Dual Ag/Co cocatalyst synergism for the highly effective photocatalytic conversion of CO2by H2O over Al-SrTiO3

Article abstract of DOI:10.1039/d1sc00206f

Loading Ag and Co dual cocatalysts on Al-doped SrTiO₃(AgCo/Al-SrTiO₃) led to a significantly improved CO-formation rate and extremely high selectivity toward CO evolution (99.8%) using H₂O as an electron donor when irradiated with light at wavelengths above 300 nm. Furthermore, the CO-formation rate over AgCo/Al-SrTiO₃(52.7 μmol h^?1) was a dozen times higher than that over Ag/Al-SrTiO₃(4.7 μmol h^?1). The apparent quantum efficiency for CO evolution over AgCo/Al-SrTiO₃was about 0.03% when photoirradiated at a wavelength at 365 nm, with a CO-evolution selectivity of 98.6% (7.4 μmol h^?1). The Ag and Co cocatalysts were found to function as reduction and oxidation sites for promoting the generation of CO and O₂, respectively, on the Al-SrTiO₃surface.

Full text of DOI:10.1039/d1sc00206f

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Dual Ag/Co cocatalyst synergism for the highly
effective photocatalytic conversion of CO by H O
2
2
Cite this: Chem. Sci., 2021, 12, 4940
over Al-SrTiO3†
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
Shuying Wang, Kentaro Teramura, *ab Takashi Hisatomi,
a
c
cd
Kazunari Domen,
ab
ab
ab
Hiroyuki Asakura,
Saburo Hosokawa
and Tsunehiro Tanaka
*
Loading Ag and Co dual cocatalysts on Al-doped SrTiO
CO-formation rate and extremely high selectivity toward CO evolution (99.8%) using H
donor when irradiated with light at wavelengths above 300 nm. Furthermore, the CO-formation rate
3
3
(AgCo/Al-SrTiO ) led to a significantly improved
2
O as an electron
ꢀ1
over AgCo/Al-SrTiO
3
(52.7 mmol h ) was a dozen times higher than that over Ag/Al-SrTiO
3
(4.7 mmol
Received 12th January 2021
Accepted 24th February 2021
ꢀ1
h
). The apparent quantum efficiency for CO evolution over AgCo/Al-SrTiO was about 0.03% when
3
ꢀ1
photoirradiated at a wavelength at 365 nm, with a CO-evolution selectivity of 98.6% (7.4 mmol h ). The
DOI: 10.1039/d1sc00206f
Ag and Co cocatalysts were found to function as reduction and oxidation sites for promoting the
rsc.li/chemical-science
2 3
generation of CO and O , respectively, on the Al-SrTiO surface.
cocatalysts for the photocatalytic conversion of CO
2
into
Introduction
chemicals such as CO and CH . Furthermore, dual cocatalysts
4
The photo-irradiative conversion of CO2 into chemicals over have been investigated to overcome the shortcomings of single
semiconductor photocatalysts typically includes three steps: (1) cocatalyst-loaded photocatalysts; i.e., to improve CO chemi-
light harvesting, (2) photoexcited electron–hole pair generation, sorption, suppress the back reaction, and enhance the
2
1
6
21
2,23
2
separation, and transfer (charge transfer), and (3) surface consumption of photogenerated holes.
Dual metal–metal
15
24,25
catalytic reactions that include reduction by electrons and alloy cocatalysts (such as Pt–Cu, and Au–Cu ) and metal–
26
1
–3
16
oxidation by holes. An urgent challenge involves preventing metal–oxide dual cocatalysts (such as Pt–MgO, Ni@NiO,
2
1,27
28
the rapid recombination of photogenerated electron–hole pairs Ag@Cr,
2
and Pt@Cu O ) have been loaded onto the surfaces
within single photocatalyst particles, which results in lower of photocatalysts to improve their photocatalytic performance
4
–6
photocatalytic performance of the semiconductor catalyst.
2
for the conversion CO into chemicals when photoirradiated.
Cocatalysts that provide essential functions during photo- Specically, a photocatalytic reaction contains two important
catalytic reactions not only promote the separation of photo- components: (1) photoexcited electrons for the reduction reac-
generated electron–hole pairs, but also reduce the activation tion, and (2) photoexcited holes for the oxidation reaction,
potential and serve as active sites for the photocatalytic evolu- based on charge balance. Normally, attention is only paid to the
1
,2,4,7–14
tion of products.
