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Preparation of Porous Tin Oxide Nanotubes Using Carbon Nanotubes as Templates and Their Gas-Sensing Properties Yong Jia,†,‡,§ Lifang He,† Zheng Guo,† Xing Chen,† Fanli Meng,† Tao Luo,† Minqiang Li,† and Jinhuai Liu*,†,‡ Key Laboratory of Biomimetic Sensing and AdVanced Robot Technology, Hefei Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China, College of Chemistry & Chemical Engineering, Anhui UniVersity, Hefei 230039, People’s Republic of China, and Department of Pharmacy, Anhui UniVersity of Traditional Chinese Medicine, Heifei 230031, People’s Republic of China ReceiVed: January 8, 2009; ReVised Manuscript ReceiVed: April 23, 2009
Porous tin oxide (SnO2) nanotubes were prepared by using multiwalled carbon nanotubes as templates. The morphology and crystal structure of the SnO2 nanotubes were characterized by field emission scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The SnO2 crystallite size was about 5 to7 nm. The as-prepared porous SnO2 nanotubes exhibited a good response and reversibility to some organic gas, such as ethanol and acetone. The sensor responses to 100 ppm ethanol and acetone were 130 and 126, respectively, at a working temperature of 200 °C. In addition, the sensors also exhibited a good response to methanol, propanol, 2-propanol, ethyl acetate, and ethyl ether. The relationship between the gas-sensing properties and the microstructure of the as-prepared SnO2 nanotubes was also discussed. 1. Introduction Tin oxide (SnO2) is a typical n-type wide band gap semiconductor (Eg ) 3.6 eV at 300 K), and has been widely utilized as a gas-sensing material. Recently, compared to SnO2-based film sensors, many researchers focused on the gas sensitivity of SnO2 nanostructured materials, such as nanoparticles,1,2 single crystalline nanowires,3,4 nanobelts,5,6 nanorods,2,7 polycrystalline nanotubes8,9 or nanowires,10,11 and hollow spheres.12 For the polycrystalline SnO2 nanomaterials, when the SnO2 crystallite size is comparable with or less than 2L (about 6 nm), where L is the depth of the space-charge layer, the sensitivity can be greatly increased.13,14 Wang et al. reported the preparation of polycrystalline SnO2 nanotubes using anodic aluminum oxide (AAO) membranes as a template, and the average crystallite size was about 15 nm, much higher than the 2L of SnO2.8 Huang et al.9 synthesized a polycrystalline thick SnO2 nanotube sheet using filter paper as a template, and the crystallite size can be controlled by the calcination temperature. However, the thick SnO2 sheet was not favorable for the diffusion of gas molecules. Since the discovery of carbon nanotubes (CNTs),15 CNTs have attracted great interest as a new carbon material due to their unique chemical and physical properties.16 In addition, CNTs can also be used as an excellent one-dimensional template for the preparation of metal oxide nanotubes,17-19 including SnO2 nanotubes. To date, the gas-sensing properties20-26 and the electrochemical performances23,27,28 of the SnO2/CNT nanocomposites have been widely reported. However, few reports focus on the gas-sensing properties of SnO2 nanotubes prepared with CNTs as template. In this paper, SnO2/CNT nanocomposites were synthesized by using the SnCl2 solution method at room temperature. Porous and polycrystalline SnO2 nanotubes were obtained after removing the CNTs by thermal treatment. * To whom correspondence should be addressed. Phone: +86-5515591142. Fax: +86-551-5592420. E-mail:
[email protected]. † Chinese Academy of Sciences. ‡ Anhui University. § Anhui University of Traditional Chinese Medicine.
