Ni growth inside ordered arrays of alumina nanopores: Enhancing the deposition rate

Article abstract of DOI:10.1016/j.electacta.2012.04.036

High aspect ratio Ni nanowires (NWs) were electrodeposited inside ordered arrays of self-assembled pores (～50 nm in diameter) in anodic alumina templates by a potentiostatic method. A systematic comparative study is here reported on Ni deposition at different applied potentials between -0.5 and -4.0 V (vs. Ag/AgCl). A detailed analysis of the structural, morphological and magnetic properties of the Ni NWs was performed using scanning electron microscopy, X-ray diffraction and vibrating sample magnetometer. Smooth, well-defined and highly-oriented (2 2 0) face-centered cubic Ni NWs were obtained for the whole range of applied potentials (E). The Ni deposition rate (η) was observed to strongly depend on E [η (μm min^-1) = 4.5 × E - 3.5], reaching ～12 μm min^-1 for E ～ -3.5 V. However, at high cathodic potentials (E ≤ -2.5 V) we found that H ₂ gas bubbles are more likely to nucleate inside the nanopores stopping further Ni deposition. Magnetic measurements revealed the high shape anisotropy of the fabricated nanostructures, and allowed us to estimate the native oxide layer thickness (～1.5 nm) formed around the Ni NWs, inside the alumina template.

Full text of DOI:10.1016/j.electacta.2012.04.036

Electrochimica Acta 72 (2012) 215–221
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Ni growth inside ordered arrays of alumina nanopores:
Enhancing the deposition rate
Mariana P. Proencaa,b, Célia T. Sousa , João Ventura , Manuel Vazquez , João P. Araujo
a
a
b
a,∗
a
IFIMUP and IN – Institute of Nanoscience and Nanotechnology and Dep. Física e Astronomia, Univ. Porto, Rua do Campo Alegre 687, 4169-007 Porto, Portugal
Institute of Materials Science of Madrid, CSIC, 28049 Madrid, Spain
b
a r t i c l e i n f o
a b s t r a c t
Article history:
High aspect ratio Ni nanowires (NWs) were electrodeposited inside ordered arrays of self-assembled
pores (∼50 nm in diameter) in anodic alumina templates by a potentiostatic method. A systematic com-
parative study is here reported on Ni deposition at different applied potentials between −0.5 and −4.0 V
Received 23 February 2012
Received in revised form 5 April 2012
Accepted 7 April 2012
(
vs. Ag/AgCl). A detailed analysis of the structural, morphological and magnetic properties of the Ni
Available online 15 April 2012
NWs was performed using scanning electron microscopy, X-ray diffraction and vibrating sample mag-
netometer. Smooth, well-defined and highly-oriented (2 2 0) face-centered cubic Ni NWs were obtained
for the whole range of applied potentials (E). The Ni deposition rate (ꢀ) was observed to strongly depend
Keywords:
Electroplating
Hydrogen diffusion
Nanowire
Nickel
Template synthesis
−1
−1
on E [ꢀ (␮m min ) = 4.5 × |E| − 3.5], reaching ∼12 ␮m min for E ∼ −3.5 V. However, at high cathodic
potentials (E ≤ −2.5 V) we found that H2 gas bubbles are more likely to nucleate inside the nanopores
stopping further Ni deposition. Magnetic measurements revealed the high shape anisotropy of the fab-
ricated nanostructures, and allowed us to estimate the native oxide layer thickness (∼1.5 nm) formed
around the Ni NWs, inside the alumina template.
©
2012 Elsevier Ltd. All rights reserved.
1
. Introduction
method (pulsed electrodeposition; PED) [12,17]. In DC deposition,
a constant potential is applied, allowing the enhancement of the
deposition rate by an adequate control of the overpotential. Opti-
mizing this process to enhance the deposition rate would then
allow the industrial production of large and organized arrays of
high quality nanostructures in an easy and fast manner.
