Iodide-Switched Deposition for the Synthesis of Segmented PdâAu

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Iodide-Switched Deposition for the Synthesis of Segmented Pd-Au-Pd Nanorods: Crystal Facet Matters Siyu Liu, Wenxin Niu, Firdoz Shaik, and Weiqing Zhang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02497 • Publication Date (Web): 06 Oct 2017 Downloaded from http://pubs.acs.org on October 9, 2017

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Iodide-Switched Deposition for the Synthesis of Segmented Pd-Au-Pd Nanorods: Crystal Facet Matters Siyu Liu,1 Wenxin Niu,2 Shaik Firdoz,2 Weiqing Zhang,1,*

1

Tianjin Key Laboratory of Advanced Functional Porous Materials, Institute for New Energy Materials & Low-Carbon Technologies, School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, China

2

Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117576

ABSTRACT Segmented metallic nanorods with well-defined shapes and controllable components play an important role on the systematic investigation of their shape-dependent catalytic, electric, and plasmonic properties of metal nanostructures. Unfortunately, the shape and composition of segmented nanorods are difficult to be precisely controlled via colloidal methods. Here, we reported the growth of Pd-Au-Pd bimetallic heterostructures by using Au five-fold twinned bipyramids (BPs) as seeds, with KI as a structure-directing reagent. Through a series of control experiments, we revealed that two parameters were identified as critical factors for the growth of segmented Pd-Au-Pd nanorods. First, five-fold twinned Au BPs with low-index end facets and high-index side facets function as a unique template for the directed growth. Second, iodide can switch the deposition of Pd on the Au BPs. A high concentration of iodide is believed to block the high-index facets 1 ACS Paragon Plus Environment

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of the Au BPs and lower the reaction kinetics to promote the selective growth of two Pd segments on the Au BPs. As a result, uniformed segmented Pd-Au-Pd nanorods were obtained. The segmented nanorods exhibit intense extinction in the near-IR range and could be a potential candidate for plasmon-based biological applications such as thermal therapy.

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INTRODUCTION One dimensional (1D) metal nanostructures have attracted considerable attention because of their aspect-ratio dependent optical and electric properties.1-3 The aspect ratio of 1D nanostructures is of paramount importance to finely tune the longitudinal surface plasmon resonance wavelength and dramatically enhance the polarizability of metals. As a novel type of 1D metal nanostructures, segmented nanorods with orderly assembled different metal segments provide richer properties for desirable applications. For example, Park et al. synthesized two-component segmented nanorods consisting of Au and Ni components and observed that Ni segments play a relaying role in the surface plasmon coupling of Au segments.4 Later they found intraparticle surface plasmon coupling in Au-Ag segmented nanorods, which shows two independent transverse modes from the Au and Ag segments, and the collective appearance of longitudinal surface plasmons (LSP) modes.5 The exceptionally tunable properties of segmented nanorods render them useful candidates for practical technology applications, such as in situ testing of

catalytic

reactions,

ultrasensitive

detection,6,7

biochemical

sensors,8-10

and

optoelectronic devices.11,12 While segmented metal nanorods have drawn considerate attention due to their potential applications, the controllable synthesis of segmented metal nanorods with precisely controlled components is still very challenging. To date, segmented metal nanorods can be precisely fabricated via selective electrodeposition using AAO template, but limited with low yield.4,5,13 To obtain segmented nanorods with large scale, developing an alternative method with wet chemistry is essential.14-16 Very recently, Wang et al. realized anisotropic overgrowth of metals on Au nanorods with the help of 3 ACS Paragon Plus Environment

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site-selective silica coating.16 Liang et al. also reported the selective growth of AgCdSe at one end and both the ends or on the side surface of Au nanorods by changing the pH value of the reaction solution.14 Unfortunately, the shape and facets of segments grown on Au nanorods are not well-defined in these works, which may play important role on the systematic investigation of their shape- and composition-dependent catalytic, electric, and plasmonic properties of these nanostructures.17-23 Therefore, it is highly desirable to develop an effective colloidal approach for the synthesis of segmented nanorods with well-defined facets. In this work, Pd-Au-Pd segmented nanorods with well-defined facets have been obtained from Au bipyramids (BPs) in the presence of KI and hexadecyltrimethylammonium bromide (CTAB). By adding different amount of KI, the Pd-Au bimetallic nanostructures evoluted from core-shell nanostructures to segmented nanorods. The formation mechanisms of the segmented Pd-Au-Pd nanorods were investigated, with a focus on unique crystal structure of the BP seeds and the facets selective deposition enabled by iodide. EXPERIMENTAL SECTION Chemicals. Diethylene glycol (DEG, Sigma-Aldrich), ethylene glycol (EG, SigmaAldrich), poly(diallyldimethylammonium chloride) (PDDA, 20 wt%, MW=200000350000, Sigma-Aldrich), 1,5-pentanediol (PD, Aldrich), chloroauric acid trihydrate (HAuCl4·3H2O, Alfa Aesar), silver nitrate (AgNO3, Sigma-Aldrich), palladium(II) chloride

