In a combined approach toward the optimization of chemical gas sensors, Fourier transform infrared spectroscopy is
used to investigate in situ the surface reactions taking place at the surface of semiconductor nanoparticles and to
simultaneously monitor the variations of the free-carrier density. The correlation between the surface reactions and the
changes in the infrared absorbance under gas adsorption/desorption cycles gives information on the chemical phenomena
responsible for electrical conductivity variations and therefore for the gas detection. Interaction of CO and NOX with tin
oxide nanoparticles is presented and discussed. While the chemical reactions leading to the increase of the electrical
conductivity under CO adsorption are relatively straightforward, the adsorption of NOX is much more complex. It is
demonstrated that, although generating a strong increase of the electrical conductivity, the NOX adsorption on a fresh tin
oxide surface is not fully reversible and actually poisons the surface. Subsequent NOX adsorptions lead to reversible
chemical reactions even though the electrical response of the sensor is weaker.
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