Cr-doped TiO₂ gas sensor for exhaust NO₂ monitoring

A set of Cr-highly doped TiO₂ samples with Cr contents ranging from 5 to 30 at.% were prepared in a sol-gel route and calcined at a temperature between 600 and 900 °C. X-ray diffraction (XRD) analyses revealed the persistence of anatase phase up to the calcination temperature of 700 °C in all samples, above which rutile phase became dominant. The segregation of Cr₂O₃ remained modest, only detectable by surface-sensitive technique like X-ray photoelectron spectra (XPS), for the 5 and 10 at.% Cr-doped samples calcined at 600 or 700 °C, suggesting incorporation of major part of doped Cr in the lattice of TiO₂. Higher calcination temperatures or higher Cr contents lead to marked segregation of Cr₂O₃. XPS spectra in the valence band region of the samples calcined at 600 °C revealed a shift of the binding energy (BE) at the band edge to the lower energy side with increasing Cr contents, suggesting a tendency for the electronic conduction to alter from n- to p-type. As tested preliminarily, the thick and thin film devices prepared with these samples exhibited p-type conduction, and, particularly, a thin film device using 10 at.% Cr-doped sample calcined at 600 °C proved promising performances in the detection of dilute NO₂ in air at 500 °C.