Synthesis, structure, and magnetic properties of Ni and Co nanoparticles encapsulated by few-layer h-BN

Article abstract of DOI:10.1016/j.jallcom.2016.07.279

Metal nanoparticles with high magnetization values and good stability in acid and base environments have potential applications in industry, biological, and environmental fields. In this paper, we demonstrated a novel approach to synthesize Ni and Co nanoparticles encapsulated by few-layer h-BN (i.e., Ni@h-BN and Co@h-BN) via annealing of freshly prepared metal ammine complexes ([Ni(NH₃)₆]Cl₂or [Co(NH₃)₆]Cl₃) and KBH₄with flowing nitrogen gas at 900 °C for 2 h. Formation of Ni@h-BN and Co@h-BN was observed from X-ray diffraction patterns, scanning electron microscopy and transmission electron microscopy. Bonding nature and elemental composition were also characterized by Fourier-transform infrared spectroscopy and Energy-dispersive X-ray spectroscope. Bonding formation process was illustrated by DSC/TGA analysis. Ni and Co NPs were found encapsulated into h-BN shells (c.a. 15 layers) with a high yield amounting to 42.6%. Additionally, the obtained saturation magnetization and coercivity of Ni@h-BN and Co@h-BN were 10.9 emu/g and 116 Oe, 83.3 emu/g and 62 Oe, respectively. Hence, the excellent performance of Co@h-BN and Ni@h-BN, especially the latter of which was prepared for the first time, could pave the way for application of h-BN encapsulation.

Full text of DOI:10.1016/j.jallcom.2016.07.279

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Journal of Alloys and Compounds 689 (2016) 153e160
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Synthesis, structure, and magnetic properties of Ni and Co
nanoparticles encapsulated by few-layer h-BN
,
Dongliang Fan a, Jin Feng a, Suyun Zhang a, Xiaomeng Lv a *, Tianyi Gao a, Jimin Xie a,
,
**
Jun Liu b
a School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013, PR China
b School of the Environment and Safety Engineering, Jiangsu University, Zhenjiang, 212013, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Metal nanoparticles with high magnetization values and good stability in acid and base environments
have potential applications in industry, biological, and environmental fields. In this paper, we demon-
strated a novel approach to synthesize Ni and Co nanoparticles encapsulated by few-layer h-BN (i.e.,
Ni@h-BN and Co@h-BN) via annealing of freshly prepared metal ammine complexes ([Ni(NH3)6]Cl2 or
[Co(NH3)6]Cl3) and KBH4 with flowing nitrogen gas at 900 ꢀC for 2 h. Formation of Ni@h-BN and Co@h-
BN was observed from X-ray diffraction patterns, scanning electron microscopy and transmission elec-
tron microscopy. Bonding nature and elemental composition were also characterized by Fourier-
transform infrared spectroscopy and Energy-dispersive X-ray spectroscope. Bonding formation process
was illustrated by DSC/TGA analysis. Ni and Co NPs were found encapsulated into h-BN shells (c.a. 15
layers) with a high yield amounting to 42.6%. Additionally, the obtained saturation magnetization and
coercivity of Ni@h-BN and Co@h-BN were 10.9 emu/g and 116 Oe, 83.3 emu/g and 62 Oe, respectively.
Hence, the excellent performance of Co@h-BN and Ni@h-BN, especially the latter of which was prepared
for the first time, could pave the way for application of h-BN encapsulation.
Received 30 March 2016
Received in revised form
22 May 2016
Accepted 26 July 2016
Available online 28 July 2016
Keywords:
Ni@h-BN
Co@h-BN
Encapsulation
Nanoparticles
Magnetic
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
Therefore, surface coating of magnetic nanoparticles is needed to
solve the problem. In addition, most magnetic materials are
Group VIII base-metal nanoparticles (NPs), i.e., Fe, Co, Ni, have
recently drawn great attention due to its low cost, fascinating d-
band electric property, and have been investigated and applied in
various energy and environmental catalysis, including Fischer-
Tropsch (FꢁT) synthesis, steam and aqueous reforming, methana-
tion, CO2 dry reforming, water-gas shift, and CO oxidation [1].
