Mesoporous tin dioxide nanopowders based sensors to selectively detect ethanol vapor

Mesoporous tin dioxide nanopowders based sensors to selectively detect ethanol vapor

Materials Science and Engineering C 31 (2011) 1369–1373 Contents lists available at ScienceDirect Materials Science and Engineering C j o u r n a l ...

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Materials Science and Engineering C 31 (2011) 1369–1373

Contents lists available at ScienceDirect

Materials Science and Engineering C j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m s e c

Mesoporous tin dioxide nanopowders based sensors to selectively detect ethanol vapor Xianzhi Guo, Yanfei Kang, Liwei Wang, Xianghong Liu, Jun Zhang, Taili Yang, Shihua Wu, Shurong Wang ⁎ Department of Chemistry, Nankai University, Tianjin 300071, China

a r t i c l e

i n f o

Article history: Received 27 May 2010 Received in revised form 30 December 2010 Accepted 4 May 2011 Available online 10 May 2011 Keywords: Mesoporous SnO2 nanopowders Synthesis Ethanol Gas sensor Selectivity

a b s t r a c t In the paper, mesoporous SnO2 nanopowders were synthesized via a simple and mild SnCl4 hydrolysis process using cationic surfactant (cetyltrime thylammonium bromide, CTAB: CH3(CH2)15N+(CH3)3Br −) as structure directing agent and ammonia as an alkali source at room temperature, combined with a subsequent calcination process. The products were characterized by X-ray diffraction analysis (XRD), thermogravimetric analysis (TGA), transmission electron microscopy (TEM) and nitrogen adsorption–desorption experiment. A gas sensor was fabricated from the as-prepared mesoporous SnO2 nanopowders and used to test the response to different concentrations of ethanol, methanol, hexane, NH3, H2 and CO at different operating temperatures. The results showed that the mesoporous SnO2 sensor exhibited high sensitivity, good selectivity and quick response–recovery characteristics to ethanol, implying the potential application of the sensor for detecting ethanol. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Tin dioxide (SnO2) is a wide-energy-gap n-type semiconductor with a bandwidth of 3.6 eV and has been extensively studied for various applications, including gas sensors [1,2], catalyst support [3], transparent conducting electrodes [4] and Li-ion battery anode materials [5,6]. As one of the most widely used semiconductor oxide gas sensors, SnO2 shows high sensitivity to many reducing gases, such as H2, CO, and alcohol [7]. Studies have proved that the properties and performances of SnO2 based sensors can be dramatically influenced by structural features. Gas sensor with a porous structure can contribute to improve gas sensing performances because of enhanced active surface area and efficient gas diffusion induced by this unique structure [8–12]. Therefore, considerable investigations have been focused on the synthesis of nanostructured SnO2 materials, such as nanowires [13,14], nanotubes [15], nanoribbons or nanobelts [16,17], macroporous films [8], mesoporous SnO2 [18–24], and hollow spheres [25]. Among these nanostructures, the mesoporous SnO2 materials have been paid great attention and several synthetic approaches, employing the supermolecular assembly of surfactant molecules as templates, have been developed to meet the ever-increasing demand in gas sensors [18–24]. For example, Hyodo et al. [18]] prepared mesoporous structure SnO2 powders by utilizing the self-assembly of a cationic surfactant

⁎ Corresponding author. Tel.: + 86 22 23505896; fax: + 86 22 23502458. E-mail address: [email protected] (S. Wang). 0928-4931/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2011.05.002

