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High Performance and Moisture Stable Humidity Sensors Based on Polyvinylidene Fluoride Nanofibers by Improving Electric Conductivity.

INTRODUCTION

Humidity sensors have been used in various fields, such as modern industry [1], agriculture [2], and smart devices [3-5], Furthermore, there is substantial interest in the precise application of such sensors in medicine [5]. Thus, humidity sensors should be high performing, such as having great linearity, high sensitivity, excellent stability, and fast response time. To improve these important characteristics, many studies have been performed with new materials such as metal oxides [6-8], ceramics [9-11], carbon materials [12-14], and polymer composites [15-18]. In addition, several approaches have been reported that possess structural advantages, such as mesoporous materials [19-21] and nanofibers [22-24], which have a huge advantage of large surface area. In particular, polymer composites are one of the most interesting materials because of their processability and reliability [25].

Polyvinylidene fluoride (PVDF) is an attractive material because of its thermal and chemical stability, but its hydrophobicity has received little attention for humidity sensors [26], In addition, an existing research about PVDF-based capacitive humidity sensors shows low sensitivity and late response and recovery time in comparison with this study [27]. Hydrophobic polymers are not sufficiently sensitive for use in humidity sensors, especially at low relative humidity (RH) levels, owing to a lack of adsorbing water molecules. Essentially, humidity sensing properties result from electric conduction, based on charge transfer processes [28]. Lithium chloride (LiCl) not only possesses good working stability but also leads to lower electric resistance with conductive ions [25]. Hence, LiCl is considered as a suitable material to improve sensing properties of humidity sensors by enhancing electric conductivity. It is known that zinc oxide (ZnO) can be easily grown in different morphologies to have a structural advantage, and it has chemically active defect sites (i.e., oxygen vacancies) that enable high sensitivity in which they are used [29]. Moreover, in terms of their good wettability and ability to mass transfer, ZnO nanostructures show promising properties for sensor applications [7,30].

Sensing performance with good hygroscopic properties of the materials, which leads to low impedance of the sensor, has been developed in most studies in this field. However, hydrophilic polymers may easily dissolve via exposure to humidity and hence fail to maintain their structural advantages. In this study, we manufactured highly sensitive and moisture-stable nanofiber humidity sensors. PVDF was used as a nanofiber matrix of the sensors for preventing dissolution via moisture, securing the stability, and sensing repeatability. The improved sensitivity with LiCl is discussed as an aspect of the electric conductivity. Fast response and recovery times were enhanced with ZnO nanoparticles. Moreover, the humidity sensing mechanism is demonstrated by complex impedance spectra.

EXPERIMENTAL

Materials

PVDF (MW 275,000: Sigma-Aldrich, St. Louis, Missouri, USA) was added into dimethylformamide (DMF; Junsei Chemical Co., Ltd., Tokyo, Japan) and magnetically stirred for 24 h at 70 [degrees]C for a concentration of 20 wt%. LiCl (Duksan Pure Chemicals Co., Ltd., Ansan, South Korea) was added into the PVDF solution to obtain LiCl-PVDF solutions with LiCl concentrations of 1-5 wt%, and the solutions were stirred for 24 h at 70 [degrees]C. ZnO (nanopowder <100 nm; Sigma-Aldrich, St. Louis, Missouri, USA) dispersed solutions were prepared by mixing the powder in ethanol for ZnO amounts of 10, 20, 30, and 40 wt%, which was then dispersed by sonication.

Methods

The various concentrations of LiCl-PVDF solution were spun via the solution-blowing spinning method onto the interdigitated electrodes (width and distance of electrodes: 0.28 mm), as shown in Fig. 1. The inner and outer nozzle diameters of the coaxial metal nozzle were 0.67 and 1.60 mm, respectively. The outer nozzle was connected to a compressed air pump, whereas the inner nozzle was connected to an injection syringe pump at a flow rate of 100 [micro]L [min.sup.-1]. The exhaust blower was positioned 60 cm from the spinneret to collect polymer fibers.

ZnO nanoparticles were additionally deposited on the LiCl-PVDF nanofiber web using an electrospraying technique. The ZnO dispersed solution was fed into the nozzle with a diameter of 0.6 mm under its own gravity. A voltage of 9.0 kV was applied to the ZnO solution, which showed the best processing stability.

