Applied Catalysis B: Environmental 199 (2016) 260–271

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Applied Catalysis B: Environmental
journal homepage: www.elsevier.com/locate/apcatb

Harnessing self-supported Au nanoparticles on layered doublehydroxides comprising Zn and Al for enhanced phenol decomposition
under solar light
Gaku Mikami a , Florentina Grosu b , Shogo Kawamura a , Yusuke Yoshida a , Gabriela Carja b,∗, Yasuo Izumi a,∗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 6 April 2016
Received in revised form 8 June 2016
Accepted 9 June 2016
Available online 17 June 2016
Keywords:
Layered double hydroxide
Gold nanoparticles
Electron transfer
PhenolPhoto-decompositiona b s t r a c t
Gold nanoparticles (AuNPs) self-coupled on semiconductors have attracted extensive attentions in theﬁeld of catalysis, however, the progress in understanding and optimizing their photocatalytic performance in response to solar light irradiation is limited. In this paper, a series of AuNPs with Zn2 Al-layered
double hydroxide (LDH) as support was fabricated via self-assembly routes at room temperature and thetuned oxidation state of AuNPs (as Au0 , Au3+ as well as mixed Au0 /Au3+ ) was revealed to have a crucial
effect on establishing their photocatalytic efﬁciency for the degradation of phenol from aqueous solution
under solar irradiation. X-ray absorption near-edge structure (XANES) and extended X-ray absorption
ﬁne structure (EXAFS) analyses permitted to characterize the speciﬁc interactions of Zn2 Al-LDH with
AuNPs and to verify that the state of Au0 /Au3+ NPs appears due to the electron transfer from Zn2 Al-LDH to
AuNPs when Zn2 Al-LDH reconstruction in the aqueous solution of Au(O2 CCH3 )3 was achieved under solar
irradiation. On the basis of these AuNPs–Zn2 Al-LDH systems, the possible roles of Au0 , Au3+ or Au0 /Au3+
in establishing synergetic effects with Zn2 Al-LDHs supports for enhancing the photocatalytic response
induced by the irradiation with solar light, for manipulating the mechanism, and the catalyst stability inphenol degradation process, are critically discussed.
© 2016 Elsevier B.V. All rights reserved.

1. Introduction
Gold nanoparticles (AuNPs) have received increasing attentions in catalysis since Hutchings et al. [1] and Haruta et al. [2]demonstrated excellent performances of gold-based catalysts inlow-temperature oxidation of CO [3]. AuNPs supported on solids
with high surface area are used for a large variety of catalytic
reactions, e.g. photocatalysis, cross coupling, sequential oxidative
addition, benzylation of aromatics, oxidative C C coupling, amination, and nucleophilic addition [4–8]. The support materials
play not only the role of stabilizing AuNPs at different oxidation state (Au␦+ , Au+ , or Au3+ ) or controlling their dispersion level(nanoparticle size) but also might act as a buffer for the electrontransfer to AuNPs, and thus to reduce the charge-carrier recombination during the photo-responsive processes [9–11]. Furthermore,∗ Corresponding authors.
E-mail addresses: carja@uaic.ro (G. Carja), yizumi@faculty.chiba-u.jp (Y. Izumi).
http://dx.doi.org/10.1016/j.apcatb.2016.06.031
0926-3373/© 2016 Elsevier B.V. All rights reserved.

(1) the particle size-dependent photoresponsive characteristics of
AuNPs, e.g. surface plasmon resonance (SPR) response, (2) thephoto-functionality of the support, and (3) the charge–transfer
characteristics from AuNPs to support materials are the recent
key issues in designing efﬁcient nano-gold composites as solarresponsive photocatalysts [12–14].
Among the numerous materials of large surface area, layered
double hydroxides (LDHs) exhibits unique features not foundin other solid supports if used as support materials for AuNPs,
thus making AuNPs/LDHs composites speciﬁc catalysts [15]. The
ﬂexibility of the LDHs composition, their semiconducting characteristics [4], and their high adsorption capacity enables totune the electronic density at the interface between AuNPs andLDHs. Furthermore, LDHs own a unique self-repairing property toreconstruct its layered structure, after being transformed throughcalcination into mixtures of metal oxides by the calcination, utilizing its structural “memory effect” to restore original layered
structure from mixed oxides. Utilizing the structural memory effectof LDHs, we recently reported a simple, one-step route to fabricate

2.5
4.8
5.3

–

–
4.7 (±2.4)4.9 (±2.1)8.5 (±3.6)4.1 (±1.0)7.3 (±2.0)

TEM
TEM

–
1.1–13.6
1.8–18.0
1.9–25.4
2.2–7.7
3.9–14.0

Mean size(nm)

2. Experimental

Particle size
distribution
(nm)

EXAFS

–
0
0
27
31
19

Exposed Au0(␮mol gcat −1 )d

97
81
59
79
54
67
–

SBET (m2 g−1 )e

5.7
3.1
2.5
2.6
–f
–f
–f

Eg (eV)

25
125
47
199
42
43
68

Photocatalytic rates(␮mol L−1 h−1 )AuNPs directly on Zn–Al–Ce LDHs [16] and Zn–Ga LDHs [17]. Thismethod used the capability of LDH matrix to form self-supported
AuNPs on LDHs, by exploiting the property of LDHs to restore its
original layered structure in the aqueous solutions of gold salts,
e.g. AuCl3 and Au(CH3 COO)3 , without using any organic additives
as surfactant and/or stabilizer for AuNPs.
In a related precedent work, it was revealed that the microscopic
morphology of LDHs has an important effect on the location and thesize of AuNPs [6]. Recently, Liu and co-workers [7] demonstrated
that the synergy between AuNPs and LDHs in AuNPs–Cr-LDHs composites increased if the Cr content was increased in the Cr-LDHs
as a support. To date, despite the signiﬁcant interests in catalysis using the AuNPs composites, maximizing their efﬁciency underirradiation with solar light still remains a meaningful challenge
and the examples in scientiﬁc literature are few [18,19]. It wasdemonstrated that the speciﬁc charge transfer between AuNPs anda semiconductor support was able to reduce the charge recombination rates and to increase the photocatalytic efﬁciency [20].
Hence, to design AuNPs–LDHs composites as active photocatalysts
under solar irradiation, one of the keys is to establish favourable
charge-transfer pathway between the supported AuNPs and LDHs.
Herein, we investigate facile self-assembly routes at room temperature to obtain novel AuNPs–Zn2 Al-LDH photocatalysts with
tuned oxidation state of AuNPs, as Au0 , Au3+ as well as a mixtureof Au0 /Au3+ . Furthermore, X-ray absorption near-edge structure(XANES) and extended X-ray absorption ﬁne structure (EXAFS)
analyses permits to demonstrate that the mixed state of Au0 /Au3+
is the result of an electron transfer from Zn2 Al-LDH to AuNPs when
Zn2 Al-LDH reconstruction in the aqueous solution of gold acetate
was achieved under solar irradiation. To benchmark the suitability of AuNP–Zn2 Al-LDH for solar photocatalysis, they were tested
for the phenol photo-decomposition. This reaction is not only aquite well-understood model reaction, but also satisﬁes an empirical requirement to eliminate a common pollutant, phenol, in thewastewater streams worldwide. We provide compelling evidence
that AuNPs are the catalytically active sites for photodegrading
phenol under irradiation with simulated solar light although thetransfer interactions between AuNPs and Zn2 Al-LDH were crucial
to enhance the photocatalytic efﬁciency.

