Fabrication and magnetic properties of nickel dodecahedra

Article abstract of DOI:10.1039/c4dt00088a

Here we report a one-pot route for the synthesis of nickel dodecahedra with 52.3 ± 0.1 emu g^-1 of saturation magnetization. The procedure is very simple, and only three chemicals (NiCl₂·6H ₂O, isopropanol and polyvinylpyrrolidone) are used throughout the entire synthetic process. During the reaction, it is believed that the application of isopropanol and the amount of polyvinylpyrrolidone play an essential role in forming the dodecahedral morphology of the final product. Furthermore, a formation process of twinning and the influence of reaction kinetic factors were proposed to explain the formation of nickel dodecahedra.

Full text of DOI:10.1039/c4dt00088a

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Fabrication and magnetic properties of
nickel dodecahedra
Cite this: Dalton Trans., 2014, 43,
5913
Lei Zhao, Guangshu Zhang and Lijun Zhao*
Here we report a one-pot route for the synthesis of nickel dodecahedra with 52.3 0.1 emu g−1 of satur-
ation magnetization. The procedure is very simple, and only three chemicals (NiCl2·6H2O, isopropanol
and polyvinylpyrrolidone) are used throughout the entire synthetic process. During the reaction, it is
believed that the application of isopropanol and the amount of polyvinylpyrrolidone play an essential
role in forming the dodecahedral morphology of the final product. Furthermore, a formation process of
twinning and the influence of reaction kinetic factors were proposed to explain the formation of nickel
dodecahedra.
Received 10th January 2014,
Accepted 16th January 2014
DOI: 10.1039/c4dt00088a
www.rsc.org/dalton
including the use of the surfactant polyvinylpyrrolidone (PVP)
or the tetramethylammonium hydroxide (TMAH) assisted
Introduction
Many scientists focus on constructing special structures for growth for Ni polyhedra.8,9 However, direct evidence for the
nanomaterials to tailor their physical or chemical properties preparation of Ni dodecahedra with geometrical shapes has
and then improve their intrinsic attributes in some applied never been reported. In this work, we report a facile solvother-
fields.1 The morphology controlled synthesis of polyhedral mal method for the preparation of Ni dodecahedra. Further-
nanocrystals with uniform geometries has been an attractive more, both the formation mechanism and magnetic
research area for the past decade, as they have broad uses in properties are investigated in detail.
catalysis, photonics, electronics, optical sensing and magnetic
applications.2–4 Essentially, for all of these applications both
the size and shape are critical parameters to control in order
to maximize their suitability. For example, the reactivity and
selectivity of a catalyst can be tailored by controlling the shape,
Materials and methods
Sample preparation
In a typical preparation procedure, the samples were syn-
thesized according to the following: 0.2 g of NiCl2·6H2O was
first dissolved in 20 mL of isopropanol (IPA). The resulting
mixture was sonicated until a yellow suspension was obtained.
The obtained solution was magnetically stirred for about
30 min followed by the addition of 0.5 g of polyvinylpyrroli-
done (PVP). The final mixture was transferred into a 30 mL
Teflon-lined autoclave. The autoclave was sealed and main-
tained at 240 °C for 12 h and then cooled to room temperature
naturally. The black product was collected and washed with
distilled water and ethanol several times. The sample was then
dried under vacuum at 60 °C for further characterization.
as it will determine the crystallographic facets exposed on the
surface of a nanocrystal and therefore the number of atoms
located at the edges or corners.5 Based on these above theo-
ries, many works have been done on polyhedral nano-
materials. For instance, Huang et al. reported the synthesis of
concave polyhedral Pt nanocrystals with high-index facets and
Lee et al. reported the synthesis of nanocrystals with variable
high-index Pd facets through the controlled heteroepitaxial
growth of trisoctrahedral Au templates.6,7 These excellent
works let us recognize that both the shape and the exposed
crystalline faces are important and can have a marked influ-
ence on activity and selectivity.
