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Synthesis, characterization and gas-sensing property for C2H5OH of SnO2 nanorods
Materials Chemistry and Physics 112 (2008) 244–248
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Synthesis, characterization and gas-sensing property for C2 H5 OH of SnO2 nanorods Heyun Zhao a,∗ , Yuehuan Li b , Liufang Yang a , Xinghui Wu a a b
Department of Materials Science and Engineering, Yunnan University, 650091 Kunming, China Life Science and Chemistry School of DaLi College, 671200 DaLi, China
a r t i c l e
i n f o
Article history: Received 25 February 2008 Received in revised form 19 May 2008 Accepted 22 May 2008 Keywords: Nanostructures X-ray diffraction Electron microscopy Catalytic properties
a b s t r a c t SnO2 nanorods with the rutile structure were prepared with annealing precursor powders produced by solid-state reaction coated surfactant at room temperature. The phase structure and morphologies of SnO2 nanorods were characterized by means of X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectrum (XPS). The results show that diameter and length of the as-made nanorods are less then 20 nm and several micrometers, respectively. The indirect-heating sensors, using the SnO2 nanorods as sensitive materials fabricated on an alumna tube with Au electrodes and platinum wires, were fabricated and investigated for the gas sensing properties. The investigation demonstrates that the sensor based on SnO2 nanorods materials has good sensitivity, high selectivity and short response and reversion time to C2 H5 OH at 300 ◦ C. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In recent years, one-dimensional (1D) nanoscale structures of metal-oxide semiconductors (MOS) such as wires, rods, belts and tubes have became the focus of intensive research owing to their unique applications in mesoscopic physics and fabrication of nanoscale devices [1–3]. Among them, SnO2 nanomaterials with 1D structures, such as nanowires or nanorods, are especially attractive due to possess novel properties in applications as gas sensors, dye-based solar cells, transistors, electrode materials, catalysts, solar cells, and electro-chromic devices, etc. Recently, some studies have been reported on the synthesis of 1D SnO2 nanomaterials (nanowires, nanorods, and nanobelts, etc.) using laser ablation [4], thermal decomposition [5], thermal evaporation [6–8], rapid thermal oxidation [9], and redox reaction [10], low-temperature chemical vapor deposition [11], solid–vapor process [12], etc. Generally, the preparation methods mentioned above involve complex procedures, sophisticated equipment, and rigorous experimental conditions. Thus, the development of mild and lost-cost synthetic routes to SnO2 nanorods was of great significance. Tin oxide is one of the earliest discovered and the most widely applied oxide gas sensing materials due to its high mobility of conduction electrons, good chemical and thermal stability under the operating conditions of sensors. Up to now, most of studies are focused on particle sintered or thin-film-based devices. More
recently, several groups have reported that the single nanowires or nanoribbons of semiconductor oxides are very promising for sensors [13–15]. The results showed that the devices based on one-dimensional nanostructures have great potential to overcome the fundamental limitations of traditional semiconductor oxides sensors based on particle sintered or thick-film such as low sensitivity, poor stability and high working temperature. The different methods and conditions in synthesizing process would produce 1D nanostructure materials with different surface state and morphology, so it is very important to study the gas-sensing properties of the 1D nanostructure materials prepared by different methods under different conditions. In this work, we report a simple and novel approach to synthesis SnO2 nanorods via annealing precursors produced by solid-state reaction between SnCl4 ·5H2 O and KBH4 in the presence of KCl + NaCl flux coated with surfactant of nonyl phenyl ether (9)/(5) (NP9/5). Gas sensors based on the as-prepared sample were fabricated and the gas-sensing properties were researched. 2. Experimental 2.1. Synthesis of SnO2 nanorods In this paper, SnO2 nanorods with the rutile structure were synthesized by annealing precursor powders produced by solid-state reaction coated with surfactant of nonyl phenyl ether (9)/(5) (NP9/5) at room temperature. The preparation of SnO2 nanorods was based on the following reaction [16]: SnCl4 ·5H2 O + 4KBH4 → 2B2 H6 + 4KCl + 5H2 O + Sn + 2H2 ↑
