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Enhancement of Energy Storage and Photoresponse Properties of Folic acid / Polyaniline Hybrid Hydrogel by in-situ Growth of Ag-Nanoparticles Sujoy Das, Priyadarshi Chakraborty, Sanjoy Mondal, Arnab Shit, and Arun K. Nandi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09468 • Publication Date (Web): 30 Sep 2016 Downloaded from http://pubs.acs.org on October 2, 2016

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ACS Applied Materials & Interfaces

Enhancement of Energy Storage and Photoresponse Properties of Folic acid / Polyaniline Hybrid Hydrogel by in-situ Growth of Ag-Nanoparticles Sujoy Das, Priyadarshi Chakraborty, Sanjoy Mondal , Arnab Shit and Arun K. Nandi* Polymer Science Unit, Indian Association for the Cultivation of Science, Jadavpur, Kolkata-700032, India *For Correspondance: A.K.Nandi, email: [email protected], Telephone No. 913324734971 Abstract: Electrically conductive hydrogels are a fascinating class of materials that exhibit multifarious applications like photoresponsive, energy storing etc and the three dimensional micro/nano fibrillar structures of the gels is the key to those applications. Herein, we have synthesized a hybrid hydrogel based on folic acid (F) and polyaniline (PANI) (FP), where F acts as a supramolecular cross-linker of PANI chains. The gels are mechanically robust and are characterized by FESEM, TEM, spectroscopic, rheological and universal testing measurements. The hybrid xerogel exhibit BET surface area 238 m2 g-1, conductivity of 0.04 S/cm, specific capacitance of 295 F/g at a current density of 1A/g and photocurrent of ~2mA under white light illumination. Silver nanoparticles (AgNPs) are in-situ grown to elegantly improve the conductivity, energy storage and photoresponse capability of the gels. The formation of AgNPs drastically improves the specific capacitances up to 646 F/g (at current density 1A/g), excellent rate capability (403 F/g at 20 A/g) and stable cycling performance with a retention ratio of 74% after 5000 cycles. The AgNPs embedded gel exhibits dramatic enhancement of photocurrent to 56 mA and its time-dependent photoillumination corroborates faster rise and decay of current compared to those of FP gel. Keywords:

PANI

hydrogel,

Conductivity,

Mechanical

Photocurrent, 1

ACS Paragon Plus Environment

properties,

Capacitance,


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Introduction: Flexible and portable electronics conductive materials are highly desired for nextgeneration flexible energy storage devices, electronics or sensors.1-4 Amongst them, conductive hydrogel shows great potential due to their large 3D pore volumes, high specific surface area, excellent solid-liquid interface, good electrical and optoelectronic properties.4-9 Because of the above mentioned magnificent properties, conductive hydrogels provide a wide range of applications, such as super capacitors,10-11 dye sensitized solar cells,12 rechargeable lithium ion batteries13 and pollutant capturing14 etc. Conducting hydrogels are usually engineered by incorporating conducting particles, such as carbon nanotubes (CNT), graphene, metal ions, conducting polymer into the gel matrix.15-18 They can also be synthesized by the in situ reduction of metal ions into the gel matrix to form metal nano particle.19,20 Conducting polymer hydrogels have demonstrated great potential due to low cost and easy synthesis, as well as unique electronic, electrochemical, and optical properties.21 Supercapacitors are very attractive power sources because they possess ultrahigh power density, excellent reversibility, very simple charging circuit and comparatively higher energy densities than usual rechargeable batteries etc. and are used in memory back-up systems, industrial power, laptops, robotic medical devices and media players etc.22,23Also, photo responsive property are used to fabricate light emitting diodes, optoelectronic memory devices, photodetectors and solar cells etc.24,25,12 Polyaniline (PANI) based conductive hydrogel is one of the most interesting materials because of its low price, good processability, excellent electrical conductivity, optoelectronics properties and various morphologies.10,13,18,26-31 These hydrogels are prepared through direct polymerization of aniline in presence of small molecules, which behaves as gelators as well as dopant.10,18,21,29,30 PANI based conductive hydrogels are promising 2


