Ionics DOI 10.1007/s11581-014-1146-8

ORIGINAL PAPER

Influence of surfactant on the morphology and supercapacitive behavior of SILAR-deposited polyaniline thin films B. H. Patil & G. S. Gund & C. D. Lokhande

Received: 9 January 2014 / Revised: 14 April 2014 / Accepted: 6 May 2014 # Springer-Verlag Berlin Heidelberg 2014

Abstract Polyaniline thin films have been prepared by the simple and inexpensive successive ionic layer adsorption and reaction (SILAR) method at room temperature in the presence of three different surfactants (Triton X-100, polyethylene glycol, and sodium dodecyl sulfate) using ammonium peroxydisulfate (APS) as an oxidizing agent. The twobeaker SILAR system is adopted with aniline in 1 M H2SO4 solution as a cationic precursor and APS solution as an anionic precursor for polymerization of aniline. The polyaniline thin films are characterized by Fourier transform infrared (FT-IR), scanning electron microscopy (SEM), and cyclic voltammetry techniques. The properties of polyaniline thin films with surfactant are compared with those of surfactant-free polyaniline thin films. The polyaniline thin films prepared with surfactants exhibited different morphologies. The electrochemical characterizations of the supercapacitor based on polyaniline thin films are studied with cyclic voltammetry and charge–discharge techniques. The polyaniline thin film with Triton X-100 exhibited high specific capacitance (C s ) of 1,040 F g−1 and good cyclic stability (85 %) as compared to the Cs of the surfactant-free polyaniline thin film (637 F g−1). Keywords Thin film . Polyaniline . Surfactant . Supercapacitor

Introduction In recent years, polymer nanostructures, particularly conducting polymer nanowires and nanofibers [1, 2], have B. H. Patil : G. S. Gund : C. D. Lokhande (*) Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur, Maharashtra 416004, India e-mail: [email protected]

received growing interest in applied and fundamental research. Among the known types of polymers, conjugated polymers have attracted a lot of attention due to their unique electrical properties, and they have been classified as a conducting polymer, having a framework of alternating single and double carbon–carbon bonds [3]. Compared with bulk, nanostructured conducting polymers are expected to display improved performance in technological applications [4]. Among the entire conducting polymer family, polyaniline is exclusive due to its inexpensive monomer, ease of synthesis, environmental stability, and simple proton doping/dedoping chemistry [5]. Polyaniline can be synthesized by template [6], enzymatic [7], chemical oxidation, or electrochemical polymerization of aniline under mild conditions [8]. The unnecessary formation of precipitation; waste of material using the above reported methods which entails high cost and is also time- and energy-consuming; and the need for sophisticated instruments which can be cumbersome for controlling various parameters may put restrictions on the commercial synthesis of materials. These can be avoided by the successive ionic layer adsorption and reaction (SILAR) method which results into the formation of thin films of the deposit at room temperature. In the literature, there are different materials such as metal oxides [9, 10], metal chalcogenites [11], and conducting polymers [12] which are deposited by the SILAR method. The small and subtle changes in reaction parameters result in the formation of different structures of polyaniline. These changes can often result in drastic differences in the polymer’s nanoscale morphology [13]. The template synthesis method is used to change the polyaniline morphology from globular particulates to nanofibers [14]. One-dimensional nanostructured polyaniline, including nanofibers, nanowires, nanobelts, nanotubes, nanorods, nanoneedles, and nanosticks, has been extensively studied recently due to its unique properties and many potential applications [15]. The recent advances in polyaniline research and its structural aspect and applications

