Titanium oxide (TiO_2) nanowires were prepared for an electrolytic capacitor application by the automatic dipping technique using a porous alumina template. The automatic dipping technique allows us to exactly control the dipping rate so that we can obtain homogenous infiltration of nanowires in the porous alumina membrane, even though the solution is very acidic. From the TEM, SEM and XRD measurements, we confirmed that anatase phase TiO2 nanowires are highly infiltrated into the porous alumina template. In addition, the electrostatic capacitance of nanowires was measured and compared with a theoretical calculation using an effective thickness (delta e). We found that the effective thickness corresponds to the mean radius of nanowires and the experimental measurements were in good agreement with the calculations

Choi, Jinsub

Choi, Sang Kyo

Chun, Hee Dong

Lee, Jaekwang

Lee, Jaeyoung

Caltech Authors - Main

INSTITUTE OF PHYSICS PUBLISHING NANOTECHNOLOGY
Nanotechnology 16 (2005) 1449–1453 doi:10.1088/0957-4484/16/9/005
Electrostatic capacitance of TiO2
nanowires in a porous alumina template
Jaeyoung Lee1,4, Jinsub Choi2, Jaekwang Lee1, Sang Kyo Choi3
and Hee Dong Chun3
1 Fuel Cell Research Center, Korea Institute of Science and Technology, Seoul 136-791,
Korea
2 Divison of Engineering and Applied Science, California Institute of Technology, Pasadena,
CA 91125, USA
3 Research Institute of Industrial Science and Technology, Pohang 790-330, Korea
E-mail: jaeyoung@kist.re.kr
Received 2 March 2005, in final form 18 April 2005
Published 29 June 2005
Online at stacks.iop.org/Nano/16/1449
Abstract
Titanium oxide (TiO2) nanowires were prepared for an electrolytic capacitor
application by the automatic dipping technique using a porous alumina
template. The automatic dipping technique allows us to exactly control the
dipping rate so that we can obtain homogenous infiltration of nanowires in
the porous alumina membrane, even though the solution is very acidic.
From the TEM, SEM and XRD measurements, we confirmed that anatase
phase TiO2 nanowires are highly infiltrated into the porous alumina
template. In addition, the electrostatic capacitance of nanowires was
measured and compared with a theoretical calculation using an effective
thickness (δe). We found that the effective thickness corresponds to the
mean radius of nanowires and the experimental measurements were in good
agreement with the calculations.
1. Introduction
During the past decades, the synthesis of nanowires with
a uniform diameter and an adjustable length has received
great attention due to their distinct properties and versatile
applications such as interconnectors and active components in
fabricating nanoscale devices [1–4]. Among various synthesis
routes for monodisperse nanowires with a high aspect ratio
(diameter of nanowires/their length), the nanoporous alumina
template-assisted method is of great interest due to its easy and
inexpensive manufacturing process. So far, the fabrication of
nanowires of metals and semiconductors by electrochemical
deposition has been extensively demonstrated [5–9]. Highly
ordered nanowires of metal oxides such as titanium oxide,
indium oxide and zinc oxide, which could usually be prepared
by the sol–gel process, have been studied as luminescence
sources, chemical sensors, photochemical catalysts, and so
on [10–13]. However, the use of a nanoporous alumina
template for the preparation of metal oxide nanowires is strictly
limited since it is very easily dissolved in a strong acidic
solution.
4 Author to whom any correspondence should be addressed.
In this study, we will show the preparation of TiO2
nanowires inside a porous alumina template by a simple
dipping method in an acidic solution. In addition, we
will discuss the effect of TiO2 nanowires on electrostatic
capacitance compared with a simple thin film of TiO2.
To the best of our knowledge, there is no report on the
electrostatic capacitance of metal oxide nanowires with a high
dielectric constant, whereas that of bulk metal oxide has been
well studied. For example, titanium oxide has a relatively
high dielectric constant, showing 48 for anatase and 110 to
117 for rutile structure [14]. In addition, it was reported by
Wihelmsen and Hurlen [15] that anodic oxide films formed on
titanium show a dielectric constant of about 60.
2. Experiment
TiO2 nanowires were infiltrated into commercially available
porous alumina membrane (Whatmann, Anodisc 25) with a
pore diameter of ca. 200 nm and the depth of 60 µm by two
successive processes: dipping and drying. The solution for
the preparation of TiO2 nanowires consists of 5 wt% TiO2
0957-4484/05/091449+05$30.00 © 2005 IOP Publishing Ltd Printed in the UK 1449
J Lee et al
i(Aluminium foil)
Figure 1. Schematic diagram of (A) dipping procedure and (B) electrostatic capacitance measurement using two electrodes.
nanoparticle solution made of 5% tetra ethyl orthosilicate
(TEOS) binder and 35% methanol, having ca. pH 1.5 [16].
In detail, the porous alumina template was impregnated
into the solution by the automatic dipping technique which
allows us to exactly control the dipping rate and the time to soak
the template in the solution (see figure 1(A)). Typical dipping
