Environmental Engineering Reference
In-Depth Information
nanotechnology. In the following sections, the approaches to make photocatalysts visible
light active and the strategies for eficient photogenerated charge carrier separation are
discussed in detail.
11.2 Electronic Band Structure Modification for Sunlight Harvesting
11.2.1 Single Semiconductor Photocatalysts
A number of semiconductors, such as TiO
2
, ZnO, WO
3
, Fe
2
O
3
, SrTiO
3
, and CdS, can act as
photoactive materials for redox/charge transfer processes owing to their electronic struc-
ture. Figure 11.4 shows the band-edge positions of several typical semiconductor photo-
catalysts and the electrochemical potentials required to trigger speciic redox reactions.
Among these semiconductors, TiO
2
is the most widely used photocatalytic material with
a band-gap energy in the 3.0-3.4 eV range. Three crystalline polymorphs of TiO
2
exist in
nature: anatase, rutile, and brookite, which are composed of a TiO
6
unit and its distortions.
The manner of the unit connections determines the lattice structure and electronic struc-
ture, and thus the bulk diffusion, and the redox potentials of photogenerated charge car-
riers. The photocatalytic activity of TiO
2
toward a speciic reaction greatly depends on its
physicochemical properties, e.g., crystalline structures, defects, and degree of aggregation.
The crystal defects exhibit a profound effect on the photocatalytic activity because they
are intimately involved in charge-trapping and recombination processes. Experimental
and theoretical results indicate that oxygen vacancies, Ti vacancies, and Ti
n
+
interstitials
are the most feasible isolated or punctual defects,
6
present both on the surface and in the
bulk of a semiconductor. These defects play a decisive role in the adsorption and sur-
face reactivity of O
2
, H
2
O, and organic molecules. The electronic states are also associated
with the defects, which cause shallow or deep donor levels that are near or below the CB,
whereas the acceptor levels are located near or above the VB. This provides opportunities
for tuning the energy band by doping.
A fundamentally intriguing issue in TiO
2
photocatalysis is the proposed synergistic
effect between anatase and rutile crystals, which may be one of the reasons for the high
-2.0
E
0
(
·
O
-
/H
2
O)
ZrO
2
KTaO
3
SrTiO
3
TiO
2
ZnS
CdS
CdSe
GaP
SiC
Si
MoS
2
WO
3
Fe
2
O
3
E
0
(HCOOH/H
2
CO
3
)
-1.0
E
0
(HCHO/H
2
CO
3
)
E
0
(H
+
/H
2
)
0
E
0
(CH
4
/CO
2
)
1.0
E
0
(O
2
/H
2
O)
2.0
3.0
4.0
5.0
3.4
3.2
3.0
3.6
2.4
1.7
2.25
3.0
1.1
1.75
2.8
2.3 eV
FIGURE 11.4
Band-edge positions of several semiconductor photocatalysts in contact with an aqueous electrolyte. (Adapted
with permission from Navarro, R. M., Alvarez-Galván, M. C., Mano, J. A. V., Al-Zahrani, S. M., Fierro, J. L. G.,
Energy Environ. Sci
., 3, 1865. Copyright 2010, Royal Society of Chemistry.)
