Synthesis and properties of Ni/ZnS magnetic luminescent bifunctional nanocomposites

Article abstract of DOI:10.1016/j.jallcom.2009.06.141

Ni/ZnS core/shell nanocomposites with both magnetic and luminescent properties were prepared via a two-step approach with an initial synthesis of Ni nanoparticles and then as templates for the deposition of ZnS nanoparticles. The structure and properties

Full text of DOI:10.1016/j.jallcom.2009.06.141

Journal of Alloys and Compounds 486 (2009) L1–L4
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Letter
Synthesis and properties of Ni/ZnS magnetic luminescent bifunctional
nanocomposites
a
a
a,b,∗
Yongbo Li , Ran Yi , Xiaohe Liu
a
Department of Inorganic Materials, Central South University, Changsha, Hunan 410083, People’s Republic of China
State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan 410083, People’s Republic of China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 May 2009
Received in revised form 22 June 2009
Accepted 22 June 2009
Available online 30 June 2009
Ni/ZnS core/shell nanocomposites with both magnetic and luminescent properties were prepared via a
two-step approach with an initial synthesis of Ni nanoparticles and then as templates for the deposition
of ZnS nanoparticles. The structure and properties of as-synthesized samples were characterized by XRD,
TEM, VSM and PL. Studies indicate that typical nanocomposites consist of Ni (core) with 80–130 nm diam-
eter and adjacent ZnS (shell) with a thickness of about 30–50 nm. The value of magnetization saturation
of nanocomposites significantly decreases after coating with ZnS as a result of the nonmagnetic coating
affecting. The emission spectrum of nanocomposites shows a small red shift as compared with that of
pure ZnS. The possible reason for this phenomenon may be ascribed to the change of size, morphology
and crystallinity of ZnS sample after coating.
Keywords:
Core/shell nanostructure
Chemical synthesis
Luminescence
Magnetic measurements
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
various approaches have been developed to synthesize magnetic
fluorescent bifunctional nanocomposites. For example, Bala et al.
Recently, there has been a great deal of interest in the fabrica-
tion of core/shell composite materials [1,2], because the physical
and chemical properties of these composite materials can be tuned
by controlling their compositions and the relative sizes of the core
and shell. The composite materials not only combine properties
of the original components but also possess novel and collective
performances not seen in the original components. For example,
Wang et al. have prepared Ag/ZnO nanocomposites through a facile
hydrothermal method; the nanocomposites enlarged the applica-
tion of single constituent and showed potential applications in
photodegradation of organic dye pollutants and destruction of bac-
teria [2].
prepared Ni/ZnS core/shell nanostructure by a sol–gel route [7].
Liu and co-workers synthesized Fe O /CdSe/ZnS magnetic fluo-
rescent bifunctional nanocomposites by depositing heterogeneous
semiconductor on magnetic nanoparticles [8]. Therefore, ZnS is an
3
4
As an important magnetic functional material, nickel has
attracted extensive attention with various applications, including
magnetic storage and catalytic fields [3,4]. However, Ni nanoparti-
cles are prone to be oxidized and unstable in acid or base due to their
large specific surface area and natural properties. A suitable coat-
ing is essential to overcome these limitations and widen its scope of
application. Zinc sulfide, a unique material for its semiconducting
and optical properties, has gained wide attention for applications
in light-emitting diodes and luminescence devices [5,6]. So far,
∗ Corresponding author at: Department of Inorganic Materials, Central South Uni-
versity, Room 236, Heiping Building, Changsha, Hunan 410083, People’s Republic of
China. Tel.: +86 731 8830543; fax: +86 731 8879815.
Fig. 1. XRD spectra of as-prepared samples: Ni nanoparticles (a) and Ni/ZnS
E-mail address: liuxh@mail.csu.edu.cn (X. Liu).
nanocomposites (b).
0
925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2009.06.141

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Y. Li et al. / Journal of Alloys and Compounds 486 (2009) L1–L4
Fig. 2. Typical TEM images of as-prepared Ni nanoparticles (a) and Ni/ZnS nanocomposites (b).
excellent and promising coating material and we select it for the
encapsulation of Ni nanoparticles to prepare magnetic fluorescent
nanocomposites.
In the present work, we have succeeded in preparing Ni/ZnS
nanocomposites and the synthesis procedure involve an initial
synthesis of magnetic material and then ZnS encapsulated on
the surface of Ni nanoparticles. The preparation process required
a relative low temperature, and no surface pretreatments were
needed to introduce new surface functional groups or addi-
tional covalent for the deposition of ZnS onto Ni nanoparticles
in our experiments. The Ni/ZnS core/shell nanocomposites are
discussed according to the structural and morphological character-
ization and property measurement including XRD, TEM, VSM and
PL.

