Sensing properties of sulfonated multi-walled carbon nanotube and graphene nanocomposites with polyaniline

Here, we discuss one of the simplest approaches for chemical functionalization of in-situ prepared polyaniline (Pani) and its nanocomposites with multi-walled carbon nanotubes (MWCNT) and graphene (GN) in chlorosulphonic acid.

Journal of Science: Advanced Materials and Devices (2019) 132e142 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Sensing properties of sulfonated multi-walled carbon nanotube and graphene nanocomposites with polyaniline Mahfoozurrahman Khan*, Tarique Anwer, Faiz Mohammad Department of Applied Chemistry, 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 27 September 2018 Received in revised form February 2019 Accepted February 2019 Available online February 2019 Here, we discuss one of the simplest approaches for chemical functionalization of in-situ prepared polyaniline (Pani) and its nanocomposites with multi-walled carbon nanotubes (MWCNT) and graphene (GN) in chlorosulphonic acid The effect of polymerization and functionalization was characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) analysis, Field emission scanning electron microscopy (FESEM) and electro-thermal analysis Results also revealed the presence of pÀp interactions between Pani and carbon allotropes leading to the formation of charge-transfer complexes This strong pÀp interaction significantly increased the resultant electrical conductivity, stabilizing them as well Further, theirs back to back sulphonation in chlorosulphonic acid significantly enhanced the solubility in one way but caused a heavy loss in conductivity conversely The thermoelectric properties of the as-prepared nanocomposites were investigated as a function of MWCNT and GN contents It was observed that as-prepared Pani/GN nanocomposites showed a greater electrical conductivity as well as an improved thermal stability in terms of DC electrical conductivity retention under isothermal and cyclic ageing conditions compared with Pani/MWCNT and Pani Finally these oxidative products were also studied for their sensing response towards amine to detect whether the particular compound is either 1, 2 , or 3 amine © 2019 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: In-situ polymerization Pani/MWCNT and Pani/GN nanocomposite Sulphonation Isothermal and cyclic ageing technique Amine identification Introduction Limitless publications on 150 years old polyaniline and young aged rapidly rising graphitic nanomaterials (i.e CNTs, graphenes, fullerenes etc.) [1,2] have been evidently shown how they have matured over the past few years with a very broad spectrum and wide application range in engineering and medical sciences as well as in their commercial & economical aspects Recently, their nanocomposites possess the combination of high stability and fairly good electrical conductivity due to the synergism between the constituents [3] Despite the stubborn improvement ensued so far, the backbone stiffness of polyaniline, limited solubility and stability of MWCNT and graphene dispersions in water remains an ongoing challenge complicating its processing, management and ultimately the scope of their applications [4] On account of these issues and to optimize their efficiency for use in various applications such as high * Corresponding author E-mail addresses: mahfooz55@gmail.com (M Khan), tariqalig001@gmail.com (T Anwer), faizmohammad54@rediffmail.com (F Mohammad) Peer review under responsibility of Vietnam National University, Hanoi strength nanofibres, sensors and nanoelectronic wires it has become of immense interest to attach some functional parts on their surface [5e7] These (covalent or non-covalent) surface modifications caused either by organic or inorganic species in general and especially by sulphonic groups highly enhanced their solubility, impart considerable stability and strong surface acidity making it highly useful for sophisticated electronic applications and excellent catalyst support for highly dispersed metal nanoparticles [8e11] That's the reason why the molecular functionalization has been the deep-seated interest of research world after the ingenious work of Yue and Epstein [12] and persistently opening the door to unprecedented materials applications Covalent functionalization is comparatively more effective and greatly alters the electrical conductivity of polyaniline It introduces