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BY 4.0 license Open Access Published by De Gruyter Open Access February 20, 2023

Ammonia gas-sensing behavior of uniform nanostructured PPy film prepared by simple-straightforward in situ chemical vapor oxidation

  • Khong Van Nguyen , Bui Ha Trung , Chu Van Tuan , Cong Doanh Sai , Tung Duy Vu , Tran Trung , Giang Hong Thai , Ho Truong Giang and Hoang Thi Hien EMAIL logo
From the journal Open Physics

Abstract

A highly uniform nanostructured polypyrrole (PPy) film prepared by a simple, straightforward in situ route of chemical vapor oxidation has been demonstrated as a sensitive substrate for NH3 gas sensing. The structure of PPy film was investigated by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). The binding characteristics of the functional groups of the PPy film were examined by Fourier transform infrared and Raman spectroscopy. NH3 sensing properties of the PPy film were evaluated by its resistive response to gas concentrations from 45 to 350 ppm at different temperatures ranging from 25 to 100°C. The sensing response maximum value was 142.6% when exposed to 350 ppm of NH3 gas at room temperature (25°C). The sensing response of PPy film shows an excellent linear relationship and high selectivity toward NH3. The NH3 sensing mechanism is due to the physisorption and chemisorption interactions of NH3 molecules and the adsorptive sites of PPy (polaron and bipolaron charging carriers).

1 Introduction

Ammonia gas is well known as one of the most widely produced compounds and has been utilized in numerous fields of the industry as an indispensable raw material [1,2,3,4]. Recently, an estimated 20% of the NH3 produced has been used for medicines, explosives, cleaning products, and refrigeration; meanwhile, the remaining 80% is used for nitrogen-based fertilizer [5]. However, the risk of NH3 gas leakage from such industrial activities into the atmosphere is a significant issue. The NH3 gas causes air pollution, which is hazardous to human health and even carries the possibility of an NH3 gas explosion. The permissible exposure limit for NH3 gas is only 25 ppm averaged over 8 h, as established by the U.S. Occupational Safety and Health Administration (OSHA) [6,7]. Moreover, NH3-related aerosols such as ammonium nitrate and ammonium sulfate can negatively impact the greenhouse balance worldwide [6,7]. Especially, the formation of synergistic particles of HNO3–H2SO4–NH3 in the upper free troposphere was found. It has been demonstrated that nitric acid, sulfuric acid, and ammonia form particles synergistically faster and in a greater order of magnitude than any two of the three mentioned components [8]. Recently, a daily breath analyzer that monitors NH3 concentration from exhaled human breath to identify lung or renal disorders in patients with non-invasiveness has been an attractive potential field for health diagnosis [9,10,11]. Therefore, the accurate measurement of NH3 gas has attracted significant attention and demand.

There are various nanomaterials and technologies used for the fabrication of ammonia gas sensors and shown in many reports [12,13,14,15]. For example, ammonia sensors have been developed using nanostructured metal oxides such as SnO2, NiO, ZnO, W18O9, and MoO3 [16]. The high surface to volume of the metal oxides nanostructured exhibits good sensitivities; nevertheless, a significant disadvantage of metal oxide NH3 sensors is the high working temperature. Therefore, to extend the practical application of NH3 sensors, conductive polymers [PANI and polypyrrole (PPy)] are considered potential candidates due to their low operating temperature. In this regard, PPy is a regularly utilized material for NH3-sensitive layers [17,18,19,20]. The charge carriers are known as soliton (having a spin of ½ and can either exist in neutral or charged states), polaron (possessing spin of ½ and charge of +1), and bipolaron (possessing spin of zero and charge of +2). They can move through several consecutive steps of odd electron extraction per step arising from the intercalation/expulsion of ions and solvent molecules or the insertion of some dopants driven by oxidation/reduction [21]. It can be considered that the states of these charge carriers are two key factors that strongly affect the NH3 gas-sensing performance. For example, Zhang et al. [22] revealed that a high-density and small-diameter (about 50 nm) of nanowire arrays of PPy had contributed to a significant increase in their surface-to-volume ratio, leading to high NH3 gas responses changing from 10 to 26% for the gas concentration increased from 1.5 to 77 ppm, respectively. However, this PPy response dependence on the NH3 gas concentration only exhibited a small linear range from 1.5 to 12 ppm, corresponding to a change from 10 to 22%. In other works, a porous network PPy thin film was synthesized by microwave-assisted bath deposition [23]. When tested toward 200 ppm, the NH3-sensing response of this PPy film was about 85%, and the response/recovery times were 70 and 110 s, respectively. Zhang et al. [24] showed two different sensors based on PPy/NS@silk-fiber and PPyNS@sponge fabricated by using a simple in situ chemical oxidation polymerization. The NH3-sensing response of the PPy@sponges (14.51%) was far lower than that of the PPy/NS@silkfibers (73.25 %) when recorded at 100 ppm under room temperature and relative humidity (RH) of 68%. The PPy/NS@silk fiber disclosed long-term stability and short response/recovery times of 24 and 69 s, respectively. These results were explained by attributing the high surface area of the flexible PPy/NS@silk fiber due to the relatively rough hill-like shapes. However, the sensing mechanisms in previous studies have already been examined and mentioned as the complex redox reactions of gas-adsorbed active sites that decreased the conductivity of these materials. Nonetheless, there has not been a fulfilled and clear explanation for the gas-sensing performance of the conducting polymers, particularly under different nanostructure morphologies.

