Influence of Humidity on a Resistive Room Temperature NO2 Dosimeter Based on Al-Doped ZnO
Authors: Wagner Ricarda , Schönauer-Kamin Daniela , Moos Ralf 
Published: May 2020

Introduction

ZnO is well suited to detect harmful gases such as NO2 at room temperature. The behavior of ZnO as a resistive concentration sensor with fast response and decay is investigated in many studies [1]. In this work, ZnO is used as a resistive NO2 dosimeter at room temperature as shown in [2]. The sensor shows accumulative behavior so that the NO2 dose, which is proportional to the time integral of the NO2 concentration, can be determined directly. This means that the dose can be determined without often error-prone (especially for small concentrations) mathematical integration of the concentration signal. In contrast to other accumulating sensors, a time-resolved sensor signal can be obtained by mathematical derivation.
Two operation phases exist for a resistive gas dosimeter, the sorption phase and the regeneration phase [2]. In the sorption phase, analyte (here: NO2) is sorbed at the functional layer (here: ZnO with 3 % Al), leading to an electrical conductivity change of the sensor layer. In absence of the analyte, the sensor signal remains constant, meaning that there is no desorption of the previously sorbed gas species. If all sorption sites are occupied, the analyte is specifically desorbed (e.g. thermally, chemically or by UV exposure [2]) in the regeneration phase before a new measuring cycle can begin. In this study for regeneration UV light (385 nm) is used to guarantee the room temperature operation of the dosimeter. Since it is known that humidity may affect the sensor signal and may promote desorption of adsorbed gas molecules [3], it will be investigated to what extent the dosimeter signal changes due to the influence of humidity.

Methods and Materials

Al-doped ZnO was produced by sol-gel synthesis as published in [4]. The functional layers were deposited on substrates with gold interdigital electrodes. The sensors were exposed to NO2 in the ppb range in dry and humid (17. 9 % r.h., 44.9 % r.h., 89.8 % r.h.) synthetic air and an impedance measurement was used to characterize the sensor behavior. Further, a water film was created by dropping water onto the sensor to check how a water film on the sensor surface affects the sensor impedance.

Results and Conclusion

In Fig. 1, the Nyquist plot (U
eff = 100 mV, f = 10 MHz -1 Hz) of the sensor for different humidity levels and for a sensor with a water film at room temperature is shown. One can see that from dry air up to 44.9 % r.h. there is a semicircle, which indicates an R||C equivalent circuit. Here, the resistance R can be calculated by eq. 1, where |Z| is the absolute value of the impedance and φ is the phase. The higher the humidity content is, the lower is the resistance, as expected. For higher humidity levels and the sample with the water film, no semicircle is found. Consequently, it is no longer possible to calculate R by eq. 1. The Nyquist plot measured in 89.9 % r.h. is similar to the one of the sensor with a water film. It can therefore be assumed that a closed water film forms on the sensor surface above a certain humidity, that dominates the electrical behavior and no dosimeter sensor signal is measurable anymore. In Fig. 2, the NO2 sensor signal is shown for dry air, 17.9 % r.h., and 44.9 % r.h., respectively. For dry air, dosimeter-type behavior can be observed. A linear correlation between sensor signal (R- R
0)/R
0 and dose D can be expected. In humidity, the sensor signal does not remain constant after NO2 exposure, but decreases a little. This means that a part of the previously adsorbed NO2 is desorbed. It is assumed, that adsorbed water molecules thus promote desorption of the adsorbed NO2, which leads to the observed loss of the dosimeter behavior. When UV light is turned on for regeneration at t = 100 min, one can see that in dry air the resistance decreases slower than in humid air. This again is a sign for a faster desorption of oxygen and NO2 in humid air. The dosimeter signal is therefore strongly affected by humidity, because desorption of adsorbed species is favored under humidity.
So it is important to find a way to reduce the influence of humidity for room temperature detection of ppb level NO2. As the desorption of NO2 at 17.9 % r.h. and 44.9 % r.h. is not so pronounced that the sensor signal returns to zero after NO2 addition, it may be assumed that the dosimeter behavior is maintained up to a certain humidity content. Since it is intended to use such a sensor without heating, the influence of humidity must also be investigated at temperatures around room temperature.

References

[1] L. Zhu et al., Room-temperature gas sensing of ZnO-based gas sensor: A review, Sensors
and Actuators A: Physical 267, 242–261 (2017); doi: 10.1016/j.sna.2017.10.021.
[2] R. Wagner et al., Novel Operation Strategy to Obtain a Fast Gas Sensor for Continuous ppb-Level NO2 Detection at Room Temperature Using ZnO-A Concept Study with Experimental Proof, Sensors 19, 4104 (2019); doi: 10.3390/s19194104.
[4] K. Maier et al., Effect of Water Vapor and Surface Morphology on the Low Temperature Response of Metal Oxide Semiconductor Gas Sensors, Materials 8, 6570–6588 (2015); doi: 10.3390/ma8095323.
[5] L. Vogel et al., Investigations on the crystal growth mechanism of one-pot-synthesized Al-doped ZnO and its UV-enhanced room temperature NO2 gas sensing characteristics, Functional Materials Letters 11 (2018); doi: 10.1142/S179360471850087X.

Figure 1