∗ Corresponding author. Tel.: +86 871 5031410; fax: +86 871 5153832. E-mail address: [email protected] (H. Zhao). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.05.039
Sn + O2 → SnO2
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SnCl4 ·5H2 O, KBH4 , NP9/5, NaCl and KCl were used as precursor materials. SnCl4 ·5H2 O, KBH4 , NaCl and KCl are of analytical purity. NP9/5 is up to 99.0% in purity. A total of 3.51 g of SnCl4 ·5H2 O, 2.00 g of NaCl and 2.55 g of KCl (according to 1:1 molar rate) were mixed with 5 ml of NP9/5 in a mortar, ground for 30 min (hereafter expressed as ‘A’); 1.54 g of KBH4 , 2.00 g of NaCl and 2.55 g of KCl were mixed with 5 ml of NP9/5 in a mortar, ground for 30 min (hereafter expressed as ‘B’). Thereafter, ‘B’ was added to ‘A’ to get mixture in a mortar, ground for 60 min. The products were washed several times by the use of acetone to remove NP9/5 and by-products, then dried in an oven at 70–80 ◦ C for 2 h. By doing so, the precursor powders were obtained. The precursor powders were annealed at 660 ◦ C, which was heating rate of 5 ◦ C min−1 , and then kept the temperature constant for 2 h in a porcelain crucible that was placed in the middle of the alumina tube. The reaction chamber is of alumina tube with a length of 1500 mm and a diameter of 60 mm. After heating treatment, it was gradually cooled to room temperature; the as-made products were washed with distilled water several times to remove NaCl, and KCl, then get pure SnO2 nanorods. 2.2. Characterization of as-synthesized samples The nanorods products were examined by X-ray diffraction (XRD) employed scanning steps of 0.02◦ /step in a 2 range from 10◦ to 100◦ , using a D/MAX-RA diffrac˚ and a graphite monochromatic crystal at tometer with Cu K␣ radiation ( = 1.5418 A) 40 kV and 50 mA. Scanning electron microscopy (SEM) photographs were obtained by XL30ESEM-TMP, Holland. Transmission electron microscopy (TEM) measurement was taken using a JEM-200CX to characterize SnO2 nanorods. The samples for TEM measurement were prepared by dispersing some products in ethanol and immersing them in an ultrasonic bath for 30 min, and then dropping a few drops of the resulting suspensions onto a copper grid coated with a layer of amorphous carbon. Meanwhile, the as-made products were also characterized by X-ray photoelectron spectroscopy (XPS), which was carried out on a PHI5500 X-ray photoelectron spectrometer, using Mg Ka X-ray as the excitation source. 2.3. Gas sensitivity test of the samples In order to prepare sensors, we elected the indirect heating structure [17]. SnO2 nanorods used as sensitive materials were fabricated on an alumina tube with Au electrodes and platinum wires. A Ni–Cr alloy crossed alumina tube was used as a resistor. This resistor ensured both substrate heating and temperature control. The gas sensing properties were examined in a chamber through which air or a sample gas was allowed to flow at a rate of 160 cm3 min−1 [17]. The sensor’s resistance was measured by using a conventional circuit in which the element was connected with an external resistor in series at a circuit voltage of 10 V. The electrical response of the sensor was measured with an automatic test system, controlled by a personal computer. The gas response ˇ was defined as the ratio of the electrical resistance in air (Ro ) to that in gas (Rg ). All of the test in gas sensing properties was carried out at a relative humidity range of 40–60%.
3. Results and discussion Fig. 1 shows the XRD pattern of as-made product of SnO2 nanorods annealing the precursor powders at 660 ◦ C in KCl + NaCl
Fig. 1. XRD pattern of as-made product of SnO2 nanorods annealing the precursor powders at 660 ◦ C in KCl and NaCl flux medium.
Fig. 2. SEM image of the SnO2 nanorods coated on the surface of alumina tube.