In this study, the PPy nanoparticle film with a highly uniform morphology was investigated. The PPy film significantly improves the NH3-sensing performance operating at room temperature. Influences of RH and operating temperature (25–100°C) on the sensing performance of the films were evaluated in detail. Furthermore, the mechanisms of polaron and bipolaron states’ formation and adsorptive interactions between NH3 molecule and adsorptive sites were also proposed.

2 Experimental

Reagents pyrrole monomer, ferric chloride (FeCl3·6H2O), and ethanol purchased from Sigma-Aldrich were used without further treatment. The Al2O3 substrates (6.3 mm × 6.3 mm × 0.25 mm) with two Pt electrodes were used for fabricating the nanostructured PPy films as shown in Figure 1. A Pt micro-heat stove integrated on the bottom of the Al2O3 substrate [25] was used to modulate the operating temperature.

Figure 1 
               Schematic representation of Pt/Al2O3 substrate (a) and SEM image of Al2O3 surface (b).
Figure 1

Schematic representation of Pt/Al2O3 substrate (a) and SEM image of Al2O3 surface (b).

The PPy nanostructured film was synthesized by a chemical vapor-phase polymerization using FeCl3 as an oxidant; the fabrication procedure was described in a previous publication [25]. In detail, ferric chloride salt FeCl3·6H2O was dissolved into 50% ethanol and 50% distilled water to obtain a 0.02 M FeCl3 solution. The Pt/Al2O3 substrates were covered with FeCl3 oxidant by a spray-coating technique (electrospinning machine NTEC, Nantong) with a spraying flow of 0.2 ml h−1, a distance from the injector to the surface of the Al2O3 substrate of 15 cm, and a spray duration within 30 min at 45°C. The FeCl3-coated Al2O3 substrate was then transferred to a polymerization chamber (1 l) containing a pyrrole monomer source. The vapor-phase polymerization process took place for 60 min at atmospheric pressure. The sample was washed with ethanol and distilled water several times to remove contaminants and then dried in an oven at 50°C for 1 h to receive the nanostructured PPy film.

Morphologies and analyzing elements components of the synthesized PPy film were investigated using a scanning electron microscope with energy-dispersive X-ray spectroscopy (SEM-4800 Hitachi). Crystal structural and bonding characteristics of the film were analyzed via X-ray diffraction (SIEMENS-D5005), spectra of Fourier transform infrared (FTIR) (Shimazu Fourier transform infrared spectrometer, IRAffinity-1S), and Raman scattering spectra (Horiba Raman spectrometer, Labram HR 800).

