Gas Sensor Devices Research Articles

Sensitivity of Newly Prepared Nanomaterials to NO2 Gas and its Importance on Environmental and Industrial Safety

Polyaniline (PANI) and manganese dioxide (MnO2) doped with nickel oxide (NiO) nanocomposite have emerged as promising materials for gas sensing applications due to their high conductivity and surface area. This study improves the synthesized PANI/MnO2:NiO nanocomposites' sensitivity towards NO2 gas. Atomic force microscopy (AFM), Fourier transfer infrared (FTIR), scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDS), and X-ray diffraction (XRD) were used to analyze the produced nanocomposites. More precise methods were investigated to identify problems associated with oxidation, corrosion and sensor activity, such as increasing the amount of added nickel oxide, changing the structure of nanocomposites, and regulating the working temperature and humidity. Based on the results of this study, it is critical to comprehend how sensitive nanomaterials are to NO2 gas because exposure to this pollutant can negatively impact the environment and human health. Strategies to reduce its effects and improve industrial and environmental safety can be devised based on how nanomaterials react to nitrogen dioxide gas. We also examine antioxidants and PANI/MnO2:NiO nanocomposites for their anti-corrosion properties to guarantee long-term stability with the sensor's conductivity. Strong antioxidant and corrosion-resistant substances should be added to the sensor to extend its lifespan and maintain its performance under challenging operating conditions. Integrating these sensitive nanomaterials into gas-sensing devices can significantly improve environmental monitoring and industrial safety protocols, ultimately contributing to better air quality management and public health protection.

Scalable Melt Polymerization Synthesis of Covalent Organic Framework Films for Room Temperature Low-Concentration SO2 Detection.

The development of highly efficient sensors for low-concentration SO2 at room temperature is important for human health and fine chemistry, but it still faces critical challenges. Herein, a scalable olefin-linked covalent organic framework (COF) with an ultramicroporous structure and abundant binding sites is first developed as the SO2 sensing material. The COF can adsorb SO2 of 220 cm3/g at 1 bar and 40 cm3/g at 0.01 bar and 298 K, surpassing all reported COFs. The computational and kinetic adsorption studies deeply unveil the selective adsorption mechanism for low-concentration SO2. Furthermore, the multicomponent gas mixture breakthrough experiments confirm that the COF can specifically capture low-concentration (2000 ppm) SO2. We innovated a melt polymerization technology to fabricate COF films with adjustable substrates and film thicknesses. COF films are directly grown on the interdigital electrodes to prepare the SO2 sensor device, which possesses a low detection limit (86 ppb) and excellent selectivity for SO2 in the presence of 10 other potentially interfering gases. Compared to other reported SO2 sensors, its overall performance is among the top. Prominently, the sensor maintains a stable output signal for more than two months, and recovery can be easily achieved by simply purifying nitrogen at room temperature without heating. This study marks the first use of COFs for SO2 sensing, opening new possibilities for COFs in the detection of low-concentration toxic gases and manufacturing gas sensor devices.

Porous and Homogeneous Nanoheterojunction-Accumulating PdO@ZnO Structure for Exhaled Breath Ammonia Sensing.

The functional gas sensor device plays a pivotal role in intelligent medical treatment, among which metal oxide semiconductors are widely studied because of their inexpensiveness and ease of fabrication. However, the metal oxide sensors present a significant challenge in detecting NH3 at ppm levels within complex exhaled gases. Herein, the ZnO/PdO-x series were prepared by in situ loading palladium particles and calcining using nano-ZIF-8 as a precursor, which not only provided more transport path for ammonia adsorption but also achieved homogeneous nanoheterojunction accumulation structure. The tailor-made ZnO/PdO-2 sensor exhibits the optimum gas sensitivity, with a response value of 5.56 for 100 ppm of NH3 at 160 °C and a lower detection limit of 0.75 ppm. Particularly, it has a clear quantitative response to the actual exhaled gas of liver and kidney patients. By elucidating the intrinsic link between the in situ loading of MOF templates and the sensing mechanism, it is expected to broaden the rational design of metal-oxide sensors and thus provide an effective method for clinical detection.

