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PMMA doped with KOH as a resistive humidity sensor
Sensors and Actuators B 124 (2007) 303–308
In situ synthesized composite thin films of MWCNTs/PMMA doped with KOH as a resistive humidity sensor Pi-Guey Su ∗ , Chao-Shen Wang Department of Chemistry, Chinese Culture University, Taipei 111, Taiwan Received 14 July 2006; received in revised form 21 December 2006; accepted 21 December 2006 Available online 28 December 2006
Abstract In situ synthesized multi-walled carbon nanotubes (MWCNTs), methyl methacrylate (MMA) and KOH composite thin films on an alumina substrate were used to fabricate resistive humidity sensors. The thin films were analyzed by scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and Fourier transform infrared spectroscopy (FT-IR). The electrical characteristics of PMMA to which various amounts of MWCNTs were added were measured in detail as a function of relative humidity (RH), to elucidate the contribution of MWCNTs to the humiditysensing capacity. The humidity-sensing mechanism of the MWCNTs/PMMA composite thin films with various amounts of MWCNTs were explained by considering the composition and microstructures. The charge-transportation by the ions formed by electrolytic dissociation, their interactions with polymers, and the dimensions of the channels through which ions migrated were used to explain the sensing mechanism of the composite thin films of MWCNTs/PMMA doped with KOH. Other sensing characteristics, including linearity, sensitivity, response and recovery time, hysteresis and stability, were also investigated. © 2007 Elsevier B.V. All rights reserved. Keywords: Humidity sensor; In situ; Composite material; MWCNTs; PMMA; KOH; Sensing mechanism
1. Introduction The construction of a good humidity sensor is rather complex, because high-performance humidity sensors must meet many requirements, including linear response, high sensitivity, fast response time, chemical and physical stability, 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]. In the last two decades, organic–inorganic nanocomposite materials have been regarded as a new class of materials for various electronic, optical and magnetic applications, because many of their bulk properties are superior to those of base polymers [4]. Since discovery of carbon nanotubes (CNTs) [5], which exhibit unique physical, chemical and structural properties, and have large aspect ratios, CNTs have been commonly identified as a potential filling material in such polymeric nanocom-
∗
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.2006.12.034
posites as CNTs/epoxy [6,7], CNTs/poly(vinyl alcohol) [8], CNTs/polypyrrole [9], CNTs/poly(methyl methacrylate) (PMMA) [10,11] and CNTs/nylon-6 [12]. Recently, CNT-based nanocomposite materials, based on polymer electrolytes, have been developed for use in humidity sensors. They include multi-wall CNTs (MWCNTs)/ sodium polystyrenesulfonate and single-wall CNTs (SWCNTs)/ SiO2 /poly(2-acrylamido-2-methylpropane sulfonate) [13,14]. However, no attempt has been made to construct resistive humidity sensors using CNTs/hydrophobic polymer composite materials, including MWCNTS/PMMA, because their conductivity does not change much with RH% [15]. Pure hydrophobic polymers or composite materials are doped with inorganic acids or salts, polymer–salts or composite–salt complexes, to improve the sensitivity to humidity. For example, poly(propargyl alcohol) doped with sulfuric acid [15,16], poly(p-diethynylbenzene-co-propargyl alcohol) doped with iron trichloride [17] and poly(2-acrylamido-2-methylpropane sulfonic acid) doped with alkali salts [18] have all been used. The sensing characteristics of these humidity sensors depend on the microstructure, which is related to the fabrication process. Along with work to optimize the sensing material, research on novel
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fabrication techniques to improve humidity sensing is ongoing. The fabrication of thick films generally involves considerable manual labor and related high costs. This study describes the humidity-sensing characteristics of MWCNTs/PMMA composite thin films fabricated by in situ synthesis on an alumina substrate in a single step. The electrical characteristics of the composite thin films were studied as a function of RH. Differences in the composition and microstructure were adopted to explain the effect of adding MWCNTs on the sensing mechanism of the composite thin films. Potassium hydroxide (KOH) was also added to improve the humidity sensitivity and linearity. The humidity-sensing characteristics were then studied. 2. Experimental 2.1. Sensor preparation The MWCNTs materials were obtained from Sunnano Inc., and contained >90 at.% MWCNTs, produced via CVD process, with diameters of about 10–30 nm. The sensors were fabricated by spin-coating and subsequent in situ synthesis on an alumina substrate with a pair of comb-like electrodes, as described elsewhere [19]. The gold electrodes were provided on the alumina substrate by screen-printing. The precursor solutions were prepared as follows: KOH (10%, Merck) and azobisisobutyronitrile (AIBN, 0.01 g) were added to methyl methacrylate (MMA, 99%, Merck) in ethanol and then MWCNTs were added to the solution and sonicated to achieve a uniform dispersion of MWCNTs. Table 1 presents the various compositions. The alumina substrate was coated with the precursor solutions by spin-coating at a speed of 500 rpm for 15 s to produce a film with a thickness of about 7 m, as observed by SEM. In situ synthesis in an oven at 70 ◦ C for 6 h yielded composite thin films of MWCNTs/PMMA doped with KOH. Accordingly, resistive humidity sensors were obtained (Fig. 1). 2.2. Instruments and analysis A field emission scanning electron microscope (FE-SEM; JEOL 6500) equipped with an energy dispersive spectrometer (EDS) was adopted to study the surface morphology and composition of the films. An infrared spectrometer (Nicolet Magna-IR 860) was adopted to characterize the composite thin films. The complex impedance of the sensors was measured as a function of relative humidity (RH) using an LCR meter (Philips PM6304)
Fig. 1. Structure of humidity sensor.
