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Point-Defect Mediated Bonding of Pt Clusters on (5,5) Carbon Nanotubes Jian-guo Wang,*,†,‡ Yong-an Lv,†,‡ Xiao-nian Li,†,‡ and Mingdong Dong*,§ College of Chemical Engineering and Materials Science, Zhejiang UniVersity of Technology, Hangzhou 310032, People’s Republic of China, State Key Laboratory Breeding Base of Green-Chemical Synthesis Technology, Zhejiang UniVersity of Technology, Hangzhou 310032, People’s Republic of China, and The Rowland Institute at HarVard, HarVard UniVersity, Cambridge, Massachusetts 02142 ReceiVed: NoVember 3, 2008
The adhesion of various sizes of Pt clusters on the metallic (5,5) carbon nanotubes (CNTs) with and without the point defect has been investigated by means of density functional theory (DFT). The calculations show that the binding energies of Ptn (n ) 1-6) clusters on the defect free CNTs are more than 2.0 eV. However, the binding energies are increased more than three times on the point defective CNTs. The dramatic increase of the binding energy has been further explained by the partial density of states, deformation charge density, and two population analyses methods (Mulliken and Hirshfeld). The stronger orbital hybridization between the Pt atom and the carbon atom shows larger charge transfers on the defective CNTs than on the defect free CNTs, which allows the strong interaction between Pt clusters and CNTs. On the basis of DFT calculations, CNTs with point defect can be used as the catalyst supports for noble metal nanoparticles adhesion, which can be applied to a series of catalytic reactions, such as fuel cell, hydrogenation, etc. Introduction One-dimensional material carbon naotubes (CNTs) are attractive to a wide variety of research fields because of their unique optical, electrical, magnetic, and mechanical properties.1-11 The metal/CNTs heterostructures provide unique properties for many applications such as biosensors, nanodevices, and heterogeneous catalysis. Due to the mechanical stability and large surface area, CNTs have been regarded as a very promising catalyst support such as the reaction in a fuel cell.12-14 In fact, a lot of experimental evidence showed that noble/transition metals supported on a CNT perform better catalytic activity toward a series of reactions than those supported on the metal oxides.15-20 The dispersion of metal nanoparticles on CNTs is a vital factor in determining the catalytic activity of metal/CNTs systems. Two main approaches to produce the metals/CNTs heterostructures are either physical adhesion or chemical modifications.21,22 For the physical adhesion, the inert surface of perfect CNTs leads to weak adhesion and poor dispersion of metal particles. To produce the uniform dispersion of metal particles on CNTs surfaces, either CNTs or nanoparticle surface functionalization are required. In fact, some experimental results also showed that the high dispersion of metal particles can be prepared on these CNTs without surface functionalized. To gain further insight into the CNTs surface-mediated adhesion of novel metal nanoparticles, theoretical studies are needed. So far, there are a number of theoretical studies of small metal clusters supported on metal oxide support (such as TiO2, Al2O3, and MgO) which are helpful in investigating the interaction between * Corresponding author. E-mail:
[email protected] (J.-g.W.) and
[email protected] (M.D.). † College of Chemical Engineering and Materials Science, Zhejiang University of Technology. ‡ State Key Laboratory Breeding Base of Green-Chemical Synthesis Technology, Zhejiang University of Technology. § The Rowland Institute at Harvard, Harvard University.
