Synthesis of Ni nanoparticles by reduction of NiO prepared with a flow-through supercritical water method

Article abstract of DOI:10.1246/cl.2006.960

Synthesis of Ni nanoparticles having an average diameter of 8.1 nm by reduction of NiO nanoparticles was carried out at 673 K. HCOOH was used as a reductant, which decomposes to CO and/or H₂ at higher temperatures. NiO nanoparticles having an average diameter of 3.8 nm were obtained from Ni(NO₃)₂ + KOH aqueous solution by a flow-through supercritical water method at 673 K and 30 MPa. Effect of HCOOH concentration on particle size and saturation magnetization was investigated. Copyright

Full text of DOI:10.1246/cl.2006.960

960
Chemistry Letters Vol.35, No.8 (2006)
Synthesis of Ni Nanoparticles by Reduction of NiO Prepared
with a Flow-through Supercritical Water Method
Kiwamu Sue,ꢀ1 Akira Suzuki,2 Muneyuki Suzuki,2 Kunio Arai,2;3 Tomotsugu Ohashi,3 Keitaro Matsui,3
Yukiya Hakuta,3 Hiromichi Hayashi,3 and Toshihiko Hiaki1
1College of Industrial Technology, Nihon University, Narashino 275-8575
2Graduate School of Environmental Studies, Tohoku University, Sendai 980-8579
3Research Center for Compact Chemical Process,
National Institute of Advanced Industrial Science and Technology, Sendai 983-8551
(Received May 2, 2006; CL-060526; E-mail: k5sue@cit.nihon-u.ac.jp)
Synthesis of Ni nanoparticles having an average diameter of
of Ni nanoparticles was carried out through the reduction of the
NiO particles with HCOOH. We examined the effect of initial
HCOOH weight and reaction time on size and magnetic satura-
tion of the obtained particles.
8.1 nm by reduction of NiO nanoparticles was carried out at 673
K. HCOOH was used as a reductant, which decomposes to CO
and/or H2 at higher temperatures. NiO nanoparticles having
an average diameter of 3.8 nm were obtained from Ni(NO3)2 +
KOH aqueous solution by a flow-through supercritical water
method at 673 K and 30 MPa. Effect of HCOOH concentration
on particle size and saturation magnetization was investigated.
Solutions for NiO synthesis were prepared by dissolving
.
precise amounts of Ni(NO3)2 6H2O (Wako Pure Chemicals,
Japan) and KOH (Wako Pure Chemicals, Japan) in distilled
and deionized water (resistivity > 0.18 Mꢀ m). The concentra-
tions of Ni(NO3)2 and KOH in reactor were 0.01 and 0.02
mol/kg, respectively. HCOOH (þ99% purity) was used for Ni
synthesis.
Over the past ten years, a flow-through supercritical water
(FT-SCW) method has been widely used for continuous produc-
tion of metal oxide nanoparticles by hydrothermal synthesis.1,2
This method uses high controllability of water properties (densi-
ty, dielectric constant, and ion product) and also related factors
(metal oxide solubility, ionic equilibria, and hydrothermal
reaction rate) with temperature and pressure.1 Very recently,
we clarified the condition to obtain the nanoparticles having a
desired particle size on the basis of the supersaturation degree
evaluated by the estimated metal oxide solubility at given super-
critical condition.3 This method is basically environmentally
sustainable process and is attracting considerable attention as
an innovative technique for industrial production of metal oxide
nanoparticles. In addition to the synthesis of metal oxides, high
temperature water reaction fields, which include SCW condition,
have also great possibility for forming size-controlled metal par-
ticles because SCW forms homogeneous mixture with reducing
agents (H2 and CO) and, therefore, reduction reaction of metal
cations can be highly controlled.4 However, synthesis of metal
particles by reduction of metal cations in high-temperature water
is difficult task because hydrothermal reaction (hydrolysis of
metal cations and dehydration) occurs as a competitive reac-
tion.5–7 Some researchers have proposed methods for Ni synthe-
sis such as the complexes formation of 1,10-phenanthroline5 and
ammonia,6 and controlling pH to acidic condition7 to prevent the
hydrolysis. In addition, Ziegler et al. proposed a new method
which used organic alkanethiol capping ligands to prevent the
hydrolysis, to reduce Cu2þ, Cu2O, or CuO and to stabilize Cu
nanoparticle formation in SCW.8 This method used the specific
property that SCW forms homogeneous mixture with organic
thiols. Although these methods promise metal particle synthesis
in high-temperature water, they are complicated or require com-
plex preparation of starting solutions.
NiO synthesis was performed by the FT-SCW method as
described in our previous works.2,3 In this work, reaction temper-
ature and pressure were 673 K and 30 MPa, respectively. Reac-
tion time was 1.0 s. Ni synthesis was performed by the following
procedure at 673 K. Prepared NiO particles (1.12 g) and HCOOH
(0.37–2.76 g) were loaded into a batch reactor (7.5 cm3) made of
SUS316. Molar ratio of HCOOH to NiO was changed from ca.
0.5 to 6. The temperature was measured with a K-type thermo-
couple that was inserted into the reactor. The air in the reactor
was replaced with Ar by successive purging, and then the reactor
was sealed. The reactor was heated by immersion into a temper-
ature-controlled molten-salt bath at 673 K. Approximately 1 min
was required for the batch reactor to reach the reaction temper-
