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Nano Res
1
Bi2WO6 Quantum Dots Intercalated Ultrathin
Montmorillonite Nanostructure and its Enhanced
Photocatalytic Performance
Songmei Sun,1 Wenzhong Wang,1() Dong Jiang,1 Ling Zhang,1 Xiaoman Li,1 Yali Zheng,1 and Qi An2
Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-014-0511-2
http://www.thenanoresearch.com on June 10, 2014
© Tsinghua University Press 2014
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Nano Research
DOI 10.1007/s12274-014-0511-2
TABLE OF CONTENTS (TOC)
Bi2WO6 Quantum Dots Intercalated Ultrathin
Montmorillonite Nanostructure and its Enhanced
Photocatalytic Performance
Songmei Sun, Wenzhong Wang, * Dong Jiang, Ling
Zhang, Xiaoman Li, Yali Zheng, and Qi An
Shanghai Institute of Ceramics, Chinese Academy of
Sciences, China
Bi2WO6 quantum dots intercalated ultrathin montmorillonite
are fabricated via a facile one-pot hydrothermal synthesis
method. This intercalated ultrathin nanostructure greatly
enhanced its solar-light-driven photocatalytic performance on
contaminant degradation and water oxidation by suppressing
the electron/hole recombination process.
Provide the authors’ webside if possible.
Author 1, webside 1
Author 2, webside 2
Bi2WO6 Quantum Dots Intercalated Ultrathin
Montmorillonite Nanostructure and its Enhanced
Photocatalytic Performance
Songmei Sun,1 Wenzhong Wang,1() Dong Jiang,1 Ling Zhang,1 Xiaoman Li,1 Yali Zheng,1 and Qi An2
Received: day month year
Revised: day month year
Accepted: day month year
(automatically inserted by
the publisher)
© Tsinghua University Press
and Springer-Verlag Berlin
Heidelberg 2014
KEYWORDS
Photocatalysis, lifetime,
ammonia degradation,
water oxidation,
montmorillonite
exfoliation
ABSTRACT
The kinetic competition between electron/hole recombination and water
oxidation is a key limitation for the development of efficient solar water
splitting material. In this study, we present a solution for solving this challenge
by constructing a quantum dots intercalated nanostructure. For the first time,
we show the interlayer charge of the intercalated nanostructure could largely
inhibit the electron-hole recombination in photocatalysis. For Bi2WO6 quantum
dots (QDs) intercalated montmorillonite (MMT) nanostructure as an example,
the average lifetime of the photogenerated charge carriers was increased from
3.06 μs to 18.8 μs by constructing an intercalated nanostructure. The increased
lifetime largely improved the photocatalytic performance of Bi2WO6 both on
solar water oxidation and environmental purification. This work not only
provides a principle method to produce QDs intercalated ultrathin
nanostructure but also a general rout to design efficient semiconductor-based
photo-conversion materials for solar fuels generation and environmental
purification.
1 Introduction
Photocatalysis provides a clean and sustainable way
to solve the current environmental and energy issues
[1-8]. Searching for efficient solar light driven
photocatalysts has attracted
substantial research interests and investigation. It is
generally recognized the kinetic competition between
electron/hole recombination and photocatalytic
reaction is a key limitation for the development of
efficient photocatalyst. Designing composite
materials provides an alternative approach to
facilitate photocatalytic reaction by suppressing the
competing electron-hole recombination, making it an
interesting research topic.
Layer structured montmorillonite (MMT) is a
potential base material for fabricating
high-performance photocatalyst, which is comprised
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2 Nano Res.
of highly anisotropic phyllosilicate layers with
negatively charged interlayer surfaces [9,10]. If
photocatalyst particles were intercalated into the
MMT interlayer space, the separation efficiency of
the photogenerated charge carriers may be largely
improved by the negatively charged interlayer
surface which would facilitate the transferability of
the photo-generated holes by electronic attraction.
This idea conceives an exciting way to improve the
photocatalytic performance by fabricating MMT
based intercalated nanostructure. Previous studies
have proved the feasibility of coupling photocatalysts
with MMT to improve the photocatalytic
performance, such as the TiO2/MMT [11-14],
CdS/MMT [15], and ZnO/MMT [16] systems etc.
