BiVO4/g-C3N4 composite visible-light photocatalyst for effective elimination of aqueous organic pollutants

Article abstract of DOI:10.1016/j.molcata.2016.08.025

BiVO₄/g-C₃N₄ composite photocatalysts were successfully synthesized by sonification and the chemical structures of BiVO₄/g-C₃N₄ heterostructures are characterized systematically. The photoc

Full text of DOI:10.1016/j.molcata.2016.08.025

Accepted Manuscript
Title: BiVO /g-C N composite visible-light photocatalyst for
4
3
4
effective elimination of aqueous organic pollutants
Author: Jie Zhao Jinghui Yan Huaijie Jia Shaowei Zhong
Xinyan Zhang Lei Xu
PII:
S1381-1169(16)30360-0
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.08.025
MOLCAA 10012
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
23-6-2016
22-8-2016
24-8-2016
Please cite this article as: Jie Zhao, Jinghui Yan, Huaijie Jia, Shaowei Zhong, Xinyan
Zhang, Lei Xu, BiVO4/g-C3N4 composite visible-light photocatalyst for effective
elimination of aqueous organic pollutants, Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2016.08.025
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<
AT>BiVO
4
/g-C
3
4
N composite visible-light photocatalyst for effective elimination of
aqueous organic pollutants
<
#
#
AU>Jie Zhao, Jinghui Yan, Huaijie Jia, Shaowei Zhong, Xinyan Zhang*
#Email##zhangxinyan@hotmail.com##/Email##, Lei Xu**
#Email##xul646@163.com##/Email##
<
<
AU>
AFF>School of Chemistry and Environmental Engineering, Changchun University
of Science and Technology, Changchun 130022, PR China
<PA>*Corresponding authors: Tel/ Fax: +86-431-8558-312 <PA>**Corresponding
authors: Tel/ Fax: +86-431-8558-3152.
<
<
<
ABS-Head><ABS-HEAD>Graphical abstract
ABS-P>
ABS-P><xps:span class="xps_Image">fx1</xps:span>
<
ABS-HEAD>Highlights► BiVO
4
/g-C
3
N
4
composite photocatalysts were
/g-C catalyst exhibited
synthesized by sonification method. ► 10 wt.% BiVO
4
3
N
4
excellent degradation efficiency towards RhB or PNP. ► Enhanced activity of BiVO
4
is ascribed to better optical untilization, adsorption ability and lower electron-hole

pairs recombination rate. ► OH radical is also active species in BiVO
4
/g-C
showing great potential for aqueous
organic contaminants elimination in future applications.
3
N -based
4
visible-light system. ► 10 wt.% BiVO
4
/g-C
3
N
4
<
ABS-HEAD>Abstract
<
ABS-P>BiVO /g-C composite photocatalysts were successfully synthesized by
4
3
N
4
sonification and the chemical structures of BiVO
characterized systematically. The photocatalytic activity of BiVO
rhodamine B (RhB) and p-nitrophenol (PNP) degradation is evaluated under visible
light irradiation. It has been found that the novel BiVO /g-C composites exhibit
excellent catalytic activities towards RhB and PNP degradation, and these BiVO /g-
composites are much more active compared with that of pure g-C . Activity
enhancement after BiVO introducing is mainly attributed to the better optical
4
/g-C
3
N
4
heterostructures are
4
/g-C for
3
N
4
4
3
N
4
4
C
3
N
4
3
N
4
4
1

absorption ability over the well contacted structure in heterojunctions, good
adsorption ability of organic contaminants and lower recombination rate of electron-

hole pairs. In addition, in BiVO
active in pure g-C
degradation, which maybe another evidence for the enhanced photocatalytic activity
by BiVO introduction. Furthermore, the most active 10 wt.% BiVO /g-C
4
/g-C
3
N
4
-based visible-light system OH radical, not
3
4
N system, is also found to be active species for pollutants
4
4
3
N
4
photocatalyst exhibits enough catalytic stability in recycle test, showing great
potential as cost-effective heterogeneous visible-light photocatalyst for aqueous
organic contaminants elimination in future applications.
<
KWD>Keywords: G-C
3
N
4
; Photocatalysis; Heterostructure; BiVO ; Sonification
4
<
H1>1. Introduction
In recent times, the increasing environmental issues have become great threaten to the
modern society. Photocatalytic technology based on semiconductor photocalysis has
been regarded as a promising alternative for environmental remediation, since it has
the property of energy saving, avoidance of re-contamination and excellent
photocatalytic property for organic pollutants degradation [1]. Of all photocatalysis
reported, TiO
durability and good photocatalytic performance. Nevertheless, the wide band gap of
TiO makes it only responsive to ultraviolet light, occupying below 4% of the solar
spectrum. This drawback of TiO greatly restricts its practical applications in
2
has probably attracted most attention because of its no toxicity, high
2
2
environmental and energy area [2]. Consequently, lots of research have been carried
2

