Investigation on shape variation of Au nanocrystals

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Current Applied Physics 8 (2008) 810–813
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<strong>Investigation</strong> on shape variation of Au nanocrystals
Sung Koo Kang a , Inhee Choi a , Jeongjin Lee a , Younghun Kim b , Jongheop Yi a, *
a School of Chemical and Biological Engineering, Institute of Chemical Engineering, Seoul National University, Seoul 151-742, Republic of Korea
b Department of Chemical Engineering, Kwangwoon University, Seoul 139-701, Republic of Korea
Received 14 August 2006; received in revised form 20 December 2006; accepted 27 April 2007
Available online 1 October 2007
Abstract
A variety of shapes, such as rod, tripod, /-shape and cube, of Au nanocrystals were synthesized by employing different reaction conditions.
The nanocrystals and their shape variation were characterized by transmission electron microscopy and UV–vis spectrophotometry.
The evolution of shape was accomplished by controlling the parameters used in their synthesis, the concentration of reducing agent
and surface capping agent. The effect of synthetic parameters on shape was explored, to determine suitable conditions for producing each
shape of nanocrystals. Nanocrystals with different shapes have different plasmon bands in the visible region of the spectrum, which is a
valuable property for sensor applications.
Ó 2007 Elsevier B.V. All rights reserved.
PACS: 61.46.Df; 61.46.Hk
Keywords: Gold (Au); Nanoparticles; Nanorod; /-Shape; Shape control; Tripod
1. Introduction
Controlling the size and shape of inorganic nanoparticles
remains an important issue in the area of advanced
material development, since their unique properties are size
and shape dependent on a nanometer scale [1–5]. Rapid
progress has been made in controlling the shape of Au
nanoparticles in recent years, leading to, for example, the
production of rods [6–12], triangular platelets [13] and multipods
[14,15]. These new structures are of great interest in
terms of their optical, electric and magnetic properties for
technological applications by ‘bottom-up’ approaches in
developing new nanodevices [16,17]. Metal multipods with
branches are particularly desirable structures for applications
in areas such as interconnections of nanocircuits
because of their electrical conductivity. Although it has
been reported that semiconductor materials, such as CdS
* Corresponding author. Fax: +82 (02) 880 7438.
E-mail address: jyi@snu.ac.kr (J. Yi).
[18–20], CdSe [21,22], ZnO [23–25], and PbS [26], can be
used to control multipod-shaped structures, only a few
studies of multipod structures of metal have been reported.
Carroll et al. synthesized Au multipod nanocrystals by the
addition NaOH and silver plates to Au–CTAB mixtures
[14]. Li et al. reported on the synthesis of branched Au
nanocrystals in high yields [15]. In the findings reported
in these papers, tiny triangular or rectangular plates, which
are formed in the initial step, develop into tripod or tetrapod
structures by anisotropic growth.
In this paper we describe the synthesis of shape-controlled
Au nanoparticles using a seed-mediated method
that was originally reported by Murphy et al. [7]. This is
a well known method for preparing rod type nanoparticles.
However, we observed that tripod- and /-shaped nanoparticles
can be produced using the same procedure, by controlling
the concentrations of reducing agent and surface
capping agent used. Even though the mechanism of growth
is yet to be elucidated, it is likely that the evolution of the
shape is controlled by the kinetics associated with the
reduction of the Au ion precursors.
1567-1739/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.cap.2007.04.035

S.K. Kang et al. / Current Applied Physics 8 (2008) 810–813 811
2. Experimental details
The Au seeds used in this work were prepared as follows;
a solution 0.6 ml of 0.1 M NaBH 4 at 0 °C was added
to 20 mL of a solution that was 2.5 · 10 4 M in HAuCl 4
and tri-sodium citrate with vigorous stirring. The color of
the solution instantly turned to brown. For the growth
solution, four tubes, each of which contained 10 mL of
an aqueous solution of 2.5 · 10 4 M HAuCl 4 and
8.23 · 10 3 M cetyltrimethylammonium bromide (CTAB)
were prepared. These solutions were held at 0 °C for all
of the syntheses. Different amounts of a 0.1 M ascorbic
acid solution (AA) were added to each tube. The amounts
added were 0.1, 0.2, 0.3 and 0.4 ml, respectively. After the
color of the solution changed from yellow to white,
0.025 ml of the seed solution was added to each tube.
