Conductive Polymer (Graphene/Ppy)–Bipo4 Composite Applications in Humidity Sensors

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

Zhen Zhu 1, Wang-De Lin 2,*, Zhi-Yi Lin 3, Ming-Hong Chuang 3, Ren-Jang Wu 3 and Murthy Chavali 4

1 School of Environmental Science and Safety Engineering, Tianjin University of Technology, Tianjin 300384, China; [email protected] 2 Department of Center for General Education, St. Mary’s Junior College of Medicine Nursing and Management, Yilan 26647, Taiwan 3 Department of Applied Chemistry, Providence University, Taichung 43301, Taiwan; [email protected] (Z.-Y.L.); [email protected] (M.-H.C.); [email protected] (R.-J.W.) 4 NTRC-MCETRC and Aarshanano Composite Technologies Pvt. Ltd., Guntur District, Andhra Pradesh 522201, India; [email protected] * Correspondence: [email protected]

Abstract: 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 Citation: Zhu, Z.; Lin, W.-D.; Lin, humidity hysteresis occurred at different relative humidities. Therefore, conductive PPy/BiPO4, Z.-Y.; Chuang, M.-H.; Wu, R.-J.; as prepared in the present study, is an excellent candidate for application in humidity sensors. Chavali, M. Conductive Polymer (Graphene/PPy)–BiPO Composite 4 Keywords: graphene; polypyrrole; impedance; BiPO4; sensing response Applications in Humidity Sensors. Polymers 2021, 13, 2013. https:// doi.org/10.3390/polym13122013

Academic Editor: Oh Seok Kwon 1. Introduction Ambient humidity must be controlled, regulated, and monitored in environments such Received: 28 May 2021 as dry cabinets, warehouse storage, industrial production, and food processing facilities, Accepted: 18 June 2021 as well as in agricultural planting operations, medical diagnostic centers—both facilities Published: 20 June 2021 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 detec- Publisher’s Note: MDPI stays neutral tion of environmental humidity. Particularly, many types of humidity sensors have been with regard to jurisdictional claims in explored due to their advantages, such as high sensitivity, rapid response and recovery, published maps and institutional affil- low hysteresis, and excellent reproducibility. Humidity sensors are commonly classified iations. 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 SnO2/MoS2 hybrid-sensing nanocomposite by synthesis through a two-step hydrothermal route; the sensor with a Copyright: © 2021 by the authors. 5 µm gap had the largest sensitivity at low humidity [13]. Mallick et al. enhanced the Licensee MDPI, Basel, Switzerland. humidity-sensing properties of polyvinylidene fluoride titanium dioxide (PVDF-TiO2) This article is an open access article nanocomposites-based capacitive humidity sensors by modifying the film surface of the distributed under the terms and TiO2 [14]. Duong et al. fabricated composite multi-walled carbon nanotubes (MWCNTs) conditions of the Creative Commons and tungsten oxide (WO3) nanobricks at different mass ratios, which attained a high Attribution (CC BY) license (https:// response with 95 wt.% of MWCNT/WO3 nanocomposite [15]. Arularasu et al. devel- creativecommons.org/licenses/by/ oped a PVDF/ZnO nanocomposite that was prepared as a humidity sensor through a 4.0/).

Polymers 2021, 13, 2013. https://doi.org/10.3390/polym13122013 https://www.mdpi.com/journal/polymers Polymers 2021, 13, 2013 2 of 11

