Direct chemical synthesis of gold nanowires with 2-D network structure and relationship between the presence of gold ions and shape stability of gold nanowires

Article abstract of DOI:10.1246/cl.2004.324

Gold nanowires with a 2-D network structure were synthesized by a direct chemical synthesis method; the shape stability of gold nanowires was closely related to the presence of gold ions in the solution phase, without which the nanowires broke, and larger nanoparticles were formed.

Full text of DOI:10.1246/cl.2004.324

324
Chemistry Letters Vol.33, No.3 (2004)
Direct Chemical Synthesis of Gold Nanowires with 2-D Network Structure and Relationship
between the Presence of Gold Ions and Shape Stability of Gold Nanowires
Lihua Pei, Koichi Mori,# and Motonari Adachiꢀ
Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-0011
(Received December 12, 2003; CL-031229)
Gold nanowires with a 2-D network structure were synthe-
sized by a direct chemical synthesis method; the shape stability
of gold nanowires was closely related to the presence of gold
ions in the solution phase, without which the nanowires broke,
and larger nanoparticles were formed.
should be noted that when the solution turned blue, an almost flat
absorbance curve with a board peak was observed from 500 to
900 nm, for example, at 6 and 20 min in Figure 1A, and 10
and 180 min in Figure 1B. When the solution color turned to
wine red, the UV–vis spectrum exhibited a narrow peak at about
547 nm after 40 min at R ¼ 0:4. Thereafter, this wine red color
and UV spectrum remained constant over time, indicating the
formation of stable gold nanoparticles. However, at R ¼ 0:3,
the flat adsorption pattern (Figure 1B) lasted until 180 min which
corresponded to the blue solution.
The synthesis of metal nanowires has attracted a great deal
of attention because of their unusual roles in the fabrication of
nanoscale electronic devices1 and investigation of quantized
conductance and localization effects.2 A lot of works have been
done for the preparation of metal nanowires using various meth-
ods such as electrochemical3 and photochemical4 reduction in
aqueous surfactant media, templating from porous alumina,5
polycarbonate membrane6 or carbon nanotube,7 wet chemical
synthesis based on a seed-mediated growth mechanism,8 and so-
lution-phase method based on capping reagents.9 Although these
methods have succeeded in synthesizing monodisperse uniform
nanowires, some problems remain, for example, removal of sur-
factant, process complexity, or the search for a suitable capping
agent. The formation mechanism is not yet fully understood.
Therefore, it will be a significant challenge to simplify the syn-
thesis route and clarify the growth mechanism of the anisotropic
metal nanoparticles.
Here, we report the synthesis of gold nanowires with 2-D
network structure using a simple chemical reduction method
with a low concentration of reducing agent. The reaction system
contains the gold precursor and reducing agent only. In addition,
it was found that the gold ions played an important role in stabi-
lizing the shape of gold nanowires in the formation process.
The experimental method was a modification of convention-
Figure 1. UV–vis absorption spectra of the reaction solutions at
different reaction stages: (A) R ¼ 0:4, reaction time of 0, 6, 20,
40 min; (B) R ¼ 0:3, reaction time of 0, 10, 180 min. Tempera-
ture 80 ꢃC.
Figure 2 shows typical transmission electron microscopy
(TEM) images of samples prepared at R ¼ 0:3 and R ¼ 0:4 with
various reaction times. Figure 2A clearly shows the formation of
gold nanowires covering a 2-D space uniformly at R ¼ 0:3 with
30 min reaction. Figure 2B shows that gold nanowires with such
a 2-D network structure can also be formed at R ¼ 0:4. The net-
work structure of gold nanowires always corresponds to the flat
UV absorbance pattern shown in Figure 1. Figure 2C is a high-
resolution TEM image of one part of the gold nanowire showing
a uniform diameter of approximately 11 nm. The electron dif-
fraction of the gold nanowire (inset of the Figure 2C) shows scat-
tering points corresponding to (111), (200), (220), and (311) of
gold crystalline facets. These findings clearly indicate that gold
nanowires are formed by tightly fused polycrystalline nanoparti-
cles. In the case of R ¼ 0:3, the shape of nanowires remained af-
ter 180 min of reaction. However, for R ¼ 0:4, the nanowires
broke and aggregated into nanoparticles (Figure 2D) of 50–
60 nm after reaction of 40 min, which corresponded to the nar-
row peak in Figure 1A. There was no change in particle mor-
phology observed after the larger particles were formed.
ꢁ
al citrate reduction of AuCl4 in water developed by Frens
(1973).10 In a typical synthesis, aqueous solution of sodium tet-
rachloroaurate (NaAuCl4ꢂ2H2O, 0.25 mM) was stirred in a tem-
perature-controlled oil bath until the solution was warmed to
