- Email: [email protected]
polyaniline nanocomposites via in-situ polymerization process
Polymer 53 (2012) 2574e2582
Contents lists available at SciVerse ScienceDirect
Polymer journal homepage: www.elsevier.com/locate/polymer
Facile synthesis and morphology control of graphene oxide/polyaniline nanocomposites via in-situ polymerization process Y.F. Huang, C.W. Lin* Department of Chemical and Materials Engineering, National Yunlin University of Science and Technology, Yunlin 640, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 February 2012 Received in revised form 12 April 2012 Accepted 16 April 2012 Available online 21 April 2012
This study reports the synthesis of graphene oxide (GO)/polyaniline (PANI) nanocomposites with controllable morphologies through in-situ polymerization of aniline monomers in the presence of GO sheets. Specific reaction parameters including solution acidity, aniline concentration, and reaction temperature are used to control the final shape of the composite product. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images are used to explore the morphology of the composite. Thermogravimetric analysis (TGA), X-ray diffraction (XRD), FTeIR and UVevis spectrophotometers are utilized to characterize the intermediates and the final products of the GO/PANI composites. Experiment results reveal that the polymerization operated in low acidity and low temperature conditions inclines to form GO/PANI nanotubes. On the other hand, the polymerization operated in high acidity inclines to form either nanospheres or aligned nanofiber arrays. These different morphologies are resulted from different polymerization routes and the formation mechanisms of these different shapes of nanocomposites are explored. Among the various nanocomposites, the GO/PANI nanospheres exhibit a highest electrochemical surface area. This study provides a facile and effective strategy to control the morphology of GO/PANI nanocomposites with characteristic electrochemical property. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Graphene oxide Polyaniline Nanocomposites
1. Introduction Conducting polymer composites are attractive for a variety of applications due to their many features including low weight, low cost, corrosion resistance, and ease of processing and shaping [1,2]. To date, the combination of conducting polymer with carbon materials, such as carbon black, carbon fibers, and carbon nanotubes has been widely studied because of their outstanding properties, such as electrochemical, thermal and mechanical properties [3e5]. However, different from the mentioned carbon materials, graphene oxide (GO) nanoplatelets have created a new field of carbon-filled nanocomposites in recent years. GO is an oxidation state of graphene, grafted with various oxygen functional groups (i.e. epoxide, hydroxyl, and carboxyl groups) on its basal planes and edges. It can be produced via a chemical reduction of exfoliated graphite oxide [6]. This nanomaterial with strong hydrophilicity and good compatibility with polymers has been widely used to fabricate polymer-based nanocomposites because of their extraordinary structure [7].
* Corresponding author. Tel.: þ886 5 534 2601x4613; fax: þ886 5 531 2071. E-mail address: [email protected] (C.W. Lin). 0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2012.04.022
As a new type of functional composite, the combination of polyaniline (PANI) with GO has been widely studied to produce materials with desired electrochemical properties in recent years. Many researchers have reported that the combination of GO and PANI can promote the composite in higher electrochemical capacitance and charge-discharge cyclic stability than each individual component [8e12]. Besides, it is also found that this type of composite has higher electrical conductivity than each of the pristine PANI and GO [13,14]. Although the combination shows some advantages, most of the reported GO/PANI composites are in a two-dimensional shape with a lateral size of several micrometers [8e14]. On the other hand, the nanostructured composites exhibit excellent mechanical, electrical and optical properties endowed by confining dimension of such materials and combining bulk and surface properties to the overall behavior. The potential of materials with reduced dimensions becomes fascinating in a variety of possible applications. It has been pointed out that the morphology plays an important role in electrochemical properties of electroactive materials, such as electrical conductivity, electrochemical active surface, and interfacial charge transfer resistance [3,15]. For examples, polypyrrole nanofibers show a significantly higher effective surface area than that of polypyrrole spheres [15]. On the other hand, carbon
Y.F. Huang, C.W. Lin / Polymer 53 (2012) 2574e2582
nanotubes possess a much higher electrical conductivity and electrochemical surface than that of carbon blacks and carbon nanofibers [3]. Moreover, the shape of nanomaterials also restricts their applications. The nanomaterials with one-dimensional morphology can serve as functional compartment and interconnector in various electrochemical nanodevices [16]. In contrast, the nanomaterials with a spherical and hollow structure are considered useful for gas sensing and gas separation [17]. To promote electrochemical properties and to diversify applications of GO/PANI composites, this study intends to synthesize a series of GO/PANI composites with controllable morphologies. Several polymerization parameters, including solution acidity, aniline concentration, and reaction temperature, were used to control the final morphology of GO/PANI products. Three distinctive shapes, including nanotube with different outside diameters, aligned nanofiber array, and nanosphere, were successfully fabricated in this study. The intermediates obtained at different reaction stages were also characterized to investigate the morphological formation mechanisms. Moreover, this study also investigated the electrochemical surface area of the GO/PANI nanocomposites. To the author’s best knowledge, this is the first study reports the strategy to control the shapes of GO/PANI nanocomposites and demonstrates their shape-dependent electrochemical properties. 2. Experimental 2.1. Materials Aniline monomers (S99.5%) were purchased from Fluka. Ammonium persulfate (APS, 98%) as oxidant was obtained from Riedel-de Haën. Graphite powders (crystalline, 300 mesh, 99%) was obtained from Alfa- Aesar. Potassium permanganate (S99%) was obtained from J.T. Baker. Sodium nitrate (S99%) was purchased from SigmaeAldrich. All reagents were used as received without further purification.
2575
lyophilization to avoid the aggregation of graphene oxide during the drying process [20]. 2.3. Synthesis of GO/PANI nanocomposites The GO/PANI composites were produced using an in-situ polymerization process of aniline monomers in the presence of graphene oxide. Firstly, GO was dispersed in an aqueous solution and sonicated for at least 1 h to exfoliate the graphite oxide into graphene oxide [21]. A suitable amount of aniline monomers was then added into the solution and the solution was sonicated for at least 30 min to produce a stable mixture of graphene oxide and aniline monomers. The preparation of nanotubes typically involved mixing an aqueous solution of aniline in low acidic solutions (&0.05 M HCl) and oxidant (ammonium peroxydisulfate) solution in the same acidic solution. The solutions were then poured rapidly into a glass vial and shaken vigorously for 30 s, and then put in an ultrasonic water bath at 0 C. The weight ratio of GO to aniline in this solution is 3:7. The concentration of aniline varied from 0.1 to 0.06 M to control the diameter of resulting nanotubes. The reaction time for the synthesis of nanotubes at 0.1 and 0.06 M aniline concentration is 90 min and 5 h, respectively. The nanospheres were synthesized with the same procedure using 0.1 M aniline monomers and 0.2 M HCl solutions. The nanofiber arrays were synthesized at 50 C in 1 M H2SO4 solutions. In this case, the aniline concentration and the weight ratio of GO to aniline is 0.03 M and 5:5, respectively. The reaction time for forming nanospheres and nanofiber arrays is 90 min. A molar ratio of [ANI]/[APS] equal to 0.8:1 was applied to all reactions. The resulting product was isolated by centrifugation and washed with de-ionized water for several times. Finally, the composite product was dried in an oven at 50 C for 1 day. The polymerization conditions and the corresponding morphologies of the GO/PANI composites are summarized in Table 1 for better comparison.
2.2. Preparation of graphene oxide
2.4. Characterization of GO and GO/PANI nanocomposites
Graphene oxide was synthesized from the natural graphite powders using a modified Hummers and Offenman’s method [18,19]. In brief, 1 g of graphite, 1 g of NaNO3, and 46 mL of concentrated H2SO4 were mixed together and stirred in an ice bath for around 30 min. Then, 3 g of KMnO4 was added slowly into the solution followed by stirring at 35 C for 1 day to form thickened paste. Afterward 46 mL of de-ionized water was added slowly into the reaction solutions to avoid the reaction temperature rising to a limit of 98 C. This solution was then kept stirring for 30 min. Finally, 140 mL of de-ionized water and 10 mL of H2O2 (30%) was poured into the mixture in sequence. This solution was then filtered by gravity filtration and the filter cake was washed with de-ionized water and 3% HCl solution alternatively. This filtered cake was then dispersed and washed with de-ionized water for several times by repeated centrifugation. The resulting samples were then dried by
The X-ray diffraction (XRD) was carried out on an X-ray diffractometer (MiniFlex II, Rigaku, USA) using Cu Ka radiation (g ¼ 0.15406 nm). The diffraction patterns obtained in a scan range of 5 e100 were collected with a scan rate of 2 min1. Thermogravimetric analysis (TGA) curves were performed on a TGA Instrument (TGA 2050, TA Instrument, USA) at a heating rate of 15 C min1 in nitrogen environment. Morphologies of the GO/PANI composites were obtained by a transmission electron microscopy (JEM-2100, JEOL, Japan) and a field emission scanning electron microscopy (JSM-7401F, JEOL, Japan). Infrared spectra of the GO/ PANI composites in pellet made with potassium bromide (KBr) were performed on an FTeIR spectrophotometer (Spectrum One, Perkin Elmer, USA) with a resolution of 4 cm1 and 32 scans. UVevis spectrophotometer (Lambda 850, Perkin Elmer, USA) was used to identify the GO/PANI dispersion in de-ionized water.
