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SnO2 synthesized by a nanocasting route for high hydrogen sensing performance
Sensors and Actuators B 160 (2011) 604–608
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Ordered mesoporous Pd/SnO2 synthesized by a nanocasting route for high hydrogen sensing performance Jing Zhao, Weinan Wang, Yinping Liu, Jinming Ma, Xiaowei Li, Yu Du ∗ , Geyu Lu ∗ State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, China
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Article history: Received 28 April 2011 Received in revised form 15 August 2011 Accepted 16 August 2011 Available online 24 August 2011 Keywords: Tin dioxide Pd Nanocasting Mesoporous Gas sensor
a b s t r a c t Ordered mesoporous SnO2 and mesoporous Pd/SnO2 have been successfully synthesized via nanocasting method using the hexagonal mesoporous SBA-15 as template. Two different procedures, impregnation technique and direct synthesis, were utilized for the doping of Pd in the mesoporous SnO2 . The results of small angle X-ray diffraction (SAXD), nitrogen adsorption–desorption and transmission electron microscopy (TEM) demonstrate that the SnO2 and Pd/SnO2 display ordered mesoporous structures and high surface areas. Wide angle X-ray diffraction (WAXD) and X-ray photoelectron spectroscopy (XPS) reveal tetragonal structure of SnO2 and the existence of Pd element. The sensing properties of mesoporous SnO2 and mesoporous Pd/SnO2 for H2 were detected. The sensor utilizing mesoporous Pd/SnO2 via direct synthesis method exhibits excellent response and recovery behavior and much higher sensitivity to H2 , compared to those using mesoporous SnO2 and mesoporous Pd/SnO2 via impregnation technique. It is believed that its high gas sensing performance is derived from the large surface area, high activity and well dispersion of Pd additive, as well as high porosity, which lead to highly effective surface interaction between the target gas molecules and the surface active sites. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Tin dioxide (SnO2 ) as an n-type semiconductor with a wide band gap, is extensively used in lithium battery [1], catalysts [2], and solar cells [3]. Especially, SnO2 is an important sensing material for various gases, such as H2 [4,5], CO [6,7], and C2 H5 OH [8]. As is well known, the fundamental mechanism of semiconducting oxide type gas sensors is based on the change in electrical conductivity due to the surface–chemical interaction between the sensor material and gas molecules, which sensing performances strongly depend on surface properties including surface area of the semiconducting oxides. Therefore, much effort has been devoted to investigations of tin dioxide nanostructures with various morphologies including nanospheres [9], nanotube [10] and nanosheets [11] during the past decade. Mesoporous metal oxides are considered promising candidates in the development of gas sensors, due to their high specific surface area, high pore volume and tunable pore size [12,13]. Recently, several mesoporous metal oxides [14–17] have been successfully synthesized by nanocasting method, which is highly reproducible and avoidable from the difficulty lying in the structure collapse during the mesostructure formation and the removal of organic templates in the conventional sol–gel processes by using surfac-
∗ Corresponding authors. Tel.: +86 431 85167808. E-mail addresses: du [email protected] (Y. Du), [email protected] (G. Lu). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.08.035
tants and block copolymers. The recent investigations reveal that a few mesoporous metal oxides synthesized via nanocasting method, such as In2 O3 [18], WO3 [19] and CeO2 [20] exhibit the high thermal stability which is needed for application as gas sensors as well as strong sensitivity and fast response to gases. Wang et al. [21], reports a fast response chlorine gas sensor based on mesoporous SnO2 , but the sensitivity of the sensor is dissatisfactory. Yang group [22] used SBA-15 as a sensor support to SnO2 and the sensitivity of the resulted sensor to H2 could be enhanced by 40 times compared to pure SnO2 . However, because of the existence of massive SiO2 , the tremendous resistance (>100 G) of the sensor limits its utilizations. It is well known that additive noble metals such as Pd, Pt and Au or their oxides can enhance the sensing properties due to chemical (spillover mechanism) or electronic sensitisations (Fermi level control) [23–26]. Generally, the samples containing semiconducting oxides and noble metal additives are obtained by impregnation procedures, which may result in the inhomogeneous distribution of additives [27]. In this work, we have developed a doping route for high dispersion of Pd in mesoporous SnO2 , which is direct injection of Pd precursor along with Sn precursor before crystallization of SnO2 at high temperature. The ordered mesoporous SnO2 and mesoporous Pd/SnO2 have been successfully synthesized via nanocasting method using the hexagonal mesoporous SBA-15 as template. Furthermore, the sensing properties of them for H2 were investigated. Noticeably, the sensor utilizing mesoporous Pd/SnO2 via direct synthesis method displays outstanding response and
