In situ decoration of plasmonic Ag nanocrystals on the surface of (BiO)2CO3 hierarchical microspheres for enhanced visible light photocatalysis

Article abstract of DOI:10.1039/c4dt00427b

Novel plasmonic 0D Ag nanocrystal decorated 3D (BiO)₂CO ₃ hierarchical microspheres were fabricated with a one-pot hydrothermal method. The as-prepared samples were systematically characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, N₂ adsorption-desorption isotherms, X-ray photoelectron spectroscopy, UV-vis diffuse reflectance spectroscopy, photoluminescence spectra, ns-level time-resolved fluorescence spectra, photocurrent generation and EIS measurement. The results indicated that the 0D Ag nanoparticles were deposited on the surface of 3D (BiO)₂CO₃ hierarchical microspheres. The deposited Ag nanoparticles were reduced from Ag⁺ by the citrate ions from bismuth citrate. The photocatalytic activity of the as-prepared samples was evaluated towards the degradation of NO at ppb-level under visible light irradiation. The intermediate NO₂ was monitored on-line during the photocatalytic reaction. The pure (BiO)₂CO ₃ microspheres exhibited decent visible light photocatalytic activity because of the surface scattering and reflecting (SSR effect) resulting from the special 3D hierarchical architecture. The Ag-decorated (BiO) ₂CO₃ microspheres (Ag/BOC) exhibited greatly enhanced photocatalytic activity, photocurrent generation and promoted NO₂ oxidation compared to the pure (BiO)₂CO₃ microspheres. The enhanced photocatalytic activity and photocurrent generation of Ag/BOC was ascribed to the cooperative contribution of the surface plasmon resonance (SPR effect), efficient separation of electron-hole pairs and prolonged lifetime of charge carriers induced by Ag nanoparticles. The photocatalytic performance of Ag/BOC was dependent on the content of Ag loading. When the amount of Ag is controlled at 5%, the highest photocatalytic performance can be achieved. Further increasing the Ag loading content promotes aggregation of the Ag particles and transforms the uniform microspheres into non-uniform microspheres, which is not beneficial to improving the activity. Importantly, the as-prepared Ag/BOC composites exhibited high photochemical stability after multiple reaction runs. The concepts of enhancing the activity through the SSR and SPR effects provide a new avenue for the development of efficient noble metal/bismuth-based plasmonic photocatalysts with attractive nano/micro architectures for efficient visible light photocatalytic activity.

Full text of DOI:10.1039/c4dt00427b

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In situ decoration of plasmonic Ag nanocrystals on
the surface of (BiO)2CO3 hierarchical microspheres
for enhanced visible light photocatalysis†
Cite this: Dalton Trans., 2014, 43,
9468
Fan Dong,*a Qiuyan Li,a Ying Zhou,b Yanjun Sun,a Haidong Zhanga and
Zhongbiao Wuc
Novel plasmonic 0D Ag nanocrystal decorated 3D (BiO)2CO3 hierarchical microspheres were fabricated
with a one-pot hydrothermal method. The as-prepared samples were systematically characterized by
X-ray diffraction, scanning electron microscopy, transmission electron microscopy, N2 adsorption–deso-
rption isotherms, X-ray photoelectron spectroscopy, UV-vis diffuse reflectance spectroscopy, photo-
luminescence spectra, ns-level time-resolved fluorescence spectra, photocurrent generation and EIS
measurement. The results indicated that the 0D Ag nanoparticles were deposited on the surface of 3D
(BiO)2CO3 hierarchical microspheres. The deposited Ag nanoparticles were reduced from Ag+ by the
citrate ions from bismuth citrate. The photocatalytic activity of the as-prepared samples was evaluated
towards the degradation of NO at ppb-level under visible light irradiation. The intermediate NO2 was
monitored on-line during the photocatalytic reaction. The pure (BiO)2CO3 microspheres exhibited decent
visible light photocatalytic activity because of the surface scattering and reflecting (SSR effect) resulting
from the special 3D hierarchical architecture. The Ag-decorated (BiO)2CO3 microspheres (Ag/BOC)
exhibited greatly enhanced photocatalytic activity, photocurrent generation and promoted NO2 oxidation
compared to the pure (BiO)2CO3 microspheres. The enhanced photocatalytic activity and photocurrent
generation of Ag/BOC was ascribed to the cooperative contribution of the surface plasmon resonance
(SPR effect), efficient separation of electron–hole pairs and prolonged lifetime of charge carriers induced
by Ag nanoparticles. The photocatalytic performance of Ag/BOC was dependent on the content of Ag
loading. When the amount of Ag is controlled at 5%, the highest photocatalytic performance can be
achieved. Further increasing the Ag loading content promotes aggregation of the Ag particles and trans-
forms the uniform microspheres into non-uniform microspheres, which is not beneficial to improving the
activity. Importantly, the as-prepared Ag/BOC composites exhibited high photochemical stability after
multiple reaction runs. The concepts of enhancing the activity through the SSR and SPR effects provide a
new avenue for the development of efficient noble metal/bismuth-based plasmonic photocatalysts with
attractive nano/micro architectures for efficient visible light photocatalytic activity.
