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M050 composites films
Sensors and Actuators B 269 (2018) 110–117
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Electrical and humidity-sensing properties of flexible metal-organic framework M050(Mg) and KOH/M050 and AuNPs/M050 composites films Pi-Guey Su ∗ , Xin-Han Lee Department of Chemistry, Chinese Culture University, Taipei 111, Taiwan
a r t i c l e
i n f o
Article history: Received 1 December 2017 Received in revised form 2 April 2018 Accepted 1 May 2018 Available online 1 May 2018 Keywords: Flexible humidity sensor MOF Composite Sensing properties Complex impedance spectra
a b s t r a c t A commercial metal-organic framework (MOF) of Basosiv M050 and its composite materials potassium hydroxide/M050 (KOH/M050) and Au nanoparticles/M050 (AuNPs/M050) were coated on a polyethylene terephthalate (PET) substrate to form novel flexible impedance-type humidity sensors. The electrical properties of M050 to which were added various amounts of KOH or AuNPs were studied in detail as functions of relative humidity (RH). The contributions of the KOH or the AuNPs to the humidity-sensing (linearity and sensitivity) properties and flexibility were thus elucidated. The M050 and composites of KOH/M050 and AuNPs/M050 films that were coated on PET substrates were analyzed using scanning electron microscopy (SEM), atomic force microscopy (AFM), UV–vis spectroscopy and infrared spectrometry. The sensor that was made of KOH/M050 film with 0.05 M added KOH exhibited high flexibility, the highest sensitivity, acceptable linearity, a short response time, a low temperature coefficient and long-term stability. The humidity-sensing mechanism of the KOH/M050 film was explained using complex impedance spectra. © 2018 Elsevier B.V. All rights reserved.
1. Introduction Flexible humidity sensors are widely integrated into smart textiles, radio frequency identification (RFID) tags and the Internet of Things (IoT) to improve quality of life and industrial processes; because they are flexible, light, soft and transparent. Many polymers-based materials have been coated on flexible substrates to fabricate flexible humidity sensors [1–13]. Flexible capacitivetype humidity sensors were fabricated by using hydrophobic polymers-based materials [1–3]. Additionally, flexible impedancetype humidity sensors were produced by using hydrophilic polymers-based with COOH, NH2 and OH functional groups and their composite materials [4–13]. Many challenges should be considered to develop flexible humidity sensors including manufacture processes, the stability of their mechanical, flexibility, electrical and humidity-sensing properties under repeated bending. Metal-organic frameworks (MOFs) are mesoporous crystalline materials, which comprise metal ions connecting to organic ligands in a networked structure [14]. Recently, MOFs have attracted sub-
∗ Corresponding author. E-mail address: [email protected] (P.-G. Su). https://doi.org/10.1016/j.snb.2018.05.002 0925-4005/© 2018 Elsevier B.V. All rights reserved.
stantial interest for use in chemical sensors owing to their ultrahigh surface areas, uniform pore size, flexible structure, and high thermal and mechanical stabilities. Such sensors include the volatile organic compounds (VOCs)-based gas sensor [15,16], the alcoholbased gas sensor [17], NH3 -based gas sensor [18] and humidity sensors [19–22]. Notably, the MOFs coatings on the substrates in the cited studies were deposited on rigid substrates, such as ceramics or SiO2 /Si. However, it is difficult to integrate rigid sensor devices on flexible electronics because rigid substrates do not allow any flexibility. To the best of our knowledge, the fabrication of flexible humidity sensors that are based on MOFs and their composite materials has not been reported. The commercial MOF (Basosiv M050; magnesium formate) [23] was chose in this work because that the Mg-based MOF exhibited water stable in a highhumidity environment [24]. Alkali salts (such as LiOH and KOH) and noble nanometals (such as Pt, Pd and Au) are used as additives for improving sensing properties of sensors because they had many chemisorption sites for gas vapors owing to their small size and high local charges [25–27]. Therefore, composite materials of potassium hydroxide/M050 (KOH/M050) and Au nanoparticles/M050 (AuNPs/M050) were prepared in this study. M050 and composite materials of KOH/M050 and AuNPs/M050 were drop-coated on a polyethylene terephthalate (PET) substrate to form flexible impedance-type humidity sensors.
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Fig. 1. (a) Photograph of flexible humidity sensors on a PET substrate and structure of a humidity sensor, (b) Schematic structure of the impedance measurement of flexible humidity sensors and the humidity atmosphere controller.
2. Experimental 2.1. Materials and preparation of flexible impedance-type humidity sensors The Mg-based MOF (magnesium formate, Sigma Aldrich, trademark M050) directly used without any further purification. AuNPs were obtained using the method in the literature [28]. The AuNPs colloidal particles were prepared by adding 38.8 mM sodium citrate (Shimakyu’s Pure. Chemicals, Lancashire, United Kingdom, 99%) to boiling aqueous 1.0 mM tetrachloroauic acid (HAuCl4 , Alfa Aesar, Osaka, Japan, 99%). Then, the solution was boiled for 15 min with vigorous stirring, and finally allowed to cool to room temperature to produce 0.9 mM AuNPs colloidal solution. Fig. 1(a) schematically plots the pattern of the flexible impedance-type humidity sensor. Firstly, sputtering Cr (thickness 50 nm) and then Au (thickness 250 nm) on a flexible substrate (polyethylene terephthalate; PET) in the temperature range of 120 ∼ 160 ◦ C formed the interdigited Au electrodes (gap 0.25 mm). The substrates were firstly treated with an H2 O2 /H2 SO4 mixture (1:2, 15 mL), washed in de-ionized water (DIW) and then cleaned in acetone solution for 3 min. M050 precursor solution was prepared by adding 100 mg of M050 powder to 1 mL ethanol and stirring vigorously. Precursor solutions of KOH/M050 and AuNPs/M050 composite materials were prepared by mixing KOH and AuNPs in various ratios with the as-prepared M050 precursor solution. Table 1 presents the various compositions examined. The as-prepared M050 and composite solutions were coated onto as-prepared PET substrate, and allowed
Table 1 Compositions of M050 and its composites-based films used to prepare flexible impedance-type humidity sensors. Sensor number
MOF (mg)
KOH (M)
AuNPs (mg)
1 2 3 4 5 6
100 100 100 100 100 100
0 0.005 0.05 0 0 0
0 0 0 60 180 540
to dry at 60 ◦ C. Therefore, a flexible impedance-type humidity sensor was obtained. 2.2. Instruments and characterizations UV–vis spectroscopy (Agilent 8453, Santa Clara, USA) was used to characterize the formation of M050, AuNPs, and AuNPs/M050 composite. An infrared spectrometry (Nicolet 380, Wilmington, USA) was used to obtain the IR spectra of the samples. The surface microstructure of the films that were coated on a PET substrate was investigated using a scanning electron microscope (SEM, Ibaraki, Japan) and an atomic force microscope (AFM, Ben-Yuan, CSPM 4000, Baijing, China) in tapping mode. Fig. 1(b) shows the humidity-sensing measurement system. A divided flow humidity generator was used to produce the testing humidity, as same as previously reported [29]. The required humidity (RH values) was adjusted according to readings from a standard humidity hygrometer (accuracy of ±0.1% RH, Rotronic, Bassersdorf, Switzerland)
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ison of Fig. 3(a) with Fig. 3(b) (inset in Fig. 3) revealed a new broader absorption band at 698 nm. This phenomenon was caused by the aggregation of the AuNPs in the AuNPs/N050 composite [30,31]. AuNPs/M050 composites retained their characteristic absorption bonds of M050. The FTIR and UV–vis spectra all suggest that neither KOH nor AuNPs destroyed the structure of M050.
