Thermal Evolution of a Platinum Cluster Encapsulated in Carbon

The Journal of Physical Chemistry C .... in the (15, 15) and (20, 20) single wall carbon nanotubes (SWNTs), using the second-moment approximation of t...
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J. Phys. Chem. C 2007, 111, 1631-1637

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Thermal Evolution of a Platinum Cluster Encapsulated in Carbon Nanotubes Daojian Cheng, Wenchuan Wang,* and Shiping Huang DiVision of Molecular and Materials Simulation, Key Lab for Nanomaterials, Ministry of Education, Beijing UniVersity of Chemical Technology, Beijing 100029, P.R. China ReceiVed: September 26, 2006; In Final Form: NoVember 16, 2006

A Monte Carlo method has been performed to simulate the thermal evolution of an icosahedral Pt55 cluster encapsulated in the (15, 15) and (20, 20) single wall carbon nanotubes (SWNTs), using the second-moment approximation of the tight-binding potentials for metal-metal interactions. The metal-carbon interactions are modeled by the Lennard-Jones potential, and the carbon atoms on the SWNTs are considered to be fixed. The melting-like structural transformation is found for the icosahedral clusters encapsulated in SWNTs. The melting-like transformation temperatures of the icosahedral clusters encapsulated in SWNTs are estimated from the fluctuations of the total potential energy, which are 280 and 320 K, respectively. The simulations indicate that the melting-like transformation temperature for the encapsulated icosahedral clusters increases with the pore size of SWNTs. At higher temperatures, a stacked structure in layers is found for the encapsulated icosahedral Pt55 clusters. Simulation results reveal that SWNTs have a significant effect on the structures of the encapsulated icosahedral Pt clusters.

1. Introduction Metal-filled carbon nanotubes have received increasing attention in recent years due to their potential applications, which include nanomagnetic recording media,1-4 nanosensors,5 and nanocatalysts,6-8 in particular. The applications can be associated with the fact that the carbon nanotubes possess some excellent properties, such as uniform pores, good chemical stability, and large specific surface area. The unique properties make carbon nanotubes very useful as a catalyst support. Especially, a large number of works were focused on the deposition of Pt clusters on the surfaces of carbon nanotubes, since Pt-deposited carbon nanotubes are effective electrocatalysts for fuel cells.9-16 Matsumoto et al.11 found that the performances of the Pt-deposited carbon nanotube catalysts are superior to that of Pt-deposited carbon black catalysts for the polymer electrolyte fuel cells (PEFCs). Tang et al.12 found that the Ptdeposited carbon nanotubes are suitable catalysts for the proton exchange membrane fuel cells, and show higher electrocatalytic activity than the Pt-deposited graphite electrode. In addition, Xing13 reported that as the high loading catalysts, the Pt clusters on carbon nanotubes can be used for the cathode of the polymer electrolyte membrane fuel cells. However, the metal clusters reported were mostly supported on the exterior surfaces of carbon nanotubes, and few studies were focused on the metal clusters encapsulated inside the carbon nanotubes. The experimental results indicate that metal clusters filled in the interior of carbon nanotubes exhibit higher activities than other supports such as Y zeolite7 and activated charcoal.7,17 Also, the Pt clusters encapsulated in single-walled carbon nanohorns were prepared, and present attractive behavior in chemical reactions.18 Therefore, the Pt clusters encapsulated in carbon nanotubes would be an important topic for further investigation. Molecular simulations present physical insights into metal deposition on substrates, and have been used to study metal* Address correspondence to this author. E-mail: [email protected]. edu.cn. Fax: +86-10-64427616.

