Research Article | Open Access
Impedimetric Humidity Sensor Based on Nanohybrid Composite of Conducting Poly(diphenylamine sulfonic acid)
Interdigitated thin film humidity sensors have been prepared using organic/inorganic nanocomposites of poly(diphenylamine sulfonic acid) (PSDA), 3-mercaptopropyltrimethoxysilane (MPTMS), and nano-ZnO. Humidity sensing properties of the sensors, regarding the effect of nano-ZnO addition and the applied alternating current frequency, were studied by impedance measurements in the frequency range of 100 Hz–1 kHz. It was found that the sensing properties of the ZnO based nanocomposite sensors had better properties than those of the PSDA-MPTMS based composite due to contribution of the nanomaterial. Good sensitivity (about three orders’ magnitude change in impedance), linear response, rapid response (90 s) and recovery (60 s), and low hysteresis within 4% as well as good repeatability and stability in the range from 12% to 95% RH were obtained.
The development of humidity sensors has showed a remarkable progress because of using various types of sensing materials in recent years. The sensing materials used in humidity sensors can be classified into ceramics [1–3], polymers [4, 5], and composites [6–8]. Ceramic humidity sensors based on semiconductor metal oxides have some critical limitations such as elevated working temperature, insufficient sensitivity, and repeatability. Polymeric humidity sensors have been widely studied for ten years. Almost all of the humidity sensors based on polymers sensitively operate at room temperature. In terms of the stability of the sensors, some highly hydrophilic polyelectrolyte and functional polymer based sensors are not sufficiently resistant at high humidity conditions. However, their hydrophilicity and ionic conductivity can be easily adjusted using various methods such as copolymerization, cross-linking with relatively hydrophobic and insulating polymers. Composites in humidity sensors consist of polymer and another material which is insulating or conducting polymer , metal [10, 11], metal oxide [12, 13], or nanomaterials [14, 15]. So, these composite humidity sensors, which combine with unique property and synergistic effect of each component, have some advantages such as high sensitivity, small hysteresis, quick response, and high stability.
Nanoscaled materials have high adsorption capacity due to their small grain size and large specific surface area. Many kinds of nanometal oxides such as, TiO2 [16, 17], SnO2 [18, 19], SiO2 , ZrO2 , and ZnO [22–27] have been widely employed alone or along with other polymers in humidity sensors due to these properties. Among them, ZnO is one of the most well-known gas sensing materials, which has been extensively carried out for study of some gases and vapors such as ethanol [28–30], acetone , ammonia , NO2 , CO , H2S , and hydrogen .
Conducting polymers which are able to change in electrical conductivity with relative humidity are good candidates for humidity sensing material. They also have high sensitivity at room temperature, ease of synthesis, facile processability, and high stability . Moreover, the combination of conducting polymers with inorganic nanoparticles leads to a development on humidity sensors with superior properties. Many reports have been published about humidity sensor applications based on nanomaterial/conducting polymer composites [21, 38–40]. In our previous study, we reported that the water soluble/conductive poly(diphenylamine sulfonic acid) in its diblock copolymer and blend with poly(ethylene glycol) and poly(vinyl acetate-co-butyl acrylate) would be a good candidate as humidity sensing material . However, water-soluble properties of such composites may require the fixing of the water-soluble component into a durable matrix for measurements in highly humid or condensed water atmosphere. In this study, we aimed to enhance the stability and sensitivity of the PSDA based humidity sensors. For this purpose, 3-mercaptopropyltrimethoxysilane (MPTMS) and nano ZnO powder were chosen for matrix modification to construct the interdigitated impedimetric type humidity sensors.
It is known that reactive silanes are very efficient to improve adhesion properties at the interfaces, water durability, and long-term stability at high temperature and humidity . Among the reactive silanes, mercapto silanes can be self-assembled onto the metal surfaces such as gold and silver and the adhesion properties of coatings can be further improved . In this study, the silver based interdigitated electrodes were used. Our studies on gold based electrodes by mass sensitive quartz crystal microbalance method have been also currently conducted. Therefore, it was expected that MPTMS may be a good candidate to adjust the hydrophilicity and to enhance the stability and water durability of the PSDA based humidity sensors. Nano-ZnO was chosen due to its small grain size and high surface area for high humidity sensitivity. There is no report about impedimetric humidity sensor based on organic/inorganic nanocomposite of the conducting poly(diphenylamine sulfonic acid) in literature. The electrical properties of the composite and nanocomposite were measured by impedance measurements at various relative humidity in the frequency range of 100 Hz–1 kHz at constant temperature.
