A g-C3N4/MIL-101(Fe) heterostructure composite for highly efficient BPA degradation with persulfate under visible light irradiation

Article abstract of DOI:10.1039/c8ta07915c

Heterostructured composites with outstanding photocatalytic performance have attracted intense attention because of their great potential for use in environmental remediation. Herein, a facile hydrothermal method to obtain heterostructured composites consisting of graphitic-carbon nitride (g-C₃N₄) and an Fe-based metal organic framework (MIL-101(Fe)) is reported. The as-prepared hybrids exhibited dramatically improved photocatalytic performance for bisphenol A (BPA) degradation in the presence of persulfate (PS) under visible light irradiation. The apparent rate constant, k, for BPA degradation by g-C₃N₄/MIL-101(Fe) composites with PS was approximately 8.9 times higher than that obtained with bare g-C₃N₄. The superior performance arises from the heterojunction formed between MIL-101(Fe) and g-C₃N₄ with a favourable band position and the excellent PS activation ability of MIL-101(Fe), which not only enables the fast electron transfer from g-C₃N₄ to MIL-101(Fe) but also accelerates PS activation for SO₄^? radical production. This work provides an effective way for constructing novel photocatalytic systems with efficient activity for refractory pollutant removal.

Full text of DOI:10.1039/c8ta07915c

Journal of
Materials Chemistry A
View Article Online
View Journal | View Issue
PAPER
A g-C N /MIL-101(Fe) heterostructure composite
3
4
for highly efficient BPA degradation with persulfate
under visible light irradiation†
Cite this: J. Mater. Chem. A, 2018, 6,
3703
2
abd
bd
Hui Zhang and Xu Zhao *a
c
Yan Gong,
Bo Yang,
Heterostructured composites with outstanding photocatalytic performance have attracted intense
attention because of their great potential for use in environmental remediation. Herein, a facile
hydrothermal method to obtain heterostructured composites consisting of graphitic-carbon nitride
3 4
(g-C N ) and an Fe-based metal organic framework (MIL-101(Fe)) is reported. The as-prepared hybrids
exhibited dramatically improved photocatalytic performance for bisphenol A (BPA) degradation in the
presence of persulfate (PS) under visible light irradiation. The apparent rate constant, k, for BPA
degradation by g-C /MIL-101(Fe) composites with PS was approximately 8.9 times higher than that
obtained with bare g-C . The superior performance arises from the heterojunction formed between
3 4
MIL-101(Fe) and g-C N with a favourable band position and the excellent PS activation ability of
3
N
4
3
N
4
Received 15th August 2018
Accepted 22nd October 2018
3 4
MIL-101(Fe), which not only enables the fast electron transfer from g-C N to MIL-101(Fe) but also
DOI: 10.1039/c8ta07915c
À
accelerates PS activation for SO
4
c
radical production. This work provides an effective way for
rsc.li/materials-a
constructing novel photocatalytic systems with efficient activity for refractory pollutant removal.
Recently, g-C
3 4
N has gathered signicant interest as a novel
1
. Introduction
heterogeneous catalyst for PS activation by virtue of its visible-
Advanced oxidation processes (AOPs) have been regarded as light responsiveness, robust stability and metal-free nature.
a promising strategy for pollutant removal in the eld of waste Unfortunately, the catalytic efficiency of g-C is still unsatis-
water treatment. As one of the most effective AOPs, the sulfate factory resulting from the limited active sites, sluggish charge
radical (SO c )-based AOP through persulfate (PS) activation carrier separation, and moderate light absorption. Thus
7–10
N
3 4
1–3
À
4
has drawn extensive attention for the degradation of recalci- exploring a g-C
3 4
N based photocatalytic system with superior PS
trant pollutants in water because of advantages including activation ability is highly desirable.
strong oxidizing capability (2.5–3.1 V vs. SHE) in a wide pH
Metal organic frameworks (MOFs) consisting of metal ions/-
range with a long half-life (30–40 ms). These impressive merits clusters and organic linkers with a porous crystalline structure
À
endow the SO
4
c
with a strong oxidative capability to decom- have emerged as novel heterogeneous catalysts with promising
11–22
pose pollutants more efficiently and completely. To date, catalytic activity.
numerous methods have been devoted to activate PS for SO
production, Among these methods, PS activation triggered by abundant active sites and intensify the mass transport.
The fascinating properties such as high
porosity and large specic surface area allow them to expose
À
4
c
4–6
photocatalysis has aroused intense interest as it offers the Recently, MIL-101(Fe) has been demonstrated as a photo-
À
potential to generate SO c under mild reaction conditions and catalyst with attractive catalytic performance for PS activation to
4
18,20
using clean energy.
decompose dyes.
The existence of coordinatively unsatu-
rated iron sites in the abundant exposed surface of MIL-101(Fe)
allows strong interactions with PS, and the linker to metal
cluster charge transfer (LCCT) triggered by light can cause the
a
Key Laboratory of Drinking Water Science and Technology, Research Center for
Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, PR
3 3
reduction of Fe(III) to Fe(II) in the trimeric Fe -m -oxo cluster,
China. E-mail: zhaoxu@rcees.ac.cn
b
resulting in the high-efficiency PS activation of MIL-101(Fe).
College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen
5
c
18060, PR China
Moreover, MIL-101(Fe) with a band gap of 2.6 eV possesses
State Key Laboratory of Environmental Chemistry and Eco-toxicology, Research excellent chemical and thermal robustness properties, which
Centre for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing
make it a promising photocatalyst for PS activation.
