Formation of nickel nanoparticles by electroless deposition using NiO and Ni(OH)2 suspensions

Article abstract of DOI:10.1149/1.2948380

Nickel particles ～300 nm in diameter were fabricated by electroless deposition using hydrazine as a reducing agent in nickel hydroxide/ethylene glycol suspension at 353 K without any dispersing agent. The formation mechanism of nickel nanoparticles is discussed from the viewpoint of thermodynamics with in situ monitoring of nickel deposition and mixed potential. Specifically, in situ monitoring of mixed potential in combination with thermodynamic calculation is useful in discriminating whether or not nickel will be deposited in a reaction. The mixed potential drastically changed at the end point of the nickel deposition reaction, indicating that the cathodic reaction, which determined the mixed potential, switched from the nickel deposition reaction to hydrogen generation reaction.

Full text of DOI:10.1149/1.2948380

Formation of Nickel Nanoparticles by Electroless Deposition Using
NiO and Ni?(?OH?)? 2 Suspensions
Shunsuke Yagi, Takaaki Koyanagi, Hidetaka Nakanishi, Tetsu Ichitsubo and Eiichiro Matsubara
J. Electrochem. Soc. 2008, Volume 155, Issue 9, Pages D583-D588.
doi: 10.1149/1.2948380
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2008 ECS - The Electrochemical Society
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Journal of The Electrochemical Society, 155 ͑9͒ D583-D588 ͑2008͒
D583
0
013-4651/2008/155͑9͒/D583/6/$23.00 © The Electrochemical Society
Formation of Nickel Nanoparticles by Electroless Deposition
Using NiO and Ni„OH… Suspensions
2
,
z
Shunsuke Yagi,* Takaaki Koyanagi, Hidetaka Nakanishi,
Tetsu Ichitsubo, and Eiichiro Matsubara
Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan
Nickel particles ϳ300 nm in diameter were fabricated by electroless deposition using hydrazine as a reducing agent in nickel
hydroxide/ethylene glycol suspension at 353 K without any dispersing agent. The formation mechanism of nickel nanoparticles is
discussed from the viewpoint of thermodynamics with in situ monitoring of nickel deposition and mixed potential. Specifically, in
situ monitoring of mixed potential in combination with thermodynamic calculation is useful in discriminating whether or not
nickel will be deposited in a reaction. The mixed potential drastically changed at the end point of the nickel deposition reaction,
indicating that the cathodic reaction, which determined the mixed potential, switched from the nickel deposition reaction to
hydrogen generation reaction.
©
2008 The Electrochemical Society. ͓DOI: 10.1149/1.2948380͔ All rights reserved.
Manuscript submitted March 19, 2008; revised manuscript received May 29, 2008. Published July 11, 2008.
Ferromagnetic nanoparticles of iron, cobalt, and nickel have po-
tential for applications in high-density magnetic recording, drug de-
livery, magnetic resonance imaging, biosensing, and catalysts. Many
Inc.͒ as received. All chemicals were of reagent grade. Distilled
water and ethylene glycol ͑Nacalai Tesque, Inc.͒ or their mixture
were used as the solvent. Reaction was conducted in a Pyrex beaker
250 cm in capacity. The solution was agitated at a rate of 500 rpm
with a magnetic stirring unit.
1
-12
3
studies on the fabrication of nickel nanoparticles,
nickel
1
3
nanobelts, and nickel composite nanoparticles ͑Ni–Ag core-shell
1
4
15
nanoparticles, Pt–Ru–Ni ternary nanoparticles ͒ have been con-
ducted. Various methods have been reported to prepare nickel nano-
Preparation of reaction suspension.— Nickel oxide colloidal
suspension and nickel chloride solution were prepared by dispersing
0.060 mol of nickel oxide and dissolving 0.060 mol of nickel chlo-
ride hexahydrate in 60.0 cm of distilled water or ethylene glycol
using ultrasound. Nickel oxide consists of particles ϳ1 ␮m diam,
and nickel chloride hexahydrate consists of particles of
1
-8
particles ͑e.g., ballmilling, electroless deposition,
gas-phase
9
10
11
reduction, spray pyrolysis, chemical vapor deposition, and
12
3
laser-driven pyrolysis ͒. Electroless deposition is one of the most
promising methods for preparing nickel nanoparticles and utilizes
the oxidation-reduction reaction of nickel͑II͒ species and reducing
agents in liquid phase. For example, Chen et al. prepared nickel
nanoparticles surface-capped with a self-assembled monolayer
Ͼ10 ␮m diam. The initial pH of the solution was adjusted to 12.0 at
3
2
98 K by 10.0 mol dm− sodium hydroxide aqueous solution with a
5
of dodecanethiol using hydrazine hydrate as a reducing agent.
