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High surface area SnO2 nanoparticles: Synthesis and gas sensing properties
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Materials Chemistry and Physics 108 (2008) 232–236
High surface area SnO2 nanoparticles: Synthesis and gas sensing properties Liujiang Xi a , Dong Qian a,b,∗ , Xincun Tang a , Chunjiao Chen a a
College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, PR China b State Key Laboratory of Powder Metallurgy, Changsha 410083, PR China Received 13 April 2007; received in revised form 22 August 2007; accepted 21 September 2007
Abstract SnO2 nanoparticles with high surface area have been synthesized by a homogeneous precipitation ethanol-thermal method with CO(NH2 )2 and SnCl4 ·5H2 O as starting materials. FT-IR, XRD, BET and TEM have been employed to characterize the as-prepared samples. The results show that the homogeneous precipitation ethanol-thermal method can effectively restrain the agglomeration, by which the SnO2 powders obtained are well dispersed and less aggregated with a size of 8–9 nm and high-specific surface area over 200 m2 g−1 . On the other hand, the SnO2 powders prepared via the single homogeneous precipitation method showed lower specific surface area of 55 m2 g−1 . The results of gas sensing measurements show that the sensors fabricated by the SnO2 nanoparticles obtained by the homogeneous precipitation ethanol-thermal method exhibit excellent sensitivity behavior to methanol and ethanol gases at room temperature. When the concentration of ethanol is 80 ppm, the sensitivity is ca. 25, and the recovery time is less than 25 s at room temperature. © 2007 Elsevier B.V. All rights reserved. Keywords: Nanostructures; Semiconductors; Chemical synthesis; Chemical techniques
1. Introduction SnO2 is an n-type semiconductor with a wide band gap of 3.65 eV, and has been widely used as gas sensors [1–4], electrode materials [5], catalyst supports [6], solar cells [7,8], etc. Especially, SnO2 nanoparticles have been intensively studied for gas sensing applications not only because of their relatively low operating temperature, but also due to the fact that they can be used to detect both reducing and oxidizing gases by adding various doping elements [9–19]. Generally, SnO2 powders with high surface area are in favor of the applications of gas sensors. Therefore, more and more efforts have been focused on the preparation of SnO2 with high surface area. Song and Kim [20] synthesized SnO2 powders with surface area of 86 m2 g−1 through a water-in-oil microemulsion method. Song and Kang [21] employed a homogeneous precipitation method to prepare SnO2 powders with specific surface area of 24–44 m2 g−1 , while SnO2 powders with specific surface area of 15–18 m2 g−1 were ∗ Corresponding author at: College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, PR China. Tel.: +86 731 8879616; fax: +86 731 8879616. E-mail address: [email protected] (D. Qian).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.09.023
obtained through a precipitation method in comparison. Chen and Gao [22] obtained SnO2 nanoparticles with high-specific area of 107–169 m2 g−1 and particle size of 3 nm via a waterin-oil microemulsion-assisted hydrothermal process. Fujihara et al. [23] fabricate thermally stable SnO2 particles with specific surface area over 110 m2 g−1 by a hydrothermal route followed by annealing treatments at 400 or 500 ◦ C. In this paper, a homogeneous precipitation ethanol-thermal method has been developed to fabricate SnO2 nanoparticles with specific surface area over 200 m2 g−1 , and the sensing properties of samples have been studied at room temperature. 2. Experimental 2.1. Synthesis All the chemical reagents were analytically pure and used without further purification. SnO2 particles were prepared by a homogeneous precipitation method modified in terms of the route in literature [21] and a homogeneous precipitation ethanol-thermal method, respectively. In a typical procedure of the first method, urea (CO(NH2 )2 ) and SnCl4 ·5H2 O were dissolved in distilled water at room temperature with concentrations of 0.15 and 0.05 mol L−1 , respectively. Then, the solutions were mixed, and heated to 90 ◦ C for 5 h to promote the hydrolysis of SnCl4 and the formation of SnO2 . The precipitates obtained were separated by centrifugation, washed with deionized water until the absence of
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Table 1 Synthesis conditions, Scherrer crystal sizes and BET surface areas of SnO2 samples (a–f) Samples
Synthesis conditions
Crystal sizesa (nm)
BET surface areas (m2 g−1 )
(a) (b)
Prepared by homogeneous precipitation Prepared by homogeneous precipitation ethanol-thermal method with the concentration of SnCl4 of 0.05 mol L−1 and ethanol-thermal treatment at 140 ◦ C for 15 h Prepared by homogeneous precipitation ethanol-thermal method with the concentration of SnCl4 of 0.05 mol L−1 and ethanol-thermal treatment at 140 ◦ C for 24 h Prepared by homogeneous precipitation ethanol-thermal method with the concentration of SnCl4 of 0.05 mol L−1 and ethanol-thermal treatment at 140 ◦ C for 30 h Prepared by homogeneous precipitation ethanol-thermal method with the concentration of SnCl4 of 0.05 mol L−1 and ethanol-thermal treatment at 170 ◦ C for 24 h Prepared by homogeneous precipitation ethanol-thermal method with the concentration of SnCl4 of 0.15 mol L−1 and ethanol-thermal treatment at 140 ◦ C for 24 h
10.5 7.8
55 218
8.1
214
8.5
205
9.0
201
8.6
207
(c)
(d)
(e)
(f)
a
Average crystal size for SnO2 was estimated by XRD using the Scherrer formula based on the (1 1 0) reflection.
