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Synthesis of Highly Faceted Pentagonal- and Hexagonal-Shaped Gold Nanoparticles with Controlled Sizes by Sodium Dodecyl Sulfate Chun-Hong Kuo,† Tian-Fu Chiang,‡ Lih-Juann Chen,‡ and Michael H. Huang*,† Departments of Chemistry and Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan Received March 31, 2004. In Final Form: June 10, 2004 We report the synthesis of pentagonal- and hexagonal-shaped gold nanoparticles with controlled diameters ranging from 5 to 50 nm. These nanoparticles were prepared by a seeding growth approach. Sodium dodecyl sulfate (SDS) molecules served as the capping agent to restrict the particle size. In addition, the formation of highly faceted gold nanoparticles may be facilitated by the possibly ineffective capping interactions between the lamellar micellar structures formed by the SDS molecules and the gold nanoparticles. The crystal structure of the highly faceted particles was found to consist of mostly {111} surfaces as particle size increases, as revealed by both TEM and XRD results.
Introduction Gold nanoparticles have been widely used in nanoscience research to bind or adsorb a variety of functional molecules.1-15 These surface-capped gold nanoparticles have many useful applications. Binding of biomolecules such as DNA and carbohydrates, for example, has been extensively applied in biological assays and detections.4-7 In addition to the studies of surface functionalization of gold nanoparticles, there is also an interest in synthesizing shape-specific gold nanoparticles. Gold nanoparticles with interesting shapes including rods,16-20 triangles,21,22 cubes,23 * To whom correspondence should be addressed. E-mail:
[email protected]. † Department of Chemistry. ‡ Department of Materials Science and Engineering. (1) Levi, S. A.; Mourran, A.; Spatz, J. P.; van Veggel, F. C. J. M.; Reinhoudt, D. N.; Mo¨ller, M. Chem.sEur. J. 2002, 8, 3808. (2) Thomas, K. G.; Kamat, P. V. J. Am. Chem. Soc. 2000, 122, 2655. (3) Xu, P.; Yanagi, H. Chem. Mater. 1999, 11, 2626. (4) Taton, T. A.; Mirkin, C. A.; Lestinger, R. L. Science 2000, 289, 1757. (5) Mucic, R. C.; Storhoff, J. J.; Mirkin, C. A.; Lestinger, R. L. J. Am. Chem. Soc. 1998, 120, 12674. (6) Loweth, C. J.; Caldwell, W. B.; Peng, X.; Alivisatos, A. P.; Schultz, P. G. Angew. Chem., Int. Ed. 1999, 38, 1808. (7) 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. (8) (a) Kanehara, M.; Oumi, Y.; Sano, T.; Teranishi, T. J. Am. Chem. Soc. 2003, 125, 8708. (b) Teranishi, T.; Hasegawa, S.; Shimizu, T.; Miyake, M. Adv. Mater. 2001, 13, 1699. (c) Teranishi, T.; Sugawara, A.; Shimizu, T.; Miyake, M. J. Am. Chem. Soc. 2002, 124, 4210. (d) Wang, Z. L. Adv. Mater. 1998, 10, 13. (9) (a) He, H. X.; Zhang, H.; Li, Q. G.; Zhu, T.; Li, S. F. Y.; Liu, Z. F. Langmuir 2000, 16, 3846. (b) Zheng, J.; Zhu, Z.; Chen, H.; Liu, Z. Langmuir 2000, 16, 4409. (10) Frankamp, B. L.; Uzun, O.; Ilhan, F.; Boal, A. K.; Rotello, V. M. J. Am. Chem. Soc. 2002, 124, 892. (11) Boal, A. K.; Ilhan, F.; DeRouchey, J. E.; Thurn-Albrecht, T.; Russell, T. P.; Rotello, V. M. Nature 2000, 404, 746. (12) (a) Maye, M. M.; Chun, S. C.; Han, L.; Rabinovich, D.; Zhong, C.-J. J. Am. Chem. Soc. 2002, 124, 4958. (b) Maye, M. M.; Zheng, W.; Leibowitz, F. L.; Ly, N. K.; Zhong, C.-J. Langmuir 2000, 16, 490. (13) Novak, J. P.; Feldheim, D. L. J. Am. Chem. Soc. 2000, 122, 3979. (14) Fink, J.; Kiely, C. J.; Bethell, D.; Schiffrin, D. J. Chem. Mater. 1998, 10, 922. (15) Lin, S.-Y.; Tsai, Y.-T.; Chen, C.-C.; Lin, C.-M.; Chen, C.-H. J. Phys. Chem. B 2004, 108, 2134. (16) (a) Chang, S.-S.; Shih, C.-W.; Chen, C.-D.; Lai, W.-C.; Wang, C. R. C. Langmuir 1999, 15, 701. (b) Yu, Y.-Y.; Chang, S.-S.; Lee, C.-L.; Wang, C. R. C. J. Phys. Chem. B 1997, 101, 6661. (17) Busbee, B. D.; Obare, S. O.; Murphy, C. J. Adv. Mater. 2003, 15, 414.
