Highly sensitive and fast-responsive room-temperature LPG sensor based on hydrothermally synthesized MoTe2

In the current study, a highly efficient and affordable sensor for liquefied petroleum gas (LPG) that operates at ambient temperature was fabricated using a thin film through an easy and low-cost approach. To achieve this objective, MoTe₂ was synthesized using a hydrothermal method. The synthesized material was characterized through powder X-ray diffraction (PXRD), Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, field-emission scanning electron microscopy (FE-SEM), and energy-dispersive X-ray spectroscopy (EDS). According to the results of PXRD and Raman spectroscopy, a pure phase of layered MoTe₂ with a hexagonal structure without any notable impurities was formed. The average crystallite size of the synthesized material is ∼37 nm. Raman spectroscopy and FTIR analysis revealed the presence of Mo–Te vibrational modes. The surface morphology reveals thin, wrinkled sheets with a crumpled topology having flake-like structures. The coexistence of Mo and Te elements was confirmed using the EDS study. The optimized sample was used to prepare the thin film using a spin-coating process. The sensing properties of the MoTe₂-based thin film were investigated as a room-ambient sensor for various LPG concentrations below the lower explosive limit of 0.5–2.0 vol%. The best sensor responses were recorded at 2.0 vol% LPG, with a value of 137, and at 0.5 vol% LPG, with a value of 26. Moreover, a rapid response time of 8 s and a recovery time of 22 s were observed at 0.5 vol% of LPG. These rapid dynamics are attributed to the prompt interaction between LPG molecules and pre-adsorbed oxygen species on the film surface, enabling fast adsorption-desorption cycles at active sensing sites. The results highlight the potential of MoTe₂ thin films as effective and scalable candidates for ambient LPG detection.

1 Introduction

Unintentional release of flammable gases, such as liquefied petroleum gas (LPG), continues to be a major risk to infrastructure and public safety. Fires, explosions, and fatalities have been reported frequently in residential, commercial, and industrial settings. These incidents, which continue to occur alarmingly frequently despite advancements in safety procedures, are frequently caused by corroded pipelines, faulty fittings, poor maintenance, or user negligence. Because of their low ignition energy, quick diffusion, and accumulation in confined spaces, even minor leaks have the potential to become life-threatening situations if they are not discovered in a timely manner. This reality emphasizes how urgently effective and dependable sensing technologies that can continuously monitor environments and provide early warnings in the event of a gas release are needed.

LPG is an exceptionally flammable gas composed primarily of propane (C₃H₈) (5%–10%) and butane (C₄H₁₀) (70%–80%), along with a small amount of other hydrocarbons (Chaurasiya et al., 2023). It is predominantly used as a source of fuel at domestic and industrial levels. Its high flammability and volatility make it essential to detect leaks promptly to prevent fire hazards and explosions. The lower explosive limit (LEL) of LPG components is ∼2% by volume in air, and regulatory exposure limits set by OSHA and NIOSH mandate early detection at levels well below this threshold (Singh et al., 2021). Gas sensors are a resistive type of sensors that have emerged as a vital tool in LPG detection. These sensors operate using a mechanism where gas adsorption modulates the charge carrier density of the sensing material (n or p type), leading to measurable changes to the resistance, as shown in Table 1. The key performance parameters of any gas sensor are sensitivity, selectivity, response/recovery time, operating temperature, and long-term stability.

Table 1

Table 1. Variation of resistance of a sensor with gas and type of materials.

So far, a wide variety of materials (viz. metal oxides, carbon nanotubes, graphene, MXene, transition metal dichalcogenides, perovskites, and metal organic frameworks) have been explored for LPG sensing (Gautam et al., 2021; Xu et al., 2017; Island et al., 2017; Dai et al., 2016; Bharati et al., 2024; Tiwari et al., 2022; Tiwari et al., 2024). Metal-oxide semiconductors (SMOs) such as ZnO, TiO₂, SnO₂, NiO, Fe₂O₃, and WO₃ have been widely studied for their chemical stability, cost effectiveness, ease of fabrication, and high sensitivity. These SMOs have the same working mechanism as that of resistive type of sensors. For example, Choudhary et al. (2021) synthesized α-Fe₂O₃ and examined their LPG-sensing performance. The fabricated sensor exhibits a response and a recovery time of 52 s and 107 s, respectively, with a sensor response of 87% toward 10 ppm of LPG at 170°C. Shinde et al. (2024) reported the ZnO-based LPG sensor with a sensor response of 367 toward 50 ppm at 350°C, with a rapid response time of 10 s. However, these SMOs often face challenges, such as high operating temperatures, long recovery time, and low selectivity, limiting their use in portable or room-temperature applications.

