Conducting polymer-based sensors have short response time at room temperature besides their good electrical conductivity. However, the poor electrical conductivity retention at a higher temperature and the failing reproducibility of sensors which are based on conducting polymers are an area of concern.
Journal of Science: Advanced Materials and Devices (2020) 84e94 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Electrical conductivity and alcohol sensing studies on polythiophene/ tin oxide nanocomposites Ahmad Husain, Sharique Ahmad, Faiz Mohammad* Department of Applied Chemistry, Zakir Husain College of Engineering and Technology, Faculty of Engineering and Technology, Aligarh Muslim University, ALIGARH, 202002, India a r t i c l e i n f o a b s t r a c t Article history: Received 26 August 2019 Received in revised form 15 January 2020 Accepted 16 January 2020 Available online 25 January 2020 Conducting polymer-based sensors have short response time at room temperature besides their good electrical conductivity However, the poor electrical conductivity retention at a higher temperature and the failing reproducibility of sensors which are based on conducting polymers are an area of concern To this end, we are reporting the preparation of polythiophene (PTh) and polythiophene/Tin oxide (PTh/ SnO2) nanocomposites by an in-situ chemical oxidative polymerisation The as-prepared materials were characterized by FTIR, SEM, UV-vis absorbance spectroscopy, TEM and XRD techniques PTh/SnO2-3 (i.e PTh/SnO2 nanocomposite containing 15% SnO2 nanoparticles) showed the highest DC electrical conductivity (9.82 Â 10À3 S,cmÀ1) in addition to a maximal stability as a function of DC electrical conductivity retention under accelerated isothermal and cyclic ageing conditions We utilized PTh/SnO2-3 to fabricate a novel pellet-shaped sensor for the selective detection of some of the higher alcohols, such as butan-1-ol (1 alcohol), butan-2-ol (2 alcohol), and 2-methyl propanol (3 alcohol) at room temperature PTh/SnO2-3 exhibited the highest response in terms of variation in DC electrical conductivity and maximal reproducibility for butan-1-ol Finally, the sensing mechanism was explained by the adsorption edesorption process of alcohol vapours on the large surface area of the PTh/SnO2 nanocomposites where electronic interactions between the lone pairs of electrons of alcohol molecules with the polarons of PTh cause the change in the DC electrical conductivity © 2020 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: PTh/SnO2 nanocomposites Electrical conductivity Alcohol sensor Introduction Nowadays, the development of highly efficient chemical/gas/ vapour sensors becomes essential because of the increasing concern about environmental protection [1e6] An ideal sensor should be reliable, selective and reversible in order to be employed practically in various applications A perfect sensor exhibits good selectivity, i.e it responds to the target analyte only in the presence of additional interfering species [7e10] One of the most common types of sensors is the chemiresistor, whose electrical resistance is highly sensitive to the different chemical environments The advantages of using the four-point interdigital electrodes are the minimisation of the contact resistance and the enhancement of the sensing response of the chemiresistor [7] The most conventional * Corresponding author E-mail address: faizmohammad54@rediffmail.com (F Mohammad) Peer review under responsibility of Vietnam National University, Hanoi sensors are based on inorganic semiconductor metal oxides, such as SnO2, TiO2, ZnO, WO3 etc because of their good sensing response, cost-effective design and simple sensing mechanism [8,9] Usually, these types of sensors could only be used at higher temperatures which cause a variation in the sensing response due to the possibility of structural deformation Also, working at high temperature is power consuming and may cause safety problems during the detection of flammable gases which might catch fire at elevated temperatures [9,10] Therefore, enormous efforts have been employed for the fabrication of a sensor working at low temperature or room temperature with a high sensitivity and a short recovery time [5e7,10] Recently, conducting polymers (CPs), employed as gas sensors at room temperature conditions, allow the detection and monitoring of various analytes, which can be a safer alternative as compared to sensors working at elevated temperatures [7] The interaction with the analyte species significantly influences the redox characteristics of the conducting polymers resulting in a modification in their work function, resistance and electrochemical potential [1,7,10] Safer detection of several combustible https://doi.org/10.1016/j.jsamd.2020.01.002 2468-2179/© 2020 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) A Husain et al / Journal of Science: Advanced Materials and Devices (2020) 84e94 gases at room temperatures is possible if we employ sensors based on conducting polymers This field today include several conducting polymers, such as polyacetylene, polyaniline, polypyrrole, polyparaphenylenes, polythiophenes These polymers consist of a spine of the extended