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Conductometric semiconducting metal oxide gas sensors have been widely used and investigated in the detection of gases. Investigations have indicated that the gas sensing process is strongly related to surface reactions, so one of the important parameters of gas sensors, the sensitivity of the metal oxide based materials, will change with the factors influencing the surface reactions, such as chemical components, surface-modification and microstructures of sensing layers, temperature and humidity. In this brief review, attention will be focused on changes of sensitivity of conductometric semiconducting metal oxide gas sensors due to the five factors mentioned above.

Metal oxide gas sensors: Sensitivity and influencing factors

High-performance gas sensors prepared using p-type oxide semiconductors such as NiO, CuO, Cr2O3, Co3O4, and Mn3O4 were reviewed. The ionized adsorption of oxygen on p-type oxide semiconductors leads to the formation of hole-accumulation layers (HALs), and conduction occurs mainly along the near-surface HAL. Thus, the chemoresistive variations of undoped p-type oxide semiconductors are lower than those induced at the electron-depletion layers of n-type oxide semiconductors. However, highly sensitive and selective p-type oxide-semiconductor-based gas sensors can be designed either by controlling the carrier concentration through aliovalent doping or by promoting the sensing reaction of a specific gas through doping/loading the sensor material with oxide or noble metal catalysts. The junction between p- and n-type oxide semiconductors fabricated with different contact configurations can provide new strategies for designing gas sensors. p-Type oxide semiconductors with distinctive surface reactivity and oxygen adsorption are also advantageous for enhancing gas selectivity, decreasing the humidity dependence of sensor signals to negligible levels, and improving recovery speed. Accordingly, p-type oxide semiconductors are excellent materials not only for fabricating highly sensitive and selective gas sensors but also valuable additives that provide new functionality in gas sensors, which will enable the development of high-performance gas sensors.

Highly sensitive and selective gas sensors using p-type oxide semiconductors: Overview

The paper critically reviews the state of the art in the field of experimental techniques possible to be applied to the study of conductometric gas sensors based on semiconducting metal oxides. The used assessment criteria are subordinated to the proposed R&D approach, which focuses on the study, and subsequent modelling, of sensors’ performance in realistic operation conditions by means of a combination of phenomenological and spectroscopic techniques. With this viewpoint, the paper presents both the to-date achievements and shortcomings of different experimental techniques, describes – by using selected examples – how the proposed approach can be used and proposes a set of objectives for the near future.

Metal oxide-based gas sensor research: How to?

This review paper encompasses a detailed study of semiconductor metal oxide (SMO) gas sensors. It provides for a detailed comparison of SMO gas sensors with other gas sensors, especially for ammonia gas sensing. Different parameters which affect the performance (sensitivity, selectivity and stability) of SMO gas sensors are discussed here under. This paper also gives an insight about the dopant or impurity induced variations in the SMO materials used for gas sensing. It is concluded that dopants enhance the properties of SMOs for gas sensing applications by changing their microstructure and morphology, activation energy, electronic structure or band gap of the metal oxides. In some cases, dopants create defects in SMOs by generating oxygen vacancy or by forming solid solutions. These defects enhance the gas sensing properties. Different nanostructures (nanowires, nanotubes, heterojunctions), other than nanopowders have also been studied in this review. At the end, examples of SMOs are given to illustrate the potential use of different SMO materials for gas sensing.

Semiconductor metal oxide gas sensors: A review

XPS study of the L-CVD deposited SnO2 thin films exposed to oxygen and hydrogen

Water–oxygen interplay on tin dioxide surface: Implication on gas sensing

The fundamental mission of materials science is the description of matter over all length scales. In this review, we apply this concept to particle research. Based on metal oxides, we show that every size range offers its specific features, and every size range had its era, when it was in the center of the research activities. In the first part of the review, we discuss on three metal oxides as examples, how and why the research focus changed its targeted size regime from the micrometer to the nanometer scale and back to the macroscopic world. Next, we present the distinct advantages of using nanoparticles over micrometer-sized particles in selected devices and we point out how such a shift in the size regime opens up new research directions. Finally, we exemplify the methods to introduce nanoparticles into macroscopic objects to make functional ceramics.

25th Anniversary Article: Metal Oxide Particles in Materials Science: Addressing All Length Scales

A successful strategy to obtain self-supporting (100 μm), UV-absorbing, and, in the visible region, highly transparent TiO2−poly(methyl methacrylate) (PMMA) films was developed. The 15 nm large anatase TiO2 nanocrystals were prepared in a nonaqueous sol−gel approach involving the mixing of Ti(OiPr)4 and benzyl alcohol. The surfaces of the resulting particles were modified with minute amounts of organic ligands in order to make the particles easily dispersible in nonpolar media like xylene and dichloromethane and compatible with PMMA, a polymer of high optical transparency and considerable technical importance. The empirical optimization process of composite fabrication was supplemented by fundamental studies of the crystallization and growth mechanism of anatase particles in a nonaqueous medium. After the preparation of corresponding nanocomposites, the materials were investigated with respect to their UV absorption capability, optical transparency in the visible-wavelength region, and photodegradation.

Dorota Koziej

Papers

Metal oxide-based gas sensor research: How to?

XPS study of the L-CVD deposited SnO2 thin films exposed to oxygen and hydrogen

Water–oxygen interplay on tin dioxide surface: Implication on gas sensing

25th Anniversary Article: Metal Oxide Particles in Materials Science: Addressing All Length Scales

Nonaqueous TiO2 Nanoparticle Synthesis: a Versatile Basis for the Fabrication of Self-Supporting, Transparent, and UV-Absorbing Composite Films