in a cell in which humidity was controlled by mixing dry and wet air using mass flow controllers (Hastings), as described elsewhere [20]. The frequency varied between 1 and 100 kHz; %RH varied at an interval of 10% RH between 30 and 90% RH, and the testing temperature was 15, 25 and 35 ◦ C. The standard deviation (presented as error bars) was obtained by extraction in three repeated experiments. 3. Results and discussion 3.1. IR spectra and microstructure of composite thin films The composite thin films of MWCNTs and PMMA as well as of pristine PMMA were analyzed using an FT-IR spectrometer, as presented in Fig. 2. Jia et al. [11] found that a new peak at 1650 cm−1 was observed from the composite because AIBN initiated the opening of the -bonds of CNTs, indicating that CNTs
Table 1 Composition of the composite films used to prepare humidity sensors Sample number
MMA (ml)
MWCNTs (g)
KOH (10%) (ml)
1 2 3 4 5 6 7
3 3 3 3 3 3 3
0 0.0005 0.0024 0.004 0.0024 0.0024 0.0024
0 0 0 0 0.01 0.05 0.1
Fig. 2. IR spectra of PMMA and MWCNTs/PMMA composite thin films.
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305
Table 2 EDS analysis results (wt.%) of the surfaces of PMMA and MWCNTs/PMMA/KOH composite thin films
Sample 1 Sample 7
C
O
K
Au
4.72 45.36
1.55 4.30
– 4.00
93.74 46.35
the MWCNTs were embedded in the PMMA-based material. Additionally, the K+ ions were also embedded in the composite thin film of MWCNTs/PMMA/KOH. 3.2. Humidity-sensing characteristics
Fig. 3. FE-SEM micrographs of MWCNTs/PMMA composite thin films (compositions as shown in Table 1): (a) Sample 3 and (b) Sample 4.
