Stepwise Growth of Decahedral and Icosahedral Silver Nanocrystals

Oct 30, 2009 - Chemistry - A European Journal 2018 11, .... Shape evolution of decahedral and icosahedral Ag flags and their intermediates from Ag nan...
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DOI: 10.1021/cg9009042

Stepwise Growth of Decahedral and Icosahedral Silver Nanocrystals in DMF

2010, Vol. 10 296–301

Masaharu Tsuji,*,†,‡ Masatoshi Ogino,‡ Ryoichi Matsuo,‡ Hisayo Kumagae,† Sachie Hikino,† Taegon Kim,‡ and Seong-Ho Yoon†,‡ †

Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga 816-8580, Japan, and Department of Applied Science for Electronics and Materials, Graduate School of Engineering Sciences, Kyushu University, Kasuga 816-8580, Japan

‡

Received August 1, 2009; Revised Manuscript Received September 21, 2009

ABSTRACT: Silver nanocrystals were synthesized by reducing AgNO3 in N,N-dimethylformamide (DMF) solution. Besides decahedral and icosahedral nanoparticles, a series of their intermediate particles, which consist of a combination of two and more tetrahedra, are obtained. It was found that decahedral and icosahedral nanoparticles are not formed through assembling of tetrahedra formed separately but produced through the stepwise growth of tetrahedral units on specific facets in DMF. A simple combination model of tetrahedral units suggested that the growth position of the fourth tetrahedral unit determines whether a decahedron or icosahedron is finally produced. In the formation of icosahedron, the crystal growth occurs inside of decahedral units. No further growth from decahedron to icosahedron was observed, indicating that there is a large energy barrier for the addition of a tetrahedron unit to a decahedron. Our study gives new information on the stepwise growth mechanism of decahedra and icosahedra in DMF solution.

Introduction Recently, control of metallic nanostructures has been the focus of intensive research because of their shape-dependent chemical and physical properties. Among them, nanostructured Au and Ag have attracted considerable attention mainly as a result of their remarkable optical properties and numerous applications in fields such as catalysts, surface plasmonics, surface-enhanced Raman scattering, and chemical and biological sensing.1-4 Different chemical and physical properties of metallic crystals arise from different crystal surface orientations. For example, the {111}, {100}, and {110} surfaces of face centered cubic (fcc) metals such as gold and silver have very different surface atom densities, electronic structures, and chemical reactivities. Therefore, the controllable preparation of nanocrystals with different shapes and exposed surfaces is very important and challenging. The past decade has witnessed the successful synthesis of gold and silver nanocrystals in a variety of shapes such as sphere, spheroid, cube, cuboctahedron, octahedron, tetrahedron, right bipyramid, decahedron, icosahedron, thin plate, and rod/wire.1-10 An equilibrium condition is not established during solution-phase syntheses. Under such a condition, multiple-twin decahedron and icosahedron, which are different from the equilibrium Wulff polyhedral shape,11 are often produced because the total free energies of decahedron and icosahedron with the twin defects are lower than that of a single crystal Wulff polyhedron. Due to the fan-out configurations of decahedron and icosahedron, the defected region will keep increasing in area as the decahedral and icosahedral seeds are enlarged laterally, making the total free energy of the system go up. As a result, these multiply twinned nanoparticles are only favored by thermodynamics at relatively small sizes. Thus, it is generally *To whom correspondence should be addressed. E-mail: tsuji@cm. kyushu-u.ac.jp. pubs.acs.org/crystal

Published on Web 10/30/2009

difficult for syntheses of large sizes of decahedral and icosahedral particles. Compared with extensive studies on syntheses of decahedral and icosahedral gold particles,1g,5-10 little work has been performed for silver. This is because cubes, bipyramids, and rods/wires with {100}-type facets and triangular or hexagonal platelike particles with {111}-type facets are preferentially formed,1,3,4 and it is generally difficult to prepare decahedral and icosahedral particles having {111} facets. Recently, Gao et al.12 found that tetrahedral and decahedral Ag nanocrystals having {111} facets could be prepared in high yields when AgNO3 was reduced in N,N-dimethylformamide (DMF) solution with the assistance of PVP (Mw = 1,300,000) under the condition of conventional oil-bath heating. On the basis of SEM and TEM observations of obtained products, they proposed that decahedral Ag nanocrystals are assembled from five tetrahedra step by step. This is an unusual growth mechanism of multiple-twin particles, because decahedral particles are generally grown from small quasi-spherical fivetwin particles1g and icosahedra are grown either by a shell-byshell mode on a small-size stable icosahedron or by a complete structural transformation from a decahedron to a metastable icosahedron on the basis of molecular-dynamics simulations of silver clusters.13 Thus, the stepwise growth of multiple twin particles in DMF gives new fundamental information on the crystal growth mechanisms of multiple-twin crystals. An important suggestion in the work of Gao et al.12 is that favorable facets of Ag nanocrystals prepared in DMF solution can be changed from {100}-type to {111}-type in DMF. Although they proposed the stepwise growth mechanism of decahedral Ag nanocrystals, little information on the growth mechanism of icosahedral Ag nanocrystals was obtained. We have recently studied the synthesis of Au core and Ag shell, denoted by Au@Ag, nanocrystals in DMF.14 We succeeded in the preparation of triangular or hexagonal platelike, octahedral, and decahedral Au@Ag nanocrystals r 2009 American Chemical Society