out to improve the solar light utilization level of photocatalysts, for instance to
develop materials with visible light sensitivity and thus good photocatalysts activities
for pollutants degradation under visible light irradiation [3]. In this aspect, g-C
3
N
4
(
graphite-like carbon nitride) emerges and has drawn increasing concern because it
has high visible light absorption ability, good stability, and excellent visible-light
photocatalytic activity on organic dyes removal [3-5]. Furthermore, the chemical
component of g-C
easy to implement [5]. All these features facilities its practical applications in organic
pollutants elimination. However, g-C still suffers from high recombination rate of
3
N
4
is inexpensive carbon and nitrogen and its preparation route is
3
N
4
photo-generated electron-hole pairs, which is an inherent and unavoidable defect of
semiconductor based photocatalysis and this drawback greatly restricted the
improvement in photocatalytic performance of g-C
3
4
N [6]. Consequently, great efforts
have been made to enhance its charge separation efficiency and hence to increase its
visible light photocatalytic activity. Several methods have been developed, including
preparation of g-C
3
4
N with different morphology [7], doption of impurities
components [8-9] and formation of composite structures [10-12]. Compared with
other methods, forming composite heterojunction could utilize the structure merits of
g-C
3
4
N , i.e. large specific surface area and open texture with two-dimensional planar
conjugation, and being a good supporter for other composite, thus efficient in
improving visible light response. Several kinds of semiconductors have been reported
to add to g-C
3
N
4
and form a heterojunction system, such as ZnWO
4
, Bi
2
WO and
6
CdS, etc [10-12]. It is reported that there is synergistic effects between the g-C
3
3
4
N and

other materials with suitable band position and the recombination rate of
photogenerated electron-hole pairs could be effectively suppressed in the composite
heterojunction [10-12].
On the other hand, recently, monoclinic BiVO
stable semiconductor, has attracted increasing interests for its excellent photocatalytic
performance for O evolution under visible light irradiation [13]. BiVO is found
4
, as a novel nontoxic and chemically
2
4
highly response to visible light because of its narrow band-gap of 2.42 eV[14], and
could utilize visible light directly without modification. All these properties suggest
that BiVO would be a probable material with great promise for fabricating composite
4
with other materials [4], although the photocatalytic efficiency of pure BiVO
4
in
organic pollutants degradation is usually not satisfied enough due to its poor
adsorption ability. Meanwhile, the energy levels of g-C
matched overlapping band-structures (both of the valence and construction band
levels of BiVO is lower than that of g-C ) which make it highly possible to form
BiVO doped g-C heterojunction with excellent transfer and separation ability of
electron-hole pairs in the heterojunction structure[15]. Furthermore, g-C has two-
dimensional open structure and very large specific surface area, which would provide
the BiVO anchored g-C heterojunction with great dispersity and excellent
adsorption ability, thus improved photocatalytic performance [4]. Indeed, most
recently, a C -BiVO heterojunction have been successfully prepared by mixing
and heating method of C and BiVO or hydrothermal route [16-17]. Nevertheless,
3
N
4
and BiVO
4
are well-
4
3
N
4
4
3
N
4
3
N
4
4
3
N
4
3
N
4
4
3
N
4
4
the chemical status of contact interface of the obtained heterojunction is greatly relied
4

on the heating process involved, thus influence their photocatalytic performance.
Motivated by this, we further research the preparation of BiVO loaded C
4
3
N
4
heterojunction by sonification method, which is superior in simple operation, green
chemical process and especially good dispersion of component[18]. Furthermore, till
now, there is not report on ultrasound assisted preparation of C
heterojunction yet as far as we know. The current study indicates that the novel C
BiVO composite photocatalyst could be successfully prepared by facile sonification
3
N
4
-BiVO
4
3
4
N -
4
method and exhibited excellent photocatalytic performance under visible light
irradiation, showing great potential as cost-effective heterogeneous visible-light
photocatalyst for aqueous organic contaminants elimination in future applications.
Furthermore, the possible photocatalytic reaction mechanism on the C -BiVO
3
N
4
4
heterojunction surface has been proposed based on systematical characterization
results..
<
<
H1>2. Experimental
H2>2.1. Photocatalysts preparations
The pure g-C
3
4
N was prepared according the literature reported following the typical
procedure [5]: urea was heated to 250 and kept for 1 h, to 350 and kept for 2 h and
finally to 550 and kept for 2 h in a semi-closed alumina crucible with a cover. The
product was thoroughly washed and dried at 80 overnight.
Pure Monoclinic BiVO
1.21275 g) was added into 5 mL HNO
which was named solution A. NH VO
4
was prepared by hydrothermal method: Bi(NO
3
)
3
·5H
2
O
(
3
(4 mol/L) and vigorously stirred for 30 min,
4
3
(0.2925 g) was put into concentrated 2 mol/L
5

