Gas Sensor Response Research Articles

Sensing mechanisms of CO and H2 with NiO material – DRIFTS investigations

The response of nickel oxide gas sensors towards CO and H2 and the underlying gas sensing mechanisms were investigated with special focus on the influence of ambient humidity interference. Surface reactions were tracked by using diffuse reflectance infrared Fourier transformation spectroscopy with simultaneous resistance measurements. The sensor response to both gases is barely influenced by the background humidity. Spectroscopic results reveal that the underlying processes at the surface are almost identical for CO and H2 reception and similar to the effect of the removal of oxygen. Accordingly, the detection of the analytes is based purely on the reduction and oxidation of the oxide material instead of the formation of analyte specific surface species.

Suppressed polaronic conductivity induced sensor response enhancement in Mo doped V2O5 nanowires

In this paper, we show the direct correlation between the suppression of the polaronic oxygen vacancy defect (Vo) density and gas sensor response of 1 at. % Mo-doped V2O5 nanowires (MVONWs). Doping 1 at. % Mo5+ leads to substitution at the V5+ site in V2O5 nanowires (VONWs) and, therefore, reduction in Vo defects. This, in turn, affects the charge carrier hopping sites and, subsequently, enhances the sensor response at lower temperatures (<320 °C). The Mo5+ dopants lead to the lowering of Fermi energy (EF) toward valence band maxima due to the reduced Vo donor density. The polaron suppression is confirmed with the activation energy of polaron hopping, increasing from 195 to 385 meV in VONWs and MVONWs. As a result, the response to ethanol gas enhanced as the depletion width is widened for the given cross section of the nanowires. This may lead to a large depletion controlled cross-sectional area and, therefore, better sensitivity. At about 350 °C, VONWs show a change in the slope of resistance vs temperature (MIT), which is not observed in the case of MVONWs. This is attributed to the presence of the enhanced non-stoichiometry of V ion resulting in metallic behavior and accompanied by a sudden rise in the sensor response at this temperature. Moreover, the absence of MIT may be attributed to the lack of such a sudden rise in the response in MVONWs.

ALGORITHM FOR PROCESSING GAS SENSOR’S RESPONSE KINETICS DATA USING EXTENDED EXPONENTIAL FUNCTION WITHOUT NUMERICAL DIFFERENTIATION

The features of the use of computer technologies for processing experimental data for solv-ing the problems of automation of research of materials for gas sensitive sensors are con-sidered. An algorithm for processing the kinetic dependence of the response of gas sensors based on the model of an extended exponential function are proposed, which does not use numerical differentiation operations when finding the parameters of this model. This allows to signifi-cantly reduce the influence of the presence of data spread in the coordinates of the approxi-mating diagrams that are used in calculating the model parameters, increase the accuracy of their determination and contribute to the implementation of an automated information measuring system for the process of computer processing and analysis of experimental data.

Growth and Characterization of Sputtered ZnO:ZnGa2O4 Dual-Phase Films on Sapphire Substrates for NO Gas-Sensing Applications

Nitric oxide (NO) released from combustion facilities and automobiles adversely affects the respiratory systems and environment. Hence, the development of a NO gas sensor with high sensing response to measure NO gas levels is significantly commendable. Herein, we report the growth of ZnO:ZnGa2O4 dual-phase films on the c-plane sapphire substrates using radio-frequency magnetron sputtering for NO gas sensors. The microstructures and morphology of ZnO:ZnGa2O4 films were systematically investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy. The XRD patterns revealed the polycrystalline nature of the ZnO:ZnGa2O4 film with (222) preferred orientation. The SEM micrographs showed the presence of grain boundaries in the nanostructures over the surface of the film due to the recrystallization of grains upon annealing the film at 700 °C. This ZnO:ZnGa2O4 film was employed to fabricate the NO gas sensor, which demonstrates significantly high sensor response of 28.6 at 400 °C operating temperature with the quick response time of 8 s and recovery time of 140 s upon exposure to 100 ppm NO gas concentration. The ZnO:ZnGa2O4-based gas sensor is found to be highly selective for NO gas among NO, CO2, and SO2 gases. The gas-sensing mechanism behind the high response of ZnGa2O4-based NO gas sensors is described. The results achieved in this study are particularly important in terms of the development of technologies for the early detection of poisonous and hazardous gases from polluting industries and metropolises, which is crucial for environmental security.