Received 10th February 2014,
Accepted 27th March 2014
DOI: 10.1039/c4dt00427b
www.rsc.org/dalton
fuel production and chemical synthesis. In the past few
decades, many semiconductor photocatalytic materials have
1. Introduction
Photocatalysis is a highly appealing technology with multiple been synthesized, including UV-excited and visible-excited
applications, mainly in the fields of pollution control, solar semiconductors.1–3 It is well known that the application of tra-
ditional UV-responsive photocatalysts is limited by their large
band gap, which cannot use solar energy effectively. Therefore,
the development of visible light (46% fraction of solar light)
responsive photocatalysts with narrow band gaps is urgent
aChongqing Key Laboratory of Catalysis and Functional Organic Molecules, College
of Environmental and Biological Engineering, Chongqing Technology and Business
University, Chongqing, 400067, China. E-mail: dfctbu@126.com;
and highly desirable, and could provide a potential route
for environmental pollution control and clean energy
development.1–3 Various approaches have been employed to
improve the visible light activity of photocatalysts, such as
metal doping, nonmetal doping, composites and noble-metal
deposition, sensitization and so on.4–6
Fax: +86-23-62769785-605; Tel: +86-23-62769785-605
bState Key Laboratory of Oil and Gas Reservoir and Exploitation, School of Materials
Science and Engineering, Southwest Petroleum University, Chengdu, 610500, China
cDepartment of Environmental Engineering, Zhejiang University, Hangzhou, 310027,
China
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c4dt00427b
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Recently, the noble-metal based plasmonic photocatalyst allic Ag by citrate ions from bismuth citrate. The pure
working under visible light has attracted great attention. The (BiO)2CO3 microspheres exhibited decent visible light activity
plasmonic photocatalyst consisting of a noble metal and semi- because of the surface scattering and reflecting effects induced
conductor possesses significant surface plasmon resonance by the 3D hierarchical structures. The novel Ag-decorated
(SPR) effects that enables the photocatalyst to absorb visible (BiO)2CO3 microsphere composites exhibited dramatically
light. The SPR effect can be described as the resonant photon- enhanced photocatalytic activity towards NO degradation com-
induced collective oscillation of valence electrons established pared with the pure (BiO)2CO3 microspheres because of the
when the frequency of photons matches the natural frequency cooperative contribution of the SPR effect, efficient separation
of the surface electrons oscillating against the restoring force of electron–hole pairs and prolonged lifetime of charge car-
of the positive nuclei.7–9 To date, some plasmonic photocata- riers by Ag nanoparticles. Meanwhile, the effect of Ag content
lysts have been reported, such as Ag@AgX (X = Cl, Br),10,11 on the photocatalytic performance was evaluated. The sample
M@TiO2 (M = Au, Pt, Ag),12 Pt/Bi2O3,13 Ag/AgCl/TiO2 and Ag- with 5% Ag loading exhibited the highest photocatalytic
14
AgI/Fe3O4@SiO2.15 All these plasmonic photocatalysts exhibi- activity. More importantly, this excellent photocatalyst showed
ted an enhanced photocatalytic activity compared with their high photochemical stability and durability, which is signifi-
substrate semiconductors.
cant for its practical application. By combination of the SSR
(BiO)2CO3 is a new multifunctional material and has effect and SPR effect, an efficient and durable visible light
received great research interest in recent years.16–27 For driven photocatalyst can be developed, which could provide
example, Chen et al. prepared (BiO)2CO3 nanotubes with anti- new insights into the design and synthesis of novel
bacterial performance.16 They also hydrothermally synthesized photocatalytic materials for environmental and energetic
(BiO)2CO3 nanobars, nanoplates and cube-like structures.17 applications.
Xie and coworkers synthesized various (BiO)2CO3 microstruc-
tures to degrade the aqueous dye Rhodamine B (RhB).18
(BiO)2CO3 nanosheets and flower-like structures were prepared
2. Experimental section
by Huang and coworkers.19 Cao et al. fabricated persimmon-
2.1. Synthesis of Ag-decorated (BiO)2CO3 samples
like (BiO)2CO3, which was able to efficiently degrade RhB and
eosin sodium salt under simulated solar light irradiation.20
All the chemicals employed in this study were analytical grade
and were used without further purification. Distilled water was
used in all experiments. In a typical synthesis process, sodium
carbonate (0.46 g) was first dissolved in distilled water (70 ml)
in a 100 ml autoclave Teflon vessel and stirred for 10 min.
Then 0.04 mol L−1 AgNO3 solution of an appropriate volume
was added into the solution and further stirred for 10 min.
