Biomedical Engineering Reference
In-Depth Information
intermediates with multiconcave sites on the tops. With an increase in the
reaction time, the neighboring holes merge either partially to create tube- in - tube
structures, or completely to form a single nanotube. The magnetite and maghemite
tube-in-tube structures are then prepared either by reduction (under a hydrogen/
argon gas fl ow at 300 °C for 5 h) or reduction and reoxidation (by exposing the
sample in air at 240 ° C for 2 h) using hematite tube - in - tube structures as precursors
(Figure 12.10a,b). The transformation to magnetite and maghemite phases is
complete, as no peaks of hematite phase precursor are observed in the XRD pat-
terns. Conversely, it is diffi cult to distinguish between magnetite and maghemite
only on the basis of the XRD patterns due to the same spinel structure and similar
lattice parameters (Figure 12.10c). Raman spectroscopy represents, for this
purpose, a very useful tool. In Figure 12.10d, the Raman spectra show the char-
acteristic bands of magnetite and maghemite, confi rming that the nanostructures
shown in Figure 12.10a and b can be ascribed to pure magnetite and maghemite,
respectively.
The magnetic properties of maghemite and magnetite tube-in-tube structures
were also investigated. The temperature-dependence of the magnetization
(a)
(b)
magnetite
maghemite
500 nm
500 nm
(c)
(d)
(A
1g
)
maghemite products
maghemite products
(A
1g
)
723
(E
g
)
(T
2g
)
665
(T
2g
)
345
507
390
193
263
(A
1g
)
668
magnetite products
(T
2g
)
magnetite products
(T
2g
)
536
308
200
400 600
Raman shift (cm
-1
)
800
1000
10
20
30
40
2
θ
(degree)
50
60
70
Figure 12.10
Scanning electron microscopy images of
(a) magnetite and (b) maghemite products; (c) X-ray
diffraction patterns; (d) Raman spectra. The tube - in - tube
structures are marked with circles in panels (a) and (b).
