Formation of nickel nanowires via electroless deposition under a magnetic field

Article abstract of DOI:10.1149/1.3596703

Nickel nanowires were synthesized via electroless deposition in an organic solvent (ethylene glycol) under a magnetic field. Deposition behavior of nickel particles and wires were electrochemically investigated at various concentrations of NaOH by an in-situ mixed potential measurement and voltammetry combined with quartz crystal microbalance. Based on the electrochemical investigation, a formation mechanism of nickel wires is proposed. According to the mechanism, nickel wires 100-370 nm in diameter with several dozen m of length were successfully prepared by controlling the reduction rate by varying a concentration of sodium hydroxide, trisodium citrate, and a nucleating agent, chloroplatinic acid.

Full text of DOI:10.1149/1.3596703

Journal of The Electrochemical Society, 158 (8) E79-E83 (2011)
0013-4651/2011/158(8)/E79/5/$28.00 The Electrochemical Society
E79
C
V
Formation of Nickel Nanowires via Electroless Deposition Under
a Magnetic Field
,z
Makoto Kawamori, Shunsuke Yagi,
*
**,a
and Eiichiro Matsubara
Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan
Nickel nanowires were synthesized via electroless deposition in an organic solvent (ethylene glycol) under a magnetic field. Depo-
sition behavior of nickel particles and wires were electrochemically investigated at various concentrations of NaOH by an in-situ
mixed potential measurement and voltammetry combined with quartz crystal microbalance. Based on the electrochemical investi-
gation, a formation mechanism of nickel wires is proposed. According to the mechanism, nickel wires 100–370 nm in diameter
with several dozen ꢀm of length were successfully prepared by controlling the reduction rate by varying a concentration of sodium
hydroxide, trisodium citrate, and a nucleating agent, chloroplatinic acid.
C
V
2011 The Electrochemical Society. [DOI: 10.1149/1.3596703] All rights reserved.
Manuscript submitted March 18, 2011; revised manuscript received May 10, 2011. Published June 10, 2011.
Nanowires of iron group metals (Fe, Co, Ni) and their alloys are
attractive materials due to their magnetic properties.1,2 Numerous
synthesis methods of iron group nanowires have been reported.3–8
In their methods, nanowires are formed by electrodeposition using
templates such as anodized aluminum oxide3–5 and polycarbonate
membrane.6–8 The methods with templates have notable advantages
that highly-ordered and size-controlled nanowires can be obtained.
The template methods, however, require several steps including fab-
rication and removal of templates in order to obtain bare nanowires.
On the other hand, a self-assemble electroless deposition of ferro-
magnetic nanowires under a magnetic field is a relatively simple
synthesis method without any templates.9 In addition, electroless
deposition is a powerful fabrication method with a wide variety of
compositions and sizes in a large-scale10,11 and thus, electroless
deposition is suitable for a practical application.
same amount of EG solution (27 cm3) containing 1.00 M N2H4,
0–0.6 M NaOH, and 0–10.0 mM Na3C6H5O7 was also prepared.
The compositions of reaction solutions are summarized in Table I.
The temperature of the solutions was kept at 353 K with nitrogen
gas bubbling to remove the dissolved oxygen. The metallic salt solu-
tion and the hydrazine solution were mixed at 353 K to start the
reaction. The reaction solution was agitated at a rate of 500 rpm
with a magnetic stirring unit at 353 K during the reaction for the
synthesis of nickel particles. For the synthesis of nickel wires, the
reaction solution was kept at 353 K in a water bath located inside
the two parallel neodymium magnets (100 ꢂ 100 mm) separated 100
mm apart. Magnetic field inside the magnets was about 100–200
mT, which is measured by a Tesla meter (KANETEC Co., Ltd.,
TM-601). After the reaction, nickel particles and wires were washed
several times with ethanol.