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recovery characteristics and much higher sensitivity in contrast to those based on mesoporous SnO2 and mesoporous Pd/SnO2 via impregnation technique. 2. Experimental 2.1. Material preparation SBA-15: The hexagonal SBA-15 silica was prepared in a modification according to the previously report [28]. 2 g of P123 (EO20 PO70 EO20 ) was dissolved in 60 mL of HCl (2 M) under stirring for 2 h. After addition of 4.25 g TEOS, the mixture was stirred at 40 ◦ C for another 24 h, followed by a hydrothermal treatment at 100 ◦ C for 24 h. Finally, the resulted solid was filtered, washed with deionized water, and calcined at 650 ◦ C for 6 h. Pure mesoporous SnO2 : 0.65 g of SnCl2 ·2H2 O was dissolved in 10 ml of ethanol. After stirring for 30 min, 0.4 g of SBA-15 was added to the solution and stirring at 40 ◦ C until all the ethanol was evaporated. Sequentially, the resulted powder was heated at 450 ◦ C for 3 h to convert tin chloride to tin dioxide within the pores of SBA-15 template. The filling and heating procedures were repeated twice to achieve higher loadings (the weight ratio of SnO2 to SiO2 is 7:3). Finally, the silica template was removed at room temperature using 2 M NaOH solution. Mesoporous Pd/SnO2 : Two different methods were employed for synthesis of mesoporous Pd/SnO2 (0.2 wt% of Pd). The samples synthesized via impregnation and direct methods were detonated as Pd/SnO2 -i and Pd/SnO2 -d, respectively. Pd/SnO2 -i: The pure mesoporous SnO2 was impregnated with Pd(NO3 )2 ·2H2 O solution and dried at 80 ◦ C. Finally, the sample was sintered at 450 ◦ C for 3 h. Pd/SnO2 -d: The filling and heating procedures were the same as that of pure mesoporous SnO2 mentioned above, except for the precursor composed of SnCl2 ·2H2 O and Pd(NO3 )2 ·2H2 O instead of pure SnCl2 ·2H2 O. 2.2. Material characterization X-ray diffraction (XRD) patterns were recorded on Rigaku D/MAX-2550 diffractometer using Cu K␣ radiation ( = 0.15418 nm). N2 adsorption–desorption isotherms were measured at 77 K with a Gemini VII surface area and porosity system. The specific surface area was estimated by the Brunauer–Emmett–Teller (BET) method, and the pore size was calculated by the Barrett–Joyner–Halenda (BJH) method using the adsorption branch of the isotherms. Transmission electron microscopy (TEM) images were obtained with a JEOL TEM-3010 instrument, operating at an acceleration voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) was performed by ESCALAB250 instrument.
Fig. 1. Low-angle XRD patterns of (a) pure SnO2 , (b) Pd/SnO2 -i and (c) Pd/SnO2 -d, and TEM image of Pd/SnO2 -d (inset).
(S = Ra /Rg ) was defined as the ratio of the electrical resistance in air (Ra ) to that in the target gas (Rg ). The response and recovery time were defined as the time taken by the sensor to achieve 90% of the total resistance change in the case of adsorption and desorption, respectively. 3. Results and discussion 3.1. Structure and morphology of as-prepared materials Low-angle XRD patterns of pure SnO2 , Pd/SnO2 -i and Pd/SnO2 -d are illustrated in Fig. 1, respectively. All the XRD patterns exhibit a very sharp diffraction peak and two weak peaks, corresponding to the (1 0 0), (1 1 0) and (2 0 0) reflections of ordered two-dimensional hexagonal mesostructures [29]. TEM image (Fig. 1, inset) of Pd/SnO2 -d shows a long-range periodic order mesopore structure in agreement with XRD result, indicating the successful replication from the silica template. However, compared to silica template SBA-15, the corresponding replica exhibit somewhat broader and less well-resolved diffraction peaks, which indicates a slightly lower overall nanostructural order as frequently found for replicated materials [18,30]. Wide-angle XRD patterns of the pure SnO2 , Pd/SnO2 -i and Pd/SnO2 -d in Fig. 2 display well-resolved diffraction peaks for SnO2 nanocrystals that can be indexed to a tetragonal structure (JCPDS No.: 41-1445). The broad nature of the XRD peaks indicates the presence of very small crystallites, and the particle sizes of pure
2.3. Sensor fabrication and measurement The as-synthesized samples were mixed with deionized water to form paste, and then the paste was coated onto an alumina tube on which a pair of Au electrodes was previously printed. Pt lead wires attaching to the Au electrodes were used for connecting with the measure instrument. After the devices were calcined at 450 ◦ C for 2 h, a Ni–Cr alloy coil was inserted into the alumina tube as a heater to keep the operating temperature. The sensing properties were measured using a static test system. The target gas was injected into a 1 L test chamber by an injector through a rubber plug. Once equilibrium has been established between the target gas and the air, the sensor was put into the test chamber. When the response reached a constant value, the sensor was taken out to recover in air. The electrical properties of the sensor were measured by a digital multimeter (Fluke 8846A). The sensor response
Fig. 2. Wide-angle XRD patterns of (a) pure SnO2 , (b) Pd/SnO2 -i and (c) Pd/SnO2 -d.