The as-prepared SnO2 nanotubes exhibited good gas-sensing properties to reducing gases. 2. Experimental Section 2.1. Purification of CNTs and Preparation of SnO2/CNT Nanocomposites and Porous SnO2 Nanotubes. CNTs were purchased from Shenzhen Nanotech Port Co. Ltd. The diameter of the CNTs is 20-30 nm. Before the SnO2/CNT nanocomposites were prepared, CNTs were calcined at 350 °C for 2 h to remove amorphous carbon. The calcined CNTs (1 g) were dispersed in 100 mL of 7.0 mol/L HNO3 by sonicating for 10 min, then refluxing at 120 °C for 12 h with stirring, then rinsing with distilled H2O until the pH of the solution was neutral, and finally dried. In a typical synthesis of SnO2/CNT nanocomposites, 1 g of tin(II) chloride (SnCl2 · 2H2O) was dissolved in 40 mL of distilled H2O, and then 0.25 mL of HCl (38%) was added. Subsequently, 10 mg of the as-treated CNTs was dispersed in the above solution. The mixture was sonicated for 20 min and then stirred at room temperature for 2 h. The SnO2/CNT nanocomposites were then separated by centrifugation and washed with distilled water several times until the pH of the solution was neutral.29,30 The as-prepared SnO2/CNT nanocomposites were calcinated at 350 °C for 3 h and then heated at 650 °C for 5 min in air to remove the CNTs. Finally, the porous SnO2 nanotubes were obtained. 2.2. Characterization. The purified CNTs, SnO2/CNT nanocomposites, and porous SnO2 nanotubes were characterized by field emission scanning electron microscopy (FE-SEM, Sirion 200, operated at 5 kV) and X-ray diffraction (XRD, X’Pert Pro MPD, Cu KR radiation, wavelength 1.5418 Å). The prepared SnO2/CNT nanocomposites and porous SnO2 nanotubes were characterized by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM, JEOL-2010, operated at 120 kV). The weight content of SnO2 in SnO2/CNT nanocomposites was measured by thermal gravimetric analysis (TGA, Pyris 1, heating rate 10 deg/min in flow
10.1021/jp9001719 CCC: $40.75 2009 American Chemical Society Published on Web 05/11/2009
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Figure 1. FESEM images of the purified CNTs (a), SnO2/CNT nanocomposites (b), and porous SnO2 nanotubes (c, d).
air). The oxygen content of the CNTs was investigated by X-ray photoelectron spectrometry (XPS, ESCALAB 250). 2.3. Structure and Preparation of the Sensor Device. The structures of the sensor device and the measurement system are the same as in our previous report.31 The as-prepared wet SnO2/ CNT nanocomposites were directly coated on the outer surface of the ceramic tube and dried in air, then calcined at 350 °C for 3 h. After that, the CNTs were removed by thermal treatment at 650 °C for 5 min in air, and at the same time, porous SnO2 nanotube networks were obtained on the surface of the ceramic tube. The sensor response was defined as S ) Ra/Rg, where, Ra is the resistance in dry air and Rg is that in the dry air mixed with detected gases. In addition, the response time was defined as the time required for the conductance to reach 90% of the equilibrium value after a test gas was injected, and the recovery time was the time necessary for a sensor to attain a conductance 10% above its original value in air. All gas-sensing results shown in this paper were obtained in the static state and dry air background, and the real-time response curves were obtained from the same sensor device. 3. Results and Discussion 3.1. Structures of the SnO2/CNT Nanocomposites and the SnO2 Nanotubes. To obtain the real morphologies of the SnO2/ CNT nanocomposites and the SnO2 nanotubes on the surface of the component, the as-prepared wet SnO2/CNT nanocomposites were coated on the surface of the ceramic substrate, and then characterized by SEM after thermal treatment. Figure 1a is the SEM image of the purified CNTs. Their diameter is in the range of 20 to 30 nm. Figure 1b is the SEM image of the SnO2/CNT nanocomposites coated on the ceramic substrate. The surface of the tube-like nanocomposites was obviously coarser than that of the purified CNTs. After removing the CNTs, porous SnO2 nanotubes were found on the surface of the ceramic substrate, as shown in Figure 1c,d. TEM images of the SnO2/CNT nanocomposites and the porous SnO2 nanotubes are shown in Figure 2. From Figure 2a,b, it is clear that the surface of the CNTs was coated with uniform and dense SnO2 nanoparticles. Panels c and d of Figure 2 suggest that after removing the CNTs, the SnO2 nanoparticles