Like noble metals, the synthesis of nickel (Ni) nano-
structures with well-controlled shapes has been a challenging
task. Before we started to work on this subject, some efforts
had been devoted to the chemical synthesis of Ni nanostructures,
Characterization
The phases were identified by means of X-ray diffraction (XRD)
with a Rigaku D/max 2500 pc X-ray diffractometer with Cu Kα
radiation (λ) of 1.54156 (Å) at a scan rate of 0.04° s−1. The mor-
phologies (FESEM) were characterized by a JEOL JSM-6700F
field-emission scanning electron microscope. Transmission
electron microscopy (TEM), high-resolution TEM (HRTEM),
selected area electron diffraction (SAED), and energy dispersive
Key Laboratory of Automobile Materials (Jilin University), Ministry of Education and
School of Materials Science and Engineering, Jilin University, Changchun 130022,
China. E-mail: lijunzhao@jlu.edu.cn; Fax: +86-0431-85095876
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X-ray spectroscopy (EDS) were carried out on a JEOL 2100F with No other crystalline impurities are detected, demonstrating
an emission voltage of 200 kV. IR spectra of the samples were that pure Ni particles were obtained.
characterized using a FTIR spectrophotometer (NEXUS, 670) in
In order to obtain more information about the Ni dodeca-
KBr pellets. Room temperature hysteresis loops were collected on hedra, a HRTEM investigation was carried out. Through tilting
a VSM-7300 vibrating sample magnetometer (Lakeshore, USA).
the projecting angle, a quadrilateral and a hexagon can be
observed from Fig. 2a and 2d, respectively. Fig. 2a shows a
quadrilateral projecting from the side of the particle, yet
Fig. 2d shows a hexagon projecting in front of the particle.
Because of the different viewing angles, the projections of the
dodecahedra are quadrilateral and hexagonal, as shown in the
Results and discussion
Characterization of Ni dodecahedra
As shown in Fig. 1a, the Ni product mainly consists of well- insets of Fig. 2a and 2d. The HRTEM images of Ni decahedra
defined dodecahedra with good uniformity and high sym- taken from different angles are shown in Fig. 2b and 2e. More-
metry. 834 edge lengths of 237 dodecahedra particles were over, Fig. 2c and 2f show magnified HRTEM images of the
counted by the software Nano Measurer 1.2. From the edge appointed region in Fig. 2b and 2e (signed with the same
length distribution histograms of the Ni decahedra (inset in color), the same lattice features (0.203 nm) demonstrated that
Fig. 1a), it can be seen that the sizes of the Ni dodecahedra dis- the surface is enclosed by (111) facets from two different pro-
tribute mainly in the range of 200–300 nm. A closer obser- jecting angle. The SAED patterns in Fig. 2g and 2h show that
vation of two dodecahedra which is shown in Fig. 1b reveals the Ni dodecahedra crystallized well. The SAED pattern, as
that the surfaces are very smooth. Moreover, every dodeca- shown in Fig. 2g, is taken from the dodecahedron displayed in
hedron is composed of 12 triangles. The TEM image in Fig. 1c Fig. 2a. The SAED pattern shows clear spots indexed to the
ˉ
shows the particles with representative hexagon contours if [112] zone axis of the fcc crystal. The Ni dodecahedra show
randomly oriented, as expected from their appearance in high crystallinity due to the observed highly ordered SAED
Fig. 1b. In addition, one thing to be mentioned is that the spots. Furthermore, the SAED pattern obtained from the deca-
lengths of some edges in one hexagon do not look exactly hedron in Fig. 2d is shown in Fig. 2f, which shows clear spots
alike, because they are viewed from different angles. The crys- indexed to the [011] zone axis of the fcc crystal. Two zone axes
tallinity and phase information can be observed from the XRD of Ni dodecahedra were present in the SAED, indicating that
ˉ
pattern shown in Fig. 1d. All diffraction peaks can be indexed the zone axes of [112] and [011] have a 30° rotation between
to a face centered cubic phase (fcc) of Ni (JCPDS 65-2865). the two closed-packed lattices.
Fig. 1 (a) Low-magnification SEM; (b) high-magnification SEM; (c) TEM and (d) XRD pattern of Ni dodecahedra.
5914 | Dalton Trans., 2014, 43, 5913–5919
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Fig. 2 (a–f) SEM images, HRTEM images and magnified HRTEM images of two Ni dodecahedra; (g) and (h) are the corresponding SAED patterns of
(a) and (d). Insets in (a) and (d) are the separately corresponding projections of the Ni dodecahedra.
The Ni crystals showed very good crystallinity because no twin plane. Based on the above analysis, it can be confirmed that
defects were observed in the image. From the analysis of SEM, the dodecahedron is a multiply-twinned particle. Ino et al. pro-
XRD and TEM, we concluded that highly crystalline, well-deve- posed a theoretical model for the formation and development
loped and homogeneous Ni dodecahedra were successfully of multiply-twinned particles and following researchers further
synthesized by the simple solvothermal method.
