Ultrafine Au nanoparticles anchored on Bi2MoO6 with abundant surface oxygen vacancies for efficient oxygen molecule activation

Article abstract of DOI:10.1039/c9cy00579j

Oxygen molecule activation is one of the most important processes to generate oxygen-containing active free radicals for organic pollutant photocatalytic decomposition. Herein, we demonstrated ultrafine Au nanoparticles (NPs) anchored on porous Bi₂MoO₆ (BMO) microspheres with rich surface oxygen vacancies (SOVs) planted via a simple chemical reduction-deposition method, which could effectively enhance the separation of photogenerated carriers for oxygen molecule activation, and thus lead to a more efficient photocatalytic decomposition ability of the phenol and dye. The sample (4.0% Au/Bi₂MoO₆) exhibited a remarkable photocatalytic performance for phenol, which is 15 times higher than BMO. The systematic studies indicate that the excellent photocatalytic activity of Au/Bi₂MoO₆ should be ascribed predominantly to the synergistic effect between SOVs, Au NPs and BMO. Both SOV and Au NP surface plasmonic resonance (SPR) can not only improve the separation and migration of the photogenerated electron (e^-)/hole(h⁺) pairs, but can also broaden the light responsive spectra, and thereby facilitate the oxygen molecule activation. It also reveals that the photocurrent intensity of the 4% Au/BMO sample is approximately 36-fold larger than that of pure BMO, and the ESR signal intensities of O₂^- and OH for Au/BMO exhibit an enhancing tendency compared to pure-BMO, demonstrating that SOVs and Au NPs are responsible for the promoted photocatalytic activity of the Bi₂MoO₆ nanostructure. Furthermore, the band gap position of Au/BMO was determined by employing UV-vis-DRS spectra, VB-XPS and Mott-Schottky plots, thus the enhancement mechanism of oxygen molecule activation is further elucidated.

Full text of DOI:10.1039/c9cy00579j

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Qingzhu Zhang et al.
Catalytic mechanism of C–F bond cleavage: insights from QM/MM
analysis of fluoroacetate dehalogenase
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DOI: 10.1039/C9CY00579J
Catalysis Science & Technology
ARTICLE
Ultrafine Au Nanoparticles Anchored on Bi MoO with Abundant
2
6
Surface Oxygen Vacancies for Efficient Oxygen Molecule
Activation
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a
a
a
a
Li Guo, Qiang Zhao, Huidong Shen, Xuanxuan Han , Kailai Zhang , Danjun Wang * , Feng Fu*
DOI: 10.1039/x0xx00000x
b
and Bin Xu*
The oxygen molecule activation is one of the most important processes to generate oxygen-containing active free radicals
for organic pollutants photocatalytic decomposition. Herein, we demonstrated the ultrafine Au nanoparticles (NPs)
anchored on porous Bi
reduction-deposition method, could effectively enhance the separation of photogenerated carriers for oxygen molecule
activation, and thus more efficient photocatalytic decomposition ability of phenol and dye. The sample (4.0%Au/Bi MoO
exhibited a remarkable photocatalytic performance for phenol, which is 15 times higher than BMO. The systematic studies
indicate that the excellent photocatalytic activity of Au/Bi MoO should be ascribed predominantly to the synergistic effect
between SOVs, Au NPs and BMO. Both SOVs and Au NPs surface plasmonic resonance (SPR) can not only improve the
2 6
MoO (BMO) microspheres with rich surface oxygen vacancies (SOVs) planted via a simple chemical
2
6
)
2
6
-
+
separation and migration of photogenerated electron (e )/hole(h ) pairs, but also broaden the light responsive spectra, and
thereby facilitate the oxygen molecule activation. It also reveals that the photocurrent intensity of 4%Au/BMO sample is
-
approximately 36-fold larger than that of pure BMO, and the ESR signal intensities of ·O
2
and ·OH for Au/BMO exhibit an
enhancing tendency compared to pure-BMO, demonstrating that SOVs and Au NPs are responsible for the promoted
photocatalytic activity of Bi MoO nanostructure. Furthermore, the band gap position of Au/BMO was determined by
2
6
employing UV-Vis-DRS spectra, VB-XPS and Mott-Schottky plots, thus the enhancement mechanism of oxygen molecule
activation is further elucidated.
and visible light response.7-13 Among them, Bi
MoO is a layered
2 6
1
Introduction
Aurivillius-related n-type semiconductor, which consist of
]2+ and MoO
2-
layers. Owing to its distinguished
is considered as a very promising
alternating [Bi
intrinsic properties,
2
O
2
4
For decades, photocatalysis technology has been widely studied to
relieve the increasingly serious environmental pollution by
utilization of solar energy. Generally, oxygen molecule is oxidant
in photocatalytic decomposition of pollutants, which can be
activated by binding with the photogenerated electron. The
1
4-16
Bi
2
MoO
6
photocatalyst. However, the poor visible-light utilization efficiency,
low mobility and rapid recombination of charge carrier still hinder
1
-6
1
7
its photocatalytic performance. In this regard, it is highly desirable
to find approaches to improve the separation of electrons/hole
pairs and broaden the response range of visible light for enhancing
-
+
2
superoxide radical (·O ), photogenerated hole(h ) and hydroxyl
radical (·OH) are responsible for the decomposition of pollutants.
Accordingly, the oxygen molecule activation is one of the most vital
processes to generate oxygen-containing radicals for photocatalytic
decomposition of the organic pollutants and selective organic
transformation.