Fig. 2. IR spectra of (a) M050, (b) KOH/M050 and (c) AuNPs/M050.
Fig. 3. UV–vis absorption spectra of (a) as-prepared AuNPs, (b) AuNPs/M050 and (c) M050.
by controlling the ratio of dry and to humid air. The total flow rate of 10 L/min was controlled by a mass controller. LCR meter (Philips PM6306, Eindhoven, Netherlands) was used to measure the impedance change of the flexible humidity sensors versus RH in a temperature-controlled chamber (Thermo-HAAKE, Taipei, Taiwan). The measurements were made at frequency of 1 kHz, an applied voltage of 1 V, an ambient temperature of 25 ◦ C. 3. Results and discussion 3.1. Characterization of M050 and KOH/M050 and AuNPs/M050 composites films 3.1.1. IR and UV–vis spectra Fig. 2 presents the FTIR spectra of the M050 and KOH/M050 and AuNPs/M050 composites. The characteristic vibrations of M050 (Fig. 2(a)) appear at 1580, 1351, 1382 and 780 cm−1 , corresponding to the antisymmetric (as (COO)) and symmetric (s (COO)) stretching modes of the carboxylic group, C H stretching and Mg O stretching, respectively. Moreover, the characteristic bonds at 3150 cm−1 correspond to intermolecular hydrogen (O H) stretching. The formation of KOH/M050 and AuNPs/M050 composites yielded not new peak (Fig. 2(b) and (c)). Fig. 3 shows the UV–vis absorption spectra of M050, as-prepared AuNPs and the AuNPs/M050 composite. The absorption band of as-prepared AuNPs was obtained at approximately 520 nm (Fig. 3(a)). A compar-
3.1.2. SEM and AFM analyses of the samples Fig. 4 presents the SEM images of the M050 and composite materials of KOH/M050 and AuNPs/M050 films on a PET substrate. The M050 film (sensor 1) is uniform, intact, and non-fractured film (Fig. 4(a)). M050 exhibits well-defined scalene cubic crystalline morphology and a mean particle diameter of approximately 2.0–4.0 m, as shown in the inset in Fig. 4(a). Fig. 4(b) and (c) (sensors 2 and 3) present SEM images of KOH/M050 films with added 0.005 and 0.05 M KOH, respectively. The KOH/M050 films were more compact than the M050 film because of the agglomeration of M050, which was caused by the fact that added KOH functioned as an intermediate electrostatic glue, so as the concentration of added KOH increased from 0.005 to 0.05 M, the particle size increased from 5 to 12 m. Sensor 3 had a more uniform and compact structure than sensor 2. Fig. 4(d)–(f) (sensors 4–6) show SEM images of AuNPs/M050 films with various amounts of added AuNPs. The surface of AuNPs/M050 became more solid as the amount of AuNPs that were incorporated increased because the aggregation of AuNPs resulting in more M050 particle–M050 particle agglomeration and, consequently, the formation of a more rigid film. The various morphologies resulted in various flexibility and humidity-sensing properties. Fig. 5 presents the surface roughness of the M050 and composite materials of KOH/M050 and AuNPs/M050 films on a PET substrate, analyzed using tapping-mode AFM. The size of each image is 5 m × 5 m. The root mean square (RMS) roughness values of the M050, KOH/M050, AuNPs/M050 films were 23.8, 24.6 and 23.6 nm, respectively. The values of roughness of M050, KOH/M050, AuNPs/M050 did not vary significantly (Fig. 5(a)–(c)). 3.2. Electrical and humidity-sensing properties of flexibleM050 and KOH/M050 and AuNPs/M050 composites films The variation of the impedance of flexible humidity sensors with bending was calculated using the formula Df = {[(If −Ib )/If ] × 100%}, where If and Ib are the impedance of the flat and bent flexible humidity sensors at 60% RH and at all places, respectively. Fig. 6(a) plots the effect of the amount of added KOH on the flexibility characteristics of the flexible humidity sensors that were made of KOH/M050 composite film. The deviation of the impedance of the flexible humidity sensor that was made of M050 film (sensor 1) and bent downward at an angle of up to 60◦ was approximately 7%. The flexibility of the KOH/M050 composite films (sensors 2 and 3) were better than that of M050 film because the added KOH acted as an intermediate electrostatic adhesive, strengthening the interface in the M050 matrix. Sensor 3 was the most flexible (Df < 4%) at an angle of up to 60◦ . Additionally, the flexibility of sensor 2 declined sharply with bending above 40◦ because in more dilute added KOH caused poorer mutual adhesion of M050 particles. Fig. 6(b) plots the effect of the amount of added AuNPs on the flexibility of the flexible humidity sensors that were made of AuNPs/M050 composite films (sensors 4–6). The flexibility of sensor 4 was similar to that of sensor 1. Moreover, the flexibility of the AuNPs/M050 composite films (sensors 4–6) decreased as the amount of the added AuNPs increased because solid films formed as described in Section 3.1.2. These results were related to the fact that in a more concentrated AuNPs solution, more AuNPs aggregated and became entangled with M050 and each other, making the AuNPs/M050 film
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Fig. 4. SEM images of M050 and composites of KOH/M050 and AuNPs/M050 films on PET substrate; (compositions as shown in Table 1). (a) sensor 1, (b) sensor 2, (c) sensor 3, (d) sensor 4, (e) sensor 5, (f) sensor 6. Insets present the high-magnification SEM image.