metal particles and oxide supports, allowing the mechanism of related catalytic reactions to be understood. However, there are only a few density functional theory (DFT) calculations devoted to investigate the interaction between metal clusters and CNTs.20,23-26 In addition, most of the experimental and theoretical investigations have focused on defect free CNTs with prefect honeycomb carbon arrangements. However, the structural defects such as topological defects, vacancies, and chemical modification can substantially introduce new properties of CNTs, allowing CNTs for other related applications.27 Among the noble metals, Pt (or Pt alloy)/CNTs have been extensively used as catalytic materials in fuel cell reactions. However, experimental studies have not made a consensus mechanism about the adhesion properties of Pt nanoparticles on defective CNTs surface. In this study, the adhesion of various sizes of Ptn (n ) 1-4, or 1-6) clusters on the outer wall of metallic (5,5) CNTs with and without point defect has been investigated by means of DFT calculations, respectively. The theoretical calculations show that Ptn clusters have stronger binding to the point defect (5,5) CNTs when compared to defect free CNTs. The mechanism of the stronger Pt binding to the point defective CNTs can be further explained by the stronger Pt-C orbital hybridization and the charge transfers. Computational Section The first-principles DFT calculations were performed with the DMol3 module in Materials Studio.28,29 The generalized gradient approximation (GGA) with PW91 functional30 was used to describe the exchange-correlation (XC) effects. The double numerical plus polarization (DNP) basis set was used in the expanded electronic wave functional. In this study, the (5,5) CNT was used. The periodic supercell CNT was adopted with length of a, b, and c lattices of 25, 20, and 9.76 Å, which can avoid interactions among repeating slabs (SI, Figure S1), with eight k-points generated by the Monkhorst-Pack technique. The Ptn binding energy (EBE)is calculated as follows: EBE ) EPTn +
10.1021/jp810277b CCC: $40.75 2009 American Chemical Society Published on Web 12/23/2008
Bonding of Pt Clusters on (5, 5) Carbon Nanotubes
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Figure 1. Geometries of Ptn (n ) 1-6) on the outer-side wall of CNTs.
ECNT - E(Ptn+CNT), where EPTn, ECNT, and E(PTn+CNT) present the energies of the most stable gas phase Ptn cluster, the CNT, and the combined systems of Ptn and CNT, respectively. Results and Discussions 1. Geometrical Properties of Ptn Clusters on Defect Free CNTs. The optimized geometries of small Ptn (n ) 1-6) clusters adsorbed on the outer wall of (5,5) CNTs are shown in Figure 1. Two stable motifs (Pt1a, Pt1b) have been proposed in a single Pt monomer and CNT system, where Pt atom forms the bridges configuration with two neighboring carbon atoms. The binding energy of Pt1a and Pt1b is 2.30 and 2.17 eV, respectively. For Pt2/CNT motifs, the three most stable dimmer configurations on CNT are presented as Pt2a, Pt2b, and Pt2c in Figure 1, respectively. Both Pt atoms are on the atop site of CNT in Pt2a and Pt2b motifs, which are slightly more stable than Pt2c in which only one Pt is directly adsorbed on the bridge site (2.19 eV, 2.04 eV vs 1.85 eV). The binding of Pt dimmers to the CNTs is weaker than for the Pt1 system, which suggests the unstable dimmer configuration. In the Pt3 trimer system, one, two, and three of Pt atom(s) are directly contacted with the outer wall of CNTs considered in this study. We found that the Pt3a cluster with two Pt atoms located at the bridge sites of the CNT is about 0.64 or 0.92 eV more stable than the Pt3b or Pt3c motifs, respectively. Consistent with the Pt3 system, Pt4a and Pt4b tetrahedron clusters with three or two Pt atoms with direct binding to the outer wall of CNT have stronger binding to the Pt4c sheet configuration with four Pt atoms located on the atop site of the CNT. Pt5 and Pt6 polyhedron clusters are build on either three (triangle) or four (rhombus) Pt atoms directly contacting the CNTs. We always observed that Pt5a and Pt6a in which four Pt atoms in the first layer are slight more stable than another configuration in which three Pt atoms are directly bonded with CNTs. The Pt-C bond distance is about 2.08 or 2.15 Å depending on the atop or bridge sites. The Pt-C bonding is referring to the chemical adsorption. The Pt-Pt distance is strongly dependent on cluster size and shape, which varies from 2.50 to 3.22 Å. The binding energies of Ptn clusters on the defect
