Environmental Engineering Reference
In-Depth Information
photocatalytic activity of Degussa P25. Electron paramagnetic resonance has provided
evidences for the charge transfer across the anatase-rutile interface. However, the direc-
tion of charge transfer is still in debate. Indeed, the synergistic effect is dificult to verify
because of the differences in the physicochemical properties of the pristine crystal phase
as compared with the individual components in the mixture. Moreover, O
2
transfer from
anatase to rutile is also proposed for the high photoactivity of P25.
7
The surface area and aggregation state determine the adsorption behavior of TiO
2
pho-
tocatalysts toward substrate molecules, which provide the prerequisite for many photo-
catalytic reactions. Nanoparticulated TiO
2
in solution can aggregate to form micron-sized
secondary structures, which inevitably reduce the exposed surface area to the outer envi-
ronment and decrease the photocatalytic reaction activity. However, the photocatalytic
activity of TiO
2
will be maintained at a high level if the aggregation is in a certain spe-
ciic fashion, e.g., adjacent nanoparticles are aligned in a given crystallographic orienta-
tion, which enables a strong electronic coupling between adjacent particles, the so-called
antenna effect.
8
Semiconductor ZnO may become an alternative to TiO
2
owing to its higher carrier mobil-
ity and the more negative position of the VB, which makes the photogenerated electrons
more energetic to reduce the protons, resulting in higher photocatalytic activity. In addi-
tion, the morphology of ZnO can be easily controlled by adjusting the synthetic parame-
ters resulting in various nanostructures including nanotube, nanorod, and nanotetrapods.
However, the wide-band-gap nature renders both TiO
2
and ZnO incapable of capturing
the visible spectrum of the solar light. With respect to the narrow-band-gap material, WO
3
,
Fe
2
O
3
, and CdS are very attractive. However, they suffer from several disadvantages: seri-
ous photocorrosion of CdS, and the relatively positive CB of WO
3
and Fe
2
O
3
. To overcome
these obstacles, various strategies have been made to increase the absorption of solar light.
11.2.2 Energy Band Engineering
Energy band coniguration determines the absorption spectra and the redox potential of a
semiconductor photocatalyst. Energy band engineering represents an effective approach
to redshift the absorption edge of photocatalysts and to improve sunlight-driven photocat-
alytic performance. Two typical approaches have been adopted to narrow the semiconduc-
tor band gap to make photocatalysts visible light active: (i) metal and/or nonmetal doping
for band-gap narrowing, and (ii) developing solid solution for continuous modulation of
band structure (Figure 11.5).
CB
CB
CB
E
0
(H
+
/H
2
)
E
0
(O
2
/H
2
O)
VB
VB
VB
(a)
(b)
FIGURE 11.5
(See color insert.)
Energy band bending strategies to narrow the band gap of semiconductor photocatalysts so
as to match the solar spectrum. (a) Element doping. (b) Solid solution.
