Article pubs.acs.org/Langmuir
Controlled Electrochemical Carboxylation of Graphene To Create a Versatile Chemical Platform for Further Functionalization Emil Bjerglund, Mikkel Kongsfelt, Kyoko Shimizu, Bjarke Bror Egede Jensen, Line Koefoed, Marcel Ceccato, Troels Skrydstrup, Steen U. Pedersen, and Kim Daasbjerg* Department of Chemistry and Interdisciplinary Nanoscience Center, University of Aarhus, Langelandsgade 140, 8000 Aarhus C, Denmark S Supporting Information *
ABSTRACT: An electrochemical approach is introduced for the versatile carboxylation of multi-layered graphene in 0.1 M Bu4NBF4/MeCN. First, the graphene substrate (i.e., graphene chemically vapor-deposited on Ni) is negatively charged at −1.9 V versus Ag/AgI in a degassed solution to allow for intercalation of Bu4N+ and, thereby, separation of the individual graphene sheets. In the next step, the strongly activated and nucleophilic graphene is allowed to react with added carbon dioxide in an addition reaction, introducing carboxylate groups stabilized by Bu4N+ already present. This procedure may be carried out repetitively to further enhance the carboxylation degree under controlled conditions. Encouragingly, the same degree of control is even attainable, if the intercalation and carboxylation is carried out simultaneously in a one-step procedure, consisting of simply electrolyzing in a CO2-saturated solution at the graphene electrode for a given time. The same functionalization degree is obtained for all multilayered regions, independent of the number of graphene sheets, which is due to the fact that the entire graphene structure is opened in response to the intercalation of Bu4N+. Hence, this electrochemical method offers a versatile procedure to make all graphene sheets in a multi-layered but expanded structure accessible for functionalization. On a more general level, this approach will provide a versatile way of forming new hybrid materials based on intimate bond coupling to graphene via carboxylate groups.
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INTRODUCTION The well-known honeycomb lattice structure of graphene is the foundation for the many highly praised properties of graphene. These properties include high transparency of light in the visible spectrum1 combined with ballistic charge transport,2−4 extremely high mechanical strength,5 and good barrier properties.6,7 However, these otherwise unprecedented properties of graphene will prove difficult to exploit in real applications without a well-equipped chemical toolbox for handling and modifying graphene in a controlled manner. Carboxyl groups and carboxylic acid derivatives are among the most useful functional groups exploited within organic chemistry and biochemistry in a large variety of available reactions.8 It is thus of immense importance to make such functional groups available on graphene. Two routes are commonly used to obtain carboxyl groups on graphene, i.e., chemical oxidation and radical-based reactions. The former is a classical way of modifying carbon materials based on using strongly oxidizing chemical conditions, typically comprising chromic/sulfuric acid or KMnO4/H2O2, to enable introduction of oxygen-containing functional groups, including carboxyl groups. This method is highly efficient, but the harsh conditions employed also introduce a large number of defects along with a restructuring of the surface.9,10 Hence, the resulting material obtained is changed so significantly that the properties are no longer comparable to graphene and is instead known as graphene oxide (GO). By reduction of GO, a material known as reduced graphene oxide (rGO) is obtained, still containing a © XXXX American Chemical Society
large amount of carboxylate groups. As opposed to GO, the rGO material is conducting and may be applied in electrochemical sensor applications.11 While this method is applicable to bulk carbon materials, in general, it is not useful for graphene already deposited or grown directly on a desired substrate, because the substrate will usually be heavily damaged under the harsh reaction conditions employed. Radical chemistry presents the second major route to functionalize the, otherwise, chemically inert graphene with chemical groups. A number of methods have been proposed in the literature, such as plasma functionalization,12,13 photochemistry,14,15 and aryldiazonium chemistry.16−18 In general, plasma polymerization and photochemistry provide mild functionalization pathways, resulting in less defects in the graphene structure than the strong chemical oxidation. However, side reactions derived from the manyfold kinds of surface chemistry initiated by these methods are to be expected.11 Aryl radicals derived from the reduction of aryldiazonium salts are widely used as grafting agents for obtaining a stable chemical modification by creating C−C bond attachments. For the most reactive aryldiazonium salts, such as 4-nitrobenzenediazonium, these reactions even occur spontaneously without any need for additional activation, in particular, at the edges and defect sites of graphene.16,17 For other aryldiazonium Received: April 7, 2014
A
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the GC used as a support material might have become modified itself.33 In this study, we disclose the electrochemical carboxylation of graphene sheets formed by chemical vapor deposition on a nickel substrate (denoted Ni−Gra), as shown in Scheme 1. In
salts, electroactivation may be used to induce the desired surface modification, although the formation of multi-layered and insulating aryl layers can be difficult to avoid. Recently, it was shown that the grafting density at the basal plane of graphene could be reasonably well-controlled using fast voltammetric sweeping as a technique.18 The formation of disordered multi-layered films at graphene is an inherent drawback of most radical-based techniques, with the exceptions being simple fluorination14 and chlorination processes.15 This is a serious problem when designing electronic and electrochemically based sensors, where it is essential that the sensing component be attached in close proximity to the surface to enable direct electron transfer.19 Recently, Bonanni et al. presented an interesting new radicalbased two-step approach to introduce carboxyl groups at a rGO surface, comprising first the thermal decomposition of azobis(isobutyronitrile) to functionalize graphene with isobutyronitrile groups, which is subsequently converted into the corresponding carboxyl groups.20 In addition, it was shown that such structures could serve as a platform for enhanced biosensing applications. Alternative pathways for carboxylating graphene exist, such as Diels−Alder-based reactions21 and non-covalent functionalization of graphene through π−π stacking.22,23 These procedures are attractive in many respects, but none of them results in a direct and controlled attachment of carboxyl groups to the basal plane of graphene. Hence, development of versatile methods for introducing carboxylate groups directly on pristine graphene is essential to fully exploit the capabilities of graphene in especially biosensor applications.20 In this respect, electrochemical grafting of electrophilic CO2 using graphene as the working electrode would seem to provide a straightforward pathway. The electrochemical reduction of CO2 on mainly metallic electrodes has been thoroughly studied in protic24 and aprotic25−28 media. Under aprotic conditions, the major product is oxalate, originating from dimerization of CO2• −, carbon monoxide, and formate. In aqueous solution, formate is the main product, while in methanol, further reduction of the protonated oxalate leads to significant amounts of glyoxylate and glycolate.29 Simonet and co-workers30,31 have investigated the electrochemical carboxylation of glassy carbon (GC) and highly ordered pyrolytic graphite (HOPG) electrodes by CO2 in N,Ndimethylformamide and found extensive functionalization within the bulk of the material. This conclusion was arrived at from a determination of the number of chemically active carboxylate groups, which were found to form what would be equivalent to 100 layers on the electrode surface. Such a bulk functionalization can easily be understood from the reasonable assumption that a buildup of negative charge during charging of the carbonaceous material is assisted by intercalation of tetraalkylammonium ions from the supporting electrolyte.32 This nucleophilic and electron-rich material reacts with CO2 in a carboxylation reaction that will gradually change the surface structure. In support of this interpretation was the observation that extensive functionalization in the case of GC and HOPG could only take place if the smaller Me4N+ rather than Bu4N+ was employed as the cation constituent of the supporting electrolyte.30,31 Most recently, Viatcheslav and Simonet also reported on the electrochemical carboxylation of graphene flakes deposited on GC but with only little characterization carried out; neither the nature nor the location of the modifications sites could be addressed, let alone the risk that
Scheme 1. Proposed Mechanism (Shown Using Chemical Structures and a Cartoon) for the Repetitive Two-Step Intercalation/Carboxylation of Graphene Consisting of Electrochemical Charging of Ni−Gra in the Presence of Bu4N+ (⊕) Followed by Reaction with Added CO2
contrast to GC and HOPG, the multi-layered Ni−Gra may be intercalated also by the larger Bu4N+ ion upon charging at a fixed potential in a degassed solution. In the second step, the strongly activated and nucleophilic graphene is allowed to react with added CO2 to give carboxylate groups, now being stabilized by Bu4N+. Importantly, this modification cycle can be applied repetitively to increase the functionalization in a controlled manner. On a more general level, this approach will, in addition to biosensor applications, as previously shown,20 provide a versatile way of forming new hybrid materials based on intimate bond coupling to graphene via carboxylate groups.
