Biomedical Engineering Reference
In-Depth Information
relative concentration. Ahrenstorf
et al
. have shown that the nucleation of Ni
Pt
nanoparticle growth occurs through Pt-rich species. Oleylamine is presumed to
be the surfactant most likely to stabilize the surface Pt atoms. In addition, Ni
50
Pt
50
nanoparticles demonstrate superparamagnetic character above a blocking tem-
perature of 7.4 K, below which they are ferromagnetic [59].
−
13.3
Magnetic Early Transition Metal Alloy Nanoparticles
Iron and cobalt alloys are of major interest as soft magnetic materials, due to their
high saturation of magnetization, negligible magnetocrystalline anisotropy, high
Curie temperature (
T
c
900 ° C), and high permeability; taken together, these
properties make these alloys desirable materials for magnetic recording, drug
delivery, and hyperthermia treatment applications. Wet-chemical synthesis strate-
gies of spherical Co
≈
Fe nanoparticles usually involve the reduction of dual-source
precursors by aqueous reductive agents or an alcohol-based polyol process [60, 61].
Amorphous Co
−
−
Fe alloys incorporating boron and/or phosphorus dopants [62, 63]
and Co
Fe nanoparticles supported on carbonaceous substrates or with dielectric
oxide coatings have been prepared using such methods [64, 65].
n
Co - Fe/ multiwall - carbon nanotube ( MWCNT ) nanocomposites are prepared by
fi rst acid-treating the carbon nanotubes with nitric acid to form surface-bound
functional groups, such as COOH,
−
OH, and C O, that act as nucleation sites for
metal nanoparticle growth. Surface-oxidized MWCNTs are dispersed into aqueous
solutions containing dissolved iron and cobalt nitrate salts, and are then heated to
100 °C until the water has evaporated completely. Subsequent calcination (Ar,
300 ° C) and reduction (H
2
, 400 ° C) results in Co
−
Fe nanoparticles, the size of which
is controlled by varying the loading (as wt%) of metal salts deposited onto the
carbonaceous support [65] .
Kim
et al
. recently prepared soft magnetic 3 : 7 Co
−
Fe alloy nanoparticles via a
borohydride reduction of aqueous solutions of cobalt chloride and iron chloride;
the Co
3
Fe
7
nanoparticles formed as atomically disordered naked clusters, without
need for a surface-capping agent. These as-prepared nanoparticles could be dis-
persed by sonication in a solution of oleic acid in isooctane. Upon thermal anneal-
ing, amorphous Co
3
Fe
7
nanoparticles crystallized to an ordered structure, with
only nominal sintering (Figure 13.10 ) [60] .
The reduction of cobalt and nickel acetate precursors at concentrations of 0.1-
0.2
M
by the polyol process in the presence of ruthenium seed material, generates
a wide array of shaped Co
−
Ni nanoparticles. A sodium hydroxide additive appears
to be the main directing agent for nanoparticle shape growth, dictating the forma-
tion of wires, dumbbells, diabolos, and platelets (see Figure 13.11). These unusual
particle shapes are categorized by the relative dimensions of a center column and
end-caps. The center column length increases with decreasing NaOH concentra-
tion, while end-cap diameter increases with increasing NaOH concentrations.
On-particle EDS and electron energy loss spectroscopy (EELS) characterization
−
