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Langmuir 2005, 21, 2012-2016

Synthesis of Branched Gold Nanocrystals by a Seeding Growth Approach Chun-Hong Kuo and Michael H. Huang* Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan Received September 23, 2004. In Final Form: November 19, 2004 Synthesis of branched gold nanocrystals by a seeding growth approach is described. In this process, HAuCl4 aqueous solution was supplied stepwise to grow the gold seeds (∼2.5 nm) into larger nanoparticles with a highly faceted particle structure (∼15-20 nm in diameter). Sodium dodecyl sulfate (SDS) served as a capping agent to facilitate the formation of highly faceted nanoparticles, and ascorbic acid was used as a weak reducing agent. The highly faceted nanoparticles then transformed into branched nanocrystals (∼40 nm in length) by further addition of the SDS-HAuCl4 solution and ascorbic acid for particle growth. The branched nanocrystals show bipod, tripod, tetrapod, and pentapod structures and are composed of mainly (111) lattice planes. These multipods appear to grow along the twin boundaries of the initially formed highly faceted gold nanoparticles, as the twin boundaries on the pods originate from the centers of the branched nanocrystals. The concentration of ascorbate ions in the solution was found to have a profound influence on branch formation. These branched nanocrystals are stable to storage at low temperature (that is, 4 °C), but they may slowly evolve into a multitwinned faceted crystal structure (that is, pentagonal-shaped decahedral structure) when stored at 30 °C.

Introduction Gold nanoparticles have been used in many studies to carry out specific functions by modification of the particle surface with a variety of molecules.1-3 For example, binding of molecules such as DNA4 and crown ether5 to roughly spherical gold nanoparticles has been demonstrated and may have applications in biological assay and chemical analysis, respectively. In addition to the studies on the functionalization of spherical gold nanoparticles, growth of other shape-specific gold nanoparticles has also been an active area of research interest in the past few years. Syntheses of gold nanorods,6-10 nanocubes,11 and nanoplates12 have been reported. The formation of these gold nanocrystals makes possible the examination of new optical properties associated with shape variations. Gold nanorods have also been used as building blocks for the * Author to whom correspondence should be addressed. E-mail: [email protected]. (1) (a) Loweth, C. J.; Caldwell, W. B.; Peng, X.; Alivisatos, A. P.; Schultz, P. G. Angew. Chem., Int. Ed. Engl. 1999, 38, 1808. (b) Caswell, K. K.; Wilson, J. N.; Bunz, U. H. F.; Murphy, C. J. J. Am. Chem. Soc. 2003, 125, 13914. (2) (a) Boal, A. K.; Ilhan, F.; DeRouchey, J. E.; Thurn-Albrecht, T.; Russell, T. P.; Rotello, V. M. Nature 2000, 404, 746. (b) Lin, C.-C.; Yeh, Y.-C.; Yang, C.-Y.; Chen, C.-L.; Chen, G.-F.; Chen, C.-C.; Wu, Y.-C. J. Am. Chem. Soc. 2002, 124, 3508. (c) Novak, J. P.; Feldheim, D. L. J. Am. Chem. Soc. 2000, 122, 3979. (3) (a) Levi, S. A.; Mourran, A.; Spatz, J. P.; van Veggel, F. C. J. M.; Reinhoudt, D. N.; Mo¨ller, M. Chem. Eur. J. 2002, 8, 3808. (b) Thomas, K. G.; Kamat, P. V. J. Am. Chem. Soc. 2000, 122, 2655. (4) (a) Taton, T. A.; Mirkin, C. A.; Letsinger, R. L. Science 2000, 289, 1757. (b) Mucic, R. C.; Storhoff, J. J.; Mirkin, C. A.; Letsinger, R. L. J. Am. Chem. Soc. 1998, 120, 12674. (5) Lin, S. Y.; Liu, S.-W.; Lin, C.-M.; Chen, C.-H. Anal. Chem. 2002, 74, 330. (6) Busbee, B. D.; Obare, S. O.; Murphy, C. J. Adv. Mater. 2003, 15, 414. (7) Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15, 1957. (8) Kim, F.; Song, J. H.; Yang, P. J. Am. Chem. Soc. 2002, 124, 14316. (9) Gai, P. L.; Harmer, M. A. Nano Lett. 2002, 2, 771. (10) Chang, S.-S.; Shih, C.-W.; Chen, C.-D.; Lai, W.-C.; Wang, C. R. C. Langmuir 1999, 15, 701. (11) Sun, Y.; Xia, Y. Science 2002, 298, 2176. (12) (a) Kim, J.-U.; Cha, S.-H.; Shin, K.; Jho, J. Y.; Lee, J.-C. Adv. Mater. 2004, 16, 459. (b) Ibano, D.; Yokota, Y.; Tominaga, T. Chem. Lett. 2003, 32, 574.