chloride ion, which was tested by an aqueous AgNO3 solution, and then washed with ethanol for three times. The resulting products were dried at 110 ◦ C for 3 h and then calcined at 500 ◦ C for 2 h to obtain samples. By the second method, the modified point in comparison with the first method was that the precipitates obtained after washed with ethanol were dispersed in ethanol and placed into a Teflon-lined autoclave. Ethanol-thermal treatments were carried out at 140 ◦ C for 15, 24 and 30 h and 170 ◦ C for 24 h, respectively. The SnO2 nanoparticles were obtained after centrifuging the resulting precipitates and then drying at 110 ◦ C for 3 h.
2.2. Characterization of the as-prepared samples Phase structures of the as-prepared samples were identified with X-ray powder diffraction (XRD) taken on a Rigaku-D-Max rA 12 kW diffractome˚ at an operation voltage and current ter with Cu K␣ radiation (λ = 1.54056 A) of 40 kV and 300 mA, respectively. Morphologies and particle sizes of the samples were observed with a Hitachi H-800 transmission electron microscope (TEM) operated at an acceleration voltage of 200 kV. Fourier transform infrared (FT-IR) spectra of the samples were recorded on an AVATAR 360 Fourier-IR spectrophotometer with KBr as compressed slices. The specific surface areas of the samples were analyzed on a BET surface analyzer (ASAP2010, Micro-meritics).
2.3. Gas sensing measurement The samples were completely ground in an agate mortar, and the SnO2 paste was formed through adding terpineol. Then, the resultant paste was coated on an alumina ceramic tube with already deposited gold electrodes with 1.5 mm spacing. After sintering at 420 ◦ C for 2 h and 300 ◦ C for 120 h, the sensor element was installed into a HW-C30A (Hanwei Group, Henan) gas sensing intelligent apparatus, which served as voltage sources. For consistency, a potential of 5 V was applied across the gold electrodes for all measurements, and all the gas sensing experiments were carried out in a closed glass container (10 L in volume) equipped with appropriate inlet and outlet for gas flow. The sensor sensitivity (Rair/Rgas) is defined as the ratio of the sensor resistance in air to that in the presence of ethanol or methanol gas.
