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Synthesis of sodium cholate mediated rod-like polypyrrole-silver nanocomposite for selective sensing of acetone vapor
Nano-Structures & Nano-Objects 21 (2020) 100419
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Synthesis of sodium cholate mediated rod-like polypyrrole-silver nanocomposite for selective sensing of acetone vapor Arpita Adhikari a , Pradip Kar b , Dipak Rana c , Sriparna De d , Jyotishka Nath a , ∗ Koushik Dutta a , Dipankar Chattopadhyay a , a
Department of Polymer Science and Technology, University of Calcutta, 92 A.P.C. Road, Kolkata 700 009, India Department of Chemistry, Birla Institute of Technology, Mesra, Ranchi 835 215, Jharkhand, India c Department of Chemical and Biological Engineering, Industrial Membrane Research Institute, University of Ottawa, 161 Louis Pasteur St., Ottawa, ON, K1N 6N5, Canada d Brainware University, Department of Allied Health Science, Barasat, Kolkata 700125, India b
article
info
Article history: Received 15 August 2019 Received in revised form 5 December 2019 Accepted 19 December 2019
a b s t r a c t Nanostructured conducting polymer provides higher surface area for analyte vapor sensing. This investigation aims to synthesize rod-like polypyrrole (PPY) silver nanocomposite tailored by sodium cholate surfactant as soft template for sensing of acetone vapor. During oxidative polymerization of pyrrole by ferric chloride Ag nanoparticle gets deposited on PPY by simultaneous reduction of silver nitrate. The crystal structure, surface morphology and nature of interfacial interactions between the two components of the prepared PPY-silver nanocomposite (PPY-Ag) are analyzed by standard characterization techniques. The Ag nanoparticles deposition on rod-like PPY increases with increasing silver nitrate concentration up to an optimum level. The PPY-Ag nanocomposites show chemiresistive type dynamic sensing responses toward methanol, ethanol and acetone vapor in a mixture with air. A comparative evaluation of typical vapor sensing parameters (% response, response time, recovery time, reproducibility, and selectivity) of PPY-Ag nanocomposite sensor obtained for different volatile organic compounds with those of the pristine PPY sensor is reported. Interestingly, the rod-like PPY-Ag nanocomposite is found to selectively sense acetone vapor showing better % response than that of the other analytes, viz., methanol, ethanol, propanol, water vapor and formaldehyde. The differences in their responses for the different analytes and the selectivity toward acetone vapor are established by considering the difference in their interactions with the nanocomposite. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Today conducting polymers are being explored as materials for gas sensors in the monitoring and control of environmental pollution. Due to having light weight, low cost and sensing ability at room temperature conducting polymers [1,2], have been found to be more advantageous over metal oxides [3], as materials for fabrication of gas sensors. For detection of environmentally hazardous chemical and volatile organic compounds (VOCs), ammonia, chlorine, etc., polypyrrole (PPY) and polyaniline (PANI) based gas sensors have been used [4–7], Out of many VOCs, acetone and different aliphatic alcohols are extensively used in laboratory and industrial practices. Inhaling higher concentrations of these VOCs above allowable limits leads to health hazard as well as environmental pollution. So there is a need of developing suitable materials for sensing of such VOCs including acetone. ∗ Corresponding author. E-mail address: [email protected] (D. Chattopadhyay). https://doi.org/10.1016/j.nanoso.2019.100419 2352-507X/© 2019 Elsevier B.V. All rights reserved.
On the other hand, acetone sensor can play important role in clinical diagnostics for diabetic patients since the level of acetone concentration in the exhaled breath of diabetic patients is used for judging the extent of diabetes. Therefore, quantification of acetone in the exhaled breath of diabetic patients has become a diagnostic approach in diabetes management [8]. Since acetone is known to form reversible H-bond with –NH– group of PPY, it is reported to be used in sensing of acetone [9]. Yu et al. [10] have reported PPY sensor array based gas analyzer for analyzing acetone in the exhaled breath of diabetic patient. Through principal component analysis (PCA) of the acetone sensing data, obtained from this PPY sensor array, a good discrimination of breath acetone was obtained between diabetic and healthy persons. For gas phase sensing of acetone, Do and Wang [11] synthesized PPY by chemical vapor deposition and chemical oxidation casting on the surface of Au/Al2 O3 and obtained acetone sensitivity value of 5.2 × 10−7 ppm−1 . Hamilton et al. [12] electrochemically synthesized free-standing PPY film for sensing of
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acetone, methanol, ethanol and other VOCs. With the help of PCA, discrimination of sensing ability of the polymer was also evaluated. Influence of dopant and electrical conduction mechanism of PPY in sensing of acetone vapor were studied by Ruangchuay et al. [13]. They have attributed the change of conductivity to the swelling, H-bond formation and reduction of charge carrier species in the doped PPY sensor. In paint and lacquer industries as well as in painting sites detection of highly flammable solvent like acetone is necessary with the help of portable sensor. Accordingly, conducting polymer based sensor was exploited for sensing of acetone vapor in lacquer and paint industry/application sites. Due to the poor processability of PPY, insulating polymer was used as a support material to make sensor device for the purpose of such sensing. In such an attempt α -naphthalene sulfonatedoped PPY/poly(methyl methacrylate) (PMMA) blend film was used as selective sensor, based on electrical conductivity, for acetone vapor in lacquer by Ruangchuay et al. [14]. Later in another work Ruangchuay et al. [15] gained an improvement of sensing selectivity of α -naphthalene sulfonate-doped PPY toward acetone and toluene after solution mixing of the doped polymer with insulating polymers like polyethylene oxide (PEO), PMMA, high density poly ethylene (HDPE), polystyrene (PS), acrylonitrile butadiene styrene (ABS) copolymer. Jiang et al. [16] also used poly (vinyl alcohol) (PVA) as support material for PPY in the form of composite film for sensing of methanol vapor. Brina et al. [17] have reported that the presence of metallic nanoparticles in conducting polymer creates active sites for in-situ electron trapping. Higher acetone sensitivity of PANI-silver nanocomposite compared to pure PANI was reported by Choudhury et al. [18]. They have deposited silver nanoparticles on PANI by reduction of silver nitrate with sodium borohydride and studied the effect of silver content on acetone vapor sensing characteristics, viz., dynamic responses, response time, sensitivity and reproducibility. For preparation of well dispersed silver nanoparticles people have used natural precursor, e.g., pomegranate peel for reduction of silver nitrate [19]. Some authors have prepared nanocomposite/nanoparticles of silver compounds having semiconducting properties [20,21]. Conductivity and relative permittivity of organic conducting polymers change depending on the frequency and concentration of the exposed volatile chemicals [22,23]. Utilizing the current frequency dependence on the conductivity of polymer Musio and Ferrara [24] developed PPY film based selective sensor for different analyte vapors, viz., methanol, ethanol, acetone and ethyl acetate where they have studied the response patterns in terms of resistance change by varying the a.c./d.c. frequencies and claimed higher sensing selectivity using low frequency a.c. response. Electrically conducting polyindole was used for sensing of methanol at room temperature [25]. In this work they have explored the effects of polarity, dielectric constant, H-bonding interaction of the different sensing analytes, e.g., methanol and showed highest relative sensing responses in terms of conductivity of the polymer. Anjum et al. [26] have reported selective sensing of lower concentration (100 ppm) alcohol vapors, viz., methanol, ethanol, and propanol in presence of air by graphite doped hydroxyapatite nanoceramic sensor. Although many reports are available on techniques for synthesis of different silver containing nanoparticles [27–33] but no work is reported on the soft template guided synthesis of silver deposited rod-like PPY-nanocomposite for VOC sensing application. In this context the present investigation reports a novel approach on the synthesis of rod-like PPY-Ag nanocomposite with the help of sodium cholate surfactant as soft template for sensing of VOC. In order to further explore the possibility of increased acetone sensing performance we have varied the deposited silver content on the PPY nanorod during polymerization by varying the amount of silver nitrate addition.
