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Gas sensing properties of tin dioxide coated onto multi-walled carbon nanotubes
Thin Solid Films 497 (2006) 355 – 360 www.elsevier.com/locate/tsf
Gas sensing properties of tin dioxide coated onto multi-walled carbon nanotubes Yan-Li Liu 1, Hai-Feng Yang, Yu Yang, Zhi-Min Liu, Guo-Li Shen, Ru-Qin Yu * State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, P.R. China Received 23 November 2004; received in revised form 22 July 2005; accepted 1 November 2005
Abstract A compound material of multi-walled carbon nanotubes (MWNTs) coated with SnO2 was synthesized at ambient conditions. The Raman spectra, X-ray diffraction (XRD) and transmission electron microscopy (TEM) results confirmed that the existence of SnO2 nanoparticles were on the outside of the MWNTs. The gas sensing properties of the material were studied. It was found that the sensor exhibited nice sensing responses to liquefied petroleum gas (LPG) and ethanol gas (C2H5OH) and a fast response and recovery within seconds. Furthermore, the gas sensor responses increased linearly with the increment of the gas concentrations of LPG and ethanol. D 2005 Published by Elsevier B.V. Keywords: MWNTs; SnO2 nanoparticles; Gas sensing response
1. Introduction Following discovery of carbon nanotubes (CNTs) [1], they have been studied extensively in versatile fields because of their novel structural characteristics: nanometer hollow tubes, high surface area with a narrow pore size distribution and high stability. They are considered as promising candidates for many materials such as composite and field emission materials, catalysts, hydrogen storage materials and electrochemical supercapacitors [2– 5]. Besides, CNTs can be served as a nanometer-sized capillary, mould or template in material fabrication because of their hollow interior and high specific surface area. A number of modifications by functionalizing, doping or coating, either in part or completely, have been suggested to further improve the properties of CNTs [6 –14]. On one hand, the introduction of foreign elements can be performed via several methods in which capillarity forces induce the filling with a molten material. Ajayan et al. [9] used the surface-tension effect to fill CNTs with vanadium oxide at relatively high temperature, i.e. 750 -C. Xu et al. [10] have * Corresponding author. Fax: +86 731 882 2782. E-mail address: [email protected] (R.-Q. Yu). 1 Present address: College of Materials Science and Engineering, Hunan University, Changsha 410082, P.R. China. 0040-6090/$ - see front matter D 2005 Published by Elsevier B.V. doi:10.1016/j.tsf.2005.11.018
reported the filling of single-walled carbon nanotubes with lanthanide halides by heating an intimate mixture of carbon nanotubes and halides above the melting point of the later. However, the high yield filling of CNTs by foreign elements under mild conditions still remains a difficult goal that numerous groups are trying to attain. On the other hand, the high surface area of CNTs with a narrow pore size distribution is very important to applications used as substrates for external coating. And it should be stressed that the external coating selectivity is relatively high compared to the filling. Kim and Sigmund [12] have reported the morphology of zinc oxide nanostructures grown on multi-walled carbon nanotubes (MWNTs). Ultrathin films, quantum dots and nanowires/ nanorods of ZnO can be grown on the outer shells of MWNTs by heat treatment without any catalysts. SnO2 quantum dots were also coupled to individual single-walled carbon nanotubes [13]. SnO2 is an n-type semiconductor oxide with wide band gap energy and is always applied as gas sensing material to detect combustible, toxic and pollutant gases [15]. The advantages of the sensor fabricated by SnO2 include high sensitivity, simple design and low cost. There have been many attempts to improve the sensitivity and selectivity of tin-oxide gas sensors for the realization of intelligent gas sensors [16 – 20]. Since the sensing mechanism of these devices relies on the chemisorption reactions that take place at the surface of the
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2. Experimental details 2.1. Multi-walled carbon nanotubes
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MWNTs were produced by the catalytic pyrolysis of methane on Ni catalyst at 450 -C [25]. The MWNT materials were submitted to various chemical –physical procedures to remove graphitic particles, amorphous carbon and catalyst impurities prior to utilization [26 –28]. Following one of the purification methodologies, 100 mg of MWNTs were oxidized at 350 -C for 2 h. To eliminate metal oxide catalysts, the oxidized amount of MWNTs was dispersed in 60 ml of 7.0 mol/L HNO3 and refluxed at 140 -C for 12 h under stirring, then rinsed with distilled H2O until the pH of the solution was neutral, and finally dried.
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Fig. 1. Raman spectra for the (A) MWNTs, (B) MWNTs coated with SnO2 before heat treatment and (C) the sample after heat treatment.
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metal oxide, it is believed that sensor sensitivity can be improved by increasing the sensitive material surface areas in order to provide more surface sites available for more oxygen species to be adsorbed on these sites and to make contact with the surrounding gases [21,22]. So it can be concluded that the sensing response can be greatly scaled up with the use of nanosized semiconducting materials [23,24]. In some preliminary experiments, it has been observed that SnO2 nanoparticles could be coated onto MWNTs fully to form a thin and uniform layer. And the materials possess excellent gas sensing responses to reducing gases. The obtained materials have been structurally characterized by Raman spectra, powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). This paper reports the experimental results of aforementioned investigations.
