Simultaneous Reduction of Graphene Oxide and Polyaniline: Doping

ARTICLE pubs.acs.org/JPCC

Simultaneous Reduction of Graphene Oxide and Polyaniline: Doping-Assisted Formation of a Solid-State Charge-Transfer Complex Cristina Vall
es, Pablo Jim
enez, Edgar Mu~noz, Ana M. Benito, and Wolfgang K. Maser* Department of Chemical Processes and Nanotechnology, Instituto de Carboquímica ICB-CSIC, C/Miguel Luesma Cast
an 4, E-50018 Zaragoza, Spain

bS Supporting Information ABSTRACT: We report the formation of a solid-state charge-transfer complex upon simultaneous reduction of a graphene oxide
polyaniline (GO-PANI) composite consisting of GO sheets coated by a thin layer of PANI. The reduced R(GO-PANI) material exhibits an unprecedented donor
acceptor interaction at the interface between RGO sheets and the thin PANI layer coating. A conceptual explanation is proposed in which RGO plays a dual role as electron acceptor and as large counterion stabilizing an atypical intermediate oxidation state of PANI. Moreover, the donor
acceptor interactions are responsible for superior materials characteristics, such as excellent water dispersibility, high environmental (chemical and thermal) degradation stability, and enhanced electric conductivity as high as 2600 S/m. These results may enable further opportunities for the development of novel electroactive materials based on graphene and intrinsically conducting polymers and the fabrication of corresponding flexible electronic devices through traditional solution processing techniques.

’ INTRODUCTION Graphene, a single-atom-thick sheet of hexagonally arranged sp2-hybridized carbon atoms, has attracted enormous research interest in recent years owing to its high electrical and thermal conductivities, great mechanical strength, and large specific surface area.1 Graphene is used as promising constituent in various types of applications ranging from nanoelectronics2 to advanced composites3 and novel electrode materials for batteries,4 supercapacitors,5 and optoelectronic devices.6 Solution-based strategies involving the exfoliation and reduction of graphite oxide offer promise for high volume production of single-layered graphene (reduced graphene oxide) sheets and for the versatile assembly and processing of functional graphene-based composite materials.7 Of particular relevance is the creation of favorable interactions with graphene’s highly conjugated structure and delocalized electron system by using for example aromatic molecules8 or conjugated polymers.9 Polyaniline (PANI), a prominent member of the family of intrinsically conducting polymers,10 is a further promising candidate for developing highly functional graphene-based composite materials. In the focus here are PANI’s extended π-conjugated structure, the presence of highly delocalized charge carriers in its doped, electrically conducting and hydrophilic state (emeraldine salt state, ES), and, moreover, its unique ability to undergo reversible doping processes as efficient means for controllable switching between neutral and charged states (see Supporting Information). Unfortunately, degradation issues related to low environmental (chemical and thermal) stability and processing problems are a drawback when it comes to the development of reliable and devices with long-term operational r 2011 American Chemical Society

functionality. Combining PANI with graphene thus should lead, similar as for carbon nanotubes,11 to significant performance and processing improvements of interest for further progress in flexible plastic and wearable electronics. Recently, various types of graphene
PANI composites were developed showing enhanced electrochemical characteristics as electrode materials for energy storage devices.12 Most works employed in situ polymerization approaches of aniline in the presence of reduced graphene oxide (RGO)12a,c or graphene oxide (GO).12d,13 While conducting RGO only shows limited dispersion possibilities, water dispersions of GO are highly compatible with the hydrophilic character of PANI in its conducting ES state and thus of advantage when using in situ polymerization processes. A subsequent reduction step transforming GO in the composite into its conducting RGO form followed by an oxidation step, aimed to recover PANI’s ES state, resulted in the enhancement of the operational functionality of the composites as electrode materials for supercapacitors.12d,13 While the focus of these works clearly laid on achieving device performance improvements, certain results,12d nevertheless, point to serious degradation problems of PANI in the corresponding composites, probably caused by overoxidation or hydrolysis. This is a strong indication that the important issue of PANI’s chemistry frequently is underestimated or not fully addressed when synthesizing graphene
PANI composites. Putting under closer scrutinity the rich scenery of oxidation and Received: February 23, 2011 Revised: April 25, 2011 Published: May 05, 2011 10468