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materials for supercapacitors owing to easy synthesis, atmospheric friendliness and pseudocapacitance behaviour of PANI.32,33 The supercapacitor and photocurrent properties of PANI based systems are reported by several research groups10, 32-34. Pan et al. have prepared PANI hydrogels using phytic acid with a specific capacitance (Cs) of 480 F/g.10 Recently, Zhao at al. have synthesized pectin - PANI aerogel showing a Cs value of 184 F/g.34 Thus fabrication of new PANI hydrogel based supercapacitors with better electrochemical performance at higher current density and cyclic stability is a subject of major research interest. Although PANI exhibits a number of interesting properties its photoresponse is very poor and there are few reports of photoresponse properties of PANI30,35-38. Polyaniline film generates a photocurrent by forming a liquid junction on dipping into an aqueous LiClO4 solution and it increases with increase of LiClO4 concentration.35 Yang at al. have prepared TiO2-PANI

core-shell

nanofibers

that

exhibits

low

photocurrent

value.37A

N-Fluorenylmethoxycarbonyl phenylalanine-PANI (FP-PANI) hydrogel has also been reported from our laboratory that exhibits moderate photocurrent properties.30 So, it would be very interesting to fabricate a new PANI based hydrogel exhibiting better Cs value with improved photoresponse property. Herein, we describe the formation of a novel 3D conducting folic acid-polyaniline hydrogels (FP), where F acts as a crosslinker to PANI chains directly forming a conducting polymer network. FP hydrogels are mechanically robust and show excellent electrical conductivity of 0.04 Scm-1. Because of 3D interconnected pores with high specific surface area and good electrical conductivity, FP hydrogels show good specific capacitance (295 F/g at 1 A/g). To further improve the electrical conductivity, Cs and photocurrent properties, we have elegantly grown silver nanoparticles (AgNPs) inside the gel matix. A magnificent 3



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enhancement (5 times) of electrical conductivity (0.21 Scm−1), excellent specific capacitance (646 F/g at 1 A/g), brilliant rate capability (62% retention at 20 A/g) and high cycling stability are noticed in the AgNPs embedded hydrogel. In addition, photocurrent of Ag nano embedded FP2 hydrogel has increased from ~2mA in FP2 gel to 56 mA. Possible reasons of sharp hikes of these important properties in the AgNP embedded FP hydrogels from the interaction of plasmons of AgNPs and polaroic band of doped PANI are discussed.

Scheme 1 Structure of the gelators and Schematic illustration of FPAg Xerogels formation and its cross-linking structure.

EXPERIMENTAL SECTION

Materials: Folic Acid (F) and silver nitrate (AgNO3) (SRL, Mumbai, India) were used asreceived. Ammonium persulphate (APS) and aniline monomer (MERCK Chemicals, Mumbai) were used as-received. Sodium bicarbonate was purchased from Aldrich Chemical. 4


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Co., USA and these chemicals were used without further purification. Aniline was distilled under reduced pressure prior to use. Preparation of FP and FPAg gels: A stock solution of aniline was prepared by dissolving 0.8 mL of aniline (A) in 20 mL of 0.4 N H2SO4. Folic acid (F, 20 mg ~ 0.045m mol) and NaHCO3 (7.60 mg) were dissolved in 1.5 ml and 1.0 ml water by mild heating to make two different F solutions. To these solutions, 1.5 and 2.0 ml of aniline solutions were added, respectively to make total volume 3ml. A deep yellow colored folic acid-aniline (FA) assembly was immediately produced in each case. Required quantity of ammonium persulfate (APS) (Table-1) was added to the FA assembly and was kept at 5o C for twenty four hours to achieve the polymerization of aniline. During the course of polymerization polyaniline (PANI, P) was produced and the deep yellow assembly turned into green colored, self sustaining FP gels. The gels were immersed in water for 10 days with intermittent change of water after every 24 hr to remove excess ions and oligomeric impurities which were confirmed from its colour and WAXS studies of dried supernatant liquid. 200 µL of aqueous AgNO3 solution (0.002% w/v) was added into folic acid, sodium bicarbonate, anililine and APS mixture making a total volume of 3 ml (Table-1) and was kept at 5o C for twenty four hours to concomitantly accomplish the polymerization of aniline and formation of the silver nanoparticles (AgNPs). The system forms deep greenish hydrogels (FPAg) (Scheme-1) and it was purified in the same procedure as mentioned for FP gel. To produce the xerogels of the above hybrid gels we adopted freeze drying technique where the gels are frozen in liquid nitrogen followed by drying under vacuum in a Freeze drier (Eyela,FDU-1200).

