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

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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.

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Nội dung Text: Sensing properties of sulfonated multi-walled carbon nanotube and graphene nanocomposites with polyaniline

Journal of Science: Advanced Materials and Devices 4 (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: Here, we discuss one of the simplest approaches for chemical functionalization of in-situ prepared Received 27 September 2018 polyaniline (Pani) and its nanocomposites with multi-walled carbon nanotubes (MWCNT) and graphene Received in revised form (GN) in chlorosulphonic acid. The effect of polymerization and functionalization was characterized by 2 February 2019 Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) analysis, Field emission scanning Accepted 3 February 2019 Available online 8 February 2019 electron microscopy (FESEM) and electro-thermal analysis. Results also revealed the presence of pp interactions between Pani and carbon allotropes leading to the formation of charge-transfer complexes. This strong pp interaction signiﬁcantly increased the resultant electrical conductivity, stabilizing them Keywords: In-situ polymerization as well. Further, theirs back to back sulphonation in chlorosulphonic acid signiﬁcantly enhanced the Pani/MWCNT and Pani/GN nanocomposite solubility in one way but caused a heavy loss in conductivity conversely. The thermoelectric properties of Sulphonation the as-prepared nanocomposites were investigated as a function of MWCNT and GN contents. It was Isothermal and cyclic ageing technique observed that as-prepared Pani/GN nanocomposites showed a greater electrical conductivity as well as Amine identiﬁcation 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/). 1. Introduction strength nanoﬁbres, sensors and nanoelectronic wires it has become of immense interest to attach some functional parts on Limitless publications on 150 years old polyaniline and young their surface [5e7]. These (covalent or non-covalent) surface aged rapidly rising graphitic nanomaterials (i.e. CNTs, graphenes, modiﬁcations caused either by organic or inorganic species in fullerenes etc.) [1,2] have been evidently shown how they have general and especially by sulphonic groups highly enhanced their matured over the past few years with a very broad spectrum and solubility, impart considerable stability and strong surface acidity wide application range in engineering and medical sciences as well making it highly useful for sophisticated electronic applications as in their commercial & economical aspects. Recently, their and excellent catalyst support for highly dispersed metal nano- nanocomposites possess the combination of high stability and fairly particles [8e11]. good electrical conductivity due to the synergism between the That's the reason why the molecular functionalization has been constituents [3]. Despite the stubborn improvement ensued so far, the deep-seated interest of research world after the ingenious work the backbone stiffness of polyaniline, limited solubility and stability of Yue and Epstein [12] and persistently opening the door to un- of MWCNT and graphene dispersions in water remains an ongoing precedented materials applications. Covalent functionalization is challenge complicating its processing, management and ultimately comparatively more effective and greatly alters the electrical con- the scope of their applications [4]. On account of these issues and to ductivity of polyaniline. It introduces some defect sites forming a optimize their efﬁciency for use in various applications such as high new type of nanostructures for diverse applications. On the other hand, Hua Bai et al. [13] have reported a non-covalent functional- ization process involving the formation of charge transfer com- * Corresponding author. plexes due to the strong pp interactions between quinoidal units E-mail addresses: mahfooz55@gmail.com (M. Khan), tariqalig001@gmail.com of Pani and pyrenyl rings of MWCNTs or graphene. Yue and Epstein (T. Anwer), faizmohammad54@rediffmail.com (F. Mohammad). [14] prepared self doped Pani almost two decades ago by the Peer review under responsibility of Vietnam National University, Hanoi. 