Trends in the Adsorption of 3d Transition Metal Atoms onto

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J. Phys. Chem. C 2010, 114, 14141–14153

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Trends in the Adsorption of 3d Transition Metal Atoms onto Graphene and Nanotube Surfaces: A DFT Study and Molecular Orbital Analysis Hubert Valencia, Adria` Gil, and Gilles Frapper* Groupe chimie quantique applique´e, LACCO UMR 6503-CNRS, UniVersite´ de Poitiers, 40 AV. du Recteur Pineau, 86022 Poitiers Cedex, France ReceiVed: April 16, 2010; ReVised Manuscript ReceiVed: June 11, 2010

The functionalization of graphene and (8,0) single-walled carbon nanotubes (SWCNTs) with individual 3d transition metal (TM) atoms was modeled using density functional theory (DFT) calculations. The structural geometry, magnetism, and binding energies were analyzed in terms of the density of states (DOS), Bader charges, and organometallic M(η6-C6H6) orbital molecular models. Trends in the binding energies were explained by a model, which included several contributions from the chemisorbed atoms: Coulomb interaction, 3dn4sx f 3dn+x electronic promotion energy (EPE), and occupation of the 1e2(δ), 2e1(π), and 2a1(σ) metal orbitals. 4s occupation, which causes Pauli repulsion, explained the physisorption trends of Cr, Mn, and Cu. The model was successfully extrapolated to a convex surface, such as that of (8,0) SWCNTs. The potential energy surfaces for the adatoms adsorbed on graphene were determined to evaluate the diffusion energy barriers. We found that Sc, Ti, Fe, and Co metals could be isolated on the graphene surface, whereas all other 3d TM atoms diffused (with possible aggregate formation). 1. Introduction Carbon supports are commonly used in heterogeneous catalysts (e.g., metal aggregate deposition, cationic exchange, oxide nanoaggregates).1 Among such supports, crystalline graphite, nanotubes, and fullerenes are well-characterized materials composed of 5- and/or 6-member carbon cycles. Such cycles are analogous to cyclopentadienyl and/or benzene molecular systems, implying that a bridge with organometallic homogeneous catalysts would be expected. Functionalization of such supports with organic or inorganic unsaturated substituents, in the form of either isolated or aggregated clustered transition metals or organometallic complexes MLn, can be a fascinating conceptual exercise for chemists. Metal atoms deposited onto functionalized carbon surfaces can be manipulated to report on the sorption properties of small molecules as a sensing material,2-6 to assist in hydrogen storage,7-10 to enable catalysis,11-14 to synthesize metallic nanowires,15,16 or to fabricate nanoelectronics.17 Since the discovery of carbon nanotubes18 and the characterization of isolated single graphene,19 much research interest has shifted to potential applications of single atomic metal deposition on the surface of materials composed of curved or planar graphitic sheets.20,21 Many experimental22-24 and theoretical25-49 studies have focused on the adsorption of metals onto graphitic supports. Because metals tend to diffuse on surfaces and form clusters,8,25,26 studies of the various preferential coordination sites and the activation barriers that separate sites are important for predicting the behavior of single adsorbed metal atoms on such surfaces. In the present study, we analyzed trends in the structural parameters, magnetic moments, and adsorption energies of 3d single metal atoms (Sc to Cu, nine different metal adatoms) on graphitic surfaces (graphene and nanotubes) by means of density functional theory (DFT) calculations with periodic boundary * To whom correspondence should be addressed. E-mail: gilles.frapper@ univ-poitiers.fr.

