Electrochemistry of Aqueous Colloidal Graphene Oxide on Pt

Jul 14, 2014 - On the basis of the results presented here, we propose that the observed response of GO on Pt electrodes is a result of the reduction o...
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Electrochemistry of Aqueous Colloidal Graphene Oxide on Pt Electrodes Glen D. O’Neil,† Andrew W. Weber,† Raluca Buiculescu,‡ Nikolaos A. Chaniotakis,‡ and Samuel P. Kounaves*,† †

Department of Chemistry, Tufts University, Medford, Massachusetts 02155, United States Laboratory of Analytical Chemistry, Department of Chemistry, University of Crete, 71 003 Iraklion, Crete, Greece

‡

S Supporting Information *

ABSTRACT: The electrochemical behavior of colloidal solutions of graphene oxide (GO) is described here in detail. The GO reduction is shown to exhibit near-reversible electron transfer on Pt electrodes, based on E1/2 and ΔEp values. The observed peak current is found to depend linearly on the concentration of the GO and the square root of the scan rate, suggesting that the response is diffusion-limited. The difference between the experimental and diffusion-only limited theoretical current values suggests that migration may be hindering mass transport to the electrode surface. Varying the type and concentration of the supporting electrolyte showed that mass transport is weakly influenced by the presence of negative charges on the graphene particles. The effect of pH on GO was also investigated, and it was found that the reduction peak heights were directly related to proton concentration in acidic solutions. On the basis of the results presented here, we propose that the observed response of GO on Pt electrodes is a result of the reduction of protons from the colloidal double layer. This difference is observed only because the Pt electrode surface can efficiently catalyze proton reduction.

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INTRODUCTION Graphene oxide (GO) has received considerable attention because it allows facile synthesis of large-scale graphene.1−3 GO is a two-dimensional network of carbon atoms, similar to graphene, with dispersed areas of sp2 hybridization interrupted by oxygen-bound sp3 carbon atoms.4−7 Oxygen functionalization renders GO soluble in water and several organic solvents, thus allowing for solution processing without further modification.8 However, breaking the sp2 lattice along graphene’s basal plane diminishes the material’s electronic properties. GO can be reduced to “graphene” using thermal, chemical, or electrochemical means to restore the electronic properties.9−13 The electron transfer properties of GO are thus of great interest and have been investigated either by studying the heterogeneous electron transfer between a GO electrode and a redox molecule in solution14−19 or by measuring the direct electrochemistry of GO films on various electrode substrates.20−24 With the former methodology, information about the electron transfer between a species in solution and the graphene material is obtained. This can provide heterogeneous rate constants, insight into the surface chemistry and reactivity, and general electroanalytical characteristics. These experiments are useful for understanding how the GO-based material will behave in applications such as sensing or fuel cells. Such detailed characterization of the electrochemical processes provides fundamental information about the direct electron transfer processes involved with the material itself and allows a © 2014 American Chemical Society

deeper understanding of GO, thus maximizing its potential for future applications. These types of studies are of interest in this work. The direct electrochemical reduction of GO immobilized on an electrode surface for the preparation of reduced graphene oxide (rGO) has been studied by several groups,20−24 but there are comparatively few reports documenting the colloidal electrochemistry of GO.25−28 With GO fixed to the electrode surface, the current−voltage response is governed by thin-film behavior and arises from the reduction of oxygen-containing functional groups such as epoxides, hydroxides, or peroxides. With this methodology, GO is not able to diffuse to and from the electrode surface, so information about its behavior in solution is lost. To date, only two communications show the electrochemical behavior of native graphene oxide colloids.25,28 Chen et al. showed that GO can be electrodeposited onto the surface of glassy carbon electrodes directly from colloidal solutions.25 Eng and Pumera showed that the voltammetry of GO colloids depends primarily on particle size and pH.28 However, both of these investigations were conducted using carbon electrode materials. The electrochemistry of nanoparticle solutions was first reported 20 years ago.29 Initial studies of SnO2 and TiO2 showed voltammetric behavior that was significantly different Received: May 28, 2014 Revised: July 11, 2014 Published: July 14, 2014 9599