p-conjugated structure and an arrangement of alternating single and double bonds (sp2 hybridised structure) This leads to the delocalisation of the p-electrons along the whole polymer chain, therefore providing these conducting polymers with their characteristic optical and electrical properties [11e14] The main problems related to the sensors based on CPs are their irreversibility and long-time instability To overcome these problems, nanocomposites of CPs with semiconducting inorganic nanoparticles could be a potential candidate The incorporation of nanoparticles in the conducting polymers induces high sensitivity and selectivity, which leads to the further improvement in their performance as a gas sensor [10,15e23] Offering a superior shape and size control as compared to the one-step redox process, the most commonly employed method for nanocomposite fabrication is the in-situ polymerization [10,16e19] Among the different conducting polymers available nowadays, polythiophene (PTh) based nanomaterials are ones of the most widely used contenders in the field of sensors due their outstanding electrical, optical, thermal, mechanical properties and their environmental stability [18e23] Over the years, tin oxide (SnO2) has been one of the most favourable sensing materials owing to its good sensitivity towards the most common reducing and oxidising gases besides its chemical and thermal stability and its bandgap of 3.6 eV [20,23e27] But, like other inorganic semiconductors, it also works at high temperature Hence, nanocomposites of conducting polymers with SnO2 nanoparticles have been synthesised having the properties of both the constituents, i.e low working temperature with high sensitivity, reversibility, selectivity and long-time stability [20,21,23,26] Herein, we prepared the nanocomposites of polythiophene with different weight percentage viz 5%, 10% and 15% of SnO2 nanoparticles by the cost-effective in-situ chemical oxidative technique in a chloroform solvent using anhydrous FeCl3 as both the oxidant and dopant The structural, chemical and optical properties of the PTh and PTh/SnO2 nanocomposites were examined by Fourier Transformed Infra-Red (FTIR), UV-VIS absorbance spectroscopy, Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and XRay Diffraction (XRD) techniques We have also explored DC electrical conductivity to investigate the butan-1-ol (1 alcohol), butan-2-ol (2 alcohol), and 2-methyl propan-2-ol (3 alcohol) vapour sensing behaviour of PTh and PTh/SnO2-3 along with their conductivity retaining ability in the accelerated isothermal and cyclic ageing conditions of all the samples Experimental Thiophene (E.Merck, India), Tin Oxide nanoparticles (Platonic Nanotech Pvt Ltd India) and butan-1-ol, butan-2-ol, 2-methyl propan-2-ol, chloroform, anhydrous ferric chloride, acetone, methanol were purchased from Fisher Scientific, India A simple and cost-effective in-situ chemical oxidative polymerisation technique was employed for the preparation of PTh and PTh/SnO2 nanocomposites with a varying weight percentage of SnO2 nanoparticles In this method, chloroform and anhydrous FeCl3 were utilized as the solvent and the oxidant, respectively In the usual procedure, mL (25.00 mmol) of thiophene (i.e monomer) was transferred into 40 mL of solvent (chloroform) followed by a ultrasonication process for 25 Further, a known amount of SnO2 nanoparticles (5%, 10% and 15%) was added to 85 60 mL of chloroform and then ultrasonicated for 30 After that, the solution containing the SnO2 nanoparticles was transferred into the thiophene solution Then, this mixture was subjected to the ultrasonication for the total duration of 90 During the ultrasonication process, the thiophene molecules were adsorbed on the surface of the SnO2 nanoparticles After that, 16.24 g (100 mmol) of ferric chloride was dissolved in 100 mL of chloroform and stirred for 20 untill a homogeneous suspension was made Then, the dropwise addition of the as-prepared FeCl3 suspension to the thiophene and SnO2 mixture was accompanied by constant stirring with a magnetic stirrer for 20 h Then, the resulting PTh/SnO2 nanocomposite was subjected to the filtration process in addition to being washed quite a few times with methanol and after that by distilled water and lastly using acetone In the course of washing, as soon as the methanol was added, there was a visible change in the colour of the materials from black to brown to be observed Finally, the synthesised materials were dried in a vacuum oven at 60 C for 18 h After the drying was completed, the materials were crushed into a very fine powders and kept in the desiccator for further experiments The PTh/SnO2 nanocomposites comprising 5%, 10% and 15% SnO2 were identified as PTh/SnO2-1, PTh/SnO2-2 and PTh/SnO2-3 respectively Polythiophene (PTh) nanoparticles were also synthesised without adding SnO2 nanoparticles by employing an identical process A variety of methods was employed for the investigation of the morphology, the formation and chemical composition of the PTh and PTh/SnO2 nanocomposites X-ray diffraction patterns, FTIR spectra, UV-VIS spectra, SEM and TEM studies were carried out by employing the Bruker D8 diffractometer with Cu-Ka radiation at 1.5418 