One-pot synthesis of mesoporous spherical SnO₂@graphene for high-sensitivity formaldehyde gas sensors

Abstract

A versatile SnO₂@graphene nanocomposite with a mesoporous structure was prepared by a facile and green solvothermal method. The nanocomposite exhibited high response to low-concentration HCHO (1 ppm) at low working temperature (120 °C). The response and recovery times are 1 s and 85 s for 1 ppm formaldehyde. There is an excellent linear relationship (R = 99.798%) between response and the concentration of HCHO in a large range (1–100 ppm).

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1. Introduction

Recently, detection of toxic gases has attracted considerable interest due to their existence in various areas and hazards to human health. In particular formaldehyde (HCHO), which exists widely both in daily life and industrial processes, is one of the indoor pollutants and is carcinogenic for humans.¹ Therefore, it is necessary to develop an effective method to detect and remove HCHO. In the past decades, various gas sensors based on metal oxide semiconductor (MOS), including ZnO,^2–4 SnO₂,^5–8 TiO₂,⁹ In₂O₃,¹⁰ have been developed to monitor HCHO and other toxic pollutants due to their low cost, easy preparation and detection reliability. MOS gas sensing mechanism is based on change of the conductivity of material in contact with a target gas.¹¹ Thus the sensing properties of MOS are strongly dependent on the morphology, size and surface area. So the key issue is to develop a feasible route to fabricate MOS with uniform morphology and high surface area. Among these MOS materials, SnO₂ has attracted much attraction due to its abundant and low price raw material, tunable morphology and structure. To improve sensitivity of SnO₂ gas sensors, it is important to prepare SnO₂ nanocomposites¹² or versatile SnO₂ with large surface areas and suitable pore size distribution, ensuring the easy access to detected gases.^13,14 Recently, it is reported that metal oxides such as Co₃O₄ or SnO₂ supported on carbon material show an enhanced gas-sensing properties.^15,16 Therefore, carbon materials, particularly graphene as a novel two-dimensional carbon matrix, have attracted tremendous attention due to their large specific surface area, extraordinary electron transport capabilities, electronic properties and abundant surface functional groups. These superior physicochemical properties make it be an effective material for fabrication of high performance composites, which have demonstrated their promising applications in the field of sensor, catalytic and energy storage.^17–20

Based on aforementioned, it is necessary to develop an efficient, economic and green synthetic process for fabricating mesoporous SnO₂ and graphene nanocomposites. However, so far few papers have reported on the one-pot synthetic routes to fabricate mesoporous spherical SnO₂@graphene nanocomposites with an excellent sensing performance for HCHO detection.

Here, we present a template-free green approach to synthesize mesoporous spherical SnO₂@graphene nanocomposites (mSnO₂@graphene) by one-pot solvothermal method, in which the used graphene was synthesized previously and the result is shown in Fig. S1 (ESI†). The advantage of this method is that mesoporous SnO₂ nanospheres can be well-dispersed on graphene surface. This synthetic approach provided a simple, effective, low-cost and green route to prepare mSnO₂@graphene nanocomposites. The as-synthesized mSnO₂@graphene nanocomposites show an excellent sensing performance for the detection of HCHO. For comparison, pure mSnO₂ and the mixture of mSnO₂ and graphene were also prepared.

2. Experimental

2.1 Materials

Pyrolytic graphite (HOPG SPI-3, 10 × 10 × 1 mm) was bought from SPI supplies. Graphite powders (Grade 230U) were provided by Asbury Graphite Mills Inc. The following chemicals were obtained from Sigma-Aldrich and used without further purification: lithium perchlorate (LiClO₄, 98%, powder), propylene carbonate (PC, anhydrous, 99.77%), lithium chloride (LiCl, 99%), tetramethylammonium hydroxide (TMA, aqueous, 25 wt%), ammonia (28%), pyridine (anhydrous, 99.8%), N,N-dimethylformamide (DMF), hydrochloric acid (HCl, 37%), absolute ethanol (CH₃CH₂OH), tin(IV) chloride pentahydrate (SnCl₄·5H₂O).

