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Humidity sensitive property of Li-doped mesoporous silica SBA-15
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Sensors and Actuators B 127 (2007) 323–329
Humidity sensitive property of Li-doped mesoporous silica SBA-15 Wangchang Geng a , Rui Wang b , Xiaotian Li a,∗ , Yongcun Zou c , Tong Zhang b , Jinchun Tu a , Yuan He b , Nan Li a a
Department of Material Science and Engineering. Jilin University, Changchun 130012, PR China b Department of Electronic Engineering. Jilin University, Changchun 130012, PR China c Department of Chemistry and State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130012, PR China Received 26 December 2006; received in revised form 14 April 2007; accepted 17 April 2007 Available online 20 April 2007
Abstract Mesoporous silica SBA-15 was synthesized by a sol–gel method, and different level of LiCl was doped into SBA-15 by heat-treating process at 550 ◦ C to form a series of samples. These samples and pure SBA-15 were investigated as humidity sensor materials at room temperature within the relative humidity range of 11–95%. It was found that after the doping of LiCl, the humidity sensitivity has been greatly improved, the impedance changed by more than three orders of magnitude over the whole humidity range. This was related to the hydrophilic property of the Li+ , and a possible mechanism was provided to explain the humidity sensitive properties. © 2007 Elsevier B.V. All rights reserved. Keywords: Mesoporous silica; SBA-15; Humidity sensitivity; Li doped; Sensors
1. Introduction The humidity control is necessary for various fields of environment detection and monitoring [1–3], and many sensors have been widely used in moisture-sensitive environment, such as libraries, museums, food storages and so on. Many kinds of sensor materials including metal oxides [4–7], polymers [8–10], and organic–inorganic hybrid composites [11–13] have been produced to serve these applications. Among these organic–inorganic composites, the inorganic silica, due to its good mechanical strength and high intrinsic impedance [14], has been successfully combined with different polymers [15,16] to improve the characteristics of the humidity sensors. Pure silica gel also has been studied as humidity sensors by Anderson and Parks [17]. This is because the Si–OH groups and water molecules adsorbed by silica gel play an important role in increasing the conductivity of silica gel. Wang et al. [18] investigated the electrical sensing properties of silica aerogel thin films, but the silica aerogel only showed humidity sensi-
∗
Corresponding author. Tel.: +86 431 5168445; fax: +86 431 5168444. E-mail address: [email protected] (X. Li).
0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.04.021
tivity when the relative humidity reached as high as 60%RH. In order to improve the humidity sensitive property, salt-doping was found to be an effective method [19,20]. So, in this paper, LiCl was used as a dopant to the mesoporous silica SBA-15 to improve the sensitivity. The humidity sensing property of pure SBA-15 was given for comparison. In addition, a possible mechanism was discussed to elucidate why the humidity sensitive property of Li-doped SBA-15 was better than that of pure SBA-15. 2. Experimental 2.1. Preparation of mesoporous silica SBA-15 Mesoporous silica SBA-15 was synthesized according to the literature method reported by Li and Zhao [21] using a triblock copolymer surfactant P123 (EO20 PO70 EO20 ) as a template. The detailed procedure was as follows: 2 g P123 was dissolved in 60 ml HCl solution (2 M) at room temperature. Then 4.4 g tetraethyl orthosilicate (TEOS) was added dropwise under stirring at 40 ◦ C for 24 h. Subsequently the resultant mixture was aged at 60 ◦ C for 24 h without stirring. The product was filtered and washed with distilled water, then dried at 100 ◦ C over night.
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The surfactant template was removed at 550 ◦ C for 8 h to obtain the pure mesoporous silica SBA-15. 2.2. Preparation of Li-SBA-15 The sensor materials of Li-SBA-15 were prepared from different weight ratios of SBA-15 and LiCl. The mixture of LiCl and SBA-15 was ground for 0.5 h in an agate mortar. The grounded mixture was transferred into a crucible and subsequently heated in a muffle furnace with a heating rate of 2 ◦ C/min up to 550 ◦ C, and kept at this temperature for 24 h. Then the Li-SBA-15 samples were obtained, and the products were designated as Li-SBA-15 (X), where X was the content of LiCl in 1 g of SBA15. In our experiment, the value of X was 0.1, 0.3, 0.5, 0.9, and 1.8 for five Li-SBA-15 samples, respectively. 2.3. Methods of characterization The powder XRD patterns were measured on a D8 Tools Xray diffraction instrument using the Cu K␣ radiation at 40 kV and 30 mA. N2 adsorption–desorption isotherms were measured at 77 K on a Micromeritics ASAP 2010 m instrument (Micromeritics Instrument Corp., Norcross, GA). Infrared spectra were taken on a Perkin-Elmer series with a resolution of 4 cm−1 . The samples were prepared in a form of KBr pellet, the thickness of the pellet being about 1.3 mm. Each spectrum was collected at room temperature under atmospheric pressure. The morphology of products was characterized by a JEOL JSM-6700F field emission scanning electron microscope (SEM). The sample was prepared by distributing the powder samples on a double-sided conducting adhesive tape. The characteristic curves of humidity sensitivity were measured on a ZL-5 model LCR analyzer at room temperature. The controlled humidity environments were achieved using saturated aqueous solutions of different salts: LiCl, MgCl2 , Mg(NO3 )2 , NaCl, KCl, and KNO3 in a closed glass vessel at ambient temperature, which yielded 11%, 33%, 54%, 75%, 85% and 95% relative humidity, respectively. The power samples were screen printed on a ceramic plate (1 cm × 0.5 cm) in which a pair of interdigitated gold electrodes were printed and then the humidity device was heated at 70 ◦ C for 5 h. A schematic image of this electrode is shown in Fig. 1.
