Chemical Structure of Oxidized Multilayer Epitaxial Graphene: A

Article pubs.acs.org/JPCC

Chemical Structure of Oxidized Multilayer Epitaxial Graphene: A Density Functional Theory Study Si Zhou,†,‡ Suenne Kim,‡ and Angelo Bongiorno*,† †

School of Chemistry and Biochemistry and ‡School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States ABSTRACT: Density functional theory calculations are carried out to model the structure and interpret recent X-ray photoelectron spectroscopy measurements of graphene oxide films obtained by Hummers oxidation of multilayer graphene grown epitaxially on silicon carbide. The confrontations between theory and experiment are used to gain insight into the nature and fraction of the oxygen functional groups present in the oxide films. The study shows that this type of graphene oxide films includes small amounts of intercalated water molecules, ether groups, and doubly oxidized carbon species and that the oxidized graphene sheets encompass a disordered and homogeneous distribution of epoxide and hydroxyl species. The results of our spectral analysis are corroborated by a study of the energetic stability of water in this form of graphene oxide.

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With respect to films obtained by using conventional methods,3,45 these novel graphene oxide films are uniform, homogeneous, almost completely devoid of edges and holes, and therefore lacking of carbonyl, carboxyl, and lactol species.39 Thanks to their reduced complexity, this type of oxide films opens the way to a detailed understanding of the chemical and physical properties of both graphene oxide systems and graphene derivatives with the multilayer geometry. Very recently, the authors contributed to elucidate the metastability characteristics exhibited at room temperature by graphene oxide films obtained from epitaxial graphene.39 In this recent work, experiments and DFT computations were combined to characterize and explain the origin of the spontaneous chemical and structural changes occurring in the films after their synthesis.39 The spontaneous changes were discovered to be attributed to the presence of C−H species in the films which, through their sequential reaction with epoxide and hydroxyl groups, favor the formation and then release of water molecules from the oxide films.39 In this work, we present a detailed study based on DFT calculations of the chemical structure of multilayer graphene oxide obtained from epitaxial graphene. For convenience, this type of oxide films is henceforth abbreviated as OeG, i.e., the oxide of epitaxial graphene films. In particular, we here use DFT to generate model structures of OeG and interpret a selected set of X-ray photoelectron spectroscopy (XPS) measurements of these films. Through the comparison of computed and experimental XPS spectra, we derive detailed information about the concentration in OeG of intercalated water molecules, epoxide and hydroxyl species, and ether

INTRODUCTION Graphene oxide is a material with potential applications in nanoelectronics,1−3 electromechanical systems,4,5 sensors,6 polymer composites,7−9 catalysis,10−12 energy storage devices,13−15 and optics.16 Synthesized for the first time in 1855 by Brodie,17 graphene oxide is still today prepared through the use of harsh chemical treatments, leading to films with complex, intriguing, and not yet fully understood physical and chemical properties.3,5,6,18−40 In this work, we use density functional theory (DFT) calculations to investigate the chemical structure of a novel form of graphene oxide films, obtained by chemical oxidation of multilayer epitaxial graphene on SiC substrates.41 The traditional method to synthesize graphene oxide consists of the following steps: (i) oxidation of graphite via the Hummers42 or similar methods,43,44 (ii) exfoliation of graphite oxide and dissolution of single-layer graphene oxide sheets in an aqueous solution, and (iii) filtration and deposition of the oxidized graphene layers onto a substrate.3,45 This method yields oxide films consisting of a stack of oxidized graphene platelets and presenting a complex microstructure. The films include a variety of oxygen functional groups, from hydroxyl and epoxide species on the undefected regions of the carbon planes to carbonyl, carboxyl, and lactol groups at the defects, holes, and edges of the oxidized carbon sheets.18,19,21,24,36,43,46−48 Also, these graphene oxide films are known to be hygroscopic and include a certain amount of water molecules in the structure.26 Due to their elevated structural and chemical complexity, the properties of this type of graphene oxide films are difficult to both characterize and control.22 In this work, we study the chemical structure of graphene oxide films1,2 obtained through the use of a novel strategy, that is, Hummers oxidation42 of thin epitaxial graphene films grown in vacuum and at high temperature on SiC substrates.39,41,49 © 2013 American Chemical Society

