalline features due to the presence of benzenoid and quinonoid groups with the observance of three diffraction peaks at 15.7, 20.7, and 25.2 2θ [21]. Padmapriya et al. [22] showed that the peaks at 15.7, 20.7, and 25.2 2θ correspond to (121), (113), and (322) crystal planes, thereby suggesting that most of the Pani is oriented along them. The pTSA/Pani@MoS2 showed peaks at 14.58, 29, 39.76, and 43 2θ, corresponding to MoS2 [23]. In the case of pTSA/Ag-Pani@MoS2, apart from MoS2 peaks, the peaks 38.13, and 77.67 2θ correspond to the (111) and (311) crystallographic planes of Ag nanoparticles with a cubic structure (JCPDS card No. 03-065-2871) [24]. The peaks of Pani are also not distinct, owing to the high-intensity peaks of MoS2 and Ag, which suppress the peaks of Pani along the Y axis of the graph due to the presented scale.

XRD
The XRD of pTSA/Pani, pTSA/Pani@MoS 2, and pTSA/Ag-Pani@MoS 2 is presented in Figure 5. Pure Pani shows semi-crystalline features due to the presence of benzenoid and quinonoid groups with the observance of three diffraction peaks at 15.7, 20.7, and 25.2 2θ [21]. Padmapriya et al. [22] showed that the peaks at 15.7, 20.7, and 25.2 2θ correspond to (121), (113), and (322) crystal planes, thereby suggesting that most of the Pani is oriented along them. The pTSA/Pani@MoS 2 showed peaks at 14.58, 29, 39.76, and 43 2θ, corresponding to MoS 2 [23]. In the case of pTSA/Ag-Pani@MoS 2 , apart from MoS 2 peaks, the peaks 38.13, and 77.67 2θ correspond to the (111) and (311) crystallographic planes of Ag nanoparticles with a cubic structure (JCPDS card No. 03-065-2871) [24]. The peaks of Pani are also not distinct, owing to the high-intensity peaks of MoS 2 and Ag, which suppress the peaks of Pani along the Y axis of the graph due to the presented scale.

XRD
The XRD of pTSA/Pani, pTSA/Pani@MoS2, and pTSA/Ag-Pani@MoS2 is presented in Figure 5. Pure Pani shows semi-crystalline features due to the presence of benzenoid and quinonoid groups with the observance of three diffraction peaks at 15.7, 20.7, and 25.2 2θ [21]. Padmapriya et al. [22] showed that the peaks at 15.7, 20.7, and 25.2 2θ correspond to (121), (113), and (322) crystal planes, thereby suggesting that most of the Pani is oriented along them. The pTSA/Pani@MoS2 showed peaks at 14.58, 29, 39.76, and 43 2θ, corresponding to MoS2 [23]. In the case of pTSA/Ag-Pani@MoS2, apart from MoS2 peaks, the peaks 38.13, and 77.67 2θ correspond to the (111) and (311) crystallographic planes of Ag nanoparticles with a cubic structure (JCPDS card No. 03-065-2871) [24]. The peaks of Pani are also not distinct, owing to the high-intensity peaks of MoS2 and Ag, which suppress the peaks of Pani along the Y axis of the graph due to the presented scale.

XPS
The elemental details, functional groups, and their interactions were analyzed by XPS ( Figure 6). The full-range survey scan showed the presence of C, O, N, Cl, and Ag without the observation of any other impurities ( Figure S2). The carbon peak is due to the residual carbon from the sample and the instrument. The C1s peaks can be deconvoluted into three peaks at 284.61, 285.8, and 288.21 eV, corresponding to sp2 and sp3-hybridized carbon (C-C, C=C), C-N and C=O, respectively [25][26][27]. The N1s peak shows peaks at 399, 400.28, and 402.90 eV, corresponding to the quinoid imine (-N=), benzenoid amine (-NH−), and the positively charged polaron species (N+), respectively [28]. The peaks at 229.8 and 232.5 eV correspond to Mo 4+ 3d5/2 and Mo 4+ 3d3/2, while the peaks at 161.9 and 163.2 eV are assigned to S2-2p3/2 and S2-2p1/2 of MoS 2 , respectively [29]. The Ag peak can be deconvoluted into the peaks at 367.90 and 373.90 eV and is attributed to the metallic silver Ag (0), thus confirming its successful deposition on pTSA/Ag-Pani@MoS 2 [30].

