Applied Catalysis A: General 504 (2015) 238–247

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Applied Catalysis A: Generaljournal homepage: www.elsevier.com/locate/apcata

Tailoring assemblies of plasmonic silver/gold and zinc–gallium
layered double hydroxides for photocatalytic conversion of carbon
dioxide using UV–visible light
Shogo Kawamura a , Magda C. Puscasu b , Yusuke Yoshida a , Yasuo Izumi a,∗ ,Gabriela Carja b,∗∗a

Department of Chemistry, Graduate School of Science, Chiba University, Yayoi 1-33, Inage-ku, Chiba 263-8522, Japan
Department of Chemical Engineering, Faculty of Chemical Engineering and Environmental Protection, Technical University, “Gh. Asachi” of Iasi,
Bd. Mangeron No. 71, Iasi 700554, Romaniab

a r t i c l e

i n f oArticle history:
Received 17 September 2014
Received in revised form
19 December 2014
Accepted 22 December 2014
Available online 2 January 2015
Keywords:
CO2 photoreduction
Layered double hydroxide
SilverGold
Surface plasmon resonancea b s t r a c t
In the search for novel efﬁcient photocatalysts for the conversion of CO2 into fuels, plasmonic photocatalysts based on the self-assemblies of silver or gold nanoparticles with [Zn3 Ga(OH)8 ]2 CO3 ·mH2 O layered
double hydroxide (Zn3 Ga|CO3 LDH) were prepared and tested for the photoreduction of CO2 by H2 underirradiation with UV–visible light. Ag and Au nanoparticles were obtained directly on the LDHs via theion-exchange method or the reconstruction method of the LDHs. The catalysts exhibited intense surface
plasmon resonance (SPR) effect at 411 and 555 nm attributable to Ag and Au nanoparticles, respectively.
The rate of CO2 photoreduction on Ag/Zn3 Ga|CO3 increased by a factor of 1.69 than that of Zn3 Ga|CO3
while the methanol selectivity also increased from 39 to 54 mol%. On Au/Zn3 Ga|CO3 , the reduction rate of
CO2 was 1.78 times higher than on Zn3 Ga|CO3 LDH whereas the methanol selectivity decreased from 39
to 13 mol%. Electron microscopy and UV–visible and X-ray spectroscopy detected particular interactionsof the cationic layers of Zn3 Ga|CO3 with Ag and Au nanoparticles. Results show that for Ag/Zn3 Ga|CO3 catalysts, CO2 photoreduction by H2 under visible light was promoted by the SPR effect of Ag nanoparticles
while for Au/Zn3 Ga|CO3 catalysts Au nanoparticles might act as electron-trapping active sites.
© 2014 Elsevier B.V. All rights reserved.

1. Introduction
The increasing concentration of CO2 in the atmosphere, as aresult of the combustion of the carbon-based fuels, is predicted
to result in unacceptable changes in the Earth’s climate [1,2]. The
photoreduction of CO2 to fuels using light energy can contribute
simultaneously to reduction of the major greenhouse gas and thedevelopment of sustainable energy. A key technological target toreach efﬁcient photoconversion of CO2 to fuels (i.e. artiﬁcial photosynthesis) is to develop an efﬁcient and robust photocatalyst [3].
An important factor limiting the conversion efﬁciency of almost
every active photocatalyst is the high rate of charge carrier recombination. Recently, Ingram and Linic [4] demonstrated that therecombination problem was signiﬁcantly alleviated by assemblingplasmonic nanoparticles and semiconductor supports. Moreover, it∗ Corresponding author. Tel.: +81 43 290 3696; fax: +81 43 290 2783.
∗∗ Corresponding author. Tel.: +40 232 201231; fax: +40 232 201231.
E-mail addresses: yizumi@faculty.chiba-u.jp (Y. Izumi), carja@uaic.ro (G. Carja).
http://dx.doi.org/10.1016/j.apcata.2014.12.042
0926-860X/© 2014 Elsevier B.V. All rights reserved.

has been recently reported that in plasmonic nanoparticles/support
co-catalytic systems the photo-responsive features of the metal
nanoparticles that manifest the surface plasmon resonance (SPR)
effect are able to tune and assist the photocatalytic properties ofthe support. The particular features of the plasmonic nanostructures at the support interface might reduce the charge carrier
recombination rate, and thereby enhance the visible-light-induced
photocatalytic activities.
Layered double hydroxides (LDHs) or hydrotalcite-like materials are layered porous matrices belonging to the class of anionic clay
(cationic layers intercalating anions) with many actual and potential applications in catalysis [5]. Recently, García group introduced
a novel concept of Ti, Ce, or Cr-doped semiconductors based on Zncontaining LDHs [6]. LDHs can be deﬁned by a versatile elemental
composition and the ratio, and have basic properties and a highadsorption capacity for CO2 [7]. LDH photocatalysts comprising Zn
and Ga have been reported, in our previous work, to convert CO2
into methanol or CO using H2 [8–10]. The photocatalytic reductionof CO2 using water and LDHs [11] and the combination of photooxidation catalyst and LDHs were also reported [12]. However, theS. Kawamura et al. / Applied Catalysis A: General 504 (2015) 238–247

band gap of these LDHs, e.g. 5.6 eV for [Zn3 Ga(OH)8 ]2 CO3 ·mH2 O,
corresponded to ultraviolet (UV) region and accordingly these LDHs
were primarily responsive to UV light [10]. The relatively wide band
gap was advantageous to set the conduction band (CB) minimum
enough negative for these LDHs compared to the reduction reaction
potentials of239

versus standard hydrogen electrode (SHE) [10]. It is essential to utilize visible light, 97% of the spectrum of solar radiation [13], while
utilizing the enough negative potential of CB minimum.
In this work, [Zn3 Ga(OH)8 ]2 CO3 ·mH2 O was combined with asensitizer of silver or gold nanoparticles that are responsive tovisible light. For Ag nanoparticles dispersed in liquid, the corresponded SPR peak progressively shifted from 405 nm to 510 nm
as the Ag particles grows from 10 nm to 100 nm [14]. Further,
the SPR peak progressively shifted from 515 nm to 572 nm as the
Au nanoparticles grows from 5 nm to 100 nm [15]. The hybridization of SPR of 100 nm of Ag cube and silicon nitride substrateat the interface was visualized by electron energy loss spectroscopy in a monochromated scanning transmission electron
microscopy (TEM) [16]. Plasmon-induced excited electron injection from nanoparticles, e.g. Ag and Au, to semiconductors, e.g. TiO2 ,exceeding the Schottky barrier, has been reported [17–20]. Mean
7 nm of Ag nanoparticles were combined with a LDH material of[Zn2 Al(OH)6 ]2 CO3 ·mH2 O by the reaction of aqueous solution of Ag+
salt with the LDH powder freshly calcined at 823 K [21] while mean
2.9 and 3.4 nm of Au nanoparticles were combined with a LDH of[Zn2 Al(OH)6 ]2 CO3 ·mH2 O and [Zn2 Al0.7 Ce0.3 (OH)6 ]2 CO3 ·mH2 O by
the reaction of aqueous solution of Au3+ with the LDH calcined at
823 K [22]. Based on these procedures, the anionic clay was transformed into metal oxides upon calcination at 823 K. The layered
structure LDH was then reconstructed when the calcined LDH at
823 K was introduced into the aqueous solutions of silver or gold
salts [21,22]. In this work, assemblies of Ag and Au nanoparticles and [Zn3 Ga(OH)8 ]2 CO3 ·mH2 O were obtained via ion-exchange
method at 290 K or via the structural reconstruction of the LDHs
in the speciﬁc aqueous solutions of Ag or Au. It is noteworthy
that Ag and Au nanoparticles were obtained directly on the LDH
platelets and no organic products were used during the preparationof Ag/LDH and Au/LDH assemblies via the reconstruction method.
We present here the performances of Ag/[Zn3 Ga(OH)8 ]2 CO3 ·mH2 O
and Au/[Zn3 Ga(OH)8 ]2 CO3 ·mH2 O for the photoconversion of CO2
using H2 and UV and visible light.

