Resistance-based H2S gas sensors using metal oxide nanostructures: A review of recent advances.

Microwave (MW) irradiation has obtained extensive importance in the field of synthesis and treatment of nanoparticles, because of its faster, cleaner and cost effectiveness than the other conventional and wet chemical methods. In this study, ZnO/graphene nanocomposites were prepared and subsequently post-treated by MW irradiation. Responses of the MW irradiated ZnO/graphene nanocomposites sensors were tested towards various gases including NO2, ethanol, acetone, toluene and CO and the results were compared with those of pristine ZnO and ZnO/graphene sensors without MW irradiation. It was demonstrated that the MW irradiated sensor had much higher response particularly to NO2 gas along with superior selectivity and shorter response/recovery times in comparison to unirradiated ZnO/graphene and pristine ZnO sensors. The possible underlying mechanism of this behavior is discussed in detail, mainly in terms of the MW-induced surface defects and the generation of finer ZnO nanoparticles. The results obtained demonstrated the beneficial effect of MW irradiation for enhancing the NO2-gas sensing behavior of ZnO/graphene nanocomposites, opening a new door not only to a novel synthesis of semiconductors/graphene nanocomposites, but also to a cost-effective way of improving their sensing capabilities.

Synthesis of zinc oxide semiconductors-graphene nanocomposites by microwave irradiation for application to gas sensors

Although the employment of n-p heterojunctions is among the most popular strategies to increase the performance of gas sensors, there have been a few systematic studies to determine the optimal composition in n-p heterojunctions. This paper reports the results of a systematic study of (n) xSnO 2 -(p) (1-x) Co 3 O 4 composite nanofibers (NFs) for gas sensing applications. Composite NFs were synthesized by the electrospinning method followed by annealing at 600 °C. For gas sensing studies, several gases at optimal working temperature (350 °C) were tested. Depending on the nominal composition, the sensors showed either n-or p-type behavior as well as different responses to the target gases. Furthermore, for all gases tested, the 0·5SnO 2 -0·5Co 3 O 4 sensor (nominal composition) showed the best gas sensing characteristics. The underlying gas sensing mechanism was examined in detail. The highest response observed in the 0·5SnO 2 -0·5Co 3 O 4 NFs sensor was primarily attributed to the major role of the p-Co 3 O 4 nanograins as electron reservoir. In addition, the possible substitution of Co +2 /Co +3 in Sn +4 sites, the catalytic effect of Co 3 O 4 and generation of defects were likely to be the secondary reasons. This highlights the importance of the optimal composition for achieving the maximum gas-sensing performance in n-p composite NFs.

Optimization and gas sensing mechanism of n-SnO2-p-Co3O4 composite nanofibers

In the present study, ZnO nanofibers (NFs) were synthesized by the simple electrospinning technique for gas sensing studies. ZnO NFs were irradiated with a high-energy (1 MeV) electron beam (e-beam) at different doses (50, 100, and 150 kGy) to study the effect of the e-beam dose on the sensing performance of the synthesized ZnO NFs. H2 sensing studies showed that the sensing properties of the unirradiated and 50 kGy-irradiated sensors were similar, which indicates that this e-beam dose was insufficient. However, the sensing characteristics improved with an increase in the irradiation dose to 100 and 150 kGy. The response of the optimal sensor (150-kGy-irradiated) to 10 ppm H2 was much higher than that to other (interfering) gases (e.g., C2H5OH, C6H6, C7H8, and CO). The observed high gas response of the 150 kGy-irradiated sensor was attributed to its high surface area resulting from the one-dimensional nature of the ZnO NFs, the grain size of ZnO, and the formation of surface defects by e-beam irradiation. The high selectivity of the ZnO NFs toward H2 gas was related mainly to the metallization of ZnO and the concentration gradient of carfbon across the NF surfaces. Overall, the findings demonstrate the effectiveness of high-energy irradiation in enhancing the sensing performance of ZnO NFs. We believe that this approach can be extended to other metal oxides for the enhancement of sensing performance.

Design of supersensitive and selective ZnO-nanofiber-based sensors for H2 gas sensing by electron-beam irradiation

Acetone is a well-known volatile organic compound that is widely used in different industrial and domestic areas. However, it can have dangerous effects on human life and health. Thus, the realization of sensitive and selective sensors for recognition of acetone is highly important. Among different gas sensors, resistive gas sensors based on nanostructured metal oxide with high surface area, have been widely reported for successful detection of acetone gas, owing to their high sensitivity, fast dynamics, high stability, and low price. Herein, we discuss different aspects of metal oxide-based acetone gas sensors in pristine, composite, doped, and noble metal functionalized forms. Gas sensing mechanisms are also discussed. This review is an informative document for those who are working in the field of gas sensors.

https://www.mdpi.com/1424-8220/20/11/3096/pdf

G. Neri

Papers

Highly stable and selective ethanol sensor based on α-Fe2O3 nanoparticles prepared by Pechini sol–gel method