Å, the PerkinElmer 1725 instrument on KBr pellets, the Shimadzu UVÀVIS spectrophotometer (model 1601), the JEOL-JSM, 6510-LV (Japan) and the JEM 2100, JEOL (Japan), respectively The DC electrical conductivity and sensing experiments were carried out with the help of the 4-in-line probe instrument attached with a PID controlled oven (Scientific Equipment, Roorkee, India) The following equation was employed for the evaluation of the DC electrical conductivity: s ẳ ẵln22S = Wị = ẵ2pSV = Iị (1) where: s, I, V, W and S are used for the DC electrical conductivity (in S,cmÀ1), current (in A), voltage (in V), the thickness of the pellet (in cm) and the probe spacing (in cm), respectively [28] The pellets utilised for the DC electrical conductivity and sensing measurements were made by 250 mg of materials with the support of a hydraulic pressure machine operating at a pressure of 70 kN applied for In order to assess the DC electrical conductivity retaining aptitude in the accelerated isothermal situation, the pellets were subjected to heat at 40 C, 60 C, 80 C, 100 C and 120 C in an air oven Then, the DC electrical conductivity was calculated at the particular temperature at an interval of in the accelerated ageing experiments In order to assess the stability under cyclic ageing conditions, the DC electrical conductivity experiments were carried out for four successive cycles within a wide range of temperature ranging from 40 C to 120 C Results and discussion 3.1 FTIR studies FTIR spectra of PTh, PTh/SnO2-1, PTh/SnO2-2 and PTh/SnO2-3 are presented in Fig The spectrum of PTh reveals a wide-ranging absorption band at nearby 3427.9 cmÀ1 which may be accredited to the eOH stretching vibrations The strong peaks at 1634.8 cmÀ1 and 86 A Husain et al / Journal of Science: Advanced Materials and Devices (2020) 84e94 Fig FT-IR spectra of: (a) PTh, (b) PTh/SnO2-1, (c) PTh/SnO2-2 and (d) PTh/SnO2-3 1440.6 cmÀ1 may be related to the stretching modes of vibrations of C]C and CeC present in thiophene rings, respectively The peak at 1189.8 cmÀ1 may be related to the in-plane stretching vibration of the CeH bond Out of plane bending modes of vibrations of CeH bonds are observed at 1105.4 cmÀ1 and 1024.1 cmÀ1 The band that appeared at 782.9 cmÀ1 could be assigned to a CeH out of plane deformation mode of the thiophene ring because of polymerization The bending vibration mode related to the CeS bond of thiophene ring could possibly be observed at 631.5 cmÀ1 The peak at 470.2 cmÀ1 corresponds to a deformation mode of the CeSeC bond The peaks at 2852.6 cmÀ1 and 2928.3 cmÀ1 could be ascribed to the stretching vibrations of the CeH bond A new peak observed at 699.7 cmÀ1 in the spectrum of PTh/SnO2-3 is due to a SneOeSn antisymmetric and symmetric vibration mode and indicates the presence of SnO2 nanoparticles The peaks at around 1730 cmÀ1 to 1740 cmÀ1 in all the materials may be due to the >C]O stretching vibration of acetone which was used during washing The FTIR spectra of PTh/SnO2-3 display similar bands like those of PTh with slightly shorter wavenumbers In the spectrum of PTh/SnO2-3, the bands appeared at 3427.9 cmÀ1, 2928.3 cmÀ1, 2852.6 cmÀ1, 1634.8 cmÀ1, 1440.6 cmÀ1, 1319.7 cmÀ1, 1189.8 cmÀ1, 1105.4 cmÀ1, 1024.1 cmÀ1, 782.9 cmÀ1, 631.5 cmÀ1 and 470.2 cmÀ1 and move to 3426.8 cmÀ1, 2922.1 cmÀ1, 2852.2 cmÀ1, 1630.5 cmÀ1, 1436.2 cmÀ1, 1310.9 cmÀ1, 1175.8 cmÀ1, 1102.6 cmÀ1, 1022.5 cmÀ1, 781.3 cmÀ1, 595.8 cmÀ1 and 461.8 cmÀ1, respectively, after the incorporation of the SnO2 nanoparticles into the PTh matrix showing some electronic (coulombic) interaction between PTh and SnO2 nanoparticles The shifting of the bands towards the smaller wavenumbers may be an indication of a successful polymerisation of thiophene monomers on the large surface area of the SnO2 nanoparticles The maximal shift among the peaks was observed for the bending vibration mode of CeS bonds, which was shifted from 631.5 cmÀ1 to 595.8 cmÀ1 This peak shift could be a result of the strong coulombic interaction between the lone pair on the sulphur atom of the thiophene and the Snỵ4 ions of the SnO2 nanoparticles These results were found to be consistent with the previously published literature data [12,21e23,28e30] 3.2 X-ray diffraction (XRD) analysis In Fig 2, XRD patterns of PTh, PTh/SnO2-1, PTh/SnO2-2 and PTh/SnO2-3 are revealed In the case of PTh, a wide diffraction peak observed within the range of 2q values from 10 to 20 designates the amorphous nature of the polymer [28,30] The presence of the SnO2 nanoparticles in PTh/SnO2-3 is confirmed by the peaks observed at 2q values of 26.76 , 34.06 , 38.14 , 51.98 , 54.86 , 62.6 , 66.21, 71.82 and 78.58 , which relate to the (110), (101), (200), (211), (220), (310), (301), (202) and (321) planes of the SnO2 nanoparticles, respectively [27,31] In the case of the SnO2 nanoparticle, the main peaks were observed at 2q ¼ 26.60 , 33.80 , 37.90 , 51.80 , 54.70 , 61.90 and 65.90 [27] In case of PTh/SnO2-3, the characteristic peaks of the SnO2 nanoparticle are shifted to slightly higher angles which signifies the coulombic interaction involving the lone pairs of electrons of the sulphur atom in PTh and the Snỵ4 ions of the SnO2 nanoparticle The result indicates the polymerisation of