Science in China Series G: Physics, Mechanics & Astronomy © 2009

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One-pot synthesis of crystalline SnO2 nanoparticles and their low-temperature ethanol sensing characteristics CHEN YuJin1,2,3†, ZHU ChunLing4, WANG LiJiao3 & WANG TaiHong2 1

School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China; Micro-Nano Technologies Research Center, Hunan University, Changsha 410082, China; 3 College of Science, Harbin Engineering University, Harbin 150001, China; 4 College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China 2

Crystalline SnO2 nanoparticles (NPs) with a diameter less than 6 nm are synthesized using potassium stannate trihydrate as the precursor in a basic system. The synthesized NPs can detect ethanol at a ppm level even at 100℃. Furthermore, the NPs have good selectivity to ethanol. The excellent ethanol sensing performances are attributed to the small size effect according to the space-charge model. SnO2, nanoparticles, sensing characteristics

Chemical sensors have played very important roles in the detection of pollutant, toxic, and industrially important gas species such as NOx, NH3, COx, H2, and ethanol. Because of their high sensitivity to the target gases and simplicity in fabrication, metal-oxide semiconductors (MOS) such as In2O3, ZnO, and SnO2 have been widely used as sensing materials. Among them, SnO2 materials have attracted much attention due to their high sensitivities and thermal stability. However, the chemical sensors with good performance for target gas species not only have high sensitivity and good selectivity, but also can work at low temperature. Generally, the size of materials has strong effect on their chemical and physical properties, and improved or novel properties have been observed in the system with the size close to or smaller than Debye size. Therefore, it is necessary to fabricate SnO2 nanostructures with small sizes by facile methods. Nowadays, many techniques have been developed to decrease the size of SnO2, and improve their sensing － performance[1 17]. Pang et al. synthesized SnO2 nanopar-

ticles (NPs) adsorbed onto the surface of SrCO3 NPs by a sonochemical method[4]. The diameter of SnO2 NPs still keeps at ~3.5 nm even after calcaion at 600℃ for 2 h. Nayral et al.[2] proposed a novel mechanism, combing decomposition of an organometallic precursor and controlled surface hydrolysis, as well as an oxidation process, to obtain SnO2 NPs. The methods above could reduce the size of SnO2 NPs, but the fabricating processes are relatively complicated. Recently, Pinna et al. developed a nonaqueous process to fabricate SnO2 NPs[14]. The NPs with an average diameter of 2－2.5 nm could detect CO at ppm level. However, the NP sensors work at much higher temperature (at 400℃). Hydrothermal routes have been successfully employed to grow small SnO2 nanostructures. For instance, Zhang et al.[7] synthesized SnO2 nanorods (NRs) with diameters of 8－15 nm by this approach. Recently, single crystalline SnO2 NRs with diameters of ~3.4 nm were fabricated by Cheng et al.[10]. More recently, we grew SnO2 NRs with diameters

Received February 23, 2009; accepted May 5, 2009; published online August 10, 2009 doi: 10.1007/s11433-009-0193-z † Corresponding author (email: [email protected]) Supported by the National Natural Science Foundation of China (Grant No. 50772025), the Natural Science Foundation of Heilongjiang Province, China (Grant No. F200828), the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20070217002), the China Postdoctoral Science Foundation (Grant Nos. 20060400042 and 200801044), and the Innovation Foundation of Harbin City (Grant No. RC2006QN017016)

Citation: Chen Y J, Zhu C L, Wang L J, et al. One-pot synthesis of crystalline SnO2 nanoparticles and their low-temperature ethanol sensing characteristics. Sci China Ser G, 2009, 52(10): 1601-1605, doi: 10.1007/s11433-009-0193-z

of 3－15 nm in a basic water-alcohol system. The NRs exhibit enhanced ethanol sensing characteristics at 300℃[12,13]. Chiu et al.[15] fabricated SnO2 NPs with diameters of (3.0±0.5) nm by the hydrothermal route. After thermal treatment at 300℃ for 1h under 10% H2/Ar, the diameter only increase to (3.3±0.6) nm. The obtained NPs could be used to detect 1.7 ppm ethanol at a working temperature down to 220℃. The previous study demonstrates that the hydrothermal route might be a very efficient way to fabricate small SnO2 with good sensing performance. Herein, we report one-pot synthesis of SnO2 NPs with diameters of 3.0－6.0 nm based on the hydrothermal method. The synthesis is conducted in a water-alcohol system, where potassium stannate is used as the precursor[18,19]. The sensors based on SnO2 NPs not only exhibit high sensitivity and good selectivity to ethanol, but also can work at a temperature of 100℃.

is defined as S = Ra/Rg, where Ra is the sensor resistance in air, and Rg is the resistance in target-air mixed gas.

