J. Phys. Chem. C 2007, 111, 1161-1165
1161
Controlled Synthesis of Icosahedral Gold Nanoparticles and Their Surface-Enhanced Raman Scattering Property Kihyun Kwon,†,‡ Kang Yeol Lee,† Young Wook Lee,† Minjung Kim,† Jinhwa Heo,† Sang Jung Ahn,‡ and Sang Woo Han*,† Department of Chemistry, Research Institute of Natural Science, and EnVironmental Biotechnology National Core Research Center, Gyeongsang National UniVersity, Jinju 660-701, Korea and AdVanced Industrial Technology Group, Korea Research Institute of Standards and Science, Daejeon 305-340, Korea ReceiVed: July 10, 2006; In Final Form: NoVember 10, 2006
The aqueous-phase synthesis of icosahedral gold nanoparticles with controlled diameters ranging from 10 to 90 nm is reported. The nanoparticles were prepared by a seed-mediated growth approach. Citrate-capped 3.5 nm gold nanoparticles, prepared by reduction of HAuCl4 with sodium borohydride, are used as seeds. Cetyltrimethylammonium bromide molecules served as a capping agent to restrict the nanoparticles size. The synthesized gold nanoparticles were characterized by UV-vis spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy, and X-ray diffraction (XRD). The crystal structure of the particles was found to consist of mostly {111} surfaces as revealed by both TEM and XRD results. The prepared icosahedral gold nanoparticles show efficient surface-enhanced Raman scattering properties.
Introduction In the past few years, metal nanostructured materials have been the focus of much scientific research due to their physicochemical properties that are distinctly different from their bulk counterparts, and therefore, considerable attention from both fundamental and applied research has been paid to the synthesis and characterization of these materials.1 Particular interest has been focused on the noble metal nanoparticles because of their unique optical and electrical properties, and interesting applications of the nanoparticles in catalysis2 and optics3 as well as in biological assays4 have been demonstrated. In addition, noble metallic nanostructures exhibit a phenomenon known as surface-enhanced Raman scattering (SERS) in which the scattering cross-sections are dramatically enhanced for molecules adsorbed thereon.5 In recent years, it has been reported that even single-molecule spectroscopy is possible by SERS.6 Colloidal gold and silver particles with spherical shape have been generally used as SERS substrates. Although nonspherical particles can be promising candidates for SERS substrates because of their unique shape-dependent optical properties, there are very few reports on their applications for the SERS measurements.7-9 Controlling the nanoparticle shape is technologically important since the optical, electronic, magnetic, and catalytic properties of nanomaterials depend critically on not only particle size but also particle shape.10 Several methods have been developed for preparing nanoparticles in a variety of shapes, including rods, prisms, wires, and disks.11-16 However, highyield synthesis of monodisperse nanoparticles with well-defined size and shape is still a challenge. Here we report a controlled synthesis of highly faceted gold nanoparticles using cetyltrimethylammonium bromide (CTAB) as the capping agent which * To whom correspondence should be addressed. E-mail: swhan@ gnu.ac.kr. † Gyeongsang National University. ‡ Korea Research Institute of Standards and Science.
