Simulating Synthesis of Metal Nanorods, Nanoplates, and

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Simulating Synthesis of Metal Nanorods, Nanoplates, and Nanoframes by Self-Assembly of Nanoparticle Building Blocks Daojian Cheng, Wenchuan Wang,* Dapeng Cao,* and Shiping Huang DiVision of Molecular and Materials Simulation, Key Laboratory for Nanomaterials, Ministry of Education, Beijing UniVersity of Chemical Technology, Beijing 100029, P. R. China ReceiVed: October 31, 2008; ReVised Manuscript ReceiVed: December 29, 2008

A general synthesis strategy to prepare metal nanostructures by self-assembly was proposed by molecular dynamics (MD) simulation. In this simulating synthesis strategy, the metal nanostructures were generated by the self-assembly of the amorphous nanoparticles with the attractive forces of the nanoparticle-nanoparticle interactions by the annealing MD method at high temperatures, and finally, the resulting amorphous metal nanostructures were cooled to 10 K, which could resemble the nanoparticle self-assembly in experiment. By using the simulating synthesis, we obtained the full atomistic models of the shape-controlled metal nanostructures, including Au, Ag-Au, and beaded Ag-Cu nanorods, triangular and hexagonal Ag nanoplates, triangular Ag-Au and hexagonal Au, cubic hollow Fe, and Ag-Au nanoframes. It is found that these models are in good agreement with the experimental results. Moreover, we predicted a new metal nanostructure, Au nanoporous framework architecture, which has not been reported in experiment, by self-assembly of the Au nanoparticles. The predicted architecture possesses three-dimensional periodic inner-connecting channels and cavities. It is believed that our simulating synthesis approach will help facilitate the preparation and design of novel metal nanostructures in experiment. Introduction Research in metal nanostructures has received intensive attention in recent years due to their unique physical and chemical properties, including optical,1 electronic,2 magnetic,3 and catalytic4 ones, in particular. Both the size and shape of metal nanostructures have a strong effect on their physicochemical properties. In comparison with the size control, the shapecontrolled synthesis of metal nanostructures has attracted much interest most recently.5 A number of shape-controlled metal nanostructures, such as nanorods,6 nanowires,7 nanotubes,8 nanoplates,9 nanoprisms,10 and nanoframes,11 have been synthesized by using different chemical methods. Among them, metal nanorods12 and nanoplates13 have been reported widely, since they can be used as building blocks for assembling into tailored nanosuperstructures. In practice, metal nanoframes have been applied to optical sensing,14 drug delivery,15 catalysis,16 and biomedical imaging.17 These monometallic and bimetallic nanorods, nanoplates, and nanoframes have been prepared from noble metals through a number of synthetic routes. For illustration, we present a compressive picture to explore the diversity of the nanostructures in the literature, as shown in Figure 1. Figure 1a-c shows the monometallic Au,6 bimetallic Ag-Au,18 and beaded Ag-Cu nanorods;19 Figure 1d and 1e shows the triangular9 and hexagonal20 Ag nanoplates, respectively. The triangular21 Ag-Au nanoframes and hexagonal22 Au nanoframes were synthesized by chemical methods (see Figure 1f and 1g). In addition, the cubic hollow Fe11 and Ag-Au23 nanoframes are shown in Figure 1h and 1i, respectively. A nanoparticle, being the building block of nanostructured materials, has recently become the focus of research.24-26 A variety of feasible nanstructures have been formed by the selfassembly of primary nanoparticles in experiment. For example, * To whom correspondence should be addressed. Fax: +86-10-64427616. E-mail: [email protected] or [email protected].

Tang et al.27 found that crystalline nanowires can be synthesized by the spontaneous organization of single CdTe nanoparticles upon controlled removal of the protective shell of the organic stabilizer. It is also believed that these metal nanoparticles may be assembled into the extended metal nanostructures under the attractive forces of the nanoparticle-nanoparticle interactions in experiment. In fact, some metal nanowires have been formed by the spontaneous self-assembly and fusion of discrete metal nanoparticles. A good example is the formation of Au nanowires by the self-assembly of a number of Au nanoparticles.28 However, from both experimental and theoretical points of view, little has been addressed on the formation of some other metal nanostructures, such as nanorods, nanoplates, and nanoframes (see Figure 1), by the self-assembly of metal nanoparticles. Computer simulations present physical insight into the details of the self-assembly process and have been used to study the formation mechanism of the nanostructures by assembling nanoparticles.29 By using molecular dynamics (MD) simulations, Sayle and co-workers29-31 obtained a variety of nanomaterials including CeO2 nanorods and CeO2, Ti-doped CeO2, ZnO, ZnS, MgO, CaO, SrO, and BaO nanoporous framework architectures by simulating the self-assembly of those nanoparticles. In addition, formation of small Pd nanowires by self-assembly of Pd nanoparticles was investigated by using the first-principles density functional calculations.32 Actually, the simulating synthesis of nanostructures by assembling nanoparticles based on atomistic simulations is desirable, which will help guide the chemical synthetic routes. However, a general strategy for simulating synthesis of diversified metal nanostructures has not yet been reported in the literature. In this paper we present a general strategy for simulating synthesis of metal nanostructures by self-assembly of metal nanoparticles. By using the MD simulation method, we report the formation processes of metal nanorods, nanoplates, and nanoframes (see Figure 1), which have been prepared success-

