九州大学大学院総合理工学府報告第 41 巻 第 2 号 30-37 頁 令和 2 年 2 月

Engineering Sciences Reports, Kyushu University
Vol. 41, No. 2, pp. 30-37, FEB, 2020

Synthesis of Au@Ni Core−shell Nanoparticles Using
NaBH4 or L-ascorbic Acid as a Reducing agent
Masaharu TSUJI*1,2† Kento FUKUTOMI*3 Masashi HATTORI*1
Keiko UTO*1 Jun-Ichiro HAYASHI*1,2 and Takeshi TSUJI*4
E-mail of corresponding author: tsuji@cm.kyushu-u.ac.jp†(Received January 23, 2020, accepted January 30, 2020)
Au@Ni core–shell nanoparticles were synthesized using Au nanorods (NRs) as seeds. The resultant
crystal structures were characterized using transmission electron microscopic (TEM) and TEM-energy
dispersed X-ray spectroscopic (EDS) measurements, and ultraviolet (UV)-visible (Vis)-near infrared(NIR) extinction spectra. Ni shells were grown over AuNRs seeds by reducing Ni(NO3)26H2O using
NaBH4 or L-ascorbic acid. When NaBH4 was used, AuNR@Ni nanoparticles having egg-like Ni shells
and small spherical Ni aggregate shells were produced. On the other hand, AuNR@Ni nanoparticles
having irregular shaped Ni shells were produced by using L-ascorbic acid. The nonoccurrence oflayered epitaxial growth for AuNR@Ni particles was attributed to the presence of unstable high-index{250} or {5 12 0} facets in AuNRs and a large lattice mismatch between Au and Ni (13.6%). When Ni
shells were formed over AuNRs, characteristic surface plasmon resonance bands of AuNR cores became
either weak or disappeared, and a long tail band originating from Ni-shell component was observed inthe 350–1150 nm region.
Key words: Au@Ni core-shell nanorods, Reducing agent, NaBH4, Ascorbic acid, TEM, TEM-EDS,

Surface plasmon resonance band

1.

Introduction

Core-shell
nanoparticles
which
are
constructed from more than one metallic phasehave been received especial interest because of
their higher catalytic, optical, electric, ormagnetic properties than those of individual
monometallic particles.1-14)
Since above
properties strongly depend on size, shape, andcomposition of nanoparticles, their controlled
syntheses of core-shell nanoparticles are
required. An ideal method to prepare size,
shape, and composition controlled core-shell
nanoparticles is epitaxial growth of core-shellparticles. Using a strong shape correlation
between cores and shells, shape controlled
epitaxial growth of shells over various shapesof core metals is possible.15-19) Compared withextensive studies on shape controlled syntheses*1

Institute for Materials Chemistry and Engineering,and Research and Education Center of GreenTechnology*2 Department of Applied Science for Electronics andMaterials*3 Department of Applied Science for Electronics andMaterials, Graduate Student*4 Department of Materials Science, Shimane
Universityof core-shell nanoparticles of novel metals likeAu and Ag, little work has been conducted forthe syntheses of nanoparticles involving
transition metals such as Cu and Ni. Although
it has generally been believed that the epitaxial
growth of a metal shell onto a different metal
can only take place when the lattice mismatch
between core and shell metals is below 5%,17)
we have demonstrated that Cu can be
epitaxially grown on various shapes of Au seeds
in spite of their large lattice mismatch (11.4%
for Au–Cu).20,21) As Au seeds, we used not only
octahedral, triangular and hexagonal plate-like,
decahedral, and icosahedral Au core seeds
having low-index {100} and {111} facets but also
NRs having higher-index {250} or {5 12 0} facets.
The growth rates of Cu shells on sharp cornersof Au cores were slower than those on flat {111}facets and single twin facets.
We have also studied epitaxial growth of
Au@Ni nanocrystals with higher latticemismatch (13.6% for Au–Ni) using octahedral,triangularandhexagonal
plate-like,
decahedral, and icosahedral nanocrystals as Au
seeds.22) We demonstrated that epitaxial
layered growth of Ni shells over polygonal Au
cores can also occur despite a larger lattice令和元年度