and investigated their gas-sensing properties to H2 and NOx. Wagner et al. [19] studied the sensing properties of mesoporous SnO2 to CO. Hayashi et al. [20,21] prepared mesoporous SnO2 powders by using Na2SnO3•3H2O or SnCl4•5H2O as a tin source and n-cetylpyridinium chloride monohydrate (C16PyCl: (C5H5NC16H33)Cl·H2O) or aerosol-OT (AOT: C20H37O7SNa) as a template, and investigated H2 sensing properties at 350 °C. Wang et al. [22,23] synthesized mesostructured SnO2 using CTAB as the template and SnCl4•5H2O as the inorganic precursor in acidic conditions and investigated ethanol and H2 gas sensing. Wang et al. [24] synthesized chlorine gas sensor based on mesoporous SnO2 through a hydrothermal process using tin chloride as a raw material, urea as a pore-forming agent and pH regulator. Nowadays, alcohol sensors have been in great demands for applications including food industry, breath analysis and environmental monitoring. High sensitivity, short response–recovery time and good selectivity are equally important to detect ethanol in the practical application. However, there were seldom works to be reported about an overall investigation on the mesoporous SnO2 based sensors for ethanol vapor. Therefore, in the present paper, the mesoporous SnO2 nanopowders were synthesized using a simpler and milder method compared to the previous reported works. Meanwhile, we also systematically examined the sensing performance of sensor to ethanol, including sensitivity, response–recovery characteristic and selectivity, as well as the temperature-dependent and concentration-dependent behaviors of the mesoporous SnO2 based sensors to ethanol. Moreover, a comparative study between as-prepared mesoporous SnO2 nanopowders and commercial SnO2 powders was also carried out to emphasize the favorable sensing performance.

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2. Experimental

A + Vc

2.1. Synthesis of mesoporous SnO2 nanopowders

2.2. Fabrication and analysis of gas sensor

Vh

sensor



Vout RL

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Δm / m0 (%)

Mesoporous SnO2 nanopowders were prepared via a simple and mild SnCl4 hydrolysis process using cationic surfactant (cetyltrime thylammonium bromide, CTAB: CH3(CH2)15N+(CH3)3Br −) as structure directing agent and ammonia as an alkali source under an acidic condition at room temperature, combined with subsequent calcination. In a typical synthesis, 1.6 g of CTAB was mixed with 100 ml of distilled deionized water to form a homogenous solution, and 5 g of SnCl4•5H2O dissolved with 100 ml of distilled deionized water was introduced into the above solution, followed by the addition of 1 mol/l of ammonia under stirring till the pH value of the mixture was adjusted to 10 to promote the hydrolysis process. After stirring for about 3 h, the sol was aged for 6 days. The resulted white slurry was centrifuged and washed by distilled water, and then dried at room temperature under vacuum to gain the white powders, doted as SnCTAB. Finally, mesoporous SnO2 nanopowders were obtained by calcining the SnCTAB precursor in a tube furnace at 300 and 400 °C for 2 h, respectively, and doted as Sn-300 and Sn-400. For the intention of comparison, the commercial SnO2 is selected, and doted as Sn-C.

10 -30.25% 15 20 25

The gas sensor was fabricated as follows. A proper amount of SnO2 nanopowder was mixed with several drops of water to form slurry. Then, the slurry was coated onto an alumina tube with a diameter of 1 mm and length of 4 mm, positioned with two Au electrodes and four Pt wires on each end of the tube. A Ni–Cr alloy filament was put through the tube and used as a heater by tuning the heating voltage. Gas sensing tests were performed on a static test system (HW-30A, HanWei Electronics Co., Ltd., Henan Province, China) using air as the reference and diluting gas at a relative humidity (RH) of 60%. The sensor was placed in a transparent testing chamber with a volume of 15 l and aged for several days before analysis. Target gas such as ethanol was injected into the testing chamber by a microsyringe. The sensor signal voltage (Vout) was collected by a computer at a constant test voltage of 5 V. The scheme of working principle of the gas sensing measurement system is shown in Fig. 1A. The sensor signal is defined as the ratio S = Ra/Rg, where Ra and Rg are the electrical resistance of the sensor in air and in test gas, respectively. 2.3. Characterization The products were characterized by thermogravimetric analysis (TGA, ZRY-2P, 10 °C/min), X-ray diffraction analysis (XRD, Rigaku D/max-2500, graphite monochromator, CuKα, λ = 0.15418 nm), transmission electron microscope (TEM, Philips FEI Tecnai 20ST, 200 kV), and nitrogen adsorption–desorption experiment (Quantachrome NOVA 2000e sorption analyzer). 3. Results and discussion Fig. 1B reveals the TGA curve of SnCTAB sample. It can be observed from the figure that the SnCTAB precursor undergoes a three-stage weight loss process from room temperature to 500 °C, with about 30.25% total weight loss. The first stage from room temperature to 100 °C results from the release of small amount of absorbed and crystal water in the sample. The second stage from 160 to 320 °C is attributed to the decomposition of the surfactant CTAB. The third stage from 375 to 500 °C is due to the removal of remnant carbon and other residual in the sample. The small-angle and wide-angle XRD patterns of the SnCTAB precursor, heat-treated products and commercial SnO2 are shown in Fig. 2. As small-angle XRD patterns shown (Fig. 2A), the patterns of

