Letter pubs.acs.org/NanoLett
Symmetry Breaking during Seeded Growth of Nanocrystals Xiaohu Xia and Younan Xia* The Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, School of Chemistry & Biochemistry and School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States S Supporting Information *
ABSTRACT: Currently, most of the reported noble-metal nanocrystals are limited to a high level of symmetry, as constrained by the inherent, face-centered cubic (fcc) lattice of these metals. In this paper, we report, for the first time, a facile and versatile approach (backed up by a clear mechanistic understanding) for breaking the symmetry of an fcc lattice and thus obtaining nanocrystals with highly unsymmetrical shapes. The key strategy is to induce and direct the growth of nanocrystal seeds into unsymmetrical modes by manipulating the reduction kinetics. With silver as an example, we demonstrated that the diversity of possible shapes taken by noble-metal nanocrystals could be greatly expanded by incorporating a series of new shapes drastically deviated from the fcc lattice. This work provides a new method to investigate shape-controlled synthesis of metal nanocrystal. KEYWORDS: Nanocrystal, symmetry breaking, kinetic control, silver, seed-mediated growth
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properties and applications of such nanocrystals. For instance, twined nanocrystals are usually more susceptible to oxidative etching due to the presence of twin defects on the surface.10 For the second approach, a few examples have been reported, including the formation of single-crystal Au nanorods in the presence of cetyltrimethylammonium bromide (CTAB),16 formation of single-crystal Ag or Pd nanobars/nanorods in the presence of bromide irons,17,18 and formation of unsymmetrically truncated Ag octahedrons in the presence of an etchant.19 However, most of these reported syntheses still lack a well-resolved mechanistic understanding, not mentioning the very limited number of unsymmetrical shapes. With Ag as an example, here we demonstrate an effective approach based on kinetic control to breaking the symmetry of a crystal lattice by simply manipulating the rate at which Ag atoms were generated from a precursor. Our experiments were based on seed-mediated growth that involves the use of Ag nanocubes as seeds in an aqueous system, with L-ascorbic acid (AA), AgNO3, and poly(vinyl pyrrolidone) (PVP) serving as the reductant, Ag precursor, and capping agent, respectively. The growth was initiated by adding an aqueous AgNO3 solution with a syringe pump into a mixture containing AA, PVP, and the seeds under magnetic stirring (see Supporting Information for experimental details). We found that the growth of a cubic seed, enclosed by six equivalent faces, could be directed to selectively take place on 1, 3, or 6 faces solely depending on the injection rate of the precursor. Both {111} and {100} facets were displayed on the newly formed Ag
ontrolling the shape of nanocrystals is a simple and yet effective means for tailoring their properties and optimizing their performance in various applications.1−9 To this end, noble-metal nanocrystals with a variety of different shapes (e.g., spheres, cubes, octahedrons, and plates) have been reported in the literature.10,11 However, as constrained by the inherent face-centered cubic (fcc) lattice taken by noble metals, essentially all of these shapes are highly symmetric in terms of spatial arrangements for the atoms. If the symmetry could be somehow broken in a controllable fashion, it would become feasible and even predictable to obtain nanocrystals with shapes and properties that have never been explored before. Since metals have a highly symmetric crystal structure, there is no intrinsic driving force for them to grow into nanocrystals with unsymmetrical shapes. For an fcc noble metal, the growth of a single-crystal seed tends to evolve into a cube, cuboctahedron, or octahedron depending on the relative growth rates along the ⟨100⟩ and ⟨111⟩ directions.12 Obviously, the confinement imposed by the cubic symmetry of a crystal lattice has to be lifted during the course of growth to obtain nanocrystals with other different shapes. In general, the symmetry can be broken using two different approaches: (i) incorporation of twin defects or stacking faults into the crystal lattice and (ii) induction of unsymmetrical or anisotropic growth for a single-crystal seed. Several examples have been reported for the first approach, including the 5-fold twinned nanorods or nanowires with a pentagonal cross-section13,14 and triangular/hexangular nanoplates containing stacking faults along the vertical direction.3,15 Although this approach can naturally break the symmetry of an fcc lattice, the existence of twin defects or stacking faults may negatively impact the © 2012 American Chemical Society
Received: October 31, 2012 Published: November 2, 2012 6038
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Figure 1. Three distinctive types of Ag nanocrystals that were obtained at different injection rates for AgNO3 solution. (a−d): (a) transmission electron microscopy (TEM) image, (b) scanning electron microscopy (SEM) image, (c) a representative example, and (d) 3D model of the Ag nanocrystals prepared at an injection rate of 0.7 mL/h for AgNO3 solution; (e−h): (e) TEM image, (f) SEM image, (g) representative examples, and (h) 3D model of Ag nanocrystals prepared at an injection rate of 8.0 mL/h for AgNO3 solution; (i−k): (i) TEM images, (j) SEM images, and (k) representative examples of Ag nanocrystals prepared at an injection rate of 100 mL/h for AgNO3 solution. The numbers on 3D models match the numbers on the electron micrographs. The 3D models shown in d and h can be conceived from the removal of five and three corners, respectively, from an octahedron. The 50 nm scale bars in i and j also apply to other TEM and SEM images. In each model, yellow and green colors denote {111} and {100} facets, respectively.
