ARTICLE pubs.acs.org/JPCC
On the Melting and Freezing of Au-Pt Nanoparticles Confined in Single-Walled Carbon Nanotubes Rongwei Shi,† Jingling Shao,‡ Xiaolei Zhu,*,† and Xiaohua Lu*,† †
State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, China ‡ College of Chemical and Biological Engineering, Yancheng Institute of Technology, Yancheng, Jiangsu, 224003, China
bS Supporting Information ABSTRACT: A MD simulation method is used to simulate the melting and freezing of Au-Pt nanoparticles confined in armchair single-walled carbon tubes ((n,n)-SWNTs), applying the second-moment approximation of the tight-binding potentials for metal-metal interactions and Lennard-Jones potential for the metal-carbon interactions. The carbon atoms on the SWNTs are taken to be fixed. The structures, total energies, Lindemman indices, and radial and axial density distributions are used to examine the behaviors of melting and freezing for Au-Pt nanoparticles confined in (n,n)-SWNTs. The nucleation analysis is carried out in terms of classical nucleation theory. The simulation results demonstrate that the solid Au-Pt clusters confined within (n,n)-SWNTs have multishell structures, even in a melted cluster. In addition, for the confined Au-Pt nanoparticles, Pt atoms tend to stay at the position close to the SWNT wall, which is different from free Au-Pt nanoparticles. Simulation results reveal that SWNTs and compositions of nanoparticles have significant effects on the structures and physical properties of the confined Au-Pt nanoparticles. On the other hand, some important thermodynamic and dynamic parameters are estimated based on MD simulations and compared with available theoretical and experimental results.
1. INTRODUCTION Metal nanoparticles have many potential applications in catalysis, sensors, magnetic devices, optoelectronics, and microelectronics fields due to their unique chemical, physical, and electrical properties. Since the particle size and composition of the bimetallic nanoparticles will affect their physical and chemical properties, they may exhibit different properties compared to their single-component metal nanoparticles.1-5 Therefore, the different bimetallic nanoparticles have been prepared through design and control of particle size and composition.4-7 The understanding of the structures and properties of bimetallic nanoparticles can be helpful to developing new materials. The Au-Pt nanoparticles are very critical for application in catalysis due to their unique electronic properties and larger surfacevolume ratio. On the other hand, under the influence of a substrate, nanoparticles are different from free nanoparticles.8-11 Their properties rely not only on the structure, particle size, and composition but also on the nature of metal-substrate interaction. Although molecular dynamics simulations can not directly provide the information about catalytic activity, they can be used to examine and explore the structure and dynamics of the nanoparticles at the atomic level. Experimentally, metals or metal compounds can be filled into the carbon nanotubes (CNTs) using different techniques.12-15 Since metal-filled carbon nanotubes have novel structures and properties, they have potential applications,12-25 such as semir 2011 American Chemical Society
conductor devices, nanocatalysts, fuel cells, and nanomagnetic recording media. One of the important properties, solid-liquid phase transition, has a significant influence on the synthesis of nanoparticles.26 Theoretical and experimental results suggested that the melting point of metal nanoparticle is lower than that of the bulk counterpart, and the melting point of a nanoparticle increases with the increase of particle size.27 Although many experimental techniques have been developed to investigate the melting process of nanoparticles, the understanding of this process is limited and not satisfactory due to the size and complicated structure of the nanoparticles.28 Especially, experimental investigations to understand the effect of a SWNT substrate on the properties of nanocluster are both expensive and difficult. Many MD simulation investigations of supported metal nanoparticles have been found in previous studies,29,30 which reveals the importance of MD simulations. MD simulations have been also applied to study metal-filled carbon nanotubes. For example, Kang et al.31 observed that the cylindrical ultrathin copper nanowires in carbon nanotubes have multishell packing structures and found32 that the copper nanoparticles confined in carbon nanotubes are inclined to move along the tube axis. The simulation Received: October 9, 2010 Revised: December 14, 2010 Published: February 3, 2011 2961
dx.doi.org/10.1021/jp109689m | J. Phys. Chem. C 2011, 115, 2961–2968
The Journal of Physical Chemistry C
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results by Wang33 revealed that the melting temperatures of the confined icosahedral Pt55 clusters are related to the diameters of SWNTs. For Na metal encapsulated in a carbon nanotube,34 several phases from an atomic strand to multishell packing structures with an increase of tube radius were observed. Poulikakos et al.35 reported a pioneering work on Au nanopartions encapsulated in (n,0)-SWNT based on MD simulations. In our previous work,36 we performed MD simulations on Au nanoparticles confined in (n,n)-SWNT and revealed some important features of the thermal evolution process for the confined Au nanoparticles. The experimental studies about phase transition and nucleation dynamics for Au-Pt nanoparticles confined in SWNTs have proven difficult. In the current work, we perform MD simulations to investigate the structure, phase transition, and nucleation on Au-Pt nanoparticles confined in (n,n)-SWNTs. The simulation results reveal some interesting structural features about the melting and freezing behaviors for confined Au-Pt nanoparticles, which will be helpful for the synthesis and processing of confined nanoparticles.
