Microwave-assisted polyol process for synthesis of Ni nanoparticles

Article abstract of DOI:10.1111/j.1551-2916.2006.00925.x

Ni nanoparticles of controlled size in the range 30-100 nm were synthesized with polyvinyl pyrrolidone (PVP) and dodecylamine (DDA) as protecting agents through a microwave-assisted technique using ethylene glycol both as a reducing agent and a solvent. The morphology of Ni nanoparticles was controlled by the amount of DDA added in these systems. Particle size and size distribution were controlled by the concentration of metal source and the DDA to PVP ratio. The synthesized Ni nanoparticles were characterized by transmission electron microscopy for particle size and morphology, and size distribution was calculated by Image J^TM software. Experimental UV-Vis spectra of Ni nanoparticles matched with the calculated spectrum based on Mie's theory. The microwave-assisted polyol process was found to be faster than the conventional-polyol process in Ni nanoparticle synthesis and the former technique is expected to be cost-effective compared with the latter.

Full text of DOI:10.1111/j.1551-2916.2006.00925.x

J. Am. Ceram. Soc., 89 [5] 1510–1517 (2006)
DOI: 10.1111/j.1551-2916.2006.00925.x
r 2006 The American Ceramic Society
ournal
J
Microwave-Assisted Polyol Process for Synthesis of Ni Nanoparticles
Dongsheng Li and Sridhar Komarneniw
Materials Research Institute, Materials Research Laboratory, The Pennsylvania State University, University Park,
Pennsylvania 16802
Ni nanoparticles of controlled size in the range 30–100 nm were
synthesized with polyvinyl pyrrolidone (PVP) and dodecylamine
(DDA) as protecting agents through a microwave-assisted tech-
nique using ethylene glycol both as a reducing agent and a sol-
vent. The morphology of Ni nanoparticles was controlled by the
amount of DDA added in these systems. Particle size and size
distribution were controlled by the concentration of metal source
and the DDA to PVP ratio. The synthesized Ni nanoparticles
were characterized by transmission electron microscopy for par-
ticle size and morphology, and size distribution was calculated
by Image JTM software. Experimental UV-Vis spectra of Ni
nanoparticles matched with the calculated spectrum based on
Mie’s theory. The microwave-assisted polyol process was found
to be faster than the conventional-polyol process in Ni nanopar-
ticle synthesis and the former technique is expected to be cost-
effective compared with the latter.
(DDA). Ni nanoparticles with controlled morphology were syn-
thesized using this method. This technique is expected to be
more cost-effective for large-scale production because it is sim-
pler, for example, compared with the procedure of injection of
an organometallic precursor into a hot surfactant mixture.11
II. Experimental Procedure
Nickel acetate tetrahydrate was used as a Ni metal precursor in
all experiments. Ethylene glycol and PVP were used as a reduc-
ing agent and a protecting agent, respectively. DDA was used as
a second protecting agent, which could coordinate with the Ni
ion precursor to affect the morphology of nanoparticles. Pro-
tecting agents, PVP and DDA, were dissolved in ethylene glycol.
The metal precursor was mixed with reducing agent and pro-
tecting agents and a small amount of H2PtCl6 was added in
some experiments to form the seeds for the synthesis of Ni
nanoparticles. The mixed solution was treated using a micro-
wave digestion system (MARS-5, CEM Corp., Matthews, NC);
the details of this equipment were given in several of our pre-
vious publications.12ꢀ16 Conventional-polyol (C-P) experiments
were carried out in Parr bombs heated in an oven. The micro-
wave-polyol (M-P) reactions were carried out at 1951C for 45
min in all cases while the C-P reactions were carried out for 2–17
h. The synthesized Ni particles could be kept in the reaction so-
lution with the protecting agent for months without oxidation.
Before characterization, the synthesized particles were centri-
fuged and washed in alcohol to remove most of the protecting
agents. The characterization of Ni particles was conducted
quickly afterward to avoid the oxidation of the Ni particles.
The size and morphology of synthesized particles were deter-
mined using a transmission electron microscope (TEM, Philips
420, Tungsten-based 120 Kev, Eindhoven, the Netherlands).
The energy-dispersive spectrometry (EDS) analysis was carried
out with a Cresham Sirius 30 Si (Li) X-ray detector (Cupertino,
CA) attached to the Philips 420 TEM. Synthesized Ni nanopar-
ticles were characterized by powder X-ray diffraction (XRD).