Ag nanoparticles were reported in 2011 to be good cocatalysts trons, while the H
that exhibit superior selectivities for photocatalytic products, neglected. Improving the activity of the oxidation half-reaction
Photocatalyst surfaces decorated with photocatalytic CO
2
reduction side involving photoexcited elec-
2
O oxidation process has largely been
12
such as CO from CO , when photoirradiated. In addition, should reduce photoexcited electron–hole recombination
many kinds of material, including noble metals (such as Au,
and Pd ), non-noble metals (such as Cu ), and metal of the reduction half reaction as larger numbers of electrons are
oxides (such as MgO, NiO, and Cu
2
12
inside a single photocatalyst, which would enhance the activity
15,16
17
15,18
Pt,
16
19
20
O ) have been studied as transferred for the photocatalytic reaction on the surface of the
2
photocatalyst.
a
The morphological structure of a semiconductor catalysts is
known to clearly affect photocatalytic performance, as photo-
catalytic reactions occur on its surface. Recently, Yu et al. re-
Department of Molecular Engineering, Graduate School of Engineering, Kyoto
University, Kyoto 615-8510, Japan. E-mail: teramura@moleng.kyoto-u.ac.jp;
tanakat@moleng.kyoto-u.ac.jp
b
Elements Strategy Initiative for Catalysts and Batteries, Kyoto University, Kyoto 615- ported that the photocatalytic activity for CO
reduction over
2
8
510, Japan
29
2
anatase TiO depended on the {001} : {101} facet ratio. In
c
Research Initiative for Supra-Materials, Shinshu University, 4-17-1 Wakasato, Nagano
30
particular, the crystal structures of La-doped NaTaO , anatase
3
3
d
80-8553, Japan
3
1,32
11,33,34
35
TiO ,
BiVO4,
18-facet-SrTiO3, and Al-SrTiO3 (ref. 36
2
3
The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
7
and ) have been studied as water-splitting photocatalysts, with
photogenerated electrons and holes reportedly spatially
†
Electronic supplementary information (ESI) available: Experimental details, and
characterizations. See DOI: 10.1039/d1sc00206f
4940 | Chem. Sci., 2021, 12, 4940–4948
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transferred to different exposed facets when photoirradiated, Table 1 Photocatalytic conversion of CO
2
into CO by reduction with
H O over various cocatalyst-loaded photocatalysts prepared by CR
2
which is considered to enhance photocatalytic activity. Li et al.
proposed that cocatalysts on the spatial facets of the BiVO4
photocatalyst not only serve their traditional reaction roles, but
also align the built-in electric eld vectors of the photocatalyst
particles, which maximizes the separation and transfer of
a
method
Product formation
ꢀ1
rate (mmol h
)
Selectivity
toward CO (%) e /h
ꢀ
+
11
Photocatalyst
H
2
O
2
CO
photoexcited electrons and holes. CoO
have recently been loaded on various photocatalysts as oxida-
tion cocatalysts that enhance photocatalytic oxygen-evolution Ag/Al-SrTiO3
x
and other Co species
Al-SrTiO
3
0.35
0.1
41.6
0.2
2.6
19.5
0.08 18.5
4.7
0.1
1.08
0.92
1.07
1.02
0.99
0.77
97.7
0.2
99.8
59.2
95.0
38,39
reaction (OER) activity.
that Al-SrTiO modied with the dual Rh/Cr O cocatalyst and
In particular, Domen et al. reported Co/Al-SrTiO
3
AgCo/Al-SrTiO
AgCo/Al-SrTiO
AgCo/Al-SrTiO
3
_CR
3
_PD
3
_IMP
0.1
14.1
0.2
25.8 52.7
17.5 20.5
3
2 3
CoOOH on its various facets exhibited extremely high photo-
catalytic water-splitting activity without losses due to charge
recombination, with a quantum efficiency of up to 96% at
2.8
4.1
a
Photocatalytic reaction conditions: amount of photocatalyst, 0.5 g;
amount of Ag loaded, 1.7 mol%; amount of Co loaded, 0.85 mol%;
37
wavelengths of 350–360 nm. Furthermore, the photocatalytic
activity for CO reduction has also been signicantly improved CO
volume of the reaction solution (H
2
O), 1.0 L; additive, 0.1 M NaHCO
ow rate, 30 mL min ; light source, 400 W high-pressure Hg
lamp with a Pyrex® jacket (to cutoff light at l < 300 nm).
3
;
ꢀ1
2
2
by simultaneously loading dual cocatalysts (reduction and
oxidation cocatalysts) onto different spatial facets of a photo-
22,40–42
catalyst.
