Langmuir 1999, 15, 701-709
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The Shape Transition of Gold Nanorods Ser-Sing Chang, Chao-Wen Shih, Cheng-Dah Chen, Wei-Cheng Lai, and C. R. Chris Wang* Department of Chemistry, National Chung Cheng University, Min-Hsiung, Chia-Yi 621, Taiwan, Republic of China Received July 23, 1998. In Final Form: September 14, 1998 We report a revised synthetic procedure based on an electrochemical method for preparing an aqueous solution containing suspended Au nanorods. The mean aspect ratios of the Au nanorods can be experimentally adjusted between 1 and 7. The evolution of the longitudinal surface plasmon bands shows an eminently sensitive dependence on the aspect ratios of the nanorods. Their dependence is accordingly described by classical-electrostatic-model predictions. The shape transition of the nanorod particles has been studied by varying some key influencing factors such as the wavelength, the laser fluence, and matrix effects. The nanorods were exposed to laser lines at 532 and 1064 nm, frequencies which correspond closely to the shortand long-axis plasmon resonances, respectively. A photon-induced shape transition process was evidenced, and the corresponding rod-to-sphere conversion contributed by a photoannealing process was observed in both cases. Meanwhile, we observed a new type of “φ-shaped” Au nanostructure in the case of 1064-nm irradiation, which possibly represents the early stage of the shape transition and indicates that the starting location of the atomic-scale restructuring is at the centroid of the Au nanorod. The results of laser fluencedependence measurements state that an efficient shape transition occurs via a multiphoton process. We also demonstrate the fabrication of the Au nanorod@silica nanostructures for preliminary studies of the matrix effects. As a result of the higher rigidity of the thin-silica-coating layer, the associated shape transition requires higher energy and proceeds less efficiently as compared with the cases for the micellestabilized Au nanorods.
I. Introduction Interested in many unique properties of nanostructured materials, researchers have made extraordinary interdisciplinary efforts for almost a decade. A large database has been compiled in the literature in many research fields. Among their properties, metal nanostructures exhibit many chemical and physical properties in different contexts, such as catalysis,1 surface-enhanced Raman phenomena,2-5 and applications to optical devices.6 In many of these applications, the observed electromagnetic effects depend strongly on the size and shape of the metal particles. Beyond size control in the preparation of spherical metal nanoparticles, a greater challenge usually encountered is the control of particle shape. Recently, an electrochemical method7 has been demonstrated as a unique synthetic route for preparing high yields of suspended Au nanorods stabilized by a mixed cationic surfactant system in aqueous solutions. There are several other strategies that have been reported for the high-yield fabrication of cylindrically shaped metal particles. These can be categorized as template-based methodologies, since a “template” is usually needed to define and stabilize a particular shape. Of all the * To whom correspondence should be addressed. Fax: 886-52721040. E-mail:
[email protected]. (1) For example: Wilcoxon, J. P.; Martino, A.; Baughmann, R. L.; Klavetter, E.; Sylwester, A. P. Mater. Res. Soc. Symp. Proc. 1993, 286, 131. (2) Nie, S.; Emory, S. R. Science 1997, 275, 1102. (3) Emory, S. R.; Nie, S. J. Phys. Chem. B 1998, 102, 493. (4) Freeman, R. G.; Graber, K. C.; Allison, K. J.; Bright, R. M.; Davis, J. A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. A.; Smith, P. C.; Walter, D. G.; Natan, M. J. Science 1995, 267, 1629. (5) Vlckova´; Gu, X. J.; Moskovits, M. J. Phys. Chem. B 1997, 101, 1588. (6) Toshima, N.; Yonezawa, T.; Kushihashi, K. J. Chem. Soc., Faraday Trans. 1993, 89, 2537. (7) Yu, Y. Y.; Chang, S. S.; Lee, C. L.; Wang, C. R. C. J. Phys. Chem. B 1997, 101, 6661.
