Journal of the American Chemical Society
Communication
ground state (ΔG = +0.54 eV), are thermodynamically
unfavorable. However, electron transfer should proceed
effectively from the conduction band to the excited photo-
sensitizer unit (ΔG = −1.48 eV) and/or the one-electron
oxidation state of the photosensitizer unit (ΔG = −2.03 eV).
This also strongly indicates that the photocatalytic reduction of
formic acid as a reduction product and formaldehyde as the
oxidized product of methanol. This process converts light energy
to chemical energy, with ΔG° = +83.0 kJ/mol.
ASSOCIATED CONTENT
Supporting Information
■
*
S
CO requires the excitation of both TaON and the RuBLRu′
General procedures and results, spectra, microscopy images of
the hybrid, and photophysical and electrochemical properties of
2
photosensitizer.
On the basis of the mechanistic aspects described above, we
concluded that the mechanism is as follows (and as shown in
Scheme 1): Irradiated light is absorbed mainly by TaON and the
Ag particles on TaON, and partly by the RuBLRu′ photo-
sensitizer unit (because of the absorbance differences). The
photogenerated hole in the TaON valence band can oxidize
AUTHOR INFORMATION
18
methanol (E°•
+
= +0.47 V vs Ag/AgNO3), and
Notes
CH
2
OH,H /CH
3
OH
The authors declare no competing financial interest.
the electrons accumulated in the conduction band can be
transferred to the excited or oxidized photosensitizer unit, but
cannot be transferred to the ground state. Therefore, another
photon absorbed by the photosensitizer is required for Ag/
TaON−RuBLRu′ interfacial electron transfer that produces an
OER species in the photosensitizer or catalyst unit. In the former
case, the subsequent intramolecular electron transfer proceeds
from the OER in the photosensitizer unit to the catalyst unit as a
ACKNOWLEDGMENTS
■
This work was partially supported by Japan Science and
Technology Agency (Research Seeds Quest Program) and
Toyota Motor Co. K.S. gratefully acknowledges the support of
the Japan Society for the Promotion of Science (JSPS) for
Research Fellowships for Young Scientists. The authors thank
the Center for Advanced Materials Analysis (Tokyo Institute of
Technology) for the SEM and TEM analyses.
thermodynamically favorable process. CO is reduced on the
2
catalyst unit to give HCOOH. Because producing formic acid
from CO requires a two-electron reduction, the stepwise two-
photon absorption and subsequent electron-transfer processes
2
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2
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The Ag nanoparticles loaded on TaON enhanced the
photocatalytic activity of the hybrid (entries 1 and 6, Table 1).
Although metallic Ag nanoparticles would act as electron donors,
this is implausible in the photocatalytic reaction because if the
loaded Ag acts as the main electron donor, the amount of HCHO
produced from methanol would be much smaller than the sum of
reduced products (Figure 2). Some groups reported that the
efficiency of electron−hole separation in the excitation of TiO2
(
(
2
(
was accelerated by loading Ag nanoparticles onto TiO because
2
19
(
Ag acts as an electron pool. Because a similar phenomenon
occurs for Ag/TaON, we believe that Ag loading probably
improves the efficiency of methanol oxidation and the
accumulation of electrons by Ag/TaON. Therefore, the
subsequent photochemical reduction of RuBLRu′ should be
accelerated. Electron transfer from Ag to the excited and oxidized
photosensitized unit might proceed because approximately 60%
of the TaON surface was covered by RuBLRu′ (Figure S5).
Another advantage of using Ag is that the greater potential of Ag
for proton reduction, compared to other noble metals (such as
(
(
(
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3
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Pt), suppresses the evolution of H . In fact, Pt/TaON−
2
RuBLRu′ mainly produced H under irradiation, even under a
2
(
CO atmosphere, and a very small amount of HCOOH (entry 9,
2
Table 1). We do not have clear experimental evidence for
participation of plasmon of Ag in the photocatalytic reaction.
In conclusion, we successfully synthesized the first visible-
light-driven Z-scheme for the photocatalytic reduction of CO2.
The photocatalyst Ag/TaON−RuBLRu′ mainly produces
(
D
dx.doi.org/10.1021/ja311541a | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX