Effect of Al doping on NO2 gas sensing of TiO2 at elevated temperatures

Undoped and Al-doped TiO2 layers were deposited by reactive magnetron sputtering using metallic targets and by adding oxygen to alumina sensor substrates that had previously been deposited with platinum-interdigitated circuits. The sensor behavior was investigated with the aim of determining the NO2 detection ability at temperatures between 400 °C and 800 °C. Morphological changes at elevated temperatures were recorded by scanning electron microscopy (SEM). Thin sensor layers were calcined at various temperatures between 500 °C and 1000 °C and analyzed by X-ray diffraction (XRD) to explore the anatase to rutile phase transformation. The lattice parameters, phase, and bonding constituents of the films were studied by Rietveld refinement, and the Al distribution was studied by ATEM mapping. All investigated layers consist of anatase phase on deposition by sputtering and convert to rutile on annealing at temperatures exceeding 600 °C. The sensor response of the undoped and Al-doped TiO2 sensors toward NO2 was investigated in the temperature range of 400-800 °C for concentrations varying from 50 to 100 and 200 ppm. NO2/NO thermodynamic equilibrium conditions at 600 °C and 800 °C were analyzed. The response of Al-doped TiO2 sensor layers toward NO was tested in synthetic air. The observed sensing behavior toward NO2 in the temperature range of 400-800 °C was evaluated using these analyses and compared with that of undoped TiO2 to define the effect of Al on the NO2 sensing of TiO2.