A hydrothermal reduction route to single-crystalline hexagonal cobalt nanowires

Article abstract of DOI:10.1002/ejic.200600061

Hexagonal close-packed (hcp) cobalt nanowires with diameters of 150-300 nm and lengths of up to several hundreds of micrometers have been synthesized by a hydrothermal reduction method. These cobalt nanowires were generated by the reduction of cobalt(II) citrate complexes by hypophosphite (H₂PO ₂^-) in basic solution at 160°C. High-resolution TEM and SAED reveal that the as-prepared Co nanowires have a single-crystalline structure with a [001] growth direction. The room-temperature hysteresis loop of these nanowires shows a ferromagnetic behavior with enhanced coercivity. A formation mechanism for these Co nanowires is proposed. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.

Full text of DOI:10.1002/ejic.200600061

FULL PAPER
DOI: 10.1002/ejic.200600061
A Hydrothermal Reduction Route to Single-Crystalline Hexagonal Cobalt
Nanowires
By-Qin Xie,[a,b] Yitai Qian,*[a,b] Shuyuan Zhang,[a] Shenquan Fu,[a] and Weichao Yu[b]
Keywords: Cobalt / Hydrothermal synthesis / Magnetic properties / Nanostructures
Hexagonal close-packed (hcp) cobalt nanowires with dia-
meters of 150–300 nm and lengths of up to several hundreds
of micrometers have been synthesized by a hydrothermal re-
duction method. These cobalt nanowires were generated by
the reduction of cobalt(II) citrate complexes by hypophos-
phite (H2PO2–) in basic solution at 160 °C. High-resolution
TEM and SAED reveal that the as-prepared Co nanowires
have a single-crystalline structure with a [001] growth direc-
tion. The room-temperature hysteresis loop of these
nanowires shows a ferromagnetic behavior with enhanced
coercivity. A formation mechanism for these Co nanowires is
proposed.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
nanowires obtained by electrodeposition techniques are
either very wide in diameter (380–420 nm) or polycrystal-
line.
Introduction
Magnetic nanomaterials have attracted considerable
interest due to their novel properties and many applications
in magnetic recording media, sensors and other devices.[1–4]
In particular, nanocrystalline Co has sparked intensive
studies not only due to its multiple crystal structures (hcp,
fcc, and ε), but also because of its structure-dependent
magnetic and electronic properties.[5] It has been reported
that the anisotropic high magnetic coercivity of hcp-Co is
the preferred structure for permanent magnet applications,
as compared with the more symmetric low coercivity fcc
phase used for soft magnetic applications.[6] A series of pos-
sible techniques have been applied for the structure-con-
trolled synthesis of Co nanocrystals. For example, ε-Co
nanoparticles (2–17 nm) have been synthesized in a mixture
of an organic solvent and surfactant by the pyrolysis of car-
bonyl compounds[6,7] or a solution-phase metal salt re-
duction.[8,9] Thermodynamically stable hcp-Co nanorods
have been obtained by the decomposition of [Co(η3-
C8H13)(η4-C8H12)] in the presence of oleic acid and a long
amine ligand.[10]
Wet chemical methods have proved to be very effective
for the production of single-crystalline, low-dimensional
hexagonal cobalt nanocrystals. For example, hcp-Co nano-
disks and their assembly of ribbons have been prepared by
rapid decomposition of carbonylcobalt in the presence of
trioctylphosphane (TOP) and oleic acid (OA),[16] and a su-
perlattice of monodisperse hcp-Co nanorods and nanowires
has been selectively synthesized by the decomposition of
organometallic precursors in the presence of a mixture of
long-chain amines and acid molecules.[17] A surfactant-as-
sisted hydrothermal reduction route has been used to syn-
thesize hexagonal cobalt nanobelts (diameter: 200–500 nm)
in a mixed solvent of water and ethanol.[18] Up to now, the
preparation of shape-anisotropic magnetic nanomaterials
by wet chemical methods has mainly been achieved in a hot
organic solvent containing a surfactant. However, recent
studies have shown that low-dimensional magnetic nanos-
tructures (such as Ni nanobelts[19] and nanoflowers[20] and
Fe2O3 nanorod arrays[21]) can be formed in aqueous solu-
tion under properly controlled conditions even without the
presence of surfactants. Herein, we report a facile hydro-
