"Visualization" Gas-Gas Sensors Based on High Performance Novel MXenes Materials.

The detection of toxic, harmful, explosive, and volatile gases cannot be separated from gas sensors, and gas sensors are also used to monitor the greenhouse effect and air pollution. However, existing gas sensors remain with many drawbacks, such as lower sensitivity, lower selectivity, and unstable room temperature detection. Thus, there is an imperative need to find more suitable sensing materials. The emergence of a new 2D layered material MXenes has brought dawn to solve this problem. The multiple advantages of MXenes, namely high specific surface area, enriched terminal functionality groups, hydrophilicity, and good electrical conductivity, make them among the most prolific gas-sensing materials. Therefore, this review paper describes the current main synthesis methods of MXenes materials, and focuses on summarizing and organizing the latest research results of MXenes in gas sensing applications. It also introduces the possible gas sensing mechanisms of MXenes materials on NH3 , NO2 , CH3 , and volatile organic compounds (VOCs). In conclusion, it provides insight into the problems and upcoming challenges of MXenes materials for gas sensing.

Recent developments and challenges in resistance-based hydrogen gas sensors based on metal oxide semiconductors.

In recent years, the energy crisis has made the world realize the importance and need for green energy. Hydrogen safety has always been a primary issue that needs to be addressed for the application and large-scale commercialization of hydrogen energy, and precise and rapid hydrogen gas sensing technology and equipment are important prerequisites for ensuring hydrogen safety. Based on metal oxide semiconductors (MOS), resistive hydrogen gas sensors (HGS) offer advantages, such as low cost, low power consumption, and high sensitivity. They are also easy to test, integrate, and suitable for detecting low concentrations of hydrogen gas in ambient air. Therefore, they are considered one of the most promising HGS. This article provides a comprehensive review of the surface reaction mechanisms and recent research progress in optimizing the gas sensing performance of MOS-based resistive hydrogen gas sensors (MOS-R-HGS). Particularly, the advancements in metal-assisted or doped MOS, mixed metal oxide (MO)-MOS composites, MOS-carbon composites, and metal-organic framework-derived (MOF)-MOS composites are extensively summarized. Finally, the future research directions and possibilities in this field are discussed.

Investigation of Laser-Induced Graphene (LIG) on a Flexible Substrate and Its Functionalization by Metal Doping for Gas-Sensing Applications.

Graphene materials synthesized using direct laser writing (laser-induced graphene; LIG) make favorable sensor materials because of their large surface area, ease of fabrication, and cost-effectiveness. In particular, LIG decorated with metal nanoparticles (NPs) has been used in various sensors, including chemical sensors and electronic and electrochemical biosensors. However, the effect of metal decoration on LIG sensors remains controversial; hypotheses based on computational simulations do not always match the experimental results, and even the experimental results reported by different researchers have not been consistent. In the present study, we explored the effects of metal decorations on LIG gas sensors, with NO2 and NH3 gases as the representative oxidizing and reducing agents, respectively. To eliminate the unwanted side effects arising from metal salt residues, metal NPs were directly deposited via vacuum evaporation. Although the gas sensitivities of the sensors deteriorate upon metal decoration irrespective of the metal work function, in the case of NO2 gas, they improve upon metal decoration in the case of NH3 exposure. A careful investigation of the chemical structure and morphology of the metal NPs in the LIG sensors shows that the spontaneous oxidation of metal NPs with a low work function changes the behavior of the LIG gas sensors and that the sensors' behaviors under NO2 and NH3 gases follow different principles.

Gas-Sensing Mechanisms and Performances of MXenes and MXene-Based Heterostructures.

MXenes are a class of 2D transition-metal carbides, nitrides, and carbonitrides with exceptional properties, including substantial electrical and thermal conductivities, outstanding mechanical strength, and a considerable surface area, rendering them an appealing choice for gas sensors. This manuscript provides a comprehensive analysis of heterostructures based on MXenes employed in gas-sensing applications and focuses on addressing the limited understanding of the sensor mechanisms of MXene-based heterostructures while highlighting their potential to enhance gas-sensing performance. The manuscript begins with a broad overview of gas-sensing mechanisms in both pristine materials and MXene-based heterostructures. Subsequently, it explores various features of MXene-based heterostructures, including their composites with other materials and their prospects for gas-sensing applications. Additionally, the manuscript evaluates different engineering strategies for MXenes and compares their advantages to other materials while discussing the limitations of current state-of-the-art sensors. Ultimately, this review seeks to foster collaboration and knowledge exchange within the field, facilitating the development of high-performance gas sensors based on MXenes.

