Materials Sciences and Applications, 2011, 2, 444-452
doi:10.4236/msa.2011.25059 Published Online May 2011 (
Copyright © 2011 SciRes. MSA
Surfactant-Free Production of Ni-Based
Yaser Bahari Mollamahaleh1, Davood Hosseini2, Mehdi Mazaheri3, Sayed Khatiboleslam Sadrnezhaad4*
1Institute for Nanoscience & Technology, Sharif University of Technology, Tehran, Iran; 2Department of Physics, University of
Barcelona, Barcelona, Spain; 3Swiss, Federal Institute of Technology in Lausanne (EPFL), Lausanne, Switzerland; 4Center of
Excellence for Advanced Materials, Department of Materials Science and Engineering, Sharif University of Technology, Tehran,
Received April 2nd, 2011; revised April 18th, 2011; accepted May 6th, 2011.
This paper introduces a facile surfactant-free method for fabrication of different types of Ni-based nanostructures in-
cluding metallic nickel nanoparticles (MNNP), nickel oxide nanoparticles (NONP) and chip-like nickel oxide nanof-
lakes (CNONF) by solvothermal technique at 190˚C. Nickel acetyl acetonate (Ni( was used as nickel precursor
for both MNNP and CNONF and NiCO3·2Ni(OH)2·nH2O was utilized for NONP. Organic alcohols including 1-hexanol
and benzyl alcohol were used as solvent to produce all powders. The crystallite sizes of MNNP, NONP and CNONF
were determined by X-ray diffraction (XRD) to be 30, 9 and 27 nm, respectively. Electron microscopy indicated final
particle sizes of 80 nm and 20 nm for MNNP and NONP, respectively and a thickness-layer less than 90 nm for CNONF.
Brunauer-Emmett-Teller (BET) experiment determined a high surface area of 68 m2/gr for CNONF.
Keywords: Nanoparticles, Nickel, Nickel Oxide, Chip-Like Nanoflakes, Solvothermal Technique
1. Introduction
Powder synthesis with controlled dimension, shape and
size-distribution has been a major challenge in colloid
chemistry for decades. Nanoscience research has so far
been mostly pertained to the development and synthesis
of nanopowders and nanostructures [1]. Nanostructured
particles have, in fact, engrossed many practitioners by
outstanding physical and chemical properties as well as
many fascinating behaviors [1,2]. Introducing a facile
approach for fabrication of various morphologically-
controlled nanopowders is of great significance to the
technological advancement of the subject.
Metallic nanoparticles have attracted much attention
during past two decades due to their unique properties
and widespread potential applications [3]. MNNP is a
distinct example which has recently engrossed practi-
tioners by its potential applications in magnetic sensors,
memory devices, conducting materials and catalysts [4].
Although techniques such as laser-ablation and evapora-
tion-condensation have been applied to fabrication of
metallic nanoparticles, high instrumental costs, compli-
cated vacuum operation and low production rate usually
associated with vacuum-based methods are still of major
concerns [3].
A few solution based chemical methods has been de-
veloped to prepare metallic nanoparticles [3]. Practices
such as thermal decomposition [5,6], supercritical alco-
hol treatment [3], thermolytic controlled reaction [4],
gelatin induced reduction [7] and electroless deposition
[8] have successfully been applied to produce MNNP. A
simple technique to circumvent the problem of aqueous
chemistry is to synthesize the organic solvents with ex-
clusion of water. Niederberger and his coworkers [11,12]
introduced a general method for synthesis of inorganic
nanomaterials using organic chemistry in their published
reports [1,9-12]. This method called nonaqueous or non-
hydrolytic pathway has several advantages over the
aqueous ways. Surfactant-free fabrication, high crystal-
linity of the particles, dual role of the organic solvent (as
solvent and ligand), reducing metal oxide synthesis com-
plexities and introducing size/shape controlling facilities
are some examples [1,9].