However, TiO2 could only be activated under UV
light. CdS and ZnO are instability for a
photocatalytic use because of their photocorrosion
phenomenon. Furthermore, most of the previous
studies on MMT based photocatalyst found the
improved photocatalytic performance were related to
the larger specific surface area and the higher
adsorption capacity of the composites [11-16]. There
is still no investigation on the roles of the interlayer
charges on the electron/hole recombination behavior,
which is one of the fundamental limits in
photocatalytic reactions. In addition, excellent solar
light transmission is also desirable for light
penetration to improve the solar light utilization
efficiency in an intercalated nanostructure. It is
necessary to develop a simple and low-cost method
for preparation of ultrathin MMT.
In this paper, graphene-analogue ultrathin MMT
was prepared by a facile hydration and freezing
process as an excellent base material for fabricating
high-performance photocatalysts. Bi2WO6, which is
inexpensive and robust for solar water oxidation
[17-21] and environmental purification [22-37], was
chosen to composite with MMT to construct an
intercalated nanostructure. Considering that the
photocatalytic activity of Bi2WO6 mainly depends on
the high oxidative ability of the photo-generated
holes, the MMT/Bi2WO6 intercalated nanostructure
was anticipated to exhibit enhanced photocatalytic
oxidation performance.
2 Experimental Section
2.1 Synthesis
Preparation of Na-MMT
Sodium modified MMT (Na-MMT) was prepared by
suspension method which has been widely reported
[38-40]. Typically, MMT powder (2g) was dispersed
in de-ionized water (35 mL) and Na2CO3 (0.1 g) was
added in this suspension subsequently. After
vigorous stirring for 1.5 h at 70 ºC, the obtained
Na-MMT was collected by centrifugation, washed
with de-ionized water, and dried for 12 h at 60 °C.
For comparison, dehydrated Na-MMT was also
prepared by calcination of the Na-MMT for 2h at
450 °C.
Preparation of graphene-analogue ultrathin
Na-MMT
The layered structures of Na-MMT could be
expanded and partly exfoliated into ultrathin few
layers by fully hydrated and freezing. For a typical
synthesis, Na-MMT (1 g) was dispersed in the
de-ionized water (100 mL). After vigorous stirring for
a certain time, the obtained Na-MMT dispersion was
put into a freezer for overnight. Then the freezed
Na-MMT is thawed at room temperature. After
ultrasonic treatment for 1 min, the dispersion was
centrifuged under the speed of 8000 rpm for 1 min,
removing the un-exfoliated block residues from the
dispersions. The obtained ultrathin expanded
Na-MMT was dispersed in water and freeze-dried for
further characterization.
Synthesis of Na-MMT/Bi2WO6 QDs composites
For synthesis of the MMT/Bi2WO6 QDs composites,
sodium oleate (2.2 mmol) and Bi(NO3)3·5H2O (0.4
mmol) were successively added to distilled water (20
mL). An aqueous solution (20 mL) containing
Na2WO4·2H2O (0.2 mmol) and ultrathin Na-MMT
(120 mg) was then injected into the above solution.
After vigorous stirring for 2 h, the mixture was
transferred to a 50 mL Teflon-lined autoclave, sealed
and heated at 160 ºC for 17 h. The system was then
allowed to cool down to room temperature. The
obtained solid products were collected by
centrifugation, washed with n-hexane and absolute
ethanol many times, and then freeze-dried for further
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3 Nano Res.
characterization. For comparison, pure Bi2WO6 QDs
were also prepared according to our previously
reported method [41].
2.2 Characterization
The purity and the crystallinity of the as-prepared
samples were characterized by powder X-ray
diffraction (XRD) on a Japan Rigaku Rotaflex
diffractometer using Cu Kα radiation while the
voltage and electric current were held at 40 kV and
100 mA. The transmission electron microscope (TEM)
analyses were performed by a JEOL JEM- 2100F field
emission electron microscope. X-ray photoelectron
spectroscopy (XPS) were obtained by irradiating
every sample with a 320 μm diameter spot of
monochromated aluminum Kα X- rays at 1486.6 eV
under ultrahigh vacuum conditions (performed on
ESCALAB 250, THERMO SCIENTIFIC Ltd.). UV-vis
diffuse reflectance spectra (DRS) of the samples were
measured using a Hitachi UV-3010PC UV-vis
spectrophotometer. The N2-sorption measurement
was performed on Micromeritics Tristar 3000 at 77 K.
