Functionalizing Arrays of Transferred Monolayer Graphene on

Jun 14, 2016 - Development of versatile methods for graphene functionalization is necessary before use in applications such as composites or as cataly...
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Functionalizing Arrays of Transferred Monolayer Graphene on Insulating Surfaces by Bipolar Electrochemistry Line Koefoed,†,‡ Emil Bjerglund Pedersen,†,‡ Lena Thyssen,† Jesper Vinther,‡ Thomas Kristiansen,† Steen U. Pedersen,*,†,‡ and Kim Daasbjerg*,†,‡,§ †

Department of Chemistry, Aarhus University, Langelandsgade 140, 8000 Aarhus C, Denmark Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Gustav Wieds Vej 14, 8000 Aarhus C, Denmark § Carbon Dioxide Activation Center, Aarhus University, Gustav Wieds Vej 14, 8000 Aarhus C, Denmark ‡

S Supporting Information *

ABSTRACT: Development of versatile methods for graphene functionalization is necessary before use in applications such as composites or as catalyst support. In this study, bipolar electrochemistry is used as a wireless functionalization method to graft 4-bromobenzenediazonium on large (10 × 10 mm2) monolayer graphene sheets supported on SiO2. Using this technique, transferred graphene can be electrochemically functionalized without the need of a metal support or the deposition of physical contacts. X-ray photoelectron spectroscopy and Raman spectroscopy are used to map the chemical changes and modifications of graphene across the individual sheets. Interestingly, the defect density is similar between samples, independent of driving potential, whereas the grafting density is increased upon increasing the driving potential. It is observed that the 2D nature of the electrode influences the electrochemistry and stability of the electrode compared to conventional electrografting using a three-electrode setup. On one side, the graphene will be blocked by the attached organic film, but the conductivity is also altered upon functionalization, which makes the graphene electrode different from a normal metal electrode. Furthermore, it is shown that it is possible to simultaneously modify an array of many small graphene electrodes (1 × 1 mm2) on SiO2.

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INTRODUCTION Graphene is a very interesting electrode material due to its exceptional physical properties.1 Electrochemical functionalization of pristine graphene sheets with organic functional groups has been reported, including electropolymerization,2 electrografting of diazonium salts3−5 or iodonium salts,6 Kolbe reactions,7 and electrochemical carboxylation.8 As graphene is only one atom thick, it makes for extremely thin electrodes, but its 2D nature also significantly alters the response to, e.g., electrochemical modifications compared to that of ordinary metal electrodes. The covalent attachment of organic molecules perturbs the extended aromatic character of graphene, which alters its electronic properties. Formation of a band gap by chemical functionalization is pertinent, if graphene is going to be used in nanoelectronic devices.9 Furthermore, if performed on bulk amounts of graphene, the introduction of organic moieties increases the dispersibility of graphene sheets in organic solvents, which is a crucial move toward the formation of nanocomposite materials.10 The above-mentioned functionalization methods all require an electrical contact to the graphene, either by having it on a metal substrate or by depositing a metal contact using lithography methods. These limits can be circumvented using © XXXX American Chemical Society

the wireless and contactless bipolar electrochemistry technique.11,12 Bipolar electrochemistry drives electrochemical reactions at the extremities (poles) of a conducting material, the bipolar electrode (BE). To carry out a bipolar experiment, the BE (here graphene) is placed in an electric field between two feeder electrodes.13 If sufficient voltage is applied between the feeder electrodes in a solution, the induced potentials at the extremities of the BE facilitates both oxidation and reduction reactions at equal rates at opposite ends of the BE. This important technique has already been used to electrochemically modify numerous carbonaceous surfaces, including carbon (nano)tubes,14,15 glassy carbon surfaces and beads,16,17 graphite,18 and, more recently, graphene.19 The resulting polarization of the object with respect to the solution is dependent on the magnitude of the electric field and the length of the BE. The interfacial potential difference along the BE varies due to this polarization. As a result, these polarization potentials may promote two opposite electrochemical reactions taking place at the poles of the BE.19,20 The potential Received: April 5, 2016 Revised: May 24, 2016

