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Study on humidity sensitive property of K2CO3-SBA-15 composites
Applied Surface Science 256 (2009) 280–283
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Study on humidity sensitive property of K2CO3-SBA-15 composites Qing Yuan a, Wangchang Geng a, Nan Li a, Jinchun Tu a, Rui Wang b, Tong Zhang b, Xiaotian Li a,* a b
Department of Material Science and Engineering, Key Laboratory of Automobile Materials of Ministry of Education, Jilin University, Changchun 130012, PR China State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 25 February 2009 Received in revised form 3 August 2009 Accepted 4 August 2009 Available online 11 August 2009
K2CO3-SBA-15 with different K2CO3 content was prepared by thermal dispersion. The structures of resultant powders were characterized by XRD, N2 adsorption and IR. The humidity sensing properties of the powders were also investigated. Compared with pure SBA-15, K2CO3-SBA-15 shows improved humidity sensing properties and the introducing level of K2CO3 has a great influence on the humid sensitivity of K2CO3-SBA-15 composites. The optimal mixing ratio was K2CO3-SBA-15 (0.8 g/g), which exhibited excellent linearity in the whole range of 11–95%RH, a resistance variation of about five orders of magnitude, and a rapid response time and recovery time about 15 and 50 s, respectively. The mechanism of the humidity sensitive properties was also discussed. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Humidity sensor Composites K2CO3 SBA-15
1. Introduction Humidity is a physical parameter denoting the degree of dryness in the atmosphere. Due to the importance of humidity on food processing, textile technology, storage areas, computer rooms, hospitals, museums and so on, good performance humidity sensors, which own the characters of high sensitivity, rapid response and fast recovery, are much more in need [1]. In recent years, many kinds of sensor materials like polymers, ceramics and composites have been produced to serve as humidity sensor body [2–4]. Among these materials, SiO2, because of both chemical and thermal stability, can be mixed with varied polymers as composites to heighten the humid sensitivity of sensors [5,6]. However, pure SiO2 did not show humidity sensitivity until the relative humidity reached as high as 60%, showing a poor humidity sensitive property [7]. For the sake of improving the performance of humidity sensors, salt-doping is an advisable method [8,9]. Compared with other structure, SiO2, the mesoporous silica SBA-15, owing to its uniform pore structure, the highly controllable and monodispersive nature of the large accessible pore size, a large surface area, has been often used as host materials to be filled with various materials to improve performance of guest materials [10–12]. It is known that K2CO3 possesses hydrophilic property and the stability, and sensitivity of humidity sensitive materials can be improved by adding K2CO3 during the sample preparation [13]. In
* Corresponding author. Tel.: +86 431 85168445; fax: +86 431 85168444. E-mail addresses: [email protected] (Q. Yuan), [email protected], [email protected] (X. Li). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.08.016
our work, a simple physical mixing method has been employed to prepare K2CO3-SBA-15 composites. The influence of mixing level on the sensitive property was studied and the mechanism of humidity sensing properties has also been discussed in detail. 2. Experimental 2.1. Preparation of mesoporous silica SBA-15 Mesoporous SBA-15 was synthesized according to the method reported by Zhao et al. [14]. Briefly, 2 g of Pluronic P123 (ethylene oxide (EO)–propylene oxide (PO) triblock copolymer, composition as EO20PO70EO20, Aldrich) was dissolved in 15 mL of distilled water under stirring. Subsequently, 60 mL (2 M) of HCl was added. After stirring for 30 min, 4.4 g tetraethyl orthosilicate (TEOS) was added dropwise under vigorous stirring at 40 8C and then the resultant mixture was aged at 60 8C for 24 h without stirring. The product was filtered and washed with distilled water, then dried at 100 8C over night. The surfactant was removed by calcining in air at 550 8C for 10 h to obtain pure mesoporous silica SBA-15. 2.2. Preparation of K2CO3-SBA-15 K2CO3-SBA-15 was obtained by grinding the mixture of K2CO3 and SBA-15 with different weight ratios using an agate mortar at room temperature. The resulting homogeneous powders were calcined in 823 K at a constant heating rate of 1 8C/min in air for 10 h. The obtained powders were denoted as nK-SBA-15, where n represents the mass of K2CO3 in 1 g of SBA-15 (n = 0.16, 0.48, 0.8, and 1.44 g, respectively).
