Au Nanoparticles Supported on BiVO4: Effective Inorganic Photocatalysts for H2O2 Production from Water and O2 under Visible Light

Article abstract of DOI:10.1021/acscatal.6b01187

The design of a safe and sustainable process for the synthesis of hydrogen peroxide (H₂O₂) is a very important subject from the viewpoint of green chemistry. Photocatalytic H₂O₂ production with earth-abundant water and molecular oxygen (O₂) as resources is an ideal process. A successful system based on an organic semiconductor has been proposed; however, it suffers from poor photostability. Here we report an inorganic photocatalyst for H₂O₂ synthesis. Visible light irradiation (λ >420 nm) of the semiconductor BiVO₄ loaded with Au nanoparticles (Au/BiVO₄) in pure water with O₂ successfully produces H₂O₂. The bottom of the BiVO₄ conduction band (0.02 V vs NHE, pH 0) is more positive than the one-electron reduction potential of O₂ (-0.13 V) while more negative than the two-electron reduction potential of O₂ (0.68 V). This thus suppresses one-electron reduction of O₂ and selectively promotes two-electron reduction of O₂, resulting in efficient H₂O₂ formation.

Full text of DOI:10.1021/acscatal.6b01187

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Article
Au Nanoparticles Supported on BiVO4: Effective Inorganic Photocatalysts
for H2O2 Production from Water and O2 under Visible Light
Hiroaki Hirakawa, Shingo Shiota, Yasuhiro Shiraishi,
Hirokatsu Sakamoto, Satoshi Ichikawa, and Takayuki Hirai
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01187 • Publication Date (Web): 21 Jun 2016
Downloaded from http://pubs.acs.org on June 23, 2016
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Au Nanoparticles Supported on BiVO4: Effective Inorganic Photocata-
lysts for H2O2 Production from Water and O2 under Visible Light
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Hiroaki Hirakawa,† Shingo Shiota,† Yasuhiro Shiraishi,*,†,‡ Hirokatsu Sakamoto,† Satoshi Ichikawa,ǁ
and Takayuki Hirai†
† Research Center for Solar Energy Chemistry, and Division of Chemical Engineering, Graduate School of Engineering Sciꢀ
ence, Osaka University, Toyonaka 560ꢀ8531, Japan
‡ PRESTO, JST, Saitama 332ꢀ0012, Japan
ǁ Institute for NanoScience Design, Osaka University, Toyonaka 560ꢀ8531, Japan
ABSTRACT: Design of safe and sustainable process for the synthesis of hydrogen peroxide (H2O2) is a very important subject
from the viewpoint of green chemistry. Photocatalytic H2O2 production with earthꢀabundant water and molecular oxygen (O2) as
resources is an ideal process. A successful system has been proposed based on an organic semiconductor; however, it suffers from
poor photostability. Here we report an inorganic photocatalyst for H2O2 synthesis. Visible light irradiation (λ >420 nm) of semiconꢀ
ductor BiVO4 loaded with Au nanoparticles (Au/BiVO4) in pure water with O2 successfully produces H2O2. The bottom of the Biꢀ
VO4 conduction band (0.02 V vs. NHE, pH 0) is more positive than the oneꢀelectron reduction potential of O2 (−0.13 V), while
more negative than the twoꢀelectron reduction potential of O2 (0.68 V). This thus suppresses oneꢀelectron reduction of O2 and selecꢀ
tively promotes twoꢀelectron reduction of O2, resulting in efficient H2O2 formation.
KEYWORDS: Photocatalysis · Hydrogen peroxide · Visible light · Bismuth vanadate · Gold nanoparticles
INTRODUCTION
2H2O + O2 → 2H2O2
(3)
H2O2 is a clean oxidant that emits only water as a byproduct
and is widely used for pulp bleaching and disinfection.1 It has
also received much attention as a fuel cell energy carrier alterꢀ
native to H2 because it is soluble in water and can be used in
an oneꢀcompartment cell for electricity generation.2 H2O2 is
currently manufactured by the anthraquinone method based on
hydrogenation of anthraquinone with H2 on Pdꢀbased catalyst
followed by oxidation with O2.3 Alternative to this highꢀ
energyꢀconsuming twoꢀstep process, oneꢀstep H2O2 synthesis
based on the reaction of H2 with O2 has also been studied with
Pdꢀbased catalysts.4,5 This direct synthesis quantitatively proꢀ
duces H2O2, but needs special care due to the potential exploꢀ
sive nature of H2/O2 mixed gases. A new method that produces
H2O2 without H2 is therefore desired.
