W-doped indium oxide synthetized via hydrothermal route for low temperature ozone sensing

Nano-crystalline indium oxide and tungsten-doped indium oxide were prepared by hydrothermal route and adopted as ozone sensitive materials at low temperature, comparing their performances with a commercial indium oxide. After hydrothermal synthesis, powders were calcined at 400 °C for 30 min and characterized by laser granulometry, thermal analysis, X-ray diffraction, N2 adsorption, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, and transmission electron microscopy. Sensors were developed by screen-printing the sensing materials onto α-alumina substrates with platinum interdigitated electrodes. After drying overnight, sensors were fired at 600 °C for 1 h in air. The sensor response was measured in the range 25 °C–200 °C carrying on all measurements in a flow chamber at a constant flow rate of 1 L/min of dry air in the range 200–500 ppb O3. Response time and recovery time (e.g., the times taken for the sensor to attain 90% of total impedance change from its initial impedance value) were detected, together with cross-sensitivity tests towards NH3, CH4, humidity, CO, NO2, CO2 and N2O. Sensor's response (R) was defined as the ratio between impedance of the film under gas exposure at the equilibrium and the impedance under dry air. Best results were obtained at 100 °C, with R equal to 464 under 500 ppb O3. These results are extremely encouraging and support the exploitation of tungsten-doped indium oxide as low-temperature ozone sensors.