To address these limitations, two-dimensional (2D) materials have gained attention for their high surface-to-volume ratio, tunable electronic properties, and low activation energy for gas adsorption. Graphene, MoS₂, and MXenes have been successfully deployed in sensors for gases such as NO₂, NH₃, and H₂, with improved sensitivity and faster kinetics. Nevertheless, in the context of LPG sensing, studies based on 2D materials remain scarce, with most investigations confined to metal oxide or sulfide-based systems. Within the 2D material family, transition metal tellurides (TMTs), particularly MoTe₂, have emerged as highly promising but underexplored candidates. MoTe₂ exists in both semiconducting 2H and metallic 1T′ phases, offering a unique ability to tailor surface reactivity and charge transport behavior (Zappa, 2017). MoTe₂ is a suitable material for gas sensing because the Te atoms are the most favorable site for the adsorption of O₂ molecules in MoTe₂. This is owing to the longer bond length (3.93 Å) of Te–O compared to the shorter bond length of Mo–O₂ (2.71 Å) in MoTe₂. This also explains the facile release of gas molecules from the surface of MoTe₂ and enhances the rate at which the system returns to its original state (Zhu et al., 2014). Furthermore, MoTe₂ shows a narrow energy bandgap (direct type) of approximately 1.5 eV for a single layer and an indirect energy bandgap of approximately 0.83 eV for bulk MoTe₂. This is smaller than the bandgaps of other TMDs (Zhou et al., 2015; Octon et al., 2016). Furthermore, the higher charge carrier density in MoTe₂ than in other TMDs, like MoSe₂ and WSe₂, results in increased sensitivity to gas molecules (Su et al., 2019; Varghese et al., 2015). These characteristics make it a suitable candidate for gas-sensing applications, demanding high sensitivity and rapid response, particularly under ambient conditions. For example, Yan et al. (2024) reported room-temperature NO₂ gas sensing based on MoS₂/MoO₃ heterojunctions. The sensor exhibits 18.9 sensor responses at 1 ppm, with a response time and a recovery time of 20 s and 30 s, respectively. Munusami et al. (2022) reported room-temperature LPG and H₂ gas-sensing performance of MoS₂/graphene. The sensor showed a sensor response of 12.2 at 1,000 ppm LPG, with a response time and a recovery time of 15 s and 22 s, respectively, compared to H₂ gas. Kodan et al. (2024) reported MoSe₂–WS₂ room-temperature NO₂ gas-sensing performance. The sensor gives a sensor response of 59.63% to 50 ppb concentration, with a response time and a recovery time of 68.90 and 65.68 s, respectively.

Recent studies have shown that MoTe₂-based devices, including field effect transistors (FETs), exhibit strong gas-sensing behavior toward oxidizing and reducing gases (Feng et al., 2017a; Wu et al., 2018a; Wu et al., 2018b). There are also some reports which show that other MoTe₂-based devices such as Schottky diodes exhibit even a better response and recovery time (Feng et al., 2017b; Shackery et al., 2018). Density functional theory and first-principles calculations further support MoTe₂’s sensitivity to various gas molecules (Liu et al., 2022; Cao et al., 2021; Panigrahi et al., 2019; Szary et al., 2022; Zhu et al., 2020; Feng et al., 2017a; Liu et al., 2021). Recently, Huang et al. (2025) reported layered MoTe₂/ZnO heterojunction for a sensitive TEA sensor at room temperature (RT). The sensor displayed a response time and a recovery time of 147 s and 173 s, respectively, with a sensor response of 91.9% at 19°C. Wu et al. (2025) reported a theoretical calculation using the first-principles DFT calculation for developing a ZnO–MoTe₂-based gas sensor. The results showed better adsorption and recovery performance for C₂H₆ than for CO₂ and H₂. However, a comprehensive study on MoTe₂ for LPG detection has yet to be reported.

The present work addresses this gap by investigating the LPG-sensing behavior of hydrothermally synthesized p-type MoTe₂ thin films at room temperature. The sensor exhibits high response, fast response/recovery time, and excellent selectivity toward LPG, demonstrating the potential of MoTe₂ as an effective and robust material for next-generation gas sensors for the first time.

