Synthesis and enhanced visible light photocatalytic activity of g- C3N4/BiOClxBr1-x heterojunctions with adjustable energy band structure

Article abstract of DOI:10.1016/j.jpcs.2019.05.002

The g-C₃N₄/BiOCl_xBr_1-x heterojunctions photocatalysts with different band gaps were successfully synthesized via a simple precipitation process. The photocatalytic activitys of the g-C₃N₄/BiOCl_xBr_1-x composites were evaluated through the degradation of Tetracycline Hydrochloride (TC)under visible light irradiation. Compared with pure g-C₃N₄ and BiOCl_xBr_1-x, the hybrids exhibited prominently enhanced visible light photocatalytic activity for the degradation of TC. The various photocatalytic perfprmances of g-C₃N₄/BiCl_xBr_1-x heterojunctions with same content of g-C₃N₄ were determined by different optical absorption capabilities resulting from the band structures, which could be controlling by changing the molar ratio of Br to Cl in BiOCl_xBr_1-x. The 10-CN/BiOCl_0·5Br_0.5 (g-C₃N₄ content: 10%)showed the highest photocatalytic activity toward TC, due to the optimal synergistic effect of bandgap structure and effective separation of electron-hole pairs. Furthermore, the capture experiments of 10-CN/BiOCl_0·5Br_0.5 during photocatalytic reaction were also carried out, the results indicted the holes (h⁺)and superoxide radicals (-O₂ ^-)were the main active substances causing the enhancement of photocatalytic activity. Finally, we proposed a reasonable photocatalytic degradation of this g-C₃N₄/BiOCl_xBr_1-x heterojunction photocatalysts.

Full text of DOI:10.1016/j.jpcs.2019.05.002

Journal of Physics and Chemistry of Solids 132 (2019) 222–229
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids
journal homepage: www.elsevier.com/locate/jpcs
Synthesis and enhanced visible light photocatalytic activity of g- C N /
3
4
BiOCl Br heterojunctions with adjustable energy band structure
x
1-x
Yibing Feng , Yi Du , Minxing Du , Xiuying Tian , Nan Jiang , Yang Liua
a
a,∗
a
b,∗∗
a
a
Key Laboratory of Processing and Testing Technology of Glass & Functional Ceramics of Shandong Province, School of Material Science and Engineering, Qilu University
of Technology (Shandong Academy of Sciences), Jinan, 250353, PR China
Hunan Provincial Key Laboratory of Fine Ceramics and Powder Materials, School of Materials and Environmental Engineering, Hunan University of Humanities, Science
b
and Technology, Loudi, 417000, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
The g-C
3
N
4
/BiOCl
x
Br1-x heterojunctions photocatalysts with different band gaps were successfully synthesized
composites were
G-C
Heterojunction
BiOCl Br1-x solid solutions
3 4
N
via a simple precipitation process. The photocatalytic activitys of the g-C N /BiOCl Br
3
4
x
1-x
evaluated through the degradation of Tetracycline Hydrochloride (TC) under visible light irradiation. Compared
with pure g-C and BiOCl Br1-x, the hybrids exhibited prominently enhanced visible light photocatalytic ac-
tivity for the degradation of TC. The various photocatalytic perfprmances of g-C /BiCl Br1-x heterojunctions
with same content of g-C were determined by different optical absorption capabilities resulting from the band
structures, which could be controlling by changing the molar ratio of Br to Cl in BiOCl Br1-x. The 10-CN/
BiOCl0·5Br0.5 (g-C content: 10%) showed the highest photocatalytic activity toward TC, due to the optimal
x
3
N
4
x
Photocatalysis
Tetracycline hydrochloride
3
N
4
x
3 4
N
x
3 4
N
synergistic effect of bandgap structure and effective separation of electron-hole pairs. Furthermore, the capture
experiments of 10-CN/BiOCl0·5Br0.5 during photocatalytic reaction were also carried out, the results indicted the
holes (h+) and superoxide radicals (-O
-
) were the main active substances causing the enhancement of photo-
2
catalytic activity. Finally, we proposed a reasonable photocatalytic degradation of this g-C
heterojunction photocatalysts.
3 4 x
N /BiOCl Br1-x
1
. Introduction
photocatalytic activity of bismuth compounds is attributed to the fact
that intrinsic layered structures are thought to induce effective se-
paration of photogenerated electron-hole pairs. However, the pure
BiOX (X = Cl, Br, I) have lower absorption in the visible-light region,
and their photocatalytic efficiencies must be further improved for
practical applications [7]. In addition, BiOBr [8], BiOCl [9] and BiOI
[10] can enhance the photocatalytic activity by forming different BiOX
mixed solid solutions. Compared with the pure BiOX, the nixted solid
solution has an adjustable energy band structure, large active crystal
surface exposed area, wide the visible light absorption range and the
efficient separation efficiency of electron-hole [11]. Qin et al. [12]re-
Environmental pollution is a global problem facing humanity and
other life forms, which attracts the attention of human beings for its
severe long-term consequences in recent years [1]. As known, tetra-
cycline hydrochloride (TC) as a typical antibiotic is widely used in the
treatment of infectious disease in human and animal, also used as an-
imal growth promoting agent in animal husbandry sometimes [2,3].