10.1021/jp809628w CCC: $40.75  2009 American Chemical Society Published on Web 02/12/2009

Self-Assembly of Nanoparticle Building Blocks

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Figure 1. Micrographs of metal nanostructures obtained in experiment from the literature. (a) Transmission electron microscope (TEM) images of the Au nanorod samples.6 (b) Scanning electron microscopy (SEM) images of Ag-Au heterometallic nanorods.18 (c) Energy-dispersive X-ray elemental maps for the beaded Ag-Cu nanorods.19 (d) TEM images of the triangular Ag nanoplates.9 (e) TEM and SEM (inset) images of the hexagonal Ag nanoplates.20 (f) TEM images of the triangular Ag-Au nanoframes.21 (g) TEM images of the hexagonal Au nanoframes.22 (h) TEM images of the cubic hollow Fe nanoframes.11 (i) TEM and SEM (inset) images of the cubic hollow Ag-Au nanoframes.23

fully in experiments, by the self-assembly of the corresponding monometallic or bimetallic nanoparticles. In particular, our simulating synthesis gives full atomistic models of these metal nanorods, nanoplates, and nanoframes. For demonstration, we predict a kind of new metal nanostructure, Au nanoporous framework architecture, which has not been reported experimentally. Methods Nanoparticle Building Blocks. Since these nanoparticle building blocks were recrystallized in our simulation, the initial structures of the nanoparticles have no effect on the selfassembly simulation results. For a nanoparticle, the noncrystalline icosahedral structure is related to a very high surface-tovolume ratio and stability observed in the experimental studies.33 In addition, metal nanoparticles exhibit structural magic numbers in experiment.34 Therefore, it is assumed that the selected monometallic or bimetallic nanoparticles here possess the icosahedral structure and magic size. Four magic size nanoparticles of 2057, 5083, 10 179, and 21 127 atoms were thus used for the metal nanoparticle blocks in this work. Figure 2a shows the identical 2057-atom icosahedral Ag nanoparticle. Figure 2b, 2c, and 2d shows the identical 5083-, 10 179-, and 21 127-atom icosahedral Au nanoparticles, respectively. The bimetallic nanoparticle possesses the core-shell structure, which was found commonly in bimetallic nanoparticles experimentally and

theoretically.35 Here, the selected Ag-Au bimetallic nanoparticles possess the core-shell structure, as observed in experiment,36 in which the surface layer is covered with Ag atoms and other layers are occupied by Au atoms. Figure 2e and 2f shows the identical 5083- and 10 179-atom icosahedral Ag-Au bimetallic nanoparticles, respectively, and the other two possess sizes of 2057 and 21 127 atoms. In addition, the icosahedral 2057-atom Fe, 5083-atom Ag, and 21 127-atom Cu nanoparticles were adopted in this work. It is worth pointing out that some of the nanoparticle building blocks in the size here could be not icosahedrons, and the newly crystallized structure becomes greatly different from the original one in the MD simulations. For example, after crystallization, the original icosahedral structure disappeared and cuboctahedrons emerged for the Fe nanoparticles in our simulation. However, it would not affect the final self-assembly simulation results. Potential Models and Simulation Code. All MD simulations were performed by using the DL-POLY version 2.18 code.37,38 For the simulating synthesis of the metal nanorods, nanoplates, and nanoframes, except the beaded Ag-Cu nanorods, there were no periodic boundary conditions, and the simulation cells containing the nanoparticles were adjusted to form the corresponding metal nanostructures. For the simulating synthesis of the beaded Ag-Cu nanorods, the one-dimensional periodic boundary condition was adopted. On the other hand, for the

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Figure 2. Nanoparticle building blocks: (a) 2057-atom icosahedral Ag nanoparticle; (b) 5083-, (c) 10 179-, and (d) 21 127-atom icosahedral Au nanoparticles; (e) 5083- and (f) 10 179-atom icosahedral Ag-Au bimetallic nanoparticles. Note that Ag is colored red and Au yellow.

simulating synthesis of the Au nanoporous framework architecture, the three-dimensional periodic boundary conditions were used, and the simulation cells containing the nanoparticles represent the periodic systems. In our simulation, the second-moment approximation of the tight-binding (TB-SMA) potential39 was used to describe the interaction between the metal atoms in the nanostructures as in our previous works.40-42 The total energy of a system in the TB-SMA potential is written as N

Utotal )

(

N

N

)

