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The demand for gas-sensing operations with lower electrical power and guaranteed sensitivity has increased over the decades due to worsening indoor air pollution. In this report, we develop room-temperature operational NH3 gas-sensing materials, which are activated through electron doping and crystal structure distortion effect in Fe0.2Ni0.8WO4. The base material, synthesized through solid-state synthesis, involves Fe cations substitutionally located at the Ni sites of the NiWO4 crystal structure and shows no gas-sensing response at room temperature. However, doping Na into the interstitial sites of Fe0.2Ni0.8WO4 activates gas adsorption on the surface via electron donation to the cations. Additionally, the hydrothermal method used to achieve a more than 70-fold increase in the surface area of structure-distorted Na-doped Fe0.2Ni0.8WO4 powder significantly enhances gas sensitivity, resulting in a 4-times increase in NH3 gas response (Rg/Ra). Photoluminescence and XPS results indicate negligible oxygen vacancies, demonstrating that cation contributions are crucial for gas-sensing activities in Na-doped Fe0.2Ni0.8WO4. This suggests the potential for modulating gas sensitivity through carrier concentration and crystal structure distortion. These findings can be applied to the development of room-temperature operational gas-sensing materials based on the cations. [ABSTRACT FROM AUTHOR]

The demand for gas-sensing operations with lower electrical power and guaranteed sensitivity has increased over the decades due to worsening indoor air pollution. In this report, we develop room-temperature operational NH3 gas-sensing materials, which are activated through electron doping and crystal structure distortion effect in Fe0.2Ni0.8WO4. The base material, synthesized through solid-state synthesis, involves Fe cations substitutionally located at the Ni sites of the NiWO4 crystal structure and shows no gas-sensing response at room temperature. However, doping Na into the interstitial sites of Fe0.2Ni0.8WO4 activates gas adsorption on the surface via electron donation to the cations. Additionally, the hydrothermal method used to achieve a more than 70-fold increase in the surface area of structure-distorted Na-doped Fe0.2Ni0.8WO4 powder significantly enhances gas sensitivity, resulting in a 4-times increase in NH3 gas response (Rg/Ra). Photoluminescence and XPS results indicate negligible oxygen vacancies, demonstrating that cation contributions are crucial for gas-sensing activities in Na-doped Fe0.2Ni0.8WO4. This suggests the potential for modulating gas sensitivity through carrier concentration and crystal structure distortion. These findings can be applied to the development of room-temperature operational gas-sensing materials based on the cations. [ABSTRACT FROM AUTHOR]

Triethylamine is a common volatile organic compound (VOC) that plays an important role in areas such as organic solvents, chemical industries, dyestuffs, and leather treatments. However, exposure to triethylamine atmosphere can pose a serious threat to human health. In this study, gas-sensing semiconductor materials of LaFeO3 nano materials with different Mo-doping ratios were synthesized by the sol–gel method. The crystal structures, micro morphologies, and surface states of the prepared samples were characterized by XRD, SEM, and XPS, respectively. The gas-sensing tests showed that the Mo doping enhanced the gas-sensing performance of LaFeO3. Especially, the 4% Mo-doped LaFeO3 exhibited the highest response towards triethylamine (TEA) gas, a value approximately 11 times greater than that of pure LaFeO3. Meantime, the 4% Mo-doped LaFeO3 sensor showed a remarkably robust linear correlation between the response and the concentration (R2 = 0.99736). In addition, the selectivity, stability, response/recovery time, and moisture-proof properties were evaluated. Finally, the gas-sensing mechanism is discussed. This study provides an idea for exploring a new type of efficient and low-cost metal-doped LaFeO3 sensor to monitor the concentration of triethylamine gas for the purpose of safeguarding human health and safety. [ABSTRACT FROM AUTHOR]

Co_3 O_4 metal oxide semiconductor is in principle suitable material for utilization as a resistive gas sensor. In this research, nanostructure cobalt oxide thin films were prepared by chemical spraying pyrolysis technique, by dissolving cobalt chloride with distilled water. The films were doped with Cerium Ce in different concentrations, and the structural and morphology properties of the samples were studied. A gas sensor was prepared from doped and undoped cobalt oxide material. The response properties were studied and the operating temperature and response time of the manufactured sensor were found, the effect of doping on the sensor properties was studied. [ABSTRACT FROM AUTHOR]

Herein, we prepared two different MOS-based gas sensors with integrated micro-hotplates. The two sensors were employed to detect various fragrances (cedar, mandarin orange, rose A, and rose B), exhibiting similarly great sensing performances. The gas sensing properties of the MOS-based sensor depend on the sensor's operating temperature. In addition to isothermal operation, various pulse heating modes were applied to investigate the gas sensing performances with respect to the four fragrances. Multivariate gas sensing features of the four fragrances were obtained under different operating modes, which were utilized for the recognition of fragrance odors successfully, based on the long short-term memory (LSTM) algorithm. [ABSTRACT FROM AUTHOR]

In this study, we demonstrate the emergence of NO2 gas sensing capabilities in the typically non-active NiWO4, a strongly correlated electron system (SCES), by introducing substitutional Fe at the Ni site. NiWO4 typically exhibits strong Coulombic repulsion between Ni atoms, resulting in a large band gap of over 3.0 eV and insulating behavior. This correlated behavior is clearly reflected in the significant increase of band gap when considering the Hubbard U correction for the cations, bringing the theoretical value closer to the observed value. The single-phase Fe0.5Ni0.5WO4 displays a notable shift in the [NiO6] symmetric vibration mode and an increase in magnetization. Additionally, theoretical calculations confirm the preservation of the wide band gap, with the Fe and O levels generated within the band gap. These findings indicate that Fe located in the Ni sites modulate Coulombic repulsion in NiWO4 SCES insulators. Unlike the poor gas-sensing performance of intrinsic NiWO4, Fe0.5Ni0.5WO4 exhibits a significant NO2 response (Rg/Ra) of 11 at 200°C than other gases and a limit of detection (LOD) of 46.4 ppb. This study provides a pathway for realizing gas-sensing performance in strongly correlated electron insulators with large band gaps through the introduction of dopant levels at the cation sites.