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T (oC) Fig. 1. Scheme of working principle of the gas sensing measurement system (Vh: heating voltage; Vc: circuit voltage; Vout: output signal voltage and RL: load resistor) (A) and TGA curve of SnCTAB (B).

both the SnCTAB and the Sn-300 contain the similar small angle peak characteristic (2θ ≈ 1.8°) of ordered mesostructured materials [26,27], whereas the Sn-400 and the Sn–C do not exhibit a small angle reflection. The disappearance of the small angle peak in the pattern of the Sn-400 indicates that the removal of the surfactant results in the structural collapse and the destruction of ordered mesoporous structure [27]. It can be seen from the wide-angle XRD patterns (Fig. 2B) that all the samples are well crystallized and all diffraction peaks can be well indexed to the tetragonal rutile structure of SnO2 (JCPDS 41–1445). On the basis of the (110) line widths, in the order of SnCTAB, Sn-300, Sn-400 and Sn–C, the average crystallite size is calculated to be 4.2, 4.6, 5.8 and 12.1, respectively, using Scherrer equation. It can be found that the Sn–C possesses much larger particle size than the as-prepared mesoporous SnO2. The particle sizes of the Sn-300 and Sn-400 do not come into being marked increase compared to that of the uncalcined SnCTAB precursor, which indicates that the addition of CTAB restrains the growth and coagulation of SnO2 crystal particles [27], therefore, the CTAB plays an important role in the preparation of SnO2 nanoparticles. TEM image of the mesostructured SnO2 calcined at 400 °C is presented in Fig. 3. The as-synthesized material is clearly mesostructured and displays a characteristic of a disordered wormhole-like topology, which is in good agreement with the result of the smallangle X-ray diffraction pattern. N2 adsorption–desorption isotherms and pore size distributions of the Sn-300 and Sn-400 are presented in Fig. 4. No hysteresis is observed in the isotherm of Sn-300, indicating that the pores are relatively free of constrictions and of uniform diameter [28]. The Sn-300 has a BET surface area of 347 m 2/g, with an average pore diameter of 2.0 nm and narrow pore distribution, demonstrating that the sample has very ordered mesoporous channels [27]. The isotherm of Sn-400 reveals a strong

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2Theta(deg.) Fig. 2. Small-angle (A) and wide-angle (B) XRD patterns of SnCTAB (a), Sn-300 (b), Sn-400 (c), and Sn-C (d).

hysteresis loop in the P/Po region from 0.4 to 0.9, which is related to the formation of bottleneck in the channels, with the ordered mesostructured framework collapse in the process of calcination [27–29], and the BET surface area is lowered to 108 m 2/g. Compared with the mesoporous SnO2 prepared in acidic conditions [22], the ones prepared in alkaline conditions possess slight lower BET surface area, but it is

Fig. 3. TEM image of the mesostructured SnO2 calcined at 400 °C.