shape of these nanocrystals, which can be conceived by removing five of the six corners of an octahedron (Figure 1d). In the following discussion, such a nanocrystal is referred to as a “5/6-truncated octahedron” for the purpose of simplicity. When the injection rate was increased by almost 10 times to 8.0 mL/h, a different morphology was observed for the final product (Figure 1e−g). The electron micrographs clearly show that the product had two major projected profiles: “houses” and “triangles”. By carefully analyzing a set of images (Figure 1g, Figures S2b and S3b), we could also resolve their shape, which can be considered as an anisotropically truncated octahedron with three of the six corners being removed from an octahedron (Figure 1h). Such a nanocrystal will be referred to as “3/6truncated octahedron” in the following discussion. It is worth pointing out that these 3/6-truncated octahedrons had a morphology similar to what we observed by introducing a second aliquot of AgNO3 solution at the end of a sulfide-
nanocrystals, and their proportions could be adjusted by varying the concentration of PVP, a capping agent that preferentially binds to Ag(100) surface.20,21 Combined together, a variety of Ag nanocrystals with unsymmetrical shapes were obtained from the same seeds. The Ag nanocubes of 40 nm in edge length were prepared using a polyol method;22 they were slightly truncated at all corners (see Figure S1 in the Supporting Information). Figure 1 shows electron micrographs and three-dimensional (3D) models of the Ag nanocrystals obtained from syntheses conducted at three significantly different injection rates for the AgNO3 solution. At a slow injection rate of 0.7 mL/h, the final product looked like a bar with truncation at one of the two longitudinal ends (Figure 1a and b). From a set of electron micrographs (see Figure 1c, Figures S2a and S3a for TEM images of an individual particle at different tilting angles and at a higher magnification, respectively), we could resolve the 6039
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mediated polyol synthesis of Ag nanocubes.19 However, it should be emphasized that the mechanism for the formation of such an exotic shape was never clearly resolved until the present work. When the injection rate was further increased by approximately 10 times to 100 mL/h, conventional Ag octahedrons were obtained (Figure 1i−k). A further increase of the injection rate by quickly adding the AgNO3 solution into a reaction with a glass pipet resulted in the formation of concave Ag octahedrons (Figure S4), whose structure and growth mechanism had been elucidated in our previous work.23 To gain insight into the shape evolution and thus elucidate the mechanism involved in the formation of these distinctive Ag nanocrystals, aliquots of the reaction solution were taken out from each synthesis after different volumes of the AgNO3 solution had been added, followed by examination by TEM. Figure 2 shows a summary of models and TEM images that detail the evolution pathways from the cubic seeds to nanocrystals with different morphologies. For the synthesis at a slow injection rate of 0.7 mL/h, the cubic seeds grew into short bars with an average aspect ratio slightly larger than 1 in the initial stage of the reaction (Figure 2b, after 0.2 mL of AgNO3). A careful analysis indicates that truncation occurred at four adjacent corners on one of the square faces. As the volume of the AgNO3 solution was increased to 0.4 mL (Figure 2c), bars with a larger aspect ratio and more significant truncation were observed. After 0.7 mL of the AgNO3 solution had been added (Figure 2d), the 5/6-truncated octahedrons with an average size (defined as the length of the longest edge, see Figure S5a) of ca. 55 nm were formed. These observations suggest that the growth mainly took place on one of the six faces of a cubic seed and was confined to the ⟨100⟩ direction. From TEM images of the samples taken out at different stages of syntheses with moderate (8.0 mL/h, Figure 2e−g) and fast (100 mL/h, Figure 2h−j) injection