Table 2 Sensitivity and linearity of flexible impedance-type humidity sensors that were made of M050 and composite KOH/M050 and AuNPs/M050 films. Sensor number
1 2 3 4 5 6
Sensing curve Sensitivitya (log Z/%RH)
Linearityb (R2 )
−0.0470 −0.0558 −0.0559 −0.0478 −0.0496 –
0.8734 0.9304 0.9351 0.8703 0.8253 –
a Sensitivity was defined as the slope of the logarithmic impedance versus relative humidity plot in the range 20–90% RH. b Linearity was shown as the correlation coefficient of the logarithmic impedance versus relative humidity plot in the range 20–90% RH.
more rigid and friable. Additionally, the KOH/M050 composite film was more flexible than the AuNPs/M050 composite film. Fig. 7(a) and (b) present the measured effects of adding KOH and AuNPs on the electrical responses of KOH/M050 and AuNPs/M050 composite films, respectively, at various relative humidities. Table 2 presents the sensitivity-related and linearity-related humiditysensing of the flexible humidity sensors those were made of M050 and KOH/M050 and AuNPs/M050 composites films. The measurements were made at 25 ◦ C using an AC voltage of 1 V at 1 kHz. M050 film (sensor 1) exhibited an impedance change in the range 40–90% RH, but almost no impedance change observed in the range 20–40% RH (Fig. 7(a)). M050 comprises well-defined networks of macromolecules with hydrophilic ligand groups (formate) and has an accessible pore-structure. Therefore, upon exposure to high RH (40–90% RH), multilayers of water vapor molecules accumulated on the surfaces and around the pores of the M050 film, inducing capillary condensation, forming H3 O+ ions by dissociation, causing the impedance of the M050 film to decrease with increasing RH. Upon exposure to low RH (20–40% RH), ion conduction in the
M050 film occurred mainly in the outermost layer, and the pristine M050 film exhibited poor intrinsic conductance, so its impedance in the range 20–40% RH was high. Adding KOH salt reduced the high impedance and improved the sensitivity and linearity of the M050 film, favoring its use over a wider working range of humidities, as shown in Fig. 7(a) and Table 2. The impedance of the KOH/M050 film decreased as the amount of added KOH increased. Additionally, the impedance of sensor 3 decreased as RH increased over a wider range of RH (20 ∼ 90% RH) than that of sensor 2, suggesting that sensor 3 had the highest sensitivity and the best linear response curve, as shown in Table 2. As described in Section 3.1.2, the M050, KOH/M050, and AuNPs/M050 films did not vary significantly in surface roughness. Therefore, the sensitivity of these films to humidity was not related to their surface morphology. These results are attributable to the fact that KOH is a strong electrolyte with a very high dissociation constant (6.63 × 1010 ) [32,33], dissociating easily and completely at low RH, so the impedance of the KOH/M050 composite film decreased as RH increased in the range 20–40% RH. Fig. 7(b) presents the effect of adding AuNPs on the electrical responses of AuNPs/M050 composite films (sensors 4 ∼ 6) versus RH. The impedance of the AuNPs/M050 composite films decreased as the amount of added AuNPs increased because the AuNPs in the AuNPs/M050 films formed new conducting paths, facilitating the transfer of charge carriers, improving conductivity [34]. Furthermore, as the amount of added AuNPs increased above 540 mg (sensor 6), an inverse sigmoidal dependence of the logarithmic impedance on humidity was observed. This result is attributed to the fact that the amount of added AuNPs exceeded the percolation threshold so the change in conductance of the AuNPs/M050 composite film as a result of the adsorption of water molecules was not obvious in the range 70–90% RH. Therefore, the humidity-sensing properties and humidity-sensing mechanism of sensor 3 were examined because sensor 3 had the highest sensitivity (slope = −0.0559 log Z/%RH) and linearity (R2 = 0.9351) over the humidity range 20–90% RH, and the best flexibility (<4%) (Table 2).
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Fig. 6. Flexibility of M050 and various KOH/M050 and AuNPs/M050 composite films coated on PET substrate; (compositions as shown in Table 1). (a) effect of amount of added KOH and (b) effect of amount of added AuNPs.
Fig. 5. AFM images of (a) M050, (b) KOH/M050 and (c) AuNPs/M050 films on PET substrate.
3.3. Humidity-sensing properties of KOH/M050 film
Fig. 7. Impedance versus relative humidity for M050 and various KOH/M050 and AuNPs/M050 composite films coated on PET substrate; (compositions as shown in Table 1). (a) effect of amount of added KOH and (b) effect of amount of added AuNPs, measured at 1 kHz, 1 V and 25 ◦ C.
Fig. 8(a) plots the hysteresis of sensor 3 in a desiccationto-humidification cycle. The hysteresis was less than 2.0% RH, measured over the RH range 20 ∼ 90% RH. Fig. 8(b) plots the effect of the ambient temperature on the log-impedance of sensor 3 against RH. As the temperature increased, the RH characteristic curve shifted to slightly lower impedance. The temperature coefficient at 15 ∼ 35 ◦ C was below −0.11% RH/◦ C over the humidity range 20 ∼ 90% RH. Fig. 8(c) plots the effect of the applied measure-
ment frequency on the log-impedance of sensor 3 against RH, the impedance was measured at frequencies of 0.1, 1, 11 and 100 kHz at a voltage of 1 V. The impedance of sensor 3 clearly decreased as decreased below 30% RH at 100 kHz because the adsorbed water could not be polarized [35]. Fig. 8(d) plots the response/recovery times of sensor 3, measured at 25 ◦ C and 1 kHz. The response time (Tres.95% ) is calculated as the time taken for the impedance of the sensor to change by 95% of its maximum change after humidifica-
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Time (s)
Fig. 8. (a) Hysteresis of sensor 3, (b) effect of temperature on the response of sensor 3, (c) effect of applied frequency on the response of sensor 3, (d) response/recovery properties of sensor 3, (e) long-term stability of sensor 3.
