Boosting Graphene Reactivity with Oxygen by Boron Doping: Density

Article pubs.acs.org/JPCC

Boosting Graphene Reactivity with Oxygen by Boron Doping: Density Functional Theory Modeling of the Reaction Path. Lara Ferrighi, Martina Datteo, and Cristiana Di Valentin* Dipartimento di Scienza dei Materiali, Università Di Milano-Bicocca Via Cozzi 55 20125 Milano, Italy. S Supporting Information *

ABSTRACT: Graphene (G) reactivity toward oxygen is very poor, which limits its use as electrode for the oxygen reduction reaction (ORR). Contrarily, boron-doped graphene was found to be an excellent catalyst for the ORR. Through a density functional study, comparing molecular and periodic approaches and different functionals (B3LYP vs PBE), we show how substitutional boron in the carbon sheet can boost the reactivity with oxygen leading to the formation of bulk borates covalently bound to graphene (BO3−G) in oxygen-rich conditions. These species are highly interesting intermediates for the OO breaking step in the reduction process of O2 to form H2O as they are energetically stable.

1. INTRODUCTION Doped graphene has become a hot material for catalysis since it has been recently reported to catalyze the oxygen reduction reaction (ORR) with high durability and selectivity.1,2 The development of an efficient but cheap and metal-free electrode to be used as cathode in fuel cells and other electrochemical energy devices is one of the major goals of the current research in electrocatalysis. Nitrogen-doped graphene3 is the most studied heteroatomdoped graphene system with examples of very active nanomaterials for the ORR.4−6 During the last year, various approaches to insert boron in the graphene layer have been reported,7−10 and, more importantly, boron-doped graphene was also found to be an excellent catalyst for ORR.8,11 Boron is believed to be substitutional to carbon, although in some cases oxygenated boron species were supposed to exist at the sheet edges to justify the broad feature for the B 1s peak at the X-ray photoelectron spectra (XPS).8,9 Little is yet known regarding the mechanism of ORR on doped graphene. Only a few investigations, based on density functional theory (DFT) calculations, are present in the literature,12−15 and only one focuses on the boron-doped system.15 Moreover, although several computational studies in the literature analyze molecular oxygen reactivity on pure graphene,16−21 only O2 physisorption on boron-doped graphene has been investigated.22−24 An atomic-level understanding of the processes involved in the oxygen interaction and reactivity with doped graphene systems would be of great scientific interest but could also help improving their efficiency as electrocatalysts. Herein, we compare the energetics of the various steps along the reaction path of atomic and molecular oxygen with pure graphene (G) and boron-doped graphene (BG). Our model is based on density functional theory calculations where we compare results obtained with different approaches, varying the functional (standard GGA/PBE or hybrid B3LYP), the basis set © 2013 American Chemical Society

(localized atomic Gaussian functions or planewaves), and the size and type of the model (periodic 4 × 4 or 8 × 8 sheet or molecular circumcoronene). All of the approaches lead consistently to the same conclusion that the presence of boron in the carbon layer boosts the graphene sheet reactivity toward oxygen to the formation of stable bulk borates. The stability dependence with oxygen partial pressure or chemical potential and electronic structure of the intermediates at the different stages of oxidation are also investigated.

2. COMPUTATIONAL DETAILS The periodic graphene model calculations were performed both with the CRYSTAL0925 (C09) and the QuantumEspresso26 (QE) codes. We used 4 × 4 or 8 × 8 supercells of 32 or 128 atoms (Figure 1) and 12 × 12 × 1 or 6 × 6 × 1 Monkhorst− Pack k-point grids, respectively. For the C09 calculations, B3LYP functional27,28 was used and the Kohn−Sham orbitals were described with localized Gaussian basis sets [631(1) for C,29 6211(1) for B,30 and 631(1) for O31]. The addition of oxygen was done by keeping the cell parameters fixed at the corresponding G and BG optimized geometries and also allowing them to relax in some cases. A denser 24 × 24 × 1 kpoint grid is used for the calculation of the total and projected density of states (DOS and PDOS) on the 4 × 4 supercells. For the QE calculations, the PBE functional32 and Vanderbuilt ultrasoft pseudopotentials with energy cutoffs of 30 and 240 Ry (for kinetic energy and charge density grids) were used.33 Cell parameters were kept fixed at the G and BG optimized geometries. The hexagonal lattice parameter of pure graphene is computed to be 2.463 and 2.472 Å for B3LYP and PBE, respectively. For B-doped graphene we observe a lengthening of the lattice to 2.476 and 2.482 Å for the two functionals, Received: November 7, 2013 Revised: December 18, 2013 Published: December 18, 2013 223