Gas-sensing performances of the synthesized PPy film were investigated with NH3 concentrations from 45 to 350 ppm in dry air (80% N2 + 20% O2) by flow-through mixing method [26] and at operating temperatures of 25, 45, 60, 80, and 100°C. To examine the influences of humidity on the sensing characteristics of the synthesized PPy film, a humidifier with a working principle based on the saturated water vapor pressure was used. The RH in the measuring chamber was selected with 11, 16, 33, 75, 84, and 94 %RH by using saturated salt solutions including LiCl, KOH, MgCl2, NaCl, KCl, and Sr(NO3)2, respectively. The sensing response (S) was calculated by the following formula: S = R gas R air R air × 100 % , where R air and R gas are saturating resistances of the film tested for drying air and analyzed gases or humidity, respectively.

3 Results and discussion

3.1 Material characteristics

The surface morphologies of the FeCl3-coated layer and the synthesized PPy film on the Al2O3 substrate are shown in Figure 2a and b, respectively. It could be seen that the surface morphology of the synthesized PPy film is a highly ordered and uniform structure consisting of nanoparticles with a diameter of about 30–50 nm. The result exhibited a cross-linked 3D network morphology, different from the conventional PPy structures like nanoparticles-cluster, nanofibers-matrix, or nano-slabs [27].

Figure 2 
                  SEM images of FeCl3 particles (a) and PPy nanoparticles (b) on Al2O3 substrate and EDS spectrum (c) and XRD pattern (d) of the synthesized PPy film.
Figure 2

SEM images of FeCl3 particles (a) and PPy nanoparticles (b) on Al2O3 substrate and EDS spectrum (c) and XRD pattern (d) of the synthesized PPy film.

The EDS spectrum of the synthesized PPy film (Figure 2c) showed peaks assigned for component elements of C, N, Cl, Fe, Al, and O at appropriate energy levels. Al and O elements belonged to the Al2O3 substrate, and C, N, Fe, and Cl were the components of the PPy film. An excess oxygen atom ratio (∼2.55%) compared with the stoichiometric composition ratio of 2/3 was ascribed to the absorption of water molecules into the PPy film. The EDS and XRD (showed Figure 2c and d, respectively) results of the PPy film also revealed that Fe and Cl elements could contribute as doping reagents (Fe2+ and Cl) in PPy structure, as previously reported [18,28]. A typical nearly amorphous structure of the PPy film could be seen as a broad characteristic peak centered at approximately 2θ = 23.5°, which should be assigned the inter-chain spacing order of PPy [29]. The peaks at 2θ = 32.2, 33.4, 62.8, and 40.3° in the XRD pattern (Figure 2d) indexed (104), (110), (214), and (113) could be a reflection of the FeCl2 structure (JCPDF 77-0044) and implied that existence of Fe2+ doping in the synthesized PPy film. The behavior is caused by iron ions attached to the Brönsted pyrrole monomers during the polymerization process [30].

The FTIR spectra in the range from 500 to 4,000 cm−1 of the PPy films under conditions of as synthesized, NH3 adsorbed, and NH3 desorbed are presented in Figure 3a. The characteristic vibrations of specific functional groups of PPy structure were disclosed. The peaks at 1,543 and 1,482 cm−1 attributed to C═C and C–C stretching bonds of main chains PPy [31,32]. Furthermore, the relative intensity of two peaks of 1,543/1,482 cm−1 increases from 0.95 to 1.14 for the as-synthesized PPy and the NH3-adsorbed PPy, respectively, indicating that a preferential form of the aromatic increased while the quinoid form decreased when conducted the NH3 adsorption. Consequently, doping levels of the NH3-adsorbed PPy could be reduced, and electron movement in the PPy network was hindered due to an electron trapping effect that induced NH3 adsorption.

Figure 3 
                  FTIR spectra of as-synthesized PPy, NH3-adsorbed PPy, and NH3 desorbed-PPy.
Figure 3

FTIR spectra of as-synthesized PPy, NH3-adsorbed PPy, and NH3 desorbed-PPy.