2. Experimental 2.1. Materials Pyrrole monomer was procured from Sigma-Aldrich Inc. and purified by vacuum distillation. The distilled monomer was stored at 4 ◦ C under nitrogen blanket prior to polymerization. Analytical reagent grade anhydrous ferric chloride, silver nitrate and hydrochloric acid were purchased from Merck India Ltd., and used without further purification. Analytical reagent grade acetone, methanol, ethanol, formaldehyde as sensing analytes were procured from E. Merck India Ltd., and used without further purification. Sodium cholate, for use as surfactant during polymerization of pyrrole, was obtained from Sisco Research Laboratory, Mumbai, India. Triple distilled water was used for the whole experimental work wherever required. 2.2. PPY-Ag nanocomposite synthesis Sodium cholate mediated rod-like polypyrrole-silver nanocomposite was prepared as per the formulations shown in Table 1 by varying the amount of silver nitrate addition. First sodium cholate was dispersed in an ice cold (0–4 ◦ C) acidic water and then to this dispersion pyrrole was added in stirring condition. Then 0.01 M silver nitrate solution was added followed by addition of ferric chloride solution in stirring condition. In each formulation the pyrrole to ferric chloride molar ratio was kept at 1:1. As soon as pyrrole started polymerizing, the color of the polymerizing mass turned black. Polymerization was allowed to continue in stirring condition at 0–4 ◦ C for 5 h. At the end of polymerization, the black solid mass was filtered and washed with acidic (1.25 M HCl) water followed by washing with acetone. Next the washed black solid polymer was dried and stored in a vacuum desiccator for characterization and sensing measurements. 2.3. Characterizations For studying the molecular order, X-ray diffraction analyses of different rod-like PPY-Ag nanocomposite samples were performed in an X-ray diffractometer (XRD) (Model: X-PERTPRO Panalytical diffractometer) using Cu Kα (λ = 1.5406) as X-ray source, 1◦ min−1 scanning rate, 40 kV voltage, and 30 mA current. Fourier transform infrared (FTIR) spectroscopic analyses of PPY-Ag nanocomposite powder samples dispersed in KBr (in pellet form) were done using a FTIR spectrophotometer (Model: Perkin-Elmer Spectrum Two) in the wave number range of 400– 4000 cm−1 . For studying the surface morphology, scanning electron microscopic analysis of the PPY-silver nanocomposite samples was done in a scanning electron microscope (SEM) (Model: ZEISS EVO 18). The nanocomposite samples, mounted on a specimen stub, were sputter coated with Pt prior to scanning under the microscope. The bulk morphology of PPY-Ag nanocomposite samples was also observed in a transmission electron microscope (TEM) (Model: JEOL-JEM-2100). For TEM analysis PPY-Ag nanocomposite sample was dispersed in distilled water with the help of a probe sonicator for the purpose of drop casting on carbon coated copper grid followed by air drying. To have an idea about the interfacial interaction between the PPY matrix and Ag nanoparticles, the X-ray photoelectron spectroscopic (XPS) analysis was done using a Thermo-VG Scientific ESCA Lab 250 microprobe equipped with an Al Kα monochromatic source (1486.6 eV photons) operated at 14 kV and 20 mA. The percentage of Ag dispersed within the PPY matrix was analyzed by inductively coupled plasma optical emission spectrometer (ICP-OES) using Optical 2100DV ICP-OES, Perkin Elmer,
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Table 1 Formulation used for sodium cholate mediated rod-like polypyrrole-silver nanocomposite synthesis. Sample designation
Components Pyrrole (mL)
AAPY-2 AAPY-S-1 AAPY-S-2 AAPY-S-3 AAPY-S-4 AAPY-S-5
0.20 0.20 0.20 0.20 0.20 0.20
Sodium cholate dispersion
Sodium cholate (g)
Water (mL)
Volume of 1.25 M HCl (mL)
0.344 0.344 0.344 0.344 0.344 0.344
35 30 27 23 19 15
5.52 5.52 5.52 5.52 5.52 5.52
Volume of 0.01 (M) silver nitrate solution (mL)
Volume of 0.057 M FeCl3 (mL)
0 5 8 12 16 20
9 9 9 9 9 9
earlier [34]. The temperature and the flow rates of carrier gas (air) as well as VOC vapor mixture with air were adjusted to obtain the particular concentration of VOC vapor (in ppm) in air [34]. From the sensing plot of relative resistance change of nanocomposite layer with respect to time due to exposure to the VOC vapor, the sensing parameters, viz., response time, recovery time, % response were calculated as reported earlier [34]. 3. Results and discussion 3.1. Preparation of PPY-Ag nanocomposite
Fig. 1. FTIR spectra of PPY-Ag nanocomposite.
USA. In order to perform ICP-OES analysis, the solution of samples was prepared by dissolving the powder sample of about 1 mg in 20 mL concentrated nitric acid and then diluted with water to 100 mL. 2.4. Sensing measurement To study the dynamic sensing performance of PPY-Ag nanocomposite, the change of electrical resistance of the fabricated sensing layer of the nanocomposite sample deposited on the probe was recorded after exposing the probe to the vapor of different volatile organic compounds at room temperature (25 ± 3 ◦ C). The sensing probe is made with a Bakelite support consisting of two parallel copper electrodes at a particular distance (2 mm) on one side. These two copper electrodes are connected with conducting wires to the source meter. The sensing layer of PPY-Ag nanocomposite on the probe was fabricated by drop casting the aqueous nanocomposite dispersion of the powder prepared by ultrasonicating 0.1–0.2 g sample for 30 min in 5–10 mL water. The semisolid coating was then dried in vacuum for 2–3 days. The DC-electrical resistance (R0 ) of the fabricated sensing probe was recorded at constant current (0.1 µA) using a digital source meter (Keithley 2410, USA) in air. In order to study the sensing response of the sample toward VOC vapor, the change of DC-electrical resistance (R) of the fabricated sensing probe at same constant current (0.1 µA) was recorded with particular time interval using exactly same type of sensor set-up as reported
PPY was prepared in an aqueous acidic medium in presence of sodium cholate as surfactant (soft template) and FeCl3 as oxidant. For the purpose of in-situ deposition of silver nanoparticles on the PPY nanorods, AgNO3 solution was mixed with pyrrole solution before the addition of oxidant. In order to investigate the influence of silver content in the PPY nanocomposite, pyrrole was polymerized by using ferric chloride as oxidant in presence of varying amounts of silver nitrate. For obtaining rodlike nanostructure of the polymer, dispersion of sodium cholate surfactant was used as soft template. Sodium cholate forms selfassociated micellar aggregates in aqueous dispersion and helps encapsulation of hydrophobic pyrrole favoring its oxidative polymerization by FeCl3 . Presence of ferric chloride and silver nitrate in the polymerization medium influenced the formation of rodlike morphology since we obtained a ribbon like morphology of same PPY when APS was used as oxidant (unpublished result). Ferric chloride without silver nitrate also produced rodlike morphology. During polymerization of pyrrole silver nitrate got reduced to metallic silver [35], and deposited on the PPY nanorod surface. Silver nitrate concentration was varied with an anticipation that population of in-situ deposited silver would increase with the increased concentration of silver nitrate in the polymerization medium. For complete removal of iron chloride and sodium cholate from the black polymer composite it was thoroughly washed with water. The wet PPY powder was then washed with acidified water (1.25 M HCl) for doping purpose prior to drying and storage in a vacuum desiccator. Others have prepared nanoparticles of silver and silver compounds using reducing agents [36–38]. 3.2. Spectroscopic analysis FTIR analysis was performed to investigate the chemical structure of PPY-Ag nanocomposites. FTIR spectra of five different PPY-Ag nanocomposites are shown in Fig. 1. The characteristic bands for the PPY-Ag nanocomposites appeared at 1034 cm−1 are due to C–H in plane deformation. The bands for all samples at around 3431 cm−1 are due to N–H and C–H stretching vibrations. Methylene group stretching vibrations are observed at around
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Fig. 2. XPS spectra of PPY-silver nanocomposites.
