Binary doped polypyrrole and polypyrrole/boron nitride nanocomposites: preparation, characterization and application in detection of liquefied petroleum gas leaks

Abstract

We report an electrical conductivity based rapid response liquefied petroleum gas (LPG) sensor using binary doped polypyrrole and polypyrrole/boron nitride (PPy/BN) nanocomposites as the conductive material. Binary doped PPy and PPy/BN nanocomposites have been synthesized using a chemical oxidative in situ polymerization method using FeCl₃ as an oxidant in the presence of camphorsulfonic acid (CSA). A PPy/BN nanocomposite has also been synthesized using a chemical oxidative in situ polymerization method using FeCl₃ as an oxidant. PPy/FeCl₃/CSA, PPy/BN/FeCl₃ and PPy/BN/FeCl₃/CSA were characterized using Fourier transform infrared spectroscopy, X-ray diffraction, thermogravimetric analysis and field emission scanning electron microscopy. On the basis of these results, the well-organized structural nanocomposites were successfully prepared owing to specific interactions between the PPy and the BN nanosheets. The results indicated that the morphology and electrical properties of the nanocomposites were significantly influenced by the BN nanosheet loading and CSA. Transformation of non-conducting PPy/BN/FeCl₃ into conducting PPy/BN/FeCl₃/CSA was also observed. Addition of CSA caused a significant increment in electrical conductivity due to binary doping of the nanocomposites. The as-prepared PPy/FeCl₃/CSA and PPy/BN/FeCl₃/CSA nanocomposites were studied for the change in their electrical conductivity on exposure to liquefied petroleum gas (LPG) and ambient air at room temperature with the possible use as a sensor for detection of LPG leaks.

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1. Introduction

Among various applications, conducting polymers have been used as sensors, due to their inherent electronic, optic and mechanical transduction mechanisms.^1–8 To enhance their sensitivity, considerable efforts have been focused on the fabrication of nanometer scale conducting polymer materials. The beneficial characteristics of these materials include their small dimensions, high surface area to volume ratio and amplified sensitivity for sensor-transducer applications.^9–13 Among the various morphologies of conducting polymer nanostructures, nanoparticles offer the advantages of being small-diameter particles for device fabrication, facile fabrication steps, and uniform size and uniform deposition for sensor electrode production without particle aggregation.^14–21

Among the various conducting polymers, polypyrrole (PPy) is especially promising in commercial applications because of its good environmental stability, facile synthesis and higher conductivity compared with many other conducting polymers. The use of PPy has been demonstrated in biosensors, gas sensors, wires, microactuators, antielectrostatic coatings, solid electrolytic capacitors, electrochromic windows, displays, polymeric batteries, electronic devices, functional membranes and so on.^22–29

Hexagonal boron nitride (h-BN) is a layered material consisting of two-dimensional (2D) atomically thin sheets of covalently bonded boron and nitrogen stacked together by weak van der Waals forces. Recently, researchers have been able to exfoliate bulk h-BN materials in large quantities to obtain high surface area h-BN sheets using a wet chemical approach and demonstrated its various applications.³⁰ Hexagonal boron nitride (h-BN) is an analogue of graphite and has tremendous applications in fields ranging from optical storage to medical treatment, photocatalysis and electrical insulation, due to its wide direct band gap (∼5.79 eV) as well as its excellent chemical stability and inoxidizability.^31,32 Recent reports demonstrate that lightweight nanocomposites have been prepared by incorporating thermally conductive nanomaterials such as carbon nanotubes, graphene and boron nitride (BN) nanosheets.^33–39 These individual nanomaterials have ultra-high thermal conductivity due to limited phonon scattering and high phonon velocity. BN is an electrical insulator with a dielectric constant of ∼3–4 ⁴⁰ and thus it is widely used in the thermal management of high power electronics and display applications, which is not possible by using CNTs and graphene.

