Facile Fabrication of MoS2-Modified SnO2 Hybrid Nanocomposite for

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Facile Fabrication of MoS2-Modified SnO2 Hybrid Nanocomposite for Ultrasensitive Humidity Sensing Dongzhi Zhang, Yan’e Sun, Peng Li, and Yong Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02206 • Publication Date (Web): 18 May 2016 Downloaded from http://pubs.acs.org on May 19, 2016

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ACS Applied Materials & Interfaces

Facile Fabrication of MoS2-Modified SnO2 Hybrid Nanocomposite for Ultrasensitive Humidity Sensing

Dongzhi Zhang1*, Yan’e Sun 1, Peng Li2*, Yong Zhang 1

1

College of Information and Control Engineering, China University of Petroleum

(East China), Qingdao 266580, China 2

State Key Laboratory of Precision Measurement Technology and Instruments,

Department of Precision Instruments, Tsinghua University, Beijing 100084, China

*Corresponding author: Dongzhi Zhang, Peng Li E-mail address: [email protected], [email protected] Tel: +86-532-86981813 Fax: +86-532-86981335

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Abstract An ultrasensitive humidity sensor based on molybdenum disulfide (MoS2)-modified tin oxide (SnO2) nanocomposite has been demonstrated in this work. The nanostructural,

morphological

and

compositional

properties

of

as-prepared

MoS2/SnO2 nanocomposite were characterized by scanning electron microscopy (SEM), transmission electron microscope (TEM), x-ray diffraction (XRD), energy dispersive spectrometer (EDS), nitrogen sorption analysis and Raman spectroscopy, which confirmed its successful preparation and rationality. The sensing characteristics of the MoS2/SnO2 hybrid film device against relative humidity (RH) were investigated at room temperature. The RH sensing results revealed an unprecedented response, ultrafast response/recovery behaviors and outstanding repeatability. To our knowledge, the sensor response yielded in this work was decades of times higher than that of the existing humidity sensors. Moreover, the MoS2/SnO2 hybrid nanocomposite film sensor exhibited greatly enhancement in humidity sensing performances as compared to the pure MoS2, SnO2 and graphene counterparts. Furthermore, complex impedance spectroscopy and bode plots were employed to understand the underlying sensing mechanisms of the MoS2/SnO2 nanocomposite towards humidity. The synthesized MoS2/SnO2 hybrid composite was proved to be an excellent candidate for constructing ultrahigh-performance humidity sensor towards various applications. Keywords: molybdenum disulfide, hybrid nanocomposite, hydrothermal route, humidity sensing Page 2 of 39


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1. Introduction Humidity sensor is of great importance in many fields, including environmental monitoring, industrial production, agricultural planting, aviation, medical and chemical monitoring.1,

2

Until now, various transduction techniques were used to

develop humidity sensors, such as resistance,3 capacitance,4 impedance,5 optical fiber, 6

and surface acoustic wave (SAW).7 Moreover, plenty of nanomaterials such as metal

oxides,8, 9 graphene,10 carbon nanotubes,11 and nanohybrids

12, 13

have been employed

to fabricate humidity-sensing devices. Remarkably, as one of the most important typical n-type metal oxide semiconductors, SnO2 was used as a suitable humidity sensing nanomaterial due to its excellent electrochemical stability, but suffered from an obvious drawback of low conductivity, long response/recovery time, narrow measuring range and difficulty in integration.14 Recent researches have shown that the decoration of SnO2 nanoparticles with other materials is an effective route to enhance their humidity sensing properties. Pascariu et al. investigated the doping of SnO2 with different proportions of NiO for enhancing humidity-sensing properties.15 Tomer et al. reported Ag decorated in SnO2/SBA-15 hybrid for detecting humidity, and showed good sensitivity and short response/recovery times towards humidty.16 Song et al. fabricated a KCl-doped SnO2 nanofibers-based humidity sensor via screen-printing method, exhibiting much better humidity sensing properties in comparison with the conventional SnO2 humidity sensors.17 As two dimensional (2D) nanomaterials, graphene and molybdenum disulphide (MoS2) are considered to be promising candidates owing to their outstanding virtues, Page 3 of 39



