Iridium oxide films as propane sensors

Iridium oxide thin films were deposited on glass substrates by using the spin-coating method from an isopropanol solution. Structural and chemical characterization from X-ray diffraction confirmed their nanocrystalline structure and IrO2 phase. Transmittance yielded a direct allowed optical gap between 2.75 and 3.16 eV. The fourpoint probe method and Hall measurements yielded high n-type conductivity (due to high electron concentration) and low mobility. To determine the impedance response toward several propane concentrations (250, 500, 750 and 1000 ppm), alternating current (AC) measurements in the frequency range from 1 Hz to 100 kHz were carried out. The sample was heated at different temperatures (30?C < T < 250?C). The largest resistive response with respect to the synthetic air was observed at a low temperature (60?C) and high frequency. On the contrary, the largest sensitivity, as defined by the International Union of Pure and Applied Chemistry, corresponds to both low propane concentration and frequency. The experimental AC admittance was confirmed to depend linearly on the square root of the propane pressure, like in the direct-current conductance case. The AC dynamic sensitivity turns out to be the largest at high frequencies for both, the real and imaginary parts of the admittance. The sensing mechanism proved to be that proposed for common metallic oxides.