participated in PMMA polymerization and adhered strongly at the interface with the PMMA matrix. However, no strong new peak at 1650 cm−1 was observed from the MWCNTs/PMMA composite thin film that was fabricated herein perhaps because the amount of the AIBN did not suffice to open the -bonds of CNTs linked with the PMMA matrix during compounding of the composite [11]. Fig. 3a and b presents SEM images of the surfaces of Samples 3 and 4, respectively. When the mass of MWCNTs added was less than 2.4 mg (Fig. 3a), the PMMA layers wrapped the MWCNTs and no naked MWCNTs were observed on the surface of the sample. Additionally, the bundles of MWCNTs were embedded and dispersed uniformly. When the mass of MWCNTs added exceeded 4 mg (Fig. 3b), many naked and aggregated MWCNTs were clearly observed on the surface. As revealed by FT-IR, the PMMA layers wrapped the MWCNTs during the in situ synthesis on an alumina substrate in a physical interaction. The composition of the surfaces of Samples 1 and 7 were analyzed by EDS, and Table 2 lists the results. The characteristic feature of the composite thin film of MWCNTs/PMMA/KOH was its relatively high content of carbon atom because most of
3.2.1. Effect of MWCNTs content on the electrical characteristics and sensing mechanism Fig. 4 presents the effect of adding large and small amounts of MWCNTs on the impedance of MWCNTs/PMMA composite thin films as a function of relative humidity. As presented in Fig. 4, pure PMMA (Sample 1) exhibited only a little impedance change over the range of humidity studied, undoubtedly because of its hydrophobicity. When the amount of MWCNTs added was under 2.4 mg (Samples 2 and 3), the impedance decreased as RH increased and the range of humidity sensing increased with increasing the amount of MWCNTs added. However, when the mass of MWCNTs added exceeded 4 mg (Sample 4), the impedance increased slightly with an increase in %RH (as presented in the inset). The humidity sensing of ceramic and polymer materials is known to mainly occur as a surface mechanism [3,21,22]. When less than 2.4 mg of MWCNTs were added (Samples 2 and 3), as described in Section 3.1, no naked MWCNTs were present on the surface of the composite material film, and the MWCNTs were embedded in PMMA. Therefore, the surface properties of MWCNTs may not be directly responsible for the improved in the humidity sensing of the composite films compared to that of the pure PMMA film. Therefore, two possible explanations are proposed to the slight improvement in the humidity-sensing range of the PMMA films doped with MWCNTs (Samples 2 and 3) are proposed. First, MWCNTs have been demonstrated theoretically and experimentally to exhibit mixed metallic and semiconducting behavior [23,24]. Therefore, MWCNTs render the MWCNTs/PMMA composite thin films richer in electrons, increasing the binding affinity and sticking coefficient of water vapor molecules. Secondly, MWCNTs are typically present in inner cavities, in interlayer pores and as aggregates of isolated CNTs and CNT bundles, forming aggregated pores [25]. Therefore, the slight improvement in the humidity-sensing range due to MWCNTs may be caused by capillary condensation [25]. When more than 4 mg of MWCNTs were added (Sample 4), as described in Section 3.1, many naked MWCNTs were observed on the surface of the film. Accordingly, the MWCNTs dominate the humidity-sensing properties. The adsorption of electron-donating water molecules compensates for the hole carriers in p-type MWCNTs, causing the electrical resistance of composite thin films to increase with humidity [26,27]. The low sensitivity was thought to be observed because the change in
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Fig. 4. Impedance vs. relative humidity for Samples 1–4 (compositions are shown in Table 1), measured at 1 kHz and 1 V.
the conductance of the individual CNTs due to gas adsorption was only about 5 × 10−6 −1 [23]. Additionally, the amount of added MWCNTs exceeded the percolation threshold so that the change in conductance of the sensing material due to adsorption of water molecules was not obvious. 3.2.2. MWCNTs/PMMA doped with KOH composite thin films The presented results were obtained from the devices based on MWCNTs/PMMA doped with KOH. Fig. 5 plots the effects of KOH addition on the impedance of MWCNTs/PMMA as a function of relative humidity. When the MWCNTs/PMMA was doped with KOH (above 2.5 wt.%) (Samples 6 and 7), an inverse sigmoidal dependence of the logarithmic impedance on humidity was observed. When the MWCNTs/PMMA was doped with KOH (0.5 wt.%) (Sample 5), a linear dependence of the logarithmic impedance on humidity was observed. The mechanism of the variation in conductance with %RH is considered below, based on a combination of the reports of Sakai et al. [18] and Casalbore-Miceli et al. [28], to explain the results of the MWCNTs/PMMA-doped KOH. First, the adsorption of water causes a thin liquid layer to form around the composite material chains or to fill the openings in the sensing composite films through capillary condensation or swelling. The sorbed water promotes the electrolytic dissociation of inorganic salts in the composite material–salt complexes. Finally, the sorbed water as a plasticizer increases the mobility of dissociated ions. Therefore, the dissociation behavior and activity coefficient of the inorganic salts can guide our understanding
of the variations in conductance that are associated with the KOH. When the amount of KOH dopant was high, some KOH was not trapped in the composite film, which would enable it to be easily and completely dissociated at low humidity. When the amount of KOH dopant was low, the KOH was completely trapped in the composite film completely, so that the KOH could be dissociated completely in a highly humid atmosphere. Fig. 6 plots the sensitivity, linearity and hysteresis of variations in impedance of MWCNTs/PMMA doped with KOH
Fig. 5. Impedance vs. relative humidity for Samples 5–7 (compositions are shown in Table 1), measured at 1 kHz and 1 V.
