Formation of Supported Graphene Oxide: Evidence for Enolate Species

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Formation of Supported Graphene Oxide: Evidence for Enolate Species Zbynek Novotny, Manh-Thuong Nguyen, Falko P. Netzer, VassilikiAlexandra Glezakou, Roger Rousseau, and Zdenek Dohnalek J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b12791 • Publication Date (Web): 05 Feb 2018 Downloaded from http://pubs.acs.org on February 5, 2018

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Journal of the American Chemical Society

Formation of Supported Graphene Oxide: Evidence for Enolate Species Zbynek Novotny,1,† Manh-Thuong Nguyen,1 Falko P. Netzer,2 Vassiliki-Alexandra Glezakou,1 Roger Rousseau,1,* and Zdenek Dohnálek1,3,* 1

Fundamental and Computational Sciences Directorate and Institute for Integrated Catalysis, Pacific Northwest 2 National Laboratory, P.O. Box 999, Richland, Washington 99352, United States; Surface and Interface Physics, In3 stitute of Physics, Karl-Franzens University, A-8010 Graz, Austria; Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, Washington 99163, United States KEYWORDS: supported graphene, graphene functionalization, graphene oxide, enolate, Ru(0001), scanning tunneling microscopy, density functional theory

ABSTRACT: Graphene oxides are promising materials for novel electronic devices or anchoring of the active sites for catalytic applications. Here we focus on understanding the atomic oxygen (AO) binding and mobility on different regions of graphene (Gr) on Ru(0001). Differences in the Gr/Ru lattices result in the superstructure, which offers an array of distinct adsorption sites. We employ scanning tunneling microscopy and density functional theory to map out the chemical identity and stability of prepared AO functionalities in different Gr regions. The AO diffusion is utilized to establish that in the regions that are close to the metal substrate, the terminally-bonded enolate groups are strongly preferred over bridge-bonded epoxy configurations. No oxygen species are observed on the graphene regions that are far from the underlying Ru, indicating their low relative stability. This study provides a clear fundamental basis for understanding the local structural and electronic factors and C-Ru bond strengthening/weakening processes that affect the stability of enolate and epoxy species.

INTRODUCTION The engineering of the electronic band structure via doping with atomic or molecular adsorbates1-4, essential for the use of graphene in novel electronic devices, or the tuning of the chemical reactivity by the incorporation of active chemical groups for catalytic applications,5-8 are examples of the versatile ways modified graphene can be employed in novel applications. The functionalization of graphene with oxygen, previously described as graphene oxide (GO),9 has been of particular interest. GO was first synthesized from graphite by exfoliating the individual graphene sheets by Brodie10 in 1859 and the procedure was later modified by Hummers11 and is still used today.12 With modified Hummer’s method, it is possible to synthesize large-scale GO membranes with a very high degree of oxidation and a broad range of oxygen-containing functional groups.13 Membranes of GO produced in such a way show unique properties: while they are impermeable to liquids, vapors and gases, including helium, they can be readily penetrated by water14, and very recently GO has been used to purify seawater into drinking water.15 To control covalent functionalization of such GO membranes, selective oxidation of both graphene edges and basal planes, as well as selective etching of GO to create

edges and nanopores with unique physical and chemical properties, are actively pursued.16 It is generally agreed that the chemical oxidation of graphite forms epoxide and hydroxyl groups located on the basal plane, while carbonyl and carboxyl groups are present at the edges.5-6, 17-19 While the above mentioned functional groups are most common, other functionalities, such as ether, lactone, or ketone groups near carbon vacancies were reported in studies concerning graphene etching using oxygen plasma or ozone.20 The chemistry of graphene is also strongly influenced by defects in the graphene films, as demonstrated for example for graphene zigzag edges and StoneWales defects, where nitrogen dopants were identified as active sites for oxygen reduction reaction.21-22 In heterogeneous catalysis, such oxygen-containing functional groups on graphitic carbon, graphene, and carbon nanotubes have been used as anchoring sites for catalytically active particles.23-27 Since the functionalities of GO all contain the carbon-oxygen bonds as a major structural element, understanding their character is of fundamental importance for the properties of GO-related materials. To date, the interaction of atomic oxygen with graphene has been studied mainly by theoretical means with the focus on the free-standing layer and it is generally accepted that atomic oxygen forms a bridge-bonded (epoxy)

