Integration of Kinetic Control and Lattice Mismatch To Synthesize Pd

Apr 13, 2016 - Hefei National Laboratory for Physical Sciences at the Microscale, Key Laboratory of Strongly-Coupled Quantum Matter Physics of Chinese...
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Integration of Kinetic Control and Lattice Mismatch to Synthesize Pd@AuCu Core-Shell Planar Tetrapods with Size-Dependent Optical Properties Min Meng, Zhicheng Fang, Chao Zhang, Hongyang Su, Rong He, Renpeng Zhang, Hongliang Li, Zhi-Yuan Li, Xiaojun Wu, Chao Ma, and Jie Zeng Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b00002 • Publication Date (Web): 13 Apr 2016 Downloaded from http://pubs.acs.org on April 15, 2016

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Revised ms# nl-2016-00002k.R1

Integration of Kinetic Control and Lattice Mismatch to Synthesize Pd@AuCu Core-Shell Planar Tetrapods with Size-Dependent Optical Properties

Min Meng,†,§ Zhicheng Fang,†,§ Chao Zhang,‡ Hongyang Su,§ Rong He,§ Renpeng Zhang,§ Hongliang Li,§ Zhi-Yuan Li,‡ Xiaojun Wu,§ Chao Ma,*,§ and Jie Zeng*,§

§

Hefei National Laboratory for Physical Sciences at the Microscale, Key Laboratory of

Strongly-Coupled Quantum Matter Physics of Chinese Academy of Sciences, Hefei Science Center & National Synchrotron Radiation Laboratory, Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China ‡

Laboratory of Optical Physics, Institute of Physics, Chinese Academy of Sciences, P. O. Box 603, Beijing 100190, P. R. China

*To whom correspondence should be addressed. E-mails: [email protected] (JZ), [email protected] (CM)

†These authors contributed equally.

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Abstract Planar nanocrystals with multiple branches exhibit unique localized surface plasmon resonance properties and great promise in optical applications. Here, we report an aqueous synthesis of Pd@AuCu core-shell planar tetrapods through preferential overgrowth on Pd cubic seeds. The large lattice mismatch between the Pd core and the AuCu shell is the key to induce the formation of branches under sluggish reduction kinetics. Meanwhile, the capping effect of cetyltrimethylammonium chloride on the {100} facets of Pd cubes with an aspect ratio of 1.2 can determine the growth direction of AuCu branches to form a planar structure. Through simply varying the amounts of Pd cubic seeds, the sizes of products can be well controlled in the range from 33 to 70 nm. With the manipulation of sizes, the peak position of in-plane dipole resonance can be adjusted from visible to near-infrared region. Due to the presence of tips and edges in the branches, planar tetrapods exhibited excellent surface-enhanced Raman scattering performance with an enhancement factor up to 9.0 × 103 for 70-nm Pd@AuCu planar tetrapods.

Keywords: planar tetrapods, kinetic control, lattice mismatch, localized surface plasmon resonance, surface-enhanced Raman scattering