dx.doi.org/10.1021/jp410966r | J. Phys. Chem. C 2014, 118, 223−230

The Journal of Physical Chemistry C

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respectively. For all those systems with a total odd number of electrons (B containing systems), we have allowed a starting magnetization leaving the systems to freely reach its ground state. The starting magnetization is always quenched during the wave function optimization, except in three cases which will be mentioned explicitly in the text (BG/3O2, BGOB, and BGO−O). The molecular graphene model calculations were performed with the GAUSSIAN0934 (G09) suite of programmes and the B3LYP functional. The model is a circumcoronene molecule (C54H18). The orbitals were described with Gaussian basis functions [6-311+G* for the inner six C atoms and for the O and B atoms; 6-31G* for the rest of the model]. All atoms were allowed to relax during the geometry optimization. Molecular calculations with a single determinant approach as in DFT require one to specify the spin configuration of the system in terms of the electron occupation of the molecular orbitals. All of those systems with a total odd number of electrons (B containing species) have spin multiplicity of 2 and, therefore, are considered in a doublet quantum spin state. In the following we will compare plots of spin density for the doublet molecular models with plots of projected density on the highest half-filled band for the nonmagnetic periodic ones, since these are the ground states for the respective approach.

Figure 1. Energy profile for molecular oxygen physi-/chemisorption on G. For quantitative details refer to Table 2. Top and side views of the balls and sticks 4 × 4 models.

However, depending on the study considered, the double epoxide species can be either more20,21,37,38 or less stable17 than the double ether. These discrepancies in the computational literature suggest that there can be some dependence of the results with the method or the model used. For this reason in the present work we present results obtained with three very different approaches to corroborate the overall general picture and prove it to be independent of the specific choice. In Figure 1 we report the reaction path of molecular oxygen with G, leading to the formation of oxidized G, as obtained with the three different setups. As zero energy reference we set the total energy sum of G and an isolated 3O2 molecule. In line with the existing literature just mentioned in the previous paragraph, we observed some differences among the three sets of data. The weak physisorption of molecular oxygen (no dispersion forces are included in the calculations reported; in the case of B3LYP/C09 we estimate the weak interactions to be 0.13 eV with the DFT+D* method39) as well as the highenergy peroxo intermediate, GO−O, resulting from the [2 + 2] cycloaddition adduct, were confirmed in all of the cases. We note that for a [2 + 2] cycloaddition reaction with an olefin, the excitation of the O2 molecule from the triplet ground state to the 1Δ singlet state is required. This is commonly done by making use of a sensitizer such as a porphyrine.40 The energy cost for this excitation is computed to be 1.66 eV with Gaussian09. As a further step of reaction, we considered the products of O2 dissociation: GO1O3 or GO1O4 species. The overall process is found to be endothermic in both cases and with all three computational approaches used. However, the relative stability of the two configurations (or products) is dependent on the type and the details of the calculations (Table 2). The doubly epoxy GO1O3 is more stable by 0.25 eV when using the periodic 4 × 4 supercell model (0.11 eV if lattice is fully relaxed) with B3LYP/C09, while the double ether GO1O4 is more stable by 0.13 eV when using the finite molecular model for representing the graphene sheet with B3LYP/G09. Actually, we noted that when using larger supercell models (e.g., 8 × 8; see Table 2) GO1O4 becomes favored by 0.10 eV compared to GO1O3, indicating that the higher strain associated with the larger structural deformation from planarity in the double ether species can be more easily counteracted for lower defect concentrations. 3.2. Boron-Doped Graphene Reactivity with Atomic and Molecular Oxygen. In the following we will investigate