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Figure 1. Chemical and physical characterization of graphene oxide. (a) ATR-FTIR and (b) Raman spectra of GO. (c) FE-SEM and (d) TEM images of GO.

from that observed with “true solutions”, indicating that the role of the nanoparticle was substantial.30,31 Recently, attention has turned toward collisions of individual particles with ultramicroelectrodes (UMEs),32,33 which has shown different classes of nanoparticle−electrode reactions. In broad terms, the electrochemistry of nanoparticle solutions can be classified into two distinct categories: those in which the nanoparticle undergoes electron transfer or catalyzes the electrochemical reaction32,34,35 and those in which the nanoparticle itself is oxidized or reduced.36−38 The extent to which each of these reactions occurs is dependent upon the nature of the nanoparticle, the potential used to generate electrolysis, the electrode material, and the solution conditions. Here we report a voltammetric analysis of native GO colloids in water. Although much is known about the physical structure of GO, there is still debate concerning GO’s exact chemical properties, especially its acidity in aqueous solutions.39 The electrochemistry of graphene oxide in solution is shown to be significantly different from the previously reported electrochemistry of surface-bound GO films. We show that suspensions of graphene oxide exhibit a reversible oxidation and reduction that is present under a variety of conditions. We investigate the origin of this response and provide a plausible mechanism for the electrochemical behavior of GO in solution.

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prepared using 18.2 MΩ/cm water from a NanoPure system (ThermoScientific, Barnstead, MA). Graphene oxide, which was prepared using a modified Hummers method,40,41 was purchased from NanoInnova Technologies (Madrid, Spain). Briefly, graphite powder was oxidized in a solution containing NaNO3, H2SO4, and KMnO4, followed by the subsequent addition of H2O2 and water to remove residual permanganate and MnO2. The suspension was then further purified by filtering and washing with distilled water until the pH of the solution was neutral. GO colloids were prepared by either ultrasonication or vigorous stirring. For ultrasonicated samples, GO and water were mixed and placed in an ultrasonic bath for 1 h. For the stirred samples, GO and water were combined and mixed on a magnetic stir plate for >48 h. All solutions were allowed to rest for 24 h before being used to ensure that only fully suspended particles were used in the redox reactions. There was no difference in the observed voltammetry between solutions prepared by ultrasonication or stirring. Prior to analysis, each suspension was purged with dry nitrogen for at least 5 min to remove oxygen. Electrochemical Measurements. All electrochemical measurements were taken using a standard three-electrode configuration. A saturated calomel electrode (SCE) and a Pt coil were used for all experiments as the reference and counter electrodes, respectively. The working electrode was a 4.0 mm diameter Pt disk. Prior to each experiment, the working electrode was polished using 1.0, 0.1, and 0.05 μm diameter alumina slurries. After being polished, the electrodes were rinsed copiously with distilled water and cleaned ultrasonically in distilled water for 1 min. The surface of the working electrode was refreshed between replicate runs by being polished with 0.05 μm alumina and rinsed with distilled water. All experiments were performed using a CH Instruments (Austin, TX) CHI830 electrochemical workstation at room temperature (22 ± 2 °C). All voltammograms presented here are the averages of three replicate scans.

EXPERIMENTAL SECTION

Materials. Potassium nitrate (KNO3), potassium chloride (KCl), sodium chloride (NaCl), nitric acid (HNO3), and potassium hydroxide (KOH) were all from Sigma and of analytical grade or higher. Each was used as received without further purification. All solutions were 9600

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Figure 2. (a) Cyclic voltammogram of an aqueous 1.0 mg mL−1 GO dispersion in 0.1 M KNO3 (pH 3.25). (b) Randles plot showing the dependence of peak current density of peak II on scan rate. (c) Cyclic voltammograms of three different solution concentrations of GO. (d) Fifty consecutive sweeps of a 1.0 mg mL−1 solution of GO. The arrow indicates the direction with an increasing number of sweeps.