Morphology genetic 3D hierarchical SnO2microstructures constructed by Sub 5 nm nanocrystals for highly sensitive ethanol-sensor.

SnO2is widely used for ethanol-sensing applications due to its excellent physicochemical properties, low toxicity and high sensitivity. However it is a challenge to construct 3D-hierarchical structures with sub 5 nm primary grain particle, which is the optimized size for ethanol sensor. Herein, genetic tri-level hierarchical SnO2microstructures are synthesised by the genetic conversion of 3D hierarchical SnS2flowers assembled by ultrathin nanosheets. The SnS2nanosheets are morphology genetic converted to porous nanosheets with sub 5 nm SnO2nanoparticles during the calcination process. When used for the detection of ethanol, the sensor exhibits a high sensitivity of 0.5 ppm (Ra/Rg = 6.8) and excellent gas-sensing response (Ra/Rg= 183 to 100 ppm) with short response/recovery time (12 s/11 s). The excellent gas sensing performance is much better than that of the previous reported SnO2-based sensors. The highly sensitivity is attributed to the large surface area derived from the recrystallization and volume changes, which offers more active sites during the morphology genetic conversion from SnS2to SnO2. Furthermore, the flower-like 3D structure enhances the stability of the materials and is beneficial for the mass diffusion dynamics of ethanol.

Electrospinning Hetero-Nanofibers In2O3/SnO2 of Homotype Heterojunction with High Gas Sensing Activity.

In2O3/SnO2 composite hetero-nanofibers were synthesized by an electrospinning technique for detecting indoor volatile organic gases. The physical and chemical properties of In2O3/SnO2 hetero-nanofibers were characterized and analyzed by X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), Energy Dispersive X-Ray Spectroscopy (EDX), specific surface Brunauer-Emmett-Teller (BET) and X-ray photoelectron spectroscopy (XPS). Gas sensing properties of In2O3/SnO2 composite hetero-nanofibers were measured with six kinds of indoor volatile organic gases in concentration range of 0.5~50 ppm at the operating temperature of 275 °C. The In2O3/SnO2 composite hetero-nanofibers sensor exhibited good formaldehyde sensing properties, which would be attributed to the formation of n-n homotype heterojunction in the In2O3/SnO2 composite hetero-nanofibers. Finally, the sensing mechanism of the In2O3/SnO2 composite hetero-nanofibers was analyzed based on the energy-band principle.

CO and HCHO Sensing by Single Au Atom-Decorated WS2 Monolayer for Diagnosis of Thermal Aging Faults in the Dry-Type Reactor: A First-Principles Study.

CO and HCHO are the main pyrolysis gases in long-term running dry-type reactors, and thus the diagnosis of thermal insulation faults inside such devices can be realized by sensing these gases. In this paper, a single Au atom-decorated WS2 (Au-WS2) monolayer is proposed as an original sensing material for CO or HCHO detection to evaluate the operation status of dry-type reactors. It was found that the Au atom prefers to be adsorbed at the top of the S atom of the pristine WS2 monolayer, wherein the binding force is calculated as -3.12 eV. The Au-WS2 monolayer behaves by chemisorption upon the introduction of CO and HCHO molecules, with the adsorption energies of -0.82 and -1.01 eV, respectively. The charge density difference was used to analyze the charge-transfer and bonding behaviors in the gas adsorptions, and the analysis of density of state as well as band structure indicate gas-sensing mechanisms. As calculated, the sensing responses of the Au-WS2 monolayer upon CO and HCHO molecule introduction were 58.7% and -74.4%, with recovery times of 0.01 s and 11.86 s, respectively. These findings reveal the favorable potential of the Au-WS2 monolayer to be a reusable and room-temperature sensing candidate for CO and HCHO detections. Moreover, the work function of the Au-WS2 monolayer was decreased by 13.0% after the adsorption of CO molecules, while it increased by 1.2% after the adsorption of HCHO molecules, which implies its possibility to be a work-function-based gas sensor for CO detection. This theoretical report paves the way for further investigations into WS2-based gas sensors in some other fields, and it is our hope that our findings can stimulate more reports on novel gas-sensing materials for application in evaluating the operation conditions of dry-type reactors.

Recent Advances in Structure Design and Application of Metal Halide Perovskite-Based Gas Sensor.

