Fabrication of nanoporous metal electrode by two-step replication technique

Article abstract of DOI:10.1016/j.jallcom.2006.09.084

Nanoporous Ni metal membrane was fabricated by using nano-technique (two-step replication) from porous anodic alumina (PAA) template. In the first step replication, poly(methyl methacrylate) (PMMA) with various concentrations and molecular weights was filled into PAA mother template to obtain PMMA negative-type cylindrical structure. The results indicate that the increasing concentration and molecular weight of PMMA is required to fabricate satisfactory PMMA negative-type cylindrical structure due to the higher intermolecular interaction of molecular chains. The higher intermolecular interaction provided better strength to maintain the desired PMMA negative-type cylindrical structure. In the second step replication, Ni was electrochemically deposited into the PMMA cylindrical structure. After removing PMMA by acetone, nanoporous Ni metal membrane was obtained. The morphology of nanoporous Ni membrane was similar to that of PAA mother template. A film of yittria-stabilized zirconia (YSZ) electrolyte can be finally deposited on the Ni metal membrane by EPD process. This metal membrane might be used as an electrode in the applications of solid electrochemical devices.

Full text of DOI:10.1016/j.jallcom.2006.09.084

Journal of Alloys and Compounds 441 (2007) L1–L6
Letter
Fabrication of nanoporous metal electrode by
two-step replication technique
a
a
a,∗
b
C.L. Liao , C.W. Chu , K.Z. Fung , I.C. Leu
a
Department of Materials Science and Engineering, National Cheng Kung University, No.1, Ta-Hsueh Road, Tainan 70101, Taiwan, ROC
Department of Materials Science and Engineering, National United University, No. 1, Lienda, Kungching Li, Miaoli 36003, Taiwan, ROC
b
Received 1 September 2006; received in revised form 19 September 2006; accepted 23 September 2006
Available online 25 October 2006
Abstract
Nanoporous Ni metal membrane was fabricated by using nano-technique (two-step replication) from porous anodic alumina (PAA) template.
In the first step replication, poly(methyl methacrylate) (PMMA) with various concentrations and molecular weights was filled into PAA mother
template to obtain PMMA negative-type cylindrical structure. The results indicate that the increasing concentration and molecular weight of PMMA
is required to fabricate satisfactory PMMA negative-type cylindrical structure due to the higher intermolecular interaction of molecular chains. The
higher intermolecular interaction provided better strength to maintain the desired PMMA negative-type cylindrical structure. In the second step
replication, Ni was electrochemically deposited into the PMMA cylindrical structure. After removing PMMA by acetone, nanoporous Ni metal
membrane was obtained. The morphology of nanoporous Ni membrane was similar to that of PAA mother template. A film of yittria-stabilized
zirconia (YSZ) electrolyte can be finally deposited on the Ni metal membrane by EPD process. This metal membrane might be used as an electrode
in the applications of solid electrochemical devices.
©
2006 Elsevier B.V. All rights reserved.
Keywords: PAA; PMMA; Two-step replication; Electrochemical deposition; Nanoporous structure
1
. Introduction
cation technique. Two-step replication process is capable of
forming materials with nanoporous configuration as the porous
anodic alumina (PAA) which is a typical self-organized hole-
array structure [5–9]. Basically, the two-step replication consists
of two parts: (i) the fabrication of negative-type poly(methyl
methacrylate) (PMMA, which is usually used as the filling mate-
rial due to its great chemical stability) from PAA template and
(ii) the subsequent deposition of desired materials into the cavity
of the PMMA negative-type matrix by electrochemical deposi-
tion (ECD) or electrophoresis deposition (EPD).
In this report, first, nanoporous PAA template with pore diam-
eter about 100 nm was fabricated from Al foil by anodization.
The effect of PMMA concentrations and molecular weights
on replication of negative-type PMMA was discussed. Then,
the Ni metal membranes were obtained from the negative-type
PMMA template by electroplating. After removing the PMMA
by acetone, Ni metal membrane could be obtained. Finally, a
yittria-stabilized zirconia (YSZ) film was deposited on the Ni
membrane by EPD. The 8YSZ film/nanoporous Ni metal mem-
brane structure might be applied in the applications of thin-film
SOFCs.
Nanoporous metal membrane may be used as a permeable
support for numerous applications such as gas electrodes, cat-
alysts, chemical reactors, etc. For example, in our concern, the
metalmembraneexhibitsseveraladvantagessuchas(i)thechan-
nels of porous membrane may be used as the passageway for
reactive gas, (ii) it exhibits good electrical conductivity for the
electron conduction, and (iii) it exhibits sufficient strength to
be used as a substrate to deposit solid electrolyte on, therefore,
it might be used in the applications of solid oxide fuel cells
(
SOFCs).
Recently, template-assisted method has been developed
mainly for the fabrications of nano-structures such as nanodots,
nanorods, nanowires and nanotubes [1–4]. Based on template-
assisted method, nanohole-array membranes of metal [5–8]
and semiconductor [9] can be fabricated by two-step repli-
∗
Corresponding author. Tel.: +886 6 2757575x62969; fax: +886 6 2380208.
E-mail address: kzfung@mail.ncku.edu.tw (K.Z. Fung).
0
925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2006.09.084

