Cr–Sn oxide thin films: Electrical and spectroscopic characterisation with CO, NO2, NH3 and ethanol

We have studied the electrical responses of Cr–Sn oxide thin films towards CO, NO2, NH3 and ethanol vapours at different temperatures and for different concentrations. Moreover, we employed FT-IR absorbance and UV–vis–NIR diffuse reflectance spectroscopies to study the electronic transitions caused by reducing treatments with ethanol and CO. Conductance measurements, performed with CO, NO2, NH3 and ethanol, show low responses towards CO, NO2 and NH3 but high responses towards ethanol. FT-IR spectroscopy, in qualitative agreement with electrical measurements, gives indications of a lower effect of CO with respect to ethanol in producing electronic absorptions related to surface reduction. Ethanol induces the formation of strong electronic absorptions in the medium IR (MIR) and near IR (NIR) regions of the sample spectra. Furthermore, from the study of vibration modes of the species formed in the gas phase, it was possible to propose surface reactions involved in the sensing mechanism with ethanol.