3. RESULTS AND DISCUSSION 3.1. Graphene Reactivity with Atomic and Molecular Oxygen. Oxygen reactivity with G has been investigated in the past, but contrasting data are available. The adsorption of one oxygen atom, resulting in the formation of an epoxide species (GO; see Table 1), is always computed to be an exothermic Table 1. Adsorption Energies (eV) for Different Configurations of Atomic Oxygen (O in Red) on G and BG (B in Green) Calculated against an Isolated Atom

process, although the range of energy gain values computed in the different studies is quite wide, from −1.90,16 to −1.92,35 to −2.03,36 to −2.43,37 to an unreasonable −4.738 eV. With the present setups we compute an adsorption energy of −1.37 eV with B3LYP/C09 (with both 4 × 4 and 8 × 8 supercells), −1.84 eV with PBE/QE, and −1.42 eV with B3LYP/G09. The O atom adsorption does not cause a strong distortion in the two-dimensional (2D) sheet which remains quite flat, with the two epoxy C atoms protruding out of plane by 0.3 Å. The molecular oxygen dissociation to form two epoxides (zipped configuration, GO1O3 in Figure 1) or two ether groups (unzipped configuration, GO1O4 in Figure 1), according to the relative position of the O atoms in the hexagonal C-ring, is always reported to be an endothermic process.17,20,21,37,38 224



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Table 2. Physi-/Chemisorption Energy of Molecular Oxygen on G and BG against an Isolated Molecule in the Triplet State.a 3

O2(ads)

O−O

0.00 0.00 − 0.00 −0.02

2.56 2.55 − 2.43 2.23

−0.04 −0.04 − −0.01 −0.02

1.34 1.35 − 1.12 1.02

O1O6

O1O5

O1O4

O1O3

2.37 2.21 (1.89) 1.55 2.19

2.12 2.10 (1.99) 1.68 1.65

0.17 0.09 (−0.19) −0.82 0.14

0.65 0.65 − 0.14 0.30

G B3LYP/C09

B3LYP/G09 PBE/QE B3LYP/C09

B3LYP/G09 PBE/QE a

BG −0.27 −0.33 (−0.51) −0.97 −0.42

0.62 0.61 − 0.04 0.29

Values in italics are after full lattice relaxation and in parentheses for the 8 × 8 supercell models.

breaking of one B−C bond (from 1.49 in BG to 1.82 Å) substituted by the new B−O and O−C bonds, where the O bridges the two atoms in an unzipped configuration (BGO1; see Table 1). Here, the B and C atoms protrude from the 2D sheet by 0.6 Å. In Figure 3 we report the spin density plot of BGO1

the tremendous effect that the introduction of a substitutional boron in the graphene lattice has on the interaction and the reactivity with oxygen. First we comment on the effect of substitutional boron on the G sheet (see Figure 2). From a structural point of view the

Figure 2. Spin density plot (in yellow) as computed with the molecular model at the B3LYP/G09 level (left) and projected density on the highest half-filled band (in yellow) as computed with the periodic model at the B3LYP/C09 level (right), on the balls and sticks representation of BG: B atom in green, C atoms in gray, and H atoms in black. Atomic distances in angstroms.

Figure 3. Spin density plot (in yellow) as computed with the molecular model at the B3LYP/G09 level (left) and projected density on the highest hal- filled band (in yellow) as computed with the periodic model at the B3LYP/C09 level (right), on the balls and sticks representation of BGO1: O atoms in red, B atom in green, C atoms in gray, and H atoms in black. Atomic distances in angstroms.

introduction of B lowers the symmetry of the system. The three B−C bonds in BG are 1.49 Å to compare with 1.42 Å for C−C in G with B3LYP/C09. The spin density associated with the doublet solution for the molecular model of BG (B-doped circumcoronene) is totally analogous to the projected density on the highest half-filled band in the nonmagnetic ground state of periodic B-doped graphene (see the last paragraph of the Computational Details). The p-type dopant introduces an electron depletion on the C atoms in ortho and para positions with respect to the boron atom. Those atoms are thus expected to be the most involved in the BG reactivity with oxygen. Four different atomic oxygen adsorption configurations on BG were identified (see Table 1). All of them present a higher adsorption energy than on pure G. The most stable one, with B3LYP/C09 binding energy of −2.92 eV (−2.99 eV with the 8 × 8 supercell) which is more than twice that on G, involves the

for B3LYP/G09 which closely resembles the projected density on the highest half-filled band for the periodic B3LYP/C09 (see the last paragraph of the Computational Details). Analogously to BG, in BGO1 the largest electron density is on the ortho and para C atoms, although some is also on the O and B ones. The other two configurations are epoxide species where the O atom bridges two different C−C pairs (BGO2 and BGO3; see Table 1) with B3LYP/C09 binding energies of −1.71 and −1.47 eV, respectively. Finally, the O atom adsorbs on top of B (BGOB) by −1.85 eV, with the B atom out of the 2D sheet by 0.5 Å and a residual magnetization localized on the oxygen atom. The larger adsorption energy of atomic oxygen is an important hint in the direction of a higher affinity of oxygen toward BG than G. The most striking consequence of that is the different energy change accompanying the dissociation of 225