Q. Yuan et al. / Applied Surface Science 256 (2009) 280–283
2.3. Characterization The structure of the products was characterized by X-ray diffraction (XRD) operated at 40 kV and 40 mA using Cu Ka radiation. N2 adsorption–desorption isotherms were measured at 77 K on a Micromeritics ASAP 2010 m instrument (Micromeritics Instrument Corp., Norcross, GA). Specific surface area was calculated using BET (Brunauer–Emmett–Teller) method, and pore size distribution was determined by applying the BJH (Barrett– Joyner–Halenda) model. Infrared spectra were taken on a Perkin– Elmer series with a resolution of 4 cm 1. Humidity sensitivity properties were measured on a ZL-5 model LCR analyzer (Made in shanghai, China) at 1 kHz and 1.0 V (AC) at room temperature. The controlled humidity environments were achieved using super saturation aqueous solutions of different salts of LiCl, MgCl2, Mg(NO3)2, NaCl, KCl and KNO3 in a closed glass vessel at room temperature, which yielded 11%, 33%, 54%, 75%, 85% and 95% relative humidity, respectively. 3. Results and discussion 3.1. Structure and morphology characterization 3.1.1. X-ray diffraction Fig. 1 depicts the XRD patterns of as-prepared SBA-15 and K2CO3-SBA-15 composites with different potassium contents. For pure mesoporous silica SBA-15, three well-resolved peaks shown in Fig. 1(a) are indexable to (1 0 0), (1 1 0), (2 0 0) reflections associated with p6mm hexagonal symmetry, and a broad peak centered at 22.28 of 2u shown in Fig. 1(b) demonstrates that the pore wall of SBA-15 was amorphous. For K2CO3-SBA-15 composites, the low-angle peaks of the 0.16 and 0.48 g/g samples did not change much compared with those of pure SBA-15, indicating that the mesoporous structure of SBA-15 was remained. While close inspection reveals that reflection (1 0 0) has a tendency to shift to higher angle, it becomes weaker with the introducing level increased. This phenomenon could be attributed to the loading amount; the more K2CO3 mixed in pure SBA-15, the higher angle (1 0 0) shifted, which was in agreement with other doping in
281
mesoporous materials reported in many literatures [15–17]. Furthermore, no reflection of potassium carbonate crystalline phase of the 0.16 and 0.48 g/g samples was detected, suggesting the full dispersion of K2CO3 in the host material. With increasing the mixing content (n = 0.8 g, 1.44 g), three peaks in low-angle scale were disappeared, indicating the mesoporous structure was absolutely demolished by introducing more K2CO3 into SBA-15. Moreover, the reflection of potassium carbonate crystalline phase was detected in the wide-angle scale and the characteristic peaks of crystalline potassium silicate also emerged. This is because under the heat treatment of K2CO3-SBA-15 at 550 8C, some potassium carbonate reacted with SiO2 to produce potassium silicate. 3.1.2. IR spectra The infrared spectra of all the samples were presented in Fig. 2. As is evident from the spectra, for pure SBA-15, the 1632 and 970 cm 1 band were characteristic of surface silanol group Si–OH stretching modes, the peaks at 1087 and 806 cm 1 were attributed to the asymmetric stretching and symmetric modes of Si–O–Si lattice vibrations [18], the 1632 and 3500 cm 1 band were the bending vibration of water. As can be seen, the intensity at 3500 cm 1 band of SBA-15 and all the K2CO3-SBA-15 samples is almost the same. However, as the loading amount of K2CO3 rose, the band at 1632 cm 1 progressively grew compared with SBA-15. So the increasing intensity of 1632 cm 1 band is attributed to Si– OH stretching modes. This is because that K2CO3 possesses hydrophilic property, with the introducing level of K2CO3 increased, more water molecules were adsorbed, and the Si–O– Si of SBA-15 interacted with water molecules to form the surface hydroxyls. As a consequence, the peak corresponding to the stretching mode of Si–OH group became stronger. In addition, with increasing K2CO3 adding level, the band at 1405 cm 1 that was ascribed to CO32 appeared and gradually increased, while the band at 806 cm 1 that was ascribed to Si–O–Si gradually disappeared and was replaced by a few bands arising from potassium silicate. This phenomenon proved again that some added K2CO3 reacted with SBA-15 to form potassium silicate, which was consistent with the result of wide-angle XRD.
