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J. Phys. Chem. C 2007, 111, 7256-7259
Hydrothermal Synthesis of SnO2 Nanoparticles and Their Gas-Sensing of Alcohol Hui-Chi Chiu and Chen-Sheng Yeh* Department of Chemistry, National Cheng Kung UniVersity, Tainan 701, Taiwan ReceiVed: December 21, 2006; In Final Form: March 22, 2007
A hydrothermal method was used to synthesize nanocrystalline SnO2 particles with an average particle size of 3.0 ( 0.5 nm. Thermally treated SnO2 with an average diameter of 3.3 ( 0.6 nm was obtained by the annealing of the as-synthesized SnO2, which was treated at 300 °C for 1 h under 10% H2/Ar. An atomic ratio of 2.3/1 (O/Sn) was observed for the thermally treated SnO2 compared to 1.2/1 for the as-synthesized one. However, a smaller surface area of 92 m2/g was measured for the thermally treated SnO2 as compared with 130 m2/g for the as-synthesized SnO2. Due to the occupation of the chloride ions in the oxygen sites of the as-synthesized SnO2, the thermally treated SnO2 displayed better sensing performance for ethanol when compared with as-synthesized SnO2. The sensing performance of the as-synthesized SnO2 sensor can be greatly improved by simply heating it at 350 °C for 5 min to partly remove Cl- from SnO2 nanoparticles. The thermally treated SnO2 exhibited good sensitivity to alcohol, i.e., methanol, ethanol, and propanol. The concentration detection limit can be as low as 1.7 ppm. Furthermore, the increased sensor signal was found to depend on the carbon chain number of the alcohol.
Introduction SnO2 is an n-type semiconductor with a wide band gap (Eg ) 3.6 eV at 300 K).1,2 It is well-known that gas-sensing characteristics of SnO2 can be dramatically altered by morphological and microstructural features of the sensing elements, such as particle size, shape, surface/volume ratio, and porosity. SnO2 nanostructures have been found to be promising gas sensors for detecting NO, NO2, CO, H2S, and C2H5OH.3-8 An alcohol sensor has always been in great demand in biomedical, chemical, and food industries. For example, ethanol could serve as a solvent for soluble active ingredients and has recently been found to play a crucial role as an alternative to automotive fuels. Although the significant work has studied SnO2 semiconducting thin films6 and other nanostructures, such as nanorods,7,8 nanobelts,3 for the detection of ethanol, there has been little research into using spherical-like SnO2 nanoparticles.7 The only report by Wang et al. fabricated 80-180 nm nanoparticles toward ethanol with sensing sensitivities of 4, 15, and 17 to 10, 50, and 100 ppm ethanol at a 300 °C working temperature.7 Regarding SnO2 spherical-like nanoparticle fabrication, most studies have introduced a sol-gel process involving different precursors, such as tin alkoxide and various tin salts, to prepare tin oxides.9-13 Gedanken et al. developed sonochemical methods to form nanocrystalline SnO2 with a particle size as low as ∼3 nm.14,15 The resulting SnO2 nanoparticles showed promising characteristics in reversibility, cycle life, and high capacity as Li insertion electrodes.15 A solution approach was employed by Xin et al. using SnCl2‚2H2O as starting materials heated in ethylene glycol via a reflux process.16 Recently, Fujihara et al. used a hydrothermal route to prepare mesoporous SnO2 nanocrystallines with an average particle size of ∼4 nm.17 They performed hydrolysis of SnCl4‚5H2O in deionized water under refluxing conditions to yield SnO2 precipitates, followed by a hydrothermal treatment to produce SnO2 particles. The asprepared mesoporous SnO2 nanoparticles showed good thermal * Corresponding author. E-mail:
[email protected].
