Size-controlled monodispersed nickel nanocrystals using 2-octanol as reducing agent

Article abstract of DOI:10.1039/c2ce26433a

Size-controlled nickel nanocrystals have been successfully synthesized using 2-octanol as the reducing agent. The elucidation of the reduction mechanism of nickel ions by 2-octanol was monitored by using UV-vis spectrometry, FT-IR spectrometry, X-ray diffractometry and transmission electron microscopy and the formation of intermediate layered hydroxides that subsequently undergoes reduction to metallic nickel was identified. Although, the nickel particles obtained in the 2-octanol-Ni acetate system were irregularly shaped with size in the sub-micrometer range, the addition of surfactants, hydroxyl ions and metal seed particles to the alcoholic system facilitated the synthesis of nanocrystals with controlled size and size distribution. The optimization of the synthesis conditions based on the above analytical data led to the preparation of a toluene-based suspension dispersing Ni nanoparticles with an average particle size of about 16 nm.

Full text of DOI:10.1039/c2ce26433a

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Size-controlled monodispersed nickel nanocrystals
using 2-octanol as reducing agent3
Cite this: CrystEngComm, 2013, 15,
29
7
a
a
a
b
Jhon L. Cuya Huaman, Nobuyoshi Hironaka, Shinya Tanaka, Kozo Shinoda,
a
a
Hiroshi Miyamura and Balachandran Jeyadevan*
Size-controlled nickel nanocrystals have been successfully synthesized using 2-octanol as the reducing
agent. The elucidation of the reduction mechanism of nickel ions by 2-octanol was monitored by using UV-
vis spectrometry, FT-IR spectrometry, X-ray diffractometry and transmission electron microscopy and the
formation of intermediate layered hydroxides that subsequently undergoes reduction to metallic nickel
was identified. Although, the nickel particles obtained in the 2-octanol–Ni acetate system were irregularly
shaped with size in the sub-micrometer range, the addition of surfactants, hydroxyl ions and metal seed
particles to the alcoholic system facilitated the synthesis of nanocrystals with controlled size and size
distribution. The optimization of the synthesis conditions based on the above analytical data led to the
preparation of a toluene-based suspension dispersing Ni nanoparticles with an average particle size of
about 16 nm.
Received 5th September 2012,
Accepted 6th November 2012
DOI: 10.1039/c2ce26433a
www.rsc.org/crystengcomm
tion limits their use in mass production. In contrast, aqueous-
Introduction
1
2–16
based routes
are more appropriate for scaling up,
In recent years, the synthesis of size and morphology
controlled nickel and its alloy nanoparticles have been a
subject of intensive research with the objective of enhancing
its electrical conductivity, magnetic properties and catalytic
however, the control over their sizes, size distribution,
morphology and optimization of the chemical and physical
properties are difficult.
On the other hand, the polyol method has been considered
1
–5
ability.
However, the main challenge in the synthesis of
an excellent route to synthesize high quality materials in large
nickel and other metals is the realization of particles with
different size ranges and shapes using a simple synthetic
protocol to cater the demand of different industries. For
example, in the case of nickel, while the catalysis industry
needs the smallest shape controlled nanoparticles, the
electronics industry requires sub-micrometer sized particles
with a narrow size distribution to be used in multilayer
17,18
scale.
The ability of polyol as a solvent and reducing agent
is suitable to obtain submicron sized particles; however, the
size reduction of nickel particles was not easily achieved even
through heterogeneous nucleation using metal seeds or fast
nucleation using highly reducing agents such as hydrazine or
1
9–21
sodium hydroboride.
However, the use of hydroxyl ions
have been proved to enhance the reduction capabilities of
polyol and this has been successfully demonstrated in the
3
,6,7
ceramic capacitors (MLCC).
Techniques to synthesize nickel particles with different
2
2,23
synthesis of iron and iron-based alloys.