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surrounded by Ag shells having {111} facets. We found that there are two routes for the formation of decahedral Au@Ag nanocrystals. One is the overgrowth of a decahedral Ag shell on a Au decahedral core, and the other is the stepwise growth of a decahedral Ag shell on a truncated bitetrahedral Au core. In the present study, we prepared not only decahedral Ag nanocrystals but also icosahedral Ag nanocrystals in DMF. We can observe a series of intermediate species of decahedron and icosahedron, which consist of a combination of two or more tetrahedra. On the basis of TEM and SEM observations and a simple combination model of tetrahedra, stepwisegrowth mechanisms of decahedron and icosahedron are discussed. Although Gao et al.12 reported that decahedra are formed by assembling of tetrahedra, we found that their mechanism cannot be applied under our conditions. Decahedra and icosahedra are not produced via assembling of tetrahedral units formed separately, but they are produced via stepwise growth of tetrahedral units on a specific {111} facet of intermediate species. On the basis of a simple combination model of tetrahedral units, we found that there is a key step in the stepwise growth mechanisms whether decahedral or icosahedral particles are produced. Our data provide new general information for the crystal growth mechanisms of multiple-twin crystals, which are important for the shape controlled syntheses of multiple-twin decahedral and icosahedral fcc crystals. Experimental Section AgNO3 (99.8%) and DMF (99.5%) were purchased from Kishida Chemical Industries Ltd. PVP powders (average molecular weight Mw ≈ 1,300,000 in terms of monomer units) were purchased from Sigma-Aldrich Inc. All these reagents were used without further purification. DMF was used as both reductant and solvent in this study. To study crystal growth mechanisms of pentagonal and hexagonal Ag nanostructures from AgNO3/PVP/DMF mixtures, we changed such experimental parameters as the PVP/AgNO3 ratio, the molecular weight of PVP (average molecular weight in terms of monomer units: 10,000, 40,000, 55,000, and 1,300,000), the injection rate of AgNO3, and the oil-bath and microwave heating.15 Then we found that the following conditions were best for the studies of each Ag nanostructures. For the study of pentagonal particles and their intermediates, 15 mL of DMF was heated to 140 °C, and then a mixture of AgNO3 (25 mM) and PVP (126 mM Mw = 1,300,000) resolved in 15 mL of DMF solvent was injected dropwise into the solution through a syringe pump at a flow rate of 0.3 mL/min for 50 min. After all reagents were injected, solutions were heated at 140 °C for 3 h. The final concentrations of AgNO3 and PVP in 30 mL of DMF were 12.5 and 63 mM, respectively. When hexagonal Ag nanostructures were studied, the same conditions except for the final PVP concentration (250 mM) were used. After naturally cooling down to room temperature, Ag products were separated and obtained from C2H5OH solution by centrifuging the colloidal solution at 15,000 rpm for 60 min four times for TEM (JEOL JEM-2010) and SEM (Hitachi S-4800) observations. High resolution (HR) TEM images were also measured (JEOL 2100F). Extinction spectra of the product solutions were measured in the UV-visible (vis) region using a Shimadzu UV-3600 spectrometer.

