Dispersed SnO₂ nanoparticles on MoS₂ nanosheets for superior gas-sensing performances to ethanol

Abstract

The unique properties of MoS₂ nanosheets make them a promising supporting substrate for preventing the interparticle aggregation of metal–oxide–semiconductor nanomaterials. Novel composites were successfully obtained by a two-step low temperature hydrothermal method for the synthesis of SnO₂ nanoparticles dispersing on the surfaces of MoS₂ nanosheets. The morphology and structure of the as-prepared samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). Owing to the supporting substrate of specific two-dimensional MoS₂ nanosheets and the superior gas-sensing performance offered by ultrasmall SnO₂ nanoparticles, the sensor based on SnO₂@MoS₂ composites exhibits high response and good selectivity to ethanol gas.

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Introduction

Metal–oxide–semiconductor (MOS) nanomaterials with small sizes exhibit high sensitivity for gas detection, owing to the fact that chemical interaction of gas molecules with the semiconductor's surface leads to changes in the electrical conductivity.^1–3 The n-type metal oxide semiconductor gas sensors, such as SnO₂, ZnO, and Fe₂O₃ have been employed in commercial applications.^4–6 Of all the sensing materials currently investigated, tin dioxide (SnO₂) nanomaterials have attracted considerable attention, because of their wide applications in lithium-ion batteries, gas sensors, sensitized solar cells, and catalysts.⁷ To improve the gas sensing performance, many novel SnO₂ nanostructures, such as zero-dimensional (0D) nanoparticles,⁸ one-dimensional (1D) nanowires,⁹ two-dimensional (2D) nanosheets,¹⁰ and three-dimensional (3D) hierarchical architectures,¹¹ have been reported. However, the strong interactions of metal oxides nanomaterials, especially for their nanostructured counterparts, cause them to clump and aggregate, thereby deteriorating the sensing performance. Accordingly, dispersing metal oxide nanostructures on various supporting substrates is regarded as a very promising approach for preventing the interparticle aggregation.^12–16

Among various substrates, two-dimensional (2D) supporters are appealing candidates because they provide a platform for attaching the nanostructures. Graphene, composed of monolayers of carbon atoms arranged in a honeycombed network, is a robust substrate due to its advantages of atomically smooth surfaces, transparency, nontoxicity, and structural stability.¹⁷ So far, graphene is a good supporting substrate for dispersing metal oxide nanostructures. For instance, Neri et al. investigated the sensing behavior of SnO₂/reduced graphene oxide nanocomposites toward NO₂.¹⁸ Liang et al. have shown that the synthesized α-Fe₂O₃@graphene nanocomposites behave enhanced gas-sensing property to ethanol.¹⁹ However, the lack of a band gap in graphene significantly limits its application.²⁰ In addition to graphene, MoS₂ has been attracting increasing attention because of its significant direct band gap (1.8 eV), large surface-to-volume ratio, and outstanding field-effect transistor (FET) behavior. Layered MoS₂ is one of the typical graphene analogues. Owing to the specific 2D confinement of electron motion and the absence of interlayer perturbation, the MoS₂ monolayer possesses a direct band gap.^21–23 The unique properties of MoS₂ nanosheets make them a promising candidate for high-performance sensing materials.^24–26 Surprisingly, as compared to the significant progress achieved in the graphene-based gas-sensing materials, the MoS₂-supported composites have been lingering far behind. As far as we know, few studies on the gas-sensing properties of metal oxide/MoS₂ composites have been reported. As compared to the significant progress achieved in the graphene-based gas-sensing materials, the MoS₂-supported composites have been lingering far behind.

