Tuning the selectivity toward CO evolution in the photocatalytic conversion of CO2 with H2O through the modification of Ag-loaded Ga2O3 with a ZnGa2O4 layer

Article abstract of DOI:10.1039/c5cy01280e

Stoichiometric evolutions of CO, H₂, and O₂ were achieved for the photocatalytic conversion of CO₂ with H₂O as an electron donor using Ag-loaded Zn-modified Ga₂O₃. The selectivity toward the evolution of CO over H₂ can be controlled by varying the amount of Zn species added in the Ag-loaded Zn-modified Ga₂O₃ photocatalyst. The production of H₂ gradually decreased with increasing amounts of Zn species from 0.1 to 10.0 mol%, whereas the evolution of CO was almost unchanged. The XRD, XAFS, and XPS measurements revealed that a ZnGa₂O₄ layer was generated on the surface of Ga₂O₃ by modification with Zn species. The formation of the ZnGa₂O₄ layer eliminated the proton reduction sites on Ga₂O₃, although the crystallinity, surface area, and morphology of Ga₂O₃ as well as the particle size and chemical state of Ag did not change. In conclusion, we designed a highly selective photocatalyst for the conversion of CO₂ with H₂O as an electron donor using Ag (the cocatalyst for the CO evolution), ZnGa₂O₄ (the inhibitor of the H₂ production), and Ga₂O₃ (the photocatalyst).

Full text of DOI:10.1039/c5cy01280e

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Tuning the selectivity toward CO evolution in the photocatalytic
conversion of CO2 by H2O through the modification of Ag-loaded
Ga2O3 with a ZnGa2O4 layer
Received 00th January 20xx,
Accepted 00th January 20xx
Zheng Wang,a Kentaro Teramura,*a,b,c Zeai Huang,a Saburo Hosokawa,a,b Yoshihisa Sakatad and
Tsunehiro Tanaka*a,b
DOI: 10.1039/x0xx00000x
www.rsc.org/
Stoichiometric evolutions of CO, H2, and O2 were achieved for the photocatalytic conversion of CO2 by H2O as an electron
donor using Ag-loaded Zn-modified Ga2O3. The selectivity toward the evolution of CO over H2 can be controlled by varying
the addition of Zn species in the Ag-loaded Zn-modified Ga2O3 photocatalyst. The production of H2 gradually decreased
with increasing amounts of Zn species from 0.1 to 10.0 mol%, whereas the evolution of CO was almost unchanged. The
XRD, XAFS, and XPS measurements revealed that a ZnGa2O4 layer was generated on the surface of Ga2O3 by modification
with the Zn species. The formation of the ZnGa2O4 layer eliminated the proton reduction sites on Ga2O3, although the
crystallinity, surface area, morphology of Ga2O3, the particle size, and chemical state of Ag did not change. In conclusion,
we designed a highly selective photocatalyst for the conversion of CO2 by H2O as an electron donor using Ag (cocatalyst for
the
CO
evolution),
ZnGa2O4
(inhibitor
of
the
H2
production),
and
Ga2O3
(photocatalyst).
production of H2 via the reduction of H+ is the dominant
pathway.5,8 Therefore, to achieve high selectivity in the
photocatalytic conversion of CO2 by H2O, the electrons
generated through the oxidation of H2O must be controlled to
selectively reduce CO2.
Introduction
Conversion of CO2 into useful carbon sources such as CO,
HCOOH, HCHO, CH3OH, and CH4 is currently attracting
considerable interest because of the demand for methods to
recycle CO2 as a natural resource.1-3 In order to achieve the
reduction of CO2, H2 or CH4 is usually fed as a reducing
agent.1,4 However, H2O is the preferred reductant, because it is
a rich source of hydrogen, harmless, and abundant. The
photocatalytic conversion of CO2 by H2O over heterogeneous
photocatalysts, regarded as artificial photosynthesis, is a
promising route for sustainable energy development.5-7 If H2O
does function as the electron donor then it is important to
obtain a stoichiometric ratio between the amount of O2
evolved and CO2 reduced. Moreover, the reduction of H+,
released from H2O molecules, usually competes with CO2
reduction when several heterogeneous materials are used as
photocatalysts for the reduction of CO2 by H2O. Generally, the
Although various photocatalysts such as TiO2,9,10 BiVO4,11
13
ZnGa2O4,12 and Zn2GeO4 have been reported for the
reduction of CO2 with H2O under photoirradiation, the
evolution of O2 was not observed in these reactions.
