Magnetic-dipolar-interaction-induced self-assembly affords wires of hollow nanocrystals of cobalt selenide

Article abstract of DOI:10.1002/anie.200503486

(Figure Presented) Giving cobalt some stick: A general strategy for the synthesis of 1D assemblies of hollow nanocrystals is presented. Cobalt nanostructures assembled through magnetic dipolar interactions (see scheme) are used as templates for the formation of wires of hollow nanocrystals of CoSe ₂ (and Co₃S₄, CoTe). The wires of hollow CoSe₂ nanocrystals are ferromagnetic, with a Curie temperature of 570 K.

Full text of DOI:10.1002/anie.200503486

Communications
[
1]
cal band-gap materials or high-density magnetic recording
[
2]
media, self-assembled monolayers (SAM) for nanometer-
[
3]
thick films on a variety of substrates, and nanofibers of self-
organized small molecules or oligopeptides for drug delivery
[
4]
and tissue engineering. Hydrogen bonding and/or van der
Waals interactions are usually the driving forces that result in
[
5]
self-assembly at the nanoscale. Despite extensive investiga-
tion, it is still difficult to control and predict the nano-
structures resulting from these two types of forces. Although
several recent reports have shown that magnetic dipolar
[
6,7]
interactions induce rather predictable nanoassemblies,
methods for maintaining these assemblies when the magnetic
force decreases or vanishes have been less thoroughly
investigated. Therefore, we decided to use the magnetic
[
8]
dipoles inherently associated with magnetic nanoparticles to
form 1D assemblies of hollow nanocrystals of semiconduc-
[
9]
tors.
We chose to make wires of hollow nanocrystals of CoSe2
to demonstrate the concept, because 1) the isolation of a
nanonecklace of cobalt nanocrystals on a substrate from a
[
7]
dispersion after removal of the solvent indicated that
magnetic dipolar interactions are sufficient to maintain 1D
assemblies of nanocrystals in solution; 2) the recently
reported formation of hollow CoSe nanocrystals from cobalt
[
10]
nanocrystals through the Kirkendall effect suggested that a
similar process could be used to generate hollow CoSe2
nanocrystals from 1D assemblies of cobalt nanocrystals;
3
) hollow nanostructures have received considerable atten-
tion because they exhibit properties that differ from their
[
11]
solid counterparts; and 4) nanowires of hollow nanocrystals
of CoSe are unknown. Herein, we show that when their sizes
2
Magnetic Nanowires
reach approximately 20 nm cobalt nanocrystals self-assemble
into wires in solution at temperatures as high as 455 K.
[
10,12]
DOI: 10.1002/anie.200503486
Through the nanoscale Kirkendall effect,
wires of hollow
CoSe nanocrystals form without loss of the preassembled
2
Magnetic-Dipolar-Interaction-Induced Self-
Assembly Affords Wires of Hollow Nanocrystals
of Cobalt Selenide**
nanostructure induced by the magnetic dipolar interactions of
the cobalt nanocrystals. In control experiments, when the size
of the cobalt nanocrystals is 6 nm, or when 20-nm cobalt
nanocrystals are in an alternating magnetic field, no wires of
hollow CoSe nanocrystals form. These observations confirm
that the magnetic interaction plays a key role in the formation
Jinhao Gao, Bei Zhang, Xixiang Zhang, and Bing Xu*
2
of the nanowires of hollow CoSe crystals. Several types of
2
Self-assembly, an attractive and practical methodology, allows
the formation of a wide range of nanostructures for promising
applications—for example, nanoparticle arrays for new opti-
ferromagnetic metallic nanocrystals (iron, cobalt, nickel,
FePt, or CoPt) can be prepared readily, and their self-
assembly depends on the magnetic dipolar interactions of
particles of a certain size. Therefore, the method shown
herein should be a general route to 1D assemblies of hollow
nanocrystals.