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2-Theta (°) Fig. 2. XRD patterns taken from (A) the MWNTs, (B) MWNTs coated with SnO2 before heat treatment and (C) the sample after heat treatment.
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Fig. 3. TEM micrographs of the prepared samples: (A) TEM image showing the general morphology of the MWNTs (scale bar 100 nm); (B) TEM image of SnO2 coated onto MWNTs forming a thin and fine layer after heat treatment (scale bar 100 nm).
2.2. Coating SnO2 onto carbon nanotube In a typical synthesis, one gram of tin(II) chloride (SnCl2) was dissolved in 40 mL of distilled H2O and then 1.0 mL of HCl (38%) was added. Subsequently, 10 mg of the as-treated MWNTs were dispersed in the above solution. This mixture was sonicated for 5– 10 min and then stirred for 60 min at room temperature. The precipitate was then separated from the mother liquor by centrifugation and was washed with distilled H2O for several times, and then dried at 70 -C under vacuum for 6 h. Part of the final product was calcined at 350 -C for 2 h.
50%. The responses of the sensor were studied in a sealed test chamber (300 cm3) with an inlet and an outlet. The test gas was injected into the chamber through the inlet port and the resistance was measured as a function of time till a constant value was attained. Then the chamber was purged with air and the experiments were repeated. The sensor response (S) for a given test gas is calculated as follows: S = R a / R g, where R g and R a are the electrical resistances of the sensor in test gas and in air, respectively. 3. Results and discussion
2.3. Characterization techniques
3.1. Structural characterization
Raman spectra were recorded using Raman spectrometer (Jobin Yvon LabRam-010 micro-Raman system, France) and 632 nm as the excitation source. XRD experiments of all the products were conducted on a RigaKu D/max 2550 X-ray ˚ ), Diffractometer, Japan, with Cu Ka radiation (k = 1.5418 A with which the data were collected in steps of 0.02- (2h) min 1 from 10- to 90- (2h). The database of the Joint Committee on Powder Diffraction Standards (JCPDS) was utilized for phase identification. The morphology and structure of the synthesized products were observed through a Hitachi-800 transmission electron microscopy (TEM, Japan) operated at 200 kV.
The Raman spectra of purified MWNTs and MWNTs coated with SnO2 are shown in Fig. 1. In the Raman spectra of MWNTs (Fig. 1A), the tangential mode at 1328 cm 1 is the dominant feature. Also, peaks at 1570 cm 1 and 1602 cm 1 are clearly seen in the Raman spectra of MWNTs, which may be assigned to the disorder-induced effect due to the finite size effect or lattice distortion [29]. While for the Raman spectra of
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For the fabrication of sensors, the compound materials were dispersed in terpineol which was used as a binder to form pastes. A platinum interdigitated electrode structure was made using a metal deposition technique with shadow masking on an alumina substrate. The obtained alumina substrate was dipped into the paste for several times to form gas sensing film. Then the element was annealed at 350 -C for 1 h at ambient atmosphere to evaporate the terpineol. Finally, the alumina substrate and a Ni – Cr heater were welded onto a bakelite substrate. The electrical resistances measurements of the sensor were carried out in an air ambient at a relative humidity (RH) around
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2.4. Measurement of the sensing characteristics
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MWNTs coated with SnO2 (Fig. 1B and C), peaks at around 425, 492, 626 and 762 cm 1 are attributed to the A2g, Eg, A1g and B2g vibration modes of SnO2, respectively [30]. Fig. 1B shows the spectra of the compound material before heat treatment at 350 -C and Fig. 1C shows the spectra after heat treatment. It is evident that the spectra of Fig. 1B and C have the same profile, which implies that SnO2 exists in those two samples. The structures of the samples were further confirmed by XRD presented in Fig. 2. Fig. 2A shows XRD pattern taken from the uncoated MWNTs. Both peaks are come from the 2HC (002) (JCPDS card no. 41-1487) [31]. Fig. 2B shows the XRD pattern of the MWNTs coated with SnO2 before heat treatment. The broad diffraction lines corresponding to the carbon nanotubes and SnO2 are observed. No diffraction peaks of other impurities are found in the sample. The maximal diffraction peaks of the specimen could be indexed in peak positions to SnO2 (JCPDS card no. 41-1445) [32]. The main
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peak of tetragonal SnO2 (110) almost overlaps with the main peaks of 2H-C (002). Comparing both spectra of Fig. 2A and B, it is easy to find that the broad peak is mainly contributed to the small size or the amorphous phase of SnO2. Furthermore, a sudden change in the peak intensities appears after the sample was heated (Fig. 2C). The increase in intensity amplitude and decrease in width of the peaks reflect the increased crystalline and enlarged crystallite dimensions caused by the heat treatment. A representative TEM image of the MWNTs is displayed in Fig. 3A, which shows that MWNTs were almost the only species in the purified products with a mean outer diameter of 30 nm, a mean inner diameter of 10 nm and lengths up to several micrometers. Fig. 3B presents a single MWNT coated with SnO2 after heat treatment. It is obvious that the nanosized SnO2 were coated onto the outside of the CNTs. The thickness of SnO2 layer on the MWNTs was found to be around 12.5 nm on average.
