Bi WO Quantum Dots Intercalated Ultrathin Montmorillonite ... · PDF fileBi 2 WO 6 Quantum...

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

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Nano Research

DOI 10.1007/s12274-014-0511-2

TABLE OF CONTENTS (TOC)




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





Received: day month year

Revised: day month year

Accepted: day month year

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© 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

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

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.


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)


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)


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)


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


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





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.

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(b) (a)

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