Different from traditional TiO
polyoxides such as Bi , BiVO , Bi
attracted much attention due to their nontoxicity, suitable bandgap
1
8-20
the catalytic performance of Bi
this end, the band gap of Bi MoO
morphology and employing different modification strategies to
2
MoO
6
-based photocatalysts.
To
2
6
was modulated by controlling its
1
-5
2
1-26
improve the photocatalytic activity.
Defect engineering is an effective strategy to modulate the
6
2
-based photocatalysts, bismuth
WO , Bi MoO and BiPO have
2
7
photocatalytic performance of semiconductors(SC).
Oxygen
2
O
3
4
2
6
2
6
4
vacancy is an important defect in changing the electronic/optical
properties of the photocatalysts, thus improving the photocatalytic
performance. Generally, oxygen vacancy is usually introduced
2
8-30
31-33
through high temperature annealing,
4
NaBH reduction,
a. Shaanxi Key Laboratory of Chemical Reaction Engineering, School of Chemistry
and Chemical Engineering, Yan’an University, Yan’an 716000, China. E-mail:
wangdj761118@163.com；E-mail: yadxfufeng@126.com; Tel: +86-911-2332037;
34
solvothermal synthesis, ultraviolet radiation and metal ion doping,
3
5,36
etc.
It was found that oxygen vacancy can not only led to the
b.
State Key Laboratory of Organic-Inorganic Composites Beijing Key Laboratory of
reduction of band-gap but also enhance the carrier concentration
as a shallow donor, which can extend the light toward longer
wavelength, as well as promote the separation of
Electrochemical Process and Technology for Materials, Beijing University of
Chemical Technology, Beijing 100029, China. E-mail: binxumail@163.com; Tel:
+
86-10-64434907
Electronic Supplementary Information (ESI) available. See
DOI: 10.1039/x0xx00000x
†
-
+
electrons(e )/hole(h ) pairs. On the other hand, noble metal
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ARTICLE
Journal Name
decoration is another effective modification strategy for to BMO. For comparison, the sample 2.0 % Au/BMO-B was also
View Article Online
3
7-39
DOI: 10.1039/C9CY00579J
photocatalysts.
The interaction between noble metal prepared without calcination. Following the similar procedure, a
nanoparticles (NPs) and SC is expected to form Schottky barriers,
which can effectively promote the interfacial electron transfer, and
Bi(NO
Ethylene glycol
Na MoO ·2H
Ethanol
3 3 2
) ·5H O
Au/BMO-SOVs
5
,40
2
4
2
O
thus significantly prevent the charge carriers recombination.
Meanwhile, noble metal NPs can respond visible-light owing to the
4
1, 42
HAuCl
4
Lysine
SOVs
localized surface plasmonic resonance effect (SPR).
Among
Calcination
50 ℃ 2 h
2
these noble metal NPs, gold (Au) has become the focus for its low
Au NPs
160 ℃ 12 h
4
3,44
toxicity and good environmental compatibility.
Although both the noble metal surface deposition and surface
oxygen vacancies (SOVs) planting are vital modification strategies
for improving photocatalytic activity under visible light,
integration the merit of SOVs and noble metal NPs to the surface of
semiconductor for synergistic promotion its photocatalytic
performance is still a challenge. In this work, we have successfully
Deionized water
Lysine
HAuCl
NaOH
4
NaBH
4
4
5
BMO
Ultrasonication
Ultrasonication
Scheme 1 Schematic diagram of preparation process of Au/BMO-A samples.
fabricated Au/Bi
planting via a simple chemical reduction-deposition method, which
exhibits superior photocatalytic activity than Bi MoO . The
2 6
MoO composites with rich SOVs simultaneous
series of Au/BMO composites were prepared and denoted as x
2
6
wt%Au/BMO-A (x = 0, 1, 2, 4, 6). For comparison, Bi
SOVs was also prepared without adding HAuCl solution, the
obtained sample was denoted as BMO-A. 4wt%Au/Bi MoO without
SOVs was prepared according to our previously reported photo-
2 6
MoO with
enhanced photocatalytic efficiency is ascribed to the synergistic
effect of SOVs and SPR of ultrafine Au NPs, which improve the
oxygen molecule activation process, and thereby it is propitious to
generate oxygen-containing radicals for the decomposition of the
organic pollutants. This strategy can be applied to design and
construct other photocatalysts for the improvement of
photocatalytic performance via an enhanced oxygen molecule
activation process.
4
2
6
4
7
reduction process. The obtained sample was denoted as 4wt %
Au/BMO-PR. The preparation process of Au/BMO composite is
shown in Scheme 1.
