Synthesis, characterization, and magnetic properties of flower-like nickel materials

Article abstract of DOI:10.1002/pssa.200622610

Flower-like Ni particles about 1 μm in diameter have been synthesized by means of a mild chemical solution method with a soft template of hexadecyltrimethy-lammonium bromide (CTAB). Analysis of the structures by scanning electron microscopy (SHM) and high-resolution transmission electron microscopy (HRTEM) reveal that they are composed of particles with an average diameter of 250 nm. The particles are composed of grains 20-30 nm in size. Based on a series of experiments, the growing process of the flower-like structure is described briefly. The coercivity of the as-grown product at 300 K is about 90 Oe. It is small in comparison with that of the reported flower-like microstructure composed of nickel nanotips, ~232 Oe. This is attributed to its shape anisotropy and polycrystalline structure. 2008 WILEY-VCH Verlag GmbH & Co. KGaA.

Full text of DOI:10.1002/pssa.200622610

phys. stat. sol. (a) 205, No. 5, 1109–1112 (2008) / DOI 10.1002/pssa.200622610
a
p s s
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applications and materials science
Synthesis, characterization,
and magnetic properties of flower-like
nickel materials
1
*, 1
2
**, 2
W. Zhou , L. Guo , L. He , and C. P. Chen
1
School of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100083, P.R. China
2
Department of Physics, Peking University, Beijing 100871, P.R. China
Received 22 December 2006, revised 11 October 2007, accepted 7 December 2007
Published online 26 February 2008
PACS 61.05.cp, 68.37.Hk, 68.37.Og, 75.50.Cc, 81.16.Be
*
Corresponding author: e-mail guolin@buaa.edu.cn, Phone: +86 10 82338162, Fax: +86 10 82338162
**
Corresponding author: e-mail cpchen@pku.edu.cn, Phone: +86 10 62751751
Flower-like Ni particles about 1 µm in diameter have been composed of grains 20–30 nm in size. Based on a series of
synthesized by means of a mild chemical solution method experiments, the growing process of the flower-like structure
with a soft template of hexadecyltrimethy-lammonium bro- is described briefly. The coercivity of the as-grown product at
mide (CTAB). Analysis of the structures by scanning electron 300 K is about 90 Oe. It is small in comparison with that of
microscopy (SEM) and high-resolution transmission electron the reported flower-like microstructure composed of nickel
microscopy (HRTEM) reveal that they are composed of parti- nanotips, ∼232 Oe. This is attributed to its shape anisotropy
cles with an average diameter of 250 nm. The particles are and polycrystalline structure.
©
2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1
Introduction Soft magnetic nanomaterials, such as monium bromide (CTAB) as the soft template. The mor-
Fe, Co, and Ni, have attracted much attention because of phology of the as-prepared product is different from the
their applications in sensors [1], magnetism studies [2] and above-mentioned flower-like products [14, 15]. It is com-
as catalysts [3]. Many groups have synthesized various Ni posed of Ni nanograins instead of nanotips and nanoplatelets.
nanomaterials with different morphologies using a number
of methods involving the electrochemical deposition [4],
the radiation method [5], sonochemical synthesis [6], vac- grade and used without further purification. In a typical
uum deposition [7], the sol–gel method [8], the hydro- experiment, 0.5 mmol NiCl ·6H O (Hengxin Chemical
2 Experimental All the reagents were analytical
2
2
thermal treatment [9] and other methods. Recently, a series Reagent Co., Shanghai, China, AR) and 3 mmol CTAB
of interesting Ni nanomaterials have been prepared via a (Aoboxin Biological Technology Co. Ltd., Beijing, China,
chemical solution technique. Reetz and coworkers [10] and AR) were dissolved in the solvent of 60 ml ethylene glycol
Li and coworkers [11] synthesized particular triangular Ni (EG, Shanghai Chemical Reagent Co., China, AR). The so-
nanosheets with solution phase methods. Xu and cowork- lution was stirred for 2 h to make sure that it was mixed
ers [12, 13] prepared Ni hollow submicrometer-sized and homogenously. Afterwards, 1 ml hydrazine monohydrate
nanometer-sized spheres. Qian et al. [9] fabricated Ni (50%, Tianjin Chemical Reagent Co., China) was dropped
nanobelts, flower-like Ni microstructures composed of Ni into the mixture so that the green solution turned light pur-
nanotips [14] and flower-like Ni nanostructures self-as- ple. It was slowly heated using an oil bath with mixture
sembled by hexagonal nanoplatelets [15] using a hydro- agitation until the temperature reached 120 °C. Then, it
thermal method. However, other Ni nanomaterials with was refluxed for 5 h under vigorous magnetic stirring. The
special morphologies are rarely reported from solution as-synthesized product was washed with ethanol and de-
methods.
Herein, we report a chemical solution method for creat-
ing flower-like Ni particles using hexadecyltrimethy-lam- by X-ray powder diffraction (XRD) using a Rigaku Dmax
ionized water.
The crystal structure of the product was characterized
©
2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