NaOH(5 mL) solution(named as solution B) and stirred for 30 min. Subsequently,
Solution B was added into solution A and stirred for another 1 h. After adjusting the
acidity to pH 6, the mixture was transferred into a Teflon-lined stainless steel
autoclave and heated to 150 and kept for 12 h, and then cooled down naturally. The
yellow solid was thoroughly washed and dried at 80 overnight.
The BiVO
4
/g-C
3
N
4
composite with BiVO
4
loading varied from 10-30% was prepared
and g-C were
by the typical procedure as follows: certain amounts of BiVO
4
3
N
4
mixed in appropriate amount of distilled water, which was stirred for 10 min. Then
the mixture was subjected to ultrasound irradiation for 1 h at 30±0 using a water and
ice mixture. The composites were dried thoroughly in 80 overnight. Finally, the
composites were collected and named as x wt.% BiVO
the BiVO loading.
H2>2.2. Material Characterization
The X-ray diffraction patterns were measured on a Rigaku powder diffractometer
4
/ g-C
3
4
N , where x represents
4
<
(
(
D/MAX-RB, Cu K, =1.5418 Å). FTIR spectra were obtained on the spectrometer
SHIMADZU FTIR-8400S). X-ray photoelectron spectroscopies were measured on a
VG-ADES 400 IR spectrometer. The scanning electron microscope (SEM) images
were determined by a JSM-6480 SEM spectrometer. The transmission electron
microscope (TEM) images were determined by a TEM -2100F spectrometer. Nitrogen
adsorption/desorption graphs were measured by a Micromeritics ASAP 2020M
instrument. UV–Vis/DRS were carried out by the spectrometer (Cary 500 UV-Vis-
NIR). Photoluminescence spectra were determined by a Varian Cary Eclipse
6

spectrometer. The samples were excited at a wavelength of 365 nm to measure the
emission spectrum. Photocurrent measurements were measured on a CHI 630E
electrochemical station.
<
H2>2.3. Photocatalytic measurements
Photocatalytic degradation of RhB and PNP were conducted in a quartz photoreactor.
The light source was provided by a PLS-SXE300 Xe lamp, and the light intensity was
−
2
set at 150 mW cm . Before Xe lamp irradiation, a suspension was obtained by
−1
mixing the solid catalyst (150 mg) and aqueous RhB solution (20 mg L , 100 mL) or
−1
PNP solution (10 mg L , 100 mL). The suspension was ultrasonicated for 10 mins
and then stirred in the dark for 120 mins to achieve good dispersion and adsorption-
desorption equilibrium between RhB molecules or PNP molecules and the catalyst.
The concentrations of RhB or PNP in the reaction systems were measured using a
Cary 500 UV-Vis-NIR spectrophotometer at  = 553 nm and  = 317 nm,
respectively.
<
H1>3. Results and Discussion
<
H2>3.1 Characterization of the materials
As presented in Fig. 1, X-ray diffraction (XRD) spectra of pure g-C
materials showed the typical diffraction peaks ascribed to graphitic stacking structure
of g-C and amonoclinic scheelite structure (PDF#14-0688) of BiVO , indicating
the target materials are successfully synthesized [19]. While for the series of
BiVO /g-C photocatalysts, both their characteristic structure patterns presented,
which proved that BiVO
3
N
4
and BiVO
4
3
N
4
4
4
3
N
4
4
has been successfully introduced into g-C
3
4
N with their
7