In nonaqueous synthesis, the organic solvent besides
the ordinary effect can also cap the surface of the par-
ticles [1,8,12]. This effect minimizes the amount of the
surfactant needed. Nonaqueous synthesis can also help to
Surfactant-Free Production of Ni-Based Nanostructures
Copyright © 2011 SciRes. MSA
eliminate the calcination step of the metal oxide fabrica-
tion process which usually has particle growth effect.
Surfactant-assisted methods show outstanding control
over grain size by capping of the nanoparticle surfaces
[13]. Surface capping provides several other advantages
like low agglomeration, good dispersibility, shape con-
trol and potential to tailor surface properties of the pro-
duced nanoparticles [9]. Nanomaterial toxicity due to the
surface attachments is, however, a major drawback espe-
cially in the gas sensing and catalysis practices [9].
Nickel oxide—usually considered as a model of p-type
material—is a promising antiferromagnetic semiconduc-
tor with wide band gap of 3.6 - 4.0 eV [14]. Nanocrystal-
line NiO is expected to possess improved properties
when compared to micrometer-sized NiO particles due to
quantum size, surface, volume and macroscopic quantum
tunneling effects [15]. NiO nanostructures have, indeed,
attracted vast number of researchers because of its
chemical stability, excellent and unique electrical, optical
and magnetic properties, outstanding catalysis effects and
application in gas sensing, chemical sensing (particularly
as a negative electrode in Li-ion batteries and fuel cells),
electrochromic behavior and magnetic and electrochem-
ical super-capacitance [14-17]. Various procedures such
as thermal decomposition [18,19], microemulsion [20],
precipitation [13,15,16,21], electrodeposition [22, 23]
sonochemistry [24] and nonaqueous [25] methods have
been applied for NiO fabrication.
The solvothermal method is economically feasible for
preparation of the monodispersed metal oxide particles of
various shapes and sizes [12]. It exhibits the advantages
of simplicity, high yield and straightforward particle-size
control [21]. Solvent-directed processes involve the reac-
tion of metal oxide precursor(s) with a common organic
solvent and usually take place at lower temperatures
(50˚C - 250˚C). Small number of reactants (precursor and
solvent) makes it easily possible to study the chemical
mechanisms. Solvothermal methods have exhibited great
success in synthesizing the various crystalline metal-
oxide nanomaterials for many systems but a few pub-
lished works have reported simple solvothermal method
for Ni and NiO synthesis [9]. In the present work, a gen-
eral surfactant-free nonaqueous solvothermal route is
presented for production of MNNP and NONP nanopar-
ticles. Method of synthesis of chip-like NiO nanoflakes
having nanometric thicknesses with smooth surfaces are
also presented.
2. Experimental Procedure
The schematic representation of the procedures used in
this research is illustrated in Figure 1. Both scanning
electron microscopy (SEM) and transmission electron
microscopy (TEM) are used to determine the nanocristal-
line morphology of the produced samples.
2.1. Synthesis of MNNP
Ni( nickel complex (1.0 gr) was dissolved into an
organic solvent including organic alcohols listed in Ta-
ble 1. The mixture was magnetically stirred until a ho-
mogenous solution was obtained. During stirring, the
temperature of the solution was gradually increased up to
about 70˚C. A Teflon-lined stainless steel autoclave was
Figure 1. Schematics of the procedures used in this research for production of different Ni-base nanostructures.
Surfactant-Free Production of Ni-Based Nanostructures
Copyright © 2011 SciRes. MSA
Table 1. Solvents used in MNNP fabrication tests.
Sample Benzyl alcohol
Ethylene glycol
S1 100 - -
S2 20 80 -
S3 - - 100
filled with the homogenous solution up to 75 vol% and
sealed and placed in a furnace which was preheated to
190˚C. The autoclave was kept in the furnace for about
24 h at 190˚C. The autoclave was then pulled out of the
furnace and cooled down freely in atmosphere. This step
was usually accompanied with formation of a black pre-
cipitate. The black precipitate was then filtered and re-
peatedly rinsed (5 times) with pure ethanol. It was then
dried for 24 h at 60˚C in oven. These cleaning procedures
were performed to remove the chemicals possibly ad-
sorbed on the surface of precipitates to reduce the ag-
glomeration potential.
2.2. Synthesis of NONP
NiCO3·2Ni(OH)2·4H2O (1.0 gr) was dissolved in an or-
ganic solution containing 50 cc benzyl alcohol and 10 cc
1-hexanol. The mixture was magnetically stirred until it
became homogenous. Following the same procedure as
that for MNNP, a green precipitate was deposited. The
precipitate was then filtered and repeatedly rinsed with
absolute ethanol (5 times) and dried for 24 h at 60˚C in
oven. The dried powder was finally calcined at 350˚C for
about 1 h. The product was a black powder collected for
further investigation.
2.3. Synthesis of CNONF
Nickel acetyl acetonate (Ni( (1.0 gr) was dis-
solved in an organic solvent which included 10 cc benzyl
alcohol and 40 cc 1-hexanol. Magnetic stirring of the
mixture was continued until a homogenous solution was
formed. Hydrogen peroxide was added according to the
same procedure as specified for NONP until CNONF
was precipitated.
3. Results and Discussion
The XRD pattern of the black sediment precipitated from
S1 and S2 solutions (Table 1) are illustrated in Figure 2.
All diffraction peaks are indexed as cubic Ni (a = 3.523
Å) according to the standard JCPDS card No. 04-0580.
From Figure 2, one can infer the high crystallinity of
MNNP which is consistent with the previous findings
[9,11]. The crystallite sizes obtained from the XRD pat-
terns are determined to be 25 nm and 19 nm for S1 and
S2 solutions, respectively. The average crystallite size of
Figure 2. XRD pattern of the Ni nanoparticles in: (a) S1 and
(b) S2.
the powders is determinable by applying the Scherrer
where d is the size of the crystallites, k is a constant de-
pending on the crystalline shape, λ is the wavelength, β is
the half-maximum full-width of the intensity peak and θ
is the diffraction angle of the sample.
Black color of the solutions confirmed the presence of
metallic nickel in all experiments. SEM images of the
MNNP products are depicted in Figure 3. Benzyl alcohol
results in the spherical morphology of the MNNP sedi-
ment. The powder has uniformly distributed particles of
120 ± 10 nm size with low agglomeration. Figure 3(a)
indicates MNNP particle size-control due to the benzyl
alcohol capping effect. Benzyl alcohol seems, therefore,
a good solvent of mild surfactant nature. In comparison
with S1, growth of the particles is reduced in S2 (Figure
3(b)). Majority of the particles in Figure 3(b) have a size
of about 90 ± 5 nm. It seems, therefore, that by adding
more hexanol to the solution, size control of the MNNP
sample can be improved and the particle size can dra-
matically reduce. These can be described as follow: al-
cohol molecules can surround Ni( in the solution.
When the temperature increases above 180˚C - 190˚C,
Ni( breaks into its components (Ni and organic
ligand) and Ni nucleation causes mixture change into
black color. The organic solvent acts, on the other hand,
as a surfactant and controls growth of the Ni nucleates
while preventing them from agglomeration. Organic sol-
vent and organic chain of the broken Ni( both act
as barriers for diffusion of the particles. This mechanism
is similar to the steric stabilization. Despite the spatial
conformation of hexanol, when both hexanol and benzyl
alcohol are applied to the solution, capping occurs more
effectively than sole presence of benzyl alcohol.
