Tuning the microstructure and functional properties of metal nanowire arrays via deposition potential

Article abstract of DOI:10.1016/j.electacta.2010.12.013

The present study is focused on electrochemical fabrication and characterization of the ordered arrays of one-dimensional Ni nanostructures templated by porous anodic alumina films. A possibility to monitor and control homogeneity of arrays and completeness of the pores filling in the course of potentiostatic deposition is discussed. Current efficiency is determined from independent analytical data and taken into account in the treatment of charge transients. The crystallinity of Ni nanowire arrays as well as the homogeneity and completeness of the replication process are demonstrated to be controlled by deposition potential. Just these structural features affect the magnetic properties of nanowire arrays. The advantages and drawbacks of the quantitative coulometric analysis of current transients are considered.

Full text of DOI:10.1016/j.electacta.2010.12.013

Electrochimica Acta 56 (2011) 2378–2384
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Tuning the microstructure and functional properties of metal nanowire
arrays via deposition potential
Kirill S. Napolskiia,∗, Ilya V. Roslyakova, Andrey A. Eliseeva, Dmitry I. Petukhova,
Alexey V. Lukashina, Shu-Fang Chenb, Chuan-Pu Liub, Galina A. Tsirlinac,1
a Department of Materials Science, Moscow State University, Leninskie Hills, 119991, Moscow, Russia
b Department of Materials Science and Engineering, National Cheng Kung University, 701, Tainan, Taiwan
c Department of Electrochemistry, Moscow State University, Leninskie Hills, 119991, Moscow, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 August 2010
Received in revised form 1 December 2010
Accepted 4 December 2010
Available online 15 December 2010
The present study is focused on electrochemical fabrication and characterization of the ordered arrays of
one-dimensional Ni nanostructures templated by porous anodic alumina films. A possibility to monitor
and control homogeneity of arrays and completeness of the pores filling in the course of potentiostatic
deposition is discussed. Current efficiency is determined from independent analytical data and taken
into account in the treatment of charge transients. The crystallinity of Ni nanowire arrays as well as
the homogeneity and completeness of the replication process are demonstrated to be controlled by
deposition potential. Just these structural features affect the magnetic properties of nanowire arrays. The
advantages and drawbacks of the quantitative coulometric analysis of current transients are considered.
Keywords:
Anodic aluminum oxide
Nanowire array
Templated electrodeposition
Current transients
Coulometric analysis
© 2010 Elsevier Ltd. All rights reserved.
1. Introduction
nanoparticles from the effect of external factors (e.g., from oxida-
tion in air).
Template synthesis is known as a bottom-up approach, which
is widely used for preparation of various types of nanostructures.
As compared to the top-down techniques, such as X-ray or electron
lithography, templated chemical and/or physicochemical fabrica-
tion methods are highly flexible, very fast and cheap pathways
enabling large-scale preparation of uniform nanostructure arrays.
These arrays, as well as well-characterized separate nanostruc-
tures, attract a great attention both from fundamental and practical
points of view, acting as elementary blocks in a magnetic high-
density storage devices [1,2], high efficiency emitters [3–5], sensors
[6,7], etc.
Templates, also known as matrices, generally perform sev-
eral functions. First, they play a role of structure directing agent
and determine geometric parameters of the replicated object. In
particular, uniform nanostructures can be fabricated only in tem-
plates possessing uniform porosity. Second, the matrix determines
an arrangement of nanostructures in the array, allowing to con-
struct nanocomposite material with a desired structure without
lithography equipment. Finally, template enables one to protect
The accuracy of template replication by the embedded mate-
rial strongly depends on the incorporation technique. In most
cases it is difficult to achieve a complete filling of a template with
desired material. For instance, infiltration methods with subse-
quent decomposition of precursors usually result in formation of
small clusters along the walls of the porous frameworks [8]. CVD
related techniques can be successfully utilized for the incorpora-
tion of complex materials into the channels of porous media, but
the problem of uniform filling of porous structure due to probable
pore blocking by growing phases is still far from being resolved [9].