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4. Conclusion
Fig. 6. Impedance vs. relative humidity for Sample 5 measured at 1 kHz and 1 V. () Humidification and () desiccation.
(Sample 5) against %RH for humidification and desiccation. The impedance changed from 104 to 107 and the curves of log-impedance as a function of RH were quite linear (Y = −0.0352X + 7.5329, where Y is the log-impedance and X is the RH; correlation coefficient R2 = 0.9953) in the range of 30–90% RH. The hysteresis between humidification and desiccation was less then 4% RH. The response time (humidification from 8 to 90% RH) was about 30 s, and the recovery time (desiccation from 85 to 7% RH) was about 25 s, as present in Fig. 7. Additionally, in the stability test, in which a sensor was subjected to more than 50 cycles of humidification and desiccation in the range of 30–90% RH at 25 ◦ C, the proposed humidity sensors worked normally at all times.
Humidity-sensing composite thin films of MWCNTs/PMMA and MWCNTs/PMMA doped with KOH were prepared in situ on an alumina substrate. FT-IR indicated that the PMMA layers wrapped MWCNTs during the in situ synthesis in a physical interaction. Most of the MWCNTs and K+ ions were embedded in the PMMA-based materials according to the SEM and EDS analysis. When a small amount of MWCNTs was added, the binding affinity of the MWCNTs/PMMA composite films increased along with the sticking coefficient, and the capillary condensation mechanism slightly increased the humidity-sensing range. When a large amount of MWCNTs was added, opposite variations in the impedance with relative humidity were observed, and the MWCNTs dominated the humidity-sensing characteristics. When the amount of MWCNTs added exceeded the percolation threshold, no variation in impedance versus relative humidity was clearly observed. Humidity sensors based on the composite thin films of MWCNTs/PMMA/KOH exhibited good sensitivity, linearity, small hysteresis, fast response, a short recovery time and high stability. The charge-transportation of ions generated in electrolytic dissociation, their interactions with polymers and the dimensions of the channels in which the ions migrated, were used to explain the sensing mechanism of the MWCNTs/PMMA/KOH composite thin films. Acknowledgements The authors would like to thank the National Science Council of Taiwan for financially supporting this research under Contract no. NSC 94-2216-E-034-007 and 95-2221-E-034-005. We also wish to thank Dr. B.Y. Wei of MRL/ITRI, Taiwan for kindly supplying MWCNTs. References
Fig. 7. Response–recovery characteristics of Sample 5 measured at 1 kHz and 1 V.
[1] N. Yamazoe, Y. Shimizu, Humidity sensors: principles and applications, Sens. Actuators 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] B.M. Novak, Hybrid nanocomposite materials—between inorganic glasses and organic polymers, Adv. Mater. 5 (1993) 422–433. [5] S. Iijima, Helical microtubules of graphitic carbon, Nature 354 (1991) 56–58. [6] P.M. Ajayan, O. Stephan, C. Colliex, D. Trauth, Aligned carbon nanotube arrays formed by cutting a polymer resin–nanotube composite, Science 265 (1994) 1212–1214. [7] P.M. Ajayan, L.S. Schadler, C. Giannaris, A. Rubio, Single-walled carbon nanotube–polymer composites: strength and weakness, Adv. Mater. 10 (2000) 750–753. [8] M.S.P. Shaffer, A.H. Windle, Fabrication and characterization of carbon nanotube/poly(vinyl alcohol) composites, Adv. Mater. 11 (1999) 937–941. [9] G.Z. Chen, M.S.P. Shaffer, D. Coleby, G. Dixon, W.Z. Zhou, D.J. Fray, Carbon nanotube and polypyrrole composites: coating and doping, Adv. Mater. 7 (2000) 522–526. [10] J.H. Sung, H.S. Kim, H.J. Jin, H.J. Choi, I.J. Chin, Nanofibrous membranes prepared by multiwalled carbon nanotube/poly(methyl methacrylate) composites, Macromolecules 37 (2004) 9899–9902.
