Fabrication of various nickel nanostructures by manipulating the one-step electrodeposition process

Article abstract of DOI:10.1149/1.2717501

By employing colloidal crystal templates with different colloid diameters, nickel well-ordered macroporous structures (inverse opal) with different void diameters can be fabricated via a template-mediated electrodeposition process. It was found that the different degrees of filling colloidal crystal template produces different nickel nanostructures during the electrodeposition process. Moreover, the filling behavior for electrodeposition of nickel into the template can be characterized in terms of the current transients recorded. Because of preferential nickel-filling into the interstitial spaces among the colloids, fabrication of well-ordered nickel nanorod and monolayer porous arrays are manipulated by controlling electrodeposition times accurately at 5 and 10 s, respectively. However, the macroporous structure covered with hollow sphere arrays on the surface can be formed in 120 s due to preferential nickel-coating around the colloidal surface. Therefore, various kinds of nickel-based structures derived from colloidal crystals can be constructed via a one-step, template-mediated electrodeposition process.

Full text of DOI:10.1149/1.2717501

Journal of The Electrochemical Society, 154 ͑6͒ E77-E83 ͑2007͒
E77
0013-4651/2007/154͑6͒/E77/7/$20.00 © The Electrochemical Society
Fabrication of Various Nickel Nanostructures by Manipulating
the One-Step Electrodeposition Process
Yi-Wen Chung,a Ing-Chi Leu,b,z Jian-Hang Lee,a Jung-Hsien Yen,a and
Min-Hsiung Hona
aDepartment of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan
bDepartment of Materials Science and Engineering, National United University, Miao-Li 360, Taiwan
By employing colloidal crystal templates with different colloid diameters, nickel well-ordered macroporous structures ͑inverse
opal͒ with different void diameters can be fabricated via a template-mediated electrodeposition process. It was found that the
different degrees of filling colloidal crystal template produces different nickel nanostructures during the electrodeposition process.
Moreover, the filling behavior for electrodeposition of nickel into the template can be characterized in terms of the current
transients recorded. Because of preferential nickel-filling into the interstitial spaces among the colloids, fabrication of well-ordered
nickel nanorod and monolayer porous arrays are manipulated by controlling electrodeposition times accurately at 5 and 10 s,
respectively. However, the macroporous structure covered with hollow sphere arrays on the surface can be formed in 120 s due to
preferential nickel-coating around the colloidal surface. Therefore, various kinds of nickel-based structures derived from colloidal
crystals can be constructed via a one-step, template-mediated electrodeposition process.
© 2007 The Electrochemical Society. ͓DOI: 10.1149/1.2717501͔ All rights reserved.
Manuscript submitted September 22, 2006; revised manuscript received January 4, 2007. Available electronically April 3, 2007.
Recently, there has been extensive interest in the creation of po-
rous materials with three-dimensional periodicity due to their poten-
tial applications, such as catalysts,1 chemical2 and gas3 sensors,
photonic4 and optoelectronic devices,5 thermal insulation materials,6
and membranes.7 In general, the atomic clusters,8 nanoparticles,9
and nanostructures10 of materials exhibit size-dependent properties
that are profoundly different from their corresponding bulk material
counterparts. Hence, it can be anticipated that porous material in the
nano or submicrometer scale with a larger surface area will exhibit
improved performance in applications, such as those requiring trans-
port through the pores or interaction with the surface. Moreover, it
has been suggested that metals with well-ordered porous networks
possess interesting photonic properties.11
For porous metals prepared using conventional methods, such as
powder sintering, slip casting and fiber metallurgy, it is difficult to
achieve materials with high surface areas.12 Although some
microporous13 or mesoporous14 metals exhibit high surface areas, a
significant fraction of the pores is not easily accessible to large
fillers due to the relatively small pore diameters. Thus, well-ordered
macroporous metals fabricated via a template-mediated method
have attracted a great deal of attention recently due to their efficient
process and wider variety of applications.