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configuration on defect-free basal sites of graphene.18, 28-30 For graphene supported on solid substrates, the situation is less clear and controversial. On one hand, density functional theory (DFT) calculations predicted that oxygen in a terminal, on-top carbon bonded configuration (enolate functional group) is more stable than a bridge-bonded (epoxy group) in the case of single layer graphene (Gr) on the (111) surfaces of Cu and Ni but no experimental studies to verify this prediction exist.31 On the other hand, the experimental studies on Gr on 6H-SiC(0001)32-33, Gr/Ir(111)34 and Gr/Pt(111)34 reported the formation of bridge-bonded epoxy functional groups but the possibility of the on-top bonded enolate configuration was not considered.

minimize contamination during the operation and was left running overnight prior the oxygen deposition. The atomic oxygen source was further tested on a bare Ru(0001) surface, where a well-defined (1×1)-O/Ru(0001) overlayer was prepared. As shown previously, only (2×1)O/Ru(0001) overlayer can be prepared by dosing molecular O2 and atomic oxygen is required to reach the (1×1)O/Ru(0001) overlayer36. Oxygen coverage in monolayers (ML) on Gr is defined relative to the surface density of carbon atoms (1 ML ≡ 3.71 × 1015 atoms/cm2). Processing of the STM images, acquired with resolution of 512 pixels per line, included background subtraction, noise removal from frequency domain images, and linear and non-linear distortion removal using algorithms described in Ref. 37.

Here, we employ scanning tunneling microscopy (STM) and provide a direct real-space mapping of atomic oxygen (AO) species binding and stability in different regions of single layer Gr (simply Gr from here on) supported on Ru(0001). Only defect-free regions are selected in the analysis to allow for a direct comparison with the accompanying theoretical studies. A complex behavior caused by an interplay of the registry and distance between the Gr lattice and the underlying Ru(0001) surface atoms is observed. The experimental and theoretical evidence clearly demonstrates that in the Gr regions that are close to the metal substrate, the terminally-bonded enolate groups are strongly preferred over bridge-bonded configuration. No AO species are observed on the Gr regions that are far from the underlying Ru, indicating their low relative stability. The DFT calculations further conclude that in these regions, the terminal-bonded enolate AO species are also preferred but are close in energy to the epoxy configuration. This study provides a clear fundamental basis for understanding the structural and electronic factors that affect the stability of both configurations in different Gr regions.

Theoretical Calculations. All computations were conducted with the CP2K package.38 First-principles calculations were carried out using the PBE-D3 density functional,39-40 the GTH pseudopotentials41 and the GPW hybrid basis set scheme42 in which the MOLOPT double-ζ Gaussian basis sets43 were employed to expand the valence electron states and 400 Ry cutoff for computing the electrostatic terms. The electrostatic potential along the surface normal direction was treated by the MartynaTuckerman approach.44 We adopted the system of (12×11) graphene/Ru(0001) periodicity with the Ru surface consisting of four atomic layers, of which the bottom two were fixed in all geometry optimizations. Because of the large surface unit cell vector sizes (~30.2Å), which is approximately 11 times the primitive cell vector of the Ru(0001) surface, only the Γ-point in the Brillouin zone was sampled in the self-consistent calculations.

METHODS Experimental Details. The Ru(0001) single crystal was cleaned using cycles of Ne+ sputtering, annealing in O2 at 850 K, followed by flash-annealing in ultra-high vacuum (UHV) to 1600 K in the same experimental setup as described previously.35 Partial Gr layers (~50% of surface covered with Gr flakes) were grown by chemical vapor deposition at 1100 K using toluene (C7H8) and decane (C10H22) as carbon precursors. The partial Gr layers were chosen, because it was found that the apex of the STM tip was easier to modify to give good performance on the metal-covered areas than on Gr-covered areas. Moreover, the edges of Gr were used as reference points for thermal drift compensation during the acquisition of time-lapsed STM movies at elevated temperatures. Atomic oxygen was produced with a commercial thermal gas cracker (Mantis Ltd.) directly attached to a variable-temperature STM (Omicron). The cracker was operated at a cracking power of 75 W with 1×10-8 Torr O2 background pressure. The STM tip was retracted several nm from the surface during the dose. Both STM heating stage and thermal gas cracker were outgassed for 100+ hours prior to the experiments to