inhibited particle aggregation during growth and stabilized the particles after they formed. In this study, citrate-capped, borohydride-reduced ca. 3.5 nm gold particles were used as seeds to grow larger nanoparticles in the presence of CTAB molecules. Gold particles with controlled sizes ranging from 10 to 90 nm were synthesized by a stepwise process.11,16-20 These particles generally have icosahedron structure. We also investigated the SERS activity of the prepared nanoparticles. Experimental Section HAuCl4, CTAB, and trisodium citrate were purchased from Aldrich. Other chemicals, unless specified, were reagent grade, and triply distilled water (resistivity greater than 18.0 MΩ·cm) was used when preparing aqueous solutions. Gold nanoparticle seeds were prepared according to the literature.11 Typically this involves preparation of a 20 mL aqueous solution containing 2.5 × 10-4 M HAuCl4 and 2.5 × 10-4 M trisodium citrate. To this solution was added 0.6 mL of ice cold 0.1 M NaBH4 with stirring. The solution immediately turned orange-red, indicating formation of gold nanoparticles. The average particle size measured from transmission electron microscopy (TEM) was ∼3.5 nm. Citrate serves as a capping agent in this case, and gold particles are stable for a couple of weeks. Icosahedral gold nanoparticles with increasing particle sizes were synthesized successively. Five 20 mL vials (labeled A-E) were taken. To these vials was added 9.0 mL of growth solution containing a mixture of 2.5 × 10-4 M HAuCl4 and 0.10 M CTAB solutions. To these solutions was added 50 µL of 0.10 M freshly prepared ascorbic acid, and the resulting solutions were stirred gently. Next, 1.0 mL of the seed solution was mixed with sample A. After 10 s, 1.0 mL was drawn from solution A and added to solution B. After 10 s, 1.0 mL of solution B was added to solution C. The same process was carried out to solution E. In all cases, each vial was gently stirred to homogenize the solutions. The solutions in vials were kept at
10.1021/jp064317i CCC: $37.00 © 2007 American Chemical Society Published on Web 12/22/2006
1162 J. Phys. Chem. C, Vol. 111, No. 3, 2007
Kwon et al.
Figure 1. UV-vis absorption spectra of gold seeds and nanoparticles in solutions A-E.
25 °C. After the solutions were stored for 24 h, the excess CTAB was removed by centrifugation and washing with deionized water. The concentrations of gold nanoparticles in solutions A, B, C, D, and E were calculated to be about 10.3, 5.8, 0.40, 0.040, and 0.020 nM, respectively, by considering the average diameter of the particles for each sample.1d,20 The extinction spectra were recorded with a SINCO S-3100 UV-vis spectrophotometer. TEM images were obtained with a JEOL JEM-2010 transmission electron microscope operating at 200 kV. High-resolution TEM characterizations were performed with a FEI Technai G2 F30 Super-Twin transmission electron microscope operating at 300 kV. The scanning electron micrographs of the samples were taken with a field emission scanning electron microscope (FESEM, FEI Sirion 600). X-ray diffraction (XRD) patterns were obtained with a Bruker AXS D8 DISCOVER diffractometer using Cu KR (0.1542 nm) radiation. Raman spectra were obtained using a Jobin Yvon/ HORIBA LabRAM spectrometer equipped with an integral microscope (Olympus BX 41). The 632.8 nm line of an aircooled He/Ne laser was used as an excitation source. Raman scattering was detected with 180° geometry using an air-cooled 1024 × 256 pixel CCD detector. Results and Discussion Gold nanoparticles were prepared by the synthesis of gold seeds and then growing these seeds into larger particles by introduction of CTAB, more gold source, and ascorbic acid as a reducing agent in the growth solution. When seed particles are introduced in the growth solution they act as nucleation centers, autocatalyzing the reduction of Au+ to Au0 on their surfaces. Au+ is first formed by reduction of Au3+ to Au+ by the mild reducing agent ascorbic acid.16 Due to the weak reducing strength of ascorbic acid, larger size particles can be grown in the presence of seed particles; secondary nucleation during the growth stage was inhibited.20,21 Successive 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 size and shape of the particles. For spherical gold nanoparticles, the plasmon band maximum generally falls between 520 and 530 nm.19,20,22 Figure 1 shows the UV-vis absorption spectra of the as-prepared gold seeds and the particles obtained from solutions A-E. Absorption bands are centered at 527, 531, 534, 546, and 582 nm for samples A, B, C, D, and E, respectively. The seed solution
Figure 2. TEM images of the gold nanoparticles in solutions A (a), B (b), C (c), D (d), and E (e).