2926 and 2857 cm−1 . Antisymmetric stretching vibration is found at around 1305 cm−1 . The peaks for all samples, appeared at 906 cm−1 are due to C–H out of plane vibration. The peaks at 1540 cm−1 are attributed to symmetric –C==C– stretching vibration. The bands at around 906 cm−1 are assigned for C–H out of plane vibration deformation [39,40]. XPS characterization was performed for further structural investigation of prepared pure PPY without silver deposition and PPY-Ag nanocomposite samples (Fig. 2). XPS analyses of PPY samples have provided signals for C, N, and Ag. It is seen from Fig. 2 that all the samples have exhibited intense C1s signal, N1s signal at binding energies 282 and 397 eV, respectively. The high resolution Ag 3d core level XPS spectrum represents two major peaks at binding energies of 365 eV (Ag 3d5/2 ) and 371 eV (3d3/2 ). The spin energy separation of 6.0 eV agrees well with the reported values for the elemental Ag0 [41]. In order to confirm the Ag loading in the nanocomposite the ICP-OES experimental data for metallic Ag content in the sample was compared with the theoretical values and the results are shown in Table 2. As shown in Table 2, the loading of Ag nanoparticles in the synthesized nanocomposite was increased gradually
with the increasing amount of silver nitrate addition. However, in the nanocomposite samples lower experimental loading value of silver was recorded than that of the theoretical amount used during the preparation (Table 2). This might be explained from the fact that neither all the silver nitrate was reduced to silver nanoparticles nor the entire monomer was converted to the polymers. Interestingly, the loading of silver in PPY matrix was found to have highest value of 0.0166 g/g for AAPY-S-3. In spite of increasing the theoretical amount of silver nitrate the silver loading gradually decreased in AAPY-S-4 and AAPY-S-5 samples. The fact can be explained with the help of the schematic representation of nanocomposite formation (Scheme 1). As shown in Scheme 1, the Ag+ ion of silver nitrate forms a complex with the –NH– groups of the monomer or oligomer of pyrrole in polymerization medium. One Ag+ ion can form coordination bonds with highest two numbers of –NH– groups. It might be expected that such type of complex formation with two coordination bonds was found to be highest for the AAPY-S-3 nanocomposite. Therefore, the silver loading was recorded as highest for AAPY-S-3 nanocomposite. However, below or above this concentration the loading was partial due to partial formation of coordination complex having two coordination bonds
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Scheme 1. Sodium cholate assisted PPY-Ag nanocomposite synthesis. Table 2 Amount of Ag loading in the nanocomposite analyzed by ICP-OES. Sample ID
Pyrrole (g)
Ag used (g)
Theo. [Ag] per g nanocomposite (g/g)
ICP-OES exp. [Ag] per g nanocomposite (g/g)
AAPY-2 AAPY-S-1 AAPY-S-2 AAPY-S-3 AAPY-S-4 AAPY-S-5
0.2 0.2 0.2 0.2 0.2 0.2
0 0.0085 0.0136 0.0204 0.0272 0.0304
0 0.0407 0.0637 0.0925 0.1197 0.1453
0 0.0068 0.0123 0.0166 0.0143 0.0119
3.3. Crystallinity and surface morphology The XRD patterns of different PPY-Ag nanocomposites presented in Fig. 3 have shown typical characteristic sharp peaks which match with the face centered cubic structure of the bulk silver at 28.25, 32.5, 46.44, 55.26, and 77.11◦ (2θ ) corresponding to (210), (113), (124), (240), and (226) planes, respectively. It is also reported in the literature [42,43] that same peaks were appeared for the above mentioned planes with a little shift of peak positions, viz., 27.51, 31.87, 45.57, 56.56, 66.26, and 75.25◦ . The small hump at around 23◦ is attributed to partial crystalline nature of PPY in the nanocomposite. It may be said from the above XRD nature that metallic Ag has been deposited on PPY during polymerization of pyrrole by reduction of Ag+ . It is also apparent from Fig. 3 that the intensities of the crystalline peaks due to silver have gradually increased with the increase of amount of deposited silver. It is relevant to mention that the composites
were prepared with gradual addition of higher amount of silver nitrate (Table 1). All the PPY-Ag nanocomposite samples were viewed under scanning electron microscope to investigate their surface morphology as shown in Fig. 4. Silver nanoparticles have been observed to remain attached on PPY nanorod surfaces probably as a consequence of redox reactions among pyrrole, FeCl3 and AgNO3 . It is also observed from the SEM images that increase of silver nitrate addition to the reaction medium increased the amount of silver deposition on the PPY nanorod surfaces. To visualize the bulk morphology of the PPY-Ag nanocomposites we also conducted TEM analysis. TEM micrographs of PPY-Ag nanocomposites are shown in Fig. 5. Silver nanoparticle depositions are also clearly visible in the TEM images of the PPY-Ag nanocomposite samples (Fig. 5). With the help of image analyzer the diameter of the PPY nanorod was calculated to be ∼142– 160 nm and that of silver nanoparticles (seen as dark spots) was calculated to be ∼3.20–6.98 nm. 3.4. Sensing performances The dynamic sensing performance of different PPY-Ag nanocomposite samples toward 580 ppm acetone vapor with air is shown in Fig. 6 and the data extracted from the figure is tabulated in Table 3. As shown in Fig. 6, the resistance of all the nanocomposite samples were increased upon exposure to acetone vapor. The proposed mechanism of acetone sensing by nanocomposite layer is presented in Scheme 2. The electrical conductivity
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A. Adhikari, P. Kar, D. Rana et al. / Nano-Structures & Nano-Objects 21 (2020) 100419 Table 3 Acetone (580 ppm) sensing parameters of PPY-Ag nanocomposite samples as obtained from Fig. 6. Sample
Response time (s)
Recovery time (s)
% Response
AAPY-2 AAPY-S-1 AAPY-S-2 AAPY-S-3 AAPY-S-4 AAPY-S-5
200 200 230 280 200 175
130 160 180 200 150 150
9 13 23 58 29 16
Table 4 Acetone sensing parameters of AAPY-S-3 nanocomposite as obtained from Fig. 7. Acetone in air (ppm)
Fig. 3. X-ray diffractograms of different PPY-Ag nanocomposites.
