SnS₂ nanoflakes for efficient humidity and alcohol sensing at room temperature

Abstract

We report a one step facile hydrothermal synthesis of layered SnS₂ nanoflakes. The as-synthesized nanosheets are characterized using X-ray diffraction, Raman spectroscopy and Transmission Electron Microscopy (TEM). The humidity sensing behavior of SnS₂ nanoflake sensor device were investigated in the range of 11–97% of relative humidity (RH) at room temperature. The response time of ∼85 s and recovery time of ∼6 s were observed for the SnS₂ nanoflake based humidity sensor. A maximum sensitivity of 11 300% is recorded. We also investigate the SnS₂ nanoflake based alcohol sensing properties towards methanol, ethanol and iso-propyl alcohol. An exclusive selectivity towards methanol with a response of 1580 is shown as compared to other analytes. The response time of ∼67 s and recovery time of just 5 s were observed for the SnS₂ nanoflake based methanol sensor.

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Introduction

Graphene has changed nanotechnology especially nanoelectronics since its invention.^1–3 Its two dimensional (2D) hexagonal sheet structure has been widely exploited by many researchers for various applications like photocatalysis,^4,5 optoelectronics,^6,7 supercapacitors,⁸ sensors^3,9,10 etc. But its semi-metallic nature has hindered its use in nanoelectronics and optoelectronic device applications. Recently, graphene analogous materials such as metal dichalcogenides like MoS₂,^11–14 WS₂,^15,16 MoSe₂,^17,18 WSe₂,^19–21 SnSe₂,²² MoTe₂,²³ WTe₂ ²⁴ and other 2D inorganic materials such as black phosphorus²⁵ and TiS₃,²⁶ etc. are widely studied to fill this void due to their extraordinary properties and performance in nanoelectronics devices as well as other applications such as optoelectronics, nanoelectronics, spintronics, catalysis etc.^11–26

The tin disulphide (SnS₂) is a layered material crystallized similar to CdI₂ type where the tin cations (filling the octahedral sites of the alternating layers) and sulphide anions form hexagonally closed packed structures. The layered SnS₂ is being extensively investigated due to its non-toxicity^27,28 and high stability^29,30 etc. It has been reported as an efficient material in various applications like Li-ion battery,³¹ NO₂ sensing,³² glucose sensing,³³ solar cells,³⁴ field emission³⁵ and enzyme biosensing applications^36–38 etc.

An important property which is needed for the vapor sensing is the number of gas adsorption sites available for the sensing. This mainly depends on the morphology of the material. In the past different morphologies of SnS₂ like nanoparticles,³⁹ nanotubes,³⁹ nanoflowers³⁵ and nanowires⁴⁰ were reported. Earlier reports on SnS₂ nanosheets synthesis by Gao et al.⁴¹ and Zhuo et al.⁴² employed ammonia solution and NaOH to control the surface morphology and porosity. We have reported earlier nanosheets of SnS₂ by direct mixing of precursors in an autoclave, which leds to formation of atomically thin nanosheets.³⁵ High surface area is very crucial for sensing applications as it plays an important role.

Humidity hindrance is a huge problem in the industrial sector. Similarly, low concentration alcohol detection is a challenge in industrial plant. Hence, it is necessary to develop a sensor with high sensitivity and faster response and recovery time for enhanced sensing in the fields of environmental control, biomedicine, chemical industries etc.^43,44 Recently, 2D materials such as black phosphorus,^43,45 MoS₂,⁴⁶ WS₂ ⁴⁷ etc. have been reported for alcohol vapor sensing. But most of them are synthesized using mechanical exfoliation or chemical vapor deposition techniques which are not economically feasible. Hence, we employed simple hydrothermal synthesis method for economically viable and for more yieldable synthesis. Also, the sensing measurements were carried out at room temperature and in absence of vacuum to simulate the industrial environment.

Experimental

Synthesis of SnS₂ nanoflakes

In a typical synthesis, 0.7 g of SnCl₄·5H₂O was taken in 30 ml distilled (DI) water. This was stirred with the aid of magnetic stirrer till a transparent solution was obtained. Then, 1.2 g thioacetamide (TAA) was added to this solution and stirred till a yellowish solution was obtained. Meanwhile, 1 M of NaOH solution was prepared. The solution was then transferred to a 50 ml capacity Teflon lined autoclave and subjected to hydrothermal synthesis at 200 °C for 24 h in furnace. The furnace was allowed to cool naturally at room temperature. The resultant product was then centrifuged for 50 min at 7000 rpm using DI water and ethanol followed by drying at 90 °C in vacuum oven.

