Fabrication and characteristics of nanostructures on glass by Al anodization and electrodeposition

Article abstract of DOI:10.1016/S0013-4686(03)00344-X

A novel fabrication process of metal and semiconductor nanostructure arrays directly onto glass substrates is described below. An aluminum film was sputter-deposited on a glass substrate coated with a tin-doped indium oxide (ITO) film. The film was then anodically oxidized to obtain a porous alumina template with pore diameters ranging from 5 to 120 nm. Through appropriate chemical dissolution, the barrier layer of the anodic alumina film was removed and the underlying ITO film was exposed to the electrolytes, thus making it possible to deposit metals or semiconductors in the pores of anodic alumina films by a direct current (dc) electrodeposition. In Ni electrodeposition, the porous alumina films not only define the dimensions of the Ni nanowires but also influence their crystalline orientation, showing a textured {220} orientation for φ 18 nm nanowires. In addition, through a cathodic electrosynthesis, a peroxocompound of titanium and ruthenium was deposited in the pores of anodic alumina film on ITO/glass substrate. After heating at 873 K for 2 h, a translucent TiO₂-RuO₂/Al₂O₃ composite nanostructure on the ITO/glass substrate was finally fabricated.

Full text of DOI:10.1016/S0013-4686(03)00344-X

Electrochimica Acta 48 (2003) 3147ꢀ3153
/
www.elsevier.com/locate/electacta
Fabrication and characteristics of nanostructures on glass by
Al anodization and electrodeposition
S.Z. Chu *, K. Wada, S. Inoue, S. Todoroki
Advanced Materials Laboratory, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan
Received 1 October 2002; received in revised form 2 April 2003; accepted 3 April 2003
Abstract
A novel fabrication process of metal and semiconductor nanostructure arrays directly onto glass substrates is described below. An
aluminum film was sputter-deposited on a glass substrate coated with a tin-doped indium oxide (ITO) film. The film was then
anodically oxidized to obtain a porous alumina template with pore diameters ranging from 5 to 120 nm. Through appropriate
chemical dissolution, the barrier layer of the anodic alumina film was removed and the underlying ITO film was exposed to the
electrolytes, thus making it possible to deposit metals or semiconductors in the pores of anodic alumina films by a direct current (dc)
electrodeposition. In Ni electrodeposition, the porous alumina films not only define the dimensions of the Ni nanowires but also
influence their crystalline orientation, showing a textured {220} orientation for f 18 nm nanowires. In addition, through a cathodic
electrosynthesis, a peroxocompound of titanium and ruthenium was deposited in the pores of anodic alumina film on ITO/glass
substrate. After heating at 873 K for 2 h, a translucent TiO
finally fabricated.
2 2 2 3
RuO /Al O composite nanostructure on the ITO/glass substrate was
Ã
/
#
2003 Elsevier Ltd. All rights reserved.
Keywords: Ni nanowire array; TiO RuO /Al O composite nanostructure; Anodization; Electrodeposition; Glass
2 3
2
Ã
/
2
1
. Introduction
Nanostructured materials have attracted much inter-
settled for practical devices. Therefore, from the prac-
tical viewpoint, exploring a fabrication method of
nanostructures directly on a durable substrate is essen-
tial for various purposes.
est because of their unique properties that are distin-
guished from the common metallurgical materials.
Particularly, with the development of aluminum anodiz-
ing technology [1,2] and the appearance of the commer-
cial anodisk alumina membranes [3], a variety of
nanostructures (metals and oxides) with various
morphologies (tubules, fibers or wires, rods) have been
fabricated by utilizing the porous alumina membranes
This study is aim to create a novel process to fabricate
various nanostructures, metals and oxides or semicon-
ductors, directly onto glass substrates by combining the
technologies of RF-sputtering, aluminum anodization
and electrodeposition. In our approach, a porous
alumina nanostructure with controlled pore density or
pore size was first formed by anodizing an aluminum
layer sputter-deposited on a glass substrate in an acid
solution. On removing the barrier layer, the porous
alumina nanostructure on glass was then used as a
template electrode to fabricate various nanostructures
within the nanopores through a direct current electro-
deposition. In the present paper, the formation process
of porous alumina films, Ni nanowires arrays, and
as the templates in sol-gel process [4ꢀ9], chemical vapor
deposition [10,11], electro- [12,13] and electroless de-
position [14,15]. However, owing to the limitation of the
/
areas (f 2ꢀ
/
4 cm) and the mechanical strength (Â100
/
mm thick) of the commercial membranes, the resultant
nanostrcutures are usually difficult to be handled or
TiO2Ã/RuO /Al O composite nanostructures on glass
2 2 3
substrates are described, and the microstructures and
the characteristics of various nanostructures were in-
vestigated.
*
Corresponding author. Tel.: ꢁ
060.
E-mail address: chu.songzhu@nims.go.jp (S.Z. Chu).
/
81-298-51-3354; fax: ꢁ81-298-54-
/
9
0
013-4686/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0013-4686(03)00344-X