∑ ∑ Ae-p(r ⁄r -1) - ∑ ξ2e-2q(r ⁄r -1) i

ij 0

j*i

ij 0

j*i

(1)

where rij is the distance between atoms i and j in the cluster and N is the number of metal atoms. r0 is the nearest-neighbor distance in the pure metals and taken as the average of the pure ones for the heterometallic interactions. The parameters (A, ξ, p, q) for the metal interactions used in this work are taken from the literature,39,43,44 as shown in Table 1. It is noted that the parameters have been successfully used to describe the metal alloys44 and monometallic and bimetallic nanoparticles.25,40-43 Simulating Synthesis Strategy. Two key steps in the simulating synthesis are described as follows. First, each nanoparticle was placed in a large-enough simulation cell to prevent their interactions. Then, a lot of identical simulation cells were arranged according to the shapes of the corresponding metal nanostructures. Second, the distances between the nanoparticles were gradually reduced, and then the attractive forces of nanoparticle-nanoparticle interactions took place. It is noticed that this general strategy has been used successfully by others29-31 in the study of assembling metal-oxide nanostructures. However, no report has been found in the literature yet for the assembly of transition-metal nanoparticles by using this strategy. Figure 3 shows our strategy for simulating synthesis to generate the metal nanostructures (see Figure 1) by selfassembly. The monometallic and bimetallic nanorods were

TABLE 1: Parameters of the TB-SMA Potential for the Ag-Ag, Au-Au, Cu-Cu, Fe-Fe, Ag-Cu, and Ag-Au Interactions a

Ag-Ag Au-Aua Cu-Cub Fe-Fec Ag-Ag in Ag-Cu and Ag-Aub Au-Au in Ag-Aub Ag-Cub Ag-Aub

A (eV)

ξ(eV)

p

q

r0 (Å)

0.1028 0.2061 0.0894 0.13315 0.1031

1.178 1.790 1.2799 1.6179 1.1895

10.928 10.229 10.55 10.50 10.85

3.139 4.036 2.43 2.60 3.18

2.8885 2.8843 2.5562 2.553 2.8885

0.2096 0.098 0.149

1.8153 1.2274 1.4874

10.139 10.700 10.494

4.033 2.805 3.607

2.8843 2.7224 2.8864

a Reference 39. b Reference 43. c Reference 44. The Ag-Ag interaction used in monometallic Ag systems39 is different than that used in bimetallic Ag-Cu and Ag-Au systems.43 Also, the Au-Au interaction is different for the monometallic Au39 and bimetallic Ag-Au43 systems.

generated by reducing the distances between the five identical simulation cells containing the monometallic or bimetallic nanoparticles, as shown in Figure 3a. It should be mentioned that this synthesis strategy by self-assembly has been successfully used to prepare Au28,45 and Pd46 nanorods. There was no periodic boundary condition along the axis direction of the nanorod. The building blocks are 10 179-atom icosahedral Au and Ag-Au nanoparticles for the monometallic and bimetallic nanorods, respectively. These nanoparticles were amorphized by performing the annealing MD simulations at 1000 K for 100 ps and then cooled to 10 K. Our strategy for generating the beaded Ag-Cu bimetallic nanorods is to reduce the distances between seven simulation cells, as shown in Figure 3b. The one-dimensional periodic boundary condition was applied along the axis direction of the nanorod. The big cell and six small identical cells represent the 21 127-atom icosahedral Cu nanoparticle and the six 2057-atom icosahedral Ag nanoparticles, respectively. These nanoparticles were also amorphized by performing the annealing MD simulations at 1000 K for 100 ps and then cooled to 10 K.



Figure 3. Schematic diagrams of the simulation cells for the simulating synthesis to generate the metal nanostructures by self-assembly of the corresponding nanoparticles: (a) Au and Ag-Au nanorods; (b) Beaded Ag-Cu nanorod; (c) Triangular Ag nanoplate; (d) Hexagonal Ag nanoplate; (e) Triangular Ag-Au nanoframe; (f) Hexagonal Au nanoframe; (g) Cubic hollow Fe and Ag-Au nanoframes. The symbols 1, 2, 3, and 4 in g show the four layers for each cubic hollow nanoframe.

Figure 4. Schematic diagrams of the simulation cells for the simulating synthesis to generate the predicted Au nanoporous framework architecture by self-assembly of Au nanoparticles. (a) Periodic array of Au nanoparticles positioned at cubic lattice positions. (b) Under MD simulation, the Au nanoparticles attract one another, move closer together, and aggregate, forming the three-dimensional channels. (c) Final three-dimensional Au nanoporous framework architecture.