2.2 Synthesis of sample

Preparation of graphene. The graphene were prepared according to Wang's procedure.²¹ In a typical process, HOPG (50 mg) was used as the negative electrode and electrochemically charged at a voltage of 15 ± 5 V in a 30 mg ml⁻¹ solution of LiClO₄ in propylene carbonate (PC). Graphite powder (2 g) was put in a porous plastic tube with a metal electrode Al of Ni or inserted as negative electrode. Carbon rod (or lithium flake) was used as the positive electrode. During the electrochemical charging, HCl/DMF solution was used to remove the solid by-products. Following the electrochemical charging, the expanded graphite was transferred into a glass Suslick cell (15 ml), followed by the addition of 50 mg ml⁻¹ of LiCl in DMF solution (10 ml), PC (2 ml) and TMA (1 ml). The mixture was sonicated for >10 hours with an ultrasonic intensity of ∼100 W cm⁻², then washed by HCl/DMF and several polar solvents of DMF, ammonia, water, isopropanol and THF, respectively. The gray-black graphene powder was collected by centrifugation or/and filtering during the washing.

Preparation of mSnO₂@graphene. Mesoporous spherical SnO₂@graphene nanocomposites were synthesized by one-pot solvothermal method. Briefly, graphene (2.5 mg) and SnCl₄·5H₂O (0.35 g) was dissolved in mixed solvent of absolute ethanol (16 ml) and concentrated hydrochloride acid (100 μl). The obtained mixture was ultrasonic vibrated 1 h to form a homogeneous dispersion. The solution was then transferred into a 100 ml Teflon-lined stainless steel autoclave and maintained at 150 °C for 24 h. At the end, the as-synthesized product was collected by centrifugation, rinsed thoroughly with absolute ethanol and ultrapure water and then dried at 60 °C in a vacuum box overnight.

The pure mesoporous spherical SnO₂ was synthesized by similar synthesis procedure of the report.²²

The mixture of mSnO₂ and graphene were obtained by mixing 2.5 mg graphene and as-synthesized mSnO₂, then it was ultrasonic vibrated in 16 ml ethanol for 2 h and centrifuged, rinsed thoroughly with absolute ethanol and ultrapure water, and then dried at 60 °C.

2.3 Gas sensor measurements

The gas sensing properties of as-synthesized nanocomposites were evaluated on WS-30A system (Weisheng Electronics Co. Ltd, Henan, China) according to previous work.²³ The paste was prepared by mixing the as-synthesized nanocomposites with ethanol, and then coated onto alumina ceramic tube with two gold electrodes. After being dried at 60 °C for 1 h in air, the ceramic tube was heated to 350 °C for 90 min in an electric furnace. A heater of Ni–Cr wire was inserted into the ceramic tube, which provided the working temperature of the gas sensor. The sensitivity (S) is defined as S = R_a/R_g, which R_a and R_g are the resistances of the sensor in ambient of air and target gases, respectively.

2.4 Characterizations

Specimens were characterized using scanning electron microscope (SEM) and transmission electron microscopy (TEM) on a JEOL JSM-7001F electron microscope at an accelerating voltage of 15 kV and FEI Tecnai G2F20 electron microscope operated at 200 kV, respectively. The structure characterization of nanocomposites was further performed by using XRD on Bruker D8 diffractometer with Cu K radiation. Micromeritics TriStarII 3020 sorption analyzer was used to examine the mesoporous properties of specimens at liquid nitrogen temperature. X-ray photoelectron spectroscopy (XPS) measurements were performed under vacuum (>10⁻⁷ Pa) using a spectrometer (Kratos-AXIS ULTRA DLD) equipped with a monochromatized X-ray source (Al Kα; hν = 1486.6 eV). The X-ray anode was run at 150 W.

3. Results and discussion

3.1 The structure characterization of nanocomposites

The morphology and structure of mSnO₂@graphene sample were elucidated by TEM and HRTEM. The representative TEM images of mSnO₂@graphene nanocomposites are shown in Fig. 1A. TEM image illustrates the large-scale formation of uniform monodispersed mSnO₂ nanospheres with an average diameter of ∼80 nm, which were uniformly and pervasively anchored on graphene nanosheets, resulting in the high loading of mSnO₂. The morphology and size are similar to that of pure mSnO₂ (ESI, Fig. S2†). The HRTEM image shows that the mSnO₂ nanospheres consist of primary nanoparticles and the mesoporous structure is formed by connected nanocrystals (Fig. 1B). The mSnO₂ size is about 6 nm, which is consistent with the selected area electron diffraction (SAED) results (inset image of Fig. 1B). The SAED pattern shows clearly rings characteristic of rutile mSnO₂ and the polycrystalline nature of mSnO₂ nanocrystals.