RESULTS AND DISCUSSION The appearance of the bare Gr monolayer on Ru(0001) before the oxidation is illustrated in the STM images shown in Figure 1a and b. The typical Moiré superstructure characteristic of the single Gr layer supported on Ru(0001) substrate can be readily recognized. Small bare Ru regions covered with carbon can be also seen in the top- and bottom-left corners of the image in Figure 1a. These regions are a result of the intentionally incomplete Gr overlayer45 that help us locate the same area before and after the oxidation and reliably prepare the STM tip. The high magnification, atomically resolved image in Figure 1b shows that the (25×25) Gr Moiré supercell. This (25×25) supercell is commensurate with (23×23) unit cells of the underlying Ru(0001).46-47 This large Gr supercell can be further divided into four similar (12.5×11.5) regions that can be approximated by a slightly simplified (12×11) model that is schematically depicted in Figure 1c.48 This model is subsequently used in our DFT calculations. 36 Within each of the regions, three distinct areas with different height above the Ru(0001) surface can be identified: The Mound region (brightest, largest separation) at 3.0 Å, and the FCC (medium bright) and the HCP (darkest) regions both at approximately 2.2 Å, see Supporting Information (SI), Figure S1 for additional computational details. Figure 1d shows the same area as in Figure 1a, after dosing AO at 450 K. Following the dose, a number (0.002 mono-


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layers) of new dark round features, with ~3 Å full width at half maximum diameter are observed. These dark features are preferentially found within the FCC region (77%) of the Gr Moiré cell (highlighted with dark-blue circles in Figure 1d), although a smaller number (23%) is also observed within the HCP regions (brown circles, Figure 1d) after dosing AO at 450 K. This difference in stability suggests an energy difference on the order of ~0.05 eV between the two sites. No AO–related features are observed in the Mound regions. It is important to note that the dark depressions of variable size on some of the Mound regions are unrelated to AO and have also been observed

previously on bare Gr. 48 In that study, the authors attributed the donut-like appearance of the mound regions to electronic effects. The complete Gr layer exhibited identical behavior, and the AO dose-dependent experiments showed only a linear increase in the AO coverage (see Figure S2). Depending on the tip termination, the AO features appear bright in ~50% of the acquired images relative to the surrounding Gr substrate. To avoid confusion, we present only the subset where these features appear dark. The same area images with the dark and bright appearances of the AO species are shown in Figure S3, SI.

Figure 1. (a) STM image of the Gr flake on Ru(0001) obtained at 450 K before dosing atomic oxygen. The hexagonal overlay illustrates the graphene lattice, that was superimposed using the current model for this surface using the Mound regions as a refer47 46-47 ence. (b) High magnification atomically-resolved STM image of bare Gr, indicating a large 25 over 23 coincidence unit cell highlighted by thick white lines. The unit cell is further divided into four regions that can be described by a simplified 12 over 11 48 model, (thin white lines). (b, c) Three distinct areas can be distinguished within each region: Mound, FCC, and HCP. Panel (d) shows the image of the same area as in (a) after dosing 0.002 monolayers of atomic oxygen at 450 K. Dark AO-related features are observed primarily in FCC regions (dark-blue circles), with a minority of features also found in HCP regions (brown circles). (e) Zoom-in STM image of an area with four AO features, without and with the Gr lattice overlay highlighting the AO features in the FCC regions and their size with respect to the density of C atoms in the Gr lattice. In the latter case, the Gr lattice was aligned to be compatible with enolate binding configuration of the AO. Images (a, d-e) were acquired at 450 K (Vs = +200 mV, It = 100 pA), image (b) at 300 K (Vs = +4.6 mV, It = 1 nA). The color coding of the atoms in (c): C - dark grey, surface Ru – light grey, nd rd 2 layer Ru – blue, 3 layer Ru – white.

