SnO2/SnSe2 Heterostructures for NO2 and H2 Sensing
Authors: Paolucci Valentina , D'Olimpio Gianluca , Kuo Chia-Nung , Ottaviano Luca , Lue Chin-Shan , Boukhvalov Danil W. , Cantalini Carlo , Politano Antonio 
Published: May 2020

Among layered semiconductors, tin diselenide (SnSe2) deserves particular attention thanks to its high intrinsic electron mobility (462.6 cm2 V-1 s-1 at T = 300 K) and ultralow thermal conductivity (3.82 W m-1 K-1). So far, some applications of SnSe2 have been proposed in several fields regarding superconductivity, Li- and Na- ion batteries, photodetection, photocatalysis, saturable absorbers for eye-safe lasers and thermoelectricity.
In this work we report the gas sensing performances of nanostructured SnSe2 thin film as NO2 (400-800 ppb range) and H2 (10-50 ppm range), operating at low temperature (75-150°C).
DFT calculations carried out on SnSe2 demonstrate that, while the stoichiometric single crystal is chemically inert even in air, the non-stoichiometric sample (SnSe2-x , with x~0.3) assumes a sub-nanometric SnO2 surface oxide layer, when it is exposed to ambient atmosphere. Remarkably, from theoretical results, it turns out that this self-assembled SnO2/SnSe2-x heterostructure is suitable to be particularly efficient in gas sensing.
Given such theoretical evidences, we performed gas sensing measurements to prove the suitability of the SnO2/SnSe2 heterostructure for gas sensing applications. SnSe2 crystals have been ground to obtain SnSe2 powder and gas sensing measurements have been performed using a volt-amperometric technique. SnSe2 thin films have been prepared by drop casting 10μl of ground powder suspended in Ethanol in the concentration of 1 mg/ml on Si3N4 substrates provided with 30 μm-spaced Pt interdigitated electrodes on the front side and a Pt resistor acting as a heater on the back side.
Prior to gas sensing, powderized sample has been characterized by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), as shown in Figure 1a and 1b. While the XPS measurements on the bulk crystal do not show the presence of any oxide, XPS on powder, shown in Figure 1a, reveals the presence of a huge amount of SnO2 on the surface, meaning the occurrence of a spontaneous oxidation process. Nevertheless, regarding the nature of surface layer, XRD spectrum in Figure 1b shows complete merging with crystalline SnSe2 according to ICDS card 98-004-9240 and no evidence of crystalline phase of SnO2.
Regarding experimental gas sensing response of the SnO2/SnSe2 heterostructure, Figure 1c shows the gas response to increasing concentrations of oxidizing NO2 gas in the range 400-800 ppb in the operating temperature (OT) range 75-150°C, utilizing dry air as carrier gas.
As can be observed, the material reveals a decrease of the Base line resistance (BLR i.e. the resistance in dry air) with increasing the OT, confirming its semiconducting behaviour. Moreover, it responds as an n-type semiconductor, with increasing its resistance under the exposure to oxidizing gases, according to the well-known behaviour of SnO2 [1]. Both panels show a good response to sub-ppm NO2 concentrations. In particular, Relative Response is 3.63 for 75°C OT while it is 1.08 when OT is 150°C. Even if temperature reduces somehow the ratio Rg/Ra in case of NO2 sensing, thermal activation confirms the advantages in terms of kinetics and adsorption/desorption times which are shorter increasing the operating temperature. This evidence, together with the encouraging results obtained at 75°C, can open future perspective regarding the light activation of the sensor, combining low temperature and opportune light-associated energies. These results are supported by the evidences coming out from theoretical calculation, giving SnO2/SnSe2 a potential material to be employed as low temperature gas sensor.
Moreover, SnO2/SnSe2 heterostructure has been tested as hydrogen gas sensor exposing the material to increasing concentrations of reducing H2 gas in the range 10-50 ppm. According to theoretical calculations, in the case of H2, adsorption provides smaller changes in electronic structure and charge densities and, therefore, sensitivity to H2 is smaller compared to the case of NO2, as shown in Figure 1d. However, the low detection limit for hydrogen in this work is found to be 10 ppm which is a good result compared to the existing literature.
To the best of our knowledge, this work reports for the first time the issues related to spontaneous oxidation of SnSe2 [2,3] and the lowest sensitivity of the SnO2/SnSe2 heterostructure to both NO2 and H2 gases, suggesting the possibility to further investigate the performances of this material as gas sensor.

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Figure 1