Geoscience Reference
In-Depth Information
During 1960s, the growth of TiO
2
and ZnO as bulk single crystals was very
popular, and much of the earlier literature survey shows such attempts, and
hydrothermal method was popularly used to synthesize both these metal oxides
under high-temperature
121]
. However,
researchers experienced several difficulties in the large-sized crystals of these
metal oxides, and their popularity was highly limited.
With the discovery of quantization effect during the late 1970s, the size reduc-
tion in metal oxide semiconductors became a major objective to achieve higher
efficiency in their applications. Difficulties in the growth of metal oxide nanocrys-
tals were associated with the control of size, morphology, coagulation, reproduc-
ibility of results, and dispersability. During 1990s, several such attempts were
made on the use of organic molecules as ligands or capping agents or surfactants.
Also ex situ surface modification was tried with limited success. Thus, an alter-
nate route was envisaged to obtain high-quality nanocrystals of TiO
2
and ZnO
with controlled size, morphology, dispersability, and without any coagulation
using in situ surface modification through novel solution processing consisting of
hydrothermal, solvothermal, and supercritical hydrothermal methods. The size
could be reduced significantly to a few nanometers without any coagulation, and
the organic coating was so much uniform and thin that did not alter the inherent
properties of metal oxide core. The organics that are insoluble and exist as sepa-
rate phases under ordinary conditions became homogeneous phases under hydro-
thermal/solvothermal conditions. The small size and high surface-to-volume ratio
of the individual nanoparticles impart distinct size-tunable physical and electronic
properties that have prompted some to refer to them as “artificial atoms.” A
highly controlled self-assembly of these hybrid nanocrystals, when dispersed in
organic solvents into 2D and/or 3D ordered structures or superlattice structures,
remains a relatively unexplored area. In recent years, the significant reduction in
the metal oxide semiconductors particle size has resulted in newer applications
such as QDs for biological imaging. A proper understanding of the cytotoxicity
and genotoxicity of these metal oxides would be highly useful for their applica-
tions in biological systems
[122]
. The researchers are exploring the possibilities of
their applications in food packing and storage. In finding appropriate applications
for these metal oxides, the control of size, morphology, and dispersibility to some
extent are most important. Several approaches are being adopted to achieve higher
efficiency in this respect like the synthesis methods, doping with an appropriate
metal, surface modification, and organic additives. Although several methods of
synthesis are employed for both TiO
2
and ZnO, such as sol
and high-pressure
conditions
[120
gel, microemulsion,
hydrothermal, microwave, and homogeneous precipitation, the novel solution pro-
cessing consisting of hydrothermal, solvothermal, and supercritical hydrothermal
methods are more effective owing to their controlled diffusion, higher activity of
the solvent, and the homogenization of the surfactant with the media under higher
temperature and pressure conditions
[19,20,31]
. The hydrothermal synthesis has
been clearly identified as an important technology for materials synthesis
[1,20,31]
. Hydrothermal technique is a promising alternative synthesis method
because of the low process temperature and very easy to control the crystal size.
