Synthesis and ferroelectric properties of multiferroic BiFeO3 nanotube arrays

Article abstract of DOI:10.1063/1.2076437

We report the synthesis and characterization of ordered multiferroic BiFe O3 (BFO) nanotube arrays. BFO nanotubes with diameters of about 250 nm and lengths of about 6 μm were fabricated by means of a sol-gel method utilizing nanochannel alumina templates. After postannealing at 700 °C, the BFO nanotubes exhibited a polycrystalline microstructure, and x-ray diffraction and transmission electron microscopy study revealed that they are of a perovskite crystal structure. Significant ferroelectric and piezoelectric characteristics of BFO nanotubes have been demonstrated by means of piezoresponse force microscopy measurement.

Full text of DOI:10.1063/1.2076437

Synthesis and ferroelectric properties of multiferroic BiFeO3 nanotube
arrays
X. Y. Zhang, C. W. Lai, X. Zhao, D. Y. Wang, and J. Y. Dai
Citation: Appl. Phys. Lett. 87, 143102 (2005); doi: 10.1063/1.2076437
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APPLIED PHYSICS LETTERS 87, 143102 ͑2005͒
Synthesis and ferroelectric properties of multiferroic BiFeO3 nanotube
arrays
X. Y. Zhang, C. W. Lai, X. Zhao, D. Y. Wang, and J. Y. Daia͒
Department of Applied Physics, The Hong Kong Polytechnic University, Kowloon, Hong Kong, People’s
Republic of China
͑Received 25 February 2005; accepted 25 August 2005; published online 26 September 2005͒
We report the synthesis and characterization of ordered multiferroic BiFeO3 ͑BFO͒ nanotube arrays.
BFO nanotubes with diameters of about 250 nm and lengths of about 6 ␮m were fabricated by
means of a sol-gel method utilizing nanochannel alumina templates. After postannealing at 700 °C,
the BFO nanotubes exhibited a polycrystalline microstructure, and x-ray diffraction and
transmission electron microscopy study revealed that they are of a perovskite crystal structure.
Significant ferroelectric and piezoelectric characteristics of BFO nanotubes have been demonstrated
by means of piezoresponse force microscopy measurement. © 2005 American Institute of Physics.
͓DOI: 10.1063/1.2076437͔
The prospect of a new generation of random access
memory technology to handle the growing demand for
memory is pushing several research fronts in this area,1–4
such as magnetic random access memory and ferroelectric
random access memory. Magnetism is involved with the lo-
cal spin of electrons, while ferroelectricity represents a co-
operative phenomenon that relies on the interaction of neigh-
boring permanent electric dipoles in a crystal lattice. These
two phenomena can coexist in some unusual perovskite-type
oxide materials including magnetic elements, such as
BiFeO3,5–7 BiMnO3,8,9 and TbMnO3,10,11 termed multiferro-
ics. Multiferroics have attracted much attention because of
their promise in the realization of a new type of memory by
a combination of ferroelectric and ferromagnetic properties.
As the development of nanoscale electronics approaches a
practical stage, it is quite natural to ask how the crystal struc-
ture and state of polarization are influenced by the shape and
size of multiferroic materials. The synthesis of multiferroic
nanostructures with a controllable size and shape is critical
not only in new device application, such as high-density
magnetically recorded ferroelectric memory, but also from a
fundamental point of view.
BiFeO3 ͑BFO͒ is known to have rhombohedrally dis-
torted perovskite structure. The space group R3c of BFO
allows for a ferroelectric atomic displacement of below
1083 K and a weak ferromagnetism of below a Néel tem-
perature TN of 643 K simultaneously. The enhancement of
polarization and related properties in heteroepitaxially con-
strained BFO films with a thickness of 200 nm has been
reported. The films display a room-temperature spontaneous
polarization almost of an order of magnitude higher than that
of the bulk.3,12 More recently, a giant ferroelectric polariza-
tion of beyond 150 ␮C/cm2 has been obtained in BFO thin
film deposited on a Pt/TiO2/SiO2/Si substrate at 90 K.13
Ordered BFO nanostructure arrays are of considerable inter-
est for future applications, such as a vertical magnetic re-
cording with an ultrahigh recording density. In this letter, we
report the synthesis of the BFO nanotube arrays by using a
sol-gel template method. Significant ferroelectric and piezo-
electric characteristics of the BFO nanotube arrays have been
revealed.