(PdCl2,

Alfa

Aesar),

hydrochloric

acid

(HCl,

37%,

Merck),

hexadecyltrimethylammonium bromide (CTAB, TCI), polyvinylpyrrolidone (PVP, average Mw=55000, Sigma-Aldrich), L-ascorbic acid (AA, Sigma-Aldrich), potassium iodide (KI, Fluka), sodium borohydride (NaBH4, Sigma-Aldrich) were used without 4 ACS Paragon Plus Environment

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further purification. H2PdCl4 solution (10 mM) was prepared by dissolving 17.73 mg of PdCl2 in 10 mL of HCl solution (20 mM) at 100 °C. Synthesis of Au seeds. Au BPs were synthesized according to a literature report.24 In a typical synthesis, PDDA (0.2 mL) and EG (10 mL) were added into a 20 mL vial and mixed under magnetic stirring at room temperature for 5 min. Then HAuCl4 (18.8 µL, 0.5 M), AgNO3 (10 µL, 0.5 M), H2PdCl4 (62.5 µL, 10 mM) and deionized (DI) water (430 µL) were subsequently added into the above mixture. After 10 min, this mixture was capped tightly and heated in an oil bath at 140 °C for 12 hours under constant stirring. The resulting Au BPs were washed with acetone and DI water. The synthesis details of five-fold twinned and single crystalline Au nanorods can be found in Supporting Information. Synthesis of Au-Pd bimetallic nanostructures. In a typical synthesis, 0.5 mL of 0.1 M KI solution was added to 5 mL of 0.1 M CTAB solution kept at 60 °C. A 125 µL of 10 mM H2PdCl4 solution and 15 µL of as-synthesized seed Au BPs solution were then added. Finally, a 50 µL of freshly prepared 0.1 M ascorbic acid solution was added. The solution was mixed thoroughly and then left undisturbed in water bath at 60 °C for 12 hours. The reaction was terminated by centrifugation at 6000 rpm for 5 min. The precipitates were washed twice with DI water and re-dispersed in DI water. Control experiments were also conducted by using different amounts of KI. Similar synthetic procedures were conducted for the coating of Pd on five-fold twinned Au nanorods and single-crystalline Au nanorods.


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Characterization. Transmission electron microscopy (TEM) images, high-resolution TEM (HRTEM) images, energy dispersive X-ray spectroscopy (EDX) spectra, highangle annular dark field (HAADF) and selected-area electron diffraction (SAED) were acquired using a JEOL JEM-2100F and Talos F200X operating at 200 kV. Scanning electron microscopy (SEM) images were taken using a JEOL JSM-6700F operating at 20 kV. UV-visible spectra were recorded using a Shimadzu UV-1601 spectrometer with plastic cuvettes of 1 cm path length at room temperature. RESULTS AND DISCUSSION Recently, Au BPs are popular as growth seeds to synthesize bimetallic nanostructures because they possess not only the five-fold twinned crystalline structure but also highindex side facets and low-index end facets.1,24-26 This unique structure of BPs may favor the selective deposition of the second metal on their facets if the deposition on different facets can be controlled. When Au BPs are used as seeds to grow the Pd atoms in the presence of CTAB and KI, Pd atoms reduced by ascorbic acid (AA) selectively deposit on the twin boundary of low-index end facets of Au BPs rather than the lateral facets of BPs, and then grow along the twin plane of the Au BP seeds (as shown in Figure 1a). TEM images clearly show that Pd-Au nanorod is composed of a Au BP in the center and two segmented Pd nanorods on two ends (Figure 1b and 1c). When 125 µL 10 mM H2PdCl4 is added into the growth solution, the average length of the segmented nanorods is 213 nm. The average width of Pd end, which is 34 nm, is smaller than the equatorial width of the Au BPs, which is 37 nm (Figure S1 in Supporting Information). In addition, there is apparent joint between Pd parts and Au BP, which is close the boundary of highindex side facets and low-index end facets. SEM images in Figure S2 and Figure 1d show 6 ACS Paragon Plus Environment