2.2 Characterization
Powder X-ray diffraction (XRD) was carried out on a XRD-7000 X-ray
diffractometer (Shimadzu, Japan), using Cu Ka radiation ( λ =
2
2
Experimental Section
-1
0
8
.15418 nm ) at a scanning rate of 2° min in a 2θ range of 20° to
0°. X-ray photoelectron spectroscopy (XPS) valence band (VB-XPS)
.1. Sample preparation
Pristine Bi MoO microspheres
were prepared according to our previous report. In a typical spectrometer. Field emission scanning electron microscope (FE-
procedure, Bi(NO ·5H O(1.3 mmol) was dissolved in 13.0 mL of SEM) images were recorded on a JSM-7610F scanning electron
ethylene glycol (EG), then Na MoO ·2H O(0.65 mmol) powder was microscope. Transmission electron microscopy (TEM) images were
2
6
. 2 6
The pristine Bi MoO microspheres spectra was recorded on
a PHI-5400 X-ray photoelectron
4
6
3
)
3
2
2
4
2
added under continuous stirring until completely dissolved. obtained on a JEM-2100 electron microscope at an accelerating
Thereafter, 32.5 mL of ethanol was added under magnetic stirring voltage of 200 kV(JEOL, Japan). In situ electron paramagnetic
to form a colorless clear liquid. Then, the miscible solution was resonance (EPR) measurement was taken in an Endor spectrometer
transferred into a Teflon-lined stainless steel autoclave of 65 mL (JEOL ES-ED3X) at 77 K in liquid nitrogen. The g factor was obtained
capacity. The autoclave was sealed and maintained at 160 ℃ for 12 by taking the signal of manganese as standard. The electron spin
-
h. After the reaction, the precipitate was separated, washed, and resonance (ESR) signals of ·OH and ·O2 radicals spin-trapped by
finally dried under vacuum at 60 ℃ overnight. The obtained spin-trap reagent 5, 5-dimethyl-1-pyrroline N-oxide (DMPO) in
product was Bi
Au/Bi MoO samples (Au/BMO). The Au/Bi
SOVs were prepared via a facile chemical reduction-deposition the irradiation source ( λ > 410 nm). The UV-vis diffuse reflectance
process. Lysine and NaBH were used as capping agent and spectra (UV-Vis-DRS) were measured with a UV-2550 UV-Vis
reducing agent, respectively. In a typical process, Bi MoO (0.50 g) spectrophotometer (Shimadzu, Japan). Photoluminescence (PL)
powder was dispersed in deionized water(10 mL), then 0.01 mol·L
2
MoO
6
, which was denoted as BMO.
water were examined on
a Bruker model ESR JES-FA200
2
6
2
MO
6
catalysts with spectrometer equipped with a quanta-Ray Nd: YAG laser system as
4
2
6
-1
spectra were measured on an F-4500 spectrophotometer (Hitachi,
-
1
HAuCl
4
(5.3 mL) and 0.01 mol·L lysine (6.0 mL) were slowly added Japan).
-
dropwise. The pH of the mixture was adjusted to 5.0 with 0.1 mol·L
NaOH solution. Thereafter, the suspension was subjected to
ultrasonication for 20 seconds, and 5.3 mL of NaBH (0.1 mol·L )
4
was injected during the sonication. The suspension turned dark in
color immediately for the formation of Au colloids and was
centrifugal washed with DI water for several times. The product was
dried at 60 ℃ in vacuum over night, followed by calcination at 250
for 2 h to eliminate the lysine residual. The obtained sample
were denoted as 2.0 % Au/BMO-A according to the mass ratio of Au
2
.3. Time-resolved transient PL measurement
Time-resolved fluorescence decay spectra (TR-PL) were recorded on
FLS920 fluorescence spectrometer (Edinburgh Analytical
1
-1
a
Instruments, UK) using the time-correlated single photon counting
method, excited with a picosecond diode laser (EPL-375) at 377.8
nm, and monitored at 470 nm. A two-exponential function equation
was used to fit the decay time, τav=(A ·τ +A ·τ )/(A ·τ +A ·τ ).
1 1 2 2 1 1 2 2
Where τ and A are decay time and the relative magnitude of
2
2
℃
2
| J. Name., 2012, 00, 1-3
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components, respectively, and τav is the average lifetime used for Au/BMO-A increased to 159.8 eV and 165.1, 159.4 and 164.8 eV,
View Article Online
DOI: 10.1039/C9CY00579J
an overall comparison. The results show the fast decay component respectively, which can ascribed to the interaction between Au and
5
3, 54
(
τ
1
and A
free excited states and the bound excited states, respectively.
.4. Photoelectrochemical measurement
1
) and the slow component (τ
2
and A
2
), decaying from the BMO, and the coexistence of SOVs.
According to Fig. S1b(ESI*),
the peak at 232.5 eV and 235.6 eV can be assigned to Mo 3d5/2 and
6
+ 54
Mo 3d3/2 of Mo , and the characteristic peak of 4.0wt%Au/BMO-
B and 4.0wt%Au/BMO-A shift to higher binding energy separately
to 233.3 eV and 236.3, 233 and 235.9 eV. As shown in Fig. 1(d), the
wide asymmetric O1s peaks observed can be fitted into three peaks
at 529.7, 530.5 and 531.8 eV for BMO sample, which are attributed
to the Bi-O, Mo-O and H-O or adsorbed oxygen, respectively.
However, the O1s binding energy (Mo-O) of Au/BMO-B and
Au/BMO-A have a shift from 530.5 eV to 531.2 eV and 530.95 eV,
respectively (Fig. 1d), which are ascribed to the formation of SOVs
with a high electron-attracting effect.
presented in Fig. 1Sc(ESI*) indicates that Au 4f spectra of Au/BMO-
B is fitted to two peaks at about 84.4 eV and 87.9 eV, which can be
assigned to the 4f7/2 and 4f5/2 binding energies of Au, suggesting
that the Au species in the samples before calcination are presented
2
The Mott-Schottky (MS) curves, photocurrent density versus time (i-
t) curves and electrochemical impedance spectroscopy (EIS) were
measured on a CHI660E electrochemical workstation (Chenhua
Instruments, China). The test system comprised a working electrode,
a Pt counter electrode, a Ag/AgCl reference electrode , a working
electrode and Na SO electrolyte (0.1 mol·L ). The working
2 4
electrode was prepared from samples fixed on ITO glass. In a typical
process, 20 mg photocatalyst was dispersed in 2 mL DI water to get
slurry, followed by coating the slurry onto ITO glass and drying at 60
℃
halogen lamp (with a UV cutoff filter) is employed as the visible light
source.