2. COMPUTATIONAL DETAILS We investigate melting and freezing behaviors of Au-Pt nanoparticles confined within armchair (n,n)-SWNTs based on MD simulations. The diameters of the selected (n,n)-SWNTs are 25.76, 33.90, and 40.68 Å, respectively. Each system consists of a Au-Pt cluster filled in the carbon tube and an infinitely long SWNT. Each SWNT is simulated by a box with length 22.017 nm using periodic boundary conditions along the tube axis. In MD simulations, Au-Pt nanoparticles (Au1-xPtx)N (x = 0.2, 0.4, 0.6, 0.8, 1.0) with N = 818, 1522, and 2230 are confined in (19,19)-, (25,25)-, and (30,30)-SWNTs, respectively. The second-moment approximation of the tight-binding (TBSMA-type potential function) has been successfully used to model the structures of nanoclusters37 and ultrathin nanowires,38 and this potential is confirmed to be effective.39,40 Thus, in current MD simulations, the Au-Au, Pt-Pt, and Au-Pt interactions are described with the TB-SMA potential.39 In the TB-SMA potential, metal-metal (M-M) interaction energy, EM-M, is given by X EM - M ¼ EiR þ EiB ð1Þ i
EiR
EiB are
where and the Born-Mayer ion-ion repulsion and band interactions, respectively. These two terms for an atom i can be represented based on X ARβ expð - pRβ ðrij = r0Rβ - 1ÞÞ ð2Þ EiR ¼ j
EiB
¼ -
X
ξ2Rβ
expð - qRβ ðrij = r0Rβ
- 1ÞÞ
ð3Þ
j
As shown in Table 1, the TB-SMA potential parameters for metal-metal are taken from ref 39. It is well-known that surface atoms have more dangling bonds than those in the bulk and often relax and reconstruct on extended two-dimensional surfaces. Tao et al.41 found that there is a significant contraction (∼0.13 Å) for {100} surface atoms. For the confined Au-Pt nanoparticles, there should be similar r(M-M) distance contractions. In fact, corresponding calibration methods have been proposed42 by Sun
Table 1. Parameters of TB-SMA Potential for Transition Metals in MD Simulations A (eV)
ξ (eV)
p
q
R0
Au-Au
0.2061
1.790
10.229
4.036
2.884
Pt-Pta Au-Ptb
0.2975 0.2476
2.695 2.196
10.612 10.421
4.004 4.020
2.775 2.830
M-M a
a
The parameters of TB potential obtained from ref 39. b AAu-Pt = (AAu1/2 , ξAu-Pt = (ξAu-PtξAu-Pt)1/2, pAu-Pt = (1/2)(pAu-Au þ AuAPt-Pt) pPt-Pt), qAu-Pt = (1/2)(qAu-Au þ qPt-Pt), r0(Au-Pt) ≈ (1/2)(r0(Au-Au) þ r0(Pt-Pt)).
et al. In current work, we have not made this calibration about the r(M-M) distance contraction and also approximately assumed r0(Au-Pt) ≈ (1/2)(r0(Au-Au) þ r0(Pt-Pt)) as shown in Table 1 for simplicity, which should not affect the conclusions derived from the current work. Lennard-Jones (LJ) potentials are applied to model the weak interaction between metals and SWNTs with the parameters obtained from ref 43 (σC-Au = 2.9943 Å, εC-Au = 0.01273 eV) and ref 33 (σC-Pt = 2.936 Å, εC-Pt = 0.04092 eV). It is assumed that the structures of SWNTs will not be significantly varied in the presence of transition metal.33 Therefore, in MD simulations, SWNTs are approximately taken as rigid as used in previous investigations.32,34-36,44 During phase transition simulations of the all systems studied, each Au-Pt nanoparticle encapsulated in the SWNT is melted by heating it to 2000 K for 1.2 ns. After that, slow cooling runs are performed from 2000 to 300 K with a temperature step of 20 K and time step of 3 fs. Each system is simulated at constant temperature for 20 000 time steps followed by constant energy simulation for 20 000 time steps. For constant temperature MD simulations, to reach the specified temperature, velocities are rescaled at every step.45 The final structure at 300 K obtained from the cooling process is taken as the initial configuration for the slow heating process. In the slow heating process, each confined Au-Pt nanoparticle is heated from 300 to 15002000 K with a temperature step of 20 K. On the other hand, in the nucleation simulations, the melted Pt2230 is kept in a heat bath of 2000 K, and heating is continued at 2000 K, and save a structure each running 4000 time steps, from which we can obtain 20 melted clusters with different thermal histories and structures. Then, we quench these melted Pt nanoparticles in SWNTs to specified temperatures (1150, 1200, 1250, and 1300 K) for 180 ps to investigate nucleation and crystallization.