L2
C.L. Liao et al. / Journal of Alloys and Compounds 441 (2007) L1–L6
image of 8YSZ film/Ni membrane were observed by scanning electron micro-
scope (Hitachi, S4100).
3. Results and discussion
3.1. Fabrication of PAA template
In order to fabricate porous metal membranes by using
two-step replication technique, anodizing the Al foil to form
PAA mother template is the first task. Fig. 2(a) shows the SEM
top-view image of the home-made PAA template. In previous
work, highly ordered PAA template can be obtained either by
one-step anodization of electropolished Al foil or by repeated
anodization of nonelectropolished Al foil in 0.3 M oxalic acid
solution under specific voltage (40 V) [11]. However, in order
to speed up the anodization process to obtain a PAA template
with desired thickness (∼60 ␮m), the anodization in this study
was carried out at 80 V. Therefore, the pores does not arrange
orderly in large area and show irregular shapes (Fig. 2(a)) are
Fig. 1. Schematic representation of using two-step replication technique to fab-
ricate nanoporous Ni membrane and 8YSZ/Ni membrane structure.
2
. Experimental procedure
Nanoporous metal membrane was obtained by a two-step replication tech-
nique. The whole steps of two-step replication are represented in Fig. 1. First,
the porous anodic alumina (PAA) template was obtained by anodization of Al
foil (Alfa, 99.997%) under 80 V in 0.3 M oxalic acid solution with a cooling
◦
circulation bath (13 C). The PAA template with 100 nm diametered channels
was finally separate from Al foil by HgCl2 and the barrier layer was removed
by 10 wt% phosphoric acid solution [10]. After fabricating the PAA template,
a thin Pt layer was deposited on the surface of PAA template by evaporation
for 10 min. This Pt layer was used as a current collector for the subsequent
electroplatingprocess. Inthefirststepreplication, PMMAsolutions(PMMAdis-
solved in chlorobenzene) with various PMMA molecular weights (Mw = 15,000,
7
0,000, and 996,000 g/mol) and various concentrations were dropped on the
Pt-coated PAA template to fabricate negative-type PMMA cylindrical struc-
ture. After the evaporation of the chlorobenzene and polymerization of PMMA,
the PAA template can be removed by 5 M NaOH solution and then a repli-
cated negative-type PMMA with cylindrical structure can be obtained. After
removing PAA template, the Pt layer previously coated on PAA surface can
be transferred to the replicated PMMA cylindrical structure and was used as a
current collector for the second step replication. In here, the second step repli-
cation was electroplating Ni into the cavity of PMMA negative-type cylindrical
structure in a solution consists of NiSO4·6H2O (330 g/L), NiCl2·6H2O (45 g/L),
and H3PO3 (35 g/L). The electroplating process was carried out at −0.9 V ver-
sus Ag/AgCl reference electrode. After Ni electroplating, negative-type PMMA
cylindrical structure can be removed by acetone and the nanoporous metal mem-
brane was obtained. Because of the nanoporous Ni metal membrane can be used
as the support and electrode for thin-film SOFCs. Therefore, a film of 8YSZ
was finally deposited on the metal membrane by EPD at −40 V for 5 min.
The solution for EPD is an 8YSZ suspension (10 g/L) consists of 8YSZ par-
ticles (Tosol, ∼Ø200–500 ␮m) disperse in a mixture of acetone and ethanol
(
3:1, v/v). In addition, iodine (0.6 g/L) added into the suspension is helpful
to increase the conductivity of 8YSZ suspension and enhance the deposition
rate.
In order to understand the effect of the concentration and molecular weight
(
Mw) of PMMA on the fabrication of PMMA negative-type cylindrical structure,
the viscosity of PMMA solution was conducted by Brookfield viscometer (DV-
II+Pro). The morphologies of the PAA template, replicated PMMA negative-
type cylindrical structure, nanoporous metal membrane, and the cross-sectional
Fig. 2. (a) SEM top-view image of PAA template. (b) PAA thickness plotted as
a function of anodization time.