Although Ni electroplating has been studied for many years,
a renewed interest has emerged in recent years owing to its
application in the controlled fabrication of a variety of magnetic
nanostructures. A review on the numerous theoretical and practical
studies on the electrodeposition of nickel can be found in Ref. [1]. In
particular, Ni nanowires (NWs) are potential candidates for mag-
netic storage devices, nanosensors, biomedical applications, and
are vital for the study of nanoconfinement effects [2–5]. Among
the various methods used to fabricate arrays of nanowires, tem-
plate synthesis combined with electrodeposition processes is a low
cost, high yield and industrially viable technique [6–11]. In particu-
lar, nanoporous alumina templates (NpATs) were found to be ideal
for the electrodeposition of dense nanowire arrays [12]. They are
stable at high temperature and in organic solvents, and their pore
channels are parallel and uniform, aligning perpendicular to the
membrane surface [13,14].
The electrolytes most commonly used for Ni electrodeposition
contain nickel sulphates, sulphamates, chlorides and/or boric acid
[12,17–22], with the pH adjusted between 2 and 5. In DC deposi-
tion, the most commonly applied potentials (E) range from −0.7 to
−1.3 V (vs. Ag/AgCl) [23]. One way to enhance the rate of Ni depo-
sition would be by decreasing E to lower (more negative) values
than the ones commonly used. However, a major problem when
electrodepositing at high cathodic potentials is the occurrence of
additional undesired reactions [24]. For example, the creation of Ni
hydroxides contaminates the Ni deposits, reducing its saturation
magnetization (MSat) and changing the corresponding crystallo-
graphic structure [25]. The addition of boric acid and the use
of vigorous agitation were shown to improve the quality of the
deposited material by avoiding hydroxides precipitation [24].
A powerful method to fabricate highly ordered arrays of metallic
nanowires is the use of templates with high aspect ratio nanopores
Until now, three different electrodeposition methods have been
developed aiming uniform and complete pore filling: direct cur-
rent (DC; potentiostatic mode) [7], alternating current (AC) [15,16]
or a hybrid constant potential and constant current in pulses
(
e.g. NpATs with ∼50 nm in diameter and 50 ␮m in length).
However, when depositing Ni inside pores with nanometer-sized
diameters, a more complex process occurs [26–28]. Due to the exis-
tence of narrow paths that connect the bulk electrolyte solution
(membrane’s top) to the cathode surface (membrane’s bottom),
∗ Corresponding author. Tel.: +351 22 04 02 334; fax: +351 22 04 02 406.
E-mail address: jearaujo@fc.up.pt (J.P. Araujo).
0
013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.electacta.2012.04.036

2
16
M.P. Proenca et al. / Electrochimica Acta 72 (2012) 215–221
the electrodeposition process will now be mainly influenced by
the concentration polarization of Ni ions inside the pores and H2
gas evolution. For Ni deposition the current efficiency (ꢁ) can be
estimated by:
jNi
ꢁ
= j
(1)
+ jHER
Ni
where jNi and jHER are the partial current densities of the nickel
reduction and the hydrogen evolution reaction (HER), respectively
−
2
[
29]. Previous works showed that jHER ∼ 3 mA cm at pH = 3.5 and
remains approximately constant when varying the applied poten-
tial (between −1.1 and −1.5 V vs. Ag/AgCl) [30]. However, at very
low pH values (∼1.5), ten times higher jHER values were found.
Fukunaka et al. showed that although H2 molecules are produced
while simultaneously depositing Ni, they are easily dissolved in the
electrolyte allowing a uniform Ni deposition inside the nanopores
[
31]. But they also observed that the concentration of H2 gas
rapidly increases with increasing current, reaching a supersatura-
tion value and nucleating nanosized H2 gas bubbles at the cathode
surface [31]. In fact, increasing the applied cathodic potential dur-
ing electrodeposition can raise the local deposition temperature
Fig. 1. Linear potential sweep curves during Ni deposition on an Au substrate (Ni
film 01) and in a NpAT (Ni NWs 01 and 02). The scattered points correspond to the
current density values after 60 s of Ni potentiostatic deposition.