conditions. With the help of orbital diagrams for the organometallic M(η6-C6H6) molecular models, DOS analysis, and Bader charge analysis, we have proposed a model to explain the observed trends. Several parameters were addressed in the model, such as the Coulomb interactions, electronic promotion energy (EPE), and orbital occupation. TM atom adsorption properties on different types of carbon supports have been studied from a theoretical perspective: supports have been modeled at the molecular level or as a periodic surface, and studies have focused on one or a small set of metal adatoms.31-33,38,41,42,45,48,49 Some controversy surrounds the nature of the adsorption sites, and only partial explanations for the geometric, magnetic moment, and adsorption energy trends have been proposed. Moreover, no theoretical study has systematically analyzed the 3d series (nine adatoms in total) in the context of curvature effects in the substitution of a graphene substrate with a nanotube substrate. Due to the thermodynamically driven tendency of TMs to cluster,8,25,26 diffusion of metals along graphene was studied here to lend insight into the kinetics of diffusion on graphene. Such phenomena are important for predicting the maximum hydrogen storage capacity for metal functionalized carbon support materials, the subject of a forthcoming paper.50 We expect that this systematic study will increase the understanding of 3d TMs bonded to carbon supports and the behavior and properties of these systems, such as magnetism. The study also provides insights into the diffusion kinetics of adsorbed 3d TMs on graphitic carbon supports. Before discussing the results, we present a short review of the theoretical literature on this subject. 2. Bibliographical Review On planar graphitic supports, the 3d series of TM atoms, from Sc to Ni, were modeled early on by Duffy et al. using spinpolarized DFT calculations based on cluster models and the local spin density approximation (LSDA).31,32 They studied different adsorption sites on graphite and showed that, among the 3d TM

10.1021/jp103445v  2010 American Chemical Society Published on Web 08/03/2010

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atoms, only Fe, Co, and Ni adsorbed at hollow sites. Other members of the series preferred top sites, whereas bridge positions were not stable for any metal. Carbon pz orbitals (π bonded states) were found to hybridize strongly with the d orbitals of the adsorbed TM atoms. The magnetic moment of any TM atom on the graphite surface depended strongly on the adsorption site. They discussed results in terms of 4s electronic promotion to 3d empty orbitals. BelBruno45 studied Cu, Ni, V, Co, and Cr, among other TM atoms, using half-sandwich metal atom complexes with benzene as a cluster approximation at the BPW91/TZP level of calculation. This study showed that η6 coordination on benzene was preferred for Ni, V, and Co, whereas Cr and Cu preferred η1 or η2 coordination. Analysis of the bond nature showed that the most weakly bound complexes were those in which metal atoms developed some amount of negative charge. The valence spin density of the metal atoms in the complexes showed relaxation from the 4s2dn to the 4s13dn+1 configurations, and it was concluded that substantial 3d contributions to the HOMO, which also contained contributions from the benzene ring orbitals, stabilized the hollow site complex (η6 C6V). The large 4s component of a metal atom destabilized the hollow site complex but stabilized the top or bridge site complexes, making those geometries the favored structures. More recently, Sevinc¸li et al.48 performed a theoretical study of the binding energies and electronic and magnetic properties of graphene and graphene nanoribbons functionalized with 3d TM atoms (Co, Cr, Fe, Mn, and Ti). They used a pseudopotential plane wave method with the PW91 functional in a periodic approximation of 2 × 2 and 4 × 4 cells. They found that 3d TM atoms were adsorbed on the graphene with binding energies ranging between 0.10 and 1.95 eV, depending on the species and coverage density. For almost all TM atoms considered, binding to the hollow sites was energetically more favorable. Cr preferred bridge sites, although Cr and Mn were not predicted to bind strongly in any of the tested configurations. In contrast with the results of Duffy et al.,31,32 Sevinc¸li et al. found that the top sites never yielded a ground state configuration for any TM atom. Diffusion of Co, Fe, and Ti was studied. It was concluded that the processes of diffusion and formation of clusters by adsorbed Ti were prevented by a significant potential barrier of 0.74 eV, whereas diffusion of Fe was found to be relatively easy. Also in 2008, Chan et al.49 published a study on the adsorption of 12 different metal adatoms on graphene including Ti and Fe 3d TM (PBE GGA periodic calculations). For 3d adatoms, their calculations are consistent with covalent bonding and the adsorption is characterized by a strong hybridization between adatom and graphene electronic states. The hollow site is found to be the more stable one for 3d TM, and the binding energies are 1.869 and 0.748 eV for Ti and Fe, respectively. This binding energy decrease is not explained. For convex graphitic supports (SWCNTs), Durgun et al.38,41 carried out a systematic periodic DFT study of the adsorption of single atoms, including 3d TM atoms, onto the zigzag (8,0) SWCNTs and the armchair (6,6) SWCNTs. A pseudopotential plane wave method within the generalized gradient approximation (GGA) was chosen for the study. They used the spinunpolarized formalism to find the most stable adsorption sites, and they performed spin-polarized calculations on the structures obtained from application of the unpolarized formalism. Hollow sites were preferred for all 3d TM atoms with the exception of Ni and Cu, which preferred the axial-bridge adsorption site. The authors described an interesting behavior: the binding energy trends from Sc to Zn followed a chevron shape (V-shaped