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Figure 4. Energy profile for molecular oxygen physi-/chemisorption on BG. For quantitative details refer to Table 2. Top and side views of the balls and sticks 4 × 4 models.

molecular oxygen on BG with respect to G, as described in detail in Figure 4. Molecular 3O2 very weakly physisorbs on the doped system (BG/3O2), in analogy with the undoped one (G/3O2). The residual magnetization in these systems is totally localized on the oxygen atoms (see Figure S1 in the Supporting Information for BG/3O2). More interestingly, the BGO−O intermediate, although still high in energy above the reactants, is much lower than GO−O (see Figure 2): 1.34 eV vs 2.56 eV in Table 2 with B3LYP/C09. Besides this large energy difference, there is another crucial effect due to the B-doping: since the BG system presents an odd number of electrons, the spin restriction on the reactivity of 3O2 with G to form GO−O does not hold in the case of BG. Therefore, we expect that the activation of 3O2 to 1O2 is not required and that a two-step radicalic path is followed to form the cycloaddition adduct BGO−O. We note that this adduct maintains a residual magnetization of 0.6 electron which is localized on the π state of the O−O bridge (see Figure S1 in the Supporting Information); therefore it resembles more a superoxo than a peroxo species. Now, what really makes the B-doped graphene much more interesting than the pure graphene system is the negative reaction energy for the molecular oxygen dissociation (exothermic process) in the case of the BGO1O6 product. This being true for all of the methods used (see Table 2) definitely consolidates the result. In the BGO1O6 configuration both O atoms are directly bound to B causing the breaking of two of the three B−C bonds (see Figure 5). B becomes highly oxidized. In Figure 5 we observe a high localization of the spin or electron density on those C atoms which are in ortho and para positions with respect to B. This effect is enhanced with respect to the BG case (see preceding text and Figure 2). Comparing Figure 3 and Figure 5, we note the more pronounced geometrical distortion of BGO1O6 with respect to BGO1; in particular the protrusion of B out of the molecular plane, in order to accommodate the second oxygen, is more evident. The other BGOO configurations with only one O atom bound to B (BGO1O5, BGO1O4, and BGO1O3) are also more stable with respect to reactants than the GOO counterparts (GO1O4 and GO1O3) by ∼2 eV (Figure 1 vs Figure 4). The

Figure 5. Spin density plot (in yellow) as computed with the molecular model at the B3LYP/G09 level (left) and projected density on the highest half-filled band (in yellow) as computed with the periodic model at the B3LYP/C09 level (right), on the balls and sticks representation of BGO1O6: O atoms in red, B atom in green, C atoms in gray, and H atoms in black. Atomic distances in angstroms.

associated processes are only slightly endothermic (below 0.7 eV in Figure 4) with B3LYP/C09. On the contrary, the two additional structures presented in Figure 4, where both O atoms are not directly bound to B (BGO2O4 and BGO2O5), are very high in energy in analogy with the GOO species, confirming the direct and crucial role played by the B atom in the stabilization of the oxygenated forms. The molecular model systematically produces lower energies for all of the configurations than the periodic one, since it can better counteract the structural deformation associated with the oxidation of the sheet. With this model even the formation of BGO1O4 (Figure 6) is an exothermic process by −0.82 eV (see Table 2 and Figure 4). 3.3. Effect of the Oxygen Concentration on the Stability of the Oxygenated Species. To further analyze the reactivity of G and BG with oxygen, we evaluated the curves of stability with respect to the oxygen chemical potential (μO), 226



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the 8 × 8 supercell model with the B3LYP/C09 method, of the increasingly oxygenated G and BG species as a function of μ′O, according to the formula Eform = Etot(GOn /BGOn) − [Etot(G/BG) + nμO]

where n is the number of O atoms. We observe that (i) the formation energy of oxidized BG species is always lower than that for oxidized G ones and (ii) the higher the degree of oxidation the higher the formation cost for a wide range of oxygen chemical potential. In the case of BG, however, at oxygen-rich conditions some crossing between lines appears, indicating the triply oxidized boron species (BGOOO or BO3−G; see Figure 8) as the most stable.