1
00085, PR China
It has been reported that coupling g-C N with MOFs is an
3
4
d
Shenzhen Key Laboratory of Environmental Chemistry and Ecological Remediation,
Shenzhen University, Shenzhen 518060, PR China
advisable way to improve its photocatalytic activity by means of
extending optical absorption, boosting charge carrier separa-
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c8ta07915c
23–26
tion and providing more active sites.
In this regard, the
1
This journal is © The Royal Society of Chemistry 2018
J. Mater. Chem. A, 2018, 6, 23703–23711 | 23703

View Article Online
Journal of Materials Chemistry A
Paper
combination of MIL-101(Fe) and g-C
3
N
4
would impart the states of the as-prepared samples were investigated by X-ray
following advantages: (i) efficient utilization of solar energy can photoelectron spectroscopy (XPS) using an ESCALAB 250Xi
be achieved by incorporating visible-light responsive MIL- (Thermo Fisher Scientic, USA). Fourier transform infrared
1
01(Fe) with g-C N ; (ii) the heterojunction formed between g- (FTIR) spectra were recorded on a VERTEX 70 (Bruker, Ger-
3 4
C
3 4
N and MIL-101(Fe) with a suitable band position allows the many) to investigate the surface chemistry. The light absorption
fast charge carrier transfer at the interface; and (iii) the abun- properties of the samples were evaluated on a Hitachi 3010
dant active sites on the surface of MIL-101(Fe) with excellent PS spectrophotometer. Raman spectra were recorded on a confocal
À
activation ability are favorable to yield highly active SO
Bearing these in mind, a systematic study on this g-C
4
c . Raman microscope (LabRAM HR 800, Horiba) equipped with an
3
N
4
/
Ar laser. A Bruker EMX A300-10/12 spectrometer was used to
MIL-101(Fe) hybrid with excellent PS activation ability for obtain the electron spin resonance (ESR) signals of the spin-
oxidative decomposition of refractory pollutants is of signicant trapped oxidative radicals with the spin-trapping agent
importance.
5,5-dimethyl-1-pyrroline-N-oxide (DMPO).
Herein, a facile hydrothermal method for the fabrication of
A conventional three-electrode system consisting of a Pt wire
the g-C
of MIL-101(Fe) greatly improved the PS activation and charge as the reference electrode was used to analyze the photo-
carrier separation of g-C . As a result, the as-prepared electrochemical properties of different samples on an electro-
3 4
N /MIL-101(Fe) composite is reported. The introduction as the counter electrode and a saturated calomel electrode (SCE)
3 4
N
composite exhibited superior photocatalytic performance for chemical workstation (CHI 660B, China). A Na
2
SO
4
solution
À1
BPA degradation in the presence of PS under visible light irra- (0.05 mol L ) was used as the electrolyte. The visible light
diation, with nearly 98% BPA degraded aer 60 min irradiation. source was a 300 W Xe lamp (BoPhilae Technology Co., LTD,
Beijing) with a 400 nm cutoff lter. The photocurrent test was
performed under chopped light irradiation. Electrochemical
2
. Experimental
impedance spectroscopy (EIS) was performed within the
2.1 Preparation of g-C N nanosheets and MIL-101(Fe)
5
3
4
frequency range from 10 to 0.01 Hz.
All starting materials were of reagent grade and used without
further purication. The bulk g-C N was rst prepared via the
direct heating of 5 g of melamine at 550 C for 4 h in air. Then
an ultrasonication treatment was used to exfoliate the bulk g-
3
4
2
.4 Evaluation of photocatalytic activity
ꢀ
The photocatalytic performances of samples were evaluated by
degradation of BPA (10 mg L ) under visible light irradiation,
in which a 300 W xenon lamp equipped with an optical lter (l >
À1
2
5
C
3
N
4
into nanosheets in water according to a previous study.
For the synthesis of MIL-101(Fe), a hydrothermal process was
carried out by using FeCl $6H O and benzene-1,4-dicarboxylic
acid (1,4-BDC) as precursors.
thylformamide (DMF) solution containing 2.45 mmol FeCl
6H O and 1.24 mmol BDC was sealed, and subsequently
400 nm) was used as the visible light source. Prior to irradiation
3
2
8,20
1
and addition of PS, an adsorption–desorption equilibrium
experiment was conducted with magnetic stirring of the
samples in BPA solution for 1 h in the dark. At a designated time
interval, 0.8 mL of the reaction solution was sampled out and
quenched with methanol, and then ltered for testing the
concentration of BPA. The concentration of BPA at different
reaction times was detected using a HPLC (Shimadzu, DGU-20
A3R(C), Japan) equipped with column C18 and a UV detector.
The detection wavelength of BPA was set as 280 nm, and the
In detail, 15 mL of N,N-dime-
3
-
$
2
ꢀ
heated up to 110 C and kept for 20 h. Aer cooling to room
temperature, the obtained product was consecutively puried
with ethanol and DMF several times, and dried overnight in
ꢀ
a vacuum oven at 60 C.
2.2 Preparation of the g-C
3 4
N /MIL-101(Fe) composite
ꢀ
temperature of column C18 was 30 C. The mobile phase was
methanol and water (70 : 30), and the ow rate was 1 mL min
À1
The synthesis process of g-C N /MIL-101(Fe) follows the same
3
4
.
synthetic procedure of MIL-101(Fe) except that a certain amount
of g-C nanosheets was added to the precursor solution. In
this way, a series of composites with different g-C contents
0.5%, 1%, 2%, 5% and 10%) were also prepared and denoted as
.5% g-C /MIL-101(Fe), 1% g-C /MIL-101(Fe), 2% g-C
MIL-101(Fe), 5% g-C /MIL-101(Fe) and 10% g-C /MIL-
A vario TOC analyzer (Elementar, Germany) was used to analyze
the total organic carbon (TOC) of BPA. The PS concentration
was determined by the KI colorimetric method according to
3 4
N
3 4
N
(
0
27
a previous report.
