Environmental Engineering Reference
In-Depth Information
parameter (
k
ct
, cm s
−1
), which is related to the observed bimolecular rate constant (
k
obs
) via
the following equation
2
:
1
1
1
r
D
=
+
0
(11.2)
2
k
4
π
r
k
obs
ct
dc
0
Equation 11.2 indicates that the rate constant for charge transfer at semiconductor-solution
interface is size dependent. Larger particles usually have a faster rate than the smaller ones.
11.1.3 Reactive Oxygen Species Generated in Photocatalytic Reactions
Photocatalytic degradation of environmental organic contaminants proceeds through
either direct reactions (mediated by holes and electrons) or indirect reactions (mediated
by reactive oxygen species [ROS], such as
∙
OH,
O
2
−
, and H
2
O
2
). Under light irradiation, the
excited electrons and holes can directly react with surface-adsorbed organic molecules, or
react with water and oxygen molecules to produce ROS, which then participate in various
reactions. The redox potentials of the generated ROS are largely dependent on the intrinsic
VB and CB positions of the semiconductor. Indeed, it is dificult to catalyze an oxidation
(reduction) reaction by semiconductors with
E
VB
(
E
CB
) more negative (positive) than the
reduction (oxidation) potential.
Hydroxyl radicals (
∙
OH), mainly produced by oxidation of H
2
O and OH
−
by the holes, are
recognized as one of the most oxidizing species for the photocatalytic oxidation of organic
contaminants. It is noted that the surface-bound and diffusive
∙
OH species have differ-
ent oxidation potentials, with
E
=+
( )
.
4
Electrons in the CB can be trapped by molecular oxygen to form
O
2
−
, which may further
generate
HO
2
and H
2
O
2
. These ROS may complement the direct photocatalytic oxidation
reactions, especially in the case where
E
VB
is insuficient for
∙
OH generation.
(
)
3
and
E
0
0
(
OH
)
>+
16
.
V
(
OH
)
272
.
V
OX
ads
NHE
OX
free
NHE
11.1.4 Nanomaterial Design for Enhanced Photocatalytic Activity
To design a high-eficiency, sunlight-induced semiconductor photocatalyst, enhancing
optical absorbance and improving charge carrier separation are the two main approaches.
Specially designed nanomaterials exhibit promising potentials in photocatalytic technol-
ogy. Many efforts have been made to develop new photocatalyst systems to achieve these
goals. Figure 11.3 shows the strategies for improving the photocatalytic activity by using
Photocatalysis
Light absorbance
Carrier separation
Band structure
modification
Z-scheme
system
Plasmon
field
Sensitization
Heterojunction
Schottky
junction
p-n
junction
Element
doping
Solid
solution
FIGURE 11.3
Schematic of strategies to improve photocatalytic activity.