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Time (s) Fig. 6. Measurement of transient responses of the sensor upon exposure to ethanol gas with the concentration from 10 ppm to 200 ppm at the operating temperature of 335 -C in 50% RH.
Table 1 The sensor resistance (kV) in air and in LPG gas versus gas concentrations Air
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3.2. Gas sensing characteristics
Table 2 The sensor resistance (kV) in air and in ethanol gas versus gas concentrations
Fig. 4 shows the dynamic response of the sensors of SnO2 coated on the MWNTs to C4H10, CH4 and CO, respectively, with the gas turning-on and turning-off. The gas is injected into the test chamber, at a working temperature of 335 -C and 50% relative humidity (RH) at 20 -C. Obviously, the electronic currents increase due to the introduction of gases, which is consistent with the n-type semiconducting properties of SnO2. The sensor shows little sensing to these three gases and the sensing responses were about 1.4, 1.5 and 1.8 for CO, C4H10 and CH4, respectively. Fig. 5 is the successive dynamic responses of the sensor to liquefied petroleum gas (LPG) with the concentration ranges from 100 to 1000 ppm at the same measurement conditions. It is evident that the sensing responses increase with the increment of the gas concentrations and the sensor exhibits a fast response and a good recovery. The similar changes are observed in the successive dynamic responses of the sensor to ethanol gas with the concentration ranges from 10 to 200 ppm at the same measurement condition (Fig. 6). The dependences of the gas sensing responses of the sample towards LPG gas concentrations and ethanol gas concentrations are presented in Fig. 7. The magnitudes of sensor response increased linearly with the LPG gas concentration up to 1000 ppm. There is also a similar trend for the relationship of the ethanol gas concentrations and the sensing responses. Tables 1 and 2 show the sensor resistances in air and in LPG gas and ethanol gas with different concentrations. The sensor resistances are 75.4 kV in air, 10.9 kV in 100 ppm LPG and 18.5 kV in 10 ppm ethanol gas, respectively. The resistances of the sensor designed in this experiment are much lower than the SnO2 nanobelt sensors [33]. The high sensitivity and low resistance maybe be contributed to the particular electrical transport mechanism, which is different from that of the SnO2 nanobelt sensors [34]. The gas sensing mechanism is based on the changes in the conductance of the semiconductor sensor. When the sensor is in air atmosphere, the oxygen-related species, such as O, O2 and O2, are present on SnO2 by the following reactions [35]:
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O2 ðgasÞ6O2 ðadsÞ þ e 6O 2 ðadsÞ þ e 62O ðadsÞ þ e 6O2 ðadsÞ:
This procedure would decrease the conductance of SnO2. When the sensor is in reducing gases at appropriate temperature, the reducing gas molecules react with oxygen ions, which loose electrons to the conduction band of SnO2 to improve the conductance of SnO2. When the sensor is set in air again, the oxygen molecules are adsorbed on the SnO2 again. And this process determines the gas sensing characteristic of SnO2. Furthermore, the selectivity of SnO2 gas sensor is due to the different reaction speed of different reducing gases on the semiconductor surface. The sensor resistance is dominated by the barriers between the SnO2 grains on the MWNTs. The work function of the MWNTs is similar to that of SnO2 [36,37]. So the Schottky
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barrier between them is very low. Electrons travel through the SnO2 grains into the MWNTs, and then conduct in the MWNTs with low resistance. This is the main reason the resistance of the gas sensor designed in the experiment is much lower than that of the SnO2 nanobelt. 4. Conclusion The compound material with SnO2 coated onto MWNTs was synthesized at ambient conditions. The Raman spectra and XRD confirmed the existence of SnO2 nanoparticles; TEM showed the formation of fine and thin layers of SnO2 on the MWNTs. The sensing response to reducing gases of gas sensors based on the compound material was measured and electrical characterization showed the material was sensitive to gaseous polluting species like LPG for potential environmental applications, as well as to ethanol for breath analyzers and food control applications. The gas sensor responses increase linearly with the increment of the gas concentrations of LPG and ethanol. The sensor has fast response and recovery within seconds. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 20375012, 20075006 and 29975006), the Foundation for PhD Thesis Research (No. 20010532008) and the Foundation of Scientific Committee of Hunan Province. References [1] S. Iijima, Nature 354 (1991) 56. [2] S. Frank, P. Poncharal, Z.L. Wang, W.A. de Heer, Science 280 (1998) 1744. [3] P. Sudan, A. Zuttel, Ph. Mauron, Ch. Emmenegger, P. Wenger, L. Schlapbach, Carbon 41 (2003) 2377. [4] P.G. Collins, A. Zettl, H. Bando, A. Thess, R.E. Smalley, Science 278 (1997) 100. [5] G.L. Che, B.B. Lakschmi, E.R. Fisher, C.R. Martin, Nature 393 (1998) 346. [6] P.M. Ajayan, S. Iijima, Nature 361 (1993) 333. [7] C.H. Kiang, J.S. Choi, T.T. Tran, A.D. Bacher, J. Phys. Chem., B 103 (1999) 7449. [8] S.C. Tsang, Y.K. Chen, P.J.F. Harris, M.L.H Green, Nature 372 (1994) 159. [9] P.M. Ajayan, O. Stephan, P. Redlich, C. Colliex, Nature 375 (1995) 564. [10] C. Xu, J. Sloan, G. Brown, S. Bailey, V.C. Williams, S. Friedrichs, K.S. Coleman, E. Flahaut, J.L. Hutchison, R.E. Dunin-Borkowski, M.L.H. Green, Chem. Commun. 24 (2000) 2427. [11] Y. Zhang, N.W. Franklin, R.J. Chen, H.J. Dai, Chem. Phys. Lett. 331 (2000) 35. [12] H. Kim, W. Sigmund, Appl. Phys. Lett. 81 (2002) 2085.