These yellow samples were denoted as Ag/Zn3 Ga|CO3 -IE353-15
and Ag/Zn3 Ga|CO3 -IE373-15, respectively.
The sample denoted as Ag/Zn3 Ga|CO3 -IE373-180 was obtained
via a similar procedure to that described for Ag/Zn3 Ga|CO3 -IE37315, however, in this case the time of reﬂux was extended up to 3 h.
A gray precipitate was obtained.
Next, Ag nanoparticles were obtained and organized with
Zn3 Ga|CO3 using the LDH reconstruction method [21]. 1.2 g of
Zn3 Ga|CO3 powder was calcined in an oven at 773 K for 8 h. Afterthe calcination, hot powder derived from Zn3 Ga|CO3 was directly
added into 200 mL aqueous solution of Ag(I) acetate (>99.9%, Alfa
Aesar; 0.10 g) stirring at a rate of 900 rpm. The pH of the solution was adjusted to 8.0 by the addition of NaOH aqueous solution(0.10 M). The reaction mixture was stirred at the rate of 900 rpm
and 290 K for 20 min and then at the rate of 150 rpm and 313 K for
5 h. Then, the precipitate was centrifuged at the rate of 10,000 rpm
and dried in an oven at 353 K for 1 h. The obtained light brownsample was denoted as Ag/Zn3 Ga|CO3 -reconst.
Au/Zn3 Ga|CO3 was obtained by the ion exchange method followed by the liquid phase reduction according to the similar
procedure reported in Ref. [24]. 0.022 g of hydrochloroauric acidtetrahydrate (>99.9%, Alfa Aesar) was dissolved in 10 mL of deionized water. 0.60 g of Zn3 Ga|CO3 was added to the aqueous solution
and the mixture was stirred at 290 K for 12 h. The precipitate wasﬁltered using a JGWP04700 ﬁlter, washed with deionized water(total 250 mL), and dried. Obtained powder was added to 5 mL ofdehydrated toluene (>99.5%, Wako Pure Chemical) and 0.0063 g
of NaBH4 (>95%, Wako Pure Chemical) was added to the suspension. After stirring for 10 min, 1.5 mL of ethanol was added and the
mixture was stirred at 290 K for 6 h. The obtained purple precipitate was collected by ﬁltration using a JGWP04700 ﬁlter, washedwith deionized water and ethanol (total 250 mL for each), anddried under vacuum at 290 K for 24 h. This sample was denoted
as Au/Zn3 Ga|CO3 -IE.
Au/Zn3 Ga|CO3 -reconst: in this case, the assembly of Au
nanoparticles with Zn3 Ga|CO3 was obtained using the structural
reconstruction of LDH in the aqueous solutions containing Au3+[22]. Hence, 1.20 g Zn3 Ga|CO3 powder was calcined in an oven at
773 K for 8 h. The resulted hot calcined powder was directly added
into 200 mL aqueous solution of Au(III) acetate (>99.9%, Alfa Aesar;
0.10 g) stirring at a rate of 900 rpm. The pH of the solution was
adjusted to 8.0 by the addition of NaOH aqueous solution (0.1 M).
The reaction mixture was stirred at the rate of 900 rpm and 290 K
for 20 min and then at the rate of 150 rpm and 313 K for 5 h. Then,the precipitate was centrifuged at the rate of 10,000 rpm and driedin an oven at 353 K for 1 h. The color of this catalyst was light
purple.

2. Experimental

2.2. Characterization

2.1. Sample synthesis

Optical spectroscopic measurements were performed using a
UV–visible spectrophotometer (Model V-650, JASCO) using D2
and halogen lamps for wavelengths between 200 and 340 nm
and between 340 and 800 nm, respectively. An integrating sphere(Model ISV-469, JASCO) was used for the diffuse reﬂectance (DR)measurements. The samples were set in contact with the quartz
window glass in a gas-tight DR cell. The measurements were performed at 290 K within the wavelength range 200–800 nm using
70 mg of sample. DR spectra were converted to absorption spectra on the basis of the Kubelka–Munk equation [9,10]. The bandgap(Eg ) value was evaluated on the basis of either simple extrapolationof the absorption edge or the ﬁt to the Davis–Mott equation,

CO2 + 2H+ + 2e− → CO + H2 O (−0.11 V)

(1)

and
CO2 + 6H+ + 6e− → CH3 OH + H2 O (−0.32 V)(2)

LDH compound of [Zn3 Ga(OH)8 ]2 CO3 ·mH2 O was synthesizedusing a reported procedure from metal nitrates, Na2 CO3 , and NaOH
in aqueous solutions controlled at pH 8 [10]. This compound isabbreviated as Zn3 Ga|CO3 .
For the ion exchange method, we followed the procedure
presented in Ref. [23]. Firstly, 0.50 g of Zn3 Ga|CO3 powder was
immersed in an aqueous solution of AgNO3 (>99.8%, Wako Pure
Chemical; 0.050 M, 10 mL) in a ﬂask and reﬂuxed at 353 or
373 K for 15 min, magnetically stirred at 900 rotations per minute(rpm). The yellow precipitates that were obtained were ﬁlteredusing a polytetraﬂuoroethene-based membrane ﬁlter (Omnipore
JGWP04700, Millipore) with a pore size of 0.2 ␮m and washed wellwith deionized water (<0.06 ␮S cm−1 ; total 250 mL). The precipitates that were obtained were dried in oven at 333 K for 24 h.

˛ × h ∝ (h − Eg )n(3)

in which ˛, h, and  are the absorption coefﬁcient, Planck’s constant,and the frequency of light, respectively, and n is 1/2, 3/2, 2, and240

S. Kawamura et al. / Applied Catalysis A: General 504 (2015) 238–247

Fig. 1. UV–visible absorption spectrum for as-synthesized samples (A) of Zn3 Ga|CO3 (a), Ag/Zn3 Ga|CO3 -IE353-15 (b), Ag/Zn3 Ga|CO3 -IE373-15 (c), Ag/Zn3 Ga|CO3 -IE373180 (d), Ag/Zn3 Ga|CO3 -reconst (e), Au/Zn3 Ga|CO3 -IE (f), and Au/Zn3 Ga|CO3 -reconst (g) and samples after photocatalytic tests for 5 h (B) of Ag/Zn3 Ga|CO3 -IE373-15 (c),Ag/Zn3 Ga|CO3 -reconst (e), Au/Zn3 Ga|CO3 -IE (f), and Au/Zn3 Ga|CO3 -reconst (g).