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such as great surface-area-to-volume ratio, high carrier mobility and low noise level.18-20 Compared with graphene with band gap of zero, MoS2 has attracted great interest as a graphene-liked 2D layered nanomaterial with direct band-gap of 1.8 eV and superb properties, which make it an effective nanomaterial for sensing NH3, NO2, H2 and water molecules.21-26 The results have proved that MoS2 has large potential for gas and humidity detection. However, the humidity sensor based on MoS2 modified SnO2 nanocomposite has not been investigated presently. Therefore, constructing a humidity sensor able to realize rapid detection with low power consumption, low cost, excellent performances and possibility of integration, still attract considerable attention. Herein, we demonstrated a MoS2/SnO2 hybrid-based humidity sensor via a facile hydrothermal route on a PI substrate with interdigital electrodes (IDEs). The nanostructural, morphological and compositional characteristics of the MoS2/SnO2 hybrid were fully examined by using XRD, SEM, TEM, EDS, nitrogen sorption (BET) and Raman spectroscopy. The sensing characteristics of the MoS2/SnO2 hybrid film sensor were investigated in a wide relative humidity range, and also made a comparison with that of pure MoS2, SnO2 and graphene counterparts. As the result, MoS2/SnO2 hybrid film sensor exhibits a response of approximating to 3285000% which is unprecedented over traditional humidity sensor, ultrafast response/recovery times, and acceptable repeatability. The underlying sensing mechanism of the MoS2/SnO2 hybrid film sensor towards humidity has been further investigated. 2. Experiment Page 4 of 39


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MoS2 was synthesized by using a facile hydrothermal route.27, 28 Na2MoO4·2H2O (1.0 g) and thioacetamide (1.2 g) were first dissolved into 80 mL of deionized (DI) water, and subsequently stirred for 0.5 h. Then, 0.6 g oxalic acid was added with stirring for another 0.5 h. The resulting suspension was hydrothermally treated at 200°C for 24 h. At last, the obtained sample was further anneal-treated at 700°C in argon for 2 h to get high-quality MoS2 crystalline. For the synthesis of MoS2/SnO2 nanocomposite, SnCl4·5H2O (1.05 g) and NaOH (0.84g) were added into the MoS2 solution with stirring for 0.5 h. And then, hydrothermal treatment of the above-mentioned solution was performed at 180°C for 16 h. The resulting MoS2/SnO2 suspension was reserved for further use. The above-mentioned two-step hydrothermal route of MoS2/SnO2 nanocomposite is shown in Figure 1. The sensor structure with interdigital microelectrodes on PI substrate was fabricated via lithography technology (Figure S1, Supporting Information). The sensing film of MoS2/SnO2 hybrid was prepared by drop-casting method. Moreover, reduced graphene oxide (RGO)/SnO2 hybrid was prepared using the similar route with MoS2 replaced by graphene oxide (GO), and SnO2 sample was prepared by hydrothermal synthesis without MoS2 added. These samples were reserved for making comparison in humidity sensing. 3. Results and discussion 3.1 Materials characterizations The morphologies of MoS2 and MoS2/SnO2 hybrid films were examined using field emission scanning electron microscopy (SEM; Hitachi S-4800). Figure 2a, b illustrates MoS2 has well-defined flake-shaped morphology. Figure 2c, d shows the Page 5 of 39



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MoS2/SnO2 hybrid is constructed by the MoS2 nanosheets uniformly decorated with SnO2 nanocrystals. The nanostructure of the as-prepared samples was observed by a transmission electron microscope (TEM; JEOL JEM-2100). Figure 3a shows the typical TEM image of MoS2 nanosheets with size about 50 nm. Figure 3b demonstrates the MoS2 nanosheets are coated with SnO2 nanocrystals. High-resolution TEM observation was further performed to confirm the crystallinity of MoS2 and SnO2. Figure 3c shows a typical HRTEM image taken from MoS2 layer, clearly revealing the lattice fringe spacing of 0.620 nm and 0.273 nm, attributed to the (002) and (100) plane of MoS2 nanocrystals.

29, 30

The SAED rings (inset of Figure 3c) of MoS2 nanosheets can be

indexed to the reflections of the MoS2 (002), (100), (103), and (110) planes. Figure 3d shows SnO2 nanocrystals on MoS2 nanosheets with labeled lattice fringe of 0.342 nm, attributing to the (110) plane of the SnO2 nanocrystal. Moreover, a few layers (1-4) of MoS2 sheets with an interlayer spacing of 0.640 nm were observed at the folded edges of MoS2 nanosheets. The XRD measurements of MoS2, SnO2 and MoS2/SnO2 samples were performed with an x-ray diffractometer (Rigaku D/Max 2500PC). The x-ray diffractograms (Figure 4a) for the synthesized samples display diffraction peaks appeared in a range of 10°–80°. The diffraction peaks of MoS2 nanosheet are observed at 2θ of 14.4°, 33.1°, 39.7° and 58.5°, which are according to the MoS2 nanocrystal of (002), (100), (103), and (110) planes.