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INSTRUMENTS AND PROCEDURES
Materials. Chemically vapor-deposited graphene on Ni (itself deposited on silicon as a thin film) was purchased from Graphene Supermarket. According to the specifications the samples are composed of patches of 1−7 layers with a size of 3−10 μm; average number of layers is 4. All samples were analyzed using XPS and Raman spectroscopy prior to use. Acetonitrile (anhydrous, 99.9%, Lab-Scan) was dried prior to electrochemical experiments by passing it through a column of activated (i.e., heated to 350 °C under vacuum) Al2O3 (99.99%, Sigma-Aldrich). After this treatment, the water content was determined to be 0.03 ± 0.01 wt % by Karl Fisher titration (TitraLab 980, Radiometer Analytical). CO2 was purchased from AGA and used as received. Tetrabutylammonium tetrafluoroborate (Bu4NBF4) was synthesized using a standard procedure. Electrochemical Setup. Cyclic voltammetry was carried out with CH Instruments 660B or 601C in a regular three-electrode setup using Ni−Gra as the working electrode, Ag/AgI as the reference electrode, and a platinum wire as the counter electrode. The upper 1 mm of the Ni−Gra plated was mounted in a holder, with the lower 7−8 mm dipped into the electrolyte solution, meaning that the active electrode area was ≈75 mm2. A macro feature of the CH Instruments software was exploited to have a rapid shift between different electrochemical techniques with no manual delay. All samples were rinsed with acetone and dried with nitrogen gas before analysis. Open circuit potentiometry was performed with the CH Instruments software developed for that specific purpose on a two-electrode setup comprising the Ni−Gra and Ag/AgI electrodes. The time interval between measurements was 0.1 s. B
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Polarization Modulation−Infrared Reflection Adsorption Spectroscopy (PM−IRRAS). Spectra were recorded on a Nicolet 6700 (Thermo) FTIR spectrometer equipped with a Varian external experiment module with a narrow band mercury−cadmium−telluride (MCT/A) detector cooled in liquid nitrogen. The infrared (IR) beam was directed through a gold wire polarizer and modulated between s and p polarization at a frequency of 74 kHz by a photoelastic modulator (PEM 90, Hinds Instruments, Hillsboro, OR). The PEM was adjusted so that the s polarization was linear for the wavenumber of 1500 cm−1. The Ni−Gra substrates were irradiated with an incident grazing angle of 80°, where the beam has a spot size with a diameter of 1−2 mm. The two signals, Rp − Rs and Rp + Rs, were extracted with a high-pass filter (40 kHz, EG&G model 189) and a lock-in amplifier (SR 810 DSP). The intensity signal is (Rp − Rs)/(Rp + Rs). The experimental settings were as follows: 200 scans were acquired with a resolution of 4 cm−1 at room temperature in a dried atmosphere. The intensity spectra for the modified sample were divided with the spectrum of the unmodified sample before baseline correction was performed using the facilities of the OMNIC 8.2 software. Raman Spectroscopy. Measurements were performed with a DXR Raman microscope from Thermo Scientific equipped with a 10 mW, 532 nm laser. In general, on each sample, in total 200 spectra were recorded in 10 × 10 spot arrays with an area of approximately 45 × 45 μm2 at two different locations using a spot size of ∼0.6 μm in diameter. For the data analysis, an automated MATLAB script was used, in which the peaks in each spectrum were fitted using as few Lorentzian peaks as possible on a polynomial background and, finally, averaged. X-ray Photoelectron Spectroscopy (XPS). XPS analyses were conducted using a Kratos Axis Ultra-DLD instrument operated with a monochromatic Al Kα X-ray source at a power of 150 W (area of the spot size of 0.21 mm2). Survey scans were acquired by accumulating two sweeps in the 0−1100 eV range at a pass energy of 160 eV. Highresolution scans were acquired at a pass energy of 20 eV. The pressure in the main chamber during the analysis was in the 10−11 bar range. The generated XPS data were processed using the CasaXPS software. Atomic surface concentrations were determined by fitting the core level spectra using Gaussian−Lorentzian line shapes and a Shirley background. The spectra are shown without energy-scale correction. The systematic error is estimated to be on the order of 5−10%.
or beyond caused delamination of the nickel and graphene layers from the silicon substrate, on which they were deposited. This problem was not encountered at less extreme potentials, and it was therefore decided to perform all charging experiments at E = −1.90 V versus Ag/AgI. Recording of additional voltammograms (fifth and seventh included in Figure 1) reveals an appreciable increase of i, as would be expected if the surface area became enlarged as a result of Bu4N+ intercalation during sweeping. Noteworthy, voltammograms recorded in a CO2-saturated electrolyte solution (concentration ≈ 270 mM)34 in the same potential range give, by and large, the same picture with no distinct peaks arising from a direct reduction of CO2 to its radical anion (inset of Figure 1). In some cases, a small irreversible reduction wave at −1.5 V versus Ag/AgI is discernible, which is most likely due to the reduction of CO2 at the edges of the wafer, where the underlying nickel becomes directly exposed to the solution. In contrast, on carbon substrates, the reduction is usually not observed until at least −1.7 V versus Ag/AgI,35 and at Ni−Gra, it seems to occur at even more negative potentials. Figure 2 shows the effect on the open circuit potential, Eocp, of adding CO2 to an electrolyte solution containing a Ni−Gra
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Figure 2. Recording of Eocp versus t for a charged/intercalated Ni−Gra sample in the absence () and presence (- - -) of CO2 in 0.1 M Bu4NBF4/MeCN.
Figure 1. First (), fifth (- - -), and seventh (···) cyclic voltammograms recorded at Ni−Gra in 0.1 M Bu4NBF4/MeCN using a sweep rate of 0.1 V s−1. The inset shows a corresponding set of voltammograms for a CO2-saturated electrolyte solution.
electrode, which was prior to this charged for 15 s at −1.9 V versus Ag/AgI. As seen, Eocp increases rapidly as a function of time (t) and reaches within 90 s a steady value (≈0.4 V versus Ag/AgI). In comparison, the control experiment without CO2 present shows a substantially slower change in Eocp, thereby demonstrating, first of all, that the charged Ni−Gra electrode reacts with CO2. Still, a discharging of the electrodes occurs in a pure electrolyte solution, the origin of which we attribute to the presence of adventitious electrophilic impurities. This may also explain why the initial Eocp = −0.7 V versus Ag/AgI (at t = 0 s) is 1.2 V higher than the potential applied during charging. Panels a and b of Figure 3 provide overviews of the development in i and Eocp, respectively, as the two-step modification involving charging/intercalation and carboxylation, respectively, is repeated 9 times. The former plot reveals that i or, more correctly, the total charge consumed during charging of the Ni−Gra electrode becomes increasingly larger the first 5−6 times, which we interpret in terms of an initial opening of the multi-layered graphene structure because of intercalation of Bu4N+. After the sixth repetition, the charging process becomes less efficient. The fact that the charging capacity of the Ni−Gra electrode diminishes from this point on
RESULTS AND DISCUSSION Electrochemical Intercalation/Carboxylation. Figure 1 shows the cyclic voltammogram of the current (i) versus potential (E) recorded at Ni−Gra as the working electrode in a standard three electrode setup in 0.1 M Bu4NBF4/MeCN. Two ill-defined reduction waves are visible at E = −1.65 and −1.83 V versus Ag/AgI. Charging of Ni−Gra at −2.0 V versus Ag/AgI
C
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Figure 4. High-resolution C 1s XPS spectra of (a) Ni−Gra fitted with a single asymmetric peak and (b) Ni−Gra−CO2 obtained after 6 repetitive modification cycles fitted using several components.
