Magnetic nanochains of metal formed by assembly of small nanoparticles

Article abstract of DOI:10.1039/b411311j

Ni nanochains are synthesized with diameters of 150-250 nm and lengths of 0.5-2 μm by assembly of small nanoparticles, which exhibit a magnetic coercivity over two orders of magnitude larger than that of bulk Ni.

Full text of DOI:10.1039/b411311j

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Magnetic nanochains of metal formed by assembly of small
nanoparticles{
Chen-Min Liu,a Lin Guo,*a,c Rong-Ming Wang,*b Yuan Deng,a Hui-Bin Xua and Shihe Yang*c
a School of Material Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing
100083, China. E-mail: guolin@buaa.edu.cn; Fax: 186-10-82338162; Tel: 186-10-82338162
b Electron Microscopy Laboratory and State Key Laboratory for Mesoscopic Physics, Peking University,
Beijing 100871, China. E-mail: rmwang@pku.edu.cn; Fax: 186-1062757551; Tel: 186-10-62757171
c Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay,
Kowloon, Hong Kong, P. R. China. E-mail: chsyang@ust.hk; Fax: 852-2358-1594; Tel: (852) 2358-7362
Received (in Cambridge, UK) 26th July 2004, Accepted 8th September 2004
First published as an Advance Article on the web 14th October 2004
Ni nanochains are synthesized with diameters of 150–250 nm
and lengths of 0.5–2 mm by assembly of small nanoparticles,
which exhibit a magnetic coercivity over two orders of
magnitude larger than that of bulk Ni.
Although high yield syntheses of monodispersed magnetic
nanoparticles have been achieved, simultaneous control of their
shape, surface structure, and anisotropy is still a challenge.
Currently, there is a genuine need for general approaches to
fabricating low dimensional structures of magnetic metals through
directed-assembly using nanoscale building blocks.1 Surfactant-
assisted shape-selective syntheses of nanostructured nickel colloid,
nanoparticle agglomerates, submicron hollow spheres, and cobalt
nanoparticle rings have been achieved.2 Recently, Cordente et al.3
have demonstrated the first synthesis of nickel nanorods by using
HAD (hexadecylamine) as a shape-inducing agent instead of the
conventional templates. In this communication, we report a simple
solution-phase approach to the synthesis of pure nickel nanochains
in the presence of a multidentate ligand poly(vinyl pyrrolidone)
(PVP).
The Ni nanochains were grown by reduction of Ni21 in a
solution (see also S1). In brief, 5 ml ethylene glycol (EG) was heated
to the boiling point (y195 uC) and refluxed for 5 min. Then, 1 ml
of hydrazine monohydrate (50%, Vol. A.R.) was added dropwise
to the boiling solution. 2 min later, 2.5 ml of NiCl2?6H2O solution
(0.2 mol/l, in EG) and 5 ml of PVP solution (0.3 mol/l, in EG) were
added rapidly to the boiling solution under vigorous magnetic
stirring. After refluxing for about 60 min, the final products in the
form of loose black powder were obtained by centrifugation. The
powder was rinsed repeatedly with absolute ethanol for over 6
times, followed by the removal of the residual solvent through
evaporation in vacuum at 80 uC. The yield of the Ni nanochains
was as high as 80%. We noticed that no sign of oxidation was
observed on the nickel nanochains we prepared even after aging for
over 6 months under ambient conditions. This indicates that the
nickel nanochains are very stable after surface modification with
PVP, which is very important for future applications.
Fig. 1a shows the X-ray diffraction (XRD) pattern of a typical
Ni nanochain sample. All of the diffraction peaks can be assigned
to the Ni fcc structure (JCPDS 04-0850) and no impurity phases
such as NiO or precursor compounds have been detected. The
purity of the as-prepared network of the Ni nanoparticle chains is
further verified by Energy Dispersive X-ray Spectroscopy (EDX).