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The Journal of Physical Chemistry C protonation states of PANI in combination with (reduced) graphene oxide therefore is needed not only to avoid undesired degradation effects in the respective composites but, moreover, to understand the interactions between (reduced) GO and PANI and explore further favorable opportunities that could broaden up the application spectrum for GO
PANI composites. Herein we provide a conceptual description of unprecedented interactions between graphene and PANI and demonstrate that simultaneous reduction of graphene oxide (GO) and PANI leads to the formation of a solid-state charge-transfer complex in the ground state. Because of its unique structure, reduced graphene oxide (RGO) behaves as an effective dopant of this conducting polymer playing a dual role as electron acceptor and anionic counterion with respect to the different oxidation states of polyaniline, resulting in the stabilization of an atypical intermediate oxidation state of PANI. Moreover, the resulting composite is characterized by a high environmental (chemical and thermal) degradation stability and water dispersibility accompanied by enhanced conducting properties.

’ EXPERIMENTAL SECTION Materials. Graphite oxide (GO) was prepared using a mod-

ified Hummers method from graphite powder (Sigma-Aldrich, ref: 332491) by oxidation with NaNO3, H2SO4, and KMnO4 in an ice bath as reported elsewhere.14 A suspension of graphene oxide sheets (GO) was obtained by mild sonication of the prepared graphite oxide powder in distilled water (1 mg/mL) for 30 min, followed by centrifugation to remove nonexfoliated materials, according to an experimental procedure described recently7a leading to a brown dispersion of exfoliated GO with a final concentration of ∼0.45 mg/mL, as determined from the residual weight of a freeze-dried aliquot of the corresponding dispersion. Preparation of the Composites. In situ oxidative polymerization of aniline in presence of dispersed GO was performed following the experimental conditions used in the synthesis of nanofibrilar PANI composites reported by Jim
enez et al.11d In detail, 50 mL of a 0.3 M solution of aniline (99.5% Scharlau Chemie, Spain) in aqueous 1 M HCl was added to the aqueous dispersion of GO (mass ratio GO/aniline = 1/1). After sonicating the mixture for 10 min, a solution of ammonium peroxidisulfate (APS, 98%, Sigma-Aldrich) in aqueous 1 M HCl was added at once (molar ratio aniline/APS = 3/1). Sonication was kept for 2 h at 15
20 °C during the in situ polymerization process. The aniline polymerization is appreciable as the color of this mixture turns green. This mixture was either filtered, washed, and dried (GO-PANI material) or subsequently reduced to R(GO-PANI) by addition of hydrazine hydrate (6 μL/mL of the initial GO suspension) and then stirred at 90 °C for 4.5 h. R(GO-PANI) was collected by filtration, washed thoroughly, and dried overnight. Aqueous dispersions of the synthesized GOPANI and R(GO-PANI) composites were prepared by mild sonication of powder samples in 2 mM solutions of hydrochloric acid in water, which yielded highly stable dispersions, comparable to nf-PANI and nanostructured PANI-multiwalled carbon nanotube composites.15 Characterization of Materials. UV
vis absorption spectra were recorded on a Shimadzu UV-2401 PC spectrophotometer between 200 and 900 nm on aqueous dispersions. Elemental analyses were performed in a Thermo Flash EA 1112 instrument with ∼3 mg of powder samples. Raman spectroscopy was

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Figure 1. Electronic microscopy images of the materials: SEM (a) and TEM (b) of nf-PANI; SEM of graphene oxide sheets (c); SEM (d) and TEM (e, f) of GO-PANI; SEM (g) and TEM (h, i) of R(GO-PANI).

performed using a Horiba Jobin-Yvon HRLAB HR 800 UV apparatus (excitation laser λ = 514 nm) on drop-casted films of 2 mM HCl dispersions of the studied materials on glass substrates. FT-IR spectra of materials pressed in KBr pellets were recorded on a Bruker Vertex 70 spectrometer in the 400
4000 cm
1 region. Cyclic voltammetries were performed in a three-electrode electrochemical cell with a platinum foil as counter electrode and a Ag/AgCl electrode as reference electrode, in aqueous HCl (pH = 2.7) electrolyte at 100 mV s
1 using a Gamry Reference 600 potentiostat/galvanostat. Films of the studied materials were drop-cast on platinum foil and used as working electrodes. Electrical conductivity of pressed pellets of the prepared materials was measured in a four-point Van der Pauw configuration using a Keithley 2602 System SourceMeter current source.16 Thermogravimetric analysis (TGA) was performed with a Setaram TG DTA 92 thermobalance using an air flow of 100 mL min
1 at a heating rate of 10 °C min
1. The morphology of the synthesized materials was characterized by scanning electron microscope (SEM, Hitachi S-3400N microscope) on powder samples and transmission electron microscopy (TEM, JEOL JSM-6400 microscope). One drop of diluted material dispersions was placed on Cu TEM grids with a holey carbon support film and then completely dried in air before TEM characterization was conducted. X-ray photoelectron spectroscopy (XPS) was carried out on a ESCAPlus Omicron spectrometer using a monochromatized Mg X-ray source (1253.6 eV) on drop-casted films of 2 mM HCl dispersions of the studied materials to probe their different electronic structures.