Room-Temperature Flexible CNT/Fe2O3 Film Sensor for ppb-Level H2S Detection.

Carbon nanotubes (CNTs) had room temperature response, large surface area, and excellent mechanical properties, making them favorable for the design of flexible, wearable, and portable gas sensors. However, CNTs were lacking in response and selective response to different gases, such as H2S. Here, we demonstrated a flexible H2S ppb-level gas sensor based on a carbon nanotube/amorphous Fe2O3 (CNT/Fe2O3) film at room temperature, which was fabricated via a simple one-step solvent-thermal method. The CNT/Fe2O3 film gas sensor exhibited a high selective response to H2S (with a response of 55.1% to 100 ppb H2S), rapid reversible response at room temperature (with a response time of ∼127 s to 100 ppb H2S), and low limit of detection to about 2 ppb. Additionally, the CNT/Fe2O3 film maintained good sensing performance under various bending conditions and could be further fabricated into the fiber gas sensor device via wet stretching, retaining response at the ppb level (with a response of 18.6% to 100 ppb H2S). This research on a flexible gas sensor device based on the CNT film/fiber opened up new possibilities for wearable portable electronic device applications.

First-principles study of O3 molecule adsorption on pristine, N, Ga-doped and -Ga-N- co-doped graphene

The adsorption of O3 molecules (ozone) on graphene, N-doped graphene, Ga-doped graphene, and -Ga-N- co-doped graphene with an emphasis on O3 detection was examined in this work. The physical characteristics of -Ga-N-co-doped graphene are significantly altered upon O3 adsorption, which makes it a suitable choice for O3 detection molecular sensors. The interaction between the O3 molecule and the adsorbent is explained on the basis of their adsorption energy, adsorption distance and charge transfer. It was found that the adsorption of ozone molecules on the -Ga-N- co-doped graphene was more favorable in energy than that on the pristine one, representing the superior sensing performance of -Ga-N- co-doped system. In our work, we estimated the charge transfer between the O3 molecule and doped graphene nanostructures based on Mulliken population analysis. The calculated adsorption energy value shows the ozone molecule more firmly adsorbs on the surface of -Ga-N- co-doped graphene nanostructures (Eads = –1.74 eV) than that of pristine graphene (Eads = –0.41 eV), deriving from a stronger covalent bond between the ozone molecule and the -Ga- N- co-doped graphene nanostructures. Our findings thus suggest that -Ga-N- co-doped graphene could be a highly efficient gas sensor device for O3 detection in the environment.

Enhancing NO2 sensing performance through interface engineering in Cs2AgBiBr6/SnO2/ZnO-NRs sensor

Developing sensors that operate under ambient conditions with minimal energy consumption, ensuring a robust response and efficient recovery to toxic NO2 gas, can help take appropriate and corrective measures to protect human health from exposure to hazardous environments. In this study, we have successfully designed a gas sensor device based on a sandwich structure of Cs2AgBiBr6/SnO2/ZnO-nanorods (CABB/SnO2/ZnO-NRs) for detecting NO2 at room temperature. The insertion of the SnO2 layer between the CABB and ZnO-NRs, which works as a hole blocker, not only improves significantly the sensor stability from a few hours to more than a month but also offers the energy step for electron transfer from ZnO-NRs to CABB. The CABB layer further enhances the charge separation, transport, and conductivity from the perovskite to the ZnO surface, actively participating in the gas-sensitive reaction. The response of CABB/SnO2/ZnO-NRs sensors to 100 ppm NO2 is about 9 times higher than that of ZnO-NRs, showing a significantly improved NO2 sensing performance.