Fig. 1. Low-angle (a) and wide-angle (b) XRD patterns of SBA-15, K2CO3-SBA-15 and K2CO3.
Q. Yuan et al. / Applied Surface Science 256 (2009) 280–283
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Fig. 2. IR spectra of SBA-15, K2CO3-SBA-15 and K2CO3.
3.1.3. N2 adsorption–desorption characterization Fig. 3 presents the N2 adsorption–desorption isotherms of pure SBA-15 and K2CO3-SBA-15 (n = 0.8 g). SBA-15 shows type IV isotherms with H1 hysteresis loop, which is typical for mesoporous materials with two-dimensional hexagonal structure. The average pore size calculated from adsorption branch is 4.10 nm.When K2CO3 was added, the typical type IV curve disappeared. In addition, the BET surface area reduced from 592 m2/g for pure SBA15 to 1.60 m2/g for K2CO3-SBA-15. These results indicated that the mesoporous structure was destroyed.
Fig. 4. Variations in impedance with relative humidity of SBA-15 and K2CO3-SBA-15.
The humidity sensitive properties (HSP) of SBA-15 and K2CO3SBA-15 were shown in Fig. 4. It was found that the impedance of pure SBA-15 did not change much until relative humidity increased to 65%. In the whole relative humidity range of our measurement from 11% to 95%, the resistance level changed less than one order of magnitude, suggesting its poor humidity
sensitivity. Whereas when K2CO3 was added, the sensitivity was enhanced gradually with increased K2CO3 amount. For example, when the introducing level was 0.48 g, the resistance level in the whole humidity ranges changed near upon three orders of magnitude. When the loading amount continued to increase (0.8 and 1.44 g), the change of the resistance level reached to five orders of magnitude. Among all the samples, the optimal mixing ratio is 0.8 g/g, because the sensor based on this sample had shown a very good linearity in the whole humidity range from 11%RH to 95%RH. Therefore, our study was focused on this sample. Response and recovery behavior is one of the important characteristics of humidity sensor and this measurement of K2CO3SBA-15 (n = 0.8 g) was shown in Fig. 5. For this sample, the
Fig. 3. N2 adsorption–desorption isotherms of pure SBA-15 and K2CO3-SBA-15 (0.8 g/g).
Fig. 5. Response and recovery properties of the humidity sensor.
3.2. Humidity sensitive properties
Q. Yuan et al. / Applied Surface Science 256 (2009) 280–283
283
mainly contribute to conduction. With increasing the relative humidity, the more water molecules are, the complete surface coverage is. Here, serial water layers are formed by physisorption and accelerate the transfer of H+ or H3O+. Based on the model of ion transfer mechanism of Grotthuss [20], H2O + H3O+ = H3O+ + H2O, the initial state and final state are same, the energy is also equivalent, so the transfer of ion is quite easy. Moreover, according to the model of ion transport mechanism reported by GasalboreMiceli et al. [21], K+, CO32 and SiO42 dissociated in the adsorbed water were also considered to be responsible for the conduction. In a word, the sensing principle of this material is mainly proton and ion conductivity in low and high relative humidity, respectively. 4. Conclusion K2CO3-SBA-15 were successfully prepared and investigated as humidity sensors. The sensitivity of SBA-15 was greatly improved by added K2CO3. The optimal mixing ratio was 0.8 g/g, which had the best linearity correlation between impedance and humidity in the whole humidity range. The resistance level had changed more than four orders of magnitude and the response-recovery time was about 65 s. Acknowledgements Fig. 6. The relationship of impedance and relative humidity at different frequencies of K2CO3-SBA-15 (0.8 g/g).
