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Preparation of meso-porous SnO2 fibers with enhanced sensitivity for n-butanol
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Preparation of meso-porous SnO2 fibers with enhanced sensitivity for nbutanol ⁎
Xueying Zhanga, Guogang Xua, , Huiyong Wangb, Hongzhi Cuia, Xiaoyuan Zhana, Laifa Sanga, Gaoyu Zhanga a b
College of Material Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China Shandong Inspection and Quarantine Technology Center, Qingdao 266590, China
A R T I C L E I N F O
A B S T R A C T
Keywords: SnO2 Porous materials Sensors Bio-template
Meso-porous SnO2 fibers were synthesized using a solvothermal method with metaplexis fruit as the bio-template. The products were characterized by powder X-ray diffraction, high resolution scanning electron microscopy, transmission electron microscopy and nitrogen adsorption/desorption measurements. Results show that SnO2 fibers present a high specific surface area of 73.665 m2/g and a meso-porous structure with the pore size of 7.821 nm, and the crystal size of SnO2 is about 6.5 ± 0.5 nm. The gas sensing performance of the prepared SnO2 fibers toward several volatile organic compounds was investigated. The results show that the meso-porous SnO2 fibers were highly sensitive and selective to n-butanol.
1. Introduction As an important n-type semiconductor, SnO2 is one of the earliest established gas sensing metal oxides due to its low-cost and excellent physical-chemical properties [1,2]. Many SnO2 based gas sensors have been synthesized to detect a variety of volatile organic compounds (VOCs) including alcohol ethanol, acetone, methanol, isopropanol, benzyl, n-butanol, H2 and so on [3–11]. Due to the good gas permeability, high specific surface area and more active spots, hollow porous fibers can effectively improve the gas sensitivity and catalytic activity of SnO2 materials. Up to now, many manufacturing processes have been applied to prepare hollow fibers, such as sol-gel method [12], electrospinning method [13,14] and template-directed synthesis [15]. For example, Li et al. prepared Ni-doped SnO2 nanofiber array using electrospinning technique, and the product showed excellent performance for NO2 detection at 250 °C. Jiang et al. synthesized Eu-doped SnO2 nanofibers via a simple electrospinning technique, and the sensor exhibited a good sensitivity to acetone in sub-ppm concentrations and the detection limit could extend down to 0.3 ppm. Zhang et al. fabricated mesoporous SnO2 nanotubes by templating against carbon fibers followed by calcination at 800 °C. In recent years, a lot of biological natural fibers have attracted more attentions, due to their unique structures, and hollow semiconductor metal oxide nanofibers have been prepared by biological natural materials as template-directed materials [16]. For example, Song et al. prepared biomorphic In2O3 microtubules using cotton as the bio⁎
template which exhibited a good selectivity for formaldehyde with rapid response and high sensitivity at 260 °C [17]. Hollow CuO fibers were prepared by Dong et al. using cotton fibers as the bio-template, and it exhibited a superior sensitivity and good selectivity toward lowppm-level (1–100 ppm) n-propanol [18]. Cerium-doped TiO2 mesoporous nanofibers reported by Xiao et al. were prepared by one-pot facile synthesis method using collagen fiber as the bio-template [19]. SnO2 hollow fibers were synthesized using kapok and Bombyx mori silk worm cocoon as the bio-template, respectively [20,21], and all these fiber structures can improve the performance of semiconductor metal oxide effectively. Metaplexis japonica belongs to one kind of perennial grass liana, and the chorionic villi of metaplexis fruit is one kind of natural fibrous materials which exhibits cylindrical hollow structures with smooth surface and thin fiber walls. To the best of our knowledge, no reports have been presented on the synthesis semiconductor metal oxide fibers using metaplexis fruit as the bio-template. Therefore, mesoporous SnO2 fibers were synthesized using a solvothermal method with metaplexis fruit as the bio-template. The gas sensing performances of the prepared SnO2 fibers toward several volatile organic compounds were systematically investigated. 2. Experimental 2.1. Materials and experiment process SnCl4·5H2O (AR grade, CAS#: 10026-06-9) was purchased from
Corresponding author. E-mail addresses: [email protected], [email protected] (G. Xu).
https://doi.org/10.1016/j.ceramint.2017.12.093 Received 23 November 2017; Received in revised form 13 December 2017; Accepted 13 December 2017 0272-8842/ © 2017 Published by Elsevier Ltd.
