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Novel standing Ni–Pt alloy nanocubes†
Jhon L. Cuya Huaman,a Shunya Fukao,a Kozo Shinodab and Balachandran Jeyadevan*a
Received 23rd February 2011, Accepted 28th March 2011
DOI: 10.1039/c1ce05241a
The synthesis of novel cubic shaped-standing Ni–Pt alloy nano-
particles is reported. Incorporation of a few percent of Pt atoms in
the presence of chloride ions and oleylamine, which helps to control
the growth and prevents agglomeration, facilitates the formation of
highly monodispersed cubic shaped particles. Furthermore, these
cubic-shaped particles stand on their corners, which is believed to be
due to the magnetic interaction between particles whose easy axis is
in the [111] direction. The formation of these unique shaped particles
with different sizes has been realized by using platinum particles as
seeds. It should be noted that these particles are highly reproducible
and holds great potential for catalytic applications.
Introduction
In the initial stages of nanotechnology oriented research, prime
importancewas given to the size and size distribution control ofmany
inorganic functional materials. This was mainly due to the fact that
the properties of nanoparticles deviate significantly from its bulk
counterpart and maximum gain of the functional properties are
realized when the particle size is small and their distribution are
narrow. The key to the progress in size controlledmetal nanoparticles
has been the development of various solution-phase synthesis tech-
niques that reduce noble and some transition metals relatively
easily.1–3 The above techniques do not require any specialized
experimental setup and also can be easily scaled up. In the solution-
phase synthesis metal nanocrystals are formed through nucleation of
clusters formed of reduced atoms in the solution and their subsequent
growth. In most cases, a metal salt precursor is reduced in non-
aqueous solution in the presence of a stabilizing agent, which prevents
aggregation, improves the chemical stability and in some instance the
physical property of the as-synthesized nanoparticles.4,5 Considerable
degree of success in the control over size and size distribution of the
particles have been achieved by controlling both the thermodynamic
aDepartment of Material Science, School of Engineering, The University ofShiga Prefecture, Hikone, Japan. E-mail: [email protected];[email protected]; Fax: +81 749 28 8486; Tel: +81 749 28 8352bInstitute of Multidisciplinary Research for Advanced Materials, TohokuUniversity, Sendai, Japan. E-mail: [email protected]; Fax:+81 22 217 5624; Tel: +81 22 217 5624
† Electronic supplementary information (ESI) available: TEM image oflarge cubic shaped particle (Fig. S1). XRD pattern of Ni–Pt nanocubes(Fig. S2). The profile of EXAFS spectrum of Ni (Fig. S3 and S4). SeeDOI: 10.1039/c1ce05241a
3364 | CrystEngComm, 2011, 13, 3364–3369
(e.g., temperature, reduction potential) and kinetic (e.g., reactant
concentration, diffusion, solubility, reaction rate) parameters.6,7
However, with the advent of exploitable technologies and under-
standing their limitations, the focus has shifted towards another
increasingly important feature of nanoparticles, which is the
morphological control of nanocrystals as many of their physical and
chemical properties are considerably shape dependent. Consequently,
appreciable researches are being made to control the shape of single-
and multiple-material systems motivated by the structure-function
relationship that could possibly lead to the discovery of novel func-
tional nanostructures.8–10Although recent studies have focused on the
control of crystallographic faces of noble-metal nanoparticles
through precise tuning of nucleation and growth steps, the exact
mechanisms for shape-controlled colloidal synthesis are often not
well understood or characterized.7,11–19
On the other hand, though transition metals such as Fe, Co, Ni
and their alloy nanoparticles have generated great interest in the fields
of high-density data storage, electromagnetic wave absorption,
magnetic fluids, and catalysts,20–22 their success in respective fields
have been limited due to the difficulty in reducing the same and also
the instability of these particles in oxidizing atmosphere. Only the
development of techniques to control the size, shape and composition
of these particles that influence the magnetic, electronic and catalytic
properties will facilitate rapid progress. Among magnetic particles,
nickel nanoparticles are considered for catalytic applications than
magnetic.23,24 Considerable research has been devoted to produce Ni
nanoparticles;25–27 however, recent interest has been on the synthesis
of nickel–platinum alloy catalyst that could be used in fuel cells
instead of platinum.28–31 Though nickel–platinum nanoparticles with
various size and composition were obtained by controlling the ther-
modynamic and kinetics of the reaction, less effort was made to
control the morphology.32–34
Nanosized particles of metals on the lab-scale are usually obtained
by thermal decomposition of organometallic compounds that are
mostly expensive and toxic. This method could be used as tool to
explore the potential of various metals and alloy materials, but
cannot be used for large-scale production. On the other hand, an
alternative technique using poly alcohol and often referred to as
‘polyol process’ have been used for the synthesis of noble metals for
quite some years.1,7,35,36 The term ‘polyol process’ is used in various
context, however, in real sense it should be restricted to cases where
the polyol is used as a reducing agent and not just as either solvent or
surfactant, especially in context of metal nanoparticle synthesis.