In our previous work, Al-SrTiO fabricated using
3
a ux method and modied with a Ag cocatalyst showed good
efficiency and selectivity towards CO evolution in an aqueous
solution when irradiated with light at wavelengths above
3 3 3 3
images of SrTiO and Al-SrTiO . The SrTiO and Al-SrTiO
particles exhibit irregular polyhedral morphologies, with
numerous nanosteps formed on the spatial surfaces of Al-
43
43,44
SrTiO
3
, as reported previously. The morphologies and crys-
300 nm.
In addition, the Ag cocatalyst on the surface of Al-
talline structures of the prepared samples are consistent with
SrTiO
3
prepared using a chemical reduction (CR) method was
36,43
those in previous reports.
The photocatalytic performance of Al-SrTiO
found to show good photocatalytic performance for the reduc-
tion of CO compared to those prepared by impregnation (IMP)
3
modied with
2
various cocatalysts, including single Ag and Co, and dual Ag and
Co cocatalysts prepared by the CR method are summarized in
Table 1. H , O , and CO were detected by GC as the gaseous
2 2
and photodeposition (PD) methods, because highly dispersed
44–46
metallic Ag nanoparticles were produced.
the effect of Ag and Co dual-cocatalyst loading on Al-SrTiO
AgCo/Al-SrTiO on the photocatalytic reactivity for the
into CO when photoirradiated, with water as
Herein, we report
3
products, and no liquid products were observed in the reaction
solutions by using high performance liquid chromatography
(
3
)
conversion of CO
the reductant.
2
ꢀ
+
(
HPLC). Importantly, the consumed e /h ratios were found to
be close to 1.0 over the SrTiO , Ag/SrTiO , Co/Al-SrTiO , and
3
3
3
AgCo/Al-SrTiO
were successfully loaded on the surface of the Al-SrTiO
tocatalyst. Only very small amounts of gaseous H , O , and CO
were detected over bare Al-SrTiO , and the H -formation rate
3
photocatalysts. Table S1† shows that Ag and Co
3
pho-
Results and discussion
2
2
3 3
Fig. 1(b) and (c) show the XRD patterns of SrTiO and Al-SrTiO ,
3
2
in which most peaks are assigned to the SrTiO phase (Fig. 1(a)), was much higher than the CO-formation rate with non-
3
with only a few small peaks attributable to the Sr Ti O phase. stoichiometric amounts of O produced. The main reductive
3
2
7
2
Moreover, the impurity phase was absent aer doping with Al product was found to be gaseous CO (97.7%) over Ag/SrTiO3,
using the ux method (Fig. 1(c)), with all peaks assigned to the and a stoichiometric amount of O was evolved, which indicates
2
3
Al-SrTiO phase. Furthermore, Fig. 1(d) and (e) show SEM
Fig. 1 XRD patterns of (a) the SrTiO
3 3 3 2 7 3
reference (ICSD no. 23076) and the synthesized (b) SrTiO (: Sr Ti O ) and (c) Al-SrTiO . SEM images of the
synthesized (d) SrTiO and (e) Al-SrTiO .
3
3
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2 3 3
that H O functions as an electron donor in the photocatalytic that Ag(1.7)Co(0.85)/Al-SrTiO and Ag(1.7)Co(1.275)/Al-SrTiO
reaction. The CO-formation rate was signicantly enhanced (4.7 prepared by the CR method exhibit the highest CO-formation
ꢀ
1
ꢀ1
ꢀ1
mmol h ) compared to the undoped catalyst. As mentioned in rates of 52.7 mmol h and 54.6 mmol h , respectively, with
12,13,47
previous reports,
enhances the photocatalytic activity for the reduction of CO
and is selective toward CO evolution. Furthermore, the Al-
SrTiO photocatalyst was also loaded with Co species, as Co has CO
been described to be a good cocatalyst for the photocatalytic aqueous NaHCO
oxidation of water;
Ag functions as a potential cocatalyst that good selectivities toward CO evolution. Ag(1.7)Co(0.85)/Al-
SrTiO is referred to as AgCo/Al-SrTiO _CR hereaer.