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 4 (2019) 132e142 133 chemical modiﬁcation in the presence of fuming H2SO4 and studied were stored in an airtight sample container for characterized their better solubility as well as redox activity and conductivity over further studies. a wider pH range. Deore et al. [15] prepared switchable self-doped Pani with interconversion between self-doped and non-self-doped 2.4. Characterization forms. Recently Zhang et al. [16] have reported the development of water soluble nanocomposite of sulphonated polyaniline with Surface morphologies of Pani (EB), Pani (EB)/MWCNT and Pani MWCNTs. Their electrical parameters such as conductivity, thermal (EB)/GN (gold coated) were viewed under a scanning electron mi- stability, dielectric behavior etc. have rarely been reported. In this croscope (SEM) (LEO 435-VF). Their phase composition was work, we report our continued effort to make use of chlor- analyzed by X-ray diffraction (XRD) recorded by Bruker D8 osulphonic acid in an inert solvent instead of using fuming H2SO4 diffractometer with Cu Ka radiation at 1.540 Å in the range of for sulphonation of Pani, Pani/MWCNT and Pani/GN nano- 5 2q 70 at 40 kV. The FT-IR spectra were recorded using composites. Comparative studies on sensing performance and DC Perkin-Elmer-Spectrum 2000 Spectrophotometer in KBr between electrical conductivity retention were also done to ascertain efﬁ- 400 and 4000 cm1. The electrical conductivity as well as thermal ciency for their potential applications in the latest sophisticated stability in terms of DC electrical conductivity retention were technologies. studied according to the method already reported [17] using the equation: 2. Experimental Ds ¼ ½ln 2ð2S=WÞ =½2pSðV=IÞ (1) 2.1. Materials used where I, V, W and S are the current (A), voltage (V), the thickness of Monomer “aniline” from E-Merck India Ltd. was puriﬁed by the pellet (cm) and probe spacing (cm) respectively and s is the DC distilling twice before use. The MWCNT (diameter and average electrical conductivity (S cm1) [17]. lengths were about 10e20 nm and 20 mm respectively) and gra- phene used in this study were purchased from Iljin Nano Tech, 3. Results and discussion Seoul, Korea. Potassium persulphate (PPS) and HCl (AR grade) and methanol were purchased from (CDH India Ltd.) and were used as 3.1. The formation of S-Pani, S-Pani/MWCNT and S-Pani/GN received. Double distilled water (DDW) was used in all the exper- imental procedures and washing. The proposed novel balanced chemical equation and formation scheme of Pani (EB) and its sulphonation has been summarized 2.2. Preparation of Pani, Pani/MWCNT and Pani/GN below. Here, ﬁrst of all Pani (EB) was prepared by simple oxidative nanocomposites polymerization whereas Pani (EB)/MWCNT and Pani (EB)/GN were also prepared in the same way by additional use of MWCNT and The nanocomposites of Pani/MWCNTs were prepared by in-situ graphene nanosheets. These in-situ products were dried and then oxidative polymerization. Firstly, 200 mL of 1M HCl and 5 mL an- treated with chlorosulphonic acid (HSO3Cl) at 75 C Fig. 1(a). Dark iline were added dropwise under stirring. The ultrasonicated sus- green free ﬂowing powders were obtained expecting to achieve pension of MWCNTs (0.15 gm in 100 mL of 1M HCl) was transferred 50% sulphonation. Achievement of 100% sulphonation has not been into aniline solution. The oxidant was readied by dissolving 14.8 gm reported so far to the best of our knowledge but it would be pre- K2S2O8 in 200 mL of 1 M HCl. The polymerization was effected by dicted to have considerably greater water solubility. Similarly, S- dropwise adding the oxidant solution into the aniline/MWCNTs Pani/MWCNT and S-Pani/GN nanocomposites can be produced in suspension and left for stirring continuously for about 16 hours. The Fig. 1(b,c). resultant greenish black slurry was ﬁltered and washed thoroughly with 2.5 L double distilled water to remove the excess acid as well 3.2. Conﬁrmation for sulphonation as oxidant until the ﬁltrate became colorless. Thus prepared nanocomposites was dedoped by aqueous (IM) ammonia to convert Color change test was carried out to get conﬁrmation whether as it into emeraldine base (EB) form. The Pani/MWCNTs (EB) was dried prepared products have undergone sulphonation or not. For this, around 70 C for 4 hours in an air oven, converted into ﬁne powder these chlorosulphonic acid treated products were dissolved in and was stored in a cool and dry place for further investigations. ammonia solution Fig. 2. It was observed that the color of the so- Pani (EB) as well as Pani (EB)/GN nanocomposites were also pre- lution turned blue within few seconds indicating the process of pared using the same method. undoping. When these undoped solutions were heated at 100 C, the color of the solution mixtures returned back to greenish black 2.3. Sulphonation of Pani, Pani/MWCNT and Pani/GN due to volatilization of ammonia vapors. This evidently proves that nanocomposites chlorosulphonic acid treated products had undergone sulphonation. The prepared Pani (EB) was sulphonated by using chlor- osulphonic acid 2 gm of as-prepared Pani (EB) powder was 3.3. FTIR spectroscopic studies dispersed in 200 mL of 1, 2-dichloroethane (DCE) and constantly stirred at 75 C. Thereafter, 3.5 gm of chlorosulphonic acid diluted Pani (EB), Pani (EB)/MWCNTs and Pani (EB)/GN and their sul- with 10 mL of DCE was added dropwise in 20 min and