The small peaks at 3,240 cm−1 and the shoulder at 1,612 cm−1 were assigned to the presence of the N–H deformation vibrations in secondary amine in pyrrole rings [33,34] and stretching and bending modes of vibrations of –OH groups, respectively. O–H vibrations mainly originated from absorbed water molecules on the PPy film. The characteristic peak at 3,125 cm−1 corresponded to the stretching vibrations of hydrogen-bonded surface water molecules [35,36]. Notably, the peak around 1,093 cm−1 was ascribed to in-plane bending vibration of NH 2 + groups derived from the protonated PPy chains [37], meaning that there was an existence of evolution of polaron states (possessing spin ½ and charge of +1) in the as-synthesized PPy film as well as protonations occurring at nitrogen atoms in pyrrole rings. The weak shoulder at 1,715 cm−1 was typically attributed to C═O stretching vibrations, indicating that pyrrole rings were slightly over-oxidized [31,38]. The peaks at 1,314 and 1,043 cm−1 were assigned for the ═C–H in-plane vibrations of the PPy chain, 938 cm−1 for C–H out-of-plane vibration, and 785 cm−1 for C–H wagging vibration at [39,40,41]. The shoulder at 1,372 cm−1 and the broad and blunt peak at 1,194 cm−1 corresponded to the C–N stretching vibrations induced by Cl anions doping in PPy [31,42]. The distinct peaks at 552 and 670 cm−1 should be assigned to Fe2+ or C–H vibrations [43,44]. It was believed that a reduction of Fe3+ into Fe2+ was formed in the vapor-phase oxidation of pyrrole monomers and then created doping states. As a result, it was obvious that the assigned peaks in the FTIR spectrum of the as-synthesized PPy film with a high flawless degree in the molecular structure had a better definition than those in our previous work [18]. Surprisingly, when the as-synthesized PPy film was exposed to 350 ppm NH3 at 25°C, the shoulder at 1,612 cm−1 shifted to 1,624 cm−1 position. The bidentate band with two peaks at 3,363 and 3,408 cm−1 only appeared in the spectrum of the NH3-adsorbed PPy [45], assigned respectively to physical and chemical adsorptions, and then disappeared for the NH3-desorbed PPy.

For further insight into the relevance of the PPy film, Raman spectroscopic technique was employed in a wave number range of 600–2,000 cm−1. As shown in Figure 4a, the relative intensities in regions of 924–931, 1,047–1,050, and 1,608–1,614 cm−1 could be assigned to the oxidized states, and they decreased under the NH3 adsorption (Figure 4b). Besides a sharp peak at 1,612 cm−1 belonged the characteristic of C═C in-ring vibrations of the quinoid form, there is a shoulder at 1,556 cm−1, which presents inter-ring C–C vibrations of the short conjugation length [46]. The relative intensity I Shoulder / I Peak of these vibrations was slightly decreased from 0.508 (for the as-synthesized PPy) to 0.435 (for the NH3-adsorbed PPy), which revealed that inter-ring C–C vibrations were hindered by NH3-adsorbed molecules. The other bands at 1,047, 984, and 931 cm−1 could be characteristics of asymmetric vibrations of in-plane deformed C–H and N–H bonds and C–C distorted ring, respectively, in the quinoid polaronic structure and the ring deformation of quinoid bipolaronic structure [47,48]. Notably, the peak at 1,047 cm−1 was obvious to be the occurrence at 1,084 cm−1 under the NH3 adsorbed. The relative intensity of I 1 , 047 / I 1 , 084 decreased markedly from 3.88 to 2.38. These results revealed that the vibrations of in-plane deformed C–H bonds were hindered by the adsorption of NH3 molecules, while the vibrations of in-plane deformed N–H bonds became stronger. Thus, NH3 molecules were adsorbed at positions very close to carbon atoms of the pyrrole rings, and electrons and positive holes in polaron and bipolaron quinoid structures were suggested to be located at the positions that NH3 molecules adsorbed.

Figure 4 
                  Raman spectra of as-synthesized PPy and NH3-adsorbed PPy (a), and the decrease of Raman intensities under NH3 adsorption (b).
Figure 4

Raman spectra of as-synthesized PPy and NH3-adsorbed PPy (a), and the decrease of Raman intensities under NH3 adsorption (b).

From the FTIR and Raman results, the mechanisms for the formation of the polaron and bipolaron structures; and NH 2 + groups derived from the protonated PPy chains were proposed, as illustrated in Figure 5.