九州大学大学院総合理工学報告

mismatch of Au–Ni. Favorable facet of Ni shells
was {111}, which was the same as the major facetof Au core seeds, as in the case of Au@Cu.20) The
selected area electron diffraction (SAED)
patterns indicated that single crystals of Ni
layers formed parallel to those of Au seeds. For
Au@Ni crystals, Ni shells were grown not only onflat {111} planes but also sharp corners of Au
cores with the same thickness.
On the basis of our previous studies, there isan exceptional case that epitaxial growth canoccur on sharp corners even though the latticemismatch is as large as 13.6%. The UV- Vis-NIR
spectra of Au@Ni nanoparticles gave a similar
spectrum to that of Ni particle, indicating that
Au@Ni particles have similar optical properties tothose of Ni shells.
In this study, syntheses of AuNR@Ni
nanoparticles are investigated. The purpose ofthis study is whether epitaxial growth occurs
for AuNR@Ni nanoparticles. The effects ofexperimental parameters, such as a reducing
agent, NaBH4 or L-ascorbic acid, and theirconcentrations, are examined for the crystal
growth of Ni shells over AuNRs.

2.

Experimental

2.1

Materials

For use in this study, AuNRs involving 1.5
mM Au atoms and cetyltrimethylammonium
bromide (CTAB) in an aqueous solution, were
supplied by Dai-Nippon Toryo Co. Ltd. They
were prepared using a photochemical
method,23) The aspect ratio of the AuNRs was
about 5 (50 nm length, 10 nm diameter; W4type).
NaBH4(>97.0%),Ni(NO3)26H2O(>98.0%), sodium citrate (Na3C6H5O72H2O),
CTAB (>99.0%), L-ascorbic acid (>99.6%),
distilled H2O (HPLC level), and C2H5OH(>99.5%) were purchased from Kishida
Chemical Industry Ltd. Polyethylene glycol(PEG) with an average molecular weight of 600
was obtained from Tokyo Chemical Ind. A highpurity Ar gas (>99.999%) was obtained fromJapan Fine Products. All reagents were usedwithout further purification.
2.2
Synthesis of AuNR@Ni nanoparticles
using NaBH4 as a reducing agent
Fig. 1 shows a schematic diagram ofexperimental apparatus used for the synthesisof AuNR@Ni nanoparticles. The apparatus
consists of a three-necked glass frasco,
mechanical stirrer, a syringe pump, an opticalfiber thermometer (not shown in Fig. 1), and an

第 41 巻第 2 号

31

Fig. 1. Oil-bath heating apparatus used forthe synthesis of Au@Ni nanoparticles.
Ar gas cylinder. 2 ml of Au seed aqueous
solution was mixed with 5 ml of 15 mM
Ni(NO3)26H2O aqueous solution, 3 ml of 5 mM
Na3C6H5O72H2O aqueous solution, 5 ml of 36
mM CTAB aqueous solution. After bubbling Ar
gas at a flow rate of 10 sccm for 30 min toremove O2 dissolved in a reagent solution, 5 ml
of NaBH4 aqueous solution was injected using
the syringe pump at an injection rate of 0.3
ml/min for 30 min. Ar bubbling was continued
during reaction to suppress oxidation. Threedifferent concentrations of NaBH4 solution, 60,
6, or 3 mM, were examined.
2.3
Synthesis of AuNR@Ni nanoparticles
using L-ascorbic acid as a reducing agent
2 ml of Au seed aqueous solution was mixed
with 5 ml of 15 mM Ni(NO3)26H2O aqueous
solution, 3 ml of 5 mM NaOH aqueous solution,and 5 ml of 3 mM PEG aqueous solution under
Ar gas bubbling. It was heated at 60 oC. Then,
5 ml of L-ascorbic acid was injected using thesyringe pump at a rate of 0.0417 ml/min forabout 2 h. The concentration of L-ascorbic acidwas 30 mM or 3 mM.
2.4

Characterization of product particles

AuNR@Ni particles were obtained fromC2H5OH solution by centrifuging the colloidal
solution at 15,000 rpm for 30 min three times
for TEM (JEM-2100XS; JEOL) observations.
Extinction spectra of the product solutions
were measured in the UV-Vis-NIR region using
a spectrometer (UV-3600; Shimadzu Corp.).