Therefore, the function of H2O as an electron donor in this
reaction is a controversial subject. Moreover, the synthesis gas
(syngas for short, H2 and CO), a raw feed material in the
Fischer–Tropsch synthesis and other syngas-to-syncrude
technologies, can be directly produced through the
photocatalytic conversion of CO2 using H2O as an electron
donor. The adjustment of synthesis gas composition (H2:CO
ratio) is essential for the Fischer–Tropsch technology.14 Thus,
the selectivity between CO and H2 in the photocatalytic
conversion of CO2 by H2O needs to be finely tuned. Sayama et
al. found that Cu-modified ZrO2 could produce CO and H2 with
the evolution of O2 in an aqueous solution of NaHCO3.15
However, the selectivity of the generated electrons for
reducing CO2 was quite low, indicating that the main reaction
was still the reduction of H+ into H2. Recently, Kudo et al.
reported that the modification of ALa4Ti4O15 (A = Ca, Sr, and
Ba) with Ag nanoparticles was effective for enhancing the
formation of CO from the photocatalytic reduction of CO2 with
H2O, thus improving the selectivity toward CO evolution over
H2 production.16 In addition, KCaSrTa5O15 was reported to be
active for the photocatalytic conversion of CO2 to CO utilizing
H2O as an electron donor.17,18 However, controlling the
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selectivity of the generated electrons for the reduction of CO2 ionization detector-gas chromatography (FID-GC) using
was difficult with Ag-loaded KCaSrTa5O15. Previously, we found methanizer and a Shincarbon ST column (carrier gas: N2).
a
that the selectivity toward CO evolution surpasses 90% for the
The UV-Vis diffuse reflectance spectra (UV-Vis DRS) were
photocatalytic conversion of CO2 by H2O over Ag-loaded Zn- measured by a JASCO V-670 spectrometer equipped with an
modified Ga2O3, although the production of H2 proceeded integrating sphere. Spectralon®, which was supplied by
preferentially over Ag-loaded Ga2O3.19 Addition of the Zn Labsphere Inc., was used as a standard reflection sample such
species into the Ag-loaded Ga2O3 promoted the selectivity as BaSO4. The structure and crystallinity of Zn-modified Ga2O3
toward CO evolution because of the suppression of H2 samples were characterized by X-ray diffraction (XRD) using a
production. Therefore, understanding the role of the Zn Rigaku Multi Flex powder X-ray diffractometer. The Zn K-edge
species in the highly selective photocatalytic conversion of CO2 (9659 eV) and Ga K-edge (10367 eV) X-ray absorption fine
by H2O is necessary for further photocatalyst development. In structure (XAFS) measurements were made in the transmission
this study, we achieved to control the H2 production without mode at the BL01B1 beamline of the SPring-8 synchrotron
affecting the evolution of CO from the photocatalytic radiation facility (Hyogo, Japan). The spectra were reduced
conversion of CO2 by H2O over Ag-loaded, Zn-modified Ga2O3 with the Rigaku REX2000 program Ver. 2.5.9 (Rigaku Corp.).
by simply changing the addition of Zn species. The formation The XPS measurement was acquired using an X-ray
of
a ZnGa2O4 structure on the surface of Ga2O3 was photoelectron spectrometer (ESCA 3400, Shimadzu Corp.). The
demonstrated in terms of XRD, XAFS, and XPS measurements, Brunauer–Emmett–Teller (BET) surface area was measured by
and the function of ZnGa2O4 structure on tuning the H2 N2 adsorption at 77 K using a volumetric gas adsorption
production was also discussed.
apparatus (BELmini, Bel Japan, Inc.). SEM images were
obtained from a Field Emission Scanning Electron Microscope
(FE-SEM, SU-8220, Hitachi High-Technologies) equipped with
an energy-dispersive X-ray spectroscope (EDS) at an
acceleration voltage of 3.0 kV.
Experimental Section
Ga2O3 was fabricated by the precipitation method as reported
previously.19 Ga(NO3)3•xH2O (12 g, Kojundo, 99.999%) was first
dissolved in distilled water (200 mL). Then an aqueous
ammonia solution (28 wt%) was added dropwise to the
aqueous solution of Ga(NO3)3•xH2O until a pH value of
approximately 8.9 was reached. A white precipitate was
centrifuged, washed extensively with distilled water (500 mL),
and dried at 353 K in air to obtain the gallium hydroxide
precursor. Ga2O3 was obtained by calcining the precursor at
1273 K for 6 h. Zn-modified Ga2O3 was synthesized by an
impregnation method. The as-prepared Ga2O3 was dispersed
in an aqueous solution containing various amounts of Zn(NO3)2
by ultrasonication. After drying under vacuum at room
temperature, the solid mixture of Ga2O3 and Zn(NO3)2 was
calcined at 1223 K for 6 h. The amount of Zn species varied
from 0 to 10.0 mol% of the total amount of metal species (Ga
atom and Zn atom).