Metal halide perovskites (MHPs) are emerging gas-sensing materials and have attracted considerable attention in gas sensors due to their unique bandgap structure and tunable optoelectronic properties. The past decade has witnessed significant developments in the gas-sensing field; however, their intrinsic structural instability and ambiguous gas-sensing mechanisms hamper their practical applications. Herein, we summarize the recent advances in MHP-based gas sensors. The physicochemical properties of MHPs are discussed at first. The structure design, including dimension design and engineering design, is overviewed as well as their fabrication methods, and we put forward our insights into the gas-sensing mechanism of MHPs. It is believed that enhanced understanding of gas-sensing mechanisms of MHPs are helpful for their application as gas-sensing materials, and structure design can enhance their stability, sensing sensitivity, and selectivity to target gases as gas sensors. Subsequently, the latest developments in MHP-based gas sensors are summarized according to their different application scenarios. Finally, we conclude with the current status and challenges in this field and propose future perspectives.

Recent Progress on Functionalized Graphene Quantum Dots and Their Nanocomposites for Enhanced Gas Sensing Applications.

Gas-sensing technology has witnessed significant advancements that have been driven by the emergence of graphene quantum dots (GQDs) and their tailored nanocomposites. This comprehensive review surveys the recent progress made in the construction methods and applications of functionalized GQDs and GQD-based nanocomposites for gas sensing. The gas-sensing mechanisms, based on the Fermi-level control and charge carrier depletion layer theory, are briefly explained through the formation of heterojunctions and the adsorption/desorption principle. Furthermore, this review explores the enhancements achieved through the incorporation of GQDs into nanocomposites with diverse matrices, including polymers, metal oxides, and 2D materials. We also provide an overview of the key progress in various hazardous gas sensing applications using functionalized GQDs and GQD-based nanocomposites, focusing on key detection parameters such as sensitivity, selectivity, stability, response and recovery time, repeatability, and limit of detection (LOD). According to the most recent data, the normally reported values for the LOD of various toxic gases using GQD-based sensors are in the range of 1-10 ppm. Remarkably, some GQD-based sensors exhibit extremely low detection limits, such as N-GQDs/SnO2 (0.01 ppb for formaldehyde) and GQD@SnO2 (0.10 ppb for NO2). This review provides an up-to-date perspective on the evolving landscape of functionalized GQDs and their nanocomposites as pivotal components in the development of advanced gas sensors.

Application of Titanium Carbide MXenes in Chemiresistive Gas Sensors.

The titanium carbide MXenes currently attract an extreme amount of interest from the material science community due to their promising functional properties arising from the two-dimensionality of these layered structures. In particular, the interaction between MXene and gaseous molecules, even at the physisorption level, yields a substantial shift in electrical parameters, which makes it possible to design gas sensors working at RT as a prerequisite to low-powered detection units. Herein, we consider to review such sensors, primarily based on Ti3C2Tx and Ti2CTx crystals as the most studied ones to date, delivering a chemiresistive type of signal. We analyze the ways reported in the literature to modify these 2D nanomaterials for (i) detecting various analyte gases, (ii) improving stability and sensitivity, (iii) reducing response/recovery times, and (iv) advancing a sensitivity to atmospheric humidity. The most powerful approach based on designing hetero-layers of MXenes with other crystals is discussed with regard to employing semiconductor metal oxides and chalcogenides, noble metal nanoparticles, carbon materials (graphene and nanotubes), and polymeric components. The current concepts on the detection mechanisms of MXenes and their hetero-composites are considered, and the background reasons for improving gas-sensing functionality in the hetero-composite when compared with pristine MXenes are classified. We formulate state-of-the-art advances and challenges in the field while proposing some possible solutions, in particular via employing a multisensor array paradigm.

Spectroscopic determination of the role of vanadyl oxygen in room temperature NH3 sensing by V2O5 nanoparticles.

In the present study, a multi-modal approach consisting of in-situ photoluminescence, Raman, and UV-Vis absorption spectroscopic studies is carried out along with chemiresistive sensing to unveil the mechanism of NH3 gas sensing by V2O5 nanoparticles in ambient air. V2O5 nanoparticles with an average size of 49 nm show a superior sensor response of 17 ± 1.5 % towards 1 ppm of NH3 gas with a response and recovery time of 96 s and 45 s, respectively. The photoluminescence and UV-Vis absorption studies in the presence of NH3 reveal electron doping to a new energy level at 1.84 eV, resulting in conduction band filling and increase in the optical band gap. The intensity of the photoluminescence spectrum shows an increase in the presence of NH3 gas as a result of this electron doping. The sensor response from the optical sensing carried out by in-situ photoluminescence study is 43 % for 40 ppm of NH3 gas. The vanadyl oxygen site is the most active in the sensing process, as evident by a selective enhancement in the intensity of V-O (vanadyl) bond vibration. This study gives an experimental evidence for the changes in optical and electronic properties of V2O5 on the adsorption of NH3 gas molecules.