Afterwards, bismuth citrate (1.6 g) was added into the above
solution and the mixture was stirred for 30 min to ensure that
all reagents dissolved sufficiently. The resulting precursor sus-
pension was hydrothermally treated at 160 °C for 24 h. After
being cooled down to room temperature, the resulting solid
was filtered, washed with water and ethanol four times and
dried at 60 °C for 12 h to get the final Ag-decorated (BiO)2CO3
sample. In order to investigate the effects of Ag content on the
photocatalytic activity of Ag-decorated (BiO)2CO3, the amount
of AgNO3 solution was controlled at 0.5, 2.5, 5.0 and 10.0 ml,
and the obtained samples were labeled as Ag/BOC-1%, Ag/
BOC-5%, Ag/BOC-10% and Ag/BOC-20%, respectively. Pure
(BiO)2CO3 microspheres (labeled as BOC) were prepared as
references without adding AgNO3. As a reference, (BiO)2CO3
with a nanosheet morphology (labelled as BOC nanosheet)
was prepared (Fig. S1†).25
Zhang et al. reported the morphogenesis and self-assembly of
3D (BiO)2CO3 hierarchical nanostructures for the ultrasensitive
detection of aromatic molecules.21 However, (BiO)2CO3 nano-
particles with a big band gap (3.1–3.5 eV) can only be excited
by UV light. In order to improve the activity of (BiO)2CO3
under visible light irradiation, Yu and co-workers reported
BiVO4/(BiO)2CO3 and graphene/(BiO)2CO3 nanocomposites
which exhibited enhanced visible light photocatalytic
activity.22,23 Dong et al. fabricated N-doped (BiO)2CO3 hierarch-
ical microspheres with outstanding photocatalytic activity
under visible light irradiation.24–26 Peng et al. prepared
Bi2O2CO3 nanoplate microspheres by a VPV-assisted hydro-
thermal method.27 Ag was then deposited on the surface of
Bi2O2CO3 via
a subsequent photoreduction process for
enhanced photocatalytic activity and supercapacitor perform-
ance.27 Although semiconductor compositing and N-doping
could make pure (BiO)2CO3 visible light active, there is little
reported on noble metal/(BiO)2CO3-based plasmonic photo-
catalysts with 3D hierarchical structures. On one hand, 3D
hierarchical structures could make pure (BiO)2CO3 visible light
active through surface scattering and reflecting effects (SSR
effect). On the other hand, noble metal decoration could
further enhance the visible light activity of 3D (BiO)2CO3 hier-
archical microspheres through surface plasmon resonance
effects (SPR effect).
2.2. Characterization
The X-ray diffraction patterns of the samples were collected on
In the present study, we developed a one-pot hydrothermal an X-ray diffractometer with Cu Kα radiation (XRD: model
method to in situ deposit plasmonic Ag nanocrystals on D/max RA, Rigaku Co., Japan). In order to characterize the
(BiO)2CO3 hierarchical microspheres in order to promote the morphology, structure and grain size of the obtained products,
visible light photocatalytic activity of the (BiO)2CO3 micro- scanning electron microscopy (SEM: JEOL model JSM-6490,
spheres. In the hydrothermal process, Ag+ was reduced to met- Japan), transmission electron microscopy (TEM: JEM-2010,
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Japan), and high-resolution transmission electron microscopy
(HRTEM: JEM-2010) were used to collect the SEM and TEM
images. The Brunauer–Emmett–Teller (BET) specific surface
area (SBET) and pore structure of the samples were analyzed by
nitrogen adsorption–desorption in a nitrogen adsorption appar-
atus (ASAP 2020, USA) with all samples degassed at 100 °C for
12 h prior to measurements. X-ray photoelectron spectroscopy
(XPS: Thermo ESCALAB 250, USA) measurements were carried
out to investigate the surface chemical compositions and their
chemical state using Al Kα X-rays (hν = 1486.6 eV) and the radi-
ation source operated at 150 W. The UV-vis diffuse reflection
spectroscopy (UV-vis DRS) was obtained for dry-pressed disk
samples with a Scan UV-vis spectrophotometer (UV-2450,
Shimadzu, Japan) using BaSO4 as a standard sample. The
Photoluminescence (PL: F-7000, HITACHI, Japan) was used to
investigate the optical properties of the obtained samples.
Steady and time-resolved fluorescence emission spectra were
recorded at room temperature with a fluorescence spectro-
photometer (Edinburgh Instruments, FLSP-920).
Fig. 1 XRD patterns of the samples obtained in the presence of
different amounts of Ag.
2.3. Evaluation of photocatalytic activity
with different Ag contents. It can be seen that all the samples
are well crystallized. The diffraction peaks of BOC without
adding AgNO3 can be readily indexed to the tetragonal
(BiO)2CO3 structure (JCPDS-ICDD Card no. 41-1488) and no
other impurities were detected, indicating a pure (BiO)2CO3
phase. When AgNO3 was used as a precursor in the fabrication
of the samples, new diffraction peaks at 2θ = 38° and 44° were
observed in Ag/BOC-5%, Ag/BOC-10% and Ag/BOC-20% in
addition to the characteristic peaks of the pure (BiO)2CO3
phase. These new peaks are ascribed to the deposition of Ag
nanoparticles (JCPDS-ICDD Card no. 65-2871) on the surface
of the (BiO)2CO3 microspheres. However, such new peaks were
not detected in the Ag/BOC-1% sample due to its low amount
of Ag. The Ag nanoparticles formed from the reduction of Ag+
by the citrate ions in bismuth citrate. Based on further obser-
vation, the peak intensity of the Ag nanoparticles increased
gradually with the increasing Ag content, while some diffrac-
tion peaks of (BiO)2CO3 in Ag/BOC-20% decreased, suggesting
that a high Ag content would affect the crystallinity of
(BiO)2CO3. Meanwhile, it is noteworthy to mention that no
changes in the diffraction peaks and lattice parameters of
(BiO)2CO3 in the Ag/BOC samples was detected, which implies