27, 28

No peaks for other

substance were observed in the spectrum, illustrating high-purity MoS2 was well Page 6 of 39


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crystallized. XRD spectrum of SnO2 nanocrystal shows feature peaks observed at different planes for the tetragonal rutile crystal structure (JCPDS 71-0652).

31

The

XRD pattern for the MoS2/SnO2 hybrid indicates the distinct peaks corresponding to the presence of MoS2 and SnO2 crystals, illustrating the successful preparation of MoS2/SnO2 nanocomposite through two-step hydrothermal synthesis. The Raman spectra (Figure 4b) for the MoS2 and MoS2/SnO2 samples were measured using a confocal Raman microprobe (RamLab-010). The Raman pattern of MoS2 exhibits two strong bands at 382.6 cm−1 and 404.4 cm−1 which are attributed to the modes of in-plane and out-of-plane vibration, respectively.21, 24, 32, 33 The Raman pattern of MoS2/SnO2 nanocompoiste exhibits the featured peaks of MoS2 and confirms the presence of MoS2, and the weak peak located at 633.1 cm−1 for MoS2/SnO2 composite is associated with the out-of-plane vibration of SnO2 crystal. 34 The MoS2/SnO2 hybrid was further characterized by Hitachi S-4800 equipped with an energy dispersive spectrometer (EDS). The EDS spectrum (Figure 4c) clearly revealed that only the elements Mo, S, Sn and O are detected in the MoS2/SnO2 hybrid, no other impurity element has been observed. The elemental composition in the MoS2/SnO2 hybrid indicates a reasonable distribution. Brunaue-Emmett-Teller (BET) measurement on the MoS2/SnO2 hybrid was performed by a surface area analyzer

(Micromeritics,

ASAP

2020M).

Figure

4d

shows

the

N2

adsorption–desorption isotherm measurement and pore size distribution. A distinct hysteresis loop at the P/P0 range of 0.45-0.98 is observed, and BET surface area of 117.02 m2/g for the MoS2/SnO2 hybrid is measured. The pore size distribution of the Page 7 of 39



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MoS2/SnO2 hybrid is observed around 3-6 nm, determined by nitrogen physisorption and Barrett-Joyner-Halenda (BJH) method. 3.2 Humidity-sensing performances The humidity-sensing experiment is carried out at ambient temperature of 20ºC. The saturated salt solutions were used to yield different RH levels, as reported in our previous work [35, 36]. Phosphorus pentoxide powder (P2O5) was used as desiccant to outtake the water molecules (0% RH) for the sensor recovery. Two conducting wires firmly sealed in the rubber seal were connected between the sensor under test and the measuring instrument. The impedance and capacitance of the sensor against RH were recorded using TH 2828 precision LCR meter. The complex impedance spectroscopy of the as-prepared MoS2/SnO2 nanocomposite film sensor was measured by HP 4194A impedance analyser. Normalized response (R) and sensitivity (S), determined by R=Rx-R0/R0×100% and S=∆R/∆RH, respectively, where Rx and R0 are the impedance or capacitance response of the sensor at x% RH and dry air, ∆R is the response change in impedance or capacitance for the sensor, ∆RH is the change of relative humidity, respectively. The impedance of the MoS2/SnO2 film versus different RH at various operation frequencies is given in Figure 5. It clearly displays that the impedance decreases with the raising of RH at the same applied frequency, also along with the increasing of operation frequency at each RH level. The impedance-frequency curve is inclined to be more flat at higher frequencies. This phenomenon can be interpreted as that, the induced polarization of water vapor adsorbed in the MoS2/SnO2 hybrid is difficult to Page 8 of 39