fabrication of novel nanorod assembly13 and may extend the applications of gold nanoparticles. While there are many studies on the controlled growth of gold nanocrystals using a wide range of preparation approaches, it is interesting to note that only recently have branched gold nanocrystals been reported. Chen et al. prepared bipod, tripod, and tetrapod gold nanocrystals using a large amount of capping surfactant cetyltrimethylammonium bromide (CTAB), HAuCl4, ascorbic acid, and NaOH.14 The branched nanocrystals (∼40 nm in length) are relatively flat and require a considerably long time to form (1-20 days). Hao et al. synthesized threetipped gold nanocrystals from an aqueous solution of bis(p-sulfonatophenyl) phenylphosphine dihydrate dipotassium, H2O2, sodium citrate, and HAuCl4.15 The nanocrystals have larger sizes (∼50-100 nm in length), and the particle structures may evolve after storage in a refrigerator for several days, as evidenced by a spectral shift of the particle absorption to the blue. Sau et al. prepared branched gold nanoparticles from an aqueous solution of gold seeds, CTAB, HAuCl4, and ascorbic acid. The branched particles are quite large (∼100-500 nm in length), and their crystal structure was not characterized.16 Branched gold nanoparticles can offer new insights about the relationships between the different particle morphologies and their corresponding surface plasmon resonance absorption characteristics, and they may also have applications in molecular labeling and nanoelectronics. Here we report a simple and relatively fast synthesis of branched gold nanocrystals of about 30-40 nm in length by a seeding growth approach. The seeding growth method involves a stepwise procedure for the preparation of larger gold nanoparticles of varying sizes (13) (a) Caswell, K. K.; Wilson, J. N.; Bunz, U. H. F.; Murphy, C. J. J. Am. Chem. Soc. 2003, 125, 13914. (b) Dujardin, E.; Hsin, L.-B.; Wang, C. R. C.; Mann, S. Chem. Commun. 2001, 1264. (14) Chen, S.; Wang, Z. L.; Ballato, J.; Foulger, S. H.; Carroll, D. L. J. Am. Chem. Soc. 2003, 125, 16186. (15) Hao, E.; Bailey, R. C.; Schatz, G. C.; Hupp, J. T.; Li, S. Nano Lett. 2004, 4, 327. (16) Sau, T. K.; Murphy, C. J. J. Am. Chem. Soc. 2004, 126, 8648.

10.1021/la0476332 CCC: $30.25 © 2005 American Chemical Society Published on Web 01/22/2005