Material characterizations

The structural identification was done by X-ray diffraction using a PANalytical X'pert Pro duel goniometer diffractometer equipped with filter for CuKα irradiation at λ = 1.5406 Å. The Raman spectra was recorded using Renishaw microscope at a wavelength of 632 nm. The morphology was investigated with the help of transmission electron microscope (TECNAI G2-20-TWIN, FEI, The Netherlands) operating at 200 keV.

Humidity sensing test

The saturated salt solutions of various salts for achieving relative humidity environments were prepared in close vessel as reported earlier.^25,48,49

Alcohol sensing test

To measure alcohol sensing characteristics, argon gas was bubbled at a constant flow rate of 50 sccm through a gas bubbler containing alcohol to get vapors with desired concentration into the test chamber. We investigated for two concentrations ∼40 ppm and ∼150 ppm.

Device fabrication

For the device fabrication, we used a 1 cm × 1 cm ITO substrate. A channel was made on this substrate by creating a scratch using a cutter so that the two sides of the channel act as source and drain. A channel width of 500 μm was noted. A thick paste of SnS₂ nanoflake was then drop-casted in the channel. This device was further dried under infrared lamp for 30 min and used for the further sensing experiments.

Electrical characterization

The two probe device is connected to Keithley Sourcemeter 2612A through copper clips which act as electrodes in the study. The source meter is connected to a computer with the help of GPIB 488A interface.

Results and discussion

Structural and morphological analysis

Fig. 1 shows the typical schematic of 2D structure of SnS₂ (a) top view and (b) side view respectively. Fig. 2(a) shows the typical X-ray diffraction to investigate the structural properties of SnS₂. The sharp and intense peaks confirmed the formation of hexagonal structure of SnS₂. The peaks were indexed according to JCPDS data card number 23-0677.

Fig. 1 Ball and stick model schematic of SnS₂ single sheet (a) top view and (b) side view.

Fig. 2 The as-synthesized SnS₂ nanoflakes (a) XRD pattern and (b) Raman spectrum.

The Raman spectroscopy analysis was done to study the signature of vibrational modes to confirm the formation of the SnS₂ material. In case of SnS₂, the Raman active vibrational mode A_1g corresponding to vertical out-of plane vibration between S–S and E_g correspond to non-degenerate in-plane vibration mode.^32,35 From Fig. 2(b), it is observed that the E_g mode at ∼224 cm⁻¹ and an intense A_1g mode at ∼310 cm⁻¹ for SnS₂ nanoflakes. The inset of Fig. 2(b) shows the schematic of the Raman active modes.

Fig. 3(a) and (b) shows the typical low magnification TEM images of as synthesized sample which shows the formation of 2D nanoflake like morphology. The formation of nanoflake morphology occurs due to the hydrolysis and dissociation of the precursors during the hydrothermal reaction. According to Butler et al.⁵⁰ alkali-assisted hydrolysis of TAA is quicker than acid catalysed hydrolysis. Hence, in our study we used aqueous NaOH solution to assist the hydrolysis of TAA. The proposed mechanism can be presumably explained from the below mentioned equations. Firstly, SnCl₄·5H₂O dissociates to give Sn⁴⁺ ions.

SnCl₄·5H₂O → Sn⁴⁺ + 4Cl⁻ + 5H₂O (1)

whereas TAA hydrolizes in the aqueous solution as follows:

CH₃CSNH₂ + OH⁻ → CH₃COO⁻ + HS⁻ (2)

Fig. 3 TEM analysis of SnS₂ nanoflakes (a and b) low magnification TEM images, (c) high resolution TEM image and (d) SAED pattern.

The HS⁻ will further dissociate as:

2HS⁻ → H₂S + S²⁻ (3)

Further Sn⁴⁺ and S²⁻ react to form SnS₂.

