The Morphologies of the Semiconductor Oxides and Their Gas-Sensing Properties

Study of the antimicrobial activity of zinc oxide nanostructures mediated by two morphological structures of leaf extracts of Eucalyptus radiata

The growing microbial resistance against antibiotics and the development of resistant strains has shifted the interests of many scientists to focus on metallic nanoparticle applications. Although several metal oxide nanoparticles have been synthesized using green route approach to measure their antimicrobial activity, there has been little or no literature on the use of Eucalyptus robusta Smith aqueous leaf extract mediated zinc oxide nanoparticles (ZnONPs). The study therefore examined the effect of two morphological nanostructures of Eucalyptus robusta Sm mediated ZnONPs and their antimicrobial and antifungal potential on some selected pathogens using disc diffusion method. The samples were characterized using Scanning and Transmission Electron Microscopy, Energy-Dispersive Spectroscopy and Fourier Transform Infrared Spectroscopy. From the results, the two ZnO samples were agglomerated with zinc oxide nanocrystalline structure sample calcined at 400 °C (ZnO NS400) been spherical in shape while zinc oxide nanocrystalline structure sample calcined at 60 °C (ZnO NS60) was rod-like. The sample calcined at higher temperature recorded the smallest particle size of 49.16 ± 1.6 nm as compared to the low temperature calcined sample of 51.04 ± 17.5 nm. It is obvious from the results that, ZnO NS400 exhibited better antibacterial and antifungal activity than ZnO NS60. Out of the different bacterial and fungal strains, ZnO NS400 sample showed an enhanced activity against S. aureus (17.2 ± 0.1 mm) bacterial strain and C. albicans (15.7 ± 0.1 mm) fungal strain at 50 mg/ml. Since this sample showed higher antimicrobial and antifungal activity, it may be explored for its applications in some fields including medicine, agriculture, and aquaculture industry in combating some of the pathogens that has been a worry to the sector. Notwithstanding, the study also provides valuable insights for future studies aiming to explore the antimicrobial potential of other plant extracts mediated zinc oxide nanostructures.

Facile synthesis of Ag lattice doped mesoporous In2O3 nanocubes for high performance ethanol sensing

Ag lattice doped In2O3 with a mesoporous structure was synthesized through a combination of hydrothermal and calcination methods. The structural and morphological characteristics were assessed using XRD, SEM, TEM, TGA, BET, and XPS analyses. Gas sensing measurements revealed that the 7.0 mol% Ag-doped In2O3 sensor displayed a response of 420 towards 100 ppm ethanol at 140 °C, which was 19 times higher than that of the pure In2O3 gas sensor. Density functional theory calculations indicated that Ag-doped In2O3 exhibited enhanced adsorption performance, higher adsorption energy, and electron transfer, resulting in higher sensitivity to ethanol. These findings were also supported by the electronic band structure, work function, and DOS analyses. These results indicated that the Ag doped mesoporous In2O3 has high potential for the preparation of high-performance ethanol sensors in practical applications.

Gas nanosensors for health and safety applications in mining

The ever-increasing demand for accurate, miniaturized, and cost-effective gas sensing systems has eclipsed basic research across many disciplines. Along with the rapid progress in nanotechnology, the latest development in gas sensing technology is dominated by the incorporation of nanomaterials with different properties and structures. Such nanomaterials provide a variety of sensing interfaces operating on different principles ranging from chemiresistive and electrochemical to optical modules. Compared to thick film and bulk structures currently used for gas sensing, nanomaterials are advantageous in terms of surface-to-volume ratio, response time, and power consumption. However, designing nanostructured gas sensors for the marketplace requires understanding of key mechanisms in detecting certain gaseous analytes. Herein, we provide an overview of different sensing modules and nanomaterials under development for sensing critical gases in the mining industry, specifically for health and safety monitoring of mining workers. The interactions between target gas molecules and the sensing interface and strategies to tailor the gas sensing interfacial properties are highlighted throughout the review. Finally, challenges of existing nanomaterial-based sensing systems, directions for future studies, and conclusions are discussed.

