Seedless, Surfactantless, High-Yield Synthesis of Branched Gold

May 8, 2007 - In this work, three-dimensional branched gold nanocrystals were produced at high yield by reacting an aqueous solution of chloroauric ac...
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Chem. Mater. 2007, 19, 2823-2830

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Seedless, Surfactantless, High-Yield Synthesis of Branched Gold Nanocrystals in HEPES Buffer Solution Jianping Xie,† Jim Yang Lee,*,†,§ and Daniel I. C. Wang†,‡ Singapore-MIT Alliance, National UniVersity of Singapore, 4 Engineering DriVe 3, Singapore 117576, Department of Chemical Engineering, M.I.T., 77 Massachusetts AVenue, Cambridge, Massachusetts 02139, and Department of Chemical and Biomolecular Engineering, National UniVersity of Singapore, 10 Kent Ridge Crescent, Singapore 119260 ReceiVed January 2, 2007. ReVised Manuscript ReceiVed March 22, 2007

In this work, three-dimensional branched gold nanocrystals were produced at high yield by reacting an aqueous solution of chloroauric acid with a Good’s buffer, HEPES, at room temperature. This particular method of preparation was scalable to gram-quantity. The branched nanocrystals containing one to eight tips were stable at room temperature and could be stored as a powder after freeze-drying. They were, however, unstable at higher temperatures and transformed into spherical particles upon boiling. The formation of the branched gold nanocrystals was kinetically controlled, as shown by the dependence of shapes on temperature and precursor salt concentration. The growth of branched gold nanocrystals in the HEPES buffer was monitored by microscopic and spectroscopic techniques, allowing the detection of several key intermediates in the growth process. Piperazine in HEPES molecule was identified as the principal moiety responsible for forming highly branched Au nanocrystals in the HEPES buffer.

Introduction Interest in nanomaterials is firmly founded upon their size and shape tunable properties which allow very diverse applications to be designed based on the same material.1 This has led to the discovery of various empirical methods for the control of nanoparticle morphology. Insofar as gold is concerned, although there is a large volume of work on the synthesis of nanospheres,2 nanocubes,3 nanorods,4 and nanoprisms,5 there are relatively fewer attempts to produce nanocrystals with complex or odd-shaped geometries in an isotropic liquid. The development of nanometer-scale branched nanocrystals of metals,6,7 metal oxides,8 and semiconductors9 is important for using these materials as the building blocks for the fabrication of complex, multiple-terminal devices by self-assembly10 and as the active components in new photovoltaic devices.11 Theoretical calculations by discrete dipole * To whom correspondence should be addressed. E-mail: [email protected]. † Singapore-MIT Alliance, National University of Singapore. ‡ M.I.T. § Department of Chemical and Biomolecular Engineering, National University of Singapore.

(1) (a) Burda, C.; Chen, X. B.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (b) Liz-Marzan, L. M. Langmuir 2006, 22, 32. (c) Yu, D. B.; Yam, V. W.-W. J. Am. Chem. Soc. 2004, 126, 13200. (2) Daniel, M. C.; Astruc, D. Chem. ReV. 2004, 104, 293. (3) Sun, Y. G.; Xia, Y. N. Science 2002, 298, 2176. (4) (a) Sau, T. K.; Murphy, C. J. Langmuir 2004, 20, 6414. (b) Kim, F.; Song, J. H.; Yang, P. D. J. Am. Chem. Soc. 2002, 124, 14316. (5) (a) Millstone, J. E.; Park, S. H.; Shuford, K. L.; Qin, L. D.; Schatz, G. C.; Mirkin, C. A. J. Am. Chem. Soc. 2005, 127, 5312. (b) Liu, B.; Xie, J.; Lee, J. Y.; Ting, Y. P.; Chen, J. P. J. Phys. Chem. B 2005, 109, 15256. (c) Li, C. C.; Cai, W. P.; Cao, B. Q.; Sun, F. Q.; Li, Y.; Kan, C. X.; Zhang, L. D. AdV. Funct. Mater. 2006, 16, 83. (6) (a) Teng, X. W.; Yang, H. Nano Lett. 2005, 5, 885. (b) Chen, J. Y.; Herricks, T.; Xia, Y. N. Angew. Chem., Int. Ed. 2005, 44, 2589. (c) Zettsu, N.; McLellan, J. M.; Wiley, B.; Yin, Y. D.; Li, Z. Y.; Xia, Y. N. Angew. Chem., Int. Ed. 2006, 45, 1288. (d) Hoefelmeyer, J. D.; Niesz, K.; Somorjai, G. A.; Tilley, T. D. Nano Lett. 2005, 5, 435.