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Fig. 4. XPS spectra in the Pd 3d region of (a) Pd/SnO2 -i and (b) Pd/SnO2 -d.
Fig. 3. (A) Nitrogen physisorption isotherms and (B) the corresponding pore size distributions of (a) pure SnO2 , (b) Pd/SnO2 -i and (c) Pd/SnO2 -d.
SnO2 , Pd/SnO2 -i and Pd/SnO2 -d calculated by the Scherrer equation are 5.0 nm, 5.7 nm and 5.0 nm, respectively. There is no characteristic of Pd, PdO or PdO2 diffraction peaks in the patterns of the Pd/SnO2 samples, which may be due to low amounts of Pd loaded in the mesoporous SnO2 or a homogeneous distribution of Pd particles. Fig. 3 shows nitrogen physisorption isotherms and the corresponding pore size distribution of the pure SnO2 , Pd/SnO2 -i and Pd/SnO2 -d, respectively. All the samples exhibit type-IV isotherms with a capillary condensation occurring in the relative pressure (P/P0 ) of 0.6–0.8, which also confirms the existence of uniform mesopores in their structure. Narrow pore size distributions of pure SnO2 and Pd/SnO2 -d with diameter of 3.9 nm and 3.6 nm are observed, respectively. Comparatively, Pd/SnO2 -i shows broader pore size distribution because of less order mesoporous structure during impregnation process of Pd. The textural properties of all the samples are summarized in Table 1. The decrease of surface area, pore volume and pore diameter of Pd/SnO2 -i is found in comparison with pure mesoporous SnO2 , which is due to Pd particles filling in the mesopore of SnO2 during the impregnation procedure. It is
worth noting that the increase of surface area for Pd/SnO2 -d can be observed, implying that the incorporation of palladium cations into the tin dioxide framework affects the integrity and mesostructure of Pd/SnO2 -d [31]. The investigations of the composition and the chemical state of palladium in Pd/SnO2 -i and Pd/SnO2 -d were carried out by XPS. As seen in Fig. 4, two weak signals are present in each spectrum due to low concentration of Pd loading in the mesoporous SnO2 . The Pd/SnO2 -i sample shows two peaks at 338.1 eV and 342.8 eV which were attributed to 3d5/2 and 3d3/2 of Pd4+ [32], respectively. By contrast, Pd 3d5/2 and 3d3/2 binding energies in Pd/SnO2 -d were shifted to lower energies of 337.4 eV and 342.0 eV, Ueishi et al. [33] reported such a rather similar value that they signed to Pd3+ incorporated into the framework of perovskite under oxidation atmosphere. 3.2. Sensing properties The responses of the sensors using pure SnO2 , Pd/SnO2 -i and Pd/SnO2 -d to 1000 ppm H2 were measured at various operating temperatures in order to find out the optimum operating temperature. As shown in Fig. 5, the maximum response of the sensor based on the pure SnO2 is 16.4 at 300 ◦ C. Unexpectedly, the sensitivities of Pd/SnO2 -i and Pd/SnO2 -d sensors have reached to 30.7 at 275 ◦ C,
Table 1 The textural properties of mesostructured samples. Sample
Surface area (m2 /g)
Pore size (nm)
Pore volume (cm3 /g)
Pure SnO2 Pd/SnO2 -i Pd/SnO2 -d
104 91 115
3.9 3.7 3.6
0.26 0.23 0.23
Fig. 5. Response of the sensors to 1000 ppm H2 at different operating temperatures.
J. Zhao et al. / Sensors and Actuators B 160 (2011) 604–608
Fig. 6. Response of Pd/SnO2 -d sensor to different H2 concentrations at 250 ◦ C.