rutile structure (JCPDS 21-1250). Fig. 1(a) and (b–f) exhibit the XRD patterns of samples prepared by the homogeneous precipitation and homogeneous precipitation ethanol-thermal method, respectively. It is apparent that the XRD peaks are by far sharper and narrower for the sample (a) prepared via the homogeneous precipitation method than that for the samples (b–f) prepared by the homogeneous precipitation ethanol-thermal method, which reveals that the average crystal size of SnO2 particles prepared via the former method is larger than that prepared by the later one. The average crystal sizes of SnO2 particles prepared by the above-mentioned two methods are ca. 10.5 nm and below 9.0 nm, respectively, estimated by the Scherrer formula based on the (1 1 0) reflection of XRD. With prolonging the ethanolthermal treatment times, the diffraction peaks become narrower and stronger due to the fact that the average crystal sizes grow larger and the crystallinity is improved. The average crystal sizes for samples (b–d) are 7.8, 8.1 and 8.5 nm for the SnO2 samples with the ethanol-thermal treatments at 140 ◦ C for 15, 24 and 30 h, respectively. The average crystal sizes are 8.1 and 9.0 nm for the SnO2 samples (c) and (e) with the ethanol-thermal treatment temperatures of 140 and 170 ◦ C, respectively, which implies that the crystal size grows large with the ethanol-thermal treatment temperature increasing. The peak width of the sample (f) pre-
3. Results and discussion The synthesis conditions, Scherrer crystal sizes and BET surface areas of SnO2 samples (a–f) are listed in Table 1, and their XRD patterns are shown in Fig. 1. All the diffraction peaks of the XRD can be readily indexed to tetragonal SnO2 with
Fig. 1. XRD patterns of the as-prepared SnO2 samples (a–f).
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Fig. 2. TEM images of samples (a), (c), (e) and (f) (shown in Fig. 2(a–d), respectively).
pared by 0.15 mol L−1 SnCl4 is little broader than the sample (c) prepared by 0.05 mol L−1 SnCl4 , of which the average crystal sizes are 8.6 and 8.1 nm, respectively. From Table 1, it can be seen that the SnO2 samples prepared by the homogeneous precipitation ethanol-thermal method have high BET surface areas over 200 m2 g−1 . In contrast, the sample (a) synthesized via the homogeneous precipitation method has a low surface area of 55 m2 g−1 . With increasing the ethanol-thermal treatment time and temperature together with the concentration of SnCl4 , the BET surface area decreases slightly. TEM images of the samples (a), (c), (e) and (f) are presented in Fig. 2(a–d), respectively. It is obvious that the sample (a) prepared by the homogeneous precipitation method is over
Fig. 3. FT-IR spectrum of sample (c).
10 nm in diameter with bad dispersion, while the samples (c), (e) and (f) prepared by the homogeneous precipitation ethanol-thermal method have diameters below 10 nm and good dispersions, which shows that the homogeneous precipitation ethanol-thermal method is in favor of the dispersion of SnO2 nanoparticles. The average sizes of samples (a), (c), (e) and (f) determined by TEM are in good agreement with the sizes estimated by XRD listed in Table 1. Fig. 3 shows the FT-IR spectrum of sample (c), in which the bands at around 3400 and 1630 cm−1 are due to the O–H vibrating mode of the absorbed water [24], while the band located at 1400 cm−1 is owing to the bending vibration of –CH2 , which shows that a few organic groups are
Fig. 4. Sensitivities of sample (c) to various concentrations of ethanol and methanol gases at room temperature.
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test gas with the sensor surface, while recovery time as the time taken to 90% of the initial steady value of the sensor resistance after the chamber was opened up. It can be seen that the electrical response and recovery are fast, and the response time is less than 45 s, while the recovery time is below 25 s at room temperature. 4. Conclusions
Fig. 5. Sensitivities of samples (a), (c), (e) and (f) to different ethanol gas concentrations at room temperature.
absorbed on the surface of SnO2 nanoparticles [22]. The bands at around 561 and 615 cm−1 can be attributed to the Sn–O stretching vibration and the O–Sn–O blending vibration in SnO2 . Fig. 4 shows the sensitivities of sample (c) to various concentrations of ethanol and methanol gases at room temperature, respectively. From Fig. 4, it can be seen that the sensitivity increases with the concentration of the ethanol and methanol gases increasing, which exhibits that the sensitivity of sample (c) to ethanol gas is better than that to methanol gas. When the gas concentration is up to 80 ppm, the sensitivity is 25.8 for ethanol and 11.3 for methanol. Fig. 5 shows the sensitivities of samples (a), (c), (e) and (f) to different ethanol concentrations at room temperature. It reveals that the sensitivity of sample (a) is obviously lower than that of samples (c), (e) and (f) at the same ethanol gas concentration, which may be due to the higher surface areas of samples (c), (e) and (f). The sensitivities of samples (c), (e) and (f) are comparative probably because their surface areas are almost the same. When the ethanol gas concentration is 80 ppm, the sensitivities are 25.8, 24.2 and 25.1 for samples (c), (e) and (f), and 14.8 for sample (a), respectively. Fig. 6 shows the typical response–recovery curve of sample (c) to 80 ppm ethanol gas at room temperature. Here, response time was defined as the time needed for a sensor to attain the 90% of the maximum change in resistance after the contact of
Fig. 6. Response–recovery plot of sample (c) to 80 ppm ethanol gas at room temperature.
