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

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 NO_x 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 NO_x. Both nitrous components, NO and NO₂, yield the same signal, which offers the opportunity to sense the total concentration of NO_x.

1. Introduction

NO_x 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 NO_x have been set up, for instance by the EU immission legislation Directive 2008. Currently, the emissions of NO_x by traffic in urban regions are a widely discussed topic since they exceed the regulatory limits. To enforce the limits, the detection of low NO_x concentrations at ppm- and sub-ppm level is required. Reliable and long-term stable sensing devices for the lowest NO_x 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 NO_x gas sensing technologies are discussed [1,3,4,5]. The sensors have to be accurate, selective, long-term stable, and should have low NO_x detection limits. Especially for air-quality monitoring, the sensors for low-temperature NO_x 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 NO_x 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 NO_x 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 NO_x 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 NO_x 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 NO_x. The sensor signal in synthetic air is very stable (shown are two measurement curves). The complex impedance increases strongly in presence of NO_x, even at a NO_x 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 NO_x 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 NO_x peaks, almost the same sensor response is visible for 5 ppm total NO_x (5 ppm NO₂ res. 4 ppm NO with 1 ppm NO₂). A huge sensor signal can determined even for NO_x concentrations below 1 ppm.

4. Conclusions

The SnS₂ sensors show a huge NO_x gas response even for low concentrations that needs to be investigated with respect to the behavior as a total NO_x 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 NO_x 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.