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.