Errors in ambient gas concentration measurement caused by acoustic response of electrochemical gas sensors

Electrochemical sensors are used to measure electroactive gases in ambient air monitoring applications. These sensors typically contain sulfuric acid electrolyte, and porous carbon working, reference, and counter electrodes. Current fluctuations caused by fluctuations in the meniscus contact shape or area at the 3 phase gas-electrolyte-electrode interface as a result of ambient pressure fluctuations have been suggested as a potentially significant source of error in sensor measurements. We confirm in the present work that the pressure oscillations associated with ambient sound can indeed lead to significant signals. We show, for a variety of commercial sensors for ambient nitrogen dioxide (NO2), that acoustic noise equivalent to that from a nearby motorcycle or heavy goods vehicle can cause transient current fluctuations at 2 Hz sampling rate equivalent in the sensor output to as much as that due to 100 parts per billion by volume (ppb) of NO2, and with a root mean square (RMS) variation averaged over 10 s of approximately 40 ppb equivalent. These observations indicate that electrochemical gas sensors can behave as “microphones” in response to loud noise. The impact of acoustic noise should be considered when using electrochemical sensors to measure ambient air quality in areas of significant noise pollution, particularly if the aim is to resolve local transient concentration variations.

Design of sample display system on electronic nose for synthetic flavor classification

This study aimed to design a controlled sample display system on an electronic nose and test its performance for classifying synthetic flavors. There are four primary components to the electronic nose design. They are a controlled sample display system, detector, signal conditioning and preprocessing, and pattern recognition software. The sample display system consists of six vials. The sample room temperature setpoint is set to 40 ℃. The controlled sample display system has one heater and two fans to even the room temperature. The one-time data collection process consists of flushing (120 s), collecting (180 s), and purging (180 s). The samples for the performance test were synthetic flavors with four different aromas; durian, mocca, orange, and strawberry. Data analysis of gas sensor response was done through two stages; pre-treatment data processing and principal component analysis (PCA). The four samples were clearly different from others, according to the PCA results. The scores of the PC-1, PC-2, and PC-3 cumulative variance were 98.28%.

High Gas Response Performance Based on Reduced Graphene Oxide/SnO2 Nanowires Heterostructure for Triethylamine Detection

SnO2 nanowires are locally synthesized by a simple thermal evaporation method and its growth mechanism is confirmed. Here, we present a simple strategy for realizing reduced graphene oxide (RGO)/SnO2 nanowires heterostructure. As expected, the heterostructure gas-sensing response is up to 63.3 when the gas concentration of trimethylamine (TEA) is 50 ppm, and it exhibits an excellent dynamic response with high stability at 180 °C. A low detection limit of 50 ppb level is fully realized. Compared to SnO2 nanowires, the sensing performance of the RGO/SnO2 heterostructure-based sensor is greatly enhanced, which can be ascribed to the RGO and the heterostructure. The RGO/SnO2 composite engineering poses an easy way to make full use of the advantages originating from RGO and heterostructure.

Interference suppression strategies for trace minor component of semiconductor gas sensor based on temperature modulation mode

Complex target gas environment containing trace minor components has strong interference to the detection results of gas sensing. In this paper, the performance degradation caused by minor components was solved by studying the microscopic characteristics related to gas sensitivity and response curve error in the time domain. By mastering the microscopic characteristics of gas sensitivity, cooling time domain was selected by sub-sensitivity characteristics to enhance recognition effect of the main components and to minimize the influence of minor components on the response. By mastering the microscopic characteristics of the response curve error, high temperature time domain was selected, which had a high tolerance to the error caused by the minor components. The anti-interference time domain was located synthetically based on these two time domains. Meanwhile, the rationality of this method was analyzed from the view of the gas-sensing characteristic and response curve error of dynamic response. Finally, the decision tree algorithm was used to verify the feasibility of this partition method. The proposed method can effectively solve the interference of trace minor component gas in the detection process, which is conducive to the expansion of the application of semiconductor sensor in the complex gas environment.