Valencia et al. curve). However, the authors did not give deeper insight into the binding energy behavior. They showed that most adsorbed 3d TM atoms, excluding Ni, had a magnetic ground state with a significant magnetic moment. They related this moment to the support curvature, because the magnetic moment increased as the curvature increased. They concluded that the bonding character and associated physical properties strongly depended on the type of adsorbed atom, particularly on the valence electron structure. With respect to curvature effects, Yagi et al.42 studied Fe, Co, and Ni on graphene, (4,4) SWCNTs, and (8,8) SWCNTs. A spin-polarized pseudopotential plane wave approach with the PW91 functional was used. The authors showed that curvature significantly affected the most favored binding sites and magnetic moments of the TM atoms. On graphene, the hollow site was the most stable site for all elements after optimization, whereas the top site was found to be unstable. The magnetic moments of the TM atoms on graphene were found to be reduced by up to 2 µB relative to the free atom, and the authors concluded that this property was related to the promotion of 4s electrons to the 3d states. For (4,4) SWCNTs, the curvature favored less coordination sites. Co and Ni preferred the outside top sites, whereas Fe preferred the outside hollow sites. The adsorption of Co on (8,8) SWCNTs was studied, and the preferred outside site was found to be the hollow outside site, although a larger diameter nanotube showed increased preference for the inside hollow site. The authors concluded that the binding energy not only depended on the curvature but also on a more subtle interplay between curvature, preferred coordination, and magnetization energy. Changes in the magnetic moment were explained by means of a local density of states (LDOS) analysis and were related to the promotion of 4s electrons into the 3d orbitals. Curvature also induced higher magnetic moments. The most stable state for the TM atom bonded to graphene or nanotubes was determined by the energy reduction resulting from changes in the magnetic state. Because the sp2 bonds in the (4,4) SWCNTs acquired sp3-like character, the top site on the (4,4) SWCNTs was found to be the most stable outside coordination state for Co and Ni. 3. Methods and Models Electronic structure calculations were carried out using the GGA PW9151 functional implemented in VASP code52 for both graphitic systems: graphene and nanotubes. The electron-ion interaction was described by the projector-augmented wave (PAW) scheme,53 and the electronic wave functions were expanded using plane waves up to a kinetic energy of 450 eV. The spin-polarization approach was implemented in all calculations. The Brillouin zone was described using a Monkhorst-Pack (M&P)54,55 scheme of special k-points to obtain a general convergence criteria of 5 × 10-3 eV for energies and 0.01 Å for structural variations between two different k-point grids. Atomic charges and charge transfer were calculated using the Henkelman56 algorithm formalism for the Bader57 partitioning of electronic density. Band diagrams and DOS analysis were obtained by fixing the Wigner-Seitz radius (rWIGS) for the support during integration over the number of electrons and then by setting rWIGS for the adsorbates within the radii of tangential spheres. This method allowed the accurate assignment of relevant atomic orbital attributions to a particular projected DOS peak. Both systems, graphene and nanotubes, were modeled using the supercell approach with periodically repeated slabs. The graphene model consisted of a single graphite sheet, and each supercell contained one atomic layer of carbon, the