and 151.2 at 250 ◦ C, respectively, whereas Pd amount (0.2 wt%) loading on the mesoporous SnO2 is far lower than that reported in previous literatures [34,35]. Noticeably, Pd/SnO2 -d sensor exhibits much higher sensitivity and lower optimum sensing temperature than that of Pd/SnO2 -i as well as pure SnO2 . Fig. 6 shows the response of the sensor based on Pd/SnO2 -d to H2 in the range of 10–2000 ppm at 250 ◦ C. The response to H2 increases rapidly with the increasing of gas concentration below 500 ppm. The correlation of the response and H2 concentration are approximately linear in the range of 500–2000 ppm, which indicates that the sensor is very suitable for detection of H2 in a wide range. This unsaturation phenomenon to relatively high H2 concentration may be resulted from the high surface area and large pore volume of Pd/SnO2 -d providing abundant surface active sites and accommodating large number of target gas molecules. It is well known that the response and recovery characteristics are important for evaluating the performance of gas sensor. To investigate the response and recovery behaviors of Pd/SnO2 -d sensor, the sensor was sequentially exposed to 10, 50, 100, 500 and 1000 ppm H2 at 250 ◦ C. As seen in Fig. 7, the fast response and recovery to H2 can be observed. It is worth noting that the sensor showed an obvious response to H2 concentration as low as 10 ppm, and the response and recovery times are about 1 and 10 s, respectively. The excellent response and recovery behavior can be explained from high porosity of Pd/SnO2 -d sensors. The sensing mechanism of the conventional SnO2 gas sensors was proved in previous work [36,37]. When SnO2 is exposed to
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reducing gas such as H2 , the reducing gas reacts with the adsorbed oxygen and releases the trapped electrons back to the conduction band, thus decreases the potential barrier, which eventually increases the conductivity of SnO2 sensor. In general, the beneficial effect on the sensor performance by noble metal catalysts is attributed to two kinds of mechanisms [23,24]. Chemical sensitization occurs by spill-over of active species, generated by the interaction with the catalyst over the sensor surface. On the other hand, electronic sensitization reflects that the additive in the oxidized state is strongly depletive of electrons in the oxide near the interface. When reduced by the target gas, it relaxes the space charge layer by giving back electrons to the oxide. In our case, the enhanced contribution to H2 sensing properties of SnO2 sensors actually comes from PdO. PdO2 is known to easily reduce to PdO by reducing gas. The electron flow may initially occur from the semiconducting sensing material to the PdO, again increasing the sensor resistance, which is recovered after reaction of PdO with H2 . Several reasons may contribute to the sensing performance of Pd/SnO2 -d better than that of Pd/SnO2 -i. It is well known that the specific surface plays a vital role in gas sensing characteristics of semiconductor oxides. Large specific surface of Pd/SnO2 -d facilitates high accessibility for gas molecules, resulting in good sensitivity. Furthermore, the doping in the semiconducting oxide is generally carried out by impregnation procedures, which may lead to inhomogeneous distribution of the additive, and thus restrain the positive effect of catalyst introduction on the sensing performances. The uniform dispersion of additive in Pd/SnO2 -d was achieved by direct synthesis method, allowing a remarkable improvement of the sensing properties. In addition, the beneficial effect to H2 sensing properties of Pd/SnO2 -d sensors actually derives from PdO species loading on the SnO2 . Pd3+ in the Pd/SnO2 -d established from XPS results is chemical unstable, and hence tend to reduce to PdO by reducing gas in contrast to Pd4+ presenting in the Pd/SnO2 -i, which reinforce the activity of Pd/SnO2 -d to the target gas. 4. Conclusion The ordered mesoporous SnO2 and Pd/SnO2 have been successfully synthesized via nanocasting method using the hexagonal mesoporous SBA-15 as template. A study on their gas sensing properties for H2 reveals that the sensor utilizing Pd/SnO2 via direct synthesis method displays swift response and recovery rate and much higher sensitivity to H2 compared to those based on mesoporous SnO2 and Pd/SnO2 via impregnation technique. It is supposed that the outstanding gas sensing properties of Pd/SnO2 -d sensor is arisen from the large surface area, high activity and well dispersion for Pd additive, as well as high porosity, which lead to highly effective surface reaction between the target gas molecules and the surface active sites in Pd/SnO2 . Acknowledgements This work was supported by Natural Science Foundation of China (Nos. 21001051 and 61074172). References
Fig. 7. Response and recovery characteristics of Pd/SnO2 -d sensor to 10–1000 ppm H2 at 250 ◦ C.