3 for allowed direct, forbidden direct, allowed indirect, and forbidden indirect electronic transitions, respectively [25]. The value of n
was also estimated by the ﬁt to Eq. (4).

ln(˛ × h) ∝ n ln(h − Eg )(4)

X-ray diffraction (XRD) pattern was observed using a D8
ADVANCE diffractometer (Bruker) at the Center for Analytical
Instrumentation, Chiba University, at a Bragg angle of 2 B = 3–60◦
with a scan step of 0.01◦ and a scan rate of 5 s per step. The measurements were performed at 40 kV and 40 mA using Cu K␣ emission
and a nickel ﬁlter.
Scanning electron microscopy (SEM) measurements were performed using JEOL Model JSM6510A at the Center for Analytical
Instrumentation. The samples were mounted on a conducting carbon tape and coated with gold. The electron accelerating voltage
was 15 kV and the magniﬁcation was between 20,000 and 50,000
times.
High-resolution (HR)-TEM images were observed using transmission electron microscope (Model H-7650, Hitachi) operating at
100 kV. Tungsten ﬁlament was used in the electron gun and thesamples were mounted on a conducting carbon with Cu grid mesh(150 mesh per inch) and the magniﬁcation was between 60,000
and 200,000 times.
Ag K-edge and Au L3 -edge X-ray absorption ﬁne structure(XAFS) spectra were measured at 290 K in transmission mode in
the Photon Factory Advanced Ring at the High Energy Accelerator
Research Organization (Tsukuba) on beamline NW10A. The storage
ring energy was 6.5 GeV and the ring current was 46.6–36.1 mA.
A Si (3 1 1) double-crystal monochromator and platinum-coatedfocusing cylindrical mirror were inserted into the X-ray beam path.
The X-ray intensity was maintained at 65% of the maximum ﬂux
using a piezo translator set to the crystal. The slit opening size was
1 mm (vertical) × 2 mm (horizontal) in front of the ionization chamber. The Ag K and Au L3 -edge absorption energy was calibrated to25,516.5 eV and 11,921.2 eV for the spectrum of Ag and Au metal
foil, respectively [26].
The XAFS data were analyzed using an XDAP package [27]. The
pre-edge background was approximated by a modiﬁed Victoreen
function (C2 /E2 ) + (C1 /E) + C0 . The background of the post-edge
oscillation was approximated by a smoothing spline function andcalculated by an equation for the number of data points, where k is
the wavenumber of photoelectrons.



Data Points

i=1(xi − BGi )2
exp(−0.075ki2 )≤ smoothing factor(5)

Multiple-shell curve-ﬁt analyses were performed for the
Fourier-ﬁltered k3 -weighted extended XAFS (EXAFS) data in k- andR-space using empirical amplitude extracted from the EXAFS data
for Ag metal foil, Ag2 O powder, and Au metal foil [28]. The interatomic distance (R) and its associated coordination number (N) for
the Ag–Ag, Ag–O, and Au–Au pairs were set to 0.288 9 nm with
the N value of 12 [29], 0.204 4 nm with the N value of 2 [30], and
0.288 4 nm with the N value of 12 [31], respectively. The many-body
reduction factor S0 2 was assumed to be equal for both the sample
and the reference.
2.3. Photocatalytic conversion tests for CO2
As-synthesized and preheated samples of LDHs were tested forthe photocatalytic conversion of CO2 [9,10]. The tests were conducted in a closed circulating system (171 mL) equipped with aphotoreaction quartz cell that had a ﬂat bottom (23.8 cm2 ). 100 mgof the LDH catalyst was uniformly spread in the photoreaction celland was evacuated by rotary and diffusion pumps (10−6 Pa) at 290 K
for 2 h until the desorbed gas was detected by an online gas chromatograph (GC). 2.3 kPa of CO2 (0.177 mmol) and 21.7 kPa of H2(1.67 mmol) were introduced to both intact and pretreated LDH
photocatalysts and were allowed to circulate for 30 min in contact
with the catalyst to attain sorption equilibrium before irradiation.
The photocatalyst was then irradiated with UV–visible lightfrom the 500-W xenon arc lamp (Ushio, Model UI-502Q) fromdownward through the ﬂat bottom of the quartz reactor for 5 h. The
distance between the bottom of the reactor and the lamp house exit
window was set to 20 mm. The light intensity was 42 mW cm−2 atthe center of the sample cell and 28 mW cm−2 at the periphery of
the bottom plate of the sample cell. The temperature was within
the range 305–313 K at the catalyst position during the illuminationfor 5 h. As comparison, the photocatalyst was irradiated with visible
light using a UV-cut ﬁlter that passes light () >420 nm (L42, Kenko)
between the light exit of UI-502Q and photocatalyst. Products andreactants were analyzed using packed columns of molecular sieve
13X-S and polyethylene glycol (PEG-6000) supported on Flusin P

S. Kawamura et al. / Applied Catalysis A: General 504 (2015) 238–247

241

Table 1
Physicochemical characterization of Zn3 Ga|CO3 LDHs ensembled with Ag or Au nanoparticles.
Photocatalyst(A) As fresh
Zn3 Ga|CO3
Ag/Zn3 Ga|CO3 -IE353-15
Ag/Zn3 Ga|CO3 -IE373-15
Ag/Zn3 Ga|CO3 -IE373-180
Ag/Zn3 Ga|CO3 -reconsta
Au/Zn3 Ga|CO3 -IE
Au/Zn3 Ga|CO3 -reconsta

Color

White
Yellow
Yellow
Gray
Light brown
PurpleLight purple

Plasmon peak (nm)

–
411
411
(ﬂat)
(ﬂat)
545
555

(B) After photocatalytic test in CO2 (2.3 kPa) + H2 (21.7 kPa) for 5 h
Zn3 Ga|CO3
White–
Ag/Zn3 Ga|CO3 -IE353-15
Yellow
411
Ag/Zn3 Ga|CO3 -IE373-15
Yellow
411
Ag/Zn3 Ga|CO3 -IE373-180
Gray(wide, ﬂat)
Ag/Zn3 Ga|CO3 -reconsta
Gray(wide, ﬂat)
Au/Zn3 Ga|CO3 -IE
545
PurpleAu/Zn3 Ga|CO3 -reconsta
Purple555

Ag/Au particle size (nm) Eg (eV)n [Eq. (4)]Interlattice distance (nm)5.4 (3/2)
5.4 (3/2)
5.3 (3/2)
5.4 (3/2)
3.1 (1/2)
5.5 (3/2)
3.1 (3/2, 2)

1.07
1.07
1.04
1.22
0.71
1.07
1.94

0.753
0.754
0.754
0.754
0.754
0.752
0.754

3.1 (1/2)
5.0 (3/2)
3.0 (2)

0.82
0.94
2.01

TEMb

EXAFS

Extra-polated

Eq. (3) (n)––5.4–27

1.6
3.7

5.6
5.6
5.5
5.5
3.1
5.1
3.0
5.6
5.6
5.5
5.5
3.1
5.4
3.0

3.6
3.2
2.6

1.9

–

–

2.0
1.7

a

Preheated at 373 K for 30 min. The layered structure of all the samples in this study was stable at this condition.
b
Most frequent particle size in histogram for HR-TEM images.