3
N
4
3
N
4
3 4
N /
For the active radical trapping test, ethylenediamine tetra-
acetic acid disodium salt (EDTA-2Na), p-benzoquinone (BQ), t-
butanol (TBA), and methanol (MeOH) were used to quench the
3
N
4
3 4
N
101(Fe).
+
À
photoinduced holes (h ), superoxide anion radicals (cO ),
2
À
2.3 Characterization
hydroxyl radicals (cOH), and sulfate radicals (SO c ) in the
4
reaction system, respectively.
The phase structure of the samples was recorded on an X-ray
diffractometer (X'pert Pro MPD) with Cu Ka. The FESEM and
TEM images were obtained on a Hitachi SU-8020 and Hitachi
H7500. The high-angle annular dark-eld scanning trans-
mission electron microscopy (HAADF-STEM) and energy
3
. Results and discussion
3 4
3.1 Characterization of g-C N /MIL-101(Fe) composites
dispersive X-ray elemental mapping (EDX) were performed The two-step synthetic process for the fabrication of the g-C
3 4
N /
using a JEOL JEM-2100F microscope at 200 kV. The chemical MIL-101(Fe) composite is illustrated in Fig. 1. Firstly, small
23704 | J. Mater. Chem. A, 2018, 6, 23703–23711
This journal is © The Royal Society of Chemistry 2018

View Article Online
Paper
Journal of Materials Chemistry A
much rougher, and tightly decorated with dense lamellar
nanosheets as displayed in Fig. 2b. Note that MIL-101(Fe)
retained its initial octahedral shape aer g-C N NS modica-
3
4
tion, indicating that the integration of g-C N NSs does not alter
3
4
its structure. The TEM images (Fig. 2c and e) reveal that the
smaller g-C nanosheet structures are closely attached and
3 4
N
packed on the surface of MIL-101(Fe). The corresponding high
resolution TEM image shown in Fig. 2d (indicated by a red
arrow in Fig. 2c) further illustrates that some nanosheets with
a size of about 50 nm are closely loaded on the MIL-101(Fe),
which implies the intense interaction between g-C N and MIL-
3 4
Fig. 1 Schematic illustration of the preparation of the g-C N /MIL-
1
01(Fe) composite.
3
4
g-C
bulk g-C N under ultrasonication treatment. Then the g-C N
3 4
3 4 3 4
N nanosheets (g-C N NSs) were obtained by exfoliation of
101(Fe). Moreover, the HAADF-STEM image (Fig. 2f) and the
3
4
corresponding EDX elemental mapping (Fig. 2g–j) clearly
illustrate the homogeneous distribution of the C and N
elements in the entire composite, which further veries that the
nanosheets and the precursors of MIL-101(Fe) were mixed at
a predened mass ratio and then subjected to a hydrothermal
process to synthesize the g-C N /MIL-101(Fe) hybrid.
3 4
The SEM image shown in Fig. 2a reveals that the as-synthe-
sized MIL-101(Fe) presents a morphology with a smooth
surface, and the particle size ranges from 500 nm to 2 mm. Aer
3 4
g-C N nanosheets are uniformly coated on the MIL-101(Fe).
The above results give solid evidence for the successful forma-
tion of g-C N /MIL-101(Fe) composites. The heterostructure
3
4
formed between g-C N and MIL-101(Fe) can facilitate the
3
4
3 4
the introduction of g-C N , the surface of MIL-101(Fe) becomes
charge carrier separation at the interface, which is advanta-
geous for the photocatalytic reaction.
The X-ray diffraction patterns of g-C
g-C /MIL-101(Fe) composites are presented in Fig. 3. Two
typical peaks assignable to the (100) and (002) planes of g-C
can be observed for the g-C nanosheets, which are consistent
It should be noted that the less
pronounced peak at 13.1 indicates that the planar size of layers
3 4
N , MIL-101(Fe) and
3 4
N
3 4
N
3 4
N
24,25
with previous studies.
ꢀ
25
is decreased aer ultrasonic exfoliation. As for MIL-101(Fe),
ꢀ
ꢀ
several sharp diffraction peaks in the range of 5 to 20 can be
clearly observed and match well with those of the previous
1
8,20,28
reports,
demonstrating the successful preparation of
/MIL-101(Fe), only the
MIL-101(Fe). In the case of g-C
3 4
N
diffraction peaks corresponding to MIL-101(Fe) are observed,
implying that the framework of MIL-101(Fe) is maintained aer
the introduction of g-C N . The absence of the diffraction peaks
3
4
of g-C N may be due to the low loading amount and the small
3
4
planar size of g-C
The FT-IR spectra of g-C
recorded and illustrated in Fig. 4. For bare g-C
peaks in the range from 1200 cm to 1650 cm originated
from the C,N-heterocyclic units and the peak at 812 cm
3
N
4
nanosheets.
/MIL-101(Fe) composites are
NSs, the
3 4
N
3 4
N
À1
À1
À1
assignable to the C N -based ring units conrms its graphitic
6
7
25,29
carbon nitride nature.
In the case of g-C N /MIL-101(Fe)
3 4
Fig. 2 SEM images of (a) MIL-101(Fe) and (b) g-C
c–e) TEM images of g-C /MIL-101(Fe); (f–i) HAADF-STEM/EDX Fig. 3 XRD patterns of bulk g-C
mapping of g-C /MIL-101(Fe). /MIL-101(Fe).