Subsequently, this
8
–21
sizes and morphologies have been already reported.
technique was extended to the synthesis copper nanoparticles
for the preparation of conducting inks using long alkyl chains
Though some of these techniques are adequate to investigate
the physical and chemical properties of nanoparticles, for
example, the thermal decomposition or reduction of nickel
2
4
alcohols. Though the reduction potential of alcohols were
strong enough to reduce copper ions, specific amounts of
hydroxyl ions and surfactant such as oleylamine influenced
the end products and organic solvent dispersible surfactant
coated monodispersed copper particles with an average
diameter of y10 nm were produced in a one-pot reaction.
These results outperformed the products obtained in strongly
reducing ethylene glycol and hinted the use of weakly reducing
alcohols with appropriate additives for designed synthesis of
metal particles through finer control over the reduction
reaction.
8
–11
salts in high boiling point solvents,
the high reaction
temperature or the toxic compounds generated during reac-
a
Department of Materials Science, School of Engineering, The University of Shiga
Prefecture, Hikone, Japan. E-mail: jeyadevan.b@mat.usp.ac.jp;
cuya.j@office.usp.ac.jp; Fax: +81 749 28 8486; Tel: +81 749 28 8352
b
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University,
Sendai, Japan. E-mail: shinoda@tagen.tohoku.ac.jp; Fax: +81 22 217 5624;
Tel: +81 22 217 562
3
Electronic supplementary information (ESI) available: Picture of reaction
Thus, here we report the results of a study undertaken to
further extend this synthetic process to prepare Ni particles
products (Fig. SI), XRD patterns of oleylamine effect (Fig. SII) and TEM images of
hydroxyl effect (Fig. SIII). See DOI: 10.1039/c2ce26433a
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using normal alcohols. The synthesis process was monitored
to elucidate the reactions that take place during the reduction
of nickel acetate in alcohol in the presence and absence of
additives that promote the reaction on one hand and inhibit
the growth on the other. In the absence of any additives,
irregular shaped and agglomerated nickel particles were
obtained. However, the size and size distribution controlled
nickel particles were obtained when additives such as hydroxyl
ions, metal seed particles and oleylamine were used to control
the reaction kinetics, nucleation rate and growth, and
agglomeration, respectively. Thus, under optimized conditions
oleylamine coated nickel nanoparticles with diameters less
than 20 nm were obtained. The magnetic properties of the
particles and preparation of the suspension for the fabrication
of monolayered particle film to be used in carbon nanomater-
ial synthesis in chemical vapor deposition is also reported and
discussed.
using a thermogravimetry equipment (TG, Rigaku Thermoplus
2
1
8120) at a scanning speed of 10 uC min
under N
2
atmosphere and in the temperature range of room tempera-
ture to 500 uC.
Results and discussion
Elucidation of reaction mechanism
Alcohols such as 2-octanol have rarely been used as a reducing
agent for metal particles; only the authors have used this
solvent for the synthesis of metal nanoparticles such as
2
4
copper. One of the reasons is that long chain alcohols are
believed not to possess the necessary potential to reduce non-
noble metals. However, here it is demonstrated that this
solvent could also reduce metals with low reduction potential
2
5
such as nickel (20.257 eV ) in the absence of any reducing
reagents such as hydrazine or borohydride compounds. Thus,
the synthesis of nickel particles were carried out mixing
2
-octanol and Ni(Ac) at 180 uC for 4 hours. Fig. 1 shows the
2
Experimental
XRD patterns of the precipitates collected at 120 and 180 uC for
different reaction durations. For comparison, the XRD pattern
Materials
2
of Ni(Ac) powder used in the present study is also shown in
2
(
(
-Octanol (C
Ni(Ac) , Ni(CH
Cu(Ac) , Cu(CH
6
H
13CH(OH)CH
COO) ?4H O), copper(II) acetate monohydrate
COO) ?H O), dihydrogen hexachloroplatinate
PtCl ?6H O), ethanol (C OH), sodium
hydroxide (NaOH), oleylamine (C18 37N) and toluene (PhH
3
), nickel(II) acetate tetrahydrate
Fig. 1a. As observed, the XRD pattern of Ni(Ac) is not retained
2
2
3
2
2
even at low reaction temperatures such as 120 uC (Fig. 1b)
2
3
2
2
suggesting that Ni(Ac) reacts with 2-octanol at low tempera-
2
hexahydrate (H
2
6
2
2 5
H
3
tures forming a light green precipitate (Fig. SI, ESI ); however
the diffraction pattern did not match with known nickel
compounds. This precipitate could be related to the formation
H
3
)
were purchased from Wako Pure Chemicals Ltd., Japan. The
commercially available reagents were used without further
purification.
