Surfactant-free synthesis of nickel nanoparticles in near-critical water

Article abstract of DOI:10.1016/j.jssc.2008.06.041

Nickel nanoparticles have been produced by combining two well-tested methods: (i) the continuous flow supercritical reactor and (ii) the reduction of a nickel salt with hydrazine. The normal precipitation of a nickel-hydrazine complex, which would complicate pumping and mixing of the precursor, was controlled by the addition of ammonia to the precursor solution, and production of nickel nanoparticles with average sizes from 40 to 60 nm were demonstrated. The method therefore provides some size control and enables the production of nickel nanoparticles without the use of surfactants. The pure nickel nanoparticles can be easily isolated using a magnet.

Full text of DOI:10.1016/j.jssc.2008.06.041

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Journal of Solid State Chemistry 181 (2008) 2681– 2683
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Surfactant-free synthesis of nickel nanoparticles in near-critical water
Ã
Peter Hald a, Martin Bremholm a, Steen Brummerstedt Iversen b, Bo Brummerstedt Iversen a,
a Center for Energy Materials, Department of Chemistry and Interdisciplinary Nanoscience Center, University of Aarhus, Langelandsgade 140, DK-8000 Aarhus C, Denmark
b SCF Technologies a/s, Smedeholm 13, DK-2730 Herlev, Denmark
a r t i c l e i n f o
a b s t r a c t
Article history:
Nickel nanoparticles have been produced by combining two well-tested methods: (i) the continuous
flow supercritical reactor and (ii) the reduction of a nickel salt with hydrazine. The normal precipitation
of a nickel–hydrazine complex, which would complicate pumping and mixing of the precursor, was
controlled by the addition of ammonia to the precursor solution, and production of nickel nanoparticles
with average sizes from 40 to 60 nm were demonstrated. The method therefore provides some size
control and enables the production of nickel nanoparticles without the use of surfactants. The pure
nickel nanoparticles can be easily isolated using a magnet.
Received 22 April 2008
Received in revised form
16 June 2008
Accepted 19 June 2008
Available online 22 June 2008
Keywords:
Nanoparticles
Continuous flow reactor
Near-critical fluids
Nickel
& 2008 Elsevier Inc. All rights reserved.
1. Introduction
produced nickel particles, but with a rather large size (600 nm)
and a broad size-distribution. In the present case, the nickel
nanoparticles were synthesised by hydrothermal reduction of
nickel salts with hydrazine. By mixing a stream of cold reactant
with a superheated solvent stream, mixing and heating occurs in
seconds. Besides giving control of temperature and pressure, this
rapid increase in temperature also leads to instantaneous crystal-
lisation. The process improves on ‘‘conventional’’ solvothermal
synthesis methods, where the mass or heat is poorly controlled.
The supercritical synthesis system used here [11,12] is inspired by
the work of several authors [13–15]. In the present setup, the
solvent is first pressurised and then pumped into a tube furnace
using standard air-driven liquid pumps. After reaching the desired
temperature and pressure, the solvent meets one or more reactant
solution(s) in the reactor block, and reaction then takes place
immediately upon mixing as well as during the back flow through
the furnace. The superheating of the solvent string can be
maintained all the way to the mixing point, which is temperature
controlled by resistance heaters. Once outside the furnace rapid
cooling of the reaction mixture is performed and the pressure is
lowered by means of a proportional relief valve.
Nickel and cobalt can be produced by the reduction of nickel
and cobalt ions using hydrazine, as demonstrated by Li [16], who
synthesised nickel fine powder by heating a solution of an
inorganic nickel salt and hydrazine in an autoclave. It was of
interest to explore if this synthesis route could be adapted to a
continuous flow system and the possibility of achieving control
over particle size and size distribution. Use of a continuous flow
reactor is inherently preferable for industrial production com-
pared with batch processes. As described by Li [16], a precipitate
The interest in nanoscale metal powders is motivated by the
improved characteristics of the pure metals and alloys. On a
macroscopic scale, they may exhibit significantly improved
hardness and strength, and other uses include microelectronics,
catalysis, and magnetism [1,2]. When the literature concerning
synthesis of fine nickel particles is reviewed, it falls roughly in two
categories: (i) either no surfactants are used, resulting in broad
size distributions and relatively large particles (hundreds of
nanometres) [3–5] or (ii) sol–gel processes and surfactants are
used, resulting in small particles with narrow size distributions
[6–8]. In the latter case, the particles are however coated in a layer
of surfactant, which is undesirable for some applications and
sol–gel is typically a batch process, which may complicate large-
scale synthesis.
Here, we report on the synthesis of nickel nanoparticles using a
continuous flow supercritical system that allows quick explora-
tion of synthesis parameter space (temperature, pressure, and
concentration). Synthesis of nanoparticles in supercritical fluids
has in recent years been the topic of many studies [9]. Super-
critical fluids exhibit highly tuneable properties and often the
solubility of a particular compound can be changed drastically by
small changes in supercritical pressure or temperature. Reduction
of nickel in near-critical water has been demonstrated by Adschiri
et al. [10] using gaseous hydrogen as the reducing agent. This
Ã
Corresponding author. Fax: +45 8619 6199.
E-mail address: bo@chem.au.dk (B.B. Iversen).
0022-4596/$ - see front matter & 2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2008.06.041

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P. Hald et al. / Journal of Solid State Chemistry 181 (2008) 2681–2683
of pink hydrazine–nickel complex is formed upon mixing
solutions of hydrazine and nickel ions. Hydrothermal treatment
of the precipitate produces nickel particles, but from the reported
transmission electron microscopic (TEM) images of the particles it
is impossible to provide reliable size estimates (probably between
50 and 200 nm) [16]. A suspension is not ideal for synthesis in the
present apparatus, but it was found that the precipitation of the
complex could be delayed for almost an hour by the addition of
ammonia.
the continuous flow synthesis, the synthesis of Li et al. [16]
was reproduced both with and without addition of ammonia.
A suspension of the nickel–hydrazine complex was treated in an
autoclave for 3 h at 150 1C, and this gave a pure nickel powder
with a size of 50.9(2.6) nm without NH3 addition and 44.6(1.6) nm
with NH3 addition.
3. Results and discussion
2. Experimental method
Treating the ammonia stabilized nickel–hydrazine complex
in the present continuous-flow apparatus lead to a clear effect on
the size of the particles when the synthesis temperature was
varied. As illustrated in Fig. 2, the particle size increased
with temperature, from 38.8(1.6) nm at 150 1C to 59.3(4.5) nm
at 350 1C.
At temperatures below 150 1C, the product was nickel
hydroxide, and a trace of nickel hydroxide is still visible in the
150 1C diffractogram shown in Fig. 3. At temperatures above
350 1C, no product was recovered. This can be explained in two
ways. Either as a physical effect, where the nickel particles
precipitate in the reactor tube, because they cannot be suspended
in the supercritical phase (the critical point of water is 374 1C
and 22.1 MPa), or as a chemical effect where hydrazine decom-
poses at the elevated temperature before the nickel ions are
reduced.
In total, 5.60 g of NiSO4 Á 6H2O (0.0213 mol) was dissolved in
360 mL distilled water. Around 120 mL 2 M ammonia solution
(0.24 mol) was slowly added while stirring to give a deep blue
solution. Immediately before the synthesis, 6.0 mL of hydrazine
hydrate (98%, 0.123 mol) was added, resulting in a Ni2+ concen-
tration of 0.044 M. The precursor solution and the preheated
distilled water were both pumped continuously at a rate of 10 mL/
min. The two liquids were mixed in the T-piece and the residence
time in the hot reactor tube after mixing was kept constant at 1 min.
The pressure was kept constant at 30.0 MPa after initial tests showed
no effect of pressure on the synthesis product. The synthesis
temperature was varied from 150 to 350 1C. As there was no gelling
or surfactants in the reaction mixture, the nickel particles could be
easily isolated from the reaction mixture using a neodymium
magnet through the wall of a beaker while decanting the aqueous
phase. Using the conditions listed above, 1.5 g of nickel nanoparticles
is produced in 1 h.
A two-fold reduction in the concentration at a constant
temperature of 200 1C has virtually no effect on the particle size,
whereas
a four-fold decrease of concentration leads to a
The products were characterised using powder X-ray diffrac-
tion and crystallite sizes were estimated from fitting a Voigt
function to the (100) peak. Correction for the instrumental profile
broadening was performed using the width obtained from Voigt
fit of a LaB6 standard sample [17,18]. The products were also
characterised by TEM (data not shown), but as found by Li et al.
the images reveal limited quantitative information about the
particle size. However, in Fig. 1 a scanning electron microscopic
(SEM) image of the product is shown, which confirms the size
estimates obtained by PXRD (see below). As a reference point for
significant increase in particle size. Lower concentrations are
normally expected to yield smaller particles, and the result
warrants further investigation. As seen in Fig. 3, nickel hydroxide
was also produced in the dilution experiments presumably
because of the reduced concentration of hydrazine. Other
syntheses performed with the same apparatus have all shown
a
positive correlation between the crystallite size and the
concentration of reactant. An example is the syntheses of titanium
dioxide in a supercritical isopropanol–water mixture, where
a
distinct reduction in size was observed with decreasing
70
65
60
55
50
45
40
35
Temperature series
30
25
20
Concentration x 1/2
Concentration x 1/4
Linear fit of temperature series
15
10
5
0
150
200
250
300
350
Temperature (°C)
Fig. 2. Crystallite size as a function of temperature and reactant concentration.
The size uncertainties were estimated from the peak-width uncertainties of the
(100) peak.
Fig. 1. SEM image of nickel particles obtained from flow synthesis at 350 1C. The
product is spheroid particles assembled in larger agglomerates.

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2683
Fig. 3. Powder X-ray diffraction patterns of as-synthesized particles. The arrows mark reflections from nickel hydroxide impurity.
concentration of reactants [11]. However, since the reaction
mechanisms vary, comparisons should be made cautiously.
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The hydrazine reduction route has been successfully adapted
to continuous flow synthesis of nickel nanoparticles in near-
critical water. Thus, pure nickel nanoparticles can be continuously
produced in seconds rather than hours, and the particle size can
be controlled between 40 and 60 nm by increasing the reaction
temperature from 150 to 350 1C. On the other hand, variation of
pressure or reactant concentration does not lead to a clear effect
on the particle size. The synthesis system described here is
comparatively cheap, and pressure, temperature, flow rate, and
reactant concentrations can be changed continuously. In the
present bench-top configuration with 6 mm inner diameter of
the reactor tubes, approximately 1.5 g of nickel nanoparticles can
be synthesised per hour. Since the continuous flow reactor in
principle is scalable, continuous production of nickel nanoparti-
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the apparatus. Ongoing studies concentrate on characterisation of
the particle size distributions using small angle X-ray scattering
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