Development of Tetrapod Zinc Oxide-Based UV Sensor for Precision Livestock Farming and Productivity

In order to ensure the health and welfare of livestock, there has been an emphasis on precision farming of ruminant animals. Monitoring the life index of ruminant animals is of importance for intelligent farming. Here, a wearable sensor for monitoring ultraviolet (UV) radiation is demonstrated to understand the effect of primary and secondary photosensitization on dairy animals. Thin films of wide bandgap semiconductor zinc oxide (ZnO) comprising multilevel of nanostructures from microparticles (MP) to nanoparticles (NP), and tetrapod (T-ZnO), were prepared as the UV sensing active materials. The sensitivity was evaluated by exposing the films to various radiation sources, i.e., 365 nm (UV A), 302 nm (UV B), and 254 nm (UV C), and measuring the electrical resistance change. T-ZnO is found to exhibit higher sensitivity and stable response (on/off) upon exposure to UV A and UV B radiation, which is attributed to their higher surface area, aspect ratio, porosity, and interconnective networks inducing a high density of chemical interaction sites and consequently improved photocurrent generation. A wearable sensor using T-ZnO is packaged and attached to a collar for dynamic monitoring of UV response on ruminant animals (e.g., sheep in this study). The excellent performance of T-ZnO wearable sensors for ruminant animals also holds the potential for a wider range of applications such as residential buildings and public spaces.

A Statistical Analysis of Response and Recovery Times: The Case of Ethanol Chemiresistors Based on Pure SnO2

Response and recovery times are among the most important parameters for gas sensors. Their optimization has been pursued through several strategies, including the control over the morphology of the sensitive material. The effectiveness of these approaches is typically proven by comparing different sensors studied in the same paper under the same conditions. Additionally, tables comparing the results of the considered paper with those available in the literature are often reported. This is fundamental to frame the results of individual papers in a more general context; nonetheless, it suffers from the many differences occurring at the experimental level between different research groups. To face this issue, in the present paper, we adopt a statistical approach to analyze the response and recovery times reported in the literature for chemiresistors based on pure SnO₂ for ethanol detection, which was chosen as a case study owing to its available statistic. The adopted experimental setup (of the static or dynamic type) emerges as the most important parameter. Once the statistic is split into these categories, morphological and sensor-layout effects also emerge. The observed results are discussed in terms of different diffusion phenomena whose balance depends on the testing conditions adopted in different papers.

ZnO size and shape effect on antibacterial activity and cytotoxicity profile

The aim of our work was the synthesis of ZnO nano- and microparticles and to study the effect of shapes and sizes on cytotoxicity towards normal and cancer cells and antibacterial activity toward two kinds of bacteria. We fabricated ZnO nano- and microparticles through facile chemical and physical routes. The crystal structure, morphology, textural properties, and photoluminescent properties were characterized by powder X-ray diffraction, electron microscopies, nitrogen adsorption/desorption measurements, and photoluminescence spectroscopy. The obtained ZnO structures were highly crystalline and monodispersed with intensive green emission. ZnO NPs and NRs showed the strongest antibacterial activity against Escherichia coli and Staphylococcus aureus compared to microparticles due to their high specific surface area. However, the ZnO HSs at higher concentrations also strongly inhibited bacterial growth. S. aureus strain was more sensitive to ZnO particles than the E. coli. ZnO NPs and NRs were more harmful to cancer cell lines than to normal ones at the same concentration.

