Nanostructured Materials for Room-Temperature Gas Sensors

Designing Self-Sustainable Icephobic Layer by Introducing a Lubricating Un-Freezable Water Hydrogel from Sodium Polyacrylate on the Polyolefin Surface

A convenient, environment-friendly, and cost-effective method to keep anti-icing for a long time was highly desirable. Slippery lubricant layers were regarded to be effective and promising for anti-icing on different surfaces, but the drought-out of lubricants and the possible detriments to the environment were inevitable. By combining super-high molecular weight sodium polyacrylate (H-PAAS) with polyolefin through a one-pot method, a self-sustainable lubricating layer with extremely low ice adhesion of un-freezable water hydrogel was achieved at subzero conditions. The lubricant hydrogel layer could auto-spread and cover the surface of polyolefin after encountering supercooled water, frost, or ice. Due to the reduction of storage modulus in the interface, the ice adhesion of the specimen surfaces was far below 20 kPa, varying from 5.13 kPa to 18.95 kPa. Furthermore, the surfaces could preserve the fairly low adhesion after icing/de-icing cycles for over 15 times and thus exhibited sustainable durability. More importantly, this method could be introducing to various polymers and is of great promise for practical applications.

Highly Sensitive Ammonia Gas Detection at Room Temperature by Integratable Silicon Nanowire Field-Effect Sensors

Toxic gas monitoring at room temperature (RT) is of great concern to public health and safety, where ultrathin silicon nanowires (SiNWs), with diameter <80 nm, are ideal one-dimensional candidates to achieve high-performance field-effect sensing. However, a precise integration of the tiny SiNWs as active gas sensor channels has not been possible except for the use of expensive and inefficient electron beam lithography and etching. In this work, we demonstrate an integratable fabrication of field-effect sensors based on orderly SiNW arrays, produced via step-guided in-plane solid-liquid-solid growth. The back-gated SiNW sensors can be tuned into suitable subthreshold detection regime to achieve an outstanding field-effect sensitivity (75.8% @ 100 ppm NH₃), low detection limit (100 ppb), and excellent selectivity to NH₃ gas at RT, with fast response/recovery time scales (T_res/T_rec) of 20 s (at 100 ppb NH₃) and excellent repeatability and high stability over 180 days. These outstanding sensing performances can be attributed to the fast charge transfer between adsorbed NH₃ molecules and the exposed SiNW channels, indicating a convenient strategy to fabricate and deploy high-performance gas detectors that are widely needed in the booming marketplace of wearable or portable electronics.

Oscillatory Copper Deposition on Conical Iron Electrodes in a Nonuniform Magnetic Field

We report the effect of a magnetic field on the deposition of copper ions on a conically shaped iron probe. In our setup, the magnetic forces and buoyancy are the key factors influencing the electrolyte flow and the mass transfer. Without external current, a spontaneous reduction of copper on the iron cone occurs, known as electroless deposition. Mach–Zehnder and differential interferometry indicate a variation in the concentration of copper ions near the cone. After an initial transient of about 60 s, temporal oscillations in the copper concentration are found under the effect of a magnetic field. In galvanostatic conditions, a similar oscillatory behavior of the concentration of the electrolyte is observed. Numerical simulations show that the oscillations are caused by the magnetic gradient, Lorentz force, and buoyancy force counteracting one another, and the oscillation frequency is estimated analytically based on this mechanism. Furthermore, we present a study on the oscillation frequency for both electroless and galvanostatic conditions with different current densities. The results of this study may stimulate future research aimed at the local control of the deposition rate and the realization of miniaturized, regularly structured deposits using magnetic fields.

Boosting Room Temperature Sensing Performances by Atomically Dispersed Pd Stabilized via Surface Coordination

The urgent requirement of monitoring air pollution worldwide evokes intensive research interest in developing chemiresistive gas sensing techniques. To overcome the limits in sensitivity and selectivity of room temperature (RT) chemiresistive sensing materials, a new strategy using single-atom catalysts (SACs) via surface coordination is proposed. As a proof-of-concept, single Pd atoms on TiO₂ (Pd₁-TiO₂) possess high efficiency in generating adsorbed O₂^- as well as high activity and selectivity in catalyzing CO oxidation at RT. As a result, Pd₁-TiO₂ shows record high sensitivity among the reported RT sensing materials, which is even comparable to those of the best materials working at high temperature. It also provides an approximately 1 order of magnitude lower limit of detection than the best CO sensing materials. Moreover, Pd₁-TiO₂ presents high selectivity toward 12 kinds of interference gases. This work not only paves a way to design high-performance RT gas sensing materials but also extends the application of SACs.

Magnetic gas sensing: working principles and recent developments

Gas sensors work on the principle of transforming the gas adsorption effects on the surface of the active material into a detectable signal in terms of its changed electrical, optical, thermal, mechanical, magnetic (magnetization and spin), and piezoelectric properties. In magnetic gas sensors, the change in the magnetic properties of the active materials is measured by one of the approaches such as Hall effect, magnetization, spin orientation, ferromagnetic resonance, magneto-optical Kerr effect, and magneto-static wave oscillation effect. The disadvantages of different types of gas sensors include their chemical selectivity and sensitivity to humidity and high-temperature operation. For example, in the case of chemiresistive-type gas sensors, the change in the sensor resistance can drastically vary in the real environment due to the presence of other gas species and the overall electrical effect is quite complex due to simultaneous surface reactions. Further, it is not easy to make stable contacts for powdered samples for the conventional electrical property-based gas sensors. Fire hazard is another issue for the electrical property-based hydrogen gas sensors due to their flammable nature at higher operating temperature. In this regard, to solve these issues, magnetic gas sensor concepts have emerged, in which the magnetic properties of the materials get modified when exposed to gas molecules. In this review article, the working principles, fundamentals, recent developments, and future perspectives in magnetic gas sensors are reviewed. Finally, the prospects and opportunities in these exciting fields are also commented upon based on their current progress.