We have reported some studies of electrochemical approaches
with in-situ mixed potential observation for the synthesis processes
of copper,12,13 cobalt,14,15 nickel,16 and Co-Ni alloy17 nanoparticles,
which is effective in thermodynamic oxidation-state control and
analysis of the formation process of nanowires as well as nanopar-
ticles. In the present work, the formation process of nickel nanowires
via electroless deposition under a magnetic field was electrochemi-
cally investigated. In order to control the morphology of nickel depos-
its, the deposition behavior of nickel was studied by an in-situ mixed
potential measurement; it is possible that the reduction rate of Ni(II)
species affects the morphology of nickel deposits. Furthermore, the
reduction rate and reduction potential of Ni(II) species were investi-
gated by voltammetry combined with quartz crystal microbalance
(QCM) as well as the oxidation rate and oxidation potential of hydra-
zine as a counterpart reaction.
The morphology of products was observed by a field-emission
scanning electron microscope (JEOL Ltd., JSM-6500F). The average
diameter and size distribution of the nickel particles were determined
by image analysis for randomly selected 600 particles in each sample.
During the synthesis of nickel particles, a gold-sputtered QCM elec-
trode (SEIKO EG&G, QA-A9M-AU) was immersed in the reaction
solution and the mixed potential on the QCM electrode was measured
by a potentiostat/galvanostat (Hokuto Denko Co., Ltd., HA-151)
using a Ag/AgCl (3.33 M KCl) electrode (Horiba, 2565A-10T) as a
reference electrode. Cyclic voltammograms were also measured by
the potentiostat/galvanostat using a nickel-sputtered QCM electrode
(SEIKO EG&G, QA-A9M-NI) as a working electrode, a platinum
electrode (20 ꢂ 20 mm) as a counter electrode, and a Ag/AgCl elec-
trode as a reference electrode. The weight of nickel deposited on the
QCM electrode, Dm, was calculated from the change in resonance
frequency of the QCM electrode, Df, by Sauerbrey’s equation18
pffiffiffiffiffiffiffiffiffi
Experimental
A
qqꢀq
2f02
Dm ¼ ꢁ
Df
[1]
The reaction solutions were prepared using nickel chloride hexa-
hydrate (NiCl2 ꢀ 6H2O) as a source of Ni(II) ions, ethylene glycol
(EG) as a solvent, and hydrazine monohydrate (N2H4 ꢀ H2O) as a
reducing agent. Sodium hydroxide (NaOH) was added as a source
of OHꢁ ions. Trisodium citrate dihydrate (Na3C6H5O7 ꢀ 2H2O) and
chloroplatinic acid hexahydrate (H2PtCl6 ꢀ 6H2O) were used as a
complexing agent and a nucleating agent, respectively. These
reagents are all reagent-grade (Nacarai Tesque, Inc.) and used with-
out further purification.
where f0 is the frequency of the QCM electrode before the deposi-
tion, A is the active area of the QCM electrode (0.196 cm2), qq is
the density of quartz (2.648 g cmꢁ3), and ꢀq is the shear modulus of
quartz (2.947 ꢂ 1011 g cmꢁ1 sꢁ2).
Results
First, 27 cm3 EG solution containing 0.100 M NiCl2, 0.2–1.4 M
NaOH, and 0–10.0 mM Na3C6H5O7 was prepared, where M corre-
sponds to mol dmꢁ3. For experiments with a nucleating agent,
H2PtCl6 (0.02–2.00 mM) was added as a nucleating agent. The
Figure 1 shows the size distributions and scanning electron mi-
croscopy (SEM) images of nickel particles synthesized without a
magnetic field at different concentrations of NaOH. The size of par-
ticles increases with a decrease in a concentration of NaOH. The av-
erage particle size is about 240 nm at 1.0 M NaOH and 480 nm at
0.1 M NaOH. The size distribution becomes sharper at a higher con-
centration of NaOH.
*
**
Electrochemical Society Student Member.
Electrochemical Society Active Member.
a Present address: Nanoscience and Nanotechnology Research Center, Osaka
Prefecture University, Osaka 599-8570, Japan.
Figure 2 shows the SEM images of nickel particles and wires
synthesized from the solutions under a magnetic field at different
z E-mail:kawamori@makoto.mbox.media.kyoto-u.ac.jp
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Journal of The Electrochemical Society, 158 (8) E79-E83 (2011)
Table I. Compositions of hydrazine solutions for the synthesis of nickel particles and wires.
