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Electrochimica Acta 93 (2013) 44– 52

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

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lectrochemical characterization of reduced graphene oxide-coatedolyester fabrics

. Molina, J. Fernández, J.C. Inés, A.I. del Río, J. Bonastre, F. Cases ∗,1

epartamento de Ingeniería Textil y Papelera, EPS de Alcoy, Universitat Politècnica de València, Plaza Ferrándiz y Carbonell s/n, 03801 Alcoy, Spain

r t i c l e i n f o

rticle history:eceived 17 October 2012eceived in revised form 8 January 2013ccepted 15 January 2013vailable online xxx

eywords:raphene

a b s t r a c t

Reduced graphene oxide (RGO) coated fabrics were obtained by chemical reduction of GO on polyester(PES) fabrics. Conducting fabrics that have different applications were obtained by applying several layersof RGO. Electrochemical techniques not traditionally used for the characterization of these materials wereused to test their electrical and electrochemical properties. Electrochemical impedance spectroscopy wasused to measure the electrical properties. The resistance of the original PES was more than 1011 � cm2, butwhen coated with three RGO layers, the resistance decreased to 23.15 � cm2. Phase angles changed from90◦ for PES and PES-GO (capacitative behavior) to 0◦ for all the RGO coated samples (resistive behavior).

raphene oxideonducting fabricolyestercanning electrochemical microscopy

Electro-activity was measured by cyclic voltammetry (CV) and scanning electrochemical microscopy.An increase in electro-activity was observed when the inactive GO was reduced to RGO. With CV anincrease of electro-activity was observed with an increasing number of RGO layers. The contact betweenthe different RGO sheets is responsible for the electric conduction in the fabrics. The techniques usedshowed that with only one RGO coating, the contact between the RGO sheets is not good and more

assur

coatings were needed to

. Introduction

Graphene has attracted a great deal of attention during recentears due to its electronic, mechanical and thermal properties, asell as other exciting properties [1–8]. Different methods have

een used for the production of graphene [7–9], and mechanicalxfoliation was the first method used to produce graphene layers1]. However, the production by this method is not very effective,o other methods such as chemical vapor deposition or chemicalethods have been developed [7–9]. The use of graphene compos-

tes with other materials has been also proposed as an alternativeolution to the large-scale production of graphene-based materials10].

In this paper we have used a chemical method to obtainraphene-coated fabrics, where graphene oxide (GO) is reducedhemically to obtain reduced graphene oxide (RGO) on the sur-ace of the fabrics. Textile materials have several advantages overonventional materials such as film, such as having good flexibil-ty and mechanical properties. In addition, they are light materials

hat have a large surface area. This large surface area makes these

aterials good substrates on which to deposit other materials forifferent applications. For example fabrics have been coated with

∗ Corresponding author. Tel.: +34 966528412; fax: +34 966528438.E-mail addresses: fjcases@txp.upv.es, delgaran@doctor.upv.es (F. Cases).

1 ISE member.

013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2013.01.071

e good electrical and electrochemical properties.© 2013 Elsevier Ltd. All rights reserved.

conducting polymers to produce conductive fabrics [11,12], withsilver or TiO2 nanoparticles to produce antibacterial fabrics [13],with single walled nanotubes and MnO2 [14] or graphene and MnO2[15] to produce super-capacitors.

Graphene-coated fabrics have usually been obtained by placingthe fabrics in contact with solutions containing GO [16,17]. GOsheets are adsorbed on the surface of the fabrics due to the attrac-tion forces between the functional groups of the fabrics and theoxidized groups of the GO sheets. The process is similar to a dyingprocess where an adsorption process occurs [16]. Once the fabricis coated with GO, it is then reduced to RGO using reducing agentssuch as sodium hydrosulfite [16] or hydrazine [17]. GO coated fab-rics have also generated interest due to the photo-catalytic activityof GO and also to its antibacterial activity [18]. Directly coatedgraphene fabrics have also been obtained in other works refer-enced in the bibliography [15]. Graphene fabrics have also beenproduced by depositing graphene by chemical vapor deposition ona Cu mesh, after which the Cu mesh was dissolved by an acid solu-tion and graphene fabrics were obtained [19]. Regarding graphenefibers, different methods such as wet spinning [20] or obtaininghydrothermally [21] using GO as a precursor material have beenused in the past. The fibers formed were later reduced chemically[20] or thermally [21] to obtain graphene fibers.

The fibers used to deposit graphene or GO have been polyarylate[16] or cotton [17]. Polyester (PES) is one of the most used fibers inindustry so it would be of interest to use it to study the production ofgraphene-coated fabrics. In addition, the presence of polar groups

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n its structure makes this family attractive because the adhesionf GO is better in fibers with polar groups (C O and C O groups areresent in PES).

In the present work, graphene-coated fabrics were obtained byeducing GO on the surface of the fabrics. Traditional characteri-ation techniques such as Fourier transform infrared spectroscopyith total attenuated reflection (FTIR-ATR) were used for chem-

cal characterization, while scanning electron microscopy (SEM)as used to observe the morphology of the coatings and the

amples. There is a lack of characterization of these materialsy electrochemical techniques, so electrochemical techniques notsually used for the characterization of these materials were used

n this work. Electrochemical impedance spectroscopy (EIS) wassed for the measurement of the electrical resistance (� cm2)nd the surface resistivity (�/�) of the fabrics and also to obtainhe phase angle (◦), which gives an indication of the insulat-ng/conducting behavior of the fabrics. Scanning electrochemical

icroscopy (SECM) was used to measure the electrochemical activ-ty by means of approach curves. SECM is a relatively novel (1989)nd powerful technique that is becoming more popular amongesearchers [22–24]. Its applications range for example from thetudy of corrosion of metals [25] to the study of biological sys-ems [23]. Cyclic voltammetry (CV) was also used to measure thelectro-activity of these materials.