261–
0: 100
0: 10065: 3585: 15
100: 0
–
4.8
16
4.8
2.2
1.9
4.8
Zn2 Al-LDH
Au/Zn2 Al-Rec-1c
Au/Zn2 Al-Rec-2
Au/Zn2 Al-Rec-3-Lighta,cAu/Zn2 Al-Imp-NaBH4 c
Au/Zn2 Al-Imp-Lysinec
Au/Zn2 Al-Lightc
a
b
c
d
e
f
g

Contents (wt%)

Au
Sample name

AuNPs supported on Zn2 Al-LDH was obtained exploring thestructural reconstruction of LDH in the aqueous solutions containing Au3+ ions [21]. Namely, 1.20 g Zn2 Al-LDH powder was calcinedin an oven at 773 K for 8 h. The resulted calcined powder wascooled down in the oven till 473 K and slowly added into 250 mL
aqueous solution of Au(III) acetate (0.10 g or 0.30 g; >99.9%, Alfa
Aesar) with a stirring at a rate of 900 rpm. The reaction mixture

Entry

2.2. AuNPs on Zn2 Al-LDH by the reconstruction method

Table 1
Characteristics of Au/Zn2 Al-LDH samples.

An aqueous solution of Zn nitrate hexahydrate (>99%, Wako
Pure Chemical) and Al nitrate nonahydrate (>99.9%, Wako Pure
Chemical) with the Zn/Al atomic ratio of 2.0 in deionized water(<0.06 ␮S cm−1 ) with the concentration of [Zn2+ ] + [Al3+ ] = 1.2 M,
an aqueous solution of NaOH (1.92 M; >97%, Kanto Chemical), andNa2 CO3 (0.8 M; >99.8%, Wako Pure Chemical) in deionized water
were simultaneously added to a ﬂask using a magnetic stirrer at
a speed of 900 rates per minute (rpm) for 48 h. The precipitate
was ﬁltered and washed with deionized water and dried at 333 K
for 24 h [21–23]. The resultant sample was denoted as Zn2 Al-LDH(Table 1a).

Au0 : Au3+

2.1. Synthesis of LDH

aIrradiated by Solar light simulator during LDH the reconstruction for 20 min for Au/Zn2 Al-Rec-1. b Evaluated from XANES edge jump values. c For Au/Zn2 Al-Rec-1, Au/ZnAl-Rec-3, Au/ZnAl-Imp-NaBH4 , Au/Zn2 Al-Imp-Lysine, and
Au/Zn2 Al-Light the amount of gold introduced during the sample preparation compared to the amount of support material was the same (5:100). d Estimated based on Au content in catalyst, Au0 :Au3+ ratio, mean Au size by
TEM, and the correlation between dispersion and diameter of fcc cubo-octahedron particles [41]. e BET surface area evaluated for samples after photocatalytic phenol decomposition tests for 5 h. f It was difﬁcult to determine theabsorption edge due to the overlap of broad SPR peaks.

G. Mikami et al. / Applied Catalysis B: Environmental 199 (2016) 260–271

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G. Mikami et al. / Applied Catalysis B: Environmental 199 (2016) 260–271

was stirred at the rate of 900 rpm and 290 K for 3 h. Then, the precipitate was centrifuged at a speed of 10 000 rpm and dried at
358 K. The sample is denoted as Au/Zn2 Al-Rec-1 (4.8 wt% of Au)
and Au/Zn2 Al-Rec-2 (16 wt% of Au), respectively (Table 1b, c). Separately, after adding the calcined LDH into the Au(III) acetate (0.10 g)solution, the resulted suspension was irradiated using a Solar simulator (Model US 800, Unnasol, Germany) for 20 min, stirred at thesame rate (900 rpm). The sample is denoted as Au/Zn2 Al-Rec-3Light (Table 1d).
As a comparison, 0.24 g Zn2 Al-LDH powder was mixed with
250 mL aqueous solution of Au(III) acetate (0.020 g) with a stirring at a rate of 900 rpm and irradiated using a Solar simulator
for 20 min. Then, the reaction mixture was stirred at a rate of
900 rpm and 290 K for 3 h. Then, the precipitate was ﬁltered using
JGWP04700 ﬁlter, washed using JGWP04700 ﬁlter with deionizedwater and ethanol (total 250 mL for each), and dried under vacuum at 290 K for 24 h. The sample was denoted as Au/Zn2 Al-Light(4.8 wt% Au; Table 1g).
2.3. AuNPs on Zn2 Al-LDH by impregnation of HAuCl4 followed byliquid reduction
0.044 g of hydrochloroauric acid tetrahydrate ( > 99.9%, Alfa
Aesar) was dissolved in 10 mL of deionized water. 0.42 g of Zn2 AlLDH was added to the aqueous solution and the mixture was stirred
at a rate of 900 rpm and 290 K for 12 h. The precipitate was ﬁltered using a membrane ﬁlter (JGWP04700 ﬁlter, Omnipore) andwashed with deionized water (total 250 mL), and dried under vacuum at 290 K for 24 h. Obtained powder was added to 10 mL ofdehydrated toluene ( > 99.5%, Wako Pure Chemical) and 0.0126 g of
NaBH4 ( > 95%, Wako Pure Chemical) was added to the suspension.
After stirring for 10 min, 3 mL of ethanol ( > 99.5%, Wako Pure Chemical) was added and the mixture was stirred at a rate of 900 rpm
and 290 K for 6 h. The precipitate was ﬁltered and washed using
JGWP04700 ﬁlter with deionized water and ethanol (total 250 mL
for each), and dried under vacuum at 290 K for 24 h [15]. The sample
was denoted as Au/Zn2 Al-Imp-NaBH4 (Table 1e).
Separately, 0.30 g of Zn2 Al-LDH was dispersed in 10 mL ofdeionized water. To ensure a uniform dispersion, the suspension
was ultrasonicated (430 W, 38 kHz) for 10 min. Then, 0.031 g of
HAuCl4 ·4H2 O in aqueous solution (20 mL), 0.036 g of l-lysine (98%,
Sigma–Aldrich Chemie) in aqueous solution (4.0 mL), and 0.026 g
of NaBH4 in aqueous solution (2.0 mL) were added sequentially tothe above suspension with a dropping speed of 1.0 mL min−1 andstirred at a rate of 900 rpm. Afterwards, dilute hydrochloric acid
(Wako Pure Chemical) was added to adjust the pH value to 9.5.
The mixed solution was stirred for 1 h and left standing for 24 h.
The precipitate was ﬁltered using JGWP04700 ﬁlter and washedwith deionized water and ethanol (total 250 mL for each), and driedat 333 K for 12 h [24]. The sample was denoted as Au/Zn2 Al-ImpLysine (Table 1f).
2.4. X-ray diffraction (XRD)
XRD pattern was observed using a D8 ADVANCE diffractometer(Bruker) at the Center for Analytical Instrumentation, Chiba University, at a Bragg angle ( B ) 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.

