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Humidity sensing and electrical properties of Na- and K-montmorillonite
Available online at www.sciencedirect.com
Sensors and Actuators B 129 (2008) 380–385
Humidity sensing and electrical properties of Na- and K-montmorillonite Pi-Guey Su ∗ , Ching-Yin Chen Department of Chemistry, Chinese Culture University, Taipei 111, Taiwan Received 19 April 2007; received in revised form 21 August 2007; accepted 21 August 2007 Available online 24 August 2007
Abstract Cationic clays, Na- and K-montmorillonite, were used to make a humidity sensor. The impedances of the Na- and K-montmorillonite were measured at various humidities and temperatures in a frequency range of 60 Hz–100 kHz. The results indicated that the counterions played a dominant role in conductance mechanism of the montmorillonite films. Moreover, the complex impedance spectra were adopted to elucidate the transport process by counterions in the mechanism for the conductance variation with %RH. For practical use, the K-montmorillonite film was chosen for further studying the other humidity sensing properties such as sensitivity, hysteresis, influences of applied frequency and ambient temperature, response and recovery times, and long-term stability. © 2007 Elsevier B.V. All rights reserved. Keywords: Humidity sensor; Montmorillonite; Counterion; Impedance analysis; Sensing properties
1. Introduction Humidity sensors are widely used in many fields such as improving quality of life and enhancing industrial processes. The investigation of a good humidity sensor is focused on the improvement of many requirements, including linear response, high sensitivity, fast response time, chemical and physical stability, a wide operating range of humidity, and low cost. Materials that have been studied with these qualities include polymers, ceramics, and composites, which have their own advantages and specific conditions of application [1–3]. Commercial humidity sensors are mostly based on ceramic materials (metal oxides), which offer major advantages with thermal, physical and chemical stability, mechanical strength, and quick response [3,4]. However, ceramic humidity sensors still suffer from insufficient sensitivity over a wide humidity range, as well as lack of reversibility and drift in base resistance with time due to chemisorption of water molecules. Clays can be divided into two main classes: cationic clays that have negatively charged alumino silicate layers and anionic
∗
Corresponding author. Tel.: +886 2 28610511x25332; fax: +886 2 28614212. E-mail address: [email protected] (P.-G. Su).
0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.08.032
clays that have positively charged hydroxide layers. These materials are neutralized by ions, cations, or anions depending on the clay type, in the interlayer space that balances the charge. Cationic clays, particularly montmorillonite, have many useful properties, such as most common minerals on the earth surface, high specific surface area, and/or excellent adsorptive capacity. Thus, they find applications in adsorbents, catalysts, and ion exchangers [5–7]. These last applications are particularly useful for the development of chemical sensors and biosensors in recent years [8–12]. Besse and coworkers [13] have used an anionic clay, Zn0.67 Al0.33 (OH)2 Cl0.33 ·nH2 O, as a humidity sensor by screen printing technique. However, no attempts have been made to construct resistive-type humidity sensors using cationic clays. This paper describes the electrical and humidity sensing properties of resistive-type humidity sensors based on Na- and K-montmorillonite. Differences in impedance and activation energy were compared between Na- and K-montmorillonite to explain the role of counterions in conductance mechanism of the montmorillonite films. The complex impedance spectra were used to explain the sensing mechanism of the K-montmorillonite film. Consequently, the humidity-sensing properties such as sensitivity, hysteresis, influences of applied frequency and ambient temperature, response and recovery times, and long-term stability were further investigated.
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2. Experimental 2.1. Humidity sensor preparation The preparation process of Na- and K-montmorillonite materials was similar to the method reported by Rotenberg and coworkers [14]. Na-montmorillonite was prepared by dispersing the natural montmorillonite (Acros Organics) in a NaCl (0.1 mol/l) solution, stirring for 12 h (repeated twice), and repeated washing with deionized water until complete removal of chloride anions was achieved by AgNO3 test. Kmontmorillonite was obtained by the same procedure as that of Na-montmorillonite with KCl instead of NaCl. The resulting sample was then dried at 80 ◦ C. A binding solution, hydrolytic TEOS (65%, w/w), was prepared by adding H2 O, C2 H5 OH and some drops HNO3 (0.25 ml) into TEOS (Aldrich). The precursor solution was prepared by adding 1 g of Na- or K-montmorillonite powder and 2 ml of the binding solution into 10 ml of ethanol with the help of ultrasonic vibration for about 1 h to obtain the well-mixed suspension. Then, the precursor solutions of the montmorillonites were spin-coated on an alumina substrate with a pair of comb-like electrodes and the film thickness was about 58.6 m, followed by thermal treatment at 80 ◦ C for 0.5 h in air, thereby a humidity sensor of resistive-type was obtained.
