Two-Dimensional (2D) SnS2-based Oxygen Sensor

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ScienceDirect Procedia Engineering 168 (2016) 1102 – 1105

30th Eurosensors Conference, EUROSENSORS 2016

Two-Dimensional (2D) SnS2-based Oxygen Sensor Yongxiang Lia,c, Salvatore Gianluca Leonardib, Anna Bonavitab, Giovanni Nerib, Wojtek Wlodarskic a

The Key Laboratory of Inorganic Functional Materials and Devices, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 200050 Shanghai, P.R. China. b Department of Engineering, University of Messina, 98166 Messina, Italy. c School of Engineering, RMIT University, Melbourne, VIC 3000, Australia

Abstract 2D SnS2 flakes with a hexagonal layer nanostructure have been synthesized using a wet chemical route. The synthesized SnS 2 flakes consist of interconnected hexagonal nanosheets with lateral size of about 90 nm and thickness in the range 5-10 nm. As prepared 2D SnS2 flakes have been integrated onto conductometric transducing gas sensor platforms and tested towards oxygen. 2D SnS2-based sensor, operating in dark at 150 °C, provided high and reversible responses to oxygen pulses in the range 0 to 20 % volume. The effect of UV irradiation on the oxygen sensing performance and the cross sensitivity towards other organic vapor and gases were also investigated. © by Elsevier Ltd. This is an openLtd. access article under the CC BY-NC-ND license © 2016 2016Published The Authors. Published by Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference. Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Keywords: 2D SnS2, Oxygen sensor.

1. Introduction Recent research on metal sulfide semiconductor nanostructures has rapidly expanded in the last years in many advanced fields [1]. In particular, tin disulfide (SnS2), a n-type semiconductor material with a bandgap of about 2.24 eV [2], due to its unique properties has been widely used in the fields of solar cells, photocatalysts, lithium ion batteries, filed effect transistors and for other remarkable applications [3-6]. Therefore, many different procedures have been employed to synthesize different SnS2 structures including nanoparticles, nanotubes, nanoflakes, nanobelt and so on [7-9]. In addition to the previous applications, recently, the SnS 2 is attracting particular interest as a sensitive material for gas sensors. Wang et al. [10] synthesized by Li-intercalation method, SnS2 single layer which exhibited rapid response and high sensitivity at room temperature towards NH3. Most recently, Giberti et al. [11] synthesized SnS2 nanorods based materials comparing the gas sensing performance with its oxide counterpart. They proved that tin disulfide had very interesting gas sensing properties, since it showed a very good selectivity to acetone and acetaldehyde at concentrations ranging 1–10 ppm, in both dry and wet conditions better than tin dioxide phase. Previously, we demonstrated the sensing ability of 2D SnS2 flakes developing a gas sensor with strong sensitivity and selectivity to NO2 molecules [12]. In this study, 2D SnS2 flakes were synthesized using a wet chemical route with a great potential for production scalability, by using octadecene as a solvent, oleic acid as a surfactant and oleylamine as a high boiling solvent. Here, we report a study on the oxygen sensing performance of the same material. Up to date, only a paper reports of the use of tin sulfide as oxygen sensing material [13].