response time from 11%RH to 95%RH was around 15 s, and the recovery time from 95%RH to 11%RH was around 50 s, which indicated that the sample has a quick response to humidity change. Fig. 6 shows the dependence of impedance of K2CO3-SBA-15 (n = 0.8 g) on relative humidity and the measurement frequency. It can be seen that at low relative humidity, with the operating frequency increasing, the impedance gradually decreased and at higher relative humidity, the four lines overlapped together, indicating that in this range the effect of different frequencies on impedance was small. Of all the lines, the line obtained at 100 Hz of serving frequency showed the best linearity, so 100 Hz was selected as our operating frequency. 3.3. Mechanism of humidity sensing properties The humidity sensing mechanism of the K2CO3-SBA-15 was analyzed as follows. At low relative humidity, water molecule was little. According to Anderson’s protons conductivity model [19], when no molecular water is present, surface coverage is not complete, and protons can be formed. At this time, only a few water molecules are chemisorbed. A proton transfers from a Si–OH group to a water molecular to form H3O+ and only migrates by hopping from site to site across the surface. So it is considered that protons
This work was supported by National Natural Science Foundation of China (no. 20743005), Chenug Kung Scholar Innovation Team (IRT0625), and Key Project from Education Ministry, China (grant no. 108042). References [1] T.H. Huang, J.C. Chou, T.P. Sun, S.K. Hsiung, Sens. Actuators B 134 (2008) 206. [2] Y. Li, M.J. Yang, G. Casalbore-Miceli, N. Camaioni, Synth. Met. 128 (2002) 293. [3] G. Mattogno, G. Righini, G. Montesperelli, E. Traversa, Appl. Surf. Sci. 70–71 (1993) 363. [4] K.M.S. Khalil, S.A. Makhlouf, Sens. Actuators A 148 (2008) 39. [5] P.-G. Su, S.-C. Huang, Sens. Actuators B 113 (2006) 142. [6] P.-G. Su, S.-C. Huang, Sens. Actuators B 105 (2005) 170. [7] T. Zhang, R. Wang, W.C. Geng, et al. Sens. Actuators B 128 (2008) 482. [8] L.J. Wang, D. Li, R. Wang, et al. Sens. Actuators B 133 (2008) 622. [9] X.H. Wang, J. Zhang, Z.Q. Zhu, J.Z. Zhu, Appl. Surf. Sci. 253 (2007) 3168. [10] Z.C. Liu, H.R. Chen, W.M. Huang, et al. Microporous Mesoporous Mater. 89 (2006) 270. [11] W. Wang, M. Song, Mater. Res. Bull. 41 (2006) 436. [12] H.X. Jin, L. Li, N.J. Chu, et al. Mater. Chem. Phys. 112 (2008) 112. [13] Z. Wang, C. Chen, T. Zhang, et al. Sens. Actuators B 126 (2007) 678. [14] D.Y. Zhao, J.L. Feng, Q.S. Huo, et al. Science 279 (1998) 548. [15] I.-S. Park, S.Y. Choi, J.S. Ha, Chem. Phys. Lett. 456 (2008) 198. [16] Q. Jiang, Z.Y. Wu, Y.M. Wang, et al. J. Mater. Chem. 16 (2006) 1536. [17] T.S. Jiang, W. Shen, Q. Zhao, M. Li, J.Y. Chu, H.B. Yin, J. Solid State Chem. 181 (2008) 2298. [18] C.-T. Wang, C.-L. Wu, Thin Solid Films 496 (2006) 658. [19] J.H. Anderson, G.A. Parks, J. Phys. Chem. 72 (1968) 3662. [20] F.M. Ernsberger, J. Am. Ceram. Soc. 66 (1983) 747. [21] G. Casalbore-Miceli, M.J. Yang, N. Camaioni, et al. Solid State Ionics 131 (2000) 311.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Study on humidity sensitive property of K2CO3-SBA-15 composites Qing Yuan a, Wangchang Geng a, Nan Li a, Jinchun Tu a, Rui Wang b, Tong Zhang b, Xiaotian Li a,* a b
Department of Material Science and Engineering, Key Laboratory of Automobile Materials of Ministry of Education, Jilin University, Changchun 130012, PR China State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 25 February 2009 Received in revised form 3 August 2009 Accepted 4 August 2009 Available online 11 August 2009