Furthermore, Metal nanoparticles of Fe, Co, Ni and their alloys have
an advantage of showing high magnetization, which render them
potential applications such as photocopying, medical imaging,
magnetic drug targeting and magnetic hyperthermia [1,2]. Unfor-
tunately, oxidation- and wear resistances of the metal NPs surface
are weak points when these particle sizes are turndown, and the
oxidation causes the deterioration of the magnetic properties [3].
accompanied by exothermic heat due to the eddy current loss in
high-frequency devices [4]. This drawback restricts their practical
applications and even makes investigations of their nanocrystalline
properties difficult [5].
One way to protect metal NPs is to encapsulate the NPs with a
chemically stable species, such as graphite [6], silicon dioxide [7],
organic polymer [8] and boron nitride nanotubes [9]. The encap-
sulation structure not only modulates the properties of metal NPs
but also protects metal NPs from growing larger. It has been re-
ported recently that single Fe, Co, and Ni sites embedded in a
graphite matrix showed prominent catalytic activity and selec-
tivity in the oxygen reduction and hydrogen evolution reactions
[10]. As so far, NPs encapsulated in graphite nanocages are antic-
ipated to feature in both scientific research and future device
applications such as cluster protection, nanoball bearings, nano-
optical magnetic devices, catalysis and biotechnology [11,12].
However, graphite sheets are conductive, whereas semiconductor
sheets are required for the control of electrons in future nanoscale
devices.
* Corresponding author.
** Corresponding author.
E-mail addresses: laiyangmeng@163.com (X. Lv), liujun1227@mail.ujs.edu.cn
(J. Liu).
http://dx.doi.org/10.1016/j.jallcom.2016.07.279
0925-8388/© 2016 Elsevier B.V. All rights reserved.

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D. Fan et al. / Journal of Alloys and Compounds 689 (2016) 153e160
Hexagonal boron nitride (h-BN), due to its cheap and highly
were used as received from Sigma Aldrich. Nickel nitrate hexahy-
drate, cobalt chloride, hydrochloric acid, ammonium chloride,
ethanol, hydrogenperoxidesolution, activatedcarbonand potassium
borohydride were all purchased from Sinopharm Chemical Reagent
with AR grade and used as received without further purification.
Deionizedwaterwasusedthroughoutthecourseoftheinvestigation.
stable property underextreme chemical and physicalenvironments,
is one of the best solutions for encapsulation. h-BN (also known as
white graphene) is a nonoxide ceramic material with attractive
physical properties, including outstanding thermal conductivity,
adsorption capacity, excellent electrical insulation, desirable
dielectric properties, and promising low thermal expansivity as well
as superior thermal stability [13,14]. Since the discovery of graphene
in 2004, h-BN with graphene-like structure has attracted much
attention [15]. Although h-BN shows similar properties of graphene,
its electrical properties are quite different from those of graphene;
while graphene is an electrical conductor, h-BN is a semiconductor
with a direct band-gap of 6 eV [16]. Furthermore, its potential
application covers a broad area, such as gate dielectric layers [17],
proton transport management [18], sensing device [19], and trans-
parent and protective coatings [20]. The h-BN coatings immunize
the nanoparticles against environmental degradation and therefore
retain their intrinsic nanocrystalline properties [21,22]. Further-
more, h-BN coatings can endow these nanoparticles with biocom-
patibility and a clear potential for further functionalization. It is
enormously significant for prospective biomedical applications [12].
Although some h-BN nanocapsules have been observed as by-
products from h-BN nanocage synthesis in previous work, there
are few reports on the formation of h-BN nanocapsules [9].