were assembled together and formed porous SnO2 nanotubes. It should be mentioned that the as-prepared porous SnO2 nanotubes were very brittle. So, we fabricated the gas-sensing component by in situ heating the ceramic tube, which had been coated with the SnO2/CNT nanocomposites. Figure 3a is the HRTEM image of the as-prepared SnO2/ CNT nanocomposites before thermal treatment. The average crystallite size of SnO2 nanoparticles was 4 nm. After removing the CNTs, the crystallite size of SnO2 nanoparticles was slightly increased to 5-7 nm, which was very close to the 2L of SnO2, as shown in Figure 3b. In addition, many mesopores were observed in the tubes, and the mean diameter of these pores was about 5 nm. XRD patterns shown in Figure 3c suggest the wide diffraction peaks of the as-prepared SnO2/CNT nanocomposites, which can be attributed to the small crystallite size of SnO2 nanoparticles. The diffraction peaks of SnO2 (JCPDS 770450) turned sharper after removing the CNTs, which can be attributed to the improved crystallization and the growth of the SnO2 nanoparticles, as shown in Figure 3b. TGA cures shown in Figure 3d present the weight content of SnO2 in the nanocomposites was 42.3%. 3.2. Gas-Sensing Properties of the Porous SnO2 Nanotubes. Panels a and b of Figure 4 show the real-time response curve and the sensor responses of the sensor device upon exposure to different concentrations of ethanol at a working temperature of 200 °C, respectively. The as-prepared porous SnO2 nanotubes have a good response to ethanol. At a low concentration of 5 ppm of ethanol, the relative response is about 11. When increasing the concentration, the response of the sensor also sharply increased, as shown in Figure 4b. The response to 100 ppm ethanol is up to 130, and the response time and recovery time were about 30 s. Furthermore, according to Figure 4a, we could observe that the sensor also had a good reversibility. The response of the SnO2 nanotubes is much higher than those of SnO2 nanobelts,6 nanorods,7 SnO2/CNT nanocomposites,25 and SnO2 nanotubes prepared by using an AAO template.8 Furthermore, the sensor fabricated with porous and polycrystalline SnO2 nanotubes also showed a significant response to methanol, propanol, 2-propanol, acetone, and ethyl acetate, as shown in Figures 5 and 6a. The sensor responses to
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Figure 2. TEM images of the SnO2/CNT nanocomposites (a, b) and porous SnO2 nanotubes (c, d).
Figure 3. HRTEM images of the SnO2/CNT nanocomposites (a) and porous SnO2 nanotubes (b). The arrows and circles in panel b present some mesopores and SnO2 nanoparticles, respectively. (c) XRD patterns of the SnO2/CNT nanocomposites and the porous SnO2 nanotubes. (d) TGA curves of the purified CNTs and the SnO2/CNT nanocomposites.
100 ppm of the above five gases were 66.5, 250, 115, 126, and 97, respectively. However, the response to ethyl ether is relatively low, as shown in Figure 6a. On the basis of the slope of the response curves and the signal of the sensor responses shown in Figures 5 and 6a, it could be easily found that the
sensor device presents the best response for alcohol and acetone. At the same time, an additional two sensors were fabricated under the same process. Similar gas-sensing properties were obtained, which suggests the good repeatability of the sensors. In addition, without using the CNT template, the sensor device
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Figure 4. Real-time response curve (a) and sensor responses (b) of the sensor device upon exposure to different concentrations of ethanol at a working temperature of 200 °C.
