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Proceeding Paper

2D SnS2—A Material for Impedance-Based Low Temperature NOx Sensing? †

1
Department of Functional Materials, University of Bayreuth, Bayreuth, Germany
2
School of Engineering, RMIT University, Melbourne, Australia
*
Authors to whom correspondence should be addressed.
Presented at the Eurosensors 2017 Conference, Paris, France, 3–6 September 2017.
Proceedings 2017, 1(4), 455; https://doi.org/10.3390/proceedings1040455
Published: 9 August 2017
(This article belongs to the Proceedings of Proceedings of Eurosensors 2017, Paris, France, 3–6 September 2017)

Abstract

:
The sensor signal of tin disulfide (SnS2), a two-dimensional (2D) group-IV dichalcogenide, deposited as a film on a conductometric transducer is investigated at 130 °C. The focus is on the detection of the total NOx concentration. Therefore, the sensor response to NO and NO2 at ppm- and sub-ppm level at low operating temperature is determined. The results show that the sensing device provides a high sensor signal to NO and NO2 even at concentrations of only 390 ppb NOx. Both nitrous components, NO and NO2, yield the same signal, which offers the opportunity to sense the total concentration of NOx.

1. Introduction

NOx sensing at low temperatures is still a difficult task, especially for air-quality monitoring in stationary or transportable air quality monitoring devices [1]. In the past years, strict emission and immission limits for NOx have been set up, for instance by the EU immission legislation Directive 2008. Currently, the emissions of NOx by traffic in urban regions are a widely discussed topic since they exceed the regulatory limits. To enforce the limits, the detection of low NOx concentrations at ppm- and sub-ppm level is required. Reliable and long-term stable sensing devices for the lowest NOx concentrations are necessary to meet the strict requirements of, for instance, the European legislation, with regard to quality of the data especially in real-time air quality monitoring [2]. In literature, various NOx gas sensing technologies are discussed [1,3,4,5]. The sensors have to be accurate, selective, long-term stable, and should have low NOx detection limits. Especially for air-quality monitoring, the sensors for low-temperature NOx sensing with a low power consumption are beneficial [1,2].
In this work a new material class based on 2D transition metal dichalcogenides (TMD) is discussed as sensing materials for NOx sensors [6,7]. Due to the special structure of SnS2, the charge transfer between physisorbed NO2 gas molecules and the 2D SnS2 material allows for NO2 sensing with high NO2 sensitivity and selectivity at low operating temperatures. In [6], the NO2 sensor response of SnS2 is described at 120 °C. In the present work, tin disulfide (SnS2) flakes are investigated as functional materials for NOx sensing at 130 °C with focus on the NO sensing performance of SnS2.

2. Materials and Methods

The transducer composed of an alumina substrate with a screen-printed interdigitated-electrode (IDE) structure (Au-IDE: 100 µm/100 µm). The sensitive film of 2D SnS2 was synthesized by a wet chemical route and drop-casted on the IDE-structure [6]. The structure and morphology of the SnS2 film was analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
The sensor response, the complex impedance |Z| of the SnS2-film, was determined at 130 °C in a synthetic gas test bench. As base gas, synthetic air with 2 vol.% water was used and NO and NO2 were added in a concentration range between 390 ppb and 2 ppm. The added NOx concentration was analyzed downstream the sensing device by a chemiluminescence detector (CLD).