Template-mediated technology is an important process for form-
ing special nanostructures, including one-dimensional ͑1D͒
nanofiber15 and three-dimensional ͑3D͒ well-ordered macroporous
structures.16 By utilizing a nanochannel-filling technology, a replica
of the host template can be obtained. The entire process has two key
factors. The first is the choice of filling technology. Many
methods15,17-19 for nanochannel filling have been developed in
which the filling for nanochannels is performed by chemical vapor
deposition,17 a dipping process,18 and electrophoresis,15 but the top
of the template usually encounters the issue of jamming. However,
the mechanism of filling nanochannels by using an
electrodeposition20 is from bottom to top, and the jamming of tem-
plate surfaces can be avoided.21 Hence, electrodeposition is a pow-
erful tool for the fabrication of nanostructures with high structural
quality and aspect ratio.22 The fabrication of high-quality and appro-
priate templates is the second factor for creating high-quality repli-
cas via a template-mediated process.23-25 For example, the colloidal
crystal, in which monodispersed colloids self-assemble into an or-
dered 3D artificial crystal structure, often acts as the template for
forming macroporous structures.26-31 Hence, by combining a high-
quality template and efficient nanochannel-filling technology, the
template-mediated process can be easily manipulated.
In this report we prepared a well-ordered nickel macroporous
structure via a template-mediated electrodeposition process. Be-
cause the colloidal crystal template via the capillary-enhanced pro-
cess exhibits the face-centered cubic ͑fcc͒ structure, the nickel
replica, which was constructed by using a template-mediated elec-
trodeposition process, has the same structure. In addition, according
to the relationship between current values and electrodeposition
time, the behavior of nickel electrodeposition in the interstitial
spaces of colloidal crystal template can be determined. Hence, well-
ordered nickel nanorod, nanoporous arrays, and macroporous struc-
ture can be fabricated via a one-step process. Further, the nickel
macroporous structures covered with hollow sphere arrays on the
surface are also constructed due to different interstitial-spaces-filling
mechanisms.
Experimental
Preparation of monodispersed colloidal particles.— The mono-
dispersed P͑St-co-MAA͒ microspheres were prepared by using an
emulsifier-free-emulsion copolymerization of styrene ͑St͒ and meth-
acrylic acid ͑MAA͒ based on the method proposed by Wang and
Pan.32 By controlling the concentration of MAA and reaction time,
two batches with different diameters of colloidal particles ͑180 and
300 nm with a relative standard deviation smaller than 5%͒ were
formed.
Fabrication of colloidal crystal template.— An efficient process
for preparing high-quality colloidal crystals, called capillary-
enhanced method, was used in this study.31 First, the substrate
coated with indium tin oxide ͑ITO͒ thin film was placed horizontally
at the bottom of a container which was filled with an aqueous dis-
persion of the P͑St-co-MAA͒ microspheres obtained by ultrasonica-
tion. The formation of colloidal crystals can be obtained by the
self-assembly of microspheres at 45°C and under a high-humidity
͑90%͒ condition. After evaporation was terminated in 24 h, the sub-
strate was picked up with a colloidal crystal film thereon.
Synthesisof nickel macroporous structure.— Cathodic dc elec-
trodeposition was conducted in a conventional three-electrode cell.
The aqueous solution used for Ni deposition33 was composed of
NiSO4 ͑2.13 M͒, NiCl2 ͑0.35 M͒, and H3BO3 ͑0.43 M͒. The work-
ing electrode was a conductive indium-tin oxide ͑ITO͒-glass sub-
strate with a deposited colloidal crystal film. A platinum plate and
silver/silver chloride were used as the counter and reference elec-
trodes, respectively. After finishing nickel electrodeposition, the col-
z E-mail: icleu@mail.mse.ncku.edu.tw
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Journal of The Electrochemical Society, 154 ͑6͒ E77-E83 ͑2007͒
Figure 3. SEM surface images of colloidal crystal filled with nickel via an
electrodeposition process. The inset figures indicate that colloidal crystal
templates are either unfilled or filled with nickel.