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Biographies Jing Zhao received her MS degree in Chemistry, Jilin University, China in 2010. She entered the PhD course in 2010, majored in microelectronics and solid state electronics. Presently, she is engaged in the synthesis and characterization of the functional materials and chemical sensors. Weinan Wang received his BS degree from the Electronics Science and Engineering department, Jilin University, China in 2010. Presently, he is a graduate student, majored in microelectronics and solid state electronics. Yinping Liu received her BS degree from the Electronics Information and Engineering department, Jilin University, China in 2011. Her current research interest is the synthesis and characterization of the semiconducting functional materials and gas sensors. Jinming Ma received his BS degree from the Electronics Science and Engineering department, Jilin University, China in 2009. Presently, he is a graduate student, majored in microelectronics and solid state electronics. Xiaowei Li received his BS degree from the Electronics Information and Engineering department, Jilin University, China in 2011. His current research interest is the synthesis and characterization of the semiconducting functional materials and gas sensors. Yu Du received her PhD from chemistry of Jilin University, China in 2006. After that, she had been working as the Postdoctoral at NANYANG Technological University, Singapore for about two years. She is currently an Associate Professor at the Electronics Science and Engineering department of Jilin University, China. Her current research interests are nanoscience and gas sensors. Geyu Lu received his BS and MS degree in electronic sciences from Jilin University, China in 1985 and 1988, respectively, and PhD degree in 1998 from Kyushu University in Japan. Now he is a professor of Jilin University, China. Presently, he is interested in the development of functional materials and chemical sensors.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Ordered mesoporous Pd/SnO2 synthesized by a nanocasting route for high hydrogen sensing performance Jing Zhao, Weinan Wang, Yinping Liu, Jinming Ma, Xiaowei Li, Yu Du ∗ , Geyu Lu ∗ State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, China
a r t i c l e
i n f o
Article history: Received 28 April 2011 Received in revised form 15 August 2011 Accepted 16 August 2011 Available online 24 August 2011 Keywords: Tin dioxide Pd Nanocasting Mesoporous Gas sensor
a b s t r a c t Ordered mesoporous SnO2 and mesoporous Pd/SnO2 have been successfully synthesized via nanocasting method using the hexagonal mesoporous SBA-15 as template. Two different procedures, impregnation technique and direct synthesis, were utilized for the doping of Pd in the mesoporous SnO2 . The results of small angle X-ray diffraction (SAXD), nitrogen adsorption–desorption and transmission electron microscopy (TEM) demonstrate that the SnO2 and Pd/SnO2 display ordered mesoporous structures and high surface areas. Wide angle X-ray diffraction (WAXD) and X-ray photoelectron spectroscopy (XPS) reveal tetragonal structure of SnO2 and the existence of Pd element. The sensing properties of mesoporous SnO2 and mesoporous Pd/SnO2 for H2 were detected. The sensor utilizing mesoporous Pd/SnO2 via direct synthesis method exhibits excellent response and recovery behavior and much higher sensitivity to H2 , compared to those using mesoporous SnO2 and mesoporous Pd/SnO2 via impregnation technique. It is believed that its high gas sensing performance is derived from the large surface area, high activity and well dispersion of Pd additive, as well as high porosity, which lead to highly effective surface interaction between the target gas molecules and the surface active sites. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Tin dioxide (SnO2 ) as an n-type semiconductor with a wide band gap, is extensively used in lithium battery [1], catalysts [2], and solar cells [3]. Especially, SnO2 is an important sensing material for various gases, such as H2 [4,5], CO [6,7], and C2 H5 OH [8]. As is well known, the fundamental mechanism of semiconducting oxide type gas sensors is based on the change in electrical conductivity due to the surface–chemical interaction between the sensor material and gas molecules, which sensing performances strongly depend on surface properties including surface area of the semiconducting oxides. Therefore, much effort has been devoted to investigations of tin dioxide nanostructures with various morphologies including nanospheres [9], nanotube [10] and nanosheets [11] during the past decade. Mesoporous metal oxides are considered promising candidates in the development of gas sensors, due to their high specific surface area, high pore volume and tunable pore size [12,13]. Recently, several mesoporous metal oxides [14–17] have been successfully synthesized by nanocasting method, which is highly reproducible and avoidable from the difficulty lying in the structure collapse during the mesostructure formation and the removal of organic templates in the conventional sol–gel processes by using surfac-
∗ Corresponding authors. Tel.: +86 431 85167808. E-mail addresses: du [email protected] (Y. Du), [email protected] (G. Lu). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.08.035