Conducting polymer composites are of great technical interest as they exhibit a wide range of electrical, optical and magnetic properties. Besides, conducting polymer composites are prepared with various inorganic additives for improving the thermal properties of these materials. Highly fire-resistant materials such as boron phosphate and huntite are also used as additives. Besides, Shao et al. have also reported the effect of gas exposure on the electrical response of CNTs and graphene on account of ease of functionalization at a molecular level.⁴¹ Later, Saha et al. reported that the development of CNT networks has a direct influence on the electrical properties and potential applications of composite materials.⁴² Likewise, hexagonal boron nitride (h-BN) is one of the additives used for this purpose.^43,44

Herein, we present a simple strategy for the preparation of PPy/FeCl₃/CSA and PPy/BN/FeCl₃/CSA nanocomposites by oxidative polymerization of pyrrole. The effects of the addition of BN nanosheets and binary doping on the physicochemical properties are investigated. The morphology, thermal stability and electrical conductivity of the resulting PPy/FeCl₃/CSA and PPy/BN/FeCl₃/CSA nanocomposites were also investigated. These surface engineered products were also examined for their dynamic response of electrical conductivity towards LPG using a simple 4-in-line probe electrical conductivity measurement set up.

2. Experimental

2.1 Materials

Pyrrole 99% (Spectrochem, India), ferric chloride anhydrous (Fisher Scientific, India), boron nitride (MK Nano, Canada), camphorsulfonic acid (TCI, Tokyo) and methanol (E. Merck, India) were used as received. The water used in these experiments was double distilled.

2.2 Synthesis of binary doped PPy and PPy/BN nanocomposites

Pyrrole (0.05 mol) and camphorsulfonic acid (0.01 mol) in 100 mL of distilled water were mixed. A solution of ferric chloride (0.05 mol) in 100 mL distilled water was then added dropwise into the mixture. The reaction mixture was stirred continuously for about 20 h resulting in the formation of a black coloured solid. The product (PPy/FeCl₃/CSA) thus formed was filtered, washed several times with distilled water and methanol, and dried in an air oven at 80 °C for 6 h.

PPy/BN nanocomposites were prepared by oxidative polymerization of pyrrole in the presence of BN nanosheets. In a typical preparation, pyrrole (0.05 mol) and camphorsulfonic acid (0.01 mol) in 100 mL of distilled water were mixed. A uniform suspension of BN (100 mg) in 100 mL deionized water prepared by ultra-sonication for 1 hour was then added into the mixture. A solution of ferric chloride (0.05 mol) in 100 mL distilled water was then added dropwise into the mixture at room temperature with constant stirring. The colour of the solution changed from greenish to black indicating the polymerization of pyrrole. The resulting solution was then stirred for a further 20 hours. The resultant mixture was then filtered, and washed thoroughly with distilled water to remove unused reactants and byproducts until the filtrate became colourless. It was further washed thoroughly with methanol to remove any other impurities. The product (PPy/BN/FeCl₃/CSA) was dried at 80 °C for 6 hours. PPy/BN/FeCl₃ was also prepared using the same procedure mentioned above without using CSA. The nanocomposite materials were kept in a desiccator for further experiments. For the electrical conductivity measurements, 0.30 g of material from each sample was pelletized at room temperature with the help of a hydraulic pressure instrument at 100 kN pressure for 15 min.

3. Results and discussion

3.1 Preparation of PPy and PPy/BN nanocomposites

Polypyrrole and polypyrrole/BN nanocomposites were prepared by oxidation of pyrrole in aqueous medium using FeCl₃ as an oxidant in the presence and absence of CSA. Thus, the prepared PPy/FeCl₃/CSA, PPy/BN/FeCl₃ and PPy/BN/FeCl₃/CSA were labelled as PPy-2, PPy/BN-1 and PPy/BN-2, respectively, where 1 stands for singly doped and 2 stands for binary doped samples as shown in Fig. 1.