SnO2 nanoparticles were synthesized by the homogeneous precipitation ethanol-thermal method and homogeneous precipitation method, respectively. By the former method, the SnO2 powders obtained have BET surface areas over 200 m2 g−1 and particle sizes below 10 nm, and the agglomeration of SnO2 can be effectively restrained. It was also found that the crystalline sizes increased slightly with the increases of the ethanol-thermal treatment time and temperature as well as the initial concentration of SnCl4 . In contrast, by the latter method, the SnO2 powders obtained exhibit a comparatively low BET surface area of 55 m2 g−1 and big particle size over 10 nm accompanied with serious agglomeration. The SnO2 powders prepared by the former method exhibit excellent sensitivity behavior to methanol and ethanol gases with the sensitivity of ca. 25 and the recovery time below 25 s at room temperature. Acknowledgement This work was supported by the opening subject of State Key Laboratory of Powder Metallurgy (no. 200506123105A). References [1] I. Matsubara, K. Hosono, N. Murayama, W. Shin, N. Izu, Sens. Actuators B: Chem. 108 (2005) 143. [2] H.C. Wang, Y. Li, M.J. Yang, Sens. Actuators B: Chem. 119 (2006) 380. [3] Y. Wang, X. Jiang, Y. Xia, J. Am. Chem. Soc. 125 (2003) 16176. [4] G.J. Li, X.H. Zhang, S. Kawi, Sens. Actuators B: Chem. 60 (1999) 64. [5] C. Kim, M. Noh, M. Choi, J. Cho, B. Park, Chem. Mater. 17 (2005) 3297. [6] D.Z. Wang, S.L. Wen, J. Chen, S.Y. Zhang, F.Q. Li, Phys. Rev. B 49 (1994) 14282. [7] S. Ferrere, A. Zaban, B.A. Gregg, J. Phys. Chem. B 101 (1997) 4490. [8] P.G. Harrison, M.J. Willet, Nature 332 (1988) 337. [9] C. Nayral, T. Ould-Ely, A. Maisonnat, B. Chaudret, P. Fau, L. Lescouz`eres, A. Peyre-Lavigne, Adv. Mater. 11 (1999) 61. [10] J. Zhang, L. Gao, Chem. Lett. 32 (2003) 458. [11] E.R. Leite, I.T. Weber, E. Longo, J.A. Varela, Adv. Mater. 12 (2000) 965. [12] S. Monredon, A. Cellot, F. Ribot, C. Sanchez, L. Armelao, L. Gueneau, L. Delattre, J. Mater. Chem. 12 (2002) 2396. [13] E.R. Leite, A.P. Maciel, I.T. Weber, P.N. Lisboa-Filho, E. Longo, C.O. Paiva-Santos, A.V.C. Andrade, C.A. Pakoscimas, Y. Maniette, W.H. Schreiner, Adv. Mater. 14 (2002) 905. [14] A.R. Roosen, W.C. Carter, Physica A 261 (1998) 232. [15] T. N¨utz, M. Haase, J. Phys. Chem. B 104 (2000) 8430. [16] C. Nayral, E. Viala, V. Colli`ere, P. Fau, F. Senocq, A. Maisonnat, B. Chaudret, Appl. Surf. Sci. 164 (2000) 219. [17] J. Arbiol, P. Gorostiza, A. Cirera, A. Cornet, J.R. Morante, Sens. Actuators B: Chem. 78 (2001) 57.
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[18] Z. Chen, J.K.L. Lai, C.H. Shek, H. Chen, J. Mater. Res. 18 (2003) 1289. [19] N.L. Wu, S.Y. Wang, I.A. Rusakova, Science 285 (1999) 1375. [20] K.C. Song, J.H. Kim, Powder Technol. 107 (2000) 268. [21] K.C. Song, Y. Kang, Mater. Lett. 42 (5) (2000) 283.
