Fabrication of Pd-Fe nanowires with a high aspect ratio by AAO template-assisted electrodeposition

Article abstract of DOI:10.1016/j.jallcom.2010.12.153

In this study, vertically oriented Pd_0.86Fe_0.14 nanowires have been fabricated using an anodized aluminum oxide (AAO) template by direct voltage electrodeposition at room temperature. AAO template-assisted electrodeposition of Pd-Fe was carried out in Pd(NH₃) ₂Cl₂:FeSO₄·7H₂O solution. The AAO template and the Pd_0.86Fe_0.14 nanowires were characterized by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) methods and X-ray diffraction (XRD). It was observed that the Pd _0.86Fe_0.14 nanowires were approximately 65 nm in diameter and 10 μm in length with an aspect ratio of 153 in a relatively large area of about 4 cm². The nucleation rate and the number of atoms in the critical nucleus are determined from the analysis of current transients.

Full text of DOI:10.1016/j.jallcom.2010.12.153

Journal of Alloys and Compounds 509 (2011) 3894–3898
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Fabrication of Pd–Fe nanowires with a high aspect ratio by AAO
template-assisted electrodeposition
Nevin Tas¸ altına, Sadullah Öztürka, Necmettin Kılınc¸ a, Hayrettin Yüzerb, Zafer Ziya Öztürka,b,∗
a Gebze Institute of Technology, Science Faculty, Department of Physics, 41400 Gebze, Kocaeli, Turkey
b TUBITAK Marmara Research Center, P.O. Box 21, 41470 Gebze, Kocaeli, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, vertically oriented Pd0.86Fe0.14 nanowires have been fabricated using an anodized aluminum
oxide (AAO) template by direct voltage electrodeposition at room temperature. AAO template-assisted
electrodeposition of Pd–Fe was carried out in Pd(NH3)2Cl2:FeSO4·7H2O solution. The AAO template and
the Pd0.86Fe0.14 nanowires were characterized by scanning electron microscopy (SEM), energy dispersive
X-ray (EDX) methods and X-ray diffraction (XRD). It was observed that the Pd0.86Fe0.14 nanowires were
approximately 65 nm in diameter and 10 ␮m in length with an aspect ratio of 153 in a relatively large
area of about 4 cm2. The nucleation rate and the number of atoms in the critical nucleus are determined
from the analysis of current transients.
Received 7 October 2010
Received in revised form
15 December 2010
Accepted 19 December 2010
Available online 28 December 2010
Keywords:
Pd–Fe alloy
Nanowire
© 2010 Elsevier B.V. All rights reserved.
Electrodeposition
Nucleation
AAO template
1. Introduction
template is an ideal template as it possesses many desirable char-
acteristics, including tunable pore dimensions, good mechanical
Metal nanostructures, such as nanoparticles, nanowires and
nanoarrays have attracted much attention in recent years due
to their extraordinary electronic, magnetic, optic and chemical
properties [1–3]. The controllable growth of metal nanowires has
been a topic of continuing investigation because of the need to
obtain high-performance and high-density nanoelectronic devices,
such as magnetic recording devices, solar cells, hydrogen stor-
age devices, transistors, chemical sensors, and so forth [4–8].
Over the last several years, metal nanowire arrays have been
fabricated using a variety of techniques, such as electrochemical
methods, chemical vapor deposition, sol–gel and template assisted
electrodeposition [5–8]. Recently, the template-assisted fabrica-
tion of functional nanowires has attracted considerable attention.
This is mainly owing to the possibility of controlling the length,
diameter, and density of the fabricated nanowires by varying the
template and/or deposition parameters. The template method has
been accomplished using a variety of templates, such as polycar-
bonate templates (PC), nanochannel alumina (NCA) and anodized
aluminum oxides (AAO). It has been considered that the AAO
strength, thermal stability and ordered nanotubes at a high den-
sity (109–1011 cm2) [9–11]. Furthermore, the distribution patterns
of pore size in the AAO template can directly influence the diam-
eters of the nanowires, which can be easily controlled by varying
the preparation conditions of the AAO, such as the temperature
and oxidation voltage. Thus, it is also a useful technique for the
deposition of metals with different sizes.
The fabrication of Fe–Pd alloy nanowires is very interesting to
study both in terms of the magnetic properties and the marten-
sitic transformation on the nano scale [12]. Also, high uniaxial
anisotropy also enables Fe–Pd particles to overcome thermal fluc-
tuations even for very small sizes. Moreover, the Fe–Pd alloy
exhibits the properties of the shape memory effect (SME) and the
magnetostrictive effect caused by face-centered cubic (fcc) to face-
centered tetragonal (fct) thermoelastic martensitic transformation
[13–15]. On the other hand, one-dimensional nanostructured
Pd–Fe alloys have attracted extensive attention recently because
of their physical and chemical properties; they are considered to
be promising candidate materials for room temperature hydro-
gen sensing because they have good physio-mechanical strength
and resistance to poisoning by other chemical species. Baba et al.
[16] studied the interaction of Pd and Pd alloys with hydrogen gas
and found that the electrical resistance of Pd and Pd alloys was
dependent on the absorbed hydrogen amount of the metal phase.
Although Pd and Pd alloys as H2 sensing materials were reported
∗
Corresponding author at: Gebze Institute of Technology, Science Faculty, Depart-
ment of Physics, Cayirova Campus, Istanbul St. no. 101, 41400 Gebze, Kocaeli, Turkey.
Tel.: +90 2626051306; fax: +90 2626538490.
E-mail address: zozturk@gyte.edu.tr (Z.Z. Öztürk).
0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.12.153