Figure 5 
                  Illustration of polaron and bipolaron structures and 
                        
                           
                           
                              
                                 
                                    NH
                                 
                                 
                                    2
                                 
                                 
                                    +
                                 
                              
                           
                           {\text{NH}}_{2}^{+}
                        
                      groups derived from protonated PPy chains in the polymerization process.
Figure 5

Illustration of polaron and bipolaron structures and NH 2 + groups derived from protonated PPy chains in the polymerization process.

3.2 Gas-sensing performance

To examine the gas-sensing behaviors of the PPy film, the resistance response at two temperatures of 25 and 100°C when exposed to four cycles of 350 ppm NH3 gas in dry air was measured, as shown in Figure 6. It was observed that the resistance increased when responded to NH3 gas and decreased to its initial value when recovered to dry air. The results showed that the PPy film had good reversibility and stability; thus, this material could be suitable for utilizing the NH3 gas sensor.

Figure 6 
                  Resistance of the PPy film responding to four cycles of 350 ppm NH3/dry air.
Figure 6

Resistance of the PPy film responding to four cycles of 350 ppm NH3/dry air.

NH3-sensing responses of the PPy film were investigated for different concentrations from 45 to 350 ppm at temperatures of 25, 45, 60, 80, and 100°C, as presented in Figure 7a. From the results, it was obvious that the resistance increased upon increasing the NH3 gas concentration. As shown in Figure 7a and b, the PPy film revealed a similar gas-sensing characteristic at different operating temperatures. It also demonstrated that the PPy film exhibited well temperature-driven reversibility to all the tested NH3 gas concentrations. The sensing responses at 25, 45, 60, 80, and 100°C (Figure 7b) showed that the dependence between the response and NH3 concentration is almost linear, with a standard deviation under 5%. With each NH3 gas concentration, the sensing response decreased with increasing the operating temperature and reached saturate values at the high operating temperature. For example, at 350 ppm NH3 (Figure 7c), the sensing response decreased significantly by about 80% when increased the operating temperature from 25 to 100°C.

Figure 7 
                  Resistance (a) of the PPy film responding to cycles of NH3/dry air and its sensing response dependences on NH3 concentration (b) and operating temperature (c).
Figure 7

Resistance (a) of the PPy film responding to cycles of NH3/dry air and its sensing response dependences on NH3 concentration (b) and operating temperature (c).

Figure 8 shows the response/recovery times of the PPy film. The response times exhibited a similar tendency with high in the low temperatures and low in the high temperatures for all the tested NH3 gas concentrations (Figure 8a). Nevertheless, the response times decreased according to increasing the NH3 concentration (Figure 8b). These behaviors could be considered to reveal the competition of the temperature-driven convective diffusion and the adsorption/desorption processes involving physical adsorption and chemisorption at the active sites. At 25 and 100°C, with the low NH3 concentration, the collisions of NH3 molecules per unit surface area per unit time were slow. Then, the number of the NH3 molecules with their kinetic energy having a larger adsorption enthalpy and their striking onto active sites are still low. Thus, a long time was needed to achieve a full surface coverage of the PPy film. Meanwhile, at higher NH3 concentration, NH3 molecules moved and overcame a certain thickness of the diffusion-convection layer to reach the effective thickness, where collisions between NH3 molecules the kinetic energy of NH3 molecules were kept a most constant and still larger adsorption enthalpy to make more sticking collisions, leading the decreases in response time. For each NH3 concentration (Figure 8a), when the temperature was raised, the kinetic energies of NH3 molecules and active sites were increased so that there were more collisions between them. This concluded that the mean lifetime of collisions was decreased, and the initial sticking probability of collisions was due to a decrease in the physical adsorption. With sticking collisions having a larger lifetime, they could be converted into physical adsorption states. As a result, the response time increased and reached a maximum value of 45°C. If the temperature continued to increase, the speed and kinetic energies of NH3 molecules increased intensively. Under the condition, the speed movement of the NH3 molecules increased, resulting in more frequent and more forceful collisions with the adsorptive sites. The increased sticking probability of collisions led to decreased response time for complete physical adsorption. In the high temperatures from 60 to 100°C, the initial stage of the physical adsorption occurred at the surface coverage reached a maximum, and the active interaction sites for chemisorption were activated with a high activation energy. At this stage, a number of the NH3 molecules with kinetic energies larger enthalpy of chemisorption were small. For a near-saturation coverage of the active interaction sites of chemisorption, the response time could increase and reach a maximum value at the high temperature, as observed at 80°C in this work. After that, the higher temperature, the greater fraction of sticking collisions of NH3 molecules with energies larger enthalpy; thus, the response time decreased. In contrast to a complicated response time evolution, the recovery time’s dependence on the temperature and the NH3 concentration was simple. As shown in Figure 8c, an exception from the lowest NH3 concentration (45 ppm), the recovery times at the higher concentrations decreased with increasing the operating temperature.

