Research on nickel morphology of dependence on the microwave-assisted polyol method

Article abstract of DOI:10.1002/pssa.200925175

The structures, morphology and magnetic properties of the ultrafine Ni nanoparticles prepared by microwave-assisted polyol method was investigated, using X-ray diffraction (XRD), transmission electron microscopy (TEM) and vibrating sample magnetometer (VS

Full text of DOI:10.1002/pssa.200925175

Phys. Status Solidi A 207, No. 2, 360–363 (2010) / DOI 10.1002/pssa.200925175
a
p s s
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applications and materials science
Research on nickel morphology of
dependence on the microwave-assisted polyol method
,
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*
Liu Xiansong , Gao Huamin , Qiu Shixing , Chen Luguo , Dend Yuxing , Zhou Dan ,
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and Xu Xiaobing
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Engineering Technology Research Center of Magnetic Materials, Anhui Province, School of Physics & Materials Science,
Anhui University, Hefei 230039, P. R. China
Chemistry and Material Science Department of Chaohu College, Chaohu 238000, P. R. China
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Received 9 April 2009, revised 15 September 2009, accepted 16 September 2009
Published online 16 October 2009
PACS 75.20.En, 75.50.Cc, 75.60.ꢀd, 75.75.þa, 81.07.Bc
*
Corresponding author: e-mail xiansongliu@yahoo.com.cn, Phone: þ86-555-5107237, Fax: þ86-555-5107237
The structures, morphology and magnetic properties of the successfully synthesized. It was interesting to note that the Ni
ultrafine Ni nanoparticles prepared by microwave-assisted nanoparticles are ideally spherical, which were examined by the
polyol method was investigated, using X-ray diffraction TEM. As immersed in different solutions, we can get different
(
XRD), transmission electron microscopy (TEM) and vibrating shape nickel particles with different conditions. Especially, the
sample magnetometer (VSM), respectively. The XRD results experimental results indicated that nanohair nickel particles
suggested that Ni nanoparticles with an fcc structure were were ferromagnetic.
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Introduction Nanoscale particles of ferromagnetic
Till now, nickel nanoparticles can be fabricated by a
metals, such as Fe, Co and Ni has gained wide-spread variety of physical and chemical methods including ultra-
interests in recent years, due to their unique physical sound irradiation, evaporation technique [12], ultrasonic
properties and potential applications in diverse areas, such spray pyrolysis, chemical reduction [13–16], electrochemi-
as magnetic recording media, sensors and catalysts. A cal technique and polyol method [17, 18]. It was demon-
considerable amount of work has been done in the last decade strated that the size, morphology and crystallinity of the
on various nanoparticles systems [1], such as metal particles, nickel particles were critical for their properties and
transition metal oxides, Ni ferrite [2] and transition metal- applications. Therefore, shape-controlled synthesis of met-
boron alloys, such as Fe–Ni–B and Co–Ni–B [3]. The allic structures has been a subject of intensive research in
transition metal oxide systems Fe–FeO, Co–CoO and recent years. In this paper, the microwave-assisted polyol
transition metal insulator systems [4] were studied as well, method was employed to fabricate the monodispersed nickel
such as Fe, Co, or Ni dispersed in SiO or Al O [5]. Ni particles with different shapes including cube, microspheres
2
2 3
nanoparticles addressed much attention over the past and nanohairs under different reaction conditions. The
decades because of their excellent magnetic properties, effects of surfactant concentration, the temperature and
which enable them to be applied in the magnetic sensors, reaction time were discussed. The magnetism of the nickel
memory devices and biomolecular separations [6]. Nickel powders with different morphologies was also investigated.
was widely used in many fields such as catalysis [7], fuel cell
electrodes [8] and magnetic storage media [9]. Recently,
2 Experimental The starting material was a solution
many researches on core–shell nanostructures of nickel of nickel (II) acetate tetrahydrate (99.998%, Aldrich
particles, nanoparticles of carbon and palladium encapsu- Chemical Co.) in ethylene glycol (>99%, Bio Lab Ltd.) or
lated with Ni nanoparticles used in catalysis [10] and nickel– triethylene glycol (TREG) (>99%, Bio Lab Ltd.). Following
nickel oxide complex structures used for high density the previously described process [19], a 100 ml glass flask
magnetic recording due to the magnetic exchange coupling was placed in a microwave oven (Spectra, 900W) and
between the ferromagnetic material (Ni) with the antiferro- connected to a condenser. In view of nanoscale nickel
magnetic material (NiO) [11].
