WS2 Nanorod as a Remarkable Acetone Sensor for Monitoring Work/Public Places

Here, we report the synthesis of the WS2 nanorods (NRs) using an eco-friendly and facile hydrothermal method for an acetone-sensing application. This study explores the acetone gas-sensing characteristics of the WS2 nanorod sensor for 5, 10, and 15 ppm concentrations at 25 °C, 50 °C, 75 °C, and 100 °C. The WS2 nanorod sensor shows the highest sensitivity of 94.5% at 100 °C for the 15 ppm acetone concentration. The WS2 nanorod sensor also reveals the outstanding selectivity of acetone compared to other gases, such as ammonia, ethanol, acetaldehyde, methanol, and xylene at 100 °C with a 15 ppm concentration. The estimated selectivity coefficient indicates that the selectivity of the WS2 nanorod acetone sensor is 7.1, 4.5, 3.7, 2.9, and 2.0 times higher than xylene, acetaldehyde, ammonia, methanol, and ethanol, respectively. In addition, the WS2 nanorod sensor also divulges remarkable stability of 98.5% during the 20 days of study. Therefore, it is concluded that the WS2 nanorod can be an excellent nanomaterial for developing acetone sensors for monitoring work/public places.


Introduction
The rapidly increasing industrial evolutions in the fields of agriculture, automobiles, biomedical, and food packaging have introduced significant concerns about environmental monitoring technologies, leading to the development of reliable and durable gas sensors [1]. The human exhaled breath contains numerous types of gases, such as ketones, nitric oxide, aldehydes, volatile organic compounds, acids, and hydrogen sulfide [2,3]. Therefore, exhaled human breath is a significant and rousing issue from the outlook of biomedical applications to inspect different diseases. Interestingly, the exhaled human breath contains nearly 870 volatile organic compound types, indicating exclusive evidence regarding metabolic disorders [4]. Therefore, studying exhaled human breath can provide insights into crucial results of humans' normal or abnormal metabolic states arising from psychological stress [5]. Acetone molecules have been considered hazardous to human health and the environment. Acetone is a member of a family of volatile organic compounds that can influence the human nervous system and other organs under excessive exposure to concentrations of nearly 173 ppm [6]. Acetone is a vital aspect of the human metabolic system and can be examined through blood, breath, and urine [7,8]. It has been found to be a precise biomarker to recognize individuals with diabetes type-I due to the presence of high acetone vapor in the exhaled breath compared to healthy humans [9]. Various sensors, such as electrochemical, colorimetric, and resistive chemical gas, have been studied to detect acetone [10][11][12]. The colorimetric sensor provides low accuracy and
The WS 2 nanorods were prepared using a facile and eco-friendly hydrothermal process. In the synthesis process, 0.1904 g of the tungsten(IV) chloride compound was mixed in 80 mL of deionized (DI) water under dynamic stirring to achieve a good mixture. After that, 0.1803 g of thioacetamide was put into the prepared mixture solution using magnetic stirring to prepare a well-mixed solution. Furthermore, we added 1 mL of hexamethyldisilazane (HMDS) as a surfactant to control the morphology of the desired final product. Moreover, this prepared solution of tungsten(IV) chloride and thioacetamide was transferred to the 100 mL Teflon-lined autoclave. Further, this solution-filled autoclave was put into an air oven at 180 • C for twenty-four hours. Finally, the as-prepared nanomaterials were rinsed with ethanol and DI water. Further, it was dried at 70 • C for 10 h in a vacuum oven and processed with heat treatment at 200 • C for 3 h under a vacuum.

Materials Characterizations
Transmission electron microscopy (TEM) (JEOL JEM 2100F, JEOL Ltd., Tokyo, Japan) was utilized to explore the structural, morphological, lattice spacings, and lattice planes of WS 2 nanorods. In addition, the atomic-resolution high-angle annular dark-field (HAADF) and electron energy loss spectroscopy (EELS) were investigated using a JEOL, JEM-2100F, JEOL Ltd., Tokyo, Japan, to study the elemental color mapping of WS 2 nanorods.