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keep accordance with the rapid change of electrical field at the higher frequency.36 The film impedance shows a sensitivity of 156.97 Ω/%RH at 100 Hz. Figure 6 shows the capacitance of the MoS2/SnO2 film versus different RH levels at various frequencies. The sensor capacitance shows much larger variation towards RH exposure at 100 Hz. An ultrahigh sensitivity of 12809pF/%RH is obtained at 100 Hz. In comparison with the impedance-RH characteristics, the sensor capacitance exhibited much more remarkable properties in RH measurement. The capacitance of the MoS2/SnO2 nanocomposite film sensor towards different RH levels at 100 Hz is shown in Figure 7. The RH measurement was switched by exposing the sensor between dry air and various RH levels. The time interval for response/recovery duration is 60 s. The sensor capacitance increases about five orders of magnitude from decades of pF to 1131630 pF toward RH varies from dry air to 97% RH, which ranks it among the highest ever reported values for RH detection. 36-39 As shown in Figure 8, the MoS2/SnO2 hybrid film sensor presents a swift response and recovery behavior towards RH, indicating a fast adsorption and desorption time for the film toward water vapor. The transient measurement is performed by switching the sensor from dry air to a given RH level, and then back to dry air after reaching steady. The response/recovery time of the MoS2/SnO2 hybrid film sensor is noted less than 5/13 s. Figure 9 shows the acceptable repeatability of the MoS2/SnO2 film sensor upon exposure to 33%, 52%, 85% RH from dry air, and the corresponding response of the sensor is up to 8.8×103 % (33% RH), 9×104 % (52% RH) and 2.1×106 % (85% RH), respectively. Page 9 of 39



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The humidity-sensing response of MoS2/SnO2 film sensor is compared with pure MoS2, pure SnO2 and RGO/SnO2 film sensor (Figure 10). The characterization of RGO/SnO2 film can be seen in the Supporting Information (Figure S2). The four sensors were tested under the same experimental conditions over a wide RH range of 11-97% RH. Convenient for observation, the inset plots the comparative response of the four sensors exposed to 11-52% RH. We can obviously find that the MoS2/SnO2 film sensor exhibits the highest response among the four sensors. For instance, the normalized response values measured at 75% RH are 5919%, 572318%, 681553% and 1167620% for pure MoS2, pure SnO2, RGO/SnO2 and MoS2/SnO2 sensor, respectively. Figure 11 plots the normalized response as a function of RH for the MoS2/SnO2 film sensor at 100 Hz. The sensor response Y shows an exponential fitting of the relative humidity X, which can be described by Y=359.723e-X/21.2-1617.374. The MoS2/SnO2 film sensor yields an unprecedented response up to 3285000% against humidity at room temperature. Moreover, the influence of temperature on the humidity sensing performance for the MoS2/SnO2 film sensor was further investigated in this work (Figure S3, Supporting Information), showing that the capacitance of the sensor decreases gradually with the ascending of temperature under tested RH, owing to the dielectric constant of adsorbed water molecules has a certain amount of dependence on the temperature. Table 1 presents the humidity sensing characteristics of this work in comparison with previous works.36-39 The sensing performance for the MoS2/SnO2 film sensor is comparable to those of conventional sensors by layer-by-layer self-assembly, solution Page 10 of 39


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dripping and inkjet-printing methods. The comparison highlights this work ranks the highest in terms of response and sensitivity among the existing conventional sensors. 3.3 Humidity-sensing mechanism The MoS2/SnO2 hybrid nanocomposite-based humidity sensor exhibited excellent sensing properties, including unprecedented response, ultrafast response and recovery times, and acceptable repeatability. The SnO2 nanocrystals dispersed on the MoS2 surface serve as adsorption centers, which are dominant in the interaction between the MoS2/SnO2 hybrid and water molecules. MoS2 nanosheets act as an anchor in the MoS2/SnO2 hybrid and play a dominant role in eliciting the sensor response. The sensing response of MoS2/SnO2 is better than that of RGO/SnO2 for humidity detection, indicating that MoS2 is superior to RGO in capturing water molecules. Modification of MoS2 with SnO2 can act synergistically to offer unique physicochemical and electronic properties for humidity sensing. The adsorption of water vapor on the MoS2/SnO2 hybrid is shown in Figure 12. The chemisorbed water molecules are initially formed, and the physical adsorption of water vapor will occur on the chemisorbed layer as RH increasing. H3O+ will produce as charge carriers due to the ionization of water molecules under an electrostatic field.