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Metal nanocrystals with less symmetry relative to the cubic lattice have received considerable attention owing to their enhanced properties and superior applications related to plasmonics, catalysis, sensing, and electronics.1-4 In the case of localized surface plasmon resonance (LSPR), it is known that symmetry reduction will give rise to unique LSPR properties for metal nanocrystals by altering their surface charge distribution and polarization of free electrons.5-9 Typically, compared with polyhedral nanocrystals confined in the cubic symmetry, planar nanocrystals contain extended resonance modes with both in-plane and out-of-plane dipole plasmon modes. Moreover, the oscillation mode, the frequency, and the ratio between scattering and absorption cross sections can be finely tuned by further maneuvering the geometric shapes of planar nanocrystals.10-14 For instance, Park and co-workers reported shape-dependent plasmon resonance where the peak position of in-plane dipole resonance red-shifted as the circular nanodisks transformed into hexagonal nanoplates.15 Zheng’s group also demonstrated the redshift of LSPR peaks of Pd nanosheets with the decreased thickness or the elongated edge length.16 Thanks to the efforts from many research groups, planar nanocrystals with different morphologies have been developed, including tripodal, triangular, hexagonal and circular nanoplates.17-26 Particularly, planar branched nanocrystals possess a variety of tips and edges that can serve as hot spots for large electric-field enhancement, contributing to the promising LSPR-assisted applications such as surface-enhanced Raman spectroscopy (SERS).27-33 However, the preparation of planar branched nanocrystals requires the selective growth of branches in two dimensions, and thus is of great challenge. Over the past few years, kinetic control has been demonstrated as a simple and versatile approach to induce anisotropic growth during the seeded growth process.34-36 A slow reduction rate allows for the generation of nanocrystals taking a shape with low symmetry, because adatoms tend to deposit on initially activated sites of a seed rather than to migrate on the surface so as to adopt their lowest energy configuration. For example, by controlling the reaction kinetics, Ag atoms could selectively nucleate and grow on different numbers of side faces (from one to six) on a Pd cubic seed.36 Further, the anisotropic growth mode can also be induced by the lattice mismatch between different components, which can favor the formation of branches as a manifestation of releasing the lattice strain.37,38 Skrabalak and co-workers demonstrated that a lattice mismatch of 4.0% between Pd and Au could facilitate the formation of branches to minimize the strain energy when Au atoms were deposited on Pd cores.37 3 ACS Paragon Plus Environment

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Herein, we combined the above methods to facilely synthesize planar tetrapods with a Pd cubic core and AuCu tetrapod shell (Pd@AuCu). The weak reducing power of glucose together with the coordination interaction between hexadecylamine (HDA) and Au3+ or Cu2+ ions produced sluggish reduction kinetics, which was responsible for the anisotropic growth. Meanwhile, the formation of branches was found to rely on the lattice mismatch of 4.02% between the Pd core and the AuCu shell as a result of releasing the lattice strain. Meanwhile, due to the capping effect of cetyltrimethylammonium chloride (CTAC) on the side faces of Pd cubic seeds with an aspect ratio of 1.2:1, the overgrowth of AuCu branches was mainly along four lateral edges on each seed, resulting in the formation of a planar structure. The sizes of nanocrystals could be well controlled in the range from 33 to 70 nm by simply varying the amounts of Pd seeds. Through size control, the peak position corresponding to the in-plane dipole mode shifted from visible to near-infrared region with the elongated pods, whereas that of the out-of-plane dipole mode was barely changed due to the similar thickness regardless of varied sizes. Owing to the unique branched structure with tips and edges, the obtained Pd@AuCu core-shell planar tetrapods exhibited excellent performance in SERS applications with an enhancement factor up to 9.0 × 103 for 70-nm Pd@AuCu planar tetrapods. In a typical synthesis, 9-nm Pd cubic seeds with an average aspect ratio of 1.2:1 (Fig. S1) were firstly prepared according to a reported protocol.39 Then, an aqueous solution containing CuCl2, HAuCl4, HDA, and CTAC was added in a 20-mL vial at room temperature. After the solution was magnetically stirred at room temperature overnight, the pre-synthesized Pd cubic seeds and glucose were added into the vial using a pipette with stirring for 5 min. The capped vial was then transferred into an oil bath and heated at 95 oC for 30 min under magnetic stirring. As the reaction proceeded, the solution changed its color from kelly green to dark blue, indicating the formation of products. After cooling down to room temperature, the mixture was centrifuged and washed with water three times and ethanol twice. Figure 1a shows a typical transmission electron microscopy (TEM) image of the nanocrystals, which took a cubic core and an overall tetrapod outline with an average edge length of 44 nm between the apexes of two adjacent pods. Most of the core-shell nanocrystals consisted of four pods with a length of ~26 nm and an angle of ~135o between adjacent pods. To visualize the spatial structure of as-obtained nanocrystals, a typical geometric model is presented in the inset of Figure 1a. Magnified TEM images of an individual nanocrystal recorded at three different tilting angles 4 ACS Paragon Plus Environment

