Conductometric Gas Sensors Research Articles

Engineering Nickel(II) Porphyrin-Conjugated Polymers with Different Aryl meso-Substituents for n-Type and p-Type Ammonia Sensors.

Conjugated polymers have revolutionized the field of conductometric gas sensors for sensing toxic gases arising from the fast urbanization and industrialization. In this work, we report the synthesis of a series of 5,15-diaryl Ni(II) porphyrin-conjugated polymers (pNiD(Aryl)P) and their integration as the top layer on an octafluorinated copper phthalocyanine (CuF8Pc) sublayer to construct bilayer heterojunction (BLH) devices for ammonia sensing. For the first time, we report the pioneering demonstration of polarity engineering within a BLH device by manipulating the meso-substituent of the 5,15-diaryl Ni(II) porphyrin-conjugated polymer constituting the top layer of the CuF8Pc/pNiD(Aryl)P BLH device. The BLH devices prepared from the 5,15-diaryl Ni(II) porphyrin-conjugated polymer bearing electron-donating meso-substituents as the top layer exhibit a p-type behavior, whereas an n-type behavior is observed for the BLH devices prepared from the 5,15-diaryl Ni(II) porphyrin-conjugated polymer bearing electron-withdrawing meso-substituents. Laser desorption ionization high-resolution mass spectrometry, UV/vis/NIR, and X-ray photoelectron spectroscopy studies provide evidence of a decrease in intramolecular dehydrogenative coupling in pNiD(Aryl)P bearing electron-withdrawing meso-substituents, resulting in low electrical conductivity of the thin films. Density functional theory calculations reveal noninvolvement of electron-withdrawing meso-substituents toward π-delocalization in the fused Ni(II) porphyrin tapes. Interestingly, all the CuF8Pc/pNiD(Aryl)P BLH devices exhibit remarkable sensing response toward NH3. Among all the devices, CuF8Pc/pNiDPP displays the highest sensitivity of -1.17% ppm-1 for NH3, whereas CuF8Pc/pNiDNapP and CuF8Pc/pNiDCNPP exhibit the best limit of detection for NH3, below 200 ppb. In addition, CuF8Pc/pNiDCNPP shows short response and recovery times of 13 and 255 s, respectively, making this device highly suitable for deployment in emergency services.

II-VI Semiconductor-Based Conductometric Gas Sensors: Is There a Future for These Sensors?

A review of the state of research in the development of conductometric gas sensors based on II-VI semiconductors is given. It was shown that II-VI compounds indeed have properties that are necessary for the development of highly efficient gas sensors. In this case, to achieve the required parameters, all approaches developed for metal oxides can be used. At the same time, during a detailed review, it was concluded that sensors based on II-VI compounds have no prospects for appearing on the gas sensor market. The main obstacle is the instability of the surface state, which leads to poor reproducibility of parameters and drift of sensor characteristics during operation.

Approaches for selectivity improvement of conductometric gas sensors: an overview

Conductometric gas sensors (CGS) have been extensively explored in recent decades owing to easy fabrication and miniaturization, low cost and distributable detectability.

Electronic control and measurement unit of conductometric nitrogen dioxide sensor in the atmosphere

High levels of air pollution lead to various negative consequences, including deterioration in public health. One of the main air pollutants in industrial cities is nitrogen dioxide. Nitrogen dioxide (NO2) is produced by various industrial processes. It is highly toxic and can cause various diseases such as bronchitis, asthma and other respiratory diseases. Despite all efforts to reduce NO2 emissions, the problem of air pollution remains relevant. Therefore, it is important to continue research into the development of new methods for continuous atmospheric monitoring and timely detection of emissions. One of the main tasks of the modern branch of science in the development of a monitoring system is the development of a sensitive, reliable and inexpensive gas sensor for NO2 in the air. The tasks of this work are design, construct, assemble and test a new original electronic control and measurement unit for a previously developed conductometric sensor for nitrogen dioxide in the atmosphere which is capable of recording and interpreting the signal in the form of a numerical response and displaying it on the screen. As a result of the work carried out, a new original electronic control and measurement unit was developed and constructed for the previously developed conductometric gas sensor for nitrogen dioxide in the atmosphere which is capable of recording and interpreting the signal in the form of a numerical response and displaying it on the screen. The developed device was tested in a system with a gas sensor with a p-type semiconductor gas-sensitive element. It has been established that for a model atmosphere of 50 ppm NO2 + N2, the sensor response recorded by the developed device is equal to (10,4 ± 0,2)∙10-3, and for a model atmosphere of 10 ppm NO2 + N2 – (6,2 ± 0,2)∙10-3. The obtained values can be used to determine the concentration of nitrogen dioxide under real atmospheric monitoring conditions.