To determine apparent quantum efficiencies, photocatalytic
reductions over AgCo/Al-SrTiO _CR were also carried out in
(0.1 M) using various light sources. Fig. 2(a)
-formation rates shows time courses for the evolution of CO, H , and O during
2
3
3
3
2
3
3
3
8,39
while high H
2
- and O
2
2
2
ꢀ
1
(
41.6 and 19.5 mmol h , respectively) were obtained over Co/Al- the photocatalytic conversion of CO2 by H O over AgCo/Al-
SrTiO , and that for CO formation was only 0.1 mmol h , which SrTiO _CR when irradiated at l above 300 nm for 5 h under
2
ꢀ
1
3
3
indicates that overall only water splitting took place over the Co/ typical conditions. Importantly, the same reaction (photo-
Al-SrTiO photocatalyst. Surprisingly, the highest CO-formation catalytic reduction of CO by H O) was also carried out under
rate (52.7 mmol h ), which is 10-times higher than that monochromatic light (l ¼ 365 nm) in NaHCO (0.2 L, 0.1 M), the
observed for Ag/Al-SrTiO , was delivered by the Al-SrTiO pho- results of which are shown in Fig. 2(b); the CO-formation rate
tocatalyst dual-modied with both Ag and Co by the CR method was approximately 7.4 mmol h with good selectivity toward CO
AgCo/Al-SrTiO _CR). Meanwhile, the selectivity for CO evolu- evolution (98.6%). The apparent quantum efficiency (AQE) of
3
2
2
ꢀ1
3
3
3
ꢀ1
(
3
tion was further improved to 99.8%, with only a trace of H2 0.03% was calculated using eqn (1):
evolved over AgCo/Al-SrTiO _CR. To the best of our knowledge,
3
AQE (%) ¼ (number of reacted electrons/number
of incident photons) ꢁ 100
this is the highest selectivity for CO evolution reported for
photocatalytic CO conversion using H O as a reductant under
aqueous conditions at wavelength (l) above 300 nm.
Consequently, we conclude that Co species in the photocatalyst
signicantly enhance photocatalytic performance for CO
(1)
2
2
12,13,21,23,48–51
Photocatalytic reaction conditions: amount of photocatalyst,
a) 0.5 g and (b) 0.1 g; amount of Ag loaded, 1.7 mol%; amount
2
(
conversion because they effectively promote the water-oxidation
half reaction.
of Co loaded, 0.85 mol%; volume of the reaction solution (H
2
O),
3 2
a) 1.0 L and (b) 0.2 L; additive, 0.1 M NaHCO ; CO ow rate; 30
(
The H -, O -, and CO-formation rates during the photo-
2
2
ꢀ
1
mL min
.
catalytic conversion of CO2 by H O over AgCo/Al-SrTiO
2
3
Although this AQE value is much lower than desired, it is
important to note that an AQE was obtained in this photo-
catalytic CO -reduction system without any sacricial reagent
2
prepared by the IMP, CR, and PD methods are also listed in
Table 1. The amounts of Ag and Co loaded into AgCo/Al-SrTiO
3
prepared by these methods were determined by XRF spectros-
copy (Table S1†), which suggested that the Ag and Co cocatalysts
under relatively high wavelength of photoirradiation. The
amounts of Ag and Co in the AgCo/Al-SrTiO _CR photocatalyst
3
had been successfully loaded onto the surface of the Al-SrTiO
prepared using each method.
3
2
were also determined by XRF spectroscopy aer CO had been
photoreduced for 5 h, the results of which are summarized in
Table S1,† which reveals that Ag and Co loadings are only
slightly lower compared to those of the as-prepared sample.
These results suggest that Ag and Co are durable under the
reaction conditions, as their photocatalyst loadings were stable
over the 5 h duration of the photocatalytic reaction.
The selectivity toward CO evolution was very low (59.2%) over
AgCo/Al-SrTiO _PD. In addition, AgCo/Al-SrTiO _IMP exhibited
3
3
ꢀ
1
a very low CO-formation rate of 4.1 mmol h . Undoubtedly, the
highest CO-formation rate was delivered by AgCo/Al-SrTiO
3
prepared by the CR method, with exceptionally good selectivity
toward CO evolution also observed. Compared with Ag/Al-
3 3 3
SrTiO and Co/Al-SrTiO , AgCo/Al-SrTiO prepared by the PD
and CR methods exhibited higher activities for photocatalytic
CO reduction and H O oxidation, which indicates that, apart
2
2
from Co playing an important role in enhancing the water
oxidation half reaction, the Ag and Co species function syner-
gistically. Furthermore, the amounts of the Ag and Co cocata-
lysts loaded by the CR method were also determined by XRF
spectroscopy, the results of which are summarized in Table S1.†
The XRF-determined amount of Ag on the surface of the Al-
SrTiO photocatalyst was found to be similar to the amount of
3
Ag precursor used in the reaction (Table S1†), whereas the
amount of Co was determined to be lower than the amount of
precursor used, particularly at Co precursor levels higher than
Fig. 2 Time courses for the evolution of H
red), and selectivity toward CO evolution (black) for the photocatalytic
conversion of CO by H O over AgCo/Al-SrTiO _CR using various
light sources: (a) 400 W high pressure Hg lamp with a Pyrex® jacket at
mance depends on the Ag and Co loading. These results show l > 300 nm, (b) LED lamp at l ¼ 365 nm.