Fig. 1 (A) TEM image, (B) HRTEM image of mSnO₂@graphene. The insert in (B) is the corresponding SAED pattern.

X-ray diffraction is a powerful method to explore the crystallinity and component of materials. The representative XRD patterns of graphene, mSnO₂ and mSnO₂@graphene are shown in Fig. 2. The sharp peak at about 2θ = 11° corresponds to the (002) reflection of stacked graphene sheets with an interlayer spacing of ∼0.8 nm, which is due to the oxygen-containing functional groups (e.g., carboxyl, hydroxyl or epoxy) on the graphene sheets.²⁴ The peaks at 26° and 42° are the characteristic stacking peak of graphene sheets. The XRD pattern of mSnO₂ reveals that all the diffraction peaks can be ascribed to tetragonal rutile SnO₂ (JCPDS no. 41-1445),^25,26 which is consistent with the SAED results. For mSnO₂@graphene, the typical peaks correspond to the tetragonal structure of mSnO₂, and the peaks of graphene are absent in the patterns, which indicates the disordered stacking of graphene sheets in the composites and the high density of mSnO₂ located to graphene layer, preventing the graphene nanosheets from restacking and most graphene sheets were separated by nanoparticles.²⁷

Fig. 2 XRD patterns of mSnO₂@graphene.

The porosity of mSnO₂@graphene was also investigated using nitrogen ad/desorption techniques. Fig. 3 shows a typical type-IV isotherm with a hysteresis loop in the relative pressure range (P/P₀) of 0.5–1.0, which suggests a mesoporous feature of nanocomposites. Similar characteristic was also observed from the ad/desorption result of pure mSnO₂ (ESI, Fig. S3†). An adsorption average pore size calculated from the BET (Brunauer–Emmett–Teller) (4V/A) model was about 9 nm. The stable porous structure offer an advantage in terms of high surface areas, the specific surface area of mSnO₂@graphene measured by the BET method is 110.7 m² g⁻¹. The high surface area makes them be used as an advanced material in the fields of catalysts, gas sensing and energy storage.

Fig. 3 N₂ ad/desorption isotherm of mSnO₂@graphene.

To further analyze the surface composition and chemical states of the mSnO₂@graphene, X-ray photoelectron spectroscopy (XPS) measurement was carried out. The spectrum of the mSnO₂@graphene indicates that the specimen consists of Sn, O and C atoms (Fig. 4A). Compared with graphene spectrum, no Sn signals could be detected in the spectrum of pure graphene. In the C 1s spectrum of mSnO₂@graphene (Fig. 4B), three peaks can be assigned to three types of carbon with different valences from graphene: non oxygenated ring C (284.6 eV), carbon in C–O bonds (286.8 eV), and carbonyl C (288.8 eV).²⁸ The XPS spectrum of Sn 3d exhibits two symmetric peaks centred at 495.9 eV and 487.4 eV (Fig. 4C), which is attributed to Sn 3d_3/2 and Sn 3d_5/2, respectively. The state of Sn is Sn²⁺ according to Handbook of X-ray Photoelectron Spectroscopy.²⁹ As a result, the mSnO₂ was well deposited on surface of graphene.

Fig. 4 (A) XPS spectra of graphene and mSnO₂@graphene. (B) The fit curve of C 1s and (C) the curve of Sn 3d peak of mSnO₂@graphene.

3.2 Sensing properties

In order to investigate the advantages of mSnO₂@graphene nanocomposites as sensor material, the gas sensing measurements were performed by exposing the obtained sensor to 100 ppm volatile organic compounds (formaldehyde, acetaldehyde, methanol, ethanol and acetone) and the working temperature was 120 °C, which was determined based on the high response of pure mSnO₂ at the same temperature. The results were shown in Fig. 5. mSnO₂@graphene has a more notable response to various substances compared with pure mSnO₂ and their mixture, and it shows highest selectivity to formaldehyde among other harmful chemical reagents. The enhanced response of mSnO₂@graphene nanocomposites show that graphene plays a key role due to its unique physicochemical properties.