Figure 8 
                  Response/recovery times of the PPy film depending on operating temperature (a and c) and NH3 concentration (b and d).
Figure 8

Response/recovery times of the PPy film depending on operating temperature (a and c) and NH3 concentration (b and d).

With the lowest NH3 gas concentration of 45 ppm, the thickness of the diffusion layer could be the greatest when compared to the other gas concentrations. Thus, from 25 to 45°C, NH3 molecules adsorbed on the active sites were released from the PPy surface and took a long time to exist in the diffusion layer. As a result, a gradient of the NH3 gas concentration gradually decreased, and then, the recovery time increased with increasing the operating temperature and reached a maximum. As the high operating temperature, the average speed and kinetic energy of NH3 molecules could increase intensively; thus, the NH3 molecules rapidly moved away from the diffusion layer to promote the desorption, as observed in Figure 8d. On the dependence of the recovery time on the NH3 concentration, there were two tendencies. In the low range of 25–45°C, the recovery time increased with increasing the NH3 gas concentration and reached a maximum of 90 ppm. In the high range of 60–100°C, the fashion of the recovery time and the NH3 concentration changed from convex (60–80°C) to concave (100°C). This change showed that when the NH3 gas concentration increased in the concave, the recovery time also increased and reached a constant value. In contrast, in the convex, the recovery time decreased and reached a minimum value. The NH3 molecules that were physically adsorbed on the top of the film surface were more quickly released into the diffusion layer when exposed to dry air. With the NH3 concentration increase, the number of NH3 molecules that were physically and chemically adsorbed could occur on the film surface; therefore, a layer of molecules could be physically adsorbed on top of the underlying chemisorbed layer, resulting in the recovery time increased again. However, in the higher operating temperature (100°C), the NH3 molecules’ speed that was diffused into/out of the film surface was high enough to form a higher number of chemisorbed NH3 molecules. Consequently, the recovery time increased with increasing the NH3 concentration.

The selectivity tests were carried out by evaluating the influence of some interfering gases (Figure 9a). The interfering gases included NO2, H2S, CO, and H2 with concentrations 100, 100, 1,000, and 1,000 ppm, respectively. The concentrations were many folds compared to the NH3 concentration of 45 ppm. However, the NH3 sensing response value of the PPy film was substantially greater for the other interfering gases. The results indicated that the film had an excellent specific identification capacity for NH3 gas. It was also obvious that the RH was as high as 94%, which could cause a capillary condensation on the PPy surface; the NH3-sensing response was much higher for responding to the high RH concentration. An effect of RH fluctuations on the NH3-sensing performance was conducted at various humidity concentrations of 11, 16, 33, 75, 84, and 94 %RH under the sensing response to 45 ppm NH3 (Figure 9b and c). The results showed that the sensing responses increased from 3.34 to 8.95% with the RH increase.

Figure 9 
                  Selectivity of the PPy film at 25 and 100°C (a) and its influence of humidity on NH3 gas sensing (b and c).
Figure 9

Selectivity of the PPy film at 25 and 100°C (a) and its influence of humidity on NH3 gas sensing (b and c).

3.3 NH3-sensing mechanism of PPy

In general, the interaction mechanism of PPy with proton-adsorbed H ad + and NH3 is presented by the whole equation as follows:

(1) ( ( PPy ) H ad + ) Cl + NH 3 ( ( PPy ) ( NH 4 + ) ad ) Cl .

