Controllable electrostatic self-assembly of sub-3 nm graphene quantum dots incorporated into mesoporous Bi2MoO6 frameworks: Efficient physical and chemical simultaneous co-catalysis for photocatalytic oxidation

Article abstract of DOI:10.1039/c6ta02371a

Over the past few years, the direct assembly of co-catalyst/modification materials into mesoporous photocatalysts has been considered a great challenge. Additionally, for photooxidation, the simultaneous achievement of fast charge separation, broad spectrum photocatalytic activity and higher carrier utilization efficiency (generating more active oxidizing groups) is quite necessary but has never been studied. To this end, for the first time, using sub-3 nm GQDs as co-catalyst, we have successfully achieved uniform modification for a mesoporous photocatalyst (mesoporous Bi₂MoO₆) using a novel electrostatic self-assembly method. The sub-3 nm GQDs, which were prepared from graphene nanosheets by a modified chemical oxide method, exhibit many unique physical and chemical properties, such as small size, electronic capture, up-conversion, and in particular, peroxidase-like activity. After the GQDs were modified, the resulting mesoporous hybrid photocatalyst (GQDs-BM) exhibited excellent charge separation efficiency and broad spectrum photocatalytic activity from UV to NIR light. More importantly, we found that a certain amount of H₂O₂ was produced through a photoreduction effect during the photocatalytic process. Unfavorably, for bare Bi₂MoO₆, the continuously-accumulating H₂O₂ could not efficiently convert into ·OH by a one-photoelectron reduction, which results in the indirect waste of photo-excited electrons. However, the chemical co-catalysis of GQDs could make this process (H₂O₂ → ·OH) more quick and efficient and moreover, did not need any additional photoelectrons, which means the effective enhancement of the utilization efficiency of photo-excited electrons (generating more ·OH). Additionally, for the as-prepared GQDs-BM, a sharp increase in photo-degradation activity for different target pollutants, such as BPA, MB, TC, CIP and phenol further confirmed that the simultaneous physical and chemical co-catalysis of GQDs can efficiently enhance the photocatalytic activity of mesoporous Bi₂MoO₆.

Full text of DOI:10.1039/c6ta02371a

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ARTICLE
Controllable electrostatic self-assembly of sub-3-nm graphene
quantum dots incorporated into mesoporous Bi MoO6
2
frameworks: efficient physical and chemical simultaneous co-
catalysis for photocatalytic oxidation
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Yuchen Hao, Xiaoli Dong*, Xiuying Wang, Shangru Zhai, Hongchao Ma and Xiufang Zhang
www.rsc.org/
Over the past few years, the direct assembly of co-catalyst/modification material into mesoporous photocatalysts has
been considered as a great challenge. Additionally, for photooxidation, the simultaneous achievement of fast charge
separation, broad spectrum photocatalytic activity, higher carrier utilization efficiency (generating more active oxidizing
groups) is quite necessary but has never been studied. To this end, we report here, for the first time, using sub-3-nm GQDs
2 6
as co-catalyst, successfully achieved the uniform modification for mesoporous photocatalyst (mesoporous Bi MoO ) by a
novel electrostatic self-assembly method. The sub-3-nm GQDs, which were prepared from graphene nanosheet by a
modified chemical oxide method, exhibit many unique physical and chemical properties, such as small size, electronic
capture, up-conversion, especially peroxidase-like activity. After GQDs modified, the resulting mesoporous hybrid
photocatalyst (GQDs-BM) exhibited excellent charge separation efficiency and broad spectrum photocatalytic activity from
UV to NIR light. More importantly, we found that a certain amount of H
effect in photocatalytic process. Unfavorably, for bare Bi MoO , the continuous-accumulating H
convert into ·OH by one-photoelectron reduction, which means the indirect waste of photo-excited electrons. However,
the chemical co-catalysis of GQDs could make this process (H →·OH) more quickly and efficiently, moreover, did not
2
O
2
was produced through the photoreduction
2
6
2 2
O
could not efficiently
2
O
2
need any additional photoelectron, which means the effectively enhancement of the utilization efficiency of photo-excited
electrons (generating more ·OH). Additionally, for the as-prepared GQDs-BM, the sharp increased photo-degradation
activity for different target pollutants, such as BPA, MB, TC, CIP and phenol further confirmed that the physical and
2 6
chemical simultaneous co-catalysis of GQDs can efficiently enhance the photocatalytic activity of mesoporous Bi MoO .
2
1-23
necessary.
However, difference from other bulk or sheet
1
. Introduction
photocatalyst, mesoporous photocatalyst, which possesses
abundant inner porous structure is very difficult to achieve fully
surface contact with other materials. Specifically, for constructing
nanocomposites or heterostructure, some modification materials,
Semiconductor photocatalysis has attracted worldwide attention
for many years as an effective pathway to solve environment
1
-9
deterioration. Specifically, photooxidation can provide an simple
and low-cost way to achieve the destruction of toxic organic
pollutants. Among various of photocatalysts, bismuth molybdate
2
4
25
26
27-28
such as 2D graphene , Ag
2
S , ZnO , C
3
N
4
, can only contact
with the outside surface of mesoporous materials, but cannot be
completely deposited into the porous channel, and very difficult to
achieve fully dispersion within it. In this article, using mesoporous
(Bi
2
MoO
6
), stable, innoxious and with a smaller band gap (2.5-
2.8eV), has been considered as a promising candidate in organic
1
0-15
2 6
Bi MoO as counterpart, we report a smart way to achieve the
dyes degradation and water splitting.
Specially, mesoporous
uniform modification for mesoporous photocatalyst by utilizing
ultrasmall 0D-sized quantum dots as co-catalyst.
Bi
2 6
MoO , owing to its large surface-to-volume ratios and abundant
porous structure, exhibits excellent optical absorption ability and
16-20
To enhance photocatalytic activity, referring existing research
results, two main strategies can be presented which mainly belong
to physics: (i) enhancing optical absorption; (ii) suppressing charge
surface catalytic activity.