and, more recently, branched multipods have been prepared.24,25 The wide variety in shape offers opportunities to observe novel optical properties and examine the mechanisms governing the growth of these gold nanostructures. Except for the more uniquely shaped gold nanoparticles mentioned above, the most common shape for gold nanoparticles reported in the 5-30 nm range appears to be roughly spherical without sharp facets.8b,26 To grow very small gold nanoparticles with highly faceted shapes (i.e., less than 10 nm), the use of a capping agent that has a possibly ineffective capping interaction with the nanoparticles may give the desired results, as demonstrated in this study. Highly faceted gold nanoparticles may provide the proper lattice surfaces to facilitate the growth of branched gold nanoparticles. Here we report a controlled synthesis of highly faceted gold nanoparticles using sodium dodecyl sulfate (SDS) as the capping agent. In this study, gold particles ca. 2.5 nm in diameter were used as seeds to grow larger nanoparticles in the presence of negatively charged SDS molecules. Gold particles with controlled sizes ranging from 5 to 50 nm were synthesized by a stepwise process.27-29 These particles generally have pentagonal and hexagonal shapes, (18) Jana, N. R.; Gearheart, L.; Murphy, C. J. Adv. Mater. 2001, 13, 1389. (19) Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15, 1957. (20) Kim, F.; Song, J. H.; Yang, P. J. Am. Chem. Soc. 2002, 124, 14316. (21) Me´traux, G. S.; Cao, Y. C.; Jin, R.; Mirkin, C. A. Nano Lett. 2003, 3, 519. (22) Zhou, Y.; Wang, C. Y.; Zhu, Y. R.; Chen, Z. Y. Chem. Mater. 1999, 11, 2310. (23) Sun, Y.; Xia, Y. Science 2002, 298, 2176. (24) Hao, E.; Bailey, R. C.; Schatz, G. C.; Hupp, J. T.; Li, S. Nano Lett. 2004, 4, 327. (25) Chen, S.; Wang, Z. L. Ballato, J.; Folger, S. H.; Carroll, D. L. J. Am. Chem. Soc. 2003, 125, 16186. (26) (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. (27) Jana, N. R.; Gearheart, L.; Murphy, C. J. Langmuir 2001, 17, 6782. (28) Jana, N. R.; Gearheart, L.; Murphy, C. J. Chem. Mater. 2001, 13, 2313. (29) Brown, K. R.; Walter, D. G.; Natan, M. J. Chem. Mater. 2000, 12, 306.
