Sensing gases

   Detecting the presence of toxic gases can be carried out by IR spectroscopic means, but such techniques do not lend themselves to a domestic market. Capitalizing on the ntype semiconducting properties of SnO₂ has led to its use in gas sensors, and sensors that detect gases such as CO, hydrocarbons or solvent (alcohols, ketones, esters, etc.) vapours are commercially available and are now in common use in underground car parking garages, automatic ventilation systems, fire alarms and gas-leak detectors. The presence of even small amounts of the target gases results in a significant increase in the electrical conductivity of SnO₂, and this change is used to provide a measure of the gas concentration, triggering a signal or alarm if a pre-set threshold level is detected. The increase in electrical conductivity arises as follows. Adsorption of oxygen on to an SnO₂ surface draws electrons from the conduction band. The operating temperature of an SnO₂ sensor is 450–750K and in the presence of a reducing gas such as CO or hydrocarbon, the SnO₂ surface loses oxygen and at the same time, electrons return to the conduction band of the bulk solid resulting in an increase in the electrical conductivity. Doping the SnO₂ with Pd or Pt increases the sensitivity of a detector. Tin(IV) oxide sensors play a major role in the commercial market and can be used to detect all the following gases, but other sensor materials include:

• ZnO, Ga₂O₃ and TiO₂/V₂O₅ for CH₄ detection;
•  La₂CuO₄, Cr₂O₃/MgO and Bi₂Fe₄O₉ for C₂H₅OH vapour detection;
• ZnO, Ga₂O₃, ZrO₂ and WO₃ for H₂ detection;
• ZnO, TiO₂ (doped with Al and In) and WO₃ for NO_x;
• ZnO, Ga₂O₃, Co₃O₄ and TiO₂ (doped with Pt) for CO detection;
• ZrO₂ for O₂ detection.