D210
Journal of The Electrochemical Society, 158 (4) D210-D216 (2011)
0013-4651/2011/158(4)/D210/7/$28.00 The Electrochemical Society
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Fabrication of Cobalt Nanowires by Electroless Deposition
under External Magnetic Field
Mary Donnabelle L. Balela, ,z Shunsuke Yagi,** and Eiichiro Matsubara
*
Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan
Metallic Co nanowires with a mean diameter of about 190 nm and lengths up to 160 mm are prepared by electroless deposition
(solution reduction) in propylene glycol at room temperature under external magnetic field. Co deposition behavior in propylene
glycol is investigated by in situ monitoring of mixed potentials in conjunction with linear sweep voltammetry and the oxidation-
reduction potential of the Co(II)/Co redox pair is determined to be about ꢀ0.54 V vs Ag/AgCl. When the mixed potential drops
below the oxidation-reduction potential of the Co(II)/Co redox pair, small Co nanoparticles are generated in the solution and are
magnetized in the presence of external magnetic field. Strong attractive dipolar interactions are induced along the magnetic field
direction, which results in the assembly of Co nanoparticles to nanowires. The Co nanowires exhibit ferromagnetic properties at
room temperature, with an enhanced coercivity of about 700 Oe possibly due to shape anisotropy.
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2011 The Electrochemical Society. [DOI: 10.1149/1.3545065] All rights reserved.
Manuscript submitted November 5, 2010; revised manuscript received December 29, 2010. Published February 23, 2011.
Control of the morphology of nanostructures is a critical aspect
in the fabrication of electronic or mechanical devices and functional
materials.1,2 For example, one-dimensional (1D) ferromagnetic
nanostructures, such as nanowires and nanorods, have been attract-
ing significant attention due to their large anisotropic magnetism,
which is important for applications in permanent magnets and high-
density magnetic storage media.3–5 Its large surface area is also fav-
orable for use as catalysts.6 On the other hand, Co oxide nanowires
and nanotubes are currently being explored as possible electrode
materials for lithium-ion batteries.7–9 In the past years, template-
directed synthesis routes have made considerable progress in the
fabrication of well-aligned magnetic nanowires and nanorods with
tunable diameter and length.2,10–14 However, such methods are not
suitable for large-scale production since the preparation and the sub-
sequent removal of templates are required.15–17 Against such back-
ground, the application of magnetic fields during solution reduction
is recently being utilized to fabricate anisotropic nanostructures
due to its simplicity and effectiveness. For example, Fe3O4
nanochains,18 Ni-Co alloy microwires,19,20 Co nanosheets,21 Co
nanorods,5,15,22,23 and Ni nanowires4,6,16 have already been prepared
by magnetic-field-assisted hydrothermal and solvothermal synthe-
ses. However, methods, such as hydrothermal and solvothermal
processes, require high temperature, high pressure, or long reaction
time to fabricate 1D nanostructures.24
In the present work, we report a rapid and inexpensive approach
to the preparation of Co nanowires at room temperature with a fast
reaction rate and a high yield. The synthesis of long and rigid
Co nanowires is performed by electroless deposition (solution
reduction) in propylene glycol under an external magnetic field. Co
nanowires with a mean diameter of about 190 nm and lengths up to
160 mm are fabricated at ambient conditions (normal pressure and
room temperature) without any templates. The reduction of Co(II)
ions in the reaction system is investigated by in situ mixed potential
measurement in combination with linear sweep voltammetry, and
the starting point of the reaction is determined. The formation of Co
nanowires is studied by observing the morphological and structural
evolution of the products. The effects of reaction conditions, such as
magnetic field strength, amount of nucleating agent and concentra-
tion of reducing agent, on the size of Co nanowires are examined.
(PG, C3H8O2), sodium hydroxide (NaOH), chloroplatinic acid hexa-
hydrate (H2PtCl6ꢁ6H2O), hydrazine monohydrate (N2H4ꢁH2O), lith-
ium perchlorate (LiClO4), and ethanol (C2H6O) by Nacalai Tesque.
Co(II) acetate tetrahydrate, PEG, and PG acted as Co precursor, sur-
factant, and solvent, respectively. On the other hand, H2PtCl6ꢁ6H2O
and N2H4ꢁH2O served as nucleating agent and reductant. In a typical
synthesis, two kinds of solutions, i.e., the metal ion solution and
reducing agent solution, were prepared separately. First, 10 mmol
Co(II) acetate and 0.125 mmol PEG were dissolved in 50 cm3 PG
while nitrogen gas (N2) was bubbled at 50 cm3 minꢀ1. Then, 25 cm3
of 1.0 M (mol dmꢀ3) NaOH-PG solution was added to hydrolyze
the Co(II) ions and adjust the pH to about 12. This decreased the
activity of Co(II) ions in the solution, which would favor the forma-
tion of small primary Co nanoparticles. Next, 10 cm3 of 2.5 mM
H2PtCl6-PG solution was added to the metal ion solution as a nucle-
ating agent to provide heterogeneous nucleation sites for Co. As the
reducing agent solution, 25 cm3 deaerated PG solution containing
4.0 M N2H4 was prepared. The reducing agent solution was then
mixed thoroughly with the metal ion solution in a 200 cm3 Pyrex
beaker by mechanical stirring. The total solution had a volume of
115 cm3, with final concentrations of 0.087 M Co(II) acetate,
1.1 mM PEG, 0.22 M NaOH, 0.22 mM H2PtCl6, and 0.87 M N2H4.
The final pH remained about 12, which would increase the reducing
power of N2H4. The resulting solution was placed in a supercon-
ducting magnet (Japan Magnet Technology JMTD-10T100M) for
1 h. The magnetic field strength was varied from 1 to 10 T by con-
trolling the working electric current of the superconducting magnet.
The synthesized nanowires were magnetically separated from the
solution and ultrasonicated for 10 min in ethanol. This washing pro-
cess was repeated 10 times to remove organic contaminants on the
surface of the nanowires.
The morphology of the as-prepared products was observed by a
field-emission-scanning electron microscope (JEOL JSM 6500-F)
and a transmission electron microscope (TEM, JEOL JEM 2010).
The mean diameter and lengths were determined by image analysis
of 100 nanowires in each sample. The crystalline structure was
investigated by X-ray diffraction (XRD, MAC Science M03XHF22)
using Cr Ka radiation. The magnetic properties, such as saturation
magnetization (Ms) and coercivity (Hc), were studied by a magnetic
property measurement system superconducting quantum interfer-
ence device (Quantum Design, Inc., MPMS SQUID XL) at room
temperature using an applied field of 2 T.
To investigate the reduction behavior of Co(II) ions in propylene
glycol at room temperature, linear sweep voltammetry was per-
formed using a potentiostat/galvanostat (Hokuto Denko, Co. Ltd.,
HA-151). A platinum (Pt)-sputtered quartz crystal substrate (0.196
cm2) and a Pt sheet (4 cm2) were used as working and counter elec-
trodes, respectively. The voltammogram was recorded from 0 to
ꢀ1.5 V at a potential sweep rate of 1 mV sꢀ1. For the evaluation of
Experimental
The chemical reagents used in this work were analytical grade
Co(II) acetate tetrahydrate [Co(C2H3O2)2ꢁ4H2O], poly(ethylene gly-
col) [PEG, H(OCH2CH2)nOH, Mw ¼ 20,000], propylene glycol
*
**
Electrochemical Society Student Member.
Electrochemical Society Active Member.
z E-mail: balela.m@hw8.ecs.kyoto-u.ac.jp
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