pH meter ͑Horiba D-21͒. Nickel chloride dissolved in the solution is
almost all hydrolyzed to nickel hydroxide. The temperature of the
solution was kept at reaction temperature in a water bath with nitro-
Couto et al. used sodium borohydride as the reducing agent and
7
fabricated nickel nanoparticles. Hou and Gao reported that mono-
3
−1
dispersed nickel nanoparticles ϳ3.7 nm diam were prepared by
gen gas bubbling ͑50 cm min ͒, which was started 30 min before
reduction of Ni͑acethylacetone͒ with sodium tetrahydridoborate
2
the reaction and lasted throughout the reaction to eliminate the effect
1
−
3
in hexadecylamine. Liu et al. controlled the rate of nickel deposi-
of dissolved oxygen. Next, 1.00 mol dm hydrazine aqueous or
3
tion using sodium tartrate as a complexing agent, and formed nickel
nanobelts by the reduction with sodium hypophosphite
ethylene glycol solution of 60.0 cm was kept at reaction tempera-
3
−1
ture with nitrogen gas bubbling ͑50 cm min ͒ for 30 min. The
1
3
monohydrate.
Wu and Chen formed nickel nanoparticles
initial pH of the hydrazine solution was adjusted to 12.0 by
9.2 nm diam from nickel chloride using hydrazine in ethylene gly-
−3
1
0.0 mol dm sodium hydroxide aqueous solution at 298 K before
col solution at 60°C and studied the effects of nickel chloride and
increasing the temperature. The hydrazine solution was then added
to the nickel oxide or nickel hydroxide suspension as a reducing
agent to deposit nickel particles. The total amount of the reaction
2
hydrazine concentrations on particle size. However, few discus-
sions have been conducted from the viewpoint of thermodynamics,
specifically electrochemistry.
3
suspension was 120.0 cm , and thus, the reaction suspension was
We reported a formation process of copper nanoparticles by elec-
troless deposition using hydrazine as a reducing agent in aqueous
copper oxide suspension. We found that the driving force of copper
deposition can be thermodynamically discussed with the oxidation-
reduction potential of Cu2+/Cu redox pair and mixed potential.
The most critical point of this method is that a barely soluble source
for metallic ions is used for the formation of metal nanoparticles.
This maintains the activity of a metal aquo ion at an extremely low
level and regulates the particle size. In the present work, we inves-
tigated the formation process of nickel nanoparticles by electroless
deposition using hydrazine as a reducing agent and discussed the
process thermodynamically.
−3
−3
0
.50 mol dm hydrazine solution with dispersed 0.50 mol dm
nickel oxide or nickel hydroxide. It should be noted that the reaction
suspension includes some amount of water introduced from sodium
hydroxide aqueous solution added for pH adjustment and the hy-
drate starting materials, such as nickel chloride hexahydrate and
hydrazine monohydrate, even if ethylene glycol is used as the sol-
vent.
16
Observation and analysis.— The precipitates obtained were ana-
lyzed by X-ray diffraction ͓͑XRD͒, MAC Science M03XHF22͔, us-
ing a molybdenum X-ray tube. The morphology of the precipitates
was observed by a field-emission scanning electron microscope
͓
͑SEM͒, JEOL JSM-6500F͔. The rate of nickel deposition was esti-
Experimental
mated in situ by electrochemical quartz crystal microbalance ͑QCM͒
using round 9 MHz AT-cut quartz crystal substrates ͑QCM sub-
strates͒, on both sides of which Au layers with a 5.0 mm diam were
sputtered with an underlying Ti buffer layer. The Au-plated QCM
substrate was fixed inside a dipping-type Teflon holder ͑Seiko
EG&G QCA917-21͒ with a circular window so that one surface of
the substrate was exposed to deposition solutions through the win-
dow while the other surface was isolated from the solutions; the
Reaction solutions were prepared using nickel oxide ͑NiO͒,
nickel chloride hexahydrate ͑NiCl ·6H O͒, sodium hydroxide
2
2
͑
NaOH͒, and hydrazine monohydrate ͑N H ·H O͒ ͑Nacalai Tesque,
2 4 2
*
Electrochemical Society Active Member.
Email: syagi@mtl.kyoto-u.ac.jp
z
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D584
Journal of The Electrochemical Society, 155 ͑9͒ D583-D588 ͑2008͒
(
a) Aqueous suspension, 298 K
(b) Aqueous suspension, 353 K
(a) Ethylene glycol suspension, 353 K
2 h
(b) Ethylene glycol suspension, 373 K
Ni
NiO
NiO
NiO
NiO
2
1
h
h
2
h
2
1
h
h
1 h
1 h
0
.5 h
0.5 h
0.5 h
0
0
.5 h
h
0 h
0
h
0 h
1
0
15
20
25
30
35
40 10
15
20
25
30
35
40
10
15
20
25
30
35
40 10
15
20
25
30
35
40
2
θ / degree (Mo-Kα)
2θ / degree (Mo-Kα)
2θ / degree (Mo-Kα)
2θ / degree (Mo-Kα)
Figure 1. ͑Color online͒ XRD patterns of precipitates obtained at ͑a͒ 298
and ͑b͒ 353 K in aqueous nickel oxide suspension by 0–2 h reaction.
