Graphene Oxides Exhibit Limited Cathodic Potential Window Due to

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Graphene Oxides Exhibit Limited Cathodic Potential Window Due to Their Inherent Electroactivity Her Shuang Toh, Adriano Ambrosi, Chun Kiang Chua, and Martin Pumera* Division of Chemistry & Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore ABSTRACT: Graphene oxides are considered to show great promise for the construction of high-performance electrochemical devices for sensing and energy storage applications. Herein, we show that, despite the advantages of graphene oxides, such as facile preparation in bulk quantities and enhanced electrochemical performances, these electrode materials, namely, graphene oxide, chemically reduced graphene oxide, and graphite oxide, show very limited potential windows in the cathodic region because of their inherent electroactivity. This has a profound influence on the use of these materials in electrochemical devices when the cathodic part of the electrochemical window for the reduction of electrolytes is required. The highly limited electrochemical window of these graphene oxides must be taken into account in the construction of new electrochemical devices working in the cathodic region.

’ INTRODUCTION Graphene and related materials have witnessed an enormous increase in interest from the scientific community because they exhibit many interesting properties, such as high mechanical strength,1 high elasticity and thermal conductivity, roomtemperature quantum Hall effect, very high room-temperature electron mobility,2 tunable optical properties,3,4 tunable band gap,5 and fast heterogeneous electron transfer rates,6,7 that can provide enormous benefits in applications in physics, chemistry, and materials science. One of the most promising applications is in electrochemical devices, such as batteries, supercapacitors, and sensors.8 10 For the construction of electrochemical devices, bulk quantities of graphene-related materials are needed. There are several approaches for the preparation of graphene.6,12 Physical methods, such as the original “tape exfoliation” method, typically do not produce the large quantities of graphene needed. Chemical preparation routes based on oxidation of graphite to graphite oxide and consequent exfoliation (thermal or sonochemical with subsequent reduction of graphene oxide) to a variety of graphene oxides (collectively called chemically modified graphenes, or CMGs12) are scalable and can produce bulk (gram) quantities of CMGs.11 The types of CMGs selected for this work include graphite oxide, graphene oxide (GO), chemically reduced graphene oxide (CRGO), and electrochemically reduced graphene oxide (ERGO).12 Graphene oxide is one of the most commonly used CMGs because the preparation steps are very simple, where graphite flakes are initially oxidized with strong oxidants to prepare graphite oxide. After purification, graphite oxide is consequently ultrasonicated in aqueous or organic solvents to give single- or few-layered graphene oxide.13 It is well-known that graphite and graphene oxide contain very large numbers of oxygen functionalities r 2011 American Chemical Society

(such as carboxyl, carbonyl, hydroxyl, epoxy, and peroxy moieties), with a C/O ratio of ∼2:1.13,14 On another note, typical reduction procedures (using hydrazine or sodium borohydride) applied to regenerate the graphene structure of the isolated graphene oxide sheets do not remove all of the oxygencontaining groups.15 Because most of the above-mentioned oxygen-containing groups are electroactive, the inherent electrochemical activities of the graphene oxide and related materials (not fully reduced) should be taken into account for electrochemical applications. As such, the solute of interest in electrochemical experiments should be electroactive at potentials different from that of the graphene oxide-modified electrodes. As an immediate consequence, the potential windows of such materials are limited. In general, the electrochemical (or potential) windows (that is, the ranges of potentials over which the electrodes are stable) of different materials can be limited because of (i) oxidation/ reduction (decomposition) of the solvent or (ii) oxidation/ reduction (decomposition or change in surface properties) of the electrode surface itself. This is common for Hg electrodes, where the electrode material, mercury, dissolves in the solution at potentials higher than 0 V (vs SCE), or Au electrodes, where the oxidation of Au metallic surfaces take place at high anodic potentials.16 Both cases are inherent to the electrochemical system in question and consequently limit the electrochemical window of the system.17 Chemically modified graphene materials are commonly used in the electrochemical sensing field.18,19 Although CMGs improve Received: April 25, 2011 Revised: June 24, 2011 Published: August 03, 2011 17647

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The Journal of Physical Chemistry C

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the sensitivity and detection limits of electrochemical systems, they present additional problems as well. Herein, we demonstrate that the electrochemical window of graphene oxide is very limited in the cathodic region, thus significantly limiting the potential window for the reduction of solutes of interest. This is largely due to the inherent electrochemical activity of graphene oxide itself in the cathodic region.