Results and Discussion
The colloidal crystal templates for the synthetic macroporous
structure studied in this work were prepared using an efficient tech-
nology described in Ref. 31. SEM images of the ͑111͒ surface of the
synthetic reveal a close-packed arrangement, as shown in Fig. 1a,
where some vacancies are found in this well-ordered array. A typical
result is shown in Fig. 1b, where the different facets of square and
hexagonal arrangements can be clearly observed at the facets ͑100͒
and ͑111͒, respectively. In fact, a square arrangement can only cor-
respond to a ͕100͖-type surface of a fcc structure, instead of in the
hexagonal close-packed ͑hcp͒. Hence, this colloidal crystal has a fcc
structure. According to discussion of modified Bragg’s law,34 a high-
quality colloidal crystal in a large area can diffract light and exhibits
a stop band in its transmission spectrum. Figure 2 shows that the
colloidal crystals consisted of 180 and 300 nm colloidal particles
and have absorptive peaks at 451 and 654 nm, respectively. The
result means that the colloidal crystals are of good crystallinity.
Figure 3 shows the nickel electrochemically deposited into the
colloidal crystal template from a nickel sulfate solution at −1.0 V
͑Ag/AgCl͒. Comparing the two insets, the interstitial space of col-
loidal crystal template had been filled. Because the filling behavior
of the electrodeposition process is from bottom to top, surface jam-
Figure 1. SEM images of well-ordered colloidal crystals prepared by a
capillary-enhanced method: ͑a͒ top view and ͑b͒ side view.
loidal crystal template can be removed by calcinations at 450°C for
4 h. Finally, nickel well-ordered macroporous film was formed on
the substrate.
Instrumentation and characterization.— Surface morphology
was observed using a Hitachi S4100 field emission scanning elec-
tron microscope, and the quality of colloidal crystal template was
gauged by using a UV-2001 UV-visible spectrometer from Hitachi.
Electrochemical synthesis of nickel was performed by a potentiostat
͑EG&G, model 263͒.
Figure 2. The absorptive spectra of col-
loidal crystals with the different colloid
diameters of ͑a͒ 300 and ͑b͒ 180 nm.
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Journal of The Electrochemical Society, 154 ͑6͒ E77-E83 ͑2007͒
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Figure 4. Chronoamperometric response of nickel electrodeposition in the
interstitial spaces of colloidal crystal template with 180 and 300 nm colloid
diameter.
ming is avoided. After the nickel electrodeposition process, the well-
ordered colloidal crystal was not destroyed due to the presintering
process performed at 135°C for 2 h.
Figure 4 shows a typical current-time transient for nickel depo-
sition into the colloidal crystal template with 180 and 300 nm col-
loidal diameter, respectively, which is divided into three stages ac-
cording to the filling behaviors. In the initial stage, from 0 to 15 s,
nickel ions in the electroplating solution are induced to migrate by
the applied potential and accumulate onto the working electrode,
resulting in the increase of current values. Subsequently, nickel is
formed at the bottom of the template not masked by the colloids by
a reduction reaction, as depicted in Fig. 5a. In the second stage, from
15 to 100 s, nickel metal forms in the interstitial spaces of the tem-
plate on the working electrode, as depicted in Fig. 5b. The current
values are at a stable state. This phenomenon of smooth current
curve with small oscillation has also been reported for nickel-filling
the interstitial spaces of colloidal crystal template by Sumida et al.35
In the third stage, 100–200 s, the level of nickel-filling into the
interstitial spaces almost reaches the surface of the colloidal crystal
template, as depicted in Fig. 5c. The time-dependence variation of
current differs from that in the second stage. In general, the behavior
of electrodeposition can be described explicitly by employing Fara-
day’s law.36 Hence, it can be used to explain the behavior of nickel-
filling the colloidal crystal template, which is expressed as
ItM
Figure 5. Schematic diagrams of nickel infiltration into colloidal crystal
templates at different electrodeposition stages of ͑a͒ 0–15, ͑b͒ 15–100, ͑c͒
100–200 s.