tants and block copolymers. The recent investigations reveal that a few mesoporous metal oxides synthesized via nanocasting method, such as In2 O3 [18], WO3 [19] and CeO2 [20] exhibit the high thermal stability which is needed for application as gas sensors as well as strong sensitivity and fast response to gases. Wang et al. [21], reports a fast response chlorine gas sensor based on mesoporous SnO2 , but the sensitivity of the sensor is dissatisfactory. Yang group [22] used SBA-15 as a sensor support to SnO2 and the sensitivity of the resulted sensor to H2 could be enhanced by 40 times compared to pure SnO2 . However, because of the existence of massive SiO2 , the tremendous resistance (>100 G) of the sensor limits its utilizations. It is well known that additive noble metals such as Pd, Pt and Au or their oxides can enhance the sensing properties due to chemical (spillover mechanism) or electronic sensitisations (Fermi level control) [23–26]. Generally, the samples containing semiconducting oxides and noble metal additives are obtained by impregnation procedures, which may result in the inhomogeneous distribution of additives [27]. In this work, we have developed a doping route for high dispersion of Pd in mesoporous SnO2 , which is direct injection of Pd precursor along with Sn precursor before crystallization of SnO2 at high temperature. The ordered mesoporous SnO2 and mesoporous Pd/SnO2 have been successfully synthesized via nanocasting method using the hexagonal mesoporous SBA-15 as template. Furthermore, the sensing properties of them for H2 were investigated. Noticeably, the sensor utilizing mesoporous Pd/SnO2 via direct synthesis method displays outstanding response and
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recovery characteristics and much higher sensitivity in contrast to those based on mesoporous SnO2 and mesoporous Pd/SnO2 via impregnation technique. 2. Experimental 2.1. Material preparation SBA-15: The hexagonal SBA-15 silica was prepared in a modification according to the previously report [28]. 2 g of P123 (EO20 PO70 EO20 ) was dissolved in 60 mL of HCl (2 M) under stirring for 2 h. After addition of 4.25 g TEOS, the mixture was stirred at 40 ◦ C for another 24 h, followed by a hydrothermal treatment at 100 ◦ C for 24 h. Finally, the resulted solid was filtered, washed with deionized water, and calcined at 650 ◦ C for 6 h. Pure mesoporous SnO2 : 0.65 g of SnCl2 ·2H2 O was dissolved in 10 ml of ethanol. After stirring for 30 min, 0.4 g of SBA-15 was added to the solution and stirring at 40 ◦ C until all the ethanol was evaporated. Sequentially, the resulted powder was heated at 450 ◦ C for 3 h to convert tin chloride to tin dioxide within the pores of SBA-15 template. The filling and heating procedures were repeated twice to achieve higher loadings (the weight ratio of SnO2 to SiO2 is 7:3). Finally, the silica template was removed at room temperature using 2 M NaOH solution. Mesoporous Pd/SnO2 : Two different methods were employed for synthesis of mesoporous Pd/SnO2 (0.2 wt% of Pd). The samples synthesized via impregnation and direct methods were detonated as Pd/SnO2 -i and Pd/SnO2 -d, respectively. Pd/SnO2 -i: The pure mesoporous SnO2 was impregnated with Pd(NO3 )2 ·2H2 O solution and dried at 80 ◦ C. Finally, the sample was sintered at 450 ◦ C for 3 h. Pd/SnO2 -d: The filling and heating procedures were the same as that of pure mesoporous SnO2 mentioned above, except for the precursor composed of SnCl2 ·2H2 O and Pd(NO3 )2 ·2H2 O instead of pure SnCl2 ·2H2 O. 2.2. Material characterization X-ray diffraction (XRD) patterns were recorded on Rigaku D/MAX-2550 diffractometer using Cu K␣ radiation ( = 0.15418 nm). N2 adsorption–desorption isotherms were measured at 77 K with a Gemini VII surface area and porosity system. The specific surface area was estimated by the Brunauer–Emmett–Teller (BET) method, and the pore size was calculated by the Barrett–Joyner–Halenda (BJH) method using the adsorption branch of the isotherms. Transmission electron microscopy (TEM) images were obtained with a JEOL TEM-3010 instrument, operating at an acceleration voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) was performed by ESCALAB250 instrument.
Fig. 1. Low-angle XRD patterns of (a) pure SnO2 , (b) Pd/SnO2 -i and (c) Pd/SnO2 -d, and TEM image of Pd/SnO2 -d (inset).
(S = Ra /Rg ) was defined as the ratio of the electrical resistance in air (Ra ) to that in the target gas (Rg ). The response and recovery time were defined as the time taken by the sensor to achieve 90% of the total resistance change in the case of adsorption and desorption, respectively. 3. Results and discussion 3.1. Structure and morphology of as-prepared materials Low-angle XRD patterns of pure SnO2 , Pd/SnO2 -i and Pd/SnO2 -d are illustrated in Fig. 1, respectively. All the XRD patterns exhibit a very sharp diffraction peak and two weak peaks, corresponding to the (1 0 0), (1 1 0) and (2 0 0) reflections of ordered two-dimensional hexagonal mesostructures [29]. TEM image (Fig. 1, inset) of Pd/SnO2 -d shows a long-range periodic order mesopore structure in agreement with XRD result, indicating the successful replication from the silica template. However, compared to silica template SBA-15, the corresponding replica exhibit somewhat broader and less well-resolved diffraction peaks, which indicates a slightly lower overall nanostructural order as frequently found for replicated materials [18,30]. Wide-angle XRD patterns of the pure SnO2 , Pd/SnO2 -i and Pd/SnO2 -d in Fig. 2 display well-resolved diffraction peaks for SnO2 nanocrystals that can be indexed to a tetragonal structure (JCPDS No.: 41-1445). The broad nature of the XRD peaks indicates the presence of very small crystallites, and the particle sizes of pure
2.3. Sensor fabrication and measurement The as-synthesized samples were mixed with deionized water to form paste, and then the paste was coated onto an alumina tube on which a pair of Au electrodes was previously printed. Pt lead wires attaching to the Au electrodes were used for connecting with the measure instrument. After the devices were calcined at 450 ◦ C for 2 h, a Ni–Cr alloy coil was inserted into the alumina tube as a heater to keep the operating temperature. The sensing properties were measured using a static test system. The target gas was injected into a 1 L test chamber by an injector through a rubber plug. Once equilibrium has been established between the target gas and the air, the sensor was put into the test chamber. When the response reached a constant value, the sensor was taken out to recover in air. The electrical properties of the sensor were measured by a digital multimeter (Fluke 8846A). The sensor response
Fig. 2. Wide-angle XRD patterns of (a) pure SnO2 , (b) Pd/SnO2 -i and (c) Pd/SnO2 -d.