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are reviewed by Ćirić-Marjanović [16]. Structure design is the new trend in the preparation of polyaniline (PANI) electrodes. The regular geometry of an electrode material can enhance ion diffusion and transportation which can lead into large capacity and energy power density of the supercapacitor [17, 18]. Also, polyaniline has excellent redox reversibility with good chemical stability so it is used in electrochemical energy storage devices. The growing demands for power sources of transient high-power density have stimulated a great interest in electrochemical supercapacitors with projected applications in digital communications, electric vehicles, burst power generation, memory backup devices, and other related devices which require high-power pulses [19]. A supercapacitor is an important energy storage device, which has attracted increasing attention because of its high-power density, long cycle life, and environment-friendly features [20–22]. The materials that have been used for supercapacitor electrodes are carbons, metal oxides, and conducting polymers. A conducting polymer is one of the most promising materials in polymer-based redox supercapacitor for its fast charge–discharge kinetics and fast doping–undoping process [23]. Gupta and Miura [24] reported the maximum specific capacitance of 742 F g−1 for potentiodynamically deposited polyaniline electrode. Dhawale et al. [25] reported the maximum specific capacitance of 861 F g−1 for electrodeposited polyaniline nanofibrillar electrode. Thus, the performance of a supercapacitor of polyaniline electrode may be influenced by morphology, which is closely related to the preparation methods and conditions. Recently, the soft template method has been used for the fabrication of various morphologies of conducting polymer. Generally, the surfactant molecules such as cationic, anionic, and nonionic amphiphiles form a micelle in the monomer solution, which affects the molecular and supramolecular structure of conducting polymers by altering the locus of polymerization [26]. Stejskal et al. [27] discussed about the role of nucleates on the growth of PANI and the morphology of polyaniline. In our previous research work, polyaniline thin films are synthesized by the SILAR method for supercapacitor application [12]. In the present work, the preliminary attempt has been made to study the effect of various surfactants (Triton X-100, polyethylene glycol, and sodium dodecyl sulfate) on the morphology and supercapacitive property of polyaniline thin films synthesized by the simple and inexpensive SILAR method. The polyaniline thin films are characterized by Fourier transform infrared (FT-IR) spectroscopy and scanning electron microscopy (SEM). The supercapacitive properties of polyaniline electrode are investigated by cyclic voltammetry (CV) and galvanostatic charge–discharge studies in 1 M H 2 SO 4 electrolyte.

Experimental Synthesis of polyaniline (PANI) thin films PANI thin films are synthesized by the successive ionic layer adsorption and reaction method using ammonium peroxydisulfate (APS) as an oxidizing agent. Triton X-100 (TX100), polyethylene glycol (PEG) 200, and sodium dodecyl sulfate (SDS) surfactants were used in the synthesis process for altering the nanostructures of PANI thin films. The schematic of the SILAR method is shown in Fig. 1. For the typical synthesis of PANI thin films by the SILAR method, a two-beaker system was used. The first beaker contained 0.4 M aniline dissolved in 1 M H2SO4 which served as a cationic precursor. The efficient polymerization was formed only in an acidic environment where aniline exists as anilinium cation. The surfactant (0.002 M) was dissolved in the monomer solution to form a homogenous mixture. The surfactant enhances the solubility of PANI since one part of the surfactant acts as a dopant, while the other part stabilizes the reactive medium. The second beaker contained the oxidant solution of 0.2 M APS in 1 M H2SO4 which served as an anionic precursor. The monomer to oxidizing agent ratio was kept constant throughout the experiment as 2:1. The stainless steel as well as the glass substrates used for the deposition was ultrasonically treated and well cleaned by detergent. The cleaned substrate was dipped into the aniline solution for 20 s in which anilinium cationic radicals get adsorbed onto the substrate surface. Then, the substrate was immersed in the APS solution for 10 s. The anilinium cationic radicals get oxidized by APS to form PANI. In this reaction step, greenish colored emeraldine salt of PANI was deposited on the substrate. The numbers of SILAR cycles were repeated to increase the thickness of PANI thin films. The number of SILAR cycles was varied for PANI (20 cycles), PANI/TX100 (12 cycles), PANI/PEG (9 cycles), and PANI/SDS (7 cycles) to get optimum film thickness.