2 2
(blue), O (green), CO
0
.85%. The effect of the loaded-amount of the dual Ag and Co
cocatalyst on the photocatalytic activity for CO reduction are
shown in Fig. S1–S3,† which reveal that photocatalytic perfor-
(
2
2
2
3
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Fig. 3(a) shows Ag K-edge XANES spectra of Ag/Al-SrTiO
3
and Al-SrTiO
3
surface, as shown in Fig. 4(a), while no obvious
AgCo/Al-SrTiO _CR, and those of Ag foil, Ag O, and Ag PO as nanoparticles are observed in the SEM image of Co/Al-SrTiO3
3
2
3
4
references. The absorption edges of the Ag species in Ag/Al- (Fig. 4(b)), although the XRF data indicate that Co had been
SrTiO3 and AgCo/Al-SrTiO3 prepared by the CR method are successfully loaded on the Al-SrTiO3 surface (Table S1†). In
2
1,23
consistent with that of Ag foil, at 25 528 eV;
Ag K-edge XANES spectra of Ag/Al-SrTiO
SrTiO _CR exhibit features that are similar to that of Ag foil. surface. We believe that the Co species on the Al-SrTiO
These results indicate that the Ag species in these photo- small to be observed by SEM. AgCo/Al-SrTiO _CR exhibited
catalysts are metallic, and the presence of Co does not inuence a similar morphology to that of Ag/Al-SrTiO ; the Al-SrTiO
in addition, the addition, the Co/Al-SrTiO
and AgCo/Al- (Fig. S5†) also show that Co species are present on the Al-SrTiO
are too
3
EDS map and Co 2p XPS spectrum
3
3
3
3
3
3
3
the chemical state of the Ag species. Fig. 3(b) shows the Co K- particles are well covered by nanoparticles around 2–20 nm in
edge XANES spectra of Co/Al-SrTiO and AgCo/Al-SrTiO _CR, size, with nanoparticles larger than 20 nm rarely observed on
3
3
52,53
with those of Co O , CoOOH,
Co (PO ) , and CoO included the photocatalyst surface. Fig. 4(d–f) show SEM images of Ag/Al-
3 4 2
3
4
for reference, which reveals that the Co K-edge XANES spectra of SrTiO
Co/Al-SrTiO and AgCo/Al-SrTiO _CR do not match those of toirradiation, which reveal that Ag metal nanoparticles are only
Co (PO and CoO; the AgCo/Al-SrTiO _CR absorption edge present on smooth Ag/Al-SrTiO {100} facets, with no Ag nano-
was closer to those of CoOOH and Co . Furthermore, the particles present on other facets, whereas Co species are
white-line and peak maximum (7727.6 eV) in the Co K-edge observed on nanostep Co/Al-SrTiO {110} facets; therefore, the
3 3 3
, Co/Al-SrTiO , and AgCo/Al-SrTiO _CR aer 5 h of pho-
3
3
3
4
)
2
3
3
3 4
O
3
XANES spectrum of AgCo/Al-SrTiO _CR are consistent with Ag metal and Co nanoparticles apparently moved to the smooth
3
those of the CoOOH reference, even though the spectral {100} and nanostep {110} facets during photoirradiation,
5
3,54
features are slightly different.
Co K-edge XANES spectra of AgCo/Al-SrTiO
possibly ascribable to highly dispersed Co species on the Al- fashion, are different aer photoirradiation. In addition, the Ag
SrTiO surface, as XANES spectra are reportedly affected by and Co nanoparticle cocatalysts are both observed on every facet
nanoparticle size.
lation of AgCo/Al-SrTiO
that of CoOOH rather than Co O (Fig. S4†), which indicates {110} facets prefer to be decorated with Ag metal nanoparticles
The differences between the respectively. Note that the positions of the Ag and Co species,
3
_CR and CoOOH are which were highly dispersed on the surface in a random
3
55,56
In addition, the Co K-edge EXAFS oscil- of each Al-SrTiO
3
particle in Fig. 4(f) in the case of AgCo/Al-
3
_CR is approximately consistent with SrTiO _CR, which indicates that the smooth {100} and nanostep
3
3
4
that CoOOH exists on the AgCo/Al-SrTiO _CR surface. The and Co nanoparticles, respectively, when irradiated. Moreover,
3
XANES spectra of Co/Al-SrTiO3 and AgCo/Al-SrTiO _CR also the SEM images of the AgCo/Al-SrTiO photocatalysts prepared
3
3
overlap, as shown in Fig. 3(c). The Co absorption edge of Co/Al- by the IMP and PD methods are shown in Fig. S6,† which reveals
SrTiO is slightly shied to lower energy compared to that of that the surface of AgCo/Al-SrTiO _IMP contains several large
AgCo/Al-SrTiO _CR, while the Co K-edge XANES features are cocatalyst particles (Fig. S6(a)†). Numerous Ag nanoparticles
3
3
3
also different. Consequently, the presence of Ag affects the were observed to have aggregated on {100} facets aer 5 h of
chemical structure of the Co species when Ag and Co species are photocatalytic reaction, in addition to the large cocatalyst
simultaneously loaded using the CR method.
