Conductive Polymer (Graphene/PPy)–BiPO4 Composite Applications in Humidity Sensors

In this particular experiment, a chain of conductive polymer graphene/polypyrrole (Gr/PPy) and BiPO4—or (Gr/PPy)–BiPO4—materials were prepared and used as moisture-sensitive materials. The structure and morphology of the conductive polymer (Gr/PPy)–BiPO4 materials were analyzed using an X-ray diffractometer, scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray spectroscopy. Moreover, properties such as hysteresis loop, impedance, sensing response, and response and recovery time were calculated and evaluated using an inductance–capacitance–resistance analyzer. The data expressed that PPy/BiPO4, as prepared in this study, exhibited excellent sensing properties, with impedance changing by only a few orders of range. Furthermore, the response time and time of recovery were 340 s and 60 s, respectively, and negligible humidity hysteresis occurred at different relative humidities. Therefore, conductive PPy/BiPO4, as prepared in the present study, is an excellent candidate for application in humidity sensors.


Introduction
Ambient humidity must be controlled, regulated, and monitored in environments such as dry cabinets, warehouse storage, industrial production, and food processing facilities, as well as in agricultural planting operations, medical diagnostic centers-both facilities in which high-tech instruments are employed-and shelter environments designed for managing life [1,2]. Recently, tremendous scientific effort has been devoted to the detection of environmental humidity. Particularly, many types of humidity sensors have been explored due to their advantages, such as high sensitivity, rapid response and recovery, low hysteresis, and excellent reproducibility. Humidity sensors are commonly classified into capacitive [3,4], resistive [5,6], optical [7,8], bulk acoustic wave [9,10], and quartz crystal microbalance [11,12] humidity sensors. Numerous types of humidity-sensing material have been used for humidity detection. Zhao et al. prepared a SnO 2 /MoS 2 hybrid-sensing nanocomposite by synthesis through a two-step hydrothermal route; the sensor with a 5 µm gap had the largest sensitivity at low humidity [13]. Mallick et al. enhanced the humidity-sensing properties of polyvinylidene fluoride titanium dioxide (PVDF-TiO 2 ) nanocomposites-based capacitive humidity sensors by modifying the film surface of the TiO 2 [14]. Duong et al. fabricated composite multi-walled carbon nanotubes (MWCNTs) and tungsten oxide (WO 3 ) nanobricks at different mass ratios, which attained a high response with 95 wt.% of MWCNT/WO 3 nanocomposite [15]. Arularasu et al. developed a PVDF/ZnO nanocomposite that was prepared as a humidity sensor through a

Method of Synthesis
As is typical in this procedure, 5 mmol of Bi(NO 3 ) 3 ·5H 2 O and 2.5 mmol of Na 3 PO 4 were dissolved in 46 mL of HNO 3 (0.3 M) under continuous stirring for 1 h to obtain a homogeneous aqueous solution. The resulting solution was transferred into a Teflon-lined stainless steel autoclave. Subsequently, the autoclave was tightly sealed and maintained in a temperature-controlled electric oven at 160 • C for 24 h. Then, after it was filtered and washed with deionized water and ethanol five times, the resulting white product was dried at 70 • C for 12 h to harvest BiPO 4 . Next, an amount of 0.5 g as-prepared BiPO 4 was added to 300 mL of an aqueous solution containing 1% CTAB under ultrasound for 0.5 h to form solution A, whereas solution B was formed by dissolving 0.03 mL of C 4 H 5 N in solution A under ultrasound for 10 min. Thereafter, 100 mL of 10% (NH 4 ) 2 S 2 O 8 and solution B were mixed under ultrasound for 0.5 h and maintained under temperatures ranging from 1 • C to 5 • C for 24 h to form a black precipitate. Finally, the as-obtained precipitate was centrifuged, washed with deionized water and ethanol several times, and dried at 70 • C for 2 h to obtain PPy/BiPO 4 . Additionally, precalculated amounts of as-prepared BiPO4 and graphene were added to 10 mL of 95% ethanol under ultrasound for 1 h to produce a black suspension. The obtained suspension was centrifuged and washed with ethanol and deionized water and then dried at 60 • C for 12 h to obtain graphene/BiPO 4 [34][35][36].