Figure 3. Recordings of (a) i versus t during charging/intercalation and (b) Eocp versus t for the carboxylation process in 9 repetitive modification cycles performed in 0.1 M Bu4NBF4/MeCN. The explicit number of modification cycles used is indicated on each of the plots.
prior to modification. Following the 6 modification cycles, the peak is considerably broadened as a consequence of appreciable changes to the graphene lattice and intercalation of Bu4N+ ions. A full deconvolution of the C 1s peak now requires several contributions. In addition to the asymmetric peak at 284.4 eV, new symmetric peaks arise from the introduction of sp3hybridized carbon atoms and carbon atoms with oxygen functionalities. The peaks at 285.2, 286.1, 286.9, and 288.0 eV are ascribed to C−C, C−O, CO/C−N, and O−CO, respectively.37,38 With the appearance of a distinct O−CO component, the successful outcome of the carboxylation process is thus confirmed. The relative contribution of O−CO with respect to CC is 1.3 ± 0.3%, which may be translated into a functionalization degree of 1:80 carbon atoms at the graphene lattice. This number will be further discussed below. A large proportion of the sp3 C−C component is expected to originate from the intercalation of Bu4N+, the presence of which was strongly inferred by PM−IRRAS (see Figures S1 and S2 of the Supporting Information). The XPS data presented in Figure 5 on the N/C ratio support this preposition because a steady increase is seen as the number of modification cycles is enhanced, until the sixth, where a plateau value at 0.055 is reached. The contribution to N from intercalated solvent (acetonitrile) is negligible according to PM−IRRAS (see the Supporting Information). Raman Spectroscopy. Figure 6a shows a representative Raman spectrum of a bare Ni−Gra substrate, where the main characteristics are the high-intensity narrow G peak at 1580 cm−1, the 2D peak at 2670 cm−1, and a low-intensity D peak at 1340 cm−1. While the G peak is a signature of the sp2hybridized graphene network, the D peak is mainly arising as a result of irregularities in this network because of functionaliza-
may be explained by the occurrence of an increasingly larger disruption of the graphene lattice as, in fact, would be expected to be the outcome of a carboxylation in process. Figure 3b depicts the development in Eocp for 90 s as CO2 was bubbled through the solution following the charging. Besides the observation that Eocp increases with time for all traces, the most noteworthy feature is its great dependence upon the modification cycle number. In fact, the initial value of Eocp reflects the charging behavior, in that it becomes increasingly more negative for the first cycles because of the intercalation of Bu4N+. This means that a much larger surface area is being charged and more electrophiles will be needed for the discharging. For the following modification cycles, the initial Eocp decreases, showing that the graphene structure, now being disrupted because of the introduction of carboxylate groups, becomes more difficult to charge. Note that the difference seen for the first trace in Figure 3b and the corresponding one in Figure 2 (with CO2) may serve to illustrate the typical variations observed when going from one Ni−Gra sample to another. To obtain more detailed information about these issues, Ni− Gra and Ni−Gra−CO2 samples were analyzed by means of XPS, Raman spectroscopy, and PM−IRRAS. Foremost, PM− IRRAS gives unambiguous evidence for the intercalation process involving Bu4N+, as discussed on the basis of Figures S1 and S2 of the Supporting Information. XPS. Panels a and b of Figure 4 show the high-resolution C 1s spectra of Ni−Gra and Ni−Gra−CO2 after 6 repetitive modification cycles, respectively. In the former case, a single asymmetric peak at 284.4 eV is observed, as would be expected for a graphene or graphitic-like material with semi-metallic behavior.36,37 This confirms the good quality of the substrate D
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Figure 6b shows a representative Raman spectrum recording of Ni−Gra−CO2, which contains several features worth noting. First of all, the ID/IG ratio has increased significantly, indicating the introduction of defects or functionalities in the graphene structure. Furthermore, new peaks are observed at ∼1480 and ∼3000 cm−1 (after deconvolution), where the latter has two origins, namely, Bu4N+ and second-order defect peaks from graphene. The peak at 1480 cm−1 is assigned to C−H sp3 deformations from Bu4N+, indicating the significant amount of intercalation taking place. In fact, after the first modification cycle, a remarkably constant relation is found for the ratio between the D and 1480 cm−1 peaks (see Figure S5 of the Supporting Information), showing a tight relationship between the degree of functionalization and the amount of intercalated counterions in terms of −CO2−Bu4N+ ion pair formation. Another interesting feature in this respect is that the I2D/IG intensity ratio (see Figure S6 of the Supporting Information) is slightly increased for Ni−Gra−CO2, despite the fact that the introduction of defects in the graphene sheet should broaden the 2D peak and, thus, lower its intensity.40 Once again, the explanation lies in the intercalation of Bu4N+ that separates the graphene sheets electronically and converts the structure from HOPG-like to that of turbostratic graphite or even separated single layers of graphene. Figure 7 shows a plot of ID/IG as the modification procedure is repeated. As seen, the increase in ID/IG is steady, until at least
Figure 5. Plot of the XPS-derived N/C ratio against the number of modification cycles for Ni−Gra−CO2.
Figure 6. Representative Raman spectra of (a) Ni−Gra and (b) Ni− Gra−CO2 obtained after 6 repetitive modification cycles. Deconvolution was carried out using Lorentzian fits.
Figure 7. Plot of ID/IG as a function of the number of modification cycles for Ni−Gra−CO2. Error bars represent standard deviations.
tion, edges, and holes. Thus, the intensity ratio of the D and G peaks, ID/IG, is an extremely sensitive parameter for measuring the degree of functionalization of graphene. For Ni−Gra, ID/IG is as small as 0.13 ± 0.09, which confirms the high quality of graphene. The 2D peak at ∼2700 cm−1 showed relatively large variations in both shape and intensity for different areas on the same Ni−Gra sample. Some of these data are available in Figure S3 of the Supporting Information, along with corresponding XPS data (see Table S1 and Figure S4 of the Supporting Information). From these variations,38,39 it was concluded that Ni−Gra contained 1−10 layers of graphene, with an average around four layers, in accordance with our previous work.18 To ensure that reliable data could be extracted, despite this inhomogeneity, at least 200 Raman spectra were measured and averaged for each sample. Furthermore, because of the multi-layered and inhomogeneous Ni−Gra samples, the ID/IG values were taken as the area of the peaks and not the maximum intensity, as normally used for single-layer graphene.
the sixth modification cycle, which shows the ability of the electrochemical approach to exert good control on the functionalization degree. At this cycle, a limiting value of around 1 is reached, which would correspond to a functionalization degree of 1:400 carbon atoms in graphene according to a recent model proposed for interpretation of such Raman data.41,42 This ratio is similar to that of 1:250 found in our previous study on electrografting of aryldiazonium salts at Ni−Gra.18 Unfortunately, the uncertainty on these numbers is high, because the calculation requires a number of assumptions on the exact nature of the structural defect sites in graphene. Furthermore, the validity of the model has not been shown for graphene on a metal substrate. For this reason, we put more trust in the XPS analysis, which gave a somewhat higher functionalization degree of 1:80 (vide supra). This number is also equal to the electrochemically determined 1:80 reported by Dano and Simonet on the carboxylation of graphite.30 The next question is if the individual layers of multi-layered regions are functionalized to the same extent. On the basis of the observation that ID/IG is independent of the exact I2D/IG E
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importantly, the carboxylation degree can be precisely controlled through strict control of the electrochemical charging process. Potentially, the carboxylated graphene may serve as a useful platform for further functionalization, e.g., attachment of biomolecules or nanoparticles, thus making this procedure interesting for fabrication of biosensors and hybrid materials.
