Influence of electrodeposition techniques on Ni nanostructures

Article abstract of DOI:10.1016/j.electacta.2008.03.045

Different Ni nanostructure arrays were fabricated by pulsed electrodeposition from a Watts bath inside the pores of anodic alumina membrane (AAM) templates. Under a trapezoidal waveform of potential, consisting of fast linear sweeps between 0 and -3 V (SCE) interleaved by delay times at 0 (10 s) and -3 V (0.1 s), Ni nanowires were grown. The rate of nanowires growth was constant up to 60 min of deposition. For longer times, the growth of nanowires was not uniform, and after about 180 min some nanowires reached the template surface exposed to the electrolyte. Under square potential pulses between the same potentials (pulse length 1 s), nanotubes of Ni are obtained. Morphological analysis of these nanostructures at different lengths revealed that the inner profile of nanotubes evolved from cylindrical to conical with increasing deposition time. The possibility to grow either nanowires or nanotubes in dependence of the potential waveform, as well as the growth rate of nanostructures were discussed taking into account the reaction of hydrogen evolution, occurring simultaneously with Ni electrodeposition.

Full text of DOI:10.1016/j.electacta.2008.03.045

Electrochimica Acta 53 (2008) 5766–5773
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Influence of electrodeposition techniques on Ni nanostructures
1
1
∗,1
Rosalinda Inguanta , Salvatore Piazza , Carmelo Sunseri
Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universit a` di Palermo, Viale delle Scienze, 90128 Palermo, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Different Ni nanostructure arrays were fabricated by pulsed electrodeposition from a Watts bath inside
the pores of anodic alumina membrane (AAM) templates. Under a trapezoidal waveform of potential,
consisting of fast linear sweeps between 0 and −3 V (SCE) interleaved by delay times at 0 (10 s) and −3 V
Received 21 December 2007
Received in revised form 4 March 2008
Accepted 20 March 2008
(
0.1 s), Ni nanowires were grown. The rate of nanowires growth was constant up to 60 min of deposition.
Available online 28 March 2008
For longer times, the growth of nanowires was not uniform, and after about 180 min some nanowires
reached the template surface exposed to the electrolyte. Under square potential pulses between the same
potentials (pulse length 1 s), nanotubes of Ni are obtained. Morphological analysis of these nanostructures
at different lengths revealed that the inner profile of nanotubes evolved from cylindrical to conical with
increasing deposition time. The possibility to grow either nanowires or nanotubes in dependence of the
potential waveform, as well as the growth rate of nanostructures were discussed taking into account the
reaction of hydrogen evolution, occurring simultaneously with Ni electrodeposition.
Keywords:
Ni electrodeposition
Nanowires
Nanotubes
Anodic alumina membranes
Nanostructures electrosynthesis
©
2008 Elsevier Ltd. All rights reserved.
1
. Introduction
parameters, such as surface treatment of aluminium, potential and
time of anodizing, nature and composition of electrolyte [14–24].
Besides, it is very easy and cheap to fabricate these membranes. In
this work, we used AAM as template for producing Ni nanostruc-
tures which can be employed for fabrication of recording magnetic
memory. The interest toward nanostructured magnetic materials is
due to the fact that current carriers are confined in the radial direc-
tion; consequently, transport is possible only in the nanowire axial
direction [25].
In recent years, nanostructured materials have attracted grow-
ing interest due to their specific properties, which allow application
in several fields such as photonics, nanoelectronics, thermoelec-
tronics. For example, nanostructured materials are employed for
the fabrication of field emission devices, magnetic memories,
nanoelectrodes, sensors, etc. The narrow size of nanostructured
materials causes significant modifications of properties with
respect to the bulk materials like thermodynamic stability, elec-
trical conductivity, mechanical, magnetic features, etc. [1].