W =
͓1͔
ZF
where W is the weight of electroplated material, I and t are electro-
plating current and time, respectively, M is the molecular weight of
electroplated material, Z is the electric charge, and F is the Faraday
constant. Then Eq. 1 can be modified, as shown in Eq. 2
that in the electrodeposition of polypyrrole into the voids of colloi-
dal crystal,37 where a rapid increase in current is suggested to be
caused mainly by a rapid increase of the electrochemical reaction
area when the growing surface reaches the membrane/bulk solution
interface.
After removing the colloidal crystal template by calcinations at
450°C for 4 h, nickel ͑with the presence of NiO on the surface͒
macroporous structures are obtained. Moreover, porous structure
with different pore sizes can be formed by employing a colloidal
crystal template having different colloid diameters. Figure 6a shows
the SEM image of the surface of a 3D ordered nickel replica formed
by depositing into a colloidal crystal template with 180 nm colloid
particles. It is apparent that a close-packed arrangement on the top-
most plane of the replica is exhibited. The inset of Fig. 6a shows a
symmetry indicating that the spherical holes are located above three
1 tM
A =
I
͓2͔
ͩ ͪ
␳h ZF
where ␳ is the density of electroplated material. The values of A and
h indicate the area and height of electroplated material, respectively.
According to the modified Faraday’s law ͑Eq. 2͒, the current value
͑I͒ increases with increasing area ͑A͒. When the height of nickel-
filling is above the surface of the colloidal crystal template, the
electrochemical reaction area changes from projected area of the
interstitial spaces into planes. The growth of nickel metal extends
from the spaces among the colloids toward the colloidal surface,
until colloidal particles on the template surface are covered com-
pletely. Hence, current value gradually elevates due to the gradual
increase of electrochemical reaction area. The result differs from
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Journal of The Electrochemical Society, 154 ͑6͒ E77-E83 ͑2007͒
Figure 6. SEM surface images of 3D well-ordered nickel macroporous
structure prepared by using colloidal crystal templates with the colloid di-
ameters of 180 nm at the ͑a͒ top view and ͑b͒ side view.
Figure 7. SEM surface images of 3D well-ordered nickel macroporous
structure prepared using colloidal crystal templates with the 300 nm colloid
diameters at the ͑a͒ top view and ͑b͒ side view.
neighboring hollow sites in the crystal. Each cavity formed by the
polystyrene particles has three dark spots corresponding to the con-
tact points with the three particles in the layer below. In the cross
section of Fig. 6b, the well-ordered macroporous structures are dem-
onstrated to be fabricated from bottom to top. These nickel nano-
structures are also called inverse opals.
As shown in Fig. 7a, the nickel porous arrays in a large area,
fabricated by depositing into colloidal crystal template with 300 nm
colloid particles, appear well-ordered in the free surface of the ͑111͒
texture. The inset of Fig. 7a also shows that each air sphere rests on
three neighboring spheres below along the ͓111͔ direction. More-
over, the same 3D macroporous structure consisting of many spheri-
cal void layers from the side view can be also observed, as shown in
Fig. 7b. Hence, nickel inverse opals can be obtained via a template-
mediated electrodeposition process.
almost equal to 3.47 at the same time. From the observation of Fig.
6b and 7b, the ratio between the height ͑4.1 ␮m͒ for nickel-filling
into colloidal crystal with 300 nm colloid diameter and 180 nm
͑1.2 ␮m͒ is equal to 3.42. It can be demonstrated that essentially
both areas not occupied by the colloids are the same for the case
using 180 and 300 nm colloid particles. These areas indicate the
nickel-filling sites, shown as shaded areas of Fig. 8. First, both black
trigonal regions for colloids with 180 and 300 nm diameter can be
calculated to be 1317 and 3657 nm2, respectively. The numbers of
the trigonal region for colloids with 180 and 300 nm diameter in the
In general, the height of nickel-filling into the colloidal crystal
template increases with increasing electrodeposition time. Further,
by using the colloidal crystal templates consisting of the different
colloid diameters, different kinetics relationships for nickel-filling
occurs with different electrodeposition current values. Hence, the
height of nickel-filling in the colloidal crystal with 180 and 300 nm
colloid diameter, respectively, can be discussed according to a modi-
fied Faraday’s law ͑Eq. 3͒, as depicted by the following
1 tM
Ah =
I
͓3͔
ͩ ͪ
␳ ZF
The nickel-filling volume ͑Ah͒ increases with increasing the current
values. As depicted in Fig. 4, the ratio between current values
͑−2.6 mA͒ upon filling a colloidal crystal with 300 nm colloid di-
ameter and 180 nm ͑−0.75 mA͒ is equal to 3.47 during the elec-
trodeposition time of 100 s. This also results in a ratio of nickel-
filling volume between 300 and 180 nm colloidal diameter being
Figure 8. The illustration of projected area to be filled by Ni in the intersti-
tial space among colloid particles.