Figure 3. ͑Color online͒ XRD patterns of precipitates obtained at ͑a͒ 353
and ͑b͒ 373 K in nickel oxide/ethylene glycol suspension by 0–2 h reaction.
electrochemically active area of the QCM substrate on one side was
thus 0.196 cm . The holder with an oscillating circuit was connected
corresponding to the precipitates obtained by the reaction at 373 K
for 2 h. Figure 4 shows SEM images of the precipitates obtained in
nickel oxide/ethylene glycol suspension by the reaction for 2 h.
These precipitates consist of particles ϳ1 ␮m diam and are almost
the same as the starting nickel oxide particles. Black precipitates
were included in the precipitates obtained by the reaction at 373 K
for 2 h. Figure 5 shows an SEM image and an XRD pattern of the
black precipitates separated by a magnet. Both nickel and nickel
oxide peaks are observed in the XRD pattern of the black precipi-
tates. Therefore, the agglomerated spinous particles of
ϳ1 ␮m diam, seen in Fig. 5a, consist of nickel and nickel oxide.
2
to a frequency counter ͑Seiko EG&G QCA917͒ with a coaxial
cable. Immersion potential of the QCM substrate was concomitantly
measured during several experimental runs, and it was assumed that
the immersion potential measured was almost the same as mixed
potential in reaction suspension in this reaction system. A Ag/AgCl
electrode ͑Horiba 2565A-10T͒ was used as a reference electrode.
Internal liquid of the reference electrode was separated into two
rooms by porous ceramics. The internal room involving a Ag/AgCl
electrode was always filled with 3.33 mol dm−3 KCl aqueous solu-
tion saturated with AgCl. In the measurement of mixed potential in
ethylene glycol solution, the external room neighboring the reaction
suspension was filled with the 1:1 mixture ͑weight ratio͒ of ethylene
glycol and 3.33 mol dm−3 KCl aqueous solution to minimize the
effect of liquid junction potential difference caused by the difference
Electroless deposition using nickel hydroxide suspension.—Figure
6
3
shows XRD patterns of precipitates obtained by 2 h reaction at
53 K in aqueous nickel hydroxide and nickel hydroxide/ethylene
glycol suspensions of pH 12.0. In both cases, the nickel chloride
starting material dissolved in the solutions was hydrolyzed to nickel
hydroxide before the reaction. Nickel peaks are observed in XRD
patterns of the precipitates obtained by the reaction for Ͼ1 h using
aqueous nickel hydroxide suspension. In contrast, strong nickel
peaks are observed in XRD patterns of the precipitates obtained by
the reaction for more than 30 min using nickel hydroxide/ethylene
glycol suspension. Thus, the reduction speed of nickel͑II͒ species is
higher in nickel hydroxide/ethylene glycol suspension than that in
aqueous nickel hydroxide suspension. The color of the ethylene gly-
col suspension suddenly changed from green to intense black at the
of transference numbers of cationic species and anionic species. The
external room was filled with 3.33 mol dm−3 KCl aqueous solution
in the case of the measurement in aqueous solution. The three kinds
−
3
of internal liquids ͑i.e., 3.33 mol dm KCl aqueous solution,
−
3
3
.33 mol dm KCl aqueous solution saturated with AgCl, and 1:1
−
3
mixture of ethylene glycol and 3.33 mol dm KCl aqueous solu-
tion͒ were replaced in each experimental run. In this work, discus-
sion proceeds on the basis that the effect of the liquid junction
potential difference is small enough to be ignored.
Results
Electroless deposition using nickel oxide suspension.— Figure 1
shows XRD patterns of precipitates obtained by the reaction for 2 h
at 298 and 353 K in aqueous nickel oxide suspension of pH 12.0.
The peaks derived from nickel metal are not observed, and only
peaks of nickel oxide exist, suggesting that the nickel oxide starting
material was not reduced to metallic nickel under these conditions.
The SEM images of these precipitates shown in Fig. 2 indicate that
nickel oxide is not dissolved at all because the particles are as large
as the starting nickel oxide particles, ϳ1 ␮m diam.
(a) 353 K
(b) 373 K
Using ethylene glycol as a solvent, electroless deposition was
conducted under the same conditions as aqueous suspension. Figure
5
µm
3
shows XRD patterns of precipitates obtained at 353 and 373 K.