The demand for gas-sensing operations with lower electrical power and guaranteed sensitivity has increased over the decades due to worsening indoor air pollution. In this report, we develop room-temperature operational NH3 gas-sensing materials, which are activated through electron doping and crystal structure distortion effect in Fe0.2Ni0.8WO4. The base material, synthesized through solid-state synthesis, involves Fe cations substitutionally located at the Ni sites of the NiWO4 crystal structure and shows no gas-sensing response at room temperature. However, doping Na into the interstitial sites of Fe0.2Ni0.8WO4 activates gas adsorption on the surface via electron donation to the cations. Additionally, the hydrothermal method used to achieve a more than 70-fold increase in the surface area of structure-distorted Na-doped Fe0.2Ni0.8WO4 powder significantly enhances gas sensitivity, resulting in a 4-times increase in NH3 gas response (Rg/Ra). Photoluminescence and XPS results indicate negligible oxygen vacancies, demonstrating that cation contributions are crucial for gas-sensing activities in Na-doped Fe0.2Ni0.8WO4. This suggests the potential for modulating gas sensitivity through carrier concentration and crystal structure distortion. These findings can be applied to the development of room-temperature operational gas-sensing materials based on the cations.

As a stable, low-cost, environment-friendly, and gas-sensitive material, semiconductor metal oxides have been widely used for gas sensing. In the past few years, single-atom catalysts (SACs) have gained increasing attention in the field of gas sensing with the advantages of maximized atomic utilization and unique electronic and chemical properties and have successfully been applied to enhance the detection sensitivity and selectivity of metal oxide gas sensors. However, the application of SACs in gas sensors is still in its infancy. Herein, we critically review the recent advances and current status of single-atom catalysts in metal oxide gas sensors, providing some suggestions for the development of this field. The synthesis methods and characterization techniques of SAC-modified metal oxides are summarized. The interactions between SACs and metal oxides are crucial for the stable loading of single-atom catalysts and for improving gas-sensitive performance. Then, the current application progress of various SACs (Au, Pt, Cu, Ni, etc.) in metal oxide gas sensors is introduced. Finally, the challenges and perspectives of SACs in metal oxide gas sensors are presented. [ABSTRACT FROM AUTHOR]

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Highlights Metal oxide and carbon hybrids (MOx/C) were micropatterned very rapidly and energy efficiently by direct laser writing. Metal-organic framework was the ideal precursor for fabricating homogeneous MOx/C hybrids due to regularly spaced metal ions and organic ligands. The fabricated sensor not only demonstrated broad-range gas sensing capability for ethanol gas (170-3,400 ppm) but also exhibited exceptional sensitivity, rapid response and recovery, selectivity, linearity, and thermal stability.

A series of chromium trioxide/zinc metastannate (Cr 2 O 3 /ZnSnO 3) composites with different ratios were prepared via a simple two-step hydrothermal and high-temperature sintering method. The samples underwent characterization and analysis to assess their micromorphologies, elemental distributions, chemical compositions, and specific surface areas. Furthermore, gas sensing properties were evaluated through experiment. Results showed that the cubic ZnSnO 3 block in Cr 2 O 3 /ZnSnO 3 was in close contact with the surface of nanoparticulate Cr 2 O 3 and the specific surface area increases. Its primary elements are Zn, Sn, Cr, and O. The 15 wt% Cr 2 O 3 /ZnSnO 3 sensor exhibited an optimal operating temperature of 175 °C, 75 °C lower than that of pure ZnSnO 3 (250 °C). The 15 wt% Cr 2 O 3 /ZnSnO 3 sensor exhibited the best sensing performance for formaldehyde gases with a concentration of 50 ppm. It exhibited a response of 37.8 and could detect ppb-level concentrations of formaldehyde (limit of detection = 612 ppb). Additionally, Cr 2 O 3 /ZnSnO 3 showed good selectivity for formaldehyde in the presence of six gases, that is, acetone, formaldehyde, methanol, ethanol, ammonia, and carbon monoxide. Cr 2 O 3 /ZnSnO 3 exhibited good short-term repeatability and long-term stability. Because of the excellent performance of Cr 2 O 3 /ZnSnO 3 composites, they have broad application prospects in formaldehyde gas detection. [ABSTRACT FROM AUTHOR]

