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NANOSTRUCTURING OF ALUMINIUM AND SYNTHESIS OF POROUS
ALUMINIUM MEMBRANES BY ANODISING
G.D. Sulka1,4, S. Stroobants2, V. Moshchalkov2, G. Borghs3, J-P. Celis4
1
Dept. Physical Chemistry, Fac. Chemistry, Jagiellonian University, 30060 Kraków (Poland)
2
Dept. VSM, Katholieke Universiteit Leuven, 3000 Leuven (Belgium)
3
4
IMEC, 3000 Leuven (Belgium)
Dept. MTM, Katholieke Universiteit Leuven, 3000 Leuven (Belgium)
Abstract
Nanostructures were produced on industrial aluminium foils by one-, two-, and three-step
anodising in sulphuric acid at cell potentials between 15 and 25 V. Almost perfect hexagonally
arranged nanopores were obtained. The pore diameter and the inter-pore distance vary in the
range of 19 – 33 and 41 – 69 nm respectively.
Fine membranes with a controlled size of pores were manufactured by anodising such
industrial aluminium foils. An anodising procedure was elaborated resulting in a high-density
array of pores with a controlled pore diameter, inter-pore distance, and depth of pores.
Introduction
Recently controlled anodization of aluminium in aqueous acids has been applied to
synthesis of a nm-scale porous structure with closed packed cells (Fig. 1).
Fig.1 Idealised hexagonal array of porous Al2O3 film formed by anodization of aluminium.
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Hexagonal patterns with extended long-range order can be easily obtained by cycles of
anodization and successive removal of the porous oxide (multi-step anodising process). A onestep anodising process was commonly used for manufacturing nanohole arrays on aluminium [14] till the time Masuda et al. [5, 6] discovered the two-step anodising process. They showed that
ordered nanopore arrays were formed in aluminium by stripping away the oxide layer obtained
by a first anodising step, and subsequently re-anodising it. Besides the widely used two-step
anodising process [6-10], a three-step process was recently proposed to obtain highly ordered
structures on anodised aluminium [11, 12].
As an alternative processing route Masuda et al. [13-16] developed a nano-indentation
technique leading to a pre-texturing of aluminium by imprinting with a SiC mold. This method
gives not only a perfect triangular pore arrangement over a large area [13-15] but can also be
used to get a square and triangular nanohole array architectures in anodised aluminium [16].
In this paper the anodising of aluminium in sulphuric acid solution is described. The twostep anodising process leading to a densely packed and well-ordered triangular structure in
aluminium is reported. The preparation of membranes with opened pores and height aspect ratio
is also discussed.
Experimental
Prior to anodising, a commercial aluminium sheet (99.997 % purity, 10×5×0.25 mm, Alfa
Aesar Johnson Matthey GmbH) was degreased in acetone, and electropolished at a constant
current of 500 mA·cm-1 for 1 min in an electrolyte of 10 °C containing perchloric acid (60 wt %)
and ethanol in a ratio of 1:4 by volume. The grain size of aluminium sheet was not uniform and
varied between 30 – 120 µm, but a typical value was about 100 µm. Anodising was conducted at
a constant cell potential in a 20 wt% sulphuric acid electrolyte. An overflow-cell equipped with a
pump system and a cooling unit was used [17]. The temperature of that electrolyte was 1 +/- 0.1
°C. The evolution of the current density during anodising was recorded.
Two-step anodising procedure was used. The process was started with a short first
anodising step of 10 min at a pre-selected cell potential between 15 and 25 V. Subsequently, the
oxide layer was removed by immersing the aluminium foils for 5 to 10 min in a mixture of
phosphoric acid (6 wt%) and chromic acid (1.8 wt%) at 60 °C. After chemical removal of the
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oxide layer, the aluminium was anodised again for a long duration at the same cell potential as
used during the first anodising step.
After anodising, the ordering of the pores on top of the aluminium foils was investigated
by scanning electron microscopy (SEM, Philips XL-30 – FEG). The structure of the anodic
porous alumina formed during the two-step process was also observed from the bottom side of
the anodised film. To that avail, the aluminium substrate was chemically removed after the twostep anodising procedure by immersion in a saturated HgCl2 solution.
In order to manufacture the membranes with opened pores the barrier layer at the bottom
of samples was chemically etched away in 5 wt % H3PO4 solution.
Results and discussion
Hexagonally arranged pores formed on the top of anodised aluminium are seen (Fig. 2a)
on a sample produced at the cell potential of 23 V. Similar very well-ordered pore structure on
anodised aluminium, were obtained after a second anodising in the range of 15 V to 23 V. Some
defects in the hexagonal arrangement of pores are noticeable at the boundaries between domains.
The analysis of the Fourier transform of SEM images of alumina layers as well as defects maps
known as Delaunay triangulations [18] showed that for the two-step procedure ordering degree is
independent of the cell potential.
The detail analysis of pore arrangement was described
previously in our paper [17].
a) Top view
b) Bottom view after opening of pores
Fig. 2 SEM top (a) and bottom after opening pores (b) views SEM images of porous alumina
layer formed at 23 V by a two-step anodising in 20 wt % H2SO4 at 1°C. The duration
of the first and second anodising step was 10 min and 185 min, respectively. The pore
opening time was 30 min.
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Recently the two-step anodising at 10 V was performed to obtain the triangular structure
on anodised aluminium with very fine pores.