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S.Z. Chu et al. / Electrochimica Acta 48 (2003) 3147ꢀ3153
/
2
. Experimental procedures
3. Results and discussion
A highly pure aluminum layer (99.99%, 1.5ꢀ
which was deposited on a glass substrate (25ꢂ100ꢂ
mm) coated with a tin-doped indium oxide (ITO: Â120
10 V I ) film, was used as the starting sample.
Aluminum deposition was performed by RF sputtering
/
2.0 mm),
3.1. Formation of porous anodic alumina films on glass
/
/
0.7
/
Anodization behavior of sputtered aluminum on ITO-
coated glass is different from aluminum sheets. Fig. 1
shows the relationship of the anodic current density and
the anodizing time for the sputtered aluminum speci-
mens in different solutions. For all the specimens, the
ꢃ1
nm, Â
/
ꢃ1
in a multi-cycled mode at a rate of 1.5 nm s
[16,17].
To obtain porous alumina films with different pore
sizes, the sputtered specimens were first decreased in
acetone for 10 min and then anodized in 10% (vol)
phosphoric, 3% (wt.) oxalic, and 10% (vol) sulfuric acid
solutions at 130 V and 280 K, 40 V and 293 K, and 15 V
and 276 K, respectively. After immersed in 5% phos-
current density in the i
stable stage, which corresponds to the aluminum layer,
and a fluctuating stage, which corresponds to the Alꢀ
ITO interface, and finally goes to zero. As the anodiza-
tion goes down the AlꢀITO interface (see shadowed
ꢀt curves exhibits a relatively
/
a a
/
/
regions), gas evolution and sparking phenomenon are
observed, and the appearance of the specimens changes
from opaque to transparent gradually. It was found in
preliminary experiments that the sheet resistance of the
ITO films on glass after anodization increased to several
tens to several thousands of ohms. Therefore, to
minimize the increment of the resistance of the anodized
specimens, the anodization was stopped immediately as
the characteristic variation of current density and
transparence were observed.
phoric acid solution at 303 K for 4ꢀ30 min, the
/
anodized specimens were used as template electrodes
to deposit nickel or TiO -based oxides within the
2
nanopores of the films by a direct current electrodeposi-
tion. In case of the fabrication of nanostructure arrays
on glass, the anodic alumina was selectively removed by
etching in a 1 M NaOH solution.
The Ni electrodeposition was carried out in an
electrolyte composed of 0.38 M NiSO , 0.13 M NiCl ,
4
2
0
.65 M H BO , and 60 ppm CH (CH ) OSO Na,
3 4 3 2 11 3
Fig. 2 shows the microstructures of the porous anodic
alumina films on ITO/glass substrate obtained in
different solutions. From the images of fracture sections
under the conditions of pH 5.2, 1.0ꢀ
/
1.5 V, and 303 K
for 2ꢀ10 min, with a magnetic stirring. In electrosynth-
/
esis of titanium and ruthenium oxides, the solution was
prepared by mixing 0.02 M TiCl , 200 ppm RuCl3×/
(left side), porous alumina structures with straight and
3
parallel channels perpendicular to the substrates are
acquired for all the solutions under the given conditions.
The average pore sizes and the pore interspaces are
nH O, and 0.08 M H O (30% in water) in an
2
2
2
ethanolꢀ/water (3:1 volume ratio) solvent. Cathodic
electrosynthesis was performed at a constant current
ꢃ2
density of Â
/
40 A m and 274 K for 4ꢀ10 min till the
/
deposits filled the pores completely and formed a
continuous layer on the anodic alumina films. Here,
ꢃ2
the current density (40 A m ) was the overall current
density that related to the apparent surface area of the
specimens. The porosities of the pore-widened alumina
films were estimated by preliminary experiments to be
about 0.5, 0.4, and 0.3 for the pore-widened phosphoric-
,
the corresponding net current densities were approxi-
oxalic-, and sulfate-anodized films, respectively. Thus,
ꢃ2
mately 80, 100, and 130 A m , respectively. A
platinum plate (3ꢂ
/
150ꢂ0.1 cm) was used as the
/
counter electrode. The distance between the working
electrodes and the counter electrode is fixed at 1.5 cm.
The morphology and the microstructure of the
anodized specimens were observed by a field emission
scanning electron microscope (FESEM: S-5000, Hita-
chi) with an energy dispersive X-ray analyzer (EDX)
after osmium evaporation. The crystal structures of the
specimens were analyzed by an X-ray diffractometer
(
XRD: RINT-2000/PC, Cu Ka, 40 V/40 mA). The UV-
Fig. 1. Changes of current density (i
sputtered aluminum films on glass with an ITO film in solutions of (a)
% oxalic acid (40 V, 293 K), (b) 10% sulfuric acid (15 V, 276 K), and
(c) 10% phosphoric acid (130 V, 280 K).
a a
) with anodization time (t ) for
visible transmittance spectra of the specimens were
measured by a spectrometer (U-3500, Hitachi) at a
resolution of 2 nm.
3