Please cite this article as: Zhang, X., Ceramics International (2017), https://doi.org/10.1016/j.ceramint.2017.12.093
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Sinopharm Chemical Reagent Co., Ltd, Shanghai, China. The metaplexis fruit obtained from the dry Metaplexis japonica was used as the biotemplate. Firstly, the chorionic villi of metaplexis fruit was dried at 50–70 °C for 12 h in a electric drying oven with forced convection. Secondly, the as-treated chorionic villi were put into NaOH solution (1 mol/L, 100 mL) for 3 h at 80 °C and washed with deionized water for several times. In a typical synthesis process, 2.3 g SnCl4·5H2O (the mass ratio of dry bio-template to synthesized SnO2 was 1:1) was dissolved in 20 mL ethanol, and the washed chorionic villi was added to the solution under stirring. Then, NaOH solution (1 mol/L) was added to the mixture system dropwise until the pH value of the mixture rose up to 9, and added deionized water until the volume ratio of ethanol to water is 1:1. The mixture were transferred into a Teflon-lined stainless-steel autoclave, and the autoclave was sealed and kept at 180 °C for 12 h. After reaction, the autoclave was cooled down naturally. The original precipitates were centrifuged and washed with deionized water and ethanol for several times, respectively. The resulting precipitates were dried at 40 °C and calcined in air with proper temperature-programmed to 500 °C, 600 °C and 700 °C for 2 h by a chamber electric furnace, respectively. The schematic procedure and the formation mechanism for fabrication of SnO2 fibers is shown in Fig. 1. Finally, the SnO2 gas sensors were prepared by a reported method in our previous work [22].
Fig. 2. XRD patterns of the product obtained from solvothermal process at 180 °C for 12 h (a) and after calcination at 500 °C (b), 600 °C (c) and 700 °C (d).
3. Results and discussion 3.1. Crystalline phase, morphology and structure characterization The crystalline phase structures of the as-synthesized precursors by solvothermal method and the corresponding calcined products were examined by XRD, as shown in Fig. 2. All the diffraction peaks were well indexed to the tetragonal rutile structure of SnO2 (JCPDS No. 411445), and the average crystallite size was about 4.7 nm, 5.8 nm, 6.8 nm and 7.3 nm for the products synthesized at 180 °C, 500 °C, 600 °C and 700 °C, respectively. It showed that crystallite size was growing slightly with the increasing of synthesis temperature. After the solvothermal process, hollow long fibers with smooth surfaces retained the basic surface morphology of the chorionic villi of metaplexis fruit were obtained, as shown in Fig. 3a. However, the integrity of the SnO2 fibers was destroyed after calcined at 600 °C, accompanied by the formation of rough surfaces, which may be caused by the oxidation and decomposition of the bio-carbon. The length of the fibers decreased to 50–100 µm, and the thickness of the fiber was about 100 nm, as shown in Fig. 3b. Fig. 3c and d show the TEM images of the as-synthesized SnO2 by the solvothermal process after calcination at 600 °C after grinding. As a matter of fact, the rough fiber was composed of many SnO2 nanoparticles with the size of 6–8 nm, as shown in
2.2. Characterization The phase identification of the product was performed using an Xray diffractometer (D/MAX2500PC model, Rigaku Co., Japan) using CuKα radiation at room temperature over a 2θ range of 5-90°. The microstructure of the products were observed by field emission high resolution scanning electron microscopy (Nova Nano SEM450, FEI, America) and transmission electron microscopy (Tecnai G2 F20, FEI, America). The porous characteristics of the prepared products were examined by N2 adsorption/desorption experiments at 77 K on a Micromeritics Quadrasorb-EVO (Quadrasorb-EVO, Quantachrome Corporation, America) gas adsorption apparatus. The specific surface area was measured according to the Brunauer-Emmett-Teller method, and the BJH model was used to calculate the pore size distribution. The gas sensing tests were performed on a WS-30A gas sensing measurement system (Weisheng Electronics Co., Ltd., Henan Province, China).
Fig. 1. The schematic procedure and the formation mechanism for fabrication of this fiber structure product.
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Fig. 3. Micrographs of the synthesized products before(a) and after the calcination (b) at 600 °C (c) TEM and SAED pattern of the synthesized SnO2 (d) HRTEM micrograph showing enlarged lattice images of the nano-sized SnO2.
product was calculated to be 73.665 m2/g, and the average pore size distribution was near 7.821 nm, suggesting that it has a high specific surface area with a mesoporous structure.