Relatively well-ordered nanostructure was
observed at the top as well as at the bottom of anodic porous alumnina layer. The diameter of
pores and the inter-pore distance estimated from the top view SEM images for samples obtained
from these experiments are 12.8 and 28.6 nm respectively.
The inter-pore distance and the pore diameter were evaluated on anodised aluminium
layers obtained after a second anodising step at different cell potentials. Experimental and
literature data of pore diameter and inter-pore distance are compared in Fig. 3. Both sets of data
are in very good agreement in all cases. No broadening effect of the pores with increasing length
of pores was observed. The pore diameter and the inter-pore distance in anodic porous alumina
achieved in sulphuric acid (Fig. 3) increase linearly with increasing cell potential. Ordered
porous alumina layers obtained after anodising in sulphuric acid, exhibit nanohole arrays with
finer dimensions than those obtained in oxalic or phosphoric acid solutions. Inter-pore distances
obtained by anodising of aluminium in oxalic and phosphoric acid solutions are in the range of
100 to 200 nm [5-8, 10, 19-21] and 400 to 500 nm [8-10, 14, 19-21] respectively. Whereas, the
pore diameter varies between 50 and 100 nm for anodising in oxalic solutions [6-8, 10, 19-21],
and between 200 and 250 nm in phosphoric acid solutions [8, 9, 14, 19-21].
Two-step anodising
80
60
70
[21]
60
[22]
50
50
40
40
30
30
20
20
10
10
0
Inter-pore distance (nm)
Pore diameter (nm)
70
80
Exp
0
10 12.5 15
17 18.7 21
23
25
Potential (V)
Fig. 3 Pore diameter and inter-pore distance vs. potential of anodising in sulphuric acid.
Experimental data obtained in this work are compared to literature data.
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The alumina membrane with the opened bottom of pores formed at 23 V by a two-step
anodising is shown in Fig. 2b. It can be seen that at the bottom pores are also hexagonally
arranged in the µm-size domains. The opening pore procedure results in showing defects up in
the ordered structure. Similar opened membranes were obtained after a second anodising in the
range of 15 V to 23 V. The aspect ratio of membranes formed at the cell potential of 23 and 15 V
can be more than 3 800 and 6 700 for, respectively.
In order to observe the channel structure of holes, cross sections of oxide layer formed at
15 V was investigated (Fig. 4). Additionally, a channeling test with an ion beam was performed.
A He+ ion beam was vertically transmit through the membrane with opened pores and the
dissipation of the beam after the membrane transmission was analysed. The dissipation diagram
is shown in Fig. 5. The relatively narrow high peak confirms that channels have grown parallelly
during anodization.
Fig. 4 The cross-sectional view of the oxide layer formed at 15 V by a two-step anodising in 20
wt % H2SO4 at 1°C. The duration of the first and second anodising step was 10 min
and 260 min, respectively.
The effective growth rates of the oxide layer were calculated from the thickness of oxide
layers estimated from SEM on samples treated by two-step anodising procedures. The effective
growth rate of the oxide layer on anodised aluminium achieved in our experimental conditions at
23 V, decrease exponentially with increasing anodising time (Fig. 6).
For the case of an
anodising potential of 15 V, the dependence is linear. The appropriate curve was fitted to
experimental points for both cell potentials. The curve equations are collected in Fig. 6.
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Fig. 5 Dissipation diagram of the He+ beam after transmission through the sample anodised at
15 V. The duration of the first and second anodising step was 10 min and 260 min,
respectively. The thickness of formed oxide layer was about 28 µm.
Oxide growth rate at 15 V
Oxide growth rate at 23 V
2
V (µm/min)
V (µm/h)
8
7
6
1.5
1
0.5
-0.4503
y = 6.3098x
y = -0.0003x + 6.6805
0
5
0
300
600 900 1200 1500
t (min)
0
30
60
90 120 150 180
t (min)
Fig. 6 Growth rate of the oxide layer (VΘ) vs. anodising time. Anodising was conducted at
constant cell potential in a 20 wt % sulphuric acid electrolyte operated at 1 °C.
These results can be understood from the evolution of the current density with anodising
time during a second step of anodization. The variation of current density with time was
recorded for both cell potentials of 15 and 23 V (Fig. 7). The appropriate curves were fitted to
experimental data for both anodising potentials and curves equations are presented in Fig. 7. The
evolution of current with anodising time is similar to the evolution of growth rates with anodising
time at a given cell potential (compare curves equations).
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Current at 23 V
50
8
40
2
I (mA/cm )
2
I (mA/cm )
Current at 15 V
10
6
4
2
y = -0.0003x + 4.8654
30
20
10
y = 112.84x -0.4281
0
0
0
300
600 900 1200 1500
t (min)
0
30
60
90 120 150 180
t (min)
Fig. 7 Chrono-amperograms recorded during the second anodising steps performed at either 15 V
or 23 V in a 20 wt % sulphuric acid electrolyte operated at 1 °C.
Conclusions
1. Two-step anodising results in almost perfectly arranged honeycomb structures in aluminium
at cell potentials between 15 V and 25V.
2. Anodising of aluminium at 10 V results in a relatively well-ordered nanostructure.
3. Experimental and literature data on pore diameter and inter-pore distance are in very good
agreement.
4. Fine membranes with controlled size of pores can be easy produced by anodising of
aluminium in sulphuric acid solution.
5. The pores grow as parallel straight channels and the growth rate depends on the passing
current during anodization.
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