Environmental Engineering Reference
In-Depth Information
11.1 Basic Principles of Photocatalysis
11.1.1 Processes of Semiconductor Photocatalysis
Photocatalysis reactions occurring on a semiconductor nanomaterial's surface have
attracted intensive attention with the aim to utilize solar energy and thus address the
increasing global concerns of environmental remediation and energy consumption. From
the point of view of photochemistry, photocatalysis aims to enable or accelerate the spe-
ciic reduction/oxidation reactions by the excited semiconductor. Typically, the electronic
energy structure within a semiconductor consists of three distinguished regimes: conduc-
tion band (CB), valence band (VB), and the forbidden band (band gap,
E
g
). The semicon-
ductor absorbs light and causes interband transitions if the energy of the incident photons
matches or exceeds the band gap, subsequently exciting electrons from the VB into the
CB in the
femtosecond time scale and leaving holes in the VB. This stage is referred to as
the semiconductor's “photoexcited” state. Typically, the CB electrons can act as reductants
with a chemical potential of +0.5 to −1.5 V vs. the normal hydrogen electrode (NHE), while
the VB holes exhibit an oxidative potential of +1.0 to +3.5 V vs. NHE.
1
The excited electrons
and holes in a semiconductor migrate to the surface and can be trapped by the trapping
sites there. These surface holes and electrons can oxidize and reduce surface-adsorbed
species through interfacial charge transfer and surface reactions. Figure 11.1 illustrates the
basic mechanism of a semiconductor photocatalytic process. During the migration pro-
cess, recombination of photogenerated charge carriers may occur either in the bulk or on
the surface by dissipating the energy as light or heat, thus suppressing the photocatalytic
activity. It is noted that the recombination process is usually enhanced by impurities or
defects in the crystal.
Compared with the conventional thermodynamics catalysis, photocatalysis can not only
promote spontaneous reaction (Δ
G
< 0), which is used to overcome the activation energy
so as to accelerate the photocatalytic reaction rate, e.g., the oxidation of organic contami-
nants by molecular oxygen, but also nonspontaneous reactions (Δ
G
> 0), which convert
into chemical energy, e.g., photocatalytic H
2
generation and photocatalytic CO
2
conversion
to hydrocarbons.
CB
hv
hv
H
+
O
2
-
--
O
•
-
H
2
Surface recombination
Bulk recombination
O
2
•
OH
+
+
+
H
2
O/OH
-
H
2
O
VB
FIGURE 11.1
(See color insert.)
Schematic illustration of processes involved in semiconductor photocatalysis.
