Tuning the Morphology of Gold Nanocrystals by Switching the Growth

Feb 12, 2008 - Yanjuan Xiang,† Xiaochun Wu,*,‡ Dongfang Liu,† Lili Feng,‡ Ke Zhang,‡ Weiguo Chu,‡. Weiya Zhou,† and Sishen Xie*,†. Bei...
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J. Phys. Chem. C 2008, 112, 3203-3208

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Tuning the Morphology of Gold Nanocrystals by Switching the Growth of {110} Facets from Restriction to Preference Yanjuan Xiang,† Xiaochun Wu,*,‡ Dongfang Liu,† Lili Feng,‡ Ke Zhang,‡ Weiguo Chu,‡ Weiya Zhou,† and Sishen Xie*,† Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Graduate School of the Chinese Academy of Sciences, Beijing 100080, People’s Republic of China, and National Center for Nanoscience and Technology, Beijing 100080, People’s Republic of China ReceiVed: October 31, 2007; In Final Form: NoVember 30, 2007

Single crystalline Au nanorods (Au NRs), synthesized via seed-mediated growth, show unique surface structures. Apart from the oft-observed {100} and {111} facets, unexpectedly, unstable {110} facets dominate in such nanorods due to {110} restricted growth. Unique properties have been suggested for the nanorods. One novel property, we believe, is that the the high-energy {110} endows the nanorod with a high reactivity, thus making the growth to more stable morphologies possible. Herein, by switching the growth to the {110} preference, we successfully obtained thermodynamically more stable morphologies (arrow-headed gold nanorods and gold nano-octahedra) with a high quality and yield. A blockade of selective underpotential deposition of silver is suggested to be responsible for the switching.

Introduction At present, noble metal nanocrystals have attracted much attention, especially their unique surface plasmon resonance (SPR) characteristics and their SPR-related wide potentionals.1 Because of the strong dependence of the SPR peak position, strength, and numbers on the shape, great efforts have been made concerning the shape-controlled growth of noble metal nanocrystals.2 However, it is still a great challenge to intentionally control the growth process in achieving the programmed shape control of metal nanocrystals. Nowadays, most of synthesis methods work in an empirical way. As a result, synthesis methods that are able to systematically control the shape of metal nanocrystals are few and work with limited success.2 Recently, short aspect ratio single crystalline gold nanorods (Au NRs) were synthesized with a high reproducibility and high yield,3 thus making them powerful candidates as hard templates to guide the growth of other metals. Core/shell bimetallic nanostructures based on Au NR templates have been explored.4 More interestingly, the Au NRs show unique surface structures.5 Wang et al. investigated their crystal structure using highresolution TEM. They found that the side facets of the rod are dominated by four {110} and four {100} facets, whereas the ends of the rod are enclosed by {111} and {110} planes.5a Apart from the oft-appearing {100} and {111} facets, unexpectedly, unstable {110} facets dominate in the nanorod.5 Strong interaction with surfactant molecules (cetyltrimethylammomium bromide, CTAB) is suggested to make them stable.6 Because of their rich facets and unique surface structures, novel properties have been suggested but less exploited. We believe that the exposure of the high-energy {110} facet should endow the nanorod with a high reactivity, thus making the growth to more * To whom correspondence should be addressed. E-mail: (X.W.) [email protected] or (S.X.) [email protected]. † Graduate School of the Chinese Academy of Sciences. ‡ National Center for Nanoscience and Technology.

stable morphologies possible. One example has been demonstrated for the growth of a Pd shell on the Au nanorod.7 We have succeeded in creating a growth condition where the growth rate of the {110} side facet is much faster than that of the {100} facet. Faster growth of the {110} facet finally led to their disappearance and a morphology transformation to a rectangleshaped Pd shell. It experimentally demonstrates that adjusting the growth rates of the {110} and {100} facets is a reasonable route for shape control. In addition, it is generally accepted that the shape of fcc metals is mainly determined by relative growth rates of different lattice planes.8 The well-defined facets of the nanorod make systematic study and tuning the growth behaviors of different facets possible. During the formation of the Au NRs, the addition of small amounts of Ag ions plays an important role. The preferential underpotential deposition (UPD) of silver atoms on the Au {110} facet is suggested to restrict the deposition of gold atoms on the {110} facet, thus inducing one-dimensional growth along the [100] direction.6d The growth of the gold nanorod is thus {110} restricted growth. Not only in the seed-mediated growth are tiny amounts of silver ions found to be crucial to the shape control of gold nanocrystals in other synthesis methods as well.9,10 Song et al. found that in a modified polyol process, with increasing silver/gold ratios, gold nanocrystals change their morphologies from octahedron to truncated octahedron, to cuboctahedron, and finally to cubes.9 Silver species are suggested to be selectively bound to {100} and to restrict the surface growth. Yang et al. have demonstrated that in the polyol method, truncated tetrahedra and icosahedra were predominantly obtained without silver salts.10 By introducing silver ions, cubic particles formed. As a foreign species, the tuning of silver to the shape is obvious. But, the detailed tuning mechanism is still unclear. Herein, by tuning reaction parameters, we have successfully switched the growth mode of Au NR from {110} restriction to preference. New morphologies, such as arrow-headed gold nanorods and gold nano-octahedra, are obtained with high quality and yield. To the best of our knowledge, this is the first