’ RESULTS AND DISCUSSION Having a closer look at the rich chemistry of PANI, we show in the following, step by step, that careful control of reaction conditions and morphology upon the synthesis of GO-PANI composite materials is the key to obtain specific interactions beyond the usually encountered π
π interactions, leading to enhanced materials properties and processing conditions. The preparation of R(GO-PANI) reported here includes the synthesis of a composite of GO and PANI and a subsequent 10469

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Figure 3. Raman spectra of the materials. Most relevant bands are indicated by arrows.

Figure 2. Infrared absorption spectra of the materials (a). An amplification of the region between 800 and 1800 cm
1 for GO-PANI and R (GO-PANI) is shown in (b).

controlled reductive treatment. The first step of this procedure, namely the synthesis of GO-PANI, involves the in situ polymerization of aniline in presence of well-dispersed GO. Such strategy is known to produce a coating of PANI in the emeraldine salt state on GO sheets, which act as efficient templates for aniline nucleation and polymerization.12a Both SEM and TEM (Figure 1) reveal a planar morphology of GO (Figure 1c) and GO-PANI (Figure 1d
f), indicating the formation of a thin layer coating of PANI on the surface of the well-exfoliated GO sheets. This is a direct consequence of the template effect (please note that PANI polymerized under similar conditions in absence of GO shows a nanofibrilar morphology (Figure 1a,b) as well as of the particular 1/1 reactant ratio of GO/aniline (please note that different GO/aniline reaction ratios up to 1/10 lead to thick PANI coatings). The critical second step of our synthetic procedure refers to the reaction of the formed GO-PANI with hydrazine hydrate, causing the simultaneous reduction of both GO and PANI and leading to the composite hence labeled as R(GO-PANI). Reaction conditions described in the Experimental Section such as temperature, reaction time, and hydrazine hydrate concentration have to be carefully selected in order to ensure an effective reduction of PANI and GO and avoid the decomposition of polyaniline through over-reduction, usually observed when long reaction times are employed. Our first observation is that hydrazine treatment of GO-PANI affects neither the laminar morphology of the composite nor the thickness of the PANI coating over the surface of the graphenes, as can be seen in microscopy images of R(GO-PANI) (Figure 1g
i).

Further evidence for the presence of thin coatings of PANI covering graphene sheets in both GO-PANI and R(GO-PANI) composites is obtained by elemental analyses. These provide PANI/graphene weight ratios of 0.85 for GO-PANI and 1.01 for R(GO-PANI) (details of these calculations are available in the Supporting Information). Despite the undefined composition reported for GO17 and RGO,18 the obtained data underline that a great part of the GO/RGO sheets is covered by a thin layer of PANI. Although no changes in the morphology of the composite are observable, the applied hydrazine treatment of GO-PANI causes the simultaneous reduction of both components, GO and PANI, as evidenced by spectroscopic techniques. An effective reduction of GO to RGO carried out by hydrazine19 is revealed by Raman, infrared (FT-IR), and UV
vis absorption spectroscopies. For GO-PANI, the PANI features in both FT-IR and Raman spectra show distinctive bands of an ES state, in agreement with composites of GO and PANI reported in the literature,12a,d,20 whereas UV
vis spectra point to a partially deprotonated ES state (between ES and EB). Surprisingly, upon reduction the spectroscopic features of PANI for R(GO-PANI) reveal intriguing differences with the ES state, which clearly point to the presence of an intermediate state of PANI between ES and LE. On one hand, FT-IR spectra (Figure 2) show significant changes, related to the gradual reduction of ES toward LE:21 (i) downshift of the =N(þ 3 )
H vibration band at 1140 to 1130 cm
1, (ii) decrease of the intensity of the C
N stretching band at 1305 cm
1, and (iii) upshift of the (CdC) quinoid band at 1486 to 1470 cm
1. Furthermore, Raman spectra (Figure 3) detect alterations that witness the partial reduction of the ES state toward LE:22 (i) apparent upshift of the maximum of the band for C
H in-plane bending vibrations in aromatic rings at 1172 to 1185 cm
1, (ii) decrease of relative intensity of band for C
N stretching vibrations at 1225 cm
1, and (iii) decrease of the band at 1490 cm
1 characteristic of C
C and C
N stretching in oxidized states of PANI (ES, EB, and pernigraniline), but absent in the reduced state, i.e., leucoemeraldine. X-ray photoelectron spectroscopy (XPS) of the prepared composites evidence the presence of nitrogen atoms in different 10470

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Figure 4. UV
vis absorption spectra of the materials. Characteristic absorption transitions are marked by thin arrows. Inset shows a photograph of vials containing aqueous dispersions of GO-PANI and R(GO-PANI).

chemical environments. Deconvolution of the N(1s) core level spectrum for GO-PANI (see Supporting Information) reveals the presence of quinoid imine (at a binding energy of 398.3 eV), secondary amine (399.5 eV), and radical cation nitrogen atoms (401.1 eV). The R(GO-PANI) spectrum shows an increased proportion of radical cation vs quinoid imine nitrogen atoms which is consistent with a higher number of semiquinone units (
NHþ 3
Ar
NH
) and a lower number of quinoid units (
NHþ=Ar=NHþ
), suggesting a lower overall oxidation state of the PANI chain of R(GO-PANI) compared to GOPANI, somewhere between ES and LE states. A further striking observation is that the spectral features typical for the intermediate oxidation state of PANI in freshly reduced R(GO-PANI) are not modified under air exposure and remain unaltered with time. This underlines once more the effective stabilization of PANI in an intermediate oxidation state between ES and LE in the solid state. In this sense, it is worth mentioning that a reduction of the emeraldine salt (ES) state of PANI typically would lead to its completely reduced leucoemeraldine (LE) state,21a,23 unstable under ambient conditions, especially in an aqueous environment, which progressively would oxidize back to the stable emeraldine state23 (ES or EB depending on the pH of the media). Moreover, the extremely high stability of the composites is observed not only for solid powder samples but remarkably also for water dispersed samples. In fact, samples of both GO-PANI and R(GO-PANI) composites are well dispersible in slightly acidic aqueous media, leading to dispersions stable for long periods of time. The reduction processes of GO-PANI to R(GO-PANI) can thus be easily followed by UV
vis spectra (Figure 4). Typical absorption bands of nf-PANI in the ES state are centered at 360 nm (assigned to π
π* transition), 430 nm, and 860 nm (assigned to transitions between polaron states and π molecular orbitals).24 The absorption spectrum of a dispersion of GO-PANI shows those three bands of the ES state overlapped with the GO absorption spectrum, although the “polaron band” at 860 nm is blue-shifted to 820 nm. This shift is consistent with a partial deprotonation of the ES state toward an EB state due to the dissociation of carboxylic groups present in GO and adjacent to PANI in the water dispersed composite, yielding carboxylate anions which replace the less basic chloride anions as counterions of ES. In contrast, dispersions of R(GO-PANI) are not as green

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as those of GO-PANI (Figure 4, inset), owing to a decreased intensity of the two polaron bands at longer wavelengths. As completely reduced LE would not display those polaron bands, but only an absorption band around 340 nm corresponding to the π
π* transition,25 the UV
vis spectrum of R(GO-PANI) points once again to an intermediate oxidation state of PANI between ES and LE, this time in aqueous dispersion, i.e., in a liquid phase. Taking into account that graphene is an excellent electron acceptor, analogous to other forms of carbon, such as C60 or SWNTs,26 and that aniline, on the other hand, is a very good electron donor,12a the observed unusual stability (in both solid and liquid phases) of this intermediate oxidation state compound has to be related to a specific interaction between RGO sheets and PANI, beyond the typically mentioned π
π interactions. We propose here a donor
acceptor interaction, establishing a ground state charge-transfer complex between RGO and PANI as a consistent explanation of all the aforementioned experimental observations, as depicted in Scheme 1. PANI in its LE state and RGO sheets are thought to act as the formal donor and formal acceptor, respectively, in the R(GO-PANI) composite. In this sense, in the dative (i.e., charge separated) state PANI takes the form of the positively charged ES state, whereas RGO gets negatively charged and carries out the function of anionic counterion to ES. An equilibrium between both species would be established through charge transfer along the interface of PANI and graphene, resulting in partially charged species typical of charge-transfer complexes, which easily explains the high dispersibility observed in aqueous medium. Here the well-known special redox properties of PANI enable the formation of this complex since interconversion between ES and LE states entails merely electron transfer processes with no further chemical modifications involving protonation or deprotonation of nitrogen atoms.27 On the other side, the extended conjugated backbone of RGO sheets is able to store a large amount of negative charges as it happens with graphene layers in graphite intercalation compounds.28 Additionally, the electron-accepting propensity of RGO may be enhanced by the presence of electron-withdrawing groups (such as carboxyl, carbonyl, and ester) along the periphery of the graphene layers. It is reasonable to presume that the spontaneous formation of this charge-transfer complex during the simultaneous reduction of PANI and GO with hydrazine prevents the elimination of most of these “peripheral” electron-withdrawing groups as a result of the accumulation of delocalized negative charges along the edges and through the conjugated surface of RGO sheets. The increased electron density in the conjugated system would hinder the further reduction and subsequent elimination of those moieties attached to the edges of RGO but would not hamper the elimination of functional groups on the basal plane which do not share electrons with the π-system of graphene. In fact, the FT-IR spectrum of R(GO-PANI) shows a “carbonylic band” around 1720 cm
1 that is absent in the spectrum of RGO material obtained through hydrazine reduction of a GO dispersion (Figure 2). At this point of the discussion it should be remarked that the particular morphology of R(GO-PANI) obtained through carefully controlling the experimental parameters is responsible for the formation of a solid-state charge-transfer complex between RGO and PANI. Three different aspects are to be considered: (i) The vast surface area of RGO sheets offers an extended contact area between RGO and PANI, so that the 10471

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Scheme 1. Chemical Reactions Leading to R(GO-PANI) and the Proposed Charge-Transfer Equilibrium between PANI and Graphene in R(GO-PANI)

charge-transfer processes, which happens at the contact interface between RGO and PANI, is enhanced. (ii) The thin layer of PANI grown on the surface of the RGO sheets, provided by the initial ratio (1:1) of reactants, minimizes the distance between dopant counterions RGO and PANI chains rendering the ideal distance for an effective doping of PANI. Lower GO/aniline ratios form thicker coatings of PANI covering the surface of RGO, avoiding thus the formation of a charge-transfer complex. Additionally, the formation of a thin PANI coating is also necessary for the successful reduction of GO sheets, since thicker layers would probably impede the elimination of the “basal plane” functional groups during the hydrazine treatment. (iii) The usually mentioned π
π interactions are insufficient to explain the observed results. Another outcome of this special morphology is the remarkable water dispersibility of the composites that confirms both the proper exfoliation of GO sheets and the formation of stable suspensions of the composites. Interestingly, the in situ reduction causes the transformation of the two hydrophilic components of GO-PANI, GO and ES, into two neutral, hydrophobic components (LE and RGO) in R(GO-PANI). This does not seem to affect the excellent water dispersibility of the composite, although RGO and LE are expected to yield poor water dispersions.29 Here again, the formation of a charge-transfer complex arises as the most suitable explanation: in fact, RGO and LE are partially charged species, and this charge separation improves their intrinsic hydrophilicity. As a consequence, the R(GO-PANI) composite sheets can form stable aqueous suspensions as electrostatic colloids, being a key advantage in the processing of this nanomaterial for further applications. The effective restoration of the conjugated π-system through reduction of graphene oxide is confirmed by the increased conductivity of R(GO-PANI) compared to GO-PANI. Electrical conductivity of GO-PANI samples reaches a value of 300 S/m

