Environmental Engineering Reference
In-Depth Information
e
-
H
+
, O
2
CB
e
-
E. coli
CB
O
2
•
-
, H
2
O
2
OH•
E. coli
VB
H
2
O
h
+
OH•
VB
TiO
2
Cu
2
O
figure 3.16
Scheme of inactivation mechanisms of
E. coli
by Cu
2
O/TNTs under visible-light irradiation. Reproduced by permission
from Ref. [185]. © 2012, Springer-Verlag.
photocatalyst shows much higher activity in the inactivation of
E. coli
. It is capable of complete inactivation of
E. coli
in 5 × 10
7
colony-forming units/ml within a record short disinfection time of 20 min under VL irradiation. The average
bactericidal percentages of the Cu
2
O/TNTs for
E. coli
are 20 times and 6.6 times higher than those of TNTs under the same
conditions and Cu
2
O/TNTs without light, respectively. This superior bactericidal performance is mainly attributed to the high
ability to produce ∙OH radicals by both the photogenerated electron and the hole of the prepared photocatalyst under VL. The
mechanism of Cu
2
O/TNTs for photocatalytic disinfection is proposed based on Figure 3.16, which schematically illustrates
photocatalytic generation of ∙OH at the Cu
2
O/TNTs film. Cu
2
O is a p-type semiconductor having a direct band gap and its CB
edge potential is approximately 1.2 eV more negative than that of TiO
2
, which makes it suitable as a VL sensitizer for TiO
2
[76]. The narrow band gap of Cu
2
O makes it capable of absorbing VL to generate photoelectrons and holes at the CB and VB,
respectively. The CB edge potential difference between Cu
2
O and TiO
2
serves as a motive force driving the photoelectrons
from the CB of Cu
2
O to the CB of TiO
2
. The synergetic effect results from the matched electron structure of Cu
2
O and TiO
2
facilitates the charge separation to minimize the recombination, which in turn prolongs the lifetime of holes to enhance the
production of ∙OH at the VB of Cu
2
O via an oxidative process. On the other hand, the electrons transferred to the CB of TiO
2
react with adsorbed oxygen to form active oxygen species such as O
2−
, O
2−
, and H
2
O
2
via a reductive process; ∙OH can then
be produced through further chemical reactions of the active oxygen species. The ability to produce ∙OH by both a photogen-
erated electron and a hole is believed to be an important advantage for the high concentration of ∙OH and thereafter the high
bacterial performance.
It is worth mentioning that Fenton-like reactions play an important role in photocatalytic disinfection due to Cu
+
/Cu
2+
reaction [186]. Paschoalinoa et al. [186] proposed the mechanism of Fenton-like reactions. They described CuO powders with
different specific surface areas that inactivate
E. coli
in aqueous solution under VL irradiation >360 nm. The CuO in contact
with the bacterial suspension shows a change in its surface oxidation state from Cu
2+
to Cu
+
. The outermost layer of the catalyst
(5-7 nm) becomes mainly Cu
2
O (80%) and CuO (20%), as observed by X-ray photoelectron spectroscopy (XPS). The suggested
mechanism of reaction at the CuO under light irradiation is as follows:
CuOh CuOe h
cb
+→
−
υ
(
,
+
)
(3.13)
vb
The e
cb
−
in equation 3.13 is produced from the CuO (p type) with a band-gap energy of 1.7 eV, a flat-band potential of −0.3V
saturated calomel electrode (SCe) (pH 7), and a VB at +1.4 V SCe [75]. The electron-hole pair is formed when photon energies
