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Understanding Defect-Stabilized Noncovalent Functionalization of Graphene Hua Zhou,* Ahmet Uysal, Daniela M. Anjos, Yu Cai, Steven H. Overbury, Matthew Neurock, John K. McDonough, Yury Gogotsi, and Paul Fenter* involving redox reactions.[2] Functionalizing graphene through chemical, electrochemical, and energetic treatments provides an effective synthetic route for many potential graphene applications.[3–5] Among the many possible candidates, the noncovalent assembly via dispersive, hydrophobic, or electrostatic interactions (i.e., π–π stacking) between small-molecule adsorbates and the graphene basal plane represents a simple and economic approach to introduce specific functionalities to the graphene surface and to control design-based synthesis of graphene derivatives.[5] For instance, smallmolecule aromatics like quinones have been used to functionalize carbonaceous materials and were shown to effectively enhance the performance of energy conversion and storage devices.[6–8] In particular, the recent use of redox-active quinonebased derivatives to replace precious-metal electrocatalysts, such as Ru, in flow batteries expands an intriguing direction for realizing massive electrical energy storage at greatly reduced cost.[9] The stable adsorption of quinone-based molecules (from methanolic solutions) on carbon electrodes has been demonstrated recently, but the effectiveness of this approach varied for different carbon materials.[10,11] Persistent and reversible proton coupled electron transfer (PCET) reactions observed for phenanthrenequinone (PQ) modified onion-like carbon (OLC) in cyclic voltammetry data imply that PQ is strongly bound to the carbon substrate.[7] However, this approach was less effective for the functionalization of more highly ordered carbons such as pyrolytic graphite (PG), and having little or no activity on the basal plane of highly oriented pyrolytic graphite (HOPG).[11–13] Clearly, a deeper understanding of the interactions of electrochemically active carbon materials with aromatic groups is crucial to controlling PCET processes relevant for the oxidation– reduction reactions at electrode and catalytic surfaces.[14,15] Here, we demonstrate the ability to functionalize epitaxial graphene (EG) by direct adsorption of PQ. Our experimental findings reveal that this functionalization relies on a defectmediated stabilization mechanism of PQ adsorption that nevertheless preserves its electrochemical activity to PCET reactions. Our results also reveal that the PQ molecules adsorb in a conformation resembling graphene stacking and gain further stabilization by hydrogen bonding to vicinal hydroxyl species that terminate defect sites within the graphene basal plane. This is demonstrated by the structural stability of the adsorbed PQ

The noncovalent functionalization of graphene by small molecule aromatic adsorbates, phenanthrenequinone (PQ), is investigated systematically by combining electrochemical characterization, high-resolution interfacial X-ray scattering, and ab initio density functional theory calculations. The findings in this study reveal that while PQ deposited on pristine graphene is unstable to electrochemical cycling, the prior introduction of defects and oxygen functionality (hydroxyl and epoxide groups) to the basal plane by exposure to atomic radicals (i.e., oxygen plasma) effectively stabilizes its noncovalent functionalization by PQ adsorption. The structure of adsorbed PQ molecules resembles the graphene layer stacking and is further stabilized by hydrogen bonding with terminal hydroxyl groups that form at defect sites within the graphene basal plane. The stabilized PQ/graphene interface demonstrates persistent redox activity associated with proton-coupled-electron-transfer reactions. The resultant PQ adsorbed structure is essentially independent of electrochemical potentials. These results highlight a facile approach to enhance functionalities of the otherwise chemically inert graphene using noncovalent interactions.

1. Introduction The extraordinary physical properties of graphene for many potential applications are well-known.[1] However, the chemical inertness of pristine (i.e., defect free) graphene is a challenge for its use in various energy applications, particularly those

Dr. H. Zhou, Dr. A. Uysal, Dr. P. Fenter Chemical Science and Engineering Division Argonne National Laboratory Argonne, IL 60439, USA E-mail: [email protected]; [email protected] Dr. D. M. Anjos, Dr. S. H. Overbury Chemical Science Division Oak Ridge National Laboratory Oak Ridge, TN 37831, USA Dr. Y. Cai, Prof. M. Neurock[+] Department of Chemical Engineering and Department of Chemistry University of Virginia Charlottesville, VA 22904, USA Dr. J. K. McDonough, Prof. Y. Gogotsi Department of Materials Science and Engineering and A. J. Drexel Nanomaterials Institute Drexel University Philadelphia, PA 19104, USA [+] Present Address: Department of Chemical Engineering and Materials Science, University of Minnesota.

DOI: 10.1002/admi.201500277

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under electrochemical control during potential cycling. These results highlight a facile approach to enhance functionalities of the otherwise chemically inert graphene through noncovalent functionalization, and provide a model system for understanding PCET reactions in catalytic energy conversions.