(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 spectraon the basis of the Kubelka–Munk equation [22,25,26]. The bandgap value was evaluated on the basis of either simple extrapolationof the absorption edge or the ﬁt to the equation of Davis and Mott
[22,26],
␣ × h∝ (h − E g )n(1)

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 3
for allowed direct, forbidden direct, allowed indirect, and forbidden
indirect transitions, respectively [27].
2.6. Transmission electron microscopy (TEM)
High-resolution (HR)-TEM images were observed using a Model
H-7650, Hitachi operating at 100 kV. Tungsten ﬁlament was usedin the electron gun and the samples 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 [28].
2.7. Au L3 -edge XANES and EXAFS
Au L3 -edge XANES and EXAFS spectra were measured at 290 K
in a transmission mode on beamline NW10A in the Photon Factory
Advanced Ring at the High Energy Accelerator Research Organization (Tsukuba). 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-coated focusing cylindrical mirror were inserted
into the X-ray beam path. The X-ray intensity was reduced at 65%of the maximum ﬂux using a piezo translator set to the crystal tosuppress the effects of higher harmonics other than 3 1 1 reﬂection[29]. The slit opening size was 1 mm (vertical) × 2 mm (horizontal) in front of the ionization chamber. The Au L3 -edge absorptionenergy was calibrated to 11 921.2 eV for the spectrum of Au metal
foil [30–32].
The XANES and EXAFS data were analyzed using an XDAP
package [33]. The pre-edge background was approximated by a
modiﬁed Victoreen function C2 /E2 + C1 /E + C0 . The background of
the post-edge oscillation x was approximated by a smoothing
spline function and calculated by an equation for the number ofdata points, where k is the angular wavenumber of photoelectrons.
Data Pointsi=1(xi − BGi )2exp(−0.075ki2 )≤ smoothig factor(2)

Multiple-shell curve-ﬁt analyses were performed for the
Fourier-ﬁltered k3 -weighted EXAFS data in k- and interatomic
distance (R)-space using empirical amplitude extracted from the
EXAFS data for Au2 O3 powder and Au metal foil. The R value andits associated coordination number (N) for the Au–O and Au–Au
pairs were set to 0.201 3 nm with the N value of 4 and 0.288 4 nmwith the N value of 12, respectively [17]. The many-body reduction
factor S0 2 was assumed to be equal for both the sample and thereference.

2.5. UV–visible spectroscopy

2.8. Brunauer, Emmett, and Teller (BET) surface area
measurements

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 sphereN2 adsorption isotherms were measured at 77.15 K on a Quantachrome Autosorb-1MP automated gas adsorption system. Prior
to the measurements, the samples were heated for 12 h under vacuum at 383 K. The BET speciﬁc surface area (SBET ) was calculated

G. Mikami et al. / Applied Catalysis B: Environmental 199 (2016) 260–271

263

by using the standard Brunauer, Emmett, and Teller method on thebasis of adsorption–desorption data [22,23].
2.9. Photocatalytic tests for phenol decomposition
Appropriate amounts of a catalyst were suspended in a phenol solution (50 mg L−1 ) in a batch reactor using a catalyst of
0.5 g L−1 . The temperature was kept constant at 298 K. After 30 min
in darkness to ensure the adsorption equilibrium of phenol on the
catalyst, the reactor was irradiated from a solar simulator (Model
US 800, Unnasol, Germany) for photocatalytic reaction tests. The
sampling of the solution was carried out using a 1 mL pipet every
20 min for 7 h of irradiation. The sampled 1.0 mL of reaction suspension was centrifuged to separate solution from the photocatalyst.
The solution was immediately analyzed using UV–visible spectrometer (Model V-550, JASCO) to determine the concentrationsof remained phenol and transformed compounds from phenol (p-benzoquinone, muconic acid, catechol, and hydroquinone)[34]. The UV–visible spectra for standard aqueous solution of pbenzoquinone (Wako Pure Chemical, >98%), trans,trans-muconic
acid (Tokyo Chemical Industry, 98%), phenol (Wako Pure Chemical, >99%), catechol (Wako Pure Chemical, >99%), and hydroquinone(Wako Pure Chemical, >99%) were also measured in transmission
mode at 290 K.
The peaks in the UV–visible spectra at 197 nm, 210 nm, 248 nm,
and 270 nm were assigned to ␲–␲* and n–␲* transition for aromatic ring [35], p-benzoquinone, and phenol [36], respectively, andthe spectra were deconvoluted using the four Gaussian peaks asfollows:f (x) = a1 exp+a3 exp 

x − 197−b1

 

x − 248
−
b3

2 

2 

+ a2 exp+ a4 exp 

x − 210−b2

 

x − 270−b4

2 

Fig. 1. XRD pattern for Zn2 Al-LDH (a), Au/Zn2 Al-Rec-1 (b), Au/Zn2 Al-Rec-2 (c),Au/Zn2 Al-Rec-3-Light (d), Au/Zn2 Al-Imp-NaBH4 (e), and Au/Zn2 Al-Imp-Lysine (f).

Fig. 2. UV–vis absorption spectrum of Zn2 Al-LDH (a), Au/Zn2 Al-Rec-1 (b), Au/Zn2 AlRec-2 (c), Au/Zn2 Al-Rec-3-Light (d), Au/Zn2 Al-Imp-NaBH4 (e), Au/Zn2 Al-Imp-Lysine(f), and Au/Zn2 Al-Light (g).

2 
(3)

The an and bn (n = 1, 2, 3, 4) were the absorbance and peak width
in nm, respectively. The b1 , b2 , b3 , and b4 values resulted in within
the range of 6.0–10, 11–30, 12–30, and 8.0–22 nm, respectively. To
reconﬁrm the assignment and evaluation of reactants and intermediate compounds, especially phenol and p-benzoquinone, thereaction solution was also analyzed high-pressure liquid chromatograph (HPLC; Model Prominence-i, Shimadzu) [37].
3. Results
3.1. XRDThe XRD patterns of Zn2 Al-LDH, Au/Zn2 Al-Rec-1, Au/Zn2 AlRec-2, Au/Zn2 Al-Rec-3-Light, Au/Zn2 Al-Imp-NaBH4 , and Au/Zn2 AlImp-Lysine samples are depicted in Fig. 1. The diffraction peaks
common at 2 B = 11.8◦ , 23.6◦ , 34.1◦ , 34.8◦ , 37.5◦ , 39.4◦ , 44.2◦ , 47.0◦ ,53.2◦ , and 56.6◦ were assigned to 0 0 3, 0 0 6, 1 0 1, 0 0 9, 1
0 4, 0 1 5, 1 0 7, 0 1 8, 1 0 0, and 12 1 1 (= 0 1 1̄ 1) diffractions for the regular layered structure of LDHs. For Au/Zn2 Al-Rec-1
and Au/Zn2 Al-Rec-3-Light samples, weak peaks derived from 1 0 0
and 1 0 1 reﬂections for wurtzite crystal of ZnO also appeared at
2 B = 31.9◦ and 36.4◦ (marked with * in Fig. 1), respectively, suggesting the layered structure of LDH was not fully recovered. For
Au/Zn2 Al-Rec-2 and Au/Zn2 Al-Rec-3-Light, a weak peak derived
from 1 1 1 reﬂection for face-centered cubic (fcc) crystal of metallic
Au was identiﬁed at 2 B = 38.3◦ (marked with  in Fig. 1).
For all the samples, the major peaks were originated from
LDH structure while the peaks due to Au did not appear or were
negligibly weak for Au/Zn2 Al-Rec-1, Au/Zn2 Al-Imp-NaBH4 , and