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using Ar and H2 flow at 1470 and 26 sccm, respectively, which was held constant throughout the process. The temperature was decreased to 1000 °C, and the growth was initiated by the addition of 4 sccm CH4 into the reaction chamber. The growth proceeded for 1 h, after which the tube was cooled down to room temperature. Cu/G samples were dip coated using 4 wt % poly(methyl methacrylate) (PMMA) in anisole. Drying was achieved by leaving the Cu/G/PMMA in a fume hood at room temperature in ambient air for 12 h. A two-electrode electrochemical setup consisting of Cu/G/ PMMA as working electrode and a graphite rod as counter electrode was used for the delamination process. The electrochemical cell consisted of an open container with a 1 M KCl aqueous solution. A cell voltage of −5 V was applied to the Cu/G/PMMA electrode, while it was being slowly immersed into the solution.29 Upon contact with the electrolyte solution, H2 bubbles arose immediately at the interface between Cu and the graphene layer, which promoted the delamination. Full delamination was accomplished within 10 s. The delaminated G/PMMA film was rinsed several times with water, and wet transfer to Si/SiO2 was performed. The SiO2/G/PMMA film was heated in an oven at 130 °C overnight. Subsequently, the PMMA was removed in acetone at 60 °C, and the sample was rinsed with isopropanol and water. Finally, SiO2/G was heated at 100 °C for 1 h. These samples are hereafter referred to as GL for large sample size (10 × 10 mm2). An array of small graphene pieces (1 × 1 mm2) is prepared by electron-beam lithography, where a large graphene sheet is divided into a matrix of 7 × 7 pieces. This sample is referred to as G S. Electron-Beam Lithography. Electron-beam patterning together with reactive ion etching was employed for patterning graphene into an array consisting of 1 × 1 mm2 graphene sheets. First, electron-beam resist, PMMA (950 kDa, A4), was spin coated onto the graphene sample to form a uniform 100 nm thick film of PMMA on top of the silicon oxide and graphene. Subsequently, electron-beam patterns were generated in the PMMA using an FEI Magellan 400 Scanning Electron Microscope with an ELPHY quantum attachment from Raith. Finally, the electron-beam generated pattern was transferred onto the graphene using oxygen plasma to etch the exposed graphene. The PMMA was removed using the same procedure as described above. Bipolar Electrografting. BBD (2 mM) and HQ (10 mM) were dissolved in 10 mL of MeCN. A homemade bipolar electrochemistry cell of glass was used for the grafting experiments.27 In this setup, the GL or GS sample was positioned in the center, while the right and left compartments each accommodated a graphite feeder electrode with L = 33 mm. After filling the cell with the grafting solution, the desired ΔE was applied for 2 min (8 and 12 V for GL and 60 V for GS). Finally, the grafted electrode was rinsed thoroughly with MeCN. X-ray Photoelectron Spectroscopy (XPS). XPS analysis was done using a Kratos Axis Ultra-DLD spectrometer (Kratos Analytical Ltd., Manchester, UK) with a monochromatic Al Kα X-ray source at a power of 150 W with an analysis area of 300 × 700 μm2. Survey spectra were acquired by accumulating two sweeps in the 0−1350 eV range at a pass energy of 160 eV. High-resolution scans were acquired by accumulating four sweeps at a pass energy of 20 eV. Spectral processing was carried out using CasaXPS (Casa Software Ltd., Teignmouth, UK). A Shirley background subtraction was performed, and atomic surface concentrations were determined from the survey spectra using the manufacturer’s sensitivity factors. Binding energies of the components in the spectra were determined by calibrating against the CC sp2 peak for C 1s at 284.7 eV. The systematic error is 5− 10%. Raman Spectroscopy. Measurements were performed with a Renishaw InVia Raman Microscope using a 150 mW 514 nm laser with 20× or 50× objective. The full areas of the samples were mapped at 20−50 μm spacing. For the data analysis, the software Wire 4.1 from Renishaw was used. Polynomial backgrounds were subtracted, and peaks in each spectrum were fitted using one single Lorentzian function for each peak. Areas having only little or no graphene as evidenced by a very low IG were disregarded (corresponds to ∼19% of the spectra for GL-8, ∼6% for GL-12, and ∼44% for GS-60).

difference, ΔV, between the two ends of the BE under the influence of an external field is given by eq 1. ΔV =

ΔE ·l L

(1)

Here, ΔE and L denote the potential difference and the distance between the feeder electrodes, respectively, and l is the length of the BE. The parameter ΔV directly affects the reactions occurring at the extremities of the polarized interface.12 In a first-order approximation, the anodic and cathodic polarization potentials are equal, but of different polarity around the center of the BE. The minimum potential difference, ΔVmin, needed to facilitate the reaction at the extremities of the substrate has to be equal to (or higher than) the difference between the potentials of the two redox processes involved, i.e., Ep,ox for the oxidation process and Ep,red for the reduction process (eq 2).

ΔVmin = Ep,ox − Ep,red

(2)

If ΔV > ΔVmin, oxidation will occur at the anodic pole of the substrate along with a concomitant reduction at the cathodic pole to ensure electroneutrality within the BE.21 In other words, the BE is behaving simultaneously as both a cathode and an anode. The theoretical foundations of bipolar electrochemistry are accessible in the work of Duval.22−26 In this study, we show that a large graphene electrode deposited on an insulating substrate can be electrochemically modified without the need of electrical contacts by employing bipolar electrochemistry. Recently, Zuccaro et al. carried out a study on deposition of copper and gold at the peripheries of graphene.19 Here, we investigate the coupled electrografting of 4-bromobenzenediazonium (BBD) at the cathode and the oxidation of hydroquinone (HQ) used as a sacrificial reagent27 at the anode of the graphene BE. On the basis of Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and optical microscopy, we demonstrate that the grafting process happens solely at the cathodic side of the graphene electrode, whereas the anodic side is left untouched. Because of the wireless nature of this technique, it can advantageously be used in the simultaneous one-step modification of an array of small (1 × 1 mm2) graphene electrodes.