[*] J. Gao, Prof. B. Xu
Department of Chemistry
The Hong Kong University of Science & Technology
Clear Water Bay, Hong Kong (China)
Fax: (+852)2358-1594
The synthesis of wires of hollow CoSe nanocrystals is easy
and straightforward. Cobalt nanocrystals of approximately
2
2
0 nm in diameter were prepared according to a modified
E-mail: chbingxu@ust.hk
[
13]
literature procedure and then dispersed in a solvent (for
example, 1,2-dichlorobenzene) by using trioctylphosphine
oxide (TOPO) as a surfactant. The injection of a dichloro-
benzene solution of selenium into the dispersion of cobalt
nanocrystals under reflux at 455 K resulted in a black
dispersion of CoSe2 nanocrystals after approximately
B. Zhang, Prof. X. Zhang
Department of Physics
The Hong Kong University of Science & Technology
Clear Water Bay, Hong Kong (China)
[
**] This work was partially supported by RGC (Hong Kong) and EHIA
(
HKUST).
3
0 min. Transmission electron microscope (TEM) images
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
reveal that the CoSe nanocrystals form wires of intercon-
2
1
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nected hollow spheres (Figure 1a). High-resolution TEM
images (inset, Figure 1a) and analysis of the electron
diffraction pattern of the sample suggest that these hollow
spheres are crystalline. Scanning electron microscope (SEM)
hollow CoSe nanocrystals match with the sizes and alignment
of the cobalt nanocrystals in the 1D assembly. These results
lead to the proposed mechanism for the formation of wires of
2
hollow CoSe nanocrystals (Scheme 1). Each cobalt nano-
2
Scheme 1. Formation of hollow CoSe nanocrystals from cobalt nano-
2
particles (NP) in the 1) absence and 2) presence of an alternating
magnetic field.
Figure 1. a) TEM image (inset: HR-TEM image), and b) SEM image of
the wires of hollow CoSe nanocrystals.
2
crystal behaves as a single magnetic dipole, and the cobalt
nanocrystals form chains or necklaces in solution because of
strong magnetic dipolar interactions. After being added into
the dispersion, selenium reacts with cobalt, and a thin layer of
images indicate that the wires of hollow CoSe nanocrystals
2
are the major nanostructure formed (Figure 1b). This obser-
vation suggests that the wires form in high yield, which may
allow large-scale production. The X-ray diffraction (XRD)
selenium or CoSe grows around the wires of cobalt nano-
2
crystals, without disintegration of the wires. Further reaction
between selenium and cobalt through the nanoscale Kirken-
dall process maintains the preassembled nanostructures and
pattern of the CoSe nanocrystals (Figure 2) matches that of
2
affords a 1D assembly of hollow CoSe nanocrystals. Accord-
2
ing to this mechanism, disruption (process 2) in Scheme 1) or
reduction of the magnetic dipolar interactions before the
addition of selenium should prevent the formation of the
wires of hollow CoSe nanocrystals. To illustrate this point, we
2
conducted the two following control experiments.
First, we chose cobalt nanocrystals with a smaller size
(6 nm) as starting materials, because they have weaker
magnetic dipolar interactions than cobalt nanocrystals with
diameters of 20 nm. Under the conditions described above,
the reaction between 6-nm cobalt nanocrystals and selenium
only affords monodispersed hollow nanocrystals (Figure 3a).
No 1D assemblies of nanocrystals are observed, probably
because the magnetic dipolar interactions between these
small cobalt nanocrystals are too weak to form chain or
necklace nanostructures at 455 K. Second, we synthesized
Figure 2. XRD pattern of the wires of hollow CoSe nanocrystals.
2
the primitive cubic phase of CoSe (Powder Diffraction File
hollow CoSe nanocrystals in an alternating magnetic field,
2
2
#
090234). The X-ray photoelectron spectrum (XPS) of the
CoSe2 nanocrystals shows peaks at 778.7 and 54.4 eV,
corresponding to the binding energies of the cobalt 2p and
3
/2
[
14]
selenium 3d5/2 states. In addition, X-ray fluorescence
XRF) analysis indicates that the ratio of Co:Se in the
(
sample is approximately 1:1.80, which suggests that other
cobalt selenide phases (for example, CoSe and Co Se ) may
3
4
[
15]
be present in small amounts.