Received: January 4, 2013 Revised: March 1, 2013 Published: March 5, 2013 6267

dx.doi.org/10.1021/jp400128t | J. Phys. Chem. C 2013, 117, 6267−6274

The Journal of Physical Chemistry C

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Table 1. Lattice Parameters (a and c), Bulk Modulus (B), and Intralayer Binding Energy (Eb) of Graphitea

groups and doubly oxidized carbon species. Furthermore, we carry out additional DFT calculations to corroborate our findings about the absence of intercalated water in the OeG films. Numerous computational studies have appeared so far on both graphene and graphene oxide. Among the various efforts, recent DFT works have addressed the bonding properties of oxygen functional groups on graphene,50−54 the energetic and electronic properties of regular arrangements of hydroxyl and epoxide groups on graphene,52,53,55−57 and the nature of the reduction mechanisms leading to the release of CO, CO2, and O 2 molecules from graphene oxide. 58,59 Tight-binding schemes20 and methods based on reactive force fields5,31,40 have also been used to address the chemical and thermal stability of multilayer graphene oxide20,31 and the effect of water on the mechanical properties of its lamellar structure.5,40 With respect to recent computational efforts, the present study represents an attempt to achieve a basic understanding of the chemical structure of graphene oxide through the direct confrontation with the experiment.

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DFT DFT-D2 PBE PBE-D PW91-D VdW-DF Exp.

a (Å)

c (Å)

B (GPa)

Eb (meV/atom)

2.47 2.47 2.46 2.46 2.45 2.47 2.46

8.77 6.39 9.62 6.43 6.69 7.52 6.65

2 67 1 46 − 12 41

1 78 1 54 83 24 52

a

Present results, DFT and DFT-D2, are compared to values obtained by employing density-functional schemes based on alternative technical details (PBE and PBE-D69) and approximations (PW91D67 and VdW-DF64). Experimental data are also reported.73−75

of four graphene layers stacked with the “AB” Bernal structure, each one including 7 × 8 graphene unit cells. This model structure is used to calculate equilibrium lattice parameters, bulk modulus, and interlayer binding energy of graphite (Table 1). In the case of the water dimer (Figure 1), we compute geometrical parameters and binding energy. The results of our DFT-D2 calculations are compared to recent calculated and experimental data in Table 1 and Table 2. These comparisons show that, with respect to standard DFT, the DFT-D2 scheme yields an improved description of both graphitic and OH···O hydrogen bonded systems. In this work, we use the DFT-D2 scheme to generate model structures of OeG and calculate C 1s and O 1s core-level energy shifts. To calculate these latter quantities, we use the core-excited pseudopotential technique.70,71 A core-level energy shift calculated through the use of this technique accounts for the vertical photoexcitation transition and includes core-hole relaxation effects.70,71 In detail, we use the fhi98PP package72 to generate a norm-conserving pseudopotential for both ionized C+ and O+. These pseudopotentials simulate the presence of a screened 1s electron hole in the core of the ionic species. Hence, the core-level energy shift at a C (or O) site in a molecular or material system is calculated as