Figure 4. SEM images of precipitates obtained at ͑a͒ 353 and ͑b͒ 373 K in
nickel oxide/ethylene glycol suspension by 2 h reaction.
Nickel oxide peaks only are observed except in the XRD pattern
(
a)
(b)
Ni
(
a) 298 K
(b) 353 K
NiO
1
µm
1
0
15
20
25
30
35
40
2
θ / degree (Mo-Kα)
5
µm
Figure 5. ͑Color online͒ ͑a͒ SEM image and ͑b͒ corresponding XRD pattern
of black precipitates obtained at 373 K in nickel oxide/ethylene glycol sus-
pension by 2 h reaction.
Figure 2. SEM images of precipitates obtained at ͑a͒ 298 and ͑b͒ 353 K in
aqueous nickel oxide suspension by 2 h reaction.
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Journal of The Electrochemical Society, 155 ͑9͒ D583-D588 ͑2008͒
D585
(
a) Aqueous suspension
(b) Ethylene glycol suspension
Ni
Ni
Table I. List of standard chemical potentials and stability con-
stants at 298 K considered and used for thermodynamic
calculation.
Ni(OH)2
Ni(OH)2
2
1
h
h
2
h
1 h
Chemical
species
Standard chemical
0
.5 h
0.5 h
potential ͑kJ mol−
1
͒
Ref.
0
h
0
h
H O
−237.3
−157.4
−48.26
−216.4
−453.3
−406.0
−26.6
0.0
17
17
17
17
17
20
17
17
17
2
−
1
0
15
20
25
30
35
40 10
15
20
25
30
35
40
OH
2
θ / degree (Mo-Kα)
2θ / degree (Mo-Kα)
2+
Ni
NiO
Ni͑OH͒2
Ni͑OH͒ ͑aq͒
NH3
N2
Figure 6. ͑Color online͒ XRD patterns of precipitates obtained at 353 K in
a͒ aqueous nickel hydroxide suspension and ͑b͒ nickel hydroxide/ethylene
͑
2
glycol suspension by 0–2 h reaction.
N H
127.9
2
4
reaction time of ϳ13 min. In contrast, the color of the aqueous
Nickel͑II͒ complex
formation reactions
Stability constants
suspension gradually changed from green to greenish black in the
log K
Ref.
2
h reaction. This indicates that the reduction reaction did not en-
Ni + N2H4 = ͓Ni͑N2H4͔͒2+
2
+
2.81
5.08
6.85
9.20
10.75
11.99
18
18
18
18
18
18
tirely finish in the aqueous suspension within 2 h, but finished in the
ethylene glycol suspension. Figure 7 shows SEM images of the
precipitates obtained in the aqueous nickel hydroxide suspension
and the nickel hydroxide/ethylene glycol suspension in the 2 h re-
action. Particles ϳ500 nm diam were obtained from the aqueous
suspension, and particles of ϳ300 nm diam were obtained from the
ethylene glycol suspension. These particles are nickel metal, but the
surface of the particles may be nickel hydroxide, because tiny and
broad peaks of nickel hydroxide are also seen in the corresponding
XRD patterns shown in Fig. 6.
2
+
2+
2+
2+
2+
2+
Ni + 2N2H4 = ͓Ni͑N2H4͒2͔
2
+
Ni + 3N2H4 = ͓Ni͑N2H4͒3͔
2
+
Ni + 4N H = ͓Ni͑N H ͒ ͔
2
4
2
4 4
2
+
Ni + 5N H = ͓Ni͑N H ͒ ͔
2
4
2
4 5
2
+
Ni + 6N H = ͓Ni͑N H ͒ ͔
2
4
2
4 6
Nickel oxide
Discussion
The overall nickel deposition reactions in this system can be
expressed as follows:
Nickel oxide
ln͑aNi2+͒ = 27.9 − 4.605pH
ln͑aNi2+͒ = 28.1 − 4.605pH
͓5͔
Nickel hydroxide
1
2
1
2
NiO + N H → Ni + N + H O
͓1͔
͓2͔
2
4
2
2
͓6͔
NiO + 2N H → Ni + N + 2NH + H O
2
4
2
3
2
where aNi2+ is the activity of Ni2+ aquo ions and the following equi-
librium reactions are considered, respectively:
Nickel oxide
Nickel hydroxide
1
2
1
2
Ni͑OH͒ + N H → Ni + N + 2H O
͓3͔
͓4͔
2
2
4
2
2
Ni͑OH͒ + 2N H → Ni + N + 2NH + 2H O
2
2
4
2
3
2
NiO + H O = Ni + 2OH−
2
+
͓7͔
2
In this section, these overall reactions are separated to partial reac-
tions, and the mechanism of nickel deposition reaction is thermody-
namically discussed in detail. All the data considered and used for
thermodynamic calculation are listed in Table I.