thiophene over the surface of the SnO2 nanoparticle, which enhances the DC electrical conductivity and the sensing ability of PTh 3.3 Scanning electron micrographic (SEM) studies Fig XRD patterns of: (a) PTh, (b) PTh/SnO2-1, (c) PTh/SnO2-2 and (d) PTh/SnO2-3 Fig depicts the morphologies of PTh and PTh/SnO2-3 The SEM micrographs of PTh revealed that the sample's surface consists of flat nanorods interconnected with each other resulting in a slightly porous morphology In the case of PTh/SnO2-3, the absence of free SnO2 nanoparticles may be related to the successful encapsulation of the SnO2 nanoparticles in the PTh matrix The observed morphology of PTh/SnO2-3 revealed a flaky or thin sheet-like structure interlinked with each other, which gives a highly porous surface The modification in morphology suggests that there may be some electronic interactions between PTh and SnO2 The highly porous and large surface area plays a tremendous role in the sensing mechanism of chemiresistors because the adsorption of the analyte gas/vapour is considered to be the first step, then the interaction between the polarons and the analyte takes place This electronic interaction leads to an alteration in the DC electrical conductivity due to the A Husain et al / Journal of Science: Advanced Materials and Devices (2020) 84e94 87 Fig SEM micrographs of PTh and PTh/SnO2-3 at different magnifications neutralisation or decrease/increase in the mobility of polarons depending on the types of the analyte gas/vapour [7,10] 3.4 Transmission electron micrographic (TEM) studies In Fig 4, TEM images of PTh and PTh/SnO2-3 are presented The TEM micrograph of PTh revealed the formation of nanorods with a flaky structure, which may also be seen in the SEM image of PTh The TEM micrograph of PTh/SnO2-3 indicates the successful polymerisation of thiophene on the surface of the SnO2 nanoparticles The TEM image of the PTh/SnO2-3 revealed that SnO2 nanoparticles (black coloured parts) are successfully captured within the PTh matrix (grey coloured background) 3.5 UVevisible absorbance spectroscopy The UVÀvis absorption spectra related to PTh and PTh/SnO2-3 are depicted in Fig In the case of PTh, the band detected at 348 nm may correspond to the pÀp* electronic transition of the benzenoid rings [22] For PTh/SnO2-1, PTh/SnO2-2 and PTh/SnO2-3, the characteristic band of pure PTh is red-shifted to 354, 366 and Fig TEM micrographs of PTh and PTh/SnO2-3 Fig UVevis absorbance spectra of: (a) PTh, (b) PTh/SnO2-1, (c) PTh/SnO2-2 and (d) PTh/SnO2-3 88 A Husain et al / Journal of Science: Advanced Materials and Devices (2020) 84e94 372 nm, respectively, that could be accredited to a growth in the degree of conjugation of PTh due to the creation of a well-organised network by the coulombic interaction of PTh with SnO2 nanoparticles The significant red shift in the PTh/SnO2-3 spectra may be related to the boosted DC electrical conductivity as a consequence of the easiness in the mobility of polarons (charge carriers) through the prolonged p-conjugation in PTh/SnO2-3 3.6 DC electrical conductivity studies The theory of polaron and bipolaron could explain the fundamental mechanism of the electrical conductivity in the intrinsically conducting polymers [11,12] The generation of polarons and bipolarons depends upon the intensity of the oxidation process At a lower oxidation level polarons are generated while a higher oxidation level favours the production of bipolarons These are charge carriers and behave like holes The number and the mobility of these charge carriers through the extended p-conjugated system determine the magnitude of the electrical conductivity of conducting polymers As the temperature rises, the electrical conductivity is expected to rise due to a greater mobility of the charge carriers But it was witnessed that at higher temperatures, the electrical conductivity decreases and some times is lost entirely due to the loss of dopants and the degradation of polymers which break the p-conjugation In the case of nanocomposites of conducting polymers, the electrical conductivity was found to increase even at a higher temperature and to behave like in a semiconductor because of the better alignment of the polymer chains, the thermal stability and the increase in the p-conjugation extent [13e17] In this paper, the PTh and PTh/SnO2 nanocomposites, having 5%, 10% and 15% of SnO2 nanoparticles, were examined for their initial DC electrical conductivities which were evaluated by employing a standard four-in-line probe technique The calculated DC electrical conductivities of PTh and PTh/SnO2 nanocomposites were found in a similar range to those exhibited by semiconductors The DC electrical conductivity of PTh was found to be 5.59 Â 10À4 S,cmÀ1, whereas the conductivities were found to be about 1.43 Â 10À3, 4.73 Â 10À3 and 9.82 Â 10À3 S,cmÀ1 for PTh/SnO2-1, PTh/SnO2-2 and PTh/SnO2-3, respectively The DC electrical conductivity related to PTh/SnO2 rises with the loading of SnO2 nanoparticles, as depicted in Fig 6a It may be supposed that the DC electrical conductivity of PTh was considerably enhanced after the incorporation of SnO2 nanoparticles due to the following two reasons: (1) the polymerization of thiophene