that Ag did not get into the lattice of (BiO)2CO3, but appeared
on the surface.25,26 According to the crystal structure of
The photocatalytic activity of the as-synthesized samples was
evaluated by removing NO at ppb-level in a continuous flow
reactor (Fig. S2† shows the photo of the reactor system). The
volume of the reactor was 4.5 L (30 cm × 15 cm × 10 cm), it was
made of polymeric glass and covered with Saint-Glass. There
was a commercial tungsten halogen lamp (150 W) vertically
placed 20 cm above the reactor. A UV cutoff filter (420 nm) was
applied to remove UV light for the testing of the visible light
photocatalytic activity. The as-prepared sample (0.20 g) was
dispersed in distilled water (50 ml) in a beaker by ultrasonic
treatment for 10 min, and then coated onto two glass dishes
with a diameter of 12.0 cm. The coated dishes were pretreated
at 70 °C to remove the water in the suspension and placed in
the center of the reactor after cooling down to room tempera-
ture. The NO gas was acquired from a compressed gas cylinder
at a concentration of 100 ppm of NO (N2 balance). The initial
concentration of NO was diluted to about 600 ppb by an air
stream. The flow rate of air stream and NO were controlled at
2.4 L min−1 and 15 mL min−1, respectively. Then the two gas
streams were premixed by a three-way valve. After an adsorp-
tion–desorption equilibrium was achieved, the lamp was
turned on. The concentration of NO was measured by a NOx
analyzer (Thermo Scientific, 42i-TL) every one min, which also
monitored the concentration of NO2 and NOx (NOx represents
NO + NO2). The removal ratio of NO (η) was calculated by η (%)
= (1 − C/C0) × 100%, where C is the outlet concentration of NO
after reaction for time t and C0 represents the inlet concen-
tration after achieving the adsorption–desorption equilibrium.
2−
(BiO)2CO3, the (Bi2O2)2+ layers and CO3 layers are ortho-
gonally intergrown. The internal layered structure shows a
lower growth rate along a certain axis to form a nanosheet
morphology.24,28
3.2. Morphological structure
The morphology and microstructure of the obtained samples
were characterized by SEM, TEM and HRTEM. In Fig. 2a and
2b, we can easily see that the pure (BiO)2CO3 sample consists
3. Results and discussion
3.1. Phase structure
XRD was used to characterize the phase structure of the as-pre- of many three-dimensional (3D) flower-like hierarchical
pared samples. Fig. 1 shows the XRD patterns of the products microspheres self-assembled from two-dimensional (2D)
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Fig. 2 SEM images of BOC (a, b) and Ag/BOC-1% (c, d).
nanosheets. Obviously, these microspheres have different sizes attaching on the surface (circled by dotted line). Fig. 3g shows
from 0.7 to 1.3 μm. Compared with the pure (BiO)2CO3 micro- a typical HRTEM image of Ag/BOC-5%, the lattice spacing was
spheres, the Ag/BOC-1% microspheres are more uniform due determined to be 0.254 nm and 0.235 nm, which correspond
to the addition of AgNO3 (2c, 2d). The Ag nanoparticles cannot to the (112) crystal plane of tetragonal (BiO)2CO3 and (111)
be observed on the surface of the (BiO)2CO3 microspheres due lattice plane of Ag, respectively. The SAED image (Fig. 3h) of
to the low loading content. This result is in agreement with one single nanosheet reveals two different kinds of diffraction
the XRD observation.
spots that can be indexed to tetragonal (BiO)2CO3 and Ag,
SEM, TEM, HRTEM and SAED images of the Ag/BOC-5% respectively. The regular diffraction spots also indicate that the
sample are shown in Fig. 3. The SEM images in Fig. 3a and 3b nanosheet is single-crystalline in nature.
show that these microspheres become uniform as the amount
Fig. 4 shows the SEM images of Ag/BOC-10%. Obviously,
of Ag is increased from 1% to 5%. Some zero-dimensional (0D) more Ag nanoparticles have formed on the surface of the 3D
monodispersed Ag nanoparticles (circled by dotted line) are (BiO)2CO3 microspheres compared to Ag/BOC-5% and a few
deposited on the surface of the 3D uniform (BiO)2CO3 micro- particles have begun to aggregate. In addition, for part of the
spheres with an average diameter of about 1.5 μm. The corres- microspheres, the nanosheets have assembled in a more
ponding EDS spectra (Fig. 3c and 3d) confirm the amount of compact manner with a diameter of about 200 nm. When the
Ag at the particle is 8%, while no Ag is detected beside the par- amount of AgNO3 was increased to 10.0 ml, all the flower-like
ticle, indicating that Ag was deposited on the surface of the hierarchical microspheres transformed into non-uniform hier-
microspheres instead of being doped into it. It is presumed archical microspheres (Fig. S3a†). The high-magnification
that the formation of the Ag nanoparticles derived from the SEM image (Fig. S3b†) reveals that Ag nanoparticles aggregated
reduction of Ag+ by citrate ions in bismuth citrate. Such together on the surface of the (BiO)2CO3 microspheres.
special microstructures consisting of 0D nanoparticles and 3D The gradually increased Ag aggregation on the surface of
flower-like hierarchical microspheres have never been reported the (BiO)2CO3 microspheres corresponds to the gradually
before. The morphological structures of the sample were increased peak intensity in the XRD pattern. Consequently, the
further confirmed by TEM and HRTEM. As shown in Fig. 3e amount of AgNO3 plays a fundamental role in the formation of
and 3f, the microspheres are indeed composed of self- the as-prepared samples and thus affects their photocatalytic
assembled nanosheets with monodispersed Ag nanoparticles activity.