The XRD patterns were recorded on a scintag diffractometer
(Cupertino, CA) operated at 35 kV and 30 current with CuKa
radiation. Particle size distribution was determined by Image
JTM software based on the assumption that particles are spher-
ical in size (particle size was calculated based on 95% confidence
interval). For the optical property characterization, the reaction
solutions were diluted with alcohol. The UV-Vis spectra were
recorded with samples in 1 cm cuvettes using Agilent 8453 UV-
Visible (UV-Vis) Spectrophotometer (Palo Alto, CA) at 251C, in
the spectral range of 250–1100 nm.
I. Introduction
ETAL nanoparticles are of great interest in the modern
materials world because of their special electronic, cata-
M
lytic, optical, and magnetic properties as compared with their
bulk counterparts. These special properties have been applied in
various fields ranging from microelectronics to biomedical sci-
ence. Magnetic nanoparticles, such as Fe, Co, and Ni, have at-
tracted much attention because of their unique properties and
applications in various fields. Ni nanoparticles could be used as
electrode materials in multilayer ceramic capacitors and cata-
lysts.1,2 Ni nanoparticles have been synthesized through many
methods, such as chemical/electrochemical methods, sonochem-
ical method,3 microwave plasma deposition,4 conventional po-
lyol process,5 and spray-pyrolysis method4–6 with various
reducing agents. In order to obtain dispersed nanoparticles, Ni
nanoparticles have also been synthesized in the Al2O3 matrix,
the Al-MCM41 host, or in the polymer matrix.7–9 However, Ni
nanoparticles with controlled morphology were obtained with
limited success. Binary protecting agent systems were used to
control particle shape in the synthesis of CdSe and Co nano-
particles. Manna et al.10 reported the synthesis of shape-con-
trolled CdSe nanoparticles with a binary surfactant system
containing tri-n-octylphosphine oxide (TOPO) and hex-
ylphosphonic acid (HPA). Puntes et al.11 synthesized magnetic
cobalt nanorods and nanospheres with narrow size distributions
and a high degree of shape control by the injection of an or-
ganometallic precursor into a hot surfactant mixture of TOPO
and oleic acid under an inert atmosphere.
In this paper, the microwave-assisted polyol method (M-P
process) was reported for the synthesis of Ni nanoparticles. Re-
ducing reaction was carried out in a binary protecting agent
system including polyvinyl pyrrolidone (PVP) and dodecylamine
III. Results and Discussion
(1) Theoretical Basis
For the synthesis of magnetic metal nanoparticles such as Ni
and Co nanoparticles, it is difficult to prepare isolated magnetic
nanoparticles, partially because the forces between the particles
are large. These forces are due to the high electron affinity and
the high surface tension arising from the partially filled d-orbital,
from the van der Waals forces between polarizable metal par-
ticles, and from magnetic dipole interactions. With PVP as a
N. Dudney—contributing editor
Manuscript No. 20644. Received June 7, 2005; approved December 15, 2005.
This research was supported by the Metals Program, Division of Materials Research,
National Science Foundation, under Grant No. DMR-0096527.
wAuthor to whom correspondence should be addressed. e-mail: komarneni@psu.edu
1510

May 2006
Microwave-Assisted Polyol Process for Synthesis of Ni Nanoparticles
1511
Table I. Quantities of Various Chemicals and Volumes of Ethylene Glycol Used in the Microwave-Polyol Experiments at 1951C/45
min for Low Concentrations of Metal Source, Dodecylamine (DDA), and Polyvinyl Pyrrolidone (PVP)
Metal precursor (mg)
Weight of PVP (mg)
Volume of ethylene
glycol (mL)
Sample code
Nickel acetate tetrahydrate
MW (630 K)
Weight of DDA (mg)
Amount of H2PtCl6 (mg)
1
2
3
4
7.8
7.8
7.8
7.8
120
120
120
120
—
1
5
20
20
20
20
20
0.98
0.98
0.98
0.98
protecting agent, large agglomerated Ni and Co nanoparticles
were obtained because the interaction between PVP and Ni ions
or metal particles was not strong enough to prevent the growth
and agglormeration of metal particles. Therefore, introducing a
second protecting agent that has a stronger interaction with
metal ions or metal particles in the reaction system was con-
sidered for the synthesis of Ni nanoparticles. DDA was used as a
second protecting agent in this work.
