© Copyright 1997 by the American Chemical Society
VOLUME 101, NUMBER 34, AUGUST 21, 1997
LETTERS Gold Nanorods: Electrochemical Synthesis and Optical Properties Yu-Ying Yu, Ser-Sing Chang, Chien-Liang Lee, and C. R. Chris Wang* Department of Chemistry, National Chung Cheng UniVersity, Min-Hsiung, Chia-Yi 621, Taiwan, Republic of China ReceiVed: May 19, 1997X
Aqueous solutions containing a high yield of suspended gold nanorods have been successfully synthesized via an electrochemical method. The control of preparing gold nanorods with different aspect ratios can be attained. Their absorption spectral features show a dominant surface plasma band corresponding to the longitudinal resonance, SPl, and its λmax shifts markedly to the red as the aspect ratio is increased. Meanwhile, the dependence of λmax for longitudinal resonance on the mean aspect ratio shows a deviation from the classical electrostatic model prediction at mean aspect ratios around 4 ( 1, where it limits the validity of the classical electrostatic approximation.
Introduction Nanostructured materials have drawn much interdisciplinary effort. Both chemical and physical properties have been found to be fruitful and, in many cases, fascinating in this nanosize range.1 Colloidal metal nanoparticles are of interest due to their special properties in many aspects, such as catalysis2 and applications of optical devices.3 The size effect on the catalytical efficiency is known,2 and the perspective effect on catalysis by the shapes of metal nanoparticles is anticipated and under investigation. A recurrent theme on the unique extinction spectral features of the anisotropic coinage metal nanoparticles is emerging for both theory4 and experiments5,6 due to the recent experimental breakthroughs in the preparation of shapecontrolled coinage metal nanoparticles. In general, the control of nanoparticle shape constitutes a preparative challenge. Some methodologies have been demonstrated via chemical reduction methods either by adjusting the water contents7 at certain interconnected microemulsion phases for the production of rodlike Cu nanoparticles or through using capping polymers8 * To whom correspondence should be addressed. Email address:
[email protected]. X Abstract published in AdVance ACS Abstracts, August 1, 1997.
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in the preparation of cubic/tetrahedral platinum nanoparticles. Rod-shaped gold nanoparticles have also been prepared by means of electrodeposition in porous aluminum oxide.9,10 Of all of the methodologies developed for the production of metal nanoparticles, in either physical or chemical basis,11 the electrochemical method12,13 offers an alternative simple means within reverse micelles in organic solvent systems. We have now successfully applied it to the preparation of Au nanoparticles within normal micelles in aqueous solution and have developed a unique synthetic route in preparing high yields of Au nanorods. Our synthetic approach is to control the growth by introducing a “shape-inducing” reagent into the electrochemical system in which appropriate surfactants are employed as both the supporting electrolyte and the stabilizer for the resulted cylindrical Au nanoparticles. It is known that the optical response of spherical Au nanoparticles exhibits a single absorption band attributed to the collective dipole oscillation (surface plasma resonance).14,15 However, it usually deviates from this single-band spectral feature while increasing the eccentricity of the particles. For example, the classical electrostatic model predictions15,16 of absorption cross sections for nanospheroids of both gold and silver have been demonstrated to split the dipolar resonance © 1997 American Chemical Society
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Figure 1. TEM images of Au nanorods with different mean aspect ratios: 2.6 (top) and 7.6 (bottom).