We further focus on determining the chemical identity of the AO species in both FCC and HCP regions. Two possibilities have been previously considered theoretically in the literature, the bridge-bonded, epoxy-like species bound to two neighboring carbons and the terminal, enolate-like species bound on top to a single carbon.31 It should be noted (Figure 1c) that the neighboring carbon atoms in the FCC and HCP regions are not equivalent due to their different coordination by the underlying metal atoms at the interface. As illustrated in the high magnification image in Figure 1e, the AO features are relatively large (~4 Å) and they do not change with temperature (114 – 500 K). This large size prevents us from reliably making

an accurate assignment of their position, i.e. on-top vs inbetween carbon atoms, and consequently of their chemical identity. To determine the AO binding we take advantage of their motion at elevated temperatures. The AO species became mobile above 400 K and an example of the AO trajectories at 500 K is shown in Figure 2a. (The complete STM movie can be found as an appendix ‘STM_movie500K.avi’.) The majority AO species in the FCC regions (blue trajectories) exhibit a tightly confined motion indicating that these regions represent local minima for the AOs. In contrast, the minority AO species observed in the HCP regions (orange) diffuse over longer distances and



ultimately end up in the more stable FCC regions. The orange trajectory in the middle of the Gr flake shows one example where the AO species crossed from one FFC region over the HCP region to another FCC region. The details of the AO diffusion analysis are presented in Section S4 of the SI. To resolve the AO binding configuration, we utilize the directionality of the AO motion. The conceptual nature of this analysis is summarized in Figure 2b. Two binding scenarios are possible: the AO adsorbed in a bridgebonded (epoxy) configuration and the terminal, on-top carbon-bonded (enolate) configuration. Assuming that one of the two configurations is preferred, as shown by DFT below, diffusive hopping between the same adsorption sites is expected.

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For single hops, (Figure 2b, upper part), there are three symmetrically inequivalent positions for the bridgebonded AO (red arrows). On the other hand, for terminal AO configuration (green arrows in Figure 2b), only two neighboring carbon atoms offer symmetrically inequivalent directions for single hops. Consequently, on average bridge-bonded and terminal AO species should diffuse in directions with azimuths of multiples of 60°, but offset by 30° as shown in the upper panel of Figure 2b. The lengths of the hops are 1.23 and 1.42 Å for bridge and terminallybonded AO, respectively. The azimuth angles of AO jumps between consecutive STM images can be easily related to the position of two neighboring mound regions, with 0° azimuth being parallel with a line connecting two nearest-neighbor mound regions along a selected direction (see Section S4 in the SI for details).

Figure 2. (a) The image shows the first frame from the time-lapsed STM movie (see ‘STM_movie-500K.avi’ in the SI) obtained after dosing a small amount of AO at 500 K. A set of AO trajectories highlighted with blue (FCC region) and orange (HCP region) traces obtained from the STM movie over the period of 3 hours (68 frames). Imaging conditions: Vs = +200 mV, It = 100 pA. (b) Possible directions of single (top) and double (bottom) hops of AO in bridge-bonded epoxy (red arrows) and terminal enolate (green arrows) configurations on Gr monolayer.

Figure 3. Hoping rate analysis of the AO species obtained from the time-lapsed STM images acquired in the 425-500 K range. (a) shows the fraction of hops (~6800 events) observed between consecutive images (acquired 160 s apart) at different temperatures. (b) shows the distribution of the double hop directions within the FCC region (upper part) and HCP region (lower part) of the Gr Moiré at 500 K. In the double hop analysis (266 events) we included events where the AO moved a distance within the 2.0-2.6 Å range. The analyzed distance range includes both epoxy and enolate diffusion scenarios (see Figure 2b bottom), but the diffu-


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sion directions are convincing: green bars correspond to 60° spaced azimuths expected for enolate species; red bars include additional directions that would have to be equally present if AOs binds as epoxy species.