some defect sites forming a new type of nanostructures for diverse applications On the other hand, Hua Bai et al [13] have reported a non-covalent functionalization process involving the formation of charge transfer complexes due to the strong pÀp interactions between quinoidal units of Pani and pyrenyl rings of MWCNTs or graphene Yue and Epstein [14] prepared self doped Pani almost two decades ago by the https://doi.org/10.1016/j.jsamd.2019.02.002 2468-2179/© 2019 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/) M Khan et al / Journal of Science: Advanced Materials and Devices (2019) 132e142 chemical modification in the presence of fuming H2SO4 and studied their better solubility as well as redox activity and conductivity over a wider pH range Deore et al [15] prepared switchable self-doped Pani with interconversion between self-doped and non-self-doped forms Recently Zhang et al [16] have reported the development of water soluble nanocomposite of sulphonated polyaniline with MWCNTs Their electrical parameters such as conductivity, thermal stability, dielectric behavior etc have rarely been reported In this work, we report our continued effort to make use of chlorosulphonic acid in an inert solvent instead of using fuming H2SO4 for sulphonation of Pani, Pani/MWCNT and Pani/GN nanocomposites Comparative studies on sensing performance and DC electrical conductivity retention were also done to ascertain efficiency for their potential applications in the latest sophisticated technologies Experimental 133 were stored in an airtight sample container for characterized further studies 2.4 Characterization Surface morphologies of Pani (EB), Pani (EB)/MWCNT and Pani (EB)/GN (gold coated) were viewed under a scanning electron microscope (SEM) (LEO 435-VF) Their phase composition was analyzed by X-ray diffraction (XRD) recorded by Bruker D8 diffractometer with Cu Ka radiation at 1.540 Å in the range of 5 2q 70 at 40 kV The FT-IR spectra were recorded using Perkin-Elmer-Spectrum 2000 Spectrophotometer in KBr between 400 and 4000 cmÀ1 The electrical conductivity as well as thermal stability in terms of DC electrical conductivity retention were studied according to the method already reported [17] using the equation: Ds ẳ ẵln 22S=Wị =ẵ2pSV=Iị (1) 2.1 Materials used Monomer “aniline” from E-Merck India Ltd was purified by distilling twice before use The MWCNT (diameter and average lengths were about 10e20 nm and 20 mm respectively) and graphene used in this study were purchased from Iljin Nano Tech, Seoul, Korea Potassium persulphate (PPS) and HCl (AR grade) and methanol were purchased from (CDH India Ltd.) and were used as received Double distilled water (DDW) was used in all the experimental procedures and washing 2.2 Preparation of Pani, Pani/MWCNT and Pani/GN nanocomposites The nanocomposites of Pani/MWCNTs were prepared by in-situ oxidative polymerization Firstly, 200 mL of 1M HCl and mL aniline were added dropwise under stirring The ultrasonicated suspension of MWCNTs (0.15 gm in 100 mL of 1M HCl) was transferred into aniline solution The oxidant was readied by dissolving 14.8 gm K2S2O8 in 200 mL of M HCl The polymerization was effected by dropwise adding the oxidant solution into the aniline/MWCNTs suspension and left for stirring continuously for about 16 hours The resultant greenish black slurry was filtered and washed thoroughly with 2.5 L double distilled water to remove the excess acid as well as oxidant until the filtrate became colorless Thus prepared nanocomposites was dedoped by aqueous (IM) ammonia to convert it into emeraldine base (EB) form The Pani/MWCNTs (EB) was dried around 70  C for hours in an air oven, converted into fine powder and was stored in a cool and dry place for further investigations Pani (EB) as well as Pani (EB)/GN nanocomposites were also prepared using the same method 2.3 Sulphonation of Pani, Pani/MWCNT and Pani/GN nanocomposites The prepared Pani (EB) was sulphonated by using chlorosulphonic acid gm of as-prepared Pani (EB) powder was dispersed in 200 mL of 1, 2-dichloroethane (DCE) and constantly stirred at 75  C Thereafter, 3.5 gm of chlorosulphonic acid diluted with 10 mL of DCE was added dropwise in 20 and the reaction mixture was left for hours The resultant greenish suspension was filtered, kept in 100 mL water and heated for hours at 75  C to promote its hydrolysis Finally, the filter cake was washed with L water, followed by 200 