1877-7058 © 2016 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference

doi:10.1016/j.proeng.2016.11.355

Yongxiang Li et al. / Procedia Engineering 168 (2016) 1102 – 1105

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2. Experimental part 2.1. Synthesis of 2D SnS2 flakes For the synthesis of 2D SnS2 Tin(IV) chloride (SnCl4·5H2O, > 99.9%, Sigma-Aldrich, 0.5 mM) was added to a mixture of 5 ml of oleic acid (> 90.0%, Sigma-Aldrich) and 10 ml of octadecene (> 90.0%, Sigma-Aldrich) in a 100 ml three-neck flask to produce the tin precursor. A standard Schlenk line was used to protect the reaction from oxygen and moisture under a flow of high-purity N2. The mixed solution was degassed at 120 °C for 1 h to remove the moisture and the oxygen. Subsequently, the solution was heated to 280 °C within 15 min with a vigorous stir (700 rpm). Sulfide powder (1 mM) was dispersed into 5 ml of oleylamine (> 90.0%, Sigma-Aldrich) to produce the sulfide precursor that was subsequently injected into the reaction system. The reaction was maintained at 280 °C for 30 min. After the solution was cooled to room temperature, the 2D SnS2 flakes (in powder form) were collected and separated from the solution by centrifugation. The powder was further washed two times by ethanol and hexane (1/1, v/v) and finally dispersed in ethanol. 2.2. Samples characterization and sensing tests The crystal structure was characterized from the powder using XRD (Philips PANanalytical) with Cu Kα radiation at 45 kV and 40 mA. Raman spectroscopy was carried out in situ on the sensing film casted on the alumina substrate using an Horiba XploRa spectrometer equipped with an Olympus BX40 microscope, a Peltier cooled charge coupled device (CCD) sensor and a 532 nm laser as the excitation source. Three samples treated at 150 °C, 250 °C and 400 °C respectively, were investigated by in situ Raman in order to evaluate the local thermal stability of the sensing material. Morphological and compositional analyses were carried out by SEM using a ZEISS 1540XB FE SEM instruments (Zeiss, Germany) equipped with an EDX detector. The transducing substrates were made of alumina planar substrates (3 mm × 6 mm) supplied with 2 pairs of interdigitated platinum electrodes and a platinum heating element on the back side. Five microliters of ethanol suspension containing 1 mg/ml of SnS2 powder were drop-casted on the transducing substrate within the interdigitated electodes area (3 mm × 3 mm) at a temperature of 50 °C. Measurements were performed under dry gases with total stream of 100 sccm, collecting the sensors resistance by means a Keithley 6487 Picoammeter/Voltage Source. All tests were performed in a Teflon chamber equipped with UV LED (400±10 nm) which illuminates the active surface of the sensor. For the oxygen sensing tests the sensors were allow to stabilize the baseline under a steam of dry nitrogen at the required operation temperature, and then they were exposed to oxygen pulses of different concentration in the range 2- 20 % vol. For all other tests dry air was used as reference gas then the sensors were exposed to the gases. 3. Results and discussion 3.1. Characterization A detailed analysis, carried out with different techniques such as XRD, SEM, TEM and Raman, has been performed previously to characterize the as synthesized nanostructures [12]. The presence of a homogeneous distribution of 2D hexagonal flakes with ‫׽‬6 nm thick corresponding to 10 monolayers of SnS2, as the thickness of a monolayer is ‫׽‬0.59 nm, has been verified (Fig. 1a). XRD pattern of as prepared 2D-SnS2 flakes was found in accordance with the hexagonal 2H SnS2 structure (ICDD 23-0677). An in situ characterization has been further carried out by SEM-EDX and micro-Raman spectroscopy on the 2D SnS2 layer deposited on the alumina sensor substrate. Fig. 1b shows the Raman analysis carried out on sensing layer heated at 150 °C. An intense Raman peak can be found at 314 cm-1 preceded by a broad peak ranging from 205-280 cm-1 which can be ascribed to the plane vibration mode (A1g) of Sn–S bonds of 2H SnS2 [14]. Data collected at higher treatment temperature, up to 400 °C, highlights that SnS2 phase well visible for the sample heated to 150 °C, tends to fade for the sample treated at 250 °C (not shown) and finally to disappear for the sample treated at 400 °C confirming the transformation to SnO2 phase. Therefore, to examine the sensing response of the pure SnS2 phase, without any contamination of oxidized phases, we performed further tests at the maximum temperature of 150 °C.

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Fig. 1. a) SEM image of 2D SnS2 flakes; b) Raman spectroscopy of 2D SnS2 flakes on sensor transducer treated at different temperature.

3.2. Sensing tests In order to evaluate the response towards oxygen, sensing tests were carried out in the dark and under UV irradiation. Figure 2a shows the dynamic responses towards different oxygen concentrations ranging from 0 to 20 % vol. recorded in dark at 150 °C. In these conditions the sensor shows fast and reversible responses with complete recovery of the baseline after oxygen removing. Figure 2b shows the calibration curves extracted from the previous dynamic responses. 350M

20% O2

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Fig. 2. a) Dynamic responses to different concentration of oxygen pulses in dark at 150 °C; b) calibration curve.

Then, the effects of different temperature and UV irradiation were also investigated. Figures 3 (a,b) show the dynamic responses recorded in dark and under UV illumination when sensor was exposed to 2% vol. of oxygen at 130 °C and the response at different temperatures, respectively. The results show that, in dark condition, reducing the temperature the response decreases. Although a little reduction of the response decreasing the temperature is also observed under UV illumination, the irradiation is able to compensate the thermal effect and allow to obtain high response even at lower temperature. At the same time, the UV illumination can improve the dynamic of the response reducing the time to recovery the baseline. The selectivity of the sensor was also investigated. Figure 3c shows the responses towards common other gases at specific concentrations, recorded in air at 150 °C and dark condition. The highest sensitivity, as reported earlier [12],

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1.8 1.6 1.4 1.2

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is observed for NO2. Also the response to acetaldehyde is relevant while the response to other volatile organic compounds, ammonia CO and CO2 result in negligible responses.

150

Temperature (°C)

Fig. 3. a) Dynamic responses recorded in dark and under UV illumination for not annealed sensor exposed to 2% vol. of oxygen at 130 °C, b) response at different temperatures, c) responses to other gases and vapors.

4. Conclusions 2D SnS2 flakes were synthesized using a facile wet chemical route and use to fabricate the sensitive layer of a semiconductor gas sensor. The sensing tests showed an excellent sensitivity towards oxygen by operating at relatively low temperature. Sensing performances were improved by UV irradiation which reduces the operating temperature and at the same time makes faster the dynamic of the sensor response.

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