Measurements

Images of the electrospun jet were continuously captured by a charge-coupled device camera (SCC-B2313; Samsung, Suwon, South Korea) throughout the spinning process. The corresponding jet area in each image, which represents the amount of resultant deposition, was calculated using self-programed image analysis. Humidity sensing performance was measured with an LCR meter (IM3533; Hioki, Ueda, Japan) at a frequency of 300 Hz and measuring voltage of 1 V under the various RHs. Each RH condition was achieved with saturated solutions of LiCl, Mg[Cl.sub.2], [K.sub.2]C[O.sub.3], NaBr, Cu[Cl.sub.2], NaCl, KCl, and [K.sub.2]S[O.sub.4], which yielded 11, 33, 43, 57, 67, 75, 86, and 97% RH, respectively. All tests were performed at the constant temperature of 20 [degrees]C. The morphology of the fiber-shaped sensing layer was observed by scanning electron microscopy (FE-SEM; SUPRA25, Carl Zeiss Co., Ltd., Oberkochen, Germany), to confirm the moisture stability of the manufactured sensor.

RESULTS AND DISCUSSION

Figure 2 shows the impedance response of the humidity sensors produced with varying PVDF nanofiber and ZnO mixtures. The PVDF-only sensor exhibited high impedance and extremely limited sensitivity over the entire RH range. The high impedance and low sensitivity are related to the lack of moisture adsorption because of the hydrophobicity of PVDF. To enhance moisture adsorption, we first made humidity sensors via electrospinning with a ZnO-PVDF mixed solution, which led the inorganic particles to the in-fiber position. However, the impedance change was insufficient for displaying the best linearity because the exposed area of the water adsorption site, in this case of ZnO nanoparticles, was smaller. Thus, ZnO nanoparticles were supplemented onto the PVDF fiber by the electrospraying method. As a result, the impedance value dramatically decreased in the high RH range, from 75% to 97%. However, the sensitivity under 67% RH still did not improve by simply adding ZnO nanoparticles.

It is thought that the reason for low sensitivity and high impedance value at the low and middle RH conditions is the low conductivity of the polymer fiber for enabling the charge carrier to be transported. The dominant sensing mechanism for this range can be explained by proton ([H.sup.+]) hopping, hydronium ([H.sub.3][O.sup.+]) diffusion, and the Grotthuss mechanism [28]. These three kinds of mechanisms show lower charge transfer than ionic conduction does, because they are performed through discontinuous charge transfer [25] and are affected by the density of proton hopping sites [31], as shown in Fig. 3a. To increase the impedance change at the low and middle RHs, the transportation of the charge carrier needs to be enhanced. By adding LiCl, it is expected that the number of charge hopping sites will increase because of [Li.sup.+] ions, as shown in Fig. 3b [31]. Moreover, [Li.sup.+] ions are also expected to be transported as the charge carrier [32,33], leading to a decrease in the electric resistance. The increased conductivity can be seen in Table 1.

Electrospinning method is based on the electric repulsion force of the charged polymer solution. The conductivity of the LiCl-PVDF solutions was too high to keep electric charges, so that the electrospun jet lost the driving force for producing nanofibers via electrospinning [34]. Solution-blowing spinning makes nanofibers with diameter range similar to electrospinning without electric force, because the driving force is the high speed of decompressed air [35]. Thus, solution-blowing spinning was performed to produce LiCl-PVDF nanofibers. The coaxial nozzle and the jet, which is the entire material beneath the nozzle, are shown in Fig. 4a. The change in jet area for 600 s of the spinning process was calculated by counting white pixels of the jet part, as depicted in Fig. 4b. The jet area can represent the jet amount emitted from the nozzle. The jet amount remained 0.5 [cm.sup.2] for 600 s at all concentrations of LiCl, which means the LiCl-PVDF deposition onto the electrodes was the same regardless of the solution concentration. Thus, the sensing performances can be compared without the influence of the absolute quantity of the sensing material.

The humidity sensing response of the LiCl-PVDF sensors is demonstrated in Fig. 5. The impedance value initially decreased under the high RH conditions as the LiCl amount increased. It is thought that the conductivity sensitively affects the humidity sensor property, especially under the high RH condition (over 75% RH), because the charge carriers were able to move continuously on a well-formed water layer [36], However, it is still not obvious below 67% RH for the samples of 1 and 2% LiCl, owing to the intrinsic high resistance of the polymer. When the LiCl concentration increased to 3%, the sensing performance at low RH started to improve, and the impedance response eventually displayed the best linearity at 4% LiCl by sufficiently increasing the hopping sites for charge carriers. Because of the higher conductivity, the charges could be easily transferred to the electrode, despite the relatively lower RH. Moreover, the hygroscopicity of LiCl enhances the ability to adsorb moisture of the composite fibers [25], which led to the impedance decrease over the entire RH range. Although the impedance curve of LiCl content at 5.0% displayed lower impedance, the 4.0% sample showed better performance than the 5.0% sample did, in terms of the total impedance change between 33% and 97% RH.