shows a band centered at 504 nm, and the particle size is measured to be around 3.5 nm. It is noticeable that the position of the plasmon band is gradually red shifted from sample A to E. Although we cannot rule out the possibility of shape change of nanoparticles, this absorption shift is believed to be caused by the progressive increase in particle size; larger particles show plasmon absorbance at longer wavelength.20,23 In fact, this was confirmed by TEM measurements. The actual particle structures and size distribution can be determined by TEM. Figure 2 shows the electron micrographs of CTAB-stabilized gold nanoparticles in solutions A-E. As shown in Figure 2, these particles have well-defined hexagonal shapes and their size distribution is relatively narrow. The measured mean particle diameters for samples A, B, C, D, and E are 11.0 ( 0.8, 13.3 ( 2.0, 32.2 ( 1.8, 69.0 ( 3.7, and 87.3 ( 12.1 nm, respectively. As expected in plasmon absorption measurements, the particle size progressively increases from sample A to E. This shows that size-controlled synthesis of gold nanoparticles can be achieved by successive seed-mediated synthetic protocol. Hexagonal-shaped gold nanoparticles have also been prepared in other systems,15,19,21,24-26 but such a highyield synthesis of monodisperse hexagonal gold nanoparticles with controlled particle size is unprecedented. The TEM images further reveal well-defined facets present in many of the nanoparticles. When viewing these nanoparticles from various angles, a twin boundary in the particles can usually be seen, suggesting formation of polyhedral gold structures. Since TEM can only give a projected image of the objects, we performed
Synthesis of Icosahedral Gold Nanoparticle
J. Phys. Chem. C, Vol. 111, No. 3, 2007 1163
Figure 3. SEM image of icosahedral gold nanoparticles. Inset shows clearly all (111) facets of a typical icosahedron. Figure 5. Powder XRD patterns of the gold nanoparticles obtained from solutions A (a), B (b), C (c), D (d), and E (e).
Figure 4. (a) High-resolution TEM image of an icosahedral gold nanoparticle obtained from solution D. (b) Enlarged image of the square region in a. 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.
SEM measurements to investigate the actual structure of the synthesized gold nanoparticles. Figure 3 shows a typical SEM image of the nanoparticles in solution D. SEM studies revealed that the particles can be more accurately described as icosahedra. The inset of Figure 3 shows clearly all (111) facets of a typical icosahedron. Note that icosahedral particles of face centered cubic (fcc) metals represent one of the most investigated multiply twinned particles in gas-phase experiments.26,27 However, we can prepare uniform metal particles with such welldefined structures in aqueous solution at ambient condition. For more insight into the crystal structure of the gold nanoparticles prepared in this work, structural analysis of the nanoparticles was performed by high-resolution TEM characterization. Figure 4a shows the high-resolution TEM image of an icosahedral nanoparticle from solution D. Lattice planes of the crystal can be seen from this image. The particle also contains regions of 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 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 fcc gold.21 The lattice planes are separated by a twin boundary as shown in the image. The results of high-resolution TEM measurements thus indicate that the particle contains crystalline lattice planes with the (111) planes covering a much larger fraction of the particle surface along with the presence of twin boundaries. XRD measurements also show that particle growth proceeds via adding more gold atoms along the preferential planes. Figure 5 shows the XRD patterns of the samples A-E. For each pattern
three peaks were observed which can be assigned to the (111), (200), and (220) diffraction peaks of fcc gold metal. It is noteworthy that the intensity ratios between the (200) and the (111) diffractions, I(200)/I(111), for the prepared samples are much smaller than the conventional bulk intensity ratio (∼0.53),26 and the ratio decreases as particle size increases; I(200)/I(111) ) 0.32, 0.29, 0.23, and 0.09 for samples B, C, D, and E, respectively (the value for sample A could not be obtained because of poor spectral quality). These observations further revealed that surfaces of the nanoparticles are primarily composed of {111} facets, and the particles preferentially grow along directions as particle size increases. On the other hand, the full width at half-maximum (fwhm) of the XRD peak can enable us to evaluate the size variation of the samples. The fwhm of