(GL Sciences) set in the online GC equipped with a thermal conductivity detector (Shimadzu, Model GC-8A).
3. Results
3.1. Characterization of the Ag/LDH and Au/LDH assemblies
DR UV–visible spectra for as-synthesized Ag/LDHs and Au/LDHs
composites are summarized in Fig. 1A. The absorption edge wasin UV light region for all the samples. The absorption edge was
extrapolated to give the Eg value of 5.6–5.1 eV for Zn3 Ga|CO3 ,Ag/Zn3 Ga|CO3 -IE, and Au/Zn3 Ga|CO3 -IE, and the Eg value of
3.1–3.0 eV for Zn3 Ga|CO3 reconstructed with Ag or Au (Table 1A).
The former values were consistent with previous study for the LDHs[10], but the latter values should be affected by the impurity metaloxide phase(s) (see the XRD section below). If the spectra were ﬁtted to Eq. (3), similar Eg values were given when the n value was
3/2 or 1/2 except for Au/Zn3 Ga|CO3 -reconst (Table 1A). Thus, theelectronic transition could be a direct process from O 2p to metal
4s or 4p of Zn, Ga from Zn3 Ga|CO3 layers or impurity metal oxides
except for Au/Zn3 Ga|CO3 -reconst.
In fact, when the data for the as-synthesized samples wereﬁt to Eq. (4) using the Eg values obtained by the extrapolationof the absorption edge (Table 1A), n values of 0.71–1.22 were
given suggesting a direct electronic transition process except for
Au/Zn3 Ga|CO3 -reconst (Table 1). The n value for Au/Zn3 Ga|CO3 reconst was signiﬁcantly greater (1.94), suggesting allowed indirectelectronic transition (n = 2) [25] modulated by the joining assembly of Zn3 Ga|CO3 and Au nanoparticles. This modulation implied
speciﬁc photocatalytic role of Au (see Section 3.2).
No absorption appeared for Zn3 Ga|CO3 except for theabsorption edge below 230 nm (Fig. 1A-a) [10]. In contrast,
Ag/Zn3 Ga|CO3 -IE353-15, Ag/Zn3 Ga|CO3 -IE373-15, Au/Zn3 Ga|CO3 IE, and Au/Zn3 Ga|CO3 -reconst absorbed light centered at 411, 411,
545, and 555 nm, respectively (Fig. 1A-b, c, f, g). A peak centered
at 533–553 nm was reported for 2.9–3.4 nm of Au nanoparticles on
LDH comprising Zn and Al and LDH comprising Zn, Al, and Ce [22].
The
absorption
for
Ag/Zn3 Ga|CO3 -IE373-180
andAg/Zn3 Ga|CO3 -reconst ranged whole visible light region (Fig. 1A-d,e) rather than forming a peak, revealing the formation of large
size Ag nanoparticles and/or a wider size distribution of Ag
nanoparticles.
The UV–visible spectra of the catalysts were also measured
after photocatalytic tests in CO2 (2.3 kPa) and H2 (21.7 kPa) for 5 h.

Negligible change was observed for Ag/Zn3 Ga|CO3 -IE373-15 and
Au/Zn3 Ga|CO3 -IE (Fig. 1A-c, f and B-c, f), demonstrating the stabilityof the catalysts during 5 h of irradiation under the reactants. On thecontrary, the absorption increased in the range between 550 and
800 nm by at most 1.94 times for Ag/Zn3 Ga|CO3 -reconst (Fig. 1A-e,
B-e) while the peak intensity at 555 nm of the as-synthesized sample (Fig. 1A-g) increased by 1.72 times after the photocatalytic test
(558 nm; Fig. 1B-g) for Au/Zn3 Ga|CO3 -reconst.
Hence, Ag nanoparticles in Ag/Zn3 Ga|CO3 -reconst catalyst
became larger in size due to the aggregation process. On the contrary, Au nanoparticles of Au/Zn3 Ga|CO3 -reconst preserved theirsize although their number increased due to the reduction of
Au oxide nanoparticles and/or to the aggregation of the welldispersed ionic Au3+ into metallic Au nanoparticles during the 5 hphotocatalytic tests. It is noteworthy that the aggregation of Ag
nanoparticles and the increase of the number of Au nanoparticle
were consistent with the change of the color of these photocatalysts from the light brown to gray and from light purple to purple,
respectively, after the photocatalytic test for 5 h (Table 1A and B).
The XRD pattern of Zn3 Ga|CO3 , Ag/Zn3 Ga|CO3 -IE373-15,
Ag/Zn3 Ga|CO3 -reconst, Au/Zn3 Ga|CO3 -IE, and Au/Zn3 Ga|CO3 reconst are depicted in Fig. 2. The diffraction peaks common at2 B = 11.7◦ , 23.5◦ , 33.5◦ , 34.2◦ , 37.0◦ , 38.9◦ , 43.7◦ , 46.5◦ , 52.7◦ ,56.1◦ , and 59.4◦ were assigned to 0 0 3, 0 0 6, 0 0 9, 0 1 2, 1 0 4,

Fig. 2. XRD pattern of Zn3 Ga|CO3 (a), Ag/Zn3 Ga|CO3 -IE373-15 (b), Ag/Zn3 Ga|CO3 reconst (c), Au/Zn3 Ga|CO3 -IE (d), and Au/Zn3 Ga|CO3 -reconst (e).

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S. Kawamura et al. / Applied Catalysis A: General 504 (2015) 238–247

0 1 5, 1 0 7, 0 1 8, 1 0 0, ½ 1 1 (= 0 1 1̄ 1), and 1 1 0 diffraction for
the regular layered structure of LDH, respectively. The interlayerinterval value was evaluated to 0.752–0.754 nm on the basis of the
0 0 3 diffraction angle and negligibly changed combined with Ag or
Au (Table 1). This indicates that nanosized Ag and Au formed via
the ion exchange and via the LDH reconstruction were exclusively
adsorbed on the external surface of LDHs.
For Zn3 Ga|CO3 , Ag/Zn3 Ga|CO3 -IE373-15, and Au/Zn3 Ga|CO3 IE samples, weak peaks derived from ␤-Ga2 O3 2 0 1 lattice and␣-GaO(OH) 1 2 0 lattice [32] can be also observed at 2 B = 19.6◦
and 26.8–26.9◦ , respectively (Fig. 2a, b, d). For the Ag/Zn3 Ga|CO3 reconst and Au/Zn3 Ga|CO3 -reconst samples, peaks derived fromZnO 1 0 0 and 1 0 1 reﬂection appeared at 2 B = 31.8◦ and 36.3◦
respectively [33]. As the peaks due to ZnO impurity did not appear
for Zn3 Ga|CO3 , Ag/Zn3 Ga|CO3 -IE373-15, and Au/Zn3 Ga|CO3 -IE,
minor amount of ZnO remained when once calcined oxide from
LDH was reconstructed in the speciﬁc Ag+ or Au3+ aqueous solutions. For the Ag/Zn3 Ga|CO3 -reconst sample, weak peaks derived
from ␤-Ga2 O3 0 2 0 and 2 0 1 lattices [32] and Zn(OH)2 1 1 0 lattice[34,35] were observed at 2 B = 18.7◦ , 19.4, and 20.7◦ , respectively.
The morphology of the LDH composites was studied by SEM.
Fig. 3 shows the SEM image of Ag/Zn3 Ga|CO3 -IE373-15. Flat
platelets with the lateral size between 200 and 700 nm and thethickness between 20 and 50 nm, that are typical for the layered
structure of LDH, were clearly observed. According to SEM analysis, a similar morphology of the platelets for LDH crystallites wasobserved for all the catalysts. On the other hand, the Ag nanoparticles were not observed by the spacial resolution of SEM (Fig. 3).
Thus, we further observed the HR-TEM images for studying thecomposites of the LDHs with Ag or Au nanoparticles.
Ag nanoparticles (5.4–27 nm) originating from ion-exchanged
Ag+ are presented in HR-TEM image for Ag/Zn3 Ga|CO3 -IE37315 (Fig. 4a). The number of Ag nanoparticles was very limited;
this is consistent with the low loading of silver (0.36 wt% as Ag
metal) based on the X-ray absorbance. The chemical composition based on Ag K, Zn K, and Ga K-edge (Table 2a) suggested theformula Ag0.018 [Zn3 Ga0.91 (OH)7.8 ](CO3 )0.45 ·7.1H2 O almost in consistent with ideal formula [Zn3 Ga(OH)8 ](CO3 )0.5 ·mH2 O based on