2 Experiment

2.1 Materials

Sodium molybdate dihydrate (Na₂MoO₄·2H₂O ≥ 99%), hydrazine hydrate (N₂H₄.H₂O, 98%), absolute ethanol (C₂H₅OH, 99.8%) and Tellurium metal powder (Te, 99.9% and ∼200 mesh) were purchased from Sisco Research Laboratory (SRL) India. All the chemicals were used as received.

2.2 Hydrothermal synthesis of MoTe₂

Two millimole of Na₂MoO₄.2H₂O and four millimole of Te powder were added to 140 mL of deionized (DI) water and stirred for 10 min; after that, 10 mL of N₂H₄.H₂O was added in the above mixture. The resulting mixture was stirred for half an hour. The stirred mixture was transferred into a stainless-steel autoclave having a Teflon-lined sleeve. The autoclave was transferred into an oven at a temperature of 200°C for a reaction time of 48 h. Once the reaction was complete, the autoclave was allowed to cool at room temperature. The obtained sample was rinsed with an abundant amount of DI water and ethanol, and subsequently dried for 12 h at a temperature of 60°C. The synthesis process of hydrothermally synthesized MoTe₂ is depicted in Figure 1.

Figure 1

Figure 1. Illustration of hydrothermal synthesis of layered MoTe₂ nano-material.

2.3 Thin-film preparation of pristine MoTe₂

A photo-resist spin coater was used to prepare the thin film from the synthesized p-type MoTe₂ sample on a 2 × 1 cm² glass substrate. Initially, the glass substrate was cleaned via an ultrasonic cleaner using DI water and ethanol. DI water was used to dissolve the sample for the preparation of the dilute MoTe₂ solution. The diluted solution was subjected to sonication. The resulting solution was dropped on the clean glass substrate and placed on a spin coater for 60 s at 3,000 rpm, followed by drying of the coated glass substrate at 50°C for 5 minutes. To obtain the necessary thickness of the thin film, the same procedure was repeated.

2.4 Characterization

The hydrothermally synthesized MoTe₂ was characterized using powder X-ray diffraction (PXRD). The PXRD data from the hydrothermally synthesized samples were collected in the range of 20°–70° 2$\mathrm{\theta }$ using a EMPYREAN diffractometer having Cu Kα radiation sources, with wave length λ = 1.5406 Å and operated at 40 kV. The morphology and elemental composition/mapping were examined using Nova Nano SEM 450 field emission scanning electron microscopy (FE-SEM) of USA, assembled with energy-dispersive electron X-ray spectroscopy (EDX, Team Pegasus Integrated EDS-EBSD with Octane Plus and Hikari Pro, EDX Inc.) operated at 15 kV. A Raman spectrometer (Renishaw in Via RE04) having 532 nm excitation wavelength with a resolution of 3 cm⁻¹ was used to record the data. The FTIR spectra were collected using a Thermo Nicolet Avatar 370 spectrometer. The resolution in FTIR spectra was 4 cm⁻¹.

2.5 LPG-sensing setup

The experimental evaluation of the sensor’s sensing capabilities was conducted by monitoring the fluctuation in electric current during exposure to LPG. A gas chamber constructed from borosilicate glass was designed specifically for detecting LPG gas. The chamber is equipped with inlet and outflow knobs for the purpose of inserting and removing LPG, respectively. The gas chamber consists of a concentration-measuring apparatus, in which the gas chamber was connected to a gas cylinder, along with a mass flow controller that quantified the gas admitted in vol.%. Furthermore, an outlet knob was designed to withdraw unadsorbed LPG. The contacts were made on the prepared thin film using silver paste. The sensing device was connected to a Keithley electrometer (model 6517B) manufactured by Tektronix, Beaverton, Oregon, USA. This electrometer was connected with a sensing device at one terminal and a computer system handling the measurement of electrical data, including current and resistance, at the other terminal, as shown in Figure 2.

Figure 2

Figure 2. Diagram illustrating the LPG-sensing setup.