simple hydrothermal approach in which the PVDF/ZnO provided more water molecule adsorption and desorption across the humidity sensor surface compared to pure ZnO nanoparticles [16]. Using the cast-drop and evaporation technique, Zebian et al. developed a hydrophilic-sensitive film based on Al2O3-PVA that was coated on the surface of no-core fibers; this exhibited high sensitivity to Al2O3 [17]. Ye et al. used Fe3O4 nanoparticles as an adsorption interface for the concurrent removal of gaseous benzene, toluene, ethylbenzene, m-xylene, and sulfur dioxide at different relative humidities, which indicated high removal efficiencies under dry conditions of Fe3O4 [18]. Xie et al. used the complex impedance analysis method to analyze CeO2 nanoparticles (NPs), which exhibited a rapid and reversible response characterized by a very small hysteresis [19]. Manikandan et al. synthesized nanocrystalline lithium-substituted copper ferrite (Lisingle bond CuFe2O4) nanoparticles with a high surface area, and obtained a fast response/recovery time [20]. Manikandan et al. synthesized a nanocrystalline-structured tin-substituted nickel ferrite (Sn-NiFe2O4) thin film, which exhibited excellent reproducibility, sensitivity, and response and recovery times [21]. Douani et al. synthesized bismuth ferrite nanoparticles BiFeO3 and bismuth ferrite/carbon fiber nanocomposites (BFO/CFs) by a hydrothermal process, showing that they have small hysteresis and good stability [22]. Arrizabalaga et al. reported on an accurate interferometric fiber optic humidity sensor of miniature size which showed fast, accurate, and sensitive properties as a commercial capacitive humidity sensor [23]. Hu et al. fabricated a novel, highly stable and sensitive humidity sensor based on bacterial cellulose (BC)-coated quartz crystal microbalance, which exhibited good reversible behavior and good long-term stability [24]. Ko et al. first report all-two-dimensional (2D) bis (trifluoromethane sulfonyl)-amide (TFSA)-doped graphene (GR) (TFSA-GR)/MoS2/ triethylenetetramine (TETA)-doped GR (TETA-GR) vertical-heterostructure semitransparent photodetectors (PDs) on rigid/flexible substrates, which exhibited good stability [25]. Among these, polymers are promising humidity-sensing candidates because they exhibit dramatic changes in electrical conductivity when exposed to various levels of environmental humidity [26–28]; they also have excellent properties, are low-cost to prepare, nontoxic, easy to fabricate, and are stable at room temperature. Additionally, graphene-based humidity sensors have received increased attention because they can generate electricity from the air by absorbing water molecules [29–31]. BiPO4 is an attractive material to researchers due to its economic cost, chemically stable structure, nontoxicity, photocatalytic characteristics, extraordinary optics, and electronic properties [32,33]. Graphene has attracted much attention from scientists and researchers because of its outstanding advantages, such as high surface area, excellent thermal conductivity, low cost, large-scale synthesis, stability, and electrical and mechanical properties; however, graphene cannot be applied to humidity sensors because of its hydrophobic nature. However, no attempt has been made to construct graphene/polypyrrole (Gr/PPy)– BiPO4 composite-resistive humidity sensors. In the present research, a series of Gr/PPy- modified BiPO4 was successfully synthesized, characterized, and investigated as sensing materials for humidity sensors. Moreover, the sensing mechanism, sensing properties, and morphology of as-prepared Gr/PPy-modified BiPO4 were examined.

2. Experimental Design 2.1. Materials

The 97% pure (Bi(NO3)3·5H2O) bismuth(III)nitrate pentahydrate, trisodium phos- phate (Na3PO4), pyrrole (C4H5N), cetyltrimethylammonium bromide (CTAB), ammonium persulfate (NH4)2S2O8), and polyvinyl alcohol ((C2H4O)x) were obtained from Sigma- Aldrich Co., Inc. (St. Louis, MO, USA). Graphene was purchased from National Chiayi University in Taiwan. All materials were utilized without any puriﬁcation. DI water was drawn from a system water puriﬁcation provided by the Milli-Q system, processed, and used in all experiments. Polymers 2021, 13, 2013 3 of 11

2.2. Method of Synthesis

As is typical in this procedure, 5 mmol of Bi(NO3)3·5H2O and 2.5 mmol of Na3PO4 were dissolved in 46 mL of HNO3 (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 BiPO4. Next, an amount of 0.5 g as-prepared BiPO4 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 C4H5N in solution A under ultrasound for 10 min. Thereafter, 100 mL of 10% (NH4)2S2O8 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/BiPO4. 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/BiPO4 [34–36].

2.3. 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 as- prepared materials was observed using ﬁeld emission scanning electron microscopy (JEOL JSM-7100F, Tokyo, Japan), with the microscope operated at 30 kV.

2.4. 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 mm2; 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 (Figure1). 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 thermos- hygrometer 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 Polymers 2021, 13, x FOR PEER REVIEW 4 of 11

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

Polymers 2021, 13, 2013 12% for the adsorption and desorption of water molecules [38]. H = Δfmax/ffs × 100%4 was of 11 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 impedanceof a sensor of to changea sensor by to 90% change of the by total 90% impedance,of the total whereasimpedance, the recoverywhereas the time recovery was the timetime was required the time for the required reverse for process. the reverse For dynamic process. testing, For dynamic the changes testing, in the RH changes at 12% and in RH90% at were 12% controlled and 90% bywere feeding controlled different by ratiosfeeding of airdifferent to water. ratios In the of providedair to water. experiment In the providedsetup, a hygrometer experiment (Rotronic)setup, a hygrometer was utilized (R forotronic) measuring was utilized the RH for value measuring with ±0.1% the RH RH valueaccuracy. with Sensing ±0.1% RH material accuracy. for the Sensing response material of impedance for the againstresponse humidity of impedance was explained against humidityby an analyzer was explained that could by measure an analyzer chemical that impedance could measure (DU 6010); chemical the input impedance voltage (DU and 6010);frequency the input were voltage 1 V and and 1 kHz, frequency respectively. were 1 V and 1 kHz, respectively.

FigureFigure 1. 1. SchematicSchematic diagram diagram of of the the experimental experimental setup. setup.

3.3. Results Results and and Discussion Discussion 3.1. Material Characterizations 3.1. Material Characterizations To investigate the structural features of the as-fabricated samples, XRD measurement To investigate the structural features of the as-fabricated samples, XRD measurement was performed. The XRD patterns of the samples presented in Figure2 indicate that was performed. The XRD patterns of the samples presented in Figure 2 indicate that the the main diffraction peak positions were located at the (110), (101), (202), (021), (130), main diffraction peak positions were located at the (110),◦ (101),◦ (202),◦ (021),◦ (130), and◦ (040) and (040) crystal planes of BiPO4, at 2θ values of 21.3 , 23.4 , 27.6 , 41.5 and 50.9 [39], crystalrespectively. planes of Additionally, BiPO4, at 2θ thevalues diffraction of 21.3°, peaks23.4°, 27.6°, were 41.5° observable and 50.9° at 2[39],θ values respectively. of 22.3◦ Additionally,and 26.8◦, which the diffraction corresponded peaks to were the observable crystalline at planes 2θ values of PPy of 22.3° and grapheneand 26.8°, [which40,41]. corresponded to the crystalline planes of PPy and graphene [40,41]. These results These results indicated that PPy/BiPO4 and Gr/BiPO4 were successfully synthesized. indicated that PPy/BiPO4 and Gr/BiPO4 were successfully synthesized. The diffraction The diffraction peaks of BiPO4 with the main lattice plane are (101), (200), (102), and (211) peakscompared of BiPO to JCPDS,4 with the which main demonstrate lattice plane a similarare (101), hexagonal (200), (102), phase. and (211) compared to JCPDS,Surface which morphology demonstrate playsa similar an essentialhexagonal role phase. in the humidity-sensing properties of sensing materials, as illustrated in Figure3a–c. Figure3a presents polymer diameters roughly in the range of 100 to 150 nanometers for domains in PPy. Figure3b shows the TEM image of the BiPO4 (200) lattice plane, which is about 0.329 nm in terms of lattice displacement. TEM images of BiPO4 are shown at low magnification, at a scale of 100 nm, as well as at a higher magnification. The figure also shows a TEM image of a single BiPO4 nanorod 569.53 nm in length and 97.92 nm in width. Figure3c illustrates that leaf-like nanoparticles were uniformly and thinly coated on the surface of the rod-like nanoparticles. Figure4 suggests that the elemental analysis (EDS) spectrum for PPy/BiPO 4 indicated the presence of C, O, N, P, and Bi atoms as major components in the as-prepared PPy/BiPO4.

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Figure 2. XRD pattern of BiPO4, PPy, graphene, 5 wt.% PPy/BiPO4, and 5 wt.% graphene/BiPO4 composites.