the reaction phonated products were characterized by FTIR to study the in- mixture was left for 4 hours. The resultant greenish suspension was teractions between comprising constituents. The peaks at 1620 and ﬁltered, kept in 100 mL water and heated for 6 hours at 75 C to 1560 cm1 (corresponding to quinoid and benzenoid ring respec- promote its hydrolysis. Finally, the ﬁlter cake was washed with 1 L tively), 1338 cm1 (C-N stretching), 1205 cm1 (C¼N stretching), water, followed by 200 mL methanol, dried in an air oven at 70 C and 823 cm1 (1,4-substituted phenyl ring stretching) are charac- and was transformed into sulphonated Pani (S-Pani). Similarly teristics of EB form of Pani Fig. 3 [18,19]. Likewise, FTIR spectra of sulphonated Pani/MWCNTs (S-Pani/MWCNTs) and sulphonated Pani/MWCNTs and Pani/GN are almost identical to Pani except all Pani/GN (S-Pani/GN) were prepared using the same method and their peaks have slightly shifted to higher wavenumbers. This
134 M. Khan et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 132e142 Fig. 1. 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 4 (2019) 132e142 135 Fig. 2. Color change test for self-doped S-Pani. seems to be due to the pp interaction between quinoid rings of accessibility. It seems that the sulphonation of Pani beans trans- Pani and pyrenyl rings of graphitic materials. formed them into granular structures. In case of S-Pani/MWCNTs On the other hand, if comparing these spectra with those of and S-Pani/GN (Fig. 4d,e), eSO3H groups seems to have attached their corresponding sulphonated counterparts, we observe that the with Pani chain encapsulating the MWCNTs and GN respectively sulphonation has caused the band broadening in all the cases. The [21]. presence of characteristic peaks at 1090 and 1015 cm1 matching to the asymmetric and symmetric O¼S¼O stretching vibrations 4. Electrical properties respectively verify the presence of sulfonic groups covalently bound to the polymer backbone [20]. The presence of SeO Electro-thermal studies of all as-prepared Pani, Pani/MWCNTs, stretching peak at 730 cm1 also supports the sulphonation of Pani/GN and their sulphonated products were well carried out by Pani, Pani/MWCNTs and Pani/GN. standard 4-in-line probe technique in the temperature range of 40 Ce150 C. As-prepared Pani, Pani/MWCNTs, Pani/GN (EB) are 3.4. Surface morphology doped with H2SO4 in which Hþ ions act as a dopant and sulpho- nation (covalent attachment of eSO3H). Results of the electrical The FE-SEM images of Pani, Pani/MWCNTs, Pani/GN and their conductivity measurements indicate the p-type semiconducting sulphonated products are shown in Fig. 4 at different magniﬁca- behavior of all the materials within the operating temperature tions. Pani (Fig. 4a) seems to have “Lima” bean shaped curved range of the experiment. It was also observed that the addition of structure, whereas Pani/MWCNTs have somewhat more elon- MWCNTs and GN has caused the augmentation in their electrical gated tubular morphology indicating polymerization of aniline conductivities as shown in Fig. 5. Since the Pani as well as infusing over MWCNTs (Fig. 4c). In the case of Pani/GN, ﬂaky sheets are graphitic nano ﬁllers MWCNTs and GN) are good conducting, the being observed which indicates the deposition of polyaniline over enhancement in DC electrical conductivity may be credited to the graphene nanosheets. The existence of MWCNT and GN can be additive synergism of both the constituents interacting at the evidently seen and act as a conductive pathway for electron car- molecular level. Although, both MWCNTs and GN are nearly rying which interconnects among Pani coated with Pani/MWCNTs matching in their chemical makeup and mechanical properties, and Pani/GN to hinder close stacking and improve electrolyte ions graphene has been observed to be far better than MWCNTs in Fig. 3. 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 4 (2019) 132e142 Fig. 4. 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 conductiv- temperature. On the other hand, Pani/MWCNTs showed a little ity, strength etc.) to the host polymer matrix [22]. Both the initial gain up to 80 C and thereafter a comparatively lower loss in MWCNTs and GN are derived from graphite but the mobility of conductivity with increase in temperature. This may be due to the electrons is higher in GN making it more electronically conducting inclusion of MWCNTs which may interact with Pani by different than MWCNTs. That's why the nanoﬁller loading of Pani by fashions causing somewhat irregularities in their behavior. In MWCNTs and GN have caused a promising augmentation in elec- addition, it causes also the low thermal stability and the insolubility trical conductivity. Besides, the strong pp-pp interaction between in water. This polymerization pathway is sulphonated for the for- the p-bonded surface of the carbon based nano scaled materials