a
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W. Zhou et al.: Synthesis, characterization, and magnetic properties of nickel materials
2
200 X-ray diffractometer with Cu Kα radiation (λ =
.1541 nm). The XRD specimens were prepared by means
130
0
125
120
115
110
of flattening the powder on the small slides. Field emission
scanning electron microscopy (FE-SEM) was carried out
by a field-emission scanning electron microanalyzer (Hi-
tachi S-4300, 5 kV) with the samples obtained from the
thick suspension dropping on the glass platelets. The high-
resolution transmission electron microscopy (HRTEM) in-
vestigations were carried out by a JEOL JEM-2100F with a
2
00 kV electron beam after the dispersed products were
105
00
dropped onto a carbon film supported by a copper grid.
The thermogravimetric analysis was performed using a
Diamond thermogravimetric analyzer (Perkin–Elmer in-
struments) under a stream of air. The product was heated
from 50 °C to 800 °C at a scan speed of 10 °C/min. The
magnetic properties of the samples were measured using a
quantum design SQUID magnetometer.
1
1
00 200 300 400 500 600 700 800
Temperature (
°
C)
Figure 2 Thermogravimetric curve of the Ni product.
3
Results and discussion The phase and purity of are ∼1 µm in diameter. Figure 3b is a typical SEM image
the as-produced sample are determined from the X-ray dif- of the sample, clearly showing that the product possesses a
fraction (XRD) pattern shown in Fig. 1. All the diffraction flower-like structure, formed by nanocrystallites with an
peaks can be indexed to the pure Ni with the face-centered average size of 250 nm. A magnified image of one cauli-
cubic crystal structure (JCPDS 04-0850) and no impurity flower-like particle is shown in Fig. 3c. This indicates that
phases, such as NiO or precursor compounds, were de- the particles of ∼250 nm are constituted by grains (“the
tected. The broadening of the diffraction peaks indicates particle units”) of 20–30 nm diameter, in accordance with
that the grains forming the sample are small. The crystal the calculating result with the XRD pattern by the Debye–
dimension, which can be roughly estimated by using the Scherrer formula.
well-known Debye–Scherrer formula, is about 20 nm ob-
tained from the full-width half-maximum (FWHM) based further examined by using transmission electron micros-
on the (111) reflection.
copy (TEM), selected-area electron diffraction (SAED)
The morphology and components of the product were
The thermogravimetric analysis was used to confirm and high-resolution transmission microscopy (HRTEM).
the purity of the as-synthesized product. In Fig. 2, the Figure 4a reveals a bright-field TEM image of a flower-
thermogravimetric curve of the product with the tempera- like particle. The inset is the corresponding selected-area
ture ranging from 50 °C to 800 °C shows one step at electron diffraction (SAED) pattern, showing a polycrys-
∼
280 °C with a weight increase of ∼27%. This increase talline structure. The spacing of 0.203 nm, 0.176 nm,
corresponds well to the increased ratio from pure Ni to 0.124 nm and 0.106 nm in the observed fringe pattern is
pure NiO. Meanwhile, there is no mass loss, indicating that nicely indexed to the (111), (200), (220) and (311) planes
the product is pure without organic residue [16].
of the fcc Ni, respectively. These are in accordance with
The scanning electron microscopy images of the sam- the result of the XRD pattern. Figure 4b is a higher-
ple are shown in Fig. 3. Figure 3a exhibits that the particles magnification image for the square area in Fig. 4a. It
shows the surface morphology, further demonstrating that
grains of diameter 20–30 nm constitute the particle of
∼
250 nm in the flower-like structure.
Figures 4c and d correspond to the area marked with c
and d in Fig. 4b, respectively. Figure 4c reveals one “particle
unit” of ∼20 nm size on the surface. It can be identified to
be pure Ni with an interplanar distance of 0.203 nm corre-
sponding to the nickel crystal plane of (111). Obviously,
the lattice fringes reaching to the boundary of the particle