Vasquez and coworkers prepared Ni nanowires by elec-
trodeposition inside AAM pores and found that the magnetic
behaviour changes with the aspect ratio (length to width ratio)
of nanowires [26–30]. Other authors reported that preferential
plane of nanowires growth is dependent on AAM pore diameter;
therefore, the magnetic behaviour can be controlled through this
parameter [31,32]. The importance of fabricating regular arrays
of magnetic elements, having sub-micron sizes, was emphasized
by Prida et al. [25], who highlighted that each single-domain
ferromagnetic nanowire represents one bit, depending on its mag-
netization state. In this way, data recording densities exceeding
For these reasons, extensive investigations dealing with prepa-
ration and characterization of these materials have been carried out
[
2,3]. For the fabrication of nanostructures different methods, like
lithography [4], chemical vapour deposition [5], sol–gel methods
6], elctrodeposition [7,8] and displacement deposition [9] were
[
proposed. One of the most popular techniques for preparation of
nanostructures is based on the use of a porous template, which
can be either partially or completely removed after the synthesis
[
(
10,11]. Polycarbonate membrane and anodic alumina membrane
AAM) are usually used for this purpose. In particular, AAM is
−
2
150 Gbit cm can be achieved [33,34]. For these applications wire
diameter and interwire spacing should be as small as possible
in order to increase the recording density [35–37]. However, for
other possible magnetic applications, such as those based on the
magneto-optical [38] or microwave properties [39] of the arrays,
larger wire diameters may be more useful.
an almost ideal template, due to its morphology consisting in an
ordered array of cylindrical pores perpendicular to the surface
[
12,13]. Another advantage of these membranes is the possibility
to control their morphological features by adjusting the anodizing
Despite the large amount of data reported in the literature,
at our knowledge no research has paid attention to the relation
between waveform of applied potential and morphology of the
nanostructures. Recently, the influence of electrodeposition condi-
∗ Corresponding author. Tel.: +39 0916567231; fax: +39 0916567280.
E-mail address: sunseri@dicpm.unipa.it (C. Sunseri).
1
ISE member.
0
013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2008.03.045

R. Inguanta et al. / Electrochimica Acta 53 (2008) 5766–5773
5767
tions (direct and alternating) on the growth orientation of metallic
gold and copper nanowires was investigated by Maurer et al. [40].
In this paper, we present results pertaining to the fabrication of Ni
nanostructures by electrodeposition under unipolar pulsed poten-
tial. We found that either nanowires or nanotubes can be fabricated
inside the channels of AAM, depending on the applied wave-
form. The nanostructures were characterized by scanning electron
microscopy (SEM), X-ray diffraction (XRD) and energy dispersive
spectroscopy (EDS). The role of the hydrogen evolution process,
occurring simultaneously with Ni deposition, is invoked in order to
explain the different nanostructures.
2. Experimental
Ni nanostructures were deposited within commercial AAM
(
Whatman, Anodisc 47) having an average pore diameter of
210 nm. The morphological characteristics of these membranes
∼
are presented elsewhere [41,42]. In particular, an AAM consists of a
regular bi-dimensional array of pores uniformly distributed on the
surface. Porosity of about 30% was evaluated by SEM analysis [43].
Prior to deposition, template pieces, having an overall area
2
of the order of 1 cm , were back sputtered with gold, mounted
onto an electrode holder by means of a carbon conductive paste,
and insulated laterally using an acrylic lacquer. The metallic Ni
nanostructure arrays were grown in a Watts bath having the fol-
−
1
•
−1 •
NiCl2 6H O,
2
lowing composition: 300 gL
NiSO4 6H O, 45 gL
2
−1
and 45 gL H BO [44]. Temperature and pH values were kept at
5 C and 4.5, respectively. The electrodeposition time was varied
3
3
◦
2
from 2 to 360 min. A typical three-electrode cell was used for the
pulsed electrodeposition, with a platinum foil as counter-electrode
and a standard calomel electrode (SCE) as reference. In the fol-
lowing all potentials are referred to SCE. Electrodeposition was
performed in the potentiostatic mode, using an EG&G potentio-
stat/galvanostat (mod. 273A). Two different potential waveforms
were applied between the same limits:
(
i) a trapezoidal wave, consisting of a linear ramp from 0 V to −3 V
1
−
(
SCE) at 1 V s , followed by a stop at −3 V for 0.1 s, a reverse
ramp from −3 to 0 V at the same sweep rate, and an arrest at
V for 10 s;
ii) a square wave between 0 V and −3 V (SCE), with delay times of
0 and 1 s at the two limits, respectively.
0
(
Fig. 1. SEM images of Ni nanowires fabricated by electrodeposition under trape-
zoidal potential waveform for 60 min: (a) sectional-view and (b) top-view after
partial dissolution of the template in 1 M NaOH for 1 h.