Nickel hydroxide
Ni͑OH͒ = Ni + 2OH−
2
+
͓8͔
Nickel oxide and nickel hydroxide are barely soluble to aqueous
solution and ethylene glycol. The activity of Ni2+ aquo ions dis-
solved in aqueous nickel oxide or nickel hydroxide suspension un-
der an equilibrium state at 298 K can be expressed as a function of
pH by thermodynamic calculation using the standard chemical po-
2
Equations 5 and 6 are derived by considering that both the changes
in Gibbs free energy of Reactions 7 and 8 equal zero in an equilib-
rium state. For example, Eq. 5 can be obtained by considering the
following equation and substituting ⌬G by 0
17
tential of each chemical,
(
a) Aqueous suspension
(b) Ethylene glycol suspension
Figure 7. SEM images of precipitates ob-
tained at 353 K in ͑a͒ aqueous nickel hy-
droxide suspension and ͑b͒ nickel
hydroxide/ethylene glycol suspension by
2
h reaction.
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D586
Journal of The Electrochemical Society, 155 ͑9͒ D583-D588 ͑2008͒
2
OH
0.2
0.0
aNi2+ · a
−
0
⌬
G = ⌬G + RT lnͩ ͪ
a
· a
NiO
H O
(a) Aqueous suspension, 298 K
2
0
0
OH
0
NiO
0
=
͑␮ 2+ + 2␮
−
− ␮
− ␮
͒
Ni
H O
2
2
OH
aNi2+ · a
−
+
RT lnͩ ͪ
͓9͔
(
b) Aqueous suspension, 353 K
E
Ni(II)/Ni
a
· a
H O
2
NiO
where ⌬G is the change in Gibbs free energy of the reaction, R is the
(
c) Ethylene glycol suspension, 353 K
gas constant ͑8.314 J mol−1 K ͒, T is temperature ͑298 K͒, ␮ is
the standard chemical potential of a chemical written in subscript,
and a is the activity of a chemical written in subscript. The activities
−1
0
0
20
40
60
80
100
120
Time / minute
of H O and NiO are 1 in this case.
2
2
+
The activity of Ni aquo ions drastically decreases with the
increase in pH, and aNi2+ is determined to be 1.3 ϫ 10− at pH 12
by Eq. 5. The direction of reaction can be predicted using thermo-
dynamic calculation in comparison to mixed potential measured in
this system. Nickel can be deposited when the mixed potential is
below the oxidation-reduction potential of Ni2+/Ni redox pair, i.e.,
Figure 8. ͑Color online͒ Changes in mixed potential measured during the
reaction in aqueous nickel oxide suspension at ͑a͒ 298, ͑b͒ 353 K, and ͑c͒ in
nickel oxide/ethylene glycol suspension at 353 K.
12
ENi2+/Ni
= −0.60 V vs standard hydrogen electrode ͑SHE͒ at pH 12.
sible to deposit nickel if the oxidation-reduction potential, ENi͑II͒/Ni,
Actually, the reaction of nickel deposition is far more complicated,
but it should be noted that the oxidation-reduction potentials of any
Ni͑II͒/Ni redox pairs are the same, −0.60 V in the presence of abun-
dant nickel oxide or nickel hydroxide in equilibrium. For example, it
in ethylene glycol is the same or similar as that in water. However,
nickel was not obtained as shown in Fig. 3a. This can be a kinetic
problem, and nickel can be obtained by increasing the reaction rate.
One easy way to increase the reaction rate is to increase the tem-
perature. In fact, a part of the nickel oxide was successfully reduced
to nickel metal particles by the 2 h reaction at 373 K as shown in
Fig. 5.