Results and Discussion Crystal Growth of Decahedral Particles. Figure 1 shows typical TEM images of Ag nanostructures obtained from AgNO3 (12.5 mM)/PVP (63 mM)/DMF solution after injection of all AgNO3/PVP reagents for 0, 1, 2, and 3 h. Just after injection of AgNO3/PVP (Figure 1a), triangular particles due to tetrahedral particles and intermediates of decahedra are observed as major products. With increasing the heating time from 0 to 3 h, yields of decahedra and their interme-

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Figure 1. TEM images of Ag nanoparticles obtained from AgNO3 (12.5 mM)/PVP (63 mM)/DMF after injection of AgNO3/PVP for 0, 1, 2, and 3 h at 140 °C.

diates which are composed of two or more tetrahedral units increase. After heating for 3 h (Figure 1d), yields of each product calculated from the shapes of about 500 particles in TEM images were as follows: triangular and hexagonal platelike particles (15%), decahedra and their intermediates (32%), hexagonal particles and their intermediates (27%), and spherical particles (26%). In Figure 1, there are a lot of intermediate structures of decahedra. Expanded TEM images of decahedra and their intermediates are shown in Figure 2. These TEM images clearly demonstrate that decahedral Ag nanocrystals are grown via stepwise growth of the tetrahedral component. Besides perfect decahedra, their intermediates, which consist of 1-4 tetrahedral units, are observed. Moreover, it should be noted that imperfect intermediates having truncated structures due to growing facets are obtained as shown in Figure 2b, d, f, and h. Possible growth mechanisms of decahedra are shown in Figure 3. Mechanism a is the proposed process of Gao et al.12 According to their model, at first tetrahedra are produced and then decahedra are formed by assembling of separate tetrahedral units. The formation of decahedra occurs through an edge-selective particle fusion mechanism, with five tetrahedra gradually assembling into a decahedron. In their mechanism, several tetrahedra come together in stepwise fashion to form a decahedron. They reported evidence of this mechanism was the observation of some imperfect decahedra absent of one or two tetrahedra, forming an obvious notch. Mechanism b is a new process found in this study. In our experiments decahedra are grown through one-by-one stepwise growth of tetrahedral units without stepwise fusion of twin facets. This mechanism was supported by the observation of a series of intermediate particles with imperfect tetrahedral units (see Figure 2b, d, f, and h) besides the imperfect decahedra absent of one, two, or three tetrahedra. We found that, until the formation of truncatedtetra-tetrahedron, most of all crystal growth occurs on only one facet until the formation of a new tetrahedral unit is completed, and the simultaneous growth of two or more facets was rare. On the other hand, when the fifth tetrahedron is formed to complete a decahedron, it is grown from a two faced {111} plane and it is connected in the intermediate position, as shown in Figure 2h by a red arrow. After the formation of tetra-tetrahedron, the fifth tetrahedron is grown to complete a decahedron. The crystal growth on other {111} facets, which does not lead to a decahedron,

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Figure 2. TEM images of decahedral Ag nanoparticles and their intermediate crystals. Reddish purple parts show growing structures of truncated tetrahedron.

Figure 3. Possible stepwise growth mechanisms of five-twinned decahedral Ag nanocrystals: (a) tetrahedra are initially formed and assembled subsequently into a decahedron, and (b) a decahedron is formed via one-by-one growth of tetrahedral units. Processes nt mean that the nth tetrahedron is added in each process.

cannot be observed. This implies that the crystal growth from tetra-tetrahedron (g) to decahedron (i) in Figure 2 occurs selectively via the intermediate particle (h). Figure S1 (Supporting Information) shows UV-vis extinction spectra of products observed after the injection of AgNO3/PVP for 0-3 h. A broad peak is observed in the 320-800 nm region. This peak can be ascribed to overlap of surface plasmon resonance (SPR) bands of spherical particles, triangular and hexagonal plates, tetrahedra, decahedra, icosahedra, and their intermediate particles,1-4 as observed in Figure 1. The peak intensity becomes stronger, and a broad tail band in the 500-800 nm region becomes prominent with increasing heating time. The peak shifts from 450 to 500 nm with increasing heating time. These spectral changes indicate that numbers and sizes of product nanoparticles, including decahedra and their intermediates, increase with increasing heating time, as observed in TEM images. Crystal Growth of Icosahedral Particles. Figure 4 shows TEM images of Ag nanoparticles prepared from a AgNO3 (12.5 mM)/PVP (250 mM)/DMF mixture after injection of AgNO3/PVP for 0-3 h. Under these conditions, with increasing heating time, the sizes of product particles increase and well-defined shapes of products are obtained. After heating for 3 h (Figure 4d), the yields of each product calculated from the shapes of about 500 particles in TEM and SEM (shown below) images were as follows: triangular

Figure 4. TEM images of Ag nanoparticles obtained from AgNO3 (12.5 mM)/PVP (250 mM)/DMF after injection of AgNO3/PVP for 0, 1, 2, and 3 h at 140 °C.