Highlights: Metal oxide and carbon hybrids (MOx/C) were micropatterned very rapidly and energy efficiently by direct laser writing. Metal-organic framework was the ideal precursor for fabricating homogeneous MOx/C hybrids due to regularly spaced metal ions and organic ligands. The fabricated sensor not only demonstrated broad-range gas sensing capability for ethanol gas (170-3,400 ppm) but also exhibited exceptional sensitivity, rapid response and recovery, selectivity, linearity, and thermal stability.Advancements in sensor technology have significantly enhanced atmospheric monitoring. Notably, metal oxide and carbon (MOx/C) hybrids have gained attention for their exceptional sensitivity and room-temperature sensing performance. However, previous methods of synthesizing MOx/C composites suffer from problems, including inhomogeneity, aggregation, and challenges in micropatterning. Herein, we introduce a refined method that employs a metal–organic framework (MOF) as a precursor combined with direct laser writing. The inherent structure of MOFs ensures a uniform distribution of metal ions and organic linkers, yielding homogeneous MOx/C structures. The laser processing facilitates precise micropatterning (< 2 μm, comparable to typical photolithography) of the MOx/C crystals. The optimized MOF-derived MOx/C sensor rapidly detected ethanol gas even at room temperature (105 and 18 s for response and recovery, respectively), with a broad range of sensing performance from 170 to 3,400 ppm and a high response value of up to 3,500%. Additionally, this sensor exhibited enhanced stability and thermal resilience compared to previous MOF-based counterparts. This research opens up promising avenues for practical applications in MOF-derived sensing devices. [ABSTRACT FROM AUTHOR]

The current trends of gas sensors made of metal oxide (MO) nanomaterial were widely investigated over the past years as a good candidate for an ecofriendly gas detecting technique. This research deals with prepare of zinc oxide (ZnO) nanoparticles (NPs) from ZnO powder by pulsed laser deposition (PLD) techniques under annealing temperature of 200 °C and 300 °C. Scanning electron microscopy (SEM) was used to examine the morphological characteristics. X-ray diffraction (XRD) patterns revealed the crystal structure. The transmittance was also examined using UV–visible spectrophotometers. The produced MO was then applied and examined as thin films to investigate its effect for gas sensing applications. This study focused on examining the effect of ZnO particles on photoconductivity in the sensing area, using a 365 nm UV-light beam to examine the photoconductivity of samples at annealing temperatures of 200 °C and 300 °C. The researchers found that the resistance/conductivity ratio of the sample prepared at 300 °C increased quickly upon excitation by UV rays, which resulted in the liberation of oxygen ions and free electrons from the material, further accelerating the photoconductivity process. The sensors exhibit their maximum CO response within the temperature range of 300–400 °C. In this work, the photoconductivity process, as a function of time, depended on the mechanism of recombining the carriers. [ABSTRACT FROM AUTHOR]

This study investigated the structural, morphological, and photoluminescent properties of ZnO nanopowders doped with Mo and V, which were synthesized using pulsed laser reactive technology. The nanoparticles' structure, shape, and size were determined using electron microscopy and X-ray diffractometry. The photoluminescence properties of the Mo- and V-doped ZnO nanopowders in different gas environments were studied, revealing that changes in the gas environment led to significant alterations in the intensity and deformation of the photoluminescence spectra. All samples exhibited strong emission bands in the UV range and a broad, non-elemental emission band in the visible region ranging from 410 to 600 nm. Decomposing the photoluminescence spectra into elementary bands revealed peaks at 430 and 520 nm. Chromaticity diagrams of the photoluminescence light emitted by the nanopowders were obtained, and it was found that the color coordinates varied depending on the gas environment, which could be useful in gas sensors. [ABSTRACT FROM AUTHOR]

Triethylamine is a common volatile organic compound (VOC) that plays an important role in areas such as organic solvents, chemical industries, dyestuffs, and leather treatments. However, exposure to triethylamine atmosphere can pose a serious threat to human health. In this study, gas-sensing semiconductor materials of LaFeO3 nano materials with different Mo-doping ratios were synthesized by the sol–gel method. The crystal structures, micro morphologies, and surface states of the prepared samples were characterized by XRD, SEM, and XPS, respectively. The gas-sensing tests showed that the Mo doping enhanced the gas-sensing performance of LaFeO3. Especially, the 4% Mo-doped LaFeO3 exhibited the highest response towards triethylamine (TEA) gas, a value approximately 11 times greater than that of pure LaFeO3. Meantime, the 4% Mo-doped LaFeO3 sensor showed a remarkably robust linear correlation between the response and the concentration (R2 = 0.99736). In addition, the selectivity, stability, response/recovery time, and moisture-proof properties were evaluated. Finally, the gas-sensing mechanism is discussed. This study provides an idea for exploring a new type of efficient and low-cost metal-doped LaFeO3 sensor to monitor the concentration of triethylamine gas for the purpose of safeguarding human health and safety.

Herein, we prepared two different MOS-based gas sensors with integrated micro-hotplates. The two sensors were employed to detect various fragrances (cedar, mandarin orange, rose A, and rose B), exhibiting similarly great sensing performances. The gas sensing properties of the MOS-based sensor depend on the sensor’s operating temperature. In addition to isothermal operation, various pulse heating modes were applied to investigate the gas sensing performances with respect to the four fragrances. Multivariate gas sensing features of the four fragrances were obtained under different operating modes, which were utilized for the recognition of fragrance odors successfully, based on the long short-term memory (LSTM) algorithm.