structure well preserved, respectively. Meanwhile, with the increasing of BiVO
loading from 10%-30%, the diffraction peaks of g-C weakened, which may be due
to the covering by the stronger diffraction patterns of BiVO at higher loadings.
Similar results have been reported by other research, that no typical crystalline peaks
4
3
N
4
4
of g-C
3
N
4
could be observed when the content of BiVO
4
on g-C
3
N
4
is above 20%
2
[20]. The surface areas of the BiVO
4
loaded g-C
3
N
4
were semblable (24–32 m /g) and
2
similar with that of the pure g-C
specific surface areas of pure g-C
The FT-IR spectra of BiVO
3
N
4
(42 m /g), suggesting the structure and large
3
N
4
were well sustained after BiVO
materials are shown in Fig. 2. For pure g-
, sharp peak at 810 cm was found revealing the existence of heptazine units in
4
deposition.
4
doped g-C
3
N
4
–
1
C
3
N
4
1
the prepared g-C
3
N . The bands located from 1640 to 1208 cm and the broad
4
1
absorption peaks located from 3300 to 3000 cm are ascribed to the typical stretching
modes of CN heterocycles and the stretching vibration of N-H bonds, respectively
[
21-23]. All these adsorption is the typical spectra for g-C
literatures [21-23]. On the other hand, the pure BiVO only showed one broad peak at
FT-IR spectra, corresponding
to the vibration of Bi-V bonds [24]. While for the series of BiVO /g-C
photocatalysts, all characteristic absorption bands of pure BiVO and g-C
stronger) appeared; with the increasing BiVO loading, the characteristic absorption
bands of pure BiVO increased to some extent. This IR results combined with the
XRD results, indicate that, after the adopting of BiVO on g-C by sonification the
3
N , in consistent with other
4
4
−1
7
50cm , which is also in line with the reported BiVO
4
4
3
N
4
4
3
N
4
(
4
4
4
3
N
4
structural integrity of g-C
3
N
4
and BiVO
4
remains intact in the heterostructures.
8

The microstructures of asprepared BiVO
SEM images (Fig. 3). SEM image of pure g-C
with a smooth surface (Fig. 3(a)). Obvious stacked pore structure was found between
the platelet. Also, the TEM image (Fig. 3(b)) has shown that g-C is almost
transparent, indicating its few-layer structure. On the other hand, the pure BiVO
4
deposited g-C
3
N
4
are revealed by TEM and
3
N
4
displays a platelet-like morphology
3
N
4
4
,
which was well crystallized in the autoclave, was a polyhedral irregular particles with
size of about hundred of nm to 1-2 μm as shown in Fig. 3(c) and (d). After
introducing of BiVO
nanoparticles of BiVO
confirmed the XRD finding that BiVO
4
, the sheet structure of g-C
abundantly scattered (Fig. 3(e) and (f)). This result further
have been successfully deposited on g-C
3
N keeps well with irregular
4
4
4
3
N
4
and formed compressed heterojunctions composites instead of a simple physical
mixture of each other. The well touched structure of the composites with an intimate
interface will be benefit for the efficient charge transmission between BiVO
in visible light excitation, and thus improved their photocatalytic activity.
The XPS spectra of BiVO doped g-C composites are presented in Fig. 4. The
result indicates that main surface elements are C1s, N1s for pure g-C , and Bi4f,
V2p, and O1s for pure BiVO , respectively. After BiVO introduced to g-C , 10
wt.% BiVO /g-C exhibited all of the above elements but V2p. Basically, the C1s
binding (Fig. 4B) of pure g-C and 10 wt.% BiVO /g-C is quite similar.
However, N1s spectra (Fig. 4C) of g-C and 10 wt.% BiVO /g-C presents
obvious difference. The first and the second contribution of N1s binding energy for
4
and g-
C
3
N
4
4
3
N
4
3
N
4
4
4
3
N
4
4
3
N
4
3
N
4
4
3
N
4
3
N
4
4
3
N
4
pure g-C
3
N
4
located at 398.1 eV and 399.1 eV, corresponding to sp2-hybridized
9

