Environmental Engineering Reference
In-Depth Information
The anatase-based system is one of the most broadly applied heterojunction photocata-
lysts. Typically, there are three different types:
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(i) oxide-anatase system: rutile-anatase
(TiO
2
-TiO
2
), SnO
2
-, ZrO
2
-, ZnO -, Bi
2
O
3
-, Fe
2
O
3
-, WO
3
-, Ta
2
O
5
-, and Cu
2
O -TiO
2
; (ii) chalco-
genide-anatase system: Bi
2
S
3
-, PbS-, CdS-, and CdSe-TiO
2
; and (iii) complex oxide-ana-
tase system: SrTiO
3
-, FeTiO
3
-, ZnFeO
3
-, BiFeO
3
-, and LaVO
4
-TiO
2
. For certain photocatalytic
application, e.g., H
2
evolution from water splitting, only TiO
2
coupled with ZnO, Cu
2
O,
Ta
2
O
5
, and CdS have the potential since the
e
−
occupied CB is more negative than E(H
2
/
H
2
O) = 0 V (vs. NHE, pH 0). Other heterojunction composites are usually used in the pho-
tocatalytic degradation of organic pollutants.
In addition to the band structure of the components, the photocatalytic performance of
heterojunctions is also related to the structural geometry, the nanostructure size, and the
contact interface. It usually requires a lattice match between the components to achieve a
better passivation and minimize structural defects. Typically, there are “external” phase
coupling or “internal” core-shell geometries for multiphase contact in the heterojunction
photocatalytic system. Recently, a surface anatase-rutile heterojunction was demonstrated
and exhibited unique photocatalytic activity.
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In addition, phase contact and subsequent
charge handling throughout the heterojunction interfaces are strongly size and defect
dependent. For example, a size-driven p-to-n transition for Cu
2
O was demonstrated as the
size decreased to the nanometer region, which subsequently affected the charge carrier
transfer in the Cu
2
O-based heterojunction.
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11.3.3 Sensitization
Other than the mutually photosensitive heterojunction, coupling one photosensitive semicon-
ductor with another nonsensitive semiconductor to fabricate a heterojunction, usually called
sensitization, may also have a positive effect on the photocatalytic performance. There are
two requirements for an effective sensitization process: (i) the sensitizer should have a strong
absorption for visible light; (ii) the CB of the sensitizer should be higher than that of the wide-
band-gap semiconductor (e.g., TiO
2
) to facilitate charge transfer. Typically, dye sensitization
and quantum dot (QD) sensitization are fabricated to harvest visible light and facilitate elec-
tron-hole separation for photoelectrochemical and photocatalytic applications (Figure 11.10b).
For dye sensitization, under visible light illumination, the excited dye molecule injects
e
−
into the CB of a wide-band-gap semiconductor to initiate the photocatalytic reaction, while
the
h
+
in the dyes are reduced by the sacriicial agent to regenerate the dyes and sustain the
whole reaction. Synthetic organic dyes, natural pigments, and transition metal coordina-
tion compounds have been used to sensitize semiconductor photocatalysts. To facilitate
electron transfer, dyes should be attached to the semiconductor surface directly by various
interactions, such as covalent bands, electrostatic interactions, and hydrogen bonds.
Semiconductor nanocrystals, known as QDs, have also exhibited good performance
as sensitizers in the visible light region. They have optical and electronic properties that
allow them to absorb sunlight and transfer photogenerated electrons to semiconductors.
Compared with dye molecules, QDs can harvest a wider range of solar spectrum and
exhibit the size-dependent tunability of the band gap, which provides an opportunity to
fabricate a sequential size-controlled arrangement of QDs to harvest the entire solar spec-
trum.
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To favor the electron transfer, the CB difference of QD and a wide-band-gap semi-
conductor should be large enough to overcome the interfacial resistance. Additionally,
QDs exhibit a carrier multiplication process, which can generate multiple electron-hole
pairs per photon.
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The most typical QD-sensitized photocatalyst is the CdS/TiO
2
system.
Under visible light irradiation, the photoinduced electrons in CdS QDs inject into the TiO
2