Metal Oxide Chemiresistors: A Structural and Functional Comparison between Nanowires and Nanoparticles

Metal oxide nanowires have become popular materials in gas sensing, and more generally in the field of electronic and optoelectronic devices. This is thanks to their unique structural and morphological features, namely their single-crystalline structure, their nano-sized diameter and their highly anisotropic shape, i.e., a large length-to-diameter aspect ratio. About twenty years have passed since the first publication proposing their suitability for gas sensors, and a rapidly increasing number of papers addressing the understanding and the exploitation of these materials in chemosensing have been published. Considering the remarkable progress achieved so far, the present paper aims at reviewing these results, emphasizing the comparison with state-of-the-art nanoparticle-based materials. The goal is to highlight, wherever possible, how results may be related to the particular features of one or the other morphology, what is effectively unique to nanowires and what can be obtained by both. Transduction, receptor and utility-factor functions, doping, and the addition of inorganic and organic coatings will be discussed on the basis of the structural and morphological features that have stimulated this field of research since its early stage.

In2O3 Based Hybrid Materials: Interplay between Microstructure, Photoelectrical and Light Activated NO2 Sensor Properties

In this work, organic–inorganic hybrids based on nanocrystalline indium oxide and ruthenium (II) heteroleptic complexes were used as sensitive materials for room temperature light-activated NO2 detection. In2O3 was obtained by chemical precipitation method and then annealed at three different temperatures (T = 300, 500, 700 °C) in order to investigate the influence of the microstructure of indium oxide on sensor characteristics of hybrid materials and on kinetics of the rise and fall of photoconductivity. The results of the X-ray phase analysis demonstrated that the obtained materials are single-phase with a cubic bixbyite structure. The Ru (II) heteroleptic complex, which was used as a photosensitizer, made it possible to shift the optical sensitivity range of the hybrids to the low energy region of the spectrum and to use a low-power LED (λmax = 470 nm) source for the photoactivation process. The sensor properties were investigated toward NO2 at sub-ppm range at room temperature. It was found that for pure oxides, the sensor signal correlates with a specific surface area, while for hybrid materials, both the sensor signal and photoresponse increase with increasing the matrix crystallinity. In this case, the main role is played by traps of nonequilibrium charge carriers, which are structural defects in the matrix.

Three-dimensional mesoporous ultra-thin monometallic cobalt layered double hydroxides nanomaterials as efficient NO2 gas sensor at room temperature

Monometallic cobalt layered double hydroxides (Co-LDHs) were synthesized using a simple hydrothermal method. 2-methylimidazole (MIm) was selected as a functional agent. The functionalization and optimization has access to the CCM...

Recent trends in gas sensing via carbon nanomaterials: outlook and challenges

The presence of harmful and poisonous gases in the environment can have dangerous effects on human health, and therefore portable, flexible, and highly sensitive gas sensors are in high demand for environmental monitoring, pollution control, and medical diagnosis. Currently, the commercialized sensors are based on metal oxides, which generally operate at high temperatures. Additionally, the desorption of chemisorbed gas molecules is also challenging. Hence, due to the large surface area, high flexibility, and good electrical properties of carbon nanomaterials (CNMs) such as carbon nanotubes, graphene and their derivatives (graphene oxide, reduced graphene oxide, and graphene quantum dots), they are considered to be the most promising chemiresistive sensing materials, where their electrical resistance is affected by their interaction with the analyte. Further, to increase their selectivity, nanocomposites of CNMs with metal oxides, metallic nanoparticles, chalcogenides, and polymers have been studied, which exhibit better sensing capabilities even at room temperature. This review summarizes the state-of-the-art progress in research related to CNMs-based sensors. Moreover, to better understand the analyte adsorption on the surface of CNMs, various sensing mechanisms and dependent sensing parameters are discussed. Further, several existing challenges related to CNMs-based gas sensors are elucidated herein, which can pave the way for future research in this area.