XPS
The elemental details, functional groups, and their interactions were analyzed by XPS ( Figure 6). The full-range survey scan showed the presence of C, O, N, Cl, and Ag without the observation of any other impurities ( Figure S2). The carbon peak is due to the residual carbon from the sample and the instrument. The C1s peaks can be deconvoluted into three peaks at 284.61, 285.8, and 288.21 eV, corresponding to sp2 and sp3-hybridized carbon (C-C, C=C), C-N and C=O, respectively [25][26][27]. The N1s peak shows peaks at 399, 400.28, and 402.90 eV, corresponding to the quinoid imine (−N=), benzenoid amine (−NH−), and the positively charged polaron species (N+), respectively [28]. The peaks at 229.8 and 232.5 eV correspond to Mo 4+ 3d5/2 and Mo 4+ 3d3/2, while the peaks at 161.9 and 163.2 eV are assigned to S2−2p3/2 and S2−2p1/2 of MoS2, respectively [29]. The Ag peak can be deconvoluted into the peaks at 367.90 and 373.90 eV and is attributed to the metallic silver Ag (0), thus confirming its successful deposition on pTSA/Ag-Pani@MoS2 [30].

DC Electrical Conductivity
The DC electrical conductivity of the pTSA/Pani, pTSA/Pani@MoS2, and pTSA/Ag-Pani@MoS2 was measured by a 4-in-line probe as presented in Figure 7a. The doping acids were pTSA and HCl, as CTAB cannot act as a dopant due to its large cationic size [31]. The electrical conductivity of unannealed pTSA/Pani, pTSA/Pani@MoS2, and pTSA/Ag-Pani@MoS2 at room temperature is 22.6, 23.1, and 26.4 S/cm, respectively, and thus the composites show much higher conductivity than our previously reported mineral acid doped composites [19]. The annealed samples at 150 °C showed slightly lower electrical conductivities i.e. 11.2, 14.4 and 16.1 S/cm for pTSA/Pani, pTSA/Pani@MoS2, and pTSA/Ag-Pani@MoS2 respectively which might be due to the loss of moisture and impurities. The reason for the higher electrical conductivity here in contrast to mineral acid doped Pani is due to the high charge density, owing to the presence of HCl and pTSA, decreased hopping or tunneling of charge carriers, and higher metallic regions in the composite [2]. The incorporation of MoS2 and Ag in Pani also leads to higher electrical conductivity in ternary pTSA/Ag-Pani@MoS2 due to the synergistic or additional effect of Pani, MoS2, and Ag, since both MoS2 and Ag have been reported to increase electrical conductivity. Kim et al. [16] showed that Pani@MoS2 is more conductive than pure Pani, due to the increase in the compactness of Pani on the incorporation of MoS2. The aligned Pani fibers or coating over MoS2 results in an increase in the compactness of Pani in pTSA/Pani@MoS2 and pTSA/Ag-Pani@MoS2. Another reason is the π-π* interactions between Pani and MoS2 lead to the greater mobility of charge carriers in pTSA/Pani@MoS2 and pTSA/Ag-Pani@MoS2. Similarly, Xia et al. [32] reported that Ag can successfully enhance the electrical conductivity of Pani and thus the presence of both MoS2 and Ag will significantly increase its