Fig. 1 The schematic of the SILAR method for deposition of PANI thin film

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The PANI thin films were characterized by Fourier transform infrared (FT-IR) spectroscopy and SEM techniques. The FT-IR spectra of the samples were collected using a PerkinElmer FT-IR Spectrum One unit. The surface morphology of the synthesized PANI thin films was visualized using a JEOL-6360 SEM. To measure the thickness of the deposited PANI films, a conventional weight difference method was used. The resistivities of the films were measured by a fourpoint probe measurement technique. The electrochemical supercapacitive performance of the PANI electrodes was studied by CV and galvanostatic charge–discharge (GCD) measurements using a potentiostat (263-A EG & G, Princeton Applied Research), forming an electrochemical cell comprised of platinum as a counter electrode and saturated calomel electrode (SCE) as a reference electrode in 1 M H2SO4 electrolyte.

Results and discussion PANI thin film formation PANI thin films are deposited by the SILAR method at room temperature (300 K). A two-beaker system was used for the deposition of PANI, in which the first beaker contains the anilinium cationic radical with surfactant and the second beaker contains the oxidant solution. When the substrate is dipped into the monomer solution, anilinium cation (C6H5NH3+) is deposited on the substrate due to the attractive force between the surface of the substrate and ions in the solution [12]. These cations are oxidized and form the emeraldine salt of PANI. In this chemical polymerization process of aniline, surfactants act as a surface-active agent. The surfactants affect the preparation of PANI in different ways: (1) anionic surfactants such as SDS may act as counter-ions for polyaniline cation, and the hydrophobic part of the surfactant molecules may adsorb on the polyaniline chain [28]. A small fraction of sodium ions from SDS at the micelle-water interface is free to dissociate from the interface, resulting in a net negative charge on each micelle, thus agglomeration of particles takes place. (2) Nonionic surfactants such as TX100 and PEG in which the chain of surfactant and polymer interacts with each other end to end and forms further longer chain results in the formation of PANI nanofibers [29]. The surfactants are employed as additives in the polymerization process in order to guide the chain formation [30]. Polymerization is expected to be initiated at the micelle-water interface because of the increased local aniline concentration and since our system is not agitated. Aniline dimer and higher oligomers are expected to accumulate at the micelle-water interface. These dimers and oligomers may be responsible for orchestrating fibrillar polymer growth [31]. Adsorption

of the surface-active agent on the PANI thin film is due to the hydrophobic component in the surfactants probably via a hydrogen bonding mechanism with the aniline N–H group [32]. This procedure is repeated for different surfactants such as TX100, PEG, and SDS. Different surfactants show different reaction approaches toward PANI. The presence of the hydrophobic (micellar core) and hydrophilic interface in normal micellar solution may induce orientation of the reactants in micelles, which in turn affect the region selectivity of the reaction and reaction kinetics [33]. The model for the formation of nanostructures for different surfactants is shown in Scheme 1.

FT-IR studies FT-IR provided valuable information regarding the functional group present in polyaniline. The polyaniline samples are prepared by using potassium bromide (KBr) (95 %) and polyaniline (5 %) to form a very fine powder of PANI. This powder is then compressed into a thin pellet which is used for further study. The FT-IR analysis is carried out in order to identify the characteristic peaks of PANI. Figure 2 represents the FT-IR spectra of (a) PANI, (b) PANI/TX100, (c) PANI/PEG, and (d) PANI/SDS. The observed peaks and their corresponding assignments are given in Table 1. It is observed that the surfactant influenced the intensities of the peaks. These results are probably due to the interaction between the surfactant and PANI [26]. The increase in intensity of the bands may be related to the quality of the PANI material. Increased band intensity suggests an increase in the number of functional groups, which indicates a good quality of PANI film [32]. The peaks at 1,574 and 1,496 cm−1 are attributed to C=C stretching vibrations of quinoid and benzenoid rings, respectively. The high intensity peaks at 1,574 and 1,496 cm−1 in PANI and PANI/TX100 are due to the long conjugation length of the polymer chain than for the