developed this theory.10–12 According to Ino’s theoretical model, a
scheme of the multiply-twinned particles’ growth process was
drawn and is shown in Fig. 3d. In order to make the system
Formation mechanism of Ni dodecahedra
To shed light on the formation mechanism of the Ni dodeca- obtain the minimum total surface free energy, the Ni atoms stack
hedra, a twinning mechanism was proposed. Fig. 3a shows the into a tetrahedron, which is then used as a base in each step to
TEM image of a Ni dodecahedron’s top view, from which six develop the multiply-twinned particles; initially the tetrahedron
triangle planes are clearly seen. The edge of the Ni dodeca- twins with two others, then this grain twins for a second time
hedron along the [011] axis was detected by HRTEM, as shown with a tetrahedron on another side, and so on, until a dodeca-
in Fig. 3b. An FFT pattern was obtained in the region of the do- hedron is formed by a structure composed of 6 tetrahedral
decahedron as labeled in Fig. 3b, which indicated the dodeca- twinned particles.
hedron is a twinned crystallite. The twins are related by twinning
In addition, kinetic factors were also investigated to explore
on the (111) plane and the twins are asymmetric along the the possible growth mechanism of the Ni dodecahedra, we
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Fig. 3 (a) TEM image of Ni dodecahedron’s top view; (b) HRTEM image near the edge of the Ni dodecahedron along the [011] axis; (c) FFT pattern;
(d) growth process schematic of multiply-twinned particles (N is the base grain, I is the first twinning, II is the secondary twinning, III is the third
twinning).
studied the samples (obtained with different dosages of PVP)
As the reaction proceeds, the polyhedral particles grow
using SEM, as shown in Fig. 4. In the synthesis of inorganic along different directions with different growth rates due to
nanostructures, many organic additives have been used to their different surface energies. In the present system, the sur-
modify certain crystallographic surfaces.13–15 The absorption factant PVP acts as not only a stabilizer to prevent the aggrega-
of the organic additives on those surfaces will eliminate part tion of the products but also a shape-controller to assist the
of the dangling bond resulting in the reduction of surface formation of dodecahedral Ni crystals. As a well-known
energy, and therefore suppressing growth along these surfaces capping or stabilizing agent, PVP molecules with long chains
obtaining the final required morphology. Therefore, the can be adsorbed to the Ni particle surfaces via physical and
addition of PVP is not only for reducing the surface energy but chemical bonding. When we add PVP into the reaction system,
also to form a polyhedral-like structure. When no PVP was it is believed that PVP tends to suppress the growth rate of the
introduced into the reaction system, we observe agglomerated {111} planes more than that of the {100} planes, since it
cubes with convexities (Fig. 4a). On the surfaces of cubes, interacts more strongly with the {111} facets than with the
n
P
which have higher surface energies, the existence of defects
(such as kinks and steps) leads to dislocation and subsequent
{100} facets.16,17 Based on the Curie–Wullf theory ( Aiδi =
i¼1
spiral growth to form convexities. Furthermore, particle aggre- minimum total surface free energy), the growth rate of a crystal
gation can also reduce surface energy by eliminating the high plane is proportional to its specific surface energy. Ai and δi
energy facets leading to Ni in the shape of agglomerated cubes are the separate area and specific surface free energy of the ‘i’
with convexities. With the introduction of PVP, we found that crystal plane. Under the synthesis system of IPA and PVP, n =
Ni dodecahedra gradually formed, and the uniformity and dis- 12 may make the total surface free energy minimum. The vis-
persion of the Ni dodecahedra began to improve (Fig. 4b). cosity of the solvent is also a key factor in influencing the final
When the dosage of PVP reached 0.3, the Ni dodecahedra were morphologies of crystals. Without using PVP, the viscosity of
taking shape, but their crystalline planes were not very clear IPA is relatively low. The solutes gradually adhere to the crystal
(Fig. 4c). However, when the dosage of PVP was increased to surfaces. The light solution rises, then the heavy solution
1.0 g, bread-like Ni particles with concaves in the center of the gradually sinks. Due to the effect of gravity, a vortex is gener-
surfaces were observed in Fig. 4d. From the above SEM obser- ated, which makes the supply of solutes inhomogeneous and
vation, it can be concluded that the nonionic surfactant PVP directional (Scheme 1a). Hence, crystal surfaces of the cubes
plays an important role in the formation of Ni dodecahedra. are obtained with convexities. Furthermore, the cubes are hexa-
When the reactions were carried out in the presence of other sur- hedrons which are occupied by six {100} planes. In order to
factants, such as the cationic surfactant cetyltrimethylammonium decrease the energies of the crystal planes, agglomeration
bromide (CTAB) or the anionic surfactant sodium dodecyl sulfate occurs. With increasing dosage of PVP, the viscosity of the
(SDS), disordered and irregular morphologies were observed.
solvent gradually improves. A gradual increase in viscosity will
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Fig. 4 SEM images of Ni particles obtained with different dosages of PVP: (a) 0 g, (b) 0.1 g, (c) 0.3 g and (d) 1 g; (e) SEM image of Ni polyhedrons
synthesized at 260 °C with 0.5 g of PVP.