10.1021/la049172q CCC: $27.50 © 2004 American Chemical Society Published on Web 07/27/2004
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Figure 1. UV-Vis absorption spectra of gold seeds and nanoparticles in solutions A, B, C, and D. Numbers in the brackets for each sample are the band maximum and the value of absorbance. Table 1. Data for the Gold Nanoparticles in the Four Sets of Solution Samples calculated particle size A 7.5 ( 1.5 nm B 10.6 ( 2.1 nm C 26.9 ( 2.8 nm D 42.1 ( 6.0 nm
standard surface plasmon I(111)/I(200) ratio in deviation band max the XRD pattern 20% 19.8% 10.4% 14.3%
522 nm 524 nm 527 nm 534 nm
2.47 2.38 9.47 15.70
and thus should possess decahedron and dodecahedron, or perhaps icosahedron, structures. The crystal structure of the particles with a hexagonal shape has been carefully examined. The mechanism for the particle growth is accordingly proposed. Experimental Section Synthesis of Gold Seeds. To 19.8 mL of aqueous solution containing 2.5 × 10-4 M HAuCl4, 0.2 mL of 0.025-M sodium citrate solution was added and the solution was stirred for 3 min (Solution I). Concurrently, 10 mL of 0.1 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 size of the particles was ca. 2.5 nm.30 Preparation of Growth Solution. 100 mL of 2.5 × 10-4 M HAuCl4 aqueous solution was prepared in a 200-mL conical flask. Then 1 × 10-3 mol of SDS was added to the solution with stirring until the SDS powder completely dissolved. This solution was used as the growth solution. Synthesis of Size- and Shape-Controlled Gold Nanoparticles. Gold nanoparticles with increasing particle sizes were prepared successively. Four 50-mL conical flasks were labeled A, B, C, and D. In flask A, 7.5 mL of growth solution was mixed with 2.5 mL of seed solution and stirred for 10 min. Next, 50 µL of 0.1-M ascorbic acid was added to the solution and stirred vigorously for another 10 min. The solution turned ruby red and the particles had diameters of 7.5 ( 1.5 nm. In flask B, 1 mL of seed solution was mixed with 9 mL of growth solution and stirred for 10 min. Then 50 µL of 0.1-M ascorbic acid was added with stirring for 10 min. The solution in flask B turned a ruby-red color. Average particle size in solution B was 10.6 ( 2.1 nm. To prepare larger gold nanoparticles, 9 mL of growth solution, 1 mL of the product solution in flask B, and 50 µL of 0.1-M ascorbic acid were added to flask C and stirred for 10 min. The solution in flask C turned purplish-red and the average diameter of the nanoparticles in solution C was 26.9 ( 2.8 nm. Finally, in flask (30) Jin, Y.; Kang, X.; Song, Y.; Zhang, B.; Cheng, G.; Dong, S. Anal. Chem. 2001, 73, 2843.
Figure 2. TEM images of SDS-stabilized gold nanoparticles in solutions A (Figure 2a), B (Figure 2b), C (Figure 2c), and D (Figures 2d & 2e) prepared from the gold seeds. Insets show enlarged views of the respective nanoparticles. The scale bar for the insets ) 10 nm. The images show that many particles in solutions A and B already possess distinct facets. As these nanoparticles grow larger in solution C, pentagonal-shaped (e.g., particles I and II) and hexagonal-shaped particles with highly faceted faces become very apparent. Particle III is likely to be oriented differently from Particles I and II so as to allow viewing of the faceted particles from a different angle. Figure 2f shows the selected area-electron diffraction pattern of the aggregated particles pictured in Figure 2c. D, 1 mL of the product solution in flask C was mixed with 9 mL of growth solution and 50 µL of 0.1-M ascorbic acid and stirred for 10 min. The solution color in flask D was indigo blue. The average particle diameter was 42.1 ( 6.0 nm. All the solutions were left in the dark for 20 min or more for the reaction to go to completion before taking UV-Vis absorption spectra. To analyze the samples by transmission electron microscopy, gold nanoparticles in all 4 flasks can be concentrated by centrifugation for 20 min (Hermle Z323 centrifuge). Different rotation speeds were used for the 4 sets of solution samples to produce precipitates (12 000 rpm for A and B, 5000 rpm for C, and 2000 rpm for D).