monomer on the large surface area of the SnO2 nanoparticles resulted in the formation of an efficient system in PTh which increases and stabilises the extended pconjugation; (2) the electronic interaction between the lone pairs of sulphur of polythiophene with the Snỵ4 ions causes an increase in the number and in the mobility of charge carriers (polarons) in the PTh chains (Fig 6b) Thus, a greater amount of SnO2 nanoparticles in the PTh matrix provides a greater surface area where charge carriers can move freely without any hindrance which boosts the electrical conductivity 3.6.1 Stability under isothermal ageing conditions The DC electrical conductivity retaining aptitude of PTh, PTh/ SnO2-1, PTh/SnO2-2 and PTh/SnO2-3 was examined under accelerated isothermal ageing conditions, and results are represented in Fig The following equation was employed for the calculation of the relative DC electrical conductivity at a particular temperature: sr;t ¼ st s0 (2) Fig (a) Initial DC electrical conductivities of PTh, PTh/SnO2-1, PTh/SnO2-2, PTh/ SnO2-3 nanocomposites and (b) the possible interaction between PTh and SnO2 nanoparticles in the PTh/SnO2 nanocomposite leading to the creation of additional polarons and electronic pathways vital for boosted electrical conductivity where sr,t, st and so symbolise the relative DC electrical conductivity at time t, the DC electrical conductivity at time t and the DC electrical conductivity (in S,cmÀ1) at time zero, respectively [28,30] The isothermal stability of PTh/SnO2 nanocomposites as a function of the retention of DC electrical conductivity was observed to be far better than that of PTh It is obvious from Fig 7a that PTh is fairly well stable at 40 and 60 C In the case of PTh, direct heating at high temperatures (i.e at 80, 100 and 120 C) resulted in a regular dropping of conductivity which may be caused by the damage of materials and the loss of the doping agent PTh/SnO2-1 shows good stability at 40, 60 and 80 C and behaves like a semiconductor, i.e an increase in the conductivity with the increasing temperature (Fig 7b) PTh/SnO2-2 shows a gain in conductivity with a high stability at 40, 60, 80 and 100 C as depicted in Fig 7c The most significant gain in conductivity with the highest stability at 40, 60, 80, 100 and 120 C was observed in the case of PTh/SnO2-3 (Fig 7d) The effect of the amount of SnO2 nanoparticles in the PTh matrix on the stability of DC electrical conductivity at different temperatures signifies the electronic interaction between PTh and SnO2, which increases the mobility of polarons as the temperature rises Consequently, it could be established that PTh/SnO2-3 displayed the maximal stability and the utmost gain in conductivity among the PTh/SnO2 nanocomposites expressed as a function of the DC electrical conductivity under isothermal ageing condition PTh/ SnO2-3 can be used as a semiconducting material at a temperature of 120 C Thus, the incorporation of a small amount of SnO2 nanoparticles in the PTh matrix can lead to a greater DC electrical conductivity, which is stable at a higher temperature and the material shows a stable semiconducting behaviour as compared to the pristine PTh Hence, the PTh/SnO2-3 composite can be considered a potential candidate in electrical and electronic applications within a wide range of temperature starting from room temperature to A Husain et al / Journal of Science: Advanced Materials and Devices (2020) 84e94 Fig Relative electrical conductivity versus time of (a) PTh, (b) PTh/SnO2-1, (c) PTh/SnO2-2 and (d) PTh/SnO2-3 under the isothermal ageing environments Fig Relative electrical conductivity of: (a) PTh, (b) PTh/SnO2-1, (c) PTh/SnO2-2 and (d) PTh/SnO2-3 under cyclic ageing conditions 89 RT 0.5 M Resistance change Conductivity change Filaments Pellet Polythiophene, polypyrrole, polyaniline derivatives Polystyrene/polyaniline nanoblend PTh/SnO2 nanocomposite chemical oxidative and melt processing chemical oxidative Resistance change Thin film ClO4 doped polypyrrole Electrochemical deposition Thin film butan-1-ol, butan-2-ol and 2-methyl propan-2-ol [37] 25 C e [35] 30 C and 120 C Quartz crystal microbalance (QCM) technique Resistance change Thin film Polyaniline Electrodeposition Resistance change chemical oxidative Pellet methanol, ethanol, 1-propanol, 2-propanol methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol methanol, ethanol, 1-propanol [36] [33,34] 25 C 2-17 mg L-1 and 2e60 ppm RT [32] e 3000 ppm 2000 ppm Conductivity change Pellet Polythiophene/graphene nanocomposite Polyaniline chemical oxidative 0.15e2.01% and 0.61e17.62% (mass/mass) 1600e4800 ppm [30] RT [15] butan-1-ol, butan-2-ol and 2-methyl propan-2-ol ethanol, methanol, sec-butanol, tert-butanol, iso-propanol methanol, ethanol, propanol, butanol, heptanol methanol, ethanol, 1-propanol and 2-propanol Conductivity change Pellet TiO2@PPy nanocomposite chemical oxidative Analyte Technique Type of sensor Method of Preparation of material A sensor may be defined as a device which can detect and measure a physical quantity and can give a clear output for it The DC electrical conductivities of conducting polymers (PTh) and their nanocomposites could be altered by changing the dopants as well as the