Fig. 1 Schematic presentation of the formation of PPy-2, PPy/BN-1 and PPy/BN-2.

3.2 Characterization

The morphology, structure and chemical composition of PPy and PPy/BN nanocomposites were characterized using a variety of techniques including Fourier transform infrared spectroscopy (FTIR) carried out using a Perkin-Elmer 1725 instrument, X-ray powder diffraction (XRD) and field emission scanning electron microscopy carried out using a LEO 435-VF. PPy and PPy/BN nanocomposites were studied in terms of their DC electrical conductivity retention under isothermal and cyclic ageing conditions. A four-in-line probe with a temperature controller PID-200 (Scientific Equipment, Roorkee, India) was used to measure the DC electrical conductivity and its temperature dependence. The DC electrical conductivity was calculated using the following equation:where I, V, W and S are the current (A), voltage (V), thickness of the pellet (cm) and probe spacing (cm), respectively, and σ is the conductivity (S cm⁻¹).⁴⁵ In isothermal stability testing, the pellets were heated at 50 °C, 70 °C, 90 °C, 110 °C and 130 °C and the DC electrical conductivity was measured at an interval of 10 min in the accelerated ageing experiments. In the case of the cyclic ageing technique, DC conductivity measurements were taken 5 times at an interval of about 80 min within the temperature range of 40–150 °C. In the sensing experiment, two pellets of PPy-2 and PPy/BN-2 were tested for their LPG sensing property. A cigarette lighter operated by commercially available dry LPG with a regulated flow rate of 16.66 ppm per s was used in the experiment.

3.3 FT-IR spectroscopic study

The FT-IR spectra of the PPy-2 and PPy/BN nanocomposites were recorded on KBr pellets. The FTIR spectrum of PPy-2 shows the main absorption peak at 3429 cm⁻¹ corresponding to the stretching vibration of N–H bonds as shown in Fig. 2. The characteristic absorption bands for C=C, C=N and C–N stretching frequencies were obtained at 1539, 1304 and 1172 cm⁻¹, respectively. In the case of PPy/BN nanocomposites the N–H, C=C, C=N and C–N stretching frequencies were observed at 3419, 1558, 1313 and 1192 cm⁻¹, respectively. The absorptions bands obtained at around 1380 cm⁻¹ can be attributed to BN stretching. The slight increase in the N–H, C=C, C=N and C–N stretching frequencies is probably attributed to the interaction of PPy and BN nanosheets. Although the stretching frequencies corresponding to the NH bonds of PPy were different from that in PPy/BN, their bending frequencies were observed to be at the same frequencies and were approximately obtained at 1030 cm⁻¹. The prominent absorption band of CSA was clearly observed at 1633 cm⁻¹, which evidently supports the formation of PPy-2 and PPy/BN-2 nanocomposites.

Fig. 2 FT-IR spectra of: (a) PPy-2, (b) PPy/BN-1, (c) PPy/BN-2 and (d) BN.

3.4 X-ray diffraction (XRD) analysis

The crystal structure of the as-prepared PPy-2 and PPy/BN nanocomposites was characterized by XRD. Fig. 3a shows the spectra of PPy-2. The appearance of a broad peak in the region of 2θ = 26.38° in PPy-2 suggests the presence of polypyrrole. Fig. 3b shows the XRD spectrum of the PPy/BN-1 nanocomposite. The spectrum shows the peaks corresponding to both PPy and BN. Fig. 3c shows the spectrum of PPy/BN-2. The figure shows the peaks relating to PPy and BN, which suggests that the structure of the nanocomposite is not altered by the addition of CSA. The broad peak observed at 2θ = 26.26°, suggests that the amorphous nature of polypyrrole and the crystalline nature of BN has been merged and shifted from 25.66° in the nanocomposite. Fig. 3d shows that the peaks at 2θ values of 25.66°, 37.10°, 40.83°, 43.55°, 49.24°, 54.23° and 76.81° correspond to BN.