Figure 6. Spin density plot (in yellow) as computed with the molecular model at B3LYP/G09 level (left) and projected density on the highest half-filled band (in yellow) as computed with the periodic model at the B3LYP/C09 level (right), on the balls and sticks representation of BGO1O4: O atoms in red, B atom in green, C atoms in gray, and H atoms in black. Atomic distances in angstroms.

a parameter that characterizes the oxygen environment (see the graph in Figure 7). The environment acts as a reservoir which

Figure 8. Spin density plot (in yellow) as computed with the molecular model at the B3LYP/G09 level (left) and projected density on the highest half-filled band (in yellow) as computed with the periodic model at the B3LYP/C09 level (right), on the balls and sticks representation of BGOOO: O atoms in red, B atom in green, C atoms in gray, and H atoms in black. Atomic distances in angstroms.

Analogous conclusions can be drawn from the analysis of the corresponding graph based on the B3LYP/G09 calculations for the molecular models (see Figure S2 in the Supporting Information). It is evident from Figure 8 that this species is a borate covalently anchored to the graphene sheet (BO3−G). It is similar to BGO1O6, with two O atoms in bridging positions causing the breaking of the B−C bonds underneath (which are now about 2.28 Å apart), with an additional O atom still in the bridging position between B and a third C atom, where the B− C bond is not broken (1.57 Å). The spin or electron density is more localized on the graphene sheet than on the borate group, which is heavily protruding out of the plane. We now analyze the energy differences associated with each stage of oxidation at μ′O = 0:

Figure 7. Formation energy (B3LYP/C09, 8 × 8 supercell) as a function of the oxygen chemical potential or as a function of the oxygen pressure at T = 300 K (top x-axis) for different oxygenated G and BG species. The relation between μ′O and O2 partial pressure is as follows: μ′ O = μ O (300K,p 0) + 1/ 2kT ln(p(O2)/p0), where μO(300K,p0) is taken from ref 41.

can give or take any amount of O2 without changing its temperature and pressure.41 Oxygen-poor conditions correspond to low values of μO, and conversely, oxygen-rich conditions correspond to high values of μO. By referencing μO to the energy of an O atom in the O2 molecule (μO = 1/2μO2 + μ′O), we take −0.72 ≤ μ′O ≤ 0, where μ′O = 0 corresponds to the oxygen-rich limit at which oxygen condensation will occur. For the lowest limit we take −0.72 corresponding to a negligible oxygen partial pressure at 300 K (10−15 atm; see top of Figure 7). In Figure 7 we report the formation energies, for

G + 1/2 O2 → GO

E = 1.30 eV

GO + 1/2 O2 → GO1O3

BG + 1/2 O2 → BGO BGO + 1/2 O2 → BGO1O6 227

E = 0.62 eV

E = − 0.32 eV E = − 0.19 eV


The Journal of Physical Chemistry C BGO1O6 + 1/2 O2 → BGOOO

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electron shortage, resulting in being almost fully occupied. The hole is thus delocalized on a number of C atoms with a tiny contribution of B, as evident also from Figure 2. These C atoms are in ortho and para positions with respect to the B atom, as discussed before in section 3.2. We now analyze the electronic structure modification induced by the oxidation of the BG species with one oxygen atom to form BGO1 (top right in Figure 9). We observe a band gap opening totally analogous to the one in BG. The 2p states associated with B and O have a little contribution in the valence band part above the Fermi energy; therefore, they give a little contribution to the hole state (see the inset in the top right of Figure 9). This state is mainly delocalized on some C atoms of the sheet, which can be identified by the plots reported in Figure 3. When two oxygens are bound to the B atom, as in BGO1O6, the B and O 2p contribution to the hole state is still tiny but is about the same for the two chemical species (see the inset in the bottom left of Figure 9 and the plots in Figure 5). Finally, when three oxygens are bound to B as in BGOOO, B 2p states do not contribute to the hole state any more, while O atoms still do (see the inset in the bottom right of Figure 9). The B atom is fully oxidized forming three covalent B−O bonds with almost no direct contact to the C sheet (see Figure 8).