Herein, we develop a two-step low temperature hydrothermal method for the synthesis of SnO₂ nanoparticles dispersing on the surfaces of MoS₂ nanosheets. To synthesize MoS₂ nanosheets, sodium molybdate was chosen as the precursor for molybdenum and thioacetamide was used as the sulfur source. During the subsequent hydrothermal treatment, MoO₄²⁻ anions were reduced under high temperature condition, forming MoS₂ nanosheets without adding any 2D substrate. SnO₂ nanoparticles were prepared using a simple hydrothermal method and dispersed onto MoS₂ nanosheets. A comparative study between pure SnO₂ nanoparticles and SnO₂@MoS₂ composites was performed to reveal the promotion effect of MoS₂ nanosheets on gas-sensing performance. Experimental results showed that SnO₂@MoS₂ composites exhibit superior gas-sensing performance to ethanol in comparison with pure SnO₂ nanoparticles. A possible sensing mechanism was also proposed for the SnO₂@MoS₂ composites.

Experimental

Synthesis of MoS₂ nanosheets

All the chemical reagents were analytical graded and used without further purification. The MoS₂ precursors were synthesized through a simple hydrothermal method. In a typical reaction, 2 mmol of sodium molybdate (Na₂MoO₄·2H₂O) and 9 mmol of thiocarbamide (CH₄N₂S) were mixed and dissolved in 70 mL deionized water. After stirring the solution for 30 min, 2.2 mmol of citric acid (C₆H₈O₇·H₂O) was added into the above solution. After magnetically stirring for 10 min, the homogeneous solution was transferred into a 100 mL Teflon-lined stainless steel autoclave, sealed tightly, and maintained at 200 °C for 21 h. After the hydrothermal procedure, the autoclave cooled down to room temperature spontaneously. The black precipitates were collected by centrifugation, washed several times with deionized water and absolute ethanol, respectively, and dried at 60 °C for 12 h in air.

Synthesis of SnO₂@MoS₂ composites

The synthesis of SnO₂@MoS₂ composites were used by the hydrothermal method. The as-fabricated MoS₂ (0.1 g) was dissolved in 80 mL deionized water and ultrasonic dispersion 20 min to ensure the black powders was completely dispersed in the solution. SnCl₂·5H₂O (0.438 g) and NaOH (0.3 g) dissolved in the solution. After magnetic stirring for 30 min, a suspension was obtained and transferred into a 100 mL stainless steel autoclave with Teflon-lined, which was heated at 180 °C for 18 h. After the autoclave was cooled down to room temperature, the precipitates were collected and washed several times with deionized water and ethanol, respectively. After drying at 60 °C for 6 h, the ashen powders were obtained.

Characterization

The crystal structure and phase composition of as-prepared samples were identified by X-ray diffraction (XRD, Bruker D8 Advance) using CuKα₁ radiation (λ = 0.15406 nm) at 30 kV and 40 mA at a scanning rate of 2° at 2θ min⁻¹. The morphology and nanostructure of the products were characterized using FEI Sirion 200 field emission gun scanning electron microscope (FESEM, Hitachi S4800), and transmission electron microscopy (TEM, Hitachi H-800). More details about the structure were investigated by the selected area electron diffraction (SEAD) pattern and high resolution transmission electron microscopy (HRTEM, JEOL 2010). X-ray photoelectron spectra (XPS) were measured using a PHI 5300 X-ray photoelectron spectrometer with AlKα radiation.

Fabrication and measurement of gas sensor

The obtained samples were firstly mixed with distilled water to form slurry through milling, and then pasted onto a prefabricated alumina tube (7 mm in length and 1.5 mm in diameter, attached with a pair of gold electrodes and platinum wires) by a small brush to form a thick film. The thickness of sensing films was about 100 μm. A Ni–Cr resistor (diameter = 0.5 mm, resistance = 35 Ω) in the inner alumina ceramic tube, as a heater, was used to provide the working temperature for the sensor device. After dried in the air and aged at 300 °C for 2 days, it has been an indirectly-heated gas sensor, and then the gas sensor was putted into the test chamber in a measuring system of WS-30A (Winsen Electronics Co. Ltd, Zhengzhou, China) by a static process. A typical testing procedure was as follows: the sensors were put into a glass chamber (18 L) at the beginning. When the resistances of all the sensors were stable, the calculated amount of the target gas or liquid was injected into glass chamber by a micro-injector and mixed with air. For the target gases obtained from liquid, the concentration of target gas was calculated by the following formula,