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reveal a planar tetrapod rather than a three-dimensional octapod for the as-prepared nanocrystals (Fig. 1, b-d). It is also confirmed by TEM images of the vertical tetrapod nanocrystals in Figure S2 that the growth along x and y axes was dominant compared with the negligible growth along the z axis. Figure 2a shows a high-angle annular dark-field scanning TEM (HAADF-STEM) image of an individual planar tetrapod, where the brighter cubic core was in good contrast to the four darker arms. The selected-area electron diffraction (SAED, inset in Fig. 2a) pattern, obtained with the electron beam parallel to the fourfold axis which is assigned to the [001] direction, elucidates the single-crystal nature of such planar tetrapods. The circles and boxes corresponded to electron diffraction from the {220} and {200} planes, respectively. Figure 2, b and c, shows the corresponding HAADF-STEM images of the regions marked with boxes in Figure 2a. The lattice spacings of 1.91 and 1.34 Å corresponded to {200} and {220} planes of the Pd core, respectively (Fig. 2b). Similarly, the lattice spacings of 1.99 and 1.41 Å were indexed as {200} and {220} planes of the AuCu shell, respectively (Fig. 2c). The powder X-ray diffraction (XRD) pattern reveals a face-centered cubic (fcc) structure of planar tetrapods (Fig. S3). Based on the Bragg equation, the lattice spacing assigned to {200} planes of the AuCu shell was calculated to be 2.01 Å, which is consistent with 1.99 Å measured in Figure 2c. Notably, although XRD analysis implies an alloy structure of the AuCu shell rather than an intermetallic phase due to the lack of parity mixed peaks,40 the atomic content cannot be facilely deduced from the XRD pattern. As such, it is unsuitable to apply Vegard’s law in the case of Cu and Au, because Cu might take the unusual interstitial position.41 In order to characterize the composition and elemental distribution of planar tetrapods, scanning transmission electron microscopy-energy dispersive X-ray (STEM-EDX) analysis was conducted. The existence of Pd, Au, and Cu in the final nanocrystals is revealed by the EDX spectrum in Figure S4. Moreover, line-scanning profiling analysis taken along the line marked in Figure 2a indicates the existence of a Pd core and a AuCu alloy shell (Fig. 2d). The elemental mapping images in Figure 2, e-h, further confirm that Pd was mainly located in the cubic core, while Au and Cu were homogeneously distributed throughout the whole shell. Moreover, the molar ratio of Au:Cu in the shell was determined as 3:1 by inductive coupled plasma-atomic emission spectroscopy (ICP-AES) analysis. To track the evolution of Pd@AuCu core-shell planar tetrapods, we monitored the reaction by taking aliquots of the reaction solution at different time points. As shown in Figure S5a, a homogeneous cubic shell with thickness of ~3 nm was formed outside of the Pd core at t = 2 min. 5 ACS Paragon Plus Environment

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Meanwhile, the ultraviolet-visible (UV-Vis) absorption spectra show an absorption peak at 560 nm (Fig. S6). When the reaction proceeded to t = 4 min, the growth at the edges tended to dominate the process (Fig. S5b). To better visualize the structure and composition of nanocrystals obtained at this stage, detailed TEM analysis is provided in Figure S7. TEM images of an individual nanocrystal recorded at different titling angles reveal a preliminary planar tetrapod outline (Fig. S7, a and b). As shown in Figure S7c, it is evident that the branches selectively grew along four lateral edges. A single-crystal nature is revealed by the SAED pattern in the inset of Figure S7c. The line-scanning profile and elemental mapping analyses show the existence of a Pd cubic core and a homogeneous AuCu alloy shell (Fig. S7, d-h). Moreover, the UV-Vis absorption spectra obtained at this stage show a new absorption peak at 780 nm derived from the formation of pods at the edges.13,14 The intensity of the peak located at 560 nm was stronger than that at 780 nm since the pods were still very short (Fig. S6). With the reaction prolonged to t = 8 min, the length of the pods was increased (Fig. S5c). Meanwhile, the 780-nm absorption peak red-shifted to 835 nm with a similar intensity to that of the peak at 560 nm due to the formation of longer pods (Fig. S6). When the reaction was extended to t = 30 min, further growth enlarged the planar tetrapods without obvious shape changes (Fig. S5d). Owing to the longer pods at this stage, the peak at 835 nm further red-shifted to 900 nm with a much stronger intensity compared with that at 560 nm (Fig. S6).42 Relative to the initial Pd nanocubes with Oh symmetry, the products apparently exhibited a planar structure with lower symmetry (D4h symmetry). Such symmetry reduction can be attributed to the anisotropic growth of four branches in the direction. As demonstrated in previous studies, highly anisotropic structures tended to become favorable in a slow reduction process.36 In our synthesis, the sluggish reduction kinetics originated from the weak reducing power of glucose together with the coordination interaction between HDA and Au3+ or Cu2+ ions. When we enhanced the reducing capacity of glucose by adding 20 µL of 1 M NaOH aqueous solution, spherical Pd@AuCu core-shell nanocrystals with a thick, homogeneous AuCu shell dominated the products (Fig. S8a). A similar result was also observed when we directly replaced glucose with a stronger reductant, formaldehyde (Fig. S8b). The existence of a Pd core and a AuCu alloy shell is confirmed by elemental mapping images (Fig. S8, c-g). The XRD pattern indicates an fcc structure of such spherical Pd@AuCu core-shell nanocrystals (Fig. S8h). The accelerated reduction in the above cases generated abundant supply of AuCu atoms, promoting 6 ACS Paragon Plus Environment