Fig. 3 XRD spectra of: (a) PPy-2, (b) PPy/BN-1, (c) PPy/BN-2 and (d) BN.

3.5 Thermogravimetric analysis (TGA)

The amount of weight loss and thermal stability of the PPy-2, PPy/BN-1 and PPy/BN-2 nanocomposites were determined by means of TGA in the range of 40–600 °C. From Fig. 4, it may be observed that the degradation process involved the loss of water in PPy and PPy/BN nanocomposites, the elimination of the dopant and the dedoping process of the polymer. It is clear that decomposition of PPy-2 started at approximately 191 °C, while at 226 °C for the PPy/BN-2 nanocomposite, and all had three weight loss processes. The PPy/BN and PPy/BN-2 nanocomposites show a higher thermal stability than that of PPy-2. The shift in decomposition temperature may be related to the interaction between CSA and BN. It is found that in CSA with PPy/BN, little difference could be observed in the thermal stability of the nanocomposites but with an obvious decrease in the residual weight. The nanocomposites synthesized with and without CSA showed better thermal stability than pure PPy-2 with increased temperature.

Fig. 4 TGA curves of: (a) PPy-2, (b) PPy/BN-1 and (c) PPy/BN-2.

3.6 Scanning electron micrograph studies

The morphologies and shapes of pure BN, PPy-2, PPy/BN-1 and PPy/BN-2 nanocomposites were characterized using FE-SEM and the obtained images are presented in Fig. 5.

Fig. 5 FE-SEM images of: (a) BN, (b) PPy-2, (c) PPy/BN-1 and (d) PPy/BN-2 nanocomposites showing granular/spherical structures at different magnifications.

The FE-SEM micrograph with high magnification (Fig. 5a) clearly shows the nanosheet like structure of BN. The FE-SEM image of PPy-2 nanoparticles is shown in Fig. 5b. It shows that the synthesized polypyrrole is agglomerated by several nanoparticles. The as-prepared PPy/BN-1 is composed of granules approximately 100–200 nm in diameter (Fig. 5c) and the granular structure of PPy is associated with the BN. In Fig. 5d PPy/BN-2 shows the granular/spherical structures associated with CSA. The effect of CSA on the PPy/BN nanocomposite morphology is seen clearly in Fig. 5c and d. The nanocomposite prepared with CSA exhibited a less compact morphology and seems to be more regular as evident from Fig. 5d, whereas the sample prepared without the surfactant exhibits a very dense and compact structure as shown in Fig. 5c. As the concentration of the incorporated CSA increased, less agglomeration and a better dispersion were obtained. The nanocomposites consisting of BN bind to the surface of large PPy polymer granules and their size remains unchanged due to the mild conditions of the in situ polymerization. The BN is well dispersed in the nanocomposites and no free BN nanoparticles were present, which indicates that the BN nanoparticles and CSA have a nucleating effect on the pyrrole polymerization and caused a homogeneous PPy shell around them.

4. Electrical conductivity

The DC electrical conductivity of PPy-2 and PPy/BN nanocomposites with and without CSA was measured using a 4-in-line probe technique. The electrical conductivity showed a decrease from 0.466 S cm⁻¹ to 0.234 S cm⁻¹ after loading boron nitride with CSA, but the PPy/BN-1 nanocomposites did not show electrical conductivity. In order to complete its octet, B atoms interact with the lone pair of electrons of N atoms of BN and thus B–N bonds become extremely polar. The highly polar B–N bonds strongly bind the polarons of the PPy and the counterion FeCl₄⁻ and lock them. This causes the loss of mobility in the polarons leading to the loss of electrical conductivity in PPy/BN as shown in Fig. 6. In the CSA doped PPy/BN-2 nanocomposites, the addition of BN increases the compactness of the sample, causing lesser coupling through the grain boundaries which in turn reduces the electrical conductivity. Whereas, in the absence of CSA in the PPy/BN-1 nanocomposites, no electrical conductivity was observed which may be due to the locking of positive charges on PPy by BN and therefore the electrical conductivity is lost due to non-mobile positive charges. With the addition of CSA, a higher crystallinity is obtained which facilitates an improvement in the charge transfer mechanism and hence, results in increased electrical conductivity.⁴⁶ The increase in electrical conductivity may also be due to the unlocking of polarons or bipolarons because of the interaction of CSA. The electrical conductivity is regained as positive charges become mobile, as shown in Fig. 1.