mL methanol, dried in an air oven at 70  C and was transformed into sulphonated Pani (S-Pani) Similarly sulphonated Pani/MWCNTs (S-Pani/MWCNTs) and sulphonated Pani/GN (S-Pani/GN) were prepared using the same method and where I, V, W and S are the current (A), voltage (V), the thickness of the pellet (cm) and probe spacing (cm) respectively and s is the DC electrical conductivity (S cmÀ1) [17] Results and discussion 3.1 The formation of S-Pani, S-Pani/MWCNT and S-Pani/GN The proposed novel balanced chemical equation and formation scheme of Pani (EB) and its sulphonation has been summarized below Here, first of all Pani (EB) was prepared by simple oxidative polymerization whereas Pani (EB)/MWCNT and Pani (EB)/GN were also prepared in the same way by additional use of MWCNT and graphene nanosheets These in-situ products were dried and then treated with chlorosulphonic acid (HSO3Cl) at 75  C Fig 1(a) Dark green free flowing powders were obtained expecting to achieve 50% sulphonation Achievement of 100% sulphonation has not been reported so far to the best of our knowledge but it would be predicted to have considerably greater water solubility Similarly, SPani/MWCNT and S-Pani/GN nanocomposites can be produced in Fig 1(b,c) 3.2 Confirmation for sulphonation Color change test was carried out to get confirmation whether as prepared products have undergone sulphonation or not For this, these chlorosulphonic acid treated products were dissolved in ammonia solution Fig It was observed that the color of the solution turned blue within few seconds indicating the process of undoping When these undoped solutions were heated at 100  C, the color of the solution mixtures returned back to greenish black due to volatilization of ammonia vapors This evidently proves that chlorosulphonic acid treated products had undergone sulphonation 3.3 FTIR spectroscopic studies Pani (EB), Pani (EB)/MWCNTs and Pani (EB)/GN and their sulphonated products were characterized by FTIR to study the interactions between comprising constituents The peaks at 1620 and 1560 cmÀ1 (corresponding to quinoid and benzenoid ring respectively), 1338 cmÀ1 (C-N stretching), 1205 cmÀ1 (C¼N stretching), and 823 cmÀ1 (1,4-substituted phenyl ring stretching) are characteristics of EB form of Pani Fig [18,19] Likewise, FTIR spectra of Pani/MWCNTs and Pani/GN are almost identical to Pani except all their peaks have slightly shifted to higher wavenumbers This 134 M Khan et al / Journal of Science: Advanced Materials and Devices (2019) 132e142 Fig Formation of S-Pani (a), S-Pani/MWCNTs (b) and S-Pani/GN (c) M Khan et al / Journal of Science: Advanced Materials and Devices (2019) 132e142 135 Fig Color change test for self-doped S-Pani seems to be due to the pÀp interaction between quinoid rings of Pani and pyrenyl rings of graphitic materials On the other hand, if comparing these spectra with those of their corresponding sulphonated counterparts, we observe that the sulphonation has caused the band broadening in all the cases The presence of characteristic peaks at 1090 and 1015 cmÀ1 matching to the asymmetric and symmetric O¼S¼O stretching vibrations respectively verify the presence of sulfonic groups covalently bound to the polymer backbone [20] The presence of SeO stretching peak at 730 cmÀ1 also supports the sulphonation of Pani, Pani/MWCNTs and Pani/GN 3.4 Surface morphology The FE-SEM images of Pani, Pani/MWCNTs, Pani/GN and their sulphonated products are shown in Fig at different magnifications Pani (Fig 4a) seems to have “Lima” bean shaped curved structure, whereas Pani/MWCNTs have somewhat more elongated tubular morphology indicating polymerization of aniline over MWCNTs (Fig 4c) In the case of Pani/GN, flaky sheets are being observed which indicates the deposition of polyaniline over graphene nanosheets The existence of MWCNT and GN can be evidently seen and act as a conductive pathway for electron carrying which interconnects among Pani coated with Pani/MWCNTs and Pani/GN to hinder close stacking and improve electrolyte ions accessibility It seems that the sulphonation of Pani beans transformed them into granular structures In case of S-Pani/MWCNTs and S-Pani/GN (Fig 4d,e), eSO3H groups seems to have attached with Pani chain encapsulating the MWCNTs and GN respectively [21] Electrical properties Electro-thermal studies of all as-prepared Pani, Pani/MWCNTs, Pani/GN and their sulphonated products were well carried out by