composition of fillers [16,17,28,30] The polarons act as the charge carriers and the electrical conductivity is governed by the ease of movement of these polarons along with the conjugated system of the polymer back-bone The electrical conduction could significantly be modified 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 a tunnelling/hopping mechanism The porous structure of PTh/ SnO2-3 permits the penetration of analyte gas molecules into the PTh film Then, the analyte gas molecules get adsorbed on the surface of PTh/SnO2-3 and interact with polarons of PTh, causing a change in the electrical conductivity This phenomenon occurs significantly in nanocomposite materials in which the 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 various oxidising as well as reducing gases/vapours For that reason, the highly porous structure and the large surface area of sensors which provide a greater extent of adsorption of analyte molecules on the sensor surface are beneficial because adsorption is considered to be the first step in sensing The chemical/gas/vapour sensing characteristics of PTh in terms of a change in electrical conductivity is based on the above theory [7,10,28,30] Table Comparison of our present study with other existing alcohol sensing studies based on conducting polymers 3.7 Sensing Conc analyte where: sT and s40 stand for the DC electrical conductivity (S,cmÀ1) at temperature T ( C) and at 40 C, i.e at the start of each cycle, respectively [28,30] The relative DC electrical conductivity of each sample was evaluated for four succeeding cycles In the case of PTh (see Fig 8a), the outcome reveals that the conductivity increased steadily for the initial two cycles and follows a regular rising trend in the number along with the mobility of the charge carriers (polarons and bipolarons) at elevated temperatures But, for the third and fourth cycles the conductivity decreases due to the material damage and the loss of conjugation PTh/SnO2-1 (Fig 8b) and PTh/SnO2-2 (Fig 8c) show a gain in the electrical conductivity with good stability for three successive cycles PTh/ SnO2-3 (Fig 8d) displayed the maximal upsurge in conductivity with an excellent stability and reversibility Therefore, PTh/SnO2-3 can be a potential candidate for applications in various technological fields where electrical conductivity retention for several repetitions is required even at higher temperatures As a consequence, it may be concluded that PTh/SnO2-3 presents the utmost stable semiconducting behaviour among all samples under cyclic ageing environments 1M (3) Material sT s40 S No sr ¼ Worki-ng Temp 3.6.2 Stability under cyclic ageing conditions The DC electrical conductivity retaining aptitude of PTh, PTh/ SnO2-1, PTh/SnO2-2 and PTh/SnO2-3 was also examined by a cyclic ageing method within the temperature range of 40e120 C, and is represented in Fig The following equation was employed for the calculation of the relative DC electrical conductivity (sr): RT Ref 120 C due to its higher DC electrical conductivity and excellent isothermal stability This study A Husain et al / Journal of Science: Advanced Materials and Devices (2020) 84e94 chemical oxidative 90 A Husain et al / Journal of Science: Advanced Materials and Devices (2020) 84e94 91 1-pentanol [36] Segal et al prepared a polystyrene/polyaniline nano blend and studied its sensing properties towards methanol, ethanol and 1-propanol [37] A comparative study of our work with the literature based on the method of preparation, the form of sensor, sensing techniques, analyte concentration and operating temperature etc is presented in Table-1 As evident from Table-1, the majority of reported literature data suggests that lower alcohols like methanol and ethanol were more selective than higher alcohols But, very little work has been done on the selective sensing among the higher alcohols containing different side chains viz primary, secondary and tertiary alcohols In this work, we studied the sensitivity, reversibility and selectivity of some higher alcohols containing different parent chains, i.e butan-1-ol (1 alcohol), butan-2-ol (2 alcohol) and 2-methyl propan-2-ol (3 alcohol) In our best knowledge, this is the first attempt to utilise PTh/SnO2 nanocomposite for this type of study Fig DC electrical conductivity alteration of PTh/SnO2-3 on exposure to 1, 2 and 3 alcohols vapours followed by ambient air with respect to time In our previous work, we reported that the binary polypyrrole/ titania (TiO2@PPy) nanocomposites serves as an alcohol sensor which selectively detects butan-2-ol in the presence of other alcohols like butan-1-ol and 2-methyl propan-2-ol vapours [15] In another study, we used a polythiophene/graphene nanocomposite as a highly selective and reversible ethanol sensor with a lower detection limit of 400 ppm at room temperature [30] Athawale et al utilised polyaniline and its substituted derivatives as sensing materials for the detection of alcohol vapour viz methanol, ethanol, propanol, butanol and heptanol [32] Ayad et al reported the detection of methanol, ethanol, 1-propanol and 2-propanol using polyaniline thin films by the quartz crystal microbalance technique [33,34] Babaei et al reported that PPy-ClO4 