Semiconducting, nanoporous metal oxides are particulary interesting for application as gas sensors. Large surface-to-volume ratios and uniform porosity play important roles in this field of research. Semiconducting metal oxides, such as SnO 2, ZnO, WO 3, or In 2 O 3, are frequently utilized as gas sensors (so-called chemiresistors), e.g., for the detection of hazardous gases in factory plants, automobile emission, or air control in living spaces. The underlying principle is the chemical interaction of the gas species with the surface of the sensing material which results in changes of the electronic conductivity in the surface- near regions. Hence, a large surface-to-volume ratio is naturally a prerequisite for high sensitivity (i.e., change in conductance) of the sensor. This is one reason why porosity is particularly desired, and a variety of chemical synthesis methods is nowadays available for the generation of porous metal oxides, including sol-gel syntheses, chemical vapor deposition, spray pyrolysis, or precipitation. Apart from large specific surface areas, porosity offers an additional chance for improved gas-sensing properties, as the diffusion of the respective gas molecules is strongly correlated with the pore size and pore architecture. In particular, gas transport in nanoporous materials (i.e., with pore widths of a few nanometers) is basically governed by Knudsen diffusion, with the diffusion coefficient scaling linearly with the pore size. This is why the gas-sensing properties of porous materials, especially gas selectivity, can be substantially improved by creating uniform pore sizes with deliberate control of porosity by means of chemical synthesis.

Three-electrode electrochemical sensor

Figure 1 - Toxic Gas Sensor

Sensing: CO + H 2 O CO 2 + 2H + + 2e - Counter: ½O 2 + 2H + + 2e - H 2 O And the overall reaction is: CO + ½O 2 CO 2

Figure 1. The effect of particle size on gas sensitivity for an SnO 2 oxide sensor exposed to CO and H 2 gases Figure 1. The effect of particle size on gas sensitivity for an SnO 2 oxide sensor exposed to CO and H 2 gases Figure 2. The effect of In 2 O 3 grain size on sensor sensitivity to 1.0 ppm NO 2 at 250 � C. 13 MATERIALS ISSUES IN SEMICONDUCTOR OXIDE SENSORS