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from smaller gold seeds.17 The particle-stabilizing agent used in this study is sodium dodecyl sulfate (SDS). The branched gold nanocrystals show bipod, tripod, tetrapod, and pentapod structures. The multipods appear to have grown from the smaller gold nanoparticles with highly faceted particle structures. An analysis of the crystal structures of the branched nanocrystals revealed that the branches are composed of (111) lattice planes. It was also found that ascorbate ions may promote the multipod formation. The structural evolution of these branched nanocrystals as a function of time and temperature was also followed to evaluate the stability of the particles. Experimental Section Synthesis of Gold Seeds. A volume of 0.2 mL of 0.025 M sodium citrate solution was added to 19.8 mL of aqueous solution containing 2.5 × 10-4 M HAuCl4 and stirred for three minutes (Solution I). Concurrently, 10 mL of 0.10 M NaBH4 solution (Solution II) was prepared by adding NaBH4 to 10 mL of ice-cold 0.025 M sodium citrate solution. When 0.6 mL of Solution II was added to Solution I with stirring, the mixture immediately turned orange red, indicating the formation of gold particles. The particle size was approximately 2.5 nm, based on a literature report using essentially the same preparation procedure.18 Preparation of Growth Solution. A volume of 100 mL of 1 × 10-4 M HAuCl4 aqueous solution was prepared. Then 5 × 10-3 mole of SDS was added to this solution with stirring until SDS powder was completely dissolved. The final concentration of SDS was 0.05 M. This solution was used as the growth solution. In a few experiments, 0.01 M SDS was used. Synthesis of Branched Gold Nanocrystals. The synthesis of branched nanocrystals was carried out by a stepwise procedure that began with the growth of more spherically shaped nanoparticles. Three 50-mL flasks were labeled A, B, and C. In flask A, 9 mL of growth solution was mixed with 1 mL of seed solution and stirred for 10 minutes. Next, 50 µL of 0.04 or 0.10 M ascorbic acid was added to the solution and stirred vigorously for another 10 minutes. The solution color turned ruby red. Then 1 mL of solution in flask A and 9 mL of growth solution were added to flask B and stirred for 10 minutes, followed by the addition of 50 µL of 0.04 or 0.10 M ascorbic acid with 10 minutes of stirring time. The solution color in flask B was purplish red. Solution in flask C was prepared the same way as that in flask B by extracting 1 mL of solution from flask B. The final solution color in flask C was purple. All the solutions were left in the dark for 20 minutes or more for the reaction to go to completion before taking UVvis absorption spectra. To analyze the samples by transmission electron microscopy (TEM), gold nanocrystals in all three flasks can be concentrated by centrifugation for fifteen minutes (Hermle Z323 centrifuge). Different rotation speeds were used for the three sets of solution samples to produce precipitates (10 000 rpm for A, 8000 rpm for B, and 3000 rpm for C).

Results and Discussion The synthesis of branched gold nanocrystals involves the preparation of small gold seeds (∼2.5 nm), followed by the stepwise addition of HAuCl4, SDS surfactant, and ascorbic acid for particle growth. Ascorbic acid serves as a weak reducing agent. The weak reducing strength of ascorbic acid can enhance the growth of larger size particles in the presence of seed particles; secondary nucleation during the growth stage was inhibited.17 In the process, the gold seeds grow to larger sizes and then evolve into branched nanocrystals. Gold nanoparticles exhibit strong surface plasmon resonance absorption that is strongly dependent on the particle morphology. For roughly spherically shaped gold nanoparticles, the absorption band typically falls between 520 and 535 nm.17 (17) Jana, N. R.; Gearheart, L.; Murphy, C. J. Langmuir 2001, 17, 6782. (18) Jin, Y.; Kang, X.; Song, Y.; Zhang, B.; Cheng, G.; Dong, S. Anal. Chem. 2001, 73, 2843.

Figure 1. UV-vis absorption spectra of gold nanocrystals in solutions A, B, and C.