Surface morphology plays an essential role in the humidity-sensing properties of sensing materials, as illustrated in Figure 3a–c. Figure 3a presents polymer diameters roughly in the range of 100 to 150 nanometers for domains in PPy. Figure 3b shows the TEM image of the BiPO4 (200) lattice plane, which is about 0.329 nm in terms of lattice displacement. TEM images of BiPO4 are shown at low magnification, at a scale of 100 nm, as well as at a higher magnification. The figure also shows a TEM image of a single BiPO4 nanorod 569.53 nm in length and 97.92 nm in width. Figure 3c illustrates that leaf-like nanoparticles were uniformly and thinly coated on the surface of the rod-like nanoparticles. Figure 4 suggests that the elemental analysis (EDS) spectrum for PPy/BiPO4

indicated the presence of C, O, N, P, and Bi atoms as major components in the as-prepared Figure 2. XRD pattern of BiPO4, PPy,graphene, 5 wt.% PPy/BiPO4, and 5 wt.% graphene/BiPO4 composites. FigurePPy/BiPO 2. XRD pattern4. of BiPO4, PPy, graphene, 5 wt.% PPy/BiPO4, and 5 wt.% graphene/BiPO4 composites.

Figure 3. TEM images of (a) PPy, (b) BiPO4, and (c) FESEM 5 wt.% PPy/BiPO4 composites.

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Polymers 2021, 13, 2013 6 of 11 Figure 3. TEM images of (a) PPy, (b) BiPO4, and (c) FESEM 5 wt.% PPy/BiPO4 composites.

Figure 3. TEM images of (a) PPy, (b) BiPO4, and (c) FESEM 5 wt.% PPy/BiPO4 composites.

Figure 4. EDX spectrum of the 5 wt.% PPy/BiPO4 composites.

Figure 4. EDX spectrum of the 5 wt.% PPy/BiPO4 composites. 3.2.Figure Sensing 4. EDX Humidity spectrum of the 5 wt.% PPy/BiPO4 composites. 3.2.3.2. SensingSensingThe roles HumidityHumidity 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 TheThe rolesroles of of the the as-prepared as-prepared samples samples in in the the humidity humidity control control were were measured measured and and in- the impedance values of the as-prepared samples (BiPO4, PPy, Gr, PPy/BiPO4, and vestigatedinvestigated at theat the optimal optimal operating operating frequency frequency (1 kHz) (1 kHz) (Figure (Figure5). Figure 5). Figure5 indicates 5 indicates that the that Gr/BiPO4) were inversely related to RH changes. Additionally, PPy/BiPO4 exhibited the impedancethe impedance values values of the as-preparedof the as-prepared samples (BiPOsamples4, PPy, (BiPO Gr,4, PPy/BiPO PPy, Gr,4 ,PPy/BiPO and Gr/BiPO4, and4) most significant change within the RH range, from 12% to 90% (a change of almost three wereGr/BiPO inversely4) were related inversely to RH related changes. to RH Additionally, changes. Additionally, PPy/BiPO4 PPy/BiPOexhibited4 theexhibited most sig-the orders of magnitude), suggesting that PPy/BiPO4 can absorb more water molecules and is niﬁcantmost significant change within change the within RH range,the RH from range, 12% from to 90% 12% (a to change 90% (a ofchange almost of threealmost orders three thusof magnitude), more sensitive suggesting to humidity. that PPy/BiPO can absorb more water molecules and is thus orders of magnitude), suggesting that PPy/BiPO4 4 can absorb more water molecules and is morethus more sensitive sensitive to humidity. to humidity.