Table 3 Flexible humidity sensor performance of this work compared with the literatures. Sensing material
Working range (%RH)
Sensitivity (log Z/%RH)
Hysteresis (%RH)
Flexibility (%) (Response deviation)
Response time (s)
References
KOH/M050 PAMAMa -AuNPs PMMA-PMAPTACb AuNPs/N-substituted pyrrole derivatives AuNPs/GO/MPTMOSc
20–90 30–90 20–90 30–90 20–90
0.0559 0.0450 0.0258 0.0827 0.0281
<2 <2 2 1 5
<4 2 <1 1 17
36 40 45 53 119
This work [36] [37] [38] [39]
a b c
PAMAM: polyamidoamine dendrimer. PMMA-PMAPTAC: copolymer of methyl methacrylate and [3-(methacrylamino)propyl] trimethyl ammonium chloride. MPTMOS: 3-mercaptopropyltrimethoxysilane.
tion from 10 to 90% RH. The recovery time (Trec.95% ) is calculated at the time required for the sensor to recover 95% of its maximum change in impedance after desiccation from 90 to 10% RH. The response time (Tres.95% ) and recovery (Trec.95% ) time of sensor 3 were 36 and 118 s, respectively. The slow recovery time of sensor 3 may have arisen from the micro/nanopores of the M050 film. Fig. 8(e) plots the long-term stability of sensor 3 at the tested RH values of 20, 60 and 90% RH. No obvious deviation in log-impedance was observed at least 34 days. Table 3 compares the humidity sens-
ing properties of sensor 3 herein with those of the sensors that were made of polymer and inorganic sensing materials, described in the literature [36–39]. The developed humidity sensor that was made of KOH/M050 composite film (sensor 3) exhibited larger working range, higher sensitivity, lower hysteresis and faster response time that those flexible humidity sensors that are made of PAMAMAuNPs [36] and PMMA-PMAPTAC [37]. Sensor 3 herein exhibited larger working range and faster response time than that of flexible humidity sensor that is made of AuNPs/N-substituted pyrrole
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line was observed on the complex plot at the low-frequency range (Fig. 9(b)). This phenomenon was related to the diffusion of ions across the interface between the Au electrode and the KOH/M050 film [40–43]. Finally, a straight line at 90% RH was observed on the impedance plot (Fig. 9(c)). Therefore, upon the adsorption of water, a liquid layer formed around the KOH/M050 film by capillary condensation, and H3 O+ and K+ ions were dissociated. The mobility and amount of the ions (H3 O+ and K+ ) increased with increasing relative humidity, and this effect dominated the conduction of the humidity sensor. Therefore, the ions (K+ and H3 O+ ) dominated the variation of conductance of the KOH/M050 composite film with relative humidity according to the obtained complex impedance plots. 4. Conclusions A flexible impedance-type humidity sensor that was based on M050 film had a small humidity-working range (40–90% RH) because of its poor intrinsic conductance. The sensitivity and linearity of the flexible impedance-type humidity sensor that was based on the M050 film were improved by adding KOH and AuNPs to increase the number of the conductance pathways of the KOH/M050 and AuNPs/M050 films at low RH. Moreover, the flexible impedance-type humidity sensor that was made of the KOH/M050 composite film exhibited higher sensitivity, better linearity and higher flexibility than that made of the AuNPs/M050 because KOH is a strong electrolyte. The flexible impedance-type humidity sensor that was made of the KOH/M050 composite film with added 0.05 M KOH had a wide working range of humidity (20–90% RH), a highest sensitivity (slope = −0.0559 log Z/%RH), an excellent linearity (R2 = 0.9351), a negligible hysteresis (<2%), a short response time (36 s), high flexibility (Df < 4%), a weak ambient temperature-dependence (−0.11% RH/◦ C) and high longterm stability (at least 34 days). The frequency-dependence of the impedance was observed at 100 kHz and at a humidity of less than 30%. The complex impedance plots of the KOH/M050 composite film changed from semicircular to linear as RH increased. These results reflect the domination of the conductivity of the KOH/M050 composite film by the ions (K+ and H3 O+ ). Acknowledgement The authors thank the Ministry of Science and Technology (grant no. MOST 106-2113-M-034-002) of Taiwan for support. Fig. 9. Complex impedance plots of sensor 3 at (a) 20% RH, (b) 40% RH and (c) 90% RH.
derivatives [38]. Sensor 3 herein had a lower hysteresis, shorter response time and greater flexibility than a flexible humidity sensor that is made of inorganic material (SiO2 -based) [39]. 3.4. Humidity-sensing mechanism of KOH/M050 film Impedance spectroscopy was used to identify the conduction mechanism of KOH/M050 composite film (sensor 3) and Fig. 9 plots the observed impedance spectra. The observed impedance was plotted at frequencies from 50 Hz to 100 kHz, relative humidities of 20–90% RH, an AC voltage of 1 V and a temperature of 25 ◦ C. In the impedance spectra, Zr (real part of the impedance Z) and Zi (imaginary part of Z) were plotted on the real axis and imaginary axis, respectively. When at a low humidity of 20% RH (Fig. 9(a)), a semicircular plot of film impedance was obtained. The semicircle was replicated using an equivalent circuit of a resistor and capacitor in parallel [40–42]. As RH increased (40% RH), an inclined
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Biographies Pi-Guey Su is currently a professor in Department of Chemistry at Chinese Culture University. He received his BS degree from Soochow University in Chemistry in 1993 and PhD degree in chemistry from National Tsing Hua University in 1998. He worked as a researcher in Industrial Technology Research Institute, Taiwan, from 1998 to 2002. He joined as an assistant professor in the General Education Center, Chungchou Institute of Technology from 2003 to 2005. He worked as an assistant professor in Department of Chemistry at Chinese Culture University from 2005 to 2007. He worked as an associate professor in Department of Chemistry at Chinese Culture University from 2007 to 2010. His fields of interests are chemical sensors, gas and humidity sensing materials, humidity standard technology and smart sensor system. Xin-Han Lee received a B.S. degree in chemistry from Chinese Culture University in 2016. He entered the M.S. course of chemistry at Chinese Culture University in 2016. His main areas of interest are humidity-sensing materials and humidity measurement system.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Electrical and humidity-sensing properties of flexible metal-organic framework M050(Mg) and KOH/M050 and AuNPs/M050 composites films Pi-Guey Su ∗ , Xin-Han Lee Department of Chemistry, Chinese Culture University, Taipei 111, Taiwan
a r t i c l e
i n f o
Article history: Received 1 December 2017 Received in revised form 2 April 2018 Accepted 1 May 2018 Available online 1 May 2018 Keywords: Flexible humidity sensor MOF Composite Sensing properties Complex impedance spectra
a b s t r a c t A commercial metal-organic framework (MOF) of Basosiv M050 and its composite materials potassium hydroxide/M050 (KOH/M050) and Au nanoparticles/M050 (AuNPs/M050) were coated on a polyethylene terephthalate (PET) substrate to form novel flexible impedance-type humidity sensors. The electrical properties of M050 to which were added various amounts of KOH or AuNPs were studied in detail as functions of relative humidity (RH). The contributions of the KOH or the AuNPs to the humidity-sensing (linearity and sensitivity) properties and flexibility were thus elucidated. The M050 and composites of KOH/M050 and AuNPs/M050 films that were coated on PET substrates were analyzed using scanning electron microscopy (SEM), atomic force microscopy (AFM), UV–vis spectroscopy and infrared spectrometry. The sensor that was made of KOH/M050 film with 0.05 M added KOH exhibited high flexibility, the highest sensitivity, acceptable linearity, a short response time, a low temperature coefficient and long-term stability. The humidity-sensing mechanism of the KOH/M050 film was explained using complex impedance spectra. © 2018 Elsevier B.V. All rights reserved.