Nanochannel alumina ͑NCA͒ templates were prepared
by means of anodization.14,15 High-purity ͑99.999%͒ alumi-
num foils were used as the start material. Prior to anodizing,
the aluminum was annealed at 500 °C in order to obtain
homogeneous conditions for the growth of pores over large
areas. Subsequently, the foils were electropolished in a 1:9
by volume mixture of HClO4 and C2H5OH. Anodization was
carried out for 12 h under a constant cell voltage of 180 V in
a 0.3 M H3PO4 solution, and the temperature was kept con-
stant at 0 °C. Then, the alumina was removed in a mixture of
phosphoric acid and chromic acid, and the Al sheet was an-
odized again under the same condition as above. After the
anodization, the remaining aluminum was removed in a satu-
rated HgCl2 solution. The pore bottoms were opened and
widened by chemical etching in a 10% H3PO4 solution at
50 °C. The BFO nanotubes were synthesized by a sol-gel
method utilizing the NCA templates. The BFO sol-gel pre-
cursor was prepared by dissolving bismuth nitrate
Bi͑NO3͒3.5H2O, and iron nitrate Fe͑NO3͒3.9H2O in
2-methoxyethanol C3H8O2, and followed by stirring for
30 min at room temperature. The concentration of the final
solution was adjusted to 0.3 M with a pH value of 1–2 by
adding 2-methoxyethanol and nitric acid. The NCA tem-
plates were immersed in the precursor solution for 20 min. In
order to obtain the perovskite phase, the templates containing
the precursor were subsequently heated in air at 700 °C for
60 min using a thermal annealing furnace. To measure the
piezoelectric properties of the BFO nanotubes, the two sur-
faces of the BFO-filled NCA templates were mechanically
polished to remove the BFO remaining on the surface until
the BFO nanotubes emerged. A continuous Pt layer and dot
electrodes with a thickness of about 100 nm were then
coated on the two sides of the samples separately for dielec-
tric and piezoelectric response characterizations. The struc-
ture and morphology of the BFO nanotubes were investi-
gated using x-ray diffraction ͑XRD͒ ͑D/Max 2250V͒, a
scanning electron microscope ͑SEM͒ ͑JEOL JSM-6300͒, and
a high-resolution transmission electron microscope ͑HR-
TEM͒ ͑JEOL-2010͒. The dielectric property was measured
using a precision impedance analyzer ͑Agilent 4294A͒, and
a͒
Electronic mail: apdaijy@inet.polyu.edu.hk
0003-6951/2005/87͑14͒/143102/3/$22.50
87, 143102-1
© 2005 American Institute of Physics
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143102-2
Zhang et al.
Appl. Phys. Lett. 87, 143102 ͑2005͒
FIG. 1. SEM images of the
nanochannel alumina ͑NCA͒ tem-
plate and BFO nanotubes: ͑a͒ NCA
template, ͑b͒ top view of the
nanochannel porous alumina filled
with BFO nanotubes, ͑c͒ oblique
view of BFO nanotube arrays, and
͑d͒ XRD pattern of the BFO nano-
tube arrays.
the piezoelectric property of the BFO nanotubes was mea-
sured by piezoresponse force microscopy using a commer-
cial scanning probe microscope ͑Nanoscope IV, Digital In-
struments͒ equipped with a conductive tip and a lock-in
amplifier ͑SR830, Stanford Research Systems͒.
dividual nanotube, the nanotubes are composed of equal
quantities of the elements Bi and Fe. The left-hand side inset
shows the selected area electron diffraction pattern ͑SAED͒
taken from the nanotube, which reveals the polycrystalline
structure nature of the BFO nanotubes. The perovskite struc-
ture of the BFO nanotubes was further confirmed by HR-
TEM image as shown in Fig. 3. It can be seen that the BFO
nanotubes are composed of nanoparticles of sizes ranging
from several to ten nanometers, and that the average wall
thickness of the nanotubes is about 20 nm. The well-
recognized lattice spacing of 0.395 nm corresponds to the
͕101͖ atomic planes.