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that the middle part of the segmented nanorods is wider than its two ends, which indicates the selective deposition of Pd atom on the low-index end facets over the high-index side facets. In some Au-Pd segmented nanorods, the shape of Au BPs is not axisymmetric because the twin boundary of Au BPs is not fully symmetric and parallel to the growth axis.27,28 The segmented structure is also confirmed with EDX analysis, which reveals that each nanorod is composed of a center BP and two segmented Pd ends (Figure 1e-g). The low magnification TEM and SEM images show the segmented nanorods were obtained with good monodispersity and high yield (Figure S2a and Figure 1b). UV-Vis extinction spectra show that longitudinal surface plasmon resonance (LSPR) of Au BPs strongly red shifted from 690 nm to 1060 nm after the formation of Pd-Au-Pd segmented nanorods (Figure 1h). This is because the selective deposition of Pd atoms along the longitudinal direction increases the aspect ratio.26,29


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Figure 1. (a) Growth of Pd-Au-Pd segmented nanorods with flat tips from Au BPs with the help of KI. (b-d) TEM and FESEM of Pd-Au-Pd segmented nanorods with flat tip obtained by adding 0.5 mL of 0.1 M KI solution. (e-g) HAADF-STEM image and EDX mapping a single Pd-Au-Pd segmented nanorod. (h) UV-Vis extinction spectra of i) Au BPs seeds and ii) Pd-Au-Pd segmented nanorods. TEM and the corresponding HAADF-STEM images in Figure 2a and 2b clearly reveal the overall segmented structure as well as the nanoscale distribution of the Pd and Au in a nanorod. The HAADF-STEM image shows a distinct segmented nanorod structure: the bright and gray parts are attributed to the Au and Pd elements, respectively. Pd-Au joints are very obvious and evidenced by HRTEM images obtained from the area 1 labeled in Figure 2c and 2d. The inset in Figure S2b show that the Au-Pd segmented nanorods are terminated with flat end facets. The flat end facet of Pd-Au-Pd segmented nanorod is further proved by SAED. Since Au BPs have a five-fold twinned structure along the growth direction,27,28 the flat end facets of the segmented nanorods should be {110} facets of fcc Pd.26 The SAED recorded from one end of the nanorod in Figure S3 exhibits a growth direction. The lattice planes along the growth direction of the nanorod obtained from area 2 in Figure 2c also show a d spacing of 0.14 nm, matching well with the (220) plane of fcc Pd (Figure 2c2). On the basis of these results, including SEM, HRTEM, and SAED studies, it can be concluded that the as-synthesized segmented nanorods are enclosed by {110} end facets. The appearance of {110} end facets of the segmented nanorods can be attributed to the stabilization effect of CTAB and KI. It has been reported that the {110} Pd facets is more stable relative to the {111} Pd facets in the presence of CTAB and KI.30 However, Huang et al. recently reported the synthesis of Pd8 ACS Paragon Plus Environment

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Au nanorods using Au decahedron seeds in the presence of CTAB and KI. In their work, the end of Pd-Au nanorods are terminated by 15 {111} facets.2 The different observation could be explained by the structure difference of Au BP and decahedron seeds. To investigate further, Au decahedra are used as growth seeds in our growth system. SEM and TEM images show that the Pd-Au nanorods are formed and their ends are also bounded with 15 {111} end facets (Figure S4). This result confirms the effect of the seed structures on the final morphologies of bimetallic nanoparticles. For the five-fold twinned Pd segmented ends, the side facets can be indexed to {100} facets, which parallel to the growth axis. Once nascent {100} facets appeared, I- prefers to adsorb on these {100} facets of Pd nanocrystals to stabilize them.2,31 In addition, the CTA+ cationic head groups can also form a stable bilayer on an iodide adlayer because of electrostatic attraction. Therefore, the adsorption of CTA+ cations plays a synergistic effect on the stability of Pd {100} side facets.32,33

Figure 2. (a, b) TEM and HAADF images of a Pd-Au segmented bimetallic nanorod with a flat tip, (c) HRTEM image obtained at the area i in a). The measured d spacing of 0.14 nm matches with that of the (220) plane of Pd, c) TEM of Pd-Au connection part