3.

Results and discussion

3.1 AuNR@Ni nanoparticles prepared using
NaBH4

32

Preparation of Au@Ni Core-shell Nanoparticles Using NaBH4 or L-ascorbic Acid

Figs. 2a and 3a-3f show TEM and TEM-EDS
data of AuNR@Ni nanoparticles prepared at an
NaBH4 concentration of 60 mM and a line
analysis data along a line shown in Fig. 3d.
Because the electron density of Ni is lower thanthat of Au, Ni and Au components are observed
by light contrast and dark one in TEM images,
respectively (Figs. 2a and 3a). Results showthat some AuNR@Ni nanoparticles having lowdensity Ni shells are formed, as shown in TEMEDS images (Figs. 3b-3d). The Au  Ni atomic
ratio of AuNR@Ni particle in Fig. 3 wasestimated to be 2.5  97.5% from EDS data. At
a high NaBH4 concentration, the reduction rateof Ni2+ is too fast so that Ni atoms concentration
becomes higher than its supersaturation.
Under such a condition, Ni particles are formed
not only around AuNRs but also outside AuNRs.
Therefore, the Ni component is distributed in awide range, so that the high atomic ratio of Ni
was obtained.
Figs. 2b, 2c, 4a-4f, and 5a-5f showcorresponding TEM and TEM-EDS data at 6
and 3 mM. At 6 mM, AuNR@Ni particles, whichhave a larger difference in dark (AuNR core)and light (Ni shell) contrast in TEM images
than that in 60 mM, are obtained (cf. Figs. 2aand 2b). This suggests that the density of Ni
shells on AuNRs at 6 mM is higher than that at

Fig. 2. TEM image of AuNR@Ni
nanoparticles prepared using NaBH4 at (a)60 mM, (b) 6 mM, and (c) 3 mM.

Fig. 3. TEM and TEM-EDS data of
AuNR@Ni nanoparticles prepared using
NaBH4 at 60 mM.

令和元年度

九州大学大学院総合理工学報告第 41 巻第 2 号

33

60 mM. The average thickness of Ni shells was
about 30–60 nm. Little Ni particles without
AuNR cores were observed, indicating that 6
mM is a better concentration than 60 mM forthe synthesis of AuNR@Ni particles withhigher density egg-like Ni shells as a major
product. In addition to AuNR@Ni particles
having uniform egg-like Ni shells, some
AuNR@Ni nanoparticles, which consist of
uniform aggregation of small Ni particles, areobserved as a minor product at 6 mM (e.g. Figs.
4a-4d). The average diameter of each spherical
Ni nanoparticle was about 2 nm and thethickness of Ni aggregation shell was 30–50 nm.
The Au  Ni atomic ratio of this AuNR@Ni
particles in Figs. 4a-4f was estimated to be 13 
87% from EDS data.
At 3 mM, products consist of two components,
which are also observed at 6 mM. One is
AuNR@Ni particles having aggregates of small
Ni nanoparticles. Ni aggregate shells arecomposed of two parts (Figs. 5a-5d); dense Ni
aggregate shells over AuNRs cores are coveredby low-density Ni aggregates. The thicknessesof high- and low-density Ni aggregates wereabout 30–50 and 30–50 nm, respectively.
Another component consists of AuNR@Ni
particles having low-density Ni shells without
Ni aggregates. This component was a major
product at 6 mM. The density of Ni shells at 3mM was lower than that obtained at 6 mM (cf.
Figs. 2b and 2c). The Au  Ni atomic ratio of
AuNR@Ni particle shown in Figs. 5a-5f wasestimated to be 12  88% from EDS data.
On the basis of above facts, AuNR@Ni
particles with egg-like Ni shells and Ni
aggregates are formed using NaBH4 at low
concentrations of 6 and 3 mM. No layered
epitaxial growth of Ni shells having welldefined facets (e.g. rectangular shape) was
observed.
Fig. 6 shows UV-Vis-NIR extinction spectraof AuNRs and AuNR@Ni nanoparticles. The
extinction spectrum of Au NRs gives a

Fig. 4. TEM and TEM-EDS data of
AuNR@Ni nanoparticles prepared using
NaBH4 at 6 mM.