However, the abuse of antibiotics brings serious environmental pollu-
tion problem, especially water pollution problems. Up to now, such as
photocatalytic technology, electrode catalyst technology and the
method of adsorption have been used to remove the TC [4]. Among
these methods, photocatalytic degradation has been considered as a
promising method for removal of TC from wastewater, because it is
cost-effective, nontoxic, easily available and environmentally friendly.
Recently, Bismuth based compounds have received enormous at-
tentions in the practical application of photocatalyst because of the
special layer structure and the appropriate band gap [5,6]. The higher
x
ported that the BiOCl Br1-x photocatalyst was successfully synthesized
by a simple hydrothermal method, and the efficiency of degradation of
RhB and MO was much higher than that of pure BiOCl and BiOBr.
Coincidentally, Xie [13] and Feng [14] et al. synthesized the BiOCl
and BiOBr 1-x (0 ≤ x ≤ 1) solid solution to improve photocatalytic
property. However, there are few studies on BiCl Br1-x, and the solid
solution still contains ultraviolet activated semiconductor optical
x 1-x
I
x
I
x
∗
Corresponding author.
∗∗
Corresponding author.
E-mail addresses: duyi1964@126.com (Y. Du), xiuyingt@yahoo.com (X. Tian).
https://doi.org/10.1016/j.jpcs.2019.05.002
Received 21 January 2019; Received in revised form 15 April 2019; Accepted 2 May 2019
Available online 03 May 2019
0022-3697/ © 2019 Published by Elsevier Ltd.

Y. Feng, et al.
Journal of Physics and Chemistry of Solids 132 (2019) 222–229
absorption. Therefore, the further modification of BiCl
x
Br1-x is needed
simple precipitation method at room temperature. In a typical experi-
to increase the absorption of visible light.
mental process, Bi
lution and keep stirring for 30 min. Then another distilled water
(20 mL) containing g-C were dropped into the above CH COOH
solution under continuous stirring for 60 min. Finally, the stoichio-
metric amounts of NH Cl and NaBr were added into the solution con-
taining g-C . The pure BiOCl, BiOBr and BiOCl Br1-x were synthe-
sized by the same method without g-C . The obtain samples were
2 3 3
O (1 mmol) was added to 10 mL of CH COOH so-
Recently, two-dimensional (2D) layered g-C
widespread attentions [15]. As a typical non-metallic polymer semi-
conductor material, g-C exhibits well visible light absorption, che-
3 4
N has attracted the
N
3 4
3
3 4
N
mical and thermal stability, which are of benefits for catalytic reactions
for a variety of applications, such as removal of organic pollutants [16],
4
3
N
4
x
sensor detection [17,18], water splitting [19], and reduction of CO
20]. It owns the optical wavelength edge of about 460 nm, corre-
sponding to a band gap of ∼2.7 eV. Therefore, it has good photo-
catalytic activity under visible light irradiation. So, g-C is often the
combination with other semiconductors for the preparation of compo-
site photocatalysts, such as g-C /BiO Cl [21], g-C /SnO2-x [22],
g-C /KTa0·75Nb0.25 [23], g-C /AgNbO [24], g-C /KNbO [25].
2
3 4
N
[
washed several times with deionized water and ethanol, respectively,
and collected after drying at 80 °C. In this experimental, a series of g-
3
N
4
C
3
N
4
/BiOCl
pared by the above-mentioned method. The obtained g-C
BiOCl Br1-x composites with g-C weight ratios of 5 wt%, 10 wt%,
20 wt% and 30 wt% were denoted as 5-CN/BiOCl Br1-x, 10-CN/
BiOCl Br1-x, 20-CN/BiOCl Br1-x and 30-CN/BiOCl
x
Br1-x composites (x = 0, 0.25, 0.5, 0.75 and 1) were pre-
3 4
N /
3
N
4
3
O
4
3
N
4
x
3 4
N
3
N
4
3
N
4
3
3
N
4
3
x
Construction of heterostructure is an effective and simple strategy to
x
x
x
Br1-x.
improve the photocatalytic activity, thus the fabrication of hetero-
structure photocatalysts g-C
new approach for enhancement of pure g-C
reported.