DC Electrical Conductivity
The DC electrical conductivity of the pTSA/Pani, pTSA/Pani@MoS 2, and pTSA/Ag-Pani@MoS 2 was measured by a 4-in-line probe as presented in Figure 7a. The doping acids were pTSA and HCl, as CTAB cannot act as a dopant due to its large cationic size [31]. The electrical conductivity of unannealed pTSA/Pani, pTSA/Pani@MoS 2 , and pTSA/Ag-Pani@MoS 2 at room temperature is 22.6, 23.1, and 26.4 S/cm, respectively, and thus the composites show much higher conductivity than our previously reported mineral acid doped composites [19]. The annealed samples at 150 • C showed slightly lower electrical conductivities i.e., 11.2, 14.4 and 16.1 S/cm for pTSA/Pani, pTSA/Pani@MoS 2, and pTSA/Ag-Pani@MoS 2 respectively which might be due to the loss of moisture and impurities. The reason for the higher electrical conductivity here in contrast to mineral acid doped Pani is due to the high charge density, owing to the presence of HCl and pTSA, decreased hopping or tunneling of charge carriers, and higher metallic regions in the composite [2]. The incorporation of MoS 2 and Ag in Pani also leads to higher electrical conductivity in ternary pTSA/Ag-Pani@MoS 2 due to the synergistic or additional effect of Pani, MoS 2, and Ag, since both MoS 2 and Ag have been reported to increase electrical conductivity. Kim et al. [16] showed that Pani@MoS 2 is more conductive than pure Pani, due to the increase in the compactness of Pani on the incorporation of MoS 2 . The aligned Pani fibers or coating over MoS 2 results in an increase in the compactness of Pani in pTSA/Pani@MoS 2 and pTSA/Ag-Pani@MoS 2 . Another reason is the π-π* interactions between Pani and MoS 2 lead to the greater mobility of charge carriers in pTSA/Pani@MoS 2 and pTSA/Ag-Pani@MoS 2 . Similarly, Xia et al. [32] reported that Ag can successfully enhance the electrical conductivity of Pani and thus the presence of both MoS 2 and Ag will significantly increase its conductivity, as obtained in our analyses. Another reason is that the in situ polymerization of aniline over MoS 2 provides much greater surface areas of Pani, leading to a much extended and conjugated system, which subsequently produces a much greater charge carrier density. Figure 7b represents the movement of charge carriers (through Pani, through Ag, π-π* interactions between Pani and MoS 2 , through Pani and Ag, etc.). conductivity, as obtained in our analyses. Another reason is that the in situ polymerization of aniline over MoS2 provides much greater surface areas of Pani, leading to a much extended and conjugated system, which subsequently produces a much greater charge carrier density. Figure 7b represents the movement of charge carriers (through Pani, through Ag, π-π* interactions between Pani and MoS2, through Pani and Ag, etc.).   The testing of DC electrical conductivity of pTSA/Ag-Pani@MoS 2 through the cyclic aging technique showed an increase in conductivity at higher temperatures in all cycles ( Figure 8). Conductivity decreased in all cycles, however, from the second cycle onwards, when the rise in conductivity at higher temperature was much more uniform. The reason for the loss of conductivity is the loss of dopant acids (HCl and pTSA), moisture, and other volatile components. However, it is expected that considerable ordered chain alignment occurs at higher temperatures, leading to uniformity in electrical properties. The increase in electrical conductivity with increasing temperature is due to normal thermal activation, as the mobility of charged carriers increases via the conductive chains of Pani, the π-π* interactions between Pani and MoS 2, and through conductive Ag. The loss of electrical conductivity decreased in subsequent cycles, with the third and fourth cycles showing much smaller losses (0.77 to 0.53 S/cm, respectively), thereby suggesting an increase in semiconducting properties on annealing at higher temperatures. The testing of DC electrical conductivity of pTSA/Ag-Pani@MoS2 through the cyclic aging technique showed an increase in conductivity at higher temperatures in all cycles ( Figure 8). Conductivity decreased in all cycles, however, from the second cycle onwards, when the rise in conductivity at higher temperature was much more uniform. The reason for the loss of conductivity is the loss of dopant acids (HCl and pTSA), moisture, and other volatile components. However, it is expected that considerable ordered chain alignment occurs at higher temperatures, leading to uniformity in electrical properties. The increase in electrical conductivity with increasing temperature is due to normal thermal activation, as the mobility of charged carriers increases via the conductive chains of Pani, the π-π* interactions between Pani and MoS2, and through conductive Ag. The loss of electrical conductivity decreased in subsequent cycles, with the third and fourth cycles showing much smaller losses (0.77 to 0.53 S/cm, respectively), thereby suggesting an increase in semiconducting properties on annealing at higher temperatures. The DC electrical conductivity retention of annealed pTSA/Ag-Pani@MoS2 was plotted as the change in the DC electrical conductivity divided by the duration of the experiment (30 min) [18]. The relative loss in conductivity of pTSA/Ag-Pani@MoS2 decreased with temperature, but the loss was very much similar (in the range of 17 × 10 −3 to 59 × 10 −3 ), thereby confirming the semiconducting nature of pTSA/Ag-Pani@MoS2 under aging conditions ( Figure 9).
The DC retention studies showed much higher conductivity and stability of pTSA/Ag-Pani@MoS2 at higher temperatures when compared with Pani composites doped with mineral acids (HCl, H2SO4, HNO3, etc.). Here the pTSA/Ag-Pani@MoS2 showed no loss in conductivity even at 150 °C, in comparison to previous reports that show a loss in conductivity around 120 °C. Given the minimal loss at higher temperatures, it can be interpreted that pTSA has a major effect on conductivity, leading to a nanocomposite with high electrical stability. The DC electrical conductivity retention of annealed pTSA/Ag-Pani@MoS 2 was plotted as the change in the DC electrical conductivity divided by the duration of the experiment (30 min) [18]. The relative loss in conductivity of pTSA/Ag-Pani@MoS 2 decreased with temperature, but the loss was very much similar (in the range of 17 × 10 −3 to 59 × 10 −3 ), thereby confirming the semiconducting nature of pTSA/Ag-Pani@MoS 2 under aging conditions ( Figure 9).
The DC retention studies showed much higher conductivity and stability of pTSA/Ag-Pani@MoS 2 at higher temperatures when compared with Pani composites doped with mineral acids (HCl, H 2 SO 4 , HNO 3 , etc.). Here the pTSA/Ag-Pani@MoS 2 showed no loss in conductivity even at 150 • C, in comparison to previous reports that show a loss in conductivity around 120 • C. Given the minimal loss at higher temperatures, it can be interpreted that pTSA has a major effect on conductivity, leading to a nanocomposite with high electrical stability.