The coupling of graphene, graphitic carbon nitride and cellulose to fabricate zinc oxide-based sensors and their enhanced activity towards air pollutant nitrogen dioxide.

It is desirable but challenging to sense toxic nitrogen dioxide (NO2) for it has become one of the most prominent air pollutants. Zinc oxide-based gas sensors are known to detect NO2 gas efficiently, however, the sensing mechanism and involved intermediates structures remain underexplored. In the work, a series of sensitive materials, including zinc oxide (ZnO) and its composites ZnO/X [X = Cel (cellulose), CN (g-C3N4) and Gr (graphene)] have been comprehensively examined by density functional theory. It is found that ZnO favors adsorbing NO2 over ambient O2, and produces nitrate intermediates; and H2O is chemically held by zinc oxide, in line with the non-negligible impact of humidity on the sensitivity. Of the formed composites, ZnO/Gr exhibits the best NO2 gas-sensing performance, which is proved by the calculated thermodynamics and geometrical/electronic structures of reactants, intermediates and products. The interfacial interaction has been elaborated on for composites (ZnO/X) as well as their complexes (ZnO- and ZnO/X-adsorbates). The current study well explains experimental findings and opens up a way to design and unearth novel NO2 sensing materials.

WS2 and WS2-ZnO Chemiresistive Gas Sensors: The Role of Analyte Charge Asymmetry and Molecular Size.

We investigate the interaction of various analytes (toluene, acetone, ethanol, and water) possessing different structures, bonding, and molecular sizes with a laser-exfoliated WS2 sensing material in a chemiresistive sensor. The sensor showed a clear response to all analytes, which was significantly enhanced by modifying the WS2 surface. This was achieved by creating WS2-ZnO heterojunctions via the deposition of ZnO nanoparticles on the WS2 surface with a high-throughput, atmospheric-pressure spatial atomic layer deposition system. Water and ethanol produced a much higher response compared to acetone and toluene for both the WS2 and WS2-ZnO sensing mediums. We resolved that the charge-asymmetry points in analyte molecules play a key role in determining the sensor response. High charge-asymmetry points correspond to highly polar bonds (HPBs) in a neutral molecule that have a high probability of interaction with the sensing medium. Our results indicate that the polarity of the HPBs primarily dictates the interaction between the analyte and sensing medium and consequently controls the response of the sensor. Moreover, the size of the analyte molecule was found to affect the sensing response; if two molecules have the same HPBs and are exposed to the same sensing medium, the smaller molecule is likely to produce a higher and faster response. Our study provides a comprehensive picture of analyte-sensor interactions that can help in advancing semiconductor gas sensors, including those based on two-dimensional materials.

Metal Oxide Heterostructures for Improving Gas Sensing Properties: A Review.

Metal oxide semiconductor gas sensors are widely used to detect toxic and inflammable gases in industrial production and daily life. The main research hotspot in this field is the synthesis of gas sensing materials. Previous studies have shown that incorporating two or more metal oxides to form a heterojunction interface can exhibit superior gas sensing performance in response and selectivity compared with single phase. This review focuses on mainly the synthesis methods and gas sensing mechanisms of metal oxide heterostructures. A significant number of heterostructures with different morphologies and shapes have been fabricated, which exhibit specific sensing performance toward a specific target gas. Among these synthesis methods, the hydrothermal method is noteworthy due to the fabrication of diverse structures, such as nanorod-like, nanoflower-like, and hollow sphere structures with enhanced sensing properties. In addition, it should be noted that the combination of different synthesis methods is also an efficient way to obtain metal oxide heterostructures with novel morphologies. Despite advanced methods in the metal oxide semiconductors and nanotechnology field, there are still some new issues which deserve further investigation, such as long-term chemical stability of sensing materials, reproducibility of the fabrication process, and selectivity toward homogeneous gases. Moreover, the gas sensing mechanism of metal oxide heterostructures is controversial. It should be clarified so as to further integrate laboratory theory research with practical exploitation.

The Synergistic Properties and Gas Sensing Performance of Functionalized Graphene-Based Sensors.