methodologies mentioned, these utilize various types of templates, such as porous alumina membranes,8-11 porous carbonate membranes,12 and lithographically-processed masks.13 The fabrication of the rodlike metal nanoparticles is then completed by filling the templates using different means, such as electrochemical deposition8-12 and vapor deposition.13 The particle-growth mechanism of the electrochemical technique, combined with the mixed cationic surfactant system,7 is still not fully understood. However, it is generally considered to be a template method with a dynamic micelle system serving as the template. The advantage of this method is that it offers a relatively simple preparation procedure for the fabrication of the suspended nanorod particles while preserving a good controllability in the particles’ dimensions. Unfortunately, the material of the rodlike nanoparticles which can be successfully synthesized by this method is currently limited to gold. In the consideration of possible future applications, an important concern is the stability of these Au nanorods under exposure to light or heat. The thermodynamic tendency of lowering surface energy14 makes the rodlike nanostructures potentially unstable relative to the spherical nanoparticles. The kinetics and dynamics of the shape transition for the nanostructures per se are interesting topics, but little is known about them to date. In this paper, we present a modified version of the preparation proce(8) Foss, C. A., Jr.; Hornyak, G. L.; Stockert, J. A.; Martin, C. R. J. Phys. Chem. 1994, 98, 2963. (9) Hulteen, J. C.; Martin, C. R. J. Mater. Chem. 1997, 7, 1075. (10) Al-Rawashdeh, N. A. F.; Sandrock, M. L.; Seugling, C. J.; Foss, C. A., Jr. J. Phys. Chem. B 1998, 102, 361. (11) van der Zande, B. M. I.; Bo¨hmer, M. R.; Fokkink, L. G. J.; Scho¨nenberger, C. J. Phys. Chem. B 1997, 101, 852. (12) Scho¨nenberger, C.; van der Zande, B. M. I.; Fokkink, L. G. J.; Henny, M.; Schmid, C.; Kru¨ger, M.; Bachtold, A.; Huber, R.; Birk, H.; Stauder, U. J. Phys. Chem. B 1997, 101, 5497. (13) Gotschy, W.; Vonmetz, K.; Leitner, A.; Aussenegg, F. R. Opt. Lett. 1996, 21, 1099. (14) Nagaev, E Ä . L. Sov. Phys. Usp. 1992, 35, 747.
10.1021/la980929l CCC: $18.00 © 1999 American Chemical Society Published on Web 12/09/1998
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dures described previously7 for generating suspended Au nanorods with the aspect ratios ranging from 1 to 7. A mixed surfactant micellar system is employed to stabilize the Au nanorods and takes a key role in defining their shape. The measured absorption spectra of these nanorods and the spectral dependence on particle shape are compared to the classical-electrostatic-model predictions. Measurements of shape transitions that occur as a result of the introduction of energy into the Au nanorods are then conducted. A pulsed laser output is employed to impart the excitation energies for inducing the shape transitions because of its good monochromaticity as compared with a simple heat source. The Au nanorods having a mean aspect ratio of 6.1 were chosen as the standard sample for the shape-transition studies because of the well-resolved transverse (SPtrans) and longitudinal surface plasmon bands (SPlong)7 associated with them, which are also close to the two available laser outputs of 532 and 1064 nm. Irradiations from both wavelengths can specifically cause the collective dipolar oscillations of the free electrons along either one of the two axial directions. The shape transition of the Au nanorods is studied by two parallel measurements. First, transmission electron microscopy (TEM) was used for the direct comparison of the particle’s images before and after the laser irradiation(s). Second, the laser fluence-dependence measurements of the disappearance of the Au nanorods were performed to reveal the energetic information related to the corresponding shape transitions. The shape transition associated with the Au nanorod having a specific aspect ratio has been examined for the possible dependence on several parameters, such as the excitation wavelength and the laser fluence. Also, the kinetics and the energetics of the shape transitions are expected to be matrix dependent. The micelle system used in our synthetic procedure is considered to be relatively “soft”. The fabrication of a more rigid coating of a thinsilica layer on the nanorods is also demonstrated. The subsequent fluence-dependent measurements have been conducted on these Au nanorod@silica nanostructures, and the results are reported herein. II. Experimental Section Synthesis of Suspended Au Nanorods. The electrochemical method for the fabrication of suspended Au nanorods in an aqueous solution has been described previously.7 The growth mechanism of Au nanorods is still not known at this stage; however, an improved synthetic