E = − 0.79 eV

For G both oxidation processes are endothermic with the addition of the second oxygen being less expensive than the first. On the other hand, for BG, all three oxidation steps are computed to be exothermic processes, with the third one, to a borate species (BGOOO or BO3−G), producing the largest energy gain (−0.79 eV). The conclusion that can be drawn from the analysis presented in this section is that BG sheets can be much more easily oxidized than G ones and depending on the oxygen conditions the extent of oxidation of the B species can be different but at common oxygen pressures borates are the most stable. 3.4. Electronic Structure and Properties. In this section we investigate the electronic structure of some selected systems in terms of the total (DOS) and projected density of states (PDOS) in the range between −15 and +5 eV with reference to the Fermi energy of the system, as reported in Figure 9. In all

4. CONCLUSION Concluding, the present density functional study clearly indicates that graphene reactivity with oxygen can be highly boosted by substitutional boron doping of the carbon lattice. Reaction processes are found to be exothermic with BG in contrast with pure G. Oxygen reacts directly with the boron in the lattice and forms bulk oxygenated boron species which are fully oxidized to stable borates covalently anchored to the carbon sheet in oxygen-rich conditions. These results were proven to be independent of the method and model used. The existence of oxygenated B species was proved before by XPS measurements;8,9 however, their role in the chemistry of Bdoped graphene has been totally overlooked and underestimated. On the basis of our study it is clear that these intermediates are at the basis of the reaction mechanism for the recently highlighted8 ORR catalyzed by B-doped graphene. The breaking of the OO bond is the crucial step in the O2 reduction to H2O. We have shown this to be energetically achievable when using B-doped graphene as catalyst. Further work is required to investigate the reaction steps leading to the overall formation of water. We believe that our results are not only relevant in the context of ORR but could be more generally useful for the interpretation of nonmetal doped graphene based catalysis.

Figure 9. Total (black) and projected (on O atoms in red and on B atom in green) densities of states at the B3LYP/C09 level for the 4 × 4 supercell model. The zero energy reference is set at the Fermi energy of the system and highlighted by a dotted blue line. In the inset the rectangular area is zoomed and doubled along the y-axis.

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these cases, the ground state is found to present a negligible magnetization, as mentioned in the computational details. We start from the density of states of B-doped graphene system (top left in Figure 9) where we observe some characteristic features. The insertion of a substitutional impurity atom reduces the high symmetry of the pure graphene system and causes the opening of a tiny band gap at the Dirac point where valence and conduction bands touch in the case of pure graphene (see Figure S3 in the Supporting Information). Moreover, the depletion of one electron following the substitution of a C with a B atom lowers the Fermi energy below the top of the valence band. The overall effect of B doping is to transform graphene from a semimetal to a low-gap p-type semiconductor. The boron 2p states are not located at the top of the valence band; therefore they only partly suffer the

ASSOCIATED CONTENT

S Supporting Information *

Figures showing the spin density plot of BG/3O2 and BGO−O, stability diagram from B3LYP/G09 calculations, and the DOS and PDOS of G, GO, GO1O3, and GO1O4 and tables listing PBE/QE vs PBE/C09 energies, lattice parameters, and bond lengths of the most representative structures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +390264485235. 228