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time that these two shapes for gold nanocrystals have been synthesized by seed-mediated growth in aqueous solutions. The blockade of selective UPD is suggested to be responsible for the switching and is achieved by increasing the overall coverage of silver so that the coverage difference at different facets does not exist anymore. The growth is therefore guided by surface energy minimization. Experimental Procedures Materials. Sodium borohydride (NaBH4), hydrogen tetrachloroaurate(III) trihydrate (HAuCl4‚3H2O), CTAB, silver nitrate (AgNO3), and L-ascorbic acid (AA) were purchased from Alfa and used as received. Milli-Q water (18 MΩ cm, MilliporeQ) was used for all solution preparations. All glassware used in the synthesis procedures was cleaned in a bath of piranha solution (70% H2SO4/30% H2O2 ) 7:3 v/v) and boiled for 30 min. Synthesis of Au Nanorods. The Au NRs were prepared by the seed-mediated growth method.3b,d CTAB-capped Au seeds were synthesized by the method described by Nikoobakht et al. Briefly, 7.5 mL of 0.1 M CTAB solution was mixed with 250 µL of 10 mM HAuCl4. Then, 0.6 mL of ice-cold 0.01 M NaBH4 solution was quickly added, resulting in the formation of a light-brown solution. The seed solution was vigorously stirred for 2 min and then kept at room temperature. This seed solution was used within 2-5 h. The synthesis procedure of Au NRs was adapted from Liu et al.6d The growth solution of Au NRs consisted of 100 mL of 0.1 M CTAB, 5 mL of 0.01 M HAuCl4, 2 mL of 0.5 M H2SO4, 1 mL of 10 mM AgNO3, and 800 µL of 0.1 M AA. After the color of the solution changed from orange to colorless, 240 µL of the seed solution was added to the growth solution. The resulting mixture was left undisturbed and aged overnight at room temperature. Au NRs were separated from the growth solution by centrifugation (12 000 rpm for 5 min). The precipitation was collected and redispersed in deionized water. Synthesis of Arrow-Headed Au Nanorods and Au NanoOctahedra. In a typical synthesis of the arrow-headed Au NRs, HAuCl4 (24 mM, 10 µL) was added to a solution containing the Au NRs (1 mL), CTAB (2 mL, 0.1 M), AA (0.1 M, 24 µL), and AgNO3 (20 mM, 12 µL). The ratios of AA/Au3+ and Au3+/Ag+ were kept at 10 and 1, respectively. The mixture was then diluted with deionized water to give a final volume of 6 mL and was kept in a water bath at 30 °C for 12 h. For the synthesis of Au nano-octahedra, the volumes of HAuCl4, AA, and AgNO3 were increased 6 times, while those of the Au NRs and CTAB remained unchanged. Electrochemical Measurements. A three-electrode cell was used with Ag/AgCl as the reference electrode, a Pt wire as the auxiliary electrode, and a bare glassy carbon (GC) or Au NRsmodified GC as the working electrode. To simulate the growth conditions of arrow-headed Au nanorods and Au nanooctahedra, cyclic voltammetry (CV) measurements were performed in a 0.03 M CTAB aqueous solution with or without AgNO3 (10 mM). To avoid the partial oxidation of Au NRs at 0.7 V, the potential sweeping was begun by immersing the working electrode at 0 V and using slow (2 mV/s) linear potential sweeps to move the potentials from 0 to 0.7 V and then back from 0.7 to 0 V. Instruments. SEM (Hitachi S-5200) was used to determine the shape and size of the nanorods. UV-vis-NIR absorption spectra were collected using a PerkinElmer Lambda 950 spectrophotometer. The high-resolution TEM (HRTEM) images were obtained using a JEOL model JEM-2010 microscope. XPS

