Growth of silver nanowires on metal plates by conventional redox displacement

Article abstract of DOI:10.1016/S0009-2614(03)00786-3

Silver nanowires (diameter 20-50 nm, length of the order of μm) have been synthesized in a high yield from nitric acid-treated iron plates dipped in a silver nitrate solution of an appropriate concentration. The formation of the silver nanowires is discussed in terms of silver nitrate concentration, substrate surface morphology and elemental nature of substrate.

Full text of DOI:10.1016/S0009-2614(03)00786-3

Chemical Physics Letters 374 (2003) 645–649
www.elsevier.com/locate/cplett
Growth of silver nanowires on metal plates by
conventional redox displacement
*
Zhi-Yuan Jiang, Zhao-Xiong Xie , Shu-Hong Zhang, Su-Yuan Xie,
Rong-Bin Huang, Lan-Sun Zheng
State Key Laboratory for Physical Chemistry of Solid Surfaces and Department of Chemistry, Xiamen University,
Siming Nanlu 422, Xiamen 361005, China
Received 7 January 2003; in final form 7 May 2003
Abstract
Silver nanowires (diameter 20–50 nm, length of the order of lm) have been synthesized in a high yield from nitric acid-
treated iron plates dipped in a silver nitrate solution of an appropriate concentration. The formation of the silver nanowires
is discussed in terms of silver nitrate concentration, substrate surface morphology and elemental nature of substrate.
Ó 2003 Elsevier Science B.V. All rights reserved.
1. Introduction
A recent investigation of the preparation of
silver nanowires without the use of a template
used nanocrystalline AgBr and a developer with
AgNO3 component as precursors [15]; others
[16,17] prepared silver nanorods by using wet
chemical methods. Silver nanorods have also been
prepared by an electrochemical technique [18] and
by UV photoreduction in the presence of poly-
mers and other materials [8,19–21]. In an exten-
sion of these studies, we present our research on
the synthesis of silver nanowires in which we have
used a conventional redox displacement reaction
that uses neither a porous template nor a soft
template.
One-dimensional nanostructural materials such
as nanowires, nanotubes and nanorods are of
fundamental and technological interest as they
find applications in conductance quantization and
nanodevices [1–6]. Of particular interest are silver
nanowires owing to their exceptionally high con-
ductance, and numerous advances have been made
in the fabrication silver nanowires materials in a
variety of nanofabrication and crystal growth
methods [7–10]. More accessible methods such as
electrochemical and chemical reduction syntheses
generally employ a template for the exclusive
growth of nanowires in the direction normal to the
substrate surface [11–14]; however, the removal of
the template poses practical problems.
2. Experimental
In a typical redox experiment, a 1 ꢀ 1 cm2 Fe or
* Corresponding author. Fax: +86-592-2183047.
E-mail address: zxxie@xmu.edu.cn (Z.-X. Xie).
Cu plate was first treated by dilute HNO3 (0.02 M)
0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0009-2614(03)00786-3