◦
by ∼20–40 C [32], thus decreasing the local pH and consequently
after the first anodization was performed in an aqueous solution
◦
increasing jHER. In this work we used an electrolyte with a high
concentration of Ni2+ ions (∼1.3 M), and pH of the solution ranging
between 4 and 5. According to Motoyama et al. [30], a current effi-
ciency of around 95% is to be expected for these electrodeposition
conditions.
of 0.2 M H2CrO4 and 0.4 M H3PO4 at 60 C. Second anodizations
were then performed for 20 h under the same conditions as the
first ones. For potentiostatic deposition inside the NpATs, a circle
with 1.1 cm in diameter was opened on the membrane backside by
removing the Al substrate in an aqueous solution of 0.2 M CuCl2
and 4.1 M HCl at room temperature. The remaining alumina bar-
rier layer at the bottom of the pores was then chemically etched
Previous reports on the optimization of Fe [33], Cu [34,35] and
Ni [23] deposition in NpATs referred a limited range of applied
potentials (from around −0.7 to −1.2 V vs. Ag/AgCl) for the depo-
sition of defect-free NWs. Many authors also studied the influence
of agitation [36], applied potential waveform [37], electrolyte type,
concentration, temperature and pH [38,39] on the Ni deposition in
NpATs. However, no work was yet reported on the optimization of
Ni deposition in NpATs at continuous high negative overpotentials
◦
in 0.5 M H3PO4 at 21 C for 2 h. During this process the pores were
slightly widen. Finally, a 100 nm thick Au layer was sputtered on
the opened bottom side of the porous membrane to serve as the
working electrode in a three-electrode cell. A Pt mesh was used as
counter electrode and Ag/AgCl (in 4 M KCl) as reference electrode
(0.197 V vs. standard hydrogen electrode; SHE). In the following,
all potentials are referred to Ag/AgCl. The depositions were per-
formed in the so-called Watts bath [12], consisting in a mixture
of 1.14 M NiSO4·6H2O, 0.19 M NiCl2·6H2O and 0.73 M H3BO3. Dur-
ing the electrodeposition, the solution was magnetically stirred at
(
lower than −2.0 V vs. Ag/AgCl). The main question that arises is
at what potential will the H2 supersaturation be reached and pre-
vent Ni NWs formation or influence the corresponding structural
and magnetic properties. Finding such limit would then allow the
fabrication of long and uniform Ni NWs in very short times.
In this work a systematic study of Ni DC deposition at high
cathodic potentials (down to −4.0 V vs. Ag/AgCl) in NpATs is pre-
sented. The influence of HER on the electrodeposition process is
discussed and reproducibility tests were performed to find the lim-
iting potential for the optimization of the Ni NWs growth inside
◦
250 rpm and kept at a constant temperature of 35 C within a pH
ranging between 4 and 5.
Cathodic sweep voltammetries were performed prior to the
deposition process by applying a potential from 0 to −1.8 V at
−
1
different sweep rates (ꢂ; from 5 to 40 mV s ). The electrodepo-
sitions were performed in a potentiostatic mode using a Solartron
1480 MultiStat. The deposited NWs were magnetically character-
ized using a vibrating sample magnetometer (VSM; LOT-Oriel EV7).
Morphological characterization was performed using a field emis-
sion scanning electron microscope (FESEM; FEI Nova NanoSEM
230). Structural analysis was performed by X-ray diffraction (XRD;
Panalytical X’Pert Pro) using Cu K␣1 radiation (ꢃ = 0.15406 nm) and
the Bragg–Brentano ꢄ/2ꢄ geometry.
−
1
the nanopores. A deposition rate of ∼12 ␮m min (more than 10
times larger than commonly reported values) for the fabrication
of organized arrays of long Ni NWs was obtained using E ∼ −3.5 V.
Magnetic measurements were also performed illustrating the high
shape anisotropy of the fabricated NWs and allowing the determi-
nation of an oxide layer thickness value of ∼1.5 nm around the Ni
NWs, inside the alumina template.