filled carbon nanotubes.19-22 Most of the works were devoted to the structural properties of the metal-filled carbon nanotubes. Choi et al.19 found that copper nanowires inside carbon nanotubes possess the structure of multishell packs by using the steepest descent method and molecular dynamics (MD) simulation. Hwang et al.20 showed a possible growth mechanism of the Cu-filled carbon nanotubes using a classical MD method. Kang et al.21 found that the structural phase of the sodiumfilled carbon nanotubes changes with the radius of the carbon nanotubes by an atomistic simulation method. Also, a few of the works were focused on the thermal evolution of the metalfilled carbon nanotubes. Very recently, Arcidiacono et al.22 studied the solidification and the structure of gold nanoparticles in carbon nanotubes using MD simulation, and found that the solidification temperature is higher than the corresponding unsupported clusters. However, none of these studies were addressed on the Pt clusters encapsulated in carbon nanotubes. In this work, using the Monte Carlo method, we investigate the thermal evolution of the icosahedral Pt55 clusters encapsulated in single wall carbon nanotubes (SWNTs). The effect of pore size on the thermal evolution of the icosahedral clusters encapsulated in SWNTs is addressed. The effect of the carbon nanotube substrate on the structure of the icosahedral Pt clusters is also discussed. 2. Computational Details 2.1. Initial Configuration. SWNTs are formed by rolling up grapheme sheets into cylinders. The diameter and properties of SWNTs can be specified by the chirality (n, m), where n and m denote the components of the chirality vectors. In this work, two armchair (n, n) SWNTs with the chirality of (15, 15) and (20, 20) are adopted. The diameters of these SWNTs are 2.035 and 2.714 nm, respectively. The Pt55 clusters of interest possess the icosahedral structure in the initial configurations, and the icosahedron is found to be the lowest energy structure for free Pt55 clusters using the TB-SMA potential, as shown by Baletto and co-workers.23,24

10.1021/jp066306v CCC: $37.00 © 2007 American Chemical Society Published on Web 01/05/2007

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TABLE 1: Parameters of the Lennard-Jones Potential for the Pt-C Interaction30,31 and the TB-SMA Potential for the Pt-Pt Interaction37 Pt-C

Pt-Pt

 (eV)

σ ( Å)

A (eV)

ξ (eV)

p

q

r0 (Å)

0.04092

2.936

0.2975

2.695

10.612

4.004

2.7747

Note that Baletto and co-workers23,24 found that the TB-SMA potential is not able to predict the lowest energy structure of a genetic Pt55 cluster. It is found that “rosette structures” are lower in energy than the icosahedron by using the DFT calculations. These structures belong to the “icosahedral family”, reflecting the distortion of the icosahedron at some of its vertices. In our simulations, the initial icosahedral Pt55 clusters were placed at the center of the cross-section of the SWNT. In fact, the clusters can be placed in different positions of the crosssection of the SWNTs as the initial configurations without influences on the final configurations of the clusters of interest. 2.2. Potential Models. As mentioned in the previous investigation,25 since the interactions between Pt metal atoms and SWNTs are weak and the structure of SWNTs would not be changed by Pt-doping, it is assumed that the carbon atoms on the SWNTs are fixed in their ideal lattice positions. Note that similar assumptions can also be found in atomistic study of sodium nanowires,21 cesium structures,26 and copper clusters20 encapsulated in carbon nanotubes. Two potential models were adopted in our simulations: one is the interaction of Pt atoms and C atoms on the SWNTs, the Pt-C interaction energy, EPt-C, and the other is the interaction between Pt atoms, the Pt-Pt interaction energy, EPt-Pt. The total potential energy, ETotal, can be thus defined by ETotal ) EPt-C + EPt-Pt. For the weak interactions between the carbon and Pt atoms, the Lennard-Jones (LJ) potential function was used to model the Pt-C interaction energy in this work. It is noticed that the LJ potential was used to model the Pt-C interaction, and obtained reasonable predictions of the structure27,28 and thermal behavior of metal clusters in the literature.22,29-31 The LJ potential parameters for the Pt-C interaction can be found in the literature,30,31 and are listed in Table 1. The fitted Pt-C potential parameters are obtained from the Lorentz-Berthelot mixing rules, taken from the literature.30,31 The Pt-Pt LJ interaction parameters were developed by Agrawal et al.,32 whereas the C-C interaction parameters were developed by Bhethanabotla and Steele.33-36 The second-moment approximation of the tight-binding (TBSMA) model37 was used to describe the Pt-Pt interaction energy, EPt-Pt. From the TB-SMA potential, the Pt-Pt interaction energy, EPt-Pt, can be expressed as

EPt-Pt )

∑i (E iR + E iB)

(1)

where E iB and E iR are the band and Born-Mayer ion-ion repulsion terms, respectively. Both terms for an atom i can be written as

E iR )

∑j ARβ e-P

Rβ Rβ(rij /r 0 -1)

,

E iB ) -

{∑ j

}

Rβ-1)

2 ξ Rβ e-2qRβ(rij /r 0

1/2

(2)

where A, ξ, p, q, and r0 of the TB-SMA scheme are fitted to experimental values of the cohesive energy, lattice parameters (by a constraint on the atomic volume), and independent elastic