approximation (DDA)7d and finite difference time domain (FDTD)7f have indicated strong enhancement of the electromagnetic field at the tips of the particles, suggesting that the branched particles could be of value to surface-enhanced Raman scattering (SERS) spectroscopy. Most of the reports on multibranched structures pertain to semiconductors of the like of CdSe, CdS, CdTe, MnS, and ZnO.8,9 In these cases, the coexistence of two lattice structures (e.g., zinc blende and wurtzite) within the same nanocrystal is required. The energy difference between these structures at certain temperatures will favor one structure during nucleation and the other structure during growth. As most metals only exist in one crystallographic form (typically in a highly symmetric face-centered cubic, or fcc, structure), it is more challenging to grow branched nanocrystals of metals by a homogeneous nucleation and growth process. Only platinum,6a,b rhodium,6c,d and gold7 have thus far been (7) (a) Chen, S. H.; Wang, Z. L.; Ballato, J.; Foulger, S. H.; Carroll, D. L. J. Am. Chem. Soc. 2003, 125, 16186. (b) Kuo, C. H.; Huang, M. H. Langmuir 2005, 21, 2012. (c) Wu, H. Y.; Liu, M.; Huang, M. H. J. Phys. Chem. B 2006, 110, 19291. (d) Hao, E. C.; Bailey, R. C.; Schatz, G. C.; Hupp, J. T.; Li, S. Y. Nano Lett. 2004, 4, 327. (e) Yamamoto, M.; Kashiwagi, Y.; Sakata, T.; Mori, H.; Nakamoto, M. Chem. Mater. 2005, 17, 5391. (f) Bakr, O. M.; Wunsch, B. H.; Stellacci, F. Chem. Mater. 2006, 18, 3297. (g) Nehl, C. L.; Liao, H. W.; Hafner, J. H. Nano Lett. 2006, 6, 683. (8) (a) Wang, Z. L. Annu. ReV. Phys. Chem. 2004, 55, 159. (b) Yan, H. Q.; He, R. R.; Johnson, J.; Law, M.; Saykally, R. J.; Yang, P. D. J. Am. Chem. Soc. 2003, 125, 4728. (9) (a) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2002, 124, 3343. (b) Manna, L.; Milliron, D. J.; Meisel, A.; Scher, E. C.; Alivisatos, A. P. Nat. Mater. 2003, 2, 382. (c) Lee, S. M.; Cho, S. N.; Cheon, J. W. AdV. Mater. 2003, 15, 441. (10) (a) Wang, D. L.; Lieber, C. M. Nat. Mater. 2003, 2, 355. (b) Stellacci, F. Nat. Mater. 2005, 4, 113. (11) (a) Huang, Y.; Lieber, C. M. Pure Appl. Chem. 2004, 76, 2051. (b) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425.