3
N
4
/BiOCl
x
Br1-x has been turned out to be a
, but little work has been
2.5. Characterization
3 4
N
The crystalline and crystal phase of the as-prepared composites
were identified by an X-ray diffractometer (XRD, Bruker D8) using
monochromatized Cu Ka radiation 10–80°. The surface properties of the
samples were recorded by X-ray photoelectron spectroscopy (XPS,
ESCALAB 250xi, USA). The morphology of the prepared composites was
observed by a scanning electron microscopy (SEM, Hitachi, S-4800,
Japan), transmission electron microscopy (TEM, JEOL, JEM-2100,
Japan) and high-resolution transmission electron microscopy (HRTEM,
JEM-2010F). Energy Dispersive Spectrometer (EDS, SUPRATM55) was
employed to measure the element composition of the product which
was attached to the SEM. The Brunauer-Emmett-Teller (BET,
Quantachrome 8 Autosorb-iQ, USA) method with nitrogen absorption-
desorption isotherms was employed to measure the specific surface area
of samples. The FT-IR spectra of the samples were measured on the
Fourier-transform infrared spectrophotometer (FT-IR, Avatar 370,
Thermo Nicolet). The UV–Vis diffuse reflectance spectra (DRS,
Shimadzu UV-2550) of the samples were measured on a UV–Vis spec-
Herein, we have prepared g-C
ferent energy band structures. A simple precipitation method at room
temperature can be adopted to prepare g-C /BiOCl Br1-x hetero-
junction composites with a high catalytic performance for the photo-
3 4 x
N /BiOCl Br1-x composites with dif-
3
N
4
x
degradation of TC under visible light. Compared with pure BiOCl
and g-C , the photocatalytic activitys of the g-C /BiOCl
x
Br1-x
Br1-x
3
N
4
3 4
N
x
heterojunctions were greatly enhanced under visible-light irradiation,
which might be ascribed to the heterojunction formed between
BiOCl
paration of photoinduced electron-hole pairs. In this work, we simpli-
fied the preparation method of g-C /BiOCl Br1-x, which will provide
x 3 4
Br1-x and g-C N , loading to a promoted efficiency of the se-
N
3 4
x
a useful methodology for designing hybrid photocatalysts to solve the
problem of antibiotic pollution.
2. Experimental
2.1. Materials
4
trophotometer with BaSO as a reference. The photoluminescence (PL)
spectra of photocatalysts were detected with a luminescence spectro-
meter using a Xe lamp as the excitation source.
Bismuth trioxide (Bi
2
O
3
) and Tetracycline hydrochloride (TC) were
O), Acetic Acid
Cl), Sodium bromide (NaBr)
were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd
Tianjin, China). All the reagents were used without any further pur-
ification.
purchased from Aladdin (Shanghai, China). Urea (CH
CH COOH), Ammonium chloride (NH
4 2
N
(
3
4
2.6. Photocatalytic activity measurement
(
A 500 W Xenon lamp (with 420 nm cutoff filter) was used as the
light source. In a typical experiment, the prepared samples were added
to 50 mL TC solution (40 mg/L). Then, the mixture was magnetically
stirred for 60 min to ensure the adsorption-desorption equilibrium in a
dark place. During irradiation, about solution (4 mL) was took out from
the mixture at the given time intervals. The UV–vis spectrophotometer
(Shimadzu, UV-2550, Japan) was employed to measure the residual
concentration of TC. In addition, the Shimadzu TOC-VCPH (Japan) was
measured by the total organic carbon (TOC) content degradation of TC.
2
.2. Preparation of pure g-C
3 4
N
Urea (20 g) was annealed in a crucible (50 mL) using a Muffle fur-
nace at 500 °C (3 °C/min) for 3 h and the samples were further calcined
for 2 h at 550 °C. The light-yellow product was ground thoroughly with
a mortar for characterization.
2.3. Preparation of BiOCl
x
Br1-x solid solution
2.7. Photoelectrochemical measurements
The BiOCl
cipitation method at room temperature. In a typical experimental pro-
cess, Bi (1 mmol) was added to 10 mL of CH COOH solution and
keep stirring for 30 min. Then the stoichiometric amounts of NH Cl and
NaBr were added into the solution containing Bi and keep stirring
x
Br1-x solid solution was prepared by using a simple pre-
The measure of photocurrent for the sample was carried out with an
electrochemical analyzer workstation (CHI 660B Chenhua Instru. Co.
Ltd., Shanghai). The synthesized samples (10 mg) was slurry and coated
on an ITO glass (size: 0.5 cm × 1 cm). The photo-response of the pre-
pared samples was measured under a 500 W Xe lamp (400 nm cutoff
filter) with the light on and off switches of 10 s.
2
O
3
3
4
2 3
O
for another 2 h. The obtain samples were washed several times with
deionized water and ethanol, respectively, then dried at 80 °C. In this
experimental, a series of BiOCl
x
Br1-x composites (x = 0, 0.25, 0.5, 0.75
3. Results and discussion
and 1) were prepared using the above-mentioned method.