2. Results and Discussions We used solution-based adsorption of PQ on pristine carbide derived EG films grown on silicon carbide (SiC) substrates to understand the underlying interactions that control the functionalization (see the Experimental Section for the description of the preparation of epitaxial graphene films and PQ adsorption).[11,16] Cyclic voltammetry (CV) measurements, shown in Figure 1a, indicate that the expected PQ PCET reactions are observed initially near 0.0 and 0.6 V versus Ag/AgCl (in 0.1 M HCl), demonstrating that this process leads to adsorption of electrochemically active PQ on pristine EG. However, the continuously decreasing electrochemical currents after several charge–discharge cycles reveal that the PCET reactions associated with the adsorbed PQ molecules are not stable under these conditions. It is likely that PQ molecules are detached from the EG basal plane during the electrochemical cycling. Similar results were obtained for a range of adsorbed PQ coverage and PQ solution concentrations (not shown), and suggest that the intrinsic π–π interaction between the graphene basal plane and the aromatic rings of PQ is not strong enough to irreversibly bind PQ molecules in an aqueous solution during electrochemical cycling. This observation is surprising given the relatively strong interaction of PQ with graphene (Eads = −1.08 eV) obtained from density functional theory (DFT) calculations with dispersion corrections and the relatively weak PQ–water and graphene–water interactions (e.g., PQ solubility in water is low, ≈5.5 × 10−7 M,[17] and graphene is hydrophobic).[11] It is, nevertheless, consistent with the lack of any stable functionalization of the graphite basal plane by PQ,[13] especially since the pristine EG films have a much lower defect density than any other graphitic carbon substrates and expose only the basal plane.[16,18] Our results, in comparison to the reported trends in the literature,[11] suggest that PQ-based functionalization of graphene is most effective for samples with the highest defect densities. A recent study of the high affinities of polycyclic aromatic hydrocarbons to graphene nanosheets (GNS) elucidated the important role of the sieving effect of the groove regions formed by wrinkles on GNS surfaces,[19,20] which may lead to charge inhomogeneities with high chemical activity.[21] This implies that PQ adsorption might be stabilized by the controlled introduction of structural distortions or imperfections in the pristine EG surface. Since EG film is strongly constrained by the SiC substrate, the introduction of wrinkles on the surface seems more technically challenging. Instead, creating defects on the pristine surface using gas phase atomic radicals such as oxygen plasma etching appears to be a viable approach.[3,22–26] Figure 1b shows that the water contact angle of the graphene basal plane and its surface morphology are modified by brief exposure to reactive oxygen plasma at low power (4–5 W for 15–20 s). The water contact angle of the as-etched EG decreased to less than 60°, as compared to ≈92° for pristine EG.[16,27] The decrease 1500277 (2 of 8)

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Figure 1. Defect stabilized PQ adsorption on EG. a) Cyclic voltammetry of the PQ/EG system in a 0.1 M HCl solution after PQ was adsorbed on a pristine EG from a solution of 5 × 10−3 M in methanol. The continuously decreasing PQ-related redox peaks in the CV curves indicate that the PQ adsorption on pristine EG is unstable. b) The water contact angle and surface morphology of the graphene film before and after exposure to reactive oxygen plasma (1 µm scale bars are shown for reference). c) Cyclic voltammetry of as-etched defective EG and the associated PQ/ EG system in 0.1 M HCl. Both CV curves are very stable with time, and these data are averaged over more than 30 cycles. The scan rate for the CV curves is 50 mV s−1.

in hydrophobicity of the plasma-treated graphene indicates an increase in the interaction strength between water and the structural defects introduced by the oxygen plasma. Moreover, the in-plane resistance of graphene increased from 20–30 kΩ for the pristine EG to 60–70 kΩ after the plasma treatment, signifying enhanced electronic perturbations induced by local structural distortions.[28,29] Atomic force microscopy (AFM) images of the pristine EG surface show terraces with meandering steps from the SiC substrate surface (Figure 1b, left). In comparison, images of the etched EG surface revealed a surface