2
+
is known that Ni ions form complexes with hydrazine and
1
8
ammonia. Because ammonia generated by Reactions 2 and 4
readily volatilizes at high reaction temperatures over 353 K, the
ammonium complexes can be ignored and only the complexes with
hydrazine are considered. The activity of one of the most predomi-
Another way to increase the reaction rate is to increase the area
where the reaction proceeds. For example, it can be effective to
decrease the size of reactant in order to increase the interface be-
tween the reactant and the solvent. Using nickel chloride as a start-
2
+
nant nickel͑II͒ species. ͓Ni͑N H ͒ ͔ , is calculated to be 0.023 us-
2
4 6
ing the data in Table I. The oxidation-reduction potential of
2
+
2
+
ing material, Ni ions dissolved in the solutions are almost all hy-
drolyzed to deposit minute nickel hydroxide particles by the
addition of sodium hydroxide aqueous solution. Minute nickel hy-
droxide particles deposited have a cross-bridged structure by weak
hydrogen bonding and form a network between themselves and the
solvent. As shown in Fig. 6, nickel was successfully obtained from
this minute nickel hydroxide as the reactant. Figure 9 shows the time
transient in mixed potential measured during the reaction at 353 K
using aqueous nickel hydroxide suspension and nickel hydroxide/
ethylene glycol suspension. In ethylene glycol suspension, the
mixed potential is below −0.8 V vs SHE for the first 12 min and
jumps up to approximately −0.5 V at 13 min. In contrast, the mixed
potential is always around −0.8 V during the reaction in aqueous
solvent. Nickel can be deposited at −0.8 V, and it is impossible to
͓
a
Ni͑N H ͒ ͔ /Ni
redox
2+ = 0.023. In addition, more precise nickel͑II͒ species
4 6
pair
is
also
−0.60 V
at
2
4 6
͓
Ni͑N H ͒ ͔
2
19
and revised Pourbaix diagram ͑potential-pH diagram͒ are reported
20
by Beverskog and Puigdomenech, and they mentioned an un-
charged aqueous nickel͑II͒ complex, Ni͑OH͒ ͑aq͒. The oxidation-
2
reduction potential of Ni͑OH͒ ͑aq͒/Ni is also calculated to be
2
−
0.60 V. Thus, an expression, Ni͑II͒/Ni redox pair is used here for
simplicity, where Ni͑II͒ or nickel͑II͒ includes all the divalent nickel
species present in the reaction system.
In general, the oxidation-reduction potentials of Ni2+/Ni redox
2
+
pair and ͓Ni͑N H ͒ ͔ /Ni redox pair are different because
2
4 6
2
+
2+
͓
Ni͑N H ͒ ͔ is much more stable than Ni aquo ion. For ex-
2 4 6
ample, ENi
2+/Ni
is −0.25 V and E
2+ is −0.60 V when the
͓Ni͑N H ͒ ͔
4 6
2
2
+
2+
deposit nickel at −0.5 V thermodynamically when E
is about
Ni͑II͒/Ni
activities of Ni , ͓Ni͑N H ͒ ͔ , and N H are all one in aqueous
2
4 6
2
4
−
0.6 V. Figure 10 shows XRD patterns of precipitates obtained be-
solution at 298 K. In the system of the present work, however, the
activities of Ni2+ and ͓Ni͑N H ͒ ͔2+ are different. The activities of
fore ͑at 8 min͒ and after ͑at 18 min͒ the point at which the mixed
2
4 6
Ni͑II͒ ionic species are determined by the presence of enough
amount of Ni͑II͒ ionic source ͑i.e., nickel oxide or nickel hydrox-
2
+
2+
ide͒, and all the Ni͑II͒ species such as Ni , ͓Ni͑N H ͒ ͔ , and
2
4 6
nickel oxide or hydroxide are in equilibrium. The activity of stable
Ni͑N H ͒ ͔ is much larger than that of unstable Ni aquo ion in
2
+
2+
͓
(a) Ethylene glycol suspension, 353 K
(b) Aqueous suspension, 353 K
2
4 6
(ii)
equilibrium, and this compensates the potential difference caused by
the difference in the stability of ionic species.
In ethylene glycol with some water, the oxidation-reduction po-
tential may be different from that in water; thus, the value in water
is used as a reference also in the discussion on ethylene glycol
reaction system.
E
Ni(II)/Ni
Figure 8 shows mixed potentials measured during the reaction in
aqueous and ethylene glycol suspensions. The mixed potential mea-
(i)
sured in aqueous suspension at 298 K is always above E
, and
Ni͑II͒/Ni
0
20
40
60
80
100
120
that at 353 K is almost always above E
during the reaction.
Ni͑II͒/Ni
Time / minute
Therefore, it is impossible to deposit nickel no matter how much
time is spent using aqueous suspension. In contrast, the mixed po-
tential measured in ethylene glycol suspension at 353 K is below
Figure 9. ͑Color online͒ Changes in mixed potential measured during the
reaction in ͑a͒ nickel hydroxide/ethylene glycol suspension and ͑b͒ aqueous
nickel hydroxide suspension at 353 K.
E
except for the first 8 min, and it is thermodynamically pos-
Ni͑II͒/Ni
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Journal of The Electrochemical Society, 155 ͑9͒ D583-D588 ͑2008͒
D587
Ethylene glycol suspension, 353 K
25
20
15
(a) Ethylene glycol suspension
Ni
Ni(OH)2
(ii)
1
0
5
0
(b) Aqueous suspension
(i)
0
10
20
30
40
50
60
Time / minute
1
0
15
20
25
30
35
40
Figure 12. Mass change of QCM substrate during the reaction in ͑a͒ nickel
hydroxide/ethylene glycol suspension and ͑b͒ aqueous nickel hydroxide sus-
pension at 353 K.