3 Results and discussion

3.1 PXRD

PXRD is used to examine the purity of the synthesized phase and crystalline nature of the synthesized sample. The PXRD pattern of the synthesized sample is shown in Figure 3. The diffraction peaks for pristine MoTe₂ were observed at 2$\mathrm{\theta }$ angles of 23.13⁰, 27.61⁰, 38.38⁰, 40.57⁰, 43.47⁰, 46.03⁰, 47.11⁰, 49.77⁰, 51.35⁰, 52.07⁰, 57.05⁰, 62.99⁰, 63.83⁰, 66.02⁰, and 67.71⁰ corresponding to the crystal planes (100), (011), (006), (110), (105), (003), (200), (201), (112), (110), (022), (113), (203), (116), and (023) of molybdenum ditelluride, respectively. All the observed diffraction peaks are well indexed. The PXRD pattern of the synthesized material (MoTe₂) is also matched well with JCPDS card no 01-073-1650 and 01-079-0736 (for Te) (Bhat and Nagaraja, 2019; Chen et al., 2022). The observed PXRD pattern does not contain any additional peaks associated with impurity phases or the monoclinic 1T′ polymorph. The absence of 1T′-specific reflections at ∼ 14.2°, ∼29.8°, and ∼35.5°, along with the agreement between the observed diffraction pattern and standard reference data, support the formation of the semiconducting 2H–MoTe₂ structure. This outcome confirms the validity of the synthesis technique used in this investigation and is consistent with earlier reports on hydrothermally synthesized MoTe₂ (Chen et al., 2022; Bhat and Nagaraja, 2019). The Scherrer’s equation was used to calculate the average crystallite size, as presented in Equation 1:

$D_{hkl}=\frac{0.9\lambda }{{\beta }_{hkl}\mathit{Cos}{\theta }_{hkl}}.(1)$

Figure 3

Figure 3. PXRD pattern of MoTe₂.

Here, λ is the wavelength of X-ray in radians, ${\theta }_{hkl}$ is Bragg’s angle of diffraction, and β_hkl is the full width at half maxima (FWHM) of the peaks (radians). The average size of the crystallite is found to be 37.14 nm. The percent crystallinity of synthesized nanostructured MoTe₂ is also calculated using the area of the crystalline peaks to the total area. The % crystallinity is found to be ∼95%. This shows that synthesized nano MoTe₂ material has an excellent crystallinity. The excellent crystallinity of synthesized nano MoTe₂ results from a precisely regulated synthesis process using high-purity precursors, with the possibility of the presence of weaker Te rings in synthesized nano MoTe₂. The existence of feebler Te rings serves as a center of nucleation or substrates, facilitating the growth of MoTe₂.

3.2 FTIR spectroscopy

FTIR analysis offers important information regarding the functional groups and various types of chemical bonds present in the materials. The FTIR spectrum of synthesized MoTe₂ nano-material is used to analyze the vibrational bands of synthesized nano MoTe₂ in the frequency range between 4,000 and 500 cm⁻¹. The recorded FTIR spectrum of synthesized nano MoTe₂ is depicted in Figure 4. The FTIR spectrum of the synthesized nano MoTe₂ material show a vibrational band at 1,632 cm⁻¹. The band assigned to this is attributed to the bending mode of adsorbed water, whereas the wide band at 3,447–3,499 cm⁻¹ is assigned to a O−H stretching mode of liquid or adsorbed water H−O−H (Shinde et al., 2021; Singh et al., 2017). Additionally, the vibrational band observed at 1,084 cm⁻¹ is assigned to the stretching mode of metal oxide (M−Ox) bonds (Abdullah et al., 2023; Shirpay and Mohagheghi, 2020). The vibrational bands observed at a low wave number of 602 cm⁻¹ and a vibrational peak at 735 cm⁻¹ correspond to the vibrational mode of metal-telluride for nano MoTe₂ (Abdullah et al., 2023). The vibrational frequency at 1,435 cm⁻¹ is corresponding to Te−O oxidation vibrational modes. The presence of characteristic vibrational modes of synthesized nano MoTe₂ validates the effective synthesis of the MoTe₂ material.

Figure 4

Figure 4. FTIR graph of synthesized MoTe₂.