Scheme 1 Schematic representation of evolution of different polyaniline nanostructures by the SILAR method

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Fig. 2 FTIR spectra of a PANI, b PANI/TX100, c PANI/PEG, and d PANI/SDS thin films

PANI/PEG and PANI/SDS samples. The peak at 1,299 cm−1 is due to the C–N stretching vibrations. The peaks at 1,124 and 814 cm−1 can be assigned to C–H in-plane and out-of-plane deformation, respectively. In addition, the peak located at 507 cm−1 is assigned to the vibrations of the SO3 group [33]. The peak at 2,923 cm−1 is due to vibrations associated with the NH2+ part in PANI [34]. FT-IR patterns of PANI, PANI/TX100, PANI/PEG, and PANI/SDS are consistent with previously reported results [35]. From the FT-IR study, it is observed that there is no extra peak in the FT-IR spectrum for PANI/TX100, PANI/PEG, and PANI/SDS samples than the pure PANI sample. It can be concluded that the surfactant is used to guide the chain formation and not take part in the reaction mechanism.

the morphology of PANI. Depending on the nature of the surfactant, the morphology of PANI changes from compact to a vertical porous nanostructure. Figure 3a–d exhibits the morphology of PANI, PANI/TX100, PANI/PEG, and PANI/SDS thin films, respectively. Such nanostructural morphology significantly improves the processability of PANI and its performance [36]. The differences in the structure of PANI in the presence of surfactants are clearly visible. Anionic surfactant (SDS) favors the polymerization of aniline so that the rate of polymerization is fast. On the other hand, the nonionic (TX100 and PEG) surfactant disfavors the polymerization. Because the rate of polymerization is slow, PANI could not be formed with a granular and compact morphology. Subsequently, the water pools collide with the deposited PANI, which induces fibrillar growth [37]. Surfactant-free PANI (Fig. 3a–a′) shows the compact nanofibrillar network. At the same magnification, PANI/TX100 reveals an interesting porous vertical, interconnected nanofibrillar structure (Fig. 3b–b′). The porous nanofibrillar network of the sample PANI/PEG is shown in Fig. 3c–c′. TX100 and PEG surfactants suppress the secondary growth of PANI which results in the formation of nanofibers [13]. For PANI/SDS (Fig. 3d–d′), a different morphology is observed than the other samples. It shows a smooth surface with nanograins. Thus, a highly nanostructured film of PANI is obtained in the presence of TX100 surfactant [38]. Due to the vertical and porous nanofibrillar network, PANI has a higher specific surface area which is useful for supercapacitor application [39].

Electrical conductivity Morphological studies Surface morphological studies of PANI thin films are carried out using the SEM technique. The micrographs of ×2,000 and ×10,000 magnifications show that the surfactant additive in the polymerization strongly affects Table 1 Observed peaks and their corresponding assignments of PANI, PANI/TX100, PANI/PEG, and PANI/SDS thin films

The electrical conductivities of PANI thin films are studied using the four-point probe method. Table 2 shows the variation in electrical conductivity of PANI thin films for different nanostructures. A maximum electrical conductivity of 54.10 S cm−1 is observed for

Assignment

N–H stretching vibrations of secondary amine Vibrations of NH2+ C=C stretching vibrations of quinoid ring C=C stretching vibrations of benzenoid ring C–N stretching vibrations C–H in plane deformation C–H out-of-plane deformation

Wavenumber (cm−1) PANI

PANI/TX100

PANI/PEG

PANI/SDS

3,197 2,928 1,578 1,490 1,299 1,123 818

3,198 2,923 1,574 1,496 1,299 1,124 814

3,213 2,928 1,580 1,484 1,300 1,116 815

3,215 2,933 1,579 1,489 1,297 1,116 812

Ionics Fig. 3 Scanning electron micrographs of PANI (a–a′), PANI/TX100 (b–b′), PANI/PEG (c–c′), and PANI/SDS (d–d′) thin films at ×2,000 and ×10,000 magnifications

the PANI/TX100 thin film which is much greater than other thin films. This may be due to the porous nanostructure of the PANI/TX100 thin film which increases the polaron transfer, which leads to enhance the electrical conductivity. The morphology and electrical properties are mostly related to each other for the conducting polymers. The surfactant changes the morphology of