thermal method to synthesize single-crystalline hcp-Co
nanowires, in which citrate salts have been introduced as a
complexing reagent and shape modifier to control the
growth process. The chemical reduction can be formulated
as follows:
Anisotropic magnetic nanoparticles are expected to exhi-
bit interesting properties because of their shape anisot-
ropy,[11,12] and an hcp-Co nanowire array has been fabri-
cated by an electrochemical deposition technique using
polycarbonate membranes,[13] porous AAO,[14] and meso-
porous silica films[15] as templates. However, most Co
–
[Co(C6H5O7)2]4– + H2PO2 + 3 OH– Ǟ
[a] Hefei National Laboratory for Physical Science at Microscale,
University of Science and Technology of China,
Hefei, Anhui 230026, People’s Republic of China
Fax: +86-551-360-7402
3–
2–
Co + 2 C6H5O7 + HPO3 + 2 H2O
Results and Discussion
The crystalline nature of the phase and its purity were
determined by X-ray diffraction (XRD). A representative
E-mail: ytqian@ustc.edu.cn
[b] University of Science and Technology of China, Department of
Chemistry,
Hefei, Anhui 230026, People’s Republic of China
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Single-Crystalline Hexagonal Cobalt Nanowires
FULL PAPER
XRD pattern of the as-prepared product is shown in Fig-
ure 1a. All the diffraction peaks can be well indexed to hex-
agonal-phase cobalt, with lattice constants of a = 2.506 and
c = 4.072 Å, which is consistent with the standard values
reported in JCPDS 05-0727 (space group P63/mmc). No
characteristic peaks due to the impurities of cobalt oxides
or hydroxides were detected, which indicates that the nanos-
tructures obtained by the present synthetic route consist of
a pure hcp phase. In addition, the relative intensity of the
peaks corresponding to the (002)/(100) and (002)/(101) pla-
nes is significantly higher than the standard values, which
indicates a different growth tropism of the products. X-ray
energy dispersive (EDS) microanalysis (shown in Figure 1b)
of the Co nanowires indicated that the sample is essentially
pure Co. Only a very small amount of oxygen (element ra-
tio: 0.54%) was detected, which was possibly from slight
oxidation of the surface. Considering that the experiment
was carried out in air, it is not difficult to explain this.
Figure 2. SEM and TEM images of the sample: (a) typical SEM
image of the as-prepared Co nanowires; (b) and (c) TEM images
of the Co nanowires, displaying the twist feature of the nanowire
(c).
A more detailed study of the nanowires was performed
by high-resolution TEM (HRTEM). Figure 3a shows a
TEM image of an individual Co nanowire with a diameter
of 200 nm. Figure 3b is a typical selected-area electron dif-
fraction (SAED) pattern that was recorded from this
nanowire. These pattern spots demonstrate the single-crys-
talline nature of this Co nanowire. Figure 3c is an HRTEM
image recorded near the edge of this Co nanowire. The
fringe spacing of 2.04 Å corresponding to the separation of
the {002} planes of hcp-Co can be seen from Figure 3c; this
also shows that the nanowire is single-crystalline. Further
studies of both the HRTEM image and SAED patterns
confirm that the single-crystalline nanowire grows along the
[001] direction. The preferential crystal growth direction
along the c axis is the magnetic easy axis of hexagonal co-
balt crystals.[22]
Figure 1. (a) XRD pattern of the final product obtained at 160 °C
after 20 h. (b) EDS pattern of the final product (Cr and Cu signals
can be attributed to the copper microgrid supporting the sample).
The morphologies and structures of the as-prepared
products were examined by SEM and TEM. SEM observa-
tion (in Figure 2a) shows that the products consist of a
large quantity of wire-like nanostructures. The nanowires
range from several tens to several hundreds of micrometers
in length. Figures 2b and c are TEM images that show the
morphology of the nanowires. Each nanowire has a uni-
form width over its entire length, and the typical diameter
of the wires is in the range 150–300 nm. Figure 2c displays
the twist feature of a nanowire with an “M” shape, thus
indicating the flexibility of the nanowire.
Figure 3. (a) Typical nanowire with a diameter of 200 nm. (b)
SAED pattern recorded from this nanowire. (c) HRTEM image
taken from the edge of this nanowire.