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EXPERIMENTAL SECTION

Chemicals. Acetonitrile (HPLC grade, ≥ 99.9%), poly(methyl methacrylate) (Mw ∼ 350,000), and anisole (99%) were purchased from Sigma-Aldrich. Potassium chloride (≥99.5%) was purchased from Merck and hydroquinone (≥99.9%) from May & Baker Ltd., England. 4-Bromobenzenediazonium tetrafluoroborate was synthesized as described elsewhere.28 N-Type Si(100) wafers with a 285 nm thick thermal oxide layer were obtained from Silicon Valley Microelectronics, Inc. Graphite feeder electrodes (Alfa Aesar, 1 mm thick, 97% metals basis) were cut into 4 × 1.5 cm2 pieces. Between experiments, they were polished using 1000 grit sandpaper. Cu foil (99.95+%) was purchased from Advent and chemically vapor deposited graphene on Ni (itself deposited on silicon as a thin film) from Graphene Supermarket. Graphene Growth and Transfer. Graphene on Cu (Cu/G) was obtained by growing graphene on 50 μm thick Cu foils (10 × 10 mm2) in a custom-built chemical vapor deposition (CVD) system. The Cu was sonicated in HPLC acetone, Milli-Q water, and 5:1 Milli-Q water/ HNO3 acid (10 min in each case) prior to the growth of graphene. The Cu foils were annealed in a split furnace at 1030 °C for 30 min B

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Scheme 1. Schematic Representation of the (a) Setup and Electrochemical Reactions for GL (10 × 10 mm2) on SiO2 and (b) GS Consisting of an Array of Smaller Graphene Samples (each 1 × 1 mm2), Where x Denotes the Distance in Millimeters from the Cathodic Edge

Table 1. Atomic Percentage Obtained by XPS for a Blank Graphene Substrate, GL-8, and GL-12 substrate

xa

b

blank GL-8 GL-8 GL-12 GL-12

∼0 ∼10 ∼0 ∼10

C 1s (atom %) 47 51 44 61 52

± ± ± ± ±

2 10 6 10 1

O 1s (atom %)

Si 2p (atom %)

± ± ± ± ±

16 ± 1 13 ± 3 17 ± 2 9±4 15 ± 1

36 31 39 24 33

1 8 5 7 0

Br 3p (atom %) 3.1 ± 1.1 5.3 ± 1.7

N 1s (atom %) 0.6 0.9 0.6 1.1 0.7

± ± ± ± ±

0.0 0.1 0.0 0.1 0.2

Br/C 0.06 0.09

a Denotes the distance in mm along the length direction from left (reduction site) to right (oxidation site) as indicated in Scheme 1. bPristine graphene on Si/SiO2.

Atomic Force Microscopy. GS-60 was characterized by AFM (Bruker, Dimension Icon) in dry phase using height imaging in tapping mode with silicon AFM probes (Olympus, OMCL-AC160TSR3), having a typical force constant of 26 N m−1, resonant frequency of 300 (±100) kHz, and nominally tip radius of 7 nm. Raster scanning was performed with damped amplitude resonant at 80% of the free resonant amplitude to minimize any wear and deformation of the grafted aryl multilayer by the AFM probe. Typical scanning speeds were 25 μm s−1. Data analysis was performed using the free SPM data analysis software Gwyddion.

that these potentials or that, at least, the relative potentials will be the same for graphene on SiO2. HQ is added in 5 times excess to secure the potential at the anodic end to the oxidation potential of HQ. With a redox potential that is low compared to the oxidation potential of any residual water, HQ is ideal for suppressing any O 2 production.32 Furthermore, by the addition of HQ the oxidation of the graphene sheet itself at the anodic site is avoided.19 Importantly, the reductive conditions are not sufficient to induce a reductive dehalogenation of a bromobenzene based film, which would take place at a potential of −2.6 V vs SCE.33 X-ray Photoelectron Spectroscopy. Table 1 gives the atomic composition of the GL-8 and GL-12 samples from the XPS survey spectrum. The successful reductive grafting is evidenced by XPS, where the atom % of Br for GL-8 is found to be 3.1 ± 1.1 at x = 0 mm (cathodic edge), while no Br at all is detected at x = 10 mm (anodic edge). If a pure organic film derived from bromobenzene units had been obtained as previously accomplished on Au,27 the Br/C ratio should be 1:6, which is higher than the 0.06 found herein. High-resolution C 1s spectra were acquired both before and after bipolar grafting at both sides. In all cases, an asymmetric peak was observed corresponding to sp2 hybridized carbon (Figure S1, Supporting Information). On the blank sample (pristine graphene on Si/SiO2) and at the anodic edge of grafted samples, which is unharmed by the grafting, a small shoulder is observed probably due to traces of PMMA. In addition, O and Si from the underlying substrate are observed, indicating that the film is either inhomogeneous or less than 10 nm thick (analysis depth of the XPS). Interestingly, for GL-12 a higher Br/C ratio of 0.09 is obtained along with a larger decrease in the atom % of O and Si. This indicates that thicker films are obtained as ΔE is increased. The C 1s spectra are identical to the ones obtained for GL-8 (see Figure S1, Supporting Information).