5



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Table 1- Preparation of FP1, FP2 and FP2Ag hydrogels.

Characterization: The morphology of the FP2 and FP2Ag xerogels was investigated by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). SEM images of the gels were observed through a FESEM instrument (JEOL, JSM 6700F) operating at 5 kV after platinum coating. TEM images were observed through TEM instrument (JEOL, model 2010EX) directly under a voltage of 200 kV. Moreover, Brunauer– Emmett–Teller (BET) surface area and pore volumes were investigated using nitrogen adsorption-desorption isotherms measured at 77 K using Autosorb 1C instrument (Quantachrome, USA) at 77 K. The samples were degassed at 343 K for 12 h under high vacuum conditions. The specific surface area of hydrogel was calculated from the nitrogen adsorption data at a relative pressure range of P/P0=0.05-0.30. Barrett-Joyner-Halenda (BJH) method was used to measure the pore size distribution of FP2 and FP2Ag hydrogels from the respective adsorption isotherms. The total pore volume was estimated at a relative pressure (P/P0) of 0.99. UV-vis spectra of the samples were recorded with a UV-Vis spectrophotometer (Hewlett-Packard, model 8453) in a cuvette of 0.1-cm path length. The FTIR spectra of the hydrogel were recorded using KBr pellets in a Perkin-Elmer FTIR instrument (FT-IR-8400S).Wide-angle X-ray scattering (WAXS) experiments on all xerogels were performed with a Bruker AXS diffractomer (model D8 Advance) using a Lynx Eye 6


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detector. The instrument was operated at a 40 kV voltage and at a 40 mA current. Xerogels were placed on glass slides and were scanned in the range of 2θ = 5-85° at a scan rate of 0.3 s/step with a step width of 0.02°. To understand the mechanical properties of all the gels rheological experiments were performed with advanced rheometer (AR 2000, TA Instruments) using cone plate geometry on a peltier plate. The diameter of the plate is 40 mm and the cone angle is 4° with plate gap of 121 µm. Also, the mechanical properties of gels were tested using Universal Testing Machine (Zwick Roell, Z005), fitted with a 10 N load cell. For the compression tests, the hydrogel samples (column, with a diameter of 15 mm and height of 15 mm) were placed between the self-leveling plates. The gels were compressed at a rate of 10 mm min-1 until the compression ratio reached 70%. The dc-conductivity of the xerogels was measured by two-probe method at 25 0C by casting a drop on indium–titanium oxide (ITO, 1 mm) strips, dried and sandwiched with another ITO. The conductivity of the sandwiched samples were measured by an electrometer (Keithley, model 2401) at 25 oC using the equation 1.