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To investigate the nucleation and growth process of these tions, the formation of Co nanowires can also be attributed
nanowires, the evolution of the Co nanostructures was to nucleation and continuous growth by Ostwald ripening.
studied at various growth stages of the hydrothermal reac- In this mechanism, the small particles are generated by re-
tion by XRD and SEM techniques. Figure 4 shows the duction of the cobalt() citrate complex by H2PO2– in basic
XRD patterns of samples that were taken after hydrother- solution in the initial stage of the hydrothermal process. In
mal treatment for 6, 12, and 18 h. After heating for 6 h, the a subsequent step, the larger particles will grow at the ex-
blue color of the mixture had disappeared and a large pense of the smaller nanoparticles through the Ostwald rip-
amount of grey solid product had formed. The XRD ening process. In the presence of citrate, most particles grow
pattern (shown in Figure 4a) of the sample obtained at this into nanowires. Finally, the nanoparticles are gradually
stage indicates that hexagonal-phase cobalt had been pro- consumed, resulting in the formation of long nanowires
duced. The SEM image (Figure 5a) shows that the sample (shown in Figure 2a).
is composed of cobalt particles and that they aggregate,
The presence of citrate ions is crucial for the formation
driven by their natural magnetism. When the hydrothermal of Co nanowires. Control experiments have shown that
reaction time was extended to 12 h, the diffraction peaks of without citrate ions, only Co particles (diameter ca. 500 nm)
cobalt strengthened (Figure 4b). The SEM image (Fig- are obtained (Figure 6a), and at a low concentration of cit-
ure 5b) reveals that radial nanowires were generated on the rate ions (0.04 ), the product (shown in Figure 6b) is a
particles, and the surfaces of nanowires were covered with flower-like aggregation of plates (width 200 nm). In ad-
large numbers of nanoparticles. The inset of Figure 5b dition, we also found that by keeping all other reaction con-
shows a magnified view of this radial structure. This image ditions the same, wire-like structures can be obtained in the
distinctly demonstrates that the Co nanowires grow from presence of either citric acid or potassium citrate, but can-
the particles. After 18 h, the diffraction peak of (002) planes not be obtained in the presence of other complexing rea-
has clearly strengthened, as shown in Figure 4c, which indi- gents such as tartrate and ethylenediamine tetraacetate.
cates that the sample has preferential tropism. The SEM This indicates that citrate anions play a crucial role in the
image shown in Figure 5c shows that most of the products formation of Co nanowires. Furthermore, the different sta-
evolve into nanowires with smooth surfaces. After 20 h, the bility and structures of the Co complex precursors may af-
yield of cobalt nanowires was above 90%.
fect the reaction conditions for the formation of wire-like
structures, as reported for the ligand-mediated synthesis of
shape-controlled materials.[20,24]
Citrate is an important biological ligand for metal ions
and at high concentrations (ca. 0.2 m) forms strong com-
plexes with metal ions such as Ag+, Ca2+, Mg2+, Ni2+, and
Zn2+ [25]
Citrate has been used as a shape modifier for
.
the synthesis of ZnO complex structures[26] and Ag
nanowires.[27] Citrate plays two major roles in our system.
First, citrate ions can coordinate with cobalt ions to form
[Co(C6H5O7)2]4– complexes in the excess citrate solu-
tion,[28,29] which decreases the free Co2+ concentration in
solution and results in the slow generation of Co nanopar-
ticles. The relatively slow reaction rate may be favorable for
the subsequent growth of Co nanostructures along the easy
magnetic axis of the hexagonal cobalt, namely the [001] di-
rection. In addition, citrate may bind to certain crystal faces
of the cobalt particles through its COO– and OH functions.
Figure 4. XRD patterns of the as-prepared samples at various
growth stages of the hydrothermal process: (a) 6 h, (b) 12 h, and
(c) 18 h.
On the basis of previous investigations on the evolution An FT-IR spectrum of the sample, hydrothermally treated
of cobalt nanowires, we found that this growth mode is very in 0.2  citrate solution for 12 h and unwashed before mea-
similar to the recent reports for the growth of one-dimen- surement, is shown in Figure 6c. It exhibits the characteris-
sional metals in solution.[19,23] Under the present condi- tic absorption for the carbonyl groups of the citrate carbox-
Figure 5. SEM images of the samples obtained at 160 °C after a hydrothermal treatment of (a) 6 h, (b) 12 h, and (c) 18 h.