Adsorption of 3d Transition Metal Atoms SCHEME 1

adsorbed atom, and a vacuum region of 20 Å between graphene layers to allow for adsorption of future MHx fragments (the subject of a forthcoming paper50). Calculations were performed using an optimized graphene lattice constant of 2.467 Å, i.e., a carbon-carbon distance of 1.424 Å that compared well with the experimental values 2.48558 and 1.415 Å.59 A periodic 4 × 4 surface containing 32 carbon atoms was used for the calculations (see Scheme 1a and Figure S1 of the Supporting Information). The number of k-points used to describe the irreducible portion of the Brillouin zone was 12 (M&P mesh of 9 × 9 × 1). The supercell used for (8,0) SWCNTs was twice the primitive cell of 32 atoms, and therefore, it contained 64 carbon atoms in addition to the adsorbed metal atom (see Figure S2 of the Supporting Information). The metal-metal distance in the model was 8.516 Å, and each nanotube was separated from its neighboring image by 7.62 Å. The number of k-points considered was 10 (M&P mesh of 1 × 1 × 20). All atomic positions were allowed to relax, and the elemental cell was maintained at a constant size during optimization. The metal atom was initially placed at a distance of 2.5 Å from the carbon atoms. The adsorption energy of a metal, Eads, is defined as

Eads ) E(M@support) - [E(M) + E(support)] where - support is either graphene or (8,0) SWCNTs - E(M@support) is the electronic energy for the equilibrated M@support system - E(support) is the electronic energy for the optimized support geometry - E(M) is the electronic energy for the isolated metal in its ground state, defined as a single atom in a cubic supercell of length 10 Å. Only the Γ point of the Brillouin zone was sampled in this case. The binding energy, Eb, was taken to be the opposite of the adsorption energy. Adsorption was assumed to be favored when this quantity was positive. For convenience, the binding energy will be used in place of Eads in the remaining sections. 4. Results This section begins with a presentation of the results obtained for the η6 coordination mode, in which the metal is simulta-

J. Phys. Chem. C, Vol. 114, No. 33, 2010 14143 SCHEME 2

neously coordinated to six carbon atoms of a 6-member cycle on the graphene or (8,0) SWCNT surface (hollow site, H). The discussion focuses on the structural, magnetic, and energetic aspects of binding. Subsequently, an electronic structure analysis is presented on the basis of a molecular orbital model and DOS analysis. An explanation for the variations in adsorption energy across the 3d TMs adsorbed onto the graphitic surfaces is proposed. Next, the effects of surface curvature on binding energy are addressed by examining the adsorption energy and geometry of a 3d TM single atom on (8,0) SWCNTs. Finally, the diffusion of single Ti, Mn, and Ni atoms, which showed diffusion behavior that was representative of the 3d series, was modeled by calculating the barrier heights observed for η1 to η6 (top-to-hollow transitions, T f H) and η2 to η6 (bridge-tohollow transition, B f H) pathways. 4.1. Structural Aspects. Prior to metal adsorption, covalent graphene sheets were flat (D6h local symmetry) and the optimized C-C distances were 1.424 Å. Deposition of 3d TM atoms modified the surface structure. Table 1 shows the metal-carbon (dMC) and metal-surface (h) distances (see Scheme 1b for an illustration). The adatom height (h) was defined as the difference between the z coordinate of a single metal atom and the average z coordinate of the C atoms in the graphene layer. The C-C bond distances and zi distances that corresponded to deviations in planarity are listed in Tables S1 and S2 of the Supporting Information. The maximum decrease of C-C distance was 0.007 Å (0.5%), whereas the maximum increase of C-C distance was 0.015 Å (1%). The maximum out-of-plane deviations (with respect to the z axis) were -0.033/ +0.014 Å. Thus, the geometric perturbations of the sheet produced by metal-carbon interactions were very weak. Perturbations were mainly localized on the nearest 6-membered ring and quickly vanished as the distance to the adsorption site increased. In conclusion, the graphene surface was weakly perturbed by deposition of the 3d TM atoms and, therefore, conserved its planarity. Figure 1 shows a comparison of the trends in dMC and the sum of the covalent radii rM + rC60 along the 3d TM series. The dMC distances varied from 2.102 to 2.518 Å, whereas the h parameter fell between 1.489 and 2.045 Å. The results suggested that the 3d series could be divided into two categories. The first category included metals from Sc to V and from Fe to Ni, with