1
XRD patterns were recorded soon after electrodeposition by
using a Philips generator (mod. PW 1130) and a PW goniometry
mod. 1050). All diffractograms were obtained in the 2ꢀ range from
sponding to about 220 potential cycles. In Fig. 1a we report the
sectional-view of the template with parallel Ni nanowires emerg-
ing from the channels. Fig. 1b shows the top-view of the nanowires
after immersion of the membrane in 1M NaOH solution for 1 h,
leading to partial dissolution of the template; this procedure was
adopted in order to evidence the morphology of the nanowires.
Specifically, a uniform diameter of the nanowires (about 210 nm)
can be observed.
Fig. 2 reports the XRD patterns for the sample of Fig. 1. In
agreement with the ICDD card 4-850 [45], Ni nanowires are poly-
crystalline with the most intense peak along the plane (1 1 1). By
using the Scherrer formula [46], we calculated a grain size of about
(
◦ ◦ ◦
0 to 100 with a step of 0.02 and a measuring time of 0.5 s for
1
step, using the Cu K␣ radiation (ꢁ = 1.54 A˚ ). Peaks were identified
according to ICDD database [45].
Morphology of electrodeposited nanostructures was observed,
before and after dissolution of the AAM template in 1 M NaOH
solution, by means of a SEM microscope (FEI ESEM, mod. XL30)
equipped with an EDS probe for chemical composition analysis.
Prior to SEM analysis, sample surface was coated with a sputtered
layer of gold, to avoid electrostatic charging under the electron
beam.
In order to ensure the reliability of the different characteriza-
tions, these were performed on different pieces of the same sample.
9
5.4 nm. The inset of Fig. 2 shows a Ni EDS mapping of the same
sample. Membrane is filled for a thickness of about 35 ␮m with
a quite uniform Ni distribution; however, the inner surface (that
in contact with gold, where Ni nucleation occurs) appears per-
fectly flat while the outer surface (corresponding to the direction of
nanowires growth) is slightly waved, indicating that length of the
nanowires is not perfectly uniform.
3. Results and discussion
3.1. Fabrication of nanowires
Ni nanowires were fabricated by electrodeposition applying
a trapezoidal potential waveform. Fig. 1 shows two SEM images
of these nanowires obtained after 60 min of deposition, corre-
Fig. 3 displays the potential waveform leading to the formation
of Ni nanowires (curve a), and the current response after different

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R. Inguanta et al. / Electrochimica Acta 53 (2008) 5766–5773
Fig. 2. XRD patterns for the nanowires of Fig. 1. Inset: Ni EDS mapping.
deposition times (curves b–g). The current transients during the
potential scan change with the deposition time. After 2 min (curve
−
2
b) a small cathodic wave with a peak current of about 20 mA cm
can be observed; the current wave became more pronounced with
increasing the deposition time, and after 180 min its shape fol-
lowed that of potential waveform (curve e). For longer deposition
times the shape of current wave did not change anymore, with a
−
2
cathodic peak of about 250 mA cm after 360 min (curve g). This
figure evidences some interesting issues. The progressive increase
of the cathodic current is related to the growth of nanowires. In
particular, the current response becomes similar to the potential
waveform, as soon as nanowires reach the outer surface of the tem-
plate, where a massive Ni layer was deposited (see below). Thus, the
increase of current density with time seems due to the progressive
decrease of the ohmic resistance, mostly due to the replacement of
the electrolytic solution inside the channels of the template with
metallic Ni. In fact, the proportionality between current density
and potential for long deposition times (curves e–g) suggests that
Ni deposition on the external surface occurs under ohmic control.
This kind of control could be ascribed either to the ohmic drop
through the nanowires connecting the outer surface of the tem-
plate to the inner gold layer or to the hydrogen bubbles developing
during Ni deposition. We underline that noticeable gas evolution
was observed during electrodeposition since the initial pulses.
The progressive filling of the AAM channels is shown in Fig. 4.
After 40 min of deposition the length of nanowires was about
Fig. 4. Sectional-view of Ni nanowires after different times of deposition under
trapezoidal waveform of potential: (a) 40 min, (b) 60 min, (c) 180 and (d) 180 min
(detail of (c) showing the edge between the section and the surface of the template).