rates were 20, 10 and 2 mm min−1. Subsequently, the porous
alumina template with the wet titania solution was dried at
80 ◦C in a vacuum oven under nitrogen atmosphere to remove
the solvent and make the structure more compact. Afterwards,
the porous alumina template was selectively dissolved in 0.1 M
NaOH solution for 10 h in order to obtain TiO2 nanowires.
The surface morphologies of TiO2 nanowires were
analysed by field emission scanning electron microscopy (FE-
SEM, Hitachi S-4300) and transmission electron microscopy
(TEM, Phillips CM200). Crystal phases were examined
by x-ray diffractometry (XRD, Phillips DY616). Elemental
qualitative analysis of prepared TiO2 nanowires was carried
out using energy dispersive x-ray spectroscopy (EDS) coupled
with the TEM equipment. Capacitance measurements of the
alumina template and TiO2 filled alumina were performed in
two electrode systems, as shown in figure 1(B). In order to
measure the capacitance of the Al2O3 template and the TiO2
nanowire–Al2O3 template structure, a Cu layer was deposited
on one side of the template by a direct current (DC) sputtering
technique. The counter-electrode is electrochemically etched
high-purity aluminium foil and it was placed at the inner
wall of the glass reactor. The electrolyte solution is 80 g l−1
ammonium borate and the capacitance was measured at
constant frequency of 80 Hz by LC R meter (ANDO type AG-
4303).
3. Results and discussion
3.1. Preparation of TiO2 nanowires
In an impregnation process, the dipping rate plays a very
important role in fabricating highly dense nanowires embedded
in the porous alumina template. For example, if the dipping
rate is too fast, the time to contact with a sol solution is too
short to make nanowires when it is dried. On the other hand, the
porous alumina template would be dissolved if it stayed in an
acidic solution for a long time, since the porous alumina is only
stable in a solution with the range of pH values between 3 and
10 (in this work, we used a solution with pH 1.5). Therefore,
an optimized dipping rate (or time) is an important factor in
the process.
As mentioned in the experimental section, three different
dipping rates of 20, 10 and 2 mm min−1 were performed
to obtain homogenously infiltrated TiO2 nanowires using a
porous alumina template. If the dipping rate is 20 mm min−1,
the template stays in the solution for only 3 s. As expected,
we cannot observe any TiO2 nanowires formed. Therefore, the
1450
Electrostatic capacitance of TiO2 nanowires in a porous alumina template
(A) 
(B) 
(C) 
Figure 2. Morphological observations of TiO2 nanowires in
commercial alumina template (TiO2-AAO I).
Dipping rate = 10 mm min−1. (A) is a top view and (B) and (C) are
cross-sectional views of TiO2.
dipping rate was reduced from 20 to 10 mm min−1. Figure 2
shows morphological observations of the sample prepared
at the dipping rate of 10 mm min−1. In figure 2(A), TiO2
agglomerates are observed near Al2O3 pores. Figure 2(B) is
a cross-sectional view of the alumina template, demonstrating
that TiO2 nanowires are formed inside the alumina template.
Even though not all the pores are filled with TiO2 nanowires, we
can observe nanowires with a high aspect ratio and a uniform
diameter.
In order to obtain highly infiltrated nanowires, the dipping
rate was further reduced from 10 to 2 mm min−1. TiO2–Al2O3
composite prepared at the dipping rate of 2 mm min−1 is shown
in figure 3. Figures 3(A) and (B) clearly show TiO2 nanowires
with the mean diameter of about 200 nm. In the case of
(A) 
(B) 
Figure 3. Top view of TiO2 nanowires in commercial alumina
template (TiO2-AAO II). Dipping rate = 2 mm min−1.
the dipping rate of 2 mm min−1, homogenous infiltration is
achieved in most pores since the porous alumina template is
soaked in the TiO2 solution for a longer time. However, it
should be mentioned that if the alumina template is dipped in
the solution for an extremely long time (for example 1 h) the
porous alumina template is dissolved completely and there is
no nanowire formation.
Figure 4 shows TEM image of TiO2–Al2O3 structure. The
bright areas indicate TiO2 nanowires in the porous alumina
template. In order to confirm the structural doubt, the EDX
analysis of TiO2 nanowires was performed. Interestingly, there
are four elements (Al, Si, Ti and O) detected. We can assume
that the silicon peak is originated from the silicon component
in the TiO2 solution. Al element can be explained as follows:
the low solution pH (1.5) induces a partial dissolution of the
porous alumina template and it makes a complex structure with
the TiO2 particles.
The crystal phase of TiO2 nanowires was revealed as
polycrystalline anatase phase by the XRD analysis (see
figure 5). We found that the crystal phase is mainly determined
by the annealing temperature, not depending on the dipping
rate, even though the dissolution of alumina slightly increases
as the dipping rate decreases.
3.2. Electrostatic capacitance
The electrolytic capacitor generally consists of foils separated
by thin layers of an insulating medium. Its capacitance is
1451
J Lee et al
(A)
(B)
1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00
o
Al