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Y.-L. Liu et al. / Thin Solid Films 497 (2006) 355 – 360
[13] W.Q. Han, A. Zettl, Nano Lett. 3 (2003) 681. [14] M.H. Joanne, A.H. Megan, D.K. Todd, Nano Lett. 2 (2002) 1253. [15] J.F. McAleer, P.T. Moseley, J.O.W. Norris, D.E. Williams, P. Taylor, B.C. Tofield, Mater. Chem. Phys. 17 (1987) 577. [16] N. Butta´, M. Melli, S. Pizzini, Sens. Actuators, B, Chem. 2 (1990) 151. [17] M.C. Horrillo, A. Serventi, D. Rickerby, J. Gutie´rrez, Sens. Actuators, B, Chem. 58 (1999) 474. [18] D.G. Rickerby, M.C. Horrillo, J.P. Santos, P. Serrini, Nanostruct. Mater. 9 (1997) 43. [19] G.G. Mandayo, E. Castan˜o, F.J. Gracia, A. Cirera, A. Cornet, J.R. Morante, Sens. Actuators, B, Chem. 95 (2003) 90. [20] R. Rella, A. Serra, P. Siciliano, L. Vasanelli, G. De, A. Licciulli, A. Quirini, Sens. Actuators, B, Chem. 44 (1997) 462. [21] R.M. Walton, D.J. Dwyer, J.W. Schwank, Appl. Surf. Sci. 125 (1998) 199. [22] M.I. Baraton, L. Merhari, Nanostruct. Mater. 10 (1998) 699. [23] Y.D. Wang, X.H. Wu, Y.F. Li, Z.L. Zhou, Solid-State Electron. 48 (2004) 627. [24] O.K. Tan, W. Cao, Y. Hu, W. Zhu, Ceram. Int. 30 (2004) 1127. [25] P. Chen, H.B. Zhang, G.D. Lin, Carbon 35 (1997) 1495. [26] J.F. Colomer, P. Piedigrosso, A. Fonseca, J.B. Nagy, Synth. Met. 103 (1999) 2482.
[27] S. Bandow, S. Asaka, X. Zhao, Y. Ando, Appl. Phys., A 67 (1998) 23. [28] J.E. Riggs, Z. Guo, D.L. Carroll, Y.P. Sun, J. Am. Chem. Soc. 122 (2000) 5879. [29] W. Li, H. Zhang, C. Wang, Y. Zhang, L. Xu, K. Zhu, S. Xie, Appl. Phys. Lett. 70 (1997) 2684. [30] K.N. Yu, Y.H. Xiong, Y.L. Liu, C.S. Xiong, Phys. Rev., B 55 (1997) 2666. [31] Powder Diffraction File. International Center for Diffraction Data, Newtown Square, PA (formerly Joint Committee on Powder Diffraction Standards, Swarthmore, PA, 1991): No. 41-1487. [32] Powder Diffraction File. International Center for Diffraction Data, Newtown Square, PA (formerly Joint Committee on Powder Diffraction Standards, Swarthmore, PA, 1991): No. 41-1445. [33] E. Comini, G. Faglia, G. Sberveglieri, Z.W. Pan, Z.L. Wang, Appl. Phys. Lett. 81 (2002) 1069. [34] Y.X. Liang, Y.J. Chen, T.H. Wang, Appl. Phys. Lett. 85 (2004) 666. [35] K.D. Schierbaum, U. Weimar, W. Gopel, R. Kowalkowski, Sens. Actuators, B, Chem. 3 (1991) 205. [36] H. Ago, T. Kugler, F. Cacialli, W.R. Salaneck, M.S.P. Shaffer, A.H. Windle, R.H. Friend, J. Phys. Chem., B 103 (1999) 8116. [37] M. Sinner-Hettenbach, M. Go¨thelid, T. Wei, N. Barsan, U. Weimar, H. von Schenck, L. Giovanelli, G. Le Lay, Surf. Sci. 499 (2002) 85.