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[23] J. Kong, N.R. Franklin, C. Zhou, M.G. Chapline, S. Peng, K. Cho, H. Dai, Nanotube molecular wires as chemical sensors, Science 287 (2000) 622–625. [24] S.J. Tans, R.M. Verschuren, C. Dekker, Room-temperature transistor based on a single carbon nanotube, Nature 393 (1998) 49– 52. [25] Q.H. Yang, P.X. Hou, S. Bai, M.Z. Wang, H.M. Cheng, Adsorption and capillarity of nitrogen in aggregated multi-walled carbon nanotubes, Chem. Phys. Lett. 345 (2001) 18–24. [26] O.K. Varghese, P.D. Kichambre, D. Gong, K.G. Ong, E.C. Dickey, C.A. Grimes, Gas sensing characteristics of multi-wall carbon nanotubes, Sens. Actuators B 81 (2001) 32–41. [27] P.S. Na, H. Kim, H.M. So, K.J. Kong, H. Chang, B.H. Ryu, B.K. Kim, J.J. Kim, L. Kim, Investigation of the humidity effect on the electrical properties of single-walled carbon nanotube transistors, Appl. Phys. Lett. 87 (2005) 093101–093103. [28] 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. Actuators B 97 (2004) 362– 368.
Biographies Pi-Guey Su is currently an assistant 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. He worked as a researcher in Industrial Technology Research Institute, Taiwan, in 1998–2002. He joined as an assistant professor in the General Education Center, Chungchou Institute of Technology in 2003–2005. His fields of interests are chemical sensors, gas and humidity-sensing materials and humidity standard technology. Chao-Shen Wang received a BS degree in Chemistry from Chinese Culture University in 2005. He entered the MS course of chemistry at Chinese Culture University in 2005. His main areas of interest are humidity-sensing materials.
In situ synthesized composite thin films of MWCNTs/PMMA doped with KOH as a resistive humidity sensor Pi-Guey Su ∗ , Chao-Shen Wang Department of Chemistry, Chinese Culture University, Taipei 111, Taiwan Received 14 July 2006; received in revised form 21 December 2006; accepted 21 December 2006 Available online 28 December 2006
Abstract In situ synthesized multi-walled carbon nanotubes (MWCNTs), methyl methacrylate (MMA) and KOH composite thin films on an alumina substrate were used to fabricate resistive humidity sensors. The thin films were analyzed by scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and Fourier transform infrared spectroscopy (FT-IR). The electrical characteristics of PMMA to which various amounts of MWCNTs were added were measured in detail as a function of relative humidity (RH), to elucidate the contribution of MWCNTs to the humiditysensing capacity. The humidity-sensing mechanism of the MWCNTs/PMMA composite thin films with various amounts of MWCNTs were explained by considering the composition and microstructures. The charge-transportation by the ions formed by electrolytic dissociation, their interactions with polymers, and the dimensions of the channels through which ions migrated were used to explain the sensing mechanism of the composite thin films of MWCNTs/PMMA doped with KOH. Other sensing characteristics, including linearity, sensitivity, response and recovery time, hysteresis and stability, were also investigated. © 2007 Elsevier B.V. All rights reserved. Keywords: Humidity sensor; In situ; Composite material; MWCNTs; PMMA; KOH; Sensing mechanism
1. Introduction The construction of a good humidity sensor is rather complex, because high-performance humidity sensors must meet many requirements, including linear response, high sensitivity, fast response time, chemical and physical stability, 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]. In the last two decades, organic–inorganic nanocomposite materials have been regarded as a new class of materials for various electronic, optical and magnetic applications, because many of their bulk properties are superior to those of base polymers [4]. Since discovery of carbon nanotubes (CNTs) [5], which exhibit unique physical, chemical and structural properties, and have large aspect ratios, CNTs have been commonly identified as a potential filling material in such polymeric nanocom-
∗
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.2006.12.034