C = (22.4 × ρ × d × V₁)/(M × V₂) (1)

where C (ppm) is the target gas concentration, ρ (g mL⁻¹) is the density of the liquid, d is the purity of the liquid, V₁ (μL) is the volume of the liquid, V₂ (L) is the volume of the glass chamber, and M (g mol⁻¹) is the molecular weight of the liquid. The working temperature of the sensor can be controlled by adjusting the heating voltage (V_heating) across a Ni–Cr alloy resistor inside the ceramic tube. A reference resistor (R_load) is put in series with the sensor to form a complete measurement circuit. In the test process, a working voltage of 5 V (V_working) was applied. By monitoring the voltage across the reference resistor (V_output), the response of the sensor in air or in a test gas could be measured. The sensor response was defined as,

Response = R_gas/R_air (2)

where R_air is the resistance of the sensor in air and R_gas is the resistance of sensor in the presence of the test gas.

Results and discussion

The purity and crystalline phase of as-synthesized pure MoS₂, SnO₂ nanoparticles and SnO₂@MoS₂ composites were analyzed by a powder X-ray diffractogram. Fig. 1 shows the XRD patterns of the samples, which displays diffraction peaks in the range of 10–80°. As for the pure MoS₂ sample, the detected peaks at 2θ = 14.4°, 33.1°, 39.7° and 58.5° can be assigned to the (002), (100), (103) and (110) planes in the hexagonal phase of MoS₂ (JCPDS 37-1492). Moreover, the diffraction peaks of SnO₂ can be easily indexed to a pure tetragonal rutile structured tin dioxide, which was consistent with the standard data file (JCPDS file no. 41-1445). No other diffraction peaks were observed. In additional, the crystallite size of SnO₂ is about 8.2 nm, which is estimated using the Scherrer formula,

D = Kλ/β cos θ (3)

where D is the grain size, K is the Scherrer constant, usually is 0.89, λ is the incident X-ray wavelengths, and β is the wide of the half of the diffraction peak, θ is the Bragg diffraction angle. The XRD pattern for the SnO₂@MoS₂ composites reveals the presence of individual components of SnO₂ and MoS₂. The diffraction peak at 14.5° is corresponding to the c-plane of MoS₂ and can be used to study the structure of MoS₂, which is composed of Mo atoms coordinated with S atoms to form the S–Mo–S sandwich layer.²⁷ In order to confirm the decoration of Au nanoparticles on the surface of hierarchical MoS₂ nanostructures, XPS analysis was performed and the patterns are presented in Fig. S1.† The doublets of Sn 3d_5/2 and 3d_3/2 peaks, located at 487.2 and 494.8 eV, correspond to the Sn⁴⁺ of SnO₂.²⁸ Fig. 3b clearly shows the appearance of a spin–orbit doublet at 232.9 (3d_3/2) and 229.7 eV (3d_5/2), which is attributed to the Mo⁴⁺ of MoS₂.²⁹

Fig. 1 XRD patterns of pure MoS₂, SnO₂, and SnO₂@MoS₂ composites.

