Keywords: gas sensors, metal oxide, thin films, nanocrystalline sensors, air quality, air monitoring, nanotechnology, air pollution, nitrogen oxides, NOx, carbon monoxide, zinc oxide, tungsten oxide, titanium dioxide, sensor networks, solid state microsensors, microelectronics, thin films
Application of nanocrystalline metal oxide gas sensors for air quality monitoring
Increasing concern regarding health effects caused by air pollution, together with the limited spatial resolution achievable with conventional monitoring stations, have driven efforts to develop inexpensive metal oxide gas sensors for air quality measurements. The sensing mechanism is dependent on physicochemical reactions between adsorbed gas molecules and oxygen species on the oxide surface, which modify the height of the grain boundary potential barrier and, thereby, the electrical resistance of the film. The sensitivity is dependent on the grain size and the influence of film structure on the electrical conductivity. A systematic study of the influence of deposition conditions on the structure of nanocrystalline metal oxide thin films was carried out in an attempt to understand its relation to the conductivity with the aim of improving the sensitivity to common pollutant gases such as nitrogen dioxide and carbon monoxide. Tin dioxide is the most commonly used material for gas sensor applications but also has the greatest cross sensitivity. Cross sensitivity can be reduced by the use of alternative materials such as zinc oxide, tungsten oxide, titanium dioxide or mixed metal oxides. Significant improvements in sensitivity and selectivity can be achieved by doping the oxide with a noble metal such as platinum or palladium to catalyse the reactions with adsorbed gas molecules. Solid state microsensors have been constructed by combining microelectronic fabrication methods with thin film technology. Detection of carbon monoxide, nitrogen oxides and benzene at environmentally relevant concentrations has been demonstrated. Integrated with signal processing and data transmission via the telecommunications infrastructure, sensor networks can provide real time information on air pollution with high spatial resolution.