Simple and rapid gas sensing using a single-walled carbon nanotube field-effect transistor-based logic inverter

Single-walled carbon nanotubes (SWCNTs) are promising candidates for gas sensing applications, providing an efficient solution to the device miniaturization challenge and allowing low power consumption. SWCNT gas sensors are mainly based on field-effect transistors (SWCNT-FETs) where the modification of the current flowing through the nanotube is used for gas detection. A major limitation of these SWCNT-FETs lies in the difficulty to measure their transfer curves, since the flowing current typically varies between 10^-12 and 10^-3 A. Thus, voluminous and energy consuming systems are necessary, severely limiting the miniaturization and low energy consumption. Here, we propose an inverter device that combines two SWCNT-FETs which brings a concrete solution to these limitations and simplifies data processing. In this innovative sensing configuration, the gas detection is based on the variation of an electric potential in the volt range instead of a current intensity variation in the microampere range. In this study, the proof of concept is performed using NO₂ gas but can be easily extended to a wide range of gases.

Electroless deposition of Pd/Pt nanoparticles on electrochemically grown TiO2 nanotubes for ppb level sensing of ethanol at room temperature

This work presents a comparative sensing study of three sensors based on pristine TiO₂ nanotubes, Pd loaded TiO₂ nanotubes, and Pt loaded TiO₂ nanotubes. Pristine TiO₂ nanotubes were synthesized using an electrochemical anodization method and an electroless plating method was used for the uniform deposition of noble metal nanoparticles of either Pd or Pt over the surface of TiO₂ nanotubes. The samples were thoroughly characterized by XRD, FESEM, EDS, TEM, and XPS techniques. The sensitivity of all three sensors was investigated at room temperature (300 K) for different volatile organic compounds like ethanol, methanol, 2-propanol, acetone, and benzene. The results revealed that loading of Pd and Pt nanoparticles improved the response magnitude of the sensor remarkably as these noble metals possess better oxygen dissociation capability than pristine TiO₂. The Pd-TiO₂ nanotube sensor exhibited a maximum response magnitude of 20-98% towards 100-1000 ppb of ethanol at room temperature. Notably, the formation of Pd/Pt-TiO₂ discrete heterojunctions on the surface of TiO₂ nanotubes was found to be responsible for enhanced sensitivity of the sensors.

Boosting the sensing properties of resistive-based gas sensors by irradiation techniques: a review

The ongoing need to detect and monitor hazardous, volatile, and flammable gases has led to the use of gas sensors in several fields to improve safety and health issues. Conductometric type gas sensors, which have considerable advantages over other gas sensors, have thrived in numerous gas sensing fields. The ever-present key challenges and requirements of these sensors are to achieve excellent performance, including high sensitivity, good selectivity, low working temperature, and durability. Therefore, tremendous research effort has focused on improving these properties, and various state-of-the-art techniques have been reported. This review article discusses the recent advances and utilization of various irradiation techniques, including electron-beam, microwave, ion-beam, and gamma-ray irradiation, along with their investigation of the effects on the physicochemical properties of pre-synthesized nanomaterials, sensing performances, and related gas sensing mechanisms. A review of the progress on the effects of different irradiation techniques for boosting the sensing properties can contribute to the evolution of highly reliable sensors to assess the environment and health. For researchers, who work on gas sensors, this paper provides information on the current trends on the advances in the novel state-of-art of irradiated materials and their promising application in the sensitive detection of various toxic and VOCs.

Synthesis of Cu2O-Modified Reduced Graphene Oxide for NO2 Sensors

Nowadays, metal oxide semiconductors (MOS)-reduced graphene oxide (rGO) nanocomposites have attracted significant research attention for gas sensing applications. Herein, a novel composite material is synthesized by combining two p-type semiconductors, i.e., Cu₂O and rGO, and a p-p-type gas sensor is assembled for NO₂ detection. Briefly, polypyrrole-coated cuprous oxide nanowires (PPy/Cu₂O) are prepared via hydrothermal method and combined with graphene oxide (GO). Then, the nanocomposite (rGO/PPy/Cu₂O) is obtained by using high-temperature thermal reduction under Ar atmosphere. The results reveal that the as-prepared rGO/PPy/Cu₂O nanocomposite exhibits a maximum NO₂ response of 42.5% and is capable of detecting NO₂ at a low concentration of 200 ppb. Overall, the as-prepared rGO/PPy/Cu₂O nanocomposite demonstrates excellent sensitivity, reversibility, repeatability, and selectivity for NO₂ sensing applications.

High-dispersed Fe2O3/Fe nanoparticles residing in 3D honeycomb-like N-doped graphitic carbon as high-performance room-temperature NO2 sensor

This work illustrates a simple polymer thermal treatment strategy to develop high-dispersed Fe₂O₃/Fe nanoparticles residing in honeycomb-like N-doped graphitic carbon (Fe₂O₃/Fe@N-GC). The as-prepared Fe₂O₃/Fe@N-GC composites consist of three-dimensional (3D) strutted interconnective graphitic carbon frame, which would not only refrain from restacking and facilitate the charge transfer, but also provide more reaction interface between gas molecules and materials. Benefiting from the synergistic merits of Fe₂O₃/Fe, N-doping graphitic carbon, high surface area and unique 3D architectures, the optimal Fe₂O₃/Fe@N-GC presents impressive sensitivity and selectivity for NO₂ gas detection at room temperature with the response of 25.48-100 ppm, response time of 2.13 s, recovery time of 11.73 s, detection limit of 10 ppb and as long as 60 days of stability. As a result, the present Fe₂O₃/Fe@N-GC composite with an easy fabrication method and high sensitivity, selectivity, stabitliy towards NO₂ at RT would inspire various designs based on the 3D honeycomb structure for more real applications in gas sensors.