COMPUTATIONAL METHODS

In this work, we use the QUANTUM-Espresso toolkit60 to perform our DFT calculations. We use a plane-wave energy cutoff of 65 Ry, norm-conserving pseudopotentials61 for all atomic species, and the exchange and correlation energy functional proposed by Perdew, Burke, and Ernzerhof.62 We employ large supercells and calculations sampling the Brillouin zone only at the Γ-point. To locate transition states and compute reaction and diffusion energy barriers, we use the nudged elastic band method,63 as implemented in the PWscf code of QUANTUM-Espresso.60 The majority of the calculations is carried out by using the CP code. London dispersion forces are important in layered materials such as graphite and multilayer graphene oxide.64−67 To account for this type of interactions in our semilocal DFT scheme, we adopt the semiempirical dispersion-corrected DFTD2 approach proposed by Grimme,68 as recently implemented in QUANTUM-Espresso by Barone et al.69 For completeness, this DFT-D2 scheme is here tested on two reference systems: graphite (Figure 1 and Table 1) and the water dimer (Figure 1 and Table 2). To model graphite, we use a supercell consisting

ΔC1s = E DFT‐D2[C ] − E DFT‐D2[C +]

(1)

where EDFT‑D2[C] and EDFT‑D2[C+] are the total energies computed from DFT-D2 of the neutral and ionized systems at the selected C site, respectively.70,71 To validate both technique and core-excited pseudopotentials, we consider a set of organic molecules. In all cases, the DFT-D2 calculations are carried out by using a cubic supercell with side equal to 30 Å. Computed and experimental C 1s and O 1s core-level binding energies are compared in Table 3 and Table 4. These comparisons show that our method yields C 1s and O 1s core-level energy shifts which over a wide energy interval are in good agreement with the experiment. On average, the percent deviation from the experimental data is about 3%.

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RESULTS AND DISCUSSION Typical C 1s and O 1s XPS spectra of as-synthesized OeG are shown in Figure 2.39 The O 1s XPS spectrum is featureless, while that one arising from the C 1s core levels consists of two broad peaks: one at lower energy arising from nonoxidized C atoms in an sp2-like configuration and a peak centered around 286.5 eV originating from C species bonded to O atoms. C and O atoms in carboxyl or lactone groups give rise to peaks at

Figure 1. (a) Interlayer binding energy in graphite calculated by using both our conventional DFT (black symbols) and corrected DFT-D2 (red symbols) schemes. (b) Ball-and-stick illustration of a water dimer. The values of the geometrical parameters Roo, β, θd, and θa obtained from both DFT and DFT-D2 are reported in Table 2. 6268



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Table 2. Geometrical Parameters (See Figure 1) and Binding Energy (E0) of a Water Dimera DFT DFT-D2 Exp.

Roo (Å)

θa (°)

θd (°)

β (°)

E0 (kcal/mol)

2.945 2.957 2.976

62.8 61.9 57 ± 10

−57.4 −56.3 −51 ± 10

5.3 4.3 2 ± 10

−2.58 −3.09 −5.4 ± 0.7

a

Results obtained by using our DFT and DFT-D2 schemes are compared to experimental data.76,77 E0 is calculated by referring the energy of the dimer to that of the two isolated molecules in vacuum.

Table 3. C 1s Core-Level Binding Energies (in eV) at C Atoms in Selected Organic Moleculesa formaldehyde methanol ethylene acetaldehyde dimethyl ether methylacetylene

benzene

chemical formula

ΔC1s

exp.

CH2O CH3OH C2H4 CH3C*OH C*H3COH (CH3)2O CH3CC*H CH3C*CH C*H3CCH C6H6

1.81 3.94 5.56 2.41 4.97 4.23 6.09 5.46 4.65 6.18

1.82 3.90 5.50 2.20 4.85 4.03 5.80 5.13 4.43 5.90

a

Binding energies are referred to the value obtained from eq 1 in the case of C in CO. Experimental data are taken from ref 78.

Figure 2. Experimental C 1s (top panel) and O 1s (bottom panel) XPS spectra of a graphene oxide film obtained by Hummers oxidation of epitaxial graphene grown on SiC. Experimental intensities (symbols) exclude the Shirley background obtained from fitting the raw data. The fitting analysis is also used to decompose the C 1s XPS spectrum in three peaks, one associated to carbonyl, carboxyl, and lactol groups (green line), another one to hydroxyl and epoxide species (blue line), and the last one to nonoxidized C species (red line). The O 1s spectrum is decomposed in two peaks, the first one associated to O atoms in hydroxyl, epoxide, and H2O species (magenta line) and the second one to O atoms in carbonyls, carboxyls, and lactols (green line).