Figure 1. UV-vis extinction spectra of gold nanocrystals from cylindrical rods (a) to differently sized arrow-headed rods (b-f), and finally to octahedra (g). The concentrations of HAuCl4 at growth solutions were 2 × 10-5 M (b), 4 × 10-5 M (c), 8 × 10-5 M (d), 1.6 × 10-4 M (e), 3.2 × 10-4 M (f), and 6.4 × 10-4 M (g). AA/Au3+ and Au3+/Ag+ ratios were kept constant at 10 and 1, respectively. The corresponding SEM images are shown in the upper part of the spectra with a scale bar of 60 nm.

spectra were collected using an ESCAlab220I-XL system and calibrated using a Au0 4f7/2 binding energy of 84 eV. Results and Discussion Figure 1 shows the extinction spectra of gold nanocrystals by increasing the amounts of added HAuCl4 while keeping the AA/Au3+ ratio, Au3+/Ag+ ratio, CTAB, and Au NR concentrations constant. Corresponding morphologies are presented as SEM images in the upper part of Figure 1. (1) At lower Au3+ concentrations (less than 4 × 10-5 M), the longitudinal SPR peak shows small red-shifts (ca. 10 nm), from 786 to 795 and 794 nm. The red-shift in the longitudinal SPR corresponds to an increase in the AR. Therefore, the slight increase in the AR indicates that the gold ions preferentially deposit at the two ends of the Au NR at lower Au3+ amounts. The morphology changes from the original cylinder-shaped rod with an octagonal crosssection6a to the arrow-headed rod with a square cross-section. (2) At higher Au3+ concentrations (between 8 × 10-5 and 3.2 × 10-4 M), instead of a red-shift, a blue-shift in the longitudinal SPR band occurs. There is a continuous blue-shift from 754 to 594 nm, while the transverse SPR slightly red-shifts from 510 to 538 nm (Figure 1d-f). Simultaneously, the intensities of both bands enhance. The rapid decrease in the AR indicates the favorable deposition at the side of the rod, which is reflected in their SEM images. As compared to the change in the length of the rod, the increase in width is more significant. For example, from spectra a-f in Figure 1, the length increases 0.24 times, whereas the width increases 1.2 times. At this concentration range, the morphology of the arrow-headed rod remains. Additionally, the size of the arrow-headed rod increases with increasing Au3+ amounts. (3) Finally, at a Au3+ concentration of 6.4 × 10-4 M, mainly one peak at 574 nm dominates. The corresponding SEM image shows that gold nanocrystals are mainly octahedra (Figure 1g). To understand the morphology transformation of the gold octahedra, its evolution process was monitored via UV-vis extinction spectroscopy and SEM measurements (Figure 2A,B). For SEM measurements, certain amounts of growth solution were taken out at selected times and centrifuged to stop the reaction. Because of a too high Au3+ concentration used in the growth of gold octahedra, we diluted the growth solution shown

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Figure 3. STEM image of the arrow-headed gold nanorods (a) and a high-resolution TEM image of a single rod when the electron beam is aligned in the [001] direction (b). The inset is the corresponding FFT image.

Figure 2. (A) Time evolution of UV-vis extinction spectra during the formation of gold nano-octahedra. (a) 2 min, (b) 4 min, (c) 6 min, (d) 8 min, (e) 10 min, (f) 12 min, (g) 14 min, (h) 16 min, (i) 18 min, and (j) 56 min. SEM images of gold nanocrystals with reaction times of 2 min, 8 min, 20 min, and 2 h for a diluted sample (B) and 1 min, 6 min, 12 min, and 2 h for an undiluted sample (C) are presented with a scale bar of 60 nm. Inset in the third image of panel C is a TEM image with the same scale bar.

