Preparation of free-standing bamboo-like Ni nanowire arrays

Article abstract of DOI:10.1246/cl.2009.394

Free-standing bamboo-like Ni nanowire arrays with periodic branches have been produced by electrochemical deposition in the pores of layer-by-layer anodic alumina membrane. This method could provide a possible approach to prepare free-standing nanostmctur

Full text of DOI:10.1246/cl.2009.394

394
Chemistry Letters Vol.38, No.5 (2009)
Preparation of Free-standing Bamboo-like Ni Nanowire Arrays
Wen Jun Zheng, Guang Tao Fei,ꢀ Biao Wang, and Li De Zhang
Key Laboratory of Materials Physics and Anhui Key Laboratory of Nanomaterials and Nanostructures,
Institute of Solid State Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences,
P. O. Box 1129, Hefei 230031, P. R. China
(Received January 5, 2009; CL-090026; E-mail: gtfei@issp.ac.cn)
Free-standing bamboo-like Ni nanowire arrays with peri-
dally from 23 to 53 V over 30 s, and then it decreases linearly
from 53 to 23 V over 3 min.19 After the remaining aluminum
and alumina barrier was removed, a layer of Au film was sput-
tered onto one side of the AAM to serve as the working elec-
trode.
odic branches have been produced by electrochemical deposi-
tion in the pores of layer-by-layer anodic alumina membrane.
This method could provide a possible approach to prepare
free-standing nanostructures of various functional materials with
great self-support ability and large surface area in practical ap-
plications.
In our experiment, the electrolyte for Ni nanowire prepara-
.
6.2 g/L H3BO4, and 125 g/L C6H5Na3O7 2H2O. The electrode-
tion was a mixture of 105 g/L NiSO4 6H2O, 5.8 g/L NaCl,
.
position was then performed for 4 h with a constant current of
0.5 mA. To testify the self-support ability of the nanowire arrays,
the AAM was totally removed by 1 M NaOH solution. The mor-
phologies of the AAM and nanowire arrays were observed by
field-emission scanning electron microscopy (FE-SEM, Sirion
200).
It is well-known that Ni, generally prepared by electrodepo-
sition and chemical liquor phase preparation, exhibits attractive
abilities in a variety of applications including hydrogen storage,
solid oxide fuel cells, and electrochemical oxidation of organic
molecules like methanol, ethanol, cyclohexanol, and glucose.1–8
Compared with Ni in bulk and film traditionally used with low
specific surface area, 1-D nanostructure arrays, which have at-
tracted enormous research interest recently, possess much higher
specific surface area.9–14 Among various approaches, the fabri-
cation of large-area and well-ordered Ni nanowire arrays in an-
odic alumina membrane (AAM) is outstanding.15–18 Usually, the
AAM needs to be removed for achieving as large a surface area
exposure as possible. However, without the support of AAM the
neighboring nanowires would assemble when their aspect ratio
(the length to width ratio) reaches a certain limit. This could
make the surface area of nanowires lower and the liquid harder
to flow into the interspaces of the structure especially for the
electrochemical oxidation, which may probably hinder the nano-
wire arrays from practical application to some extent. Therefore,
to make a well-standing nanowire array with great self-support
ability, adequate separation and without agglomeration becomes
very important in either experiment or practical application, and
it is still a challenge to achieve this.
In this paper, we prepared free-standing bamboo-like Ni
nanowire arrays of periodic branches using layer-by-layer
AAM by electrochemical deposition.19 Even if there is a rela-
tively high aspect ratio that the branches of each nanowire lean
against the neighboring one would still make the whole array
stand well with AAM being removed. Furthermore, the well-
aligned array could give rise to much more exposure of nanowire
surface area, which plays an important role especially in the
electrochemical oxidation for liquid which can easily and suffi-
ciently contact the surface of almost each nanowire. It is antici-
pated that our method will motivate the study of preparing prom-
ising free-standing nanomaterials network with great self-sup-
port ability and large surface area.
Figure 1a shows the profile of layer-by-layer branched
AAM, anodized for about 12 h in the second oxidation, with
main channels and branched pores growing in a fixed spatial or-