The interaction between PVP and Ni ions or Ni metal may
either be attributed to the donation of a pair of electrons from
the carbonyl oxygen to the Ni ions or to the complex formation
of the nitrogen on five-membered nitrogen-containing hetero-
cycles with the Ni ions.17 Compared with DDA, PVP leads to a
weaker interaction because of the steric hindrance from the five-
membered heterocycles. DDA provides a pair of electrons from
the nitrogen at the end of a 12-carbon chain. The steric hin-
drance from the 12-carbon chain is smaller than that from five-
membered heterocycles. The UV-Vis spectra of Ni21–PVP and
Ni21–DDA–PVP also indicate that DDA has a stronger inter-
action with Ni ions (which will be discussed below in the section
on optical properties). Therefore, DDA, which has stronger
binding with Ni metal or Ni ions, serves as a second protecting
agent to prevent the growth and agglomeration of Ni nanopar-
ticles.
Fig. 1. Transmission electron micrographs of Ni particles formed by the microwave-polyol (M-P) process with polyvinyl pyrrolidone (PVP) of
molecular weight 630 K. (a) Without dodecylamine (DDA) and (b–d) with 1, 5, and 20 mg DDA, respectively).

1512
Journal of the American Ceramic Society—Li and Komarneni
Vol. 89, No. 5
50
40
30
20
10
0
0
20
40
60
80 100 120 140 160 180
Diameter of particles (nm)
Fig. 4. Particle size distribution histogram of Ni nanoparticles of Fig.
1(c). (Size distribution was determined by counting 130 particles using
Image Jt software).
Fig. 2. Selected area electron diffraction (SAED) pattern of Ni nano-
particles shown in Fig 1c. Debye rings are assigned to {111} (d1 5 2.07
˚
˚
˚
A), {200} (d2 5 1.79 A), {220} (d3 5 1.24 A), and {311} (d4 5 1.08 A).
˚
Furthermore, according to the reduction mechanism, DDA
can also increase the reaction rate and lead to the formation of
Ni nanoparticles. Yang et al.18 and Horiuchi et al.19 reported the
mechanism of polyol reduction of metal ions. Accordingly, the
mechanism of reduction of Ni21 by ethylene glycol was provid-
ed as follows:
rate, therefore leading to the formation of Ni nanoparticles and
generating larger Ni nanoparticles. The effects of DDA on the
particle size and morphology depend on the PVP molecular
weight and the amount of DDA added in the reaction system.
(2) Effect of DDA on Morphology of Synthesized Ni
Nanoparticles
HOCH2CH2OH ! CH3CHO þ H2O
Table I lists the quantities of various chemicals and volumes of
ethylene glycol used in the M-P synthesis of Ni nanoparticles.
Figures 1 (a)–(d) shows the morphologies of Ni particles formed
by the M-P process with PVP of molecular weight 630 K, with
or without DDA. Agglomerated large Ni metal particles (larger
than 100 nm) were formed without adding DDA (shown in
Fig. 1(a)). The presence of PVP alone in ethylene glycol did not
lead to the formation of well-dispersed Ni nanoparticles, which
indicates that the interaction between PVP and Ni ion might not
be strong enough to prevent the growth and agglomeration of
Ni particles. Therefore, the addition of a second protective agent
to interact with Ni ions is a possible method to prevent the
growth and agglomeration of Ni particles based on the theoret-
ical basis (vide supra). Hence, we used DDA as a second pro-
tective agent. The interaction between DDA and Ni ions
appears to be stronger, and well-dispersed Ni nanoparticles
could be obtained in the presence of DDA. The morphology
of Ni metal was changed by adding DDA as a second protective
agent apparently because of the interaction with Ni ions. By
adding 1 mg of DDA, a small amount of Ni nanoparticles in the
form of strings was obtained (shown in Fig. 1(b)). Well-dis-
persed Ni nanoparticles with an average particle size of 63 nm
were obtained with an apparently optimum amount of DDA (5
mg), as shown in Fig. 1(c). These nanoparticles have a tendency
to form a chain-like organization. With increasing amounts of
the DDA, the size and arrangement of Ni nanoparticles chan-
ged. By increasing the amount of DDA further (20 mg), the size
of Ni nanoparticles decreased and these Ni nanoparticles had a
different arrangement and aggregated to some degree (TEM
image shown in Fig. 1(d)). Selected area electron diffraction
(SAED) pattern (Fig. 2) verifies the formation of crystallized Ni
particles (shown in Fig. 1(c)) with an FCC structure. The SAED
CH3CHO þ Ni2þ þ H2O ! CH3COOꢀ þ Ni þ 3Hþ
The amine group of DDA reacts with H1 formed from the
reduction reaction with the formation of CH3C11H22NH13 . The
reduction rate can be effectively increased because of the de-
creased amount of H1 when the amount of DDA is large
enough.