Deep-Learning-Based Blood Glucose Detection Device Using Acetone Exhaled Breath Sensing Features of α-Fe2O3-MWCNT Nanocomposites.

Owing to the correlation between acetone in human's exhaled breath (EB) and blood glucose, the development of EB acetone gas-sensing devices is important for early diagnosis of diabetes diseases. In this article, a noninvasive blood glucose detection device through acetone sensing in EB, based on an α-Fe2O3-multiwalled carbon nanotube (MWCNT) nanocomposite, was successfully developed. Different amounts of α-Fe2O3 were added to the MWCNTs by a simple solution method. The optimized acetone gas sensor showed a response of 5.15 to 10 ppm acetone gas at 200 °C. Also, the fabricated sensor showed very good sensing properties even in an atmosphere with high relative humidity. Since the EB has high humidity, the proposed sensor is a promising device to exactly detect the amount of acetone in EB with high humidity. The sensor was powered by a 3200 mAh battery with the possibility of charging using mains electricity. To increase the reliability and calibration of the sensing device, a practical test was taken to detect acetone EB from 50 volunteers, and a deep learning algorithm (DLA) was used to detect the effect of various factors on the amount of acetone in each person's acetone EB. The proposed device with ±15 errors had almost 85% correct responses. Also, the proposed device had excellent response, short response time, good selectivity, and good repeatability, leading it to be a suitable candidate for noninvasive blood glucose sensing.

Experimental and Theoretical Investigation of a Novel Acrylic Acid Gas Sensing Device Based on CuScO2 Microsheets

Experimental and Theoretical Investigation of a Novel Acrylic Acid Gas Sensing Device Based on CuScO2 Microsheets

Study the Effect of Ion Doping on ZnO Nanostructures for Room Temperature NH3 Gas Sensor

We investigated the impact of doping ion type on the performance of a ZnO-based ammonia gas sensor to show the capability of these ions to achieve high-performance gas sensing at room temperature. A sol-gel method was used to synthesize both doped and undoped ZnO nanostructures, while the gas sensor device was made by casting ZnO onto a glass substrate for a uniform thin film. Then Al electrodes were attached to the film. The characterization was carried out via field-emission scanning electron microscopy, energy-dispersive spectroscopy, X-ray diffraction, UV–vis, Pl luminescence, Brunnauer-Emmett-Teller, I-V characteristic, and gas sensor setup device. PL measurement shows an increase in green emission spectra with Ba ion shifting the peaks from VO to VO+ and VO+ to VO++ states. The gas sensor test at room temperature greatly enhances performance for certain ions. The Ba ions greatly influence gas sensor performance, increasing the response to 24 compared to 5 for undoped ZnO. The room-temperature enhancement achieved by the Ba ions could open the way to investigate more effective dopants for NH3 gas sensors.

Development of room-temperature operable TiO2-x-based hydrogen gas sensor with light irradiation

In this research work, TiO2-x-thin films are deposited through a radio-frequency magnetron sputtering technique by varying the oxygen partial pressure (pO2) of 6.0 %, 6.4 %, and 6.8 %. The X-ray diffraction analysis depicts the formation of the anatase phase of the annealed thin film with a high crystalline nature. The existence of higher oxygen vacancies and surface roughness are confirmed by the X-ray photoelectron spectroscopy and the atomic force microscopy analyses, respectively, in the 6.0 % pO2 grown thin film. At room temperature (30 °C) and dark conditions, the optimized TiO2-x-based hydrogen gas sensor device depicts a notable response of 22.9 % on exposure to 100 ppm of H2 gas. The irradiation of the ultra-violet (UV) light (λ = 365 nm) on the sensing material promotes desorption of the oxygen species and hence improves the responsivity of the sample. The highest responsivity of 87.7 %, along with faster response and recovery time of 9.4 s and 7.8 s, respectively, is achieved under the UV light irradiation and 100 ppm of H2 gas for the sample grown at 6.0 % pO2, which signifies its potential for the development of room temperature operable hydrogen gas sensor.