19 ␮m (Fig. 4a), and after 60 min the length was increased to 35 ␮m
(
Fig. 4b). Micrographs 4c and d reveal that for longer deposition
times nanowires growth is not uniform. In particular, after 180 min
some nanowires reached the outer surface, while other channels
were only partially filled. Fig. 4c displays the whole thickness of the
composite membrane, while the continuity between the nanowires
and the hemispherical islands of Ni deposited on the external sur-
face is evidenced in micrograph 4d. A careful control of nanowires
length and its uniformity are of fundamental importance for appli-
cations. The present study shows that, in the chosen conditions,
length is quite uniform up to 60 min of deposition (see inset of
Fig. 2), while for longer deposition times significant differences
in length are noticed between different channels (Fig. 4c and d).
This is probably due to the parasitic hydrogen evolution reaction,
which interferes with the reaction of Ni deposition. For deposition
times less than 60 min, the cathodic current was rather low. There-
fore, at the beginning of each negative potential sweep, membrane
channels were practically free of hydrogen, because part of it was
conveyed outside the channels and part was consumed during the
long delay time at 0 V, where the current was anodic. For longer
deposition times as the cathodic current density increased, also the
Fig. 3. Applied potential waveform for the fabrication of Ni nanowires (curve a), and
current transients at different deposition times (curves b–g).

R. Inguanta et al. / Electrochimica Acta 53 (2008) 5766–5773
5769
Fig. 6. Sectional-view of Ni nanowires grown by electrodeposition under trape-
zoidal potential waveform for 240 min. A copper tape was used to connect the gold
layer of template to the electrode holder.
The progressive emersion of nanowires determines the formation
of new Ni particles, so that the coverage of the external surface
increases. After 360 min metallic coverage of the external surface
was complete, as shown in Fig. 5c, where the coalescence of Ni
nuclei is evident.
The rate of growth of nanowires was also investigated by
changing some experimental condition, in order to explain some
non-clarified issues of our previous work [24]. In that work,
we observed a lower and poorly reproducible growth rate of Ni
nanowires, with quite different values of nanowires length for
apparently identical experiments. In this work, we have found that
as the gold layer of the template is connected to the electrode holder
through a copper tape rather than a carbon paste, the current den-
sity in otherwise identical conditions was lower, and consequently,
shorter nanowires were formed. Fig. 6 shows Ni nanowires formed
with a copper tape connection: in this case we obtained nanowires
lengths equal to 4 and 14 ␮m after 240 min of deposition, in two
apparently identical experiments, whereas with a carbon paste
connection a length of about 50 ␮m was obtained after 180 min,
with a satisfactory reproducibility. This finding suggests that the
poor reproducibility of the experiments with copper tape is due
probably to the poor quality of the electrical contact between elec-
trode holder and gold.
3.2. Fabrication of nanotubes
Ni nanotubes were fabricated by pulsed electrodeposition
Fig. 5. Morphology of the template external surface after different times of deposi-
applying a square potential waveform to the electrode. Fig. 7 reports
two SEM images of these nanotubes obtained after 30 min of depo-
sition (corresponding to 164 cycles) and etching of the template in
1 M NaOH solution for 1 h. This treatment was performed in order to
dissolve the membrane and to evidence morphology of the nanos-
tructures formed inside the AAM channels. EDS analysis confirmed
that these nanotubes consisted of pure Ni (see below). Fig. 7a shows
the top-view of the etched sample, with a quite uniform distribu-
tion of nanotubes. At higher magnification (Fig. 7b) it is possible to
distinguish the almost perfect cylindrical shape of the nanotubes
and their size. For this sample, the average internal diameter and
the wall thickness are about 170 and 20 nm, respectively.
tion: (a) and (b) 180 min, (c) 360 min.
rate of hydrogen production was higher. Consequently, some chan-
nels remained either partially or completely screened by hydrogen
bubbles, because gas production rate was higher than that of con-
suming.
The progressive coverage of the AAM external surface with Ni is
shown in Fig. 5. Fig. 5a shows some hemispherical metallic nuclei,
some of which begin to coalesce, randomly distributed over the
surface. The cause of the random distribution of these nuclei is elu-
cidated in Fig. 5b, where it is possible to distinguish four white
spots, which can be identified as sheaves of nanowires on the point
of emerging to the surface. As one sheaf reaches the external surface
of the membrane, deposition of massive Ni occurs over it, with the
formation of metallic particles. In other words, after 180 min some
nanowires have already reached the surface, and they generate the
nuclei for the formation of Ni macro-particles on the external sur-
face, while other nanowires were still growing inside the pores.