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Fig. 4. XPS spectra in the Pd 3d region of (a) Pd/SnO2 -i and (b) Pd/SnO2 -d.
Fig. 3. (A) Nitrogen physisorption isotherms and (B) the corresponding pore size distributions of (a) pure SnO2 , (b) Pd/SnO2 -i and (c) Pd/SnO2 -d.
SnO2 , Pd/SnO2 -i and Pd/SnO2 -d calculated by the Scherrer equation are 5.0 nm, 5.7 nm and 5.0 nm, respectively. There is no characteristic of Pd, PdO or PdO2 diffraction peaks in the patterns of the Pd/SnO2 samples, which may be due to low amounts of Pd loaded in the mesoporous SnO2 or a homogeneous distribution of Pd particles. Fig. 3 shows nitrogen physisorption isotherms and the corresponding pore size distribution of the pure SnO2 , Pd/SnO2 -i and Pd/SnO2 -d, respectively. All the samples exhibit type-IV isotherms with a capillary condensation occurring in the relative pressure (P/P0 ) of 0.6–0.8, which also confirms the existence of uniform mesopores in their structure. Narrow pore size distributions of pure SnO2 and Pd/SnO2 -d with diameter of 3.9 nm and 3.6 nm are observed, respectively. Comparatively, Pd/SnO2 -i shows broader pore size distribution because of less order mesoporous structure during impregnation process of Pd. The textural properties of all the samples are summarized in Table 1. The decrease of surface area, pore volume and pore diameter of Pd/SnO2 -i is found in comparison with pure mesoporous SnO2 , which is due to Pd particles filling in the mesopore of SnO2 during the impregnation procedure. It is
worth noting that the increase of surface area for Pd/SnO2 -d can be observed, implying that the incorporation of palladium cations into the tin dioxide framework affects the integrity and mesostructure of Pd/SnO2 -d [31]. The investigations of the composition and the chemical state of palladium in Pd/SnO2 -i and Pd/SnO2 -d were carried out by XPS. As seen in Fig. 4, two weak signals are present in each spectrum due to low concentration of Pd loading in the mesoporous SnO2 . The Pd/SnO2 -i sample shows two peaks at 338.1 eV and 342.8 eV which were attributed to 3d5/2 and 3d3/2 of Pd4+ [32], respectively. By contrast, Pd 3d5/2 and 3d3/2 binding energies in Pd/SnO2 -d were shifted to lower energies of 337.4 eV and 342.0 eV, Ueishi et al. [33] reported such a rather similar value that they signed to Pd3+ incorporated into the framework of perovskite under oxidation atmosphere. 3.2. Sensing properties The responses of the sensors using pure SnO2 , Pd/SnO2 -i and Pd/SnO2 -d to 1000 ppm H2 were measured at various operating temperatures in order to find out the optimum operating temperature. As shown in Fig. 5, the maximum response of the sensor based on the pure SnO2 is 16.4 at 300 ◦ C. Unexpectedly, the sensitivities of Pd/SnO2 -i and Pd/SnO2 -d sensors have reached to 30.7 at 275 ◦ C,
Table 1 The textural properties of mesostructured samples. Sample
Surface area (m2 /g)
Pore size (nm)
Pore volume (cm3 /g)
Pure SnO2 Pd/SnO2 -i Pd/SnO2 -d
104 91 115
3.9 3.7 3.6
0.26 0.23 0.23
Fig. 5. Response of the sensors to 1000 ppm H2 at different operating temperatures.
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Fig. 6. Response of Pd/SnO2 -d sensor to different H2 concentrations at 250 ◦ C.
