Environmental Engineering Reference
In-Depth Information
series of β-AgAl
1−
x
Ga
x
O
2
solid solutions were explored as visible-light-sensitive photocata-
lysts.
24
The level of CB minimum can be continuously tuned by varying the ratio of
Ga/Al and thereby allows the modulation of band gaps continuously from 2.19 to 2.83 eV
(Figure 11.8b). The synergistic effect between visible light absorption and redox potentials
is achieved optimally in the β-AgAl
0.6
Ga
0.4
O
2
samples.
Continuous modulation of both the VB and the CB provides an effective approach to
achieving both effective visible light absorption and adequate redox potential. For exam-
ple, the modulation of the (AgNbO
3
)
1−
x
(SrTiO
3
)
x
band structure depends on the extent of
both the hybridization of the Ag4
d
and O2
p
orbitals in the VB as well as the Nb4
d
and Ti2
p
orbitals in the CB, achieving the highest visible light activity with (AgNbO
3
)
0.75
(SrTiO
3
)
0.25
.
26
It is expected that the photoreactivity would be favorably promoted by changing the com-
ponents of the solid solution material owing to their lexible electronic structure.
11.3 Heterogeneous Systems for Enhanced Charge Separation
Energy band engineering can extend optical absorption of photocatalysts to the visible
light region, as discussed above. An alternative method to enhance photoreactivity is to
improve the separation eficiency of photogenerated charge carriers and reduce bulk/
surface charge recombination. Numerous strategies, such as heterojunction, plasmon-
exciton coupling, and cocatalyst modiication, have been explored to facilitate the separation
process to increase the utilization of charge carriers and obtain high photoreactivity.
11. 3.1 Met al
@
Semiconductor Schottky Junction
For metal@semiconductor composite material, a Schottky barrier (ϕ
SB
) forms at the semi-
conductor-metal interface due to the work function difference.
27
The Schottky barrier can
decrease the recombination rate of electron-hole pairs. Femtosecond diffuse relectance spec-
troscopy experiments have demonstrated the effective charge separation in the Pt/TiO
2
com-
posites for the enhanced photocatalytic activity.
28
Generally, a larger work function difference
results in a strong Schottky barrier effect, and therefore exhibits a better activity for reduction
reactions. This can explain the highest activity of Pt/TiO
2
among the metal-TiO
2
composites.
The structural geometry of the metal-semiconductor plays a critical role in the photo-
catalytic performance. Figure 11.9 illustrates two typical metal-semiconductor systems:
Semiconductor
Semiconductor
Metal
CB
hv
hv
-
-
-
-
- -
-
-
Metal
+
+
+
+ + +
VB
(a)
(b)
FIGURE 11.9
(See color insert.)
Metal@semiconductor photocatalytic systems: (a) metal particle supported on semiconductor
surface; (b) metal@semiconductor core@shell structure.