Structural and electrical conductivity studies of Polyaniline - WO3 hybrid nanocomposites for gas sensing applications

Abstract Conducting polymer – metal oxide based hybrid nanocomposites are a fascinating class of materials for miniaturized and flexible gas sensor devices. They exhibit enhanced physiochemical properties such as sensitivity, selectivity towards various volatile and hazardous chemical and bioanalytes. Our study focuses on conducting polyaniline (PANI) and WO3 nanocomposites, where different weight percentages (wt.%) of WO3 nanoparticles are embedded within the conducting PANI matrix using an in-situ oxidation polymerization synthesis technique. The surface morphology analysis indicated that the WO3 nanoparticles with an average grain size of ~200 nm are homogeneously distributed within the PANI nanofibers. The Fourier Transform Infra-Red (FTIR) spectrum analysis showed that the absorption peaks at 1111,1291, 1385, 1474, and 1560 cm−1 are typical of the conducting PANI emeraldine phase. We attribute the additional broad peak ranging between 840 to 720 cm−1 in the spectrum to WO3 phase, wherein, the intensity of the peak increases with WO3 content in case of hybrid composites. Current-voltage (I-V) characteristics for all our samples showed linear behaviour up to 1.2 volts. Temperature-dependent DC electrical conductivity (σ) studies measured from room temperature to 120°C for pure PANI, and PANI-WO3 composites showed an enhanced electrical conductivity of values up to 0.12 S/cm for PANI as compared to WO3 with σ ~ 1.4 x 10−3 S/cm. Pure PANI exhibits semiconducting behavior with an increase in electrical conductivity with temperature due to the charge carrier delocalization within the dispersed PANI backbone. The addition of higher concentrations of WO3 in composites leads to a metallic-like behavior, characterized by a decrease in electrical conductivity with temperature. These observations are attributed to the field-assisted band bending effects at the interfaces of PANI and WO3. Our composites show desired electrical characteristics suitable for gas sensing applications.

Realization of CO2 gas sensors and broadband photodetectors using metal/high-k CeO2/p-Si heterojunction

Realization of high-k dielectric oxide material-based devices such as photodetectors (PDs)/gas sensors fabricated on silicon substrate seems to be the utmost promising as a cost-effective approach to use in next generation optoelectronic field. We have explored metal/CeO2/p-Si heterojunction to construct as a broadband (BB) PD and CO2 gas sensor device. The effect of post-annealing procedure on photodetection and CO2 gas sensing characteristics were correlated using AFM, XRD, XPS, Raman and UV–Vis studies. At 0 V bias, the fabricated as-deposited Au/CeO2/p-Si PD device exhibits outstanding performance with enhanced responsivity of 85 AW−1 (at 910 nm), detectivity of 1.36×1015 Jones (at 850 nm), EQE of 1.3×104 % (at 850 nm), faster rise and fall times of 124 ms and 107 ms (at 900 nm). On the other hand, the photodetection characteristic parameters were slightly deteriorated as a function of annealing with respect to the as-deposited device. One of the causes for this fact was attributed to the increase in rms surface roughness (AFM) and decrease in absorbance of CeO2 thin film (UV–Vis). Additionally, Ag/CeO2/p-Si heterojunction was studied as a CO2 gas sensor and evaluated response/recovery times as a function of CO2 gas concentration. Sensing responses of CeO2 as-deposited, CeO2-400, CeO2-500, CeO2-600 and CeO2-700 °C were around 58, 60, 62, 81 and 90 %, respectively, when exposed to 200 ppm of CO2. Thus, we validate that the prospect of integrating enhanced performance in BB PDs and CO2 gas sensors using metal/CeO2/p-Si heterojunction could serve as a basic building block in near future optoelectronic applications. Furthermore, the effect of post-annealing temperature on the morphological, structural, BB photodetection and CO2 gas sensing properties were discussed in detail.

