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Carbon 95 (2015) 888e894

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Carbon

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Directed self-assembly of graphene oxide on an electrospun polymerfiber template

T.P. Vinod a, Xiuxiu Yin a, Jürgen Jopp b, Raz Jelinek a, b, *

a Department of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva 8410501, Israelb Ilse Katz Institute for Nanoscale Science and Technology, Ben-Gurion University of the Negev, Beer-Sheva 8410501, Israel

a r t i c l e i n f o

Article history:Received 24 July 2015Received in revised form31 August 2015Accepted 5 September 2015Available online 8 September 2015

* Corresponding author. Department of Chemistry,Negev, Beer-Sheva 8410501, Israel.

E-mail address: razj@bgu.ac.il (R. Jelinek).

http://dx.doi.org/10.1016/j.carbon.2015.09.0210008-6223/© 2015 Elsevier Ltd. All rights reserved.

a b s t r a c t

Development of self-assembly routes for long-range organization of graphene oxide (GO) is an activefield of research due to the enormous potential and diverse applications of this carbonaceous material.We present a simple strategy for producing anisotropic GO assemblies through utilizing electrospunpolymer fibers as a physical template. We demonstrate that intertwined uniform electrospun microfiberscomprising a mixture of polyethylenimine (PEI) and polyvinylpyrrolidone (PVP) constituted docking sitesfor GO flakes through electrostatic attraction between the negatively-charged GO and positive amine-displaying polymer fibers. The reduced GO (r-GO) network formed after chemical reduction exhibitedelectrical conductivity and optical transparency, opening the way for varied electro-optical applications.

© 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Graphene, an atomic scale honey-comb lattice of sp2 hybridizedcarbon atoms, has attracted significant interest due to its uniqueelectronic, optical, mechanical, and chemical properties [1e6].Graphene-based materials display novel and improved function-alities compared to other substances and exhibit potential for awide variety of applications [7,8]. Transparent conductive films(TCFs) in particular, which are essential components in variedelectronic and photonic applications, have been prepared fromgraphene using varied strategies [7,9e11]. TCF applications benefitfrom the excellent charge carrier mobility of graphene and thefeasibility of producing flexible films [7,12].

High quality graphene sheets can be prepared through variedmethods such as exfoliation, chemical vapor deposition (CVD),epitaxial growth, and others [13e16]. While those techniques havebeen widely used for graphene fabrication, synthesis of water-soluble graphene oxide (GO) and its subsequent reduction(through thermal or chemical means) to reduced graphene oxide(r-GO) is considered a promising route towards production ofgraphene and its use in practical applications and devices. Theadvantages of GO emanate from the use of inexpensive reagents,

Ben-Gurion University of the

simple procedures not requiring sophisticated instrumentation,scalability, and wide range of potential applications [17e20]. Anessential prerequisite for utilizing GO in TCFs and other applica-tions is the development of methods for arranging the GO sheets intwo- and three-dimensions [21]. Varied methodologies have beenproposed to achieve this goal, including self-assembly of GO at theliquideliquid and liquideair interfaces [22], electrophoretic depo-sition [23,24], spin coating [25,26], dip coating [27,28], transferprinting [29,30] and others.

Fibers and nanowires of metals have been employed as platformsfor TCFs, primarily due to the long-range conductivity achievedthrough linkage among individual wire elements, as well as theabundant empty spaces within the conductive network, enablingoptical transparency [12,31e33]. Electrospun polymer fibers consti-tute useful templates or etch masks for metal assemblies andconstruction of TCFs exhibiting excellent electrical conductivity andoptical transparency [34e36]. Electrospinning is a powerful generictechnology for production of uniform fibers, particularly usingpolymer blends [37,38]. Previous studies have reported incorpo-ration of GO inside or outside electrospun fibers and their applica-tions in sensing [39,40], and for improving physical scaffolding ofthe fibers [41]. Here we show that electrospun fibers comprisingpolyethylenimine (PEI) and polyvinylpyrrolidone (PVP) served as aversatile template for construction of transparent conductive GOfilms. Importantly, the assembly process was based upon theelectrostatic attraction between the negatively-charged GO and thePEI/PVP fibers (which are positively-charged due to the surface-

T.P. Vinod et al. / Carbon 95 (2015) 888e894 889

displayed amine residues). Following chemical reduction, aninterspersed r-GO network was produced, yielding transparent/conductive films.

2. Experimental

2.1. Materials

Doubly purified water with 18.3 MU cm resistivity from aBarnstead D7382water purification system (Barnstead Thermolyne,Dubuque, IA) was used for all the experiments. Natural graphiteflakes (~325 mesh), sodium nitrate, and polyvinylpyrrolidone(Mw ¼ 1300000) were purchased from Alfa Aesar (USA). Branchedpolyethylenimine (b-PEI) with averageMw ~25,000 and conductivesilver paint were purchased from Sigma Aldrich (USA). Potassiumpermanganate was obtained from Solufix (Israel). Hydrogenperoxide (30%) was obtained from SANAD for Science and Tech-nology (Israel). Sulfuric acid was purchased from Bio-Lab (Israel).Hydrochloric acid (32%) was purchased from Daejung (Korea).Hydrazine hydrate (55%) was purchased from Acros (USA). Ethanolwas purchased from JT Baker and DMFwas obtained from Frustrom(Israel). All the chemicals were used as delivered by the manufac-turer without further purification.

2.2. Synthesis of graphene oxide

Graphene oxide was synthesized according to a modifiedHummers method [42,43]. Briefly,1 g of graphite flakes were addedto a round bottom flask containing 46 mL of H2SO4 cooled in an icebath. The mixture was then sonicated for 1 h to disperse thegraphite. After shifting the flask to a water bath, 1 g of NaNO3 and6 g of KMnO4 were successively added very slowly (within 15 min).The temperature of the water bath was increased to 40 �C and thereaction mixture was stirred for 3 h. 50 mL of deionized water wasslowly added to the reaction mixture, followed by 5 mL H2O2.Stirring was continued for another 10 min. The reaction mixturewas then centrifuged (5000 rpm, 20 min) to separate the precipi-tate. Centrifugation was repeated until the supernatant showed aneutral pH. Resulting brown colored residue of graphene oxide wasthen dried through lyophilization and stored at 4 �C.

2.3. Electrospinning of PEI/PVP fibers

A solution of the polymers was prepared in ethanol such thatthe combined weight percentage of polymers in the solution was8% and the ratio of weight percentages of PEI to PVP was 3:1.Electrospinning was carried out using an experimental setup

Fig. 1. Experimental scheme. Schematic representation of (a) Electrospinning and (b) assemonline).

shown in Fig. 1. The polymer solution was placed in a syringe, onwhich pressure was applied from a syringe pump. High voltage wasapplied between the syringe needle and a grounded metallic col-lector. Glass substrates were placed on the substrate to collect theelectrospinning fibers. We observed that uniform and continuousfiber formation were obtained when we use a flow rate of 0.5 mL/hin the syringe pump, 13 kV voltage between the needle and col-lector, and 15 cm separation between them. Thickness of the fiberscan be varied by using needles of different diameters and theamount of fiber deposited on the substrates can be controlled byusing different deposition times. Typically, electrospinning times ofbetween 30 s and 2 min were used to prepare samples withdifferent densities of fibers on the surface.

2.4. Assembly of GO on PEI/PVP fibers

GO solutionwas prepared by dissolving GO flakes (8mg/mL) in asolvent mixture of DMF and water (9:1) [44]. The mixture wassonicated for 3 h to ensure effective exfoliation and dispersion.Glass substrates containing electrospun PEI/PVP fibers on the sur-face were then kept immersed in the GO solution for 12 h. Thesubstrates were subsequently rinsed with water two times toremove unbound GO and dried in air.

2.5. Reduction of GO on the PEI/PVP fiber template

Reduction of GO was based upon a published method [45] withminor modifications. Specifically, substrates containing GO on PEI/PVP fibers were placed inside a closed container along with hy-drazine hydrate. The container was then heated in low pressure at100 �C for 3 h, as these conditions were found to produce the mostoptimal reduction. After cooling, the substrates were rinsed withwater and dried in air.

2.6. Instruments and characterization

Scanning electron microscopy (SEM) experiments were con-ducted with a JEOL (Tokyo, Japan) JSM-7400F scanning electronmicroscope. X-ray photoelectron spectroscopy (XPS) analysis wasconducted using a Thermo Fisher ESCALAB 250 instrument with abasic pressure of 2� 10�9mbar. The samples were irradiated in twodifferent areas using monochromatic Al Ka, 1486.6 eV X-rays, usinga beam size of 500 mm. Conductivity measurementswere performedon a Keithley 2400 sourcemeter in a two-probe configuration. Forthe sample preparation of electrical measurements, the substratecontaining the gold filmwas mounted in a thermal evaporator witha stainless steel shadowmask attached. Cr (10 nm) and Au (90 nm)

bly and reduction of GO on PEI/PVP fibers. (A colour version of this figure can be viewed

T.P. Vinod et al. / Carbon 95 (2015) 888e894890

were evaporated onto the substrates to obtain the electrodepatches at a predefined spacing. Raman spectroscopymeasurementswere carried out on a Jobin-Yvon LabRam HR 800 micro-Ramansystem, equipped with a Synapse CCD detector. The excitationsource was an Argon laser (514.5 nm) with a power of 3 mWon thesample. In most of the measurements, the laser power was reducedby 10 using ND filters. The laser was focusedwith an�100 objectiveto a spot of about 1 mm. The grating used was a 600 gmm�1 and themeasurement was done using a confocal hole of 100 mm with anexposure time of 1 min. Atomic force microscopy (AFM) andconductive-AFM (C-AFM) were conducted on MFP-3D-Bio setupfrom Asylum Research. A dual-gain ORCA holder (106 and 109 V/A),suitable for current sensing in the range of 1 pAe10 mAwas used forthe simultaneous measurements of topography and current incontact mode. A Si probe with Pt coated tip (AC240TM) was usedfor the imaging. For the measurements, the sample substrate con-taining r-GO/PEI/PVP networkwas placed on an AFM specimen disc(Ted Pella) made of steel and the top surface of the sample wasconnected to the specimen disc with conductive Ag paint. Spec-imen disc and the AFM tip were connected to a voltage source andthe circuit is closed when the tip is in contact with the conductiveregions of the sample. Current from this circuit is measured forcurrent mapping and IeV measurements.

3. Results

The new GO template-deposition strategy, designed to producea fiber network enabling electronic transport and optical trans-parency, is depicted in Fig. 1. A polymer mixture comprising PEI andPVP was used to generate uniform microfibers through electro-spinning (Fig. 1a). The rationale for using both PEI and PVP in thiswork was our observation that this particular polymer blend wasamenable for production of uniform and stable fibers via electro-spinning, and the fact that both polymers display positive and polarnitrogen-containing functional groups. Accordingly, electrostaticattraction between the negatively-charged GO flakes and the amineresidues would promote formation of graphene network upon thePEI/PVP template. Indeed, previous studies have demonstrated thatPEI could induce self-assembly of GO through the electrostaticattraction between the polymer amine groups and the carboxylicacid moieties of GO [44].

Following the electrospinning procedure, the fibers were placedupon a transparent substrate, and subsequently immersed in asolution of GO flakes synthesized by modified Hummer's method[42,43] (Fig. 1b). The GO flakes exhibited lateral dimensions ofbetween 100 nm and 10 mm, thickness of 1 nme1.5 nm, as isgenerally the case in chemical exfoliation procedures for GO syn-thesis [16] and their consistency and purity were confirmed

Fig. 2. Selective attachment of GO to the polymer fibers. SEM images of (a) PEI/PVP fib

through application of microscopy and Raman spectroscopy(Fig. S1, supplementary information). As depicted in Fig. 1b, sinceGO has negative surface charge due to the carboxylic groups presentat the edges [22], the GO flakes are electrostatically attracted to thepositive functional groups (amines and other nitrogen-containingunits) upon the PEI/PVP fibers. In the final stage, the fiber-attached GO underwent chemical reduction upon brief exposureto hydrazine vapor, producing a conductive network of reduced-GO(r-GO) attached to the electrospun polymer template.

The GO/fiber networkwasmicroscopically and spectroscopicallycharacterized (Figs. 2e3). Fig. 2 depicts representative scanningelectronmicroscopy (SEM) images of the PEI/PVP electrospun fibersbefore and after r-GO assembly. Fig. 2a shows the bare polymer fi-bers, exhibiting diameters of between 500 nm and 2 mm. SelectiveattachmentofGO to thefibers is evident in the SEM imagepresentedin Fig. 2b (additional SEM results are shown in Fig. S2, supplemen-tary information). While the GO particulates exhibit varied sizes,Fig. 2b demonstrates that the GO flakes trace the fiber template andalmost no GO is present in the empty areas in between fibers. Thedark appearance of the coated fibers in the SEM image in Fig. 2b andinterfacial contacts between adjacent GO sheets indicates that acontinuous GO network was dormed upon the fibers. Specificattachment of GO to the polymerfiber template is further confirmedby AFM analysis (Fig. S3, supplementary information).

Fig. 3 presents spectroscopic data confirming the trans-formation of GO to r-GO on the polymer fiber template. Fig. 3adepicts Raman spectroscopy data displaying the characteristic Gband corresponding to the first order scattering of the E2g at1590 cm�1 and the D band corresponding to the lattice distortionsat 1350 cm�1 [46,47]. The increase in the ratio between the D and Gpeaks (i.e. increase in the intensity ratio ID/IG), apparent in Fig. 3a, isindicative of the reduction of GO to r-GO [46,47]. Raman analysisfurther confirmed that GO/r-GO flakes were present exclusivelyupon the polymer fibers (Fig. S4, supplementary information).

X ray photoelectron spectroscopy (XPS) data (Fig. 3b) provideadditional evidence for the reduction of GO upon the PEI/PVP fibertemplate. Specifically, the C 1s spectrum of the PEI/PVP/GO fibersystem (Fig. 3b, left spectrum) can be deconvoluted into fourcomponents [48] corresponding to CeC (284.8 eV), CeO (286.3 eV),C]O (287.6 eV), and C(O)O (289.0 eV). Similarly, as depicted inFig. 3b (right spectrum), the C 1s spectra of PEI/PVP/r-GO wasdeconvoluted to five components [48] corresponding to CeC(284.8 eV), C]N (285.4 eV), CeO (286.4 eV), C]O (287.9 eV), andC(O)O (288.9 eV). Importantly, the XPS spectral analysis demon-strates that the relative intensities of the peaks corresponding tooxygen-bonded C atoms were lower compared to the deconvolutedCeC XPS peak, consistent with the reduction of GO to r-GO [46,48].Indeed, significant increase in carbon-to-oxygen ratio, calculated

ers, and (b) r-GO on PEI/PVP fibers. The scale bars represent 20 mm in both images.

Fig. 3. Spectroscopic characterization. (a) Raman spectra of PEI/PVP coated with GO (red) and r-GO (black). (b) XPS spectra of the C 1s region of PEI/PVP/GO and PEI/PVP/r-GO. Theblack spectra were the experimental results and the colored component peaks correspond to the carbon atoms in CeC (blue), CeO (red), C]O (magenta), C(O)O (green) and C]N(purple). (A colour version of this figure can be viewed online).

Fig. 4. Electrical conductivity of a single r-GO/polymer fiber. Conductive atomic force microscopy (C-AFM) measurements depicting current mapping of r-GO assembled upon PEI/PVP fibers (top image), and IeV curves measured at two locations indicated by arrows (bottom). (A colour version of this figure can be viewed online).

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T.P. Vinod et al. / Carbon 95 (2015) 888e894892

from the intensities of C1s peaks and O1s peaks in the XPS exper-iments, further confirms the reduction of GO to r-GO (Table S1,supplementary information). XPS spectra of bare PEI/PVP fibers(without attached graphene) show distinct positions of the carbonatoms in PEI and PVP that were different than in the GO-attachedfiber samples (Fig. S5, supplementary information). Notably, thefact that no underlying polymer peaks were recorded in the GO/PEI/PVP samples (Fig. 3b) implies that the graphene coating isrelatively thick. This conclusion is further supported by the absenceof polymer peaks in the Raman spectrum (Fig. 3a).

Since r-GO is electrically conductive, the r-GO/fiber network isconducive to production of transparent/conductive films. Figs. 4and 5 explores the electrical and optical properties of the films.Fig. 4 depicts results of conductive atomic force microscopy (C-AFM) experiment. To record the C-AFM image, the tip and thesample were electrically linked, providing simultaneous currentand topography mapping in the contact mode. The representativeC-AFM image together with linear IeV curve reflecting Ohmicbehavior of electron transport, which was recorded at the indicatedlocation upon the fiber (Fig. 4i), indicate that the r-GO assemblyupon the polymer scaffold endows conductivity to the individual r-GO/fiber. Note the lack of electrical conductivity outside of the r-GO/fiber (IeV curve in Fig. 4ii). Current mapping experiment done onPEI/PVP fibers (Fig. S6, supplementary information) showednegligible contrast between the background and the fibers,providing further evidence that the conductivity in our samples isdue to the presence of r-GO on the fibers.