The XRD analysis is used to characterize the phase structure of the
catalyst. Fig. 1(a) shows the XRD patterns of the prepared BiOBr,
BiOCl0·25Br0.75, BiOCl0·5Br0.5, BiOCl0·75Br0.25 and BiOCl samples. The
peaks of BiOCl and BiOBr can be indexed to tetragonal phase BiOCl
2
.4. Preparation of g-C /BiOCl
3
N
4
x
Br1-x heterojunction
The g-C /BiOCl Br1-x heterojunction was prepared by using a
3
N
4
x
223

Y. Feng, et al.
Journal of Physics and Chemistry of Solids 132 (2019) 222–229
x x
Fig. 1. (a) The XRD diffraction patterns of the BiOCl Br1-x solid solutions; (x = 0, 0.25, 0.5, 0.75 and 1) (b) The XRD patterns of the synthesized 10-CN/BiOCl Br1-x
composites; (c) The FT-IR spectra of the g-C
3
N
4
, BiOCl0·5Br0.5 and 30-CN/BiOCl0·5Br0.5
.
(
JCPDS No. 82–0485) and BiOBr (JCPDS No. 78–0348). The results
show that the diffraction peaks shift slightly to the bigger angle with
−
decreasing amount of Cl, which indicates that Cl has a smaller ionic
−
radius than Br (0.181 versus 0.196 nm). This is basically consistent
with previous reports [26]. Compared with BiOBr, the diffraction peaks
of the prepared samples gradually shift implying BiOCl
lutions are formed. Fig. 1(b) shows the diffraction patterns of the pre-
pared 10-BiOCl Br1-x solid solutions and pure g-C (JCPDS card
number 87–1523). The CN/BiOCl Br1-x composites present the similar
diffraction peaks with the pristine BiOCl Br1-x. With the couple of g-
, the peak intensity and position of the BiOCl Br1-x solid solutions
do not significantly change, demonstrating no obvious influence of g-
on the phase structure of BiOCl Br1-x solid solutions. Any sig-
nificant g-C characteristic diffraction peaks is observed on these
diffractograms, which may be attributed to the lower intensity of g-
and surface coverage of BiOCl Br1-x. Similarly, the XRD diffraction
peaks of 10-CN/BiOCl Br1-x composites shift gradually to a bigger angle
x
Br1-x solid so-
x
3 4
N
x
x
C
N
3 4
x
C
N
3 4
x
3 4
N
C
N
3 4
x
x
with the decreasing amount of Cl.
Fig. 1(c) shows the FT-IR spectra of as-synthesized samples. The
−
1
spectral bands of 1630 cm in all spectra correspond to the stretching
3 4
and bending vibrations of O–H or N–H. In the FT-IR spectrum of g-C N ,
−
1
the broad band at ∼3100 cm
[
results from N–H stretching vibration
−
1
27]. The peaks at 1240, 1319 and 1405 cm
matic C–NH–C stretching. The band near 807 cm is attributed to out-
of-plane bending modes of C–N heterocycles. In addition, the broad
are attributed to aro-
3 4
Fig. 2. SEM images of the as-synthesized samples: (a) g-C N ; (b) BiOCl0·5Br0.5;
(c)–(d) 10-CN/BiOCl0·5Br0.5 and (e)–(j) elemental mappings of 10-CN/
BiOCl0·5Br0.5 composites.
−1
−
1
absorption bands at 3435 and 1658 cm
of the BiOCl0·5Br0.5, are re-
lated to the O–H stretching and bending vibrations, respectively. And
−
1
the small band centered at 1044 cm
X = Cl, Br) band symmetry stretching vibration peak. In the FT-IR
spectra, the main characteristic peaks of g-C and BiOCl0·5Br0.5 cor-
can also be designated as Bi-X
(
3 4
N
respond to the characteristic peaks of 10-CN/BiOCl0·5Br0.5 composites
photocatalyst.
The morphology and microstructure of the samples were char-
3 4
acterized by SEM and TEM. The g-C N displays a typically accumu-
lation lamellar structure morphology with a large size (Fig. 2(a)). The
pure BiOCl0·5Br0.5 are the micro-flower morphology assembled by a lot
of nanoflakes in Fig. 2(b). And Fig. 2(c) shows the microstructure of the
3 4
10-CN/BiOCl0·5Br0.5 composite. The heterojunction of g-C N with
BiOCl0·5Br0.5 microspheres results in the formation of the more compact
micro-flower-like structures.
Furthermore, the EDS in Fig. 2(e–j), clearly demonstrates that Bi, Br,
Cl, O, C and N uniformly distribute in the red box area of the nanoflakes
(
Fig. 2(d)), further confirming the uniform formation of 10-CN/
BiOCl0·5Br0.5 photocatalyst.
Fig. 3. TEM images of (a) g-C
3 4
N ; (b) BiOCl0·5Br0.5; (c) 10-CN/BiOCl0·5Br0.5 and
3 4
The morphology of samples (g-C N , BiOCl0·5Br0.5 and 10-CN/
(d) HRTEM image of 10-CN/BiOCl0·5Br0.5 composites.