Table 4. O 1s Core-Level Binding Energies (in eV) of O Atoms in Selected Organic Moleculesa carbon dioxide water formaldehyde acetaldehyde acetic acid methanol ethanol dimethyl ether dimethyl carbonate

chemical formula

ΔO1s

exp.

CO2 H2O CH2O CH3COH CH3CO*OH CH3COO*H CH3OH CH3CH2OH (CH3)2O (CH3O)2CO* (CH3O*)2CO

1.27 2.65 3.13 4.17 4.45 2.62 3.53 3.91 4.86 4.75 3.09

1.29 2.69 3.15 4.18 4.46 2.67 3.67 3.97 4.84 4.55 2.90

configurations, release excess stress, and obtain a fully optimized model structure of the film (Figure 3). Through the use of this strategy, we generate a total of seven models of

a

Binding energies are referred to the value obtained from eq 1 in the case of O in CO. Experimental data are taken from ref 78.

higher and lower energies in the C 1s and O 1s XPS spectra, respectively (Figure 2).22 However, standard spectral analyses show (see Figure 2) that these peaks are small, indicating that the OeG films include, with respect to conventional graphene oxide films,22 a minimal amount of these oxygen functional groups, or else holes and edges. Overall, the XPS spectra in Figure 2 show that the oxidized graphene layers of OeG encompass a distribution of hydroxyl and epoxide species, and standard analyses can be used to deduce an approximate value of both the O:C ratio of the films and PC−O, the fraction of oxidized C atoms.39 To gain further insight, we here use DFT calculations. To generate model structures of the OeG films, we adopt the following strategy. We use a supercell of graphite including four layers and 5 × 6 unit cells of graphene per layer. We insert a pseudorandom distribution of oxygen atoms, O−H radicals, and H2O molecules near and in-between the carbon planes. Finally, we carry out a variable-cell Car−Parrinello damped molecular dynamics simulation60 to eliminate unfavorable

Figure 3. (a) Ball-and-stick illustration of a model structure of OeG generated from DFT-D2. Gray, red, and white colors are used to indicate C, O, and H atoms, respectively. (b) Selected region of a model structure of OeG showing, from left to right, a hydroxyl group, a water molecule intercalated between two oxidized graphene layers, and an epoxide group. (c) Selected region of a model structure of OeG showing an ether group and, indicated by the arrow, a doubly oxidized C atom, shared by the ether and an epoxide group. 6269



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OeG, four with an O:C ratio equal to 0.44 and modeling the structure of the as-synthesized films and three with an O:C ratio equal to 0.38 and modeling films aged at room temperature for more than two months.39 For each O:C ratio, the model structures present selected and different distributions of hydroxyl (C−OH), epoxide (C−O−C), and H2O species (Figure 3). Composition, structural parameters, and mean interlayer binding energy of the OeG models are reported in Table 5 and Table 5. Chemical, Geometrical, and Energetic Information of the Model Structures of As-Synthesized OeG Films Generated from DFT-D2a model

C−O−C

C−OH

H2O

d (Å)

Δd (Å)

Eb (meV/Å2)

A B C D

0.26 0.26 0.30 0.31

0.18 0.16 0.13 0.06

0.00 0.02 0.02 0.07

4.43 4.48 4.45 4.63

0.30 0.30 0.35 0.38

44.2 45.1 32.5 33.4

Figure 4. Density of the electronic states of selected model structures of OeG generated from DFT-D2 (see legend and tables). Solid lines have been obtained by smearing the Kohn−Sham single-electron energies with a Gaussian function of width 0.3 eV. The Fermi level is set at 0 eV.

a The fractions of C−O−C, C−OH, and H2O species are referred to the total number of C atoms. Mean interlayer separation, average intralayer vertical buckling, and mean interlayer binding energy are indicated by d, Δd, and Eb, respectively. The models have an O:C ratio of 0.44.