Scheme 1 Scheme of the function of PVP during the process of Ni formation.
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Fig. 5 (a) STEM image, (b) EDS spectrum, and (c) nickel element mapping image of Ni dodecahedra. (d) FT-IR spectrum and (e) room-temperature
hysteresis loop of Ni decahedrons.
hinder the presence of the vortex, and then the supply of structure during the solvothermal process. Before we reported
solute can diffuse (Scheme 1b). Consequently, the crystal this work, a lot of experiments were done on the application of
grows in this manner with a more complete structure and solvents including alkaline solutions, deionized water,
uniform morphology, and then decahedrons are obtained. ethanol, dimethylformamide (DMF), and so on. However, it
However, with a further increase in viscosity, diffusion of the was found that these solvents can only influence the prepa-
solute becomes harder. The crystal forms under conditions of ration of noble metal polyhedra (Au, Ag, Pd, Pt) or some kinds
a lack of solute supply (Scheme 1c). Because the edges and of metal oxide,19–23 but they have no effect on the synthesis of
corners of the crystal easily accept the solute and grow quickly, Ni polyhedra. Based on these reported works, we explored and
the clear crystal planes of the dodecahedra gradually disappear applied a new volatile solvent (IPA). During the reaction, IPA
and bread-like Ni particles form.
plays two roles: solvent and reducing agent. The reduction
Apart from the above kinetic factors, temperature also has a process of Ni is accompanied by the dehydration and oxidation
strong influence on the final morphology. One thing to be of IPA at high temperature.24
mentioned is that the reaction temperature is vital for the for-
To depict the nature of the prepared Ni dodecahedra,
mation of Ni dodecahedra. If the reaction temperature is lower elemental mapping analysis of the Ni dodecahedra was carried
than 220 °C, no Ni products can be obtained. On the contrary, out. Fig. 5b shows the EDS spectra of the Ni dodecahedra.
if the reaction temperature is higher than 220 °C, for example Combined with the STEM images (Fig. 5a) and the nickel
240 °C, the dodecahedra will disappear, replaced by large element mappings (Fig. 5c) it was found that no O signals
amounts of polyhedral-like particles with a size of about were detected therefore the Ni dodecahedra were composed of
500 nm, as shown in Fig. 4e. If we observe the SEM image care- pure nickel. The surface state of the Ni dodecahedra was
fully, it can be seen that there are some dodecahedra and further checked by FT-IR spectroscopy. No obvious Ni–O bond
icosahedra. Thus, as dictated by thermodynamics, we can is seen from Fig. 5d, which proves that the surface of Ni is not
speculate that the shapes of the products evolve to a spherical- oxidized.
like structure to reduce the surface energy with increasing
temperature.18
Magnetic properties
On the basis of the above experimental observations, we Nickel is a typical ferromagnetic material. The magnetic pro-
found that the entire reaction process is very simple with a perties of the Ni dodecahedra are very important for its future
high repeatability because of the few reactants. Furthermore, potential application. Fig. 5e presents the hysteresis loop of
IPA is indispensable for the formation of the dodecahedral the Ni dodecahedra measured at room temperature. The
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saturation magnetization (Ms) of the dodecahedra (52.3
0.1
Acknowledgements
emu g−1) is very close to bulk Ni (55 emu g−1). The symmetrical
structure and uniform shape of the dodecahedra may be the The financial support from the Major Science and Technology
reason for the high Ms value. The atomic magnetic moment of Projects Research Plan of Changchun City (13KG75) is appreciated.
iron group elements is mainly due to the contribution of elec-
tron spin. However, the contribution of electron spin orbit is
small or even zero. Hence, the magnetic moment can generally References
be calculated by the formula μs = 2√(S(S + 1)) (S is spin
1 S. Mann and G. A. Ozin, Nature, 1996, 382, 313.
2 N. L. Rosi and C. A. Mirkin, Chem. Rev., 2005, 105, 1547.
3 A. R. Tao, S. Habas and P. D. Yang, Small, 2008, 4, 310.
4 Y. Xia, Y. J. Xiong and B. Lim, Angew. Chem., Int. Ed., 2009,
quantum number).25 The crystal symmetry is mainly com-
posed of equivalent crystal faces, and the crystal edges and
corners are repeated regularly. This is because it has a regular
grid structure, which is a reflection of its cyclical repetition in
48, 60.
three-dimensional space. Ni dodecahedra have high crystal
5 R. Narayanan and M. A. El-Sayed, Nano Lett., 2004, 4, 1343.
6 X. Q. Huang, et al., J. Am. Chem. Soc., 2011, 133, 4718.
7 Y. Yu, Q. B. Zhang, B. Liu and J. Y. Lee, J. Am. Chem. Soc.,
2010, 132, 18258.