S.Z. Chu et al. / Electrochimica Acta 48 (2003) 3147ꢀ
/3153
3149
Fig. 2. FESEM images of vertical fracture sections (left side) and surface morphology (right side) of porous anodic alumina films on ITO/glass
substrate obtained in (a, b) phosphoric, (c, d) oxalic, and (e, f) sulfuric acid solutions. Before imaging of surface morphology, the specimens were
3 4
immersed in a 5% H PO solution at 303 K for 30 min (b), 20 min (d), and 4 min (f), respectively.
approximately 120 and 350 nm, 30 and 110 nm, and 5ꢀ
/
8
strates can be removed by an appropriate chemical
dissolution while still preserving the porous nanostruc-
tures on the substrates. This feature is of significant
meaning in our approach, because it makes it possible to
expose the conductive ITO film to the electrolytes for
the dc electrodeposition. Fig. 3 shows the morphologies
of nickel nanowire arrays embedded in porous alumina
films on ITO/glass substrates (left side) and after being
removed the alumina films (right side). The Ni deposi-
tion generates on the ITO film and continues in the
pores of the alumina films, leading to Ni nanowire
arrays standing on the ITO/glass substrate. The nickel
nanowires possess uniform diameters upright same as
the pores and take the contours of the porous alumina
films. Almost all of the pores of alumina films are
uniformly filled with nickel nanowires, even though the
height of the nanowires are uneven (see arrows in Fig. 3a
and c), which is reflected as black regions in the top view
of nanowire array (see arrows in Fig. 3d). The average
and 45 nm for the as-anodized phosphoric, oxalic, and
sulfuric films, respectively. More importantly, the re-
sultant alumina films have an arched barrier layer with a
thin thickness that is only 1/4ꢀ1/3 of the pore walls (see
/
arrows in Fig. 2a and c), which could be caused by the
local increases of temperature and/or acidity at the pore
bases during the anodization at the Alꢀ/ITO interface
[
17]. Moreover, from the surface morphologies of the
specimens (Fig. 2, right side), the pore distribution of the
anodic films are uniform throughout the layers even
without pretreatment like electro-polishing or pre-in-
dentation, possibly due to the uniform {111} texture of
the sputtered aluminum layers [17].
3
.2. Fabrication of Ni nanowire arrays on glass
Owing to the thin thickness, the barrier layer of
anodic alumina films formed on the ITO/Glass sub-