But, to further optimize its
photocatalytic performance, some modification are quite
2
9-32
recombination.
Specifically, although the mesoporous structure
can enhance light absorption ability, but the optical window cannot
School of Light Industry and Chemical Engineering, Dalian Polytechnic University,
#
Electronic Supplementary Information (ESI) available: [details of
any supplementary information available should be included here].
See DOI: 10.1039/x0xx00000x
3
3-34
1 Qinggongyuan, Dalian 116034, P R China E-mail: dongxiaoli65@163.com
be efficiently expanded.
Accordingly, broadening optical
spectrum and making the photocatalyst possesses broad spectrum
photocatalytic activity form UV to Near-Infrared light is quite
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necessary. Besides that, the mesoporous photocatalysts possess OD-sized material as co-catalyst, and facilitate the utilization of
large surface area, which not only can provide more activity sites GQDs as co-catalyst in photocatalytic fields.
but also means more surface trapping states, leading to surface
carrier recombination, so, constructing a nanocomposite and
2
. Experimental
achieving the fast transfer of photo-generated carriers is also very
Synthesis of hierarchical mesoporous Bi
In a typical procedure, 242 mg Bi(NO
Na MoO ·2H O were dissolved in 10 mL of ethylene glycol (EG)
2
MoO
6
hollow (M-BM)
3
5-36
important.
However, to the best of our knowledge,
3
)
3
·5H O and 60 mg
2
simultaneously achieved above two aspects is still possesses
challenges.
2
4
2
respectively under magnetic stirring. After completely dissolved,
the two solutions were mixed together, then the mixture solution
were dropwised into 30 ml isopropanol, follow by stirring for 20
min, and the resulting solution was transferred into a 50 mL Teflon
stainless steel autoclave, and heated to 160 °C for 24 h. After the
autoclave was air-cooled to room temperature, the mixture was
For photocatalytic oxidation, besides light absorption and carrier
separation, the conversion from photo-generated carriers to strong
oxidizing groups is also very important but lack of attention. As is
known, the essence of photooxidation is utilizing strong oxidizing
-
groups (such as ·OH, ·O
2
) and holes to achieve the oxidation of
pollutants (such as dye, BPA, MB), which means if a fixed number of
photo-excited carriers can generate more strong oxidizing groups,
the oxidation ability of photocatalyst will be further enhanced,
centrifugaled, washed with distilled water and ethanol, and dried at
o
8
0 C in air.
3
7-42
leading to excellent photooxidaiton activity.
Difference from
Synthesis of sub-3-nm ox-GQDs
00 mg of graphite oxide (synthesized through chemical
exfoliation of graphite powders using a modified Hummers’
holes, the photo-excited electrons do not possess oxidation ability,
2
+
-
and, as a multi-electron reduction reaction (2H + O
2
+ 2e → H
2 2
O +
-
-
5
1-52
e →·OH + OH ), the conversion from electrons to oxidizing groups is
method
ultrasound (200 W, 40 kHz) for 2.5 h, the mixture was centrifuged at
000 rpm to collect the supernatant liquid and vacuum drying to
) were dispersed in 500 mL of deionized water and
4
3-44
quite complex.
certain amount of H
reduction reactions, but, for bare Bi
constantly accumulate and cannot efficiently convert into
strong oxidizing groups (·OH), which means the indirect waste of
photo-excited electrons (generating H ). It was hypothesized that,
if a novel chemical co-catalysis can achieve the effective and quick
conversion from photogenerated H to ·OH, even do not need
Additionally, in this work, we found that a
can be produced through the photoelectron
MoO , these photogenerated
2 2
O
6
2
6
obtain small size graphene oxide sheets. Then, 30 mL as-prepared
small size graphene oxide sheets solution (0.5mg/mL) was first
mixed with 8 ml concentrated nitric acid and 3 mL sulphuric acid,
after ultrasound for 1h, the mixture was then transferred to a 100
2 2
H O
2
O
2
o
C oil bath and stirred for 24 h. After cooling to room temperature,
2
O
2
the mixture was placed under the mild ultrasonication for 20 min.
additional photo-excited electrons, the photo-excited electrons will
generate more oxidizing groups (·OH), which is beneficial for further
enhancing the photooxidation activity compared with only physical
co-catalysis.
2 3
Then sodium carbonate (Na CO ) was added to the mixture solution
until pH=8. Finally, the mixture were filtered through a 0.22μm
microporous membrane and the final product solution was further
dialyzed in a dialysis bag (retained molecular weight: 1000 Da), and
light yellow solution of ox-GQDs were obtained.
Graphene quantum dots (GQDs), a graphene based material with
size less than 10nm, has attracted increasing attention owing to its
4
5-48
unique physical and chemical properties.
Specifically, in
Fabrication of GQDs-BM nanocomposite
photocatalytic fields, the electrical conductivity of GQDs has been
2 6
The GQDs/M-Bi MoO hybrid (GQDs-BM) was prepared through
4
9
considered as an efficient way to suppress carrier recombination.
a surface charge modification method and a simple hydrothermal
treatment. The M-BM powders were functionalized by APTES.
Briefly, 0.5 g of M-BM powder was dispersed in 250 mL of ethanol
and ultrasound for 30 min. Next, 1.6 mL of APTES was added, and
the mixture solution was treated in refluxing ethanol solution under
But, the systematically study of utilizing the up-conversion property
of GQDs to broaden the optical window of photocatalyst was still
absent. Moreover, GQDs also possess high chemical catalysis
activity (peroxidase-like activity), which can catalyze the
5
0
decomposition of H
2
O
2
, generating ·OH under dark. And this
new direction to enhance the
7
0 °C for 4 h. After that, the powder was collected, washed with
property can provide
a
ethanol and water three times, respectively. After dried at 60 °C in
air, 0.4 g of as-prepared APTES-modified M-BM was added into 32
mL of deionized water and ultrasound for 20 min. 8 mL of the as-
synthesized ox-GQDs solution (1mg/mL) were added to the as-
obtained M-BM suspension and stirred for 2 h, follow by ultrasound
for 30 min. The suspension was then transferred to a Teflon-sealed
photooxidation performance by the chemical co-catalysis which is
proposed for the first time in this article.