Table 2 Values of electrical conductivity and corresponding morphologies of PANI, PANI/TX100, PANI/PEG, and PANI/SDS thin films

PANI films and this is reflected in the changes in electrical conductivity [28]. Supercapacitive study The supercapacitive performance of nanofibrillar PANI electrodes is tested by using CV. For the purpose of deposition, a

Sr. no.

Sample

Morphology

Electrical conductivity (S cm−1)

1 2

PANI PANI/TX100

3.98 54.10

3 4

PANI/PEG PANI/SDS

Compact nanofibrillar network Porous vertical, interconnected nanofibrillar structure Porous nanofibrillar network Nanograins

18.57 17.32

Ionics Fig. 4 CV curves of a PANI, b PANI/TX100, c PANI/PEG, and d PANI/SDS electrode for 5 mV s−1 scan rate in 1 M H2SO4 electrolyte

Fig. 5 The CV curves of a PANI, b PANI/TX100, c PANI/PEG, and d PANI/SDS electrode at different scanning rates with a 1-M fixed H2SO4 electrolyte concentration

Ionics Fig. 6 Variation of specific capacitance with scan rate

Fig. 7 Galvanostatic charge–discharge curves of a PANI, b PANI/TX100, c PANI/PEG, and d PANI/SDS with current densities in 1 M H2SO4 electrolyte

Ionics Table 3 Values of specific capacitance for PANI, PANI/TX100, PANI/PEG, and PANI/SDS thin films calculated from CV and GCD Sr. no. Sample

Specific capacitance (F g−1) calculated through: CV

1

PANI

2 3 4

PANI/TX100 1,040 PANI/PEG 376 PANI/SDS 447

637

Cs ¼

GCD 515

∫I

dV =dt

ð1Þ

The interfacial capacitance (Ci) is calculated as Ci ¼

C A

C W

ð3Þ

where I is the average current in ampere, dV/dt is the scan rate, A is the area of the film dipped into the electrolyte, and W is the weight of the PANI film dipped into the electrolyte. From the charge–discharge studies, specific capacitance is calculated as

979 211 356

4×1-cm2 stainless steel substrate was used, out of which 3× 1 cm2 was coated with the polyaniline film and 1×1 cm2 was used for the electrochemical study. The capacitance (C) is calculated by the following relation:

C¼

The specific capacitance (Cs) of the PANI electrode is calculated by the equation:

ð2Þ

Cs ¼

Id  Td ΔV  W

ð4Þ

where V is the potential, Id is the discharging current density, and Td is the discharging time. Cyclic voltammetry study Figure 4 shows the overlay of CV curves of PANI, PANI/TX100, PANI/PEG, and PANI/SDS thin films for the scan rate of 5 mV s−1. The result shows the two pairs of characteristic peaks (A1/B1, A2/B2). The transition of PANI

Fig. 8 The CV curves for stability of a PANI, b PANI/TX100, c PANI/PEG, and d PANI/SDS electrodes at the 2nd and 1,000th cycles in 1 M H2SO4 electrolyte for 50 mV s−1 scan rate