Therefore, DDA can prevent the growth and agglomeration
of Ni nanoparticles and thus lead to well-dispersed Ni nano-
particles because of a stronger binding with Ni ions or Ni metal.
On the other hand, DDA also increases the reduction reaction
˚
pattern shows Debye rings assigned to {111} (d1 5 2.07 A),
˚
{200} (d2 5 1.79 A), {220} (d3 5 1.24 A), and {311} (d4 5 1.08
˚
˚
A). The d-values were calculated from the SAED pattern. Figure
3 presents the EDS spectrum of Ni nanoparticles shown in
Fig. 1(c), which also verifies the formation of Ni metal. Peaks
corresponding to Cu and C in the EDS spectrum resulted from
Fig. 3. Energy dispersive spectrometry of Ni nanoparticles shown in
Fig. 1(c).

May 2006
Microwave-Assisted Polyol Process for Synthesis of Ni Nanoparticles
1513
500
450
400
350
300
250
200
150
100
50
75
111
70
65
60
200
220
55
a
b
50
c
0
45
40
50
60
70
80
0
200
400
600
800 1000 1200 1400
PVP molecular weight (103)
Degree
Fig. 5. X-ray diffraction (XRD) patterns of Ni nanoparticles synthe-
sized with different amounts of dodecylamine (DDA) added in the re-
action solutions. (a) Without DDA, (b) with 5 mg DDA, and (c), with 20
mg DDA. The inset shows the two small peaks of hexaganol structure
(010 and 002) and one peak of FCC structure (111) of Ni nanoparticles
synthesized by adding 20 mg DDA.
Fig. 6. Relationship of polyvinyl pyrolidone (PVP) molecular weight
and synthesized Ni nanoparticles.
(4) Contribution of DDA to the Formation of Ni
Nanoparticles
Besides its contribution to the control of Ni nanoparticle mor-
phology, DDA can also be helpful in the formation of Ni nano-
particles when a Pt metal source is not used (Pt was used to
nucleate Ni nanoparticles). At a high concentration of metal
source and PVP, as listed in Table III, Ni metal particles cannot
be produced without adding DDA or with a small amount of
DDA.
With a high concentration of PVP, the steric hindrance of
PVP around Ni ions prevents the nucleation and growth of Ni
particles. By increasing the amount of DDA, Ni nanoparticles
were synthesized. This is probably related to the mechanism of
the reduction as discussed before. The particle size and size
distribution were changed by increasing the amount of DDA
further. Figures 7 (a)–(c) show the TEM images of Ni nano-
particles synthesized with different ratios of DDA to PVP re-
peating unit. At a DDA to PVP ratio of 0.04, nonuniform Ni
nanoparticles were produced. The average particle size is ap-
proximately 83 nm, and the particle size distribution is broad,
ranging from about 20 to 120 nm. Uniform Ni nanoparticles of
about 106 nm on average were synthesized with a DDA to PVP
ratio of 1. Particle size ranged mainly from 100 to 120 nm. At a
DDA to PVP ratio of 2, Ni nanoparticles with a bimodal size
(60 and 120 nm) distribution were obtained.
the copper grid with carbon film. In this spectrum, a very small
oxygen peak also appeared. This might be due to the surface
oxidation of Ni nanoparticles, or more likely, because of the
residue of surfactant around the nanoparticles. A size distribu-
tion histogram of synthesized Ni nanoparticles (shown in
Fig. 1(c)) was calculated by Image Jt software based on the
assumption that particles are spherical in size, as shown in
Fig. 4. Figure 5 shows the XRD pattern of Ni nanoparticles
synthesized with different amounts of DDA added in the reac-
tion solutions (Fig. 5(a), without DDA; Fig. 5(b), with 5 mg
DDA; and Fig. 5(c), with 20 mg DDA). Powder XRD patterns
(Fig. 5) prove the formation of Ni metal particles. Without or
with a small amount of DDA (5 mg) in the synthesis system, Ni
particles with cubic structure were synthesized (Figs. 5(a) and
(b)). With 20 mg DDA added in the synthesis system, Ni par-
ticles with both cubic and hexagonal structures were synthe-
sized. Inset shows the two small peaks corresponding to a
hexagonal structure (010 and 002) and one peak corresponding
to a cubic structure (111) as shown in Fig. 5(c).