of layered nickel hydroxide, Ni(OH)22x(Ac)
brucite-like structure as was described by Fievet et al.,
obtained during the hydrolysis of Ni(Ac) in a mixture of
x 2
?nH O with a
2
6,27
Synthesis of Ni nanoparticles
2
propylene glycol, diethylene glycol, or ethanol and water.
However, the XRD patterns they obtained differ markedly with
the one observed in Fig. 1. The absence of additional water in
the system may have influenced the hydrolysis of nickel
acetate and led to the formation of layered hydroxide
The typical procedure to synthesize Ni particles is as follows:
2
0.1 M Ni(Ac) was mixed with 2-octanol into a 300 mL three-
necked flask. The solution was pre-heated at 120 uC for 2 hours
in order to remove volatile compounds. The reduction reaction
was carried out at 180 uC for 4 hours under constant
2
mechanical stirring and N purging. After cooling, the
particles were collected by centrifugation, washed with ethanol
and dried under vacuum for 2 hours. Then, the particles were
dispersed in organic solvent and stored.
Characterization
The optical properties of the reactants were analyzed using an
ultraviolet-visible spectrophotometer (UV-vis, Hitachi U2000)
using quartz cells in the range of 250 to 1000 nm and a Fourier
transform infrared spectrometer (FT-IR, Thermolite Scientific
2
1
Nicolet-IS5) in the range 500 to 4000 cm . The crystalline
phases of the intermediates and final precipitates were
identified by an X-ray diffractometer (XRD, Philips-Xpert)
using Cu Ka radiation. The size and morphology of the
intermediates as well as the final precipitates were determined
by using a transmission electron microscope (TEM, Hitachi
H8100, 200 kV). The room temperature magnetic measure-
ments were made using the vibrating sample magnetometer
(VSM, Tamagawa Seisakusho) under an applied field of 1 T
and the instrument was calibrated using Ni metal. Besides, the
thermal behaviour of the solid powders were examined by
Fig. 1 XRD patterns of Ni compounds reduced by 2-octanol at different reaction
temperatures and durations.
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30 | CrystEngComm, 2013, 15, 729–737
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2
8
structure. This green precipitate was very stable even at 180
uC for a reaction duration less than 2 h as observed in Fig. 1c–
e. However, after 2 h at 180 uC, a small peak that could be
assigned to the (111) plane of metal nickel was detected at
agglomerated with an average diameter of 460 nm (Fig. 2d).
The agglomeration is believed to have been promoted due to
the absence of surfactant in the system. Likewise, the SAED
pattern (inset in Fig. 2d) shows only the rings corresponding to
the (111), (200), (220) and (311) planes for pure polycrystalline
nickel.