WS 2 NRs-Based Sensor Fabrication and Measurements
The following process was used to fabricate the WS 2 nanorods-based acetone sensor: The first binder solution of 0.5 g of polyvinylidene fluoride (PVDF) was prepared using solvent N-Methyl-2-pyrrolidone (NMP) in a drop-wise manner. After that, we slowly added WS 2 nanorod powder to the binder solution and mixed it well to obtain the desired solution for sensor fabrication. The WS 2 nanorod gluey solution was coated on the glass substrate using drop-casting and dried slowly at 40 • C. Further, the silver paste was used to make contact on both sides of the film (deposited on glass) for the electrical connection. The gas-sensing measurements were conducted using the Keithley-2100 multimeter; however, the Motwane-454 multimeter was used to maintain the temperature inside the test chamber. The acetone gas-sensing measurements were investigated at 25 • C-100 • C for 5 ppm, 10 ppm, and 15 ppm concentrations. The acetone concentrations were injected into the test gas chamber using a Hamilton micro-syringe. In addition, the volume of the acetone concentrations (C, ppm) was estimated using Equation (1) [47]: where C (ppm) is the desired acetone concentration, ρ (g L −1 ) is the liquid acetone density, V l (µL) is the volume of the liquid acetone, M (g mol −1 ) is the molecular weight of the acetone, T ( • C) is the working temperature of acetone sensing, and V is the acetone gas test chamber.  Figure 1 shows the schematic depiction of synthesis, the crystal structure, and the acetone-sensing properties of the WS 2 nanorods. It depicts the solution preparation of tungsten and thioacetamide with the HMDS surfactant. It also reveals the hydrothermal reaction conditions at 180 • C for 24 h. Furthermore, it verifies the successful synthesis of WS 2 nanorods via TEM images. It shows the crystallographic illustration of the WS 2 crystal structure and the visualization of the acetone-sensing mechanism. The detailed synthesis procedure was discussed in the material synthesis section. The detailed synthesis procedure was discussed in the acetone-sensing mechanism section.
where C (ppm) is the desired acetone concentration, ρ (g L −1 ) is the liquid acetone density, Vl (μL) is the volume of the liquid acetone, M (g mol −1 ) is the molecular weight of the acetone, T (°C) is the working temperature of acetone sensing, and V is the acetone gas test chamber. Figure 1 shows the schematic depiction of synthesis, the crystal structure, and the acetone-sensing properties of the WS2 nanorods. It depicts the solution preparation of tungsten and thioacetamide with the HMDS surfactant. It also reveals the hydrothermal reaction conditions at 180 °C for 24 h. Furthermore, it verifies the successful synthesis of WS2 nanorods via TEM images. It shows the crystallographic illustration of the WS2 crystal structure and the visualization of the acetone-sensing mechanism. The detailed synthesis procedure was discussed in the material synthesis section. The detailed synthesis procedure was discussed in the acetone-sensing mechanism section.   Figure 2a,b depict the agglomerations of the WS2 nanorods with various sizes and lengths. It seems to be overlapped, crossed WS2 nanorods, which can improve oxygen molecules' conduction mechanism and interactions with the nanorods and gas-sensing properties. Figure 2c displays the WS2 nanorods whose length varies from nearly 20 nm to 200 nm. It also elucidates the WS2 nanorods whose widths fluctuate from 3 nm to 6 nm. Figure 2d shows several attached WS2 nanorods, forming various intersections between nanorods. It also reveals a thickness of 3 nm to 6 nm and a length of 20 nm to 200 nm of WS2 nanorods. It seems to develop several solid connections/attachments between the nanorods, which offer a large surface area, more adsorption, and chemisorption of atmospheric oxygen and gas molecules.   