Additional TEM analysis of the dispersion of cobalt
nanocrystals prior to the addition of selenium showed that the
cobalt nanocrystals assemble into wires (see Supporting
Information), which are similar to the assemblies of hollow
CoSe nanocrystals. We also observed a small amount of
necklace structures in both the samples of cobalt nanocrystals
2
Figure 3. a) TEM image (inset: HR-TEM image) of the monodispersed
hollow CoSe nanospheres synthesized from 6-nm cobalt nanocrystals;
2
and hollow CoSe nanocrystals (see Supporting Information).
Furthermore, the sizes and alignment of the holes of the
2
b) TEM image of the hollow CoSe
alternating magnetic field.
2
nanostructures synthesized in an
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Communications
B(t) = B sin(wt), where B = 35 G and w = 2pf with f = 50 Hz,
0
0
generated by a Helmholtz coil. In this field, the reaction of 20-
nm cobalt nanocrystals with selenium gives only a small
number of individual hollow nanocrystals of CoSe , in
2
addition to a small number of selenium nanowires (Fig-
[
16]
ure 3b). This result is consistent with process 2) proposed
in Scheme 1. Because of the head-to-tail arrangement of the
magnetic dipoles of the nanoparticles, it is difficult to “switch”
all of the dipoles at the same time. The breaking of the chain is
due to the vibrational magnetic torque applied, which disrupts
the dipolar interactions between the particles. Once the chain
is broken, the alternating magnetic torque causes the particles
to rotate and prevents them from reconnecting. This mech-
anism agrees with our observations. These two experiments
confirm that magnetic dipolar interactions play a key role in
Figure 5. The hysteresis loopfor the wires of hollow CoSe 2 nano-
crystals at 50 K (inset: temperature-dependence of the ZFC (lower)
and FC (upper) magnetization of the wires of hollow CoSe nano-
2
the formation of 1D assemblies of hollow CoSe nanocrystals.
2
crystals in a magnetic field of 100 Oe).
The use of magnetic dipolar interactions, which can be easily
modified through the application of an alternating magnetic
field, in the formation of nanostructures provides a useful
alternative to the use of electronic dipolar interactions (as
applied in the assembly of nanowires of CdSe and PbTe ).
To establish the generality of this procedure, we replaced
selenium by sulfur or tellurium in the reaction with 20-nm
cobalt nanocrystals. As shown in Figure 4, we obtained wires
netism. Similar M(T) curves were observed for 20-nm cobalt
nanocrystals (see Supporting Information). The magnetic
[
5]
[17]
behavior of the wires of hollow CoSe nanocrystals suggests
2
that these nanocrystals may actually form a polycrystalline
wire with holes. The small peak at 14 K in the ZFC curve
(Figure 5, inset) could be due to the unblocking of super-
paramagnetic CoSe nanocrystals, which is evidenced by the
2
FC data from 5 to 30 K. To exclude the possibility that the
ferromagnetism solely originates from residual cobalt nano-
crystals, we measured the magnetization of the wires of
hollow CoSe nanocrystals in a 1000-Oe field from 5 to 674 K
2
(Figure 6). We used a SQUID magnetometer to collect the
Figure 4. TEM images (insets: HR-TEM images) of the wires of hollow
nanocrystals of a) Co S and b) CoTe.
3
4
of hollow Co S and CoTe nanocrystals, which are similar in
3
4
morphology to the hollow CoSe nanocrystals. The XPS of the
2
Co S wires shows peaks at 778.9 and 162.2 eV, corresponding
3
4
to the binding energies of the cobalt 2p3/2 and sulfur 2p states.