[22] D.L. Chen, L. Gao, J. Colloid Interface Sci. 278 (2004) 137. [23] S. Fujihara, T. Maeda, H. Ohgi, E. Hosono, H. Imai, S.-H. Kim, Langmuir 20 (2004) 6476. [24] C.P. Sbu, S.R. Kumar, P. Mukundan, K.G.K. Warrier, Chem. Mater. 14 (2002) 2876.
Materials Chemistry and Physics 108 (2008) 232–236
High surface area SnO2 nanoparticles: Synthesis and gas sensing properties Liujiang Xi a , Dong Qian a,b,∗ , Xincun Tang a , Chunjiao Chen a a
College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, PR China b State Key Laboratory of Powder Metallurgy, Changsha 410083, PR China Received 13 April 2007; received in revised form 22 August 2007; accepted 21 September 2007
Abstract SnO2 nanoparticles with high surface area have been synthesized by a homogeneous precipitation ethanol-thermal method with CO(NH2 )2 and SnCl4 ·5H2 O as starting materials. FT-IR, XRD, BET and TEM have been employed to characterize the as-prepared samples. The results show that the homogeneous precipitation ethanol-thermal method can effectively restrain the agglomeration, by which the SnO2 powders obtained are well dispersed and less aggregated with a size of 8–9 nm and high-specific surface area over 200 m2 g−1 . On the other hand, the SnO2 powders prepared via the single homogeneous precipitation method showed lower specific surface area of 55 m2 g−1 . The results of gas sensing measurements show that the sensors fabricated by the SnO2 nanoparticles obtained by the homogeneous precipitation ethanol-thermal method exhibit excellent sensitivity behavior to methanol and ethanol gases at room temperature. When the concentration of ethanol is 80 ppm, the sensitivity is ca. 25, and the recovery time is less than 25 s at room temperature. © 2007 Elsevier B.V. All rights reserved. Keywords: Nanostructures; Semiconductors; Chemical synthesis; Chemical techniques
1. Introduction SnO2 is an n-type semiconductor with a wide band gap of 3.65 eV, and has been widely used as gas sensors [1–4], electrode materials [5], catalyst supports [6], solar cells [7,8], etc. Especially, SnO2 nanoparticles have been intensively studied for gas sensing applications not only because of their relatively low operating temperature, but also due to the fact that they can be used to detect both reducing and oxidizing gases by adding various doping elements [9–19]. Generally, SnO2 powders with high surface area are in favor of the applications of gas sensors. Therefore, more and more efforts have been focused on the preparation of SnO2 with high surface area. Song and Kim [20] synthesized SnO2 powders with surface area of 86 m2 g−1 through a water-in-oil microemulsion method. Song and Kang [21] employed a homogeneous precipitation method to prepare SnO2 powders with specific surface area of 24–44 m2 g−1 , while SnO2 powders with specific surface area of 15–18 m2 g−1 were ∗ Corresponding author at: College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, PR China. Tel.: +86 731 8879616; fax: +86 731 8879616. E-mail address: [email protected] (D. Qian).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.09.023
obtained through a precipitation method in comparison. Chen and Gao [22] obtained SnO2 nanoparticles with high-specific area of 107–169 m2 g−1 and particle size of 3 nm via a waterin-oil microemulsion-assisted hydrothermal process. Fujihara et al. [23] fabricate thermally stable SnO2 particles with specific surface area over 110 m2 g−1 by a hydrothermal route followed by annealing treatments at 400 or 500 ◦ C. In this paper, a homogeneous precipitation ethanol-thermal method has been developed to fabricate SnO2 nanoparticles with specific surface area over 200 m2 g−1 , and the sensing properties of samples have been studied at room temperature. 2. Experimental 2.1. Synthesis All the chemical reagents were analytically pure and used without further purification. SnO2 particles were prepared by a homogeneous precipitation method modified in terms of the route in literature [21] and a homogeneous precipitation ethanol-thermal method, respectively. In a typical procedure of the first method, urea (CO(NH2 )2 ) and SnCl4 ·5H2 O were dissolved in distilled water at room temperature with concentrations of 0.15 and 0.05 mol L−1 , respectively. Then, the solutions were mixed, and heated to 90 ◦ C for 5 h to promote the hydrolysis of SnCl4 and the formation of SnO2 . The precipitates obtained were separated by centrifugation, washed with deionized water until the absence of