particles on the surfaces of the photocatalyst particles. The
Fig. 4 shows SEM images of the Ag/Al-SrTiO , Co/Al-SrTiO , unusual size of the cocatalyst on AgCo/Al-SrTiO _IMP negatively
3
3
3
and AgCo/Al-SrTiO _CR photocatalysts. Ag nanoparticles affects photocatalytic performance. Fig. S6(b)† shows that the
3
approximately 2–25 nm in size are uniformly dispersed on the Ag and Co dual cocatalyst is spatially located on the {100} facets
Fig. 3 (a) Ag K-edge XANES spectra of Ag/Al-SrTiO
3
(black), AgCo/Al-SrTiO
3
_CR (red), Ag foil (blue), Ag
2
O (green), and Ag PO
3
4
(yellow). (b) and
(
(
c) Co K-edge XANES spectra of AgCo/Al-SrTiO
yellow).
3
_CR (red), Co/Al-SrTiO (black), CoOOH (purple), Co
3
3
O
4
(green), Co (PO ) (blue), and (f) CoO
3
4 2
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Fig. 4 SEM images of various photocatalysts: (a, d) Ag/Al-SrTiO
3
prepared using CR method, (b, e) Co/Al-SrTiO
3
prepared using CR method, and
(
3
c, f) AgCo/Al-SrTiO _CR; (a–c) show fresh samples and (d–f) show samples after 5 h of photoirradiation.
1
1,34
and {110} facets when the PD method was used. In addition, the PbO ) are selectively loaded on various BiVO
and 18-facet-
2
4
35
Ag nanoparticles on the {100} facets are much larger than those SrTiO3 facets when irradiated under aqueous conditions. In
obtained using the CR method. Thus, the main reason for the addition, the Rh/Cr hydrogen-evolution reaction (HER) cocata-
poor selectivity of AgCo/Al-SrTiO
3
_PD is likely to be the over- lyst and the CoOOH OER cocatalyst were also reported to be
sized Ag nanoparticles on its surface, as it is known that the Ag spatially photodeposited on the {100} and {110} facets of Al-
3
7
cocatalyst plays an important role during the photocatalytic SrTiO
3
, respectively. Selective photodeposition was proposed
11,34
35
37
reduction of CO
2
to CO. BiVO
4
,
3 3
18-facet-SrTiO , Al-SrTiO ,
to be due to the charge-rectication effect inside individual
and KTaO3, which have exposed anisotropic facets, were also semiconductor photocatalyst particles.
40
37,57
Moreover, an aniso-
reported to exhibit similar properties. Li et al. reported that tropically deposited cocatalyst was reported to play a positive
metals (Ag, Pt, and Au) and metal oxides (CoO , MnO , and charge-separation and transfer role inside individual
x
2
3
Fig. 5 TEM images of AgCo/Al-SrTiO _CR photocatalysts: (a) before reaction and (b–d) after 5 h of photocatalysis. (c) Ag nanoparticles on
a smooth Al-SrTiO facet and (d) enlarged region showing AgCo/Al-SrTiO
3
3
_CR nanostep facet.
4944 | Chem. Sci., 2021, 12, 4940–4948
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photocatalyst particles by aligning the electric elds built in the
cocatalyst-loaded photocatalyst. Therefore, Ag and Co species investigated depositing various metals, such as Ag, Pt, Au, and
3
To further study the anisotropic properties of Al-SrTiO , we
11
possibly appear on different Al-SrTiO3 facets aer photo- metal oxides, such as MnO , PbO , and Co O , on the Al-SrTiO
3
2
2
3
4
irradiation because the {100} and {110} facets of Al-SrTiO
surface by the PD method. Interestingly, metal nanoparticles
(i.e., Ag, Pt, and Au), which were observed by EDS (Fig. S7†),
3
2
9,33,37
exhibit different band structures and band-edge positions.
