Film Gas Sensors Research Articles

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.

Regulation of receptor function in NiCo2O4-SnO2 heterojunction for H2S detection at room temperature

In recent years, the World Health Organization has increasingly emphasized air quality in production environments, leading to heightened societal demands for monitoring of pollutant gases at room temperature. Traditional semiconductor gas-sensitive materials, such as tin oxide (SnO2), have their gas-sensing performance largely limited by their receptor functions, resulting in common issues like low response and poor recovery at room temperature. Spinel-type bimetallic oxides, such as nickel cobaltate (NiCo2O4), offer a unique solution due to their rich adsorption sites on the surface, which provide distinctive receptor functions for detecting toxic gases at room temperature. Herein, NiCo2O4 is synthesized via a one-step hydrothermal method, with a porous spherical cluster structure, and combined with SnO2 to form a heterojunction. The NiCo2O4-SnO2 heterojunction film gas sensor exhibits excellent gas-sensing performance for H2S at room temperature, including high response, short response time, good repeatability, and selectivity. Additionally, the unique receptor functions of the NiCo2O4 were analyzed through first-principles calculations, revealing a semiconductor p-n conversion phenomenon in the presence of H2S gas. The composite also demonstrates a conversion from p-n heterojunction to n-n homojunction during the sensing process, enhancing its gas-sensing performance. This work not only addresses the receptor function limitations of traditional gas-sensitive semiconductor but also provides a feasible approach for controlling carrier types in semiconductors.

Quantum Confinement and End-Sealing Effects for Highly Sensitive and Stable Nitrogen Dioxide Detection: Homogeneous Integration of Ti3C2Tx-Based Flexible Gas Sensors.

The real-time and room-temperature detection of nitrogen dioxide (NO2) holds significant importance for environmental monitoring. However, the performance of NO2 sensors has been hampered by the trade-off between the high sensitivity and stability of conventional sensitive materials. Here, we present a novel fully flexible paper-based gas sensing structure by combining a homogeneous screen-printed titanium carbide (Ti3C2Tx) MXene-based nonmetallic electrode with a MoS2 quantum dots/Ti3C2Tx (MoS2 QDs/Ti3C2Tx) gas-sensing film. These precisely designed gas sensors demonstrate an improved response value (16.3% at 5 ppm) and a low theoretical detection limit of 12.1 ppb toward NO2, which exhibit a remarkable 3.5-fold increase in sensitivity compared to conventional Au interdigital electrodes. The outstanding performance can be attributed to the integration of the quantum confinement effect of MoS2 QDs and the conductivity of Ti3C2Tx, establishing the main active adsorption sites and enhanced charge transport pathways. Furthermore, an end-sealing effect strategy was applied to decorate the defect sites with naturally oxygen-rich tannic acid and conductive polymer, and the formed hydrogen bonding network at the interface effectively mitigated the oxidative degradation of the Ti3C2Tx-based gas sensors. The exceptional stability has been achieved with only a 1.8% decrease in response over 4 weeks. This work highlights the innovative design of high-performance gas sensing materials and homogeneous gas sensor techniques.

Sensing performance of lead monoxide-doped tin oxide thick film gas sensor

Sensing performance of lead monoxide-doped tin oxide thick film gas sensor

Determining the influence of relative humidity on the working parameters of a gas sensor based on zinc oxide

This paper investigates the dependence of operating parameters of a gas sensor based on zinc oxide obtained by the method of direct current magnetron sputtering. The production of gas sensor films was carried out using a VUP-5M vacuum unit with an original material-saving magnetron. The study of the dependence of the working parameters of the gas sensor was carried out under standard conditions. It was found that with increasing humidity, the electrical resistance of the gas sensor decreases and, accordingly, the sensitivity to the target gas decreases. A significant reaction of the gas sensor to an increase in humidity was observed in the range of 65–80 % relative humidity. The mechanism of influence of relative humidity on the sensitivity of a gas sensor based on ZnO was investigated. The change in resistance of the gas sensor is caused by trapped electrons on adsorbed oxygen molecules on the surface of the sensitive layer. The capture of electrons from the conduction zone leads to bending of the conduction zone and an increase in the space charge zone, respectively, to a change in the resistance of the sensitive layer of the gas sensor. In the atmosphere, when O2 molecules are adsorbed on the ZnO surface, they remove an electron from the conduction band. The reaction of oxygen adsorbed on the ZnO surface with reducing gases and the replacement of adsorbed oxygen with other molecules changes the bending of the conduction band and reduces the area of space charge. Adsorption of water on the surface of zinc oxide occurs according to the dissociation mechanism, which consists in the adsorption of steam molecules or hydroxyl groups with the subsequent displacement of previously adsorbed oxygen and free electrons and, accordingly, leads to a decrease in the sensitivity of the gas sensor. In addition, the adsorption of water vapor (H2O) molecules leads to less chemisorption of oxygen species on the ZnO surface due to the reduction of the surface area, which is responsible for the sensor response. Approaches to reduce the influence of relative humidity on the sensitivity of a gas sensor based on zinc oxide have been proposed