films could be employed as an aliphatic alcohol vapour sensor The sensor showed the highest sensitivity for methanol as compared to ethanol, 1-propanol and 2-propanol [35] Hatfield et al used polythiophene, polypyrrole and polyaniline derivatives to fabricate an n-alcohol sensor, i.e methanol, ethanol, 1-propanol, 1-butanol and Fig 10 DC electrical conductivity alteration of PTh on exposure to 1, 2 and 3 alcohols vapours followed by ambient air with respect to time 3.7.1 Sensing response as a function of change in DC electrical conductivity The sensitivity of PTh and PTh/SnO2-3 towards higher alcohols such as butan-1-ol (1 alcohol), butan-2-ol (2 alcohol) and 2-methyl propan-2-ol (3 alcohol) was investigated by quantifying the alterations in their DC electrical conductivity at room temperature through exposing them in the environment of alcohol vapour for 50 s followed by ambient air for another 50 s (Fig & Fig 10) The concentration of the aqueous solutions of each of the alcohols tested was 0.5 M When PTh/SnO2-3 was exposed in the environment of alcohol vapour, a decrease in the overall DC electrical conductivity was detected with the increasing exposure time As soon as the pellet was removed from the alcohol vapour environment and kept in the ambient air, the conductivity started to increase with respect to time Different kinds of alcohols displayed different behaviours in the alcohol vapours environment and air In the case of 1 alcohol, the DC electrical conductivity change was found to be maximal while it was minimal in the case of 2 alcohol The DC electrical conductivity change in the environment of 1 alcohol, 2 alcohol and 3 alcohol was found to be 7.7 Â 10À3 S,cmÀ1, 5.6 Â 10À3 S,cmÀ1 and 2.5 Â 10À3 S,cmÀ1, respectively The decrease in the electrical conductivity may be attributed to the charge transfer between alcohol (i.e the lone pairs of electrons of alcohol molecules) and the polarons of PTh When the lone pairs of electrons on the oxygen atom in alcohol molecules interact with the polarons of PTh, the mobility of the polarons (charge carriers) decreases and some polarons may also get neutralised which causes a decrease in the DC electrical conductivity As soon as the pellet was kept in ambient air, the molecules of alcohol get desorbed, and the electrical conductivity started to rise When the PTh pellet was exposed to the environment of 1 alcohol, 2 alcohol and 3 alcohol, the change in electrical conductivity was detected to be 3.2 Â 10À4 S,cmÀ1, 2.5 Â 10À4 S,cmÀ1 and 1.2 Â 10À4 S,cmÀ1, respectively In the case of PTh/SnO2-3, the change in the electrical conductivity (sensing response) in the environment of 1 alcohol, 2 alcohol and 3 alcohol was 24.06, 22.4 and 20.8 times of the change in conductivity of PTh, respectively Thus, PTh/SnO2-3 exhibits superior sensing efficacy compared to PTh The significant improvement in the sensing efficiency of PTh/SnO2-3 may be attributed to the higher electrical conductivity and highly porous and large surface area which provides a greater number of active sites in which the adsorption of analyte vapours takes place 3.7.2 Reproducibility test The sensors based on PTh and PTh/SnO2-3 were also tested for their reproducibility The reproducibility of the sensing response of 92 A Husain et al / Journal of Science: Advanced Materials and Devices (2020) 84e94 Fig 11 Reversibility of PTh/SnO2-3 on alternate exposure to 1, 2 and 3 alcohols vapours followed by ambient air with respect to time PTh and PTh/SnO2-3 was measured by first exposing the pellet of the sample in alcohol vapours for 30 s and after that 30 s in ambient air for a total duration of 180 s (Fig 11) PTh/SnO2-3 shows excellent reproducibility in the environment of 1 alcohol due to the greater extent of adsorption and complete desorption of the analyte vapour in the ambient air While in the environment of 2 alcohol and 3 alcohol, PTh/SnO2-3 shows relatively low reversibility because of the considerably lower extent of adsorption and partial desorption In contrast, PTh exhibits poor reproducibility in the environment of 1 alcohol, 2 alcohol as well as 3 alcohol due to the slow rate of adsorption and the very poor rate of desorption However, PTh shows some reversible nature in 1 alcohol vapour due to the ease of adsorption and the interaction between polarons and lone pairs of the oxygen atom of the alcohol molecules (Fig 12) The sensor-based on PTh/SnO2-3 shows the highest electrical conductivity change (sensing response) and reproducibility in case of 1 alcohol which may be explained by the following points: (1) due to the linear structure of 1 alcohol, the extent of adsorption is very high and the lone pairs of electrons of the oxygen atom of the alcohol molecules can freely interact with the polarons of PTh; (2) this interaction causes a decrease in the mobility as well as the Fig 12 Reversibility of PTh on alternate exposure to 1, 2 and 3 alcohols vapours followed by ambient air with respect to time A Husain et al / Journal of Science: Advanced Materials and Devices (2020) 84e94 Fig 13 Recommended mechanism of interaction of different alcohols with PTh/SnO2 nanocomposites neutralisation of the polarons, which is accountable for a decrease in the DC