4
5.07u. With prolonged reaction duration of 3 h, this peak
gradually shifted to lower angles but the value was still higher
than 44.5u given in the JCPDS No. 04-0850 for FCC nickel
In order to understand the reaction mechanism, FT-IR
spectrometry was used to analyse the liquid and solid
products. The spectra of the liquid products were compared
with those of pure 2-octanol (Fig. 3a). The O–H stretching
(Fig. 1f). This deviation is suggested to have been caused by
the structural defects generated by hydroxide impurities in the
2
9
structure of nickel metal. In addition, a background profile
was also detected from y20u toward higher angles caused by
the hydroxide dissolution in favour of the formation of nickel
2
1
vibration band at 3336 cm , the symmetric and asymmetric
2
1
CH
2
stretching vibration peaks between 2959 and 2856 cm
bending vibration at 1458 and
375 cm were clearly seen in the spectra obtained for the
2
8
metal. However, the profile obtained after 4 h of reaction was
free of structural defects and the presence of sharp and high
intensity peak corresponding to the (111) plane of pure large
nickel particles was observed (Fig. 1g).
and those corresponding to CH
1
3
2
1
reacted liquid at different reaction temperatures and dura-
tions (Fig. 3b–g). It should be noted that the above peaks
Fig. 2 shows the TEM micrographs of the precipitates
corresponding to the XRD peaks shown in Fig. 1. The obtained
photographs of layered structures are in good agreement with
the results obtained by XRD. Then, the green powders
obtained at 120 and 180 uC were found to be constituted by
layered structures (Fig. 2a–c). After 2 hours of reaction,
isolated small particles with diameters y15 nm, which could
be ascribed to metallic nickel, were also observed together
with the layered structures (Fig. 2c). At the end of the reaction
correspond mainly to 2-octanol. Additionally, when Ni(Ac)
was introduced to the solvent and then heated at 120 uC, a
small peak was observed at 1713 cm (Fig. 3b). The intensity
of this peak increased with the progression of the reaction
2
2
1
(Fig. 3c–g). This peak could be assigned to CLO groups.
However, further analysis is necessary to determine whether it
is a consequence of the liberation of acetate ions from Ni
source or a product of esterification reaction of 2-octanol as
30
was observed in the synthesis of ZnO with benzyl alcohol.
(180 uC for 4 h), the particles were irregular shaped and
Additionally, the generation of 2-octanone, the main oxidizing
product of 2-octanol in presence of nickel salts, could also be
3
1
another possibility.
The FTIR spectra of the solid products are shown in Fig. 4.
Ni(Ac) , which was used as a reference, shows a sharp peak at
2
2
1
21
3
466 cm , as well as a broad band centered at 3099 cm
characteristic of O–H stretching vibrations due to structural
2
water content, and the symmetric and asymmetric COO
2
1
vibrations of acetate ligand at 1504 and 1414 cm (Fig. 4a).
The spectrum of the precipitate obtained by reacting nickel
acetate with 2-octanol at 120 uC exhibited several peaks at
2
1
3
559, 3437, 3276 and 3029 cm assigned to O–H stretching
Fig. 2 TEM micrographs of the product obtained at (a) 120 and 180 uC for
different reaction durations: (b) 0, (c) 2 and (d) 4 hours. Inset in (d) shows the
SAED pattern of metallic nickel particles.
Fig. 3 FT-IR spectra of the liquid solutions collected at different reaction
temperatures and durations.
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Fig. 4 FT-IR spectra of the liquid solutions collected at different reaction
temperatures and durations.
Fig. 5 UV-vis spectra of the suspension obtained at 180 uC for different reaction
durations: (a) 0, (b) 1, (c) 2, (d) 3 and (e) 4 hours.
vibrations. These peaks are related to different O–H bonding
2
7
in layered nickel hydroxides. The presence of water in the
2
1 27
system can also be observed at 1608 cm
hand, strong CH stretching vibration peaks corresponding to
-octanol detected at 2958 and 2857 cm in Fig. 3a did not
.