Fig. 3c. The diameter of the particles agrees well with the observation of SEM images and the XRD results. The above results allow deduction of the potential formation mechanism of the mesoporous structure SnO2, as shown in Fig. 1. Initially, Sn4 + was absorbed onto the smooth surface of the fibers when the SnCl4·5H2O solution was mixed with the dry chorionic villi of metaplexis fruit. Next, Sn(OH)4 precursors were synthesized by hydrolysis when the NaOH solution was added. Then, nano-sized SnO2 crystallite can be synthesized during the solvothermal process, as indicated by the XRD analysis. At the same time, the chorionic villi of metaplexis fruit will be carbonized after the solvothermal process, which can replicate the architecture of the fiber structure of the chorionic villi. Finally, after calcination, bio-carbon was oxidized and decomposed to form pores between the nanoparticles, and the pore size can be verified by nitrogen adsorption test, as shown in Fig. 4. The specific surface area of the
3.2. Gas sensing performance Both the high specific surface area and the mesoporous structure have beneficial effects on the as-prepared SnO2 as an excellent gassensing material. Hence, the gas sensing performance of the as-prepared SnO2 was investigated. As shown in Fig. 5a, the response values of this produced SnO2 by calcination at 600 °C to 300 ppm n-butanol, ethanol, methanol, acetone, ammonia, acetic acid and methane at 200 °C were approximately 415.313, 165.316, 13.06, 24.116, 6.115, 61.463, 3.844, respectively. These results indicated that the as-prepared SnO2 exhibited high sensitivity and selectivity to n-butanol. As we all know, nbutanol is a flammable, stimulating and narcotic liquid with characteristic odor, which is widely present in industrial waste streams and can be released into the atmosphere when used as a solvent for paints, coatings, varnishes, resins, vegetable oils. It is also deleterious for human health. It can cause headache, dizzy, dermatitis and discomfort of eyes and nose when people exposed to it. Therefore, it is necessary to fabricate gas sensors with high sensitivity, selectivity, and stability toward n-butanol [29–31]. As a result, the gas-sensing properties of the as-prepared SnO2 were further investigated using n-butanol as the major target gas. Fig. 5b shows the sensitivity-temperature curves of the sensors by calcination at different temperatures (500 °C, 600 °C, 700 °C) towards 300 ppm nbutanol. The results indicate that the sensor materials by calcination at the temperature of 600 °C shows the highest sensitivity to n-butanol, and the optimum operating temperature of the biomorphic SnO2 to nbutanol is 150 °C. As the response/recovery time was too long at the temperature of 150 °C (287 s/133 s when exposed to 10 ppm n-butanol), the rest of gas sensing tests were studied at the temperature of 200 °C. Fig. 5c shows the response-recovery curves and the response of the
Fig. 4. The nitrogen adsorption–desorption isotherms and the pore size distribution curve (inset) of the synthesized product obtained from solvothermal process after calcination.
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Fig. 5. (a) The response of biomorphic SnO2 to 300 ppm different measured gases at 200 °C. (b) The response of SnO2 obtained from solvothermal process after calcination at different temperatures to 300 ppm n-butanol at different testing temperatures (c) The typical response curve of biomorphic SnO2 at 200 °C exposed to 10–500 ppm n-butanol (d) The typical response curve of biomorphic SnO2 at 200 °C exposed to 300 ppm n-butanol (e) the response repeatability curve of biomorphic SnO2 exposed to 300 ppm n-butanol at 200 °C (f) the response stability curve of biomorphic SnO2 to 300 ppm n-butanol at 200 °C.
Fig. 5e presents six reversible response cycles of the prepared SnO2 hollow fibers to 300 ppm n-butanol at 200 °C on a day, and Fig. 5f shows the responses of the prepared SnO2 hollow fibers to 300 ppm nbutanol at 200 °C more than a week. All the results confirmed that the SnO2 hollow fibers produced using metaplexis fruit as the bio-template had a stable and repeatable characteristic as the sensing material. All the results show that the prepared mesoporous nano-sized SnO2 with hollow fiber structure is a promising material for n-butanol sensors. To date, a variety of semiconductor metal oxide gas sensors have been prepared to detect butanol, as shown in Table 1. Compared with all these literatures, the present mesoporous SnO2 based sensor showed
sensor to n-butanol over a range of concentrations from 10 ppm to 500 ppm at 200 °C. The synthesized SnO2 porous fibers possessed a hollow structure, allowing diffusion of the target gases to the inner surface of the fibers and the pores of the fibrous surface, resulting in a significant improvement of gas sensing response relative to traditional semiconductor sensors. In addition, the large specific surface area of the mesoporous SnO2 may also contribute to the increased response of the sensor. Additionally, the response and recovery time of this SnO2 porous fibers were improved to 64 s and 36 s towards 300 ppm n-butanol as shown in Fig. 5d. In addition, good stability is one important properties of gas sensors. 4
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Table 1 comparison of the sensing performances of multiple materials based gas sensors toward butanol. Materials
Operating temperature (°C)
Centration (ppm)
Response
Response time/Recovery time (s)
Refs.