The same analysis can be extended to double hops (Figure 2b, lower panel). For bridge-bonded AO diffusion, there are three symmetrically inequivalent directions (red arrows), resulting in hops with azimuths of multiples of 30°. For the terminal AO, all carbon atoms are equivalent in terms of possible azimuths of double hops, and multiples of 60° are expected (green arrows). The lengths of the double hops are 2.13 and 2.46 Å for bridge-bonded AO and 2.46 Å for terminally-bonded AO. The large difference in distances between single and double hops allows us to easily distinguish them apart as illustrated in Section S4 of the SI. The evaluation of longer (triple and quadruple) hops is more complex as the range of possible distances is larger. Nonetheless, by selecting relatively low temperatures (< 500 K) for the analysis, their contribution is fairly small. As demonstrated above, both single and double AO hops should allow us to determine the nature of the AO binding in both FCC and HCP regions. To this end, we have performed a number of experiments between 425 and 500 K and analyzed the diffusion distances and azimuths of the AO species as shown in Figure 3. To our initial surprise, the analysis presented in Figure 3a revealed that at all temperatures the single hops are highly unfavorable compared to double hops. Similarly, the fraction of quadruple hops is larger than the fraction of triple hops. As shown in DFT calculations presented below, the absence of the single hops is only consistent with terminal binding of the enolate species (but not the epoxy).

Figure 4. Arrhenius plot of the temperature dependent AO hopping rates in the HCP and FCC regions. The plotted hooping rates are corrected to account for the fact that some hops can appear as hops of a different length (e.g., statistically only 1/2 of the true quadruple hops are observed as quadruple hops, 1/3 appears as double hops, and 1/6 as absence of hops, see Section S4 in the SI for more details).

Further independent experimental evidence for the terminal binding of the AO species comes from the analysis of the azimuths of the double hops in Figure 3b. The results from 266 events obtained at a single temperature of 500 K are plotted as a fraction within the FCC and HCP

regions of the Gr Moiré. The analysis shows that the vast majority of double hops (94%) follows the azimuths separated by 60° (green) both within the FCC and HCP regions. As outlined in the discussion of Figure 2b, this scenario is only consistent with terminal binding (enolate) of the AO species in both regions. The fact that the probabilities along all 60° spaced azimuths (green) are identical (within the error of the experiment) also demonstrates that diffusion is not affected by the tip scanning direction. A small fraction, 6%, of the double hop events (shown in red) is observed in the directions that are offset from the green azimuths by 30°. While these azimuths could, in principle, be assigned to a small fraction of epoxy double hops, the collective evidence presented in this study strongly suggests that these are simply errors in the analysis, i.e., inaccurate distance measurements due to small thermal drift in the images. The details of the hopping distance and direction analysis are presented in Section S4 of the SI. While Figure 3 clearly demonstrates the dominant contribution of double hops along the azimuths that correspond to the enolate type of configuration, the hopping rates presented in Figure 3a can be further used to quantify the kinetic parameters of the diffusion as shown in Figure 4. The diffusion energy obtained from Figure 4 are 0.9 and 1.2 eV in the HCP and FCC regions, respectively. The corresponding pre-exponential factors are 7 × 107 and 2 × 1010 s-1. The implications of these values are discussed below together with the values determined theoretically. It should be noted that the hopping rates used in this analysis were corrected to account for the fact that some hops can appear as hops of a different length. For example, the observed zero hops have a contribution from true zero hops as well as a sequence of two back and forth double hops that are missed during the acquisition of two consecutive frames. Similarly, the observed double hops contain a contribution from quadruple hops on the same carbon ring. Such corrections are small and reach only 16 % at the highest temperature of 500 K (see Section S4 of the SI for further details). To gain insight into the binding of the AO species in different regions we turn to theory. Adsorption configurations on different regions are shown in Figure 5, with the energetic and structural properties presented in Table 1. In all three regions, three distinct structures are possible: two enolate and one epoxy group. In agreement with the experiment, we find that the enolate species (labeled C1 enolate) in the FCC and HCP regions are preferred and that their energy is slightly lower (by 0.12 eV) in the FCC region as compared to HCP region. Moreover, the stability of the C1 enolate is much lower in the Mound region. Closer inspection of the bond length and partial atomic charges of the C1-O bond indeed closely resemble those of alkoxide ions as further discussed in Section S5 of the SI. Additionally, the spin-density of the C1 enolate shows that AO in this configuration is not a radical species (see Section S6 of the SI).