Some TiO2ꢀbased inorganic photocatalysts have been proꢀ
posed,6–9 but are inactive for H2O2 production from water and
O2. This is because they mainly promote oneꢀelectron reducꢀ
tion of O2, producing a superoxide (●OOH) radical (eq. 4) raꢀ
ther than twoꢀelectron reduction of O2 (eq. 2).9 Another reason
is that these TiO2ꢀbased catalysts have insufficient activity for
water oxidation (eq. 1);10 therefore, sacrificial electron donors
such as alcohols are necessary. In addition, these catalysts
require UV light (~400 nm) for photoexcitation; the formed
H2O2 is subsequently decomposed by absorbing UV light.11,12
The design of photocatalysts that promote water oxidation and
twoꢀelectron reduction of O2 under visible light irradiation (λ
>400 nm) is necessary.
O2 + H+ + e– → ●OOH
(4)
Catalytic H2O2 synthesis with earthꢀabundant water and O2
as resources is therefore ideal. Photocatalysis is one promising
method for this purpose. The basic concept is as follows: the
valence band holes (VB h+) formed by photoexcitation of the
catalyst oxidize water and produce O2 and H+ (eq. 1). Twoꢀ
electron reduction of O2 by the conduction band electrons (CB
e–) produces H2O2 (eq. 2). As a result of this, H2O2 can be proꢀ
duced from water and O2 (eq. 3).
Earlier, we reported that an organic semiconductor based on
graphitic carbon nitride (gꢀC3N4) promotes these reactions (eqs.
1 and 2) under visible light and successfully produces mM levꢀ
els of H2O2.13 This organic system, however, suffers from poor
photostability, as often observed for related materials.14,15 The
design of robust inorganic system is therefore desired for susꢀ
tainable H2O2 synthesis.
2H2O + 4h+ → O2 + 4H+
O2 + 2H+ + 2e– → H2O2
(1)
(2)
One powerful approach for the design of inorganic photoꢀ
catalyst for H2O2 synthesis is the use of Au nanoparticles. Terꢀ
anishi et al. prepared TiO2 loaded with Au particles.16 The
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Au/TiO2 catalyst, when irradiated by UV light in an EtOH
amount of Au loaded [x (wt %) = Au/BiVO4 × 100]. As shown
solution, produces relatively larger amount of H2O2 than the
early reported inorganic systems.6–9 As shown in Scheme 1a,
the photoformed CB e– of TiO2 is trapped by the Au particles.
These e– promote twoꢀelectron reduction of O2 more selectiveꢀ
ly as compared to bare TiO2 or TiO2 loaded with other metals.
This results in relatively efficient production of H2O2. The
Au/TiO2 catalyst is, however, inactive for H2O2 formation in
water. This is because TiO2 is less active for water oxidation10
and UV irradiation is required for its photoexcitation inevitaꢀ
bly decomposes the formed H2O2.11,12 In addition, as shown in
Scheme 1a, the bottom of TiO2 CB (−0.19 V vs. NHE, pH 0)17
is more negative than the potentials for oneꢀelectron reduction
of O2 (−0.13 V) and twoꢀelectron reduction of O2 (0.68 V).18
This thus promotes both reduction reactions and decreases the
selectivity for H2O2 formation. The loading of Au particles
onto the semiconductors that are active for water oxidation
under visible light and have relatively positive CB would be
effective for photocatalytic H2O2 production.
in Figure 2, transmission electron microscopy (TEM) image of
Au0.2/BiVO4 prepared by calcination at 673 K shows Au partiꢀ
cles with an average diameter 7.2 nm. The lattice spacings of
the Au particles (0.24 nm) and BiVO4 (0.31 nm and 0.26 nm)
agree with those of standards Au(111) (JCPDS 01ꢀ1174, 0.235
nm), BiVO4(103) (JCPDS 01ꢀ074ꢀ4894, 0.312 nm), and Biꢀ
VO4(200) (0.260 nm). As shown in Figure 1, Au0.2/BiVO4
shows characteristic absorption at ca. 600 nm assigned to the
surface plasmon resonance (SPR) of Au particles.22
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Figure 1. Absorption spectra of BiVO4 and Au0.2/BiVO4 prepared
by calcination at 673 K. Plots show the action spectrum for H2O2
formation on Au0.2/BiVO4. The apparent quantum yields were
calculated with the equation [ΦAQY (%) = ([H2O2 formed] × 2) /
(photon number entered into the reaction vessel) × 100]. The
spectra for other catalysts are summarized in Figure S5 (Supportꢀ
ing Information).
Scheme 1. Energy diagrams for (a) Au/TiO2 and (b) Au/BiVO4,
and reduction potential of O2.