Metal oxide nanowires (NWs) with a high surface area, ease of fabrication, and precise control over diameter and chemical composition are among the best candidates for the realization of resistive gas sensors. Among the different techniques used for the synthesis of materials with NW morphology, approaches based on the vapor–liquid–solid (VLS) mechanism are very popular due to the ease of synthesis, low price of starting materials, and possibility of branching. In this review article, we discuss the gas-sensing features of metal oxide NWs grown by the VLS mechanism, with emphasis on the growth conditions and sensing mechanism. The growth and sensing performance of SnO2, ZnO, In2O3, NiO, CuO, and WO3 materials with NW morphology are discussed. The effects of the catalyst type, growth temperature, and other variables on the morphology and gas-sensing performance of NWs are discussed. [ABSTRACT FROM AUTHOR]

As a stable, low-cost, environment-friendly, and gas-sensitive material, semiconductor metal oxides have been widely used for gas sensing. In the past few years, single-atom catalysts (SACs) have gained increasing attention in the field of gas sensing with the advantages of maximized atomic utilization and unique electronic and chemical properties and have successfully been applied to enhance the detection sensitivity and selectivity of metal oxide gas sensors. However, the application of SACs in gas sensors is still in its infancy. Herein, we critically review the recent advances and current status of single-atom catalysts in metal oxide gas sensors, providing some suggestions for the development of this field. The synthesis methods and characterization techniques of SAC-modified metal oxides are summarized. The interactions between SACs and metal oxides are crucial for the stable loading of single-atom catalysts and for improving gas-sensitive performance. Then, the current application progress of various SACs (Au, Pt, Cu, Ni, etc.) in metal oxide gas sensors is introduced. Finally, the challenges and perspectives of SACs in metal oxide gas sensors are presented.

Effective detection of ammonia gas is of great importance due to its detrimental effects on human health, environment, and ecosystem. High-performance composite gas sensors are vital in accomplishing this goal. Herein, we investigate the performance of an ammonia (NH 3) gas sensor fabricated via dip-coating the silver interdigitated electrode for PMMA/RGO/ZnO (PRZ) nanocomposite solution with acetone as a solvent. The PRZ ternary nanocomposite was synthesized using the in-situ solution polymerization method and the resistive properties of the films assembled on the interdigitated electrode were analyzed, with respect to the fixed and varying ammonia gas concentrations, using LCR meter. When the sensor is operated in the controlled chamber containing ammonia gas at room temperature, the sensor responds rapidly to ammonia with a fast recovery of 13.02 s at a gas concentration of 350 ppm. The PRZ sensor exhibits high sensing percentage response (527%), excellent repeatability (four times), high sensitivity at low concentrations (less than 10 ppm), swift response and recovery times (1.94 s/13.02 s), and long-term stability (up to 90 days) with fluctuation of 3.2%, which signifies PRZ composite as a potential material for ammonia gas sensor. Aspects such as simplicity of the synthesis process and fabrication, excellent sensing performance, as well as fast response-recovery time at a particular gas concentration are noteworthy in this study. These features can be utilized for the detection of ammonia gas in chemical and biological fields. [Display omitted] [ABSTRACT FROM AUTHOR]

Chemiresistive gas sensors were produced by functionalizing graphene with a ~3 nm layer of mixed oxide xCu2O⸱yMnO using pulsed laser deposition (PLD) from a hopcalite CuMn2O4 target. Sensor response time traces were recorded for strongly oxidizing (NO2, O3) and reducing (NH3, H2S) poisonous gases at ppb and ppm levels, respectively. The morphology of the MOX layer was modified by growth temperature during PLD, resulting in the optimization of the sensor response. Differences in decomposition or oxidation rates on catalytically active metal oxide (MOX) were utilized to achieve partial selectivity for pairs of gases that have similar adsorption and redox properties. The predominant selectivity towards ozone in most samples at different measuring conditions remained difficult to suppress. A distinct selectivity for H2S emerged at higher measurement temperatures (100–150 °C), which was assigned to catalytic oxidation with O2. Several gas–MOX interaction mechanisms were advanced to tentatively explain the sensor behavior, including reversible electron transfer in the simplest case of NO2, decomposition via ionic transients for O3, and complex catalytic oxidative transformations for NH3 and H2S. [ABSTRACT FROM AUTHOR]

The study of structural, morphological and photoluminescent properties of magnesium oxide, gallium trioxide nanopowders and their mixes obtained using pulsed laser reactive technology is carried out. The main emission peak for MgO located at about 430 nm can be assigned to recombination processes as a result of the formation of oxygen vacancy complexes. For Ga2O3 main emission band located at about 440 nm. Changing the gas environment (air, CO2, H2) leads to a significant change in the intensity of the spectra and its deformation for both metal oxides, that can be used in the gas sensors. The use of mixed metal oxides (MgO/Ga2O3) leads to improve sensitivity, enhanced adsorption ability, extensive catalytic activity, and high thermodynamic stability which is of considerable interest to the gas sensing. [ABSTRACT FROM AUTHOR]

To mimic the human olfactory system, an electronic nose (E-nose, also known as artificial olfactory) has been proposed based on a multiple gas sensor array and a pattern recognition algorithm. Detection of volatile organic components (VOCs) has many potential applications in breath analysis, food quality estimation, and indoor and outdoor air quality monitoring, etc. In this study, a facile single-needle electrospinning technology was applied to develop the four different semiconductor metal oxide (MOS) nanofibers sensor arrays (SnO2, CuO, In2O3 and ZnO, respectively). The array shows a smooth surface and constant diameter of nanofiber (average of 150 nm) resulting in high sensitivity to multiple target analyte gases. Five human health related VOCs gases were measured by fabricated E-nose and different response patterns were obtained from four MOS nanofibers sensors. Combined with feature extraction from the response curves, a principal component analysis (PCA) algorithm was applied to reduce the dimension of feature matrix, Thus, the fabricated E-nose system successfully discriminated five different VOCs gases. Real-time and non-invasive gas monitoring by E-nose is very promising for application in human health monitoring, food monitoring, and other fields. [ABSTRACT FROM AUTHOR]

In this study, we have proposed and analyzed a theoretical model for NO2 gas sensors. We have developed a realistic model for variation in sensor response as a function of the gas concentration, temperature, humidity, and sensing film thickness and derived a general expression relating to the sensor response. Tungsten trioxide (WO3)-based gas sensors were adopted because of their physical and chemical properties, low cost, and ease of preparation. Superior sensing characteristics were obtained for 0.42 eV activation energy, where the device exhibited a sensor response of 14.41 toward 1 ppm NO2 gas concentration at room temperature. To validate the applicability of the proposed model, a comparative analysis was conducted, and an excellent match was obtained between the proposed model and experimental data. The analysis carried out in this work offers a feasible and promising approach for a wide variety of sensing materials. [ABSTRACT FROM AUTHOR]

The structural, optical and gas detection characteristics of Mn-doped ZnO thin films synthesised by the Successive Ionic Layer Adsorption and Reaction (SILAR) process with varied doping concentrations (4, 6 and 8%) were examined. X-ray diffraction data was used to investigate the structural properties and confirm the hexagonal wurtzite crystal structure. The surface morphology and the existence of all elements were verified using a field emission scanning electron microscope and energy dispersive X-ray spectroscopy, respectively. To explore the vibrational bands present in Mn-doped ZnO films, we used FTIR spectra at ambient temperature. Ultraviolet-visible analysis reveals the absorption edge, which is found in the ultraviolet area, with a minor shift towards high wave length, whereas the energy bandgap decreases as Mn concentrations increase. At 200°C operating temperature, the gas sensing properties of Zn0.92Mn0.08O showed good sensitivity, selectivity and shorter response time when compared to other prepared films. [ABSTRACT FROM AUTHOR]

Cadmium-tin oxide (CSO) thin films were fabricated by spray pyrolysis method at different substrate temperatures (Ts). The results showed a significant effect of Ts on the surface morphology and the electrical properties, which in turn has a significant effect on sensor gas sensitivity. The sample prepared at 400 °C appeared in the form of a donut shape, which has the best ozone sensitivity. No ozone sensitivity appeared in-dark, while showed good sensitivity when illuminated with UV at room temperature (RT). The study showed that the photon exposure method can substitute for the conventional method of sensors heating. [ABSTRACT FROM AUTHOR]

For a comparative study of the gas sensing performance to dissolved volatile organic compounds (VOCs), the SnO2-powders have been prepared using two different fabrication routes; the flame-spray-pyrolysis (FSP) and the sol-gel (SG) route; and were admixed with the same additive-powders (alumina, YSZ, and NASICON). The morphology of the two different SnO2/additive families were investigated by ESEM analysis and Energy-dispersive X-Ray Spectroscopy (EDS). Both SnO2/additive material families were separately deposited as thick-film-layers on two four-fold sensor-chips which were simultaneously thermo-cyclically operated in a measurement cell combined with a carrier gas probe, which enables sensing tests with evaporated VOCs (acetic acid, propionic acid, ethanol, acetone) dissolved in water. The resulting Conductance-over-Time-Profiles (CTPs) highlight better sensitivities of most of the FSP-layers to all the analytes compared to the SG-prepared layers. Furthermore, the CTP shapes of the FSP layers show clearly enhanced specificity representing the individual analyte components. This was interpreted to be the consequence of the extremely fine, scarcely agglomerated grain morphology of FSP-prepared powders and their very narrow grain size distribution which provide better conditions for enhanced gas specific surface reactions. Results promise a better chemical analysis capability of dissolved VOCs by numerical analysis of the CTP of FSP-prepared gas sensitive layers.

Nanostructured semiconducting metal oxides (SMOs) hold the potential for playing a critical role in gas-sensing applications due to their interconnected nanograins, large surface area, and shorter diffusion length. The rational introduction of ubiquitous porosity in SMOs can enhance gas/air diffusion as well as the accessibility of nanograins and reactive sites. Although much work has been researched for adopting highly porous nanostructured SMOs to enhance gas sensing, no comprehensive review on porosity control has yet been presented. In this review, the latest porosity control methods for SMOs, structural properties, and their gas-sensing results are reported. Finally, perspective on porosity control methods and future directions are provided for further development and research of porous SMO-based gas-sensing nanomaterials. [ABSTRACT FROM AUTHOR]

With the emerging of the Internet of Things, chemiresistive gas sensors have been extensively applied in industrial production, food safety, medical diagnosis, and environment detection, etc. Considerable efforts have been devoted to improving the gas‐sensing performance through tailoring the structure, functions, defects and electrical conductivity of sensitive materials. Among the numerous sensitive materials, mesoporous semiconductor metal oxides possess unparalleled properties, including tunable pore size, high specific surface area, abundant metal–oxygen bonds, and rapid mass transfer/diffusion behavior (Knudsen diffusion), which have been regarded as the most potential sensitive materials. Herein, the synthesis strategies for mesoporous metal oxides are overviewed, the classical functionalization techniques of sensitive materials are also systemically summarized as a highlight, including construction of mesoporous structure, regulation of micro‐nano structure (i.e., heterojunctions), noble metal sensitization (e.g., Au, Pt, Ag, Pd) and heteroatomic doping (e.g., C, N, Si, S). In addition, the structure–function relationship of sensitive materials has been discussed at molecular‐atomic level, especially for the chemical sensitization effect, elucidating the interface adsorption/catalytic mechanism. Moreover, the challenges and perspectives are proposed, which will open a new door for the development of intelligent gas sensor in various applications. [ABSTRACT FROM AUTHOR]