agglomeration [20], so-called surfactants or dispersants with
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Phys. Status Solidi A 207, No. 2 (2010)
361
polyvinylpyrrolidone (PVP, average molecular weight
40 000, Sigma Chemicals) had to be added. Before the
reaction, 50 ml of a solution of 0.100–0.250 M Ni(Ac) in
2
ethylene glycol (or TREG) was purged by argon for 30 min,
then the microwave oven was turned on at the power level of
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0–100% with a continued flow of the gas. The reaction was
stopped as soon as the black suspension appeared, and
immediately cooled in ice water. The resulting solid product
was washed thoroughly with ethanol and centrifuged. All
these processes were repeated six times. The nickel particles
were dried under vacuum and kept in a glove box.
The powder X-ray diffraction (XRD) patterns were
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collected on a Bruker AXSD Advanced Powder X-ray
diffractometer (using Cu Ka radiation l ¼ 0.15418 nm). The
particle morphology and nature of Ni were studied by
transmission electron microscopy (TEM) employing a
JEOL-JEM 100 SX microscope working at 100 kV. After a
sonication bath for 20 min in absolute ethanol, samples for
TEM were prepared by placing a drop of the sample
suspension on a copper grid (400 meshes, Electron
Microscopy Sciences) coated with carbon film. Magnetic
measurements at room temperature were conducted using an
Oxford Instrument, vibrating sample magnetometer (VSM).
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3
Results and discussion
.1 Morphology and structural characterization
by TEM In order to reveal the morphology and structure of
Ni nanoparticles, the TEM experiment was performed for the
sample prepared under a solution of 0.250 M Ni(Ac) in
2
TREG. A typical TEM image of the Ni sample is shown
in Fig. 1a, which shows Ni grains of a polygon structure with
the grain sizes in the range of 500–800 nm. The particle
morphology is very similar to that given in literature [18].
Its growth process was usually attributed to the Ostwald
ripening. The formation of quasi-spheral clusters in the
TREG solution must result from an even quicker aggregation
during both nucleating and growing stage at the boiling
temperature (285 8C).
Figure 1 TEMimagesofsampleswithtwophotosforeachsample:
a) Ni particles (800 nm) in TREG at the power level of 100% of the
microwave oven; (b) Ni particles (10 nm)in ethylene glycol with 4 g
PVP, HRTEM is given at the bottom; (c) Ni (spherical) in TREG
at the power level of 70% of the microwave oven; and (d) Ni
(
The conventional polyol method was previously used to
prepare metallic nickel particles, but the particles of the (nanohairs) in ethylene glycol at the power level of 75% of the
corresponding reaction under microwave radiation were microwave oven.
slightly large and rather uniform. During the current
experiments in a solution of 0.100 M Ni(Ac) in ethylene
2
glycol, some PVP were added (4 g PVP). It is found that the particle from growing and several particles from coming
nickel particles have roughly particles about 10 nm in together in ethylene glycol solution added adequate PVP.
diameter and a few particles are connected together. It is
The morphology of the particles was further revealed by
clear to see that the size and morphology of nickel particles in TEM measurements. These particles are synthesized in a
Fig. 1b are different from that in Fig. 1a. We found that a solution of nickel (II) acetate tetrahydrate (99.998%, Aldrich
suitable amount of PVP is needed to stabilize the nanoscale Chemical Co.) in TREG (>99%, Bio Lab Ltd.). Before the
cluster produced during the microwave-assisted reaction and reaction, 50 ml of a solution of 0.100 M Ni(Ac) in TREG
2
prevent the agglomeration. It indicates that the average was purged by argon for 20 min, after which the microwave
particle size of the sample obtained with the addition of the oven was turned on at a power level of 70% with a continued
surfactant is relatively smaller than that without the addition flow of the gas. The reaction was stopped as soon as a black
of the surfactant. It can be thought that the surfactant has two suspension appeared which was immediately cooled in ice
essential effects: the absorption and the decrease of surface water. The resulting solid product was washed thoroughly
tension, and the colloid effect. Therefore, the surfactant with ethanol and centrifuged. TEM demonstrated that the
molecules coat the nickel particles to prevent the single morphology of Ni nanoparticles is predominantly ideally
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L. Xiansong et al.: Research on nickel morphology
Figure 3 Themagnetichysteresisloopsofpolygonnickelparticles
by VSM at room temperature.