N. Tas¸altın et al. / Journal of Alloys and Compounds 509 (2011) 3894–3898
3895
by many research groups [17–23], further improvement is still
required with respect to selectivity, sensitivity and the lifetime of
the sensors. To date, only a limited number of reports describing
the fabrication of Fe–Pd alloy nanowires has appeared in the lit-
erature [12,24–26], and there is no report about the fabrication of
Pd–Fe nanowires which have low percentage of Fe content. Fei et al.
[12] fabricated Fe0.3Pd0.7 and Fe0.6Pd0.4 nanowire arrays in 20 nm
diameters by alternating current electrodeposition into an AAO
template for the investigation of structural and magnetic proper-
ties. Then, Hu et al. [25] fabricated an AAO template approximately
60 nm in diameter and 40 ␮m in length. Afterwards, they fabricated
a Fe–Pd nanowire array by dc electrodeposition using FeCl2 and
PdCl2 with a pH value of three. Thus, Fe0.95Pd0.05 nanowires were
fabricated for magnetic applications. Furthermore, Prida et al. [26]
fabricated Fe0.7Pd0.3 nanowires approximately 35 nm in diameter
and 4 ␮m in length using an AAO template by using alternating
pulsed electrodeposition. When a chlorinated solution is used for
electrodeposition (for instance, FeCl2), the AAO template can be
deformed or dissolved due to the high acidic media and pH value,
unfortunately. Further studies and the development of the AAO
template-assisted fabrication of nanowires are obviously of sig-
nificant importance; however, there is an important lacking point
in the literature about the fabrication of vertically free-standing
Pd–Fe nanowires. Therefore, in this study, the fabrication of a
Pd–Fe nanowire array with a high aspect ratio was reported in
detail. Moreover, we reported new impressive results on the mech-
anism and kinetics of the initial stages of Pd–Fe electrodeposition
in the AAO template, nucleation and growth kinetics of the Pd–Fe
nanowires.
2. Experimental
AAO templates were fabricated by the two-step anodization of high-purity alu-
minum foils in 0.3 M oxalic acid solution under a constant dc voltage of 40 V [27,28].
A thin gold film was evaporated onto one surface of the as-prepared AAO template,
which serves as the working electrode that has a silver epoxy attached to the Ti
foil as a substrate. The used Ti foil was polished and cleaned beforehand. Then, the
remaining aluminum of the AAO template, which was on the opposite side of the Au
film, was etched in 5 wt% HgCl2 for complete removal. After that, the AAO template
was treated again in a 5 wt% phosphoric acid solution at 35 ◦C for 18 min to remove
the AAO barrier layer. A schematic diagram of the preparation stages of the AAO
template is given in Fig. 1.
Electrodeposition was carried out in a Pd(NH3)2Cl2:FeSO4·7H2O solution with a
pH value of six. Firstly, the Pd(NH3)2Cl2 solution was prepared. PdCl2 was added to
hydrochloric acid and mixed until it completely dissolved the solution and it became
brown and transparent. Pink-colored Pd(NH3)2Cl2 appeared when the solution was
dropped into dilute ammonia. The solution was stirred at 70 ◦C for 1 h. Then, the pH
value of the solution was adjusted to seven by the addition of dilute sulfuric acid.
Thus, the Pd(NH3)2Cl2 solution was obtained. After that, we added FeSO4·7H2O and
C7H6O6S·2H2O to the Pd(NH3)2Cl2 solution and adjusted its pH value to about six
by adding dilute ammonia. Finally, the Pd–Fe electrolyte was obtained.
Electrodeposition was carried out at room temperature (25 ◦C) using a three-
electrode potentiostatic system with a saturated calomel electrode as a reference
electrode and a graphite counter electrode. Pd–Fe was deposited using a poten-
tial of −1 V with a 1800-s deposition time by the direct voltage electrodeposition
method. Then, the sample was annealed at 500 ◦C for 1.5 h. The annealing pro-
cess provides Pd–Fe nanowires with a crystalline structure. To obtain free-standing
Pd–Fe nanowires, the AAO template was dissolved with a dilute NaOH solution, and
then carefully rinsed away with deionized water.
Fig. 1. Schematic diagram of the preparation stages of the AAO template.
153. The AAO nanotube density is approximately 1.75 × 1014 cm−2
[27]. Fig. 2b shows the bottom of the AAO template after Al is
removed with HgCl2, and Fig. 2c shows the opened AAO barrier
layer of template in phosphoric acid solution. It is clearly seen from
comparison of Fig. 2a and c that the pore diameters of template are
approximately same. So, the complete removal of the oxide barrier
layer from the pore bottom of AAO is succeeded.
Fig. 3 shows the current behavior as a function of potential dur-
ing the electrodeposition of the Pd–Fe nanowires.
Electrodeposition was carried out in a Pd(NH3)2Cl2:FeSO4·7H2O
solution with a pH value of six. Firstly, the Pd(NH3)2Cl2 solution
was prepared by adding PdCl2 into hydrochloric acid and mixing
Scanning electron microscopy, energy dispersive X-ray (SEM; EDX, Jeol JSM
6335) methods and X-ray diffraction (XRD, Cu K␣, ꢀ = 0.154 nm) were used to study
the crystalline structure and morphologies of the fabricated Pd–Fe nanowires.
3. Results and discussion
The morphologies of the fabricated AAO template and the Pd–Fe
nanowires were studied by SEM. Fig. 2a shows SEM image of the
fabricated AAO template after the second anodization process.
According to the SEM image, the fabricated hexagonally straight
AAO nanotubes were approximately 70 nm in diameter, 10 ␮m in
length, and had 90-nm interpore distances and an aspect ratio of