stability, and the specific surface area was more than 110 m2/g after annealing. In the present work, a hydrothermal method for the preparation of the spherical-like SnO2 nanoparticles is presented. SnO2 nanoparticles with an average particle size of 3.0 ( 0.5 nm were obtained in a solution containing alcohol and water. For gas sensing, the as-synthesized SnO2 nanoparticles were exposed to 50 ppm ethanol with a sensitivity (Rair/Rgas) up to 37 at a 220 °C working temperature. Furthermore, the resultant particles exhibited good thermal stability after annealing. The thermally treated SnO2 exhibited a promising feature: good sensitivity to alcohol such as methanol, ethanol, and propanol. Experimental Section SnO2 nanoparticles were prepared using the hydrothermal method. SnCl4‚5H2O was introduced into a mixed solution of 2-propanol and distilled water with a ratio of 4:1. The solutions were adjusted to pH 12 by the addition of NaOH, and then transferred to a stainless autoclave, and heated at 150 °C for 24 h. The particle morphology and size were obtained using a field emission transmission electron microscopy (FETEM, FEIE.O Tecnai F20 G2 MAT S-TWIN at 200 kV) image. TEM measurements were prepared by adding one drop of 5 µL solution on a copper grid coating with a carbon film, and then left to dry in a vacuum. Multipurpose X-ray thin-film diffractometer (thin-film XRD) analysis was recorded on a Rigaku D/MAX2500 diffractometer using Cu KR radiation, (λ) 1.5406 Å) at 40kV and 100 mA. The specific surface area was measured by the Brunauer-Emmett-Teller method at 77K using a Micromeritics ASAP 2010 system and the pore size distribution was calculated by Barret-Joyner-Halenda (BJH) under 77 K nitrogen adsorption. The resulting SnO2 nanoparticles were used for sensing measurements. The alumina blank substrate includes two pairs of gold wire, where one pair on the backside uses as a resistive heater to control the operation temperature and the other pair
10.1021/jp0688355 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/27/2007
Hydrothermal Synthesis of SnO2 Nanoparticles
Figure 1. (a) HRTEM image of the as-synthesized SnO2 and (b) EDS analysis of the as-synthesized SnO2. Peaks of Cu and C resulting from the carbon-covered copper grid.
on the topside is connected to the sample and receives the resistance signal changing during the sensor test. The sample was spread on the alumina blank substrate (2 × 4 mm) to form thin films of the SnO2 powders. The sensors were introduced into the test chamber for the evaluation of their detection sensitivity to the alcohol. All the measurements of the gas sensing properties were carried out in air. Concentrated gas was introduced by an injection needle into the chamber homogenized by a fan installed inside the chamber. The reducing gas concentration inside the test chamber was indicated by using a commercial digital meter with a valid calibration. The experiments were recorded by measuring the electrical current of the sensor devices under a voltage of 5 V using an electrometer. Results and Discussion High-resolution transmission (HRTEM) image in Figure 1a shows the resulting nanocrystalline SnO2 using hydrothermal synthesis with a particle size of 3.0 ( 0.5 nm. The HRTEM image clearly shows a single crystalline structure with a lattice fringe of 0.334 nm corresponding to a crystal plane of (110). These small nanoparticles displayed a certain degree of aggregation. Interestingly, the energy dispersive data (EDS) shows an atomic ratio of 1:1.2 for Sn:O (Figure 1b) accompanied by the existence of a small Cl peak. The chloride ions embedded in the SnO2 nanoparticles prepared by the hydrolysis of SnCl4‚5H2O at 95 °C have been observed by Fujihara et al.17 They have demonstrated that the chloride ions can be removed by annealing SnO2 nanocrystals. As-synthesized SnO2 was treated at 300 °C for 1 h under 10%
J. Phys. Chem. C, Vol. 111, No. 20, 2007 7257
Figure 2. (a) HRTEM image of SnO2 after being thermally treated at 300 °C for 1 h under 10% H2/Ar and (b) EDS analysis of the thermally treated SnO2 nanoparticles.
