DSpace at VNU: First-principles modeling of C-60-Cr-graphene nanostructures for supporting metal clusters

DSpace at VNU: First-principles modeling of C-60-Cr-graphene nanostructures for supporting metal clusters tài liệu, giáo...

View Article Online PCCP View Journal Accepted Manuscript This article can be cited before page numbers have been issued, to this please use: H M Le, H Hirao, Y Kawazoe and D Nguyen-Manh, Phys Chem Chem Phys., 2013, DOI: 10.1039/C3CP53529K Volume 12 | Number 43 | 2010 This is an Accepted Manuscript, which has been through the RSC Publishing peer review process and has been accepted for publication ence Physical Chemistry Chemical Physics www.rsc.org/pccp Volume 12 | Number 43 | 21 November 2010 | Pages 14369–14636 PCCP chers 090856 Accepted Manuscripts are published online shortly after acceptance, which is prior to technical editing, formatting and proof reading This free service from RSC Publishing allows authors to make their results available to the community, in citable form, before publication of the edited article This Accepted Manuscript will be replaced by the edited and formatted Advance Article as soon as this is available To cite this manuscript please use its permanent Digital Object Identifier (DOI®), which is identical for all formats of publication ding scientific C has once rst major journal More information about Accepted Manuscripts can be found in the Information for Authors m to advance the n, conversion and the sustainable China ue now! Charity Number 207890 Pages 14369–14636 org/ees ISSN 1463-9076 COMMUNICATION Soler-Illia et al COVER ARTICLE Electrical Conductivity in Hore et al Patterned Silver-Mesoporous Water structure at solid surfaces and its Titania Nanocomposite Thin Films: implications for biomolecule adsorption Towards Robust 3D Nano-Electrodes 1463-9076(2010)12:43;1-V Please note that technical editing may introduce minor changes to the text and/or graphics contained in the manuscript submitted by the author(s) which may alter content, and that the standard Terms & Conditions and the ethical guidelines that apply to the journal are still applicable In no event shall the RSC be held responsible for any errors or omissions in these Accepted Manuscript manuscripts or any consequences arising from the use of any information contained in them www.rsc.org/pccp Registered Charity Number 207890 Physical Chemistry Chemical Physics Dynamic Article Links ► Physical Chemistry Chemical Physics View Article Online DOI: 10.1039/C3CP53529K ARTICLE TYPE Cite this: DOI: 10.1039/c0xx00000x Published on 01 October 2013 Downloaded by Rensselaer Polytechnic Institute on 01/10/2013 19:03:10 www.rsc.org/xxxxxx First-Principles Modeling of C60–Cr–Graphene Nanostructures for Supporting Metal Clusters Hung M Le,*1,2 Hajime Hirao,1 Yoshiyuki Kawazoe,3 and Duc Nguyen-Manh4 10 15 Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X DOI: 10.1039/b000000x We present a first-principles modeling study of a new class of nanomaterials in which buckminsterfullerene (C60) and graphene (G) are bridged by Cr via coordination bonds Two nanostructures denoted as G(C54)–Cr–C60 and G(C150)–Cr–C60 are investigated, which share many similarities in the configuration geometries but differ in the distribution densities of Cr–C60 on the graphene surface The binding energies between C60 and the rest of the system in these complexes are calculated to be 2.59 and 2.10 eV, respectively, indicative of their good structural stability Additional spin-polarized calculations indicate that G(C54)–Cr–C60 is weakly ferromagnetic, which is chiefly due to the contribution from the 3d shell of Cr We then investigate three model complexes of C60–Cr–G(C54) and a metal cluster (Ni4, Pd4, or Pt4) The binding energies of these three nanostructures are significantly large (3.57, 2.38, and 4.35 eV, respectively) Electron density analysis along the Ni–C, Pd–C, and Pt–C bonds consistently affirms that the Pt–C bond is the strongest while the Pd–C bond is the weakest The strong Pt–C bond is attributed to the effective overlap of 5d (Pt) and 2pz (C) orbitals Partial density of z 20 states analysis indicates that Ni4 and Pd4 substantially contribute to the strong ferromagnetism of the complexes, whereas Pt4 is observed to be non-magnetic even when the spin-orbit coupling is taken into account H2 dissociation on the Ni4 complex is also examined, and the estimated reaction barrier is relatively low (0.76 eV) I Introduction 25 30 35 40 45 Graphene is a newly discovered two-dimensional material composed of sp2-hybridized carbon It has been proved experimentally that a graphene nanostructure (i.e., at the nanometer scale) is extremely stable and exhibits superconductivity.1 Importantly, this two-dimensional material is considered as a zero-gap semiconductor, despite being entirely made of a non-metal element.2 It has been demonstrated that the combination of graphene and metal has the potential to find useful applications in electronic and spintronic devices.3, As such, the interactions between graphene and metals have continuously attracted a great deal of attention in the research community, and the interesting features of graphene have been widely explored both experimentally and theoretically.5-15 On one hand, many experimental efforts have been made to explore and exploit the interactions between graphene and metal nanoparticles/surfaces There is in fact abundant interest in the roles of metals such as Ni,5 Au, Fe, and Cr,6-8 and thus various experimental techniques have been applied to examine how these metals participate in interlayer interactions Interestingly enough, there is a gas-sensing application of a metal-graphene structure when the surface of graphene is decorated with Ag nanoparticles.9 On the other hand, the establishment of high-performance This journal is © The Royal Society of Chemistry [year] 50 55 60 65 computing systems and the development of density functional theory (DFT) 16, 17 have enabled theoretical and computational investigations of larger nanomaterials, and such first-principles studies have made substantial contributions to the research area of graphene-metal interactions By employing DFT, Nakada and Ishii systematically investigated the atomic decorations of graphene with most of the elements on the periodic table and reported the favored binding site of each element as well as its corresponding binding energy.11 It was revealed that a specific metal element had a tendency to bind preferentially to either the hexagonal (H), top (T), or bridge (B) site, as illustrated in Fig Particularly, Cr, the 3d transition metal of interest in this work, preferentially binds to the H site of graphene with high chemical stability In another study, Giovannetti et al examined the interactions between graphene and noble metals such as Cu, Au, Ag, and Pt, and suggested that there was a shift