Gas-Sensing Properties and Mechanisms of 3D Networks Composed of ZnO Tetrapod Micro-Nano Structures at Room Temperature.

Metal oxide semiconductors (MOSs) hold great promise for electronic devices such as gas sensors. The utilization of ZnO as a conductometric gas sensor material can be traced back to its early stages; however, its application has primarily been limited to high-temperature environments. A gas sensor based on highly porous and interconnected 3D networks of ZnO tetrapod (ZnO-T) micro-nano structures was fabricated via an easy chemical vapor deposition (CVD) method. Homemade instruments were utilized to evaluate the gas-sensing of the sample at room temperature. It exhibited good gas-sensing at room temperature, particularly with a response of up to 338.80% toward 1600 ppm ethanol, while also demonstrating remarkable repeatability, stability, and selectivity. Moreover, the unique gas-sensing properties of ZnO-T at room temperature can be reasonably explained by considering the effect of van der Waals forces in physical adsorption and the synergistic effect of carrier concentration and mobility. The aforementioned statement presents an opportunity for the advancement of gas sensors utilizing ZnO at room temperature.

Amorphous Pt-decorated α-Fe2O3 sensor with superior triethylamine sensing performance prepared by a one-step impregnation method

Noble metals are widely used additives to improve the sensing performance of conductometric metal oxide gas sensors. Although amorphous noble metal particles are expected to further improve the gas sensor response, common techniques often result in crystalline nanoparticles. Herein, an amorphous Pt nanoparticle-decorated α-Fe2O3 sensor was prepared via a temperature-controlled facile one-step impregnation method without the use of a reductant, surfactant, or subsequent thermal reduction. The amorphous Pt-functionalized α-Fe2O3 exhibited superior sensing performance to triethylamine, achieving 100 °C and 140 °C decreases in the optimal working temperature, and had better selectivity, with 22-fold and 14-fold improvements in gas response when compared to those of crystallized Pt-functionalized α-Fe2O3 and pristine α-Fe2O3, respectively. The as-prepared amorphous Pt-decorated α-Fe2O3 sensor showed great promise as a promising triethylamine sensor, and the newly developed facile impregnation method was shown to be an effective strategy to synthesize amorphous noble metal-decorated metal oxide gas sensors.

Deposition of Composite Nanoparticle-Based Thin Films for Gas Sensing

Working principle of conductometric gas sensors is based on modulation in electrical conductivity of the sensing material due to adsorption-desorption reactions between the target gas and the sensor surface. Nano-structuring the sensing material increases the effective surface area for interaction between the target gas and the sensing material thereby enhancing the sensorial response. Metal oxide semiconductors (MOS) have shown remarkable sensing performance to oxidizing and reducing gases. The combination of different MOS has often demonstrated to synergistically improve the gas sensing performance via the formation of highly sensitive heterojunctions at their interface.In this work, we deposited thin films composed of nanoparticles (NPs) of p-type CuOx and n-type WOx to enhance the effective surface area of the material and to realize the maximum number of nano p-n heterojunctions in the material. The films were prepared using the Nanogen-Trio NP source equipped with three 1′′ magnetrons, mounted on a custom-built deposition chamber and sputtered by a DC power supply. Depositions were executed in Ar+O2 gas mixture. The O2 admixture may significantly enhance the mass flux of NPs. Furthermore, the O2 admixture allows one to tune the desired stoichiometry of the respective metal oxides. On the other hand, the pronounced hysteresis complicates the deposition of WOx NPs. The deposition process was controlled using an in-house developed software to prepare thin films composing a mixture of NPs with a defined volumetric ratio of the individual materials.This work demonstrates the versatility of our custom-built gas aggregation source that facilitates the preparation of multi-composite NP-based thin film giving the best sensorial response.

