Detection of toxic gases such as hydrogen sulfide (H2S) at low temperatures is of high importance as it reduces power consumption, increases long-term stability, and lowers the danger of explosion in the presence of flammable gases. Among different gas sensors, chemiresistive metal oxide semiconductors sensors are the most popular type due to their low cost, high sensitivity, compact size, ease of use, and high thermal and mechanical stability. However, these sensors generally require a high operating temperature (>100 °C), attributed to their insufficient surface area. Hence, their pristine bulk structures cannot meet the requirements of low-temperature gas sensing. To overcome these limitations, the bulk structure of metal oxide sensors can be modified either geometrically or chemically. Herein, we comprehensively reviewed a wide range of metal oxide-based sensors for low-temperature H2S gas sensing. We highlighted the recent advances in the modifications of these sensors for enhanced gas sensing properties, such as high selectivity/sensitivity and fast response/recovery time.

A review of low-temperature H2S gas sensors: fabrication and mechanism

Upgrading the gas sensing performance of metal oxide semiconductor (MOS) to meet the practical demands of MOS sensor for detecting various harmful gases is a significant issue in the field of environmental protection. Herein, we propose a sensitization strategy to improve the gas sensing performance of ZnO by in-situ construction of ZnS-ZnO n-n heterojunction. To expound this strategy, nanoscale ZnS-ZnO heterojunctions were homogeneously modified on self-synthesized ZnO porous hollow spheres (PHSs) by sulfurating them under hydrothermal condition. The achieved hybrid ZnS/ZnO PHSs are about 1–1.5 µm in diameter, whose shell is assembled by secondary nanoparticles with the size of 40–70 nm. The results of gas sensing tests indicated that after modification with ZnS-ZnO heterojunctions, the gas sensing performances of the ZnO PHS sensor to triethylamine (TEA) were remarkably improved, especially in terms of faster response speed (4 s for 100 ppm TEA), higher sensitivity (0.520/ppm for 1–30 ppm TEA) and better selectivity (S TEA/acetone = 8.8). The ZnS-ZnO heterojunction-related sensitization mechanism was discussed in detail. • In situ ZnS-ZnO nanoheterojunctions were modified on ZnO PNSs. • The ZnS/ZnO PNS sensor shows boosted gas sensing performances to TEA. • The sensitization effects of ZnS-ZnO heterojunction on ZnO were discussed. 

Boosting TEA sensing performance of ZnO porous hollow spheres via in situ construction of ZnS-ZnO heterojunction

Metal-organic frameworks (MOFs), a newer class of crystalline nanoporous materials, have been in the limelight owing to their exceptional tunability for structures and physicochemical properties, and have found successful applications in gas storage, gas separation, and catalysis. The mesmerizing properties of MOFs, especially the extensive and tunable porosity and chemical selectivity, also make them an excellent candidate class as chemo-sensory materials. Moreover, MOF-based sensors have attracted considerable attention in the past decade. Recent literature reviews focused on the progress of MOF-based electronic sensors and luminescent MOF sensors, while sensors exploiting the dielectric properties (refractive index) of MOFs were also demonstrated and discussed very recently. The motivation of this report is to provide, for the first time, a general review on such MOF sensors with a particular focus on miniature optical fiber (OF) based MOF sensors and to demonstrate the promising potential of MOFs as dielectric coatings on OF for highly sensitive chemical sensing. The fundamental principle of OF-MOF sensors relies on the tunability of the refractive index of a MOF, which is dependent on the amount and type of adsorbed guest molecules in the MOF pores. MOF sensors based on different optical sensing principles are reviewed; challenges and perspectives on further research into the field of OF-MOF sensors are also discussed.

Metal-Organic Framework Materials Coupled to Optical Fibers for Chemical Sensing: A Review

Single-wall carbon nanotubes (SWCNTs) have a high aspect ratio, large surface area, good stability and unique metallic or semiconducting electrical conductivity, they are therefore considered a promising candidate for the fabrication of flexible gas sensors that are expected to be used in the Internet of Things and various portable and wearable electronics. In this review, we first introduce the sensing mechanism of SWCNTs and the typical structure and key parameters of SWCNT-based gas sensors. We then summarize research progress on the design, fabrication, and performance of SWCNT-based gas sensors. Finally, the principles and possible approaches to further improving the performance of SWCNT-based gas sensors are discussed.

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Gas Sensors Based on Single-Wall Carbon Nanotubes

Transition metals Fe3+, Ni2+ modified titanium dioxide (TiO2) film sensors fabricated by CPT method to sense some toxic environmental pollutant gases

Gas sensor technology is widely utilized in various areas ranging from home security, environment and air pollution, to industrial production. It also hold great promise in non-invasive exhaled breath detection and an essential device in future internet of things. The past decade has witnessed giant advance in both fundamental research and industrial development of gas sensors, yet current efforts are being explored to achieve better selectivity, higher sensitivity and lower power consumption. The sensing layer in gas sensors have attracted dominant attention in the past research. In addition to the conventional metal oxide semiconductors, emerging nanocomposites and graphene-like two-dimensional materials also have drawn considerable research interest. This inspires us to organize this comprehensive 2020 gas sensing materials roadmap to discuss the current status, state-of-the-art progress, and present and future challenges in various materials that is potentially useful for gas sensors.

Yude Wang

Papers

Gas sensing materials roadmap.