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Pore diameter (nm) Fig. 4. N2 adsorption–desorption isotherms (A) and pore size distributions (B) of asprepared mesoporous SnO2.

much higher than the commercial SnO2 powders do (38 m2/g). In addition, the sample exhibits an average pore diameter of 3.8 nm after calcined at 400 °C and a broad pore size distribution. This suggests that as the calcination temperature increases, the number of pores decreases as a result of sintering while at the same time the pore size increases, therefore, the surface area decreases with the increase of the calcination temperature [27]. Gas-sensing experiments were performed at different temperatures in order to determine the optimum operating temperature for ethanol detection, and shown in Fig. 5. It is obvious that the operating temperature has a great influence on the response of SnO2 sensor to 0.1 vol.% ethanol in air. As shown in Fig. 5, the response of as-prepared SnO2 sensors to ethanol increases with the increase of the operating temperature and attains a maximum value at 300 °C, followed by a decrease with a further increase of the operating temperature. At the optimum operating temperature of 300 °C, the sensor signal of Sn-300 and Sn-400 to 0.1 vol.% ethanol is 70.3 and 100.4, respectively. According to the XRD results (Fig. 1), the Sn-300 sample possesses smaller crystal size and more ordered mesoporous structure than the Sn-400 sample, which is advantageous to the adsorption of the ethanol vapor. However, as shown in the TG curve (Fig. 2), there is small amount of undecomposed CTAB and other residuals, which is disadvantageous to the adsorption of ethanol vapor and results in the descend in the response to ethanol. Compared with as-prepared SnO2 sensors, commercial SnO2 sensor exhibits a much lower response at 300 °C, and it can reach the highest sensor signal (54.3) to ethanol at higher operating temperature of 350 °C, which can be related to the lower specific surface and larger crystal size of commercial SnO2.

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Fig. 6. Sensitivities of the Sn-400 sensor to 0.1 vol.% various gases at different operating temperatures (A) and comparison of sensor signal at 300 °C (B).

adsorption promotes the sensing reaction between ethanol molecules and oxygen species. On the other hand, the mesoporous structure of the SnO2 powder is responsible for the high specific surface area. The inner surface of mesoporous structure can contribute extra active sites for ethanol detection but might not be effectively utilized for H2 or CO detection, which results in much higher sensing properties to ethanol vapor ethanol than to other gases [33]. The response–recovery characteristic is equally important to evaluate the overall performance of a sensor. Fig. 8 illustrates the typical response–recovery characteristics of the as-prepared mesoporous SnO2 based sensor to 0.1 vol.% ethanol vapor at different operating 160 140

Sensitivity (Ra/Rg)

As has been reported, SnO2 is a typical n-type semiconductor, and its gas-sensing mechanism belongs to the surface-controlled type, and the change of resistance is dependent on the species and the amount of chemisorbed oxygen on the surface [30]. When the sensor is in air, the surface of SnO2 is covered by plenty of oxygen adsorbates, such as O 2−, O −, and O2− [31]. The formation of the oxygen adsorbate layer leads to a decrease in the electron density on the sensor surface due to the transfer of electrons from the sensor surface to the adsorbate layer. When the sensor is exposed to ethanol vapor, the ethanol gas reacts with the oxygen ions on the surface, which results in the release of free electrons to the sensor. This leads to the change in resistance of the SnO2 sensor. As a typical surface-sensitive material, the amount of oxygen and test gas on the surface of SnO2 is strongly dependent on the microstructure of the SnO2, namely, the specific area, particle size, and the porosity. Therefore, the as-prepared mesoporous SnO2 sensors with a high surface area and porosity show high response to ethanol. For practical use, the selectivity of the sensor is a necessary consideration. Hence, we also examined the response of the Sn-400 based sensor to other gases including methanol, hexane, H2, NH3, and CO. The results are presented in Fig. 6. It is clear that the sensor exhibits without exception the highest response to ethanol at different operating temperatures from 160 to 350 °C. Specially at 300 °C, the response to ethanol is significantly higher than to methanol, hexane, NH3, H2 and CO, with the selectivity of S0.1vol.% ethanol/S0.1vol.% methanol = 16.0, S0.1vol.% ethanol/S0.1vol.% hexane = 17.0, S0.1vol.% ethanol/S0.1vol.% NH3 = 33.2, S0.1vol.% ethanol/S0.1vol.% H2 = 43.5, and S0.1vol.% ethanol/S0.1vol.% CO = 98.0, which implies the good selectivity of the sensor to ethanol. Furthermore, Fig. 7 shows the response of the Sn-400 sensor to various gases of 0.05–0.3 vol.% at 300 °C. It can be obviously seen that in the full concentration range from 0.05 to 0.3 vol.%, the Sn-400 sensor represents considerable higher response to ethanol than to other tested gases. Therefore, according to the experimental results, the as-prepared mesoporous SnO2 sensor can selectively detect ethanol gas with the interference of other gases. It is well accepted that the high specific surface area can enhance the sensor sensitivity, for it can supply a large amount of surfaceactive sites for both oxygen adsorption and surface reaction, leading to large changes of the surface properties of the sensors [32]. Above results have shown that the mesoporous SnO2 sensor has much higher response to ethanol than to methanol, hexane, NH3, H2 and CO under the same test condition. This may be related to the microstructure of the mesoporous SnO2, which results in the selectively adsorption of different gases. The mesoporous structure of SnO2 is propitious to the adsorption of ethanol molecules, suggesting that the enhanced