3 4
N /MIL-101(Fe);
(
N
3 4
3 4 3 4
N , g-C N NSs, MIL-101(Fe) and g-
3
N
4
3 4
C N
This journal is © The Royal Society of Chemistry 2018
J. Mater. Chem. A, 2018, 6, 23703–23711 | 23705

View Article Online
Journal of Materials Chemistry A
Paper
and MIL-101(Fe) exhibit fundamental absorption edges at 440
and 460 nm, demonstrating the visible-light responsive char-
acteristic and the wider light absorption region of MIL-101(Fe).
Obviously, compared with pristine g-C N NSs, the enhanced
3
4
light absorption in the visible light region of the g-C
MIL-101(Fe) composite suggests that coupling of g-C
MIL-101(Fe) can broaden the optical absorption of g-C
3 4
N /
with
NSs,
3
N
N
3 4
4
which is benecial for improving the photocatalytic perfor-
mance. In addition, the bandgaps of g-C NS and g-C
MIL-101(Fe) obtained from the Tauc plot are determined to be
.8 and 2.75 eV, respectively. The narrowed bandgap energy also
implies the efficiently broadened light harvesting range of
pristine g-C
3
N
4
3 4
N /
Fig. 4 FT-IR spectra of g-C
01(Fe).
3 4 3 4
N NSs, MIL-101(Fe) and g-C N /MIL-
2
1
3 4
N .
3 4
composites, both the characteristic peaks assigned to g-C N
and MIL-101(Fe) can be observed, demonstrating the coexis- 3.2 Photocatalytic activity
tence of g-C N and MIL-101(Fe) in the composites.
An XPS study was carried out to investigate the chemical
components and bonding characteristics of g-C N /MIL-
3
4
The photocatalytic performances in BPA degradation over
g-C and g-C /MIL-101(Fe) composites were rst evaluated
3
N
4
3 4
N
3
4
under visible light irradiation. The blank experiment result
shown in Fig. 7 indicates that the photodecomposition of BPA is
extremely low under visible light irradiation. Note that the
g-C N /MIL-101(Fe) composite exhibits improved photocatalytic
101(Fe). The high-resolution C 1s and N 1s XPS signals of g-
3 4
C N
/MIL-101(Fe) are measured and shown in Fig. 5. Three
deconvoluted peaks centered at 284.6, 286.3, and 288.3 eV in the
C 1s spectrum of Fig. 5a are assigned to the impurity carbon,
3
4
activity under visible light irradiation compared with bare g-
C N NSs and MIL-101(Fe), implying that the coupling of g-C N
3 4
primary or secondary amines (C–NH
x
) on the edges of heptazine
3
4
2
units, and the sp -hybridized carbon of the g-C N , respec-
3
4
with MIL-101(Fe) can enhance the photocatalytic activity of
g-C NS. More signicantly, the addition of PS can further
dramatically improve the photocatalytic activity of the g-C
MIL-101(Fe) system and almost 98% BPA can be degraded aer
0 min irradiation. This may be attributed to the synergistic
effect of g-C N and MIL-101(Fe), which can promote the
23,30
tively.
Meanwhile, the N 1s spectra of Fig. 5b can be tted
3 4
N
into three deconvoluted peaks at 398.7 eV, 399.8 eV and
3 4
N /
2
4
01.1 eV, which are ascribed to the sp -hybridized aromatic N in
23,30
C–N]C, tertiary N in N–C
3
and N–H groups,
with MIL-101(Fe) to
3 4
N /MIL-101(Fe) composites.
further veri-
6
fying the successful integration of g-C
construct the g-C
3 4
N
3
4
generation of active free radicals for BPA degradation. Addi-
tionally, it is noteworthy that the content ratio of g-C N has an
Fig. 6a and b display the optical absorption spectra and the
corresponding calculated bandgaps of the samples. Both g-C N
3
4
3
4
evident impact on the BPA degradation effect of the samples. As
depicted in Fig. 8, upon increasing the content of g-C
3
N
4
from
0.5% to 2%, the BPA degradation efficiency increases gradually
and reaches the highest value of 98% for the 2% g-C
3
N
4
/MIL-
101(Fe) composite. However, further increasing the proportion
of g-C N content from 2% to 10% leads to a deterioration of the
3
4
photocatalytic performance of the composite. This may be due
to the fact that the excess amount of g-C N loaded on the MIL-
3
4
101(Fe) shielded the light absorption of MIL-101(Fe), resulting
in the decreased amount of photoinduced electron–hole pairs
in the MIL-101(Fe) which could participate in reactions.
Furthermore, the reaction kinetics analysis shown in Fig. 9
indicates the pseudo-rst-order kinetics behavior of this
3 4
Fig. 5 High-resolution XPS of (a) C 1s and (b) N 1s spectra of g-C N /
MIL-101(Fe).
Fig. 6 (a) UV-Vis diffuse reflectance spectra of g-C
N
3 4
3 4
NS and g-C N /
Fig. 7 Photocatalytic activities in BPA degradation over different
MIL-101(Fe) photocatalysts and (b) the corresponding Tauc plot.
samples under visible light irradiation.
23706 | J. Mater. Chem. A, 2018, 6, 23703–23711
This journal is © The Royal Society of Chemistry 2018

View Article Online
Paper
Journal of Materials Chemistry A
À1
À1
catalyst dosage from 0.1 g L
to 0.5 g L . However, the
degradation of BPA decreases with further increasing the cata-
À1
À1
lyst dosage from 0.5 g L to 1 g L . The reduced activity may
arise from the shielding of light absorption by the excessive
dosage, which results in less photoinduced charge carrier
generation for participating in the surface reaction.