The sensitivity enhancement of TiO2-based VOCs sensor decorated by gold at room temperature

Detection of hazardous toxic gases for air pollution monitoring and medical diagnosis has attracted the attention of researchers in order to realize sufficiently sensitive gas sensors. In this paper, we fabricated and characterized a Titanium dioxide (TiO2)-based gas sensor enhanced using the gold nanoparticles. Thermal oxidation and sputter deposition methods were used to synthesize fabricated gas sensor. X-ray diffraction analysis was used to determine the anatase structure of TiO2 samples. It was found that the presence of gold nanoparticles on the surface of TiO2 enhances the sensitivity response of gas sensors by up to about 40%. The fabricated gas sensor showed a sensitivity of 1.1, 1.07 and 1.03 to 50 ppm of acetone, methanol and ethanol vapors at room temperature, respectively. Additionally, the gold nanoparticles reduce 50 s of response time (about 50% reduction) in the presence of 50 ppm ethanol vapor; and we demonstrated that the recovery time of the gold decorated TiO2 sensor is less than 40 s. Moreover, we explain that the improved performance depends on the adsorption-desorption mechanism, and the chemical sensitization and electronic sensitization of gold nanoparticles.

Investigating the properties of nickel oxide thin films prepared via DC reactive magnetron sputtering for potential application in gas sensing

Nanocrystalline nickel oxide (NiO) thin films were developed on Si substrate using DC reactive magnetron sputtering. The goal of our study was to investigate the impact of O2/Ar gas ratios (varied from 10% to 20%) on the properties of sputtered NiO thin films. Further, after the deposition, all the samples were annealed at 400 ℃ in air. The prepared thin films were characterized by GIXRD, UV–visible spectroscopy, and FESEM to investigate the structural, optical, and morphological properties, respectively. The GIXRD results demonstrated the formation of the polycrystalline nature of sputtered NiO films with the FCC phase. As the O2 ratio was increased, the preferred orientation of NiO thin film was found to change from (111) to (200) plane with an average crystallite size of ∼ 14 nm. The UV–visible spectroscopy measurements show the semi-transparent nature of the deposited NiO films with a decreasing trend in the transmittance value from ∼ 83% to ∼ 77% with an increase in the O2 to Ar gas ratio. This behavior could be due to the poor crystallization of the film. However, the change in the calculated band gap of NiO films with respect to O2 partial pressure was found to be negligible and lay between 3.82 eV and 3.84 eV. The surface morphology was studied using FESEM, which depicted the uniform and homogenous distribution of grains with cracks on the film that improve the gas permeability; hence it may prove beneficial for gas sensing applications. Besides, the observed contact angle, as measured by the wettability test for the NiO films at different oxygen partial pressure, was greater than 90°, which shows that all the deposited NiO thin films are hydrophobic. Additionally, we have tested the CO gas sensor responses of the NiO sample deposited at a 15% O2/Ar ratio due to the high hydrophobic nature among all three ratios.

