Metal oxide gas sensors: sensitivity and influencing factors

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.

Figure 1.

Schematic diagram of band bending after chemisorptions of charged species (here the ionosorption of oxygen) E_C, E_V, and E_F 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 eV_surface denotes the potential barrier. The conducting electrons are represented by e⁻ and + represents the donor sites (adapted from [23]).

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.

The response of single oxide and composite sensors to 5 ppm ethanol vapour at 100% RH (adapted from [32]).

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.

Two representative TEM photographs of Pd particles loaded on the surface of SnO2 (adapted from [40]).

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.

Effect of particle size on gas sensitivity for CO (adapted from [50]).

Figure 8.

Variation in sensitivity with average particle size (adapted from [51]).

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.

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.

Correlation between sensing temperature and sensitivity to 1,000 ppm H₂ of different types of SnO₂ gas sensors (adapted from [65]).

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.

Response of the Sm₂O₃-doped SnO₂ sensor to different concentrations of C₂H₂ at different RH (adapted from [72]).

Figure 14.

Gas response versus operating temperature of porous ZnO nanoplate sensor to 100 ppm chlorobenzene and ethanol (adapted from [76]).