Characteristic Methods
Structural properties of as-prepared material were analyzed using the powder X-ray diffractometer (XRD) method-the Shimadzu XRD-6000 (operated at 35 kV and 35 mA) with a (λ = 1.5404 Å) Cu source. The particle size of the as-obtained prepared photocatalysts was averaged and explained using the Debye-Scherrer equation. Transmission electron microscopy (TEM) images were acquired using a JEOL JEM2010 transmission electron microscope (JEOL, Japan) at an accelerating voltage of 200 kV. The samples for TEM analysis were suspended in ethanol with the assistance of ultrasound and dispersed on a copper grid. The chemical composition of the as-synthesized samples was analyzed using an energy-dispersive X-ray spectroscope (EDS). The morphology of different asprepared materials was observed using field emission scanning electron microscopy (JEOL JSM-7100F, Tokyo, Japan), with the microscope operated at 30 kV.

Sensor Fabrication and Humidity Testing
A solution of 10% PVA was used as the binder. The sensing chips were fabricated by dip-coating them on a pair of comb-like gold electrodes on an alumina substrate (10 × 5 mm 2 ; rotational speed, 1000 rpm). The chips were subsequently heated at 80 • C for 0.5 h and calcined at 300 • C for 4 h.
The humidity parameter and its response were calculated in a flow system that worked dynamically [37], wherein an airtight glass chamber was developed to preserve the sensors and store them ( Figure 1). In the sensor setup, air was injected into the water to generate water vapor, which subsequently filled the testing chamber. Various levels of specific relative humidity (RH) were maintained for 15 min to enable the humidity sources to reach equilibrium. The chamber temperature was controlled at 25 • C ± 2 • C; a thermoshygrometer was connected to the testing chamber for RH measurement. Taiwan's Center of Measurement Standard/ITRI provides the standard concentration which was used to calibrate the commercial sensor for humidity. The RH response (S) sensor was calculated as S equals the ratio of Rd and Rh. The parameters of Rd under dry conditions were measured as a value of resistance (12% RH) for the developed sensor, while Rh was measured according to how much resistance would be exerted at a specific humidity [36]. The humidity hysteresis properties were 12% and 90%, and they were then decreased to 12% for the adsorption and desorption of water molecules [38]. H = ∆fmax/ffs × 100% was evaluated as humidity hysteresis error (H) using the equation provided above, wherein ∆fmax was the error of hysteresis at its maximum and ffs was the output for the response of the full scale. The response or recovery time was defined as the time required for the impedance of a sensor to change by 90% of the total impedance, whereas the recovery time was the time required for the reverse process. For dynamic testing, the changes in RH at 12% and 90% were controlled by feeding different ratios of air to water. In the provided experiment setup, a hygrometer (Rotronic) was utilized for measuring the RH value with ±0.1% RH accuracy. Sensing material for the response of impedance against humidity was explained by an analyzer that could measure chemical impedance (DU 6010); the input voltage and frequency were 1 V and 1 kHz, respectively. measured as a value of resistance (12% RH) for the developed sensor, while Rh was measured according to how much resistance would be exerted at a specific humidity [36]. The humidity hysteresis properties were 12% and 90%, and they were then decreased to 12% for the adsorption and desorption of water molecules [38]. H = Δfmax/ffs × 100% was evaluated as humidity hysteresis error (H) using the equation provided above, wherein Δfmax was the error of hysteresis at its maximum and ffs was the output for the response of the full scale. The response or recovery time was defined as the time required for the impedance of a sensor to change by 90% of the total impedance, whereas the recovery time was the time required for the reverse process. For dynamic testing, the changes in RH at 12% and 90% were controlled by feeding different ratios of air to water. In the provided experiment setup, a hygrometer (Rotronic) was utilized for measuring the RH value with ±0.1% RH accuracy. Sensing material for the response of impedance against humidity was explained by an analyzer that could measure chemical impedance (DU 6010); the input voltage and frequency were 1 V and 1 kHz, respectively.

