General information on chemical gas sensors

Chemical (gas) sensors based on semiconducting metal oxides (SMOX) are currently one of the most investigated groups of gas sensors. They have attracted the attention of many users and scientists interested in gas sensing under atmospheric conditions due to the: low cost and flexibility associated to their production; the simplicity of their use; the large number of detectable gases/possible application fields. The state of the art sensors are realized by depositing a SMOX poly-crystalline, thick, porous film over a substrate provided with electrodes and a heater. The former are used for the readout of the resistance of the sensing layer, which depends on the composition of the ambient atmosphere. The latter allows for heating at a temperature in the range between 200 to 400°C, which is needed to speed up the surface reactions and minimize the influence of the humidity. In most cases, minute quantities of noble metal additives (Pt, Pd and Au) are added at the surface in order to tune the selectivity, lower the operation temperature and improve the response time. In air, at the surface of the SMOX, in the case of a n-type conduction, the ionosorption of oxygen decreases the concentration of the free charge carriers, which are trapped at the surface; this is causing an overall increase of the sensor resistance. In the case in which a reducing gas (e.g. CO) appears in the atmosphere, its reaction with the pre-adsorbed oxygen decreases the negative surface charge with an overall effect of sensor resistance decrease. A cartoon representation of the described surface phenomena is shown in Figure 1. The dependence of the sensor’s resistance/conductance on the concentration of the target gas is not linear, the reason being the way in which the charge transfer processes associated to the surface reaction is translated into a change of the concentration of free charge carriers taking part to the conduction in the sensing layer; an example is provided in Figure 2.