Chemiresistors Based on Li-Doped CuO–TiO2 Films

Chemiresistors based on thin films of the Li-doped CuO–TiO2 heterojunctions were synthesized by a 2-step method: (i) repeated ion beam sputtering of the building elements (on the Si substrates and multisensor platforms); and (ii) thermal annealing in flowing air. The structure and composition of the films were analyzed by several methods: Rutherford Backscattering (RBS), Neutron Depth Profiling (NDP), Secondary Ion Mass Spectrometry (SIMS), and Atomic Force Microscopy (AFM), and their sensitivity to gaseous analytes was evaluated using a specific lab-made device operating in a continuous gas flow mode. The obtained results showed that the Li doping significantly increased the sensitivity of the sensors to oxidizing gases, such as NO2, O3, and Cl2, but not to reducing H2. The sensing response of the CuO–TiO2–Li chemiresistors improved with increasing Li content. For the best sensors with about 15% Li atoms, the detection limits were as follows: NO2 → 0.5 ppm, O3 → 10 ppb, and Cl2 → 0.1 ppm. The Li-doped sensors showed excellent sensing performance at a lower operating temperature (200 °C); however, even though their response time was only a few minutes, their recovery was slow (up to a few hours) and incomplete.

Synthesis of ZnAl2O4 and Evaluation of the Response in Propane Atmospheres of Pellets and Thick Films Manufactured with Powders of the Oxide

ZnAl2O4 nanoparticles were synthesized employing a colloidal method. The oxide powders were obtained at 300 °C, and their crystalline phase was corroborated by X-ray diffraction. The composition and chemical structure of the ZnAl2O4 was carried out by X-ray and photoelectron spectroscopy (XPS). The optical properties were studied by UV-vis spectroscopy, confirming that the ZnAl2O4 nanoparticles had a direct transition with bandgap energy of 3.2 eV. The oxide's microstructures were microbars of ~18.2 nm in size (on average), as analyzed by scanning (SEM) and transmission (TEM) electron microscopies. Dynamic and stationary gas detection tests were performed in controlled propane atmospheres, obtaining variations concerning the concentration of the test gas and the operating temperature. The optimum temperatures for detecting propane concentrations were 200 and 300 °C. In the static test results, the ZnAl2O4 showed increases in propane response since changes in the material's electrical conductance were recorded (conductance = 1/electrical resistance, Ω). The increases were ~2.8 at 200 °C and ~7.8 at 300 °C. The yield shown by the ZnAl2O4 nanoparticles for detecting propane concentrations was optimal compared to other similar oxides categorized as potential gas 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.

Recent advances in energy-saving chemiresistive gas sensors: A review

With the tremendous advances in technology, gas-sensing devices are being popularly used in many distinct areas, including indoor environments, industries, aviation, and detectors for various toxic domestic gases and vapors. Even though the most popular type of gas sensor, namely, resistive-based gas sensors, have many advantages over other types of gas sensors, their high working temperatures lead to high energy consumption, thereby limiting their practical applications, especially in mobile and portable devices. As possible ways to deal with the high-power consumption of resistance-based sensors, different strategies such as self-heating, MEMS technology, and room-temperature operation using especial morphologies, have been introduced in recent years. In this review, we discuss different types of energy-saving chemisresitive gas sensors including self-heated gas sensors, MEMS based gas sensors, room temperature operated flexible/wearable sensor and their application in the fields of environmental monitoring. At the end, the review will be concluded by providing a summary, challenges, recent trends, and future perspectives.

Nanomaterial-Based CO2 Sensors

The detection of carbon dioxide (CO2) is critical for environmental monitoring, chemical safety control, and many industrial applications. The manifold application fields as well as the huge range of CO2 concentration to be measured make CO2 sensing a challenging task. Thus, the ability to reliably and quantitatively detect carbon dioxide requires vastly improved materials and approaches that can work under different environmental conditions. Due to their unique favorable chemical, optical, physical, and electrical properties, nanomaterials are considered state-of-the-art sensing materials. This mini-review documents the advancement of nanomaterial-based CO2 sensors in the last two decades and discusses their strengths, weaknesses, and major applications. The use of nanomaterials for CO2 sensing offers several improvements in terms of selectivity, sensitivity, response time, and detection, demonstrating the advantage of using nanomaterials for developing high-performance CO2 sensors. Anticipated future trends in the area of nanomaterial-based CO2 sensors are also discussed in light of the existing limitations.

