Modified TiO-Based Photocatalytic Systems for the Removal of Emerging Contaminants from Water - Aquananotechnology: Global Prospects - page 491

CB - Ti 3d -

CB Ti 3d

Eg 2.5 eV

Eg 3.2 eV

VB + O 2p VB + O 2p

TiO 2 N-doped TiO 2

FIGURE 25.3
Reaction mechanism of N-doped TiO 2 . (From Asahi, R. et al., Science , 293, 269, 2001.)

hv

e - e -

CB CB

Eg = 2.5 eV

Eg = 3.2 eV

VB

VB

h +
O 2p

TiO 2

CdS

FIGURE 25.4
Coupling of TiO 2 with CdS.

25.7.4 Coupling with Semiconductors

Coupling with other semiconductors also increases photocatalytic eficiency by shifting
the absorption to the visible region. In this method, a large band-gap semiconductor is
coupled with a small band-gap semiconductor with a more negative conduction band
level. The small band-gap semiconductor can be excited under visible light. The photogen-
erated electrons are transferred to the conduction band of the large band-gap semiconduc-
tor (Malato et al., 2009). The mechanism is shown in Figure 25.4.

25.7.5 Metal Ion Implantation

In this method, TiO 2 is bombarded with high-energy transition metals by applying a high
voltage. This process changes the electronic structure of TiO 2 and modiies its photocata-
lytic activity to the visible region up to 600 nm (Malato et al., 2009). Metal ions such as Cr,
Fe, Ni, and V are used for this method. Metal ion-implanted TiO 2 possesses a lot of advan-
tages and it is known as the second-generation photocatalyst (Anpo, 2000).