posites as CNTs/epoxy [6,7], CNTs/poly(vinyl alcohol) [8], CNTs/polypyrrole [9], CNTs/poly(methyl methacrylate) (PMMA) [10,11] and CNTs/nylon-6 [12]. Recently, CNT-based nanocomposite materials, based on polymer electrolytes, have been developed for use in humidity sensors. They include multi-wall CNTs (MWCNTs)/ sodium polystyrenesulfonate and single-wall CNTs (SWCNTs)/ SiO2 /poly(2-acrylamido-2-methylpropane sulfonate) [13,14]. However, no attempt has been made to construct resistive humidity sensors using CNTs/hydrophobic polymer composite materials, including MWCNTS/PMMA, because their conductivity does not change much with RH% [15]. Pure hydrophobic polymers or composite materials are doped with inorganic acids or salts, polymer–salts or composite–salt complexes, to improve the sensitivity to humidity. For example, poly(propargyl alcohol) doped with sulfuric acid [15,16], poly(p-diethynylbenzene-co-propargyl alcohol) doped with iron trichloride [17] and poly(2-acrylamido-2-methylpropane sulfonic acid) doped with alkali salts [18] have all been used. The sensing characteristics of these humidity sensors depend on the microstructure, which is related to the fabrication process. Along with work to optimize the sensing material, research on novel
304
P.-G. Su, C.-S. Wang / Sensors and Actuators B 124 (2007) 303–308
fabrication techniques to improve humidity sensing is ongoing. The fabrication of thick films generally involves considerable manual labor and related high costs. This study describes the humidity-sensing characteristics of MWCNTs/PMMA composite thin films fabricated by in situ synthesis on an alumina substrate in a single step. The electrical characteristics of the composite thin films were studied as a function of RH. Differences in the composition and microstructure were adopted to explain the effect of adding MWCNTs on the sensing mechanism of the composite thin films. Potassium hydroxide (KOH) was also added to improve the humidity sensitivity and linearity. The humidity-sensing characteristics were then studied. 2. Experimental 2.1. Sensor preparation The MWCNTs materials were obtained from Sunnano Inc., and contained >90 at.% MWCNTs, produced via CVD process, with diameters of about 10–30 nm. The sensors were fabricated by spin-coating and subsequent in situ synthesis on an alumina substrate with a pair of comb-like electrodes, as described elsewhere [19]. The gold electrodes were provided on the alumina substrate by screen-printing. The precursor solutions were prepared as follows: KOH (10%, Merck) and azobisisobutyronitrile (AIBN, 0.01 g) were added to methyl methacrylate (MMA, 99%, Merck) in ethanol and then MWCNTs were added to the solution and sonicated to achieve a uniform dispersion of MWCNTs. Table 1 presents the various compositions. The alumina substrate was coated with the precursor solutions by spin-coating at a speed of 500 rpm for 15 s to produce a film with a thickness of about 7 m, as observed by SEM. In situ synthesis in an oven at 70 ◦ C for 6 h yielded composite thin films of MWCNTs/PMMA doped with KOH. Accordingly, resistive humidity sensors were obtained (Fig. 1). 2.2. Instruments and analysis A field emission scanning electron microscope (FE-SEM; JEOL 6500) equipped with an energy dispersive spectrometer (EDS) was adopted to study the surface morphology and composition of the films. An infrared spectrometer (Nicolet Magna-IR 860) was adopted to characterize the composite thin films. The complex impedance of the sensors was measured as a function of relative humidity (RH) using an LCR meter (Philips PM6304)
Fig. 1. Structure of humidity sensor.
in a cell in which humidity was controlled by mixing dry and wet air using mass flow controllers (Hastings), as described elsewhere [20]. The frequency varied between 1 and 100 kHz; %RH varied at an interval of 10% RH between 30 and 90% RH, and the testing temperature was 15, 25 and 35 ◦ C. The standard deviation (presented as error bars) was obtained by extraction in three repeated experiments. 3. Results and discussion 3.1. IR spectra and microstructure of composite thin films The composite thin films of MWCNTs and PMMA as well as of pristine PMMA were analyzed using an FT-IR spectrometer, as presented in Fig. 2. Jia et al. [11] found that a new peak at 1650 cm−1 was observed from the composite because AIBN initiated the opening of the -bonds of CNTs, indicating that CNTs
Table 1 Composition of the composite films used to prepare humidity sensors Sample number
MMA (ml)
MWCNTs (g)
KOH (10%) (ml)
1 2 3 4 5 6 7
3 3 3 3 3 3 3
0 0.0005 0.0024 0.004 0.0024 0.0024 0.0024
0 0 0 0 0.01 0.05 0.1
Fig. 2. IR spectra of PMMA and MWCNTs/PMMA composite thin films.