2.2. Instruments and analysis A field emission scanning electron microscope (FE-SEM; JEOL 6500) was used to investigate the surface morphology. A laser scattering particle size distribution analyzer (Horiba LA-910) was used to measure the particle size. Complex impedance of a sensor was measured with an LCR meter (Philips PM6306) in a test chamber under the conditions of a measurement frequency of 1 kHz, an applied voltage of 1 V, an ambient temperature of 25 ◦ C, and different humidity levels in the range of 30–90%RH. A frequency range from 60 to 100 kHz, a RH range from 30 to 90% at 25 ◦ C and an applied voltage of 1 V were chosen for the complex impedance analysis. The humidity in the test chamber was controlled by mixing dry and wet air through mass flow controllers (Hastings), as described elsewhere [15]. The RH values were measured with a calibrated hygrometer (Rotronic) with an accuracy of ±0.1%RH.
3. Results and discussion 3.1. Microstructure of montmorillonite films Fig. 1a and b presents SEM images of the surfaces of Na- and K-montmorillonite, respectively. Both Na- and Kmontmorillonite had porous structure. Moreover, no obvious difference in particle size was observed between Na- and Kmontmorillonite. The mean particle size distribution was about 10.58 m.
Fig. 1. FE-SEM micrographs of (a) Na-montmorillonite and (b) K-montmorillonite.
3.2. Electrical properties and sensing mechanism of Naand K-montmorillonite The impedances of the Na- and K-montmorillonite were measured at various relative humidities as shown in Fig. 2. The measurement was made at 25 ◦ C and an ac voltage of 1 V, and 1 kHz. The impedance decreased with increasing RH for both Na- and K-montmorillonite. Moreover, the impedance of Na-montmorillonite was higher than that of K-montmorillonite. This order is the same as that of the equivalent conductance of the alkali ions at an infinite concentration. The resistance measurements were carried out in the temperature range from 15 to 35 ◦ C at humidities from 30 to 90%RH and an ac voltage of 1 V, and 1 kHz. The data were plotted as the measured resistance versus 1/T at humidities from 30 to 90%RH, and the activation energy for conduction was obtained from the Arrehenius plot of resistance. The obtained activation energy was plotted against the relative humidity in Fig. 3. The activation energy decreased with increasing RH for both Naand K-montmorillonite. Moreover, the activation energy of Namontmorillonite was higher than that of K-montmorillonite. The activation energy for conduction is the potential barrier from one
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Fig. 2. Impedance vs. relative humidity plots of Na- and K-montmorillonite, measured at 1 V, 1 kHz and 25 ◦ C.
stable position to another [16]. Thus, this figure shows that the potential barrier for electric conduction of K-montmorillonite is lower than that of Na-montmorillonite. This figure also suggests that the majority carrier of the montmorillonite film in this sorption range is the ions. In addition the protons from water could also become the carrier at high humidity [17]. As described above, these observations were thought to indicate that the counterions play a dominant role in conductance mechanism of the montmorillonite films. Impedance spectroscopy is a powerful technique to understand the conduction mechanisms of humidity sensors. Therefore, the impedance plot was adopted to elucidate the transport process by ions in the conduction mechanism of the montmorillonite films. The impedance measurements were carried out in the frequency range from 60 Hz to 100 kHz at humidities from 30 to 90%RH, an ac voltage of 1 V and 25 ◦ C. The typical complex impedance spectra of the K-montmorillonite film at different humidities are shown in Fig. 4. At low humidity, a quasi-semicircle was observed. As RH increased, a line appeared in the low frequency range. Besse and coworkers [13] and Rotenberg and coworkers
Fig. 4. Complex impedance plots of K-montmorillonite at 40 and 80%RH.