Figure 2a,b depict the agglomerations of the WS 2 nanorods with various sizes and lengths. It seems to be overlapped, crossed WS 2 nanorods, which can improve oxygen molecules' conduction mechanism and interactions with the nanorods and gas-sensing properties. Figure 2c displays the WS 2 nanorods whose length varies from nearly 20 nm to 200 nm. It also elucidates the WS 2 nanorods whose widths fluctuate from 3 nm to 6 nm. Figure 2d shows several attached WS 2 nanorods, forming various intersections between nanorods. It also reveals a thickness of 3 nm to 6 nm and a length of 20 nm to 200 nm of WS 2 nanorods. It seems to develop several solid connections/attachments between the nanorods, which offer a large surface area, more adsorption, and chemisorption of atmospheric oxygen and gas molecules. Figure 3a-e show the HRTEM and fast Fourier transform (FFT) patterns to explore the crystal structure, lattice spacing, and lattice planes of the WS 2 nanorods. Figure 3a displays the HRTEM image of the WS 2 nanorods with 2 to 5 nm thickness, and the length varies from 20 nm to 100 nm. Figure 3b,c reveal the magnified HRTEM images of the WS 2 nanorods, indicating lattice spacings of 0.28 nm corresponding to the (100) lattice plane. Further, Figure 3d,e exhibit the FFT patterns of the WS 2 nanorods to study the lattice plane and spacing, signifying the growth of the (100) lattice plane corresponding to the lattice spacing Figure 3a-e show the HRTEM and fast Fourier transform (FFT) patterns to explore the crystal structure, lattice spacing, and lattice planes of the WS2 nanorods. Figure 3a displays the HRTEM image of the WS2 nanorods with 2 to 5 nm thickness, and the length varies from 20 nm to 100 nm. Figure 3b,c reveal the magnified HRTEM images of the WS2 nanorods, indicating lattice spacings of 0.28 nm corresponding to the (100) lattice plane. Further, Figure 3d,e exhibit the FFT patterns of the WS2 nanorods to study the lattice plane and spacing, signifying the growth of the (100) lattice plane corresponding to the lattice spacing of 0.28 nm. The FFT pattern justifies the lattice spacing results, as shown in Figure  3a. The literature reports discussed similar HRTEM and FFT results of WS2 nanostructures [48,49]. In addition, the elemental information of the WS2 nanorods was studied using the HAADF image consistent with the color mapping of tungsten and sulfur elements. Figure  4a-c unveil the dark-field TEM (HAADF) image and corresponding elemental mapping of tungsten and sulfur elements of the WS2 nanorods. Figure 4a divulges the HAADF image of the WS2 nanorods to examine the elements' composition and presence in the desired In addition, the elemental information of the WS 2 nanorods was studied using the HAADF image consistent with the color mapping of tungsten and sulfur elements. Figure 4a-c unveil the dark-field TEM (HAADF) image and corresponding elemental mapping of tungsten and sulfur elements of the WS 2 nanorods. Figure 4a divulges the HAADF image of the WS 2 nanorods to examine the elements' composition and presence in the desired area. It also shows short to long WS 2 nanorods with thin diameters. Figure 4b,c reveal the color mapping of tungsten and sulfur elements of the WS 2 nanorods from the selected area (as shown in Figure 4a). It exhibits the presence of tungsten and sulfur elements over the selected area, confirming the successful formation of the WS 2 composition. These results are well-matched and supported by the HRTEM and FFT results of the WS 2 . In addition, the elemental information of the WS2 nanorods was studied using the HAADF image consistent with the color mapping of tungsten and sulfur elements. Figure  4a-c unveil the dark-field TEM (HAADF) image and corresponding elemental mapping of tungsten and sulfur elements of the WS2 nanorods. Figure 4a divulges the HAADF image of the WS2 nanorods to examine the elements' composition and presence in the desired area. It also shows short to long WS2 nanorods with thin diameters. Figure 4b,c reveal the color mapping of tungsten and sulfur elements of the WS2 nanorods from the selected area (as shown in Figure 4a). It exhibits the presence of tungsten and sulfur elements over the selected area, confirming the successful formation of the WS2 composition. These results are well-matched and supported by the HRTEM and FFT results of the WS2.