L. Xi et al. / Materials Chemistry and Physics 108 (2008) 232–236
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Table 1 Synthesis conditions, Scherrer crystal sizes and BET surface areas of SnO2 samples (a–f) Samples
Synthesis conditions
Crystal sizesa (nm)
BET surface areas (m2 g−1 )
(a) (b)
Prepared by homogeneous precipitation Prepared by homogeneous precipitation ethanol-thermal method with the concentration of SnCl4 of 0.05 mol L−1 and ethanol-thermal treatment at 140 ◦ C for 15 h Prepared by homogeneous precipitation ethanol-thermal method with the concentration of SnCl4 of 0.05 mol L−1 and ethanol-thermal treatment at 140 ◦ C for 24 h Prepared by homogeneous precipitation ethanol-thermal method with the concentration of SnCl4 of 0.05 mol L−1 and ethanol-thermal treatment at 140 ◦ C for 30 h Prepared by homogeneous precipitation ethanol-thermal method with the concentration of SnCl4 of 0.05 mol L−1 and ethanol-thermal treatment at 170 ◦ C for 24 h Prepared by homogeneous precipitation ethanol-thermal method with the concentration of SnCl4 of 0.15 mol L−1 and ethanol-thermal treatment at 140 ◦ C for 24 h
10.5 7.8
55 218
8.1
214
8.5
205
9.0
201
8.6
207
(c)
(d)
(e)
(f)
a
Average crystal size for SnO2 was estimated by XRD using the Scherrer formula based on the (1 1 0) reflection.
chloride ion, which was tested by an aqueous AgNO3 solution, and then washed with ethanol for three times. The resulting products were dried at 110 ◦ C for 3 h and then calcined at 500 ◦ C for 2 h to obtain samples. By the second method, the modified point in comparison with the first method was that the precipitates obtained after washed with ethanol were dispersed in ethanol and placed into a Teflon-lined autoclave. Ethanol-thermal treatments were carried out at 140 ◦ C for 15, 24 and 30 h and 170 ◦ C for 24 h, respectively. The SnO2 nanoparticles were obtained after centrifuging the resulting precipitates and then drying at 110 ◦ C for 3 h.
2.2. Characterization of the as-prepared samples Phase structures of the as-prepared samples were identified with X-ray powder diffraction (XRD) taken on a Rigaku-D-Max rA 12 kW diffractome˚ at an operation voltage and current ter with Cu K␣ radiation (λ = 1.54056 A) of 40 kV and 300 mA, respectively. Morphologies and particle sizes of the samples were observed with a Hitachi H-800 transmission electron microscope (TEM) operated at an acceleration voltage of 200 kV. Fourier transform infrared (FT-IR) spectra of the samples were recorded on an AVATAR 360 Fourier-IR spectrophotometer with KBr as compressed slices. The specific surface areas of the samples were analyzed on a BET surface analyzer (ASAP2010, Micro-meritics).
2.3. Gas sensing measurement The samples were completely ground in an agate mortar, and the SnO2 paste was formed through adding terpineol. Then, the resultant paste was coated on an alumina ceramic tube with already deposited gold electrodes with 1.5 mm spacing. After sintering at 420 ◦ C for 2 h and 300 ◦ C for 120 h, the sensor element was installed into a HW-C30A (Hanwei Group, Henan) gas sensing intelligent apparatus, which served as voltage sources. For consistency, a potential of 5 V was applied across the gold electrodes for all measurements, and all the gas sensing experiments were carried out in a closed glass container (10 L in volume) equipped with appropriate inlet and outlet for gas flow. The sensor sensitivity (Rair/Rgas) is defined as the ratio of the sensor resistance in air to that in the presence of ethanol or methanol gas.
