zinc oxide nanocomposite

Materialia 9 (2020) 100599

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DC electrical conductivity and liquefied petroleum gas sensing application of polythiophene/zinc oxide nanocomposite Ahmad Husain a, Mohd Urooj Shariq b, Faiz Mohammad a,∗ a b

Department of Applied Chemistry, Zakir Husain College of Engineering and Technology, Aligarh Muslim University, Aligarh 202002, India Department of Chemistry, Faculty of Science, Aligarh Muslim University, Aligarh 202002, India

a r t i c l e

i n f o

Keywords: Polythiophene/zinc oxide nanocomposites Liquefied petroleum gas sensor DC electrical conductivity

a b s t r a c t Despite being a versatile fuel, liquefied petroleum gas (LPG) is extremely flammable even at very low concentration of 1000 ppm. Therefore, detection of LPG below the 1000 ppm is necessary for safety point of view. Herein, we are reporting the synthesis of polythiophene (PTh) and polythiophene/zinc oxide nanocomposite (PTh/ZnO) through in-situ chemical oxidative polymerization method. The structure and morphology of as-synthesized materials were determined by various techniques such as SEM, FTIR, XRD and TEM. The stability of DC electrical conductivity of PTh and PTh/ZnO nanocomposites under an accelerated isothermal as well as cyclic ageing conditions were evaluated. Both the conductivity and stability of PTh significantly improved by incorporating a small amount of ZnO nanoparticles. We also studied LPG sensing performances of pelletized form sensors based on PTh and PTh/ZnO-3 by alternate exposure in LPG atmosphere and ambient air. The lower detection limit of PTh/ZnO-3 was found to be 600 ppm. PTh/ZnO-3 based sensor exhibited about 1.58 and 4.9 times greater sensing efficiency in terms of sensing response and reversibility as compared to the pristine PTh based sensor at 2400 ppm.

1. Introduction Liquefied petroleum gas (LPG) is extensively used as a fuel in households throughout the world. It mainly comprises of a blend of hydrocarbons primarily the saturated ones like butane and propane. Ethyl mercaptan, an odorizing agent, added to LPG for safety purposes causes irritation and respiratory troubles. LPG is vulnerable to fire even at scarce concentrations, although being a versatile fuel. National Institute for Occupational Safety and Health (NIOSH, USA) and the Occupational Safety and Health Administration (OSHA, USA) recommended that maximum permissible exposure of LPG should be 1000 ppm. Upon its leakage, it can lead to brutal hazards to human lives and may result in a disaster. As it is used extensively, it needs to be selectively and rapidly detected below 1000 ppm to prevent any catastrophe from occurring [1–7]. Over the past two decades, nanostructured materials have introduced a new offshoot to material science. This branch of material science has been developed and studied to a great extent during the course of these years. These materials display a vast array of excellent optical, electrical and mechanical properties [8–11]. They have been engaged to manufacture energy storage devices [12], sensors [13–15], electrochromic and electro-optic devices [16–18]. Nowadays, the nanocomposites of conductive polymers and inorganic materials are amongst the most studied categories of materials. The encapsulation/amalgamation ∗

of inorganic nanoparticles into the fundamentally conducting polymers is the method widely used for synthesizing these nanocomposites [19–23]. By incorporating nanoparticles in the conducting polymers, high selectivity and sensitivity are induced in them, which further enhances their performance when employed in gas sensing applications [24]. There are many conducting polymers available nowadays such as polythiophene, polyaniline, polyacetylene, polypyrrole and polyindole exhibiting excellent qualities for instance high electrical conductivity, good chemical as well as environmental stability [25–32]. Amongst the different conducting polymers present today, nanomaterials based on polythiophene are one of the most widely studied and used in various technological applications due to their environmental stability and outstanding electrical, optical, thermal and mechanical properties [33–40]. Over the years, owing to outstanding electrical, mechanical and optical properties (a bandgap of about 3.3 eV), zinc oxide (ZnO) nanoparticle has been found extremely valuable in the numerous technological fields like photocatalysis, sensors, biomedical applications, solar cells and supercapacitors [3,5,6,41–44]. Recently various reports have been published on LPG sensors based on nanocomposites of conducting polymers. A PTh/SnO2 nanocomposite based sensor was synthesized by Barkade et al. which exhibited high sensitivity while sensing LPG at ambient temperatures [45]. In another study, Barkade et al. employed the in-situ mini-emulsion polymerization technique