Fig. 3. SEM image for Ag/Zn3 Ga|CO3 -IE373-15.
Table 2
Chemical composition (wt%) of LDH composite samples based on X-ray absorption
edge jump values.a
Entry

Sample

Ag

Au

Zn

Ga

(O, C, H)

(a)
(b)
(c)
(d)

Ag/Zn3 Ga|CO3 -IE373-15
Au/Zn3 Ga|CO3 -IE
Ag/Zn3 Ga|CO3 -reconst
Au/Zn3 Ga|CO3 -reconst

0.36
–
10.2
–

–
2.2
–
4.1

35.7

11.5

(52.4)a

XAFS spectra at Ag K, Zn K, Ga K, and Au L3 -edges.

introduced amounts of precursors. Ag particles (2.2–8.0 nm, themean size 3.6 nm) assembled with LDH by reconstruction method(Fig. 4b) and the loading of Ag was 10.2 wt% (Table 2c). The population of Ag nanoparticles were apparently higher than that for
Ag/Zn3 Ga|CO3 -IE373-15 (Fig. 4a).
2.2–10.6 nm of Au particles (the mean size 3.2 nm) wereobserved in the HR-TEM image for Au/Zn3 Ga|CO3 -IE (Fig. 4c).
The distribution was quite homogeneous. In the HR-TEM image
for Au/Zn3 Ga|CO3 -reconst, Au nanoparticles (0.9–5.5 nm, themean size 2.6 nm) during LDHs reconstruction process in theFig. 4. TEM image for Ag/Zn3 Ga|CO3 -IE373-15 (a), Ag/Zn3 Ga|CO3 -reconst (b), Au/Zn3 Ga|CO3 -IE (c), and Au/Zn3 Ga|CO3 -reconst (d).

S. Kawamura et al. / Applied Catalysis A: General 504 (2015) 238–247

243

Fig. 5. (A) Normalized Ag K-edge XANES spectra for Ag metal foil (a), Ag/Zn3 Ga|CO3 -IE373-15 (b), sample b after photocatalytic test in CO2 and H2 for 5 h (c; dotted line),
Ag/Zn3 Ga|CO3 -373-180 (d), and Ag2 O (e). (B) Normalized Au L3 -edge XANES spectra for Au metal foil (a), Au/Zn3 Ga|CO3 -IE (b), sample b after photocatalytic test in CO2 andH2 for 5 h (c; dotted line), and Au2 O3 + boron nitride (d).

presence of Au3+ were clearly observed (Fig. 4d). The loading of
Au was between 2.2 and 4.1 wt% for these samples (Table 2band d). This evidenced that for Au/Zn3 Ga|CO3 -IE Au nanoparticles
are distributed more homogeneous than that for Au/Zn3 Ga|CO3 reconst, suggesting special interaction between the Au3+ and oxides
derived from Zn3 Ga|CO3 during the structural reconstruction
process.
Ag K-edge X-ray absorption near-edge structure (XANES) spectra for Ag/Zn3 GaCO3 -IE373-15 and Ag/Zn3 GaCO3 -IE373-180 weredepicted in Fig. 5A-b and d, respectively. The spectrum patterns
in the XANES region for both LDH composite samples were quite
similar to that for Ag metal (a) rather than AgI 2 O (e). Thus, Ag
nanoparticles combined with LDH via the ion exchange method
consisted of Ag0 atoms. The metallic Ag0 state of Ag/Zn3 Ga|CO3 IE373-15 negligibly changed after the photocatalytic test in CO2(2.3 kPa) and H2 (21.7 kPa) for 5 h (Fig. 5A-c) in comparison to spectrum b for the as-synthesized sample. Similarly, metallic Au0 stateof fresh Au/Zn3 GaCO3 -IE remained after the photocatalytic test in
CO2 and H2 for 5 h (Fig. 5B-b, c).
The k3 -weighted EXAFS function was depicted in Fig. 6a for
Ag/Zn3 GaCO3 -IE373-15 (row 1) and Ag/Zn3 GaCO3 -IE373-180 (row2). The associated Fourier transform is shown in the column b. The
peaks at 0.27 nm (phase shift uncorrected) in the Fourier transform were nicely curve-ﬁt by the parameters of Ag–Ag interatomic
pair (Fig. 6c and d) and the ﬁt results were listed in Table 3. The N
value for Ag/Zn3 GaCO3 -IE373-15 was 7.6 (particle size ∼1.6 nm),signiﬁcantly smaller than N of 10.1 (particle size ∼3.7 nm) for
Ag/Zn3 Ga|CO3 -IE373-180 (Table 1A) [36], demonstrating the gradual growth of Ag nanoparticles during the reﬂux procedure at 373 K
between 15 and 180 min.

Fig. 6. Ag K-edge EXAFS for Ag/Zn3 Ga|CO3 -IE373-15 (1) and Ag/Zn3 Ga|CO3 -IE373-180 (2) and Au L3 -edge EXAFS for Au/Zn3 Ga|CO3 -IE (3). k3 -weighted EXAFS oscillation (a),its associated Fourier transform (b), and best-ﬁt results in k-space (c) and R-space (d).

244

S. Kawamura et al. / Applied Catalysis A: General 504 (2015) 238–247

Table 3The curve-ﬁt analysis results of Ag K and Au L3 -edge EXAFS for Ag/Zn3 Ga|CO3 -IE and
Au/Zn3 Ga|CO3 -IE samples.a
SampleAg–Ag or Au–Au
R (nm)
N 2 (10−5 nm2 )Goodness of ﬁt(a)
Ag/Zn3 Ga|CO3 -IE373-15

0.283 (±0.003)
7.6 (±0.7)
3.4 (±0.9)

7.0 × 1030.2895 (±0.0007)
10.1 (±0.3)
2.1 (±2.1)

2.9 × 1030.2878 (±0.003)
8.3 (±0.3)
1.0 (±0.2)

5.4 × 102(b)
Ag/Zn3 Ga|CO3 -IE373-180(c)
Au/Zn3 Ga|CO3 IEaThe values in parentheses are evaluated ﬁt errors.