Nanoarchitectured porous carbons derived from ZIFs toward highly sensitive and selective QCM sensor for hazardous aromatic vapors

Metal-organic frameworks (MOFs) are a versatile source of carbon nanoarchitectures in gas sensing applications (Torad et al., 2019). Herein, several types of nanoporous carbons (NPCs) have been prepared by in-situ carbothermal treatment of zeolitic imidazolate frameworks (ZIFs) under different inert atmospheres to achieve a highly sensitive discrimination of vaporized aromatic compounds. In this study, we demonstrate how different carbonization conditions under the flow of N₂ or H₂ gases affect the surface area and the degree of graphitization of the resulting NPCs polyhedrons, and their consequent effect on the sensing performance in terms of sensitivity and selectivity toward toxic volatile hydrocarbons. A growth of carbon nanotubes (CNTs) is observed on the surface of polyhedral NPCs after careful carbonization of ZIF crystals under H₂ atmosphere. The fabricated quartz crystal microbalance (QCM) sensor with CNT-containing NPCs demonstrates increased sensitivity and selectivity towards toxic volatile aromatic hydrocarbons over the aliphatic analogues, suggesting the rich growth of hairy graphitic-like CNTs on the surface of carbon framework act as highly selective sensing antennae for vapor molecular discrimination of toxic aromatic hydrocarbons. Despite of increased selectivity towards volatile aromatic compounds, however, the surface area of CNT-rich NPCs derived from hybrid ZIFs and ZIF-67 is greatly sacrificed as compared to CNT-free NPCs from ZIF-8 polyhedron. In the case of Co-containing ZIF-67, the rich growth of hair-like CNTs, which is induced by the presence of Co, is observed during carbothermal reduction under a flow of H₂ gas, thus allowing ultra-selective detection of aromatic hydrocarbons in the vapor phase, such as benzene (C₆H₆) and toluene (C₆H₅CH₃) over their aliphatic analogue, c-hexane (c-C₆H₁₂) of same molecular mass, size and vapor pressure.

Highly Selective Detection of Benzene and Discrimination of Volatile Aromatic Compounds Using Oxide Chemiresistors with Tunable Rh-TiO2 Catalytic Overlayers

Volatile aromatic compounds are major air pollutants, and their health impacts should be assessed accurately based on the concentration and composition of gas mixtures. Herein, novel bilayer sensors consisting of a SnO2 sensing layer and three different xRh-TiO2 catalytic overlayers (x = 0.5, 1, and 2 wt%) are designed for the new functionalities such as the selective detection, discrimination, and analysis of benzene, toluene, and p-xylene. The 2Rh-TiO2/SnO2 bilayer sensor shows a high selectivity and response toward ppm- and sub-ppm-levels of benzene over a wide range of sensing temperatures (325-425 °C). An array of 0.5Rh-, 1Rh-, and 2Rh-TiO2/SnO2 sensors exhibits discrimination and composition analyses of aromatic compounds. The conversion of gases into more active species at moderate catalytic activation and the complete oxidation of gases into non-reactive forms by excessive catalytic promotion are proposed as the reasons behind the enhancement and suppression of analyte gases, respectively. Analysis using proton transfer reaction-quadrupole mass spectrometer (PTR-QMS) is performed to verify the above proposals. Although the sensing characteristics exhibit mild moisture interference, bilayer sensors with systematic and tailored control of gas selectivity and response provide new pathways for monitoring aromatic air pollutants and evaluating their health impacts.

Highly Sensitive Humidity Sensors Based on Polyethylene Oxide/CuO/Multi Walled Carbon Nanotubes Composite Nanofibers

Polymer composites are favorite materials for sensing applications due to their low cost and easy fabrication. In the current study, composite nanofibers consisting of polyethylene oxide (PEO), oxidized multi-walled carbon nanotubes (MWCNT) and copper oxide (CuO) nanoparticles with 1% and 3% of fillers (i.e., PEO–CuO–MWCNT: 1%, and PEO–CuO–MWCNT: 3%) were successfully developed through electrospinning for humidity sensing applications. The composite nanofibers were characterized by FTIR, XRD, SEM and EDX analysis. Firstly, they were loaded on an interdigitated electrode (IDE), and then the humidity sensing efficiency was investigated through a digital LCR meter (E4980) at different frequencies (100 Hz–1 MHz), as well as the percentage of relative humidity (RH). The results indicated that the composite nanofibers containing 1% and 3% MWCNT, combined with CuO in PEO polymer matrix, showed potent resistive and capacitive response along with high sensitivity to humidity at room temperature in an RH range of 30–90%. More specifically, the PEO–CuO–MWCNT: 1% nanocomposite displayed a resistive rapid response time within 3 s and a long recovery time of 22 s, while the PEO–CuO–MWCNT: 3% one exhibited 20 s and 11 s between the same RH range, respectively.