2
. Experimental
3. Results and discussion
In this work Ni was electrodeposited on top of an Au sub-
3.1. Linear sweep voltammetry
strate and inside self-assembled pores of anodic alumina templates
(
∼
pore diameter ∼50 nm; interpore distance ∼105 nm and thickness
Prior to Ni potentiostatic deposition inside the nanopores we
used linear sweep voltammetry (LSV) to find the Ni deposition
potentials, which are known to depend on the agitation conditions,
electrolyte composition, temperature and pH.
50 ␮m). The Au substrate was prepared by sputtering a layer of
∼
60 nm in thickness (3 min at 40 mA) on top of a Cu foil tape, using
a Q150R S sputter coater. The membranes where fabricated by a
◦
two-step anodization process in 0.3 M oxalic acid at 40 V and 5 C
For an accurate comparison of the charge-controlled and diffu-
sion limited processes we performed LSVs on an Au substrate and
[
13,40]. Prior to the first anodization, Al discs (99.999% purity) were
−
1
degreased with acetone and ethanol, followed by electropolishing
in a 75% ethanol and 25% perchloric acid solution for 2 min at 20 V
in the NpATs, at 50 and 20 mV s (Fig. 1). The Ni deposition on
the Au substrate (Ni film 01 in Fig. 1), performed from 0 to −8.0 V,
clearly shows a charge-controlled process, as expected for metal
[
41]. First anodizations were carried out for 24 h. Alumina removal

M.P. Proenca et al. / Electrochimica Acta 72 (2012) 215–221
217
Fig. 3. Selected current density transients during Ni electrodeposition in NpATs
performed at different constant potentials for 1 min. Inset evidences the H2 gas
bubble formation and release during an anomalous Ni deposition at −3.5 V.
Fig. 2. Linear potential sweep curves during Ni deposition on an Au substrate
−1
(
Ni film at 50 mV s ) and in NpATs at different scan rates (1st to 4th scans). Inset
plots the cathodic peak current vs. the square root of sweep rate.
solution (mol cm−3). This allowed us to estimate the diffusion coef-
ficient of Ni2+ ions in the electrolyte used. The obtained value
layer deposition [42]. However, Ni deposition inside the nanopores
(
Ni NWs 01 and 02 in Fig. 1), performed from 0 to −5.0 V, illustrates a
−
7
2
−1
of D ∼ (1.0 ± 0.1) × 10 cm s is in good agreement with values
combination of charge and diffusion controlled processes, together
with the nucleation of H2 gas bubbles at high cathodic potentials.
The LSV curves during Ni deposition inside the nanopores follow the
same linear trend as the ones obtained during Ni deposition on the
Au substrate, indicating the presence of a similar charge controlled
process. However, a cathodic and anodic peak corresponding to the
deposition and dissolution of Ni, respectively, are also clearly seen
in Fig. 2. The cathodic peak, that illustrates a diffusion-controlled
process, is only present during Ni deposition inside the nanopores.
This evidences the existence of a mixed (charge and diffusion) con-
trolled process for Ni deposition inside the nanopores, while only
charge-controlled when deposited on a flat substrate. As for the
quick drop of current density to almost zero at high cathodic poten-
tials in Fig. 1, this most likely results from the nucleation of H2
gas bubbles inside the nanopores [30,31,43]. When applying high
cathodic potentials there is a local increase of temperature, result-
ing in a pH decrease, and therefore higher H2 evolution. Due to
reported in the literature [45].
The results in Fig. 2 also allowed us to determine the equilibrium
potential (Eeq) of Ni deposition reaction for the used electrolyte
2+
and deposition conditions [Eeq(Ni /Ni) ∼ −0.46 V vs. Ag/AgCl]. In
◦
an aqueous solution at 25 C one has Eeq ∼ −0.257 V vs. SHE, cor-
responding to ∼−0.454 V vs. Ag/AgCl [24,44], in good agreement
with our results. Ni reduction can thus only occur if a potential
lower than −0.46 V is applied. However, one cannot assume that
the potential at the maximum cathodic peak corresponds to the
optimum potential value for Ni deposition. A systematic study is
therefore required to obtain the limiting Ni deposition potential
that leads to the growth of smooth and defect-free Ni NWs at a
high rate.