procedure has been developed in terms of both the yields of the nanorods and the controllability of the length of the Au nanorods: The synthesis is conducted within a simple two-electrode-type electrochemical cell, as shown in the schematic diagram of Figure 1. A gold metal plate (3 × 1 × 0.05 cm) is used as the anode and the cathode is a platinum plate (3 × 1 × 0.05 cm); both electrodes are immersed in the electrolytic solution to a depth of about 1.5 cm during the electrolysis. Both electrodes are fixed in place by a Teflon spacer. The spacing between them is kept at ca. 0.25 cm. Typically, 3 mL of the electrolytic aqueous solution is used, which contains 0.08 M cationic surfactant, hexadecyltrimethylammonium bromide (C16TABr, 99%; Sigma), and 12.6 mg of a much more hydrophobic cationic cosurfactant, tetradodecylammonium bromide (TC12ABr, >98%; Fluka). The C16TABr serves not only as the supporting electrolyte but also as the stabilizer for nanoparticles, to prevent their further growth. The glass electrolytic cell containing the mixed solution is then placed into an ultrasonic cleaner (Branson, model 1210) and the temperature is controlled manually for 5 min to allow the solution to reach a temperature equilibrium with the ultrasonic bath. The temperature is maintained roughly at 36.0 °C throughout the preparation. Right before the electrolysis, 65 µL of acetone and 45 µL of cyclohexane are added into the electrolytic solution.
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Figure 1. Schematic diagram of the setup for preparation of Au nanorods. The electrochemical system contains the following: VA, power supply; G, glassware electrochemical cell; T, Teflon spacer and the electrode holder; S, electrolytic solution; U, ultrasonic cleaner; A, anode (Au); C, cathode (Pt). Acetone is used for loosening the micellar framework, and cyclohexane is necessary for enhancing the formation of the elongated rodlike C16TABr micelle.15 The fabrication of the Au nanorods is performed under a constant-current mode. The typical current setting is 5 mA, and the preparation time is about 20 min. During the synthesis, the bulk gold metal is converted from the anode to form gold nanoparticles, most likely at the interfacial region of the cathodic surface and within the electrolytic solution, which allows for subsequent growth of the cylindrical shape. Another important factor influencing the control of the aspect ratio of the Au nanorods is the presence of a silver plate inside the electrolytic solution. One (or two) silver plate, typically 3 × 1 × 0.05 cm in dimension, is gradually immersed into the solution behind the Pt electrode. The redox reaction between the gold ions, generated from the anode, and the silver metal, allows the formation of the silver ions. We found that the amount of the silver ions and the speed of their release could efficiently affect the control of, mainly, the major-axis length of the nanorod. The detailed growth mechanism and the role of silver ions in it, unfortunately, are still not known at the present stage. However, the total area of immersed silver plate at the end of the electrolysis will roughly determine the rod length. The larger the area, the longer the Au nanorod prepared will be. Following the synthesis, a centrifugation (Hettich, D-78532; 6500 rpm at 25 °C for 20 min) is helpful to further increase the yield of Au nanorods. The upper portion of the centrifuged solution is then transferred to another microcentrifuge tube for further centrifugation (12 000 rpm at 25 °C for 20 min) to obtain flocculent precipitate at the bottom of the tube. The flocculates of the Au nanorods are then redispersed by deionized water to a desired concentration for storage and other usage, such as TEM analysis (see below) or further surface modification, such as addition of a thin layer of silica coating. Preparation of Au Nanorod@Silica Core-Shell Structured Particles. The core-shell structure of Au nanosphere@silica particles has been prepared previously.16 An analogous coating procedure was employed for a Au nanorod embedded in a thin-silica layer. Briefly, a redispersed Au nanorod solution (1 mL) is prepared according to the above-mentioned procedures. The concentration of the Au nanorods is properly set to a value by adjusting the absorbance at λmax of SPlong. For example, an absorbance equal to ca. 2 is used for Au nanorods whose λmax of longitudinal SP resonance is ca. 900 nm (corresponding to a mean aspect ratio of ca. 5). This Au nanorod solution is then transferred into a bottle (PTFE, 5 mL) prepared for the reaction. The preparation of the MPTMS ((3-mercaptopropyl)trimethoxysilane, 97%; Fluka) ethanol reagent is done by a direct (15) To¨rnblom, M.; Henriksson, U. J. Phys. Chem. B 1997, 101, 6028. (16) Liz-Marza´n, L. M.; Giersig, M.; Mulvaney, P. Langmuir 1996, 12, 4329.