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morphology that is dominated by defects such as cavities and etch pits a few nanometers in size, as observed elsewhere.[30,31] However, the total coverage of vacancy-type defects remains small (approximately a few percent). In addition to these vacancy-type defects, atomic-scale sp3-type defects, in which oxygen adatoms bind to the graphene basal plane as hydroxyl, epoxide, or carbonyl functional groups are expected to be significant.[19,20,31–33] A recent study by high-resolution transmission electron microscopy of defect structures in suspended exfoliated graphene after oxygen plasma etching illustrated the coexistence of both sp3-type and vacancy-type defects on the basal plane under experimental conditions (i.e., plasma etching rate and exposure time) that are comparable to those used here.[24] Subsequent electrochemical measurements confirm that the introduction of extrinsic defects is an effective approach to stabilize PQ adsorption on graphene basal planes. Figure 1c shows cyclic voltammetry of as-etched defective EG before and after PQ adsorption with stable behavior observed for more than 30 cycles. The CV curve for the as-etched EG without adsorbed PQ is essentially featureless, having a shape suggesting primarily capacitive charge transfer, and exhibits only a very weak redox feature at ≈0.3 V (which may be due to the redox reactions of attached functional groups). In contrast, the CV curve of PQ adsorbed on defective EG shows a clear signature of the protonation/deprotonation associated with PCET reactions. This Faradic charge transfer due to protonation of the adsorbed PQ (pseudocapacitance) doubles the total capacitance of the functionalized graphene system (the contribution from the conventional electric double layer capacitance remains about the same).[7] The demonstration that PQ adsorption is stabilized by structural defects introduced on the EG basal plane immediately raises the question: how do these defects stabilize PQ adsorption? This information can be obtained through high-resolution in situ X-ray reflectivity (XR) measurements of the PQ/graphene structure. XR data of oxygen plasma-treated EG sample are shown in Figure 2a (blue circles). The data are very similar to that from as-grown pristine EG films grown on SiC, whose complete structural analysis has been reported previously.[16,34] Relevant features of the XR data include the strong tails of the SiC Bragg peaks (near L = 6, 12), the substantially weaker few-layer graphene Bragg peaks (near L = 3.8 and 8.5), and the continuous variation of the reflectivity signal which is sensitive to the graphene/SiC interface structure. These data were analyzed using model-dependent nonlinear least squares fitting to derive the interfacial structures, in particular including the height and coverage for each of the incomplete graphene layers. The derived electron density profile of the as-etched defective EG (Figure 2b) is very similar to that of the previously reported pristine EG,[16] showing a few graphenic carbon layers (Gn) with variable coverages ranging from ≈50% for G1, with decreasing occupancies for G2 and G3 (see Table 1). These layers reside on top of a 6 3 × 6 3 R30° reconstructed carbon buffer layer (G0) that is structurally commensurate with the SiC substrate.[35] These results indicate that the modest oxygen plasma etching did not disrupt the structural motif of the EG/SiC interfacial system.[24,25] The decreasing coverage of the G1 through G3 layers implies that the plasma-treated graphene retains the morphology found for the pristine sample, in which the exposed surface is distributed between the partial graphene

Figure 2. Molecular-scale structures of as-etched EG sample before and after PQ adsorption, as seen by X-ray reflectivity. a) X-ray reflectivity data of an EG sample after oxygen plasma treatment (blue circles) and the same sample after PQ adsorption (red squares). b) XR derived total electron density profile for the as-etched EG sample, which consists of a trilayer graphene film (G1, G2, and G3) and the interfacial carbon layer (G0) epitaxially grown on an SiC substrate. c) The total electron density profile for the same defective EG sample after PQ adsorption. Adsorbed PQ molecules, resembling graphitic carbon, reside on consecutive layers (G1, G2, and G3) of the trilayer graphene film, as demonstrated by their increased integrated densities and widths, indicating the addition of PQ. Both XR measurements were carried out in a dry He environment.

layers. These data do not, however, distinguish between partial coverages due to domains or localized vacancies in the graphene domains. However, given that the O2-plasma treatment led to changes in water contact angles and stable PQ adsorption, it is apparent that a significant number of defects have been introduced. XR data of the same defective EG sample, both before and after PQ adsorption (Figure 2a, red squares), are compared. A few features can be seen clearly from a comparison of these

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Table 1. Interfacial structural parameters of defective epitaxial graphene before and after PQ adsorption, obtained from the model-dependent best-fit models of the X-ray reflectivity data. Conditions Parameters Graphene layer occupancy (fractional coverage with respect to ideal graphene layer)

Intrinsic r.m.s. width for graphene layer [Å]

Before PQ adsorption

After PQ adsorption

G0: 0.90 ± 0.01

G0: 0.90 ± 0.01

G1: 0.53 ± 0.02

G1: 0.53 ± 0.01

G2: 0.14 ± 0.01

G2 + PQ1: 0.30 ± 0.02

G3: 0.03 ± 0.01

G3 + PQ2: 0.16 ± 0.02

–

PQ3: 0.04 ± 0.01

G0: 0.21 ± 0.01

G0: 0.22 ± 0.01

G1: 0.14 ± 0.02

G1: 0.1 ± 0.01

G2: 0.1

G2 + PQ1: 0.55 ± 0.05

G3: 0.1

G3 + PQ2: 0.3 ± 0.03

–

PQ3: 0.3 ± 0.03

Si: 0.1 ± 0.01

Si: 0.09 ± 0.01

C: 0.18 ± 0.01

C: 0.18 ± 0.02

Si: 0.60 ± 0.06

Si: 0.60 ± 0.02

C: 1.25 ± 0.06

C: 1.25 ± 0.06

G0 to Si dist. [Å]

2.17 ± 0.01

2.17 ± 0.01 3.30 ± 0.02

Last Si–C bilayer disp. from bulk [Å] Last Si–C bilayer occupancy

G1 to G0 [Å]

3.29 ± 0.02

G2 (PQ1) to G1 [Å]

3.72 ± 0.03

3.41 ± 0.03

G3 (PQ2) to G2 [Å]

3.25 ± 0.10

3.74 ± 0.06

PQ3 to G3 [Å]

–

3.51 ± 0.15

Robinson roughness β

0

0

2.94 (4.5%)

7.75 (9.0%)