Fig. 6 Initial DC electrical conductivity of: (a) PPy-2, (b) PPy/BN-1 and (c) PPy/BN-2 nanocomposites.

4.1 Isothermal stability studies

The stability, in terms of the DC electrical conductivity retention, of the PPy-2 and PPy/BN-2 nanocomposites was studied under isothermal ageing conditions as shown in Fig. 7. The relative electrical conductivity was plotted against time for each temperature as given in the equation below:where σ_r,t = relative electrical conductivity at time t, σ_t = electrical conductivity at time t, and σ₀ = electrical conductivity at time zero.

Fig. 7 Change in the relative electrical conductivity of: (a) PPy-2 and (b) PPy/BN-2 nanocomposites under isothermal ageing conditions.

To investigate the stability, in terms of the DC electrical conductivity retention, of these materials, the best method is to compare the relative electrical conductivity with respect to time at different temperatures for different samples. The DC conductivity of the samples (5 readings of each sample were taken at an interval of 5 min) was measured at the temperatures 50, 70, 90, 110 and 130 °C. Fig. 7a shows that the electrical conductivity of PPy-2 is fairly stable at 50 °C, 70 °C and 90 °C. In the case of PPy/BN-2, the electrical conductivity is fairly stable at 50 °C, 70 °C and 110 °C, as shown in Fig. 7b. The instability shown by PPy/BN-2 at 90 °C seems to be due to instrumental deviation. Thus, PPy-2 is observed to be more stable than PPy/BN-2 in terms of electrical conductivity under the isothermal ageing condition.

4.2 Stability under cyclic ageing

The stability, in terms of the DC electrical conductivity retention, of the PPy-2, PPy/BN-1 and PPy/BN-2 nanocomposites was also studied using a cyclic ageing technique, also within the temperature range of 40 °C to 130 °C, as shown in Fig. 8. The conductivity measurements were also recorded for subsequent cycles and it was observed that the conductivity decreased gradually from the first to fifth cycle, showing a regular trend in all cases. The relative electrical conductivity was calculated using the following equation:where σ_r is the relative electrical conductivity, σ_T is the electrical conductivity at temperature T (°C) and σ₄₀ is the electrical conductivity at 40 °C.

Fig. 8 The relative electrical conductivity of: (a) PPy-2 and (b) PPy/BN-2 nanocomposites under cyclic ageing conditions.

The decrease in electrical conductivity with the introduction of BN in the nanocomposite structure is supposed to be due to the insulating behaviour of boron nitride, because its outer electrons are bound by nitrogen atoms, thus the hindrance in the transport of carriers between different molecular chains of PPy and the interaction at the interface of PPy and boron nitride probably led to the reduction of the conjugation length of PPy in the nanocomposites. In the case of the PPy/BN-2 nanocomposites the electrical conductivity increases due to the doping of CSA and increase in the number of charge carriers, which can be connected to the delocalization effect of doping and formation of the polarons or bipolarons in the nanocomposite structure, thus enhancing the electrical conductivity of the nanocomposites.