Table 1 Polymerization conditions and corresponding morphologies of GO/PANI nanocomposites. Morphology
Aniline concentration
Reaction temperature
Solution
Weight ratio of aniline to GO
Reaction time
Nanotubes (250 nm in O.D.) Nanotubes (45 nm in O.D.) Nanofiber arrays Nanospheres
0.1 M
0 C
0.05 M HCl
7:3
90 min
0.06 M
0 C
0.01 M HCl
7:3
5h
0.1 M 0.1 M
50 C 0 C
0.2 M HCl 1 M H2SO4
5:5 7:3
90 min 90 min
2576
Y.F. Huang, C.W. Lin / Polymer 53 (2012) 2574e2582
The cyclic voltammetry (CV) curves of the GO/PANI composites were conducted with a three-electrode setup. The working electrode was prepared as follows. The GO/PANI products were firstly dispersed in isopropanol and then a suitable amount of NafionÒ solution was added to the dispersion. The content of Nafion in the mixture is 10 wt% and the amount of GO/PANI in the electrode is 1 mg cm2. This ink was then spread on a carbon cloth and dried at 60 C. Ag/AgCl and platinum wire was used as a reference and counter electrode, respectively. The cyclic voltammetry curves were obtained in a 0.5 M H2SO4 solution with a scan rate of 50 mV s1. The scan range was from 0.2 to 1.0 V versus Ag/AgCl. 3. Results and discussion 3.1. Morphology characterization of GO/PANI nanocomposites Fig. 1 presents the SEM images of the GO/PANI samples obtained from different polymerization conditions. TEM images are additionally provided as inset in Figure (a) and (b) for better distinction. As can be seen, several polymerization parameters affect greatly the resulted morphologies of the products. As polymerization was performed in low acidic solution (i.e. & 0.05 M HCl) and low temperature (i.e. 0 C), a shape of nanotube with an outer diameter of 250 nm and an inner diameter of 150 nm could be obtained, as shown in Fig. 1(a). However, with the decrease of aniline concentration from 0.1 to 0.06 M, the outer diameter of the nanotubes decreases significantly to only 45 nm, as can be seen in Fig. 1(b). Obviously, aniline concentration is a critical factor determining the diameter of the nanotubes. Apart from aniline concentration, solution acidity was also found to be an important factor leading to a morphological change of the composite. With the increase of solution acidity from 0.05 M to 0.2 M, the polymerization reaction led to a shape of nanosphere with a diameter of around 100 nm, as shown in Fig. 1(c).
Interestingly, the nanospheres are connected with a bound of nanowires. The inset image in Fig. 1(c) reveals that the diameter of the nanowires is less than 10 nm, significantly thinner than those reported in the literature (i.e. 30e100 nm) [22,23]. Now, we turn our attention to the effect of reaction temperature. It is noted that the morphology changes again with the increase of reaction temperature from 0 C to 50 C. The formation of aligned nanofiber arrays with a diameter of only 30 nm can be observed in Fig. 1(d) from the reaction solution consisting of 1 M H2SO4 and 0.03 M aniline monomers. Based on above, a temporary conclusion can be drawn as that the final GO/PANI morphology is changeable through some specific polymerization parameters including solution acidity, aniline concentration, and reaction temperature. In order to further verify the coexistence of GO and PANI, derivative thermogravimetric analysis (DTA) and XRD techniques were used to identify the hybrid nature of these obtained products. Fig. 2 shows the DTA curves of the pristine GO and PANI (as an insert in Figure), as well as the GO/PANI nanostructures with various shapes. As can be seen, all samples exhibit a mass loss peak below 100 C, associated to the loss of absorbed water in the samples. Along with temperature rising, another two mass loss peaks at around 200 C and 500 Ce700 C can be observed. These peaks can be ascribed respectively to the decomposition of oxygencontaining groups on GO sheets [24] and the decomposition of the polyaniline backbone [25]. These results confirm the coexistence of GO and polyaniline in the products. This verification can be reinforced with the observation from the XRD patterns presented in Fig. 3(a), in which the composite samples exhibit two characteristic peaks at 8e11 and 25 . The former one is corresponding to the grafts of various functional groups (i.e. epoxide and carboxyl) on the GO sheets [26], and the later one is associated to the amorphous structure of PANI [27]. However, by a further comparison of the XRD patterns of the GO/PANI nanocomposites, it is noted that that the peak located at
Fig. 1. SEM images of the GO/PANI nanostructures: (a) nanotubes obtained from 0.1 M aniline solution, (b) nanotubes obtained from 0.06 M aniline solution, (c) nanospheres, and (d) aligned nanofiber arrays. The reaction temperature is 0 C for both nanotubes and nanospheres while is 50 C for nanofiber arrays. The insets appeared in (a) and (b) are TEM images of the nanotubes.
Y.F. Huang, C.W. Lin / Polymer 53 (2012) 2574e2582
2577
Fig. 2. DTA curves of GO, PANI, and GO/PANI nanocomposites.
8e11 shifts along with the change of the product shapes. The pristine GO sample shown in Fig. 3(b) exhibits a peak at 10.9 which can be assigned to an interlayer distance of 0.81 nm between GO sheets. However, with the formation of GO/PANI nanotubes, the diffraction peak gradually shifts to a smaller angle close to 8.7,
Fig. 4. (a) FTeIR spectra and (b) XRD patterns of the various intermediates obtained at a reaction time of 30 min.
indicating an increase of GO interlayer distance up to 1.02 nm. It can be ascribed to the intercalation of PANI chains between GO sheets [26]. On the other hand, with the formation of the aligned nanofiber arrays, the corresponding peak remains at 10.9 , suggesting the nanofiber arrays may primarily grow on the GO surfaces rather than intercalating between GO sheets. Moreover, it is worthy to note that this peak become almost invisible with the formation of GO/PANI nanospheres, reflecting a full exfoliation of the GO sheets in this product [28]. In this condition, the as-synthesized nanosphere may serve as a spacer to prevent the re-stacking of GO sheets during the reaction process [29]. According to above results, it can be summarized that the parameter modulation of aniline monomers polymerized in presence of GO sheets affects not only the final morphology but also the layer structure of GO sheets within the GO/PANI nanocomposites. Succeeded in controlling GO/PANI morphology, the investigation was turned to explore the formation mechanism of the GO/PANI morphology through characterizing the intermediates obtained in different reaction stages. 3.2. Exploration of the formation mechanisms of the GO/PANI nanocomposites
Fig. 3. XRD patterns of the pristine GO, PANI, and GO/PANI nanocomposites.
3.2.1. Characteristics of the GO/PANI intermediates formed in the initial reaction stage Fig. 4 shows the FTeIR spectra of the initially-formed GO/PANI samples obtained from different reaction solutions. As can be seen, the absorbance bands of the GO/PANI intermediates obtained at a reaction time of 30 min changed with the variation of the reaction parameters. In a condition to form nanotubes (&0.05 M HCl), phenazine-like aniline units can be obtained in the initial reaction stage, verified by the peak observed at 1414 cm1 which represents totally symmetrical stretching of the phenazine heterocyclic ring [30,31]. The band at 1622 cm1 corresponds to the absorption of the C]C ring stretching vibration in newly-formed substituted phenazine-like segments [31,32]. In an acidity condition to form nanospheres and aligned nanofiber arrays (S0.2 M HCl), the characteristic bands associated to emeraldine polyaniline units can be observed. The CeH out-of-plane bending vibration assigned to a para-substitution pattern appears at 829 cm1, and the N]Q]N absorption peak (with Q representing the quinoid ring) at 1141 cm1 are also visible [27].
2578
Y.F. Huang, C.W. Lin / Polymer 53 (2012) 2574e2582
These different molecular structures of the initially-formed intermediates are ascribed to be resulted from different polymerization routes with different reaction parameters. For the formation of nanotubes at low acidity, the oxidation of aniline molecules produces phenazine-like units through an ortho-coupling route [31e34]. However, when producing nanospheres and nanofiber arrays at high acidity, the aniline molecules might firstly convert to anilinium cations and then polymerized through a para-coupling route to form emeraldine polyaniline chains [33,34]. 3.2.2. The interaction between aniline monomers and GO sheets Now, we turn our attention to the interaction between aniline monomers and GO sheets. Researchers have reported that the attractive electrostatic interactions formed between the negatively charged surface of oxidized carbon black and anilinium cations can promote adsorption of anilinium cations on oxidized carbon black [35]. Therefore, in the cases of forming nanospheres and nanofiber arrays (i.e. in higher acidity), the anilinium cations formed initially would adsorb well on the GO surfaces and the subsequent in-situ growth of polyaniline chains would block the re-stacking of the GO sheets [36]. On the other hand, in the case of forming nanotubes (i.e. in lower acidity), the reduction of p electron density on the graphene after the oxidation process would weaken the adsorption capacity of graphene oxide to neutral aniline monomers [35]. In this condition, the oxidation of neural aniline monomers may firstly produce the self-standing phenazine-like aniline units in the reaction solution and then the p-p electron interaction formed between the phenazine units and the GO sheets may promote the re-stacking of GO sheets. Consequently, a phenazine unitsintercalated GO structure can be established. The pep stacking interactions can be formed between two relatively nonpolar aromatic rings, such as carbon nanotubes, graphene oxide, and aromatic molecules (i.e. dopamine and phenazine units) [37,38]. Zhang et al. [39], have demonstrated the stacking of new layered structure existing in hybrid through the intermolecular pep
interactions formed between two different kinds of moieties, such as phthalocyanine derivatives and graphene oxide. Very recently, it was discovered that key nanosheet structural changes, occurring within the first minutes of aniline polymerization, affect the self-assembly course of the subsequent PANI nanostructure [40]. In order to further verify the effect of the reaction parameters on the structure of the GO/PANI nanocomposites, XRD was utilized to identify the ordered structure of these initially-formed GO/PANI complexes and the patterns are shown in Fig. 4(b). Although conventional analysis techniques struggle at this early stage due to the reason that they require relatively large amounts of sample for analysis. However, the variation of the detectable GO layered structures still can provide helpful evidence for supporting our inference. Regarding the nanotubes, the diffraction peak can be observed to shift gradually to a smaller angle at 8.7, suggesting the intercalation of PANI between GO sheets [27]. However, in the cases of preparing nanospheres and nanofiber arrays, the intensity of the peak at w10 decreases significantly at this reaction stage due to the disordered layer structures of the GO sheets [36]. Accordingly, it can be summarized that different polymerization routes of aniline monomers would result in different structures of the GO/PANI complexes formed at the initial reaction stage. Apart from the variation of the molecular structure of PANI and the ordered structure of GO, it can be noted that the initiallyformed intermediates also exhibit different morphologies. Fig. 5 (a) and (b) show the deposition of tree-like objects on the GO surfaces at the initial reaction stage to form nanotubes. On the contrary, corresponding to the formations of nanospheres and aligned nanofiber arrays, Fig. 5 (c) and (d) present the growth of rod-like protuberances on the GO surfaces at the initial reaction stage. These observations reveal that a large number of PANI nuclei are initiated on the GO surfaces at the early stage and the functional groups on GO sheet are thought to serve as anchoring sites for the subsequent in-situ polymerization of aniline monomers [12].