2. Experimental Procedure
ZnO nanoparticles (grain size <100 nm) were supplied from ABCR. Sodium salts of diphenylamine sulfonic acid and hydrochloric acid were analytical grade Aldrich products. The electrochemical polymerization of diphenyl amine sulfonic acid (PSDA) was carried out by a method described previously . 3-Mercaptopropyltrimethoxysilane (MPTMS, purity 95%, ABCR) was used without further purification.
2.2. Instruments and Analysis
Vibrational spectra of the nanocomposite materials were obtained using a Perkin Elmer Spectrum One FT-IR spectrometer. Scanning electron microscope (SEM) images of prepared sensor films on precleaned microscope slides were characterized by a Philips XL 30-ESEM-FEG/EDAX instrument. Electrical characteristics of the nanocomposites as a function of relative humidity (RH) were measured with LCR meter (HIOKI 3522-50) in a test chamber at constant temperature (°C) and 0.2 V from 1 kHz to 100 Hz. All the measurements were carried out at different relative humidity levels in the range of 10–95% RH. The RH levels were generated by saturated salt solution method and controlled by a Carl Roth P330 capacitive type of commercial humidity sensor. This humidity sensor has a 0–99% relative humidity measuring range with an accuracy of ±3% RH.
2.3. Preparation of Humidity Sensors Based on Nanocomposite
80 μL of 10 wt.% PSDA in 0.1 M HCl solution and 2 μL of pure MPTMS were mixed. The mixture was stirred and dispersed by ultrasonic method. After ultrasonication, 5 μL of sol-gel solution was carefully dropped onto the epoxy based substrate with a circular silver coated copper interdigitated electrode (thickness of the digits was 20 μm). The prepared films were allowed to dry at room temperature. Finally, the as-coated electrode was heated at 105°C for 2 hours in an oven to fabricate humidity sensor based on conducting polymer as shown in Figure 1.
To prepare humidity sensors based on nanocomposite, nanometer ZnO particles were added to sol-gel solution which was described above. The mixture was sonicated to achieve a uniform dispersion of nanoparticles. Different PSDA-MPTMS nanocomposites were prepared and the same casting, drying, and curing procedures mentioned above were used to obtain nanocomposite films. Nanocomposites were prepared by taking into consideration the weight percentage of nanoparticle used in the sensors. The composition of the prepared sensors is shown in Table 1. The upper edges of the silver digits were completely covered by the films after curing and the film thicknesses were estimated to be ca. 25–30 μm from SEM images.
3. Results and Discussion
3.1. FTIR Spectroscopy Results
Figure 2 shows the FTIR spectra of the PSDA-MPTMS (S1), nanometer ZnO, and PSDA-MPTMS-ZnO (S4) nanocomposite. All characteristic peaks of PSDA  appear in the FTIR spectrum of PSDA-MPTMS composite. The broad band at around 2500 cm−1 which is assigned to the –SH group of MPTMS disappeared in the ZnO based nanocomposite . Further, the magnitude of the distinct stretching vibration band of ZnO near 480 cm−1 decreases in PSDA-MPTMS-ZnO nanocomposite. These results indicate the interaction of the –SH group with ZnO [46–48]. We must note here that the interactions among weak acidic thiol groups on polysilane, hydrolyzed silanols, and deprotonated basic amine groups on PSDA should also be taken into account [49, 50]. The broad band around 3400 cm−1 in each spectrum corresponds to O-H stretching of silanol groups and adsorbed water.
3.2. Microstructure Characteristics of PSDA-MPTMS and PSDA-MPTMS-ZnO Nanocomposite Thin Films
The SEM images of the PSDA-MPTMS (S1) and PSDA-MPTMS-ZnO (S4) nanocomposite are shown in Figures 3(a) and 3(b), respectively. As shown from the SEM images of PSDA-MPTMS film in Figure 3(a), PSDA is uniformly dispersed, like microdroplets, in the highly ordered polysilane network. It is shown from Figure 3(b) that ZnO nanoparticles were almost homogeneously distributed in the matrix. Although some cracks were observed in the ZnO based composite film, there was not any obstacle such as physical disintegration of the films in the whole sensors for the measurements at dry and humid atmosphere. It seems that a more stable structure in terms of durability under high humidity conditions is provided by adding MPTMS into the composite .