rates, it is not difficult to figure out that the growth in these two cases took place on three adjacent faces and all six faces of a cubic seed, respectively. Both the 3/6-truncated octahedrons and octahedrons were measured to be ca. 80 nm along the longest edge (Figure S5b and c). This value was in agreement with the size of an octahedron produced via exclusive growth on the {100} faces of a 40 nm cubic seed (Figure S6), indicating that the growth in both cases was also confined to the {100} faces. In the aforementioned syntheses, the AgNO3 was supposed to be immediately reduced into Ag atoms upon contacting with AA.23,24 Therefore, the concentration of Ag atoms in the reaction solution was mainly determined by the injection rate of AgNO3 that could be tightly controlled through the use of a syringe pump. When the injection rate was slow enough, the concentration of Ag atoms around a cubic seed was too low to ensure deposition on all six faces of a cubic seed. Instead, only one of the six faces of a seed was deposited with the newly formed Ag atoms, leading to the formation of 5/6-truncated octahedrons. A similar mechanism was involved when AgNO3 was injected at a moderate rate. In this case, the small number of Ag atoms generated from the precursor were only enough to cover three of the six faces of a cubic seed, and a 3/6-truncated octahedron was thus formed. The reason why Ag preferred to deposit on three adjacent faces instead of other combinations is still not clear and might be related to the collision pattern of Ag atoms with the seed, as well as the surface diffusion of Ag adatoms.25 At a fast injection rate, the sufficiently high concentration of Ag atoms gave each face of a cubic seed the
Figure 2. Growth pathways for cubic Ag seeds in the standard syntheses at a relatively low concentration (1.0 mg/mL) for PVP. (a) Schematic showing the morphological changes for Ag nanocrystals formed under three different injection rates for AgNO3 solution. The red arrows indicate the directions of growth. (b−j) TEM images showing the evolution of shape for Ag nanocrystals with the growth occurring on: (d) one, (e−g) three, and (h−j) six faces of a cubic seed. The volume of AgNO3 injected was (b) 0.2, (c) 0.4, and (d) 0.7 mL at a rate of 0.7 mL/h; (e) 0.5, (f) 1.1, and (g) 1.6 mL at a rate of 8.0 mL/ h; and (h) 1.2, (i) 2.5, (j) 3.5 mL at a rate of 100 mL/h, respectively. The numbers on 3D models match the numbers on the TEM images.
same chance to receive the newly formed Ag atoms, and thus conventional octahedrons were produced. In another set of experiments, we increased the concentration of PVP in the reaction solution from 1.0 to 30 mg/mL, while all other parameters were kept the same. We found that, in the initial stages of the syntheses at slow, moderate, and fast injection rates, the 40 nm cubic seeds had grown into Ag bars with an average aspect ratio of 1.2 (Figure S7b), 55 nm cubes (Figure S7e), and 52 nm cubes (Figure S7h), respectively. As the amount of AgNO3 solution was increased, the bars and cubes started to show truncations at the corners (as shown in Figure S7c, f, and i, respectively), and eventually evolved into 75 nm 5/6-truncated octahedrons, 100 nm 3/6-truncated octahedrons, and 110 nm conventional octahedrons as shown in Figure S7d, g, and j, respectively. To elucidate the 6040
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mechanisms involved in these growth pathways, we have to take into account the factor of thermodynamic control. In the absence or with a relatively low concentration of PVP, the surface free energies of low-index facets of a Ag nanocrystal increase in the order of γ{111} < γ{100} < γ{110}.26 Immediately after the growth of a cubic seed, the {100} facets will be gradually replaced with the more stable {111} facets, which is consistent with the observations shown in Figure 2 (with PVP at a low concentration of 1.0 mg/mL). In the presence of PVP at a relatively high concentration (30 mg/mL), however, the order of γ{111} and γ{100} would be reversed since PVP binds more