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Fig. 3 SEM (a, b), EDS (c, d), TEM (e, f), HRTEM (g) and SAED (h) images of Ag/BOC-5%.
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Fig. 4 SEM images of Ag/BOC-10%.
3.3. BET surface areas and pore structure
the large mesopores are ascribed to the pores formed between
the microspheres. The BET surface areas of BOC, Ag/BOC-1%,
Ag/BOC-5%, Ag/BOC-10% and Ag/BOC-20% were measured to
be 46, 45, 44, 41 and 37 m2 g−1, indicating that the surface de-
coration of Ag nanocrystals exerts little impact on the BET
surface areas of BOC. In general, porous structures are
expected to be useful in the photocatalytic process by offering
efficient transport pathways for reactants and products and
facilitating the adsorption of pollutants for degradation.33
The nitrogen adsorption–desorption isotherms and corres-
ponding pore size distribution curves of the as-prepared
samples are displayed in Fig. 5. According to the Brunauer–
Deming–Deming–Teller (BDDT) classification, the majority of
physisorption isotherms can be classified into six types. Typi-
cally, the Ag/BOC-5% and pure BOC both have an isotherm of
type IV, indicating the presence of mesopores (Fig. 5a). Ag/
BOC-5% displays higher absorption at relatively high pressure
compared with BOC, suggesting the presence of more large
mesopores in Ag/BOC-5%. This result is also demonstrated by
3.4. Chemical composition by XPS
the pore-size distribution curves. The increased amount of The XPS measurements were carried out to elucidate the
large mesopores are probably due to the addition of AgNO3
that changes the morphologies of the samples. Furthermore,
chemical state of the elements on the surface of the Ag/
BOC-10% sample, as shown in Fig. 6. The two strong peaks at
the shape of the hysteresis loops is of type H3, implying the 159.1 and 164.4 eV in the spectra (Fig. 6a) are assigned to
existence of slit-like pores.29,30 This result is consistent with
the SEM observations. The corresponding pore size distri-
Bi4f7/2 and Bi4f5/2, respectively, which is the feature of Bi3+ in
(BiO)2CO3.34 Fig. 6b shows the C1s spectra of (BiO)2CO3. The
bution curves of the samples are shown in Fig. 5b. It can be peak at 284.78 eV is caused by the adventitious carbon species,
−2
seen that the pore-size distribution range for the samples is
broad in the range of 2 to 80 nm and bimodal with small
while the peak at 288.7 eV can be ascribed to the CO3 in
(BiO)2CO3. The O1s peak (Fig. 6c) at 530.99 eV is characteristic
mesopores (3 nm) and larger ones (10 nm), confirming the for- of Bi–O in (BiO)2CO3. The spectra of Ag3d3/2 and Ag3d5/2
mation of mesopores.31,32 The small mesopores can be attribu-
(Fig. 6d) were recorded, and the peaks were identified at 374.1
ted to the pores formed among the aggregated nanosheets and and 368.2 eV, indicating that Ag+ was reduced to metallic Ag by
Fig. 5 N2 adsorption–desorption isotherms (a) and pore size-distribution curves (b) of Ag/BOC-5% and BOC.
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Fig. 6 XPS spectra of Ag/BOC-5%, Bi4f (a), C1s (b), O1s (c), Ag3d (d).
citrate ions in bismuth citrate.35 The result has also been
demonstrated by XRD and SEM analysis.
3.5. Light absorption, charge separation and carrier lifetime
It is well known that the optical properties of semiconductors
are closely related to their photocatalytic activity. The UV-vis
DRS spectra of the Ag/BOC and pure (BiO)2CO3 samples are
shown in Fig. 7. In the UV region, all the samples containing
Ag display higher absorption compared with pure (BiO)2CO3,
probably due to the addition of AgNO3. In addition, the
(BiO)2CO3 microspheres exhibit higher absorption than
(BiO)2CO3 nanosheets both in the UV and visible light region,
which is due to their special 3D hierarchical structure (SEM
Fig. 2). This special hierarchical structure could generate mul-
tiple light scattering and reflecting effects, thus greatly increas-
ing the effective optical path-length of
a photon and
Fig. 7 UV-vis DRS spectra of the samples with different amounts of Ag
and pure (BiO)2CO3.