XRF analysis indicates that the ratio of Co:S in the sample is
Figure 6. Temperature-dependence of the ZFC (lower) and FC (upper)
magnetization of the wires of hollow CoSe nanocrystals in a magnetic
1
:1.32, nearly 3:4, which agrees with the ratios of the
2
field of 1000 Oe.
corresponding precursors. For the CoTe sample, XRFanalysis
indicates that the ratio of Co:Te is approximately 1:1 (see
Supporting Information).
data from 5 to 300 K and a vibrating sample magnetometer to
collect the data from 300 to 674 K. The Curie temperature
was determined by the position of the maximum of j dM/dTj
to be 570 K, which is roughly 1000 K lower than the Curie
temperature of cobalt. This result suggests that the ferromag-
Magnetization measurements indicate that the wires of
hollow CoSe nanocrystals are ferromagnetic. As shown in
2
Figure 5, the field-dependent magnetization (M(H)) of the
wires of hollow CoSe nanocrystals at 50 K displays hysteresis
2
typical of a ferromagnet. The coercivity (H ) of the wires of
netism of the wires may arise from CoSe , although the
c
2
hollow CoSe nanocrystals is approximately 600 Oe, roughly
detailed mechanism has yet to be established.
2
half that of the 20-nm cobalt nanocrystals (approximately
In summary, we have demonstrated that the nanoscale
Kirkendall effect can be applied to a magnetically preassem-
bled nanostructure to form 1D assemblies of hollow nano-
crystals of cobalt chalcogenides at elevated temperatures.
1
100 Oe; see Supporting Information). Standard zero-field
cooling (ZFC) and field cooling (FC) temperature-dependent
magnetization (M(T)) measurements indicate bulk ferromag-
1
222
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This procedure provides an alternative route to nanowires
with a unique morphology (interconnected hollow nano-
crystals) that is difficult to achieve using other means. This
[12] K. N. Tu, U. Gosele, Appl. Phys. Lett. 2005, 86, 09311.
[
[
[
13] V. F. Puntes, K. M. Krishnan, A. P. Alivisatos, Science 2001, 291,
2115.
14] H. van der Heide, R. Hemmel, C. F. van Bruggen, C. Haas, J.
Solid State Chem. 1980, 33, 17.
15] C. E. M. Campos, J. C. de Lima, T. A. Grandi, K. D. Machado, V.
Drago, P. S. Pizani, Solid State Commun. 2004, 131, 265.
“
magnetic-guiding” strategy is also compatible with other
[
9,18]
methodologies for creating nanostructures.
In addition,
the magnetic nanostructures produced may have potential
applications in the fabrication of nanodevice components. We
are currently investigating this possibility.
[16] B. Gates, Y. Yin, Y. Xia, J. Am. Chem. Soc. 2000, 122, 12582.
[17] K.-S. Cho, D. V. Talapin, W. Gaschler, C. B. Murray, J. Am.
Chem. Soc. 2005, 127, 7140.
[
18] a) H. W. Gu, R. K. Zheng, X. X. Zhang, B. Xu, J. Am. Chem.
Soc. 2004, 126, 5664; b) H. W. Gu, Z. M. Yang, J. H. Gao, C. K.
Chang, B. Xu, J. Am. Chem. Soc. 2005, 127, 34.
Received: October 3, 2005
Revised: November 21, 2005
Published online: January 18, 2006
Keywords: chalcogenides · cobalt · magnetic properties ·
.
nanostructures · self-assembly
[
1] a) S. H. Park, D. Qin, Y. Xia, Adv. Mater. 1998, 10, 1028; b) B.
Gates, Y. Xia, Appl. Phys. Lett. 2001, 78, 3178.
[
2] a) S. Sun, C. B. Murray, D. Weller, L. Folks, A. Moser, Science
2000, 287, 1989; b) V. F. Puntes, P. Gorostiza, D. M. Aruguete,
N. G. Bastus, A. P. Alivisatos, Nat. Mater. 2004, 3, 263; c) T.
Hyeon, Chem. Commun. 2003, 927.
[
3] J. C. Love, L. A. Estroff, J. K. Kriebel, R. G. Nuzzo, G. M.
Whitesides, Chem. Rev. 2005, 105, 1103.
