Fabrication of Various Conducting Polymers Using Graphene Oxide

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Chemistry of Materials

Fabrication of Various Conducting Polymers Using Graphene Oxide as a Chemical Oxidant Minkyu Kim, Choonghyeon Lee, Young Deok Seo, Sunghun Cho, Jihoo Kim, and Jyongsik Jang*

School of Chemical and Biological Engineering, College of Engineering, Seoul National University (SNU), 599 Gwanangno, Gwanak-gu, Seoul, 151-742 (Korea)

RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to)

* Tel: +82-2-880-7069; Fax: +82-2-888-7295; e-mail: [email protected]

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ABSTRACT Recently, using graphene oxide (GO) for “carbocatalysis” has great attraction as a novel application of graphene-based nanomaterials and expected to opens a host of possibilities for chemical synthesis because of the abundance of natural carbon sources, as well as the low density, extensive chemical functionalization, hydrophilicity, low cost, and ease of preparation. Here, we demonstrate that the GO can play a role as a chemical oxidant for various CPs (polythiophene, polyaniline, and polypyrrole), and diverse graphene-CP composites (graphenepolythiophene, graphene-polyaniline, and graphene-polypyrrole) can simply and rapidly be synthesized by using the GO as both graphene precursor and chemical oxidant. The UV-vis analysis confirms that the GO has successfully polymerized the CPs and been transformed to reduced graphene oxide (RGO). The SEM and TEM analyses show that the CPs have successfully been coated on the few-layered graphene sheets. Raman analysis and series of FTIR analyses have been conducted to survey what functional group in the GO polymerized the monomers, and they reveal that hydroxyl and epoxy groups in the GO polymerized the monomers. Finally, plausible polymerization mechanisms have specifically and deeply proposed based on the IR result, classical radical polymerization mechanisms of the CPs, and widely adopted thermal reduction mechanism of the GO.

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BRIEFS Various conducting polymers (polythiophene, polyaniline, and polypyrrole) have successfully been polymerized by using graphene oxide as a chemical oxidant, and plausible polymerization mechanisms have specifically and deeply proposed.

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The conducting polymers (CPs), prepared by typical oxidative polymerization method,1-3 has attracted great attention in vast fields of energy storage/conversion,4-8 electronic,9-11 and biological

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applications due to their fascinating properties such as unique redox behavior,

tunable electrical conductivity, inherent biocompatibility, and high flexibility.4-16 Despite a large number of advantages, however, inferior properties of CPs (e.g., low electrochemical stability, weak mechanical strength, etc.) often hinder their practical usage.12,17,18 The need to overcome this limitation has led to the growing interest in hybridization of CPs with other additives.12,17,18 For example, many kinds of nanostructured carbon materials (e.g., carbon nanotube, mesoporous carbon, etc.) have been incorporated with CPs to produce composites with desirable properties (e.g., enhanced thermal stability, increased electrochemical stability, or improved mechanical strength).17-22 In particular, of the various nanostructured carbon additives, graphene has recently drawn significant attention because of its exceptional electrical, thermal, electrochemical, and mechanical properties.17,18,23 With the burgeoning interest in the graphene additives, indeed, combination of graphene and CP has extensively been studied and the produced graphene-CP composites have shown the superior properties compared with other carbon-CP composites through the remarkable properties of graphene and synergistic effects between CP and graphene.24-28 Majorly, the graphene-CP composites have been fabricated by following two methods; (i) mixing of preformed CP with reduced graphene oxide (RGO)17,29,30 and (ii) in situ oxidative polymerization of monomer in the presence of RGO.24-28 Between the two approaches, especially, the (ii) in situ polymerization method have more widely been utilized than (i) mixing method because the composites produced by in situ polymerization method exhibited much higher surface area and electrical conductivity than composites synthesized by mixing