Figure 5. Real-time response curves of the sensor device upon exposure to different concentrations of methanol (a), propanol (b), 2-propanol (c), and acetone (d) at a working temperature of 200 °C. The insets show the corresponding sensor response curves.
was fabricated by using the same method. The sensor response to 100 ppm of ethanol is only 17.5 at a working temperature of 200 °C, much lower than those of porous polycrystalline SnO2 nanotubes. For the semiconductor oxide sensors, working temperature is an important factor. Figure 6b presents the relationship between the sensor responses and the working temperature. In
the range of 120 to 160 °C, the sensor response was sharply increased with increasing working temperature, and up to 360 at 160 °C. Then, the sensor response was decreased at higher working temperature. However, the recovery times of the sensor device were longer than 30 s when the working temperature was lower than 200 °C. For instance, the recovery time was 75 s at a working temperature of 160 °C. So, the optimum
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Figure 6. (a) Sensor responses of the sensor device upon exposure to different concentrations of ethyl acetate and ethyl ether at a working temperature of 200 °C. (b) Sensor responses of the sensor device upon exposure to ethanol (100 ppm) at different working temperatures. The relationship in panel b is between the sensor responses and the preparation times of the SnO2/CNT nanocomposites.
TABLE 1: Comparison of Gas-Sensing Characteristics of SnO2 Gas Sensors Fabricated by Different Approaches
fabrication approach SnO2 thin-film SnO2 hollow spheres single SnO2 nanobelt SnO2 nanorods SnO2 porous films SnO2 nanotubes SnO2 nanotubes
combustion chemical vapor deposition carbon spheres as template carbon spheres as template thermal evaporation hydrothermal route hydrothermal route PS spheres as template AAO as template CNTs as template
ethanol concentrated (ppm) 500 1000 100 250 300 100 100 100 100 100
working temperature of the sensor device was 200 °C. The sensor responses shown in Figure 6b are the mean value of three time measurements, and the errors of sensor responses are also shown in the figure. The results suggest the good reproducibility of the sensors. The comparison of sensor responses of various pure SnO2 gas sensors for ethanol sensing is summarized in Table 1. Accordingly, these results definitely show that the porous SnO2 nanotubes gas sensor has achieved better sensing properties than that of other SnO2 nanostructured materials. The gas-sensing stability of the sensor device is very important for its further application. So we measured the sensor responses exposed to 100 ppm of ethanol and acetone one month after the previous measurement. The results suggest the fluctua-
temp (°C)/rel humidity (%) 200/dry air 300/57% RH 250/in air 400/30% RH 300/25% RH 300/25% RH 300/dry air 200/40-50% RH 200/dry air 160/dry air
sensor response S ) Rair/Rgas
response time/recovery time (s)
50.5
122/39
32
75 119 41.6 83.8 13.9 17.3 7 130 360
4/10 10/30 -/-/1/1 36/5/5 30/30 30/75
12 33 6 2 7 34 8 this work
ref
tion of the sensitivity is less than 4%. We also fabricated the sensor device using the traditional method.31 The as-prepared SnO2 nanotubes were dispersed into an appropriate amount of ethanol under ultrasonication. Then the suspension solution formed was coated on the outer surface of the ceramic tube and dried in air. The gas-sensing properties were measured, and the results suggested the sensor response exposed to 100 ppm of ethanol was 80 at a working temperature of 200 °C. However, the sensor response was decreased to 65 one month later. The fluctuation of the sensitivity is about 20%. So, in situ fabrication of the SnO2 nanotube networks on the surface of the ceramic tube can greatly improve the gas-sensing stability of the sensor device.
Figure 7. SEM images of the SnO2 nanotubes derived from the SnO2/CNT nanocomposites synthesized with 1 (a), 3 (b), and 4 h (c) of reaction time.
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Figure 8. (a) The oxygen contents in CNTs after refluxing in HNO3 with different times. (b) Sensor responses of the sensor device upon exposure to ethanol (100 ppm) at different working temperatures. The relationship in panel b is between the sensor responses and the refluxing times of the CNTs in HNO3.