Fig. 1. A schematic image of the humidity sensor.
Fig. 2. Lower angle XRD patterns of SBA-15 and Li-SBA-15(0.1). The curves for other Li-SBA-15(X) samples were the same as that of Li-SBA-15(0.1).
3. Results and discussion 3.1. Structure and morphology 3.1.1. X-ray diffraction The lower angle XRD patterns of SBA-15 and Li-SBA-15(X) are shown in Fig. 2. We could see that the three peaks attributed to (1 0 0), (1 1 0), and (2 0 0) of SBA-15 were disappeared in LiSBA-15(X), indicating that the mesoporous structure of SBA-15 was destroyed after introducing LiCl into SBA-15. In fact, it was because that LiCl and SBA-15 reacted to form new phases as follows. Fig. 3 shows the wide angle XRD patterns of SBA-15, pure LiCl, and Li-SBA-15 (X = 0.1, 0.3, 0.5, 0.9, and 1.8). As could be seen, a broad peak centered at 22.2◦ of 2θ was observed for the pure SBA-15, indicating that the pore wall of SBA-15 was amorphous. In the pattern of LiCl shown on the top of this
Fig. 3. Wide angle XRD spectra of SBA-15, Li-SBA-15(X) of X = 0.1, 0.3, 0.5, 0.9 and 1.8, and pure LiCl.
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Fig. 4. Nitrogen adsorption–desorption isotherms of (a) mesoporous SBA-15 and (b) Li-SBA-15(0.1).
figure, the diffraction peaks appeared at 2θ = 30.1◦ and 34.9◦ corresponded to the (1 1 1) and (2 0 0) of LiCl, respectively. The peaks at 2θ = 23.2◦ , 32.9◦ , 40.7◦ and 47.4◦ were ascribed to the (2 0 0), (2 2 0), (2 2 2), and (4 0 0) of the lithium chloride hydrate, indicating that some water molecules were absorbed by LiCl to form LiCl(H2 O). For the samples of Li-SBA-15, we can see that the broad peak of SBA-15 became weaker and weaker till disappeared with an increase of LiCl content. And it was found that after the heat treatment of Li-SBA-15(X) at 550 ◦ C, new phases were formed. As seen from the XRD curves, the peaks of LiCl or LiCl(H2 O) disappeared and replaced by the characteristic peaks of lithium silicate. The peaks at 2θ = 18.8◦ , 26.9◦ , 33.0◦ and 38.7◦ were corresponding to the planes of (2 0 0), (1 1 1), (0 2 0), and (0 0 2) of Li2 SiO3 , respectively, and other peaks at 2θ = 24.3◦ , 25.0◦ and 37.6◦ were ascribed to the (0 4 0), (1 1 1), and (0 0 2) of Li2 Si2 O5 . 3.1.2. N2 adsorption–desorption characterization In order to characterize the mesoporous structure of SBA-15 and Li-SBA-15 further, the N2 adsorption–desorption isotherm of SBA-15 and the represent isotherm curve of Li-SBA-15
(X = 0.1) are shown in Fig. 4. As could be seen from Fig. 4a, the curve was of type IV, indicating the mesoporous structure of SBA-15. However, after Li was doped, the typical curve of type IV was disappeared, as could be seen from Fig. 4b, the inflection of the step shifted to a higher relative P/P0 (>0.9) and the hysteresis ring was very small, suggesting that the mesoporous structure was destroyed in Li-SBA-15. This was consistent with the result of low-angle XRD. The pore size distributions, calculated from the desorption isotherms, are shown in Fig. 5. The centered pore size for SBA-15 was 7.22 nm. However, for LiSBA-15, this peak was replaced by three weak peaks of 2.83, 3.80, and 6.43 nm (the corresponding adsorbed volumes were below 0.03 cm3 /g), as shown in Fig. 5b. The inset in Fig. 5b was an amplified curve of these three weak peaks. This result also demonstrated the serious loss of mesoporous structure in LiSBA-15. In addition, the BET surface area also decreased from 421 cm2 /g for SBA-15 to 34.4 cm2 /g for Li-doped SBA-15. 3.1.3. IR spectrum IR spectra of all the samples are shown in Fig. 6. Curves (a)–(g) correspond to the samples of SBA-15, Li-SBA-15(0.1),
Fig. 5. Pore size distribution curves of (a) mesoporous SBA-15 and (b) Li-SBA-15(0.1).