Corresponding author. E-mail address: faizmohammad54@rediffmail.com (F. Mohammad).

https://doi.org/10.1016/j.mtla.2020.100599 Received 21 October 2019; Accepted 22 January 2020 Available online 23 January 2020 2589-1529/© 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

A. Husain, M.U. Shariq and F. Mohammad

Materialia 9 (2020) 100599

for synthesizing PPy/ZnO nanocomposite [1]. They observed that the response time required for sensing the LPG was reduced to a minimum owing to the controlled dimension of the particles of the as-prepared nanocomposites. Sultan et al. synthesized a PPy/BN nanocomposite based highly efficient LPG sensor [46]. In our recent work, we have reported an ethene gas sensor based on PTh/ZrO2 nanocomposites which showed good sensitivity, reversibility and selectivity [47]. In another study, we have synthesized PTh/Graphene nanocomposite by in-situ chemical oxidative polymerization method. PTh/Graphene nanocomposite has been utilized to fabricate a novel ethanol sensor [48]. Encouraged by these prospects, we have prepared PTh and its nanocomposites with ZnO nanoparticles consisting of different weight per cent of ZnO viz. 3%, 6% and 9%. In our best knowledge, this the first attempt to utilized PTh/ZnO nanocomposite based sensor for the detection of LPG leakage at room temperature. We have investigated the sensing response of PTh/ZnO-3 at different concentrations of LPG such as 600 ppm, 1200 ppm, 1800 ppm and 2400 ppm. 2. Experimental 2.1. Materials Thiophene (E. Merck), Zinc oxide nanoparticles, APS 10–30 nm (US Research Nanomaterials Inc.), Chloroform (Fisher Scientific), anhydrous ferric chloride (CDH), methanol (E. Merck) and acetone (E. Merck) were utilized in their original form. Double distilled water was utilized in carrying out the experiments. 2.2. Preparation of polythiophene (PTh) and polythiophene (PTh)/zinc oxide (ZnO) nanocomposites The method employed for preparing PTh and PTh/ZnO nanocomposites was in-situ chemical oxidative polymerization [47]. During this process, thiophene monomers were polymerized on the surface of ZnO nanoparticles utilizing chloroform as a solvent along with anhydrous FeCl3 acting as an oxidant. In the usual procedure, 2 mL (25.00 mmol) thiophene monomers were dissolved in 40 mL of chloroform and then ultrasonicated for 25 min. Further, a known amount of ZnO nanoparticles (3%, 6% and 9%) were added to 60 mL of chloroform and then ultrasonicated for 30 min. After that, the solution containing ZnO nanoparticles was transferred into the thiophene solution. Then, this mixture was subjected to ultrasonication for the total duration of 90 min. During the ultrasonication process, the thiophene molecules were adsorbed on the surface of ZnO nanoparticles. After that in 100 mL of chloroform, 16.24 g (100 mmol) of ferric chloride was dissolved and was stirred for 20 min till a homogeneous suspension was made. This was followed by the dropwise addition of the as-prepared FeCl3 suspension to the thiophene and ZnO mixture accompanied with uninterrupted stirring for 20 h on the magnetic stirrer. Then the as-synthesized PTh/ZnO nanocomposite was filtered along with washing several times utilizing methanol subsequently by double distilled water and in the end with the acetone. In the course of washing, as soon as the methanol was added, there was a visible change in the color of the materials from deep black to dark brownish. Then the synthesized materials were retained for drying in a vacuum oven at 60 °C for the duration of 24 h. Finally, these materials were converted to a fine powder by crushing in a mortar and pestle. The PTh/ZnO nanocomposites comprising 3%, 6% and 9% by weight of ZnO nanoparticles have been recognized as PTh/ZnO-1, PTh/ZnO-2 and PTh/ZnO-3 respectively. The pristine PTh nanoparticles were prepared by employing an identical process. 2.3. Morphological and structural characterization Morphological and structural characterizations of PTh and PTh/ZnO3 were carried out by XRD, FTIR, SEM and TEM techniques with the help of the Bruker D8 diffractometer with Cu K𝛼 radiation at 1.5418 Å,