and hexagonal platelike particles (16%), decahedra and their intermediates (22%), hexagonal particles and their intermediates (41%), and spherical particles (21%). This shows that the yield of hexagonal particles and their intermediates at higher concentration of PVP is higher than that in Figure 1. To examine the shapes of the hexagonal particles, we measured TEM images from different angles and HRTEM

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Figure 5. SEM images of Ag nanoparticles observed from various view angles and HRTEM images. Ag nanparticles were obtained from AgNO3 (12.5 mM)/PVP (250 mM)/DMF after injection of AgNO3/PVP for 3 h at 140 °C.

images (Figure S2 in Supporting Information). Most hexagonal particles have edge lines due to twin planes, and hexagonal structures are maintained by rotating view angles within (30°. The HRTEM images show that the intervals of each lattice fringe agree well with intervals of Ag{111} planes. Thus, hexagonal particles have {111} facets as dominant facets in each triangular component from the top view. To obtain more information on the crystal structures of hexagonal crystals, SEM images of products were measured (Figure 5). SEM images indicated that products consist of tetrahedra, triangular and hexagonal plates, decahedra, intermediates of polyhedra, and hexagonal particles (Figure 5). The bright vertexes and sharp edges of each Ag polyhedra and their intermediates are observed distinctly. From SEM images of hexagonal particles (Figure 5f), we had definite evidence that nonplatelike hexagonal particles were icosahedral particles and their intermediates which arise from combination of tetrahedral units. It is expected that the formation of an icosahedron proceeds through one-by-one stepwise growth of tetrahedral units as in the case of decahedron. However, the simultaneous growth of multiple facets was observed at the final stage for the formation of a complete icosahedron, as shown by red arrows in Figure 5f. Thus, the crystal growth of more than two facets occurs at the final stage for the full cover of the notch and cavity of the decahedron and icosahedron. Figure S3 (Supporting Information) shows UV-vis extinction spectra of products observed after heating for 0-3 h. The spectra were similar to those shown in Figure S1. The peak at 460 nm is observed more clearly than that in Figure S1. These spectral changes indicate that numbers and sizes of product nanoparticles including icosahedra and their intermediates increase with increasing heating time, as observed in TEM images (Figure 4). Stepwise Growth Mechanisms of Decahedral and Icosahedral Ag Nanostructures. Decahedra and icosahedra consist of

5 and 20 tetrahedra sharing one and three common edges, respectively, although some defects are involved in these multiple twin particles.1g It was found that decahedral and icosahedral Ag nanoparticles were produced through the stepwise growth as shown in Figures 3 and 6, where tetrahedral units are grown one-by-one from a AgNO3/PVP (Mw = 1,300,000) mixture in DMF. In Figure 6 are also shown typical SEM images of tetrahedron, decahedron, icosahedron, and intermediates of these polyhedra and their growth mechanisms. When the bitetrahedron is grown from a tetrahedron, the second tetrahedron can be grown on four equivalent {111} facets of the tetrahedron (process 2t in Figure 3). The resulting bitetrahedron has the same structure, independent of the position of four {111} growing facets. When the tritetrahedron is grown from a bitetrahedron, the third tetrahedron can be grown on six equivalent {111} facets of the bitetrahedron (process 3t in Figure 3). As in the case of the bitetrahedron, the tritetrahedron has the same structure with each other, independent of the position of the six {111} facets of the bitetrahedron. The above condition changes for the formation of tetra-tetrahedra. When tetra-tetrahedra are grown from a tritetrahedron by the addition of one more tetrahedron, three different shapes of tetra-tetrahedra are produced (4Ta, 4Tb, and 4Tc in Figure 6), depending on the position of growing facets as shown in reddish purple color. Figure 7 shows SEM and TEM images and crystal structures of three types of tetratetrahedra observed from various view angles. There are eight {111} facets in the tritetrahedron, and two, two, and four of them belong to types 3a, 3b, and 3c in Figure 6, respectively. From the growth on type 3a facets, a type 4Ta tetra-tetrahedron is produced. On the other hand, types 4Tb and 4Tc tetra-tetrahedra are produced from the growth on types 3b and 3c facets, respectively. It should be noted that decahedron 5T can only be produced through the addition of the fifth tetrahedron to type 4Ta tetra-tetrahedron, and its

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Figure 6. Stepwise growth mechanisms of decahedron and icosahedra and SEM images of various products. nT means that particles consist of n pieces of tetrahedra.