For insulating matrices, more promise is given by electrodeposition
method which basically allows almost complete filling of the pores
due to continuous movement of the growth front through porous
structure. Another advantage of this approach appears when elec-
trocrystallization takes place, which is typical for metals and many
other solids (including binary semiconductors and complex phases
[10]). In this case the precise in situ control and monitoring of
incorporation process is possible under potentiostatic deposition
mode. Electrode potential presents an instrument to fix oversatu-
ration and, correspondingly, nucleation-growth conditions. As for
monitoring, it is based on measuring current density and electric
charge transients, providing information about nucleation-growth
kinetics and filling degree. A number of complications arise when
deposition is accompanied by parallel gas evolution process (i.e.
∗
Corresponding author. Tel.: +7 495 9395248; fax: +7 495 9390998.
E-mail address: napolsky@inorg.chem.msu.ru (K.S. Napolskii).
ISE member.
1
0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.12.013

K.S. Napolskii et al. / Electrochimica Acta 56 (2011) 2378–2384
2379
hydrogen evolution is unavoidable when depositing less noble met-
als). There is also a general problem: additional technological steps
are required to arrange the electric contact at one side of template.
This step is naturally well-taken when fabricating the arrays for
applications requiring contacts.
The electrochemical deposition of nanowire arrays was first
reported in 1970 [11], where tracked mica films were used as a
template. Since then, this method has been successfully applied to
prepare nanowires of metals [12,13], alloys [14], semiconductors
[15], and conductive polymers [16,17] in various matrices with 1D
channels.
Porous anodic aluminum oxide (AAO) films are known as a
promising template for preparation of ordered arrays of nanowires.
In comparison to other porous materials with 1D channels (such
as track-etched membranes, porous silicon, block copolymer films,
etc.), anodic alumina possesses well defined arrangement of pores,
with structural parameters (pores diameter, interpore distance and
membrane thickness) easily varied by tuning anodization condi-
tions [18].
Nowadays, examination of homogeneity and completeness of
the replication process in templates, is mostly addressed using
scanning electron microscopy technique [14,19–21], which is
known for its locality and can be applied only post factum. Small
angle scattering techniques, such as small angle X-ray scattering,
should also be mentioned as a useful tool giving an information
about homogeneity of the filling, but these techniques are rather
expensive (especially if applied in situ to monitor deposition) [22].
Despite electrodeposition approach is widely used for growth of
various nanostructures in porous templates, its resources to moni-
tor easily the replication process by electrochemical responses are
still lacking. The examples of using current transients as indicators
to reveal several stages of metal electrodeposition into the channels
of track-etched membranes can be found in Refs. [23,24]. The rapid
increase of current density occurs when the growth of hemispheri-
cal caps starts at external surface of porous template, i.e. when some
faster growing nanowires reach this surface. Unfortunately, nearly
no attention has been paid to a quantitative analysis of j(t) curves
and corresponding deposition charge. Our estimations based on the
deposition charge data provided in Ref. [24] result in the filling
factor below 50%.
In what follows we present the quantitative characterization of
charge transients recorded in the course of potentiostatic depo-
sition in combination with solid state characterization data. We
demonstrate that this approach can shed a light to the growth
process of metal nanostructures and allows us to find optimal depo-
sition conditions. We observed that homogeneous filling of the
porous films by nickel can be achieved only in a narrow range
of deposition potentials (Ed): lower absolute values of potentials
lead to very slow growth rates, while the higher rates result in a
rapid decrease of pores filling factors. We also illustrate a profound
effect of the deposition potential on the crystallinity and magnetic
properties of Ni nanowire arrays.
electrode cell in 0.3 M oxalic acid (98%, Aldrich) at constant applied
potential difference of 40 V using Pt wire as a counter electrode. The
electrolyte was pumped through the cell by a peristaltic pump, and
its temperature was kept in the range of 2–4 ◦C during anodization.