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Fig. 6. IR spectra of (a) SBA-15 and (b)–(f) Li-SBA-15(X) of X = 0.1, 0.3, 0.5, 0.9, and 1.8, respectively. The curve of LiCl (g) is given for reference.
Li-SBA-15(0.3), Li-SBA-15(0.5), Li-SBA-15(0.9), Li-SBA15(1.8), and LiCl, respectively. As can be seen, for mesoporous silica SBA-15, the peaks at 1092 and 808 cm−1 were attributed to the asymmetric stretching and symmetric modes of Si–O–Si lattice vibrations, respectively. Characteristic peaks of Si–OH were observed at 1635 and 950 cm−1 . All the bands were similar to the results of literature [18,22,23]. It was interesting that the peak of 950 cm−1 was very weak for SBA-15 compared to other samples. This was because that the number of surface hydroxyls on the pore walls of SBA-15 decreased after the sample was heat-treated at 550 ◦ C for 8 h. After the introduction of LiCl, the characteristic peak of Si–OH at 950 cm−1 became stronger and stronger with increasing the LiCl level up to 0.5 g. This can be explained as follows: when the level of LiCl increased, more water molecules were adsorbed due to the hydrophilic property of LiCl. Water molecules interacted with the Si–O–Si of SBA-15 to form the surface hydroxyls. So the intensity of the peak became stronger. However, when the content of LiCl was beyond 0.5 g, more water molecules were adsorbed, and some of the hydroxyls began to release the H+ , resulting in a decreased
Fig. 8. Humidity sensitive properties of pure SBA-15 and Li-SBA-15(X) of X = 0.1, 0.3, 0.5, 0.9, and 1.8.
amount of the surface hydroxyls, so that the intensity of this peak trended to reduce. In addition, the band at 808 cm−1 that was ascribed to Si–O–Si gradually disappeared with increasing the LiCl level and was replaced by a few bands of lithium silicate. This phenomenon proved again that the dopant LiCl reacted with SBA-15 to form a new phase. This was in agreement with the result of wide angle XRD. 3.1.4. SEM image The typical morphology of SBA-15 and a representative image of Li-SBA-15(X) are shown in Fig. 7. As can be seen from Fig. 7a, mesoporous SBA-15 was consisted of some short rods. The diameter of these rods was about 300 nm. After the doping of LiCl, the morphology of SBA-15 was maintained, as can be observed in Fig. 7b, although it was slightly different from that of SBA-15. The partial loss of the morphology may be
Fig. 7. SEM images of (a) mesoporous SBA-15 and (b) Li-SBA-15(0.1).
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Fig. 9. Variations of impedance of Li-SBA-15(0.1) as a function of relative humidity at different frequencies of 100 Hz, 1 kHz, 10 kHz, and 100 kHz.
related to the reaction between LiCl and SBA-15, as confirmed by XRD patterns and IR spectra. 3.2. Humidity sensitive property The results of resistance measurements as a function of %RH at room temperature are shown in Fig. 8. It can be seen that the drop of the resistance for pure SBA-15 was smaller in the whole %RH range, showing lower humidity sensitive properties. After the doping of LiCl, the resistance of all the Li-SBA-15 samples greatly decreased with an increase in relative humidity by more than three orders of magnitude over the range of 11–95%RH, showing higher humidity sensitive properties. However, as can
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be seen, when the LiCl content was below 0.5 g/g, the curves presented good linearity. When the content of LiCl was 0.9 g/g and 1.8 g/g, however, the linearity of the resistance versus %RH was relatively poor over a wide humidity range. So, it was not suitable for Li-SBA-15(0.9) and Li-SBA-15(1.8) to serve as humidity sensors. The sample of Li-SBA-15(0.1) exhibiting the maximum humidity sensitive property and linearity was selected for the evaluation of hysteresis, response–recovery time, and so on. Fig. 9 shows the dependence of impedance of Li-SBA15(0.1) on relative humidity and the measurement frequency. As can be seen, when the frequency was 100 Hz, the impedance decreased greatly with increasing the relative humidity. However, when a higher frequency was used, such as 1 kHz, 10 kHz, and even 100 kHz, the range of impedance change became less with increasing the %RH. Namely, at low %RH, the influence of frequency on the impedance was larger than that at high %RH. In order to study the sensing behavior, the complex impedance plots are shown in Fig. 10. As could be seen, at low humidity, a semicircle was observed, which was mainly resulted from the intrinsic impedance of the materials [24]. With an increase of humidity, the semicircle radius gradually decreased and a straight line was appeared at low frequencies. This straight line was considered to be the contribution of ionic species in the Li-doped mesoporous silica. The response and recovery curve is shown in Fig. 11. According to the literature [25], the time taken by a sensor to achieve 90% of the total impedance change is defined as the response time or recovery time. For the sample of Li-SBA-15(0.1), the response time was about 21 s when increasing from 11% to 95%RH, and the recovery time was 51 s when decreasing from 95% to 11%. Fig. 12 reveals the hysteresis data of Li-SBA15(0.1). The hysteresis of the sensing material was about 6%.