Where ‘R’ is the resistance, ‘a’ is the area of the electrolytes,‘d’ is the thickness of the samples. Cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) experiments of the hydrogels were performed using a three-electrode system at room temperature using electrochemical station (CHI600E). Here, modified glassy carbon electrode (GCE) as working electrode, saturated Ag/AgCl as reference electrode, Pt wire as counter electrode and 0.5 M H2SO4 as an electrolyte were used. Glassy carbon electrodes of 3.0 mm in diameter were carefully polished with 1, 0.3 and 0.05 µm alumina powder, then washed by water with 7



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sonication and was dried in air for use. All xerogels were coated on GCE by drop casting and were dried under vacuum. Electrochemical impedance spectroscopy (EIS) of the cells were recorded using a Solartron SI 1260 impedance analyzer (Solarton, U.K.) at 300 C over the frequency range from 1 Hz to 1 MHz with an ac perturbation of 25 mV at the 0 V dc level. Photocurrent measurement. For photocurrent measurement, the FP2 and FP2Ag conducting xerogel was dispersed in a 7:3 mixture of m-cresol and chloroform and were sonicated for 1 hr. It was then spin-coated at 3000 rpm for 30 s over the ITO and was dried under vacuum at 60 °C for 4 hr. Thin films were produced over the ITO. Aluminium was deposited over the sample, and was used as another electrode. This device was used for I-V characteristics and photosensitivity measurement. The Keithley model 2401 source meter was used for current measurement and 150 W xenon lamp source (Newport Corp.,Springfield, OH; model 67005) was used for illumination of light. Result and discussion: The formation of hybrid hydrogels is illustrated in scheme 1 where the structures of folic acid (F), aniline and polyaniline (P) in emeraldine salt (ES) form are presented. Folic acid is sparingly soluble in water which prohibits the formation of a hydrogel by the usual heating-cooling technique. To avoid this problem we have mixed sodium bicarbonate with F in water and it converts F into its sodium salt making soluble in water. On addition of aniline solution to this F solution, an ionic interaction between the carboxylate ions of F and protonated aniline occurs resulting an assembly formation between the components. This assembly when polymerized by APS produces a green coloured gel. PANI in the emeraldine base(EB) form consists of a mixture of benzonoid and quinonoid structures but in the doped state (emeraldine salt, ES) the radical cations are generated. F contains two carboxylic acid groups which become ionized due to presence of the base NaHCO3. The in-situ produced 8


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PANI is in the doped state due to presence of sulphuric acid in the solution and the radical cations of PANI interact with the carboxylate anion of F. This ionic interaction might result in the development of a lamellar-like structure containing PANI chains at both the sides and the folic acid molecule at the intermediate as in PANI-dinonyl sulfonic acid and PANIdinonyl disulfonic acid gels39 that finally self assemble producing nanodimensional fibrillar network morphology entrapping a large amount of water within it due to surface forces (Scheme-1). Morphology: To investigate the morphologies of the gels, FESEM and HRTEM experiments are carried out on the hybrid xerogels. It is evident from the FESEM images (Figure1a) that FP2 gel exhibits 3-dimensional hierarchical porous structures consisting of coral-like dendritic nanofibers. In Figure S1a the FESEM image at a lower magnification clearly indicates the presence of similar morphology throughout the whole sample and the histogram indicates a most probable value of the fibre diameter ~65nm in the xerogel state. A porous fibrillar network structure of FP2 gel is also manifested in the TEM images (Figure 2a, Figure S2a). Because of the presence of large open channels of pores, this type of interconnected continuous network would be more efficient than other morphologies for electrochemical applications.

(a)

(b)

AgNPs

200 nm

200 nm

Figure 1. FESEM images of (a) FP2 gel and (b) FP2Ag gel. 9



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(a)

(b)

(c)

(d)

0.236 nm

1 nm

Figure 2. (a) TEM image of FP2 Hydrogels. (b) TEM image of Ag NPs embedded into F-PANI NF in FP2Ag hydrogel. (c) HRTEM image of FP2Ag hydrogel, (inset, fringe pattern of a AgNP). (d) EDX pattern of FP2Ag xerogel. Also, this interconnected continuous network can create three-dimensional percolative path for charge carriers along the doped PANI chains, resulting good electrical conductivity. The bright, prominent spots observed in the FESEM image of the FP2Ag gel (Figure1b, Figure S1b), indicate the AgNPs with high electron densities adhered on the fibre surface PANI of fibrillar network matrix. TEM micrographs of FP2Ag xerogel (Figure 2b, Figure S2(b,c)) exhibits that AgNPs (black spots) are homogeneously spread over the fibre surface of the gel matrix. The enlarged TEM images in Figure S2(b,c) also illustrates that the AgNPs are spherical with an isotropic distribution showing most probable diameter of ~10 10