Composition of the deposited material was investigated, as
shown in Fig. 8, where XRD patterns and EDS spectrum (inset) are
reported. Like in the previous case, the deposit was polycrystalline,
with a grain size (calculated from the main peak, by means of the
◦
Sherrer’s equation [46]) of about 18 nm. The wave from 2ꢀ = 20 to
◦
30 in the XRD patterns is due to the amorphous nature of anodic
alumina. The presence of Al O3 in the XRD patterns is due to the
2

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R. Inguanta et al. / Electrochimica Acta 53 (2008) 5766–5773
Fig. 9. Applied potential waveform for the fabrication of Ni nanotubes (curve a) and
current response (curves b–g) at different electrodeposition times.
fact that this analysis was performed prior to dissolution of the tem-
plate in NaOH. Also EDS spectrum show in the inset of the figure
was taken before dissolution, as confirmed by the presence of O and
Al peaks. EDS spectra were recorded also after template dissolution,
and they are equal to that reported apart the absence of O peak and
the low intensity of Al one. Au peaks, present in both spectra, are
due to the gold sputtered on the back side of the template (XRD
patterns) and on the AAM front surface before SEM analysis (EDS).
In Fig. 9, the potential waveform (curve a) and the current
response after different times (curves b–g) are reported. The cur-
−
2
rent transients show an initial spike of about −170 mA cm
,
independent of electrodeposition time, followed by a fast decay
toward an almost constant value, dependent on the deposition
time. After 2 min (curve b of Fig. 9), the value of current density
at the end of the square wave is close to 0 mA cm , while after
60 min it is about 130 mA cm (curve g). This behaviour is typical
−
2
−
2
3
of a potentistatic transient. The cathodic current spike is likely due
to double layer charging following the potential sudden change. As
the double layer is fully charged, current arises from the faradaic
reactions and becomes almost constant. The current value at the
end of transient increases with deposition time. This indicates a
progressive decrease of the resistance inside the channels, anal-
ogous to the case of nanowires growth. Visual inspection of the
electrode revealed hydrogen evolution during the electrodeposi-
tion process, even if outer gas bubbling was less intense than in
the previous case in the presence of almost the same current den-
sity. This suggests a higher hydrogen trapping inside the AAM pores
during the growth of nanotubes.
Fig. 7. SEM images with different magnifications of Ni nanotubes fabricated by elec-
trodeposition for 60 min under square waveform of potential. Template was partially
dissolved by immersion in 1 M NaOH for 60 min.
The length of nanotubes after different deposition times is
shown in the sectional-views of Fig. 10. Fig. 10a is relative to nan-
otubes grown for 30 min, while Fig. 10b, which is relative to 360 min
of electrodeposition, shows a detail of nanotubes close to the exter-
nal surface of the membrane. From this last picture, a quite uniform
length can be observed, indicating that nanotubes grow in each
pore at about the same rate. Fig. 11 shows a linear dependence of
the average length of nanotubes on deposition time. From this plot
−
1
a growth rate equal to 0.164 ␮m min can be inferred.
The shape of nanotubes is more evident after partial dissolu-
tion of AAM template in 1 M NaOH for 120 min. Following this
treatment, the morphology of nanotubes grown after 360 min of
deposition is shown in Fig. 12 at different magnifications. Fig. 12b
(magnification of the squared area of Fig. 12a) shows that the inner
diameter of the nanotubes, fractured at about half of their length,
is about 60 nm, corresponding to a wall thickness of 75 nm, while
after 30 min of deposition the wall thickness was 20 nm (see Fig. 7).
Moreover, from Fig. 12a it can be seen that the wall thickness of
the same nanotubes at the extremity close to the external surface
Fig. 8. XRD patterns of a membrane filled with Ni nanotubes fabricated by elec-
trodeposition under square potential pulses for 60 min. Inset: EDS spectrum for the
same specimen.