(3) Effect of PVP Molecular Weight on Morphology of
Synthesized Ni Nanoparticles
(5) Effect of the Amount of DDA Added on the Particle Size
of Synthesized Ni Nanoparticles
To prepare well-separated Ni nanoparticles, the amount of
DDA can be decreased with increasing molecular weight of
PVP because PVP of higher molecular weight has a stronger
ability to prevent the growth and agglomeration of metal par-
ticles. Table II lists the amount of DDA needed for the forma-
tion of well-dispersed Ni nanoparticles with PVP of different
molecular weights. Figure 6 shows that the Ni nanoparticle size
decreased with increasing molecular weight of PVP. This is be-
cause of the stronger interaction of PVP of higher molecular
weight with Ni.
Table IV lists the different amounts of DDA added and the size
of Ni nanoparticles synthesized with different PVP molecular
weights. Figure 8 presents the relationship between particle size
and the amount of DDA added. Initially, particle size increased
with a small increase of DDA. By increasing the amount of
DDA further, the particle size decreased. In the case of PVP
molecular weight of 1300 K, Ni nanoparticles did not form. The
possible mechanism is as follows: with a small amount of DDA,
DDA interacts with Ni ions and Ni metal particles first and
Table II. Optimum Amount of Dodecylamine (DDA) Needed for the Formation of Well-Dispersed Ni Nanoparticles with Polyvinyl
Pyrrolidone (PVP) of Different Molecular Weights
Metal precursor (mg)
Weight of PVP (mg)
Sample code Ni(CH3CO2)2 ꢁ 4H2O MW (40 K) MW (630 K) MW (1300 K) Weight of DDA (mg) Volume of ethylene glycol (mL) Amount of H2PtCl6 (mg)
5
3
6
7.8
7.8
7.8
120
—
—
—
120
—
—
—
120
8
5
1
20
20
20
0.98
0.98
0.98

1514
Journal of the American Ceramic Society—Li and Komarneni
Vol. 89, No. 5
a
30
25
20
15
10
5
0
0
20 40 60 80 100 120 140 160 180
Diameter of particles (nm)
b
70
60
50
40
30
20
10
0
0
20 40 60 80 100 120 140 160 180
Diameter of particles (nm)
c
40
35
30
25
20
15
10
5
0
0
20 40 60 80 100 120 140 160 180
Diameter of particles (nm)
Fig. 7. Transmission electron microscope (TEM) images and particle size distributions of Ni nanoparticles synthesized. (a) sample 9, (b) sample 10, and
(c) sample 11 [see tables]).
Table III. Quantities of Various Chemicals and Volumes of Ethylene Glycol Used in the Microwave-Polyol Experiments at 2001C/60
min for High Concentrations of Metal Source, Polyvinyl Pyrrolidone (PVP), and Dodecylamine (DDA) and without Pt Seeds
Metal precursor (mg)
Weight of PVP (mg)
Ã
Sample code
Nickel acetate tetrahydrate
MW (10 K)
Weight of DDA (mg)
Volume of ethylene glycol (mL)
Ratio
Formation of Ni metal
7
8
9
10
11
Ã
99.2
99.2
99.2
99.2
99.2
600
600
600
600
600
—
20
40
1000
2000
20
20
20
20
20
0
No
No
Yes
Yes
Yes
0.02
0.04
1
2
Ratio: molar ratio of DDA to PVP repeating unit.

May 2006
Microwave-Assisted Polyol Process for Synthesis of Ni Nanoparticles
1515
Table IV. List of Different Amounts of Dodecylamine (DDA) Added, Different Molecular Weights of Polyvinyl Pyrrolidone (PVP)
Used, Particle Size and Quantities of Various Chemicals Used in the Microwave-Polyol Experiments at 1951C/45 min for Low
Concentrations of Metal Source, PVP, and DDA
Metal precursors (mg)
Weight of PVP (mg)
Nickel acetate
tetrahydrate
MW
MW
MW
Weight of
Volume of ethylene
glycol (mL)
Amount of
Particle
size nm
Sample code
(1300 K)
(630 K)
(40 K)
DDA (mg)
H2PtCl6 (mg)
12
13
14
15
16
17
18
19
20
21
7.8
7.8
7.8
7.8
7.8
7.8
15.6
7.8
7.8
7.8
120
120
120
1
5
10
5
10
20
100
5
10
20
20
20
20
20
20
20
10
20
20
20
0.98
0.98
0.98
0.98
0.98
0.98
1.96
0.98
0.98
0.98
51
83
—
63
107
56
32
61
68
50
120
120
120
120
120
120
120
prevents growth and agglomeration and with a small increase of
DDA, the extra amount of DDA interacts with H1 formed by
this reduction reaction, and thus increases the reaction rate.