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method.17,30,31-36 However, when the graphene-CP composites synthesized by in situ polymerization method are used as active materials in practical applications, they often show limited performances due to the agglomeration of graphene sheets.27,33,37 For instance, Cheng and co-workers synthesized the graphene-polyaniline (PANI) composite using in situ polymerization method and obtained specific capacitance as high as 233 F g-1 by applying it as electrode material for supercapacitor.27,33,37 However, the capacitance was mostly ascribed to the pseudo capacitance from the PANI, and electric double layer capacitance from the graphene was not highly utilized because of the agglomerated morphology of the graphene sheets.27,33,37 To polymerize the CP on single layer or few layers graphene with circumventing agglomeration of graphene sheets, significant research efforts have been made using diverse approaches. However, despite such extensive efforts, it still remains a great challenge to polymerize the CP on single layer or few layers graphene in an effective, easier, and straightforward manner. For example, many researchers have ultrasonicated the RGO solution before the in situ polymerization step to decrease the restacking of RGO.38,39 Although there has been obvious decrease in restacking of RGO sheets, the considerable amount of agglomerated RGO was still observed in the composites even after this pre-ultrasonic treatment.38,39 Meanwhile, non-covalently functionalized RGO28 or graphene oxide (GO)25,27 have also been utilized for producing few-layered graphene sheet-CP composite. While CPs were successfully polymerized on few layered graphene sheets by using the non-covalently functionalized RGO or GO, these approaches often need extensive and complicated synthetic steps, which hinders practical applications.25,27 Furthermore, in the case the GO is used for preparing graphene-PANI composite, essentially required post-reduction/dedoping and redoping treatments leads to

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decrease in the doping degree of the PANI and the structure change of the composite, resulting in decrease in performances of the composite.27 Recently, Bielawski and co-works demonstrated the ability of the GO to act as oxidant for various alcohols and alkenes,40 which is a relatively new application area of the GO with outstanding potential.41 In this work, the GO underwent reduction during the organic material is oxidized.40 Inspired by this result, we envisioned that aforementioned chllenge may be simply solved by using the GO as oxidative initiator for CPs. Herein, we show that the GO can play role as chemical oxidant for various CPs (polythiophene; PTh, PANI, and polypyrrole; PPy), and diverse RGO-CP composites (RGO-PTh, RGO-PANI, and RGO-PPy) can rapidly and simply be synthesized by using the GO as a chemical oxidant. The successful polymerizations of CPs by the GO were clearly confirmed by the UV-vis, SEM, and TEM analysis. Raman analysis and series of FT-IR analyses were also conducted to survey how the GO initiated the observed polymerizations. Lastly, the polymerization mechanisms were specifically and deeply proposed based on the IR result, classical radical polymerization mechanisms of the CPs,3,42-46 and widely adopted thermal reduction mechanism of the GO.47,48

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RESULTS AND DISCUSSION The synthetic procedure of RGO-CP composites (RGO-PTh, RGO-PANI, and RGO-PPy) are schematically illustrated in Figure 1a. The RGO-CP composites were simply and rapidly synthesized by using the GO as initiator. Firstly, the monomers (thiophene, aniline, and pyrrole) were added to GO aqueous solution, respectively, and then heated at 90 oC for 10 min., leading to RGO-PTh, RGO-PANI, and RGO-PPy, respectively. It should be noted that hydrochloric acid was additionally added to the GO solution in the case of producing RGO-PANI composite to increase doping level of the PANI.42,49 In the experiment, the heating temperature was carefully set considering following fact. It has been reported that thermal deoxygenation of GO initiates at temperatures larger than 70 oC, and at temperatures lower than 70 oC, dissociation of oxygen functionalities in GO become inefficient or die off.50 To maximize utilization of oxygen functional groups under relatively mild heating condition below 100 oC, we set the reaction temperature as 90 oC. Figure 1b illustrates UV-vis spectra of the GO and RGO-CP composites. The GO shows its characteristic absorption peak at 228 nm.51 However, this peak has completely red-shifted to 250 nm in the spectra of the RGO-CP composites, indicating that the GO has been transformed to RGO51 after the GO is heated with monomers, respectively. Interestingly, the GO peak has more red-shifted in spectra of the RGO-CP composites (from 228 nm to 250 nm) than in that of hydrothermally reduced GO (HRGO; prepared by heating GO solution at 90 oC for 10 min. in the absence of the monomer, Figure S1) (from 228 nm to 240 nm), indicating that the GO is more reduced when the GO solution is heated with the monomer in comparison with the case where the GO solution is solely annealed. In addition with the RGO peak, the intrinsic peaks of the CPs are also observed in the spectra of RGO-CP composites: 1) the absorption band at 447 nm in the spectrum of the RGO-PTh 7



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corresponds to π–π* electronic transition within highly π-conjugated PTh backbone;52,53 2) in the spectrum of the RGO-PANI, absorption peaks at 310, 420, and 800-1000 nm are assigned to π– π* transition of the benzenoid rings (310 nm) and polaron band transition (420 and 800-1000 nm) in emeraldine salt form of PANI (acid-doped state);54 3) the absorption band at 477 nm in the spectrum of the RGO-PPy is attributed to π–π* transition within the PPy backbone.55 These peaks reveal that the monomers have successfully been polymerized into the CPs after the monomers are thermally annealed with the GO, respectively. As control experiment, only the monomers were heated in water without the GO, respectively (other synthetic conditions were identical to those of the RGO-CP composites). As shown in the Figure 1c, no reactions occurred in the absence of the GO, indicating that the GO played role as initiator for polymerizing the PTh, PANI, and PPy.

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Figure 1. a) Facile synthetic procedure of RGO-CP composites, b) UV-vis spectra of GO, RGOPTh, RGO-PANI, and RGO-PPy, and c) UV-vis spectra of monomers (thiophene, aniline, and pyrrole) and heated monomers (H-thiophene, H-aniline, and H-pyrrole).

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Figure 2 displays FE-SEM images of the GO and RGO-CP composites. Figure 2a and 2b exhibit as-prepared GO possesses wrinkled structure and smooth surface related with intrinsic nature of the GO.56,57 The SEM images of RGO-CP composites (Figure 2c-h) clearly show that polymer particles are coated on the RGO sheet. The Figure 2c and 2d illustrate that agglomerated PTh clusters have been formed both on the RGO sheet and at the outside of the RGO, and the surface of the RGO is covered with PTh nanowhiskers. The SEM images of RGOPANI (Figure 2e and 2f) depict that PANI protuberances coat the whole surface of the RGO, and those have been much formed at the periphery than the center of the RGO. As for the RGOPPy composite (Figure 2g and 2h), uniform and entire coating of granular PPy particles is observed.

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Figure 2. SEM images of a, b) GO, c, d) RGO-PTh, e, f) RGO-PANI, and g, h) RGO-PPy.

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The morphologies of GO and RGO-CP composites were further investigated by TEM, as shown in the Figure 3a-h. Figure 3a and 3b depicts the transparent and wrinkled paper-like structure of the GO, implying that single or few layers GO has been successfully prepared.56 Such thin and two-dimensional characteristics of the GO may bring the favorable effect on polymerizing CPs by supplying a large number of polymerization sites to the monomers. In the cases of the RGO-CP composites, it is clearly observed that newly formed polymer enwraps the wrinkled single or few layers RGO sheet (Figure 3c-h).

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Figure 3. TEM image of a, b) GO, c, d) RGO-PTh, e, f) RGO-PANI, and g, h) RGO-PPy.

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Having confirmed the GO has polymerized the CPs and RGO-CP composites were successfully fabricated, we monitored the evolution of the GO’s functionalities in absence and presence of the monomer using IR spectroscopy to investigate what functional group in GO has polymerized the monomers. Figure 4a shows the FT-IR spectra of the GO and HRGO. The IR spectra of the GO displays distinct vibrational modes of epoxide (C-O-C) (870 cm-1), ethers (CO) (900-1100 cm-1), epoxide (C-O-C) (1170 cm-1), epoxide and hydroxyl (C-O-C and C-OH) (1226 cm-1), ketone (C=O) (1280-1500 cm-1), sp2-hybridized C=C (1500-1600 cm-1), and ketone (C=O) (1620, 1735, and 1750-1850 cm-1).58 After mild heat treatment without the monomer (HRGO), the intensities of the absorption bands at 870 cm-1 (C-O-C), 1170 cm-1 (C-O-C), and 1226 cm-1 (C-O-C and C-OH) substantially weakened, whereas the absorbance intensities of the 1280-1500 cm-1 (C=O), 1500-1600 cm-1 (sp2-hybridized C=C), 1620, 1735, and 1750-1850 cm-1 (C=O) peaks significantly increased. These spectral changes obviously reveal that the epoxide and hydroxyl groups were desorbed from the GO and ketone (C=O) and sp2-hybridized C=C groups were newly formed in the basal plane during the thermal treatment. For the C-O band (900-1100 cm-1), it exhibits only slight increases in the peak intensity compared with the bands for the C=O and sp2-hybridized C=C groups, implying that ether groups were formed only a few in comparison to the C=O and sp2-hybridized C=C groups. Surprisingly, in the cases that the GO was heated with the monomer (Figure 4b; RGO-PTh, RGO-PANI, and RGO-PPy), the C=O peak maintained same intensity with that of the GO, instead the peaks of the CP were newly appeared (peaks of PTh, PANI, and PPy were summarized in Table S1, S2, and S3, respectively). For the cases that only the monomer was heated in water at 90 o C without the GO, no reaction was observed (Figure S2). In conjunction with FT-IR analysis, Raman analysis was also conducted to provide a more complete chemical

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bonding structure of the RGO-CP composites. As shown in Figure S3, the Raman spectra of the RGO-PTh, RGO-PANI, and RGO-PPy represent the characteristic peaks of the PTh, PANI, and PPy, respectively. These results indicate that the ketone group (C=O) is not formed in the basal plane, instead the CPs are newly formed. The shape and intensity of sp2-hybridized C=C peak was hard to confirm changes because of overlapping of CP peaks. In the case of the C-O peak, it shows slight increases in the IR spectra of the RGO-PTh and RGO-PANI, and negligible decrease in the IR spectrum of the RGO-PPy. Previous reported studies demonstrated that interactions between the adjacent epoxy and hydroxyl groups on the basal plane of GO during the thermal annealing result in formation of ketone (C=O) on the graphene sheet.47,48 Therefore, the obtained result that the ketone (C=O) group was not formed, instead the CPs were newly formed when the GO and monomer heated together implies that the hydroxyl and epoxy groups in the basal plane of the GO did not react with neighboring epoxy and hydroxyl groups, instead they interacted with the monomer and then polymerize the CPs when the GO is heated with the monomer. As shown in Figure S4-S6, we have since studied the polymerizations of monomers with respect to the heating temperature (60 and 80 oC) and the oxidation level of GO. At annealing temperature of 60 oC, CPs were not formed (Figure S4). When the temperature increased to 80 o

C, the polymerizations of monomers were observed (Figure S4), indicating that heating

temperature plays an important role in polymerizations of monomers by the GO. Figure S5 illustrates FT-IR spectra of the heated GO (H-GO 60 and 80; prepared by heating GO solution at 60 and 80 oC for 10 min. in the absence of the monomer). The H-GO 60 shows marginal changes compared with the GO, indicating that the dissociation of epoxy and hydroxyl groups in GO are feeble at heating temperature of 60 oC. In the IR spectrum of the H-GO 80, peaks at 870, 1170,

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and 1226 cm-1 corresponding to the epoxide (870 and 1170 cm-1) and epoxide and hydroxyl (1226 cm-1) noticeably decreased, whereas peaks at 1280-1500, 1500-1600, and 1620, 1735, and 1750-1850 cm-1 corresponding to the ketone (1280-1500, 1620, 1735, and 1750-1850 cm-1) and sp2-hybridized C=C (1500-1600 cm-1) prominently increased in comparison with pristine GO, demonstrating that hydroxyl and epoxy groups were decomposed from the GO and ketone and sp2-hybridized C=C groups were newly generated at heating temperature of 80 oC. Overall, the SEM and IR results reveal that monomers are polymerized by the GO at the heating temperatures that epoxy and hydroxyl groups are decomposed (in this research, heating temperatures larger than 80 oC). To investigate the effect of the oxidation level of the GO on polymerizations of the monomers, the oxidation level of the GO was controlled. Firstly, the atomic ratio of oxygen to carbon (O/C ratio) of the GO used above was surveyed, and the O/C ratio was confirmed to ca. 0.4, which is referred to below as the GO 2 (Figure S6e). On the basis of the O/C ratio of GO 2, the O/C ratio of the GO was further controlled as ca. 0.3 (GO 1) and 0.5 (GO 3) (Figure S6i and S6a). When the O/C ratio of the GO was ca. 0.3, the polymerizations of the monomers were not observed (Figure S6j, S6k, and S6l). However, when the O/C ratio of the GO increased to ca. 0.4 and 0.5, the monomer was polymerized by the GO (Figure S6f, S6g, S6h, S6b, S6c, and S6d). This result indicates that the oxidation level of the GO paly important role in polymerizations of the monomers and the monomers are polymerized by the GO when the O/C ratio of the GO is larger than ca. 0.4. We reasoned that the GO is unable to sufficiently initiate the polymerization at the O/C ratio of the GO lower than ca. 0.3.

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Figure 4. FT-IR absorbance spectrum of a) GO and HRGO and b) GO, RGO-PTh, RGO-PANI, and RGO-PPy.

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Based on IR result, classical radical polymerization mechanisms of the CPs,3,42-46 and widely accepted thermal reduction mechanism of the GO,47,48 we proposed the possible polymerization mechanisms. As described above, experimental and theoretical results have revealed that the interplay between the neighboring epoxy and hydroxyl groups during the thermal annealing lead to formation of the ketone (C=O) on the graphene sheet.47,48 More specifically, first-principles calculations and molecular dynamics simulations have demonstrated that the ketone (C=O) group is formed by following three different mechanisms: 1) interplay between two adjoining epoxides on the opposite sides of the sheet, 2) interactions of hydroxyl and epoxy groups connected to the two sides of the basal plane, and 3) interactions of pairs of hydroxyls attached to the opposite sides of the graphene surface.47 Taking these studies into account, for each polymer, we have proposed polymerization mechanisms as following three cases: case 1) polymerization initiated by two neighboring epoxides on the opposite sides of the sheet, case 2) polymerization started by hydroxyl and epoxy groups adsorbed on the two sides of the sheet, and case 3) polymerization commenced by pairs of hydroxyls located on the opposite sides of the plane. In each case, the initial configuration of GO which reacts with the monomer has designed as simple as possible to suit the case; for case 1): GO (E) consisted of two neighboring epoxides on the opposite sides of the sheet, for case 2): GO (EH) composed of hydroxyl and epoxy groups adsorbed on the two sides of the sheet, and for case 3): GO (H) comprised of pairs of hydroxyls located on the opposite sides of the plane. The peripheries of the GO (E), GO (EH), and GO (H) have been designed to refer to widely adopted Gao model (Figure S7).59 Figure 5 illustrates polymerization mechanism of the PANI initiated by two neighboring epoxides on the opposite sides of the graphene sheet. In the first step, anilinium ion approaches to epoxide in GO (E) due to electrostatic static attraction between positively charged H in

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anilinium ion and negatively charged O of epoxide (dipole-dipole interaction). Immediately after, new O-H bond is formed, and simultaneously one C-O bond of epoxide and N-H bond of anilinium ion are cleaved, yielding hydroxyl and radical on the GO (E) and aniline radical cation. As soon as the initial aniline radical cation is formed in solution, it reacts with another aniline radical cation, resulting in p-amino-diphenylamine (PADPA) dication.42 At the same time, anilinium ion moves toward newly created hydroxyl on the GO (E), and then, new O-H bond forms, and concurrently one C-O bond of epoxide and N-H bond of anilinium ion are broken, giving water molecule, sp2-hybridized C=C restored RGO (E) without any epoxy groups and ketones (C=O), and new aniline radical cation. In the third step, the chloride ions remove protons of PADPA dication, producing neutral PDAPD and hydrochloric acid molecule.42 At the same time, newly formed aniline radical cation reacts with another aniline radical cation, yielding new PADPA dication. Next, protons of dication are transferred to chloride ions, forming hydrochloric acid molecule and neutral PDAPD. The neutral PDAPD is pronated by the hydrochloric acid in solution, and then, protonated PDAPD ion and anilinium ion are oxidized by epoxide in GO (E) (analogous to step 1), resulting in PDAPD radical cation and aniline radical cation. Then, the PDAPD radical cation couples with aniline radical cation (analogous to step 2), resulting in trimer dication. Next, protons of dication are transferred to chloride ions (analogous to step 3), forming neutral trimer. Continuous this process produce the pernigraniline.42,45 In the final step, the pernigraniline is protonated by unreacted anilinium chloride, resulting in green emeraldine salt form of PANI.42,45 The additional polymerization mechanisms of PANI are illustrated in Figure S8 and S9.

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Figure 5. Polymerization mechanism of the PANI initiated by two neighboring epoxides on the opposite sides of the graphene sheet.

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Figure 6 depicts polymerization mechanism of PTh initiated by two neighboring epoxides on the opposite sides of the graphene sheet. At the beginning of the reaction, the thiophene monomer approaches to epoxide on GO (E) 1 due to the electrostatic attraction between positively charged H in thiophene and negatively charged O in the epoxide (quadrupole60-62dipole interaction),63-65 and then, the new O-H bond starts to form and simultaneously the C-H bond of thiophene and one C-O bond of epoxide are partially broken (transition state). After this, the new O-H bond forms fully, and concurrently one C-O bond in epoxide and C-H bond of the thiophene are broken completely, giving hydroxyl and radical on the GO (E) 1 and thiophene radical. This reaction step is similar to previously reported ring opening reaction of epoxy group on the GO, leading to formation of hydroxyl and radical on the sheet.66 Since the generation of radical on the GO or on the monomer is energetically unfavorable, this reaction step would have been endothermic. In the second step, the newly formed thiophene radical attacks neutral thiophene monomer, resulting in radical dimer. At the same time, another neutral thiophene monomer approaches to hydroxyl on the GO (E) 1 via electrostatic interaction between positively charged H in thiophene and negatively charged O in the hydroxyl (quadrupole60-62-dipole interaction).63-65 Shortly after, a new O-H bond forms, and the C-O bond in hydroxyl and the C-H bond in thiophene are cleaved, yielding water molecule, sp2-hybridized C=C restored RGO (E) 1 without any epoxides and ketones (C=O), and new thiophene radical. This process is analogous to theoretically and experimentally observed recombinative desorption of the hydrogen atom on graphene and graphite surfaces, which results in the disappearance of radical.66 Next, the radical dimer formed via step 2 bounds to epoxide on the GO (E) 2 due to the electrostatic attraction between positively charged H in radical dimer and negatively charged O

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in epoxide. Soon after, new O-H bond forms and the C-H bond in radical dimer and one C-O bond in epoxide are broken, yielding radical and hydroxyl on GO (E) 2 and neutral dimer. Meanwhile, the thiophene radical created via step 2 reacts with the neutral thiophene monomer, leading to formation of radical dimer. Next, the radical dimer generated through step 3 approaches to hydroxyl group on GO (E) 2 due to electrostatic interaction between positively charged H in radical dimer and negatively charged O in hydroxyl group. After that, O-H bond forms, and C-O bond in hydroxyl and C-H bond in radical dimer are cleaved, forming water molecule, sp2-hybridized C=C restored RGO (E) 2 without any epoxides and ketones (C=O), and new neutral dimer. The dimer is oxidized by the epoxide in GO (E) (analogous to step 1), and then, radical dimer reacts with neutral thiophene monomer (analogous to step 2), forming radical trimer. Then, the one hydrogen atom of the radical trimer are transferred to epoxide in GO (E) (analogous to step 3), resulting in neutral trimer. Continuation of this process yields the PTh.3,43 The additional polymerization mechanisms of PTh are illustrated in Figure S10 and S11. Polymerization mechanism (initiation, propagation, and termination) of the PPy and PTh are basically the same.43 Thus, the polymerization mechanisms of PPy by the GO (E), GO (EH), and GO (H) are same with those of PTh (Figure S12-S14).

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Figure 6. Polymerization mechanism of the PTh initiated by two adjoining epoxides on the opposite sides of the sheet.

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CONCLUSION Since mechanically exfoliated single-layer graphene is firstly reported by Geim and co-works in 2004, graphene has extensively impacted the areas of modern physics, chemistry, and material science, and a tremendous effort has been made to apply the graphene or chemically modified graphene in various fields.41,67-80 Recently, using the GO for “carbocatalysis” has great attraction as a novel application of graphene-based nanomaterials and expected to opens a host of possibilities for chemical synthesis because of the abundance of natural carbon sources, as well as the low density, extensive chemical functionalization, hydrophilicity, low cost, and ease of preparation.41,66 In this study, we have demonstrated that the GO can be utilized as a chemical oxidant for various CPs, and diverse graphene-CP composites can simply and rapidly be fabricated by using the GO as the initiator. Additionally, taking a step forward, we have revealed that the hydroxyl and epoxy groups in the GO polymerized the monomers through the systemic series of FT-IR analyses, and have deeply and specifically proposed the possible polymerization mechanisms based on the IR result, classical radical polymerization mechanisms of CPs, and widely adopted thermal reduction mechanism of GO. We believe that this research would be the valuable and informative report that extends the application fields of the GO and provides an easy and quick method for preparing diverse graphene-CP composites.

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MATERIALS AND METHODS Materials. Graphite flake (325 mesh) and sodium nitrate (NaNO3, 99 %) were purchased from the Sigma-Aldrich Co. Potassium persulfate (K2S2O8, 99 %) was purchased from the Kanto Chemical Co. Sulfuric acid (H2SO4, 95 %), hydrogen peroxide (H2O2, 30~35.5 %), hydrochloric acid (HCl, 35~37 %) were purchased from the Samchun Chemical Co. Potassium permanganate (KMnO4, 99.3 %) and phosphorus pentoxide (P2O5, extra pure) were acquired from the Junsei Chemical Co. Thiophene (C4H4S, ≥ 99 %) and aniline (C6H5NH2, ≥ 99.5 %) were purchased from the Sigma-Aldrich Co. Pyrrole (C4H4NH, 99 %) and hydrochloric acid (HCl, 35~37 %) were purchased from the Samchun Chemical Co. Preparation of GO. GO powder was prepared by modified Hummers’ method.59,81 At first, graphite flake (5 g), K2S2O8 (2.5 g) and P2O5 (2.5 g) were dissolved in H2SO4 solution (30 ml) in a flask and the whole solution was heated at 80 o C for 6 h for pre-oxidation. The solution was filtered through cellulose acetate filter (Whatman) with pure water and remaining flake was kept in vacuum oven for 24 h. The dried graphite flake was poured into well-mixed NaNO3/H2SO4 solution in ice bath with vigorous stirring. Then KMnO4 (15g) was slowly poured into the solution for 30 min in ice bath system. After 30 min, ice bath was removed and whole solution was heated at 45 o C for 12 h with vigorous stirring, turning the solution to brownish gray paste. Then deionized water (230 ml) was slowly poured into the paste, keeping the temperature below 40 o C. Deionized water (700 ml) was poured again and H2O2 (25 ml) was slowly added to the solution, forming bright yellow graphitic oxide solution. This solution was washed with 10 wt % HCl solution for three times and deionized water for several times until the solution became neutral (pH 7). This solution was ultra-sonicated for 1 h to exfoliate graphitic oxide into GO. Then the GO solution was centrifuged at 4000 rpm for 30 min to exclude residue. The solution