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Notes

(17) Li, J.-L.; Kudin, K. N.; McAllister, M. J.; Prud’homee, R. K.; Aksay, I. A.; Car, R. Oxygen-Driven Unzipping of Graphitic Materials. Phys. Rev. Lett. 2006, 96, 176101. (18) Boukhvalov, D. W.; Katsnelson, M. I. Chemical Functionalization of Graphene. J. Phys.: Condens. Matter 2009, 21, 344205. (19) Yan, J.-A.; Chou, M. Y. Oxidation Functional Groups On Graphene: Structural And Electronic Properties. Phys. Rev. B 2010, 82, 125403. (20) Larciprete, R.; Fabris, S.; Sun, T.; Lacovig, P.; Baraldi, A.; Lizzit, S. Dual Path Mechanism in the Thermal Reduction of Graphene Oxide. J. Am. Chem. Soc. 2011, 133, 17315−17321. (21) Nguyen, M.-T. An ab initio study of oxygen on strained graphene. J. Phys.: Condens. Matter 2013, 25, 395301. (22) Dai, J.; Yuan, J.; Giannozzi, P. Gas Adsorption on Graphene Doped with B, N, Al, and S: A Theoretical Study. Appl. Phys. Lett. 2009, 95, 232105. (23) Dai, J.; Yuan, J. Adsorption of Molecular Oxygen on Doped Graphene: Atomic, Electronic, and Magnetic Properties. Phys. Rev. B 2010, 81, 165414. (24) Ferro, Y.; Allouche, A.; Marinelli, F.; Brosset, C. Theoretical Study of Oxygen Adsorption on Boron-Doped Graphite. Surf. Sci. 2004, 559, 158−168. (25) Dovesi, R.; Saunders, V. R.; Roetti, C.; Orlando, R.; ZicovichWilson, C. M.; Pascale, F.; Civalleri, B.; Doll, K.; Harrison, N. M.; Bush, I. J.; et al. CRYSTAL09 User’s Manual; University of Torino: Torino, Italy, 2009. (26) Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I.; et al. Quantum Espresso: A Modular and Open-Source Software Project for Quantum Simulations of Materials. J. Phys.: Condens. Matter 2009, 21, 395502. (27) Becke, A. D. Density-functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652. (28) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785−789. (29) Demichelis, R.; Noël, Y.; D′Arco, P.; Rérat, M.; ZicovichWilson, C. M.; Dovesi, R. Properties of Carbon Nanotubes: An ab Initio Study Using Large Gaussian Basis Sets and Various DFT Functionals. J. Phys. Chem. C 2011, 115, 8876−8885. (30) Peintinger, M. F.; Oliveira, D. V.; Bredow, T. Consistent Gaussian Basis Sets of Triple-Zeta Valence with Polarization Quality for Solid-State Calculations. J. Comput. Chem. 2013, 34, 451−459. (31) Gatti, C.; Saunders, V. R.; Roetti, C. Crystal Field Effects on the Topological Properties of the Electron Density in Molecular Crystals: The Case of Urea. J. Chem. Phys. 1994, 101, 10686−10696. (32) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865. (33) Few checks of comparison were done with PBE/QE and PBE/ C09 methods (see Supporting Information). (34) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, Revision D.01; Gaussian: Wallingford, CT, USA, 2009. (35) Sun, T.; Fabris, S.; Baroni, S. Surface Precursors and Reaction Mechanisms for the Thermal Reduction of Graphene Basal Surfaces Oxidized by Atomic Oxygen. J. Phys. Chem. C 2011, 115, 4730−4737. (36) Šljivančanin, Z.; Milošević, A. S.; Popović, Z. S.; Vukajlović, F. R. Binding of Atomic Oxygen on Graphene from Small Epoxy Clusters to a Fully Oxidized Surface. Carbon 2013, 54, 482−488. (37) Topsakal, M.; Ciraci, S. Domain Formation on Oxidized Graphene. Phys. Rev. B 2012, 86, 205402. (38) Yan, J.-A.; Chou, M. Y. Oxidation Functional Groups on Graphene: Structural and Electronic Properties. Phys. Rev. B 2010, 82, 125403. (39) Civalleri, B.; Zicovich-Wilson, C. M.; Valenzano, L.; Ugliengo, P. B3LYP Augmented with an Empirical Dispersion Term (B3LYPD*) as Applied to Molecular Crystals. CrystEngComm 2008, 10, 405− 410.

The authors declare no competing financial interests.

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ACKNOWLEDGMENTS We thank Lorenzo Ferraro for his technical help. This work was supported by the Italian MIUR through the national grant Futuro in Ricerca 2012 RBFR128BEC ″Beyond graphene: Tailored C-layers for novel catalytic materials and green chemistry″ and by CINECA supercomputing center though the ISCRA-B Grant 2013 ″IsB06_BGFORCAT″.