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Journal of The Electrochemical Society, 154 ͑6͒ E77-E83 ͑2007͒
colloidal crystal ͑of substrate area 0.503 cm2͒ are equal to 3.58 and
E81
1.3 ϫ 109, respectively. Finally, the product of the area and the
number per unit area, i.e., the entire projected area for nickel-filling
͑A͒, is equal to 4715 ϫ 109 and 4754 ϫ 109 nm2 for the colloid
with 180 and 300 nm diameter, respectively. The area ͑A͒ between
the 180 and 300 nm colloidal system is almost the same. Because
both cases possess the same area for nickel-filling, the ratio of their
heights for nickel filling can be estimated according to the ratio of
their electrodeposition currents in the semiquantitative analysis.
Nickel ions in the solution migrate toward the working electrode
with the colloidal crystal template in the electrodeposition process.
Subsequently, nickel metal reduces and deposits into the interstitial
spaces of the colloidal crystal template. Because the numbers of
180 nm colloid particles filled in the constant volume is larger than
the ones of the 300 nm colloid particles filled in the same volume,
the nanochannels composed of interstitial spaces of 180 nm colloi-
dal stacks include more turns. These turns cause a larger resistance
for ionic fluid near the electrical double layers in the fluid system.38
Therefore, the migration speed of nickel ions in the interstitial
spaces of colloidal crystal with 180 nm diameter is lower than for
the case of 300 nm colloid particles. According to the semiquantita-
tive analysis, the current values in the 300 nm colloidal system are
larger than ones in the 180 nm colloidal system.
In the initial stage of the nickel electrodeposition process, from 0
to 15 s, the different morphologies of the porous array can be ob-
tained by manipulating electrodeposition time. Figure 9a shows a
monolayer of nickel porous array fabricated via an electrodeposition
process for 10 s. Each air hole is surrounded by six spherical voids,
which is a hcp network of circular holes. The pattern of pore array
indicates a replica of colloidal crystal in 2D. The inset of Fig. 9a
shows a cross-sectional image of a nickel macroporous monolayer
in which only one layer of nickel macropore exists. When elec-
trodeposition time is decreased to 5 s, nanodot arrays can be ob-
tained as shown in Fig. 9b. This structural arrangement is similar to
the porous arrays in a close-packed network. The inset figure with a
high magnification shows that one hole is surrounded by six nickel
nanodots. These sites occupied by nickel nanodots are the interstitial
spaces among three neighboring colloidal particles. Because these
sites covered by ITO thin film are exposed to the electrodeposition
solution, they possess a large free surface area for the nickel reduc-
tion and deposition. Hence, the nickel ions in the electrodeposition
solution migrate toward these sites and more easily produce nan-
odots of nickel metal via an electrochemical reduction reaction.
Further, the phenomenon of nickel-filling interstitial spaces at the
template bottom can be described according to the chronoampero-
metric response at short time, as shown in Fig. 9c. From 0 to 3 s,
because of the supply of a negative potential, nickel ions with posi-
tive charge near the electrode in the solution migrate fast and accu-
mulate onto the ITO-glass substrate not covered by colloids. Subse-
quently, nickel metal is reduced at these sites, resulting in a rapid
increase of the electrodeposition current. Because of the presence of
time-dependent diffusion regime,39 the increasing trend of elec-
trodeposition current became smaller from 3 to 8 s. Nevertheless,
these nickel ions are reduced to metal gradually at the same sites.