Although the best linearity of impedance response was unexpectedly achieved without inorganic particles, ZnO nanoparticles were additionally deposited by electrospraying them onto 3% LiCl-PVDF, which did not show the best linearity, to confirm the influence of ZnO. As shown in Fig. 6, the impedance values dropped as the content of ZnO increased because the moisture adsorption was enhanced. The impedance response at the low RH also improved in comparison to Fig. 2 owing to the increased conductivity by LiCl, showing that the best linearity was achieved at the ZnO concentration of 40%.

Moisture stability is an essential property for working life, especially for fiber-structure sensors. This property was investigated via exposure of the 4% LiCl-PVDF sensor and 40% ZnO3% LiCl-PVDF sensor, which shows the best linearity of impedance response, to environments with 11% and 97% RH alternated repeatedly for 10 s each for 600 s. The impedance change between 11 % and 97% RH was consistent throughout the testing period, as shown in Fig. 7. However, the change for the ZnO-mixed sensor was greater than that for the LiCl-PVDF sensor because of the good wettability of ZnO nanoparticles [30].

The nanofibers of the 4% LiCl-PVDF and 40% ZnO-3% LiCl-PVDF, before the humidity testing, were shown in Fig. 8a and b. A strong and directional driving force of the solution-blowing spinning made the nanofibers agglomerated but did not produce polymer beads. The average diameter of the as-spun nanofibers was 73.4 [+ or -] 15.2 nm. As shown in Fig. 8c and d, the fiber shapes of the 4% LiCl-PVDF and 40% ZnO-3% LiCl-PVDF sensors, respectively, were well preserved, even though they were exposed numerous times to alternating extreme humidity conditions. Furthermore, the ZnO nanoparticles in the on-fiber position were still attached to the fiber surface, as shown in Fig. 8d and e. The nanoparticles under 100 nm formed agglomerated mass, as shown in Fig. 8f. This shows that PVDF-based sensors are suitable for enhancing moisture stability.

The response and recovery times for the sensors that presented the best linearity of impedance response were measured. The times taken to achieve 90% of the total change between 11 and 97% RH were defined as the response and recovery times [37], The response and recovery times of the 4% LiCl-PVDF sensor were 7.4 and 32.7 s, respectively, as shown in Fig. 9a, whereas the 40% ZnO-mixed sensor showed quick response and recovery times, 1.7 and 16.1 s, respectively, as shown in Fig. 9b. According to previous research, response and recovery times can be attributed to the aggregation of water molecules, which can restrict the mobility of these molecules from the polymer chains [15]. However, ZnO nanoparticles can easily diffuse water molecules [7], which results in the fast response and recovery of the sensors.

To determine the sensing mechanism under the various RH conditions, complex impedance plots are presented in Fig. 10. The real (Z') and imaginary (Z") parts of the impedance were magnified by proper factors so they were in the same plane for the sake of comparison. Complex impedance is generally reduced with an increase in RH, owing to the increase of permittivity through water adsorption. At the low RHs of 33 and 43%, the impedance had large values, forming small parts of semicircles. The intrinsic resistance of PVDF mainly contributed the high impedance, because few water molecules are adsorbed onto the sensing layer and the protons were transferred only by hopping from site-to-site (Fig. I la). As the RH increased to the middle RHs of 57 and 67%, more water molecules were adsorbed and ionized under the electric field as [H.sub.3][O.sup.+] ions, which can transfer charges through the Grotthuss mechanism (Fig. 11b) [36], Thus, the impedance curve is a semicircle with decreased values. When the RH further increased to over 75%, water molecules began to accumulate at the interface between the sensing layer and the electrodes. Therefore, the impedance curve includes a short straight line in the low-frequency region, which is caused by the diffusion of ions or charge carriers at the interface (Fig. 11c) [38], In general, as the RH increases further to the high region, more and more water molecules are adsorbed and form bulk liquid water. If dissociated free ions of LiCl increase in the bulk liquid water, the semicircle tends to be gradually weakened and subsequently disappears, and the straight line is emerged because of the increasing contribution of an electrolyte solution to the conductivity of the sensing layer [19]. However, the semicircle is still observed and begins almost from the origin, even though the RH increased up to 97%. It is although that a multilayer of water for dissolving the Li ions did not from because of the hydrophobicity of PVDF, so that the solution resistance did not appear. This demonstrates the moisture stability of the ZnO-LiCl-PVDF humidity sensors.