The specific surface area and the pore size
distribution were calculated using the
Brunauer–Emmett–Teller (BET) and
Barrett–Joyner–Halenda (BJH) methods, respectively.
Time-resolved fluorescence spectroscopy (HORIBA
Scientific FluoroCube) was employed to monitor the
fluorescence emission decay of photoexcited carriers.
This instrument adopts a principle of the time
correlated single photon counting (TCSPC) technique.
All of the measurements were conducted at room
temperature in air.
2.3 Photocatalytic Test
Photocatalytic activities of the samples were
evaluated by aqueous ammonia degradation and
water oxidation under simulated solar light
irradiation of a 500 W Xe lamp. For aqueous
ammonia degradation, the reaction cell was put in a
sealed black box of which the top was opened. In
each experiment, 0.05 g of the MMT/Bi2WO6 QDs
(53.6 wt% Bi2WO6) or 0.026g pure Bi2WO6 QDs
photocatalyst was added into 200 mL of NH4Cl
solution (100 mg/L). The pH value of the NH4Cl
solution was adjusted to 10.5 by aqueous NaOH. The
concentration of NH4+/NH3 was estimated using
distillation-neutralization-titration method. For the
O2 production from water oxidation, 0.05 g of the
MMT/Bi2WO6 QDs (53.6 wt% Bi2WO6) or 0.026g pure
Bi2WO6 QDs photocatalyst was dispersed in
de-ionized water (200 mL) in a Pyrex cell with a
quartz window on top. The amount of evolved O2
was determined with on-line gas chromatography
equipped with a thermal conductivity detector (NaX
zeolite column, N2 carrier). Nitrogen was purged
through the cell before reaction to remove oxygen.
3 Results and Discussion
Scheme 1 illustrated the whole synthesis procedure
of the MMT/Bi2WO6 intercalated nanostructure,
including the synthesis process of the ultrathin
Na-MMT base material. First of all, the original
Na-MMT base material was fully hydrated in water.
In this process, the diffusion of water molecules to
interlayer spaces of the MMT widened the layer
spacing. From the XRD pattern in Fig. 1a, the
hydrated Na-MMT has an interlayer spacing of d =
1.2 nm with 2θ = 7.3°. The interlayer spacing is
Scheme 1 The schematic illustration of the whole synthesis procedure for the MMT/Bi2WO6 QDs including the swelling process of
the MMT base material.
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4 Nano Res.
increased about 0.2 nm compared with that of the
dehydrated Na-MMT sample. The layered structure
of hydrated Na-MMT was further expanded to a
partly exfoliated ultrathin nanostructure by a
freezing process. As shown in Fig. 1a, the further
expanded Na-MMT has an increased interlayer
d-spacing of 1.26 nm with 2θ =7.0°. Furthermore, the
(001) peak of the ultrathin Na-MMT becomes
relatively stronger than the other XRD peaks after
freezing, revealing that the freezing process
enhanced the ordering of the crystal structure along c
axis, leading to the formation of high-quality 2D
ultrathin nanostructure. The expanded interlayer
spacing in the swelling process facilitates the
following Bi2WO6 intercalation by hydrothermal
synthesis.
Figure 1 (a) XRD patterns of Na-MMT base material, (b) XRD
patterns of MMT/Bi2WO6 QDs and pure Na-MMT after
hydrothermal treatment.
Fig. 1b shows the XRD patterns of pure Na-MMT
and the MMT-Bi2WO6 nanocomposites after
hydrothermal treatment under the same conditions.
Without adding Bi2WO6 precursor, the original MMT
sample exhibited a clearly (001) diffraction peak at 2θ
= 7.0° with an interlayer d-spacing of 1.26 nm, which
is the same as that of the untreated one. After
integrating with Bi2WO6, the basal spacing of MMT
increased from 1.26 nm to 2.1 nm, demonstrating that
certain amount of Bi2WO6 substances were
intercalated into the interlayer space. Furthermore,
the (001) diffraction peak of MMT became broader in
the composite material, indicating that the layered
structure become disorder and is further exfoliated
by the Bi2WO6 intercalation. Besides the similar
characteristic (001) diffraction peak of MMT, the
nanocomposites also exhibit distinctive diffraction
peaks for orthorhombic Bi2WO6 (JCPDS card 73-2020).