As the particles elongate into rods, a second absorption band appears that is red-shifted from the 520-535 nm band and is dependent on the aspect ratios of the nanorods.7,10 Figure 1 shows the UV-vis absorption spectra of gold nanocrystals in solutions A, B, and C. The spectra were taken using a Jasco V-570 spectrometer. The absorption band maximum for the gold seeds is at 510 nm (data not shown). Nanoparticles in solution A should have roughly spherical structure, as indicated by an absorption band at 524 nm. As particle size increases in solution B, the absorption band is slightly red-shifted to 538 nm. A more significant shift in the absorption band occurs in solution C with a band maximum at 560 nm. The results suggest that the nanoparticles in solution C should possess shapes that are appreciably different from spherical shape, and some particle elongation or distortion should have occurred. The spectral shifts cannot be attributed to nanoparticle aggregation, which can also give similar shifts, because almost all the particles existed as separate particles. A mixture of spherical and branched nanoparticles could also give a similar spectrum. However, most of the nanoparticles in solution C were not spherical in shape, but were branched or distorted. Such red-shifting of the absorption band has been observed in other branched nanoparticles, but the reported band maxima appeared at 670-700 nm.14,15 The difference in the positions of the UV-vis absorption bands may be attributed, as will be shown below, to the possibly shorter branches formed in solution C as compared to those synthesized in the two previous reports.14,15 Longer branches may have the same effect as nanorods do on the shifts of the UV-vis absorption bands to longer wavelengths. The actual particle morphologies were obtained by transmission electron microscopy (TEM) examination. TEM images were acquired with a JEOL JEM-2010 electron microscope operating at 200 kV. Figure 2 shows the TEM images of nanoparticles in solutions B (Figure 2a) and C (Figure 2b). Gold nanoparticles in solution A had diameters of around 5 nm (data not shown). In solution B, the average particle size had increased to 15-20 nm in diameter. Most of the nanoparticles exhibited a highly faceted crystal structure. Many of them had pentagonal and hexagonal shapes, suggesting the formation of decahedral and dodecahedral (or possibly icosahedral) gold structures, respectively. The presence of well-defined crystal faces with sharp twin boundaries in these nanoparticles may be facilitated by the selection of SDS as the capping agent;19 use of other capping surfactants had resulted in the formation of mainly spherically shaped


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Figure 2. TEM images of nanocrystals in solutions B (Figure 2a) and C (Figure 2b). Nanoparticles in solution B exhibit highly faceted crystal structure with well-defined faces. Inset shows an enlarged view of the highly faceted nanoparticles in the upper left portion of Figure 2a. Nanoparticles in solution C had developed multiple branches. Inset is a close-up view of some representative branched nanocrystals found in solution C. Scale bar in both insets ) 10 nm. The corresponding selected area electron diffraction pattern for the nanoparticles in solution C is shown in Figure 2c.

Figure 3. TEM images of branched gold nanocrystals. These nanocrystals possess very obvious twin boundaries on the pods that appear to originate from the centers of the particles. They have several different shapes including (a) V-shaped bipod or asymmetric tripod, (b) quasi-tetrapod, (c) pentapod, and (d) asymmetric tetrapod structures.

Au nanoparticles without sharp facets when the particle size was relatively small (that is, less than 30 nm).20 With the addition of more gold source and SDS molecules,

extensive growth of branched nanocrystals was observed in solution C (Figure 2b). The average particle size had increased to ∼40 nm. Figure 2c shows a selected area electron diffraction pattern of the branched gold nanoparticles. The diffraction rings can be indexed to the facecentered cubic unit cell structure of gold. Further examination of the branched nanoparticles revealed a variety of particle morphologies. Figure 3 shows the TEM images of a variety of branched gold nanocrystals found in solution C. The nanocrystals exhibited bipod, tripod, tetrapod, and pentapod structures with distinct twin boundaries. The twin boundaries extend from the centers of the particles to the ends of the pods. The same kind of twin boundaries was also present in the highly faceted nanoparticles of solution B. On the basis of these observations, it can be reasonably assumed that the formation of branched nanocrystals is strongly related to the presence of sharp twin boundaries in the highly faceted gold nanoparticles. High-resolution TEM images of selected branched nanocrystals were taken to further study the structures of these branched nanoparticles. Figure 4a displays the high-resolution TEM image of a quasi-tetrapod nanocrystal. The lattice fringe spacings of the entire parts of this nanoparticle were determined. Figures 4b and 4c show the enlarged views of the square regions in Figure 4a to reveal the lattice fringes and the twin boundaries. An interplanar spacing of 2.36 Å was measured for all the lattice fringes of the pods, as well as the rest of the

Figure 4. (a) High-resolution TEM image of a quasi-tetrapod of gold. There appear to be undeveloped branch features on top of this nanocrystal. A crystal structure analysis was performed over all parts of this nanoparticle. The two square regions in this image are enlarged to show the lattice planes constituting this nanocrystal and are shown in (b) (end of one pod) and (c) (edge portion of another pod). In both regions, (111) lattice fringes with an interplanar spacing of 2.36 Å were measured for all lattice planes. Twin boundaries can be clearly seen in (b).