. Experimental

.1. Reagents and materials

All reagents used were of analytical grade. Sodium dithion-te (Na2S2O4) and sulphuric acid (H2SO4) were supplied by

erck. Hexaammineruthenium (III) chloride (Ru(NH3)6Cl3), 98%nd potassium chloride (KCl), were used as received from Acrosrganics and Merck respectively.

Monolayer GO powders were supplied by Nanoinnova Tech-ologies S.L. (Spain).

PES fabrics were supplied by Viatex S.A. and their characteris-ics were: fabric surface density, 140 g m−2; warp threads per cm,0 (warp linear density, 167 dtex); weft threads per cm, 60 (weft

inear density, 500 dtex). These are specific terms used in the field ofextile industry and their meaning can be consulted in a textile glos-ary [26]. The estimated diameter of polyester fibers was around7 �m (estimated employing SEM). The effective area of the fabricas also estimated employing the values of density (1.37 g cm−3),

abric surface density and diameter of the fibers. The value obtainedas around 24 cm2 per geometrical cm2 of the fabric.

Where necessary, solutions were deoxygenated by bubblingitrogen (N2 premier X50S). Ultrapure water was obtained fromn Elix 3 Millipore-Milli-Q Advantage A10 system with a resistivityear to 18.2 M� cm.

.2. Synthesis of RGO on PES fabrics

RGO coated fabrics were obtained with the same method usedy Fugetsu et al. [16]. Solutions of 3 g L−1 of GO were prepared byixing monolayer GO powders with water in an ultrasound bath

or 30 min. Then the PES fabric was placed in contact with the GOolution for 30 min to allow the dyeing of the fabric. Fabrics werellowed to dry for 24 h. After this time, they were placed for 30 minn a solution containing sodium dithionite at approximately 90 ◦Co perform the reduction of GO to RGO. Finally, the fabrics were

ashed several times with ultrapure water. RGO powders from

olution were also filtered to perform FTIR-ATR measurements inrder to assure the reduction of GO to RGO. Several samples with aifferent number of RGO coatings (1, 2, 3, 4) were obtained (PES-G1,

ca Acta 93 (2013) 44– 52 45

PES-G2, PES-G3, PES-G4) repeating the same procedure mentionedabove. One sample coated with GO (PES-GO) was also obtained forcomparison of the different results.

2.3. FTIR-ATR

FTIR-ATR with horizontal mono-rebound attenuated totalreflection was performed with a Nicolet 6700 Spectrometerequipped with deuterated triglycine sulfate detector. An accessorywith pressure control was used to equalize the pressure in thedifferent solid samples. A prism of ZnSe was used. Spectra werecollected with a resolution of 4 cm−1 and 100 scans were averagedfor each sample. GO and RGO powders were characterized. PES fab-rics uncoated and coated with GO and RGO were also characterizedby FTIR-ATR.

2.4. SEM

A Jeol JSM-6300 scanning electron microscope was used toobserve the morphology of the samples. SEM analyses were per-formed using an acceleration voltage of 10 kV. Samples for SEMmeasurements were coated with Au using a sputter coater Bal-TecSCD 005.

2.5. Electrical measurements

An Autolab PGSTAT302 potentiostat/galvanostat was used toperform EIS analyses. EIS measurements were performed in the105–10−2 Hz frequency range. The amplitude of the sinusoidalvoltage used was ±10 mV. Measurements were carried out in atwo-electrode arrangement. Two types of configuration were usedto carry out the measurements. In the first one, the sample waslocated between two round copper electrodes (A = 1.33 cm2), whilein the second configuration, two rectangular copper electrodes(0.5 cm × 1.5 cm) separated by 1.5 cm and pressed on the fabricsample were used. The measured area of the fabric with this config-uration was a square of 1.5 cm so the measured impedance modulus(�) was equal to the surface resistivity (�/�) [27,28].

2.6. CV

An Autolab PGSTAT302 potentiostat/galvanostat was used toperform CV measurements of the RGO coated fabrics in 0.5 M H2SO4solutions. The conducting fabric sample was located between twoTi plates to connect the sample with the potentiostat/galvanostat.The measurements were performed in a three-electrode arrange-ment. The counter electrode used was made of stainless steel; thepre-treatment consisted of polishing, degreasing with acetone in anultrasonic bath and washing with water in the ultrasonic bath. Theworking electrode was made by cutting a strip of the RGO coatedconducting fabric, the exposed area of the fabric to the solutionwas 1 cm2. Potential measurements were referred to Ag/AgCl (3 MKCl) reference electrode. Oxygen was removed from solution bybubbling nitrogen gas for 10 min and then a N2 atmosphere wasmaintained during the measurements. The ohmic potential dropwas measured and introduced in the Autolab software (GPES). Themeasurements were done between −0.2 V and +0.8 V. The charac-terization by means of CV has been taken at different scan rates asprevious authors have corroborated the influence of this param-eter on the electrochemical response obtained [11,27]. The scanrates used were 50 mV s−1, 5 mV s−1 and 1 mV s−1.