The result is shown in Fig. 1b, which exhibits appreciable signals
only for Ni.
Fig. 1 a. XRD pattern of the as-synthesized nickel nanochain networks; b.
EDX profile of a nickel nanochain of the as-synthesized sample.
and they are in close contact with each other, forming branched,
necklace-like chains with lengths of 0.5–2 mm. This can be seen
more clearly from the bright field TEM image in Fig. 2b. The
corresponding nano-beam electron diffraction (NBED) pattern of
the nickel nanochains network is displayed in the inset of the TEM
image, which indicates that the nanoparticle network is polycrystal-
line. In fact, the individual nanoparticles are also found to be
polycrystalline. The observed ED rings (see the inset of Fig. 2b) are
converted to the lattice spacings of 0.203, 0.176, 0.124, and
0.106 nm, which are nicely indexed to the (111), (200), (220), and
(311) planes of the fcc nickel, respectively. This is in complete
agreement with the inference above from the XRD result.
However, it is noticed that the NBED rings are not continuous
but composed of discrete spots, which suggests preferential
orientations of the Ni nanoparticles in the chain network.
Fig. 2c is a lattice-resolved HRTEM image taken near the
contact region of two contiguous Ni nanoparticles (see also a more
magnified view in Fig. S5). Obviously, the lattice fringes prolongate
across the two nanoparticles without interruption by an apparent
boundary. To our knowledge, such nearly epitaxially attached and
fused Ni nanoparticle chains in a highly-branched network are
unprecedented.
Information on the morphologies and structures of the Ni
nanochains networks can be gleaned from Fig. 2. As shown in
Fig. 2a, all of the Ni nanochains are in the size range of 150–250 nm
Regarding the mechanism for the growth of highly branched Ni
nanoparticle chains, we believe that both N2H4?H2O and PVP play
important roles. Most probably, N2H4?H2O serves both as a ligand
and a reducing agent during the reaction. To start with, Ni21 in the
solution reacted with hydrazine to form sky-blue complex,
{ Electronic supplementary information (ESI) available: synthesis, mag-
netic measurements, Ni nanoparticle to nanochain evolution, enlarged
HRTEM image of Fig. 2c. See http://www.rsc.org/suppdata/cc/b4/
b411311j/
2 7 2 6
C h e m . C o m m u n . , 2 0 0 4 , 2 7 2 6 – 2 7 2 7
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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Fig. 2 a. SEM image of a typical as-prepared Ni nanochain sample; b. A bright field TEM image. The inset shows the corresponding nano-beam electron
diffraction (NBED) pattern; c. A lattice-resolved HRTEM image taken near the joint region of two contiguous Ni nanoparticles.
[Ni(N2H4)3]21, which is very stable at ambient temperature.4 At the
boiling point of ethylene glycol (about 197 uC), the excessive
hydrazine acted as a reducing agent and converted [Ni(N2H4)3]21
to Ni through homogeneous nucleation. The Ni nanoparticle
formation is likely to involve the following chemical reactions:
and the adjacent antiferromagnetic NiO would otherwise result in a
shift of the hysteresis loop. Again, this indicates that the samples
have no significant oxidization during and after the preparation.
In conclusion, a self-assembled network of pure Ni nanochains
has been successfully prepared by a simple solution-phase method.
Our synthetic approach has the virtues of simplicity, high yield,
product stability, and generality for metallic systems. The uniform
Ni nanochains are oriented and fused together like a bracelet with
little lattice mis-orientation and apparent boundary. A significantly
enhanced magnetic coercivity has been obtained for the nanochains
compared to the bulk metal. The oriented attachment chain growth
mechanism has been proposed for the formation of the unique
nickel nanochain network structures prepared in solution. The
excellent stability, the uniform size, the hierarchical self-assembled
structure, and the peculiar magnetic properties of the bracelet-
like metal nanochains lend a model system for fundamental
investigations and promising applications in various fields of
nanotechnology.