2 6
Herein, for the first time, using mesoporous Bi MoO as
counterpart, sub-3-nm GQDs as co-catalyst, we provided a novel
electrostatic self-assembly method to achieve the incorporation of
o
2 6
ultrasmall GQDs into mesoporous Bi MoO frameworks (GQDs-BM).
autoclave and maintained at 180 C for 6 h to achieve GQDs-BM
More importantly, the mechanism of the physical (electronic
capture and up-conversion property) and chemical (peroxidase-like
activity) co-catalysis of GQDs have been systematically investigated
in this paper. Moreover, the photodegradation experiments of
various target pollutants further ensured that the GQDs as co-
catalyst can efficiently enhance the photooxidation activity of
mesoporous bismuth molybdate. We believe, our work can provide
a smart way to modify mesoporous photocatalyst using ultrasmall
hybrid material. Finally, obtained products were collected, washed
with water, and freeze drying.
Characterization and measurements
The TEM analyses were performed by a JEOL JEM-2100F
transmission electron microscope. SEM images were taken using a
field-emission scanning electron microscope (JSM-7800F, JEOL). The
crystallinity and the purity of the as-prepared samples were
2
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characterized by powder X-ray diffraction (XRD) analysis on a in quartz reactor which contained 50 mL pollutants aqueous
Shimadzu XRD-6100 diffractometer at 40 kV and 40 mA with Cu Ka solution. Before irradiation, the mixture was stirred in the dark for
radiation. Data were recorded at a scan rate of 0.07°/s in the 45 min to ensure the absorption-desorption equilibrium between
o
o
2
Theta range 10 -70 . X-ray photoelectron spectroscopy (XPS) the model pollutants and the photocatalyst. In the process of
measurement by a Thermo VG ESCALAB-250 system with Al-Kα and photocatalytic degradation, a certain volume of suspension (about
Mg-Kα source operated at 15 kV. Light absorption property was 3 mL) was sampled every 10 min, then centrifuged immediately to
evaluated by UV-vis diffuse reflectance spectra (UV-vis DRS, CARY remove the particles. The filtrates were analyzed for residual
1
00&300, VARIAN), BaSO
PL spectra of the photocatalysts were analysis using Hitachi F-7000 spectrophotometer and normalized concentration changes (C/C
fluorescence spectrophotometer. The N -sorption measurement RhB, BPA, CIP, TC, MB and phenol were obtained based on the
4
was used as a reflectance standard. The pollutants concentration, the absorbance was measured by UV-vis
0
) of
2
was performed by using Micromeritics Tristar 3000 at 77 K, the maximum absorption at 554 nm, 278 nm, 276 nm, 356 nm, 664 nm
poresizedistribution were estimated using the Barrett-Joyner- and 268 nm, respectively.
Halenda (BJH) methods.
3
. Results and discussion
Electrochemical and photocatalytic test
3
.1 Co-assembly process and morphological studies.
Photoelectrochemical measurements were carried out on a
CHI660E electrochemical workstation (Shanghai Chenhua, China)
using a three-electrode configuration with the as-prepared samples
as working electrodes. The working electrodes were prepared by
dip-coating method, 5 mg of sample was suspended in 0.15 mL of
ultrapure water to produce slurry, which was then dip-coated onto
a glass carbon electrode and dry under an alcohol environment. All
investigated electrodes show a similar thickness. The light source is
a 300 W Xe lamp (PLS-SXE 300, Beijing Perfect Light Co., Ltd.). The
current J-V curves of the working electrodes were obtained by the
linear sweep voltammogram (LSV) with a scan rate of 10 mV/s. The
photocurrent response was obtained by potentiostatic (current vs.
time, I-t) measurements under intermittent illumination at a bias of
In the co-assembly process, to ensure the sub-3-nm GQDs can
uniformly and stably deposited on the outside surface/inner
channels of mesoporous Bi MoO (M-BM), two mainly strategies
were proposed: (i) ultrasound-assisted dipping; (ii) electrostatic self-
assembly. Specifically, the mesoporous Bi MoO (M-BM) was
2
6
2
6
prepared via a solvothermal approach, after being modified with
APTES, the surface of M-BM holds amine functional groups which
were positively charged. Additionally, the ox-GQDs synthesized
from GO nanosheet were negatively charged. Through the
ultrasound-assisted dipping process, the ox-GQDs solution fully
entered into the channels and achieved sufficient contact with M-
BM, afterwards, under the effect of electrostatic attraction, the ox-
GQDs were driven and anchored on the surface of M-BM. And
finally, the ox-GQDs were reduced to the GQDs through
hydrothermal effect, and the GQDs-BM hybrid was successfully
obtained (see Scheme 1). Compared with physical mixture, the
electrostatic self-assembly can make the GQDs contact with M-BM
more fully, and enhance the stability of GQDs-BM hybrid, leading to
more excellent photooxidation performance (Fig. S1). Moreover,
the optimal loading level was investigated by the photodegradation
experiments with different samples as photocatalyst (Fig. S2).
0.7 V vs. SCE. Electrochemical impedance spectroscopy (EIS) was
conducted with an alternating current signal (10mV) in the
5
frequency range of 0.1-10 Hz at open circuit potential (OCP). Mott-
Schottky plots were measured at a frequency of 100 Hz in the dark.
The incident photon to current conversion efficiency (IPCE) was
calculated by the following formula.
