Photocatalyst þ hv ! h
þ
þ e
À
ð1:1Þ
At the semiconductor surface, electrons then migrate from the valence band to the
conduction band, forming holes (h
+
) in the valence band. The three main active
species that usually partake in the photocatalytic reactions are the superoxide radical
anion (O 2
À•
), hydroxyl (HO
•
) radical, and h
+ with the HO
• radical as the main
oxidant involved in the photodegradation of contaminant in an aqueous medium.
The thermodynamics of the photocatalytic reduction and oxidation reactions are
determined using the band edge positions of a given photocatalyst. Therefore, during
the photodegradation reaction, it is essential to consider the reduction and oxidation
potentials of the substrate and several other intermediate reactions that will occur.
The electrons can combine with the oxygen (O 2 ) molecule to produce the O 2
À•
,
which further forms the hydroxyl radicals only if the reaction is thermodynamically
favorable (Eqs. 1.2, 1.3, and 1.5).
e
À
þ O 2 ! O 2
À •
ð1:2Þ
O 2
À •
þ H
þ
! HOO
•
ð1:3Þ
2HOO
•
! O 2 þ H 2 O 2
ð1:4Þ
H 2 O 2 þ hv ! 2HO
•
ð1:5Þ
Concurrently, the h
+ then interacts with water (H 2 O) or hydroxyl ion (OH
À ) to
generate the HO
• radicals (Eqs. 1.6 and 1.7).
h
þ
þ HO
À
! HO
•
ð1:6Þ
h
þ
þ H 2 O ! HO
•
þ H
þ
ð1:7Þ
The resulting HO
• radical serves as a strong oxidizing agent, which then combine
with the organic pollutants in the water matrices to form an intermediate product,
which then produces CO 2 , H 2 O, and other products (Eq. 1.8).
Pollutant þ HO
•
! CO 2 þ H 2 O þ Degradation product s
ð Þ
ð1:8Þ
Furthermore, depending on the oxidation conditions and catalyst type, the
photogenerated holes are broadly regarded as an oxidant to directly degrade the
organic pollutants (Chong et al. 2010). Thus, in the absence of hole or electron
scavenger, the photoinduced electrons can recombine with holes after their generation. Therefore, the presence of specific scavengers is essential for restraining the
charge recombination rate to enhance the photocatalytic efficiency.
To develop a semiconductor photocatalyst material capable of utilizing a wider part
of the solar energy effectively, the semiconductor must have a (1) smaller band gap to
absorb a broad region of the electromagnetic spectrum, (2) a favorable VB edge
position for the generation of h
+ and hydroxyl radicals (Casbeer et al. 2012), (3) high
stability, and (4) efficient charge separation and migration (Qu and Duan 2013).
1 Nanotechnology for Water and Wastewater Treatment Using Graphene. . .
5
þ
þ e
À
ð1:1Þ
At the semiconductor surface, electrons then migrate from the valence band to the
conduction band, forming holes (h
+
) in the valence band. The three main active
species that usually partake in the photocatalytic reactions are the superoxide radical
anion (O 2
À•
), hydroxyl (HO
•
) radical, and h
+ with the HO
• radical as the main
oxidant involved in the photodegradation of contaminant in an aqueous medium.
The thermodynamics of the photocatalytic reduction and oxidation reactions are
determined using the band edge positions of a given photocatalyst. Therefore, during
the photodegradation reaction, it is essential to consider the reduction and oxidation
potentials of the substrate and several other intermediate reactions that will occur.
The electrons can combine with the oxygen (O 2 ) molecule to produce the O 2
À•
,
which further forms the hydroxyl radicals only if the reaction is thermodynamically
favorable (Eqs. 1.2, 1.3, and 1.5).
e
À
þ O 2 ! O 2
À •
ð1:2Þ
O 2
À •
þ H
þ
! HOO
•
ð1:3Þ
2HOO
•
! O 2 þ H 2 O 2
ð1:4Þ
H 2 O 2 þ hv ! 2HO
•
ð1:5Þ
Concurrently, the h
+ then interacts with water (H 2 O) or hydroxyl ion (OH
À ) to
generate the HO
• radicals (Eqs. 1.6 and 1.7).
h
þ
þ HO
À
! HO
•
ð1:6Þ
h
þ
þ H 2 O ! HO
•
þ H
þ
ð1:7Þ
The resulting HO
• radical serves as a strong oxidizing agent, which then combine
with the organic pollutants in the water matrices to form an intermediate product,
which then produces CO 2 , H 2 O, and other products (Eq. 1.8).
Pollutant þ HO
•
! CO 2 þ H 2 O þ Degradation product s
ð Þ
ð1:8Þ
Furthermore, depending on the oxidation conditions and catalyst type, the
photogenerated holes are broadly regarded as an oxidant to directly degrade the
organic pollutants (Chong et al. 2010). Thus, in the absence of hole or electron
scavenger, the photoinduced electrons can recombine with holes after their generation. Therefore, the presence of specific scavengers is essential for restraining the
charge recombination rate to enhance the photocatalytic efficiency.
To develop a semiconductor photocatalyst material capable of utilizing a wider part
of the solar energy effectively, the semiconductor must have a (1) smaller band gap to
absorb a broad region of the electromagnetic spectrum, (2) a favorable VB edge
position for the generation of h
+ and hydroxyl radicals (Casbeer et al. 2012), (3) high
stability, and (4) efficient charge separation and migration (Qu and Duan 2013).
1 Nanotechnology for Water and Wastewater Treatment Using Graphene. . .
5