Fig. 5. TEM and TEM-EDS data of
AuNR@Ni nanoparticles prepared using
NaBH4 at 3 mM.

34

Preparation of Au@Ni Core-shell Nanoparticles Using NaBH4 or L-ascorbic Acid
1.0

Absorbance

0.8

Au NRs
60 mM
6 mM
3 mM

0.6
0.4
0.2
0.0
200

400

600

800

1000

1200

Wavelength / nm
Fig. 6. UV-VIS-NIR spectra of AuNRs and
AuNR@Ni particles prepared using NaBH4.
longitudinal surface plasma resonance (LSPR)
band at about 910 nm with a full width at half
maximum (FWHM) of about 200 nm and aweak transverse SPR band at about 510 nm.
When Ni shells are formed at NaBH4
concentration of 60, 6, and 3 mM, the 910 nmband becomes weak and slight peak shifts toshorter wavelength (about 10 nm) at 60 mM
and to longer wavelength (about 30 nm) at 6
and 3 mM are observed. In addition to peaks at
900–940 nm and 510 nm region originatingfrom Au core components, weak tail bands areobserved in the 300–750 nm region. These
weak tail bands are attributable to the Nicomponents of AuNR@Ni and Ni particles ineach condition.22) The relative intensity of thetail bands to that of the 910 nm AuNR main
bands becomes weak with decreasing the
NaBH4 concentration. This shows that therelative amount of Ni component in the
products decreases with decreasing the NaBH4
concentration.
3.2 AuNR@Ni nanoparticles prepared using
L-ascorbic acidFigs. 7a and 8a-8f show TEM and TEM-EDS
data of AuNR@Ni nanoparticles prepared at a
L-ascorbic acid concentration of 30 mM and aline analysis data along a line shown in Fig. 8d.
Results show that thin irregular shapes of Ni
shells are overgrown on AuNRs and theirdistributions are slightly high around the
AuNRs. Ni shells over AuNRs are not flat
shape, indicating that layered epitaxial Ni shell
growth does not occur in this condition. No Ni
aggregate shells were observed using Lascorbic acid. The Au  Ni atomic ratio of thisAuNR@Ni particle in Figs. 8a-8f was estimatedFig. 7. TEM image of AuNR@Ni
nanoparticles prepared using L-ascorbic
acid at (a) 30 mM, and (b) 3 mM.
to 75  25% from EDS data.
Figs. 7b and 9a-9f show corresponding data of
AuNR@Ni at a lower L-ascorbic acidconcentration of 3 mM. Although layered Nishells are not formed, the formation of low
density of Ni shells are observed around
AuNRs. The Au  Ni atomic ratio of AuNR@Ni
particles in Figs. 9a-9f was estimated to be 75 
25% from EDS data. Ni component was
distributed in a wider space range than that at
30 mM. Therefore, a similar Au  Ni atomic
ratio was obtained even though the density of
Ni shells around AuNRs was low.
Fig. 10 show UV-Vis-NIR extinction spectraof AuNRs and AuNR@Ni nanoparticles
obtained using L-ascorbic acid at 30 and 3 mM.
At 30 mM, a long tail band of Ni component isobserved in the 350–1150 nm region and no Au
peak at 910 nm is observed. This suggests that
AuNRs are fully covered by Ni component inthis condition. At a lower concentration of 3
mM a weak AuNR component and a weak tailband of Ni component appear. These spectralfeatures are consistent with the observations

