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Review
. 2010;10(3):2088-106.
doi: 10.3390/s100302088. Epub 2010 Mar 15.

Metal oxide gas sensors: sensitivity and influencing factors

Chengxiang Wang  1 , Longwei Yin, Luyuan Zhang, Dong Xiang, Rui Gao
Affiliations
Review

Metal oxide gas sensors: sensitivity and influencing factors

Chengxiang Wang et al. Sensors (Basel). 2010.

Abstract

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.

Keywords: gas sensors; metal oxide; sensitivity; surface reaction.

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Figures

Figure 1.
Figure 1.
Schematic diagram of band bending after chemisorptions of charged species (here the ionosorption of oxygen) EC, EV, and EF denote the energy of the conduction band, valence band, and the Fermi level, respectively, while Λair denotes the thickness of the space-charge layer, and eVsurface denotes the potential barrier. The conducting electrons are represented by e− and + represents the donor sites (adapted from [23]).
Figure 2.
Figure 2.
Structural and band models of conductive mechanism upon exposure to reference gas. (a) with or (b) without CO (adapted from [23,24]).
Figure 3.
Figure 3.
The response of single oxide and composite sensors to 5 ppm ethanol vapour at 100% RH (adapted from [32]).
Figure 4.
Figure 4.
Temperature dependence of CO sensitivity (200 ppm) of SnO2, ZnO and ZnO-SnO2 composites. 20 ZT means 20 mol% ZnO–80 mol% SnO2 sample and others are the similar (adapted from [26]).
Figure 5.
Figure 5.
Two representative TEM photographs of Pd particles loaded on the surface of SnO2 (adapted from [40]).
Figure 6.
Figure 6.
(a) Schematic depiction of the major process taking place at a SnO2 nanowire/nanobelt surface when exposured to O2. (b) Band diagram of the pristine SnO2 nanostructure and in the vicinity (and beneath) a Pd nanoparticle. The radius of the depletion region is determined by the radius of the spillover zone (adapted from [46]).
Figure 7.
Figure 7.
Effect of particle size on gas sensitivity for CO (adapted from [50]).
Figure 8.
Figure 8.
Variation in sensitivity with average particle size (adapted from [51]).
Figure 9.
Figure 9.
Typical SEM images of the as-prepared ZnSnO3 products: (a) CTAB = 0.15 M; (b) CTAB = 0.4 M; (c) CTAB = 0.75 M. (d) The corresponding XRD patterns of the as-prepared ZnSnO3 polyhedra, A: octahedra; B: truncated octahedra; C: 14-faceted polyhedra (adapted from [60]).
Figure 10.
Figure 10.
Typical response curves of 14-faceted polyhedral (line 1) and octahedral (line 2) ZnSnO3 and ZnSnO3 powder (line 3) gas sensors to (a) H2S, (b) HCHO, (c) C2H5OH with increasing concentrations. (d) The response curves of 14-faceted polyhedra (line1) and octahedra (line 2) to H2S (70 ppm) at room temperature (adapted from [60]).
Figure 11.
Figure 11.
Correlation between sensing temperature and sensitivity to 1,000 ppm H2 of different types of SnO2 gas sensors (adapted from [65]).
Figure 12.
Figure 12.
Gas sensing mechanism of Sm2O3-doped SnO2 in the atmosphere of (a) C2H2 and (b) C2H2 and humidity (adapted from [74]).
Figure 13.
Figure 13.
Response of the Sm2O3-doped SnO2 sensor to different concentrations of C2H2 at different RH (adapted from [72]).
Figure 14.
Figure 14.
Gas response versus operating temperature of porous ZnO nanoplate sensor to 100 ppm chlorobenzene and ethanol (adapted from [76]).
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