Vapor Sensing Studies
The vapor sensing of pTSA/Ag-Pani@MoS2 for ammonia and methanol was studied in the range of 0.1-1 M for 40 s (Figure 10). A sharp decrease in the conductivity was observed at high concentration but the sensitivity decreased with the decrease in concentration. This response results suggests that a concentration of ammonia and methanol even lower than 0.1 M can be efficiently sensed by pTSA/Ag-Pani@MoS2. The percentage sensing response (SR) was calculated by [33]: SR (%) = (Change in electrical conductivity/Initial conductivity) × 100 (5) The percentage sensing response of pTSA/Ag-Pani@MoS2 and pTSA/Pani@MoS2 for 1 M ammonia was 70.48 and 53.70% and for methanol, 51.17 and 38.62%, respectively. This suggests that both pTSA/Ag-Pani@MoS2 and pTSA/Pani@MoS2 are more responsive towards ammonia, with pTSA/Ag-Pani@MoS2 showing much higher sensitivity towards both ammonia and methanol.

Vapor Sensing Studies
The vapor sensing of pTSA/Ag-Pani@MoS 2 for ammonia and methanol was studied in the range of 0.1-1 M for 40 s (Figure 10). A sharp decrease in the conductivity was observed at high concentration but the sensitivity decreased with the decrease in concentration. This response results suggests that a concentration of ammonia and methanol even lower than 0.1 M can be efficiently sensed by pTSA/Ag-Pani@MoS 2 . The percentage sensing response (SR) was calculated by [33]: SR (%) = (Change in electrical conductivity/Initial conductivity) × 100 (5)