Goodness of fit χ2 (R factor)

data before we discuss the associated density profile. First, the characteristic graphene (001) thin-film Bragg peak (as marked by the right arrow, Figure 2a) shifts toward L = 4.5 upon PQ adsorption, which is the peak position for multilayer graphene (graphite). Second, the graphene (002) peak at L = 9 becomes slightly narrower after PQ adsorption. Third, the intensity dip near L = 2 shifts toward lower L with the addition of PQ (indicated by the leftward arrow on Figure 2a). All of these features in XR data imply that additional “graphene-like” layers have been added to the existing EG layer upon adsorption of PQ. This form of stacking has also been observed in self-assembled organic monolayers on EG grown using vacuum method.[36,37] The derived electron density profile after PQ is adsorbed on defective EG (Figure 2c) reveals how PQ molecules interact with the defective EG. The increase in integrated density of individual “graphene” layers indicates the incorporation of PQ into the existing graphene layers. While the density profiles for the G0 and G1 layer show very minor changes, the G2 and G3 layer peaks show an increase in their integrated areas and a broader peak width. Moreover, an additional layer is found on top of G3 layer which was not present on the defective EG without PQ. These features indicate that the adsorbed PQ molecules adsorb in a manner that is consistent with π–π 1500277 (4 of 8)

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stacking interaction between graphene and PQ, and resembles a graphenic carbon layer, residing mainly on top of consecutive layers of G1, G2, and G3 of the trilayer graphene film, but with negligible adsorption on the G0 layer. Comparison of the structural models before and after PQ adsorption (Table 1) reveals that: (1) the height difference between adsorbed PQ layer and the underlying graphene is ≈3.4 Å, suggesting a lying down configuration of the PQ molecule, as was expected from the above qualitative analysis; (2) the mixed graphene/PQ layers have a broader intrinsic rms width compared to the highly ordered graphene layer underneath. These widths are enlarged by a factor 3–5 due to the PQ adsorption, presumably due to the detailed differences in the structure of PQ and graphene (e.g., a distribution of heights and/or tilts of PQ with respect to the Gn layers), and possibly with contributions from thermal disorders; (3) the increase in electron density of PQ adsorbed in all layers (PQ1 + PQ2 + PQ3) is equivalent to 33% of a full graphene layer, which corresponds to the effective PQ areal density of 0.7 molecules nm−2 (a full van der Waals monolayer of PQ has an areal density close to 1.1 molecules nm−2).[38] One puzzling aspect of these results is the observed PQ coverage. It might be expected that the PQ coverage on defective EG surface would be controlled by its direct interaction with a defect if the associated structural distortions modulated the PQ–graphene interactions that enhance the dispersive or π–π attractions. In this sense, the surface coverage of adsorbed PQ should be limited by the defective site coverage observed by AFM (i.e., few percent of the surface area), as implied in the experimental observation of a peak adsorption of aromatic compounds onto GNS as a function of adsorbate coverage.[19,20] Instead, the observed PQ coverage covers ≈64% of the geometric surface. Neither classical hydrophobic effects nor dispersion forces are significant for aromatic group stacking interactions in aqueous solution.[39] This observation suggests that the PQ adsorption is nonlocally stabilized, consistent with a previous scanning tunneling microscopy study of quinone adsorption on HOPG,[13] in which electrostatic attractions with partial charges at HOPG defect sites were identified as a controlling factor to mediate adsorption. This also argues against a model in which the PQ adsorption is only stabilized by direct covalent bonding (in a parallel configuration) through the O atoms at step edges of the graphene sheet.[40,41] In particular, it was found from a recent study that the adsorption and electroactivity of quinone-based derivative molecules on graphene is independent of step edge density.[42] The schematic illustration in Figure 3 summarizes our structural model of PQ adsorption on defective EG in which adsorption is distributed widely across the surface terraces with only minor contribution from the step edges. Broadly speaking, the nonlocal effect due to structural perturbations also suggests an effective approach to tailor the electronic coupling between adsorbed functional organic molecules and graphene for electronic applications.[36,37,43] To understand the defect stabilized adsorbate–graphene interaction, we carried out first principle DFT calculations, which examine the adsorption of PQ on the pristine graphene surface as well as on functionalized defect sites on model graphene surfaces. Previously, we showed that the adsorption of PQ on the pristine graphene surface without defects occurs via

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Figure 3. Structure model of PQ adsorbed on defective EG. Illustration of PQ adsorption on few-layer EG with multiple partial graphene layers, after the introduction of defects (here shown as carbon vacancies) to the basal planes. The adsorbed PQ molecules resemble that of a graphitic carbon layer, residing on top of an EG layer with an average vertical spacing of 3.4 Å. An adsorption mode relying on direct covalent interactions with O atoms at step edges of the graphene sheet, as indicated by the dashed circle, is not found to be significant.