of 0.5 eV in the graphene Fermi level upon the formation of weak bonding interactions.12 Maassen et al performed a spintronic investigation of graphene–Co(111) and graphene–Ni(111) interactions using first-principles modeling, and calculated the spin filtering efficiencies of those two models to be greater than 80 and 60%, respectively.4 An intercalation structure and vibrational modes of graphene–alkali earth metals–graphene were also investigated.13 The adsorption of [journal], [year], [vol], 00–00 | Physical Chemistry Chemical Physics Accepted Manuscript Page of 10 Physical Chemistry Chemical Physics Page of 10 View Article Online 20 Published on 01 October 2013 Downloaded by Rensselaer Polytechnic Institute on 01/10/2013 19:03:10 25 Fig Three possible adsorption sites of a metal atom on a honeycomb unit in graphene 30 35 that it can be employed to carry metal clusters In fact, there has been experimental work in which graphene was utilized as a ligand to carry complex structures of Cr–benzene.26 In addition, the successful attachment of C60 to bis(benzene) chromium27 also demonstrates the possibility of connecting C60 and graphene using a bridging transition-metal atom (Cr) These successful experimental efforts have highly motivated us to design a complex of graphene and C60 More specifically, we employ DFT calculations to investigate a model in which buckminsterfullerene and graphene are bridged by a Cr atom via coordination bonds Then, interplays between structural stability and magnetic properties are deliberately discussed on the basis of electronic structure data In addition, such a nanostructure may be considered as an excellent candidate for supporting metal nanoparticles and may be employed in the future design and development of nanocatalysts To examine this feature more realistically, we test the possibility of decorating the steadied buckminsterfullerene complex with a small metal cluster (of Ni, Pd, or Pt) and investigate the energetic stability as well as its electronic and magnetic properties II Buckminsterfullerene –Cr–graphene models 40 45 50 Fig (a) The theoretical model of G(C54)–Cr–C60, where C60 is attached on the graphene monolayer surface via coordination bonds with Cr In this unit cell, the graphene layer contains 54 C atoms (b) The illustrative model of G(C150)–Cr–C60, which contains C60 attached on Cr–graphene In this unit cell, the graphene layer contains 150 C atoms 10 15 Pt4 on graphene and boron-substituted graphene was presented by Wu and co-worker.15 Recently, Bui et al.14 have performed an investigation of graphene–Cr–graphene nanostructures using the local-spin-density approximation (LSDA)18 of DFT, and discussed the interplays between stability and ferromagnetism based on electronic structure data Buckyball buckminsterfullerene (C60) is made of carbon atoms and has a spherical shape This interesting molecular structure was first discovered by Kroto and co-workers in 1985.19, 20 There have also been proposals as to the synthesis of buckminsterfullerene-based materials for catalytic purposes.21-24 Duffe et al.25 decorated C60 monolayers with Ag clusters, and demonstrated that the resultant structures were stable and worked very efficiently as catalysts when they were supported by gold sink or graphite With that result in mind, in this study, we suggest and examine a different way to steady C60 on a surface so | Journal Name, [year], [vol], 00–00 55 60 65 70 According to the theoretical results obtained from our previous study,14 Cr is expected to bind to the H site (indicated in Fig 1) of a honeycomb hexagonal unit on graphene as well as buckminsterfullerene In addition, our previous study of graphene–Cr–graphene structures also showed that the distribution density of an attached structure (i.e Cr–C60 in this case) on the graphene surface has a significant impact on the stability, magnetism, and active surface area of buckminsterfullerene The curved surface of C60 should be beneficial in improving the reacting efficiency of attached metal nanoparticles that are to act as catalysts in chemical reactions, and its attachment on graphene would enhance the recoverability of the catalysts Two graphene–Cr–C60 models are investigated in this study In the first model, a periodic sheet of graphene containing 54 C atoms per unit cell is decorated with Cr–C60 (Fig 2(a)) In the second model, we attempt to reduce the distribution density of Cr–C60 by extending the area of the periodic graphene sheet so that one Cr–C60 complex exists per 150 C atoms (Fig 2(b)) For convenience, let us denote these two investigated nanostructures as G(C54)–Cr–C60 and G(C150)–Cr–C60, respectively In both cases, the investigated systems are two-dimensional slabs, and for the vacuum treatment in the z direction, we employ a unit cell with the c lattice parameter of 30 Bohr (15.86 Å) More specifically, the thickness of vacuum layer in G(C54)–Cr–C60 and G(C150)–Cr–C60 is approximately 6.34 Å III Computational details All calculations are executed using the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional28, 29 as implemented in the Quantum Espresso package.30 The employed ultrasoft pseudopotentials include the 2s, 2p valence wavefunctions for C and the 3s, 3p, 4s, 3d, and 4p valence wavefunctions for Cr.31, 32 The generalized gradient approximation with spin polarization is This journal is © The Royal Society of Chemistry [year] Physical Chemistry Chemical Physics Accepted Manuscript DOI: 10.1039/C3CP53529K Page of 10 Physical Chemistry Chemical Physics View Article Online DOI: 10.1039/C3CP53529K 10 15 20 25 30 35 40 employed to investigate the metal-aromatic ring (from buckminsterfullerene and graphene) interactions The k-point mesh for most calculations is selected as (6 × × 1), which enables computations with reasonable computational cost However, for the studies of G(C150)–Cr–C60 and a chemical reaction involving the G(C54)–Cr–C60 complex, we perform calculations at the Γ point because the computational demand is extremely high The kinetic-energy cutoff for plane-wave expansion has a significant impact on energy convergence and computing time; in this study, we employ 45 Rydberg for all calculations The semi-empirical correction option for van der Waals interactions33, 34 is also activated in our calculations in order to improve the description of the interaction between C60 and graphene Since the investigated systems have two-dimensional extension, we employ a 2D-like unit cell for the computational treatment That is, the structures are thought of as periodic in the x and y directions, while the z direction is assumed to be isolated with a vacuum layer As mentioned earlier, the c lattice parameter is initially chosen as 30 Bohr (15.86 Å) The Broyden-FletcherGoldfarb-Shanno35 (BFGS) algorithm is employed to optimize the equilibrium structures with an energy-convergence criterion of 