Black phosphorus nanosheets-sensitized Zn-doped α-Fe2O3 nanoclusters for trace acetone detection

Acetone vapor not only poses a significant threat to the ecological environment and human health but serves as a specific volatile organic compounds (VOCs) biomarker within the exhaled breath to distinguish individual diabetes, thereby urgently necessitating a sensitive and selective acetone detection. To this end, conductometric gas sensors featuring black phosphorus nanosheets (BP)-decorated Zn-doped hematite (α-Fe2O3) nanoclusters as the sensitive material were prepared. By subtly modulating the componential ratio and operation temperature, the optimal Zn-doped α-Fe2O3/BP sensor delivered a prominent response of 5.32 and short response/recovery times of 8/68 s toward 0.5 ppm acetone at 160 °C as well as a large sensitivity of 5.3/ppm over the concentration range of 0.1–1.5 ppm. Moreover, favorable selectivity, repeatability, and long-term stability were demonstrated. The superior sensing performance after incorporating a tiny but moderate amount of BP over Zn-doped α-Fe2O3 sensors could be attributed to the consequent enlarged specific surface area, increased oxygen vacancies, and numerous p-n heterojunctions. Furthermore, additional polylactic acid (PLA) encapsulation membrane endowed the as-prepared sensors with a remarkably improved humidity tolerance, showcasing a huge application potential in non-invasive diabetes diagnosis from exhaled breath featuring high-humidity conditions in the future.

Conductometric Gas Sensor Based on MoO3 Nanoribbon Modified with rGO Nanosheets for Ethylenediamine Detection at Room Temperature.

An ethylenediamine (EDA) gas sensor based on a composite of MoO3 nanoribbon and reduced graphene oxide (rGO) was fabricated in this work. MoO3 nanoribbon/rGO composites were synthesized using a hydrothermal process. The crystal structure, morphology, and elemental composition of MoO3/rGO were analyzed via XRD, FT-IR, Raman, TEM, SEM, XPS, and EPR characterization. The response value of MoO3/rGO to 100 ppm ethylenediamine was 843.7 at room temperature, 1.9 times higher than that of MoO3 nanoribbons. The MoO3/rGO sensor has a low detection limit (LOD) of 0.235 ppm, short response time (8 s), good selectivity, and long-term stability. The improved gas-sensitive performance of MoO3/rGO composites is mainly due to the excellent electron transport properties of graphene, the generation of heterojunctions, the higher content of oxygen vacancies, and the large specific surface area in the composites. This study presents a new approach to efficiently and selectively detect ethylenediamine vapor with low power.

Conductometric NO2 gas sensors achieved by design of Ti3C2Tx @ZnO heterostructures for application around 80 °C

Low-temperature, stable and highly selective behaviors are the essential indicators for evaluating gas sensors. Although ZnO has attractive stability as a sensitive material, it is still an enormous challenge to construct highly sensitive gas sensors with high selectivity at low temperatures employing ZnO. Herein, we have achieved a high-selectivity and low-temperature stable nitrogen dioxide (NO2) gas sensor by composite of Ti3C2Tx flakes with ZnO nanorods by facile hydrothermal process. As a results, Ti3C2Tx doped with 3 mmol ZnO exhibits the highest sensitivity (190%, 50 ppm), outstanding selectivity and stability for NO2 gas, due to the presence of multiple Schottky barriers (SB), which are originated from the heterostructure of Ti3C2Tx and ZnO. The metal-like properties of Ti3C2Tx can provide multiple electron transfer channels and accelerate the electron transfer rate. Moreover, the adsorption of more NO2 molecules can also enhance the sensitivity of sensitive material as ZnO, which is rich in adsorption sites, can form interactions with NO2. The first-principles calculation supported a preferential adsorption of NO2 on ZnO nanorods. Our investigation facilitates the appropriate design of high-performance gas sensors at lower operating temperature.