Figure 5. Experimental (circles) and calculated (solid lines) C 1s XPS spectra of as-synthesized OeG. Experimental intensities exclude the Shirley background. Models are labeled as in Table 5.

Table 6. Chemical, Geometrical, and Energetic Information of the Model Structures of Aged OeG Generated from DFTD2a model

C−O−C

C−OH

H2O

d (Å)

Δd (Å)

Eb (meV/Å2)

E F G

0.10 0.25 0.18

0.27 0.05 0.20

0.01 0.08 0.00

4.55 4.69 4.55

0.28 0.38 0.31

49.5 38.0 37.3

tration in the models B, C, and D (Figure 5). The origin of this disagreement can be understood as follows. At a fixed O:C ratio, an increase in the amount of intercalated water molecules in the film leads necessarily to a decrease in the total number of epoxide and hydroxyl groups oxidizing the carbon layers. To reach an accord with the experiment, PC−O, i.e., the fraction of of C−O bonds present in the model structure, must remain close to the experimental estimate of 0.64. To this end, the solution is decreasing the number of C−OH species and increasing the number of C−O−C groups in the structure (Table 5). Unfortunately, our DFT calculation shows that this operation is detrimental to the XPS spectrum. In fact, the calculations show that fractions of C−O−C larger than 0.26 lead to the occurrence of a significant number of both ether groups and doubly oxidized C atoms. As shown in Figure 6, these two species give rise to C 1s chemical shifts which are, respectively, smaller and larger than those associated to epoxide and hydroxyl groups and therefore to a deterioration of the agreement between theory and experiment. OeG films aged at room temperature exhibit a O:C ratio of about 0.38 and the XPS spectra shown in Figure 7.39 To elucidate the chemical structure of these films, we consider three model structures of OeG with a O:C ratio of 0.38 and selected distributions of C−O−C, C−OH, and H2O species (Table 6). The experiment shows that the fraction of carbonyl species in aged OeG amounts to 0.02−0.04.39 These species are not included in our model structures of OeG. The comparison between experiment and theory in Figure 7 shows that, in contrast to the case of the as-synthesized OeG films, it is difficult to reach firm conclusions about the fractions of C−O−C, C−OH, and H2O species present in the aged films. Models E, F, and G present, in fact, well-selected but different fractions of C−O−C, C−OH, and H2O species, and they all

a

The models have an O:C ratio of 0.38. See Table 5 for column naming definitions.

Table 6. Binding energies are between 32 and 50 meV/Å2, which is around the value obtained for graphite (Table 1). The average interlayer distance and the out-of-plane root-meansquare deviation from the mean of the C atoms are found to lie in the intervals 4.4−4.7 Å and 0.28−0.38 Å, respectively. The density of the electronic states of selected model structures of OeG is also shown in Figure 4. In accord with the experiment,1 we find that OeG is gapless and exhibits a finite density of electronic states at the Fermi level. Figure 5 shows the comparison between calculated and experimental C 1s XPS spectra of as-synthesized OeG.39 Model A shows the best agreement with the experiment. This atomistic model includes no intercalated H2O molecules and a fraction of C−O−C and C−OH speciesrelative to the total amount of Cequal to 0.26 and 0.18, respectively. In model A, the total fraction PC−O of singly oxidized C atoms is 0.66, very close to the value of 0.64 obtained from the analysis of the XPS measurements.39 Models B, C, and D contain up to 7% of water molecules intercalated in-between the layers and a number of epoxide and hydroxyl species yielding a value for PC−O equal to 0.64, 0.65, and 0.61, respectively. These values of PC−O are in good agreement with the experimental estimate. Nonetheless, the comparison in Figure 5 shows that the accord with the experiment worsens steadily for increasing the water concen6270