After first anodization for 48 h alumina film was selectively etched
away in a 0.6 M H3PO4 + 0.2 M CrO3 solution at 70 ◦C. After second
anodization under the same conditions for 24 h, the ca. 50 ␮m thick
oxide layers were obtained. Then the oxide films were separated
from the aluminum substrate by selective metal dissolution in a
mixture of Br2 and CH3OH (1:10 vol.). Subsequently, the pore bot-
toms were opened by chemical etching in a 5 vol.% H3PO4 aqueous
solution at 60 ◦C for 5 min.
In order to prepare metal nanowire arrays by electrodeposition
technique, a continuous layer of Au (ca. 100 nm thick) was ther-
mally sputtered onto one side of the AAO template to serve as
working electrode. Current densities and deposition charges per
geometric surface area of this electrode are reported below. Cop-
per plates were used as current collectors and mechanical supports
for thin AAO/Au porous templates. Back side of working electrode
was isolated with silicone wax. Electrodeposition of nickel was car-
ried out in three-electrode cell from aerated 0.6 M NiSO4 + 0.1 M
NiCl2 + 0.3 M H3BO3 electrolyte (pH 4.2) at room temperature. Plat-
inum wire was used as a counter electrode. Saturated Ag/AgCl
electrode connected with a cell via Luggin capillary served as the
reference electrode. Potentials hereinafter are given versus satu-
rated (KCl) Ag/AgCl electrode (EAg/AgCl − ESHE = 0.195 V). Deposition
was performed under potentiostatic mode controlled by Autolab
PGSTAT 302 instrument in the potential interval Ed = −0.7 to −2.0 V.
In order to prevent the initial blocking of pores with air bubbles,
vacuum pre-treatment procedure was carried out prior to metal
electrodeposition [25].
2.2. Samples characterization
Electrochemical characterization of nanowires growth inside
porous matrix was carried out by analysis of current transients
recorded during electrodeposition using Autolab PGSTAT 302
potentiostat. All electrochemical experiments were performed in
aerated electrolytes. Current efficiencies were determined by ele-
mental analysis (Elan DRC II Perkin Elmer ICP mass spectrometer)
of samples dissolved in hot aqua regia.
Scanning electron microscopy (SEM) images were recorded on
a Supra 50 VP instrument (LEO). High resolution transmission elec-
tron microscopy (HRTEM) and selected-area electron diffraction
measurements were performed using JEM-2100F (JEOL) instru-
ment. The absence of the foreign phases in the prepared samples
was confirmed by XRD analysis using the MAR diffractometer at
the BM01A beam line (SNBL) at ESRF.
The magnetic properties of Ni/AAO nanocomposites were stud-
ied with a Cryogenics S-600 SQUID magnetometer.
3. Results and discussion
2. Experimental
According to SEM observations, anodic alumina membranes
prepared according to the abovementioned protocol possess a peri-
odic hexagonal arrangement of pores with an average interpore
distance Dint ≈ 105 nm on a large area (Fig. 1). The AAO film has
a narrow pore size distribution with an average pore diameter
Dp ≈ 40 nm. The cross-sectional image illustrates a constant pore
diameter all over the thickness of AAO film (ca. 50 ␮m). The porosity
of the AAO calculated from Dint and Dp equals 13%.
To estimate the Ni deposition potential range, cyclic voltam-
metry measurements were performed in 0.6 M NiSO4 + 0.1 M
NiCl2 + 0.3 M H3BO3 electrolyte (pH 4.2) using gold plate as a work-
ing electrode (Fig. 2(a)). An equilibrium potential for nickel in this
2.1. Samples preparation
The anodic alumina porous templates were prepared by two-
step anodization technique [18]. Prior to anodization high purity
aluminum foils (99.999%, 0.5 mm thick, Goodfellow) were annealed
at 550 ◦C for 24 h in air in order to remove the mechanical stress and
to enhance the grain size in the metal. Subsequently, the Al plates
were mechanically polished by diamond suspensions (Struers) to
a mirror finish and cleaned repeatedly with acetone and deionized
water under sonication. The anodization was carried out in two-

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K.S. Napolskii et al. / Electrochimica Acta 56 (2011) 2378–2384
Fig. 1. SEM images of AAO porous film: top view (a) and cross-section (b). The inset shows the schematic cross-section of the porous alumina structure: Dint is interpore
distance, Dp is pore diameter.