the (111) peak was gradually decreased from 0.6 (sample A) to 0.4 (sample E). As confirmed by TEM measurements, this result implies that the size of the nanoparticles increases from sample A to E.28 In a study on the synthesis of multiple shapes of gold nanoparticles, Sau and Murphy reported that the morphology and dimension of the nanoparticles depend on the relative concentrations of the seeds and stabilizer as well as gold source and reducing agent.15 They obtained nanorods and triangular or square particles for an ascorbic acid concentration 1.6 times the gold ion concentration at 1.6 × 10-2 M CTAB and 2.0 × 10-4 M gold ions. On increasing the ascorbic acid concentration, particles with hexagonal and cubic shapes were formed. Formation of nanoparticles with different shapes was explained by interplay between the variation of growth rate at different planes of the particles and faceting tendency of the stabilizers.15,29 In a report on the synthesis of gold nanocrystals with a modified polyol process, Kim et al. also prepared icosahedral gold nanoparticles by adjusting the relative concentration of gold precursors and surface-regulating polymer (poly(vinyl pyrrolidone), PVP).26 On the basis of these previous results, we can conclude tentatively that the experimental condition used in this study, i.e., relative concentrations of reactants, is suitable for formation of icosahedral gold nanoparticles. As mentioned in the Introduction, nonspherical particles can be useful for SERS substrates due to their unique optical properties. Regarding this fact, we examined SERS efficiency of the icosahedral gold nanoparticles. As a typical example we show the Raman scattering spectra of 4-nitrobenzenethiol (4-
1164 J. Phys. Chem. C, Vol. 111, No. 3, 2007
Kwon et al.
Figure 6. SERS spectra of 1 × 10-4 M 4-NBT in aqueous Au sols obtained from solutions A (a), B (b), C (c), D (d), and E (e). Inset shows the UV-vis absorption spectra of solutions c and d.
NBT) in aqueous Au sols in Figure 6. The overall spectral features were very reproducible and consistent with those of the previously reported SERS spectra.30 The concentration of 4-NBT was 1 × 10-4 M, and at this concentration no Raman scattering was observed in the absence of colloid under the same experimental conditions; this indicates that the spectra in Figure 6 must be SERS spectra. An interesting observation is that SERS efficiency depends on the nanoparticle size. SERS intensity of sample C is the strongest among the prepared particles. From a comparison of the Raman spectrum of an aqueous solution of 4-NBT (0.1 M), we can estimate surface enhancement factors of sample C to be 2.5 × 104. We also obtained similar results with other molecules such as 2-mercaptopyridine (2-MP) and rhodamine 6G (R6G). The observed size-dependent SERS activity is not associated with the extent of aggregation of Au nanoparticles. The inset of Figure 6 shows the UV-vis absorption spectra of solutions c and d. The absorbance of solution d is larger than that of solution c at 700-900 nm. If the extent of aggregation of Au nanoparticles has an effect on the SERS activity, the SERS intensity of solution d must be larger than that of solution c. As shown in Figure 6, that is not the case. In fact, the SERS activity is related to the particle size. For example, Nie et al. found that 60 nm gold particles showed the most efficient SERS intensity when the excitation wavelength is about 650 nm.31 It is widely accepted that the SERS effect is the result of a combination of enhancement of localized electromagnetic field incident on an adsorbed molecule at metallic nanostructures and chemical effect.5,32,33 Intense localized fields arising from surface plasmon resonance may not be related to the observed experimental results because the relative SERS intensities are not correlated with relative absorbance of the particles at the excitation wavelength (see Figure 1). There are some reports on the fact that efficient Raman enhancement is not directly correlated with surface plasmon absorption.8,31 This means that other size-dependent effects might be present, such as size-dependent charge-transfer enhancement. The detailed mechanism of the size-dependent SERS activity is under further investigation. Besides the size of the nanoparticles, the morphology of the particles also has a great effect on the SERS activities of organic molecules.7-9 In relation to this fact, we measured SERS spectra of various adsorbates with common colloidal substrate for SERS
Figure 7. SERS spectra of (a) 1 × 10-4 M 4-NBT, (b) 1 × 10-4 M 2-MP, and (c) 1 × 10-6 M R6G obtained with both icosahedral (upper trace) and spherical (lower trace) particles.