Gas sensing properties of tin dioxide coated onto multi-walled carbon nanotubes Yan-Li Liu 1, Hai-Feng Yang, Yu Yang, Zhi-Min Liu, Guo-Li Shen, Ru-Qin Yu * State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, P.R. China Received 23 November 2004; received in revised form 22 July 2005; accepted 1 November 2005
Abstract A compound material of multi-walled carbon nanotubes (MWNTs) coated with SnO2 was synthesized at ambient conditions. The Raman spectra, X-ray diffraction (XRD) and transmission electron microscopy (TEM) results confirmed that the existence of SnO2 nanoparticles were on the outside of the MWNTs. The gas sensing properties of the material were studied. It was found that the sensor exhibited nice sensing responses to liquefied petroleum gas (LPG) and ethanol gas (C2H5OH) and a fast response and recovery within seconds. Furthermore, the gas sensor responses increased linearly with the increment of the gas concentrations of LPG and ethanol. D 2005 Published by Elsevier B.V. Keywords: MWNTs; SnO2 nanoparticles; Gas sensing response
1. Introduction Following discovery of carbon nanotubes (CNTs) [1], they have been studied extensively in versatile fields because of their novel structural characteristics: nanometer hollow tubes, high surface area with a narrow pore size distribution and high stability. They are considered as promising candidates for many materials such as composite and field emission materials, catalysts, hydrogen storage materials and electrochemical supercapacitors [2– 5]. Besides, CNTs can be served as a nanometer-sized capillary, mould or template in material fabrication because of their hollow interior and high specific surface area. A number of modifications by functionalizing, doping or coating, either in part or completely, have been suggested to further improve the properties of CNTs [6 –14]. On one hand, the introduction of foreign elements can be performed via several methods in which capillarity forces induce the filling with a molten material. Ajayan et al. [9] used the surface-tension effect to fill CNTs with vanadium oxide at relatively high temperature, i.e. 750 -C. Xu et al. [10] have * Corresponding author. Fax: +86 731 882 2782. E-mail address: [email protected] (R.-Q. Yu). 1 Present address: College of Materials Science and Engineering, Hunan University, Changsha 410082, P.R. China. 0040-6090/$ - see front matter D 2005 Published by Elsevier B.V. doi:10.1016/j.tsf.2005.11.018
reported the filling of single-walled carbon nanotubes with lanthanide halides by heating an intimate mixture of carbon nanotubes and halides above the melting point of the later. However, the high yield filling of CNTs by foreign elements under mild conditions still remains a difficult goal that numerous groups are trying to attain. On the other hand, the high surface area of CNTs with a narrow pore size distribution is very important to applications used as substrates for external coating. And it should be stressed that the external coating selectivity is relatively high compared to the filling. Kim and Sigmund [12] have reported the morphology of zinc oxide nanostructures grown on multi-walled carbon nanotubes (MWNTs). Ultrathin films, quantum dots and nanowires/ nanorods of ZnO can be grown on the outer shells of MWNTs by heat treatment without any catalysts. SnO2 quantum dots were also coupled to individual single-walled carbon nanotubes [13]. SnO2 is an n-type semiconductor oxide with wide band gap energy and is always applied as gas sensing material to detect combustible, toxic and pollutant gases [15]. The advantages of the sensor fabricated by SnO2 include high sensitivity, simple design and low cost. There have been many attempts to improve the sensitivity and selectivity of tin-oxide gas sensors for the realization of intelligent gas sensors [16 – 20]. Since the sensing mechanism of these devices relies on the chemisorption reactions that take place at the surface of the
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Y.-L. Liu et al. / Thin Solid Films 497 (2006) 355 – 360
A
2. Experimental details 2.1. Multi-walled carbon nanotubes
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Intensity (a.u.)
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MWNTs were produced by the catalytic pyrolysis of methane on Ni catalyst at 450 -C [25]. The MWNT materials were submitted to various chemical –physical procedures to remove graphitic particles, amorphous carbon and catalyst impurities prior to utilization [26 –28]. Following one of the purification methodologies, 100 mg of MWNTs were oxidized at 350 -C for 2 h. To eliminate metal oxide catalysts, the oxidized amount of MWNTs was dispersed in 60 ml of 7.0 mol/L HNO3 and refluxed at 140 -C for 12 h under stirring, then rinsed with distilled H2O until the pH of the solution was neutral, and finally dried.
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Fig. 1. Raman spectra for the (A) MWNTs, (B) MWNTs coated with SnO2 before heat treatment and (C) the sample after heat treatment.
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metal oxide, it is believed that sensor sensitivity can be improved by increasing the sensitive material surface areas in order to provide more surface sites available for more oxygen species to be adsorbed on these sites and to make contact with the surrounding gases [21,22]. So it can be concluded that the sensing response can be greatly scaled up with the use of nanosized semiconducting materials [23,24]. In some preliminary experiments, it has been observed that SnO2 nanoparticles could be coated onto MWNTs fully to form a thin and uniform layer. And the materials possess excellent gas sensing responses to reducing gases. The obtained materials have been structurally characterized by Raman spectra, powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). This paper reports the experimental results of aforementioned investigations.
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2-Theta (°) Fig. 2. XRD patterns taken from (A) the MWNTs, (B) MWNTs coated with SnO2 before heat treatment and (C) the sample after heat treatment.
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Fig. 3. TEM micrographs of the prepared samples: (A) TEM image showing the general morphology of the MWNTs (scale bar 100 nm); (B) TEM image of SnO2 coated onto MWNTs forming a thin and fine layer after heat treatment (scale bar 100 nm).
2.2. Coating SnO2 onto carbon nanotube In a typical synthesis, one gram of tin(II) chloride (SnCl2) was dissolved in 40 mL of distilled H2O and then 1.0 mL of HCl (38%) was added. Subsequently, 10 mg of the as-treated MWNTs were dispersed in the above solution. This mixture was sonicated for 5– 10 min and then stirred for 60 min at room temperature. The precipitate was then separated from the mother liquor by centrifugation and was washed with distilled H2O for several times, and then dried at 70 -C under vacuum for 6 h. Part of the final product was calcined at 350 -C for 2 h.