K2CO3-SBA-15 with different K2CO3 content was prepared by thermal dispersion. The structures of resultant powders were characterized by XRD, N2 adsorption and IR. The humidity sensing properties of the powders were also investigated. Compared with pure SBA-15, K2CO3-SBA-15 shows improved humidity sensing properties and the introducing level of K2CO3 has a great influence on the humid sensitivity of K2CO3-SBA-15 composites. The optimal mixing ratio was K2CO3-SBA-15 (0.8 g/g), which exhibited excellent linearity in the whole range of 11–95%RH, a resistance variation of about five orders of magnitude, and a rapid response time and recovery time about 15 and 50 s, respectively. The mechanism of the humidity sensitive properties was also discussed. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Humidity sensor Composites K2CO3 SBA-15
1. Introduction Humidity is a physical parameter denoting the degree of dryness in the atmosphere. Due to the importance of humidity on food processing, textile technology, storage areas, computer rooms, hospitals, museums and so on, good performance humidity sensors, which own the characters of high sensitivity, rapid response and fast recovery, are much more in need [1]. In recent years, many kinds of sensor materials like polymers, ceramics and composites have been produced to serve as humidity sensor body [2–4]. Among these materials, SiO2, because of both chemical and thermal stability, can be mixed with varied polymers as composites to heighten the humid sensitivity of sensors [5,6]. However, pure SiO2 did not show humidity sensitivity until the relative humidity reached as high as 60%, showing a poor humidity sensitive property [7]. For the sake of improving the performance of humidity sensors, salt-doping is an advisable method [8,9]. Compared with other structure, SiO2, the mesoporous silica SBA-15, owing to its uniform pore structure, the highly controllable and monodispersive nature of the large accessible pore size, a large surface area, has been often used as host materials to be filled with various materials to improve performance of guest materials [10–12]. It is known that K2CO3 possesses hydrophilic property and the stability, and sensitivity of humidity sensitive materials can be improved by adding K2CO3 during the sample preparation [13]. In
* Corresponding author. Tel.: +86 431 85168445; fax: +86 431 85168444. E-mail addresses: [email protected] (Q. Yuan), [email protected], [email protected] (X. Li). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.08.016
our work, a simple physical mixing method has been employed to prepare K2CO3-SBA-15 composites. The influence of mixing level on the sensitive property was studied and the mechanism of humidity sensing properties has also been discussed in detail. 2. Experimental 2.1. Preparation of mesoporous silica SBA-15 Mesoporous SBA-15 was synthesized according to the method reported by Zhao et al. [14]. Briefly, 2 g of Pluronic P123 (ethylene oxide (EO)–propylene oxide (PO) triblock copolymer, composition as EO20PO70EO20, Aldrich) was dissolved in 15 mL of distilled water under stirring. Subsequently, 60 mL (2 M) of HCl was added. After stirring for 30 min, 4.4 g tetraethyl orthosilicate (TEOS) was added dropwise under vigorous stirring at 40 8C and then the resultant mixture was aged at 60 8C for 24 h without stirring. The product was filtered and washed with distilled water, then dried at 100 8C over night. The surfactant was removed by calcining in air at 550 8C for 10 h to obtain pure mesoporous silica SBA-15. 2.2. Preparation of K2CO3-SBA-15 K2CO3-SBA-15 was obtained by grinding the mixture of K2CO3 and SBA-15 with different weight ratios using an agate mortar at room temperature. The resulting homogeneous powders were calcined in 823 K at a constant heating rate of 1 8C/min in air for 10 h. The obtained powders were denoted as nK-SBA-15, where n represents the mass of K2CO3 in 1 g of SBA-15 (n = 0.16, 0.48, 0.8, and 1.44 g, respectively).