3.3. Humidity Sensing Behaviors of PSDA-MPTMS and PSDA-MPTMS ZnO Nanocomposites
3.3.1. Complex Impedance Characteristics
Impedance spectroscopy is a powerful technique to understand the conduction mechanisms of humidity sensors . Complex impedance spectrum is measured by applying a single voltage to the interface and measuring the phase shift and amplitude of the electrical parameters for the frequency () range explored. The change in total impedance can be represented as a Nyquist plot, in which the imaginary part of the material’s impedance, , or its reactance, is plotted against the real part of the impedance, , or resistance for each excitation frequency. Another representation of the frequency variation of impedance can be found through Bode plots. In Bode-diagrams, the absolute value of the total impedance or the imaginary part of the impedance and the phase angle are plotted as a function of the angular frequency (), in a logarithmic and semilogarithmic plot, respectively. This yields information about the electrode structure. Planar electrodes will result in plots where the linear part of the curve has a slope of −1, while the slope will be −1/2 for a porous electrode. Extrapolation of this line to gives , where is the double layer capacitance.
The complex impedance spectra of PSDA-MPTMS (S1) and PSDA-MPTMS-ZnO (S4) nanocomposites at low (~10% RH) and moderate humidity (~60% RH) are shown in Figures 4(a) and 4(b), respectively, in a form of Nyquist plot. As can be seen from the figures, PSDA-MPTMS and PSDA-MPTMS-ZnO sensors have similar impedance behavior at low humidity levels in the frequency range studied. At moderate humidity levels, they gave semicircles, indicating the decrease in their capacitive reactance and resistance values. With further increasing of humidity, the semicircle radius reduced gradually and a straight line appeared at lower frequencies and high humidified conditions (data not shown). It can be obviously shown in the Figure 4 that PSDA-MPTMS-ZnO sensor has lower impedance, that is, lower resistance and capacitive reactance than that of PSDA-MPTMS sensor at moderate humidity levels. Furthermore, it can be seen from the plots that the imaginary part of the impedance, rather than the resistance, is more affected in the presence of water. PSDA-MPTMS-ZnO sensor seemed to have a higher absorption capability of water molecules. It can also be claimed from the slopes of Bode plots in Figures 5(a) and 5(b) that PSDA-MPTMS and PSDA-MPTMS-ZnO sensors behave as planar electrodes, and they have similar capacitances at low humidity conditions. However, PSDA-MPTMS-ZnO sensor has a lower slope than that of PSDA-MPTMS sensor and exhibits a more porous electrode structure and a higher capacitance at moderate humidity levels. It is known that cracks create porosity on the surfaces . Higher porosity and nanosized ZnO grains increases surface to volume ratio of the materials and enhances diffusion ability of water into or out of the porous structure. It can be concluded that both the capacitive and ionic charge transfer processes contribute to the sensor’s response but the capacitive reactance mainly determines the total impedance changes. With an increase of humidity through saturation, the semicircles almost disappeared and the similar straight lines were obtained at low frequency levels for both of them. This might be due to the contribution of ionic species in the composites.
3.3.2. The Effect of ZnO on Humidity-Impedance Characteristics
The effects of the nanometer ZnO content on the impedance responses and humidity sensing of the PSDA-MPTMS-ZnO nanocomposite thin films were measured at various RH, and the results are shown in Figure 6. PSDA-MPTMS based sensor was also studied for comparison.
As can be seen from Figure 6, the impedance of PSDA-MPTMS decreased exponentially from 4.2 × 104 kΩ to 1.9 × 101 kΩ for humidity change from 10 to 91% RH. It can be calculated that the impedance changed approximately by more than three orders of magnitude over the measured RH range. It was not sensitive to low humidity (10–50% RH), while the further increase of RH results in sharp decrease in impedance. The adding of MPTMS was expected to lead to increasing stability of the sensors in high RH levels. It must be noted here that MPTMS did not have any significant response towards humidity in the preliminary studies.
Similar to PSDA-MPTMS sensor, as shown from Figure 6, the impedance of the all ZnO based sensors changed by more than three orders of magnitude over the same RH range. Among them, PSDA-MPTMS-ZnO (S4) showed the best linearity with a correlation coefficient of 0.9832 in the range of 12% to 95% RH. Therefore, S4 nanocomposite was selected for further research. It was concluded that all of humidity sensors are sensitive to water molecules. This is considered due to the hydrophilic groups on ZnO surface and ionic groups and doped ions in PSDA to interact with water molecules. Consequently, the impedance of the sensors decreased with the increasing absorption of water molecules. As mentioned in Section 3.3.1, the high surface-to-volume ratio and porous structure of PSDA-MPTMS-ZnO sensors allow for more water absorption at moderate humidity ranges. One thing to note here from Figure 6 is that the S4 sensor has a higher impedance than the others at highest humidity (95% RH), while it has a significantly lower impedance at moderate humidity levels. This behavior confirms the increasing surface area and porosity of the films but the decreasing the quantity of the ionic carriers with the increase of ZnO content. Since water has very high dielectric constant at room temperature, the effective dielectric constant and the capacitance of the film will increase accordingly. At low and moderate humidity, the coverage of water on the surface did not seem to be continuous; the electrolytic conduction is difficult. At high humidity, several water layers are formed, which accelerate the transfer of H+ or H3O+. In this condition, ions in PSDA also dissolve in the adsorbed water. As a result, ionic charge transfer process appeared at high humidity.