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Figure 5. PXRD patterns of the nanoparticles in solutions B and C. Here 0.10 M ascorbic acid concentration was used to promote a slightly larger amount of pod formation, which facilitates the PXRD characterization. The strong intensity of the (111) diffraction peak suggests that the multipod formation is largely growth along the (111) lattice planes.

nanoparticle. Thus, the multipods form single crystals with (111) lattice planes. This result is reasonable, since the highly faceted gold nanoparticles formed in solution B also have dominant (111) lattice planes (see data below). Considering that the multipods typically exhibit twin boundaries that originate from the center of a nanoparticle, the multipods are likely to be formed by adding gold ions over the {111} facets adjacent to the twin boundaries in the final growth process. Thus, the faceted particle structure may facilitate the formation of multiply twinned branched nanocrystals. The fact that the branched nanocrystals have essentially {111} faces, including the base regions between two adjacent branches, is in contrast to the explanations offered in some reports on the formation of gold nanorods and multipods, where strong interactions of capping surfactant molecules with the {110} faces of gold nanoparticles promote the preferential growth of nanoparticles along 〈111〉 directions.9,14 To further check that the multipods of the branched nanocrystals grow predominantly along (111) planes, powder X-ray diffraction (PXRD) patterns of the nanoparticles in solutions B and C were taken. PXRD patterns were performed using a Rigaku MXP3 diffractometer with a Cu KR radiation at λ ) 1.5418 Å. Figure 5 shows the PXRD patterns of the highly faceted nanoparticles (trace B) and the branched nanoparticles (trace C). Here the samples used were prepared using a slightly higher concentration of ascorbic acid (0.10 M), which was found to show a larger amount of branch formation for a better PXRD characterization. The highly faceted nanoparticles gave a dominant (111) diffraction peak, and the branched nanocrystals continued to show a very strong (111) diffraction peak compared to the (200) peak (Figure 5, trace C). The same trend was observed for the samples prepared using 0.04 M ascorbic acid in the growth solution. The results are consistent with those obtained by the TEM characterization that the multipods grow by adding the incoming gold ions to existing (111) lattice planes. The formation of branched nanocrystals may also be related to the concentration of ascorbate ions, in addition to the contribution of SDS to the formation of particles with highly faceted structures. Thus, it is desirable to determine the effects ascorbate ions have on multipod formation. Only highly faceted Au nanoparticles without branches were obtained in solution C, using 0.01 M SDS and 0.04 M sodium ascorbate as the reducing agent. Upon increasing the sodium ascorbate concentration, a dramatic

Figure 6. Temperature and time effects on the morphology of the branched gold nanocrystals. (a) TEM image of the highly branched gold nanocrystals before storage. The arrow points at one of the branches. (b) Keeping the freshly prepared branched gold nanocrystals at 30 °C for 10 days resulted in the formation of faceted gold nanoparticles with pentagonal or other more well-defined shapes, as pointed by the arrows. The long and slender multipods had largely disappeared. (c) Keeping the branched nanocrystal solution at 4 °C for 10 days did not show appreciable structural changes. Slender multipods were still present, as indicated by the arrow. For this set of study, branched nanocrystals were prepared using 0.10 M ascorbic acid and 0.05 M SDS. Scale bar ) 20 nm in all three figures.

morphological change occurred. Use of 0.10 M sodium ascorbate during particle growth resulted in the formation of extensively branched nanoparticles. The branches appeared slender and uniform in diameter (∼5 nm) (see Supporting Information, Figure 1). The results suggest that ascorbate ions have a pronounced effect on the formation of branched nanocrystals. The weak reducing strength of ascorbic acid (or ascorbate ions) may be the key to the formation of branches. This observation is consistent with reports on the preparation of high-aspectratio gold nanorods, in which the ascorbate monoanion was found to play a key role in the formation of long gold nanorods from 4-nm gold seeds.6,21 In our case, the (19) Kuo, C.-H.; Chiang, T.-F.; Chen, L.-J.; Huang, M. H. Langmuir 2004, 20, 7820. (20) (a) Fleming, D. A.; Williams, M. E. Langmuir 2004, 20, 3021. (b) Liu, Y.; Male, K. B.; Bouvrette, P.; Luong, J. H. T. Chem. Mater. 2003, 15, 4172. (c) Walker, C. H.; St. John, J. V.; Wisian-Neilson, P. J. Am. Chem. Soc. 2001, 123, 3846. (d) Teranishi, T.; Hasegawa, S.; Shimizu, T.; Miyake, M. Adv. Mater. 2001, 13, 1699. (21) Gole, A.; Murphy, C. J. Chem. Mater. 2004, 16, 3633.