measurements, i.e., spherical gold nanoparticles, and compared them with those obtained with the icosahedral gold nanoparticles (sample C). The spherical gold nanoparticles were prepared by citrate reduction method34 and have similar average size with icosahedral particles. Figure 7 shows the SERS spectra of 4-NBT (1 × 10-4 M), 2-MP (1 × 10-4 M), and R6G (1 × 10-6 M) molecules obtained with both icosahedral (upper trace) and spherical (lower trace) particles in aqueous solutions. It is noticeable that icosahedral gold nanoparticles give much stronger signals. SERS intensities obtained with the icosahedral particles are at least 4 times greater than those with spherical particles. One of the possible reasons for the observed difference in SERS activities between particle shapes is different surface chemistry of the particles. Icosahedral and spherical particles are stabilized with CTAB and citrate, respectively. Therefore, the net surface charge and molecular nature of the capping group are different. This different environment can thus affect the access of the adsorbates to the surface of the nanoparticle. However, we believe that this effect is not a dominant factor responsible for the higher SERS activity of icosahedral particles because the icosahedral nanoparticles show higher SERS efficiencies for all the adsorbates tested which have different polarities. Furthermore, we obtained similar results with spherical Au nanoparticles prepared in the presence of CTAB itself, instead of citrate. The enhanced SERS intensities of icosahedral particles may then be ascribed to intrinsic structural properties of the particles. Large enhancement of the local electromagnetic field is generally observed near sharp surface features.7-9 These active regions can serve as hot sites for surface plasma enhancement.6,8 Icosahedral particles have more well-defined edges and corners and generally sharper surface features than do spheres. Then, this morphological characteristic should result in greater localized field enhancement for icosahedral particles than spherical ones.
Synthesis of Icosahedral Gold Nanoparticle Conclusions We demonstrated that by using a seeding growth approach, icosahedral gold nanoparticles with sizes ranging from 10 to 90 nm can be produced with narrow particle size distribution. The interesting finding is that icosahedral particles show sizedependent SERS activities and give much stronger signals than do spherical particles. Since the synthesized nanoparticles have perfect symmetry for spatial arrangement and also show high SERS activity, they can be useful in interesting research such as fabrication of high-order nanostructures with tunable optical and optoelectronic properties and optical sensors with high efficiency. Acknowledgment. This work was supported by a grant from the MOST/KOSEF to the Environmental Biotechnology National Core Research Center (grant no. R15-2003-012-010010) and Technology Development Program of the Ministry of Agriculture and Forestry, Republic of Korea. References and Notes (1) (a) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257. (b) In Metal Nanoparticles: Synthesis, Characterization and Applications; Feldheim, D. L., Foss, C. A., Jr., Eds.; Marcel Dekker: New York, 2002. (c) Daniel, M.-C.; Astruc, D. Chem. ReV. 2004, 104, 293. (d) Schmid, G. Chem. ReV. 1992, 92, 1709. (2) (a) Narayanan, R.; El-Sayed, M. A. Nano Lett. 2004, 4, 1343. (b) Lewis, L. N. Chem. ReV. 1993, 93, 2693. (3) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729. (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. (c) Loweth, C. J.; Caldwell, W. B.; Peng, X.; Alivisatos, A. P.; Schultz, P. G. Angew. Chem., Int. Ed. 1999, 38, 1808. (5) Chang, R. K.; Furtak, T. E. Surface Enhanced Raman Scattering; Plenum Press: New York, 1982. (6) (a) Jiang, J.; Bosnick, K.; Maillard, M.; Brus, L. J. Phys. Chem. B 2003, 107, 9964. (b) Futamata, M.; Maruyama, Y.; Ishikawa, M. Vib. Spectrosc. 2002, 30, 17. (7) Orendorff, C. J.; Gole, A.; Sau, T. K.; Murphy, C. J. Anal. Chem. 2005, 77, 3261.