The same procedure was repeated with respect to CTAB
concentration, i.e., 2.74 · 10 3 and 1.37 · 10 3 M.
3. Results and discussion
The evolution of the shape of Au nanoparticles was
tracked by means of TEM. The results show that the shape
of the Au nanoparticles is dependent on the concentration
of the reducing agent, ascorbic acid (AA), and the surfactant,
cetyltrimethylammonium bromide (CTAB) that are
used. These two parameters were found to be interrelated;
different combinations resulted in various shapes. Fig. 1
Fig. 1. TEM images of Au nanoparticles synthesized using different reaction conditions, as a function of AA and CTAB concentration. (AA) and (CTAB)
were (a) 1 and 8.31 mM, (b) 2 and 8.31 mM, (c) 3 and 8.31 mM, (d) 4 and 8.31 mM, (e) 3 and 2.74 mM, and (f) 3 and 1.37 mM, respectively.

812 S.K. Kang et al. / Current Applied Physics 8 (2008) 810–813
shows TEM images of Au nanoparticles obtained when different
amounts of AA and CTAB were added to the solutions.
Fig. 1a–d clearly shows a variation in shape as the
addition of AA is gradually increased at a constant CTAB
concentration (8.23 · 10 3 M). Particles synthesized using
1 mM AA were spherical and rod-shaped (Fig. 1a). When
the concentration of AA in the solution was gradually
increased, a transition from rod-shaped particles with a
lower aspect ratio (Fig. 1b, 2 mM of AA), through tripods
(Fig. 1c, 3.00 mM of AA), to irregular shapes (Fig. 1d,
4 mM of AA) was observed. Since the minimum concentration
of AA required for the reduction of Au ions was 1 mM
in these systems (minimum amount of color change from a
yellow Au(III)–CTAB solution to a white Au(I)–CTAB
solution), these results clearly indicate that the use of an
excess of reducing agent was a major factor in the shape
transitions.
Interestingly, the concentration of the surface capping
agent, CTAB, also led to a variation in the shape of the
Au nanocrystals. When the concentration of CTAB was
decreased to three times (2.74 · 10 3 M) and the other
parameters of Fig. 1c were held constant, their shape was
altered from tripods to /-shaped particles (Fig. 1e). In
addition, particles synthesized using a 1.37 · 10 3 M
CTAB solution, with the other parameters of Fig. 1c held
constant, were nanocubes, as well as other shapes.
The formation of various shapes is likely due to an interplay
between the adsorption of surface capping agent and
growth kinetics induced by the reducing agent. CTAB molecules
have a stronger affinity for unstable {1 10} facets
than other facets [27]. Because of this property of CTAB,
the growth of Au nanorods is thought to involve the longitudinal
surface, CTAB strongly adsorbs to the {110} facets
of Au nanorods, thus conferring stability. When the AA
concentration is increased with the values of other parameters
held constant, tripod type nanoparticles are generated
and the morphology of the tripod is likely to be related to
kinetic factors. It should be noted that in the case of the Au
tripod, growth occurred when the {1 10} and {1 11} facets
were both favorably stabilized at the same time, assuming
the growth mechanism reported by Manna et al. [21]. A
higher AA concentration favors the more rapid formation
of all facets and a sufficient number of CTAB molecules
can simultaneously stabilize both the {110} and {1 11} facets,
thereby producing a tripod.