1
240 j(λ)
E(λ)λ
IPCE(
λ
) =
×100
Where λ is the wavelength (nm), j is the photocurrent density,
and E is the incident power of the monochromatic light. The E was
measured with a photometer, and the photocurrent density (j) was
determined by measuring current versus time curve at a constant
potential (1.0 VSCE ).
Photocatalytic activity measurements
Scheme 1. The schematic diagram of the co-assembly process.
The photocatalytic performance were evaluated by the
decomposition of RhB, bisphenol A (BPA), ciprofloxacin (CIP),
tetracycline hydrochloride (TC), methylene blue (MB) and phenol
under simulated sunlight irradiation. And the pH value was not
adjusted when the reaction was conducted. The light source for
photocatalytic reaction was a 300 W Xe lamp (PLS-SXE 300, Beijing
Perfect Light Co., Ltd.). The distance between the Pyrex glass and
the light source was about 10 cm. The photocatalytic activity of the
prepared catalysts under simulated sunlight was estimated by
measuring the degradation rate of RhB (10 mg/L), BPA (10 mg/L),
CIP (10 mg/L), TC (10 mg/L), MB (10 mg/L) and phenol (10 mg/L) in
an aqueous solution. Specifically, 0.2 g/L photocatalyst were added
The high-magnfication SEM image of GQDs-BM hybrid material
(
Fig. 1a) can help us to observe subtle structure. It clearly shows
that the hollow sphere was consisted of many nanosheets which
were of several nanometers in thickness. The cross-linking of
nanosheets formed hierarchical flower-like mesoporous spheres
(
the mesoporous structure of GQDs-BM is supported by the
nitrogen sorption isotherms and pore size distributions curves, Fig.
S3). And we can easily obtain that the spheres have hollow
structure and the shell thickness is about 200 nm. Moreover, the
EDX elemental mapping results (Fig. S4) reveal an uniform
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distribution of C, O, Mo and Bi elements throughout the material, domains in the two-dimensional hexagonal lattice structure. The
-
1
suggesting the successful deposition of GQDs on the surface of M- amorphous D band peak appeared at 1370 cm , with an I
D G
ratios
/I
5
6-57
BM. Fig. 1b shows one complete hollow sphere of GQDs-BM hybrid, about 0.97, which is similar with previous work.
it can clearly reveals that many nanosheets reciprocal chiasma form for FT-IR spectrum (Fig. 2c), the absorption bands at 567 cm and
Besides that,
-1
-1
3
D mesoporous hierarchical structure and an obvious contrast 734 cm can be considered as the asymmetric stretching mode and
-1
6
between the dark edge and the bright center confirmed their the bending vibration of the MoO , the absorption band at 842 cm
hollow structure. Fig. 1c and d shows the deposition of GQDs on be identified based on asymmetric and symmetric vibration modes
6
−
mesoporous Bi
only deposited on the external surface of mesoporous Bi
also inner the porous channels, indicative of the formation of GQDs- bare GQDs, the existence of C-O stretching peak at 1120 cm , C-H
2
MoO
6
, many ultrasmall black spots (sub-3-nm) not (Mo-O stretching) of the corner sharing (MoO
6
) octahedron which
2
MoO , but can prove existence of pure Bi MoO . For the GQDs-BM hybrid and
6
2
6
-1
-
1
BM hybrid material (also shown in Fig. S5). The better deposition of bending peak at 1390 cm and a weak absorption of C=O
-1
GQDs on M-BM can be interpreted as the sufficient dipping process vibrational stretch was observed at 1726 cm belong to GQDs,
and the electrostatic interaction effect. As shown by the HETEM which can also ensure the successfully synthesis of GQDs-BM hybrid
image (inset, Fig. 1c), we can easily find two different lattice photocatalyst.
distance of 0.23 nm and 0.31 nm, which can be attributed to the
200) spacing of GQDs and (131) spacing of Bi MoO
Moreover, to further to identify the surface chemical
composition of M-BM and GQDs-BM hybrid photocatalyst, and the
interaction between M-BM and GQDs, the XPS analysis was
performed. Specifically, for high-resolution C1s spectrum (Fig. 2d),
(
2
6
.
2
the peak at 284.2 eV corresponds to the C-C coordination with sp
orbital and the peak at 286.2 eV belongs to the C-O coordination of
GQDs. The weaker peaks around 287.5 eV (C=O) and 290 eV (-
58-60
COOH) belong to the surface functional groups of GQDs.
Furthermore, the high-resolution Bi 4f spectrum (Fig. S7a) exhibits
obvious peaks at 158.0 eV (Bi4f7/2) and 162.9 eV (Bi4f5/2), ascribed
3
+
to the Bi in crystal structure. Additionally, after the deposition of
GQDs, the Bi4f peak exhibits an obviously shift compared to the Bi
4
f peak of bare M-BM, which indicates that the surface chemical
6
1
environment in GQDs-BM hybrid has changed. The Mo3d peaks
are centered at 232.4 eV and 235.5 eV are ascribed Mo3d5/2 and
Mo3d3/2, respectively (Fig. S7b). The high-resolution O1s spectrum
(
Fig. S7c) shows three different peaks at 530 eV, 531.8 eV and 532.8
eV which belong to the lattice oxygen in crystalline Bi MoO , C-O
2
6
and C=O groups from GQDs, respectively. The XPS analysis indicates
the codependent of GQDs and M-BM, which is consistent with the
Raman spectrum and FT-IR analysis.
Fig. 1 (a) SEM image, (b)-(d) TEM images and HRTEM image (inset in Figure (c)) of
prepared GQDs-BM hybrid material.