Au/Zn2 Al-Imp-Lysine above the detection limit of XRD apparatus used. The XRD results indicated that the samples obtained byimpregnation followed by liquid reduction (Au/Zn2 Al-Imp-NaBH4
and Au/Zn2 Al-Imp-Lysine) did not comprise [mixed] metal oxides
of Zn and/or Al (Fig. 1e, f).
3.2. UV–visible spectra
UV–visible absorption spectra for the Au-LDHs photocatalysts
are depicted in Fig. 2. In the spectra for Au/Zn2 Al-Rec-1, Au/Zn2 AlRec-2, and Au/Zn2 Al-Rec-3-Light, a peak appeared due to SPR of
AuNPs at 539 nm, 540 nm, and 547 nm, respectively (Fig. 2b–d).
SPR peaks centered at 533–553 nm were previously reported for
2.9–3.4 nm size of AuNPs embedded on LDH [16]. The red shiftof the SPR peak from 539 nm (for Au/Zn2 Al-Rec-1) to 547 nm (forAu/Zn2 Al-Rec-3-Light) suggested the quantum size effect of AuNPs
leading to lower-energy plasmonic near-ﬁeld when AuNPs grew
during LDH reconstruction under UV–visible light irradiation.
The band gap values for support materials were evaluatedbased on Davis–Mott equation (Table 1). The band-gap values for
Zn2 Al-LDH (5.7 eV; Table 1a) apparently decreased to 3.1–2.5 eV
for AuNP–LDHs via the reconstruction route (Table 1b–d). It was
not due to the change of electronic property of LDH support but
can be ascribed to the presence of minor metal oxide phase, e.g.
ZnO and/or ZnAl2 O4 and/or the synergetic effects of AuNPs with
ZnO/ZnAl2 O4 /Zn2 Al-LDH. In the UV–visible spectra of Au/Zn2 AlImp-NaBH4 and Au/Zn2 Al-Imp-Lysine (Fig. 2e, f), the characteristic
absorption edge at ∼400 nm did not appear. This absorption edge
is due to ZnO and/or spinel metal oxides: ZnAl2 O4 . Thus, both the
XRD and UV–vis absorption results suggested the jointed phases ofmajor LDHs and minor mixed oxides for AuNPs–LDHs composites

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G. Mikami et al. / Applied Catalysis B: Environmental 199 (2016) 260–271

Fig. 3. Normalized Au L3 -edge XANES spectra for Au/Zn2 Al-Rec-1 (a), Au/Zn2 Al-Rec2 (b), Au/Zn2 Al-Rec-3-Light (c), Au/Zn2 Al-Imp-NaBH4 (d), Au/Zn2 Al-Imp-Lysine (e),Au metal foil (f), and Au2 O3 (g).

Scheme 1. Proposed reaction mechanism of photocatalytic phenol decomposition
on gold species formed on LDH via reconstruction (A) or impregnation (B). Indirect
path via p-benzoquinone also took place for B, but the rates were lower than those
for A. The reaction paths from o-benzoquinone or muconic acid to CO2 are unknown.

Fig. 4. Comparison of the Au L3 -edge XANES spectra for Au/Zn2 Al-Rec-3-Light (a)and Au/Zn2 Al-Imp-NaBH4 (b) to the combination of standard spectra of Au metal foil
(Au00 ) and Au2 O3 (Au3+ ). The mixing ratio of Au00 :Au3+ was progressively varied:
0:100 (c), 55:45 (d), 65:35 (e), 75:25 (f), 85:15 (g), 95:5 (h), and 100:0 (i).

obtained by the LDH reconstruction method. The special environment provided by the transformation of the mixed oxides (ZnO
and/or ZnAl2 O4 ) into the layered LDH structure might give rise to
speciﬁc photo-responsive properties for the AuNPs–LDHs composites. For Au/Zn2 Al-Imp-NaBH4 and Au/Zn2 Al-Imp-Lysine, the SPR
peak due to Au was located at 536 nm and 537 nm, respectively(Fig. 2e, f). The UV–visible spectrum for Au/Zn2 Al-Light was ratherfeatureless and a weak, broad SPR peak appeared at 552 nm.
As will be described based on Au L3 -edge XANES (see section 3.3),
Au–LDHs comprising Au0 sites (Au/Zn2 Al-Rec-3-Light, Au/Zn2 AlImp-NaBH4 , and Au/Zn2 Al-Imp-Lysine) exhibited relatively intense
SPR peaks (Fig. 2d–f) in clear contrast to weaker SPR peaks for thecomposites comprising exclusive Au3+ sites (Au/Zn2 Al-Rec-1 and
Au/Zn2 Al-Rec-2; Fig. 2b, c).
3.3. Au L3 -edge XANES
Au L3 -edge XANES spectra measured for Au–LDH composites are
depicted in Fig. 3. The spectrum patterns for both Au/Zn2 Al-Rec1 and Au/Zn2 Al-Rec-2 (spectra a, b) were quite similar to that of
Au2 O3 (spectrum g) while the spectrum e for Au/Zn2 Al-Imp-Lysine
was similar to that of Au metal foil (spectrum f). On the other hand,the post-edge pattern for Au/Zn2 Al-Rec-3-Light and Au/Zn2 Al-ImpNaBH4 (spectra c and d) was that between that of Au00 and Au3+ ,demonstrating that the Au sites in these two catalysts were themixture of Au0 and Au3+ . The spectra for Au/Zn2 Al-Rec-3-Light and
Au/Zn2 Al-Imp-NaBH4 were ﬁtted with the standard spectra f andg of Au metal (Au0 ) and Au2 O3 (Au3+ ) by progressively changing
the mixing ratio of the two spectra (Fig. 4). The spectra of Fig. 3cand d most resembled mixed spectra with the mixing ratio of Au0 :Au3+ = 65:35 (Fig. 3e) and 85:15 (Fig. 3 g), respectively.

The Au sites were reduced to Au0 in Au/Zn2 Al-Imp-NaBH4 and
Au/Zn2 Al-Imp-Lysine by the effects of NaBH4 and/or lysine. On theother hand, the formation of Au0 in Au/Zn2 Al-Rec-3-Light was theconsequence of the electron transfer from Zn2 Al LDH to AuNPs
resulted from the charge separation occurred under the irradiation of UV–visible light. Previous results showed that the chargetransfer between TiO2 and the supported AuNPs [38] and between
ZnO and the supported Cu ions [28] should suppress the charge
recombination thus to maximize its photocatalytic response at surface. Based on these results, the speciﬁc charge transfer processes
between AuNP and Zn2 Al-LDH in the Au/Zn2 Al-Rec-3-Light sample
might improve the photocatalytic efﬁciency.

3.4. TEM
Fig. 5 shows representative TEM images and the particle size
distribution for AuNPs. Table 1 listed the mean diameter values.
TEM images indicated that almost spherical NPs were densely distributed on the Zn2 Al-LDH surface. The mean sizes of the AuNPs
were 4.7 nm and 4.9 nm for Au/Zn2 Al-Rec-1 and Au/Zn2 Al-Rec2, respectively, while mean size for AuNPs increased to 8.5 nm
for Au/Zn2 Al-Rec-3-Light (Fig. 5a–c and Table 1b–d). Furthermore,
together with XANES and TEM results, it was suggested thatparticle size distribution of AuNPs consisted of Au3+ for Au/Zn2 AlRec-3-Light negligibly change from that of Au/Zn2 Al-Rec-1 while
under solar irradiation greater AuNPs consisted of Au0 than ∼5 nm
were formed. Thus, particle size distribution for the AuNPs comprising mixed valences of Au00 & Au3+ in Au/Zn2 Al-Rec-3-Light
became wider (Fig. 5c). This result reveals an intimate interactionof Au3+ NPs with the Zn2 Al-LDH surface during the restoration of
the LDH in which Au3+ NPs supported on Zn2 Al-LDH would be able
to accept electrons quite effectively from the LDH surface to form
Au00 NPs (Scheme 1A). To obtain more detailed information about
the interactions between AuNPs and LDH surface, Au L3 -edge EXAFS
was employed to monitor the local structure for Au (see Section
3.5).
For AuNP–LDHs composites obtained by impregnation method
the mean diameter of self-supported AuNPs were 4.1 nm for
Au/Zn2 Al-Imp-NaBH4 and 7.3 nm for Au/Zn2 Al-Imp-Lysine (Fig. 5d,e and Table 1e, f).

