Environmental Engineering Reference
In-Depth Information
metal particle supported on semiconductors and metal@semiconductor core@shell struc-
tures. In the metal-supported semiconductor photocatalysts, the photogenerated electrons
are injected into the metal nanoparticle causing reduction reactions, while the remaining
photogenerated holes in the semiconductor cause oxidation reactions. For metal@semicon-
ductor core@shell structures, photogenerated electrons will accumulate and store in the
metal core, e.g., about 66 electrons in one Ag@TiO
2
particle under UV light irradiation.
29
These stored electrons in the metal core can transfer to the outside to react with an electron
acceptor, such as O
2
and C
60
. Although the photocatalytic reactivity is limited, core-shell
structures can signiicantly eliminate the corrosion or dissolution of metal particles.
The loading amount, size distribution, and chemical states of metals are all crucial fac-
tors for the photoreactivity of metal@semiconductor junctions. Although impregnation
and photodeposition are often adopted to deposit metal nanoparticles on semiconductors,
they fail in controlling the particle size and dispersion. Experimental results demonstrated
that well-dispersed metal nanoparticles provided a large metal-semiconductor interface
and promoted the separation of electron-hole pairs. Moreover, noble metals may exist as
several chemical states, e.g., Pt can exist as Pt
0
, Pt
II
, and Pt
IV
, which may exhibit different
activities in the photocatalytic reaction.
30
More careful and systematic studies are needed
to clarify the active components of the metals in photocatalytic reactions.
11.3.2 Semiconductor/Semiconductor Heterojunction
Compared with single-phase semiconductor, multisemiconductor hybrid systems have sig-
niicant advantages in promoting photocatalytic activity: (i) to extend the photoresponse
region by coupling semiconductors with different band gaps; (ii) to promote electron-hole
pair separation by the vectorial charge transfer in the heterojunction; and (iii) to improve
the selectivity due to the reduction and oxidation reactions at different sites.
To be used under solar irradiation, a series of visible light photosensitive p-n and n-n
heterojunctions
31
have been investigated: p-Si/n-TiO
2
, p-CuMnO
2
/n- Cu
2
O, p-CuFeO
2
/
n-SnO
2
, p-ZnFe
2
O
4
/n-SrTiO
3
, and p-Si/n-TiO
2
. A p-Cu
2
O/n -WO
3
coupling system was
developed to avoid back-reactions of photoinduced charges for photocatalytic H
2
produc-
tion.
32
In all these p-n and n-n heterojunction photocatalysts, a much higher visible light
photoreactivity was obtained owing to the effective separation of photogenerated elec-
trons and holes compared with the single p- or n-components (Figure 11.10a). However, an
obvious drawback of this mechanism is that a portion of the redox energy of the electrons/
holes is released after the transfer process in the heterojunction.
E
C
Red
2
Dye*
e
Red
e
h
E
C
E
C
e
Red
hv
Ox
2
Dye
hv
Red
1
Ox
Ox
h
E
V
Ox
1
E
V
E
V
h
(a)
(b)
FIGURE 11.10
(See color insert.)
Schematic diagram illustrating the principle of charge separation between two semiconduc-
tor nanoparticles: (a) heterojunction; (b) sensitization.