Nanocomposites of Quasicrystal Nanosheets and MoS2 Nanoflakes for NO2 Gas Sensors

Nitrogen dioxide (NO2) is one of the air pollutant gases that harms the environment, human health, and the world ecosystem. The main source of NO2 emission is the combustion engine, and therefore, it requires a gas-sensing device to monitor the emission. Herein, we report a highly sensitive and selective NO2 sensor by low-cost and scalable fabrication methods using a hybrid nanocomposite of two-dimensional (2D) Al70Co10Fe5Ni10Cu5 quasicrystal (QC) nanosheets and MoS2 nanoflakes. The 2D-QC nanosheets combined with MoS2 nanoflakes form a heterostructure, which improves the gas sensor’s performance. The Al70Co10Fe5Ni10Cu5/MoS2 heterostructure showed an excellent gas-sensing response (ΔR/Ra %) of ∼66%, which is about 2.27 times higher than that of the pristine MoS2 nanoflakes-based sensor for 100 ppm NO2 at 100 °C. In addition, the hybrid nanocomposite gas sensor device also exhibited high selectivity and good sensitivity. The improvement in the gas sensor performance is explained using the density functional theory analysis, which illustrates that the 2D-QC provides a lot of active sites for the adsorption of NO2 molecules, explained using the adsorption energy, charge difference density, and Bader charges analysis. These gas-sensing results emphasize the significance of the integration of the 2D Al70Co10Fe5Ni10Cu5 QC with MoS2 nanoflakes, being useful to the design and development of NO2-based gas-sensing technology and holding great promise in environmental protection.

A CNN-Based E-Nose Using Time Series Features for Food Freshness Classification

In this article, a convolutional neural network (CNN)-based electronic nose system is proposed, which utilizes time series features to classify the freshness of food. The electronic nose is composed of three elements: a highly sensitive miniaturized micro-electromechanical systems (MEMS) metal–oxide (MOX)–semiconductor gas sensor array, a complementary metal–oxide–semiconductor (CMOS) integrated circuit, and a CNN-based classification unit. The gas sensors exhibit exceptional sensitivity, low power consumption, and a small size. The interface circuit for sensor readout is fabricated using a standard 180-nm CMOS process, with an area of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.5\times1.5$ </tex-math></inline-formula> mm. The interface chip mainly consists of an analog-to-digital converter (ADC), an electrically erasable programmable read-only memory (EEPROM), an oscillator (OSC), power management, a heater driver, and a digital control circuit. Fixed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${R}{(}{t}{)}$ </tex-math></inline-formula> exposures are taken at specific intervals under gas input and output conditions. Time series features are extracted from a diverse set of sensor signals, which allows the subtle differences in various food odors at different freshness levels to be identified. The incorporation of time series feature extraction extends the data features, resulting in enhanced classification accuracy with a limited number of sensors. An abstract odor map input to the CNN is formed by combining the steady-state and transient information of the gas sensor response. The final freshness classification accuracy of 20 types of foods is 97.3%. The accuracy is improved by 6.5% after the implementation of time series feature extraction, demonstrating the significance of instantaneous fluctuation information for classification accuracy.

Repetitive Direct Comparison Method for Odor Sensing.

Olfactory sensors are one of the most anticipated applications of gas sensors. To distinguish odors-complex mixtures of gas species, it is necessary to extract sensor responses originating from the target odors. However, the responses of gas sensors tend to be affected by interfering gases with much higher concentrations than target odor molecules. To realize practical applications of olfactory sensors, extracting minute sensor responses of odors from major interfering gases is required. In this study, we propose a repetitive direct comparison (rDC) method, which can highlight the difference in odors by alternately injecting the two target odors into a gas sensor. We verified the feasibility of the rDC method on chocolates with two different flavors by using a sensor system based on membrane-type surface stress sensors (MSS). The odors of the chocolates were measured by the rDC method, and the signal-to-noise ratios (S/N) of the measurements were evaluated. The results showed that the rDC method achieved improved S/N compared to a typical measurement. The result also indicates that sensing signals could be enhanced for a specific combination of receptor materials of MSS and target odors.

Highly Sensitive Chemiresistive H2S Detection at Subzero Temperature over the Sb-Doped SnO2@g-C3N4 Heterojunctions under UV Illumination.