Figure 1. Snapshots of the icosahedral Pt cluster encapsulated in the (15, 15) and (20, 20) SWNTs at 100 K. Views: Perpendicular to the tube axis on the left and parallel to the tube axis on the right.

constants for the reference crystal structure at T ) 0 K. The Pt-Pt interaction parameters for the TB-SMA potential can be found in the literature,37 as shown in Table 1. 2.3. Monte Carlo Method. The canonical Monte Carlo method was used to study the thermal evolution of the icosahedral Pt55 clusters encapsulated in SWNTs. The periodic boundary condition was applied along the SWNT axis. Temperature increased from 100 to 600 K in an increment of 50 K, but the increment was reduced to 10 K near the melting-like transformation temperature. In our simulations, 3.3 × 107 MC steps were run at each temperature point. The first 1.1 × 107 steps were used to reach the equilibrium, where the fluctuation of the total energy was less than 0.2%, and the last 2.2 × 107 steps were used for the average of various physical quantities. The equilibration configurations of the clusters after the MC steps at a given temperature were used as the starting configuration for the MC run at the higher temperatures. A similar method has been used to investigate the melting properties of Cu-Au bimetallic clusters in our previous work38 with satisfactory results. 3. Results and Discussion 3.1. Thermal Evolution of the Encapsulated Icosahedral Cluster. The atomic configurations of the icosahedral Pt clusters encapsulated in the (15, 15) and (20, 20) SWNTs at 100 K are presented in Figure 1. It is found in Figure 1 that the icosahedral Pt clusters stay near the inner surfaces of both the (15, 15) and (20, 20) SWNTs at 100 K, moving away from the centers of cross-sections of the SWNTs in the initial configurations. Apparently, this observation is due to the fact that the icosahedral Pt clusters present a rather weak cluster-nanotube (PtC) interaction, when encapsulated in SWNTs. In addition, the configurations in Figure 1 reveal that the Pt clusters preserve the icosahedral structure encapsulated in both the (15, 15) and (20, 20) SWNTs at 100 K. This means that the simulation steps at low temperature are sufficient to equilibrate the system within this icosahedral structure. Figure 2 gives the temperature dependences of the total potential energy, ETotal, Pt-Pt interaction energy, EPt-Pt, and Pt-C interaction energy, EPt-C, for the icosahedral Pt clusters encapsulated in the (15, 15) and (20, 20) SWNTs, respectively. As is found from Figure 2, the total potential energies, ETotal, decline at higher temperatures, which is different from the results for the free clusters39 or graphite-supported clusters.31 This

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Figure 2. The total potential energy, ETotal, Pt-Pt interaction energy, EPt-Pt, and Pt-C interaction energy, EPt-C, as a function of temperature: (a) the icosahedral Pt cluster encapsulated in the (15, 15) SWNT and (b) the icosahedral Pt cluster encapsulated in the (20, 20) SWNT. Tm represents the melting-like structural transformation temperature.

observation was also reported for an argon cluster confined in zeolite NaCaA by Monte Carlo simulation,40 where the total potential energy of the system decreases with the increase of temperature at higher temperatures. These phenomena may mainly be due to the fact that the icosahedral Pt clusters at low temperatures are not the most favorable structure for the whole (cluster encapsulated in SWNT) system, so that the rearrangements and transformations of the icosahedron occur at higher temperatures. Therefore, the Pt-C interaction energies, EPt-C, at higher temperatures decrease significantly, attributed to the rearrangements and transformations of the icosahedral clusters. Consequently, this leads to the lowering of the total potential energy, ETotal, at higher temperatures, as shown in Figure 2. In Figure 2, sharp decreases in the total potential energy curves, ETotal, as a function of temperature for the icosahedral clusters encapsulated in both the (15, 15) and (20, 20) SWNTs are observed, corresponding to the melting-like structural transformations of the icosahedral clusters encapsulated in the SWNTs. To identify the melting-like transformation temperature, we define the heat capacity CV per atom as a function of the potential energy E fluctuation, given by41-43

CV )

(〈E2〉 - 〈E〉2) nkBT2

(3)

where E is the potential energy, kB is the Boltzman constant, n is the total number of atoms in the cluster, and T is the temperature. It should be mentioned that in this definition of the heat capacity CV per atom, the contribution to fluctuations of the coupling between the cluster and the carbon nanotube is