standard 4-in-line probe technique in the temperature range of 40  Ce150  C As-prepared Pani, Pani/MWCNTs, Pani/GN (EB) are doped with H2SO4 in which Hỵ ions act as a dopant and sulphonation (covalent attachment of eSO3H) Results of the electrical conductivity measurements indicate the p-type semiconducting behavior of all the materials within the operating temperature range of the experiment It was also observed that the addition of MWCNTs and GN has caused the augmentation in their electrical conductivities as shown in Fig Since the Pani as well as infusing graphitic nano fillers MWCNTs and GN) are good conducting, the enhancement in DC electrical conductivity may be credited to the additive synergism of both the constituents interacting at the molecular level Although, both MWCNTs and GN are nearly matching in their chemical makeup and mechanical properties, graphene has been observed to be far better than MWCNTs in Fig FTIR Spectra of: (a) Pani(EB) (b) S-Pani (c) Pani(EB)/MWCNT (d) S-Pani/MWCNT (e) Pani(EB)/GN and (f) S-Pani/GN 136 M Khan et al / Journal of Science: Advanced Materials and Devices (2019) 132e142 Fig FE-SEM images of (a) Pani (b) S-Pani (c) Pani/MWCNTs (d) S-Pani/MWCNTs (e) Pani/GN and (f) S-Pani/GN contributing its exceptional attributions (like electrical conductivity, strength etc.) to the host polymer matrix [22] Both the MWCNTs and GN are derived from graphite but the mobility of electrons is higher in GN making it more electronically conducting than MWCNTs That's why the nanofiller loading of Pani by MWCNTs and GN have caused a promising augmentation in electrical conductivity Besides, the strong pp-pp interaction between the p-bonded surface of the carbon based nano scaled materials and the conjugated structure of Pani also imparts the enhancement of the conductivity up to some extent But contrary to that, sulphonation (covalent attachment of eSO3H) to Pani and its nanocomposites with either MWCNTs or GN one, has caused a promising cutback in electrical conductivity of all the three S-Pani, S-Pani/MWCNTs as well as S-Pani/GN This seems to be because of the low extent of doping 4.1 Isothermal ageing The isothermal investigation in terms of DC electrical conductivity retention of prepared materials would be a key component in analyzing their thermal stability From Fig 6, it may be seen that the as-prepared Pani as well as Pani/GN showed a significant increase in loss in DC electrical conductivity with increasing the temperature On the other hand, Pani/MWCNTs showed a little initial gain up to 80  C and thereafter a comparatively lower loss in conductivity with increase in temperature This may be due to the inclusion of MWCNTs which may interact with Pani by different fashions causing somewhat irregularities in their behavior In addition, it causes also the low thermal stability and the insolubility in water This polymerization pathway is sulphonated for the formation of the soluble and thermally stable in a common polar solvents and water It was observed that the S-Pani/GN showed a significant decrease in the magnitude of either loss/gain leading towards the better thermal stabilization as compared with the S-Pani/MWCNTs This seems to be because of the fact that GN has (a) the flat geometry, (b) the high mobility of charge carriers and (c) the very high density of surface defects etc as compared to MWCNTs These extraordinary properties of GN may facilitate the interaction of Pani with incoming sulphonic groups and thus strengthening the thermal stability of as-prepared nanocomposites much more than that of MWCNTs The change in the relative electrical conductivity in each experiment was divided by the pratical duration (20 min) to get electrical conductivity loss/gain per minute of heating as given by the following equation: M Khan et al / Journal of Science: Advanced Materials and Devices (2019) 132e142 137 Fig Initial DC electrical conductivity of: (a) Pani (b) Pani/MWCNTs (c) Pani/GN (d) S-Pani (e) S-Pani/MWCNTs and (f) S-Pani/GN Ds ¼ sf À si Dt Sensing studies (2) where Ds ¼ change in relative electrical conductivity/min, sf ¼ final relative electrical conductivity at temperature T, si ¼ initial relative electrical conductivity at temperature T and Dt is duration of experiment (20 min) Hence, it may be inferred that the S-Pani/GN is thermally more stable as compared to S-Pani/MWCNTs and has better solubility in water holding greater promise in