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TABLE 1: Calculated Metal-Carbon (dMC) and Metal-Surface (ha) Distances for Metals Adsorbed onto Graphene and Metal-Carbon (dMC1 and dMC2)b Distances for Metals Adsorbed onto (8,0) SWCNTs (All Distances Are in Å)

a

systems

Sc

Ti

V

Cr

Mn

Fe

Co

Ni

Cu

dMC h dMC1 dMC2

2.435 1.922 2.204 2.476

2.331 1.791 2.129 2.341

2.331 1.791 2.159 2.389

2.518 2.045 2.290 2.539

2.511 2.022 2.229 2.538

2.105 1.489 2.038 2.231

2.102 1.490 2.028 2.234

2.117 1.510 2.020 2.238

2.513 2.044 2.238 2.532

See Scheme 1b. b See Scheme 2.

M-C bond lengths very close to the sum of the covalent radii, and the maximum difference for V was only 10%. Thus, the M-C bond character was easily qualified as covalent, and such TM atoms were considered chemisorbed. The second category included Cr, Mn, and Cu, with M-C bond lengths more than 20% higher than the sum of the covalent radii. Such atoms were very weakly bonded to the carbon surface and were considered to be physisorbed. Note that the 3d shells of the Cr, Mn, and Cu atoms were half-occupied (Cr and Mn) or totally occupied (Cu) in the ground state. We next consider metal atoms adsorbed onto the curved graphitic surface of (8,0) SWCNTs. The curvature of the nanotube favored a boat conformation for the 6-membered rings. Consequently, M-C distances were divided into two sets: the shortest dMC1 corresponded to the average distance between the TM atom and the (two) carbon atoms with maximum height, and the longest dMC2 corresponded to the average distance between the TM atom and the four other C atoms of the 6-membered ring (see Scheme 2). Table 1 reports the M-C distances (dMC1, dMC2) for the nanotube. The C-C distances and the r distance corresponding to the radius of the M@(8,0)SWCNT system are listed in Table S3 of the Supporting Information. Figure 2 compares the trends in the optimized distances with the sum of the covalent radii rM + rC.60 The M-C distances in (8,0) SWCNT varied from 2.020 Å (Ni) to 2.290 Å (Cr) for the M-C shortest distance (dMC1) and from 2.231 Å (Fe) to 2.539 Å (Cr) for the longer distance (dMC2). Note that dMC2 distances were clearly comparable to the graphene dMC distances for all metals except Fe, Co, and Ni. The intermediate lengths observed for these atoms proved that they were sufficiently small that the metal penetrated the carbon ring (see Scheme 3). All distances were in good agreement with previous results reported in the literature.37,38,41 A comparison of the dMC1 and dMC2 distances with the sum of the covalent radii60 in Figure 2 showed that the distances could be divided into the categories described for graphene. The M-C bonds

Figure 2. dMC1 and dMC2 distances (Å) corresponding to each 3d TM atom adsorbed onto (8,0) SWCNTs. The dashed line represents the sum of the covalent radii rC and rM for C and TM.

SCHEME 3

were clearly covalent, except for Cr, Mn, and Cu. These three atoms produced a dMC2 bond length of 2.5 Å, similar to that observed in graphene, and they were considered physisorbed. The global structural properties of metal adsorption on both planar and curved graphitic surfaces were similar with respect to the metal-carbon bond type. 4.2. Magnetic and Charge Aspects. The total magnetic moment µ (in µB) of each adsorbed TM is given in Table 2 and compared with the quantity predicted for an isolated metal atom in its ground state. For adsorption onto graphene, µ increased continuously and monotonically from Sc to Cr/Mn with an increment of 1.1 µB. From Fe to Ni, µ decreased monotonically by one unit, similar to the trend observed in free atoms. In moving from Sc to V, the difference between M@graphene and M was +1, whereas this difference was -2 for Fe, Co, and Ni. We will propose an explanation for these variations in section TABLE 2: Total Magnetic Moment (µ) Calculated for the Systems M@graphene and M@(8,0)SWCNTa systems

Figure 1. dMC distances (Å) corresponding to each 3d TM atom adsorbed onto graphene. The dashed line represents the sum of the covalent radii rC and rM for C and TM.

