Geoscience Reference
In-Depth Information
alkaline earth titanates.
Table 10.5
lists fine powders synthesized under hydrother-
mal conditions.
In recent years, glycothermal and solvothermal syntheses are popular to prepare
fine powders of nitrides, alumina, iron oxide, hexaferrites, and so on
[178
181]
.
These methods are very useful in obtaining shaped and sized particles under rela-
tively lower pressure
temperature conditions.
10.6.9 Synthesis of II
VI Semiconductor Nanoparticles
There is a growing interest in the synthesis of II
VI semiconductor nanoparticles
with a precise control over their shape and size owing to their unique properties,
which make them useful in solar cells, light-emitting diode, nonlinear optical materi-
als, optoelectronic, and electronic devices, and also for medical and biorelated appli-
cations. Further, these compounds exhibit varying structures such as zincblende,
wurtzite, and halite. Prior to the usage of the term QDs, these compounds were
known as colloidal particles, or semiconductor cluster molecules. These are also
popularly known as chalcogenides, which include sulfides, selenides, and tellurides.
The synthesis and properties of these II
VI semiconductors have been extensively
studied especially after their promising applications in biomedical science. There
are several reviews published on these materials
[182
185]
. Several methods of
synthesis have been attempted on these materials to prepare them as fine particles
since the discovery of quantum size effect in 1980s. CdS and ZnS are the earliest
chalcogenides prepared, both uncapped and polymer-capped nanoparticles through
wet chemical methods. The success of the earlier methods depended essentially on
the ability to stop the crystal growth process immediately after nucleation begins by
controlling the equilibrium between solid CdS (ZnS) and solvated metal ions in the
solution
[186]
. Later, SCF technology was also employed to obtain bulk quantities
of these chalcogenides. However, several problems were faced by the earlier work-
ers as the products were aggregated and not uniformly distributed. Nonaqueous sol-
vents were employed to prepare fine particles of
these chalcogenides under
subcritical to supercritical conditions
[187
189]
. Heating of CdCl
2
and S, Se, or Te
for 24 h in an autoclave filled with ethylenediamine (80% of the total volume)
resulted in CdS, CdSe, or CdTe nanorods, respectively
[190]
. These compounds are
especially interesting for their semiconducting and thermoelectric properties. There
are several hundreds of reports on these sulfides, such as CdS, CdSe, PbS, ZnS,
CuS, NiS, NiS
2
, NiS
7
,Bi
2
S
3
, AgIn
5
S
8
, MoS, FeS
2
, InS, and Ag
2
S, prepared with or
without capping agents/surfactants/additives to alter their morphologies and sizes as
desired. Most of them are known for their high-performance photovoltaic solar cells
based on nontoxic and earth abundant materials.
Among II
S,
Se, Te), CdS is an important one. Li et al.
[191]
have used thioacetamide as the sul-
fide source, as it easily releases sulfide ions, which are beneficial to lower the reac-
tion rate and shorten the reaction period. The experiments are usually carried out in
the temperature range 150
VI group semiconductor nanomaterials, AX (A
5
Cd, Pb, Zn; X
5
200
C using nonaqueous solvents, whose critical tem-
peratures are generally within this range.
