Geoscience Reference
In-Depth Information
cases were very fine grained and submicroscopic in size without any X-ray
data. But today, it has evolved as one of the most efficient methods of soft
chemistry in processing the advanced materials like nanomaterials with a con-
trolled size, shape, and physical characteristics.
A great variety of nanomaterials have been obtained using the hydrothermal
method. Amongst them the native elements, metal oxides, hydroxides, silicates,
carbonates, phosphates, sulfides, tellurides, nitrides, and selenides both as particles
and nanostructures like nanotubes, nanowires, and nanorods are the most common
ones. The method has been popularly used for the synthesis of a variety of nano-
forms of carbon like sp
2
,sp
3
, and intermediate types.
Several researchers
[37
40]
have pioneered the low temperature soft hydrother-
mal processing technique for preparing highly oriented, dispersed, self-assembled
nanoparticles with a great control over size and morphology, for a wide variety of
compounds ranging from native metals, sulfides, selenides, metal oxides, hydro-
xides, ferrites, PZTs, carbonates, silicates, tantalates, titanates, vanadates, and car-
bon polymorphs. Their approach is unique, and soft processing methods have been
employed through an appropriate solvent and precursor selection. Also some of
them have used the external energy like microwave, sonar, electrochemical,
mechanochemical, milling, and magnetic with hydrothermal to prepare some of
the above-said variety of nanomaterials. Similarly, Adschiri and Arai
[42,44]
have pioneered the hydrothermal technique under supercritical conditions for
processing a wide range of metal oxides, hydroxides, and so on. They have
extensively studied the theoretical and experimental aspects of this technique and pro-
posed a systematic mechanism for the formation of various metal oxide nanoparticles
under supercritical hydrothermal conditions. In recent years, Adschiri and coworkers
have
extended this
technique
for
a wide
range of other materials
like
organic
inorganic hybrid materials with a perfect control over the particle formation,
size, morphology, and self-assembly
[42
44]
. Since, the subject of nanomaterials
processing under hydrothermal conditions is a vast one in the present day
context, here the authors discuss only a selected group of nanomaterials processing to
show the present trend and the future direction in hydrothermal processing of materi-
als
[31]
.
The technique is especially handy for the preparation of nanoparticles of
native elements, metal oxides, ceramics (PZT, alumina, zirconia, yttria, ceria,
and several bioceramics), epitaxial growth of crystalline thin films, composites,
fine, ultrafine, and nanoparticles with a desired shape, size, and dispersability.
There are several thousands of publications on the processing of advanced mate-
rials, using hydrothermal technology, and it is impossible to discuss in detail
every aspect of it. Accordingly, the processing of a few important selected
advanced materials like native elements, oxides, carbon-based materials, sele-
nides, tellurides, and sulfides, and so on including their nanostructures will be
selected. Also the processing of composites, coatings, thin films, fine particles,
ultra-fine particles, nanoparticles,ceramics, whiskers, and reinforcement under
hydrothermal, solvothermal and supercritical hydrothermal conditions will be
discussed.