There are no other impurity phases from the XRD
profile. The average grain size of the Bi2WO6 phase is
calculated to be around 3 nm based on the Scherrer
equation.
Figure 2 Low (a, b) and high (c, d) magnified TEM images of
the MMT/Bi2WO6 QDs nanocomposites.
More information on the microstructure of the
composite material was investigated by TEM images.
The low magnified TEM images in Fig. 2a shows a
(a)
(b)
(b) (a)
(c) (d)
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5 Nano Res.
panoramic morphology of the MMT-Bi2WO6
composite material. One can observe that the Bi2WO6
quantum dots (QDs) are uniformly spread over the
MMT base material. These QDs located both on the
surface and in the interlayers, resulting in an obvious
diffraction contrast in TEM image. A closer TEM
image (Fig. 2b) reveals that the MMT base material
has an ultrathin sheet-like structure and some
nanosheets were crumpled. A higher magnified TEM
image (Fig. 2c) clearly shows that both the edge of
the ultrathin MMT base material and the
nanostructure of the Bi2WO6 QDs (marked with
arrows). The sizes of the Bi2WO6 QDs on the surface
were estimated about 2~3 nm. This value is close to
that calculated from the Scherrer formula based on
XRD. Besides, some smaller Bi2WO6 QDs of low
contrast with a size around 1 nm were also observed
in the interlayer regions. Further insight into the
morphology of the composite material is provided
from high magnified TEM image shown in Fig. 2d,
which presents a view of several stacking layers of
MMT nano-sheets, indicating that the MMT base
material kept the basic layered structure after
intercalation. Analyses of the interlayer d-spacing in
Fig. 2d further confirmed the intercalation of the
Bi2WO6 QDs in the interlayer regions. The TEM
image in Fig. 2d demonstrates a nonuniform
interlayer spacing of the MMT base material after
intercalation. The wider interlayer d-spacing was
estimated about 2.1~2.2 nm, which is similar with
that calculated from XRD pattern. The narrower
interlayer d-spacing was estimated to be around 1.16
nm that is smaller than the interlayer d-spacing of the
pure MMT material. This indicates that the
intercalation of Bi2WO6 QDs in the interlayer of MMT
augments the interlayer space and decreases the
neighbor layer space at the same time.
The nitrogen adsorption-desorption isotherms
measured on the MMT and the composite material
were shown in Fig. S1 in the Electronic
Supplementary Material. Both
adsorption–desorption profiles displayed IV-type
isotherm with a H3 type hysteresis loop according to
Brunauer-Deming-Deming-Teller (BDDT)
classification, indicating the presence of mesopores
with a slit-like shape [42]. The amounts of N2
Figure 3 XPS analysis of pure Bi2WO6 QDs and MMT/Bi2WO6
QDs nanocomposites: (a) Bi 4f, (b) W 4f.
absorbed on the composite material were much
larger than that on pure MMT (Fig. S1a).This
indicates that the porous structure was largely
developed by QDs intercalation. The mesoporous
structures of the prepared samples were also
indicated by the pore size distributions which are
shown in Fig. S1b. The pore diameter at the peak of
pure MMT was 1.89 nm, while the MMT/Bi2WO6
composites exhibited multi-peaks that are located at
2.35 nm, 4.36 nm, and 6.07 nm. The increased pore
size in the MMT/Bi2WO6 composite material further
proved the intercalation of QDs in the interlayer of
MMT. The multi-peak pore size distribution was
resulted from the intercalated and partially exfoliated
structures, which had been confirmed from its XRD
pattern and TEM images above. The specific surface
area of the pure MMT sample was calculated to be
(a)
(b)
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6 Nano Res.
47.44 m2g-1 according to the BET method. The total
pore volume was 0.102 cc/g at a relative pressure
(P/P0) of 0.994. Under the same conditions, the BET
surface area and the total pore volume of
MMT/Bi2WO6 QDs were increased to 87.24 m2/g and
0.8635 cc/g, respectively. The greatly increase of BET
surface area and total pore volume also is an
evidence to prove the intercalation of the QDs into
the interlayer of MMT.