BiOCl0·5Br0.5) was further characterized by TEM in Fig. 3. Fig. 3(a)
shows that g-C sample is a two-dimensional (2D) flake-like struc-
ture. Compared with flower-like BiOCl0·5Br0.5 sample (Fig. 3(b)), the g-
nanosheets coat onto the edges of BiOCl0·5Br0.5 in Fig. 3(c), which
3 4
N
3 4
C N
224

Y. Feng, et al.
Journal of Physics and Chemistry of Solids 132 (2019) 222–229
Fig. 4. XPS spectra of g-C
3
N
4
, BiOCl0·5Br0.5, 10-CN/BiOCl0·5Br0.5 (a–f) High revolution XPS spectra: (a) Bi 4f, (b) O 1s, (c) Cl 2p, (d) Br 3d, (e) C 1s, (f) N 1s spectra.
is consistent with the SEM observation. In addition, HRTEM images
Fig. 3(d)) confirm the two phases of g-C and BiOCl0·5Br0.5 are
closely contacted to form a close interface. The HRTEM image of g-C
shows lattice planes in regular intervals and plane intervals of 0.326 nm
corresponding to the 002 planes of g-C [28]. In addition, the ob-
binding energy values of Bi–O bonds and the hydroxyl groups on the
surface in the BiOCl0·5Br0.5 are observed at 533.1 eV and 530.68 eV,
which are slightly lower than those for 10-CN/BiOCl0·5Br0.5 hetero-
junction. Such shifts behind may be attributed to the interaction be-
(
3 4
N
3 4
N
3
N
4
3 4
tween BiOCl0·5Br0.5 and g-C N . Fig. 4(c) shows the binding energy
served lattice space is 0.352 nm in HRTEM, which corresponding to the
crystallographic plane (011) of the tetragonal BiOCl0·5Br0.5 crystal. The
good coupling of g-C N and BiOCl0·5Br0.5 facilitates the charge trans-
3 4
mission and separation photogenerated electron-hole pair, loading to
the enhancing photocatalytic activities.
XPS measurements are used to determine the chemical compositions
and oxidation states of as-synthesized sample. Fig. 4 shows the XPS
spectra of Bi 4f, N 1s, O 1s, Br 3d, Cl 2d and C 1s in the surface of 10-
values of Cl 2p1/2 and Cl 2p3/2 in the 10-CN/BiOCl0·5Br0.5 heterojunc-
tions are observed at 199.65 eV and 198.05 eV. It also can be seen from
the XPS spectra, the binding energy values of 10-CN/BiOCl0·5Br0.5
heterojunction are higher than that of BiOCl0·5Br0.5. The peaks at 68.6
and 69.62 eV are ascribed to Br 3d5/2 and Br 3d3/2 in 10-CN/
BiOCl0·5Br0.5 heterojunction. It also high than that the BiOCl0·5Br0.5.
Fig. 4(e) shows the C 1s XPS spectra of g-C and 10-CN/BiOCl0·5Br0.5
3
N
4
heterojunction. The C 1s spectrum of g-C N has two peaks located at
3 4
2
CN/BiOCl0·5Br0.5 heterojunction, g-C
reveals the peaks located at 159.25 and 164.55 eV are assigned to Bi
7/2 and Bi 4f5/2, indicating a trivalent oxidation state for Bi element,
3
N
4
and BiOCl0·5Br0.5. Fig. 4(a)
287.6 and 284.0 eV, which correspond to sp hybridized carbon in N-
2
containing aromatic ring (N–C]N) and sp adventitious C–C bonds. For
4f
the10-CN/BiOCl0·5Br0.5 heterojunction, the C 1s peaks at 283.4 and
which are very close to XPS results provide by previous reported for
BiOCl0·5Br0.5. However, it was observed that the binding energy of Bi
287.2 eV, indicating the existence of pure g-C
3 4
N [29]. For N 1s spec-
2
trum of g-C , the main peak at 398.0 eV is ascribed to sp hybridized
3 4
N
4
f
7/2 and Bi 4f5/2 peaks was slightly higher than that of pure
BiOCl0·5Br0.5 in the 10-CN/BiOCl0·5Br0.5 heterojunction. Such shifts of
peaks behind may be attributed to the interaction between g-C and
aromatic nitrogen bonded to carbon atoms (C]N–C) in Fig. 4(f), [30].
The other two peaks at 400.6 and 399.7 eV corresponds to N–H groups
and the tertiary nitrogen bonds to carbon atoms (N-(C) ), respectively.
3
3
N
4
BiOCl0·5Br0.5.10-CN/BiOCl0.5Br0.5 heterojunction displays two O1s
peaks at 533.15 eV and 530.73 eV in the (Fig. 4(b)). However, the
The XPS spectra show that the formation of 10-CN/BiOCl0·5Br0.5 com-
posite material is consistent with the above analysis results.
x x
Fig. 5. Degradation profiles of RhB using (a) BiOCl Br1-x and (b) 10-CN/BiOCl Br1-x under UV–visible light irradiation.
225

Y. Feng, et al.
Journal of Physics and Chemistry of Solids 132 (2019) 222–229
Table 1
Surface properties of g-C
photocatalytic degradation of TC. However, when the scavenger am-
monium oxalate (AO) and benzoquinone (BQ) are added, the de-
3
N
4
, BiOCl0·5Br0.5 and 10-CN/BiOCl0·5Br0.5.