Green Valorization of Waste Plastics to Graphene as an Upcycled Eco-Friendly Material for Advanced Gas Sensing

The valorization technique successfully transformed waste polyethylene terephthalate (PET) into valuable carbon nanomaterial (CN)/graphene, while doped and undoped ZnO nanopowders were synthesized via sol–gel methods. Utilizing XRD, BET, TEM, EDX, FTIR, and TGA analyses, the synthesis of sp2 2D sheet, pristine, and doped ZnO nanostructures was confirmed. Solid-state gas sensor devices, tested under 51% relative humidity (RH), 30 °C ambient temperature, and 0.2 flow rate, exhibited a 3.4% enhanced response to H2 gas compared to CO2 at 50 ppm concentrations over time. Notably, the ZnO/CN sensor surpassed CN and ZnO alone, attributed to CN dopant integration with decreasing order of response performance as ZnO/CN > CN > ZnO. This study underscores the efficacy of valorization techniques in generating high-value carbon nanomaterials and their efficacy in bolstering gas sensor performance, with ZnO/CN demonstrating superior response capabilities.

Enhancing the Responsiveness of Thermoelectric Gas Sensors with Boron-Doped and Thermally Annealed SiGe Thin Films via Low-Pressure Chemical Vapor Deposition.

Thermoelectric gas sensor (THGS) devices with catalysts and Si0.8Ge0.2 thin films of different boron doping levels of 1018, 1019, and 1020 cm-3 were fabricated, and their transport properties are investigated. SiGe films were deposited on Si3N4/SiO2 multilayers on Si substrates using low-pressure chemical vapor deposition (LPCVD) and thermally annealed at 1050 °C. The Seebeck coefficients of the SiGe films were increased after thermal annealing, ranging from 191 to 275 μV/K at temperatures of 74 to 468 °C in air, and reaching the highest power factor of 6.78 × 10-4 W/mK2 at 468 °C. The thermal conductivity of the SiGe films varied from 2.4 to 3.0 W/mK at 25 °C. The THGS detection performance was tested for the H2 gas in air from 0.01 to 1.0%, and compared to the thermoelectric properties of the SiGe films. The high-temperature annealing treatment process was successful in enhancing the thermoelectric performance of both the SiGe films and sensor devices, achieving the best THGS performance with the sensor device fabricated from the annealed SiGe film with 1018 cm-3 boron-doped Si0.8Ge0.2.

Carbon gas sensors synthesized using acacia auriculiformis tree branches to detect methanol, ethanol, acetone vapors and ammonia gas

Metal oxides and carbon are the prime candidates of gas sensor devices. For the inaugural instance, carbonaceous particulate matter was meticulously derived from the branches of Acacia auriculiformis tree indigenous to Sri Lanka to detect vapor and gas. It is worth noting that biomass derived carbon had hitherto remain unexplored in the realm of vapor and gas detection. Carbon films were fabricated via the doctor blade method. The resistivity characteristic of these carbon samples exhibited discernible variations upon exposure to methanol, ethanol, acetone vapors, and ammonia gas. Notably, the gas-sensing properties of sample derived from the central core and the outer shell of Acacia auriculiformis tree branches were independently assessed in these vapors and gas at ambient temperature. Various analytical techniques, including X-ray diffraction (XRD), UVvisible absorption spectroscopy, Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM), were judiciously employed to scrutinize the structural attribute, optical band gap, chemical composition and surface morphology of the aforementioned carbon film samples, respectively. XRD analyses unequivocally confirmed the successful formation of carbonaceous films. FTIR spectra conspicuously delineated the presence of oxygen within the carbon films. Importantly, the optical band gap of our carbon films manifested within the range characteristic of organic carbon. Furthermore, the resistance of the carbon films demonstrated a significant reduction upon adsorption of each vapor. It is noteworthy that while gas sensitivities and response times were superior for carbon derived from the central core of the Acacia tree branches, the recovery times were notably expedited for carbon synthesized from the outer shell of these branches. The zenith of gas sensitivity, an impressive 187%, was recorded in response to 1000 ppm of methanol vapor, when employing carbon sourced from the central core of the acacia tree branches.