Sc

Ti

V

Cr

Mn

Fe

Co

Ni

Cu

2.24 3.32 4.41 5.55 5.40 2.00 1.03 0.00 0.87 µM@graphene µM@(8,0)SWCNT 0.70 1.80 3.43 5.09 5.04 2.18 1.23 0.12 0.77 µM 1 2 3 6 5 4 3 2 1 a

Values are given in µB, and those for isolated ground state metals (M) are presented for comparison.

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TABLE 3: Metal Formal Charge (QM) in M@graphene and M@(8,0)SWCNT Systems systems

Sc

Ti

V

Cr

Mn

Fe

Co

Ni

Cu

QM(M@graphene) QM(M@(8,0)SWCNT)

1.30 1.51

1.15 1.39

0.95 1.09

0.58 0.72

0.61 0.67

0.79 0.79

0.59 0.60

0.55 0.48

0.22 0.32

TABLE 4: Binding Energies of 3d TM Atoms on Graphene and (8,0) SWCNTs (Energies Are Given in eV) systems

Sc

Ti

V

Cr

Mn

Fe

Co

Ni

Cu

M@graphene M@(8,0)SWCNT difference

1.51 1.86 +0.35

1.74 1.86 +0.12

1.09 1.08 -0.01

0.17 0.13 -0.04

0.15 0.31 +0.16

1.21 1.07 -0.14

1.59 1.64 +0.05

1.55 1.32 -0.23

0.07 0.03 -0.04

5. The magnetic moments of Cr, Mn, and Cu remained more or less constant, similar to the isolated atoms, which confirmed the assumption that these TM atoms were physisorbed on the carbon surface. No noticeable changes in the electronic configurations were observed. Similar trends were observed for adsorption onto (8,0) SWCNTs. µ increased from 0.7 µB to 5.1 µB in moving from Sc to Cr/Mn. µ then decreased monotonically by one unit from Fe to Ni. The main difference between the carbon-based surfaces was observed for adsorption of Sc, Ti, and V, which showed decreases in µ (graphene to nanotube) of 1.54, 1.52, and 0.98 µB, respectively. For these three atoms, µ in M@(8,0)SWCNT was almost identical to its value in the isolated metal. This result disagreed with the assessment of the M-C distances, which classified these atoms as chemisorbed. The other TMs agreed well (µ differed by Eb(Ni) > Eb(Fe), which is consistent with our results. 7. Conclusion A systematic DFT study of the 3d TM interactions with planar and curved graphitic surfaces was performed. Structurally, both graphene and (8,0) SWCNT surfaces presented η6 hollow adsorption sites with metals that mainly bound covalently and ionicity degrees that varied as expected with the metal electronegativity (charge transfer from metal to surface). It is clear that 3d TM atoms, with the exception of Cr, Mn, and Cu, were chemisorbed onto graphene with an η6 hollow geometry (Eb ) 1.09-1.74 eV). Half-filled Cr and Mn and filled Cu 3d atoms were physisorbed on these surfaces, and no energetically favored adsorption sites were predicted. In general, the binding energy increased as one moved in either direction away from the Cr/ Mn couple of the 3d series. The chevron shape of the metal-surface binding energies was explained in terms of the DOS, Bader charge, and molecular orbital diagram analysis. To understand this trend, we considered several parameters. Coulombic attractive interactions were important for the most electropositive metals on the left-hand side of the 3d series (Sc, Ti, and V). TM atoms with half or fully occupied 3d orbitals remained in the electronic state of the isolated metal (the EPE of chemisorption was too high). The 4s level remained occupied, and the metal-surface π repulsion was sufficiently high that