Room-Temperature Hydrogen-Sensing Capabilities of Pt-SnO2 and Pt-ZnO Composite Nanoceramics Occur via Two Different Mechanisms

Impressive room-temperature gas-sensing capabilities have been reported for nanomaterials of many metal oxides, including SnO₂, ZnO, TiO₂, WO₃, and Fe₂O₃, while little attention has been paid to the intrinsic difference among them. Pt-SnO₂ and Pt-ZnO composite nanoceramics have been prepared through convenient pressing and sintering. The former shows strong and stable responses to hydrogen in 20% O₂-N₂ (synthetic air) at room temperature, while the responses to hydrogen in N₂ cannot be stabilized in limited times; the latter shows strong and stable responses to hydrogen in N₂, while the responses to hydrogen in synthetic air are greatly depressed. Further analyses reveal that for Pt-ZnO, the responses result from the reaction between hydrogen and oxygen chemisorbed on ZnO; while for Pt-SnO₂, the responses result from two reactions of hydrogen, one is that with oxygen chemisorbed on SnO₂ and the other is hydrogen chemisorption on SnO₂. These results reveal two different room-temperature hydrogen-sensing mechanisms among MOXs, which results in highly contrasting room-temperature hydrogen-sensing capabilities attractive for sensing hydrogen in oxygen-contained and oxygen-free environments, separately.

Sustainable NO removal and its sensitive monitoring at room temperature by electrogenerated Ni (I) electron mediator

Online monitoring of gas pollutants in the gas phase at room temperature using an electrochemical macro gas flow sensor is challenging and important for the pollutant treatment process. In this work, for the first time, we tried to explore the homogeneous and heterogeneous application of Ni(II) (CN)₄^2- in the KOH environment for the removal and monitoring of toxic nitric oxide gas. The homogeneous electrogenerated Ni(I) (CN)₄^3- was effectively removing the toxic nitric oxide gas by electro scrubbing method and the novel Ni(II) (CN)₄^2- and KOH modified electrode used for heterogeneous sensor application with high sensitivity, and reliability toward Nitric oxide gas. The sensor showed enhanced gas diffusion and high sensitivity. Scanning electron microscopy and X-ray diffraction confirmed the modification of the carbon felt electrode. In a high concentrated KOH environment, the active mediator stabilized the sensor for a long time compared to the neutral environment. The Ni(II) (CN)₄^2- fabricated carbon felt was used to monitor the concentration of nitric oxide gas pollutant; the calculated sensitivity was approximately -0.33 mA ppm^-1 cm^-2. The current increased linearly with increasing nitric oxide concentration up to 12 ppm and was validated by online gas chromatography. The developed electrochemical gas flow sensor successfully monitored the unremoved nitric oxide gas at the exit from the MER electro-scrubbing process; the concentration was calculated using a calibration plot.

Enhancing room-temperature NO2 gas sensing performance based on a metal phthalocyanine/graphene quantum dot hybrid material

Metal phthalocyanine (MPc) has a great saturation response value, but its low conductivity and slow response speed limit its practical application. A novel hybrid material composed of graphene quantum dots (GQDs) and metal phthalocyanine derivatives has been obtained. GQDs can be anchored onto the surface of MPc nanofibers through π-π stacking. The response to NO2 can be significantly enhanced under certain component proportion matching, which is much better than their respective response to NO2. The introduction of GQDs greatly increases the conductivity of phthalocyanine fibers, leading to a faster response of the hybrid material. In addition, the reproducibility, selectivity and stability of the hybrid materials are excellent, and the minimum response concentration can reach 50 ppb. Ultra-low-power laser irradiation was used to solve the problem of slow recovery of metal phthalocyanine. Overall, we present the advantages of combining MPc nanofibers with GQDs and pave a new avenue for the application of MPc-GQD hybrids in the gas sensing field.

Berlin Green Framework-Based Gas Sensor for Room-Temperature and High-Selectivity Detection of Ammonia

Ammonia detection possesses great potential in atmosphere environmental protection, agriculture, industry, and rapid medical diagnosis. However, it still remains a great challenge to balance the sensitivity, selectivity, working temperature, and response/recovery speed. In this work, Berlin green (BG) framework is demonstrated as a highly promising sensing material for ammonia detection by both density functional theory simulation and experimental gas sensing investigation. Vacancy in BG framework offers abundant active sites for ammonia absorption, and the absorbed ammonia transfers sufficient electron to BG, arousing remarkable enhancement of resistance. Pristine BG framework shows remarkable response to ammonia at 50-110 °C with the highest response at 80 °C, which is jointly influenced by ammonia's absorption onto BG surface and insertion into BG lattice. The sensing performance of BG can hardly be achieved at room temperature due to its high resistance. Introduction of conductive Ti₃CN MXene overcomes the high resistance of pure BG framework, and the simply prepared BG/Ti₃CN mixture shows high selectivity to ammonia at room temperature with satisfying response/recovery speed.

Regulation of electronic properties of ZnO/In2O3 heterospheres via atomic layer deposition for high performance NO2 detection

Heterogeneous In₂O₃/ZnO spheres designed by atomic layer deposition manifest high response to NO₂ detection.

ZnO nanoarrays via a thermal decomposition–deposition method for sensitive and selective NO2 detection

ZnO nanoarrays deposited by a thermal decomposition–deposition (TDD) strategy demonstrate a high response for NO₂ detection.

Enhanced room-temperature ethanol sensing performance of porous MoO3/V0.13Mo0.87O2.935 heterostructures self-assembled with 2D nanosheets

Porous MoO₃/V_0.13Mo_0.87O_2.935 heterostructures self-assembled with 2D nanosheets have been primarily prepared by a facile method for effectively detecting ethanol at room temperature.