Improved ammonia sensitivity and broadband photodetection by using PBTTT-C14/WS2-QDs nano-heterojunction based thin film transistor

An ambient temperature broadband photo-detector and ammonia (NH3) gas sensor devices are developed with PBTTT-C14/WS2-QDs heterojunction thin film. This composite thin film of PBTTT-C14 conjugate polymer and hydrothermally synthesized WS2-QDs have been deposited by high yield and cost-effective ‘floating film transfer method’ (FTM), which works as a channel (p-type) of the thin film transistor (TFT). To determine the NH3 sensing behavior of the TFT, different concentrations of analyte (0.5–20 ppm) have been exposed on the channel of the TFT, which shows the response of ∼ 50 % at 10 ppm ammonia concentration that is significantly higher than PBTTT-C14 only TFT. This improvement is made possible by the NH3-responsive depleted layer of polymer/QDs heterojunction, which varies widely in the presence of ammonia. As a consequence, the ON/OFF ratio, carrier mobility and threshold voltage (Vth) of the device changes in a wider range. Besides, this TFT based gas sensor is also highly selective to the other interfering gases. In addition to NH3 gas sensitivity, This TFT also shows very high photosensitivity under white light illumination that enhances ‘ON’ and ‘OFF’ current of the device by ∼ 2.2 and 70 times, respectively under one sun white light illumination, whereas Vth of the device is shifted by ∼ 22 V. Besides, the device shows quite fast response/ recovery time under NH3 gas exposure and light illumination.

Room Temperature-Built Gas Sensors from Green Carbon Derivative: A Comparative Study between Pristine SnO2 and GO-SnO2 Nanocomposite

Room temperature-built gas sensors were fabricated from graphene oxide (GO), pristine and doped SnO2 nanostructures. The as-synthesized green carbon derivative (GO) nanomaterials were prepared from waste plastic precursor using Modified Hummer’s methodology. Pristine SnO2 and GO-SnO2 nanocomposite were synthesized employing a wet synthesis technique known as co-precipitation. The as-prepared nanoparticles were investigated for structural crystallographic and morphological features using X-ray diffractometry (XRD) and Transmission electron microscopy (TEM) analytical techniques. High-angle annular dark field (HAADF) and elemental quantifications of the nanopowders were investigated with the Energy dispersive X-ray spectroscopy (EDX). Textural features were determined with the assistance of Brunauer-Emmett-Teller (BET) analyzer. Thermogravimetric analysis (TGA) was performed to ascertain the material stability and degradability of the synthetic materials. Functional group and bond structure analysis was conducted using Fourier-transform infrared (FTIR) spectroscopy. Gas sensor devices were tested for responses towards CH4, H2, LPG, and CO2 gases at 20 ppm concentrations of each. GO-SnO2 nanocomposite sensing device showed optimal detection response towards the respective analyte gases with values of 5.00, 5.08, 4.90 and 3.41 respectively. The prepared nanocomposite showed stability and selectivity towards the target gases in an order of magnitude of H2 > CH4 > LPG > CO2. The optimal gas sensor device’s dynamic gas sensing response was ascribed to the GO doping effect which relatively increased its surface area (46.48 m2g-1) and absorption sites.

A Dual-Channel MoS2-Based Selective Gas Sensor for Volatile Organic Compounds.