the fields of thermoelectric race 4.2 Cyclic ageing A bar graph showing cyclic ageing process of Pani, Pani/ MWCNTs, Pani/GN and their sulphonated derivatives have been demonstrated in Fig It was observed that loss/gain in conductivities of sulphonated products was much lesser than those samples which were not treated with chlorosulphonic acid It seems that the sulphonation (covalent attachment of eSO3H) has replaced the free charge carriers in S-Pani, S-Pani/MWCNTs and S-Pani/GN and remain covalently intact with them causing enhancement in thermal stability The fundamental and principle enabling Pani to be used as chemosensory is how either dopants or other reagents interact with it's producing charge carriers responsible for electrical conduction This interaction at the molecular level affects the number and the movement of charge carriers along the chain Design of a simple, low cost and portable novel gas sensing device based on small changes in conductivity in response to the binding of analysts has been reported here Ansari et al [17] in their nanocomposite of polyaniline with TiO2 has described the sensing response towards ammonia with a good response as well as a fast recovery Presently, we are continuing to deal with this interaction not limiting to ammonia only but also for amines 1, 2 or 3 5.1 Treatment of film for sensing The films for sensing were prepared by dissolving powders of Pani, Pani/MWCNTs and Pani/GN in the N-Methyl-2-pyrrolidone (NMP) solution by a sonicator Then, it is casted on a round shape Petridis and is put in the oven at 70  C till the solvent (NMP) evaporated After that it is taken out in the form of film Fig Changes in the relative electrical conductivity/min of: (a) Pani (b) Pani/MWCNTs (c) Pani/GN (d) S-Pani (e) S-Pani/MWCNTs and (f) S-Pani/GN under isothermal ageing 138 M Khan et al / Journal of Science: Advanced Materials and Devices (2019) 132e142 Fig Changes in the electrical conductivity/cycle of: (a) Pani, (b) Pani/MWCNTs, (c) Pani/GN, (d) S-Pani, (e) S-Pani/MWCNTs and (f) S-Pani/GN under cyclic ageing by a very cheap and best methodology As-prepared films doped in a hydrochloride solution (1M) Furthermore, they were taken out in hrs and then dried at room temperature These films were then put inside the test box of 4-probe and, were alternately exposed to the ambient air, vapors of ammonia and corresponding amines for deprotonation In this case, notable changes in response were observed There are a lot of articles reported the increase of the conductivity of polyaniline with the acid added and the decrease in the basic atmosphere But herein, we have observed an unusual phenomenon in juxtaposition to the above mentioned assumption This seems to be because of the fact that the preparation of polyaniline was carried out in the acidic medium and thus obtained products were usually emeraldine salts which are good conducting in nature When these emeraldine salts are allowed to come in contact with basic atmosphere, there are possibilities for the formation of several redox forms of Pani, such as leucoemeraldine base (non conducting), emeraldine base (half-oxidized form), conducting emeraldine salt (half-oxidized and protonated form), and pernigraniline base (fully oxidized form) [23] Therefore, its mechanism of gas sensitivity exposed to vapor molecules would obviously be complex [24] The sensing mechanism depends upon the types of interactions which are involved, i.e strong chemical bond formation or weak hydrogen bonding, van der Waals force etc., between the sensing film and adsorbed vapor molecules It is well known that for a strong interaction system, the recovery is generally very difficult, but in case of weak interactions, the recovery is easy even at room temperature Since we have observed here both the recovery as well as the reproducibility but with a constant decrease in magnitude That's the reason that there are possibilities of both types of interactions In brief, it can be concluded that there are two processes which are in operation The first one is the irreversible acid-base compensation or electrical neutralization of the Pani backbone and the second one is the reversible chemisorptions of amines with Pani In case of the irreversible acid/base neutralization the mechanism is simple (based on the protonation and deprotonation process) and the conductivity decreases in basic conditions and gets increased in ambient air