ZnO hollow spheres Al-doping porous ZnO Au-WO3 Au-WO3 nanofiber mesoporous In-(TiO2/WO3) Porouscoral-like Co3O4 leaf-like -Fe2O3 ZnO-decorated α-Fe2O3 Mesoporous SnO2 3D hierarchical SnO2 Copolymer-assisted SnO2 Pt-3D SnO2 Au-3D SnO2 Mesoporous SnO2 fibers Mesoporous SnO2 fibers
385 300 250 250 200 120 260 225 150 320 140 100 340 150 200
500 100 100 100 50 1000 100 100 10 100 100 50 150 300 300 100
292 751.95 14.35 229.7 127.2 27.7 8 57 630 49.5 44.3 44 55.5 556.5 415.3 122.0
36/9 25/23 10/35 55/13 2.2/3 42/199 34/21 55/no giving 11/far from satisfactory 10/20 8/14 No giving No giving 195/100 64/36
[23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] This work This work
O2- (ads) + e-↔2O- (ads) (＞150 °C)
(3)
O (ads) + e ↔O (ads) (＞150 °C) -
-
2-
-
CH3CH2CH2CH2OH + 6O2 (ads)→4CO2 + 5H2O + 6e -
(4) -
CH3CH2CH2CH2OH + 12O (ads)→4CO2 + 5H2O + 12e 2-
(5) -
CH3CH2CH2CH2OH + 12O (ads)→4CO2 + 5H2O + 24e
(6) -
(7)
4. Conclusion Mesoporous SnO2 fibers were fabricated by a solvothermal method using metaplexis fruit as a bio-template. Results show that the fibers presented a high specific surface area of 73.665 m2/g with nano-sized pores, and the crystallite size was about 6.5 ± 0.5 nm. The gas sensing results show that the mesoporous SnO2 fibers was highly sensitive and selective to n-butanol, and the optimum operating temperature was about 150 °C. The sensitivities to 300 ppm n-butanol were 556.5 and 415.3 at 150 °C and 200 °C, respectively, which indicated that the prepared SnO2 exhibited high sensitivity and selectivity to n-butanol and has high potential application in the gas sensor field.
Fig. 6. The schematic diagram of the proposed sensing mechanism of the biomorphic SnO2.
high sensitivity, good selectivity to n-butanol, but response and recovery time should be modified in the future study.
Acknowledgements We gratefully acknowledge the financial supports from SDUST Research Fund (2015JQJH102), National Natural Science Foundation of China (No. 51772176), Distinguished Taishan Scholars in Climbing Plan (No. tspd20161006) and Science and Technology Planning Project of AQSIQ (2017IK104).