Great progress has been achieved in theoretical aspects of their
structures and properties [23e25] and BN nanocages have been
found as by-products, especially in violent syntheses such as arc
discharge [26]. However, experimental progress is still limited due
to harsh synthesis condition. Zhu et al. once reported a high-
temperature (1750 ꢀC) synthesis of BN nanocages with diameters
below 200 nm by using a homemade B-N-O precursor [27]. Takeo
et al. have put forward a facile route to synthesize silver nano-
particles encapsulated within h-BN nanocages produced from
mixtures of boric acid, urea and silver nitrate upon reduction at
700 ꢀC in hydrogen for 7 h [28]. Ichihito et al. applied the annealing
method to prepare h-BN nanocapsules encaging Fe nanoparticles
by using Fe4N/B powders as raw materials [29]. Using most of these
methods, however, the yield of produced h-BN nanocapsules was
very small, and it was difficult to evaluate their optical, magnetic or
electronic properties. Clearly, synthesis of a larger amount of h-BN
nanocapsules was required.
The purpose of the present work is to synthesize metal nano-
particles encapsulated within h-BN layers by annealing of metal
ammine complexes with KBH4, investigate their structure and
magnetic properties and provide method of scalable production of
h-BN encapsulating metal nanoparticles. Herein, we report a freshly
prepared amine complexes route for the synthesis of Ni@h-BN and
Co@h-BN, based on an anion coordination protocol. First, ammonia
(the precursor of h-BN) ligand-based molecular was designed to
bind the surface of the pre-formed primary metal core ion before
reduction, i.e., an appropriate amount of ammonium chloride was
added into hexaammine nickel ligand or hexaammine cobalt ligand
aqueous solution. Second, solution was filtered after complete
precipitation, and dried in the vacuum oven after properly cleaned
process. Then, the precursors and potassium borohydride (KBH4)
were annealed at 900 ꢀC in N2 for 2 h and final products were ob-
tained. Finally, we explored the unique magnetic properties
exhibited by the final products in a remarkable improvement of the
saturation magnetization and remanent magnetization.
2.2. Encapsulation of Ni and Co nanoparticles by h-BN
In a typical procedure, metal ammine complexes ([Ni(NH3)6]Cl2
or [Co(NH3)6]Cl3) were freshly prepared as follows. First, 9 g nickel
nitrate hexahydrate was added into 5 mL of deionized water in
100 mL conical flask, and stirred to complete dissolution. Then,
after adding 40 ml of concentrated aqueous ammonia, the dark blue
solution was then added 10 g of ammonium chloride solid under
stirring until dissolved. The blue precipitate crystals were obtained,
and the crude product was purified by vacuum filtration, washed
with a small amount of concentrated aqueous ammonia, ethanol,
ether and dried in air to obtain hexaammine nickel chloride
products [Ni(NH3)6]Cl2.
As to the synthesis of [Co(NH3)6]Cl3, 9 g finely cobalt chloride,
6 g of solid ammonium chloride and 10 ml of deionized water were
mixed and heated to dissolve before adding 0.5 g of activated car-
bon. After cooled to room temperature, 20 ml of concentrated
aqueous ammonia was added, and further cooled to below 10 ꢀC
before slowly adding 20 ml of 6% hydrogen peroxide. The mixture
was heated in a water bath at 60 ꢀC for 20 min. After cooling down,
filtered precipitate was redissolved in 3 ml of concentrated hy-
drochloric acid containing 80 ml boiling water, and the hot solution
filtered. After filtration,10 ml of concentrated hydrochloric acid was
slowly added into the filtrate with ice cooling until complete pre-
cipitation of crystals. After vacuum drying, hexaammine cobalt
chloride [Co(NH3)6]Cl3 as an orange-yellow solid powder was
obtained.
The freshly prepared amine complexs ([Ni(NH3)6]Cl2 (or
[Co(NH3)6]Cl3)) and KBH4 powders with molar ratio of 3:1 were
well mixed by triturator, and then set on an alumina boat. Since
KBH4 powder was easily deliquesced in the atmosphere, all ma-
nipulations were done in an inert gas. The samples were annealed
with flowing nitrogen (N2) gas (100 sccm) at 900 ꢀC for 2 h, and
cooled down to room temperature in the furnace. The schematic
of synthesis process is shown in Fig. 1. Annealed samples were
well washed and filtrated several times in deionized water and
ethanol.