C.L. Liao et al. / Journal of Alloys and Compounds 441 (2007) L1–L6
L3
Fig. 3. SEM top-views of replicated negative type of PMMA (Mw = 15,000 g/mol) that fabricated with various concentrations: (a) 10 wt% and (b) 15 wt%. SEM
top-views of replicated negative-type structures of PMMA (Mw = 70,000 g/mol) that fabricated with various concentrations: (c) 10 wt% and (d) and (e) 15 wt%.
attributed to the higher anodization voltage. After anodization,
the PAA thickness was plotted as a function of anodization time
3.2. Fabrication of PMMA cylindrical structure from PAA
template
(Fig. 2(b)). In this figure, the anodization could be divided into
two regions. The anodization rate in the initial 10 min is about
In order to obtain the same porous structure as PAA template,
two-step replication process was accepted. In the first step repli-
cation, it is performed by filling PMMA into the pore structure
of PAA mother template. In order to obtain satisfactory PMMA
negative-type cylindrical structure, PMMA solutions with vari-
ous PMMA molecular weights and concentrations were tried to
fill the PAA template.
Fig. 3(a) and (b) shows the SEM micrographs of replicated
PMMA negative-type structures by using various concentrations
(10 and 15 wt%) of PMMA (Mw = 15,000 g/mol). After remov-
ing the PAA template by 5 M NaOH, the remaining PMMA
2
1
.4 ␮m/min. And then, the anodization rate reduces to about
␮m/min when anodization period extends to 10–60 min. The
decreasing of anodization rate is due to the increasing length
of PAA channel which increases the diffusion path of the
dissolve species and then suppresses the further anodization
rate. This result indicates that the anodization rate at initial
stage is not strongly affected by the length of PAA channel.
However, when the length of PAA channel is longer than
about 24 ␮m, the anodization rate is suppressed into a steady
state.

L4
C.L. Liao et al. / Journal of Alloys and Compounds 441 (2007) L1–L6
does not show cylindrical structure but shows discontinuous
and porous structure instead of the desired cylindrical structure.
This structure could not be used for the second step replication
to electroplate nanoporous metal membrane. Thus, PMMA Mw
was increasing to 70,000 g/mol to prepare the PMMA solutions.
Fig. 3(c)–(e)showstheSEMobservationsofreplicatednegative-
type PMMA obtained from PMMA (70,000 g/mol) solutions
with various concentrations. In Fig. 3(c), the PMMA cylindrical
structure is still not observed by filling 10 wt% PMMA solu-
tion into PAA template. But as the PMMA (Mw = 70,000 g/mol)
concentration increasing to 15 wt%, the wire-like structure grad-
ually forms in some area (Fig. 3(d) and (e)). In these figures,
because of the Van der Waals’ force, the aggregation of PMMA
wires can be observed. The significant improvement of repli-
cated PMMA cylindrical structure is observed when the PMMA
Mw increases from 15,000 to 70,000 g/mol, and, this result
indicates that the increasing PMMA Mw is beneficial for the
fabrication of replicated PMMA cylindrical structure. However,
the cylindrical structure is only observed in some area and is still
not satisfactory for the subsequent metal-electroplating process.
In order to obtain the desired cylindrical structure in a large
area, PMMA Mw was finally increased to 996,000 g/mol. Fig. 4
Fig. 4. Top-view images of PMMA nano-arrays fabricated at PMMA concentration of: (a) 10 wt%, (b) 13 wt%, (c) 15 wt%, cross-section images (d) 10 wt%, (e)
3 wt%, and (f) 15 wt% (PMMA Mw = 965,000 g/mol).
1

C.L. Liao et al. / Journal of Alloys and Compounds 441 (2007) L1–L6
L5
Table 1
The dependence of PMMA solution viscosities on various concentrations and
molecular weights
PMMA concentration
wt%)
PMMA molecular weight (g/mol)
(
16,000
70,000
996,000
1
1
1
0
3
5
5.94
7.38
8.64
15.13
24.92
37.56
117.67
401.91
892.32
Viscosity unit: cP.
shows the replicated negative-type PMMA cylindrical struc-
ture fabricated by using PMMA with Mw is 996,000 g/mol.
Fig. 4(a)–(c) are top-view and Fig. 4(d)–(f) are cross-sectional
images of replicated negative-type PMMA obtained from 10
to 15 wt% PMMA solutions, respectively. From the top-view
observations, the aggregation of PMMA nanowires is observed
due to the Van der Waals’ force between long PMMA nanowires.
Thecross-sectionalimagesshowthatthePMMAnanowireswith
diameter of 100 nm was successful obtained by filling PMMA
(
996,000 g/mol) with various concentrations into PAA template.
Because the large-area negative-type PMMA cylindrical struc-
ture was successfully obtained by using 996,000 g/mol PMMA,
the subsequent electroplating (the second step replication) was
carried out by using these replicated negative-type PMMA cylin-
drical structures as templates.
It is known that the viscosity of polymer solutions is involving
many interacting variables, such as the polymer concentration
and molecular weight [12,13]. In general, the viscosity can
be regarded as a simple reference for the degree of molecu-
lar entanglement of polymers. Therefore, in other words, the
entanglement of polymer molecules is relative to the polymer
concentration and molecular weight. The viscosity of PMMA
solutions with various concentrations and molecular weights
was measured and shown in Table 1. In this table, the vis-
cosity increases with the increasing PMMA concentration and
molecular weight can be observed. The increasing PMMA vis-
cosity with the increasing polymer concentration is attributed
to the more molecules dissolve in the solvent that increases the
chance of entanglement of molecules. Thus, the high-degree
entanglement of molecules leads to the higher viscosity and
higher strength to maintain the cylindrical structure. Therefore,
the increasing concentration of PMMA solution is benefic to
obtain satisfactory negative-type cylindrical structure.
Moreover, the molecular weight is generally based on the
mass of polymer chain unit [13]. Therefore, the length of PMMA
chain is relative and increasing with the Mw of PMMA. As a
result of the low Mw PMMA exhibits short-chain molecules and
shows low degree of PMMA entanglement, the intermolecular
interaction of PMMA is too weak to maintain the cylindri-
cal structure. Thus it forms intermittent and porous structure.
As the Mw of PMMA increased, the viscosity is increasing
due to the high-degree entanglement of PMMA molecules with
long chain. Consequently, the stronger intermolecular interac-
tion of PMMA is benefic to sustain the cylindrical structure, and,
the desired negative-type PMMA cylindrical template can be
obtained.
Fig. 5. The current transient curves for (a) long time and (b) short time (<3000 s)
of Ni metal electroplating into the negative-type PMMA.
3.3. Fabrication of nanoporous Ni membrane and
fabrication of 8YSZ film on Ni membrane
The second replication was performed by electroplating the
desired material into the negative-type PMMA cylindrical struc-
ture (as a template) obtained previously. Fig. 5(a) shows the
current transient curve of electroplating Ni membrane for the
duration of 8000 s. In order to view closely within the initial
electroplating period of 3000 s, region I of Fig. 5(a) is enlarged
and shown in Fig. 5(b). The initial current transient (region I)
shows a conventional electroplating behavior for filling mate-
rials into template [1,14]. The electroplating current quickly
increases from −0.03 to −0.05 mA in the beginning and then
drops back to −0.02 mA due to the consumption of electro-
plating species on the surface. Subsequently, the current slowly
increases due to the electroplating species gradually diffuse to
the electroplating surface. Finally, the current maintains at a

