Nanoionic Materials as Gas Sensors

Although the fact that significant changes occurred in the resistance of a semiconductor when a gas was adsorbed onto its surface was first reported over 55 years ago [47], the use of a metal oxide semiconductor was not suggested until 1962 [48]. Since that time, many commercial devices exploiting this phenomenon have been developed [49], and whilst several different oxides [47, 50-58] have been investigated in the role, tin oxide has been the most widely used. 

It can be seen from the above discussion that the use of nanoionic materials for gas sensors is a natural extension to the findings already reported. Nanosized materials offer advantages in terms of improved sensor response due to the much higher surface areas available. However, the definition of a nano ionic sensor material can be very broad the nanoionic component of the sensor material might refer to the bulk majority phase, but alternatively it could refer to a dispersed catalytic or dopant phase, or even a combination of both. 

The resistance of the nanocomposite film was also greater than that of the pure tin oxide film, most likely because the presence of the CNTs had led to the introduction of pores. Nonetheless, this illustrates the potential of these nanocomposite-type materials - where both phases are nanocrystalline - in the field of gas sensors. 

Although the above-described examples have all been based on relatively simple nanocrystalline metal oxides, additional phases might have been introduced. The use of more complex metal oxides has also been investigated, with nanocrystalline thick films of both barium titanate [96] and cobalt titanate [97] having been considered as possible sensor materials. When the response of such barium titanate films doped with 10% CuO and 10% CdO was studied with respect to CO, LPG, H2S, and H2 [96], sensor selectivity was improved for LPG over the other gases at 250 °C. However, the addition of 0.3 wt% Pd resulted in an even greater selectivity to LPG at a lower temperature, of 225 °C. 

Finally, complex nanocrystalline oxide-based materials such as Gdo gSro jCoO [98] 

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