3.3. Gas sensing mechanism It is well known that SnO2 belongs to the surface-resistance-control semiconductor material, and the resistance of the SnO2 will be increased during the adsorption of oxygen on the surface, while decreased during the reaction of detected gas with the adsorption of oxygen. The process and gas sensing mechanism are shown in Fig. 6. In the air ambient, oxygen molecules are first adsorbed onto the surface of SnO2 to form single or double oxygen ions, depending on the temperature. When operating temperature is below 150 °C, the amount of chemisorbed O2- in tin oxide is much more than the amount of other O ion species according to Eq. (2). When operating temperature is higher than 200 °C, O2- and O- play a main role according to Eqs. (3) and (4). As a result, the thickness of electron depletion layer increased with conduction channel becoming narrow while the sensor exposed to the air, leading to the increase of sensor resistance. When the sensor exposed to n-butanol, the thickness of electron depletion layer decreased with conduction channel becoming broad according to Eqs. (5)–(7), leading to the reduction of sensor resistance, as shown in Fig. 6. O2 (g)↔O2 (ads) O2 (ads) + e
-
↔O2-
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Preparation of meso-porous SnO2 fibers with enhanced sensitivity for nbutanol ⁎
Xueying Zhanga, Guogang Xua, , Huiyong Wangb, Hongzhi Cuia, Xiaoyuan Zhana, Laifa Sanga, Gaoyu Zhanga a b
College of Material Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China Shandong Inspection and Quarantine Technology Center, Qingdao 266590, China
A R T I C L E I N F O
A B S T R A C T
Keywords: SnO2 Porous materials Sensors Bio-template
Meso-porous SnO2 fibers were synthesized using a solvothermal method with metaplexis fruit as the bio-template. The products were characterized by powder X-ray diffraction, high resolution scanning electron microscopy, transmission electron microscopy and nitrogen adsorption/desorption measurements. Results show that SnO2 fibers present a high specific surface area of 73.665 m2/g and a meso-porous structure with the pore size of 7.821 nm, and the crystal size of SnO2 is about 6.5 ± 0.5 nm. The gas sensing performance of the prepared SnO2 fibers toward several volatile organic compounds was investigated. The results show that the meso-porous SnO2 fibers were highly sensitive and selective to n-butanol.
1. Introduction As an important n-type semiconductor, SnO2 is one of the earliest established gas sensing metal oxides due to its low-cost and excellent physical-chemical properties [1,2]. Many SnO2 based gas sensors have been synthesized to detect a variety of volatile organic compounds (VOCs) including alcohol ethanol, acetone, methanol, isopropanol, benzyl, n-butanol, H2 and so on [3–11]. Due to the good gas permeability, high specific surface area and more active spots, hollow porous fibers can effectively improve the gas sensitivity and catalytic activity of SnO2 materials. Up to now, many manufacturing processes have been applied to prepare hollow fibers, such as sol-gel method [12], electrospinning method [13,14] and template-directed synthesis [15]. For example, Li et al. prepared Ni-doped SnO2 nanofiber array using electrospinning technique, and the product showed excellent performance for NO2 detection at 250 °C. Jiang et al. synthesized Eu-doped SnO2 nanofibers via a simple electrospinning technique, and the sensor exhibited a good sensitivity to acetone in sub-ppm concentrations and the detection limit could extend down to 0.3 ppm. Zhang et al. fabricated mesoporous SnO2 nanotubes by templating against carbon fibers followed by calcination at 800 °C. In recent years, a lot of biological natural fibers have attracted more attentions, due to their unique structures, and hollow semiconductor metal oxide nanofibers have been prepared by biological natural materials as template-directed materials [16]. For example, Song et al. prepared biomorphic In2O3 microtubules using cotton as the bio⁎
template which exhibited a good selectivity for formaldehyde with rapid response and high sensitivity at 260 °C [17]. Hollow CuO fibers were prepared by Dong et al. using cotton fibers as the bio-template, and it exhibited a superior sensitivity and good selectivity toward lowppm-level (1–100 ppm) n-propanol [18]. Cerium-doped TiO2 mesoporous nanofibers reported by Xiao et al. were prepared by one-pot facile synthesis method using collagen fiber as the bio-template [19]. SnO2 hollow fibers were synthesized using kapok and Bombyx mori silk worm cocoon as the bio-template, respectively [20,21], and all these fiber structures can improve the performance of semiconductor metal oxide effectively. Metaplexis japonica belongs to one kind of perennial grass liana, and the chorionic villi of metaplexis fruit is one kind of natural fibrous materials which exhibits cylindrical hollow structures with smooth surface and thin fiber walls. To the best of our knowledge, no reports have been presented on the synthesis semiconductor metal oxide fibers using metaplexis fruit as the bio-template. Therefore, mesoporous SnO2 fibers were synthesized using a solvothermal method with metaplexis fruit as the bio-template. The gas sensing performances of the prepared SnO2 fibers toward several volatile organic compounds were systematically investigated. 2. Experimental 2.1. Materials and experiment process SnCl4·5H2O (AR grade, CAS#: 10026-06-9) was purchased from
Corresponding author. E-mail addresses: [email protected], [email protected] (G. Xu).
https://doi.org/10.1016/j.ceramint.2017.12.093 Received 23 November 2017; Received in revised form 13 December 2017; Accepted 13 December 2017 0272-8842/ © 2017 Published by Elsevier Ltd.