Herein, when Pani is exposed to low concentration of amines, the positive charge carrying nitrogens in the emeraldine salt play some acid-base chemistry leading to the undoping of the Pani The lone pair of electrons existed on amines thus interacts with the positive site of Pani This causes the decrease in the intensities of the positive charge carriers (holes) as well as their mobility resulting in the decrease in electrical conductivity [17] Iin case of reversible chemisorption process it can be inferred that when nitrogen of ammonia comes in contact with the emeraldine salt of Pani, it forms a temporarily unstable complex leading to the decrease in electrical conductivity Whereas if the process is reversed and emeraldine salt form of Pani is provided at the ambient environment, then the previously formed temporary complex breaks down into their constituents causing regain in electrical conductivity But the unusual electrical behavior of Pani observed here, in some cases of our studies, seems to be attributed by several factors like the presence of electron releasing groups, steric hindrance, the van der Waals interactions etc 5.2 Effects of ammonia and their derivatives (1, 2 and 3 amines) on the electrical response of the Pani, Pani/MWCNTs and Pani/GN sensor 5.2.1 The selectivity The effects of ammonia, methylamine (Ma), dimethylamine (Dma), and trimethylamine (Tma) on the electrical response of the sensor are shown in Fig From this figure, it seems that the response magnitude as well as the response rate decrease in some cases and in a few cases they increases with time Thus the different electrical response of the same Pani sensor towards vapor molecules of different derivatives of ammonia was observed On account of these differences in results, Pani chemosensor might be used to distinguish ammonia and some similar vapors of their derivatives (1, 2 and 3 amines) with the help of so designed sensor arrangement [25] 5.2.2 Factors affecting electrical response It is well established that in methylamine, dimethylamine and trimethylamine, one, two and/or three electron releasing groups M Khan et al / Journal of Science: Advanced Materials and Devices (2019) 132e142 139 Fig Conductivity variation of the in-situ polymerized Pani film on exposure to (a) ammonia (b) methylamine (c) dimethylamine and (d) trimethylamine (-CH3) are attached respectively which creates different electron density at the incoming nitrogen atom This difference in electron density causes the dissimilarity in the interaction of sensor film with adsorbed gas molecules, resulting in the variation of observed conductivity This is the effect of electron releasing groups (ỵI effect) Effect of steric hindrance: There are the so-called the effect of steric hindrance There, the nitrogen atom responsible for interaction are more hindered in 3 amine than in 2 and 1 We also know that the N-atom of the emeraldine salt is radical cation and is very small in size which causes polarization and will create induced dipole moments in the incoming vapor molecules and there would be possibilities of strong van der Waal's interaction between them It was also observed that increment in electrical conductivity also depends upon the degree of polarizing power of the cation radical and polarizability of the incoming electron rich molecule The higher the polarity of the vapor the more is the conductivity increased This is the effect of polarization These all above mentioned reasons are seemed to be the possible cause for out of the ordinary behavior in electrical conductivity of Pani sensor film in the basic environment and ambient air But in our viewpoint, the unusual behavior of the electrical conductivity largely depends upon the strong interaction of the van der Waal's force resulting from the grain boundary effects 5.2.3 Reproducibility The reversible chemisorption process based on the physical adsorption and the desorption process causes a change in the conductivity of the sensor film attributing to its reproducibility The noteworthy changes in conductivities of as-prepared materials on the exposure to different concentrations of aqueous ammonia and amines at room temperature as a function of time are observed We 140 M Khan et al / Journal of Science: Advanced Materials and Devices (2019) 132e142 Fig Interaction and conductivity variation of Pani/MWCNTs film on exposure to (a) ammonia (b) methylamine (c) dimethylamine