mation of the soluble and thermally stable in a common polar and the conjugated structure of Pani also imparts the enhancement solvents and water. of the conductivity up to some extent. It was observed that the S-Pani/GN showed a signiﬁcant But contrary to that, sulphonation (covalent attachment of decrease in the magnitude of either loss/gain leading towards the eSO3H) to Pani and its nanocomposites with either MWCNTs or GN better thermal stabilization as compared with the S-Pani/MWCNTs. one, has caused a promising cutback in electrical conductivity of all This seems to be because of the fact that GN has (a) the ﬂat ge- the three S-Pani, S-Pani/MWCNTs as well as S-Pani/GN. This seems ometry, (b) the high mobility of charge carriers and (c) the very to be because of the low extent of doping. high density of surface defects etc. as compared to MWCNTs. These extraordinary properties of GN may facilitate the interaction of Pani 4.1. Isothermal ageing with incoming sulphonic groups and thus strengthening the ther- mal stability of as-prepared nanocomposites much more than that The isothermal investigation in terms of DC electrical conduc- of MWCNTs. tivity retention of prepared materials would be a key component in The change in the relative electrical conductivity in each analyzing their thermal stability. From Fig. 6, it may be seen that the experiment was divided by the pratical duration (20 min) to get as-prepared Pani as well as Pani/GN showed a signiﬁcant increase electrical conductivity loss/gain per minute of heating as given by in loss in DC electrical conductivity with increasing the the following equation:
M. Khan et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 132e142 137 Fig. 5. 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. 5. Sensing studies sf si Ds ¼ (2) Dt The fundamental and principle enabling Pani to be used as where Ds ¼ change in relative electrical conductivity/min, chemosensory is how either dopants or other reagents interact sf ¼ ﬁnal relative electrical conductivity at temperature T, with it's producing charge carriers responsible for electrical con- si ¼ initial relative electrical conductivity at temperature T and Dt is duction. This interaction at the molecular level affects the number duration of experiment (20 min). and the movement of charge carriers along the chain. Design of a Hence, it may be inferred that the S-Pani/GN is thermally more simple, low cost and portable novel gas sensing device based on stable as compared to S-Pani/MWCNTs and has better solubility in small changes in conductivity in response to the binding of analysts water holding greater promise in the ﬁelds of thermoelectric race. 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, 4.2. Cyclic ageing we are continuing to deal with this interaction not limiting to ammonia only but also for amines 1, 2 or 3 . A bar graph showing cyclic ageing process of Pani, Pani/ MWCNTs, Pani/GN and their sulphonated derivatives have been demonstrated in Fig. 7. It was observed that loss/gain in conduc- 5.1. Treatment of ﬁlm for sensing tivities of sulphonated products was much lesser than those sam- ples which were not treated with chlorosulphonic acid. It seems The ﬁlms for sensing were prepared by dissolving powders of that the sulphonation (covalent attachment of eSO3H) has replaced Pani, Pani/MWCNTs and Pani/GN in the N-Methyl-2-pyrrolidone the free charge carriers in S-Pani, S-Pani/MWCNTs and S-Pani/GN (NMP) solution by a sonicator. Then, it is casted on a round and remain covalently intact with them causing enhancement in shape Petridis and is put in the oven at 70 C till the solvent thermal stability. (NMP) evaporated. After that it is taken out in the form of ﬁlm Fig. 6. 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 4 (2019) 132e142 Fig. 7. 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 gets increased in ambient air. Herein, when Pani is exposed to low in a hydrochloride solution (1M). Furthermore, they were taken concentration of amines, the positive charge carrying nitrogens in out in 2 hrs and then dried at room temperature. These ﬁlms the emeraldine salt play some acid-base chemistry leading to the were then put inside the test box of 4-probe and, were alter- undoping of the Pani. The lone pair of electrons existed on amines nately exposed to the ambient air, vapors of ammonia and thus interacts with the positive site of Pani. This causes the decrease corresponding amines for deprotonation. In this case, notable in the intensities of the positive charge carriers (holes) as well as changes in response were observed. their mobility resulting in the decrease in electrical conductivity There are a lot of articles reported the increase of the conduc- [17]. Iin case of reversible chemisorption process it can be inferred tivity of polyaniline with the acid added and the decrease in the that when nitrogen of