Q. Yuan et al. / Applied Surface Science 256 (2009) 280–283
2.3. Characterization The structure of the products was characterized by X-ray diffraction (XRD) operated at 40 kV and 40 mA using Cu Ka radiation. N2 adsorption–desorption isotherms were measured at 77 K on a Micromeritics ASAP 2010 m instrument (Micromeritics Instrument Corp., Norcross, GA). Specific surface area was calculated using BET (Brunauer–Emmett–Teller) method, and pore size distribution was determined by applying the BJH (Barrett– Joyner–Halenda) model. Infrared spectra were taken on a Perkin– Elmer series with a resolution of 4 cm 1. Humidity sensitivity properties were measured on a ZL-5 model LCR analyzer (Made in shanghai, China) at 1 kHz and 1.0 V (AC) at room temperature. The controlled humidity environments were achieved using super saturation aqueous solutions of different salts of LiCl, MgCl2, Mg(NO3)2, NaCl, KCl and KNO3 in a closed glass vessel at room temperature, which yielded 11%, 33%, 54%, 75%, 85% and 95% relative humidity, respectively. 3. Results and discussion 3.1. Structure and morphology characterization 3.1.1. X-ray diffraction Fig. 1 depicts the XRD patterns of as-prepared SBA-15 and K2CO3-SBA-15 composites with different potassium contents. For pure mesoporous silica SBA-15, three well-resolved peaks shown in Fig. 1(a) are indexable to (1 0 0), (1 1 0), (2 0 0) reflections associated with p6mm hexagonal symmetry, and a broad peak centered at 22.28 of 2u shown in Fig. 1(b) demonstrates that the pore wall of SBA-15 was amorphous. For K2CO3-SBA-15 composites, the low-angle peaks of the 0.16 and 0.48 g/g samples did not change much compared with those of pure SBA-15, indicating that the mesoporous structure of SBA-15 was remained. While close inspection reveals that reflection (1 0 0) has a tendency to shift to higher angle, it becomes weaker with the introducing level increased. This phenomenon could be attributed to the loading amount; the more K2CO3 mixed in pure SBA-15, the higher angle (1 0 0) shifted, which was in agreement with other doping in
281
mesoporous materials reported in many literatures [15–17]. Furthermore, no reflection of potassium carbonate crystalline phase of the 0.16 and 0.48 g/g samples was detected, suggesting the full dispersion of K2CO3 in the host material. With increasing the mixing content (n = 0.8 g, 1.44 g), three peaks in low-angle scale were disappeared, indicating the mesoporous structure was absolutely demolished by introducing more K2CO3 into SBA-15. Moreover, the reflection of potassium carbonate crystalline phase was detected in the wide-angle scale and the characteristic peaks of crystalline potassium silicate also emerged. This is because under the heat treatment of K2CO3-SBA-15 at 550 8C, some potassium carbonate reacted with SiO2 to produce potassium silicate. 3.1.2. IR spectra The infrared spectra of all the samples were presented in Fig. 2. As is evident from the spectra, for pure SBA-15, the 1632 and 970 cm 1 band were characteristic of surface silanol group Si–OH stretching modes, the peaks at 1087 and 806 cm 1 were attributed to the asymmetric stretching and symmetric modes of Si–O–Si lattice vibrations [18], the 1632 and 3500 cm 1 band were the bending vibration of water. As can be seen, the intensity at 3500 cm 1 band of SBA-15 and all the K2CO3-SBA-15 samples is almost the same. However, as the loading amount of K2CO3 rose, the band at 1632 cm 1 progressively grew compared with SBA-15. So the increasing intensity of 1632 cm 1 band is attributed to Si– OH stretching modes. This is because that K2CO3 possesses hydrophilic property, with the introducing level of K2CO3 increased, more water molecules were adsorbed, and the Si–O– Si of SBA-15 interacted with water molecules to form the surface hydroxyls. As a consequence, the peak corresponding to the stretching mode of Si–OH group became stronger. In addition, with increasing K2CO3 adding level, the band at 1405 cm 1 that was ascribed to CO32 appeared and gradually increased, while the band at 806 cm 1 that was ascribed to Si–O–Si gradually disappeared and was replaced by a few bands arising from potassium silicate. This phenomenon proved again that some added K2CO3 reacted with SBA-15 to form potassium silicate, which was consistent with the result of wide-angle XRD.
Fig. 1. Low-angle (a) and wide-angle (b) XRD patterns of SBA-15, K2CO3-SBA-15 and K2CO3.
Q. Yuan et al. / Applied Surface Science 256 (2009) 280–283
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Fig. 2. IR spectra of SBA-15, K2CO3-SBA-15 and K2CO3.