NASA has detected H2S in the persistently shadowed region of the lunar South Pole through NIR and UV/vis spectroscopy remotely, but in situ detection is generally considered to be more accurate and convincing. However, subzero temperatures in space drastically reduce chemisorbed oxygen ions for gas sensing reactions, making gas sensing at subzero temperature something that has rarely been attempted. Herein, we report an in situ semiconductor H2S gas sensor assisted by UV illumination at subzero temperature. We constructed a g-C3N4 network to wrap the porous Sb doped SnO2 microspheres to form type II heterojunctions, which facilitate the separation and transport of photoinduced charge carriers under UV irradiation. This UV-driven technique affords the gas sensor a fast response time of 14 s and a response value of 20.1 toward 2 ppm H2S at -20 °C, realizing the sensitive response of the semiconductor gas sensor at subzero temperature for the first time. Both the experimental observations and theoretical calculation results provide evidence that UV irradiation and the formation of type II heterojunctions together promote the performance at subzero temperature. This work fills the gap of semiconductor gas sensors working at subzero temperature and suggests a feasible method for deep space gas detection.

Evaluation of Red Wine Acidification Using an E-Nose System with Venturi Tool Sampling.

The quality of wine is checked both during the production process and upon consumption. Therefore, manual wine-tasting work is still valuable. Due to the nature of wine, many volatile components are released, and it is therefore difficult to determine which elements need to be controlled. Acetic acid is one of the substances found in wine and is a crucial substance for wine quality. Gas sensor systems may be a potential alternative for manual wine tasting. In this work, we have developed a TGS2620 gas sensor module to analyze acetic acid levels in red wine. The gas sensor module was refined according to the Venturi effect along with signal slope analysis, providing promising results. The example included in this paper demonstrates that there is a direct relationship between the slope of the MOS gas sensor response and the acetic acid concentration. This relationship is useful to evaluate the ethanol oxidation in acetic acid in red wine during its production process.

ZnO-MEMS sensor-cell prepared by inkjet printing for low concentration acetone detection

In this study, inkjet printing tactics is employed to deposit sensitive materials, which is a nest-like array vertically assembled by petal-shaped structure composed of nanoparticles. It overcomes the disadvantages of cumbersome and unstable operation of traditional deposition technology for gas sensor sensitive materials based on MEMS. The mesoporous ZnO-sensor shows high gas-sensing response to low concentration acetone. The excellent performance of mesoporous ZnO-sensors lies in the uniform small nanoparticles, their multi-dimensional assembly structures and rich oxygen vacancy defects.

Temperature and Ambient Band Structure Changes in SnO2 for the Optimization of Hydrogen Response

Tin dioxide (SnO2) is one of the most used materials for sensing applications operating at high temperatures. Commonly, “undoped SnO2” is made by precursors containing elements that can have a deleterious impact on the operation of SnO2 sensors. Here, we employ experimental and theoretical methods to investigate the structural properties and electronic structure of the rutile bulk and surface SnO2, focusing on unintentional doping due to precursors. Unintentional doping from precursors as well as intrinsic doping can play an important role not only on the performance of gas sensors, but also on the properties of SnO2 as a whole. The theoretical calculations were performed using density functional theory (DFT) with hybrid functionals. With DFT we examine the changes in the electronic properties of SnO2 due to intrinsic and unintentional defects and we then discuss how these changes affect the response of a SnO2-based gas sensor. From an experimental point of view, we synthesized low-cost SnO2 thin films via sol–gel and spin-coating processes. To further enhance the performance of SnO2, we coated the surface with a small amount of platinum (Pt). The crystalline structure of the films was analyzed using x-ray diffraction (XRD) and scanning electron microscopy (SEM), while for the determination of the elements contained in the sample, X-ray photoelectron spectroscopy (XPS) measurements were performed. Furthermore, we investigated the effect of temperature on the band structure of SnO2 in air, in a vacuum and in nitrogen and hydrogen chemical environments. To optimize the response, we used current–voltage characterization in varying environments. The aim is to associate the response of SnO2 to various environments with the changes in the band structure of the material in order to gain a better understanding of the response mechanism of metal oxides in different pressure and temperature environments. We found that the resistance of the semiconductor decreases with temperature, while it increases with increasing pressure. Furthermore, the activation energy is highly affected by the environment to which the thin film is exposed, which means that the thin film could respond with lower energy when exposed to an environment different from the air.