Acetone-Sensing Characteristics
The temperature and test gas concentration mainly affect the gas-sensing properties of the sensor. Therefore, it is necessary to find the optimum working temperature for a specific gas concentration of the chemical gas sensors. In light of this, we investigated the acetone-sensing properties of the WS2 nanorod sensor at 25 °C-100 °C for 5 ppm, 10 ppm, and 15 ppm concentrations. The desired acetone concentration of the WS2 nanorod sensor was evaluated with the help of Equation (1). In addition, the acetone gas sensitivity [S (%)] of the WS2 nanorod sensor was estimated using Equation (2) [50]:

Acetone-Sensing Characteristics
The temperature and test gas concentration mainly affect the gas-sensing properties of the sensor. Therefore, it is necessary to find the optimum working temperature for a specific gas concentration of the chemical gas sensors. In light of this, we investigated the acetone-sensing properties of the WS 2 nanorod sensor at 25 • C-100 • C for 5 ppm, 10 ppm, and 15 ppm concentrations. The desired acetone concentration of the WS 2 nanorod sensor was evaluated with the help of Equation (1). In addition, the acetone gas sensitivity [S (%)] of the WS 2 nanorod sensor was estimated using Equation (2) [50]: where R a and R acetone are the resistances measured under air and different acetone gas concentrations at different operating temperatures. Figure 5a shows the sensitivity vs. temperature plots of the WS 2 nanorod sensor for 5, 10, and 15 ppm acetone concentrations. It is found that the sensitivity increases with the increasing acetone concentration and operating temperature of the WS 2 nanorod sensor.
The WS 2 nanorod sensor shows the highest sensitivity of 94.5% for 15 ppm of acetone at 100 • C. However, the WS 2 nanorod sensor reveals the lowest sensitivity of 18.5% for 5 ppm of acetone at 25 • C. The high sensitivity observed at 100 • C compared to the low sensitivity at 25 • C, 50 • C, and 75 • C for increasing concentrations may be attributable to the following reasons: (i) The high thermal energy at 100 • C (as compared to 25 • C) allows more thermally excited electrons to reach the conduction band, which can easily interact with the oxygen molecules to form the active site on the WS 2 nanorod sensor surface; (ii) the large surface area of the 2D WS 2 nanorod provides more interactions of oxygen molecules to form a large amount of active site on the sensor surface; (iii) the high electronic/ionic conductivity of WS 2 ; (iv) high electronic and chemical responsiveness; and (v) rapid adsorption/desorption and extremely high diffusion of acetone molecules, leading to fast and outstanding sensitivity [51][52][53]. Figure 5b manifests the sensitivity vs. acetone concentration plots of the WS 2 nanorod sensor at two operating temperatures of 25 • C and 100 • C. It is perceived that the WS 2 nanorod sensor divulges low sensitivities of 18.5%, 24.7%, and 32.5% at an operating temperature of 25 • C for 5, 10, and 15 ppm of acetone, respectively. However, the WS 2 nanorod sensor displays high sensitivities of 64.5%, 82.4%, and 94.5% at an operating temperature of 100 • C for 5, 10, and 15 ppm of acetone, respectively. It is also elucidated that the sensitivity increases with the acetone concentration at 25 • C and 100 • C. Many factors influence the sensitivity of the WS 2 nanorod sensor due to the following reasons: (i) The high diffusion rate of gas molecules on the WS 2 nanorod sensor surface due to the high concentration gradient of acetone molecules. The concentration gradient is proportional to the diffusion rate; therefore, acetone sensitivity increases with the acetone concentration at 25 • C and 100 • C. Furthermore, (ii) the acetone molecules formed a significant dipole moment due to the presence of the C-C=O group. It encourages the chemical adsorption/desorption and redox reaction capability of the WS 2 nanorod sensor material [3,54].