carbon.43 Interpretation of the ATR-FTIR spectrum indicates that the as-received GO is heavily functionalized and has qualities similar to those of materials that have been synthesized using identical methodologies. Raman spectroscopy was used to characterize the structural properties and specifically the degree of oxidation of the GO (Figure 1b).44 The Raman spectrum obtained shows the characteristic D- and G-bands, the ratio of which is indicative of the quality of graphitic materials. The D-band (1330 cm−1) is caused by defects within the graphite structure, while the Gband (1575 cm−1) is related to in-plane CC bond stretching. The Raman spectrum of GO displays previously reported characteristics, indicating a high degree of disorder in the material,.42 The intensity ratio of the D- and G-bands is 1.5:1, a value that indicates a low degree of defects within the graphene plane. The morphology of GO was investigated using both SEM and TEM. The SEM micrograph in Figure 1c shows >1 μm crumpled sheets. The surfaces of GO are quite rough and disordered, as expected from oxidized graphene. The TEM image in Figure 1d was prepared from GO suspensions and shows that the nanosheets are well-dispersed and can be individually resolved.45,46 The SEM and TEM images are consistent with those previously reported, indicating the high quality of the material. Electrochemical Characterization of GO Colloids. The electrochemical behavior of graphene oxide colloids was

Material Characterization. Fourier transform infrared (FTIR) spectra were recorded on a Thermo-Electron Nicolet 6700 FT-IR optical spectrometer with a DTGS KBr detector at a resolution of 4 cm−1. Raman measurements were performed at room temperature using a Nicolet Almega XR Raman spectrometer with a 473 nm blue laser as an excitation source. The beam was focused on the sample through a confocal microscope equipped with a 50× objective. Highresolution transmission electron microscopy (HR-TEM) micrographs were acquired on a JEOL JEM-2100 electron microscope, operating at 80 kV. The samples for HR-TEM analysis were prepared by evaporation of droplets (0.5 mg mL−1 suspension of GO in water) placed on Formvar/carbon-coated TEM grids (Analytical Instruments S.A.). The scanning electron microscopy (SEM) images were recorded using a field emission Jeol 7000F scanning electron microscope. For SEM experiments, GO was dispersed in water and dried before being mounted.

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RESULTS AND DISCUSSION Graphene Oxide Characterization. The GO was characterized using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, Raman spectroscopy, SEM, and TEM prior to electrochemical analysis. The ATRFTIR absorption spectrum (Figure 1a) of as-received GO shows a very broad feature between ∼2500 and 3750 cm−1 corresponding to C-OH, COOH, and H2O stretches.42 The spectrum also shows several absorption bands in the region between 2000 and 900 cm−1, which have been previously assigned to epoxides, alcohols, carboxyls, ketones, and sp2 9601

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Figure 3. pH dependence on the voltammetry of GO. (a) Cyclic voltammograms of a 1.0 mg mL−1 GO suspension at various pH values. The native GO suspension has a pH of 3.25. (b) Plots of experimental and theoretical peak current density vs hydrogen ion activity. Theoretical values were calculated using the Randles−Sevcik equation as described in the Supporting Information.

number of sweeps. This indicates that there are no major structural or reorganizational effects of the colloid and no fouling on the electrode surface. This is contrary to the current understanding of graphene oxide reduction47 and indicates that the redox activity of the native colloidal solutions on Pt electrodes is not driven by reductions of oxygen-containing functional groups. After this initial assessment, further experiments were performed to understand the features of the GO voltammograms. Effect of pH on GO Voltammetry. Several reports have shown that surface-bound GO exhibits pH-dependent voltammetry.22,27 Some colloidal systems, such as SnO2, have also been known to show a pH-dependent voltammetric response.29,30 Using these studies as a framework, we investigated the pH dependence on graphene oxide by adding small volumes of concentrated (8.0 M) nitric acid or 1.0 M potassium hydroxide to a stock solution containing 1.0 mg mL−1 GO in 0.1 M KNO3. Nitric acid and potassium hydroxide were chosen to maintain identical counterions in the electrolyte. Note that the initial graphene oxide suspension with the supporting electrolyte has a pH of 3.25. Figure 3a shows the cyclic voltammograms of GO in solution at various pH values. When nitric acid is added to the stock solution, the peak current increases linearly with the activity of the hydrogen ion (aH+) (Figure 3b, black curve). In a separate experiment, the pH was increased upon addition of KOH. Soon after the pH started to increase, the mass transfer-limited peak disappeared. It should be stressed here that when the pH was decreased upon addition of nitric acid, there was only a small shift in the potential of this peak. These observations suggest that the electrochemical behavior of GO is related to decreases in the pH values. This behavior is in sharp contrast to previous studies, which have mainly been conducted using different types of carbon electrodes.22,27 We suspect the reason for this is that the Pt surface is catalytically active toward a number of reactions, notably the reduction of H+ to H2. In fact, H+ reduction on Pt electrodes has been well studied and compares very well with the data presented herein.48−50 In GO suspensions, protons are known to originate from complex interactions between GO particles and the solvent.39 As a control experiment, we