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the dominance of isotropic growth mode and thus the formation of a homogeneous AuCu shell on each seed. The reduction kinetics was also sensitive to the amount of HDA. The amine group in HDA was expected to interact with Au3+ and Cu2+ ions, thus driving down the reduction potentials of metal precursors. Specifically, it is documented that the standard reduction potentials of AuCl4-/Au and Cu2+/Cu pairs are 1.00 V and 0.34 V, respectively. After coordination with amine groups, the standard reduction potentials of [Au(NH3)4]3+/Au and [Cu(NH3)4]2+/Cu pairs turn out to be 0.33 V and 0 V, respectively.43 To validate the existence of coordination interaction between amine groups and Au3+ or Cu2+ ions, fourier transform infrared (FT-IR) measurements were conducted. As shown in Figure S9, the absorption peak of HDA around 3338 cm-1 was attributed to N-H stretching vibration. When HDA was mixed with HAuCl4 or CuCl2, this peak would exhibit a shift in location, confirming the existence of coordination interaction between amine and Au3+ or Cu2+ ions. Relative to the standard synthesis, a uniform AuCu shell was formed on each Pd cubic seed when the amount of HDA was halved to 45 mg (Fig. S10a). In this case, owing to the suppressed expression of coordination, the reduction was accelerated and non-selective growth dominated the synthetic process. By contrast, with 180 mg of HDA added, the anisotropic growth was more prominent, leading to the formation of multipods with long branches (Fig. S10b). In addition to the kinetic control, the large lattice mismatch between the core and shell components can also contribute to the dominance of anisotropic growth mode. Specifically, the lattice mismatch between the Pd core and the AuCu shell as high as 4.02% should be able to facilitate the growth of branches as a means of minimizing the lattice strain. In order to validate this hypothesis, 12-nm Au3Cu nanocubes (Fig. S11) were prepared to replace Pd cubic seeds for the seeded growth of AuCu shells.44 In this case, the lattice mismatch was weakened to 0.55%, leading to the generation of octapodal core-shell nanocrystals with a single-crystal nature instead of planar tetrapods (Fig. S12). Similar tendency was also observed by Yang and co-workers.45 Exemplified by the Pt@Pd system, they found a small lattice mismatch between Pd and Pt (0.77%) promoted the formation of conformal core-shell nanocrystals. By contrast, the deposition of Au onto Pt cubic seeds resulted in the formation of anisotropic rods owing to the large lattice mismatch between Au and Pt (4.08%). Meanwhile, the capping effect of CTAC on the {100} facets of Pd cubes with an aspect 7 ACS Paragon Plus Environment