The microstructure and morphology of the as-prepared samples were further characterized by FESEM and TEM. As shown in Fig. 2a, the surface morphology of pure MoS₂ could be clearly observed from typical FESEM image. It can be seen that hierarchical MoS₂ is flower-like nanosphere with a diameter of about 1.5 μm. A high magnification FESEM image of MoS₂ is shown in Fig. S2a,† indicating large amount of uniform MoS₂ nanosheets. In good agreement with the FESEM image, a low-magnification TEM image (Fig. S2b†) of a single MoS₂ nanoflower. It can be seen that the flower-like nanostructure is constructed by relatively densely packed nanosheets. Under the examination of TEM (Fig. S1c†), the MoS₂ nanosheet is fully transparent, showing the extremely small thickness of this 2D structure. With the wrinkles and scrolling, the morphology of the MoS₂ nanosheet is similar to that of a single graphene nanosheet. Moreover, the corresponding SAED pattern (Fig. S1d†) confirms the hexagonal structure of hierarchical MoS₂ nanostructures and presents well-defined rings that can be well indexed to the XRD patterns. The as-synthesized MoS₂ nanosheets were further decorated with SnO₂ nanoparticles via another hydrothermal process. As shown in Fig. 2b, SnO₂ nanoparticles dispersed on the MoS₂ nanoflower. A few SnO₂ nanoparticles aggregations are observed in FESEM image. The intriguing structure is also elucidated under TEM to provide further insight about the morphology and microstructure. Fig. 2c is the TEM image of typical MoS₂ nanosheets inlayed with several dispersive SnO₂ nanoparticles. It can be seen that the sizes of the SnO₂ nanoparticles are less than 10 nm, which is in accord with the results of XRD patterns. Under a higher magnification, the ultrasmall SnO₂ nanoparticles attached on MoS₂ nanosheets can be well observed. The cross-sections of MoS₂ nanosheets can be observed. As shown in Fig. 2d, the distance between two adjacent atomic planes were calculated to be around 0.64 nm, corresponding to the interplanar distance of (002) plane of hexagonal MoS₂ crystalline structure. The lattices of SnO₂ nanoparticles can be also clearly observed with the interplanar distance of 0.34 nm, matching the (110) plane of cubic SnO₂ crystalline structure (JCPDS 41-1445).

Fig. 2 (a) FESEM image of pure MoS₂, (b) FESEM image and (c and d) TEM images of SnO₂@MoS₂ composites.

On the basis of the results stated above, the formation process of SnO₂@MoS₂ composites in hydrothermal system is concluded. First, sodium molybdate was chosen as the precursor for molybdenum and thioacetamide was used as the sulfur source. During the hydrothermal treatment, MoO₄²⁻ anions were reduced under high temperature condition, forming MoS₂ nanoparticles.^30,31 Second, these nanoparticles grow up into nanosheet structures in order to reduce the high surface energy through the process known as oriented aggregation. As the reaction further proceeds, the nanosheets tend to merge together to well-defined MoS₂ nanoflowers through a self-assembly process.^32,33 In addition, the formation of SnO₂ nanoparticles adhering on the MoS₂ can be expressed as follows:^34,35

Sn⁴⁺ + 6OH⁻ → Sn(OH)₆²⁻ (4)

Sn(OH)₆²⁻ → SnO₂ + 2H₂O (5)

To explore the advantages of the SnO₂@MoS₂ composites, the as-prepared product was evaluated as sensing material to investigate its gas sensing performance. For comparison, the sensing properties of the pure SnO₂ nanoparticles prepared by hydrothermal method were also studied. It is well known that the gas response of semiconductor gas sensors is greatly influenced by the operating temperature. In order to determine the optimum operating temperatures of sensors based on SnO₂@MoS₂ composites and pure SnO₂ nanoparticles, the response of two sensors to 200 ppm ethanol were tested as a function of operating temperature. As shown in Fig. 3, both sensors exhibit peak-shaped dependence on the operating temperature. It can be clearly observed that the ethanol response first increased with working temperature, and then gradually decreased when the temperature further increases. The relative optimum working temperature can be explained as follows: normally, the reactivity between the target gas and adsorption oxygen needs certain activation energy, which is provided by increasing reaction temperature. At low working temperature, the adsorbed methanol molecules are not activated enough to overcome the activation energy barrier to react with the adsorption oxygen species, while at high temperatures the gas adsorption is too difficult to be adequately compensated for the increased surface reactivity.^36–38 Moreover, it can be observed that the pure SnO₂ nanoparticles have the maximum gas response at 340 °C, whereas SnO₂@MoS₂ composites have the maximum gas response at 280 °C. Compared with pure SnO₂ nanoparticles, SnO₂@MoS₂ composites exhibit superior gas-sensing performances to ethanol.