and 151.2 at 250 ◦ C, respectively, whereas Pd amount (0.2 wt%) loading on the mesoporous SnO2 is far lower than that reported in previous literatures [34,35]. Noticeably, Pd/SnO2 -d sensor exhibits much higher sensitivity and lower optimum sensing temperature than that of Pd/SnO2 -i as well as pure SnO2 . Fig. 6 shows the response of the sensor based on Pd/SnO2 -d to H2 in the range of 10–2000 ppm at 250 ◦ C. The response to H2 increases rapidly with the increasing of gas concentration below 500 ppm. The correlation of the response and H2 concentration are approximately linear in the range of 500–2000 ppm, which indicates that the sensor is very suitable for detection of H2 in a wide range. This unsaturation phenomenon to relatively high H2 concentration may be resulted from the high surface area and large pore volume of Pd/SnO2 -d providing abundant surface active sites and accommodating large number of target gas molecules. It is well known that the response and recovery characteristics are important for evaluating the performance of gas sensor. To investigate the response and recovery behaviors of Pd/SnO2 -d sensor, the sensor was sequentially exposed to 10, 50, 100, 500 and 1000 ppm H2 at 250 ◦ C. As seen in Fig. 7, the fast response and recovery to H2 can be observed. It is worth noting that the sensor showed an obvious response to H2 concentration as low as 10 ppm, and the response and recovery times are about 1 and 10 s, respectively. The excellent response and recovery behavior can be explained from high porosity of Pd/SnO2 -d sensors. The sensing mechanism of the conventional SnO2 gas sensors was proved in previous work [36,37]. When SnO2 is exposed to
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reducing gas such as H2 , the reducing gas reacts with the adsorbed oxygen and releases the trapped electrons back to the conduction band, thus decreases the potential barrier, which eventually increases the conductivity of SnO2 sensor. In general, the beneficial effect on the sensor performance by noble metal catalysts is attributed to two kinds of mechanisms [23,24]. Chemical sensitization occurs by spill-over of active species, generated by the interaction with the catalyst over the sensor surface. On the other hand, electronic sensitization reflects that the additive in the oxidized state is strongly depletive of electrons in the oxide near the interface. When reduced by the target gas, it relaxes the space charge layer by giving back electrons to the oxide. In our case, the enhanced contribution to H2 sensing properties of SnO2 sensors actually comes from PdO. PdO2 is known to easily reduce to PdO by reducing gas. The electron flow may initially occur from the semiconducting sensing material to the PdO, again increasing the sensor resistance, which is recovered after reaction of PdO with H2 . Several reasons may contribute to the sensing performance of Pd/SnO2 -d better than that of Pd/SnO2 -i. It is well known that the specific surface plays a vital role in gas sensing characteristics of semiconductor oxides. Large specific surface of Pd/SnO2 -d facilitates high accessibility for gas molecules, resulting in good sensitivity. Furthermore, the doping in the semiconducting oxide is generally carried out by impregnation procedures, which may lead to inhomogeneous distribution of the additive, and thus restrain the positive effect of catalyst introduction on the sensing performances. The uniform dispersion of additive in Pd/SnO2 -d was achieved by direct synthesis method, allowing a remarkable improvement of the sensing properties. In addition, the beneficial effect to H2 sensing properties of Pd/SnO2 -d sensors actually derives from PdO species loading on the SnO2 . Pd3+ in the Pd/SnO2 -d established from XPS results is chemical unstable, and hence tend to reduce to PdO by reducing gas in contrast to Pd4+ presenting in the Pd/SnO2 -i, which reinforce the activity of Pd/SnO2 -d to the target gas. 4. Conclusion The ordered mesoporous SnO2 and Pd/SnO2 have been successfully synthesized via nanocasting method using the hexagonal mesoporous SBA-15 as template. A study on their gas sensing properties for H2 reveals that the sensor utilizing Pd/SnO2 via direct synthesis method displays swift response and recovery rate and much higher sensitivity to H2 compared to those based on mesoporous SnO2 and Pd/SnO2 via impregnation technique. It is supposed that the outstanding gas sensing properties of Pd/SnO2 -d sensor is arisen from the large surface area, high activity and well dispersion for Pd additive, as well as high porosity, which lead to highly effective surface reaction between the target gas molecules and the surface active sites in Pd/SnO2 . Acknowledgements This work was supported by Natural Science Foundation of China (Nos. 21001051 and 61074172). References
Fig. 7. Response and recovery characteristics of Pd/SnO2 -d sensor to 10–1000 ppm H2 at 250 ◦ C.
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Biographies Jing Zhao received her MS degree in Chemistry, Jilin University, China in 2010. She entered the PhD course in 2010, majored in microelectronics and solid state electronics. Presently, she is engaged in the synthesis and characterization of the functional materials and chemical sensors. Weinan Wang received his BS degree from the Electronics Science and Engineering department, Jilin University, China in 2010. Presently, he is a graduate student, majored in microelectronics and solid state electronics. Yinping Liu received her BS degree from the Electronics Information and Engineering department, Jilin University, China in 2011. Her current research interest is the synthesis and characterization of the semiconducting functional materials and gas sensors. Jinming Ma received his BS degree from the Electronics Science and Engineering department, Jilin University, China in 2009. Presently, he is a graduate student, majored in microelectronics and solid state electronics. Xiaowei Li received his BS degree from the Electronics Information and Engineering department, Jilin University, China in 2011. His current research interest is the synthesis and characterization of the semiconducting functional materials and gas sensors. Yu Du received her PhD from chemistry of Jilin University, China in 2006. After that, she had been working as the Postdoctoral at NANYANG Technological University, Singapore for about two years. She is currently an Associate Professor at the Electronics Science and Engineering department of Jilin University, China. Her current research interests are nanoscience and gas sensors. Geyu Lu received his BS and MS degree in electronic sciences from Jilin University, China in 1985 and 1988, respectively, and PhD degree in 1998 from Kyushu University in Japan. Now he is a professor of Jilin University, China. Presently, he is interested in the development of functional materials and chemical sensors.