Article pubs.acs.org/Langmuir
Simple Reductant Concentration-Dependent Shape Control of Polyhedral Gold Nanoparticles and Their Plasmonic Properties Miharu Eguchi,†,‡ Daisuke Mitsui,† Hsin-Lun Wu,† Ryota Sato,† and Toshiharu Teranishi*,†,§,∥ †
Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571, Japan PRESTO-JST, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571, Japan § Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan ∥ CREST-JST, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571, Japan ‡
ABSTRACT: We report a facile seed-mediated method for the synthesis of monodisperse polyhedral gold nanoparticles, with systematic shape evolution from octahedral to trisoctahedral structures. The control over the particle growth process was achieved simply by changing the concentration of the reductant in the growth solution, in the presence of small spherical seed nanoparticles. By progressively increasing the concentration of the reductant used in the growth solution (ascorbic acid), while keeping the amount and type of added surfactant constant, the morphology of the gold nanoparticles was varied from octahedral to truncated octahedral, cuboctahedral, truncated cubic, cubic, and finally trisoctahedral structures. These nanoparticles were monodisperse in size, possessed similar volumes, and were naturally oriented so that their larger crystal planes were face down on quartz substrates when deposited from the solution. By adjusting the volume of gold seed nanoparticle solution added to a growth solution, the size of the simplest gold nanoparticles (with a highly symmetric cubic morphology) could be tuned from 50 ± 2.1 to 112 ± 11 nm. When other seed nanoparticles were used, the size of the cubic Au nanoparticles reached 169 ± 7.0 nm. The nanoparticle growth mechanism and the plasmonic properties of the resulting polyhedral nanoparticles are discussed in this paper. region.15,16 Past studies on the shape-control of such NPs have typically employed the selective adsorption of surfactants on specific crystal planes, which was believed to be essential for control over the shape of metal NPs.17 Very recently, Huang and co-workers demonstrated that the role of surfactants in directing the morphology of polyhedral NPs was less important than previously thought; they illustrated this using the systematic shape evolution of Au NPs (from rhombic dodecahedral to octahedral structures), produced in a seedmediated and iodide-assisted synthesis.18,19 Here, we report the development of a seed-mediated method for the synthesis of monodisperse Au NPs, where a systematic shape evolution from octahedral to trisoctahedral structures is achieved simply by changing the concentration of the reductant. The monodisperse spherical Au seed NPs (smaller than 10 nm) evolved through various polyhedral structures, including octahedral, truncated octahedral, cuboctahedral, truncated cubic, cubic, and trisoctahedral structures. These
I. INTRODUCTION Localized surface plasmons are collective free carrier (electron or hole) oscillations in conductive and semiconductor nanoparticles (NPs) that are excited by incident light waves. Two important phenomena associated with localized surface plasmons are chemical enhancement and plasmon energy propagation, which can be exploited by making use of the subwavelength confinement of electromagnetic energies around the NPs.1−3 This chemical enhancement can be used to improve photochemical processes such as surface enhanced Raman scattering (SERS),4 the enhancement of nonlinear excitation processes in target molecules or semiconductor NPs,5 and the electron injection from plasmonic metals into semiconductors;6 it is believed that plasmon energy propagation has the potential to allow the realization of optics and photonics beyond the diffraction limit of light.7−12 For these purposes, conductive and semiconductor NPs must be applied as nanoantennas; an ability to precisely control the size and shape of these NPs13,14 is therefore necessary if the desired results are to be achieved. To date, much effort has been dedicated to this goal. Gold (Au) and silver (Ag) NPs with controlled shapes have large extinction coefficients (∼109 M−1 cm−1), and well-defined LSPR absorption features that can be systematically tuned in the visible-near-infrared (vis-NIR) © 2012 American Chemical Society
Special Issue: Colloidal Nanoplasmonics Received: January 14, 2012 Revised: March 7, 2012 Published: March 8, 2012 9021
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structures all had similar volumes; when the concentration of ascorbic acid in the growth solution was varied, a larger number of gold atoms fed to a single Au seed NP accelerated the growth rate of the {111} crystal planes, rather than the {100} planes. In this work, we demonstrated the first successful growth of highly symmetric cubic Au NPs (the simplest of the polyhedra produced here) to a size of over 150 nm; these particles are useful for both the easy manipulation and the light scattering observation of single NPs.8,20 We also investigated the plasmonic features of these polyhedral Au NPs.