Results and Discussion Our preparation of gold nanoparticles was carried out by the synthesis of gold seeds, and then growing these seeds into larger particles by the introduction of a surfactant SDS, more gold salt source, and a small amount of ascorbic acid as a weak reducing agent in the growth solution. 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.27 HAuCl4 was used as the gold source for both the synthesis of seeds and their growth into larger particles. Aqueous HAuCl4 solution showed a very light
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Figure 3. Size distribution histograms of gold nanoparticles in solutions A, B, C, and D.
yellow color. Reduction of HAuCl4 by NaBH4 in the presence of sodium citrate, a capping agent for the seed formation, changed solution color to orange-red indicating the formation of gold seed particles. Mixing of precise amounts of the seed solution and the growth solution allowed the preparation of nanoparticles with a high degree of control of particle size distribution. Gold nanoparticles exhibit strong plasmon resonance absorption that is dependent on the particle size and shape. For roughly spherical gold nanoparticles, the plasmon band maximum generally falls between 520 and 530 nm.27,28,31 Figure 1 shows the UV-Vis absorption spectra of the gold seeds and the particles obtained from solutions A, B, C, and D (Hitachi U-3300 spectrophotometer). These spectra were taken for freshly prepared samples. Absorption bands for all samples are centered between 520 and 535 nm. The absorbance increase is due to the progressive increase in particle size; larger particles have larger molar extinction coefficient values.27,32 The seed solution shows a band centered at 510 nm and the particle size is believed to be around 2.5 nm. Jin et al.30 reported the preparation of ∼2.5-nm gold seeds using essentially the same procedure, and the absorption band maximum appeared at 508 nm. Table 1 summarizes the particle diameters and the plasmon band maxima for the four sets of samples. The actual particle structures and size distribution can be determined by transmission electron microscopy. Figures 2 and 3 show the electron micrographs (JEOL JEM-2010 electron microscope operating at 200 kV) and histograms, respectively, of SDS-stabilized gold nanoparticles in solutions A, B, C, and D. In solutions A (Figure 2a) and B (Figure 2b), the particle size distribution is (31) Shimizu, T.; Teranishi, T.; Hasegawa, S.; Miyake, M. J. Phys. Chem. B 2003, 107, 2719. (32) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410.
relatively narrow. The images reveal well-defined facets present in many of the nanoparticles. Insets of Figures 2a and 2b show the enlarged views of the respective faceted nanoparticles. Particles that show this highly faceted structure can be as small as about 5-10 nm in diameter. These particles are developing into well-defined pentagonal and hexagonal shapes from the projections of decahedra and dodecahedra. Such a high concentration of highly faceted gold nanoparticles in this size range was not usually observed or not obvious enough to be discussed in reports using other surfactants (e.g., cetyltrimethylammonium bromide or CTAB)27,28 as the particle-stabilizing agents. As particle size increases further in solution C (Figure 2c), almost all of the particles show highly faceted structures and the majority of the particles have pentagonal (e.g., particle I and II in Figure 2c) or hexagonal shape. The particles still keep very good size uniformity. Some particles have a truncated triangular shape. When viewing these nanoparticles from various different angles, a twin boundary in the middle of the particles can usually be seen (e.g., particle III in Figure 2c), suggesting the formation of decahedral and dodecahedral gold structures. Hexagonal-shaped gold nanoparticles have also been prepared in other systems, but they appear to have substantially larger diameters (i.e., 30-100 nm).22,28,33 Figures 2d & 2e show the TEM images of the particles obtained from solution D. These larger particles mostly exhibit a well-defined hexagonal shape. Figure 2f shows the selected-area electron diffraction pattern of the particles in solution C. The diffraction pattern taken over several nanoparticles reveals a ring pattern that can be indexed as derived from the (111), (200), (220), (311), and (222) lattice planes of gold. (33) Kim, B.; Tripp, S. L.; Wei, A. J. Am. Chem. Soc. 2001, 123, 7955.