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RESULTS AND DISCUSSION Scheme 1a shows the setup for the electrografting procedure on the GL substrate, which is an electrode consisting of graphene deposited on Si/SiO2. To prepare GL, graphene was first grown on 10 × 10 mm2 polycrystalline Cu foils in a homemade CVD system followed by electrochemical transfer using a twoelectrode setup.29,30 On Si/SiO2 (285 nm SiO2) graphene is optically visible,31 and the absence of Raman peaks from SiO2 with υ̅ > 1100 cm−1 facilitates Raman analysis and mapping. Furthermore, transfer to an insulating substrate ensures that graphene is the only conducting material when performing bipolar electrochemistry. Therefore, any electrochemical reactions must be driven by the potential difference experienced by the graphene. Electrografting of BBD was carried out by immersing GL into the solution and applying ΔE = 8 or 12 V for 2 min (samples denoted GL-8 and GL-12, respectively). The values of ΔE selected arose from the following consideration: with the peak potentials of the reduction of BBD, Ep,red = −0.32 V vs SCE, and oxidation of HQ, Ep,ox = 1.09 V vs SCE, already known from cyclic voltammetry,27 ΔVmin = 1.41 V ensues from eq 2. Consequently, the polarization would have to provide ΔE = 4.7 V (using L = 33 mm and l = 10 mm) according to eq 1. In fact, this value should be considered a lower limit since additional potential drops would occur at both feeder electrode/solution interfaces. Note that since the reported peak potentials are measured at glassy carbon as electrode material27 we assume C

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Figure 1. XPS profile for (a) GL-8 and (b) GL-12 showing the atomic percentages of Si (pink), C (black), and Br (blue), recorded every 0.5 mm across each sample.

followed by a very fast dissociation of nitrogen.35 In the case of graphene, it is known that the grafting occurs at defect sites and at the basal plane, depending on the graphene quality, but also that already grafted molecules can be attacked.3 If a metal is used as substrate, the metal would be able to maintain its high conductivity, even if the surface should become blocked by an organic film. Noteworthy, this would not be the case for the 2D graphene material which, with no substrate underneath to carry the current, will lose its conductivity, once the sp2 hybridization becomes sufficiently disrupted. To test this hypothesis, bipolar electrografting with ΔE = 8 V was performed on multilayered graphene (four layers in average)8 deposited on a Ni substrate, having no conductivity issue whatsoever. This Ni/G sample was analyzed by line-scan XPS (Figure S2, Supporting Information). Once again, the atom % of Br is increasing, when going toward x = 0 mm, with a concomitant decrease in the atom % of Ni. Interestingly, in this case the Br/C ratio reaches 0.16, equal to the 1:6 ratio expected for a pure organic film derived from bromobenzene units indicating the formation of a thick layer. Exactly the same result was obtained for bipolar electrografting of BBD on Au.27 In conclusion, the multilayered graphene electrode thus behaves like a metal electrode and is not altered to the same extent as the GL electrodes were. Although the structure of the outer graphene layer is disrupted, the underlying graphene layers along with the Ni are still capable of transferring electrons, until the point is reached, where the grafted film is so thick that it becomes blocking. The surface concentration, Γ, of grafted molecules can be obtained from the XPS measurements according to eq 3.

In addition, it is observed that the atom % of O and Si for both GL samples at the anodic edges corresponds to that of blank graphene as shown in entry 1 in Table 1. Concerning the content of nitrogen, it is 0.6% for the blank sample, indicating that the amount of adventitious nitrogen present is nonnegligible. Yet, the two grafted samples see increases to 0.9% and 1.1% on the cathodic site which may be attributed to the formation of azo bonds during the diazonium salt grafting process. To get further insight into the grafting features, XPS linescans were made for GL-8 and GL-12 for x going from 0.5 to 9 mm (Figure 1). Figure 1a depicts the evolution of the atom % of Si, C, and Br as a function of x for GL-8. Noteworthy, at x = 9 mm the Si and C contents are dominant and resemble entry 1 in Table 1 for unmodified graphene on SiO2; no Br is observed. Moving toward smaller x, the Si constituent decreases along with a concomitant increase in the C and Br contents, until they reach, by and large, a plateau at x = 5 mm. Interestingly, a small decrease is observed in the atom % of C going toward x = 0 mm, whereas the atom % of Br is constant. These results are somewhat in contrast to a previous study on bipolar grafting of BBD on Au substrates, where the grafting area moves upon increasing ΔE. This latter phenomenon was attributed to the kinetics related to the competition between the reduction of BBD and the concomitant reduction of the grafting agent, the aryl radical.27 In this context, it is important to have in mind that the exact position on the electrode having zero overpotential with respect to the solution, x0, may shift during an experiment to keep the total rates of the oxidation and reduction processes equal at all time. In fact, in our case this is likely to occur, considering that the electrochemical rate of the grafting process at the cathodic site most likely decreases because of the surface passivation. Ultimately, this may lead to a decrease in the active length, l, and the grafting will come to a halt, once ΔV becomes too small to drive the two opposite redox reactions. Hence, both the grafting efficiency and electron transfer blocking properties of the grafted layers play central roles in bipolar electrochemistry. The results indicate that the underlying substrate plays a crucial role for the exact grafting behavior. In general, electroreduction of aryldiazonium salts generates highly reactive aryl radicals in close proximity to the electrode, either via a dissociative electron transfer34 or a stepwise electron transfer

Γ = 6.5 × 10−9mol cm−2

In this expression,

atom %Br atom %Cgraphene

atom % Br atom % Cgraphene

(3)

denotes the ratio of the

intensities of the Br 3p peak and the contribution of graphene to the C 1s peak. The latter may be obtained as the difference of atom % Ctotal − 6 × atom % Br, considering that each Br is attached to 6 C atoms in the phenyl ring. The factor 6.5 × 10−9 mol cm−2 is the carbon atom surface density based on the unit cell of graphene.36 It then follows that Γ = 6.2 × 10−10 mol cm−2 for GL-8 and Γ = 1.2 × 10−9 mol cm−2 for GL-12 at the cathodic edge (x = 0 mm), which is similar to previously D

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Figure 2. Raman maps of GL-8 and GL-12 before and after bipolar electrografting, displaying (a) I2D/IG and (b) ID/IG. Samples are mapped with 50 μm spacing.