’ EXPERIMENTAL SECTION Materials. 2-Nitrotoluene (2-NT) and 4-amino-2-nitrotoluene (4-A-2-NT) were obtained from Alfa Aesar (Ward Hill, MA). Sodium tetraborate decahydrate, methyl orange (MO), uric acid (UA), graphite, and all other chemicals were obtained from Sigma-Aldrich (St. Louis, MO). The glassy carbon electrode (3 mm in diameter), Pt auxiliary electrode (Pt wire), and Ag/AgCl (with porous Teflon tip) reference electrode were obtained from CH Instruments (Austin, TX). Methods. Graphite oxide was synthesized from graphite according to Staudenmaier’s method.14 Specifically, 17.5 mL of H2SO4 and 9 mL of HNO3 (fuming) were mixed in a roundbottom flask at 0 °C for 15 min. Subsequently, 1 g of graphite was added to the mixture under vigorous stirring to obtain a homogeneous solution. Then, 11 g of KClO3 was added slowly into the mixture at 0 °C over 15 min. After the KClO3 had dissolved, the flask was capped loosely to allow the escape of ClO2 gas and left to stir vigorously for 96 h at room temperature. Upon completion, the mixture was then poured into 1 L of deionized water and filtered to give a residue of graphite oxide. Graphite oxide was redispersed and washed repeatedly with 5% HCl to remove sulfate ions. Graphite oxide was finally washed with deionized water until the filtrate was observed to be of neutral pH. The graphite oxide slurry was then dried at 60 °C for 48 h in a vacuum oven. Graphene oxide (GO) was obtained by ultrasonicating (37 kHz) a 1 mg/mL suspension of graphite oxide in N,N-dimethylformamide (DMF) for 1 h.20 Chemically reduced graphene oxide (CRGO) was prepared by mixing 10 mg of graphite oxide with 20 mL of 50 mM aqueous NaBH4 for 1 h. Thereafter, the mixture was ultrasonicated (37 kHz) for 1 h. The CRGO dispersion was then washed repeatedly with deionized water to remove the remaining NaBH4 and dried at 60 °C for 48 h in a vacuum oven.21 Electrochemically reduced graphene oxide (ERGO) was made from GO using the amperometry method. After modification of the glassy carbon (GC) working electrode surface with a GO film, a potential of 1.2 V (vs Ag/AgCl) was applied for 900 s in a phosphate buffer solution (pH 7.2).22 All materials were characterized by transmission electron microscopy (TEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS); these results were published in our previous publication.23 Briefly, the G/D Raman signal ratios were 1.12, 1.05, 1.11, and 1.08 for graphite oxide, graphene oxide, chemically reduced graphene oxide, and electrochemically reduced graphene oxide, respectively. The C/O ratios determined by XPS were 1.9, 2.8, 2.9, and 5.1 for graphite oxide, graphene oxide, CRGO, and ERGO, respectively. The chemically modified graphene materials (such as graphene oxide, graphite oxide, and electrochemically reduced graphene oxide) were prepared in DMF at a concentration of 1 mg/mL, and 1 μL of the suspension was used to drop-coat the glassy carbon electrodes. The ERGO-modified electrode was

Figure 1. Cyclic voltammograms of a blank 20 mM borate buffer solution (pH 9.3) with different CMG-modified GC and bare GC electrodes. Bare GC, black dotted line; graphene oxide, red line; graphite oxide, blue line; chemically reduced graphene oxide, orange line; electrochemically reduced graphene oxide, green line. Conditions: scan rate of 0.1 V/s, buffer solution purged for 15 min with N2 before measurements.