3.0x107

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7 BiPO 1.0x101.5x107 4

5 % PPy/BiPO

Impedance ( Impedance 4 BiPO 7 5 % Graphene4 /BiPO 1.0x106 4 5.0x10 5 % PPy/BiPO Impedance ( Impedance PPy 4 Graphene 5 % Graphene/BiPO 5.0x106 4 0.0 PPy Graphene 0.00 20406080100 Relative humidity (%) 0 20406080100 Figure 5. Discrepancies inRelative impedance humidity relative (%) to the variations in relative humidity (%) for varying Figure 5. Discrepancies in impedance relative to the variations in relative humidity (%) for varying (Gr/PPy)-BiPO44 compositecomposite contents. contents. Figure 5. Discrepancies in impedance relative to the variations in relative humidity (%) for varying (Gr/PPy)-BiPO4 composite contents. Figure 66aa suggestssuggests thatthat thethe responseresponse ofof thethe as-preparedas-prepared samplesample changedchanged graduallygradually with increasingincreasing RHRH underunder humidity humidity levels levels lower lower than than 50% 50% RH, RH, whereas whereas the the response response of the of sensorFigure increased 6a suggests sharply that with the higher response ranges of ofthe RH. as-prepared Moreover, sample PPy/BiPO changedexhibited gradually the the sensor increased sharply with higher ranges of RH. Moreover, PPy/BiPO4 4 exhibited with increasing RH under humidity levels lower than 50% RH, whereas the response of thehighest highest humidity humidity in response in response (S = (S 24.6) = 24.6) to changes to changes in RH in RH (Figure (Figure6). 6). the sensor increased sharply with higher ranges of RH. Moreover, PPy/BiPO4 exhibited Humidity hysteresis, defined as the maximum difference in measured values of RH the highest humidity in response (S = 24.6) to changes in RH (Figure 6). when the humidity sensor is exposed to adsorption and desorption processes, is an Humidity hysteresis, defined as the maximum difference in measured values of RH essential parameter for evaluating the reliability of a humidity sensor. Figure 6b suggests 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

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that PPy/BiPO4 had minor humidity hysteresis loss and a relatively large hysteresis loop Polymers 2021, 13, x FOR PEER REVIEW 7 of 11 (0.70%) that occurred at RH levels less than 50%; this indicated that a fast equilibrium Polymers 2021, 13, 2013 7 of 11 could be achieved between the adsorption and desorption process for PPy/BiPO4.

that PPy/BiPO4 had minor humidity hysteresis loss and a relatively large hysteresis loop

(0.70%) that occurred at RH levels less9 than 50%; this indicated that a fast equilibrium 25 (a) 10 could be achieved between the adsorption and(b) desorption process for PPy/BiPO4. 108 20 PPy Graphene 107 ) BiPO4 Ω 15 6 5 % PPy/BiPO4 910 25 (a) 10 adsorption

5 % Graphene/BiPO4 5 (b) desorption 10 8 10 10 Response 20 PPy 4 Graphene 710 5 ( Impedance 10 ) BiPO4 15 Ω 3 610 5 % PPy/BiPO4 10 0 adsorption