3.2. Electrodepositions at different potentials
Ni was electrodeposited in the NpATs at constant potentials (E)
the high NpATs homogeneity, the nucleation of H gas bubbles will
2
between −0.5 and −4.0 V for 1 min. Fig. 3 shows selected current
transients during the deposition process. After the first few seconds,
that correspond to the initial Ni nucleation on the Au contact [34],
Ni is deposited at a constant rate for E between −1.0 and −3.5 V. At
E ≥ −0.7 V, no significant amounts of Ni were found inside the pores
after 1 min of deposition. On the other hand, the current transient
for the deposition at −4.0 V shows a completely different behavior,
with a rapid drop to almost zero after the first 10 s. The same drop
of the current density to almost zero was also observed during LSV
inside the nanopores, occurring at −2.8 and −4.0 V (Fig. 1). For a
better comparison, the current values after 1 min of Ni deposition
were plotted in Fig. 1, showing a good agreement with the linear
sweep voltammetry curves.
Due to the thorough process of template preparation and
accurate control of the several experimental parameters of elec-
trodeposition, three reproducibility experiments were performed
for each E. The samples where the current density dropped to
almost zero after a few seconds are here called anomalous sam-
ples, while those where the deposition current density remained
approximately constant during the 1 min of deposition are called
normal samples. These reproducibility tests were performed at E
between −1.0 and −4.0 V. For Ni deposition at E ≥ −2.0 V all sam-
ples presented the expected current density transient during the
1 min experiment, so that one obtained 100% of normal samples.
For E ≤ −2.5 V the percentage of normal samples linearly dropped
to 0% at −4.0 V (Fig. 4). When increasing the cathodic potential
occur at approximately the same time inside all pores, uniformly
stopping further Ni deposition. Therefore, there is a limiting applied
potential for the deposition of Ni NWs due to the existence of a
template with nanochannels instead of a flat metallic substrate.
Additionally, we studied the LSV curves obtained during Ni
deposition inside the nanopores at different sweep rates (ꢂ; from 5
−1
−1
to 40 mV s ; Fig. 2). In the first scan (at 5 mV s ) a small cathodic
peak occurred with a maximum at ∼−0.7 V. During this scan Ni
deposition already took place, so that, when performing the second
scan, one also observes an anodic peak at ∼−0.05 V, correspond-
ing to Ni reduction (Fig. 2). In the subsequent scans performed at
−
1
higher voltage sweep rates (ꢂ = 10, 20 and 40 mV s ) the cathodic
peak is more evident and has its maximum at ∼−0.8, −0.9 and
−
0.95 V, respectively. The inset of Fig. 2 shows that the peak cur-
rent ip increases linearly with the square root of the sweep rate
1
/2
ꢂ
, following the Randles–Sevcik equation [44]:
ꢀ
ꢁ
1
/2
F3
RT
ip = 0.4463
n3/2AD1/2cꢂ1/2
(2)
where F is the Faraday constant (96,485 C mol−1), R is the univer-
−1
−1
sal gas constant (8.314 J mol
K ), T is the absolute temperature
(
K), n is the number of electrons involved in the electrochemical
2
reaction, A is the surface area of the electrode (cm ), D is the dif-
fusion coefficient of the electroactive species (cm s ) and c is
2
−1
the bulk concentration of electroactive species in the electrolyte

2
18
M.P. Proenca et al. / Electrochimica Acta 72 (2012) 215–221
Fig. 4. Percentage of normal samples obtained at different applied potentials during
Ni deposition inside nanopores.