3.2 Phase and chemical composition
We performed XRD analysis to determine the structural phases of
as-prepared GQDs-BM hybrid and M-BM. For Fig. 2a, the distinct
diffraction peaks of different samples at 28.3°, 32.9°, 47.0°, 56.0°,
o
5
8.5 , and 68.8° corresponding to (131), (002), (062), (133), (262),
2 6
and (400) crystal planes of orthorhombic Bi MoO (JCPDS 84-0787),
respectively. As expected, after the deposition of GQDs, a very
o
weak diffraction peak around 24.7 which belong to GQDs can be
found, indicating that the GQDs-BM hybrid was successfully
5
3-55
synthesized.
Additionally, no other diffraction peaks from
impurities can be found, indicating the as-prepared samples possess
high purity.
The samples were further characterized by Raman spectrum. As
indicated in Fig. 2b, for GQDs-BM and M-BM, the obvious peaks at Fig. 2 (a) XRD patterns (b) Raman spectrum (c) FT-IR spectrum of GQDs-BM, M-
-
1
BM and bare GQDs; (d) high-revolution C1s XPS spectrum of GQDs-BM hybrid.
Note that the Raman spectrum of GQDs-BM and GQDs was obtained by
deducting the background of GQDs owing to its strong photoluminescence effect
2
94, 344, 808 and 844 cm can be considered from pure Bi
2 6
MoO .
But for GQDs-BM and bare GQDs, the G band peak of the GQDs at
-1
1
590 cm are related to the E2g vibrational mode of aromatic during characterization.
4
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3
.3 Investigating the physical co-catalysis of GQDs
efficiently suppress surface charge recombination, which is
beneficially for enhancing the photocatalytic performance of
3.3.1 Suppressing surface recombination
The suppression of surface charge recombination by the mesoporous photocatalyst.
electronic capture effect of GQDs were investigated through
photoluminescence spectrum (PL), electrochemical impedance
spectroscopy (EIS), transient photocurrent (I-T) and IPCE curves.
From PL spectrum (Fig. 3a), the GQDs-BM hybrid and M-BM exhibit
strong emission peak at approximately 472 nm, which is mainly
assigned to the emission of the band-gap transition. By contrast, we
can easily obtain that the weaker peak intensity around 472nm for
6
2-63
GQDs-BM which indicates higher carrier separation efficiency.
Additionally, through EIS spectrum we can profound insights into
6
4-65
the carrier transport in the heterojunction system.
each arc on the EIS Nyquist plot reflects the charge transfer process
occurring at the contact interface between the
The radius of
electrode/electrolyte interface, and the smaller radius represents
the lower charge transfer resistance. So, the GQDs-BM hybrid
exhibits smaller charge transfer resistance compare with M-BM (Fig.
3b). Suggesting that the GQDs-BM hybrid exhibits faster interfacial
charge transfer between the electrode and electrolyte after the
GQDs deposited, which mainly due to the effective separation of
photo-generated electron-hole pairs. As is known, considering the Fig. 3 Photoluminescence (PL) spectra (a), EIS Nyquist plots (b), transient
photocurrents versus time plotted (c), incident photon to current conversion
surface charge recombination, the carrier separation efficiency can
efficiency plotted was measured in 0.5M Na
2
SO
4 2 4
solution (d), 0.5 M Na SO
takes into account efficiencies for two fundamental processes
which can be expressed by Eq. 1.
2 3
solution together with 0.5M Na SO solution as hole-capture (e) of as-prepared
GQDs-BM and M-BM.
η
=
ηsep ×(1−ηrec
)
(1)
3
.3.2 Broadening optical window
The optical absorption of M-BM and GQDs-BM hybrid were
Where η is the total carrier separation efficiency, ηsep is the bulk
carrier separation efficiency (inner the photocatalyst), and ηrec is the
surface charge recombination rate. Considering the deposition of
GQDs cannot change the morphological structure and essential
investigated by DRS spectrum (Fig. 4a). The absorption edge of the
M-BM was about 480nm, indicating that the M-BM can only harvest
6
8
little scope of visible light which agrees with the previous report.
But, after the GQDs deposited, the light absorption ability obviously
enhanced (UV light), which is beneficial for the light harvesting.
However, more importantly, the optical window of photocatalyst
further broadened which mainly owing to the up-conversion
property of GQDs. Specifically, from the DRS spectrums of bare
GQDs (Fig. S9b) and GQDs-BM hybrid, an obvious absorption peak
from 600 nm to 800 nm can be found, which means GQDs-BM
hybrid can achieve the absorption of long-wavelength visible light
and NIR light after GQDs deposited. Furthermore, the up-
conversion property of GQDs can be evidenced by PL emission. As
shown in Fig. 4b, the strong emission peaks of GQDs in the range of
2 6
attribute of matrix material (mesoporous Bi MoO ), the bulk carrier
separation efficiency (ηsep) before and after GQDs deposited nearly
the same. Consequently, the PL and EIS spectrum can indirect
indicate that the GQDs-BM hybrid exhibits less surface
recombination.
For deepen understanding the role of GQDs for promoting
carriers separation, we investigated the surface recombination
66-67
process in M-BM and GQDs-BM photocatalyst.
Transient
photocurrent is a good tool to gain information on surface
recombination process. From Fig. 3c we can clearly obtain that the
photocurrent transient of M-BM is much more conspicuous than
that of GQDs-BM, which means more surface carrier recombination
in M-BM than that in GQDs-BM. Furthermore, by comparing the
incident photon to current conversion efficiency (IPCE) before and
after GQDs loaded, we also obtain that the deposition of GQDs can
effectively suppressing the surface recombination. Specifically, in
Fig. 3d, the IPCE of M-BM is far less than that of GQDs-BM hybrid.