2 (high defect density). In many spectra, the D′ and G peaks become so wide that they overlap as seen, e.g., for the grafted graphene in Figure 3. This indicates the onset of stage 2, where ID starts to decrease again.41 On the basis of these considerations, a defect density of Γ = 6.8 × 10−11 mol cm−2 is found for GL-12, corresponding to 1 defect per 95 carbon atoms (see Supporting Information). This value appears very low compared to the grafting density found with XPS (Γ = 1.2 × 10−9 mol cm−2). However, it is wellknown that electrografting of diazonium salts can be used to form thick films.42−45 The defect density calculated from Raman spectroscopy is only representative of the graphene layer, while the value from XPS is based on Br throughout the film. It is, therefore, not surprising to find a value 18 times lower using Raman spectroscopy. Moreover, diazonium grafting often results in mushroom-like structures with few anchoring points (defects in the graphene). This would result in a low defect density as observed by Raman spectroscopy but a substantial grafting as observed by XPS. It may also be noted that the general stability of these films stored under ambient conditions seems to be good since no spectroscopic changes were observed, even after a two month period. The ratios of ID/IG reported here are very large compared to those obtained in our previous studies of electrografting of diazonium salts on graphene (ID/IG < 1.34).3 As mentioned above, in the same study it was observed that diazonium grafting occurs mainly at graphene edges and defects. Since the ID/ID′ ratio indicates that the major defect type is from edges, this could suggest that the bipolar electrografting effectively increases the intensity of defects to the point, where the graphene sheet can no longer function as one large electrode. Interestingly, GL-8 shows a feature ∼0.5 mm in size, which is unchanged after bipolar electrografting (see Figure 2a). This is a piece of graphene, which is disconnected from the full sheet and which, because of its small size, is experiencing ΔV < ΔVmin (see eq 1). Yet, the modification should be possible even on pieces of this small size, simply by employing a higher ΔE. To investigate this, a new sample, GS, was prepared, consisting of an array of small (1 × 1 mm2) graphene electrodes. The reason that bipolar electrochemistry should allow for the concurrent functionalization of such an array is that each piece of graphene acts as an individual BE. In a conventional electrochemical three-electrode setup, it would be difficult to modify small graphene pieces, let alone modify an entire array of graphene electrodes simultaneously. In Figure 4, Raman maps of GS-60 (ΔE = 60 V) demonstrate similar features to the GL samples, namely, grafting of the

reported results using a normal three-electrode setup for electrografting of BBD.37 Raman Spectroscopy. Figures 2 and 3 show the analysis of the transferred graphene by Raman spectroscopy. For GL

Figure 3. Representative Raman spectra of transferred graphene (red) and the anodic (green, x = 10 mm) and cathodic (blue, x = 0 mm) sites of the graphene sheet for GL-12 after bipolar grafting.

samples, a spectrum was obtained for every 50 μm over the entire surface, both before and after bipolar electrografting (Figure 2). Before electrografting, the sample exhibits the characteristic features for high-quality monolayer graphene with large I2D/IG ratio (∼2), where I2D denotes the intensity of the 2D peak and IG the intensity of the G peak, and no visible D peak (Figure 3 and Figure S3, Supporting Information). The Raman maps after bipolar electrografting clearly show very abrupt changes in both I2D/IG and ID/IG across the surface upon decreasing x. This indicates that grafting occurs in the interval at x = 0−6 mm for GL-8 and x = 0−7.5 mm for GL-12, which is in accordance with the XPS profiles (see Figure 1). Specifically, the anodic side of the sheets is largely unaffected showing no D peak and with I2D/IG = 1.96 ± 0.48, while the cathodic side exhibits a large D peak (ID/IG = 2.14 ± 0.32) and a significant decrease in I2D/IG (= 0.60 ± 0.13). The values reported here are for GL-12, but those for GL-8 are comparable. In the grafted area, ID/ID′ = 4.30 ± 0.63, indicating that the main defect type is related to graphene edges.38 From the values of ID/IG, the defect density for covalently attached functional groups on graphene may be extracted as described elsewhere (see Supporting Information).39,40 This approach gives a defect density for stage 1 (low defect density) and stage E

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Figure 4. Raman maps of arrays of 1 × 1 mm2 graphene pieces on GS60 before and after bipolar electrografting, displaying (a) I2D/IG and (b) ID/IG. The sample is mapped with 20 μm spacing. Figure 5. Optical bright field images of the GS-60 array after bipolar grafting showing (a) one of the graphene electrodes and the (b) interface between two graphene squares. Corresponding dark field images are displayed in (c) and (d). AFM images of GS-60 showing the (e) cathodic edge with film formation and the (f) anodic edge (no organic film) with corresponding height profiles.