[
4] a) T. C. Holmes, S. de Lacalle, X. Su, G. S. Liu, A. Rich, S. G.
Zhang, Proc. Natl. Acad. Sci. USA 2000, 97, 6728; b) G. A. Silva,
C. Czeisler, K. L. Niece, E. Beniash, D. A. Harrington, J. A.
Kessler, S. I. Stupp, Science 2004, 303, 1352; c) B. G. Xing, C. W.
Yu, K. H. Chow, P. L. Ho, D. G. Fu, B. Xu, J. Am. Chem. Soc.
2002, 124, 14846; d) Z. M. Yang, H. W. Gu, Y. Zhang, L. Wang,
B. Xu, Chem. Commun. 2004, 208.
[
5] a) Z. Y. Tang, N. A. Kotov, M. Giersig, Science 2002, 297, 237;
b) Z. Y. Tang, N. A. Kotov, Adv. Mater. 2005, 17, 951.
6] a) J. C. Love, A. R. Urbach, M. G. Prentiss, G. M. Whitesides, J.
Am. Chem. Soc. 2003, 125, 12696; b) A. R. Urbach, J. C. Love,
M. G. Prentiss, G. M. Whitesides, J. Am. Chem. Soc. 2003, 125,
[
12704.
[
[
7] a) S. L. Tripp, S. V. Pusztay, A. E. Ribbe, A. Wei, J. Am. Chem.
Soc. 2002, 124, 7914; b) S. L. Tripp, R. E. Dunin-Borkowski, A.
Wei, Angew. Chem. 2003, 115, 5749; Angew. Chem. Int. Ed. 2003,
42, 5591.
8] a) T. Hyeon, S. S. Lee, J. Park, Y. Chung, H. Bin Na, J. Am.
Chem. Soc. 2001, 123, 12798; b) J. Park, K. J. An, Y. S. Hwang,
J. G. Park, H. J. Noh, J. Y. Kim, J. H. Park, N. M. Hwang, T.
Hyeon, Nat. Mater. 2004, 3, 891; c) J. Park, E. Lee, N. M. Hwang,
M. S. Kang, S. C. Kim, Y. Hwang, J. G. Park, H. J. Noh, J. Y. Kini,
J. H. Park, T. Hyeon, Angew. Chem. 2005, 117, 2932; Angew.
Chem. Int. Ed. 2005, 44, 2872.
[
9] Y. D. Yin, A. P. Alivisatos, Nature 2005, 437, 664.
[
[
10] Y. D. Yin, R. M. Rioux, C. K. Erdonmez, S. Hughes, G. A.
Somorjai, A. P. Alivisatos, Science 2004, 304, 711.
11] a) J. Chen, F. Saeki, B. J. Wiley, H. Cang, M. J. Cobb, Z. Y. Li, L.
Au, H. Zhang, M. B. Kimmey, X. D. Li, Y. Xia, Nano Lett. 2005,
5, 473; b) Y. Sun, Y. Xia, J. Am. Chem. Soc. 2004, 126, 3892; c) L.
Rapoport, N. Fleischer, R. Tenne, Adv. Mater. 2003, 15, 651; d) L.
Rapoport, Y. Bilik, Y. Feldman, M. Homyonfer, S. R. Cohen, R.
Tenne, Nature 1997, 387, 791; e) N. A. Dhas, K. S. Suslick, J. Am.
Chem. Soc. 2005, 127, 2368; f) D. Goll, A. E. Berkowitz, H. N.
Bertram, Phys. Rev. B 2004, 70; g) R. Tenne, Prog. Inorg. Chem.
2001, 50, 269; h) Y. G. Sun, B. Mayers, Y. N. Xia, Adv. Mater.
2003, 15, 641; i) X. M. Sun, Y. D. Li, Angew. Chem. 2004, 116,
3915; Angew. Chem. Int. Ed. 2004, 43, 3827; j) X. Wang, Y. D. Li,
Angew. Chem. 2003, 115, 3621; Angew. Chem. Int. Ed. 2003, 42,
497.
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