3.3 Raman spectroscopy

Raman spectroscopy is a powerful method for investigating vibrational modes, detecting defects, and assessing structural phase purity. A LASER source of 532 nm was used to record the Raman spectrum of the MoTe₂ nano-crystalline material. Figure 5 presents the Raman spectrum of MoTe₂ recorded between 1,000 and 80 cm⁻¹. MoTe₂ exhibits all the Raman active modes in the investigated range. These three prominent modes of vibration correspond to the characteristic vibrational modes of MoTe₂. The vibrational mode at 224 cm⁻¹ corresponds to the conspicuous in-plane E¹_2g mode of vibration. The E¹_2g peak signifies the in-plane vibration induced by two tellurium atoms in relation to a molybdenum atom (Grzeszczyk et al., 2016). The existence of this mode of vibration indicates the formation of the layered MoTe₂ structure. The vibrational mode at 175 cm⁻¹ corresponds to the out-of-plane A_1g mode. The in-plane E_1g mode is observed at 118 cm⁻¹. The presence of the weaker A_1g mode also confirms few numbers of layers in the synthesized MoTe₂. All the observed vibrational modes of Raman corresponding to E¹_2g, A_1g, and E_1g modes are in good agreement with the 2H phase of MoTe₂. The absence of additional bands that typically arise in the monoclinic 1T′ phase—characterized by complex multiple structures due to the reduced symmetry—confirms that the synthesized sample is structurally consistent with the semiconducting 2H phase. These vibrational characteristics are in strong agreement with previously reported Raman profiles for hexagonal 2H–MoTe₂. This result is consistent with the PXRD analysis, confirming the formation of the pure 2H–MoTe₂ phase (Zhou et al., 2015; Guo et al., 2015; Lv et al., 2017).

Figure 5

Figure 5. Raman spectra of synthesized layered MoTe₂ nano-material.

3.4 FE-SEM

The morphology and structure of the MoTe₂ material were analyzed using FE-SEM. Figure 6a, b depicts the FE-SEM micro-images of MoTe₂ synthesized using the hydrothermal technique. Figure 6a shows a disordered and highly textured surface with prominent flake-like and platelet structures. These flakes appear randomly oriented and aggregated into loosely packed clusters, suggesting a layered morphology. The lateral dimensions of the flakes range from ∼200 to 800 nm, with thicknesses consistent with few-layered to multilayered structures. Additionally, several elongated features are observed, which may indicate anisotropic growth behavior, possibly driven by the directional energy minimization along specific crystallographic planes during hydrothermal synthesis. Figure 6b shows thin, wrinkled sheets with a crumpled topology, consistent with exfoliated or few-layered 2D layers. These morphological features are indicative of weak van der Waals interactions between the layers, facilitating delamination and restacking. Notably, numerous ultrafine nanoparticles can be observed uniformly distributed on or near the MoTe₂ flakes. These particles are attributed to a nucleation center. This morphology is consistent with the crystallographic features identified in PXRD analysis. The diffraction peaks observed at 2θ values of 38.38°, 43.47°, 52.07°, 63.83° and 66.02° correspond to the (006), (105), (110), (203) and (116) planes of hexagonal 2H–MoTe₂, respectively, confirming the formation of a layered structure. Raman observations also complemented these findings by revealing a few-layer configuration, which aligns with the thin-sheet morphology observed in the SEM. Together, the SEM, PXRD, and Raman results confirm that the synthesized MoTe₂ possesses a layered, few-sheet architecture, with high crystallinity and pure 2H symmetry, which is ideal for applications requiring efficient surface interaction and charge transport.

Figure 6

Figure 6. FE-SEM images of synthesized MoTe2 (a) lower magnification, (b) higher magnification.

Furthermore, the EDS analysis, as shown in Figure 7a, was conducted to determine the composition of the elements present in the synthesized MoTe₂ nano-material. The results confirm that the material primarily consists of two elements: molybdenum (Mo) and tellurium (Te). Figures 7b–d display the elemental distribution and color mapping of MoTe₂, revealing a uniform dispersion of Mo and Te throughout the sample. It is also worth noting that the color mapping indicates a Te-rich pattern, which aligns with the PXRD observation and is attributed to the presence of weaker Te rings (Bhat and Nagaraja, 2019).

Figure 7

Figure 7. EDS spectra of synthesized MoTe₂. (a) Elemental mapping of Mo (b), elemental mapping of Te (c), and elemental mapping of MoTe₂ (d).