into two SP bands, in which the induced dipole oscillates along and transverse to the spheroidal axis. A strong dependence of the λmax for the longitudinal resonance, the SPl band, on the aspect ratios is predicted, while the position of the transverse resonance, the SPt band, shows only relatively insignificant change as the aspect ratio varies. Our series of studies have confirmed these and have revealed the relationship between the SPl band positions and the mean aspect ratios of the gold nanorods. The trend of a shift for λmax’s is then compared to the classical electrostatic model prediction. Electrochemical Synthesis of Gold Nanorods Gold nanorods have been prepared via electrochemical oxidation/reduction within a simple two-electrode type cell analogous to the system described previously.12,13 A gold metal plate (3 × 1 × 0.05 cm) is used as the anode and a platinum plate (3 × 1 × 0.05 cm) is used as the cathode in our electrochemical cell. Both electrodes are immersed in an electrolytic solution consisting of a cationic surfactant, hexadecyltrimethylammonium bromide (C16TAB, 99%; Sigma), and a rod-inducing cosurfactant. The C16TAB serves not only as the supporting electrolyte but also as the stabilizer for nanoparticles to prevent their further growth. During the synthesis, the bulk gold metal is converted from the anode to form gold nanoparticles most probably at the interfacial region of the cathodic surface and within the electrolytic solution. A controlled-current electrolysis is used throughout the process for a typical current of 3 mA and a typical electrolysis time of
Letters 30 min. The synthesis is conducted under an ultrasonication and a controlled temperature, typically at 38 °C. An appropriate amount of acetone added into the electrolytic solution is necessary. The growth mechanism of Au nanorods is still not known at this stage; however, evidences suggest that the role of acetone is to facilitate the incorporation of cylindricalshaped-inducing cosurfactant into the C16TAB micellar framework and inducing the cylindrical growth to form the AuC16TAB-TC8AB system. The typical rod-inducing cosurfactant employed is an overall much more hydrophobic cationic surfactant: tetraoctylammonium bromide (TC8AB, >98%; Fluka). Furthermore, the rod-inducing capability has also been found on several analogous surfactants, such as TC10AB. The resulted dispersions of gold nanorods having different mean aspect ratios are then subjected to the measurements of both their absorption spectra and transmission electron micrographs. The former reflects the particle shape-dependence of the surface plasma resonances, and the latter determines the size and shape distributions of these gold nanoparticles. Absorption spectra were collected on a HP 8453 ultraviolet-visible photodiode array spectrophotometer using a 1 cm quartz cell. High-resolution transmission electron microscope (TEM) data were acquired on a Hitachi HF-2000 field emission TEM operated at 200 kV accelerating voltage. Samples containing Au nanoparticles were prepared by dip coating of colloidal solution on formvar/carbon film Cu grids (200 mesh; 3 mm, Agar Scientific Ltd.). Energy dispersive X-ray analyses also confirm that no other metallic impurities coexist in the Au nanoparticles. Figure 1 shows TEM images of Au nanorods having two different mean aspect ratios. Their shapes are clearly distinguishable from the spheroidal shape. Of all the samples containing suspended gold nanorods, much narrower distributions of transverse diameters were normally obtained compared to the distributions of the longitudinal length. The mean transverse diameters of thus prepared gold nanorods are typically equal to ca. 10 nm. The control for obtaining a high yield of Au nanorods with different aspect ratios can be achieved by carefully manipulating the experimental parameters. Absorption Spectral Features of Gold Nanorods It is known that the main feature of the absorption spectra for metallic nanoparticles is of the SP resonance band(s). From one up to three SP bands can be observed corresponding to three polarizability axes of the metallic nanoparticles. The optical properties of metallic particles ranging from microclusters14,17 to nanoparticles6,18,19 have been investigated mainly on the size effects concerning the shift of the SP resonance and the variation of the SP bandwidth. However, it has been demonstrated both theoretically4,15,16,20 and experimentally6,21 that the SP resonances of the coinage metal nanoparticles depend much more sensitively on the particle shapes than on the sizes. The absorption spectra of gold nanorods are characterized by the dominant SPl band (at longer wavelength) corresponding to longitudinal resonance, as shown in Figure 2A-C, and a much weaker transverse resonance (at shorter wavelength, ca. 520 nm) is evidenced. An additional contribution to the intensity of the SPt band is possible by the amount of Au spheres existing in the dispersions of gold nanorods. The typical spectrum of Au nanospheres is superimposed in Figure 2A, and its band position coincides with the SPt band. Meanwhile, the position of SPl band is subject to a shift due to different mean aspect ratios. The evolution of longitudinal SP resonances is evidenced, and the corresponding distributions of aspect ratios and their mean values are exhibited in Figure 2D-F. An
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Figure 2. Aspect ratio dependent absorption spectra of gold nanorods. (A-C) Absorption spectra of suspended gold nanorods solutions with increasing mean aspect ratios. (D-F) Distributions of aspect ratios analyzed from the corresponding TEM micrographs; mean/fwhm ) 1.8/0.9 (D), 3.0/1.9 (E), 5.2/3.0 (F).