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ratio of 1.2 plays a pivotal role in directing the growth of AuCu branches along four lateral edges to form a planar structure. Due to the selective adsorption of CTAC on the {100} facets of Pd nanocrystals, it is energetically favorable for AuCu atoms to deposit on the edges of Pd cubic seeds. Among all the twelve edges, four lateral edges are apparently longer relative to the others, because the aspect ratio of as-obtained Pd nanocubes was 1.2:1 (Fig. S1b). The four longer edges provided more sites, which were more likely to be activated by the initially deposited AuCu atoms. After the activation, deposition of the newly formed atoms was largely confined to these activated sites, resulting in the preferential growth of AuCu branches along four longer edges in the direction. In comparison, without the addition of CTAC, the AuCu branches grew randomly on the surface of Pd cubic seeds (Fig. S13a). The elemental mapping images reveal the existence of a Pd core and a AuCu alloy shell (Fig. S13, b-e). The XRD pattern indicates an fcc structure of such Pd@AuCu core-shell nanocrystals (Fig. S13f). Furthermore, in order to validate the role played by an aspect ratio of 1.2:1, well-defined Pd cubes with an aspect ratio of ~1.0 were successfully prepared according to Xu’s work and then employed as the seeds (Fig. S14a).46 In this case, the formation of Pd@AuCu core-shell planar tetrapods was no longer favored, and three-dimensional Pd@AuCu multipods occupied the majority of products (Fig. S14, b-h). Based on the standard synthesis, we realized size control of the products by adding different amounts of 9-nm Pd seeds with the other conditions unchanged. As shown in Figure S15, 70-, 44-, and 33-nm planar tetrapods with a uniform size distribution were synthesized with the addition of 0.1, 0.3, and 0.5 mL of Pd seeds, respectively. When more Pd seeds were injected into the solution, more deposition sites were provided for Au and Cu atoms. As a consequence, the elongation of branches was expected to become less obvious, leading to the formation of short-branched nanocrystals. The size control associated with the unique structure of Pd@AuCu core-shell planar tetrapods enables an ideal platform to explore the size-dependence of LSPR and SERS properties. In the extinction spectra within the range of 300-1300 nm (Fig. 3a), two typical LSPR peaks of 33-nm planar tetrapods were observed at 560 and 735 nm, respectively. When the sizes of tetrapods were increased to 44 and 70 nm, the longer-wavelength absorption peak was shifted to the near-infrared region at 917 and 1083 nm, respectively. As such, with the pods getting longer, surface polarization, such as charge separation, would change accordingly. It is generally believed that the increased charge separation induced by the accumulation of surface charges at 8 ACS Paragon Plus Environment

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sharp corners weakens the restoring force for electronic oscillation, giving rise to a redshift of the longer-wavelength resonance peak.47,48 Interestingly, the shorter-wavelength resonance peak, associated with the vertical charge separation, was barely changed owing to the similar thickness regardless of varied sizes. Then we applied the discrete dipole approximation (DDA) method to simulate extinction properties of Pd@AuCu core-shell planar tetrapods with different sizes. The coordinate system used in the simulation is presented in the inset of Figure 3b. As shown in Figure 3b and Figure S16, the simulated spectra matched well with the experimental results in terms of the peak numbers and positions. A slight shift between the experimental and simulated spectra was observed, which can be attributed to the differences between idealized structural models used in the simulation and real samples. After all, there is an inevitable degree of heterogeneity in the prepared samples with regard to the size and shape. For reference, simulated extinction spectra of Pd@AuCu planar tetrapods excitated by polarized light along the x- and z-axis direction are provided in Figure S17. Moreover, to discern the resonance mode in the extinction spectra, near-field distributions of an individual Pd@AuCu core-shell planar tetrapod with different sizes were also calculated (Fig.3, c-f and Figs. S18-S19). For all the samples, the shorter-wavelength resonance peak can be assigned to the out-of-plane dipole mode, while the longer-wavelength resonance peak corresponds to the in-plane dipole mode. In comparison with their isotropic counterparts, branched nanocrystals often exhibit enhanced SERS efficiency because of the large number of surface atoms deposited at tips and edges which can serve as hot spots for large electric-field enhancement.49-54 Figure 4 compares the SERS spectra of crystal violet (CV) adsorbed on the surface of Pd@AuCu planar tetrapods with different sizes. Based on the intensity of the peak located at around 1620 cm-1, the SERS enhancement factors were estimated to be 9.0 × 103, 1.5 × 103, and 5.2 × 102 for 70-nm, 44-nm, and 33-nm Pd@AuCu core-shell planar tetrapods, respectively (see Fig. S20 for the detailed calculation), indicating that the nanocrystals with longer branches exhibited a higher enhancement factor. In summary, Pd@AuCu core-shell planar tetrapods were synthesized through a facile seed-mediated growth method. The success of this synthesis relies on the integration of kinetic control, lattice mismatch, and capping effect. In addition, through simply changing the amounts of Pd seeds, the sizes of planar tetrapods varied from 33 to 70 nm, resulting in the adjustment of in-plane dipole plasmon peak from visible to near-infrared region. The obtained Pd@AuCu 9 ACS Paragon Plus Environment