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and 30% smaller than in as-synthesized OeG, while model E, which shows the best agreement with the experiment, includes 50% and −61% of these same species, respectively. To accommodate, on the contrary, 8% of H2O in the structure, an accord with the experiment can be obtained by including a fraction of C−O−C equal to 0.25 and a fraction of C−OH as little as 0.05. These fractions are 4% and 71% smaller than those of model A. Also in the case, however, and in agreement with ref 39 the amount of H in the structure appears to be larger than that present in as-synthesized OeG. We finally note that the fractions of ether groups in model E, F, and G are 0.01, 0.04, and 0.03, respectively. The fraction of ethers deduced from the experimental O 1s XPS spectrum shown in Figure 7 is around 0.01, indicating that model E indeed presents a chemical structure in very good agreement with the experiment and thus that also in aged OeG intercalated water molecules are present in minimal concentrations. To support the results discussed above, we here address the issue of the stability and hence the presence of water in OeG. To tackle this complex problem, we here simply consider the interaction of two hydroxyl species on both a clean and oxidized graphene layer. In particular, we employ a periodic layer consisting of 5 × 6 unit cells of graphene. In a first case we use DFT calculations to sample the energy landscape of two reacting hydroxyl species on the pristine layer, while in the second case we extract an oxidized layer from model A and compute the energies involved in the reaction of two nearestneighboring hydroxyl groups. The results of these calculations are reported in Figure 8 and Figure 9.

Figure 6. (a) and (b) C 1s XPS spectra obtained from DFT-D2 (solid black lines) by using models A and C. Open circles show the experimental XPS spectra of as-synthesized OeG. The spectra computed from DFT-D2 are decomposed in four peaks: one peak arising from nonoxidized C atoms in a sp2-like configuration (red dashed line), the second one associated to C atoms in epoxide and hydroxyl species (blue dashed line), the third one arising from ether groups (light-blue dashed line), and the last peak associated to doubly oxidized C atoms (pink dashed line). (c) C−O−C bond angle distribution extracted from the model structures of OeG. The distribution shows that epoxide and ether groups are clearly distinguishable on the basis of this geometrical parameter.

Figure 8. Scheme showing selected configurations and energies of two hydroxyl groups adsorbed on a pristine graphene layer. Energy values are expressed with respect to the energy of two isolated hydroxyl groups on graphene at infinite distance from each other. Segments show the energy of the ball-and-stick illustrations reported either above or below. For clarity, numerical values are also shown. Cyan colored segments indicate the energy of transition states, while the red colored segment shows the energy of reaction product. Figure 7. Experimental (circles) and calculated (solid lines) C 1s (top panel) and O 1s (bottom panel) XPS spectra of a OeG film aged at room temperature for more than two months. Experimental intensities exclude the Shirley background. Models are labeled as in Table 6.

Figure 8 shows that hydroxyl groups on graphene tend to agglomerate to form energetically very stable complexes. When adsorbed on the same side of the graphene layer, the most

give rise to C 1s and O 1s XPS spectra in relatively good agreement with the experiment. However, the values of PC−O in the models E, F, and G are 0.44, 0.51, and 0.52, respectively. The experimental estimate is 0.45, and therefore model E is the one showing the best agreement with the experiment. Table 6 and Figure 7 show that, under the assumption of no or little water in aged OeG, the agreement with the experiment can be reached only by considering a structure which, with respect to the as-synthesized films, is enriched in C−OH species and deprived in C−O−C groups. Model G contains a fraction of C−OH and C−O−C species which are 11% larger

Figure 9. Scheme showing the energies and configurations involved in the reaction of two hydroxyl groups in OeG (circled on the left) to form a water molecule and an epoxide group (right panels). Ball-andstick illustration of the transition state is also shown in the center. 6271