Results and Discussion
The photocatalytic reaction including the photodeposition Fig. 1 Rates of CO (black), O2 (white), and H2 (gray) evolution and selectivity
toward CO evolution in the photocatalytic conversion of CO2 by H2O over Ag-
loaded Zn-modified Ga2O3 with various amounts of Zn species. Reaction
of Ag cocatalyst was carried out in a flow system using an
inner-irradiation-type reaction vessel at room temperature
conditions: Amount of catalyst, 1.0 g; cocatalyst: 1.0 wt%; volume of water, 1.0 L;
flow rate of CO2, 30 mL min−1; concentration of NaHCO3, 0.1 mol L−1; light source,
and ambient pressure. The Zn-modified Ga2O3 or Ga2O3
high-pressure mercury lamp (400 W).
photocatalyst (1.0 g) was dispersed in ultra-pure water (1.0 L)
containing AgNO3 (9.36×
10-5 M), and CO2 (99.999%) was
Figure 1 shows the formation rates of CO, H2, and O2 and
selectivity toward CO evolution for the photocatalytic
conversion of CO2 by H2O over Ag-loaded Zn-modified Ga2O3
with various amounts of Zn species. All photocatalysts tested
could stoichiometrically produce CO, H2, and O2 under
photoirradiation. Other reduction products, such as CH4 in the
gas phase or HCOOH, HCHO, and CH3OH in the solution, were
not detected by FID-GC and HPLC. These results indicate that
H2O acted as an electron donor and the number of
photogenerated electrons used in the reduction of CO2 and H+
was similar to that of photogenerated holes consumed by the
oxidation of H2O. However, the selectivity of the generated
bubbled into the solution at a flow rate of 30.0 mL min-1 to
purge the air in the reaction system. The Ag cocatalyst (1.0
wt%) was deposited on photocatalyst under UV light
irradiation of a 400 W high-pressure mercury lamp for 4 h.
Then NaHCO3 (0.1 mol L−1) was added to the same solution,
and the suspension was irradiated under the same mercury
lamp with a quartz filter connected to a cooling water system.
Gaseous products such as H2, O2, and CO were analyzed by
thermal conductivity detector-gas chromatography (TCD-GC)
using a GC-8A chromatograph (Shimadzu Corp.) equipped with
a Molecular Sieve 5A column (carrier gas: Ar), and by flame
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Fig. 3 (A) XRD patterns of bare Ga2O3 (a) and Zn-modified Ga2O3 with 0.1 mol%
(b), 0.5 mol% (c), 1.0 mol% (d), 3.0 mol% (e), 5.0 mol% (f), and 10.0 mol% (g) of Zn
species; (B) The enlarged XRD patterns of all samples at 2θ ranged from 36.0° to
40.0°. (C) The change of crystallite size in Zn-modified Ga2O3 with various
amounts of Zn species.
electrons toward the reduction of CO2 was quite different, and
depended on the amount of Zn species. The formation rate of
H2 over the Ag-loaded Ga2O3 without the Zn species was much
higher than that of CO, hence the selectivity of the generated
electrons toward the reduction of CO2 was only 28.5%. When
Ga2O3 was modified with 0.1 mol% of Zn species, the
production of H2 was suppressed. The formation rate of H2
decreased proportionally with the increase in the amount of
Zn species, and was negligible for Zn species concentrations
higher than 3.0 mol%. In contrast, the formation rate of CO
was not influenced by the amount of Zn species added.
Eventually, the selectivity for CO evolution over Ag-loaded Zn-
modified Ga2O3 gradually increased with elevated amounts of
Zn species. When the Zn content was more than 3.0 mol%, the
selectivity for CO evolution approached 100%, implying that
only the reduction of CO2 and oxidation of H2O proceeded on
the Ag-loaded Zn-modified Ga2O3. On the other hand, the rate
of O2 evolution declined with the increased Zn content, which
accompanied with the decrease of H2 production.
