Gas sensors based on nickel oxide come in various types. The gas sensors can be either optical or electrical. The principle of most gas sensors is that a reaction of the sensor with the gas causes changes in the surface of the sensor, which results in changes of the optical and/or electrical properties.
Optical sensors
In optical sensors, the colour of the sensor varies with the concentration of the detected gas. For instance, Ando et. al. described an optical gas sensor for the detection of CO. [1]
Electrical sensors come in two types. In the first type the resistance of the sensor varies with the concentration of the gas. In the other type the output voltage depends on the concentration. The resistance-type sensors operate due to a reaction of the gas with chemisorbed oxygen at the surface of the sensor. This changes the amount of electrons at the surface and thereby the conductivity. [2] These sensors have to work at higher temperatures ranging from 130°C to several hundred degrees.
The voltage-type sensors use nickel oxide as thermoelectric (TE) material. By applying a temperature gradient to a thermoelectric material, a small voltage is generated. This temperature gradient can be provided by a chemical reaction at one side of the TE material.
Figure 1. Principle of a thermoelectric gas sensor. Adapted from [3].
For instance, by coating the nickel oxide with Pt, hydrogen gas can be detected, since platinum catalyses the combustion of hydrogen already at room temperature. When H2 flows over the Pt surface, the combustion reaction generates heat and thus a voltage. With other catalysts, different gases may be detected in this way.
Figure 2. Example of H2 gas sensor. Adapted from [4].
Figure 3. Temperature difference and Voltage of the H2 gas sensor as a function in time. Adapted from [5].
by students.chem.tue.nl