Figure 2 X-ray powder diffraction pattern of the samples: (a) Ni
polygon particles (100 nm), (b) Ni particles (10 nm), (c) Ni (spher-
ical), and (d) Ni (nanohairs).
spherical with smooth surfaces and uniform size. Figure 1c
presents a magnified TEM image.
Some hairy zones in the Ni nanoparticles, the
monomorphic and uniform hair-like Ni nanoparticles, can
be detected for a solution of 0.200 M Ni(Ac) in ethylene
2
glycol at the power level of 75% of the microwave oven.
Furthermore, with increasing thetemperature, the additionof
a little amount of PVP (1 g) was beneficial to the formation of
hair-like Ni nanoparticles with a small diameter and
narrower distribution, shown in Fig. 1d. This indicates that
the as-synthesized Ni nanohairs were actual monodispersive
self-organization by hundreds of smaller primary nanolines,
which can be regarded as a secondary nanostructure. This
self-organization should be ascribed to the mutual attraction
of magnetic forces, and with the synthesis time increased, the
collision and coalescence rates of Ni nucleus would get Figure 4 ThemagnetichysteresisloopsofNinanohairsbyVSMat
room temperature.
higher.
3
.2 Structural characterization by XRD Figure 2
2
shows the XRD patterns for the precipitated powder at magnetization s ¼ 44.5 A m /kg in Fig. 3, it is found that the
s
different reaction conditions. The position of the diffraction coercivity is higher than that of the polygon structure
peaks matches closely the reported JCPDS (4-850) data. In (110 Oe). It can be thought that the interaction between
Fig. 2, all the peaks at different angles are from fcc Ni(111), particles would decrease the coercivity. However, the M–H
(200) and (220). This indicates that the single phase fcc Ni is hysteresis loop of the nanohair samples shows that it is not a
successfully synthesized by the microwave-assisted polyol saturation magnetization curve absolutely under the applied
method.
field with 20 000 Oe. Our results indicate that the magnetic
hysteresis loops of the spherical nickel particles shown in
3.3 Magnetic properties In order to get more Fig. 1c is very similar to that in Fig. 3 and the coercivity Hc
insight into the nature of the as-prepared samples, the and saturation magnetization s is not obviously different.
s
magnetic state of the as-prepared samples has been checked For the sample in Fig. 1b, we cannot get the magnetic
at room temperature, measured by VSM in Figs. 3 and 4. It is hysteresis loop in exact definition because it is only a
worth noting that the Ni nanohairs at 300 K are ferromag- superparamagnetic curve.
netic as shown in Fig. 4. The magnetic hysteresis loops of Ni
nanohairs are given with the coercivity H ¼ 335 Oe and
c
2
saturation magnetization s ¼ 12.9 A m /kg. By comparison
4 Conclusions Our experiment shows that the single
s
with the results of the coercivity H ¼ 110 Oe and saturation phase fcc Ni by XRD is successfully synthesized by the
c
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Original
Paper
Phys. Status Solidi A 207, No. 2 (2010)
363
microwave-assisted polyol method. Under different reaction
conditions, we can get different shape nickel particles by
TEM such as polygon, microsphere, hair, etc. The concen-
tration of PVP played the key role in the formation of the
novel structure in ethylene glycol or TREG solution.
Especially, the experimental results indicate that nanohair
nickel particles are ferromagnetic.
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Acknowledgements This work was supported by the
National Natural Science Foundation of China under Grant No.
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