4 LPG-sensing performance of the MoTe₂-sensing film

The LPG-sensing capabilities are assessed by investigating the change in electrical resistance of the prepared MoTe₂ thin-film sensor at 0.5–2.0 volume % of LPG concentration at an ambient temperature. Figure 8 illustrates the response and recovery properties of the MoTe₂-sensing film after exposure to a range of concentrations from 0.5 to 2.0 vol%, below the LEL value. First, the value of sensor’s current is set fixed at a constant value in air, and then, the static resistance was observed in air. This is taken as R_a. Second, the thin film was exposed through LPG, and the change in resistance (R_g) was recorded. To check the all-inclusive sensor’s parameters, such as response and recovery time, selectivity, and sensitivity, relevant tests were conducted. Figure 8 shows the response of the sensor at various concentrations of LPG. A strong correlation between the detection capabilities of the sensor and the gas concentration is shown by the fact that as the LPG concentration increases from 0.5 to 2.0 vol%., it is interesting to observe that the resistance of the sensing element increases with the LPG concentration. The sensor response of the prepared thin-film sensor is determined by the ratio of the measured resistance (when exposed to the target gas) to the resistance in the air. The response of the thin-film sensor at different concentrations of LPG was determined using Equation 2 (Verma et al., 2022; Singh et al., 2021) and presented in Figure 9a.

$S.R=\frac{R_g}{R_a},(2)$

where R_g is the resistance after LPG exposure and R_a is the resistance in the air. Figure 9a shows that the sensor response at a low concentration of LPG (0.5 vol%) is 26. In contrast, the largest sensor response was determined as 137 at 2.0 vol% LPG concentration. It is demonstrated that with the increase in the concentration of LPG, the sensor response increases linearly (Figure 9a), indicating that a smaller amount of the gas interacts with a smaller portion of the sensing film surface. However, when the LPG concentration increases, the area covered by the gas also increases proportionately; therefore, an increased surface contact is established between the LPG and the thin-film sensor, consequently increasing the sensor response. The sensitivity of the LPG sensor is defined as the ratio of the change in the resistance to the change in the concentration (Verma et al., 2022).

$\mathrm{S}=\frac{\Delta \mathrm{R}}{\Delta \mathrm{C}}=\frac{\text{change}\text{in}\text{resistance}}{\text{change}\text{in}\text{concentration}}.(3)$

Figure 8

Figure 8. Response transient of the MoTe₂ thin film for varying LPG concentrations (0.5–2.0 vol%).

Figure 9

Figure 9. (a) LPG volume % vs. response of sensor of the MoTe₂ thin-film sensor. (b) LPG concentration-dependence time of response and time of recovery. (c) Sensing response curve at 0.5 volume % LPG. (d) Reproducibility for 1.5 volume % LPG.

To find out the value of the sensitivity of the thin-film sensor, a linear regression analysis was carried out. The value of sensitivity is obtained from the slope of linear fitting of LPG vol% vs. sensor response. The evaluated value of sensitivity was 77 sensor response per vol%. Moreover, the R² factor value was 0.97, indicating that even the device operates well below the LEL. This sensitivity value and R² generally describe the reliability of the LPG sensor.

The fabricated MoTe₂ thin film has a faster response and recovery time for low-concentration gas than for high-concentration LPG. The determination of response and recovery time is derived using the characteristic graph (Figure 8). The response and recovery curves of the sensing film at different LPG concentrations are depicted in Figure 9b. The time of response of the sensing films was determined by measuring how long it takes for their resistance to change by 90% after exposure to LPG. Similarly, the time of recovery of the sensing films was determined by measuring how long it takes for their resistance to change by 90% and return to its initial value after the removal of the LPG. The characteristic curve of the sensor response for 0.5 vol% of LPG is presented in Figure 9c. An excellent sensor response and a recovery time of 8 and 22 s, respectively, were achieved for 0.5 vol% LPG. Table 2 shows the sensor response and time of response and recovery at different concentrations of LPG.

Table 2

Table 2. Sensor parameters for MoTe₂ at different concentrations of LPG.

Repeatability is an essential sensor parameter that refers to the ability to consistently produce the similar result over a specific time interval for similar measurements. The recurrence graph for 1.5 vol% LPG using the MoTe₂ sensor film is shown in Figure 9d, which displays four consecutive cycles. Here, we found that the reported sensor yielded a result of 99.40%, with an error of only 0.6%. This is possibly due to the presence of moisture, which reduces the absorption capacity (Hu et al., 2012).