3. Results and Discussion

The microstructure of the SnS2 film is shown in Figure 1. The SEM image (Figure 1a) of the deposited film shows SnS2 particles with a hexagonal shape which appear to from flake like structures. This is proven by the TEM image (Figure 1b). The shape of the particle is a hexagonal plate, with an average diameter of 100 nm and a thickness of less than 10 nm. This planar 2-dimensional flake structure was selected due to its very high active surface area resulting in a high adsorption capability for physisorbed NOx molecules.
Initial impedance spectra of a SnS2 sensor, shown in Figure 2 in the form of Nyquist-plots (frequency between 1 MHz and 1 Hz, root-mean-square value of the amplitude 200 mV, temperature 130 °C), present a semi-circular behavior with a high sensor signal when exposed to 390 ppb NOx. The sensor signal in synthetic air is very stable (shown are two measurement curves). The complex impedance increases strongly in presence of NOx, even at a NOx concentration in the sub-ppm range.
As stated in [6] for NO2 exposure, the strong resistance increase can be explained by the effect that the adsorbed NO2 gas molecules act as electron acceptors. Charge is transferred from the SnS2 flakes to the adsorbed NO2 and the SnS2 flakes deplete with charge carriers. The reduced number of free electrons leads to the increasing resistance.
For further measurements, we selected a constant frequency of 1 Hz. The sensor was exposed to varying NO and NO2 concentration steps, and the resulting |Z| is presented in Figure 3 and Figure 4. The impedance of the SnS2 sensor is around 600 MΩ in synthetic air, and increases when exposed to 1 ppm NO or NO2 to 2.2 GΩ with the same sensor signal for NO and NO2. The oscillating of the sensor signal is due to temperature fluctuations of the furnace (around 10 °C). The response time is quite good, but the signal recovery is relatively slow. The sensor seems to be a total NOx sensing device.
To investigate this more in detail, Figure 4 includes the NO and NO2 concentrations determined by a CLD gas analyzer. Comparing the first two NOx peaks, almost the same sensor response is visible for 5 ppm total NOx (5 ppm NO2 res. 4 ppm NO with 1 ppm NO2). A huge sensor signal can determined even for NOx concentrations below 1 ppm.

4. Conclusions

The SnS2 sensors show a huge NOx gas response even for low concentrations that needs to be investigated with respect to the behavior as a total NOx sensor. The developed sensing device provides high impedance values. The impedance changes strongly when exposed to low concentrations of NO or NO2 and the sensor seems to be suitable for sub-ppm level NOx detection. The dependence of the resistance on the thickness of the SnS2 film is an interesting task for further investigations. Additionally, the concentration dependent sensor response has to be analyzed and the response and the recovery time need to be improved.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Figure 1. (a) SEM image of SnS2 particles; (b) TEM image of SnS2 particles.
Figure 1. (a) SEM image of SnS2 particles; (b) TEM image of SnS2 particles.
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Figure 2. Impedance spectra of a SnS2 sensor at 130 °C in synthetic air and with 390 ppb NOx in the Nyquist-plot representation; spectra determined with Ueff = 200 mV and between 1 Hz and 10 MHz.
Figure 2. Impedance spectra of a SnS2 sensor at 130 °C in synthetic air and with 390 ppb NOx in the Nyquist-plot representation; spectra determined with Ueff = 200 mV and between 1 Hz and 10 MHz.
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Figure 3. Complex impedance |Z| signal of the SnS2 sensor determined at f = 1 Hz at 130 °C during NOx exposure to 1 ppm, 2 ppm and 0.5 ppm NO res. NO2.
Figure 3. Complex impedance |Z| signal of the SnS2 sensor determined at f = 1 Hz at 130 °C during NOx exposure to 1 ppm, 2 ppm and 0.5 ppm NO res. NO2.
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Figure 4. Complex impedance |Z| of the SnS2 sensor determined at f = 1 Hz at 130 °C during NOx exposure and the added NOx concentration analyzed by CLD downstream the sensor device.
Figure 4. Complex impedance |Z| of the SnS2 sensor determined at f = 1 Hz at 130 °C during NOx exposure and the added NOx concentration analyzed by CLD downstream the sensor device.
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MDPI and ACS Style

Schönauer-Kamin, D.; Li, Y.; Wlodarski, W.; Ippolito, S.; Moos, R. 2D SnS2—A Material for Impedance-Based Low Temperature NOx Sensing? Proceedings 2017, 1, 455. https://doi.org/10.3390/proceedings1040455

AMA Style

Schönauer-Kamin D, Li Y, Wlodarski W, Ippolito S, Moos R. 2D SnS2—A Material for Impedance-Based Low Temperature NOx Sensing? Proceedings. 2017; 1(4):455. https://doi.org/10.3390/proceedings1040455

Chicago/Turabian Style

Schönauer-Kamin, Daniela, Yongxiang Li, Wojtek Wlodarski, Samuel Ippolito, and Ralf Moos. 2017. "2D SnS2—A Material for Impedance-Based Low Temperature NOx Sensing?" Proceedings 1, no. 4: 455. https://doi.org/10.3390/proceedings1040455

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