Controllable synthesis of nickel dendritic crystals induced by magnetic field

Article abstract of DOI:10.1016/j.materresbull.2007.09.006

We demonstrated in this paper a simple and easy method for the preparation of dendritic nickel crystals in an external magnetic field in boiling ethylene glycol (EG) solution. The structural features and morphology of the sample were investigated using powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The values of saturation magnetization (M_s) and coercivity (H_c) of the dendritic crystals characterized by using a vibrating sample magnetometer (VSM) are 170.3 emu g^-1 and 50.7 Oe, respectively. It was clear that the external magnetic field was the most important factor for controlling the morphology of the product.

Full text of DOI:10.1016/j.materresbull.2007.09.006

Materials Research Bulletin 43 (2008) 2100–2104
www.elsevier.com/locate/matresbu
Controllable synthesis of nickel dendritic crystals induced
by magnetic field
a,b,c
a,*
Xia Zhang
, Weimin Liu
a
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics,
Chinese Academy of Sciences, Lanzhou 730000, PR China
b
Laboratory of Special Functional Materials, Henan University, Kaifeng 475001, PR China
c
Graduate School of Chinese Academy of Science, Beijing 100039, PR China
Received 1 December 2006; received in revised form 7 August 2007; accepted 18 September 2007
Available online 22 September 2007
Abstract
We demonstrated in this paper a simple and easy method for the preparation of dendritic nickel crystals in an external magnetic
field in boiling ethylene glycol (EG) solution. The structural features and morphology of the sample were investigated using powder
X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The values of
saturation magnetization (M ) and coercivity (H ) of the dendritic crystals characterized by using a vibrating sample magnetometer
s
c
À1
(
VSM) are 170.3 emu g and 50.7 Oe, respectively. It was clear that the external magnetic field was the most important factor for
controlling the morphology of the product.
2007 Published by Elsevier Ltd.
#
Keywords: A. Magnetic materials; A. Nanostructures; B. Chemical synthesis; B. Crystal growth; C. X-ray diffraction
1
. Introduction
Developing ways of tailoring the structure of materials on specific morphology has been one of the important goals
of material scientists. The shape and size of inorganic materials are well-known to have great effect on their widely
varying properties [1]. For example, the carrier motion of 1D magnetic Fe O is restricted in two directions so that they
2
3
are expected essentially to improve photochemical, photophysical, and electron-transport properties [2]. In recent
years, much effort has been devoted to the fabrication of different magnetic materials with various shapes, such as
nanoparticles, nanorods, nanotubes, microflowers, etc. [3–7]. Among the current shape-controlling techniques, there
are many factors affecting the shape of the product, such as the internal structure of the product, external conditions of
solvents, temperature, concentration of the reactants, the kinds of templates, etc. [8]. Up to now, to the best of our
knowledge, few investigations have been reported about the synthesis of dendritic magnetic materials due to an
external magnetic field used for the control of their morphologies.
A general method for the preparation of nano-scaled magnetic particles is through the rapid decomposition of
corresponding metal carbonyl compounds in organic solvents, while using amine and oleic acid as surfactants to
*
Corresponding author. Tel.: +86 931 4968166; fax: +86 931 8277088.
E-mail address: wmliu@lzb.ac.cn (W. Liu).
0025-5408/$ – see front matter # 2007 Published by Elsevier Ltd.
doi:10.1016/j.materresbull.2007.09.006