II. EXPERIMENTAL SECTION
Figure 1. TEM image of 8.7 ± 0.3 nm CTAC-Au seed NPs.
Synthesis of Shape-Controlled Polyhedral Au NPs. The control over the shape of the Au NPs was achieved using a seedmediated method, which involved the synthesis of spherical Au seed NPs, followed by the growth of the seed NPs. The spherical Au seed NPs were synthesized as follows: 0.6 mL of 10 mM ice-cold NaBH4 aq. was added to a mixture of 9.55 mL of 0.1 M cetyltrimetylammonium bromide (CTAB) aq. and 0.05 mL of 50 mM HAuCl4·4H2O aq. under vigorous stirring. This was followed by stirring at 28 °C for 1 h to obtain the small spherical Au NPs, with sizes ranging from 3 to 5 nm. A total of 300 μL of the small Au NP aq. was then added to a mixture of 6 mL of 0.2 M cetyltrimethylammonium chloride (CTAC) aq., 6 mL of 0.5 mM HAuCl4·4H2O aq., and 8 mL of ion-exchanged water to obtain the CTAC-protected Au (CTAC-Au) seed NPs. The growth of the CTAC-Au seed NPs into well-faceted polyhedral NPs was achieved by adding 0.03 mL of the CTAC-Au seed NP solution to 9.97 mL of an aqueous solution (growth solution) containing ascorbic acid, CTAB (10 mM), CTAC (90 mM), and HAuCl4·4H2O (0.2 mM). The concentration of ascorbic acid was varied from 0.6 to 10 mM, to obtain different shapes such as octahedra, truncated octahedra, cuboctahedra, truncated cubes, cubes, and trisoctahedra. Synthesis of Large Cubic Au NPs. Large cubic Au NPs were also synthesized using a two-step seed-mediated method, which involved the synthesis of small Au NPs, the sysnthesis of CTAB-protected Au (CTAB-Au) seed NPs, and the growth of the CTAB-Au seed NPs. To obtain the small Au NPs, 0.6 mL of 10 mM ice-cold NaBH4 aq. was added to a mixture of 7.75 mL of 0.1 M CTAB aq. and 0.05 mL of 50 mM HAuCl4·4H2O aq. under vigorous stirring; this was followed by stirring at 28 °C for 1 h. To obtain the CTAB-Au seed NPs, 5 μL of a 10-fold diluted small Au NP (4.6 ± 1.3 nm) aqueous solution was then added to a mixed solution containing 1.2 mL of 0.1 M ascorbic acid, 1.6 mL of 0.1 mM CTAB aq., and 40 μL of 50 mM HAuCl4·4H2O. A desired amount of the CTAB-Au seed NP aq. was then added to an aqueous solution (growth solution) containing ascorbic acid, CTAB, and HAuCl4·4H2O, to grow the Au seed NPs into the cubic NPs. The total volume of the solution was 10 mL, and the final concentrations of ascorbic acid, CTAB, and HAuCl4·4H2O in the aqueous solution were 11 mM, 25 mM, and 190 μM, respectively. The concentrations of CTAB-Au seed NPs as Au atoms employed were 44, 20, 10, 5.0, and 2.3 μM. Measurements. Transmission electron microscopy (TEM) images were taken using a JEOL JEM-1011 electron microscope at an acceleration voltage of 100 kV. TEM samples were prepared by dropcasting a sample solution on a TEM grid and allowing the solvent to evaporate in air. Scanning electron microscopy (SEM) observations were carried out using an S-4800 (HITACHI) scanning electron microscope at an acceleration voltage of 25 kV. X-ray diffraction (XRD) patterns were collected using a PANalytical X’Pert Pro MPD instrument with Cu Kα radiation (λ = 1.542 Å), operating at 45 kV and 40 mA. XRD samples were prepared by depositing the sample solution on a quartz plate. UV−vis−NIR extinction spectroscopy was conducted using a U-4100 spectrophotometer (HITACHI).
