Environmental Engineering Reference
In-Depth Information
To represent the addition of a lower valency cation to the ZnO lattice, one
may consider the solution of Li
2
O in ZnO. This will also affect the defect concen-
tration of the parent ZnO lattice in two ways:
1.
Two Li
ions can be accommodated in two normal Zn
2
sites when only one
anionic site would be occupied. The second anionic site could be filled by
the incorporation of
1
/
2
O
2
(g) from the gas phase with simultaneous capture
of electrons from the conduction band in order to ionize the gas as per the
following defect equilibrium:
1
2
O
2
(g) s 2Li
Li
2
O
2e
′
Zn
2O
x
O
(6.5)
2.
Since removal of electrons from the conduction band upsets the equilibrium
condition of Eq. (6.2), an alternative scheme for Li
2
O incorporation would
be to displace one Zn
2
ion from the normal Zn site to interstitial site as per
the following defect equilibrium:
Li
2
O
ZnO s 2Li
Zn
Zn
••
i
2O
x
O
(6.6)
Therefore, the net result of doping ZnO with Li
2
O would be to increase the
concentration of interstial Zn ions and reduce the concentration of electrons in
the conduction band. As a consequence, there will be an increase in the cationic
conductivity (
σ
e′
). Such doping
would therefore lead to an increased oxidation rate of Zn containing a small
amount of Li. The lattice defect models of pure ZnO along with its modified
versions due to doping by Cr
2
O
3
and Li
2
O are presented in Fig. 6.1.
σ
i
) and a reduction in electronic conductivity (
6.2.2 p-Type Oxide (NiO)
The intrinsic defect structure of such compounds involves cation vacancies and
complementary electron holes in the valence band. In the case of an NiO lattice,
the incorporation of O
2
from the gas phase into the oxide lattice can be repre-
sented as:
1
2
O
2
(g) s O
x
O
V
Ni
2
h
•
(6.7)
for which one can write the following expression of equilibrium constant at a
temperature,
T
:
[
V
Ni
]
p
2
P
1/2
O
2
K
(
T
)
(6.8)
where [
V
Ni
] and
p
are the concentrations of cation vacancies and positive holes,
respectively, per m
3
of NiO lattice.