2
8
À
À
42À
À
8
O c
S
2
O
+ SO
4
c
/ SO
+ S
2
(1)
Radical quenching experiments were rst carried out to
identify the contribution of active species involved in the g-
C N /MIL-101(Fe)/Vis/PS system. EDTA-2Na and BQ were used
3 4
3 4
Fig. 8 Photocatalytic activities in BPA degradation over g-C N /MIL-
1
3 4
01(Fe) with different contents of g-C N under visible light irradiation.
+
À
as scavengers to quench the h and cO
2
species, respectively.
À
While the cOH and SO
4
c
can be captured by MeOH due to
similar reactivity rates. To differentiate the different contri-
33
À
bution of cOH and SO c , TBA was employed to scavenge the
4
À
34–36
targeted cOH instead of SO c .
Fig. 10a and b show the
4
inuence of different scavengers on the BPA degradation effi-
ciency. Obviously, the degradation of BPA decreased markedly
À
with the addition of BQ, suggesting the dominant role of cO
2
species in the photocatalytic reaction. In the presence of
EDTA-2Na, a moderate decrease of the BPA degradation effi-
+
ciency can be observed, indicating that h are also responsible
for BPA degradation in this reaction system. Note that the
Fig. 9 (a) Comparison of kinetic constants of BPA degradation with
different systems; (b) the calculated corresponding k of different
systems.
inhibition of performance with the addition of MeOH is obvi-
À
ously higher than that with TBA, implying that the SO
responsible for the BPA degradation.
4
c
is
Furthermore, an ESR technique was employed to charac-
terize the active radicals in the photocatalytic reaction systems.
As illustrated in Fig. 11a, the ESR signals corresponding to
DMPO–cOH adducts with relative intensities of 1 : 2 : 2 : 1 and
reaction, and the apparent rate constant (k) achieved with the
À1
optimal 2% g-C N /MIL-101(Fe)/Vis/PS system is 0.0589 min
,
3
4
which is nearly 8.9 times higher than that with the g-C N /Vis/
3
4
À1
PS system (0.0066 min ). As shown in Fig. S1,† the BPA TOC
removal rate is about 82% in the g-C /MIL-101(Fe)/Vis/PS
system aer 60 min reaction, which is remarkably higher than
that of the g-C /Vis/PS system. This result indicates the
excellent mineralization capability of the composite. In addi-
tion, the g-C /MIL-101(Fe)/Vis/PS system exhibits the most
À
DMPO–SO4c adducts can be clearly observed for the g-C N /
3
4
3 4
N
MIL-101(Fe)/Vis/PS and g-C N /MIL-101(Fe)/PS systems, veri-
3
4
À
4
c radicals in these reaction
fying the existence of cOH and SO
systems. Note that the intensity of the cOH signal for the g-C
MIL-101(Fe)/Vis/PS system is much stronger than that of g-
/MIL-101(Fe)/PS, indicating that visible light irradiation
3 4
N
3 4
N /
3 4
N
3 4
C N
efficient PS decomposition activity (Fig. S2†), which further
veries its superior PS activation ability. The above results
demonstrate that the MIL-101(Fe) with excellent PS activation
triggers much more cOH radical production. The produced cOH
À
radicals may originate from the reaction between SO
4
c
and
À
7,33
À
H
O/OH .
2
To highlight the vital role of MIL-101(Fe) in PS
ability can promote SO
4
c
production to remarkably improve
/MIL-101(Fe).
The impacts of PS concentration, BPA concentration and
catalyst dosage on the degradation of BPA over the g-C
activation in this reaction, the ESR signals of g-C N /
3
4
3 4
the photocatalytic performance of g-C N
MIL-101(Fe)/Vis, MIL-101(Fe)/Vis, MIL-101(Fe)/Vis/PS, and
g-C /Vis/PS systems were also investigated. Both g-C /MIL-
01(Fe)/Vis and g-C /Vis/PS systems show a negligible cOH
3
N
4
3 4
N
3
N
4
/
1
3 4
N
MIL-101(Fe) composite were investigated and are shown in
Fig. S3–S5.† An increased degradation efficiency is observed
with increasing the PS concentration from 0.1 to 1.0 mM.
However, further increasing the PS concentration results in
a slightly decreased activity (Fig. S3†). This is possibly due to the
excess PS under high concentration conditions, which can react
À À
31,32
with SO c to produce the less active S O c through eqn (1).
4 2 8
The degradation efficiency decreases upon increasing the BPA
concentration (Fig. S4†). This can be explained by the fact that
the amount of generated active species involved in the reaction
is not enough to decompose the large amount of BPA. In
addition, the catalyst dosage greatly affects the catalytic activity
Fig. 10 (a) Reactive species trapping experiments of g-C
3 4
N /MIL-
3 4
of the g-C N /MIL-101(Fe) composite as shown in Fig. S5.† The
1
01(Fe) with PS under visible light irradiation; (b) the corresponding
degradation efficiency increases gradually with increasing the reaction constants.
This journal is © The Royal Society of Chemistry 2018
J. Mater. Chem. A, 2018, 6, 23703–23711 | 23707

View Article Online
Journal of Materials Chemistry A
Paper
Fig. 12 Repeat experiments with g-C
N
3 4
/MIL-101(Fe) for BPA
degradation.
heterojunction formed between MIL-101(Fe) and g-C
explore the relative band alignment of g-C and MIL-101(Fe),
the band positions were rst estimated by the Mott–Schottky
M–S) and DRS measurement. As illustrated in Fig. 13a, the
3 4
N . To
3 4
N
(
positive slope observed in the M–S curve indicates the n-type
nature of g-C N . The at-band position derived from the M–S
3
4
plot is determined to be À1.1 V versus SCE. Because the CB edge
potential of an n-type semiconductor is close to the at-band
37–39
potential,
À1.1 V. Combining the band gap (E
3 4
Fig. 6b), the valence band (VB) edge potential of g-C N is
the CB potential of g-C
3 4
N is estimated to be
2À
Fig. 11 ESR signals of (a) DMPO–cOH and (b) DMPO–cO in various
g
) value of 2.8 eV for g-C N
3 4
photocatalytic systems.