For the polycrystalline SnO2 nanomaterials, the crystallite size of SnO2 nanoparticles plays a very important role in the gassensing properties. The crystallite size of the as-prepared SnO2 nanoparticles was 5 to 7 nm, which is very close to the 2L of SnO2. In addition, the porous nanostructure is also advantageous to improving the gas-sensing properties.18,19 From Figure 6b, we also find the preparation time of the SnO2/CNT nanocomposites has a great influence on the gas-sensing properties of the final SnO2 nanotubes. At a working temperature of 160 °C, all of the sensor devices show the best responses to 100 ppm of ethanol. The optimum preparation time of the SnO2/CNT nanocomposites was 2 h. To clarify its cause, the morphology of the as-prepared SnO2 nanotubes was studied by SEM, and the results are shown in Figure 7. From Figures 7 and 1d, it is clear that SnO2/CNT nanocomposites synthesized with 2 h of reaction time resulted in the formation of SnO2 nanotubes which have the best morphology, including the long SnO2 nanotubes and the uniform distributing of pores. Shorter reaction time brings on the short SnO2 nanotubes, as shown in Figure 7a. When the reaction time was 3 h, the number of pores is decreased compared to the number at 2 h (Figure 1d). Furthermore, few pores can be observed in the SnO2 tubes which derived from the SnO2/CNT nanocomposites which were produced with 4 h of reaction time, as shown in Figure 7c, and as a result, the gas-sensing properties of the SnO2 nanotubes were decreased to a large scale, as shown in Figure 6b. So, the small crystallite size of SnO2 nanoparticles and the uniform distribution of the mesopores are the reasons for the high gassensing properties of the SnO2 nanotubes. It was well accepted that treatment with oxidizing acid will produce oxygen-containing functional groups on the side wall of CNTs which can act as sites for the tin oxide coating.24,30 The effect of the refluxing time in HNO3 solution on the oxygen content of the CNTs was investigated by XPS, and the results are shown in Figure 8a. The oxygen content in the untreated CNTs was 1.21%. After refluxing in HNO3 for 6 h, the oxygen content was increased to 3.29%. Longer refluxing time created more oxygen-containing functional groups. However, when the refluxing time was longer than 12 h, the oxygen content was only slightly increased. The results suggest the refluxing time in HNO3 has a great influence on the content of oxygencontaining functional groups on the side wall of CNTs.
Furthermore, the relationship between the sensor responses to 100 ppm of ethanol and the refluxing time of the CNTs in HNO3 was also studied, and the results are shown in Figure 8b. In addition, the gas-sensing properties of the product with untreated CNTs as template were also investigated, and the results are also shown in Figure 8b. It is very interesting to find that, at a working temperature of 200 °C, the sensor response was similar to those of the SnO2 nanotubes prepared with the refluxed CNT template. However, the sensor responses were much lower when the working temperature was lower than 200 °C. After refluxing 6 h, the gas-sensing properties of the prepared SnO2 nanotubes were greatly improved compared with the ones derived from the untreated CNTs. The improved gassensing properties have a close relationship to the increase of the oxygen content. However, in the range of 120 to 180 °C, it seems that longer refluxing time does not have much influence on the sensor responses. When the refluxing time was up to 18 h, the sensor responses were increased again at the higher working temperature. The results suggest that more oxygencontaining functional groups in CNTs are more favorable for the preparation of SnO2 nanotubes with better gas-sensing properties. 4. Conclusions Porous and polycrystalline SnO2 nanotubes were prepared by using CNTs as a template. The SnO2 crystallite size in the tubes was about 5 to 7 nm, and the porous SnO2 nanotubes exhibited high sensitivity and good reversibility toward some reducing gases. The sensing responses to 100 ppm of ethanol and acetone were 130 and 126, respectively, at a working temperature of 200 °C. The good gas-sensing properties were attributed to the small size of SnO2 nanoparticles and the porous microstructure of the SnO2 nanotubes. Acknowledgment. This work was supported by the Knowledge Innovation Program of the Chinese Academy of Sciences, the National High Technology Research and Development Program of China (Grant No. 2007AA022005), the National Basic Research Program of China (Grant No. 2007CB936603), and the National Natural Science Foundation of China (Grant Nos. 60604022, 10635070, and 60801021).
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