and (d) trimethylamine have systematically investigated several samples of Pani, Pani/ MWCNTs and Pani/GN for their sensing response to ammonia as well as amines which can be looked into one by one in the Figs and 10 It can be seen that almost all as-prepared materials showed the worth mentioning response but with different paces The conductivity could also be recovered upon flushing with the ambient air Along with this reproducibility, they were observed to have excellent reversibility, stability and selectivity toward NH3 gas over their derivatives which are of immense importance for their potential applications in designing of efficient and novel portable sensing devices [26,27] To investigate the reversibility of the gas sensor so designed, first of all, the as-prepared Pani was repeatedly exposed to ammonia gas It was observed that on exposure of Pani to ammonia vapor, the process of chemisorption starts occurring Here in, when nitrogen of ammonia comes in contact with the emeraldine salt of Pani, it forms a temporarily unstable complex leading to the decrease in electrical conductivity But when the process is reversed and emeraldine salt form of Pani is provided at the ambient environment then the previously formed temporary complex breaks down into their constituents causing regain in electrical conductivity M Khan et al / Journal of Science: Advanced Materials and Devices (2019) 132e142 141 Fig 10 Conductivity variation of in-situ polymerized Pani/GN film on exposure to: (a) ammonia (b) methylamine (c) dimethylamine and (d) trimethylamine It was observed that the extent of the recovery was constantly decreasing with increase in time as indicated by the continuous decrease in the amplitude of damped oscillation This regular decrease in the recovery of gas sensor may be attributed to the regular consumption of active sites of Pani with constant rate and lastly because of the insufficient numbers of reacting sites available for ammonia moiety to reform the complex structure required for obtaining the recovery response From the decrease in amplitude as shown in the graph, it can be inferred that with an increase in time slight irreversibility starts occurring which is due to the electrical compensation of the Pani backbone by ammonia Conclusion In summary, we have successfully prepared Pani and its nanocomposite with MWCNTs and GN via the oxidative polymerization Reinforcement of both MWCNTs and GN in Pani has dazzlingly enhanced their DC electrical conductivity but the result was more fruitful for graphene Furthermore, effective supplementation of functional groups -SO3H to the polyaniline surface of these in-situ products has brightly improved their solubility thus openined new possibilities for their prospective technological applications However, ironically it has caused a heavy cutback in conductivity of all the products This may probably due to the replacement of ionic charge carriers by the covalently induced -SO3H After detailed studies of characteristics and sensing properties, finally, the authors came to the conclusion that the infusion of graphene is much more effective than MWCNTs Thus, it may be postulated that mingling of graphene can be thought out as an universal approach to prepare nanocomposites with enhanced conductivity and better solubility, which may find more realistic applications in modern electronic devices and seems to be a replaceable alternate even for metals in next generation We are looking forward to continued explosive growth in this field 142 M Khan et al / Journal of Science: Advanced Materials and Devices (2019) 132e142 Conflict of interest The authors have declared no conflict of interest Acknowledgements One of the author's Dr Mahfoozurrahman Khan acknowledges with thanks the financial support from Department of Science & Technology, India (DST PURSE-II) References [1] B Dong, B.L He, C.L Xu, H.L Li, Preparation and electrochemical characterization of polyaniline/multi-walled carbon 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M.H Cho, F Mohammad, Ammonia vapor sensing and electrical properties of fibrous multi-walled carbon nanotube/ polyaniline nanocomposites prepared in presence of cetyl-trimethylammonium bromide,... extraordinary properties of GN may facilitate the interaction of Pani with incoming sulphonic groups and thus strengthening the thermal stability of as-prepared nanocomposites much more than that of MWCNTs