H2/Ar. The thermally treated SnO2 nanoparticles retained a particle size with an average diameter of 3.3 ( 0.6 nm and remained as a single crystalline structure, as seen in Figure 2a. However, the chloride peak disappeared and the oxygen ratio rose up to 1/2.3 for Sn/O (Figure 2b). The suppression of particle growth despite high-temperature annealing indicates the thermal stability and uniformity in the particle size. Previous results reported by Kudryavtseva et al.18 and Fujihara et al.17 have shown that SnO2 particles with a narrow size distribution developed slowly in crystal growth in the course of the annealing process. It is worth mentioning that our synthesis strategy was similar to the reported hydrothermal method by Samulski et al.,19 where they prepared SnO2 nanorods in a solution containing alcohol and water (pH ∼12). Instead of obtaining sphericallike nanoparticles, one-dimensional SnO2 nanorods with ∼20 nm rod length and ∼5 nm in width were produced in their experiments. The distinct results of these two experiments may be attributed to the alcohol precursor species and the mixing ratio of alcohol to water introduced. Unfortunately, detailed experimental parameters were not specified in their report.19 X-ray diffraction (XRD) measurements were conducted to identify the overall phase compositions of the resulting SnO2 particles, as seen in Figure 3. Both diffraction peaks represented the as-synthesized (Figure 3a) and thermally treated (Figure 3b) samples and can be attributed to the tetragonal SnO2 structure. The peaks have a broad contour, which can be attributed to the small size of both SnO2 particles. Figure 4 shows the typical adsorption-desorption isothermal plot for the nitrogen adsorption-desorption of the as-synthesized and thermally treated SnO2. The as-synthesized nanoparticles displayed a mesoporosity of type IV with a distinct hysteresis
7258 J. Phys. Chem. C, Vol. 111, No. 20, 2007
Figure 3. XRD patterns of the (a) as-synthesized and (b) thermally treated SnO2.
Figure 4. Nitrogen adsorption-desorption isotherm of the (a) assynthesized and (b) thermally treated SnO2.
loop in the range of 0.4-1.0 P/Po, indicating an H2-type hysteresis. The presence of H2-type hysteresis represents the disordered and inhomogeneous distribution of pore size and indicates an interconnected network of pores. The BarrettJoyner-Halenda (BJH) analysis shows an inhomogeneity with a pore size of 4.2 nm and Brunauer-Emmett-Teller (BET) surface area of 130 m2/g. On the other hand, the thermally treated SnO2 basically followed the type I isotherm feature, corresponding to a microporous structure with pore sizes less than 2 nm, although a slight hysteresis loop was observed. The BET analysis indicated that the surface area was reduced to 92 m2/g. The resulting SnO2 nanoparticles could be potential candidates for gas-sensing devices. The sensing experiment of the obtained SnO2 nanoparticles of 25 ppm ethanol was carried out in a temperature range between 100 and 400 °C in order to determine the optimum temperature. Both as-synthesized and thermally treated SnO2 have the fastest response/recovery time, which is defined as the time needed to reach 90% of total signal change, at 220 °C. Thus, further detection was chosen at 220 °C. Figure 5 shows the response in a repetitive run involving ethanol sensing (25 ppm) at 220 °C. The sensor signal is given as the
Chiu and Yeh
Figure 5. Repetitive response of the (a) as-synthesized and (b) thermally treated SnO2 to ethanol (25 ppm) at 220 °C.