2
θ / degree (Mo-Kα)
Figure 10. ͑Color online͒ XRD patterns of precipitates obtained by the
reaction for ͑i͒ 8 and ͑ii͒ 18 min in nickel hydroxide/ethylene glycol suspen-
sion at 353 K.
ential to the hydrogen generation reaction in the early stage of the
reaction; thus, the mixed potential is defined by the oxidation reac-
tions of hydrazine and the reaction of nickel deposition. After most
of the nickel͑II͒ species have been consumed, the hydrogen genera-
tion reaction becomes mainly a cathodic reaction and the mixed
potential is defined by the oxidation reactions of hydrazine and the
hydrogen generation reaction. In our preliminary experiment, nickel
hydroxide was not completely reduced to nickel by the reaction in
potential suddenly increased, i.e., 13 min. Before this point, the
peaks of nickel are not observed, but strong peaks of nickel are
observed after the point.
This change of the mixed potential can be explained by taking
into account that mixed potential is defined as a potential where the
total current of anodic reactions counterbalances the total current of
cathodic reactions. In this system, anodic reactions are mainly the
oxidation reactions of hydrazine as follows
−
3
ethylene glycol solvent at 353 K when 1 mol dm sodium hydrox-
−
3
ide aqueous solution was used to adjust pH instead of 10 mol dm
sodium hydroxide aqueous solution. Figure 12 shows the mass
change of QCM substrate during the reaction in aqueous nickel hy-
droxide suspension and nickel hydroxide/ethylene glycol suspension
at 353 K. Although the mass simply increases in both cases, the
increase in mass stops after 13 min using the nickel hydroxide/
ethylene glycol suspension. The increase in mass is due to nickel
deposition, and this also confirms that the main cathodic reaction
was switched from the nickel deposition reaction to the hydrogen
generation reaction at 13 min in ethylene glycol suspension al-
though the main cathodic reaction was still the reaction of nickel
deposition in aqueous suspension.
It is highly possible that the difference in the mixed potentials, as
seen in Fig. 8 and 9, is also attributed to the difference in the balance
of each partial reaction ͑e.g., nickel deposition reaction, hydrogen
generation reaction, and the oxidation reaction of hydrazine͒. When
the balance is changed by the kind of nickel precursor, mixed po-
tential can vary even if the precursor has almost the same chemical
potential and there is little difference in Gibbs free energy change of
the total reaction. Considering the present system, the standard
chemical potentials of nickel oxide and nickel hydroxide are almost
the same, but the reaction path and activation energy of nickel depo-
sition might be different. This difference can change the balance of
the cathodic partial currents of nickel deposition and that of hydro-
gen generation in the total cathodic current, resulting in the differ-
ence in mixed potential. It is interesting that mixed potential is de-
termined kinetically although the value of mixed potential itself can
be treated as a thermodynamic parameter. Thermodynamic consid-
eration performed in this work can also be applied to other reaction
systems to predict whether or not metal nanoparticles will be depos-
ited by electroless deposition.
−
N H + 4OH → N + 4H O + 4e
͓10͔
͓11͔
2
4
2
2
−
1
2
N H + OH → N + NH + H O + e
The oxidation-reduction potentials of N /N H and N -NH /N H
redox pairs in aqueous solution at 298 K can be calculated as a
function of pH using the standard chemical potential of each
2
4
2
3
2
2
2
4
2
3
2
4
1
7
chemical
E
͑/V vs SHE͒ = − 0.327 − 0.0591pH
͑/V vs SHE͒ = − 1.584 − 0.0591pH
N -NH /N H
2 4
͓12͔
N /N H
2 4
2
E
͓13͔
2
3
At pH 12, E
and E
are −1.04 and −2.29 V, respec-
N -NH /N H
2 3 2 4
N /N H
2
2
4
tively. Meanwhile, cathodic reactions can be mainly the reaction of
nickel deposition and hydrogen generation reaction as follows
Ni͑II͒ + 2e → Ni
͓14͔
−
2
H O + 2e → H + 2OH
As illustrated in Fig. 11, the reaction of nickel deposition is prefer-
͓15͔
2
2
Current I
Ia, total
Increase in mixed potential
Potential E
Conclusion
Consumption of nickel(II) species
We have investigated the electrochemical formation process of
nickel nanoparticles by electroless deposition in aqueous and ethyl-
ene glycol suspensions. Through the present work, we have obtained
the following results.
Nickel particles approximately 300 nm diam were obtained by
the reaction using hydrazine as a reducing agent in nickel hydroxide/
ethylene glycol suspension at 353 K without any dispersing agent.