electrical conductivity; (3) due to the branched structure of 2 alcohol and 3 alcohol, the rate of adsorption becomes relatively slow, and the interaction between the polarons and the lone pairs of alcohol molecules decreases due to a greater steric hindrance The greater the crowding, the poorer will be the electrondonating capacity That is why, in the case 3 alcohols, the sensing response is minimal Thus, PTh/SnO2-3 nanocomposite may be used to fabricate a highly sensitive, selective and reproducible butan-1-ol (1 alcohol) sensor 3.7.3 Proposed Mechanism for sensing The sensing response (i.e change in DC electrical conductivity) of the tested pellets as a function of the difference in the DC electrical conductivity of PTh/SnO2-3 to different alcohol vapours viz 1 alcohol, 2 alcohol and 3 alcohol is based on the decline in the DC electrical conductivity by exposing to the analyte vapour and the return to the approximately initial value as soon as exposed to the ambient air The mechanism involved in the sensing aptitude of PTh/SnO2-3 was defined by means of the variation in the DC electrical conductivity (i.e sensing response) through an easily explicable adsorptionedesorption method of alcohol vapours at ambient temperature on the surface of PTh/SnO2-3, as depicted in Fig 13 In the presence of the alcohol molecules, which is a source of electrons, the lone pairs on the oxygen atom of the alcohol molecules interact with the polarons of PTh/SnO2-3 Consequently, the mobility of the polarons retards, which spontaneously diminishes the DC electrical conductivity As soon as the pellet was kept in the ambient air atmosphere, the alcohol molecules started to desorb from pores of PTh/SnO2-3 Subsequently, the electrical conductivity started to revert towards the original value So, the adsorptionedesorption method of alcohol molecules on the highly porous and large surface area of PTh/SnO2-3 significantly alters the mobility of the polarons which is the reason for the drop and rise in the electrical conductivity in the environment of the alcohol vapour and the ambient air in that order Conclusion In this study, PTh and PTh/SnO2 nanocomposites were synthesised with different weight percentage viz 5%, 10% and 15% of SnO2 nanoparticles by the in-situ chemical oxidative method Different instrumental techniques such as FTIR, SEM, TEM, UV-vis absorbance spectroscopy and XRD were utilised for the 93 characterization of the as-synthesised materials We evaluated the electrical properties comprehensively by studying the DC electrical conductivity retention performances of all the materials under accelerated isothermal as well as cyclic ageing conditions Among all materials, PTh/SnO2-3 was found to be the best semiconductor showing an initial DC electrical conductivity of 9.82 Â 10À3 S,cmÀ1 The PTh/SnO2-3 based sensor showed the best sensitivity, selectivity as well as reproducibility towards butan-1-ol (1 alcohol) as compared to butan-2-ol (2 alcohol), and 2-methyl propan-2-ol (3 alcohol) In the case of PTh/SnO2-3, the change in DC electrical conductivity (sensing response) in the environment of 1 alcohol, 2 alcohol and 3 alcohol was found to be 24.06, 22.4 and 20.8 times the change in conductivity of PTh, respectively The sensing mechanism was successfully cited by the adsorption of the alcohol vapours on the large surface area of PTh/SnO2 nanocomposites followed by an electronic interaction between the polarons and the lone pairs of electrons of the oxygen atoms of alcohol molecules which cause a decrease in the DC electrical conductivity Thus, this study suggests that these PTh/SnO2 nanocomposites could be utilised as a semiconducting material in various electrical and electronic applications besides their sensing capability towards different alcohol vapours tested in this study 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 Acknowledgements Ahmad Husain thankfully acknowledges USIF, AMU References [1] M Khan, T Anwer, F Mohammad, Synthesis and sensing properties of sulfonated multi-walled carbon nanotube and graphene nanocomposites with polyaniline, J Sci Adv Mater Dev (2019) 132e142, https://doi.org/10.1016/ j.jsamd.2019.02.002 [2] N Abraham, R.R Krishnakumar, C Unni, D Philip, Simulation studies on the responses of ZnO-CuO/CNT nanocomposite based SAW sensor to various volatile organic chemicals, J Sci Adv Mater Dev (2018) 125e131, https:// doi.org/10.1016/j.jsamd.2018.12.006 [3] C.M Hung, D Thi, T Le, N Van Hieu, On-chip growth of semiconductor metal oxide nanowires for gas sensors: a review, J Sci Adv Mater Dev (2017) 263e285, 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Table Comparison of our present study with other existing alcohol sensing studies based on conducting polymers 3.7 Sensing Conc analyte where: sT and s40 stand for the DC electrical conductivity. .. creation of additional polarons and electronic pathways vital for boosted electrical conductivity where sr,t, st and so symbolise the relative DC electrical conductivity at time t, the DC electrical. .. PTh/SnO2-3, the change in the electrical conductivity (sensing response) in the environment of 1 alcohol, 2 alcohol and 3 alcohol was 24.06, 22.4 and 20.8 times of the change in conductivity of PTh,