Grain Boundary Control of Organic Semiconductors via Solvent Vapor Annealing for High-Sensitivity NO2 Detection

The microstructure of the organic semiconductor (OSC) active layer is one of the crucial topics to improve the sensing performance of gas sensors. Herein, we introduce a simple solvent vapor annealing (SVA) process to control 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) OSC films morphology and thus yields high-sensitivity nitrogen organic thin-film transistor (OTFT)-based nitrogen dioxide (NO₂) sensors. Compared to pristine devices, the toluene SVA-treated devices exhibit an order of magnitude responsivity enhancement to 10 ppm NO₂, further with a limit of detection of 148 ppb. Systematic studies on the microstructure of the TIPS-pentacene films reveal the large density grain boundaries formed by the SVA process, improving the capability for the adsorption of gas molecules, thus causing high-sensitivity to NO₂. This simple SVA processing strategy provides an effective and reliable access for realizing high-sensitivity OTFT NO₂ sensors.

Gold-carbonaceous materials based heterostructures for gas sensing applications

The carbon nanostructures such as carbon nanotubes and graphene decorated with gold nanoparticles exhibit promising gas sensing applications with enhanced sensitivity.

Facile Fabrication of an Ammonia-Gas Sensor Using Electrochemically Synthesised Polyaniline on Commercial Screen-Printed Three-Electrode Systems

Polyaniline (PANI) is a conducting polymer, widely used in gas-sensing applications. Due to its classification as a semiconductor, PANI is also used to detect reducing ammonia gas (NH3), which is a well-known and studied topic. However, easier, cheaper and more straightforward procedures for sensor fabrication are still the subject of much research. In the presented work, we describe a novel, more controllable, synthesis approach to creating NH3 PANI-based receptor elements. The PANI was electrochemically deposited via cyclic voltammetry (CV) on screen-printed electrodes (SPEs). The morphology, composition and surface of the deposited PANI layer on the Au electrode were characterised with electron microscopy, Fourier-transform infrared spectroscopy and profilometry. Prior to the gas-chamber measurement, the SPE was suitably modified by Au sputtering the individual connections between the three-electrode system, thus showing a feasible way of converting a conventional three-electrode electrochemical SPE system into a two-electrode NH3-gas detecting system. The feasibility of the gas measurements' characterisation was improved using the gas analyser. The gas-sensing ability of the PANI-Au-SPE was studied in the range 32-1100 ppb of NH3, and the sensor performed well in terms of repeatability, reproducibility and sensitivity.

Metal oxide-based gas sensors for detection of exhaled breath markers

Exhaled breath test is a typical disease monitoring method for replacing blood and urine samples that may create discomfort for patients. To monitor exhaled breath markers, gas biomedical sensors have undergone rapid progress for non‐invasive and point‐of‐care diagnostic devices. Among gas sensors, metal oxide‐based biomedical gas sensors have received remarkable attention owing to their unique properties, such as high sensitivity, simple fabrication, miniaturization, portability and real‐time monitoring. Herein, we reviewed the recent advances in chemoresistive metal oxide‐based gas sensors with ZnO, SnO₂ and In₂O₃ as sensing materials for monitoring a range of exhaled breath markers (i.e., NO, H₂, H₂S, acetone, isoprene and formaldehyde). We focused on the strategies that improve the sensitivity and selectivity of metal oxide‐based gas sensors. The challenges to fabricate a functional gas sensor with high sensing performance along with suggestions are outlined.

Correlation between Microstructure and Chemical Composition of Zinc Oxide Gas Sensor Layers and Their Gas-Sensitive Properties in Chlorine Atmosphere

In this article, we present results concerning the impact of structural and chemical properties of zinc oxide in various morphological forms and its gas-sensitive properties, tested in an atmosphere containing a very aggressive gas such as chlorine. The aim of this research was to understand the mechanism of chlorine detection using a resistive gas sensor with an active layer made of zinc oxide with a different structure and morphology. Two types of ZnO sensor layers obtained by two different technological methods were used in sensor construction. Their morphology, crystal structure, specific surface area, porosity, surface chemistry and structural defects were characterized, and then compared with gas-sensitive properties in a chlorine-containing atmosphere. To achieve this goal, scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and photoluminescence spectroscopy (PL) methods were used. The sensing properties of obtained active layers were tested by the temperature stimulated conductance method (TSC). We have noticed that their response in a chlorine atmosphere is not determined by the size of the specific surface or porosity. The obtained results showed that the structural defects of ZnO crystals play the most important role in chlorine detection. We demonstrated that Cl2 adsorption is a concurrent process to oxygen adsorption. Both of them occur on the same active species (oxygen vacancies). Their concentration is higher on the side planes of the zinc oxide crystal than the others. Additionally, ZnO sublimation process plays an important role in the chlorine detection mechanism.

Insight the process of hydrazine gas adsorption on layered WS2: a first principle study

The process of hydrazine gas adsorption on layered WS₂ has been systematically studied from first principle calculations. Our results demonstrate that this adsorption process is exothermic, and hydrazine molecules are physically adsorbed. The layer-dependent adsorption energy and interlayer separation induced by van der Waals interaction exerted by hydrazine molecules lead to the difficulty in desorbing hydrazine molecules from layered WS₂ as the number of layers increases. The most interesting finding is the emergence of localized impurity states below the Fermi level upon the hydrazine adsorption, irrespective of the number of WS₂ layers, resulting in a significant effect on the band structures and subsequently changing its electrical conductivity. Furthermore, a layer-dependent small charge transfer occurs between hydrazine and layered WS₂, leading to a charge redistribution and considerable polarization in the adsorbed systems. The existence of defects and the humidity, on the other hand, influences the sensitivity of layered WS₂ to the hydrazine adsorption. Obtained results show that a perfectly layered WS₂ might be a promising candidate as an efficient nanosensor to detect such toxic gas in dry environment.

