Geoscience Reference
In-Depth Information
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 tem-
perature, and display devices. Moreover, ZnO quantum dots with very low toxicity,
high photostability, biofriendly, and biodegradable have been demonstrated
[50
52]
. Zincite crystals may be grown by different methods: From gas phase (it
produces needles of ZnO), from high-temperature solution using PbF
2
(it produces
platelike crystals), and hydrothermally. It is interesting to note that the properties
of crystals grown by the first two methods are technologically unsatisfactory owing
to the imperfections in the crystal, and to unsuitable habits. Therefore, the most
promising method of producing isometric zinc oxide crystals of good quality is the
hydrothermal method
[53]
. Hence, the process of crystallization represents an
important technique for many applications. In this respect, understanding, control-
ling, and optimizing crystal morphology is of fundamental importance in those
fields where an incorrectly defined crystal morphology may have an impact upon a
number of important technological applications. Therefore, the hydrothermal tech-
nique is found to be the most suitable one for the growth of ZnO crystals. Seeded
crystal growth of zincite has been carried out using NaOH and KOH solutions and,
similarly, aqueous solutions of chlorides
[54
58]
. The most useful solvent
is
2
10 M KOH, in which the growth takes place within a wide temperature interval
200
500
C and P
600 atm. During 1970's, Lobachev and his group have
studied the solubility of zincite in detail
[53,54]
. The solubility is less dependent
upon the temperature when the concentration of the solution is
150
5
3 M, but for high-
er concentrations, it is distinct.
Figure 9.6
shows the solubility of zincite in KOH
solution. With a raise in the concentration of KOH, the solubility of zincite in
KOH solution increases nonlinearly, but with a raise in the concentration of KOH
at lower temperature (
,
200
C), it becomes linear.
,
5
4
1.2
3
0.8
2
0.4
1
0
100
200
300
400
T (°C)
Figure 9.6 Solubility of zincite in KOH solution
[54]
.