(
calculated to be 1.7 V. For the MIL-101(Fe), the estimated value
of at-band potential is À0.7 V versus SCE, which corresponds
to a CB of À0.7 V. Taking into account the band gap value of
signal. This result evidently suggests that the MIL-101(Fe) plays
an important role in PS activation and the formed cOH radicals
2
.6 eV, the VB potential is determined to be 1.9 V. As depicted in
À
stem from the SO
4
c . As expected, compared with the MIL-
3 4
Fig. 14, upon integration of MIL-101(Fe) with g-C N to form
101(Fe)/Vis system, the distinguishable cOH signals observed in
the MIL-101(Fe)/Vis/PS system further demonstrate the excellent
PS activation ability of MIL-101(Fe). In addition, no DMPO–cOH
can be observed in the g-C
cating the poor PS activation ability of g-C
clearly demonstrated that the coupling of g-C N with MIL-
3 4 3 4
N /Vis and g-C N /PS systems, indi-
3 4
N . The above results
3
4
1
01(Fe) greatly facilitates the activation of PS, which is benecial
À
to yield SO
4
c
radicals. Meanwhile, as shown in Fig. 11a and b,
À
a distinct sextet characteristic signal of the DMPO–cO
2
adduct is
observed in the ESR spectrum of the g-C
system, demonstrating the generation of cO
tion. Note that the signal intensity of the g-C
3
N
4
/MIL-101(Fe)/Vis/PS
À
2
during the reac-
/MIL-101(Fe)/Vis/
3
N
4
3 4
Fig. 13 Mott–Schottky plots of (a) g-C N and (b) MIL-101(Fe).
PS system is obviously stronger than that of the g-C
3 4
N /Vis/PS
À
counterpart, implying the improved cO2 production ability of
the g-C N /MIL-101(Fe)/Vis/PS system.
3
4
3 4
Moreover, the stability of the g-C N /MIL-101(Fe) composite
was further evaluated through ve consecutive operations.
From Fig. 12, no noticeable degradation of activity is observed
3 4
in 5 repeat runs, implying that the g-C N /MIL-101(Fe)
composite is stable during the photocatalytic reactions.
Furthermore, no obvious structural variation is observed from
the XRD patterns (Fig. S6†) and the SEM image (Fig. S7†) of the
g-C N /MIL-101(Fe) composite aer ve cycles, which further
3
4
3 4
demonstrates the robust stability of g-C N /MIL-101(Fe).
3
.3 Mechanism of photocatalytic activity enhancement
The above results indicate that the remarkably improved pho-
tocatalytic activity of the composite may arise from the between g-C
Fig. 14 Schematic diagram of the band structure and charge transfer
3
N
4
and MIL-101(Fe).
23708 | J. Mater. Chem. A, 2018, 6, 23703–23711
This journal is © The Royal Society of Chemistry 2018

View Article Online
Paper
Journal of Materials Chemistry A
a heterojunction, the appropriate band alignment between g-
C N and MIL-101(Fe) satises well the thermodynamic
3
4
requirements for the charge carrier separation at the interface.
The photoinduced electrons and holes can transfer rapidly
across the interface and accumulate on the surface of
MIL-101(Fe) and g-C N , respectively, which results in the
3 4
enhancement of photogenerated charge carrier separation.
In order to elucidate the above charge transfer mechanism of
3 4
the g-C N /MIL-101(Fe) composite, photo-chronoamperometry
and electrochemical impedance spectroscopy (EIS) were carried
out. The transient photocurrent densities of g-C N , MIL-
Fig. 16 ESR spectra of MIL-101(Fe) under different conditions.
3
4
1
01(Fe), and g-C N /MIL-101(Fe) are compared and shown in
3 4
Fig. 15a. Apparently, g-C
higher transient current density than bare g-C
enhancement of transient photocurrent directly reects the
efficient charge carrier separation ability of g-C /MIL-101(Fe).
3
N
4
/MIL-101(Fe) presents a much
MIL-101(Fe). Moreover, the high-resolution XPS spectra of Fe 2p
3 4
N
. The observed
of the g-C
3 4
N /MIL-101(Fe) composite before and aer reaction
were also recorded and are depicted in Fig. 17. Note that the
3 4
N
2
+
existence of the peak corresponding to Fe in the g-C N /
3
4
Moreover, as observed from the Nyquist plot of the EIS spec-
trum shown in Fig. 15b, g-C N /MIL-101(Fe) exhibits a smaller
MIL-101(Fe) sample aer reaction further supports the above
3
4
17,41
ESR results.
semicircular radius than g-C N , which implies the lower
3
4
Moreover, in situ Raman spectroscopy was used to clarify the
charge-transfer resistance and more efficient interfacial charge
transfer in g-C /MIL-101(Fe). The above results evidently
demonstrate that the introduction of MIL-101(Fe) into g-C
interaction between MIL-101(Fe) and PS. As depicted in Fig. 18,
3 4
N
À1
the two bands centred at 835 and 1075 cm in the spectrum of
3 4
N
PS solution correspond to the S–O stretching and the symmetric
efficiently promotes the dissociation of photogenerated elec-
tron–hole pairs, which allows more photoinduced charge
carriers to participate in the surface catalytic redox reaction.