While the C-AFM analysis in Fig. 4 reveals electrical conductivityat the level of single r-GO/polymer fibers, we also examined themacroscopic electrical and optical properties of the r-GO/fibernetwork (Fig. 5). Fig. 5a presents currentevoltage (IeV) curves

Fig. 5. Electrical conductivity and optical transparency of r-GO/fiber films. (a) IeV curvesrecorded at a different location within the film. (b) Percentage transmittance curves of r-GO/PEI/PVP film. The dotted rectangle indicates the location of the r-GO/PEI/PVP film (magnifiedexternal voltage, confirming current passage through the r-GO/PEI/PVP film.

measured after evaporating gold contacts at a distance of 100 mm(through a shadow mask) on r-GO/polymer fiber film deposited ona glass substrate. The linear appearance of the IeV curves indicatescontinuous electron pathways and effective electric conductancethrough the fiber-template r-GO network (i.e. Ohmic properties).Linear IeV curves were recorded even in case of longer electrodedistances (Fig. S7, supplementary information). Electrical conduc-tivity of the films varied according to the density/coverage of fiberspresent. Films with greater concentrations of fibers exhibitedhigher conductivity due to more abundant electron transportpathways. Sheet resistances in the range of 10 kU/sq to 100 kU/sqwere recorded in films having different densities of r-GO/fibers.

Fig. 5b depicts transmittance spectra recorded for samplesprepared with different fiber concentrations. Representative pho-tographs of r-GO/fiber films spectra are provided in Fig. S8, sup-plementary information. The transmittance spectra and images inFig. 5b reveal that denser r-GO/fiber films gave rise to lowertransparency (e.g. 65% transmittance at 550 nm). Overall, werecorded transmittance of between 65% and 90% (at 550 nm) in ther-GO/PEI/PVP films having different fiber concentrations. Theseresults are ascribed to the fact that higher concentrations of thefibers attenuated light passage through the film and correspond-ingly limited transparency, as apparent in Fig. 5b. Table S2 corre-lates sheet resistance and transmittance in several films exhibitingdifferent fiber densities. To investigate the influence of the GOflakes' sizes upon conductivity, we carried out size separation of GO(dissolved in DMF:H2O at a 9:1 ratio) using centrifugation. SmallerGO flakes in the supernatant and bigger GO flakes in the residuewere separately incubated with the polymer fibers. Notably, whilethe fibers coated with the larger GO sheet fraction were electricallyconductive, samples prepared using the smaller GO sheet exhibited

recorded for r-GO/PEI/PVP films using electrode separation of 100 mm. Each curve wasPEI/PVP films having different fiber densities. (c) An electrical circuit containing a r-GO/view indicated). The red LED bulb is turned on (right photograph) upon application of

T.P. Vinod et al. / Carbon 95 (2015) 888e894 893

negligible conductivities. This result is attributed to the reportedobservations that smaller r-GO display low conductivity [49,50].

The new r-GO/polymer fiber films can be readily used forpractical uses. The photographs in Fig. 5c depict a closed electricalcircuit in which the r-GO/fiber film was positioned as a componentwithin the circuit. A switch was then used to turn on/off an externalvoltage and a red LED bulb (top right) was employed to reportpassage of current in the circuit. The right photograph in Fig. 5cnicely demonstrates that the bulb could be turned on by applyingvoltage to the electrical circuit (i.e. turning on an electric switch),confirming effective charge transport through the r-GO/fiber film.

4. Discussion

Wepresent a newapproach for self-assembly of graphene oxide,using a physical template comprising electrospun polymer fibers.The thrust of the assembly process is the electrostatic attractionbetween the negatively-charged GO sheets and positive residuesupon the polymer fibers. The fiber-template GO sheets can be easilyand effectively reduced, producing a network of conductive r-GOnetwork. Importantly, optical transparency of the r-GO films wasdirectly related to the specific attachment of GO to the polymer fiberframework (rather than randomly upon the film surface) e

generating abundant empty regions within the films which enablelight transmission.

While diverse methodologies for production of “bottom-up” (i.e.self-assembled) graphene films have been reported, most suchtechniques face significant hurdles, particularly with regard to thetradeoff between electrical conductivity and optical transparency[25,51e53]. Specifically, to attain good conductivity - i.e. usefulsheet resistances - multiple layers of r-GO have been usuallyrequired, resulting in significant reduction of light transmittance.For example, in a widely-cited publication reporting fabrication ofself-assembled r-GO films through spin coating and subsequentreduction, sheet resistances in the range of 103 U sq�1 wererecorded for samples of maximal 40% transparency at 550 nm[25,51]. Indeed, in that study sheet resistance increased byapproximately two orders-of-magnitude for films exhibitingtransmittance of around 90% [25,51]. This conundrum highlightsthe fact that conductivity can be improved by increasing thethickness of r-GO while severely compromising the optical trans-parency. The system reported here, however, partly surmounts thisconductivity/transparency tradeoff since graphene does not uni-formly coat the surface but rather the GO sheets only trace thepolymer fiber network, thereby generating abundant empty spacesthat enable light passage (i.e. optical transparency). Indeed, the useof electrospun polymer fibers as a physical template for GO as-sembly provides a means for conductivity/transparency modula-tion through varying the density of deposited fibers.

