Facile Synthesis of Wormhole-Like Mesoporous Tin Oxide via Evaporation-Induced Self-Assembly and the Enhanced Gas-Sensing Properties
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Wormhole-like mesoporous tin oxide was synthesized via a facile evaporation-induced self-assembly (EISA) method, and the gas-sensing properties were evaluated for different target gases. The effect of calcination temperature on gas-sensing properties of mesoporous tin oxide was investigated. The results demonstrate that the mesoporous tin oxide sensor calcined at 400 °C exhibits remarkable selectivity to ethanol vapors comparison with other target gases and has a good performance in the operating temperature and response/recovery time. This might be attributed to their high specific surface area and porous structure, which can provide more active sites and generate more chemisorbed oxygen spices to promote the diffusion and adsorption of gas molecules on the surface of the gas-sensing material. A possible formation mechanism of the mesoporous tin oxide and the enhanced gas-sensing mechanism are proposed. The mesoporous tin oxide shows prospective detecting application in the gas sensor fields.
KeywordsEvaporation-induced self-assembly (EISA) method Mesoporous tin oxide Semiconductor gas sensor Gas-sensing properties
Average pore diameter
Joint Committee Powder Diffraction Standards
Specific surface areas
Tansmission electron microscopy
Total pore volume
Among semiconducting metal oxides, tin dioxide (SnO2), a wide band gap semiconductor (3.6 eV) with a rutile-type crystal structure, has been attracting much attention for various potential applications in the fields of anode materials of lithium-ion batteries , dye-sensitized solar cells , photocatalysis [3, 4, 5], conductive materials , and gas sensors  owing to its large band gap, nonstoichiometric nature, excellent electronic mobility, and stability. Nowadays, gas sensors are playing very important roles in the monitoring of environmental pollution , indoor air quality, public health, non-invasive disease diagnosis, and industrial applications. Many semiconducting metal oxides like ZnO , Co3O4 , WO3 [11, 12, 13, 14, 15], NiO [16, 17], and SnO2 [18, 19, 20, 21, 22, 23] have been used for gas-sensing applications because of the excellent response, high sensitivity, good reliability, and low cost. Among them, SnO2 has been extensively investigated for gas sensors with a great sensitivity toward several gases, including acetone , nitrogen dioxide , toluene , ethanol , formaldehyde [28, 29], and methanol .
The properties of SnO2 directly depend on its structural and morphological state, such as the phase, particle size, and band gap. Therefore, many efforts were made to synthesize SnO2 into useful nanostructured morphologies to tailor its chemical and physical properties [17, 31, 32]. So, various SnO2 nanostructures with different morphologies have been obtained, which exhibited good sensing properties to many test gases. Meanwhile, SnO2 with mesoporous structure possesses high specific surface area and narrow pore size distribution, which can provide more in-situ active sites for superior interaction of SnO2 powders with analyte gas and easy gas diffusion into the porous sensing layers; it could further enhance the gas-sensing properties. Mesoporous SnO2 has been previously prepared through various methods including sol-gel and sonochemical methods utilizing supramolecular templates. However, the literatures relating to the preparation of SnO2 indicate that a simple and economic method to synthesize mesoporous SnO2 still poses a challenge and further improvement is necessary. Furthermore, evaporation-induced self-assembly is a pretty effective method for the synthesis of porous nanocrystals and has the advantages of homogeneous pore sizes, controllable morphologies, and mild reaction conditions [33, 34].
In this paper, a facile evaporation-induced self-assembly process was employed to synthesize SnO2 mesostructure under mild conditions for effective gas sensor application. The microstructure, morphology, and the sensing properties of the mesoporous SnO2 were systematically investigated. The test results about gas-sensing properties showed the as-prepared mesoporous SnO2 had a good sensitivity at an appropriate operating temperature, and the enhanced gas-sensing properties were closely related to their interconnected pores and exposed facets. Furthermore, the possible mechanism of enhanced gas-sensing properties was also discussed.