[14] have explained that the quasi-semicircle is due to a kind of polarization and can be modeled by an equivalent circuit of parallel resistor (material resistance) and capacitor (overall geometric capacitance of the device). On the opposite, the straight line at low frequencies corresponds to the formation of induced dipoles by the motion of counterions along the clay sheets with montmorillonite/electrode interface process at high humidity. As described above, a model schematically shown in Fig. 5, based on the combination of the reports of Ito et al. [18], Casalbore-Miceli et al. [19], Marry et al. [20], and Ferrage et al. [21], was proposed to explain that the counterions played an important role in the mechanism for the conductance variation with %RH. Firstly, with the adsorption of water, a sort of thin liquid layer forms around the montmorillonite sheets or fills the openings in the sensing montmorillonite films through capillary condensation or swelling. The counterions are solvated in the sorbed water in the montmorillonite sheets. Finally, the sorbed water acts as a plasticizer to increase the mobility of the solvation of counterions. Therefore, the dissociation behavior, diffusion and activity coefficients of the counterions can guide our understanding of the variations in conductance. 3.3. Humidity sensing properties of K-montmorillonite
Fig. 3. Activation energy vs. relative humidity for Na-montmorillonite and Kmontmorillonite, measured at 1 V, 1 kHz.
The humidity sensing properties of Na- and Kmontmorillonite are summarized in Table 1. K-montmorillonite had slightly better linearity and hysteresis but smaller sensitivity than Na-montmorillonite. However, the Na-montmorillonite
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Fig. 5. Schematic drawing to show the humidity response of montmorillonite. The squares indicate the montmorillonite sheets.
Table 1 Comparison of humidity sensing characteristics between Na- and Kmontmorillonite Sensing characteristics
Na-montmorillonite
K-montmorillonite
Sensitivitya Linearityb Hysteresisc
−0.0283 log Z/%RH 0.9981 3.32%RH
−0.0179 log Z/%RH 0.9993 2.29%RH
a The sensitivity was defined as the slope of the logarithmic impedance vs. relative humidity plot in the range 30–90%RH. b The linearity was shown as the correlation coefficient of the logarithmic impedance vs. relative humidity plot in the range 30–90%RH. c The hysteresis was evaluated by the difference in logarithmic impedance between humidification and desiccation processes in the range of 30–90%RH.
film had high resistance at low humidity, so it is not suitable for practical use. Therefore, the K-montmorillonite was chosen for further studying the other humidity sensing properties for practical use. Fig. 6 shows the plot of impedance of the sensor versus RH as function of measurement frequency at a voltage of 1 V. The frequency obviously influenced the humidity dependence of the impedance of the sensor. The impedance decreased with increasing the frequency, and the best linearity of the impedance versus RH curve appeared at 1 kHz. The logarithmic impedance was
Fig. 6. Impedance vs. relative humidity plots of a montmorillonite sensor at various frequencies.
Fig. 7. Impedance vs. relative humidity plots of a montmorillonite sensor at various temperatures, measured at 1 V and 1 kHz.
almost flat above 10.2 kHz. Thus, with increasing the frequency the impedance became independent of the humidity, and was controlled mainly by the geometric capacitance of the sensor [22]. Fig. 7 shows the impedance of the sensor depended on the ambient temperature. When the temperature increased, the RH
Fig. 8. Response-recovery properties of a montmorillonite sensor, measured at 1 V, 1 kHz, and 25 ◦ C.