In the case of a slightly lower CTAB concentration with
the same AA concentration as was used for a tripod, /-
shaped particles are produced in high yield (Fig. 1d). The
preparation of /-shaped Au particles has been reported
and a mechanism that explains the growth mechanism
has been proposed by the El-Sayed et al. [28]. The transition
in shape from rod to /-shaped particles appears to
be originated from local melting by the laser pulses, followed
by surface reconstruction. This surface reconstruction
is driven by a decrease in the surface energy with a
reduction in the {110} surface area and an increase in
the {111} facets. Based on the fact that a decrease in the
concentration of CTAB led to the production of various
shapes, we reasoned that an insufficient adsorption of
CTAB on the facets of the Au nanocrystals occurred, thus
causing the shapes to be varied via surface migration of Au
atoms during the growth.
Under the condition of lowest CTAB with the same AA
concentration at conditions for producing a tripod induces
an increase in the rate of disappearance of unstable {110}
facets, because the insufficient capping of CTAB on {110}
facets. As a result, the production of cubes with hexagon
and triangle particles are produced, which surround the relatively
stable {111} and {100} facets.
Fig. 2 shows UV–vis absorption spectra of Au nanoparticles
that correspond to the TEM images shown in Fig. 1.
In the case of rod type Au nanoparticles (Fig. 1a–b), two
peaks appear for the Au surface plasmon band. One
appears at around at 540 nm which is a conventional plasmon
band for spherical particles (transverse) and the other
appears in the longer wavelength region, at 1100 nm
(aspect ratio = 7, Fig. 1a) and 900 nm (aspect ratio = 5,
Fig. 1b), corresponding to the longitudinal plasmon band
for rod-shaped particles. In general, the position of the longitudinal
absorption band is dependent on the aspect ratio
of the rod-shaped particles [13]. The aspect ratio was
decreased with increasing AA concentration, and the longitudinal
absorption band was gradually blue-shifted in this
system. Interestingly, these two split peaks were merged in
the range of 630–650 nm for tripod particles (Fig. 2c and
d). This result is consistent with data reported in a previous
study of the optical properties Au tripods. Hao et al.
showed that the color of a branched Au tripod was blue
and the peak position of the absorption spectrum varied
in the range from 650 to 700 nm [15]. In addition, the characteristics
of /-shaped particles were intermediate in
Fig. 2. Absorption spectra of Au nanocrystals synthesized using different
reaction conditions, as a function of (AA) and (CTAB). (AA) and (CTAB)
were (a) 1 and 8.31 mM, (b) 2 and 8.31 mM, (c) 3 and 8.31 mM, (d) 4 and
8.31 mM, (e) 3 and 2.74 mM, and (f) 3 and 1.37 mM, respectively.

S.K. Kang et al. / Current Applied Physics 8 (2008) 810–813 813
behavior between nanorods and tripods with regard to
peak splitting and the position of peaks (Fig. 2e). That is,
the spectrum of /-shaped particles showed two split peaks,
transverse (600 nm) and longitudinal (1000 nm). The position
of transverse peak, however, is shifted to 600 nm,
intermediate between peak position of nanorods and tripods.
The position of the longitudinal peak implies that
/-shaped particles have optical properties that are similar
to rod-typed particles. Furthermore, colloidal solutions of
tripod and /-shaped particles are blue. Finally, the peak
for nanocubes at 530 nm is similar to that for spherical particles
(Fig. 2f). Based on these results, absorption spectra of
the solutions containing Au nanoparticles of various
shapes show clear differences in optical properties.
4. Conclusion
In summary, a variety of Au nanoparticle shapes were
produced in aqueous media, using a seed-mediated
method. Their shape could be controlled by the appropriate
adjustment of the concentrations of reducing agent
and surface capping agent used. From the results, we conclude
that the morphological control of Au nanoparticles is
related to an interplay between the adsorption of the surface
capping agent and the kinetics of reduction of Au ions.
In addition, their unique optical properties permitted us to
classify their morphology.
Acknowledgment
This work was supported by Grant No. (R01-2006-000-
10239-0) from the Basic Research Program of the Korea
Science & Engineering Foundation.
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