investigated using cyclic voltammetry (CV). A representative cyclic voltammogram of a 1.0 mg mL−1 graphene oxide suspension in a 0.1 M KNO3 supporting electrolyte is presented in Figure 2a. The voltammogram is dominated by two waves in the forward and reverse scans. The shoulder that appears prior to the main reduction peak (I) is likely caused by adsorption onto the Pt electrode surface, which appears to be a reversible process as it is also reproduced on the oxidation scan (I′). A plot of the peak current of peak I versus scan rate shows a linear relationship, confirming that peak I is a deposition (Figure S1 of the Supporting Information). The reduction and oxidation peaks (II and II′, respectively) are centered with an E1/2 of −394 ± 3 mV versus the SCE, indicating a reversible reaction with a ΔEp of 71 ± 4 mV, and the maximal current observed at the reduction peak (jp) is at 144 ± 6 μA cm−2. An interesting feature of the GO cyclic voltammogram is that both redox peaks remain at the same potential and height (current) independent of the starting point or scan direction, which indicates that GO is in an equilibrium state. The reversibility is also a very important characteristic, suggesting that the electron transfer kinetics are very fast on this electrode surface. Figure 2b shows a Randles plot of the peak current of peak II versus the square root of the scan rate (v) between 25 and 500 mV s−1. The linear relationship between the peak current density (jp) and v1/2 indicates that the current is diffusionlimited over the scan rates studied and that the peak current should also be proportional to concentration. Calculation of the diffusion coefficient of the GO from the Randles plot was not possible because the exact molecular weight of the GO particles is not known and cannot be determined. Figure 2c shows cyclic voltammograms of GO at concentrations of 1.0, 3.0, and 5.0 mg mL−1 in 100 mM KNO3. The peak current scales linearly with concentration over this region, with a linear regression coefficient of 0.995. Also evident from Figure 2c is the increased ΔEp, which is likely caused by an Ohmic drop in the electrochemical cell caused by the increased concentration of GO. The cyclic voltammograms of GO are remarkably reproducible over multiple scans, as is shown in Figure 2d. During multisweep CV, there is no change in the cathodic peak current and a slight decrease in anodic peak current with an increasing 9602