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nm. HRTEM image (Figure 2c) shows the fringe pattern of AgNPs and from the enlarged figure at the upper inset the d-spacing value is calculated to be 0.236 nm corresponding to 111 plane of Ag crystal. The selected-area electron diffraction pattern of FP2Ag (Figure S2d) has demonstrated the concentric diffraction rings of bright spots representing the dhkl values of 0.236, 0.204 and 0.144nm corresponding to the presence of (111), (200) and (220) Miller planes of AgNPs. The EDX pattern (Figure 2d) of FP2Ag xerogel shows the presence of carbon, oxygen, nitrogen, and Ag corresponding to all the components present in the trihybrid gel. The porous properties of FP2 and FP2Ag xerogels, studied by nitrogen adsorptiondesorption measurements, exhibit type IV adsorption isotherms (Figure 3a).40 It is to be noted that there is difference in the adsorption-desorption isotherms of FP2 and FP2Ag xerogel. The former does not exhibit any hysteresis between adsorption and desorption isotherms but FP2Ag exhibits the characteristics with H2-type hysteresis and it may indicate the existence of interconnected and ink - bottle type of pores.41-43 Also at similar relative pressure the volume of adsorbant gas is higher for FP2Ag than that of FP2 xerogel. The presence of AgNPs may be attributed to both the differences because of the large surface area of AgNPs. (a)

0.006

3

150

FP2 FP2Ag

0.008

dV/dD (cm /g/nm)

-1 3

(b)

FP2 FP2Ag

200

Quantity Adsorbed (cm g STP)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100

0.004

0.002

50 0.000

0.0

0.2

0.4

0.6

0.8

Relative Pressure (P/P0)

0

1.0

5

10

15

20

25

30

35

Pore Diameter (nm)

Figure 3. (a) Nitrogen adsorption/desorption isotherm of FP2 and FP2Ag xerogel (b) DFT (density function theory) pore size distributions of FP2 and FP2Ag.

11



The BET specific surface area (SBET) of FP2 and FP2Ag xerogels are 238 and 287 m2 g-1, respectively. BJH calculations reveal the pore volumes of 0.22 and 0.26 cm3 g-1 with the pore diameter distribution in the range 1.7-15 nm (Figure 3b) indicating the presence of meso pores. The xerogels also have macropores due to the removal of entrapped solvents in the gels.44 The high SBET of the xerogels would result in better active sites for the Faradic reaction of PANI to exhibit better electrochemical performance. Structural Analysis: FTIR spectra of the xerogels are presented in Figure 4a. Characteristic stretching bands at 1567-1565 cm-1, 1503-1495 cm-1, 1292-1290 cm-1, 825-824 cm-1 in the FP1, FP2 and FP2Ag gels are ascribed to γC=C for quinoid rings, γC=C for benzenoid rings, γC-N for the secondary aromatic amine, and γC-H aromatic out of plane deformation for the 1,4disubsituted benzene, respectively definitely supports the formation of PANI.45 PANI (EB) has a peak at 1160 cm-1 which corresponds to the vibration mode of quinonoid(Q) structure (N=Q=N) of emeraldine base form. In FP1 the peak occurs at 1154 cm-1 indicating the double bond character of quinonoid structure is lost due to polaron(Q=N+H-B or B-N+H-B) formation on doping.46 In the FP2 and FP2Ag gels this peak has shifted to lower energy with the highest shift observed in the FP2Ag gel.

(a)

(b ) 1142

1148

2500

FP2Ag FP2 FP1 2000

824 1567 1292 1503

1500

FP2A g FP2 FP1

Intensity (a.u.)

