ZnO thick films for NO₂ detection: effect of different nanostructures on the sensors' performances

In this paper, the great sensitivity and selectivity to NO₂ detection at low temperature down to ppb level of zinc oxide (ZnO) thick-film sensors are reported. Sensor performances of ZnO films prepared by screen-printing technique were evaluated comparing two different ZnO nano-powders in wurtzite crystal structure. Powders were synthesized by hydrothermal route (HT-ZnO) and by auto-combustion sol–gel synthesis (AC-ZnO). After proper characterization of the nano-powders, the thick-film sensors were fabricated by screen-printing technique onto α-alumina substrates equipped with Pt interdigitated electrodes, followed by a thermal treatment. The sensor response was studied in the range 50–250 °C. Best results were reached at 150 °C, with sensor response R (Z_g/Z₀)—defined as the ratio between impedance under NO₂ (Z_g) and impedance under dry air (Z₀)—equal to 42.92 for HT-ZnO and to 23.18 for AC-ZnO under 0.5 ppm of NO₂ in dry conditions. Finally, response and recovery time were measured, and selectivity of the sensors was determined by exposing the film toward O₃, CO₂, CH₄, N₂O and humidity at the best working temperature. Both sensors showed great sensitivity for NO₂ detection, supporting the exploitation of these sensors as NO₂ detectors at ppb level.