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from leucoemeraldine to its conducting emeraldine state is responsible for peaks A1/B1, and the other peaks A2/B2 are due to transition from emeraldine to pernigraniline [37]. It is observed that there is a significant variation in the specific capacitance for the different morphologies of PANI thin films. The calculated specific capacitance values at 5 mV s−1 scan rate are 637, 1,040, 376, and 447 F g−1 for PANI, PANI/TX100, PANI/PEG, and PANI/SDS, respectively. For the PANI/TX100 thin film, a maximum specific capacitance is observed. The enhancement in the specific capacitance for the synthesis of the PANI thin film in the presence of surfactant TX100 is due to the longer conjugated length of PANI which exhibited higher electrical conductivity [33, 40]. The CV curves of PANI, PANI/TX100, PANI/PEG, and PANI/SDS thin films at scan rates of 5, 10, 20, and 50 mV s−1 are shown in Fig. 5a–d within the potential window −0.2 to +0.8 V/SCE. The inner active sites of the electroactive material cannot complete the redox transition at a higher scan rate so that 5, 10, 20, and 50 mV s −1 scan rates are selected for the cyclic voltammetric studies. It shows that with the increase in scan rate, redox current increases, clearly indicating a good rate ability of PANI thin films [41]. With the increase in scan rate, the shift in the redox peaks at higher positive and negative potentials is observed and this is due to the diffusion effect of protons within the electrode [42]. The redox peaks observed in the CV curve are due to the reduction and oxidation of the electrochemically active species at the electrode. The appearance of redox peaks shows the pseudocapacitive behavior of polyaniline electrode. The specific capacitance values decrease as the scan rate increases from 5 to 50 mV s−1 as shown in Fig. 6.

Galvanostatic charge–discharge measurement The effect of surfactants on the charging–discharging behavior of PANI thin films is studied using the GCD measurement technique at current densities of 1, 2, and 4 mA cm−2 within the potential window −0.2 to + 0.8 V/SCE in 1 M H 2SO 4 electrolyte. Figure 7a–d shows the typical galvanostatic charge–discharge curves of PANI electrodes for different nanostructures. The nonlinear discharge curves indicate the pseudocapacitive behavior of the PANI electrode. The discharging time of PANI samples with surfactant is higher than the surfactant-free PANI sample. Table 3 shows the specific capacitance values calculated using CV and GCD curves. The PANI/TX100 electrode shows better supercapacitive performance than the other electrodes. This is due to the highly porous nanofibrillar structure of the electrode surface which provides maximum surface area at the electrode–electrolyte interface.

Cyclic stability study Figure 8a–d shows the cyclic stability of PANI electrodes synthesize with and without the surfactant at the scan rate of 50 mV s−1 for the 1,000 cycles. From the figure, it is seen that the specific capacitance values decreased from (a) 268 to 176 F g−1, (b) 284 to 241 F g−1, (c) 115 to 67 F g−1, and (d) 143 to 100 F g−1 of PANI, PANI/TX100, PANI/PEG, and PANI/SDS, respectively, in the first 1,000 cycles. The redox peaks in the CV curve of polyaniline electrodes become weak and disappear after the 1,000 cycles, and this is due to the reduced pseudocapacitor of PANI [43]. The PANI/TX100 sample shows 85 % cyclic stability which is much greater than those of PANI/PEG (59 %), PANI/SDS (70 %), and pure PANI (66 %) electrodes. The stability of the PANI electrode is higher than the earlier reported values [25, 44]. These results indicate that a material of this kind has long-term electrochemical stability and good electrochemical reversibility [45].

Conclusions In this study, we have demonstrated that various PANI nanostructures could be synthesized by varying the micellar environments during chemical polymerization. It is also found that the electrical conductivity and the electrochemical properties depend on the morphology of the PANI thin films. The conductivity of PANI is enhanced by surfactant additives, and the highest conductivity (54.10 S cm−1) is observed for the thin film prepared in the presence of surfactant TX100. From the results, it is observed that the specific capacitance of the PANI/TX100 film is higher (1,040 F g−1) than that of other (PANI/PEG, PANI/SDS, and PANI) thin films with 85 % cyclic stability. Hence, the PANI prepared in the presence of Triton X-100 is found to be the suitable electrode materials for redox supercapacitors. Acknowledgments The authors are grateful to the University Grant Commission (UGC), New Delhi (India) through the DSA-I Program and to the Department of Science and Technology for financial support through the DST-PURSE scheme and FIST programs.

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