Selectivity is a sensor’s ability to differentiate the gas of interest from other analyte gases. High selectivity indicates a stronger response of the sensor to the target gas than to other analyte gases. In this study, variety of gases are used to evaluate the MoTe₂-sensing film’s selectivity using methanol (CH₃OH), LPG, ethanol (C₂H₅OH), acetone (CH₃COCH₃), and carbon dioxide (CO₂) for 0.5 vol% at room temperature, as illustrated in Figure 10a. The sensor responses to methanol, LPG, ethanol, acetone, and CO₂ are recorded as 12, 26, 10, 5, and 3, respectively. Additionally, the coefficient of selectivity (K) of all gases used in this study is evaluated from Equation 4:

$K=\frac{{S.R}_{LPG}}{{S.R}_{Gas}},(4)$

Figure 10

Figure 10. (a) Selectivity of the MoTe₂ sensor for different target gases at 0.5 vol%. (b) Effect of humidity on LPG sensing.

where S.R_LPG and S.R_Gas represent the sensor response for LPG and other gases, respectively. The thin film exhibited a selectivity coefficient of 2.16 for methanol, 2.6 for ethanol, 5.2 for acetone, and 8.6 for CO₂. A higher K-value indicates greater selectivity in LPG detection than in the detection of other gases used in this study. For example, a K-value of 2.16 for methanol describes that the response of the sensor toward LPG is 2.16 times greater than that of methanol. These results indicate that the MoTe₂-sensing film exhibits the highest selectivity coefficient toward LPG of all other analyte gases. The performance of the fabricated thin-film sensor based on the synthesized MoTe₂ nanomaterial is compared with the recently investigated materials for LPG gas detection, as shown in Table 3. This table clearly demonstrates that the material (MoTe₂) used in this study exhibits a rapid response time (8 s) and excellent recovery time (22 s) at room temperature compared to other materials for LPG sensing. It is interesting to note that the present investigation is carried out at room temperature, whereas others are reported at higher temperature. Furthermore, it is also clear from Table 3 that the present work has a much better response time (8 s) than the other materials reported at room temperature for the detection of LPG. Therefore, the MoTe₂ nanomaterial is competitive and superior to many of the reported materials that often require elevated operating temperatures or exhibit slower dynamics. This demonstrates the potential of MoTe₂ as a promising material for future development of low-power, high-performance gas sensors operating at room temperature.

Table 3

Table 3. Gas-sensing performance of various materials with varying concentrations of LPG.

The influence of humidity on the sensing performance of MoTe₂ thin films has also examined in the presence of 0.5 vol% LPG. The humidity within the chamber increases to 30%–100% using a humidifier with a K₂SO₄-saturated solution, as shown in Figure 10b. In the lower humidity range (30%–45%), the sensor exhibits a strong response, whereas in the mid-range (50%–65%), only minor fluctuations are detected; at elevated humidity levels (70%–90%), the sensor response reduces because at higher humidity, physisorption is saturated, and contact between the sensing film and LPG molecules is weaker, resulting in a reduced sensing performance. The MoTe₂ thin film is very appropriate for the mid-humidity spectrum.

5 Gas-sensing mechanism

The mechanism of the gas sensing of the MoTe₂ sensor shows fluctuation of the charge carrier on thin film surface upon encountering LPG. Thin film surfaces have adsorbed ambient oxygen via charge transfer. Both the molecular (O₂⁻) and atomic oxygen (O⁻ and O²⁻) species are ionized as surface oxygen. The existence of negative oxygen species is essential for LPG absorption as it has an affinity for O₂⁻, which enhances receptor activation. The processes governing oxygen absorption on thin-film surfaces are presented in Equations 5–7:

${\mathrm{O}}_{2\left(\text{atmospheric}\right)}\to {\mathrm{O}}_{2\left(\text{adsorbed}\right),}(5)$

${\mathrm{O}}_{2\left(\text{adsorbed}\right)}+{\mathrm{e}}^{-}\to {\mathrm{O}}_{2\left(\text{adsorbed}\right)}^{-},(6)$

${\mathrm{O}}_{2\left(\text{adsorbed}\right)}^{-}+{\mathrm{e}}^{-}\to {2\mathrm{O}}_{\left(\text{adsorbed}\right).}^{-}(7)$

In a LPG-containing environment, LPG adsorbed on the thin film’s surface interacts through pre-ionized oxygen species and forms hydrogen bonds with the surface molecules and produces changes in electrical resistance. The interaction of LPG and the adsorbed oxygen species is described using chemical reactions outlined in Equations 8 and 9, which illustrate the possible mechanisms underlying the LPG-sensing response. When LPG interacts with chemisorbed oxygen species, a dehydration process occurs, resulting in an unstable intermediate complex (C_nH_2n:O) and release of CO₂ and H₂O. The probable reactions are as follows:

${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}+2}+{20}^{-}\to {\mathrm{H}}_2\mathrm{O}+{\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}}:\mathrm{O}+\text{electron},(8)$

${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}}:\mathrm{O}+{\mathrm{O}}^{-}\to {\text{CO}}_2{+\mathrm{H}}_2\mathrm{O}+\text{electron}.(9)$

Here, n is equal to 3 and 4.