value (see Figure S7 of the Supporting Information), the conclusion is that this is indeed the case, the origin of which, presumably, is the opening of the entire graphene structure as a response to the intercalation of Bu4N+. Hence, the electrochemical method offers a versatile procedure to make all graphene sheets in a multi-layered but expanded structure accessible for functionalization. Concerning the current efficiency for these carboxylation reactions, it is extremely low, namely, 0.02%, calculated on the basis of the charge consumed after 6 modification cycles (=0.104 C) and the number of functionalized carbon atoms from XPS (=1:80). This indicates that most of the charge apparently goes to reducing CO2 to its radical anion, followed by its dimerization to oxalate, as described in studies using mainly metals as working electrodes.25−28 In fact, this circumstance turns out to be a great advantage for the carboxylation approach proposed herein, in that a too efficient grating process would have made it very difficult to control the functionalization degree. Besides the high controllability that this two-step modification procedure offers, it also gives valuable information about each of the individual steps, i.e., the charging and carboxylation. However, for future use as a carboxylation tool, it would be advantageous if the overall intercalation/carboxylation modification could be accomplished in a simple one-step continuous electrolysis process without the intermediate need of purging with argon and charging the graphene. Encouragingly, a few such experiments performed on saturated CO2 solutions at E = −1.9 V versus Ag/AgI gave comparable carboxylation patterns, as obtained in the two-step procedure, indicating that a complete functionalization could be accomplished within 2 min under such conditions. In general, the electrolysis current was 5−6 times higher than the charging current recorded in the previous experiments, which again may be attributed to the concomitant reduction of CO2 to its radical anion. Importantly, the functionalization degree may be fully controlled by the electrolysis time, making this carboxylation procedure recommendable in most situations, if nothing else for the sake of convenience and less time consumption. This being said, it should be emphasized that the Bu4N+-intercalated graphene structures generated in the two-step procedure are enormously interesting entities in themselves because of their high nucleophilicity, which should make it possible to functionalize them with a variety of electrophiles.31 In this manner, they would become a versatile platform for creating high-area expanded graphene materials possessing multifunctionality.
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ASSOCIATED CONTENT
S Supporting Information *
PM−IRRAS, XPS, and Raman analysis of Ni−Gra and Ni− Gra−CO2. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The Danish Agency for Science, Technology and Innovation and the High Technological Foundation are gratefully acknowledged for financial support. Newtec A/S is thanked for carrying out Raman measurements.
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REFERENCES
(1) Nair, R. R.; Blake, P.; Grigorenko, A. N.; Novoselov, K. S.; Booth, T. J.; Stauber, T.; Peres, N. M. R.; Geim, A. K. Fine structure constant defines visual transparency of graphene. Science 2008, 320, 1308. (2) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. Twodimensional gas of massless Dirac fermions in graphene. Nature 2005, 438, 197−200. (3) Zhang, Y. B.; Tan, Y. W.; Stormer, H. L.; Kim, P. Experimental observation of the quantum hall effect and Berry’s phase in graphene. Nature 2005, 438, 201−204. (4) Blake, P.; Yang, R.; Morozov, S. V.; Schedin, F.; Ponomarenko, L. A.; Zhukov, A. A.; Nair, R. R.; Grigorieva, I. V.; Novoselov, K. S.; Geim, A. K. Influence of metal contacts and charge inhomogeneity on transport properties of graphene near the neutrality point. Solid State Commun. 2009, 149, 1068−1071. (5) Lee, C.; Wei, X.; Kysar, J. W.; Hone, J. Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science 2008, 321, 385−388. (6) Bunch, J. S.; Verbridge, S. S.; Alden, J. S.; van der Zande, A. M.; Parpia, J. M.; Craighead, H. G.; McEuen, P. L. Impermeable atomic membranes from graphene sheets. Nano Lett. 2008, 8, 2458−2462. (7) Compton, O. C.; Kim, S.; Pierre, C.; Torkelson, J. M.; Nguyen, S. T. Crumpled graphene nanosheets as highly effective barrier property enhancers. Adv. Mater. 2010, 22, 4759−4763. (8) Georgakilas, V.; Otyepka, M.; Bourlinos, A. B.; Chandra, V.; Kim, N.; Kemp, K. C.; Hobza, P.; Zboril, R.; Kim, K. S. Functionalization of graphene: Covalent and non-covalent approaches, derivatives and applications. Chem. Rev. 2012, 112, 6156−6214. (9) Gómez-Navarro, C.; Meyer, J. C.; Sundaram, R. S.; Chuvilin, A.; Kurasch, S.; Burghard, M.; Kern, K.; Kaiser, U. Atomic structure of reduced graphene oxide. Nano Lett. 2010, 10, 1144−1148. (10) Mkhoyan, K. A.; Contryman, A. W.; Silcox, J.; Stewart, D. A.; Eda, G.; Mattevi, C.; Miller, S.; Chhowalla, M. Atomic and electronic structure of graphene oxide. Nano Lett. 2009, 9, 1058−1063. (11) Zeng, L.; Wang, W.; Liang, J.; Wang, Z.; Xia, Y.; Lei, D.; Ren, X.; Yao, N.; Zhang, B. The changes of morphology, structure and optical properties from carbon nanotubes treated by hydrogen plasma. Mater. Chem. Phys. 2008, 108, 82−87.
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CONCLUSION Multi-layered graphene on a nickel substrate can be converted into a strongly nucleophilic species through electrochemical charging and intercalation of tetrabutylammonium ions. The nucleophilic charged graphene is capable of reacting with an electrophile, such as carbon dioxide, resulting in the functionalization of graphene with carboxylate groups. In contrast to other carboxylation techniques, the electrochemical method gives assembling directly at the graphene surface. Moreover, the initial intercalation of tetrabutylammonium ions opens the entire graphene structure, through which all of the graphene layers become accessible to carboxylation. The charging and carboxylation steps can be carried out separately or concomitantly in simple experimental setups, where, F