ature. Reaction time was 5–60 min, which includes heating up
time. The reactor was quenched in a water bath, which was kept
at room temperature.
The crystal structures of the products were analyzed by pow-
der X-ray diffractometry (XRD, RINT-2200, Rigaku), using
Cu Kꢀ radiation. Observation of these products was performed
by a transmission electron microscope (TEM, TECNAI-G2,
FEI Co). Particle size distribution and average particles size with
standard deviation (S.D.) were determined on the basis of the
diameter of about 150–200 particles measured from TEM
results. Concentrations of remaining Ni ion in the recovered
aqueous solution were measured by inductively coupled plasma
(ICP) emission spectroscopy (SPS-7800, Seiko). Conversion of
Ni ion to NiO was defined as (1 ꢁ C=C0Þ ꢂ 100, where C and
C0 are molal concentrations of the metal species in the recovered
and starting solutions, respectively. The magnetic properties
of the Ni particles at room temperature were measured by
a vibrating sample magnetometer (VSM, TEM-WF86R-153,
Toei Kogyo Co. Ltd.).
In this work, we investigated a relatively simple route to Ni
nanoparticles. NiO nanoparticles were prepared by the FT-SCW
method. Considering that decomposition of HCOOH at higher
temperatures produces reducing agents, CO and H2,9 preparation
Experimental conditions and results are summarized in
Table 1. Figures 1–3 show XRD patterns, typical TEM images,
and typical VSM results of the products, respectively. NiO nano-
particles having an average size of 3.8 nm was prepared by the
Copyright Ó 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.8 (2006)
961
Table 1. Experimental conditions and results
40
20
0
Reaction
time
/min
Average
size
/nm
Saturation
magnetization
/emu gꢁ1
HCOOH Crystal
phasea
S.D.
/nm
Run No.
/g
1
2
3
4
5
6
7
—
5
—
NiO
Ni, (NiO)
Ni, (NiO)
Ni
3.8
4.9
5.2
6.1
5.0
6.7
8.1
8.1
1.6
1.2
2.1
1.3
1.5
3.8
—
0.69
0.69
0.69
0.37
1.38
2.76
18.3
33.5
37.8
2.1
30
60
30
30
30
a)
-20
-40
Ni, (NiO)
Ni, (NiO)
Ni
d)
b)
c)
32.5
36.3
aSpecies within parentheses indicate minor product.
-5
0
5
Magnetic field/ kOe
Figure 3. Typical VSM results of the obtained particles by re-
duction of NiO a) for 5 min, b) for 30 min, c) for 60 min with
0.69 g of HCOOH, and d) with 2.76 g of HCOOH for 30 min.
g)
f)
intensity of the Ni phase also increased and Ni single phase par-
ticles were obtained for 60 min. The increase in reaction time
leads to the increase in particle size and saturation magnetization
from 4.9 to 6.1 nm and from 18.3 to 37.8 emu/g, respectively.
The result suggests that the Ni synthesis occurs along the follow-
ing steps. Because crystallinity of the primary produced NiO
particles was relatively low, NiO particles grew up during heat-
ing-up period. After that reduction of NiO proceeded in parallel
with HCOOH decomposition and Ni nanoparticles with higher
saturation magnetization could be obtained.
In conclusion, we have proposed a relatively simple synthe-
sis method to obtain Ni nanoparticles. Ni particles were synthe-
sized by reduction of NiO with HCOOH at 673 K, which was
prepared by a flow-through supercritical water method. A de-
tailed study of the preparation mechanism of Ni particles includ-
ing decomposition behavior of HCOOH in the presence of NiO,
which acts as a catalyst for HCOOH decomposition,10 as well as
the applicability for the synthesis of other metals is currently in
progress.
e)
d)
c)
b)
a)
40
60
/ CuK / degree
80
2
θ
α
Figure 1. XRD patterns of the obtained particles by a) FT-SCW
method and reduction of NiO b) for 5 min, c) for 30 min, d) for
60 min with 0.69 g of HCOOH, e) with 0.37 g of HCOOH, f)
with 1.38 g of HCOOH, and g) with 2.76 g of HCOOH for
30 min. Circle and triangle denote Ni and NiO, respectively.
a)
b)
This research was partially supported by the Ministry of
Education, Culture, Sports, Science and Technology, Japan,
Grant-in-Aid for Scientific Research (B) and for Promotion of
Multi-disciplinary Research Projects.
References
1
2
3
Y. Hakuta, H. Hayashi, K. Arai, Curr. Opin. Solid State Mater.
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K. Sue, M. Suzuki, K. Arai, T. Ohashi, H. Ura, K. Matsui, Y.
Hakuta, H. Hayashi, M. Watanabe, T. Hiaki, Green Chem.
2006, 8, 634.
20 nm
20 nm
Figure 2. Typical TEM images of the obtained particles by
a) FT-SCW method and b) reduction of NiO for 60 min with
0.69 g of HCOOH.
4
5
T. M. Seward, E. U. Franck, Ber. Bunsen-Ges. Phys. Chem. 2981,
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FT-SCW method as shown in Figures 1a and 2a. Conversion in
this experiment was ca. 92%.
In the experiment of Ni synthesis, with increasing HCOOH
weight from 0.37 to 2.76 g for 30 min, the peak intensity of the
Ni phase increased and the peak of the NiO was disappeared
at the condition using 2.76 g of HCOOH. In addition, the in-
crease in HCOOH weight leads to the increase in particle size
and saturation magnetization from 5.0 to 8.1 nm and from 2.1
to 36.3 emu/g, respectively. Then, with increasing reaction time
from 5 to 60 min at constant HCOOH weight of 0.69 g, the peak
6
7
8
´
J. Wodka, W. A. Charewicz, Hydrometallurgy 1991, 27, 191.
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Published on the web (Advance View) July 22, 2006; doi:10.1246/cl.2006.960