Rational Design of Semiconductor-Based Chemiresistors and their Libraries for Next-Generation Artificial Olfaction

Artificial olfaction based on gas sensor arrays aims to substitute for, support, and surpass human olfaction. Like mammalian olfaction, a larger number of sensors and more signal processing are crucial for strengthening artificial olfaction. Due to rapid progress in computing capabilities and machine-learning algorithms, on-demand high-performance artificial olfaction that can eclipse human olfaction becomes inevitable once diverse and versatile gas sensing materials are provided. Here, rational strategies to design a myriad of different semiconductor-based chemiresistors and to grow gas sensing libraries enough to identify a wide range of odors and gases are reviewed, discussed, and suggested. Key approaches include the use of p-type oxide semiconductors, multinary perovskite and spinel oxides, carbon-based materials, metal chalcogenides, their heterostructures, as well as heterocomposites as distinctive sensing materials, the utilization of bilayer sensor design, the design of robust sensing materials, and the high-throughput screening of sensing materials. In addition, the state-of-the-art and key issues in the implementation of electronic noses are discussed. Finally, a perspective on chemiresistive sensing materials for next-generation artificial olfaction is provided.

Ammonia Gas Sensor Response of a Vertical Zinc Oxide Nanorod-Gold Junction Diode at Room Temperature

Conventional metal oxide semiconductor (MOS) gas sensors have been investigated for decades to protect our life and property. However, the traditional devices can hardly fulfill the requirements of our fast developing mobile society, because the high operating temperatures greatly limit their applications in battery-loaded portable systems that can only drive devices with low power consumption. As ammonia is gaining importance in the production and storage of hydrogen, there is an increasing demand for energy-efficient ammonia detectors. Hence, in this work, a Schottky diode resulting from the contact between zinc oxide nanorods and gold is designed to detect gaseous ammonia at room temperature with a power consumption of 625 μW. The Schottky diode gas sensors benefit from the change of barrier height in different gases as well as the catalytic effect of gold nanoparticles. This diode structure, fabricated without expensive interdigitated electrodes and displaying excellent performance at room temperature, provides a novel method to equip mobile devices with MOS gas sensors.

Stretchable, Stable, and Room-Temperature Gas Sensors Based on Self-Healing and Transparent Organohydrogels

Conductive hydrogels have emerged as promising candidate materials for fabricating wearable electronics because of their fascinating stimuli-responsive and mechanical properties. However, the inherent instability of hydrogels seriously limits their application scope. Herein, the stable, ultrastretchable (upon to 1330% strain), self-healing, and transparent organohydrogel was exploited as a novel gas-responsive material to fabricate NH₃ and NO₂ gas sensors for the first time with extraordinary performance. A facile solvent substitution method was employed to convert the unstable hydrogel into the organohydrogel with a remarkable moisture retention (avoid drying within a year), frost resistance (freezing point below -130 °C), and unimpaired mechanical and gas sensing properties. First-principles simulations were performed to uncover the mechanisms of antidrying and antifreezing effects of organohydrogels and the interactions between NH₃/NO₂ and organohydrogels, revealing the vital role of hydrogen bonds in enhancing the stability and the adsorption of NH₃/NO₂ on the organohydrogel. The organohydrogel gas sensor displayed high sensitivity, ultralow theoretical limit of detection (91.6 and 3.5 ppb for NH₃ and NO₂, respectively), reversibility, and fast recovery at room temperature. It exhibited the capabilities to work at a highly deformed state with nondegraded sensing performance and restore all the electrical, mechanical, and sensing properties after mechanical damage. The gas sensing mechanism was understood by considering the gas adsorption on functional groups, dissolution in the solvent, and the hindering effect on the transport of ions.

Au Nanoparticles Decorated Mesoporous SiO2 -WO3 Hybrid Materials with Improved Pore Connectivity for Ultratrace Ethanol Detection at Low Operating Temperature

Semiconducting metal oxides-based gas sensors with the capability to detect trace gases at low operating temperatures are highly desired in applications such as wearable devices, trace pollutant detection, and exhaled breath analysis, but it still remains a great challenge to realize this goal. Herein, a multi-component co-assembly method in combination with pore engineering strategy is proposed. By using bi-functional (3-mercaptopropyl) trimethoxysilane (MPTMS) that can co-hydrolyze with transition metal salt and meanwhile coordinate with gold precursor during their co-assembly with PEO-b-PS copolymers, ordered mesoporous SiO₂ -WO₃ composites with highly dispersed Au nanoparticles of 5 nm (mesoporous SiO₂ -WO₃ /Au) are straightforward synthesized. This multi-component co-assembly process avoids the aggregation of Au nanoparticles and pore blocking in conventional post-loading method. Furthermore, through controlled etching treatment, a small portion of silica can be removed from the pore wall, resulting in mesoporous SiO₂ -WO₃ /Au with increased specific surface area (129 m²  g^-1 ), significantly improved pore connectivity, and enlarged pore window (>4.3 nm). Thanks to the presence of well-confined Au nanoparticles and ε-WO₃ , the mesoporous SiO₂ -WO₃ /Au based gas sensors exhibit excellent sensing performance toward ethanol with high sensitivity (R_a /R_g = 2-14 to 50-250 ppb) at low operating temperature (150 °C).