Accelerating the Gas-Solid Interactions for Conductometric Gas Sensors: Impacting Factors and Improvement Strategies.

Metal oxide-based conductometric gas sensors (CGS) have showcased a vast application potential in the fields of environmental protection and medical diagnosis due to their unique advantages of high cost-effectiveness, expedient miniaturization, and noninvasive and convenient operation. Of multiple parameters to assess the sensor performance, the reaction speeds, including response and recovery times during the gas-solid interactions, are directly correlated to a timely recognition of the target molecule prior to scheduling the relevant processing solutions and an instant restoration aimed for subsequent repeated exposure tests. In this review, we first take metal oxide semiconductors (MOSs) as the case study and conclude the impact of the semiconducting type as well as the grain size and morphology of MOSs on the reaction speeds of related gas sensors. Second, various improvement strategies, primarily including external stimulus (heat and photons), morphological and structural regulation, element doping, and composite engineering, are successively introduced in detail. Finally, challenges and perspectives are proposed so as to provide the design references for future high-performance CGS featuring swift detection and regeneration.

Pyrochlore cerium stannate (Ce2Sn2O7) for highly sensitive NO2 gas sensing at room temperature

Gas sensing at low temperatures remains a notable challenge at present. Owing to their strong catalytic activity, oxygen defects, and unique size, pyrochlore-based oxides have attracted considerable attention for preparing conductometric gas sensors that are effective at low temperatures. In this study, we prepared a pyrochlore-Ce2Sn2O7 structure using a facile hydrothermal method. The structure, morphology, and surface valance states of the prepared sample were analyzed. Additionally, we investigated the gas-sensing properties, such as the selectivity, response, response and recovery time, repeatability, stability, limit of detection, and influence of relative humidity on the sensing performance for NO2 gas molecules at room temperature (30 ± 2 ℃). The Ce2Sn2O7 exhibited outstanding selectivity for NO2 gas molecules compared with the interfering gas molecules such as N2O, CO, CO2, SO2, NH3,NO and H2S at room temperature (30 ± 2 ℃.). Moreover, the response and recovery time values were 4 s and 52.5 s, respectively, and an outstanding response of 125% was achieved toward 50 ppm NO2 gas molecules. The Ce2Sn2O7 sensor device exhibited excellent repeatability and stability over 90 days. The excellent gas-sensing performance was attributable to the high specific surface area, presence of oxygen vacancies, and surface-adsorbed oxygen molecules.

An ultrasonically catalyzed conductometric metal oxide gas sensor system with machine learning-based ambient temperature compensation

The ultrasonic catalysis method provides a new means of enhancing sensing performance of some types of gas sensors. But generating the ultrasound consumes electric energy and increases the system mass. Thus, it is significant to lower the energy consumption and mass of the ultrasonic excitation structure as much as possible, while enhancing the sensing performance. In this work, a thin conical metal shell excited by a piezoelectric disk has been proposed, serving as the focusing ultrasonic transducer and protection case for a SnO2 gas sensor. The ultrasonic transducer’s total mass is only 11 g. The experiments show that the ultrasonic transducer can significantly reduce the low detection limit (LDL), and measured LDL of the sonicated sensor for H2 can be as low as 107 ppb (1/3 of the same sensing element not sonicated) with a response of 4.6%. In this case, the ultrasonic transducer’s power consumption and temperature rise is 146 mW and 3 ℃, respectively. The low temperature rise indicates that the LDL may be further decreased by increasing the ultrasonic vibration. The effect of ambient temperature change on predicted H2 concentration is compensated by two convolutional neural networks for the low (1–100 ppm) and ultralow (≤ 1 ppm) concentration ranges, and the relative error of concentration prediction in these two concentration ranges is 5.7% and 12.3%, respectively.