To investigate the surface chemical states of the Bi
and W elements in the MMT/Bi2WO6 composite
material, XPS measurements were conducted on it
and the sample of pure Bi2WO6 QDs was also
selected as a contrast. As shown in Fig. 3a, the
binding energies of Bi 4f5/2 and Bi 4f7/2 peaks are at
164.75 and 159.43 eV in pure Bi2WO6 QDs. This
revealed a trivalent oxidation state for bismuth.
However, in the case of MMT/Bi2WO6 QDs, the
binding energies of the Bi 4f5/2 and Bi 4f7/2 peaks are
decreased to 164.53 and 159.23 eV, respectively. This
indicates a change of the chemical state in the
composite material. Besides, the binding energies of
W 4f5/2 and W 4f7/2 in the composite material were
also obvious decreased comparing with that in the
pure Bi2WO6 QDs (Fig. 3b). The lower binding energy
of Bi and W elements in the composite material may
be attributed to its intercalated nanostructure, where
the negative charge of interlayer reduces the
oxidation states of the surface Bi and W elements.
Figure 4 Comparison of UV-vis diffuse reflectance spectra for
the pure Bi2WO6 QDs and MMT/Bi2WO6 QDs nanocomposite.
The optical absorption property of the prepared
MMT/Bi2WO6 QDs composite material was
investigated by UV−vis diffuse reflectance spectrum,
and the result was shown in Fig. 4. The pure Bi2WO6
QDs exhibited an intense UV-vis light adsorption
within a wavelength shorter than 410 nm. Because of
the quantum confinement effect, the absorption edge
of the Bi2WO6 QDs was obviously blue shifted
compared with an ordinary nanoscale Bi2WO6 of
which the absorption edge usually located at 450 nm.
More remarkable, the absorption edge of the
composite material is shifted to shorter wavelengths
compared with the pure QDs sample. The further
blue shift of the absorption edge may be ascribed to
the smaller grain size of the intercalated QDs in the
composite material.
Figure 5 (a) Photocatalytic O2 evolution from pure water under
simulated solar light irradiation over the as-synthesized
MMT/Bi2WO6 QDs and pure Bi2WO6 QDs. (b) Comparison of
the photo-degradation efficiency of aqueous ammonia in the
presence of MMT/Bi2WO6 QDs and pure Bi2WO6 QDs.
(a)
(b)
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7 Nano Res.
Scheme 2 The schematic illustration of charge carrier transfer
process in the MMT/Bi2WO6 composite material.
The photocatalytic activity of the MMT/Bi2WO6
QDs composite material was evaluated by the water
oxidation and ammonia degradation. The O2
evolution from pure water under simulated solar
light irradiation was examined over the as-prepared
samples. As shown in Fig. 5a, the composite material
showed an average O2 evolution rate of 67 μmol/h
within 5 h, which was nearly 1.4-fold higher than
that of pure Bi2WO6 QDs. No O2 was found for pure
MMT under the same conditions. In addition to the
O2 evolution from pure water, the photocatalytic
performance was further investigated by the
degradation of aqueous ammonia, which is a major
nitrogen-containing pollutant in natural environment.
It has been reported NH4+ is prone to reside in the
interlayer space of the MMT based material as an
exchangeable ion [43]. Therefore, photocatalytic
oxidation of aqueous ammonia could better reveal
the influence of interlayer charge on the
photocatalytic performance. An initial concentration
of 100 mg/L NH4+/NH3 was used throughout the
photocatalytic test. As shown in Fig. 5b, the
concentration of NH4+/NH3 decreases from the initial
100 mg/L to an approximately of 30.5 mg/L after
photocatalytic oxidation by MMT/Bi2WO6 QDs for
1.5 h. Nearly 70% of the ammonia was degraded by
this composite material. Under the same conditions,
the degraded ammonia by pure Bi2WO6 QDs was
about only 24%, indicating an obvious deficient
performance compared with the MMT/Bi2WO6 QDs
composite material. Note that comparing with the
Figure 6 Fluorescence emission decay spectra of the
MMT/Bi2WO6 QDs and pure Bi2WO6 QDs sample, excited at
280 nm and monitored at 400 nm.
improved efficiency on water oxidation, the
MMT/Bi2WO6 QDs exhibited better performance on
ammonia oxidation than that of pure Bi2WO6 QDs. To
ensure that the improved photocatalytic oxidation of
aqueous ammonia by the composite material is not
ascribed to the adsorption of MMT or the
volatilization of NH3, a MMT/Bi2WO6 mediated blank
experiment was conducted in dark under the same
conditions. It was found the concentration of
ammonia decreased about only 10 % after 1.5 h. This
suggests that the largely decreased concentration of
NH4+/NH3 in the MMT/Bi2WO6 suspension under
irradiation is mainly resulted from a photocatalytic
process.