nitrogen (C=N–C) and tertiary nitride (N-C
BiVO /g-C composite, the N-C binding energy shifts to higher level (about 1.2
eV). This may be caused by the interaction with BiVO inducing C1s and N1s orbitals
of g-C to the inner shift [20]. Furthermore, Bi 4f spectra over g-C and 10 wt.%
BiVO /g-C (Fig. 4D) clearly presented the Bi 4f binding energies of pure BiVO
greatly shift to lower binding energy (about 0.6 eV) after BiVO deposition on g-
. These XPS analysis confirmed the above XRD and TEM results and clearly
proved that compressed heterojunctions composites have been formed between g-
and BiVO instead of a simple physical mixture.
Fig. 5 illustrates the UV-Vis spectra of the BiVO -g-C
absorption edge of g-C is about 460 nm, and the bandgap is estimated to be 2.7
eV, in accordance with the literature results [26]. Pure BiVO have much stronger
absorption bands in visible-light and UV regions with steeper shape compared with g-
, indicating a direct energy band gap [27]. As estimated, the absorption edge of
pure BiVO is around 525 nm, and its band gap is calculated to be 2.36 eV, also in
accordance with the literature [28]. Meanwhile, the series of BiVO -doped g-C
shows strong absorption in visible-light and UV regions, with the adsorption range
between absorption edge of pure BiVO (525 nm) and g-C 4 (460 nm). All of the
band gaps of BiVO -doped g-C were slightly less than that of pure g-C (2.7
eV); the band gaps of the 10 wt.%, 20 wt.%, and 30 wt.% BiVO /g-C are
estimated to be 2.67, 2.63, and 2.60 eV, respectively. These results indicate that after
introducing of BiVO , the absorption edges of BiVO /g-C composites are shifted
3
) group of g-C
3
N
4
[25]. As for 10 wt.%
4
3
N
4
3
4
3
N
4
3
N
4
4
3
N
4
4
,
4
C
3
N
4
C
3
N
4
4
4
3
N composites. As shown, the
4
3
N
4
4
C
3
N
4
4
4
3
4
N all
4
3
N
4
3
N
4
3
N
4
4
3
N
4
4
4
3
N
4
10

to longer wavelength i.e. extend towards visible light range, which implies better
optical absorption ability.
3
.2Photocatalytic performance
The photocatalytic activities of BiVO
degradation are presented in Fig.6. As shown, all the samples reached the adsorption-
desorption equilibrium for RhB molecules at about 30 min and the 10 wt.% BiVO /g-
sample has shown the highest adsorption rate of RhB (ca. 20% ), almost twice
of that of other photocatalysts (all at about 10%). After light on, pure BiVO showed
negligible low activity for RhB degradation, and pure g-C showed moderate high
activity with RhB 50% degraded within 35 min. However, the BiVO deposited g-
, with 10 wt.% BiVO /g-
and 10 wt.% BiVO /g-C almost equally
could
materials, and 10 wt.% BiVO /g-
exhibit the super photocatalytic activity with RhB 50% degraded within 10 min
and complete degradation within 40 min. To further illustrate performance of the trace
amount of BiVO on the photocatalytic activity improvement of g-C materials and
found out the optimized doping level of BiVO4, the photocatalytic activity of g-C
samples with lower BiVO doping level than 10%, such as 5% and 7% are evaluated.
As shown in Fig. S1, 10 wt.% BiVO /g-C still exhibited the highest activity among
the three catalysts, followed by 7 wt.% BiVO /g-C and 5 wt.% BiVO /g-C
respectively. This result indicates that 10% BiVO4 is the optimized doping on the
4
/g-C
3
4
N materials for RhB and PNP
4
C
3
N
4
4
3
N
4
4
C
3
3
N
N
4
4
materials all exhibited much higher activity than C
3
N
4
4
C
highest and 20 wt.% BiVO /g-C
4
3
N
4
4
3
N
4
followed. This result indicate that the introducing of small amount of BiVO
greatly increased the catalytic activities of C
4
3
N
4
4
C
3
N
4
4
3
N
4
3
N
4
4
4
3
N
4
4
3
N
4
4
3
4
N ,
11

photocatalytic activity improvement of g-C
3
4
N materials under the experiment
condition, and it has showed high photocatalytic activity and great potential for
further application, hence the followed discussion mainly concentrated on 10 wt.%
BiVO
As shown in Fig. 6d, the characteristic adsorption of RhB (at 513 nm) over 10 wt.%
BiVO /g-C under visible-light irradiation decreased constantly with extended
4
/g-C
3
N
4
material.
4
3
N
4
irradiation time, with the RhB adsorption hardly observed after 40 min visible-light
irradiation. Furthermore, this result is further confirmed by the RhB degradation rate
calculated (Fig. 6c). The degradation rate of the catalysts followed the order: 10 wt.%
-
1
-1
BiVO
BiVO
4
4
/g-C
3
3
N
N
4
4
(0.09307 min ) > 20 wt.% BiVO
4
/g-C
3
N
4
(0.07172 min ) > 30 wt.%
-
1
-1
-1
/g-C
(0.0658 min ) > g-C
/g-C has about 4.8 times of degradation rate of pure g-C
proved the acitivity enhancement effect of BiVO introducing.
For better evaluation of the visible-light catalytic property of these composites,
Fig.6B showed photocatalytic performance of BiVO /g-C materials for the
degradation of PNP, which is UV light insensitive. It is turned out the PNP
degradation performance over these BiVO deposited g-C catalysts is very similar
with that of RhB degradation process. As shown in Fig. 6(b), with the introducing of
BiVO , the photocatalysts exhibited much higher PNP conversion compared with
pure g-C . Also especially, 10 wt.% BiVO /g-C exhibited the highest
photocatalytic activity, with PNP completely degraded within 120 min. Combined
these degradation results of BiVO and PNP, it could be concluded that the
3
N
4
(0.02463 min ) >BiVO
4
(0.0425 min ). The
1
0 wt.% BiVO
4
3
N
4
3
4
N clearly
4
4
3
N
4
4
3
N
4
4
3
N
4
4
3
N
4
4
12