However, an obvious improvement of IPCE performance of M-BM is
observed (Fig. 3e) after decoupling surface recombination by adding
3
50 to 500nm which was motivated with 660 nm to 780 nm light
can be found. It is indicate that the GQDs exhibit excellent up-
conversion property which can achieve the conversion from NIR
light to visible light. Besides that, to investigate the effect of up-
conversion property on the practical photocatalytic performance,
the J-V curves and photodegradation curves under 650nm-cut
simulated sunlight were obtained. The J-V curve of different
samples clearly reveals that the GQDs-BM hybrid exhibits a
significant photocurrent response, but, for bare M-BM, no obvious
photocurrent response can be found (Fig. 4c). This phenomenon
clearly indicates that the GQDs-BM hybrid can utilize long-
wavelength/NIR light to produce active electrons and holes.
Moreover, the broad spectrum photocatalytic activity is also
supported by the obvious photodegradation performance of GQDs-
2 3
the hole scavenger Na SO (in the presence of which surface
3
6
recombination is suppressed due to its rapid and facile oxidation ).
However, the IPCE of GQDs-BM only changed a little, which means
2 3
the GQDs played the same role as Na SO : suppressing surface
recombination. Accordingly, the GQDs as electron traps can
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BM hybrid under 650nm-cut simulated sunlight (Fig. 4d), which can the H
2 2
O detected in the photodegradation process was very little,
decompose 28% of pollutants (BPA) within 140 min. In summary, and did not found obvious accumulation. On the basis of
the GQDs as co-catalyst can absorb long-wavelength/NIR light and experimental, we can suggest that the photogenerated H
convert it to visible light, which can be utilized by mesoporous quickly decomposed by the co-catalysis of GQDs.
2 2
O can be
2 6
Bi MoO to produce photogenerated carriers and achieve the
ꢀ
ꢀ
photodegradation of pollutants.
O + e →·O
(2)
(3)
(4)
(5)
2
2
ꢀ
ꢀ
+
·
O + e + 2H → H O
2
2
2
+
−
O + 2H + 2e → H O
2
2
2
ꢀ
ꢀ
H O + e →·OH + OH
2
2
2 2
Fig. 5 (a) Mott-Schottky plot of as-prepared M-BM, (b) production of H O under
simulated sunlight irradiation (in the presence of BPA as pollutant).
Fig. 4 (a) UV-vis absorption spectra of M-BM and GQDs-BM (red curve) catalysts
and digital photograph of catalyst grains (inset); (b) up-conversion PL spectra of
GQDs under different excitation wavelength; J-V curves (c) and photodegradation
curves (d) of different samples under 650nm-cut simulated sunlight.
In order to further confirmed the catalyst activity of GQDs for
2 2
H O decomposition, the electrochemical behavior of different work
electrodes made by M-BM, GQDs and GQDs-BM were measured
using Linear sweep voltammetry (LSV) in PBS buffer solution
2 2
containing H O (Fig. 6a). We can clearly obtain that the GQDs-BM
and GQDs work electrodes exhibit higher reduction current density,
which means both GQDs-BM hybrid material and GQDs possess
3
.4 Investigating the chemical co-catalysis of GQDs
The specific peroxidase-like activity of GQDs can create possibility
2 2
strong catalytic activity toward H O decomposition. Comparing the
for further enhancing the conversion efficiency from photo-excited
electrons to strong oxidizing groups (·OH), leading to more excellent
photocatalytic oxidation activity of hybrid photocatalyst. From
Mott-Schottky plot (Fig. 5a), we can obtain that the flat band
potentials (Vfb) of M-BM is about 0.01 V vs. NHE. And the
conduction band position of M-BM is estimated to be -0.09 V vs.
NHE because the conduction band edge is generally more negative
lower current density of M-BM, we can also suggest that the
deposition of GQDs can make the hybrid material exhibit catalyst
2 2 2 2
activity for H O . Furthermore, to explore the product of H O
catalytic decomposition, fluorescence experiments were taken to
detect ·OH during the reaction. According to the previous
50
research , Terephthalic acid (TA) was used as a fluorescence probe
for tracking of ·OH because it could capture ·OH and generate 2-
hydroxy terephthalic acid (TAOH), which emitted unique
fluorescence around 435 nm. Fig. 6b shows the change of
6
9
by -0.1 V than Vfb for n-type semiconductor. This values of ECB is
-
similar with the one-electron reduction potential of O
2
(E
o 2 2
(O /·O ) =
-
0.057 V vs NHE) and more negative than two-electron reduction of
7
0,44
2 2
fluorescence intensity in the mixed solution of TA, H O and GQDs,
O
2
(E
o
(O
2
/H
2
O
2
= 0.70 V vs NHE).
Consequently, for bare M-BM
the presence of ·OH can be proved through found a remarkable
fluorescence enhancement at 435 nm relative to control
experiments after 2h reaction (the respectively PL intensity shown
inner the Fig. 6b). The above experiments fully illustrates that the
and GQDs-BM hybrid, in the process of photocatalytic, the photo-
-
generated electrons can generate some ·O
Besides that, some H also can be produced by two main routes
which are one-electron reduction of ·O
2 2
by reducing O (Eq. 2).
2
O
2
-
2
(Eq. 3) and two-electrons
This conclusion can be supported by Fig.
b, which examined the possible formation of by a
When using bare M-BM as
was very obvious, and
with the increases of reaction time, the production of H
increased consecutively. On the other hand, for bare M-BM, the
generated H can transformed into ·OH by trapping one electron
Eq. 5) , but, the continuous accumulation of H indicates that
this process was very inefficient, which means the indirect waste of
photoelectrons (generating H ). However, after GQDs deposited,
7
1-74
presence of ·OH during the catalytic decomposition of H
GQDs or GQDs-BM hybrid under dark.
2 2
O by
2
reduction of O (Eq. 4).
5
2 2
H O
7
5
To deep understand the contribution of peroxidase-like activity
of GQDs and the coordination mechanism between M-BM and
GQDs in the photooxidation process. The J-V curves of GQDs-BM
hybrid and M-BM work electrodes in the ultrapure water without
any other to add were conducted (Fig. 6c). It can clearly reveals that
colorimetric DPD method.