Fig. 5. XRD pattern of the Ni NWs electrodeposited in a NpAT at −2.5 V. Inset shows
a detail of the (2 2 0) Ni peak for selected samples.
applied we are raising the amount of Ni2+ and H+ ions at the cath-
ode surface. Therefore jNi will be higher, but as a result of the local
deposition temperature raise and thus local decrease of pH, jHER
will also increase [30]. Consequently, there is a higher probability
of the nucleation of H2 gas bubbles to occur at the cathode surface,
stopping Ni deposition and leading to the sudden decrease of the
current density to almost zero. Nevertheless, one notes that jHER
was reported to be around 5% of the total current density during Ni
deposition in these experimental conditions [30]. This value is high
enough for the local nucleation of H2 gas bubbles to occur at high
cathodic potentials.
deposition density up to 75 mA cm 2, above which no significant
effect is observed [50]. We can thus state that the applied poten-
tials between −1.0 and −4.0 V (combined with the conditions here
used) produce fcc Ni NWs with a preferred orientation along [2 2 0].
This control of the crystallographic structure of the Ni NWs is very
important, since it strongly affects the corresponding magnetic
properties.
−
Ni NWs were also characterized by FESEM cross-sectional views,
after breaking the alumina samples. Fig. 6a–h shows that a uni-
form deposition of well-defined and defect-free Ni inside the
nanopores can be obtained for all normal samples. Although some
fractured NWs can be seen, these likely resulted from the crack-
ing of the alumina templates during FESEM samples preparation.
In the anomalous samples we found some areas where the Ni
NWs electrodeposited had an inhomogeneous length distribution
(Fig. 6i). As previously explained, at higher cathodic potentials,
small variations in the nanopores’ diameter, local temperature and
pH become more relevant. The probability of H2 gas bubbles to
nucleate and stop subsequent Ni deposition increases with the
applied cathodic potential. These only affect the NWs length and
consequently their size distribution throughout the membrane, and
at very high cathodic potentials (E = −4 V) Ni deposition stops in
almost all nanopores after a few seconds (Fig. 6j).
Additionally, note that the use of a two-step anodization process
for the preparation of NpATs leads to ordered hexagonal arrays of
nanopores with a diameter dispersion of ∼6% [41], although the
boundaries between adjacent ordered domains present pores with
larger diameters (∼3% of the total number of pores). When increas-
ing the applied cathodic potential one enlarges the probability that
small deviations in the nanopore diameters affect the nucleation
of H2 gas bubbles, stopping further Ni deposition, and resulting
in an inhomogeneous NWs length distribution. This phenomenon
can easily be seen in the fluctuations of the current density tran-
sients during Ni deposition (inset of Fig. 3), corresponding to the
formation and release of H2 gas bubbles inside a small fraction of
nanopores. At very high cathodic potentials (E = −4 V), Ni deposi-
tion will stop in almost all nanopores after a few seconds, keeping
Ni deposition only in ∼3% of the total number of pores (correspond-
ing to the larger pores present at the ordered domain boundaries).
This is in good agreement with the decrease of ∼97% observed in
Fig. 7 shows the dependence of the deposition rate (ꢀ) on the
applied potential for Ni deposition, estimated using the normal
samples. The average length of the Ni NWs linearly increases with
|E|. Our experiments then allowed the estimation of the rate of
Ni deposition inside the nanopores for any applied potential: ꢀ
j
, as shown in Fig. 3.
total
−
1
−1
(
␮m min ) = 4.5 × |E| − 3.5, reaching ∼12 ␮m min for E ∼ −3.5 V.
Most reports on Ni electrodeposition at constant potentials have
3.3. Structural and morphological properties
so far been performed at around −1 V, which corresponds to a
−
1
deposition rate more than 10 times smaller (ꢀ ∼ 0.8 ␮m min ).
XRD measurements confirmed the presence of Ni for all depo-
sitions at E ≤ −1.0 V. These samples show similar patterns of
face-centered cubic (fcc) Ni strongly textured along the [2 2 0]
direction (Fig. 5), mainly due to the nanopores’ geometrical con-
finement, in good agreement with previous works [46,47]. This
preferential growth can also be attributed to the high overpoten-
tials used that kinetically favor the (2 2 0) face formation [39,48].