Fig. 5. SEM images of the various intermediates obtained at a reaction time of 30 min from the reaction condition to form (a) nanotube with a diameter of 250 nm, (b) nanotube with a diameter of 45 nm, (c) nanospheres, and (d) aligned nanofiber arrays.
Y.F. Huang, C.W. Lin / Polymer 53 (2012) 2574e2582
Based on above results, it can be inferred that different reaction conditions will lead to different morphologies of the initiallyformed GO/PANI intermediates. Obviously, these variations are responsible for the final shape of the GO/PANI composite developed in the succeeding reaction stages. To fully explore the formation mechanisms of GO/PANI products with various shapes, a further characterization for the intermediates obtained at extended reaction time is essential. 3.2.3. The formation mechanism of GO/PANI nanotubes Fig. 6(a) compares the FTeIR spectra of the GO/PANI intermediates obtained from different reaction stages under the polymerization conditions to form nanotubes. As can be seen, samples obtained in the early stage exhibit the characteristic peaks corresponding to the formation of phenazine-like aniline units at 1414 and 1622 cm1. However, as the reaction time advances to 60 min, the band at 1414 cm1 assigned to the totally symmetrical stretching of the phenazine ring disappears. Contrarily, the CeH out-of-plane bending vibrations band from the N]Q]N absorption peak at 1142 cm1 becomes apparent due to the reason that the head-to-tail couplings of aniline monomers are newly formed since the reaction time advances longer than 30 min.
Fig. 6. (a) FTeIR spectra and (b) UVevis spectra of the intermediates obtained at different reaction stages under the reaction conditions to form nanotubes with a diameter of 250 nm.
2579
It is inferred that the increase of solution acidity during the polymerization process may be responsible for the molecular structural changes of the products. Several studies reported the hydrogen atoms abstracted during the oxidation of amino groups and benzene rings as protons increased the solution acidity during the polymerization process [41e43]. The increased solution acidity will consequently cause the change of the polymerization route of aniline from ortho-coupling to para-coupling and thus leads to the variation of PANI molecular structure during the polymerization process [31e34]. UVevis analysis was also conducted to further analyze the GO/ PANI intermediates obtained at different reaction stages and the spectra are presented in Fig. 6(b), in which the intermediates exhibit an absorption band at about 430 nm. This band is due to the formation of phenazine-like structures in this stage [44]. However, as the reaction prolongs, the characteristic absorption bands at wavelengths of 320e360 nm, 420e440 nm and 800e900 nm become visible at a reaction time of 60 min. These bands are assigned to the formation of emeraldine salt polyaniline (ES-PANI). The first absorption band corresponds to the pep* electron transition within the benzenoid segments. The second and the third bands correspond to the acid-doped state and the polaron formation in polyaniline, respectively [23]. Besides, the absorbance band at 420e440 nm, assigned to the acid-doped state, becomes apparent after the reaction time of 60 min. This result verifies the increase of doping level of PANI as the reaction prolonged. Fig. 7 presents the TEM image of the intermediates obtained at the reaction time of 60 min. It clearly demonstrates the transformation of the GO/PANI composite through a semi-curling process and subsequently the formation of the nanotubes appeared at the reaction time of 90 min, as shown in Fig. 1(a). The morphological transition of the product into nanotubes may be ascribed to the changes of molecular structure and doping state of the intercalated polyaniline molecules during the polymerization process. In this condition, the phenazine-like units with a nonlinear structure formed at initial reaction stage may serve as the axis of curling into nanotubes [45e47]. Besides, the conformation change of the linear emeraldine aniline units after the protonation process may provide the driving force for the initially-formed GO/ PANI sheets to curl into nanotubes [45e47]. A gradual change of polyaniline conformation from an expanded coil to a compact coil
Fig. 7. TEM image of the intermediates obtained at the reaction time of 60 min from the reaction solution to form nanotubes with a diameter of 250 nm.
2580
Y.F. Huang, C.W. Lin / Polymer 53 (2012) 2574e2582
after the protonation process is probably due to the combination of the negative ions to the imine nitrogen atoms, which releases the repulsive forces between polyaniline chains [48,49]. Accordingly, it is believed that the synergic effect of the changes of both molecular structure and doping state of the intercalated PANI molecules make GO/PANI complexes transit to nanotube morphology. 3.2.4. The formation mechanisms of GO/PANI nanospheres and aligned nanofiber array Now, we turn our attention to the formation mechanisms of the GO/PANI nanospheres and the GO/PANI aligned nanofiber array. Fig. 8 is the FTeIR spectra of the intermediates obtained at different reaction stages under the conditions of forming nanospheres and aligned nanofiber array. The characteristic peaks related to the formation of linear emeraldine polyaniline chains at 829 and 1141 cm1 appear during the entire polymerization process. In this condition, the intrinsic tendency of the initially-formed linear emeraldine polyaniline chains may lead to the growth of rod-like objects on the GO surfaces, as shown in Fig. 5(c) and (d). It has been reported that the polymerization of aniline monomers in high acidic environment can intrinsically form nanofibers [23,50e52]. The linear nature of the emeraldine polyaniline chains formed at the initial reaction stage can thus serve as a template to form nanofibers [51]. These arguments are satisfactorily consistent with our observations in the SEM images of the growth of the rod-like
objects on the GO surfaces at the initial reaction stage (Fig. 5(c)e(d)) and in the FTeIR spectra (Fig. 8). It is therefore inferred that the linear aniline units formed at the initial reaction stage can serve as the template to grow the rod-like protuberances on the GO surfaces. As the polymerization reaction prolonged, the initially-formed rod-like protuberances may grow continuously through different routes. At lower reaction temperature, the initially-formed rod-like protuberances turn their morphology into nanospheres (Fig. 1(c)). On the other hand, if polymerization proceeds at higher temperature and lower aniline concentration, the initially-formed protuberances continue to grow into nanofibers on GO surfaces and thus result in the formation of the aligned nanofiber arrays (Fig. 1(d). There are three possible routes existing in the polymerization process following the formation of the primary PANI nanofibers, including (i) the continuing formation of the primary PANI nanofibers, (ii) growing the primary nanofibers into thicker fibers with uneven surfaces, and (iii) the growth and aggregation of the thicker fibers leading to irregular particles [52]. The polymerization follows the routes (ii) and (iii) in conventional procedures all produce irregular particles. Polymerization rate is a critical factor to inhibit the development of the second and the third routes for PANI nanofiber growth [52,53]. Moreover, to lower aniline concentration can also efficiently inhibit the secondary growth of the primary nanofibers [23,54]. It is therefore inferred that the reaction carried out under high reaction temperature and low aniline concentration may efficiently depress the secondary growth of the initiallyformed rod-like objects on GO surfaces and consequently lead to the formation of aligned nanofiber array. At this moment, we summarize the growth mechanisms of GO/ PANI nanotubes, nanospheres and aligned nanofiber arrays as follows. (i) At low solution acidity, the oxidation of aniline monomers produced intercalated polyaniline chains with a head of phenazine-like aniline unit and a tail of para-linked aniline unit. The phenazine-like unit can serve as the axis and the protonation of emeraldine aniline unit can provide the driving force for the formation of a nanotube through a self-curling process. (ii) At high solution acidity, aniline monomers formed linear polyaniline chains at the initial reaction stage and these linear chains may serve as template for the growth of rod-like objects on GO surfaces. As the reaction extends, these rod-like objects may continue to grow and turn into nanospheres if low reaction temperature is applied. On the other hand, if high reaction temperature and low aniline concentration are applied, the secondary growth of the rod-like objects can be effectively inhibited and aligned nanofiber array will be formed consequently. A comprehensive scheme illustrating polymerization condition and corresponding morphology of GO/ PANI composite is presented in Fig. 9. 3.3. The shape-dependent electrochemical properties of the GO/ PANI nanocomposites
Fig. 8. FTeIR spectra of the intermediates obtained from the reaction solution to form (a) nanospheres, (b) aligned nanofiber arrays.