Apart from the efficient charge carrier separation ability of
the g-C N /MIL-101(Fe) composite, the subsequent surface
2À
42–44
S]O stretching vibration mode of S O , respectively.
In
2
8
the case of MIL-101(Fe), the four bands observed in the Raman
45,46
spectrum coincide with those of previous reports.
Obviously,
both the peaks assigned to PS and MIL-101(Fe) can be observed
in the spectrum of the MIL-101(Fe)/PS system, which gives
direct evidence of the interaction between MIL-101(Fe) and PS.
3
4
catalytic redox reaction also plays a vital role in the efficient
photocatalytic activity. From the aforementioned experimental
results, the introduction of MIL-101(Fe) in the composite greatly
accelerates the PS activation. To gain further insights into the
reactive centers of MIL-101(Fe), the ESR tests were rst per-
formed. As shown in Fig. 16, a typical ESR signal is observed in
3
+
the dark condition, which could be ascribed to Fe in octahe-
19,40
dral FeO of the MIL-101(Fe).
Upon visible light irradiation,
6
the above signal is almost totally quenched, which should result
2
+
2À
3+
from the formation of Fe by a charge transfer from O to Fe
in the photoexcited MIL-101(Fe). Notably, the ESR signal of Fe
3
+
can be regenerated again by further addition of PS into the
2
+
above system, demonstrating that the formed Fe is involved in
the activation of PS. The ESR results evidently suggest that the
3 4
Fig. 17 The high-resolution XPS spectra of Fe 2p of g-C N /MIL-
metal center (Fe) is the active site for the PS activation over 101(Fe) before and after reaction.
Fig. 15 (a) Transient photocurrent density of g-C
3
N4, MIL-101(Fe) and
g-C /MIL-101(Fe) samples with light on/off cycles under visible light
3 4
N
irradiation; (b) EIS Nyquist plots of g-C
MIL-101(Fe).
3 4
N , MIL-101(Fe) and g-C
N
3 4
/
Fig. 18 Raman spectra of PS solution, MIL-101(Fe), and MIL-101(Fe) in
PS solution.
This journal is © The Royal Society of Chemistry 2018
J. Mater. Chem. A, 2018, 6, 23703–23711 | 23709

View Article Online
Journal of Materials Chemistry A
Paper
photocatalytic activity for BPA degradation in the presence of
PS. The apparent rate constant k achieved with the optimized
À1
2
8
1
% g-C N /MIL-101(Fe) sample is 0.0589 min , which is about
3
4
.9 times higher than that with g-C N . The introduction of MIL-
01(Fe) into g-C N not only facilitates interfacial charge carrier
3 4
3
4
separation by the formed heterojunction with appropriate band
alignment but also greatly accelerates the PS activation for
À
SO
4
c
production, thus resulting in the superior activity for
photocatalytic BPA decomposition. This work provides a prom-
ising strategy for designing other heterostructure hybrids with
outstanding photocatalytic performance for pollutant removal.
Fig. 19 Proposed photocatalytic mechanism of g-C
N
3 4
/MIL-101(Fe).
À1
2À
More importantly, a new peak at 980 cm assignable to SO4
Conflicts of interest
2À
is observed in this spectrum, indicating the formation of SO
during the reaction. The formed SO
might arise from the There are no conicts to declare.
catalytic decomposition of PS by MIL-101(Fe) through eqn (2)
Note that the peak intensity of SO
increases as the reaction time prolongs, suggesting more SO
4
2À
4
47,48
2À
and (3).
4
progressively
2À
Acknowledgements
4
production and PS decomposition during the reaction. In
addition, the aforementioned result of the decomposition rate
of PS shown in Fig. S2† reveals that nearly 89% of PS decom-
posed in the MIL-101(Fe)/PS system, which further supports
this explanation and manifests the energetically favorable
interaction between MIL-101(Fe) and PS.
This work was nancially supported by the National Natural
Science Foundation of China (21777106), the National Major
Science and Technology Program for Water Pollution Control
and Treatment (2017ZX07202).
Notes and references
2
8
À
À
8
Fe(III) + S O
/ Fe(II) + S O c
(2)
(3)
2
2
1
D. B. Miklos, C. Remy, M. Jekel, K. G. Linden, J. E. Drewes
u¨ bner, Water Res., 2018, 139, 118–131.
2
8
À
À
42À
+ SO
Fe(II) + S
2
O
/ Fe(III) + SO
4
c
and U. H
U. I. Gaya and A. H. Abdullah, J. Photochem. Photobiol., C,
008, 9, 1–12.
2
2
According to the above discussion, a tentative mechanism
for the outstanding photocatalytic activity of g-C N /MIL-
3 R. Andreozzi, V. Caprio, A. Insola and R. Marotta, Catal.
Today, 1999, 53, 51–59.
3
4
1
01(Fe) with PS is proposed and illustrated in Fig. 19. Beneting
from the well matched band structure and intimate contact
between g-C and MIL-101(Fe), the photogenerated electrons
in the CB of g-C could transfer to the CB of MIL-101(Fe), and
the photogenerated holes would tend to migrate from the VB of
MIL-101(Fe) to the VB of g-C at the interface, resulting in the
4 S. Wacławek, H. V. Lutze, K. Gr u¨ bel, V. V. T. Padil, M. Cˇ ern ´ı k
and D. D. Dionysiou, Chem. Eng. J., 2017, 330, 44–62.
5 R. Y. Xiao, Z. H. Luo, Z. S. Wei, S. Luo, R. Spinney, W. C. Yang
and D. D. Dionysiou, Curr. Opin. Chem. Eng., 2018, 19, 51–58.