Meanwhile, the nickel nanorod is fabricated in 5 s. Due to the ex-
istence of a colloidal crystal template, the nickel metals cannot grow
freely. The actual electrode area is the cross section of the interstitial
spaces of the template, which leads to a stable current during the
period of Ni filling into the template. Hence, current is about the
same from 8 to 15 s. The nickel macroporous monolayer also forms
in 10 s via the template-mediated process. Therefore, three types of
current variation during electrodeposition respectively lead to the
fabrication of nanorods and a macroporous monolayer.
Figure 9. SEM images of a 2D nickel nanostructure based on ͑a͒ a
macroporous monolayer and ͑b͒ nanodot arrays prepared by a template-
mediated electrodeposition process for 10 and 5 s, respectively. ͑c͒ Chrono-
amperometric response of nickel electrodeposition in the interstitial spaces of
colloidal crystal template with 300 nm colloid diameter for 15 s.
When the electrodeposition time proceeds for 120 s, a novel
nickel nanostructure consisting of macroporous structure covered
with a hollow sphere array can be formed after removing colloidal
crystal template, as shown in Fig. 10a. From the figure inset, the
nickel hollow spherical nanostructures can be observed clearly. The
behavior of nickel-filling interstitial spaces on the top surface of
colloidal crystal template is different from the nanostructure at the
bottom and middle. At the template bottom and middle, nickel metal
is reduced and grows at the interstitial spaces among the colloidal
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E82
Journal of The Electrochemical Society, 154 ͑6͒ E77-E83 ͑2007͒
macroporous structure covered with hollow sphere arrays is fabri-
cated, resulting from preferential adsorption coating of nickel
around the colloids. Because this nanostructure possesses both a
large surface area and a 3D well-ordered arrangement, it has poten-
tial for many applications, including magnetic devices,41 sensors,42
and surface-enhanced Raman spectroscopy.43
Conclusions
A high-quality, colloidal crystal template can be fabricated via
the capillary-enhanced method. The mechanism for nickel-filling
into the interstitial spaces of colloidal crystal templates can be di-
vided into four stages according to the current values during the
electroplating process. Further, the ratio of height of nickel-filling
colloidal crystal with 300 and 180 nm diameter can be estimated
using the ratio of their current values in the second stage of the
electrodeposition process. In addition, well-ordered nanorods and
nanoporous arrays can be formed in 5 and 10 s, respectively, and the
nickel hollow spherical nanostructures can be obtained on the sur-
face for an electrodeposition time of 120 s. Therefore, various kinds
of nickel nanostructure can be produced via the one-step elec-
trodeposition process. Moreover, their forming mechanism can be
described according to the relation of current-time responses.
Acknowledgments
Financial support from the National Science Council, Taiwan,
through contract no. NSC93-2216-E-006-039 is greatly appreciated.
National Cheng Kung University assisted in meeting the publication
costs of this article.
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particles according to previous results, as depicted in the left of Fig.
10b. This mechanism results in a well-ordered, macroporous array
on the top of the nanostructure, as shown in Fig. 7b. However, they
are produced and coated around the colloidal surface, instead of
interstitial spaces, on the template top. The mechanism of forming a
hollow sphere array on the top is depicted in the right of Fig. 10b.
This is attributed to an electrostatics-induced effect between colloi-
dal particles and the nickel ions.40 The zeta-potential of
P͑St-co-MAA͒ colloidal particles is measured to be negative in our
preliminary study ͑not shown here͒. A great number of positive
nickel ions in the solution are attracted toward the working electrode
by the applied constant potential ͓−1 V to ͑Ag/AgCl͔͒ during the
electrodeposition process. This phenomenon results in many nickel
ions being adsorbed onto the colloidal surface at the template top,
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reach the top of the template. Finally, nickel metal is reduced pref-
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the interstitial spaces among the colloids. On the contrary, a
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