Valencia et al. these TM atoms were physisorbed. The 3a1(s) level was found to be virtual when the EPE was low (Fe, Co, and Ni), leading to a high binding energy. Finally, 2e1(π) occupation by electrons carrying the same spin moment led to energetic destabilization (case of V). The computed magnetism in each of the M@graphene and M@(8,0)SWCNT periodic systems was explained by assigning an electronic configuration using both qualitative molecular orbital diagrams and charge transfer considerations. The (8,0) SWCNT surface presented sp3 hybridization, which interacted more strongly with the d orbitals; the lowest occupied d bands overlapped strongly with the lowest unoccupied sp3 band. Therefore, the magnetic moments were one unit less than the magnetic moments in graphene for systems containing Sc, Ti, or V. The potential energy surfaces for M@graphene suggest that knowledge of the three principal adsorption sites was sufficient to estimate the diffusion energy. From our results, Sc, Ti, Fe, and Co are predicted to be stable as isolated atoms on any graphitic carbon surface. All other metals are predicted to freely diffuse and eventually form aggregates. With these results in mind, we undertook a theoretical study of hydrogen adsorption onto 3d TM-functionalized carbon surfaces.67 This study will be the subject of a forthcoming paper.50 Acknowledgment. This research was financially supported in part by the Re´gion Poitou-Charentes (Ph.D. grant to H.V. and visiting fellowship to A.G.). Calculations were supported in part at the “Centre Informatique National de l’Enseignement Supe´rieur’ (C.I.N.E.S.) at Montpellier (France). Supporting Information Available: Structural parameters for M@surface (M ) Sc to Cu and surface ) graphene or (8,0) SWCNT), density of states (DOS), qualitative interaction diagram between the DOS on the graphitic surfaces and single 3d adatom orbitals, comparison of the binding energies in the cation and neutral M@C6H6 species and the M@surface system, and the MO diagram of the M+C6H6. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kinoshita, K. Carbon: electrochemical and physicochemical properties; John Wiley & Sons, Inc.: New York, 1988. (2) Kong, J.; Chapline, M. G.; Dai, H. AdV. Mater. 2001, 13, 1384. (3) Kong, J.; Franklin, N. R.; Zhou, C.; Chapline, M. G.; Peng, S; Cho, K.; Dai, H. Science 2000, 287, 622. (4) Collins, P. G.; Bradley, K.; Ishigami, M.; Zettl, A. Science 2000, 287, 1801. (5) Chang, H.; Lee, J. D.; Lee, S. M.; Lee, Y. H. Appl. Phys. Lett. 2001, 79, 3863. (6) Mota, R.; Fagan, S. B.; Fazzio, A. Surf. Sci. 2007, 601, 4102. (7) Yildirim, T.; I´n˜iguez, J.; Ciraci, S. Phys. ReV. B 2005, 72, 153403. (8) Sun, Q.; Jena, P.; Wang, Q.; Marquez, M. J. Am. Chem. Soc. 2006, 128, 9741. (9) Chandrakumar, K. R. S.; Ghosh, S. K. Nano Lett. 2008, 8, 13. (10) Durgun, E.; Ciraci, S.; Yildirim, T. Phys. ReV. B 2008, 77, 085405. (11) Planeix, J. M.; Coustel, N.; Coq, B.; Brotons, V.; Kumbhar, P. S.; Dutartre, R.; Geneste, P.; Bernier, P.; Ajayan, P. M. J. Am. Chem.Soc. 1994, 116, 7935. (12) Luo, J. Z.; Gao, L. Z.; Leung, Y. L.; Au, C. T. Catal. Lett. 2000, 66, 91. (13) Lordi, V.; Yao, N.; Wei, J. Chem. Mater. 2001, 13, 733. (14) Li, W.; Liang, C.; Zhou, W.; Qiu, J.; Zhou, Z.; Sun, G.; Xin, Q. J. Phys. Chem. B 2003, 107, 6292. (15) Ajayan, P. M.; Iijima, S. Nature 1993, 361, 333. (16) Zhang, Y.; Dai, H. Appl. Phys. Lett. 2000, 77, 3015. (17) Javey, A.; Guo, J.; Farmer, D. B.; Wang, Q.; Wang, D.; Gordon, R. G.; Lundstrom, M.; Dai, H. Nano Lett. 2004, 4, 447. (18) Iijima, S. Nature 1991, 354, 56. (19) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Yiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666.

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