Biomedical Engineering Reference
In-Depth Information
After a 3 h reaction, mostly spherical
γ
- Fe
2
O
3
nanoparticles were formed, whereas
uniform
- Fe
2
O
3
nanorods of 500 nm length and 20 nm width were formed after
4 h, but with an aspect ratio in excess of 25 : 1. The particles formed after a 5 h
reaction had undergone a phase transformation from
γ
- Fe
2
O
3
. The rod-
like shape was derived (as shown by Tulpar and Ducker [107]) by the surfactant
molecules being adsorbed onto preferential crystalline planes of the iron oxide
nuclei; this prevented them from further grow and consequently induced an
anisotropic growth process.
The magnetic hysteresis curves of the 3 h, 4 h, and 5 h samples, when
measured at room temperature, showed that the spherical and rod-like
γ
- Fe
2
O
3
to
α
- Fe
2
O
3
particles had comparable saturation magnetization, but differed in their coercivity
values, from 0.28 Oe for spherical particles up to 0.7 kOe for the rods. This
high coercivity value was ascribed to the contribution of the shape anisotropy
to the total magnetic anisotropy. A different magnetic behavior was observed,
however, for the sample prepared by a 5 h reaction. In this case, TEM analysis
revealed the particles to be almost spherical, whereas the XRD spectrum
showed a phase change from maghemite (
γ
γ
- Fe
2
O
3
) to the more stable, antiferro-
magnetic hematite (
- Fe
2
O
3
). Later, by increasing the SDS surfactant concentra-
tion 10-fold, from 1.8 mmol to 18 mmol, and after a 3 h reaction, multipod
α
- Fe
2
O
3
particles were isolated. In this study the authors ascribed the anisometric morphol-
ogy to an unbalanced distribution of SDS molecules on the different crystalline
faces of the
γ
- Fe
2
O
3
nuclei, which resulted in some facets showing preferential
growth compared to others. The hysteresis loops of maghemite aggregated
nanoparticles and nonaggregated multipod structures showed the same saturation
magnetization value (
M
s
γ
0.02 emu g
− 1
), while the coercivity and remanent
magnetization values for the aggregated nanoparticle sample (
H
c
= 0.28 kOe;
M
r
= 0.0078 emu g
− 1
) were higher than those for the sample with multipod mor-
phology (
H
c
= 0.09 kOe;
M
r
= 0.0026 emu g
− 1
). This means that the increased scale
of branches grown along uneasy magnetic axes makes the
≈
γ
- Fe
2
O
3
magnetization
more diffi cult.
In this example, it was shown clearly that a control on nanoparticle shape
anisometry would lead to a wide alteration of magnetic properties, thus opening
perspectives for the design of new magnetic building blocks for different applica-
tions [11, 108] .
12.4.2
Core - Shell Nanoparticles
Core-shell particles often exhibit improved physical and chemical properties over
their single-component counterparts, and hence are potentially useful in a broader
range of applications.
Indeed, the shell can alter the charge, functionality, and reactivity of the particle
surface; magnetic, optical or catalytic functions may be readily imparted to the
dispersed colloidal matter, depending on the properties of the coating. Encasing