The triangular Ag nanoplates, hexagonal Ag nanoplates, triangular Ag-Au nanoframes, and hexagonal Au nanoframes were generated by modifying the distances between those

monometallic or bimetallic nanoparticles, as shown in Figure 3c, 3d, 3e, and 3f, respectively. No periodic boundary condition was applied in the simulations, and the four


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Figure 5. Structure evolution during the self-assembly of five 10 179-atom icosahedral Au nanoparticles to form the Au nanorod at 0, 5, 25, 50, and 100 ps.

building blocks were composed of 5083-atom Ag, Ag, Ag-Au, and Au nanoparticles for the four nanostructures, respectively. These nanoparticles were amorphized as mentioned above. The cubic hollow Fe and Ag-Au nanoframes were generated by reducing the size of the simulation cells containing Fe or Ag-Au nanoparticles. There are four layers for each cubic hollow nanoframe with the symbols 1, 2, 3, 4, as shown in Figure 3g. Again, no periodic boundary condition was applied in the simulation, and the two building

blocks were composed of 2057-atom Fe and Ag-Au nanoparticles. It is interesting to trace the formation of the rather complicated cubic hollow Fe nanoframe in a larger time scale. These Fe nanoparticles were therefore amorphized by performing the annealing MD simulations at 1000 K for up to 500 ps and then cooled to 10 K. Figure 4 shows the strategy for our simulating synthesis to generate the predicted Au nanoporous framework architectures, which have not been reported by self-assembly in experiment. The strategy is to modify the simulation cell to



Figure 6. Structure evolution during the self-assembly of six 2057-atom icosahedral Ag nanoparticles, and a 21 127-atom icosahedral Cu nanoparticle to form the beaded Ag-Cu nanorod at 0, 5, 25, 50, and 100 ps. Note that Ag is colored red and Cu green.

ensure that the Au nanoparticles could interact with each other in all three dimensions. A similar strategy has been used to generate the Ti-doped CeO2 nanoporous framework architectures by self-assembly of the corresponding nanoparticles.29 The building block was the icosahedral 21 127-atom Au nanoparticle, as shown in Figure 2d. The threedimensional periodic boundary conditions were used in the simulation. Figure 4a shows the Au nanoparticles within the periodic simulation cells in the initial condition, and each simulation cell consisting of the Au nanoparticle is small

enough to ensure the aggregation between the nanoparticles. These nanoparticles were amorphized by performing the annealing MD simulations at 1000 K for 50 ps. In the MD simulations, the nanoparticles attracted, moved closer between each other, and aggregated, forming the three-dimensional channels, as shown in Figure 4b. Then, the nanoparticles continued to aggregate, forming the three-dimensional Au nanoporous framework architecture (see Figure 4c). After 50 ps, the Au nanoporous framework architecture was cooled to 10 K.


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Figure 7. Final structure at 10 K of (a) the Au nanorod, (b) the Ag-Au nanorod, and (c) the beaded Ag-Cu nanorod. Note that Ag is colored red, Au yellow, and Cu green.

Figure 8. Evolution processes and final results of the triangular and hexagonal Ag nanoplates. A, B, C, and D represent the structures at 0, 5, 25, and 50 ps, respectively, and E represents the final structure at 10 K for the triangular Ag nanoplate. F, G, H, and I represent the structures at 0, 5, 25, and 50 ps, respectively, and J represents the final structure at 10 K for the hexagonal Ag nanoplate.



Figure 9. Evolution processes and final results of the triangular Ag-Au and hexagonal Au nanoframes. A, B, C, and D represent the structures at 0, 5, 25, and 50 ps, respectively, and E represents the final structure at 10 K for the triangular Ag-Au nanoframe. F, G, H, I, and J represent the structures at 0, 5, 25, 50, and 100 ps, respectively, and K represents the final structure at 10 K for the hexagonal Au nanoframe. Note that Ag is colored red and Au yellow.

Figure 10. Evolution process of the cubic hollow Fe nanoframe. A and B represent the three-dimensional and X-Z-view initial structure of the system, respectively. C, D, E, F, G, H, and I represent the X-Z-view structures at 5, 50, 100, 200, 300, 400 and 500 ps, respectively. J represents the three-dimensional structure of the cubic hollow Fe nanoframe at 500 ps.

Results Nanorods. Typical structures during the self-assembly of five 10 179-atom icosahedral Au nanoparticles are shown in Figure 5. The structure at 0 ps in Figure 5 shows the initial condition of the five Au nanoparticles. Then, these nanoparticles were amorphized by performing MD simulation, resulting in interaction with their neighbors, as shown by the structure at 5 ps in Figure 5. The nanoparticles aggregated with each other after 25 ps and assembled into the Au nanorod after 50 ps. At last, a perfect Au nanorod was formed by the five Au nanoparticles, as given by the structure at 100 ps in Figure 5. Similar to the Au nanorod, the Ag-Au nanorod was formed by the selfassembly of five 10 179-atom identical Ag-Au nanoparticles. Each Ag-Au nanoparticle possesses the core-shell structure in which Ag atoms occupy the surface shell, covering completely the Au atoms in the core. The evolution process of forming the Ag-Au nanorod is similar to the self-assembly of the Au nanoparticles into the Au nanorod. The beaded Ag-Cu bimetallic nanorod was formed by the self-assembly of six 2057-atom icosahedral Ag nanoparticles and a 21 127-atom icosahedral Cu nanoparticle. The evolution process of forming the beaded Ag-Cu bimetallic nanorod is shown in Figure 6. The Cu nanoparticle is placed in the center, and the six Ag nanoparticles are arranged in two sides, as shown by the initial structure at 0 ps. Then, the whole system was amorphized by performing the annealing MD simulation at 1000

