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the [001] direction, and the following dissolution progress also takes place along the [001]
direction and simultaneously nanorings can be formed. In addition, other approaches,
such as a colloidal-crystal-assisted-lithography strategy [55], thermal transformation pro-
cesses [23], or miniemulsion polymerization [56], can also be applied for the synthesis of
Fe 3 O 4 nanorings.
Fe 3 O 4 nanosheets can be obtained by oxidizing Fe substrates in acidic solution in a hot
plate at 70°C [57]. The fabrication of Fe 3 O 4 nanoprisms has been reported by Hou's and
Zhang's groups using solvothermal or hydrothermal processes [58,59]. The morphology of
the Fe 3 O 4 nanoprisms is controllable by varying the volume ratio between ethylene glycol
(EG) and 1,3-propanediamine because of the coordination between the -NH 2 group and
Fe 3+ [41]. Fe 3 O 4 nanoplates can be prepared by using supercritical luid methods [60]. Lu et
al. reported the fabrication of γ-Fe 2 O 3 nanoplates and then converting to Fe 3 O 4 nanoplates
via a reduction process in the presence of poly(vinylpyrrolidone) [61]. In addition, hydro-
thermal or solvothermal processes have also been developed for the synthesis of Fe 3 O 4
nanoprisms or nanoplates [62].
14.2.2.2 Hierarchical Superstructures
Recently, many efforts have been devoted to the self-assembly of nanoscale building blocks
into 2-D and 3-D hierarchical superstructures for preventing the agglomeration and pro-
viding more tunable and unique properties [63]. In addition, Fe 3 O 4 hierarchical superstruc-
tures can partially overcome the “superparamagnetic limit,” which is the conlict between
reducing the magnetic energy barrier and decreasing the size [50].
Quite a few self-assembled Fe 3 O 4 chains have been synthesized (Figure 14.6a) on the
basis of the nanoscale Kirkendall effect, in which Fe nanoparticles are irst self-assembled
into chainlike structures and then solid Fe spheres can be gradually oxidized into Fe 3 O 4
hollow nanospheres [64-66]. The hierarchical and porous structure of Fe 3 O 4 hollow submi-
crospheres with Fe 3 O 4 nanoparticles have been fabricated by using a solvothermal method
[67]. In this system, the formation of Fe 3 O 4 submicrospheres composed of Fe 3 O 4 nanoparti-
cles with diameters ranging from 20 to 30 nm (Figure 14.6b) can be attributed to reduction
and Ostwald ripening. Hierarchical lower-like Fe 3 O 4 superstructures are another impor-
tant class of hierarchical superstructures studied [63,68-70]. Zhong et al. have reported
the fabrication of lower-like Fe 3 O 4 superstructures using an EG-mediated self-assembly
(a)
(b)
(c)
1 µm
1 µm
200 nm
FIGURE 14.6
(a) TEM images of the chainlike arrays of Fe 3 O 4 hollow nanospheres. (From Gong J, Li S, Zhang D et al., Chemical
Communications , 46, 3514, 2010.) (b) SEM image of the hierarchical porous Fe 3 O 4 hollow submicrospheres. (From
Wang Y, Zhu Q, Tao L, CrystEngComm , 13, 4652, 2011.) (c) SEM and TEM images of the lower-like Fe 3 O 4 super-
structures. (From Han L, Chen Y, Wei Y, CrystEngComm , 14, 4692, 2012.)
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