The inset of Fig. 5 shows a detail of the most intense Ni peak. The
grain size of the Ni NWs was estimated using the Scherrer equa-
tion [49], with results varying between 80 and 100 nm. We found
an overall slight decrease of the NWs grain size with decreasing
E down to −3.2 V. In fact, it was previously reported that in Ni
thin films the grain size decreases rapidly with increasing current
3.4. Magnetic properties
Most of Ni NW array applications exploit the corresponding
magnetic properties, particularly the high shape anisotropy [51,52].
One important step to improve the magnetic properties of Ni NWs
is the optimization of their fabrication process. High aspect ratio
Ni NWs have sharp magnetization reversal, high coercive field (Hc)
and large squareness (S) for an applied magnetic field (H) along
the wire long axis (//direction). On the other hand, for H perpen-
dicular to the wire axis (⊥ direction), hysteresis is much reduced
leading to lower values of Hc and S. The easy axis of magnetization is

M.P. Proenca et al. / Electrochimica Acta 72 (2012) 215–221
219
Fig. 6. FESEM cross-sectional images of Ni NWs deposited in NpATs for 1 min at a constant potential of: (a) −1.0 V (L ∼ 830 nm); (b) −1.5 V (L∼2.5 ␮m); (c) −2.0 V (L ∼ 4.5 ␮m);
(
d) −2.5 V (L∼7.5 ␮m); (e) −2.8 V (L ∼ 12 ␮m); (f) −3.0 V (L∼11.5 ␮m); (g) −3.2 V (L ∼ 8.5 ␮m); (h) −3.5 V (L∼11.8 ␮m); (i) −2.5 V (anomalous sample); (j) −4.0 V (anomalous
sample).
therefore oriented parallel to the long wire axis as previously
reported [51,53].
anisotropy in the parallel direction (Fig. 8c). Perpendicular to the
wire axis, squareness is small (around 0.05 for normal samples and
around 0.1 for the anomalous ones).
It is important to note that squareness and coercivity depend
on the aspect ratio of the Ni NWs. However, all the results plot-
ted in Fig. 8 correspond to Ni NWs with the same diameter and
aspect ratios between 16 and 240. For very high aspect ratio values
Hysteresis loops of the Ni NWs grown in the NpATs were
measured using VSM (Fig. 8a). Similar magnetic properties were
/
/
observed for all samples: Hc ꢀ H and S// ꢀ S . The coercive
⊥
⊥
c
/
/
field measured along the wire axis (Hc ) ranged between 800 and
1
100 Oe, showing the same overall trend with E for both normal
(>10) only small variations of S are expected [54], so that the coer-
and anomalous samples (Fig. 8b). As expected, the values of the
⊥
civity and squareness are not expected to depend much on wire
length in our samples, as observed. Magnetic measurements also
show no evidence of the presence of impurities or defects in the
fabricated Ni NWs. If Ni hydroxides were present, the coercive field
and squareness along the long axis would be much smaller [33,55].
The fact that similar magnetic measurements were obtained for all
samples allowed us to infer that no significant amount of impurities
is present in the anomalous samples when compared to the normal
ones.
coercive field measured perpendicular to the wire axis (H ) were
c
/
/
found to be one order of magnitude lower than Hc (between 100
and 170 Oe), showing however the same overall trend with E for all
samples.
The squareness is defined as the ratio between remanent (MR)
and saturation (MSat) magnetization. Along the wire axis, square-
ness is close to 1 for all samples, revealing the high magnetic
The magnetic texture, measured as the ratio between the rema-
nent magnetization perpendicular and parallel to the wire axis
⊥
M /M ) [33], is summarized in Fig. 8d. The values obtained
R R
//
(
ranged between 0.05–0.1 for the normal samples and 0.1–0.3 for
the anomalous ones. The smaller magnetic texture corresponds to
a better orientation of the magnetization along the wire axis. The
higher value observed for the anomalous samples should be related
with an inhomogeneous distribution of the NWs length.