50%. The responses of the sensor were studied in a sealed test chamber (300 cm3) with an inlet and an outlet. The test gas was injected into the chamber through the inlet port and the resistance was measured as a function of time till a constant value was attained. Then the chamber was purged with air and the experiments were repeated. The sensor response (S) for a given test gas is calculated as follows: S = R a / R g, where R g and R a are the electrical resistances of the sensor in test gas and in air, respectively. 3. Results and discussion
2.3. Characterization techniques
3.1. Structural characterization
Raman spectra were recorded using Raman spectrometer (Jobin Yvon LabRam-010 micro-Raman system, France) and 632 nm as the excitation source. XRD experiments of all the products were conducted on a RigaKu D/max 2550 X-ray ˚ ), Diffractometer, Japan, with Cu Ka radiation (k = 1.5418 A with which the data were collected in steps of 0.02- (2h) min 1 from 10- to 90- (2h). The database of the Joint Committee on Powder Diffraction Standards (JCPDS) was utilized for phase identification. The morphology and structure of the synthesized products were observed through a Hitachi-800 transmission electron microscopy (TEM, Japan) operated at 200 kV.
The Raman spectra of purified MWNTs and MWNTs coated with SnO2 are shown in Fig. 1. In the Raman spectra of MWNTs (Fig. 1A), the tangential mode at 1328 cm 1 is the dominant feature. Also, peaks at 1570 cm 1 and 1602 cm 1 are clearly seen in the Raman spectra of MWNTs, which may be assigned to the disorder-induced effect due to the finite size effect or lattice distortion [29]. While for the Raman spectra of
10 C4H10 CH4 CO
4
ou t
6
as
as
G in
For the fabrication of sensors, the compound materials were dispersed in terpineol which was used as a binder to form pastes. A platinum interdigitated electrode structure was made using a metal deposition technique with shadow masking on an alumina substrate. The obtained alumina substrate was dipped into the paste for several times to form gas sensing film. Then the element was annealed at 350 -C for 1 h at ambient atmosphere to evaporate the terpineol. Finally, the alumina substrate and a Ni – Cr heater were welded onto a bakelite substrate. The electrical resistances measurements of the sensor were carried out in an air ambient at a relative humidity (RH) around
G
2.4. Measurement of the sensing characteristics
Output Voltage (V)
8
2
0 0
20
40
60
80
100
120
Time (s) Fig. 4. Transient responses of the sensor to 500 ppm C4H10, 500 ppm CH4 and 500 ppm CO operating at 335 -C in 50% RH.
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22
9
20 1000 ppm Gas Out
500 ppm
18
7
16
Sensitivity
200 ppm 6 100 ppm 5 4
14 12 10
as
3
G
Output Voltage (V)
8
A
In
8
2
6
1 0
0 0
100
200
300
400
500
600
700
100 200 300 400 500 600 700 800 900 1000 1100
800
Concentration (ppm)
Time (s) 30
Fig. 5. Measurement of transient responses of the sensor upon exposure to LPG with the concentration from 100 ppm to 1000 ppm at the operating temperature of 335 -C in 50% RH.
10 9 100 ppm
7
200 ppm Gas Out
50 ppm
6 5
20 ppm 10 ppm
4 3
Gas In
Output Voltage (V)
8
2
Sensitivity
MWNTs coated with SnO2 (Fig. 1B and C), peaks at around 425, 492, 626 and 762 cm 1 are attributed to the A2g, Eg, A1g and B2g vibration modes of SnO2, respectively [30]. Fig. 1B shows the spectra of the compound material before heat treatment at 350 -C and Fig. 1C shows the spectra after heat treatment. It is evident that the spectra of Fig. 1B and C have the same profile, which implies that SnO2 exists in those two samples. The structures of the samples were further confirmed by XRD presented in Fig. 2. Fig. 2A shows XRD pattern taken from the uncoated MWNTs. Both peaks are come from the 2HC (002) (JCPDS card no. 41-1487) [31]. Fig. 2B shows the XRD pattern of the MWNTs coated with SnO2 before heat treatment. The broad diffraction lines corresponding to the carbon nanotubes and SnO2 are observed. No diffraction peaks of other impurities are found in the sample. The maximal diffraction peaks of the specimen could be indexed in peak positions to SnO2 (JCPDS card no. 41-1445) [32]. The main
B 25
20
15
10
5 0
20
40
60
80
100 120 140 160 180 200 220
Concentration (ppm) Fig. 7. Sensitivity of the sensor to (A) LPG and (B) ethanol.
peak of tetragonal SnO2 (110) almost overlaps with the main peaks of 2H-C (002). Comparing both spectra of Fig. 2A and B, it is easy to find that the broad peak is mainly contributed to the small size or the amorphous phase of SnO2. Furthermore, a sudden change in the peak intensities appears after the sample was heated (Fig. 2C). The increase in intensity amplitude and decrease in width of the peaks reflect the increased crystalline and enlarged crystallite dimensions caused by the heat treatment. A representative TEM image of the MWNTs is displayed in Fig. 3A, which shows that MWNTs were almost the only species in the purified products with a mean outer diameter of 30 nm, a mean inner diameter of 10 nm and lengths up to several micrometers. Fig. 3B presents a single MWNT coated with SnO2 after heat treatment. It is obvious that the nanosized SnO2 were coated onto the outside of the CNTs. The thickness of SnO2 layer on the MWNTs was found to be around 12.5 nm on average.