Effect of the Morphology and Electrical Property of Metal-Deposited ZnO Nanostructures on CO Gas Sensitivity

The development of a highly sensitive gas sensor for toxic gases is an important issue in that it can reduce the damage caused by unexpected gas leaks. In this regard, in order to make the sensor accurate and highly responsive, we have investigated which morphology is effective to improve the sensitivity and how the deposited nanoparticle affects the sensitivity by controlling the morphology of semiconductor oxides—either nanorod or nanoplate—and depositing metal nanoparticles on the semiconductor surface. In this study, we compared the CO gas sensitivity for sensors with different morphology (rod and plate) of ZnO nanostructure with metal nanoparticles (gold and copper) photodeposited and investigated the correlation between the gas sensitivity and some factors such as the morphology of ZnO and the properties of the deposited metal. Among the samples, Au/ZnO nanorod showed the best response (~86%) to the exposure of 100 ppm CO gas at 200 °C. The result showed that the electrical properties due to the deposition of metal species also have a strong influence on the sensor properties such as sensor response, working temperature, the response and recovery time, etc., together with the morphology of ZnO.

Edge-exposed MoS2 nanospheres assembled with SnS2 nanosheet to boost NO2 gas sensing at room temperature

SnS₂ nanosheets (NSs) have become an ideal candidate for high performance gas sensors due to their unique sensing properties. However, the restacking and aggregation in the process of sensor manufacturing have great influence on the gas sensing performance. In this study, we synthesized a novel heterojunction of the flower-like porous SnS₂ NSs with edge exposed MoS₂ nanospheres via a facile hydrothermal method and sensitive response has achieved at room temperature (27℃). After functionalization, the SMS-Ⅱ showed excellent response (Ra/Rg = 25.9-100 ppm NO₂), which is 22.3 times higher than that of the pristine SnS₂ NSs. The sensor also has the characteristics of short response time of 2 s, excellent base line recovery (28.2 s), long-term stability and reliability within 16 weeks, good selectivity and low detection concentration of only 50 ppb. The p-n heterojunction formed between the edge-exposed spherical MoS₂ and the 3D flower-like SnS₂ NSs has a synergistic effect, providing a highly active sites for the adsorption of NO₂ gas, which greatly enhance the sensitivity of the sensor. Simple fabrication and excellent gas sensing performance of the SnS₂/MoS₂ heterostructure nanomaterials (NMs) will highly effective for commercial gas sensing application.

WO3 and Ionic Liquids: A Synergic Pair for Pollutant Gas Sensing and Desulfurization

This review deals with the notable results obtained by the synergy between ionic liquids (ILs) and WO3 in the field of pollutant gas sensing and sulfur removal pretreatment of fuels. Starting from the known characteristics of tungsten trioxide as catalytic material, many authors have proposed the use of ionic liquids in order to both direct WO3 production towards controllable nanostructures (nanorods, nanospheres, etc.) and to modify the metal oxide structure (incorporating ILs) in order to increase the gas adsorption ability and, thus, the catalytic efficiency. Moreover, ionic liquids are able to highly disperse WO3 in composites, thus enhancing the contact surface and the catalytic ability of WO3 in both hydrodesulfurization (HDS) and oxidative desulfurization (ODS) of liquid fuels. In particular, the use of ILs in composite synthesis can direct the hydrogenation process (HDS) towards sulfur compounds rather than towards olefins, thus preserving the octane number of the fuel while highly reducing the sulfur content and, thus, the possibility of air pollution with sulfur oxides. A similar performance enhancement was obtained in ODS, where the high dispersion of WO3 (due to the use of ILs during the synthesis) allows for noteworthy results at very low temperatures (50 °C).

Computational Study of the Substituent Effects for the Spectroscopic Properties of Thiazolo[5,4-d]thiazole Derivatives

Inspired by the structure and optical properties of N,N'-dialkylated/dibenzylated 2,5-bis(4-pyridinium)thiazolo[5,4-d]thiazole, we proposed a series of disubstituted thiazolo[5,4-d]thiazole derivatives as promising materials for multifunctional optoelectronic, electron transfer sensing, and other photochemical applications. Density functional theory study of the electronic structures and transition properties of those newly proposed molecules indicates that the electron-donating and electron-withdrawing groups introduced to the peripheral pyridyl ligands extend the distributions of molecular frontier orbitals, increase the electron density in thiazolo[5,4-d]thiazolea, and therefore lead to remarkable red-shifts of their absorption and emission peaks.