55-58
-1
59, 60
The Au 4f XPS spectrum
2
for 6 h. The area of the electrodes is about 1×2 cm . A 400 W
6
1
in the compound state. However, Au 4f binding energy peaks of
Au/BMO-A sample have a shift to 84.1 eV and 87.7 eV, which
revealed that Au species in the samples were reduced to metallic
2
.5. Photocatalytic activity
The photocatalytic activity of the samples were evaluated by the
degradation of phenol, methyl orange (MO) and rhodamine (RhB).
A 300 W halogen lamp was used as sunlight source with 420 nm
cutoff filters. First, photocatalyst (200 mg) was dispersed in 200 mL
51, 62
state after the calcination process.
-1
-1
-1
of phenol (10 mg·L ), MO (10 mg·L ) or RhB (10 mg·L ),
respectively. The mixture were stirred in dark for 30 min to
establish an adsorption/desorption equilibrium. Then, the light
source was turned on. During the photocatalytic reaction, 10 mL
supernate was sampled at given time intervals, and then
photocatalysts was separated through centrifugation. The
concentration of pollutant solution was detected by a UV-visible
spectrophotometer (Japan, Shimadzu 2550) at the absorption
4
9,50
maximum.
Total organic carbon analyzer (VARIO TOC,
Elemental, Germany) was employed for the determining the TOC of
phenol solutions.
3
Results and discussion
3
.1. Structure and morphology
Fig. 1a shows the XRD patterns of Au/BMO-A heterostructures with
different Au contents. The pristine BMO and Au/BMO-A samples
are identified as orthorhombic BMO (JCPDS card No.76-2388),47
indicating that Au NPs have negligible influence on structure of
BMO. Two weak diffraction peaks at 38.2 ° and 44.3 ° are observed,
which are indexed as (111) plane and (200) plane of Au (JCPDS No.
Fig.1 (a) XRD pattern of as-prepared BMO and the series of Au/BMO-A
heterostructures, (b) XRD patterns in a 2θ range of 26.5º-30º, (c) XPS spectra of Bi
4
f and (d) O 1s for BMO, 4wt%Au/BMO-B and 4wt%Au/BMO-A were before and
after calcined at 250℃ for 2 h, respectively.
5
1
8
0-0019), respectively. Furthermore, with the increasing of the Au
content, the (131) peak of BMO slightly shifts to a lower angle (Fig.
b), which may be ascribed to the interaction of Au NPs and BMO
surface.
The composition and chemical state of the BMO and Au/BMO
1
heterostructures were confirmed by XPS. Fig. S1a(ESI*) reveals that
Au/BMO composite contains Bi, Mo, O, Au and trance amount C
element, respectively. The C element should be attributed to the
5
2
adventitious of XPS instruments itself. Fig. 1c reveal that two
characteristic peaks of BMO at around 159 eV and 164.4 eV
3
+
corresponds to Bi 4f7/2 and Bi 4f5/2 of Bi . Compared with pristine
BMO, the binding energy of 4.0wt% Au/BMO-B and 4.0wt%
This journal is © The Royal Society of Chemistry 20xx
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Fig. 2 FE-SEM images of as-prepared (a~c) hollow BMO microsphere, (d~f) Au/BMO-A samples exhibit the similar morphology with different
View Article Online
DOI: 10.1039/C9CY00579J
4
wt% Au/BMO-A heterostructures.
Au loading (Fig. S2). The elemental mapping as shown in Fig. 3 (a-b)
reveals that 4%Au/BMO-A heterostructure are well agreement with
XPS analysis. As shown in Fig. 3(c~g), all the elements are clearly
observable and uniformly distributed, revealing that the Au NPs
were successfully anchored and highly dispersed onto the surface of
BMO microspheres.
(
a )
( b )
( c )
2
.5 μm
2
.5 μm
Bi Mα1
( d ) Mo Lα1
( e ) O Kα1
( f ) Au Mα1
( g )
To further identify the microstructure of the as-synthesized
BMO and Au/BMO-A composite, TEM and HR-TEM were performed.
According to Fig.4(a~b), the BMO microspheres consist of many
nanosheets with a thickness of about 10 nm. However, HR-TEM
images indicate the surface of Au/BMO is relatively smooth (Fig.
2
.5 μm
2.5 μm
2.5 μm
2.5 μm
Fig. 3 (a) SEM image of 4wt%Au/BMO microsphere, (b) EDX spectrum of sample
Au/BMO, (c) The spatial distribution of all elements in 4wt%Au/BMO-A, and the
elemental mapping of (d) Bi, (e) Mo, (f) O and (g) Au in sample 4wt%Au/BMO-A.
4
c). The SAED patterns (insert of Fig.4b and Fig.4e) reveal the
polycrystalline nature of the BMO and 4wt% Au/BMO-A with the
same symmetry. The measured lattice fringes with spacing of 0.317
(
a )
( d )
53,54
nm ascribe to the (131) plane of BMO.
In addition, the
interplanar lattice spacing of 0.21 nm and 0.238 nm correspond to
the Au (200) and (111) atomic planes, respectively (insert picture in
6
3, 64
Fig. 4f).
It further indicates that Au NPs was deposited on the
secondary nanosheets of BMO microspheres.
200 nm
200 nm
Although the pristine BMO reveals the perfect lattice fringe
(
Fig. 4c), the edge of lattice fringe for 4.0wt%Au/BMO-A becomes
(
b )
( e )
blurred (Fig. 5a), implying that its surface structure is damaged and
SOVs are formed.