Sn⁴⁺ + S²⁻ → SnS₂ (4)

But at lower temperature, the production of S²⁻ is very slow. Hence, subjecting to higher temperature and prolonged time was done for easier and effective nucleation, growth and formation of SnS₂ nanoflakes.^35,50

Fig. 3(c) shows the high resolution TEM image of SnS₂ nanoflake. This was taken to study the lattice fringes and the grain boundaries. A d-spacing of ∼0.32 nm was observed from the HRTEM image as shown in inset of Fig. 3(c). This value can be attributed to (100) plane of SnS₂. The SAED pattern of SnS₂ nanoflake sample were recorded and shown in Fig. 3(d) which confirms that the synthesized SnS₂ sample belongs to the hexagonal crystal structure.

Humidity and alcohol sensor

The humidity sensing analysis was done at relative humidity ranging from 11–97%. Fig. 4(a) shows the schematic of a two-probe SnS₂ sensor device used to test the humidity sensing performance. The current–voltage (I–V) measurements were taken from −3 V to +3 V and the resistance values were calculated from the slope of the plot. Fig. 4(b) shows the plot of typical resistance of the sensor device against the relative humidity values. From this plot we observed that the resistance decreased with increasing humidity, establishing that the conductivity increased as the relative humidity increased. These resistance values were also used to calculate the sensitivity of the device. Fig. 4(c) shows the sensitivity vs. relative humidity plot. The % sensitivity has been calculated using the formula:where, R₁₁ is the resistance of the sensor device in 11% RH and R_ΔRH is the resistance of the sensor device in change in relative humidity. The maximum sensitivity of ∼11 300% in 97% relative humidity was observed. The repeated cycles of the two-probe sensor device in 11% and 97% relative humidity at a bias voltage +3 V were recorded to study the response and recovery time and cyclic stability of the device. Fig. 4(d) shows the current–time (I–t) plot through which the response and recovery time were noted as ∼85 s and ∼6 s respectively. The fabricated device displayed a quick response upon exposure to the water molecules at room temperature. Note that the response time was much greater than the recovery time. The response of the sensors device is limited by strong adsorption/desorption of analyte molecules at room temperature. The thermodynamic adsorption of analyte molecules may not be much favorable during the response time and during the recovery time analyte molecules easily get desorbs due to low absorption energy, that's why response time was much greater than the recovery time. Further, the response of sensor device may be improved by operating device at slight higher temperature or by light irradiation. The sensitivity can also be modulated using the micro/nano structures of SnS₂ nanoflake, smaller the thickness, higher will be the sensitivity due to large surface area. The sensor device was exposed to water molecules at higher relative humidity until a saturation point was achieved. Once the device reached the saturation, the flow of water molecules were turned off by again exposing it to the lower humidity level, so as to attend the original resistance. Though the response time is not the best, the recovery kinetics was much better than many 2D layered materials reported in the literature.^{11,25,26,48,50–52} This observation might be related to the thickness dependant properties of the SnS₂ nanosheets. The comparison of sensing characteristics with other materials has been elaborated in Table 1.

Fig. 4 Humidity sensing performance of SnS₂ nanoflakes (a) schematic of device, (b) resistance vs. relative humidity plot, (c) sensitivity vs. relative humidity plot and (d) typical I–t cycle plot.

Table 1 Comparison of response and recovery time of reported humidity sensor materials with this work of SnS₂ nanoflakes

Materials	Synthesis method	Response time (s)	Recovery time (s)	Reference
MoS2	Mechanical exfoliation	9	17	12
WS2	Hot wire-CVD	12	13	16
VS2	Liquid exfoliation	30–40	12–50	52
Black phosphorus	Liquid exfoliation	255	10	48
Black phosphorus	Electrochemical exfoliation	101	26	25b
V2O5	Hydrothermal method	240	300	49
SnS2	Hydrothermal method	85	6	This work

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The sensing mechanisms were explained as below. The resistance of the sensor device was recorded at a lower humidity level (11% RH) as the base value resistance. Typically the sensor was then exposed at a different humidity levels from 11% RH to 97% RH with constant flow rate of 10 sccm Ar. When the SnS₂ sensor device was exposed to the water molecules, a charge transfer between the water molecules and SnS₂ nanosheet takes place which results in the resistance change of the sensor device. The strong interactions between the water molecules and SnS₂ nanosheets results into the significant change in conductivity of SnS₂ sensor device.⁵¹ Water molecule is an electron donating molecule, it possesses n-type characteristics and upon exposure to SnS₂ nanosheet sensor device, it injects charges to the surface of sheet and which results into the shifting of Fermi level towards the conduction band of SnS₂ leading to the decrease in the resistance of the sensor device.⁵¹ Also, the sensing mechanism here is dominant by proton conduction. When the device is exposed to the humid environment, water molecules in its atmosphere get adsorbed on the channel material. At low humidity, less water molecules are adsorbed causing lesser surface interactions with the material and therefore less proton transfer. At higher humidity, due to the adsorption of multiple water layers, the transfer of protons easier and more in number. Hence, the conductivity increases as the proton conduction increases i.e. as relative humidity increases.^25,53,54