Fig. 1. FT-IR spectra of: (a) PTh, (b) ZnO nanoparticles, (c) PTh/ZnO-1, (d) PTh/ZnO-2 and (e) PTh/ZnO-3.

Perkin-Elmer 1725 instrument on KBr pellets, JEOL, JSM, 6510-LV (Japan) and JEM 2100, JEOL (Japan) respectively. DC electrical conductivity and sensing experiments were performed by four-in-line probe instrument attached with the PID controlled oven manufactured by Scientific Equipment, Roorkee, India. All the synthesized materials were evaluated for their thermal stability as a function of their DC electrical conductivity under isothermal and cyclic ageing environments. The following equation was utilized for the evaluation of conductivity: [ ] [ ] 𝜎 = ln 2(2S∕W) ∕ 2𝜋S(V∕I) (1) where: V, S, W and I stands for the voltage (V), probe spacing (cm), the pellet- thickness (cm), current (A) and 𝜎 represents the DC electrical conductivity (Scm−1 ) respectively [47,48]. 250 mg of each sample was converted to pellet form at room temperature by employing a hydraulic pressure instrument operating at a pressure of 70 kN for 3 min. The pellet- thickness of PTh, PTh/ZnO-1, PTh/ZnO-2, PTh/ZnO-3 and ZnO nanoparticles were 0.94 cm, 0.96 cm, 0.96 cm, 0.95 cm and 1.03 cm respectively. To test the isothermal stability, the pellets were heated at temperatures of 40 °C, 60 °C, 80 °C, 100 °C and 120 °C. The DC electrical conductivities of all the samples were evaluated at a specific temperature at an interval of every 4 min. The DC electrical conductivity calculations were carried out four times inside the temperature range of 40–120 °C for the evaluation of stability under cyclic ageing environments. 3. Results and discussion 3.1. FT-IR studies The FTIR spectra related to PTh, PTh/ZnO-1, PTh/ZnO-2, PTh/ZnO3 and ZnO nanoparticles are presented in Fig. 1. For PTh, the wide peak at nearby 3401 cm−1 may be recognized as the O–H stretching vibration. The peaks at 1622 cm−1 and 1319 cm−1 may pertain to the C=C, and C–C stretching mode of vibration of the thiophene rings correspondingly. The peaks at 1196 cm−1 , 1105 cm−1 and 1024 cm−1 may be assigned to the C–H bending vibration of PTh. The peak at around 783 cm−1 reveals the out of plane deformation mode related to the C–H in thiophene as a consequence of polymerization taking place. The peak at 639 cm−1 was as a result of the bending mode of vibration of the C-S bond present in thiophene ring. The band at 470 cm−1 resembles C-S-C ring deformation mode. The peaks at 2852 cm−1 and 2928 cm−1 may be

A. Husain, M.U. Shariq and F. Mohammad

Materialia 9 (2020) 100599

3.2. X-ray diffraction analysis Fig. 2 reveals the XRD patterns related to PTh, PTh/ZnO-1, PTh/ZnO2, PTh/ZnO-3 and ZnO nanoparticles. In the case of PTh, the wide diffraction peak detected in the range of 2𝜃 =12° to 18° designate the amorphous nature of the polymer [47,48]. In case of ZnO nanoparticles, the peaks observed at 2𝜃 = 31.68°, 34.36°, 36.20°, 47.68°, 56.70°, 62.94°, 67.84° and 69.08° may correspond to the (100), (002), (101), (102), (110), (103), (112) and (201) planes respectively [49,50]. In the case of PTh/ZnO nanocomposites, the maxima of this broad peak of PTh moved to a greater angle with the incorporation of ZnO nanoparticles due to electronic interaction of PTh with ZnO nanoparticles. The presence of ZnO nanoparticles in the PTh/ZnO nanocomposites can be confirmed by the presence of the characteristic peaks of ZnO nanoparticles. 3.3. Scanning electron microscopic (SEM) studies