10-5 eV/cell and a gradient-convergence criterion of 10-4 eV/Å/cell In order to optimize structures efficiently, we impose a constraint to perform constant-volume optimizations (the fixed volume is sufficiently large), but three unit-cell axes are fully adjusted to reduce the energy In the subsequent investigation of decorating Ni, Pd, and Pt nanoparticles, the structural optimizations are executed within a fixed unit cell The ultrasoft pseudopotentials are employed which describe the valence wavefunctions of (4s, 4p, 3d), (5s, 5p, 4d), and (6s, 6p, 5d) for Ni, Pd, and Pt, respectively The c lattice parameter in these cases is extended to 40 Bohr (21.17 Å) The PBE-derived results for G(C54)–Cr–C60 are validated by performing additional PW91 calculations.36, 37 Upon the availability of reliable ultrasoft potentials, we later validate the calculation on a complex between G(C54)–Cr–C60 and a Pt4 cluster (validation of Ni and Pd complexes is excluded because of unavailability of reliable pseudopotentials for PW91 calculations) 50 60 E binding = E graphene −Cr + E C60 − E structure In this high-densed model, there are 54 C atoms on the graphene sheet per Cr–C60 The geometry optimization is performed on this model using the PBE functional In order to obtain a sufficiently relaxed structure, we set the energy conversion criterion as 10-6 eV and the gradient conversion criterion as 10-4 eV/Å in all three dimensions In addition, the unit cell is simultaneously optimized during the relaxation of atoms It is noticed from the optimized structure that those six C atoms coordinating to Cr are slightly pushed down from the original graphene surface In the This journal is © The Royal Society of Chemistry [year] (1) where Egraphene-Cr and EC represent the total energy of the 60 65 70 75 80 85 IV Results and discussion IV.1 The G(C54)–Cr–C60 nanostructure 45 55 equilibrium structure, we observe that the Cr–C(graphene) bond has a length of 2.16 Å In a previous study of the graphene–Cr– graphene nanostructure,14 one of the most stable structures (namely 1-12 GMG) had an equilibrium Cr–C bond length of 2.18 Å, which is slightly longer than the Cr–C(graphene) bond in the current case The Cr–C(C60) bond has a distance of 2.20 Å, which is slightly longer than the Cr–C(graphene) distance The Cr–graphene layer distance is found to be 1.60 Å, while the Cr– C60 distance is 1.67 Å The structural stability of a particular nanostructure is evaluated by its corresponding binding energy: 90 95 graphene–Cr complex and C60 in the unit cell, respectively, while Estructure is the total energy of the equilibrium nanostructure In a previous study,11 the graphene–Cr complex was shown to be stable; therefore, we employ the total energy of such a complex in binding energy calculations instead of using the total energy of graphene and Cr separately A positive binding energy indicates that the nanostructure of concern is energetically stable, whereas a negative binding energy is an indication of structural instability By adopting equation (1), the binding energy of G(C54)–Cr– C60 is calculated as 2.59 eV, which indicates that it is an energetically stable nanostructure The binding energies of the previously-investigated graphene–Cr–graphene nanostructures (namely 1-12 GMG) were shown to be less than 2.09 eV.14 Hence, the current case study of G(C54)–Cr–C60 suggests that steadying C60 on the graphene–Cr complex provides enhanced stability (by 0.5 eV) compared to the cases of graphene–Cr– graphene intercalated structures The important distances discussed above and binding energy of G(C54)–Cr–C60 (as well as G(C150)–Cr–C60) are summarized in Table I along with the validation data obtained from PW91 calculations for G(C54)–Cr– C60 The difference in geometry configuration predicted by PBE and PW91 are negligible The binding energy predicted by PW91 is lower than the previous calculation by 16.2%; however, its magnitude is still relatively high, which indicates the structural stability of the complex Besides analyzing the theoretical stability, we also perform electronic structure analysis to characterize the spin polarization and magnetic behavior of the nanostructure From the DFT calculations of G(C54)–Cr–C60, two useful quantities, i.e., the total and absolute magnetizations, can be derived from densityof-state (DOS) analysis For convenience, we denote them as MT and MA, respectively The mathematical formulas of MT and MA are expressed as follows: (2) M T = (nup − ndown )d r ∫ M A = ∫ nup − ndown d r (3) In the above equations, nup and ndown respectively describe the Journal Name, [year], [vol], 00–00 | Physical Chemistry Chemical Physics Accepted Manuscript Published on 01 October 2013 Downloaded by Rensselaer Polytechnic Institute on 01/10/2013 19:03:10 Table I The important interatomic distances and binding energies for G(C54)–Cr–C60 and G(C150)–Cr–C60 Cr–C(graphene) Cr–C(C60) Cr–graphene Cr–C60 Binding bond (Å) bond (Å) separation (Å) separation (Å) energy (eV) 2.16 2.20 1.60 1.67 2.59 G(C54)–Cr–C60 (PBE) 2.16 2.21 1.61 1.69 2.17 G(C54)–Cr–C60 (PW91) G(C150)–Cr–C60 (PBE) 2.15 2.21 1.61 1.68 2.10 Physical Chemistry Chemical Physics Page of 10 View Article Online PBE PW91 2s Published on 01 October 2013 Downloaded by Rensselaer Polytechnic Institute on 01/10/2013 19:03:10 PBE PW91 0.002 0.001 Table II Main orbital contributions (µB) to total magnetization in G(C54)–Cr–C60 Graphene C60 Cr -0.065 -0.130 0.750 -0.072 -0.122 0.748 3d x − y s+p 2px 2px 3d z 2pz 2s 2pz orbitals (2py) (2py) (3dxy) -0.075 0.004 -0.048 -0.068 -0.029 0.008 0.440 0.115 -0.080 0.004 -0.004 -0.068 -0.027 0.008 0.426 0.120 30 35 3dzx (3dzy) 0.036 0.041 the ferromagnetic moment, while both graphene and C60 contribute to the anti-ferromagnetic moment Overall, when we sum up all magnetic terms, we are left with a weak ferromagnetic moment as a result of cancellation of the two opposing effects In addition, the electron distribution of Cr around the Fermi level (located at 0) indicates conducting behavior, suggesting that the nanostructure is metallic When we further analyze the 3d shell of Cr and 2pz orbitals of C (from graphene and C60), it is shown that all 3d spins align ferromagnetically (Fig and Table II) In particular, we observe that the 3d orbital of Cr exhibits the z 40 largest ferromagnetic contribution The 2pz subshells of C, which mainly donate electrons toward the 3d orbitals of Cr to form coordination bonds, align anti-ferromagnetically with respect to the 3d shell of metal (Table II) Indeed, its conducting behavior can be examined in this plot, where the 3d contribution to the z 45 Fig Spin-polarized PDOS of the G(C150)–Cr–C60 nanostructure and PDOS of Cr 3d and C60 2pz subshells The Fermi level is located at It can be obviously seen that the ferromagnetism is mainly caused by the spin polarization of Cr 10 15 20 25 numbers of spin-up and spin-down states