Detection of Different Molecules

The response to hydrocarbons can be explained by the dissociated hydrogen atoms forming a polarized layer at the insulator surface. However, an observed response to carbon monoxide cannot be explained so readily. A careful investigation has been carried out into the response to CO at 600°C by Nakagomi et al. [20]. It was observed that the response to hydrogen and CO showed an additive effect. It was also observed that the gas response to both CO and H was considerably lowered by the presence of water vapor in the atmosphere. Nakagomi suggests three possibilities for the CO response. 

The response to CO in argon may actually be a response to background hydrogen, since the sensor is observed to become extremely sensitive to hydrogen once the CO has consumed all the oxygen atoms on the sensor surface [39]. 

In argon, the CO molecules may simply reduce the oxide layer on the Pt surface, which gives rise to an increased response due to the difference in 

In an oxygen atmosphere CO sometimes gives a direct gas response for a porous metal film. This indicates that the CO molecule may be detected when adsorbed at a site where the dipole moment of CO is able to influence the mobile carriers in the semiconductor. 

The details in the CO response continue to be the subject of many discussions and a full explanation will need further studies. These can include spectroscopy studies such as diffuse reflectance transform infrared spectroscopy (DRIFT), which can be performed under realistic conditions [43,44], and theoretical modeling [45]. However it seems likely that not only hydrogen gives rise to charged or polarized complexes on the insulator surface. Equation (2.3) may now be written as 

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