The effect of etching parameters for removal of AAO template as
Cu2O/Cu nanowires fabricated by electrochemical deposition
Y. H. Leea, I. C. Leub, M. T. Wua, J. H. Yena, M. H. Hon aand K. Z. Funga*
a
Department of Materials Science and Engineering
National Cheng Kung University
b
Department of Electronic Engineering
Kun Shan University of Technology
a
No.1 Ta-Hsueh road, Tainan 701
Taiwan
Abstract: -This article reports our recent results of electrochemically deposited Cu 2O/Cu nanowires by AAO
template assisted method. The deposited Cu2O/Cu nanowires were observed by SEM and TEM under different
removal condition. The results indicated that the composition and surface morphologies strongly affected by
the condition as the removal of AAO template. We find that the well-controlled condition of AAO removal is a
key point to maintain the nano-structure. The optimum condition for AAO removal is decided as pH = 12 of
NaOH solution.
Key-Words: - Cu2O, Cu, Electrochemical deposition, Nano-wires, AAO template
1
Introduction
Recently, the nano-sized materials have received
much attention due to their excellent physical and
chemical properties. The electrochemical deposition
was regarded as an inexpensive and easy method to
obtain nano-structure without post heat-treatment
and vacuum. The AAO （ Anodized Aluminum
Oxide ） template assisted method is a newly
developed nano-fabrication technique for aligned
nano-arrays. Our group has reported several results
of the combination of electrochemical deposition
and AAO template assisted method【1-4】. Cu2O and
its related material has been noticed for its particular
optical and electrical properties【5-7】. Furthermore,
Cu2O has also been reported to be a promising
anode material for lithium ion battery【8-10】. We
successfully synthesized Cu2O/Cu composite
nanowires by electrochemical deposition combined
with the assistance of AAO template. After the
removal of AAO template by NaOH, it was found
that the morphologies of nanowires were pH
dependent.
2
Experimental procedure
2.1 AAO template preparation
The high purity Au and Al films were deposited on
to Si wafer by E-beam evaporation. The anodization
of Al was first performed in a bath consisting of
0.3M oxalic acid solution under 40V at 13℃. After
removing the oxide in a mixture of 12wt% H3PO4
and 6wt% H2Cr2O4, the remaining Al film was again
oxidized till it was all converted to alumina. The
samples were then immersed into 5wt% H3PO4
solution in order to remove the barrier layer in the
Al/Au interface. The evaporated Au layer was used
as a conductive layer for the electrochemical
deposition.
2.2 Deposition parameters
The Cu2O/Cu nanowires were synthesized from an
aqueous solution prepared from 0.6M Cu（Ⅱ）and
3M lactate that contained copper （ Ⅱ ） sulfate
pentahydrate （ Showa ） and lactic acid
（Riedel-deHaën）. After adding certain amount of
5M NaOH, the solution was then stirred overnight
with a magnetic stirplate. The stabled solution was
then adjusted to desired pH 10. The electrochemical
deposition system employed is a conventional
three-electrode cell. An AAO mounted by epoxy
was used as working electrode. The counter
electrode is Pt foil while an Ag/AgCl was used as
reference electrode in a 3M NaCl solution. The
deposition
was
performed
with
an
potentiostat/galvanostat （EG&G, Model 273A）
under a constant current density. The current density
was set at 0.05 mA/cm2. The deposition was
terminated when the channels of AAO were filled.
The deposited specimen was then cleaned with
deionized water to remove the remaining impurities.
2.3 The removal of AAO template
The deposited samples were immersed in NaOH
solution to remove AAO template. The pH of NaOH
varied from 11.0 to 13.5. The duration of immersion
was determined to be 30 minutes. After immersion,
the specimens were rinsed by deionized water to
clean remaining NaOH.
2.3 The morphological observation and
structural analysis
The porous alumina substrate and deposited
nanowires were then observed by Scanning Electron
Microscope（Philips, XL-40FEG） and Transmission
Electron Microscope （JEOL, JEM-3010）. The
structural analysis was conducted using TEM and
glancing angle x-ray diffractometer （ D-Max,
Rigaku）.