Compositions of metallic salt solutions
Compositions of metallic salt solutions
[NiCI2]
(M)
[NaOH]
(M)
[Na3C6H5O7]
(mM)
[N2H4]
(M)
[NaOH]
(M)
[Na3C6H5O7]
(mM)
(i)
0.100
0.100
0.100
0.100
0.100
0.100
1.4
0.3
0.2
1.4
1.4
1.4
—
—
1.00
1.00
1.00
1.00
1.00
1.00
0.6
0.1
—
0.6
0.6
0.6
—
—
(ii)
(iii)
(iv)
(v)
—
—
5.0
7.5
10.0
5.0
7.5
10.0
(vi)
concentrations of NaOH. At 1.0 M NaOH, nickel particles about
250 nm in diameter were obtained. Beads-like nickel wires were
obtained at 0.2 M NaOH. Nickel wires approximately 500 nm in di-
ameter with a smooth surface were obtained at 0.1 M NaOH. It
should be noted that there is no large change of the each particle
size with and without a magnetic field at the same concentration of
NaOH. These results also indicate that a magnetic field strongly
affects the morphology of nickel.
NaOH. The oxidation potential of hydrazine positively shifts from
ꢁ1.06 to ꢁ0.85 V vs Ag/AgCl and the slope of the anodic current
density becomes smaller with the decrease in a concentration of
NaOH. The conductivity of the solution becomes lower with
decreasing a concentration of NaOH. The conductivities measured
by a conductivity electrode (Horiba, 9382-10D) are 2.12, 1.27, and
0.72 S mꢁ1 at 1.0, 0.2, and 0.1 M NaOH, respectively. The large
change of anodic current density at different concentrations of
NaOH cannot be obviously explained only by the change of the so-
lution resistance. Considering the increased ohmic drop due to the
solution resistance, the slope of the I/V curve is still smaller at a
lower concentration of NaOH. In other words, the current density at
a potential is smaller at a lower NaOH concentration. This indicates
that a reduction ability of hydrazine becomes weaker at a lower con-
centration of NaOH.
In the present process, the following hydrazine oxidation reac-
tions mainly occur as anodic reactions
N2H4 þ 4OHꢁ ! N2 þ 4H2O þ 4e
[2]
1
N2H4 þ OHꢁ
!
N2 þ NH3 þ H2O þ e
[3]
2
Figure 3 shows anodic polarization curves measured on nickel sub-
strates in 120 cm3 EG containing 0.50 M N2H4 and 0.1–1.0 M
Both a weight of deposited metal and a total current density are
simultaneously measured by voltammetry combined with QCM. The
Figure 1. Size distributions and SEM
images of nickel particles synthesized
from solutions containing 0.050 M NiCl2,
0.50 M N2H4, (a) 1.0 M, (b) 0.2 M, and
(c) 0.1 M NaOH. Average diameter dav is
also indicated.
Figure 2. SEM images of nickel nanopar-
ticles and nanowires synthesized from sol-
utions containing 0.050 M NiCl2, 0.50 M
N2H4, (a) 1.0 M, (b) 0.2 M, and (c) 0.1 M
NaOH under a magnetic field.
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Journal of The Electrochemical Society, 158 (8) E79-E83 (2011)
E81
Figure 5. Time dependence of mixed potential in solutions containing 0.050
M NiCl2, 0.50 M N2H4, (a) 1.0 M, (b) 0.2 M, and (c) 0.1 M NaOH.
Figure 3. Anodic polarization curves measured at 1 mV sꢁ1 in EG contain-
ing 0.50 M N2H4, (a) 1.0 M, (b) 0.2 M, and (c) 0.1 M NaOH.
and 0.1 M NaOH, respectively. Therefore, it is clarified that the re-
dox potential of Ni(II)/Ni redox couple shifts to the negative direc-
tion by increasing the concentration of NaOH. The reduction current
density of Ni(II) species is much smaller at 1.0 and 0.2 M compared
with that at 0.1 M NaOH. This is because Ni(II) ions in the metallic
salt solution were hydroxylated to form green nickel hydroxide and/
or nickel alkoxide sol by the addition of NaOH, which results in the
low current efficiency of the nickel deposition as shown in Fig. 4.