4.3 I–V studies

The current versus voltage plots of the PPy-2, PPy/BN-1 and PPy/BN-2 nanocomposites are shown in Fig. 9. The I–V characteristics of these samples were recorded at 80 °C. The PPy-2 and PPy/BN-2 nanocomposites showed that current increases with the increase in voltage while PPy/BN-1 does not show this effect. From Fig. 9, it is observed that a higher electrical conductivity is shown by PPy-2 than by PPy/BN-2, as electrical conductivity is inversely proportional to voltage. The PPy-2 and PPy/BN-2 showed ohmic variations which are fairly regular with respect to the applied voltage. This linear increase in current with the applied voltage is related to the conduction mechanism of PPy and its nanocomposites. PPy/BN-1 showed the I–V behaviour of an insulator.

Fig. 9 I–V characteristics of: (a) PPy-2, (b) PPy/BN-1 and (c) PPy/BN-2 nanocomposites.

5. LPG sensing

Though LPG is known as one of the most common domestic fuels, its leakage even at ppm levels may cause irritation and breathing problems in human beings. LPG is comprised mainly of butane and traces of ethyl mercaptan for its characteristic odor that causes irritation and breathing problems. To study this, we became interested in examining LPG leaks using the materials prepared, in view of –SH groups of ethyl mercaptan having a strong tendency to interact with molecules like PPy and BN at room temperature (∼25 °C). The source of LPG was a commercial lighter with a regulated flow rate, and the amount of LPG used for the sensing study was 2 g.

Dhawale et al. have synthesized a polyaniline/ZnO nanocomposite by electrodepositing polyaniline on a chemical bath deposited ZnO film. They investigated liquefied petroleum gas (LPG) detection using their as prepared nanocomposite, and compared this with N₂ and CO₂ gases and LPG exhibited the maximum response upon exposure of LPG.⁴⁷ Later, S. Barkade et al. synthesized a PPy/ZnO nanocomposite using an in situ miniemulsion polymerization of pyrrole, which is suitable for liquefied petroleum gas (LPG) sensor development. They observed that the controlled size of the hybrid particles using this synthesis strategy minimizes the response time for sensing LPG significantly.⁴⁸ However, we have observed that the effect of ambient air exposure to the polypyrrole decreases the electrical conductivity in our previous work.⁴⁹ But, herein, we have tried to forward our continued efforts and examine the effect of LPG molecules on the electrical conductivity (Fig. 10).

Fig. 10 Real set up of the gas sensor unit and schematic of the four probe measurement unit.

The LPG gas sensitivity of PPy-2 was analysed by measuring the changes in the electrical conductivity at room temperature. The response time and sensing intensity are the two different factors by which the gas sensitivity of PPy-2 was investigated. Gas sensitivity was measured for 60 seconds, after which the pellet was exposed to air for a further 60 seconds. When PPy-2 was exposed to the gas, it was observed that the electrical conductivity decreased with the increase in time. The reason for this observed decrease may be due to the nucleophilic property of the thiol group of ethyl mercaptan, the inherent component of LPG which donates its electron density to electron deficient polypyrrole. This decreases the mobility of charge carriers leading to a decrease in the electrical conductivity. This lowering in charge carriers along with decreased delocalization causes the drop in electrical conductivity. When the pellet was exposed to air, the electrical conductivity started to increase with time and reached its maximum value after 60 seconds, due to the desorption of ethyl mercaptan from the surface of PPy-2 (Fig. 11).

Fig. 11 Effect on the DC electrical conductivity of PPy-2 on exposure to LPG with respect to time.

The DC electrical conductivity was measured in order to evaluate the reversibility response of PPy-2. This reversibility was measured by first keeping the sample in gas for 10 s followed by 10 s in air for a total duration of 120 seconds. As observed from Fig. 12 the material shows good reversibility.

Fig. 12 Variation in the DC electrical conductivity of PPy-2 on alternating exposure to LPG and air.