strongly to Ag(100) than Ag(111) surface and thus reducing γ{100}.20,21 In this case, growth on 1 and 3 or 6 faces of a cubic seed would result in the formation of bars (Figure S7b) and enlarged cubes (Figure S7e and h), respectively, with no development of {111} facets. As the growth proceeded, the concentration of PVP in the solution would drop due to the increase in total area for the {100} facets. When the concentration fell below a critical value, {111} facets will start to appear on the products.20 Therefore, further growth of the bars and enlarged cubes would eventually produce the elongated 5/6-truncated octahedrons (Figure S7d), 3/6truncated octahedrons (Figure S7g), and conventional octahedrons (Figure S7j), respectively. In a sense, it was a combination of both thermodynamic and kinetic controls that gave rise to a larger variety of nanocrystals with unsymmetrical shapes. It is worth pointing out that the size of the exotic Ag nanocrystals and Ag octahedrons could all be easily controlled by starting with cubic Ag seeds with different sizes. For example, 25 nm 5/6-truncated octahedrons (Figure 3b), 33 nm 3/6-truncated octahedrons (Figure 3c), and 35 nm octahedrons (Figure 3d) were obtained by using Ag nanocubes with an average edge length of 20 nm (Figure 3a) as the seeds. Remarkably, the morphologies were still maintained for the final products regardless of the particle size. Our analyses on a large number (>50) of particles indicate that the yields of the 25 nm 5/6-truncated octahedrons (Figure 3b) and 33 nm 3/6truncated octahedrons (Figure 3c) were slightly lower than those of the 5/6- and 3/6-truncated octahedrons (Figure 1) grown from 40 nm cubic seeds (∼70% vs >90%). The relatively low yields can be attributed to the broader size distribution of the 20 nm cubic seeds (Figure 3a) as compared with the 40 nm cubic seeds (Figure S1). The unsymmetrical structures intrinsic to the Ag nanocrystals prepared in this work make them attractive for the investigation of localized surface plasmon resonance (LSPR) properties. Figure 4 compares normalized UV−vis spectra recorded from aqueous suspensions of the 5/6-truncated octahedrons (Figure 1a), 3/6-truncated octahedrons (Figure 1e), and the corresponding 40 nm cubic Ag seeds (Figure S1) from which these two anisotropically truncated octahedrons were grown. The major LSPR peaks of the 5/6- and 3/6-truncated octahedrons were 10 and 40 nm, respectively, red-shifted from that of the cubic seeds as would be expected from a size increase. The cubic seeds showed two LSPR peaks, which had been assigned in our previous work.27 In comparison, a third peak emerged for both the 5/6- and 3/6-truncated octahedrons. This observation agrees well with the results obtained from the discrete dipole approximation (DDA) calculations where the number of resonance peaks exhibited by a nanostructure was found to increase with the number of way in which it can be polarized (i.e., when the structure has a lower symmetry).28,29
Figure 3. Extension of the strategy to Ag nanocrystals with smaller sizes. (a) TEM image of Ag cubes with an average edge length of 20 nm that were used as the seeds. (b−d) TEM images of the corresponding nanocrystals grown from the 20 nm seeds. All of the reaction conditions were kept the same as those in the standard synthesis except for the difference in size (20 nm) and concentration (ca. 7.0 × 109 particles/mL) for the seeds. The volume of AgNO3 injected and the injection rate were (b) 0.5 and 0.7 mL/h; (c) 1.2 and 8.0 mL/h; and (d) 2.6 and 100 mL/h, respectively. The insets show TEM images of individual nanocrystals at a higher magnification. The scale bar in the inset of d is 10 nm and applies to all other insets.
Figure 4. LSPR properties of the two different types of unsymmetrical Ag nanocrystals. The red, blue, and dashed black curves correspond to normalized UV−vis spectra taken from the samples shown in Figure 1 (a and e) and Figure S1, respectively (as indicated in the plots and the corresponding models). The peak positions are labeled on each spectrum.
