Geoscience Reference
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dopant atoms that locate on interstitial sites. The defect compensates for the
substitutional impurity level through the formation of a deep level trap. In some
cases, strong lattice relaxations can drive the dopant energy level deeper within the
gap. In other systems, one may simply have a low solubility for the chosen dopant
limiting the accessible extrinsic carrier density. Each dopant has a different charac-
ter in this respect. A wide range of dopants like silver, indium, tin, tungsten, palla-
dium, chromium, manganese, iron, antimony, neodymium, and in some cases
coupled doping, have been introduced to achieve strong lattice relaxations, which
in turn result in a large-scale atomic level defects that enhances the application
potential of these compounds. The SCF method is the most popular preparative
method for titania nanoparticles. TiO
2
(rutile or anatase) nanoparticles have been
prepared frequently starting from stabilized TiCl
4
solutions. Some researchers have
used the combination of hydrolysis and polycondensation of titanium tetra-isoprop-
oxide, Ti(OR)
4
, used it in the presence of tetramethyl ammonium hydroxide, with a
preliminary heat treatment under reflux. Usually, the titania prepared in supercriti-
cal ethanol exhibits a higher degree of crystallinity and contains less hydroxide.
The smaller particles have proved to be the best candidates for the photocatalytic
and biological applications owing to the larger surface area. However, the conven-
tional hydrothermal methods yield larger particles, and hence the SCF technology
is the most viable one for producing nanoparticles with a shortest residence time
[120]
. Also the properties of titania particles could be easily enhanced through a
systematic approach to the synthesis methods. Mousavand et al.
[43,125]
have
done in situ surface modification during supercritical hydrothermal synthesis of
titania particles. A Ti(SO
4
)
2
solution was heated to 200
C or 400
C in the presence
of hexaldehyde, which resulted in a perfect dispersion of the synthesized nanoparti-
cles in isooctane, implying more efficient immobilization of hexaldehyde on the
titania nanoparticles for the in situ surface modification during the SCF synthesis
than for postsurface modification. The binding of the organic molecules used for
surface modification might not be physical adsorption, but rather covalent binding.
The titania particles without hexaldehyde were aggregated with a broad size distri-
bution, whereas those synthesized with hexaldehyde were more dispersed and with
a size range up to 10 nm. The immobilization of the organic molecules immedi-
ately after the nucleation of the particles probably suppressed the growth nuclei,
and in addition, the formation of the organic layer also suppressed the aggregation
of nanoparticles in the solution. Using in situ surface modification, the particles'
surfaces can be changed to hydrophilic or hydrophobic, and also the surface energy
can be altered, which are the key factors in the biomedical application of these
nanoparticles.
Figure 10.28
shows the TEM images of the TiO
2
particles synthe-
sized at 400
C in the presence of hexaldehyde. The titania particles without hexal-
dehyde in the reaction were aggregated and the size distribution was broad ranging
from several nanometers to 70 nm, while with the hexaldehyde, the particles were
relatively dispersed on the grid, and the size was in the range of several nanometers
to 10 nm. It should be noted that the addition of the organic molecules resulted in
the reduction of particle size and the dispersion of the particles. This clearly
implies that
the immobilization of the organic molecules must have occurred
