Geoscience Reference
In-Depth Information
The hydrothermal process has several advantages over other growth processes
such as: The use of simple equipment, ease of operation, catalyst-free growth, low
cost, and environmentally friendly process. The low reaction temperatures make
this method an attractive one for microelectronics and plastic electronics
[6]
. Also
the method is especially useful for the synthesis of nanotubes, nanowires, nanorib-
bons, and other nanostructures of these two metal oxides, which find some special
applications in the nanostructure forms. In spite of the fact that doping has been
one of the oldest approaches crystal growers adopted in the earlier times to mod-
ify the crystal morphology, it is still popular, even in the processing of nanoparti-
cles. The researchers are also employing coupled doping to alter the
semiconductor properties of these metal oxides, and it has become an active area
of research. Several parameters are being worked out theoretically in this respect
[123,124]
. Further, the use of in situ surface modification has yielded enormous
success in the processing of these metal oxide nanoparticles under hydrothermal
conditions
[125]
. Some researchers are discussing the advantages of organic addi-
tives over the surface modifiers not only to control the size and morphology but
also to enhance the reaction kinetics.
Contrary to the growth of TiO
2
and ZnO metal oxide bulk single crystals, the
synthesis of TiO
2
and ZnO nanoparticles is usually carried out under mild hydrother-
mal and solvothermal conditions (usually T
120
180
C, but in some cases up
5,
to 250
C and P
autogeneous) and at around 400
C in case of supercritical hydro-
thermal method. Generally, the experiments are carried out in general purpose
autoclaves using Teflon liners or batch reactors using platinum or gold liners or
capsules.
Figure 10.27
shows the flow chart of the experimental methodology
adopted in our laboratory. A wide range of solvents and surfactants has been used
for both TiO
2
and ZnO nanoparticle fabrication. Also the molar concentration of the
raw material TiO
2
and ZnO was varied to investigate the quality of the resultant pro-
ducts. The experimental duration was varied from 24 h to just 4 h depending upon
the precursors used, especially the organic additives and surfactants. The dopants
were introduced in the solution form in the desired mole concentration, and in some
cases they were introduced as metals directly into the hydrothermal and solvother-
mal system. After the experimental run, the products were washed and freeze-dried.
5
Solvents
Acidic (HNO
3
, HCl, and HCOOH), alkaline (KOH and NaOH), and organic
solvents (methanol, diethylene glycol, n-butyl alcohol, and diphenylether) were
used as solvents, and the pH of the system was measured before and after the
experimental run to check the pH variation. The organic solvents were very active
under hydrothermal conditions, and the experimental temperature was significantly
low when organic solvents were used.
Also the organic additives played a prominent role in the crystallization mechanism
of these two metal oxides, and in controlling their particle size and agglomeration.
Hexylamine has been used as additive in small quantity. The role of organic solvents
has not been understood precisely, although there are some excellent publications