Fig. 2. XRD patterns of: (a) PTh, (b) ZnO nanoparticles, (c) PTh/ZnO-1, (d) PTh/ZnO-2 and (e) PTh/ZnO-3.

credited to C–H stretching vibrations. All the peaks observed in the spectrum of PTh were similar to the previously published reports [47,48]. In the spectrum of ZnO nanoparticle, the peaks at 1706 cm−1 , 1628 cm−1 , 1556 cm−1 , 1410 cm−1 , 1090 cm−1 , 816 cm−1 , 632 cm−1 and 486 cm−1 were similar to the previously published literature [49]. In the case of PTh/ZnO nanocomposites, the characteristic peaks of both the PTh and ZnO were observed. Also, a slight shifting of most of the characteristic peaks of both the PTh and ZnO may be due to the existence of some electronic interaction between PTh with ZnO nanoparticles, as shown in Fig. 5b.

Fig. 3 displays the morphology related to PTh, PTh/ZnO-1, PTh/ZnO-2 and PTh/ZnO-3.The SEM image of PTh (Fig. 3a) displays that the surface is composed of thick flakes with several small tubes like structure which provide slightly porous morphology which is considered very beneficial for sensing applications. All three PTh/ZnO nanocomposites showed similarity in morphology. The morphological modifications between PTh and PTh/ZnO-3 could be realized which in case of PTh/ZnO-3 appears to be composed of flakes and sheet-like structures making it highly porous (Fig. 3d). Also, in the case of PTh/ZnO-3, no free ZnO nanoparticles were detected, which shows that ZnO nanoparticles were successfully covered by PTh matrix. 3.4. Transmission electron microscopic (TEM) studies In Fig. 4, the TEM images of PTh and PTh/ZnO-3 at different magnifications are presented. The flaky and tubular morphology of PTh could Fig. 3. SEM micrographs of: (a) PTh, (b) PTh/ZnO-1, (c) PTh/ZnO-2 and (d) PTh/ZnO-3.

A. Husain, M.U. Shariq and F. Mohammad

Materialia 9 (2020) 100599

Fig. 4. TEM micrographs of: (a) PTh and (b, c and d) PTh/ZnO-3.

be observed in the TEM image of PTh as depicted in Fig. 4a. In the TEM images of PTh/ZnO-3 (Fig. 4b, c & –d), the PTh matrix and ZnO nanoparticles are indicated by grey background and dark black colored parts, respectively. These TEM images showed that thiophene monomers were successfully and homogeneously polymerized over the surface of ZnO, i.e. ZnO got incorporated or encapsulated into the matrix of PTh.It may also be established that ZnO nanoparticles were evenly dispersed inside the PTh matrix. The polymerization of thiophene monomers over the vast surface of ZnO nanoparticles provide a highly efficient 𝜋-conjugated system facilitating an extra movement of charge carriers that is why PTh/ZnO-3 shows significantly improved DC electrical conductivity and sensing ability towards LPG. 3.5. DC electrical conductivity The methodology used for calculating the initial DC electrical conductivity of PTh, PTh/ZnO nanocomposites and ZnO nanoparticles was the standard four-in-line-probe technique. The calculated conductivities related to PTh, PTh/ZnO-1, PTh/ZnO-2, PTh/ZnO-3 and ZnO nanoparticles were about 0.000568, 0.000924, 0.00371, 0.00893 and 0.9185 Scm−1 respectively. It was evident from Fig. 5a that upon increasing the loading of ZnO nanoparticles, there was a rise in the electrical conductivity of the PTh/ZnO nanocomposites. It may be inferred that upon loading of ZnO nanoparticles in the PTh the electrical conductivities of nanocomposites were considerably boosted courtesy to the following reasons: (1) the creation of an efficient arrangement of the 𝜋conjugated system of PTh on the large surface area of ZnO nanoparticles.