+
2
3
gradation curve of TC is obviously inhibited, which means that h
Samples
BET surface area (m /g)
Total pore volume (cm /g)
−
2
and·O
are the main active species of the TC degradation. The results
+
g-C
BiOCl0·5Br0.5
0-CN/BiOCl0·5Br0.5
3
N
4
90.9 ± 0.5
70.6 ± 0.6
39.3 ± 0.8
0.23
0.178
0.200
of photocatalytic mechanism reveal that h and ·OH are the primary
−
−
active species, while e and·O
2
play a role in the degradation of TC.
1
Furthermore, the stability and reusability of 10-CN/BiOCl0·5Br0.5
composite was further evaluated by cyclic experiments on the de-
gradation of TC. As shown in Fig. 6(d), the photocatalytic activity of 10-
CN/BiOCl0·5Br0.5 is still maintained over 85% after four recycling, in-
dicates that the 10-CN/BiOCl0·5Br0.5 composite have good stability.
The optical absorption properties of the prepared samples were
The photocatalytic performance of synthesized samples is evaluated
by the decompose of the TC under simulated solar light. Fig. 5(a) shows
the photodegradation curves of BiOCl Br1-x (x = 1, 0.75, 0.5, 0.25 and
photocatalyst towards TC under the same conditions. The
BiOCl0·5Br0.5 photocatalyst exhibits more superior photocatalytic ac-
tivity than another BiOCl Br1-x (x = 1, 0.75, 0.25 and 0).
The optimal CN/BiOCl0·5Br0.5 photocatalyst was changed by ad-
justing the percentage of g-C in CN/BiOCl0·5Br0.5. Fig. 5(b) reveals
the photocatalytic experiments on the CN/BiOCl0·5Br0.5 heterojunctions
with different content of g-C at 5%, 10%, 20% and 30%. The pure g-
can degrade about 40% of TC, and only 65% of TC can be re-
x
0
)
x
studied by UV–Vis DRS. The UV–Vis DRS of the 10-CN/BiOCl Br1-x
composites are shown in Fig. 7(a). It is found that 10-CN/BiOBr, 10-
CN/BiOCl0·25Br0.75, 10-CN/BiOCl0·5Br0.5, 10-CN/BiOCl0·75Br0.25 and 10-
CN/BiOCl composites exhibit the similar absorbance edges. However,
x
3 4
N
the redshift of the 10-CN/BiOCl
obvious. In addition, from Fig. 7(a) and (c), it can be seen that 10-CN/
BiOCl Br1-x composites exhibits a significant redshift phenomenon
compared to pure BiOCl Br1-x. This is due to the introduction of g-C
which enhances the visible light absorption capacity of BiOCl Br1-x but
also makes light absorption characteristics similar to 10-CN/BiOCl Br1-x
composites. As Fig. 7(b) presents the UV–Vis DRS of g-C
BiOCl0·5Br0.5 with different content of g-C (5%, 10%, 20% and
0%). The 10-CN/BiOCl0·5Br0.5 has the longest absorption band in these
composites among these samples. Fig. 7(c) shows the DRS of the pure g-
and BiOCl Br1-x solid solutions. The pure g-C reveals an es-
sential absorption edge rising at about 430 nm, displaying that the pure
g-C can be active by the visible light. At the same time, the
BiOCl Br1-x solid solution has a steep absorption band and nearly par-
x
Br1-x composites absorption edge is not
3 4
N
x
3 4
C N
x
3 4
N ,
moved in presence of BiOCl0·5Br0.5 after 150 min irradiation under
visible light. The degradation ratio of TC on the 10-CN/BiOCl0·5Br0.5
composite reaches almost 80% after 150 min irradiation. The results
x
x
3 4
N /
3 4
show that the amount of g-C N has a significant effect on the photo-
3 4
N
catalytic activity of the heterojunction. The optimum percentage of CN/
BiOCl0·5Br0.5 composite is 10%, which has the highest photocatalytic
activity for the decomposition of TC. According to the above discussion,
it can be concluded that 10-CN/BiOCl0·5Br0.5 possesses the best pho-
tocatalytic performance under visible light irradiation, attributed to the
composite could separate the photogenerated carriers effectively.
Table 1 demonstrates the specific surface area data of as-synthesized
3
C
3
N
4
x
3 4
N
3 4
N
x
allel light absorption edge characteristics, indicating that the band
transition in BiOCl Br solid solution depends on the ratio of Cl to Br.
1
0-CN/BiOCl0·5Br0.5, g-C
3 4
N and BiOCl0·5Br0.5 samples. Compared to g-
2
2
x
1-x
3 4
C N (90.9 m /g) and BiOCl0·5Br0.5 (70.6 m /g), the specific surface
2
It can be seen that the redshift of absorption wavelength increases with
the decrease in Cl concentration, indicating that the formation of
BiOCl Br1-x solid solution is not physical mixing of BiOCl and BiOBr.
x
Furthermore, the optical band-gap is estimated through the following
equation
area (39.3 m /g) of 10-CN/BiOCl0·5Br0.5 sample is not the largest, but it
has the best photocatalytic activity. Generally, the photocatalytic ac-
tivity of samples with large specific surface area is relatively high.