2.7. SECM

SECM measurements were carried out with a Sensolyticsscanning electrochemical microscope. The three-electrode cell

4 chimica Acta 93 (2013) 44– 52

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onfiguration consisted of a 100-�m-diameter Pt SECM-tip, a Ptire auxiliary electrode and an Ag/AgCl (3 M KCl) reference elec-

rode. The solution selected for the measurements was an aqueousolution of 0.01 M Ru(NH3)6Cl3 (redox mediator) with 0.1 M KCl. Allhe experiments were carried out in an inert nitrogen atmosphere.he substrates were the different fabrics obtained, and theseubstrates were then glued to microscope slides with epoxy resin.he dimensions of the samples employed were 0.5 cm × 0.5 cm0.25 cm2). The SECM-tip was moved in z-direction and the tip cur-ent was recorded to obtain the approach curves. Approach curvesive us an indication of the electro-activity of the surface. Theseurves were compared to the theoretical ones (positive and nega-ive feedback models). Experimental approach curves were shiftedo adjust with the theoretical ones to calculate the real distanceetween the sample and the SECM-tip. After that the electrode wasositioned at the desired height to carry out the constant-heightECM images in the plane xy. The substrate surfaces in all theeasurements were at their open circuit potential (OCP).

. Results and discussion

.1. FTIR-ATR

FTIR-ATR measurements were performed to characterize GOowders as well as the RGO powders obtained after chemicallyeducing GO. Fig. 1 shows the spectra of GO and RGO. GO pow-ers present different bands arising from oxidized groups. Theand around 1720 cm−1 was attributed to stretching vibrationsrom C O [29–31]. The peak at 1613 cm−1 was attributed to C Crom unoxidized sp2 bonds [29–31]. The band at 1220 cm−1 wasssigned to C OH stretching vibrations [29], and the band around

050 cm−1 arises from C O stretching vibrations [29,31]. When GOas reduced to RGO, all the peaks arising from oxidized groupsere substantially reduced, indicating a reduction of the oxygen

ontent in the sample, although they were not completely removed.

Fig. 2. Micrographs of (a) PES-1G (×10,000), (b) PES-1G (×

Fig. 1. FTIR-ATR spectra of GO and RGO powders. Resolution 4 cm−1, 100 scans.

The reduction of oxygen below a certain level is not really feasi-ble because it is difficult to remove remaining hydroxyl groups [9].RGO powders showed a band at 1562 cm−1; this band has beenattributed to skeletal vibration of graphene sheets [32]. When GOand RGO were deposited on PES fabrics, no substantial variationwas observed in the original spectra of PES; only a slight decreaseof the different bands of PES was observed (figures not shown). Themajority of bands of GO and RGO coincide with that of PES and thedifferent contributions cannot be properly discerned.

3.2. SEM

SEM was used to observe the morphology of the coatingsobtained. Fig. 2 shows the different micrographs obtained for thedifferent RGO coated fabrics. Fig. 2a and b shows micrographs ofthe PES-1G sample. In these two micrographs RGO sheets deposited

on PES fibers can be seen. In general, it is very difficult to observethese sheets; however the folds created during the deposition pro-cess in some of these sheets help to locate them. The dimensionsof the RGO sheets are about 4–6 �m in length. An increase in the

10,000), (c) PES-4G (×100) and (d) PES-4G (×2000).

J. Molina et al. / Electrochimi

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ig. 3. Bode plots for PES, PES-GO, PES-1G, PES-2G, PES-3G and PES-4G. Sampleocated between two copper electrodes. Frequency range from 105 Hz to 10−2 Hz.

umber of coatings (samples PES-2G, PES-3G, PES-4G) producedn increase in the RGO content, however it was more difficult toocate the RGO sheets since they could not be distinguished fromhe fibers. Fig. 2c and d shows micrographs of the PES-4G sample,here the presence of some particles can be observed on the sur-

ace of the fibers. The values of the impedance modulus obtainedith the electrical measurements (Section 3.3) indicated that theeposition of 4 coatings of RGO produced a decrease of more than

orders of magnitude when compared with PES. So the fibers with coatings would be completely coated with RGO, despite our noteing able to observe them. Fig. 2c shows also the structure of theabric, with the warp and weft at different heights. The estimatediameter of the fibers by SEM micrographs was around 17 �m.

.3. Electrical measurements

EIS was used to measure the electrical resistance (� cm2) of theifferent samples obtained as well as their phase angle (◦); the lat-er measurement gives an indication of the insulating/conductingehavior of the fabrics. Fig. 3 shows the Bode plots for the dif-erent fabrics measured. Samples of PES and PES coated with GOere also measured to compare the different values obtained. Con-ucting fabrics with 1, 2, 3 and 4 RGO deposition processes wereeasured.In Fig. 3a, the impedance modulus (|Z|) at the different frequen-

ies for the different samples can be observed. PES fabrics presentalues of |Z| higher than 1011 � cm2 for low frequencies. Whenhe sample was coated with GO, |Z| was not substantially modi-ed and similar values were obtained. GO is an insulating material

ue to the disrupted sp2 bonding networks [33], however whenhe sample of PES coated with GO was reduced, a decrease of |Z|f about seven orders of magnitude was obtained (26,014 � cm2