Figure 3. The heat capacity CV per atom as a function of temperature: (a) the icosahedral Pt cluster encapsulated in the (15, 15) SWNTand (b) the icosahedral Pt cluster encapsulated in the (20, 20) SWNT.

neglected. The heat capacity CV per atom curves for the icosahedral clusters encapsulated in the (15, 15) and (20, 20) SWNTs are plotted in Figure 3, parts a and b, respectively. Consistent with the literature,41-43 the melting-like transformation temperature is defined as that at the maximum of the peak in the heat capacity CV per atom. As is seen from Figures 2 and 3, the maximum values in the heat capacity CV per atom curves correspond to the temperatures, where the sharp decreases in the total potential energy curves for the icosahedral clusters encapsulated in both the (15, 15) and (20, 20) SWNTs take place. Therefore, we estimate that the melting-like transformation temperatures are 280 and 320 K for the icosahedral clusters encapsulated in the (15, 15) and (20, 20) SWNTs, respectively. Furthermore, the results suggest that the melting-like transformation temperature of the icosahedral clusters encapsulated in the SWNTs increases with the pore size of the SWNTs. It is useful to explore the melting-like transformation process by the snapshots and pair correlation functions of the icosahedral clusters at the temperatures before and after the melting-like transformation temperature. The definition of the pair correlation function can be found elsewhere in our previous work.44,45 Figure 4 shows the snapshots and pair correlation functions at 270, 280, and 290 K for the icosahedral cluster encapsulated in the (15, 15) SWNT. The initial icosahedral shape of the Pt55 cluster encapsulated in the (15, 15) SWNT remains unchanged before the melting-like structural transformation at 270 K (see Figure 4a). The peaks in the pair correlation function g(r), corresponding to the coordination shells of Pt atoms, are clearly distinguished at 270 K (see Figure 4b). At the melting-like

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Figure 4. Snapshots (views: perpendicular to the tube axis on the left and parallel to the tube axis on the right) and pair correlation functions of the icosahedral Pt cluster encapsulated in the (15, 15) SWNT at 270, 280, and 290 K: (a) snapshots and (b) pair correlation functions.

transformation temperature of 280 K, not the icosahedral structure but the stacked structure is found for the Pt55 cluster encapsulated in the (15, 15) SWNT, as shown in Figures 4a. The peaks in the pair correlation function disappear, corresponding to the disorder structure of the Pt55 cluster encapsulated in the (15, 15) SWNT at the melting-like transformation temperature of 280 K (see Figure 4b). At the higher temperatures after the melting-like structural transformation, e.g., T ) 290 K, the Pt55 cluster encapsulated in the (15, 15) SWNT keeps the stacked structure, as shown in Figure 4a. No peaks are observed in the pair correlation function at 290 K, indicating that the Pt cluster is disordered after the melting-like structural transformation (see Figure 4b). Figure 5 shows the snapshots and pair correlation functions at 310, 320, and 330 K for the icosahedral cluster encapsulated in the (20, 20) SWNT. The melting-like structural transformation of the icosahedral cluster encapsulated in the (20, 20) SWNT is similar to that in the (15, 15) SWNT. As is seen from Figures 5a, the initial icosahedral structure for the Pt55 cluster encapsulated in the (20, 20) SWNT is still preserved before the melting-like structural transformation at 310 K, but the icosahedral structure is transformed into the stacked structure at the melting-like transformation temperature of 320 K. The peaks in the pair correlation function g(r) disappear at 320 K, which

Cheng et al.

Figure 5. Snapshots (views: perpendicular to the tube axis on the left and parallel to the tube axis on the right) and pair correlation functions of the icosahedral Pt cluster encapsulated in the (20, 20) SWNT at 310, 320, and 330 K: (a) snapshots and (b) pair correlation functions.

indicates that the cluster is not structured with the icosahedron at the melting-like transformation temperature of 320 K (see Figure 5b). At the higher temperature 330 K after the meltinglike structural transformation, the Pt55 cluster encapsulated in the (20, 20) SWNT remains in the stacked structure, and the peaks in the pair correlation function g(r) also disappear, as shown in Figure 5a,b. 3.2. Effect of the Carbon Nanotube Substrate. Aiming at exploring the effects of the carbon nanotube substrate on the structure of the icosahedral Pt cluster before and after the melting-like structural transformation, we calculated the radial density of the Pt atoms along the radial direction, i.e., perpendicular to the tube axis of SWNTs, F(r), where r is the radial distance from the center of the SWNTs. F(r) is calculated from the trajectories of the MC simulation after equilibrium, given by

F(r) )

〈N(r)〉 2πr∆rL

(4)

where N(r) is the ensemble average of the number of Pt atoms

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Figure 6. The reduced radial density functions, F*(r), of the icosahedral Pt cluster encapsulated in the (15, 15) SWNT at (a) 100, (b) 270, (c) 280, and (d) 600 K.