CONCLUSIONS

In summary, we manufactured PVDF nanofiber-based humidity sensors. As a result of humidity sensing performance, the conductivity of the sensing material and its ability to adsorb moisture played an important role in the sensors. The sensors with PVDF and ZnO-PVDF did not show good sensing performance, owing to their low conductivity, especially under low and middle RH conditions. Hence, the conductivity was improved by adding LiCl, and the 4% LiCl-PVDF and 40% ZnO-3% LiCl-PVDF sensors presented the best linearity for impedance response with RH, because the charge transfer was enhanced. Moreover, the PVDF fiber successfully guaranteed the moisture stability and repeatability of the sensor. Meanwhile, ZnO improved the response and recovery times of the sensors because of its moisture desorption ability. This work proves that high-performance humidity sensors are able to be produced with hydrophobic-based materials, increasing the advantage of moisture stability. It is expected that similar humidity sensors can be fabricated using other hydrophobic polymers and inorganic salts, if a suitable solvent is selected for dissolving the polymer and ionizing the salt to increase charge transfer.

ACKNOWLEDGMENTS

This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials (grant number 10050946) funded by the Ministry of Trade, Industry and Energy, Republic of Korea
ABBREVIATIONS

DMF    Dimethylformamide
LiCl   Lithium chloride
PVDF   Polyvinylidene fluoride
RH     Relative humidity
SEM    Scanning electron microscopy
ZnO    Zinc oxide


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Sejin Choi, Hu Min Lee, Han Seong Kim [iD]

Department of Organic Material Science and Engineering, Pusan National University, Busan, 46241, Republic of Korea

Correspondence to: H. S. Kim; e-mail: hanseongkim@pusan.ac.laContract grant sponsor: Korea Evaluation Institute of Industrial Technology; contract grant number: 10050946.

DOI 10.1002/pen.24905

Caption: FIG. 1. Schematic image of the solution-blowing spinning setup.

Caption: FIG. 2. Impedance response of PVDF, in-fiber ZnO-PVDF, and on-fiber ZnO-PVDF humidity sensors with different relative humidities (measuring frequency: 300 Hz, voltage: 1 V).

Caption: FIG. 3. Scheme of charge transfer mechanism for (a) low and (b) high conductivity sensing materials.

Caption: FIG. 4. (a) Jet image and (b) change in jet area during the solution-blowing spinning process for the various concentrations of LiCl-PVDF solution.

Caption: FIG. 5. Impedance response of LiCl-PVDF sensors with various LiCl amounts (measuring frequency: 300 Hz, voltage: 1 V).

Caption: FIG. 6. Impedance response of ZnO-3% LiCl-PVDF sensors with various ZnO amounts (measuring frequency: 300 Hz, voltage: 1 V).

Caption: FIG. 7. Impedance change during repeatability testing of (a) 4% LiCl-PVDF and (b) 40% ZnO-3% LiCl-PVDF sensors (measuring frequency: 300 Hz, voltage: 1 V).

Caption: FIG. 8. SEM images of (a, c) 4% LiCl-PVDF and (b, d) 40% ZnO-3% LiCl-PVDF sensors before and after repeatability testing, respectively. The inset image of (b) shows 3% LiCl-PVDF fiber before electrospraying ZnO nanoparticles. (e) Larger area and (f) high magnification image of the 40% ZnO-3% LiCl-PVDF sensor.

Caption: FIG. 9. Response and recovery of (a) 4% LiCl-PVDF and (b) 40% ZnO-3% LiCl-PVDF sensors (measuring frequency: 300 Hz, voltage: 1 V).

Caption: FIG. 10. Complex impedance curve of the 40% ZnO-3% LiCl-PVDF sensor for the humidity range of 33%- 97% RH, in the frequency range of 10-200 kHz.

Caption: FIG. 11. The schematic image of humidity sensing mechanism at (a) low, (b) middle and (c) high RH condition.
TABLE 1. Conductivity of LiCl-PVDF composite
amounts.

LiCl concentration (wt%)   Conductivity (mS)

0 (PVDF only)                   0.0044
1                                 1.5
2                                 2.3
3                                 2.9
4                                 3.5
5                                 4.0
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Author:Choi, Sejin; Lee, Hu Min; Kim, Han Seong
Publication:Polymer Engineering and Science
Article Type:Technical report
Date:Feb 1, 2019
Words:3672
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