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Z.-Y. Jiang et al. / Chemical Physics Letters 374 (2003) 645–649
and then immersed in 800 ml AgNO3 solution
(5 ꢀ 10ꢁ4 M) at room temperature. After 1 h, the
material growing on surface was collected, washed
with water and ethanol, and dried in vacuum. The
material was characterized by powder X-ray dif-
fraction (XRD) on a Rigaku D/Max-rC X-ray
diffractometer equipped with Cu-Ka radiation.
The transmission electron microscopy (TEM) im-
ages were recorded with a JEM-100CX II micro-
scope at an accelerating voltage of 100 kV, and
scanning electron microscopy (SEM) images with
a JEM5600LV microscope (JEOL) operating at
15 kV. The UV–Vis absorption spectrum was
recorded on a UV-2501 PC (SHIMADZU) in-
strument.
3. Results and discussion
3.1. Characterization of silver nanowires
More than 90% of the product of the redox
reaction of the Fe plate that had been immersed in
a 5 ꢀ 10ꢁ4 M AgNO3 for 1 h exists as silver
nanowires with diameter of 20–50 nm and length
of the order of lm (Fig. 1a). Nanowires longer
than 10 lm could be grown by this procedure
(Fig. 1b). Of the remaining 10%, some were
branched in a tree motif. A minor amount of
nanoparticles was also present that were probably
the precursor of the nanowires. The nanowires are
single-crystalline, as supported by the electron
diffraction pattern (inset in Fig. 1a) recorded by
directing the electron beam on to a selected posi-
tion of a particular nanowire (marked by the ar-
row). The electron diffraction pattern suggested a
face-centred cubic (fcc) Ag crystallite as the elec-
tron beam was set perpendicular to the (1 1 1)
plane of the crystallite. The diffractogram (Fig. 1c)
is interpreted in terms of a fcc Ag phase as
it matches that the JCPDS standard (JCPDS
4-0783). The diffractogram did not suggest the
presence of possible impurities, such as Ag2O,
AgNO3 and Fe.
Fig. 1. (a) TEM image of the as-prepared silver nanowires from
a 5 ꢀ 10ꢁ4 M AgNO3 solution. Inset: electron diffraction pat-
tern recorded by directing the electron beam at a selected point
(arrowed) of the nanowire. (b) TEM image of a silver nanowire
that is more than 10 lm. (c) XRD pattern of as-prepared silver
nanowires.
The nanowires displayed a UV–Vis spectrum
similar to those reported [9,17,18,22,23]. The UV–
Vis spectrum of a suspension of as-prepared silver
nanowires in ethanol showed a broad peak at ca.
390 nm (Fig. 2) that could arise from surface
plasmon excitation of the silver nanostructures
[22,23].

Z.-Y. Jiang et al. / Chemical Physics Letters 374 (2003) 645–649
647
Fig. 2. The UV–Vis absorption spectrum of a suspension of
silver nanowires in ethanol.
3.2. Parameters influencing the formation of the
nanowires
3.2.1. AgNO3 concentration
The optimum concentration for the preparation
of the silver nanowires fell into a narrow range,
from 5 ꢀ 10ꢁ4 to 1 ꢀ 10ꢁ3 M (for the Fe plates) as
the higher concentrations led to the formation of
unwanted dendrites and some much larger nano-
wires. At 1 ꢀ 10ꢁ3 M, the nanowires exhibited a
diameter of 40–100 nm, as characterized by the
TEM image (Fig. 3a); however, the use of higher
concentration led to a reduction of yield by 20–
30%. A comparison of the product from 5 ꢀ 10ꢁ4 M
solution (Fig. 1) with the product from the 1 ꢀ 10ꢁ3
M solution reveals the presence of nodes on the
nanowires at the higher concentration (Fig. 3a). At
much higher concentrations (e.g., 5 ꢀ 10ꢁ3 M),
dendrites were formed (Fig. 3b). On the other hand,
lowering the concentration (e.g., 1 ꢀ 10ꢁ4 M) led to
irregularly shaped nanoparticles (Fig. 3c), their si-
zes ranging from 3 to 50 nm (average about 20 nm).
The uniformity of the nanowires was ensured by
the use of a constant concentration of AgNO3, and
this was effected by using a large volume (800 ml) of
the reagent. Uniform nanowires could be prepared
in about an hour.
Fig. 3. TEM images of silver nanostructures grown on
the surface of an Fe plate that had been immersed in an
AgNO3 solution. (a) 1 ꢀ 10ꢁ3 M, (b) 5 ꢀ 10ꢁ3 M, and (c) 1 ꢀ
10ꢁ4 M.
treatment affected the yield. For example, an
Fe plate that had been only polished mechani-
cally (SEM image shown in Fig. 4a) when reacted
with 5 ꢀ 10ꢁ4 M AgNO3 afforded a filmy product
with a thickness of a nanometer scale along with
nanoparticles. The surface of the plates after the
acid treatment (SEM image shown in Fig. 4b)
was much rougher; a rougher surface appears to
improve the yield of the nanowires.
3.2.2. Surface morphology of the metal substrate
The plates used in the synthesis of nanowires
had to be nitric acid-treated plates as such a