solution can be roughly estimated as −0.46 V (unfortunately the
activity of nickel ions remains unknown). A low current plateau
was found for potentials higher than −0.7 V indicating a very low
Ni deposition rate under this mode. At more negative potentials
current density increases, and a typical “nucleation loop” is notice-
able. Thedissolution ofNi is observedatthe potentials above −0.3 V.
All voltammetric features and characteristic potential intervals are
very similar to voltammetric details reported earlier for Watts type
electrolytes [26].
In case of AAO working electrode the picture changes drasti-
cally (Fig. 2(b)). The voltammogram indicates a broad maximum at
−1.2 V, and no loop is observed. Current density is ca. 50 times
lower as compared to deposition on bare Au support. Thus the
decrease of current when going from bare to partly screened surface
is much stronger than the simultaneous decrease of electrochem-
ically available surface area (<10 times). This difference can be
assigned to much slower diffusion of Ni2+ through nanochannels
to metal/electrolyte interface and, correspondingly, to higher dif-
fusion contribution to mixed deposition current. Manifestations of
higher Ohmic drop are also evident, induced by high solution resis-
tance in very thin pores, hence some distortion of potential values
is unavoidable, especially at high deposition currents. The deposi-
tion rate at saturation conditions (E < −1.2 V) can be calculated from
Faraday’s law according
dꢀ
dt
ꢁ
dq
=
(1)
nFSpores dt
ꢁ is current efficiency, n = 2, F is Faraday constant (96,500 C mol−1),
q is charge density per geometric surface area, and Spores is a rel-
ative electrochemically available surface area in AAO film. The
dimensionless value of Spores for anodic alumina film is equal
to its porosity, and can be easily obtained from the geometric
parameters of the structure: for 40 nm pores and pore density
of 1.05 × 1010 cm−2 Spores = 0.13. Experimental current efficiencies
were obtained by elemental analysis of samples electrodeposited
at constant potential. Using ꢁ ≈ 0.5 for E form −1.5 to −1.2 V one
can evaluate the metal growth rate as ∼4.0 × 10−8 mol cm−2 s−1
.
Fig. 2. Cyclic voltammograms recorded for 0.6 mol dm−3 NiSO4 + 0.1 mol dm−3
NiCl2 + 0.3 mol dm−3 H3BO3 on a bare Au plate (a) and on AAO/Au porous electrode
(b) at 20 mV s−1. The inset on panel (a) presents the enlarge part of voltammogram
in the region from −0.9 to 0.3 V.
2+
From the other hand a flux of Niaq
from the solution to
electrode can be very roughly estimated by applying Fick’s law
2+
j = − D ∇ C = D(Celectr − Cext)/d with Cext equals to Niaq
concen-

K.S. Napolskii et al. / Electrochimica Acta 56 (2011) 2378–2384
2381
tration in electrolyte volume, Celectr ꢀ Cext, and d equals to AAO
layer thickness. To find the order of magnitude, not the value, we
assume steady-state diffusion to planar surface. We also neglect the
role of electroosmotic and convectional flow (taking into account
the blocking of pores basements by the support, as well as small
pore diameter). Electromigration of nickel ions can play a certain
role, however it is also negligible due to a high concentration of
electrolyte and low electric field applied during electrodeposition.
2+
Therefore the main contribution to Niaq flux goes from chemical
diffusion. One should also take into account that an effective diffu-
sion of cations in AAO channels is suppressed as compared to bulk
diffusion, because of the positive charge of the alumina surface in
acidic medium [27–29].