in Figure 1g 3 times to be able to measure the extinction spectra properly. With the proceeding of the reaction, the longitudinal SPR continuously shifted to shorter wavelengths to a final position of 570 nm. An ultimate blue-shift of about 280 nm was achieved. In comparison with Figure 1, this blue-shift trend is quite similar. From SEM images, we found that the dominate morphology before 8 min was the arrow-headed rod and that after 8 min it became the (truncated) octahedron (Figure 2B). For comparison, the shape evolution of the undiluted sample is also presented by SEM images. The TEM image in the inset shows regular hexagons, indicating that the particles aligned their C3 axes normal to the substrate. The SEM images demonstrate that the process of shape evolution is similar but much faster for the undiluted sample. In both cases, the final products are octahedra. This means that the relative growth rates of different facets are constant, thus inducing the same final shape. This leaves us the space for further optimizing the growth conditions by tuning overall deposition rates. With respect to shape evolution, obviously, the arrow-headed rod is the intermediate of the octahedron, and two of the octahedral vertexes come from the two arrow-heads that develop from the two endcaps of the original cylindrical rod. As the original endcaps have both {111} and {110} facets, this process proposes the disappearance of four {110} facets during the formation of these two octahedral vertexes. Figure 3 is a typical STEM image of arrow-headed nanorods. Most nanorods should lie on the carbon film with one side surface parallel to it. In this way, the electron beam is easily aligned in the direction normal to this surface, inducing the distribution of the atoms on it being imaged. This is shown in the HRTEM image of a single rod (Figure 3b). The correspond-

Figure 4. Schematics illustrating the formation process of the gold octahedron: (A) side view and (B) top view.

ing fast Fourier transform (FFT) pattern corresponds to only one set of diffraction spots:{002}. This indicates that the imaged surface is the {100} facet. Considering its square cross-section, the four side surfaces for the arrow-headed rod must be {100} facets. This suggests that the four {110} side facets gradually become reduced during the formation of the arrow-headed rod and finally disappear. According to the previous analysis, we propose the following formation process for the gold octahedron (Figure 4). The crystal structures of the starting Au NR seeds have been described previously in detail5 and are schematically shown here in Figure 4A,B for the side and top views, respectively. For simplicity, we divided the growth of the rod into two parts: the endcap and the middle part. At the endcap, the four unstable {110} facets grow more quickly, become reduced, and finally disappear. Instead, four {111} facets become enlarged, meet at the top, and finally form an apex there. The newly formed endcap therefore has four {111} facets with an arrow-head at top and a square at the bottom. Note that the original cylindrical endcap has an octagonal cross-section. At the side of the rod, gold atoms deposit also more quickly along the {110} planes than along the {100} planes, thus leading to an enlargement of the {100} side surfaces accompanying a reduction of {110}. The final consequence is also the disappearance of all four {110} side surfaces. The cross-section of the rod at the middle part becomes square terminated with four more stable {100} side surfaces (see Figure 4B, dashed squares). The square cross-section of the arrow-head (dotted squares in Figure 4B) forms a 45° included angle with respect to that of the middle part (dashed

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Figure 5. SEM images of original Au NRs (a) and newly formed Au NRs grown with Ag+/Au3+ ratios of (b) 1:5, (c) 1:2, (d) 1:1, (e) 2:1, and (f) 5:1. The AA/Au3+ ratios are 10:1 in all cases. The scale bars are 90 nm.

squares in Figure 4B). Between the interconnections of {111} at the arrow-heads and {100} at the sides, new small triangular {110} facets appear. Owing to the lower packing density of CTAB molecules at the endcaps, more gold atoms deposit there. The arrow-heads look, therefore, a little larger than the middle part of the rod. In conclusion, the arrow-headed nanorods form through competing growth between the {111} and the {110} facets at the endcaps and between {110} and {100} at the sides. This is a thermodynamically driven growth process because unstable {110} facets become reduced. Although the arrowheaded nanorods are more stable than cylindrical nanorods, the existence of newly formed {110} and large {100} side surfaces make them form a more stable morphology. The newly emerged {110} facets continue to grow more quickly, inducing the further enlargement of the arrow-heads. When the two {110} facets at the two ends of the rod knock together and finally disappear, a Au octahedron forms. The quadrilateral {111} side surface in the arrow-head becomes a triangle due to the disappearance of {110}. The minimization of the surface energy is therefore the driving force of the shape evolution. To understand the formation mechanism of the arrow-headed Au NRs and the Au nano-octahedra, three key reaction parameters (Ag+/Au3+, AA/Au3+, and CTAB concentration) were investigated. The Ag+/Au3+ ratio was found to play a crucial role. For example, arrow-headed nanorods are formed at Ag+/Au3+ ratios g 1:2. The ARs of the rods reduce with increasing Ag+/Au3+ ratio (Figure 5). In the synthesis of cylindrical Au NRs, Ag ions play an important role as well.5b To obtain them with high yields, Ag+/Au3+ ratios are lower than 1:5. The ARs of the rods increase from 2 to 6 by increasing the Ag+/Au3+ ratios (1:50- to 1:5). When Ag+/Au3+ ratios are higher than 1:5, the ARs become reduced again, accompanied by an increase in the amount of spherical nanoparticles. As mentioned previously, cylindrical Au NRs are obtained with {110} restricted growth, which is favored by lower Ag+/Au3+ ratios. The selective UPD of silver atoms on {110} has been suggested to be responsible for the restricted growth of {110}: 6d Selective UPD of silver on the (110) surface of the Au seed changed Ag+ to Ag0. The formation of Ag0 restricted the growth of Au along the direction. After that, Ag0 was oxidized again to Ag+ by a galvanic exchange reaction with AuCl4-, thus realizing the morphology tuning Au NRs. Our preliminary CV measurement supported the existence of Ag UPD on the Au NR. As shown in Figure 6a, the UPD of silver can be seen clearly at 0 V. For the desorption sweeping, one peak at 0.63