Fig. 10. Complex impedance plots of Li-SBA-15(0.1) at different relative humidity.
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Fig. 11. Response and recovery characteristic of Li-SBA-15(0.1).
3.3. Mechanism of the humidity sensitive properties Here, we try to give a possible mechanism qualitatively to explain the humidity sensitive properties of our samples. For pure mesoporous SBA-15, due to its highly resistive nature and the decrease of hydroxyl groups from the surface during the heattreating process, it showed a poor humidity sensitive property. Similar results have been reported in literature [14,18]. After the doping of LiCl, the concentration of surface hydroxyl groups increased, as indicated by the IR spectra. These hydroxyl groups provide the adsorption sites for water and play an important role to the humidity sensitive properties. At low relative humidity, only a few water molecules are adsorbed. The coverage of water on the surface is not continuous. A proton may be transferred from a Si–OH group to a water molecule to form H3 O+ [17], and
the proton migrates only by hopping from site to site across the surface. The transfer of H+ or H3 O+ is difficult on this discontinuous water layer, so that the impedance is relatively high at low %RH. When the %RH is increased to the middle region, one or several serial water layers are formed. The serial water layer accelerates the transfer of H+ or H3 O+ , according to the ion transfer mechanism of Grotthuss [26], H2 O + H3 O+ → H3 O+ + H2 O. The initial and final states are the same. The energy is also equivalent, so that the transfer of ions is quite easy. In this process, some Li+ come from the dissociation of lithium silicate also attend the transfer process. Based on the model of ion transport mechanism reported by Casalbore-Miceli et al. [27], Li ions could dissolve in the adsorbed water, and the dielectric constant, which is a function of the adsorbed water, makes them free from the interaction of the opposite charges. So these ions could transfer freely. The quick transfer of ions on the water layer results in a sharp decrease of the impedance. When the %RH increases to the high region, more lithium silicate dissociates into free Li+ and the Li+ ions dominate the ion transfer process. So, the impedance continuously decreases by more than three orders of magnitude compared to the initial impedance. 4. Conclusions Mesoporous silica SBA-15 and Li-doped SBA-15 were studied as humidity sensor materials. Their structure was characterized by low-angle XRD, wide-angle XRD, N2 adsorption–desorption, and IR spectroscopy. These tests indicated that the ordered mesoporous structure of SBA-15 was lost by the doping of LiCl, and it was found that lithium silicate was formed by the interaction of LiCl and SBA-15. The abundant dissociation of the lithium silicate into free Li+ ions at high relative humidity played an important role in improving the humidity sensitive properties of these materials. Of all these samples, Li-SBA-15(0.1) presented the maximum humidity sensitive property, better linearity and shorter response–recovery time. Acknowledgements The authors acknowledge Nailin Yue, who is studying in Department of Chemistry and State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, for her generous help in SEM, and we are grateful to Dr. Tong Zhang for her timely help in the field of humidity sensors. This work was supported by the National Science Foundation (Grant No. 20151001) and Natural Science Foundation of Jilin province (Grant No. 20040505). References
Fig. 12. Hysteresis of Li-SBA-15(0.1).
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Biographies Wangchang Geng received his BS degree in Department of Material Science, Jilin University, in 2003. In 2005, he entered the PhD course ahead of schedule after 2 year’s MS course in Jilin University. His main areas of interest are the synthesis of mesoporous materials, and the humidity sensitive property of host–guest composite materials. Rui Wang received her BS degree from the College of Electronics Science and Engineering, Jilin University, Changchun, China in 2005. Presently she is a graduate student, majored in microelectronics and solid state electronics, and engaged in novel sensing materials and humidity sensors. Xiaotian Li is a professor of Department of Material Science in Jilin University. He received a BS, MS degree in Department of Electronic Science in Jilin University in 1989 and 1992, respectively. He obtained his PhD degree in the Department of Chemistry and State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, in 2000. His main research interests are inorganic chemistry. Yongcun Zou received his BS degree in Department of Entironment Science, Jilin University, in 2001. His research area is instrument analysis. Tong Zhang received her MS degree in major of Semiconductor Materials in 1992 and PhD degree in the field of Microelectronics and Solid State Electronics in 2001 from Jilin University. She was appointed a full professor in College of Electronics Science and Engineering, Jilin University in 2001. Now, she is interested in the field of sensing functional materials, gas sensors and humidity sensors. Jinchun Tu received a BS degree in Department of Material Science, Jilin University, in 2003. Now, he is studying for the MS course, and his major interests are mesoporous metal oxides and chemical sensor technology. Yuan He received her BS degree from the College of Electronics Science and Engineering, Jilin University, Changchun, China in 2006. Presently she is a graduate student, majored in microelectronics and solid state electronics, and engaged in novel sensing materials and humidity sensors. Nan Li received her BS degree in Department of Chemistry, Jilin University, in 1999. Then she obtained her PhD from Jilin University in 2004. Her main research interests are inorganic chemistry and chemical sensors.