Although the instability of Ag and Au species in Ag/Zn3 Ga|CO3 reconst and Au/Zn3 Ga|CO3 -reconst under the photocatalytic
conditions was already suggested in UV–visible spectra (Fig. 1),
tentative curve-ﬁt analyses for fresh Ag/Zn3 Ga|CO3 -reconst and
Au/Zn3 Ga|CO3 -reconst samples after photocatalytic tests in
CO2 + H2 irradiated by UV–visible light for 5 h provided N value of
8.4 (particle size ∼2.0 nm) for the Ag–Ag interatomic pair and N
value of 7.7 (particle size ∼1.7 nm) for the Au–Au interatomic pair
(Table 1B). These mean particle sizes for Ag or Au were consistent
with that obtained based on HR-TEM (3.6 and 2.6 nm, respectively; Table 1A). Furthermore, N(Au–Au) value of 8.3 (particle size
∼1.9 nm) for fresh Au/Zn3 Ga|CO3 -IE was also consistent with mean
particle size of 3.2 nm obtained based on HR-TEM (Table 1A). The
detailed analyses for structure change of the Ag and Au sites duringreconstruction step and photocatalytic tests are in progress [37].
3.2. Photocatalytic reduction of CO2 using Ag/LDH and Au/LDH
assemblies
The photocatalytic tests under CO2 + H2 and UV–visible
light using Ag/Zn3 Ga|CO3 -IE373-15 produced mainly methanol(54 mol%) and minor CO as the reaction products (Table 4A-b
and Fig. 7b). The total formation rate was 220 nmol h−1 gcat −1 ;thus improved by a factor of 1.69 times compared to that using
Zn3 Ga|CO3 (130 nmol h−1 gcat −1 ; Table 4A-a) [10]. On the contrary,
Ag/Zn3 Ga|CO3 -IE373-180 exhibited lower photocatalytic activity

giving rise only to CO (67 nmol h−1 gcat −1 ) and less than the detection limit of methanol (Table 4A-c and Fig. 7c). Based on the EXAFS
curve ﬁt analyses and XANES results, the increase of the mean diameter of Ag particles from ∼1.6 nm to ∼3.7 nm was demonstrated forthe photocatalysts (Tables 1 and 3). The lower catalytic activity of
Ag/Zn3 Ga|CO3 -IE373-180 might indicate a direct inﬂuence of theAg nanoparticle size on the CO2 photoreduction reactivity.
The photocatalytic performance of Ag/Zn3 Ga|CO3 -reconst wassuperior to that of Ag/Zn3 Ga|CO3 -IE373-180 (Fig. 7d), althoughdecreased in comparison to that of Ag/Zn3 Ga|CO3 -IE373-15
or undoped Zn3 Ga|CO3 (Table 4A-a–d). The gray color of
Ag/Zn3 Ga|CO3 -IE373-180 and Ag/Zn3 Ga|CO3 -reconst was maintained after the photocatalytic tests (Table 1B). Accordingly, a ﬂat,wide absorption region in the visible light range was observed forthese samples (Fig. 1d and e).
Major products distribution switched to CO (13–14 mol%) when
Au/LDHs composites were tested as photocatalysts. Au/Zn3 Ga|CO3 IE produced CO at a rate of 162 nmol h−1 gcat −1 and thecorresponded selectivity was equal to 86 mol% (Table 4A-e andFig. 7e) in comparison to 46–68 mol% of CO using Zn3 Ga|CO3 and
Zn3 Ga|CO3 composites with Ag nanoparticles (Table 4A-a–d). The
CO formation rate was even higher using Au/Zn3 Ga|CO3 -reconst(201 nmol h−1 gcat −1 ; Fig. 7f) than that using Au/Zn3 Ga|CO3 -IE(Table 4A-e, f). The Au/Zn3 Ga|CO3 -reconst photocatalyst exhibitedthe best performance on the total formation rate. Total product formation rate was 231 nmol h−1 gcat −1 , improved by a factor of 1.78
times compared to that using Zn3 Ga|CO3 (Table 4f).
The stoichiometry of formed molar amount (n) of methanol, CO,and water following Eqs (1) and (2) (nmethanol + nCO = nwater ) was
consistent using Ag/Zn3 Ga|CO3 -IE-373-15, Ag/Zn3 Ga|CO3 -reconst,
and Au/Zn3 Ga|CO3 -reconst (Fig. 7b, d, f). Further, 74% excessive
water was obtained on Ag/Zn3 Ga|CO3 -IE373-180 (Fig. 7c), probably due to structural water desorbed from the interlayer space of
Zn3 Ga|CO3 LDH. On the contrary, the formed water was 48 mol% of
the total amount of methanol and CO formed when Au/Zn3 Ga|CO3 IE were tested as a photocatalyst (Fig. 7e). This could be theconsequence that part of the formed water might be adsorbed on
the photocatalyst.
To conﬁrm the role of the SPR effect and/or electron trap sitesof Ag or Au nanoparticles, the photoreduction of CO2 by H2 undervisible light irradiation ( > 420 nm) were also performed (Fig. 7b
and Table 4B) and the results were compared to that obtained when
UV–visible light were used for irradiation (see Table 4A).
Under visible light only, Zn3 Ga|CO3 did not produce any
products above the detection limit of GC. On the contrary,
Ag/Zn3 Ga|CO3 -IE373-15 exhibited 31 and 86% of methanol and CO
formation rates, respectively, compared to the corresponding rates

Table 4
Photocatalytic rates of CO2 reduction with H2 using the nanoparticles/LDH catalysts.a
Entry

Photocatalyst

Formation rate (nmol h−1 gcat −1 )Selectivity to CH3 OH (mol%)

Reference

130
220
67
94
187
231

39
54
<15
32
14
13

[10]
This work
This work
This work
this work
This work

<12
124
<12
<12

<33
29
<33
<33

[10]
This work
This work
This work

CH3 OHCO(A) Irradiated by UV–visible light
Zn3 Ga|CO3
ab
Ag/Zn3 Ga|CO3 -IE373-15
Ag/Zn3 Ga|CO3 -IE373-180cdAg/Zn3 Ga|CO3 -reconstb
Au/Zn3 Ga|CO3 -IEe
Au/Zn3 Ga|CO3 -reconstb
f

51
118
<12
30
26
30

80
102
67
64
162
201

(B) Irradiated by visible light ( > 420 nm)
Zn3 Ga|CO3
ab
Ag/Zn3 Ga|CO3 -IE373-15
Au/Zn3 Ga|CO3 -IEe
Au/Zn3 Ga|CO3 -reconstb
f

<4
36
<4
<4

<8
88
<8
<8

a
b

In CO2 (2.3 kPa) + H2 (21.7 kPa). The catalyst amount was 100 mg. Values in parentheses are evaluated ﬁt errors.
Preheated at 373 K for 30 min.