To prove this suggestion we have performed a direct measure-
2+
ment of Niaq flux through AAO membrane with a thickness of
50 ␮m under electric field. The AAO membrane with through pores
was fixed in a H-cell filled by 20 ml of electrolyte for nickel elec-
trodeposition from one side, and by 20 ml of indifferent electrolyte
of close ionic strength (0.6 M Na2SO4 + 0.2 M NaCl + 0.3 M H3BO3)
from another (permeate) side. Working and counter electrodes
were placed into Ni-containing and Ni-free solutions correspond-
ingly. The potential of −1.0 V versus saturated Ag/AgCl electrode
was applied. Nickel concentration in permeate was directly mea-
sured by ICP mass spectrometry. These measurements gave a value
2+
of Niaq
diffusion flux of 2.8 × 10−8 mol cm−2 s−1, which stays
rather close to nickel growth rate estimated from the current
density. The above mentioned calculations were performed under
assumption of all pores being involved in the transport process.
Thus the estimated nickel ions flux should be considered as the
2+
lower limit of Niaq flux through single pore.
2+
The calculated diffusion coefficient of Niaq in AAO channels
(∼2.0 × 10−7 cm2 s−1) is found to be far less than bulk diffusion coef-
ficient of nickel in aqueous solutions (∼0.6 × 10−5 cm2 s−1) [30].
According to literature [27] this fact is attributed to suppression of
diffusion by electric double layer in nanochannel.
One should note that high potentials can also result in blocking
of pores by hydrogen gas generated inside the pores. Indeed, at
potentials above −1.2 V hydrogen generation is clearly visible with
unaided eye. Basing on cyclic voltammetry results the potential
range starting from −0.7 V was chosen for a detailed examination
of the electrodeposition potential effect on the microstructure and
morphology of Ni nanowires grown in AAO templates.
Fig. 3(a) demonstrates a typical current transient recorded in the
course of potentiostatic metal deposition into porous AAO mem-
brane. There are several characteristic stages of electrodeposition
process [23]. At first, maximum of cathodic current correspond-
ing to nucleation process is observed (region I, see insert as well)
followed by a nearly steady-state regime of electrodeposition at
constant current on the second stage, which corresponds to nickel
growth within AAO channels (region II). Next at a third stage, a rapid
current increase occurs, indicating nanowires approaching anodic
film surface and growth of hemispherical caps at the end of each
wire (region III). Expansive growth of metal surface at this stage
turns into coalesce of hemispherical caps to form planar and con-
tiguous metallic top layer. Growth of metallic layer at the fourth
stage takes place at a constant current (region IV).
To provide an accurate analysis of metal deposition process, cur-
rent efficiencies (ꢁ) were calculated for Ni electrodeposited into
AAO templates at various potentials. Total quantities of nickel in
Ni/AAO nanocomposites were determined by elemental analysis of
samples dissolved in hot aqua regia. Current efficiencies of 82–87%
were found for −0.8 to −1.1 V interval, in satisfactory agreement
with current efficiency for galvanostatic nickel deposition on bare
supports under usual galvanostatic modes known from galvanics.
At more negative potentials, the efficiencies fall down and reach ca.
10% at −2.0 V. This means that when Ni nucleation in some pores is
Fig. 3. Time (a) and charge (b) dependences of Ni electrodeposition current (poten-
tiostatic mode) for AAO/Au porous template. The inset in panel (a) shows initial
region of j–t curve. I–IV correspond to deposition stages. Arrow indicates the tran-
sition from II to III deposition stages. The curves obtained at various deposition
potentials (b), as well as the degree of pores filling (c), are presented after correction
for current efficiency.
blocked, hydrogen evolution continues at this portion of electrode
surface.
It should be noted that both duration and current densities
at different stages of electrodeposition depend strongly on the
applied potential. Fig. 3(b) illustrates a plot of current densities

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K.S. Napolskii et al. / Electrochimica Acta 56 (2011) 2378–2384
currents are more probable, because of slow diffusion inside the
pores.