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(12) Bon, S. B.; Valentini, L.; Verdejo, R.; Garcia Fierro, J. L.; Peponi, L.; Lopez-Manchado, M. A.; Kenny, J. M. Plasma fluorination of chemically derived graphene sheets and subsequent modification with butylamine. Chem. Mater. 2009, 21, 3433−3438. (13) Baraket, M.; Stine, R.; Lee, W. K.; Robinson, J. T.; Tamanaha, C. R.; Sheehan, P. E.; Walton, S. G. Aminated graphene for DNA attachment produced via plasma functionalization. Appl. Phys. Lett. 2012, 100, 233123. (14) Zhang, L.; Zhou, L.; Yang, M.; Liu, Z.; Xie, Q.; Peng, H.; Liu, Z. Photo-induced free radical modification of graphene. Small 2013, 9, 1134−1143. (15) Li, B.; Zhou, L.; Wu, D.; Peng, H.; Yan, K.; Zhou, Y.; Liu, Z. Photochemical chlorination of graphene. ACS Nano 2011, 5, 5957− 5961. (16) Bekyarova, E.; Itkis, M. E.; Ramesh, P.; Berger, C.; Sprinkle, M.; de Heer, W. A.; Haddon, R. C. Chemical modification of epitaxial graphene: Spontaneous grafting of aryl groups. J. Am. Chem. Soc. 2009, 131, 1336−1337. (17) Sharma, R.; Baik, J. H.; Perera, C. J.; Strano, M. S. Anomalously large reactivity of single graphene layers and edges toward electron transfer chemistries. Nano Lett. 2010, 10, 398−405. (18) Lillethorup, M.; Kongsfelt, M.; Ceccato, M.; Jensen, B. B. E.; Jørgensen, B.; Pedersen, S. U.; Daasbjerg, K. High- versus low-quality graphene: A mechanistic investigation of electrografted diazoniumbased films for growth of polymer brushes. Small 2014, 10, 922−934. (19) Liang, B.; Fang, L.; Yang, G.; Hu, Y.; Guo, X.; Ye, X. Direct electron transfer glucose biosensor based on glucose oxidase selfassembled on electrochemically reduced carboxyl graphene. Biosens. Bioelectron. 2013, 43, 131−136. (20) Bonanni, A.; Chua, C. K.; Pumera, M. Rational design of carboxyl groups perpendicularly attached to a graphene sheet: A platform for enhanced biosensing applications. Chem.Eur. J. 2014, 20, 217−222. (21) Bian, S.; Scott, A. M.; Cao, Y.; Liang, Y.; Osuna, S.; Houk, K. N.; Braunschweig, A. B. Covalently patterned graphene surfaces by a forceaccelerated Diels−Alder reaction. J. Am. Chem. Soc. 2013, 135, 9240− 9243. (22) Yinjie, K.; Jinhua, C.; Xingliang, Z.; Xiaohua, Z.; Qionghua, Z.; Cuihong, L. Synthesis of carboxylate-functionalized graphene nanosheets for high dispersion of platinum nanoparticles based on the reduction of graphene oxide via 1-pyrenecarboxaldehyde. Nanotechnology 2013, 24, 395604. (23) Kodali, V. K.; Scrimgeour, J.; Kim, S.; Hankinson, J. H.; Carroll, K. M.; de Heer, W. A.; Berger, C.; Curtis, J. E. Nonperturbative chemical modification of graphene for protein micropatterning. Langmuir 2011, 27, 863−865. (24) Fischer, J.; Lehmann, T.; Heitz, E. The production of oxalic acid from CO2 and H2O. J. Appl. Electrochem. 1981, 11, 743−750. (25) Gennaro, A.; Isse, A. A.; Severin, M.-G.; Vianello, E.; Bhugun, I.; Savéant, J.-M. Mechanism of the electrochemical reduction of carbon dioxide at inert electrodes in media of low proton availability. J. Chem. Soc., Faraday Trans. 1996, 92, 3963−3968. (26) Bernard, G.; Simonet, J. Mixed electrochemical reduction of graphite and organic electrophiles: A possible method of building modified carbon electrodes? J. Electroanal. Chem. Interfacial Electrochem. 1980, 112, 117−125. (27) Costentin, C.; Robert, M.; Savéant, J.-M. Catalysis of the electrochemical reduction of carbon dioxide. Chem. Soc. Rev. 2013, 42, 2423−2436. (28) Amatore, C.; Savéant, J.-M. Mechanism and kinetic characteristics of the electrochemical reduction of carbon dioxide in media of low proton availability. J. Am. Chem. Soc. 1981, 103, 5021−5023. (29) Eggins, B. R.; Ennis, C.; McConnell, R.; Spence, M. Improved yields of oxalate, glyoxylate and glycolate from the electrochemical reduction of carbon dioxide in methanol. J. Appl. Electrochem. 1997, 27, 706−712. (30) Dano, C.; Simonet, J. Cathodic reactivity of graphite with carbon dioxide: An efficient formation of carboxylated carbon materials. J. Electroanal. Chem. 2004, 564, 115−121.
(31) Simonet, J. Conditions for a large cathodic sequestration of carbon dioxide into glassy carbon. Generation of a carbon polycarboxylate versatile material. Electrochem. Commun. 2012, 21, 22−25. (32) Simonet, J.; Lund, H. Electrochemical behaviour of graphite cathodes in the presence of tetraalkylammonium cations. J. Electroanal. Chem. Interfacial Electrochem. 1977, 75, 719−730. (33) Jouikov, V.; Simonet, J. Efficient cathodic carboxylation of graphene: Building a new versatile material. Electrochem. Commun. 2014, 43, 67−70. (34) Tomita, Y.; Teruya, S.; Koga, O.; Hori, Y. Electrochemical reduction of carbon dioxide at a platinum electrode in acetonitrile− water mixtures. J. Electrochem. Soc. 2000, 147, 4164−4167. (35) Christensen, P. A.; Hamnett, A.; Muir, A. V. G.; Freeman, N. A. CO2 reduction at platinum, gold and glassy carbon electrodes in acetonitrile: An in-situ FTIR study. J. Electroanal. Chem. Interfacial Electrochem. 1990, 288, 197−215. (36) Wallace, P. R. The band theory of graphite. Phys. Rev. 1947, 71, 622−634. (37) Yamada, Y.; Yasuda, H.; Murota, K.; Nakamura, M.; Sodesawa, T.; Sato, S. Analysis of heat-treated graphite oxide by X-ray photoelectron spectroscopy. J. Mater. Sci. 2013, 48, 8171−8198. (38) Nolan, H.; Mendoza-Sanchez, B.; Ashok Kumar, N.; McEvoy, N.; O’Brien, S.; Nicolosi, V.; Duesberg, G. S. Nitrogen-doped reduced graphene oxide electrodes for electrochemical supercapacitors. Phys. Chem. Chem. Phys. 2014, 16, 2280−2284. (39) Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; Geim, A. K. Raman spectrum of graphene and graphene layers. Phys. Rev. Lett. 2006, 97, 187401. (40) Eckmann, A.; Felten, A.; Mishchenko, A.; Britnell, L.; Krupke, R.; Novoselov, K. S.; Casiraghi, C. Probing the nature of defects in graphene by Raman spectroscopy. Nano Lett. 2012, 12, 3925−3930. (41) Wang, Q. H.; Jin, Z.; Kim, K. K.; Hilmer, A. J.; Paulus, G. L. C.; Shih, C.-J.; Ham, M.-H.; Sanchez-Yamagishi, J. D.; Watanabe, K.; Taniguchi, T.; Kong, J.; Jarillo-Herrero, P.; Strano, M. S. Understanding and controlling the substrate effect on graphene electrontransfer chemistry via reactivity imprint lithography. Nat. Chem. 2012, 4, 724−732. (42) Lucchese, M. M.; Stavale, F.; Ferreira, E. H. M.; Vilani, C.; Moutinho, M. V. O.; Capaz, R. B.; Achete, C. A.; Jorio, A. Quantifying ion-induced defects and Raman relaxation length in graphene. Carbon 2010, 48, 1592−1597.
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Controlled Electrochemical Carboxylation of Graphene To Create a Versatile Chemical Platform for Further Functionalization Emil Bjerglund, Mikkel Kongsfelt, Kyoko Shimizu, Bjarke Bror Egede Jensen, Line Koefoed, Marcel Ceccato, Troels Skrydstrup, Steen U. Pedersen, and Kim Daasbjerg* Department of Chemistry and Interdisciplinary Nanoscience Center, University of Aarhus, Langelandsgade 140, 8000 Aarhus C, Denmark S Supporting Information *
ABSTRACT: An electrochemical approach is introduced for the versatile carboxylation of multi-layered graphene in 0.1 M Bu4NBF4/MeCN. First, the graphene substrate (i.e., graphene chemically vapor-deposited on Ni) is negatively charged at −1.9 V versus Ag/AgI in a degassed solution to allow for intercalation of Bu4N+ and, thereby, separation of the individual graphene sheets. In the next step, the strongly activated and nucleophilic graphene is allowed to react with added carbon dioxide in an addition reaction, introducing carboxylate groups stabilized by Bu4N+ already present. This procedure may be carried out repetitively to further enhance the carboxylation degree under controlled conditions. Encouragingly, the same degree of control is even attainable, if the intercalation and carboxylation is carried out simultaneously in a one-step procedure, consisting of simply electrolyzing in a CO2-saturated solution at the graphene electrode for a given time. The same functionalization degree is obtained for all multilayered regions, independent of the number of graphene sheets, which is due to the fact that the entire graphene structure is opened in response to the intercalation of Bu4N+. Hence, this electrochemical method offers a versatile procedure to make all graphene sheets in a multi-layered but expanded structure accessible for functionalization. On a more general level, this approach will provide a versatile way of forming new hybrid materials based on intimate bond coupling to graphene via carboxylate groups.
