Geoscience Reference
In-Depth Information
Figure 10.22 TEM photographs
of CeO
2
crystals obtained under
(a) subcritical conditions and (b)
supercritical conditions.
Source: Photographs courtesy:
Prof. T. Adschiri, Tohoku
University, Japan.
source, reaction temperature, and duration. The UV
Vis absorption and photolumines-
cence spectra of CeO
2
nanorods show unusual redshift and enhanced light emission
respectively, compared with that of bulk CeO
2
. This might be due to the abundant
defects in CeO
2
nanorods and the shape-dependent effect.
Recently, a simple and green chemistry approach based on the organic ligand assis-
tance to achieve the shape control and self-assembly of the ceria particles under super-
critical conditions has been proposed
[43,44]
. A simple strategy for the synthesis of
metal oxide nanocrystals in the organic ligand-assisted SCF technique is shown in
Figure 10.23
.
Figure 10.24
shows the TEM photographs of the ceria nanoparticles obtained
through such an approach. The morphology and the particle size of the ceria vary
with the mole ratios of decanoic amine to decanoic acid, which suggests that the
self-assembly is influenced by nanocrystals shape, especially, the strong interactions
from the exposed crystal planes can control the superlattice pattern, when the aver-
age size of nanocrystals and surface ligand molecules are similar. Even the carbox-
ylic acid can be a good surfactant for ceria particle growth under supercritical
hydrothermal conditions.
Figure 10.25
shows the pronounced effect of organic ligand molecules on the
morphology of the nanocrystals formed by the SCF technique. The transformation
of the shape of the ceria nanocrystals from truncated octahedral to cubic was
mostly caused by the suppression of crystal growth on the (001) surface as the
organic ligand molecules were likely to interact preferentially with the (001)
surface (
Figure 10.25
).
A wide range of metal oxide nanoparticles have been obtained under supercriti-
cal hydrothermal conditions for applications not only in ceramics, coatings, cata-
lysts, sensors, semiconductors, magnetic data storage, solar energy devices,
ferrofluids but also in medical fields such as hyperthermia, bioimaging, cell label-
ing, special drug delivery system (DDS), and so on. However, their application
potential is dictated by their surface nature, particle size, and also shape. Although
the synthesis of metal oxides is not new, their applications were limited. With the
discovery of size quantization effect in these materials during 1980s, there is a
seminal progress in the synthesis of these metal oxides with desired properties for