A flame spray pyrolysis (FSP) technique was applied to obtain pure and Nb(V)-doped nanocrystalline β-Ga2O3, which were further studied as gas sensor materials. The obtained samples were characterized with XRD, XPS, TEM, Raman spectroscopy and BET method. Formation of GaNbO4 phase is observed at high annealing temperatures. Transition of Ga(III) into Ga(I) state during Nb(V) doping prevents donor charge carriers generation and hinders considerable improvement of electrical and gas sensor properties of β-Ga2O3. Superior gas sensor performance of obtained ultrafine materials at lower operating temperatures compared to previously reported thin film Ga2O3 materials is shown. [ABSTRACT FROM AUTHOR]

n -butanol detection is crucial to environmental protection and mankind's health because of its toxicity and irritation to the respiratory system. Mono-metal oxide semiconductors (such as SnO 2) often face disadvantages such as low sensitivity, insufficient selectivity, and a high detection limit. Constructing heterojunctions has been considered an efficient way to enhance the performance of MOS gas sensors. This work presents the synthesis of a series of β -CdSnO 3 -SnO 2 nanorod-like heterostructures with good n -butanol sensitivity using a hydrothermal and post-calcination approach. The optimized CSO/SO-8 shows the highest response value about 90–100 ppm n -butanol at 190 °C, which is 15 times greater than that of SnO 2 , and 6.5 times more than that of β -CdSnO 3. Meanwhile, the CSO/SO-8 also exhibits a short response and recovery time (1–2 s/63 s). More importantly, the detection limit is reduced to ppb level (50 ppb). The improved gas-sensitive properties can be explained by the formation of n - n heterojunctions between β -CdSnO 3 and SnO 2 , which increases the content of chemisorbed oxygen, thus enhancing the performance of the gas-sensitive. Our work not only provides a material with n - n heterostructures but also the strategy for constructing heterojunctions to improve gas-sensitive properties. [Display omitted] • The nanorod-like β -CdSnO 3 -SnO 2 heterojunctions were prepared successfully. • CSO/SO-8 exhibits the optimal gas sensing performance to n -butanol. • CSO/SO-8 enables rapid detection (1–2 s) as well as trace n -butanol detection. • The improved performance can be attributed to the formation of heterojunctions that produce more chemisorbed oxygen. [ABSTRACT FROM AUTHOR]

Developing fast, accurate and sensitive triethylamine gas sensors with low detection limits is paramount to ensure the safety of workers and the public. However, sensors based on single metal oxide materials still suffer from drawbacks such as low response sensitivity and long response and recovery times. To address these challenges, in this work, a series of mesoporous CdO/CdGa 2 O 4 microspheres were synthesized. We optimized the sensor's sensing performance to triethylamine by fine-tuning the ratio of CdO to CdGa 2 O 4. Among them, CdO:3CdGa 2 O 4 -based sensor demonstrates a rapid response time of 2 s to detect 100 ppm of triethylamine, with a high response value of 211 and exceptional selectivity. Furthermore, it exhibits a low detection limit of 20 ppb for triethylamine, making it suitable for practically testing fish freshness. Crucially, electron transfer between the heterojunctions increases the chemically adsorbed oxygen on the materials' surface, thereby enhancing the sensor's response sensitivity to triethylamine. This discovery provides new insights and methodologies for the design of highly efficient triethylamine gas sensors. [Display omitted] • A novel triethylamine gas sensor based on mesoporous CdO/CdGa 2 O 4 microspheres has been successfully synthesized. • The sensor based on this material has fast response and low detection limit. • Electron transfer between the heterojunctions increases chemically adsorbed oxygen on the material's surface. • The sensor shows excellent selectivity for triethylamine and can be used in complex environments. [ABSTRACT FROM AUTHOR]

Metal oxide nanowires (NWs) with a high surface area, ease of fabrication, and precise control over diameter and chemical composition are among the best candidates for the realization of resistive gas sensors. Among the different techniques used for the synthesis of materials with NW morphology, approaches based on the vapor–liquid–solid (VLS) mechanism are very popular due to the ease of synthesis, low price of starting materials, and possibility of branching. In this review article, we discuss the gas-sensing features of metal oxide NWs grown by the VLS mechanism, with emphasis on the growth conditions and sensing mechanism. The growth and sensing performance of SnO2, ZnO, In2O3, NiO, CuO, and WO3 materials with NW morphology are discussed. The effects of the catalyst type, growth temperature, and other variables on the morphology and gas-sensing performance of NWs are discussed.

Metal oxides are ideal for the fabrication of gas sensors: they are sensitive to many gases while allowing the device to be simple, tiny, and inexpensive. Nonetheless, their lack of selectivity remains a limitation. In order to achieve good selectivity in applications with many possible interfering gases, the sensors are inserted into an electronic nose that combines the signals from nonselective sensors and analyzes them with multivariate statistical algorithms in order to obtain selectivity. This review analyzes the scientific articles published in the last decade regarding electronic noses based on metal oxide nanowires. After a general introduction, Section 2 discusses the issues related to poor intrinsic selectivity. Section 3 briefly reviews the main algorithms that have hitherto been used and the results they can provide. Section 4 classifies the recent literature into fundamental research, agrifood, health, security. In Section 5, the literature is analyzed regarding the metal oxides, the surface decoration nanoparticles, the features that differentiate the sensors in a given array, the application for which the device was developed, the algorithm used, and the type of information obtained. Section 6 concludes by discussing the present state and points out the requirements for their use in real-world applications.