On the other
below 200 uC and registering a weight loss of 31.9%. This value
is comparable to the theoretical structural water content in the
nickel source (29.0%). The second step is due to the
decomposition of acetate, which occurs between 250 and 395
uC. A weight loss of 38.1% was registered in this step, which is
lower than the theoretical value of 47.5% to get metal Ni. This
accounts for a total weight loss of 70%, which is closer to the
theoretical value to obtain NiO, in contrast to 76.4% for nickel
metal. On the other hand, the TG curve of the sample collected
at 120 uC is shown in Fig. 6b. This exhibited three steps: (1) at
temperatures below 120 uC, the dehydration of sample
occurred with a weight variation of 5.7%; (2) between 120 uC
2
2
1
2
appear in the solid. This was believed due to the removal of
alcohol from the surface of precipitates during the washing
step. Whereas peaks ascribed to symmetric and asymmetric
COO vibrations are more intense and shifted towards 1557
and 1398 cm due to the coordination of acetate anions to
nickel hydroxide layers. The hydroxyl and acetate peaks start
to lose intensity after 3 hours of reaction; however, they
remained even at the end of reaction as impurities on metal
surfaces.
Fig. 5 shows the UV-vis spectra of suspensions obtained by
reducing nickel acetate at 180 uC with 2-octanol for different
reaction durations. In this analysis, 2-octanol, which was used
to dilute the reactant containing the precipitate, was used as a
blank. At 180 uC, the light is absorbed in almost all the
analyzed range and four absorption peaks at 305, 395, 419 and
65 nm were observed. Except for the peaks at 305 and 419
cm , the rest could be ascribed to the Ni(II) in octahedral
As the reaction progressed, these
remained and only a reduction in intensity was observed. At
the end of reaction, only the peaks at 305 and 419 cm were
detected, but slightly shifted to lower wavelengths, 291 and
05 cm (Fig. 5e). These absorption peaks could be related to
the generation of aldehyde or ketone that corresponds to the
UV-vis fingerprints found at y300 nm. On the other hand,
2
2
1
3
2
2
6
and 220 uC, continuous dehydroxylation accounted for an
additional weight loss of 7.2% and (3) at higher temperatures,
the decomposition of acetate led to a weight loss of 44.5%. The
results are in good agreement with the formation of layered
Ni(OH) (Ac) ?nH O compounds where interlamellar water,
x
y
2
hydroxyl ions and acetate are removed at different steps.
Likewise, this behaviour was also observed in the samples
obtained at 180 uC until 2 h of reaction (Fig. 6c). After 3 h of
reaction, the dehydroxylation step is not clearly observed but
the sample showed 14.5% weight loss, which is comparable to
the already reported value (Fig. 6d). These phenomena could
be associated to the dissolution of the layered hydroxides. For
the sample obtained after 4 hours of reaction, the thermal
behaviour did not show any considerable changes and was
practically stable until 250 uC; after that a weight loss of 2.6%,
which is due to impurities also observed in FT-IR analysis.
The results revealed that Ni acetate reacted at low
temperatures exhibiting a partial dehydration and subsequent
hydrolysis of the sample. It is believed that the layered
morphology whose composition could be Ni(OH) Ac ?nH O,
6
2
1
2
6,33,34
coordination.
2
1
2
1
4
3
5
3
4
peaks corresponding to metallic nickel (y330 nm) were not
observed due to the formation of few number of particles at
the initial stages and growth as the reaction time progressed.
Fig. 6 represents the thermal behaviour of Ni acetate and the
solid precipitates obtained at various reaction temperatures.
In the case of nickel acetate (Fig. 6a), the reaction could be
x
y
2
where x , y, which was roughly estimated from the TG. The
high amount of acetate anions in the structure could be
ascribed to the absence of water for hydrolysis. This
3
6
described in two steps. The first step could be assigned to
the dehydration of the sample occurring at temperatures
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reaction time which facilitated the particle growth. Thus if we
intend to prepare monodispersed nickel nanoparticles, on one
hand, the nucleation rate has to be increased and the
agglomeration has to be inhibited on the other. Thus, we
attempted the enhancement and control of the nucleation rate
by introducing hydroxyl ions and metallic seed particles and
inhibited the particle growth by introducing surface active
agent such as oleylamine. The results of the study carried out
to control the size and morphology of nickel particles are
reported in the following.