prepared from a GO-modified electrode by the electrochemical reduction procedure described above. The nitroaromatic compounds were prepared as individual 1 mg/mL stock solutions and subsequently diluted to the desired concentrations. The 20 mM borate buffer was purged with nitrogen gas for 15 min prior to each measurement. Apparatus. All of the electrochemical experiments were performed using an μAutolab type III instrument (Eco Chemie BV, Utrecht, The Netherlands) connected to a personal computer and controlled by General Purpose Electrochemical Systems v. 4.9 software. The experiments were carried out at room temperature using a three-electrode configuration in a 5 mL glass cell (CH Instruments).

’ RESULTS AND DISCUSSION We examined the electrochemical behavior of chemically modified graphenes, namely, graphite oxide, graphene oxide, chemically reduced graphene oxide, and electrochemically reduced graphene oxide, using cyclic voltammetry. In particular, we focused on their electrochemical windows in the cathodic region. Figure 1 shows the cyclic voltammograms of all of the abovementioned CMGs. The voltammetry scans were carried out from +1.0 V (vs Ag/AgCl) in the negative direction and reversed at 1.4 V. It can be clearly seen that the ERGO and bare glassy carbon electrodes do not exhibit any sign of decomposition of the electrolyte or electrochemical reaction of the electrode material. In contrary, graphite oxide, GO, and CRGO exhibit large reduction currents starting at 0.75 V and increasing to 1.4 V. Such reduction currents can be assigned to the reduction of the oxygen-containing groups of graphite oxide, graphene oxide, and chemically reduced graphene oxide. These reduction waves are normally observed for functional groups such as expoxide,24 aldehydes,25 and peroxides.26 The reduction of graphene oxide was previously studied by Zhou et al.,22 who clearly demonstrated that oxygen-containing groups are responsible for the reduction current. It should be pointed out that the reason we did not observe reduction currents at 0.75 V on the electrochemically reduced graphene oxide is because the ERGO was previously prepared from the reduction of GO at 1.2 V for 15 min. Therefore, all reducible oxygen-containing groups had been removed prior to the electrochemical experiments. It should also 17648

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Figure 2. Cyclic voltammograms of 2-nitrotoluene with different CMG-modified GC and bare GC electrodes. Bare GC, black dotted line; graphene oxide, red line; graphite oxide, blue line; chemically reduced graphene oxide, orange line; electrochemically reduced graphene oxide, green line. Conditions: scan rate of 0.1 V/s, 20 mM borate buffer (pH 9.3), buffer solution purged for 15 min with N2 before measurements, 20 ppm 2-nitrotoluene.

be pointed out that glassy carbon electrode, which is used as a “standard” electrode, does not exhibit any reduction signal in this potential window. We also investigated the scope of expansion of the electrochemical cathodic windows in the case of ERGO and bare GC. We found that there is a slight increment of the background current at 1.3 V and a steeper increment at 1.65 V for ERGO as compared to bare glassy carbon, which exhibits the cathodic end of its potential window at approximately 1.65 V. The electrochemical reduction of the above-mentioned CMGs (GO, CRGO, and graphite oxide) might affect the ability to detect potential compounds that are reducible at potentials close to or below the potential where the particular CMGs are reduced. The large signals from the reductions of GO, CRGO, and graphite oxide could likely overlap the reduction signals given by the solute of interest. To illustrate this possibility, we chose two compounds that are relevant to the environmental analysis of nitroaromatic explosives, namely, 2-nitrotoluene (2-NT) and 4-amino-2-nitrotoluene (4-A-2-NT). Figure 2 shows the reduction of 20 ppm 2-NT at bare GC and CMG-modified electrodes. The reduction of 2-NT at the bare GC electrode provides a voltammetric peak starting at 0.5 V with a maximum at 0.82 V with a clear tail. A similar voltammogram of 2-NT is observed at ERGO, with a peak potential at 0.82 V. However, the reduction peaks for the inherent oxygen moieties on graphite oxide, graphene oxide, and chemically reduced graphene begin at 0.75 V, which results in the reduction signal of 2-NT (with a peak potential of 0.82 V) being superposed on the large background slope; see Figure 2 (red line for GO, orange line for CRGO, and blue line for graphite oxide). This would definitely obscure the physicalchemical information for the reduction of 2-NT on GO, CRGO, and graphite oxide electrode surfaces. As ERGO exhibits better performance than the other CMGs, we investigated the sensitivities of ERGO and GC electrodes upon reduction of 2-NT in the concentration range of 20 200 ppm. We found out that the sensitivities did not differ significantly: that of GC was 206 nA/ ppm, whereas that of ERGO was found to be 250 nA/ppm. To demonstrate that this issue affects more than just one compound, we extended this study to two other compounds in