Si Ti
Figure 4. EDX analysis and TEM image of TiO2 nanowires in
porous alumina.
(This figure is in colour only in the electronic version)
determined by both electrode surface area and the thickness of
dielectric layer and the permittivity of dielectric material.
C = εε0 A/δ (1)
where ε0 is the vacuum permittivity, ε is the relative dielectric
constant of the oxide film, A is the surface area, and δ is the
film thickness.
In order to increase the electrostatic capacitance of the
capacitor, it is necessary to increase the surface area (A) of
the electrodes, to decrease the gap (δ) between the electrodes,
or to provide an insulator with a high dielectric constant (ε)
between the electrodes. In a conventional aluminium-based
capacitor, the surface area could be increased by a factor of
1000 by electrochemical etching of aluminium. Aluminium is
electrochemically dissolved at pitting sites of aluminium oxide
film when it is exposed to aggressive anions such as Cl−. As
a result of metal dissolution, the aluminium becomes rough,
with higher surface area [17–20].
In a capacitor based on nanowires with a high aspect ratio,
we expect that the interface area between the electrolyte and
the dielectric metal oxide (Ae/o) can be increased, whereas
that between the dielectric metal oxide and the metal (Ao/m)
is almost the same or less. In this study, the effect of the
unbalanced surface areas on the capacitance is investigated.
Assuming that the titanium oxide is the anatase phase with
the relative dielectric constant of 48 and the thickness of 60 µm
as shown in figure 6(A), we can calculate that the electrolytic
capacitance of the film is around 7.0 × 10−4 µF cm−2 from
equation (1). However, the measured capacitance of the TiO2
Figure 5. X-ray diffractometry (XRD) analysis of TiO2 nanowires.
It shows that the crystal phase is polycrystalline anatase structure.
(A)
60µm
(B)
60µm
Figure 6. Schematic diagram of (a) TiO2 thin film capacitor and (b)
TiO2 nanowire capacitor.
nanowires in the porous alumina is about 143 µF cm−2, which
is 3.5 × 105 times higher than the calculation based on the
simple thin film. In fact, the real geometry of our sample is
complicated, as shown in figure 6(B). The surface areas of Ae/o
and Ao/m are enlarged by a factor of 174 and 0.5, respectively,
compared with those in figure 6(A), if we suppose that the
density of pores in the porous alumina with the pore diameter
of 200 nm is 4.6×108 cm−2 [21]. Now, an effective thickness
(δe) is derived for the calculation of the capacitance based on
nanowires with the high aspect ratio since the films discontinue
in the case of nanowires.
Equation (1) can be rationalized as
C/A = (εε0/δe) × F (2)
where F is the surface enlargement factor, e.g., unity for
figure 6(A). In addition, we assume that F is 174 in figure 6(B),
since Ae/o  Ao/m.
Equation (2) would be described as
δe (cm) = (εε0 × F)/CA (3)
where CA (=C/A) is the capacitance per unit area (cm2).
The CA of TiO2 nanowires can be roughly estimated as
around 93 µF cm−2, that is the deduction of 50 µF cm−2
(porous alumina template with the Cu electrode only, measured
1452
Electrostatic capacitance of TiO2 nanowires in a porous alumina template
value) from 143 µF cm−2 (TiO2 nanowires/porous alumina
composite with the Cu electrode, measured value).
From equation (3), δe of the nanowire capacitor is about
90 nm, which corresponds to the mean radius of nanowires
(r ≈ 100 nm). Since the core of the nanowires is the farthest
away from the electrolyte, it is reasonable to assume that the
radius of the nanowires is the effective thickness, δe, for the
calculation of capacitance based on nanowires. We believe
that the small difference between the calculated δe and the
actual radius r is due to the facts that the nanowires are not
a perfectly circular shape and that there is a deviation of the
density of nanowires. Even though a carefully theoretical study
is needed to deeply understand the capacitance of nanowires,
we believe that this study shows the basis for the development
of a nanowire capacitor.
4. Conclusion
In summary, highly ordered TiO2 nanowires were infiltrated
into a porous alumina template by a simple dipping technique.
Since the solution was very acidic, the dipping rate was
the critical factor to obtain the homogenous infiltration of
TiO2 nanowires. We found that the optimized dipping
rate is 2 mm min−1 and the prepared TiO2 nanowires are
polycrystalline anatase phase, which is determined not by the
dipping rate but by the annealing process. The electrostatic
capacitance of the nanowires was measured and compared with
a theoretical calculation. Considering the effective thickness
(δe) of nanowire structured TiO2, the capacitance calculation
of TiO2 nanowires supported experimental measurement. We
expect that this paper will be a reference for the development
of nano-scale dielectric capacitance systems.
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Electrostatic capacitance of TiO_2 nanowires in a porous alumina template

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Electrostatic capacitance of TiO_2 nanowires in a porous alumina template

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