The UV-Vis diffuse reflectance spectra of bare Ga2O3 and
Zn-modified Ga2O3 with various amounts of Zn species are
shown in Figure 2. The bare Ga2O3 exhibited an absorption
edge at 265 nm and the bandgap energy of Ga2O3 was
estimated as 4.6 eV according to the Davis–Mott’s equation
([F(R)*hv]2).20 Modification of Ga2O3 with different amounts of
Zn species did not change the absorption edge of the spectra,
thus the addition of Zn species did not contribute to the band
structure of Ga2O3 to alter selectivity toward CO evolution in
the photocatalytic conversion of CO2
Zn-modified Ga2O3 was treated at high temperature after
the impregnation of Ga2O3 with Zn(NO3)2, which means that
the Zn ions should either form a new compound at the surface
of Ga2O3 or be incorporated into the bulk structure of Ga2O3.
Figure 3(A) shows the XRD patterns of bare Ga2O3 and Zn-
modified Ga2O3 with various amounts of Zn species. Ga2O3 was
calcined at 1273 K before the introduction of Zn species, thus
all the samples exhibited the same monoclinic structure, β-
Ga2O3, which is
a thermally stable phase of the five
polymorphic phases of Ga2O3. For the addition of Zn species
from 0.1 mol% to 1.0 mol%, the XRD patterns of Zn-modified
Ga2O3 were similar to that of bare Ga2O3. However, a
diffraction peak representing the (311) facet of the ZnGa2O4
structure was observed in the XRD pattern of 3.0 mol% Zn-
modified Ga2O3. This indicates that the Zn species react with
Ga2O3 to generate ZnGa2O4, and the crystallinity of the
ZnGa2O4 structure in Zn-modified Ga2O3 is promoted by adding
higher amounts of Zn species. Furthermore, the diffraction
peak of ZnGa2O4 became sharper and higher as the amount of
Zn species was increased. When the amount of Zn species
reached 10.0 mol%, the peaks for the (311), (400), (422), and
(440) facets of ZnGa2O4 were clearly observed. Therefore,
based on the XRD results, the formation of highly crystalline
ZnGa2O4 may be responsible for the highly selective
photocatalytic conversion of CO2 by H2O, because the 3.0
mol%, 5.0 mol%, and 10.0 mol% Zn-modified Ga2O3 exhibited
much higher selectivity toward CO evolution than the other
samples.
Fig. 2 UV-Vis DRS of Zn-modified Ga2O3 with various Zn loading amount.
When a foreign metal ion is inserted into a crystal
structure, the substitution of the original lattice ion or the
residual stress from a new mixed compound usually results in
the peak shift of the XRD pattern associated with the change in
the interplanar distance.21-23 Figure 3(B) shows the magnified
XRD patterns of all samples in the 2θ range from 36.0° to
40.0°. The diffraction peaks of Zn-modified Ga2O3 with various
amounts of Zn ions did not shift significantly in comparison to
those of bare Ga2O3. The ionic radius of Zn2+ (0.074 nm) is
larger than that of Ga3+ (0.062 nm);21,22 however, the
unaltered peak position in the XRD pattern implies that the Zn
ion does not act as a dopant in the bulk Ga2O3 lattice.
Furthermore, this result indicates that the formation of
ZnGa2O4 in the Zn-modified Ga2O3 with larger amounts of Zn
ions did not generate enough residual stress to distort the
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Ga2O3 structure.23 The crystallite sizes of bare Ga2O3 and Zn- references, thus the chemical state of the Zn species is divalent.
modified Ga2O3 derived from Scherrer’s equation using the However, the spectrum of Zn-modified Ga2O3 samples with 0.1
FWHM (Full Width of Half Maximum) of the XRD peaks are mol% Zn species was quite different from those of ZnO and
displayed in Figure 3(C). The crystallite size of bulk Ga2O3 ZnGa2O4. Therefore, we speculate that the introduction of a
remained in spite of the added amount of Zn ions, thus the tiny amount of Zn species by the impregnation method
crystallinity of Ga2O3 was stable during the process of resulted in the isolated dispersion of Zn ions in the surface
modification with Zn species. Based on these results, we layer of Ga2O3 and a different local structure for the Zn species
concluded that the Zn species are located on the surface of compared to that of ZnGa2O4. Moreover, as the amount of Zn
Ga2O3, and ZnGa2O4 is generated when a large amount of Zn increased, the XANES spectra became increasingly similar to
species is added.
that of ZnGa2O4, indicating that further addition of Zn ions
could cause a gradual change in the local structure of the Zn
species. For samples with more than 3.0 mol% of Zn species,
the spectra were identical to that of ZnGa2O4. This result is in
agreement with the XRD measurements. In the corresponding
FT of the EXAFS spectra shown in Figure 5(B), the first
coordination peaks are attributed to the Zn-O shell for both
the ZnO and ZnGa2O4 references (ca. 1.98 Å). The position of
the second coordination peak is assigned to the Zn-Ga shell for
the ZnGa2O4 reference (ca. 3.46 Å), and is longer than that of
the Zn-Zn shell peak for the ZnO reference (ca. 3.22 Å). All
samples of Zn-modified Ga2O3 exhibited a Zn-O shell peak at
the same characteristic position as the ZnO and ZnGa2O4
samples, and the intensity of the Zn-O shell peak increased
with increasing Zn addition. A weak shell peak was observed in
the spectrum of 0.1 mol% Zn-modified Ga2O3 at the same
position as the Zn-Zn shell peak of the ZnO reference, implying
that the highly dispersed Zn species mainly exist as ZnO on the
surface of Ga2O3. However, the second shell peak of the 0.5
mol% sample appeared at the characteristic distance of the Zn-
Ga shell, which indicates that the local structure of ZnGa2O4
was formed in Zn-modified Ga2O3 with 0.5 mol% Zn addition.
The peak corresponding to the Zn-Ga shell became higher and
sharper with the addition of Zn up to 3.0 mol%, and then
remained unchanged with further increases in the amount of
Zn. The Zn-K edge XAFS results confirm that the introduction of
a tiny amount of Zn species (>0.1 mol%) into Ga2O3 could
result in the formation of the ZnGa2O4 structure, which was
not detected by XRD measurements.
Fig. 4 The Zn K-edge XANES spectra of Zn foil (a), ZnO (b), ZnGa2O4 (c) and Zn-
modified Ga2O3 with different amount of Zn species: 0.1 mol% (d), 0.5 mol% (e),
1.0 mol% (f), 3.0 mol% (g), 5.0 mol% (h), and 10.0 mol% (i).
On the other hand, Figure 6 and 7 exhibit the Ga K-edge
XANES, EXAFS, and FT of the EXAFS spectra of Zn-modified
Ga2O3 samples with Ga2O3 and ZnGa2O4 as the references. The
XANES spectra of Zn-modified Ga2O3 with various amounts of
Zn species were identical to that of bulk Ga2O3. For the FT of
the EXAFS spectra, the spectral shape of Zn-modified Ga2O3
was also consistent with that of bulk Ga2O3 and the intensity of
the two coordination peaks were similar for each sample.
These results suggest that the local structure of Ga ions in the
Zn-modified Ga2O3 was not disturbed by the addition of Zn
species.
Fig. 5 The Zn K-edge EXAFS (A) and Fourier transforms of EXAFS (B) spectra of Zn
foil (a), ZnO (b), ZnGa2O4 (c) and Zn-modified Ga2O3 with different amount of Zn
species: 0.1 mol% (d), 0.5 mol% (e), 1.0 mol% (f), 3.0 mol% (g), 5.0 mol% (h), and
10.0 mol% (i).
To examine the relationship between the selectivity
toward CO evolution and the Zn modification, especially for
the addition of a small amount of Zn species, the chemical
state and local structure of the Zn species were analyzed by
XAFS measurements. Figure 4 and 5 show the Zn K-edge
XANES, EXAFS, and Fourier transforms (FT) of k3-weighted
EXAFS spectra of Zn-modified Ga2O3 with various amounts of
Zn species. Zinc foil, ZnO, and ZnGa2O4 were used as
references. The ZnGa2O4 reference was fabricated by calcining
a mixture of ZnO and Ga2O3 at 1473 K for 20 h, and it showed
the typical XRD pattern of spinel type ZnGa2O4. The absorption
edge of Zn-modified Ga2O3 samples in the Zn-K edge XANES
spectra (Figure 4) are similar to those of the ZnO and ZnGa2O4
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on the ratio of the mixture in the starting material for Zn-
modified Ga2O3. When the amount of Zn species added
increased from 0.1 mol% to 1.0 mol%, the surface ratio of Zn
atoms to total metal atoms sharply increased from 2.4 mol%
to 4.8 mol%. Further increase in the amount of Zn species
resulted in a continuous rise in the surface ratio of Zn atoms to
total metal atoms. Therefore, the high-temperature treatment
of the Zn(NO3)2-impregnated Ga2O3 starting material promotes
the surface reaction between Zn(NO3)2 and Ga2O3 to form the
ZnGa2O4 layer at the surface of Ga2O3 when the Zn content is
higher than 0.1 mol%. The coverage and thickness of the
ZnGa2O4 layer on the surface of Ga2O3 would increase with
greater loadings of Zn(NO3)2. In addition, the specific surface
area of Zn-modified Ga2O3 measured by the BET method is also
shown in Figure 8. The surface area of the modified Ga2O3 did
not vary proportionally with the generation of the ZnGa2O4
layer on the surface of Ga2O3; thus, the thin ZnGa2O4 layer
must grow evenly on the surface of Ga2O3.