(
i) Ic, Ni(II)/Ni
ii) Ic, H O/H
(
2
2
Figure 11. ͑Color online͒ Schematic diagram of mixed potential measured in
this system.
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D588
Journal of The Electrochemical Society, 155 ͑9͒ D583-D588 ͑2008͒
2
3
4
. S.-H. Wu and D.-H. Chen, J. Colloid Interface Sci., 259, 282 ͑2003͒.
The reaction rate using nickel hydroxide was much faster than that
using nickel oxide as a starting material. In situ monitoring of mixed
potential in combination with thermodynamic calculation revealed
the reaction possibility. Mixed potential drastically increased ap-
proximately from −0.8 to −0.5 V vs SHE on reaching the end point
of the nickel deposition reaction using nickel hydroxide/ethylene
glycol suspension at 353 K because the cathodic reaction that deter-
mined the mixed potential switched from the nickel deposition re-
action to the hydrogen generation reaction. Thus, real-time monitor-
ing of mixed potential can be used to find the end point of the
reaction. Mass change measurement on QCM substrate during the
reaction also supported this result.
. R.-Y. Chen and K.-G. Zhou, Trans. Nonferrous Met. Soc. China, 16, 1223 ͑2006͒.
. P. Migowski, G. Machado, S. R. Texeira, M. C. M. Alves, J. Morais, A. Traverse,
and J. Dupont, Phys. Chem. Chem. Phys., 9, 4814 ͑2007͒.
5. L. Chen, J. Chen, H. Zhou, D. Zhang, and H. Wan, Mater. Sci. Eng., B, 452–453,
262 ͑2007͒.
6
7
. G.-P. Jin, Y.-F. Ding, and P.-P. Zheng, J. Power Sources, 166, 80 ͑2007͒.
. G. G. Couto, J. J. Klein, W. H. Schreiner, D. H. Mosca, A. J. A. de Oliveira, and A.
J. G. Zarbin, J. Colloid Interface Sci., 311, 461 ͑2007͒.
8. Y.-X. Zhang, W.-J. Fu, and X.-Q. An, Trans. Nonferrous Met. Soc. China, 18, 212
͑2008͒.
9
. E. Kauffeldt and T. Kauffeldt, J. Nanopart. Res., 8, 477 ͑2006͒.
0. K. Y. Jung, J. H. Lee, H. Y. Koo, Y. C. Kang, and S. B. Park, Mater. Sci. Eng., B,
37, 10 ͑2007͒.
1
1
1
1. V. V. Bakovets, V. N. Mitkin, and N. V. Gelfond, Chem. Vap. Deposition, 11, 368
͑
2005͒.
Acknowledgments
1
1
2. Y. He, X. Li, and M. T. Swihart, Chem. Mater., 17, 1017 ͑2005͒.
3. Z. Liu, S. Li, Y. Yang, S. Peng, Z. Hu, and Y. Qian, Adv. Mater. Res., 15, 1946
This research was supported by a Grant-in-Aid for Knowledge
Cluster Initiative ͑Kyoto Nanotech Cluster͒, a Grant-in-Aid for the
Global COE Program ͑International Center for Integrated Research
and Advanced Education in Materials Science͒, and a Grant-in-Aid
for Young Scientists ͑Start-up no. 19860041͒, all from the Japan
Society for the Promotion of Science, Ministry of Education, Cul-
ture, Sports, Science and Technology of Japan.
͑
2003͒.
1
1
4. D.-H. Chen and S.-R. Wang, Mater. Chem. Phys., 100, 468 ͑2006͒.
5. X. Zhang, F. Zhang, R.-F. Guan, and K.-Yu Chan, Mater. Res. Bull., 42, 327
͑2007͒.
6. S. Yagi, H. Nakanishi, E. Matsubara, S. Matsubara, T. Ichitsubo, K. Hosoya, and Y.
Matsuba, J. Electrochem. Soc., 155, D474 ͑2008͒.
7. W. M. Latimer, The Oxidation States of the Elements and their Potentials in Aque-
ous Solutions, 2nd ed., Prentice-Hall, Englewood Cliffs, NJ ͑1959͒.
8. R. M. Smith and A. E. Martell, Editors, Critical Stability Constants, Plenum Press,
New York ͑1976͒.
9. M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, Cebelcor,
Brüssels ͑1966͒.
1
1
1
1
Kyoto University assisted in meeting the publication costs of this article.
References
1
. Y. Hou and S. Gao, J. Mater. Chem., 13, 1510 ͑2003͒.
20. B. Beverskog and I. Puigdomenech, Corros. Sci., 39, 969 ͑1997͒.
Downloaded on 2012-11-25 to IP 150.108.161.71 address. Redistribution subject to ECS license or copyright; see www.esltbd.org