were synthesized using the seed-mediated method, where the structural changes were induced simply by changing the reductant (ascorbic acid) concentration. The first, small Au NPs (synthesized in the NaBH4 reduction of HAuCl4·4H2O) had sizes in the 3−5 nm range. These small Au NPs were subjected to mild growth, which was achieved via the ascorbic acid reduction of HAuCl4·4H2O; this produced the CTAC-Au seed NPs, whose TEM image is shown in Figure 1. The size of these seed NPs was 8.7 ± 0.3 nm, and no twinned crystalline structures were observed. Such monodispersity and single crystallinity was very important to allow well-faceted single crystalline polyhedral NPs to be obtained in the next growing step. Various polyhedral Au NPs with similar volumes were formed simply by changing the concentration of the reductant (ascorbic acid) in the growth solution. Figure 2 shows SEM images of the resulting Au NPs, synthesized with various concentrations of ascorbic acid. Octahedron (Figure 2a), truncated octahedron (Figure 2b), cuboctahedron (Figure 2c), truncated cube (Figure 2d), cube (Figure 2e), and trisoctahedron (Figure 2f) structures were obtained at ascorbic acid concentrations of 0.6, 1.2, 1.4, 1.8, 2.0, and 10 mM, respectively. The sizes of these NPs (as indicated in Figure 3) were 84.2 ± 2.5 nm (octahedron), 64.8 ± 4.2 nm (truncated octahedron), 53.0 ± 1.3 nm (cuboctahedron), 50.9 ± 1.7 nm (truncated cube), 49.9 ± 2.1 nm (cube), and 76.1 ± 3.4 nm (trisdoctahedron). Assuming that the number of NPs before and after the seed-mediated growth reaction was the same, the volume of each NP should have remained the same. In fact, the volumes of the octahedra, truncated octahedra, cuboctahedra, truncated cubes, and cubes calculated in consideration of their sizes and shapes were 1.00 × 105, 1.19 × 105, 1.24 × 105, 1.27 × 105, and 1.24 × 105 nm3, respectively, demonstrating that a similar number of gold atoms were deposited on the CTAC-Au seed NPs, regardless of the concentration of ascorbic acid. The yields (the ratios of the number of desired NPs to the number of whole NPs) were 75.2% (octahedron), 75.4% (truncated octahedron), 81.3% (cuboctahedron), 84.1% (truncated cube), 96.0% (cube), and 91.6% (trisoctahedron). In addition, when adopting the concentration of ascorbic acid used from cubic (2.0 mM) to trisoctahedral (10 mM) structures, the Au NPs have intermediate structures between cube and trisoctahedron. The shape evolution of the Au NPs (depending on the concentration of ascorbic acid) is schematically illustrated in Figure 3, where the {111}, {100}, and {221} crystal planes are indicated in white, red, and gray, respectively.17,21 The octahedral, cubic, and trisoctahedral structures consisted of eight {111}, six {100}, and 24 {221} planes, respectively. The truncated octahedral, cuboctahedral, and truncated cubic structures were composed of six {100} and eight {111} planes,
III. RESULTS AND DISCUSSION Shape Evolution of Au NPs, Induced by Changes in the Reductant Concentration. Various polyhedral Au NPs 9022
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Figure 2. SEM images of well-faceted polyhedral Au NPs. The shape and the concentration of ascorbic acid for each NP type were (a) octahedron, 0.6 mM; (b) truncated octahedron, 1.2 mM; (c) cuboctahedron, 1.4 mM; (d) truncated cube, 1.8 mM; (e) cube, 2.0 mM; and (f) trisoctahedron, 10 mM. Insets show the enlarged SEM images.