6 J. L. Wang and S. Z. Wang, Chem. Eng. J., 2018, 334, 1502–
1517.
3 4
N
3 4
N
3 4
N
improved charge carrier separation. On one hand, the accu-
mulation of photo-induced electrons on the surface of MIL-
7 Y. B. Wang, X. Zhao, D. Cao, Y. Wang and Y. F. Zhu, Appl.
Catal., B, 2017, 211, 79–88.
1
01(Fe) with plentiful active sites accelerates the activation of
8 H. J. Dong, M. Y. Wei, J. Li, J. Fang, L. Gao, X. X. Li and
A. H. Xu, RSC Adv., 2016, 6, 70747–70755.
À
the adsorbed PS for the production of SO4c radicals. On the
other hand, the photo-induced electrons on the surface of g-
9 M. Y. Wei, L. Gao, J. Li, J. Fang, W. X. Cai, X. X. Li and
A. H. Xu, J. Hazard. Mater., 2016, 316, 60–68.
À
C
3
N
4
can reduce the adsorbed O
2
to yield cO
2
radicals. These
formed active radicals with strong oxidative capability can 10 Y. F. Tao, Q. Ni, M. Y. Wei, D. S. Xia, X. X. Li and A. H. Xu,
decompose BPA rapidly. Meanwhile, the holes accumulated on
the surface of g-C can oxidize BPA. As a result, the g-C
MIL-101(Fe) composite presented outstanding photocatalytic
performance for BPA degradation with PS.
RSC Adv., 2015, 5, 44128–44136.
3
N
4
3
N
4
/
11 J. Lee, O. K. Farha, J. Roberts, K. A. Scheidt, S. T. Nguyen and
J. T. Hupp, Chem. Soc. Rev., 2009, 38, 1450–1459.
12 A. Dhakshinamoorthy, A. M. Asiri and H. Garc ´ı a, Angew.
Chem., Int. Ed., 2016, 55, 5414–5445.
1
3 Y. Li, H. Xu, S. X. Ouyang and J. H. Ye, Phys. Chem. Chem.
Phys., 2016, 18, 7563–7572.
4
. Conclusion
In summary, a heterostructured g-C
was fabricated through a facile hydrothermal method. By virtue
of the advantages of increased light absorption capacity, 15 K. G. M. Laurier, F. Vermoortele, R. Ameloot, D. E. De Vos,
3
N
4
/MIL-101(Fe) composite 14 B. Li, H. M. Wen, Y. J. Cui, W. Zhou, G. D. Qian and
B. L. Chen, Adv. Mater., 2016, 28, 8819–8860.
efficient charge carrier separation, and abundant surface
active sites, the composite exhibited remarkably improved
J. Hoens and M. B. J. Roeffaers, J. Am. Chem. Soc., 2013,
135, 14488–14491.
23710 | J. Mater. Chem. A, 2018, 6, 23703–23711
This journal is © The Royal Society of Chemistry 2018

View Article Online
Paper
Journal of Materials Chemistry A
16 Y. H. Fu, D. R. Sun, Y. J. Chen, R. K. Huang, Z. X. Ding, 32 T. A. Bruton and D. L. Sedlak, Chemosphere, 2018, 206, 457–
X. Z. Fu and Z. H. Li, Angew. Chem., Int. Ed., 2012, 51,
364–3367.
7 C. Gao, S. Chen, X. Quan, H. T. Yu and Y. B. Zhang, J. Catal.,
017, 356, 125–132.
8 X. H. Li, W. L. Guo, Z. H. Liu, R. Q. Wang and H. Liu, Appl.
Surf. Sci., 2016, 369, 130–136.
464.
3
33 T. Zeng, X. L. Zhang, S. H. Wang, H. Y. Niu and Y. Q. Cai,
Environ. Sci. Technol., 2015, 49, 2350–2357.
34 X. G. Duan, H. Q. Sun, Y. X. Wang, J. Kang and S. B. Wang,
ACS Catal., 2015, 5, 553–559.
35 G. L. Wang, S. Chen, X. Quan, H. T. Yu and Y. B. Zhang,
Carbon, 2017, 115, 730–739.
36 G. S. Liu, S. J. You, Y. Tan and N. Q. Ren, Environ. Sci.
Technol., 2017, 51, 2339–2346.
37 S. W. Cao, B. J. Shen, T. Tong, J. W. Fu and J. G. Yu, Adv.
Funct. Mater., 2018, 28, 1800136.
38 Y. Gong, X. Quan, H. T. Yu and S. Chen, Appl. Catal., B, 2017,
219, 439–449.
39 Z. F. Jiang, W. M. Wan, H. M. Li, S. Q. Yuan, H. J. Zhao and
P. K. Wong, Adv. Mater., 2018, 30, 1706108.
40 M. Nishikawa, Y. Mitani and Y. Nosaka, J. Phys. Chem. C,
2012, 116, 14900–14907.
41 D. Wilson and M. A. Langell, Appl. Surf. Sci., 2014, 303, 6–
13.
42 C. Q. Yang and X. H. Gu, J. Appl. Polym. Sci., 2001, 81, 223–
228.
43 T. Zhang, Y. Chen, Y. R. Wang, J. L. Roux, Y. Yang and
J.-P. Crou ´e , Environ. Sci. Technol., 2014, 48, 5868–5875.
44 H. H. Eysel and G. Rosinger, J. Raman Spectrosc., 1987, 18,
123–127.
45 M. J. Duan, Z. Y. Guan, Y. W. Ma, J. Q. Wan, Y. Wang and
Y. F. Qu, