1018
Chemistry Letters Vol.38, No.11 (2009)
Hydrothermal-reduction Synthesis of Ni Nanoparticles
by Superrapid Heating Using a Micromixer
Kiwamu Sue,ꢀ1 Akira Suzuki,2 Yukiya Hakuta,3 Hiromichi Hayashi,3 Kunio Arai,2
Yoshihiro Takebayashi,1 Satoshi Yoda,1 and Takeshi Furuya1
1Nanotechnology Research Institute, AIST, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba 305-8565
2Graduate School of Environmental Studies, Tohoku University, Aramaki Aza Aoba-07, Sendai 980-8579
3Research Center for Compact Chemical Process, AIST, 4-2-1 Nigatake, Miyagino-ku, Sendai 983-8551
(Received July 27, 2009; CL-090697; E-mail: k.sue@aist.go.jp)
Heater
Single-phase, high-crystalline, and single-crystalline Ni
High
pressure
pump
HCOOH
(1.59 mm o.d., 0.18 mm i.d.)
SUS316 tube
a)
b)
c)
Mixing
point
P
nanoparticles under 20 nm were continuously prepared through
hydrothermal reduction by superrapid heating of Ni(HCOO)2
solution using a micromixer in a homogeneous mixture field
of H2O and H2 at 673 K and 30 MPa over 0.5 s.
T
Ni(HCOO)2
Micromixer
P
SUS316 Union Tee
(2.3 mm i.d.)
Reactor
Cooler
Teflon-lined
SUS316 tube
(1.7 mm i.d.)
H2O or HCOOH
aq. soln.
Ni
HCOOH
SUS316 tube
Mixing
point
(3.18 mm o.d., 1.74 mm i.d.)
Ni nanoparticles have great potential in several technologi-
cal applications such as internal electrodes in multilayer ceramic
capacitors and as material with high hydrogen storage capacity
because of lower cost compared with noble metals.1 There are
a number of studies of the synthesis of Ni nanoparticles.2 How-
ever, these methods often require multistep treatment, complex
preparation of starting solutions, large amounts of organic sol-
vents, and relatively expensive surfactants for obtaining single-
phase, high-crystalline, and single-crystalline Ni nanoparticles.
Supercritical water (SCW) has several specific features such
as drastic changes of dielectric constant and ionic product by
manipulating temperature and pressure while maintaining a sin-
gle phase without any organics, acids, or bases, and also forma-
tion of homogeneous mixtures with organics and gases.3 Be-
cause of these features, SCW has high controllability of redox
and crystallization reactions, which are major reactions for the
synthesis of the Ni nanoparticles4 and could lead to environmen-
tally benign innovative chemical processes. Until now, a few
studies of Ni nanoparticle synthesis in SCW have been reported.5
In these studies, a continuous synthesis method6 was applied for
heating a starting solution to reaction temperature. However, the
products did not meet the above-mentioned characteristics be-
cause homogeneous nucleation could not be achieved due to
mainly the slow heating rate of starting solution.
Ni(HCOO)2
T
P
Filter
Back
pressure
regulator
H2O
Ni(HCOO)2
aq. soln.
SUS316 Union Tee
(3.18 mm o.d., 2.3 mm i.d.)
Ni
Figure 1. (a) Schematic diagram of apparatus. (b) Newly con-
structed micromixer. (c) Conventional mixer.
mixer are shown in Figures 1a, 1b, and 1c, respectively. The
starting solution (fed from the left branch at 20 g minꢁ1
,
298 K) was mixed with a preheated HCOOH solution (fed from
the upper branch at 80 g minꢁ1, 702 K) in the micromixer, which
was mainly composed of a microtube (0.18 mm i.d.) and a union
tee (2.3 mm i.d.). Velocities of the starting solution before the
mixing in the newly constructed micromixer and the convention-
al mixer are 13 and 0.08 m sꢁ1, respectively. Reynolds number
(Re) of the solution in the reactor after the mixing was
2:1 ꢂ 104, and the Re values were the same for both mixers.
By this construction, the starting solution was fed to a mixing
point without preheating before mixing because of the high ve-
locity, and it was rapidly heated to the reaction temperature of
673 K after the mixing due to the high Re. System pressure
was controlled at 30 MPa by a back-pressure regulator. The res-
idence time, ꢀ, was calculated using the total flow rate, the reac-
tor volume, and water density at 673 K and was varied from 0.5
to 2.0 s by changing the reactor length. Products were recovered
as a slurry solution, removed using a membrane filter, and dried
at 333 K in an electric oven for 24 h. In the case using the con-
ventional mixer, the products could not be recovered because
of the plugging in the left branch tube.
In this study, we introduce a newly constructed micromixer
for continuous synthesis through superrapid heating of a starting
solution and try to directly synthesize single-phase, high-crystal-
line, and single-crystalline Ni nanoparticles in SCW and H2.
A starting solution was prepared by dissolving precise
.
amounts of Ni(HCOO)2 2H2O (Purity >92%, Wako Pure
Chemicals) in distilled and deionized water (Resistivity >0:18
Mꢀ m). HCOOH solution was prepared by dissolving HCOOH
(Purity >99%, Wako Pure Chemicals) in water. Concentrations
of Ni(HCOO)2 and HCOOH (CHCOOH) were 0.1 and 0.0–
2.5 mol kgꢁ1, respectively. HCOOH was used to produce H2 as
a reductant produced by hydrothermal decomposition.7 These
solutions were continuously purged with Ar during each experi-
ment to remove dissolved O2.
The crystal structures were analyzed by XRD. The crystal-
lite sizes, DXRD, were calculated from FWHM by the Scherrer
equation. Morphological observation was performed by TEM.
The particle size distribution and the average particle size,
DTEM, with the standard deviation (SD) were determined on
the basis of a TEM image. The magnetic properties at room tem-
perature was measured with VSM for the determination of the
saturation magnetization, MS, and the coercivity, HC. Concentra-
tion of remaining nickel ion in the recovered solution was
measured by ICP-AES to evaluate conversion of nickel ion into
solid product, X.
Continuous synthesis was performed using a newly con-
structed micromixer. A schematic diagram of the experimental
apparatus, a newly constructed micromixer, and a conventional
Copyright ꢀ 2009 The Chemical Society of Japan