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305
Table 2 EDS analysis results (wt.%) of the surfaces of PMMA and MWCNTs/PMMA/KOH composite thin films
Sample 1 Sample 7
C
O
K
Au
4.72 45.36
1.55 4.30
– 4.00
93.74 46.35
the MWCNTs were embedded in the PMMA-based material. Additionally, the K+ ions were also embedded in the composite thin film of MWCNTs/PMMA/KOH. 3.2. Humidity-sensing characteristics
Fig. 3. FE-SEM micrographs of MWCNTs/PMMA composite thin films (compositions as shown in Table 1): (a) Sample 3 and (b) Sample 4.
participated in PMMA polymerization and adhered strongly at the interface with the PMMA matrix. However, no strong new peak at 1650 cm−1 was observed from the MWCNTs/PMMA composite thin film that was fabricated herein perhaps because the amount of the AIBN did not suffice to open the -bonds of CNTs linked with the PMMA matrix during compounding of the composite [11]. Fig. 3a and b presents SEM images of the surfaces of Samples 3 and 4, respectively. When the mass of MWCNTs added was less than 2.4 mg (Fig. 3a), the PMMA layers wrapped the MWCNTs and no naked MWCNTs were observed on the surface of the sample. Additionally, the bundles of MWCNTs were embedded and dispersed uniformly. When the mass of MWCNTs added exceeded 4 mg (Fig. 3b), many naked and aggregated MWCNTs were clearly observed on the surface. As revealed by FT-IR, the PMMA layers wrapped the MWCNTs during the in situ synthesis on an alumina substrate in a physical interaction. The composition of the surfaces of Samples 1 and 7 were analyzed by EDS, and Table 2 lists the results. The characteristic feature of the composite thin film of MWCNTs/PMMA/KOH was its relatively high content of carbon atom because most of
3.2.1. Effect of MWCNTs content on the electrical characteristics and sensing mechanism Fig. 4 presents the effect of adding large and small amounts of MWCNTs on the impedance of MWCNTs/PMMA composite thin films as a function of relative humidity. As presented in Fig. 4, pure PMMA (Sample 1) exhibited only a little impedance change over the range of humidity studied, undoubtedly because of its hydrophobicity. When the amount of MWCNTs added was under 2.4 mg (Samples 2 and 3), the impedance decreased as RH increased and the range of humidity sensing increased with increasing the amount of MWCNTs added. However, when the mass of MWCNTs added exceeded 4 mg (Sample 4), the impedance increased slightly with an increase in %RH (as presented in the inset). The humidity sensing of ceramic and polymer materials is known to mainly occur as a surface mechanism [3,21,22]. When less than 2.4 mg of MWCNTs were added (Samples 2 and 3), as described in Section 3.1, no naked MWCNTs were present on the surface of the composite material film, and the MWCNTs were embedded in PMMA. Therefore, the surface properties of MWCNTs may not be directly responsible for the improved in the humidity sensing of the composite films compared to that of the pure PMMA film. Therefore, two possible explanations are proposed to the slight improvement in the humidity-sensing range of the PMMA films doped with MWCNTs (Samples 2 and 3) are proposed. First, MWCNTs have been demonstrated theoretically and experimentally to exhibit mixed metallic and semiconducting behavior [23,24]. Therefore, MWCNTs render the MWCNTs/PMMA composite thin films richer in electrons, increasing the binding affinity and sticking coefficient of water vapor molecules. Secondly, MWCNTs are typically present in inner cavities, in interlayer pores and as aggregates of isolated CNTs and CNT bundles, forming aggregated pores [25]. Therefore, the slight improvement in the humidity-sensing range due to MWCNTs may be caused by capillary condensation [25]. When more than 4 mg of MWCNTs were added (Sample 4), as described in Section 3.1, many naked MWCNTs were observed on the surface of the film. Accordingly, the MWCNTs dominate the humidity-sensing properties. The adsorption of electron-donating water molecules compensates for the hole carriers in p-type MWCNTs, causing the electrical resistance of composite thin films to increase with humidity [26,27]. The low sensitivity was thought to be observed because the change in
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Fig. 4. Impedance vs. relative humidity for Samples 1–4 (compositions are shown in Table 1), measured at 1 kHz and 1 V.