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S.Z. Chu et al. / Electrochimica Acta 48 (2003) 3147ꢀ3153
/
Fig. 3. Nickel nanowire arrays standing on ITO/glass substrates fabricated in anodic alumina films obtained in phosphoric (a, b), oxalic (c, d), and
sulfuric acid solutions (e, f). Before imaging, the specimens were selectively etched in a solution of 10% NaOH at room temperature for 10 s to 5 min.
diameters of the Ni nanowires are 150, 50 and 18 nm for
the anodic films formed in phosphoric, oxalic, and
sulfuric acid solutions, respectively. Based on the max-
each other due to the elasticity and the high surface
energy. In the case of catalysts, the Ni nanowire arrays
with large surface areas would give a high catalytic
effect.
imum thickness (Â
/
3.3 mm) of the three anodic films
Fig. 2), the possible maximum aspect ratios of the
(
It is very interesting that the pore sizes of the anodic
oxide films not only define the dimension of the Ni
nanowires but also affect the crystal orientation and the
grain sizes of the Ni nanowires. Fig. 4 gives the X-ray
diffraction patterns of a flat Ni film (a) and Ni nanowire
arrays deposited in sulfuric-anodized alumina films (b,
c) on the ITO/glass substrate. The flat Ni film has a
polycrystalline fcc structure with a strong {111} diffrac-
tion peak (a). Whereas the nanowires obtained from the
sulfuric films (b, f 18 nm) exhibit a nearly equal
intensity of {111} and {220} reflections in the pattern,
indicating that the {220} texture has been developed in
the Ni nanowires, that is, parallel to the axis of the
nanowires. Based on the calculated results of integral
intensity, I111/I220, the crystal orientation of Ni deposits
resultant Ni nanowires are calculated as 20, 66, and 183,
respectively.
After being removed the anodic alumina films, the
independent Ni nanowire arrays with different aspect
ratios are achieved on ITO/glass substrate. The Ni
nanowires with relatively low aspect ratios or large
diameters that are fabricated in the phosphoric (Fig.
3
b*
/
A.R.ꢄ
/
20, f 150 nm) and oxalic anodized films
(
Fig. 3b*
/
A.R.ꢄ
/
66, f 50 nm) can stand sturdily on the
substrates because of their rigid structures and the broad
roots that lead to a good adhesion to the substrates (see
arrows in Fig. 3a). While for the nanowires formed in
the sulfuric-anodized films (Fig. 3f*
nm), the Ni nanowires usually get together or lean on
/
A.R.ꢄ183, f 18
/

S.Z. Chu et al. / Electrochimica Acta 48 (2003) 3147ꢀ
/3153
3151
3
.3. Electrochemical synthesis of TiO2Ã
/RuO /Al O
2 2 3
composite nanostructures on glass
The electrosynthesis of titanium and ruthenium
oxides nanostructures on glass were also attempted by
the present approach. Fig. 5 shows the FESEM images
of fracture sections, and surface and bottom morphol-
ogies of as-electrodeposited specimens upon the differ-
ent anodic alumina films. The as-electrodeposits, which
composed of tiny grains from 10 to 50 nm, first fill the
pores and then form a continuous layer on the anodic
alumina films. The as-deposits in the anodic alumina
films are sandy and easily fall off when preparing the
samples for FESEM observation. Fig. 6 shows an
energy-dispersive X-ray (EDX) spectrum corresponding
to the specimens in Fig. 5c. The Ti Ka and Ru La peaks
are detected, indicating the co-deposition of titanium
and ruthenium. The atomic ratio of ruthenium to
titanium in the electrodeposits is estimated as Â
Tiꢀ14 at.% Ru). In addition, according to the XPS
analysis to the as-electrodeposits (not shown), titanium
/1:6
(
/
4
ꢁ
is identified to be the oxide (Ti ) and ruthenium is in
the metallic state.
Fig. 4. X-ray diffraction patterns of (a) a flat Ni film (2 mm)
electrodeposited on an ITO/Glass substrate, (b) Ni nanowire arrays
and (c) an over-electrodeposited film/nanowires layer fabricated in
sulfuric-anodized alumina films. Inserts are schemes of the Ni
nanostructures.
Fig. 7 shows UV-Vis transmittance spectra of various
nanostructures on ITO/glass substrate. The average
transmittance of the anodized specimen is close to that
of the ITO/glass substrate (dot line), indicating that the
aluminum layer is completely anodized into the oxide
and the anodic alumina film is transparent through UV-
visible light range. After electrodeposition, the transmit-
tance of the specimen decreases greatly, due to the filling
changes from the {111} texture of the flat film (I111
I220
/
24) to the {220} texture of nanowires with a
ꢄ
/
lateral dimension of f 18 nm (I111/I220 0.96). More
ꢄ
/
interestingly, the {220} texture of the f 18 nm nano-
wires is preserved and even enhanced as Ni deposits
emerge beyond the surface of the anodic film and form a
of the TiO2Ã
/
Ru in the pores of alumina film and the
increase of the absorbance by metallic ruthenium. After
heated at 837 K for 2 h, the color of the electrodeposits
change from gray to milky, showing a relatively high
transmittance.
continuous layer in Â
/
1 mm thick (pattern c, I111/I220
ꢄ
/
0
.09). In addition, the Ni nanowire arrays obtained from
Unlike the common alloy electrodeposition, the
electrosynthesis of titanium and ruthenium oxide is
complex and the mechanism on the formation of
titanium and ruthenium oxide has still not been
elucidated. It is found that, during electrodeposition,
the depositing potential always increases with the
deposition time. For the specimens in Fig. 5, the
deposition potentials finally reached 250 V (a, b), 57 V
the phosphoric (f 150 nm) and oxalic films (f 50 nm)
were also measured (not shown) and similar XRD
patterns to the flat Ni film were obtained. Furthermore,
1
based on the calculation from the Scherrer formula , the
average sizes of the Ni crystallites perpendicular to the
substrate planes are approximately estimated as 143 nm
(
a), 31 nm (b), and 48 nm (c), respectively. This indicates
that the porous alumina templates also affect the
orientation and the dimension of the Ni crystallites in
nanowires. Particularly, the crystal dimension of f 18
nm Ni nanowires is much larger in the direction parallel
to the axis of the nanowires, indicating that the porous
film with small pore size promotes the growth of nickel
crystallites in the Z-axis direction.
(c, d), and 51 V (e, f) as the deposition was stopped.
According to Zhitomisky [19], during electrodeposition,
a peroxocompound of titanium, instead of titanium, is
formed through the hydrolysis of peroxocomplex of
titanium by electrogenerated base. The titanium perox-
ocompound is of poor conductivity, thus leading to the
abnormal increase of the deposition potential. Never-
theless, the reduced ruthenium and/or the ruthenium
oxide in the electrodeposits are conductive [19], thus
enabling the continuous electrodeposition in direct
current mode. In addition, it is also observed that, as
just being lifted from the electrolyte, the as-deposited
1
Scherrer formula[18]: 0.9l/b cos u. Where bꢄFWHM of the
/
˚
220} reflection; lꢄ1.5405 A.
{
/

