phys stat sol (b) 243, No 13, 3472 – 3475 (2006) / DOI 10.1002/pssb.200669166 Structural and electronic properties of Ptn (n = 3, 7, 13) clusters on metallic single wall carbon nanotube Nguyen Thanh Cuong1, Dam Hieu Chi*, 2, 3, Yong-Tae Kim1, and Tadaoki Mitani1 Materials Science School, Japan Advanced Institute of Science and Technology, 1-1, Asahidai, Tatsunokuchi, Ishikawa, Japan Center for Strategic Development of Science and Technology, Japan Advanced Institute of Science and Technology, 1-1, Asahidai, Tatsunokuchi, Ishikawa, Japan Faculty of Physics, Hanoi University of Science, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam Received 20 April 2006, revised 17 August 2006, accepted 18 August 2006 Published online 11 October 2006 PACS 61.46.Bc, 61.46.Fg, 71.15.Mb, 73.22.–f A systematic study of Ptn (n = 3, 5, 7) clusters adsorbed on the metallic (5, 5) single wall carbon nanotube was carried out using theoretical calculations within Density Functional Theory The geometrical and electronic structure and interaction between the Pt clusters and the single wall carbon nanotube were investigated The bridge adsorption sites on the outer wall of the carbon nanotube are found favorable for Pt atom We found that the average C – Pt and Pt – Pt bond length, binding energy, and the amount of charge transfers from the Pt cluster toward the nanotube increase with the size of cluster The calculated densityof-states suggest a mixing of ionic and covalent character for the binding nature of this system © 2006 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim Introduction Nowadays, the catalysis plays an innovative role in the development of new technologies and electrocatalyst designs, a key factor for enhancing catalysis performances, become big issues accompany the industrialization Nanotechnology is believed to be important in heterogeneous catalysis due to its peculiar properties and potential applications The carbon nanotube with beautiful tubular structure and a large effective surface which could facilitate the adsorption of small catalyst particles, has received much attention recently Several studies [1, 2] were carried out that deal with the applications of carbon nanotubes as supports for catalyst in fuel cell In these researches, Pt and Pt–Ru materials are found to be the best catalysts of fuel cell to enhance the cathode oxygen reduction reaction, and it is believed that dispersity and cluster size of catalyst particles mainly affect the properties of the electro-catalyst Therefore, many consequent studies [3, 4] on dispersion and size control of clusters on carbon nanotube supports for electrocatalysts have been conducted to investigate the effect of cluster size on electro-catalytic activity Recently, we have succeeded in establishing a new method based on a fundamental bottom-up approach to synthesize highly dispersed and size controlled Pt clusters on carbon nanotube supports, which is called atom-to-cluster (SAC) approach [5] The introduction of thiol groups to the surface of carbon nanotube resulted in the extreme single-atom dispersion, and a finite size control of clusters from the dispersed single atoms was archived by heating process In this paper, continuing the flow of our previous papers which reported experimental results, we report our first-principles study on the interaction between a Pt single atom and Ptn (n = 3, 7, 13) clusters, and a metallic (5, 5) single wall carbon nanotube (SWNT) We will focus on the adsorption of the Pt * Corresponding author: e-mail: dam@jaist.ac.jp © 2006 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim Original Paper phys stat sol (b) 243, No 13 (2006) 3473 clusters on the SWNT, particularly the charge transfer and the binding nature between Pt and SWNT, which could help us to better understand the experimental observations and could benefit the experimental study on catalysis Computational methods We performed periodic calculations based on the density functional theory (DFT) [6, 7] by employing a linear combination of localized pseudoatomic orbitals (LCPAO) method [8] For the exchange and correlation terms, the generalized gradient approximation (GGA) is used as described by Perdew, K Burke, and M Ernzerhof (PBE) [9] Double valence plus single polarization orbitals were used as a basis set with cutoff radii of 7.0 a.u and 4.5 a.u for Pt and C respectively [10] Troullier–Martins type pseudopotentials [11] with a partial core correction are used to replace the deep core partial