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Figure 3. Dimensions of Au nanorods vs corresponding λmax’s of SPlong resonances. Error bars stand for standard deviations of particle dimensional distributions. Dotted curves are arbitrary and provided only to indicate general trend.
Figure 2. TEM images of Au nanorods with two different mean aspect ratios: 2.7 (top) and 6.1 (bottom). Statistical data associated with dimensions of two samples are briefly described as follows Top: minor axis ) 13.8 ( 2.1 nm, major axis ) 35.8 ( 4.9 nm, aspect ratio ) 2.7 ( 0.6, number of particles ) 1184. Bottom: minor axis ) 10.6 ( 2.3 nm, major axis ) 62.6 ( 15.8 nm, aspect ratio ) 6.1 ( 2.2, number of particles ) 1418. Scale bars represent 50 nm. mixing of a 1:100 volume ratio (VMPTMS:VEtOH) solution. Ten microliters of MPTMS in ethanol are added to the Au nanorod solution, and the mixture is allowed to stand for 20 min under slow stirring to ensure a proper coverage of the thiol attaching onto the nanorod surface. An aqueous silicate solution was freshly prepared by mixing 0.24 g of sodium silicate solution (Na2O(SiO2)3-5, 27 wt % SiO2; Aldrich27) with 50 mL of deionized water. The coating step for the silica polymerization on the particle surface was conducted by adding 40 µL of the fresh aqueous silicate solution (pH ≈ 10.0) to the MPTMS modified Au nanorod solution, again under slow stirring. The mixture (pH ≈ 7.5) was allowed to stand for 24 h before a 2:1 dilution with deionized water was made, to be used for subsequent measurements. Photon-Induced Shape Transition and Laser FluenceDependence Measurements. A pulsed laser beam, either 1064 or 532 nm, from the Nd:YAG laser (Continuum, SLII-10) output with a pulse duration of ca. 6 ns, was directed into an optical cell having a path length of 1 mm. A small amount of the Au nanorod solution was transferred into the cell to about 1 cm height. The laser beam was expanded to roughly 3.0 cm2 using a home-built beam expander (Galilean type, 3×) and was aligned carefully to ensure the full coverage of the sample by the laser beam. The energies of the laser pulses were adjusted by directly varying the flashlamp-discharge voltages. The area of the laser beam was monitored for each measurement to calculate the laser fluence.
For laser fluence-dependence measurements, the absorption spectra were then recorded before and after the laser irradiation for various laser fluences. On the basis of these results, certain laser conditions, in terms of the laser fluence, were chosen to prepare TEM samples for the particle-shape analyses. Characterization of the Au Nanostructures. The resulting dispersions of (silica-coated) Au nanorods and the nanospheres produced from the shape transition before and after laser illumination were subjected to the measurement of both their optical absorption spectra and their structural images. The former reflects the particle shape dependence of the surface plasmon resonances and provides a rough measure for the mean aspect ratios of the nanorods. The latter determines the shape and the dimension distributions of these Au nanoparticles and thus provides direct evidence of the shape transition. Absorption spectra were collected on an HP 8453 UV-vis photodiode array spectrophotometer using a 1-cm (or 1-mm) path-length quartz cell. The shape and dimension distributions were measured by either a JEOL JEM-1200EX transmission electron microscope (TEM) or a Hitachi HF-2000 field emission TEM operated at 200 kV accelerating voltage. Samples containing Au nanoparticles were prepared by dip-coating Formvar/carbon-film Cu grids (200 mesh, 3 mm; Agar Scientific Ltd.) with the redispersed colloidal solution. Energy-dispersive X-ray (EDS) analyses have been conducted on many individual Au nanorods to verify any possible coexisting metallic impurities and have also been conducted on each silica-coated Au nanorod to verify the presence of the Si and S.