rutile structure (JCPDS 21-1250). Fig. 1(a) and (b–f) exhibit the XRD patterns of samples prepared by the homogeneous precipitation and homogeneous precipitation ethanol-thermal method, respectively. It is apparent that the XRD peaks are by far sharper and narrower for the sample (a) prepared via the homogeneous precipitation method than that for the samples (b–f) prepared by the homogeneous precipitation ethanol-thermal method, which reveals that the average crystal size of SnO2 particles prepared via the former method is larger than that prepared by the later one. The average crystal sizes of SnO2 particles prepared by the above-mentioned two methods are ca. 10.5 nm and below 9.0 nm, respectively, estimated by the Scherrer formula based on the (1 1 0) reflection of XRD. With prolonging the ethanolthermal treatment times, the diffraction peaks become narrower and stronger due to the fact that the average crystal sizes grow larger and the crystallinity is improved. The average crystal sizes for samples (b–d) are 7.8, 8.1 and 8.5 nm for the SnO2 samples with the ethanol-thermal treatments at 140 ◦ C for 15, 24 and 30 h, respectively. The average crystal sizes are 8.1 and 9.0 nm for the SnO2 samples (c) and (e) with the ethanol-thermal treatment temperatures of 140 and 170 ◦ C, respectively, which implies that the crystal size grows large with the ethanol-thermal treatment temperature increasing. The peak width of the sample (f) pre-
3. Results and discussion The synthesis conditions, Scherrer crystal sizes and BET surface areas of SnO2 samples (a–f) are listed in Table 1, and their XRD patterns are shown in Fig. 1. All the diffraction peaks of the XRD can be readily indexed to tetragonal SnO2 with
Fig. 1. XRD patterns of the as-prepared SnO2 samples (a–f).
234
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Fig. 2. TEM images of samples (a), (c), (e) and (f) (shown in Fig. 2(a–d), respectively).
pared by 0.15 mol L−1 SnCl4 is little broader than the sample (c) prepared by 0.05 mol L−1 SnCl4 , of which the average crystal sizes are 8.6 and 8.1 nm, respectively. From Table 1, it can be seen that the SnO2 samples prepared by the homogeneous precipitation ethanol-thermal method have high BET surface areas over 200 m2 g−1 . In contrast, the sample (a) synthesized via the homogeneous precipitation method has a low surface area of 55 m2 g−1 . With increasing the ethanol-thermal treatment time and temperature together with the concentration of SnCl4 , the BET surface area decreases slightly. TEM images of the samples (a), (c), (e) and (f) are presented in Fig. 2(a–d), respectively. It is obvious that the sample (a) prepared by the homogeneous precipitation method is over
Fig. 3. FT-IR spectrum of sample (c).
10 nm in diameter with bad dispersion, while the samples (c), (e) and (f) prepared by the homogeneous precipitation ethanol-thermal method have diameters below 10 nm and good dispersions, which shows that the homogeneous precipitation ethanol-thermal method is in favor of the dispersion of SnO2 nanoparticles. The average sizes of samples (a), (c), (e) and (f) determined by TEM are in good agreement with the sizes estimated by XRD listed in Table 1. Fig. 3 shows the FT-IR spectrum of sample (c), in which the bands at around 3400 and 1630 cm−1 are due to the O–H vibrating mode of the absorbed water [24], while the band located at 1400 cm−1 is owing to the bending vibration of –CH2 , which shows that a few organic groups are
Fig. 4. Sensitivities of sample (c) to various concentrations of ethanol and methanol gases at room temperature.
L. Xi et al. / Materials Chemistry and Physics 108 (2008) 232–236
235
test gas with the sensor surface, while recovery time as the time taken to 90% of the initial steady value of the sensor resistance after the chamber was opened up. It can be seen that the electrical response and recovery are fast, and the response time is less than 45 s, while the recovery time is below 25 s at room temperature. 4. Conclusions
Fig. 5. Sensitivities of samples (a), (c), (e) and (f) to different ethanol gas concentrations at room temperature.
