In recent years, gas sensors have been increasingly used in industrial production and daily life. Metal oxide semiconductor gas sensing materials are favoured for their outstanding physical and chemical properties, low cost and simple preparation methods. However, the gas sensing mechanisms of metal oxide semiconductors have not been considered by researchers, resulting in omissions and errors in the interpretation of gas sensing mechanisms in many articles. This review organizes and introduces several common gas sensing mechanisms of metal oxide semiconductors in detail and classifies them into two categories. The scope and relationship of these mechanisms are clarified. In addition, this review selects four strategies for enhancing the gas sensing properties of metal oxide semiconductors and analyses the gas sensing mechanisms to highlight the importance of the gas sensing mechanism. Finally, some perspectives for future investigations on the gas sensing mechanisms of metal oxide semiconductors are discussed as well.

Gas sensing mechanisms of metal oxide semiconductors: a focus review

The properties and applications of molybdenum oxides are reviewed in depth. Molybdenum is found in various oxide stoichiometries, which have been employed for different high-value research and commercial applications. The great chemical and physical characteristics of molybdenum oxides make them versatile and highly tunable for incorporation in optical, electronic, catalytic, bio, and energy systems. Variations in the oxidation states allow manipulation of the crystal structure, morphology, oxygen vacancies, and dopants, to control and engineer electronic states. Despite this overwhelming functionality and potential, a definitive resource on molybdenum oxide is still unavailable. The aim here is to provide such a resource, while presenting an insightful outlook into future prospective applications for molybdenum oxides.

Molybdenum Oxides – From Fundamentals to Functionality

https://www.econstor.eu/bitstream/10419/244006/1/1693663112.pdf

Enhanced sensing performance of ZnO nanostructures-based gas sensors: A review

Metal oxide semiconductor (MOS) gas sensors possess extensive applications due to their high sensitivity, low cost, and simplicity To boost their excellent sensing performance and meet the growing demand for applications, a series of strategies have been developed, such as the surface morphology engineering and function manipulation Recently, the controlled morphology with exposed high-energy facets and the facet-dependent sensing properties have attracted much attention Because of its abundant unsaturated active sites, the crystal planes with high surface energy usually serve as promising platform for gas sensing After a lot of survey of literature, the authors provide a review of recent efforts on engineering crystal structures with exposed high-energy facets of MOS nanomaterials and their improved gas-sensitive performance, emphasis on six kinds of common gas-sensitive MOS including ZnO, SnO2, TiO2, α-Fe2O3, NiO and Cu2O Also, the relationship between dangling bonds density and gas-sensing properties has been systematically discussed and used as one significant factor to evaluate superior sensing surface of MOS According to the research and calculation, surface engineering by selectively exposing high-energy facets provides an effective way to obtain MOS gas-sensitive materials with superior performance The understanding of the facet-dependent properties of MOS will assist in and guide the fabrication of more excellent gas sensors in the future

An overview: Facet-dependent metal oxide semiconductor gas sensors

The alarming rise of indoor pollution and the need to combat the associated negative effects have promoted increasing attention in modernizing the chemical sensing technologies by newly designed materials with rich and tunable functionalities at atomic or molecular levels. With the appealing physical, chemical, optical, and electronic properties for various potential applications, the state-of-art gas-sensing nanomaterials and their future perspectives are well-documented and summarized in this paper. Specifically, the key performance attributes are addressed in detail such as the sensitivity, selectivity, reversibility, operating temperature, response time, and detection limit. As such, this review provides both critical insights in exploring and understanding various gas sensing nanomaterials and points out limitations and opportunities for further developments, such as morphology control, doping and surface alteration, atomic-scale characterization, and applications in different fields. Finally, the challenges and outlooks are discussed on the basis of the current developments.

Functional gas sensing nanomaterials: A panoramic view

In this paper, an ultra-high response and fast response/recovery triethylamine (TEA) gas sensor is fabricated by growing Ag nanoparticles on the surface of hierarchical 3D porous ZnO microspheres (Ag-ZnO) via a green and facile precipitation method. The morphological characterizations reveal that the hierarchical 3D ZnO microspheres are assembled by large numbers of thin and porous nanosheets. The sizes of the microspheres are 3–5 μm and the thickness of these porous nanosheets is about 20 nm. Gas sensing measurements show that the Ag-ZnO gas sensor exhibits superior TEA sensing performance. Its response to 100 ppm TEA is 6043, which is 14.8 times higher than that of pure ZnO sensor. Besides, its optimal working temperature decreases from 230 to 183.5 °C compared with the pure one. And the Ag-ZnO gas sensor also exhibits fast response/recovery rate (about 1 s and 60 s, respectively). Furthermore, the Ag-ZnO gas sensor shows no obvious degradation through a testing period of 30 days. The superior TEA sensing performances could be attributed to the structural advantage of hierarchical 3D porous microsphere, as well as the synergistic effects of Ag nanoparticles and ZnO microspheres.

On the high response towards TEA of gas sensors based on Ag-loaded 3D porous ZnO microspheres

High sensitive and fast formaldehyde gas sensor based on Ag-doped LaFeO3 nanofibers

High performance humidity sensor based on metal organic framework MIL-101(Cr) nanoparticles

In this work, α-MoO 3 and Ni-doped α-MoO 3 were synthesized through a facile solvothermal method and various techniques were employed to study the phase and morphological properties. The gas sensing measurements showed that 5 mol% Ni-doped α-MoO 3 had superior formaldehyde sensing capability compared with pure α-MoO 3 . The maximum response value approached 41–100 ppm formaldehyde at 255 °C, which was about 4.1 times bigger than that of pure α-MoO 3 . In addition, the 5 mol% Ni-doped α-MoO 3 gas sensor was also found to have lower detection limit and better selectivity to formaldehyde. The mechanism involved in gas sensing performance of Ni-doped α-MoO 3 was also discussed.

Chuan Chen

Papers

On the high response towards TEA of gas sensors based on Ag-loaded 3D porous ZnO microspheres

High sensitive and fast formaldehyde gas sensor based on Ag-doped LaFeO3 nanofibers

High performance humidity sensor based on metal organic framework MIL-101(Cr) nanoparticles

Synthesis of Ni-doped α-MoO3 nanolamella and their improved gas sensing properties

One-step synthesis and gas sensing properties of hierarchical Fe doped Co3O4 nanostructures