K, resulting in the assembly by the nanoparticle-nanoparticle interaction. At last, the seven nanoparticles assembled into the beaded Ag-Cu bimetallic nanorod, as given by the structure at 100 ps in Figure 6. The final structure of the Au nanorod at 10 K is shown in Figure 7a. Figure 7b shows the final structure of the Ag-Au nanorod from the self-assembly of the five Ag-Au nanoparticles at 10 K. Unlike the Au nanoparticle, the Ag-Au nanoparticle possesses the special core-shell structure. Therefore, the AgAu nanorod also presents the core-shell structure after the evolution, in which the Ag atoms stay mainly in the surface region and the Au atoms occupy the internal sites, forming the core-shell nanorod, as shown in Figure 7b. The final structure at 10 K of the beaded Ag-Cu nanorod is shown in Figure 7c. It is found in Figure 7c that the Cu nanoparticles are connected with the Ag nanorods, forming the beaded Ag-Cu nanorod. In the beaded Ag-Cu nanorod, the big Cu nanoparticle and the small Ag nanorod are clearly arranged alternately. A basic unit of the beaded Ag-Cu nanorod is also shown in Figure 7c. From this basic unit, we can see that some Ag atoms diffuse onto the surface of the Cu nanoparticle, making the beaded Ag-Cu nanorod more rigid. Nanoplates. The triangular and hexagonal Ag nanoplates were generated by self-assembly of the identical 5083-atom Ag nanoparticles. Figure 8 shows the evolution processes and final triangular and hexagonal Ag nanoplates. Structures A, B, C,


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Figure 11. Evolution process of the cubic hollow Ag-Au nanoframe. A and B represent the three-dimensional and X-Z-view initial structure of the system, respectively. C, D, and E represent the X-Z-view structures at 5, 25, and 50 ps, respectively. F represents the three-dimensional structure of the cubic hollow Ag-Au nanoframe at 50 ps. Note that Ag is colored red and Au yellow.

Figure 12. Final structures at 10 K of the cubic hollow (a) Fe and (b) Ag-Au nanoframes. B-B, C-C, and D-D represent the cross-sections of Fe nanoframe. E-E, F-F, and G-G represent the cross-sections of Ag-Au nanoframe. Note that Ag is colored red, Au yellow, and Fe green.

and D in Figure 8 represent the evolution at 0, 5, 25, and 50 ps for the triangular Ag nanoplate, respectively. The initial configuration at 0 ps is shown Figure 8A. Then, the 10 identical 5083-atom Ag nanoparticles were amorphized by performing the annealing MD simulation, forming the triangular Ag nanoplate, as shown in parts B and C of Figure 8. A new structure approaching the triangular Ag nanoplate was formed at 50 ps (see Figure 8D). Figure 8E gives the final structure at 10 K of the triangular Ag nanoplate. Similar to the formation process of the triangular Ag nanoplate, the hexagonal Ag nanoplate was also formed by selfassembly of the identical 5083-atom Ag nanoparticles. F-I of Figure 8 show the evolution of self-assembling the 14 identical 5083-atom Ag nanoparticles into the hexagonal nanoplate at 0, 5, 25, and 50 ps. The initial structure of these nanoparticles is shown in Figure 8F. Then, the hexagonal Ag nanoplate was formed gradually by aggregation of the 14 identical 5083-atom

Ag nanoparticles from 5 to 50 ps (see G-I of Figure 8). Figure 8J demonstrates the final structure at 10 K of the hexagonal Ag nanoplate. Nanoframes. The triangular Ag-Au nanoframe was generated by self-assembly of 12 identical 5083-atom Ag-Au nanoparticles. Figure 9A shows the initial configuration of 12 identical 5083-atom Ag-Au nanoparticles at 0 ps. It should be mentioned that the initial 5083-atom Ag-Au nanoparticles possess the core-shell structure with Ag occupying the surface layer and Au staying in the inner core, as shown in Figure 9A. Then, these Ag-Au nanoparticles were amorphized by performing the annealing MD simulation at 1000 K, and the structures at 5, 25, 50 ps in the evolution process are shown in B, C, D of Figure 9, respectively. It is found in Figure 9D that the 12 identical 5083-atom Ag-Au nanoparticles aggregated into a new structure with the triangular Ag-Au nanoframe at 50 ps. After 100 ps, the triangular Ag-Au nanoframe was



Figure 13. Snapshots with multiple simulation cells for the Au nanoporous framework architecture aiming to show the periodic channels at (a) 0.5, (b) 5, (c) 10, (d) 15, (e) 25, and (f) 50 ps, respectively.

