Gas Sensors for Monitoring Air Pollution

The air pollution caused by exhaust gases from automobiles has become a critical issue. In some regions, fossil fuel combustion is a problem as well. The principal gases that cause air pollution from automobiles are nitrogen oxides, NOx (NO and NO2), and carbon monoxide (CO). Because NOx gases with sulfur oxides (SOx) emitted from coal fired plants cause acid rain and global warming and produce ozone (O3) that leads to serious metropolitan smog from photochemical reaction, they must be detected and reduced [1-5]. In addition, as greater amounts of oil organic compounds are currently being produced by applied construction materials and households, the number of people who develop various symptoms after moving into a new apartment (e.g., tickle, vertigo, headache, skin trouble) is increasing [6,7]. The principal gases that cause this phenomenon (called “sick-building syndrome”) are formaldehyde (HCHO) and volatile organic compounds (VOCs) [8]. Especially, formaldehyde is the most dangerous among indoor pollutants as it could harm all kinds of organisms. Considering these, the allowed concentration of formaldehyde in the Netherlands and Germany is only 0.1 ppm [9,10]. Therefore, gas sensors with excellent reactivity and stability are needed. The first decade of the 21st century has been labeled by some as the “Sensor Decade.” A sensor is a device that converts a physical phenomenon into an electrical signal. As such, sensors represent part of the interface between the physical world and the world of electrical devices, such as computers. In recent years, sensors have received people’s attention as one of the important devices in electronic systems and enormous capability for information processing has been developed within the electronics industry. Of all sensors, gas sensors and light sensors have been most actively studied [11-13]. The final goal of gas sensor development is to establish the array technology of multifunctional gas sensors that can monitor air pollution with low cost, and is to fabricate the electronic nose using this technology. Gas sensors are defined as a device that can substitute for human olfaction, and there are many researches being conducted to monitor air pollution by using these gas sensors. Gas sensors can be classified into semiconductor-type, solid electrolyte-type, electrochemicaltype and catalytic combustion-type. Among these, the semiconductor-type gas sensor, the most well-known, is operated by changing its conductivity when it is exposed to gas. The semiconductor-type gas sensor has the advantages of rapid reactivity, efficiency, and gas selectivity when suitable additives are applied to it [14,15]. Sensors made of inorganic materials are the most commonly used, especially ceramics. One reason is that many sensors are used in very severe conditions such as high temperature, reactive or corrosive atmosphere and high humidity, and ceramics are most reliable materials in these conditions. Another reason may be that the microstructure of ceramics can be controlled by process