Though the potential of polyol was good enough even to reduce iron
This journal is ª The Royal Society of Chemistry 2011
Fig. 1 TEM images of nickel particles synthesized using 1-heptanol as
reducing agent and oleylamine as surfactant under the following
hydroxyl ion and dihydrogen hexachloroplatinate concentrations (a)
0 mM, 0 mM, (b) 2.25 mM, 0 mM, (c) 0 mM, 0.2 mM (d) 0 mM, 1 mM (e)
0 mM, 2 mM, (f) 0 mM, 10 mM. Scale bar 100 nm.
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ions, the reaction rate was not high enough to create a super-satu-
ration concentration for the formation of clusters that are larger than
the critical size of nucleus and consequent growth. Recently, the use
of hydroxyl ions has proved effective to accelerate the reaction to
facilitate super-saturation concentrations of transition metal ions
including iron and the formation of Fe and Fe-based alloys particle
have been realized.37–39 The above results suggested that the synthesis
of transition metals could also be achieved using low reducing
potential alcohols.
Experimental
Materials
Nickel salts such as nickel(II) chloride hexahydrate (NiCl2$6H2O,
98%), and nickel(II) acetate tetrahydrate (Ni(CH3CO2)2$4H2O, 98%),
platinum salts such as dihydrogen hexachloroplatinate hexahydrate
(H2PtCl6$6H2O, 98.5%) and platinum(II) acetylacetonate (Pt
(C5H7O2)2, 50.6% Pt), 1-heptanol (C7H15OH, 98%), methanol
(CH3OH, 99.8%), toluene (PhH3, 99.5%), oleylamine (CH3(CH2)7-
CH]CH(CH2)7CH2NH2, 70%), and sodium hydroxide (NaOH,
97%) were purchased from Wako Pure Chemicals Ltd., Japan. The
commercially available reagents were used without further
purification.
Synthesis of Ni–Pt nanocubes
In a typical procedure to synthesize Ni and Ni–Pt nanoparticles,
38 mM nickel salt was totally dissolved in 5 mL methanol using
ultrasonication. The above solution was mixed with 100 mL 1-hep-
tanol containing 0.42 M oleylamine and a specific dihydrogen hex-
achloroplatinate concentration and, then, heated at 448K for 60min.
The particles were recovered by using a magnet. Then, the particles
were washed with a mixture of methanol and toluene in order to
remove unreacted compounds and excess oleylamine. Finally, the
particles were dispersed in toluene.