5 % Graphene/BiPO 4 2 desorption 10 10510 0 20406080100 Response 0 20406080100 104 5 ( Impedance Relative humidity (%) Relative humidity (%) 103 0 (a) (b) 102 FigureFigure 6. 6.(a )(a Discrepancies) Discrepancies in in response response relative relative to to the the variations variations in relativein0 relative humidity 20406080100 humidity (%) (%) for for varying varying (Gr/PPy)-BiPO (Gr/PPy)-BiPO4 0 20406080100 4 compositecomposite contents; contents; (b )(b hysteresis) hysteresis characteristics characteristics of of 5 wt.%5 wt.% PPy/BiPO PPy/BiPOcomposite.4 compositeRelative. humidity (%) 4 Relative humidity (%) (a) HumidityCrucial parameters hysteresis, deﬁnedsuch as astime the of maximum recovery difference (andb) response in measured data are values vital offor RHevaluating when the the humidity performance sensor of is humidity exposed tosens adsorptionors. These and parameters desorption represent processes, humidity is an Figure 6. (a) Discrepancies essentialinsensors’ response parameter speed relative in to formeasuring the evaluating variatio RHns the inin relative reliabilityvarious humidity environments. of a humidity (%) for varyingFigure sensor. (Gr/PPy)-BiPO7 Figure illustrates6b suggests that4 the composite contents; (b) hysteresis characteristics of 5 wt.% PPy/BiPO4 composite. thatresponse PPy/BiPO and 4recoveryhad minor times humidity for PPy/BiPO hysteresis4 were loss 340 and s and a relatively 60 s between large 12% hysteresis RH and loop 90% (0.70%)RH, respectively. that occurred The at RHobserved levels lesssensing than parameters 50%; this indicated were compared that a fast with equilibrium the previously could Crucial parameters such as time of recovery and response data are vital for bereported achieved sensors between [4,42–45] the adsorption listed in and Table desorption 1. Overall, process the PPy/BiPO for PPy/BiPO4 sensor4. exhibited an evaluating the performance of humidity sensors. These parameters represent humidity excellentCrucial sensing parameters response such across as time the of recoverywhole humidity and response range; data however, are vital its for response evaluating and sensors’ speed in measuring RH in various environments. Figure 7 illustrates that the therecovery performance times were of humidity longer than sensors. those of These other sensors, parameters as the represent PPy/BiPO humidity4 sensor was sensors’ tested response and recovery times for PPy/BiPO4 were 340 s and 60 s between 12% RH and 90% speedusing in materials measuring with RH different in various sensitivities. environments. Selectivity Figure is7 anillustrates essential that parameter the response for the RH, respectively. The observed sensing parameters were compared with the previously andpractical recovery application times for of PPy/BiPO humidity4 weresensors. 340 The s and cross-sensitivity 60 s between 12%of PPy/BiPO RH and4 90% has RH,been reported sensors [4,42–45] listed in Table 1. Overall, the PPy/BiPO4 sensor exhibited an respectively.studied for Thevarious observed gases sensing at room parameters temperatur werees, such compared as carbon with monoxide the previously (CO), reported ammonia excellent sensing response across the whole humidity range; however, its response and sensors(NH3), [4nitrogen,42–45] listed dioxide in Table (NO12.), Overall, and nitric the PPy/BiPOoxide (NO).4 sensor As Figure exhibited 8 illustrates, an excellent the recovery times were longer than those of other sensors, as the PPy/BiPO4 sensor was tested sensingPPy/BiPO response4 sensor across had the the highest whole humidityselectivity range; for humidity however, and its the response lowest responsiveness and recovery using materials with different sensitivities. Selectivity is an essential parameter for the timesto other were types longer of thangases. those of other sensors, as the PPy/BiPO4 sensor was tested using practicalmaterials application with different of sensitivities.humidity sensors. Selectivity The iscross-sensitivity an essential parameter of PPy/BiPO for the4 has practical been studiedapplication for various of humidity gases at sensors. room temperatur The cross-sensitivityes, such as carbon of PPy/BiPO monoxide4 has (CO), been ammonia studied 3 2 (NHfor various), nitrogen gases dioxide at room temperatures,(NO ), and nitric such asoxide carbon (NO). monoxide As Figure (CO), 8 ammonia illustrates, (NH the3), PPy/BiPOnitrogen dioxide4 sensor (NOhad 2the), andhighest nitric selectivity oxide (NO). for humidity As Figure and8 illustrates, the lowest the responsiveness PPy/BiPO 4 tosensor otherhad types the of highest gases. selectivity for humidity and the lowest responsiveness to other types of gases.

Figure 7. Response and recovery characteristics of 5 wt.% PPy/BiPO4 composites.

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Figure 7. Response and recovery characteristics of 5 wt.% PPy/BiPO4 composites.

TableTable 1. 1.ComparativeComparative humidity-sensing humidity-sensing performanceperformance of of modiﬁed modified conductive conductive polymer-based polymer-based sensors sensorswith with previously previously published published reports. reports.

Measurement Range Response/RecoveryResponse/ Sensing Material Measurement Range References Sensing Material (% RH) TimeRecovery (s) References (% RH) MCM-41/PPy 11–95 −915/Time−100 (s) [42]

RGO/SnOMCM-41/PPy2 11–9711–95 102/6−915/−100 [42] [43 ] TrianglamineRGO/SnO2 11–97 102/6 [43] 5–95 720/300 [44] Trianglaminehydrochloride hydrochloride 5–95 720/300 [44] MCM-41/PEDOTMCM-41/PEDOT 11–9511–95 165/115165/115 [4] [4 ] PPyPPy 11–9511–95 41/12041/120 [45] [45 ] PPy/BiPO4 12–90 340/60 This work PPy/BiPO4 12–90 340/60 This work