The TEM images of AgCo/Al-SrTiO
and Co cocatalysts were deposited on the Al-SrTiO
shown in Fig. 5, with nanoparticles around 2–20 nm in size; aqueous solutions of AgNO
these nanoparticles exhibit a lattice distance of 0.242 nm, which to Ag, Pt, and Au nanoparticles on the {100} facets by H
3
_CR reveal that the Ag were spatially deposited on the {100} facets of Al-SrTiO
surface, as shown in Fig. 7(a)–(c), which suggests that the metal cations in
, H PtCl , and HAuCl are reduced
3
, as
3
3
2
6
4
2
O as the
58,59
is attributable to the Ag {111} facet.
Ag nanoparticles are electron donor, as shown by the following equations:
aggregated on the {100} facets of single photocatalyst particles
and much larger particles were observed aer 5 h of photo-
irradiation, with the largest around 20 nm in size. In addition,
Fig. 5(b) and (d) reveal the appearance of numerous small
nanoparticles on the {110} facets of the photocatalyst. As
mentioned earlier, the XANES spectrum (Fig. 3) suggests that
+
ꢀ
Ag + e / Ag
(2)
(3)
(4)
(5)
3
+
ꢀ
Au + 3e / Au
4
+
ꢀ
Pt + 4e / Pt
highly dispersed CoOOH was generated on the Al-SrTiO
3
+
+ 4H +4e
ꢀ
2
H
2
O / 2O
2
surface; however, there are no clear CoOOH fringe patterns
visible in Fig. 5(d), although the fringe pattern of the {110} facet
of metallic Ag is observed, which indicates that the main Co
where eqn (2)–(4) show the reductions of metal ions and eqn (5)
shows the oxidation of water. Meanwhile, Fig. 7(d)–(f) reveal
that metal oxide nanoparticles (i.e., MnO , PbO , and Co O ) are
spatially located on the {110} facets when NaIO
hole donor, according to eqn (6)–(9):
species in AgCo/Al-SrTiO
The UV-Vis DR spectra of the Al-SrTiO
SrTiO , and AgCo/Al-SrTiO _CR cocatalysts prepared by the CR
method are shown in Fig. 6. Al-SrTiO exhibits an absorption
edge at approximately 390 nm, which is consistent with that
3
_CR is amorphous CoOOH.
2
2
3
3 4
3
, Ag/Al-SrTiO
3
, Co/Al-
was used as the
35,62
3
3
3
2
+
+
Mn + 2h + 2H
+
2
O / MnO
2
+ 4H
(6)
36,43
reported previously.
However, broad absorption bands,
2
+
+
+
3
Co + 3h + 4H O / Co O + 8H
(7)
which are assignable to surface plasmon resonance (SPR), are
evident in the spectra of Ag/Al-SrTiO and AgCo/Al-SrTiO _CR,
2
3
4
3
3
2
+
+
Pb + 2h + 2H
+
2
O / PbO
2
+ 4H
(8)
(9)
3
but not Co/Al-SrTiO ; these characteristic bands correspond to
60
Ag nanoparticles. In addition, spectra f and g in Fig. 6 reveal
that that the SPR bands are slightly redshied aer use, which
indicates that much larger nanoparticles are generated during
photocatalysis. We conclude that cocatalyst nanoparticles
ꢀ
ꢀ
+
ꢀ
IO3 + 4e + 6H / I + 3H O
2
where eqn (5)–(7) show metal-ion oxidation and eqn (8) corre-
sponds to the reduction of iodate.
The abovementioned results suggest that photogenerated
electrons and holes selectively transfer to different facets of the
photocatalyst because the electric elds within single photo-
migrate and aggregate on the Al-SrTiO surface during photo-
3
irradiation, as shown in the SEM and TEM images (Fig. 4 and
61
5).
11,37
catalyst particles are aligned;
therefore, reductive and
oxidative sites exist on different Al-SrTiO3 facets. Table S2†
summarizes the photocatalytic activities of various single-
cocatalyst- loaded Al-SrTiO
3
samples prepared by the PD
method, which reveals that only the Ag metal nanoparticle
cocatalyst exhibited good selectivity toward CO evolution
(
92.2%) by CO
2
reduction, even though the CO-formation rate
ꢀ1
was only 3.2 mmol h . The Pt-, Au-, Co O -, MnO -, and PbO -
3
4
2
2
loaded Al/SrTiO3 only split water, with poor CO-evolution
selectivities.