The saturation magnetic moment (mSat) follows the mass dis-
tribution of the deposited Ni (Fig. 9). The Ni mass electrodeposited
was estimated using Faraday’s laws of electrochemistry. From the
magnetic and structural characterizations already presented we
concluded that no impurities were present and pure Ni was elec-
trodeposited. Since the concentration of Ni2+ ions in the electrolyte
used is very high, and for this electrolyte the current efficiency is
9
5% (as discussed before), one does not expect a large contribution
from HER to the overall electrodeposition current (jHER ≈ 0.05 × j).
We can thus estimate the charge involved in the Ni deposition
reaction as the integral of the current transients, considering a cur-
rent efficiency of ∼95% [Eq. (1)]. One should note that, even for
the anomalous Ni deposition current transients, where j suddenly
Fig. 7. Ni deposition rate ꢀ after 1 min of deposition at different applied poten-
tials. The error bars were estimated considering several samples fabricated at each
potential.

2
20
M.P. Proenca et al. / Electrochimica Acta 72 (2012) 215–221
Fig. 8. (a) Selected hysteretic cycles of Ni NWs measured by VSM, with the magnetic field applied parallel (solid line) and perpendicular (dashed line) to the nanowires
axis. Influence of deposition potential on: (b) the coercive field parallel (squares) and perpendicular (triangles) to the wire axis; (c) the squareness parallel (diamonds) and
perpendicular (triangles) to the wire axis; and (d) the magnetic texture (circles); for normal (filled symbols) and anomalous (open symbols) samples.
NpATs. The Ni deposition rate was observed to increase with the
−
1
cathodic applied potential, ꢀ (␮m min ) = 4.5 × |E| − 3.5, reaching
−
1
∼
12 ␮m min for E ∼ −3.5 V. This rate is approximately 10 times
higher than at −1 V (the value usually reported in the literature).
Such negative applied potentials allow the faster growth of long
Ni NWs. However, it also increases the probability of small devi-
ations in the pore diameter, local temperature and pH to affect
the nucleation of H2 gas bubbles, stopping subsequent Ni depo-
sition, and resulting in an inhomogeneous NW length distribution.
For E ≥ −2.0 V all the samples showed normal current density tran-
sients. However, for E ≤ −2.5 V the probability of H gas bubbles to
2
stop deposition increased linearly, reaching 100% at −4.0 V. Mag-
netic measurements showed high shape anisotropy for all samples,
makingthem prospectivecandidatesfor magneticmemory devices.
Moreover, the optimization of the DC deposition here described can
improve the industrial production of large and organized arrays of
high quality nanostructures making this a much easier and faster
process.
Fig. 9. Saturation magnetic moment at a field of 15,000 Oe vs. the Ni estimated
mass (considering a deposition current efficiency of 95% [30]). The linear fit allows
extracting the saturation magnetization value of around 49.5 emu g for Ni NWs.
−
1
Acknowledgements
M.P. Proen c¸ a and C.T. Sousa are thankful to FCT for
doctoral and post-doctoral grants SFRH/BD/43440/2008 and
SFRH/BPD/82010/2011, respectively. J. Ventura acknowledges
financial support through FSE/POPH. M. Vázquez thanks the Span-
ish Ministry of Economia y Competitividad, MEC, under project
MAT2010-20798-C05-01. J.P. Araújo also thanks the Funda c¸ ão Gul-
benkian for its financial support within the ‘Programa Gulbenkian
de Estímulo à Investiga c¸ ão Científica’. The authors acknowledge
funding from FCT through the Associated Laboratory – IN.
drops to almost zero, mSat follows the same linear trend with the
calculated Ni mass (Fig. 9). The expected linear relationship allowed
−1
us to extract MSat for Ni NWs (∼49.5 emu g ), which compares well
−
1
with other reported values (46–50 emu g ) [17,56], being slightly
−1
lower than MSat of bulk Ni (∼55 emu g ) [52,57]. The lower value
of MSat for Ni NWs can be attributed to the presence of a thin nickel
oxide layer of ∼1.5 nm thick at the surface of the nanowires.
4
. Conclusions
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allow the electrodeposition of smooth and well-defined Ni NWs in
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