(2) then an electronic interaction between the lone pairs of electrons on the sulphur atoms of PTh with Zn2+ ions of ZnO nanoparticles leads to an increase in the number of polarons in PTh.(3) As a consequence of this phenomenon, the polarons present in PTh show an increase in their mobility over the vast surface area of ZnO nanoparticles, resulting in increased DC electrical conductivity. 3.5.1. Retention of DC electrical conductivity under isothermal ageing condition The as-synthesized PTh and PTh/ZnO nanocomposites were investigated for their stability with respect to DC electrical conductivity retaining ability under isothermal ageing conditions, as depicted in Fig. 6. The following equation was utilized for the calculation of the relative DC electrical conductivity (𝜎 r, t ) at a specific temperature: σr,t =

σt σ0

(2)

In this equation, 𝜎 t and 𝜎 o are representative of the DC electrical conductivities at time t and zero, respectively [47]. It is pretty apparent from Fig. 6 (a and b) that PTh and PTh/ZnO-1 exhibited good stability at temperatures up to 60 °C and 80 °C respectively. In the case of PTh, direct heat at the temperature above 60 °C resulted in a regular dropping of conductivity which may be caused by damage of materials as well as the loss of the doping agent. While it is evident from Fig. 6c that the PTh/ZnO-2 appeared to show stability at 40 °C, 60 °C, 80 °C and 100 °C however the stability of PTh/ZnO-3 is greatly enhanced up to 120 °C as illustrated in Fig. 6d. PTh/ZnO-3 performs like a semi-

A. Husain, M.U. Shariq and F. Mohammad

Materialia 9 (2020) 100599

Fig. 5. (a) Initial DC electrical Conductivity of PTh, PTh/ZnO-1, PTh/ZnO-2, PTh/ZnO-3 and ZnO nanoparticles and (b) the possible interaction between PTh and ZnO nanoparticles in the PTh/ZnO nanocomposite leading to the formation of extra polarons and electronic pathways vital for increased DC electrical conductivity.

Fig. 6. Relative DC electrical conductivity vs time of (a) PTh, (b) PTh/ZnO-1, (c) PTh/ZnO-2 and (d) PTh/ZnO-3 under isothermal ageing environments.

A. Husain, M.U. Shariq and F. Mohammad

Materialia 9 (2020) 100599

Fig. 7. Relative DC electrical conductivity of: (a) PTh, (b) PTh/ZnO-1, (c) PTh/ZnO-2 and (d) PTh/ZnO-3 under cyclic ageing conditions.

conductor at all the tested temperatures up to 120 °C, i.e. showing an increase in the conductivity with increasing temperature. Also, the effect of incorporation of ZnO nanoparticles in the PTh on the stability at different temperatures strongly suggests about an electronic interaction between PTh and ZnO which cause an increase in the mobility of polarons with rising temperature. Henceforth, PTh/ZnO-3 revealed the maximum stability along with the highest gain in DC electrical conductivity amongst the PTh and PTh/ZnO nanocomposites. These results showed that PTh/ZnO-3 could be a promising semiconducting material in various electrical and electronic applications even at 120 °C due to its outstanding isothermal stability. 3.5.2. Retention of dc electrical conductivity under cyclic ageing condition The relative DC electrical conductivity (𝜎 r ) was calculated by making use of the following equation: σ σr = T (3) σ40 Where: 𝜎 T and 𝜎 40 are representative of the DC electrical conductivities at temperature T and 40 °Ci.e. at the start of every cycle, respectively [47]. Then the conductivity of all the prepared samples was recorded for four continuous cycles. These results disclosed that DC electrical conductivity rose gradually for every single cycle and it followed an organized pattern in case of all the prepared samples that may be attributed to the rise in the quantity along with the mobility of polarons at higher temperatures. But in the case of PTh (Fig. 7a), for the third and fourth cycle, electrical conductivity decreases due to material damage and loss of conjugation. PTh/ZnO-1 (Fig. 8b) and PTh/ZnO-2 (Fig. 8c) displayed the gain in conductivity along with good stability and reversibility up to three successive cycles. PTh/ZnO-3 presented the maximum enhanced conductivity along with excellent stability (Fig. 8d). Accordingly, it can be established that PTh/ZnO-3 is the best semiconductor as compared