introducing of BiVO
and 10 wt.% BiVO /g-C
photocatalytic activity for the organic pollutant degradation.
The high photocatalytic activity of 10 wt.% BiVO /g-C for the organic pollutant
4
have greatly improved the catalytic activity of C
3
4
N materials
4
3
N
4
photocatalyst exhibited the extraordinarily high
4
3
N
4
degradation motivated us to test its durability, which is also a crucial factor for the
practical applications. Recycling reactions profiles (Fig. 7) showed that RhB can be
completely degraded within 40 min in each cycle and no obvious activities decrease
in catalytic activity was observed in the recycling reactions. Extraordinarily, the high
photocatalytic activity and good stability of 10 wt.% BiVO
degradation were comparable to or even superior to the most applied TiO
photocatalysts [29-30]. As shown in the Fig. S2, under the same experiment
conditions, the activity of 10 wt.% BiVO /g-C is much higher than pure TiO
photocatalysts. Thus, 10 wt.% BiVO /g-C seems great promising as a cost-
effective photocatalyst for organic pollutant degradation in future application.
H2>3.3 Role of BiVO in photocatalytic activity enhancement on g-C
The above characteristic results including XRD, FTIR and TEM/SEM clearly
confirmed that BiVO have been successfully deposited on the surface of g-C
formed well touched heterojunctions composites. The intimate interface between
BiVO and g-C in the composites greatly benefits for their UV-Vis absorption. As
shown in UV-VIS DRS, the absorption edges of BiVO /g-C heterojunctions are
4
/g-C
3
N
4
for RhB
2
based
4
3
N
4
2
4
3
N
4
<
4
3
N
4
4
3
4
N and
4
3
N
4
4
3
N
4
prolonged towards visible light range, usually representative of better optical
absorption ability [31]. The RhB decomposition results confirmed that the introducing
13

of BiVO
4
have greatly improved the catalytic activity of C
3
4
N catalysis. Thus, better
optical absorption ability due to the well contacted structure in heterojunctions
composites may play the first and most important role in the improvement of
photocatalytic activity of g-C
3
N
4
. Furthermore, the heterojunctions composites,
2
especially 10 wt.% BiVO /g-C
4
3
N
4
has much larger BET-surface area (32.0 m /g) than
2
that of pure BiVO
greatly increased its adsorption of organic contaminants and thus improve its
photocatalytic activity. However, the BET-surface area could not be the only factor
deciding the adsorption performance of BiVO /g-C materials. As shown in Fig.6a,
before light irradiation, the 10 wt.% BiVO /g-C sample, although has lower BET
area than pure g-C , already has shown much higher RhB adsorption rate (almost
twice) compared with that of pure g-C and their RhB adsorption results all in
4
(1.9 m /g), indicating that the deposition of BiVO
4
on g-C
3
N
4
4
3
N
4
4
3
N
4
3
N
4
3
N
4
consistent with their final photocatalytic performance under visible-light irradiation.
Thus, we could deduce that there must be additional special adsorption effect over 10
wt.% BiVO
g-C , BiVO
contaminants, which would be another reason contributed for its high activity.
Finally, the photoluminescence (PL) spectra of BiVO deposited g-C is taken, as it
4
/g-C
3
N
4
besides simple physical adsorption. Hence, compared with pure
3
N
4
4
deposited g-C
3
N
4
exhibited better adsorption ability of organic
4
3
N
4
is demonstrated that the separation efficiency of electron-hole pairs of the
photocatalysts, revealed by fluorescence intensity in the PL spectra, is usually a very
important factor influencing their activities [32]. Higher photocatalytic activity was
usually associated with the photocatalysis exhibited lower fluorescence intensity,
14