Effect of surfactant: oleylamine
In general, the function of oleylamine in the synthesis of
nanoparticles is to inhibit the growth and agglomeration
through the coordination of their N atoms with the metal,
3
8–40
oxides, and sulfide surfaces.
Oleylamine can also passivate
the metallic surface avoiding their oxidation and ease their
dispersion in organic solvents when particle sizes are small
4
1
enough.
Thus, the synthesis of nickel particles was
Fig. 6 Thermal behaviour of Ni(Ac)
2
?4H
2
O and their products synthesized in
attempted by introducing 0.4 M oleylamine in the Ni(Ac) –2-
2
2
-octanol.
octanol system and their TEM micrographs are shown in
Fig. 8. At 120 uC, a small quantity of green precipitate was
observed in the bottom of the flask. Here too, the layered
structures were found at 120 and 180 uC (Fig. 8a and b).
Likewise, small particles that could be ascribed to metallic
nickel nanoparticles were also observed at the borders of the
compound was very stable at high temperatures and remained
for long periods of reaction. Likewise, changes in the valence
state of nickel compounds or formation of new phases were
not observed. Therefore, it is believed that the intermediate
product dissolved slowly liberating Ni ions and subsequently
reduced to give nickel nuclei. The presence of products such as
2-octanone, is an indirect evidence for the reduction of nickel
ions through the oxidation of 2-octanol. This mechanism
could be considered similar to the oxidation of ethylene glycol
3
4,37
to aldehydes during the reduction of nickel ions.
However,
further analysis is necessary to confirm this. With prolonged
reaction time, these few nuclei grew in the absence of
surfactant. Thus, it could be concluded that it is possible to
synthesize nickel particles with 2-octanol and the reduction of
nickel ions is carried out in the following sequence as shown
schematically in Fig. 7: (1) dehydration and hydrolysis forming
intermediate layered nickel hydroxide, (2) dissolution and
metal nickel nucleation and (3) particle growth.
Synthesis of monodispersed Ni nanoparticles
The nickel particles thus produced are in the sub-micrometer
range and show evidence of agglomeration. This could be due
to small number of metallic nuclei generated and prolonged
Fig. 8 TEM images of samples synthesized in 2-octanol with 0.4 M oleylamine at
(a) 120 and 180 uC for different reaction durations: (b) 1, (c) 2 and (d) 4 hours.
Fig. 7 Synthesis scheme of nickel particles using 2-octanol as a reducing agent.
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agglomerated layers. Further prolongation of the reaction time
to 2 h, increased the population of small nickel nanoparticles
(Fig. 8c). The particles looked monodispersed with an average
diameter of 32 nm and beside uniformly distributed on TEM
grid surface. Increasing the reaction time led to further growth
of particles through the reduction of un-reacted Ni-complex
and nickel particles with an average size of 44 nm were
detected (Fig. 8d). Although the presence of layered structures
was confirmed, the structure and probably the composition
are different from the ones observed in Fig. 2. The XRD
analysis of these powders revealed a phase that could be
indexed with structures similar to a-Ni(OH) (JCPDS No. 22-
2
3
0444) as shown in Fig. SII, ESI. Likewise, it was noted that the
hydroxide began to disappear after 1 h of reaction at 180 uC
and at the same time the appearance of the metal nickel phase
was also detected. The reaction progressed with time and pure
metal Ni was found after 2 hours of reaction. Additional
reaction time only enhanced the crystallinity and purity of the
product.
The results showed marked enhancement in the reaction
kinetic due to the fact that at room temperature Ni(Ac)2
reacted with the surfactant forming oleylamine–Ni acetate
complexes, which was exemplified by the change in colour of
the solution and also showing high solubility in 2-octanol.