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repeated this experiment on a glassy carbon electrode, which cannot catalyze the reduction of H+ to H2. These results are shown in Figure S2 of the Supporting Information, and the lack of a reduction peak over an identical scan window suggests that the observed electrochemistry is indeed catalyzed by the platinum surface. There are two possible explanations for the observed interactions between the GO particle and excess acid in solution. The first is that the increased acidity is causing chemical reactions with GO and forming more electroactive byproducts. When the structure and chemistry of GO are taken into account, this would most likely be the protonation of carboxylic acids. We believe this is unlikely to be significant as the pKa values of most carboxylic acids are between 4 and 5, suggesting that at pH 3.25 the vast majority of carboxylic acids would already be protonated. In addition, considering that carboxylic acids are able to form only at the edge plane flake edges without causing significant damage to the GO colloid, it is unlikely that the magnitude of the observed response (∼1250 μA/cm2) can be entirely caused by the reduction of carboxylic acids.39 If a large percentage of carboxylic acids were to form under these conditions, it would mean significant structural changes to GO that were not observed using electron microscopy (e.g., Figure 1c,d and ref 39). While we do not entirely rule out carboxylic acids contributing to the observed response, we do not believe that carboxylic acids are the main source of the electroactivity. A second possible explanation is that the observed response is caused by proton reduction at the GO/Pt surface, similar to the situation for other colloids such as SnO2.29,30 This mechanism seems more likely, when the composition of the colloidal double layer is considered. In a native solution, the GO colloids are negatively charged1 and must be stabilized by a layer of positively charged ions to maintain colloidal stability. When no other species are present during exfoliation, the ions that make up the double layer are protons.39 In either case, on the basis of our results, it appears that the reversible wave centered at approximately −400 mV versus the SCE is likely the reversible reduction and subsequent oxidation of H+ to H2. To gain a further understanding of the pH-dependent response, we compared the measured peak current densities (black squares) to theoretical values calculated using the Randles−Sevcik equation (red circles in Figure 3b). Interestingly, the current densities of the two data sets and slopes of the two lines do not match, which would be expected if the H+ was freely diffusing to the electrode surface,51 which would occur only if chemical reactions between GO and the solution produced a significant amount of free acid. At the low end of the pH range, the experimental peak current density was measured to be 1260 μA cm−2 while the theoretical value predicts the current to be 2990 μA cm−2. Similarly, in the high pH range, the experimental peak current density was 6.1 μA cm−2 while the theoretical value was calculated to be 335 μA cm−2. This disparity could indicate that another form of mass transport is impeding the flux of H+ to the electrode surface. To investigate if migration negatively impacted the flux of GO, we performed linear sweep voltammetry for the reduction of GO on a 10 μm Pt UME (Figure 4). In these experiments, GO was first dispersed in H2O and the supporting electrolyte was added immediately before the voltammograms were collected, taking care to minimize the time between the addition of the supporting electrolyte and analysis, which was shown to prevent the exchange of the double-layer protons

Figure 4. Steady state cyclic voltammograms of 1.0 mg mL−1 GO suspensions at varying concentrations of the supporting electrolyte, added by standard addition after the GO had been dispersed in pure water.

with K+ in solution.39 Note that the supporting electrolyte concentration was changed by the addition of a small volume of concentrated KNO3 so that the overall volume of the solution was changed by 50 mM) concentration of NaCl. Figure 7 shows cyclic voltammograms of 1.0 mg mL−1 GO

Figure 5. Comparison of two GO suspensions of equal acidity (pH 2.0). The black curve shows the response of a 3.0 mg mL−1 suspension of GO, while the red curve shows the response of a 1.0 mg mL−1 suspension acidified with HNO3.

difference in measured current, is caused by proton equilibrium with the colloid’s double layer. This is compounded by the effects of migration of GO particles to the electrode surface because of the negative charge on the colloid. In essence, the 3.0 mg mL−1 solution contains more “sites” with which H+ can associate and therefore shows a decreased response because of migration. Importantly, if freely dissociated protons are the source of the redox signal, then the removal of the GO flakes from solution by filtration should yield little or no change in the measured current. Figure 6 shows the results of a 1.0 mg mL−1

Figure 7. Cyclic voltammograms showing the response of GO solutions dispersed in varying concentrations of the supporting electrolyte.

solutions that were prepared in solutions containing different amounts of NaCl. All concentrations of NaCl were greater than 50 mM to eliminate migration. (Note that in Figure 2 the supporting electrolyte was added after the GO had been dispersed in nanopure water. Because the samples were immediately analyzed, there was not sufficient time for the K+ to displace the H+ at the colloidal double layer.) As the concentration of the supporting electrolyte is increased from 50 mM NaCl to 1 M NaCl, the peak current density increases. The difference in cathodic peak currents between the samples containing 50 and 100 mM NaCl is significant, increasing from 163 to 214 μA/cm2. For concentrations of 0.1, 0.5, and 1 M, there are also increases (from 214 to 238 μA/cm2), but they are less prevalent. This result is most likely due to the total replacement of the protons within the colloidal double layer when ∼100 mM NaCl is added. On the basis of these results, we suggest the following CE mechanism for colloidal graphene oxide reduction on platinum electrodes:

Figure 6. Electrochemical response of GO suspensions before (red) and after (black) filtration.