1 0 0
120
240
360
480
600
720
840
960
Time (s) Fig. 6. Measurement of transient responses of the sensor upon exposure to ethanol gas with the concentration from 10 ppm to 200 ppm at the operating temperature of 335 -C in 50% RH.
Table 1 The sensor resistance (kV) in air and in LPG gas versus gas concentrations Air
Concentrations of LPG (ppm) 100
200
500
1000
75.4
10.9
7.8
4.9
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3.2. Gas sensing characteristics
Table 2 The sensor resistance (kV) in air and in ethanol gas versus gas concentrations
Fig. 4 shows the dynamic response of the sensors of SnO2 coated on the MWNTs to C4H10, CH4 and CO, respectively, with the gas turning-on and turning-off. The gas is injected into the test chamber, at a working temperature of 335 -C and 50% relative humidity (RH) at 20 -C. Obviously, the electronic currents increase due to the introduction of gases, which is consistent with the n-type semiconducting properties of SnO2. The sensor shows little sensing to these three gases and the sensing responses were about 1.4, 1.5 and 1.8 for CO, C4H10 and CH4, respectively. Fig. 5 is the successive dynamic responses of the sensor to liquefied petroleum gas (LPG) with the concentration ranges from 100 to 1000 ppm at the same measurement conditions. It is evident that the sensing responses increase with the increment of the gas concentrations and the sensor exhibits a fast response and a good recovery. The similar changes are observed in the successive dynamic responses of the sensor to ethanol gas with the concentration ranges from 10 to 200 ppm at the same measurement condition (Fig. 6). The dependences of the gas sensing responses of the sample towards LPG gas concentrations and ethanol gas concentrations are presented in Fig. 7. The magnitudes of sensor response increased linearly with the LPG gas concentration up to 1000 ppm. There is also a similar trend for the relationship of the ethanol gas concentrations and the sensing responses. Tables 1 and 2 show the sensor resistances in air and in LPG gas and ethanol gas with different concentrations. The sensor resistances are 75.4 kV in air, 10.9 kV in 100 ppm LPG and 18.5 kV in 10 ppm ethanol gas, respectively. The resistances of the sensor designed in this experiment are much lower than the SnO2 nanobelt sensors [33]. The high sensitivity and low resistance maybe be contributed to the particular electrical transport mechanism, which is different from that of the SnO2 nanobelt sensors [34]. The gas sensing mechanism is based on the changes in the conductance of the semiconductor sensor. When the sensor is in air atmosphere, the oxygen-related species, such as O, O2 and O2, are present on SnO2 by the following reactions [35]:
Air
Concentrations of ethanol (ppm) 10
20
75.4
18.5
13.4
O2 ðgasÞ6O2 ðadsÞ þ e 6O 2 ðadsÞ þ e 62O ðadsÞ þ e 6O2 ðadsÞ:
This procedure would decrease the conductance of SnO2. When the sensor is in reducing gases at appropriate temperature, the reducing gas molecules react with oxygen ions, which loose electrons to the conduction band of SnO2 to improve the conductance of SnO2. When the sensor is set in air again, the oxygen molecules are adsorbed on the SnO2 again. And this process determines the gas sensing characteristic of SnO2. Furthermore, the selectivity of SnO2 gas sensor is due to the different reaction speed of different reducing gases on the semiconductor surface. The sensor resistance is dominated by the barriers between the SnO2 grains on the MWNTs. The work function of the MWNTs is similar to that of SnO2 [36,37]. So the Schottky
50 6.7
100 4.7
200 3.1
barrier between them is very low. Electrons travel through the SnO2 grains into the MWNTs, and then conduct in the MWNTs with low resistance. This is the main reason the resistance of the gas sensor designed in the experiment is much lower than that of the SnO2 nanobelt. 4. Conclusion The compound material with SnO2 coated onto MWNTs was synthesized at ambient conditions. The Raman spectra and XRD confirmed the existence of SnO2 nanoparticles; TEM showed the formation of fine and thin layers of SnO2 on the MWNTs. The sensing response to reducing gases of gas sensors based on the compound material was measured and electrical characterization showed the material was sensitive to gaseous polluting species like LPG for potential environmental applications, as well as to ethanol for breath analyzers and food control applications. The gas sensor responses increase linearly with the increment of the gas concentrations of LPG and ethanol. The sensor has fast response and recovery within seconds. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 20375012, 20075006 and 29975006), the Foundation for PhD Thesis Research (No. 20010532008) and the Foundation of Scientific Committee of Hunan Province. References [1] S. Iijima, Nature 354 (1991) 56. [2] S. Frank, P. Poncharal, Z.L. Wang, W.A. de Heer, Science 280 (1998) 1744. [3] P. Sudan, A. Zuttel, Ph. Mauron, Ch. Emmenegger, P. Wenger, L. Schlapbach, Carbon 41 (2003) 2377. [4] P.G. Collins, A. Zettl, H. Bando, A. Thess, R.E. Smalley, Science 278 (1997) 100. [5] G.L. Che, B.B. Lakschmi, E.R. Fisher, C.R. Martin, Nature 393 (1998) 346. [6] P.M. Ajayan, S. Iijima, Nature 361 (1993) 333. [7] C.H. Kiang, J.S. Choi, T.T. Tran, A.D. Bacher, J. Phys. Chem., B 103 (1999) 7449. [8] S.C. Tsang, Y.K. Chen, P.J.F. Harris, M.L.H Green, Nature 372 (1994) 159. [9] P.M. Ajayan, O. Stephan, P. Redlich, C. Colliex, Nature 375 (1995) 564. [10] C. Xu, J. Sloan, G. Brown, S. Bailey, V.C. Williams, S. Friedrichs, K.S. Coleman, E. Flahaut, J.L. Hutchison, R.E. Dunin-Borkowski, M.L.H. Green, Chem. Commun. 24 (2000) 2427. [11] Y. Zhang, N.W. Franklin, R.J. Chen, H.J. Dai, Chem. Phys. Lett. 331 (2000) 35. [12] H. Kim, W. Sigmund, Appl. Phys. Lett. 81 (2002) 2085.