to other samples under cyclic ageing conditions. Thus, PTh/ZnO-3 could be a promising material in numerous technological fields which demand stable DC electrical conductivity for several repetitions even at higher temperatures. 3.6. Sensing The DC electrical conductivities of conducting polymers and their nanocomposites are easily mutable, which can be done by altering the composition of fillers and dopants [19,20,22,51,52]. They behave like ptype semiconductors in which polarons and bipolarons (similar to holes) perform as the charge carriers. The electrical conductivity is governed by the number of polarons and bipolarons and their mobility along with the extended 𝜋-conjugated system. The electrical conduction could considerably be adjusted by any type of interaction with the polymer chain that can affect the quantity and the mobility of these charge carriers either alongside the polymer chain or by tunnelling/hopping mechanism. Generally, adsorption of analyte gas/chemical on the sensor surface is considered to be the initial step in the sensing process which could be achieved through the highly porous and vast surface area of the sensor providing a greater number of adsorption sites [53]. As soon as analyte molecules adsorb on the sensor-surface, they interact with polarons of PTh, and therefore, a change in electrical conductivity could be observed. This phenomenon happens significantly in the nanocomposite of conducting polymers in which electrical conductivity is explained through the transfer of electrons between fillers (nanoparticles) and polymers. Therefore, robust sensor consequences are detected for conducting polymer nanocomposites with the various oxidizing as well as reducing gases/vapours [24,54,55]. The chemical/gas/vapour sensing features of PTh as a function of change in DC electrical conductivity is centered on the above principle.

A. Husain, M.U. Shariq and F. Mohammad

Materialia 9 (2020) 100599

Fig. 8. (a) Instrumental setup and (b) Schematic presentation of LPG sensor by the four-in-line-probe.

For the calculation of% sensing response (S), the following formula was used: Δσ 𝑆= × 100 (4) σi where: 𝜎 i and Δ𝜎 represent the initial DC electrical conductivity and change in DC electrical conductivity during complete exposure of gas, i.e. for 120 s, respectively. 3.6.1. Sensing response Owing to the highest conductivity along with better stability under isothermal and cyclic ageing conditions, PTh/ZnO-3 was selected for sensing studies. The pristine PTh was also tested as a sensor for a comparison point of view. The PTh and PTh/ZnO-3 were converted to pellet form and associated with the four-in-line probes and kept in a sealed compartment comprising an inlet section for the introduction of the gas to be sensed, i.e. LPG. Firstly LPG was introduced to the inlet compartment for 120 s. After the removal of the pellet from the compartment, it was exposed to air for another 120 s. Upon exposure to LPG, the electrical conductivity of PTh/ZnO-3 decreased with respect to time. This practical decrease in the DC electrical conductivity may be there in consequences of the polarons of polythiophene being slowed down along with some may be neutralized by the lone pairs of the electrons on the sulphur atoms in ethyl mercaptan (C2 H5 SH), a fundamental constituent of LPG. Upon exposure of the pellet to ambient air for 120 s, the conductivity initially started to spike with time and then regressed near to its initial conductivity as a result of the ethyl mercaptan being desorbed from the pores of PTh/ZnO-3. As evident in Fig. 9, the conductivity of pristine PTh pellet declined by exposing in the LPG. After stopping the flow of gas, the sample was introduced to ambient conditions for 120 s upon which there was a rise in DC electrical conductivity with time. The sensing responses were found to be 71.98% and 45.56% for PTh/ZnO-3 and PTh at 2400 ppm of LPG, respectively. Thus, the PTh/ZnO-3 based sensor showed about 1.58 times greater sensing response when compared to the response of the pristine PTh based sensor towards LPG at 2400 ppm (Table 1). The sensing response of PTh/ZnO-3 was calculated at 600 ppm, 1200 ppm, 1800 ppm and 2400 ppm and found to be 43.22%, 55.69%, 66.14% and 71.98% respectively. As the concentration of LPG increases, the number of ethyl mercaptan molecules increase, which interact with a large number of polarons and reduces their mobility. As a result, a more significant change in conductivity was observed at higher concentrations (Fig. 10). 3.6.2. Reversibility test The sensor pellets of PTh and PTh/ZnO-3 were tested for their reversibility by keeping them firstly in an environment of LPG till the 30 s