Please cite this article as: Zhang, X., Ceramics International (2017), https://doi.org/10.1016/j.ceramint.2017.12.093
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Sinopharm Chemical Reagent Co., Ltd, Shanghai, China. The metaplexis fruit obtained from the dry Metaplexis japonica was used as the biotemplate. Firstly, the chorionic villi of metaplexis fruit was dried at 50–70 °C for 12 h in a electric drying oven with forced convection. Secondly, the as-treated chorionic villi were put into NaOH solution (1 mol/L, 100 mL) for 3 h at 80 °C and washed with deionized water for several times. In a typical synthesis process, 2.3 g SnCl4·5H2O (the mass ratio of dry bio-template to synthesized SnO2 was 1:1) was dissolved in 20 mL ethanol, and the washed chorionic villi was added to the solution under stirring. Then, NaOH solution (1 mol/L) was added to the mixture system dropwise until the pH value of the mixture rose up to 9, and added deionized water until the volume ratio of ethanol to water is 1:1. The mixture were transferred into a Teflon-lined stainless-steel autoclave, and the autoclave was sealed and kept at 180 °C for 12 h. After reaction, the autoclave was cooled down naturally. The original precipitates were centrifuged and washed with deionized water and ethanol for several times, respectively. The resulting precipitates were dried at 40 °C and calcined in air with proper temperature-programmed to 500 °C, 600 °C and 700 °C for 2 h by a chamber electric furnace, respectively. The schematic procedure and the formation mechanism for fabrication of SnO2 fibers is shown in Fig. 1. Finally, the SnO2 gas sensors were prepared by a reported method in our previous work [22].
Fig. 2. XRD patterns of the product obtained from solvothermal process at 180 °C for 12 h (a) and after calcination at 500 °C (b), 600 °C (c) and 700 °C (d).
3. Results and discussion 3.1. Crystalline phase, morphology and structure characterization The crystalline phase structures of the as-synthesized precursors by solvothermal method and the corresponding calcined products were examined by XRD, as shown in Fig. 2. All the diffraction peaks were well indexed to the tetragonal rutile structure of SnO2 (JCPDS No. 411445), and the average crystallite size was about 4.7 nm, 5.8 nm, 6.8 nm and 7.3 nm for the products synthesized at 180 °C, 500 °C, 600 °C and 700 °C, respectively. It showed that crystallite size was growing slightly with the increasing of synthesis temperature. After the solvothermal process, hollow long fibers with smooth surfaces retained the basic surface morphology of the chorionic villi of metaplexis fruit were obtained, as shown in Fig. 3a. However, the integrity of the SnO2 fibers was destroyed after calcined at 600 °C, accompanied by the formation of rough surfaces, which may be caused by the oxidation and decomposition of the bio-carbon. The length of the fibers decreased to 50–100 µm, and the thickness of the fiber was about 100 nm, as shown in Fig. 3b. Fig. 3c and d show the TEM images of the as-synthesized SnO2 by the solvothermal process after calcination at 600 °C after grinding. As a matter of fact, the rough fiber was composed of many SnO2 nanoparticles with the size of 6–8 nm, as shown in
2.2. Characterization The phase identification of the product was performed using an Xray diffractometer (D/MAX2500PC model, Rigaku Co., Japan) using CuKα radiation at room temperature over a 2θ range of 5-90°. The microstructure of the products were observed by field emission high resolution scanning electron microscopy (Nova Nano SEM450, FEI, America) and transmission electron microscopy (Tecnai G2 F20, FEI, America). The porous characteristics of the prepared products were examined by N2 adsorption/desorption experiments at 77 K on a Micromeritics Quadrasorb-EVO (Quadrasorb-EVO, Quantachrome Corporation, America) gas adsorption apparatus. The specific surface area was measured according to the Brunauer-Emmett-Teller method, and the BJH model was used to calculate the pore size distribution. The gas sensing tests were performed on a WS-30A gas sensing measurement system (Weisheng Electronics Co., Ltd., Henan Province, China).
Fig. 1. The schematic procedure and the formation mechanism for fabrication of this fiber structure product.
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Fig. 3. Micrographs of the synthesized products before(a) and after the calcination (b) at 600 °C (c) TEM and SAED pattern of the synthesized SnO2 (d) HRTEM micrograph showing enlarged lattice images of the nano-sized SnO2.
product was calculated to be 73.665 m2/g, and the average pore size distribution was near 7.821 nm, suggesting that it has a high specific surface area with a mesoporous structure.