absorbed on the surface of SnO2 nanoparticles [22]. The bands at around 561 and 615 cm−1 can be attributed to the Sn–O stretching vibration and the O–Sn–O blending vibration in SnO2 . Fig. 4 shows the sensitivities of sample (c) to various concentrations of ethanol and methanol gases at room temperature, respectively. From Fig. 4, it can be seen that the sensitivity increases with the concentration of the ethanol and methanol gases increasing, which exhibits that the sensitivity of sample (c) to ethanol gas is better than that to methanol gas. When the gas concentration is up to 80 ppm, the sensitivity is 25.8 for ethanol and 11.3 for methanol. Fig. 5 shows the sensitivities of samples (a), (c), (e) and (f) to different ethanol concentrations at room temperature. It reveals that the sensitivity of sample (a) is obviously lower than that of samples (c), (e) and (f) at the same ethanol gas concentration, which may be due to the higher surface areas of samples (c), (e) and (f). The sensitivities of samples (c), (e) and (f) are comparative probably because their surface areas are almost the same. When the ethanol gas concentration is 80 ppm, the sensitivities are 25.8, 24.2 and 25.1 for samples (c), (e) and (f), and 14.8 for sample (a), respectively. Fig. 6 shows the typical response–recovery curve of sample (c) to 80 ppm ethanol gas at room temperature. Here, response time was defined as the time needed for a sensor to attain the 90% of the maximum change in resistance after the contact of
Fig. 6. Response–recovery plot of sample (c) to 80 ppm ethanol gas at room temperature.
SnO2 nanoparticles were synthesized by the homogeneous precipitation ethanol-thermal method and homogeneous precipitation method, respectively. By the former method, the SnO2 powders obtained have BET surface areas over 200 m2 g−1 and particle sizes below 10 nm, and the agglomeration of SnO2 can be effectively restrained. It was also found that the crystalline sizes increased slightly with the increases of the ethanol-thermal treatment time and temperature as well as the initial concentration of SnCl4 . In contrast, by the latter method, the SnO2 powders obtained exhibit a comparatively low BET surface area of 55 m2 g−1 and big particle size over 10 nm accompanied with serious agglomeration. The SnO2 powders prepared by the former method exhibit excellent sensitivity behavior to methanol and ethanol gases with the sensitivity of ca. 25 and the recovery time below 25 s at room temperature. Acknowledgement This work was supported by the opening subject of State Key Laboratory of Powder Metallurgy (no. 200506123105A). References [1] I. Matsubara, K. Hosono, N. Murayama, W. Shin, N. Izu, Sens. Actuators B: Chem. 108 (2005) 143. [2] H.C. Wang, Y. Li, M.J. Yang, Sens. Actuators B: Chem. 119 (2006) 380. [3] Y. Wang, X. Jiang, Y. Xia, J. Am. Chem. Soc. 125 (2003) 16176. [4] G.J. Li, X.H. Zhang, S. Kawi, Sens. Actuators B: Chem. 60 (1999) 64. [5] C. Kim, M. Noh, M. Choi, J. Cho, B. Park, Chem. Mater. 17 (2005) 3297. [6] D.Z. Wang, S.L. Wen, J. Chen, S.Y. Zhang, F.Q. Li, Phys. Rev. B 49 (1994) 14282. [7] S. Ferrere, A. Zaban, B.A. Gregg, J. Phys. Chem. B 101 (1997) 4490. [8] P.G. Harrison, M.J. Willet, Nature 332 (1988) 337. [9] C. Nayral, T. Ould-Ely, A. Maisonnat, B. Chaudret, P. Fau, L. Lescouz`eres, A. Peyre-Lavigne, Adv. Mater. 11 (1999) 61. [10] J. Zhang, L. Gao, Chem. Lett. 32 (2003) 458. [11] E.R. Leite, I.T. Weber, E. Longo, J.A. Varela, Adv. Mater. 12 (2000) 965. [12] S. Monredon, A. Cellot, F. Ribot, C. Sanchez, L. Armelao, L. Gueneau, L. Delattre, J. Mater. Chem. 12 (2002) 2396. [13] E.R. Leite, A.P. Maciel, I.T. Weber, P.N. Lisboa-Filho, E. Longo, C.O. Paiva-Santos, A.V.C. Andrade, C.A. Pakoscimas, Y. Maniette, W.H. Schreiner, Adv. Mater. 14 (2002) 905. [14] A.R. Roosen, W.C. Carter, Physica A 261 (1998) 232. [15] T. N¨utz, M. Haase, J. Phys. Chem. B 104 (2000) 8430. [16] C. Nayral, E. Viala, V. Colli`ere, P. Fau, F. Senocq, A. Maisonnat, B. Chaudret, Appl. Surf. Sci. 164 (2000) 219. [17] J. Arbiol, P. Gorostiza, A. Cirera, A. Cornet, J.R. Morante, Sens. Actuators B: Chem. 78 (2001) 57.
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