10.1021/cm0700100 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/08/2007

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demonstrated to form branched nanocrystals. Chen et al.7a reported a seeded process to synthesize gold multipods (1-4 branches) by using preformed silver plates as seeds in an excess of cetyltrimethylammonium bromide (CTAB) capping agent. The formation process was slow, requiring a considerably long period of time (1-20 days) for completion. Kuo and Huang7b reported a similar multistep process by using small gold seeds and sodium dodecyl sulfate (SDS) capping agent. Gold nanocrystals with random branches and poorly defined structures were obtained. Branched gold nanoparticles could also be formed by a seedless route. For example, Wu et al.7c prepared branched gold nanocrystals by a CTAB process7a without silver seeds and studied the structural stability of the branched nanocrystals. Hao et al.7d reported the synthesis of gold nanocrystals with one to three branches using bis(p-sulfonatophenyl)phenylphosphine dipotassium salt (BSPP) as the stabilizer and no nanoparticle seeds. The product was asymmetric gold nanoparticles, which underwent structural transformation after storage in a refrigerator for a few days to produce a blue shift of light absorption in the UV-vis region. Another seedless synthesis was recently reported by Yamamoto et al.,7e who obtained star-shaped gold nanoparticles in the presence of poly(vinylpyrrolidone) (PVP). The yield of the star-shaped nanoplates was low (7%) as spherical particles were the main product (89%). All of these published efforts involved invariably a complex multistep seeded process or the use of soft directing agents, for example, surfactants and polymers. Directing agents such as CTAB and PVP are difficult to remove from the nanoparticle surface, requiring harsh conditions or multiple washings. Microstructural characterizations have shown that well-defined shapes such as tripods, tetrapods,7a and star shapes7e have a two-dimensional planar structure. The synthesis of three-dimensional multibranched gold nanoparticles is a more recent development. For example, urchinlike gold nanoparticles were produced by Bakr et al.7f using a two-step process including first the reduction of gold ions by H2O2 and subsequently the addition of a thiolated capping agent. The particles had random branches but their crystal structure was not reported. Less random three-dimensional gold nanoparticles with star shapes were recently synthesized by Nehl et al.7g using growth conditions that would normally produce gold nanorods. In their preparation the authors replaced the often-used surfactant-stabilized gold seeds by a commercially available colloid (10 nm diameter gold colloid). The optical properties of ca. 100 nm star-shaped gold nanocrystals were then studied by single-particle spectroscopy. The synthesis of well-defined three-dimensional branched gold nanocrystals and the understanding of the underlying growth mechanism are still lacking in the literature. Herein we report a simple, one-pot synthesis of threedimensional branched gold nanocrystals using a relatively “green” chemical, 2-[4-(2-hydroxyethyl)-1-piperazinyl]ethanesulfonic acid (HEPES), where HEPES is serving as both reducing and shape-directing agents. HEPES is a popular pH buffer used extensively in chemistry and biochemistry laboratories and in tissue culturing.12 This is because it has a pKa close to the physiological pH (7.5), maximum aqueous

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solubility and minimum solubility in other solvents, low permeability to cell membranes, minimal salt and temperature effects, high chemical stability, and supposedly low affinities for metal ions.12 Recently, HEPES was used to buffer peptide solutions for the preparation of gold nanoparticles.13 The authors assumed that peptides were responsible for the reduction of gold ions (AuCl4-) and neglected to consider the possible influence of HEPES. However, a recent study has shown that the piperazine ring in HEPES is able to generate nitrogen-centered free radicals, which can reduce gold ions (AuCl4-) even in low concentration (∼6 ppm) of the latter.14 In this study, we found that HEPES is more than a capable reducing agent. It can also guide the subsequent growth of gold nanoparticles in aqueous solution to stable branched structures at very high yield (ca. 92%). Transmission electron microscopy (TEM) indicated that the branched gold nanocrystals have complex three-dimensional structures. A kinetically controlled process for the growth of branched gold nanocrystals is proposed and the effects of environmental factors such as precursor concentration and temperature were studied. The growth process of branched gold nanocrystals in the HEPES buffer solution was reconstructed by following the changes in the UV-vis spectra of the reaction mixture, and the size and shape of the product particles, with time. Furthermore, comparative experiments have also been conducted by using other Good’s buffers to identify the functional group(s) in HEPES active for promoting the anisotropic growth of gold nanocrystals. Presented below are the details of this investigation. Experimental Section All chemicals were purchased from Sigma Aldrich and used asreceived. Ultrapure Millipore water (18.2 MΩ) was used as the solvent. Examinations of nanoparticle morphology and size by field emission scanning electron microscopy (FESEM) and by transmission electron microscopy (TEM) made use of a JEOL JSM-6700F microscope at 25 kV with a transmission electron detector (TED) and a JEOL JEM-2010 microscope at 200 kV (or a JEOL JEM 2010FE at 200 kV for high-resolution images), respectively. UVvis spectroscopy and powder X-ray diffraction (XRD) were carried out on a Shimadzu UV-2450 spectrometer operating at 1 nm resolution and on a Bruker D8 Discover with GADDS and Cu KR radiation at λ ) 0.15418 nm, respectively. The percentage conversion of HAuCl4 to Au was determined by atomic emission spectroscopy (AES). The AES measurements were performed on a Perkin-Elmer Optima 3000DV atomic emission spectrometer with inductively coupled plasma (ICP). The emission line at 242.795 nm was used to measure the concentration of elemental Au. Nanoparticle Synthesis. All glassware was washed with Aqua Regia (HCl:HNO3 in a 3:1 ratio by volume) and rinsed with ethanol and ultrapure water. (Caution! Aqua Regia is a Very corrosiVe oxidizing agent which should be handled with great care.) Aqueous stock solution of HEPES with concentration of 100 mM was prepared with ultrapure water, and pH was adjusted to 7.4 ( 0.1 (12) (a) Good, N. E.; Winget, G. D.; Winter, W.; Connolly, T. N.; Izana, K.; Singh, R. M. M. Biochemistry 1966, 5, 467. (b) Good, N. E.; Izawa, K. Methods Enzymol. 1972, 24, 43. (13) (a) Slocik, J. M.; Stone, M. O.; Naik, R. R. Small 2005, 1, 1048. (b) Slocik, J. M.; Govorov, A. O.; Naik, R. R. Supramol. Chem. 2006, 18, 415. (14) Habib, A.; Tabata, M.; Wu, Y. G. Bull. Chem. Soc. Jpn. 2005, 78, 262.