Fig. 5 Responses of mSnO₂@graphene, mSnO₂ and the mixture of mSnO₂ and graphene sensor to the harmful chemical substances with 100 ppm at 120 °C.

The responses of the mSnO₂@graphene, mSnO₂ and the mixture of mSnO₂ and graphene sensors versus different formaldehyde concentrations have been further investigated at operating temperature of 120 °C. As shown in Fig. 6, mSnO₂@graphene sensor has obvious response to 1 ppm formaldehyde. Especially, with formaldehyde concentration increasing from 1 to 400 ppm, the responses of mSnO₂@graphene increase promptly. For 1 ppm formaldehyde, the sensor response (R_a/R_g) of mSnO₂@graphene is 4.9, which is about 1.7 times more than that of pure mSnO₂. It is also obvious that the response–recovery property of mSnO₂@graphene sensor is different from that of the mSnO₂ sensors and their mixture, and it exhibited a very remarkable response. Because graphene provides high surface area to absorb formaldehyde molecules and the planar structure facilitates gas diffusion, which results in the fast and high response. The response and recovery times are 1 s and 85 s for 1 ppm formaldehyde, and the corresponding values are 1 s and 90 s for 400 ppm respectively. The fast recovery of the mSnO₂ and mSnO₂@graphene sensor show the formaldehyde and product molecules released quickly and sensor gets back to the original state. However, the response and recovery of the mixture of mSnO₂ and graphene were unsatisfactory on exposal to formaldehyde with different concentrations (results are shown in Fig. 6), this could be owing to the gas and electron poor transfer compared to mSnO₂@graphene nanocomposites. The more importance is that there is good linearity (R = 99.798%) between concentration and response to formaldehyde in the large concentration range from 1 to 100 ppm at 120 °C (Fig. 7). These results indicate that mSnO₂@graphene may serve not only as a sensitive material for formaldehyde detection but also a powerful tool to qualitatively test formaldehyde with possible concentration of in our daily environment.

Fig. 6 Responses of mSnO₂@graphene, mSnO₂ and the mixture of mSnO₂ and graphene sensor to formaldehyde gas with various concentrations at 120 °C.

Fig. 7 Linear dependence of the response to formaldehyde concentration (1 to 100 ppm).

The interaction of gas molecules with mSnO₂ or mSnO₂@graphene follows the general gas-sensing mechanism, which involves the gas adsorption, charge transfer, and desorption process.³⁰ When sensing materials was exposed to the detected gases, the reductive gas molecules such as formaldehyde will react with the ionized oxygen species (O₂⁻, O²⁻, or O⁻) adsorbed on the surface of materials. Thus resistance change by releasing and trapping electrons results in the conductivity change. Graphene in mSnO₂@graphene promotes electron transfer between SnO₂ and gas molecules. Therefore, the sensing selectivity can be obtained by the detection of different reductive compounds due to their different reduction ability.

4. Conclusions

In summary, we developed a facile solvothermal strategy to in situ prepare mesoporous spherical SnO₂@graphene nanocomposites. This novel nanocomposite exhibits a great improvement in chemical sensing performance as a gas sensor material for HCHO detection, such as low operating temperature, high sensitivity, good selectivity, excellent linear regression relation and low detection limit in comparison with pure mesoporous SnO₂ and the mixture of SnO₂ and graphene. The superior gas sensor property can be attributed to the presence of graphene substrate and the mesoporous structure of SnO₂. The simple and controllable method may offer an effective technique to synthesize materials for detection of formaldehyde in practice applications. Furthermore, this synthesis method also provides a reference for preparation of other graphene based mesoporous metal oxides, which can be applied in the fields of sensor, photoelectron and catalysis.

Acknowledgements

The authors thank partial financial support by National Natural Science Foundation of China (51572185 and 21403274), Natural Science Foundation of Shanxi Province (2014011016-4) and Guangdong Innovative and Entrepreneurial Research Team Program (2014ZT05N200).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra00857g

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