Thus, Ni(Ac)2 complex dissociated at shorter reaction time
2
than those observed for Ni(Ac) , which subsequently got
reduced to nickel metal. In other systems such as Fe(III)–
oleylamine and Au(II)–oleylamine, oleylamine has been found
to assist the reduction reaction and the formation of Fe
3
O
4
Fig. 9 Influence of oleylamine concentration in the synthesis of Ni particles. (a)
4
2,43
and metal gold has been detected.
Thus, we believe that
2
1, (b) 5, (c) 10 and (d) 20 molar ratio between oleylamine to Ni(Ac) .
the reducing capability of oleylamine has helped the synthesis
of nickel particles at shorter reaction durations. On the other
hand, the oleylamine adsorption on the surface of nanopar-
ticle inhibited the growth process. When the concentration of
oleylamine is insufficient the particles tend to grow and
agglomerate, too.
irregular and thin agglomerated particles. When the solution
reached 180 uC (Fig. 10b), new phases were detected and one
of them corresponded to nickel oxide (JCPDS No. 44-1159) and
the rest to metallic nickel (JCPDS No. 04-0850). However,
prolonged reaction duration resulted in intensity enhance-
ment of peaks corresponding to metallic nickel (Fig. 10c–e).
In order to control the size, different oleylamine concentra-
tions were added to Ni(Ac) –2-octanol system by varying the
2
molar ratio between oleylamine to Ni(Ac)
0. It was found that the particle size of nickel decreased
monotonically with increasing molar ratio; e.g. 96, 36, 23, and
8 nm for the molar ratios of 1, 5, 10 and 20, respectively
Fig. 9). Agglomerated particles were not observed even when
2
in the range of 1 to
2
1
(
the molar ratio was 1; however stable suspensions in toluene
were obtained only when the particles were less than 30 nm in
diameter.
Effect of hydroxyl ions: NaOH
In polyols, the addition of hydroxyl ions has been widely used
to improve the kinetics of the reduction reaction to produce
metallic nanoparticles with much lower reduction potential
2
4 44
compared to nickel such as Fe (20.44 eV ). In this regard,
NaOH dissolved in methanol (molar ratio between hydroxyl to
nickel was 2) was used to enhance the reduction of Ni(Ac) in
2
2
-octanol. At 120 uC, a green precipitate that could be ascribed
to the formation of b-Ni(OH) was obtained. However, some
peaks did not exactly match with the known XRD profiles of
b-Ni(OH) (Fig. 10a) according to the JCPDS No. 14-0117 and
the microphotographs of these samples (Fig. SIII, ESI ) shows
2
2
Fig. 10 XRD patterns of Ni particles obtained by adding NaOH to 2-octanol.
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Slow transformation of nickel oxide to metallic nickel was
observed, however, the oxide phase remained even after
reaction duration of 4 hours (Fig. 10d). It is likely that at high
hydroxyl ion concentration, the system is destabilized and the
reaction corresponding to the dissolution of metallic nickel,
formation of nickel hydroxide and consequent reduction to
metallic nickel occur repeatedly. The addition of NaOH
accelerates the nucleation of nickel nanoparticles, generating
nickel nuclei at shorter reaction time. This resulted in the
reduction of the grain size of nickel particles. TEM pictures
took at these conditions showed well-crystallized particles with
a broad size distribution and cubic morphology (Fig. SIII,
3
ESI ). The particles have a strong tendency to agglomerate
because of the absence of surfactant. Progression of reaction
did not show any considerable changes; although individual
irregular particles with size larger than 80 nm could be found.
Although the addition of hydroxyl ions to the system
enhanced the reduction kinetics of Ni(Ac) , the absence of
2
surfactant eased particle agglomeration. In addition, inter-
mediate phases such as Ni(OH) and NiO that are totally
2
different to those analyzed in previous cases were found. An
optimal hydroxyl ion concentration will be needed to get pure
materials on one hand and the addition of surfactants will be
also necessary to prevent their agglomeration on the other. On
the other hand, the use of nucleating agents could be
considered to obtain oxide free nickel nanoparticles.