An olfactory figure-ground segregation: The resistance fluctuation analysis of acetone gas for acetone/random gas mixtures recognition

Recognizing the disease-specific odor in human breath is a feasible non-invasive disease detection strategy. The most critical issue of olfactory diagnosis lies in recognizing the target gas among complex respiration compounds accurately. This paper demonstrated a novel approach to binary gas mixture recognition, exploring nature-related features of the target “odor object” (acetone gas) regardless of its “noisy background” (ethanol or methanol gases). Those features were extracted from time-series resistance fluctuations collected with one conventional SnO2 thin film gas sensor (TF) or our sensitivity-controllable lines gas sensor (TL). 6 features (variance, root mean square, band power, relative band power, entropy, and the number of values crossing mean value) were selected and analyzed with k-Nearest Neighbor regressor (KNN) and Support Vector Regressor (SVR). 18 types of acetone/ethanol mixtures and 3 acetone/methanol mixtures were tested with the proposed models and further explained with SHAP (SHapley Additive exPlanations). The model of TL and KNN achieved the optimal performance of regressing acetone concentrations with the coefficient of determination (R2) of 0.97 and 0.95 in individual gas and binary gas mixtures, respectively. Entropy was found to be the key feature that was linked to sensor properties. Finally, acetone gas data mixed with simulated gas and random noise data were experimented to evaluate our models’ potential for acetone/random gas mixture recognition and robustness.

Electrophoretically fabricated tubular Taguchi type ZnFe2O4-based thick film gas sensor

A facile method has been developed to fabricate tubular chemi-resistive type thick film gas sensors. The method includes the preparation of a stable suspension of the sensing material followed by fabrication of a uniform sensing layer by electrophoretic deposition (EPD) technique on the tubular Taguchi type alumina sensing element. Prior to fabrication of the sensing film, the non-conducting alumina tube is in-situ coated with a conducting polymer (polypyrrole). EPD grown Taguchi sensors demonstrate excellent sensing capability, with responses of 87 % and 46 % to 400 and 12.5 ppm acetone, respectively. Moreover, these sensors exhibit an impressive fast response time of approximately 20–30 s. The developed process can be extended to various other sensing materials to develop Taguchi type sensors in a repetitive manner.

Ultrasensitive and highly selective NO2 gas sensing of porous MXene nanoribbon assemblies

Nitrogen dioxide (NO2), a potential source for acid rain and photochemical smog, is known to harm human health, plant growth and crop yield, and the environment. Therefore, designing a room temperature operable NO2 gas sensor with exceptional sensitivity and selectivity is extremely important. Herein, we report that Ti3C2Tx MXene (Tx = –OH, –O, –F, etc.) nanoribbons containing inbuilt anatase TiO2 on their surfaces achieve highly sensitive detection of NO2 gas at room temperature with negligible cross-responses to other gas analytes. The MXene nanoribbons' ability to form porous gas sensor film facilitates the efficient diffusion of gas molecules and the sufficient utilization of surface-active sites, leading to a high response of −87.5 % to 50 ppm NO2 gas exposure with a response time of 9.2 s at room temperature. Signifying the low concentration NO2 detection abilities, MXene nanoribbons displayed a response of −4.4 % towards 500 ppb NO2 exposure. MXene nanoribbons' NO2 gas sensing ability is carrier gas dependent and declines monotonically as operating temperature increases. In addition, the performance comparison experiments conducted at 50 ppm NO2 gas exposure suggest that the MXene nanoribbons' response is 14.3, 1.6, and 45.3 times higher than the responses of multilayer MXene, MXene-TiO2 nanoplate, and MXene-TiO2 nanoparticle structures, respectively. The explicit selectivity and high sensitivity of MXene nanoribbons toward NO2 gas are expected to provide exciting opportunities for personal safety and environmental monitoring.