ammonia comes in contact with the emer- basic atmosphere. But herein, we have observed an unusual phe- aldine salt of Pani, it forms a temporarily unstable complex leading nomenon in juxtaposition to the above mentioned assumption. to the decrease in electrical conductivity. Whereas if the process is This seems to be because of the fact that the preparation of poly- reversed and emeraldine salt form of Pani is provided at the aniline was carried out in the acidic medium and thus obtained ambient environment, then the previously formed temporary products were usually emeraldine salts which are good conducting complex breaks down into their constituents causing regain in in nature. When these emeraldine salts are allowed to come in electrical conductivity. But the unusual electrical behavior of Pani contact with basic atmosphere, there are possibilities for the for- observed here, in some cases of our studies, seems to be attributed mation of several redox forms of Pani, such as leucoemeraldine by several factors like the presence of electron releasing groups, base (non conducting), emeraldine base (half-oxidized form), steric hindrance, the van der Waals interactions etc. conducting emeraldine salt (half-oxidized and protonated form), and pernigraniline base (fully oxidized form) [23]. Therefore, its 5.2. Effects of ammonia and their derivatives (1, 2 and 3 amines) mechanism of gas sensitivity exposed to vapor molecules would on the electrical response of the Pani, Pani/MWCNTs and Pani/GN obviously be complex [24]. sensor The sensing mechanism depends upon the types of interactions which are involved, i.e. strong chemical bond formation or weak 5.2.1. The selectivity hydrogen bonding, van der Waals force etc., between the sensing The effects of ammonia, methylamine (Ma), dimethylamine ﬁlm and adsorbed vapor molecules. It is well known that for a (Dma), and trimethylamine (Tma) on the electrical response of the strong interaction system, the recovery is generally very difﬁcult, sensor are shown in Fig. 8. From this ﬁgure, it seems that the but in case of weak interactions, the recovery is easy even at room response magnitude as well as the response rate decrease in some temperature. Since we have observed here both the recovery as cases and in a few cases they increases with time. Thus the different well as the reproducibility but with a constant decrease in electrical response of the same Pani sensor towards vapor mole- magnitude. That's the reason that there are possibilities of both cules of different derivatives of ammonia was observed. On account types of interactions. of these differences in results, Pani chemosensor might be used to In brief, it can be concluded that there are two processes which distinguish ammonia and some similar vapors of their derivatives are in operation. The ﬁrst one is the irreversible acid-base (1, 2 and 3 amines) with the help of so designed sensor compensation or electrical neutralization of the Pani backbone arrangement [25]. and the second one is the reversible chemisorptions of amines with Pani. In case of the irreversible acid/base neutralization the 5.2.2. Factors affecting electrical response mechanism is simple (based on the protonation and deprotonation It is well established that in methylamine, dimethylamine and process) and the conductivity decreases in basic conditions and trimethylamine, one, two and/or three electron releasing groups
M. Khan et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 132e142 139 Fig. 8. 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 electron rich molecule. The higher the polarity of the vapor the density at the incoming nitrogen atom. This difference in electron more is the conductivity increased. This is the effect of polarization density causes the dissimilarity in the interaction of sensor ﬁlm These all above mentioned reasons are seemed to be the with adsorbed gas molecules, resulting in the variation of observed possible cause for out of the ordinary behavior in electrical con- conductivity. This is the effect of electron releasing groups (þI ductivity of Pani sensor ﬁlm in the basic environment and ambient effect). air. But in our viewpoint, the unusual behavior of the electrical Effect of steric hindrance: There are the so-called the effect of conductivity largely depends upon the strong interaction of the van steric hindrance. There, the nitrogen atom responsible for interac- der Waal's force resulting from the grain boundary effects. tion are more hindered in 3 amine than in 2 and 1. We also know that the N-atom of the emeraldine salt is radical 5.2.3. Reproducibility cation and is very small in size which causes polarization and will The reversible chemisorption process based on the physical create induced dipole moments in the incoming vapor molecules adsorption and the desorption process causes a change in the and there would be possibilities of strong van der Waal's interac- conductivity of the sensor ﬁlm attributing to its reproducibility. The tion between them. It was also observed that increment in elec- noteworthy changes in conductivities of as-prepared materials on trical conductivity also depends upon the degree of polarizing the exposure to different concentrations of aqueous ammonia and power of the cation radical and polarizability of the incoming amines at room temperature as a function of time are observed. We