S. Kawamura et al. / Applied Catalysis A: General 504 (2015) 238–247

245

Fig. 7. Time course of photocatalytic tests under CO2 (2.3 kPa) and H2 (21.7 kPa) using UV–visible light and Ag/Zn3 Ga|CO3 -IE373-15 (b), Ag/Zn3 Ga|CO3 -IE373-180 (c),Ag/Zn3 Ga|CO3 -reconst (d), Au/Zn3 Ga|CO3 -IE (e), and Au/Zn3 Ga|CO3 -reconst (f). Time course of comparison tests under CO2 (2.3 kPa) and H2 (21.7 kPa) using visible light( > 420 nm) and Ag/Zn3 Ga|CO3 -IE373-15 (b ). CO2 (; diamond), H2 O ( ; triangle), CH3 OH ( ; square), and CO ( ; circle).

under UV–visible light (Tables 4A-b and 3B-b). The stoichiometry
following Eqs. (1) and (2) was preserved when irradiation by visible light was used (Fig. 7b ). This might suggest that the LDHs isable to utilize UV light for the photocatalysis while Ag nanoparticles are able to promote the CO2 photoreduction by utilizing the
SPR response. It is worthy to note that Au based catalysts were
not active when visible light ( > 420 nm) was used for irradiation(Table 4B-e, f).
4. Discussion
4.1. Structure of Ag/LDH and Au/LDH catalysts
Regular layered structures with the interlattice distance
between 0.752 and 0.754 nm were conﬁrmed for Zn3 Ga|CO3 andall the assemblies of Zn3 Ga|CO3 LDH with Ag or Au nanoparticles
tested in this study (Table 1). The interlattice distance corresponds to the LDH containing carbonate and water molecules in
the interlayers [10] revealing that Ag and Au nanoparticles might
be located only on the surface of the platelets of Zn3 Ga|CO3 (thatare deﬁned by sizes between 200 and 700 nm and the thickness between 20 and 50 nm, see Fig. 3). Ag nanoparticles of
5.4–27 nm were observed by the HR-TEM for Ag/Zn3 Ga|CO3 -IE37315 (Fig. 4a). The loading of Ag nanoparticles was low (0.36 wt%
as Ag metal, Table 2a), and the N value obtained by the EXAFS
curve ﬁt analysis for the sample was 7.6 (Table 3a) suggestinga mean particle size deﬁned by a mean diameter of ∼1.6 nm(Table 1A). This discrepancy of particle size might be the consequence to the limited number of Ag nanoparticles, but also the
difﬁculty to observe very small Ag nanoparticles in our HR-TEM
analysis.
Au nanoparticles with a diameter inside the range of [0.9–5.5]
nm were observed in the HR-TEM for Au/Zn3 Ga|CO3 -reconst(Fig. 4d). Hence for this photocatalyst, the mean diameter of Au
nanoparticles is equal to 2.6 nm (see Table 1A). This result is consistent with the SPR peak situated at 555 nm (Fig. 1A) that, inagreement with the previous studies correspond to Au nanoparticles of 2–3 nm [14].
The mean particle sizes of Ag or Au for Ag/Zn3 Ga|CO3 -reconst(3.6 nm), Au/Zn3 Ga|CO3 -IE (3.2 nm), and Au/Zn3 Ga|CO3 -reconst(2.6 nm) obtained based on HR-TEM (Table 1A and Fig. 4) werein consistent with values (2.0, 1.9, and 1.7, respectively) based on
EXAFS (Tables 1A, B and 3 and Fig. 6).
4.2. Roles in CO2 photoreduction of the photocatalyst
components: LDH and nanoparticles of Ag/Au
Zn3 Ga|CO3 photocatalyst photoreduced CO2 into CO andmethanol using H2 (Table 4A-a) [10]. The formation rates of CO
and methanol both increased for Ag/Zn3 Ga|CO3 -IE373-15. In fact,
the total formation rate of Ag/Zn3 Ga|CO3 -IE373-15 increased by afactor of 1.69 times compared to that of Zn3 Ga|CO3 . The changeof methanol selectivity from 39 to 54 mol% was enough moderate(Table 4A-b).
For Au based catalysts, the total formation rates of CO
and methanol using Au/Zn3 Ga|CO3 -IE and Au/Zn3 Ga|CO3 -reconst
increased by 1.44 and 1.78 times, respectively, in comparison to
Zn3 Ga|CO3 . Hence, the selectivity to CO increased from 61 mol% for
Zn3 Ga|CO3 to 86–87 mol% (Table 4A-e, f) for Au/Zn3 Ga|CO3 , suggesting different promoting mechanisms/sites.
The energy diagram and proposed electron ﬂows during thephotocatalytic process are illustrated in Scheme 1 considering thework function (WF) of Ag (4.52–4.74 eV) and Au (5.31–5.47 eV)[38,39], wavelength of SPR peak top (411 nm for Ag/Zn3 Ga|CO3 373-15, 555 nm for Au/Zn3 Ga|CO3 -reconst; Fig. 1A and Table 1),
bandgap values of the Zn3 Ga|CO3 LDH (Table 1A), and the reaction
potential values for CO2 reduction to CO (−0.11 V versus SHE) andfor CO2 reduction to methanol (−0.32 V versus SHE) [10].
Based on the above results, we assume that the photoreductionof CO2 by H2 (Table 4), under irradiation with UV and visible lightmight induce both bandgap excitation of Zn3 Ga|CO3 (5.6 eV) andthe surface plasmon excitation (2.23–3.02 eV), respectively. Due tothe difference in the values of WF and SPR energy for Au (3.1–3.2 eV)
and Ag (1.5–1.7 eV), excited electrons at conduction band (CB) for
Zn3 Ga|CO3 might be transferred to Au whereas the SPR-excitedelectrons at Ag nanoparticles might migrate to Zn3 Ga|CO3 LDH (seeScheme 1).
The higher selectivity to CO using Au/Zn3 Ga|CO3 -reconst and
Au/Zn3 Ga|CO3 -IE (86–87 mol%) might suggest that the contribution of Au sites via the electron transfer derived from the band
gap excitation of the Zn3 Ga|CO3 support. On the contrary, for
Zn3 Ga|CO3 (CO selectivity: 61 mol%), Ag/Zn3 Ga|CO3 -IE373-15, and246

S. Kawamura et al. / Applied Catalysis A: General 504 (2015) 238–247

Scheme 1. The energy diagram and proposed electron ﬂows in Ag/Zn3 Ga|CO3 -IE373-15 (right) and Au/Zn3 Ga|CO3 -reconst (left) during photocatalytic reduction of CO2 .

Ag/Zn3 Ga|CO3 -reconst (46–68 mol%), Ag nanoparticles on the LDH
surface might be involved as major electron-donating sites during CO2 photoreduction. This agrees well to the previous reported
results for Au nanoparticles deposited on glass (to form methane,
formaldehyde, ethane, and methanol) showing the direct contribution of Au in CO2 photoreduction [19]. It should be noted that
excited electrons due to SPR of nanosized Ag or Au were not always
inserted to wide-Eg LDH Zn3 Ga|CO3 (5.6 eV), in comparison to theinsertion from Au [17–19] and Ag nanoparticles [17] to TiO2 (Eg :3.0–3.2 eV).
Therefore, results points out that Ag/Zn3 Ga|CO3 -IE373-15 catalyzed the photoreduction of CO2 in a similar way to Zn3 Ga|CO3 ,boosted by Ag sensitization by the electron transfer from thenanoparticles of Ag to LDH either (i) by the hot electron transfer,(ii) thermally excited electron transfer, or (iii) plasmonic resonant energy transfer [17–20]. This dependence on wavelength ofirradiated light was similar to the behavior of photooxidation of
methanol/ethanol using Au/TiO2 [18].
Furthermore, the assemblies of the LDH with Au nanoparticles
were inert when irradiated by visible light ( > 420 nm) (Table 4B-e,f). The electrons created by SPR of Au are not able to photoreduce
CO2 by visible light, whereas Eg -excited electrons in LDH by UV light
was trapped in Au and then transferred to CO2 -derived species. As
the potential of excited electrons at Au is lower (more positive)
than that of excited electrons at the CB of Zn3 Ga|CO3 , energeticallyfavorable CO formation would have a priority rather than methanol
formation in the presence of UV light (Scheme 1).
5. Conclusions
The present results illustrate the potential of the plasmonicnanostructured assemblies of Au/Zn3 Ga(OH)8 ]2 CO3 ·mH2 O and
Ag/Zn3 Ga(OH)8 ]2 CO3 ·mH2 O as photocatalysts for CO2 reduction by
H2 using both UV and visible light. The CO2 photoreduction rateusing Ag/Zn3 Ga|CO3 -IE catalyst was higher (220 nmol h−1 gcat −1 )by a factor of 1.69 in comparison to that of Zn3 Ga|CO3 . Under
visible light irradiation, the photoreduction rate of Ag/Zn3 Ga|CO3 IE decreased to 56% of corresponding rate under irradiation
with UV–visible light. Moreover, Au/Zn3 Ga|CO3 -reconst irradiated by UV–visible light showed the best catalytic performances(231 nmol h−1 gcat −1 ), although these catalysts were CO selective(87 mol%). All Au/Zn3 Ga|CO3 were found to be inert for CO2 photoreduction by H2 when irradiated by visible light ( > 420 nm). We
present evidence that Ag nanoparticles might assist the metal/LDH
assemblies’ photoreduction activity by the electron injection due tothe SPR effect while the Au nanoparticles might act as electron trap
excited in the LDH matrix irradiated by UV light. The optimized

