Cr2O3, WO3 single and Cr/W binary oxide prepared by physical methods for gas sensing applications

Thin films have been prepared by thermal evaporation of high purity Cr2O3, WO3 powders in vacuum on Si/Si3N4 substrates provided with Pt interdigital electrodes and annealed between 300 and 600 °C for different times ranging from 1 to 24 h. Mixed oxides Cr/W films have been prepared by thermal evaporation of Cr2WO6 powders, previously synthesized by solid state reaction at 1200 °C from Cr2O3 and WO3 precursor oxides. The electrical response has been measured exposing the films to sub-ppm NO2 concentrations (100–300 ppb in dry air) at different operating temperatures ranging between 25 and 250 °C. The best response to NO2 has been found to be at 150 °C. Cr/W mixed oxides films have shown enhanced gas sensitivity as respect to WO3 and Cr2O3. Gas selectivity to NO2 in NOX mixtures (10 ppm NO and 0.7 ppm NO2) were also improved by the presence of Cromium.