140 M. Khan et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 132e142 Fig. 9. 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/ To investigate the reversibility of the gas sensor so designed, MWCNTs and Pani/GN for their sensing response to ammonia as ﬁrst of all, the as-prepared Pani was repeatedly exposed to well as amines which can be looked into one by one in the Figs. 9 ammonia gas. It was observed that on exposure of Pani to ammonia and 10. It can be seen that almost all as-prepared materials vapor, the process of chemisorption starts occurring. Here in, when showed the worth mentioning response but with different paces. nitrogen of ammonia comes in contact with the emeraldine salt of The conductivity could also be recovered upon ﬂushing with the Pani, it forms a temporarily unstable complex leading to the ambient air. Along with this reproducibility, they were observed to decrease in electrical conductivity. But when the process is have excellent reversibility, stability and selectivity toward NH3 gas reversed and emeraldine salt form of Pani is provided at the over their derivatives which are of immense importance for their ambient environment then the previously formed temporary potential applications in designing of efﬁcient and novel portable complex breaks down into their constituents causing regain in sensing devices [26,27]. electrical conductivity.
M. Khan et al. / Journal of Science: Advanced Materials and Devices 4 (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 enhanced their DC electrical conductivity but the result was more decreasing with increase in time as indicated by the continuous fruitful for graphene. Furthermore, effective supplementation of decrease in the amplitude of damped oscillation. This regular functional groups -SO3H to the polyaniline surface of these in-situ decrease in the recovery of gas sensor may be attributed to the products has brightly improved their solubility thus openined new regular consumption of active sites of Pani with constant rate and possibilities for their prospective technological applications. How- lastly because of the insufﬁcient numbers of reacting sites available ever, ironically it has caused a heavy cutback in conductivity of all for ammonia moiety to reform the complex structure required for the products. This may probably due to the replacement of ionic obtaining the recovery response. From the decrease in amplitude as charge carriers by the covalently induced -SO3H. After detailed shown in the graph, it can be inferred that with an increase in time studies of characteristics and sensing properties, ﬁnally, the authors slight irreversibility starts occurring which is due to the electrical came to the conclusion that the infusion of graphene is much more compensation of the Pani backbone by ammonia. effective than MWCNTs. Thus, it may be postulated that mingling of graphene can be thought out as an universal approach to prepare 6. Conclusion nanocomposites with enhanced conductivity and better solubility, which may ﬁnd more realistic applications in modern electronic In summary, we have successfully prepared Pani and its nano- devices and seems to be a replaceable alternate even for metals in composite with MWCNTs and GN via the oxidative polymerization. next generation. We are looking forward to continued explosive Reinforcement of both MWCNTs and GN in Pani has dazzlingly growth in this ﬁeld.
142 M. Khan et al. / Journal of Science: Advanced Materials and Devices 4 (2019) 132e142 Conﬂict of interest [13] H. Bai, Y. Xu, L. Zhao, C. Li, G. Shi, Non-covalent functionalization of graphene sheets by sulfonated polyaniline, Chem. Commun. 13 (2009) 1667e1669. [14] J. Yue, A.J. Epstein, Synthesis of self-doped conducting polyaniline, J. Am. The authors have declared no conﬂict of interest. Chem. Soc. 112 (1990) 2800e2801. [15] B.A. Deore, I. Yu, M.S. Freund, A switchable self-doped polyaniline: intercon- version between self-doped and non-self-doped forms, J. Am. Chem. Soc. 126 Acknowledgements (2004) 52e53. [16] H. Zhang, H.X. Li, H.M. Cheng, Water-soluble multiwalled carbon nanotubes functionalized with sulfonated polyaniline, J. Phys. Chem. B 110 (2006) One of the author's Dr. Mahfoozurrahman Khan acknowledges 9095e9099. with thanks the ﬁnancial support from Department of Science & [17] M.O. Ansari, F. 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