at a certain energy level For a particular orbital, a positive spin polarization increases the total magnetic moment and thereby contributes to ferromagnetism Conversely, a negative spin polarization contributes to anti-ferromagnetism Therefore, MT is always less than or equal to MA When the total and absolute magnetizations are similar, all polarization terms are positive, and we can conclude that all orbitals contribute to ferromagnetism From the orbital analysis of PBE data, the total magnetization of G(C54)–Cr–C60 is estimated as 0.55 µB/cell, which indicates ferromagnetic behavior, while the absolute magnetization is 1.09 µB/cell When such magnetic moments are compared to the magnetic moments of graphene–Cr–graphene intercalated nanostructures examined in a previous study,14 the ferromagnetism exhibited by G(C54)–Cr–C60 can be considered fairly weak The absolute magnetization is almost twice as large as the total magnetic moment due to the significant effect of antiferromagnetic moment, which helps to reduce the overall ferromagnetism of the nanostructure To have a more pictorial understanding of the origin of magnetism exhibited by this structure, we analyze the total DOS and partial DOS (PDOS) of G(C54)–Cr–C60 The bonding interaction between six-membered carbon rings (from either graphene or C60) and a transition-metal element is dominantly formed by coordination bonds, in which the 2pz orbitals from C have a tendency to donate electrons toward an unfilled 3d orbital of the transition metal As a result, the C 2pz and metal 3d orbitals would play important roles in the bond formation and spin polarization In fact, Cr contributes strongly to | Journal Name, [year], [vol], 00–00 50 55 60 65 70 75 spin-down state is dominant at the Fermi energy, demonstrating metallic behavior of this nanostructure The magnetic contributions of individual subshells from PW91 are found to agree excellently with the PBE data, as shown in Table II In this first structure, we have successfully attached C60 on top of Cr in the graphene–Cr complex, and the resulting nanostructure is seen to be very stable (when compared with the previously studied graphene–Cr–graphene intercalation nanostructures14) Also, from the spin-polarized analysis, it is found that G(C54)–Cr–C60 exhibits a weak ferromagnetic moment In the experimental synthesis of such nanostructures, another possibility should be considered that C60 can attach directly to the graphene surface via van der Waals interaction,38 while Cr may accidentally bind to C60 without establishing any coordination interactions with graphene Therefore, a testing case is performed, in which we consider a direct decoration of graphene with the C60–Cr complex (also illustrated in Fig 2(a)) At convergence of geometry optimization, Cr does not occupy the H site on C60; in fact, it prefers to bind to the B site of two C The honeycomb unit in C60 is not superposed with that in graphene like the previous case (G(C54)–Cr–C60) According to our binding energy calculation, the new structure (G(C54)–C60–Cr) is stable with a positive binding energy of 1.40 eV; however, this is still lower than the binding energy of G(C54)–Cr–C60 by 54% Interestingly, a strong ferromagnetic moment is observed (4.54 µB/cell) In this paper, we not perform further analysis of magnetism for this less stable structure The distribution density of the Cr–C60 group on graphene may vary depending upon the experimental conditions; therefore, we believe that it is beneficial to explore the configurational and structural stability of a less-densed G–Cr–C60 nanostructure IV.2 The G(C150)–Cr–C60 nanostructure In this low-densed G–Cr–C60 nanostructure, the graphene sheet has one Cr–C60 unit per 150 C atoms in the two-dimensional unit cell, which means that the distribution density of the Cr–C60 This journal is © The Royal Society of Chemistry [year] Physical Chemistry Chemical Physics Accepted Manuscript DOI: 10.1039/C3CP53529K Page of 10 Physical Chemistry Chemical Physics View Article Online 10 Published on 01 October 2013 Downloaded by Rensselaer Polytechnic Institute on 01/10/2013 19:03:10 15 20 Fig Decoration of the (C60–Cr–graphene) complex with a tetrahedral metal cluster B1 is the bond between two inner metal atoms, while B2 is the bond between the vertex and a 2nd-layer metal atoms 25 30 35 40 Fig Spin-polarized PDOS of the Ni4–(C60–Cr–graphene) complex and PDOS of Ni4 3d and C60 2pz subshells From the plot, all 3d orbitals are shown to be ferromagnetic, while 2pz of C60 is weakly anti-ferromagnetic Table III Metal–C, metal–metal distances and binding energies in X4–C60–Cr–G(C54) (X = Ni/Pd/Pt) Distances (Å) X Binding energy (eV) X–C B 1* B 2* Ni 2.01 2.38 2.32 3.57 Pd 2.20 2.74 2.67 2.38 Pt 2.14 2.72 2.65 4.35 * See Fig cluster is lower than that in the former case The C60–Cr cluster is similarly attached to the central H site of the graphene monolayer (Fig 1) The geometry optimization of G(C150)–Cr–C60, however, is computationally too demanding Therefore, to achieve convergence within a reasonable computational time, we lower the energy and gradient convergence thresholds to 10-5 eV and 10-4 eV/Å, respectively 45 50 55 60 65 This journal is © The Royal Society of Chemistry [year] In the converged geometry, the average equilibrium Cr– C(graphene) bond length is 2.15 Å, which is almost the same as that in G(C54)–Cr–C60 The bond length between Cr and C in C60 (2.21 Å) is also observed to be very similar to that of the previous nanostructure (2.20 Å) When we only consider the z direction, the Cr–C60 separation is found to be 1.68 Å, while the Cr– graphene separation is approximately 1.61 Å In general, even when we extend the hosting graphene sheet (or in other words, we reduce the distribution density of Cr–C60), the key geometrical features are similar in the two nanostructures The theoretical calculations of total energies for G(C150)–Cr– C60 at Γ point produce a binding energy of 2.10 eV according to equation (1) As a consequence, we conclude such a nanostructure to be less energetically stable than the previously investigated structure (G(C54)–Cr–C60) Nevertheless, the magnitude of binding energy is still relatively large, which suggests good structural stability IV.3 Attachments of a metal cluster on G(C54)–Cr–C60 The decoration of a graphene monolayer surface with C60–Cr complexes may offer many promising applications, especially in chemical catalysis The role of G–Cr–C60 structures in chemical reactions remains uncertain, and we not seek for a definite conclusion in this paper However, such structures are capable of supporting one or multiple nanoscaled metal clusters, which might be active enough to catalyze chemical processes, especially organic reactions.39-42 In this study, we demonstrate the possibility of attaching nano-clusters of three active metals in the Nickel group (group 10), i.e., Ni, Pd, and Pt Owing to the high computational demand, the G(C54)–Cr–C60 complex will be decorated with only one metal cluster (of Ni/Pd/Pt) There have been a number of first-principles modeling studies of Ni, Pd, or Pt nanoparticles Ni nanoparticles were found to assist the initial growth of carbon nanotubes.43 In a previous study reported by Kacprzak and co-workers,44 γ-alumina was decorated with Pd9 clusters Nanoscaled Pt, an active catalytic material, was intensively investigated by Kumar and Kawazoe using first-principles modeling methods.45 There are, in addition, two other studies in which spin-orbit coupling was taken into account by non-collinear calculations to describe spin polarization more accurately.46, 47 In this study, three metal clusters of reasonably small size, Ni4, Pd4, and Pt4, are chosen for illustration purposes From our preliminary DFT calculations, it is suggested that Ni4 and Pd4 structures favorably adopt the tetrahedral configuration rather than the planar rhombus structure, while Pt4 favorably adopts the rhombus configuration However, when binding to C60, the rhombus structure of Pt4 is distorted, and it is then converted to tetrahedral Hence, we will consider the attachment of tetrahedral Ni4/Pd4/Pt4 on the C60–Cr–G complex Geometry optimization is performed to explore binding stability and electronic/magnetic properties It should be noticed that during the optimizations of metal cluster–(C60–Cr–G) complexes, we neglect the change in unit cell to simplify the optimization and reduce the computational cost Interestingly, we learn from the optimized result on Ni4–(C60–Cr–G) that three Ni atoms have direct interactions with C60 (in the C60–Cr–G complex, as shown in Fig 4) More specifically, each of the three Ni atoms at the Ni4/C60 interfacial layer (i.e., bottom layer of the tetrahedron) favorably assumes the B positions (described in Fig Journal Name, [year], [vol], 00–00 | Physical Chemistry Chemical Physics Accepted Manuscript DOI: 10.1039/C3CP53529K Physical Chemistry Chemical Physics Page of 10 View Article Online Table V Orbital contributions to the total magnetization from C60 2pz and a metal cluster (Ni4/Pd4) C60 Metal cluster d x −y dzx dz 2pz s (dzy) (dxy) Ni4 (4s and 3d) on -0.19 0.03 0.68 0.56 0.31 (C60–Cr–G) Pd4 (5s and 4d) on -0.04 0.07 0.71 0.21 0.19 (C60–Cr–G) 2 Published on 01 October 2013 Downloaded by Rensselaer Polytechnic Institute on 01/10/2013 19:03:10 15 20 The binding energy of Ni4–(C60–Cr–G) structure is then examined in order to evaluate its theoretical stability From earlier calculations in this study, it is expected that the C60–Cr–G complex is energetically stable and can be synthesized experimentally Therefore, we will systematically use the total energy of the whole C60–Cr–G base structure for the calculations of binding energy, which is expressed as follows: Ebinding = EC60 −Cr −G + Emetal − Emetal −(C60 −Cr −G ) (4) where EC −Cr −G represents the total energy of the C60–Cr–G base 60 structure, Emetal is the total energy of the investigated metal cluster (Ni4 in this case), and E metal −( C −Cr −G ) is the total energy of 60 25 30 Fig Spin-polarized DOS of the Pd4–(C60–Cr–G) complex and PDOS of Pd4 4d and C60 2pz subshells The latter shows that all 4d orbitals align ferromagnetically, while 2pz of C60 contributes a small anti-ferromagnetic moment 35 40 the produced structure The binding energy of Ni4–(C60–Cr–G) is estimated to be 3.57 eV, which indicates that the complex is energetically stable The total and absolute magnetizations for Ni4–(C60–Cr–G) are computed as 2.82 and 3.85 µB/cell, respectively It is Ni4 that significantly increases these magnetization values for the complex Recall that from the base structure optimization of G(C54)–Cr–C60, the total magnetization is only 0.55 µB/cell From spin-polarized calculations, the Ni4 cluster aligns ferromagnetically, while C60 behaves as a weak antiferromagnetic residue (as shown in Fig 5) Also, it is observed that Cr and graphene have little influence on the overall spin polarization For convenience, we summarize the ferromagnetic/anti-ferromagnetic moments of graphene, Cr, C60, and Ni4 (plus Pd4 and Pt4) and show the details in Table IV The spin polarizations of Ni4 3d, 4s and C60 2pz are carefully examined, and the main orbital contributions to total magnetization are shown in Table V The 4s shell of Ni4 only contributes little (0.03 µB/cell) to the ferromagnetism, while the 3d shells (especially 3d ) account for the majority of the overall z 45 ferromagnetism As shown in detail in Table V, all orbital contributions of 3d subshells align ferromagnetically More specifically, the polarization of 3d contributes 0.68 µB/cell z (27.9% of the 3d polarization), while 3dzx and 3dzy are observed to make identical contributions of 0.31 µB/cell (12.9% of the 3d polarization), and 3d 2 and 3dxy exhibit similar contributions x −y 50 Fig Spin-polarized DOS of the Pt4–(C60–Cr–G) complex It is shown that there is no spin polarization in Pt4 10 1) and coordinates to two C atoms in the honeycomb ring Closer inspection shows that the Ni–C bond length is approximately 2.01 Å, which is shorter than the previous Cr–C bond (2.20 Å) The bond between two Ni atoms at the Ni4/C60 boundary (B1 as indicated in Fig 4) is somewhat affected by the interaction with C60, and the bond length is consequently 2.38 Å When there is no effect from C60, the distance between the vertex Ni and a bottom-layer Ni (B2) is slightly more compressed (2.32 Å) In Table III, we summarize the calculated bonding distances for all complexes in which three metal clusters (Ni4, Pd4, and Pt4) are individually attached on C60–Cr–G | Journal Name, [year], [vol], 00–00 55 of 0.56 µB/cell (23.2% of the 3d polarization) Orbital contributions to the total magnetization from Ni4 and C60 can be seen also from Fig 5, in which the PDOS diagrams of all 3d subshells of Ni4 and 2pz of C60 are shown When Pd4 is supported by C60–Cr–G, the structural configuration shares many similarities to the previously investigated Ni4-decorated structure as shown in Fig In fact, our calculations show that the Pd–C bond is 2.20 Å, which is longer than the Ni–C bond in the previous structure by 9.45% Table IV Partial magnetizations (µB) of graphene, Cr, C60, and metal cluster (Ni4/Pd4/Pt4) Metal Total Graphene Cr C60 cluster Ni4 on 2.82 -0.07 0.69 -0.22 2.42 (C60–Cr–G) Pd4 on 2.20 -0.07 0.76 -0.04 1.56 (C60–Cr–G) Pt4 on 0.58 -0.07 0.77 -0.12 0.00 (C60–Cr–G) This journal is © The Royal Society of Chemistry [year] Physical Chemistry Chemical Physics Accepted Manuscript DOI: 10.1039/C3CP53529K Page of 10 Physical Chemistry Chemical Physics View Article Online 20 Published on 01 October 2013 Downloaded by Rensselaer Polytechnic Institute on 01/10/2013 19:03:10 25 30 35 cluster, which gives rise to the major ferromagnetism exhibited by the whole structure In addition, we also