Geoscience Reference
In-Depth Information
metal oxides obtained under conventional hydrothermal conditions, wherein bulk
single crystals are formed. This sometimes leads to the specific characteristics of
the products.
Jiao et al.
[110]
have reported the hydrothermal preparation of ZrO
2
nanocrystal-
lites using organic additives. Phase-pure tetragonal and monoclinic zirconia nano-
crystallites of various particle sizes and morphologies were prepared in the presence
of polyhydric alcohols such as glycerols and di- and tri-ethanolamine, which gave a
tetragonal phase, while alkyl halides favored the formation of monoclinic ZrO
2
. The
as-prepared tetragonal zirconia particles were spherical or elliptical in shape and
B
8
30 nm in size, whereas the monoclinic zirconia particles were spindle-like and
20
40 nm in size.
B
10.6.5 Hydrothermal Processing of TiO
2
and ZnO Nanoparticles
Metal oxides such as TiO
2
and ZnO find extensive applications in modern technol-
ogy exhibiting a number of attractive characteristics such as chemical stability, non-
toxicity, low cost, and the highest oxidation rate. Both have high melting
temperatures. The unique physicochemical properties of these metal oxides also
offer an exciting spectrum of applications having some additional advantages of
being biocompatible and environmentally friendly. They have several advantages
due to their low cost, ease of handling, and high resistance to photoinduced decom-
position
[111]
. TiO
2
shows maximum light scattering with virtually no absorption. It
is nontoxic and chemically inert. It also has a special photocatalytic sterilization
function, which can be used for antibacterial applications
[112]
. It is a widely used
photoprotective component of various cosmetic products. Hence, the penetration of
titania nanoparticles through the epidermis of human foreskin to understand its pos-
sible biological effects in vivo and in vitro is an important study
[113]
. The titania
composites are most popular as implants and hence their biological response of tis-
sues with macrophagic activity to titania has been investigated extensively
[114]
.
Major breakthrough occurred during 1970s when Fujishima and Honda
[111a]
reported electrochemical photolysis of water at semiconductor electrode (TiO
2
)
[115]
. TiO
2
has several other specific applications and the common ones are as
photocatalyst, dye-sensitized solar cells, white pigment, ceramic glazes, sun screen,
and UV absorbers, also as electronic data storage, and so on. Zinc oxide is well
known as n-type wide band gap semiconductor (
3.37 eV at 300 K) with a
large exciton energy of 60 meV and thermal energy of 27 meV, and is a subject of
research owing to its unique mechanical, electrical, and optical properties with a
combination of high stability, very high melting point with valuable device potential
for piezoelectric transducers, gas sensors, optical waveguide, transparent conductive
films, varistors, solar cell windows, bulk acoustic wave devices, and so on. Due to
bright UV luminescence, ZnO is a perspective material for the manufacture of
UV-light-emitting diodes, UV lasers operating at room temperature, and display
devices. Moreover, ZnO QDs with very low toxicity, high photostability, bio-
friendly, and biodegradable have been demonstrated
[116
Δ
E
5
118]
.
