Advanced artificial algorithms integrated with p-type NiO nanofibers for health-hazardous gases classification

The detection of toxic gases by resistive gas sensors, which are mainly fabricated using semiconducting metal oxides, is of importance from a safety point of view. These sensors have outstanding electrical and sensing properties as well as are inexpensive. W18O49 (WO2.72), which is a non-stoichiometric tungsten oxide, possesses abundant oxygen vacancies, which are beneficial for the adsorption of oxygen gas molecules and act as sites for sensing reactions. Thus, through the rational design of W18O49-based gas sensors using strategies such as morphology engineering, doping, decoration, formation of composites or their combination, the fabrication of high-performance W18O49 gas sensors is feasible. Herein, we present the gas-sensing features of pristine W18O49, doped W18O49, decorated W18O49 and composite-based W18O49 sensors. Moreover, focusing on the sensing mechanism of W18O49 sensors, this review provides an in-depth understanding on the working principles of the sensing of toxic gases using W18O49.

Among the various types of chemical gas sensors, resistive gas sensors are attractive because of their high sensitivity, stability, low cost and fast response. In particular, metal oxide (MOx) semiconductors, such as SnO2, ZnO and TiO2, have been widely studied as active layer. However, performances of sensors and their selectivity are strongly dependent of the morphology, structuration and nature of the active material. In particular, due to the strong correlation between grain size and sensor response, nanostructured and/or heterostructured materials permit an enhancement of the gas sensing response. Such structuration requires versatile and well-controlled elaboration approach. Atomic layer deposition (ALD) thus appears as a technique of choice due to its simplicity, reproducibility, the atomic scale precision of the deposited thickness and high homogeneity of the obtained films. Herein, it will be shown that ALD has proven to be well-suited for the elaboration of compact thin films, nanostructures and heterostructures to be applied for the detection of a variety of analytes.[1] We aim at summarizing the most significant progresses related into the literature to gas sensors based on ALD thin films, nanostructures and heterostructures in order to highlight the peculiarity of atomic layer deposition for the fabrication of the sensing layer in resistive sensors. Particular attention will be given to heterostructures based on carbon nanotubes (CNTs) coated with ALD metal oxides investigated as sensing layers. We will also shortly discuss the nanostructure properties in parallel to the sensing mechanisms in order to try to develop clear structure–property correlations. Indeed, with their high surface area, good thermal and electric conductivity and mechanical as well as chemical stability, CNTs provide ideal properties as a support for a second material that can be deposited onto their surface either as particles or as a thin film. Due to the small dimensions, interactions between the deposited material and the tubes at the interface can significantly alter the properties of the composite. This is specifically the case for semiconducting materials when the dimensions are in the range of the Debye length. Using a non-aqueous sol-gel ALD approach and controlled surface functionalization, metal oxides such as V2O4, TiO2 and SnO2 have been precisely deposited as film or particles onto CNTs. On the one hand, combining semiconductor oxides with a conductive support permits to reduce the overall resistance of the sensitive layer. On the other hand, due to the formation of a p-n heterojunction between the p-type conductive support and the n-type thin film, an enhancement of the gas-sensing response is observed depending on the oxide morphology. In particular, V2O4-, TiO2-, ZnO- and SnO2-ALD coated nanotubes have been tested as active component in gas-sensing devices.[2-4] [1] C. Marichy, N. Pinna [2] C. Marichy, N. Donato, M.-G. Willinger, M. Latino, D. Karpinsky, S.-H. Yu, G. Neri, N. Pinna, Advanced Functional Materials 2011, 21, 658-666. [3] M. G. Willinger, G. Neri, A. Bonavita, G. Micali, E. Rauwel, T. Herntrich, N. Pinna, Physical Chemistry Chemical Physics 2009, 11, 3615-3622. [4] C. Marichy, N. Donato, M. Latino, M. G. Willinger, J. P. Tessonnier, G. Neri, N. Pinna, Nanotechnology 2015, 26, 024004.

In recentyears, 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.

Resistive gas sensors have attracted significant attention due to their simple architecture, low cost, and ease of integration, with widespread applications in environmental monitoring, industrial safety, and healthcare diagnostics. This review provides a comprehensive overview of recent advances in resistive gas sensors, focusing on their fundamental working mechanisms, sensing material design, device architecture optimization, and intelligent system integration. These sensors primarily operate based on changes in electrical resistance induced by interactions between gas molecules and sensing materials, including physical adsorption, charge transfer, and surface redox reactions. In terms of materials, metal oxide semiconductors, conductive polymers, carbon-based nanomaterials, and their composites have demonstrated enhanced sensitivity and selectivity through strategies such as doping, surface functionalization, and heterojunction engineering, while also enabling reduced operating temperatures. Device-level innovations—such as microheater integration, self-heated nanowires, and multi-sensor arrays—have further improved response speed and energy efficiency. Moreover, the incorporation of artificial intelligence (AI) and Internet of Things (IoT) technologies has significantly advanced signal processing, pattern recognition, and long-term operational stability. Machine learning (ML) algorithms have enabled intelligent design of novel sensing materials, optimized multi-gas identification, and enhanced data reliability in complex environments. These synergistic developments are driving resistive gas sensors toward low-power, highly integrated, and multifunctional platforms, particularly in emerging applications such as wearable electronics, breath diagnostics, and smart city infrastructure. This review concludes with a perspective on future research directions, emphasizing the importance of improving material stability, interference resistance, standardized fabrication, and intelligent system integration for large-scale practical deployment.