Y. Li et al. / Journal of Alloys and Compounds 486 (2009) L1–L4
L3
2
.
Experimental details
Fig. 3 shows magnetic properties of as-obtained products mea-
sured with a VSM at room temperature. The value of magnetization
saturation (Ms) of Ni nanoparticles is 53.6 emu/g, which is higher
than that of 49 emu/g reported in [7]. This variation may be as
a result of anisotropy of nanoparticles and dipolar interaction
among nanoparticles. The Ms of Ni/ZnS nanocomposites is about
2.1. Synthesis
NiCl2·6H2O (0.2377 g), poly(vinyl pyrrolidone) (0.5 g), absolute ethanol (1 ml)
and N2H4·H2O (8 ml 80 wt%) and NaOH (2 g) were all added into deionized water
◦
(
20 ml). The above mixture was heated to 80 C and the reaction lasted for 1 h.
Then a simple two-step chemical solution reaction was employed to build core/shell
nanostructure, which was similar to the work of Shim and co-workers [9]. Firstly,
the obtained Ni (0.5 mmol) nanoparticles and thioacetamide (4 mmol) were put into
47.8 emu/g, lower than that of Ni nanoparticles. The reasonable
explanation for this phenomenon is that the nonmagnetic coating
layer on the surface of magnetic nanoparticles reduced the mag-
netism of magnetic materials. It can be found that the Ms of Ni/ZnS
nanocomposites is larger as compared to the previous report, this
result may be related to the larger Ms of core material and the shell
thickness. The covercivity (Hc) of magnetic nanoparticles is deter-
mined by the strength of magnetic dipoles in magnetic domains,
as well as the relations between adjacent magnetic domains [11].
However, it can be found that the value of Hc has almost no change
after ZnS coating from Fig. 3, indicating that the encapsulation has
no effect on the structure of Ni nanoparticles.
PL study is a powerful tool to investigate the optical proper-
ties of magnetic luminescent bifunctional nanostructures. We have
performed the room-temperature PL to show the optical proper-
ties of nanocomposite over its individual component (ZnS). The
preparation procedure of ZnS nanoparticles was similar to that of
Ni/ZnS nanocomposites. The only difference was that there was
no Ni nanoparticles as seeds involved during the preparation. The
room-temperature luminescence spectra of samples are presented
in Fig. 4. In Fig. 4, ZnS nanoparticles have a strong emission band
at 397 nm which is similar to other reports [12,13]. For nanocom-
posites, a strong emission is observed at 399 nm and the maximum
emission peaks of two samples are so close and show a small red
shift as compared with pure ZnS, and the similar phenomenon is
alsoobservedinTang’swork[14]. However, itisdifferentfromBala’s
work in which Ni/ZnS nanocomposites present a lager blue shift
compared to pure ZnS [7]. The possible reason of the small red shift
is that the size, morphology and crystallinity of ZnS sample may
change after Ni nanoparticles as seeds in synthesis procedure of
nanocomposites, and the alteration may make PL properties of two
samples relatively approach because PL property of ZnS nanomate-
rials is generally sensitive to the synthetic conditions, crystal size,
and shape. Furthermore, compared to pure ZnS nanoparticles, the
nanocomposite particles become larger after the coating so as to
result in a red shift of emission band. Compared with pure ZnS, it
is noted that the fluorescent intensity of nanocomposite decreases
obviously, this significant decrease indicates that the presence of Ni
nanoparticles in nanocomposite strongly reduces the fluorescence
◦
deionized water (20 ml) under vigorous mechanical stirring under 60 C for 2 h. Sub-
sequently, Zn(AC)2·H2O (2 mmol) was added to the above solution and the mixture
◦
was irradiated by ultrasonic waves under 60 C for 30 min. The final products were
centrifuged and washed with absolute ethanol and distilled water several times
respectively, and dried in a vacuum at 60 C for 4 h.
◦
2.2. Characterization
The obtained samples were characterized by X-ray powder diffractometer (XRD)
with Cu K␣ radiation (ꢀ = 1.5418 Å). The size and morphology of the products were
characterized by a JEM-200CX transmission electron microscope (TEM) at 160 kV.
Magnetic properties of the samples were carried out by using a LDJ-9600 vibrating
sample magnetometer (VSM). Photoluminescence (PL) measurements were carried
out at room temperature using 260 nm as the excitation wavelengths with a lumi-
nescence spectrometer (Hitachi F-4500).
3
. Results and discussion
During the sample preparation, the molar ratio of Zn(AC) ·H O
2
2
to thioacetamide was selected to be 0.5 in order to obtain the
better density of ZnS particles on the surface of Ni nanoparti-
cles, which was testified by a literature report [10]. Fig. 1 shows
the XRD spectra of as-prepared samples. The diffraction peaks at
◦ ◦ ◦
2
ꢁ = 44.4 , 51.8 and 76.4 can be well indexed to the (1 1 1), (2 0 0),
and (2 2 0) planes of cubic fcc-type Ni crystals (JCPDS 65-2865).
As for nanocomposites, the diffraction peaks of ZnS phase can be
clearly distinguished and the data matches well those for pure cubic
phase ZnS (JCPDS 80-0020), which indicates that the ZnS phase
exists in the nanocomposites. The crystallite size of ZnS sample can
be calculated according to the Scherrer equation (d = 0.89ꢀ/ˇ cos ꢁ),
and the average size is about 2.1 nm.
Typical TEM micrographs of the pure Ni nanoparticles and
Ni/ZnS nanocomposites are shown in Fig. 2(a) and (b). As revealed
by TEM images, the Ni nanoparticles are almost spherical with
8
0–130 nm diameter. From Fig. 2(b), it can be seen that Ni nanopar-
ticles are completely encapsulated in a ZnS shell with a thickness
of about 30–50 nm.
The magnetic properties of nanocomposites were examined
and compared with the magnetic properties of Ni nanoparticles.
Fig. 3. Magnetization versus applied magnetic field for Ni nanoparticles (a) and
Ni/ZnS nanocomposites (b).
Fig. 4. PL spectra of the samples: ZnS sample (a) and Ni/ZnS nanocomposites (b).

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Y. Li et al. / Journal of Alloys and Compounds 486 (2009) L1–L4
intensity. The reason may be due to absorption of Ni nanoparticles,
the so-called quantum dots and static quenching [15].
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In conclusion, we have prepared Ni/ZnS core/shell nanocom-
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nanoparticles. The typical nanocomposites consist of Ni (core) with
8
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Financial support of this work by National Natural Science Foun-
dation of China (No. 50504017), and Hunan Provincial Key Science
and Technology Project of China (No. 2007FJ3008) is gratefully
acknowledged.