of the nanocomposite was observed due to having induced doping interactions [44] between electronegative –NH– groups of polymer matrix with the electropositive silver metal nanoparticles (Scheme 2). In order to explain the sensing mechanism, the interactions between the silver nanoparticles and polypyrrole matrix were considered to be decreased due to effective interactions with the acetone vapor. As a result, the resistance of the nanocomposite layer increased by decreasing effective doping influence or dedoping effect of silver nanoparticles on the polypyrrole matrix. The main type of interactions of the acetone vapor was explained as dielectric interactions of both the components of nanocomposite layer (Scheme 2). Moreover, the effective interactions with the analyte vapor were increased by increasing the percentage of silver nanoparticles as the polarity is further increased by the nanoparticles. Silver nanoparticles at higher percentage were also found to have more number of effective interaction sites to influence the de-doping effect of silver nanoparticles on the polypyrrole matrix. Therefore, the response was recorded as very poor for the sample AAPY-2 without having any silver nanoparticle. The increasing sensing performances of the nanocomposites were observed with gradual increase of the silver nanoparticles and the increasing effect reached the highest value for the sample AAPY-S-3 (Fig. 6). For the AAPY-S-3 significantly highest sensing response (58%) was recorded with highest response time and recovery time of 280 and 200 s, respectively for 580 ppm acetone (Table 3). The reason might be due to the effective adsorption/desorption of acetone vapor on AAPY-S-3 nanocomposite layer having higher percentage as well as better distribution with minimum aggregations of silver nanoparticles. Thus the AAPY-S-3 nanocomposite was selected as best candidate for the sensing of acetone vapor. Fig. 7 represents the dynamic responses of AAPY-S-3 nanocomposite upon the exposure to acetone vapor with air at different concentrations, viz. 580, 420, 270, and 195 ppm. The
Cycle
Response time (s)
Recovery time (s)
Response (%)
195
1 2 3
180 175 160
170 150 150
20.0 19.8 20.7
270
1 2 3
310 300 305
240 230 220
34.4 35.6 34.1
420
1 2 3
300 280 290
210 200 200
38.8 39.5 38.7
580
1 2 3
295 260 250
180 160 190
57.5 57.6 57.5
acetone sensing parameters obtained from the plot are shown in Table 4. A good sensing reproducibility up to consecutive third cycle and increasing % response of the AAPY-S-3 nanocomposite layer were observed with increasing concentration of acetone vapor in air (Fig. 7). In addition, the almost repeatable response time and recovery time obtained for all three cycles in same concentration were slightly influenced by the change of concentration of acetone vapor in air (Table 4). A polynomial curve, shown in Fig. 8, was fitted for the % response of AAPY-S-3 nanocomposite layer toward varying concentrations of acetone vapor with air. Very good polynomial fitting was confirmed from the R2 value of 0.9448 and the % response for the unknown concentration might be calculated from the equation shown on the Fig. 8. The theoretical response limit was determined as 25 ppm simply by extrapolating the nonlinear polynomial curve up to X -axis (Fig. 8). The dynamic sensing responses of AAPY-S-3 nanocomposite layer toward methanol and ethanol vapors with different concentrations in air are shown in Fig. 9. As shown in Fig. 9, the sensing response has increased with gradual increase of the analyte concentration and methanol vapor has shown little higher % response than that of the ethanol vapor at same concentration (Fig. 9). The response and recovery times for the sensing of methanol and ethanol vapors were found to be within the range ∼160–300 s, which was observed comparable with that of the acetone vapor. However, the observed value of % response was noted as significantly low for methanol and ethanol vapors compared to that of the acetone vapor. 3.5. Acetone selectivity The selectivity response of acetone over the other VOCs, viz., methanol, ethanol, propanol, water, formaldehyde is shown in Fig. 10. As shown in Fig. 10, the AAPY-S-3 nanocomposite has shown highest response (∼56%) toward acetone vapor at 580 ppm concentration. Methanol and ethanol vapors have shown ∼40 and ∼30% responses, respectively (Fig. 10), whereas very low response values were recorded for propanol, water and formaldehyde vapors at the same concentration. The selectivity can be
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Fig. 4. SEM images of PPY-silver nanocomposite.
Fig. 5. TEM images of PPY-silver nanostructures.
Fig. 6. 580 ppm acetone sensing properties for different PPY-Ag nanocomposite samples.
explained form the respective dielectric constant and dipole moment of the analytes (Table 5). As shown in Table 5, the dielectric constants of the good solvents such as N-methyl-2-pyrrolidone (NMP) and dimethyl sulfoxide (DMSO) are about 32.6 and 47.2, respectively. The dipole moment of these two solvents is almost
∼4 D. In the explanation, the dielectric constant was assumed to be responsible for close interaction with the sensing layer by solvation and the dipole moment was considered to have effective dipolar de-doping interactions with the components of the nanocomposite sensing layer (Scheme 2). Therefore, the analyte
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Scheme 2. Proposed mechanism of acetone sensing by PPY-Ag nanocomposite layer.
Fig. 7. Acetone sensing properties of AAPY-S-3 nanocomposite at different concentrations.
Fig. 8. Polynomial fit of the curve to obtain detection limit of acetone.
Fig. 9. Methanol and ethanol sensing properties of AAPY-S-3 nanocomposite at different concentrations.
having those values close to the NMP or DMSO should have better sensing performances through better interactions [45]. Thus the interactions of sensing layer with acetone vapor, having highest dipole moment (2.88 D) and moderately low dielectric constant (21), should be considered as highest compared to those of the other analytes like methanol, ethanol, propanol, water and formaldehyde. The interactions of the sensing layer with methanol and ethanol by solvation were supposed to be higher than that of acetone due to having dielectric constant values of 33 and 25.3, respectively. However, the sensing responses of methanol and ethanol having dipole moments 1.70 and 1.69 D, respectively were observed as lower than that of the acetone vapor. This is because of poor effective dipolar de-doping interactions with the components of the nanocomposite sensing layer. For the analytes, propanol and formaldehyde the dielectric constants as well as dipole moments are very low and thus poor sensing response was recorded due to having poor interactions with the nanocomposite layer. In addition, the water having very high dielectric constant also failed to interact effectively with the sensing layer.
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CRediT authorship contribution statement Arpita Adhikari: Conceptualization, Formal analysis, Methodology, Visualization, Writing - original draft. Pradip Kar: Investigation, Methodology, Resources, Writing - review & editing. Dipak Rana: Supervision. Sriparna De: Formal analysis. Jyotishka Nath: Methodology. Koushik Dutta: Data curation. Dipankar Chattopadhyay: Funding acquisition, Resources, Supervision, Writing - review & editing. Acknowledgments The first author A. Adhikari likes to thank the West Bengal DST, for her fellowship. S. De gratefully acknowledged DST-SERB, Govt. of India, for providing Post-Doctoral fellowship under NPDF Scheme. K. Dutta likes to thank West Bengal DST, for fellowship. We also acknowledge the Center for Research in Nanoscience and Nanotechnology (CRNN), University of Calcutta, for instrumental facilities.