observe a small ferromagnetic-moment contribution from Cr Graphene and C60 unsurprisingly align anti-ferromagnetically; however, such opposing behavior is not significant enough to surpass the ferromagnetism of the overall structure The ferromagnetic/antiferromagnetic moments of graphene, Cr, C60, and Pd4 are listed in Table IV We then examine the orbital-resolved spin polarization of the 4d and 5s shells of Pd4 as well as the 2pz orbital of C60 The main orbital contributions to total magnetization from Pd 4d, 5s and C60 2pz are shown in Table V In the case of Pd4, it is recognized that the ferromagnetic moment resulting from 5s (0.07 µB/cell) is somewhat higher than that from the 4s orbital of Ni4 (previously reported as 0.03 µB/cell), but it is still considerably small The PDOS of 4d 2 is very similar to that of 4dxy, while the PDOS x −y of 4dzx and 4dzy are almost identical In the 4d shells of Pd4, we observe some major distinctions from the 3d shells of Ni4 More specifically, the contributions from d 2 (dxy) and dzx (dzy) are x −y Fig Spin-polarized PDOS of one M–C pair (M = Ni/Pd/Pt) 40 relatively small when compared to those for Ni4 In fact, the ferromagnetic contributions from d 2 (dxy) and dzx (dzy) are 0.21 x −y µB/cell (13.8 % of the 4d polarization) and 0.19 µB/cell (12.6%), respectively, while the highest and dominant contribution comes from d , which is 0.71 µB/cell (47.1%) For illustration z 45 50 55 Fig Electron density distribution on the plane containing the Ni–C/Pd–C/Pt–C bond 10 15 The bond between two Pd atoms at the the Pd4/C60 boundary (labeled B1 in Fig 4) is 2.74 Å, which is significantly longer than the corresponding Ni–Ni bond (2.38 Å) by 15.13% Likewise, the B2 distance in the Pd4 complex (2.67 Å) is also longer than the corresponding Ni–Ni distance (2.32 Å) Subsequently, the binding energy is calculated using equation (4) to evaluate the theoretical stability of this structure The binding energy is calculated as 2.38 eV, which reveals that the decoration of C60– Cr–G with Pd4 results in a complex less stable than the Ni4decorated complex Still, this binding energy is relatively large, which indicates good stability of Pd4–(C60–Cr–G) The total and absolute magnetizations computed for this nanostructure are 2.20 and 2.90 µB/cell, respectively Compared to the previous case of Ni4, the structure resulting from Pd4 attachment on C60 tends to exhibit smaller spin polarization and therefore produces a lower total magnetic moment with ferromagnetic alignment From the DOS analysis of spin polarization, it can be obviously seen that there is a large difference between spin-up and spin-down states of the Pd4 This journal is © The Royal Society of Chemistry [year] 60 65 70 75 purposes, the plot of PDOS of all 4d subshells of Pd4 and 2pz of C60 is shown in Fig The optimized structure of Pt4–C60–Cr–G is observed to be similar to that of Ni4–C60–Cr–G and Pd4–C60–Cr–G in the previous cases From our DFT calculations, the Pt–C bond distance is 2.14 Å The bond distance between two bottom-layer Pt atoms (B1 in Fig 4) is 2.72 Å, which is close to the Pd–Pd bond distance in the previous case The B2 bond distance is 2.65 Å, which is slightly shorter than the corresponding Pd–Pd bond (2.67 Å) The binding energy is calculated as 4.35 eV, and we can thus conclude that Pt4–(C60–Cr–G) is the most stable nanostructure of the three examined cases Inspection of the total and absolute magnetizations reveals that the Pt4 is a weakly ferromagnetic material (0.58 and 1.12 µB/cell, respectively) Compared to the previous two cases, this structure exhibits a smaller degree of spin polarization with ferromagnetic alignment As shown in Fig 7, the spin-up and spin-down DOS of the Pt4 cluster (we also examine the spin-up and spin-down states of each Pt atom) virtually coincide, indicating that this cluster is purely non-magnetic As mentioned earlier, the total and absolute magnetizations of the G(C54)–Cr–C60 base structure are 0.55 and 1.08 µB/cell, respectively Therefore, we can conclude from our DFT calculations on Pt4–(C60–Cr–G) that the ferromagnetism comes mainly from the base structure (G(C54)– Cr–C60), rather than from the metal cluster (Pt4) In the case of the Pt4 cluster, it is important to perform noncollinear calculations with spin-orbit coupling to account for the relativistic effect due to 6s-5d hybridization Such calculations were previously reported in two theoretical investigations of Ptn nanoclusters.46, 47 In those studies, tetrahedral Pt4 itself was shown to be antiferromagnetic with a total magnetic moment of 2.71 µB Our benchmark calculations for Pt4 show that the antiferromagnetic moment is 2.52 µB, which is in good agreement with the previous result However, for the large-sized Pt4–(C60– Journal Name, [year], [vol], 00–00 | Physical Chemistry Chemical Physics Accepted Manuscript DOI: 10.1039/C3CP53529K Physical Chemistry Chemical Physics Page of 10 View Article Online Published on 01 October 2013 Downloaded by Rensselaer Polytechnic Institute on 01/10/2013 19:03:10 10 15 20 25 30 Cr–G), the non-collinear calculations with spin-orbit interactions become prohibitive Therefore, we instead choose to examine the Pt4–C60 structure in order to verify the reported magnetic property Interestingly enough, when C60 is decorated with tetrahedral Pt4, the non-collinear calculations with spin-orbit coupling once again indicate that Pt4–C60 is non-magnetic Such observations are sufficient to verify our previous calculations, and we conclude that the weak ferromagnetic moment of Pt4– (C60–Cr–G) mainly originates from Cr, while the Pt4–C60 complex is observed to be non-magnetic A validation check is also made between PBE and PW91 calculations for the Pt4 complex From the PW91 data, we can conclude that the difference in geometric configuration is small, and we also observe a weak magnetic moment (0.58 µB/cell) given by Pt4–(C60–Cr–G) The binding energy given by PW91 calculations suggests that Pt4–(C60–Cr–G) is very stable with a positive energy of 4.81 eV, which is 10.5% higher than the PBEpredicted binding energy At this point, we have observed from binding energy calculations that Ni4–(C60–Cr–G) is more stable than Pd4–(C60– Cr–G) by 1.19 eV (3.57 eV versus 2.38 eV), while the total magnetic moment of Ni4–(C60–Cr–G) is larger than that of Pd4– (C60–Cr–G) by 0.62 µB Therefore, we believe that there is an interplay between binding energies and ferromagnetic moments when C60, which is supported by Cr–G, is decorated with Ni4 and Pd4 The decoration of Pt4, on the other hand, exhibits weak ferromagnetism (in fact, spin-polarization calculations indicate that Pt4 is non-magnetic) while its binding energy is surprisingly the largest of the three investigated cases In order to clarify this unexpected observation, we carefully examine the PDOS plots of three C–M pairs (with M being Ni, Pd, or Pt) as shown