We also investigated the alcohol sensing behavior of the SnS₂ nanoflakes for three analytes methanol, ethanol and isopropyl alcohol (IPA). Fig. 5(a) and (b) shows the response and resistance of the material when the test analyte molecules were introduced in the test chamber. The response was calculated using the following formula,

where R_a referred to the resistance of the device in Argon and R_g represented the resistance in presence of test analytes. From the figures we observed that methanol had the lowest resistance and hence, a high response of 1580 at ∼150 ppm followed by ethanol and then IPA. The inset of Fig. 5(a) shows the response at two different concentrations ∼40 ppm and ∼150 ppm respectively. An exclusive selectivity towards methanol was observed as compared to ethanol and IPA. Further I–t plot at a bias voltage +3 V were shown in Fig. 5(c) which was taken at a concentration of 150 ppm for all the test analytes to study the response and recovery time characteristics. In case of methanol the response and recovery time was calculated as 67 s and 5 s respectively. The response and recovery of ethanol and IPA have been depicted in Fig. 5(d). Table 2 summarizes the sensing parameters such as response factor, response time and recovery time for all three analytes studied. The response and recovery time for SnS₂ based alcohol sensor device are found to be better than that of graphene sensor.⁹

Fig. 5 Alcohol sensing performance of SnS₂ nanoflakes (all alcohols at 150 ppm) at 25 °C (a) response vs. alcohols and inset shows the response vs. concentration, (b) resistance vs. alcohols, (c) typical I–t plot and (d) bar diagram of response and recovery time of the test alcohols.

Table 2 Comparison of response, response time and recovery time for various alcohols based on SnS₂ nanosheet sensor device

Alcohols	Response (Ra/Rg) at 150 ppm	Response time (s)	Recovery time (s)
IPA	85	50	23
Ethanol	211	69	4
Methanol	1580	67	5

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We have thus confirmed the highest selectivity for the methanol, which is probable due to the fact that dissimilar dielectric constants values of these alcohols. For example for the case of methanol the dielectric constant value is ∼32.7, for ethanol it is ∼24.6 and for IPA its ∼18.6.^55–57 Thus, it is expected that the response of SnS₂ nanosheet sensor device slightly differs upon the adsorption of methanol, ethanol and IPA vapors, thus getting different responses.

Conclusion

In conclusion, we have successfully synthesized 2D SnS₂ nanoflakes by one-step hydrothermal synthesis method. The XRD, Raman spectroscopy, TEM, HRTEM and SAED confirmed the hexagonal crystal structure and nanoflake like morphology of SnS₂. Further, humidity sensing performance for the 2D SnS₂ nanoflakes showed the maximum sensitivity of ∼11 300% in 97 RH%. Our humidity sensor device shows fast response time of ∼85 s and record low recovery time of ∼6 s respectively. We have also studied the alcohol sensing properties of SnS₂ nanoflakes for methanol, ethanol and IPA. Compared to ethanol and IPA, methanol showed higher response of ∼1580 at 150 ppm due to high dielectric constant value. The SnS₂ based alcohol sensor device shows a response time of ∼67 s and quick recovery time of ∼5 s in case of methanol.

Conflict of interest

The author declares no competing financial interest.

Acknowledgements

The research work was supported by Department of Science and Technology (Government of India) under Ramanujan Fellowship (Grant No. SR/S2/RJN-130/2012), NCL-MLP project grant 028626, DST-SERB Fast-track Young scientist project Grant No. SB/FT/CS-116/2013, Broad of Research in Nuclear Sciences (BRNS) Grant No. 34/14/20/2015 (Government of India), partial support by INUP IITB project sponsored by DeitY, MCIT, Government of India.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra21252b

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