with the oxygen molecules to form the active site on the WS2 nanorod sensor surface; (ii) the large surface area of the 2D WS2 nanorod provides more interactions of oxygen molecules to form a large amount of active site on the sensor surface; (iii) the high electronic/ionic conductivity of WS2; (iv) high electronic and chemical responsiveness; and (v) rapid adsorption/desorption and extremely high diffusion of acetone molecules, leading to fast and outstanding sensitivity [51][52][53]. Figure 5b manifests the sensitivity vs. acetone concentration plots of the WS2 nanorod sensor at two operating temperatures of 25 °C and 100 °C. It is perceived that the WS2 nanorod sensor divulges low sensitivities of 18.5%, 24.7%, and 32.5% at an operating temperature of 25 °C for 5, 10, and 15 ppm of acetone, respectively. However, the WS2 nanorod sensor displays high sensitivities of 64.5%, 82.4%, and 94.5% at an operating temperature of 100 °C for 5, 10, and 15 ppm of acetone, respectively. It is also elucidated that the sensitivity increases with the acetone concentration at 25 °C and 100 °C. Many factors influence the sensitivity of the WS2 nanorod sensor due to the following reasons: (i) The high diffusion rate of gas molecules on the WS2 nanorod sensor surface due to the high concentration gradient of acetone molecules. The concentration gradient is proportional to the diffusion rate; therefore, acetone sensitivity increases with the acetone concentration at 25 °C and 100 °C. Furthermore, (ii) the acetone molecules formed a significant dipole moment due to the presence of the C-C=O group. It encourages the chemical adsorption/desorption and redox reaction capability of the WS2 nanorod sensor material [3,54].    Figure 6a shows the transient characteristic of the WS 2 nanorod sensor at 25 • C and 100 • C for 5, 10, and 15 ppm of acetone. It shows a fast response and recovery at all acetone concentrations. It also exhibits that as the acetone concentration increases, the recovery and the response time decrease. This may be because of the enhanced chemisorption rate of acetone molecules on the WS 2 nanorod sensor's surface [55,56]. Figure 6b illustrates the response and recovery time plot of acetone molecules of the WS 2 nanorod sensor. The response and recovery times were estimated when the WS 2 nanorod sensor attained 90% of its maximum value and recovered 90% of its minimum value [57]. The WS 2 nanorod sensor displays a quick response time of 3.02 min and a recovery time of 3.41 min at 100 • C for the 15 ppm concentration. This may be due to the high surface area of WS 2 nanorods, rapid adsorption/desorption, and the chemisorption process. On the other hand, the reduction in the depletion layer and potential barrier height is due to more thermally excited electrons in the conduction band, leading to quick response and recovery [58,59].
Interestingly, the selectivity of the gas sensor plays a vital role in distinguishing the specific gas from various other gases. Here we investigated the selectivity of the WS 2 nanorod sensor to six gases, including acetone. Figure 7a shows the sensitivity vs. test gas plots of the WS 2 nanorod sensor at 100 • C for 15 ppm concentrations of various gas to disclose the selectivity behavior. It is detected that the WS 2 nanorod sensor reveals the maximum sensitivity to acetone (94.5%) compared to the other gas, such as ethanol (46.4%), methanol (33.2%), ammonia (25.7%), acetaldehyde (21.2%), and xylene (13.2%) at 100 • C for 15 ppm concentrations. In addition, the selectivity coefficient (C s ) to quantify the sensitivity of the WS 2 nanorod sensor was calculated using the following Equation (3) [60]. C s = S acetone S other gas (3) and the response time decrease. This may be because of the enhanced chemisorption rate of acetone molecules on the WS2 nanorod sensor's surface [55,56]. Figure 6b illustrates the response and recovery time plot of acetone molecules of the WS2 nanorod sensor. The response and recovery times were estimated when the WS2 nanorod sensor attained 90% of its maximum value and recovered 90% of its minimum value [57]. The WS2 nanorod sensor displays a quick response time of 3.02 min and a recovery time of 3.41 min at 100 °C for the 15 ppm concentration. This may be due to the high surface area of WS2 nanorods, rapid adsorption/desorption, and the chemisorption process. On the other hand, the reduction in the depletion layer and potential barrier height is due to more thermally excited electrons in the conduction band, leading to quick response and recovery [58,59]. Interestingly, the selectivity of the gas sensor plays a vital role in distinguishing the specific gas from various other gases. Here we investigated the selectivity of the WS2 nanorod sensor to six gases, including acetone. Figure 7a shows the sensitivity vs. test gas plots of the WS2 nanorod sensor at 100 °C for 15 ppm concentrations of various gas to disclose the selectivity behavior. It is detected that the WS2 nanorod sensor reveals the maximum sensitivity to acetone (94.5%) compared to the other gas, such as ethanol (46.4%), methanol (33.2%), ammonia (25.7%), acetaldehyde (21.2%), and xylene (13.2%) at 100 °C for 15 ppm concentrations. In addition, the selectivity coefficient (Cs) to quantify the sensitivity of the WS2 nanorod sensor was calculated using the following Equation (3) [60].