flux medium. All diffraction peaks were quite similar to those of bulk SnO2 , which could be indexed as the tetragonal structure SnO2 and the diffraction data were in good agreement with the reported values from the Joint Committee on Powder Diffraction Standards card (JCPDS 88-0287). No characteristic peaks of impurities, such as KBH4 , NaCl and KCl, were observed. It demonstrates that the as-made product SnO2 nanorods are pure phase of rutile SnO2 . SEM image revealed that there were solid rod-like structures in the as-prepared and calcined samples. Fig. 2 shows SEM image of the SnO2 nanorods on alumina tube. Most rod-like structures have regular morphology, and the lengths are in the range of 1 and several micrometers. The general TEM morphologies are shown in Fig. 3(a) and (b). They showed the typical TEM images of the as-made nanorods after annealing at 660 ◦ C for 2 h. It can be seen that the product mainly consists of solid rod-like structures with diameters 20 nm and lengths up to tens of micrometers. Fig. 3(c) shows a highresolution transmission electron micrograph (HRTEM) of a SnO2 nanorod and structure uniform and clear lattice fringe. It illustrates that the single SnO2 nanorod is high quality single rutile crystals without extended defects inside. Fig. 4 shows XPS spectra taken from Sn and O regions of sample. The peaks at about 494.5 and 486.0 eV are attributed to Sn3d3/2 and Sn3d5/2 (Fig. 4(a)), respectively, which are approximately the same as the standard spectrum of Sn3d. The gap between the Sn3d3/2 and Sn3d5/2 level is 8.5 eV, which is close to the data for Sn3d in SnO2 . Fig. 4(b) shows that the O1s XPS is asymmetric, indicating that at least two oxygen species are presented in the nearby region. The peak at about 529.9 eV is due to oxygen in the SnO2 crystal lattice, which corresponds to Sn–O bonds. Whereas the peak at about 531.0 eV is due to chemisorbed oxygen caused by surface hydroxyl, which corresponds to H–O bonds. Clearly, these results prove that the sample is SnO2 with rutile structure. Peak area of the Sn3d and O1s cores are measured and used to calculate the chemical composition of the sample. The areas are determined by fitting the curves using a nonlinear least squares curve fitting program. The atomic composition of Sn and O is calculated by using peak area sensitivity factors and the atomic ratio of Sn/O is nearly 1:1.42, which is strongly deviated from stoichiometry of SnO2 composition. The results show that the atomic ratio of Sn/O is changed from 1:1 to 1:2. The deviation of composition from stoichiometry caused by oxygen vacancies
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Fig. 4. The XPS analysis of the SnO2 nanorods: (a) Sn region and (b) O region.
Fig. 3. Low-magnification TEM image (a); TEM image of a single SnO2 nanorod (b). Scale bar: 50 nm. Top inset: select area electron diffraction (SAED) pattern of the SnO2 nanorod; HRTEM image of the SnO2 nanorod (c). Scale bar: 5 nm.
strongly affect the gas-sensing property of sensor based on SnO2 nanorods. The effect of operating temperature on gas sensing properties of sensor was investigated, and the results are shown in Fig. 5. It is clear that the operating temperature has an obvious influence on the response of sensor to 1000 ppm C2 H5 OH. The response increases with a rise in operating temperature. The nanorods tin oxides-based sensor exhibited the highest sensitivity to C2 H5 OH at 300 ◦ C. If the temperature increases again, the response decreases. This behavior can be explained in analogy with the mechanism of gas adsorption and desorption on SnO2 [18]. An n-type metaloxide can adsorb oxygen from the atmosphere both in the O2− and in the O− species. It is showed from above results of XPS that much oxygen was adsorbed on surface of SnO2 nanorods. The adsorption of O− is the most interesting process in sensors, because this oxygen ion is the more reactive and thus makes the material more sensitive to the presence of reducing gases. At relatively low temperature the surface preferentially adsorbs O2− and the sensitivity of the material is consequently very small. As the temperature increases, the dominant process becomes the adsorption of O− , and then the response increases too. If the temperature increases too much, progressive adsorption of all oxygen ionic
H. Zhao et al. / Materials Chemistry and Physics 112 (2008) 244–248
Fig. 5. The effects of the operating temperature on the response of sensor at 1000 ppm ethanol.
species previously adsorbed occurs and the response decreases [19]. We examined the gas sensing behavior of sensor based on SnO2 nanorods materials with respect to various reducing gases such as ethanol, toluene, NH3 , petroleum ether, acetone, formaldehyde, benzene, and the testing results are shown in Fig. 6. The tin oxide nanorods-based sensor exhibited the highest response to 1000 ppm ethanol at operating temperature of 300 ◦ C. The response is considerably larger than those recently reported for ethanol gas sensors [20–23]. Experiments have been carried out for the gas sensitivity of various reducing gases under the different gas concentrations, too. It is clear that the responses of the gas sensor based on SnO2 nanorods show good dependence on gas concentrations. It can be seen that the sensor exhibits very low response to toluene, NH3 , petroleum ether, acetone, formaldehyde, benzene, but good response to C2 H5 OH. The response is more than 120 at 1000 ppm concentration for C2 H5 OH. The selectivity is ˇ1000ppm(methanol) /ˇ1000ppm(acetone) = 17.3, and ˇ1000ppm(methanol) /ˇ1000ppm(formaldehyde) = 21.1, ˇ1000ppm(methanol) /ˇ1000ppm(other gases) > 35, respectively. This gas sensor showed good selectivity to other reducing gases, such as toluene, NH3 , petroleum ether, benzene. The response and recovery to C2 H5 OH at operating temperature of 300 ◦ C were investigated, and the curve is shown in Fig. 7.