Figure 7. The reduced radial density functions, F*(r), of the icosahedral Pt cluster encapsulated in the (20, 20) SWNT at (a) 100, (b) 310, (c) 320, and (d) 600 K.

in a cell of 2πr∆rL, and L is the length of the simulation box. Here, we define F*(r) ) 2πr∆rLF(r), and then the reduced radial density function F*(r) is given by F*(r) ) 〈N(r)〉. Figure 6 shows the reduced radial density functions, F*(r), for the icosahedral Pt cluster encapsulated in the (15, 15) SWNT at 100, 270, 280, and 600 K, respectively. Many peaks are observed in F*(r) for the Pt cluster encapsulated in the (15, 15) SWNT at 100 and 270 K, due to the fact that the Pt cluster remains the icosahedral structure before the melting-like structural transformation (see Figure 6a,b). At the melting-like transformation temperature of 280 K, the density of Pt atoms

increases gradually along the radius of the SWNT (from the center to surface), and achieves the maximum near the inner surface of the (15, 15) SWNT, corresponding to the stacked structure at 280 K (see Figure 6c). Moreover, F*(r) for the Pt cluster encapsulated in the (15, 15) SWNT displays four peaks at 600 K, where each peak corresponds to one layer, indicating that the Pt cluster possesses a four-layer stacked structure at 600 K (see Figure 6d). Figure 7 shows the reduced radial density functions, F*(r), for the icosahedral Pt cluster encapsulated in the (20, 20) SWNT at 100, 310, 320, and 600 K, respectively. The results are similar

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Cheng et al. 20) SWNTs at 600 K. It is found in Table 2 that the average distances between the SWNTs and the first layers of the Pt atoms are 3.02 and 2.98 Å for the (15, 15) and (20, 20) SWNT at 100 K, respectively, which are larger than the distances between the layers from the first to third one. 4. Conclusions

Figure 8. The layer-by-layer structure of the Pt cluster encapsulated in SWNTs at 600 K: (a) (15, 15) SWNT and (b) (20, 20) SWNT. The first layer is the layer that is the closest to the inner surface of SWNT. Some atoms are disconnected from each other due to the fact that their bond length exceeds a fixed threshold. Views: Parallel to the tube axis on top and perpendicular to the tube axis on the bottom.

TABLE 2: The Average Distance between the SWNT Inner Surface and the First Layer and Average Distances between Layers, in Å, for the Pt55 Clusters Encapsulated in the (15, 15) and (20, 20) SWNTs at 600 K SWNT

from SWNT to 1st layer

from 2nd layer to 1st layer

from 3rd layer to 2nd layer

from 4th layer to 3rd layer

(15, 15) (20, 20)

3.02 2.98

2.16 2.21

2.26 2.26

2.25 2.04

to that in the (15, 15) SWNT. At the temperatures of 100 and 310 K before the melting-like structural transformation, F*(r) for the Pt cluster displays several peaks, corresponding to the icosahedral structure of the Pt cluster before the melting-like structural transformation (see Figure 7a,b). Only four peaks, increasing with the radius, are found in F*(r) for the Pt cluster at the melting-like transformation temperature of 320 K, meaning that the icosahedral structure is transformed into the stacked structure upon the melting-like structural transformation (see Figure 7c). The Pt55 cluster encapsulated in the (20, 20) SWNT possesses a four-layer structure at 600 K, which is verified by the four sharp peaks in F*(r) for the Pt cluster encapsulated in the (20, 20) SWNT (see Figure 7d). Parts a and b of Figure 8 show the layer-by-layer structures of the Pt clusters encapsulated in the (15, 15) and (20, 20) SWNTs at 600 K, respectively. As is seen from Figure 8a,b, the atoms of each layer possess the hexagonal lattice, which is in agreement with the observation of Pt6, Pt13, and Pt100 deposited on graphite by using molecular dynamics simulations.28 It is found in Figure 8a,b that the number of atoms for each layer decreases gradually from the first layer, which is the closest to the inner surface of the SWNTs, to the fourth layer for the Pt clusters encapsulated in both the (15, 15) and (20, 20) SWNTs. This implies that the structure is a favorite arrangement in terms of the interaction between the Pt clusters and the inner surface of the SWNTs at the temperature of 600 K. Table 2 gives the average distances between the SWNT inner surface and the first layer, and the average distances between layers for the Pt55 clusters encapsulated in the (15, 15) and (20,