X. Zhang, W. Liu / Materials Research Bulletin 43 (2008) 2100–2104
2101
improve size dispersion and nanocrystal stability. This method can give high quality monodisperse magnetic particles
9–13]. Herein, we present a new one-pot reaction route to synthesize a type of magnetic material in boiling ethylene
[
glycol solution. Induced by an external magnetic field, large-scale Ni dendritic crystals could be controllably
synthesized in boiling ethylene glycol solution.
2
. Experimental procedure
All chemicals used were of analytical grade and used as received without further purification. In a typical synthesis,
1
mmol Ni(CH COOH) Á4H O was dissolved in 50 mL EG and was heated to 60 8C, then 0.25 mmol poly(vinyl
3
2
2
pyrrolidone) (PVP, MM = 30,000) was added to the reaction mixture as a stabilizer. The reaction solution was heated
slowly to the boiling point (197 8C). After refluxing about 15 min the color of the solution turned from green to grey
black, indicating the formation of Ni crystals. The mixture was refluxed about 1 h in total and cooled to room
temperature quickly. In addition, the overall experimental process was performed in an external magnetic field with a
value of 280 Oe. The obtained black precipitate was centrifuged and washed with water and ethanol several times, then
dried in air.
The as-prepared powder sample was characterized by XRD on an XRO-PERT-PRO (Netherlands) X-ray
diffractometer with Cu Ka radiation (l = 1.5406 Å). The morphology was determined by using a scanning electron
microscopy (SEM) (JSM-5600LV) and a transmission electron microscopy (TEM) (JEOL JEM-2010EX). Magnetic
properties of the product were investigated by using a vibrating sample magnetometer (Lake shore 7304 VSM).
3
. Results and discussion
Fig. 1 shows an XRD pattern of the resulting product. The XRD spectrum of the as-prepared powder exhibits
multiple intense peaks, and all the peaks can be perfectly indexed to crystalline Ni, not only in peak position but also in
their relative intensity, indicting that the as-prepared powders are crystalline Ni. These peaks at the scattering angles
(
2u) of 44.548, 51.888 and 76.428 correspond to the reflections from the 1 1 1, 2 0 0 and 2 2 0 crystal planes,
respectively, of the cubic phase of Ni. The lattice parameter for the unit cell of the Ni sample is calculated as
a = 3.524 Å in good agreement with the known lattice parameter for crystalline Ni (a = 3.523 Å, JCPDS No. 87-0712).
The crystallite size estimated from each diffraction peak using the Scherrer equation is D1 1 1 = 26.7 nm,
D2 0 0 = 12.2 nm, D2 2 0 = 12.9 nm.
The morphology of the sample was recorded by SEM and TEM with different magnifications as shown in Fig. 2.
Fig. 2a shows a low magnification view of the as-prepared product. Fig. 2b shows that the product prepared in a
magnetic field has an evident dendritic morphology, with an average diameter of 300–450 nm and an average length of
several micrometers. The TEM image (Fig. 2c) shows a representative pattern of the resulting product, further
revealing the dendritic structure. Careful observation finds the dendritic crystal is composed of nanocontacted
Fig. 1. Powder X-ray diffraction (XRD) pattern of the as-obtained Ni dendritic nanocrystals.

2102
X. Zhang, W. Liu / Materials Research Bulletin 43 (2008) 2100–2104
Fig. 2. SEM and TEM images of the sample prepared in an external magnetic field. SEM images of dendritic Ni crystal (a) and (b), TEM image of
dendritic Ni (c), and SAED pattern (d).
subunits, which look a bit like pearls on a string. The selected-area electron diffraction pattern (SAED) from a single
strand shown in Fig. 2d exhibits multiple intense rings composed of discrete spots, corresponding to the (1 1 1), (2 0 0)
and (2 2 0) diffraction planes of cubical phase of Ni, which are consistent with the XRD results above. The multiple
intense rings prove that Ni has a poly-crystalline nature.
In this experiment, EG itself acts not only as a solvent in the whole process but also as a reductant. At the boiling
point, nickel acetate was slowly reduced into nickel nuclei. Since the freshly formed nuclei in the solution are unstable,
they have a tendency to grow and were larger as the reaction time increased. In an external magnetic field, freshly
formed nuclei grow along the direction of the applied magnetic field due to the dipolar interactions, leading to a
dendritic structure. Remarkably, it is believed that the dendritic growth is kinetically controlled [14]. We have carried
out a similar experiment without using an external magnetic field, and the spherical structures of pure Ni crystals are
Fig. 3. SEM image of the sample prepared without using an external magnetic field.

X. Zhang, W. Liu / Materials Research Bulletin 43 (2008) 2100–2104
2103
Fig. 4. Magnetic hysteresis loop of the resultant nickel measure at room temperature: (a) dendritic nickel and (b) spherical nickel.
obtained, as shown in Fig. 3. These results prove that the external magnetic field is a critical factor which influences the
morphology of Ni crystals. In addition, it is noted that the concentration of the reactants do not have much effect on the
size of final product, we suppose that the size of product is somewhat dependent on the different type surfactant [15],
and detailed investigations are still being performed in our group.
Nickel is widely used as magnetic material, and its magnetic properties are greatly affected by the size, shape,
crystallinity, etc. Fig. 4a shows the magnetic hysteresis loop of the resultant dendritic nickel measured at room
temperature in an applied field of 10,000 Oe, showing a coercive force (H ), saturation magnetism (M ) and remnant
c
s
À1
À1
magnetism (M ) of ca. 170.3 Oe, 50.7 emu g , 13.3 emu g , respectively. The magnetic behavior of the dendritic
r
structure is strongly dependent on the orientation between the applied magnetic field and a single strand (not known
À1
À1
here). Comparing that with spherical nickel (Fig. 4b) (146.1 Oe, 48.8 emu g , 8.9 emu g ), a better enhanced
coercive force and remnant magnetism are seen for the dendritic nickel. While, relative to that of the bulk nickel
À1
À1
(
100 Oe, 55 emu g , 2.7 emu g ), the reduced value of M for both samples prepared in and not in a magnetic field
s
could be due to the inclusion of some non-magnetic residues from the production process as well as due to the
increased interactions between the nanocontacted subunits, which reduce the total magnetic moment [16].
4
. Conclusion
In summary, we have described a shape-controlled synthesis of dendritic nickel crystal in boiling ethylene glycol
solution. Induced by an external magnetic field, the morphology of the product could be perfectly controlled and the
results are highly reproducible in our experiment. It is expected that other nano-sized magnetic materials can also be
prepared by this simple and versatile method.
Acknowledgments
The authors were grateful for the financial supports from the ‘973’ Program of China (Grant No. 2007CB607601),
and the Innovative Group Foundation from NSFC (Grant No. 50421502).
References
[
[
[
[
[
[
[
1] H.L. Xu, W.Z. Wang, W. Zhu, J. Phys. Chem. B 110 (2006) 13829.
2] C.Z. Wu, P. Yin, X. Zhu, C.Z. OuYang, Y. Xie, J. Phys. Chem. B 110 (2006) 17806.
3] Y.W. Chu, J.H. Hu, W.I. Yang, C.C. Wang, J. Phys. Chem. B 110 (2006) 3135.
4] X.M. Ni, Q.B. Zhao, H.G. Zheng, D.G. Zhang, Chem. Lett. 10 (2005) 1408.
5] L. Liu, H.Z. Kou, W.L. Mo, Y.Q. Wang, J. Phys. Chem. B 110 (2006) 15218.
6] L.Q. Xu, J. Du, Y.T. Qian, J. Phys. Chem. B 110 (2006) 3871.
7] B.D. Mao, Z.H. Kang, E.B. Wang, S.Y. Lian, Mater. Res. Bull. 41 (2006) 2226.

2104
X. Zhang, W. Liu / Materials Research Bulletin 43 (2008) 2100–2104
[
[
8] H. Zhang, D. Yang, X. Ma, Y. Ji, J. Xu, D. Que, Nanotechnology 15 (2004) 622.
9] S.H. Sun, H. Zeng, D. Robinson, S. Raoux, G.X. Li, J. Am. Chem. Soc. 126 (2004) 273.
[
[
[
[
[
[
[
10] S. Peng, C. Wang, J. Xie, S.H. Sun, J. Am. Chem. Soc. 128 (2006) 10676.
11] G.J. Cheng, D. Romero, A.R. Hight Walker, Langmuir 21 (2005) 12055.
12] V.F. Puntes, D. Zanchet, C.K. Erdonmez, J. Am. Chem. Soc. 124 (2002) 12874.
13] T.J. Daou, G. Pourroy, C. Leuvrey, G. Rogez, Chem. Mater. 18 (2006) 4399.
14] X.W. Lou, C.L. Yuan, A. Archer, Chem. Mater. 18 (2006) 3921.
15] M.B. Sigman, A.E. Saunders, B.A. Korgel, J. Am. Chem. Soc. 125 (2003) 16050.
16] N. Cordente, C. Amiens, B. Chaudret, M.J. Casanove, J. Appl. Phys. 94 (2003) 6358.