design of the plasmonic nanoparticles-LDH self-assemblies canopen the way for further development of novel promising catalysts
for CO2 photoreduction.
Acknowledgments
The authors are grateful for the ﬁnancial supports from theGrant-in-Aid for Scientiﬁc Research C (26410204, 22550117) fromJapan Science Promotion Agency and from the Romanian National
Authority for Scientiﬁc Research, CNCS-UEFISCDI (PN-II-IDEI-PCE75/2013). X-ray absorption experiments were conducted under
the approval of the Photon Factory Proposal Review Committee(2014G631, 2011G033).
References[1] A. Corma, H. García, J. Catal. 308 (2013) 168–175.
[2] Y. Izumi, Coord. Chem. Rev. 257 (2013) 171–186.
[3] S.C. Roy, O.K. Varghese, M. Paulose, C.A. Grimes, ACS Nano 4 (2010) 1259–1278.
[4] D.B. Ingram, S. Linic, J. Am. Chem. Soc. 133 (2011) 5202–5205.
[5] F. Cavani, F. Triﬁrò, A. Vaccari, Catal. Today 11 (1991) 173–301.
[6] C. Gomes Silva, Y. Bouizi, V. Fornés, H. García, J. Am. Chem. Soc. 131 (2009)
13833–13839.
[7] Z. Yong, A.E. Rodrigues, Energy Convers. Manag. 43 (2002) 1865–1876.
[8] M. Morikawa, N. Ahmed, Y. Yoshida, Y. Izumi, Appl. Catal. B144 (2014) 561–569.
[9] N. Ahmed, M. Morikawa, Y. Izumi, Catal. Today 185 (2012) 263–269.
[10] N. Ahmed, Y. Shibata, T. Taniguchi, Y. Izumi, J. Catal. 279 (2011) 123–135.
[11] K. Teramura, S. Iguchi, Y. Mizuno, T. Shishido, T. Tanaka, Angew. Chem. Int. Ed.
51 (2012) 8008–8011.
[12] M. Morikawa, Y. Ogura, N. Ahmed, S. Kawamura, G. Mikami, S. Okamoto, Y.
Izumi, Catal. Sci. Technol. 4 (2014) 1644–1651.
[13] R.E. Blankenship, D.M. Tiede, J. Barber, G.W. Brudvig, G. Fleming, M. Ghirardi,
M.R. Gunner, W. Junge, D.M. Kramer, A. Melis, T.A. Moore, C.C. Moser, D.G.
Nocera, A.J. Nozik, D.R. Ort, M.W. Parson, R.C. Prince, R.T. Sayre, Science 332(2011) 805–809.
[14] http://www.cytodiagnostics.com/silver nanoparticle properties.php[15] http://www.cytodiagnostics.com/gold nanoparticle properties.php[16] O. Nicoletti, F. de la Pena, R.K. Leary, D.J. Holland, C. Ducati, P.A. Midgley, Nature
502 (2013) 80–84.
[17] C. Clavero, Nat. Photonics 8 (2014) 95–103.
[18] Y. Tian, T. Tatsuma, J. Am. Chem. Soc. 127 (2005) 7632–7637.
[19] W. Hou, W.H. Hung, P. Pavaskar, A. Goeppert, M. Aykol, S.B. Cronin, ACS Catal.
1 (2011) 929–936.
[20] S.K. Cushing, J. Li, F. Meng, T.R. Senty, S. Suri, M. Zhi, M. Li, A.D. Bristow, N. Wu,
J. Am. Chem. Soc. 134 (2012) 15033–15041.
[21] G. Carja, Y. Kameshima, A. Nakajima, C. Dranca, K. Okada, Int. J. Antimicrob.
Agent 34 (2009) 534–539.
[22] G. Carja, M. Birsanu, K. Okada, H. Garcia, J. Mol. Mater. Chem. A 1 (2013)
9092–9098.
[23] C. Chen, P. Gunawan, X.W. Lou, R. Xu, Adv. Funct. Mater. 22 (2012) 780–787.
[24] L. Wang, J. Zhang, X. Meng, D. Zheng, F.S. Xiao, Catal. Today 175 (2011) 404–410.
[25] F. Wooten, Optical Properties of Solids, Academic Press, New York, USA, 1972,
pp. 142.
[26] J.A. Bearden, Rev. Mod. Phys. 39 (1967) 78–124.
[27] M. Vaarkamp, H. Linders, D. Koningsberger, XDAP version 2.2.7, XAFS Services
International, Woudenberg, The Netherlands, 2006.

S. Kawamura et al. / Applied Catalysis A: General 504 (2015) 238–247[28] Y. Izumi, T. Itoi, S. Peng, K. Oka, Y. Shibata, J. Phys. Chem. C 113 (2009)
6706–6718.
[29] L.G. Liu, W.A. Bassett, J. Appl. Phys. 44 (1973) 1475–1479.
[30] C. Petitjean, D. Horwat, J.F. Pierson, J. Phys. D 42 (2009) 025304.
[31] S. Fujii, U. Akiba, M. Fujihira, J. Am. Chem. Soc. 124 (2002) 13629–13635.
[32] X. Liu, G. Qiu, Y. Zhao, N. Zhang, R. Yi, J. Alloys Compd. 439 (2007) 275–278.
[33] Y. Yoshida, Y. Mitani, T. Itoi, Y. Izumi, J. Catal. 287 (2012) 190–202.
[34] A.E. Rakhshani, A. Bumajdad, J. Kokaj, S. Thomas, Appl. Phys. A 97 (2009)759–764.

247[35] K. Kusaba, T. Yagi, J. Yamaura, H. Gotou, T. Kikegawa, J. Phys. Conf. Ser. 215(2010) 012001.
[36] B.J. Kip, F.B.M. Duivenvoorden, D.C. Koningsberger, R. Prins, J. Catal. 105 (1987)
26–38.
[37] S. Kawamura, G., Carja, Y. Izumi, in preparation.
[38] R.C. Weast, CRC Handbook of Chemistry and Physics, vol. 82, CRC Press, Boca
Raton, USA, 2001, pp. 12–130.
[39] Y. Ogura, S. Okamoto, T. Itoi, Y. Fujishima, Y. Yoshida, Y. Izumi, Chem. Commun.
50 (2014) 3067–3070.