Vapor Sensing Studies
The vapor sensing of pTSA/Ag-Pani@MoS2 for ammonia and methanol was studied in the range of 0.1-1 M for 40 s (Figure 10). A sharp decrease in the conductivity was observed at high concentration but the sensitivity decreased with the decrease in concentration. This response results suggests that a concentration of ammonia and methanol even lower than 0.1 M can be efficiently sensed by pTSA/Ag-Pani@MoS2. The percentage sensing response (SR) was calculated by [33]: SR (%) = (Change in electrical conductivity/Initial conductivity) × 100 (5) The percentage sensing response of pTSA/Ag-Pani@MoS2 and pTSA/Pani@MoS2 for 1 M ammonia was 70.48 and 53.70% and for methanol, 51.17 and 38.62%, respectively. This suggests that both pTSA/Ag-Pani@MoS2 and pTSA/Pani@MoS2 are more responsive towards ammonia, with pTSA/Ag-Pani@MoS2 showing much higher sensitivity towards both ammonia and methanol. The percentage sensing response of pTSA/Ag-Pani@MoS 2 and pTSA/Pani@MoS 2 for 1 M ammonia was 70.48 and 53.70% and for methanol, 51.17 and 38.62%, respectively. This suggests that both pTSA/Ag-Pani@MoS 2 and pTSA/Pani@MoS 2 are more responsive towards ammonia, with pTSA/Ag-Pani@MoS 2 showing much higher sensitivity towards both ammonia and methanol.
On account of the higher sensitivity of pTSA/Ag-Pani@MoS 2 in relation to both ammonia and methanol, its stability was tested for long-term exposure to vapors. Figure 11 shows that on exposure to ammonia and methanol, pTSA/Ag-Pani@MoS 2 attained a steady state in~45 s, with conductivity in the case of ammonia decreasing from 16.64 to 4.12 S/cm and for methanol from 16.68 to 7.88 S/cm. On further exposure over a subsequent 60 s, very little drop in conductivity was observed, as indicated by a nearly horizontal line in the graph. Furthermore, on flushing the sample with air, the conductivity acquired close to its original value in both ammonia-and methanol-exposed samples. However, it can be observed that in the case of ammonia-and methanol-exposed samples, a 7.3 and 2.5% loss in conductivity was observed, respectively. Nanomaterials 2023, 13, x FOR PEER REVIEW 11 of 16 Figure 10. Exposure of pTSA/Pani@MoS2 to ammonia (a) and methanol (c); exposure of pTSA/Ag-Pani@MoS2 to ammonia (b) and methanol (d).
On account of the higher sensitivity of pTSA/Ag-Pani@MoS2 in relation to both ammonia and methanol, its stability was tested for long-term exposure to vapors. Figure 11 shows that on exposure to ammonia and methanol, pTSA/Ag-Pani@MoS2 attained a steady state in ~45 s, with conductivity in the case of ammonia decreasing from 16.64 to 4.12 S/cm and for methanol from 16.68 to 7.88 S/cm. On further exposure over a subsequent 60 s, very little drop in conductivity was observed, as indicated by a nearly horizontal line in the graph. Furthermore, on flushing the sample with air, the conductivity acquired close to its original value in both ammonia-and methanol-exposed samples. However, it can be observed that in the case of ammonia-and methanol-exposed samples, a 7.3 and 2.5% loss in conductivity was observed, respectively. For the reproducibility studies, the pTSA/Ag-Pani@MoS2 sample was exposed to vapors for 45 s, followed by exposure to air, and conductivity was recorded [34] (Figure 12). It can be seen from the figure that pTSA/Ag-Pani@MoS2 possesses high reversibility in relation to both ammonia and methanol. The conductivity changes of 5.24 and 3.63 S/cm for ammonia and methanol, respectively, after 45 s suggests the higher sensitivity of pTSA/Ag-Pani@MoS2 towards ammonia. However, a loss of sensitivity of 1.44 and 0.18 S/cm, respectively, in the ammonia and methanol-exposed samples during the first cycle suggests much greater electrical compensation in the case of ammonia. It can also be seen that the loss of ~15 and ~2% conductivity was observed for ammonia-and methanol-exposed samples after three cycles. From the observations, it can be concluded that although pTSA/Ag-Pani@MoS2 showed much higher sensitivity towards ammonia, it can also be used in longer runs for methanol sensing. For the reproducibility studies, the pTSA/Ag-Pani@MoS 2 sample was exposed to vapors for 45 s, followed by exposure to air, and conductivity was recorded [34] (Figure 12). It can be seen from the figure that pTSA/Ag-Pani@MoS 2 possesses high reversibility in relation to both ammonia and methanol. The conductivity changes of 5.24 and 3.63 S/cm for ammonia and methanol, respectively, after 45 s suggests the higher sensitivity of pTSA/Ag-Pani@MoS 2 towards ammonia. However, a loss of sensitivity of 1.44 and 0.18 S/cm, respectively, in the ammonia and methanol-exposed samples during the first cycle suggests much greater electrical compensation in the case of ammonia. It can also be seen that the loss of~15 and~2% conductivity was observed for ammonia-and methanol-exposed samples after three cycles. From the observations, it can be concluded that although pTSA/Ag-Pani@MoS 2 showed much higher sensitivity towards ammonia, it can also be used in longer runs for methanol sensing. Nanomaterials 2023, 13, x FOR PEER REVIEW 12 of 16 . Figure 12. Reversibility studies of pTSA/Ag-Pani@MoS2 on ammonia and methanol exposure.