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was dried in vacuum oven and GO powder was obtained. The GO powder was kept in vacuum oven for further use and obtained GO was referred to GO 2. To investigate the effect of the oxidation level of the GO on polymerizations of the monomers, the oxidation level of the GO was controlled by adjusting the feeding weight ratio of KMnO4 to graphite was changed as 1:1 and 5:1, and the obtained GO was named as GO 1 and GO 3, respectively. Preparation of RGO-CP composites (RGO-PTh, RGO-PANI, and RGO-PPy). A 70 mL vial was charged with GO (90 mg), monomer (thiophene, aniline, or pyrrole; 30 mg), deionized water (30 mL), and a magnetic stir bar. The vial was then sealed with a Teflon-lined cap under ambient atmosphere and the resulting mixture was stirred at 90 °C for 10 min. The reaction mixture was then cooled to room temperature, vacuum filtered, and washed with deionized water. The filtered solid was collected and the water solvent was evaporated. For aniline monomer, HCl aqueous solution (11.5 M, 0.3 mL) was additionally added to the vial before vial is sealed with a Teflon-lined cap. The RGO-CP composites obtained are referred to as RGOPTh, RGO-PANI, and RGO-PPy, respectively. To investigate effect of the heating temperature on polymerizations of monomers by the GO, a series of control experiments was performed wherein GO solution (90 mg GO in 30 mL of deionized water) was annealed with the monomer (thiophene, aniline, or pyrrole; 30 mg) at heating temperatures of 60 and 80 oC for 10 min. To investigate effect of the oxidation level of the GO on polymerizations of monomers, a series of control experiments was performed wherein GO 1, 2, and 3 solution (90 mg GO in 30 mL of deionized water) was annealed with the monomer (thiophene, aniline, or pyrrole; 30 mg), respectively, at heating temperatures of 90 oC for 10 min. Preparation of HRGO. A 70 mL vial was charged with GO (90 mg), deionized water (30 mL), and a magnetic stir bar. The vial was then sealed with a Teflon-lined cap under ambient

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atmosphere and the GO solution was stirred at 90 °C for 10 min. The solution was then cooled to room temperature, vacuum filtered, and washed with deionized water. The filtered solid was collected and the water solvent was evaporated. Preparation of H-GO 60 and H-GO 80. A 70 mL vial was charged with GO (90 mg), deionized water (30 mL), and a magnetic stir bar. The vial was then sealed with a Teflon-lined cap under ambient atmosphere and the GO solution was stirred at 60 °C for 10 min. The solution was then cooled to room temperature, vacuum filtered, and washed with deionized water. The filtered solid was collected and the water solvent was evaporated. For the H-GO 80, only the heating temperature was changed to 80 oC (other synthetic conditions were identical to those of H-GO 60). Preparation of heated monomers (H-thiophene, H-aniline, H-pyrrole). A 70 mL vial was charged with monomer (thiophene, aniline, or pyrrole; 30 mg), deionized water (30 mL), and a magnetic stir bar. The vial was then sealed with a Teflon-lined cap under ambient atmosphere and the monomer solution was stirred at 90 °C for 10 min. The solution was then cooled to room temperature. For aniline monomer, HCl aqueous solution (11.5 M, 0.3 mL) was additionally added to the vial before vial is sealed with a Teflon-lined cap. The heated monomers are referred to as H-thiophene, H-aniline, and H-pyrrole, respectively. Preparation of pristine CPs (PTh, PANI, PPy). A 70 mL vial was charged with ammonium persulfate (90 mg), monomer (thiophene, aniline, or pyrrole; 30 mg), deionized water (30 mL), and a magnetic stir bar. The vial was then sealed with a Teflon-lined cap under ambient atmosphere and the solution was stirred at 90 °C for 10 min. The resulting product was then cooled to room temperature, vacuum filtered, and washed with deionized water. The filtered solid was collected and the water solvent was evaporated. For aniline monomer, HCl aqueous