J. Phys. Chem. C, Vol. 111, No. 3, 2007 1165 (8) Zhang, J.; Li, X.; Sun, X.; Li, Y. J. Phys. Chem. B 2005, 109, 12544. (9) Orendorff, C. J.; Gearheart, L.; Jana, N. R.; Murphy, C. J. Phys. Chem. Chem. Phys. 2006, 8, 165. (10) (a) Dirix, Y.; Bastiaansen, C.; Caseri, W.; Smith, P. AdV. Mater. 1999, 11, 223. (b) Dickson, R. M.; Lyon, L. A. J. Phys. Chem. B 2000, 104, 6095. (11) Jana, N. R.; Gearheart, L.; Murphy, C. J. J. Phys. Chem. B 2001, 105, 4065. (12) Jin, R.; Cao, Y.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. Science 2001, 294, 1901. (13) Sun, Y.; Mayers, B.; Herricks, T.; Xia, Y. Nano Lett. 2003, 3, 955. (14) Maillard, M.; Huang, P.; Brus, L. Nano Lett. 2003, 3, 1611. (15) Sau, T. K.; Murphy, C. J. J. Am. Chem. Soc. 2004, 126, 8648. (16) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J.; Gou, L.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857. (17) Gole, A.; Murphy, C. J. Chem. Mater. 2004, 16, 3633. (18) Brown, K. R.; Walter, D. G.; Natan, M. J. Chem. Mater. 2000, 12, 306. (19) Jana, N. R.; Gearheart, L.; Murphy, C. J. Chem. Mater. 2001, 13, 2313. (20) Jana, N. R.; Gearheart, L.; Murphy, C. J. Langmuir 2001, 17, 6782. (21) Kuo, C.-H.; Chiang, T.-F.; Chen, L.-J.; Huang, M. H. Langmuir 2004, 20, 7820. (22) Shimizu, T.; Teranishi, T.; Hasegawa, S.; Miyake, M. J. Phys. Chem. B 2003, 107, 2719. (23) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410. (24) Zhou, Y.; Wang, C. Y.; Zhu, Y. R.; Chen, Z. Y. Chem. Mater. 1999, 11, 2310. (25) Kim, B.; Tripp, S. L.; Wei, A. J. Am. Chem. Soc. 2001, 123, 7955. (26) Kim, F.; Connor, S.; Song, H.; Kuykendall, T.; Yang, P. Angew. Chem., Int. Ed. 2004, 43, 3673. (27) (a) Marks, L. D. Rep. Prog. Phys. 1994, 57, 603. (b) Yacaman, M. J.; Ascencio, J. A.; Liu, H. B.; Gardea-Torresdey, J. J. Vac. Sci. Technol. B 2001, 19, 1091. (28) Suryanarayana, C.; Norton, M. G. X-ray diffraction; Plenum Press: New York, 1998. (29) Petroski, J. M.; Wang, Z. L.; Green, T. C.; El-Sayed, M. A. J. Phys. Chem. B 1998, 102, 3316. (30) Han, S. W.; Lee, I.; Kim, K. Langmuir 2002, 18, 182. (31) Krug, J. T.; Wang, G. D.; Emory, S. R.; Nie, S. M. J. Am. Chem. Soc. 1999, 121, 9208. (32) Moskovits, M. ReV. Mod. Phys. 1985, 57, 783. (33) Kim, K. L.; Lee, S. J.; Kim, K. J. Phys. Chem. B 2004, 108, 9216. (34) Lee, K. Y.; Kim, M.; Hahn, J.; Suh, J. S.; Lee, I.; Kim, K.; Han, S. W. Langmuir 2006, 22, 1817.