An additional notable feature of the fiber-template graphenefilms is the simplicity of the assembly process, both conceptuallyand technically. Essentially, the technique utilizes conventionally-synthesized GO sheets that are electrostatically-targeted to thesurface of polymer fibers produced through electrospinning e apowerful and easy to apply technology for generating large quan-tities of elongated fibers. Utilization of electrospinning for assem-bling the GO template points to potential applications of thetechnique for construction of transparent and conductive filmseven upon non-planar surfaces, since electrospun polymer fiberscan be deposited on such substrates [54,55].

As a consequence of the use of readily available building blocks,which furthermore involve coating only a fraction of the surfacearea, very small reagent quantities are needed. The GO solution, infact, can be reused several times as only a small concentration ofthe material is required for effective deposition upon the fibers.

Very few studies successfully integrated polymer fibers and gra-phene systems [22]. While electrospun polymer fibers have beenpreviously used as templates for graphene [39,41], such fibers havebeen used as physical scaffolds for GO, not as a platform for creatingtransparent, conductive films. Recent studies reporting the use ofelectrospun fibers for generating GO-based conductive filmsrequired chemical derivatization of the fiber surface [40] or GO[56], both constitute significantly more complex and elaborateschemes compared to the assembly approach presented here.

Several important aspects in the GO/polymer self-assemblyprocess should be highlighted. Specifically, the absence of PEI/PVP-associated peaks in the Raman and XPS data in Fig. 2, com-plemented by the AFM analysis in Fig. S3, indicate a relatively thickGO coating upon the polymer fibers, likely comprising severallayers of GO sheets. Such multi-layered coating might appear sur-prising since electrostatic attraction (which is inversely propor-tional to distance) is presumed to induce GO attachment. It hasbeen reported, however, that PEI, which comprises of the amine-displaying component within the electrospun polymer fibers,gave rise to diffusion-induced self-assembly of GO, similarly givingrise to multiple layers of GO upon the polymer surface [44]. Wehypothesize that this mechanism also plays a significant role in theassembly process of GO upon the polymer fibers.

Several experimental parameters were shown to affect theconductivity and transparency of the GO/polymer fiber films. Thedegree of GO coating, determined by both graphene concentrationand incubation time of GO with the polymer fibers, intimatelyaffected the conductivity, likely due to the extent of coating andthickness of the GO layer attached to the polymer surface.Furthermore, the conductivity/transparency profiles of the GOfilms (e.g. Fig. 5) highlight the significance of electrospinning time asa primary parameter. Specifically, the duration of electrospinningaffects fiber densitywhich constitutes a major factor responsible forboth enabling electron transport (i.e. inducing conductivity), aswell as optical transparency (as light can be transmitted throughthe empty spaces between the surface-deposited fibers).

5. Conclusions

In conclusion, we present a new methodology for directed self-assembly of graphene oxide (GO), using electrospun polymer fibersas a physical template. Driving the assembly process is the elec-trostatic attraction between the negatively-charged GO (specif-ically the carboxylic moieties displayed upon the GO edges) and thepositive surface charge upon the polymer fibers (through aminesand nitrogen-containing groups). Following chemical reduction ofthe fiber-templated GO assembly, we demonstrated formation of acontinuous network of reduced GO (r-GO) which traced the elec-trospun polymer fibers. The r-GO network exhibited electricalconductivity from the level of single r-GO-coated fibers to themacroscale e yielding large area conductive films. Such films werefurthermore transparent, on account of the fiber organization dis-playing sizeable empty surface areas enabling light transmittance.Importantly, both conductivity and transparency could be modu-lated by the density of deposited polymer fibers. The new template-directed self-assembly approach is easy to apply, does not requireexpensive reagents nor sophisticated instrumentation, and couldopen the way to varied electro-optic applications.

Acknowledgments

We are grateful to Dr. Leila Zeiri for help with the Raman mea-surements, Dr. Natalya Froumin for XPS measurements, and DvirElmakayes for helping with LED circuit.

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Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.carbon.2015.09.021.

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