% Transmittance (a.u.)

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1154

1000

500

20

-1

Wavenumber (cm )

40

60

80

(2 θ ) d egree

Figure 4. (a) FTIR spectra of FP1, FP2 and FP2Ag xerogels. (b) XRD pattern of FP1, FP2 and FP2Ag xerogels. 12


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To elucidate the mechanism of cross-linking of the PANI chains by F, we have exploited FTIR spectroscopy. F encompasses two carboxylic acid groups that can interact with more than one PANI chains causing a cross-linking effect. During the preparation of FP gels the carboxylic acid groups convert to folate anion in presence of NaHCO3 and would interact with anilinium ions produced from the H2SO4 and aniline. There would be an equilibrium where the folate anions become partially protonated with the protons of anilinium ions. During the in-situ polymerization, doped PANI chains are produced and the radical cations of doped PANI interact with folate anions causing a cross-linking effect. The formation of radical cations are confirmed from XPS analysis. Also the carboxylic acid group of partly protonated folic acid would interact with the aminic hydrogen of doped PANI chains via hydrogen bonding. This type of non-covalent cross- linking of PANI chains were previously observed where phytic acid was used as a gelator and dopant.10 We have strengthen this proposition by comparing the FTIR spectra of pure F powder with the FP xerogels (Figure S3). The F powder shows peaks at 3543, 3416 and 3324 cm-1, the first one is attributed to the vibration of –OH group of carboxylic acids of F and the later two are due to the vibration peaks of amino groups. In the FP2 gel these peaks broaden with a peak centred at 3410 cm-1. The –C=O stretching vibration of –COOH groups of F appears at 1694 cm−1 as a sharp peak in the pure F powder.31This peak shifts towards lower frequency (1692 cm−1) and also becomes broadened indicating H-bonding interactions between –COOH groups of F with NH group of PANI. Thus it is inevitable that the both the -COOH groups of F interact with PANI simultaneously by ionic interaction and by H-bonding interactions. This indeed causes a cross-linking between the PANI chains by the folic acid. Wide angle X-ray diffraction (WAXS) patterns of all the xerogels are shown in Figure 4b. It is observed that all FP1 and FP2 xerogels are mostly amorphous in nature. Both xerogels exhibit two broad bands centred peak at 2θ=20 (d=0.443nm) and 25.2o (d=0.353nm), ascribed 13



to the periodicity in the directions parallel and perpendicular to the amorphous PANI chains respectively.47 However, FP2Ag xerogel shows sharp crystalline peaks ascribed to the crystals of AgNPs, retaining the characteristic peaks of the FP systems. FP2Ag xerogel exhibits two amorphous peak at 2θ =20.9, 25.10, and crystalline peaks at 2θ = 38°, 44°, 64°, 77°, and 82°, representing Bragg’s reflections from the (111), (200), (220), (311), and (222) planes of AgNPs.48 The WAXS results therefore indicates that the gel structure remains intact and the Ag NPs do not influence the gel structure. This is because the AgNPs are formed and stabilized at the fibre surface of the FP2Ag gel. Furthermore, the formation of PANI, and AgNPs, in FP2Ag hydrogels are confirmed form XPS study (Figure 5a). The strong XPS signals at 286, 399 and 552 eV are assigned to C(1s), N(1s) and O(1s) respectively, supporting the formation of PANI.49 The binding energies of the AgNPs in the XPS spectrum are 367 and 372.9 eV, owing to Ag 3d5/2 and Ag 3d3/2 energy levels shown in figure 5b.49 The deconvoluted spectra of carbon and nitrogen characterizing different nature of bondings are shown in figure S4a,b. The deconvoluted spectra (Figure S4b) clearly indicate the presence of radical cations at the nitrogen atoms indicating the doped state of PANI.

(a)

FP2Ag

(b)

FP2Ag

C1s

100

Ag 3d N1s

200

300

400

500

600

Ag(3d5/2)

Intensity (a.u.)