Fig. 3c. The diameter of the particles agrees well with the observation of SEM images and the XRD results. The above results allow deduction of the potential formation mechanism of the mesoporous structure SnO2, as shown in Fig. 1. Initially, Sn4 + was absorbed onto the smooth surface of the fibers when the SnCl4·5H2O solution was mixed with the dry chorionic villi of metaplexis fruit. Next, Sn(OH)4 precursors were synthesized by hydrolysis when the NaOH solution was added. Then, nano-sized SnO2 crystallite can be synthesized during the solvothermal process, as indicated by the XRD analysis. At the same time, the chorionic villi of metaplexis fruit will be carbonized after the solvothermal process, which can replicate the architecture of the fiber structure of the chorionic villi. Finally, after calcination, bio-carbon was oxidized and decomposed to form pores between the nanoparticles, and the pore size can be verified by nitrogen adsorption test, as shown in Fig. 4. The specific surface area of the
3.2. Gas sensing performance Both the high specific surface area and the mesoporous structure have beneficial effects on the as-prepared SnO2 as an excellent gassensing material. Hence, the gas sensing performance of the as-prepared SnO2 was investigated. As shown in Fig. 5a, the response values of this produced SnO2 by calcination at 600 °C to 300 ppm n-butanol, ethanol, methanol, acetone, ammonia, acetic acid and methane at 200 °C were approximately 415.313, 165.316, 13.06, 24.116, 6.115, 61.463, 3.844, respectively. These results indicated that the as-prepared SnO2 exhibited high sensitivity and selectivity to n-butanol. As we all know, nbutanol is a flammable, stimulating and narcotic liquid with characteristic odor, which is widely present in industrial waste streams and can be released into the atmosphere when used as a solvent for paints, coatings, varnishes, resins, vegetable oils. It is also deleterious for human health. It can cause headache, dizzy, dermatitis and discomfort of eyes and nose when people exposed to it. Therefore, it is necessary to fabricate gas sensors with high sensitivity, selectivity, and stability toward n-butanol [29–31]. As a result, the gas-sensing properties of the as-prepared SnO2 were further investigated using n-butanol as the major target gas. Fig. 5b shows the sensitivity-temperature curves of the sensors by calcination at different temperatures (500 °C, 600 °C, 700 °C) towards 300 ppm nbutanol. The results indicate that the sensor materials by calcination at the temperature of 600 °C shows the highest sensitivity to n-butanol, and the optimum operating temperature of the biomorphic SnO2 to nbutanol is 150 °C. As the response/recovery time was too long at the temperature of 150 °C (287 s/133 s when exposed to 10 ppm n-butanol), the rest of gas sensing tests were studied at the temperature of 200 °C. Fig. 5c shows the response-recovery curves and the response of the
Fig. 4. The nitrogen adsorption–desorption isotherms and the pore size distribution curve (inset) of the synthesized product obtained from solvothermal process after calcination.
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Fig. 5. (a) The response of biomorphic SnO2 to 300 ppm different measured gases at 200 °C. (b) The response of SnO2 obtained from solvothermal process after calcination at different temperatures to 300 ppm n-butanol at different testing temperatures (c) The typical response curve of biomorphic SnO2 at 200 °C exposed to 10–500 ppm n-butanol (d) The typical response curve of biomorphic SnO2 at 200 °C exposed to 300 ppm n-butanol (e) the response repeatability curve of biomorphic SnO2 exposed to 300 ppm n-butanol at 200 °C (f) the response stability curve of biomorphic SnO2 to 300 ppm n-butanol at 200 °C.
Fig. 5e presents six reversible response cycles of the prepared SnO2 hollow fibers to 300 ppm n-butanol at 200 °C on a day, and Fig. 5f shows the responses of the prepared SnO2 hollow fibers to 300 ppm nbutanol at 200 °C more than a week. All the results confirmed that the SnO2 hollow fibers produced using metaplexis fruit as the bio-template had a stable and repeatable characteristic as the sensing material. All the results show that the prepared mesoporous nano-sized SnO2 with hollow fiber structure is a promising material for n-butanol sensors. To date, a variety of semiconductor metal oxide gas sensors have been prepared to detect butanol, as shown in Table 1. Compared with all these literatures, the present mesoporous SnO2 based sensor showed
sensor to n-butanol over a range of concentrations from 10 ppm to 500 ppm at 200 °C. The synthesized SnO2 porous fibers possessed a hollow structure, allowing diffusion of the target gases to the inner surface of the fibers and the pores of the fibrous surface, resulting in a significant improvement of gas sensing response relative to traditional semiconductor sensors. In addition, the large specific surface area of the mesoporous SnO2 may also contribute to the increased response of the sensor. Additionally, the response and recovery time of this SnO2 porous fibers were improved to 64 s and 36 s towards 300 ppm n-butanol as shown in Fig. 5d. In addition, good stability is one important properties of gas sensors. 4
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Table 1 comparison of the sensing performances of multiple materials based gas sensors toward butanol. Materials
Operating temperature (°C)
Centration (ppm)
Response
Response time/Recovery time (s)
Refs.