High-Temperature Hydrogen Sensing Performance of Ni-Doped TiO2 Prepared by Co-Precipitation Method

This work deals with the substantially high-temperature hydrogen sensors required by combustion and processing technologies. It reports the synthesis of undoped and Ni-doped TiO2 (with 0, 0.5, 1 and 2 mol.% of Ni) nanoparticles by a co-precipitation method and the obtained characteristics applicable for this purpose. The effect of nickel doping on the morphological variation, as well as on the phase transition from anatase to rutile, of TiO2 was investigated by scanning electron microscopy, X-ray diffraction and Raman spectroscopy. The resistive sensors prepared with these powders were tested toward H2 at 600 °C. The results indicate that 0.5% Ni-doped TiO2 with almost equal amounts of anatase and rutile shows the best H2 sensor response (ΔR/R0 = 72%), response rate and selectivity. The significant improvement of the sensing performance of 0.5% Ni-doped TiO2 is mainly attributed to the formation of the highest number of n-n junctions present between anatase and rutile, which influence the quantity of adsorbed oxygen (i.e., the active reaction site) on the surface and the conductivity of the material.

Adjustment of oxygen vacancy states in ZnO and its application in ppb-level NO2 gas sensor

Oxygen vacancy (V_O) is long believed as a key factor influencing the gas sensing properties. However, the concentration of V_O is generally focused while the V_O state is neglected, which masks the inherent mechanism of gas sensor. Using a post annealing process, the influence of V_O states on the response of ZnO nanofilm to NO₂ gas is investigated in this study. The systematical analysis of the results obtained by different methods indicates a transformation of V_O from the neutral to the doubly ionized state during post annealing treatment. The results also imply that the gas sensing properties is not directly correlated with the V_O concentration. And due to the competitive adsorption of ambient O₂, the neutral V_O is majorly occupied by the adsorbed O₂ while the V_O in doubly ionized state can promote the adsorption of NO₂. Consequently, the transition of V_O from the neutral to the doubly ionized state can lead to a dramatic increase of the response to NO₂, from 733 to 3.34 × 10⁴ for 100 ppm NO₂. Guided by this mechanism, NO₂ gas sensing in ppb-level is also achieved: the response reaches 165% to 25 ppb (0.025 ppm) NO₂ with a good repeatability.

Ultrasensitive and Stretchable Conductive Fibers Using Percolated Pd Nanoparticle Networks for Multisensing Wearable Electronics: Crack-Based Strain and H2 Sensors

The need for wearable electronic devices continues to grow, and the research is under way for stretchable fiber-type sensors that are sensitive to the surrounding atmosphere and will provide proficient measurement capabilities. Currently, one-dimensional fiber sensors have several limitations for their extensive use because of the complex structures of the sensing mechanisms. Thus, it is essential to miniaturize these materials with durability while integrating multiple sensing capabilities. Herein, we present an ultrasensitive and stretchable conductive fiber sensor using PdNP networks embedded in elastomeric polymers for crack-based strain and H₂ sensing. The fiber multimodal sensors show a gauge factor of ∼2040 under 70% strain and reliable mechanical deformation tolerance (10,000 stretching cycles) in the strain-sensor mode. For H₂ sensing, the fiber multimodal sensors exhibit a wide sensing range of high sensitivity: -0.43% response at 5 ppm (0.0005%) H₂ gas and -27.3% response at 10% H₂ gas. For the first time, we demonstrate highly stretchable H₂ sensors that can detect H₂ gas under 110% strain with mechanical durability. As demonstrated, their stable performance allows them to be used in wearable applications that integrate fiber multimodal sensors into industrial safety clothing along with a microinorganic light-emitting diode for visual indication, which exhibits proper activation upon H₂ gas exposure.

Co-FeS2/CoS2 Heterostructured Nanomaterials for pH Sensing

Biosensors are widely used in production and life, and can be used in medicine, industrial production, and scientific research. Among them, the detection of pH has always received extensive attention. In this study, we demonstrate the use of a one-step hydrothermal method to prepare Co-FeS2/CoS2 nanomaterials as pH sensor (pH vs. overpotential) for the first time. The proposed pH sensor exhibits outstanding performance in KOH solutions via electrochemical methods with good stability. Overall, the results of this study not only add to the non-noble transition metal electrocatalysis research, but also identify important sensing characteristics for electrocatalysts.

Mechanism and Suppression of Physisorbed-Water-Caused Hysteresis in Graphene FET Sensors

Hysteresis is a problem in field-effect transistors (FETs) often caused by defects and charge traps inside a gate isolating (e.g., SiO₂) layer. This work shows that graphene-based FETs also exhibit hysteresis due to water physisorbed on top of graphene determined by the relative humidity level, which naturally happens in biosensors and ambient operating sensors. The hysteresis effect is explained by trapping of electrons by physisorbed water, and it is shown that this hysteresis can be suppressed using short pulses of alternating gate voltages.

Platinum-Supported Cerium-Doped Indium Oxide for Highly Sensitive Triethylamine Gas Sensing with Good Antihumidity

Triethylamine is extremely harmful to human health, and chronic inhalation can lead to respiratory and hematological diseases and eye lesions. Hence, it is essential to develop a triethylamine gas-sensing technology with high response, selectivity, and stability for use in healthcare and environmental monitoring. In this work, a simple and low-cost sensor based on the Pt- and Ce-modified In₂O₃ hollow structure to selectively detect triethylamine is developed. The experimental results reveal that the sensor based on 1% Pt/Ce₁₂In exhibits excellent triethylamine-sensing performance, including its insusceptibility to water, reduced operating temperature, enhanced response, and superior long-term stability. This work suggests that the enhancement of sensing performance toward triethylamine can be attributed to the high relative contents of O_V and O_C, large specific surface area, catalytic effect, the electronic sensitization of Pt, and the reversible redox cycle properties of Ce. This sensor represents a unique and highly sensitive means to detect triethylamine, which shows great promise for potential applications in food safety inspection and environmental monitoring.