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Acknowledgements The authors would like to thank the National Science Council of Taiwan for financially supporting this research under Contract Nos. 95-2221-E-034-005 and 96-2221-E-034-004. References
Fig. 9. The long-term stability of a montmorillonite sensor, measured at 1 V, 1 kHz, and 25 ◦ C.
characteristic curve shifted to the lower impedance side may be because the thermal energy is applied to activate the mobile charge carriers within a stable water layer. The average temperature coefficient between 15 and 25 ◦ C was −0.23%RH/◦ C in the humidity range of 30–90%RH. Fig. 8 shows the response and recovery properties of the sensor, measured at 25 ◦ C and 1 kHz. The response time (humidification from 13 to 90%RH) was about 40 s, including the equilibration time of the water vapor inside the testing chamber. Therefore, the real response time of the sensor should be much shorter, and the recovery time (desiccation from 83 to 8%RH) was about 120 s. This phenomenon could be due to the slow desorption rate of water molecules inside the montmorillonite material. The long-term stability is shown in Fig. 9. The sensor impedance had no obvious deviation at three testing points of 30, 60, and 90%RH for 37 days at least. 4. Conclusion It was found that K-montmorillonite had lower resistance and activation energy than Na-montmorillonite. Moreover, the activation energy of both Na- and K-montmorillonite was reduced with water adsorption. These behaviors reflected that the conductivity of montmorillonite was contributed by counterions. The plots of the complex impedance of the montmorillonite material in different RH showed that the shapes of the curves changed from a semicircle to a line with an increase in RH. Therefore, there are different sensing mechanisms at low and high RH. The humidity sensor made of K-montmorillonite showed an acceptable linearity (Y = −0.0179X + 7.1379; R2 = 0.9993) and a small hysteresis within 2.29%RH. The average temperature coefficient between 15 and 25 ◦ C was −0.23%RH/◦ C in the humidity range of 30–90%RH. The response and recovery time of the sensor were 40 and 120 s, respectively. The frequency dependence of the impedance was bigger at low RH than at high RH. The operation stability over the time makes the montmorillonite film potentially viable for practical use in ceramics integrated humidity sensors.
[1] N. Yamazoe, Y. Shimizu, Humidity sensors: principles and applications, Sens. Actuator 10 (1986) 379–398. [2] J.G. Fagan, V.R.W. Amarakoon, Reliability and reproducibility of ceramic sensors. Part III. Humidity sensors, Am. Soc. Bull. 72 (1993) 119–130. [3] B.M. Kulwicki, Humidity sensors, J. Am. Ceram. Soc. 74 (1991) 697– 708. [4] E. Traversa, Ceramic sensors for humidity detection: the state-of-the-art and future developments, Sens. Actuator B 23 (1995) 135–156. [5] A. Vaccari, Preparation and catalytic properties of cationic and anionic clays, Catal. Today 41 (1998) 147–162. [6] A. Vaccari, Clays and catalysis: a promising future, Appl. Clay Sci. 14 (1999) 161–198. [7] L.G. Yan, X.Q. Shan, B. Wen, S. Zhang, Effect of lead on the sorption of phenol onto montmorillonites and organo-montmorillonites, J. Colloid Interface Sci. 308 (2007) 11–19. [8] U. Guth, S. Brosda, J. Schomburg, Applications of clay minerals in sensor techniques, Appl. Clay Sci. 11 (1996) 229–236. [9] L. Telli, B. Brahimi, A. Hammouche, Study of a pH sensor with MnO2 and montmorillonite-based solid-state internal reference, Solid State Ionics 128 (2000) 255–259. [10] C. Mousty, Sensors and biosensors based on clay-modified electrodes-new trends, Appl. Clay Sci. 27 (2004) 159–177. [11] M. Darder, M. Colilla, E. Ruiz-Hitzky, Chitosan–clay nanocomposites: application as electrochemical sensors, Appl. Clay Sci. 28 (2005) 199–208. [12] S. Yao, S. Yuan, J. Xu, Y. Wang, J. Luo, S. Hu, A hydrogen peroxide sensor based on colloidal MnO2 /Na-montmorillonite, Appl. Clay Sci. 33 (2006) 35–42. [13] J.E. Moneyron, A. de Roy, C. Forano, J.P. Besse, Realization of humidity sensors based on a screen-printed anionic clay, Appl. Clay Sci. 10 (1995) 163–175. [14] A. Cad`ene, B. Rotenberg, S. Durand-Vidal, J.C. Badot, P. Turq, Dielectric spectroscopy as a probe for dynamic properties of compacted smectites, Phys. Chem. Earth 31 (2006) 505–510. [15] P.G. Su, I.C. Chen, R.J. Wu, Use of poly(2-acrylamido-2-methylpropane sulfonate) modified with tetraethyl orthosilicate as sensing material for measurement of humidity, Anal. Chim. Acta 449 (2001) 103–109. [16] J.F. Shackelford, Introduction to Materials Science for Engineers, Macmillan, New York, 1992, pp. 138–157. [17] Z. Chen, C. Lu, Humidity sensors: a review of materials and mechanisms, Sens. Lett. 3 (2005) 274–295. [18] S. Ito, M. Washio, I. Makino, N. Koura, K. Akashi, Role of potassium ions in humidity sensitivity of K+ --ferrite, Solid State Ionics 86–88 (1996) 1005–1011. [19] G. Casalbore-Miceli, M. Yang, Y. Li, N. Camaioni, A. Martelli, A. Zanelli, Effect of the doping level on the conductance of polymer-salts complexes in the presence of humidity, Sens. Actuator B 97 (2004) 362– 368. [20] V. Marry, P. Turq, T. Cartailler, D. Levesque, Microscopic simulation of structure and dynamics of water and counterions in a monohydrated montmorillonite, J. Chem. Phys. 117 (2002) 3454–3463. [21] E. Ferrage, B. Lanson, N. Malikova, A. Plancon, B.A. Sakharov, V.A. Drits, New insights on the distribution of interlayer water in bi-hydrated semectite from X-ray diffraction profile modeling of 00l reflections, Chem. Mater. 17 (2005) 3499–3512. [22] V. Bondarenka, S. Grebinskij, S. Mickevicius, V. Volkov, Thin films of polyvanadium-molybdenum acid as starting materials for humidity sensors, Sens. Actuator B 28 (1995) 227–231.
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Biographies Pi-Guey Su is currently an associate professor of Department of Chemistry at Chinese Culture University. He received his BS degree at Soochow University in chemistry in 1993 and PhD degree in chemistry at National Tsing Hua University in 1998. His fields of interests are chemical sensors, gas and humidity sensing materials, and humidity standard technology.
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Ching-Yin Chen received a BS degree in chemistry from Chinese Culture University in 2006. He entered the MS course of chemistry at Chinese Culture University in 2006. His main areas of interest are humidity sensing materials.
Sensors and Actuators B 129 (2008) 380–385
Humidity sensing and electrical properties of Na- and K-montmorillonite Pi-Guey Su ∗ , Ching-Yin Chen Department of Chemistry, Chinese Culture University, Taipei 111, Taiwan Received 19 April 2007; received in revised form 21 August 2007; accepted 21 August 2007 Available online 24 August 2007
Abstract Cationic clays, Na- and K-montmorillonite, were used to make a humidity sensor. The impedances of the Na- and K-montmorillonite were measured at various humidities and temperatures in a frequency range of 60 Hz–100 kHz. The results indicated that the counterions played a dominant role in conductance mechanism of the montmorillonite films. Moreover, the complex impedance spectra were adopted to elucidate the transport process by counterions in the mechanism for the conductance variation with %RH. For practical use, the K-montmorillonite film was chosen for further studying the other humidity sensing properties such as sensitivity, hysteresis, influences of applied frequency and ambient temperature, response and recovery times, and long-term stability. © 2007 Elsevier B.V. All rights reserved. Keywords: Humidity sensor; Montmorillonite; Counterion; Impedance analysis; Sensing properties
1. Introduction Humidity sensors are widely used in many fields such as improving quality of life and enhancing industrial processes. The investigation of a good humidity sensor is focused on the improvement of many requirements, including linear response, high sensitivity, fast response time, chemical and physical stability, a wide operating range of humidity, and low cost. Materials that have been studied with these qualities include polymers, ceramics, and composites, which have their own advantages and specific conditions of application [1–3]. Commercial humidity sensors are mostly based on ceramic materials (metal oxides), which offer major advantages with thermal, physical and chemical stability, mechanical strength, and quick response [3,4]. However, ceramic humidity sensors still suffer from insufficient sensitivity over a wide humidity range, as well as lack of reversibility and drift in base resistance with time due to chemisorption of water molecules. Clays can be divided into two main classes: cationic clays that have negatively charged alumino silicate layers and anionic
∗
Corresponding author. Tel.: +886 2 28610511x25332; fax: +886 2 28614212. E-mail address: [email protected] (P.-G. Su).
0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.08.032
clays that have positively charged hydroxide layers. These materials are neutralized by ions, cations, or anions depending on the clay type, in the interlayer space that balances the charge. Cationic clays, particularly montmorillonite, have many useful properties, such as most common minerals on the earth surface, high specific surface area, and/or excellent adsorptive capacity. Thus, they find applications in adsorbents, catalysts, and ion exchangers [5–7]. These last applications are particularly useful for the development of chemical sensors and biosensors in recent years [8–12]. Besse and coworkers [13] have used an anionic clay, Zn0.67 Al0.33 (OH)2 Cl0.33 ·nH2 O, as a humidity sensor by screen printing technique. However, no attempts have been made to construct resistive-type humidity sensors using cationic clays. This paper describes the electrical and humidity sensing properties of resistive-type humidity sensors based on Na- and K-montmorillonite. Differences in impedance and activation energy were compared between Na- and K-montmorillonite to explain the role of counterions in conductance mechanism of the montmorillonite films. The complex impedance spectra were used to explain the sensing mechanism of the K-montmorillonite film. Consequently, the humidity-sensing properties such as sensitivity, hysteresis, influences of applied frequency and ambient temperature, response and recovery times, and long-term stability were further investigated.
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2. Experimental 2.1. Humidity sensor preparation The preparation process of Na- and K-montmorillonite materials was similar to the method reported by Rotenberg and coworkers [14]. Na-montmorillonite was prepared by dispersing the natural montmorillonite (Acros Organics) in a NaCl (0.1 mol/l) solution, stirring for 12 h (repeated twice), and repeated washing with deionized water until complete removal of chloride anions was achieved by AgNO3 test. Kmontmorillonite was obtained by the same procedure as that of Na-montmorillonite with KCl instead of NaCl. The resulting sample was then dried at 80 ◦ C. A binding solution, hydrolytic TEOS (65%, w/w), was prepared by adding H2 O, C2 H5 OH and some drops HNO3 (0.25 ml) into TEOS (Aldrich). The precursor solution was prepared by adding 1 g of Na- or K-montmorillonite powder and 2 ml of the binding solution into 10 ml of ethanol with the help of ultrasonic vibration for about 1 h to obtain the well-mixed suspension. Then, the precursor solutions of the montmorillonites were spin-coated on an alumina substrate with a pair of comb-like electrodes and the film thickness was about 58.6 m, followed by thermal treatment at 80 ◦ C for 0.5 h in air, thereby a humidity sensor of resistive-type was obtained.
2.2. Instruments and analysis A field emission scanning electron microscope (FE-SEM; JEOL 6500) was used to investigate the surface morphology. A laser scattering particle size distribution analyzer (Horiba LA-910) was used to measure the particle size. Complex impedance of a sensor was measured with an LCR meter (Philips PM6306) in a test chamber under the conditions of a measurement frequency of 1 kHz, an applied voltage of 1 V, an ambient temperature of 25 ◦ C, and different humidity levels in the range of 30–90%RH. A frequency range from 60 to 100 kHz, a RH range from 30 to 90% at 25 ◦ C and an applied voltage of 1 V were chosen for the complex impedance analysis. The humidity in the test chamber was controlled by mixing dry and wet air through mass flow controllers (Hastings), as described elsewhere [15]. The RH values were measured with a calibrated hygrometer (Rotronic) with an accuracy of ±0.1%RH.
3. Results and discussion 3.1. Microstructure of montmorillonite films Fig. 1a and b presents SEM images of the surfaces of Na- and K-montmorillonite, respectively. Both Na- and Kmontmorillonite had porous structure. Moreover, no obvious difference in particle size was observed between Na- and Kmontmorillonite. The mean particle size distribution was about 10.58 m.
Fig. 1. FE-SEM micrographs of (a) Na-montmorillonite and (b) K-montmorillonite.