An activity of Ni2þ ions decreases with an increase of the concentra-
tion of NaOH and the redox potential of Ni(II)/Ni shifts to the nega-
tive direction according to the Nernst equation.
Figure 5 shows the time transition in the mixed potential on the
QCM substrate during the synthesis. Just after mixing the metallic
salt solution and the hydrazine solution, the mixed potential rapidly
dropped below the redox potentials of Ni(II)/Ni in each condition,
indicating that nickel deposition is thermodynamically possible. At
a lower concentration of NaOH, the mixed potential positively
shifts, which corresponds to the result that the current density of hy-
drazine oxidation declines with decrease in a concentration of
NaOH. An increase of potential with time can be mainly attributed
to a consumption of hydrazine, because the mixed potential is
mainly determined by the balance of hydrazine oxidation reactions
and a decomposition of large amount of solvent.
following reduction reaction of Ni(II) species was separately evaluated
from the weight change of nickel deposited on the QCM substrate
NiðIIÞ þ 2e ! Ni
[4]
where Ni(II) represents all the nickel(II) species, e.g. Ni2þ, NiOHþ,
Ni(OH)2, HNiO2ꢁ, and NiO22ꢁ 19
The contribution of the reductive
.
decomposition of the solvent to the total current density is dominant
in this process, and the current density due to nickel deposition is
hardly observed by a normal voltammetry. Figure 4 shows cyclic
voltammograms measured in 120 cm3 EG containing 0.050 M
NiCl2 and 0.1–1.0 M NaOH. At 1.0 M NaOH (Fig. 4a, dotted line),
the cathodic current of hydrogen generation is observed below about
ꢁ0.80 V vs Ag/AgCl during the cathodic sweep, and the oxidization
current of hydrogen adsorbed on the QCM substrate is observed
above ꢁ0.87 V. In a cyclic voltammogram drawn by QCM result
(solid line), the nickel reduction was observed below about ꢁ0.80 V
during the cathodic sweep, and the dissolution of nickel was seen
above ꢁ0.80 V during the anodic sweep. Therefore, the redox
potential of Ni(II)/Ni redox couple is about ꢁ0.80 V vs Ag/AgCl
at 1.0 M NaOH. Similarly, the redox potential of Ni(II)/Ni redox
couple can be determined to be about ꢁ0.70 and ꢁ0.45 V at 0.2 M
Figure 4. Cyclic voltammograms measured by potentiostat (dotted line) and QCM (solid line) at 1 mV sꢁ1 in EG containing 0.050 M NiCl2, (a) 1.0 M, (b) 0.2
M, and (c) 0.1 M NaOH.
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E82
Journal of The Electrochemical Society, 158 (8) E79-E83 (2011)
smooth surface. Therefore, the necessary condition for the formation
of nickel wires is that the alignment of particles occurs prior to the
end of the reduction reaction. At 1.0 M NaOH, the reduction ability
of hydrazine is so high that large amount of nickel nuclei are rapidly
formed, and the nickel deposition has finished before the alignment
of particles. The nickel particles are not tightly connected under a
magnetic field because of their fast reduction rate at 1.0 M NaOH. It
is likely that the nickel wires easily broke down during washing in
ethanol as shown in Fig. 2a. At a lower concentration of NaOH, the
reduction ability of hydrazine is not so high that there is enough
time for the particles to align along the magnetic field and are con-
nected by preferential deposition at the necks between them. Thus,
the nickel wires are obtained at a lower concentration of NaOH as
shown in Figs. 2b and 2c. The wires are beads-like shape at 0.2 M
NaOH and become smooth at 0.1 M NaOH.
In order to verify this formation mechanism of nickel wires, the
nickel wires were fabricated by adding a complexing agent,
Na3C6H5O7. Generally, complexing agents such as citrate, whose
complex formation constant with Ni(II) ions is high, are used for a
control of the activity of Ni(II) ions and the deposition rate.20,21
Figure 7 shows the SEM images of nickel deposits synthesized
from the reaction solutions containing 0.050 NiCl2, 0.50 M N2H4,
1.0 M NaOH, and 5.0–10.0 mM Na3C6H5O7 under a magnetic
field. While nickel particles were obtained without Na3C6H5O7
(Fig. 2a), beads-like nickel wires whose diameter is about 320 nm
were formed at 5.0 mM Na3C6H5O7 (Fig. 7a). The surface of
nickel wires becomes smoother at a higher concentration of
Na3C6H5O7 (Figs. 7b and 7c). Consequently, the formation of
wires becomes easier at a slower reaction rate as it is expected
from the discussion above.