Here we report that the answer may lie in the use of semiꢀ
conductor bismuth vanadate (BiVO4), active for water oxidaꢀ
tion under visible light (~550 nm).19 We found that BiVO4
loaded with Au particles (Au/BiVO4) successfully produces
H2O2 from water and O2 under visible light; this is a first exꢀ
ample of allꢀinorganic photocatalyst for H2O2 synthesis. As
shown in Scheme 1b, the bottom of BiVO4 CB (0.02 V)20 is
more positive than the oneꢀelectron reduction potential of O2
(−0.13 V), while more negative than the twoꢀelectron reducꢀ
tion potential of O2 (0.68 V).18 Photoexcitation of Au/BiVO4
therefore selectively promotes twoꢀelectron reduction of O2,
successfully producing H2O2.
RESULTS AND DISCUSSION
Catalyst preparation and characterization. BiVO4 was
prepared by stirring KV3O8 and Bi(NO3)3·5H2O in water.19 As
shown in Figure 1, the obtained yellow powders (average size,
1.2 ꢁm; BET surface area, 1.5 m2 g–1) absorb visible light at
<550 nm. As shown in Figure S1 (Supporting Information), Xꢀ
ray diffraction (XRD) analysis of the powders shows distincꢀ
tive diffractions for monoclinic BiVO4.21 Au particles were
loaded on BiVO4 by the deposition–precipitation method:22
The BiVO4 powders were stirred in water (pH 7) containing
HAuCl4⋅4H2O at 353 K and calcined at different temperatures,
affording dark yellow powders of Aux/BiVO4, where x is the
Figure 2. (a) TEM image of Au0.2/BiVO4 prepared by calcination
at 673 K and (b) size distribution of the Au particles. (c, d) Highꢀ
resolution images.
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Photocatalytic reaction. Photoreactions were performed by
as the fresh one, indicating that the catalyst is reusable without
the loss of activity.
photoirradiation of pure water (30 mL) containing respective
catalysts (50 mg) by a Xe lamp (λ >420 nm) with magnetic
stirring under O2 (1 atm) at 298 K. Table 1 summarizes the
results obtained by 10 h reaction. Bare BiVO4 (entry 1) scarceꢀ
ly produces H2O2 (<5.0 ꢁM, less than the detection limit). In
contrast, Au loading onto BiVO4 significantly enhances H2O2
formation (40.2 ꢁM, entry 2). Other metal particles loaded on
BiVO4 (entries 3–7) are ineffective (<12.4 ꢁM). In addition,
50
Au0.2/BiVO4
40
30
20
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other semiconductors such as TiO2,16 WO3, and AgNbO3
loaded with Au particles (entries 8–10) are also inactive
(<13.9 ꢁM) even under UV irradiation. These data suggest that
Au/BiVO4 specifically produces H2O2 very efficiently.
Au0.2/TiO2
10
10
0
0
5
t / h
Table 1. Results of photocatalytic H2O2 production. a
metal
particle
light
irradiation
[nm]
H2O2
Figure 3. Timeꢀdependent changes in the H2O2 concentrations
during photoreaction on Au0.2/TiO2 (λ >300 nm) and Au0.2/BiVO4
prepared by calcination at 673 K (λ >420 nm). Reaction condiꢀ
tions are identical to those in Table 1.