or 0.01 Hz). Fig. 4 shows the dependence of |Z| on ac frequency for

ca Acta 93 (2013) 44– 52 47

the different samples so that the changes can be properly observed.When another layer of RGO was deposited on the fabric (PES-2G), |Z|lowered to 700 � cm2 for 0.01 Hz. The third layer of RGO producedalso a decrease of |Z| to 23.15 � cm2 for 0.01 Hz. However the fourthcoating did not produce a decrease in |Z| and around 29.4 � cm2 for0.01 Hz was measured. As mentioned previously, Fig. 4 shows thedependence of |Z| on ac frequency for the different samples of PEScoated with RGO. The values of |Z| can be observed with the vary-ing frequency. It is worth mentioning that the values of |Z| increasewith the decreasing frequency for the PES-1G sample. This wouldindicate a poor contact between the RGO sheets (slight capacitativebehavior). On the other hand, the samples with more coatings ofRGO (PES-2G, PES-3G, PES-4G) showed a decrease of |Z| with thedecreasing frequency, this would indicate a slight inductive behav-ior. From the first to the third coating there is an increase of theconductivity of the fabrics due to the improved contact between theRGO sheets. In the first coating, part of the RGO layers may not be incontact so that electrical conduction is more difficult. Another layerof RGO allows the contact between the isolated sheets. CV measure-ments showed similar results, confirming the better contact withmore coatings of RGO (Section 3.4). After three deposits the limitof conductivity was reached and more layers did not produce anincrease in the conductivity.

Fig. 3b shows the data for the phase angle at the different fre-quencies. PES presents a phase angle of about 90◦, a typical valueof an insulating material that acts as a capacitor (data at low fre-quencies is not presented since noise due to the large values ofimpedance modulus was observed). For the sample of PES coatedwith GO, the same phase angle was obtained indicating no sub-stantial modification of its electrical properties by the GO layer,the PES-GO sample also acted as an electrical insulator. However,for all the samples coated with RGO (PES-1G, PES-2G, PES-3G andPES-4G), 0◦ phase angles were obtained in the majority of the fre-quency range, indicating that the fabrics behaved like conductingmaterials, showing a resistive behavior.

The values of surface resistivity (�/�) were obtained withthe second electrode configuration; two rectangular copper elec-trodes (0.5 cm × 1.5 cm) separated by 1.5 cm and pressed on thefabric sample. The values of surface resistivity obtained were1.6 × 107 �/� (PES-1G), 2.94 × 105 �/� (PES-2G), 11.06 × 103 �/�(PES-3G) and 20.85 × 103 �/� (PES-4G). The tendency obtainedwas similar to that observed with the first electrode configurationwith a decrease of the surface resistivity until the third coating pro-cess. These values of surface resistivity would make these fabricsappropriate as antistatic materials. Static charging on the surface ofthe fabrics is excluded with surface resistivity below 5 × 109 �/�[34], so even the sample coated with one layer of RGO would beadequate for this purpose.

3.4. CV

Fig. 5 shows the voltammograms obtained for PES-1G and PES-3G samples. Fig. 5a shows the voltammogram obtained for thePES-1G sample. With only one deposition process the voltammo-gram obtained showed a linear behavior (indicating a resistiveresponse) with a current density value of about −1.4 nA cm−2 at−0.2 V for 50 mV s−1. Different scan rates were also used to test theelectro-activity of the fabrics (50, 5 and 1 mV s−1), obtaining thesame resistive voltammogram for all the scan rates used. Fig. 5bshows the voltammograms obtained for the PES-3G sample. Thehigher scan rate (50 mV s−1) produced a current density of around−30 �A cm−2 at −0.2 V. The current density obtained with 3 depo-

sition processes (PES-3G) was more than 4 orders of magnitudehigher than that obtained for the sample with only one deposi-tion process (PES-1G). In this case it can be seen that the scan ratehas some influence on the electrochemical behavior. With the scan

48 J. Molina et al. / Electrochimica Acta 93 (2013) 44– 52

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ig. 4. Magnified dependence of the impedance modulus on ac frequency for (alectrodes. Frequency range from 105 Hz to 10−2 Hz.

ate of 1 mV s−1, the form of the voltammogram was more similaro voltammograms obtained for graphene [35]. Also the electricalharge and current densities of the voltammogram increased withhe decreasing scan rate. This behavior has not been observed in theibliography since the normal trend is an increase of the charge andurrent density with the increasing scan rate [35,36]. The expla-ation for this fact could be the nature of the sample. Samples ofraphene are normally studied in isolation [35,36] or are depositedn conducting substrates such as metals where the charge trans-er between the metal and graphene is instantaneous. However inonducting fabrics, RGO is deposited on PES, which is an insulatingaterial. So charge transfer has to be produced on the RGO sheets.

he electrical contact from the top to the bottom of the samples produced through the contact of the different RGO sheets. Forxample for a 1 cm length sample, if the average size of the RGOheets were 10 �m (the real size is lower as has been observed inEM micrographs), at least 1000 RGO sheets would be needed to

roduce the electrical contact between the top and the bottom ofhe fabric (the case for a monolayer smooth sample). This numberncreases in the case of the fabrics which present a 3D structure,

here different fibers and yarns compose the fabrics. The charge

ig. 5. Cyclic voltammograms of (a) PES-1G without ohmic compensation, (b) PES-3G whird scan for all measurements. 0.5 M H2SO4 (pH ∼ 0). Scan rates used: 50, 5 and 1 mV s−

1G, (b) PES-2G, (c) PES-3G and (d) PES-4G. Sample located between two copper

transfer is slow since it has to proceed through the RGO sheets. Ifthe scan rate is too fast (50 mV s−1), there is no sufficient time toproduce the oxidation and reduction of RGO sheets and a resistiveresponse was obtained. When the scan rate decreased to 5 mV s−1,there was more time to produce the oxidation and reduction of theRGO sheets and this is why there was an increase of the electri-cal charge and current density in the voltammograms. The lowestscan rate (1 mV s−1) produced a higher increase of the electricalcharge and current densities reached since there was even moretime to produce the oxidation and reduction of RGO sheets. Withonly one deposition process (PES-1G), it was not possible to observethe redox processes even with the lowest scan rate (1 mV s−1). Thiswas attributed to the fact that there were not enough RGO sheets inthe coating to produce a proper contact between them and only thecharacteristic resistive response was observed. The results obtainedin CV were in agreement with those observed in EIS, where the sametrend was observed.