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INTRODUCTION The well-known honeycomb lattice structure of graphene is the foundation for the many highly praised properties of graphene. These properties include high transparency of light in the visible spectrum1 combined with ballistic charge transport,2−4 extremely high mechanical strength,5 and good barrier properties.6,7 However, these otherwise unprecedented properties of graphene will prove difficult to exploit in real applications without a well-equipped chemical toolbox for handling and modifying graphene in a controlled manner. Carboxyl groups and carboxylic acid derivatives are among the most useful functional groups exploited within organic chemistry and biochemistry in a large variety of available reactions.8 It is thus of immense importance to make such functional groups available on graphene. Two routes are commonly used to obtain carboxyl groups on graphene, i.e., chemical oxidation and radical-based reactions. The former is a classical way of modifying carbon materials based on using strongly oxidizing chemical conditions, typically comprising chromic/sulfuric acid or KMnO4/H2O2, to enable introduction of oxygen-containing functional groups, including carboxyl groups. This method is highly efficient, but the harsh conditions employed also introduce a large number of defects along with a restructuring of the surface.9,10 Hence, the resulting material obtained is changed so significantly that the properties are no longer comparable to graphene and is instead known as graphene oxide (GO). By reduction of GO, a material known as reduced graphene oxide (rGO) is obtained, still containing a © XXXX American Chemical Society
large amount of carboxylate groups. As opposed to GO, the rGO material is conducting and may be applied in electrochemical sensor applications.11 While this method is applicable to bulk carbon materials, in general, it is not useful for graphene already deposited or grown directly on a desired substrate, because the substrate will usually be heavily damaged under the harsh reaction conditions employed. Radical chemistry presents the second major route to functionalize the, otherwise, chemically inert graphene with chemical groups. A number of methods have been proposed in the literature, such as plasma functionalization,12,13 photochemistry,14,15 and aryldiazonium chemistry.16−18 In general, plasma polymerization and photochemistry provide mild functionalization pathways, resulting in less defects in the graphene structure than the strong chemical oxidation. However, side reactions derived from the manyfold kinds of surface chemistry initiated by these methods are to be expected.11 Aryl radicals derived from the reduction of aryldiazonium salts are widely used as grafting agents for obtaining a stable chemical modification by creating C−C bond attachments. For the most reactive aryldiazonium salts, such as 4-nitrobenzenediazonium, these reactions even occur spontaneously without any need for additional activation, in particular, at the edges and defect sites of graphene.16,17 For other aryldiazonium Received: April 7, 2014
A
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the GC used as a support material might have become modified itself.33 In this study, we disclose the electrochemical carboxylation of graphene sheets formed by chemical vapor deposition on a nickel substrate (denoted Ni−Gra), as shown in Scheme 1. In
salts, electroactivation may be used to induce the desired surface modification, although the formation of multi-layered and insulating aryl layers can be difficult to avoid. Recently, it was shown that the grafting density at the basal plane of graphene could be reasonably well-controlled using fast voltammetric sweeping as a technique.18 The formation of disordered multi-layered films at graphene is an inherent drawback of most radical-based techniques, with the exceptions being simple fluorination14 and chlorination processes.15 This is a serious problem when designing electronic and electrochemically based sensors, where it is essential that the sensing component be attached in close proximity to the surface to enable direct electron transfer.19 Recently, Bonanni et al. presented an interesting new radicalbased two-step approach to introduce carboxyl groups at a rGO surface, comprising first the thermal decomposition of azobis(isobutyronitrile) to functionalize graphene with isobutyronitrile groups, which is subsequently converted into the corresponding carboxyl groups.20 In addition, it was shown that such structures could serve as a platform for enhanced biosensing applications. Alternative pathways for carboxylating graphene exist, such as Diels−Alder-based reactions21 and non-covalent functionalization of graphene through π−π stacking.22,23 These procedures are attractive in many respects, but none of them results in a direct and controlled attachment of carboxyl groups to the basal plane of graphene. Hence, development of versatile methods for introducing carboxylate groups directly on pristine graphene is essential to fully exploit the capabilities of graphene in especially biosensor applications.20 In this respect, electrochemical grafting of electrophilic CO2 using graphene as the working electrode would seem to provide a straightforward pathway. The electrochemical reduction of CO2 on mainly metallic electrodes has been thoroughly studied in protic24 and aprotic25−28 media. Under aprotic conditions, the major product is oxalate, originating from dimerization of CO2• −, carbon monoxide, and formate. In aqueous solution, formate is the main product, while in methanol, further reduction of the protonated oxalate leads to significant amounts of glyoxylate and glycolate.29 Simonet and co-workers30,31 have investigated the electrochemical carboxylation of glassy carbon (GC) and highly ordered pyrolytic graphite (HOPG) electrodes by CO2 in N,Ndimethylformamide and found extensive functionalization within the bulk of the material. This conclusion was arrived at from a determination of the number of chemically active carboxylate groups, which were found to form what would be equivalent to 100 layers on the electrode surface. Such a bulk functionalization can easily be understood from the reasonable assumption that a buildup of negative charge during charging of the carbonaceous material is assisted by intercalation of tetraalkylammonium ions from the supporting electrolyte.32 This nucleophilic and electron-rich material reacts with CO2 in a carboxylation reaction that will gradually change the surface structure. In support of this interpretation was the observation that extensive functionalization in the case of GC and HOPG could only take place if the smaller Me4N+ rather than Bu4N+ was employed as the cation constituent of the supporting electrolyte.30,31 Most recently, Viatcheslav and Simonet also reported on the electrochemical carboxylation of graphene flakes deposited on GC but with only little characterization carried out; neither the nature nor the location of the modifications sites could be addressed, let alone the risk that
Scheme 1. Proposed Mechanism (Shown Using Chemical Structures and a Cartoon) for the Repetitive Two-Step Intercalation/Carboxylation of Graphene Consisting of Electrochemical Charging of Ni−Gra in the Presence of Bu4N+ (⊕) Followed by Reaction with Added CO2
contrast to GC and HOPG, the multi-layered Ni−Gra may be intercalated also by the larger Bu4N+ ion upon charging at a fixed potential in a degassed solution. In the second step, the strongly activated and nucleophilic graphene is allowed to react with added CO2 to give carboxylate groups, now being stabilized by Bu4N+. Importantly, this modification cycle can be applied repetitively to increase the functionalization in a controlled manner. On a more general level, this approach will, in addition to biosensor applications, as previously shown,20 provide a versatile way of forming new hybrid materials based on intimate bond coupling to graphene via carboxylate groups.
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INSTRUMENTS AND PROCEDURES
Materials. Chemically vapor-deposited graphene on Ni (itself deposited on silicon as a thin film) was purchased from Graphene Supermarket. According to the specifications the samples are composed of patches of 1−7 layers with a size of 3−10 μm; average number of layers is 4. All samples were analyzed using XPS and Raman spectroscopy prior to use. Acetonitrile (anhydrous, 99.9%, Lab-Scan) was dried prior to electrochemical experiments by passing it through a column of activated (i.e., heated to 350 °C under vacuum) Al2O3 (99.99%, Sigma-Aldrich). After this treatment, the water content was determined to be 0.03 ± 0.01 wt % by Karl Fisher titration (TitraLab 980, Radiometer Analytical). CO2 was purchased from AGA and used as received. Tetrabutylammonium tetrafluoroborate (Bu4NBF4) was synthesized using a standard procedure. Electrochemical Setup. Cyclic voltammetry was carried out with CH Instruments 660B or 601C in a regular three-electrode setup using Ni−Gra as the working electrode, Ag/AgI as the reference electrode, and a platinum wire as the counter electrode. The upper 1 mm of the Ni−Gra plated was mounted in a holder, with the lower 7−8 mm dipped into the electrolyte solution, meaning that the active electrode area was ≈75 mm2. A macro feature of the CH Instruments software was exploited to have a rapid shift between different electrochemical techniques with no manual delay. All samples were rinsed with acetone and dried with nitrogen gas before analysis. Open circuit potentiometry was performed with the CH Instruments software developed for that specific purpose on a two-electrode setup comprising the Ni−Gra and Ag/AgI electrodes. The time interval between measurements was 0.1 s. B
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Polarization Modulation−Infrared Reflection Adsorption Spectroscopy (PM−IRRAS). Spectra were recorded on a Nicolet 6700 (Thermo) FTIR spectrometer equipped with a Varian external experiment module with a narrow band mercury−cadmium−telluride (MCT/A) detector cooled in liquid nitrogen. The infrared (IR) beam was directed through a gold wire polarizer and modulated between s and p polarization at a frequency of 74 kHz by a photoelastic modulator (PEM 90, Hinds Instruments, Hillsboro, OR). The PEM was adjusted so that the s polarization was linear for the wavenumber of 1500 cm−1. The Ni−Gra substrates were irradiated with an incident grazing angle of 80°, where the beam has a spot size with a diameter of 1−2 mm. The two signals, Rp − Rs and Rp + Rs, were extracted with a high-pass filter (40 kHz, EG&G model 189) and a lock-in amplifier (SR 810 DSP). The intensity signal is (Rp − Rs)/(Rp + Rs). The experimental settings were as follows: 200 scans were acquired with a resolution of 4 cm−1 at room temperature in a dried atmosphere. The intensity spectra for the modified sample were divided with the spectrum of the unmodified sample before baseline correction was performed using the facilities of the OMNIC 8.2 software. Raman Spectroscopy. Measurements were performed with a DXR Raman microscope from Thermo Scientific equipped with a 10 mW, 532 nm laser. In general, on each sample, in total 200 spectra were recorded in 10 × 10 spot arrays with an area of approximately 45 × 45 μm2 at two different locations using a spot size of ∼0.6 μm in diameter. For the data analysis, an automated MATLAB script was used, in which the peaks in each spectrum were fitted using as few Lorentzian peaks as possible on a polynomial background and, finally, averaged. X-ray Photoelectron Spectroscopy (XPS). XPS analyses were conducted using a Kratos Axis Ultra-DLD instrument operated with a monochromatic Al Kα X-ray source at a power of 150 W (area of the spot size of 0.21 mm2). Survey scans were acquired by accumulating two sweeps in the 0−1100 eV range at a pass energy of 160 eV. Highresolution scans were acquired at a pass energy of 20 eV. The pressure in the main chamber during the analysis was in the 10−11 bar range. The generated XPS data were processed using the CasaXPS software. Atomic surface concentrations were determined by fitting the core level spectra using Gaussian−Lorentzian line shapes and a Shirley background. The spectra are shown without energy-scale correction. The systematic error is estimated to be on the order of 5−10%.