entry
catalyst
[ꢀM] c
size [nm] b
1
2
BiVO4
>420 g
>420 g
>420 g
>420 g
>420 g
>420 g
>420 g
>420 g
>420 g
>300 h
>420 g
>420 g
<5.0
40.2
39.2
40.0
12.4
<5.0
<5.0
<5.0
<5.0
<5.0
13.9
<5.0
d
Au0.2/BiVO4
1st reuse e
2nd reuse e
Ag0.2/BiVO4
Pd0.2/BiVO4
7.2 ± 2.2
The photoexcited Au/BiVO4 produces H2O2 via water oxiꢀ
dation and twoꢀelectron reduction of O2 (eqs. 1−3). This is
confirmed by the half reactions: the O2 reduction with EtOH
as an electron donor (Figure S2, Supporting Information) and
the water oxidation with AgNO3 as an electron acceptor (Figꢀ
ure S3, Supporting Information) successfully occur on the
Au0.2/BiVO4 catalyst. It is noted that O2 reduction with EtOH
by 10 h photoreaction produces much larger amount of H2O2
(257 ꢁM) than that formed in pure water (40 ꢁM, Figure 3). It
is also noted that the initial rate of water oxidation with Agꢀ
NO3 on Au0.2/BiVO4 (8.6 ꢁmol h−1) is larger than those on
Au0.2/TiO2, Au0.2/WO3, and Au0.2/AgNbO3 (<3.5 ꢁmol h−1);
this is consistent with the activity tendency for H2O2 producꢀ
tion (Table 1, entries 2 and 8−10). These findings suggest that
water oxidation (eq. 1) is the rateꢀdetermining step for H2O2
production from water and O2; the Au/BiVO4 catalyst with
higher activity for water oxidation exhibits the best catalytic
performance.24
7.4 ± 1.8
7.0 ± 2.0
f
f
3
4
5
6
7
8
9
f
Pt0.2/BiVO4
Co0.2/BiVO4
Ni0.2/BiVO4
f
f
d
Au0.2/TiO2
d
Au0.2/WO3
d
10
Au0.2/AgNbO3
a
Reaction conditions: water (30 mL), catalyst (50 mg), O2 (1
b
atm), temperature (298 K), time (10 h). Determined by TEM
observations (Figure S6, Supporting Information). Determined
by redox titration with KMnO4 (detection limit, 5.0 ꢁM). Preꢀ
c
d
pared by the deposition–precipitation followed by calcination at
e
673 K. Catalysts were reused after simple washing with water
f
As shown in Figure 3, during the reaction on Au0.2/BiVO4,
the rate of H2O2 formation decreases with photoirradiation
time. This is due to the subsequent decomposition of the
formed H2O2 by photocatalytic reactions. Figure S4 (Supportꢀ
ing Information) shows the results for H2O2 decomposition
experiments on Au/BiVO4. In the dark condition, H2O2 is
scarcely decomposed. In contrast, visible light irradiation of
the solution even with EtOH as an electron donor or with
NaIO3 as an electron acceptor under Ar atmosphere decreases
the concentration of H2O2. This indicates that, as usually obꢀ
served in related systems, H2O2 is decomposed by the reducꢀ
tion with CB e– (eq. 5)16 and by the oxidation with VB h+ (eq.
6).25.
followed by drying in vacuo. Prepared with sodium formate as a
h
reductant. g Light intensity at 420–500 nm is 26.9 W m−2. Light
intensity at 300–450 nm is 27.3 W m−2.
Figure 3 shows the timeꢀdependent changes in the H2O2
concentrations during reactions on Au0.2/TiO2 and Au0.2/BiVO4.
Au0.2/TiO2 scarcely produces H2O2 (<5.0 ꢀM). In contrast, on
Au0.2/BiVO4, H2O2 concentration increases with time. This
suggests that photoexcited Au/BiVO4 promotes water oxidaꢀ
tion (eq. 1) and O2 reduction (eq. 2), producing H2O2 (eq. 3).
Figure 1 shows the action spectrum for H2O2 formation on
Au0.2/BiVO4. Change in the apparent quantum yields (ΦAQY
)
agree well with the absorption spectrum of bare BiVO4. This
indicates that bandꢀgap excitation of BiVO4 promotes the reacꢀ
tions, where the SPR absorption of the Au particles does not
affect the reaction. As shown in Table 1 (entry 2), the catalyst
recovered after the reaction exhibits almost the same activity
H2O2 + e– → ●OH + OH–
H2O2 + 2h+ → O2 +2H+
(5)
(6)
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Selective two-electron reduction of O2. Oneꢀelectron reꢀ
B = 0.2nFν–1/6CD2/3
(8)
duction of O2 (eq. 4) is suppressed on the Au/BiVO4 catalyst.
As shown in Scheme 1b, the BiVO4 CB (0.02 V)20 lies beꢀ
tween the potentials for oneꢀelectron reduction of O2 (–0.13 V)
and twoꢀelectron reduction of O2 (0.68 V),18 although those of
many other semiconductors such as TiO2 lie on more negative
positions (Scheme 1a).17 This positive CB level of BiVO4
therefore suppresses oneꢀelectron reduction of O2 (eq. 4). This
is confirmed by electron spin resonance (ESR) analysis with
5,5ꢀdimethylꢀ1ꢀpyrroline Nꢀoxide (DMPO) as a spin trapping
reagent. An EtOH/water mixture (9/1 v/v, 5 mL) containing
DMPO (0.1 mmol) was photoirradiated with Au0.2/BiVO4 or
Au0.2/TiO2 catalyst (20 mg). Figure 4 shows the ESR spectra
of the solutions recovered after 3 min photoirradiation. The
Au0.2/BiVO4 sample (red) shows almost no signal, but the
Au0.2/TiO2 sample (black) exhibits distinctive signals assigned
F is the Faraday constant (96485 C mol–1), ν is the kinetic
viscosity of water (0.01 cm2 s–1), C is the bulk concentration of
O2 in solution (1.3 × 10–6 mol cm–3), and D is the diffusion
coefficient of O2 (2.7 × 10–5 cm2 s–1), respectively.33 The n for
Au0.2/TiO2 (black) is determined to be 1.6, suggesting that
oneꢀelectron reduction of O2 (n = 1) indeed occurs mainly on
Au/TiO2, as supported by the ESR data (Figure 4). This indiꢀ
cates that, as shown in Scheme 1a, the negative CB level of
TiO2 promotes oneꢀelectron reduction of O2. In contrast, the n
for Au0.2/BiVO4 (red) is 2.5, indicating that the e− on the Au
particles mainly promotes twoꢀelectron reduction of O2 (n = 2).
