Conductometric Sensors Research Articles

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.

Defect engineering of nanostructured ZnSnO3 for conductometric room temperature CO2 sensors

Slow response/recovery kinetics and high limit of detection (LOD) are crucial challenges for practical room-temperature carbon dioxide (CO2) gas sensor platforms. Herein, visible light-excited ZnSnO3 three-dimensional nanostructures are utilized for highly sensitive and selective detection of CO2. The sensing performance reveals that fine-controlled oxygen vacancy (OV) and highly-active electron transition of defect-rich nanomaterials are obtained via modulating the hydrogen treatment duration, conducive to the prominently enhanced ppm-level CO2 response with low LOD (3.05 ppm), ultrafast response time (16.814 s) and improved stability (14.658 ± 2.339 @ 400 ppm for 14 days) among all the reported ternary metal oxide semiconductor (TMOS) -based gas sensors. The photophysical enhancement mechanism of analyte is discussed in detail by combining experimental and theoretical investigation through the effect of photoexcitation on both adsorbed CO2 molecules and sensing materials, the role of pre-adsorbed oxygen for regulating targeted gas interaction, the changes of electronic transition in the bandgap, the impact of in-plane OV and bridging OV on CO2 sensing, and the function of OV for modifying the coordination sites and geometry of metal cations at the surface. From a broader perspective, the fabricated sensor provides a remarkably facile and effective technique for rapid and repeatable CO2 monitoring.

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.

Effect of Fractal Topology on the Resistivity Response of Thin Film Sensors.

We discuss the effect of topological inhomogeneity of very thin metallic conductometric sensors on their response to external stimuli, such as pressure, intercalation, or gas absorption, that modify the material's bulk conductivity. The classical percolation model was extended to the case in which several independent scattering mechanisms contribute to resistivity. The magnitude of each scattering term was predicted to grow with the total resistivity and diverge at the percolation threshold. We tested the model experimentally using thin films of hydrogenated palladium and CoPd alloys where absorbed hydrogen atoms occupying the interstitial lattice sites enhance the electron scattering. The hydrogen scattering resistivity was found to grow linearly with the total resistivity in the fractal topology range in agreement with the model. Enhancement of the absolute magnitude of the resistivity response in the fractal range thin film sensors can be particularly useful when the respective bulk material response is too small for reliable detection.

Synthesis, Structural and Sensor Properties of Nanosized Mixed Oxides Based on In2O3 Particles.

The paper considers the relationship between the structure and properties of nanostructured conductometric sensors based on binary mixtures of semiconductor oxides designed to detect reducing gases in the environment. The sensor effect in such systems is determined by the chemisorption of molecules on the surface of catalytically active particles and the transfer of chemisorbed products to electron-rich nanoparticles, where these products react with the analyzed gas. In this regard, the role is evaluated of the method of synthesizing the composites, the catalytic activity of metal oxides (CeO2, SnO2, ZnO), and the type of conductivity of metal oxides (Co3O4, ZrO2) in the sensor process. The effect of oxygen vacancies present in the composites on the performance characteristics is also considered. Particular attention is paid to the influence of the synthesis procedure for preparing sensitive layers based on CeO2-In2O3 on the structure of the resulting composites, as well as their conductive and sensor properties.

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.

Hierarchical NiO/TiO2 heterojuntion-based conductometric hydrogen sensor with anti-CO-interference

Constructing a heterojunction is an effective way to improve the gas sensing properties of metal oxide semiconductors. In this work, a hydrogen sensor containing hierarchical and porous NiO/TiO2 heterojunction bilayer structure was designed. The top layer is formed by the NiO nanowalls grown along the [220] direction with exposed (111) facets, and the bottom layer is formed by the [002] oriented TiO2 nanorod arrays. The exposed high energy crystal surfaces of NiO and TiO2, together with the adapted surface depletion region, induced by the formation of the heterojunction, enable the hydrogen sensor to have both a wide detection range and a much higher response to hydrogen than single layer materials. With the optimized size of the pores existing between the walls, the hydrogen sensor shows excellent prevention to interference from CO. Both the gas diffusion regulated by the porous structure and the surface interaction between H2/CO and O2- controlled by the heterojunction barrier contribute to the distinctive detection of H2 and CO at room temperature (25 ℃ ± 2 ℃).