Sensors and Actuators B 127 (2007) 323–329
Humidity sensitive property of Li-doped mesoporous silica SBA-15 Wangchang Geng a , Rui Wang b , Xiaotian Li a,∗ , Yongcun Zou c , Tong Zhang b , Jinchun Tu a , Yuan He b , Nan Li a a
Department of Material Science and Engineering. Jilin University, Changchun 130012, PR China b Department of Electronic Engineering. Jilin University, Changchun 130012, PR China c Department of Chemistry and State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130012, PR China Received 26 December 2006; received in revised form 14 April 2007; accepted 17 April 2007 Available online 20 April 2007
Abstract Mesoporous silica SBA-15 was synthesized by a sol–gel method, and different level of LiCl was doped into SBA-15 by heat-treating process at 550 ◦ C to form a series of samples. These samples and pure SBA-15 were investigated as humidity sensor materials at room temperature within the relative humidity range of 11–95%. It was found that after the doping of LiCl, the humidity sensitivity has been greatly improved, the impedance changed by more than three orders of magnitude over the whole humidity range. This was related to the hydrophilic property of the Li+ , and a possible mechanism was provided to explain the humidity sensitive properties. © 2007 Elsevier B.V. All rights reserved. Keywords: Mesoporous silica; SBA-15; Humidity sensitivity; Li doped; Sensors
1. Introduction The humidity control is necessary for various fields of environment detection and monitoring [1–3], and many sensors have been widely used in moisture-sensitive environment, such as libraries, museums, food storages and so on. Many kinds of sensor materials including metal oxides [4–7], polymers [8–10], and organic–inorganic hybrid composites [11–13] have been produced to serve these applications. Among these organic–inorganic composites, the inorganic silica, due to its good mechanical strength and high intrinsic impedance [14], has been successfully combined with different polymers [15,16] to improve the characteristics of the humidity sensors. Pure silica gel also has been studied as humidity sensors by Anderson and Parks [17]. This is because the Si–OH groups and water molecules adsorbed by silica gel play an important role in increasing the conductivity of silica gel. Wang et al. [18] investigated the electrical sensing properties of silica aerogel thin films, but the silica aerogel only showed humidity sensi-
∗
Corresponding author. Tel.: +86 431 5168445; fax: +86 431 5168444. E-mail address: [email protected] (X. Li).
0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.04.021
tivity when the relative humidity reached as high as 60%RH. In order to improve the humidity sensitive property, salt-doping was found to be an effective method [19,20]. So, in this paper, LiCl was used as a dopant to the mesoporous silica SBA-15 to improve the sensitivity. The humidity sensing property of pure SBA-15 was given for comparison. In addition, a possible mechanism was discussed to elucidate why the humidity sensitive property of Li-doped SBA-15 was better than that of pure SBA-15. 2. Experimental 2.1. Preparation of mesoporous silica SBA-15 Mesoporous silica SBA-15 was synthesized according to the literature method reported by Li and Zhao [21] using a triblock copolymer surfactant P123 (EO20 PO70 EO20 ) as a template. The detailed procedure was as follows: 2 g P123 was dissolved in 60 ml HCl solution (2 M) at room temperature. Then 4.4 g tetraethyl orthosilicate (TEOS) was added dropwise under stirring at 40 ◦ C for 24 h. Subsequently the resultant mixture was aged at 60 ◦ C for 24 h without stirring. The product was filtered and washed with distilled water, then dried at 100 ◦ C over night.
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The surfactant template was removed at 550 ◦ C for 8 h to obtain the pure mesoporous silica SBA-15. 2.2. Preparation of Li-SBA-15 The sensor materials of Li-SBA-15 were prepared from different weight ratios of SBA-15 and LiCl. The mixture of LiCl and SBA-15 was ground for 0.5 h in an agate mortar. The grounded mixture was transferred into a crucible and subsequently heated in a muffle furnace with a heating rate of 2 ◦ C/min up to 550 ◦ C, and kept at this temperature for 24 h. Then the Li-SBA-15 samples were obtained, and the products were designated as Li-SBA-15 (X), where X was the content of LiCl in 1 g of SBA15. In our experiment, the value of X was 0.1, 0.3, 0.5, 0.9, and 1.8 for five Li-SBA-15 samples, respectively. 2.3. Methods of characterization The powder XRD patterns were measured on a D8 Tools Xray diffraction instrument using the Cu K␣ radiation at 40 kV and 30 mA. N2 adsorption–desorption isotherms were measured at 77 K on a Micromeritics ASAP 2010 m instrument (Micromeritics Instrument Corp., Norcross, GA). Infrared spectra were taken on a Perkin-Elmer series with a resolution of 4 cm−1 . The samples were prepared in a form of KBr pellet, the thickness of the pellet being about 1.3 mm. Each spectrum was collected at room temperature under atmospheric pressure. The morphology of products was characterized by a JEOL JSM-6700F field emission scanning electron microscope (SEM). The sample was prepared by distributing the powder samples on a double-sided conducting adhesive tape. The characteristic curves of humidity sensitivity were measured on a ZL-5 model LCR analyzer at room temperature. The controlled humidity environments were achieved using saturated aqueous solutions of different salts: LiCl, MgCl2 , Mg(NO3 )2 , NaCl, KCl, and KNO3 in a closed glass vessel at ambient temperature, which yielded 11%, 33%, 54%, 75%, 85% and 95% relative humidity, respectively. The power samples were screen printed on a ceramic plate (1 cm × 0.5 cm) in which a pair of interdigitated gold electrodes were printed and then the humidity device was heated at 70 ◦ C for 5 h. A schematic image of this electrode is shown in Fig. 1.