Succeeded in morphology control and mechanism exploration, our investigation was extended to the electrochemical properties of these three types of GO/PANI nanostructure. Several studies reported that the combination of polyaniline and GO enabled the composites to exhibit higher electrochemical properties than each individual component [8e12]. However, most of these studies did not pay attention to the relationship between the composite morphology and the measured electrochemical properties. This study therefore tries to build up the shape-dependent electrochemical properties of the GO/PANI nanocomposites. Fig. 10 compares the CV curves of the GO/PANI nanocomposites with various shapes. As can be seen, the GO/PANI nanotubes with a diameter of 250 nm exhibit a peak current density of 2.9 A g1,
Y.F. Huang, C.W. Lin / Polymer 53 (2012) 2574e2582
2581
Fig. 9. Scheme of the formation of GO/PANI nanostructures with various shapes through an in-situ polymerization process.
quite close to that obtained from the nanofiber arrays (ca. 2.5 A g1). However, as the nanotube diameter largely reduces to 45 nm, the peak current density of the GO/PANI nanotubes increases significantly to 4.0 A g1. The higher current density stands for a higher electrochemical active surface of the electroactive materials [15]. This result suggests that the decrease of nanotube diameter is advantageous in electrochemical active surface. It is worthy to note that the nanospheres exhibit a highest current density (6.9 A g1) among the investigated nanocomposites. This excellent characteristic in terms of electrochemical active surface is ascribed to the fully exfoliation state of the GO sheets in the nanospheres as has been well identified in previous section (i.e. Fig. 2(b)). The fully exfoliated structure of GO sheets is widely reported to possess higher specific surface area [55]. This
preliminary result directs a future work to explore possible applications of GO/PANI composites with specific morphologies in various electrochemical devices. 4. Conclusions This study demonstrates an efficient method to synthesize a series of GO/PANI nanocomposites with various shapes through in-situ polymerization of aniline monomers in the presence of GO sheets. The selected reaction parameters determine the polymerization routes and thus result in different morphologies of GO/PANI nanocomposites. At low solution acidity, the synergistic effect of structural change of polyaniline and doping state during the polymerization process leads to the formation of nanotubes through a self-curling process, driven by the intercalated polyaniline in the GO sheets. As the solution acidity increases, the in-situ polymerization produces linear polyaniline chains at the initial reaction stage, which serve as templates for the growth of the rod-like objects on GO surfaces. If the reaction proceeds in a low temperature condition, the initially-formed rod-like objects may consequently grow into nanospheres. However, if the secondary growth of these rod-like objects can be efficiently inhibited by carrying out the reaction under low aniline concentration and high reaction temperature, then GO/PANI with aligned nanofiber arrays can be obtained. Among the various investigated nanocomposites, the nanospheres exhibit a highest electrochemical active surface. This work provides a facile and effective method to control GO/PANI nanocomposites with desired shapes for the possible applications in various electrochemical devices. Acknowledgement
Fig. 10. Cyclic voltammogram curves of the GO/PANI nanocomposites measured in 0.5 M H2SO4 solutions.
The authors are grateful to the National Science Council of Taiwan (ROC) for the financial support of this work under grant no. NSC 99-2221-E-224-069-MY2.
2582
Y.F. Huang, C.W. Lin / Polymer 53 (2012) 2574e2582
References [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]
Lu X, Zhang W, Wang C, Wen T, Wei Y. Prog Polym Sci 2011;36(5):671e712. Hatchett DW, Josowicz M. Chem Rev 2008;108(2):746e69. Al-Saleh MH, Sundararaj U. Composites Part A 2011;42(2):2126e42. Wu KH, Ting TH, Wang GP, Ho WD, Shih CC. Polym Degrad Stab 2008;93(2): 483e8. Deng J, Ding X, Zhang W, Peng Y, Wang J, Long X, et al. Eur Polym J 2002; 38(12):2497e501. Dreyer DR, Park S, Bielawski CW, Ruoff RS. Chem Soc Rev 2010;39:228e40. Stankovich S, Dikin D, Piner RD, Kohlhaas KA, Kleinhammes A, Jia Y, et al. Carbon 2007;45(7):1558e65. Xu LQ, Liu YL, Neoh K, Kang E, Fu GD. Macromol Rapid Commun 2011;32(8): 684e8. Wang D, Li F, Zhao J, Ren W, Chen Z, Tan J, et al. ACS Nano 2009;3(7):1745e52. Wang H, Hao Q, Yang X, Lu L, Wang X. Electrochem Commun 2009;11(6): 1158e61. Murugan AV, Muraliganth T, Manthiram A. Chem Mater 2009;21(21):5004e6. Xu J, Wang K, Zu S, Han B, Wei Z. ACS Nano 2010;4(9):5019e26. Gao Y, Yip H, Chen K, O’Malley KM, Acton O, Sun Y, et al. Adv Mater 2011; 23(16):1903e8. Yang N, Zhai J, Wan M, Wang D, Jiang L. Synth Met 2010;160(15e16): 1617e22. Xu P, Han X, Zhang B, Mack NH, Jeon S, Wang HL. Polymer 2009;50(12): 2624e9. Fu G, Li G, Neoh KG, Kang ET. Prog Polym Sci 2011;36(1):127e67. Zhang J, Wang S, Wang Y, Xu M, Xia H, Zhang S, et al. Sensor Actuat B-Chem 2009;139(2):411e7. Hummers WS, Offeman Jr RE. J Am Chem Soc 1958;80(6). 1339e1339. Becerril HA, Mao J, Liu Z, Stoltenberg RM, Bao Z, Chen Y. ACS Nano 2008;2(3): 463e70. Xin Y, Liu J, Zhou Y, Liu W, Gao J, Xie Y, et al. J Power Sources 2011;196(3): 1012e8. Li D, Muller MB, Gilje S, Kaner RB, Wallace GG. Nat Nanotechnol 2008;3:101e5. Huang J, Kaner RB. J Am Chem Soc 2004;126(3):851e5. Chiou NR, Epstein AJ. Adv Mater 2005;17(13):1679e83. Park S, An J, Jung I, Piner RD, An SJ, Li X, et al. Nano Lett 2009;9(4):1593e7. Abell L, Pomfret SJ, Adams PN, Monkman AP. Synth Met 1997;84(1e3): 127e32. Zhang K, Zhang LL, Zhao XS, Wu J. Chem Mater 2010;22(4):1392e401.
[27] Zhang Z, Wei Z, Wan M. Macromolecules 2002;35(15):5937e42. [28] Bissessur R, Liu PKY, White W, Scully SF. Langmuir 2006;22(4):1729e34. [29] Park S, Shao Y, Wan H, Rieke PC, Viswanathan VV, Towne SA, et al. Electrochem Commun 2011;13(3):258e61. e nková I, Trchová M, Sapurina I, Cieslar M, [30] Konyushenko EN, Stejskal J, Sed et al. Polym Int 2006;55(1):31e9. e nková I, Konyushenko EN, Stejskal J, Holler P, [31] Trchová M, Sed Ciri ceMarjanovi c G. J Phys Chem B 2006;110(19):9461e8. [32] Stejskal J, Sapurina I, Trchová M, Konyushenko EN, Holler P. Polymer 2006; 47(25):8253e62. [33] Zujovic ZD, Laslau C, Bowmaker GA, Kilmartin PA, Webber AL, Brown SP, et al. Macromolecules 2010;43(2):662e70. [34] Ciri ceMarjanovi c G, Trchová M, Stejskal J. Collect Czech Chem Commun 2006; 71(10):1407. [35] Radovic LR, Silva IF, Ume JI, Menéndez JA, Leon-Leon CA, Scaroni AW. Carbon 1997;35(9):1339e48. [36] Chen G, Shau S, Juang T, Lee R, Chen C, Suen S, et al. Langmuir 2011;27(23): 14563e9. [37] Laslau C, Zujovic Z, Travas-Sejdic J. Prog Polym Sci 2010;35(12):1403e9. [38] Liu J, Tao L, Yang W, Li D, Boyer C, Wuhrer R, et al. Langmuir 2010;26(12): 10068e75. [39] Zhang X, Feng Y, Tang S, Feng W. Carbon 2010;48(1):211e6. [40] Zujovic ZD, Laslau C, Bowmaker GA, Kilmartin PA, Webber AL, Brown SP, Travas-Sejdic J. Macromolecules 2010;43(2):662e70. [41] Fu Y, Elsenbaumer RL. Chem Mater 1994;6(5):671e7. [42] Gospodinova N, Mokreva P, Terlemezyan L. Polymer 1993;34(11):2438e9. [43] Neoh KG, Kang ET. Polymer 1993;34(18):3921e8. [44] Salma B, Rudolf H. Electrochim Acta 2007;52(17):5346e56. [45] Huang YF, Lin CW. Polym Int 2010;59(9):1226e32. [46] Huang YF, Lin CW. Synth Met 2009;159(17e18):1824e30. [47] Huang YF, Lin CW. Polymer 2009;50(3):775e82. [48] MacDiarmid AG, Epstein AJ. Synth Met 1995;69(1e3):85e92. [49] Avlyanov JK, Min Y, MacDiarmid AG, Epstein AJ. Synth Met 1995;72(1):65. [50] Huang J, Kaner RB. Angew Chem Int Ed 2004;43(43):5817e21. [51] Li W, Zhu M, Zhang Q, Chen D. Appl Phys Lett 2006;89:103110e2. [52] Jing XL, Wang YY, Wu D, She L, Guo Y. J Polym Sci Part A Polym Chem 2006; 44(2):1014e9. [53] Li D, Kaner RB. J Mater Chem 2007;17:2279e82. [54] Chiou N, Epstein AJ. Synth Met 2005;153(1e3):69e72. [55] Frelink T, Visscher W, Veen J. J Electroanal Chem 1995;382(1e2):65e72.
