Synthesis of silver nanotubes by electroless deposition in porous anodic aluminium oxide templates

Article abstract of DOI:10.1039/b315931k

An electroless deposition method has been employed for the synthesis of silver nanotubes using porous anodic aluminium oxide as templates, by which high-yield silver nanotubes with length over ten microns have been synthesized.

Full text of DOI:10.1039/b315931k

Synthesis of silver nanotubes by electroless deposition in porous anodic
aluminium oxide templates
Shu-Hong Zhang, Zhao-Xiong Xie,* Zhi-Yuan Jiang, Xin Xu, Juan Xiang, Rong-Bin Huang and
Lan-Sun Zheng
State Key Laboratory for Physical Chemistry of Solid Surfaces & Department of Chemistry, Xiamen
University, Xiamen 361005, China. E-mail: zxxie@xmu.edu.cn; Fax: +86-592-2183047
Received (in Cambridge, UK) 8th December 2003, Accepted 17th March 2004
First published as an Advance Article on the web 5th April 2004
An electroless deposition method has been employed for the
synthesis of silver nanotubes using porous anodic aluminium
oxide as templates, by which high-yield silver nanotubes with
length over ten microns have been synthesized.
Therefore, the as-prepared nanotubes are Ag nanotubes. It should
be pointed out that the Ag nanotubes are unstable and collapse in a
few seconds under the irradiation of electron beam in the TEM
chamber, making it difficult to take high quality TEM images. The
collapse of Ag nanotubes is due to the re-crystallization of Ag
caused by the heat generated from electron beam irradiation, since
the silver nanotube may suffer from a high strain.
The presence of Ag nanotube can be further confirmed by using
X-ray diffraction (XRD). Fig. 2 shows the XRD pattern of the as-
prepared Ag nanotube. All the peaks can be attributed to the fcc Ag
(JCPDS Card No. 4–783) and are obviously broadened, owing to
the nanosize of the Ag crystals.
Fig. 3a shows the UV-visible absorption spectroscopy of as-
prepared products. There is an absorption peak around 390 nm,
which is caused by the surface plasmon resonance of one-
dimensional Ag nanostructures.8,9 The position of the absorption
peak is in agreement with the diameter of the Ag nanotubes. In Fig.
3b, a fluorescence spectrum of Ag nanotubes from the ethanol
suspension was presented, which shows a peak at 354 nm.
Luminescence of noble metal nanoclusters was previously assigned
to the radiative recombination of Fermi level electrons and sp- or d-
band holes.10 We speculate that the same basic mechanism also
works for the fluorescence of Ag nanotubes observed here.
Nanostructures with hollow interiors have been the focus of much
current research due to their potential uses in many areas. Such
materials may serve as extremely small containers for encapsula-
tion, as host materials to load catalysts, and as small templates to
build other nanostructural systems, etc..1–3 Owing to their unique
properties in comparison with their bulk counterparts,4 noble
metals of hollow nanostructures are of particular interest. Great
effort has been devoted to the synthesis of such one-dimensional
(1D) nanomaterials and there have appeared some successful
examples for the synthesis of various noble metal materials with
hollow nanostructures.3,5,6 To our knowledge, however, synthesis
of silver nanotubes has not been reported yet. In this communica-
tion, we report a high-yield synthesis of silver nanotubes by
electroless deposition in porous anodic aluminium oxide (AAO)
templates, a very simple, low-cost and green chemical method.
The AAO templates with ordered nanopore arrays used in our
work have a thickness of about 15 mm.7 Diameters of the
nanochannels in the templates range from 25 nm to 60 nm. Before
the electroless deposition, the AAO templates were first kept under
low pressure ( ~ 9.9 Torr) for more than 8 hours. Then the AAO
templates were immersed in a freshly prepared electroless bath
solution (pH = 8–9) containing silver complex (4.25 3 1023
M
+
Ag(NH3)2 ) and a reductant consisting of 2.27 3 1022 M glucose
and 2.67 3 1023 M tartaric acid. After 24 hours at the temperature
of 15–20 °C, AAO templates were taken out from the plating bath
and immersed in de-ionized water to remove the reactants, and then
the templates were dissolved in 4 M NaOH for more than 12 hours.
After dissolving the AAO templates, the raw products were
centrifuged and rinsed with de-ionized water several times. Finally,
black solids were collected and ultrasonically dispersed in ethanol
to get a suspended solution for further analysis.
Figs. 1a and 1b show the typical transmission electron micro-
scope (TEM, JEM-100CX) images of the products. As can be seen,
the products consist of a tubular 1D nanostructure. The nanotubes
spread uniformly over a narrow range of diameters, being about 25
to 60 nm for the outer diameters and 10–30 nm for the inner
diameters. The outer diameters correspond well to the pore size of
the AAO templates used, and it is found the outer diameters can be
adjusted by choosing other AAO templates of different pore size.
The nanotubes may grow over ten microns long with open tops. The
short nanotubes could be straight, while the long nanotubes are
usually bent. Further evidence of the nature of the nanotubes comes
from the high-resolution transmission electron microscope
(HRTEM, Tecnei F30) images. Figs. 1c and 1d show the HRTEM
images of the walls of nanotubes. In Fig. 1c, the fringe spacing
along the perpendicular direction to the long axis of the tube was