To summarize, thermal evolution of the icosahedral Pt55 clusters encapsulated in the (15, 15) and (20, 20) SWNTs is investigated by canonical Monte Carlo simulations, based on the TB-SMA potentials for metal-metal interactions and LJ potential for the metal-carbon interactions. The melting-like structural transformation is found for the icosahedral Pt55 clusters encapsulated in SWNTs. The melting-like transformation temperatures of the icosahedral clusters encapsulated in both the (15, 15) and (20, 20) SWNTs are mainly identified by the heat capacities CV per atom which are 280 and 320 K, respectively. The results indicate that the melting-like transformation temperature of the icosahedral Pt55 clusters encapsulated in SWNTs increases with the pore size of SWNTs. On the other hand, the Pt clusters encapsulated in the (15, 15) and (20, 20) SWNTs possess the stacked structures at temperatures higher than the melting-like transformation temperatures, which is attributed to the effect of the carbon nanotube substrate. Especially, the fourlayer stacked structures are found for the encapsulated Pt clusters at 600 K. Strictly speaking, the total potential energy for a cluster encapsulated in SWNTs should also encompass the C-C interactions. It is noticed that since the nanotubes here are considered as rigid, their fluctuations and their contribution to entropy are neglected with the consequence that the statistical ensemble used is not canonical. As a result, some slight errors of the transition temperatures would appear by using eq 3 in this work. Therefore, it would be better that a more accurate model is introduced to describe the C-C interactions. In addition, the Tersoff-Brenner potentials have been used for modeling the C-C atomic interactions in SWNTs and would be a better choice to understand the encapsulation process of metal clusters in SWNTs.19,20,46,47 These will be addressed in our future work. Acknowledgment. This work is supported by the National Natural Science Foundation of China (Nos. 20476004 and 20236010) and the National Basic Research Program of China (Grant No. G2003CB615807). References and Notes (1) Liu, S.; Zhu, J.; Mastai, Y.; Felner, I.; Gedanken, A. Chem. Mater. 2000, 12, 2205. (2) Yang, C. K.; Zhao, J. J.; Lu, J. P. Phys. ReV. Lett. 2003, 90, 257203. (3) Kang, Y. J.; Choi, J.; Moon, C. Y.; Chang, K. J. Phys. ReV. B 2005, 71, 115441. (4) Borowiak-Palen, E.; Mendoza, E.; Bachmatiuk, A.; Rummeli, M. H.; Gemming, T.; Nogues, J.; Skumryev, V.; Kalenczuk, R. J.; Pichler, T.; Silva, S. R. P. Chem. Phys. Lett. 2006, 421, 129. (5) Yang, M.; Yang, Y.; Liu, Y.; Shen, G.; Yu, R. Biosens. Bioelectron. 2006, 21, 1125. (6) Rajesh, B.; Ravindranathan Thampi, K.; Bonard, J. M.; Xanthopoulos, N.; Mathieu, H. J.; Viswanathan, B. J. Phys. Chem. B 2003, 107, 2701. (7) Zhang, A. M.; Dong, J. L.; Xu, Q. H.; Rhee, H. K.; Li, X. L. Catal. Today 2004, 347, 93-95. (8) Cui, H. F.; Ye, J. S.; Liu, X.; Zhang, W. D.; Sheu, F. S. Nanotechnology 2006, 17, 2334. (9) Li, W.; Liang, C.; Zhou, W.; Qiu, J.; Zhou, Z. H.; Sun, G.; Xin, Q. J. Phys. Chem. B 2003, 107, 6292. (10) Li, W.; Liang, C.; Zhou, W.; Qiu, J.; Li, H.; Sun, G.; Xin, Q. Carbon 2004, 42, 436.

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