due to the conductive pathway provided by the continuous layers of conducting PANI-ES deposited on the insulating GO sheets. The higher conductivity value of 2600 S/m observed for R(GOPANI) should be attributed to the presence of conducting RGO sheets in the composite, as reduction (or partial reduction) of the ES state is known to decrease its conductivity.30 In addition, R(GO-PANI) displays a higher thermal stability than GO-PANI, which is revealed by thermogravimetric analyses of the samples. It can be clearly observed that the weight loss attributed to the simultaneous oxidation of PANI and graphene sheets in the case of R(GO-PANI) is shifted to higher temperatures by about 50 °C compared to GO-PANI (see Supporting Information). This also underlines the stabilization of PANI in that intermediate oxidation state explained by the proposed donor
acceptor interaction with RGO. The environmental stability of R(GO-PANI) toward oxidation in aqueous media should also be attributed to the R(GOPANI) charge-transfer complex formation. Capacitive and redox characteristics of GO, RGO, nf-PANI, GO-PANI, and R(GO-PANI) obtained by cyclic voltammetry (Figure 5) provide further insight into the interactions between GO and PANI. First, the increased area of RGO compared to GO (Figure 5a) clearly points out RGO’s higher capacity due to its enhanced electrical conductivity and large surface area. Second, comparing the voltammograms of GO-PANI and nf-PANI (Figure 5b) shows that the peaks corresponding to the ES
LE transition (centered around 0.2 V vs Ag/AgCl) are shifted to higher potential values. This shift is explained by the partial deprotonation of ES in the GO-PANI composite in aqueous media, also observed in UV
vis absorption spectroscopy and caused by carboxylate groups of GO. Potential peak values for this ES
LE redox transition are strongly pH dependent, and the partial conversion of ES into EB is analogous to a “local” increased pH value of the CV media, causing the observed potential shift. Third, cyclic voltammetry of R(GO-PANI) in acidic media (Figure 5b) reveals the capacitative contribution of conducting RGO, appreciable in the rectangular shape of the 10472

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the subsequent simultaneous reduction step with hydrazine. These conditions ensure the thin layer growth of PANI on GO and enable the successful formation of a R(GO-PANI) product which shows donor
acceptor interactions at the interface of RGO and PANI. Here RGO behaves as effective dopant of PANI carrying out a double function, namely as electron acceptor of reduced PANI, i.e., leucoemeraldine (LE), and as large anionic counterion of doped PANI, i.e., emeraldine salt (ES). The donor
acceptor interaction is noticed as a remarkable stabilization of PANI in an intermediate oxidation state between LE and ES. The formation of a charge-transfer complex explains the hydrophilic character of R(GO-PANI) and its excellent water dispersibility as electrostatic colloids. It also is responsible for improved materials properties such as enhanced conductivity up to 2600 S/m and superior environmental (chemical and thermal degradation) stability. These charge-transfer phenomena not only provide important insights on synergetic interactions between graphene and PANI species,but also should stimulate the development of novel functional materials based on graphene and conducting polymers through improved processing routes of interest for further progress in flexible plastic and wearable electronics.

’ ASSOCIATED CONTENT

bS

Supporting Information. Polyaniline: structure and oxidation states (S1); details of elemental analyses (S2.1); X-ray photoelectron spectroscopy (XPS) (S2.2); and thermogravimetric analyses (TGA) (S2.3). This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 5. Cyclic voltammograms of the materials: GO and RGO (a); nf-PANI, GO-PANI, and R(GO-PANI) (b). Redox peaks corresponding to the LE
ES transition are indicated by arrows.

curve, superimposed on the redox peaks caused by changes in the oxidation state of PANI. Regarding the redox behavior of PANI in R(GO-PANI), there is a potential shift of 24 mV in the ES
LE transition toward lower potential values, for both oxidation and reduction sweeps, with respect to PANI. Moreover, this potential shift remains stable throughout several oxidation
reduction cycles, confirming the redox stability of this kind of PANI and supporting the existence of a specific interaction between PANI and RGO in the material. The observed charge-transfer phenomenon presented here, a consequence of the special morphology of R(GO-PANI) composite, gives rise to enhancement effects of some specific properties and, additionally, offers favorable processing characteristics from aqueous media. These findings provide enhanced processing possibilities for this type of electroactive material by traditional solution casting/printing/stamping techniques over large areas and thus are of great interest for the development of flexible electronic devices (e.g., low-cost memories, optical displays, and supercapacitors) on various types of supports such as plastics, paper, or textiles.

’ CONCLUSIONS Simultaneous reduction of GO sheets covered by a thin layer of PANI leads to the doping assisted formation of a solid-state charge-transfer complex between RGO and PANI. The one-pot synthetic strategy comprises two critical issues: the use of a specific 1:1 GO to aniline ratio in an in situ polymerization and

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by Spanish Ministry of Science and Innovation (MICINN) and the European Regional Development Fund (ERDF) under Project MAT2010-15026 and the Government of Aragon (DGA) under Project DGA-T66 CNN. C.V. acknowledges MICINN for her postdoc contract under Juan de la Cierva Programme. P.J. thanks Fundaci
on Ram
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