Improved ppb-level NO2 conductometric sensor induced by trace Au on SnO2 nanosheet

As a main exhaust emission from vehicles, nitrogen dioxide is harmful to environment and human health. Gas sensors with high performance for nitrogen dioxide detection is essential. In this work, SnO2 nanosheet aggregates loaded with Au were used to fabricate a NO2 gas sensor. Pristine SnO2 was synthesized via a solvothermal method. Subsequently, the ultraviolet (UV) light-assisted reduction method was utilized to load Au nanoparticles and improve the sensing properties. The SnO2 modified with a trace of Au nanoparticles (0.5 wt%) shows high response to ppb-level NO2 (35–100 ppb) at a low temperature (90 °C). Besides excellent selectivity and long-term stability, the 0.5% Au/SnO2 based sensor can detect 2 ppb NO2 with the response value of 1.2. The enhanced sensing properties can be attributed to the interface interaction between Au and SnO2, causing the enhanced absorption capability for oxygen and NO2 molecules. Additionally, trace amount of loading Au can maintain small size of Au nanoparticles, making the active sites on SnO2 more effective. This work provides a convenient method for preparing a ppb-level NO2 gas sensor with high performance.

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.

Synthesis of conducting water-dispersible polyaniline particles and its template-guided patterning

Organic conductors are a promising material for future electronics. Although, its processing is a big challenge. In this manuscript, we demonstrate micropatterning of polyaniline within the pores of a polycarbonate membrane to fabricate a device for a conductometric sensor. The aqueous dispersion of polyaniline particles was prepared by single pot oxidative polymerization of aniline monomer using dodecylbenzene sulfonic acid as a surfactant as well as a dopant, and the polyaniline from its aqueous dispersion was then precipitated within the pores of polycarbonate membrane by adding solvents of varying polarities. The phase diagram of precipitation was drawn by titrating the polyaniline dispersion with tetrahydrofuran solvent. The dispersion was characterized by Fourier transform infrared spectroscopy and UV-Vis spectroscopy, as well as a transmission electron microscope. The molecular weight of the synthesized polyaniline was measured by gel permission chromatography. The morphology of polyaniline has been compared through transmission electron microscopy in its dispersed form and after precipitation. Field emission scanning electron microscopy was used to see the growth of channel formation of precipitated polyaniline within the pores of the polycarbonate membrane. The electroactivity of the precipitated polyaniline was measured by in-situ conductance measurement as a function of the electrochemical state.

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.

Effect of {010} crystal facets of Bi2MoO6 and 1D/2D heterostructures for conductometric room temperature NH3 gas sensors

High working temperature and the insufficient limits of detection limit the broad applications of semiconductor chemiresistive gas sensors. Herein, pure Bi2MoO6 nanosheets and a series of one-dimensional/two-dimensional (1D/2D) multi-walled CNTs/Bi2MoO6 nanocomposites were developed via a facile hydrothermal route for room-temperature ammonia monitoring. The as-synthesized samples were characterized by various analytical techniques. 0.5 wt% MWCNTs/Bi2MoO6 nanocomposites showed the best sensing properties to 10–50 ppm NH3, including low limit of detection (157 ppb), high response (Ra/Rg = 44.2 @ 50 ppm), good selectivity, reproductivity, and anti-humidity sensing ability. The enhanced gas sensing mechanism was proposed based on the synergetic effect of high-energy crystal facets, modified surface characteristics and p-n heterojunction. Density functional theory (DFT) studies were also carried out to further clarify the gas sensing mechanisms. This work provides a practical approach to design and fabricate high-precision gas sensors working at room temperature.

An electronic nose using conductometric gas sensors based on P3HT doped with triflates for gas detection using computational techniques (PCA, LDA, and kNN)

An electronic nose using conductometric gas sensors based on P3HT doped with triflates for gas detection using computational techniques (PCA, LDA, and kNN)

Room temperature operated hydrogen sensor using palladium coated on tapered optical fiber