which is representative of less recombination rate of photo-induced electron-hole
pairs [20]. As shown in Fig. 8, PL emission intensities of the 10 wt.% BiVO /g-C
is significantly lower than that of pure g-C , suggesting the charge carriers
recombination process of g-C can be suppressed in the presence of BiVO , and this
4
3
N
4
3
N
4
3
N
4
4
may be another important factor for the improved photocatalytic activity after BiVO
4
introducing. This deduction is also directly proved by photoelectrochemistry test,
which could intuitionally display the complicated processes of generation, separation
and migration of electrons and holes pairs [20]. During photoelectrochemistry test, the
one with prompt photocurrent response corresponded to fast generation, separation
and transportation of the photogenerated electrons in the light system [20]. As shown
in Fig.9, all tested materials (as working electrodes) present sharp increased
photocurrent responses immediately after light irradiation is on, and the photocurrents
intensity are kept well during three cycles of light on-off intermittent irradiation. The
1
0 wt.% BiVO
indicating the lowest electron-hole recombination rate. This result combined the
photoluminescence spectra of BiVO deposited g-C (Fig. 8) clearly demonstrated
that low electron-hole recombination rate plays an important role in the photoactivity
enhancement after BiVO introducing.
H2>3.4 Possible photocatalytic mechanism of BiVO
4
/g-C
3
N
4
/ITO electrodes present the highest photocurrent intensity,
4
3
N
4
4
<
4
/g-C
3
N
4
The high photocatalytic activity and stability of 10wt.% BiVO
4
/g-C
3
N
4
for RhB
degradation motivated us to investigate its photocatalytic mechanism in depth for
15

better application and understanding its activity enhancement further. Accordingly,
free radical and hole captive test were carried out to identified the active species in the
photocatalytic process of RhB degradation over pure g-C
. As demonstrated by previous literatures [33,34], 1,4-benzoquinone (BQ),
ethylene diamine tetraacetic acid (EDTA) and isopropyl alcohol (IPA) are used as
3
N
4
and 10wt.% BiVO /g-
4
C
3
N
4
O , hVB and OH scavengers, respectively.
+

2


For the process over pure g-C
3
N
4
(Fig.10(a)), the presence of BQ ( O scavenger) and
2
+
EDTA (hVB scavenger) have great inhibition effect on the RhB degradation rate,

while the presence of IPA ( OH scavenger) has no obvious change for RhB


+
degradation. These results indicate that O
2
and hVB play a major and minor role,
+
respectively, in the photocatalytic RhB oxidization over g-C
3
N
4
, while hVB hardly
contributes. On the other hand, these scavengers displayed quite different influence on
RhB degradation over the 10 wt.% BiVO
4
/g-C
3
N
4
. As shown in Fig.10(b), besides
photocatalytic process, which also
/g-C
Based on the above analysis, a possible photocatalytic mechanism of the BiVO
composites is proposed (Fig. 11). Both the redox potential of the valence band
VB = 2.36 eV) and the conduction band (ECB = 0 eV) of BiVO are more positive
than those of g-C (EVB = 1.4 eV and ECB = -1.3 eV). The pure g-C and BiVO
can both be excited and produce photo-excited electrons and holes under visible light
irradiation. Since the CB potential of g-C is lower than that of BiVO , the
electrons in the CB of the g-C would transfer to the CB of BiVO . Similarly, the
O

and hVB+, OH is also active for BiVO
/g-C
N
2
4
3
4
corroborate the enhanced photocatalytic activity of BiVO
4
3
4
N .
4
/g-
C
3
N
4
(
E
4
3
N
4
3
N
4
4
3
N
4
4
3
N
4
4
16

VB potential of BiVO
would migrate to the VB of g-C
between BiVO and g-C could effectively restrain the recombination of
4
is higher than that of g-C
3
N
4
, the holes in the VB of BiVO
4
3
N
4
. As a consequence, the matching band potentials
4
3
N
4
photogenerated electron–hole and improve the separation efficiency, which greatly
promoted the photocatalytic degradation of RhB.
The electrons in the process of transferring from the CB of g-C
3
4
N to the CB of

, which together with the holes contributed on RhB
BiVO
4
could reduce O
2
to O
2

degradation. Meanwhile, in the presence of water OH radical is generated as the
following possible reactions, and also plays an important role in RhB degradation:
+


h
VB +OH → OH (1)
e+O
→ O

; O

+H →HO
+
; 2HO 
→H
; H
+O 
→ OH+OH +O (2)