free CNTs surface are between 2.19 and 3.22 eV. For all Ptn systems, DFT calculations show consistent results, where the bridge between the Pt atom and the carbon atom in CNT in the Pt cluster proves stronger binding due to the stronger Pt-C chemical absorption involved. 2. Geometrical Properties of Pt Clusters on CNTs with the Point Defect. Structural defects are always existed to CNTs. One of structural defects of interest in CNTs is the vacancies. The vacancy defects may occur in the as-grown nanotubes, or they can be generated by several methods like chemical treatment or irradiation with charged particles.27 With a point (single atom) defect, we investigated several small clusters Ptn(n ) 1-4) on (5,5) CNTs. The Pt1 atom is located on the hollow site of CNT with Pt-C bond distances of 1.94, 1.94, and 2.03 Å, providing 7.38 eV of binding energy. For Ptn(n ) 2-4), one of the Pt atoms in the clusters always adsorbed on the hollow site as shown in Figure 2. The binding energy (Pt1c) is only 2.28 eV when Pt is located on the bridge site far from the point vacancy. For Pt2, the same conclusion was drawn. Pt2c, located on the atop site of carbon far from the point vacancy, is much less stable than Pt2a and Pt2b, in which one Pt atom is located at the hollow site of the point vacancy. For Pt3 triangular configurations, which all adsorbed at the point vacancy, the stability decreasing order is three, two, and one Pt atoms bonded with CNTs. For the polyhedron cluster, by changing the Pt4 clusters adsorbing side on to CNTs (Figure 2), the binding energy of the Pt4a tetrahedron cluster, in which three Pt atoms bonded with CNTs, is more stable by about 0.51 and 0.65 eV for Pt4b and Pt4c, in which only two or one Pt atom bonded with CNTs. By comparing with free defect CNTs, we found that the vacancy on defective CNTs surface is always the anchor site for Ptn clusters absorption. The point defect plays a great role for stabilizing the Pt clusters on CNTs surfaces. For instance for the same type cluster Pt4a, the binding energies of most stable Pt4a clusters on defective CNTs are three times more than the one in the defect free CNTs. For the Ptn(1-3) system, the defect can enhance also
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Figure 2. Geometries of Ptn on the outer-side wall of CNT with point defect.
more than three time stronger binding comparing to defect free CNT for Pt cluster binding. 3. Electronic Properties of Pt Clusters on CNTs. To gain insight into the bonding mechanism between Pt clusters and CNTs, partial density of states (DOS) of Pt1 and carbon atoms of CNTs without and with defect is shown in Figure 3, parts a and b, respectively. It clearly shows that the DOS of Pt on (5,5) defective CNTs moves downward compared with Pt1 on the perfect CNTs. Parts c and d of Figure 3 show the deformation charge differences of Pt1/CNTs and Pt1/ CNTs(va). It can be seen that the orbital hybridization of the Pt atom on the CNTs with vacancy is much stronger than the one on perfect CNTs. The deformation charge density also shows that more charge transfer from Pt clusters to defective CNTs is observed compared to the perfect CNT surface. In addition, two population analysis methods (Mulliken and Hirshfeld) have been further carried out in the Ptn(1-4) system both for defective free and point-defect CNTs. Charge transfers from Pt clusters to defective CNTs is more than 0.30 and 0.15 electron than to defective free CNTs based on Mulliken and Hirshfeld analysis (Figure 4). The quantitative population analysis further explains that the point defect CNTs introduces more electronic charge in the metal/CNTs systems allowing for the enhancement of the binding. Conclusions We have carried out systematic studies on the defect-mediated Pt cluster absorption on CNTs using DFT calculations. Although the binding energies of Pt clusters on defect free CNTs are all larger than 2.0 eV, by introducing point defect on the CNT surface, the theoretical results show the binding energies of Pt clusters on CNTs are increasing dramatically. The stronger binding on the vacancy defective CNTs can be explained by
Figure 3. Partial DOS of Pt and carbon of (a) Pt1/CNTs and (b) Pt1/ CNTs(va). Deformation charge differences of (c) Pt1/CNTs and (d) Pt1/CNTs(va).
Figure 4. The electron charge of Ptn (n ) 1-4) adsorbed on (5, 5) CNTs with or without point defect.
the stronger orbital hybridization between Pt atoms and carbon. In addition, the much larger charge transfers also occurred on the defect CNTs. Our studies indicate that defective CNTs can be used as a template for noble metal cluster assembly without requiring surface chemical modifications, which are applied as a catalyst support for a series of catalyst reactions, such as fuel cell and hydrogenation. The electronic properties of interface between Pt clusters and CNTs can be turned by introducing point defect. By changing the vacancy density of CNTs, one can produce novel heterostructures with certain configurations which can potentially be used for applications in electromechanical nanodevices.