3. RESULTS AND DISCUSSION The structures of (Au1-xPtx)818/(25,25)-SWNT (x = 0.2, 0.4, 0.6, 0.8, and 1.0) at 300 K are displayed in Figure 1. It is found from Figure 1 that the solid Au-Pt nanoparticles confined in SWNTs exhibit cylindrical multishelled structures, and the atoms of each layer possess the hexagonal lattice, which are in agreement with previous results on Au cluster/(n,0)-SWNT by Poulikakos et al.35 and on Au cluster/(n,n)-SWNT by Shao et al.36 and are different from those of free Au-Pt clusters46or bulk gold or platinum. During MD simulations on Pt55 confined in (15,15)-SWNT and (20,20)-SWNT at 600 K, Wang et al.33 also found that the Pt55 nanoparticle has multilayer stacked structures. Obviously, the multishelled structures of confined Au-Pt clusters are closely related to the confinement environment of SWNT. The structures of the confined Au-Pt clusters are relatively more ordered than those of free Au nanowires as 2962
dx.doi.org/10.1021/jp109689m |J. Phys. Chem. C 2011, 115, 2961–2968
The Journal of Physical Chemistry C expected.47 It is due to the fact that the metal-SWNT interactions improve the stability of Au-Pt clusters. On the other hand, for free Au-Pt nanoparticles,48 Pt-core/Au-shell structures are most stable. However, for the confined Au-Pt nanoparticles,
Figure 1. Images of (Au1-xPtx)818 confined in (19,19)-SWNTs parallel to the tube axis of SWNTs ((a), (b), (c), (f),and (g)), perpendicular to the tube axis (d), and three-dimensional picture (e) at 300 K. (a) x = 0.2; (b) x = 0.4; (c) x = 0.6; (d) x = 0.6; (e) x = 0.6; (f) x = 0.8; (g) x = 1.0. The blue, yellow, and pink balls represent carbon, gold, and platinum atoms, respectively.
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since Pt-SWNT interaction is stronger than Au-SWNT interaction, Pt atoms tend to distribute from tube wall to tube center with an increase of Pt composition, while Au atoms are inclined to distribute from the tube center to the tube wall with increasing Au composition at 300 K as shown in Figure 1. Figure SI-1 (Supporting Information) shows the relationship between the Pt probability in the outermost layer and Pt composition in the confined (Au1-xPtx)1522 (x = 0.2, 0.4, 0.6, 0.8, and 1.0). Obviously, the Pt probability in the outermost layer is significantly larger than Pt composition in the confined (Au1xPtx)1522, which confirms that Pt atoms tend to stay on the outer layers of confined Au-Pt nanoparticles. The structure of the capping regions on the top and bottom of the nanoparticles has a cylindrical multishelled structure with atom defect. The relative numbers of atoms in the capping regions are about 10%. It is worth noting that in the capping region the number of Au atoms is significantly larger than that of Pt atoms as shown in Figure SI-1(c) (Supporting Information). To gain insight into the phase transition process, the structures of (Au0.2Pt0.8)1522 inside SWNTs are represented in Figure 2. During slow cooling and heating processes, the (Au0.2Pt0.8)1522 clusters have multishelled structures (Figure 2 (c), (d), and (e)) with ordered structure before melting. However, after melting, the (Au0.2Pt0.8)1522 exhibits disordered structures (Figure 2(a), (b), (f), and (g)). In addition, during quenching runs, the confined Au-Pt clusters exhibit similar structures for all systems studied. Figure 3 shows the relationship between the total energy and temperature for Au-Pt nanoparticles filled in SWNTs during slow heating processes. As shown in Figure 3, energy jumps in the total energy curves imply the phase transitions or structural disorders of metal nanoparticles. To examine whether the phase transition is a real melting, the Lindemann index is computed and analyzed. We divide a confined Au-Pt cluster into several shells in radial direction. The Lindemann index49 of each layer δi is given by δi ¼ 2=ðNi ðNi - 1ÞÞ
X qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2 Ærjk2 æ - Ærjk æ = Ærjk æ
ð4Þ
j