π–π stacking and dispersive interactions between the carbon and oxygen atoms of PQ and graphene surface, as shown in Figure 4a.[11] The distance between the PQ and graphene surface was calculated to be 3.4 Å which is consistent with the X-ray reflectivity data reported above and previous results from neutron scattering.[11] The binding energy of PQ was calculated to be −1.08 eV which is reasonably strong but significantly less than that estimated from thermogravimetric analysis (TGA) data (≈−1.8 eV) for PQ on the more defective onion-like-carbon surfaces.[7,44] This is consistent with results reported herein as well as by Anjos et al. which indicated that PQ is more weakly adsorbed on the pristine graphene and graphite surfaces and can be readily removed upon potential cycling.[11] As discussed above, the defect sites that result from oxygen plasma etching likely form hydroxyl or carbonyl intermediates when exposed to water. DFT simulations indicate that both hydroxyl and atomic oxygen bind very strongly to the defect sites on the basal plane of graphene. In aqueous solutions, the resulting hydroxyl or carbonyl groups can pull the first water layer 0.1 Å closer to the graphene surface via hydrogen bonding thus increasing the local hydrophilicity of the graphene surface. This is consistent with the result reported above from the oxygen plasma etching that shows an increase in the hydrophilicity of the surface and an increased interaction between water and the extrinsic defects. The increase in the binding energy of PQ on the plasmatreated surfaces indicates that the oxygen-functionalized defects enhance the adsorption of PQ but do not appreciably change its adsorption structure. To examine the influence of defects on the interactions with PQ, we optimized the structure of a PQ molecule directly above an OH-functionalized defect site in the basal plane of graphene. The results shown in Figure 4b clearly reveal that PQ is too far away from the surface to maintain the π–π interactions and bonds to the surface solely through weaker hydrogen bonds that form between the surface hydroxyl intermediates and the carbonyl groups on PQ, resulting in a much weaker adsorption of PQ at −0.56 eV. PQ, however, can also adsorb onto the basal plane at the edges of the defect sites thus maintaining the strong π–π interactions between the aromatic PQ backbone and the graphene basal plane while also allowing the vicinal surface hydroxyl groups to further anchor the PQ to the surface via hydrogen bonding as shown in Figure 4c. The adsorption of PQ in this combined π–π stacking and H-bonding mode increases its adsorption energy

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Figure 4. DFT-calculated adsorption structures and energies for PQ on graphene. a) The adsorption of PQ on the nondefect pristine graphene surface is dominated by π–π interactions. b) The adsorption of PQ at the hydroxylated defect sites on PQ where the predominant interaction is through hydrogen bonding. c) The adsorption of PQ parallel to graphene surface via π–π interactions adjacent to vicinal OH defect sites which stabilizes the adsorption through hydrogen bonding with the carbonyl groups on PQ.

to −1.46 eV, which is close to the value estimated from TGA experiments for defective onion-like carbons.[7,44] In this sense, each defect site can potentially accommodate a few rather than a single PQ molecule to bind strongly onto the graphene basal plane. This appears to reconcile the observation that the effective PQ coverage exceeds significantly the geometric area of defects on the graphene basal plane. The simulation results reported here are consistent with the experimental findings in that PQ retains its structural attributes (with a distance of 3.4 Å between PQ and the graphene surface), as it is still dictated by π–π stacking and van der Waals interactions once stabilized on EG surface.[36,37] The increased hydrophilicity of graphene due to the presence of defects[16] enables

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water to come closer to the adsorbed PQ and form hydrogen bonds to carbonyl groups on it, thus resulting in the similar combined π–π stacking and hydrogen bonding interaction shown in Figure 4c. The more significant role that hydrogen bonding to water plays in the aqueous electrochemistry of quinones with respect to proton transfer has been pointed out.[45] On the surface of defective EG, hydrogen bonding simply acts to further stabilize the PQ molecule and aid in its redox properties. This adsorption mechanism combining π–π stacking and hydrogen bonding interactions enhancing noncovalent functionalization of graphene was not discussed in previous adsorption studies of aromatic compounds on GNS.[19,20] Further insights into the PCET reaction can be obtained by observing the potential-dependent structural changes in the adsorbed PQ layer under electrochemical control. Representative XR data at static potentials, below, at, and above the redox peak (also in 0.1 M HCl solution), are shown in Figure 5. These

XR data are essentially indistinguishable. In particular, the scattering intensities that are directly sensitive to graphene/PQ layers (for 3 < L < 5) show negligible changes over a wide range of potentials (from −100 to 1000 mV). This demonstrates the strong structural conformity and robustness of the adsorbed PQ layer during the PCET-redox process. These data imply that the electron and proton transfer associated with the oxidation/ reduction reactions do not lead to any significant changes in the surface bonding or configuration of the adsorbed PQ.

3. Conclusion In summary, the persistent redox activity of PQ adsorbed on plasma-treated graphene in aqueous solutions, a model for proton coupled electron transfer reactions, was used to demonstrate the successful noncovalent functionalization of graphene by small molecule aromatic adsorbates. The present results reveal the unstable electrochemical redox behavior of PQ adsorbed on pristine graphene (consistent with that found for other highly ordered carbons) due to an unexpectedly weak interaction with the graphene basal plane. A simple approach is demonstrated for functionalizing pristine graphene through the introduction of defects (both sp3-type, vacancy-type, and oxygencontaining) via exposure to modest oxygen plasmas. These defects further stabilize π–π adsorption of PQ on graphene by a combination of hydrogen bonding and nonlocal interactions. Such an approach is expected to be widely applicable to many functional adsorbate/2D-crystal systems.[36,47,48] The interfacial structures and processes illuminated in this work lay a foundation for understanding relevant interactions between aromatic functional groups with graphene via our in situ experimental approach, providing unique insights into the atomistic interactions at fluid–solid interfaces for catalytic energy systems.