Figure 6. Schematic of the formation process of nickel wires.
Discussion
The concentration of NaOH significantly affects on both the hy-
drazine oxidation reactions and the nickel deposition reaction. With
decreasing the amount of NaOH, the reduction ability of hydrazine
becomes weaker, and as a result, the mixed potential becomes
higher as shown in Figs. 3 and 5. Thus, the driving force of nickel
deposition tends to be larger and homogeneous nucleation of nickel
is favorable at a higher concentration of NaOH. On the other hand,
the deposition current density of nickel is much smaller at 1.0 M
NaOH than that at 0.1 M NaOH as shown in Fig. 4, which indicates
that the particle growth is suppressed at a higher NaOH concentra-
tion. Consequently, the finer nickel particles with a narrower size
distribution are achieved at 1.0 M NaOH as shown in Fig. 1a.
The morphological change in nickel wires is explained by the
formation mechanism of nickel wires schematically drawn in Fig. 6.
First, nickel particles are formed in the reaction solution in the ini-
tial reaction stage, and are aligned along a magnetic field due to
magnetic interaction. Then, nickel preferentially deposits at the
necks between particles to decrease the interfacial energy. The par-
ticles are tightly connected and finally form rigid wires with a
Figure 7. SEM images of nickel nano-
wires synthesized from solutions contain-
ing 0.050 M NiCl2, 0.50 M N2H4, 1.0 M
NaOH, (a) 5.0 mM, (b) 7.5 mM, and (c)
10.0 mM Na3C6H5O7 under a magnetic
field.
Figure 8. SEM images of nickel nano-
wires synthesized from solutions contain-
ing 0.050 M NiCl2, 0.50 M N2H4, 1.0 M
NaOH, 10.0 mM Na3C6H5O7, (a) 0.01
mM, (b) 0.10 mM, and (c) 1.00 mM
H2PtCl6 under a magnetic field.
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Journal of The Electrochemical Society, 158 (8) E79-E83 (2011)
E83
In the present results, it is important that the nickel wires are
formed after the formation of nickel particles. Therefore, we can apply
the same size-control method as particles synthesis by electroless dep-
osition for the diameter control of nickel wires. To control the nickel
particle size, chloroplatinic acid (H2PtCl6) is often used as a nucleat-
ing agent.22,23 It is considered that the nucleating agent, whose redox
potential is much higher than that of the objective metal, provides
many heterogeneous nucleation sites, resulting in the formation of
abundant and fine particles. The formation of platinum nuclei divides
the nucleation and growth stages during the formation of nickel par-
ticles, which gives us a sharper size distribution. Figure 8 shows the
SEM images of nickel wires synthesized from the solutions containing
0.050 NiCl2, 0.50 M N2H4, 1.0 M NaOH, 10.0 mM Na3C6H5O7, and
0.01–1.00 mM H2PtCl6. The diameter of nickel wires becomes
smaller with an increase in a concentration of H2PtCl6; the average
diameters were 280, 160, and 100 nm at 0.01, 0.10, and 1.00 mM
H2PtCl6, respectively. In this way, the wire diameter can be easily
controlled by the additive amount of nucleating agents.
from the Japan Society for the Promotion of Science, Ministry of Edu-
cation, Culture, Sports, Science and Technology of Japan. This
research was also supported by the Kurata Memorial Hitachi Science
and Technology Foundation and Shorai Foundation for Science and
Technology.
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Acknowledgments
This research was supported by a Grant-in-Aid for JSPS fellows,
Grant-in-Aid for Knowledge Cluster Initiative (Kyoto Nanotechnol-
ogy Cluster), Grant-in-Aid for the Global COE Program (International
Center for Integrated Research and Advanced Education in Materials
Science), and Grant-in-Aid for Young Scientists (B 20760505), all
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