6
0, 65
To further verify the existence of SOVs and
monitor the various behaviors of the SOVs, in-situ EPR spectra were
performed, as shown in Fig. 5b. It indicate that BMO shows a weak
EPR signal at g value of 1.991, which may be ascribed to the
formation of the SOVs during the solvothermal process. The EPR
signal at g = 2.002 for 4wt%Au/BMO-A and 4wt%Au/BMO-B are
much higher than pure BMO, as shown in Fig. 5, which is attributed
1
00 nm
100 nm
0
.317nm
0.21nm
0.238nm
111)
Au NPs
(
c )
(200) ( f )
(
(131)
2
nm
2 nm
2
nm
4
0, 60, 65-67
to the unpaired electrons trapped by SOVs.
BMO, 4wt%Au/BMO-B also exhibits an obvious increased SOVs
signal, indicating that NaBH reduction increase the number of
SOVs. During the reduction of HAuCl , the excessive NaBH acts as
Compared to
4
5
nm
5 nm
4
4
oxygen scavenger for producing the active hydrogen. The strong
reduction capability of active hydrogen can remove the oxygen
Fig. 4 TEM and HR-TEM images of BMO (a~c) and (e~f) 4wt%Au/BMO-A.
6
8
atoms from the surface of BMO, thus forming SOVs. Moreover,
the EPR signal of 4wt%Au/BMO-A is slightly lower than that of
4wt%Au/BMO-B due to its calcination in air-rich atmosphere for
(
b )
(
a )
6
6
elimination of lysine. Therefore, it can be concluded that
wt%Au/BMO-A possess rich SOVs, which can serve as the active
4
sites for improving the carrier transfer efficiency as well as oxygen
molecule activation. Also, it could gain the synergistic effect of Au
NPs and SOVs in Au/BMO heterostructure.
5
nm
3
.2. Optical properties
Fig. 5 (a) HR-TEM image of Au/BMO, (b) EPR spectra of BMO, 4wt%Au/BMO-B
and 4wt% Au/BMO-A.
The UV-Vis-DRS were employed to investigate the optical properties
of BMO and Au/BMO heterostructures. Clearly, the color of
Au/BMO changes from yellow to gray and black with the increasing
of Au deposition amount (Fig. 6). The BMO microsphere exhibited
absorption edge at about 470 nm, corresponding to the intrinsic
band-gap transition. When Au NPs were loaded on BMO, the
absorption of Au/BMO within visible light range were enhanced and
absorption edge became red shift, indicating that the photocatalyst
could absorb more photons and be favorable for photocatalytic
reaction. In addition, Au/BMO-A samples exhibited an extra
absorption peak at about 580 nm (Fig. 6a) due to the absorption of
According to SEM images shown in Fig. 2(a~c), BMO exhibits a
uniform hollow microspheres (insert of Fig. 2a) , which consists of
many nanosheets with the thickness of about 10 nm. Likewise, the
morphology of 4wt%Au/BMO-A ( Fig. 2d~e ) is quite similar to that
of the hollow BMO microspheres. However, the surface of
nanosheets is damaged and blurred after the deposition of Au NPs
and calcination at 250 ℃ for 2h (Fig. 2d), which results in the
formation of SOVs. In addition, many Au NPs are monodispersed on
the secondary nanosheets of BMO microspheres (Fig. 2f). Also, all
4
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the Au SPR effect.40,68,69-71 Furthermore, the intensity of the concentration and transport of the photoexcited charge carriers can
View Article Online
DOI: 10.1039/C9CY00579J
be effectively promoted and the superiority of synergism of SOVs
absorption of the Au SPR effect strengthen with the increasing of
(
a )
( b )
Fig. 6 Optical properties of samples. (a) UV-Vis-DRS spectra; (b)band gap (E
BMO and series of Au/BMO samples.
g
) of
Fig. 7 Charge-carriers separation efficiency of BMO an X wt%Au/BMO(X=1, 2, 4,
Au NPs from 1wt% to 4wt%. However, when the deposition amount
is more than 5%, the absorption peak change is not obvious. The
6
). (a) Photoluminescence spectra, (b) time-resolved fluorescence decay spectra,
c) transient photocurrent response under visible light irradiation and (d)
electrochemical impedance spectroscopy (EIS) under visible light.
(
band-gap energies (E
g
) of BMO samples are estimated from a plot
1
/2
of (αhν) versus hν, as displayed in Fig. 6b. The calculated E values
g
of BMO and 4wt% Au/BMO-A are around 2.33 and 1.70 eV,
respectively (Fig. 6b). The 4wt% Au/BMO-A with Au NPs and SOVs
shows more intense light absorption and narrower band gap,
indicating that both the planted SOVs and the anchored Au NPs
enhance the visible light harvesting.
Table 1 Lifetimes (τ
1
and τ
2
) and relative intensities of pure BMO and Au/BMO
samples
τ
1
(ns)/Int.(%)
τ
2
(ns)/Int.(%)
τ
av(ns)
χ2
BMO
0.19/86.86
0.22/84.39
1.68/13.14
1.70/15.61
0.386
0.451
1.110
1.201
4.0%Au/BMO
3
.3. Charge-carriers separation efficiency
and Au can be verified. The electrochemical impedance
spectroscopy (EIS) Nyquist plots of pure-BMO and Au/BMO with
different amounts of Au are shown in Fig.7(d). It can be seen that
4%Au/BMO-A presents the smallest arc radius, indicating its
Due to the photoluminescence (PL) emission originating from the
recombination of charge carriers, PL spectrum are often used to
investigate the separation and transfer efficiency of e /h pairs.