Characterization
The size and morphology of the particles were analyzed by using
transmission electron microscope (FE-TEM Hitachi HF-2000). The
samples for the TEMmeasurements were prepared by depositing the
toluene dispersed copper particles on the amorphous carbon-coated
grids. The metal and crystalline oxide components of the particle
samples were identified using the XRD method. The apparatus used
in this was a Rigaku RINT2000 diffractometer, in which the Cu-Ka
radiation was used as the incident X-rays. In the XRD experiments,
the powder of sample was filled into a recess with 5mmdiameter and
0.1 mm depth in single crystalline silicon plate. Crystal structural
parameters and crystalline phase composition was determined by
using a program TOPAS ver. 3 produced by Bruker axs. In order to
analyze the local atomic environmental structure around certain
element, the XAFS spectra of the samples at Ni K and Pt L3
absorption edges were recorded using an in-house X-ray absorption
spectrophotometer, namely, Rigaku R-XAS Looper. In the spec-
trometer, the demountable X-ray tube with Mo target as the white
X-ray source and the Si(400) Johansson-type bent single crystal as the
monochromator crystal were used. The experiments at Ni K and Pt
L3 absorption edges were carried out in the transmission mode using
the samples diluted with BN powder and pelletized and in the
This journal is ª The Royal Society of Chemistry 2011
fluorescence yield mode using the samples pressed without dilution,
respectively. The data processing of the measured X-ray absorbance
spectra was carried out by using the program REX2000 ver. 2.5.9
produced by Rigaku. The magnetic measurements were made at
room temperature using the vibration sample magnetometer (VSM)
of Tamagawa Seisakusho, under an applied field of 1T and calibrated
with standard Ni.
Results and discussion
Based on experimental knowledge gathered on above studies,37–39 we
have attempted the synthesis of nickel metal and alloy nanoparticles
using alcohols such as 1-heptanol. Alcohols have been used generally
to synthesize a variety of oxides through the esterification reaction
between acetate and alcohols giving hydroxide nuclei, which are
posteriori reduced to give oxides.40 As reported here, these nuclei
could be engineered to form metal nanoparticles by introducing
additives such as hydroxyl ions to derive metal nanoparticles, in this
case metal nickel. However, the reduction of nickel acetate occurs too
fast generating big nickel particles with a tendency to form compli-
cated dendritic structures composed of platelet needles joined
together at the center (Fig. 1a). On the other hand, when the nucle-
ation and the growth process is influenced by adding hydroxyl ions
the needle-like structure of nickel particles are retained, although, the
particle size became smaller, decreasing from 280 to 90 nm (Fig. 1b).
Here, the diameter of dendritic particles were determined by
measuring the circle that covers the whole particle and the values
reported represents the average diameter of particles observed in
TEM. This may be due to the change in the degree of super-satu-
ration induced by the promotion of the reduction reaction by the
addition of hydroxyl ion, which has already been observed in the
synthesis of Fe and FeCo.37–39 On the other hand when seed forming
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salt such as dihydrogen hexachloroplatinate is introduced, the nickel
particles lost their original morphology with progressive reduction in
size from 280 to 55 nm, and took elongated, ‘boomerang’ and ‘ninja
knife’ shapes (Fig. 1c–e). This is due the forced nucleation resulting
from the reduction in the threshold of free energy change necessary
for the growth of clusters besides other factors such as surfactant
concentration, solvent type, etc.13,41 At this stage of the synthesis,
the physical properties of the particles are influenced very much by
the nucleation process rather than the growth. Consequently, the
morphological changes we have observed in this system are the result
of inhibition of growth due insufficient source of metal ions.
When the concentration of seed forming salts was increased by
a factor of five, expecting to generate appropriate conditions for burst
nucleation, in contrast to our expectation cubic-shaped particles were
synthesized as shown in Fig. 1f. The details of the morphological and
structural studies of the sample are shown in Fig. 2. Though the
particles look like hexagonal-shaped platelets at first sight (Fig. 2a),
a careful study of the same using STEM observation confirmed their
cubic morphology as shown in Fig. 2b. Furthermore, the electron
diffraction pattern shown in Fig. 2c suggested that the particles are
single crystal in nature. The d-spacing of (111) plane measured from
the high resolution TEM (HRTEM) micrograph of the Ni-rich NiPt
inserted in Fig. 2(a) was 0.2150 nm. The hexagonal shape of the plan
view is the consequence of the cubes are standing at their corners
having their diagonal axis vertical. Though the reason for this
phenomenon is not clear it is believed that the magnetic interaction
between particles whose magnetic easy axis is in the [111] direction
could be considered plausible.42 To make certain that these particles
Fig. 2 (a) Magnified TEM images of Ni nanocubes observed in Fig. 1f
(the inset shows the lattice fringes of the crystal), (b) STEM image of Ni
nanocubes and (c) electron diffraction pattern of Ni nanocubes Scale bar
25 nm.