Fig. 1. A schematic image of the humidity sensor.
Fig. 2. Lower angle XRD patterns of SBA-15 and Li-SBA-15(0.1). The curves for other Li-SBA-15(X) samples were the same as that of Li-SBA-15(0.1).
3. Results and discussion 3.1. Structure and morphology 3.1.1. X-ray diffraction The lower angle XRD patterns of SBA-15 and Li-SBA-15(X) are shown in Fig. 2. We could see that the three peaks attributed to (1 0 0), (1 1 0), and (2 0 0) of SBA-15 were disappeared in LiSBA-15(X), indicating that the mesoporous structure of SBA-15 was destroyed after introducing LiCl into SBA-15. In fact, it was because that LiCl and SBA-15 reacted to form new phases as follows. Fig. 3 shows the wide angle XRD patterns of SBA-15, pure LiCl, and Li-SBA-15 (X = 0.1, 0.3, 0.5, 0.9, and 1.8). As could be seen, a broad peak centered at 22.2◦ of 2θ was observed for the pure SBA-15, indicating that the pore wall of SBA-15 was amorphous. In the pattern of LiCl shown on the top of this
Fig. 3. Wide angle XRD spectra of SBA-15, Li-SBA-15(X) of X = 0.1, 0.3, 0.5, 0.9 and 1.8, and pure LiCl.
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Fig. 4. Nitrogen adsorption–desorption isotherms of (a) mesoporous SBA-15 and (b) Li-SBA-15(0.1).
figure, the diffraction peaks appeared at 2θ = 30.1◦ and 34.9◦ corresponded to the (1 1 1) and (2 0 0) of LiCl, respectively. The peaks at 2θ = 23.2◦ , 32.9◦ , 40.7◦ and 47.4◦ were ascribed to the (2 0 0), (2 2 0), (2 2 2), and (4 0 0) of the lithium chloride hydrate, indicating that some water molecules were absorbed by LiCl to form LiCl(H2 O). For the samples of Li-SBA-15, we can see that the broad peak of SBA-15 became weaker and weaker till disappeared with an increase of LiCl content. And it was found that after the heat treatment of Li-SBA-15(X) at 550 ◦ C, new phases were formed. As seen from the XRD curves, the peaks of LiCl or LiCl(H2 O) disappeared and replaced by the characteristic peaks of lithium silicate. The peaks at 2θ = 18.8◦ , 26.9◦ , 33.0◦ and 38.7◦ were corresponding to the planes of (2 0 0), (1 1 1), (0 2 0), and (0 0 2) of Li2 SiO3 , respectively, and other peaks at 2θ = 24.3◦ , 25.0◦ and 37.6◦ were ascribed to the (0 4 0), (1 1 1), and (0 0 2) of Li2 Si2 O5 . 3.1.2. N2 adsorption–desorption characterization In order to characterize the mesoporous structure of SBA-15 and Li-SBA-15 further, the N2 adsorption–desorption isotherm of SBA-15 and the represent isotherm curve of Li-SBA-15
(X = 0.1) are shown in Fig. 4. As could be seen from Fig. 4a, the curve was of type IV, indicating the mesoporous structure of SBA-15. However, after Li was doped, the typical curve of type IV was disappeared, as could be seen from Fig. 4b, the inflection of the step shifted to a higher relative P/P0 (>0.9) and the hysteresis ring was very small, suggesting that the mesoporous structure was destroyed in Li-SBA-15. This was consistent with the result of low-angle XRD. The pore size distributions, calculated from the desorption isotherms, are shown in Fig. 5. The centered pore size for SBA-15 was 7.22 nm. However, for LiSBA-15, this peak was replaced by three weak peaks of 2.83, 3.80, and 6.43 nm (the corresponding adsorbed volumes were below 0.03 cm3 /g), as shown in Fig. 5b. The inset in Fig. 5b was an amplified curve of these three weak peaks. This result also demonstrated the serious loss of mesoporous structure in LiSBA-15. In addition, the BET surface area also decreased from 421 cm2 /g for SBA-15 to 34.4 cm2 /g for Li-doped SBA-15. 3.1.3. IR spectrum IR spectra of all the samples are shown in Fig. 6. Curves (a)–(g) correspond to the samples of SBA-15, Li-SBA-15(0.1),
Fig. 5. Pore size distribution curves of (a) mesoporous SBA-15 and (b) Li-SBA-15(0.1).