Selectivity
For the selectivity studies, pTSA/Ag-Pani@MoS2 was exposed to different analytes under similar conditions (1M of 30 mL of analytes). The sensitivity towards ammonia was the highest, followed by alcohols, aldehydes, benzene, and phenol ( Figure 13). It can be interpreted that the change in conductivity depends on various factors, such as electron donation capacity, vapor pressure, size of the analyte, as well as electrical compensation capacity towards pTSA/Ag-Pani@MoS2. Both ammonia (owing to its high basicity and compensation effect) and methanol (among other alcohols) interact much more efficiently with imine nitrogen than ethanol and propanol, due to their much smaller size. Benzene and phenol showed the least change in conductivity, due to their much lower vapor pressure and larger size, which resulted in the least interaction between charged nitrogen and Pani.

Selectivity
For the selectivity studies, pTSA/Ag-Pani@MoS 2 was exposed to different analytes under similar conditions (1M of 30 mL of analytes). The sensitivity towards ammonia was the highest, followed by alcohols, aldehydes, benzene, and phenol ( Figure 13). It can be interpreted that the change in conductivity depends on various factors, such as electron donation capacity, vapor pressure, size of the analyte, as well as electrical compensation capacity towards pTSA/Ag-Pani@MoS 2 . Both ammonia (owing to its high basicity and compensation effect) and methanol (among other alcohols) interact much more efficiently with imine nitrogen than ethanol and propanol, due to their much smaller size. Benzene and phenol showed the least change in conductivity, due to their much lower vapor pressure and larger size, which resulted in the least interaction between charged nitrogen and Pani. . Figure 12. Reversibility studies of pTSA/Ag-Pani@MoS2 on ammonia and methanol exposure.

Selectivity
For the selectivity studies, pTSA/Ag-Pani@MoS2 was exposed to different analytes under similar conditions (1M of 30 mL of analytes). The sensitivity towards ammonia was the highest, followed by alcohols, aldehydes, benzene, and phenol ( Figure 13). It can be interpreted that the change in conductivity depends on various factors, such as electron donation capacity, vapor pressure, size of the analyte, as well as electrical compensation capacity towards pTSA/Ag-Pani@MoS2. Both ammonia (owing to its high basicity and compensation effect) and methanol (among other alcohols) interact much more efficiently with imine nitrogen than ethanol and propanol, due to their much smaller size. Benzene and phenol showed the least change in conductivity, due to their much lower vapor pressure and larger size, which resulted in the least interaction between charged nitrogen and Pani.

Sensing Mechanism
The sensing behavior of Pani is recorded as the change in its electrical conductivity. The interaction of vapors with positively charged nitrogen hinders the movement of charge carriers (solitons, polarons, or bipolarons), which results in an increase/decrease in electrical conductivity. This is the main principle employed in the fabrication of Pani-based gas sensors.
The sensing mechanism involves the chemisorption-desorption of ammonia/methanol onto pTSA/Ag-Pani@MoS 2 and the electrical compensation phenomenon. On exposure to lower concentrations of vapors, the lone pair of ammonia/methanol interacts with the positive nitrogen in Pani, which results in the lowered mobility of charge carriers in Pani and hence a decrease in conductivity. However, as soon as the chemisorption of ammonia/ethanol occurs, the valency of nitrogen and oxygen increases, and an unstable configuration is obtained, as shown in Figures 14 and 15. Thus, possible desorption occurs, leading to the restoration of electrical conductivity. On exposure to higher concentrations of ammonia, the acid-base reaction between pTSA and ammonia predominates, leading to the electrical neutralization of Pani into an emeraldine base and hence a loss of sensing properties. In contrast, due to little or no compensation by methanol, it can be suggested that pTSA/Ag-Pani@MoS 2 composite can be used for the sensing of methanol for a longer number of cycles as well as at higher concentrations. The improved sensing performance of pTSA/Ag-Pani@MoS 2 , when compared with pTSA/Pani@MoS 2 , is due to the greater surface area and more conductive pathways in the composite. The surface area of pTSA/Pani@MoS 2 and pTSA/Ag-Pani@MoS 2 is presented in Figure S3 and Table S1.