36, 37, 40

As the water

molecules adsorption continues, the multi-layer physisorption of water molecules exhibit liquid-like behavior, the protons hopping transport via ionic conductivity, which was brought about by H2O + H3O+ = H3O+ + H2O.41, 42 At high RH conditions, the free water can interpenetrate into the interlayer of MoS2/SnO2 film, which is contributed to a large enhancement in dielectric constant and sensor response. Page 11 of 39



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To further discover the underlying sensing mechanism of the MoS2/SnO2 hybrid film, the complex impedance spectroscopy (CIS) combining with bode plots are alternative and effective approaches.36 Figure 13 shows the CIS plots of the MoS2/SnO2 hybrid film sensor and corresponding equivalent circuit (EC) models. The CIS plot (Figure 13a) at low humidity (11, 23 and 33%RH) is similar to a line, indicating a “non-Debye” effect resulted by the intrinsic impedance of the MoS2/SnO2 hybrid film. The EC of such CIS is represented by a resistor and a constant phase element (CPE) in parallel (Figure 13d). 8, 43 Here Rct is the charge transfer resistance, and CPE1 is the impedance of MoS2/SnO2 hybrid film. It can be demonstrated using the corresponding bode plot of the MoS2/SnO2 hybrid film sensor at 11% RH (Figure 14a), in which the phase angle looks flat at high frequency part. In this case, the ion transfer is difficult to across MoS2/SnO2 hybrid film due to the fact that the adsorbed water is discontinuous. As RH increases (43, 52 and 67%RH), the CIS plots are shown in Figure 13b. Each curve is composed by a short line at the end of a circular arc. The short line in the low frequency region represents Warburg behavior. 5, 44, 45 The EC of such CIS plots in this case is shown in Figure 13e. Herein CPE2 is introduced to represent the Warburg impedance at electrode-film interface for the MoS2/SnO2 film device. The bode plot at 43%RH (Figure 14b) indicates that the phase angle ascends with the rising of operation frequency, which is different with that at 11% RH. In the situation, the film surface will absorb much more water molecules as RH increasing and form physisorbed water layer. The protons hopping transport caused by Grotthuss chain Page 12 of 39


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reaction takes important role in this case. At high humidity (75, 85, 95% RH), the CIS curves (Figure 13c) indicate the line at the tail end of the circular arc becomes longer as the raising of RH while part of the circular arc becomes invisible. The corresponding EC is shown in Figure 13f, Rf is introduced as film resistance mainly caused by the penetrated free water into the film. Figure 14c shows the bode plot of the MoS2/SnO2 hybrid film sensor under 85% RH. At this case, multi-layer physisorption of water vapor promotes the protons transfer and penetration into the MoS2/SnO2 hybrid film, leading to a great decrease in the sensor impedance and a sharp increase in the sensor capacitance. 4. Conclusions In summary, this paper demonistrated a high-performance humidity sensor based on MoS2/SnO2 hybrid, which is decades of times higher than that of the existing conventional sensors. The synthesized MoS2/SnO2 sample was characterized in terms of its nanostructural, morphological and compositional features by XRD, SEM, TEM, BET, EDS and Raman spectroscopy. The humidity sensing properties of MoS2/SnO2 hybrid film sensor was investigated over a wide RH range at room temperature. The results highlight that the MoS2/SnO2 hybrid film sensor has not only ultrahigh sensitivity at room temperature, but also rapid response/recovery times and good repeatability. The sensing performance is greatly better than that of pure MoS2, pure SnO2, and RGO/SnO2 film counterparts.

ASSOCIATED CONTENT Supporting information Page 13 of 39



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Materials, sensor fabrication process, characterization of RGO/SnO2 nanocomposite, temperature influence on the humidity sensing.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China under Grant 51407200 and 51405257, the Science and Technology Plan Project of Shandong Province under Grant 2014GSF117035, the Fundamental Research Funds for the Central Universities of China under Grant 15CX05041A, and the Science and Technology Project of Huangdao Zone, Qingdao, China, under Grant No. 2014-1-51.

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Table 1 Comparison in sensing properties towards various humidity sensors. Type

Fabrication

Meas. range

Response

Sensitivity

Ref.