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Figure 5. TEM image of the same hexagonal-shaped gold nanoparticle shown in Figure 4a, showing the distribution of {100} and {111} facets.
Figure 4. (a) High-resolution TEM image of a hexagonalshaped gold nanoparticle obtained from solution D. Lattice planes of the crystal can be seen. The scale bar is equal to 10 nm. (b) Enlarged image of the square region in Figure 4a. A line is drawn to indicate a twin boundary bisecting two adjacent facets. A d-spacing between adjacent lattice planes of 2.36 Å corresponds to the (111) planes. (c) Selected-area electron diffraction pattern of the nanoparticle. The diffraction pattern corresponds to a superposition of square [001] and rectangular [112] zone patterns of face-centered cubic gold. The remaining spots are from double-diffraction reflections.
To further understand the crystal structure of the highly faceted gold nanoparticles prepared in this study, a detailed structural analysis of the hexagonal-shaped nanoparticles was performed by high-resolution TEM characterization (JEOL JEM-4000EX electron microscope operating at 400 kV). Figure 4a shows the high-resolution TEM image of a hexagonal-shaped nanoparticle obtained from solution D. Lattice planes of the crystal can be seen from this image. The particle also contains regions of curved twin boundaries. This type of structural feature is typical of many of the nanoparticles obtained in this study. Figure 4b gives the enlarged image of the square region in Figure 4a to identify the lattice planes forming this nanoparticle. A d-spacing of 2.36 Å for adjacent lattice planes corresponds to the (111) planes of face-centered cubic gold. The lattice planes are separated by a twin boundary as indicated by a line on the image. The selectedarea electron diffraction pattern of the crystal is shown in Figure 4c. The diffraction pattern can be indexed to a superposition of two individual diffraction patterns, one with a square symmetry corresponding to the [001] zone axis and the other with a rectangular symmetry corresponding to the [112] zone axis. The remaining diffrac-
tion spots are a result of the double-diffraction reflections.34 This diffraction pattern matches well with those reported for pentatwinned gold and silver nanorods, suggesting essentially the same crystal structures are present for multi-twinned gold and silver nanoparticles and nanorods.34,35 The crystal structure of the entire area of this particle was carefully examined by enlarging the high-resolution TEM image for all parts of the particle. The result is shown in Figure 5. In addition to the presence of twin boundaries, the particle contains (111) and (200) lattice planes with the (111) planes covering a much larger fraction of the particle surface. Separation between adjacent (200) lattice planes was measured to be ∼2.07 Å, very close to the 2.04-Å d-spacing of the (200) diffraction planes in the standard X-ray diffraction pattern for gold. Thus, the particle is composed of {111} and {100} surfaces. This observation is supported by a number of studies on both gold and silver nanorods showing the stabilization by surfactant molecules over the {110} or {100} surface along the nanorod axis, allowing the growth along {111} facets of the pentatwinned ends of the nanorods.34-36 To determine whether particle growth indeed proceeds via adding more gold atoms along the preferential (111) planes, XRD patterns of all solution samples were taken. The XRD patterns for nanoparticles in solutions B, C, and D are shown in Figure 6. The XRD pattern for particles in solution A is very similar to that for solution B. In all samples, as expected, the (111) and (200) diffraction peaks are observed. The diffraction peak for the (220) lattice planes is also present but its intensity is very weak. Interestingly, there is an obvious increase in the diffraction intensity of the (111) peak relative to that of the (200) peak in going from particles in solution B to C and then to D (see Table 1). This is a strong evidence of the preferential growth of the particles along 〈111〉 directions as particle size increases. (34) Sun, Y.; Mayers, B.; Herricks, T.; Xia, Y. Nano Lett. 2003, 3, 955. (35) Johnson, C. J.; Dujardin, E.; Davis, S. A.; Murphy, C. J.; Mann, S. J. Mater. Chem. 2002, 12, 1765. (36) Gai, P. L.; Harmer, M. A. Nano Lett. 2002, 2, 771.