2
2
2
2
2
2
O
2
+O
2
2
O
2
2
2
O +hVB+→H
O
2
+2H ; H
+
O
+hVB →2 OH (3)
+

H
2
2
2
2
<
H1>4. Conclusions
BiVO /g-C heterostructure were facilely fabricated by a simple sonification
method and were found to be highly active towards RhB and PNP degradation.
Moreover, compared with the pure g-C , these BiVO /g-C composites are much
more active. The enhanced photocatalytic activity after BiVO
4
3
N
4
3
N
4
4
3
N
4
4
introducing to g-C
3
N
4
is mainly attributed to the better optical absorption ability over the well contacted
structure in heterojunctions, good adsorption ability of organic contaminants and
lower recombination level of electron-hole pairs. In addition, over BiVO
4
/g-C
3
N
4
,


+
besides O
2
and hVB species (both active in BiVO
4
/g-C
3
N
4
and pure g-C
3
4
N system),
17

OH radical is also found to be active species for pollutants degradation; while in pure
g-C system it hardly contributes. This result also corroborates the improved
catalytic activity of 10 wt.% BiVO /g-C . Furthermore, the most active 10 wt.%
BiVO /g-C photocatalyst exhibits enough catalytic stability, showing great
3
N
4
4
3
N
4
4
3
N
4
potential as cost-effective heterogeneous visible-light-driven photocatalyst for the
purification of aqueous organic pollutants in future applications.
Acknowledgments
This work was financially supported by the Science and Technology Development
Plan of Jilin Province (20140101162JC; 20120346; 20140520150JH) and National
Natural Science Funds for Young Scholar (21307007).
<
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PW12O40 under Visible
<
<
/BIBL>
Figure>Fig. 1 XRD patterns of g-C
3
N
4
, BiVO
, BiVO
(a), BiVO
(d), 10 wt.% BiVO
4
and BiVO
4
/g-C
3
N
4
materials.
materials.
<
<
Figure>Fig. 2 FT-IR spectra of g-C
Figure>Fig. 3 SEM images of g-C
(b), BiVO
3
N
4
4
and BiVO
4
/g-C
3
N
4
3
N
4
4
(c), 10 wt.% BiVO
4
/g-C
3
4
N (e)
and TEM pictures of g-C
3
N
4
4
4
/g-C
3
N
4
(f).
20

<
Figure>Fig. 4 High-resolution XPS of (A) g-C
; (B) C 1s, (C) N 1s and (D) Bi 4f binding energy regions.
Figure>Fig. 5 UV-Vis/DRS (a) and the band gaps (b) of g-C
BiVO /g-C materials.
Figure>Fig. 6 Adsorption and photocatalytic property of BiVO
3
N
4
、BiVO
4
and 10 wt.% BiVO /g-
4
C
3
N
4
<
3
N
4
, BiVO and
4
4
3
N
4
<
4
deposited g-C
3
N
4
materials towards the degradation of RhB (a) and PNP (b); 400 nm  λ  780 nm. (c)
is the 10 wt.% BiVO /g-C corresponding reactive constant towards the
4
3
N
4
degradation of RhB. (d) are the UV-Vis absorption spectra of RhB solution as a
function of irradiation time during the photocatalytic process over the 10 wt.%
-
1
1
BiVO
Figure>Fig. 7 Recycling experiments over the 10 wt.% BiVO
degradation of under visible-light irradiation. Catalyst amount 100 mg; c
4
/g-C
3
N
4
. Catalyst amount 100 mg; c
0
= 20 mg L , 10 mg L ; volume 100 mL.
/g-C for RhB
= 20 mg
<
4
3
N
4
0

1
L ; volume 100 mL; 400 nm  λ  780 nm.
Figure>Fig. 8 Room temperature PL spectra of g-C
under the excitation wavelength of 356 nm.
Figure>Fig. 9 Photocurrent responses of g-C
<
3
N
4
and 10 wt.% BiVO
4
/g-C
3
N
4
<
3
N
4
and 10 wt.% BiVO
electrolyte solution under Xe irradiation.
Figure>Fig. 10 The effect of scavengers on catalytic activity of g-C (a) and 10
wt.% BiVO /g-C (b) towards the degradation of RhB. Catalyst amount 100 mg; c
4
/g-C
3
N
4

1
electrodes in 0.01 mol L Na
2
SO
4
<
3
N
4
4
3
N
4
0

1
=
<
20 mg L ; volume 100 mL; 400 nm < λ < 780 nm.
Figure>Fig. 11 Proposed reaction scheme for photocatalytic process over BiVO
4
deposited g-C
3
4
N composites under visible-light irradiation.
TDENDOFDOCTD
21