Conductometric gas sensor based on p-type GaN hexagonal pits /PANI for trace-level NH3 detection at room temperature

Herein, we demonstrate two ammonia (NH3) gas sensors based on p-type Gallium nitride (GaN) hexagonal pits (GaN-HP)/ Polyaniline (PANI) heterostructure prepared by wet etching process and in situ polymerization. The morphologies, structures of p-type GaN-HP/PANI sensors based on planer p-type GaN with etching time for 20 min and 30 min (GaN-HP/PANI-1 and GaN-HP/PANI-2) have been characterized. The responses of GaN-HP/PANI-1 and GaN-HP/PANI-2 are 3.2 and 6.3 times higher than that of pure PANI at room temperature, which is primarily due to the gas sensing improvement effect of p-p junction between p-type GaN-HP and PANI. Moveover, the GaN-HP/PANI-2 shows 1.94-fold enhancement in the gas sensing response toward 100 ppm NH3 and lower detection limits than that of GaN-HP/PANI-1, indicating that its performance is highly dependent on the etching time. Compared to GaN-HP/PANI-1, the excellent improvement of gas sensing for GaN-HP/PANI-2 can be attributed to the higher specific surface area, the stronger protonation degree of PANI and synergistic effects of the p-p junction effect. Furthermore, the selectivity, repeatability and the effect of relative humidity on the gas sensing performances of the two both sensors have also been explored. This work can provide a helpful reference for development of high-performance GaN-HP/PANI based gas sensors.

Conductometric methane gas sensors based on ZnO/Pd@ZIF-8: Effect of dual filtering of ZIF-8 to increase the selectivity

Selectivity remains the biggest challenge for metal oxide semiconductor (MOS)-based gas sensors, especially for inactive gases such as CH4. In this work, the core-shell structured ZnO/Pd@ZIF-8 was fabricated via the self-templated method and the sensor based on the ternary hybrid showed excellent CH4 selectivity against NH3, CO and NO2. ZIF-8 was critical for the selectivity enhancement via dual filtering effects: a) size-induced sieving, where NO2 with a larger kinetic diameter of 4.5 Å can be hindered by ZIF-8 with an effective aperture size of 4.0–4.2 Å; b) polarity-induced sieving, where NH3 (2.6 Å) and CO (3.76 Å) with larger polarity showed stronger interaction with ZIF-8 and their diffusion is therefore retarded. Therefore, the selectivity of nonpolar CH4 (3.8 Å) is improved. The present insight about the dual filtering effects of ZIF-8 may provide a new insight to improve MOS sensing selectivity by rational designing MOF structures.

Conductometric ethanol gas sensor based on a bilayer film consisting of SnO2 film and SnO2/ZnSnO3 porous film prepared by magnetron sputtering

As one important type of the sensing material, the metal oxide film can realize accurate detection to the low concentration of ethanol. In this paper, a double-layer nano film was prepared by sputtering a SnO2 bottom layer and co-sputtering a SnO2/ZnSnO3 porous top layer with SnO2 and ZnO targets. Firstly, the effects of Zn content on the properties of the single-layer SnO2/ZnSnO3 porous film were investigated by changing sputtering power of ZnO and keep sputtering power of SnO2 constant. When the sputtering power of ZnO is 25 W, the single-layer SnO2/ZnSnO3 porous film has the best response to ethanol. To expose more adsorption sites to air, the SnO2/ZnSnO3 porous layer was deposited on a SnO2 thin film with thickness of 10 nm to form the double-layer sensing material. The double-layer thin film has response of 11.5–50 ppm ethanol at 290 °C, which is 2.1 times that of the single-layer SnO2/ZnSnO3 porous layer, and the response and recovery time of the double layer are 56 s and 666 s to 50 ppm ethanol at 290 °C, which are shortened by 63 s and 641 s compared to those of the single layer, respectively. In addition, the double-layer porous film also exhibits excellent long-term stability and good selectivity to ethanol. The n-n heterojunction formed between SnO2 and ZnSnO3 and unique double-layer porous structure are the main reasons for the excellent sensing performance.