3366 | CrystEngComm, 2011, 13, 3364–3369
are not made to stand by the magnetic lens in the transmission
electronmicroscope, observations of these particles were alsomade in
transmission electron microscope with shielded magnetic coils, and
the particles were found to remain standing even in the above case.
We also attempted to apply a magnetic field during microscopic
observation, but the field was not strong enough to influence the
particle orientation. But, it was interesting to note that the only
circumstance under which the particles fall flat waswhen a large cubic
shaped particle, which could not stand on its own due its size, was
surrounded by particles throughmagnetic interaction as shown in the
ESI (Fig. S1).† The measurement of magnetic properties made on
these particles reveal that the saturationmagnetization is only 32 emu
g�1 and lower compared to the value of bulk nickel, which is 55 emu
g�1. The degree of reduction inmagnetization suggests the presence of
41.8% of non-magnetic core, in this case Pt. However, the weight
percent of Pt determined through chemical and TEM-EDX analysis
have been found to be around 5 wt. % and the reduction in
magnetization is larger than anticipated by purely considering the
non-magnetic core. This suggests that Pt atom is incorporated in the
structure of nickel lattice and the structural analysis is necessary to
understand the distribution of the Pt atoms in the particles. Thus in
order to analyze the local atomic environmental structure around Pt,
the XAFS spectra of the samples at Pt L3 absorption edges were
recorded using an in-house X-ray absorption spectrophotometer,
namely, Rigaku R-XAS Looper.43
Fig. 3 Fourier transform profile of EXAFS spectrum measured at Pt L3
absorption edge for the sample and calculated nearest-neighbouring
correlation using FEFF code for Pt (top) and Ni (bottom) as nearest
neighbouring elements.
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Fig. 3 shows the Fourier transformprofile of the EXAFS spectrum
measured at the Pt L3 absorption edge for the sample. Fourier
transforms for the nearest neighbouring correlation calculated by
using FEFF 8.20 code under assumption of Pt or Ni are drawn by
broken lines in this figure. FEFF calculated nearest neighbouring
correlation peak profile was in good agreement with the experimental
profile in case of the assumption of Ni as the first nearest neigh-
bouring element and the distance was determined as 0.2535 nm closer
than 0.2775 nm in pure Pt metal. This indicates that platinum atoms
distribute homogeneously in the alloy lattice without forming a Pt-
rich cluster. The XRD pattern of Ni–Pt nanocubes and EXAFS
spectra at Ni K-absorption edge are given in the ESI (Fig. S2–S4).†
However from the magnetic property point of view, further studies
are necessary to understand the influence of Pt atoms in the nickel
face-centered lattice on the magnetic properties of Ni–Pt alloy.
On the other hand, to understand the formation of crystallo-
graphically controlled Ni particles, we monitored the growth of the
particles over the reaction time. Fig. 4a–f shows the gradual growth
of irregular and faceted shaped 4 nm particles obtained for a reaction
time of 60 s to cubic-shaped 28 nm particles after a reaction time of
60 min. Noble metals, which adopt a face-centered cubic (fcc) lattice,
possess different surface energies for different crystal planes14–19 and
the formation of crystallographic shapes has been discussed based on
crystallinity of the seeds.44 But, if we have a close look at the shape of
the particles formed at different duration of the reaction, we could
observe that the growth process is quite different to what has been
already reported in the literature.44 The 4 nm particles formed after
60 s form units consisting of 3–4 particles in the next three minutes of
the reaction time. Then, with the progression of the reaction time, the
adatoms diffuse to the surface of aggregated particles and the parti-
cles continue to grow in size forming multi-armed nanostar struc-
tures. Associated with the growth, the grain boundaries between
particles that formed the aggregates at the early stages of the reaction
Fig. 4 TEM photographs of samples taken at different times to evaluate
the formation of Ni–Pt nanocubes (a) 1, (b) 4, (c) 6, (d) 10, (e) 20 and (f)
60 min. Scale bar 20 nm.