the conductance of the individual CNTs due to gas adsorption was only about 5 × 10−6 −1 [23]. Additionally, the amount of added MWCNTs exceeded the percolation threshold so that the change in conductance of the sensing material due to adsorption of water molecules was not obvious. 3.2.2. MWCNTs/PMMA doped with KOH composite thin films The presented results were obtained from the devices based on MWCNTs/PMMA doped with KOH. Fig. 5 plots the effects of KOH addition on the impedance of MWCNTs/PMMA as a function of relative humidity. When the MWCNTs/PMMA was doped with KOH (above 2.5 wt.%) (Samples 6 and 7), an inverse sigmoidal dependence of the logarithmic impedance on humidity was observed. When the MWCNTs/PMMA was doped with KOH (0.5 wt.%) (Sample 5), a linear dependence of the logarithmic impedance on humidity was observed. The mechanism of the variation in conductance with %RH is considered below, based on a combination of the reports of Sakai et al. [18] and Casalbore-Miceli et al. [28], to explain the results of the MWCNTs/PMMA-doped KOH. First, the adsorption of water causes a thin liquid layer to form around the composite material chains or to fill the openings in the sensing composite films through capillary condensation or swelling. The sorbed water promotes the electrolytic dissociation of inorganic salts in the composite material–salt complexes. Finally, the sorbed water as a plasticizer increases the mobility of dissociated ions. Therefore, the dissociation behavior and activity coefficient of the inorganic salts can guide our understanding
of the variations in conductance that are associated with the KOH. When the amount of KOH dopant was high, some KOH was not trapped in the composite film, which would enable it to be easily and completely dissociated at low humidity. When the amount of KOH dopant was low, the KOH was completely trapped in the composite film completely, so that the KOH could be dissociated completely in a highly humid atmosphere. Fig. 6 plots the sensitivity, linearity and hysteresis of variations in impedance of MWCNTs/PMMA doped with KOH
Fig. 5. Impedance vs. relative humidity for Samples 5–7 (compositions are shown in Table 1), measured at 1 kHz and 1 V.
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4. Conclusion
Fig. 6. Impedance vs. relative humidity for Sample 5 measured at 1 kHz and 1 V. () Humidification and () desiccation.
(Sample 5) against %RH for humidification and desiccation. The impedance changed from 104 to 107 and the curves of log-impedance as a function of RH were quite linear (Y = −0.0352X + 7.5329, where Y is the log-impedance and X is the RH; correlation coefficient R2 = 0.9953) in the range of 30–90% RH. The hysteresis between humidification and desiccation was less then 4% RH. The response time (humidification from 8 to 90% RH) was about 30 s, and the recovery time (desiccation from 85 to 7% RH) was about 25 s, as present in Fig. 7. Additionally, in the stability test, in which a sensor was subjected to more than 50 cycles of humidification and desiccation in the range of 30–90% RH at 25 ◦ C, the proposed humidity sensors worked normally at all times.
Humidity-sensing composite thin films of MWCNTs/PMMA and MWCNTs/PMMA doped with KOH were prepared in situ on an alumina substrate. FT-IR indicated that the PMMA layers wrapped MWCNTs during the in situ synthesis in a physical interaction. Most of the MWCNTs and K+ ions were embedded in the PMMA-based materials according to the SEM and EDS analysis. When a small amount of MWCNTs was added, the binding affinity of the MWCNTs/PMMA composite films increased along with the sticking coefficient, and the capillary condensation mechanism slightly increased the humidity-sensing range. When a large amount of MWCNTs was added, opposite variations in the impedance with relative humidity were observed, and the MWCNTs dominated the humidity-sensing characteristics. When the amount of MWCNTs added exceeded the percolation threshold, no variation in impedance versus relative humidity was clearly observed. Humidity sensors based on the composite thin films of MWCNTs/PMMA/KOH exhibited good sensitivity, linearity, small hysteresis, fast response, a short recovery time and high stability. The charge-transportation of ions generated in electrolytic dissociation, their interactions with polymers and the dimensions of the channels in which the ions migrated, were used to explain the sensing mechanism of the MWCNTs/PMMA/KOH composite thin films. Acknowledgements The authors would like to thank the National Science Council of Taiwan for financially supporting this research under Contract no. NSC 94-2216-E-034-007 and 95-2221-E-034-005. We also wish to thank Dr. B.Y. Wei of MRL/ITRI, Taiwan for kindly supplying MWCNTs. References
Fig. 7. Response–recovery characteristics of Sample 5 measured at 1 kHz and 1 V.
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Biographies Pi-Guey Su is currently an assistant 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. He worked as a researcher in Industrial Technology Research Institute, Taiwan, in 1998–2002. He joined as an assistant professor in the General Education Center, Chungchou Institute of Technology in 2003–2005. His fields of interests are chemical sensors, gas and humidity-sensing materials and humidity standard technology. Chao-Shen Wang received a BS degree in Chemistry from Chinese Culture University in 2005. He entered the MS course of chemistry at Chinese Culture University in 2005. His main areas of interest are humidity-sensing materials.