Figure 3. Schematics of polyhedral NPs. {111}, {100}, and {221} crystal planes are indicated in white, red, and gray, respectively.
Figure 5. Extinction spectra of aqueous solutions of polyhedral Au NPs.
formed by supplying a small amount of gold atoms to each CTAC-Au seed NP per unit time. In contrast, at high concentrations of ascorbic acid, the rapid growth of the seed NPs produced a thermodynamically unstable structure, owing to the accelerated growth rate of the {111} lattice planes (compared with the {100} planes). In this case, the catalytically active {111} planes would play an important role in forming the Au NPs surrounded by higher surface energy lattice planes. These results indicated that the role of surfactants in directing the polyhedral NP morphology is less significant than the feeding rate of gold atoms to a single Au seed NP. The structure-dependent optical properties of the Au NPs obtained with this method can be studied, because the effects of surfactants on the physicochemical properties of the NP are avoided. X-ray Diffraction Analysis. It is likely that the polyhedral Au NPs were oriented on the quartz substrate with their larger crystal planes down when deposited from the solution; this was confirmed in the powder X-ray diffraction analysis. Figure 4
Figure 4. Powder X-ray diffraction patterns for various well-faceted polyhedral Au NPs.
with increasing surface area ratio of {100} to {111} planes in the order given. Figures 2 and 3 indicate that the final shape (the exposed surface planes) of the Au NPs was strongly affected by the concentration of ascorbic acid; that is, the use of a higher concentration of ascorbic acid resulted in the Au NPs being surrounded by higher surface energy lattice planes ({111} < {100} < {221} for a face-centered-cubic structure). At low concentrations of ascorbic acid, the formation of a thermodynamically stable structure was favored; the structure 9023
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Figure 6. SEM images of cubic Au NPs with sizes of (a) 50 ± 2.1 nm, (b) 69 ± 2.4 nm, (c) 92 ± 6.1 nm, and (d) 112 ± 11 nm.
similar surface areas, their X-ray diffraction patterns clearly showed both (111) and (200) diffraction peaks. Taking the ratio of the intensities of the (111) and (200) planes into consideration, it was found that the cuboctahedral NPs were preferentially laid on the substrate with their {100} planes down. In the case of the trisoctahedral NPs, the intensity ratio of the (111) to the (200) planes was close to that of bulk Au, indicating that the trisoctahedral NPs had no preferential orientation on the substrate. As calculated using the Scherrer formula, the crystallite volumes of the octahedral and cubic Au NPs (as extreme examples) calculated from their crystallite sizes (octahedron, 9.42 × 104; cube, 1.40 × 105 nm3) were in good agreement with the volumes obtained from the SEM observations (octahedron, 1.00 × 105; cube, 1.24 × 105 nm3), meaning that the polyhedral NPs synthesized using our method were single crystals. Plasmonic Properties of Various Polyhedral Au NPs. To investigate the shape-dependent plasmonic properties of the polyhedral Au NPs with similar volumes, the extinction spectra of aqueous solutions of six types of the polyhedral Au NPs were measured in the visible region (Figure 5). All of the NP samples exhibited a localized surface plasmon resonance (LSPR) peak at around 550 nm, which was associated with a dipole mode. Five of the polyhedral Au NP types (the sixth type being the octahedral NPs) showed an LSPR peak at approximately 540 nm; this similarity between the peak wavelengths occurred because the free electron oscillation lengths between the facing (100) planes in these five NPs were identical to each other, owing to the similar sizes of the particles. A slight red-shift in the LSPR peaks for the cuboctahedron, truncated cube, and cube structures (with the red-shift increasing for each morphology, in the order given) was in a good agreement with the theoretical expectation that a spectral shift (of the extinction) to shorter wavelengths should take place as the cube was truncated to the cuboctahedron structure.22 The octahedral Au NPs showed an LSPR peak at a longer wavelength of 562 nm. It is likely that the increased size (oscillation length) of the
Figure 7. Extinction spectra of cubic Au NPs with sizes of 50, 69, 92, and 112 nm.