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Results and discussion

Figure 1 shows XRD pattern of as-synthesized SnO2 NPs. All the indicating peaks can be indexed to tetragonal rutile structure of SnO2, which agrees well with the reported values from JCPDS card No. 41-1445 (space group: P42/mnm, a = 0.4738 nm and c = 0.3187 nm). No peaks corresponding to other impurity are observed in the pattern, indicating high purity of the NPs. Compared with those of the bulk counterpart, the peaks are relatively broadened, which indicates that the NPs have very small crystal sizes.

1 Experiment For hydrothermal synthesis of SnO2 NPs, 0.75 g of urea and 0.115 g of potassium stannate trihydrate (K2SnO3· 3H2O, 95%) are added to 32 ml of water-alcohol (38 vol% alcohol) solution. After stirring for 5－10 min, the formed suspension is transferred into 50 ml Teflon-lined stainless-steel autoclave heated to 170℃ for 36 h. After cooling to room temperature, the obtained precipitates are washed several times with absolute ethanol and distilled water. Products are characterized with X-ray powder diffraction (XRD, D/MAX2500, Cu Kα radiation), scanning electron microscopy (SEM, JEOL JSM6700F), transmission electron microscopy (TEM, JEOL 2010). The fabrication process of the sensors based on the － products has been described elsewhere[12,13,20 22]. The sensitivity (S) of the sensors to target gases in this paper

Figure 1 XRD pattern of the as-synthesized SnO2 NPs. The broadened peaks reveal that the NPs have very small size.

Figure 2(a) is a typical SEM image of as-synthesized SnO2 NPs. The size of SnO2 can not be exactly determined by the image because they are very small, but it is still found that they are of particle-like morphology. The morphology and size of the NPs are further characterized by transmission electron microscope. Figure 2(b)

Figure 2 (a) A typical SEM image; (b) TEM image; (c) HRTEM image of the as-synthesized SnO2 NPs. 1602

Chen Y J et al. Sci China Ser G-Phys Mech Astron | Oct. 2009 | vol. 52 | no. 10 | 1601-1605

displays a typical TEM image of the NPs. It is clearly observed that SnO2 NPs have uniform size, and their diameters are in range of 3.0－6.0 nm, which agrees well with the measurement result of XRD. High-resolution TEM (HRTEM) reveals the fine structure of the NPs, as shown in Figure 2(c). The clear lattice fringes in the HRTEM image reveal the single crystal nature of the SnO2 NPs. The indexed lattice spacing is about 0.334 nm, corresponding to a crystal plane of the (110) plane of SnO2. All the above results demonstrate that crystalline SnO2 NPs have uniform and small size, and are potential candidates for gas-sensing devices. The sensing characteristics of the obtained SnO2 NPs of ethanol are carried out in the temperature range of between 80 and 300℃ in order to determine the optimum temperature. Figure 3(a) shows sensor response under 10－500 ppm ethanol exposure at a working temperature of 300℃. The response time defines the time taken for the sensor to reach the saturation value after the NPs are exposed to ethanol vapor, and the recovery time for recovery of the resistance to 98% of the initial level after removal of ethanol vapor. It is found that both the response time and the recovery time of the NPs are less than 30 s. It is convenient when the sensors are required to continuously detect the target gases. Therefore, the NP sensors with a very short recovery time are more promising for further applications. The sensitivities are 2.8－19.4 to 10－500 ppm of ethanol, respectively, as shown in the left inset in Figure 3(a). The results are comparable to those of small-sized 1D SnO2 nanostructures[21,24], which reveals that the NPs fabricated in this work exhibit a very high sensitivity to ethanol gas. Interestingly, the sensitivities of the SnO2 NPs to ethanol in the whole tested concentration ranges are increased sharply with the decrease of the working temperature, as shown in Figure 3(b). As the working temperature is reduced to 220℃, the sensitivity for 500 ppm ethanol is increased to 69.0. The phenomenon is further observed at lower temperature. For example, the sensitivity could be up to 94.4 at 100℃, and exhibits enhanced performance by a factor of ~4.9 compared with the results at 300℃. The results above suggest that the SnO2 NPs may work at a temperature below 100℃. Unfortunately, the sensor signal is not detected at temperature lower than 100℃, i.e. 80℃ because of too high resistance of the NPs at the

Figure 3 (a) Sensor responses to ethanol vapor at 220℃; (b) comparison of ethanol sensitivities at different temperatures; (c) the sensitivities under 100 ppm ethanol exposure over three repeated cycles at 300℃.

temperature. Figure 3(c) shows the sensitivity of the nanoparticles to 100 ppm ethanol over three repeated cycles at 300℃. Small deviation of the value of S reveals a good reproducibility of the sensor based on the SnO2 NPs. In addition, the sensing properties of the nanoparticles are measured several weeks after the previous measurement. The fluctuation of the sensitivity is less than ±3%, which indicates that the sensing properties of the NPs are stable.