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Fig. 7. Response and recovery times of the sensor towards ethanol with 1000 ppm operated at an operating temperature of 300 ◦ C.
The response and recovery times (time for 90% of total response change) were about 10–15 s and 25–50 s for the 1000 ppm C2 H5 OH, respectively. These times are short enough for practical use. When the sensor are exposed to air, adsorb oxygen on the nanorods surface would trap electrons from the conduction band of SnO2 due to the strong electron negativity of the oxygen atom, and . Therefore, the concentration of produce adsorbed oxygen O2− (ads) electrons in the conduction band would decrease and the resistance of the material would increase. Introducing a reductive gas such as ethanol, a chemical reaction would occur between C2 H5 OH and O2− on the surface of SnO2 nanorods. It was found that C2 H5 OH (ads) was decomposed to ether, CO, H2 O et al in the range of 40–420 ◦ C. The possibility of reaction can be explained as: C2 H5 OH(gas) + O2− → C2 H5 O− + OH− (ads) (gas) (ads) C2 H5 O− → (C2 H5 )2 O(ads) + O− + e− (ads) (ads) C2 H5 OH(gas) + O2− + hole → CO2 + H2 O + V0 •• (ads) where V0 •• is a doubly charged oxygen vacancy. These reactions infuse electrons into the SnO2 nanorods material. Electrons produced from the reaction would decrease the resistance of the material, result in the output voltage increased when the gas sensor was exposed in the test gas. The analysis result is according with the curve of the response and recovery. 4. Conclusion In summary, the SnO2 nanorods with diameters of 20 nm and lengths of several micrometers have been successfully prepared with annealing precursor powders produced by solid-state reaction at room temperature. The results of XRD, TEM and XPS confirmed the morphologies and structure of the nanorods. The sensor fabricated from the nanorods exhibited excellent ethanol sensing properties with good response, high selectivity and short response and reversion. The experimental results indicate the potential applications of using SnO2 nanorods for fabricating gas sensing. Acknowledgements
Fig. 6. Variation in sensor response to different tested gases (ethanol, HCHO, acetone, toluene, ammonia, and petroleum ether) at an operating temperature of 300 ◦ C.
This work was supported by the Project of National Natural Science Foundation of China (Grant No. 50662006), and the Natural Science Foundation of Education Department of Yunnan
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Province (Grant No. 07Z11021), and the Natural Science Foundation of Yunnan Province (Grant No. 2007E173M), and the Major Teacher Training Foundation of Yunnan University. References [1] Y. Huang, X.F. Duan, Y. Cui, L.J. Lauhon, K.H. Kim, C.M. Lieber, Science 294 (2001) 1313. [2] M.H. Hunag, S. Mao, H. Feik, H.Q. Yan, Y.Y. Wu, H. Kind, E. Weber, R. Russo, P.D. Yang, Science 292 (2001) 1897. [3] C.P. Collier, T. Vossmeyer, J.R. Heath, Annu. Rev. Phys. Chem. 49 (1998) 371. [4] Z.Q. Liu, D.H. Zhang, H. Song, L. Chao, Adv. Mater. 15 (2003) 1754. [5] C.K. Xu, G.D. Xu, Y.K. Liu, X.L. Zhao, G.H. Wang, Scr. Mater. 46 (2002) 789. [6] Z.R. Dai, J.L. Gole, J.D. Stout, Z.L. Wang, J. Phys. Chem. B 106 (2002) 1274. [7] S.H. Sun, G.W. Meng, Y.W. Wang, T. Gao, M.G. Zhang, Y.T. Tian, X.S. Peng, L.D. Zhang, Appl. Phys. A 76 (2003) 287. [8] J.S. Lee, S.K. Sima, B. Min, K. Cho, S.W. Kim, S. Kim, J. Crystal Growth 267 (2004) 145. [9] J.Q. Hu, X.L. Ma, N.G. Shang, Z.Y. Xie, N.B. Wong, C.S. Lee, S.T. Lee, J. Phys. Chem. B 106 (2002) 3823.