The much enhanced photocatalytic performance
was ascribed to the MMT based intercalated
nanostructure which has negatively charged
interlayer surface. It is well known that
photocatalytic activity is closely related to the
behavior of photogenerated charge carriers. The
negatively charged interlayer surface of MMT is
expected to facilitate the separation and transfer of
the photo-generated charge carriers. More
specifically (as shown in Scheme 2), under simulated
solar light irradiation, the
photogenerated holes could migrate to the
surface quickly under the electronic attraction from
the negatively charged interlayer surface. The
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8 Nano Res.
effective hole transfer to the surface further improved
the separation rate of the charge carriers once they
were generated in the Bi2WO6 photocatalyst. For
Bi2WO6 photocatalyst, the photocatalytic
performance mainly depends on the high oxidative
ability of its photogenerated holes. In the composite
material, both the improved charge separation and
the enhanced hole migration increased the number of
the photogenerated holes that take part in
photocatalytic reaction on surface, resulting in the
largely enhanced photocatalytic performance on
solar water oxidation and ammonia degradation. To
prove this viewpoint, we investigated the decay
behavior of the photogenerated charge carriers in the
composite material. Fig. 6 shows the time-resolved
fluorescence emission decay spectra of the
MMT/Bi2WO6 QDs and pure Bi2WO6 QDs samples.
The decay curves of these two samples are obviously
multi-exponential including nonradiative and
radiative decay. It is clear that the decay kinetics of
the MMT/Bi2WO6 QDs is much slower than that of
pure Bi2WO6 QDs. The average fluorescence lifetimes
obtained via multi-exponential decay fitting are 18.8
and 3.06 μs for the MMT/Bi2WO6 QDs and pure
Bi2WO6 QDs, respectively. This proves that the
migration of photogenerated holes to the negatively
charged interlayer surface must exist, which can
retard the recombination of the charge carriers and
increase their lifetime.
4 Conclusions
The results presented in this study highlight the
significance of an intercalated nanostructure on the
electron/hole separation in a semiconductor-based
photo-conversion process. We have demonstrated the
feasibility of fabricating MMT based interacted
nanostructure to improve the photocatalytic activity
of Bi2WO6 QDs. The as-fabricated MMT based
composite material exhibited extremely enhanced
photocatalytic performance both on solar water
oxidation and ammonia degradation. The largely
improved photocatalytic performance was mainly
ascribed to the interlayer charge induced inhibition
of electron-hole recombination. The better
understanding of the charge transfer behavior in the
intercalated nanostructure is of great importance for
developing other similar high-performance
photo-conversion material.
Acknowledgements
This work was financially supported by the National
Basic Research Program of China (Grant Nos.
2010CB933503, 2013CB933203), National Natural
Science Foundation of China (Grant Nos. 51102262,
51272269), and the Science Foundation for Youth
Scholar of State Key Laboratory of High Performance
Ceramics and Superfine Microstructures (Grant Nos.
SKL201204).
Electronic Supplementary Material: Supplementary
material (N2 adsorption-desorption isotherm and the
corresponding pore-size distribution analysis) is
available in the online version of this article at
http://dx.doi.org/10.1007/s12274-***-****-*
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Nano Res.
Electronic Supplementary Material
Bi2WO6 Quantum Dots Intercalated Ultrathin
Montmorillonite Nanostructure and its Enhanced
Photocatalytic Performance
Songmei Sun,1 Wenzhong Wang,1() Dong Jiang,1 Ling Zhang,1 Xiaoman Li,1 Yali Zheng,1 and Qi An2
Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)
Figure S1 (a) Typical N2 adsorption-desorption isotherm of pure MMT and MMT/Bi2WO6 QDs; (b) the corresponding pore-size
distribution.
Address correspondence to [email protected]
(b) (a)