Material Characterizations
To investigate the structural features of the as-fabricated samples, XRD measurement was performed. The XRD patterns of the samples presented in Figure 2 indicate that the main diffraction peak positions were located at the (110) Figure 3c illustrates that leaf-li nanoparticles were uniformly and thinly coated on the surface of the rod-li nanoparticles. Figure 4 suggests that the elemental analysis (EDS) spectrum for PPy/BiP indicated the presence of C, O, N, P, and Bi atoms as major components in the as-prepar PPy/BiPO4.   Figure 3c illustrates that l nanoparticles were uniformly and thinly coated on the surface of the r nanoparticles. Figure 4 suggests that the elemental analysis (EDS) spectrum for PPy indicated the presence of C, O, N, P, and Bi atoms as major components in the as-p PPy/BiPO4.

Sensing Humidity
The roles of the as-prepared samples in the humidity control were measured and investigated at the optimal operating frequency (1 kHz) ( Figure 5). Figure 5 indicates that the impedance values of the as-prepared samples (BiPO4, PPy, Gr, PPy/BiPO4, and Gr/BiPO4) were inversely related to RH changes. Additionally, PPy/BiPO4 exhibited the most significant change within the RH range, from 12% to 90% (a change of almost three orders of magnitude), suggesting that PPy/BiPO4 can absorb more water molecules and is thus more sensitive to humidity.  Figure 6a suggests that the response of the as-prepared sample changed gradually with increasing RH under humidity levels lower than 50% RH, whereas the response of the sensor increased sharply with higher ranges of RH. Moreover, PPy/BiPO4 exhibited the highest humidity in response (S = 24.6) to changes in RH ( Figure 6).
Humidity hysteresis, defined as the maximum difference in measured values of RH when the humidity sensor is exposed to adsorption and desorption processes, is an essential parameter for evaluating the reliability of a humidity sensor. Figure 6b suggests

Sensing Humidity
The roles of the as-prepared samples in the humidity control were measured and investigated at the optimal operating frequency (1 kHz) ( Figure 5). Figure 5 indicates that the impedance values of the as-prepared samples (BiPO 4 , PPy, Gr, PPy/BiPO 4 , and Gr/BiPO 4 ) were inversely related to RH changes. Additionally, PPy/BiPO 4 exhibited the most significant change within the RH range, from 12% to 90% (a change of almost three orders of magnitude), suggesting that PPy/BiPO 4 can absorb more water molecules and is thus more sensitive to humidity.