O1s

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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700

Ag(3d3/2)

365

Binding energy (eV)

370

375

380

Binding energy (eV)

Figure 5. (a) XPS spectra obtained from FP2Ag xerogel. (b) XPS core level spectra of the Ag3d regions.

14


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UV-Vis spectra of all gels, obtained by dispersing the gels in water are displayed in Figure 6. The absorption bands at 362 and 367 nm for the FP1, and FP2 gels are ascribed π-π* transition in the benzenoid rings, while the peak at 428 nm in all the gels are attributed to the polaron -π* band transition of PANI, indicating the PANI is produced in the doped state. FP1 FP2 FP2Ag

0.4

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.2

0.0 300

400

500

600

700

800

900

1000

Wavelength (nm)

Figure 6. UV-Vis spectra of FP1, FP2 and FP2Ag hydrogels diluted with water. In FP2Ag gels these peaks are not prominent, rather a new peak is observed at 421 nm which may be attributed to the plasmon band of AgNPs.50 The characteristic peaks of the AgNPs (421 nm) overlap with the polaron to -π*(428 nm) band transition peak of PANI resulting in a broad peak. It is interesting to note that in all the cases there is a peak at ~810 nm in all the gels / hybrid gels originating from π- band to polaron band transition of PANI. So, from the UV-vis study it is proved that the PANI molecules are present in the doped state i.e in the PANI (ES) form. This is similar to the nanocomposite of AgNP and polyaniline doped with dinonylnapthalene disulfonic acid where polaronic bands are observed.50 Similar in situ growth of AgNPs is also reported from an one-step synthesis in nitric acid medium using APS as initiator for AgNPs embedded over PANI nanofibers.51 It may therefore be argued that the Ag nanoparticles are produced during the oxidation of ANI to PANI where Ag+ become reduced to AgNP producing FP2Ag gel. We have also made a reference experiment without adding any APS, but no AgNP formation was detected.

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It is now interesting to compare the peak positions for the transition of π band to polaroic bands of FP1, FP2 and FP2Ag xerogels. The FP1 has the polaron band peak at 805nm, FP2 at 831 and FP2Ag has peak at 797 nm. The red shift of polaronic band of FP2 from that of FP1 may be attributed to the increased π- stacking interaction of higher amount of PANI with the folic acid where polarons get delocalized through the aromatic rings of folic acid moiety. The blue shift after AgNP formation in the FP2Ag gel may be attributed to the use of PANI chains to stabilize the AgNPs through its nitrogen atoms when the PANI chains become somewhat skewed showing lower conjugation length. This stabilization process may cause an interaction between the polarons of AgNps with the polaronic band of doped PANI. Mechanical properties: All gels are viscoelastic materials and they exhibit properties of both viscous and elastic materials. Mechanical properties of the gels are analyzed by the rheological parameters viz. storage modulus (G′) and loss modulus (G″). The storage modulus (G′) correspond to the energy stored during the shear process of a viscoelastic material (i.e. solid-like nature) and the loss modulus (G″) signifies the energy dissipated during the shear process (flow or liquid-like nature) and in the gel state G′ > G″. Frequency sweep experiments are carried out on all the gels and are presented in Figure.7a. It is apparent from the figure that storage modulus is linear for a wide range of frequency and G′ > G″ in all the gels characterizing the viscoelastic (solid like) nature of the gels. The gels are mechanically robust in nature which is evident from the magnitude of G′ (~104 Pa). Such high mechanical properties stem from the formation of continuous network structure with rigid backbones. The covalent linkages of PANI chains also facilitate easy storage and dissipation of energy.31 This is further established from the fact that the modulus values of the FP2 and FP2Ag systems are higher than FP1 because of the increased PANI content in them. Elasticity (G′-

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G″) of the gels are calculated from the absolute magnitudes of G′ and G″ which increases in the order FP1 gel

Enhancement of Energy Storage and ... - ACS Publications

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