Figure 14. Final three-dimensional structures at 10 K of (a) one Au framework unit cell, (b) the other unit cell of the Au nanoporous framework architecture, and (c) the unit cell with the “top” sliced off.

formed. The final structure at 10 K of the triangular Ag-Au nanoframe is presented in Figure 9E, in which most of the Ag atoms in the final triangular Ag-Au nanoframe stay in the surface region. It means that the surface segregation phenomenon of Ag appears even in the triangular Ag-Au nanoframe. Similar results are found in the formation process of the hexagonal Au nanoframe. Structures F, G, H, and I in Figure 9 represent the evolution at 0, 5, 25, and 50 ps for the hexagonal Au nanoframe, respectively. The initial configuration at 0 ps of the 12 identical 5083-atom Au nanoparticles is shown in Figure 9F. Then, the hexagonal Au nanoframe was formed by aggregation of the 12 identical 5083-atom Au nanoparticles from 5 to 50 ps by performing the annealing MD simulation gradually (see G-I of Figure 9). At 50 ps, a hexagonal Au nanoframe, Figure 9I, was found. The structure of the Au nanoframe at 100 ps is shown in Figure 9J. It is found that the Au nanoframe possesses the ring-like structure, indicating that the hexagonal Au nanoframe was changed into the ring-like Au nanoframe at 100 ps. The final structure at 10 K of the ring-like Au nanoframe is demonstrated in Figure 9K. Figure 10 shows the formation process of the cubic hollow Fe nanoframe by self-assembly of 32 identical 2057-atom Fe

nanoparticles. Figure 10A shows the three-dimensional initial structure of the Fe nanoparticles at 0 ps, and Figure 10B shows the X-Z view of the structure. Then, these Fe nanoparticles were amorphized by performing the annealing MD simulation and the structures at 5, 50, 100, 200, 300, 400, and 500 ps in the evolution process are shown in C, D, E, F, G, H, and I of Figure 10 with the X-Z view, respectively. It is found in B-E of Figure 10 that the hollow Fe nanoframe shrinks but keeps its cubic shape unchanged from 0 to 100 ps. When the Fe nanoframe was amorphized from 100 to 200 ps, its cubic shape was changed into a rhombohedron-like one, as shown in E and F of Figure 10. At 300 ps, the rhombohedron-like hollow Fe nanoframe was formed, as shown in Figure 10G. It is found in G-I of Figure 10 that the hollow cavity in the Fe nanoframe retains its size and rhombohedron-like shape unchanged from 300 to 500 ps. The three-dimensional structure of the hollow Fe nanoframe at 500 ps is shown in Figure 10J. Similar to the Fe nanoframe, the cubic hollow Ag-Au nanoframe was generated by self-assembly of 32 identical 2057atom Ag-Au nanoparticles, as shown in Figure 11. The threedimensional initial structure and its X-Z view are shown in A and B of Figure 11, respectively. It is found in Figure 11B that


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Figure 15. Final three-dimensional structure at 10 K of the Au nanoporous framework architecture: (a) 1 × 5 × 5 simulation cells; (b) 3 × 3 × 3 simulation cells; (c) 2 × 2 × 2 fully atomistic model; (d) 2 × 2 × 2 fully atomistic model with the “top” sliced off.

the 2057-atom Ag-Au nanoparticle possesses the core-shell structure with Ag atoms occupying the surface layer and Au atoms staying in the core. After 5 ps, these Ag-Au nanoparticles merged due to the attractive forces of nanoparticle-nanoparticle interactions, as shown by Figure 11C. Then, the cubic hollow Ag-Au nanoframe was formed gradually (see D and E in Figure 11). Figure 11F shows the three-dimensional structure of the cubic hollow Ag-Au nanoframe at 50 ps. Figure 12a and 12b presents the final structures at 10 K of the cubic hollow Fe and Ag-Au nanoframes, respectively. It is found in Figure 12b that most of the Ag atoms in the final hollow Ag-Au nanoframe stay in the surface region of the nanoframe, indicating that the surface segregation of Ag also occurs in the hollow Ag-Au nanoframe. To show it more clearly, we also give the structures for different cross-sections. B-B, C-C, and D-D of Figure 12 represent the cross-sections of the hollow Fe nanoframe. Clearly, the channels and cavities are found for the Fe nanoframe. It is found in Figure 12a that the final Fe nanoframe possesses the rhombohedron-like structure. E-E, F-F, and G-G of Figure 12 show the cross-sections of the hollow Ag-Au nanoframe. Similarly, the inner channels and cavities are found in the Ag-Au nanoframe. Predicted Nanoporous Framework Architecture. Figure 13a-f shows the snapshots with multiple simulation cells for the Au nanoporous framework architecture with the periodic channels taken during the annealing MD simulation at 0.5, 5, 10, 15, 25, and 50 ps, respectively. It is found in Figure 13a that the Au nanoparticles still maintained the initial shape after 0.5 ps. Then, the Au nanoparticles began to aggregate with each other, forming the channels gradually, as shown in Figure 13b-d. After 25 ps, the spherical channels were found for the system due to aggregation of the Au nanoparticles, as shown in Figure 13e. The Au nanoporous framework architecture was formed with the periodic channels at 50 ps, as shown in Figure 13f. Figure 14a shows the final three-dimensional structure of one Au framework unit cell at 10 K, which was evolved from the Au nanoparticle. Similarly, the other Au framework unit cell, which reveals a cavity interconnected by perpendicular channels, is shown in Figure 14b, and the same unit cell with the “top” sliced off is shown in Figure 14c. It is found in Figure 14b and 14c that inner channels and cavities are in the shifted unit cell of the three-dimensional Au nanoporous framework architecture. The final three-dimensional structure at 10 K of the Au nanoporous framework architecture with the 1 × 5 × 5