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X. Guo et al. / Materials Science and Engineering C 31 (2011) 1369–1373

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Time (s) Fig. 8. Dynamic response–recovery curves of gas sensor based on Sn-400 to 0.1 vol.% ethanol at different operating temperatures.

temperatures from 160 to 350 °C. It can be observed from the figure that the sensor shows very fast response and recovery characteristics at the operating temperature from 160 to 350 °C. Fig. 9 shows the typical response–recovery characteristics of the as-prepared mesoporous SnO2 sensor to ethanol vapor with the concentrations of 0.05, 0.1, 0.15, 0.2, 0.25 and 0.3 vol.%. It can be seen that the sensor shows very fast response and recovery characteristics and the response of the sensor increases with the increase of the ethanol vapor concentration. The response and recovery times (time for 90% of total sensor signal change) are about 20–36 and 54–90 s, respectively, which is short enough for practical use. 4. Conclusion In summary, mesoporous SnO2 nanopowders were prepared via a simple SnCl4 hydrolysis process using cationic surfactant (cetyltrime thylammonium bromide, CTAB: CH3(CH2)15N+(CH3)3Br −) as structure directing agent and ammonia as an alkali source at room temperature, followed by a calcination process. The products were characterized by X-ray diffraction analysis (XRD), thermogravimetric analysis (TGA), and nitrogen adsorption–desorption. The XRD results indicate that all the samples are well crystallized, and both the SnO2 containing CTAB and calcined at 300 °C exhibit ordered mesoporous structures, with a mean particle size of 4.2 and 4.6 nm, respectively. Although the further calcination at 400 °C results in the destruction of ordered mesoporous structure and the decrease of the surface area, it does not bring a marked

increase of mean particle size (5.8 nm), which indicates the addition of CTAB restrains the growth and coagulation of SnO2 crystal particle, therefore, the CTAB plays an important role in the preparation of SnO2 nanoparticles. Gas-sensing experiment results show that, at the test temperature of 300 °C, the as-prepared SnO2 sensors possess more higher response to 0.1 vol.% ethanol than to commercial SnO2 sensor (50.2), and the SnO2 sensor calcined at 400 °C shows higher response (100.4) to ethanol than to the one calcined at 300 °C (70.3). Meanwhile, the as-prepared mesoporous SnO2 sensor exhibits without exception the higher response to ethanol at different operating temperatures from 160 to 350 °C than to other gases including methanol, hexane, NH3, H2 and CO. Specially at 300 °C, the response to ethanol is significantly higher than to methanol, hexane, NH3, H2 and CO, and in the full concentration range from 0.05 vol.% to 0.3 vol.%, the sensor all represents considerable higher response to ethanol than to other tested gases. This implies the good selectivity of the sensor to ethanol. The sensor shows very fast response and recovery characteristics at the operating temperature from 160 to 350 °C and the ethanol concentrations from 0.05 vol.% to 0.3 vol.%, suggesting the potential application of the as-prepared mesoporous SnO2 sensor for detecting ethanol. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 20871071) and the Applied Basic Research Programs of Science and Technology Commission Foundation of Tianjin (09JCYBJC03600 and 10JCYBJC03900). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

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