This suggests that, as shown in Scheme 1b, the relatively posiꢀ
tive CB of BiVO4 selectively promotes twoꢀelectron reduction
of O2 on the Au particles. This thus facilitates efficient producꢀ
tion of H2O2.
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β
to the DMPO–●OOH spin adduct (αN = 13.5 G; αH = 9.2 G, g
= 2.0066).26,27 This clearly suggests that Au/BiVO4 suppresses
oneꢀelectron reduction of O2.
a
0
DMPO−•OOH
g = 2.0066
Au0.2/TiO2
α
N = 13.5
Au0.2/BiVO4
αHβ = 9.2
−1
a. Au0.2/BiVO4
b. Au0.2/TiO2
Rotating speed
400 rpm
800 rpm
1200 rpm
1600 rpm
−2
−0.6
−0.4
−0.2
0
0.2
E / V vs. Ag/AgCl
b
1
Au0.2/TiO2
n = 1.6
3400
3420
3440
3460
Magnetic Field / G
Figure 4. ESR spectra measured at 298 K for the solutions obꢀ
tained by (a) λ >420 nm light irradiation of Au0.2/BiVO4 or (b) λ
>300 nm light irradiation of Au0.2/TiO2 in an EtOH/water/O2 sysꢀ
tem with DMPO (0.1 mmol). The catalysts were prepared by the
deposition–precipitation method followed by calcination at 673 K.
0.5
0
Au0.2/BiVO4
n = 2.5
0
0.02
0.04
ω−1/2 / rpm−1/2
0.06
0.08
Photoexcited Au/BiVO4 promotes selective twoꢀelectron reꢀ
duction of O2 on the Au particles. Electrochemical analysis
with a rotating disk electrode confirms this. Figure 5a shows
the linearꢀsweep voltammograms of the catalysts measured in
a buffered water (pH 7) under O2 (1 atm) at different rotating
speeds.28,29 At negative forward voltages, the e− transferred
from the semiconductor CB to the Au particles promote O2
reduction.30 The Koutecky–Levich plots of the data obtained at
the constant electrode potential (–0.3 V) are shown in Figure
5b. The slope of the plots obtained by linear regression was
used to estimate the average number of electrons (n) involved
in the overall O2 reduction.31,32 The plots were interpreted on
the following equations, where j is the measured current densiꢀ
ty, jk is the kinetic current density, and ω is the rotating speed
(rpm), respectively:
Figure 5. (a) Linearꢀsweep voltammograms of Au0.2/TiO2 and
Au0.2/BiVO4 prepared by calcination at 673 K, measured on a
rotating disk electrode at different rotating speeds. (b) The Kouꢀ
teckyꢀLevich plots of the data obtained at the constant potential (–
0.3 V).
Effect of Au particle size and amount of Au loaded. The
size of Au particles strongly affects the catalytic activity. The
Au0.2/BiVO4 catalysts were prepared at different calcination
temperatures to create Au particles with different sizes. As
shown in Figure 6a (circle), the size of Au particles increases
with a rise in temperature because the particle migration leads
to their sintering;22 the treatment at 623, 673, 773 and 873 K
produces Au particles with 6.5, 7.2, 11.1 and 13.5 nm diameꢀ
ters, respectively.34 These catalysts were used for photocatalytꢀ
j–1 = jk–1 + B–1ω–1/2
(7)
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ic H2O2 production. The bar data in Figure 6a show the H2O2
The amount of Au loaded also affects the catalytic activity.