simulation cells is shown in Figure 15a. It is found in Figure 15a that the Au nanoporous framework architecture possesses the inner-connecting channels. Similarly, a surface-rendered model of the Au nanoporous framework architecture with the 3 × 3 × 3 simulation cells at 10 K is shown in Figure 15b to reveal clearly the Au nanoporous framework architecture with the inner-connecting channels in three dimensions. To show the inner cavities in the Au nanoporous framework architecture, a 2 × 2 × 2 fully atomistic model of the Au nanoporous framework architecture is shown in Figure 15c. Moreover, the 2 × 2 × 2 fully atomistic model with the “top” sliced off is shown in Figure 15d. Figure 15c and 15d demonstrates that there are three-dimensional periodic inner cavities in the Au nanoporous framework architecture. Conclusion The synthesis of metal nanostructures such as nanorods, nanoplates, and nanoframes has attracted steadily growing attention due to their extensive applications in chemical and biological sensing, separation, and catalysis in particular. In this work, we describe a potential simulating synthesis strategy to prepare the metal nanostructures by the self-assembly of the metal nanoparticles. On the basis of this synthesis strategy, we can prepare not only the metal nanorods, nanoplates, and nanoframes (shown in experiment) but also the metal nanoporous framework architecture (not shown in experiment). These metal nanostructures were generated in two respects: one is to modify the distances between the metal nanoparticles to ensure the attractive forces of the nanoparticle-nanoparticle interactions; another is to allow the amorphous nanoparticles to selfassemble by the annealing MD method, in which the amorphous nanoparticles were obtained at 1000 K and the metal nanostructures were formed after cooling to 10 K. The final atomistic structures of the metal nanostructures, including Au, Ag-Au, and beaded Ag-Cu nanorods, triangular and hexagonal Ag nanoplates, triangular Ag-Au nanoframes and hexagonal Au nanoframes, cubic hollow Fe, and Ag-Au nanoframes, are in good agreement with the experimental results.6,9,11,18-23 We can conclude that these metal nanostructures could be synthesized by self-assembly of the corresponding nanoparticles by simulating synthesis. In addition, we also present the predicted Au nanoporous framework architecture by the self-assembly of Au nanoparticles. It is found that the

Self-Assembly of Nanoparticle Building Blocks predicted metal nanoporous framework architecture possesses the three-dimensional periodic inner-connecting channels and cavities. In summary, we present here the strategy for simulating synthesis by self-assembly of the metal nanoparticles to generate the metal nanostructures. The results, which are in good agreement with experiment in several cases, show that our simulation strategy is rational and reliable. It is noticed that computational limitations enble us to make some approximations in our simulation inevitability, although these will be overcome in the future. It is believed that the simulating synthesis of metal microstructures will help guide the design and preparation of novel metal nanomaterials with the desired chemical properties, in particular. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Nos. 20736002 and 20776005), the National Basic Research Program of China (Grant No.G2003CB615807), the Program of NCET of Ministry of Education (NCET-06-0095), and “Chemical Grid Program” of BUCT. References and Notes (1) Haes, A. J.; Van Duyne, R. P. J. Am. Chem. Soc. 2002, 124, 10596– 10604. (2) Park, S.-J.; Taton, T. A.; Mirkin, C. A. Science 2002, 295, 1503– 1506. (3) Tanase, M.; Bauer, L. A.; Hultgren, A.; Silevitch, D. M.; Sun, L.; Reich, D. H.; Searson, P. C.; Meyer, G. J. Nano Lett. 2001, 1, 155–158. (4) Chen, M.; Goodman, D. W. Acc. Chem. Res. 2006, 39, 739–746. (5) Sun, Y.; Xia, Y. Science 2002, 298, 2176–2179. (6) Ni, W.; Kou, X.; Yang, Z.; Wang, J. ACS Nano 2008, 2, 677–686. (7) Tan, Y. N.; Lee, J. Y.; Wang, D. I. C. J. Phys. Chem. C 2008, 112, 5463–5470. (8) Sun, Y.; Xia, Y. AdV. Mater. 2004, 16, 264–268. (9) Bonacina, L.; Callegari, A.; Bonati, C.; vanMourik, F.; Chergui, M. Nano Lett. 2006, 6, 7–10. (10) Bae, Y.; Kim, N. H.; Kim, M.; Lee, K. Y.; Han, S. W. J. Am. Chem. Soc. 2008, 130, 5432–5433. (11) Kim, D.; Park, J.; An, K.; Yang, N. K.; Park, J. G.; Hyeon, T. J. Am. Chem. Soc. 2007, 129, 5812–5813. (12) Nie, Z. H.; Fava, D.; Rubinstein, M.; Kumacheva, E. J. Am. Chem. Soc. 2008, 130, 3683–3689. (13) Zhu, Y. C.; Geng, W. T. J. Phys. Chem. C 2008, 112, 8545–8547. (14) Sun, Y.; Xia, Y. Anal. Chem. 2002, 74, 5297–5305. (15) Portney, N.; Ozkan, M. Anal. Bioanal. Chem. 2006, 384, 620– 630. (16) Kim, S. W.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc. 2002, 124, 7642–7643.