=
(3) The estimated values of Cs of the WS2 nanorod-based acetone sensor are 2.0 (ethanol), 2.9 (methanol), 3.7 (ammonia), 4.5 (acetaldehyde), and 7.1 (xylene). These Cs values indicate that the sensitivity of the WS2 nanorod sensor to acetone is 7.1, 4.5, 3.7, 2.9, and 2.0 times higher than xylene, acetaldehyde, ammonia, methanol, and ethanol, respectively. Therefore, it is concluded that the WS2 nanorod sensor is most suitable for acetone detection compared to other tested gases at 100 °C with 15 ppm concentrations. The lowest unoccupied orbital energy has different values for different gases [61]. Figure 7b depicts the stability plot of the WS2 nanorod sensor for 20 days for 15 ppm concentrations at 100 °C. The sensitivity of the WS2 nanorod sensor slowly reduces with time from 94.5% (on the 1st day) to 93.0% (on the 20th day). It also exhibits the excellent stability of the WS2 nanorod sensor of 98.5% over twenty days for a 15 ppm acetone concentration at 100 °C. The high stability of the WS2 nanorod sensor may be due to the excellent electrical and thermal conductivity of WS2. On the other hand, the nanorod's large surface area also provides high exposure to acetone molecules and rapid interactions with the adsorbed oxygen-active ions (O − ), leading to excellent stability. In addition, a comparison between the various TMD-based acetone sensors has been discussed here with our observed results of the WS2 nanorod sensor. The MoS2-CuO nanocomposite sensor exhibited a high response of 16.21 for 10 ppm acetone at room temperature. It also showed a fast response of 61 s and recovery of 85 s [62]. The WS2/WO3 sensor has demonstrated a prolonged response time of 823 s and recovery time of 1093 s at 100 °C for 20 ppm acetone [42]. The decoration of Co3O4 on ZnS nanorods has been discussed regarding the acetone-sensing characteristics, which elucidated a high response of 1650% for 500 ppm acetone at room temperature under 2.2 mW cm −2 UV illumination [63]. A 2D SnS nanoflakes-based sensor displayed a high response of 1000%, a response time of ~35 s, and a recovery time of ~45 s at 100 °C for 10 ppm acetone [64]. SnS2-based sensors have demonstrated a high response of ~25, a response time of ~210 s, and a recovery time of ~600 s at 300 °C for 10 ppm acetone [65]. We found very limited reports on the WS2-based acetone sensor in the literature. Therefore, it is concluded that the WS2 nanorods could be a promising nanomaterial for an acetone sensor.