Fig. 5c shows the voltammograms obtained for the PES-3G sam-ple at different scan rates, however in this case the ohmic resistancewas measured and compensated. It was observed that this compen-sation is necessary for materials that have an ohmic resistance such

ithout ohmic compensation and (c) PES-3G with ohmic resistance compensation;1.

chimica Acta 93 (2013) 44– 52 49

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of PES sample. We have to take into account that GO is also an insu-lating material due to the disrupted sp2 structure [33] as we haveseen previously in EIS measurements. The negative feedback model

J. Molina et al. / Electro

s conducting polymer-coated fabrics [11,27]. PES-1G sample haduch a high resistance that it was not possible to compensate it, andor this reason it was not compensated in Fig. 5a. Fig. 5b was alsoone without compensation to compare it with Fig. 5a under theame conditions. Comparing the voltammograms without (Fig. 5b)nd with compensation (Fig. 5c), an increase in the electrical chargend the current density compared to Fig. 5b can be seen.

.5. SECM

Approach curves were obtained by SECM measurements to testhe electro-activity of the conducting fabrics. Approach curves given indication of the electro-activity of the sample surface. TheECM-tip is negatively polarized to reduce the oxidized (Ox) formf the redox mediator at a diffusion controlled rate (i∞). The reduc-ion current obtained (i) is measured at the SECM-tip. In our casee used Ru(NH3)6

3+ (Ox) as redox mediator and it was reducedo Ru(NH3)6

2+ (Red) at the SECM-tip at a potential of −0.4 V (vs.g/AgCl 3 M KCl). There can be different outcomes, depending on

he conductivity of the substrate and the distance between theECM-tip and the substrate:

When the SECM-tip is sufficiently far from the substrate there isno impediment for the diffusion of oxidized species (Ox) to bereduced on the surface of the SECM-tip, the current measured inthis case is the diffusion current (i∞).If the surface is non conductive, when the SECM-tip approachesthe substrate there is a hindrance to the diffusion of oxidizedspecies (Ox) to be reduced on the tip. This is caused by the insu-lating substrate and causes a decrease of the current measured,in this case the current measured i < i∞. This situation is knownas negative feedback [22].On the other hand, if the electrode is conductive, when the elec-trode approaches the surface, there is an increase in oxidizedspecies (Ox) of the mediator. The surface of the conducting mate-rial is able to oxidize the reduced form (Red) of the mediator. Thiscauses an increase in oxidized species (Ox) and the consequentincrease of the reduction current measured at the SECM-tip, inthis case i > i∞. This case is known as positive feedback [22].

n approach curves, the normalized current registered at the SECM-ip (I) is represented vs. the normalized distance (L). The normalizedurrent is defined as follows: I = i/i∞. i is the current measured atach normalized distance L. i∞ is the diffusion current defined as

∞ = 4nFDaC, in which n is the number of electrons involved in theeaction, F is the Faraday constant, D is the diffusion coefficient, as the radius of the SECM-tip and C is the concentration of the reac-ant. The normalized currents depend on RG (RG = Rg/a, where Rgs the radius of the insulating glass surrounding the Pt tip of radiusa”) and the normalized distance L, where L = d/a (d is the SECM-ip–substrate separation). The RG of the SECM-tip used in this workas RG ≥ 20. According to Rajendran et al. [37], Pade’s approxima-

ion gives a close and simple equation with less relative error forll distances and is valid for RG > 10. The approximate expressionf the steady-state normalized current assuming positive feedbackor finite conductive substrate together with finite insulating glasshickness is:

C =[

1 + 1.5647L + 1.316855

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](1)

he selection of the expression for the normalized tip current

ssuming negative feedback was based on the equation for a RG = 20nd L range 0.4–20 [38]:

INS =[

10.3554 + (2.0259/L) + 0.62832 × exp(−2.55622/L)

](2)

Fig. 6. Cyclic voltammogram obtained with a 100 �m Pt SECM-tip in a 0.01 MRu(NH3)6

3+ and 0.1 M KCl solution; scan rate 50 mV s−1.

Approach curves predicted for pure positive and negative feedbackcalculated from Eqs. (1) and (2) have been also included in the dif-ferent figures to compare the empirical data with the theoreticalmodels.

A 100 �m Pt SECM-tip held at a potential of −0.4 V (vs. Ag/AgCl3 M KCl) was used as the working electrode. According to thevoltammograms obtained with the SECM-tip in the working solu-tion (Fig. 6), this potential was selected to reduce the oxidizedform of the mediator, Ru(NH3)6

3+, at a diffusion-controlled rate.The approach (I–L) curves obtained for PES and PES-GO are shownin Fig. 7. As can be seen, as the SECM-tip approaches the surface ofthe PES sample, there is a decrease in the normalized current (I < 1),indicating negative feedback. PES is an insulating material and is notable to reoxidize the reduced form of the mediator. GO approachcurves have not been obtained in previous works to the best of ourknowledge. When GO was deposited on PES fabrics similar behav-ior was observed. Three measurements obtained in different partsof the fabrics are shown and in all of them negative feedback wasobtained. The deposit of GO did not significantly affect the nature

Fig. 7. Approaching curves for PES (· · ·), PES-GO (continuous lines) and theoreticalnegative feedback model (�). Obtained with a 100 �m diameter SECM-tip in 0.01 MRu(NH3)6

3+ and 0.1 M KCl. The tip potential was −400 mV (vs. Ag/AgCl) and theapproach rate was 10 �m s−1.