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Fig. 6. IR spectra of (a) SBA-15 and (b)–(f) Li-SBA-15(X) of X = 0.1, 0.3, 0.5, 0.9, and 1.8, respectively. The curve of LiCl (g) is given for reference.
Li-SBA-15(0.3), Li-SBA-15(0.5), Li-SBA-15(0.9), Li-SBA15(1.8), and LiCl, respectively. As can be seen, for mesoporous silica SBA-15, the peaks at 1092 and 808 cm−1 were attributed to the asymmetric stretching and symmetric modes of Si–O–Si lattice vibrations, respectively. Characteristic peaks of Si–OH were observed at 1635 and 950 cm−1 . All the bands were similar to the results of literature [18,22,23]. It was interesting that the peak of 950 cm−1 was very weak for SBA-15 compared to other samples. This was because that the number of surface hydroxyls on the pore walls of SBA-15 decreased after the sample was heat-treated at 550 ◦ C for 8 h. After the introduction of LiCl, the characteristic peak of Si–OH at 950 cm−1 became stronger and stronger with increasing the LiCl level up to 0.5 g. This can be explained as follows: when the level of LiCl increased, more water molecules were adsorbed due to the hydrophilic property of LiCl. Water molecules interacted with the Si–O–Si of SBA-15 to form the surface hydroxyls. So the intensity of the peak became stronger. However, when the content of LiCl was beyond 0.5 g, more water molecules were adsorbed, and some of the hydroxyls began to release the H+ , resulting in a decreased
Fig. 8. Humidity sensitive properties of pure SBA-15 and Li-SBA-15(X) of X = 0.1, 0.3, 0.5, 0.9, and 1.8.
amount of the surface hydroxyls, so that the intensity of this peak trended to reduce. In addition, the band at 808 cm−1 that was ascribed to Si–O–Si gradually disappeared with increasing the LiCl level and was replaced by a few bands of lithium silicate. This phenomenon proved again that the dopant LiCl reacted with SBA-15 to form a new phase. This was in agreement with the result of wide angle XRD. 3.1.4. SEM image The typical morphology of SBA-15 and a representative image of Li-SBA-15(X) are shown in Fig. 7. As can be seen from Fig. 7a, mesoporous SBA-15 was consisted of some short rods. The diameter of these rods was about 300 nm. After the doping of LiCl, the morphology of SBA-15 was maintained, as can be observed in Fig. 7b, although it was slightly different from that of SBA-15. The partial loss of the morphology may be
Fig. 7. SEM images of (a) mesoporous SBA-15 and (b) Li-SBA-15(0.1).
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Fig. 9. Variations of impedance of Li-SBA-15(0.1) as a function of relative humidity at different frequencies of 100 Hz, 1 kHz, 10 kHz, and 100 kHz.
related to the reaction between LiCl and SBA-15, as confirmed by XRD patterns and IR spectra. 3.2. Humidity sensitive property The results of resistance measurements as a function of %RH at room temperature are shown in Fig. 8. It can be seen that the drop of the resistance for pure SBA-15 was smaller in the whole %RH range, showing lower humidity sensitive properties. After the doping of LiCl, the resistance of all the Li-SBA-15 samples greatly decreased with an increase in relative humidity by more than three orders of magnitude over the range of 11–95%RH, showing higher humidity sensitive properties. However, as can
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be seen, when the LiCl content was below 0.5 g/g, the curves presented good linearity. When the content of LiCl was 0.9 g/g and 1.8 g/g, however, the linearity of the resistance versus %RH was relatively poor over a wide humidity range. So, it was not suitable for Li-SBA-15(0.9) and Li-SBA-15(1.8) to serve as humidity sensors. The sample of Li-SBA-15(0.1) exhibiting the maximum humidity sensitive property and linearity was selected for the evaluation of hysteresis, response–recovery time, and so on. Fig. 9 shows the dependence of impedance of Li-SBA15(0.1) on relative humidity and the measurement frequency. As can be seen, when the frequency was 100 Hz, the impedance decreased greatly with increasing the relative humidity. However, when a higher frequency was used, such as 1 kHz, 10 kHz, and even 100 kHz, the range of impedance change became less with increasing the %RH. Namely, at low %RH, the influence of frequency on the impedance was larger than that at high %RH. In order to study the sensing behavior, the complex impedance plots are shown in Fig. 10. As could be seen, at low humidity, a semicircle was observed, which was mainly resulted from the intrinsic impedance of the materials [24]. With an increase of humidity, the semicircle radius gradually decreased and a straight line was appeared at low frequencies. This straight line was considered to be the contribution of ionic species in the Li-doped mesoporous silica. The response and recovery curve is shown in Fig. 11. According to the literature [25], the time taken by a sensor to achieve 90% of the total impedance change is defined as the response time or recovery time. For the sample of Li-SBA-15(0.1), the response time was about 21 s when increasing from 11% to 95%RH, and the recovery time was 51 s when decreasing from 95% to 11%. Fig. 12 reveals the hysteresis data of Li-SBA15(0.1). The hysteresis of the sensing material was about 6%.