All chemicals used in the experiments were analytical-grade reagents purchased from Sinopharm Chemical Reagent Co. Ltd. and used without further purification. In a typical procedure, 0.42 g SnCl4·5H2O and 0.336 g citric acid were first dissolved in 10 mL of deionized water. 0.144 g of structure-directing agent (template) (EO)20(PO)70(EO)20 (P123) was dissolved in 10 mL ethanol, and 1 mL of nitric acid was added as a condensation inhibitor. P123 solution was then added into the tin solution with vigorous stirring. The formed mixture was covered with PE film, stirred at 60 °C in water bath for 2 h, and then put into a drying oven at 60 °C to undergo solvent evaporation process. The as-formed solid was calcined in air for 3 h to remove the template and finally produce the mesoporous SnO2. The mesoporous SnO2 calcined at 350, 400, and 450 °C were named SnO2-350 °C, SnO2-400 °C, and SnO2-450 °C, respectively.
The phase analysis was performed at the D/MAX2550VB+ X-ray diffractometer with an acceleration voltage of 40 kV and an emission current of 300 mA, Cu Kα radiation (λ = 1.5405 Å) as radiation source, and graphite as monochromator; 2θ ranged from 0.5° to 80° was detected at a scanning rate of 0.02 °/s. Transmission electron spectroscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images of the products were taken by a Tecnai G2-20ST electron microscopy at 220 kV. The N2 adsorption-desorption isotherms were recorded at 77 K and analyzed using an ASAP 2020 Surface Area analyzer. The specific surface areas were calculated using the Brunnauer-Emmett-Teller (BET) equation, and estimates of the pore size distributions were deduced by means of Barrett-Joyner-Halenda (BJH) methods. Fourier-transform infrared (FTIR) spectra of the samples were recorded on a Nicolet Nexus 670 FTIR spectrophotometer using KBr pellets, and the mixture was pressed into a pellet for IR measurement. The photoluminescence (PL) spectrum was measured on a HITACHI FL-4500 at room temperature using a Xe lamp with a wavelength of 310 nm as the excitation source.
Firstly, the powders of mesoporous SnO2 were mixed with terpineol saturated with methylcellulose to form diluted slurry. Then, the slurry was coated onto an alumina ceramic tube which was printed with a pair of gold electrodes and four Pt wires. After being dried under ambient conditions, the ceramic tube was heated at 350 °C for 3 h. Finally, a small Ni-Cr alloy coil was inserted into the tube as a heater to provide the operating temperature.
The gas-sensing test was performed on a WS-30A system (Weisheng Electronics Co., Ltd., China). Before the measurements, the device was aged at 350 °C for 48 h in air to improve stability. The response was defined as Ra/Rg, where Ra and Rg were the resistances of the sensor exposed in air and in reducing atmosphere, respectively. The response and recovery times were defined as the time taken by the sensor to achieve 90% of the total resistance change in the case of adsorption and desorption, respectively. The humidity-sensing properties of mesoporous SnO2 sensors were studied at the optimum operating temperature under four different relative humidity (RH) (24, 43, 75, and 97%) using saturated solutions of CH3COOK, K2CO3, NaCl, and K2SO4, respectively. The testing principle of the gas sensors was similar to that described in the literature .