Contents lists available at SciVerse ScienceDirect
Polymer journal homepage: www.elsevier.com/locate/polymer
Facile synthesis and morphology control of graphene oxide/polyaniline nanocomposites via in-situ polymerization process Y.F. Huang, C.W. Lin* Department of Chemical and Materials Engineering, National Yunlin University of Science and Technology, Yunlin 640, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 February 2012 Received in revised form 12 April 2012 Accepted 16 April 2012 Available online 21 April 2012
This study reports the synthesis of graphene oxide (GO)/polyaniline (PANI) nanocomposites with controllable morphologies through in-situ polymerization of aniline monomers in the presence of GO sheets. Specific reaction parameters including solution acidity, aniline concentration, and reaction temperature are used to control the final shape of the composite product. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images are used to explore the morphology of the composite. Thermogravimetric analysis (TGA), X-ray diffraction (XRD), FTeIR and UVevis spectrophotometers are utilized to characterize the intermediates and the final products of the GO/PANI composites. Experiment results reveal that the polymerization operated in low acidity and low temperature conditions inclines to form GO/PANI nanotubes. On the other hand, the polymerization operated in high acidity inclines to form either nanospheres or aligned nanofiber arrays. These different morphologies are resulted from different polymerization routes and the formation mechanisms of these different shapes of nanocomposites are explored. Among the various nanocomposites, the GO/PANI nanospheres exhibit a highest electrochemical surface area. This study provides a facile and effective strategy to control the morphology of GO/PANI nanocomposites with characteristic electrochemical property. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Graphene oxide Polyaniline Nanocomposites
1. Introduction Conducting polymer composites are attractive for a variety of applications due to their many features including low weight, low cost, corrosion resistance, and ease of processing and shaping [1,2]. To date, the combination of conducting polymer with carbon materials, such as carbon black, carbon fibers, and carbon nanotubes has been widely studied because of their outstanding properties, such as electrochemical, thermal and mechanical properties [3e5]. However, different from the mentioned carbon materials, graphene oxide (GO) nanoplatelets have created a new field of carbon-filled nanocomposites in recent years. GO is an oxidation state of graphene, grafted with various oxygen functional groups (i.e. epoxide, hydroxyl, and carboxyl groups) on its basal planes and edges. It can be produced via a chemical reduction of exfoliated graphite oxide [6]. This nanomaterial with strong hydrophilicity and good compatibility with polymers has been widely used to fabricate polymer-based nanocomposites because of their extraordinary structure [7].
* Corresponding author. Tel.: þ886 5 534 2601x4613; fax: þ886 5 531 2071. E-mail address: [email protected] (C.W. Lin). 0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2012.04.022
As a new type of functional composite, the combination of polyaniline (PANI) with GO has been widely studied to produce materials with desired electrochemical properties in recent years. Many researchers have reported that the combination of GO and PANI can promote the composite in higher electrochemical capacitance and charge-discharge cyclic stability than each individual component [8e12]. Besides, it is also found that this type of composite has higher electrical conductivity than each of the pristine PANI and GO [13,14]. Although the combination shows some advantages, most of the reported GO/PANI composites are in a two-dimensional shape with a lateral size of several micrometers [8e14]. On the other hand, the nanostructured composites exhibit excellent mechanical, electrical and optical properties endowed by confining dimension of such materials and combining bulk and surface properties to the overall behavior. The potential of materials with reduced dimensions becomes fascinating in a variety of possible applications. It has been pointed out that the morphology plays an important role in electrochemical properties of electroactive materials, such as electrical conductivity, electrochemical active surface, and interfacial charge transfer resistance [3,15]. For examples, polypyrrole nanofibers show a significantly higher effective surface area than that of polypyrrole spheres [15]. On the other hand, carbon
Y.F. Huang, C.W. Lin / Polymer 53 (2012) 2574e2582
nanotubes possess a much higher electrical conductivity and electrochemical surface than that of carbon blacks and carbon nanofibers [3]. Moreover, the shape of nanomaterials also restricts their applications. The nanomaterials with one-dimensional morphology can serve as functional compartment and interconnector in various electrochemical nanodevices [16]. In contrast, the nanomaterials with a spherical and hollow structure are considered useful for gas sensing and gas separation [17]. To promote electrochemical properties and to diversify applications of GO/PANI composites, this study intends to synthesize a series of GO/PANI composites with controllable morphologies. Several polymerization parameters, including solution acidity, aniline concentration, and reaction temperature, were used to control the final morphology of GO/PANI products. Three distinctive shapes, including nanotube with different outside diameters, aligned nanofiber array, and nanosphere, were successfully fabricated in this study. The intermediates obtained at different reaction stages were also characterized to investigate the morphological formation mechanisms. Moreover, this study also investigated the electrochemical surface area of the GO/PANI nanocomposites. To the author’s best knowledge, this is the first study reports the strategy to control the shapes of GO/PANI nanocomposites and demonstrates their shape-dependent electrochemical properties. 2. Experimental 2.1. Materials Aniline monomers (S99.5%) were purchased from Fluka. Ammonium persulfate (APS, 98%) as oxidant was obtained from Riedel-de Haën. Graphite powders (crystalline, 300 mesh, 99%) was obtained from Alfa- Aesar. Potassium permanganate (S99%) was obtained from J.T. Baker. Sodium nitrate (S99%) was purchased from SigmaeAldrich. All reagents were used as received without further purification.
2575
lyophilization to avoid the aggregation of graphene oxide during the drying process [20]. 2.3. Synthesis of GO/PANI nanocomposites The GO/PANI composites were produced using an in-situ polymerization process of aniline monomers in the presence of graphene oxide. Firstly, GO was dispersed in an aqueous solution and sonicated for at least 1 h to exfoliate the graphite oxide into graphene oxide [21]. A suitable amount of aniline monomers was then added into the solution and the solution was sonicated for at least 30 min to produce a stable mixture of graphene oxide and aniline monomers. The preparation of nanotubes typically involved mixing an aqueous solution of aniline in low acidic solutions (&0.05 M HCl) and oxidant (ammonium peroxydisulfate) solution in the same acidic solution. The solutions were then poured rapidly into a glass vial and shaken vigorously for 30 s, and then put in an ultrasonic water bath at 0 C. The weight ratio of GO to aniline in this solution is 3:7. The concentration of aniline varied from 0.1 to 0.06 M to control the diameter of resulting nanotubes. The reaction time for the synthesis of nanotubes at 0.1 and 0.06 M aniline concentration is 90 min and 5 h, respectively. The nanospheres were synthesized with the same procedure using 0.1 M aniline monomers and 0.2 M HCl solutions. The nanofiber arrays were synthesized at 50 C in 1 M H2SO4 solutions. In this case, the aniline concentration and the weight ratio of GO to aniline is 0.03 M and 5:5, respectively. The reaction time for forming nanospheres and nanofiber arrays is 90 min. A molar ratio of [ANI]/[APS] equal to 0.8:1 was applied to all reactions. The resulting product was isolated by centrifugation and washed with de-ionized water for several times. Finally, the composite product was dried in an oven at 50 C for 1 day. The polymerization conditions and the corresponding morphologies of the GO/PANI composites are summarized in Table 1 for better comparison.
2.2. Preparation of graphene oxide
2.4. Characterization of GO and GO/PANI nanocomposites
Graphene oxide was synthesized from the natural graphite powders using a modified Hummers and Offenman’s method [18,19]. In brief, 1 g of graphite, 1 g of NaNO3, and 46 mL of concentrated H2SO4 were mixed together and stirred in an ice bath for around 30 min. Then, 3 g of KMnO4 was added slowly into the solution followed by stirring at 35 C for 1 day to form thickened paste. Afterward 46 mL of de-ionized water was added slowly into the reaction solutions to avoid the reaction temperature rising to a limit of 98 C. This solution was then kept stirring for 30 min. Finally, 140 mL of de-ionized water and 10 mL of H2O2 (30%) was poured into the mixture in sequence. This solution was then filtered by gravity filtration and the filter cake was washed with de-ionized water and 3% HCl solution alternatively. This filtered cake was then dispersed and washed with de-ionized water for several times by repeated centrifugation. The resulting samples were then dried by
The X-ray diffraction (XRD) was carried out on an X-ray diffractometer (MiniFlex II, Rigaku, USA) using Cu Ka radiation (g ¼ 0.15406 nm). The diffraction patterns obtained in a scan range of 5 e100 were collected with a scan rate of 2 min1. Thermogravimetric analysis (TGA) curves were performed on a TGA Instrument (TGA 2050, TA Instrument, USA) at a heating rate of 15 C min1 in nitrogen environment. Morphologies of the GO/PANI composites were obtained by a transmission electron microscopy (JEM-2100, JEOL, Japan) and a field emission scanning electron microscopy (JSM-7401F, JEOL, Japan). Infrared spectra of the GO/ PANI composites in pellet made with potassium bromide (KBr) were performed on an FTeIR spectrophotometer (Spectrum One, Perkin Elmer, USA) with a resolution of 4 cm1 and 32 scans. UVevis spectrophotometer (Lambda 850, Perkin Elmer, USA) was used to identify the GO/PANI dispersion in de-ionized water.