Fig. 6 The Ga K-edge XANES spectra of Ga2O3 (a), ZnGa2O4 (b) and Zn-modified
Ga2O3 with different amount of Zn species: 0.1 mol% (c), 0.5 mol% (d), 1.0 mol%
(e), 3.0 mol% (f), 5.0 mol% (g), and 10.0 mol% (h).
Figure 9 shows the SEM images of Ag-loaded Ga2O3 with
and without modification by Zn species. As shown, formation
of ZnGa2O4 layer on the surface of Ga2O3 with various amounts
of Zn species caused insignificant changes in the particle size
compared to bare Ga2O3. Moreover, the morphology of each
Zn-modified Ga2O3 with different amounts of Zn species was
similar to that of bare Ga2O3. These results are consistent with
XRD and BET measurement. In addition, the Ag particles by the
photodeposition method were aggregated on the Ga2O3 and
Zn-modified Ga2O3 photocatalysts observed in the SEM images
(Figure 9 and S2), and the size and the dispersion of Ag
particles were similar for each sample. The weak surface
plasmon absorptions at the visible region in the UV-Vis DRS of
Ag-loaded Ga2O3 and Ag-loaded Zn-modified Ga2O3 (Figure S3)
were attributed to the aggregation and poor dispersion of Ag
particles. The chemical state of Ag particles prepared by the
photodeposition method was proved to be metallic Ag shown
in Figure S4 and S5. The atomic ratio of Ag was nearly
constant in the Ag-loaded Zn-modified Ga2O3 samples with
different Zn content (Table S1). It is clear that the physical and
chemical properties of Ag particles deposited on the Ga2O3 and
Zn-modified Ga2O3 photocatalysts were not dominated by the
variation of Zn addition. Therefore, the improvement of the
selectivity towards CO evolution over Ag-loaded Zn-modified
Ga2O3 is related to the formation of the ZnGa2O4 layer on the
surface of Ga2O3.
Fig. 7 The Ga K-edge EXAFS (A) and Fourier transforms of EXAFS (B) spectra of
Ga2O3 (a), ZnGa2O4 (b) and Zn-modified Ga2O3 with different amount of Zn
species: 0.1 mol% (c), 0.5 mol% (d), 1.0 mol% (e), 3.0 mol% (f), 5.0 mol% (g), and
10.0 mol% (h).
Fig. 8 The surface ratios of Zn ions to total metal ions (Zn and Ga) and the specific
surface area of Zn-modified Ga2O3 with various Zn contents.
Since the introduced Zn species could react with Ga2O3 to
generate the ZnGa2O4 crystal structure with the bulk phase of
Ga2O3 remaining unchanged, the surface of Ga2O3 is expected
to be covered with a layer of ZnGa2O4. XPS measurement is a
surface-sensitive technique because the probing depth of XPS
usually varies between 1.5 and 6 nm depending on the kinetic
energy of the photoelectrons.24 Figure
8 displays the
dependence of the surface ratio of Zn atoms to total metal
atoms (Zn and Ga), estimated by XPS measurement (Figure S1),
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The function of the ZnGa2O4 layer on the surface of Ga2O3
was further confirmed by the reaction using bare Ga2O3, pure
phase ZnGa2O4, and the physical mixture of Ga2O3 and
ZnGa2O4 as shown in Table 1. The extremely high formation
rate of H2 and low rate of CO evolution over bare Ga2O3
without Ag loading and Zn modification clarifies that a large
number of active sites exist for the reduction of H+ on the
surface of bare Ga2O3. In comparison, the loading of metallic
Ag particles on bare Ga2O3 did not decrease the H2 production
but enhanced the evolution of CO, indicating that the
deposition of Ag particles created the preferable active sites
for the conversion of CO2 without affecting the reduction
reaction of H+. The obvious suppression of H2 production by
the introduction of Zn species into Ag-loaded Ga2O3 catalyst
proves that the generation of the ZnGa2O4 layer on the surface
of Ga2O3 could erase the active sites for the reduction reaction
of H+. Furthermore, the evolution of CO, H2, and O2 in a
stoichiometric ratio was obtained over the Ag-loaded ZnGa2O4
photocatalyst, and the selectivity for CO evolution over H2 was
similar to that over Ag-loaded Zn-modified Ga2O3 (Table 1).