A possible mechanism for the photoreduction of CO
CO over AgCo/Al-SrTiO _CR using H O as the reductant is
depicted in Scheme 1. As mentioned above, different cocatalyst
2
into
3
2
+
3+
are spatially deposited on anisotropic facets, with Ag , Au , and
4
+
2+
Pt reduced and loaded onto smooth {100} facets, and Mn ,
2
+
2+
Co , and Pb oxidized and loaded on {110} facets. We propose
that photogenerated electrons and holes are transferred to
different Al-SrTiO3 facets; therefore, reduction and oxidation
35,37
Fig. 6 UV-Vis DR spectra: (a) Al-SrTiO
Al-SrTiO , and (d, g) AgCo/Al-SrTiO _CR. (b–d) As-prepared samples
and (e–g) samples after 5 h of photocatalysis.
3
, (b, e) Co/Al-SrTiO
3
, (c, f) Ag/ occur at different facets.
Even though the Ag and Co species
_CR, these
3
3
were dispersed well on the surface of AgCo/Al-SrTiO
3
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3 3 4 2
Fig. 7 SEM images of various of cocatalyst-loaded Al-SrTiO samples prepared by the PD method: (a) Ag, (b) Pt, (c) Au, (d) Co O , (e) MnO , and
(
2
f) PbO .
Scheme 1 Plausible photoirradiation mechanisms: (a) cocatalyst loading by the PD method and (b) the photocatalytic conversion of CO
CO over Al-SrTiO modified with Ag and Co by the CR method using water as the reductant.
2
into
3
Ag and Co were redeposited onto different crystal facets of the 300 nm, which is ten-times higher than that observed for Ag/Al-
ꢀ
1
ꢀ1
Al-SrTiO
irradiation as shown in Fig. 4 and 5. Moreover, based on these evolved when irradiated with LED light (365 nm), with an
results, CO is photocatalytically converted into CO at Ag apparent quantum efficiency of 0.03%. In addition, this study
cocatalyst particles on the {100} facets of Al-SrTiO , whereas Co demonstrated that the reductive and oxidative sites are
3 3
during the photocatalytic reaction under photo- SrTiO (4.7 mmol h ). Furthermore, 7.4 mmol h of CO gas was
2
3
cocatalyst particles oxidize water on the {110} facets. Photo- distributed on the {100} and {110} facets of Al-SrTiO , respec-
3
catalytic activity is signicantly enhanced aer dual cocatalyst tively. Therefore, photocatalytic CO2 reduction and water
loading because photogenerated electrons and holes move to oxidation occur separately on different Al-SrTiO facets. Syner-
3
different facets and are quickly captured by the Ag and Co gism between the Ag and Co dual cocatalysts effectively
cocatalysts, respectively.
enhances the photocatalytic conversion of CO
with H O as the electron donor.
2 3
over Al-SrTiO
2
Conclusions
Conflicts of interest
Loading Ag and Co onto Al-SrTiO
activity for the photocatalytic conversion of CO
3
signicantly improved its
by H O (as the
The authors declare no competing nancial interest.
2
2
electron donor), with extremely high selectivity toward CO
evolution (99.8%), in which Ag and Co enable CO reduction
2
Acknowledgements
and H O oxidation on the Al-SrTiO surface, respectively. A CO-
2
3
ꢀ
1
evolution rate of up to 52.7 mmol h was observed over AgCo/Al- This research was partially supported by the Program for
SrTiO _CR when irradiated with light at wavelengths above Elements Strategy Initiative for Catalysts and Batteries,
3
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commissioned by the Ministry of Education, Culture, Sports, 26 C.-W. Tsai, H. M. Chen, R.-S. Liu, K. Asakura and T.-S. Chan,
Science, and Technology (MEXT) of Japan. This work was also J. Phys. Chem. C, 2011, 115, 10180–10186.
supported by JSPS KAKENHI grant number 18H01788. Shuying 27 R. Pang, K. Teramura, H. Asakura, S. Hosokawa and
Wang thanks the State Scholarship of the China Scholarship T. Tanaka, ACS Sustainable Chem. Eng., 2018, 7, 2083–2090.
Council, which is affiliated with the Ministry of Education of the 28 Q. Zhai, S. Xie, W. Fan, Q. Zhang, Y. Wang, W. Deng and
People's Republic of China. Takashi Hisatomi thanks the Japan Y. Wang, Angew. Chem., Int. Ed., 2013, 52, 5776–5779.
Society for the Promotion of Science (JSPS) for a Grant-in-Aid for 29 J. Yu, J. Low, W. Xiao, P. Zhou and M. Jaroniec, J. Am. Chem.
Scientic Research on Innovative Areas (grant no. 18H05156).
Soc., 2014, 136, 8839–8842.
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0 H. Kato, K. Asakura and A. Kudo, J. Am. Chem. Soc., 2003,
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