The theoretical results presented in Figure 5 and Table 1 further allow us to understand the relative stability of the enolate and epoxy species. As shown in Figure 5, the enolate can be located at the C1 (on top of Ru hollow, Figure 5d-f) site or C2 (on top of Ru atom, Figure 5j-l) site, while the epoxy is bound to both C1 and C2 sites (Figure 5g-i). In all regions, we find that the stability decreases from C1 enolate to epoxy and to C2 enolate and that the destabilization is correlated with a weakening of the C2-Ru bond. In the FCC regions, before AO adsorption, the C2-Ru bond is 2.21 Å (see Table 1). Upon AO adsorption at C1, the C2Ru bond becomes 0.06 Å shorter. On the other hand, the AO adsorption at C2/C1C2-bridge weakens the C2-Ru in-

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teraction, as evidenced by the 0.36/0.23 Å elongation of this bond. As a result, the C1 enolate site is 0.64/0.37 eV more stable than the C2/C1C2 sites. This also holds true for the HCP region, where the C1 site is 0.67/0.38 eV more stable that C2/C1C2 sites. More details about how the relaxation of C-Ru bonds affects the AO adsorption are provided in Section S8 of the SI. This difference in stability also provides a natural explanation of the observed preponderance for double hops observed in the STM. Given that each carbon ring has three C1 atoms separated by three C2 atoms, two C1 adsorption sites are separated by a “double” hop for AO diffusion, see Section S12 for more details on the diffusion of AO in the three Gr regions.

Figure 5. Side views of the DFT structures of graphene on Ru(0001) along the [1,1] direction before and after AO adsorption. In each region, oxygen was adsorbed at the C1 site (d,e,f), C1C2 bridge site (g,h,i), and C2 site (j,k,l). The adsorption energies (eV) 3 are referenced to the bare Gr and O atom in the gas phase. (m) The potential energy surfaces in the corresponding regions, see the SI, Section S11 for details of energy barrier calculations.


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Table 1. Adsorption energy, ∆Ea, (eV) and bond lengths (Å) for Gr on Ru(0001) before and after AO adsorption. FCC C1-C2 C2-Ru Bare Gr

1.47

2.21

C1 Enolate O-C1

1.54

2.15

Epoxy C1-O-C2

1.48

2.44

C2 Enolate O-C2

1.53

2.57

C1/2-O

HCP C1-C2

C2-Ru

1.47

2.21

-2.28

1.55

2.15

1.44/1.48 -1.91

1.50

2.43

1.54

2.56

1.29

1.30

∆Ea

-1.64

C1/2-O

Mound C1-C2

C2-Ru

1.43

3.20

-2.16

1.54

2.28

1.44/1.49 -1.78

1.47

2.71

1.52

2.74

1.30

1.30

∆Ea

-1.49

C1/2-O

∆Ea

1.30

-1.78

1.43/1.48 -1.62

1.30

-1.48

3

The adsorption energies are referenced to the bare supported Gr and atomic O in the gas phase. For comparison, the adsorption energy of the epoxy oxygen on unsupported Gr is -1.83 eV (see the SI, Section S7). See SI for the discussion of the accuracy of the calculations.

In the Mound region carbon atoms are located at Ru hollow sites, making the distances between Ru and C atoms larger than in the FCC and HCP regions. We found that upon oxygen adsorption in the Mound region, the Gr sheet becomes deformed (see SI, Section S9). Although AO is still stabilized in the C1 enolate configuration, the potential energy surface is much flatter compared to that in the FCC or HCP regions with the AO adsorption energy at C1, C12, C2 of 1.78,-1.62, and -1.48 eV, respectively. Upon AO adsorption in the C1 enolate configuration, the C2-Ru bond becomes shorter, by as much as 0.92 Å, which stabilizes this configuration relative to the C2 enolate and epoxy. Despite this stabilization, the C2-Ru bond (2.28 Å) is still significantly longer than that in FCC and HCP regions (both 2.15 Å). Considering that the interface between Ru and Gr is identical for FCC and HCP within the first metal layer, we examine the effect of the symmetry of the second metal layer on the stability of AO binding configurations. Based on the electronic properties, adsorption at FCC is preferred. First, the local work function at FCC C1 is slightly lower than that at HCP C1 (by 0.1 eV, see SI, Section S10), indicating a higher reduction strength of the FCC sites. Second, we note that the bonding charge density between Gr and Ru(0001) in the HCP region is higher than in the FCC region (see SI, Section S11), implying a stronger GrRu bonding in the HCP area. The stronger intramolecular Gr-Ru interactions in HCP lead to weaker intermolecular AO and Gr-Ru interactions. In Figure 5m, we show the potential energy surfaces of AO in the three regions which corroborate our assignment of the primary mode of diffusion as a double hop. A double hop would involve motion from C1, over C2, to another C1 site whereas a single hop would only involve motion from C1 to C2 sites. The former can occur with a total calculated energy barrier of around ~0.8 eV (compare with 0.9-1.2 eV from the experimental data in Figure 4) and is hence facile. Conversely, a single hop would have a similar energy but for those sites adjacent to the