On the surface of the thin film, as the concentration of LPG increases, the interaction between LPG and chemisorbed oxygen species also increases. Increased LPG content leads to higher interaction between LPG and chemisorbed oxygen species, resulting in the release of electrons (Equations 8, 9). These electrons then recombine with holes in the p-type material, leading to a reduction in hole concentration. This reduction in majority carriers increases the resistance of the MoTe₂ film. The released electron creates a diffusion pathway by reducing the space charge region, facilitating better interaction between gas molecules and the sensor film, which enhances the sensor’s performance.

As the LPG concentration increases, the interaction between LPG and chemisorbed oxygen species also increases. As elaborated in Equations 8, 9, the release of electrons increases the height and width of the barrier potential. At saturation, resistance remains constant, and the decrease in resistance upon LPG removal is due to the recovery of surface-adsorbed oxygen species, which re-establish the charge transfer equilibrium and restore hole-rich conditions in the p-type MoTe₂ material. The mechanism is schematically depicted in Figure 11. In this study, the MoTe₂ thin film serves as an efficient room-temperature LPG sensor capable of detecting trace amounts of LPG with rapid response and fast recovery while exhibiting remarkably high sensor responsivity and sensitivity.

Figure 11

Figure 11. Gas-sensing mechanism of MoTe₂ thin-film LPG sensing.

6 Conclusion

In conclusion, a simple and easy hydrothermal method was used for successful synthesis of the MoTe₂ nano-materials. The PXRD analysis confirmed the existence of the MoTe₂ hexagonal phase, with an average crystallite size of 37.14 nm and percentage crystallinity of 95%. Microstructural, morphological, and elemental analyses of the synthesized MoTe₂ revealed thin, wrinkled sheets with a crumpled topology and flake-like structures, along with the presence of Mo and Te elements. Finally, the MoTe₂ thin-film sensor was fabricated and tested for room-temperature LPG detection from LPG concentration of 0.5–2.0 vol%. The fabricated sensor demonstrated excellent sensing capabilities with very quick time of response as 8 s and fast time of recovery as 22 s at 0.5 vol% of LPG. The sensitivity of the sensor was found to be 77 sensor responses per concentration. The linear concentration dependence of the sensor’s response was found for LPG, with a maximum value of 137 at 2.0 vol% of LPG. The repeatability test was carried out for 1.5 vol% LPG using the MoTe₂ thin film, in which four consecutive cycles were performed, and the fabricated sensor showed a repeatability of 99.40% with an error of 0.6%. The fabricated sensing film shows excellent selectivity toward LPG measurements with various analyte gases, such as methanol, LPG, ethanol, acetone, and CO₂, at a concentration of 0.5 vol% and at room temperature. The sensor responses to methanol, LPG, ethanol, acetone, and CO₂ were found to be 12, 26, 10, 5, and 3, respectively. These results indicate that the MoTe₂-sensing film exhibits the highest sensitivity to LPG of all other gases. As the findings provide shorter response times and greater LPG sensitivity, this cost-effective synthesized MoTe₂ with elongated nano-needle and sheet-like structures shows excellent performance in LPG detection. These features establish MoTe₂ as a promising candidate for the next generation of low-power, miniaturized gas sensors, with potential extension to other volatile organic compounds, smart home safety systems, and industrial leak detection networks, and it has the potential to boost the sensing market.

Data availability statement

The original contributions presented in the study are included in the article/Supplementary Material; further inquiries can be directed to the corresponding author.

Author contributions

AS: Investigation, Methodology, Writing – original draft, Formal Analysis, Visualization. AK: Visualization, Investigation, Formal analysis, Writing – review and editing. NC: Writing – review and editing, Visualization, Resources, Validation. AR: Resources, Data curation, Validation, Writing – review and editing. MG: Resources, Data curation, Writing – review and editing, Validation. BY: Resources, Validation, Writing – review and editing. MS: Resources, Formal Analysis, Project administration, Funding acquisition, Methodology, Supervision, Conceptualization, Writing – review and editing.

Funding

The author(s) declare that no financial support was received for the research and/or publication of this article.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Generative AI statement

The author(s) declare that no Generative AI was used in the creation of this manuscript.

Publisher’s note

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