Fabrication and characterization of smooth high aspect ratio zirconia nanotubes

Article abstract of DOI:10.1016/j.cplett.2005.05.065

In the present work, we report formation of high aspect ratio zirconia nanotubes by electrochemical anodization of zirconium in a 1 M (NH ₄)₂SO₄ electrolyte containing 0.5 wt% NH ₄F. Highly self-organized zirconia nanotubes can be formed with a diameter of ≈50 nm and a length of ≈17 μm, i.e. with an aspect ratio of more than 300. The nanotubes show a distinct smooth and straight morphology. XRD investigation reveals that the nanotubes have a cubic crystalline structure directly after anodization, that is, without any further annealing.

Full text of DOI:10.1016/j.cplett.2005.05.065

Chemical Physics Letters 410 (2005) 188–191
www.elsevier.com/locate/cplett
Fabrication and characterization of smooth high aspect ratio
zirconia nanotubes
*
Hiroaki Tsuchiya, Jan M. Macak, Luciano Taveira, Patrik Schmuki
Department of Materials Science, Institute for Surface Science and Corrosion (LKO), University of Erlangen-Nuremberg,
Martensstrasse 7, D-91058 Erlangen, Germany
Received 13 April 2005; in final form 17 May 2005
Available online 14 June 2005
Abstract
In the present work, we report formation of high aspect ratio zirconia nanotubes by electrochemical anodization of zirconium in
a 1 M (NH4)2SO4 electrolyte containing 0.5 wt% NH4F. Highly self-organized zirconia nanotubes can be formed with a diameter of
ꢀ50 nm and a length of ꢀ17 lm, i.e. with an aspect ratio of more than 300. The nanotubes show a distinct smooth and straight
morphology. XRD investigation reveals that the nanotubes have a cubic crystalline structure directly after anodization, that is, with-
out any further annealing.
Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction
high aspect ratio pore arrays, however the pores that
were formed showed a wavy and somewhat irregular
wall morphology as apparent in Fig. 1. Anodization
done in HF electrolytes also leads to tubular feature that
are however comparably irregular and wavy [14].
In this Letter, we show a pronounced improvement of
the method, that is, the formation of smooth and
straight high aspect ratio zirconia nanotubes.
One-dimensional nanoscale oxides such as nanotubes
and nanowires have attracted much attention due to
their potential applications in nanotechnology. In par-
ticular, zirconium oxide, so-called zirconia has been
extensively used as a catalyst or as a catalyst support
[1,2], as a sensor [3] and as a solid-electrolyte [4–6].
Zirconia has excellent technological properties and char-
acteristics such as chemical and thermal stability,
mechanical strength, wear resistance, low electrical con-
ductivity and a high degree of biocompatibility. Re-
cently, much research efforts have focused on the
synthesis of zirconia nanotubes by using templating
techniques [7–9] and atomic layer deposition [10]. Re-
cently, we reported a simple electrochemical approach
to form extremely thick nanoporous zirconia [11] and
nanotubular zirconia layers [12,13]. An example is
shown in Fig. 1. These porous layers have been formed
in H2SO4/NaF electrolytes [12,13]. The treatment led to
2. Experimental
Samples were zirconium foils (99.8% purity, Good-
fellow, England) with 0.1 mm thickness. Prior to the
electrochemical treatment, the samples were ultra-soni-
cated in acetone, isopropanol and methanol succes-
sively, followed by rising with deionized (DI) water
and finally drying with nitrogen. The electrochemical
cell consisted of a conventional three-electrode arrange-
ment with a platinum gauze as a counter electrode and a
Huber-Luggin capillary with a Ag/AgCl (1 M KCl) elec-
trode as a reference electrode. The samples were pressed
against an o-ring in the electrochemical cell leaving
*
Corresponding author. Fax: +49 9131 852 7582.
E-mail address: schmuki@ww.uni-erlangen.de (P. Schmuki).
0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2005.05.065