50 J. Molina et al. / Electrochimica Acta 93 (2013) 44– 52

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ig. 8. Approaching curves for (a) PES-1G (continuous lines), theoretical positive fe�). Obtained with a 100 �m diameter Pt SECM-tip in 0.01 M Ru(NH3)6

3+ and 0.1 M

s also presented for comparison of the experimental curves withhe theoretical one.

Fig. 8 shows different approach curves obtained in differentarts of the fabric for PES-1G (a) and PES-4G (b) samples. As can beeen in Fig. 8a, the behavior changes completely from PES-GO. Inhis case when the SECM-tip approaches the surface of the sample,here is an increase in the normalized current (I > 1). This indicateshat the sample is electro-active, since the sample is able to reoxi-ize the reduced form of the redox mediator. Averaged values ofositive feedback around 1.3 were obtained for L = 0.6. In Fig. 8bnalyses on the sample of PES-4G are shown. In this case posi-ive feedback was also obtained and averaged values of 1.4 werebtained for L = 0.6. The positive feedback model is also presented

or comparison with the theoretical model. There is little differencehen the sample is coated 1, 2, 3 or 4 times with the same procedure

PES-1G, PES-2G, PES-3G, PES-4G). In this case the experimentsere done at OCP (sample is not polarized), so only the degree

ig. 9. 3D (a and c) and 2D (b and d) constant height SECM images of PES-1G sample (a anaken with a 100 �m diameter Pt SECM-tip, in 0.01 M Ru(NH3)6

3+ at a constant height. Thf x and y lines were 1500 �m × 1500 �m with increments of 50 �m.

k model (�) and (b) PES-4G (continuous lines), theoretical positive feedback modelhe tip potential was −400 mV (vs. Ag/AgCl) and the approach rate was 10 �m s−1.

of coverage of the sample by RGO can have influence on the elec-trochemical response obtained. The electrical contact between thedifferent RGO sheets does not play a role in this case, however, it hadsignificant effect on the electrical conductivity measured by EIS andthe electro-activity measured by CV as was commented previously.There has been little previous work on graphene materials usingSECM [39–41]. Similar values of positive feedback were obtainedfor graphene obtained by vapour deposition using a FeMeOH redoxmediator, which has a high heterogeneous electron transfer rate[39]. Future work is currently being carried out to study theelectrochemical activity of the samples by SECM in polarizationconditions.

The main reason why the 100 �m diameter tip was selected

to carry out the measurements was the irregular topography ofthe fabric. The employment of Pt SECM-tips of 100 �m of diam-eter allows to be positioned at a distance sufficiently high (andinside the influence of the field) to perform an array scan and at

d b) and PES-GO sample (c and d). 0.25 cm2 geometrical area sample; images weree tip potential was −400 mV; the scan rate was 200 �m s−1 in comb mode; lengths

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J. Molina et al. / Electro

he same time avoid the impact of the SECM-tip with the weft. Theeasured thickness of the fabric (with a micrometer) was 170 �m,

o the difference of height between the warp and weft would beround 57 �m (170 �m/3). For instance, if we take the positiveeedback model, to produce a sufficiently intense current signaln the approach curve (I around 1.4), values of L < 1 are needed.f we had employed a 10 �m diameter tip, the distance betweenhe tip and the sample would be lower than 10 �m. This small gapould produce the impact between the tip and the fabric. Even smallariations of height between the different parts of the fabric whenluing the sample to the glass slide could cause the tip to touchhe sample. In addition, if we had employed lower diameter tips,e would have not been able to detect the signal produced by thearp, due to the big difference of height between warp and weft.

hen the 100 �m diameter SECM-tip was the best solution for thetudy of the fabrics.

One main application of the SECM technique is scanning sur-aces to obtain 2D and 3D images of the electrochemical activity ofhe surface of the samples [22,39]. To obtain these representationshe surface of the sample is scanned at a constant height within thenfluence of the substrate at an initial height of 40 �m above theample. Our aim was not to characterize individually the fibers ofhe fabric (the diameter of the SECM-tip was 100 �m, so fibers can-ot be resolved properly since its diameter is around 17 �m). Theurpose was to observe if RGO was well distributed on the surface ofhe fabrics. And finally make a comparison between samples coatedith RGO and GO to see the different behavior (positive feedback

s. negative feedback, respectively).Fig. 9a and b shows the 3D and 2D representation, respectively,

f the electro-activity of the PES-1G sample. As can be seen, allhe parts of the fabric present normalized current values I > 1 (pos-tive feedback) indicating that the sample is completely coated

ith RGO. The difference between darker and lighter zones can bettributed to the morphology that the fabric presents, with zonesf different height (warp and weft). In this case, the topography ofhe fabric has more influence than local differences of electroactiv-ty. Fig. 10 shows a superposition of the 2D SECM array and a SEM

icrograph for a PES-1G sample. As can be seen, the darker zonesoincide with the weft (zones with higher elevation) and the lighterones with the warp (zones with lower height). The difference isbvious if we compare the results of PES-1G sample with the sam-le of PES-GO (Fig. 9c and d). In the latter case, normalized currentalues I < 1 (negative feedback) can be seen on the whole fabric. This

ndicates the insulating nature of the sample. In the case of PES-GO,he more elevated parts of the fabric (weft) produce the lowest val-es of negative feedback since they are closer to the SECM-tip andhe diffusion of redox species is hindered.

ig. 10. Superposition of the 2D SECM array and a SEM micrograph for a PES-1Gample.