Sensing Humidity
The roles of the as-prepared samples in the humidity control were measured and investigated at the optimal operating frequency (1 kHz) ( Figure 5). Figure 5 indicates that the impedance values of the as-prepared samples (BiPO4, PPy, Gr, PPy/BiPO4, and Gr/BiPO4) were inversely related to RH changes. Additionally, PPy/BiPO4 exhibited the most significant change within the RH range, from 12% to 90% (a change of almost three orders of magnitude), suggesting that PPy/BiPO4 can absorb more water molecules and is thus more sensitive to humidity.  Figure 6a suggests that the response of the as-prepared sample changed gradually with increasing RH under humidity levels lower than 50% RH, whereas the response of the sensor increased sharply with higher ranges of RH. Moreover, PPy/BiPO4 exhibited the highest humidity in response (S = 24.6) to changes in RH ( Figure 6).
Humidity hysteresis, defined as the maximum difference in measured values of RH when the humidity sensor is exposed to adsorption and desorption processes, is an essential parameter for evaluating the reliability of a humidity sensor. Figure 6b suggests  Figure 6a suggests that the response of the as-prepared sample changed gradually with increasing RH under humidity levels lower than 50% RH, whereas the response of the sensor increased sharply with higher ranges of RH. Moreover, PPy/BiPO 4 exhibited the highest humidity in response (S = 24.6) to changes in RH ( Figure 6). that PPy/BiPO4 had minor humidity hysteresis loss and a relatively large hysteresis loop (0.70%) that occurred at RH levels less than 50%; this indicated that a fast equilibrium could be achieved between the adsorption and desorption process for PPy/BiPO4. Crucial parameters such as time of recovery and response data are vital for evaluating the performance of humidity sensors. These parameters represent humidity sensors' speed in measuring RH in various environments. Figure 7 illustrates that the response and recovery times for PPy/BiPO4 were 340 s and 60 s between 12% RH and 90% RH, respectively. The observed sensing parameters were compared with the previously reported sensors [4,[42][43][44][45] listed in Table 1. Overall, the PPy/BiPO4 sensor exhibited an excellent sensing response across the whole humidity range; however, its response and recovery times were longer than those of other sensors, as the PPy/BiPO4 sensor was tested using materials with different sensitivities. Selectivity is an essential parameter for the practical application of humidity sensors. The cross-sensitivity of PPy/BiPO4 has been studied for various gases at room temperatures, such as carbon monoxide (CO), ammonia (NH3), nitrogen dioxide (NO2), and nitric oxide (NO). As Figure 8 illustrates, the PPy/BiPO4 sensor had the highest selectivity for humidity and the lowest responsiveness to other types of gases. Humidity hysteresis, defined as the maximum difference in measured values of RH when the humidity sensor is exposed to adsorption and desorption processes, is an essential parameter for evaluating the reliability of a humidity sensor. Figure 6b suggests that PPy/BiPO 4 had minor humidity hysteresis loss and a relatively large hysteresis loop (0.70%) that occurred at RH levels less than 50%; this indicated that a fast equilibrium could be achieved between the adsorption and desorption process for PPy/BiPO 4 .
Crucial parameters such as time of recovery and response data are vital for evaluating the performance of humidity sensors. These parameters represent humidity sensors' speed in measuring RH in various environments. Figure 7 illustrates that the response and recovery times for PPy/BiPO 4 were 340 s and 60 s between 12% RH and 90% RH, respectively. The observed sensing parameters were compared with the previously reported sensors [4,[42][43][44][45] listed in Table 1. Overall, the PPy/BiPO 4 sensor exhibited an excellent sensing response across the whole humidity range; however, its response and recovery times were longer than those of other sensors, as the PPy/BiPO 4 sensor was tested using materials with different sensitivities. Selectivity is an essential parameter for the practical application of humidity sensors. The cross-sensitivity of PPy/BiPO 4 has been studied for various gases at room temperatures, such as carbon monoxide (CO), ammonia (NH 3 ), nitrogen dioxide (NO 2 ), and nitric oxide (NO). As Figure 8 illustrates, the PPy/BiPO 4 sensor had the highest selectivity for humidity and the lowest responsiveness to other types of gases.
Polymers 2021, 13, x FOR PEER REVIEW 7 of 11 that PPy/BiPO4 had minor humidity hysteresis loss and a relatively large hysteresis loop (0.70%) that occurred at RH levels less than 50%; this indicated that a fast equilibrium could be achieved between the adsorption and desorption process for PPy/BiPO4. Crucial parameters such as time of recovery and response data are vital for evaluating the performance of humidity sensors. These parameters represent humidity sensors' speed in measuring RH in various environments. Figure 7 illustrates that the response and recovery times for PPy/BiPO4 were 340 s and 60 s between 12% RH and 90% RH, respectively. The observed sensing parameters were compared with the previously reported sensors [4,[42][43][44][45] listed in Table 1. Overall, the PPy/BiPO4 sensor exhibited an excellent sensing response across the whole humidity range; however, its response and recovery times were longer than those of other sensors, as the PPy/BiPO4 sensor was tested using materials with different sensitivities. Selectivity is an essential parameter for the practical application of humidity sensors. The cross-sensitivity of PPy/BiPO4 has been studied for various gases at room temperatures, such as carbon monoxide (CO), ammonia (NH3), nitrogen dioxide (NO2), and nitric oxide (NO). As Figure 8 illustrates, the PPy/BiPO4 sensor had the highest selectivity for humidity and the lowest responsiveness to other types of gases.