2 2
photocatalyst, the photogeneration of H O
2 2
O
2
O
2
4
3
the GQDs-BM hybrid achieves a higher reduction current density (-
(
2 2
O
2
0
.71 mA/cm -0.8V vs. SCE) which is about 3.2 folds larger than that
2
of M-BM (-0.22 mA/cm -0.8V SCE). The improvement of current
2
O
2
6
| J. Name., 2012, 00, 1-3
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density are more remarkable than that of oxidizing current density do not need photoelectrons, which means the competition
about 2.2 times, Figure 3c), which indicates that part of photo- between O and H for photoelectrons will no longer exist.
reduction reactions in GQDs-BM hybrid did not need Accordingly, more photoelectrons will be used to produce H or
can be further convert into ·OH by
decomposition, we can suggest that there are two the chemical co-catalysis of GQDs.
(
2
2 2
O
2
O
2
-
photoelectrons. Considering the special chemical catalytic activity ·O
of GQDs for H
2 2 2
, and the as-generated H O
2
O
2
types of reduction reactions occur on the surface of the GQDs-BM
hybrid in the photocatalytic process: one is the photoelectrons
participated, the other is the GQDs participated (no light is needed).
To further verify the chemical co-catalysis of GQDs, single factor
experiments were conducted which can efficiently avoid the
interference from surface charge recombination and other factors
such as light absorption performance. As is known, after the GQDs
deposited, the higher light-harvesting performance and expanded
optical window will make the hybrid photocatalyst generate more
photoelectrons, so, to investigate the chemical co-catalysis of GQDs
more accurate, the different photoelectric conversion performance
should be avoided. To this end, firstly, the J-V plots of GQDs-BM
and M-BM in ultrapure water without any other to add under
monochromatic light were obtained (Fig. S10). Unlike above
experiments, we use the relative current density which can
2 4
estimate by dividing the current density by IPCE (in Na SO , no
holes capture; Fig. 3d) to reflect the degree of reduction reactions
on different samples. The Fig. 6d and e shows the relative current
density under 380nm and 420nm monochromatic light,
respectively. It can clearly reveals that the J/IPCE for GQDs-BM is
about 1.4 times (380nm, -0.85 V vs. SCE) and 1.5 times (420 nm, -
0.85V vs. SCE) higher than bare M-BM, which means more
reduction reactions occurred on GQDs-BM surface. Similarly, using
Terephthalic acid (TA) as fluorescence probe, the relative
fluorescence intensity which obtained by dividing the fluorescence
intensity (in ultrapure water contained 0.5M Na
to avoid the interference from the ·OH produced by holes) by the
IPCE (in Na SO and 0.5M Na SO ; Fig. 3e) under monochromatic solution (PH=7.0) containing H O (50 mM) in dark. The working electrodes were
light can be used to detect ·OH. Through the images inner the Fig.
d and e, the ΔPL/IPCE for GQDs-BM is higher than M-BM, which
means the GQDs-BM hybrid exhibits higher efficiency for generating
2 3
SO as hole capture
Fig. 6 (a) Current-potential plots for GQDs, GQDs-BM and M-BM in PBS buffer
2
4
2
3
2
2
prepared by dip-coating, 20 μL GQDs solution (1mg/mL), GQDs-BM and M-BM
solution (5 mg of sample was suspended in 0.15 mL of ultrapure water to
produce slurry) was dip-coated onto glass carbon electrodes and dry under an
alcohol environment, respectively. (b) Fluorescence spectra of the PBS solution
6
·
OH by the chemical co-catalysis of GQDs. Moreover, Fig. 6f shows (pH = 7.4) include only TA; TA and H
2
O
2
; only GQDs; TA and GQDs; TA, GQDs and
H
2
O
2
after 2 h reaction. The concentrations of TA, H and GQDs were 0.5 mM, 1
2 2
O
the PL intensity of GQDs-BM and M-BM at different reaction time in
the whole photocatalytic process, we can obtain that the PL
mM and 100 μg/mL, respectively. Histograms of relative PL intensity (inset)
showed the catalysis effect of GQDs. (c) Current-potential plots for GQDs-BM and
intensity of GQDs-BM is about 1.85 times higher than M-BM, which M-BM in ultrapure water without any other to add under the simulated sunlight.
d), (e) Relative current-potential plots for GQDs-BM and M-BM in ultrapure
water without any other to add under 380nm and 420nm monochromatic light.
Relative PL intensity (inset Fig. (d),(e)) in ultrapure water with 0.5M Na SO
intensity of GQDs-BM hybrid increased consecutively and did not solution as hole-capture and 0.5 mM TA as fluorescence probe under 380nm and
(
is more remarkable than the improvement of IPCE (Fig. 3e, about
.25 times). Additionally, with the increases of reaction time, the PL
1
2
3
4
20nm light after 2 h reaction. (f) PL intensity-time plots for GQDs-BM and M-BM
in ultrapure water with 0.5M Na SO solution as hole-capture and 0.5 mM TA as
fluorescence probe under simulated sunlight irradiation.
reach saturation, which can indicates that the chemical co-catalysis
of GQDs is stable, durable and high-efficiency.