der in the direction perpendicular to the surface of AAM.
Figure 1b is the side view of Ni nanowire arrays with AAM com-
pletely removed. We can observe that the nanowires have suc-
cessfully replicated the structure of the AAM. Along the main
channel, each nanowire has periodic branches that are perfectly
natural brackets separating one nanowire from the neighboring
ones, which guarantees enough room between these nanowires,
and plentiful surface area exposed to air. The periodic appear-
ance of branches on nanowires corresponds to the periodic
occurrence of branched pores of the channels in our unique
AAM. Figures 1c and 1d are large-scale SEM images of nano-
wire array in side view and top view, respectively. It can be seen
that the whole array stands upright very well even without the
support of AAM, in sharp contrast to straight nanowire prepared
using ordinary AAM with cylindrical pores. The length of the
nanowires can be adjusted by varying the electrodeposition time.
Even when the aspect ratio reaches about 60:1 seen from
Figure 1c, the array still possesses great self-support ability as
At first, high-purity aluminum foils (99.999%) were
annealed and electrochemically polished. The layer-by-layer
branched AAM was prepared by adjusting the anodizing voltage
periodically during electrochemical anodization. A cell voltage
period procedes as follows: the voltage first increases sinusoi-
Figure 1. The SEM images of layer-by-layer AAM (a), and
free-standing bamboo-like nanowire array in side view (b), (c)
and top view (d).
Copyright Ó 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.5 (2009)
395
Figure 3. The SEM image of bamboo-like nanowire array pre-
pared under a comparatively high electric current.
Figure 2. The schematic illustration of the growth process of
bamboo-like nanowires.
open another possibility in the fabrication of free-standing nano-
structure with great self-support ability and large surface area
exposure out of a variety of materials, even metallic oxide by
using sol–gel methods. We believe this could bring benefits in
many applications, such as electrocatalytic oxidation, surface-
enhanced Raman scattering, surface plasmon resonance, and
photocatalysis. It is expected that our solution will develop the
large-area nanostructures of functional materials in practical ap-
plications.
a whole. This high aspect ratio could possibly enhance the elec-
troactivity of Ni nanowire arrays as a catalyst in electrocatalytic
oxidation.
Figure 2 schematically illustrates the growth process of
nanowires. Initially, by receiving electrons, Ni2þ ions become
Ni atoms being deposited into the main channels, and the stem
of nanowires grows as ordinary straight wires do (Figure 2a).
When the top of a nanowire reaches the branching position of
the AAM, the branches of nanowires begin to stretch out forward
into the branched pores and meanwhile the stem continues to
grow upward (Figure 2b). At the preliminary stage, the tips of
these neonatal branches heading to the branched pores develop
dimensionally in all directions owing to the loss of confinement
from the walls of pores (Figure 2c). The deposition of Ni atoms
at the top of nanowires and at the brim of branched pores occurs
simultaneously. In this case, the Ni grows into the branched
pores gradually (Figure 2d). Meanwhile, Ni2þ ions in main chan-
nels of AAM diffuse into branched pores to balance the differ-
ence in concentration. Since the supplement could not satisfy
the exhaustion of Ni2þ ions in branched pores in time, the growth
speed of stem is higher than that of branches. With the entrance
to the branched pores closed down by the stem and the left Ni2þ
ions exhausted, the branches will finally stop growing. Subse-
quently, the nanowires will still develop along the main channels
(Figure 2e). With several deposition periods being repeated, the
nanowires successfully replicate the geometry of channels in our
layer-by-layer AAM.
This work was supported by the National Natural Science
Foundation of China (Nos. 50671099, 50172048, 10374090,
and 10274085), Ministry of Science and Technology of China
(No. 2005CB623603), and Hundred Talent Program of Chinese
Academy of Sciences.
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Published on the web (Advance View) March 28, 2009; doi:10.1246/cl.2009.394