4. Experimental Section

Figure 5. Structural robustness during PCET process of PQ adsorbed on defective EG. a) Cyclic voltammetry of the defective EG adsorbed with PQ in a 0.1 M HCl solution. The scan rate is 2 mV s−1. (The smaller voltage separation of the peaks in the CV curve with respect to that in Figure 1 is due to the use of a slower scan rate,[46] as shown in Figure S1 (Supporting Information).) b) In situ XR measurements at five static electrochemical potentials, as marked by respective colors. The negligible changes in the XR signal, and in particular in the specific region sensitive to graphene and adsorbed PQ structure implies a structural conformity and robustness during the PCET-redox process that is minimally perturbed by the applied potential.

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Epitaxial Graphene Growth and Characterizations: EG film was grown on on-axis cut 6H-SiC (0001) wafer (350 µm thick, nitrogen doped; Cree Inc., USA) by thermal decomposition in a vacuum oven (Solar Atmospheres, PA) at 1200–1500 °C in a high vacuum of 10−6 Torr (heating rate: 10 K min−1). The quality of as-grown EG samples (both structural integrity and intrinsic contact angles) was characterized by Raman spectroscopy and water contact angle measurements.[16] The EG samples after oxygen plasma etching were also characterized by contact angle measurements to demonstrate the change of surface energy due to the etching process. Raman spectra were recorded with an inVia Renishaw (Gloucestershire, UK) microRaman spectrometer. An Ar-ion laser with horizontal polarization was operated at 514.5 nm in a backscattering geometry. The spectral resolution was 1.7 cm−1 (1800 lines mm−1 grating) and the lateral resolution was 0.7 µm. The contact angle of water on EG films was measured under the ambient environment. The measurements were carried out on a two-axis goniometer. Samples were horizontally mounted on the goniometer and a 5 µL drop was deposited on the sample’s surface with an Eppendorf pipette. Determination of the contact angles was performed using the ImageJ software package fitting an ellipsoidal shape to the digital images of sessile drops. The uncertainty of the contact angle measurements is no larger than 4°. The surface morphologies of the EG films at as-grown and plasma-etched conditions were examined by AFM (Asylum Research MFP-3D).

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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements The authors thank Sang Soo Lee, Tim T. Fister, Francesco Bellucci, and Nouamane Laanait for technical and intellectual support at various stages of the experiments. This effort was supported as part of the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the U. Department of Energy, Office of Science, Office of Basic Energy Sciences. Use of the beamlines 6ID and 33ID at the Advanced Photon Source was supported by DOE-SC-BES under Contract No. DEAC02–06CH11357 to UChicago Argonne, LLC as operator of Argonne National Laboratory. The calculations reported herein used resources from the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the US Department of Energy under contract no. DE-AC02-05CH11231. Received: May 28, 2015 Revised: July 26, 2015 Published online:

[1] A. H. C. Neto, F. Guinea, N. M. R. Peres, K. S. Novoselov, A. K. Geim, Rev. Mod. Phys. 2009, 81, 109. [2] Y. Q. Sun, Q. Wu, G. Q. Shi, Energy Environ. Sci. 2011, 4, 1113. [3] J. E. Johns, M. C. Hersam, Acc. Chem. Res. 2012, 46, 77. [4] A. Hirsch, J. M. Englert, F. Hauke, Acc. Chem. Res. 2012, 46, 87. [5] J. A. Mann, W. R. Dichtel, J. Phys. Chem. Lett. 2013, 4, 2649. [6] D. Hulicova-Jurcakova, M. Seredych, G. Q. Lu, T. J. Bandosz, Adv. Funct. Mater. 2009, 19, 438. [7] D. M. Anjos, J. K. McDonough, E. Perre, G. M. Brown, S. H. Overbury, Y. Gogotsi, V. Presser, Nano Energy 2013, 2, 702. [8] H. Y. Liu, G. Q. Zhang, Y. F. Zhou, M. M. Gao, F. L. Yang, J. Mater. Chem. A 2013, 1, 13902. [9] B. Huskinson, M. P. Marshak, C. W. Suh, S. Er, M. R. Gerhardt, C. J. Galvin, X. D. Chen, A. Aspuru-Guzik, R. G. Gordon, M. J. Aziz, Nature 2014, 505, 195. [10] G. Pognon, T. Brousse, D. Bélanger, Carbon 2011, 49, 1340. [11] D. M. Anjos, A. I. Kolesnikov, Z. L. Wu, Y. Cai, M. Neurock, G. M. Brown, S. H. Overbury, Carbon 2013, 52, 150. [12] K. Kano, B. Uno, Anal. Chem. 1993, 65, 1088. [13] M. T. McDermott, R. L. McCreery, Langmuir 1994, 10, 4307. [14] S. Hammes-Schiffer, Acc. Chem. Res. 2009, 42, 1881. [15] A. Sarapuu, K. Helstein, K. Vaik, D. J. Schiffrin, K. Tammeveski, Electrochim. Acta 2010, 55, 6376. [16] H. Zhou, P. Ganesh, V. Presser, M. C. F. Wander, P. Fenter, P. R. C. Kent, D. E. Jiang, A. A. Chialvo, J. McDonough, K. L. Shuford, Y. Gogotsi, Phys. Rev. B 2012, 85, 035406. [17] P. Karásek, J. Planeta, M. Roth, J. Chem. Eng. Data 2009, 54, 1457. [18] M. Sprinkle, P. Soukiassian, W. A. de Heer, C. Berger, E. H. Conrad, Phys. Status Solidi RRL 2009, 3, A91. [19] J. Wang, Z. M. Chen, B. L. Chen, Environ. Sci. Technol. 2014, 48, 4817. [20] K. J. Yang, J. Wang, B. L. Chen, J. Mater. Chem. A 2014, 2, 18219. [21] O. Glukhova, M. Slepchenkov, Nanoscale 2012, 4, 3335. [22] D. C. Kim, D. Y. Jeon, H. J. Chung, Y. S. Woo, J. K. Shin, S. Seo, Nanotechnology 2009, 20, 375703. [23] L. G. Cancado, A. Jorio, E. H. M. Ferreira, F. Stavale, C. A. Achete, R. B. Capaz, M. V. O. Moutinho, A. Lombardo, T. S. Kulmala, A. C. Ferrari, Nano Lett. 2011, 11, 3190.