Fig. 9. The effect on the conductivity of PTh and PTh/ZnO-3 upon exposure to LPG at 2400 ppm followed by ambient air with respect to time.

followed by the ambient air for another 30 s up to three cycles as revealed in Fig. 11. The response related to the reversibility of PTh/ZnO-3 as a function of DC electrical conductivity was found to be excellent as compared to pristine PTh up to three cycles. In the case of both the PTh and PTh/ZnO-3, the electrical conductivity remains to decrease after every cycle, which may be due to slow and incomplete desorption of LPG from the surface of the sensors. The PTh and PTh/ZnO-3 were evaluated for their reversibility as a function of the per cent change, i.e. loss of DC electrical conductivity after three complete cycles. After completion of third cycle, the change (i.e. loss) in electrical conductivity was found to be 0.0015 Scm−1 (i.e. 16.8%) and 0.00046 Scm−1 (i.e. 82.5%) for PTh/ZnO-3 and PTh respectively. Thus, sensor-based on PTh/ZnO3 exhibited 4.9 times greater response in terms of reversibility when compared to pristine PTh.

3.6.3. Stability The stability of a sensor working at room temperature is a very important parameter for long term use. Herein, we determined the stability of the sensor for 15 days at the highest and lowest concentration, i.e. at 2400 ppm and 600 ppm (Fig. 12). The result showed that the sensing response decreased gradually with respect to the number of days. After completion of 15 days, the sensing response was decreased by 19.20% and 15.61% at 2400 ppm and 600 ppm, respectively.

A. Husain, M.U. Shariq and F. Mohammad

Materialia 9 (2020) 100599

Table 1 The comparative study of the present work with the existing literature reports on LPG sensors based on ZnO. S. no.

Material used

Concentration of LPG

Working temperature

% Response/sensitivity

Ref.

1. 2.

PANI/ZnO PPy/ZnO

1000 ppm 1000–1800 ppm

30–90 °C RT

[5] [1]

3. 4. 5. 6. 7. 8.

ZnO/PPy/PbS QDs Al Doped ZnO ZnO ZnO ZnO with Pt catalyst PTh/SnO2

200–15,000 ppm 1.6 vol% 0.1–0.7 vol% 200 ppm 1000–9000 ppm 0.5–3.0 vol%

RT 325 °C 300–450 °C 200 °C 200–400 °C RT

9. 10.

PTh/Sn-TiO2 PTh/ZnO

– 600–2400 ppm

50–400 °C RT

8.73% at 36 °C 21.6% (1000 ppm) 34.5% (1800 ppm) 85.5% (15,000 ppm) 87% — 1.04 54% (6000 ppm at 250 °C) 9.5% (0.5 vol%) 56.2% (2.5 vol%) Maximum at 300 °C 71.98% (2400 ppm) 66.14% (1800 ppm) 55.69% (1200 ppm) 43.22% (600 ppm)

[7] [4] [3] [2] [6] [45] [56] This work

Fig. 12. The stability of the PTh/ZnO-3 based sensor at 600 ppm and 2400 ppm. Fig. 10. The sensing responses of PTh/ZnO-3 on exposure to different concentration of LPG for 120 s.