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Figure 1. (A) FESEM image of branched gold nanocrystals. (B-I) Representative TEM images of branched gold nanocrystals with different number of tips: (B) one, (C) two, (D) three, (E) four, (F) five, (G) six, (H) seven, and (I) eight branches. All scale bars are 10 nm. (J) Yield of branched gold nanocrystals as a function of the number of tips. (K) UV-vis spectrum of branched gold nanocrystals. The inset shows a digital picture of the aqueous solution of branched gold nanocrystals.

at 25 °C by adding 1 M NaOH solution. In a typical experiment, 2 mL of 100 mM HEPES (pH 7.4) was mixed with 3 mL of deionized water, followed by the addition of 50 µL of 20 mM HAuCl4 solution. Without shaking, the color of solution changed from light yellow to slightly purple and finally to greenish blue within 30 min. This process could be scaled up to 500 mL volume without changes in the optical properties, morphology, and size of the products. The syntheses of gold nanoparticles in other Good’s buffers (4-(2-hydroxyethyl) piperazine-1-(2-hydroxypropane-sulfonic acid) (HEPPSO, pKa ) 7.50); 1,4-piperazine-diethanesulfonic acid (PIPES, pKa ) 6.80); 4-morpholinepropanesulfonic acid (MOPS, pKa ) 7.2); 3-morpholino-2-hydroxypropanesulfonic acid (MOPSO, pKa ) 6.90)) were carried out under similar conditions except for the replacement of HEPES by the other Good’s buffer. All products were collected after 1 h of reaction.

Results and Discussion Figure 1A shows a typical FESEM image of gold nanocrystals recovered after 1 h of reaction. The product was a mixture of variously shaped particles including spheres (∼8%) and branched particles (∼92%) with one to eight tips. The yields of the branched gold nanocrystals were calculated by analyzing hundreds of particles in more than one FESEM images. For each particle, the number of tips was counted so that zero tip indicates a spherical nanoparticle and one, two, three, four, five, six, seven, and eight tips correspond to increasingly complex branched particles. It should be pointed out that the analysis might undercount the number

of tips on the branched gold nanocrystals because not all of the tips were visible in the two-dimensional projection of three-dimensional structures. With this in mind a histogram showing the yield analysis of a typical preparation is given in Figure 1J, with Figures 1B-1I showing the typical TEM images of gold nanocrystals with different numbers of tips. The length of the tips ranged from 15 to 25 nm, with width of ca. 8 nm (the length of the tip was defined as the distance between the tip and the center of the particle and the width was measured at the midpoint of the tip length). In contrast to the characteristic red color of spherical gold nanoparticles, the colloidal solution of branched gold nanocrystals was greenish blue, as shown in the inset of Figure 1K. The absorption spectrum in Figure 1K shows two surface plasmon resonance (SPR) peaks (518 and 658 nm). It has been known both theoretically15 and experimentally16 that elongated gold nanoparticles feature a longitudinal plasmon resonance that red-shifts from ca. 520 nm (the value for spherical gold nanoparticles) by an amount proportional to their aspect ratio. The peak at 658 nm in Figure 1K is therefore a longitudinal plasmon resonance due to the elongated tips of the nanocrystals. The peak at ca. 518 nm could be the transverse plasmon resonance of the tips, which was also found in previous work on gold nanorods.15,16 The (15) Link, S.; EI-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410. (16) Nikoobakht, B.; EI-Sayed, M. A. Chem. Mater. 2003, 15, 1957.

