A Low Temperature NO2 Sensor Based on SnSe2/SnO2 Heterojunction
Authors: Huang Baoyu , Li Xinyu , Li Xiaogan 
Published: July 2021

Introduction
Nitrogen dioxide (NO2) is one of the main air pollutants. A Higher concentration of NO2 may cause serious damage to respiratory system and even death. Therefore, it is necessary to develop a low-energy consumption gas sensor which can sensitive and accurate analysis of NO2 in the atmosphere.
In this aspect, two dimensional (2D) layered nanostructured materials such as metal dichalcogenides (MX2, M=Mo, Sn etc..; X=S, Se etc..) have received widespread attentions in NO2 detection due to their high specific surface area and unique electrical properties[1]. For example, SnS2 demonstrated excellent NO2 sensing properties [2]. Moreover, as reported in literature [3], the formation of SnO2/SnS2 heterostructure is beneficial to the further improvement of sensing performance. However, the gas sensing properties of SnSe are rarely reported. Therefore, the gas sensing properties of SnSe2 and SnSe2/SnO2 heterojunctions have investigated in this work.
Experiment
The SnSe2 powder was purchased from Chengdu Alfa Metal Material Co. Ltd. After grinding for half an hour, the SnSe2 powder was dispersed in an alcohol solution to obtain suspension by ultrasonic treatment. The paste was deposited onto the alumina substrates with Au interdigital electrodes. Then, the sensors were dried at 70 °C for 12 h. The SnO2/SnSe2 heterojunction based sensor was obtained by in-situ oxidation SnSe2 at 500 °C for 1 h in simulation air which is composed of 84.8 % dry nitrogen and 15.2 % dry oxygen. For comparison, the SnO2 sensor was also prepared by in-situ oxidation SnSe2 at 750 °C.

Results and conclusions

1. Structure and Morphology
Fig.1a shows the XRD patterns of SnSe2, SnO2 and SnO2/SnSe2, respectively. For the SnO2/SnSe2 sample, the diffraction peaks located at (001), (002), (003), (004), (005) lattice planes are assigned to the hexagonal SnSe2 (JCPDS Card No. 89-2939) [4], and the other diffraction peaks are consistent with tetragonal SnO2 (JCPDS No. 41-1445). No third phase was observed in the SnO2/SnSe2 sample indicating the product with high purity. This is consistent with the HRTEM analysis. As shown in Fig. 1(b), the fringe spacing of 0.296 nm and 0.192 nm are corresponded to the d-spacing of (002) and (003) crystal planes of SnSe2, whereas the fringe spacing of 0.340 nm and 0.265 nm are assigned to the d-spacing of (110) and (101) crystal planes of SnO2. Additionally, the EDX mapping images and EDX spectrum of SnO2/SnSe2 sample are shown in Fig.1c-d. It can be noticed that there only exist Sn, Se, and O elements in the composites material, and Sn (yellow), Se(red), and O (green) elements are uniformly distributed in the material, which confirming that the formation of SnO2 nanocrystals on the surface of SnSe2 and the oxidation of SnSe2 to SnSe2/SnO2 is partial.
2. Gas-Sensing Properties
Fig. 2a shows the dynamic response and recovery curve of sensors to 8 ppm NO2 at 120 °C. Compared to pristine SnSe2 and SnO2 based sensor, the SnSe2/SnO2 based sensor exhibits the highest response value (5.6), which is approximately 10 times higher than that of SnSe2 based sensor (0.54). In contrast, since SnO2 didn’t work at the optimum temperature (usually at 200-400 °C), the redox reactions to induce a sensor response were almost inertia at 120 °C. The selectivity of the SnSe2/SnO2 based sensor to several VOCs, including ethanol, ammonia, formaldehyde, acetone and methanol at 8 ppm in Fig. 2b. The response towards 8 ppm NO2 is much higher compared to the other tested gases indicating a possible excellent selectivity towards NO2. The above results fully prove that the formation of SnSe2/SnO2 heterojunctions can enhance the sensing performance of materials.

References

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Figure 1