[10] Y.K. Liu, C.L. Zheng, W.Z. Wang, Y.J. Zhan, G.H. Wang, J. Crystal Growth 233 (2001) 8. [11] D. Calestani, M. Zha, A. Zappettini, L. Lazzarini, G. Salviati, L. Zanotti, G. Sberveglieri, Mater. Sci. Eng. C 25 (2005) 625. [12] R.S. Yang, Z.L. Wang, J. Am. Chem. Soc. 128 (2006) 1466. [13] E. Comini, G. Faglia, G. Sberveglieri, Z.W. Pan, Z.L. Wang, Appl. Phys. Lett. 81 (2002) 1869. [14] M. Law, H. Kind, B. Messer, F. Kim, P.D. Yang, Angew. Chem. 41 (2002) 2405. [15] A. Kolmakov, Y.X. Zhang, G.S. Cheng, M. Moskovits, Adv. Mater. 15 (2003) 997. [16] S.C. Chen, Important Reaction of Inorganic Chemistry, Shanghai Science Press, 1986. [17] X.H. Wu, Y.D. Wang, Y.F. Li, Z.L. Zhou, Mater. Chem. Phys. 77 (2002) 588. [18] N. Yamazoe, J. Fuchigama, M. Kishikawa, T. Seiyama, Surf. Sci. 86 (1979) 335. [19] D. Manno, G. Micocci, R. Rella, A. Serra, A. Tayrino, A. Tepore, J. Appl. Phys. 82 (1997) 54. [20] N. Yamazoe, Sens. Actuators B 5 (1991) 7. [21] T. Maekawa, J. Tamaki, N. Miura, N. Yamazoe, S. Matsushima, Sens. Actuators B 9 (1992) 63. [22] Y.J. Chen, L. Nie, X.Y. Xue, Y.G. Wang, T.H. Wang, Appl. Phys. Lett. 88 (2006) 083105. [23] E.T. Hayes, B.K. Bellingham, H.B. Mark, A. Galal, Electrochim. Acta 41 (1996) 337.
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Synthesis, characterization and gas-sensing property for C2 H5 OH of SnO2 nanorods Heyun Zhao a,∗ , Yuehuan Li b , Liufang Yang a , Xinghui Wu a a b
Department of Materials Science and Engineering, Yunnan University, 650091 Kunming, China Life Science and Chemistry School of DaLi College, 671200 DaLi, China
a r t i c l e
i n f o
Article history: Received 25 February 2008 Received in revised form 19 May 2008 Accepted 22 May 2008 Keywords: Nanostructures X-ray diffraction Electron microscopy Catalytic properties
a b s t r a c t SnO2 nanorods with the rutile structure were prepared with annealing precursor powders produced by solid-state reaction coated surfactant at room temperature. The phase structure and morphologies of SnO2 nanorods were characterized by means of X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectrum (XPS). The results show that diameter and length of the as-made nanorods are less then 20 nm and several micrometers, respectively. The indirect-heating sensors, using the SnO2 nanorods as sensitive materials fabricated on an alumna tube with Au electrodes and platinum wires, were fabricated and investigated for the gas sensing properties. The investigation demonstrates that the sensor based on SnO2 nanorods materials has good sensitivity, high selectivity and short response and reversion time to C2 H5 OH at 300 ◦ C. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In recent years, one-dimensional (1D) nanoscale structures of metal-oxide semiconductors (MOS) such as wires, rods, belts and tubes have became the focus of intensive research owing to their unique applications in mesoscopic physics and fabrication of nanoscale devices [1–3]. Among them, SnO2 nanomaterials with 1D structures, such as nanowires or nanorods, are especially attractive due to possess novel properties in applications as gas sensors, dye-based solar cells, transistors, electrode materials, catalysts, solar cells, and electro-chromic devices, etc. Recently, some studies have been reported on the synthesis of 1D SnO2 nanomaterials (nanowires, nanorods, and nanobelts, etc.) using laser ablation [4], thermal decomposition [5], thermal evaporation [6–8], rapid thermal oxidation [9], and redox reaction [10], low-temperature chemical vapor deposition [11], solid–vapor process [12], etc. Generally, the preparation methods mentioned above involve complex procedures, sophisticated equipment, and rigorous experimental conditions. Thus, the development of mild and lost-cost synthetic routes to SnO2 nanorods was of great significance. Tin oxide is one of the earliest discovered and the most widely applied oxide gas sensing materials due to its high mobility of conduction electrons, good chemical and thermal stability under the operating conditions of sensors. Up to now, most of studies are focused on particle sintered or thin-film-based devices. More
