Electrohydrodynamic atomization (EHDA) assisted wet chemical synthesis of nickel nanoparticles

Article abstract of DOI:10.1016/j.materresbull.2012.03.055

In this study nickel nanoparticles were prepared via chemical reduction of nickel acetate using sodium borohydride using electrohydrodynamic atomization (EHDA) technique. This technique was used to spray a finely dispersed aerosol of nickel precursor solution into the reductive bath. Obtained particles were characterized by means of X-ray diffraction (XRD), UV-Visible spectroscopy, and transmission electron microscopy (TEM). Results confirmed the formation of nickel nanoparticles and showed that applying EHDA technique to chemical reduction method results in producing smaller particles with narrower size distribution in comparison with conventional reductive precipitation method.

Full text of DOI:10.1016/j.materresbull.2012.03.055

Materials Research Bulletin 47 (2012) 1666–1669
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Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Electrohydrodynamic atomization (EHDA) assisted wet chemical synthesis
of nickel nanoparticles
*
M. Barzegar Vishlaghi, M. Farzalipour Tabriz , O. Mohammad Moradi
Department of Mechanical Engineering, University of Tabriz, Tabriz, Iran
A R T I C L E I N F O
A B S T R A C T
Article history:
In this study nickel nanoparticles were prepared via chemical reduction of nickel acetate using sodium
borohydride using electrohydrodynamic atomization (EHDA) technique. This technique was used to
spray a finely dispersed aerosol of nickel precursor solution into the reductive bath. Obtained particles
were characterized by means of X-ray diffraction (XRD), UV–Visible spectroscopy, and transmission
electron microscopy (TEM). Results confirmed the formation of nickel nanoparticles and showed that
applying EHDA technique to chemical reduction method results in producing smaller particles with
narrower size distribution in comparison with conventional reductive precipitation method.
ß 2012 Elsevier Ltd. All rights reserved.
Received 28 November 2011
Accepted 21 March 2012
Available online 29 March 2012
Keywords:
A. Metals
B. Chemical synthesis
C. Electron microscopy
1. Introduction
are inevitable [17,18]. However, oxidation of metal nanoparticles
during their synthesis cannot be completely avoided [19]. Shape
In recent years, transition metal nanoparticles have attracted
much interest due to their unique electrical, optical, chemical, and
magnetic properties which makes them useful in various fields
such as microelectronics, optoelectronics, catalysis, and magnetic
information storages [1,2]. Due to high dependence of physical and
chemical properties of metal nanoparticles on their shape and size
distribution [3–5], extensive studies have been done on controlling
these parameters in various synthesis methods. However, achiev-
ing nanoparticles with smaller size and narrow size distribution is
still a challenging issue. Among transition metal nanoparticles,
nickel nanoparticles have attracted considerable attention which is
mostly due to their magnetic and catalytic properties [4,6–8].
Nickel nanoparticles have a wide range of applications such as in
rechargeable batteries [5], chemical catalysts [9], and multilayer
capacitors [9,10]. Various chemical methods have been developed
for the preparation of Ni nanoparticles such as chemical vapor
deposition [11], electroless plating [12], polyol process [13],
hydrothermal method [14], wet chemical reduction [15], and
microemulsion method [16]. In comparison, chemical reduction
method has many advantages. It is a simple, inexpensive and the
most versatile method for preparing metallic nanoparticles. In
chemical reduction method, metal ions in metal salt solution are
reduced to metal particles by a reducing agent. Due to high
tendency of transition metal nanoparticles to oxidation and
agglomeration, using appropriate surfactants or capping agents
and size distribution of particles in chemical reduction method can
be easily controlled by adjusting synthesis parameters, e.g. solvent
composition, reactants concentration, reaction temperature, sur-
factant type and concentration [20–23]. One of the most effective
parameters on particle size in this synthesis method is the size of
reactant droplets. The droplets with smaller size contain fewer
reactant ions which lead to generation of smaller nanoparticles in
reductive bath. However, decreasing droplet size by simple
pouring or dripping of solutions in conventional chemical
reduction method is limited due to technological reasons. Recently
the advanced technique of electrospray has introduced new routes
to nanotechnology. Electrospraying, which is also referred as
electrohydrodynamic atomization (EHDA), is a technique which
uses strong electric fields for dispersing liquids into fine droplets.
In this technique, the liquid flows out of a highly charged capillary
nozzle and is forced to be dispersed into fine droplets by electric
field. Due to repulsive forces between charged droplets, they are
self dispersing in space which prevents droplets coagulation.
Electrospray droplets have narrow size distribution and depending
on electric field magnitude, their size can range from hundreds of
micrometers down to several tens of nanometer [24,25]. EHDA
technique can be easily applied to chemical reduction method. By
electrospraying the metal precursor solution into reductive bath,
very fine nanoparticles with narrow size distribution can be
obtained. Recently, Valvo et al. reported the synthesis of tin
nanoparticles and Sn–Co compound for Li-ion batteries via
electrospraying [26,27]. The precursor droplets size, their velocity
and dispersion can be controlled by adjusting EHDA parameters
such as voltage magnitude and the flow rate of the liquid [24–27].
* Corresponding author. Tel.: +98 9147620971; fax: +98 4113356026.
E-mail address: meisam.fa@gmail.com (M. Farzalipour Tabriz).
0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.materresbull.2012.03.055

M. Barzegar Vishlaghi et al. / Materials Research Bulletin 47 (2012) 1666–1669
1667
In this paper, we present a novel wet chemical reduction
method with the assistance of electrospraying technique to
produce fine nickel nanoparticles with narrow size distribution.
We also investigated the effects of electrospraying on decreasing
the size of obtained Ni nanoparticles.
spraying used in this study are shown in Fig. 2. Droplet and jet
diameters were estimated to be about 2000 m (2 mm) and
30–40 m, respectively. A strong black color was observed
immediately after spraying started, which is corresponded to
the reduction of nickel ions into metallic nickel. Reduction reaction
could be expressed as:
m
m
2. Experimental
NiðCH3COOÞ þ 2NaBH4 ! Ni # þ 2CH3COONa #
2 þ 2BH3 " þ H2 "
(1)
2.1. Materials
Nickel acetate (Ni(Ac)2ꢀ4H2O), sodium borohydride (NaBH4),
cetyltrimethylammonium bromide (CTAB), methanol and butanol
were purchased from Merck and used without further purification.
Precipitates were separated from solution by centrifugation,
washed with methanol and acetone several times and dried at
70 8C.
2.2. Synthesis
2.3. Characterization
Nickel nanoparticles were synthesized by reduction of Ni2+
cations dissolved in the sprayed droplets of nickel precursor
solution. Different solvents were tried as precursor/reductive
solutions medium in order to achieve suitable solutions for
electrospraying and reduction processes. Butanol and methanol
were chosen as synthesis media due to their high stability in
electrospray. No water is used because it prevents forming a stable
electrospray due to its high electrical conductivity and also it
causes the formation of nickel hydroxide instead of Ni nanopar-
ticles. As precursor solution, nickel acetate was dissolved in a
mixture of methanol/butanol with volume ratio of 1:1. Concen-
tration of solution was 0.12 M. For preparing reductive bath,
cationic surfactant CTAB was dissolved in methanol and after
stirring, NaBH4 was added to the solution as a reducing agent. Due
to the spontaneous reaction of NaBH4 with methanol, about 40 wt%
more than calculated stoichiometric amount of NaBH4 was added
to the bath to ensure complete reduction of precursor. The
precursor solution was injected into the reductive bath through an
electrified capillary nozzle with inner diameter of 0.6 mm using a
syringe pump at a constant flow rate of 30 ml/h. A ring with
diameter of 2 cm made of stainless steel used as a counter
electrode. Capillary nozzle was fixed at the center of ring with no
distance between the end of the nozzle and ring center. The
schematics of the experimental set-up are shown in Fig. 1. In order
to achieve EHDA in multi-jet mode, a high voltage of 6.3 kV was
applied between the nozzle and ring. Dripping mode obtained by
applying no voltage to produce Ni nanoparticles by simple
reductive precipitation. The experiments were carried out in air
and at room temperature. Images of two different modes of
Obtained particles were characterized by X-ray diffraction using
a
D-5000 X-ray diffractometer with Cu-K
a
radiation
(k = 0.154178 nm). Particles shape and size were studied by Philips
EM208S (100 kV) Transmission Electron Microscopy. UV–Vis
Fig. 1. Schematics of experimental set-up.
Fig. 2. Images of (a) dripping and (b) multi-jet mode.

1668
M. Barzegar Vishlaghi et al. / Materials Research Bulletin 47 (2012) 1666–1669
spectra of nanoparticles were recorded by double-beam Shimadzu
UV-2450 Scan UV–Vis spectrophotometer.
X-ray diffraction patterns of synthesized nanoparticles are
shown in Fig. 4. No crystalline peak is detectable in diffraction
patterns of samples which is the typical feature of the amorphous
materials. This can be attributed both to the effectsof reducing agent
and high surface to volume ratio of particles produced with the
assistance of electrospraying. As it have been reported earlier
[21,28], reduction precipitation of metal nanoparticles by NaBH4
generally leads to generation of weakly ordered particles with small
sizes and largesurfaceareas. Thisisdueto the fastreducingnatureof
NaBH4 which causes instant formation of solid particles. On the
other hand, as indicated in TEM images, using EHDA is effective in
reducing particles size which results in increasing the contribution
of surface atoms and enhancing the surface reactivity. Thus,
formation of surface amorphous oxides and residual contaminants
are expected during the processing of materials. Similar results have
been reported recently by Valvo et al. for tin nanoparticles [26] and
Sn–Co compound [27] produced via electrospraying. From these
results it can be concluded that using electrospraying in reductive
precipitation and NaBH4 as reducing agent result in generating
small nanoparticles with increased surface areas which leads to
amorphization of collected precipitates.
3. Results and discussion
Morphology of synthesized Ni nanoparticles was investigated
by TEM and results are shown in Fig. 3. It can be seen that particles
produced in multi-jet mode (Fig. 3b) are spherical and have a
narrow size distribution with an average size of 2–4 nm. Although
particles are agglomerated, they are still discernable and no big
particle is observed. Particles produced in dripping mode (Fig. 3a)
are larger and less homogeneous in size distribution and shape in
comparison with particles produced in multi-jet mode. Size of the
particles in dripping mode is in the range of 10–20 nm. As it was
expected, the results show that applying electric field to dripping
mode clearly affects the morphology of nanoparticles and yields a
homogeneous distribution of smaller particles. SAED pattern
recorded from these nanoparticles is consistent with fcc structure
of crystalline nickel (inset at a).
UV–Visible spectroscopy of collected powders dispersed in
methanol was also carried out to confirm the formation of nickel
nanoparticles. Fig. 5 shows the UV–Visible absorption spectrum of
synthesized Ni nanoparticles produced in multi-jet mode. The
Fig. 4. XRD patterns of nickel nanoparticles synthesized in dripping and multi-
jet mode.
Fig. 3. TEM image of nickel nanoparticles synthesized in (a) dripping and (b) multi-
Fig. 5. UV–Visible absorption spectrum of Ni nanoparticles synthesized in multi-jet
jet mode and SAED pattern of particles (inset at a).
mode dispersed in methanol.

M. Barzegar Vishlaghi et al. / Materials Research Bulletin 47 (2012) 1666–1669
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