III. Results and Discussion Shape and Dimension Analyses of Au Nanorods. Figure 2 shows TEM images of Au nanorods having two different mean aspect ratios. The EDS analyses on individual Au nanorods confirm that no other detectable metallic impurities are present. Their shapes are clearly distinguishable from the spheroidal shape. The mean transverse diameter of a Au nanorod is typically equal to ca. 10 nm, while the mean longitudinal length is variable. For example, in the case of Figure 2 (bottom), the mean diameter is 10.6 ( 2.3 nm for a total count of 1418 rods. The long axis of the nanorod shows a greater distribution, of 62.2 ( 15.8 nm. The mean aspect ratio is analyzed to be 6.1 ( 2.2. Figure 3 exhibits the major and minor lengths of the experimentally prepared Au nanorods having different aspect ratios vs the corresponding λmax’s of the
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SPlong resonances. The larger the aspect ratio is, the more red shifted the SPlong resonance is (see below). The results in Figure 3 indicate that, by carefully controlling the preparation parameters, we can manipulate the length of the rod along the symmetry axis. The rod length is tunable from ca. 15 to ca. 70 nm while the minor diameter is kept between 15 and 9 nm by this preparation procedure. We are now able to prepare Au nanorods in yields as high as 90-95% routinely. Meanwhile, the analysis of the HRTEM data reveals that the Au nanorod crystals have the fcc lattice structure. In addition, no specific lattice plane dominates the growth along the longitudinal direction. Both (010) and (001) were found perpendicular to the major axis. More detailed characterizations, to gain better knowledge of the nanocrystalline structure are ongoing. Evolution of Surface Plasmon Resonances. It is known that a main feature of the absorption spectra for metallic nanoparticles arises from SP resonances, which are sensitive to the particle shape. In many cases, the size-dependence becomes insignificant in comparison with the dramatic shape effects. From one to as many as three SP bands can be observed corresponding to three polarizability axes of the metallic nanoparticles. The absorption spectra of Au nanorods are characterized by the dominant SPlong band (at longer wavelength) corresponding to the longitudinal resonance, as shown in Figure 4, and the very weak SPtrans band (at shorter wavelength, ca. 520 nm).7,9-11 An additional contribution to the intensity of the SPtrans band is possible depending upon the amount of coexisting Au nanospheres. The dependence of the absorption spectral features on the shape parameter, mean aspect ratio, is summarized in Figure 4. The evolution of the SPlong resonances is clearly demonstrated. The observed red shift of the SPlong resonance with an increasing aspect ratio is exactly what electrostatic17,18 and electrodynamic19 theories predict for prolate Au nanospheroids. To make a comparison between the experiment and the classical electrostatic prediction, a calculation, based on the classical electrostatic model (Mie/ Drude formalism), further modified by the size-dependent terms, is conducted for the prolate Au nanoparticles having different aspect ratios corresponding to the experimental spectra shown in Figure 4. The interaction between the incident electromagnetic wave and a nonspherical metal nanoparticle can be described by the radiation of a lossy dipole induced at the center of the particle. Assuming the scattering and higher order absorption terms are negligible, the expression of the mean absorption cross section, averaged over all orientations, for prolate Au nanoparticles was calculated in the dipole approximation:20
σ(ω) ) -(8π2/3λ) × Im(Rl + 2Rt)
(1)
where Rl and Rt are the polarizabilities for the longitudinal and transverse components, respectively. They can be expressed, under a rough assumption that the sizedependent dielectric constant of the prolate can be treated separately, by
Rl,t ) V[l,t(ω) - 1]/{4π + [l,t(ω) - 1]Pl,t}
(2)
where V is the volume of the particle, l,t(ω) is the (17) Creighton, J. A.; Eadon, D. G. J. Chem. Soc., Faraday Trans. 1991, 87, 3881. (18) Wang, D.-S.; Kerker, M. Phys. Rev. 1981, B24, 1777. (19) Zeman, E. J.; Schatz, G. C. J. Phys. Chem. 1987, 91, 634. (20) Kerker, M. The Scattering of Light and Other Electromagnetic Radiation; Academic Press: New York, 1969.