Aux/BiVO4 catalysts with different Au loadings (x = 0.2, 0.5,
1.0, and 2.0 wt %) were prepared by calcination at 673 K. As
shown by the circles in Figure 6b, the sizes of Au particles
increase with an increase in the Au loadings;22 the average
diameters for x = 0.2, 1.0 and 2.0 catalysts are 7.2, 11.9 and
12.5 nm, respectively. As shown by the bar data in Figure 6b,
the activities of these catalysts decrease with the Au loadings,
although the loading of larger amount of Au increases the net
surface areas of Au particles. This is due to the suppression of
e– injection from the photoexcited BiVO4 to Au particles. The
metal/semiconductor heterojunction creates a Schottky barrier
concentration obtained by 10 h reaction. The catalyst prepared
at 623 K shows lower activity than that of the catalyst preꢀ
pared at 673 K. This is probably because, as observed in relatꢀ
ed systems,35–37 calcination at lower temperature leads to
weaker adhesion of metal particles onto the semiconductor
surface and suppresses electron transfer from photoexcited
BiVO4 to Au particles. In contrast, the calcination at higher
temperature creates stronger interaction between BiVO4 and
Au particles, but the catalysts prepared at higher temperature
(723, 773, and 823 K) show very low activity. This is because
they contain larger Au particles, which have smaller surface
areas and decrease the number of active sites for O2 reduction.
Photocurrent response analysis confirms this. Figure 7 shows
the photocurrent response of the catalysts loaded on a FTO
glass electrode, obtained under visible light irradiation.
Au0.2/BiVO4 prepared at 673 K (red) generates current larger
than that of bare BiVO4 (black), suggesting that Au particles
mediate the e– migration from BiVO4 to FTO glass.38
Au0.2/BiVO4 prepared at 873 K (blue) that contains larger Au
particles shows smaller current enhancement, indicating that
the decreased number of surface Au atoms suppresses e– miꢀ
gration from BiVO4 to FTO glass. These findings suggest that
smaller Au particles have larger number of reduction sites for
O2 and efficiently produce H2O2 (Figure 6a).
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(
φ
B) that strongly affects the efficiency for e– injection. The
increase in the amount of metal loaded onto the semiconductor
leads to an increase in the height of B, because of the decrease
φ
in the Fermi level of the semiconductor.39 The decreased cataꢀ
lytic activity for larger Au loading catalysts is therefore beꢀ
cause the increased φB suppresses the e– injection from the
photoexcited BiVO4 to Au particles. As shown in Figure 7,
Au2.0/BiVO4 (green) generates current much smaller than that
of Au0.2/BiVO4 (red), suggesting that the e– injection is indeed
suppressed. The above findings indicate that loading of smallꢀ
er numbers of small Au particles (ca. 7 nm) exhibits best cataꢀ
lytic performance for the H2O2 production.
on
a
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10
0
20
15
10
5
Au0.2/BiVO4 (673 K)
dAu = 7.2 nm
2
1
0
off
Au2.0/BiVO4 (673 K)
dAu = 12.5 nm
Au0.2/BiVO4 (873 K)
dAu = 13.5 nm
0
623 673 723 773
Calcination Temp. / K
873
Bare BiVO4
400
b
0
100
200
300
50
40
30
20
10
0
20
15
10
5
t / s
Figure 7. Photocurrent response of bare BiVO4, Au0.2/BiVO4,
Au2.0/BiVO4 prepared by calcination at 673 K, and Au0.2/BiVO4
prepared by calcination at 873 K, obtained under visible light
irradiation (λ >420 nm) at a bias of 0.5 V. The average sizes of
Au particles on the respective catalysts (dAu) are denoted in the
figure.
0
0.2 0.5
1.0
2.0
CONCLUSION
Au Loading, x / wt %
We found that Au/BiVO4 promotes photocatalytic H2O2
production from water and O2 under visible light irradiation,
via the promotion of water oxidation and selective twoꢀ
electron reduction of O2. At present, the catalytic activity is
insufficient; the ΦAQY at 420 nm is 0.24%, which is oneꢀtenth
of that of the organic semiconductor photocatalysts (2.6%).13
Activity improvement is therefore necessary for practical apꢀ
plications. Nevertheless, the inorganic system presented here
based on water oxidation and selective twoꢀelectron reduction
Figure 6. Effects of (a) calcination temperature of Au0.2/BiVO4
and (b) amount of Au loaded (x) for Aux/BiVO4 on the H2O2 conꢀ
centration formed by 10 h photoreaction. Reaction conditions are
identical to those in Table 1. Circles denote the average diameters
of Au particles on the catalysts. The calcination temperature for
the catalysts (b) is 673 K. Typical TEM images of catalysts and
size distributions of their Au particles are summarized in Figure
S6 (Supporting Information).
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of O2 may contribute to the design of green, stable and susꢀ
tainable H2O2 synthesis based on photocatalysis.
centration in solution was determined by the redox titration
with KMnO4.
