Environmental Engineering Reference
In-Depth Information
100
80
60
TiO
2
lm
Cu
2
O film,
Composite film
Composite lm made
by previous method
40
20
0
0
1
2
3
4
5
6
Time (h)
figure 3.11
Photodegradation of methylene blue by TiO
2
/Cu
2
O composite film (with the ratio of TiO
2
and Cu
2
O 1:1 (w/w)), the composite
film made by previous method (with only 3 wt.% Cu
2
O), TiO
2
film and Cu
2
O film, respectively, in the same condition. pH 4, [FeSO
4
] 0.10 g/l,
[edTA] 0.05 g/l. Reproduced by permission from Ref. [141]. © 2007, American Chemical Society.
is about −0.2 eV [158]. The difference between rutile TiO
2
and anatase TiO
2
is not under consideration here. Cu
2
O is one of the
semiconductors that have the highest CB. The band gap of Cu
2
O is about 2.0 eV, and the CB of Cu
2
O is −1.4 eV [159]. Coupling
of the two kinds of semiconductors allows the vectorial displacement of electrons from Cu
2
O to TiO
2
. Thus, under VL excita-
tion, photogenerated electrons resulting from Cu
2
O accumulate in the lower-lying CB of TiO
2
to form the Ti
3+
electron center,
whereas holes can accumulate in the VB Cu
2
O to form the hole center, which can be consumed during weak oxidation.
Accumulated electrons in the CB of TiO
2
can be transferred to oxygen adsorbed on the surface to form O
2−
or O
2
2−
, which com-
bines with H
+
to form H
2
O
2
. Therefore, the probability of photogenerated electrons in TiO
2
to recombine with the holes in the
Cu
2
O VB is very low, which leads to the enhancement of the photocatalytic activity of Cu
2
O. The photocatalytic reaction paths
for the TiO
2
/Cu
2
O composite system are shown in equations 3.2-3.7. Highly mobile electrons first transfer from the CB of
Cu
2
O to that of TiO
2
. The hole stays in the Cu
2
O VB for weak oxidation reaction. Finally, the hydrocarbon resulting from the
photodegradation of a pollutant (methylene blue) and Fe
2+
(which is a kind of effective hole scavenger) causes hole
consumption.
),
(3.2)
Cu Oh h
2
+→ +
υ
(
+
e
−
vb
cb
e
−
+>
TiOH iOH
IV
→>
III
(3.3)
cb
h
vb
+
+
holescavenger eak oxidation
→
(3.4)
Ti
3+
acts like e
cb
−
with the reaction of O
2
.
Ti OH OO Ti OH
III
+→ +>
•
−
IV
(3.5)
2
2
As H
2
O
2
is evolved from O
2
•−
, with the existence of an electronic donor, the following equations can be driven.
OHHO
2
•
−
+→
•
(3.6)
2
HO He HO
cb
•
++→
+
−
(3.7)
2
2
2
Because the VB of Cu
2
O is not high enough, the oxidation ability of holes from Cu
2
O under VL to form the peroxide radical is
limited. When Fe
2+
is present in the solution, Fenton reagent with H
2
O
2
, which evolves according to equations 3.5, 3.6, and 3.7,
is formed, and the advanced oxidation process can start. This is why the TiO
2
/Cu
2
O composite reported earlier [76] has good
photocatalytic activity only under UV light without the presence of Fe
2+
and edTA. This photocatalytic reaction path works