Fig. 10. Bar-diagram for selectivity of acetone over the other VOCs at 580 ppm concentration. Table 5 Dielectric constant and dipole moment of different analytes [46]. Analyte
Boiling point (◦ C)
Dielectric constant
Dipole moment (D)
Acetone Methanol Ethanol Propanol Water Formaldehyde DMSO NMP
56.2 64.7 78.4 97.3 100.0 −19.0 189.0 202.0
21.0 33.0 25.3 20.8 80.1 2.5 47.2 32.6
2.88 1.70 1.69 1.55 1.85 2.33 3.96 4.10
4. Conclusions Rod-like PPY-Ag nanocomposite was synthesized through insitu single step polymerization of pyrrole using sodium cholate surfactant as soft template in presence of silver nitrate. The nature of interfacial interactions of two components was discussed briefly from the various standard characterizations in order to establish the electronic properties of the synthesized nanocomposite. The coating of PPY/Ag nanocomposites was explored as chemiresistive type sensor for methanol, ethanol and acetone vapor mixture with air. The optimized silver loading in the nanocomposite was estimated as 1.6% and it was also found to have better sensing performances toward acetone than that of the other nanocomposite samples with different silver loadings. Though the nanocomposite layer was responding well for the methanol, ethanol and acetone vapors, the observed value of % response was noted as significantly low for methanol and ethanol vapor as that of the acetone vapor. The nanocomposite might be considered as selective sensing layer for acetone vapor over the methanol, ethanol, propanol, water, and formaldehyde vapor. The sensing mechanism toward acetone vapor was proposed and the selectivity was explained considering the respective dielectric constant and dipole moment of the analytes. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Nano-Structures & Nano-Objects journal homepage: www.elsevier.com/locate/nanoso
Synthesis of sodium cholate mediated rod-like polypyrrole-silver nanocomposite for selective sensing of acetone vapor Arpita Adhikari a , Pradip Kar b , Dipak Rana c , Sriparna De d , Jyotishka Nath a , ∗ Koushik Dutta a , Dipankar Chattopadhyay a , a
Department of Polymer Science and Technology, University of Calcutta, 92 A.P.C. Road, Kolkata 700 009, India Department of Chemistry, Birla Institute of Technology, Mesra, Ranchi 835 215, Jharkhand, India c Department of Chemical and Biological Engineering, Industrial Membrane Research Institute, University of Ottawa, 161 Louis Pasteur St., Ottawa, ON, K1N 6N5, Canada d Brainware University, Department of Allied Health Science, Barasat, Kolkata 700125, India b
article
info
Article history: Received 15 August 2019 Received in revised form 5 December 2019 Accepted 19 December 2019
a b s t r a c t Nanostructured conducting polymer provides higher surface area for analyte vapor sensing. This investigation aims to synthesize rod-like polypyrrole (PPY) silver nanocomposite tailored by sodium cholate surfactant as soft template for sensing of acetone vapor. During oxidative polymerization of pyrrole by ferric chloride Ag nanoparticle gets deposited on PPY by simultaneous reduction of silver nitrate. The crystal structure, surface morphology and nature of interfacial interactions between the two components of the prepared PPY-silver nanocomposite (PPY-Ag) are analyzed by standard characterization techniques. The Ag nanoparticles deposition on rod-like PPY increases with increasing silver nitrate concentration up to an optimum level. The PPY-Ag nanocomposites show chemiresistive type dynamic sensing responses toward methanol, ethanol and acetone vapor in a mixture with air. A comparative evaluation of typical vapor sensing parameters (% response, response time, recovery time, reproducibility, and selectivity) of PPY-Ag nanocomposite sensor obtained for different volatile organic compounds with those of the pristine PPY sensor is reported. Interestingly, the rod-like PPY-Ag nanocomposite is found to selectively sense acetone vapor showing better % response than that of the other analytes, viz., methanol, ethanol, propanol, water vapor and formaldehyde. The differences in their responses for the different analytes and the selectivity toward acetone vapor are established by considering the difference in their interactions with the nanocomposite. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Today conducting polymers are being explored as materials for gas sensors in the monitoring and control of environmental pollution. Due to having light weight, low cost and sensing ability at room temperature conducting polymers [1,2], have been found to be more advantageous over metal oxides [3], as materials for fabrication of gas sensors. For detection of environmentally hazardous chemical and volatile organic compounds (VOCs), ammonia, chlorine, etc., polypyrrole (PPY) and polyaniline (PANI) based gas sensors have been used [4–7], Out of many VOCs, acetone and different aliphatic alcohols are extensively used in laboratory and industrial practices. Inhaling higher concentrations of these VOCs above allowable limits leads to health hazard as well as environmental pollution. So there is a need of developing suitable materials for sensing of such VOCs including acetone. ∗ Corresponding author. E-mail address: [email protected] (D. Chattopadhyay). https://doi.org/10.1016/j.nanoso.2019.100419 2352-507X/© 2019 Elsevier B.V. All rights reserved.
On the other hand, acetone sensor can play important role in clinical diagnostics for diabetic patients since the level of acetone concentration in the exhaled breath of diabetic patients is used for judging the extent of diabetes. Therefore, quantification of acetone in the exhaled breath of diabetic patients has become a diagnostic approach in diabetes management [8]. Since acetone is known to form reversible H-bond with –NH– group of PPY, it is reported to be used in sensing of acetone [9]. Yu et al. [10] have reported PPY sensor array based gas analyzer for analyzing acetone in the exhaled breath of diabetic patient. Through principal component analysis (PCA) of the acetone sensing data, obtained from this PPY sensor array, a good discrimination of breath acetone was obtained between diabetic and healthy persons. For gas phase sensing of acetone, Do and Wang [11] synthesized PPY by chemical vapor deposition and chemical oxidation casting on the surface of Au/Al2 O3 and obtained acetone sensitivity value of 5.2 × 10−7 ppm−1 . Hamilton et al. [12] electrochemically synthesized free-standing PPY film for sensing of
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acetone, methanol, ethanol and other VOCs. With the help of PCA, discrimination of sensing ability of the polymer was also evaluated. Influence of dopant and electrical conduction mechanism of PPY in sensing of acetone vapor were studied by Ruangchuay et al. [13]. They have attributed the change of conductivity to the swelling, H-bond formation and reduction of charge carrier species in the doped PPY sensor. In paint and lacquer industries as well as in painting sites detection of highly flammable solvent like acetone is necessary with the help of portable sensor. Accordingly, conducting polymer based sensor was exploited for sensing of acetone vapor in lacquer and paint industry/application sites. Due to the poor processability of PPY, insulating polymer was used as a support material to make sensor device for the purpose of such sensing. In such an attempt α -naphthalene sulfonatedoped PPY/poly(methyl methacrylate) (PMMA) blend film was used as selective sensor, based on electrical conductivity, for acetone vapor in lacquer by Ruangchuay et al. [14]. Later in another work Ruangchuay et al. [15] gained an improvement of sensing selectivity of α -naphthalene sulfonate-doped PPY toward acetone and toluene after solution mixing of the doped polymer with insulating polymers like polyethylene oxide (PEO), PMMA, high density poly ethylene (HDPE), polystyrene (PS), acrylonitrile butadiene styrene (ABS) copolymer. Jiang et al. [16] also used poly (vinyl alcohol) (PVA) as support material for PPY in the form of composite film for sensing of methanol vapor. Brina et al. [17] have reported that the presence of metallic nanoparticles in conducting polymer creates active sites for in-situ electron trapping. Higher acetone sensitivity of PANI-silver nanocomposite compared to pure PANI was reported by Choudhury et al. [18]. They have deposited silver nanoparticles on PANI by reduction of silver nitrate with sodium borohydride and studied the effect of silver content on acetone vapor sensing characteristics, viz., dynamic responses, response time, sensitivity and reproducibility. For preparation of well dispersed silver nanoparticles people have used natural precursor, e.g., pomegranate peel for reduction of silver nitrate [19]. Some authors have prepared nanocomposite/nanoparticles of silver compounds having semiconducting properties [20,21]. Conductivity and relative permittivity of organic conducting polymers change depending on the frequency and concentration of the exposed volatile chemicals [22,23]. Utilizing the current frequency dependence on the conductivity of polymer Musio and Ferrara [24] developed PPY film based selective sensor for different analyte vapors, viz., methanol, ethanol, acetone and ethyl acetate where they have studied the response patterns in terms of resistance change by varying the a.c./d.c. frequencies and claimed higher sensing selectivity using low frequency a.c. response. Electrically conducting polyindole was used for sensing of methanol at room temperature [25]. In this work they have explored the effects of polarity, dielectric constant, H-bonding interaction of the different sensing analytes, e.g., methanol and showed highest relative sensing responses in terms of conductivity of the polymer. Anjum et al. [26] have reported selective sensing of lower concentration (100 ppm) alcohol vapors, viz., methanol, ethanol, and propanol in presence of air by graphite doped hydroxyapatite nanoceramic sensor. Although many reports are available on techniques for synthesis of different silver containing nanoparticles [27–33] but no work is reported on the soft template guided synthesis of silver deposited rod-like PPY-nanocomposite for VOC sensing application. In this context the present investigation reports a novel approach on the synthesis of rod-like PPY-Ag nanocomposite with the help of sodium cholate surfactant as soft template for sensing of VOC. In order to further explore the possibility of increased acetone sensing performance we have varied the deposited silver content on the PPY nanorod during polymerization by varying the amount of silver nitrate addition.
2. Experimental 2.1. Materials Pyrrole monomer was procured from Sigma-Aldrich Inc. and purified by vacuum distillation. The distilled monomer was stored at 4 ◦ C under nitrogen blanket prior to polymerization. Analytical reagent grade anhydrous ferric chloride, silver nitrate and hydrochloric acid were purchased from Merck India Ltd., and used without further purification. Analytical reagent grade acetone, methanol, ethanol, formaldehyde as sensing analytes were procured from E. Merck India Ltd., and used without further purification. Sodium cholate, for use as surfactant during polymerization of pyrrole, was obtained from Sisco Research Laboratory, Mumbai, India. Triple distilled water was used for the whole experimental work wherever required. 2.2. PPY-Ag nanocomposite synthesis Sodium cholate mediated rod-like polypyrrole-silver nanocomposite was prepared as per the formulations shown in Table 1 by varying the amount of silver nitrate addition. First sodium cholate was dispersed in an ice cold (0–4 ◦ C) acidic water and then to this dispersion pyrrole was added in stirring condition. Then 0.01 M silver nitrate solution was added followed by addition of ferric chloride solution in stirring condition. In each formulation the pyrrole to ferric chloride molar ratio was kept at 1:1. As soon as pyrrole started polymerizing, the color of the polymerizing mass turned black. Polymerization was allowed to continue in stirring condition at 0–4 ◦ C for 5 h. At the end of polymerization, the black solid mass was filtered and washed with acidic (1.25 M HCl) water followed by washing with acetone. Next the washed black solid polymer was dried and stored in a vacuum desiccator for characterization and sensing measurements. 2.3. Characterizations For studying the molecular order, X-ray diffraction analyses of different rod-like PPY-Ag nanocomposite samples were performed in an X-ray diffractometer (XRD) (Model: X-PERTPRO Panalytical diffractometer) using Cu Kα (λ = 1.5406) as X-ray source, 1◦ min−1 scanning rate, 40 kV voltage, and 30 mA current. Fourier transform infrared (FTIR) spectroscopic analyses of PPY-Ag nanocomposite powder samples dispersed in KBr (in pellet form) were done using a FTIR spectrophotometer (Model: Perkin-Elmer Spectrum Two) in the wave number range of 400– 4000 cm−1 . For studying the surface morphology, scanning electron microscopic analysis of the PPY-silver nanocomposite samples was done in a scanning electron microscope (SEM) (Model: ZEISS EVO 18). The nanocomposite samples, mounted on a specimen stub, were sputter coated with Pt prior to scanning under the microscope. The bulk morphology of PPY-Ag nanocomposite samples was also observed in a transmission electron microscope (TEM) (Model: JEOL-JEM-2100). For TEM analysis PPY-Ag nanocomposite sample was dispersed in distilled water with the help of a probe sonicator for the purpose of drop casting on carbon coated copper grid followed by air drying. To have an idea about the interfacial interaction between the PPY matrix and Ag nanoparticles, the X-ray photoelectron spectroscopic (XPS) analysis was done using a Thermo-VG Scientific ESCA Lab 250 microprobe equipped with an Al Kα monochromatic source (1486.6 eV photons) operated at 14 kV and 20 mA. The percentage of Ag dispersed within the PPY matrix was analyzed by inductively coupled plasma optical emission spectrometer (ICP-OES) using Optical 2100DV ICP-OES, Perkin Elmer,
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Table 1 Formulation used for sodium cholate mediated rod-like polypyrrole-silver nanocomposite synthesis. Sample designation
Components Pyrrole (mL)
AAPY-2 AAPY-S-1 AAPY-S-2 AAPY-S-3 AAPY-S-4 AAPY-S-5
0.20 0.20 0.20 0.20 0.20 0.20
Sodium cholate dispersion
Sodium cholate (g)
Water (mL)
Volume of 1.25 M HCl (mL)
0.344 0.344 0.344 0.344 0.344 0.344
35 30 27 23 19 15
5.52 5.52 5.52 5.52 5.52 5.52
Volume of 0.01 (M) silver nitrate solution (mL)
Volume of 0.057 M FeCl3 (mL)
0 5 8 12 16 20
9 9 9 9 9 9
earlier [34]. The temperature and the flow rates of carrier gas (air) as well as VOC vapor mixture with air were adjusted to obtain the particular concentration of VOC vapor (in ppm) in air [34]. From the sensing plot of relative resistance change of nanocomposite layer with respect to time due to exposure to the VOC vapor, the sensing parameters, viz., response time, recovery time, % response were calculated as reported earlier [34]. 3. Results and discussion 3.1. Preparation of PPY-Ag nanocomposite
Fig. 1. FTIR spectra of PPY-Ag nanocomposite.
USA. In order to perform ICP-OES analysis, the solution of samples was prepared by dissolving the powder sample of about 1 mg in 20 mL concentrated nitric acid and then diluted with water to 100 mL. 2.4. Sensing measurement To study the dynamic sensing performance of PPY-Ag nanocomposite, the change of electrical resistance of the fabricated sensing layer of the nanocomposite sample deposited on the probe was recorded after exposing the probe to the vapor of different volatile organic compounds at room temperature (25 ± 3 ◦ C). The sensing probe is made with a Bakelite support consisting of two parallel copper electrodes at a particular distance (2 mm) on one side. These two copper electrodes are connected with conducting wires to the source meter. The sensing layer of PPY-Ag nanocomposite on the probe was fabricated by drop casting the aqueous nanocomposite dispersion of the powder prepared by ultrasonicating 0.1–0.2 g sample for 30 min in 5–10 mL water. The semisolid coating was then dried in vacuum for 2–3 days. The DC-electrical resistance (R0 ) of the fabricated sensing probe was recorded at constant current (0.1 µA) using a digital source meter (Keithley 2410, USA) in air. In order to study the sensing response of the sample toward VOC vapor, the change of DC-electrical resistance (R) of the fabricated sensing probe at same constant current (0.1 µA) was recorded with particular time interval using exactly same type of sensor set-up as reported
PPY was prepared in an aqueous acidic medium in presence of sodium cholate as surfactant (soft template) and FeCl3 as oxidant. For the purpose of in-situ deposition of silver nanoparticles on the PPY nanorods, AgNO3 solution was mixed with pyrrole solution before the addition of oxidant. In order to investigate the influence of silver content in the PPY nanocomposite, pyrrole was polymerized by using ferric chloride as oxidant in presence of varying amounts of silver nitrate. For obtaining rodlike nanostructure of the polymer, dispersion of sodium cholate surfactant was used as soft template. Sodium cholate forms selfassociated micellar aggregates in aqueous dispersion and helps encapsulation of hydrophobic pyrrole favoring its oxidative polymerization by FeCl3 . Presence of ferric chloride and silver nitrate in the polymerization medium influenced the formation of rodlike morphology since we obtained a ribbon like morphology of same PPY when APS was used as oxidant (unpublished result). Ferric chloride without silver nitrate also produced rodlike morphology. During polymerization of pyrrole silver nitrate got reduced to metallic silver [35], and deposited on the PPY nanorod surface. Silver nitrate concentration was varied with an anticipation that population of in-situ deposited silver would increase with the increased concentration of silver nitrate in the polymerization medium. For complete removal of iron chloride and sodium cholate from the black polymer composite it was thoroughly washed with water. The wet PPY powder was then washed with acidified water (1.25 M HCl) for doping purpose prior to drying and storage in a vacuum desiccator. Others have prepared nanoparticles of silver and silver compounds using reducing agents [36–38]. 3.2. Spectroscopic analysis FTIR analysis was performed to investigate the chemical structure of PPY-Ag nanocomposites. FTIR spectra of five different PPY-Ag nanocomposites are shown in Fig. 1. The characteristic bands for the PPY-Ag nanocomposites appeared at 1034 cm−1 are due to C–H in plane deformation. The bands for all samples at around 3431 cm−1 are due to N–H and C–H stretching vibrations. Methylene group stretching vibrations are observed at around
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Fig. 2. XPS spectra of PPY-silver nanocomposites.