3
Results and discussion
Shown in Fig.1 is the top-view SEM observation of
an AAO template. From the micrograph, the
well-aligned porous alumina were fabricated onto
the Au coated Si substrate. The pore diameter is
about 60 nm and length is 300 nm. The density of
pores is approximate 1*1011 per cm2. The ratio of
exposed pore to overall exposed area is about 0.33.
The AAO template was then immerse in the
Cu-containing lactate solution. Subsequently, the
electrochemical deposition was conducted on the
AAO template. The Cu and Cu2O were
electrochemically deposited into the pore of AAO
template. The Cu2O/Cu filled AAO was then
immerse in NaOH solution with varied pH values.
Fig. 2(a) shows the surface of deposited sample after
etched with NaOH solution (pH=11.0). This figure
indicated that AAO still remained intact and was not
etched by NaOH solution. When the pH of NaOH
solution increased to 11.5, AAO template was
partially removed as shown in Fig. 2(b). At the same
time, the Cu2O/Cu nanowires were exposed. As the
pH increased to 12, all of AAO template was
removed by NaOH solution. Thus, well-aligned
Cu2O/Cu nanowires with diameter of 60nm and
length of 300nm were clearly observed in Fig. 2(c).
When the pH of NaOH solution increased to higher
values as shown in Fig.2 (d), the removal of
deposited Cu2O/Cu was locally seen. Therefore, the
deposited Cu2O/Cu nanowires was not stable in the
very basic solution.
The deposited naowires were analyzed by a glancing
angle XRD. The scanning rate was set at 4°/min
from 20 ° to 80 ° of 2 θ angles. The result of
diffraction was plotted in Fig.3. The reflected peak
has been identified as the (110) (111) (200) (220)
reflective planes of Cu2O and (111) (200) (220) of
Cu. The reflection marked with the star symbol is
from the substrate. This result shows that the
nanowires consisted of Cu2O and Cu composite
were fabricated by combined electrcochemical
deposition and AAO template assisted methods.
The deposited nanaowires were also observed by
TEM observation. The bright-field image is shown
in Fig. 4. The diameter of nanowires is in a good
agreement with the observation by SEM.
Furthermore, the image also shows the
polycrystalline of Cu2O/Cu composite. The
well-defined diffraction rings （not shown here）also
indicated the several responsible reflection planes of
Cu2O and Cu.
4
Conclusion
Based on these results, Cu2O/Cu nanowires were
successfully
fabricated
using AAO-assisted
electrochemical deposition method. However, the
pH value of NaOH etching solution plays an
important role in the fabrication of Cu2O/Cu
nanowires. When the pH of NaOH etching solution
was less than 12, Cu2O/ Cu nanowires were not
completely exposed due to the partially dissolution
of AAO template. Under pH=12, AAO template was
entirely removed without the dissolution of
Cu2O/Cu nanowires. The well-aligned Cu2O/Cu
nanowires with the diameter of 60nm and length of
300nm were obtained.
Acknowledgement
The authors gratefully acknowledge the financial
support by National Science Council of Taiwan.
（Grant No. NSC 92-2120-M-006-003）
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1.2μm
Fig.1 The top-view SEM
observation of porous AAO
template
514nm
Fig.2 (a) The morphology of
deposited nanowires after etched
in NaOH solution (pH=11.0)
2.25μm
Fig.2 (b) The morphology after
etched in NaOH solution
(pH=11.5)
1.8μm
Fig.2 (c) The morphology after
etched in NaOH solution
(pH=12.0)
Fig.4 The bright-field image of
deposited Cu2O/Cu nanowires
1.8μm
(220)
(200)
20
30
40
50
2
Fig.3 The XRD pattern of
deposited nanowires
(220)
(200)
Relative intensity
Cu2O (110)
Cu (111)
(111)
Fig.2 (d) The morphology after
etched in NaOH solution
(pH=12.5)
60
70
80