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Figure 2. TEM images of a gold nanocrystal with five branches tilted at different angles to the axis indicated by the arrow: (A) -30°, (B) -15°, (C) 0°, (D) +15°, and (E) +30°. (F) HRTEM image of a tip of the gold nanocrystal showing single crystallinity and growth in the 〈111〉 direction.

peak at 658 nm is strong and relatively narrow. The relatively narrow full-width at half-maximum (fwhm ) 98 nm with a standard deviation of 3 nm) of the peak may seem counter to intuition, given the marked structural heterogeneity of the nanocrystals in our samples (Figure 1A). This can however be understood from the simulation study on metal tripods. According to the calculations by Hao et al.,7d the optical properties of branched particles are dominated by dipole excitation from the particle tips, which is sensitive to the length and sharpness of the tip. Therefore, heterogeneity in the number of the branches should only have a minor effect on the spectrum. Consequently, the narrow absorption peak of the longitudinal plasmon resonance could be attributed to the similar sharpness and the relatively narrow distribution of the aspect ratios of the tips (ca. 2-3) of variously branched gold nanocrystals. The location of ca. 658 nm also agrees well with theoretical predictions15 and experimental work on gold nanorods,16 which showed nanorods with aspect ratios from 2 to 3 giving rise to longitudinal plasmon absorption centering at ca. 600-700 nm. The branched gold nanocrystals obtained in this study have distinctively smaller sizes and more complex three-dimensional structures than those previously reported.7 The dark spots in the TEM images of branched gold nanocrystals (Figures 1E, 1G, 1H, and 1I) are tips radiating either inward or outward from the planes of the figures. The threedimensionality of the branched gold nanocrystals was confirmed by tilting a nanocrystal sample at different angles relative to the electron beam, as shown in Figures 2A-2E. The arrow in Figure 2A shows the axis about which the TEM grid was rotated from -30° to +30°. From these images we concluded that this particular gold nanocrystal has a threedimensional structure with five tips of about the same length. Figure 2F is the high-resolution TEM image of the tip showing both single crystallinity and the 〈111〉 growth direction. The lattice spacing between the (111) planes, 0.236 nm, is the same as that of bulk Au. This finding is consistent with previous observations of Pt nanorods,17 Pt multipods,6b and Rh multipods6c,d elongated in the 〈111〉 direction. It has

Figure 3. HRTEM images of two gold nanocrystals with different branched structures: (A) five branches; (B) seven branches. The HRTEM image of the former shows growth of all tips in the 〈111〉 direction, indicating that the particle is single crystalline throughout. The nanocrystal with seven branches has a twin plane in the middle.

been reported that the formation of metal multipods and smooth planar shapes could be associated with the competitive growth between (111) and (100) planes.7a,18 Interestingly, gold nanocrystals with other numbers of branches (data not shown) also showed tip growth along the 〈111〉 directions. The oriented growth suggests very weak or no adsorption of HEPES molecules on the {111} family of planes. At the current stage of method development, the final product is usually a mixture of branched gold nanocrystals containing one to eight tips. Some of them are single crystals while some are twinned crystals, as shown by the HRTEM images in Figure 3. Figure 3A shows the HRTEM image of a gold nanocrystal with five branches, which is single crystalline. In contrast, in the HRTEM image of the gold nanocrystal with seven branches in Figure 3B, a twin plane that bisects the particle into two halves is clearly visible around the middle. The progress of reaction and product evolution was followed by time-course measurements of UV-vis spectra (Figure 4A) and TEM images (Figure 4B-4D). The solution was light yellow at the start and faded slowly to colorless in 5 min before the color returned as light pink and deepened (17) Chen, J. Y.; Herricks, T.; Geissler, M.; Xia, Y. N. J. Am. Chem. Soc. 2004, 126, 10854. (18) Petroski, J. M.; Wang, Z. L.; Green, T. C.; EI-Sayed, M. A. J. Phys. Chem. B 1998, 102, 3316.