recently, several groups have reported that the single nanowires or nanoribbons of semiconductor oxides are very promising for sensors [13–15]. The results showed that the devices based on one-dimensional nanostructures have great potential to overcome the fundamental limitations of traditional semiconductor oxides sensors based on particle sintered or thick-film such as low sensitivity, poor stability and high working temperature. The different methods and conditions in synthesizing process would produce 1D nanostructure materials with different surface state and morphology, so it is very important to study the gas-sensing properties of the 1D nanostructure materials prepared by different methods under different conditions. In this work, we report a simple and novel approach to synthesis SnO2 nanorods via annealing precursors produced by solid-state reaction between SnCl4 ·5H2 O and KBH4 in the presence of KCl + NaCl flux coated with surfactant of nonyl phenyl ether (9)/(5) (NP9/5). Gas sensors based on the as-prepared sample were fabricated and the gas-sensing properties were researched. 2. Experimental 2.1. Synthesis of SnO2 nanorods In this paper, SnO2 nanorods with the rutile structure were synthesized by annealing precursor powders produced by solid-state reaction coated with surfactant of nonyl phenyl ether (9)/(5) (NP9/5) at room temperature. The preparation of SnO2 nanorods was based on the following reaction [16]: SnCl4 ·5H2 O + 4KBH4 → 2B2 H6 + 4KCl + 5H2 O + Sn + 2H2 ↑
∗ Corresponding author. Tel.: +86 871 5031410; fax: +86 871 5153832. E-mail address: [email protected] (H. Zhao). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.05.039
Sn + O2 → SnO2
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SnCl4 ·5H2 O, KBH4 , NP9/5, NaCl and KCl were used as precursor materials. SnCl4 ·5H2 O, KBH4 , NaCl and KCl are of analytical purity. NP9/5 is up to 99.0% in purity. A total of 3.51 g of SnCl4 ·5H2 O, 2.00 g of NaCl and 2.55 g of KCl (according to 1:1 molar rate) were mixed with 5 ml of NP9/5 in a mortar, ground for 30 min (hereafter expressed as ‘A’); 1.54 g of KBH4 , 2.00 g of NaCl and 2.55 g of KCl were mixed with 5 ml of NP9/5 in a mortar, ground for 30 min (hereafter expressed as ‘B’). Thereafter, ‘B’ was added to ‘A’ to get mixture in a mortar, ground for 60 min. The products were washed several times by the use of acetone to remove NP9/5 and by-products, then dried in an oven at 70–80 ◦ C for 2 h. By doing so, the precursor powders were obtained. The precursor powders were annealed at 660 ◦ C, which was heating rate of 5 ◦ C min−1 , and then kept the temperature constant for 2 h in a porcelain crucible that was placed in the middle of the alumina tube. The reaction chamber is of alumina tube with a length of 1500 mm and a diameter of 60 mm. After heating treatment, it was gradually cooled to room temperature; the as-made products were washed with distilled water several times to remove NaCl, and KCl, then get pure SnO2 nanorods. 2.2. Characterization of as-synthesized samples The nanorods products were examined by X-ray diffraction (XRD) employed scanning steps of 0.02◦ /step in a 2 range from 10◦ to 100◦ , using a D/MAX-RA diffrac˚ and a graphite monochromatic crystal at tometer with Cu K␣ radiation ( = 1.5418 A) 40 kV and 50 mA. Scanning electron microscopy (SEM) photographs were obtained by XL30ESEM-TMP, Holland. Transmission electron microscopy (TEM) measurement was taken using a JEM-200CX to characterize SnO2 nanorods. The samples for TEM measurement were prepared by dispersing some products in ethanol and immersing them in an ultrasonic bath for 30 min, and then dropping a few drops of the resulting suspensions onto a copper grid coated with a layer of amorphous carbon. Meanwhile, the as-made products were also characterized by X-ray photoelectron spectroscopy (XPS), which was carried out on a PHI5500 X-ray photoelectron spectrometer, using Mg Ka X-ray as the excitation source. 2.3. Gas sensitivity test of the samples In order to prepare sensors, we elected the indirect heating structure [17]. SnO2 nanorods used as sensitive materials were fabricated on an alumina tube with Au electrodes and platinum wires. A Ni–Cr alloy crossed alumina tube was used as a resistor. This resistor ensured both substrate heating and temperature control. The gas sensing properties were examined in a chamber through which air or a sample gas was allowed to flow at a rate of 160 cm3 min−1 [17]. The sensor’s resistance was measured by using a conventional circuit in which the element was connected with an external resistor in series at a circuit voltage of 10 V. The electrical response of the sensor was measured with an automatic test system, controlled by a personal computer. The gas response ˇ was defined as the ratio of the electrical resistance in air (Ro ) to that in gas (Rg ). All of the test in gas sensing properties was carried out at a relative humidity range of 40–60%.
