Selective epichlorohydrin-sensing performance of Ag nanoparticles decorated porous SnO₂ architectures

Abstract

Epichlorohydrin (ECH) is mainly an industrial raw material, while it is toxic and dangerous for human health, so finding a simple and effective method for detecting ECH gas is challenging work. Porous SnO₂ structures are successfully obtained through a hydrothermal method using SnCl₄·5H₂O as the tin source and water as the solvent. Ag nanoparticle (NPs) decorated porous SnO₂ structures have also been prepared by the impregnation method. The morphology and structure of the as-prepared products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and the nitrogen adsorption–desorption technique. The as-synthesized nanostructures, with diameters ranging from 4–5 μm, are composed of porous nanosheets with an average thickness of 60 nm. The gas sensing properties of the Ag NPs decorated porous SnO₂ structures for ECH gas were investigated in detail, and the results showed that the 10% Ag NPs decorated porous SnO₂ structures gas sensor was the optimum sensor, demonstrating a response (S = 50) for 100 ppm of ECH gas, while the working temperature was decreased by about 180 °C. In addition, it showed a low detection level (0.5 ppm) for ECH gas, good reproducibility and repeatability, and long-term stability, implying a promising application in detecting ECH gas.

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Introduction

Semiconductor gas sensors have been applied in the detection of volatile organic compounds (VOCs) due to their fast, high sensitivity and low cost characteristics.^1–4 Tin dioxide, as a well-known wide band gap semiconductor (E_g = 3.6 eV at 300 K), is considered the most promising functional material due to its highly sensitive gas-sensing.^5–8 However, SnO₂ can react with several gases simultaneously, so the poor selectivity hinders its practical application.⁹ Doping with noble metal nanomaterials is an important and effective approach to improve the gas-sensing selectivity and high response of semiconductors.^10–13 Kim et al. reported Pd nanoparticle-loaded SnO₂ nanofibers showing excellent sensitivity and selectivity to H₂ gas, which can be attributed to the catalytic effect of the PdO nanocrystallites in promoting the oxidation of H₂ to H₂O and enhancing electron depletion at the surface of the SnO₂ crystallites.¹⁴ Ju's group investigated 0.08 wt% Pt doped SnO₂ nanofiber sensors on micro heater platforms and exhibited superior H₂S gas sensing characteristics, which are explained by the catalytic promotion effect of Pt.¹⁵

Epichlorohydrin (1-chloro-2,3-epoxypropane, ECH, the structural formulae of epichlorohydrin can be observed in Fig. S1 in the ESI†) is mainly used in the production of epoxy resins, synthetic glycerol, and elastomers.^16,17 However, ECH is toxic by inhalation and dermal and oral absorption and can be dangerous for the central nervous system.^18,19 Exposure to ECH in the workplace can rapidly irritate the eyes, skin and respiratory tract of workers. At high levels of exposure, nausea, vomiting, labored breathing, inflammation of the lungs, pulmonary edema, and renal lesions may emerge.²⁰ In particular, people who drink water containing high levels of ECH over a long period of time could engender stomach problems and may have an increased risk of getting cancer.^21,22 The Occupational Safety and Health Administration (OSHA) recommend that the exposure limit is 19 mg m⁻³ (5 ppm) for 8-hour time weighted average (TWA) standard.²³ The National Institute of Occupational Safety and Health's (NIOSH) recommended exposure limit is to ensure that a worker can escape from an exposure condition that is likely to cause death or immediate or delayed permanent adverse health effects or prevent escape to the environment.²⁴ As a result, the requirement for the fast detection of low concentrations of ECH is becoming more and more urgent. Gas chromatography, gas chromatography–mass spectrometry, solid phase microextraction and headspace gas chromatography methods have been developed and validated for the determination for ECH.^25–30 Metal oxide based chemiresistive gas sensors are easy to incorporate into portable real-time detectors of ECH gas because of their miniaturized size, low cost, simplicity of fabrication, and fast operation.^31–33

In the present study, Ag NPs decorated porous SnO₂ structures as low concentration ECH gas sensing detectors were studied. The phase structures and morphologies of the as-prepared product were analyzed by X-ray diffraction, SEM, TEM and nitrogen adsorption–desorption isotherms. The effects of the Ag content and working temperature on the ECH gas sensing properties were investigated in detail. The results indicated that the 10% Ag NPs catalyst on porous SnO₂ structures sensor can be applied in an ECH detector and public safety investigation.