in Fig We can see that in the unpolarized Pt–C case, both spin-up and spin-down states of Pt 5d and C Fig 10 H–H bond cleavage on Ni4–(C60–Cr–G) The activation energy for this reaction is estimated as 0.76 eV The pure dissociation of H–H bond requires about 4.52 eV 60 65 70 z 35 2pz significantly overlap, which implies that there is a strong donor-acceptor interaction, or a stable coordination bond A comparison shows that DOS distributions in the Ni and Pd cases are very similar to each other (Fig 8) However, there is a distinction because the 3d 2 subshell of Ni tends to receive 75 x −y 40 45 50 greater electron density from C 2pz Indeed, this behavior would cause not only an increase in spin polarization but also formation of a stronger bond between Ni and C Another piece of evidence can be found that supports the bond-strength argument, when we inspect the electron density distribution on the plane that contains a metal-carbon bond (Fig 9) It is shown that the highest electron density is observed in the middle of a Pt–C bond, while the lowest electron density is observed in the middle of Pd–C, which is consistent with the previous binding energy observations In addition, there is a significant electron density around Ni due to the acceptance of many electrons from C60 This observation implies the strong overlap of Ni 3d–C 2pz, which causes strong ferromagnetism IV.4 Assessment of catalytic capability 55 As stated earlier, one goal of developing the C60–Cr–graphene base structure in this study is the use in chemical reaction catalysis In order to illustrate the catalytic capability of the proposed nanostructure, we investigate a dissociation reaction of H2 on Ni4–(C60–Cr–G(C54)) It is reported elsewhere that the cleavage of H–H bond requires as much as 104.2 kcal/mol (4.52 eV).48 In this | Journal Name, [year], [vol], 00–00 80 85 90 95 illustration, we will consider a simple mechanism in which the H2 molecule initially bonds to Ni (by a Ni–H bond), then another Ni attracts the remaining H atom and forms Ni–H interaction At the end, H–H completely dissociates, and two H atoms bind to different sites of the Ni4 cluster Because of the high structural complexity (119-atom catalyst complex with an addition of H atoms), we only perform Γ calculations to examine the potential energy profile The initial reactant structure of (H2–Ni4 complex) and the final product structure (in which each H is connected to one Ni) are first optimized, and the nudged-elastic-band (NEB) numerical method49 is employed to locate the transition state As shown in Fig 10, the final product is less stable than the initial structure (H2–Ni4 complex) by 0.73 eV The transition state is 0.03-eV higher in energy than the final product, which means that it requires about 0.76 eV to break the H–H bond and the presence of Ni4 in this case assists in lowering the original reaction barrier Therefore, we believe that the nanostructures developed in this work are potentially capable of activating chemical reactions V Summary In this study, we present a first-principles modeling study of a new class of two-dimensional materials, in which buckminsterfullerene (C60) is attached to the surface of graphene via Cr coordination bonds We investigate two nanostructures, G(C54)–Cr–C60 (more densed) and G(C150)–Cr–C60 (less densed), which differ in the distribution ratio of the Cr–C60 complex on the surface, as shown in Fig Sharing many similarities in geometry and having relatively high binding energies, the two investigated structures are both predicted to be energetically stable Spin polarization analysis shows that G(C54)–Cr–C60 is weakly ferromagnetic, to which the 3d shells of Cr make a dominant magnetic contribution The 2pz spins of C from both graphene and C60, on the other hand, align anti-ferromagnetically (contributing negative magnetic moments) Following the successful optimization of graphene–Cr–C60 structures, we then examine the potential decoration of C60 with small metal clusters, which might confer catalytic activity to the This journal is © The Royal Society of Chemistry [year] Physical Chemistry Chemical Physics Accepted Manuscript DOI: 10.1039/C3CP53529K Page of 10 Physical Chemistry Chemical Physics View Article Online DOI: 10.1039/C3CP53529K 10 15 z 20 25 30 35 40 45 50 coordination bonds due to interactions between both spin-up and spin-down states To deliver more evidence to confirm the high stability of Pt4 attachment, we perform electron density analysis on the planes containing the Ni–C, Pd–C, or Pt–C bonds Our theoretical data (illustrated in Fig 9) actually imply that the Pt–C bond is stronger than the other ones, while the Pd–C bond is the weakest Nevertheless, all calculated binding energies for the three investigated cases are still relatively large, which leads us to believe that the suggested complexes are thermodynamically stable The Ni4 and Pd4 clusters contribute strong ferromagnetic moments to the resultant complexes Classified as a moderately stable structure, Ni4–(G(C150)–Cr–C60) exhibits the strongest ferromagnetism, while the computed total magnetic moment of Pd4–(G(C150)–Cr–C60) is lower than that in the Ni case Overall, we find that the 3d shells of Ni4 as well as the 4d shells of Pd4 strongly contribute to the magnetism On the other hand, Pt4 is observed to be non-magnetic (from calculations with/without spin-orbit coupling), and the total magnetic moment of Pt4– (G(C150)–Cr–C60) is found to be very similar to that of the G(C150)–Cr–C60 base structure; thus, the magnetism is produced mainly from the spin polarization of the Cr 3d shells To validate the theoretical prediction given by PBE calculations, we additionally perform PW91 calculations for C60–Cr–G(C54) and its decoration with Pt4 Overall, very good consistency is observed, and we conclude that PBE calculations are reliable In experimental synthesis, a metal cluster may be employed to steady C60 on the graphene surface rather than a single bridging transition-metal atom (Cr) considered in this study Such nanostructures may have good structural stability, and we believe that it is beneficial to extend further investigations for such complexes in the future An illustrative example is presented when we employ Ni4– (C60–Cr–G) to activate H2 dissociation According to the reaction barrier estimation, it only requires 0.76 eV to break the stable H– H bond, which indicates potential applications of such nanostructures in chemical catalysis 60 65 We thank Vietnam National University in Ho Chi Minh City (VNU-HCM) for their support in this work, and also acknowledge supercomputing support from the Institute for Materials Research, Tohoku University, Japan during the course of this research H.H thanks a Nanyang Assistant Professorship This journal is © The Royal Society of Chemistry [year] Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371, Singapore