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Compared with our previous reports on ethanol sensing characteristics,[12,13,20] the NPs in the present work exhibit significantly improved sensing performances, as shown in Table 1. Such enhanced performances are evident of two aspects. Firstly, the sensitivities of the NPs are comparable or improved. For example, the sensitivity of 4－15 nm NRs to 100 ppm ethanol is 13.9[12], while it is increased to 30.5 for the NPs in the present work; the sensitivity of 2－8 nm NPs loaded on carbon nanotubes to 500 ppm ethanol is 41.2[20], while it is up to 94.4 for the NPs in this work. Secondly, the working temperature is significantly decreased to 100℃. The low working temperature allows the NPs to be fabricated to the sensors with low power consumption. Table 1 Comparisons of ethanol sensing performances of the NPs in this work with other different SnO2 nanostructures (Note: ref. [12], NRs with a diameter of 4－15 nm; ref. [13], NRs with a diameter of 3－12 nm, NPs loaded on the surface of carbon nanotubes with a diameter of 2－8 nm) Working C (ppm) 10 100 300 500 temperature (℃) S in ref. [12] 4.2 13.9 31.4 300 S in ref. [13] 30.7 83.8 300 S in ref. [20] 11.1 42.2 350 S in this work 10.2 30.5 52.6 94.4 100

Such good sensing performances of the NPs are attributed to their small sizes. For n-type MOS, spacecharge layer (its thickness, Ld) will always be formed on their surfaces because electrons are trapped by adsorbed oxygen species. When MOS is under a reducing molecule exposure, such as ethanol, part of the trapped electrons will be released back to the bulk, leading to an increase of conductance of MOS. If the diameter of MOS is close to or smaller than 2Ld, its sensitivity will be ex－ ponentially improved[12,13,20 22]. In this work, the diameter of SnO2 NPs blows 6 nm. Therefore, the NPs should exhibit very high sensitivity to ethanol gas according to the space-charge model. As for the reduction of the operating temperature, the reasons are also related to the small size effect. As the grain size is reduced to a scale comparable to the space charge length, the surface activities will be increased greatly. In this case, the operating temperature will also decrease remarkably. It was reported that the operating temperature decreased from 330 to 250℃ due to reduced size of tin oxide NRs[23]. The SnO2 NPs with 3.6 nm could detect ethanol vapor at a optimum working temperature of 220℃[15]. This phenomenon is also observed in other MOS sensing materi1604

als. For example, 4 nm WO2.75 NRs were found to be able to can detect 100 ppm NH3 at room temperature[24]. Recently, we successfully fabricate ZnO NRs with a diameter of less than 15 nm[22]. The working temperature of NRs is decreased from 300℃ to 140℃. Thus, the reduction of the size of sensing materials is a very efficient way for gas sensors with low operating temperature. Gas sensors for practical applications not only require high sensitivities and low working temperature, but also very good selectivity to target molecules. Here we choose H2 and CH4 for determining the selectivity of the SnO2 NPs to ethanol. Figure 3 shows the sensor sensitivities under 50－500 ppm H2 exposure at different working temperature. The sensitivity to 500 ppm at 300℃ is only 10.8, 8.7 times lower than that of the NPs to 500 ppm ethanol at 100℃. Moreover, with a decrease in working temperature, the sensitivity to H2 displays an inverse change compared with ethanol, that is, it is also decreased, as shown in Figure 4. The sensitivities are reduced to 3.7 and 2.8 at 220℃ and 100℃, respectively. As the NP sensors exposure to 10－1000 ppm CH4, any signal is not observed, which reveals that they have no response to CH4 with concentration in the tested range.

Figure 4 Hydrogen sensing characteristics of the sensors at different temperatures.

3 Conclusion In summary, crystalline SnO2 NPs with a diameter less than 6 nm are successfully synthesized. They show excellent ethanol sensing performances at relatively low working temperature. Our results demonstrate that the NPs are very promising materials for fabricating ethanol sensors.

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