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Figure 4. (A) UV-vis spectra recorded as a function of time of reaction between HEPES and chloroaurate ions in aqueous solutions. Curves 1-6 correspond to spectra recorded at 0, 5, 10, 15, 20, and 60 min into the reaction. (B-D) Representative TEM images of the products recovered after (B) 10, (C) 15, and (D) 20 min of reaction. The insets in (C) and (D) show two typical branched gold nanocrystals with seven and three tips, respectively. All scale bars are 10 nm.

Figure 5. (A and B) FESEM images of gold nanoparticles: (A) after freeze-drying and re-dispersion in water; (B) harvested immediately after synthesis (1 h reaction) and boiled for 5 min. (C) UV-vis spectra of gold nanoparticles in water: spectrum 1 was taken right after synthesis (1 h reaction); after the measurement, the sample was kept at room temperature for a week (spectrum 2); after the measurement, the sample was freeze-dried and stored at room temperature for a month before it was redispersed in water for optical measurements (spectrum 3); after the measurement, the sample was brought to boiling for 5 min (spectrum 4). (D) XRD patterns and intensity ratios: spectrum 1, gold nanoparticles were collected right after synthesis; spectrum 4, gold nanoparticles obtained after 5 min of boiling the solution collected right after synthesis.

gradually to greenish blue finally. The light pink solution after 10 min of reaction had a weak absorption peak at ca. 548 nm (spectrum 3, Figure 4A) attributable to the gold nanoparticles. The TEM image of the solid product recovered at that time showed only tiny gold nanoparticles with size less than 10 nm (Figure 4B). These nanocrystals grew into multipods (Figure 4C) within the next 5 min, concomitant with the gradual change of the solution color from light pink to blue, and the appearance of a new SPR peak at ca. 636 nm in the UV-vis spectrum (spectrum 4, Figure 4A). The

branched gold nanocrystals grew in size for the next 5 min, which was also indicated by the red shifting of SPR to the near-infrared (NIR) region (from ca. 636 to 658 nm, spectra 4 and 5 of Figure 4A). Two typical branched geometries of gold nanocrystals were selected to track the evolution, one was the seven-branched star and the other was the tripod (see inset of Figures 4C and 4D). The growth of the branched nanoparticles was estimated by the increase in the average tip length. For the star-shaped nanocrystals, the average tip length of ∼11 nm at 15 min stabilized to ∼23 nm at 20

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Figure 6. TEM images showing the effects of temperature and concentration of the precursor salt on the formation of branched gold nanocrystals: (A) 0.2 mM HAuCl4 at 100 °C; (B) 0.1 mM HAuCl4 at room temperature; (C) 0.5 mM HAuCl4 at room temperature. (D) UV-vis spectra of gold nanoparticles synthesized under different conditions: spectrum 1, 0.2 mM HAuCl4 and room temperature; spectrum 2, 0.2 mM HAuCl4 and 100 °C; spectrum 3, 0.1 mM HAuCl4 and room temperature; spectrum 4, 0.5 mM HAuCl4 and room temperature. All samples were collected at t ) 1 h into the reaction.

min. The corresponding change for the tripod was from 10 nm at 15 min to 24 nm at 20 min. The absorption at 658 nm measured as a function of time (see inset of Figure 4A) showed an evolutionary pattern: an induction period (∼10 min) for nucleation, a rapid growth of the branched gold nanocrystals thereafter, and the termination of growth at ca. 20 min (the complete reduction of Au(III) ions was confirmed by AES analysis). The as-synthesized nanocrystals (recovered after 1 h of reaction) were stable in solution and could be stored at room temperature or at 4 °C for at least a week without changes. Alternatively, the solvent could be removed by freeze-drying and the nanoparticles stored in the solid form for a month and more, and redispersed whenever needed. There was no significant difference in optical properties between the nanoparticles recovered after 1 h of reaction (spectrum 1, Figure 5C) and those after 1 week of storage at room temperature (spectrum 2, Figure 5C). However, the spectrum of the redispersed freeze-dried particles (spectrum 3, Figure 5C) showed noticeable broadening in the NIR region typical of particle aggregation. It should be stated that the drying and redispersion process could be repeated several times without changes to the spectral features (data not shown). The TEM image of redispersed gold nanocrystals (Figure 5A) show that the particles had maintained their branched morphology, and aside from some interparticle agglomeration, there were no significant changes in their shape or size distribution (compared to Figure 1A of gold nanocrystals collected after 1 h of reaction). While the branched structures were stable at room temperature and robust to the freeze-drying process, they underwent changes at higher temperatures. When the solution of the gold nanocrystals collected after 1 h of reaction was brought to boiling, the color of the solution changed from