HCP/FCC region it would be uphill in energy by ca 0.64 eV and thus be improbable by a Boltzmann factor of exp[0.64/kBT] ~ 10-10. Finally, we discuss the lack of AO in the Mound regions. Upon dosing, AO will hit all three regions of the Moiré structure. However, no AO is observed in the Mound region, implying that after landing on this region, AO has to diffuse to the FCC or HCP regions. This supposition is supported by the small energy difference between different binding configurations in the Mound region and a low diffusion barrier of 0.3 eV, as shown in Table 1 and Figure 5.

CONCLUSIONS In this work, we mapped out the stability of atomic oxygen (AO) species within different regions of metalsupported graphene (Gr). We provide experimental and theoretical proof of the existence of terminally-bonded enolate functional groups for Gr supported on Ru(0001). The bridge-bonded epoxy species that represent the preferred binding motif on free standing Gr and on graphite are found to be significantly less stable. DFT calculations highlight the importance of metal-graphene interactions and their critical role in stabilizing atomic oxygen. This implies that the local chemistry of anchor groups, such as oxygen, can be controlled by a templating effect of the underlying graphene substrate. We have demonstrated that the preferred enolate binding sites are periodic in nature with ~ 3 nm separation. Such preferential binding can be possibly utilized in the future for engineering of periodic arrays of well-defined electronic and catalytic materials. These principles open up the possibility of controlled synthesis of spatially uniform nanoparticle arrays on such redox-active surfaces.

ASSOCIATED CONTENT Supporting Information: Section S1: Details for DFT calculations for graphene on Ru(0001); Section S2: STM images of a single Gr flake as a function of increasing AO dose; Section



S3: STM images of AO species with dark and bright appearance; Section S4: Hopping rate analysis; Section S5: Comparison of the enolate species with CO bonds in alkoxide molecules and ions; Section S6: Spin density of enolates; Section S7: Oxygen on unsupported graphene; Section S8: Adsorption of AO with fixed and relaxed Gr/Ru; Section S9: Atomic O adsorption in the Mound region; Section S10: Computation of the local electrostatic potential; Section S11: Bonding charge pattern induced by the Gr/Ru contact. Section 12: Diffusion of oxygen in FCC, HCP, and Mound regions; Section 13: STM movie ‘STM_movie-500K.avi’ of the AO diffusion at 500 K. Section 14: DFT errors.

AUTHOR INFORMATION Corresponding Authors *[email protected] *[email protected]

Present Addresses †Department of Physics, University of Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland

Author Contributions ZN performed experimental work and analysis of the data. ZD designed the experiments. ZD and FN guided the data analysis. MTN, VAG and RR performed DFT simulations. The manuscript was written by all the authors.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We thank Michael Schmid for development of a custom ImageJ macros, Ryan T. Frederick for assistance during the acquisition of small portion of the experimental data and Rentao Mu for numerous discussions in the early stages of this project, Marcella Iannuzzi for sharing DFT graphene/Ru(0001) structures. This work was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences and performed in EMSL, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is a multiprogram national laboratory operated for DOE by Battelle. Computational Resources were provided by a user proposal at the NERSC user facility located at Lawrence Berkley National Laboratory, FPN acknowledges the award of an Alternate Sponsored Fellowship at PNNL and financial support of the University of Graz.

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Formation of Supported Graphene Oxide: Evidence for Enolate Species

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