GO solution before (red trace) and after filtration using a 200 nm filter (Thermo 190-9920; red trace). The characteristic voltammogram for GO is observed in the case in which GO is present, while there is a complete lack of Faradaic activity when the nanomaterial is removed by filtration. This suggests that the protons available for electron transfer are likely present at the adsorbed double layer of the colloid. Finally, the reduction of H+ is catalyzed by the Pt electrode surface, and changing the electrode material should significantly reduce the response (Figure S2 of the Supporting Information). There is nearly no

{GO}H n ↔ {GO}n − + nH+

(1)

1 H2 (2) 2 where the braces indicate a graphene oxide particle. In the first step (eq 1), protons are dissociated from the colloid within the diffusion layer of the electrode. Once dissociated, the protons are free to undergo electron transfer at the surface of the Pt electrode (eq 2). This mechanism is similar to what has been H + + e− ↔

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the National Science Foundation (OCE-1060945), and GSRT Aristia II 3271.

reported for the reduction of weak acids on platinum electrodes,48−50,53 with the main difference arising because of the nature of the chemical step before the electron transfer. For weak acids, this step is governed by the acid−base equilibria, while for GO reduction, the chemical step is related to the stability of the diffuse double layer of the colloid. This study can be placed into context by comparing the results with those of three other significant reports by Chen et al.,25 Guo et al.,54 and Eng and Pumera,28 which describe the colloidal electrochemistry of GO on carbon electrodes. Chen et al. observed a deposition peak during the cathodic scan and subsequently used the deposition as a method for preparing rGO-modified electrodes.25 While this report is the first presenting a voltammogram of freely diffusing GO nanoparticles, it lacked an investigation of how parameters such as pH, electrolyte concentration, and electrode material affect electrodeposition. The recent communication by Eng and Pumera further develops the ideas put forth by Chen et al. but still does not include a thorough investigation of an electrode material other than GC.28 An investigation of electrodeposition of GO on Au, GC, ITO, and Pt was discussed in the paper by Guo et al., and it was noticed that Pt performed poorly compared to the other substrates.54 It is plausible that the Pt electrode used in that work was inhibited by the catalytic reduction of protons from the GO’s double layer.