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Figure 3. Cyclic voltammograms of 4-amino-2-nitrotoluene with different CMG-modified GC and bare GC electrodes. Bare GC, black dotted line; graphene oxide, red line; graphite oxide, blue line; chemically reduced graphene oxide, orange line; electrochemically reduced graphene oxide, green line. Conditions: scan rate of 0.1 V/s, 20 mM borate buffer (pH 9.3), buffer solution purged for 15 min with N2 before measurements, 20 ppm 4-amino-2-nitrotoluene.

Figure 4. Cyclic voltammograms of (A) 4.5 mM methyl orange and (B) 10 mM uric acid in phosphate buffer (50 mM, pH 7.2) on different CMG-modified GC and bare GC electrodes. Bare GC, black dotted line; graphene oxide, red line; graphite oxide, blue line; chemically reduced graphene oxide, orange line; electrochemically reduced graphene oxide, green line. Inset in A shows detailed area of reduction of methyl orange. Conditions: scan rate of 0.1 V/s, buffer solution purged for 15 min with N2 before measurements.

the cathodic area. Figure 3 shows the reduction of 4-amino-2nitrotoluene (4-A-2-NT), which is another related compound of nitroaromatic explosives, at CMG and bare GC surfaces. The results shown in Figure 3 are analogous to the results shown above. 4-A-2-NT exhibits well-defined peaks at the bare GC and 17649

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The Journal of Physical Chemistry C ERGO electrodes (Figure 3, black dotted and green lines, respectively) at a potential 0.86 V. However, at the GO, CRGO, and graphite oxide surfaces (Figure 3, red, orange, and blue lines, respectively), the reductions of these three CMGs occur at more positive potentials (starting at 0.75 V), and therefore, the reduction peak of 4-A-2-NT is superimposed on the slope of the background current. Such voltammetric signals of 4-A-2-NT are strongly distorted, and it is thus not viable to use GO, CRGO, and graphite oxide materials for physical-chemical analysis or analytical chemistry purposes for this compound. Another compound for which the reduction is affected in the cathodic area is methyl orange (MO). The reduction of methyl orange on all investigated electrode surfaces starts at 0.45 V with a peak at 0.65 V (Figure 4A). However, in the cases of GO, graphite oxide, and CRGO, this peak is already superimposed on the reduction wave of the oxygen moieties of these CMGs, similarly to the case of nitroaromatic compounds (Figure 4A, inset). In addition, as an example, we found that the anodic area is not affected by the inherent reduction wave of CMGs for the oxidation of the important biomarker uric acid (Figure 4B).

’ CONCLUSIONS In summary, we have demonstrated that the electrochemical potential windows of graphene oxide, chemically reduced graphene oxide, and graphite oxide are limited in the cathodic region. This is due to the fact that these chemically modified graphenes contain groups that are reducible at relatively mild negative potentials. Such a limitation has a profound influence on the usability of these chemically modified graphenes in both sensing and energy storage applications. It is necessary to say that the electrochemistry at graphene oxides is more complex than originally assumed, as one must take into account the electrochemistry of graphene oxides as well. For use in electrochemical devices, it might be beneficial to use electrochemically reduced graphene, as its prior reduction completely removes electrochemically reducible oxygen-containing groups resulting in a very stable graphene material with a large electrochemical window in the cathodic region.

