1.2.2 Fundamentals of Semiconductor-Based Photocatalyst
Materials
Thermodynamics of Heterogeneous Semiconductor Photocatalyst
Thermodynamically, the relative energy bands of graphene and semiconductors play a
key role in attaining enhanced photocatalytic performance. The reduction and oxidation reactions at the semiconductor surface are driven by the photoinduced charge
carriers only if their potentials straddle the CB and the VB edge positions. Thus, to
achieve a high photodegradation of a given pollutant, the photoinduced holes and
electrons must have a favorable oxidation and reduction ability to combine with the
species adsorbed on the semiconductor surface to generate free radicals, which can act
as reactive species during the photodegradation process (Chen et al. 2010). Therefore,
the more positive valence band edges are helpful for an oxidation reaction, while the
more negative conduction band edges are favorable for a reduction reaction. The
potentials of selected species are shown in Table 1.1 (Bard et al. 1985).
Kinetics of Heterogeneous Semiconductor Photocatalyst
Generally, the thermodynamic properties, such as narrow band gap and suitable
band edge positions, do not allow high performance of the semiconductor
photocatalysts since the overall photocatalytic activity is affected by other factors.
These include interface/surface morphology, structure at nano- and microlevels,
crystallinity, composition, and adsorption capacity of the materials (Li et al. 2015).
The complicated surface reaction kinetics and charge carrier dynamics in the multistep process of photocatalysis result in a low quantum yield (Pasternak and Paz
2013). The underlying mechanism of photocatalysis comprises (Pasternak and Paz
2013) (1) light harvesting, (2) charge separation, (3) charge migration/recombination, and (4) charge utilization. Kinetically, the rapid recombination rate of charge
carriers is a limiting factor in achieving enhanced photocatalytic activity (Li et al.
Table 1.1 Standard redox potential for selected active species involved in the photodegradation of
pollutants (Bard et al. 1985)
Reaction
E
0 (V vs. normal hydrogen electrode at pH ¼ 0)
e
À + O 2 ! O 2
À•
À0.330
2e
À + O 2 + H 2 O ! HO 2
À + OH
À
À0.065
e
À + O 2 + H
+ ! HO 2
•
À0.046
e
À + HO 2
À + H 2 O ! HO
• + OH
À
0.184
e
À + O 2
À + H 2 O ! HO 2
À + OH
À
0.200
2e
À + O 2 + 2H
+ ! H 2 O 2
0.695
4h
+ + 2H 2 O ! O 2 + 4H
+
1.229
h
+ + OH
À ! HO
•
2.690
4h
+ + 4OH
À ! O 2 + 2H 2 O
0.401
6
F. Opoku et al.
Materials
Thermodynamics of Heterogeneous Semiconductor Photocatalyst
Thermodynamically, the relative energy bands of graphene and semiconductors play a
key role in attaining enhanced photocatalytic performance. The reduction and oxidation reactions at the semiconductor surface are driven by the photoinduced charge
carriers only if their potentials straddle the CB and the VB edge positions. Thus, to
achieve a high photodegradation of a given pollutant, the photoinduced holes and
electrons must have a favorable oxidation and reduction ability to combine with the
species adsorbed on the semiconductor surface to generate free radicals, which can act
as reactive species during the photodegradation process (Chen et al. 2010). Therefore,
the more positive valence band edges are helpful for an oxidation reaction, while the
more negative conduction band edges are favorable for a reduction reaction. The
potentials of selected species are shown in Table 1.1 (Bard et al. 1985).
Kinetics of Heterogeneous Semiconductor Photocatalyst
Generally, the thermodynamic properties, such as narrow band gap and suitable
band edge positions, do not allow high performance of the semiconductor
photocatalysts since the overall photocatalytic activity is affected by other factors.
These include interface/surface morphology, structure at nano- and microlevels,
crystallinity, composition, and adsorption capacity of the materials (Li et al. 2015).
The complicated surface reaction kinetics and charge carrier dynamics in the multistep process of photocatalysis result in a low quantum yield (Pasternak and Paz
2013). The underlying mechanism of photocatalysis comprises (Pasternak and Paz
2013) (1) light harvesting, (2) charge separation, (3) charge migration/recombination, and (4) charge utilization. Kinetically, the rapid recombination rate of charge
carriers is a limiting factor in achieving enhanced photocatalytic activity (Li et al.
Table 1.1 Standard redox potential for selected active species involved in the photodegradation of
pollutants (Bard et al. 1985)
Reaction
E
0 (V vs. normal hydrogen electrode at pH ¼ 0)
e
À + O 2 ! O 2
À•
À0.330
2e
À + O 2 + H 2 O ! HO 2
À + OH
À
À0.065
e
À + O 2 + H
+ ! HO 2
•
À0.046
e
À + HO 2
À + H 2 O ! HO
• + OH
À
0.184
e
À + O 2
À + H 2 O ! HO 2
À + OH
À
0.200
2e
À + O 2 + 2H
+ ! H 2 O 2
0.695
4h
+ + 2H 2 O ! O 2 + 4H
+
1.229
h
+ + OH
À ! HO
•
2.690
4h
+ + 4OH
À ! O 2 + 2H 2 O
0.401
6
F. Opoku et al.