FT-IR and UV-Vis-NIR characterisation of pure and mixed MoO3 and WO3 thin films for gas sensors

We report a spectroscopic characterisation of MoO3, WO3 and a MoO3–WO3 mixed oxide thin films deposited on alumina and silicon substrates. Absorbance FT-IR and diffuse reflectance UV-Vis-NIR spectra were recorded after treatments in vacuum and after interaction with O2, NO2/O2, CO/O2 or pure CO at increasing temperatures up to 673 K. For all the films, reducing treatments (vacuum, CO or CO/O2) cause the increase of a variety of broad absorptions both in the Vis-NIR and medium IR regions. These absorptions decrease in intensity after contact with oxidising atmospheres (O2 or NO2/O2), so that they are all assignable to electronic transitions due to the presence of a variety of donor levels related to oxygen defects.