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(4) Shi, Y. M.; Fang, W. J.; Zhang, K. K.; Zhang, W. J.; Li, L. J. Small 2009, 5, 2005. (5) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183. (6) Pumera, M. Chem. Rec. 2009, 9, 211. (7) Kampouris, D. K.; Banks, C. E. Chem. Commun. 2010, 46, 8986. (8) Pumera, M. Chem. Soc. Rev. 2010, 39, 4146. (9) Ratinac, K. R.; Yang, W.; Gooding, J. J.; Thordarson, P.; Braet, F. Electroanalysis 2011, 23, 803. (10) Pumera, M. Energy Environ. Sci. 2011, 4, 668. (11) Ang, P. K.; Wang, S.; Bao, Q. L.; Thong, J. T. L.; Loh, K. P. ACS Nano 2009, 3, 3587. (12) Dreyer, D. R.; Ruoff, R. S.; Bielawski, C. W. Angew. Chem., Int. Ed. 2010, 49, 9336. (13) Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. Chem. Soc. Rev. 2010, 39, 228. (14) Staudenmaier, L. Ber. Dtsch. Chem. Ges. 1898, 31, 1481. (15) Gao, W.; Alemany, L. B.; Ci, L. J.; Ajayan, P. M. Nat. Chem. 2009, 1, 403. (16) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980. (17) Arning, M. D.; Minteer, S. D. Electrode Potentials. In Handbook of Electrochemistry; Zoski, C., Ed.; Elsevier Science: Amsterdam, 2007. (18) Zuo, X.; He, S.; Li, D.; Peng, C.; Huang, Q.; Song, S.; Fan, C. Langmuir 2010, 26, 1936. (19) Pumera, M.; Ambrosi, A.; Bonanni, A.; Chng, E. L. K.; Poh, H. L. Trends Anal. Chem. 2010, 29, 954. (20) Paredes, J. I.; Villar-Rodil, S.; Martinez-Alonso, A.; Tascon, J. M. D. Langmuir 2008, 24, 10560. (21) Shin, H. J.; Kim, K. K.; Benayad, A.; Yoon, S. M.; Park, H. K.; Jung, I. S.; Jin, M. H.; Jeong, H. K.; Kim, J. M.; Choi, J. Y.; Lee, Y. H. Adv. Funct. Mater. 2009, 19, 1987. (22) Zhou, M.; Wang, Y. L.; Zhai, Y. M.; Zhai, J. F.; Ren, W.; Wang, F. A.; Dong, S. J. Chem.—Eur. J. 2009, 15, 6116. (23) Ambrosi, A.; Bonanni, A.; Pumera, M. Chem. Eur. J., published online Aug 11, 2011, http://dx.doi.org/10.1002/chem.201101117. (24) (a) Xiao, L.; Wildgoose, G. G.; Compton, R. G. Sens. Actuators B 2009, 138, 397. (b) Lee, L.; Villalba, M. M.; Smith, R. B.; Davis, J. Electrochem. Commun. 2009, 11, 1555. (25) Doherty, A. P.; Brooks, C. A. Electrochim. Acta 2004, 49, 3821. (26) Stuart, E. J. E.; Pumera, M. J. Phys. Chem. C 2010, 114, 21296.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was partially supported by Grant MINDEF NTUJPP/10/07 from the Ministry of Defense, Singapore, and an NAP startup fund (Grant M58110066) provided by NTU. We also acknowledge funding support for this project from the Nanyang Technological University under the Undergraduate Research Experience on Campus (URECA) program. ’ REFERENCES (1) Bunch, J. S.; van der Zande, A. M.; Verbridge, S. S.; Frank, I. W.; Tanenbaum, D. M.; Parpia, J. M.; Craighead, H. G.; McEuen, P. L. Science 2007, 315, 490. (2) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666. (3) Nair, R. R.; Blake, P.; Grigorenko, A. N.; Novoselov, K. S.; Booth, T. J.; Stauber, T.; Peres, N. M. R.; Geim, A. K. Science 2008, 320, 1308. 17650

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