This journal is ª The Royal Society of Chemistry 2011
disappeared. After 20 min of reaction time, cubic crystals with a side
length of about 25 nm were formed. The shape and size of the
particles remained as it is for a reaction time of about 60 min. Even
though Pt salts could react forming Pt nanoparticles at low ion
concentration and act as seeds in the synthesis of nickel nanoparticles,
their role at higher platinum concentration and the consequent
formation of cubic shaped particles is not clear.
On the other hand, though we introduced platinum salts to facil-
itate the formation of Pt seeds for nano-sized nickel particles, the
formation of Ni–Pt alloy with the platinum salts remaining in the
solution cannot be ruled out. Thus experiments were carried out to
form nickel nanoparticles using platinum seeds (Fig. 5) prepared
separately reducing dihydrogen hexachloroplatinate in 1-heptanol.
The Pt seeds were introduced into reactionmediums having (a) nickel
salts and (b) amixture of nickel and platinum salts in addition to 0.42
M of oleylamine. In the case of nickel salts, irregular shaped particles
were observed (Fig. 5a); in this case Pt only acts as a nucleating agent
but it did not control the final shaped of nickel nanoparticles. On the
other hand, in cases where different concentrations of platinum salts
were present, the shape of the particle changed from polydispersed
polyhedral to monodispersed cubic with the gradual increase in
platinum ion concentration as shown in Fig. 5b–c. These results show
that the presence of Pt4+ in the solution is important to get nickel
nanocubes and it could follow a similar mechanism to obtain Pd
nanocubes through of Fe3+ ions.45 As against the Pd nanocube case
where Fe is not incorporated, here we have the Pt atoms incorporated
in the system but the concentration of the same is very low. The true
role of Pt in the formation of Ni–Pt nanocubes is yet to be
understood.
There is another important factor considered by some researchers
to get metal nanocubes as for example the type of precursors. It has
been reported that Pt nanocrystal is only formed when dihydrogen
hexachloroplatinate is used as the source of platinum but not plat-
inum acetylacetonate.46,47They claim that the presence of chloride ion
is vital in the formation of cubes. Thus, to investigate the influence of
chloride ions in our system, the synthesis of nickel nanoparticles was
attempted using platinum acetylacetonate. The shape of the particles
obtained in the above case was irregular and elongated as shown in
Fig. 6a. However, the particle size is small suggesting that under this
condition Pt acetylacetonate only acts as a nucleating agent and
assists the formation of Ni–Pt alloys. However, when we change the
nickel salts from acetate to chloride and use Pt nanoparticles as seeds
and conditions similar to Fig. 5a, the particles take different
Fig. 5 Effect of platinum salt concentrations during the synthesis of
Ni–Pt nanocubes using constant amount of Pt seed particles. (a) Pt seeds
(2 mM Pt), (b) Pt seeds (2 mM), Pt salt (2 mM), and (c) Pt seeds (2 mM),
Pt salts (8 mM). Scale bar 50 nm.
CrystEngComm, 2011, 13, 3364–3369 | 3367
Fig. 6 Effect of chloride in the synthesis of Ni–Pt nanocubes. (a) Ni
acetate (38 mM), Pt acetylacetonate (10 mM), (b) Ni chloride (38 mM),
Pt seeds (2 mM Pt), and (c) Ni chloride (38 mM), Pt acetylacetonate
(8 mM) and Pt seeds (2 mM Pt). Scale bar 100 nm.
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morphologies, spherical, triangle and hexagonal shapes but not cubes
(Fig. 6b). Therefore, formation of faceted shapes could be associated
with the presence of chloride ions.14,48 On the other hand, when Pt
acetylacetonate was added to the system to facilitate the presence of
Pt ions, the particles with cubic morphology is observed as shown in
Fig. 1f (Fig. 6c).
As a consequence, we believe that the presence of both Pt and Cl
ions are necessary for the formation of cubic shaped Ni–Pt particles.
