Biomedical Engineering Reference
In-Depth Information
pyridine or other nonpolar solvents such as hexane. These 2 - D and 3 - D super-
lattices of cobalt nanocrystals coated with lauric acid chains were found to be
highly stable, with samples exposed to air appearing not to be oxidized even after
a period of several weeks.
Furthermore, ordered fcc “supercrytals” or disordered 3-D assemblies consisting
of 7.5 nm cobalt nanoparticles were produced by the slow evaporation of a colloidal
solution on a highly oriented pyrolitic graphite (HOPG) substrate and optimiza-
tion of the substrate temperature and evaporation rate [24, 25]. Grazing incidence
small-angle X-ray scattering (GISAXS) diffractograms were used to study the dif-
ference between the ordered and disordered structures and magnetic properties
of well-characterized disordered and fcc-ordered 3-D assemblies were also inves-
tigated. Signifi cant differences were seen to arise from the mesoscopic order, due
to the differences in anisotropy and distribution of dipolar interaction energies in
the two systems. Moreover, the soft annealing of fcc cobalt nanoparticles at 125 ° C
induced the structural transition into ferromagnetic hcp cobalt nanocrystals
without changing the size, size distribution, and passivating layer [26].
In addition to anionic AOT-assisted surfactant, the control of cobalt nanoparticle
production of sizes ranging about 4 nm to 9 nm has been achieved using a germ-
growth method during inverse micelle synthesis with the cationic surfactant,
didodecyldimethylammonium bromide [27] . Mixed platinum - cobalt nanoparticles
with an average size of 3-4 nm were prepared by using nonionic Triton X-100 as
surfactant and 2 - propanol as cosurfactant [28] . Haeiwa
et al
. reported an approach
to encapsulate Co nanoparticles with SiO
2
capsule based on a reverse micellar
technique. For this, cationic tetra-alkyl ammonium bromide (CTAB) was used as
the surfactant to prepare cobalt ion micelles, after which the reductant was added
to form cobalt nanoparticles, followed by the addition of tetraethylorthosilicate
(TEOS). The hydrolysis and polymerization of silane occurred in water droplets
and then formed a hollow silica shell. The water content was shown to affect the
reduction of cobalt salt as well as the hydrolysis rate of TEOS, and thus to control
the thickness of the shell [29]. Ganguli
et al
. prepared cobalt oxalate nanorods by
fi rst using a reverse micellar process, and then decomposing the rods thermally
to cobalt in a H
2
atmosphere [30] .
16.3.3.3.2
Chemical Reduction in Aqueous Solution
The reduction of inorganic
cobalt salts in aqueous solution with powerful reducing reagents (e.g., borohy-
drides, such as NaBH
4
or KBH
4
) is a simple, convenient and cost-effective approach
for the preparation of cobalt nanoparticles. For this, cobalt ions were reduced to
metallic cobalt by borohydride, according to the following two-step reaction:
2
Co
2
+
+
4
BH
−
+
9
H O
→+
Co B
12 5
.
H
+
3
B OH
(
)
4
2
2
2
3
4
Co B
+→ +
3
O
8
Co
2
B O
2
2
2
3
When cobalt salts (e.g., cobalt chloride) were mixed rapidly with NaBH
4
, ultra-
fi ne particles of Co
2
B would fi rst be formed, and these would react further with