shows powder X-ray diffraction patterns for the obtained polyhedral Au NPs deposited on a quartz glass substrate. The amount of deposited NPs was carefully controlled to form a monolayer (or less) on the substrate. The octahedral and truncated octahedral Au NPs exhibited a strong diffraction peak at 2θ = 38.2°, which was assigned to the (111) reflection; the weaker peak at 2θ = 44.4° was assigned to the (200) reflection (a peak intensity ratio of (111)/(200) = 1/0.49 for bulk Au), indicating that these NPs were laid on the substrate surface with their {111} planes down. In contrast, the cuboctahedral, truncated cubic, and cubic Au NPs showed stronger diffraction peaks that were associated with the (200) plane, rather than the (111) plane. The relative intensity of the two peaks was dependent on the orientation of the deposited nanoparticles, demonstrating that these NPs were arranged with their {100} planes facing the substrate. This was also supported by the SEM observations (see Figure 2d and e). In the case of the cuboctahedral NPs, because the {111} and {100} planes had 9024
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Figure 8. TEM images of cubic Au NPs with sizes of (a) 43 ± 1.9 nm, (b) 67 ± 3.8 nm, (c) 89 ± 4.6 nm, and (d) 169 ± 7.0 nm.
whole NPs) were 96.0% (50 ± 2.1 nm), 76.2% (69 ± 2.4 nm), 64.2% (92 ± 6.1 nm), and 64.3% (112 ± 11 nm). The extinction spectra of aqueous solutions of these cubic Au NPs were measured in the visible−near-infrared region (see Figure 7). The cubic NPs smaller than 100 nm exhibited a single LSPR peak, which was attributed to a dipole mode; this peak showed a red-shift as the particle size increased. For the cubic NPs larger than 100 nm, two major peaks appeared. The peak at longer wavelengths (which was attributed to a dipole mode) also showed red-shift with increasing particle size; the other peak emerged at shorter wavelengths and resulted from higher multipolar charge distributions.22 Plasmonic Properties of Large Cubic Au NPs. The largest size shown by the cubic Au NPs produced using the CTAC-Au seed-mediated method was 112 ± 11 nm. The use of CTAB-Au seed NPs in place of CTAC-Au seed NPs was effective in generating larger cubic NPs. Figure 8 shows TEM images of the CTAB-Au seed NPs (Figure 8a) and the resulting cubic Au NPs synthesized using the CTAB-Au seed-mediated method (Figure 8b−d). The size of the CTAB-Au seed NPs used was 43 ± 1.9 nm. The size of the cubic NPs grown could be tuned from 67 ± 3.8 to 169 ± 7.0 nm by decreasing the concentration of CTAB-Au seed NPs from 44 to 2.3 μM. Similarly, as for the CTAC-Au seeded cubic Au NPs, this indicated that when a smaller volume of CTAB-Au seed NPs was used, a larger number of gold atoms were available for each seed NP to generate larger cubic NPs. The 169 nm cubic NPs obtained in the present research are larger than any previously reported cubic Au NPs. The extinction spectra of these cubic NPs (shown in Figure 9) showed a tendency similar to that observed in Figure 7. The cubic NPs smaller than 100 nm exhibited a single LSPR peak, which was attributed to a dipole LSPR resonance. As the cubic NPs grow larger than 100 nm, two additional peaks appeared at shorter wavelength, accompanying the red-shift of the peak assigned to the dipole LSPR mode. These new peaks might be derived from higher multipolar charge distributions.22
Figure 9. Extinction spectra of cubic Au NPs with different sizes.