Effect of doping on the structural, optical and electrical properties of TiO2 thin films for gas sensor

Effect of doping on the structural, optical and electrical properties of TiO2 thin films for gas sensor

Investigation of SnO2-based Thick Film Gas Sensors through Optimization of Artificial Neural Networks Parameters for Hazardous Gas Detection

In the present study, a systematic approach for optimizing artificial neural network parameters for the detection of a greenhouse gas carbon di-oxide (CO2) through SnO2-based thick film gas sensors for a wide range of concentrations is presented. The un-doped and Pd-doped SnO2 thick film is used as the sensor. The approach includes dataset preparation, input and output variable selection, artificial neural network architecture selection, training, validation, hyperparameter tuning, and testing. The input variables were selected based on their relevance to the problem at hand, such as the concentrations of the gas. The output variables were the changes in resistance of the sensor in response to the corresponding input variables. The artificial neural network architecture was carefully chosen, considering factors like the number of layers, neurons per layer, activation functions, and learning rate. Using the prepared dataset, the artificial neural network was trained by adjusting the connection weights between neurons to minimize the disparity between actual and predicted sensor responses. After training, the accuracy of the artificial neural network and generalization ability were assessed using a separate dataset for validation. The approach in the presented study can be used for different types of gases, and can be utilized in various applications

Real-Time Monitoring of Toxic Gases using Artificial Neural Networks with SnO2-based Thick Film Gas Sensors

The present study aims to develop an advanced system for real-time identification and measurement of harmful gases using Pd-doped SnO2-based thick film gas sensors integrated with Artificial Neural Networks. These sensors, renowned for their exceptional sensitivity and selectivity, undergo testing in a meticulously controlled gas chamber environment. Here, gas concentrations, temperature, and humidity are meticulously controlled. The gas chamber setup allows for mixing gases like carbon monoxide, nitrogen dioxide, and sulfur dioxide with nitrogen gas to achieve desired concentrations ranging from to . Temperature is kept at and relative humidity is maintained between to . The sensitivity analysis of the sensor demonstrates its adeptness in detecting low concentrations of target gases, with sensitivity increasing as gas concentrations rise, also the selectivity assessments highlight their ability to accurately differentiate between target gases and common interferents, ensuring precise detection even in complex gas mixtures. Response time testing indicates rapid detection capabilities, crucial and useful for emergencies. The Artificial Neural Network model, trained via the backpropagation algorithm, demonstrates remarkable accuracy, precision, recall, and F1 scores in predicting gas concentrations. The findings indicate that this integrated system offers a reliable and efficient solution for toxic gas detection, with potential applications in industrial safety and environmental monitoring.

A photoactivated gas sensor of SnO2 ordered spherical pore array: Electrodeposition-based construction and application in room temperature detection of ethanol gas

The photoactivated semiconductor film gas sensor has received increasing attention in the room temperature detection of environmental gases, but the uncontrolled surface morphology and the associated light area still limit the application and the exploitation of the sensing response potential. Herein, a monolayer colloid crystal template strategy based on electrodeposition was introduced to in situ construct SnO2 ordered spherical macropore array (OSPA) sensors used for ethanol gas detection under a UV-LED. By adjusting the electrodeposition time, various OSPAs with regularly variable heights of pore-wall were obtained. The light area of the array could then be systematically regulated based on the spherical pore-walls’ curve characteristics. The OSPAs showed a nearly 5-fold change in response in detection of 100 ppm ethanol gas under a 265 nm UV-LED, corresponding to the light area from small to large. The highest sensing response is even higher than that of almost all SnO2-based room temperature sensors that have previously been reported. Moreover, the sensor has excellent selectivity and linearity, a 1 ppm lower detection limit, and exceptional long-term stability. These finds are corroborated by analyses of the electrodeposition mechanism, the sensing mechanism, optical simulations, and various characterizations. The strategy provides an efficient way to tune and enhance the sensing response of a photoactivated gas sensor, and is beneficial for exploiting the sensing potential of a semiconductor.

Cu-doped LaFe1-xCuxO3 perovskites nano-crystallites for enhanced VOCs detection

Highly sensitive, and versatile metal oxide semiconductors, including perovskites, are used in gas sensors for environmental, and safety applications. This study described the substitution of Cu for Fe in LaFe1-xCuxO3 nanostructured perovskites, denoted as Cx’s, using Pechini sol-gel method at 600 °C, to detect the volatile organic compounds (VOCs). Characterization techniques, including TGA/DTA, XRD, BET, FE-SEM/EDS, Raman, PL, XPS, UV-DRS, FTIR, TPD-O2 and ethanol chemisorption showed significant enhancements in physicochemical, and gas sensing properties of LaFeO3 perovskites, upon copper doping in its structure. The responses to ethanol improved, the average crystallite and nanoparticle sizes decreased, and the electrical conductivity increased as the perovskite copper content increased to x = 0.6. The crystallite size of 20.5 nm for LaFeO3 decreased to 12.7 nm for LaFe0.4Cu0.6O3. The SC0.6 thick film gas sensor showed high potential in the VOCs detection, with best responses of 17 for ethanol, 18 for acetone, and 16 for acetaldehyde when exposed to each of 300 ppm at 300 °C. At these conditions, the response and recovery times of highly stable SC0.6 sensor for ethanol gas were 4 and 11 min, respectively.