Significant progress has been made in two-dimensional material-based sensing devices over the past decade. Organic vapor sensors, particularly those using graphene and transition metal dichalcogenides as key components, have demonstrated excellent sensitivity. These sensors are highly active because all the atoms in the ultra-thin layers are exposed to volatile compounds. However, their selectivity needs improvement. We propose a novel gas-sensing device that addresses this challenge. It consists of two side-by-side sensors fabricated from the same active material, few-layer molybdenum disulfide (MoS₂), for detecting volatile organic compounds like alcohol, acetone, and toluene. To create a dual-channel sensor, we introduce a simple step into the conventional 2D material sensor fabrication process. This step involves treating one-half of the few-layer MoS₂ using ultraviolet-ozone (UV-O3) treatment. The responses of pristine few-layer MoS₂ sensors to 3000 ppm of ethanol, acetone, and toluene gases are 18%, 3.5%, and 49%, respectively. The UV-O3-treated few-layer MoS₂-based sensors show responses of 13.4%, 3.1%, and 6.7%, respectively. This dual-channel sensing device demonstrates a 7-fold improvement in selectivity for toluene gas against ethanol and acetone. Our work sheds light on understanding surface processes and interaction mechanisms at the interface between transition metal dichalcogenides and volatile organic compounds, leading to enhanced sensitivity and selectivity.

Polycrystalline hollow MOF derived Co3O4 semiconductor to achieve room-temperature ammonia detection in human exhaled breath

Functional gas-sensing devices, on their portable and non-invasive merits, have stimulated a new exploring in clinical detection of gaseous biomarkers for critical diseases such as hepatopathy and nephropathy. Metal oxide semiconductor (MOS) sensors owing to their high sensitivity and facile manufacture appears to be the most promising for integration into illness surveillance devices. However, the high operating temperature of MOS poses great challenge for their realistic application. Herein, MOF-derived Co3O4 sensor series are prepared by facile manipulation of the morphological structure and crystalline state of their maternal ZIF-67. It shows that polycrystalline hollow ZIF-67 (PH-ZIF-67) derived PH-Co3O4 exhibits the optimum response and selectivity to trace NH3 at room-temperature sensing. PH-Co3O4 has a low limit of detection and well moisture resistance, which further behaves a significant quantitative response to actual exhaled breath of patients with hepatic encephalopathy. Furthermore, the scientific link between performance enhancement and structural adjustment is revealed by XPS, EPR, etc. It is found that the construction of the polycrystalline structure increases the oxygen vacancies generated during calcination, and additionally, the hollow structure provides more chemisorbed oxygen on the extra inner surface, which coupled to enhance the number of reactive oxygen sites responsible for the response improvement.

Flexible and Disposable Gas Sensors Based on Two-Dimensional Materials

Transition metal dichalcogenides (TMDCs) nanomaterials, in particular Molybdenum disulfide (MoS2), have been employed frequently as a basis for flexible gas sensors due to their extreme sensitivity to gas molecules, super mechanical and electrical properties, and large surface area. This work aims to study the behavior of the flexible gas sensor made of 2D-MoS2 under exposure to nitrogen dioxide (NO2) gas at the part per million (ppm) level. The mono-layered MoS2 was successfully synthesized by Chemical Vapor Deposition (CVD). The formation of MoS2 layers was confirmed by Raman spectroscopy and Photoluminescence (PL). Two different gas-sensing devices were fabricated by transferring two MoS2 samples (obtained from two positions inside the CVD tube) onto paper substrates. Specifically, upstream sample Sup was obtained from an area near the MoO3 source, and downstream sample Sdown was obtained from an area far from the MoO3 source. Both sensors showed a good response to a concentration as low as (1.5 ppm) of NO2. Although a high response of 62.8% along with a fast response of 9 sec were recorded by Sdown, the sensor showed a slow recovery time of 42 sec. On the other hand, Sup showed good stability with an appropriate response of 36.8% along with a reasonable response time and recovery times of 20 and 27 sec, respectively. Such behavior could be accredited to the difference in the reactivity in both MoS2 samples. This work opens the way for further improvements in manufacturing MoS2-based flexible gas sensors.