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265

Fig. 5. TEM images and analysis for Au/Zn2 Al-Rec-1 (a), Au/Zn2 Al-Rec-2 (b), Au/Zn2 Al-Rec-3-Light (c), Au/Zn2 Al-Imp-NaBH4 (d), and Au/Zn2 Al-Imp-Lysine (e).

3.5. Au L3 -edge EXAFS
The coordination environment of AuNPs in AuNP–Zn2 Al-LDH
was investigated based on the Au-L3 edge EXAFS. The k3 -weighted
EXAFS function was depicted in Fig. 6. The peaks at 0.16 nm and0.28 nm (phase shift uncorrected) in the Fourier transform were
curve-ﬁt with the parameters of Au–O and Au–Au interatomic pairs,
respectively (Fig. 6b, d).
The best-ﬁt results were listed in Table 2. The N value of Au–O for
Au/Zn2 Al-Rec-3-Light was 1.6 and that for Au–Au was 9.0 (Fig. 63 and Table 2c), signiﬁcantly smaller than N values for models of
Au2 O3 (4; Table 2f) and Au metal (12; Table 2 g). Namely, the N valueof Au–O for Au/Zn2 Al-Rec-3-Light was 38–43% of N value for model
Au2 O3 crystallines taking the ﬁt error values into account. Similarly,
the N value for Au–Au in Au/Zn2 Al-Rec-3-Light was 65–85% of thatfor Au metal foil taking the ﬁt error values into account.
On the other hand, the N values of Au–O for Au/Zn2 Al-Rec-1 and
Au/Zn2 Al-Rec-2 samples (both 3.7, Table 2a, b) was much closer tothat of Au–O in Au2 O3 . These results demonstrated that the fabrication of the catalyst under irradiation from solar simulator decreased
the N values of both Au–Au and Au–O shells. Previous studies
reported that the decreased N(Au–Au) values in AuNPs–LDHs composites indicate stronger interactions between the nanosized Au
and LDHs surface [20]. Thus, Zn2 Al-LDH reconstruction in the aqueous solution of Au acetate under the irradiation of UV–visible lightshould give rise to closer interactions between Zn2 Al-LDH and the
supported Au nanoparticles.
The mean particle size for Au suggested by N(Au–Au) value
for Au/Zn2 Al-Rec-3-Light (9.0; Table 2c), Au/Zn2 Al-Imp-NaBH4(11.3; Table 2d), and Au/Zn2 Al-Imp-Lysine (11.8; Table 2e) were
2.5 nm, 4.8 nm, and 5.3 nm, respectively, assuming fcc crystalline
packing [39]. These values were signiﬁcantly smaller than thoseobtained using TEM images (8.5 nm, 4.1 nm, and 7.3 nm, respectively, Table 1d–f). The reasons of this discrepancy would be that
smaller NPs were not observable in TEM and thus TEM tends
to overestimate the nanoparticle size and/or that the size of Au

oxides was neglected in the estimation based on N(Au–Au) valuesof EXAFS. In fact, the mean values by TEM (7.3 nm) and by EXAFS(5.3 nm) were relatively close for exclusive Au00 NPs in Au/Zn2 AlImp-Lysine (Table 1f).
Only Au–Au peak was observed for Au/LDHs obtained byimpregnation method (Fig. 6-4, 6-5 and Table 2d, e). Thus, EXAFS
results provided further evidence for direct interactions of the surface self-supported AuNPs and Zn2 Al-LDH layers.
3.6. BET surface area
The SBET value was 97 m2 g−1 for Zn2 Al-LDH (Table 1a) in
consistent with reported values for LDHs of similar composition (33–83 m2 g−1 ) [22,23]. The value decreased by a factor of
0.61–0.84 times for reconstructed LDH samples in the presence ofAu (Table 1b–d) or by a factor of 0.56–0.69 times for impregnated
Au/LDHs (Table 1e, f). The decrease was not simply correlated tothe preparation method of catalysts or Au contents (Table 1).
3.7. Photocatalytic tests
Phenol was chosen as a molecule to represent toxic organic compounds with low degradability. The suitability of AuNPs–Zn2 AlLDHs for photocatalytic decomposition of phenol was evaluatedfrom aqueous solutions under irradiation with simulated solar
light. Fig. 7 shows the evolution of the UV–visible absorption proﬁles for reaction solution during the course of irradiation time.
Blank tests without photocatalyst or solar light revealed negligible photocatalytic activities less than 99% degradation of phenol,
verifying that phenol degradation reaction was driven via photocatalytic process.
For all the AuNPs–Zn2 Al-LDH catalysts, the characteristic
absorbance at 270 nm for phenol decreased as the photo-reaction
time elapsed. Unsupported Zn2 Al-LDH exhibited only 7.6% ofdecrease at 5 h of irradiation from the phenol absorbance at 0 min(Fig. 7a), demonstrating that only LDH was insufﬁcient for degrad-

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Fig. 6. Au L3 -edge EXAFS for Au/Zn2 Al-Rec-1 (1), Au/Zn2 Al-Rec-2 (2), Au/Zn2 Al-Rec-3-Light (3), Au/Zn2 Al-Imp-NaBH4 (4), and Au/Zn2 Al-Imp-Lysine (5). k3 -weighted EXAFS
 oscillation (a), its associated Fourier transform (b), and best-ﬁt results in k-space (c) and R-space (d). The red (or thin) line represents the experimental values, and the blue
(or thick) line represents calculated values in panels b and d. The solid line represents the magnitude and the dotted line represents the imaginary part in panels c and d. (Forinterpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)ing phenol under solar light. This fact indicates the essential
contribution of AuNPs to the photocatalytic process.
It was established that phenol photodecomposition could follow different pathways following either a direct oxidative opening
process of the aromatic ring, in which muconic acid is formed fromo-benzoquinone, or an indirect ring cleavage process to hydroxylated phenolic intermediates e.g. p-benzoquinone/hydroquinone
as primary intermediate species [36,39]. The peaks at 246, 263,
270, 275, and 289 nm were reported to appear in the UV absorption region for p-benzoquinone, trans,trans-muconic acid, phenol,
catechol, and hydroquinone [36,40] and in fact conﬁrmed for
4.3–74 ppm of standard aqueous solutions (Fig. 7h). In this regard,differences were observed between the Au–LDH composite catalysts obtained by reconstruction and impregnation methods.
Namely, for Au/Zn2 Al-Rec-1, Au/Zn2 Al-Rec-2 and Au/Zn2 Al-Rec3-Light (Fig. 7b–d), formation of p-benzoquinone, identiﬁed as acharacteristic absorption at 248 nm, showed that phenol degradation underwent through hydroxylated phenolic intermediates(Fig. 8B) versus both indirect and direct opening process of aromatic using Zn2 Al-LDH and impregnated Au photocatalysts on LDH(Fig. 7a, e–g). The peak intensity was evaluated based on peakdeconvolution using equation 3 as a function of wavelength. Quan-tum mechanically, it would be strict to deconvolute peaks as afunction of wave number, i.e. the dimension of energy. The timecourse in Fig. 7d was replotted as a function of wave number (Fig.
S1). It looks almost identical to Fig. 7d except for the direction ofhorizontal axis is inverse and we believe that the ﬁt using equation
3 as a function of wavelength makes sense.
For reconstructed Au–LDHs, the peak intensity at 248 nm
reached a maximum at 40–60 min from the start of irradiation(Fig. 8B-b–d), however, corresponding peak due to hydroquinone
at 288 nm was not detected during the time course probably due
to the quick oxidative conversion to thermodynamically morestable p-benzoquinone. Among them, Au/Zn2 Al-Rec-3-Light was
the most active based on the decrease of phenol absorbance at
270 nm (Fig. 8A-d). Interestingly, for Au/Zn2 Al-Rec-2, the peakintensity of p-benzoquinone decreased for initial 4 h but insteadslightly increased after 5 h of irradiation (Fig. 8B-c). It appears thatmost likely cause was the balance of the rates of phenol photooxidation and the redox equilibrium between hydroquinone andp-benzoquinone:
HO(surface) + h+ → • OH

(4)

Phenol + • OH + h+ → Hydroquinone + H+

(5)

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267

Table 2The curve-ﬁt analysis results of Au L3 -edge EXAFS for Au/Zn2 Al LDH samples.
Entry

SampleAu–O
R (nm)
N 2 (10−5 nm2 )a

Au/Zn2 Al-Rec-1b

Au/Zn2 Al-Rec-2

c

Au/Zn2 Al-Rec-3-Lightd

Au/Zn2 Al-Imp-NaBH4

0.197 (±0.001)
3.7 (±0.9)
0.8 (±0.1)
0.204 (±0.002)
3.7 (±0.3)
0.3 (±0.1)
0.195 (±0.002)
1.6 (±0.1)
0.1 (±0.4)
0.199 (±0.005)
0.3 (±0.2)
0.1 (±0.5)e

Au/Zn2 Al-Imp-Lysine

Modelsf

Au2 O3g

Au metal

Au–Au

4.1 × 103

1.0 × 103

0.2888 (±0.0004)
9.0 (±1.2)
0.3 (±0.1)
0.287 (±0.001)
11.3 (±0.6)
0 (±0.1)
0.287 (±0.001)
11.8 (±0.7)
0.18 (±0.04)

0.2013
4–(6)

p-Benzoquinone + 6O2 → . . . → 6CO2 + 2H2 O.

(7)

8.2 × 102

2.8 × 102

1.0 × 102–

0.2884
12–

Hydroquinone + 2 h+ → p-Benzoquinone + 2H+

Goodness of ﬁt–

Phenol + ·OH + h+ → Catechol + H+

(8)

Catechol + 2 h+ → o-Benzoquinone + 2H+(9)o-Benzoquinone + 2·OH → muconicacid(10)10 mM sodium phosphate buffer as a carrier. However, the calibration of the amount only for p-benzoquinone was difﬁcult inthe standard mixed solutions of p-benzoquinone (0.1–2.0 ppm),
trans,trans-muconic acid (0.1–2.0 ppm), phenol (5.0–100 ppm),
catechol (5.0–100 ppm), and hydroquinone (5.0–100 ppm) presumably due to the equilibrium between p-benzoquinone andhydroquinone (equation 6). The calibration of hydroquinone
using HPLC was conﬁdent due to the difference of sensitivity (molar absorption coefﬁcient) between p-benzoquinone andhydroquinone (Fig. 7h). This equilibrium may be also related
to the fact that no hydroquinone peak was observed in thephoto-decompsition monitoring using UV–visible spectroscopy(Fig. 7a–g). Improved HPLC analyses will be published elsewhere[41].
Furthermore, photodecomposition of acetophenone (25 mg L−1 )was also monitored to check the availability of UV–visible monitoring for the other catalytic reaction (Supplementary data). The
reactivity order was

muconicacid + 11/2O2 → . . . → 6CO2 + 3H2 O.

(11)

Au/Zn2 Al-Rec-3-Light > Au/Zn2 Al-Rec–1 > Au/Zn2 Al-Light >For Au/Zn2 Al-Imp-NaBH4 and Au/Zn2 Al-Imp-Lysine, we clearlyobserved the increase in the broadness of the absorption around270 nm. This suggests the formation of muconic acid (260 nm) andcatechol (276 nm) in addition to phenol (270 nm) in the reaction
solution. Thus in the proposed reaction path for the second group, in
addition to initial indirect ring opening via p-benzoquinone basedon the 248-nm peak maximum at 20–40 min from the start of irradiation (Fig. 8B-a, e–g), the oxidation of catechol to o-benzoquinone
and the direct opening of the aromatic ring of o-benzoquinone tomuconic acid also occurred [36]:Thus, different reaction paths were revealed between these two
types of catalysts. Fig. 8 shows the results for measured absorbanceof each deconvoluted peak using Gaussian functions. The activity sequence of the catalysts after 160 min of irradiation underUV–visible light was in the order (phenol degradation ratio in theparenthesis):
Au/Zn2 Al-Rec-3-Light(100%) > Au/Zn2 Al-Rec-1(70.1%) >Au/Zn2 Al-Light(52.3%) > Au/Zn2 Al-Rec-2(25.4%)∼ Au/Zn2 Al-Imp-NaBH4 (26.0%) ∼ Au/Zn2
Al-Imp-Lysine(22.5%) > Zn2 Al-LDH(17.6%).

(12)

Au/Zn2 Al-Rec-3-Light was the most active in degrading phenol under simulated solar irradiation. The major reaction routes
via equations 4–7 based on the monitoring using UV–visible spectroscopy should be conﬁdent with the conﬁrmation of peaks ofphenol and p-benzoquinone by HPLC equipped with a column
Shim-pack VP-ODS using the mixed solution of acetonitrile and

Au/Zn2 Al-Imp-NaBH4 ,(13)

similar as in equation 12.
Au/Zn2 Al-Rec-1 showed slightly less catalytic activity with 70%
phenol degradation ratio of that using Au/Zn2 Al-Rec-3-Light for
160 min of reaction. Note that Au/Zn2 Al-Rec-3-Light, Au/Zn2 AlRec-1, and Au/Zn2 Al-Light have essentially identical chemicalcompositions in LDH and also Au content although chemical
states were different in AuNPs: a mix state of Au0 and Au3+
in Au/Zn2 Al-Rec-3-Light versus exclusive Au3+ in Au/Zn2 Al-Rec-1.
Hence, balancing the oxidation state of AuNPs should clearly inﬂuence the photocatalytic performance.
Increasing Au content in Au/Zn2 Al-Rec-2 steadily declined thephenol degradation ratio to 25% for 160 min of reaction (Fig. 8Ac) and p-benzoquinone was accumulated (Fig. 8B-c). Furthermore,
Au–Zn2 Al catalysts obtained by impregnation-reduction method
showed slightly less photocatalytic activity, with 26–23% phenol
degradation over the same time frame. For quantitative comparison
between the catalysts, speciﬁc rates (␮mol L−1 h−1 ) were calculated
based on the derivative of phenol amount as a function of time

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Fig. 7. UV–vis absorption spectrum of phenol photo degradation time course for Zn2 Al-LDH (a), Au/Zn2 Al-Rec-1 (b), Au/Zn2 Al-Rec-2 (c), Au/Zn2 Al-Rec-3-Light (d), Au/Zn2 AlImp-NaBH4 (e), Au/Zn2 Al-Imp-Lysine (f), Au/Zn2 Al-Light (g), p-benzoquinone (4.3 ppm; h1), trans,trans-muconic acid (5.0 ppm; h2), phenol (67 ppm; h3), catechol (74 ppm;h4), and hydroquinone (68 ppm; h5).

(Table 1):
Au/Zn2 Al-Rec-3-Light(199) > Au/Zn2 Al-Rec-1(125) >Au/Zn2 Al-Light(68) > Au/Zn2 Al-Rec-2(47) ∼ Au/Zn2
Al-Imp-NaBH4 (42) ∼ Au/Zn2
Al-Imp-Lysine(43) > Zn2 Al-LDH(25).