increase in the mean aspect ratios, 1.8, 3.0, and 5.2, results in a red-shift of the SPl bands: 600, 710, and 873 nm. The same absorption behavior has also been observed previously in aqueous dispersions of rod-shaped gold nanoparticles.10 In contrary to the obvious shift of the longitudinal resonances, the absorption bands of the transverse resonance is relatively unchanged and located at ca. 520 nm. It is partially due to the fact that the transverse diameters of thus prepared gold nanorods do not vary significantly from 10 nm. Also, the size effect of the resonant shift is believed to be not as important as the shape effect according to our experience on the spectral studies of spherical gold nanoparticles prepared under similar systems. We summarize the results of our series studies for the meanaspect-ratio-dependent λmax of the SPl band in Figure 3. The extreme shift of the longitudinal resonances to the longer wavelengths with increasing the aspect ratio is clearly observed and is exactly what the theories by both electrostatic15,16 and electrodynamic20 approaches predict for a similar case of gold prolate nanospheroids. To make a comparison between the experiment and the classical electrostatic prediction, the calculation of electrostatic theory is conducted for prolate gold
nanoparticles. No size effect due to the spatial dispersion of the free electron density22 is considered in our calculation, and the bulk optical parameters23 of the gold are used. The calculated result for the electrostatic model prediction is superimposed in Figure 3 with our experimental data. The agreement between experiment and this electrostatic model prediction is good within the low aspect ratio regime. Interestingly, a discontinuity in the trend of SPl band shift is indicated at a mean aspect ratio ) ca. 4 ( 1. The implication of this discontinuity is that the dimensions of the elongated nanorods along the symmetry axis beyond this point are sufficiently large so that the electrostatic approximation no longer applies.24 In addition, Figure 3 provides a good measure for the aspect ratio of gold nanorods for our synthesis process. Two interesting questions arise: Where does the SPl band end while the aspect ratio keeps increasing? Will the redshift of the SPl band level off, or will it switch to the opposite direction? We are currently improving our synthetic method for preparing such gold nanorods having much larger aspect ratios to gain a direct proof on them.
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Letters Acknowledgment is made to the National Science Council (NSC85-2113-M-194-006 and NSC85-2732-M-194-001) for the financial support as well as to the National Chung Cheng University (Type-A research grant) for partial support of this research. Contributions from Mr. Yao, S.Y. for HRTEM measurements in Department of Materials Science and Engineering, National Cheng Kung University, are also gratefully acknowledged. References and Notes
Figure 3. Spectral shift of the longitudinal resonance band, SPl, vs mean aspect ratio of the gold nanorods. Points are experimentally determined values, and the line shows the trend predicted by the classical electrostatic theory.
Summary Suspended colloidal aqueous systems of gold nanorods with different mean aspect ratios are synthesized by a newly developed electrochemical method with the aid of ionic surfactants as both the supporting electrolytes and the main constituents of the micellar framework. The absorption spectral features of the nanorods consist of two surface plasma bands corresponding to transverse and longitudinal resonances due to the anisotropy of the shape. A strong dependence of the longitudinal resonances on the mean aspect ratio is observed and matches the theories from both electrostatic and electrodynamic approaches. In addition, a discontinuity in the trend of the shift for the longitudinal resonances is observed. The implication probably states that the availability of the classical electrostatic prediction is limited beyond this point.
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