H ad + resulted from polymerizing pyrrole monomers into PPy and adsorbed on polypyrrole chains [49]. Furthermore, it is well known that NH3 gas with an electronic configuration having a pair of 2pz 2 electrons plays as an electronic donating molecule [50]. While PPy modified by dopant possesses, it has conformational and structural changes of charge carriers such as soliton, polaron, and bipolaron structures [21], which NH3 molecules can be chemically adsorbed by bonding between the nitrogen atoms of NH3 molecular and C or/and N atoms in pyrrole rings having unoccupied orbitals (positive hole sites in polaron and bipolaron structures). In this adsorption, the 2pz 2 electron is partially transferred from NH3 molecules into positive hole sites and NH3 +• radicals. On the other path, NH3 molecules are physically adsorbed at C or/and N atoms having an odd electron or π electrons of conjugated systems via hydrogen bonds, having an electrostatic attraction between these electrons and hydrogen atoms in NH3 molecules. These adsorptions reduce the movement of electrons and holes through the π-conjugated system of PPy backbone chains and then increase the electric resistance of the PPy chains. During the desorption process, the electrons return from PPy chains to NH3 molecules until almost all NH3 molecules are removed the PPy resistance can be totally or partially recovered to its original value [51,52].

In more detail, Figure 10 represents the adsorption and desorption of NH3 molecules on adsorptive sites within the PPy backbone. Herein, the adsorptive interactions were depicted by dashed lines and arrows that presented the movement of electrons. The physical adsorption of NH3 molecules by the electrostatic interaction of hydrogen atoms of NH3 molecule and π electrons of the conjugated system decrease π electrons’ density at the adsorbed sites, so the double bonds were represented by one slight line and one bold line (before the adsorption represented by two bold lines). The NH3 chemisorption was conducted at the positive holes in polaron and bipolaron structures of the PPy chains.

Figure 10 
                  Illustration of mechanism of adsorption/desorption of NH3 molecules at adsorptive sites of PPy with formation of 
                        
                           
                           
                              
                                 
                                    
                                       (
                                       
                                          
                                             
                                                NH
                                             
                                             
                                                4
                                             
                                          
                                       
                                       )
                                    
                                 
                                 
                                    ad
                                 
                                 
                                    +
                                 
                              
                           
                           {({\text{NH}}_{4})}_{\text{ad}}^{+}
                        
                     .
Figure 10

Illustration of mechanism of adsorption/desorption of NH3 molecules at adsorptive sites of PPy with formation of ( NH 4 ) ad + .

4 Conclusion

The PPy film with highly ordered and uniform nanoparticles with the diameter of about 30–50 nm was successfully prepared by a chemical vapor-phase polymerization using FeCl3 oxidant. The sensing responses of the PPy film to different concentrations of NH3 gas were tested at the operating temperatures from 25 to 100°C. The PPy film displayed the highest sensing response of 143% for 350 ppm NH3 at 25°C, and the lowest sensing response of 37% for 45 ppm NH3 at 100°C. The results demonstrated a good linear relationship between the sensing response and the NH3 concentration. For the result of other interfering gases, the sensing response of the PPy film was significantly greater and exhibited an outstanding specific identification capacity for detecting NH3 gas.

The interaction of PPy with NH3 molecules was revealed when examining electron transfer between the adsorptive sites of PPy chains and NH3 molecules, caused increasing the film’s resistance. The interaction mechanism between NH3 molecules and the adsorptive sites of the PPy chains was considered to be the formation of polaron and bipolaron structures, as well as the mechanism of adsorption/desorption of NH3 molecules.

Acknowledgments

The authors are thankful for financial support by the project (code UTEHY.L.2020.09) from Hung Yen University of Technology and Education Technology.

  1. Funding information: This work was financially supported by the project (code UTEHY.L.2020.09) from Hung Yen University of Technology and Education Technology.

  2. Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.

  3. Conflict of interest: The authors state no conflict of interest.

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Received: 2022-12-09
Revised: 2023-01-30
Accepted: 2023-02-01
Published Online: 2023-02-20

© 2023 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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