In order to demonstrate the ferroelectric characteristics
of the BFO nanotubes, piezoresponse d33 hysteresis loop of
an individual BFO nanotube was measured using the con-
ductive atomic force microscope tip applied with a 16.5 kHz
ac electric field plus a swept dc voltage, and the result is
shown in Fig. 4. The details of the piezoresponse measure-
ment have been reported earlier, where the piezoelectric
properties of Pb͑Zr0.53Ti0.47͒O3 nanowires were studied.16
The decrease in d33 at high electric field, as shown in Fig. 4,
Figures 1͑a͒ and 1͑b͒ show the SEM images of the
closely packed porous NCA template and one filled with
BFO nanotubes, respectively. The pore diameters and the
thickness of the NCA template are about 250 nm and 6 ␮m,
respectively; and it can be seen that almost all of the pores
are filled with BFO nanotubes. Figure 1͑c͒ is the correspond-
ing SEM image of the free-standing BFO nanotube arrays
after the alumina was etched away using a 4 M NaOH solu-
tion. The XRD spectrum of the BFO nanotubes after the
alumina was dissolved away is shown in Fig. 1͑d͒. The re-
flection peaks are clearly distinguishable, and can be per-
fectly indexed as a rhomohedrally distorted perovskite BFO
structure. No obvious reflections that would indicate the ex-
istence of second phases were detected.
Figure 2͑a͒ shows the transmission electron microscope
͑TEM͒ image of BFO nanotubes after the alumina was dis-
solved away; it is apparent that the BFO nanotubes are of a
uniform diameter. Figure 2͑b͒ shows the TEM image of a
single BFO nanotube. The right-hand side inset shows the
energy dispersive x-ray ͑EDX͒ spectrum taken from the in-
FIG. 2. ͑a͒ TEM image of the BFO nanotubes after completely dissolving
the NCA template, and ͑b͒ TEM image of an isolated BFO nanotube, the left
inset shows the corresponding SAED pattern, the right inset shows the EDX
spectrum.
FIG. 3. HRTEM image of the BFO nanotubes, the arrows indicate the ͕101͖
atomic planes with lattice spacing around 0.395 nm.
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143102-3
Zhang et al.
Appl. Phys. Lett. 87, 143102 ͑2005͒
of the dielectric constant at low and intermediate frequen-
cies, which has been explained by the phenomenon of dipole
relaxation; while the variation in dielectric loss with fre-
quency represents the relaxation absorption of the
dielectric.17 The dielectric constant and dielectric loss were
90 and 0.04, respectively, at 100 kHz. The dielectric constant
value matches well with the values reported in the
literature.18,19
In summary, ordered BFO nanotube arrays were fabri-
cated using sol-gel synthesis within hexagonal closely
packed nanochannel alumina templates. These BFO nano-
tube arrays exhibit significant piezoelectric characteristics
and may have potential application in fabrication of magneti-
cally recorded ferroelectric memory, and lead-free piezoelec-
trics for sensors and actuators.
This work was supported by a Postdoctoral Fellow
project of the Hong Kong Polytechnic University ͑G-YX17͒.
FIG. 4. Piezoelectric hysteresis loop of a single BFO nanotube measured by
piezoresponse force microscopy.
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Figure 5 shows the dielectric constant and dielectric loss
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as a function of frequencies in the range of 1 kHz to 1 MHz.
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