Sensing Mechanism
The sensing behavior of Pani is recorded as the change in its electrical conductivity. The interaction of vapors with positively charged nitrogen hinders the movement of charge carriers (solitons, polarons, or bipolarons), which results in an increase/decrease in electrical conductivity. This is the main principle employed in the fabrication of Panibased gas sensors.
The sensing mechanism involves the chemisorption-desorption of ammonia/methanol onto pTSA/Ag-Pani@MoS2 and the electrical compensation phenomenon. On exposure to lower concentrations of vapors, the lone pair of ammonia/methanol interacts with the positive nitrogen in Pani, which results in the lowered mobility of charge carriers in Pani and hence a decrease in conductivity. However, as soon as the chemisorption of ammonia/ethanol occurs, the valency of nitrogen and oxygen increases, and an unstable configuration is obtained, as shown in Figures 14 and 15. Thus, possible desorption occurs, leading to the restoration of electrical conductivity. On exposure to higher concentrations of ammonia, the acid-base reaction between pTSA and ammonia predominates, leading to the electrical neutralization of Pani into an emeraldine base and hence a loss of sensing properties. In contrast, due to little or no compensation by methanol, it can be suggested that pTSA/Ag-Pani@MoS2 composite can be used for the sensing of methanol for a longer number of cycles as well as at higher concentrations. The improved sensing performance of pTSA/Ag-Pani@MoS2, when compared with pTSA/Pani@MoS2, is due to the greater surface area and more conductive pathways in the composite. The surface area of pTSA/Pani@MoS2 and pTSA/Ag-Pani@MoS2 is presented in Figure S3 and Table S1.

Conclusions
In summary, pTSA/Ag-Pani@MoS2 was synthesized by in situ polymerization of aniline with hydrothermally synthesized MoS2 and the subsequent deposition of silver nanoparticles over it. Morphological studies observed Pani-coated MoS2 Ag spheres and tubes well anchored to the surface. Structural analysis showed the presence and interactions between Pani, MoS2, and Ag, thereby suggesting the successful synthesis of the ternary composite. The ternary composite showed the highest electrical conductivity of 16.1 S/cm, due to the Pani and Ag offering multiple conduction pathways, i.e., through Pani, through Ag, π-π* interactions between Pani and MoS2, through Pani and Ag, etc. Cyclic aging studies demonstrated a much lower loss of conductivity during the third and fourth cycles, i.e., 0.77 to 0.53 S/cm, respectively, while isothermal measurement showed a similar loss at (in the range of 17 × 10 −3 to 59 × 10 −3 ), thereby suggesting the semiconducting nature of the composite. The pTSA/Ag-Pani@MoS2 showed better sensing responses of 70.48 and 51.17%, respectively, for ammonia and methanol, in contrast to 53.70 and 38.62% for that of pTSA/Pani@MoS2. The sensing mechanism showed that chemisorption-desorption of ammonia/methanol onto pTSA/Ag-Pani@MoS2 occurs, while the compensation phenomenon also takes place in the case of ammonia.

Conclusions
In summary, pTSA/Ag-Pani@MoS 2 was synthesized by in situ polymerization of aniline with hydrothermally synthesized MoS 2 and the subsequent deposition of silver nanoparticles over it. Morphological studies observed Pani-coated MoS 2 Ag spheres and tubes well anchored to the surface. Structural analysis showed the presence and interactions between Pani, MoS 2 , and Ag, thereby suggesting the successful synthesis of the ternary composite. The ternary composite showed the highest electrical conductivity of 16.1 S/cm, due to the Pani and Ag offering multiple conduction pathways, i.e., through Pani, through Ag, π-π* interactions between Pani and MoS 2 , through Pani and Ag, etc. Cyclic aging studies demonstrated a much lower loss of conductivity during the third and fourth cycles, i.e., 0.77 to 0.53 S/cm, respectively, while isothermal measurement showed a similar loss at (in the range of 17 × 10 −3 to 59 × 10 −3 ), thereby suggesting the semiconducting nature of the composite. The pTSA/Ag-Pani@MoS 2 showed better sensing responses of 70.48 and 51.17%, respectively, for ammonia and methanol, in contrast to 53.70 and 38.62% for that of pTSA/Pani@MoS 2 . The sensing mechanism showed that chemisorption-desorption of ammonia/methanol onto pTSA/Ag-Pani@MoS 2 occurs, while the compensation phenomenon also takes place in the case of ammonia.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.