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Adsorption of Phenanthrenequinone on Epitaxial Graphene: Commercial 9,10-phenanthrenequinone with a purity of 95% was obtained from Aldrich and used without further purification. The PQ was adsorbed on the pristine and defective EG sample surfaces by adsorption from a 0.01 mol L−1 PQ solution in methanol for 20–30 min, followed by subsequent rinsing the electrode with a large amount of deionized water and then rinsed again with methanol (the last step was performed to eliminate any residual PQ microcrystals on the EG surface that might interfere with intrinsic redox of adsorbed PQ monolayer). Oxygen Plasma Etching of Epitaxial Graphene: Oxygen plasma etching was utilized to introduce defects on the EG surface. The EG samples were placed in a vacuum oxygen plasma chamber. The plasma power was stabilized at 5 W for etching. The partial pressure of oxygen is controlled in the range of 0.1–1 Torr with background pressure of a few mTorr in the chamber with an exposure time of 15–20 s. Water contact angles were measured for the as-etched EG samples promptly after being removed from the vacuum chamber. Interfacial Structure Probed Using High-Resolution X-Ray Reflectivity: The interfacial structure of PQ adsorbed on EG was probed by measuring the specular XR signal at the 6-ID and 33-ID beamlines of the Advanced Photon Source. A thin film sample cell and a Roper CCD X-ray detector were mounted on a six-circle goniometer (a Huber psi-C diffractometer at 6-ID and a Newport Kappa diffractometer at 33-ID). The incidence beam, defined by a pair of slits [with apertures of 0.05–0.4 mm (vertical) × 0.5–2 mm (horizontal)], was reflected from the sample. The specular crystal truncation rod was recorded as a function of vertical momentum transfer, Q = (4π/λ)sin(2θ/2) (where λ is the X-ray wavelength 0.9501 Å, and 2θ is the scattering angle). This is also written in terms of the index L in reciprocal lattice units (r.l.u.) as L = Qc/2π, where c is the vertical lattice spacing of the SiC substrate. In Situ Electrochemical Characterization and X-Ray Study: A custom electrochemical cell with a thin liquid film configuration was used for both electrochemical characterization and in situ X-ray studies with simultaneous potential control. The cell is equipped with the standard three-electrode setup. A thin Au foil is placed in direct contact with the EG sample working electrode via a conductive silver paste. The contact area was sealed with epoxy to isolate any possible direct contact between the electrolyte and Au foil. A Pt wire is used for the counter electrode. A standard Ag/AgCl couple is used for the reference electrode. The electrochemical characterizations were performed with an expanded fluid layer using a Gamry Instruments Reference 600 potentiostat. The cyclic voltammetry measurements were done in 0.1 M HCl solution unless specified otherwise. In the process of in situ X-ray measurements, the thickness of the sealed fluid layer was reduced down to a few micrometers by applying a negative pressure using a syringe to reduce absorption of the X-ray beam through the cell. Ab Initio Density Functional Theory Calculations: First principle periodic density functional theory calculations along with dispersion (DFT+D) were carried out using the Vienna Ab Initio Software Program (VASP) to calculate the adsorption of PQ on different graphene surfaces in order to probe the strong bonding of PQ on the more defective graphene surfaces.[49–52] All of the calculations were carried out within the generalized gradient approximation using ultrasoft pseudopotentials to describe the core–electron interactions and the Perdew–Wang 91 exchange correlation functional.[51–53] A cutoff energy of 400 eV was used to terminate the plane wave basis set. The electronic structures were converged to within 1 × 10−6 eV in all of the simulations. A 3 × 3 × 1 Monkhorst–Pack mesh was used to sample the surface Brillouin zone. The geometric structures were optimized until the forces on all of the atoms were found to be less than 0.02 eV Å−1. The weak dispersion interactions were calculated using the methods by Grimme.[54,55] The nondefective graphene surface was modeled using a 7 × 7 unit cell and 98 carbon atoms where all of the atoms were allowed to relax. The simulations examined the adsorption of PQ on the pristine graphene surface and the influence of local defect sites and OH functionalized defects on the binding of PQ.