absorption probability.40 It is noteworthy to mention that
there is a broad absorption in the visible light region from
400 nm to 600 nm for Ag/BOC-5%, Ag/BOC-10% and
Ag/BOC-20%, centered at 415 nm. Such absorption ability in
the visible light region may be attributed to the SPR effect of
such a phenomenon due to the low content of Ag. The above
results are in agreement with the XRD and SEM analysis. The
samples exhibit increased absorption in the visible light
region with increasing Ag content. This can be ascribed to the
stronger SPR effect caused by more deposited Ag nanoparti-
cles, therefore absorbing more solar light.36,37 In general, the
spatially confined electrons in metallic Ag nanoparticles.36,37
previous report has demonstrated that Ag nanoparticles with
100 nm diameters could exhibit a SPR absorption centered at
415 nm.38 However, the Ag/BOC-1% sample does not exhibit
A
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(BiO)2CO3 samples with an excitation wavelength of 280 nm. It
can be seen that the fluorescence emission peaks are mainly
centered at 325–500 nm, which contains both the UV and
visible light regions. This result reveals that the as-prepared
samples are able to absorb both UV and visible light. The
absorption of visible light can be ascribed to the special hier-
archical structures of the (BiO)2CO3 microspheres and the SPR
effect of the Ag nanoparticles, which have been demonstrated
by UV-vis DRS analysis. Furthermore, the peak intensity of
BOC is stronger than that of all the Ag/BOC samples,
suggesting that the recombination rate of electron–hole pairs
on Ag/BOC samples is lower than BOC. This result is sup-
ported by the fact that the deposited Ag nanoparticles can act
as electron traps increasing electron–hole separation.39,40
Meanwhile, the peak intensity decreased with increasing Ag
content, which suggests that more Ag nanoparticles can trap
more electrons and reduce electron–hole recombination. In a
word, the presence of metal Ag nanoparticles plays a crucial
role in separating electron–hole pairs.
Fig. 8 PL spectra of the Ag/BOC samples with different amounts of Ag
and pure (BiO)2CO3.
To investigate the photophysical properties of the photo-
excited charge carriers of BOC and Ag/BOC-5%, the ns-level
time-resolved fluorescence decay spectra were recorded, as
shown in Fig. 9a and 9b. The radiative lifetime can be deter-
mined by fitting the decay spectra, as summarized in Table 1.
The short lifetime (τ1) of BOC is 0.35 ns. After Ag decoration,
the lifetime of Ag/BOC-5% increased to 0.54 ns. The percen-
tage of charge carriers with a short lifetime of the two samples
special 3D hierarchical structure enables (BiO)2CO3 to absorb
more visible light through the light reflecting and scattering.
The deposition of Ag nanoparticles on the surface of
(BiO)2CO3 microspheres can further significantly enhance the
visible light absorption.
The recombination of electron–hole pairs was measured by
PL. Fig. 8 shows the PL spectra of the Ag/BOC and pure
Fig. 9 The ns-level time-resolved fluorescence spectra monitored at 440 nm under 375 nm excitation at room temperature for the BOC (a) and
Ag/BOC-5% (b) samples.
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light irradiation with the (BiO)2CO3 as the reference. The pure
(BiO)2CO3 with a nanosheet morphology (see Fig. S1†) shows
little activity under visible light irradiation. However, a decent
removal ratio of 22% is observed on the pure (BiO)2CO3 with a
microsphere morphology (see Fig. 2a) under the same con-
ditions. This fact could be ascribed to the special hierarchical
structure of the (BiO)2CO3 microspheres (SEM in Fig. 2), which
could induce effects of multiple light surface scattering and
reflecting (SSR effects), thus greatly increasing the effective
optical path-length of a photon and absorption probability
(Fig. 7).41 A similar phenomenon has been observed for BiOCl
hierarchical nanostructures.41 From BET analysis, it is clear
the hierarchical structures can also offer efficient transport
pathways for reactants and products to accelerate the reaction
rate. As expected, the Ag/BOC samples exhibit outstanding
photocatalytic activity, the photocatalytic activity increases
from 40% to 54% with increasing the Ag content from 1.0 to
5.0%, much higher than that of the (BiO)2CO3 microspheres,
C-doped TiO2 (25%),42 N-doped TiO2 (36%),42 C3N4 (33%)43
and BiOBr (21%).44 Further increasing the Ag loading could
result in a decrease in photocatalytic activity. A drastic
decrease in photocatalytic activity was observed for Ag/
BOC-20%, which can be ascribed to the serious aggregation of
Ag nanoparticles and non-uniform morphology of the
(BiO)2CO3 microspheres (Fig. S3a†). Aggregated Ag nanoparti-
cles are not favorable for SPR effects and the non-uniform
morphology of (BiO)2CO3 is not beneficial for SSR effects,
which, in all, resulted in the decreased activity of Ag/BOC-20%.
The reaction intermediate of NO2 was monitored during
the photocatalytic oxidation of NO on-line in the outlet, as
shown in Fig. 10b. The fraction of NO2 generated over pure
(BiO)2CO3 was relatively high during the reaction. For the Ag
decorated samples, the fraction of NO2 over the samples
decreased significantly, especially for the samples with low
contents of Ag. When the loading content of Ag increased
from 5% to 20%, the fraction of NO2 markedly increased
because of Ag particle aggregation and the destruction of the
Table 1 The kinetics of emission decay parameters of the BOC and Ag/
BOC-5% samplesa
Lifetime
(ns)
Relative
percentage (%)
Samples
BOC
Component
χ2
τ1
τ2
τ1
τ2
0.35
4.25
0.54
5.54
95.57
4.43
95.38
4.62
1.021
1.062
Ag/BOC-5%
N
P
a The decay spectra were fitted by the equation f ðtÞ ¼ A þ Bieꢀt=τ
.
i
i¼1
shows little change. The long lifetime (τ2) of the charge car-
riers increased from 4.25 ns for BOC to 5.54 ns for Ag/
BOC-5%. The percentage of charge carriers with a long lifetime
also increased slightly after Ag decoration. These results imply
that the radiative lifetime of all charge carriers increased due
to the formation of the Ag/BOC composites. The photoexcited
electron transfer from (BiO)2CO3 to the Ag nanoparticles is
responsible for the prolonged lifetime of the charge carriers.