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REFERENCES

(1) Zhu, C.; Dong, S. Recent Progress in Graphene-Based Nanomaterials as Advanced Electrocatalysts Towards Oxygen Reduction. Nanoscale 2013, 5, 1753−1767. (2) Yang, Z.; Nie, H.; Chen, X.; Chen, X.; Huang, S. Recent Progress in Doped Carbon Nanomaterials as Effective Cathode Catalysts for Fuel Cell Oxygen Reduction Reaction. J. Power Sources 2013, 236, 238−249. (3) Urban, K. W. Electron microscopy: The Challenges of Graphene. Nat. Mater. 2011, 10, 165−166. (4) Gong, K.; Du, F.; Xia, Z.; Durstock, M.; Dai, L. Nitrogen-Doped Carbon Nanotube Arrays with High Electrocatalytic Activity for Oxygen Reduction. Science 2009, 323, 760−764. (5) Qu, L.; Liu, Y.; Baek, J.-B.; Dai, L. Nitrogen-Doped Graphene as Efficient Metal-Free Electrocatalyst for Oxygen Reduction in Fuel Cells. ACS Nano 2010, 4, 1321−1326. (6) Shao, Y.; Zhang, S.; Engelhard, M. H.; Li, G.; Shao, G.; Wang, Y.; Liu, J.; Aksayc, I. A.; Lin, Y. Nitrogen-Doped Graphene and its Electrochemical Applications. J. Mater. Chem. 2010, 20, 7491−7496. (7) Kim, Y. A.; Fujisawa, K.; Muramatsu, H.; Hayashi, T.; Endo, M.; Fujimori, T.; Kaneko, K.; Terrones, M.; Behrends, J.; Eckmann, A.; Casiraghi, C.; Novoselov, K. S.; Saito, R.; Dresselhaus, M. S. Raman Spectroscopy of Boron-Doped Single-Layer Graphene. ACS Nano 2012, 6, 6293−6300. (8) Sheng, Z.-H.; Gao, H.-L.; Bao, W.-J.; Wang, F.-B.; Xia, X.-H. Synthesis of Boron Doped Graphene for Oxygen Reduction Reaction in Fuel Cells. J. Mater. Chem. 2012, 22, 390−395. (9) Cattelan, M.; Agnoli, S.; Favaro, M.; Garoli, D.; Romanato, F.; Meneghetti, M.; Berinov, A.; Dudin, P.; Granozzi, G. Microscopic View on a Chemical Vapor Deposition Route to Boron-Doped Graphene Nanostructures. Chem. Mater. 2013, 25, 1490−1495. (10) Wang, H.; Zhou, Y.; Wu, D.; Liao, L.; Zhao, S.; Peng, H.; Liu, Z. Synthesis of Boron-Doped Graphene Monolayers Using the Sole Solid Feedstock by Chemical Vapor Deposition. Small 2013, 9, 1316−1320. (11) Yang, L.; Jiang, S.; Zhao, Y.; Zhu, L.; Chen, S.; Wang, X.; Wu, Q.; Ma, J.; Ma, Y.; Hu, Z. Boron-Doped Carbon Nanotubes as MetalFree Electrocatalysts for the Oxygen Reduction Reaction. Angew. Chem. 2011, 123, 7270−7273. (12) Yu, L.; Pan, X.; Cao, X.; Hu, P.; Bao, X. Oxygen Reduction Reaction Mechanism on Nitrogen-Doped Graphene: A Density Functional Theory Study. J. Catal. 2011, 282, 183−190. (13) Yan, H. J.; Xu, B.; Shi, S. Q.; Ouyang, C. Y. First-Principles Study of the Oxygen Adsorption and Dissociation on Graphene and Nitrogen Doped Graphene for Li-Air Batteries. J. Appl. Phys. 2012, 112, 104316. (14) Kaukonen, M.; Krasheninnikov, A. V.; Kauppinen, E.; Nieminen, R. M. Doped Graphene as a Material for Oxygen Reduction Reaction in Hydrogen Fuel Cells: A Computational Study. ACS Catal. 2013, 3, 159−165. (15) Fan, X.; Zheng, W. T.; Kuo, J.-L. Oxygen Reduction Reaction on Active Sites of Heteroatom-Doped Graphene. RSC Adv. 2013, 3, 5498−5505. (16) Sorescu, D. C.; Jordan, K. D. Theoretical Study of Oxygen Adsorption on Graphite and the (8,0) Single-walled Carbon Nanotube. J. Phys. Chem. B 2001, 105, 11227−11232. 229



Article

(40) Turro, N. J. Modern Molecular Photochemistry; Benjamin/ Cummings: Menlo Park, CA, USA, 1978. (41) Reuter, K.; Scheffler, M. Composition, Structure and Stability of RuO2(110) as a Function of Oxygen pressure. Phys. Rev. B 2001, 65, 035406.

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Boosting Graphene Reactivity with Oxygen by Boron Doping: Density

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