1. Introduction Flexible humidity sensors are widely integrated into smart textiles, radio frequency identification (RFID) tags and the Internet of Things (IoT) to improve quality of life and industrial processes; because they are flexible, light, soft and transparent. Many polymers-based materials have been coated on flexible substrates to fabricate flexible humidity sensors [1–13]. Flexible capacitivetype humidity sensors were fabricated by using hydrophobic polymers-based materials [1–3]. Additionally, flexible impedancetype humidity sensors were produced by using hydrophilic polymers-based with COOH, NH2 and OH functional groups and their composite materials [4–13]. Many challenges should be considered to develop flexible humidity sensors including manufacture processes, the stability of their mechanical, flexibility, electrical and humidity-sensing properties under repeated bending. Metal-organic frameworks (MOFs) are mesoporous crystalline materials, which comprise metal ions connecting to organic ligands in a networked structure [14]. Recently, MOFs have attracted sub-
∗ Corresponding author. E-mail address: [email protected] (P.-G. Su). https://doi.org/10.1016/j.snb.2018.05.002 0925-4005/© 2018 Elsevier B.V. All rights reserved.
stantial interest for use in chemical sensors owing to their ultrahigh surface areas, uniform pore size, flexible structure, and high thermal and mechanical stabilities. Such sensors include the volatile organic compounds (VOCs)-based gas sensor [15,16], the alcoholbased gas sensor [17], NH3 -based gas sensor [18] and humidity sensors [19–22]. Notably, the MOFs coatings on the substrates in the cited studies were deposited on rigid substrates, such as ceramics or SiO2 /Si. However, it is difficult to integrate rigid sensor devices on flexible electronics because rigid substrates do not allow any flexibility. To the best of our knowledge, the fabrication of flexible humidity sensors that are based on MOFs and their composite materials has not been reported. The commercial MOF (Basosiv M050; magnesium formate) [23] was chose in this work because that the Mg-based MOF exhibited water stable in a highhumidity environment [24]. Alkali salts (such as LiOH and KOH) and noble nanometals (such as Pt, Pd and Au) are used as additives for improving sensing properties of sensors because they had many chemisorption sites for gas vapors owing to their small size and high local charges [25–27]. Therefore, composite materials of potassium hydroxide/M050 (KOH/M050) and Au nanoparticles/M050 (AuNPs/M050) were prepared in this study. M050 and composite materials of KOH/M050 and AuNPs/M050 were drop-coated on a polyethylene terephthalate (PET) substrate to form flexible impedance-type humidity sensors.
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Fig. 1. (a) Photograph of flexible humidity sensors on a PET substrate and structure of a humidity sensor, (b) Schematic structure of the impedance measurement of flexible humidity sensors and the humidity atmosphere controller.
2. Experimental 2.1. Materials and preparation of flexible impedance-type humidity sensors The Mg-based MOF (magnesium formate, Sigma Aldrich, trademark M050) directly used without any further purification. AuNPs were obtained using the method in the literature [28]. The AuNPs colloidal particles were prepared by adding 38.8 mM sodium citrate (Shimakyu’s Pure. Chemicals, Lancashire, United Kingdom, 99%) to boiling aqueous 1.0 mM tetrachloroauic acid (HAuCl4 , Alfa Aesar, Osaka, Japan, 99%). Then, the solution was boiled for 15 min with vigorous stirring, and finally allowed to cool to room temperature to produce 0.9 mM AuNPs colloidal solution. Fig. 1(a) schematically plots the pattern of the flexible impedance-type humidity sensor. Firstly, sputtering Cr (thickness 50 nm) and then Au (thickness 250 nm) on a flexible substrate (polyethylene terephthalate; PET) in the temperature range of 120 ∼ 160 ◦ C formed the interdigited Au electrodes (gap 0.25 mm). The substrates were firstly treated with an H2 O2 /H2 SO4 mixture (1:2, 15 mL), washed in de-ionized water (DIW) and then cleaned in acetone solution for 3 min. M050 precursor solution was prepared by adding 100 mg of M050 powder to 1 mL ethanol and stirring vigorously. Precursor solutions of KOH/M050 and AuNPs/M050 composite materials were prepared by mixing KOH and AuNPs in various ratios with the as-prepared M050 precursor solution. Table 1 presents the various compositions examined. The as-prepared M050 and composite solutions were coated onto as-prepared PET substrate, and allowed
Table 1 Compositions of M050 and its composites-based films used to prepare flexible impedance-type humidity sensors. Sensor number
MOF (mg)
KOH (M)
AuNPs (mg)
1 2 3 4 5 6
100 100 100 100 100 100
0 0.005 0.05 0 0 0
0 0 0 60 180 540
to dry at 60 ◦ C. Therefore, a flexible impedance-type humidity sensor was obtained. 2.2. Instruments and characterizations UV–vis spectroscopy (Agilent 8453, Santa Clara, USA) was used to characterize the formation of M050, AuNPs, and AuNPs/M050 composite. An infrared spectrometry (Nicolet 380, Wilmington, USA) was used to obtain the IR spectra of the samples. The surface microstructure of the films that were coated on a PET substrate was investigated using a scanning electron microscope (SEM, Ibaraki, Japan) and an atomic force microscope (AFM, Ben-Yuan, CSPM 4000, Baijing, China) in tapping mode. Fig. 1(b) shows the humidity-sensing measurement system. A divided flow humidity generator was used to produce the testing humidity, as same as previously reported [29]. The required humidity (RH values) was adjusted according to readings from a standard humidity hygrometer (accuracy of ±0.1% RH, Rotronic, Bassersdorf, Switzerland)
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ison of Fig. 3(a) with Fig. 3(b) (inset in Fig. 3) revealed a new broader absorption band at 698 nm. This phenomenon was caused by the aggregation of the AuNPs in the AuNPs/N050 composite [30,31]. AuNPs/M050 composites retained their characteristic absorption bonds of M050. The FTIR and UV–vis spectra all suggest that neither KOH nor AuNPs destroyed the structure of M050.
Fig. 2. IR spectra of (a) M050, (b) KOH/M050 and (c) AuNPs/M050.
Fig. 3. UV–vis absorption spectra of (a) as-prepared AuNPs, (b) AuNPs/M050 and (c) M050.
by controlling the ratio of dry and to humid air. The total flow rate of 10 L/min was controlled by a mass controller. LCR meter (Philips PM6306, Eindhoven, Netherlands) was used to measure the impedance change of the flexible humidity sensors versus RH in a temperature-controlled chamber (Thermo-HAAKE, Taipei, Taiwan). The measurements were made at frequency of 1 kHz, an applied voltage of 1 V, an ambient temperature of 25 ◦ C. 3. Results and discussion 3.1. Characterization of M050 and KOH/M050 and AuNPs/M050 composites films 3.1.1. IR and UV–vis spectra Fig. 2 presents the FTIR spectra of the M050 and KOH/M050 and AuNPs/M050 composites. The characteristic vibrations of M050 (Fig. 2(a)) appear at 1580, 1351, 1382 and 780 cm−1 , corresponding to the antisymmetric (as (COO)) and symmetric (s (COO)) stretching modes of the carboxylic group, C H stretching and Mg O stretching, respectively. Moreover, the characteristic bonds at 3150 cm−1 correspond to intermolecular hydrogen (O H) stretching. The formation of KOH/M050 and AuNPs/M050 composites yielded not new peak (Fig. 2(b) and (c)). Fig. 3 shows the UV–vis absorption spectra of M050, as-prepared AuNPs and the AuNPs/M050 composite. The absorption band of as-prepared AuNPs was obtained at approximately 520 nm (Fig. 3(a)). A compar-
3.1.2. SEM and AFM analyses of the samples Fig. 4 presents the SEM images of the M050 and composite materials of KOH/M050 and AuNPs/M050 films on a PET substrate. The M050 film (sensor 1) is uniform, intact, and non-fractured film (Fig. 4(a)). M050 exhibits well-defined scalene cubic crystalline morphology and a mean particle diameter of approximately 2.0–4.0 m, as shown in the inset in Fig. 4(a). Fig. 4(b) and (c) (sensors 2 and 3) present SEM images of KOH/M050 films with added 0.005 and 0.05 M KOH, respectively. The KOH/M050 films were more compact than the M050 film because of the agglomeration of M050, which was caused by the fact that added KOH functioned as an intermediate electrostatic glue, so as the concentration of added KOH increased from 0.005 to 0.05 M, the particle size increased from 5 to 12 m. Sensor 3 had a more uniform and compact structure than sensor 2. Fig. 4(d)–(f) (sensors 4–6) show SEM images of AuNPs/M050 films with various amounts of added AuNPs. The surface of AuNPs/M050 became more solid as the amount of AuNPs that were incorporated increased because the aggregation of AuNPs resulting in more M050 particle–M050 particle agglomeration and, consequently, the formation of a more rigid film. The various morphologies resulted in various flexibility and humidity-sensing properties. Fig. 5 presents the surface roughness of the M050 and composite materials of KOH/M050 and AuNPs/M050 films on a PET substrate, analyzed using tapping-mode AFM. The size of each image is 5 m × 5 m. The root mean square (RMS) roughness values of the M050, KOH/M050, AuNPs/M050 films were 23.8, 24.6 and 23.6 nm, respectively. The values of roughness of M050, KOH/M050, AuNPs/M050 did not vary significantly (Fig. 5(a)–(c)). 3.2. Electrical and humidity-sensing properties of flexibleM050 and KOH/M050 and AuNPs/M050 composites films The variation of the impedance of flexible humidity sensors with bending was calculated using the formula Df = {[(If −Ib )/If ] × 100%}, where If and Ib are the impedance of the flat and bent flexible humidity sensors at 60% RH and at all places, respectively. Fig. 6(a) plots the effect of the amount of added KOH on the flexibility characteristics of the flexible humidity sensors that were made of KOH/M050 composite film. The deviation of the impedance of the flexible humidity sensor that was made of M050 film (sensor 1) and bent downward at an angle of up to 60◦ was approximately 7%. The flexibility of the KOH/M050 composite films (sensors 2 and 3) were better than that of M050 film because the added KOH acted as an intermediate electrostatic adhesive, strengthening the interface in the M050 matrix. Sensor 3 was the most flexible (Df < 4%) at an angle of up to 60◦ . Additionally, the flexibility of sensor 2 declined sharply with bending above 40◦ because in more dilute added KOH caused poorer mutual adhesion of M050 particles. Fig. 6(b) plots the effect of the amount of added AuNPs on the flexibility of the flexible humidity sensors that were made of AuNPs/M050 composite films (sensors 4–6). The flexibility of sensor 4 was similar to that of sensor 1. Moreover, the flexibility of the AuNPs/M050 composite films (sensors 4–6) decreased as the amount of the added AuNPs increased because solid films formed as described in Section 3.1.2. These results were related to the fact that in a more concentrated AuNPs solution, more AuNPs aggregated and became entangled with M050 and each other, making the AuNPs/M050 film
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Fig. 4. SEM images of M050 and composites of KOH/M050 and AuNPs/M050 films on PET substrate; (compositions as shown in Table 1). (a) sensor 1, (b) sensor 2, (c) sensor 3, (d) sensor 4, (e) sensor 5, (f) sensor 6. Insets present the high-magnification SEM image.
Table 2 Sensitivity and linearity of flexible impedance-type humidity sensors that were made of M050 and composite KOH/M050 and AuNPs/M050 films. Sensor number
1 2 3 4 5 6
Sensing curve Sensitivitya (log Z/%RH)
Linearityb (R2 )
−0.0470 −0.0558 −0.0559 −0.0478 −0.0496 –
0.8734 0.9304 0.9351 0.8703 0.8253 –
a Sensitivity was defined as the slope of the logarithmic impedance versus relative humidity plot in the range 20–90% RH. b Linearity was shown as the correlation coefficient of the logarithmic impedance versus relative humidity plot in the range 20–90% RH.
more rigid and friable. Additionally, the KOH/M050 composite film was more flexible than the AuNPs/M050 composite film. Fig. 7(a) and (b) present the measured effects of adding KOH and AuNPs on the electrical responses of KOH/M050 and AuNPs/M050 composite films, respectively, at various relative humidities. Table 2 presents the sensitivity-related and linearity-related humiditysensing of the flexible humidity sensors those were made of M050 and KOH/M050 and AuNPs/M050 composites films. The measurements were made at 25 ◦ C using an AC voltage of 1 V at 1 kHz. M050 film (sensor 1) exhibited an impedance change in the range 40–90% RH, but almost no impedance change observed in the range 20–40% RH (Fig. 7(a)). M050 comprises well-defined networks of macromolecules with hydrophilic ligand groups (formate) and has an accessible pore-structure. Therefore, upon exposure to high RH (40–90% RH), multilayers of water vapor molecules accumulated on the surfaces and around the pores of the M050 film, inducing capillary condensation, forming H3 O+ ions by dissociation, causing the impedance of the M050 film to decrease with increasing RH. Upon exposure to low RH (20–40% RH), ion conduction in the
M050 film occurred mainly in the outermost layer, and the pristine M050 film exhibited poor intrinsic conductance, so its impedance in the range 20–40% RH was high. Adding KOH salt reduced the high impedance and improved the sensitivity and linearity of the M050 film, favoring its use over a wider working range of humidities, as shown in Fig. 7(a) and Table 2. The impedance of the KOH/M050 film decreased as the amount of added KOH increased. Additionally, the impedance of sensor 3 decreased as RH increased over a wider range of RH (20 ∼ 90% RH) than that of sensor 2, suggesting that sensor 3 had the highest sensitivity and the best linear response curve, as shown in Table 2. As described in Section 3.1.2, the M050, KOH/M050, and AuNPs/M050 films did not vary significantly in surface roughness. Therefore, the sensitivity of these films to humidity was not related to their surface morphology. These results are attributable to the fact that KOH is a strong electrolyte with a very high dissociation constant (6.63 × 1010 ) [32,33], dissociating easily and completely at low RH, so the impedance of the KOH/M050 composite film decreased as RH increased in the range 20–40% RH. Fig. 7(b) presents the effect of adding AuNPs on the electrical responses of AuNPs/M050 composite films (sensors 4 ∼ 6) versus RH. The impedance of the AuNPs/M050 composite films decreased as the amount of added AuNPs increased because the AuNPs in the AuNPs/M050 films formed new conducting paths, facilitating the transfer of charge carriers, improving conductivity [34]. Furthermore, as the amount of added AuNPs increased above 540 mg (sensor 6), an inverse sigmoidal dependence of the logarithmic impedance on humidity was observed. This result is attributed to the fact that the amount of added AuNPs exceeded the percolation threshold so the change in conductance of the AuNPs/M050 composite film as a result of the adsorption of water molecules was not obvious in the range 70–90% RH. Therefore, the humidity-sensing properties and humidity-sensing mechanism of sensor 3 were examined because sensor 3 had the highest sensitivity (slope = −0.0559 log Z/%RH) and linearity (R2 = 0.9351) over the humidity range 20–90% RH, and the best flexibility (<4%) (Table 2).
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Fig. 6. Flexibility of M050 and various KOH/M050 and AuNPs/M050 composite films coated on PET substrate; (compositions as shown in Table 1). (a) effect of amount of added KOH and (b) effect of amount of added AuNPs.
Fig. 5. AFM images of (a) M050, (b) KOH/M050 and (c) AuNPs/M050 films on PET substrate.
3.3. Humidity-sensing properties of KOH/M050 film
Fig. 7. Impedance versus relative humidity for M050 and various KOH/M050 and AuNPs/M050 composite films coated on PET substrate; (compositions as shown in Table 1). (a) effect of amount of added KOH and (b) effect of amount of added AuNPs, measured at 1 kHz, 1 V and 25 ◦ C.
Fig. 8(a) plots the hysteresis of sensor 3 in a desiccationto-humidification cycle. The hysteresis was less than 2.0% RH, measured over the RH range 20 ∼ 90% RH. Fig. 8(b) plots the effect of the ambient temperature on the log-impedance of sensor 3 against RH. As the temperature increased, the RH characteristic curve shifted to slightly lower impedance. The temperature coefficient at 15 ∼ 35 ◦ C was below −0.11% RH/◦ C over the humidity range 20 ∼ 90% RH. Fig. 8(c) plots the effect of the applied measure-
ment frequency on the log-impedance of sensor 3 against RH, the impedance was measured at frequencies of 0.1, 1, 11 and 100 kHz at a voltage of 1 V. The impedance of sensor 3 clearly decreased as decreased below 30% RH at 100 kHz because the adsorbed water could not be polarized [35]. Fig. 8(d) plots the response/recovery times of sensor 3, measured at 25 ◦ C and 1 kHz. The response time (Tres.95% ) is calculated as the time taken for the impedance of the sensor to change by 95% of its maximum change after humidifica-
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Time (s)
Fig. 8. (a) Hysteresis of sensor 3, (b) effect of temperature on the response of sensor 3, (c) effect of applied frequency on the response of sensor 3, (d) response/recovery properties of sensor 3, (e) long-term stability of sensor 3.