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S.Z. Chu et al. / Electrochimica Acta 48 (2003) 3147ꢀ3153
/
Fig. 5. FESEM images of vertical fracture sections and morphology of TiO
2
Ã
/
2 3
Ru/Al O composite nanostructures on ITO/glass substrates. The
porous alumina films are obtained in (a, b) phosphoric, (c, d) oxalic, and (e, f) sulfuric acid solutions under the conditions as in Fig. 1.
specimens were opaque with a metallic luster. However,
after dried in the air for a few minutes, the deposits lose
the metallic luster and change to translucent gradually
dehydration or oxidation of the as-deposited titanium
peroxocompound with unstable nature. The transmit-
tance change for the specimen after the heat treatment
could be explained by the thermal decomposition of
titanium peroxocompound and/or the oxidization of
ruthenium.
(
Fig. 7). This phenomenon may be ascribed to the
4. Conclusions
1)
Various nanostructures, metals and oxides, are
fabricated directly on glass substrates by combining
technologies of RF-sputtering, aluminum anodiza-
tion, and dc electrodeposition.
2)
Porous alumina films (f 5ꢀ/120 nm) formed by
anodizing aluminum layers sputtered on ITO/glass
have a thinner barrier layer compared with the pore
walls which can be selectively removed through a
chemical dissolution. This makes it possible to
expose the underlying conductive ITO film to the
Fig. 6. An EDAX spectrum of TiO
ture on ITO/glass shown in Fig. 5a.
2 2 3
ÃRu/Al O composite nanostruc-
/

S.Z. Chu et al. / Electrochimica Acta 48 (2003) 3147ꢀ
/3153
3153
could be ascribed to the thermal decomposition of
as-deposited titanium peroxocompound and/or the
oxidization of ruthenium.
Acknowledgements
This work is the part of Japan Millennium Project of
‘Search and Creation of a Catalyst for Removing
‘
Harmful Chemical Substances’’. The authors wish to
thank the Asahi Glass Co., LTD for providing glass
substrates with ITO films and Kyodo International
Company for assisting aluminum sputtering deposition.
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(
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continuous
1
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/
2
2
3
tructure is fabricated the ITO/glass substrate, which