Table 1 Polymerization conditions and corresponding morphologies of GO/PANI nanocomposites. Morphology
Aniline concentration
Reaction temperature
Solution
Weight ratio of aniline to GO
Reaction time
Nanotubes (250 nm in O.D.) Nanotubes (45 nm in O.D.) Nanofiber arrays Nanospheres
0.1 M
0 C
0.05 M HCl
7:3
90 min
0.06 M
0 C
0.01 M HCl
7:3
5h
0.1 M 0.1 M
50 C 0 C
0.2 M HCl 1 M H2SO4
5:5 7:3
90 min 90 min
2576
Y.F. Huang, C.W. Lin / Polymer 53 (2012) 2574e2582
The cyclic voltammetry (CV) curves of the GO/PANI composites were conducted with a three-electrode setup. The working electrode was prepared as follows. The GO/PANI products were firstly dispersed in isopropanol and then a suitable amount of NafionÒ solution was added to the dispersion. The content of Nafion in the mixture is 10 wt% and the amount of GO/PANI in the electrode is 1 mg cm2. This ink was then spread on a carbon cloth and dried at 60 C. Ag/AgCl and platinum wire was used as a reference and counter electrode, respectively. The cyclic voltammetry curves were obtained in a 0.5 M H2SO4 solution with a scan rate of 50 mV s1. The scan range was from 0.2 to 1.0 V versus Ag/AgCl. 3. Results and discussion 3.1. Morphology characterization of GO/PANI nanocomposites Fig. 1 presents the SEM images of the GO/PANI samples obtained from different polymerization conditions. TEM images are additionally provided as inset in Figure (a) and (b) for better distinction. As can be seen, several polymerization parameters affect greatly the resulted morphologies of the products. As polymerization was performed in low acidic solution (i.e. & 0.05 M HCl) and low temperature (i.e. 0 C), a shape of nanotube with an outer diameter of 250 nm and an inner diameter of 150 nm could be obtained, as shown in Fig. 1(a). However, with the decrease of aniline concentration from 0.1 to 0.06 M, the outer diameter of the nanotubes decreases significantly to only 45 nm, as can be seen in Fig. 1(b). Obviously, aniline concentration is a critical factor determining the diameter of the nanotubes. Apart from aniline concentration, solution acidity was also found to be an important factor leading to a morphological change of the composite. With the increase of solution acidity from 0.05 M to 0.2 M, the polymerization reaction led to a shape of nanosphere with a diameter of around 100 nm, as shown in Fig. 1(c).
Interestingly, the nanospheres are connected with a bound of nanowires. The inset image in Fig. 1(c) reveals that the diameter of the nanowires is less than 10 nm, significantly thinner than those reported in the literature (i.e. 30e100 nm) [22,23]. Now, we turn our attention to the effect of reaction temperature. It is noted that the morphology changes again with the increase of reaction temperature from 0 C to 50 C. The formation of aligned nanofiber arrays with a diameter of only 30 nm can be observed in Fig. 1(d) from the reaction solution consisting of 1 M H2SO4 and 0.03 M aniline monomers. Based on above, a temporary conclusion can be drawn as that the final GO/PANI morphology is changeable through some specific polymerization parameters including solution acidity, aniline concentration, and reaction temperature. In order to further verify the coexistence of GO and PANI, derivative thermogravimetric analysis (DTA) and XRD techniques were used to identify the hybrid nature of these obtained products. Fig. 2 shows the DTA curves of the pristine GO and PANI (as an insert in Figure), as well as the GO/PANI nanostructures with various shapes. As can be seen, all samples exhibit a mass loss peak below 100 C, associated to the loss of absorbed water in the samples. Along with temperature rising, another two mass loss peaks at around 200 C and 500 Ce700 C can be observed. These peaks can be ascribed respectively to the decomposition of oxygencontaining groups on GO sheets [24] and the decomposition of the polyaniline backbone [25]. These results confirm the coexistence of GO and polyaniline in the products. This verification can be reinforced with the observation from the XRD patterns presented in Fig. 3(a), in which the composite samples exhibit two characteristic peaks at 8e11 and 25 . The former one is corresponding to the grafts of various functional groups (i.e. epoxide and carboxyl) on the GO sheets [26], and the later one is associated to the amorphous structure of PANI [27]. However, by a further comparison of the XRD patterns of the GO/PANI nanocomposites, it is noted that that the peak located at
Fig. 1. SEM images of the GO/PANI nanostructures: (a) nanotubes obtained from 0.1 M aniline solution, (b) nanotubes obtained from 0.06 M aniline solution, (c) nanospheres, and (d) aligned nanofiber arrays. The reaction temperature is 0 C for both nanotubes and nanospheres while is 50 C for nanofiber arrays. The insets appeared in (a) and (b) are TEM images of the nanotubes.
Y.F. Huang, C.W. Lin / Polymer 53 (2012) 2574e2582
2577
Fig. 2. DTA curves of GO, PANI, and GO/PANI nanocomposites.
8e11 shifts along with the change of the product shapes. The pristine GO sample shown in Fig. 3(b) exhibits a peak at 10.9 which can be assigned to an interlayer distance of 0.81 nm between GO sheets. However, with the formation of GO/PANI nanotubes, the diffraction peak gradually shifts to a smaller angle close to 8.7,
Fig. 4. (a) FTeIR spectra and (b) XRD patterns of the various intermediates obtained at a reaction time of 30 min.
indicating an increase of GO interlayer distance up to 1.02 nm. It can be ascribed to the intercalation of PANI chains between GO sheets [26]. On the other hand, with the formation of the aligned nanofiber arrays, the corresponding peak remains at 10.9 , suggesting the nanofiber arrays may primarily grow on the GO surfaces rather than intercalating between GO sheets. Moreover, it is worthy to note that this peak become almost invisible with the formation of GO/PANI nanospheres, reflecting a full exfoliation of the GO sheets in this product [28]. In this condition, the as-synthesized nanosphere may serve as a spacer to prevent the re-stacking of GO sheets during the reaction process [29]. According to above results, it can be summarized that the parameter modulation of aniline monomers polymerized in presence of GO sheets affects not only the final morphology but also the layer structure of GO sheets within the GO/PANI nanocomposites. Succeeded in controlling GO/PANI morphology, the investigation was turned to explore the formation mechanism of the GO/PANI morphology through characterizing the intermediates obtained in different reaction stages. 3.2. Exploration of the formation mechanisms of the GO/PANI nanocomposites
Fig. 3. XRD patterns of the pristine GO, PANI, and GO/PANI nanocomposites.
3.2.1. Characteristics of the GO/PANI intermediates formed in the initial reaction stage Fig. 4 shows the FTeIR spectra of the initially-formed GO/PANI samples obtained from different reaction solutions. As can be seen, the absorbance bands of the GO/PANI intermediates obtained at a reaction time of 30 min changed with the variation of the reaction parameters. In a condition to form nanotubes (&0.05 M HCl), phenazine-like aniline units can be obtained in the initial reaction stage, verified by the peak observed at 1414 cm1 which represents totally symmetrical stretching of the phenazine heterocyclic ring [30,31]. The band at 1622 cm1 corresponds to the absorption of the C]C ring stretching vibration in newly-formed substituted phenazine-like segments [31,32]. In an acidity condition to form nanospheres and aligned nanofiber arrays (S0.2 M HCl), the characteristic bands associated to emeraldine polyaniline units can be observed. The CeH out-of-plane bending vibration assigned to a para-substitution pattern appears at 829 cm1, and the N]Q]N absorption peak (with Q representing the quinoid ring) at 1141 cm1 are also visible [27].
2578
Y.F. Huang, C.W. Lin / Polymer 53 (2012) 2574e2582
These different molecular structures of the initially-formed intermediates are ascribed to be resulted from different polymerization routes with different reaction parameters. For the formation of nanotubes at low acidity, the oxidation of aniline molecules produces phenazine-like units through an ortho-coupling route [31e34]. However, when producing nanospheres and nanofiber arrays at high acidity, the aniline molecules might firstly convert to anilinium cations and then polymerized through a para-coupling route to form emeraldine polyaniline chains [33,34]. 3.2.2. The interaction between aniline monomers and GO sheets Now, we turn our attention to the interaction between aniline monomers and GO sheets. Researchers have reported that the attractive electrostatic interactions formed between the negatively charged surface of oxidized carbon black and anilinium cations can promote adsorption of anilinium cations on oxidized carbon black [35]. Therefore, in the cases of forming nanospheres and nanofiber arrays (i.e. in higher acidity), the anilinium cations formed initially would adsorb well on the GO surfaces and the subsequent in-situ growth of polyaniline chains would block the re-stacking of the GO sheets [36]. On the other hand, in the case of forming nanotubes (i.e. in lower acidity), the reduction of p electron density on the graphene after the oxidation process would weaken the adsorption capacity of graphene oxide to neutral aniline monomers [35]. In this condition, the oxidation of neural aniline monomers may firstly produce the self-standing phenazine-like aniline units in the reaction solution and then the p-p electron interaction formed between the phenazine units and the GO sheets may promote the re-stacking of GO sheets. Consequently, a phenazine unitsintercalated GO structure can be established. The pep stacking interactions can be formed between two relatively nonpolar aromatic rings, such as carbon nanotubes, graphene oxide, and aromatic molecules (i.e. dopamine and phenazine units) [37,38]. Zhang et al. [39], have demonstrated the stacking of new layered structure existing in hybrid through the intermolecular pep
interactions formed between two different kinds of moieties, such as phthalocyanine derivatives and graphene oxide. Very recently, it was discovered that key nanosheet structural changes, occurring within the first minutes of aniline polymerization, affect the self-assembly course of the subsequent PANI nanostructure [40]. In order to further verify the effect of the reaction parameters on the structure of the GO/PANI nanocomposites, XRD was utilized to identify the ordered structure of these initially-formed GO/PANI complexes and the patterns are shown in Fig. 4(b). Although conventional analysis techniques struggle at this early stage due to the reason that they require relatively large amounts of sample for analysis. However, the variation of the detectable GO layered structures still can provide helpful evidence for supporting our inference. Regarding the nanotubes, the diffraction peak can be observed to shift gradually to a smaller angle at 8.7, suggesting the intercalation of PANI between GO sheets [27]. However, in the cases of preparing nanospheres and nanofiber arrays, the intensity of the peak at w10 decreases significantly at this reaction stage due to the disordered layer structures of the GO sheets [36]. Accordingly, it can be summarized that different polymerization routes of aniline monomers would result in different structures of the GO/PANI complexes formed at the initial reaction stage. Apart from the variation of the molecular structure of PANI and the ordered structure of GO, it can be noted that the initiallyformed intermediates also exhibit different morphologies. Fig. 5 (a) and (b) show the deposition of tree-like objects on the GO surfaces at the initial reaction stage to form nanotubes. On the contrary, corresponding to the formations of nanospheres and aligned nanofiber arrays, Fig. 5 (c) and (d) present the growth of rod-like protuberances on the GO surfaces at the initial reaction stage. These observations reveal that a large number of PANI nuclei are initiated on the GO surfaces at the early stage and the functional groups on GO sheet are thought to serve as anchoring sites for the subsequent in-situ polymerization of aniline monomers [12].