Therefore, particle size increases. By increasing the amount of
DDA further, because of the large amount of DDA surrounding
Ni ions and Ni particles formed, the effect of steric hindrance
dominates and the particle size decreases. When PVP of molec-
ular weight 1300 K was used, Ni nanoparticles were not formed
even with a small amount of DDA because of the stronger steric
hindrance of high molecular weight PVP.
were carried out at 1951C for 2–17 h in an oven by the former
method. Table V lists the reaction times and the quantities of
chemicals of reaction solutions. Ni nanoparticles did not form
with a reaction time of 2 h. On increasing the reaction time to 5
and 17 h, Ni particles were synthesized. Figure 9 shows the TEM
images of synthesized Ni nanoparticles. With the conventional
method, larger agglomerated Ni particles were synthesized. This
is probably because with conventional heating, the reaction so-
lution cannot quickly reach the high temperature that the reac-
tion needs. A large number of nuclei cannot form for the growth
of Ni nanoparticles. Therefore, the particle size is large and par-
ticles agglomerate. These results clearly show that the formation
of Ni nanoparticles by the M-P process is faster than that with
the C-P process probably because of the generation of localized
high temperatures in the former.
(6) Comparison of the Microwave-Assisted Technique with
the C-P Method
Ni nanoparticles were synthesized by the C-P method and com-
pared with those of the M-P method. The reduction reactions
(7) Optical Properties of Synthesized Ni Nanoparticles
UV-Vis absorbance spectra were obtained in ethanol. Ni21 and
PVP mixture and Ni21, PVP, and DDA mixture were dissolved
in ethanol and followed by the UV-Vis experiment. The reaction
mixture of synthesized Ni nanoparticles in ethylene glycol was
diluted with ethanol and followed by the UV-Vis experiment.
According to Mie’s theory, the extinction coefficient, K, for
small particles, is approximately calculated by the following
equation20:
M.W. of PVP (630K)
110
M.W. of PVP (1300K)
M.W. of PVP (40K)
100
90
80
70
60
50
40
2
K ¼ ð18pNVn3=2=lÞe2=½ðe1 þ 2n2Þ þ e22ꢂ
Where N is the particle concentration, V the volume of the
particles, n the index of refraction of the dielectric medium, l the
wavelength of incident light, and e1 and e2 are the real and im-
aginary parts of the complex dielectric constant of the particles.
This equation is based on the assumptions that N is low and the
particles have uniform sizes and spherical shapes. Based on this
theory, Creighton and Eadon21 have reported the calculated
optical spectrum of various metal nanoparticles in water and in
vacuum and Ni nanoparticles exhibit surface plasmon resonance
0
5
10
15
20
Weight of DDA (mg)
Fig. 8. Relationship of the amounts of dodecylamine (DDA) added
and particle size of Ni nanoparticles synthesized with different polyvinyl
pyrrolidone (PVP) molecular weights.
Table V. List of Reaction Times and Quantities of Chemicals in Reaction Solutions Heated at 1951C by Conventional Hydrothermal
Process
Metal precursor (mg)
Weight of PVP (mg)
Sample code Nickel acetate tetrahydrate
MW (630 K)
Weight of DDA (mg) Volume of ethylene glycol (mL) Amount of H2PtCl6 (mg) Reaction time (h)
22
23
24
7.8
7.8
7.8
120
120
120
5
5
5
20
20
20
0.98
0.98
0.98
2
5
17
DDA, dodecylamine; PVP, polyvinyl pyrrolidone

1516
Journal of the American Ceramic Society—Li and Komarneni
Vol. 89, No. 5
Fig. 9. Transmission electron microscope (TEM) images of Ni nanoparticles synthesized with the conventional method. Reactions were carried out in
an oven for (a) 5 and (b) 17 h.
(SPR) absorption between 300 and 400 nm. It has been reported
that Ni-implanted silica glass exhibited absorptions at 354 nm.20
Figure 10 presents the calculated UV-Vis spectrum of Ni
nanoparticles according to the above equation, showing that the
SPR absorption of Ni nanoparticles is at about 330 nm. The
optical constants as a function of wavelength for the bulk metal
(Ni) were taken from the article of Johnson and Christy22 for the
calculation of the UV-Vis spectrum of Ni nanoparticles.