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Towards Epitaxial Graphene Electronics. Phys. Rev. Lett. 2008, 101, 026801. (3) Eda, G.; Fanchini, G.; Chhowalla, M. Large-Area Ultrathin Films of Reduced Graphene Oxide as a Transparent and Flexible Electronic Material. Nat. Nanotechnol. 2008, 3, 270−274. (4) Robinson, J. T.; Zalalutdinov, M.; Baldwin, J. W.; Snow, E. S.; Wei, Z.; Sheehan, P.; Houston, B. H. Wafer-Scale Reduced Graphene Oxide Films for Nanomechanical Devices. Nano Lett. 2008, 8, 3441− 3445. (5) Medhekar, N. V.; Ramasubramaniam, A.; Ruoff, R. S.; Shenoy, V. B. Hydrogen Bond Networks in Graphene Oxide Composite Paper: Structure and Mechanical Properties. ACS Nano 2010, 4, 2300−2306. (6) Robinson, J. T.; Perkins, F. K.; Snow, E. S.; Wei, Z. Q.; Sheehan, P. E. Reduced Graphene Oxide Molecular Sensors. Nano Lett. 2008, 8, 3137−3140. (7) Potts, J. R.; Dreyer, D. R.; Bielawski, C. W.; Ruoff, R. S. Graphene-Based Polymer Nanocomposites. Polymer 2011, 52, 5−25. (8) Putz, K. W.; Compton, O. C.; Palmeri, M. J.; Nguyen, S. T.; Brinson, L. C. High-Nanofiller-Content Graphene Oxide−Polymer Nanocomposites via Vacuum-Assisted Self-Assembly. Adv. Funct. Mater. 2010, 20, 3322−3329. (9) Dreyer, D. R.; Jarvis, K. A.; Ferreira, P. J.; Bielawski, C. W. Graphite Oxide as a Dehydrative Polymerization Catalyst: a One-Step Synthesis of Carbon-Reinforced Poly(Phenylene Methylene) Composites. Macromolecules 2011, 44, 7659−7667. (10) Dreyer, D. R.; Bielawski, C. W. Carbocatalysis: Heterogeneous Carbons Finding Utility in Synthetic Chemistry. Chem. Sci. 2011, 2, 1233−1240. (11) Dreyer, D. R.; Jia, H.; Bielawski, C. W. Graphene Oxide: A Convenient Carbocatalyst for Facilitating Oxidation and Hydration Reactions. Angew. Chem., Int. Ed. 2010, 49, 6813−6816. (12) Jia, H.; Dreyer, D. R.; Bielawski, C. W. Graphite Oxide as an Auto-Tandem Oxidation−Hydration−Aldol Coupling Catalyst. Adv. Synth. Catal. 2011, 353, 528−532. (13) Gao, W.; Singh, N.; Song, L.; Liu, Z.; Reddy, A. L. M.; Ci, L.; Vajtai, R.; Zhang, Q.; Wei, B.; Ajayan, P. M. Direct Laser Writing of Micro-Supercapacitors on Hydrated Graphite Oxide Films. Nat. Nanotechnol. 2011, 6, 496−500. (14) Wang, D.; Kou, R.; Choi, D.; Yang, Z.; Nie, Z.; Li, J.; Saraf, L. V.; Hu, D.; Zhang, J.; Graff, G. L.; et al. Ternary Self-Assembly of Ordered Metal Oxide−Graphene Nanocomposites for Electrochemical Energy Storage. ACS Nano 2010, 4, 1587−1595. (15) Kim, T. Y.; Lee, H. W.; Stoller, M.; Dreyer, D. R.; Bielawski, C. W.; Ruoff, R. S.; Suh, K. S. High-Performance Supercapacitors Based on Poly(Ionic Liquid)-Modified Graphene Electrodes. ACS Nano 2011, 5, 436−442. (16) Loh, K. P.; Bao, Q.; Eda, G.; Chhowalla, M. Graphene Oxide as a Chemically Tunable Platform for Optical Applications. Nature Chem. 2010, 2, 1015−1024. (17) Brodie, B. Note Sur un Nouveau Procédé pour la Purification et la Désagrégation du Graphite. Ann. Chim. Phys. 1855, 45, 351−353. (18) Schniepp, H. C.; Li, J.-L.; McAllister, M. J.; Sai, H.; HerreraAlonso, M.; Adamson, D. H.; Prud’homme, R. K.; Car, R.; Saville, D. A.; Aksay, I. A. Functionalized Single Graphene Sheets Derived from Splitting Graphite Oxide. J. Phys. Chem. B 2006, 110, 8535−8539. (19) Gómez-Navarro, C.; Weitz, R. T.; Bittner, A. M.; Scolari, M.; Mews, A.; Burghard, M.; Kern, K. Electronic Transport Properties of Individual Chemically Reduced Graphene Oxide Sheets. Nano Lett. 2007, 7, 3499−3503. (20) Paci, J. T.; Belytschko, T.; Schatz, G. C. Computational Studies of the Structure, Behavior upon Heating, and Mechanical Properties of Graphite Oxide. J. Phys. Chem. C 2007, 111, 18099−18111. (21) Gao, W.; Alemany, L. B.; Ci, L.; Ajayan, P. M. New Insights into the Structure and Reduction of Graphite Oxide. Nature Chem. 2009, 1, 403−408. (22) Yang, D.; Velamakanni, A.; Bozoklu, G.; Park, S.; Stoller, M.; Piner, R. D.; Stankovich, S.; Jung, I.; Field, D. A.; C., A. V., Jr.; et al. Chemical Analysis of Graphene Oxide Films after Heat and Chemical