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ca Acta 93 (2013) 44– 52 51

4. Conclusions

Polyester (PES) fabrics were coated with graphene oxide (GO)and later it was reduced through a chemical method to obtainreduced graphene oxide (RGO). Samples with a different numberof coatings were obtained and analyzed. Fourier transform infraredspectroscopy showed the diminution of the bands attributed tooxidized groups after reducing GO to RGO. Scanning electronmicroscopy allowed the observation of sheets of RGO deposited onPES fibers, although in general it was very difficult to observe thecoatings formed. Electrochemical impedance spectroscopy (EIS),cyclic voltammetry (CV) and scanning electrochemical microscopy(SECM) showed the effective reduction of GO to RGO. With EISa decrease of the electrical resistance of about 7 orders of mag-nitude was observed when the fabric of PES-GO was reduced toPES-1G. EIS measurements have also shown a progressive reduc-tion in electrical resistance and surface resistivity with the numberof coatings. With three coatings the minimum electrical resistance(23.15 � cm2) and surface resistivity (1.1 × 104 �/�) were reached.More coatings did not produce a significant improvement in theelectrical properties. The surface resistivity obtained with only oneRGO coating (1.6 × 107 �/�) would be adequate for antistatic pur-poses where values below 5 × 109 �/� are needed. As in EIS, CV alsoshowed an increase in the electro-activity with the number of RGOlayers. With only one RGO coating, the contact between the dif-ferent RGO sheets is not satisfactory and more layers are needed toimprove this contact and obtain a material with good electrical andelectrochemical properties. Scan rate is also a key parameter in thecharacterization of these materials and only low scan rates allowthe best observation of redox processes. Approach curves and mapsof electro-activity obtained by SECM showed the clear differencebetween levels of electro-activity of PES-GO and PES-RGO sam-ples. The electrochemical techniques used for the characterizationindicate the need to apply several RGO coatings during the synthe-sis to obtain materials with the best electrical and electrochemicalproperties.

Acknowledgements

The authors wish to thank the Spanish Ministerio de Cienciae Innovación (contracts CTM2010-18842-C02-02 and CTM2011-23583) and Universitat Politècnica de València (Vicerrectorado deInvestigación PAID-06-10 contract 003-233) for their financial sup-port. J. Molina is grateful to the Conselleria d’Educació, Formació iOcupació (Generalitat Valenciana) for the Programa VALi+D post-doctoral fellowship. A.I. del Río is grateful to the Spanish Ministeriode Ciencia y TecnologRía for the FPI fellowship.

References

[1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V.Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon film, Sci-ence 306 (2004) 666.

[2] A.K. Geim, K.S. Novoselov, The rise of graphene, Nature Materials 6 (2007) 183.[3] A.K. Geim, Graphene: status and prospects, Science 324 (2009) 1530.[4] A.H. Castro Neto, F. Guinea, N.M.R. Peres, K.S. Novoselov, A.K. Geim, The

electronic properties of graphene, Reviews of Modern Physics 81 (2009)109.

[5] H.B. Heersche, P. Jarillo-Herrero, J.B. Oostinga, L.M.K. Vandersypen, A.F. Mor-purgo, Bipolar supercurrent in graphene, Nature 446 (2007) 56.

[6] M.J. Allen, V.C. Tung, R.B. Kaner, Honeycomb carbon: a review of graphene,Chemical Reviews 110 (2010) 132.

[7] C. Soldano, A. Mahmood, E. Dujardin, Production, properties and potential ofgraphene, Carbon 48 (2010) 2127.

[8] V. Singh, D. Joung, L. Zhai, S. Das, S.I. Khondaker, S. Seal, Graphene based mate-

rials: past, present and future, Progress in Materials Science 56 (2011) 1178.

[9] S. Park, R.S. Ruoff, Chemical methods for the production of graphenes, NatureNanotechnology 4 (2009) 217.

10] J.R. Potts, D.R. Dreyer, C.W. Bielawski, R.S. Ruoff, Graphene-based polymernanocomposites, Polymer 52 (2011) 5.

5 chimi

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

2 J. Molina et al. / Electro

11] J. Molina, M.F. Esteves, J. Fernández, J. Bonastre, F. Cases, Polyaniline coatedconducting fabrics. Chemical and electrochemical characterization, EuropeanPolymer Journal 47 (2011) 2003.

12] J. Molina, A.I. del Río, J. Bonastre, F. Cases, Chemical and electrochemical poly-merisation of pyrrole on polyester textiles in presence of phosphotungstic acid,European Polymer Journal 44 (2008) 2087.

13] S. Gowri, L. Almeida, T. Amorim, N. Carneiro, A.P. Souto, M.F. Esteves, Poly-mer nanocomposites for multifunctional finishing of textiles – a review, TextileResearch Journal 80 (2010) 1290.

14] L. Hu, M. Pasta, F. La Mantia, L. Cui, S. Jeong, H.D. Deshazer, J.W. Choi, S.M.Han, Y. Cui, Stretchable, porous, and conductive energy textiles, Nano Letters10 (2010) 708.

15] G. Yu, L. Hu, M. Vosgueritchian, H. Wang, X. Xie, J.R. McDonough, X. Cui,Y. Cui, Z. Bao, Solution-processed graphene/MnO2 nanostructured textilesfor high-performance electrochemical capacitors, Nano Letters 11 (2011)2905.

16] B. Fugetsu, E. Sano, H. Yu, K. Mori, T. Tanaka, Graphene oxide as dyestuffs forthe creation of electrically conductive fabrics, Carbon 48 (2010) 3340.