2
3
In summary, after the deposition of GQDs, the photogenerated
2 2
H O can fully and efficiently convert into ·OH, which can avoid the
indirect waste of the photo-excited electrons. Additionally, this
3
3
.5 Photocatalysis mechanism, performance and stability
.5.1 Active species trapping and photocatalysis mechanism
To determine the contribution of each active species in the
chemical co-catalysis process do not need any photoelectrons,
+
which means the photoelectrons reduction reaction from H and O
2
to ·OH only need two photoelectrons (three photoelectrons before
GQDs loaded). Moreover, in the photocatalytic process, the
photocatalytic system, trapping experiments were conducted. In
the trapping experiment, different radical scavengers, such as
ammonium oxalate (AO), p-benzoquinone (BQ) and tert-butanol
reduction of O
reduction of H
2
to H
2
O
2
[E
0
(O
(H
2
/H
2 2
O = 0.70 V vs. NHE)] and
2
O
2
to ·OH [E
0
2
O
2
4
/·OH) = 0.71 V vs. NHE] have
(
TBA) were added into the degradation system, respectively. As
4
similar standard redox potentials. Then O
for photoelectrons in photocatalytic process. But after the
deposition of GQDs, it can quickly reduce H to produce ·OH but
2
would compete with
shown in Fig. S11, when TBA (·OH scavenger) was added, the
photocatalytic oxidation rate was obviously decreased, which is
more remarkable than other conditions. It is validating that the ·OH
2 2
H O
2
O
2
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play the most important role in the photooxidation process. with is almost 2.3 times higher than bare M-BM. Moreover, as co-
+
Additionally, after AO (h scavenger) added, the photo-degradation catalyst, the bare GQDs do not have obvious photocatalytic activity,
+
rate was apparently decreased, which means that the h also play but, the physical and chemical simultaneous co-catalysis of GQDs
important role in the photoreaction process. Moreover, after BQ can effectively enhance the photocatalytic performance of
-
(·O
2
scavenger) was added, the photodegradation rate was mesoporous photocatalyst (M-BM). On the other hand, in order to
decreased but not obvious. By contrast, we can suggest that the generalize the effectiveness of our photocatalyst, we investigate
-
·
OH were not only generated by the oxidation of OH , but also the photocatalytic performance of GQDs-BM photocatalyst for
produced by the reduction of H degradation a diverse range of pollutants, such as MB, TC, CIP and
2
2
O .
Basis on the experimental, the schematic elaboration of phenol under simulated sunlight irradiation which shown in Fig. 7d.
photocatalysis mechanism was shown in Fig. 7a. Firstly, the As results, the degradation of phenol exhibits the best effect,
incorporation of GQDs enhanced the light-harvesting performance demonstrating the unique catalytic behavior under our
range from UV to NIR light, which is beneficial for generating more experimental conditions.
active electrons and holes on the surface of mesoporous Bi
2
MoO
6
.
The reusability and stability of photocatalysts are very important
Secondly, the excellent electronic capture property of GQDs could to the practical application. To evaluate the reusability and stability
perform as electron traps, which is beneficial for suppressing of GQDs-BM hybrid photocatalyst, recycling experiment are
surface carrier recombination. Afterwards, the separated electrons performed for the decomposition of BPA under simulated sunlight
+
-
2 2 2 2 2 2
will react with O and H and generate ·O and H O , the H O will irradiation (Fig. S13a). After five cycles, the photoreactivity of
quickly convert into ·OH by the chemical co-catalysis of GQDs (no GQDs-BM not found significant decrease, indicating this hybrid
+
photoelectrons are needed). On the other hand, the h on the material with high stability. The XRD spectrum of GQDs-BM hybrid
−
+
surface of the M-BM can oxidize the OH into ·OH. At last, the h , material before and after the photocatalytic reactions are shown in
-
2
·O and ·OH can oxidize pollutants and achieve the degradation of Fig. S13b, the crystal structures of the GQDs-BM nanocomposite is
pollutions.
no obvious change. Moreover, the XPS spectrum and SEM images of
GQDs-BM after photooxidation reactions can further ensure the
stability of GQDs-BM hybrid photocatalyst (Fig S14).
4
. Conclusion
In conclusion, we have presented a smart co-assembly strategy
to incorporate ultrasmall (sub-3-nm) graphene quantum dots into
the porous structure of mesoporous photocatalyst (M-BM) by a
novel electrostatic self-assembly process. This strategy (ultrasmall
0
D-sized material as co-catalyst, electrostatic self-assembly method)
is also applicable to other mesoporous photocatalysts, which can
make the modification material fully contact with the mesoporous
photocatalyst and achieve the well-dispersed within the
mesoporous structure. Besides that, the physical and chemical co-
catalysis of GQDs, such as suppressing surface recombination,
enhancing optical absorption ability from UV to NIR light and
improving the utilization efficiency of photo-excited electrons, were
systematically and comprehensively investigated. This co-catalysis
of GQDs is favorable to a broad range of broad-spectrum
photocatalysts, especially some photocatalysts which can only
2 2
generate H O by photoreduction, besides charge separation and
Fig.
7 Schematic representation of the photocatalysis mechanism (a);
optical absorption, the peroxidase-like activity of GQDs can
enhance the utilization efficiency of photo-excited electrons for
photocatalytic degradation of RhB (b) and BPA (c) in the presenc
e of pure M-BM, GQDs-BM and GQDs under visible light irradiation; (d)
photocatalytic decomposition efficiency of various pollutants using GQDs-BM as photooxidation in nature. At last, we believe, this work will provide
photocatalyst (within 30 min).
a new idea for the skilled application of GQDs in the photocatalytic
fields and paves an ingenious way to modify mesoporous
photocatalysts.
3.5.2 Photocatalytic activity and stability
The photooxidation performance before and after GQDs
deposited were firstly evaluated by the degradation of RhB and BPA
in the aqueous solution under the simulated sunlight. As presented
in Fig. 7b, when using bare M-BM as photocatalyst, only 42% of RhB
can be decomposed with 30 min. But, after the deposition of GQDs,
nearly 90% of the RhB can be degraded within 30 min. Similarly,
when using BPA as target pollutant (Fig. 7c), the GQDs-BM
photocatalyst can decomposed 69% of the pollutants within 30 min,
Acknowledgements
This research was supported by the National Natural Science
Foundation of China (Grant no. 21476033) and Cultivation Program
for Excellent Talents of Science and Technology Department of
Liaoning Province (no. 201402610).
8
| J. Name., 2012, 00, 1-3
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3
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