Fig. 5. SEM images of the various intermediates obtained at a reaction time of 30 min from the reaction condition to form (a) nanotube with a diameter of 250 nm, (b) nanotube with a diameter of 45 nm, (c) nanospheres, and (d) aligned nanofiber arrays.
Y.F. Huang, C.W. Lin / Polymer 53 (2012) 2574e2582
Based on above results, it can be inferred that different reaction conditions will lead to different morphologies of the initiallyformed GO/PANI intermediates. Obviously, these variations are responsible for the final shape of the GO/PANI composite developed in the succeeding reaction stages. To fully explore the formation mechanisms of GO/PANI products with various shapes, a further characterization for the intermediates obtained at extended reaction time is essential. 3.2.3. The formation mechanism of GO/PANI nanotubes Fig. 6(a) compares the FTeIR spectra of the GO/PANI intermediates obtained from different reaction stages under the polymerization conditions to form nanotubes. As can be seen, samples obtained in the early stage exhibit the characteristic peaks corresponding to the formation of phenazine-like aniline units at 1414 and 1622 cm1. However, as the reaction time advances to 60 min, the band at 1414 cm1 assigned to the totally symmetrical stretching of the phenazine ring disappears. Contrarily, the CeH out-of-plane bending vibrations band from the N]Q]N absorption peak at 1142 cm1 becomes apparent due to the reason that the head-to-tail couplings of aniline monomers are newly formed since the reaction time advances longer than 30 min.
Fig. 6. (a) FTeIR spectra and (b) UVevis spectra of the intermediates obtained at different reaction stages under the reaction conditions to form nanotubes with a diameter of 250 nm.
2579
It is inferred that the increase of solution acidity during the polymerization process may be responsible for the molecular structural changes of the products. Several studies reported the hydrogen atoms abstracted during the oxidation of amino groups and benzene rings as protons increased the solution acidity during the polymerization process [41e43]. The increased solution acidity will consequently cause the change of the polymerization route of aniline from ortho-coupling to para-coupling and thus leads to the variation of PANI molecular structure during the polymerization process [31e34]. UVevis analysis was also conducted to further analyze the GO/ PANI intermediates obtained at different reaction stages and the spectra are presented in Fig. 6(b), in which the intermediates exhibit an absorption band at about 430 nm. This band is due to the formation of phenazine-like structures in this stage [44]. However, as the reaction prolongs, the characteristic absorption bands at wavelengths of 320e360 nm, 420e440 nm and 800e900 nm become visible at a reaction time of 60 min. These bands are assigned to the formation of emeraldine salt polyaniline (ES-PANI). The first absorption band corresponds to the pep* electron transition within the benzenoid segments. The second and the third bands correspond to the acid-doped state and the polaron formation in polyaniline, respectively [23]. Besides, the absorbance band at 420e440 nm, assigned to the acid-doped state, becomes apparent after the reaction time of 60 min. This result verifies the increase of doping level of PANI as the reaction prolonged. Fig. 7 presents the TEM image of the intermediates obtained at the reaction time of 60 min. It clearly demonstrates the transformation of the GO/PANI composite through a semi-curling process and subsequently the formation of the nanotubes appeared at the reaction time of 90 min, as shown in Fig. 1(a). The morphological transition of the product into nanotubes may be ascribed to the changes of molecular structure and doping state of the intercalated polyaniline molecules during the polymerization process. In this condition, the phenazine-like units with a nonlinear structure formed at initial reaction stage may serve as the axis of curling into nanotubes [45e47]. Besides, the conformation change of the linear emeraldine aniline units after the protonation process may provide the driving force for the initially-formed GO/ PANI sheets to curl into nanotubes [45e47]. A gradual change of polyaniline conformation from an expanded coil to a compact coil
Fig. 7. TEM image of the intermediates obtained at the reaction time of 60 min from the reaction solution to form nanotubes with a diameter of 250 nm.
2580
Y.F. Huang, C.W. Lin / Polymer 53 (2012) 2574e2582
after the protonation process is probably due to the combination of the negative ions to the imine nitrogen atoms, which releases the repulsive forces between polyaniline chains [48,49]. Accordingly, it is believed that the synergic effect of the changes of both molecular structure and doping state of the intercalated PANI molecules make GO/PANI complexes transit to nanotube morphology. 3.2.4. The formation mechanisms of GO/PANI nanospheres and aligned nanofiber array Now, we turn our attention to the formation mechanisms of the GO/PANI nanospheres and the GO/PANI aligned nanofiber array. Fig. 8 is the FTeIR spectra of the intermediates obtained at different reaction stages under the conditions of forming nanospheres and aligned nanofiber array. The characteristic peaks related to the formation of linear emeraldine polyaniline chains at 829 and 1141 cm1 appear during the entire polymerization process. In this condition, the intrinsic tendency of the initially-formed linear emeraldine polyaniline chains may lead to the growth of rod-like objects on the GO surfaces, as shown in Fig. 5(c) and (d). It has been reported that the polymerization of aniline monomers in high acidic environment can intrinsically form nanofibers [23,50e52]. The linear nature of the emeraldine polyaniline chains formed at the initial reaction stage can thus serve as a template to form nanofibers [51]. These arguments are satisfactorily consistent with our observations in the SEM images of the growth of the rod-like
objects on the GO surfaces at the initial reaction stage (Fig. 5(c)e(d)) and in the FTeIR spectra (Fig. 8). It is therefore inferred that the linear aniline units formed at the initial reaction stage can serve as the template to grow the rod-like protuberances on the GO surfaces. As the polymerization reaction prolonged, the initially-formed rod-like protuberances may grow continuously through different routes. At lower reaction temperature, the initially-formed rod-like protuberances turn their morphology into nanospheres (Fig. 1(c)). On the other hand, if polymerization proceeds at higher temperature and lower aniline concentration, the initially-formed protuberances continue to grow into nanofibers on GO surfaces and thus result in the formation of the aligned nanofiber arrays (Fig. 1(d). There are three possible routes existing in the polymerization process following the formation of the primary PANI nanofibers, including (i) the continuing formation of the primary PANI nanofibers, (ii) growing the primary nanofibers into thicker fibers with uneven surfaces, and (iii) the growth and aggregation of the thicker fibers leading to irregular particles [52]. The polymerization follows the routes (ii) and (iii) in conventional procedures all produce irregular particles. Polymerization rate is a critical factor to inhibit the development of the second and the third routes for PANI nanofiber growth [52,53]. Moreover, to lower aniline concentration can also efficiently inhibit the secondary growth of the primary nanofibers [23,54]. It is therefore inferred that the reaction carried out under high reaction temperature and low aniline concentration may efficiently depress the secondary growth of the initiallyformed rod-like objects on GO surfaces and consequently lead to the formation of aligned nanofiber array. At this moment, we summarize the growth mechanisms of GO/ PANI nanotubes, nanospheres and aligned nanofiber arrays as follows. (i) At low solution acidity, the oxidation of aniline monomers produced intercalated polyaniline chains with a head of phenazine-like aniline unit and a tail of para-linked aniline unit. The phenazine-like unit can serve as the axis and the protonation of emeraldine aniline unit can provide the driving force for the formation of a nanotube through a self-curling process. (ii) At high solution acidity, aniline monomers formed linear polyaniline chains at the initial reaction stage and these linear chains may serve as template for the growth of rod-like objects on GO surfaces. As the reaction extends, these rod-like objects may continue to grow and turn into nanospheres if low reaction temperature is applied. On the other hand, if high reaction temperature and low aniline concentration are applied, the secondary growth of the rod-like objects can be effectively inhibited and aligned nanofiber array will be formed consequently. A comprehensive scheme illustrating polymerization condition and corresponding morphology of GO/ PANI composite is presented in Fig. 9. 3.3. The shape-dependent electrochemical properties of the GO/ PANI nanocomposites
Fig. 8. FTeIR spectra of the intermediates obtained from the reaction solution to form (a) nanospheres, (b) aligned nanofiber arrays.