IV. Conclusions
Well-dispersed Ni nanoparticles were synthesized with a binary
protecting agent system of PVP and DDA. The morphology of
Ni nanoparticles was dependent on the amount of DDA added
2.0
1-Ni nanoparticles (~63 nm)
2-Ni2+−DDA−PVP
3-Ni2+−PVP
Figure 11 shows the UV-Vis absorbance spectra from Ni21
–
PVP, Ni21–PVP–DDA, and Ni nanoparticles stabilized with
PVP and DDA in ethanol. Solutions of Ni ions exhibit two ab-
sorption bands assigned to crystal field transition of octahedral
Ni21: one strong band at around 400 nm and one weak band at
about 600–800 nm. The two absorption bands of Ni21–DDA–
PVP solution are slightly shifted toward shorter wavelengths,
indicating that DDA has stronger binding with Ni ions. The
UV-Vis absorption of reaction solution of Ni nanoparticles sta-
bilized with PVP and DDA shows one peak at around 300–350
nm attributed to SPR absorption of Ni nanoparticles, which
proves the formation of Ni nanoparticles.
1.5
1.0
0.5
0.0
1
2
3
Figures 12 and 13 show the UV-Vis spectra of Ni nanopar-
ticles synthesized with PVP of different molecular weights (630
and 1300 K). Compared with the UV-Vis absorption of Ni
nanoparticles synthesized with PVP molecular weight of 630 K,
SPR absorption bands of Ni nanoparticles produced with PVP
molecular weight of 1300 K are broad and weak. This is prob-
ably because of the effect of PVP surrounding Ni nanoparticles
on the SPR absorption. The interaction between PVP and Ni
ions broadens the SPR absorption band. PVP of higher
molecular weight have more N or O atoms to bind with
Ni nanoparticles and thus lead to broadening of the SPR
absorption band.
300
400
500
600
700
800
900
Wavelength (nm)
Fig. 11. UV-Vis absorbance spectra from ethanol solution of Ni21
–
PVP, Ni21–PVP–DDA, and synthesized Ni nanoparticles stabilized with
polyvinyl pyrrolidone (PVP) and dodecylamine (DDA) (sample 15).
2.0
1.5
1.0
0.08
0.06
0.04
0.02
0
Sample 15 (~63 nm)
0.5
Sample 18 (~32 nm)
0.0
300
400
500
600
700
800
900
Wavelength (nm)
200
400
600
wavelength (nm)
Fig. 10. Calculated UV-Vis spectrum of Ni nanoparticles.
800
1000
1200
Fig. 12. UV-Vis absorbance spectra from ethanol solution of synthe-
sized Ni nanoparticles stabilized with dodecylamine (DDA) and polyvinyl
pyrrolidone (PVP) with a molecular weight of 630 K (samples 15 and 18).

May 2006
Microwave-Assisted Polyol Process for Synthesis of Ni Nanoparticles
1517
3Y. Koltypin, G. Katabi, X. Cao, R. Prozorov, and A. Gedanken, ‘‘Sonochem-
ical Preparation of Amorphous Nickel,’’ J. Non-Cryst. Solids, 201 [1–2] 159–62
(1996).
1.0
4J. R. Brenner, J. B. L. Harkness, M. B. Knickelbein, G. K. Krumdick, and C.
L. Marshall, ‘‘Microwave Plasma Synthesis of Carbon-Supported Ultrafine Metal
Particles,’’ Nanostruct. Mater., 8 [1] 1–17 (1997).
0.8
0.6
0.4
0.2
0.0
5T. Hinotsu, B. Jeyadevan, C. N. Chinnasamy, K. Shinoda, and K. Tohji, ‘‘Size
and Structure Control of Magnetic Nanoparticles by Using a Modified Polyol
Process,’’ J. Appl. Phys., 95 [11] 7477–9 (2004).
6W. N. Wang, Y. Itoh, I. W. Lenggoro, and K. Okuyama, ‘‘Nickel and Nickel
Oxide Nanoparticles Prepared from Nickel Nitrate Hexahydrate by a Low Pres-
sure Spray Pyrolysis,’’ Mater. Sci. Eng. B—Solid State Mater. Adv. Technol., 111
[1] 69–76 (2004).
Sample 12 (~51 nm)
Sample 13 (~83 nm)
7J.-S. Jung, K.-H. Choi, W.-S. Chae, Y.-R. Kim, J.-H. Jun, L. Malkinski, T.
Kodenkandath, W. Zhou, J. B. Wiley, and C. J. O’Connor, ‘‘Synthesis and Char-
acterization of Ni Magnetic Nanoparticles in AlMCM41 Host,’’ J. Phys. Chem.
Solids, 64 [3] 385–90 (2003).
8D. Kumar, S. J. Pennycook, A. Lupini, G. Duscher, A. Tiwari, and J. Narayan,
‘‘Synthesis and Atomic-Level Characterization of Ni Nanoparticles in Al Matrix,’’
Appl. Phys. Lett., 81 [22] 4204–6 (2002).
300
400
500
600
700
800
900
9R. R. Rakhimov, E. M. Jackson, J. S. Hwang, A. I. Prokof’ev, I. A. Ale-
xandrov, A. Y. Karmilov, and A. I. Aleksandrov, ‘‘Mechanochemical Synthesis of
Co, Ni, Fe Nanoparticles in Polymer Matrices,’’ J. Appl. Phys., 95 [11 II] 7133–5
(2004).
Wavelength (nm)
Fig. 13. UV-Vis absorbance spectra from ethanol solution of synthe-
sized Ni nanoparticles stabilized with dodecylamine (DDA) and polyvi-
nyl pyrrolidone (PVP) with a molecular weight of 1300 K (samples 12
and 13).
10L. Manna, E. C. Scher, and A. P. Alivisatos, ‘‘Synthesis of Soluble and Proc-
essable Rod-, Arrow-, Teardrop-, and Tetrapod-Shaped CdSe Nanocrystals,’’ J.
Am. Chem. Soc., 122 [51] 12700–6 (2000).
11V. F. Puntes, K. M. Krishnan, and A. P. Alivisatos, ‘‘Colloidal Nanocrystal
Shape and Size Control: The Case of Cobalt,’’ Science, 291 [5511] 2115–7 (2001).
12H. Katsuki, S. Furuta, and S. Komarneni, ‘‘Microwave Versus Conventional-
Hydrothermal Synthesis of NaY Zeolite,’’ J. Porous Mater., 8 [1] 5–12 (2001).
13B. L. Newalkar, S. Komarneni, and H. Katsuki, ‘‘Microwave-Hydrothermal
Synthesis and Characterization of Barium Titanate Powders,’’ Mater. Res. Bull.,
36 [13–14] 2347–55 (2001).
in these systems. The interaction between DDA and Ni ions and
Ni metal particles is stronger than that between PVP and Ni ions
and Ni metal particles. PVP with higher molecular weight has
stronger ability to coordinate with Ni ions or Ni metal particles.
Therefore, the amount of DDA can be decreased in the reaction
system by increasing the molecular weight of PVP.
Adding large amounts of DDA was also helpful for the for-
mation of Ni nanoparticles without Pt seeding. The particle size
and size distribution of Ni metal nanoparticles were controlled
by changing the DDA to PVP ratio.
14B. L. Newalkar, J. Olanrewaju, and S. Komarneni, ‘‘Microwave-Hydrother-
mal Synthesis and Characterization of Zirconium Substituted SBA-15 Mesopo-
rous Silica,’’ J. Phys. Chem. B, 105 [35] 8356–60 (2001).
15S. Komarneni, D. Li, B. Newalkar, H. Katsuki, and A. S. Bhalla, ‘‘Micro-
wave—Polyol Process for Pt and Ag Nanoparticles,’’ Langmuir, 18 [15] 5959–62
(2002).
16H. Katsuki and S. Komarneni, ‘‘Microwave-Hydrothermal Synthesis of
Monodispersed Nanophase a-Fe2O,’’ J. Am. Ceram. Soc., 84 [10] 2313–7 (2001).
17M. Liu, X. Yan, H. Liu, and W. Yu, ‘‘Investigation of the Interaction Between
Polyvinylpyrrolidone and Metal Cations,’’ 44 [1] 55–64 (2000).
18J. Yang, T. C. Deivaraj, H.-P. Too, and J. Y. Lee, ‘‘Acetate Stabilization of
Metal Nanoparticles and its Role in the Preparation of Metal Nanoparticles in
Ethylene Glycol,’’ Langmuir, 20 [10] 4241–5 (2004).
The microwave-assisted method produced well-dispersed Ni
nanoparticles, and the microwave-assisted polyol process was
faster than the conventional-polyol process in the synthesis of Ni
nanoparticles.
19S. Horiuchi, T. Fujita, T. Hayakawa, and Y. Nakao, ‘‘Three-Dimensional
Nanoscale Alignment of Metal Nanoparticles Using Block Copolymer Films as
Nanoreactors,’’ Langmuir, 19 [7] 2963–73 (2003).
20T. Isobe, S. Y. Park, R. A. Weeks, and R. A. Zuhr, ‘‘The Optical and Mag-
netic Properties of Ni1-Implanted Silica,’’ J. Non-Cryst. Solids, 189 [1–2] 173–80
(1995).
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&