8 H. J. Song, et al., CrystEngComm, 2012, 14, 405.
9 L. J. Zhao, Y. J. Wang and Q. Jiang, Mater. Lett., 2010, 64,
215.
10 S. Ogawa and S. Ino, J. Cryst. Growth, 1972, 13, 48.
11 B. J. M. Wiley, et al., Chem.–Eur. J., 2005, 11, 454.
12 K. Yagi, et al., J. Cryst. Growth, 1975, 28, 117.
13 B. Wiley, Y. G. Sun, B. Mayers and Y. N. Xia, Chem.–Eur. J.,
2005, 11, 454.
14 P. Toneguzzo, et al., Adv. Mater., 1998, 10, 1032.
15 Y. J. Xiong, et al., Langmuir, 2006, 22, 8563.
symmetry; hence, the orbital angular momentum may just be
partly frozen. Furthermore, for the 3dn of Ni, n = 8, the
ground state in the dodecahedron has dual degeneracy.25 The
orbital angular momentum is partly frozen. Therefore, the
magnetic moment of Ni can be calculated using the formulae
μJ = gJ√(J(J + 1))μB (J = L + S, J is the total quantum number;
L is the orbital quantum number). When n is larger than 5,
gJ is larger than 2.25 Based on the above analysis, we have
explained why the Ni dodecahedra have a high Ms value. More-
over, the Ni decahedra exhibit greatly enhanced coercivity
(155.4 1 Oe) compared with bulk Ni (100 Oe).26 It is known
that the coercivity of magnetic materials depends strongly
on various types of anisotropy (crystal anisotropy, shape
anisotropy, stress anisotropy, externally induced anisotropy,
16 M. Tsuji, R. Matsuo, P. Jiang, N. Miyamae, D. Ueyama,
and exchange anisotropy), among which shape anisotropy is
et al., Cryst. Growth Des., 2008, 8, 2528.
predicted to produce the largest coercive forces.27 In addition,
with a decrease in particle size, the formation of domain walls
becomes energetically unfavorable and the coherent rotation
of spins is required instead of domain wall motion for the
changes in the magnetization, resulting in larger coercivity.
Therefore, a higher coercivity compared with bulk nickel was
17 H. Cölfen and S. Mann, Angew. Chem., Int. Ed., 2003, 42, 2350.
18 Y. J. Xiong and Y. N. Xia, Adv. Mater., 2007, 19, 3385.
19 M. Leng, et al., J. Am. Chem. Soc., 2010, 132, 17084.
20 S. D. Sun, et al., CrystEngComm, 2011, 13, 2217.
21 C. K. Tsung, et al., J. Am. Chem. Soc., 2009, 131, 5816.
22 J. Zhang, et al., J. Am. Chem. Soc., 2010, 132, 14012.
23 X. Q. Huang, et al., J. Am. Chem. Soc., 2009, 131, 13916.
24 R. J. Joseyphus, et al., J. Solid State Chem., 2007, 180, 3008.
25 D. F. Wan and X. L. Ma, Magnetic Physics, University of Elec-
observed. Moreover, similar values of coercive fields are
reported by many previous researchers.28–31
tronic Science and Technology Publishing House, 1994,
p. 154.
Conclusions
In summary, Ni dodecahedra have been fabricated by a simple 26 J. H. Hwang, et al., J. Mater. Res., 1997, 12, 1076.
solvothermal method. The presence of a twinning structure 27 L. L. Diandra and D. R. Reuben, Chem. Mater., 1996, 8, 1770.
makes for the formation of Ni dodecahedra. In addition, the 28 D. Q. Zhang, G. S. Li and J. C. Yu, Cryst. Growth Des., 2009,
dosage of PVP has a direct influence on the viscosity of the IPA
system. An appropriate viscosity can facilitate the formation of 29 W. Hui, Z. Rui, L. Dandan and P. Wei, Chem. Mater., 2007,
Ni dodecahedra. Due to the symmetric structure and uniform 19, 3506.
morphology of Ni dodecahedra, they have a high Ms value 30 X. M. Liu and S. Y. Fu, J. Cryst. Growth, 2007, 306, 428.
9, 2812.
which is comparable to bulk Ni.
31 Y. H. Leng, et al., Nanotechnology, 2006, 17, 1797.
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