While the surface area of g-C
than that of 10-CN/BiOCl0·5Br0.5, but the photocatalytic activity of g-
and BiOCl0·5Br0.5 samples is lower than that of 10-CN/
3 4
N and BiOCl0·5Br0.5 samples are bigger
C
N
3 4
)n/2
αhν = A(hν - E
g
(1)
BiOCl0·5Br0.5. Therefore, we speculate that BET is not the main factor
affecting photocatalytic activity in this system.
Where a, h, n, Eg and A are the absorption coefficient, Planck constant,
the light frequency, the band gap, and a constant, respectively. And in
this formula, n (n = 1 is direct transition, n = 4 is indirect transition)
decides the characteristics of the transition in semiconductor. For this
The enhanced photocatalytic activity of the 10-CN/BiOCl0·5Br0.5
composite photocatalyst can be further evaluated by mineralization.
Fig. 6(a) displays the TC degradation in terms of TOC removal. It is
clearly revealed that the photocatalytic reaction of TC is more than that
of the degradation percentage of TOC, which is owing to the miner-
alization of TC molecules in photocatalytic process. In addition, the
decrease of TOC implies that TC is degraded into the final products,
reaction the value of n is 4 for the indirect transition. Fig. 7(d) reveals
the plot of (ahv)1 versus hv. And the E
/2
values of pure BiOCl0·5Br0.5, g-
g
C
3
N
4
and 10-CN/BiOCl0·5Br0.5 are calculated at 2.65 eV, 2.82 eV and
2.40 eV, respectively.
3 4
The photoluminescence (PL) spectra of g-C N , BiOCl0·5Br0.5 and 10-
such as H
2
O and CO
2
.
As shown in Fig. 6(b), the intermediate products produced by
photocatalytic degradation of TC are confirmed by the HPLC technique.
With the photocatalytic reaction proceeding, the peak area of TC de-
creased continuously at 2.41 min, indicating that TC gradually de-
graded. The results show that the photocatalytic performance obtained
by TOC or HPLC is similar to that obtained by UV–Vis absorption
spectra. The HLPC analysis provided solid evidence for photodegrada-
tion the RhB and the intermediates during the degradation reaction in
the presence of 10-CN/BiOCl0·5Br0.5 composite photocatalytic systems.
On behalf of better concluded the possible mechanism of photo-
CN/BiOC0·5Br0.5 were measured to compare the separation ability of
photogenerated carriers in the prepared samples. As shown in Fig. 8(a),
the emission peaks centered at 460 nm is observed on g-C N , which
3 4
can be ascribed to the band gap recombination of electron-hole pairs.
The pure BiOCl0·5Br0.5 shows relatively lower emission peak at 450 nm.
3 4
Compared with pure g-C N and BiOCl0·5Br0.5, the 10-CN/BiOC0·5Br0.5
presents the lowest intensity in the PL spectra, suggesting higher se-
paration efficiency of charge carriers. Therefore, the photogenerated
charge carriers in the 10-CN/BiOC0·5Br0.5 sample have the longest
lifetime, which helps more excited electrons to participate in the pho-
tocatalytic reaction rather than recombining with the holes.
3 4
catalytic activity, the capture experiments of 10-CN/g-C N /
BiOCl0·5Br0.5 composites during the photocatalytic reaction were mea-
sured to determine the main active substances that cause enhanced
In order to understand the excellent photoelectrochemical proper-
ties, g-C N , BiOCl0·5Br0.5 and 10-CN/BiOCl0·5Br0.5 were measured by
3 4
photocatalytic activity. Fig. 6(c) reveals that the photocatalytic de-
gradation of TC is not affected by the addition of AgNO (e scavenger)
3
transient photocurrent. Generally, a higher photocurrent means the
photo-carriers can be generated and separated more efficiently, thus
representing higher photoactivity. As shown in Fig. 8(b), the 10-CN/
BiOCl0·5Br0.5 exhibits higher photocurrent density than these of pure g-
−
and IPA (·OH scavenger). Fig. 6(c) reveals that the addition of AgNO
3
−
(
e
scavenger) and IPA (·OH scavenger) has no effect on the
226

Y. Feng, et al.
Journal of Physics and Chemistry of Solids 132 (2019) 222–229
Fig. 6. (a) TOC removal and (b) HPLC chroma-
tograms of TC solution at different time in pre-
sence of 10-CN/BiOCl0·5Br0.5 under visible light
irradiation. (c) Photocatalytic degradation of TC
over 10-CN/BiOCl0·5Br0.5 in the presence of dif-
ferent scavengers under visible light irradiation.
(d) The photocatalytic stability of 10-CN/
BiOCl0·5Br0.5 composite in recycling reactions.