Effect of metallic seeds: Cu and Pt
Fig. 11 Size-effect of metallic seed particles on the synthesis of Ni nanoparticles
Heterogeneous nucleation has been applied in the synthesis of
metal nanoparticles by using polyol as a route to get smaller
sizes. In the case of nickel, the size reductions have been
realized by adding metallic particles or highly reducible metal
in oleylamine–2-octanol system. (a) and (c) Cu(Ac)
2
at MR: 40 and 4, respectively
and (b) and (d) H PtCl at MR: 40 and 4, respectively.
2
6
3
4,37
salts such as Ru, Pt, Pd, Ag, etc. into polyol.
Recently, the
on silicon wafers at different particle concentration and
speeds as shown in Fig. 12b–d. The construction of monolayer
films composed of nickel nanoparticles with total coverage
authors have applied this technique to prepare Ni–Pt
4
5
nanoparticles in 1-heptanol. The results showed that Ni–Pt
nanocubes with a particle size of y30 nm could be obtained
by using a specific concentration of Pt salts. Here, Pt was
observed to form at low temperatures and then these particles
2 2 6
acted as nuclei. In the present study, Cu(Ac) and H PtCl were
added as nucleating agents for the size reduction of nickel
nanoparticles in 2-octanol; here, the molar ratio between Ni to
metal seed particles (MR) was selected as 40. In order to avoid
the agglomeration of the particles, oleylamine was added to
the alcoholic system and molar ratio with nickel was set at 8.
Fig. 11a and b show particles slightly smaller than that
observed in Fig. 9c; thus, we conclude that those metallic salts
help to reduce the size of nickel nanoparticles. The average
particle sizes of these samples were found to be 24 and 21 nm
for Cu and Pt salts, respectively. In addition, the effect of
concentration of metal seed particles was evaluated by
2 2 6
increasing the concentration of Cu(Ac) and H PtCl at a
molar Ni/Cu or Pt equal to 4. The TEM images shown in
Fig. 11c and d revealed a marked reduction of size, 18 and 16
nm on average for Cu and Pt seeds, respectively.
Finally, the nickel nanoparticles synthesized under the
optimum conditions were successfully dispersed in toluene
as shown in Fig. 12a and nanostructures for the synthesis of
Fig. 12 The photograph of (a) nickel particle dispersion in toluene and SEM
micrographs of nanoparticle array on silicon wafer prepared by spin coating the
4
6
2
1
21
carbon nanotubes (CNTs) using chemical vapor deposition
CVD) technique were prepared by spin coating the suspension
suspension at different conditions: (b) 25 g L , 2000 rpm, (c) 12.5 g L , 3000
2
1
(
rpm and (d) 25 g L , 3000 rpm.
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Conclusions
Nickel particles with sizes ranging between 16 to 460 nm have
been synthesized by using 2-octanol as solvent and reducing
agent and nickel acetate as the metal source. The evolution of
nickel nanoparticles has been through the formation and
consequent dissolution of layered hydroxides to produce free
metal ions, which subsequently get reduced by the alcohol
leading to nucleation and growth of metallic nickel particles.
The addition of hydroxyl ions, metallic seeds, and especially
oleylamine were found to control the particle size and
distribution. Overall, the results suggest that 2-octanol could
be a potential reducing agent to prepare metallic nickel
powders for different applications such as catalysts, MLCC or
metallic seeds for CNTs.
Fig. 13 Magnetization of nickel products obtained at different reaction
Acknowledgements
durations: (a) 2, (b) 3 and (c) 4 h.
This study was supported by Grant-in Aid for Basic Research
#(B) 22310064 from the Ministry of Education, Science,
2
1
Culture and Sport of Japan.
was possible when the particle concentration was 25 g L
.
The formation of semiconducting single-walled carbon nano-
tubes (SWNTs) using such templates has been confirmed and
a detailed report will be communicated soon.
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