ZnO hollow spheres Al-doping porous ZnO Au-WO3 Au-WO3 nanofiber mesoporous In-(TiO2/WO3) Porouscoral-like Co3O4 leaf-like -Fe2O3 ZnO-decorated α-Fe2O3 Mesoporous SnO2 3D hierarchical SnO2 Copolymer-assisted SnO2 Pt-3D SnO2 Au-3D SnO2 Mesoporous SnO2 fibers Mesoporous SnO2 fibers
385 300 250 250 200 120 260 225 150 320 140 100 340 150 200
500 100 100 100 50 1000 100 100 10 100 100 50 150 300 300 100
292 751.95 14.35 229.7 127.2 27.7 8 57 630 49.5 44.3 44 55.5 556.5 415.3 122.0
36/9 25/23 10/35 55/13 2.2/3 42/199 34/21 55/no giving 11/far from satisfactory 10/20 8/14 No giving No giving 195/100 64/36
[23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] This work This work
O2- (ads) + e-↔2O- (ads) (＞150 °C)
(3)
O (ads) + e ↔O (ads) (＞150 °C) -
-
2-
-
CH3CH2CH2CH2OH + 6O2 (ads)→4CO2 + 5H2O + 6e -
(4) -
CH3CH2CH2CH2OH + 12O (ads)→4CO2 + 5H2O + 12e 2-
(5) -
CH3CH2CH2CH2OH + 12O (ads)→4CO2 + 5H2O + 24e
(6) -
(7)
4. Conclusion Mesoporous SnO2 fibers were fabricated by a solvothermal method using metaplexis fruit as a bio-template. Results show that the fibers presented a high specific surface area of 73.665 m2/g with nano-sized pores, and the crystallite size was about 6.5 ± 0.5 nm. The gas sensing results show that the mesoporous SnO2 fibers was highly sensitive and selective to n-butanol, and the optimum operating temperature was about 150 °C. The sensitivities to 300 ppm n-butanol were 556.5 and 415.3 at 150 °C and 200 °C, respectively, which indicated that the prepared SnO2 exhibited high sensitivity and selectivity to n-butanol and has high potential application in the gas sensor field.
Fig. 6. The schematic diagram of the proposed sensing mechanism of the biomorphic SnO2.
high sensitivity, good selectivity to n-butanol, but response and recovery time should be modified in the future study.
Acknowledgements We gratefully acknowledge the financial supports from SDUST Research Fund (2015JQJH102), National Natural Science Foundation of China (No. 51772176), Distinguished Taishan Scholars in Climbing Plan (No. tspd20161006) and Science and Technology Planning Project of AQSIQ (2017IK104).
3.3. Gas sensing mechanism It is well known that SnO2 belongs to the surface-resistance-control semiconductor material, and the resistance of the SnO2 will be increased during the adsorption of oxygen on the surface, while decreased during the reaction of detected gas with the adsorption of oxygen. The process and gas sensing mechanism are shown in Fig. 6. In the air ambient, oxygen molecules are first adsorbed onto the surface of SnO2 to form single or double oxygen ions, depending on the temperature. When operating temperature is below 150 °C, the amount of chemisorbed O2- in tin oxide is much more than the amount of other O ion species according to Eq. (2). When operating temperature is higher than 200 °C, O2- and O- play a main role according to Eqs. (3) and (4). As a result, the thickness of electron depletion layer increased with conduction channel becoming narrow while the sensor exposed to the air, leading to the increase of sensor resistance. When the sensor exposed to n-butanol, the thickness of electron depletion layer decreased with conduction channel becoming broad according to Eqs. (5)–(7), leading to the reduction of sensor resistance, as shown in Fig. 6. O2 (g)↔O2 (ads) O2 (ads) + e
-
↔O2-
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