Carbone Monoxide (CO) Detection Device Based on the Nickel Antimonate Oxide and a DC Electronic Circuit

Carbon monoxide (CO) is very toxic to health. CO gas can cause intoxication and even death when the concentration is high or there are long exposure times. To detect atmospheres with CO gas concentration detectors are placed. In this work, a novel CO detection device was proposed and applied for CO detection. For its implementation, four stages were developed: Synthesis of nickel antimonite (NiSb2O6) oxide powders, physical characterization of NiSb2O6 powders, Pellet fabrication and sensitivity test in CO atmospheres and electronic circuit implementation where signal adaptation and signal amplification were considered. Experimentally, a chemical sensor was built and characterized, its signal adaptation circuit was implemented and also it was proved using CO concentrations from 1 to 300 ppm with the operating temperatures of 100, 200, and 300 °C. Its optimal operation was at 300 °C. From the experimental results, the CO detection device had excellent functionality because the chemical sensor based on the nickel antimonite oxide had high sensitivity and good electrical response, whereas the DC electronic circuit had good performance.

Selective-detection NO at room temperature on porous ZnO nanostructure by solid-state synthesis method

High sensitivity and selectivity detection of NO at room temperature has always been full of challenges. In this work, a kind of porous ZnO with coralline-like nanostructure was prepared by a rapid and simple solid-state synthesis strategy, using zinc acetate and oxalic acid as precursors. Structural analysis and morphological investigations of the ZnO powder showed that it has a large specific surface area (32.75 m² g^-1) and many nanometer-sized channels between ZnO nanoparticles. This is beneficial to the adsorption and desorption of NO, which is an important reason for the selective detection of NO by the ZnO powder at room temperature. So based on the ZnO powder, a gas sensor was fabricated and its gas-sensing properties were investigated. It exhibited outstanding response (23.59) and fast response time (331 s) to 40 ppm of NO at room temperature (21 ± 2 °C). As the relative humidity study changed from 17% to 80% at 10 ppm of NO, the sensitivity of the sensor changed little, only decreased from 1.43 to 1.12. The stability study was also carried out. Under the concentration of 5 ppm of NO, the relative standard deviation was 0.33% within 8 days, which indicates that the obtained sensor is suitable for practical application.

Semiconductor Metal Oxides as Chemoresistive Sensors for Detecting Volatile Organic Compounds

Volatile organic compounds (VOCs), which originate from painting, oil refining and vehicle exhaust emissions, are hazardous gases that have significant effects on air quality and human health. The detection of VOCs is of special importance to environmental safety. Among the various detection methods, chemoresistive semiconductor metal oxide gas sensors are considered to be the most promising technique due to their easy production, low cost and good portability. Sensitivity is an important parameter of gas sensors and is greatly affected by the microstructure, defects, catalyst, heterojunction and humidity. By adjusting the aforementioned factors, the sensitivity of gas sensors can be improved further. In this review, attention will be focused on how to improve the sensitivity of chemoresistive gas sensors towards certain common VOCs with respect to the five factors mentioned above.

Development of a potential LPG sensor based on a PANI–Co3O4 nanocomposite that functions at room temperature

Nanostructured Co₃O₄ was synthesized by a sol–gel technique while 30% & 40% Co₃O₄–PANI (polyaniline) nanocomposites were successfully prepared employing an in situ polymerization technique.