by the normconserving soft potentials in a factorized separable form with multiple projectors proposed by Blochl Also the real space grid techniques are used with the energy cut-off of 150 Ry in numerical integrations and the solution of the Poisson equation using the Fast Fourier Transformations (FFT) technique All the DFT calculations were performed by using OpenMX code [12], which is designed for the realization of large-scale DFT calculations The infinite one-dimensional (5, 5) metallic single wall carbon nanotube is simulated We have applied the supercell with 25.0 Å for a lattice and 16.0 Å for b lattice, which are large enough to neglect the interaction between the nanotube and its periodic images The c lattice (17.07 Å) aligned with the axis of the nanotube is tuned to match the periodic condition The structure optimization was stopped when the forces due to displacements of an atom in the unit cell converged within 0.002 Ha/Å Results and discussion We carefully optimized the geometrical structures, from initial structures constructed by putting a Pt single atom and Ptn (n = 3, 7, 13) cluster on all the nonequivalent high symmetry sites of the outer wall of the (5, 5) SWNT The best adsorption configuration structures are presented in Fig When a Pt single atom adsorb on the SWNT, the bridge sites is most preferred The underlying C–C bonds are drawn upward slightly, and the lengths are elongated consequently Similarly, Pt3 and Pt7 adsorbed in the form in which the contact Pt atoms (to the SWNT) are located on the bridge sites In the case Pt13 cluster, there are Pt atoms have contact with outer wall of nanotube Two of these are stabilized at the bridge sites, while the other Pt atom is located at nearly the bridge sites with some stress due to the geometrical structure of Pt13 Consequently, the adsorption led to a slight deformation in the geometrical structure of Pt13 a c b d Fig (online colour at: www.pss-b.com) Optimized geometrical structures of the Pt atom/Pt clusters adsorbed on the (5, 5) SWNT Small grey balls represent carbon atoms, and small blue balls represent Pt atoms (a) Pt single atom, (b) Pt3, (c) Pt7, and (d) Pt13 clusters adsorbed on the (5, 5) SWNT www.pss-b.com © 2006 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim N T Cuong et al.: Structural and electronic properties of Ptn (n = 3, 7, 13) clusters 3474 Table Binding energies (eV) of the free Pt clusters and Pt clusters adsorbed on the (5, 5) SWNT and distances (Å) of the closest C – Pt and Pt – Pt in the clusters adsorbed on the (5, 5) SWNT Eb1 Eb2 dC–Pt dPt–Pt Pt3 Pt7 Pt13 2.17 2.94 2.17 2.57 2.99 2.99 2.18 2.57–3.42 3.32 4.81 2.2 2.61–3.91 For further understanding of the binding nature of the adsorption, we adopted formulas Eb1 = nEPt - EPt n ; n Eb2 = EPt n + ESWNT - EPt n /SWNT DOS (states/eV) to evaluate the cohesive energy of free Pt clusters, and the binding energy of the Pt clusters on the (5, 5) SWNT Here, EPt , EPt n, and ESWNT are the total energies for a free standing Pt single atom, Pt clusters, and a bare SWNT, respectively EPt n /SWNT is the total energy for the configuration with the Pt clusters adsorbed on the SWNT Eb1 are cohesive energies and Eb2 are binding energies computed with respect to isolated metal clusters Table shows the binding energies for the best adsorption sites for Ptn (n = 3, 7, 13) clusters One can see that the cohesive energies and binding energies increases with the size of Pt cluster On the other hand, the average bond length of C–Pt and Pt–Pt also increase with the size of Pt cluster, in consistent with our previous experimental observations [5] The binding energies of Pt clusters adsorbed on graphene sheet were also carried out for comparison, and revealed that the curvature of the tube has an obvious effect on the interaction between the Pt and the tube This fact raises the question of the nature of the interaction between Pt clusters and the SWNT One can postulate three possibilities for the binding nature in this system One is the ionic binding in which the Coulomb interaction plays the central role The second is the covalent binding in which strong covalent bonding between Pt and C is essential in the adsorption process, and the third is a mixing of ionic and covalent for the bonding character One can easily find 100 (a) Ef = –5.98 eV Ef = –5.05 eV 80 60 40 20 DOS (states/eV) 100 (b) Fig (online colour at: www.pss-b.com) Density of states for the isolated Pt13 cluster (a – blue line) and the bare (5, 5) SWNT (a – red line), and the projected DOS for the adsorbed Pt13 cluster (b – blue line) and the (5, 5) SWNT (b – red line) The red, blue, and black vertical dotted lines denote the Fermi levels for the bare (5, 5) SWNT, the isolated Pt13 cluster, and the adsorbed system, respectively Ef = –5.63 eV 80 60 40 20 -12 -10 -8 -6 -4 -2 Energy (eV) © 2006 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim www.pss-b.com Original Paper phys stat sol (b) 243, No 13 (2006) Table 3475 Total electron transfer from the Pt clusters to the (5, 5) SWNT total charge transfer Pt Pt3 Pt7 Pt13 0.53 0.81 1.23 1.86 that the binding energy is depending much on the number of C–Pt bonding, which suggests the covalence in the binding nature For clarifying, the binding nature in this system, we investigated the electronic structures of the Pt clusters adsorbed on the (5, 5) SWNT The density of state (DOS) for the adsorbed Pt clusters and the isolated metal clusters with the same structure were analyzed (Fig 2) The Mulliken charge analyses also were carried out for the evaluation of the total electron transfers from the Pt clusters toward the carbon nanotube (Table 2) The DOS of the adsorbed Pt13 cluster was slightly deformed from the DOS of the isolated Pt13 cluster, and shifted by ca 0.6 eV toward a lower energy region This shift can be explained as the increase in effective Coulomb potential due to the loss of charge These results not only confirm the tendency of charge transfer from the Pt clusters toward the carbon nanotube, but also suggest the ionicity in binding nature Finally, the deformation of the DOS of adsorbed clusters, compare with that of isolated clusters, strongly suggests an orbital mixing between metal cluster and carbon nanotube at near Fermi level From all of evidence, we can conclude that the binding between the Pt clusters and the SWNT in this system has both ionic and covalent characters Conclusion We performed a first-principles study on the interaction between a single Pt atom and Ptn (n = 3, 7, 13) clusters, and a metallic (5, 5) SWNT The best adsorption site of single Pt atom on the outer wall of SWNT is the bridge-type site and the curvature of the carbon nanotubes does affect the adsorption The binding energies between the Pt clusters and the SWNT increases with the size of Pt cluster Charge density analyses confirm a charge transfer from Pt clusters toward the carbon nanotube, and the amount of charge transfers increases linearly with the size of Pt cluster The calculated density-of-states suggest a mixing of ionic and covalent characters for the binding nature of the systems Acknowledgements This work has been partly supported by the 21st COE (Center of Excellence) Program “Study of Scientific Knowledge Creation” of JAIST, funded by Ministry of Education, Culture, Sports, Science and Technology (MEXT, Japan) and the HJK Computation for Materials Science project, funded by JAIST And one of us, N T Cuong, thanks Komatsu Seiren Co., Ltd for the financial support References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] E Antolini et al., Appl Catal B, Environ 63, 137 – 149 (2005) E Frackowiak et al., Electrochem Commun 8, 129 – 132 (2005) Zhibin He et al., Mater Chem Phys 85, 396 – 401 (2004) T Matsumoto et al., Catal Today 90, 277 – 281 (2004) Yong-Tae Kim, K Ohshima, K Higashimine, T Uruga, M Takata, H Suematsu, and T Mitani, Angew Chem Int Ed 45, 407 – 411 (2005) P Hohenberg and W Kohn, Phys Rev 136, B864 (1964) W Kohn and L J Sham, Phys Rev 140, A1133 (1965) T Ozaki, Phys Rev B 67, 155108 (2003) J P Perdew, K Burke, and M Ernzerhof, Phys Rev Lett 77, 3865 (1996) T Ozaki and H Kino, Phys Rev B 69, 195113 – 195113 (2004) N Troullier and J L Martine, Phys Rev B 43, 1993 (1991) DFT OpenMX code is available on the web site http://staff.aist.go.jp/t-ozaki/openmx/ www.pss-b.com © 2006 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim ... realization of large-scale DFT calculations The infinite one-dimensional (5, 5) metallic single wall carbon nanotube is simulated We have applied the supercell with 25.0 Å for a lattice and 16.0... clusters, and a metallic (5, 5) SWNT The best adsorption site of single Pt atom on the outer wall of SWNT is the bridge-type site and the curvature of the carbon nanotubes does affect the adsorption... structures constructed by putting a Pt single atom and Ptn (n = 3, 7, 13) cluster on all the nonequivalent high symmetry sites of the outer wall of the (5, 5) SWNT The best adsorption configuration structures