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N. Tas¸altın et al. / Journal of Alloys and Compounds 509 (2011) 3894–3898
Fig. 3. The current transient during the electrodeposition of Pd–Fe in the AAO tem-
plate.
obtained. After that, we dissolved FeSO4·7H2O and C7H6O6S·2H2O
into three containers of distilled water and mixed that with a
Pd(NH3)2Cl2 solution and adjusted its pH value to about six by
adding dilute ammonia. Finally, the Fe–Pd electrolyte was obtained.
The role of C7H6O6S·2H2O in electrodeposition is to prevent
Fe2+, from forming into Fe3+. In previous studies, the electrodepo-
Fig. 2. SEM images of the fabricated AAO template: (a) top view, (b) bottom view,
and (c) after removing of the alumina barrier layer.
it until the solution was completely dissolved and became brown
and transparent as the following reaction shows:
PdCl2 + HCl → [PdCl4]2− + 2H+
(1)
Pink-colored Pd(NH3)2Cl2 appeared when the solution was
dropped into dilute ammonia as follows:
[PdCl4]2− + 4NH3 → [Pd(NH3)4]2+Cl2
(2)
The solution was stirred at 70 ◦C for 1 h. Then, the pH value of
the solution was adjusted to seven by the addition of dilute sulfuric
acid as follows:
[Pd(NH3)4]Cl2 + H2SO4 → [Pd(NH3)2]Cl2 + (NH4)2SO4
(3)
The (NH4)2SO4 increases the electroconductivity of the elec-
trodeposition solution. Thus, the Pd(NH3)2Cl2 solution was
Fig. 4. Log(current density)–log(time) transients of increasing current density dur-
ing Pd–Fe electrodeposition on the Au electrode.