Fig. 3 Correlation between gas response to 200 ppm ethanol and the operating temperature for the sensors based on SnO₂@MoS₂ composites and pure SnO₂ nanoparticles.

The response and recovery time is an important factor to evaluate the gas sensing properties of the sensor. The response time was defined as the time required for the variation in resistance to reach 90% of the equilibrium value after a test gas was injected, and the recovery time as the time necessary for the sensor to return to 10% above the original resistance in air after releasing the test gas. Fig. 4a shows the dynamic response transient of the SnO₂@MoS₂ composites gas sensor to different concentrations of ethanol at its optimum operating temperature of 280 °C. It is clear that the response curves of the sensor increases sharply with increasing concentration of ethanol and then returns to the baseline quickly with the ethanol exhausted out in the closed testing chamber, indicating their quick and reversible response and recovery time. Moreover, it can be seen that the response of the sensors increases with the increasing of the ethanol concentrations. The sensor response can be empirically represented as S = a(C)^b + 1, where a and b are the constants and S is the gas response, C is the concentration of the test gas.^39,40 Fig. 4b shows a chart of logarithm of the response of the sensor (S − 1) versus the logarithm of methanol concentration (C). It can be found that the response of the sensor based on SnO₂@MoS₂ composites has a good linear relationship with the ethanol concentration (50–1000 ppm range) in logarithmic forms. As shown in Fig. 3c, the straight line is the calibration curve ant the experimental data were fitted as: y = 0.4717 × x + 0.9561, where y is log(S − 1) and x is log(C). The results highlighted the potential applications of SnO₂@MoS₂ composites in monitoring ethanol gas. The selectivity of gas sensors is the ability that a sensor can distinguish different kinds of gases, which is also important for the gas sensing properties. Fig. 5 demonstrated that the gas sensor show responses to various gases of 200 ppm, including ammonia, ethanol, formaldehyde, and acetone. Significantly, the sensor based on SnO₂@MoS₂ composites displayed a much higher response and better selectivity to ethanol as opposed to any other test gases at the working temperature of 280 °C.

Fig. 4 (a) Dynamic response transient of the SnO₂@MoS₂ composites gas sensor to different concentrations of ethanol at 280 °C. (b) Dilogarithm fit curve of the response of the sensor to the concentration of ethanol.

Fig. 5 Response of the sensor based on SnO₂@MoS₂ composites to various test gases.

SnO₂ is well-known as an n-type gas sensing material and its gas sensing mechanism belongs to the surface-controlled type, the change of resistance is due to the species and the amount of chemisorbed oxygen on the surface. Meanwhile, the tests of gas sensing properties can be explained by the gas sensing mechanism of SnO₂ samples, with the changing in resistance of the sensor upon exposure to different gas atmospheres. When the sensors were exposed to air, the resistance of SnO₂ is controlled by the concentration of adsorbed oxygen species (O₂⁻, O⁻ or O²⁻) that trap electrons and act as scattering centers effectively reducing the semiconductor conductivity. When the sensor is exposed to ethanol vapor at higher temperature, ethanol reacts with the adsorbed oxygen ions reducing their concentration and thereby increasing the semiconductor conductivity. The possible reactions took place on the surface of tin oxide as follows:^41–43

CH₃CH₂OH + 6O⁻ → 2CO₂ + 3H₂O + 6e⁻ (6)