Optimization of the hydrogen response characteristics of halogen-doped SnO2

The increasing demand for efficient sensing devices with facile low-cost fabrication has attracted a lot of scientific research effort in the recent years. In particular, the scientific community aims to develop new candidate materials suitable for energy-related devices, such as sensors and photovoltaics or clean energy applications such as hydrogen production. One of the most prominent methods to improve materials functionality and performance is doping key device component(s). This paper aims to examine in detail, both from a theoretical and an experimental point of view, the effect of halogen doping on the properties of tin dioxide (SnO2) and provide a deeper understanding on the atomic scale mechanisms with respect to their potential applications in sensors. Density Functional Theory (DFT) calculations are used to examine the defect processes, the electronic structure and the thermodynamical properties of halogen-doped SnO2. Calculations show that halogen doping reduces the oxide bandgap by creating gap states which agree well with our experimental data. The crystallinity and morphology of the samples is also altered. The synergy of these effects results in a significant improvement of the gas-sensing response. This work demonstrates for the first time a complete theoretical and experimental characterization of halogen-doped SnO2 and investigates the possible responsible mechanisms. Our results illustrate that halogen doping is a low-cost method that significantly enhances the room temperature response of SnO2.

Pt/MoS2/Polyaniline Nanocomposite as a Highly Effective Room Temperature Flexible Gas Sensor for Ammonia Detection.

A Pt/MoS2/polyaniline (Pt/MoS2/PANI) nanocomposite is successfully synthesized by the hydrothermal process combined with the in situ polymerization method, and then Pt particles are decorated on its surface. The Pt/MoS2/PANI nanocomposite is deposited on a flexible Au-interdigitated electrode of a polyimide (PI) film. The flexible sensor exhibits a higher response value and fast response/recovery time to NH3 at room temperature (RT). It results in 2.32-fold and 1.13-fold improvement in the gas-sensing response toward 50 ppm NH3 compared to those of PANI and MoS2/PANI-based gas sensors. The detection limit is 250 ppb. The enhancement sensing mechanisms are attributed to the p-n heterojunction and the Schottky barrier between the three components, which has been confirmed by the current-voltage (I-V) curves. A satisfactory selectivity to NH3 against trimethylamine (TMA) and triethylamine (TEA) is obtained according to density functional theory (DFT), Bader's analysis, and differential charge density to illustrate the adsorption behavior and charge transfer of gas molecules on the surface of the sensing materials. The sensor retains the excellent sensing response value even under high relative humidity and sensing stability at higher bending angle/numbers to NH3 gas. Hence, Pt/MoS2/PANI can be regarded as a promising sensing material for high-performance NH3 detection at room temperature applied in flexible wearable electronics.

Highly efficient reduced tungsten oxide-based hydrogen gas sensor at room temperature

Reduced tungsten oxide nanoparticles (WO2.72) was synthesized from tungsten trioxide by calcination method. The prepared samples were systematically characterized via field-emission scanning electroscope (FESEM), Raman spectroscopy, and X-ray diffraction (XRD). Then, the prepared WO2.72 was spin coated on Si/SiO2 substrate, and the sensor was developed with interdigital electrode. The gas sensing property of the developed sensor toward H2 gas was investigated. The result shows that the fabricated WO2.72 sensor possesses excellent H2 gas sensor response of 27% at room temperature. Moreover, the WO2.72 sensor exhibited outstanding long-term stability and repeatability at 500 ppm. Besides, it exhibited stable sensor response in wide range of humidity reaveviling its potential candidacy for real application of H2 gas monitoring. Therefore, this study paves way for development of effective, stable and fast responsing H2 gas sensor at room temperature that is very important in environmental remediation.