greenish blue to red within minutes. TEM showed the particles in the solution after 5 min of boiling to be completely gold nanospheres (Figure 5B). The size of the nanocrystals also decreased in the process, from ca. 30-50 nm for the branched gold nanocrystals to ca. 15 nm spherical gold nanoparticles. The transformation of shapes from branched structures to spheres was also witnessed by UVvis spectroscopy, which showed only one peak at ca. 526 nm for the boiled solution (spectrum 4, Figure 5C). The transformation of shapes at elevated temperatures could be explained by Ostwald ripening. Similar to other fcc metals, the surface energy of gold decreases in the order of (110), (100), and (111).19 In an effort to lower the total surface energy, the tip was dissolved to reduce the (110) and (100) exposure and the resultant Au atoms were re-deposited on the central part of the nanocrystal to increase the percentage of (111) facets on the surface. The powder X-ray diffraction (XRD) patterns of branched gold nanocrystals immediately after synthesis (spectrum 1, Figure 5D) and gold nanospheres obtained after 5 min of boiling (spectrum 4, Figure 5D) showed five reflections in the 2θ range of 30-85°, indexable to the (111), (200), (220), (311), and (222) reflections of fcc Au (JCPDS, file no. 4-0784).20 There was a noticeable increase in the I(111)/I(110) intensity ratio from 5.08 to 7.46 due to boiling (see table in the inset of Figure 5D). Since truncated octahedron is the most thermodynamically stable shape for a fcc metal particle, the branched structures had to be a kinetically controlled product. The branched structures were stable at low temperatures (e.g., at room (19) Jiang, Q.; Lu, H. M.; Zhao, M. J. Phys.: Condens. Matter 2004, 16, 521. (20) Bayliss, P. Mineral Powder Diffraction File Date Book; JCPDS: Swarthmore, PA, 1986.

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Figure 7. FESEM images of gold nanoparticles synthesized under conditions similar to those used for Figure 1A, except for the replacement of HEPES by other Good’s buffers: (A) HEPPSO; (B) PIPES; (C) MOPS; and (D) MOPSO. All samples were collected after 1 h of reaction. (E) UV-vis spectra of gold nanoparticles synthesized by using different Good’s buffers. Curves 1-5 correspond to the Good’s buffer of HEPES, HEPPSO, PIPES, MOPS, and MOPSO, respectively. (F) Molecular structures of the five Good’s buffers.

temperature or 4 °C), but would transform into the more thermodynamically stable form (spheres) if sufficient thermal energy was provided for atomic reorganization. The formation of branched gold nanocrystals through the kinetically controlled process was also evidenced by systematically varying environmental factors which have strong influence on the reaction kinetics. When the nanocrystal preparation was carried out at 100 °C, the results (Figure 6A and spectrum 2 in Figure 6D) were the same as boiling the branched nanocrystals produced at room temperature: formation of only gold nanospheres. When the reaction conditions were fixed at room temperature and 1 h of reaction, a 2-fold decrease in the HAuCl4 concentration (to 0.1 mM) resulted in branched gold nanocrystals with higher aspect ratios for the tips, as shown in the TEM image of Figure 6B, and the red shifting and broadening of the NIR peak (from 658 to 688 nm, spectrum 3, Figure 6D). Conversely, an increase in the HAuCl4 concentration by 2.5 times (to 0.5 mM, Figure 6C and spectrum 4 in Figure 6D) led to the proliferation of irregularly shaped and spherical gold nanoparticles in the products. The mechanism for the formation of the branched gold structures is apparently different from the model developed by Peng21 for CdSe, where highly anisotropic nanostructures (branched and one(21) Peng, X. G. AdV. Mater. 2003, 15, 459.