When 400 nm was used as the excitation wavelength, the PL
spectrum of BMO exhibits a strong emission peak at approximately
-
+
72-75
-
+
strongest ability to separate and transfer photogenerated e /h
7
9,80
pairs.
4
70 nm(Fig. 7a). However, the PL intensity is dramatically weakened
3
.4. Photocatalytic performance
when the Au NPs were deposited. However, with the Au NPs
loading amount increasing from 1wt% to 4wt%, the emission peak
decreased slightly. Especially, the 4%Au/BMO-A exhibits the
weakest PL intensity. It suggests that Au NPs anchored on BMO as
well as SOVs can greatly improve the separation efficiency of the
photogenerated e /h pairs.
To further investigate the e /h pairs recombination process
for BMO and Au/BMO, the recombination kinetics of
photogenerated e /h pairs were investigated by TR-FL spectra, as
shown in Fig. 7(b). A two-exponential function equation was used to
fit the decay time, as listed in Table 1. The photoelectron lifetimes
of BMO and 4%Au/BMO are 0.386 and 0.451 ns, respectively,
indicating that the synergistic effect of Au NPs and SOVs makes the
photoelectron lifetime of the Au/BMO photocatalyst increase.
Furthermore, the effect of Au NPs and SOVs on the carrier
concentration and transport BMO was investigated by photocurrent
The visible-light-responsive activity of BMO and Au/BMO were
evaluated by monitoring the degradation of phenol, RhB, and MO.
Fig. 8(a) indicated that the degradation of MO was negligible and
RhB was about 5% in a blank experiment. However, BMO exhibits
photogegradation rate of RhB and MO are 30% and 10% in 25 min,
respectively. However, when 4.0wt%Au/BMO-A was employed as
photocatalyst, the photocatalytic degradation rate was significantly
enhanced, and 100% degradation of MO and RhB could be finished
within 100 min and 25 min, respectively. Using BMO and
-
+
-
+
-
+
4
.0wt%Au/BMO-A as photocatalysts, the calculated rate constants
-1
-1
(kapp/ min ) are 0.00389, 0.04613 min for MO, and 0.01515, 0.2229
-1
min for RhB, respectively (Fig. 8b). The degradation activities of
.0 % Au/BMO-A to RhB and MO are approximately 11.9 and 14.7
4
times higher than BMO, respectively. Furthermore, phenol was
used as another model pollutant to investigate the photocatalytic
performance of 4.0%Au/BMO-A. Fig. 8c shows the variation of
7
6-78
measurements.
Fig. 7(c) shows the photocurrent density versus
0
phenol concentration (C/C ) over photocatalysts against reaction
time (i-t) curves of the samples under visible light (λ≥420 nm)
without bias. Evidently, the 4wt%Au/BMO-A composite exhibits a
remarkably enhanced photocurrent response in contrast with
BMO.The photocurrent intensity of the 4%Au/BMO-A composite is
approximately 36-fold larger than that of BMO, revealing that the
time. The degradation efficiency of phenol was negligible in blank
test (without any photocatalyst) and the BMO exhibits 9 %
degradation rate after 3 h. However, with the increase of Au
loading from 1wt% to 4wt% in the Au/BMO, the degradation rate of
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Journal Name
phenol was significantly enhanced. But when further increased the Au/BMO-A (right), (f) TOC changes during the courVsieew AortficlepOhnelninoel
photodecomposition in the presence of BMO and 4 wD t O% AI : u1 /0B . M1 0 O3 -9A /C u9 nCd Ye 0r 0v 5i s 7i b9 l Je
light radiation.
Au loading, the photocatalytic activity decreased. It is because the
more Au deposition conversely occupies the active sites of the
catalyst surface, which results in the decreasing of the
8
1
photocatalyst activity. According to the first-order kinetics model,
-1
the apparent rate constant(kapp/min ) are 0.0339, 0.2087, 0.4212,
0
2
8
.5095 and 0.2424 min-1 for pure-BMO, 1wt%Au/BMO,
wt%Au/BMO, 4wt%Au/BMO and 6wt%Au/BMO, respectively (Fig.
d). It can be seen that the kapp value of 4wt%Au/BMO-A is much
higher than others, which is approximately 15 times higher than
pure-BMO. Fig. 8(e) indicated that the absorption characteristic of
phenol peak at 270 nm obviously decreases after 180 min
irradiation. As shown in Fig. S3, both SOVs planting and Au NPs
anchoring can enhance the photocatalytic activity of pristine
Bi
with SOVs(4wt%Au/BMO-A) exhibited the better photocatalytic
activity for phenol degradation than that of 4wt%Au/Bi MoO
without SOVS(4wt%Au/BMO-PR) and Bi MoO with SOVs(BMO-A).
2 6 2 6
MoO for phenol degradation. Furthermore, 4wt%Au/Bi MoO
2
6
2
6
The enhanced photocatalytic activity of 4wt%Au/BMO-A is
attributed to the synergistic effect between the SOVs, Au NPs and
2 6
Bi MoO , which not only narrows the band gap, improving the Fig. 9 DMPO spin-trapping ESR spectra of photocatalysts in water for (a~b) BMO;
visible light response ability, but also facilitates the formation of (c~d)4wt%Au/BMO-A.
Schottky barriers between Au NPs and Bi
interface allows only the movement of electrons from the Bi
2
MoO
6
. The Au/Bi
2
MoO
MoO
6
2
6
to Au NPs and hinders electron transfer back across the Schottky
barrier, and thereby prevents from the recombination of the
5
4
photogenerated electron-hole pairs.