octahedral NPs (corresponding to an increased free electron oscillation length between the facing apexes in Figure 3) compared with the other five NP types was responsible for the red-shift of the LSPR peak. Simple and low-symmetric structures are most suitable for the construction of dark mode-based plasmon waveguides. We therefore focused our attention on the size-control and plasmonic properties of the cubic Au NPs. Control over the size of the cubic Au NPs was achieved by changing the amount of CTAC-Au seed NPs added to the growth solution. Figure 6 shows SEM images of the resulting cubic Au NPs, synthesized with different volumes of the CTAC-Au seed NPs. The size of the cubic NPs could be tuned from 50 ± 2.1 nm to 112 ± 11 nm by decreasing the concentration of CTAC-Au seed NPs from 0.55 to 0.018 μM. This indicated that when a smaller volume of CTAC-Au seed NPs was used, larger cubic NPs were generated; this was because a larger number of gold atoms were available to each seed NP. The yields (where the yield was taken as the ratio of the number of cubic NPs to the number of 9025
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(12) Novotny, L.; van Hulst, N. Antennas for light. Nat. Photonics 2011, 5, 83−90. (13) Kanehara, M.; Koike, H.; Teranishi, T. Indium tin oxide nanoparticles with compositionally tunable surface plasmon resonance frequencies in the near-IR region. J. Am. Chem. Soc. 2009, 131, 17736− 17737. (14) Luther, J. M.; Jain, P. K.; Ewers, T.; Alivisatos, A. P. Localized surface plasmon resonances arising from free carriers in doped quantum dots. Nat. Mater. 2011, 10, 361−366. (15) Wiley, B.; Sun, Y.; Xia, Y. Synthesis of silver nanostructures with controlled shapes and properties. Acc. Chem. Res. 2007, 40, 1067− 1076. (16) Tao, A. R.; Habas, S.; Yang, P. Shape control of colloidal metal nanocrystals. Small 2008, 4, 310−325. (17) Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Shape-controlled synthesis of metal nanocrystals: simple chemistry meets complex physics? Angew. Chem., Int. Ed. 2009, 48, 60−103. (18) Wu, H.-L.; Kuo, C.-H.; Huang, M. H. Seed-mediated synthesis of gold nanocrystals with systematic shape evolution from cubic to trisoctahedral and rhombic dodecahedral structures. Langmuir 2010, 26, 12307−12313. (19) Chung, P.-J.; Lyu, L.-M.; Huang, M. H. Seed-mediated and iodide-assisted synthesis of gold nanocrystals with systematic shape evolution from rhombic dodecahedral to octahedral structures. Chem.Eur. J. 2011, 17, 9746−9752. (20) Yang, S.-C.; Kobori, H.; He, C.-L.; Lin, M.-H.; Chen, H.-Y.; Li, C.; Kanehara, M.; Teranishi, T.; Gwo, S. Plasmon hybridization in individual gold nanocrystal dimers: direct observation of bright and dark modes. Nano Lett. 2010, 10, 632−637. (21) Ma, Y.; Kuang, Q.; Jiang, Z.; Xie, Z.; Huang, R.; Zheng, L. Synthesis of trisoctahedral gold nanocrystals with exposed high-index facets by a facile chemical method. Angew. Chem., Int. Ed. 2008, 120, 8901−8904. (22) Noguez, C. Surface plasmons on metal nanoparticles: the influence of shape and physical environment. J. Phys. Chem. C 2007, 111, 3806−3819.
IV. CONCLUSION In summary, we developed a facile seed-mediated method for the synthesis of monodisperse Au NPs, where a systematic shape evolution from octahedral to trisoctahedral structures was achieved simply by changing the concentration of reductant. Using the same surfactant, our method produced six types of polyhedral Au NPs, which all showed similar particle volumes. For the first time, simple and highly symmetric cubic NPs were grown to a size of over 150 nm. The six types of polyhedral CTAC-Au NPs have the potential to be useful for the study of electric enhancement factors, both in the vicinity of the NPs and in the nanogaps between NPs; this could be achieved without considering the different surrounding refractive index. The large cubic Au NPs could also be applied as building blocks for plasmon energy propagation beyond the diffraction limit of light.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was partially supported by a KAKENHI Grant-Aid for Scientific Research A (No. 23245028) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan (T.T.).
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REFERENCES
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