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planar tetrapods were found as promising candidates for SERS, with an enhancement factor up to 9.0 × 103 in the case of 70-nm Pd@AuCu planar tetrapods. This approach represents an important contribution to the rational design and controllable synthesis of nanocrystals with planar branched structures.

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(35) Chen, S.; Jenkins, S. V.; Tao, J.; Zhu, Y.; Chen, J. J. Phys. Chem. C. 2013, 117, 8924-8932. (36) Zhu, C.; Zeng, J.; Tao, J.; Johnson, M. C.; Schmidt-Krey, I.; Blubaugh, L.; Zhu, Y.; Gu, Z.; Xia, Y. J. Am. Chem. Soc. 2012, 134, 15822-15831. (37) DeSantis, C. J.; Skrabalak, S. E. J. Am. Chem. Soc. 2012, 135, 10-13. (38) Chiu, C. Y.; Huang, M. H. Angew. Chem. Int. Ed. 2013, 52, 12709-12713. (39) Jin, M.; Liu, H.; Zhang, H.; Xie, Z.; Liu, J.; Xia, Y. Nano Res. 2011, 4, 83-91. (40) Sra, A. K.; Ewers, T. D.; Schaak, R. E. Chem. Mater. 2005, 17, 758-766. (41) He, R.; Wang, Y. C.; Wang, X.; Wang, Z.; Liu, G.; Zhou, W.; Wen, L.; Li, Q.; Wang, X.; Chen, X.; Zeng, J.; Hou, J. G. Nat. Commun. 2014, 5, 4327. (42) Jones, M. R.; Mirkin, C. A. Angew. Chem. Int. Ed. 2013, 52, 2886-2891. (43) Inzelt, G. In Encyclopedia of Electrochemistry; Bard, A. J.; Stratmann, M.; Scholz, F.; Pickett, Ch., Eds.; Wiley-VCH: Weinheim, 2006; Vol. 7, 43-46. (44) Liu, Y.; Walker, A. R. Angew. Chem. Int. Ed. 2010, 122, 6933-6937. (45) Habas, S. E.; Lee, H.; Radmilovic, V.; Somorjai, G. A.; Yang, P. Nature Mater. 2007, 6, 692-697. (46) Niu, W.; Li, Z. Y.; Shi, L.; Liu, X.; Li, H.; Han, S.; Chen, J.; Xu, G. Cryst. Growth Des. 2008, 8, 4440-4444. (47) Rycenga, M.; Cobley, C. M.; Zeng, J.; Li, W.; Moran, C. H.; Zhang, Q.; Qin, D.; Xia, Y. Chem. Rev. 2011, 111, 3669-3712. (48) Zhuo, X.; Zhu, X.; Li, Q.; Yang, Z.; Wang, J. ACS Nano 2015, 9, 7523-7535. (49) Niu, W.; Chua, Y. A. A.; Zhang, W.; Huang, H.; Lu, X. J. Am. Chem. Soc. 2015, 137, 10460-10463. (50) Cheng, L.; Ma, C.; Yang, G.; You, H.; Fang, J. J. Mater. Chem. A 2014, 2, 4534-4542. (51) Mettela, G.; Siddhanta, S.; Narayana, C.; Kulkarni, G. U. Nanoscale 2014, 6, 7480-7488. (52) Sanz-Ortiz, M. N.; Sentosun, K.; Bals, S.; Liz-Marzán, L. M. ACS Nano 2015, 9, 10489-10497. (53) Liang, H.; Li, Z.; Wang, W.; Wu, Y.; Xu, H. Adv. Mater. 2009, 21, 4614-4618. (54) Yang, Y.; Liu, J.; Fu, Z. W.; Qin, D. J. Am. Chem. Soc. 2014, 136, 8153-8156.