1
0
.0
.8
( a )
0
.6
RhB No catalyst
RhB BMO
RhB 4wt% Au/BMO
MO No catalyst
MO BMO
0
0
0
.4
.2
.0
MO 4wt% Au/BMO
0
20
40
60
80
100
Fig. 10 Photogenerated carrier trapping in the system of photodegradation of
phenol by 4wt%Au/BMO-A under visible light irradiation (λ> 420 nm).
Irridiation/min
To further investigate the mineralization rate of phenol, the
total organic carbon (TOC) was detected (Fig. 8f). The mineralization
yield of 4% Au/BMO-A reaches as high as 45% after 3h of
irradiation, while that of BMO is only 1.3%, proving the enhanced
photocatalytic performance of the former.
No catalyst
BMO
1wt% Au/BMO
2wt% Au/BMO
4
wt% Au/BMO
wt% Au/BMO
6
0
0
.20
.15
0.0 h
0.5 h
3
.5. Enhanced Oxygen molecule activation mechanism
(
e )
1.0 h
1.5 h
2.0 h
2.5 h
3.0 h
To further reveal the possible mechanism of oxygen molecule
activation, the ESR spin trapping technique were performed to
identify the main active species under visible light irradiation as
shown in Fig. 9. The intensity of ·OH and ·O
enhanced with the irradiation time prolonging, indicating that the
0
0
.10
.05
-
2
radical signals is
BMO
40 250 260 270 280
4
wt% Au/BMO
0
.00
2
250 260 270 280 290
Wavelength(nm)
formation of radical during the photodegradation process.
-
Fig. 8 (a) Photocatalytic degradation efficiency of RhB or MO by BMO and 4wt% Importantly, ESR signal of ·OH and ·O intensities for 4%Au/BMO-A
2
-1
Au/BMO, (b) The apparent rate constant (kapp/min ) of the photodegradation of
RhB and MO using different catalysts, (c) Photocatalytic degradation efficiency of
phenol by BMO and xwt%Au/BMO-A(x=1, 2, 4, 6), (d) The apparent rate
constants(kapp/h ) of the photodegradation of phenol using different catalysts,
and (e) The change of the absorption spectra of photodegradated phenol with
increasing irradiation time under visible light using BMO ( left ) and 4wt%
are significantly enhanced compared to BMO. It reveals that Au NPs
deposition and SOVs can increase the separation efficiency of e /h
pairs, and thereby promotes the oxygen activation efficiency as well
as photocatalytic activity of BMO. Moreover, the active species
trapping experiments during the photodegradation of phenol over
-
+
-1
4
%Au/BMO-A were also conducted. Ethylenediaminetetraacetic
6
| J. Name., 2012, 00, 1-3
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CP al et aa lsy es i ds oS c ni eo nt c ae d &j uTs et c mh na or gl oi ng sy
Journal Name
ARTICLE
acid disodium salt (EDTA-2Na), isopropanol (IPA) and benzoquinone the enhanced photocatalytic activity under visible light is suggested.
View Article Online
+
-
DOI: 10.1039/C9CY00579J
(
BQ), were utilized as the scavengers of h , ·OH and ·O
2
,
SOVs is a shallow defect, which may near the minimum conduction
8
2
81,86
respectively. As shown in Fig. 10, BQ and IPA hindered the band (CBM) or above the valence band maximum (VBM).
As
photocatalytic degradation of phenol, while EDTA-2Na evidently manifested in our previous works, the existence of SOVs would
8
8
suppressed the photocatalytic degradation efficiency of phenol. induce the formation of subbands above the VB of BMO. The
-
Thus, it could be concluded that both ·OH and ·O
2
were main active enhancement of photocatalytic activity for the Au/BMO
species in the phenol photodecomposition process. Meanwhile, heterostructures compared to pure BMO can be interpreted as
+
phenol could be direct oxidized degradation by photogenerated h
of Au/BMO.
follows. During the visible light irradiation, the Au/BMO can be
excited to produce e /h pairs and the photogenerated e in the CB
-
+
-
In order to further elucidate the oxygen molecule activation could transfer to Au NPs,because the Fermi level of BMO is -0.11 V
3
1
48
mechanism, conduction band (CB) and valence band (VB) positions vs NHE, lower than that of gold (0.45 V vs NHE). Consequently, a
of all the samples were examined, as shown in Fig. S4(ESI*) and Fig. Schottky barrier height of 0.56 V is generated at the Au/BMO
S5(ESI*). From Fig. S4(ESI*), it can be seen that both BMO and interface. Additionally, when Au NPs are deposited on the surface
Au/BMO show a positive slop, indicating that they are all n-type of BMO, due to the Fermi level equilibrium between BMO and Au
6
4,89
semiconductors. The flat band potentials of BMO and Au/BMO are NPs, Schottky barriers formed at a Au-BMO junction.
-
This
-
0.33 and -0.28 V, respectively, versus the Ag/AgCl electrode, which unique Au-BMO interface allows the e transfer from BMO to Au
corresponds to -0.11 and -0.06 V versus the normal hydrogen NPs and hinders its transfer back across the Schottky barrier, and
8
3,84
-
+
electrode (NHE).
Fig. S5(ESI*) exhibits the VB-XPS spectrum of thereby prevents the recombination of the photogenerated e /h
-
BMO and Au/BMO. It reveals that the energy gap between Fermi pairs. Thus, the photogenerated e from the CB of BMO tend to
level ( E
f
) and valence band are 2.00 and 1.32 V for BMO and 4wt% transfer to the Au surface due to the surface electric near field
8
5
-
+
Au/BMO, respectively. It has been reported that the flat band produced by the SPR effect of Au NPs, which facilitates the e /h
potential of n-type semiconductor equals the Fermi level.