Design of an economic, portable, compact, and indigenous instrument setup for measuring sensing characteristics of thin film gas sensors.

Thin film gas sensor characterization is very demanding for various applications because of technical design trade-offs in commercially available gas sensors. For gas sensing characterization, a suitable gas-testing experimental setup is very much needed in this context. Various factors in the experimental setup can affect a thin film gas sensor's response beyond gas exposure. These factors include the test chamber's volume, relative humidity, uniform operating temperature, uniform pressure, uniform gas density, uniform gas distribution, uniform gas concentration in the gas chamber, and uniform relative gas flow velocity over the surface of the sensor. All these environmental parameters, not being so predictive in nature, induce an inherent design trade-off in the experimental setup design. Although all the commercially available gas testing experimental setups are equally good considering the dedicated purpose for which they are made. However, all of them are for generic applications but not for specific applications because of their inherent trade-offs in their usability features. Those trade-offs always provide an opportunity to introduce a new setup with its own unique advantages. Hence, in this article, we have presented a portable, compact, indigenous gas sensing experimental setup for studying the performance of gas sensors. We have characterized and tested the setup using a ZnO based thin film gas sensor when exposed to CO2 gas at concentrations ranging from 1445 to 4631 ppm. The proposed gas sensing setup's compact size offers unique advantages, including portability and compatibility for uniform environmental conditions.

Advanced self-convergent calibration for selenized two-dimensional film gas sensors

This research presents a gas sensor system featuring a selenized two-dimensional (2D) film as its primary sensing material, integrated with metal-gate-coupled floating gate devices to enable self-convergent calibration. The inherent variability in resistance levels of 2D gas-sensing materials across different devices has been a significant challenge, resulting in substantial deviations of the output signal within the sensing circuit. To address this issue, we introduce a novel self-convergent operational technique, which effectively mitigates the impact of resistance variations thereby enhancing the precision and reliability of gas-sensing outcomes. The proposed gas sensor system promises to deliver consistent and accurate results, even with device-to-device resistance variations, making it a valuable contribution to gas-sensing technology. This work holds substantial potential for various applications requiring highly precise gas detection and quantification.

Advancements in Zinc Oxide (ZnO) thin films for photonic and optoelectronic applications: A focus on doping and annealing processes

This study focuses on the potential of Zinc oxide (ZnO) as a versatile material for photonic and optoelectronic applications, owing to its direct wide bandgap (Eg ≈ 3.175 eV) and significant excitonic energy. ZnO, both in pure and doped forms, exhibits promise in various domains, including solar cells, photoelectrochemical cells, thin film transistors, gas sensors, and nanogenerators. The manuscript delves into the methodologies for producing ZnO:B films, including reactive evaporation, evaporation from two sources, and flash evaporation, each addressing the challenges of achieving the desired film composition and structure. The investigation reveals that the optimized ZnO:B films possess crystalline phases with hexagonal lattice structures, demonstrating significant enhancements in electrical conductivity upon specific annealing treatments. The research underscores the impact of doping and microstructure modifications on the optoelectronic properties of ZnO films, contributing to advancements in semiconductor-based thin films and powders.

Study of rectifying properties and true Ohmic contact on Sn doped V2O5 thin films deposited by spray pyrolysis method