Fig. 7. UV–vis DRS spectrum of the as-fabricated (a) 10-CN/BiOCl
x
Br1-x, (b) CN/BiOCl
x
Br1-x with different g-C
3 4 x 3 4
N content, (c) BiOCl Br1-x and g-C N samples,
1/2
respectively; (d) Plot of the (αhν)
versus hν of BiOCl0·5Br0.5, 10-CN/BiOCl0·5Br0.5 and g-C N samples.
3 4
C
3
N
4
and BiOCl0·5Br0.5, which are consistent with their photocatalytic
activities.
Additionally, the electrochemical impedance spectroscopy (EIS)
BiOCl0·5Br0.5 and g-C
the Nyquist diagram of 10-CN/BiOCl0·5Br0.5 composite is smaller than
that of pure BiOCl0·5Br0.5 and g-C , indicating a separation of the
3 4
N . As shown in Fig. 8(c), the radius of the arc on
3 4
N
measurement was carried out to further prove the superior charge
carrier transfer of 10-CN/BiOCl0·5Br0.5 compared with pure
photogenerated electron-hole pairs and interface charge transfer is
improved. These results are consistent with the results of the transient
227

Y. Feng, et al.
Journal of Physics and Chemistry of Solids 132 (2019) 222–229
3 4
Fig. 8. (a) Photoluminescence spectra,(b) Transient photocurrent responses curves and (c) EIS Nyquist plots of pure g-C N , BiOCl0·5Br0.5 and 10-CN/BiOCl0·5Br0.5.
Fig. 9. Proposed mechanisms for photocatalytic performance enhancement of 10-CN/BiOCl0·5Br0.5 composites.
photocurrent response.
significantly increased active site and the formation of Z-scheme het-
The possible mechanism of TC degradation caused by CN/
BiOCl0·5Br0.5 heterojunction is shown in Fig. 9. Any factor that inhibits
photoelectron electron-hole pair recombination can enhance photo-
erojunction. These can improve the photoelectron and hole separation
−
+
efficiency and extend the life of electrons-holes. O
proved to be the main active substances leading to TC degradation by
x
the trapping experiment results. In addition, the g-C /BiOCl Br1-x
2
and H were
catalytic activity. According to previous reports, the g-C
visible light effectively [31]. Meanwhile, it should be clear that the
BiOCl Br1-x solid solutions can absorb ultraviolet and visible light.
3
N
4
can absorb
3
N
4
composites presented good repeatability, reproducibility and stability
after recycling. Therefore, as a highly efficient and stable visible light
photocatalyst, it will be a useful hybrid photocatalysts to solve the
problem of antibiotic pollution.
x
Therefore, the possible mechanism can be explained as follows. As
shown in Fig. 9, with the irradiation of visible light, electrons and holes
are generated in the conduction band (CB) and valence band (VB) of
BiOCl0·5Br0.5 and g-C
(
(
are more easily transfers to the CB of g-C
accumulation on the CB of g-C
3
N
4
, respectively. Since the CB values of g-C
−1.21 eV) [32]are more negative than that of BiOCl0·5Br0.5
+0.27 eV) [33], the photoinduced electrons in the CB of BiOCl0·5Br0.5
. which results in electrons
. The electrons react with O on the
radicals can
O and other molecules. At same time, the
holes accumulating on the VB of BiOCl0·5Br0.5 will reduce the TC to
CO , H O and other molecules. The heterojunction formed between
BiOCl0·5Br0.5 and g-C can facilitate the effective separation of the
3 4
N
Acknowledgments
This work was supported by the Natural Science Foundation of
Hunan Province (Grant No. 2018JJ3251), Project Supported by
Scientific Research Fund of Hunan Provincial Education Department
3 4
N
3
−
2
N
4
2
−
surface of g-C
oxidize the TC to CO
3
N
4
to yield •O
, H
radicals, the creative •O
2
(
Grant No. 17B138), the Open Foundation of Hunan Provincial Key
2
2
Laboratory of Fine Ceramics and Powder Materials (Grant No.
TC201701), Foundation Project for Young Talents of Hunan University
of Humanities, Science and Technology (Grant No. 2016QN11) and the
Planed Science and Technology Project of Hunan Province (Grant
No.2016TP1028).
2
2
3 4
N
photoinduced electron-hole pair, which is benefit for enhancing pho-
tocatalytic activity. The photocurrent experiments (Fig. 8(b)) further
confirms this possible mechanism.
Appendix A. Supplementary data
4
. Conclusions
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.jpcs.2019.05.002.
The g-C
by a simple precipitation method at room temperature. The resulting g-
/BiOCl Br1-x composites demonstrated an enhanced photocatalytic
performance towards degradation of TC under visible light irradiation.
The enhanced activity of g-C /BiOCl Br1-x can be attributed to the
3 4 x
N /BiOCl Br1-x Z-scheme heterojunction was synthesized
C
N
3 4
x
Notes
3
N
4
x
The authors declare no competing financial interest.
228

Y. Feng, et al.
Journal of Physics and Chemistry of Solids 132 (2019) 222–229
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