Fig. 13. The selectivity of PTh/ZnO-3 based sensor at 600 ppm and 2400 ppm.

importantly, at 600 ppm, the sensor was found to be exclusively selective for LPG as other tested compounds did not show any sensing response, i.e. change in conductivity. Fig. 11. The reversibility of PTh and PTh/ZnO-3 on alternate exposure to LPG at 2400 ppm, followed by ambient air with respect to time.

3.6.4. Selectivity To determine the selectivity of the sensor towards LPG, the sensor was exposed in different volatile organic compounds (VOCs) such as benzene, toluene, chlorobenzene and hexane for 120 s at 2400 ppm and 600 ppm (Fig. 13). At 2400 ppm, benzene, toluene, chlorobenzene and hexane showed very low response as compared to LPG. Most

3.6.5. Sensing mechanism The adsorption and desorption mechanism of LPG on the sensorsurface at room temperatures was exploited for the explanation of the sensing mechanism of PTh/ZnO-3, as depicted in Fig. 14. In PTh/ZnO3 nanocomposite, upon the electronic interaction of ZnO with the lone pairs of electrons of the sulphur atoms, a large number of polarons with high mobility in polythiophene chain are generated resulting in amplified electrical conductivity. As we already know that LPG comprises of mainly propane, butane and ethyl mercaptan in trace amounts. As the

A. Husain, M.U. Shariq and F. Mohammad

Materialia 9 (2020) 100599

Fig. 14. The suggested sensing mechanism of PTh/ZnO nanocomposites involving the interaction of polarons of PTh with LPG.

hydrocarbon molecules are neutral at room temperature, they did not show any electronic interaction with polarons of PTh/ZnO-3. Ethyl mercaptan may be the only constituent that could be responsible for alteration in the conductivity in the case of PTh/ZnO-3 nanocomposite upon its exposure to LPG. After adsorption of LPG on the sensor surface, the lone pairs of electrons on the sulphur atom of ethyl mercaptan electronically interact and get bind with polarons of PTh/ZnO-3 which in turn slows down the speed of polarons resulting in a reduction in the conductivity. Ethyl mercaptan molecules upon exposure to the ambient air get desorbed from the pores of PTh/ZnO-3 based sensor, and this causes the conductivity to revert to its approximate initial value. Thus, it can be inferred from the above results that the mobility of polarons is governed by the easily explainable adsorption-desorption process of ethyl mercaptan into the pores of the PTh/ZnO-3. Accordingly, the above phenomena can be successfully exploited to explain the sensing mechanism of PTh/ZnO-3 towards LPG.

Declaration of Competing Interest None.

Funding This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Acknowledgments “We thankfully acknowledge the Department of Physics, AMU, Aligarh for providing XRD facilities.”

4. Conclusion In this study, we reported the synthesis, characterization, DC electrical conductivity and LPG sensing studies of PTh and PTh/ZnO nanocomposite. PTh/ZnO nanocomposites showed remarkably improved DC electrical conductivity with greater stability under accelerated isothermal and cyclic ageing conditions. The sensor-based on PTh/ZnO-3 showed a quick response, excellent reversibility along with a lower detection limit of 600 ppm of LPG at room temperature. The sensing response of PTh/ZnO-3 based sensor was found to be 44.22%, 55.69%, 66.14% and 71.98% at 600 ppm, 1200 ppm, 1800 ppm and 2400 ppm, respectively. This study revealed that PTh/ZnO nanocomposites could be utilized in numerous electronic and electrical applications as a semiconducting material where stable electrical conductivity even at higher temperature is a primary requirement besides their excellent LPG sensing capability. Also, this sensing study at room temperature with good sensing response (43.22%) even at very low concentration (600 ppm) could be very vital because of highly combustible nature of LPG at higher concentrations and temperatures. In future, more work on the stability and the selectivity of the sensor can improve its overall performance.

Supplementary material Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.mtla.2020.100599.

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