3. Results and discussion Fig. 1 shows the XRD pattern of as-made product of SnO2 nanorods annealing the precursor powders at 660 ◦ C in KCl + NaCl
Fig. 1. XRD pattern of as-made product of SnO2 nanorods annealing the precursor powders at 660 ◦ C in KCl and NaCl flux medium.
Fig. 2. SEM image of the SnO2 nanorods coated on the surface of alumina tube.
flux medium. All diffraction peaks were quite similar to those of bulk SnO2 , which could be indexed as the tetragonal structure SnO2 and the diffraction data were in good agreement with the reported values from the Joint Committee on Powder Diffraction Standards card (JCPDS 88-0287). No characteristic peaks of impurities, such as KBH4 , NaCl and KCl, were observed. It demonstrates that the as-made product SnO2 nanorods are pure phase of rutile SnO2 . SEM image revealed that there were solid rod-like structures in the as-prepared and calcined samples. Fig. 2 shows SEM image of the SnO2 nanorods on alumina tube. Most rod-like structures have regular morphology, and the lengths are in the range of 1 and several micrometers. The general TEM morphologies are shown in Fig. 3(a) and (b). They showed the typical TEM images of the as-made nanorods after annealing at 660 ◦ C for 2 h. It can be seen that the product mainly consists of solid rod-like structures with diameters 20 nm and lengths up to tens of micrometers. Fig. 3(c) shows a highresolution transmission electron micrograph (HRTEM) of a SnO2 nanorod and structure uniform and clear lattice fringe. It illustrates that the single SnO2 nanorod is high quality single rutile crystals without extended defects inside. Fig. 4 shows XPS spectra taken from Sn and O regions of sample. The peaks at about 494.5 and 486.0 eV are attributed to Sn3d3/2 and Sn3d5/2 (Fig. 4(a)), respectively, which are approximately the same as the standard spectrum of Sn3d. The gap between the Sn3d3/2 and Sn3d5/2 level is 8.5 eV, which is close to the data for Sn3d in SnO2 . Fig. 4(b) shows that the O1s XPS is asymmetric, indicating that at least two oxygen species are presented in the nearby region. The peak at about 529.9 eV is due to oxygen in the SnO2 crystal lattice, which corresponds to Sn–O bonds. Whereas the peak at about 531.0 eV is due to chemisorbed oxygen caused by surface hydroxyl, which corresponds to H–O bonds. Clearly, these results prove that the sample is SnO2 with rutile structure. Peak area of the Sn3d and O1s cores are measured and used to calculate the chemical composition of the sample. The areas are determined by fitting the curves using a nonlinear least squares curve fitting program. The atomic composition of Sn and O is calculated by using peak area sensitivity factors and the atomic ratio of Sn/O is nearly 1:1.42, which is strongly deviated from stoichiometry of SnO2 composition. The results show that the atomic ratio of Sn/O is changed from 1:1 to 1:2. The deviation of composition from stoichiometry caused by oxygen vacancies
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Fig. 4. The XPS analysis of the SnO2 nanorods: (a) Sn region and (b) O region.
Fig. 3. Low-magnification TEM image (a); TEM image of a single SnO2 nanorod (b). Scale bar: 50 nm. Top inset: select area electron diffraction (SAED) pattern of the SnO2 nanorod; HRTEM image of the SnO2 nanorod (c). Scale bar: 5 nm.
