Environmental Engineering Reference
In-Depth Information
TABLE 11.1
Typical Z-Scheme Photocatalyst for Water Splitting under Visible Light
Irradiation
Activity
(
μ
mol h
−1
)
H
2
Evolution
Photocatalyst
O
2
Evolution
Photocatalyst
Electron
Mediator
Reference
H
2
O
2
Pt/SrTiO
3
:Cr, Ta
Pt/WO
3
IO
3−
/I
−
16
8
[39]
Pt/TaON
RuO
2
/TaON
IO
3−
/I
−
3
1.5
[40]
Pt/CaTaO
2
N
Pt/WO
3
IO
3−
/I
−
6.6
3.3
[41]
Pt/TaON
Pt/WO
3
IO
3−
/I
−
24
12
[42]
Pt/SrTiO
3
:Rh
BiVO
4
Fe
3+
/Fe
2+
15
7.2
[43]
Pt/SrTiO
3
:Rh
Bi
2
MoO
6
Fe
3+
/Fe
2+
19
8.9
[43]
Pt/ZrO
2
/TaON
Pt/WO
3
IO
3−
/I
−
52
26.6
[44]
mediator redox couple is critical to boosting effective electron relay and suppressing the
backward reactions. Compared with ionic redox couples in which electron transfer process
takes place between two isolated photocatalysts, the direct coupling of the components
through a solid electron mediator is more favorable in retarding back reactions, which is
also called a direct Z-scheme photocatalyst system. A typical example of a direct Z-scheme
system is a site-selective Au@CdS/TiO
2
nanojunction, which exhibits a higher photocata-
lytic activity than single or two-component systems as a result of the vectorial electron
transfer (TiO2→Au→CdS) driven by the two-step excitation process.
45
Indeed, an excellent
solid-state electron mediator should possess the ability to achieve a dynamic equilibrium
between electron accepting and donating processes. Recently, the reduced graphene oxide
as a solid electron mediator for a Z-scheme photocatalytic water splitting system is dem-
onstrated.
46
Another crucial factor for a Z-scheme system is the PS I/mediator/PS II contact
interface, which should ensure a continuous electron low between the photocatalysts.
In contrast to the Z-scheme process involving a redox mediator, the weaker oxidative
hole and reductive electron can also be directly quenched at the solid heterojunction inter-
face while keeping the stronger oxidative hole and reductive electron isolated on different
semiconductors. This coniguration appears to work without a redox mediator, such as
the ZnO-CdS coupled system.
47
In relation to this, the Ru-SrTiO
3
:Rh-BiVO
4
photocatalyst
without a redox mediator was demonstrated to perform overall water splitting through a
Z-scheme mechanism.
48
These results suggest that an intimate contact between two com-
ponents is necessary in Z-scheme systems without electron mediators.
Although the catalytic activities and eficiencies of the current available Z-scheme sys-
tems are still quite low, the versatility and capability will make the Z-scheme system the
focus of future research on photocatalytic applications. The challenges lie in the construc-
tion of adequate interface contacts to realize eficient electron transfer in Z-scheme systems.
11.3.5 Cocatalyst Modification
Cocatalyst coupling with semiconductors can reduce the overpotential of the photocata-
lytic reaction and act as a surface reduction or oxidation site, thus facilitating the reactions
(Figure 11.12). To date, the most widely used cocatalysts include noble metals (e.g., Pt, Pd,
Ru, Rh, Au, Ir), metal oxides (e.g., NiO, Rh
x
Cr
1−
x
O
3
), and composites (e.g., Ni/NiO, Rh/
Cr
2
O
3
).
49
Thus, it is imperative to develop low-cost cocatalysts for replacement. Recently, a
series of sulide cocatalysts, such as MoS
2
, WS
2
, and PbS, have been explored. It is noted that
