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Herein, we report the fabrication of hydrogen gas sensors with enhanced sensitivity and excellent selectivity. The sensor device is based on the strategic combination of ZnO nanowires (NWs) decorated with palladium nanoparticles (Pd NPs) and a molecular sieve metal-organic framework (MOF) nanomembrane (ZIF-8). The Pd NPs permit the sensors to reach maximal signal responses, whereas the ZIF-8 overcoat enables for an excellent selectivity. Three steps were employed for the fabrication: (i) coating of a miniaturized sensor with vapor-grown ZnO NWs, (ii) decoration of these NWs with Pd NPs by atomic layer deposition, and (iii) partial solvothermal conversion of the tuned NWs surface to ZIF-8 nanomembrane. The microstructure and composition investigations of the ZIF-8/Pd/ZnO nanostructured materials confirmed the presence of both metallic Pd NPs and uniform ZIF-8 thin membrane layer. The integration of these nanomaterials within a miniaturized sensor device enabled the assessment of their performance for H detection at concentrations as low as 10 ppm in the presence of various gases such as CH, CH, CHOH, and CHCOCH. Remarkably high-response signals of 3.2, 4.7, and 6.7 ( R/ R) have been measured for H detection at only 10, 30, and 50 ppm, whereas no noticeable response toward other tested gases was detected, thus confirming the excellent H selectivity obtained with such a sensor design. The results obtained showed that the performance of gas sensors toward H gas can be greatly increased by both the addition of Pd NPs and the use of ZIF-8 coating, acting as a molecular sieve membrane. Furthermore, the presented strategy could be extended toward the sensing of other species by a judicious choice of both the metallic NPs and MOF materials with tuned properties for specific molecule detection, thus opening a new avenue for the preparation of highly selective sensing devices.

Gas sensors are essential for industry and for a wide range of applications. They are for examples applied in public safety, pollution monitoring, and various industrial processes. Among the different gas sensing technologies, semiconducting metal oxide-based gas sensors are the most popular because of their low price, high sensitivity, short response time, high stability and simple operation. In these gas sensors, because gas adsorption has a direct relationship with the surface area of the sensing material, a higher surface area will result in a higher sensing response. Therefore, along with simple synthesis methods, nanowires (NWs) have recently gained special attention for the realization of gas sensors. In this tutorial review, the synthesis of metal oxide NWs, the fabrication of gas sensors and their sensing mechanisms are discussed. Different gas sensors such as single NW, noble metal functionalized NWs, heterojunctions NWs, self-heating NWs, UV-activated NWs and core-shell NWs are presented. This tutorial review aims to provide a broad vision for the researchers and students working in this upcoming field. 2 I. Toxic gases and vaporsGases are intimately linked to life, as most of the living species continuously need to breathe air, which is basically a mixture of oxygen, nitrogen, argon, and other gases. In addition, many gases are used in our industrial era. For example, liquefied petroleum gas (LPG) is widely used in industry, as well as for cooking and heating purposes. 1 Even though LPG is not toxic, it is highly explosive. 2 Also, hydrogen gas is seen as the next "green fuel" and is currently used in fuel cells, although it is highly explosive. 3,4 In addition to explosive gases, the sources of toxic and pollutant gases have been significantly increased in the recent years, and there are many toxic gases in our atmosphere. 5 Toxic gases can cause harm in low levels over long periods of time (chronic exposure) or in higher concentrations over short periods of time (acute exposure). The threshold limit value (TLV) has been defined as the maximum concentration of a gas, which is allowed for repeated exposure without resulting in adverse health effects. 6 For example, the TLV values for CO, NO2 and H2S gases are 50, 3 and 10 ppm, respectively. 6 Based on the WHO (World Health Organization), air pollution is mainly due to toxic gases and caused around seven million premature deaths in 2012. 7 There are many toxic gases in our surrounding atmosphere. For example, carbon monoxide (CO) poisoning results in over 5000 deaths in the USA. 8 In Denmark, from 1995 to 2015, several hundred people passed away due to CO poisoning. 9 Also, in Iran, as a typical developing country, 836 deaths occurred in 2016due to CO poisoning. 10 CO has not any color, odor and taste, 11 and it has 240 times greater affinity for hemoglobin in comparison with oxygen. It forms carboxyhemoglobin, which leads to a reduced oxygen delivery to tissues and can cause tissue hypoxia. 8,12 Also, CO easily binds to cytochrome oxidase and leads to...

We propose a novel approach to improve the gas-sensing properties of n-type nanofibers (NFs) that involves creation of local p-n heterojunctions with p-type reduced graphene oxide (RGO) nanosheets (NSs). This work investigates the sensing behaviors of n-SnO2 NFs loaded with p-RGO NSs as a model system. n-SnO2 NFs demonstrated greatly improved gas-sensing performances when loaded with an optimized amount of p-RGO NSs. Loading an optimized amount of RGOs resulted in a 20-fold higher sensor response than that of pristine SnO2 NFs. The sensing mechanism of monolithic SnO2 NFs is based on the joint effects of modulation of the potential barrier at nanograin boundaries and radial modulation of the electron-depletion layer. In addition to the sensing mechanisms described above, enhanced sensing was obtained for p-RGO NS-loaded SnO2 NFs due to creation of local p-n heterojunctions, which not only provided a potential barrier, but also functioned as a local electron absorption reservoir. These mechanisms markedly increased the resistance of SnO2 NFs, and were the origin of intensified resistance modulation during interaction of analyte gases with preadsorbed oxygen species or with the surfaces and grain boundaries of NFs. The approach used in this work can be used to fabricate sensitive gas sensors based on n-type NFs.

Nanostructured materials have attracted considerable research interest over the recent decades because of their potential applications in nanoengineering and nanotechnology. On the other hand, the developments in nanotechnology are strongly dependent on the availability of new materials with novel and engineered morphologies. Among the novel nanomaterials reported thus far, composite nanofibers (NFs) have attracted considerable attention in recent years. In particular, metal oxide NFs have great potential for the development of gas sensors. Highly sensitive and selective gas sensors can be developed by using composite NFs owing to their large surface area and abundance of grain boundaries. In composite NFs, gas sensing properties can be enhanced greatly by tailoring the conduction channel and surface properties by compositional modifications using the synergistic effects of different materials and forming heterointerfaces. This review focuses on the gas sensing properties of composite NFs synthesized by an electrospinning (ES) method. The synthesis of the composite NFs by the ES method and the sensing mechanisms involved in different types of composite NFs are presented along with the future perspectives of composite NFs.