Oxygen Active Site Formation and Acetone Molecule Detection Mechanism
The acetone recognition mechanism of the WS2 nanorod sensor essentially depends on the change in sensor resistance during gas sensing. The chemisorption reaction among the adsorbed active sites (O − ) on the WS2 nanorod surface and acetone molecules determines the gas-sensing process [66]. It regulates the concentration of oxygen molecules with the WS2 nanorod sensor surface and modulates the sensor resistance [67]. Equations (4)-(6) represent the interaction reactions of atmospheric oxygen molecules and the crea- The estimated values of C s of the WS 2 nanorod-based acetone sensor are 2.0 (ethanol), 2.9 (methanol), 3.7 (ammonia), 4.5 (acetaldehyde), and 7.1 (xylene). These C s values indicate that the sensitivity of the WS 2 nanorod sensor to acetone is 7.1, 4.5, 3.7, 2.9, and 2.0 times higher than xylene, acetaldehyde, ammonia, methanol, and ethanol, respectively. Therefore, it is concluded that the WS 2 nanorod sensor is most suitable for acetone detection compared to other tested gases at 100 • C with 15 ppm concentrations. The lowest unoccupied orbital energy has different values for different gases [61]. Figure 7b depicts the stability plot of the WS 2 nanorod sensor for 20 days for 15 ppm concentrations at 100 • C. The sensitivity of the WS 2 nanorod sensor slowly reduces with time from 94.5% (on the 1st day) to 93.0% (on the 20th day). It also exhibits the excellent stability of the WS 2 nanorod sensor of 98.5% over twenty days for a 15 ppm acetone concentration at 100 • C. The high stability of the WS 2 nanorod sensor may be due to the excellent electrical and thermal conductivity of WS 2 . On the other hand, the nanorod's large surface area also provides high exposure to acetone molecules and rapid interactions with the adsorbed oxygen-active ions (O − ), leading to excellent stability.
In addition, a comparison between the various TMD-based acetone sensors has been discussed here with our observed results of the WS 2 nanorod sensor. The MoS 2 -CuO nanocomposite sensor exhibited a high response of 16.21 for 10 ppm acetone at room temperature. It also showed a fast response of 61 s and recovery of 85 s [62]. The WS 2 /WO 3 sensor has demonstrated a prolonged response time of 823 s and recovery time of 1093 s at 100 • C for 20 ppm acetone [42]. The decoration of Co 3 O 4 on ZnS nanorods has been discussed regarding the acetone-sensing characteristics, which elucidated a high response of 1650% for 500 ppm acetone at room temperature under 2.2 mW cm −2 UV illumination [63]. A 2D SnS nanoflakes-based sensor displayed a high response of 1000%, a response time of 35 s, and a recovery time of~45 s at 100 • C for 10 ppm acetone [64]. SnS 2 -based sensors have demonstrated a high response of~25, a response time of~210 s, and a recovery time of 600 s at 300 • C for 10 ppm acetone [65]. We found very limited reports on the WS 2 -based acetone sensor in the literature. Therefore, it is concluded that the WS 2 nanorods could be a promising nanomaterial for an acetone sensor.

Oxygen Active Site Formation and Acetone Molecule Detection Mechanism
The acetone recognition mechanism of the WS 2 nanorod sensor essentially depends on the change in sensor resistance during gas sensing. The chemisorption reaction among the adsorbed active sites (O − ) on the WS 2 nanorod surface and acetone molecules determines the gas-sensing process [66]. It regulates the concentration of oxygen molecules with the WS 2 nanorod sensor surface and modulates the sensor resistance [67]. Equations (4)- (6) represent the interaction reactions of atmospheric oxygen molecules and the creation of active sites (O − ) on the WS 2 nanorod surface at different operating temperatures [51,68,69].
These active oxygen ions/sites are responsible for interacting with the acetone molecules. Therefore, the possible reaction between the adsorbed active sites (O − ) and acetone molecules on the WS 2 nanorod sensor surface is discussed in Equation (7) [59,70].