It is interesting to note that the utilized synthesis me-
thod is not restricted to a specific organic solvent. While
the MNNP particle size is crucially affected by the length
of the organic solvent. The mean particle size of S3
(Figure 3(c)) produced in ethylene glycol is, for ex-
Surfactant-Free Production of Ni-Based Nanostructures
Copyright © 2011 SciRes. MSA
(a) (b)
(c) (d)
Figure 3. SEM images of: (a) S1, (b) S2, (c) S3 and (d) NONP sediments. All S1-S3 samples indicate spherical mor phology. S2
has the smallest particle size.
ample, more than 200 nm. This can be related to the short
length of the ethylene glycol chain. Song et al. [17] have
reported that ethylene glycol can serve as a ligand react-
ing with metal ions to form linear coordination complex-
es. They have clearly observed that this cannot lead to a
nanoscale powder. It is worth mentioning that all the
samples have had a spherical morphology. Ni particles
have, in fact, had a tendency to form spherical shapes in
the organic solvent to minimize their surface free energy.
In spite of the long aging time, Ostwald ripening would
have occurred leading to a nearly uniform particle size
distribution [26]. In the Ostwald ripening mechanism,
large particles grow at the expense of the smaller par-
ticles which are more soluble [26]. Niderbeger has re-
ported on the use of benzyl alcohol as a solvent [9]. This
has resulted in formation of metal oxide nanoparticles [9].
Benzyl alcohol here has followed a different way which
has resulted in the formation of the metallic nanoparticles.
Surfactant-Free Production of Ni-Based Nanostructures
Copyright © 2011 SciRes. MSA
It has been proposed, additionally, that the use a solvent
with a longer chain can effectively lower the size of the
Present work shows a facile single-step nonaqueous
route for synthesis of the MNNP particles. With previous
synthesis methods, if Ni( is replaced with
NiCO3·2Ni(OH)2·4H2O, a green precipitate is obtained.
The XRD pattern shows no obvious diffraction peak in-
dicating this precipitate (Figure 4(a)). The FTIR spec-
trum of the green precipitate is shown in Figure 5. The
broad absorption band centered at 3400 cm1 is attributed
to the O-H band stretching vibrations. The band at 1615
cm1 attributes to (H-O-H) bending mode. Wave num-
bers of 1457 cm1 and 1383 cm1 can be assigned to
bending vibrations of the CH3 group, δ(CH3). The band
at 1383 cm1 is also primarily related to the banding vi-
bration of the ionic 2
CO . The strong band at 420 cm1
corresponds to the stretching mode of the NiO particles.
Figure 6 shows the TG-DTA decomposition curve of
the green precipitate. From room temperature to 260˚C,
the weight loss can be caused by desorption of water
from the surface of the green precipitate. The major
weight loss happens from 260˚C to 300˚C which accom-
panies a sharp DTA peak. The weight loss value attri-
buted to this transformation is close to the thermal de-
composition or the dehydration of Ni(OH)2 to form NiO
particles. Based on characterization investigations, one
can conclude that the green precipitate is Ni(OH)2 with
low crystallinity. According to JCPDS Nu. 44-1159, all
XRD peaks of the calcined product correspond to the
NiO structure (Figure 4(b)). Applying the Debye-Scher-
rer formula, the crystallite size of the as-prepared NiO is
calculated to be 9 nm.
Figure 3(d) shows highly agglomerated NONP par-
ticles. Substitution of nickel-organic substance with an
inorganic nickel compound has resulted in a totally dif-
ferent effect on conformation of the molecules rather
Figure 4. XRD pattern of the green NONP precipitate: (a)
before calcination and (b) after calcination at 350˚C.
Figure 5. FTIR spectrum of the green NONP precipitate
before calcination.
Figure 6. TG-DTA curve of the green NONP precipitate.
Surfactant-Free Production of Ni-Based Nanostructures
Copyright © 2011 SciRes. MSA
than their pathway and composition. Not only the solvent
is, hence, important in determining the property of the
final product, the initial precursor composition is also
significant. A totally different chemistry is, hence, do-
minant for each product. The nonaqueous synthesis is a
one-step process for preparation of the MNNP; while the
(a) (b)
Figure 7. TEM NONP: (a) nanostructural image and (b) diffraction pattern.