It should be noted that the synthesis was carried out under nitrogen
atmosphere and etching reaction using O2/Cl� system is not operable
in this system. However it should be remembered that in a solution-
phase synthesis, other factors such as capping agents or impurities
can also alter the order of free energies of different crystal facets
through their physical or chemical interaction with metal surface.7,49
Thus, it is necessary to investigate the effect of surfactant on the
formation of the cubic crystals. The function of oleylamine during the
synthesis of nickel nanoparticles is not only to stabilize and avoid the
coalescence of the particles but also for the control their shape.
Hence, we evaluated the effect of oleylamine concentration on the
formation of nickel nanocubes (Fig. 7). It was found that the growth
of the particles is partially inhibited at lower concentrations and the
particles become irregular and spherical in shape which is reported to
be more stable structure than simple cubes (Fig. 7a). However, when
the concentration reached 0.42 M the growth of {100} planes were
inhibited and nanocubes were obtained (Fig. 7b). When the
concentration was increased further, the growth was inhibited
strongly and the particles were small and took the shape of tetrahe-
dron (Fig. 7c).We conclude that either the nucleation process itself or
Fig. 7 Effect of oleylamine concentration in the formation of Ni–Pt
nanocubes. (a) 0.21, (b) 0.42, and (c) and 0.84 M oleylamine. Scale bar 50
nm.
3368 | CrystEngComm, 2011, 13, 3364–3369
increased ligand-capping is favoured at higher oleylamine concen-
trations resulting in the formation of small particles. However, it
should be noted that in the case of pure nickel ions the formation of
cubes was not realized under any of the oleylamine concentrations
used in the above study.
The experimental investigations have suggested that the formation
of the cubic-shapedNiPt nanoparticles depends on the presence of (i)
specific concentration of Pt ions, (ii) chloride ions and (iii) oleyl amine
concentration. The influence of chloride ions and oleylamine on the
formation of various shapes has been already established by
researchers working on the synthesis metal nanoparticles.7,14,49
However, the influence of Pt ion concentration on shape of Ni-rich
NiPt nanoparticles has not been reported or discussed in the past.
In the present system, the reducing agent 1-heptanol has the
necessary potential to reduce both Pt and Ni ions. However, the Pt
ions will get reduced easily compared to Ni if we consider the
reduction potential of these elements and this may play a vital role in
initiating the formation Ni rich NtPt alloy nanoparticles. Further-
more, the EXAFS analysis also suggests that the reduction of both
elements occurs simultaneously and the nearest neighbour for the Pt
atom is not Pt and is Ni. In addition, the chemical analyses of the
solids obtained at different reaction times have shown that the Ni to
Pt ratio in the solid precipitates has been constant throughout the
reaction. This suggests that the formation of Ni95Pt5 alloys is ener-
getically favoured over the other compositions. According to the
recent Ni–Pt phase diagram, Ni could form solid solution with Pt at
various concentrations and the lower limit has been reported to be
around 5 atomic percent.50Thus, we believe that the Pt concentration
in the NiPt alloy is also a necessary condition for the formation of
cubic-shaped particles. However, the degree of influence of each of
these parameters is not clear at present. Further investigation is
necessary to elucidate the mechanism for the formation of cubic-
shaped NiPt alloy nanoparticles.
Conclusions
The synthesis of nickel particles with sizes ranging between 280 and
28 nm of nickel and nickel-platinum alloy nanoparticles were
synthesized using alcohol reduction process. Especially, we have been
successful in the synthesis of novel standing [111] Ni–Pt alloy nano-
particles by co-reduction of nickel and platinum salts in 1-heptanol
with high reproducibility through of heterogeneous nucleation. The
experimental investigations have confirmed that the factors that
facilitate the formation of this unique cubic shaped Ni–Pt alloy are
platinum ion and surfactant concentrations and the presence of
chloride ions.
Acknowledgements
This study was supported by Grant-in Aid for Basic Research #(B)
22310064 from theMinistry of Education, Science, Culture and Sport
of Japan. The authors would like to acknowledge Mr. K. Motomiya
of Graduate School of Environmental Studies-Tohoku University,
for high-resolution TEM measurements.
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