Current Trends in Nanomaterials for Metal Oxide-Based Conductometric Gas Sensors: Advantages and Limitations—Part 2: Porous 2D Nanomaterials

This article discusses the features of the synthesis and application of porous two-dimensional nanomaterials in developing conductometric gas sensors based on metal oxides. It is concluded that using porous 2D nanomaterials and 3D structures based on them is a promising approach to improving the parameters of gas sensors, such as sensitivity and the rate of response. The limitations that may arise when using 2D structures in gas sensors intended for the sensor market are considered.

Synthesis of SnO2 quantum dot sensitized LaFeO3 for conductometric formic acid gas sensors

Quantum dots (QDs) with small size are also expected to improve sensing properties of complex metal oxides semiconductors. Herein, SnO2 QDs was adopted to dope and sensitize LaFeO3 for formic acid (HCOOH) gas sensing. In detail, SnO2 QDs was pre-synthesized by a solvothermal reaction and dispersed with LaFeO3 sol-gel precursor followed by calcination treatment. Characterization results found that SnO2 QDs doping enabled LaFeO3 become porous, which was beneficial for the diffusion of gas molecular. The sensing properties of composites of LaFeO3 doped with different amounts of SnO2 QDs were investigated and compared with that of pure LaFeO3 and LaFeO3/SnO2 composites. SnO2 QDs doping largely improve the sensitivity of LaFeO3 to HCOOH. The sensor based on LaFeO3 doped with 2.5 wt% SnO2 QDs exhibited much better sensing performance to HCOOH, and a high response value of 31.5–100 ppm HCOOH and a low detection limit of 1 ppm can be reached at 210 ℃. Its high HCOOH sensing properties might be contributed to the formation of heterojunctions between SnO2 QDs and LaFeO3 and the small size of SnO2 QDs. This work confirms the advantages of QDs sensitization in semiconductor gas sensing.

Conductometric nitrogen dioxide gas sensor based on gallium nitride quantum dots film

Gallium nitride (GaN) has some challenges for gas sensing, such as high detection lower limit (LOD) or complicated epitaxy of nanostructures. To address these issues, we exclusively designed a GaN quantum dots (QDs) film to enhance the gas interaction capability which fabricated by metal-organic chemical vapor deposition (MOCVD). The GaN QDs film gas sensor exhibits a low LOD (5 ppb) to nitrogen dioxide (NO2) at room temperature (RT), which has superior gas performance than other GaN-based sensors. The results of morphology and structure characterization manifest that many ultra-small QDs around 2 nm in diameter were stacked to form QDs of larger size, and confirm the existence of vacancies for the GaN QDs film. And the GaN QDs film sensor displays a ultra-high electron mobility of 1237.37 cm2 V−1s−1 at RT, which is greatly beneficial for gas sensing. The sensing mechanisms were analyzed based on its abundant active sites, ultrahigh carrier mobility, high surface energy and chemical activity, grain boundary barrier and good electrical conductivity. Moreover, the sensor processes high selectivity, repeatability, stability and reusability based on MOCVD technology which can realize precise control of structure and morphology, thus this sensor is expected to achieve rapid industrialization.

Recent Advances in Gas Sensing Technology Using Non-Oxide II-VI Semiconductors CdS, CdSe, and CdTe

In recent years, there has been an increasing need and demand for gas sensors to detect hazardous gases in the atmosphere, as they are indispensable for environmental monitoring. Typical hazardous gas sensors that have been widely put to practical use include conductometric gas sensors, such as semiconductor gas sensors that use the change in electrical resistance of metal oxide semiconductors, catalytic combustion gas sensors, and electrochemical gas sensors. However, there is a growing demand for gas sensors that perform better and more safely, while also being smaller, lighter, less energy-demanding, and less costly. Therefore, new gas sensor materials are being explored, as well as optical gas sensor technology that expresses gas detection not electrically but optically. Cadmium sulfide (CdS), cadmium selenide (CdSe), and cadmium telluride (CdTe) are typical group II-VI non-oxide semiconductors that have been used as, for example, electronic materials. Recently, they have attracted attention as new gas sensor materials. In this article, recent advances in conductometric and optical gas sensing technologies using CdS, CdSe, and CdTe are reviewed.