Humidity-Sensing Mechanism
The humidity-sensing mechanism can be explained by the chemical and physical adsorption of water molecules on the PPy/BiPO4 surface, as illustrated in Figure 9. At low humidity, the probability of contact between water molecules and PPy/BiPO4 composite was low, so only the outer particles came into contact with the water molecules, as shown in Figure 9a. Since water molecules could not form a continuous water layer in this process, the transfer of H2O or H3O + onto the discontinuous water layer was challenging [46]. Therefore, in addition to its use of high-conductor graphene, the sensor impedance is extremely high. Figure 5 shows that, when the RH was 12% in the pure PPy, pure graphene and pure BiPO4, Gr/BiPO4, and PPy/BiPO4 composite, the impedance was 2.15 × 10 7 Ω, 9.18 × 10 -1 Ω, 2.71 × 10 7 Ω, 2.03 × 10 7 Ω, 1.49 × 10 7 Ω, respectively. As the RH level increased, the physisorption of water molecules occurred on the chemisorbed layer through hydrogen bonding, and the formation of a water multilayer, as illustrated in Figure 9b. The serial water layers accelerated the transfer of H2O or H3O + . The ion transfer mechanism presented by Grotthuss [47] and Casalbore-Miceli et al. [48] involves the transfer of H2O or H3O + on serial water layers H2O + H3O + → H3O + + H2O, as shown in Figure 9c. An ionic transfer is the main conduction mode in this process, and the impedance decreases as the relative humidity increases. The rapid transfer of ions on the water layer sharply reduces impedance. The PPy/BiPO4 composites exhibited humiditysensing properties that are more favorable than those of the pure PPy or BiPO4 samples.

Humidity-Sensing Mechanism
The humidity-sensing mechanism can be explained by the chemical and physical adsorption of water molecules on the PPy/BiPO 4 surface, as illustrated in Figure 9. At low humidity, the probability of contact between water molecules and PPy/BiPO 4 composite was low, so only the outer particles came into contact with the water molecules, as shown in Figure 9a. Since water molecules could not form a continuous water layer in this process, the transfer of H 2 O or H 3 O + onto the discontinuous water layer was challenging [46]. Therefore, in addition to its use of high-conductor graphene, the sensor impedance is extremely high. Figure 5 shows that, when the RH was 12% in the pure PPy, pure graphene and pure BiPO 4 , Gr/BiPO 4 , and PPy/BiPO 4 composite, the impedance was 2.15 × 10 7 Ω, 9.18 × 10 -1 Ω, 2.71 × 10 7 Ω, 2.03 × 10 7 Ω, 1.49 × 10 7 Ω, respectively. As the RH level increased, the physisorption of water molecules occurred on the chemisorbed layer through hydrogen bonding, and the formation of a water multilayer, as illustrated in Figure 9b. The serial water layers accelerated the transfer of H 2 O or H 3 O + . The ion transfer mechanism presented by Grotthuss [47] and Casalbore-Miceli et al. [48] Figure 9c. An ionic transfer is the main conduction mode in this process, and the impedance decreases as the relative humidity increases. The rapid transfer of ions on the water layer sharply reduces impedance. The PPy/BiPO 4 composites exhibited humidity-sensing properties that are more favorable than those of the pure PPy or BiPO 4 samples.

Conclusions
In this paper, various polymer (Gr/PPy)-BiPO 4 materials were successfully synthesized and used to detect RH. The structure and morphology samples were characterized by XRD, TEM, FESEM, and EDX. In this study, a humidity sensor based on Gr/BiPO 4 and PPy/BiPO 4 was prepared under room temperature conditions. The experimental results show that the PPy/BiPO 4 composite exhibited excellent humidity-sensing capabilities, including negligible humidity hysteresis, sensing response (S = 24.6), and high selectivity. Moreover, the response was 340 s and the recovery time was 30 s. Compared to pure PPy, pure graphene, and pure BiPO 4 , as well as Gr/BiPO 4 and PPy/BiPO 4 composites, these results indicate that PPy/BiPO 4 -based humidity sensors are good candidates for humidity sensor applications.