MoS2/SnO2

Hydrothermal

0-97%RH

3285000%

12809pF/%RH

This work

Graphene oxide/polyelectrolyte

Layer-by-layer self-assembly

11-97%RH

265640%

1552.3pF/%RH

36

Graphene oxide

Solution dripping

15%-95%RH

37800%

46.253pF/%RH

37

polyimide

Inkjet-printing

16%-90%RH

155500%

24.5pF/%RH

38

Au/PVA

Drop-casting

11.3%-97%RH

80.86%

11.38pF/%RH

39

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Figure captions Figure 1. Two-step hydrothermal route for MoS2/SnO2 nanocomposite. Figure 2. SEM characterization of (a) and (b) MoS2, (c) and (d) MoS2/SnO2 nanocomposite at different scale. Figure 3. TEM images of (a) MoS2 nanosheets, and (b) MoS2/SnO2 hybrid. HRTEM images of (c) MoS2 nanocrystals, and (d) MoS2/SnO2 hybrid. Figure 4. (a) X-ray diffractograms of MoS2, SnO2, and MoS2/SnO2 samples. (b) Raman patterns for MoS2 and MoS2/SnO2 hybrid. (c) EDS spectrum and elemental composition of MoS2/SnO2 hybrid. (d) N2 adsorption–desorption isotherm and pore diameter distribution of MoS2/SnO2 hybrid. Figure 5. Impedance versus relative humidity for the MoS2/SnO2 hybrid film under varying operation frequencies. Figure 6. Capacitance versus relative humidity for the MoS2/SnO2 hybrid film under various operation frequencies. Figure 7. Capacitance of the MoS2/SnO2 hybrid film sensor upon exposure to various RH levels at 100 Hz. Figure 8. Response/recovery characteristics of the MoS2/SnO2 hybrid film sensor towards various RH levels. Figure 9. Repeatability performance of the MoS2/SnO2 hybrid film sensor exposed to 33% RH, 52% RH, and 85% RH from 0%RH.

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Figure 10. Comparative results of MoS2/SnO2 hybrid film sensor in sensing response with pure MoS2, pure SnO2 and RGO/SnO2 hybrid film. Inset: the comparative normalized response of the four sensors exposed to11-52% RH. Figure 11. The sensor response as a function of RH in the range of 11-97%. Inset: the sensor response towards 11-52% RH. Figure 12. Schematic of humidity sensing at the MoS2/SnO2 hybrid film. Figure 13. Complex impedance plots at (a) low, (b) medium, and (c) high humidity, and equivalent circuits of the MoS2/SnO2 hybrid film sensor at (d) low, (e) medium, and (f) high humidity (ReZ: real part; ImZ: imaginary). Figure 14. Bode plots of the MoS2/SnO2 hybrid sensor towards (a) low humidity (11% RH), (b) medium humidity (43% RH), and (c) high humidity (85% RH).

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Figure 1. Two-step hydrothermal route for MoS2/SnO2 nanocomposite.

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(a)

(b)

(c)

(d)

Figure 2. SEM characterization of (a) and (b) MoS2, (c) and (d) MoS2/SnO2 nanocomposite at different scale.

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Figure 3. TEM images of (a) MoS2 nanosheets, and (b) MoS2/SnO2 hybrid. HRTEM images of (c) MoS2 nanocrystals, and (d) MoS2/SnO2 hybrid.

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Figure 4. (a) X-ray diffractograms of MoS2, SnO2, and MoS2/SnO2 samples. (b) Raman patterns for MoS2 and MoS2/SnO2 hybrid. (c) EDS spectrum and elemental composition of MoS2/SnO2 hybrid. (d) N2 adsorption–desorption isotherm and pore size distribution of MoS2/SnO2 hybrid.

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Figure 5. Impedance versus relative humidity for the MoS2/SnO2 hybrid film under varying operation frequencies.

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Figure 6. Capacitance versus relative humidity for the MoS2/SnO2 hybrid film under various operation frequencies.

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Figure 7. Capacitance of the MoS2/SnO2 film sensor upon exposure to various RH levels at 100 Hz.

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Figure 8. Response/recovery characteristics of the MoS2/SnO2 hybrid film sensor towards various RH levels.

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Figure 9. Repeatability performance of the MoS2/SnO2 hybrid film sensor exposed to 33% RH, 52% RH, and 85% RH from dry air.

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Figure 10. Comparative results of MoS2/SnO2 hybrid film sensor in sensing response with pure MoS2, pure SnO2 and RGO/SnO2 hybrid film. Inset: the comparative normalized response of the four sensors exposed to11-52% RH.

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Figure 11. The sensor response as a function of RH in the range of 11-97%. Inset: the sensor response towards 11-52% RH.

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Figure 12. Schematic of humidity sensing at the MoS2/SnO2 hybrid film.

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Figure 13. Complex impedance plots at (a) low, (b) medium, and (c) high humidity, and equivalent circuits of the MoS2/SnO2 hybrid film sensor at (d) low, (e) medium, and (f) high humidity (ReZ: real part; ImZ: imaginary).

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Figure 14. Bode plots of the MoS2/SnO2 hybrid sensor towards (a) low humidity (11% RH), (b) medium humidity (43% RH), and (c) high humidity (85% RH).

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Graphical Abstract

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Facile Fabrication of MoS2-Modified SnO2 Hybrid Nanocomposite for

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