Figure 8a-f show the schematic drawings of the oxygen adsorption reaction, depletion layer formation, the creation of a potential barrier, and reaction mechanisms of acetone molecules with the oxygen active sites (O − ) on the WS 2 nanorod surface. Figure 8a-c depict the schematic sketch of the creation of active sites on the WS 2 nanorod sensor surface and depletion region in the electronic band structure. Firstly, atmospheric oxygen [O 2 (atmospheric)] was adsorbed on the WS 2 nanorod sensor surface (O 2 (adsorbed)) using the process discussed in Equation (4). Further, it is expected that, below 100 • C, the adsorbed oxygen (O 2 (adsorbed)) interacts with the electrons in the conduction band and creates the active sites (O 2 − ) on the WS 2 surface, as discussed in Equation (5). After that, active sites (O 2 − ) take more thermally excited electrons from the conduction band of the WS 2 nanorod, finally creating the active sites (O − ) on the WS 2 nanorod sensor surface, as discussed in Equation (6). Figure 8b portrays a schematic view of the emergence of a depletion layer around the WS 2 nanorod sensor surface during the adsorption process, which plays an intensive role in acetone sensing. Figure 8c describes the electronic band structure of the WS 2 nanorod sensor following the various steps as discussed in Equations (4)- (6). It illustrates that the depletion layer and potential barrier are created during the adsorption and active site (O − ) formation on the WS 2 nanorod sensor. Similar reports have studied and discussed the exploration of the concept of the creation of active oxygen ions/sites in the literature [71][72][73]. Figure 8d reveals the graphical visualization of the interaction between acetone molecules and the active site (O − ) on the WS 2 nanorod sensor surface, following the process discussed in Equation (7), showing the liberation of CO 2 gas, H 2 O, and electrons in the conduction band of the WS 2 nanorod. Figure 8e discloses an illustration of the acetone-sensing mechanism (as discussed in Equation (7)) on the WS 2 nanorod sensor surface and the release of carbon dioxide, water, and electrons. Figure 8f unveils the electronic band structure of the chemisorption of acetone molecules on the WS 2 nanorod sensor (as discussed in Equation (7)). It is observed that the declining depletion region and the height of the potential barrier are created during the adsorption and creation of the active site (O − ) on the WS 2 nanorod surface. It also frees electrons in the conduction band of the WS 2 nanorod sensor during the release of the carbon oxide and water molecules. The sensitivity of the acetone increases with increasing temperature and concentration due to the reduction in the depletion region and potential barrier heights, as schematically illustrated in Figure 8a-f, which supports the results discussed in Figures 5 and 6. Similar reports of acetone-sensing mechanisms have been studied and discussed in the literature [73][74][75][76].
trates that the depletion layer and potential barrier are created during the adsorption and active site (O − ) formation on the WS2 nanorod sensor. Similar reports have studied and discussed the exploration of the concept of the creation of active oxygen ions/sites in the literature [71][72][73]. Figure 8d reveals the graphical visualization of the interaction between acetone molecules and the active site (O − ) on the WS2 nanorod sensor surface, following the process discussed in Equation (7), showing the liberation of CO2 gas, H2O, and electrons in the conduction band of the WS2 nanorod. Figure 8e discloses an illustration of the acetone-sensing mechanism (as discussed in Equation (7)) on the WS2 nanorod sensor surface and the release of carbon dioxide, water, and electrons. Figure 8f unveils the electronic band structure of the chemisorption of acetone molecules on the WS2 nanorod sensor (as discussed in Equation (7)). It is observed that the declining depletion region and the height of the potential barrier are created during the adsorption and creation of the active site (O − ) on the WS2 nanorod surface. It also frees electrons in the conduction band of the WS2 nanorod sensor during the release of the carbon oxide and water molecules. The sensitivity of the acetone increases with increasing temperature and concentration due to the reduction in the depletion region and potential barrier heights, as schematically illustrated in Figure 8a-f, which supports the results discussed in Figures 5 and 6. Similar reports of acetone-sensing mechanisms have been studied and discussed in the literature [73][74][75][76].

Conclusions
In conclusion, we studied an acetone gas-sensing application based on WS 2 nanorods (NRs). The WS 2 nanorod sensor shows the highest sensitivity of 94.5% at 100 • C for 15 ppm acetone. It also discloses the admirable selectivity of acetone compared to other gases, such as xylene, methanol, ammonia, acetaldehyde, and ethanol at 100 • C with a 15 ppm concentration. Further, it demonstrates fantastic stability over 20 days at 100 • C for a 15 ppm concentration. Consequently, it is concluded that the WS 2 nanorod can offer a new choice for fabricating reliable, low-cost, environmentally friendly acetone sensors for observing workplace safety.