Succeeded in morphology control and mechanism exploration, our investigation was extended to the electrochemical properties of these three types of GO/PANI nanostructure. Several studies reported that the combination of polyaniline and GO enabled the composites to exhibit higher electrochemical properties than each individual component [8e12]. However, most of these studies did not pay attention to the relationship between the composite morphology and the measured electrochemical properties. This study therefore tries to build up the shape-dependent electrochemical properties of the GO/PANI nanocomposites. Fig. 10 compares the CV curves of the GO/PANI nanocomposites with various shapes. As can be seen, the GO/PANI nanotubes with a diameter of 250 nm exhibit a peak current density of 2.9 A g1,
Y.F. Huang, C.W. Lin / Polymer 53 (2012) 2574e2582
2581
Fig. 9. Scheme of the formation of GO/PANI nanostructures with various shapes through an in-situ polymerization process.
quite close to that obtained from the nanofiber arrays (ca. 2.5 A g1). However, as the nanotube diameter largely reduces to 45 nm, the peak current density of the GO/PANI nanotubes increases significantly to 4.0 A g1. The higher current density stands for a higher electrochemical active surface of the electroactive materials [15]. This result suggests that the decrease of nanotube diameter is advantageous in electrochemical active surface. It is worthy to note that the nanospheres exhibit a highest current density (6.9 A g1) among the investigated nanocomposites. This excellent characteristic in terms of electrochemical active surface is ascribed to the fully exfoliation state of the GO sheets in the nanospheres as has been well identified in previous section (i.e. Fig. 2(b)). The fully exfoliated structure of GO sheets is widely reported to possess higher specific surface area [55]. This
preliminary result directs a future work to explore possible applications of GO/PANI composites with specific morphologies in various electrochemical devices. 4. Conclusions This study demonstrates an efficient method to synthesize a series of GO/PANI nanocomposites with various shapes through in-situ polymerization of aniline monomers in the presence of GO sheets. The selected reaction parameters determine the polymerization routes and thus result in different morphologies of GO/PANI nanocomposites. At low solution acidity, the synergistic effect of structural change of polyaniline and doping state during the polymerization process leads to the formation of nanotubes through a self-curling process, driven by the intercalated polyaniline in the GO sheets. As the solution acidity increases, the in-situ polymerization produces linear polyaniline chains at the initial reaction stage, which serve as templates for the growth of the rod-like objects on GO surfaces. If the reaction proceeds in a low temperature condition, the initially-formed rod-like objects may consequently grow into nanospheres. However, if the secondary growth of these rod-like objects can be efficiently inhibited by carrying out the reaction under low aniline concentration and high reaction temperature, then GO/PANI with aligned nanofiber arrays can be obtained. Among the various investigated nanocomposites, the nanospheres exhibit a highest electrochemical active surface. This work provides a facile and effective method to control GO/PANI nanocomposites with desired shapes for the possible applications in various electrochemical devices. Acknowledgement
Fig. 10. Cyclic voltammogram curves of the GO/PANI nanocomposites measured in 0.5 M H2SO4 solutions.
The authors are grateful to the National Science Council of Taiwan (ROC) for the financial support of this work under grant no. NSC 99-2221-E-224-069-MY2.
2582
Y.F. Huang, C.W. Lin / Polymer 53 (2012) 2574e2582
References [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]
Lu X, Zhang W, Wang C, Wen T, Wei Y. Prog Polym Sci 2011;36(5):671e712. Hatchett DW, Josowicz M. Chem Rev 2008;108(2):746e69. Al-Saleh MH, Sundararaj U. Composites Part A 2011;42(2):2126e42. Wu KH, Ting TH, Wang GP, Ho WD, Shih CC. Polym Degrad Stab 2008;93(2): 483e8. Deng J, Ding X, Zhang W, Peng Y, Wang J, Long X, et al. Eur Polym J 2002; 38(12):2497e501. Dreyer DR, Park S, Bielawski CW, Ruoff RS. Chem Soc Rev 2010;39:228e40. Stankovich S, Dikin D, Piner RD, Kohlhaas KA, Kleinhammes A, Jia Y, et al. Carbon 2007;45(7):1558e65. Xu LQ, Liu YL, Neoh K, Kang E, Fu GD. Macromol Rapid Commun 2011;32(8): 684e8. Wang D, Li F, Zhao J, Ren W, Chen Z, Tan J, et al. ACS Nano 2009;3(7):1745e52. Wang H, Hao Q, Yang X, Lu L, Wang X. Electrochem Commun 2009;11(6): 1158e61. Murugan AV, Muraliganth T, Manthiram A. Chem Mater 2009;21(21):5004e6. Xu J, Wang K, Zu S, Han B, Wei Z. ACS Nano 2010;4(9):5019e26. Gao Y, Yip H, Chen K, O’Malley KM, Acton O, Sun Y, et al. Adv Mater 2011; 23(16):1903e8. Yang N, Zhai J, Wan M, Wang D, Jiang L. Synth Met 2010;160(15e16): 1617e22. Xu P, Han X, Zhang B, Mack NH, Jeon S, Wang HL. Polymer 2009;50(12): 2624e9. Fu G, Li G, Neoh KG, Kang ET. Prog Polym Sci 2011;36(1):127e67. Zhang J, Wang S, Wang Y, Xu M, Xia H, Zhang S, et al. Sensor Actuat B-Chem 2009;139(2):411e7. Hummers WS, Offeman Jr RE. J Am Chem Soc 1958;80(6). 1339e1339. Becerril HA, Mao J, Liu Z, Stoltenberg RM, Bao Z, Chen Y. ACS Nano 2008;2(3): 463e70. Xin Y, Liu J, Zhou Y, Liu W, Gao J, Xie Y, et al. J Power Sources 2011;196(3): 1012e8. Li D, Muller MB, Gilje S, Kaner RB, Wallace GG. Nat Nanotechnol 2008;3:101e5. Huang J, Kaner RB. J Am Chem Soc 2004;126(3):851e5. Chiou NR, Epstein AJ. Adv Mater 2005;17(13):1679e83. Park S, An J, Jung I, Piner RD, An SJ, Li X, et al. Nano Lett 2009;9(4):1593e7. Abell L, Pomfret SJ, Adams PN, Monkman AP. Synth Met 1997;84(1e3): 127e32. Zhang K, Zhang LL, Zhao XS, Wu J. Chem Mater 2010;22(4):1392e401.
[27] Zhang Z, Wei Z, Wan M. Macromolecules 2002;35(15):5937e42. [28] Bissessur R, Liu PKY, White W, Scully SF. Langmuir 2006;22(4):1729e34. [29] Park S, Shao Y, Wan H, Rieke PC, Viswanathan VV, Towne SA, et al. Electrochem Commun 2011;13(3):258e61. e nková I, Trchová M, Sapurina I, Cieslar M, [30] Konyushenko EN, Stejskal J, Sed et al. Polym Int 2006;55(1):31e9. e nková I, Konyushenko EN, Stejskal J, Holler P, [31] Trchová M, Sed Ciri ceMarjanovi c G. J Phys Chem B 2006;110(19):9461e8. [32] Stejskal J, Sapurina I, Trchová M, Konyushenko EN, Holler P. Polymer 2006; 47(25):8253e62. [33] Zujovic ZD, Laslau C, Bowmaker GA, Kilmartin PA, Webber AL, Brown SP, et al. Macromolecules 2010;43(2):662e70. [34] Ciri ceMarjanovi c G, Trchová M, Stejskal J. Collect Czech Chem Commun 2006; 71(10):1407. [35] Radovic LR, Silva IF, Ume JI, Menéndez JA, Leon-Leon CA, Scaroni AW. Carbon 1997;35(9):1339e48. [36] Chen G, Shau S, Juang T, Lee R, Chen C, Suen S, et al. Langmuir 2011;27(23): 14563e9. [37] Laslau C, Zujovic Z, Travas-Sejdic J. Prog Polym Sci 2010;35(12):1403e9. [38] Liu J, Tao L, Yang W, Li D, Boyer C, Wuhrer R, et al. Langmuir 2010;26(12): 10068e75. [39] Zhang X, Feng Y, Tang S, Feng W. Carbon 2010;48(1):211e6. [40] Zujovic ZD, Laslau C, Bowmaker GA, Kilmartin PA, Webber AL, Brown SP, Travas-Sejdic J. Macromolecules 2010;43(2):662e70. [41] Fu Y, Elsenbaumer RL. Chem Mater 1994;6(5):671e7. [42] Gospodinova N, Mokreva P, Terlemezyan L. Polymer 1993;34(11):2438e9. [43] Neoh KG, Kang ET. Polymer 1993;34(18):3921e8. [44] Salma B, Rudolf H. Electrochim Acta 2007;52(17):5346e56. [45] Huang YF, Lin CW. Polym Int 2010;59(9):1226e32. [46] Huang YF, Lin CW. Synth Met 2009;159(17e18):1824e30. [47] Huang YF, Lin CW. Polymer 2009;50(3):775e82. [48] MacDiarmid AG, Epstein AJ. Synth Met 1995;69(1e3):85e92. [49] Avlyanov JK, Min Y, MacDiarmid AG, Epstein AJ. Synth Met 1995;72(1):65. [50] Huang J, Kaner RB. Angew Chem Int Ed 2004;43(43):5817e21. [51] Li W, Zhu M, Zhang Q, Chen D. Appl Phys Lett 2006;89:103110e2. [52] Jing XL, Wang YY, Wu D, She L, Guo Y. J Polym Sci Part A Polym Chem 2006; 44(2):1014e9. [53] Li D, Kaner RB. J Mater Chem 2007;17:2279e82. [54] Chiou N, Epstein AJ. Synth Met 2005;153(1e3):69e72. [55] Frelink T, Visscher W, Veen J. J Electroanal Chem 1995;382(1e2):65e72.