Figure 8. XRD pattern of CNONF green precipitate: (a) before calcination and (b) after calcination at 350˚C.
Figure 9. TG-DTA curve of CNONF green precipitate.
Surfactant-Free Production of Ni-Based Nanostructures
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two-step NiO preparation is based on the organic-inor-
ganic reaction which accompanies thermal decomposi-
tion. Figure 7 illustrates the TEM image of the NONP
sample. The size of the relatively low agglomerated par-
ticles is estimated about 20 nm.
BET analysis showed the surface area of the NONP
particles to be 33 m2/gr. Assuming that the particles are
same-sized spheres with smooth surfaces, the surface
area can be related to the average equivalent particle size
by d = 6000/(ρ·S) (in nm), where d is the average diame-
ter of the spherical particles in nanometer; ρ is the theo-
retical density of NiO (6.67 g/cm3) and S represents the
measured surface area of the powder in m2/g. The spe-
cific surface area is determined to be 33 m2/g; while the
equivalent average particle size evaluated by TEM is ~
27 nm. The particle sizes obtained from BET and TEM
are, thus, in good agreement.
The third product is CNONF. The initial procedure for
its production is similar to S1 except that 10 cc hydrogen
peroxide is added to the solution which results in a green
precipitate like NONP; but different in its appearance for
being agglomerated. The XRD pattern of the green pre-
cipitate is shown in Figure 8(a) indicating Ni(OH)2 (03-
0177) with crystallite size of 27 nm determined by De-
(a) (b)
(c) (d)
Figure 10. SEM image morphologies of CNONF at magnification: (a) 2 kx, (b) 4 kx, (c) 10 kx and (d) 30 kx.
Surfactant-Free Production of Ni-Based Nanostructures
Copyright © 2011 SciRes. MSA
Figure 11. TEM CNONF: (a and b) images and (c) diffrac-
tion pattern.
bye-Scherrer correlation. In the TG-DTA graph (Figure
9), one can observe the simple decomposition of Ni(OH)2
to NiO below 300˚C without any other pertinent weight
loss effect. Diffraction peaks of Figure 8(b) belong to
NiO structure with crystallite size of 30 nm according to
JCPDS Nu. 44-1159. Morphology of the as-produced
NiO nanostructure is illustrated in Figures 10 and 11.
SEM images of platelet chip-like structures belonging to
NiO (Figure 10) is confirmed by TEM images shown in
Figure 11(a) having less than 90 nm thickness (Figure
11(b)) and some crystallinity presence (Figure 11(c)).
BET analysis showed surface area of 68 m2/gr for the
as-produced CNONF which was about two times that of
NONP. Due to its high surface area, the prepared struc-
ture seemed useful for catalytic application.
4. Conclusions
Different chemistries are applied to similar solvothermal
procedure to achieve three nickel-based nanostructures:
MNNP, NONP and CNONF. With the aid of nonaqueous
chemistry, a spherical uniform morphology is obtained
for metallic nickel. The starting solvent has crucial role on
the particle size of the powder. Based on the SEM results,
the particle sizes of the sediments precipitated from S1
and S3 are 110, 90 and 300 nm. A long chain organic
compound mixed with (or even without) benzyl alcohol
seems to be able to effectively reduce these particle sizes.
By application of the aqueous-organic chemistry into the
system, the NiO nanostructure can be obtained as small as
20 nm. Using an oxidant such as H2O2, chip-like nickel
oxide nanoflakes less than 90 nm in thickness and having
an interesting morphology is obtained. BET results show
highly active surface areas of 68 m2/g for CNONF and 33
m2/g for NONP synthesized powders.
5. Acknowledgements
The authors wish to thank the financial support of the
Iran National Science Foundation.
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