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branches may be considered as short nanorods, and the highly faceted gold nanoparticles in solution B may serve as the seeds. Their multifaceted structures facilitated the formation of multipods rather than long nanorods. It is interesting to study the structural stability of these branched nanocrystals and see whether structural evolution might occur. A solution of highly branched gold nanocrystals using 0.10 M ascorbic acid (Figure 6a) was divided into two sets and stored at either 30 or 4 °C for 10 days to follow the structural changes as a function of time and temperature. Due to a greater extent of branch formation under this preparation condition, a single absorption peak at 612 nm was measured for the freshly prepared solution C. There was a slow, but progressive, blue-shift in the absorption band from 612 to 574 nm after 240 h of storage at 30 °C (see Supporting Information, Figure 2). On the other hand, the spectrum was basically unchanged after 240 h of storage at 4 °C. Direct evidence of the morphological changes occurring in the two samples was examined by TEM images using a JEOL CX-200 electron microscope operating at 200 kV. Figures 6b and 6c show that there is a clear difference in the morphology of the two samples. After particle storage at 30 °C for 10 days, the long and slender multipods had mostly disappeared, and many nanoparticles had evolved into highly faceted pentagonal or other more structurally well-defined shapes. The structural evolution may continue by raising the temperature slightly. In contrast, the branched nanocrystals still maintained their multipod structure under a storage temperature of 4 °C for 10 days. Clearly, at a temperature of 30 °C, there is enough thermal energy to reorganize the gold atoms to form new particle structures. A highly faceted multiply twinned particle structure should be thermodynamically more stable under most conditions and is more frequently observed.22 Hence, the branched nanocrystals may be a more kinetically (22) (a) Yacaman, M. J.; Ascencio, J. A.; Liu, H. B.; Gardea-Torresdey, J. J. Vac. Sci. Technol. B 2001, 19, 1091. (b) Marks, L. D. Rep. Prog. Phys. 1994, 57, 603.

Kuo and Huang

driven product; however, over time these particles would evolve into a more thermodynamically stable structure, given enough thermal energy is supplied for the atoms to reorganize. However, if these branched nanocrystals were kept at a relatively low temperature (that is, in a refrigerator), structural integrity can be well preserved. Conclusions In summary, a simple seeding growth method for the synthesis of branched gold nanocrystals has been demonstrated. Highly faceted nanocrystals (∼15-20 nm in diameter) were synthesized, using SDS as a capping agent. As more gold ions were added to the multitwinned particle surfaces consisting of predominantly (111) lattice planes during particle growth, branched nanocrystals were produced. Bipod, tripod, tetrapod, and pentapod nanocrystals have been observed. The concentration of ascorbate ions in the solution was found to play a key role in promoting the multipod formation. The branched nanocrystals were stable to long-time storage at 4 °C, but they can slowly evolve into more thermodynamically stable highly faceted structures above room temperature. Branched gold nanocrystals with longer pods can be prepared by continuing the seeding growth process, and we are continuing our effort toward this direction. If the branched nanocrystals with sufficiently long arms can be controllably grown, their potential use as interconnects in the fabrication of nanoscale electronic devices may be considered. Acknowledgment. We thank the National Science Council of Taiwan for the support of this work (NSC922113-M-007-042) and the startup fund provided by the Department of Chemistry, National Tsing Hua University. Supporting Information Available: TEM images of nanoparticles prepared using sodium ascorbate as the reducing agent and time-dependent UV-vis absorption spectra of branched gold nanocrystal solutions stored at 4 and 30 °C. This material is available free of charge via the Internet at LA0476332