Table 1. Summary of Product Morphology from Different Good’s Buffers functional groupsa Good’s buffer HEPES HEPPSO PIPES MOPS MOPSO

ability to direct the anisotropic hydroxyl sulfonate piperazine morpholine growthb O OO O

O O OO O O

O O O O O

+ + + -

a O indicates number of functional groups in the molecule. b The ability to direct the anisotropic growth of gold nanocrystals is based on analyses of FESEM and UV-vis spectra of the as-synthesized gold nanoparticles.

dimensional) were obtained at high monomer concentrations, and spherical CdSe nanoparticles were formed at low monomer concentrations. In our case, highly anisotropic gold nanostructures were formed under condition of slow reduction where the supersaturation of Au0 was low. Only isotropic, spherical gold nanoparticles were formed when the reduction proceeded quickly and the supersaturation was high. The HRTEM results (Figure 2F), which showed that branched gold nanocrystals were formed by selective tip growth in the 〈111〉 directions, suggest relatively weak or no adsorption of HEPES on the {111} planes compared with

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the {100} planes. It is not clear at this moment why HEPES molecules affined strongly to the {100} planes. There are three functional groups in the HEPES molecule, namely, hydroxyl, sulfonate, and piperazine. A comparative study was therefore carried out to determine the relative contributions of these groups in the anisotropic growth of gold nanocrystals. Four other Good’s buffers (HEPPSO, PIPES, MOPS, and MOPSO, see Figure 7F for molecule structures) were used to synthesize gold nanoparticles under conditions similar to the synthesis by HEPES. The resulting gold nanoparticle products were characterized by FESEM (Figures 7A-7D) and by UV-vis spectroscopy (Figure 7E), with Table 1 providing a quick summary of the experimental observations. Branched gold nanocrystals were obtained in HEPPSO (Figure 7A) and PIPES (Figure 7B) buffers, whereas only gold nanospheres were formed in MOPS (Figure 7C) and MOPSO (Figure 7D) buffers. The different outcomes also resulted in two SPR peaks for the nanocrystals formed in HEPPSO (ca. 520 and 694 nm, spectrum 2, Figure 7E) and PIPES (ca. 521 and 600 nm, spectrum 3, Figure 7E) and only one SPR peak for the nanocrystals synthesized in MOPS (ca. 557 nm, spectrum 4, Figure 7E) and MOPSO (ca. 537 nm, spectrum 5, Figure 7E). The analysis in Table 1 shows that piperazine should be the most accountable for the formation of branched gold nanocrystals, without which spherical gold nanoparticles were formed exclusively. It is reasonable to infer that the presence of piperazine in HEPES, HEPPSO, and PIPES is responsible for the adsorption of these molecules on different gold facets, with the binding on the {111} planes being the weakest. A computational study on the adsorption of piperazine on various surfaces of a gold nanoparticle facetted by {111} and {100} planes is currently underway, which may help to provide further information.

Xie et al.

Conclusions A simple room-temperature one-pot synthesis of threedimensional gold nanocrystals has been developed. The highyield synthesis, which was based on the reduction and shapedirecting ability of a Good’s buffer, HEPES, can be easily scaled up to gram-quantity. Small branched gold nanoparticles with one to eight tips, tip length of ca. 15-25 nm and tip width of ca. 8 nm, were produced as the major products (ca. 92%). Strong absorption due to transverse and longitudinal plasmon resonances of the tips was detected in the UV-vis region. It was found that the tips grew selectively in the 〈111〉 directions. These highly branched nanocrystals were stable at room temperature and could be stored as a powder after freeze-drying. They were, however, unstable at elevated temperatures where simple boiling would convert them into spherical particles. The formation of the branched gold nanocrystals was kinetically controlled, as shown by the dependence of shapes on parameters such as temperature and precursor salt concentration. The growth of branched gold nanocrystals in HEPES buffer was then reconstructed by monitoring the evolution of SPR signals with UV-vis spectroscopy and morphological changes with transmission electron microscopy. A long induction period for nucleation and a rapid growth of branched gold nanocrystals before termination were detected. A comparative study using other Good’s buffers implicated piperazine as the functional group most responsible for forming the highly branched Au nanocrystals in HEPES, HEPPSO, and PIPES. Acknowledgment. One of the authors (J. P. Xie) is financially supported under the Singapore/MIT Alliance (SMA) program. CM0700100