Figure 4. Evolution of SPlong resonances for Au nanorods as a function of the mean aspect ratios. Corresponding mean aspect ratios from top to bottom are 1.0, 2.5, 3.5, 4.7, 5.9, and 6.5 nanospheres, respectively, and transverse diameters used in calculations are fixed to 10 nm. Superimposed spectra (dotted curves) are the predicted absorption spectra by the classical electrostatic model for Au nanoparticles in H2O. Absorbances of the calculated spectra are arbitrary and adjusted for easy comparison. In the case of Au nanospheres, a radius of 10 nm is used in the model calculation.
frequency-dependent complex dielectric constant of the particle, and Pl,t are the depolarization factors for the longitudinal and transverse components17
Pl ) [4π/(1 - d2)]{[d/(d2 - 1)1/2]ln[d - (d2 - 1)1/2] + 1} (3) Pt ) (4π - Pl)/2
(4)
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where d () a/b) is the aspect ratio of the prolate particle and a and b are the semimajor and semiminor lengths, respectively. The size-dependent corrections to l,t(ω) are expressed by
l,t(ω) ) 1,l,t(ω) + i2,l,t(ω)
(5)
and
1,l,t(ω) ) ∞ - [ωp2/(ω2 + ωd,l,t2)] + B1(ω)
(6)
2,l,t(ω) ) [ωp2ωd,l,t/ω(ω2 + ωd,l,t2)] + B2(ω)
(7)
where ∞ is the value of the bulk 1(ω) at infinite frequency (∼1 for most metals), ωp is the plasma frequency21 (equal to 2.18 × 1015 Hz for Au), and B1,2(ω) are the interband terms contributed by the bound electrons, which can be determined from the bulk optical parameters22 of gold. Both B1 and B2 terms are assumed to be unchanged from the bulk, and ωd,l,t is the damping (or relaxation) frequency. Since the dependencies on both shape and size of the absorption cross-section spectra for nanoprolates are expected, we attempted herein to treat the damping frequency of ωd separately in terms of the longitudinal and transverse components, ωd,l and ωd,t, so that the real and imaginary parts of the particle’s dielectric constant can be calculated correspondingly using eqs 6, 7, and 5. The polarizability spectra for both directions can be simulated using eq 2, and the final size- and shapedependent absorption cross-section spectrum is then calculated. Both ωd,l and ωd,t can be described by
ωd,l ) SvF/leff,l and ωd,t ) SvF/leff,t
(8)
where leff,l,t is the effective mean free path of the conduction electrons, vF is the velocity of electrons at the Fermi energy (1.39 × 106 m/s for Au21), S is the slope parameter that is assumed to be isotropic and 0 < S e 1, and lbulk is the electronic mean free path of its bulk (31.0 nm for Au21). As the particle dimensions become less than lbulk, scattering of the free electrons at the particle boundary acts to lower the mean free path. The effective mean free path quantities corresponding to the two relevant particle dimensions become size-dependent and are estimated using the following relations:
1/leff,l ) 1/lbulk + 1/a
(9a)
1/leff,t ) 1/lbulk + 1/b
(9b)
They are treated separately for both the longitudinal and the transverse directions and are the terms where the size dependence actually enters. The resulting calculated absorption spectra are plotted against the corresponding experimental spectra for Au nanorods with various mean aspect ratios. The classical model predicts that the position of the SPlong band is strongly dependent on the aspect ratios of the Au nanoprolates and is only weakly dependent on its overall size for a given aspect ratio. Also, the size correction broadens the bandwidths of the SP bands without affecting the positions significantly (16-nm red-shifted for the prolate with a ) 5 nm, b ) 30 nm, and aspect ratio ) 6), thus yielding calculated spectra in general accord with (21) Ashcroft, N. W.; Mermin, N. D. Solid State Physics; CBS Publishing Asia Ltd., 1988. (22) Lide, D. R. CRC Handbook of Chemistry and Physics, 74th ed.; CRC Press: Boca Raton, FL; pp 12-113.
our experimental results. In comparing the bandwidths of the experimental and model spectral bands in Figure 4, it is apparent that they agree only qualitatively. It is realized that an assumption of monodispersed particles is adopted in the calculations; however, the experimental sample contains polydispersed Au nanorods. The slope parameter S may be another key factor for this discrepancy, where S ) 1 was used throughout the model calculations in Figure 4. A proper value of S should be