Microhotplates for Metal Oxide Semiconductor Gas Sensor Applications-Towards the CMOS-MEMS Monolithic Approach

The recent development of the Internet of Things (IoT) in healthcare and indoor air quality monitoring expands the market for miniaturized gas sensors. Metal oxide gas sensors based on microhotplates fabricated with micro-electro-mechanical system (MEMS) technology dominate the market due to their balance in performance and cost. Integrating sensors with signal conditioning circuits on a single chip can significantly reduce the noise and package size. However, the fabrication process of MEMS sensors must be compatible with the complementary metal oxide semiconductor (CMOS) circuits, which imposes restrictions on the materials and design. In this paper, the sensing mechanism, design and operation of these sensors are reviewed, with focuses on the approaches towards performance improvement and CMOS compatibility.

Ni-Doped ZnS Nanospheres Decorated with Au Nanoparticles for Highly Improved Gas Sensor Performance

Novel Ni-doped wurtzite ZnS nanospheres decorated with Au nanoparticles (Au NPs⁻ZnS NSs) have been successfully fabricated using a simple method involving vacuum evaporation followed by an annealing process. This transition metal-doped gas sensor had high responsivity, extremely fast response and recovery time, and excellent selectivity to formaldehyde at room temperature. The response and recovery time are only 29 s and 2 s, respectively. Since ZnS is transformed into ZnO at a high temperature, superior room temperature-sensing performance can improve the stability and service life of the sensor. The improvement in sensing performance could be attributed to the reduced charge-transfer distance resulting from the creation of a local charge reservoir layer, and the catalytic and spillover effect of Au nanoparticles. The rough and porous spherical structure can also facilitate the detection and diffusion of gases. The as-prepared Au NPs⁻ZnS NSs are considered to be an extremely promising candidate material for gas sensors, and are expected to have other potential applications in the future.

Recent Advances of SnO2-Based Sensors for Detecting Fault Characteristic Gases Extracted From Power Transformer Oil

Tin oxide SnO₂-based gas sensors have been widely used for detecting typical fault characteristic gases extracted from power transformer oil, namely, H₂, CO, CO₂, CH₄, C₂H₂, C₂H₄, and C₂H₆, due to the remarkable advantages of high sensitivity, fast response, long-term stability, and so on. Herein, we present an overview of the recent significant improvement in fabrication and application of high performance SnO₂-based sensors for detecting these fault characteristic gases. Promising materials for the sensitive and selective detection of each kind of fault characteristic gas have been identified. Meanwhile, the corresponding sensing mechanisms of SnO₂-based gas sensors of these fault characteristic gases are comprehensively discussed. In the final section of this review, the major challenges and promising developments in this domain are also given.

Sensitivity Tests of Pellets Made from Manganese Antimonate Nanoparticles in Carbon Monoxide and Propane Atmospheres

Nanoparticles of manganese antimonate (MnSb₂O₆) were prepared using the microwave-assisted colloidal method for its potential application as a gas sensor. For the synthesis of the oxide, manganese nitrate, antimony chloride, ethylenediamine and ethyl alcohol (as a solvent) were used. The precursor material was calcined at 800 °C in air and analyzed by X-ray diffraction. The oxide crystallized into a hexagonal structure with spatial group P321 and cell parameters a = b = 8.8054 Å and c = 4.7229 Å. The microstructure of the material was analyzed by scanning electron microscopy (SEM), finding the growth of microrods with a size of around ~10.27 μm and some other particles with an average size of ~1.3 μm. Photoacoustic spectroscopy (PAS) studies showed that the optical energy band (Eg) of the oxide was of ~1.79 eV. Transmission electron microscopy (TEM) analyses indicated that the size of the nanoparticles was of ~29.5 nm on average. The surface area of the powders was estimated at 14.6 m²/g by the Brunauer⁻Emmett⁻Teller (BET) method. Pellets prepared from the nanoparticles were tested in carbon monoxide (CO) and propane (C₃H₈) atmospheres at different concentrations (0⁻500 ppm) and operating temperatures (100, 200 and 300 °C). The pellets were very sensitive to changes in gas concentration and temperature: the response of the material rose as the concentration and temperature increased. The results showed that the MnSb₂O₆ nanoparticles can be a good candidate to be used as a novel gas sensor.