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Another possible reason contributing to the ineffective capping interactions is that the concentration of SDS in solution A (7.5 mM) is slightly lower than the critical micelle concentration (cmc) of SDS (cmc of SDS ) 8.1 mM).40 Under this condition, SDS molecules may not effectively cap the small nanoparticles formed in solution A. Concentrations of SDS in solutions B, C, and D are 9.0 mM, 9.9 mM, and 10.0 mM, respectively. The SDS concentration in solution B is not much higher than the cmc of SDS, so the argument of ineffective capping by the SDS micelles should still be valid. Once the particle structure is defined, additional gold ions are added to the existing highly faceted surfaces and grow into larger particles. The SDS molecules do not have a sufficient ability to alter the overall faceted shapes of the nanoparticles when these particles reach a certain size. Summary Figure 6. Powder X-ray diffraction patterns of the gold nanoparticles obtained from solutions B, C, and D. Insets show the enlarged XRD patterns in the (200) peak region. A significant increase in the intensity of the (111) diffraction peak relative to that of the (200) diffraction peak as particle size increases is evident.
The formation of these highly faceted gold nanoparticles is attributed to the possibly ineffective capping interactions between the SDS molecules and the small gold nanoparticles (i.e., less than 10 nm). In contrast to many surfactants such as CTAB that are typically described to form spherically shaped micelles at lower surfactant concentrations,37 SDS molecules may prefer to form lamellar micellar structures.38 SDS molecules form lamellar mesostructured sol-gel thin films under both low and high surfactant concentrations.38 When gold nanoparticles are less than 10 nm in diameter, the lamellar micellar structures formed by the SDS molecules may not effectively cover all surfaces of the growing nanoparticles. This facilitates a more unrestricted growth of the nanoparticles into their preferred highly faceted multipletwinned structure with mainly {111} and {100} facets.39 (37) (a) Brinker, C. J.; Lu, Y.; Sellinger, A.; Fan, H. Adv. Mater. 1999, 11, 579. (b) Raman, N. K.; Anderson, M. T.; Brinker, C. J. Chem. Mater. 1996, 8, 1682. (38) (a) Huang, M. H.; Dunn, B. S.; Zink, J. I. J. Am. Chem. Soc. 2000, 122, 3739. (b) Huang, M. H.; Dunn, B. S.; Soyez, H.; Zink, J. I. Langmuir 1998, 14, 7331. (39) Marks, L. D. Rep. Prog. Phys. 1994, 57, 603.
We have demonstrated that the use of sodium dodecyl sulfate as the capping agent can produce highly faceted gold nanoparticles. By using a seeding growth approach, highly faceted gold nanoparticles with sizes ranging from 5 to 50 nm can be achieved with a relatively narrow particle size distribution. These nanoparticles typically exhibit pentagonal and hexagonal shapes and should possess decahedral and dodecahedral structures. Nanoparticles that are as small as 5-10 nm in diameter already show well-defined faceted structures. The crystal structure of individual nanoparticles was determined to possess mostly {111} facets and smaller regions of {100} facets as particle size increases. The formation of these highly faceted nanoparticles is attributed to the possibly ineffective capping interactions between the lamellar micellar structures formed by the SDS molecules and the nanoparticles. It is envisioned that the use of a capping agent such as SDS can be explored to prepare other interesting and potentially useful nanostructures, such as branched gold particles and faceted silver nanoparticles, as suggested by our preliminary results. Acknowledgment. Support for this work is by a grant from the National Science Council of Taiwan (NSC 912119-M-007-006). The starting fund was provided by the Department of Chemistry, National Tsing Hua University. LA049172Q (40) Everett, D. H. Basic Principles of Colloid Science; Royal Society of Chemistry: London, 1988; p156.