measured to be 0.21 nm; while that along the long axis of the tube
was measured to be 0.29 nm. The former corresponds to the
interplanar (100) distance; while the latter corresponds to the (110)
distances of Ag (fcc). Thus the long axis of the tube is along the
å110Å direction. In Fig. 1d, the fringe spacing of 0.24 nm was
measured along the direction perpendicular to the long axis of the
tube. This corresponds well to the interplanar (111) distance.
Fig. 1 (a, b) Typical TEM images of silver nanotubes synthesized by
electroless deposition in AAO templates. (c, d) HRTEM images of the wall
of silver nanotubes; the left sides are the core of the tube for their relatively
dark color. åhklÅ represents the [hkl] or its equivalent directions.
Fig. 2 The XRD pattern of the as-prepared silver nanotubes.
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Previous reports showed that a large quantity of alumina
nanotubes/nanowires was obtainable by etching porous alumina
membranes in an aqueous NaOH solution at room temperature.11
Thus special attention has to be paid not to mistake alumina
nanotubes for deposited metal nanotubes. In the present work,
about 5 times the calculated volume of NaOH solution was used to
dissolve the corresponding amount of AAO templates and the
dissolving time was kept long enough (more than 12 hours).
Template-directed synthesis of metal 1D nanostructures may be
the most promising route in terms of cost and productivity. Owing
to its hexagonally ordered and nearly parallel porous structures,
AAO has caused a great deal of excitement as template in the
synthesis of various nanorods.12–16 Some metal tubular structures
in micron or nanometer scale have also been successfully
synthesized.17–20 The key to the formation of these tubular
structures in templates was believed to be due to the presence of
“molecular anchors” on the pore wall.19 In the present work, no
external “molecular anchors” were introduced.
The reaction used for the formation of Ag nanotubes by the
electroless deposition in templates can be viewed as being
borrowed from the well-known Tollens’ test in sugar chemistry,
where the silver ion is reduced to metallic silver, which deposits in
the form of a mirror on the wall of the test tube.21 The detailed
mechanism of the formation of the nanotubes could be very
complicated,22 but we proposed that it may follow a self-catalytic
reduction route as described below. First, the pore wall of AAO
templates might have some defect (active) sites, serving as original
nucleation sites for the reduction of silver complex ions. Once the
nuclei are formed, the silver species serves as the precursor or
catalyst for the subsequent reduction of silver ions and the silver
grows in the form of tubes owing to the geometrical confinement of
the nano-channels. We infer that the subsequent reduced silver
atoms deposit onto the top of the tubes, resulting in 1D growth of
the tubes. The HRTEM images show that the long axis is along the
å110Å direction and the side walls are made of (111) and (100)
planes. This growth direction is the same as found for some Ag
nanowires.9 As it is known that the (110) facet has a relatively high
chemical potential, the growth rate on this facet should be much
faster than that on any other facets, which results in the preferential
growth of Ag nanotubes along the å110Å direction. The (111) and
(100) surfaces are the most and the next most stable surfaces,
respectively. Hence these surfaces form the ending walls of the
nanotubes. We believe that the preferential crystal growth of silver
is a controlling factor for the growth of Ag nanotubes. This
accounts for the observation that nanotubes might grow out of the
templates. The scanning electron microscope (SEM) image of a
cross section of a template after the electroless deposition for 2
hours (relatively short deposition time) shows that some nanotubes
have already grown out of the templates as marked by arrows in
Fig. 4.
Fig. 4 SEM image of a cross section of a template filled with silver
nanotubes, which shows some silver nanotubes have grown out of the AAO
template.
electroless deposition because the solutions enter small pores by the
capillary effect. The pH value is another key factor to obtain high
quality nanotubes. To avoid corrosion of AAO templates in strong
alkali or strong acid, the resulting pH value of the electroless bath
has to be adjusted to around 8 or 9.
In summary, high yield silver nanotubes with diameters
corresponding to the AAO template pore sizes and of length up to
tens of microns have been successfully synthesized using electro-
less deposition. This work should be of interest as a successful
example of the fabrication of silver nanotubes and the electroless
deposition in templates may be applied to the synthesis of
nanotubes of other metals.
This work is supported by the NSF of China (Grant No.
20021002, 20173046), the Ministry of Science and Technology of
China (Grant No. 2001CB610506, 2002CCA01600), the NSF of
Fujian Province (Grant No. E0310004) and the Fok Ying-Tung
Educational Foundation.
Notes and references
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Fig. 3 (a) UV-vis absorption spectra of an ethanol suspension of silver
nanotubes. (b) Room temperature fluorescence emission spectra of silver
nanotubes in ethanol at the exciting wavelength of 285 nm.
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