九州大学大学院総合理工学報告

Fig. 8. TEM image of AuNR@Ni
nanoparticles prepared using L-ascorbic
acid at 30 mM.
from TEM and TEM-EDS images, in which
AuNRs at 3 mM are covered by lower density
Ni shells than those at 30 mM. On the basis of
SPR data, more dense Ni shells, which
suppress the optical property of AuNR corecomponent, are formed by using L-ascorbic acid.
Thus, the optical property of AuNR@Ni
particles can be controlled by changing the Lascorbic acid concentration.
3.3
Growthnanoparticles

mechanismof

AuNR@Ni

We have previously found rectangular
AuNR@Pd particles having {100} facets are
dominantly formed when Pd shells wereovergrown on AuNRs having {250}, {5 12 0},{100}, and {110} facets (Fig. 11).21,25) If similar
epitaxial growth occurs, rectangular shaped
AuNR@Ni particles will be formed (Fig. 11a).
However, such rectangular shapes of AuNR@Ni

第 41 巻第 2 号

35

Fig. 9. TEM image of AuNR@Ni
nanoparticles prepared using L-ascorbic
acid at 3 mM.

3.0
2.5

Absorbance

令和元年度

Au NRs

2.0

30 mM
3 mM

1.5
1.0
0.5
0.0
200

400

600

800

1000

1200

Wavelength / nm
Fig. 10. UV-VIS-NIR spectra of AuNRs and
AuNR@Ni particles prepared using Lascorbic acid.

36

Preparation of Au@Ni Core-shell Nanoparticles Using NaBH4 or L-ascorbic Acidparticles were not obtained in this work. When
NaBH4 was used as a strong reducing agent,
AuNR@Ni nanoparticles having egg-like Nishells (Fig. 11b) and small spherical Ni
aggregates (Fig. 11c) were produced. On theother hand, AuNR@Ni nanoparticles having
irregular shaped Ni shells were produced byusing L-ascorbic acid as a weak reducing agent(Fig. 11d). In our previous study on Au@Ni,22)
Ni shells were epitaxially grown overoctahedral, triangular and hexagonal plate-like,
decahedral, and icosahedral Au core seeds,
even though lattice mismatch between Au andNi is as large as 13.6%. However, such epitaxial
growth of Ni shells was not observed for AuNR
core seeds in this study. A major reason for nonobservation of epitaxial growth of Ni shells willbe the existence of high-index {250} or {5 12 0}
facets in AuNRs. Epitaxial layered growth oversuch high-index facets having high surface
energies is more difficult than that over stablelow-index {100} and {111} facets having low
surface energies. This will be a major reason
why the layered growth of Ni shells was notobserved for AuNRs. Although epitaxial
layered growth was observed for AuNR@Cu,21)
it was not found for AuNR@Ni using the same
AuNRs with high-index {250} or {5 12 0} facetsin this study. The lattice mismatch between Au
and Ni (13.6%) is larger than that between Au
and Cu (11.4%). The large lattice mismatch
between Au core and Ni shell atoms will be
another reason for non-observation of epitaxial
growth of AuNR@Ni particles.

4.

Summary and Conclusion

AuNR@Ni core-shell nanorods were preparedby reducing Ni(NO3)26H2O using NaBH4 or Lascorbic acid as strong or weak reducing agent,respectively. When NaBH4 was used, egg-likeNi shells and Ni aggregate shells were formed.
On the other hand, AuNR@Ni particles having
irregular shapes of Ni shells were produced byusing L-ascorbic acid. Although Ni shells were
epitaxially grown over octahedral, triangularand hexagonal plate-like, decahedral, andicosahedral Au core seeds having low-index{100} and {111} facets in our previous study,22)
no epitaxial growth of rectangular AuNR@Ni
particles was observed using AuNR seeds inthis study. The lack of epitaxial growth using
AuNRs was explained by the presence of highindex {250} or {5 12 0} facets in AuNRs andlarge lattice mismatch between Au core and Ni
shell atoms (13.6%). In this work, AuNR@Ni
particles having egg-like Ni shells, Ni
aggregate shells, and/or irregular shaped Nishells were obtained, although their yields
depended on the reducing agent and its
concentration. These new shaped AuNR@Ni
may give unique catalytic and magnetic
properties.
Acknowledgments
We thank Dai-Nippon Toryo Co. Ltd. forsuppling us AuNRs used in this study. This
work was supported by JSPS KAKENHI Grant
number 25286003.
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Fig. 11. Four types of AuNR@Ni particles.

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