This result means the Ag-loaded ZnGa2O4 catalyst itself
possesses suitable active sites for the conversion of CO2 and
oxidation of H2O except for the reduction of H+. When ZnGa2O4
was physically mixed with Ga2O3, the evolution of H2 still
dominated because the proton reduction sites on the surface
of Ga2O3 are not covered by ZnGa2O4. Therefore, the ZnGa2O4
layer modified on the surface of Ga2O3 has a characteristic role
to inhibit the production of H2.
Fig. 9 SEM images of Ag-loaded bare Ga2O3 (a) and Ag-loaded Zn-modified Ga2O3
with 0.1 mol% (b), 0.5 mol% (c), 1.0 mol% (d), 3.0 mol% (e), 5.0 mol% (f), and 10.0
mol% (g) of Zn species.
Table 1. The formation rate of gas products and selectivity toward CO
evolution in the photocatalytic conversion of CO2 by H2O over various
photocatalysts[a]
Formation rate (μmol h-1
)
Selectivity toward
CO evolution (%)
Photocatalysts[b]
H2
O2
CO
5.4
111
[c]
Bare Ga2O3
268
280
139
181
2.0
Ag/Ga2O3
28.4
[d]
Ag/ZnGa2O4/Ga2O3
8.9
2.1
60.6
28.9
165
108
50.8
109
92.4
96.0
31.9
[e]
Ag/ZnGa2O4
Ag/(Ga2O3+ZnGa2O4)[f]
233
[a] Reaction conditions: Amount of catalyst, 1.0 g; volume of water, 1.0 L; flow rate
of CO2, 30 mL min−1; concentration of NaHCO3, 0.1 mol L−1; light source, high-
pressure mercury lamp (400 W). [b] Ag cocatalyst was loaded by the
photodeposition method and the loading amount was 1.0 wt%. [c] Ga2O3 without
Ag cocatalyst and Zn species modification. [d] The modification amount of Zn
species was 3.0 mol%. [e] ZnGa2O4 was synthesized by the solid state reaction with
calcination at 1223 K. [f] The physical mixture with the same composition as the
ZnGa2O4/Ga2O3 photocatalyst.
Fig. 10 A proposed mechanism for the photocatalytic conversion of CO2 in and by
H2O over Ag-loaded Ga2O3 (A), Ag-loaded Zn-modified Ga2O3 with low Zn content
(B), and Ag-loaded Zn-modified Ga2O3 with high Zn content (C).
6 | J. Name., 2012, 00, 1-3
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Based on the above described results, it can be proposed a industry. Especially, since it is very difficult to adjust the ratio
functioning mechanism of the modification of Ag-loaded Ga2O3 of H2 by CO (so-called, H2/CO ratio) depending on products of
with ZnGa2O4 layer in the photocatalytic conversion of CO2 by the Fischer–Tropsch technology. In this study, the synthesis
H2O in Figure 10. Undoubtedly, the photocatalytic reaction on gas was produced via the photocatalytic conversion of CO2
Zn-modified Ga2O3 is triggered by the absorption of photons, with H2O as an electron donor over Ag-loaded Zn-modified
leading to the generation of electrons and holes in the bulk of Ga2O3 photocatalysts and the ratio of H2 to CO was finely
photocatalyst. The photoexcited electrons subsequently controlled by the straightforward alteration of Zn addition.
migrate to the surface active sites to participate in the
reduction of CO2 and H+, while the holes oxidize H2O into O2.
Conclusions
The metallic Ag particle deposited on Ga2O3 and Zn-modified
Ga2O3 photocatalysts with the similar particle size and
Stoichiometric amounts of CO, H2, and O2 were evolved in the
photocatalytic conversion of CO2 by H2O as an electron donor
using an Ag-loaded Zn-modified Ga2O3 photocatalyst. The
production of H2 gradually decreased with increasing amounts
of Zn species from 0.1 to 10.0 mol%, whereas the evolution of
CO was almost unchanged. Consequently, the selectivity
toward CO evolution increased to almost 100%.