ratio of electrical resistance in air (Rair, at a relative humidity of 48%) to that in the testing gas (Rgas), Rair/Rgas. Both as-synthesized and thermally treated SnO2 sensors had a slightly decreased sensitivity after two successive runs. Interestingly, the thermally treated SnO2 displayed better sensing performance of ethanol as compared with the as-synthesized SnO2. The response/recovery time was measured as 30/148 and 18/ 44 s for the as-synthesized and thermally treated SnO2, respectively. The response/ recovery time observed in thermally treated SnO2 was consistently faster than that of as-synthesized SnO2 over air/ethanol cycles. Generally, a large BET surface area leads to good sensing properties. Comparing the BET surface area of the as-synthesized SnO2 (130 m2g-1) with the thermally treated SnO2 (92 m2g-1), the latter had less surface area but had a better gas response. This puzzle can be clarified by the mechanisms of sensing ethanol, which involves the interaction between ethanol and the coverage of the chemisorbed oxygen ions, such as O2-, O-, and O2-, on the surface of the semiconducting tin dioxides. Because electrons from sensing materials are trapped by adsorbed oxygen species, a space-charge region is formed on the surface of the metal oxides. The ethanol reacts with ionic oxygen species, and then the electrons trapped by the oxygen adsorbents are released to the metal oxide, leading to an increased conductivity of the oxides. As mentioned earlier, the thermally treated SnO2 had a higher atomic ratio (2.3/1 (O/ Sn)) than that of the as-synthesized SnO2 (1.2/1 (O/Sn)). The higher ratio of oxygen suggests that more oxygen species existed as lattice oxygen in the metal oxide or as surface-adsorbed oxygen in the thermally treated SnO2. Therefore, more electrons produced from defect reactions increased the conductivity of the thermally treated SnO2 upon exposure to ethanol. On the other hand, the existence of the chloride ions, as seen in the as-synthesized SnO2, could have occupied the oxygen sites leading to a lower oxygen atomic ratio, although the assynthesized SnO2 had a higher surface area. As mentioned earlier, annealing behavior can effectively remove Cl- from SnO2 nanoparticles. A control experiment was carried out by heating the assynthesized and thermally treated SnO2 nanomaterials at 350 °C for 5 min immediately after the ethanol sensing experiments, with results shown in Figure 5. Subsequently, both SnO2 sensors were used to test the ethanol again at 220 °C as the concentration
Hydrothermal Synthesis of SnO2 Nanoparticles
J. Phys. Chem. C, Vol. 111, No. 20, 2007 7259 TABLE 1: Sensing Performance of the Thermally Treated SnO2 Nanoparticles to Methanol, Ethanol, and Propanol as the Concentrations Varied from 1.7 to 500 ppm at 220 °C sensing gas con. 1.7 ppm 17 ppm 50 ppm 100 ppm 200 ppm 500 ppm CH3OH C2H5OH C3H7OH a
2 4 6
7 17 32
16 37 96
28 65 106
52 110 140
103 167 NAa
NA: not determined.
is yet to be answered, thermally treated SnO2 nanoparticles could have technological applications in sensing alcohols. Conclusions
Figure 6. Dynamic response of the (a) as-synthesized and (b) thermally treated SnO2 sensors at 220 °C to ethanol gas with concentration of 1.7-500 ppm. (Both as-synthesized and thermally treated SnO2 nanomaterials were heated at 350 °C for 5 min immediately after the ethanol sensing performance of Figure 5.)
varied from 1.7 to 500 ppm, as shown in Figure 6. At operating temperature of 220 °C, both materials now displayed a comparable sensitivity and ethanol concentrations as low as 1.7 ppm could be detected with a sensor signal greater than 4. As a matter of fact, the used as-synthesized SnO2 enhanced the performance by a factor of 2, with the sensor signal up to 26 for 25 ppm of ethanol as compared with the sensitivity value of 13 determined in Figure 5. The increased sensitivity of the as-synthesized sensor can be attributed to the thermal curing of the as-synthesized SnO2 sensor to partly remove Cl-, leaving more oxygen sites exposed. After heating for 5 min at 350 °C, EDS measurements showed the removal of Cl- ranging from ∼29% to ∼68% from different samples (as-synthesized SnO2). The oxygen atomic ratios were found to increase up to ∼24%. However, some results did not show an appreciable change when the amount of Cl- decreased. The short heating period (5 min) could have caused an incomplete oxidation process. The thermally treated SnO2 nanoparticles exhibited good sensing toward ethanol compared to other reported SnO2 nanoparticle-based sensing materials. Although significant work has been done to study SnO2 semiconducting thin films6 or other nanostructures, such as nanorods,7,8 nanobelts3 to detect ethanol, there has been little research into using SnO2 spherical-like nanoparticles. Recently, Wang et al. have fabricated 80-180 nm of SnO2 nanoparticles and the resulting nanoparticles exhibited sensitivities of 4, 15, and 17 to 10, 50, and 100 ppm ethanol at a 300 °C working temperature.7 Finally, the thermally treated SnO2 nanoparticles were also tested for other alcohols, i.e., methanol and propanol, as well as ethanol. Table 1 lists the sensor signal for the three alcohols as a function of concentration from 1.7 to 500 ppm. The sensing tests were all carried out at 220 °C. The thermally treated SnO2 sensor displayed good sensitivity for all three alcohols, with a detection limit as low as 1.7 ppm. Most interestingly, the sensing performance seems to be related to the number of the carbon chain, as thermally treated SnO2 consistently exhibited higher sensitivity to alcohol with longer carbon chains across the testing concentration. While the detailed reasons for the dependence on the number of the carbon chain
Nanocrystalline SnO2 particles with a size of around ∼3 nm have been fabricated using a hydrothermal process. Due to the existence of Cl-, the as-synthesized and thermally treated SnO2 exhibited different atomic ratio of O/Sn. The different atomic ratio is related to the different number of the oxygen species in SnO2 and strongly affects the interactions between the targeting gas and oxygen species. It was found that used as-synthesized SnO2 sensors can improve their sensing performance by being heated at 350 °C for 5 min to partly remove Cl-. The thermally treated SnO2 was as an effective gas sensor for alcohol and the detection limit can be as low as 1.7 ppm. The present SnO2 offers a good subject for practical applications as an alcohol sensor. Acknowledgment. We would like to thank the National Science Council of Taiwan for financially supporting this work. References and Notes (1) Shukla, S.; Ludwig, L.; Parrish, C.; Seal, S. Sens. Actuators, B 2005, 104, 223. (2) Hu, J.; Bando, Y.; Liu, Q.; Golberg, D. AdV. Funct. Mater. 2003, 13, 493. (3) Comini, E.; Faglia, G.; Sberveglieri, G.; Pan, Z.; Wang, Z. L. Appl. Phys. Lett. 2002, 81, 1869. (4) Chowdhuri, A.; Gupta, V.; Sreenivas, K.; Kumar, R.; Mozumdar, S.; Patanjali, P. K. Appl. Phys. Lett. 2004, 84, 1180. (5) Wang, Y.; Jiang, X.; Xia, Y. J. Am. Chem. Soc. 2003, 125, 16176. (6) Liu, Y.; Koep, E.; Liu, M. Chem. Mater. 2005, 17, 3997. (7) Chen, Y. J.; Nie, L.; Xue, X. Y.; Wang, Y. G.; Wang, T. H. Appl. Phys. Lett. 2006, 88, 083105. (8) Chen, Y. J.; Xue, X. Y.; Wang, Y. G.; Wang, T. H. Appl. Phys. Lett. 2005, 87, 233503. (9) Cabot, A.; Dieguez, A.; Romano-Rodriguez, A.; Morate, J. R.; Barsan, N. Sens. Actuators, B 2001, 79, 98. (10) Rumyantseva, M. N.; Gaskov, A. M.; Rosman, N.; Pagnier, T.; Morante, J. R. Chem. Mater. 2005, 17, 893. (11) Bose, A. C.; Kalpana, D.; Thangadurai, P.; Ramasmy, S. J. Power Sources 2002, 107, 138. (12) Santos, L. R. B.; Chartier, T.; Pagnoux, C.; Baumard, J. F.; Santillii, C. V.; Pulcineli, S. H.; Larbot, A. J. Eur. Ceram. Soc. 2004, 24, 3713. (13) Briois, V.; Belin, S.; Zucolotto Chalaca, M.; Santos, R. H. A.; Antilli, C. V. S.; Pulcinelli, S. H. Chem. Mater. 2004, 16, 3885. (14) Pang, G.; Chen, S.; Koltypin, Y.; Zaban, A.; Feng, S.; Gedanken, A. Nano Lett. 2001, 1, 723. (15) Zhu, J.; Lu, Z.; Aruna, S. T.; Aurbach, D.; Gedanken, A. Chem. Mater. 2000, 12, 2557. (16) Jiang, L.; Sun, G.; Zhou, Z.; Sun, S.; Wang, Q.; Yan, S.; Li, H.; Tian, J.; Guo, J.; Zhou, B.; Xin, Q. J. Phys. Chem. B 2005, 109, 8774. (17) Fujihara, S.; Maeda, T.; Ohgi, H.; Hosono, E.; Imai, H.; Kim, S-H. Langmuir 2004, 20, 6476. (18) Kudryavtseva, S. M.; Vertegel, A. A.; Kalinin, S. V.; Oleynikov, N. N.; Ryabova, L. I.; Meshkov, L. L. S.; Nesterenko, N.; Rumyantseva, M. N.; Gaskov, A. M. J. Mater. Chem. 1997, 7, 2269. (19) Cheng, B.; Russell, J. M.; Shi, W.; Zang, L.; Samulski, T. E. J. Am. Chem. Soc. 2004, 126, 5972.