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solution (11.5 M, 0.3 mL) was additionally added to the vial before vial is sealed with a Teflonlined cap. Characterization. Scanning electron microscope (SEM) and transmission electron microscope (TEM) images were obtained using a JSM-6701F (JEOL, Japan) and LIBRA 120 (Carl Zeiss, Germany), respectively. Ultraviolet-visible (UV-vis) and Fourier transform infrared (FT-IR) spectra were recorded on a S-3100 (Scinco, Korea) and FT-IR/NIR Frontier (Perkin Elmer, USA) spectrophotometer, respectively. Raman spectra were taken using a Horiba Jobin-Yvon LabRam Aramis spectrometer with a 514.5 nm Ar-ion laser as the excitation source. The X-ray photoelectron spectra (XPS) were measured using Sigma probe (ThermoVG, U.K). UV-vis Measurements. To obtain the UV-vis spectra of the GO, HRGO, RGO-CP composites (RGO-PTh, RGO-PANI, and RGO-PPy), monomers (thiophene, aniline, and pyrrole) and heated monomers (H-thiophene, H-aniline, and H-pyrrole) samples, quartz cuvette was firstly charged with deionized water. Next, the samples were added to the cuvettes, respectively. Then, UV-vis spectra of the samples were taken with a quartz cuvette on a Scinco S-3100 UV-vis scanning spectrophotometer. In the case of measuring UV-vis spectrum of aniline monomer, hydrochloric acid was additionally added to the cuvette before UV-vis spectrum is taken. FT-IR Measurements. The spectra of the GO, HRGO, RGO-CP composites (RGO-PTh, RGO-PANI, and RGO-PPy), CPs (PTh, PANI, and PPy), monomers (thiophene, aniline, and pyrrole), heated monomers (H-thiophene, H-aniline, and H-pyrrole), H-GO 60, and H-GO 80 were measured in attenuated total reflectance (ATR) mode using a Perkin Elmer FT-IR/NIR Frontier spectrometer.

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Raman Measurements. The Raman scattered light signals of the pristine CPs (PTh, PANI, and PPy) and RGO-CP composites (RGO-PTh, RGO-PANI, and RGO-PPy) were collected in a backscattering geometry using the x50 microscope objective lens. XPS Measurements. The atomic % of oxygen and carbon and O/C ratio of the GO were determined from the obtained XPS spectra by considering the integrated intensities of the O1s and C1s XPS peaks and sensitivity factors.

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Supporting Information. UV-spectra of GO and HRGO, FT-IR spectra of CPs (PTh, PANI, and PPy), monomers (thiophene, aniline, and pyrrole), and heated monomers (H-thiophene, Haniline, and H-pyrrole), Raman spectra of CPs (PTh, PANI, and PPy) and RGO-CP composites (RGO-PTh, RGO-PANI, and RGO-PPy), TEM images of the materials obtained by heating the GO solution for 10 min. at 80 oC with thiophene, aniline, pyrrole and at 60 oC with thiophene, aniline, pyrrole, FT-IR spectra of GO, H-GO 60, and H-GO 80, XPS spectra of GO 1, GO 2, and GO 3 and TEM images of the materials obtained by heating the GO 1 solution for 10 min. at 90 o

C with thiophene, aniline, pyrrole and the GO 2 solution for 10 min. at 90 oC with thiophene,

aniline, pyrrole, and the GO 3 solution for 10 min. at 90 oC with thiophene, aniline, pyrrole, structural model of GO as proposed by Gao and co-workers, polymerization mechanisms of the PANI initiated by hydroxyl and epoxy groups on GO, polymerization mechanisms of the PANI initiated by hydroxyl groups on GO, polymerization mechanism of the PTh initiated by hydroxyl and epoxy groups on GO, polymerization mechanism of the PTh initiated by hydroxyl groups on GO, polymerization mechanism of the PPy initiated by epoxide groups on GO, polymerization mechanism of the PPy initiated by hydroxyl and epoxy groups on GO, polymerization mechanism of the PPy initiated by hydroxyl groups on GO, table of FT-IR peaks for PTh, table of FT-IR peaks for PANI, and table of FT-IR peaks for PPy. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgment. This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (no. 2011-0017125).

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Synopsis TOC

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Fabrication of Various Conducting Polymers Using Graphene Oxide

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