NASICON-based NO2 device attached with metal oxide and nitrite compound for the low temperature operation

NASICON (Na3Zr2Si2PO12)-based potentiometric NO2 devices, such as Au-attached, (ITO, NaNO2)-attached and (ITO, NaNO2–Li2CO3)-attached types, were fabricated and their NO2 sensing properties were investigated in the range of 2–7 ppm NO2 under 1–50% relative humidity (RH) between 30 °C and 150 °C. The Au-attached device had a NO2 sensing capability under wet condition (30% RH) without an auxiliary phase, although it showed no EMF response under dry condition (below 1% RH). This result suggests that NO2 sensing reaction of the device is closely related with water vapor. To enlarge the NO2 sensitivity at room temperature, a (metal, nitrite)-attached device was fabricated by combining ITO with NaNO2. The EMF change of the (ITO, NaNO2)-attached device was estimated to be 43.6 mV under 30% RH in the range of 2–7 ppm NO2 and enlarged remarkably in comparison with 17.7 mV of the Au-attached device. However, the NO2 sensitivity of the (ITO, NaNO2)-attached device was not only fluctuated by the change of relative humidity, but the EMF values shifted upward with increasing humidity at a fixed NO2 concentration. It is considered that this originates from the fact that a nitrite auxiliary phase is easy to dissolve in water. Therefore, Li2CO3 with durability to water vapor was melted together with NaNO2 and used as the auxiliary phase of the (ITO, NaNO2)-attached device. As the result, it was found that the EMF change of the (ITO, NaNO2–Li2CO3)-attached device was constant almost without depending on humidity. The EMF values of the device were also proportional to the logarithm of NO2 concentration and well consistent with theoretical values obtained from the Nernstian equation.