Published on the web June 5, 2010
717
Chemical Synthesis of Silver Nanowires Using N,N-Dimethyldodecylamine Oxide
Hideki Matsune, Yudai Kuramitsu, Sakae Takenaka, and Masahiro Kishida*
Department of Chemical Engineering, Graduate School of Engineering, Kyushu University,
744 Moto-oka, Nishi-ku, Fukuoka 819-0395
(Received April 9, 2010; CL-100346; E-mail: kishida@chem-eng.kyushu-u.ac.jp)
:
tertiary amines
This paper describes a new approach for the synthesis
of uniform silver nanowires (AgNWs) using zwitterionic
amphiphile, N,N-dimethyldodecylamine oxide (DDAO). Heat-
ing of AgCl and DDAO in the presence of silver nanoparticles
at 135 °C for 3 h produces longer AgNWs. The DDAO serves
as a source of both a reducing and capping agent in the
process.
growth
AgCl
+
135°C
(100)
(111)
DDAO
Ag nanoparticle
AgNW
Figure 1. Schematic illustration of the experimental procedure
used to produce AgNWs. Three-dimensional morphologies of
AgNW showing {111} end faces and {100} side faces. The
common fivefold axis of elongation is h110i.
In the past decade, considerable efforts have been devoted to
producing inorganic nanowires because of their unique proper-
ties and potential application in electronic, photonic, magnetic,
and sensing devices.1-5 Silver nanowires (AgNWs) especially
have attracted much attention in many technological areas
because of the high electrical and thermal conductivities of bulk
silver.6-8 The one-dimensional (1-D) silver nanostructures also
have unique optical and catalytic properties.7-9
Various chemical strategies have been applied for the
synthesis of 1D inorganic nanostructure.10 However, most
strategies reported for the synthesis of AgNWs have applied a
polyol method.7,11-13 The polyol method for AgNW synthesis
was originally demonstrated by Figlarz and co-workers through
the reduction of AgNO3 with ethylene glycol in the presence of a
protective agent, poly(vinylpyrrolidone) (PVP).14 Xia and co-
workers have improved the method to obtain uniform, longer
AgNWs in a high yield using a small amount of additives
(including PtCl2 or Fe(II)/Fe(III) species), and also revealed the
continuous, simultaneous injection of AgNO3 and PVP into the
vessel during the reaction was effective for the better controlling
of the resulting product morphology of AgNWs.6,7
Despite a number of reports for the standard polyol method
to produce AgNWs, attempts to find alternative AgNW
preparation methods have had limited success, although they
are needed in order to exploit their peculiar properties and
unique application. As capping agents instead of polymers,
amphiphilic molecules such as cetyltrimethylammonium bro-
mide or Aerosol OT have been used to prepare anisotropic
nanocrystals of silver or other metal;15-18 however, the resulting
morphologies were rod-like, and high aspect ratio wire-like
nanocrystals were scarce.
We report here the synthesis of AgNWs using N,N-
dimethyldodecylamine oxide (DDAO) for the first time. The
zwitterionic amphiphile, DDAO having a head group with a
dipole (N¼O) is used as a protective agent of Ag nanoparticles,
because the negatively charged oxygen atom of the head group
will show an affinity for metal surfaces. The DDAO molecules
also serve as a source of both a reducing and capping agent in
the heating process for the 1-D crystal growth of silver. AgCl is
used as a silver source of AgNWs. Our method can be conducted
at a relatively high concentration of silver (>350 mM) compared
to that of the standard polyol method (tens mM).
Our procedure for the synthesis of AgNWs is illustrated in
Figure 1. The AgNWs were produced by heating a mixture of
DDAO (0.7 g) and AgCl (20 mg) without solvent at 135 °C,
where a small amount of silver nanoparticles (<2 wt %) were
contained in order to promote the seed-mediated growth of
silver.24 Upon heating, the DDAO immediately changed to oil,
and then gray, silvery precipitate of AgNWs gradually appeared
in the oil. After 3 h, the reaction vessel was cooled to room
temperature. The precipitate was collected and washed repeat-
edly with CHCl3 on a filter (0.45 ¯m in pore size) to remove
by-products such as nanoparticles and organics (The purified
product contained <5 wt % organics, as estimated from TG
analysis.). A weight of the purified AgNW was 17 mg, which
corresponds to a yield of 84% based on AgCl addition.
Figures 2a and 2b show the scanning electron microscopy
(SEM) images of the product prepared by our procedure. A large
number of wire-like nanostructures more than tens of micro-
meters in length were found through the SEM observation. The
nanowires have a smooth surface and a uniform diameter (ca.
70 nm) along the length, corresponding to an aspect ratio of
more than ca. 1000. Figure 2c shows the powder X-ray
diffraction (XRD) pattern of the nanowires. The peaks at 38.1,
44.3, 64.5, and 77.5° were assigned to the diffraction of a face-
centered cubic ( fcc) unit cell. The unit cell parameter
(a = 0.409 nm) calculated from the peak positions corresponds
closely to the value reported for bulk silver (a = 0.4086 nm,
JCPDS file No. 04-0783). The XRD pattern shows no AgCl
peaks (silver source), indicating its complete reduction to
metallic silver. In addition, the higher intensity ratio of 4.3 for
the (111) to (200) peaks (as compared to that of 2.5 for the bulk
silver), indicates enrichment of the (111) crystalline plane in the
AgNWs. Figure 2d shows the scattering spectrum obtained from
the AgNW powder embedded in a thin quartz cell. The spectrum
shows a broad peak with a maximum at 388 nm and a shoulder
at 365 nm from the transverse plasmon modes of the 1D
nanostructure of silver.19 This differs from a plasmon peak of
around 420 nm for spherical silver nanoparticles tens of nano-
meters in size. This also indicates that the wire-like nano-
structures shown in Figures 2a and 2b were composed of
metallic silver.
Chem. Lett. 2010, 39, 717-719
© 2010 The Chemical Society of Japan