The detection of NOx in combustion and exhaust gases requires higher selectivity towards NO and NO2 at temperatures exceeding 500 °C. As the gas temperature increases above 500 °C, the thermodynamic equilibrium for NO2/NO indicates that NO-content increases gradually reaching to 100% NO at 800 °C. Most common impurity gases in combustion environment are oxygen, CO and water vapor requiring a low cross-sensitivity in detection of nitrogen oxides.
High temperature stability of TiO2 makes it a promising sensing material for detection of CO, H2 and NO2 at elevated temperatures. While n-type undoped TiO2 yields very good sensor properties towards H2, its doping with suitable trivalent cations increases the sensitivity and selectivity towards nitrogen oxides. This work investigates the NO/NO2-sensing behavior of Cr3+ and Co3+-doped TiO2 layers at temperatures up to 900°C. Cr-doped TiO2 layer is deposited by reactive magnetron sputtering from titanium targets under oxygen/argon flow, while Co-doped TiO2 is synthetized from powders through wet chemistry method. An alteration of semiconductor behavior of TiO2 layers from n- to p-type by Cr-doping and high sensitivity towards NO2 as temperature increases from 400 °C to 800 °C. 2.5% Cr-doped TiO2 forms anatase Phase at heat-treatment temperatures around 700°C, above which the rutile phase formation starts. Whereas, 2% Co-doped TiO2 layers yield fully rutile phase and CoTiO3 phases on heat-treatment at temperatures around 700 °C. The morphology of the sputtered layers alters with temperature, showing some grain formation at 800°C and extreme grain growth at 1000 °C. The layer produced through the powder shows some sintering and partly core-shell strcture although the grain size remains fine. This work compares the sensing behavior of these two differently doped TiO2 layers at high-temperatures towards nitrogen oxides and analyses the sensor properties such as cross-sensitivity towards CO, sensitivity and selectivity changes on introduction of humidity at temperatures exceeding 500 °C.