© 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.
S1089-5647(97)01656-8 CCC: $14.00
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
6662 J. Phys. Chem. B, Vol. 101, No. 34, 1997
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
Letters
J. Phys. Chem. B, Vol. 101, No. 34, 1997 6663
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.
6664 J. Phys. Chem. B, Vol. 101, No. 34, 1997
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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