E-mail: hung.m.le@hotmail.com Faculty of Materials Science, University of Science, Vietnam National University, Ho Chi Minh City, Vietnam New Industry Creation Hatchery Centre, Tohoku University, 6-6-4, Aramaki, Aoba, Sendai, 980-8579, Japan Theory and Modeling Department, Culham Centre for Fusion Energy, United Kingdom Atomic Energy Authority, Abingdon, OX14 3DB, UK 70 75 80 85 90 10 11 12 13 14 95 15 100 16 17 18 19 105 20 21 22 23 110 24 25 Acknowledgement 55 Notes and references 26 115 27 R Prasher, Science, 2010, 328, 185-186 A K Geim and K S Novoselov, Nat Mater., 2007, 6, 183-191 S M Avdoshenko, I N Ioffe, G Cuniberti, L Dunsch and A A Popov, ACS Nano, 2011, 5, 9939-9949 J Maassen, W Ji and H Guo, Nano Lett., 2010, 11, 151-155 M Weser, Y Rehder, K Horn, M Sicot, M Fonin, A B Preobrajenski, E N Voloshina, E Goering and Y S Dedkov, Appl Phys Lett., 2010, 96, 012504 R Muszynski, B Seger and P V Kamat, J Phys Chem C, 2008, 112, 5263-5266 W Hong, H Bai, Y Xu, Z Yao, Z Gu and G Shi, J Phys Chem C, 2010, 114, 1822-1826 R Zan, U Bangert, Q Ramasse and K S Novoselov, Nano Lett., 2011, 11, 1087-1092 Y Hajati, T Blom, S H M Jafri, S Haldar, S Bhandary, M Z Shoushtari, O Eriksson, B Sanyal and K Leifer, Nanotechnology, 2012, 23, 505501 J Wintterlin and M L Bocquet, Surf Sci., 2009, 603, 1841-1852 K Nakada and A Ishii, Solid State Commun., 2011, 151, 13-16 G Giovannetti, P A Khomyakov, G Brocks, V M Karpan, J van den Brink and P J Kelly, Phys Rev Lett., 2008, 101, 026803 R A Jishi, D M Guzman and H M Alyahyaei, Adv Stud Theor Phys., 2011, 5, 703-716 V Q Bui, H M Le, Y Kawazoe and D Nguyen-Manh, J Phys Chem C, 2013, 117, 3605-3614 H.-Y Wu, X Fan, J.-L Kuo and W.-Q Deng, J Phys Chem C, 2011, 115, 9241-9249 P Hohenberg and W Kohn, Phys Rev., 1964, 136, B864-B871 W Kohn and L J Sham, Phys Rev., 1965, 140, A1133-A1138 J P Perdew, A Ruzsinszky, J Tao, V N Staroverov, G E Scuseria and G I Csonka, J Chem Phys., 2005, 123, 062201 H W Kroto, J R Heath, S C O'Brien, R F Curl and R E Smalley, Nature, 1985, 318, 162-163 H W Kroto, Nature, 1987, 329, 529-531 P R Birkett, A J Cheetham, B R Eggen, J P Hare, H W Kroto and D R M Walton, Chem Phys Lett., 1997, 281, 111-114 T Braun, M Wohlers, T Belz, G Nowitzke, G Wortmann, Y Uchida, N Pfänder and R Schlögl, Catal Lett., 1997, 43, 167-173 T Braun, M Wohlers, T Belz and R Schlögl, Catal Lett., 1997, 43, 175-180 J Niemeyer and G Erker, ChemCatChem, 2010, 2, 499-500 S Duffe, N Gronhagen, L Patryarcha, B Sieben, C Yin, B von Issendorff, M Moseler and H Hovel, Nat Nano, 2010, 5, 335-339 S Sarkar, S Niyogi, E Bekyarova and R C Haddon, Chem Sci., 2011, 2, 1326-1333 D V Konarev, S S Khasanov, A Y Kovalevsky, G Saito, A Otsuka and R N Lyubovskaya, Dalton Trans., 2006, 3716-3720 Journal Name, [year], [vol], 00–00 | Physical Chemistry Chemical Physics Accepted Manuscript Published on 01 October 2013 Downloaded by Rensselaer Polytechnic Institute on 01/10/2013 19:03:10 complex The metal clusters of interest in this study are tetrahedral Ni4, Pd4, and Pt4 At convergence of optimizations, three metal atoms from the cluster (Ni4/Pd4/Pt4) coordinatively bind to a honeycomb ring in C60 as shown in Fig The binding energy of each nanostructure is then evaluated to examine the theoretical stability of the complex Our DFT calculations show that the attachment of Ni4 and Pt4 is more favorable than the attachment of Pd4 to G(C54)–Cr–C60 This result is consistent with a previous study showing that Pd has a tendency to interact weakly with a graphene surface in comparison to atomic Ni and Pt.11 For the Ni4 decoration, the strong binding energy is more correlated with the strong magnetic behavior of Ni 3d orbitals In the Pt4 case, even though the whole complex is non-magnetic, we observe the highest binding energy of the three cases, and accordingly conclude that the hybridization effect between 2pz (from C60) and 5d (from Pt) is very effective in forming Physical Chemistry Chemical Physics Page 10 of 10 View Article Online DOI: 10.1039/C3CP53529K 10 15 20 25 30 35 40 45 10 | Journal Name, [year], [vol], 00–00 Physical Chemistry Chemical Physics Accepted Manuscript Published on 01 October 2013 Downloaded by Rensselaer Polytechnic Institute on 01/10/2013 19:03:10 28 J P Perdew, K Burke and M Ernzerhof, Phys Rev Lett., 1996, 77, 3865-3868 29 J P Perdew, K Burke and M Ernzerhof, Phys Rev Lett., 1997, 78, 1396-1396 30 P Giannozzi, S Baroni, N Bonini, M Calandra, R Car, C Cavazzoni, D Ceresoli, G L Chiarotti, M Cococcioni, I Dabo, A D Corso, S d Gironcoli, S Fabris, G Fratesi, R Gebauer, U Gerstmann, C Gougoussis, A Kokalj, M Lazzeri, L MartinSamos, N Marzari, F Mauri, R Mazzarello, S Paolini, A Pasquarello, L Paulatto, C Sbraccia, S Scandolo, G Sclauzero, A P Seitsonen, A Smogunov, P Umari and R M Wentzcovitch, J Phys Condens Matter, 2009, 21, 395502 31 D Vanderbilt, Phys Rev B, 1990, 41, 7892-7895 32 A Dal Corso, Phys Rev B, 2001, 64, 235118 33 S Grimme, J Comput Chem., 2006, 27, 1787-1799 34 V Barone, M Casarin, D Forrer, M Pavone, M Sambi and A Vittadini, J Comput Chem., 2009, 30, 934-939 35 D F Shanno, J Optim Theory Appl., 1985, 46, 87-94 36 J P Perdew, J A Chevary, S H Vosko, K A Jackson, M R Pederson, D J Singh and C Fiolhais, Phys Rev B, 1992, 46, 66716687 37 J P Perdew, J A Chevary, S H Vosko, K A Jackson, M R Pederson, D J Singh and C Fiolhais, Phys Rev B, 1993, 48, 49784978 38 C S Liu, G Kumar and V K Tripathi, J Appl Phys., 2006, 100, 013304 39 H.-Y Tuan, D C Lee, T Hanrath and B A Korgel, Nano Lett., 2005, 5, 681-684 40 L Wu, J Ling and Z.-Q Wu, Adv Synth Catal., 2011, 353, 14521456 41 T.-L Ho, Fiesers' Reagents for Organic Synthesis, John Wiley & Sons, Inc., Hoboken, NJ, 2013 42 L Hu, X Cao, L Shi, F Qi, Z Guo, J Lu and H Gu, Org Lett., 2011, 13, 5640-5643 43 Y.-H Shin and S Hong, Appl Phys Lett., 2008, 92, 043103 44 K A Kacprzak, I Czekaj and J Mantzaras, Phys Chem Chem Phys., 2012, 14, 10243-10247 45 V Kumar and Y Kawazoe, Phys Rev B, 2008, 77, 205418 46 P Blonski, S Dennler and J Hafner, J Chem Phys., 2011, 134, 034107 47 H K Yuan, H Chen, A L Kuang and B Wu, J Magn Magn Mater., 2013, 331, 7-16 48 G Herzberg and A Monfils, J Mol Spectrosc., 1961, 5, 482-498 49 D Sheppard, R Terrell and G Henkelman, J Chem Phys., 2008, 128, 134106 This journal is © The Royal Society of Chemistry [year] ... dissociates, and two H atoms bind to different sites of the Ni4 cluster Because of the high structural complexity (119-atom catalyst complex with an addition of H atoms), we only perform Γ calculations... is located at It can be obviously seen that the ferromagnetism is mainly caused by the spin polarization of Cr 10 15 20 25 numbers of spin-up and spin-down states at a certain energy level For. .. candidate for supporting metal nanoparticles and may be employed in the future design and development of nanocatalysts To examine this feature more realistically, we test the possibility of decorating