Abstract With the emerging of the Internet of Things, chemiresistive gas sensors have been extensively applied in industrial production, food safety, medical diagnosis, and environment detection, etc. Considerable efforts have been devoted to improving the gas‐sensing performance through tailoring the structure, functions, defects and electrical conductivity of sensitive materials. Among the numerous sensitive materials, mesoporous semiconductor metal oxides possess unparalleled properties, including tunable pore size, high specific surface area, abundant metal–oxygen bonds, and rapid mass transfer/diffusion behavior (Knudsen diffusion), which have been regarded as the most potential sensitive materials. Herein, the synthesis strategies for mesoporous metal oxides are overviewed, the classical functionalization techniques of sensitive materials are also systemically summarized as a highlight, including construction of mesoporous structure, regulation of micro‐nano structure (i.e., heterojunctions), noble metal sensitization (e.g., Au, Pt, Ag, Pd) and heteroatomic doping (e.g., C, N, Si, S). In addition, the structure–function relationship of sensitive materials has been discussed at molecular‐atomic level, especially for the chemical sensitization effect, elucidating the interface adsorption/catalytic mechanism. Moreover, the challenges and perspectives are proposed, which will open a new door for the development of intelligent gas sensor in various applications.

The realization of an unobtrusive and effective technology able to track fish freshness in real time and inform on its edibility is highly demanded, but still unachieved. In the present paper, we address this issue through a single metal oxide gas sensor working in temperature modulation mode. The system can work without an external reference air source, which is an appealing feature for its possible integration in domestic refrigerators. Tests were carried out using fresh sea bream fillets as case study and working both inside the refrigerator and at room temperature. Parallel gas chromatography–mass spectrometry and microbiological characterization indicated the marked dependence of both the microbiological condition and the gas-phase composition from the individual sample and from the storage temperature. Despite such a large variability, which may be expected in real applications, the proposed system provided similar responses whenever the total bacterial population approached and exceeded the edibility threshold of 107 CFU/g. [ABSTRACT FROM AUTHOR]

Methanol, naturally present in small quantities in the distillation of alcoholic beverages, can lead to serious health problems. When it exceeds a certain concentration, it causes blindness, organ failure, and even death if not recognized in time. Analytical techniques such as chromatography are used to detect dangerous concentrations of methanol, which are very accurate but also expensive, cumbersome, and time-consuming. Therefore, a gas sensor that is inexpensive and portable and capable of distinguishing methanol from ethanol would be very useful. Here, we present a resistive gas sensor, based on tin oxide nanowires, that works in a thermal gradient. By combining responses at various temperatures and using machine learning algorithms (PCA, SVM, LDA), the device can distinguish methanol from ethanol in a wide range of concentrations (1–100 ppm) in both dry air and under different humidity conditions (25–75% RH). The proposed sensor, which is small and inexpensive, demonstrates the ability to distinguish methanol from ethanol at different concentrations and could be developed both to detect the adulteration of alcoholic beverages and to quickly recognize methanol poisoning. [ABSTRACT FROM AUTHOR]

Monitoring environmental hazards and pollution control is vital for the detection of harmful toxic gases from industrial activities and natural processes in the environment, such as nitrogen dioxide (NO2), ammonia (NH3), hydrogen (H2), hydrogen sulfide (H2S), carbon dioxide (CO2), and sulfur dioxide (SO2). This is to ensure the preservation of public health and promote workplace safety. Graphene and its derivatives, especially reduced graphene oxide (rGO), have been designated as ideal materials in gas-sensing devices as their electronic properties highly influence the potential to adsorb specified toxic gas molecules. Despite its exceptional sensitivity at low gas concentrations, the sensor selectivity of pristine graphene is relatively weak, which limits its utility in many practical gas sensor applications. In view of this, the hybridization technique through heterojunction configurations of rGO with metal oxides has been explored, which showed promising improvement and a synergistic effect on the gas-sensing capacity, particularly at room temperature sensitivity and selectivity, even at low concentrations of the target gas. The unique features of graphene as a preferential gas sensor material are first highlighted, followed by a brief discussion on the basic working mechanism, fabrication, and performance of hybridized rGO/metal oxide-based gas sensors for various toxic gases, including NO2, NH3, H2, H2S, CO2, and SO2. The challenges and prospects of the graphene/metal oxide-based based gas sensors are presented at the end of the review. [ABSTRACT FROM AUTHOR]

There is an increasing need for the development of low-cost and highly sensitive gas sensors for environmental, commercial, and industrial applications in various areas, such as hazardous gas monitoring, safety, and emission control in combustion processes. Considering this, resistive-based gas sensors using metal oxide semiconductors (MOSs) have gained special attention owing to their high sensing performance, high stability, and low cost of synthesis and fabrication. The relatively low final costs of these gas sensors allow their commercialization; consequently, they are widely used and available at low prices. This review focuses on the important MOSs with different morphologies, including quantum dots, nanowires, nanofibers, nanotubes, hierarchical nanostructures, and other structures for the fabrication of resistive gas sensors. [ABSTRACT FROM AUTHOR]