(14)

It was also apparent from the rates that Au/Zn2 Al-Rec-3-Light
showed a maximum rate for phenol degradation and the order ofthe rates was essentially identical to the order of degradation extent
in equation 12. The results showed that Au0 -based catalysts wereapproximately four orders of magnitude less active than Au/Zn2 AlRec-3-Light comprising a mixed valence state of Au0 /Au3+ .
In fact, the numbers of surface-exposed Au0 sites were
estimated for Au/Zn2 Al-Rec-3-Light, Au/Zn2 Al-Imp-NaBH4 , and
Au/Zn2 Al-Imp-Lysine based on mean Au particle size by TEM
and cubo-octahedron model [42] (Table 1d–f). The values
27 ␮mol gcat −1 , 31 ␮mol gcat −1 , and 19 ␮mol gcat −1 , respectively,
show no correlation to photocatalytic rates of phenol decomposition. Furthermore, BET surface area was evaluated for catalysts
after phenol photodecomposition tests for 5 h (Table 1). The activity
order is written again with the BET surface area in the parenthesis:

G. Mikami et al. / Applied Catalysis B: Environmental 199 (2016) 260–271

269

Fig. 8. The time course for peak at 270 nm (phenol; A) and at 248 nm (p-benzoquinone; B) evaluated based on Gaussian deconvolution during phenol photo-degradation for
Zn2 Al-LDH (a), Au/Zn2 Al-Rec-1 (b), Au/ZnAl-Rec-2 (c), Au/Zn2 Al-Rec-3-Light (d), Au/Zn2 Al-Imp-NaBH4 (e), Au/Zn2 Al-Imp-Lysine (f), and Au/Zn2 Al-Light (g).

Fig. 9. The photocatalytic activity for six cycles of phenol decomposition. Before the
6th cycle, the catalyst was kept during 1 h under solar irradiation using Au/Zn2 AlRec-3-Light.

Au/Zn2 Al-Rec-3-Light(79 m2 g−1 ) > Au/Zn2 Al-Rec-1(81 m2 g−1 ) > Au/Zn2 Al-Rec-2(59 m2 g−1 )∼ Au/Zn2 Al-Imp-NaBH4 (54 m2 g−1 ) ∼ Au/Zn2 AlImp-Lysine(67 m2 g−1 ) > Zn2 Al-LDH(97 m2 g−1 ).

(14 )

It was difﬁcult to ﬁnd the correlation between the BET surface areaand he photocatalytic activity order. Thus, Au0 sites and speciﬁc
surface area were directly not related to photocatalytic activity.
We also noticed that Au/Zn2 Al-Rec-3-Light showed the highest
intensity for the SPR response of AuNPs (see Fig. 2d), although a systematic correlation between the SPR intensity of the self-supported
AuNPs and the photocatalytic efﬁciency of the studied catalysts
was not found mostly due to the difference of SPR response
between Au0 and Au3+ sites. Furthermore, as shown in Table 1b,c, and e, there was no remarkable difference in AuNPs mean
particles sizes of Au/Zn2 Al-Rec-1, Au/ZnAl-Rec-2, and Au/Zn2 AlImp-NaBH4 (4.1–4.9 nm) based on TEM images. On the other
hand, a slight variation was observed for Au/Zn2 Al-Rec-3-Light and
Au/Zn2 Al-Imp-Lysine with a mean size of AuNPs of 8.5 and 7.3 nm,
respectively (Table 1d, f). Thus, small extent of increase of AuNPs
mean size affected no impact on the photocatalytic activity.
In view of its application, the stability of the photocatalyst isalso important and needs to be evaluated. The best Au/Zn2 AlRec-3-Light photocatalyst was tested for the reusability. Fig. 9shows the recycling experiments of Au/Zn2 Al-Rec-3-Light for phenol degradation under solar irradiation. The degradation rates

using photocatalysts were 100%, 91.2%, 86.8%, 78.9%, and 72.8% in
the ﬁrst, second, third, fourth, and ﬁfth cycle of tests (160 minirradiation for each cycle). The catalyst efﬁciency still remained
at 73% of initial activity after the ﬁfth successive recycle tests.
Furthermore, the used Au/Zn2 Al-Rec-3-Light sample was separated from the reactor after the ﬁfth cycle and irradiated for 1 h
under UV–visible light. After the light treatment, the catalyst wastested again for phenol photo-decomposition. Conﬁdently, the efﬁciency for phenol degradation increased to 80.9% of initial activity,
increased from 72.8% in the ﬁfth cycle. This behavior might berelied on the particular charge transfers in the intimately mixed
phases of AuNPs and Zn2 Al-LDHs evolved under the irradiation
with UV–visible light.
AuNPs leaching, a possible explanation for the decrease in
efﬁciency was independently ruled out by centrifuging the reaction mixture after each cycle. No further reaction of phenol wasobserved when the supernatant was mixed with the fresh phenol
solution under the solar irradiation, demonstrating a minimal/nocontribution from leached AuNPs. Furthermore, the UV–visible
analysis of the supernatant did not detect SPR effect of AuNPspointing out that nano-sized gold species was not present in the
supernatant. The leaching of Zn or Al from the LDH layers mightoccur only at acidic pH (lower than 3) while the pH of the phenol
aqueous solutions used during the experiments (containing also
the catalyst) was around 7.5.

4. Discussion
4.1. Electronic and structure characteristics of AuNPs on
AuNPs/LDHs interface
Photocatalytic phenol decomposition rates followed the orderof equation 12. Basically, photocatalysts comprising Au embedded
during the reconstruction of LDH layers were superior to impregnated Au on LDH using chemical reductant and/or surfactant or
unsupported LDH (Fig. 8). Typical difference between these twogroups was the valence state of Au atoms in AuNPs: major valence
state was Au3+ in the former AuNP–LDHs via reconstruction routeversus major metallic Au0 state in the latter Au photocatalysts
impregnated on LDH based on Au L3 -edge XANES.
In the photocatalytic activity order of phenol decompositionin equation 12, Au/Zn2 Al-Rec-3-Light was especially active and30 mg L−1 of phenol was completely decomposed to lower thandetection limit within 160 min of UV–visible light irradiation(Figs. 7d and 8d). The valence state of Au was evaluated to Au0 :Au3+ of 65: 35 based on the comparison to the mixture of Au L3 -

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G. Mikami et al. / Applied Catalysis B: Environmental 199 (2016) 260–271