The electric charge necessary for homogeneous filling of all the
AAO pores by nickel (at ꢁ = 1) can be valued by Faraday’s law:
nFꢂV
qtheor
=
(2)
M
where ꢂ = 8.9 g cm−3, M = 59 g mol−1 are density and atomic weight
of nickel and V is the volume of the pores. For 50 ␮m thick film with
40 nm pores and a pore density of 1.05 × 1010 cm−2 the expected
charge equals 18.9 C cm−2, which is notably higher than the exper-
imental values. An analysis of the pores filling factors (qexp/qtheor),
clearly indicates the tendency of rapid decrease of nickel loading at
deposition potential shift from −0.8 V to more negative values (the
trend is illustrated in Fig. 3(c)). Therefore to provide homogeneous
filling of pores by metal one needs to apply less negative potential,
but still corresponding to reasonable growth time. In our experi-
ments it is surely −0.8 V, not −0.7 V because of too low growth rate
in the latter case.
An analysis of growth front homogeneity by SEM imaging of
Ni/AAO composites cross-sections illustrates more or less uniform
growth of nickel wires in different pores if the potential is not too
negative (Fig. 4(a)), i.e. the majority of pores are filled simultane-
ously with close rates. Under these favorable deposition conditions
the length of nanowires depends directly on the electric charge
and can be varied controllably by Faraday law. On the other hand,
as the total charge spent before the moment when nanowires
approach the surface depends pronouncedly on deposition poten-
tial, we should conclude that electrodeposition does not take place
in a number of pores, which stay partly or totally unfilled (either
from the very beginning, or starting from some moment after initial
growth of a short wire). This may happen by a number of reasons.
First, for very small channels the nucleation fails to start due to
absence of active sites or blocking of metal/electrolyte contact by
an air persistent in the pores and not deleted before the deposition.
Second, the growth can stop at the early stage if the reagent diffu-
sion to gold electrode is limited by pore coning/narrowing at some
point. Third, at high negative potential the growth of nanowire can
be eventually stopped at any moment due to the blocking of pore
by a hydrogen bubble. An efficiency of all these process (especially
diffusion and hydrogen generation) depends significantly on the
applied potential, thus leading to an essential decrease of metal
loading. This is clearly seen in Fig. 4(b), where the majority of
wires are still very short, when some certain wires (see arrows) are
already many times longer. This inhomogeneity of the growth front
correlates with the portion of completely empty pores and explains
why the filling factor can be below 1% even when the metallic layer
already starts to grow at the external surface.
Fig. 4. SEM images of cross-sections of Ni/AAO nanocomposites obtained at various
deposition potentials Ed: (a) −0.8 V, (b) −1.2 V. Deposition charge equals 10 and
9 C cm−2 for samples presented on panel (a) and (b), respectively. Arrows in (b) show
the rare wires being much longer than the majority of growing nanostructures.
as a function of electric charge spent for metal deposition. Here
and below current density is corrected to the current efficiency.
One can clearly see that the charge spent before the moment
when nickel nanowires approach the external surface of AAO
(transition from region II to region III) decreases gradually with
the shift of electrodeposition potential to more negative values
(Fig. 3(c)). Thus, the potential affects not only current density (that
is equivalent to the growth rate), but also metal loading value. In
particular, the potential change from −0.8 to −0.9 V leads to the
reduction of the charge passed till the moment of metal growth
at the membrane surface nearly twice from 13.7 to 7.2 C cm−2
.
According to TEM studies of samples after matrix dissolution,
all of them consist of metal nanowires with diameters very close
to channel diameter in AAO template (Fig. 5). However the crys-
tallinity of nanowires from different samples was found to be
completely different. At low overpotentials the wires have poly-
crystalline structure, in the middle potential range of −0.9 to
This interval of the least negative potentials at which we were
able to deposit Ni with a reasonable rate is close to potential
interval studied in classical Fleischmann’s work [31]. According to
[31], in this region nickel deposition on a flat support takes place
under kinetic control, when in our membrane configuration mixed
Fig. 5. TEM images of Ni nanowires extracted from the oxide matrix. Insets present selected area electron diffraction patterns recorded from single nanowires.