Figure 8. Selectivity towards relative humidity and various gas species of 5 wt.% PPy/BiPO4 composites. Figure 8. Selectivity towards relative humidity and various gas species of 5 wt.% PPy/BiPO4 composites.3.3. Humidity-Sensing Mechanism The humidity-sensing mechanism can be explained by the chemical and physical 3.3.adsorption Humidity-Sensing of water Mechanism molecules on the PPy/BiPO4 surface, as illustrated in Figure9. At low humidity,The humidity-sensing the probability mechanism of contact between can be waterexplained molecules by the and chemical PPy/BiPO and4 physicalcomposite adsorptionwas low, of so water only themolecules outer particles on the PPy/BiPO came into4 contactsurface, with as illustrated the water in molecules, Figure 9. At as shownlow humidity,in Figure the9a. probability Since water of molecules contact between could not water form molecules a continuous and water PPy/BiPO layer in4 composite this process, + wasthe low, transfer so only of the H2 outerO or Hparticles3O onto came the into discontinuous contact with water the water layer molecules, was challenging as shown [46 ]. in Therefore,Figure 9a. inSince addition waterto molecules its use of could high-conductor not form a graphene, continuous the water sensor layer impedance in this is extremely high. Figure5 shows that, when the RH was 12% in the pure PPy, pure graphene process, the transfer of H2O or H3O+ onto the discontinuous water layer was challenging 7 [46].and Therefore, pure BiPO in4 ,addition Gr/BiPO to4 ,its and use PPy/BiPO of high-conductor4 composite, graphene, the impedance the sensor was impedance 2.15 × 10 Ω, × –1 × 7 × 7 × 7 is extremely9.18 10 high.Ω, 2.71 Figure10 5 showsΩ, 2.03 that,10 whenΩ, 1.49the RH10 wasΩ, 12% respectively. in the pure As thePPy, RH pure level increased, the physisorption of water molecules occurred on the chemisorbed layer through graphene and pure BiPO4, Gr/BiPO4, and PPy/BiPO4 composite, the impedance was 2.15 × hydrogen bonding, and the formation of a water multilayer, as illustrated in Figure9b. 107 Ω, 9.18 × 10–1 Ω, 2.71 × 107 Ω, 2.03 × 107 Ω, 1.49 × 107 Ω, respectively. As the RH level The serial water layers accelerated the transfer of H O or H O+. The ion transfer mechanism increased, the physisorption of water molecules occurred2 3 on the chemisorbed layer presented by Grotthuss [47] and Casalbore-Miceli et al. [48] involves the transfer of H2O or through+ hydrogen bonding, and the formation+ of +a water multilayer, as illustrated in H3O on serial water layers H2O + H3O → H3O + H2O, as shown in Figure9c. An ionic Figure 9b. The serial water layers accelerated the transfer of H2O or H3O+. The ion transfer transfer is the main conduction mode in this process, and the impedance decreases as the mechanism presented by Grotthuss [47] and Casalbore-Miceli et al. [48] involves the relative humidity increases. The rapid transfer of ions on the water layer sharply reduces transfer of H2O or H3O+on serial water layers H2O + H3O+ → H3O+ + H2O, as shown in impedance. The PPy/BiPO composites exhibited humidity-sensing properties that are Figure 9c. An ionic transfer 4is the main conduction mode in this process, and the more favorable than those of the pure PPy or BiPO samples. impedance decreases as the relative humidity increases.4 The rapid transfer of ions on the water layer sharply reduces impedance. The PPy/BiPO4 composites exhibited humidity- sensing properties that are more favorable than those of the pure PPy or BiPO4 samples.

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Figure 9.9.Scheme Scheme of of humidity-sensing humidity-sensing mechanism: mechanism: (a) low (a) humidity, low humidity, (b) high humidity,(b) high (humidity,c) adsorption (c) adsorptionmodels for PPy/BiPOmodels for4 PPy/BiPOcomposites.4 composites.