2.3. Characterization
The crystal structure and phase purity of final products were
analyzed by X-ray diffraction (XRD), using a D8 Advanced XRD
(Bruker AXS, Germany) equipped with Cu-Ka radiation
(l
¼ 1.5406 Å) and the scanning rate was 7ꢀ minꢁ1 in the 2
q range
from 5ꢀ to 80ꢀ. Fourier-transform infrared (FTIR) spectroscopy was
performed with a Nicolet Nexus 470 IR to determine the specific
functional groups presenting on the surface. Scanning electron
microscopy (SEM) was performed with a Hitachi (Japan) S-4800 II,
operated at an acceleration voltage of 10 kV, to characterize the
morphology of the prepared samples. Elemental analyses were
performed with a Vario El Cube elemental analyzer (Elementar,
Germany). The morphology and particle size of the products were
also examined by transmission electron microscopy (TEM) with a
Jeol (Japan) JEM-2010 operated at 200 kV. Magnetic properties
were measured by vibrating sample magnetometer system (VSM)
under 10000 Oe field.
2. Experimental section
2.1. Materials
Hexagonal boron nitride (AR grade) flakes (1e2 mm in diameter)

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D. Fan et al. / Journal of Alloys and Compounds 689 (2016) 153e160
155
3. Results and discussion
(a)
(b)
(c)
h-BN
Ni@h-BN
Co@h-BN
26.75°
(002)
3.1. Crystal structure and functional groups analysis
44.26°
(111)
26.56°
(002)
51.52°
(200)
Fig. 2 shows the XRD pattern of pristine BN powders (a), Ni@h-
BN (b) and Co@h-BN (c) measured using X-ray diffractometer with
75.90°
(220)
CuK
a
radiation (
l
¼ 1.54 Å). Four characteristic diffraction peaks at
44.51°
(111)
26.75ꢀ, 41.58ꢀ, 54.94ꢀ and 76.08ꢀ in Fig. 2(a) can be readily indexed
to (002), (100), (004) and (110) planes of h-BN (JCPDS 45e0893)
[30,31]. As exhibited in Fig. 2(b), the peaks located at 44.51ꢀ, 51.85ꢀ
and 76.37ꢀ correspond to Ni (JCPDS 04e0850), arising from (111),
(200) and (220) planes, respectively [32,33]. Fig. 2(c) reveals the
XRD pattern of the as-synthesized Co@h-BN. The small reflection
peaks visible at 44.26ꢀ, 51.52ꢀ and 75.90ꢀ, were corresponding to
(111), (200) and (220) reflections of the crystalline planes of Co
species, respectively [34,35], which can be indexed to Co (JCPDS
15e0806) with space group Pnma.
51.85°
(200)
26.86°
(002)
41.42°
(100)
76.37°
(220)
41.58°
(100)
54.94°
(004)
76.08°
(110)
10
20
30
40
50
60
70
80
FT-IR spectroscopy is used as a primary tool to determine the
surface functional groups and bonding nature in h-BN. In Fig. 3, FT-
IR spectra of pristine h-BN powder (a), Ni@h-BN (b) and Co@h-
BN(c) were compared in 500e4000 cmꢁ1. The pristine h-BN pow-
2θ (degree)
Fig. 2. XRD pattern of (a) pristine h-BN powder (b) Ni@h-BN (c) Co@h-BN.
der exhibits two sharp absorption peaks at 1405 and 804 cmꢁ1
,
corresponding to in-plane BN stretching vibration of sp2-bonded h-
BN and out of plane B-N-B bending vibration, respectively [36,37].
The Ni@h-BN (b) also shows two sharp absorption peaks at 1384
and 804 cmꢁ1, however, the relative peak intensity of in-plane B-N
stretching and out of plane B-N-B bending vibration is decreased
significantly compared with pristine h-BN powder. It could be
attributed to the smaller thickness of Ni@h-BN compared with h-
BN powder [38]. It should be noted that peaks at 1376 cmꢁ1 and
804 cmꢁ1 are also found in the spectrum of Co@h-BN(c), which
correspond to characteristic peak of spectra of pristine h-BN (a).