K.S. Napolskii et al. / Electrochimica Acta 56 (2011) 2378–2384
2383
deposition process and the structure of Ni/AAO nanocomposites.
The complete and homogeneous filling of the porous template
with nickel can be achieved only in a narrow range of deposi-
tion potentials: lower deviations from equilibrium potential lead to
very slow growth rates, while more negative potentials result in a
rapid decrease of pores filling factors. The deposition potential also
defines the crystallinity and magnetic properties of Ni nanowire
arrays.
Our attempts to obtain the initial regions of current transients
(region I) reproducibly, in order to analyze nucleation kinetics, were
less successful at this stage. Most probably the reason is nucleation
statistical nature, as the size of bottom of a single pore can corre-
spond to very few nucleation centers. One cannot also exclude less
reproducible blocking with the residual air and/or initially formed
hydrogen bubbles. Surprisingly, despite of this very arbitrary nucle-
ation behavior the potential-dependent growth of metal in the
regions II and III is quite reproducible if the potential is not too
negative.
Monitoring mentioned above in Introduction can be arranged
in the form of plots presented in Fig. 3(b). These plots provide the
reliable information about pores filling in a wide potential range,
in which current efficiency remains constant. As for controllability,
it is the highest for very narrow potential interval supporting more
or less homogeneous growth front.
Fig. 6. Dependence of the coercivity of Ni/AAO nanocomposites in longitudinal
direction on deposition potential.
−1.2 V single crystalline wires are formed, while higher overpo-
tentials again lead to polycrystalline wires formation. One can
note that the tendency for crystallinity is similar to the trend
for current efficiencies. Indeed as Ni2+ → Ni and 2H+ → H2 reac-
tions take place simultaneously at the metal/electrolyte interface,
they should be considered as concurrent processes. The oxygen
reduction process O2 + 4H+ → 2H2O can also play a role, especially
at low absolute values of deposition potential, as we are deal-
ing with aerated solutions. In case Ni2+ → Ni is dominant one the
continuous single-crystalline wires are formed, while in case of
the advantage of 2H+ → H2 process hydrogen microbubbles form
on metal/electrolyte interface resulting in secondary nucleation of
nickel and polycrystalline nanowire structure. Additional factor of
structural disorder can be nickel hydride formation [32], with sub-
sequent withdrawal of hydrogen in air. For pH above 2 concentrated
␤-hydride is expected to form only at very high negative poten-
tials. We can assume that non-monotonous trend observed for
polycristallinity results also from the adsorption of anionic react-
ing species (Ni(II) aquachloride complexes). This phenomenon can
be strongly affected by the electrode charge, and can support the
formation of additional nucleation centers.
Crystalline structure of metal nanowire influences effectively
the functional properties of Ni/AAO composites. In particular the
coercivity of Ni/AAO in longitudinal direction falls more than twice
when going from single-crystalline to polycrystalline structures
(Fig. 6). One should note that the effect cannot arise from magnetic
dipole interactions between the neighboring wires, as magneto-
static interactions are known to decrease with distance as l−3 in
the direction perpendicular to magnetic dipole orientation [1]. This
indicates a decrease of an interaction field with decreasing pore
filling factors, which is contrary to the observed tendency. On the
other hand grain boundaries in nanowire can act as magnetic rever-
sal nucleation sites, leading to sharp decrease in coercive force and
remanence magnetization by decreasing demagnetization field [1].
Thus the magnetic properties are more sensitive to the crystallinity
of nanowires than to filling factors, leading to sharp decrease of
nanocomposite coercivity with increasing metal deposition over-
potential.
Acknowledgments
This work is partially supported by the Russian Ministry
of Education and Science (grants nos. 02.513.11.3485 and
14.740.11.0301), the Russian Foundation for Basic Research (grants
nos. 09-03-01123, 09-03-12246 and 10-02-00634) and RF Pres-
ident Grant MK-6626.2010.3. The authors are grateful to D.Yu.
Chernyshov (ESRF) for his help with XRD analysis.
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