Novel standing Ni–Pt alloy nanocubes

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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

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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


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.


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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