(C)
(b)
Co@h-BN
Ni@h-BN
(a)
h-BN
B-N bending
3.2. Morphology and particle size analysis
Scanning electron microscopy (SEM), energy-dispersive X-ray
spectrum (EDX) and transmission electron microscopy (TEM) were
employed to characterize the morphology, particle size and identify
the atomic composition of the Ni@h-BN and Co@h-BN samples.
Fig. 4 and Fig. 5 show the low- and high-magnification TEM images
of the Ni@h-BN and Co@h-BN samples, respectively. The low-
B-N stretching
500
1000 1500 2000 2500 3000 3500 4000
-1
Wavenumber
(
cm
)
Fig. 3. FT-IR spectra of (a) pristine h-BN powder (b) Ni@h-BN (c) Co@h-BN.
Fig. 1. The schematic of synthesis process.

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D. Fan et al. / Journal of Alloys and Compounds 689 (2016) 153e160
Fig. 4. TEM image of (a,b,c) Ni@h-BN. HRTEM image of (d) Ni@h-BN.
magnification TEM image in Fig. 4a, b, c presents the overall
morphology of the Ni@h-BN, which indicates that the Ni@h-BN
powders consist of metal nanoparticles (NPs) completely coated by
h-BN shells. The metal NPs have a lateral size of generally
20e200 nm. According to the statistical analysis of HRTEM (Fig. 4
d), the h-BN shells on the Ni NPs are 10e40 layers, and most of
the h-BN shells (>90%) consist of 15 layers. The inter-lattice dis-
tance of the core is 0.20 nm, which is in good agreement with that
of fcc-Ni (111) planes. Fig. 5 displays a typical TEM image of the
Co@h-BN sample. Co nanoparticles are dispersed in h-BN matrix
with different shapes, such as spheres, ellipsoids, etc. Overall,
except of a few large particles with diameter size of 200 nm, most
Co nanoparticles with small size varied from 5 to 20 nm are shown
in Fig. 5a, b, c. The HRTEM (Fig. 5 d) images displayed cobalt NPs
that are completely encapsulated by h-BN shells, and the h-BN
shells on the cobalt NPs are 5e10 layers. The interlayer distance of
the encapsulation is 0.33 nm, corresponding to h-BN (002) planes.
The inter-lattice distance of the core is calculated to be 0.21 nm,
which is consistent with that of fcc-Co (111) planes. Although Co, Ni
particles are prepared under the same processing conditions, the
size of the formed particles differs with respect to the type of ele-
ments, which means the properties of the elements themselves
such as the differences in melting points or vapor pressures may
play a role in size variations [39].
hybrid composites. As can be seen in Fig. 6a, b, the Ni@h-BN
nanoparticles sample consists of h-BN-encapsulated nanoparticles
and h-BN sheets. Several nanoparticles trapped at the tips of h-BN
sheets can also be observed from above image. Most of the nano-
particles are in the size range of 50e200 nm and displays well-
defined coreeshell structure of nanoparticles with shell thickness
of about 15 nm, which is agreed with the TEM result. The SEM
images of Co@h-BN sample are shown in Fig. 6d, e. It can be seen
that the nanoparticles are spherical with size in the range of
20e200 nm and completely encapsulated by h-BN shells, and the h-
BN shells on the encapsulation NPs are c.a. 10 nm.
Ni, B, and N element can be found in the corresponding EDX
pattern of Ni@h-BN (Fig. 6c). The nitrogen peak is as high as that of
boron peak, indicating the approximate ratio of 1:1. Fig. 6f shows
the EDX pattern Co@h-BN powders analysis confirming the exis-
tence of Co, B, and N element, and the boron peak is as high as that
of nitrogen peak, indicating the approximate ratio of 1:1.
Combining with the synthesis methology, the characterization re-
sults indicate that the growth of metal nanocapsules coated with h-
BN layers were formed synchronously.
3.3. Chemical reactions involved
In order to reveal the reaction process of the Ni@h-BN or Co@h-
BN and investigate the influence of thermal stability and oxidation
behavior, a DSC-TGA measurement was carried out as shown in
Fig. 6 clearly showed a typical low magnification SEM image and
energy-dispersive X-ray spectrum (EDX) of Ni@h-BN and Co@h-BN