360
Y.-L. Liu et al. / Thin Solid Films 497 (2006) 355 – 360
[13] W.Q. Han, A. Zettl, Nano Lett. 3 (2003) 681. [14] M.H. Joanne, A.H. Megan, D.K. Todd, Nano Lett. 2 (2002) 1253. [15] J.F. McAleer, P.T. Moseley, J.O.W. Norris, D.E. Williams, P. Taylor, B.C. Tofield, Mater. Chem. Phys. 17 (1987) 577. [16] N. Butta´, M. Melli, S. Pizzini, Sens. Actuators, B, Chem. 2 (1990) 151. [17] M.C. Horrillo, A. Serventi, D. Rickerby, J. Gutie´rrez, Sens. Actuators, B, Chem. 58 (1999) 474. [18] D.G. Rickerby, M.C. Horrillo, J.P. Santos, P. Serrini, Nanostruct. Mater. 9 (1997) 43. [19] G.G. Mandayo, E. Castan˜o, F.J. Gracia, A. Cirera, A. Cornet, J.R. Morante, Sens. Actuators, B, Chem. 95 (2003) 90. [20] R. Rella, A. Serra, P. Siciliano, L. Vasanelli, G. De, A. Licciulli, A. Quirini, Sens. Actuators, B, Chem. 44 (1997) 462. [21] R.M. Walton, D.J. Dwyer, J.W. Schwank, Appl. Surf. Sci. 125 (1998) 199. [22] M.I. Baraton, L. Merhari, Nanostruct. Mater. 10 (1998) 699. [23] Y.D. Wang, X.H. Wu, Y.F. Li, Z.L. Zhou, Solid-State Electron. 48 (2004) 627. [24] O.K. Tan, W. Cao, Y. Hu, W. Zhu, Ceram. Int. 30 (2004) 1127. [25] P. Chen, H.B. Zhang, G.D. Lin, Carbon 35 (1997) 1495. [26] J.F. Colomer, P. Piedigrosso, A. Fonseca, J.B. Nagy, Synth. Met. 103 (1999) 2482.
[27] S. Bandow, S. Asaka, X. Zhao, Y. Ando, Appl. Phys., A 67 (1998) 23. [28] J.E. Riggs, Z. Guo, D.L. Carroll, Y.P. Sun, J. Am. Chem. Soc. 122 (2000) 5879. [29] W. Li, H. Zhang, C. Wang, Y. Zhang, L. Xu, K. Zhu, S. Xie, Appl. Phys. Lett. 70 (1997) 2684. [30] K.N. Yu, Y.H. Xiong, Y.L. Liu, C.S. Xiong, Phys. Rev., B 55 (1997) 2666. [31] Powder Diffraction File. International Center for Diffraction Data, Newtown Square, PA (formerly Joint Committee on Powder Diffraction Standards, Swarthmore, PA, 1991): No. 41-1487. [32] Powder Diffraction File. International Center for Diffraction Data, Newtown Square, PA (formerly Joint Committee on Powder Diffraction Standards, Swarthmore, PA, 1991): No. 41-1445. [33] E. Comini, G. Faglia, G. Sberveglieri, Z.W. Pan, Z.L. Wang, Appl. Phys. Lett. 81 (2002) 1069. [34] Y.X. Liang, Y.J. Chen, T.H. Wang, Appl. Phys. Lett. 85 (2004) 666. [35] K.D. Schierbaum, U. Weimar, W. Gopel, R. Kowalkowski, Sens. Actuators, B, Chem. 3 (1991) 205. [36] H. Ago, T. Kugler, F. Cacialli, W.R. Salaneck, M.S.P. Shaffer, A.H. Windle, R.H. Friend, J. Phys. Chem., B 103 (1999) 8116. [37] M. Sinner-Hettenbach, M. Go¨thelid, T. Wei, N. Barsan, U. Weimar, H. von Schenck, L. Giovanelli, G. Le Lay, Surf. Sci. 499 (2002) 85.