Action spectrum analysis. Photoreactions were carried out
in water (30 mL) with Au0.2/BiVO4 (50 mg) within a borosiliꢀ
cate glass bottle (φ 35 mm; capacity, 50 mL). After ultrasoniꢀ
cation and O2 bubbling, the bottle was photoirradiated using a
Xe lamp for 10 h, where the incident light was monochromatꢀ
ed by the bandꢀpass glass filters (Asahi Techno Glass Co.).43
The fullꢀwidth at halfꢀmaximum (fwhm) of the lights was
11−16 nm.
ESR analysis. The spectra were recorded at the Xꢀband usꢀ
ing a Bruker EMXꢀ10/12 spectrometer with a 100 kHz magꢀ
netic field modulation at a microwave power level of 10.5
mW, where microwave power saturation of the signals does
not occur.44 The magnetic field was calibrated with a 1,1′ꢀ
diphenylꢀ2ꢀpicrylhydrazyl (DPPH) as a standard. The measꢀ
urements were carried out as follows: catalyst (20 mg) was
suspended in an EtOH/water mixture (9/1 v/v, 5 mL) containꢀ
ing DMPO (0.1 mmol) within a Pyrex glass tube (φ 12 mm;
capacity, 20 mL), and the tube was sealed with a rubber sepꢀ
tum cap. After ultrasonication (3 min) and O2 bubbling (5
min), the solution was photoirradiated with magnetic stirring
for 3 min. After the irradiation, the catalyst was recovered by
centrifugation, and the resulting solution was subjected to
analysis.
Electrochemical analysis. Rotating disk electrode analysis
was performed on a computerꢀcontrolled CHI600D advanced
electrochemical system with a threeꢀelectrode cell. An
Ag/AgCl electrode and a Pt wire electrode were used as the
reference and counter electrode, respectively. The working
electrode was prepared according to literature procedure:45
catalysts (50 mg) were dispersed in EtOH (2 mL) with Nafion
(50 mg) as an ionomer by ultrasonication. The suspension (20
µL) was put onto a Pt disk electrode and dried at room temꢀ
perature. The linearꢀsweep voltammograms were obtained in
O2ꢀsaturated 0.1 M phosphate buffer solution (pH 7) with a
scan rate 10 mV s–1 at different rotating speeds (400–1600
rpm). After each scan, O2 was bubbled through the electrolyte
for 5 min to saturate O2.
Photocurrent response was measured in a conventional
threeꢀelectrode cell using an electrochemical analyzer (SI
1280B, TOYO Corp.). The working electrode was prepared
using a fluoride tin oxide (FTO) glass. Catalysts (50 mg) were
dispersed in EtOH (1 mL) with pꢀethylene glycol (60 mg), and
the suspension was ultrasonicated for 1 h. The resultant slurry
(10 µL) was put onto a FTO glass. After drying in air, the
glass was annealed at 623 K for 30 min under air flow for
strong adhesion of the catalyst.46 An water containing 0.5 M
Na2SO4 was used as the electrolyte. N2 gas was bubbled
through the solution 10 min. The working electrode was imꢀ
mersed in the solution with a Pt sheet and a Ag/AgCl electrode
as the counter and reference electrodes, respectively. The
working electrode was photoirradiated (λ >420 nm) from the
back side (FTO glass/semiconductor interface) at a potential of
0.5 V. The exposed area under photoirradiation was 0.25 cm2.
Other analysis. XRD patterns were measured on a Philips
X′PertꢀMPD spectrometer. Diffuseꢀreflectance (DR) UVꢀvis
spectra were measured on an UVꢀvis spectrophotometer
(JASCO Corp.; Vꢀ550) equipped with Integrated Sphere Apꢀ
EXPERIMENTAL SECTION
Materials. All of the reagents used were purchased from
Wako, Tokyo Kasei, and SigmaꢀAldrich and used without
further purification. Water was purified by the Milli Q system.
TiO2 (JRCꢀTIOꢀ4) was kindly supplied from the Catalyst Soꢀ
ciety of Japan (Japan Reference Catalyst). WO3 was purchased
from Kojundo Chemical Laboratory Co..
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Catalyst preparation. BiVO4 was prepared as follows:19
a
mixture of K2CO3 (115 mg) and V2O5 (455 mg) was calcined
in air at 730 K for 10 h, producing KV3O8. The obtained
KV3O8 was added to water (50 mL) with Bi(NO3)3·5H2O (2.3
g) and stirred at 298 K for 72 h. The solids were recovered by
centrifugation, washed with water, and dried in vacuo.