Experimental section

Materials

All the reagents were of analytical grade and used without further purification: L-cysteine (C₃H₇NO₂S, Aladdin), stannic chloride, pentahydrate (SnCl₄·5H₂O, Sinopharm Chemical Reagent Co., Ltd. Shanghai, China), silver nitrate (AgNO₃, Sinopharm Chemical Reagent Co., Ltd. Shanghai, China).

Synthesis of the porous SnO₂ structures

Here, we used a one-step hydrothermal route by simply controlling the temperature and dosage to get porous SnO₂ structures.³⁴ In a typical experiment, L-cysteine (0.4840 g, 2 mmol) was dispersed in deionized water (25 mL) to form a clear solution, then SnCl₄·5H₂O (0.7010 g, 2 mmol) was added into the above solution under vigorous stirring. After being stirred for 15 min, the solutions were transferred to a 50 mL Teflon-sealed autoclave and maintained at 180 °C for 10 h. The black products were isolated by centrifugation, washed with deionized water and ethanol three times and finally dried at 80 °C for 8 h. Finally, porous SnO₂ structures were obtained by annealing the precursor at 700 °C for 2 h in an air atmosphere.

Synthesis of the Ag nanoparticle decorated porous SnO₂ structures

Ag-doped porous SnO₂ structures were prepared as reported by the impregnation method.³⁵ A mixture of SnO₂ (0.2220 g) and AgNO₃ (0.0175, 0.0350, 0.0525 g, w–w 5%, 10%, 15%) in 5 mL of deionized water was stirred magnetically for 24 h at room temperature, and the resulting precipitated solid was dried at 100 °C.

Characterization

The as-synthesized products were characterized by an X-ray diffraction instrument (Rigaku Dmax-rB, Cu–Kα radiation, λ = 0.1542 nm, 40 KV, 100 mA). The morphology and structure of the as-synthesized SnO₂ structures were studied using a JEOL JSM-6700F field emission scanning electron microscope (SEM), and high-resolution transmission electron microscope (HRTEM JEM-2100, 200 kV). The specific surface area was estimated using the Brunauer–Emmett–Teller (BET) method based on the nitrogen adsorption isotherm (77 K) using a NAVA 2000e surface area and pore size analyzer (Quantachrome instruments, USA), and the pore sizes were calculated using Barrett–Joyner–Halenda (BJH) models from the adsorption branches of the adsorption–desorption isotherms. The sample was degassed at 200 °C for 12 h in a vacuum prior to the measurements. The gas-sensing properties of the samples were determined using a China WS-30A gas sensitivity instrument.

Fabrication and measurement of the gas sensors

The gas sensor was fabricated as follows: the powders were mixed with water, and then coated onto an Al₂O₃ tube on which two platinum wires had been installed at each end. The structure of the sensor is shown in Scheme 1. The operating temperature was controlled by adjusting the heating power using a Ni–Cr alloy coil placed through the tube to heat the sensing material. The sensing element was aged at 300 °C for 7 days in order to improve its stability and repeatability. The gas sensitivity, S, was defined as the ratio R_a/R_g, where R_a was the resistance measured in air and R_g was that measured in the tested gas atmosphere. The response and recovery times were defined as the time to reach 90% of the final signal. The relation between S and the voltage is as follows:

Scheme 1 Schematic structure of the gas sensor.

U _c is the testing voltage (U_c = 5 V), R_L is the load resistance, I_a, U_a, and R_a are the electrical parameters in air, I_g, U_g, and R_g are the electrical parameters in the gas.

Results and discussion

Fig. 1 shows a typical XRD pattern of the Ag NPs decorated porous SnO₂ structures with different Ag (0–15%) doping content. All the diffraction peaks of the pure SnO₂ structures can be indexed to the tetragonal rutile SnO₂ structure (JCPDS card no. 41-1445, α₀ = 4.738 Å and c₀ = 3.188 Å). No characteristic peaks were observed for other impurities, revealing the high purity of the prepared SnO₂ microstructures. Because the amount of Ag NPs doped in the SnO₂ structures is too small, the XRD pattern of 5% Ag NPs doped SnO₂ shows almost no change compared with that of the pure SnO₂ products. However, in the cases of the 10% and 15% Ag-doped SnO₂ products, the Ag peaks can be clearly observed at angles of around 38.1, 44.3 and 81.5 (JCPDS no. 87-0717), further indicating the formation of Ag NPs decorated porous SnO₂ products.