3.3.3. The Effect of Frequency on Response
The dependence of impedance response on the applied frequency was measured as shown in Figures 7(a) and 7(b) for PSDA-MPTMS (S1) and PSDA-MPTMS-ZnO (S4) based sensors, respectively. The effect of frequency on the impedance is greater at low and moderate RHs compared to the state at high RH. The impedance decreases when the frequency increases. Polarization of the water molecules is higher in the case of low frequencies. This leads to higher electrostatic attractive forces between water molecules and sensitive layer at low alternating current frequencies and consequently higher impedance changes than those of higher frequencies . S1 and S4 display almost identical behaviors towards applied frequencies. The linearity of the responses is getting better toward lower alternating current frequencies and the best one was obtained for S4 based sensor at 1 kHz. Applied frequency and voltage were thus kept constant at 1 kHz and 0.2 V, respectively.
3.3.4. Repeatability, Hysteresis, and Long-Term Stability
Figure 8 illustrates a typical time dependent response of PSDA-MPTMS-ZnO (S4) sensor measured by exposing it to cycle of 10% RH and 95% RH to investigate sensor repeatability. The relative standard deviations in impedance values at low and high humidity levels in this experiment were less than 1%. It can be inferred that the humidity sensing process is extremely reversible and repeatable for the nanocomposite sensor. Furthermore, for one month at least, sensor responses do not have significant deviation by measuring the impedance change at various % RH over this period even in the highest RH. The measurements were repeated for S4 sensor every 7 days for 1 month. The impedance variation is less than 4% RH at each humidity region. This shows the long-term stability of the PSDA-MPTMS-ZnO (S4) sensor at various humidity. Also, hysteresis was studied to determine the repeatability of the nanocomposite sensor. As shown in Figure 9, the response behavior of the sensor did not change during the humidity measurements from low to high RHs, and then in the reverse direction between 12% RH and 95% RH. It can be regarded that the nanocomposite sensor exhibits a low hysteresis (<4% RH) in the whole humidity range. Compared with the nanocomposite sensor, PSDA-MPTMS sensor shows relatively high hysteresis at moderate humidity levels (the maximum hysteresis was 10% RH at 60% RH).
3.3.5. Response and Recovery Time
It is well known that response and recovery behavior are an important characteristic for evaluating the performance of the sensing materials. The response and recovery times of the humidity sensors have been measured by adjusting the humidity between low and high levels. The response or recovery time is the time required to achieve 90% of the steady response when the humidity is increased or decreased between two levels, respectively . The response (adsorption) and recovery (desorption) times of the sensors were measured by suddenly changing humidity from 10% to 95% RH and from 95% to 10% RH, respectively. Figure 10 represents the response and recovery behavior of PSDA-MPTMS-ZnO (S4) sensor. The response time of the adsorption process is 90 s, and the recovery time of the desorption process is 60 s. Also S1, S2, and S3 sensors have shown good response and recovery times, but, in comparison with S4, they have a narrow linear humidity range (as shown in Figure 6) and/or high hysteresis (data not shown).
The above experimental results indicated that the ZnO nanocomposite of PSDA-MPTMS showed satisfying humidity sensing properties such as a good repeatability, low hysteresis and high sensitivity in a wide humidity range.
In the present work, humidity sensitive PSDA-MPTMS and PSDA-MPTMS-ZnO nanocomposite were prepared by a simple mechanical mixing method and their humidity sensing characteristics were investigated. The PSDA-MPTMS-ZnO nanocomposite humidity sensor showed the best linearity over a wide humidity range in comparison with the sensor composed of PSDA-MPTMS alone. Due to water soluble nature of the PSDA, MPTMS was added into the composites to adjust the hydrophilicity and the stability of the sensing layer and availed itself. Addition of ZnO into the composite enhanced the sensitivity of the sensors at low and moderate humidity levels. The nanocomposite with 50 wt.% of ZnO sensor displays good sensitivity, linear response, rapid response (90 s) and recovery (60 s), and low hysteresis within 4% as well as good repeatability in 12–95% RH. Compared with the similar humidity sensors in literature, the thin film sensors of PSDA have similar or better properties [22–27, 41, 50]. Consequently, PSDA-MPTMS-ZnO nanocomposite could be a promising candidate for humidity sensing in practical applications.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
This research was supported by Yildiz Technical University Scientific Research Projects Coordination Department (Project no. 2011-01-02-DOP05).
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