2926 and 2857 cm−1 . Antisymmetric stretching vibration is found at around 1305 cm−1 . The peaks for all samples, appeared at 906 cm−1 are due to C–H out of plane vibration. The peaks at 1540 cm−1 are attributed to symmetric –C==C– stretching vibration. The bands at around 906 cm−1 are assigned for C–H out of plane vibration deformation [39,40]. XPS characterization was performed for further structural investigation of prepared pure PPY without silver deposition and PPY-Ag nanocomposite samples (Fig. 2). XPS analyses of PPY samples have provided signals for C, N, and Ag. It is seen from Fig. 2 that all the samples have exhibited intense C1s signal, N1s signal at binding energies 282 and 397 eV, respectively. The high resolution Ag 3d core level XPS spectrum represents two major peaks at binding energies of 365 eV (Ag 3d5/2 ) and 371 eV (3d3/2 ). The spin energy separation of 6.0 eV agrees well with the reported values for the elemental Ag0 [41]. In order to confirm the Ag loading in the nanocomposite the ICP-OES experimental data for metallic Ag content in the sample was compared with the theoretical values and the results are shown in Table 2. As shown in Table 2, the loading of Ag nanoparticles in the synthesized nanocomposite was increased gradually
with the increasing amount of silver nitrate addition. However, in the nanocomposite samples lower experimental loading value of silver was recorded than that of the theoretical amount used during the preparation (Table 2). This might be explained from the fact that neither all the silver nitrate was reduced to silver nanoparticles nor the entire monomer was converted to the polymers. Interestingly, the loading of silver in PPY matrix was found to have highest value of 0.0166 g/g for AAPY-S-3. In spite of increasing the theoretical amount of silver nitrate the silver loading gradually decreased in AAPY-S-4 and AAPY-S-5 samples. The fact can be explained with the help of the schematic representation of nanocomposite formation (Scheme 1). As shown in Scheme 1, the Ag+ ion of silver nitrate forms a complex with the –NH– groups of the monomer or oligomer of pyrrole in polymerization medium. One Ag+ ion can form coordination bonds with highest two numbers of –NH– groups. It might be expected that such type of complex formation with two coordination bonds was found to be highest for the AAPY-S-3 nanocomposite. Therefore, the silver loading was recorded as highest for AAPY-S-3 nanocomposite. However, below or above this concentration the loading was partial due to partial formation of coordination complex having two coordination bonds
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Scheme 1. Sodium cholate assisted PPY-Ag nanocomposite synthesis. Table 2 Amount of Ag loading in the nanocomposite analyzed by ICP-OES. Sample ID
Pyrrole (g)
Ag used (g)
Theo. [Ag] per g nanocomposite (g/g)
ICP-OES exp. [Ag] per g nanocomposite (g/g)
AAPY-2 AAPY-S-1 AAPY-S-2 AAPY-S-3 AAPY-S-4 AAPY-S-5
0.2 0.2 0.2 0.2 0.2 0.2
0 0.0085 0.0136 0.0204 0.0272 0.0304
0 0.0407 0.0637 0.0925 0.1197 0.1453
0 0.0068 0.0123 0.0166 0.0143 0.0119
3.3. Crystallinity and surface morphology The XRD patterns of different PPY-Ag nanocomposites presented in Fig. 3 have shown typical characteristic sharp peaks which match with the face centered cubic structure of the bulk silver at 28.25, 32.5, 46.44, 55.26, and 77.11◦ (2θ ) corresponding to (210), (113), (124), (240), and (226) planes, respectively. It is also reported in the literature [42,43] that same peaks were appeared for the above mentioned planes with a little shift of peak positions, viz., 27.51, 31.87, 45.57, 56.56, 66.26, and 75.25◦ . The small hump at around 23◦ is attributed to partial crystalline nature of PPY in the nanocomposite. It may be said from the above XRD nature that metallic Ag has been deposited on PPY during polymerization of pyrrole by reduction of Ag+ . It is also apparent from Fig. 3 that the intensities of the crystalline peaks due to silver have gradually increased with the increase of amount of deposited silver. It is relevant to mention that the composites
were prepared with gradual addition of higher amount of silver nitrate (Table 1). All the PPY-Ag nanocomposite samples were viewed under scanning electron microscope to investigate their surface morphology as shown in Fig. 4. Silver nanoparticles have been observed to remain attached on PPY nanorod surfaces probably as a consequence of redox reactions among pyrrole, FeCl3 and AgNO3 . It is also observed from the SEM images that increase of silver nitrate addition to the reaction medium increased the amount of silver deposition on the PPY nanorod surfaces. To visualize the bulk morphology of the PPY-Ag nanocomposites we also conducted TEM analysis. TEM micrographs of PPY-Ag nanocomposites are shown in Fig. 5. Silver nanoparticle depositions are also clearly visible in the TEM images of the PPY-Ag nanocomposite samples (Fig. 5). With the help of image analyzer the diameter of the PPY nanorod was calculated to be ∼142– 160 nm and that of silver nanoparticles (seen as dark spots) was calculated to be ∼3.20–6.98 nm. 3.4. Sensing performances The dynamic sensing performance of different PPY-Ag nanocomposite samples toward 580 ppm acetone vapor with air is shown in Fig. 6 and the data extracted from the figure is tabulated in Table 3. As shown in Fig. 6, the resistance of all the nanocomposite samples were increased upon exposure to acetone vapor. The proposed mechanism of acetone sensing by nanocomposite layer is presented in Scheme 2. The electrical conductivity
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A. Adhikari, P. Kar, D. Rana et al. / Nano-Structures & Nano-Objects 21 (2020) 100419 Table 3 Acetone (580 ppm) sensing parameters of PPY-Ag nanocomposite samples as obtained from Fig. 6. Sample
Response time (s)
Recovery time (s)
% Response
AAPY-2 AAPY-S-1 AAPY-S-2 AAPY-S-3 AAPY-S-4 AAPY-S-5
200 200 230 280 200 175
130 160 180 200 150 150
9 13 23 58 29 16
Table 4 Acetone sensing parameters of AAPY-S-3 nanocomposite as obtained from Fig. 7. Acetone in air (ppm)
Fig. 3. X-ray diffractograms of different PPY-Ag nanocomposites.