N. Tas¸altın et al. / Journal of Alloys and Compounds 509 (2011) 3894–3898
3897
Fig. 6. EDX spectra from the surface of the as-deposited Pd–Fe nanowire array.
trodeposition kinetics of Pd–Fe on the Au electrode in the AAO
template, decreasing and then beginning to increase the current
density, occurred over a very short time period. The Pd2+ and Fe2+
solutions that were used for electrodeposition filled a nanotube
of the AAO template that was 5 × 106 nm3 in volume (70 nm). To
form an atomic Pd0 and Fe0 on the Au film at the bottom of the
AAO nanotube, the process required Pd2+ and Fe2+ ions. As the
required process was diffusion-controlled, double layers at the AAO
nanotubes of the template could form after 30 s, which is a very
short time period for this process. After forming the double lay-
ers, the continuously increasing current densities had a nonlinear
characteristic.
The log(current density)–log(time) transient of increasing cur-
rent density during the Pd–Fe electrodeposition on the Au electrode
is shown in Fig. 4.
The slope of the log(current density)–log(time) transient in
Fig. 4 is approximately 0.5 A cm−2 s−1/2, which indicates that the
electrodeposition kinetics follow a parabolic relationship. There-
fore, the experimentally found value for the number representing
the Pd–Fe nuclei density (N) can be calculated by using the follow-
ing relation:
Fig. 5. (a) High magnification SEM image of the Pd–Fe nanowires in the AAO tem-
plate. (b) SEM image of the Pd–Fe nanwires after removing the AAO template. (c)
High magnification top view of the Pd–Fe nanowires.
ꢁ1/2
N =
kP
(6)
zF ꢂ(2Dc)3/2M1/2
sition performed for equal molar Pd and Fe and for the Fe/Pd ratio
in fabricated nanowires was up to 25%. In this study, for the elec-
trodeposition of low ferric Fe2+ (<20) consisting of the Pd–Fe alloy,
it easier to form the metal phase than a Pd2+ ion. Instead of equal
molar Pd and Fe amounts, we used an electrolyte that consists of
twice as little Fe2+ molarity in proportion to Pd2+ molarity. When
voltage is applied the number of [Pd(NH3)2]2+ ions in proportion
to the number of the Fe2+ ions at the cathode are two times higher.
The Fe ratio in the formed alloy is low, as expressed below:
Here the value of the parabolic rate constant (kP) was obtained
from the slope of current density–t1/2 transient. The elec-
trodeposition current (I) usually obeys the following growth
relation.
tion of metallic ions in the solution, zF is the effective molar
charge of the electrodepositing species, and is time.
D is the diffusion coefficient, c is the concentra-
t
M
and ꢁ are the molecular weight and density of the metallic
species deposited, respectively. We take the values of z = 2,
[Pd(NH3)2] + 2e− → Pd0 + 2NH3
Fe2+ + 2e− → Fe0
(4)
(5)
F = 96,500 C mol−1
,
D = 6.70 × 10−6 cm2 s−1
,
M = 106.42 g mol−1
,
c = 2.087 × 10−6 mol cm−3, and ꢁ = 12.023 g cm−3. Thus, our Pd–Fe
nanodeposition calculated value of N is 1.367 × 109.
As shown in Fig. 3, the electrodeposition of the Pd–Fe nanowires
is not a steady state process. According to the Pd–Fe and Au inter-
action, the current–time transient shows a sharp drop in the initial
current density due to the charging of the double layer, which was
followed by a nonlinear increase in the current density with elec-
trodeposition time. Such double layer charging was also observed
during the electrodeposition of various metals [29–33]. The elec-
Fig. 5 shows the SEM images of the as-deposited Pd–Fe
nanowires before and after dissolving the AAO template. As shown
in Fig. 5a, the Pd–Fe nanowires completely filled the AAO. Fig. 5b
and c shows that the Pd–Fe nanowires are roughly parallel to each
other, vertically aligned to form an array. Fabricated vertical Pd–Fe
nanowires are approximately 65 nm in diameter and 10 ␮m in
length.

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N. Tas¸altın et al. / Journal of Alloys and Compounds 509 (2011) 3894–3898
Acknowledgements
This study was supported by The Scientific and Technologi-
cal Research Council of Turkey. The project title is “Investigation
and Development of Nanotechnologic Hydrogen Sensors” and the
project number is “106T546”.
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4. Conclusions
A highly ordered Pd–Fe nanowire array that has a high surface
area was fabricated at room temperature by AAO template-assisted
electrodeposition. The morphology, structure and growth mecha-
nisms of these Pd–Fe nanowires were reported in detail. The initial
stages of the electrodeposition of the Pd–Fe nanowires on Au were
studied using potentiostat. The nucleation rate and the number
of atoms in the critical nucleus are determined from the analysis
of current transients. Discussions of the underlying principle that
affects the properties of Pd–Fe in the present work can be help-
ful for further understanding the other alloy-structured nanowires.
We believe that understanding the nucleation and growth mecha-
nisms of the Pd–Fe nanowires gives rise to potential applications,
especially in the field of sensors.