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as alkyltrimethylammonium bromides and cetylpyridinium
chloride have been investigated and used for synthesizing
gold nanorods [15]. The results demonstrate that the aspect
ratio of the resulting gold nanorods increased with the
length of the surfactant chain. The surfactant binds as a
bilayer to the growing nanorods and promotes the elonga-
tion of the nanorods via a ‘zipping’ mechanism. Moreover,
multifield 2H relaxation has been used to quantify the effect
of solubilization of alkanes on the size and shape of the
micelles in aqueous solutions of haxadecyltrimethylammo-
nium bromide, C16TAB [16]. Recently we had described a
new approach, based on the seed-mediated method, which
could extend the length of the resultant products using a
simple procedure [17]. Since the local structures of nanoma-
terials are greatly important for future applications.
Although TEM has been shown to be a powerful tool for
the analysis of nanoparticles, it is limited to study the chem-
ical bonds and structural characteristics of multicomponent
materials. However, X-ray absorption spectroscopy (XAS)
can provide information about the nanostructure (e.g. coor-
dination number, interatomic distance, and oxidation state
of absorption atoms). This structural information is aver-
aged over all of the X-ray-excited atoms in the entire sample,
but since a large percentage of the atoms in nanoparticles
are at the interface. Extended X-ray absorption fine struc-
ture (EXAFS) was useful for identifying the random or pre-
ferred occupation of sites around specific atoms, many
studies about nanostructural analysis have been reported
using EXAFS [18–22]. Thus, the local structures of gold
nanowires was demonstrated in present study.
0.2, 2, 20 and 200 mL of growth solutions, respectively.
After ascorbic acid had been added, the growth solution
became colorless as gold ions were reduced to gold atoms.
These three colorless solutions were added to the gold seed
solution one by one at 45 s intervals. The solutions turned
from pink to violet within 10 min.
2.3. Characterization of gold nanowires
The surface morphology of the samples was examined
by TEM (JEM-2000EX, 200 kV). The specimens were
obtained by placing several drops of the colloidal solution
onto a Formvar-covered copper grid and evaporating it in
air at room temperature. Before the specimen was pre-
pared, the colloidal solutions were sonicated for 1 min to
improve the particle dispersion on the copper grid. A series
of X-ray diffraction (XRD) patterns were recorded at room
temperature using BL01C2 of the National Synchrotron
Radiation Research Center (NSRRC), Taiwan (k =
0.07749 nm). The flux of BL01C2 is ꢀ1010 which is about
four orders of magnitude than in-house X-ray sources.
As a result, it is possible to perform XRD analysis even
with very small amount of sample.
In the XAS measurement, the gold nanowires solution
was centrifuged for 10 min at a speed of 4000 rpm. The
upper part of the colorless solution was removed and the
solid portion was collected. The nanowires powder was
mounted in aluminum cells and pasted on a plastic tape.
Au L3-edge X-ray absorption spectra were recorded on
the Wiggler-C beam line of the National Synchrotron
Radiation Research Center, in Taiwan, which was designed
for such experiments. The data were collected in transmis-
sion mode using gaseous nitrogen and argon-filled ioniza-
tion chambers as detectors. Energy calibrations were
undertaken using Au metal foils, designating the first
inflection point as 11,918 eV. X-ray absorption spectra
for Au metal foils were measured simultaneously in each
measurement when the metal foils were positioned before
the window of the third ion chamber to eliminate any
energy shift problem. The EXAFS data analyses were
carried out by the standard procedure [23]. The normalized
k3-weighted EXAFS spectra, k3x(k), were Fourier trans-
2. Experimental
2.1. Materials
Hydrogen tetrachloraurate(III) hydrate, trisodium cit-
rate dehydrate (99%), silver nitrate (99%), ascorbic acid
(99%), cetyltrimethylammonium bromide (CTAB) (99%)
were obtained from Across Organics and used without fur-
ther purification. The water used throughout this work was
reagent-grade water produced by a Milli-Q SP ultrapure-
water purification system of Nihon Millipore Ltd., Tokyo.
formed in the k range from 3.0 to 13 Aꢁ1 to show the con-
˚
2.2. Preparation of gold nanowires
tribution of each bond pair to the Fourier transform (FT)
peak. The S20 (amplitude reduction factor) was fixed at 0.8
for the Au atoms to determine the structural parameters of
each bond pair. The structural parameters, such as the
coordination number, the interatomic distance and the
Debye–Waller, were adjustable parameters in the fitting
of the EXAFS spectra.
In a typical synthesis, 300 mL of 0.025 mM HAuCl4
aqueous solution was prepared, and then 9 g of solid
CTAB was added to the gold salt solution. The solution
was heated to 40 ꢁC with stirring to dissolve the CTAB.
The solution was used as a growth solution after it was
cooled to room temperature. The growth solution was used
within 1 min of its preparation. After 10 min, gold clusters
formed in the solution. 3–4 nm gold seed particles were pre-
pared, as already reported by other researchers, using the
trisodium citrate capping method.3 0.02 mL of gold seeds
were placed in a beaker. 0.1, 1, 10 and 100 mL of freshly
prepared ascorbic acid (10 mM) solutions were mixed with
3. Results and discussion
Nanowires with aspect ratios 40–70 were synthesized by
our previous report [17]. However, synthesis of nanowires
alone was not possible, and the fact that particles form in
mixed shapes suggests that those gold seeds that can grow
Chen