of the nanocomposite was observed due to having induced doping interactions [44] between electronegative –NH– groups of polymer matrix with the electropositive silver metal nanoparticles (Scheme 2). In order to explain the sensing mechanism, the interactions between the silver nanoparticles and polypyrrole matrix were considered to be decreased due to effective interactions with the acetone vapor. As a result, the resistance of the nanocomposite layer increased by decreasing effective doping influence or dedoping effect of silver nanoparticles on the polypyrrole matrix. The main type of interactions of the acetone vapor was explained as dielectric interactions of both the components of nanocomposite layer (Scheme 2). Moreover, the effective interactions with the analyte vapor were increased by increasing the percentage of silver nanoparticles as the polarity is further increased by the nanoparticles. Silver nanoparticles at higher percentage were also found to have more number of effective interaction sites to influence the de-doping effect of silver nanoparticles on the polypyrrole matrix. Therefore, the response was recorded as very poor for the sample AAPY-2 without having any silver nanoparticle. The increasing sensing performances of the nanocomposites were observed with gradual increase of the silver nanoparticles and the increasing effect reached the highest value for the sample AAPY-S-3 (Fig. 6). For the AAPY-S-3 significantly highest sensing response (58%) was recorded with highest response time and recovery time of 280 and 200 s, respectively for 580 ppm acetone (Table 3). The reason might be due to the effective adsorption/desorption of acetone vapor on AAPY-S-3 nanocomposite layer having higher percentage as well as better distribution with minimum aggregations of silver nanoparticles. Thus the AAPY-S-3 nanocomposite was selected as best candidate for the sensing of acetone vapor. Fig. 7 represents the dynamic responses of AAPY-S-3 nanocomposite upon the exposure to acetone vapor with air at different concentrations, viz. 580, 420, 270, and 195 ppm. The
Cycle
Response time (s)
Recovery time (s)
Response (%)
195
1 2 3
180 175 160
170 150 150
20.0 19.8 20.7
270
1 2 3
310 300 305
240 230 220
34.4 35.6 34.1
420
1 2 3
300 280 290
210 200 200
38.8 39.5 38.7
580
1 2 3
295 260 250
180 160 190
57.5 57.6 57.5
acetone sensing parameters obtained from the plot are shown in Table 4. A good sensing reproducibility up to consecutive third cycle and increasing % response of the AAPY-S-3 nanocomposite layer were observed with increasing concentration of acetone vapor in air (Fig. 7). In addition, the almost repeatable response time and recovery time obtained for all three cycles in same concentration were slightly influenced by the change of concentration of acetone vapor in air (Table 4). A polynomial curve, shown in Fig. 8, was fitted for the % response of AAPY-S-3 nanocomposite layer toward varying concentrations of acetone vapor with air. Very good polynomial fitting was confirmed from the R2 value of 0.9448 and the % response for the unknown concentration might be calculated from the equation shown on the Fig. 8. The theoretical response limit was determined as 25 ppm simply by extrapolating the nonlinear polynomial curve up to X -axis (Fig. 8). The dynamic sensing responses of AAPY-S-3 nanocomposite layer toward methanol and ethanol vapors with different concentrations in air are shown in Fig. 9. As shown in Fig. 9, the sensing response has increased with gradual increase of the analyte concentration and methanol vapor has shown little higher % response than that of the ethanol vapor at same concentration (Fig. 9). The response and recovery times for the sensing of methanol and ethanol vapors were found to be within the range ∼160–300 s, which was observed comparable with that of the acetone vapor. However, the observed value of % response was noted as significantly low for methanol and ethanol vapors compared to that of the acetone vapor. 3.5. Acetone selectivity The selectivity response of acetone over the other VOCs, viz., methanol, ethanol, propanol, water, formaldehyde is shown in Fig. 10. As shown in Fig. 10, the AAPY-S-3 nanocomposite has shown highest response (∼56%) toward acetone vapor at 580 ppm concentration. Methanol and ethanol vapors have shown ∼40 and ∼30% responses, respectively (Fig. 10), whereas very low response values were recorded for propanol, water and formaldehyde vapors at the same concentration. The selectivity can be
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Fig. 4. SEM images of PPY-silver nanocomposite.
Fig. 5. TEM images of PPY-silver nanostructures.
Fig. 6. 580 ppm acetone sensing properties for different PPY-Ag nanocomposite samples.
explained form the respective dielectric constant and dipole moment of the analytes (Table 5). As shown in Table 5, the dielectric constants of the good solvents such as N-methyl-2-pyrrolidone (NMP) and dimethyl sulfoxide (DMSO) are about 32.6 and 47.2, respectively. The dipole moment of these two solvents is almost
∼4 D. In the explanation, the dielectric constant was assumed to be responsible for close interaction with the sensing layer by solvation and the dipole moment was considered to have effective dipolar de-doping interactions with the components of the nanocomposite sensing layer (Scheme 2). Therefore, the analyte
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Scheme 2. Proposed mechanism of acetone sensing by PPY-Ag nanocomposite layer.
Fig. 7. Acetone sensing properties of AAPY-S-3 nanocomposite at different concentrations.
Fig. 8. Polynomial fit of the curve to obtain detection limit of acetone.
Fig. 9. Methanol and ethanol sensing properties of AAPY-S-3 nanocomposite at different concentrations.
having those values close to the NMP or DMSO should have better sensing performances through better interactions [45]. Thus the interactions of sensing layer with acetone vapor, having highest dipole moment (2.88 D) and moderately low dielectric constant (21), should be considered as highest compared to those of the other analytes like methanol, ethanol, propanol, water and formaldehyde. The interactions of the sensing layer with methanol and ethanol by solvation were supposed to be higher than that of acetone due to having dielectric constant values of 33 and 25.3, respectively. However, the sensing responses of methanol and ethanol having dipole moments 1.70 and 1.69 D, respectively were observed as lower than that of the acetone vapor. This is because of poor effective dipolar de-doping interactions with the components of the nanocomposite sensing layer. For the analytes, propanol and formaldehyde the dielectric constants as well as dipole moments are very low and thus poor sensing response was recorded due to having poor interactions with the nanocomposite layer. In addition, the water having very high dielectric constant also failed to interact effectively with the sensing layer.
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CRediT authorship contribution statement Arpita Adhikari: Conceptualization, Formal analysis, Methodology, Visualization, Writing - original draft. Pradip Kar: Investigation, Methodology, Resources, Writing - review & editing. Dipak Rana: Supervision. Sriparna De: Formal analysis. Jyotishka Nath: Methodology. Koushik Dutta: Data curation. Dipankar Chattopadhyay: Funding acquisition, Resources, Supervision, Writing - review & editing. Acknowledgments The first author A. Adhikari likes to thank the West Bengal DST, for her fellowship. S. De gratefully acknowledged DST-SERB, Govt. of India, for providing Post-Doctoral fellowship under NPDF Scheme. K. Dutta likes to thank West Bengal DST, for fellowship. We also acknowledge the Center for Research in Nanoscience and Nanotechnology (CRNN), University of Calcutta, for instrumental facilities.
Fig. 10. Bar-diagram for selectivity of acetone over the other VOCs at 580 ppm concentration. Table 5 Dielectric constant and dipole moment of different analytes [46]. Analyte
Boiling point (◦ C)
Dielectric constant
Dipole moment (D)
Acetone Methanol Ethanol Propanol Water Formaldehyde DMSO NMP
56.2 64.7 78.4 97.3 100.0 −19.0 189.0 202.0
21.0 33.0 25.3 20.8 80.1 2.5 47.2 32.6
2.88 1.70 1.69 1.55 1.85 2.33 3.96 4.10
4. Conclusions Rod-like PPY-Ag nanocomposite was synthesized through insitu single step polymerization of pyrrole using sodium cholate surfactant as soft template in presence of silver nitrate. The nature of interfacial interactions of two components was discussed briefly from the various standard characterizations in order to establish the electronic properties of the synthesized nanocomposite. The coating of PPY/Ag nanocomposites was explored as chemiresistive type sensor for methanol, ethanol and acetone vapor mixture with air. The optimized silver loading in the nanocomposite was estimated as 1.6% and it was also found to have better sensing performances toward acetone than that of the other nanocomposite samples with different silver loadings. Though the nanocomposite layer was responding well for the methanol, ethanol and acetone vapors, the observed value of % response was noted as significantly low for methanol and ethanol vapor as that of the acetone vapor. The nanocomposite might be considered as selective sensing layer for acetone vapor over the methanol, ethanol, propanol, water, and formaldehyde vapor. The sensing mechanism toward acetone vapor was proposed and the selectivity was explained considering the respective dielectric constant and dipole moment of the analytes. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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