Figure 7. SEM, TEM, and crystal structures of three types of tetratetrahedra observed from various view angels. Growth facets of type 4Tb and 4Tc structures leading to icosahedron are shown in reddish purple color.

formation pathways through 4Tb and 4Tc are closed. On the other hand, an icosahedron and its intermediates are produced though the stepwise addition of tetrahedra to types 4Tb and 4Tc tetra-tetrahedra via such intermediates as 7T, 8T, and 10T in Figure 6. From 4Tb and 4Tc tetra-tetrahedra, the same 10T is formed via 8T and 7T. Figure 8 shows crystal structures of septa-, octa-, and deca-tetrahedra observed from various view angles as typical intermediates of icosahedra. In the stepwise growth of icosahedra, although the addition of tetrahedral units occurs to inner facets of intermediates as shown for 4T, 7T, 8T, and 10T in Figures 7 and 8 (reddish purple color), the addition of tetrahedral units to outside facets of intermediates (silver color) does not occur. This suggests that once types 4Tb and 4Tc in Figure 6 are formed, they grow to icosahedron by the stepwise inner growth via such intermediates as 7T, 8T, and 10T. Although an icosahedron has decahedral units, nanocrystals having

Figure 8. Growth facets (reddish purple) and nongrowth facets (silver) of septa-, octa-, and deca-tetrahedra leading to icosahedron.

one additional tetrahedron on a decahedron cannot be obtained. This suggests that the decahedral structure is a stable structure so that it can survive as a final product. Further growth from a decahedron to an icosahedron is unfavorable and an icosahedron is grown through a different growth process in the fourth step. Differences in surface energies of each {111} facet in intermediate species determine whether decahedral or icosahedral particles are produced preferentially in this stepwise growth. We found that the yields of decahedron and icosahedron changed by changing the concentration of PVP. Assuming that the probabilities of the addition of a tetrahedron to eight facets of a tritetrahedra are equal, the formation ratio of decahedra/icosahedra becomes 1:3, because the number of type 3a facets is one-third of type 3b þ 3c (see Figures 6 and 7). We found that the decahedra/icosahedra ratio was about 1:1

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at a PVP concentration of 64 mM, while it was about 1:2 at a higher PVP concentration of 250 mM. These results indicate that the overgrowth of a tetrahedron on tritetrahedra occurs more statistically at a high covering of PVP on the precursor intermediate. On the other hand, the overgrowth of a tetrahedral unit on type 3a facets in the notch part of tritetrahedra takes place at a higher probability than the statistic model at a lower PVP concentration, because growth facets can be more open for the addition of Ag atoms. Thus, the kinetic control of the growth process by the differences in the protection probability of each {111} facet by PVP can be used as a promising experimental parameter for the shape controlled synthesis of such multiple-twin polyhedra as decahedra and icosahedra. Conclusion The stepwise growth mechanisms of decahedral and icosahedral silver nanocrystals were studied by reducing AgNO3 in DMF solution. From the detailed TEM and SEM observations of products, we found that decahedral and icosahedral nanoparticles are not formed through assembling of tetrahedra formed separately but produced through stepwise growth of tetrahedral units on specific facets. A simple assembly model of tetrahedral units indicated that the growth position of the fourth tetrahedral unit determines whether decahedra or icosahedra are finally produced. Thus, a kinetic control of the fourth process by changing such an experimental parameter as the concentration of PVP is important for the shapeselective syntheses of silver polyhedra. Our study gives new information on the stepwise growth mechanism of decahedra and icosahedra in DMF solution. In order to clarify detailed growth mechanisms of decahedra and icosahedra through stepwise growth, further detailed studies including theoretical calculations are required. Acknowledgment. This work was supported by Joint Project of Chemical Synthesis Core Research Institutions, Grant-in-Aid for Scientific Research on Priority Areas “unequilibrium electromagnetic heating”, Grant-in-Aid for Scientific Research (B) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan (Nos. 19033003 and 19310064), and the Kyushu University GCOE program “Novel Carbon Resource Sciences”. We thank Prof. P. Jiang of National Center for Nanoscience and Technology, Beijing, for his helpful discussion at the beginning of this work, Prof. H. Ago and Dr. N. Ishigami in our lab for the use of their SEM and help in SEM observations, and Mr. T. Tanaka in our research institute for his help in HRTEM measurements.

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Supporting Information Available: UV-vis data and TEM images of hexagonal Ag nanoparticles observed from various view angles and their HRTEM images. This material is available free of charge via the Internet at http://pubs.acs.org.

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