or beyond caused delamination of the nickel and graphene layers from the silicon substrate, on which they were deposited. This problem was not encountered at less extreme potentials, and it was therefore decided to perform all charging experiments at E = −1.90 V versus Ag/AgI. Recording of additional voltammograms (fifth and seventh included in Figure 1) reveals an appreciable increase of i, as would be expected if the surface area became enlarged as a result of Bu4N+ intercalation during sweeping. Noteworthy, voltammograms recorded in a CO2-saturated electrolyte solution (concentration ≈ 270 mM)34 in the same potential range give, by and large, the same picture with no distinct peaks arising from a direct reduction of CO2 to its radical anion (inset of Figure 1). In some cases, a small irreversible reduction wave at −1.5 V versus Ag/AgI is discernible, which is most likely due to the reduction of CO2 at the edges of the wafer, where the underlying nickel becomes directly exposed to the solution. In contrast, on carbon substrates, the reduction is usually not observed until at least −1.7 V versus Ag/AgI,35 and at Ni−Gra, it seems to occur at even more negative potentials. Figure 2 shows the effect on the open circuit potential, Eocp, of adding CO2 to an electrolyte solution containing a Ni−Gra
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Figure 2. Recording of Eocp versus t for a charged/intercalated Ni−Gra sample in the absence () and presence (- - -) of CO2 in 0.1 M Bu4NBF4/MeCN.
Figure 1. First (), fifth (- - -), and seventh (···) cyclic voltammograms recorded at Ni−Gra in 0.1 M Bu4NBF4/MeCN using a sweep rate of 0.1 V s−1. The inset shows a corresponding set of voltammograms for a CO2-saturated electrolyte solution.
electrode, which was prior to this charged for 15 s at −1.9 V versus Ag/AgI. As seen, Eocp increases rapidly as a function of time (t) and reaches within 90 s a steady value (≈0.4 V versus Ag/AgI). In comparison, the control experiment without CO2 present shows a substantially slower change in Eocp, thereby demonstrating, first of all, that the charged Ni−Gra electrode reacts with CO2. Still, a discharging of the electrodes occurs in a pure electrolyte solution, the origin of which we attribute to the presence of adventitious electrophilic impurities. This may also explain why the initial Eocp = −0.7 V versus Ag/AgI (at t = 0 s) is 1.2 V higher than the potential applied during charging. Panels a and b of Figure 3 provide overviews of the development in i and Eocp, respectively, as the two-step modification involving charging/intercalation and carboxylation, respectively, is repeated 9 times. The former plot reveals that i or, more correctly, the total charge consumed during charging of the Ni−Gra electrode becomes increasingly larger the first 5−6 times, which we interpret in terms of an initial opening of the multi-layered graphene structure because of intercalation of Bu4N+. After the sixth repetition, the charging process becomes less efficient. The fact that the charging capacity of the Ni−Gra electrode diminishes from this point on
RESULTS AND DISCUSSION Electrochemical Intercalation/Carboxylation. Figure 1 shows the cyclic voltammogram of the current (i) versus potential (E) recorded at Ni−Gra as the working electrode in a standard three electrode setup in 0.1 M Bu4NBF4/MeCN. Two ill-defined reduction waves are visible at E = −1.65 and −1.83 V versus Ag/AgI. Charging of Ni−Gra at −2.0 V versus Ag/AgI
C
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Figure 4. High-resolution C 1s XPS spectra of (a) Ni−Gra fitted with a single asymmetric peak and (b) Ni−Gra−CO2 obtained after 6 repetitive modification cycles fitted using several components.
Figure 3. Recordings of (a) i versus t during charging/intercalation and (b) Eocp versus t for the carboxylation process in 9 repetitive modification cycles performed in 0.1 M Bu4NBF4/MeCN. The explicit number of modification cycles used is indicated on each of the plots.