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Single-Crystalline Hexagonal Cobalt Nanowires
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Figure 7. TEM images of the samples prepared at different NaOH
concentrations: (a) 1  NaOH solution, (b) 4  NaOH solution.
The M/H hysteresis loop (Figure 8) of the powdered Co
nanowires measured at room temperature (300 K) shows a
ferromagnetic behavior. The coercivity (Hc) and saturation
magnetization (Ms) values are 166.8 Oe and 133.1 emug–1,
respectively. The saturation magnetization is reduced rela-
tive to the bulk value (168 emug–1).[30] Surface oxidation at
grain boundaries has been shown to cause a decrease in
saturation magnetization.[30,31] Under the present experi-
mental conditions, such a surface oxidation could be the
reason for the decrease of Ms. In addition, although the
Figure 6. TEM images of products from syntheses with no sodium
citrate (a) and 0.04  sodium citrate solution (b). (c) FT-IR spec-
trum of the sample hydrothermally treated in 0.2  citrate solution
for 12 h and unwashed before measurement.
ylate ligands in the asymmetric and symmetric vibration re-
gions. The asymmetric stretching vibrations νas-COO– appear
at 1597 cm–1 and the symmetric stretching vibration νs-COO–
at 1407 cm–1. The bands are shifted to lower frequencies
compared to those of free citric acid, which suggests some
interaction between metal and citrate ligand. These interac-
tions could force the nanoparticles to grow along only one
axis. In this case, citrate plays a similar role to that in the
formation of Ag nanowires.[27]
Other reaction conditions, such as the concentration of
NaOH in the reaction system and the reaction temperature,
are also important factors in determining the shape of the
final product. We found that a low concentration of NaOH
solution (1 ) led to the formation of bamboo-like nanos-
tructures with an average diameter of 300 nm (shown in
Figure 7a). The reaction at a higher concentration of
NaOH solution (4 ) produced Co dendrites (Figure 7b).
Furthermore, if the other reaction conditions were main-
tained and the reaction temperature was reduced to less
than 140 °C, the product was only aggregated particles. A
higher temperature (such as 180 °C) resulted in wider wire-
like structures and even some dendritic Co crystals. A tem-
perature between 140 and 160 °C is therefore optimal for
the growth of Co nanowires.
Figure 8. (a) Magnetic hysteresis loop of as-prepared cobalt
nanowires measured at room temperature (300 K). (b) Enlargement
of (a) at low field.
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B.-Q. Xie, Y. Qian, S. Zhang, S. Fu, W. Yu
FULL PAPER
perature on KBr mulls. The magnetic measurement of the sample
was carried out with a superconducting quantum interference de-
vice (SQUID) magnetometer (Quantum Design MPMS XL-7) with
6.4 mg of sample sealed in a small cylindrical vessel.
sample was washed before the measurement, a very small
quantity of citrate molecules adsorbed on the Co nanowires
could quench the magnetic moment through electron ex-
change between the ligand and surface atoms, which would
partly result in the reduction of saturation magnetiza-
tion.[32] Compared to the value for bulk material (a few tens
of oersteds),[33] Co nanowires exhibit enhanced coercivity,
which may be attributed to the small size and anisotropic
shape of the nanowires. However, the coercivity value is
much lower than that of the 2D and 3D superlattice of
nanorods (740, 3200, and 7200 Oe)[17] or nanobelts
(410.6 Oe)[18] with smaller sizes. Because a higher coercivity
Acknowledgments
Financial support from the National Natural Science Foundation
of China and the 973 Project of China are appreciated. The authors
thank Dr. Li Pi for recording the SQUID experiment.
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in 40 mL of distilled water, and then NaOH (10 , 10 mL) aqueous
solution was added with constant magnetic stirring. The final con-
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citrate, 0.2  NaH2PO2, and 2  NaOH. The mixture was stirred
for 10 min and then transferred into a 55-mL Teflon-lined auto-
clave. The autoclave was sealed and maintained in a furnace at
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Received: January 21, 2006
Published Online: April 11, 2006
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