In this paper, V2O5 thin-films were deposited on glass substrates by using the ultrasonically nebulized spray pyrolysis technique. Doping concentration of 1 %, 2 %, and 7 % of Sn, in V2O5 thin-films is studied by using metal–semiconductor-metal (MSM) based device structure. X-ray diffractometer (XRD) and field emission scanning electron microscope (FESEM) were used to analyze the surface morphology of the thin films. The XRD spectroscopic analysis shows the crystal size of above-mentioned samples, to be respectively 39 nm, 58 nm, 60 nm and 72 nm. The FESEM images showed the enhancement of crystal size with increase in Sn doping concentration. The optical properties of Sn doping on V2O5 thin film were studied by using UV–Vis spectrometer. The UV–Vis spectroscopic analysis shows that the absorption coefficient values decrease with increase in concentration of Sn metal doping in V2O5 thin film. The activation energies of all thin-film samples were calculated from Arrhenius plots and were found to be 0.938 eV, 1.101 eV, 1.11 eV and 1.169 eV, respectively, for all the thin-film samples mentioned above. The MSM based structure was fabricated by using a shadow mask and thermal evaporation. Later, the I-V characteristics of all the thin films were obtained by using semiconductor parameter analyzed at a biased voltage between −50 V and + 50 V with a step size of 1 V. Rectification ratio of the V2O5 films is significantly enhanced as the doping concentration increases. It was found that the rectification ratio of undoped V2O5 thin films increased linearly from 1.018 to 1.059 with an increase in temperature from room temperature to 130 °C. Similar trend was followed for 1 % (from 1.079 to 1.198), 2 % (from 1.081 to 1.224) and 7 % (from 1.095 to 1.311) Sn doped films. These results show the potential application of V2O5 thin films in the field of optoelectronics and thin film gas sensors.

Adjusting surface coverage of Pt nanocatalyst decoration for selectivity control in CMOS-integrated SnO2 thin film gas sensors.

Smart gas-sensor devices are of crucial importance for emerging consumer electronics and Internet-of-Things (IoT) applications, in particular for indoor and outdoor air quality monitoring (e.g., CO2 levels) or for detecting pollutants harmful for human health. Chemoresistive nanosensors based on metal-oxide semiconductors are among the most promising technologies due to their high sensitivity and suitability for scalable low-cost fabrication of miniaturised devices. However, poor selectivity between different target analytes restrains this technology from broader applicability. This is commonly addressed by chemical functionalisation of the sensor surface via catalytic nanoparticles. Yet, while the latter led to significant advances in gas selectivity, nanocatalyst decoration with precise size and coverage control remains challenging. Here, we present CMOS-integrated gas sensors based on tin oxide (SnO2) films deposited by spray pyrolysis technology, which were functionalised with platinum (Pt) nanocatalysts. We deposited size-selected Pt nanoparticles (narrow size distribution around 3 nm) by magnetron-sputtering inert-gas condensation, a technique which enables straightforward surface coverage control. The resulting impact on SnO2 sensor properties for CO and volatile organic compound (VOC) detection via functionalisation was investigated. We identified an upper threshold for nanoparticle deposition time above which increased surface coverage did not result in further CO or VOC sensitivity enhancement. Most importantly, we demonstrate a method to adjust the selectivity between these target gases by simply adjusting the Pt nanoparticle deposition time. Using a simple computational model for nanocatalyst coverage resulting from random gas-phase deposition, we support our findings and discuss the effects of nanoparticle coalescence as well as inter-particle distances on sensor functionalisation.

A V2O5/MWCNTs composite thin film gas sensor for the fast detection of ammonia

In the present study, we develop a new ammonia gas sensor by using sol-gel method and post annealing process. The structure, morphology, and composition of the gas sensor have been characterized by SEM, XRD, XPS, and Raman. The results prove that the V2O5/MWCNTs (multi-walled carbon nanotubes) composite thin film was successfully deposited on alumina substrate. By comparing the responses of the gas sensor to ammonia gas at different temperatures, we find out that the sensor shows good response and excellent response time to ammonia gas at 200 ℃. At this temperature, the sensor exhibits a response of 20% for 100 ppm of ammonia gas with a response time of only 8 s. We find its response time is better than that of ammonia sensors based on single V2O5 or MWCNTs materials. The outstanding sensitivity and unique response curve can be mainly attributed to the semiconductor properties of the two materials and the formation of P-N heterojunction at their interface.

Exploration of annealing effect on physical properties of Indium oxide films for gas sensors

In the present work, an influence of annealing on physical properties of Indium oxide (In2O3) thin films is investigated for H2S gas sensing applications. The In2O3 films of thickness 450 nm are grown employing physical vapor deposition based electron beam evaporation technique followed by annealing at different temperatures viz. 250 °C, 325 °C and 400 °C for 1 h. XRD analysis indicated crystallization of annealed films in cubic phase having (222) predominant reflection where crystallite size of all the In2O3 layers is tuned in range of 17–26 nm. Optical analysis unveiled higher transmittance in ultraviolet and visible regions whereas absorbance of films is fluctuated with thermal annealing. The 2D AFM images of In2O3 films indicated distribution of nano-spherical grains and these In2O3 films also indicated Ohmic nature where resistivity is detected to be enhanced with annealing. Optimized In2O3 films based gas sensors showed gas response of 0.85 and sensor response of 15.38 % towards toxic H2S gas.