R. Inguanta et al. / Electrochimica Acta 53 (2008) 5766–5773
5771
Fig. 12. (a) Micrograph of Ni nanotubes after 360 min of electrodeposition and par-
tial dissolution of the template in 1M NaOH for 120 min and (b) magnification of
nanotubes section at about half length.
shape of Ni nanotubes. The evolution of the inner shape from cylin-
drical to conical could be explained by assuming that the rate of
growth along the axis of the channels is higher than the rate of
growth in radial direction. Consequently, the basal plane is pro-
gressively occluded as deposition time increases. In order to obtain
Ni nanotubes open on both sides (top and bottom), it is necessary
to stop the electrodeposition prior to the closure of the bottom.
Therefore, nanotubes length is limited by the occurrence of this
Fig. 10. Sectional-view of nanotubes after different times of electrodeposition: (a)
0 min and (b) 360 min.
3
of the template is about 100 nm (lower left part of the micro-
graph).
This finding suggests that nanotubes are externally cylindrical
but with an inner conical profile. Fig. 13 sketches a possible inner
Fig. 11. Average nanotube length vs. deposition time.
Fig. 13. Possible evolution of the inner profile of Ni nanotubes with time.

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R. Inguanta et al. / Electrochimica Acta 53 (2008) 5766–5773
event. This means that it is possible to control the length and the
inner diameter of the nanotubes by adjusting the deposition time
and, probably, the potential limits of the square pulse.
The different rates of Ni deposition along the two channel direc-
tions (longitudinal and radial) could be due to the interference of
hydrogen evolution, that probably influences also the shape of the
nanostructures (either nanowires or nanotubes) associated to the
potential pulse waveform.
Other authors invoked the occurrence of this parasitic reaction
in order to explain the different shapes of nanostructures fabri-
cated by using polycarbonate membrane as template [47]. In that
work nanostructures were fabricated at constant current density,
while our experiments were carried out imposing different poten-
tial pulse waveforms. When a trapezoidal potential pulse is applied,
the increase of current during the cathodic potential scan, even if
fast, is progressive, leading to the formation of small hydrogen bub-
bles that do not hinder the simultaneous deposition of Ni inside
the channels. The surface of deposition is periodically renewed,
because part of hydrogen is conveyed outside the channels and part
is oxidized during the polarization at 0 V. Probably this mechanism
leads to the formation of nanowires.
time wereexplainedbothinvokingthescreening effectbyhydrogen
bubbles that are formed simultaneously to Ni deposition.
In the case of trapezoidal waveform, small bubbles of hydro-
gen are formed leaving sufficient free surface for deposition of Ni,
which consequently, occurs over the entire inner surface of the
channels. On the contrary, a square potential pulse leads to forma-
tion of bigger gas bubbles, that screen the bottom surface of pores;
thus, the deposition of Ni is confined into the gap between bubbles
and channel wall.
Further work is in progress in order to better controlling the
growth rate of the nanostructures in dependence on potential
waveform shape and limits, pH and composition of the deposition
bath.
Acknowledgments
This work was supported financially by Universit a` di Palermo-
APQ Ricerca della Regione Siciliana delibera CIPE n. 17/2003,
“Laboratorio dell’innovazione nel settore dei beni culturali: Sper-
imentazione di Nanotecnologie e Nanomateriali”.
On the contrary, when a square potential pulse is applied,
cathodic current increases abruptly when potential steps from 0 to
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In this work we investigated the fabrication of nanostructures
(
by electrodeposition of Ni inside the channels of AAM under pulsed
potential perturbations. It was found that the shape of nanostruc-
tures depends on the potential waveform. Under a trapezoidal
wave, ranging between 0 and −3 V (SCE), Ni nanowires were
formed, whose length increased with deposition time. Their growth
rate was constant up to 60 min of deposition, while for longer times
non-uniform lengths were observed in different channels. After
about 180 min, some nanowires reached the template external sur-
face, over which deposition of massive Ni began. Hemispherical Ni
particles were formed, that progressively coalesced leading to the
formation of a compact metal layer. As the nanowires reached the
external template surface, the shape of current transients modi-
fied, becoming equal to that of the potential pulse. This indicates
an ohmic control of the deposition process.
Under a square potential waveform, nanotubes of Ni were fab-
ricated inside the channels of AAM. Up to 30 min of deposition,
nearly cylindrical nanotubes were formed. For longer times, the
inner shape of nanotubes evolved from cylindrical to conical, due
to the progressive shutting of the bottom. The formation of different
nanostructures (either nanowires or nanotubes) and the modifica-
tion of the inner shape of nanotubes with increasing the deposition
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