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Supporting Information. Experimental details, TEM image and size distribution of Pd cubic seeds with an aspect ratio of ~1.2, TEM image and geometric model of Pd@AuCu core-shell planar tetrapods, XRD pattern of Pd@AuCu core-shell planar tetrapods, EDX spectrum of Pd@AuCu core-shell planar tetrapods, TEM images of Pd@AuCu core-shell planar tetrapods obtained at different reaction time points, UV-Vis spectra of the reaction solution obtained at different reaction time points, TEM analysis of an individual Pd@AuCu nanocrystal obtained at t = 4 min, TEM analysis of products prepared using different reductants, FT-IR spectra, TEM images of nanocrystals obtained with the addition of different amounts of HDA, TEM image of Au3Cu cubic seeds, TEM analysis of nanocrystals prepared using Au3Cu cubes as the seeds, HAADF-STEM images and XRD pattern of nanocrystals prepared without the addition of CTAC, TEM image of Pd cubes with an aspect ratio of ~1.0, HAADF-STEM images of three-dimensional Pd@AuCu multipods, TEM images and size distributions of nanocrystals obtained with the addition of different amounts of Pd cubic seeds, coordinate system and calculated extinction spectra of Pd@AuCu core-shell planar tetrapods, near-field distributions, and the normalized Raman spectrum. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

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This work was supported by Collaborative Innovation Center of Suzhou Nano Science and Technology, MOST of China (2014CB932700), NSFC under Grant Nos. 21573206, 51371164, 51132007, and 11434017, Strategic Priority Research Program B of the CAS under Grant No. XDB01020000, Hefei Science Center CAS (2015HSC-UP016), and Fundamental Research Funds for the Central Universities.

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Figure 1. (a) TEM image of Pd@AuCu core-shell planar tetrapods prepared through the standard procedure. The inset image in (a) shows a model of an individual planar tetrapod. (b-d) TEM images of an individual Pd@AuCu core-shell planar tetrapod recorded at different tilting angles: (b) -23o, (c) 0o, and (d) 23o. The scale bar in (b) also applies to (c) and (d).

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Figure 2. (a) HAADF-STEM image of a typical Pd@AuCu core-shell planar tetrapod. The inset shows the corresponding SAED pattern with the electron beam directed along the [001] axis. The circles correspond to electron diffractions from the {220} planes while the boxes correspond to those from the {200} planes. (b,c) Corresponding HAADF-STEM images of the regions marked in (a). (d) Elemental line-scanning profiles along the direction marked by a white line in (a). (e-h) STEM-EDX elemental mapping of (e) Pd, (f) Cu, (g) Au, and (h) merged images of an individual Pd@AuCu core-shell planar tetrapod.

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Figure 3. (a) Normalized UV-Vis spectra of aqueous suspensions of Pd@AuCu core-shell planar tetrapods with different sizes. (b) Comparison of experimental and calculated UV-Vis spectra of 44-nm Pd@AuCu core-shell planar tetrapods. The inset image in (b) shows the coordinate system used in the DDA calculation. (c, d) Calculated near-field amplitude distributions at the interface of (c) xz and (d) xy for an individual 44-nm Pd@AuCu core-shell planar tetrapod at the 870-nm resonance peak. (e, f) Calculated near-field amplitude distributions at the interface of (e) xz and (f) xy for an individual 44-nm Pd@AuCu core-shell planar tetrapod at the 500-nm resonance peak.

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Figure 4. The SERS spectra of 10 µL of 1.0 × 10-6 M CV solution adsorbed on the Pd@AuCu core-shell planar tetrapods with different sizes on the glass slide. The SERS spectra were obtained with λex = 514 nm excitation, Plaser = 1 mW, and t = 10 s. The scale bar is 50 cps.

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