AgNbO3 was prepared according to literature procedure:23
a
mixture of Ag2O (232 mg) and Nb2O5 (266 mg) was calcined
in air at 1173 K for 12 h.
Aux/BiVO4 [x (wt %) = 0.2, 0.5, 1.0, and 2.0] were prepared
as follows:22 HAuCl4·4H2O (2.1, 5.2, 10.5, or 20.9 mg) was
added to water (50 mL). The pH of the solution was adjusted
to 7 by an addition of 1 M NaOH. BiVO4 (0.5 g) was added to
the solution and stirred vigorously at 353 K for 3 h. The solids
were recovered by centrifugation, washed with water, and
calcined at the disignated temperature (673, 723, 773, or 873
K) for 2 h under air flow. Au0.2/TiO2, Au0.2/WO3, and
Au0.2/AgNbO3 were prepared in a similar manner.
M0.2/BiVO4 [M = Ag, Pd, Pt, Co, and Ni] were prepared as
follows:40 BiVO4 (0.5 g) was added to water (20 mL) containꢀ
ing AgNO3 (1.6 mg), Pd(NO3)2 (2.2 mg), H2PtCl6·6H2O (2.7
mg), Ni(NO3)2·6H2O (5.0 mg), or CoCl2·6H2O (4.0 mg), and
evaporated under stirring at 393 K for 12 h. The resultant was
dried at 673 K for 2 h. The powders were added to a sodium
formate solution (0.1 M, 20 mL) and stirred at 313 K for 2 h
under Ar (1 atm). The particles were recovered by centrifugaꢀ
tion, washed with water, and dried in vacuo.
Photoreaction. Catalyst (50 mg) was added to water (30
mL) within a borosilicate glass bottle (φ 35 mm; capacity, 50
mL), and the bottle was sealed with a rubber septum cap. The
catalyst was dispersed well by ultrasonication for 5 min, and
O2 was bubbled through the solution for 15 min. The bottle
was immersed in a temperatureꢀcontrolled water bath (298 K)
and photoirradiated at λ >420 nm using a 2 kW Xe lamp
(USHIO Inc.) with magnetic stirring.41 Photocatalytic O2 reꢀ
duction with EtOH as sacrificial electron donor was performed
with catalyst (50 mg) in an EtOH/water (9/1 v/v) mixture (30
mL) under O2 atmosphere.42 Photocatalytic water oxidation
was performed with catalyst (50 mg) in water (30 mL) conꢀ
taining AgNO3 (0.05 M) under Ar atmosphere.13 Decomposiꢀ
tion experiment of H2O2 was performed with catalyst (50 mg)
in solution (30 mL) containing H2O2 (10 mM) under Ar atꢀ
mosphere, where water, EtOH/water (9/1 v/v), and aqueous
NaIO3 solution (0.05 M) were used as a solvent. Spectral irraꢀ
diance for the Xe lamp was measured with a spectroradiomeꢀ
ter USRꢀ40 (USHIO Inc.). After the reaction, the gasꢀphase
products were analyzed by GCꢀTCD (Shimadzu; GCꢀ14B).
The catalyst was recovered by centrifugation, and H2O2 conꢀ
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paratus ISVꢀ469, using BaSO4 as a reference. The spectra for
catalysts are summarized in Figure S5 (Supporting Inforꢀ
mation). TEM observations were performed using an FEI
Tecnai G2 20ST analytical electron microscope operated at
200 kV.47 Typical TEM images of catalysts and size distribuꢀ
tions of their metal particles are summarized in Figure S6
(Supporting Information).
ASSOCIATED CONTENT
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XRD patterns of catalysts (Figure S1), timeꢀprofiles for H2O2
formation in an EtOH/water/O2 system (Figure S2), timeꢀprofiles
for water oxidation with AgNO3 (Figure S3), results of H2O2 deꢀ
composition on Au/BiVO4 (Figure S4), DR UVꢀvis spectra of
catalysts (Figure S5), and TEM images of catalysts and size disꢀ
tribution of metal particles (Figure S6). This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
shiraish@cheng.es.osakaꢀu.ac.jp
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was supported by the GrantꢀinꢀAid for Scientific Reꢀ
search (No. 26289296) from the Ministry of Education, Culture,
Sports, Science and Technology, Japan (MEXT), and by the Preꢀ
cursory Research for Embryonic Science and Technology
(PRESTO) from Japan Science and Technology Agency (JST).
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Int. Ed. 2013, 52, 9686–9690.
(32) Lin, L.; Zhu, Q.; Xu, A. J. Am. Chem. Soc. 2014, 136, 11027–
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