ConspectusGas sensors are used in various applications to sense toxic gases, mainly for enhanced safety. Resistive sensors are particularly popular owing to their ability to detect trace amounts of gases, high stability, fast response times, and affordability. Semiconducting metal oxides are commonly employed in the fabrication of resistive gas sensors. However, these sensors often require high working temperatures, bringing about increased energy consumption and reduced selectivity. Furthermore, they do not have enough flexibility, and their performance is significantly decreased under bending, stretching, or twisting. To address these challenges, alternative materials capable of operating at lower temperatures with high flexibility are needed. Two-dimensional (2D) materials such as MXenes and transition-metal dichalcogenides (TMDs) offer high surface area and conductivity owing to their unique 2D structure, making them promising candidates for realization of resistive gas sensors. Nevertheless, their sensing performance in pristine form is typically weak and unacceptable, particularly in terms of response, selectivity, and recovery time (trec). To overcome these drawbacks, several strategies can be employed to enhance their sensing properties. Noble-metal decoration such as (Au, Pt, Pd, Rh, Ag) is a highly promising method, in which the catalytic effects of noble metals as well as formation of potential barriers with MXenes or TMDs eventually contribute to boosted response. Additionally, bimetallic noble metals such as Pt-Pd and Au/Pd with their synergistic properties can further improve sensor performance. Ion implantation is another feasible approach, involving doping of sensing materials with the desired concentration of dopants through control over the energy and dosage of the irradiation ions as well as creation of structural defects such as oxygen vacancies through high-energy ion-beam irradiation, contributing to enhanced sensing capabilities. The formation of core-shell structures is also effective, creating numerous interfaces between core and shell materials that optimize the sensing characteristics. However, the shell thickness needs to be carefully optimized to achieve the best sensing output. To reduce energy consumption, sensors can operate in a self-heating condition where an external voltage is applied to the electrodes, significantly lowering the power requirements. This enables sensors to function in energy-constrained environments, such as remote or low-energy areas. An important advantage of 2D MXenes and TMDs is their high mechanical flexibility. Unlike semiconducting metal oxides that lack mechanical flexibility, MXenes and TMDs can maintain their sensing performance even when integrated onto flexible substrates and subjected to bending, tilting, or stretching. This flexibility makes them ideal for fabricating flexible and portable gas sensors that rigid sensors cannot achieve.

Preparation of NiO nanofibers by electrospinning and their application for electro-catalytic oxidation of ethylene glycol

• Hollow Sn-doped NiO nanofibers are synthesized through a facile electrospinning approach. • The Sn-doped NiO sensor with suitable Sn content (6 at%) shows the highest gas response to triethylamine. • Sn doping concentrations can significantly influence the resistance of sensors in air and in target gas under different RH. • The resistances of the sensor change slightly in target gas under different RH with suitable Sn doping content (6 at%). High stable triethylamine gas sensors under different relative humidity are highly desirable in order to correctly detect the concentrations of target gas. In this study, a series of Sn-doped NiO hollow nanofibers were prepared through a facile electrospinning process followed by heat treatment. Sn doping could inhibit the crystal growth, and the crystal sizes would decrease with the increase of Sn doping concentration. Gas sensing investigation indicates that Sn doping could significantly enhance the gas response towards triethylamine at a relative low temperature. Especially, the gas sensor exhibits the highest response to triethylamine when the doping content of Sn reaches to 6 at%. The response value is about 16.6–100 ppm triethylamine, and it is ∼9.2 times higher than that of pure NiO nanofibers at the same operating temperature. In addition, the resistances of the gas sensors with different doping contents of Sn would change differently in air or in target gas under variable relative humidity. The resistances in target gas are almost unchanged with the increase of relative humidity with the Sn doping content of 6 at%. It is reasonable to speculate that Sn doping can heavily alter the surface state of NiO nanofibers, which is beneficial for the improvement of the gas response and humidity dependence properties.

Resistive gas sensors are considered promising candidates for gas detection, benefiting from their small size, ease of fabrication and operation convenience. The development history, performance index, device type and common host materials (metal oxide semiconductors, conductive polymers, carbon-based materials and transition metal dichalcogenides) of resistive gas sensors are firstly reviewed. This review systematically summarizes the functions, functional mechanisms, features and applications of seven kinds of guest materials (noble metals, metal heteroatoms, metal oxides, metal–organic frameworks, transition metal dichalcogenides, polymers, and multiple guest materials) used for the modification and optimization of the host materials. The introduction of guest materials enables synergistic effects and complementary advantages, introduces catalytic sites, constructs heterojunctions, promotes charge transfer, improves carrier transport, or introduces protective/sieving/enrichment layers, thereby effectively improving the sensitivity, selectivity and stability of the gas sensors. The perspectives and challenges regarding the host–guest hybrid materials-based gas sensors are also discussed.