• The α -Fe 2 O 3 nanomaterials were prepared using a simple one-step hydrothermal method. • S1-250 sensor has a lower operating temperature (225 °C) and the highest response(88.47/100 ppm). • The long-term stability, selectivity and resistance to moisture of the S1-250 sensor were better than that of the S2-350 sensor. • The structure-activity relationship between material structure and gas-sensitive properties is studied to provide new ideas for the design of gas-sensitive materials. In different environments, high concentration of n-butanol will have certain harm to the human senses and nervous system, meanwhile the electrochemical sensor limits its widespread use due to its high power consumption, so it is very meaningful to develop a semiconductor n-butanol sensor with low energy consumption. In this paper, α-Fe 2 O 3 nanorods were prepared by one-step hydrothermal method and then assembled into a n-butanol sensor capable of detecting n-butanol, and the effects of two different calcination temperatures on the performance of the sensor were investigated. Due to its higher Fe3+ content, higher oxygen vacancy content and larger specific surface area, S1-250 provided more active sites for gas adsorption, which making the response of S1-250 to 100 ppm n-butanol at 215 °C reached to 88.4. Finally, the effect of the calcination temperature on the sensor and the response mechanism were discussed. This paper offers promising applications for low-energy n-butanol sensors assembled from a single material α −Fe 2 O 3. [ABSTRACT FROM AUTHOR]

[Display omitted] • Figures of merit of light-activated gas sensors, operable at room temperature are addressed. • Role of light irradiation on the sensing material to elevate gas-sensing performances has been discussed. • 2-D material-based gas sensor shows significant response under visible light activation. • Discussed the future scopes towards the development of light-activated gas sensors. In this modern era, the necessity of a safe environment with a swift detection of even minute concentrations of hazardous and combustible gases has spurred significance in the advancement of gas sensor technology. In this aspect, the room temperature operable gas sensors have marked their importance by ensuring the safe detection of combustible gases. Nonetheless, the incomplete recovery of such gas sensors requires thermal activation, which entails several limitations. Therefore, the light-activation of gas sensors has garnered considerable attention owing to its compactness and cost-effective operations. The light-activation generates the electron-hole pairs which activate the sensing surface and modulate the charge carrier concentration, thereby enhancing the gas-sensing performances. In this review, the gas-sensing performances of various photoactive sensing materials including metal oxides and two-dimensional materials under light irradiation have been discussed. The gas sensors based on metal oxide and two-dimensional materials have shown significant performance in terms of response, as well as sharp response and recovery times under both ultra-violet and visible light illumination. Finally, this review emphasizes the challenges and future scopes associated with the light-activated room temperature operable gas sensors, which could lead a pathway toward the development of an ultrafast gas sensor. [ABSTRACT FROM AUTHOR]

Among the existing metal-oxide gas sensors, cobalt oxide has the flexibility to revise the morphology through Cr-dopant to enhance sensing properties. Sensitive-surface of the chromium-doped cobalt oxide has proven its effective sensing nature to hydrogen sulfide gas. Interestingly, chromium-dopant increases the surface area, leading to particle size reduce and produces the more active sites for gas molecules. Also, the dopant creates impurity phases on the material which extends the sites for more reaction. To confirms these characteristics, the photoluminescence spectra showed intense peak that mimics the faster transport of electron to accelerate the sensing reaction. According to sensing measurement, the doped sensor is showing three-fold increase of response to 10 ppm gas and also, it detects the 2 ppm efficiently. The doped sensor warrants the stable response to gas due to higher reproducibility. Notably, the doped sensor detects the 1 ppm of gas at 120 s and recovery itself around 200 s. The doped sensor imparts response at room-temperature, affirming sensitive-surface. The doped sensor has shown the capable under humidity environment through response. [Display omitted] • The Cr-dopant increases the charge transfer between the material during the reaction. • The dopant increases the actives sites through their secondary phase formation. • The doped Co3O4 sensor shows rich metallic surface of higher conductivity due to the exposure of H 2 S gas. [ABSTRACT FROM AUTHOR]

The researches structural, morphological and photoluminescent properties of zinc oxide nanopowders doped by Mg and Ca obtained using pulsed laser reactive technology is carried out. Electron microscopy and X-ray diffractometry were conducted to determine the structure, shape, and size of the nanoparticles. Investigation of the peculiarities of photoluminescence in different ambient of Ca- and Mg-doped ZnO nanopowders was carried out. The decomposition of luminescence spectra into elementary bands shows the presence of four elementary peaks. The most intense two bands in visible region which are sensitive to ambient gas, a blue-band centered at 430 nm and stronger in intensity the band around 520 nm. The influence of the impurity and the different ambient on the redistribution of elementary luminescence bands intensities was investigated. That can be used in the gas sensors. The visible emissions observed may be ascribed to the intrinsic defects in particular oxygen vacancies. Large surface area and surface defects are the most important variables to determine the sensory properties of ZnO nanopowders and making them are the most favorable material for gas sensors. [ABSTRACT FROM AUTHOR]

Sensor stability is defined as the ability to maintain a relatively stable and repeatable signal over a sufficient period. Long-term stability for gas sensors is an essential capability for carrying out long-term data collection of human exhaled breath, environmental monitoring and other gas detection in the modern electronic information age. This article reviews the research advances on the stability of metal oxide semiconductor gas sensors in the past five years. The impact of structure, environment, toxicity and sensor array on the sensor stability are discussed. Then, the improvement schemes of existing materials and structure design are summarized. The achievements of structure doping, humidity, anti-poisoning and photoactivation are overviewed. Finally, the great significance of elucidating the sensing mechanism and carrying out the life acceleration test for future research and development is pointed out. [ABSTRACT FROM AUTHOR]

Abstract 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.