3.2. Electrical properties and sensing mechanism of Naand K-montmorillonite The impedances of the Na- and K-montmorillonite were measured at various relative humidities as shown in Fig. 2. The measurement was made at 25 ◦ C and an ac voltage of 1 V, and 1 kHz. The impedance decreased with increasing RH for both Na- and K-montmorillonite. Moreover, the impedance of Na-montmorillonite was higher than that of K-montmorillonite. This order is the same as that of the equivalent conductance of the alkali ions at an infinite concentration. The resistance measurements were carried out in the temperature range from 15 to 35 ◦ C at humidities from 30 to 90%RH and an ac voltage of 1 V, and 1 kHz. The data were plotted as the measured resistance versus 1/T at humidities from 30 to 90%RH, and the activation energy for conduction was obtained from the Arrehenius plot of resistance. The obtained activation energy was plotted against the relative humidity in Fig. 3. The activation energy decreased with increasing RH for both Naand K-montmorillonite. Moreover, the activation energy of Namontmorillonite was higher than that of K-montmorillonite. The activation energy for conduction is the potential barrier from one
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Fig. 2. Impedance vs. relative humidity plots of Na- and K-montmorillonite, measured at 1 V, 1 kHz and 25 ◦ C.
stable position to another [16]. Thus, this figure shows that the potential barrier for electric conduction of K-montmorillonite is lower than that of Na-montmorillonite. This figure also suggests that the majority carrier of the montmorillonite film in this sorption range is the ions. In addition the protons from water could also become the carrier at high humidity [17]. As described above, these observations were thought to indicate that the counterions play a dominant role in conductance mechanism of the montmorillonite films. Impedance spectroscopy is a powerful technique to understand the conduction mechanisms of humidity sensors. Therefore, the impedance plot was adopted to elucidate the transport process by ions in the conduction mechanism of the montmorillonite films. The impedance measurements were carried out in the frequency range from 60 Hz to 100 kHz at humidities from 30 to 90%RH, an ac voltage of 1 V and 25 ◦ C. The typical complex impedance spectra of the K-montmorillonite film at different humidities are shown in Fig. 4. At low humidity, a quasi-semicircle was observed. As RH increased, a line appeared in the low frequency range. Besse and coworkers [13] and Rotenberg and coworkers
Fig. 4. Complex impedance plots of K-montmorillonite at 40 and 80%RH.
[14] have explained that the quasi-semicircle is due to a kind of polarization and can be modeled by an equivalent circuit of parallel resistor (material resistance) and capacitor (overall geometric capacitance of the device). On the opposite, the straight line at low frequencies corresponds to the formation of induced dipoles by the motion of counterions along the clay sheets with montmorillonite/electrode interface process at high humidity. As described above, a model schematically shown in Fig. 5, based on the combination of the reports of Ito et al. [18], Casalbore-Miceli et al. [19], Marry et al. [20], and Ferrage et al. [21], was proposed to explain that the counterions played an important role in the mechanism for the conductance variation with %RH. Firstly, with the adsorption of water, a sort of thin liquid layer forms around the montmorillonite sheets or fills the openings in the sensing montmorillonite films through capillary condensation or swelling. The counterions are solvated in the sorbed water in the montmorillonite sheets. Finally, the sorbed water acts as a plasticizer to increase the mobility of the solvation of counterions. Therefore, the dissociation behavior, diffusion and activity coefficients of the counterions can guide our understanding of the variations in conductance. 3.3. Humidity sensing properties of K-montmorillonite
Fig. 3. Activation energy vs. relative humidity for Na-montmorillonite and Kmontmorillonite, measured at 1 V, 1 kHz.
The humidity sensing properties of Na- and Kmontmorillonite are summarized in Table 1. K-montmorillonite had slightly better linearity and hysteresis but smaller sensitivity than Na-montmorillonite. However, the Na-montmorillonite
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Fig. 5. Schematic drawing to show the humidity response of montmorillonite. The squares indicate the montmorillonite sheets.
