Semiconductor Metal Oxides as Chemoresistive Sensors for Detecting Volatile Organic Compounds

Abstract

Volatile organic compounds (VOCs), which originate from painting, oil refining and vehicle exhaust emissions, are hazardous gases that have significant effects on air quality and human health. The detection of VOCs is of special importance to environmental safety. Among the various detection methods, chemoresistive semiconductor metal oxide gas sensors are considered to be the most promising technique due to their easy production, low cost and good portability. Sensitivity is an important parameter of gas sensors and is greatly affected by the microstructure, defects, catalyst, heterojunction and humidity. By adjusting the aforementioned factors, the sensitivity of gas sensors can be improved further. In this review, attention will be focused on how to improve the sensitivity of chemoresistive gas sensors towards certain common VOCs with respect to the five factors mentioned above.

Keywords: gas sensor; metal oxide; semiconductor; sensitivity; volatile organic compounds.

Figure 1

A schematic diagram of reaction mechanism of SnO₂-based sensor to HCHO: (a) in air, (b) in VOC. Reprinted from [61] with permission.

Figure 2

Schematic model of the effect of the crystallite size on the sensitivity of semiconductor metal oxide gas sensors: (a) D >> 2 L; (b) D ≥ 2 L; (c) D < 2 L. Reprinted from [28] with permission.

Figure 3

(a) N₂ adsorption-desorption isotherms of Metal-Organic Frameworks-derived Co₃O₄ structures; (b) Responses of CS-, PCS-, PPC-, and NP-Co₃O₄-based sensors at different operating temperature toward 200 ppm acetone; Reprinted from [75] with permission.

Figure 4

(a) SEM image of meso-macroporous SnO₂; (b) Real-time responses of the sensors based on meso-macroporous SnO₂ and traditional SnO₂, respectively. Reprinted from [77] with permission.

Figure 5

(a) Schematic illustration of the contact potential barrier under the electron transfer between two neighboring ZnO nanosheets viewed from the side with less and more defects; (b) The normalized defect content, the oxygen species content and the normalized response to 200 ppm acetone vapor at 300 °C. Reprinted from [78] with permission.

Figure 6

Schematic energy band diagrams of Pt and α-Fe₂O₃ before/after contact. Reprinted from [80] with permission.

Figure 7

Schematic diagram showing the possible band structures at (a) p–n junction; (b) n–n junction. Reprinted from [25] with permission.

Figure 7

Schematic diagram showing the possible band structures at (a) p–n junction; (b) n–n junction. Reprinted from [25] with permission.

Figure 8

(a) Effect of relative humidity on the base line resistance; (b) Schematic representation of water molecule adsorption on ZnO nanotube surface. Reprinted from [83] with permission.

Figure 9

Demonstration of the beer monitoring using the mesoporous semi-blooming SnO₂ nanoflowers-based sensor: (a) the beer was not poured; (b) the beer was poured; (c) the green diode turned on; (d) the red diode turned on. Reprinted from [102] with permission.

Figure 10

(a) Response of the sensor based on ZnO/ZnFe₂O₄ heterostructures to different gases (200 ppm) at different working temperatures; (b) Resistance transient of the sensor towards 20 ppm acetone. Reprinted from [154] with permission.

Figure 11

(a) SEM image of SnO₂ microtubes; (b) Responses of the sensor to 50 ppm CO, C₆H₆, C₆H₇N, NH₃, HCHO, C₃H₆O and C₂H₅OH operated at 92 °C. Reprinted from [168] with permission.

Figure 12

Dynamic toluene sensing transients of SnO₂ products with (a) solid cubes; (b) single-shell structures; (c) yolk-shell structures to toluene with different concentrations. The right insets show the corresponding response time (res) and recovery time (recov) examined to 20 ppm toluene, respectively. Reprinted from [201] with permission.

Figure 13

TEM images of (a) Ag-loaded hierarchical ZnO-reduced graphene oxide hybrid; (b) Ag-loaded ZnO-reduced graphene oxide hybrid; Transient response at different humidity concentrations of (c) Ag-loaded hierarchical ZnO-reduced graphene oxide hybrid; (d) Ag-loaded ZnO-reduced graphene oxide hybrid. Reprinted from [207,215] with permission.