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obtained at the area ii in a), d) SAED of one end of a nanorod (area iii), indicating its growth direction is . The structural evolution of the Pd-Au-Pd segmented nanorods was followed by terminating reaction at different reaction times. In early reaction period, the deposition of Pd atoms started at the end facets (Figure 3a and b). With the reaction proceeding, Pd atoms continue to deposit at the ends of Au BPs, leading to irregular particles connected to the ends of Au BPs (Figure 3c and d). When additional Pd atoms are deposited, the size and shape of Pd particles at the ends of Au BPs became larger and more regular (Figure 3e and f). At this stage of reaction, the Pd atoms start to across the boundary of high-index side facets and low-index end facets. This may be ascribed to either the deposition of Pd with slow rate or the diffusion of Pd atoms during the reaction. Eventually, Pd-Au-Pd segmented nanorod with the two regular Pd ends on a Au BP was formed (Figure 3g and h). The regular Pd ends are finally bounded by {110} facets perpendicular to the growth axis.29,32


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Figure 3. TEM images of Pd-Au-Pd segmented nanorods obtained by terminating reaction at different reaction times: (a-b) 15 min; (c-d) 1 h; (e-f) 2 h; (g-h) 6 h. The formation mechanisms of the segmented Pd-Au-Pd nanorods were investigated, with a focus on the selective deposition enabled by facets preferential adsorption of I- and a slow reaction kinetics caused by the strong complexing capacity of I- with H2PdCl4. As well-known that halide ions (I-, Br- and Cl-) are used extensively to control the shapes of noble metal nanoparticles due to their different facet affinity.34,35 In addition, halide ions, especially I- ions, are strong complexing agent. When I- ions complex with H2PdCl4, the reaction kinetics of Pd2+ ions reduction is largely decreased due to the low reduction potential of Pd/PdI42-.2,31 A slow reaction kinetics usually facilitates the preferential deposition on a specific facets of growth seeds.36,37 To understand the effects of I- ions on the formation of segmented nanorods, the structural evolution of the Au-Pd bimetallic nanostructures was followed by adding different amounts of KI to the growth solution. A clear shape evolution of as-synthesized nanocrystals with different KI concentration was observed (Figure 4). In the absence of KI, Au@Pd core-shell nanorods were obtained (Figure 4a). In this case, the deposition of Pd occurred on all the surface of Au BP seeds. When 25 µL of 1 mM KI solution was added (KI/H2PdCl4=1:50), peanut-like nanocrystals were obtained (Figure 4b). It can be seen clearly that with the introduction of I- ions, the selective deposition of Pd on the {111} end facets gradually started. As the concentration of KI is very low, the effects of I- ions on both the facet surface energies and reaction kinetics are not significant. With more KI solution introduced into growth solution (KI/H2PdCl4=2:1), dumbbell shaped nanocrystals were obtained (Figure 4c). In this case, more newly reduced Pd atoms are deposited on the end of BPs and more 11 ACS Paragon Plus Environment

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defined facets of Pd appeared compared with the structures as shown in Figure 4b. Once 0.5 mL of 0.1 M KI was added (KI/H2PdCl4=40:1), the Pd-Au-Pd segmented nanorods with well-defined facets were obtained (Figure 4d). In the presence of high concentration of KI, the growth of Pd atoms rarely occurred on the high-index facets of Au BPs, suggesting the high-index facets of Au BPs is more inert for Pd deposition than that of {111} end facets. Besides, the fully complexing of I- with H2PdCl4 gives rise to a very slow reaction kinetics, which plays important role on the Pd selective deposition. These results demonstrate that the introduction of KI in the growth solution can block the highindex facets of Au BPs and slow down the reaction kinetics to promote the selective deposition of Pd on the low-index facets of BP seeds.38

Figure 4. TEM images of Au-Pd bimetallic nanorods obtained by adding different amount of KI: (a) 0 µL, (b) 25 µL 1 mM, (c) 25 µL 0.1 M, (d) 0.5 mL 0.1 M.