stable complexes are formed when the two hydroxyl groups reside at either the ortho or para positions of a hexagonal ring. In these configurations, the two hydroxyl groups can reach very favorably to form a free water molecule and an epoxide group on graphene. This hydrogen-bonded complex is isoenergetic with the most stable dimer formed when the two hydroxyl groups are adsorbed on the opposite sides of the graphene layer in ortho positions. Similarly, Figure 8 shows that, on a fully oxidized graphene layer, hydroxyl groups form motifs whose energetic stability is superior to that of water. Our nudgedelastic-band DFT calculations show, in particular, that the process of breaking the hydroxyl trimer in Figure 9 to form a water molecule and an epoxide group requires an energy of about 1.7 eV and leads to an energy increase of about 1.3 eV. Overall, in agreement with the results of the spectral analysis based on DFT, these simple calculations show that, in dilute amounts, water molecules in OeG may be energetically less stable than tightly bound hydroxyl complexes adsorbed on the graphene layer.

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CONCLUSIONS In this work, we have used DFT calculations to model the structure and interpret selected XPS measurements of graphene oxide films obtained by oxidizing epitaxial graphene films grown on SiC (OeG). The confrontations between computed and experimental XPS spectra have led to the following results. First, as-synthesized OeG films exhibit an O:C ratio of about 0.44, and they include a fraction of epoxide and hydroxyl groups equal to about 0.26 and 0.18, respectively. These films are devoid of intercalated water molecules, ether groups, and doubly oxidized C atoms. Second, OeG films aged at room temperature present an O:C ratio of about 0.38 and a structure enriched in hydroxyl groups and still poor in intercalated water molecules and ether groups. Third, the absence of intercalated water molecules in the OeG films is consistent with the results of DFT calculations showing that tightly bound hydroxyl complexes on graphene are energetically stable against water desorption. The results presented in this work apply to oxide films obtained from epitaxial graphene grown on SiC. Due to different processing and layer stacking, conventional graphene oxide obtained from graphite is indeed known to exhibit chemical characteristics deviating from those reported in this work.22,26

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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 All the authors acknowledge the support of the National Science Foundation, grants CMMI-1100290 and DMR0820382. S.Z. and A.B. also acknowledge the support of the NSF grant CHE-0946869.

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Chemical Structure of Oxidized Multilayer Epitaxial Graphene: A

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