80 ꢃC. Then a controlled amount of trisodium citrate was added
rapidly into this solution. After the initiation of the reaction, the
solution was taken out at specific time intervals for UV–vis spec-
troscopy and TEM observation. To monitor the concentration of
gold ions during the reaction, 3 mL 48% HBr acid was injected
into the 5-mL aliquots taken from the reaction flask, followed
by centrifugation for 15 min at 6000 rpm. The supernatant layer
was used to measure the UV–vis absorbance. Then, the concen-
tration of gold ions was calculated from the peak absorbance at
381 nm. The initial molar concentration ratio of citrate/NaAuCl4
(denoted as R) was controlled at 0.4 and 0.3, respectively.
Figure 1 shows the UV–vis absorption spectra taken from
solutions at different reaction stages for R ¼ 0:4 and R ¼ 0:3.
Upon the addition of sodium citrate, the initially light yellow so-
lution became colorless, indicating the reduction of AuCl4ꢁ. It
Time-resolved measurement of the concentration of gold
Copyright ꢀ 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.3 (2004)
325
Figure 3. TEM images of as-synthesized gold nanowires (A)
and after treatment with H2 (B). The sample of A was prepared
at R ¼ 0:3 for 10 min. The TEM grid of B was obtained by put-
ting the TEM grid A in the atmosphere of H2 gas for 1 hour at
room temperature.
This work was supported by a Grant-in-aid for Scientific Re-
search from the Ministry of Education, Culture, Sports, Science
and Technology of Japan. The authors gratefully acknowledge
Professor S. Isoda and Mr. Y. Murada for assistance with the
TEM observation. Professor T. Yonezawa is also appreciated
for helpful discussion.
Figure 2. TEM images of gold nanowires taken from the reac-
tion solution at different reaction stages: (A) R ¼ 0:3, 30 min;
(B) R ¼ 0:4, 20 min; (C) HRTEM and ED, R ¼ 0:4, 20 min;
(D) R ¼ 0:4, 40 min. The TEM samples were obtained by drop-
ping the solutions onto carbon-coated copper grids placed on a
filter paper for rapid removal of the liquid.
References and Notes
ions showed that the gold ions were completely reduced after
40 min when R was controlled at 0.4. However, the reduction re-
action was much slower for R ¼ 0:3 and a certain amount of
gold ions remained in the solution after 180 min. The UV–vis
spectra and TEM observations indicated that transformation
from gold nanowires to particles always coincided with disap-
pearence of the gold ions, indicating that gold ions played an im-
portant role in stabilizing the shape of gold nanowires during the
formation process. In order to verify the decisive effect of gold
ions on the shape stability, a TEM grid with as-synthesized
nanowires was treated with H2 gas at room temperature then ob-
served again. Figure 3 shows that the beautiful nanowires (Fig-
ure 3A) broke after the H2 treatment and larger particles were
formed (Figure 3B). This result indicates that the gold ions cap-
ped on the surface of gold nanowires were reduced by H2,11 re-
sulting in a breakdown of the nanowire shape.
#
1
2
Present address: Solution and Technology Dept. of AIR WA-
TER INC., Japan.
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3
4
5
ꢁ
The citrate reduction of AuCl4 has been used for synthesis
of gold nanoparticles.10 The sodium citrate acts as both reducing
agent and capping agent in the reaction. However, when the mo-
lar ratio of the sodium citrate/NaAuCl4 was reduced to 0.4 or
0.3, the preliminary gold nanoparticles formed by the chemical
6
7
ꢁ
reduction of AuCl4 were considered to be thermodynamically
unstable because of the insufficient capping of citrate.ꢁFurther-
more, as presented in the study by Biggs et al., AuCl4 can be
adsorbed preferentially onto the gold surface even in the pres-
ence of excess citrate ions and initiate an attractive force be-
tween the gold surfaces.12 Thus we can hypothesize that the for-
mation of gold nanowires was initiated by the jump in of gold
nanoparticles when they approached below 10 nm,12 followed
by deposition of newly-formed gold atoms onto the concave re-
gions of the connected particles. Successively, the relatively ex-
8
9
ꢁ
cess of AuCl4 adsorbed on the surfꢁace of nanowires maintain-
10 G. Frens, Nature (London), Phys. Sci., 241, 20 (1973).
11 C. K. Tan, V. Newberry, T. R. Webb, and C. A. McAuliffe, J.
Chem. Soc., Dalton Trans., 1987, 1299.
12 S. Biggs, P. Mulvaney, C. F. Zukoski, and F. Grieser, J. Am.
Chem. Soc., 116, 9150 (1994); J. F. Wall, F. Grieser, and C.
F. Zukoski, J. Chem. Soc., Faraday Trans., 93, 4017 (1997).
ing their shapes. When the AuCl4 had disappeared by the
reactions, the nanowires broke and aggregated into thermody-
namically stable gold nanoparticles. The formation mechanism
of gold nanowires and its relation to the gold ions are still under
investigation.
Published on the web (Advance View) February 14, 2004; DOI 10.1246/cl.2004.324