prior to modification. Following the 6 modification cycles, the peak is considerably broadened as a consequence of appreciable changes to the graphene lattice and intercalation of Bu4N+ ions. A full deconvolution of the C 1s peak now requires several contributions. In addition to the asymmetric peak at 284.4 eV, new symmetric peaks arise from the introduction of sp3hybridized carbon atoms and carbon atoms with oxygen functionalities. The peaks at 285.2, 286.1, 286.9, and 288.0 eV are ascribed to C−C, C−O, CO/C−N, and O−CO, respectively.37,38 With the appearance of a distinct O−CO component, the successful outcome of the carboxylation process is thus confirmed. The relative contribution of O−CO with respect to CC is 1.3 ± 0.3%, which may be translated into a functionalization degree of 1:80 carbon atoms at the graphene lattice. This number will be further discussed below. A large proportion of the sp3 C−C component is expected to originate from the intercalation of Bu4N+, the presence of which was strongly inferred by PM−IRRAS (see Figures S1 and S2 of the Supporting Information). The XPS data presented in Figure 5 on the N/C ratio support this preposition because a steady increase is seen as the number of modification cycles is enhanced, until the sixth, where a plateau value at 0.055 is reached. The contribution to N from intercalated solvent (acetonitrile) is negligible according to PM−IRRAS (see the Supporting Information). Raman Spectroscopy. Figure 6a shows a representative Raman spectrum of a bare Ni−Gra substrate, where the main characteristics are the high-intensity narrow G peak at 1580 cm−1, the 2D peak at 2670 cm−1, and a low-intensity D peak at 1340 cm−1. While the G peak is a signature of the sp2hybridized graphene network, the D peak is mainly arising as a result of irregularities in this network because of functionaliza-
may be explained by the occurrence of an increasingly larger disruption of the graphene lattice as, in fact, would be expected to be the outcome of a carboxylation in process. Figure 3b depicts the development in Eocp for 90 s as CO2 was bubbled through the solution following the charging. Besides the observation that Eocp increases with time for all traces, the most noteworthy feature is its great dependence upon the modification cycle number. In fact, the initial value of Eocp reflects the charging behavior, in that it becomes increasingly more negative for the first cycles because of the intercalation of Bu4N+. This means that a much larger surface area is being charged and more electrophiles will be needed for the discharging. For the following modification cycles, the initial Eocp decreases, showing that the graphene structure, now being disrupted because of the introduction of carboxylate groups, becomes more difficult to charge. Note that the difference seen for the first trace in Figure 3b and the corresponding one in Figure 2 (with CO2) may serve to illustrate the typical variations observed when going from one Ni−Gra sample to another. To obtain more detailed information about these issues, Ni− Gra and Ni−Gra−CO2 samples were analyzed by means of XPS, Raman spectroscopy, and PM−IRRAS. Foremost, PM− IRRAS gives unambiguous evidence for the intercalation process involving Bu4N+, as discussed on the basis of Figures S1 and S2 of the Supporting Information. XPS. Panels a and b of Figure 4 show the high-resolution C 1s spectra of Ni−Gra and Ni−Gra−CO2 after 6 repetitive modification cycles, respectively. In the former case, a single asymmetric peak at 284.4 eV is observed, as would be expected for a graphene or graphitic-like material with semi-metallic behavior.36,37 This confirms the good quality of the substrate D
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Figure 6b shows a representative Raman spectrum recording of Ni−Gra−CO2, which contains several features worth noting. First of all, the ID/IG ratio has increased significantly, indicating the introduction of defects or functionalities in the graphene structure. Furthermore, new peaks are observed at ∼1480 and ∼3000 cm−1 (after deconvolution), where the latter has two origins, namely, Bu4N+ and second-order defect peaks from graphene. The peak at 1480 cm−1 is assigned to C−H sp3 deformations from Bu4N+, indicating the significant amount of intercalation taking place. In fact, after the first modification cycle, a remarkably constant relation is found for the ratio between the D and 1480 cm−1 peaks (see Figure S5 of the Supporting Information), showing a tight relationship between the degree of functionalization and the amount of intercalated counterions in terms of −CO2−Bu4N+ ion pair formation. Another interesting feature in this respect is that the I2D/IG intensity ratio (see Figure S6 of the Supporting Information) is slightly increased for Ni−Gra−CO2, despite the fact that the introduction of defects in the graphene sheet should broaden the 2D peak and, thus, lower its intensity.40 Once again, the explanation lies in the intercalation of Bu4N+ that separates the graphene sheets electronically and converts the structure from HOPG-like to that of turbostratic graphite or even separated single layers of graphene. Figure 7 shows a plot of ID/IG as the modification procedure is repeated. As seen, the increase in ID/IG is steady, until at least
Figure 5. Plot of the XPS-derived N/C ratio against the number of modification cycles for Ni−Gra−CO2.
Figure 6. Representative Raman spectra of (a) Ni−Gra and (b) Ni− Gra−CO2 obtained after 6 repetitive modification cycles. Deconvolution was carried out using Lorentzian fits.
Figure 7. Plot of ID/IG as a function of the number of modification cycles for Ni−Gra−CO2. Error bars represent standard deviations.
tion, edges, and holes. Thus, the intensity ratio of the D and G peaks, ID/IG, is an extremely sensitive parameter for measuring the degree of functionalization of graphene. For Ni−Gra, ID/IG is as small as 0.13 ± 0.09, which confirms the high quality of graphene. The 2D peak at ∼2700 cm−1 showed relatively large variations in both shape and intensity for different areas on the same Ni−Gra sample. Some of these data are available in Figure S3 of the Supporting Information, along with corresponding XPS data (see Table S1 and Figure S4 of the Supporting Information). From these variations,38,39 it was concluded that Ni−Gra contained 1−10 layers of graphene, with an average around four layers, in accordance with our previous work.18 To ensure that reliable data could be extracted, despite this inhomogeneity, at least 200 Raman spectra were measured and averaged for each sample. Furthermore, because of the multi-layered and inhomogeneous Ni−Gra samples, the ID/IG values were taken as the area of the peaks and not the maximum intensity, as normally used for single-layer graphene.
the sixth modification cycle, which shows the ability of the electrochemical approach to exert good control on the functionalization degree. At this cycle, a limiting value of around 1 is reached, which would correspond to a functionalization degree of 1:400 carbon atoms in graphene according to a recent model proposed for interpretation of such Raman data.41,42 This ratio is similar to that of 1:250 found in our previous study on electrografting of aryldiazonium salts at Ni−Gra.18 Unfortunately, the uncertainty on these numbers is high, because the calculation requires a number of assumptions on the exact nature of the structural defect sites in graphene. Furthermore, the validity of the model has not been shown for graphene on a metal substrate. For this reason, we put more trust in the XPS analysis, which gave a somewhat higher functionalization degree of 1:80 (vide supra). This number is also equal to the electrochemically determined 1:80 reported by Dano and Simonet on the carboxylation of graphite.30 The next question is if the individual layers of multi-layered regions are functionalized to the same extent. On the basis of the observation that ID/IG is independent of the exact I2D/IG E
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importantly, the carboxylation degree can be precisely controlled through strict control of the electrochemical charging process. Potentially, the carboxylated graphene may serve as a useful platform for further functionalization, e.g., attachment of biomolecules or nanoparticles, thus making this procedure interesting for fabrication of biosensors and hybrid materials.
value (see Figure S7 of the Supporting Information), the conclusion is that this is indeed the case, the origin of which, presumably, is the opening of the entire graphene structure as a response to the intercalation of Bu4N+. Hence, the electrochemical method offers a versatile procedure to make all graphene sheets in a multi-layered but expanded structure accessible for functionalization. Concerning the current efficiency for these carboxylation reactions, it is extremely low, namely, 0.02%, calculated on the basis of the charge consumed after 6 modification cycles (=0.104 C) and the number of functionalized carbon atoms from XPS (=1:80). This indicates that most of the charge apparently goes to reducing CO2 to its radical anion, followed by its dimerization to oxalate, as described in studies using mainly metals as working electrodes.25−28 In fact, this circumstance turns out to be a great advantage for the carboxylation approach proposed herein, in that a too efficient grating process would have made it very difficult to control the functionalization degree. Besides the high controllability that this two-step modification procedure offers, it also gives valuable information about each of the individual steps, i.e., the charging and carboxylation. However, for future use as a carboxylation tool, it would be advantageous if the overall intercalation/carboxylation modification could be accomplished in a simple one-step continuous electrolysis process without the intermediate need of purging with argon and charging the graphene. Encouragingly, a few such experiments performed on saturated CO2 solutions at E = −1.9 V versus Ag/AgI gave comparable carboxylation patterns, as obtained in the two-step procedure, indicating that a complete functionalization could be accomplished within 2 min under such conditions. In general, the electrolysis current was 5−6 times higher than the charging current recorded in the previous experiments, which again may be attributed to the concomitant reduction of CO2 to its radical anion. Importantly, the functionalization degree may be fully controlled by the electrolysis time, making this carboxylation procedure recommendable in most situations, if nothing else for the sake of convenience and less time consumption. This being said, it should be emphasized that the Bu4N+-intercalated graphene structures generated in the two-step procedure are enormously interesting entities in themselves because of their high nucleophilicity, which should make it possible to functionalize them with a variety of electrophiles.31 In this manner, they would become a versatile platform for creating high-area expanded graphene materials possessing multifunctionality.
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ASSOCIATED CONTENT
S Supporting Information *
PM−IRRAS, XPS, and Raman analysis of Ni−Gra and Ni− Gra−CO2. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The Danish Agency for Science, Technology and Innovation and the High Technological Foundation are gratefully acknowledged for financial support. Newtec A/S is thanked for carrying out Raman measurements.
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REFERENCES
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CONCLUSION Multi-layered graphene on a nickel substrate can be converted into a strongly nucleophilic species through electrochemical charging and intercalation of tetrabutylammonium ions. The nucleophilic charged graphene is capable of reacting with an electrophile, such as carbon dioxide, resulting in the functionalization of graphene with carboxylate groups. In contrast to other carboxylation techniques, the electrochemical method gives assembling directly at the graphene surface. Moreover, the initial intercalation of tetrabutylammonium ions opens the entire graphene structure, through which all of the graphene layers become accessible to carboxylation. The charging and carboxylation steps can be carried out separately or concomitantly in simple experimental setups, where, F
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