3
6-38
-
separation on the BMO. Therefore, photogenerated e can survive
Hence, the calculated VB positions of BMO and Au/BMO are 1.89 for a period of time in the current research, which will play a vital
and 1.26 V, respectively. The CB positions of both BMO and role in the photocatalytic process. Additionally, due to the lower CB
4
wt%Au/BMO are calculated based on the band gaps to be -0.44 (-0.44 eV vs NHE) voltage of BMO than the redox potential of
-
86,87
-
eV, respectively, which are higher than O
Moreover, the band diagrams of BMO and Au/BMO are shown in form ·O
Fig. S6(ESI*).
2
/·O
2
(-0.33 V vs NHE).
O
2
/·O
2
( -0.33 eV vs NHE), which can be trapped by O
2
to further
-
2
and it further binds to the proton to form ·OH. In
+
addition, the phenol could be directly degraded by the h from the
Based on the above results, a schematic diagram for the VB of BMO. Hence, for Au/BMO, synergistic effect of SOVs and
charge separation and oxygen molecule oxygen mechanism over surface plasmonic Au greatly accelerate its catalytic performance
SOVs and plasmonic Au NPs co-modified BMO photocatalyst is for the phenol degradation under visible light irradiation.
proposed and illustrated in Scheme 2, thereby, the mechanism for
Pollutant
O2
CO  H O ...
2
2


O2
-2

OOH  OH
e-
e-0
Au
e-
e-

-
1
0
1
2
e-
H
CMB(-0.44eV)
e- e- e- e- e- e- e- e-
Bi M O
6
2
O
VMB(1.26eV)
VMB(1.89eV)
Pollutant
Band gap position of BMO
h+ h+ h+ h+ h+ h+ h+ h+
Surface oxygen-vacancy states
Band gap position of Au/BMO
3
4
CO  H O ...
2 2
Scheme
2
Schematic illustration of hole/electron separation and transfer process for the Au/BMO photocatalysts under visible light irradiation.
facile reduction–deposition method. Simultaneously planting SOVs
Conclusions
and depositing Au NPs on the surface of BMO can greatly improve
the separation efficiency of photogenerated carriers as well as
enlarge the visible-light responsive region, thereby enhance the
In summary, SOVs and Au NPs co-modification BMO photocatalyst
has been rationally designed and successfully fabricated via a
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CP al et aa lsy es i ds oS c ni eo nt c ae d &j uTs et c mh na or gl oi ng sy
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ARTICLE
Journal Name
oxygen molecule activation efficiency. The as-fabricated Au/BMO
heterostructure exhibited superior tphotocatalytic performance
for phenol and dyes degradation, in which the 4% Au/BMO
composite exhibits the optimal efficiency under visible light
irradiation. The SPR effect of Au NPs and surface oxygen vacancy
plays the important roles in the enhancement of photocatalytic
activity. Dual synergistic effect of the surface oxygen vacancy, the
SPR of Au nanoparticles, and the unique metal-semiconductor
junction effectively improves the separation and migration of
photogenerated electrons and holes and simultaneously represses
the recombination of electrons and holes, resulting in the high
photocatalytic performance of Au/BMO. The present work
provides an effective strategy to improve the oxygen molecule
activation efficiency through dual synergistic effect of SOVs and
metal NPs of SPR effect.
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Conflicts of interest
There are no conflicts to declare.
[
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4
This work was supported by the National Natural Science
Foundation of China (No. 21663030, 21666039) and the Open
Project of State Key Laboratory of Organic-Inorganic
Composites Beijing Key Laboratory, Beijing University of
Chemical Technology Beijing(No.oic-201901009) and the
Project of Science & Technology Office of Shaanxi Province (No.
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This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 9
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CP al et aa lsy es i ds oS c ni eo nt c ae d &j uTs et c mh na or gl oi ng sy
Page 10 of 11
ARTICLE
Journal Name
View Article Online
DOI: 10.1039/C9CY00579J
A table of contents entry
Ultrafine Au Nanoparticles Anchored on Bi MoO with Abundant Surface Oxygen
2
6
Vacancies for Efficient Oxygen Molecule Activation
a
a
a
a
a
a
a
b
Li Guo, Qiang Zhao, Huidong Shen, Xuanxuan Han , Kailai Zhang , Danjun Wang * , Feng Fu* and Bin Xu*
a
Shaanxi Key Laboratory of Chemical Reaction Engineering, School of Chemistry & Chemical Engineering, Yan’an
University, Yan’an 716000, China.
b
State Key Laboratory of Organic-Inorganic Composites Beijing Key Laboratory of Electrochemical Process and
Technology for Materials, Beijing University of Chemical Technology, Beijing 100029, China.
Summary:
Au NPs were anchored on Bi MoO with rich SOVs to improve O activation for photocatalytic degradation of phenol
2
6
2
and dye.
Pollutant
O2
CO  H O ...
2
2


O2
-2
e-
OOH  OH
e-0

-
1
0
1
2
e-
H
Au
CMB(-0.44eV)
e- e- e- e- e- e- e- e-
e-
e-
Bi M O
6
2
O
VMB(1.26eV)
VMB(1.89eV)
Pollutant
Band gap position of BMO
h
+
h
+
h
+
h
+
h
+
h
+
h
+
h
+
Surface oxygen-vacancy states
Band gap position of Au/BMO
3
4
CO  H O ...
2 2
1
0 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C9CY00579J
Catalysis Science & Technology
ARTICLE
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