Materials Letters 64 (2010) 2652–2654

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Materials Letters
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t

Facile synthesis of networked gold nanowires based on the redox

characters of aniline
Xu Fan a,1, Lianke Liu b,1, Zhirui Guo c, Ning Gu a,⁎, Lina Xu a, Yu Zhang a
a
State Key Laboratory of Bioelectronics and Jiangsu Laboratory for Biomaterials and Devices, Southeast University, Nanjing 210096, China
b
Cancer Center, The First Affiliated Hospital of Nanjing Medical University, Nanjing 210029, China
c
Research Institute of Cardiovascular Disease, The First Affiliated Hospital of Nanjing Medical University, Nanjing 210029, China

a r t i c l e i n f o a b s t r a c t

Article history: Metal nanowires are often used in the study of electron transport properties, electronic device technology
Received 7 July 2010 and many other fields. We have reported a facile while effecting an approach to networked gold nanowires
Accepted 26 August 2010 by taking advantage of the redox characters of aniline at different pH values. The effects of different synthetic
Available online 8 September 2010
parameters, such as mole ratio of HAuCl4 to aniline, pH value, on the shape and size of the final gold products
are investigated. Based on the experimental data, a possible growth mechanism is also proposed. This work
Keywords:
Crystal growth
probably contributes to not only the fundamental research of shape-controlled nanostructures but also new
Nanomaterial methods of producing nanoscaled metal wires for nanodevice study.
© 2010 Elsevier B.V. All rights reserved.

1. Introduction and sodium hydroxide was purchased from Shanghai Jiuyi Chemical
Ltd. Both of them were used as received. The networked nanowires
Metal nanostructures have been quite attractive due to its unique were synthesized as following. Briefly, the chloroauric acid was
optical [1,2], electronic [3,4], magnetic [5] and other properties. And the dissolved in water at 25 μM with pH adjusted by sodium hydroxide to
physical and chemical properties are highly dependent on the shape of 7.0, and heated to 80 °C during a course of 30 min. To 100 ml of this
the nanostructures [6,7]. As a result, shape-controlled synthesis of metal solution, 1 ml of 7.5 mM aniline in water was added under vigorous
nanostructures is becoming one of the hottest fields in the recent years. stirring. The solution was stirred at 80 °C for 2 h. Then the resulting
Metal nanowires are often used in the study of the electron transport suspension was centrifuged several times and the rinsed samples
properties and electronic device technology [8,9]. Many methods to were redispersed in DI water for further characterization.
obtain metal nanowires reported before were based on physical method The resulting gold nanostructures were characterized by TEM
which is costly and greatly depends on the apparatus [10]. As a contrast, (JEM-2000EX, JEOL) operating at 120 kV. HRTEM images and SAED
wet chemical method plays an important role in shape-controlled pattern were characterized by JEOL JEM-2010 TEM operating at
synthesis of nanostructures in virtue of high output, multiplex routes 200 kV. XRD spectra was recorded by D/Max RA circumgyration X-ray
[11–13], less cost, etc. Hence, to develop a facile wet chemical synthesis diffraction instrument (Cu target, λ = 1.5405 Å). Ultraviolet-visible-
method of metal nanowires will be of great significance. near-IR (UV–vis–NIR) absorption spectra were recorded by a
Herein, based on the redox characters of aniline at different pH SHIMADZU UV-3150 spectrophotometer.
value, we reported a facile while effective method of synthesizing
networked gold nanowires by aniline reducing chloroauric acid. This 3. Results and discussion
work will contribute to the fundamental research of shape-controlled
nanostructures besides developing new methods to produce nanos- During the addition of sodium hydroxide, the chloroauric acid
caled conducting wires for nanodevice study. solution faded gradually. After the aniline was added, the reactant
turned to salmon pink immediately and then gray blue bit by bit. It is
2. Materials and methods showed distinctly by the TEM micrograph that the nanostructure is
crooked nanowires with a 2D networked fabric (Fig. 1a). HRTEM image
Aniline was purchased from Shanghai Lingfeng Chemical Ltd. and of part of the nanowire reveals that it comprises caudate gold
was purified by atmospheric vacuum distillation. Chloroauric acid nanoparticles with crystal facets in different directions, and gold
HAuCl4•4H2O was obtained from Shanghai NO.1 Chemical Factory atoms depositing in the clearance of particles to form nanowires
(Fig. 1e). Corresponding SAED pattern shows the annular diffraction
⁎ Corresponding author. Tel.: + 86 25 83272476; fax: + 86 25 83272476.
spots of {111}, {200}, {220}, {311} and {420}, validating the nanowires
E-mail address: guning@seu.edu.cn (N. Gu). have a polycrystalline structure (Fig. 1b). Fig. 1c throws out the identical
1
The authors contributed equally to this work. powder XRD pattern of the gold nanostructures. The five Bragg

0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.matlet.2010.08.072
 X. Fan et al. / Materials Letters 64 (2010) 2652–2654 2653