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[24] A. Zandiatashbar, G. H. Lee, S. J. An, S. W. Lee, N. Mathew, M. Terrones, T. Hayashi, C. R. Picu, J. Hone, N. Koratkar, Nat. Commun. 2014, 5, 4186. [25] Z. M. Xu, Z. M. Ao, D. W. Chu, A. Younis, C. M. Li, S. Li, Sci. Rep. 2014, 4, 6450. [26] S. P. Surwade, S. N. Smirnov, I. V. Vlassiouk, R. R. Unocic, G. M. Veith, S. Dai, S. M. Mahurin, Nat. Nano 2015, 10, 459. [27] Y. J. Shin, Y. Y. Wang, H. Huang, G. Kalon, A. T. S. Wee, Z. X. Shen, C. S. Bhatia, H. Yang, Langmuir 2010, 26, 3798. [28] S. G. Walton, S. C. Hernández, M. Baraket, V. D. Wheeler, L. O. Nyakiti, R. L. Myers-Ward, C. R. Eddy Jr., D. K. Gaskill, Mater. Sci. Forum 2012, 717, 657. [29] C. Kocabas, S. Suzer, Anal. Chem. 2013, 85, 4172. [30] S. P. Koenig, L. Wang, J. Pellegrino, J. S. Bunch, Nat. Nano 2012, 7, 728. [31] A. Eckmann, A. Felten, A. Mishchenko, L. Britnell, R. Krupke, K. S. Novoselov, C. Casiraghi, Nano Lett. 2012, 12, 3925. [32] C. Gómez-Navarro, J. C. Meyer, R. S. Sundaram, A. Chuvilin, S. Kurasch, M. Burghard, K. Kern, U. Kaiser, Nano Lett. 2010, 10, 1144. [33] N. Leconte, J. Moser, P. Ordejón, H. H. Tao, A. Lherbier, A. Bachtold, F. Alsina, C. M. Sotomayor Torres, J. C. Charlier, S. Roche, ACS Nano 2010, 4, 4033. [34] H. Zhou, M. Rouha, G. Feng, S. S. Lee, H. Docherty, P. Fenter, P. T. Cummings, P. F. Fulvio, S. Dai, J. McDonough, Y. Gogotsi, ACS Nano 2012, 6, 9818. [35] K. V. Emtsev, F. Speck, T. Seyller, L. Ley, J. D. Riley, Phys. Rev. B 2008, 77, 155303.

1500277 (8 of 8)

wileyonlinelibrary.com

[36] Q. H. Wang, M. C. Hersam, Nat. Chem. 2009, 1, 206. [37] J. D. Emery, Q. H. Wang, M. Zarrouati, P. Fenter, M. C. Hersam, M. J. Bedzyk, Surf. Sci. 2011, 605, 1685. [38] A. D. Rae, A. C. Willis, Z. Kristallogr. 2003, 218, 221. [39] L. F. Newcomb, S. H. Gellman, J. Am. Chem. Soc. 1994, 116, 4993. [40] M. T. McDermott, R. L. McCreery, Langmuir 1994, 10, 4307. [41] M. T. McDermott, K. Kneten, R. L. McCreery, J. Phys. Chem. 1992, 96, 3124. [42] G. H. Zhang, P. M. Kirkman, A. N. Patel, A. S. Cuharuc, K. McKelvey, P. R. Unwin, J. Am. Chem. Soc. 2014, 136, 11444. [43] H. Huang, S. Chen, X. Gao, W. Chen, A. T. S. Wee, ACS Nano 2009, 3, 3431. [44] S. M. Chathoth, D. M. Anjos, E. Mamontov, G. M. Brown, S. H. Overbury, J. Phys. Chem. B 2012, 116, 7291. [45] M. Quan, D. Sanchez, M. F. Wasylkiw, D. K. Smith, J. Am. Chem. Soc. 2007, 129, 12847. [46] R. G. Keil, J. Electrochem. Soc. 1986, 133, 1375. [47] D. Z. Sun, W. H. Lu, D. Le, Q. Ma, M. Aminpour, M. A. Ortigoza, S. Bobek, J. Mann, J. Wyrick, T. S. Rahman, L. Bartels, Angew. Chem., Int. Ed. 2012, 51, 10284. [48] G. Y. Gou, B. C. Pan, L. Shi, ACS Nano 2010, 4, 1313. [49] G. Kresse, J. Furthmuller, Phys. Rev. B 1996, 54, 11169. [50] G. Kresse, J. Furthmuller, Comput. Mater. Sci. 1996, 6, 15. [51] D. Vanderbilt, Phys. Rev. B 1990, 41, 7892. [52] J. P. Perdew, Y. Wang, Phys. Rev. B 1986, 33, 8800. [53] J. P. Perdew, Y. Wang, Phys. Rev. B 1992, 45, 13244. [54] S. Grimme, J. Comput. Chem. 2006, 27, 1787. [55] S. Grimme, J. Comput. Chem. 2004, 25, 1463.

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