Low Working Temperature of ZnO-MoS2 Nanocomposites for Delaying Aging with Good Acetylene Gas-Sensing Properties

The long-term stability and the extension of the use time of gas sensors are one of the current concerns. Lowering the working temperature is one of the most effective methods to delay aging. In this paper, pure MoS₂ and ZnO-MoS₂ nanocomposites were successfully prepared by the hydrothermal method, and the morphological characteristics were featured by scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Pure MoS₂ and ZnO-MoS₂ nanocomposites, as a comparison, were used to study the aging characteristic. The sensing properties of the fabricated gas sensors with an optimal molar ratio ZnO-MoS₂ (Zn:Mo = 1:2) were recorded, and the results exhibit a high gas-sensing response and good repeatability to the acetylene detection. The working temperature was significantly lower than for pure MoS₂. After aging for 40 days, all the gas-sensing response was relatively attenuated, and pure MoS₂ exhibits a faster decay rate and lower gas-sensing response than nanocomposites. The better gas-sensing characteristic of nanocomposites after aging was possibly attributed to the active interaction between ZnO and MoS₂.

Enhancement of Low-Temperature Gas-Sensing Performance Using Substoichiometric WO3-x Modified with CuO

To verify the effect of oxygen vacancy on gas sensitivity, we have systematically investigated the gas-sensing performance of copper oxide/substoichiometric tungsten oxide (CuO/WO_3-x) nanocomposite sensors. Oxygen deficiency in WO_3-x facilitates the reaction of hydrogen sulfide (H₂S) gas with chemisorbed oxygen species (i.e., O₂^-, O^-, and O^2-) at low temperature. The oxygen/sulphur exchange reaction between CuO and H₂S in the sensing process can achieve room temperature operation of gas sensors. After the WO_3-x nanorods were modified by a low content of CuO nanoparticles (Cu:W = 1:20), the sensors present an n-type sensing behavior. Their best working temperatures drop from 289 °C (or 386 °C) to 99 °C (or 70 °C) at which the responses are improved by 14 to 163 times for different x values. Among them, CuO(L)/W₅O₁₄ shows the highest sensitivity of 1575.7 to 10 ppm H₂S at 99 °C and 171.5 to 10 ppm H₂S at room temperature. Once WO_3-x were loaded with a high concentration of CuO nanoparticles (Cu:W = 1:2), they exhibit a p-type behavior, and the optimal working temperatures reduce suddenly to room temperature at which CuO(H)/W₁₈O₄₉ displays the most sensitive response of 7.2 even toward trace amounts of H₂S as low as 100 ppb. In addition, p-type CuO weakens the metal-like characteristics of W₁₈O₄₉ and such weakening effect enhances with an increase in the CuO content. Therefore, the sensing performance of the CuO/W₁₈O₄₉ composite is the best among the four CuO/WO_3-x sensors. The two designs for low and high Cu/W molar ratios all achieve enhanced room-temperature H₂S gas response, with a fast recovery time of ∼60 s under heating pulse, as well as an excellent selectivity, which makes the sensors a promising candidate for practical applications. Moreover, the micro-Raman spectra confirmed CuS formation and the thermal effect on the decomposition of CuS in the sensing process was studied.

Treatment-dependent surface chemistry and gas sensing behavior of the thinnest member of titanium carbide MXenes

MXenes, a rapidly developing family of two-dimensional materials possessing tunable electronic properties and abundant surface functional groups, are promising gas-sensing materials. Ti₂CT_x, with a thinner unit cell thickness compared to its compositional analogue Ti₃C₂T_x and thus more profound surface-dependent properties, has been less explored over the past years. Herein, by etching the precursor Ti₂AlC with a concentrated HF or LiF/HCl mixture, semiconducting Ti₂CT_x (HF) nanosheets and metallic Ti₂CT_x (LiF/HCl) nanosheets were obtained, respectively, arising from their treatment-dependent surface functionalization. In addition, the resulting metallic nanosheets were partially oxidized into TiO₂/Ti₂CT_x (LiF/HCl) hybrid, which exhibited superior sensitivity toward NH₃ gas as compared with Ti₂CT_x (HF) and Ti₂CT_x (LiF/HCl). Detailed analysis suggests that a high concentration of surface oxygen containing species, such as -O_x, -(OH)_x and Ti-O-Ti, is generally beneficial for NH₃ sensing, and a relatively higher -O_x concentration allows rapid gas desorption and sensor recovery.

Room-Temperature Gas Sensors Under Photoactivation: From Metal Oxides to 2D Materials

Room-temperature gas sensors have aroused great attention in current gas sensor technology because of deemed demand of cheap, low power consumption and portable sensors for rapidly growing Internet of things applications. As an important approach, light illumination has been exploited for room-temperature operation with improving gas sensor's attributes including sensitivity, speed and selectivity. This review provides an overview of the utilization of photoactivated nanomaterials in gas sensing field. First, recent advances in gas sensing of some exciting different nanostructures and hybrids of metal oxide semiconductors under light illumination are highlighted. Later, excellent gas sensing performance of emerging two-dimensional materials-based sensors under light illumination is discussed in details with proposed gas sensing mechanism. Originated impressive features from the interaction of photons with sensing materials are elucidated in the context of modulating sensing characteristics. Finally, the review concludes with key and constructive insights into current and future perspectives in the light-activated nanomaterials for optoelectronic gas sensor applications.