J. Phys. Chem. C, Vol. 113, No. 10, 2009 3997 (17) Chen, J.; Saeki, F.; Wiley, B. J.; Cang, H.; Cobb, M. J.; Li, Z. Y.; Au, L.; Zhang, H.; Kimmey, M. B.; Li, X. D.; Xia, Y. Nano Lett. 2005, 5, 473–477. (18) Seo, D.; Yoo, C. I.; Jung, J.; Song, H. J. Am. Chem. Soc. 2008, 130, 2940–2941. (19) Walter, E. C.; Murray, B. J.; Favier, F.; Penner, R. M. AdV. Mater. 2003, 15, 396–399. (20) Xie, J.; Lee, J. Y.; Wang, D. I. C.; Ting, Y. P. ACS Nano 2007, 1, 429–439. (21) Metraux, G. S.; Cao, Y. C.; Jin, R.; Mirkin, C. A. Nano Lett. 2003, 3, 519–522. (22) Jiang, L. P.; Xu, S.; Zhu, J. M.; Zhang, J. R.; Zhu, J. J.; Chen, H. Y. Inorg. Chem. 2004, 43, 5877–5883. (23) Lu, X.; Au, L.; McLellan, J.; Li, Z. Y.; Marquez, M.; Xia, Y. Nano Lett. 2007, 7, 1764–1769. (24) Cheng, D.; Huang, S.; Wang, W. Phys. ReV. B 2006, 74, 064117. (25) Cheng, D.; Wang, W.; Huang, S. J. Phys. Chem. B 2006, 110, 16193–16196. (26) Glotzer, S. C.; Solomon, M. J. Nat. Mater. 2007, 6, 557–562. (27) Tang, Z.; Kotov, N. A.; Giersig, M. Science 2002, 297, 237–240. (28) Xie, J.; Zhang, Q.; Lee, J. Y.; Wang, D. I. C. J. Phys. Chem. C 2007, 111, 17158–17162. (29) Sayle, D. C.; Feng, X.; Ding, Y.; Wang, Z. L.; Sayle, T. X. T. J. Am. Chem. Soc. 2007, 129, 7924–7935. (30) Sayle, D. C.; Seal, S.; Wang, Z.; Mangili, B.; Price, D. W.; Karakoti, A. S.; Kuchibhatla, S. V. T. N.; Hao, Q.; Mobus, G.; Xu, X.; Sayle, T. X. T. ACS Nano 2008, 2, 1237–1251. (31) Sayle, D. C.; Mangili, B. C.; Klinowski, J.; Sayle, T. X. T. J. Am. Chem. Soc. 2006, 128, 15283–15291. (32) Vines, F.; Illas, F.; Neyman, K. M. Angew. Chem., Int. Ed. 2007, 46, 7094–7097. (33) Marks, L. D. Rep. Prog. Phys. 1994, 57, 603. (34) Martin, T. P. Phys. Rep. 1996, 273, 199. (35) Ferrando, R.; Jellinek, J.; Johnston, R. L. Chem. ReV. 2008, 108, 845–910. (36) Shibata, T.; Bunker, B. A.; Zhang, Z.; Meisel, D.; Vardeman, C. F.; Gezelter, J. D. J. Am. Chem. Soc. 2002, 124, 11989–11996. (37) Smith, W. J. Mol. Graphics 1987, 5, 71–74. (38) Smith, W.; Forester, T. R. J. Mol. Graphics 1996, 14, 136–141. (39) Cleri, F.; Rosato, V. Phys. ReV. B 1993, 48, 22–33. (40) Cheng, D.; Wang, W.; Huang, S. J. Phys. Chem. C 2007, 111, 8037–8042. (41) Cheng, D.; Wang, W.; Huang, S. J. Phys. Chem. C 2007, 111, 1631–1637. (42) Cheng, D.; Wang, W.; Huang, S.; Cao, D. J. Phys. Chem. C 2008, 112, 4855–4860. (43) Rapallo, A.; Rossi, G.; Ferrando, R.; Fortunelli, A.; Curley, B. C.; Lloyd, L. D.; Tarbuck, G. M.; Johnston, R. L. J. Chem. Phys. 2005, 122, 194308. (44) Stanek, J.; Marest, G.; Jaffrezic, H.; Binczycka, H. Phys. ReV. B 1995, 52, 8414. (45) Huang, Y.; Li, D.; Li, J. Chem. Phys. Lett. 2004, 389, 14–18. (46) Shi, Z.; Wu, S.; Szpunar, J. A. Nanotechnology 2006, 2161.

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Simulating Synthesis of Metal Nanorods, Nanoplates, and

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