Fig. 10. Complex impedance plots of Li-SBA-15(0.1) at different relative humidity.
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Fig. 11. Response and recovery characteristic of Li-SBA-15(0.1).
3.3. Mechanism of the humidity sensitive properties Here, we try to give a possible mechanism qualitatively to explain the humidity sensitive properties of our samples. For pure mesoporous SBA-15, due to its highly resistive nature and the decrease of hydroxyl groups from the surface during the heattreating process, it showed a poor humidity sensitive property. Similar results have been reported in literature [14,18]. After the doping of LiCl, the concentration of surface hydroxyl groups increased, as indicated by the IR spectra. These hydroxyl groups provide the adsorption sites for water and play an important role to the humidity sensitive properties. At low relative humidity, only a few water molecules are adsorbed. The coverage of water on the surface is not continuous. A proton may be transferred from a Si–OH group to a water molecule to form H3 O+ [17], and
the proton migrates only by hopping from site to site across the surface. The transfer of H+ or H3 O+ is difficult on this discontinuous water layer, so that the impedance is relatively high at low %RH. When the %RH is increased to the middle region, one or several serial water layers are formed. The serial water layer accelerates the transfer of H+ or H3 O+ , according to the ion transfer mechanism of Grotthuss [26], H2 O + H3 O+ → H3 O+ + H2 O. The initial and final states are the same. The energy is also equivalent, so that the transfer of ions is quite easy. In this process, some Li+ come from the dissociation of lithium silicate also attend the transfer process. Based on the model of ion transport mechanism reported by Casalbore-Miceli et al. [27], Li ions could dissolve in the adsorbed water, and the dielectric constant, which is a function of the adsorbed water, makes them free from the interaction of the opposite charges. So these ions could transfer freely. The quick transfer of ions on the water layer results in a sharp decrease of the impedance. When the %RH increases to the high region, more lithium silicate dissociates into free Li+ and the Li+ ions dominate the ion transfer process. So, the impedance continuously decreases by more than three orders of magnitude compared to the initial impedance. 4. Conclusions Mesoporous silica SBA-15 and Li-doped SBA-15 were studied as humidity sensor materials. Their structure was characterized by low-angle XRD, wide-angle XRD, N2 adsorption–desorption, and IR spectroscopy. These tests indicated that the ordered mesoporous structure of SBA-15 was lost by the doping of LiCl, and it was found that lithium silicate was formed by the interaction of LiCl and SBA-15. The abundant dissociation of the lithium silicate into free Li+ ions at high relative humidity played an important role in improving the humidity sensitive properties of these materials. Of all these samples, Li-SBA-15(0.1) presented the maximum humidity sensitive property, better linearity and shorter response–recovery time. Acknowledgements The authors acknowledge Nailin Yue, who is studying in Department of Chemistry and State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, for her generous help in SEM, and we are grateful to Dr. Tong Zhang for her timely help in the field of humidity sensors. This work was supported by the National Science Foundation (Grant No. 20151001) and Natural Science Foundation of Jilin province (Grant No. 20040505). References
Fig. 12. Hysteresis of Li-SBA-15(0.1).
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Biographies Wangchang Geng received his BS degree in Department of Material Science, Jilin University, in 2003. In 2005, he entered the PhD course ahead of schedule after 2 year’s MS course in Jilin University. His main areas of interest are the synthesis of mesoporous materials, and the humidity sensitive property of host–guest composite materials. Rui Wang received her BS degree from the College of Electronics Science and Engineering, Jilin University, Changchun, China in 2005. Presently she is a graduate student, majored in microelectronics and solid state electronics, and engaged in novel sensing materials and humidity sensors. Xiaotian Li is a professor of Department of Material Science in Jilin University. He received a BS, MS degree in Department of Electronic Science in Jilin University in 1989 and 1992, respectively. He obtained his PhD degree in the Department of Chemistry and State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, in 2000. His main research interests are inorganic chemistry. Yongcun Zou received his BS degree in Department of Entironment Science, Jilin University, in 2001. His research area is instrument analysis. Tong Zhang received her MS degree in major of Semiconductor Materials in 1992 and PhD degree in the field of Microelectronics and Solid State Electronics in 2001 from Jilin University. She was appointed a full professor in College of Electronics Science and Engineering, Jilin University in 2001. Now, she is interested in the field of sensing functional materials, gas sensors and humidity sensors. Jinchun Tu received a BS degree in Department of Material Science, Jilin University, in 2003. Now, he is studying for the MS course, and his major interests are mesoporous metal oxides and chemical sensor technology. Yuan He received her BS degree from the College of Electronics Science and Engineering, Jilin University, Changchun, China in 2006. Presently she is a graduate student, majored in microelectronics and solid state electronics, and engaged in novel sensing materials and humidity sensors. Nan Li received her BS degree in Department of Chemistry, Jilin University, in 1999. Then she obtained her PhD from Jilin University in 2004. Her main research interests are inorganic chemistry and chemical sensors.