indicate that there is no oxide layer on the surface. The in-
set shows the fast Fourier transform (FFT) of the HRTEM
image, and it suggests that the grain is a single crystal.
That is, the “particle units” of ∼20 nm constituting the
polycrystalline flower-like particles are single crystals.
Figure 4d is a lattice-resolved HRTEM image taken near
the contact region of two “particle units”. The stripes in
different directions correspond to two “particle units”, and
4
0
50
60
70
80
90
100
2θ
(deg.)
Figure 1 XRD pattern of the as-obtained product.
©
2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Original
Paper
phys. stat. sol. (a) 205, No. 5 (2008)
1111
Figure 3 SEM images of the Ni flower-like nanostructures prepared by the mild chemical solution method. (a) Low-magnification
SEM image of the as-prepared product. (b) A typical image showing a flower-like nanostructure. (c) Magnified SEM image of one par-
ticle showing its cauliflower-like structure constituted by grains of 20–30 nm.
it is seen from the coalescent region that the two “particle (S3) of diameter ∼500 nm is forming as exhibited in
units” have grown together.
To explore the growth process of the flower-like parti- These rough surfaces capped with CTAB make it easy to
cles, a series of experiments were performed. The forma- combine these particles.
tion process is illustrated in three steps a, b and c in Fig. 5.
Figure 6 shows the M–H data measured at 300 K. An
Fig. 5c. This might contribute to the structure in Fig. 5b.
And the images of Sample 1 (S1), Sample 2 (S2) and open hysteresis loop is observed in the low-field region.
Sample 3 (S3) correspond to the steps a, b and c, respec- The remnant magnetization is approximately 5 emu/g, so
tively. At the first step, excessive hydrazine acts as a re- the remnant ratio is calculated as roughly 0.09. The coer-
2+
ducing agent and converts the precursor [Ni(N H ) ] [17] civity field, H , is determined as 90 Oe, which is smaller
C
2
4 3
than the 232 Oe obtained in flower-like microstructures of
nickel nanotips [14]. The polycrystalline structure of the
particle, as shown in Fig. 4a, will average out the contribu-
tion of the intrinsic magnetocrystalline anisotropy. In addi-
tion, the spherical shape of the particle will result in a
small demagnetization factor and corresponding negligible
shape anisotropy [18]. Thus, a much reduced coercivity in
comparison with the flower-like nanotips is expected. The
saturation magnetization determined at high field is
to Ni grains with an average diameter of 20 nm (S1). Sec-
ond, the Ni grains aggregate into bigger ones of diameter
150 nm (S2), and these particles are capped with CTAB,
∼
showing rough surfaces. Finally, the flower-like structure
55 emu/g, ∼0.58µ /Ni atom, as shown in the inset. It is
B
close to the bulk value, ∼0.6µ /Ni atom. The Ni nanocrys-
B
tallites of 20–30 nm is comparable with the coherent
length of 25 nm [19, 20]. The magnetization reversal mode
of the nanocrystallines should be coherent rotational and
superparamagnetism (SPM) is expected to exist above the
blocking temperature. A general and detailed discussion on
Figure 4 TEM and HRTEM images of one typical flower-like
particle. (a) TEM image of one particle and the corresponding
SAED pattern inserted. (b) TEM image with a higher magnifica-
tion for the square area in Fig. 4a. (c) HRTEM image of a single Figure 5 Illustration of the formation process of Ni flowers with
crystal for the box marked with c in Fig. 4b. The inset is the rele- CTAB as soft templates. Images of Sample 1 (S1), Sample 2 (S2)
vant FFT pattern. (d) HRTEM image of the coalescence of two and Sample 3 (S3) correspond to the process of a, b and c, respec-
single crystals for the area marked with d in Fig. 4b.
tively.
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W. Zhou et al.: Synthesis, characterization, and magnetic properties of nickel materials
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©
2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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