cathodic side of the graphene flake to a point, where the ID/IG and I2D/IG ratios are uniform across the grafted area. In comparison with the GL samples, the GS sample initially exhibits a somewhat lower graphene quality, and the anodic area is slightly altered after bipolar electrografting (see Figure S4, Supporting Information), mostly notable evidenced by the decrease in I2D/IG. However, there is no simultaneous increase in ID/IG in the anodic area, meaning that there is no aryl radical grafting taking place here. Overall, this demonstrates that by increasing ΔE, small graphene BEs can simultaneously be electrochemically modified. This is highly interesting since it is well-known that BEs can act as electroanalytical sensors by utilizing electrochemiluminescence (ECL),20 e.g., as a DNA sensor,46 and it has, furthermore, been shown that ECL can be used as an optical readout for an array of BEs.47 With proper control of the experimental conditions, electrochemical functionalization should even be possible to carry out on small flakes suspended in solvents. Optical Microscopy and Atomic Force Microscopy. Figure 5 displays optical microscopy and AFM images of the GS-60 array after bipolar electrografting. It is evident that the GS-60 array of electrodes has turned blue at x = 0 mm due to the film formation. In Figure 5a, the bright field optical image of one of the graphene electrodes in the array is shown. While the blue color is prominent at x = 0 mm, it is fading out as x increases. In Figure 5b, the interface between two graphene electrodes is shown, where the left edge corresponds to x = 0 mm, and the right edge corresponds to x = 1 mm. It is seen that the unmodified graphene (x = 1 mm) and the modified graphene (x = 0 mm) have different colors. The same trend is observed in the dark field images shown in Figure 5c and d. The observed brightening of the electrografted part of the

electrode is attributed to an increased scattering of light. Furthermore, the presence of wrinkles is observed. Figure 5e−f shows AFM images of the edges of graphene on GS-60. The anodic edge consistently shows a height of ∼3 nm, which is consistent with previously reported values for monolayer graphene with residues of PMMA.48 The grafted edge is 15−20 nm, clearly showing that a thick organic film has been grafted on the samples.

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CONCLUSIONS In this study, a versatile grafting procedure based on the wireless bipolar electrochemistry technique is introduced to deposit organic films on both a large graphene electrode and arrays of small graphene electrodes. Using this technique, transferred graphene can be electrochemically functionalized without the need of a metal support or the deposition of physical contacts. XPS analysis clearly shows the successful grafting of only the cathodic side of the graphene, while the anodic side is left undamaged. The entire electrode is mapped by Raman spectroscopy visualizing the abrupt change from pristine graphene to electrografted graphene depending on the size of the applied potential, ΔE. Also, AFM shows that films are grafted on the cathodic site, whereas the anodic site is left undamaged. It appears that the 2D nature of the graphene electrode has significant influence on the reactivity, when compared to multilayer graphene or graphene on conducting F

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(6) Chan, C. K.; Beechem, T. E.; Ohta, T.; Brumbach, M. T.; Wheeler, D. R.; Stevenson, K. J. Electrochemically Driven Covalent Functionalization of Graphene from Fluorinated Aryl Iodonium Salts. J. Phys. Chem. C 2013, 117, 12038−12044. (7) Sarkar, S.; Bekyarova, E.; Haddon, R. C. Reversible Grafting of αNaphthylmethyl Radicals to Epitaxial Graphene. Angew. Chem., Int. Ed. 2012, 51, 4901−4904. (8) Bjerglund, E.; Kongsfelt, M.; Shimizu, K.; Jensen, B. B. E.; Koefoed, L.; Ceccato, M.; Skrydstrup, T.; Pedersen, S. U.; Daasbjerg, K. Controlled Electrochemical Carboxylation of Graphene To Create a Versatile Chemical Platform for Further Functionalization. Langmuir 2014, 30, 6622−6628. (9) Kuila, T.; Bose, S.; Mishra, A. K.; Khanra, P.; Kim, N. H.; Lee, J. H. Chemical functionalization of graphene and its applications. Prog. Mater. Sci. 2012, 57, 1061−1105. (10) Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Graphene-based composite materials. Nature 2006, 442, 282−286. (11) Fosdick, S. E.; Knust, K. N.; Scida, K.; Crooks, R. M. Bipolar Electrochemistry. Angew. Chem., Int. Ed. 2013, 52, 10438−10456. (12) Loget, G.; Zigah, D.; Bouffier, L.; Sojic, N.; Kuhn, A. Bipolar Electrochemistry: From Materials Science to Motion and Beyond. Acc. Chem. Res. 2013, 46, 2513−2523. (13) Fleischmann, M.; Ghoroghchian, J.; Rolison, D.; Pons, S. Electrochemical Behavior of Dispersions of Spherical Ultramicroelectrodes. J. Phys. Chem. 1986, 90, 6392−6400. (14) Loget, G.; Lapeyre, V.; Garrigue, P.; Warakulwit, C.; Limtrakul, J.; Delville, M.-H.; Kuhn, A. Versatile Procedure for Synthesis of JanusType Carbon Tubes. Chem. Mater. 2011, 23, 2595−2599. (15) Warakulwit, C.; Nguyen, T.; Majimel, J.; Delville, M.-H.; Lapeyre, V.; Garrigue, P.; Ravaine, V.; Limtrakul, J.; Kuhn, A. Dissymmetric Carbon Nanotubes by Bipolar Electrochemistry. Nano Lett. 2008, 8, 500−504. (16) Koefoed, L.; Shimizu, K.; Pedersen, S. U.; Daasbjerg, K.; Kuhn, A.; Zigah, D. One-step preparation of bifunctionalized surfaces by bipolar electrografting. RSC Adv. 2016, 6, 3882−3887. (17) Loget, G.; Roche, J.; Kuhn, A. True Bulk Synthesis of Janus Objects by Bipolar Electrochemistry. Adv. Mater. 2012, 24, 5111− 5116. (18) Bradley, J.-C.; Ma, Z. Contactless Electrodeposition of Palladium Catalysts. Angew. Chem., Int. Ed. 1999, 38, 1663−1666. (19) Zuccaro, L.; Kuhn, A.; Konuma, M.; Yu, H. K.; Kern, K.; Balasubramanian, K. Selective Functionalization of Graphene Peripheries by using Bipolar Electrochemistry. ChemElectroChem 2016, 3, 372−377. (20) Mavré, F.; Chow, K.-F.; Sheridan, E.; Chang, B.-Y.; Crooks, J. A.; Crooks, R. M. A Theoretical and Experimental Framework for Understanding Electrogenerated Chemiluminescence (ECL) Emission at Bipolar Electrodes. Anal. Chem. 2009, 81, 6218−6225. (21) Loget, G.; Kuhn, A. Shaping and exploring the micro- and nanoworld using bipolar electrochemistry. Anal. Bioanal. Chem. 2011, 400, 1691−1704. (22) Duval, J. F. L.; Huijs, G. K.; Threels, W. F.; Lyklema, J.; van Leeuwen, H. P. Faradaic depolarization in the electrokinetics of the metal−electrolyte solution interface. J. Colloid Interface Sci. 2003, 260, 95−106. (23) Duval, J. F. L.; Minor, M.; Cecilia, J.; van Leeuwen, H. P. Coupling of Lateral Electric Field and Transversal Faradaic Processes at the Conductor/Electrolyte Solution Interface. J. Phys. Chem. B 2003, 107, 4143−4155. (24) Duval, J. F. L.; van Leeuwen, H. P.; Cecilia, J.; Galceran, J. Rigorous Analysis of Reversible Faradaic Depolarization Processes in the Electrokinetics of the Metal/Electrolyte Solution Interface. J. Phys. Chem. B 2003, 107, 6782−6800. (25) Duval, J. F. L. Electrokinetics of the amphifunctional metal/ electrolyte solution interface in the presence of a redox couple. J. Colloid Interface Sci. 2004, 269, 211−223.