The third peak for 5/6-truncated octahedrons is a broad shoulder in the range of 500−560 nm. Since the 5/6-truncated octahedron has an overall one-dimensional anisotropy, this broad peak could be attributed to the surface polarization along the longitudinal axis, which is similar to what was calculated for Ag nanobars with roughly the same size.17,30 While the third 6041
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(11) Tao, A. R.; Habas, S.; Yang, P. Small 2008, 4, 310−325. (12) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153−1175. (13) Murphy, C. J.; Jana, N. R. Adv. Mater. 2002, 14, 80−82. (14) Sun, Y.; Gates, B.; Mayers, B.; Xia, Y. Nano Lett. 2002, 2, 165− 168. (15) Malikova, N.; Pastoriza-Santos, I.; Schierhorn, M.; Kotov, N. A.; Liz-Márzan, L. M. Langmuir 2002, 18, 3694−3697. (16) Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15, 1957− 1962. (17) Wiley, B. J.; Chen, Y.; McLellan, J. M.; Xiong, Y.; Li, Z.; Ginger, D.; Xia, Y. Nano Lett. 2007, 4, 1032−1036. (18) Xiong, Y.; Cai, H.; Wiley, B. J.; Wang, J.; Kim, M. J.; Xia, Y. J. Am. Chem. Soc. 2007, 129, 3665−3675. (19) Cobley, C. M.; Rycenga, M.; Zhou, F.; Li, Z.; Xia, Y. Angew. Chem., Int. Ed. 2009, 48, 4824−4827. (20) Xia, X.; Zeng, J.; Oetjen, L. K.; Li, Q.; Xia, Y. J. Am. Chem. Soc. 2012, 134, 1793−1801. (21) Al-Saidi, W. A.; Feng, H.; Fichthorn, K. A. Nano Lett. 2012, 12, 997−1001. (22) Zhang, Q.; Li, W.; Wen, L. P.; Chen, J.; Xia, Y. Chem.Eur. J. 2010, 16, 10234−10239. (23) Xia, X.; Zeng, J.; McDearmon, B.; Zheng, Y.; Li, Q.; Xia, Y. Angew. Chem., Int. Ed. 2011, 50, 12542−12546. (24) Métraux, G. S.; Cao, Y. C.; Jin, R.; Mirkin, C. A. Nano Lett. 2003, 3, 519−522. (25) Zhang, Q.; Ge, J.; Pham, T.; Goebl, J.; Hu, Y.; Lu, Z.; Yin, Y. Angew. Chem., Int. Ed. 2009, 48, 3516−3519. (26) Vitos, L.; Ruban, A. V.; Skriver, H. L.; Kollar. J. Surf. Sci. 1998, 411, 186−202. (27) Zhou, F.; Li, Z.; Xia, Y. J. Phys. Chem. C 2008, 112, 20233− 20240. (28) Yang, W. H.; Schatz, G. C.; Van Duyne, R. P. J. Chem. Phys. 1995, 103, 869−875. (29) Wiley, B. J.; Im, S. H.; Li, Z.; McLellan, J.; Siekkinen, A.; Xia, Y. J. Phys. Chem. B 2006, 110, 15666−15675. (30) Zhang, Q.; Moran, C. H.; Xia, X.; Rycenga, M.; Li, N.; Xia, Y. Langmuir 2012, 28, 9047−9054.
peak for 3/6-truncated octahedrons is located at 390 nm, a location between the two peaks seen in the spectrum for the 40 nm cubic seeds. Previous DDA calculations indicated that the major LSPR peak of 40 nm Ag octahedrons is located between the two peaks of 40 nm cubes as we see in this case.29 In this regard, the 3/6-truncated octahedron could be considered as a hybrid between a cube and an octahedron, and therefore the third peak at 390 nm could be assigned to the dipole modes concentrating at the corners of its octahedral portion. In comparison, the LSPR peaks of the unsymmetrical Ag nanocrystals with smaller sizes (25−33 nm, as shown in Figure 3b,c) were relatively narrow and less distinguishable (Figure S8). In summary, we have demonstrated a versatile approach to breaking the symmetry of crystal lattice and thus obtain novel nanocrystals with unsymmetrical shapes and distinctive optical properties. The key to the success was to control the rate at which a precursor was introduced and reduced to selectively promote the growth along different directions of a cubic Ag seed. We have also demonstrated that, with the assistance of thermodynamic control, the types of the unsymmetrical Ag nanostructures could be further enriched. This work represents the first attempt for breaking the symmetry of crystal lattice in a controlled manner and thus providing a simple and robust route to a vast variety of new shapes for nanocrystals. We believe that the approach reported here could also be extended to other systems involving different types of seeds and other noble metals or inorganic materials.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
S Supporting Information *
Additional information and figures. This material is available free of charge via the Internet at http://pubs.acs.org.
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported in part by the NSF (DMR 1104614 and 1215034) and startup funds from Georgia Institute of Technology.
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REFERENCES
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