Results and Discussion
The textural characteristics of all samples
Figure 6b shows the relationship curves tested at 200 °C between responses and ethanol concentration for the mesoporous SnO2 sensors calcined at different temperatures. It shows that the optimum ethanol concentration is 200 ppm for mesoporous SnO2 calcined at different temperatures. Mesoporous SnO2 calcined at 400 °C exhibits the highest response, and its response to 200 ppm ethanol reaches 41.6, which is much higher than that calcined at 350 and 450 °C. Figure 6c displays the response-recovery curves of the mesoporous SnO2 sensors for ethanol, which are tested under the same conditions (the operating temperature is 200 °C) in order to make a comparison. It revealed that the response speed of the SnO2-400 °C sensor is higher than SnO2-350 °C and SnO2-450 °C. The response and recovery time of the SnO2-400 °C sensor was 31 and 2 s, respectively. With the ethanol concentration increasing from 10 to 200 ppm, the gas-sensing properties curves show an increasing tendency, and the maximum response was 41.6 at 200 ppm. However, when the concentration of ethanol continuously increased to 400 ppm, their sensitivity are decreased and shows a leveling off from 400 to 2000 ppm, because the sensitivity of the sensors was saturated. Moreover, the responses of SnO2-350 °C and SnO2-450 °C show the similar varying tendency, but the responses are much lower than those of SnO2-400 °C. Selectivity is another important parameter to evaluate the sensing ability of a gas sensor [51, 52]. Figure 6d shows a bar graph of the mesoporous SnO2 sensors with different calcined temperatures to 200 ppm of ethanol, methanal, methanol, and acetone at the operating temperature of 200 °C. As shown in Fig. 6d, the sensors exhibit the highest response to ethanol against other target gases. In addition, the sensors are less sensitive to acetone. Meanwhile, the response of the mesoporous SnO2 calcined at 350, 400, and 450 °C to 200 ppm of ethanol is 9.3, 41.6, and 30.5, respectively. It can also be observed that the responses of the SnO2-350 °C sensor to 200 ppm of ethanol, methanal, acetone, and methanol are less than 10 at 200 °C. These results demonstrate that the as-prepared mesoporous SnO2 sensors can selectively detect ethanol vapors with the interference of other gases and have a good performance in the operating temperature and response/recovery time.
Based on the results of gas-sensing properties for the mesoporous SnO2 sensors with different calcined temperatures, it was revealed that the mesoporous SnO2-400 °C sensor has the best comprehensive performance, which can be attributed to the high surface area and pore volume formed through the induction of self-assembly process. It shows a slight decrease in textural and gas-sensing properties when the calcined temperature rises from 400 to 450 °C, indicating that mesoporous SnO2 has good chemical stability and thermal stability. In addition, the decrease is due to the slight collapse of the mesostructure. The mesoporous SnO2-350 °C sensor has the worst overall performance, which is attributed to the channel plugging by the residual organic template. When the calcined temperature rose to 400 °C, the organic template was removed completely and may form the interconnected pore channels to enhance the gas-sensing performance further.
Sensing performances of mesoporous SnO2 to ethanol in this work and previously reported sensing materials
NiO/SnO2 thin film
100 ppm, 250 °C
100 ppm, 100 °C
100 ppm, 25 °C
2 ppm, 150 °C
hollow SnO2 nanoparticles
100 ppm, 300 °C
50 ppm, 133 °C
mesoporous SnO2-350 °C
200 ppm, 200 °C
mesoporous SnO2-400 °C
200 ppm, 200 °C
mesoporous SnO2-450 °C
200 ppm, 200 °C
In summary, the SnO2 with mesoporous nanostructures were successfully fabricated by means of evaporation-induced self-assembly technique, using triblock copolymer P123 as the template and tin (IV) chloride pentahydrate as the metal precursor, and calcined at different temperatures. The results revealed that the mesoporous SnO2 have good chemical and thermal stability. In the gas-sensing studies, the mesoporous SnO2 exhibited enhanced gas-sensing properties, such as fast response/recovery time, high sensitivity, and good sensing selectivity to ethanol. Mesoporous SnO2 calcined at 400 °C exhibits the highest response, and its response to 200 ppm ethanol reaches 41.6. This might be attributed to their high specific surface area and interconnected pores structure, which can provide more active sites and generate more chemisorbed oxygen spices to promote the diffusion of ethanol molecules and their adsorption on the surface of the SnO2. We believe that the mesoporous SnO2 could have excellent detecting application in the field of pollution detecting, medical diagnosis, and industrial security.
This work was supported by the National Natural Science Foundation of China (51704030), the China Postdoctoral Science Foundation (2017 M610617, 2017 M623182), Shaanxi Postdoctoral Science Foundation (2017BSHEDZZ10), the Special Fund for Basic Science Research of Central Colleges of Chang’an University (310831171002), and the Training Program of Innovationand Entrepreneurship for Undergraduates (201710710273).
XL and KP developed the concept. XL and KP conceived the project and designed the experiments. XL wrote the final paper. XL and KP wrote the initial drafts of the work. XL designed the experiments and synthesized and characterized the materials. XL and KP analyzed the data. All authors discussed the results and commented on the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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