Bonding of Pt Clusters on (5, 5) Carbon Nanotubes Acknowledgment. This work was supported by the “Qianjiang Distinguished Professor” program of Zhejiang Province, People’s Republic of China. Supporting Information Available: Figure showing the unit cell used in the calculations. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Andrews, R.; Jacques, D.; Qian, D.; Rantell, T. Acc. Chem. Res. 2002, 35, 1008. (2) Avouris, P. Acc. Chem. Res. 2002, 35, 1026. (3) Dai, H. Acc. Chem. Res. 2002, 35, 1035. (4) Dresselhaus, M. S.; Dresselhaus, G.; Jorio, A.; Souza Filho, A. G.; Pimenta, M. A.; Saito, R. Acc. Chem. Res. 2002, 35, 1070. (5) Fischer, J. E. Acc. Chem. Res. 2002, 35, 1079. (6) Niyogi, S.; Hamon, M. A.; Hu, H.; Zhao, B.; Bhowmik, P.; Sen, R.; Itkis, M. E.; Haddon, R. C. Acc. Chem. Res. 2002, 35, 1105. (7) Ouyang, M.; Huang, J. L.; Lieber, C. M. Acc. Chem. Res. 2002, 35, 1018. (8) Rao, C. N. R.; Govindaraj, A. Acc. Chem. Res. 2002, 35, 998. (9) Sloan, J.; Kirkland, A. I.; Hutchison, J. L.; Green, M. L. H. Acc. Chem. Res. 2002, 35, 1054. (10) Sun, Y. P.; Fu, K.; Lin, Y.; Huang, W. Acc. Chem. Res. 2002, 35, 1096. (11) Zhou, O.; Shimoda, H.; Gao, B.; Oh, S.; Fleming, L.; Yue, G. Acc. Chem. Res. 2002, 35, 1045. (12) Liang, Z. X.; Zhao, T. S. J. Phys. Chem. C 2007, 111, 8128.
J. Phys. Chem. C, Vol. 113, No. 3, 2009 893 (13) Xing, Y. J. Phys. Chem. B 2004, 108, 19255. (14) Mu, Y.; Liang, H.; Hu, J.; Jiang, L.; Wan, L. J. Phys. Chem. B 2005, 109, 22212. (15) Liu, Z. T.; Wang, C. X.; Liu, Z. W.; Lu, J. Appl. Catal., A 2008, 344, 114. (16) 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. (17) Corma, A.; Garcia, H.; Leyva, A. J. Mol. Catal. A: Chem. 2005, 230, 97. (18) Pham-Huu, C.; Keller, N.; Charbonniere, L. J.; Ziessle, R.; Ledoux, M. J. Chem. Commun. 2000, 1871. (19) Kongkanand, A.; Vinodgopal, K.; Kuwabata, S.; Kamat, P. V. J. Phys. Chem. B 2006, 110, 16185. (20) Guo, S.; Dong, S.; Wang, E. J. Phys. Chem. C 2008, 112, 2389. (21) Kim, Y. T.; Mitani, T. J. Catal. 2006, 238, 394. (22) Lordi, V.; Yao, N.; Wei, J. Chem. Mater. 2001, 13, 733. (23) Zhao, J. x.; Ding, Y. h J. Phys. Chem. C 2008, 112, 2558. (24) Sung Jin, K.; Yong Jin, P.; Eun Ju, R.; Ki Kang, K.; Kay Hyeok, A.; Young Hee, L.; Jae Young, C.; Chan Ho, P.; Seok Kwang, D.; Min Ho, P.; Cheol Woong, Y. Appl. Phys. Lett. 2007, 90, 023114. (25) Maiti, A.; Ricca, A. Chem. Phys. Lett. 2004, 395, 7. (26) Yang, S. H.; Shin, W. H.; Lee, J. W.; Kim, S. Y.; Woo, S. I.; Kang, J. K. J. Phys. Chem. B 2006, 110, 13941. (27) Charlier, J. C. Acc. Chem. Res. 2002, 35, 1063. (28) Delley, B. J. Chem. Phys. 1990, 92, 508. (29) Delley, B. J. Chem. Phys. 2000, 113, 7756. (30) Perdew, J. P.; Wang, Y. Phys. ReV. B 1992, 45, 13244.
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