17] W. Gu, Y. Zhao, Graphene modified cotton textiles, Advanced MaterialsResearch 331 (2011) 93.

18] K. Krishnamoorthy, U. Navaneethaiyer, R. Mohan, J. Lee, S.J. Kim, Grapheneoxide nanostructures modified multifunctional cotton fabrics, AppliedNanoscience 2 (2012) 119.

19] X. Li, P. Sun, L. Fan, M. Zhu, K. Wang, M. Zhong, J. Wei, D. Wu, Y. Cheng,H. Zhu, Multifunctional graphene woven fabrics, Scientific Reports 2 (2012)395.

20] Z. Xu, C. Gao, Graphene chiral liquid crystals and macroscopic assembled fibres,Nature Communications 2 (2011) 571.

21] Z. Dong, C. Jiang, H. Cheng, Y. Zhao, G. Shi, L. Jiang, L. Qu, Facile fabricationof light, flexible and multifunctional graphene fibers, Advanced Materials 24(2012) 1856.

22] P. Sun, F.O. Laforge, M.V. Mirkin, Scanning electrochemical microscopy in the21st century, Physical Chemistry Chemical Physics 9 (2007) 802.

23] M.V. Mirkin, B.R. Horrocks, Electroanalytical measurements using the scanningelectrochemical microscope, Analytica Chimica Acta 406 (2000) 119.

24] A.L. Barker, M. Gonsalves, J.V. Macpherson, C.J. Slevin, P.R. Unwin, Scanningelectrochemical microscopy: beyond the solid/liquid interface, Analytica Chim-ica Acta 385 (1999) 223.

25] R.M. Souto, J.J. Santana, L. Fernández-Mérida, S. González, Sensing electro-

chemical activity in polymer coated metals during the early stages of coatingdegradation – effect of the polarization of the substrate, Electrochimica Acta56 (2011) 9596.

26] Complete textile glossary, available from: http://www.celaneseacetate.com/textile glossary filament acetate.pdf, 2001 (accessed 15.10.12).

[

ca Acta 93 (2013) 44– 52

27] J. Molina, J. Fernández, A.I. del Río, R. Lapuente, J. Bonastre, F. Cases, Stabilityof conducting polyester/polypyrrole fabrics in different pH solutions. Chemicaland electrochemical characterization, Polymer Degradation and Stability 95(2010) 2574.

28] J. Molina, J. Fernández, A.I. del Río, J. Bonastre, F. Cases, Chemical,electrical and electrochemical characterization of hybrid organic/inorganicpolypyrrole/PW12O40

3− coating deposited on polyester fabrics, Applied SurfaceScience 257 (2011) 10056.

29] E.Y. Choi, T.H. Han, J. Hong, J.E. Kim, S.H. Lee, H.W. Kim, S.O. Kim, Noncova-lent functionalization of graphene with end-functional polymers, Journal ofMaterials Chemistry 20 (2010) 1907.

30] D.C. Marcano, D.V. Kosynkin, J.M. Berlin, A. Sinitskii, Z. Sun, A. Slesarev, L.B.Alemany, W. Lu, J.M. Tour, Improved synthesis of graphene oxide, ACS Nano 4(2010) 4806.

31] Y. Si, E.T. Samulski, Synthesis of water soluble graphene, Nano Letters 8 (2008)1679.

32] P. Lian, X. Zhu, S. Liang, Z. Li, W. Yang, H. Wang, Large reversible capacity ofhigh quality graphene sheets as an anode material for lithium-ion batteries,Electrochimica Acta 55 (2010) 3909.

33] D.R. Dreyer, S. Park, C.W. Bielawski, R.S. Ruoff, The chemistry of graphene oxide,Chemical Society Reviews 39 (2010) 228.

34] T. Textor, B. Mahltig, A sol–gel based surface treatment for preparation of waterrepellent antistatic textiles, Applied Surface Science 256 (2010) 1668.

35] A.T. Valota, I.K. Kinloch, K.S. Novoselov, C. Casiraghi, A. Eckmann, E.W. Hill,R.A.W Dryfe, Electrochemical behavior of monolayer and bilayer graphene, ACSNano 5 (2011) 8809.

36] W. Li, C. Tan, M.A. Lowe, H.D. Abruna, D.C. Ralph, Electrochemistry of individualmonolayer graphene sheets, ACS Nano 5 (2011) 2264.

37] L. Rajendran, S.P. Ananthi, Analysis of positive feedback currents at the scanningelectrochemical microscope, Journal of Electroanalytical Chemistry 561 (2004)113.

38] A.J. Bard, M.V. Mirkin, Scanning Electrochemical Microscopy, Marcel DekkerInc., New York, 2001.

39] C. Tan, J. Rodríguez-López, J.J. Parks, N.L. Ritzert, D.C. Ralph, H.D. Abruna, Reac-tivity of monolayer chemical vapor deposited graphene imperfections studiedusing scanning electrochemical microscopy, ACS Nano 6 (2012) 3070.

40] A.G. Güell, N. Ebejer, M.E. Snowden, J.V. Macpherson, P.R. Unwin, Structuralcorrelations in heterogeneous electron transfer at monolayer and multilayergraphene electrodes, Journal of the American Chemical Society 134 (2012)

7258.

41] J. Rodríguez-López, N.L. Ritzert, J.A. Mann, C. Tan, W.R. Dichtel, H.D. Abruna,Quantification of the surface diffusion of tripodal binding motifs on grapheneusing scanning electrochemical microscopy, Journal of the American ChemicalSociety 134 (2012) 6224.