strongly affect the gas-sensing property of sensor based on SnO2 nanorods. The effect of operating temperature on gas sensing properties of sensor was investigated, and the results are shown in Fig. 5. It is clear that the operating temperature has an obvious influence on the response of sensor to 1000 ppm C2 H5 OH. The response increases with a rise in operating temperature. The nanorods tin oxides-based sensor exhibited the highest sensitivity to C2 H5 OH at 300 ◦ C. If the temperature increases again, the response decreases. This behavior can be explained in analogy with the mechanism of gas adsorption and desorption on SnO2 [18]. An n-type metaloxide can adsorb oxygen from the atmosphere both in the O2− and in the O− species. It is showed from above results of XPS that much oxygen was adsorbed on surface of SnO2 nanorods. The adsorption of O− is the most interesting process in sensors, because this oxygen ion is the more reactive and thus makes the material more sensitive to the presence of reducing gases. At relatively low temperature the surface preferentially adsorbs O2− and the sensitivity of the material is consequently very small. As the temperature increases, the dominant process becomes the adsorption of O− , and then the response increases too. If the temperature increases too much, progressive adsorption of all oxygen ionic
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Fig. 5. The effects of the operating temperature on the response of sensor at 1000 ppm ethanol.
species previously adsorbed occurs and the response decreases [19]. We examined the gas sensing behavior of sensor based on SnO2 nanorods materials with respect to various reducing gases such as ethanol, toluene, NH3 , petroleum ether, acetone, formaldehyde, benzene, and the testing results are shown in Fig. 6. The tin oxide nanorods-based sensor exhibited the highest response to 1000 ppm ethanol at operating temperature of 300 ◦ C. The response is considerably larger than those recently reported for ethanol gas sensors [20–23]. Experiments have been carried out for the gas sensitivity of various reducing gases under the different gas concentrations, too. It is clear that the responses of the gas sensor based on SnO2 nanorods show good dependence on gas concentrations. It can be seen that the sensor exhibits very low response to toluene, NH3 , petroleum ether, acetone, formaldehyde, benzene, but good response to C2 H5 OH. The response is more than 120 at 1000 ppm concentration for C2 H5 OH. The selectivity is ˇ1000ppm(methanol) /ˇ1000ppm(acetone) = 17.3, and ˇ1000ppm(methanol) /ˇ1000ppm(formaldehyde) = 21.1, ˇ1000ppm(methanol) /ˇ1000ppm(other gases) > 35, respectively. This gas sensor showed good selectivity to other reducing gases, such as toluene, NH3 , petroleum ether, benzene. The response and recovery to C2 H5 OH at operating temperature of 300 ◦ C were investigated, and the curve is shown in Fig. 7.
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Fig. 7. Response and recovery times of the sensor towards ethanol with 1000 ppm operated at an operating temperature of 300 ◦ C.
The response and recovery times (time for 90% of total response change) were about 10–15 s and 25–50 s for the 1000 ppm C2 H5 OH, respectively. These times are short enough for practical use. When the sensor are exposed to air, adsorb oxygen on the nanorods surface would trap electrons from the conduction band of SnO2 due to the strong electron negativity of the oxygen atom, and . Therefore, the concentration of produce adsorbed oxygen O2− (ads) electrons in the conduction band would decrease and the resistance of the material would increase. Introducing a reductive gas such as ethanol, a chemical reaction would occur between C2 H5 OH and O2− on the surface of SnO2 nanorods. It was found that C2 H5 OH (ads) was decomposed to ether, CO, H2 O et al in the range of 40–420 ◦ C. The possibility of reaction can be explained as: C2 H5 OH(gas) + O2− → C2 H5 O− + OH− (ads) (gas) (ads) C2 H5 O− → (C2 H5 )2 O(ads) + O− + e− (ads) (ads) C2 H5 OH(gas) + O2− + hole → CO2 + H2 O + V0 •• (ads) where V0 •• is a doubly charged oxygen vacancy. These reactions infuse electrons into the SnO2 nanorods material. Electrons produced from the reaction would decrease the resistance of the material, result in the output voltage increased when the gas sensor was exposed in the test gas. The analysis result is according with the curve of the response and recovery. 4. Conclusion In summary, the SnO2 nanorods with diameters of 20 nm and lengths of several micrometers have been successfully prepared with annealing precursor powders produced by solid-state reaction at room temperature. The results of XRD, TEM and XPS confirmed the morphologies and structure of the nanorods. The sensor fabricated from the nanorods exhibited excellent ethanol sensing properties with good response, high selectivity and short response and reversion. The experimental results indicate the potential applications of using SnO2 nanorods for fabricating gas sensing. Acknowledgements
Fig. 6. Variation in sensor response to different tested gases (ethanol, HCHO, acetone, toluene, ammonia, and petroleum ether) at an operating temperature of 300 ◦ C.
This work was supported by the Project of National Natural Science Foundation of China (Grant No. 50662006), and the Natural Science Foundation of Education Department of Yunnan
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