Figure 6. CV images of Au NRs-modified GC electrode in (a) 0.03 M CTAB and 1 mM AgNO3, (b) 0.03 M CTAB without AgNO3, and (c) bare GC electrode in 0.03 M CTAB and 1 mM AgNO3. Scan rate was 2 mV/s.

V and a shoulder around 0.53 V appeared. However, the corresponding readsorption features of Ag did not occur for the following back sweeping, which was most likely due to the partial oxidation of the Au NRs at 0.7 V. This oxidation prohibited the further UPD of Ag. The relatively high noise level in the voltammogram is mainly due to the poor conductivity of CTAB (as seen from a similar noise level for the GC electrode). In contrast, these two features did not appear in the cyclic voltammograms of two control experiments (i.e., the Au NRs-modified GC electrode in the electrolyte not containing Ag ions (Figure 6b) and the bare GC electrode in the electrolyte containing Ag ions (Figure 6c)). The 0.53 V potential is close to one of the UPD potentials of Ag on Au (111)11 and Au (100)12, whereas the 0.63 V potential is 0.1 V more positive. The data of Ag UPD on Au (110) have been unavailable up to now experimentally. According to the calculations of Sanchez et al., the UPD shifts for the Au/Ag+ system are 0.12, 0.17, and 0.28 V for Au (111), Au (100), and Au (110), respectively.13 Considering that the dominant surfaces for Au NR are the {110} facets, the 0.63 V dissolution peak should reflect the UPD potential of Ag on Au (110), while the weaker shoulder should come from the contributions of Au (111) and Au (100). Our results therefore support the existence of selective Ag UPD on the different lattice planes of Au. This selective UPD can interpret the restricted growth of {110} for the Au NRs and that of {100} for Song et al.’s and Yang et al.’s cases but does not work for the case we present here. Other scenarios should exist because the growth of {110}, instead of being restricted, is obviously enhanced here. One large difference between our case and the others is that we employ quite high Ag+/Au3+ ratios. If the coverage of silver at the Au surface can be increased by increasing silver/gold ratios and is higher than full coverage for all exposed facets, the selective UPD cannot be held anymore. The control experiment verified this assumption (Figure 7). For the a series in Figure 7, the growth conditions of cylindrical Au NRs were used. The shape of the Au NRs remained unchanged (see the inset SEM image).

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Figure 7. Relationship between Ag/Au molar ratios in the rod by EDX analysis (averaged for five measurements) and the added Ag+/AuCl4molar ratios in the growth solutions. The only difference between a and b series is the AA/AuCl4- ratio employed in the growth solution. For a series, this ratio is 1.1, whereas it changes to 10 for b series. The inserted SEM images show the difference in morphology of the obtained nanorods caused by the AA/AuCl4- ratio. The scale bar is 60 nm. Solid lines are used to guide the eyes.