L6
C.L. Liao et al. / Journal of Alloys and Compounds 441 (2007) L1–L6
carried out by EPD. Fig. 6(b) shows the cross-sectional image
of the structure of 8YSZ film/Ni membrane. The thickness of
the YSZ film is about 15 ␮m after EPD for 5 min. The YSZ film
deposited by EPD is extremely dense and well adheres to the Ni
metal membrane. The 8YSZ film is just coated on the surface of
Ni membrane but not deposited into the channels. This result can
be attributed to the 8YSZ particles are larger than the diameter
of channel. The structure of YSZ film/Ni metal membrane may
be used for SOFCs applications. The electrochemical property
of this unique structure will be discussed in the near future.
4. Conclusion
PAA mother template fabricated by anodization Exhibits
00 nm diametered nanoporous structure. Low Mw (15,000 and
0,000 g/mol) PMMA cannot build up satisfactory negative-
1
7
type cylindrical structure. The negative-type PMMA cylindrical
structure was successfully obtained by increasing the molecu-
lar weight of PMMA to 996,000 g/mol. The higher molecular
weight exhibits long-chain molecules and increases the degree
of intermolecular entanglement. Consequently, the higher inter-
molecular interaction of polymers maintains the desired cylin-
drical structure. Nanoporous Ni metal membrane was obtained
by electroplating Ni into the PMMA cylindrical template. The
morphology of metal membrane is similar to that of PAA mother
template. The nanoporous metal membrane may be used as elec-
trode in SOFCs applications since the 8YSZ electrolyte can be
deposited on the Ni metal membrane by EPD technique and
shows well adhesive to the Ni membrane.
Fig. 6. (a) SEM top-view observation of Ni membrane fabricated by electroplat-
ing. (b) SEM cross-sectional image of 8YSZ film deposited on Ni membrane by
EPD.
steady state due to the balance of consumption and supplement
of the electroplating species. As a result, the metal is gradually
deposited into the cavity of PMMA cylindrical-structured tem-
plate. In region II (Fig. 5(a)), the rapid increasing current results
from the interconnecting of deposited metal through the perco-
lation point. The cavity of the replicated PMMA is filled with
metal and the metal begins to form a thin and continuous metal
layer over the top of replicated PMMA cylindrical-structured
template. Thus, the conducting area for further electroplating is
greatly enhanced and the current increases obviously. Finally,
the current remains steady at ∼−8 mA as shown in region III.
Based on the current transient of electroplating metal into the
negative-type PMMA cylindrical-structured template, the thick-
ness of metal hole-array membrane could be controlled.
By using the two-step replication technique, the Ni metal
membrane is successfully fabricated as shown in Fig. 6(a). The
nanopores observed from Ni membrane are similar to that of
PAA mother template. However, some channels with diameter
slightly larger than that of PAA template is due to the aggrega-
tion of PMMA wires in the negative-type cylindrical template.
The nanoporous Ni membrane may be used as the substrate
for YSZ thin-film deposition in SOFCs applications. The sub-
sequent deposition of 8YSZ electrolyte on Ni membrane was
References
[
1] M.T. Wu, I.C. Leu, J.H. Ten, M.H. Hon, Electrochem. Solid-State Lett. 7
2004) c61.
(
[2] M.S. Dresselhaus, Y.M. Lin, O. Rabin, M.R. Black, G. Dresselhaus,
Nanowires, in: Springer Handbook of Nanotechnology, Springer, Berlin,
2004.
[
[
[
3] G.E. Possin, Rev. Sci. Instrum. 41 (1970) 772.
4] C.R. Martin, Science 266 (1994) 1961.
5] H. Masuda, K. Nishio, N. Baba, Thin Solid Films 223 (1993) 1.
[6] H. Masuda, T. Mizuno, N. Baba, T. Ohmori, J. Electroanal. Chem. 368
(1994) 333.
[
[
[
7] H. Masuda, K. Fukuda, J. Electroanal. Chem. 473 (1999) 240.
8] H. Masuda, K. Fukuda, Science 268 (1995) 1466.
9] P. Hoyer, H. Masuda, J. Mater. Sci. Lett. 15 (1996) 1228.
[
10] M.T. Wu, I.C. Leu, M.H. Hon, J. Vac. Sci. Technol. B 20 (2002) 776.
[11] L. Zhang, H.S. Cho, F. Li, R.M. Metzger, W.D. Doyle, J. Mater. Sci. Lett.
17 (1998) 291.
[
12] M. Bohdaneck y´ , J. Kuv a´ ˇr , in: A.D. Jenkins (Ed.), Polymer Science Library
: Viscosity of Polymer Solutions, Elsevier, 1982, p. 14.
2
[
13] M. Joan Comstock (Ed.), ACS Symposium Series: Polymer Science
Overview, Stahl, 1981, pp. 74–88.
[14] T.M. Whitney, J.S. Jiang, P.C. Searson, C.L. Chien, Science 261 (1993)
1316.