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(1) Li, D.; Müller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Processable aqueous dispersions of graphene nanosheets. Nat. Nanotechnol. 2008, 3, 101−105. (2) Eda, G.; Fanchini, G.; Chhowalla, M. Large-area ultrathin films of reduced graphene oxide as a transparent and flexible electronic material. Nat. Nanotechnol. 2008, 3, 270−274. (3) Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Kim, K. S.; Ahn, J.-H.; Kim, P.; Choi, J.-Y.; Hong, B. H. Large-scale pattern growth of graphene films for stretchable transparent electrodes. Nature 2009, 457, 706−710. (4) Nakajima, T.; Mabuchi, A.; Hagiwara, R. A New Structure Model of Graphite Oxide. Carbon 1988, 26, 357−361. (5) Mermoux, M.; Chabre, Y.; Rousseau, A. FTIR and 13C NMR study of graphite oxide. Carbon 1991, 29, 469−474. (6) Lerf, A.; He, H.; Forster, M.; Klinowski, J. Structure of Graphite Oxide Revisited. J. Phys. Chem. B 1998, 102, 4477−4482. (7) Szabó, T.; Berkesi, O.; Forgo, P.; Josepovits, K.; Sanakis, Y.; Petridis, D.; Dékány, I. Evolution of Surface Functional Groups in a Series of Progressively Oxidized Graphite Oxides. Chem. Mater. 2006, 18, 2740−2749. (8) Paredes, J. I.; Villar-Rodil, S.; Martínez-Alonso, A.; Tascón, J. M. D. Graphene oxide dispersions in organic solvents. Langmuir 2008, 24, 10560−10564. (9) Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. The chemistry of graphene oxide. Chem. Soc. Rev. 2010, 39, 228−240. (10) Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Graphene-based composite materials. Nature 2006, 442, 282−286. (11) An, S. J.; Zhu, Y.; Lee, S. H.; Stoller, M. D.; Emilsson, T.; Park, S.; Velamakanni, A.; An, J.; Ruoff, R. S. Thin Film Fabrication and Simultaneous Anodic Reduction of Deposited Graphene Oxide Platelets by Electrophoretic Deposition. J. Phys. Chem. Lett. 2010, 1, 1259−1263. (12) Park, S.; Ruoff, R. S. Chemical methods for the production of graphenes. Nat. Nanotechnol. 2009, 4, 217−224. (13) Ramanathan, T.; Abdala, A. A.; Stankovich, S.; Dikin, D. A.; Herrera-Alonso, M.; Piner, R. D.; Adamson, D. H.; Schniepp, H. C.; Chen, X.; Ruoff, R. S.; Nguyen, S. T.; Aksay, I. A.; Prud’Homme, R. K.; Brinson, L. C. Functionalized graphene sheets for polymer nanocomposites. Nat. Nanotechnol. 2008, 3, 327−331. (14) Ambrosi, A.; Pumera, M. Electrochemistry at CVD Grown Multilayer Graphene Transferred onto Flexible Substrates. J. Phys. Chem. C 2013, 117, 2052−2058. (15) Ritzert, N. L.; Rodríguez-López, J.; Tan, C.; Abruña, H. D. Kinetics of Interfacial Electron Transfer at Single Layer Graphene Electrodes in Aqueous and Non-Aqueous Solutions. Langmuir 2013, 29, 1683−1694. (16) Pumera, M. Graphene-based nanomaterials and their electrochemistry. Chem. Soc. Rev. 2010, 39, 4146−4157. (17) Brownson, D. A. C.; Munro, L. J.; Kampouris, D. K.; Banks, C. E. Electrochemistry of graphene: Not such a beneficial electrode material? RSC Adv. 2011, 1, 978−988. (18) Brownson, D. A. C.; Kampouris, D. K.; Banks, C. E. Graphene electrochemistry: Fundamental concepts through to prominent applications. Chem. Soc. Rev. 2012, 41, 6944−6976. (19) Brownson, D. a. C.; Lacombe, A. C.; Gómez-Mingot, M.; Banks, C. E. Graphene oxide gives rise to unique and intriguing voltammetry. RSC Adv. 2012, 2, 665−668. (20) Chng, E. L. K.; Pumera, M. Solid-state electrochemistry of graphene oxides: Absolute quantification of reducible groups using voltammetry. Chem.Asian J. 2011, 6, 2899−2901. (21) Ramesha, G. K.; Sampath, S. Electrochemical Reduction of Oriented Graphene Oxide Films: An in Situ Raman Spectroelectrochemical Study. J. Phys. Chem. C 2009, 113, 7985−7989.

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CONCLUSIONS Our in-depth study of the electrochemistry of graphene oxide colloids in aqueous solutions using platinum electrodes shows that GO displays highly reversible diffusion-limited voltammograms, which are linearly dependent on pH in the acidic range. This response is significantly different from that exhibited by surface-bound GO films. Detailed investigation of the counterion effect suggests that the voltammetric behavior of the GO colloid is adversely affected by migration because of the presence of a large number of negative charges on the graphene surface. We conclude that the basis of the observed response is the oxidation and/or reduction of protons located at the graphene surface. Comparisons between the expected Randles−Sevcik peak currents and the experimental currents show a significant deviation, suggesting that the redox chemistry of graphene oxide is strongly linked to interactions of the GO with the hydrogen ions and the electrode material.

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ASSOCIATED CONTENT

* Supporting Information S

(1) Dependence of scan rate on Peak I from Figure 1(a), (2) voltammetric response of GO on a glassy carbon electrode, and (3) theoretical peak current calculations used in Figure 3b. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was in part supported by grants from the National Aeronautics and Space Administration (ASTID-SC1407258), 9605

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