The prolonged lifetime of the charge carriers could increase its
probability of being captured by reactive substrates to initiate
the following photocatalytic reactions.
3.6. Photocatalytic activity and photocurrent generation
NO in concentrations at the ppb level is a representative
indoor and outdoor air pollutant. The obtained Ag/BOC
samples were employed in the photocatalytic removal of NO in
air to demonstrate their potential ability for air pollutant
degradation. As previous reported, NO is very stable and
cannot be photolyzed under light irradiation without photo-
catalysts.34 However, in the presence of photocatalytic materials,
NO reacts with the photogenerated reactive radicals to produce
the NO2 intermediate and transforms into the final HNO3
product.25,34
Fig. 10a shows the variation in NO concentration (C/C0%)
with irradiation time over the Ag/BOC samples under visible
Fig. 10 (a) Visible light photocatalytic activity of Ag-decorated (BiO)2CO3 and pure (BiO)2CO3 samples for the removal of NO in air. (B) Monitoring
of the fraction of NO2 intermediate over the samples during photocatalytic reaction (continuous reactor, NO concentration: 600 ppb).
9476 | Dalton Trans., 2014, 43, 9468–9480
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Fig. 11 Photocurrent generation (a) and Nyquist plots (b) for the BOC nanosheet, BOC microsphere and Ag-BOC-5% electrodes under visible light
irradiation (λ > 420 nm, [Na2SO4] = 0.5 M).
microspheres (Fig. S3b†). However, the high fraction of NO2 increased visible light absorption of the plasmonic Ag
over Ag/BOC-20% is still lower than that over pure (BiO)2CO3. nanocrystal.45,46
This fact implies that Ag decoration could promote the oxi-
The separation efficiency of the photo-generated electron–
−
dation of the intermediate NO2 to the final product NO3 as hole pairs is a key factor for enhancement of the photocatalytic
shown in the following reactions.25,34 The final oxidation pro- activity and photocurrent. The interface charge separation
ducts (nitric acid or nitrate ions) can simply be washed away efficiency was investigated by EIS and is shown in Fig. 11b.
by water.
Fig. 11b shows the EIS Nyquist plots of the BOC nanosheet,
BOC microsphere and Ag-BOC-5% electrodes with and without
irradiation. The radius of the arc in the EIS spectra indicates
the reaction rate occurring at the surface of the electrode.45,46
The smaller arc radius on the EIS Nyquist plot of Ag-BOC-5%
under visible light irradiation implies that a more efficient
separation of photo-generated electron–hole pairs and a faster
interfacial charge transfer occurred in the interface of the Ag
nanocrystals and (BiO)2CO3 microspheres. It was also noted
that the arc radius on the EIS Nyquist plot of Ag-BOC-5% was
smaller than that of the pure (BiO)2CO3 microspheres without
irradiation. This fact suggests that the surface decoration of Ag
nanocrystals could change the distribution of charge carriers
on the (BiO)2CO3 microspheres and make the charge transfer
easier.45,46 These results imply that the coupling of Ag nano-
crystals with the (BiO)2CO3 microspheres can effectively
improve the separation efficiency of photo-generated electron–
hole pairs, consistent with the results from PL (Fig. 8) and
time-resolved fluorescence spectra (Fig. 9).
NO þ 2•OH ! NO2 þ H2O
ð1Þ
ð2Þ
ð3Þ
ð4Þ
•
NO2 þ OH ! NO3ꢀ þ Hþ
NO þ NO2 þ H2O ! 2 HNO2
•
ꢀ
ꢀ
NO þ O2 ! NO3
Considering the enhanced activity and inhibited NO2 gene-
ration, a 5% Ag content is the optimum to achieve the highest
photocatalytic performance. The above results indicate that
the appropriate amount of Ag loading could play a pivotal role
in enhancing the visible light photocatalytic performance of
(BiO)2CO3 microspheres.
Photocurrent generation was carried out for the BOC
nanosheet, BOC microsphere and Ag-BOC-5% electrodes to
evaluate the electronic interaction between Ag and the
(BiO)2CO3 microspheres and is shown in Fig. 11a. It can be
seen that steady and prompt photocurrent generation is
obtained during the on and off cycles of visible light illumina-
tion. The BOC nanosheet sample shows almost no photocur-
3.7. Mechanism of activity enhancement by plasmonic Ag
decoration
rent response due to its large band gap. The BOC As previous studies have shown, the enhancement in photo-
microspheres show a decent photocurrent under visible light catalytic performance can be ascribed to the synergetic effects
due to the SSR effects, consistent with their photocatalytic of many factors, such as hierarchical structure, surface area,
activity. The charge transport in this as-prepared sample pro- interfacial charge transfer and efficient separation of photo-
ceeds quickly, making the changes in “on” and “off” currents induced electrons and holes.42,47–52 According to the above
nearly vertical. It is significant to observe that the photo- characterizations, the enhanced photocatalytic activity of Ag/
current of the Ag-BOC-5% electrode is higher than that of the BOC can be attributed to the synergetic effects of the follow-
pure BOC microsphere electrode shown in Fig. 11a. The photo- ing factors. Firstly, the SPR effect of spatially confined elec-
current enhancement of Ag-BOC can be ascribed to the trons in the deposited Ag nanoparticles propels the Ag/BOC
enhanced photo-generated electron–hole separation and the samples to absorb more visible light, which has been demon-
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Fig. 13 Repeated visible light photocatalytic runs of Ag/BOC-5% for
the removal of NO in air.