Fig. 1. (a) TEM image, (b) the corresponding SAED pattern, (c) XRD pattern, (d) UV–vis–NIR absorption spectra, and (e) HRTEM image of the gold nanowires obtained at 80 °C, pH
7.0. (f) TEM image of the paramecium-like gold nanoaggregates obtained at 80 °C, pH 5.6. The inset of (f) is a magnified image of the paramecium-like gold nanoaggregates, bar
50 nm.

reflections clearly correspond to the face-centered cubic structure of able to be oxidized to N-phenyl-1, 4-benzquinonediimine (C6H5―N C6-
metallic gold (JPCDS file, card No. 04-0784). This has already been H4 NH, PBQ) in the whole pH range, but only when pHb 2, PBQ could
confirmed by the SAED pattern for the sample as shown in Fig. 1b. The continue being oxidized to polyaniline [18,19]. As reported before [20], in
broad X-ray reflection peaks testify the wires were consisted of small situ produced polyaniline is able to effectively adsorb on the {111} facets
gold particles [14]. The mean diameter of the nanoparticles is calculated and prevents growth on these facets through its multiplex nitrogenous
to be about 6 nm from the line width of the {111} plane diffraction groups, while PBQ does not have these properties. Based on the characters
according to Scherre equation [15]. of aniline and the results of the experiment above, we deem that the
When sodium hydroxide was added, [AuCl4]− reacted with OH− to nanowires formed follow this principle, as shown in Fig. 2. Firstly, PBQ
[Au(OH)nCl4-n]− [16] and the color of the solution changed as a result. created by oxidation of aniline will not continue polymerizing with aniline
We have studied the resultant nanoparticles when pH of the reaction to form polyaniline because the pH of the system is 7, which is much
system was 5.6 and 9.0 for contrast. When the pH was 5.6, the reaction higher than 2. Thus, no polymer preferentially adsorbs any existent facets
speed increased obviously. As a result, amounts of gold crystal nuclei and the resultant nanoparticles run to spheroids, which is stable in
were produced in a short time and aggregated quickly. In this condition, thermodynamics. In this pH, there are little [AuCl4]− and lots of [Au
paramecium-like nanoaggregates were produced (Fig. 1f). But when pH (OH)nCl4 − n]− in the solution, and [AuCl4]− is constantly reduced to gold
was 9.0, no reaction was observed. We conclude that the reduction crystal nuclei. At the same time, aniline is oxidized to PBQ and releasing
potential of [Au(OH)nCl4 − n]− is much lower than [AuCl4]−. H+, which sets [AuCl4]− free from [Au(OH)nCl4 − n]−. Then, [AuCl4]− goes
In our synthesis system, aniline serves as reducing agent as well as on being reduced to gold atoms. However, it needs a long time for the pH
stabilizer which supplies positive surface charge of gold nanostructures of the solution to decrease in less than 2 despite of the releasing of H+.
[17]. According to the observation of Gospodinova Terlemezyan, aniline is Thus, the main components of the solution are PBQ and remanent aniline,

Fig. 2. Schematic of the possible formation process of the networked gold nanowires.
2654 X. Fan et al. / Materials Letters 64 (2010) 2652–2654

Fig. 3. TEM images of gold products obtained at pH 7.0 with molar ratio (aniline to gold) of (a) 6:1, (b) 1.5:1 and (c) 0.5:1, respectively.

whose adsorption ability is much lower than polyaniline. The gold the reaction. Then, the gold crystal nuclei assemble to lines and the
nanoparticles are unstable due to small size and weak stabilizing ability of slowly later-born gold atoms filled in the concaves between neighbor
PBQ and aniline, and these unstable particles aggregate to linear assembly nuclei. Finally, networked gold nanowires are obtained. In this synthesis
on account of brownian motion and van der waals attractive force. The system, the reaction properties of aniline are used effectively. This work
linear assembly process is also described in Fig. 2. Firstly, the dimmers of provides a convenient approach to synthesize networked gold nano-
gold nanoparticles are formed, which induce the asymmetric distribution wires, and probably contributes to the study of electron transport and
of the surface charges on the nanoparticles. That is, the surface charge electronic devices. Moreover, the work is an example of designed
density at the joint of a dimmer is much higher than that of the other parts nanoscaled synthesis strategy and may arouse other inspirations in this
of a dimmer. Thus, a third particle tends to attack to the head or end face and other fields.
but not side face. This process continues and results in the formation of
linear assemblies of gold nanoparticles. At the same time, the concaves
Acknowledgments
between neighbor particles are filled with later-born gold atoms.
Afterward, particles in lines form wires [21,22], and at last, nanowires
This work is supported by the National Natural Science Foundation
with a 2D networked fabric are produced.
of China (NSFC, 30870679, 30970787, 81000664) and China Postdoc-
To further substantiate the condition of producing gold nanowires in
toral Science Foundation (20090451236).
this system, we studied the resultant nanostructures when changing the
quantum of aniline in the original system. We doubled and halved the
quantum of aniline while other reactants that remain unchanged, and References
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