Ni21 1 N2H4 A [Ni(N2H4)3]21
[Ni(N2H4)3]21 1 N2H4 A NiE 1 4NH3F 1
2N2F 1 H2F 1 2H1
(2-1 )
At this time forward, the PVP molecules behaved like soft-
templates. Initially, very small Ni nanoparticles (y10 nm) were
formed (see Fig. S4). With the increase of the refluxing time,
presumably, the small Ni nanoparticles diffused and aggregated to
form larger nanoparticles due to the magnetic dipole–dipole
interaction and effects of the PVP templates. The larger Ni
nanoparticles were then assembled into necklace-like chains with
multiple branches because of the stronger anisotropic magnetic
forces. At the growth temperature, there was a thermodynamic
driving force for aggregated growth because the surface energy is
reduced substantially when the interface is eliminated.5,6 The little
lattice mis-orientation between the bordering Ni nanoparticles
(Fig. 2c) suggests that an oriented attachment mechanism may also
play a role at certain stages of the assembly as found for other
systems.7,8 In the synthesis of Ni nanochains, we often encountered
trimmed Cayley Trees,9 which can well explain the formation of the
highly branched nanochains network. In the Cayley Trees, some of
the branches have dead ends. This so happens when two growing
branches stumble upon each other, blocking further growth.10 In
the highly-branched network of the Ni nanochains we prepared,
such dead ends can be clearly seen, as indicated by arrows in
Fig. 2b.
This project was supported by the National Natural Science
Foundation of China (No. 20373004). L. Guo thanks BUAA for
financial support through the Outstanding Youth Award (I). SY
acknowledges support from the Research Grants Council of Hong
Kong and HKUST.
Notes and references
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B. Chaudret, Chem. Mater., 1999, 11, 526; (c) J. Bao, Y. Liang, Z. Xu
and L. Si, Adv. Mater., 2003, 15, 1832; (d) S. L. Tripp, S. V. Pusztay,
A. E. Ribbe and A. Wei, J. Am. Chem. Soc., 2002, 124, 7914.
3 N. Cordente, M. Respaud, F. Senocq, M. J. Casanove, C. Amiens and
B. Chaudret, Nano Lett., 2001, 1, 565.
4 Y. D. Li, L. Q. Li, H. W. Liao and H. R. Wang, J. Mater. Chem., 1999,
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5 A. P. Alivisatos, Science, 2000, 289, 736.
6 R. L. Penn and J. F. Banfield, Geochim. Cosmochim. Acta, 1999, 63,
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D. R. Veblen, J. Phys. Chem. B, 2001, 105, 2177.
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11 R. M. Bozorth, Ferromagnetism, D. Van Nostrand Company, Inc.,
Toronto, 1951.
It should be mentioned that similar Co nanochains had also been
prepared successfully using the same methods in our laboratory
(see Fig. S3). This strongly suggests that our method is rather
general for the synthesis of branched metal nanochain networks.
Magnetic properties of the branched Ni nanochains were
investigated by a VSM (vibrating sample magnetometer) technique
(see S2). The coercivity value amounts to 16 mT for Ni
nanoparticle chains, which is over two orders of magnitude
larger than that of bulk Ni (0.07 mT).11 One can expect that the
reduced size and dimensionality of the Ni nanostructures may
change the magnetization reversal mechanism, leading to the large
enhancement of coercivity. This is interesting for future application
of magnetic recording devices. Second, the magnetization hysteresis
loop is symmetric with respect to the zero field. This suggests that
there is no exchange biasing effect12 caused by, say, NiO (TN
520 K),11 because the exchange coupling between ferromagnetic Ni
~
12 J. Nogues and I. K. Schuller, J. Magn. Magn. Mater., 1999, 192, 203.
C h e m . C o m m u n . , 2 0 0 4 , 2 7 2 6 – 2 7 2 7
2 7 2 7