Fig. 12 Photocatalytic mechanism scheme of Ag-decorated (BiO)2CO3
under visible light irradiation.
spheres instead of accumulating on the surface, thus effec-
tively preventing the photocatalyst from deactivating.
strated by UV-vis DRS analysis. Also, owing to the SPR effect,
the Ag nanoparticles can be photoexcited, thus enhancing the
surface electron excitation and interfacial electron
transfer.53–55 Second, since the Fermi level of metallic Ag
4. Conclusion
(0.4 eV) is lower than the conduction band of (BiO)2CO3 In summary, novel 0D Ag nanocrystal decorated 3D (BiO)2CO3
(0.20 eV), the photogenerated electrons would probably trans- hierarchical microspheres were fabricated by a one-pot hydro-
fer from (BiO)2CO3 to the deposited Ag nanoparticles (as thermal method for the first time. In the hydrothermal
shown in Fig. 12),56 creating a Schottky barrier at the interface process, Ag+ was in situ reduced to metallic Ag nanocrystals
that reduces the recombination of electron–hole pairs and by citrate ions and then deposited on the surface of the
increases the lifetime of the charge carriers (Fig. 8 and 9). (BiO)2CO3 microspheres. The pure (BiO)2CO3 microspheres
After the separation of electrons and holes, these two kinds of exhibited decent visible light photocatalytic performance
photogenerated charge carriers would be transformed into because of their multiple light reflecting and scattering effects
active species (•OH) that are responsible for the degradation of induced by the special 3D hierarchical architecture. The
pollutants. As the redox potential of O2/OH− is 0.401 eV, the Ag/BOC samples exhibited drastically enhanced visible light
photoexcited electrons of Ag together with the electrons trans- photocatalytic activity and promoted NO2 oxidation towards
ferred from (BiO)2CO3 can reduce O2 to OH−. Meanwhile, the NO removal over the (BiO)2CO3 microspheres, which can be
potential of the holes at the VB of (BiO)2CO3 (3.53 eV) is more ascribed to the cooperative contribution of the SSR effect, SPR
positive than the redox potential of OH−/•OH (1.99 eV), and effect, efficient separation of electron–hole pairs and pro-
therefore the holes can oxidize OH− to •OH. The OH− radicals, longed lifetime of the charge carriers by the Ag nanoparticles.
as a major reactive oxidation species, could oxidize the NO to Among the as-prepared samples, Ag/BOC-5% with a well-main-
the final NO3− product.
tained morphology showed the highest photocatalytic activity,
In order to use the as-prepared samples in practical appli- which suggested that the amount of Ag loading plays a key role
cations, the photocatalytic stability of the samples was tested. in enhancing the visible light photocatalytic activity of the
An ideal photocatalyst should maintain photochemical stabi- (BiO)2CO3 microspheres. Further increasing the Ag loading
lity and durability under long irradiation times so that it can content could promote the aggregation of Ag particles and
be used repeatedly.34 Fig. 13 shows the long term photo- transform the (BiO)2CO3 into non-uniform microspheres. In
catalytic activity of Ag/BOC-5% under visible light irradiation. addition, the as-prepared Ag/BOC-5% sample exhibited high
It is significant to note that the photocatalytic efficiency of photochemical stability and durability. The present results
Ag/BOC-5% does not exhibit any obvious loss after five demonstrate that combining the SSR and SPR effects could
recycles, indicating that Ag/BOC is relatively stable and does make the Ag/BOC composite a powerful and durable visible
not photo-corrode during the photocatalytic process. The light driven photocatalyst for environmental pollution control.
photochemical stability of Ag/BOC can probably ascribed to The present work could provide new insights into the in situ
the fact that the reactants and products can transfer quickly in fabrication of noble metal/bismuth-based plasmonic photo-
the special hierarchical structures of the (BiO)2CO3 micro- catalysts with novel nano/micro structures.
9478 | Dalton Trans., 2014, 43, 9468–9480
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21 X. Zhang, Y. Zheng, D. G. McCulloch, L. Y. Yeo, J. R. Friend
and D. R. MacFarlane, J. Mater. Chem. A, 2014, 2, 2275–
2282.
Acknowledgements
This research was financially supported by the National
Natural Science Foundation of China (51108487, 51102245), 22 P. Madhusudan, J. R. Ran, J. Zhang, J. G. Yu and G. Liu,
the Natural Science Foundation of Chongqing Appl. Catal., B, 2011, 110, 286–295.
(cstc2013jcyjA20018), and the Innovative Research Team Devel- 23 P. Madhusudan, J. G. Yu, W. G. Wang, B. Cheng and
opment Program in University of Chongqing (KJTD201314).
G. Liu, Dalton Trans., 2012, 41, 14345–14353.
24 F. Dong, W. K. Ho, S. C. Lee, Z. B. Wu, M. Fu, S. C. Zou and
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25 F. Dong, S. C. Lee, Z. B. Wu, Y. Huang, M. Fu, W. K. Ho,
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27 S. J. Peng, L. L. Li, H. T. Tan, Y. Z. Wu, R. Cai, H. Yu,
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