Acetone is a main respiratory marker for diabetic patients. In this paper, P-type NiO nanofibers were prepared by electrospinning and used for the detection of acetone gas. NiO nanofibers were characterized by SEM and XRD. The uniform NiO nanofibers with face-centered cubic structure was obtained. The working temperature of NiO nanofibers was optimized, and the optimal operating temperature is 220°C. The response-recovery curve was tested, and the response and recovery time is 24.6 s and 610 s respectively. The response to different concentrations of acetone was also analyzed, and the detection limit was 100 ppb. These results show that NiO nanofibers based on electrospinning have potential applications in the respiratory testing of diabetes.

With a series of widespread applications, resistive gas sensors are considered to be promising candidates for gas detection, benefiting from their small size, ease-of-fabrication, low power consumption and outstanding maintenance properties. One-dimensional (1-D) nanomaterials, which have large specific surface areas, abundant exposed active sites and high length-to-diameter ratios, enable fast charge transfers and gas-sensitive reactions. They can also significantly enhance the sensitivity and response speed of resistive gas sensors. The features and sensing mechanism of current resistive gas sensors and the potential advantages of 1-D nanomaterials in resistive gas sensors are firstly reviewed. This review systematically summarizes the design and optimization strategies of 1-D nanomaterials for high-performance resistive gas sensors, including doping, heterostructures and composites. Based on the monitoring requirements of various characteristic gases, the available applications of this type of gas sensors are also classified and reviewed in the three categories of environment, safety and health. The direction and priorities for the future development of resistive gas sensors are laid out.

In this work, novel NiO–CdO composite nanofibers with excellent conductivity and electrocatalytic property are reported. The hybrid nanofibers are fabricated by a facile two-step synthetic route consisting of electrospinning and subsequent calcination at 500 °C. EDX element mapping indicates the homogeneous distribution of CdO and NiO along the hybrid nanofiber. The incorporation of CdO into the NiO nanofibers is found to significantly suppress the grain size of the NiO crystallites. HRTEM images clearly reveal the dislocation areas with the displacement of lattice lines between CdO and NiO, which might contribute to the significantly improved properties of NiO nanofibers after CdO-doping. The as-prepared NiO–CdO nanofibers are further applied to the non-enzymatic glucose detection and their glucose response is around 9-fold and 1650-fold higher than those obtained with pristine NiO nanofibers and CdO nanofibers, respectively, in the cyclic voltammetry studies. The simple strategy of doping conductive CdO into a semi-conductive metal oxide with dislocation feature opens a new route to generate novel conductive metal oxide composite nanofibers with outstanding functions, which may have great potential applications ranging from sensors to energy area.

Hydrogen sulfide (H2S) is a colorless, flammable, and highly toxic gas that underscores the need for cost-effective, energy-efficient, simple, convenient, and durable detection methods. Resistive gas sensors based on inorganic conductive materials and organic conductive polymers can effectively address these requirements. This review discusses the hazards of H2S gas and reviews sensors capable of detecting H2S at room temperature, including those based on metal oxides, MXene/carbon materials, and p-type conductive polymers. It explores the mechanisms behind their enhanced response at room temperature, such as utilizing special structures (e.g., porous/hollow nanospheres, nanowires, nanotubes, and nanocapsules) to increase the effective surface area of the sensing materials, employing metal particles sensitization to improve gas adsorption, and leveraging heterojunctions to amplify the response. Additionally, this review highlights the limitations of these sensors and provides insights for the further development of low-power resistive H2S gas sensors.

Pure and 0.18–13.2 at.% Fe-doped NiO nanofibers were prepared by electrospinning and their gas sensing characteristics and microstructural evolution were investigated. The responses ((Rg − Ra)/Ra, where Rg is the resistance in gas and Ra is the resistance in air) to 5 ppm C2H5OH, toluene, benzene, p-xylene, HCHO, CO, H2, and NH3 at 350–500 ° C were significantly enhanced by Fe doping of the NiO nanofibers, while the responses of pure NiO nanofibers to all the analyte gases were very low ((Rg − Ra)/Ra = 0.07–0.78). In particular, the response to 100 ppm C2H5OH was enhanced up to 217.86 times by doping of NiO nanofibers with 3.04 at.% Fe. The variation in the gas response was closely dependent upon changes in the base resistance of the sensors in air. The enhanced gas response of Fe-doped NiO nanofibers was explained in relation to electronic sensitization, that is, the increase in the chemoresistive variation due to the decrease in the hole concentration induced by Fe doping.

Explanation of the low oxygen sensitivity of thin film phthalocyanine gas sensors

1D hierarchical CdS NPs/NiO NFs heterostructures with enhanced photocatalytic activity under visible light irradiation