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The unique structure of Au BPs also plays an important role on the selective deposition of Pd atoms on the seeds. To demonstrate the importance of seeds, fivefold twinned Au nanorods and single-crystalline nanorods (Figure S4 and S5) were also explored as seeds for the growth of Au-Pd bimetallic nanostructures. When five-fold twinned nanorods enclosed only by low-index {111} and {100} facets were used as growth seeds and other reaction conditions were kept the same, the obtained heterostructures were Au@Pd core-shell nanorods. TEM image clearly shows the formation of Au@Pd core-shell nanostructures, confirming that Pd deposited on the whole surface of five-fold twinned nanorods (Figure 5a and b). This result demonstrates the structure effect of Au BPs with both low- and high-index facets is crucial for the selective deposition of Pd. In addition, the Au@Pd core-shell nanorods grew in length but maintained a constant width, which is the same observation with Au-Pd segmented nanorods. In both cases, the deposition of Pd along the twin plane rather than on the lateral surface of five-fold twinned Au nanocrystals will minimize the lattice strain.39 Compared with five-fold twinned Au nanocrystals, single-crystalline Au nanocrystals may result in the different deposition behavior of the second metal in order to minimize the total strain energy. Once single-crystalline nanorods were used as seeds instead of Au five-fold twinned structures, the Au@Pd core-shell nanorods with a rectangular shape were observed (Figure 5c and d). This deposition of Pd occurred on both the end and side facets of the single-crystalline Au nanorods, which is different with that of Pd on the fivefold twinned Au nanocrystals.


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Figure 5. TEM images of Au@Pd nanorods obtained by using other Au nanocrystals as growth seeds. The volume of 0.1 M KI solution was kept at 500 µL, (a-b) five-fold twinned nanorods, (c-d) single-crystalline nanorods. Generally, the high-index side facets of Au BPs possess a high lattice distortion energy, the second metal, like Ag, will preferentially deposit on them. In this study, Pd atoms selectively deposited on the low-index end facets of Au BPs instead of the highindex side facets when Au BPs were used as growth seeds in the presence of CTAB and KI. In short, the formation of Pd-Au-Pd segmented nanorods with {110} end facets can be primarily attributed to the following reasons: (1) selective adsorption of I- ions on high-index side facets of Au BPs reduces their surface energy and blocks the preferential deposition of Pd atoms;26,40 (2) the complexing reaction of I- ions with H2PdCl4 significantly lowers the reduction kinetics of Pd2+ ions, which is beneficial for the preferential deposition of Pd atoms;2, 31, 36,37 (3) selective adsorption of I- ions on nascent


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{100} facets stabilizes the {110} end facets and {100} side facets of Pd segmented parts;2,31 (4) the unique structure of Au BP provides new variables for experimentally maneuvering the deposition model of second metals on the surface of growth seeds.26 CONCLUSION In summary, Pd-Au-Pd segmented nanorods with flat tips have been synthesized using Au BPs as growth seeds. Through a series of control experiments, we revealed that two parameters were identified as critical factors for the growth of segmented Pd-Au-Pd nanorods. It is found that, in the presence of a high concentration of I- ions, Pd atoms selectively deposited on the low-index end facets of Au BPs but not on the high-index side facets, while Pd atoms grew on the whole surface of other Au seeds including fivefold twinned nanorods and single crystalline nanorods. The switched deposition of Pd atoms were attributed to the unique structure of Au BPs, selective adsorption of I- ions and slow reaction kinetics. These results manifest the role of seed structures on the growth modes of second metals, which is important for gaining mechanistic insights into the control synthesis of bimetallic heterostructures with controllable components and tailorable optical properties. ASSOCIATED CONTENT Supporting Information Experimental details for the synthesis of Au growth seeds, additional TEM, HRTEM, SAED images. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION 15 ACS Paragon Plus Environment

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Corresponding Author E-mail: [email protected]. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT We are thankful for financial support from the Tianjin Municipal Science and Technology Commission (16JCYBJC41600) and the Fundamental Research Funds of Tianjin University of Technology. REFERENCES (1) Zhuo, X.; Zhu, X.; Li, Q.; Yang, Z.; Wang, J., Gold NanobipyramidDirectedGrowth of Length-Variable Silver Nanorods with Multipolar Plasmon Resonances. ACS Nano 2015, 9 (7), 7523-7535. (2) Xu, L.; Wang, K.; Jiang, B.; Chen, W.; Liu, F.; Hao, H.; Zou, C.; Yang, Y.; Huang, S., Competitive Effect in The Growth of Pd–Au–Pd Segmental Nanorods. Chem. Mater. 2016, 28 (20), 7394-7403. (3) Li, C.; Sun, L.; Sun, Y.; Teranishi, T., One-Pot Controllable Synthesis of Au@Ag Heterogeneous Nanorods with Highly Tunable Plasmonic Absorption. Chem. Mater. 2013, 25 (13), 2580-2590. (4) Kim, S.; Shuford, K. L.; Bok, H.-M.; Kim, S. K.; Park, S., Intraparticle Surface Plasmon Coupling in Quasi-One-Dimensional Nanostructures. Nano Lett. 2008, 8 (3), 800-804.