substrates. The single-layer 2D graphene electrode may be blocked by the grafting of an organic layer, but the conductivity is at the same time altered. Therefore, this 2D electrode behaves differently compared to a conventional metal electrode. Analysis of the pertinent Raman spectra indicates that the large graphene sheets are damaged at the edges, effectively creating smaller domains, which are no longer properly electrically connected. Furthermore, it is shown that it is possible to modify each individual graphene electrode (1 × 1 mm2) of a larger array on SiO2. This suggests that this technique may find use in the modification of graphene flakes in suspension.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b01309. High-resolution C 1s core level spectra, line-scan XPS of grafted Ni/G, Raman spectra, and calculation of defect density from the value of ID/IG (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(S.U.P.) E-mail: [email protected]. *(K.D.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Danish Council for Strategic Research (DA-GATE DSF 12-131827) and the Innovation Fund Denmark (NIAGRA 582012-4) are acknowledged for financial support. We appreciate the generous financial support from the Danish National Research Foundation (grant no. DNRF118).

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ABBREVIATIONS BBD, 4-bromobenzenediazonium; BE, bipolar electrode; Cu/ G, graphene on Cu; Ni/G, graphene on Ni; HQ, hydroquinone; GL-8, large (10 × 10 mm2) graphene electrode modified using ΔE = 8 V; GL-12, large (10 × 10 mm2) graphene electrode modified using ΔE = 12 V; GS-60, small (1 × 1 mm2) graphene electrode modified using ΔE = 60 V

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REFERENCES

(1) Geim, A. K.; Novoselov, K. S. The rise of graphene. Nat. Mater. 2007, 6, 183−191. (2) Zuccaro, L.; Krieg, J.; Desideri, A.; Kern, K.; Balasubramanian, K. Tuning the isoelectric point of graphene by electrochemical functionalization. Sci. Rep. 2015, 5, 11794. (3) 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. (4) Gan, L.; Zhang, D.; Guo, X. Electrochemistry: An Efficient Way to Chemically Modify Individual Monolayers of Graphene. Small 2012, 8, 1326−1330. (5) Mooste, M.; Kibena, E.; Kozlova, J.; Marandi, M.; Matisen, L.; Niilisk, A.; Sammelselg, V.; Tammeveski, K. Electrografting and Morphological Studies of Chemical Vapour Deposition Grown Graphene Sheets Modified by Electroreduction of Aryldiazonium Salts. Electrochim. Acta 2015, 161, 195−204. G

DOI: 10.1021/acs.langmuir.6b01309 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