3.1.3. N2 adsorption–desorption characterization Fig. 3 presents the N2 adsorption–desorption isotherms of pure SBA-15 and K2CO3-SBA-15 (n = 0.8 g). SBA-15 shows type IV isotherms with H1 hysteresis loop, which is typical for mesoporous materials with two-dimensional hexagonal structure. The average pore size calculated from adsorption branch is 4.10 nm.When K2CO3 was added, the typical type IV curve disappeared. In addition, the BET surface area reduced from 592 m2/g for pure SBA15 to 1.60 m2/g for K2CO3-SBA-15. These results indicated that the mesoporous structure was destroyed.
Fig. 4. Variations in impedance with relative humidity of SBA-15 and K2CO3-SBA-15.
The humidity sensitive properties (HSP) of SBA-15 and K2CO3SBA-15 were shown in Fig. 4. It was found that the impedance of pure SBA-15 did not change much until relative humidity increased to 65%. In the whole relative humidity range of our measurement from 11% to 95%, the resistance level changed less than one order of magnitude, suggesting its poor humidity
sensitivity. Whereas when K2CO3 was added, the sensitivity was enhanced gradually with increased K2CO3 amount. For example, when the introducing level was 0.48 g, the resistance level in the whole humidity ranges changed near upon three orders of magnitude. When the loading amount continued to increase (0.8 and 1.44 g), the change of the resistance level reached to five orders of magnitude. Among all the samples, the optimal mixing ratio is 0.8 g/g, because the sensor based on this sample had shown a very good linearity in the whole humidity range from 11%RH to 95%RH. Therefore, our study was focused on this sample. Response and recovery behavior is one of the important characteristics of humidity sensor and this measurement of K2CO3SBA-15 (n = 0.8 g) was shown in Fig. 5. For this sample, the
Fig. 3. N2 adsorption–desorption isotherms of pure SBA-15 and K2CO3-SBA-15 (0.8 g/g).
Fig. 5. Response and recovery properties of the humidity sensor.
3.2. Humidity sensitive properties
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mainly contribute to conduction. With increasing the relative humidity, the more water molecules are, the complete surface coverage is. Here, serial water layers are formed by physisorption and accelerate the transfer of H+ or H3O+. Based on the model of ion transfer mechanism of Grotthuss [20], H2O + H3O+ = H3O+ + H2O, the initial state and final state are same, the energy is also equivalent, so the transfer of ion is quite easy. Moreover, according to the model of ion transport mechanism reported by GasalboreMiceli et al. [21], K+, CO32 and SiO42 dissociated in the adsorbed water were also considered to be responsible for the conduction. In a word, the sensing principle of this material is mainly proton and ion conductivity in low and high relative humidity, respectively. 4. Conclusion K2CO3-SBA-15 were successfully prepared and investigated as humidity sensors. The sensitivity of SBA-15 was greatly improved by added K2CO3. The optimal mixing ratio was 0.8 g/g, which had the best linearity correlation between impedance and humidity in the whole humidity range. The resistance level had changed more than four orders of magnitude and the response-recovery time was about 65 s. Acknowledgements Fig. 6. The relationship of impedance and relative humidity at different frequencies of K2CO3-SBA-15 (0.8 g/g).
response time from 11%RH to 95%RH was around 15 s, and the recovery time from 95%RH to 11%RH was around 50 s, which indicated that the sample has a quick response to humidity change. Fig. 6 shows the dependence of impedance of K2CO3-SBA-15 (n = 0.8 g) on relative humidity and the measurement frequency. It can be seen that at low relative humidity, with the operating frequency increasing, the impedance gradually decreased and at higher relative humidity, the four lines overlapped together, indicating that in this range the effect of different frequencies on impedance was small. Of all the lines, the line obtained at 100 Hz of serving frequency showed the best linearity, so 100 Hz was selected as our operating frequency. 3.3. Mechanism of humidity sensing properties The humidity sensing mechanism of the K2CO3-SBA-15 was analyzed as follows. At low relative humidity, water molecule was little. According to Anderson’s protons conductivity model [19], when no molecular water is present, surface coverage is not complete, and protons can be formed. At this time, only a few water molecules are chemisorbed. A proton transfers from a Si–OH group to a water molecular to form H3O+ and only migrates by hopping from site to site across the surface. So it is considered that protons
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