The SnO₂ sensors change to the initial electronic structure when exposed to air again. The SnO₂@MoS₂ composites exhibit superior gas-sensing performances to ethanol, which is better than that of its counterpart of SnO₂ nanoparticles. That is, there exists a beneficial effect of dispersing SnO₂ nanoparticles on MoS₂ nanosheets. As shown in Fig. 6, the improvement of sensing performance of SnO₂@MoS₂ composites may be attributed to the following reasons. Firstly, MoS₂ nanosheet with high surface area provides a platform for attaching the SnO₂ nanoparticles, preventing their interparticle aggregation. This kind of nanostructure can provide a large specific surface area, which is of great benefit to numerous oxygen molecules adsorbed onto SnO₂ nanoparticles, and facilitate the diffusion of ethanol gas, improving the reaction of the ethanol gas with surface adsorbed oxygen.^44,45 Besides, the operating temperature of SnO₂@MoS₂ composite sensor has a significant decrease, compared to the pure SnO₂ nanoparticles, indicating the surface reaction occurred at a lower operating temperature, which can be attributed to that the activation energy of the surface reaction is lowered by mixing MoS₂ nanosheets. In this work, MoS₂ nanosheet with high surface area provides a platform for attaching the SnO₂ nanoparticles, preventing their interparticle aggregation. The crystallite size of SnO₂ is less than 10 nm, which is estimated by XRD patterns and TEM image. Under this conditions, very low activation energy for the grain growth has been calculated for SnO₂ nanocrystallites within the size range of 3–20 nm. While, above this size range (20–300 nm), the activation energy for the grain growth increases exponentially.^46,47 Secondly, the significant increase of the gas-sensing performance for SnO₂@MoS₂ composites can be attributed to the active site provided by the MoS₂ nanosheets and also the good interaction between the two materials,⁴⁸ therefore improving the electron transfer rate, and thus enhancing gas-sensing response. Thirdly, the MoS₂ nanosheet in this work exhibits a p-type semiconducting behavior in air, which is similar to literatures.^49–53 Thus, at the interface between the SnO₂ nanoparticles and the MoS₂ nanosheets, there forms a p–n junction, which will result in the MoS₂ and SnO₂ having a same Fermi energy level at the interface. Thus a staggered band offset and a built-in internal electric field was formed near the interface. When SnO₂@MoS₂ composites are exposed to ethanol vapor at higher temperature, electrons generate in the reaction can easily cross the interface and transfer to the conductive band of SnO₂ nanoparticles because the conduction band and the valence band of SnO₂ both lie below the energy band of MoS₂ in this composite. As a result, the gas-sensing performance can thus be improved eventually in the composites of dispersing SnO₂ nanoparticles on the surfaces of MoS₂ nanosheets. Furthermore, to reveal the influence of the content of SnO₂ in composites, the study of morphology and gas response of SnO₂@MoS₂ composites with different reaction conditions was investigated by changing the MoS₂/SnO₂ molar ratios, while temperature and reaction time were kept at 200 °C and 21 h, respectively (Fig. S3†).

Fig. 6 Schematic of gas-sensing mechanism of a MoS₂ nanosheet decorated with SnO₂ nanoparticles.

Conclusions

In summary, a simple solution route was successfully promoted to synthesize novel gas-sensing composite by dispersing SnO₂ nanoparticles on the surfaces of MoS₂ nanosheets. Importantly, it is found that MoS₂ nanosheets play an important role for enhancing the gas-sensing performance. The SnO₂@MoS₂ composite sensor exhibits better gas-sensing performance in comparison with pure SnO₂ nanoparticles, indicating the potential applications as gas sensor material toward ethanol detection.

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (No. 61102006 and 51172095), Natural Science Foundation of Shandong Province, China (No. ZR2015EM019 and ZR2014EL006), and Shandong Province Higher Educational Science and Technology Program (No. J15LA56).

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Footnote

† Electronic supplementary information (ESI) available: XPS spectra, FESEM and TEM images of MoS₂ nanosheets, and FESEM images and gas response of SnO₂@MoS₂ composites at different molar ratio. See DOI: 10.1039/c5ra15019a

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