edge XANES spectra for Au metal and Au2 O3 by changing the mixing
ratio of the two (Fig. 4a, e).
The coordination of Au sites in Au/Zn2 Al-Rec-3-Light was further investigated by Au L3 -edge EXAFS analyses (Fig. 6-3). BothAu–Au and Au–O peaks in the Fourier transform were clearlyobserved only for this sample among the AuNP–LDH samples via
reconstruction. In fact, both Au–O peak with the N value of 1.6 (4for Au2 O3 crystallites) and Au–Au peak with the N value of 9.0 (12for Au metal) were given based on the curve-ﬁt analysis (Table 2c).
These reduced N values compared to bulk crystallites/crystals suggested mean 8.5 nm of AuNPs embedded on LDH surface (TEM,
Fig. 5c). The reconstruction of LDH was taking place in a complex
environment in which the mixture of ZnO and ZnAl2 O4 was able
to reconstruct the brucite-like Zn2 Al-LDH layers. Such a speciﬁc
environment might facilitate particular phases junctions between
ZnO/ZnAl2 O4 /Zn2 Al-LDH and the evolved AuNPs where Au3+ ionsshould bind with their surface to constitute thin Au2 O3 interface
phase and diffused electrons originating from the charge separation
by UV–visible light irradiation during the reconstruction process
should reduce surface Au sites to Au0 (Scheme 1A). The interfaceof Au2 O3 phase between metallic Au0 and LDH might facilitate
the electron transfer from semiconducting LDH to Au sites for O2
reduction during aerobic phenol photo-decomposition whereas themetallic Au0 NPs were not effectively embedded on the LDH surface
for impregnated AuNP–LDH samples thereby the electron transfer
at the interface was slower and the charge separation efﬁciency
was lower.
Furthermore, the comparison of Debye–Waller factors (␴2 )obtained in EXAFS analysis revealed that Au0 had a higher
disorder in Au/Zn2 Al-Rec-3-Light (␴2 Au–Au: 0.3 × 10−5 nm2 )than in the AuNP–LDHs obtained via impregnation (␴2 Au–Au:
0 × 10−5 nm2 and 0.18 × 10−5 nm2 ) due to speciﬁc interaction
between AuNP and LDH during reconstruction [43] while the
Au–O showed the lowest disorder in Au/Zn2 Al-Rec-3-Light (␴2
0.1 × 10−5 nm2 ) compared to other reconstructed AuNP–LDHs(␴2 0.3 × 10−5 –0.8 × 10−5 nm2 ) comprising dominant AuIII sites
probably due to the difference between the Au oxide interface sites
and Au oxide NP sites.
4.2. Proposed reaction mechanism of phenol photodecomposition
utilizing various states of Au on LDH
The other difference between the ﬁrst group, AuNP–LDHs via
reconstruction, and the second group, obtained by impregnation of Au on LDH, was the photocatalytic reaction pathway:
via indirect ring cleavage mechanism to p-benzoquinone and/or
other hydroxylated phenolic intermediates versus both indirect anddirect oxidative ring opening processes, respectively. Thermodynamically more stable p-benzoquinone compared to hydroquinone
would be formed via reaction of phenol with a hole and hydroxy
radical (produced from a hole & surface hydroxy; equation 4) tohydroquinone (equation 5) on LDH surface under the irradiationof UV–visible light followed by further two electron oxidation byholes to p-benzoquinone (equation 6). These oxidation reaction
steps would be accelerated by the effective charge transfer at theinterface between AuNPs and LDH and the resultant better charge
separation efﬁciency as the assembly. Using anatase-type TiO2 ,boron-doped anatase TiO2 , and Au and/or Pd-doped TiO2 underUV–visible light, a peak at 289 nm due to hydroquinone gradually
increased associated with the decrease of peak decrease at 270 nm
due to phenol, but any clear peak at 248 nm due to p-benzoquinone
was not observed [31,44]. This difference suggested that the photooxidation from hydroquinone to p-bezoquinone was favorable
over LDH surface (Scheme 1A) versus signiﬁcant photo-reduction
from p-benzoquinone to hydroquinone over TiO2 -based photocatalysts. The stabilization of p-benzoquinone was also pointed out for

CeO2 –ZnTi-LDH photocatalyst via LDH reconstruction route [36].
However, the photocatalytic contribution of impurity level of metaloxide phases, e.g. ZnO and ZnAl2 O4 , detected for the ﬁrst group,
AuNP–LDHs via reconstruction, as detected for active photocatalysts in XRD (Fig. 1b, d) cannot be totally excluded because thecharge separation efﬁciency using metal oxides is in general superior to that using metal hydroxides.
In the second group of photocatalysts, the peak due to pbenzoquinone at 248 nm appeared after 20–40 min suggesting
indirect ring opening, however the peak was weaker (Fig. 8B-a,e–g) than that in the ﬁrst group, especially for Au/Zn2 Al-Rec-2
and Au/Zn2 Al-Rec-3-Light (Fig. 8B-c, d), suggesting that direct ringcleavage mechanism also proceeded. In fact, the minor contribution of muconic acid (260 nm) and catechol (276 nm) was suggested
based on the broader peak width centered at 270 nm (phenol),
especially for Au/Zn2 Al-Imp-NaBH4 and Au/Zn2 Al-Imp-Lysine during the time course of phenol decomposition irradiated by light(Fig. 7a, e–g). It should be noted that p-benzoquinone was detected
after 40–60 min for the ﬁrst group but that the phenol decomposition rates to CO2 and H2 O (equation 7) was faster using ﬁrst
group, especially using Au/Zn2 Al-Rec-3-Light: total mineralization
at 160 min (Fig. 7d). Thus, the rates from phenol to p-benzoquinone(Scheme 1A) should be signiﬁcantly accelerated for the ﬁrst groupcompared to the rates from phenol to muconic acid (Scheme 1B)
due to greater amount of diffused holes owing to the effective
charge separation for reconstructed AuNP–LDH assembly.
This study provides experimental evidence that to design efﬁcient AuNPs–LDHs for solar energy photocatalysis the tailored
construction for speciﬁcally designed the characteristics of thephase junctions between AuNPs and LDH is required.

5. Conclusions
AuNPs formed via reconstruction route of LDH in Au(III)acetate aqueous solution was compared to AuNPs impregnated on LDH and reduced by NaBH4 and/or lysine. The LDH
reconstruction was performed in the presence/absence ofUV–visible light. AuNPs impregnated in LDH in the presenceof UV–visible light were also prepared from comparison.
The activity of phenol photo-decomposition was in theorder
Au/Zn2 Al-Rec-3-Light<span> Au/Zn2 Al-Rec–1 > Au/Zn2 AlLight > Au/Zn2 Al-Rec–2 ∼ Au/Zn2 Al-Imp-NaBH4 ∼ Au/Zn2 AlImp-Lysine > Zn2 Al-LDH. Thus, AuNPs were essential for thephotocatalysis and reconstructed AuNP–LDHs were basicallysuperior to impregnated AuNP–LDHs. Among them, Au/Zn2 Al-Rec3-Light prepared via LDH reconstruction irradiated by UV–visible
light was the best and totally photo-decomposed 50 mg L−1 ofphenol in 160 min. In the catalyst, approximately 5 nm of Au0 NPs
were embedded on LDH surface through Au2 O3 layers as revealedby Au L3 -edge XANES and EXAFS. Charges (electrons and holes)in the Au–LDH assembly separated by light were effectively diffused to Au0 NPs and LDH surface and reduced O2 and proceededindirect ring-opening mechanism to hydroquinone and then top-benzoquinone, respectively. As the unsupported LDH negligibly
photo-decomposed phenol, AuNPs were essential and directly
incorporated in the catalysis. AuNPs impregnated on LDH under
the UV–visible irradiation (not reconstruction route) or reduced
using NaBH4 and/or lysine were less active and decomposed
phenol via both indirect mechanism via p-benzoquinone anddirect ring-opening mechanism to catechol, o-benzoquinone, andthen to muconic acid.

G. Mikami et al. / Applied Catalysis B: Environmental 199 (2016) 260–271

Acknowledgments
The authors are grateful for the ﬁnancial supports from theGrant-in-Aid for Scientiﬁc Research C (26410204, 22550117) fromJapan Science Promotion Agency, Japan and from the RomanianNational Authority for Scientiﬁc Research, CNCS-UEFISCDI (PN-IIID-PCE-75/2013), Romania. X-ray absorption experiments wereconducted under the approval of the Photon Factory Proposal
Review Committee (2014G631, 2012G683, 2011G033), Japan. The
authors thank Shimadzu Corporation for the HPLC analyses.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,in the online version, at http://dx.doi.org/10.1016/j.apcatb.2016.
06.031.
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