Fig. 1 The XRD patterns of 0%, 5%, 10%, and 15% Ag NPs doped porous SnO₂ samples. The XRD peaks of SnO₂ (JCPDS no. 41-1445) are marked with “*”, and Ag (JCPDS no. 87-0717) are marked with “#”.

The morphology and size of the as-synthesized SnO₂ products are characterized by SEM. A typical low magnification SEM image is shown in Fig. 2a, revealing a flower-like structure with average diameters of 4–5 μm, and large quantities of porous SnO₂ products can be observed in Fig. S2 in the ESI.† In addition, these nanostructures consist of some nanoporous nanosheets with average thicknesses of 60 nm, as shown in Fig. 2b. The porous character may be attributed to the fact that the SnO₂ nanoparticles have a strong tendency to aggregate with each other to minimize their surface energy in the annealing process.³⁶

Fig. 2 Morphology of the as-synthesized porous SnO₂ products. (a, b) SEM images at different magnifications. (c) SEM images of the Ag NPs engineered porous SnO₂ products. (d) Energy-dispersive spectrum (EDS) recorded from the Ag NPs engineered porous SnO₂ products.

After impregnating with a AgNO₃ aqueous solution, the flower-like structures are coarser than those of pure SnO₂. It could also be seen from Fig. 2c that there are many Ag NPs on the surface of the SnO₂ products. The as-synthesized nanostructure was examined using the energy dispersive X-ray spectrum (EDS), as shown in Fig. 2d. It can be clearly seen that three elements (Ag, Sn, and O) exist in this area. Similar EDS results have also been obtained in many different areas, which further confirm that the Ag NPs are uniformly distributed in the whole sample.

Further detailed structural analysis of the porous SnO₂ products was characterized using TEM. The typical TEM image in Fig. 3a shows that the shape of the as-synthesized product was similar to that of the SEM observation. A high-magnification TEM image (Fig. 3b) revealed numerous pores and that the nanosheets consist of many nanoparticles. A high resolution TEM image of the porous SnO₂ product shows resolved fringes separated by 0.335 nm and 0.265 nm, corresponding to the (110) and (101) lattice spacing of a tetragonal SnO₂ crystal, respectively. From the inset of Fig. 3c, it is clear that the nanosheets exhibit an almost polycrystalline diffraction pattern.

Fig. 3 (a, b) Low TEM images, (c) HRTEM image (inset: the corresponding SAED pattern).

To further examine the porous structure of the SnO₂ products, nitrogen adsorption–desorption isotherms and the corresponding Barrett–Joyner–Halenda (BJH) pore diameter distribution were measured to obtain information about the nature of the porosity, as shown in Fig. 4. It reveals that the isotherm of the as-synthesized SnO₂ products is a typical isotherm with H₃-type hysteresis loops³⁷ and the porous SnO₂ products possess a mesoporous (2–50 nm) structure. The BET specific surface area is 26.60 m³ g⁻¹. The pore size distribution demonstrates the material contains a mesoporous structure with an average pore size centered at about 22.41 nm. Due to the large surface area and porous structure, the nanostructures have an advantage for gas adsorption–desorption and gas molecular diffusion, indicating potential applications in gas sensors with enhanced gas-sensing performance.

Fig. 4 Nitrogen adsorption–desorption isotherm and pore diameter distribution of the samples.

In order to evaluate the unique characteristics of the porous SnO₂ products with the catalytic role of the Ag NPs, we investigated the Ag NPs decorated porous SnO₂ products gas sensing performance to detect some dangerous gases in production processes. Fig. 4a shows the different Ag NPs doped content sensors responses to 100 ppm of ECH gas at different operating temperatures. The optimal working temperature and the corresponding response for the 0%, 5%, 10%, 15% Ag NPs decorated porous SnO₂ product gas sensors are 220 °C and 34, 260 °C and 33, 180 °C and 50, and 220 °C and 38.1, respectively. The result indicates that the highest response (S = 50) is for the 10% Ag NPs doped porous SnO₂ products at an optimum operation temperature of about 180 °C. In addition, Fig. S3 in the ESI† shows an XRD pattern of the Ag NPs doped porous SnO₂ products before and after the heat treatment at 180 °C for 24 h, which is the optimum temperature of the gas sensing detection. The Ag phase doesn't transform before and after the heat treatment.

To further confirm the sensing properties of the as-synthesized Ag NPs doped porous SnO₂ product sensors, five typical gases (ECH, ammonia, methylbenzene, formaldehyde, and chloroform in production processes) were selected as target gases to investigate the gas response at 180 °C, where the concentration of all the tested gases is 100 ppm. The results shown in Fig. 5b demonstrate that the Ag NPs doped SnO₂ product sensors show excellent selectivity to ECH, whereas they present low responses to the other typical interference gases at the same temperature, especially the 10% Ag NPs doped SnO₂ product sensor. This indicates that the sensor has quite excellent selectivity towards ECH gas, which can be attributed to the catalytic promotion effect of the Ag NPs. Fig. 6a presents the response transients of different gas sensors exposed to 100 ppm of ECH gas at an operating temperature of 180 °C. The response time and recovery time of the 0%, 5%, 10% and 15% Ag NPs doped SnO₂ nanostructure gas sensors were calculated to be 17s and 287s, 16s and 238s, 16s and 186s, and 15s and 183s, respectively. Furthermore, the 10% Ag doped SnO₂ sensor exhibits the highest response and a good response time and recovery time among all the samples. The real-time response curve of the 10% Ag-doped porous SnO₂ sensors at 180 °C for different concentrations of ECH is shown in Fig. 6b. It is obvious that the structures have a good response to ECH. At a low concentration of 0.5 ppm of ECH, the response value is about 7.38, and the sensitivity increases at elevated concentrations of the target gases. So the OSHA's exposure limit of ECH can be detected easily.²³

Fig. 5 (a) The response of different Ag NPs (0–15%) doped content porous SnO₂ product gas sensors to 100 ppm of ECH gas as a function of the operating temperature. (b) The selectivity towards ECH, ammonia, methylbenzene, formaldehyde, and chloroform at 180 °C (all gas concentrations were 100 ppm).

Fig. 6 (a) Different Ag NPs (0–15%) doped content porous SnO₂ product gas sensors to 100 ppm of ECH at 180 °C. (b) Real-time response curves of the 10% Ag NPs doped SnO₂ product gas sensors upon exposure to different concentrations of ECH at a working temperature of 180 °C.

The reproducibility and repeatability of the sensor, as an important element,³⁸ was measured. Fig. 7a shows the repeating response of the 10% Ag NPs doped porous SnO₂ product based sensor. The sensor was tested 8 times with 100 ppm of ECH gas at 180 °C. No obvious variations in the sensor sensitivity were observed. The sensor finally recovered fully after removing the ECH gas, indicating the good reproducibility and repeatability of the sensor to ECH gas. The stability of the sensors was also investigated. The sensing property of the 10% Ag NPs engineered porous SnO₂ product sensor was tested at 180 °C for 100 ppm of ECH in 50 days. Fig. 7b showed that the sensor exhibited good long-term stability, fulfilling the requirements for gas sensor applications. These results indicate that the sensor can potentially be applied in ECH detectors and public safety investigation.

Fig. 7 (a) Continued cycles of the response and recovery plot of the 10% Ag NPs doped SnO₂ product gas sensors to 100 ppm of ECH at 180 °C. (b) Plot showing the stability of the 10% Ag NPs doped SnO₂ product gas sensors to 100 ppm of ECH at 180 °C.

Conclusion

In summary, a simple hydrothermal technique was demonstrated for the synthesis of porous SnO₂ products with a diameter of 4–5 μm, which were composed of porous nanosheets with an average thicknesses of 60 nm. Ag NPs decorated porous SnO₂ products were obtained by the impregnation method. The obtained nanostructures exhibited an excellent gas sensing performance towards ECH gas. The 10% Ag NPs engineered SnO₂ product gas sensor shows the best performance, demonstrating a high response, good selectivity, short response and recovery time and a very low detection limit (0.5 ppm) for ECH gas. Additionally, it shows the sensor has good reproducibility and repeatability, and long-term stability for ECH gas, proving to be a promising material for ECH detection in practical applications.

Acknowledgements

This work was supported by the Foundation for Key Project of Ministry of Education, China (no. 211046), the Program for New Century Excellent Talents in Heilongjiang Provincial University (1252-NCET-018), the Scientific Research Fund of Heilongjiang Provincial Education Department (12531179) and the Program for Scientific and Technological Innovation Team Construction in Universities of Heilongjiang (no. 2011TD010).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ce41478g

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