Table 3 Flexible humidity sensor performance of this work compared with the literatures. Sensing material
Working range (%RH)
Sensitivity (log Z/%RH)
Hysteresis (%RH)
Flexibility (%) (Response deviation)
Response time (s)
References
KOH/M050 PAMAMa -AuNPs PMMA-PMAPTACb AuNPs/N-substituted pyrrole derivatives AuNPs/GO/MPTMOSc
20–90 30–90 20–90 30–90 20–90
0.0559 0.0450 0.0258 0.0827 0.0281
<2 <2 2 1 5
<4 2 <1 1 17
36 40 45 53 119
This work [36] [37] [38] [39]
a b c
PAMAM: polyamidoamine dendrimer. PMMA-PMAPTAC: copolymer of methyl methacrylate and [3-(methacrylamino)propyl] trimethyl ammonium chloride. MPTMOS: 3-mercaptopropyltrimethoxysilane.
tion from 10 to 90% RH. The recovery time (Trec.95% ) is calculated at the time required for the sensor to recover 95% of its maximum change in impedance after desiccation from 90 to 10% RH. The response time (Tres.95% ) and recovery (Trec.95% ) time of sensor 3 were 36 and 118 s, respectively. The slow recovery time of sensor 3 may have arisen from the micro/nanopores of the M050 film. Fig. 8(e) plots the long-term stability of sensor 3 at the tested RH values of 20, 60 and 90% RH. No obvious deviation in log-impedance was observed at least 34 days. Table 3 compares the humidity sens-
ing properties of sensor 3 herein with those of the sensors that were made of polymer and inorganic sensing materials, described in the literature [36–39]. The developed humidity sensor that was made of KOH/M050 composite film (sensor 3) exhibited larger working range, higher sensitivity, lower hysteresis and faster response time that those flexible humidity sensors that are made of PAMAMAuNPs [36] and PMMA-PMAPTAC [37]. Sensor 3 herein exhibited larger working range and faster response time than that of flexible humidity sensor that is made of AuNPs/N-substituted pyrrole
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line was observed on the complex plot at the low-frequency range (Fig. 9(b)). This phenomenon was related to the diffusion of ions across the interface between the Au electrode and the KOH/M050 film [40–43]. Finally, a straight line at 90% RH was observed on the impedance plot (Fig. 9(c)). Therefore, upon the adsorption of water, a liquid layer formed around the KOH/M050 film by capillary condensation, and H3 O+ and K+ ions were dissociated. The mobility and amount of the ions (H3 O+ and K+ ) increased with increasing relative humidity, and this effect dominated the conduction of the humidity sensor. Therefore, the ions (K+ and H3 O+ ) dominated the variation of conductance of the KOH/M050 composite film with relative humidity according to the obtained complex impedance plots. 4. Conclusions A flexible impedance-type humidity sensor that was based on M050 film had a small humidity-working range (40–90% RH) because of its poor intrinsic conductance. The sensitivity and linearity of the flexible impedance-type humidity sensor that was based on the M050 film were improved by adding KOH and AuNPs to increase the number of the conductance pathways of the KOH/M050 and AuNPs/M050 films at low RH. Moreover, the flexible impedance-type humidity sensor that was made of the KOH/M050 composite film exhibited higher sensitivity, better linearity and higher flexibility than that made of the AuNPs/M050 because KOH is a strong electrolyte. The flexible impedance-type humidity sensor that was made of the KOH/M050 composite film with added 0.05 M KOH had a wide working range of humidity (20–90% RH), a highest sensitivity (slope = −0.0559 log Z/%RH), an excellent linearity (R2 = 0.9351), a negligible hysteresis (<2%), a short response time (36 s), high flexibility (Df < 4%), a weak ambient temperature-dependence (−0.11% RH/◦ C) and high longterm stability (at least 34 days). The frequency-dependence of the impedance was observed at 100 kHz and at a humidity of less than 30%. The complex impedance plots of the KOH/M050 composite film changed from semicircular to linear as RH increased. These results reflect the domination of the conductivity of the KOH/M050 composite film by the ions (K+ and H3 O+ ). Acknowledgement The authors thank the Ministry of Science and Technology (grant no. MOST 106-2113-M-034-002) of Taiwan for support. Fig. 9. Complex impedance plots of sensor 3 at (a) 20% RH, (b) 40% RH and (c) 90% RH.
derivatives [38]. Sensor 3 herein had a lower hysteresis, shorter response time and greater flexibility than a flexible humidity sensor that is made of inorganic material (SiO2 -based) [39]. 3.4. Humidity-sensing mechanism of KOH/M050 film Impedance spectroscopy was used to identify the conduction mechanism of KOH/M050 composite film (sensor 3) and Fig. 9 plots the observed impedance spectra. The observed impedance was plotted at frequencies from 50 Hz to 100 kHz, relative humidities of 20–90% RH, an AC voltage of 1 V and a temperature of 25 ◦ C. In the impedance spectra, Zr (real part of the impedance Z) and Zi (imaginary part of Z) were plotted on the real axis and imaginary axis, respectively. When at a low humidity of 20% RH (Fig. 9(a)), a semicircular plot of film impedance was obtained. The semicircle was replicated using an equivalent circuit of a resistor and capacitor in parallel [40–42]. As RH increased (40% RH), an inclined
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Biographies Pi-Guey Su is currently a professor in Department of Chemistry at Chinese Culture University. He received his BS degree from Soochow University in Chemistry in 1993 and PhD degree in chemistry from National Tsing Hua University in 1998. He worked as a researcher in Industrial Technology Research Institute, Taiwan, from 1998 to 2002. He joined as an assistant professor in the General Education Center, Chungchou Institute of Technology from 2003 to 2005. He worked as an assistant professor in Department of Chemistry at Chinese Culture University from 2005 to 2007. He worked as an associate professor in Department of Chemistry at Chinese Culture University from 2007 to 2010. His fields of interests are chemical sensors, gas and humidity sensing materials, humidity standard technology and smart sensor system. Xin-Han Lee received a B.S. degree in chemistry from Chinese Culture University in 2016. He entered the M.S. course of chemistry at Chinese Culture University in 2016. His main areas of interest are humidity-sensing materials and humidity measurement system.