4. Conclusions In this paper, various polymer (Gr/PPy)–BiPO materials were successfully synthe- In this paper, various polymer (Gr/PPy)–BiPO4 4 materials were successfully synthesizedsized and used and to detectused to RH. detect The structureRH. The and structure morphology and morphology samples were samples characterized were characterizedby XRD, TEM, by FESEM, XRD, TEM, and EDX. FESEM, In this and study, EDX. a In humidity this study, sensor a humidity based on sensor Gr/BiPO based4 and on PPy/BiPO4 was prepared under room temperature conditions. The experimental results Gr/BiPO4 and PPy/BiPO4 was prepared under room temperature conditions. The show that the PPy/BiPO4 composite exhibited excellent humidity-sensing capabilities, experimental results show that the PPy/BiPO4 composite exhibited excellent humidity- including negligible humidity hysteresis, sensing response (S = 24.6), and high selectivity. sensing capabilities, including negligible humidity hysteresis, sensing response (S = 24.6), Moreover, the response was 340 s and the recovery time was 30 s. Compared to pure and high selectivity. Moreover, the response was 340 s and the recovery time was 30 s. PPy, pure graphene, and pure BiPO4, as well as Gr/BiPO4 and PPy/BiPO4 composites, Compared to pure PPy, pure graphene, and pure BiPO4, as well as Gr/BiPO4 and these results indicate that PPy/BiPO4-based humidity sensors are good candidates for PPy/BiPO4 composites, these results indicate that PPy/BiPO4-based humidity sensors are humidity sensor applications. good candidates for humidity sensor applications. Author Contributions: Conceptualization, W.-D.L.; methodology, W.-D.L.; software, W.-D.L.; formal Author Contributions: Conceptualization, W.-D.L.; methodology, W.-D.L.; software, W.-D.L.; analysis, W.-D.L.; investigation, W.-D.L.; data curation, M.-H.C. and Z.-Y.L.; writing—original formal analysis, W.-D.L.; investigation, W.-D.L.; data curation, M.-H.C. and Z.-Y.L.; writing— draft preparation, Z.Z. and W.-D.L.; writing—review and editing, W.-D.L. and M.C.; visualization, original draft preparation, Z.Z. and W.-D.L.; writing—review and editing, W.-D.L. and M.C.; R.-J.W.; project administration, R.-J.W. All authors have read and agreed to the published version of visualization, R.-J.W.; project administration, R.-J.W. All authors have read and agreed to the the manuscript. published version of the manuscript. Funding: This research received no external funding. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data sharing not applicable. Data Availability Statement: Data sharing not applicable. Acknowledgments: Department of Applied Chemistry is thanked by the authors of Providence Acknowledgments: Department of Applied Chemistry is thanked by the authors of Providence University, Sha-Lu, Taichung, for the humidity sensor fabrication. University, Sha-Lu, Taichung, for the humidity sensor fabrication. Conflicts of Interest: The authors declare that they have no known competing financial interests or Conflicts of Interest: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. personal relationships that could have appeared to influence the work reported in this paper. References References 1. Duan, Z.; Jiang, Y.; Yan, M.; Wang, S.; Yuan, Z.; Zhao, Q.; Sun, P.; Xie, G.; Du, X.; Tai, H. Facile, Flexible, Cost- 1. Saving,Duan, Z.; and Jiang, Environment-Friendly Y.; Yan, M.; Wang, Paper-BasedS.; Yuan, Z.; HumidityZhao, Q.; Sun, Sensor P.; forXie, Multifunctional G.; Du, X.; Tai, Applications.H. Facile, Flexible,ACS Cost-Saving, Appl. Mater. Interfacesand Environment-Friendly2019, 11, 21840–21849. [Paper-BasedCrossRef] Humidity Sensor for Multifunctional Applications. ACS Appl. Mater. 2. Zhang,Interfaces J.; 2019 Wang,, 11 X.-X.;, 21840 Zhang,–21849. B.; Ramakrishna, S.; Yu, M.; Ma, J.-W.; Long, Y.-Z. In Situ Assembly of Well-Dispersed Ag Nanopar- 2. ticlesZhang, throughout J.; Wang, Electrospun X.-X.; Zhang, Alginate B.; NanofibersRamakrishna, for Monitoring S.; Yu, M.; Human Ma, J.-W.; Breath—Smart Long, Y.-Z. Fabrics. In SituACS Appl.Assembly Mater. of Interfaces Well- 2018Dispersed, 10, 19863–19870. Ag Nanoparticles [CrossRef ][throughoutPubMed] Electrospun Alginate Nanofibers for Monitoring Human Breath—Smart 3. Ali,Fabrics. S.; Jameel, ACS Appl. M.A.; Mater. Gupta, Interfaces A.; Langford, 2018, 10 S.J.;, 19863 Shafiei,–19870. M. Capacitive humidity sensing performance of naphthalene diimide derivatives at ambient temperature. Synth. Met. 2021, 275, 116739. [CrossRef]

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