For the b series in Figure 7, the growth conditions of arrowheaded Au NRs were employed. Arrow-headed nanorods were obtained at Ag+/AuCl4- g 1:2 (see the inset SEM image). For the cylindrical gold nanorods, the measured Ag/Au ratios by EDX analysis are around 3% statistically. In contrast, for the arrow-headed nanorods, the measured Ag/Au ratios are higher than 8% statistically. For a rough calculation, the Ag/Au ratio of a cylindrical gold nanorod (61.2 nm length vs 14.3 nm width) with one monolayer coverage of silver is around 7%. Above this limit, the selective UPD should not work. Because of the rapid overall growth rate, the growth is guided by surface energy minimization, thus {110} preferential growth. To gain more insight into the distribution and final valences of silver species in the Au nano-octahedra, a surface-sensitive XPS measurement was performed. From the survey spectrum (Figure 8a), the Ag/Au atomic ratio was 0.48, much higher than 0.18 (averaged for 6 measurements) obtained from EDX analysis. This difference indicates that the Ag species mainly exist on the surface of the Au nano-octahedron, therefore shielding the underlying Au surface from the XPS detection. Both the binding energy of Ag 3d and the Auger parameter of silver (724.5 eV),14 especially the latter, indicate that the final valence of Ag on the surface of the rod is +1. From the survey spectrum, the Br/Ag atomic ratio is 0.86, suggesting that Ag+ ions mainly exist in the form of AgBr. Apart from Ag+/Au3+ ratios, the AA/Au3+ ratio is another important factor for the formation of the arrow-headed Au NRs and Au nano-octahedra. In comparison to the Ag+/Au3+ ratio, the role of the AA/Au3+ ratio is more straightforward. Welldefined arrow-headed rods form at AA/AuCl4- ratios > 7.5 (Figure 9). Higher AA/Au3+ ratios guarantee a higher Ag/Au ratio in the obtained nanorod (Figure 7) and an overall fast deposition rate. The former blockades the selective UPD, while the latter guides the growth to a thermodynamically favored mode. The more AA is added, the bigger the {111} surfaces are. When AA is 20 times the Au3+ ion concentration, the apexes of the arrow are very sharp. This also forms an interesting contrast to the formation of cylindrical Au NRs, where low AA concentrations, AA/AuCl4- ratios between 1.1 and 1.6, are

Figure 8. XPS spectra of the Au nano-octahedra (a) from a survey scan and (b) from the Ag 3d core level.

Figure 9. SEM images of original Au NRs (a) and newly formed Au NRs grown with a AA/Au3+ ratio of (b) 1.25:1, (c) 1.5:1, (d) 3:1, (e) 7.5:1, and (f) 30:1. The Ag+/Au3+ ratio is 1:1. The scale bars are 90 nm.

employed to keep the deposition under kinetically controlled conditions. In addition to Ag+/Au3+ and AA/Au3+ ratios, the concentration of CTAB in the growth solution is another important factor for the formation of the arrow-headed Au NRs and Au nanooctahedra. At CTAB concentrations lower than 0.07 M, welldefined arrow-headed rods form (Figure 10). The sizes of the {111} facet increase with decreasing CTAB concentrations. Note that increasing the AA amount in the growth solution leads to a similar trend. This indicates that lower CTAB concentrations

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Xiang et al. Acknowledgment. Prof. Z. Y. Tang from the National Center for Nanoscience and Technology is thanked for cyclic voltammetry measurements. This work was supported by the National Natural Science Foundation of China (Grants 10334060 and 20773032) and the “973” National Key Basic Research Program of China (Grants 2005CB623602, 2006CB705600, 2006CB932602, and 2006AA03Z326). References and Notes

Figure 10. SEM images of original Au NRs (a) and newly formed Au NRs grown with CTAB concentrations of (b) 0.01 M, (c) 0.03 M, (d) 0.05 M, (e) 0.07 M, and (f) 0.1 M. The Ag+/Au3+ and AA/Au3+ ratios are 1:1 and 10:1, respectively. The scale bars are 90 nm.

favor faster deposition of gold, therefore a thermodynamically preferred growth mode. At a CTAB concentration of 0.1 M, dogbone-shaped nanorods dominate (Figure 10f). Conclusion In this study, we demonstrated the shape-controlled synthesis of Au nanocrystals using single crystalline Au NRs as seeds. New morphologies (here the arrow-headed gold nanorods and gold nano-octahedra) were obtained with high quality and yield. Considering the high controllability of cylindrical gold nanorods, we believe that tailoring the growth of metal on the Au NRs will be a promising way to achieve the programmed shape control of metal nanocrystals, not only for single component metal but also for two (or more) components. Au nano-octahedra have been reported by a modified polyol process,9 not yet by the seed-mediated method. As to the arrowheaded nanorods, similar shapes were reported for Au core/Ag shell bimetallic nanorods and were considered as one special morphology of the dumbbell.4b,d,15,16 The crystal structure and formation mechanism, however, have not been elucidated.4b,d,15,16 A potential application for these two structures is surface enhanced Raman scattering as the sharp nanoarrows would make the huge electric field enhancement possible. In addition, their strong SPR peak can be easily tuned from the visible to the near-IR region of the spectrum, especially for the arrow-headed rods. Furthermore, their special morphologies and rich spectral characteristics are expected to endow them with more applications.

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