(45) Mooste, M.; Kibena-Põldsepp, E.; Marandi, M.; Matisen, L.; Sammelselg, V.; Tammeveski, K. Electrochemical Properties of Gold and Glassy Carbon Electrodes Electrografted with an Anthraquinone Diazonium Compound using the Rotating Disc Electrode Method. RSC Adv. 2016, 6, 40982−40990. (46) Chow, K.-F.; Mavré, F.; Crooks, R. M. Wireless Electrochemical DNA Microarray Sensor. J. Am. Chem. Soc. 2008, 130, 7544−7545. (47) Chow, K.-F.; Mavré, F.; Crooks, J. A.; Chang, B.-Y.; Crooks, R. M. A Large-Scale, Wireless Electrochemical Bipolar Electrode Microarray. J. Am. Chem. Soc. 2009, 131, 8364−8365. (48) Ciuk, T.; Pasternak, I.; Krajewska, A.; Sobieski, J.; Caban, P.; Szmidt, J.; Strupinski, W. Properties of Chemical Vapor Deposition Graphene Transferred by High-Speed Electrochemical Delamination. J. Phys. Chem. C 2013, 117, 20833−20837.

(26) Duval, J. F. L.; Buffle, J.; van Leeuwen, H. P. Quasi-reversible Faradaic Depolarization Processes in the Electrokinetics of the Metal/ Solution Interface. J. Phys. Chem. B 2006, 110, 6081−6094. (27) Koefoed, L.; Pedersen, S. U.; Daasbjerg, K. Grafting of Aryl Diazonium, Iodonium, and Sulfonium Salts in Unusual Patterns by Exploiting the Potential Gradient in Bipolar Electrochemistry. ChemElectroChem 2016, 3, 495−501. (28) Shu, X.-z.; Zhang, M.; He, Y.; Frei, H.; Toste, F. D. Dual Visible Light Photoredox and Gold-Catalyzed Arylative Ring Expansion. J. Am. Chem. Soc. 2014, 136, 5844−5847. (29) Koefoed, L.; Kongsfelt, M.; Ulstrup, S.; Č abo, A. G.; Cassidy, A.; Whelan, P. R.; Bianchi, M.; Dendzik, M.; Pizzocchero, F.; Jørgensen, B.; Bøggild, P.; Hornekær, L.; Hofmann, P.; Pedersen, S. U.; Daasbjerg, K. Facile electrochemical transfer of large-area single crystal epitaxial graphene from Ir(111). J. Phys. D: Appl. Phys. 2015, 48, 115306. (30) Wang, Y.; Zheng, Y.; Xu, X.; Dubuisson, E.; Bao, Q.; Lu, J.; Loh, K. P. Electrochemical Delamination of CVD-Grown Graphene Film: Toward the Recyclable Use of Copper Catalyst. ACS Nano 2011, 5, 9927−9933. (31) Blake, P.; Hill, E. W.; Castro Neto, A. H.; Novoselov, K. S.; Jiang, D.; Yang, R.; Booth, T. J.; Geim, A. K. Making graphene visible. Appl. Phys. Lett. 2007, 91, 063124. (32) Loget, G.; Kuhn, A. Electric field-induced chemical locomotion of conducting objects. Nat. Commun. 2011, 2, 535. (33) Vase, K. H.; Holm, A. H.; Norrman, K.; Pedersen, S. U.; Daasbjerg, K. Covalent Grafting of Glassy Carbon Electrodes with Diaryliodonium Salts: New Aspects. Langmuir 2007, 23, 3786−3793. (34) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson, J.; Savéant, J.-M. Covalent Modification of Carbon Surfaces by Aryl Radicals Generated from the Electrochemical Reduction of Diazonium Salts. J. Am. Chem. Soc. 1997, 119, 201−207. (35) Daasbjerg, K.; Sehested, K. Reduction of Substituted Benzenediazonium Salts by Solvated Electrons in Aqueous Neutral Solution Studied by Pulse Radiolysis. J. Phys. Chem. A 2002, 106, 11098−11106. (36) Peres, N. M. R. Colloquium: The transport properties of graphene: An introduction. Rev. Mod. Phys. 2010, 82, 2673−2700. (37) D’Amour, M.; Bélanger, D. Stability of Substituted Phenyl Groups Electrochemically Grafted at Carbon Electrode Surface. J. Phys. Chem. B 2003, 107, 4811−4817. (38) 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. (39) 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. (40) 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. (41) Cançado, L. G.; Jorio, A.; Ferreira, E. H. M.; Stavale, F.; Achete, C. A.; Capaz, R. B.; Moutinho, M. V. O.; Lombardo, A.; Kulmala, T. S.; Ferrari, A. C. Quantifying Defects in Graphene via Raman Spectroscopy at Different Excitation Energies. Nano Lett. 2011, 11, 3190−3196. (42) Ceccato, M.; Bousquet, A.; Hinge, M.; Pedersen, S. U.; Daasbjerg, K. Using a Mediating Effect in the Electroreduction of Aryldiazonium Salts to Prepare Conducting Organic Films of High Thickness. Chem. Mater. 2011, 23, 1551−1557. (43) Bélanger, D.; Pinson, J. Electrografting: a powerful method for surface modification. Chem. Soc. Rev. 2011, 40, 3995−4048. (44) Bousquet, A.; Ceccato, M.; Hinge, M.; Pedersen, S. U.; Daasbjerg, K. Redox Grafting of Diazotated Anthraquinone as a Means of Forming Thick Conducting Organic Films. Langmuir 2012, 28, 1267−1275. H

DOI: 10.1021/acs.langmuir.6b01309 Langmuir XXXX, XXX, XXX−XXX