Gaseous pollutants such as hydrogen gas (H2) are present in daily human activities and have been studied extensively due to their high explosive and widespread use in many fields. A common H2 gas detector is electrically based. Although these electrical or conductometric sensors attain high sensitivity, they suffer from drawbacks, including poor selectivity, high operating temperature, and susceptibility to electromagnetic interference, which the optical-based sensor can improve. This study describes the development of a palladium-coated (Pd) optical fiber for the room temperature (H2) hydrogen application process. To improve the evanescent light field that propagates through the fiber, a multimode fiber was used to fabricate a transducing channel with cladding and core diameters of 125 μm and 62.5 μm respectively. The multimode optical fibers were tapered from 125 μm cladding diameters to 20 μm diameter, 10 mm waist-length, 5 mm to the up and down tapered region, and coated with Pd by using the drop-casting technique. Various characterization techniques have been used to characterize palladium, such as Field Emission Scanning Electron Microscopy (FESEM), X-ray energy scattering (EDX), X-ray Diffraction (XRD), and atomic force microscopy (AFM). The fabricated Pd-based sensor operates safely at room temperature with a gas concentration of 0.125 % to 2.00 % H2. The measured sensitivity, response, and recovery time were 18,645 %, 50 s, and 230 s, respectively. The selectivity of the fabricated sensor toward H2 was affirmed with no response to other gases such as ammonia (NH3) and methane (CH4). However, this study demonstrates reliable, efficient, and reproducible H2 detection using a simple and cost-effective method.

Conductometric H2 sensors at room temperature based on palladium nanoparticles on ceria

The rapid detection of hydrogen (H2) at 25 ℃ was of important. Here, A strategy was used to construct a hydroxyl-oxygen vacancy model by functionalizing palladium nanoparticles (Pd NPs) on cubic, rod and spherical CeO2, through a simple reduction process to obtain the Ce4+/Ce3+ transited and the generated of Pd. The results have shown that the 5.0 wt% Pd NPs /CeO2-C sensor exhibited sensitive H2 sensing performance (response value Ra/Rg of 1322 for 1% H2), and fast sensing speed (response time of only 3 s for 1% H2 and 2 s for 3% H2) in the air at 25 °C, the actual detection limit of the instrument is only 50 ppm. Probably due to the reduction of ascorbic acid, the chemisorbed oxygen and Ce3+ were raised by a factor of 2.95 and 1.62. The synergistic effect of the hydroxyl-oxygen vacancy model and Pd presents a new idea for the detection of hydrogen at 25 ℃.

Introducing oxygen vacancies on sodium titanate via NaBH4 treatment for conductometric hydrogen gas sensors

Sodium titanate (NTO) has drawn great attentions in various applications, but few works focus on using NTO as gas sensors. The implication of NTO as gas sensors requires a simple synthesis route to fabricate NTO with large surface areas and oxygen vacancies. Herein, Pd-decorated NTO microscale hierarchical spheres (Pd-NTO MHSs) is synthesized using Ti foil as a precursor·H2O2 is added to promote the preparation of NTO MHSs under a low base-concentration and low temperature condition, making the synthetic route more eco-friendly than previously methods. Additionally, we demonstrate NaBH4 reduction is a one-step process that simultaneously decorates Pd nanoparticles and brings abundant oxygen vacancies on the NTO surface, but too high NaBH4 concentration deteriorates the hydrogen sensing performance. At the optimized condition, Pd-NTO MHSs have a response time of ∼1.8 s to 1% H2 and a wide detection range of 10 ppm to 10% H2 at 25 °C. This study highlights that NaBH4 treatment is an effective strategy for regulating surface chemical properties and introducing oxygen vacancy on NTO, enabling Pd-NTO MHSs an excellent sensing property. It is also helpful for the rational design of NTO as a versatile sensing material with abundant oxygen vacancies to sense other gases.

Conductometric butanone gas sensor based on Co3O4 modified SnO2 hollow spheres with ppb-level detection limit

Butanone is a precursor chemical and detrimental to human body, as a consequence, it is significant for social stability to accurately detect trace butanone. In this paper, Co3O4/SnO2 is prepared by modifying Co3O4 nanoparticles on the surface of SnO2 hollow spheres, demonstrating outstanding sensitivity to butanone. SEM, TEM, HRTEM, EDS, XRD, XPS, EPR and BET characterize the morphology, composition, and properties of Co3O4/SnO2. The response of 0.5 at% Co3O4/SnO2 to 100 ppm butanone is 3900 at 210 °C, moreover, the detection limit can reach ppb level (response is 7.2 to 1 ppb), with excellent repeatability and stability. Since Cox+ can replace Sn2+, the content of oxygen vacancy on the surface is adjusted by modifying Co3O4 with different content to improve the gas sensitivity. Of course, the specific surface area and pore size change correspondingly. The role of p-n heterojunction, as well as the synergistic sensitization principle of catalytic and small size effect, are also analyzed in detail. The gas sensing results show that 0.5 at% Co3O4/SnO2 is an excellent gas sensing material.

Stretchable graphene conductor based on fluid dynamics and its application to flexible conductometric sensor

Stretchable graphene conductor based on fluid dynamics and its application to flexible conductometric sensor