The detection of toxic gases has long been a priority in industrial manufacturing, environmental monitoring, medical diagnosis, and national defense. The importance of gas sensing is not only of high benefit to such industries but also to the daily lives of people. Graphene-based gas sensors have elicited a lot of interest recently, due to the excellent physical properties of graphene and its derivatives, such as graphene oxide (GO) and reduced graphene oxide (rGO). Graphene oxide and rGO have been shown to offer large surface areas that extend their active sites for adsorbing gas molecules, thereby improving the sensitivity of the sensor. There are several literature reports on the promising functionalization of GO and rGO surfaces with metal oxide, for enhanced performance with regard to selectivity and sensitivity in gas sensing. These synthetic and functionalization methods provide the ideal combination/s required for enhanced gas sensors. In this review, the functionalization of graphene, synthesis of heterostructured nanohybrids, and the assessment of their collaborative performance towards gas-sensing applications are discussed.

Exploring optical and electrical gas detection based on zinc-tetra-phenyl-porphyrin sensitizer.

We developed optical waveguide (OWG), ultraviolet-visible spectrophotometry (UV-vis), and electrically operated gas sensors utilizing zinc-tetra-phenyl-porphyrin (ZnTPP) as sensitizer. Strikingly, ZnTPP thin-film/K+-exchanged glass OWG sensing element exhibits a superior signal-to-noise ratio of 109.6 upon 1 ppm NO2 gas injection, which is 29.5 and 3.8 times larger than that of UV-vis (absorbance at wavelength of 438 nm) and ZnTPP electrical sensing elements prepared on an alumina ceramic tube, respectively. Further results on Fourier infrared spectra and UV-vis spectra, confirm a strong chemical adsorption of NO2 gas on ZnTPP. Therefore, our studies highlight the selection of suitable detection technique for analyte sensing with ZnTPP.

Improving Gas-Sensing Performance Based on MOS Nanomaterials: A Review.

In order to solve issues of air pollution, to monitor human health, and to promote agricultural production, gas sensors have been used widely. Metal oxide semiconductor (MOS) gas sensors have become an important area of research in the field of gas sensing due to their high sensitivity, quick response time, and short recovery time for NO2, CO2, acetone, etc. In our article, we mainly focus on the gas-sensing properties of MOS gas sensors and summarize the methods that are based on the interface effect of MOS materials and micro-nanostructures to improve their performance. These methods include noble metal modification, doping, and core-shell (C-S) nanostructure. Moreover, we also describe the mechanism of these methods to analyze the advantages and disadvantages of energy barrier modulation and electron transfer for gas adsorption. Finally, we put forward a variety of research ideas based on the above methods to improve the gas-sensing properties. Some perspectives for the development of MOS gas sensors are also discussed.

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.

Oxygen-Plasma-Assisted Enhanced Acetone-Sensing Properties of ZnO Nanofibers by Electrospinning.

In this Article, ZnO nanofibers were prepared by electrospinning. The as-prepared ZnO electrospun fibers were treated with plasma. The morphology, structure, and element content of the ZnO nanofibers greatly changed after treatment with different plasmas. The test results indicated that the acetone-sensing performance was remarkably improved for oxygen-plasma-assisted ZnO nanofibers. Furthermore, the density function theory (DFT) calculation results revealed that the acetone adsorption energy of ZnO nanofibers treated with oxygen plasma was 2 times greater than that of untreated ZnO nanofibers, and the electrons transferred between ZnO nanofibers and acetone molecules produced a more remarkable change in electronic structure for the oxygen-plasma-treated ZnO nanofibers. Our work demonstrates that the oxygen plasma treatment method can help improve the acetone-sensing performance of ZnO nanofibers.

Volatile Organic Compounds Gas Sensors Based on Molybdenum Oxides: A Mini Review.

As a typical n-type semiconductor, MoO3 has been widely applied in the gas-detection field due to its competitive physicochemical properties and ecofriendly characteristics. Volatile organic compounds (VOCs) are harmful to the atmospheric environment and human life, so it is necessary to quickly identify the presence of VOCs in the air. This review briefly introduced the application progress of an MoO3-based sensor in VOCs detection. We mainly emphasized the optimization strategies of a high performance MoO3, which consists of morphology-controlled synthesis and electronic properties functional modification. Besides the general synthesis methods, its gas-sensing properties and mechanism were briefly discussed. In conclusion, the application status of MoO3 in gas-sensing and the challenges still to be solved were summarized.