Table 1 Comparison of humidity sensing characteristics between Na- and Kmontmorillonite Sensing characteristics
Na-montmorillonite
K-montmorillonite
Sensitivitya Linearityb Hysteresisc
−0.0283 log Z/%RH 0.9981 3.32%RH
−0.0179 log Z/%RH 0.9993 2.29%RH
a The sensitivity was defined as the slope of the logarithmic impedance vs. relative humidity plot in the range 30–90%RH. b The linearity was shown as the correlation coefficient of the logarithmic impedance vs. relative humidity plot in the range 30–90%RH. c The hysteresis was evaluated by the difference in logarithmic impedance between humidification and desiccation processes in the range of 30–90%RH.
film had high resistance at low humidity, so it is not suitable for practical use. Therefore, the K-montmorillonite was chosen for further studying the other humidity sensing properties for practical use. Fig. 6 shows the plot of impedance of the sensor versus RH as function of measurement frequency at a voltage of 1 V. The frequency obviously influenced the humidity dependence of the impedance of the sensor. The impedance decreased with increasing the frequency, and the best linearity of the impedance versus RH curve appeared at 1 kHz. The logarithmic impedance was
Fig. 6. Impedance vs. relative humidity plots of a montmorillonite sensor at various frequencies.
Fig. 7. Impedance vs. relative humidity plots of a montmorillonite sensor at various temperatures, measured at 1 V and 1 kHz.
almost flat above 10.2 kHz. Thus, with increasing the frequency the impedance became independent of the humidity, and was controlled mainly by the geometric capacitance of the sensor [22]. Fig. 7 shows the impedance of the sensor depended on the ambient temperature. When the temperature increased, the RH
Fig. 8. Response-recovery properties of a montmorillonite sensor, measured at 1 V, 1 kHz, and 25 ◦ C.
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Acknowledgements The authors would like to thank the National Science Council of Taiwan for financially supporting this research under Contract Nos. 95-2221-E-034-005 and 96-2221-E-034-004. References
Fig. 9. The long-term stability of a montmorillonite sensor, measured at 1 V, 1 kHz, and 25 ◦ C.
characteristic curve shifted to the lower impedance side may be because the thermal energy is applied to activate the mobile charge carriers within a stable water layer. The average temperature coefficient between 15 and 25 ◦ C was −0.23%RH/◦ C in the humidity range of 30–90%RH. Fig. 8 shows the response and recovery properties of the sensor, measured at 25 ◦ C and 1 kHz. The response time (humidification from 13 to 90%RH) was about 40 s, including the equilibration time of the water vapor inside the testing chamber. Therefore, the real response time of the sensor should be much shorter, and the recovery time (desiccation from 83 to 8%RH) was about 120 s. This phenomenon could be due to the slow desorption rate of water molecules inside the montmorillonite material. The long-term stability is shown in Fig. 9. The sensor impedance had no obvious deviation at three testing points of 30, 60, and 90%RH for 37 days at least. 4. Conclusion It was found that K-montmorillonite had lower resistance and activation energy than Na-montmorillonite. Moreover, the activation energy of both Na- and K-montmorillonite was reduced with water adsorption. These behaviors reflected that the conductivity of montmorillonite was contributed by counterions. The plots of the complex impedance of the montmorillonite material in different RH showed that the shapes of the curves changed from a semicircle to a line with an increase in RH. Therefore, there are different sensing mechanisms at low and high RH. The humidity sensor made of K-montmorillonite showed an acceptable linearity (Y = −0.0179X + 7.1379; R2 = 0.9993) and a small hysteresis within 2.29%RH. The average temperature coefficient between 15 and 25 ◦ C was −0.23%RH/◦ C in the humidity range of 30–90%RH. The response and recovery time of the sensor were 40 and 120 s, respectively. The frequency dependence of the impedance was bigger at low RH than at high RH. The operation stability over the time makes the montmorillonite film potentially viable for practical use in ceramics integrated humidity sensors.
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Biographies Pi-Guey Su is currently an associate professor of Department of Chemistry at Chinese Culture University. He received his BS degree at Soochow University in chemistry in 1993 and PhD degree in chemistry at National Tsing Hua University in 1998. His fields of interests are chemical sensors, gas and humidity sensing materials, and humidity standard technology.
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Ching-Yin Chen received a BS degree in chemistry from Chinese Culture University in 2006. He entered the MS course of chemistry at Chinese Culture University in 2006. His main areas of interest are humidity sensing materials.