experiment was adopted by using visible light as the light source; the result of no
product yield proved that the electron transfer was not followed by the previous one.
Furthermore, when the EDTA was added as the hole scavengers, the result shows an
improved CO and H 2 yield but decreased O 2 yield which further reflects the
enhanced electron-hole separation (Fig. 12.7b).
Compared with the PS–PS Z-scheme, the interface between two solids usually
contains many defects, which may inhibit the charge transfer. Therefore, the PS-CPS Z-scheme with a conductor insertion could reduce the electron transfer resistance
and thus improve the CO2PR efficiency. He et al. [25] reported an Ag 3 PO 4 /C 3 N 4
composite with enhanced CO2PR activity. Since the Ag 3 PO 4 is not stable, therefore,
Ag NPs were formed in situ within the composites under light irradiation and acted
as the electron mediator. Taking into account of the CB level of Ag 3 PO 4 (0.45 eV), if
the composites followed the double-charge transfer mechanism, the introduction of
Ag 3 PO 4 cannot promote the CO2PR. So it is reasonable to believe that the charge
transfer route followed Z-scheme mechanism. In this way, Ag accepts the photogenerated electrons from Ag 3 PO 4 and recombines with holes from C 3 N 4 ; subsequently, the photo-generated electrons with more negative reduction potential could
be used into the CO2PR reaction. Wei et al. [44] developed a PS–C–PS Z-scheme
structure photocatalyst that contains CdS (shell), Pt (core), and TiO 2 (support),
which show enhanced CO 2 photoreduction activity and selectivity (36.8 μmol/g.h
CH 4 yield and 98.1% CH 4 selectivity). The location of reduction sites and electron
transfer route was confirmed by Ag photo-deposition method. The Ag NPs selectively deposited on the shell of CdS instead of TiO 2 surface which clearly demonstrates that the CdS acts as the reduction site and the electron transfer follows the
TiO 2 ! Pt ! CdS route. In order to present consolidate proof to Z-scheme charge
transfer behavior, Li et al. [45] first adopt Kelvin probe force microscopy to detect
surface potential change of In 2 S 3 -Au-WO 3 . Compared with WO 3 /In 2 S 3 , the SPV
image (reflect the concentration of photo-generated holes) of WO 3 /Au/In 2 S 3 shows
significant change from 10 mV to 30 mV; this difference vividly reflects the efficient
charge separation and the role of Au as the electron mediator.
Another way to improve the charge separation efficiency in photocatalysis is the
construction of double cocatalysts (usually refers to the electron trapping agent and
hole collect agent) with spatial separated configurations. Domen and coworkers [62]
first developed Ta 3 N 5 photocatalyst hollow shell with Pt and CoO x deposited inside
and outside of the shell, respectively. Followed by this pioneered work, similar
strategies have been proposed such as the thin heterojunction Pt–
TiO 2 @In 2 O 3 @MnO x hollow shell structure, porous TiO 2 tube, or hollow C 3 N 4
shell with spatial separated Pt and CoO x NPs [63–65], etc. Recently, our group
developed a new strategy to construct spatial configuration by introducing Pt NPs
and CoO x NPs outside and inside of the skeleton of hierarchical TiO 2 -SiO 2 (HTSO)
[8], abbreviated as Pt/HCTSO. The HR-TEM image clearly indicates that the Pt NPs
and CoO x NPs separated by the HTSO skeleton (Fig. 12.8a); on the other hand,
EDS-mapping image shows the Pt and Co species are well-dispersed throughout the
framework of HTSO and no aggregation happened (Fig. 12.8b). The CO 2 photoreduction evaluation result revealed that the 0.8% Pt/HCTSO (0.8%) shows enhanced
12.4 Roles and Properties of Different Cocatalysts
295
product yield proved that the electron transfer was not followed by the previous one.
Furthermore, when the EDTA was added as the hole scavengers, the result shows an
improved CO and H 2 yield but decreased O 2 yield which further reflects the
enhanced electron-hole separation (Fig. 12.7b).
Compared with the PS–PS Z-scheme, the interface between two solids usually
contains many defects, which may inhibit the charge transfer. Therefore, the PS-CPS Z-scheme with a conductor insertion could reduce the electron transfer resistance
and thus improve the CO2PR efficiency. He et al. [25] reported an Ag 3 PO 4 /C 3 N 4
composite with enhanced CO2PR activity. Since the Ag 3 PO 4 is not stable, therefore,
Ag NPs were formed in situ within the composites under light irradiation and acted
as the electron mediator. Taking into account of the CB level of Ag 3 PO 4 (0.45 eV), if
the composites followed the double-charge transfer mechanism, the introduction of
Ag 3 PO 4 cannot promote the CO2PR. So it is reasonable to believe that the charge
transfer route followed Z-scheme mechanism. In this way, Ag accepts the photogenerated electrons from Ag 3 PO 4 and recombines with holes from C 3 N 4 ; subsequently, the photo-generated electrons with more negative reduction potential could
be used into the CO2PR reaction. Wei et al. [44] developed a PS–C–PS Z-scheme
structure photocatalyst that contains CdS (shell), Pt (core), and TiO 2 (support),
which show enhanced CO 2 photoreduction activity and selectivity (36.8 μmol/g.h
CH 4 yield and 98.1% CH 4 selectivity). The location of reduction sites and electron
transfer route was confirmed by Ag photo-deposition method. The Ag NPs selectively deposited on the shell of CdS instead of TiO 2 surface which clearly demonstrates that the CdS acts as the reduction site and the electron transfer follows the
TiO 2 ! Pt ! CdS route. In order to present consolidate proof to Z-scheme charge
transfer behavior, Li et al. [45] first adopt Kelvin probe force microscopy to detect
surface potential change of In 2 S 3 -Au-WO 3 . Compared with WO 3 /In 2 S 3 , the SPV
image (reflect the concentration of photo-generated holes) of WO 3 /Au/In 2 S 3 shows
significant change from 10 mV to 30 mV; this difference vividly reflects the efficient
charge separation and the role of Au as the electron mediator.
Another way to improve the charge separation efficiency in photocatalysis is the
construction of double cocatalysts (usually refers to the electron trapping agent and
hole collect agent) with spatial separated configurations. Domen and coworkers [62]
first developed Ta 3 N 5 photocatalyst hollow shell with Pt and CoO x deposited inside
and outside of the shell, respectively. Followed by this pioneered work, similar
strategies have been proposed such as the thin heterojunction Pt–
TiO 2 @In 2 O 3 @MnO x hollow shell structure, porous TiO 2 tube, or hollow C 3 N 4
shell with spatial separated Pt and CoO x NPs [63–65], etc. Recently, our group
developed a new strategy to construct spatial configuration by introducing Pt NPs
and CoO x NPs outside and inside of the skeleton of hierarchical TiO 2 -SiO 2 (HTSO)
[8], abbreviated as Pt/HCTSO. The HR-TEM image clearly indicates that the Pt NPs
and CoO x NPs separated by the HTSO skeleton (Fig. 12.8a); on the other hand,
EDS-mapping image shows the Pt and Co species are well-dispersed throughout the
framework of HTSO and no aggregation happened (Fig. 12.8b). The CO 2 photoreduction evaluation result revealed that the 0.8% Pt/HCTSO (0.8%) shows enhanced
12.4 Roles and Properties of Different Cocatalysts
295