The gas sensitivity of PPy/BN-2 was analysed by measuring the changes in the electrical conductivity at room temperature. Gas sensitivity was measured for 60 seconds, after which the pellet was exposed to air for a further 60 seconds. When PPy/BN-2 was exposed to the gas, it was observed that the electrical conductivity sharply decreased with the increase in time, and then levelled off for the reasons mentioned in previous paragraph. When the pellet was exposed to air, the electrical conductivity started to increase with time, and then decreased and became constant. It may be suggested that the nanocomposite is composed of two components viz. binary doped PPy and BN salt with CSA. The two components interact with the lone pair of ethyl mercaptan differently i.e. a reversible interaction with the binary doped polypyrrole and an irreversible interaction with the BN salt with CSA (Fig. 13).

Fig. 13 Effect on the DC electrical conductivity of PPy/BN-2 on exposure to LPG with respect to time.

The DC electrical conductivity of the PPy/BN-2 composite was also measured in order to evaluate the reproducibility response as was done in the previous case. The reproducibility was measured by first keeping the sample in gas for 10 s followed by 10 s in air for a total of 120 seconds. It may be observed from Fig. 14 that the nanocomposite showed poor reversibility compared to PPy-2, which confirms that no desorption of gas molecules from the surface occurs.

Fig. 14 Variation in the DC electrical conductivity of PPy/BN-2 on alternating exposure to LPG and air.

5.1 Proposed mechanism for LPG sensing

The sensing mechanism of LPG gas was explained on the basis of adsorption and desorption processes through the DC electrical conductivity at room temperature. A variation occurs in the DC electrical conductivity after exposure to LPG. As the PPy-2 is exposed to LPG, the electrical conductivity decreases. The decrease in electrical conductivity is attributed to the decrease in charge carriers as the LPG molecules reach the surface of PPy-2. When the pellet is then exposed to air, the electrical conductivity reaches back to the starting value. In the case of the PPy/BN-2 nanocomposite, the DC electrical conductivity decreases after exposure to LPG and remains almost the same after exposure to air. The change in DC electrical conductivity may be due to physico-chemical adsorption between the nanocomposite and absorbed gas molecules (Scheme 1).

Scheme 1 Proposed interaction between LPG gas and (a) PPy-2 and (b) PPy/BN-2.

The chemical composition of LPG is butane, propane, traces of ethyl mercaptan and other hydrocarbons. At room temperature, only ethyl mercaptan is reactive, butane, propane and the other hydrocarbons do not react. In the case of PPy-2, the ethyl mercaptan is adsorbed on the surface via interactions between the lone pairs of sulfur and the polaron of the polypyrrole ring. This interaction leads to the localization of polypyrrole ring electrons which in turn causes the decrease in the mobility of charge carriers, thereby decreasing the electrical conductivity.

In the case of the PPy/BN-2 nanocomposite, the lone pairs of nitrogen and the polarons of the polypyrrole ring interact with the electron deficient nitrogen and negatively charged boron atoms of the boron nitride ring, respectively. As the LPG gas interacts with this nanocomposite, the lone pairs of the sulfur atoms of ethyl mercaptan bind strongly with the polarons of the polypyrrole ring, which decreases the electrical conductivity. After the composite is exposed to air, the irreversible nature of conductivity is observed because of the strong interaction of ethyl mercaptan with the polarons of the polypyrrole ring.

6. Conclusion

The PPy/BN nanocomposite with and without CSA was successfully synthesized using an in situ polymerization method. FTIR and XRD analysis confirmed the presence of BN and PPy. FESEM analysis shows the uniform dispersion of the BN in the polypyrrole and the effect of CSA can be clearly seen. DC electrical conductivity was observed with the addition of CSA in the PPy/BN nanocomposite. The results highlighted that this material can be applied in the gas sensing field to develop LPG sensors with performances suitable for practical application. Therefore, this sensor based on PPy/BN-2 may be useful for single shot investigations i.e. as a dosimeter.

Acknowledgements

Adil Sultan gratefully acknowledges Mr. Nayeem Ahmed for helpful discussions.

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