H. Tsuchiya et al. / Chemical Physics Letters 410 (2005) 188–191
189
3. Results and discussion
Various electrolytes and electrochemical treatments
were tested to improve the morphology of the porous
ZrO2 layers from previous works [11–13].
In order to obtain electrochemical information, cur-
rent–time transients proved to be a valuable tool. Fig.
2 shows such current–time curves recorded during anod-
ization of zirconium in different electrolytes after a po-
tential sweep from OCP to 20 V with a sweep rate of
1 V sÀ1. The H2SO4/NaF electrolyte was used in previ-
ous work and is included as a reference for a case where
the fluoride concentration and other parameters were
optimized to achieve self-organized pore formation. In
the previous work tube formation was carried out in a
neutral (NH4)2SO4/NH4F buffer to suppress some disad-
vantages of the acidic electrolyte [15]. Evidently the
(NH4)2SO4 containing 0.5 wt% NH4F shows a very sim-
ilar characteristics as the H2SO4/NaF electrolyte. Both
curves in fluoride-containing electrolytes deviate clearly
from the curve in fluoride-free (NH4)2SO4. The higher
current density in the fluoride-containing electrolytes
indicates an additional dissolution process takes place.
This can be attributed to a dissolution of the oxide layer
as soluble fluoro-complexes. As pointed out in previous
work this enhanced solubility by complex formation is a
key factor to achieve pore formation on valve metals. In
the fluoride-free electrolyte and as a very well estab-
lished in literature, only a compact oxide layer was
observed.
Fig. 1. SEM images of anodic porous zirconia layer. (a) Top-view and
(b) cross-sectional of the porous layer formed on Zr in 1 M
H2SO4 + 0.2 wt% NaF electrolyte at 20 V in analogy to [12,13].
Fig. 3 shows SEM images of the surface layer formed
in 1 M (NH4)2SO4 + 0.5 wt% NH4F at 20 V for 1 h after
a potential sweep from OCP to 20 V with a scan rate of
1 V sÀ1 – shown are (a) a top-view, (b) a cross-section
and (c) a bottom-view. The cross-section and bottom-
1 cm2 of the samples surface exposed to the electrolyte.
The electrolyte was 1 M (NH4)2SO4 + 0.5 wt% NH4F
and was prepared from analytical grade chemicals and
DI water.
Electrochemical treatments were carried out using a
high-voltage potentiostat Jaissle IMP 88 PC connected
to a digital multimeter interfaced to a computer. The
electrochemical treatments consisted of a potential
sweep from an open-circuit potential (OCP) to 20 V with
the sweep rate of 1 V sÀ1, followed by holding the poten-
tial at 20 V for 1 h. After the treatment, the samples
were rinsed with DI water and then dried with nitrogen
stream.
The structure and morphology of the resulting layer
were characterized with a Hitachi SEM FE S4800
field-emission scanning electron microscopy (FE-
SEM). Thickness information was obtained by direct
SEM cross-sectional observation on samples that were
bent until the oxide layer cracked off. Structural infor-
mation of the layer was obtained using a X-ray diffrac-
tometer (Phillips XÕpert-MPD PW3040). The chemical
composition of the layer was obtained by X-ray photo-
electron spectroscopy (PHI 5600 XPS).
Fig. 2. Current–time curves recorded for different electrolytes during
anodization of Zr at 20 V after a potential sweep from OCP to 20 V
with a sweep rate of 1 V sÀ1
.

190
H. Tsuchiya et al. / Chemical Physics Letters 410 (2005) 188–191
most remarkable difference is that the nanotubes in
(NH4)2SO4 + 0.5 wt% NH4F are extremely straight
and the irregularities of the pore walls (wavy morphol-
ogy) does not occur any more. With length of
ꢀ17 lm, the aspect ratio of the nanotube layer is more
than 300. Although the surface morphology is generally
very uniform, on some locations, parts of the nanotube
layers bended down in a heap, which might be due to the
extremely high aspect ratio. Furthermore, by comparing
Fig. 1b and Fig. 3b, it becomes clear that the cross-
sectional feature is relatively different from that in
H2SO4 + 0.2 wt% NaF. A key factor to form the straight
nanotube layers might be buffering effect of (NH4)2SO4/
NH4F system, thus damping possible alterations in the
local pH within the pores – this is analogy with previous
work on TiO2 nanotube formation [15] where we
showed that the pH-profile within a growing tube can
strongly determine the final morphology.
Fig. 4 shows an X-ray diffraction pattern of the nano-
tube layer shown in Fig. 3. It is clear that the layer has a
crystalline structure; the peaks corresponding to a cubic
zirconia are detected. This is in line with previous work
on compact oxide layers formed in a wide variety of
electrolytes where the cubic structures were predomi-
nant [16]. The observed crystalline structure is particu-
larly noteworthy as porous layers formed on other
valve metals such as Ti, W, Nb and Ta typically have
an amorphous structure [17]. Therefore – if a crystalline
structure is desired – the layer on such valve metals must
be transformed to a specific crystalline structure by
annealing. However, the fact that the zirconia nano-
tubes have a cubic crystalline structure directly after
anodization without annealing is of a particular advan-
tage in applications that are sensitive to thermal anneal-
ing. Additional XPS investigation also showed that the
Fig. 3. SEM images of the zirconia nanotube layer formed on Zr in
1 M (NH4)2SO4 + 0.5 wt% NH4F at 20 V after a potential sweep from
OCP to 20 V: (a) top-view; (b) cross-section; (c) bottom-view.
view were obtained from a mechanically cracked sam-
ple. It is clear from Fig. 3a that the layer consists of
self-organized nanotube arrays of ꢀ50 nm in diameter.
It is also evident from Fig. 3a and c that the nanotubes
are open at the top-end, while they are closed at the bot-
tom. The closed bottom-ends correspond to the barrier
oxide layers (essentially the high-field oxide). The fea-
tures of the top-view and bottom-view of the nanotube
layer formed in (NH4)2SO4 + 0.5 wt% NH4F is similar
to that in H2SO4 + 0.2 wt% NaF [12,13]. However, the
Fig. 4. XRD patterns for the zirconia nanotube layer formed on Zr in
1 M (NH4)2SO4 + 0.5 wt% NH4F.

H. Tsuchiya et al. / Chemical Physics Letters 410 (2005) 188–191
191
structure essentially consists of a ZrO2 though only
traces of F could be detected in the nanotube layer.
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Smooth high aspect ratio zirconia nanotubes were
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