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Z.-Y. Jiang et al. / Chemical Physics Letters 374 (2003) 645–649
number of active sites (such as surface defects)
are probably present, which should expedite the
reduction of Agþ and also serve as nucleation
sites for the reduction of Agþ. Therefore, much
more Ag nuclei should be formed on the treated
Fe surface. The silver nuclei, Fe plate and the
AgNO3 solution comprise primary cells: the Ag
deposits continuously onto silver islands while
Fe simultaneously goes into solution as Fe2þ
(or Fe3þ). For the reaction Fe + Agþ ! Fe2þ (or
Fe3þ) + Ag#, the difference of the electropotential
is large enough to give rise to a non-equilibrium
system at a high concentration of Agþ. Non-
equilibrium conditions generally leads to fractal
growth and the formation of dendrites [24]. This is
shown by the dendrites (Fig. 3b) that are formed
at a 5 ꢀ 10ꢁ3 M concentration; however, if the
concentration of Agþ ions is drastically lowered,
the deposition of the Ag would deplete all the Agþ
ions near the surface of the Fe plate. Those Agþ
ions that diffuse from the bulk solution will move
only to the top of the silver particles to result in
the formation of one-dimensional wire; the wires
do not grow laterally. In one case, the growth of
the dendrites is restricted, but nodes on the
nanowires could still be seen (Fig. 3a). The uni-
formity of the silver nanowires parallels the de-
crease of the Agþ concentration (Fig. 1). The
electron diffraction pattern (Fig. 1a) shows the
growth of the Ag nanowire in the [1 1 0] direction.
As the nanowires crystallize in the fcc phase, the
(1 1 0) face is expectedly a less compact face that
must be more chemically active in the growth
process. As the reduced Ag atoms preferentially
deposit on this face, one-dimensional growth in
the [1 1 0] direction takes place. If, on the other
hand, the concentration of Agþ ions is too low,
the reaction and deposition rate will be corre-
spondingly low, so that the growth is hindered
even before the particles can grow into the wires
as they can adsorb impurities or react with the
oxygen in solution.
Fig. 4. SEM image of the surface of an Fe plate that had been
(a) polished mechanically, and (b) treated with dilute HNO3
solution.
3.2.3. Elemental nature of the metal plate
The yield of the nanowires was highest with Fe
compared with Cu, Al and Mg. For example, only
a maximum yield of 10% could be obtained with
Cu at all concentrations. In fact, at low concen-
trations, the product comprised different nano-
structures but few silver nanowires whereas at high
concentration, large visible crystallites were
formed. Mg and Al plates gave only a minor
quantity of Ag dendrites.
3.3. Mechanism of formation of the nanowires
In the displacement reaction between a metal
and Ag, the metal loses electrons and goes into the
solution as ions; the Agþ ions in the solution then
accept the electrons and Ag metal is deposited on
the surface of the metal. The process is represented
by the following redox equation:
On a smooth surface that is more even, the Ag
aggregates cover almost the entire surface by the
displacement reaction, but this prevents the further
reduction of Agþ ions. When the reaction is al-
lowed to proceed for a longer time, a film of silver
is formed along with particles of silver. When
M þ nAgþ ¼ Mnþ þ nAg
On the rough surface after the Fe plate had
been treated by dilute HNO3 solution, larger

Z.-Y. Jiang et al. / Chemical Physics Letters 374 (2003) 645–649
649
mechanically polished Fe plates are used, silver
films rather than nanowires are formed.
of Education, and the Natural Science Foundation
of Fujian Province.
Ideally, a redox reaction requires two metals
whose potentials are different. However, in a
practical situation, in addition to this requirement
of a potential difference of the standard electrode
potentials, the over-potential and concentrations
of the different ions also affect the reaction. The
relatively high electropositivity of Fe permits the
redox reaction in non-equilibrium condition to
take place over a wide range of concentrations of
Agþ ions, an outcome that can be exploited for the
production of nanowires. However, with a less
active metal such as copper, the process of build-
ing up a non-equilibrium system is difficult to
control owing to the smaller electrode potential
difference, and the structures and sizes of the
product are difficult to predict. Finally, with either
magnesium or aluminium, both of which are much
more active than iron, the redox displacement
takes place at an elevated rate. Although non-
equilibrium growth builds up readily for the
growth of the dendrites, the metal plates are
themselves very active and they react with rapidly
water or air, so that the part of the surface is
quickly coated with the metal oxide.
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