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(5) Kim, S.; Kim, S. K.; Park, S., Bimetallic Gold−Silver Nanorods Produce Multiple Surface Plasmon Bands. J. Am. Chem. Soc. 2009, 131 (24), 8380-8381. (6) Tang, L.; Li, S.; Xu, L.; Ma, W.; Kuang, H.; Wang, L.; Xu, C., Chirality-based Au@Ag Nanorod Dimers Sensor for Ultrasensitive PSA Detection. ACS Appl. Mater. Interfaces 2015, 7 (23), 12708-12712. (7) Xu, S.; Ouyang, W.; Xie, P.; Lin, Y.; Qiu, B.; Lin, Z.; Chen, G.; Guo, L., Highly Uniform Gold Nanobipyramids for Ultrasensitive Colorimetric Detection of Influenza Virus. Anal. Chem. 2017, 89 (3), 1617-1623. (8) Alkilany, A. M.; Lohse, S. E.; Murphy, C. J., The Gold Standard: Gold Nanoparticle Libraries To Understand the Nano–Bio Interface. Acc. Chem. Res. 2013, 46 (3), 650-661. (9) Smith, K. W.; Zhao, H.; Zhang, H.; Sánchez-Iglesias, A.; Grzelczak, M.; Wang, Y.; Chang, W.-S.; Nordlander, P.; Liz-Marzán, L. M.; Link, S., Chiral and Achiral Nanodumbbell Dimers: The Effect of Geometry on Plasmonic Properties. ACS Nano 2016, 10 (6), 6180-6188. (10) Lu, J.; Chang, Y.-X.; Zhang, N.-N.; Wei, Y.; Li, A.-J.; Tai, J.; Xue, Y.; Wang, Z.Y.; Yang, Y.; Zhao, L.; Lu, Z.-Y.; Liu, K., Chiral Plasmonic Nanochains via the SelfAssembly of Gold Nanorods and Helical Glutathione Oligomers Facilitated by Cetyltrimethylammonium Bromide Micelles. ACS Nano 2017, 11 (4), 3463-3475. (11) Rao, W.; Li, Q.; Wang, Y.; Li, T.; Wu, L., Comparison of Photoluminescence Quantum Yield of Single Gold Nanobipyramids and Gold Nanorods. ACS Nano 2015, 9 (3), 2783-2791.


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Figure 1. (a) Growth of Pd-Au-Pd segmented nanorods with flat tips from Au bipyramids with the help of KI. (b-d) TEM and FESEM of Pd-Au-Pd segmented nanorods with flat tip obtained by adding 0.5 mL of 0.1 M KI solution. (e-g) HAADF-STEM image and EDX mapping a single Pd-Au-Pd segmented nanorod. (h) UV/Vis extinction spectra of i) Au BPs seeds and ii) Pd-Au-Pd segmented nanorods. 98x102mm (300 x 300 DPI)

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Figure 2. (a-d) TEM and HAADF image of a Pd-Au segmented bimetallic nanorod. (c1 and c2) HRTEM images obtained at the areas 1 and 2 in c), the measured d spacing of 0.14 nm matches with that of the (220) plane of Pd. (d1) HAADF image obtained at the area 1 in d) 85x40mm (300 x 300 DPI)


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Figure 3. TEM images of Pd-Au-Pd segmented nanorods obtained by terminating reaction at different reaction times: (a-b) 15 min; (c-d) 1 h; (e-f) 2 h; (g-h) 6 h. 121x80mm (300 x 300 DPI)


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Figure 4. TEM images of Au-Pd bimetallic nanorods obtained by adding different amount of KI: (a) 0 µL, (b) 25 µL 1 mM, (c) 25 µL 0.1 M, (d) 0.5 mL 0.1 M. 80x64mm (300 x 300 DPI)


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Figure 5. TEM images of Au@Pd nanorods obtained by using other Au nanocrystals as growth seeds. The volume of 0.1 M KI solution was kept at 500 µL, (a-b) five-fold twinned nanorods, (c-d) single-crystalline nanorods. 80x64mm (300 x 300 DPI)


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98x63mm (300 x 300 DPI)


Iodide-Switched Deposition for the Synthesis of Segmented PdâAu

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