Characterization using UV-Vis DRS, XRD, XAFS, XPS, BET and
SEM measurements confirmed that a ZnGa2O4 layer was
generated on the surface of Ga2O3 by the addition of Zn
species, and the band structure, crystallinity, surface area,
morphology of Ga2O3, the particle size, dispersion, and
chemical state of Ag cocatalyst were not affected. The ZnGa2O4
layer has a special function to suppress the reduction of H+ in
the photocatalytic conversion of CO2 by H2O over Ag-loaded
Zn-modified Ga2O3 photocatalyst. Since the generation ratio of
H2 to CO can be finely tuned by using the Ag-loaded Zn-
modified Ga2O3 photocatalyst, it is a promising way to produce
synthesis gas through the photocatalytic conversion of CO2 by
H2O.
dispersion functions as
a cocatalyst to capture the
photoexcited electrons for the reduction of CO2. Thus, similar
activity for the evolution of CO over the Ag-loaded Zn-modified
Ga2O3 with different Zn contents (0 mol% to 10 mol%) is
attributable to the same extraction efficiency of electrons from
Ga2O3 and Zn-modified Ga2O3 by the Ag particles. Moreover,
the crystallinity and surface area of Ga2O3 are almost
unaltered after modification with the ZnGa2O4 layer, which
maintains the formation rate of CO in the photocatalytic
conversion of CO2 by H2O.
Due to the existence of a large number of active sites for
the reduction of H+ on the surface of Ga2O3, the evolution of
H2 is the main reaction over Ag-loaded Ga2O3 (Figure 10(A)).
The selectivity toward CO evolution against the H2 production
over Ag-loaded Zn(0.1 mol%)-modified Ga2O3 is still lower than
50%, indicating that tiny amount of Zn species with ZnO
structure is insufficient to block the active sites for the
reduction of H+. The modification of Ga2O3 with Zn species
higher than 0.5 mol% causes the surface reaction between
Zn2+ and Ga2O3 to generate ZnGa2O4 structure, thus the active
sites for the H2 production on the surface of Ga2O3 are
eliminated (Figure 10(B)). The growth of the ZnGa2O4 layer on
the surface of Ga2O3 with the increase in Zn species addition
results in the further decrease of the number of active sites for
H2 production (Figure 10(C)). Moreover, the band gap of Ga2O3
(4.6 eV) is larger than that of ZnGa2O4 (4.2 eV, estimated by
Acknowledgements
This study was partially supported by a Grant-in-Aid for
Scientific Research on Innovative Areas "All Nippon Artificial
Photosynthesis Project for Living Earth" (No. 2406) of the
Ministry of Education, Culture, Sports, Science, and
Technology (MEXT) of Japan, the Precursory Research for
Embryonic Science and Technology (PRESTO), supported by
the Japan Science and Technology Agency (JST), and the
Program for Elements Strategy Initiative for Catalysts &
Batteries (ESICB), commissioned by the MEXT of Japan.
[F(R)*hv]1/2 25
) and the valence band potential of Ga2O3 is 0.8
eV more positive than that of ZnGa2O4 (Figure S6). For this
reason, the conduction band of ZnGa2O4 is located at more
negative potential than that of Ga2O3. The combination of
ZnGa2O4 and Ga2O3 heterostructure could facilitate the
photogenerated electrons transfer from ZnGa2O4 to Ga2O3,22
which diminishes the electron density in the ZnGa2O4 structure
for the reduction of H+. Therefore, the evolution of H2 is
suppressed with the increase of Zn content in Ag-loaded Zn-
modified Ga2O3. In addition, stoichiometric amount of O2
evolved over Ag-loaded Zn-modified Ga2O3 photocatalysts
reveals that the formation of ZnGa2O4 layer on the surface of
Ga2O3 keeps the H2O oxidation sites remaining, because higher
valence band of ZnGa2O4 structure leads to the migration of
holes from Ga2O3 to ZnGa2O4. The decrease of O2 evolution
with the introduction of Zn species is owing to the suppression
of H2 production which consumes the photoexcited electrons
donated from the oxidation of H2O.
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Tuning the selectivity toward CO evolution in the photocatalytic
conversion of CO2 by H2O through the modification of Ag-loaded
Ga2O3 with a ZnGa2O4 layer
Zheng Wang, Kentaro Teramura,* Zeai Huang, Saburo Hosokawa, Yoshihisa Sakata and
Tsunehiro Tanaka*
The formation and growth of ZnGa2O4 layer on the Ag-loaded Zn-modified Ga2O3
photocatalyst effectively controlled the generation ratio of CO to H2 in the
photocatalytic conversion of CO2 by H2O.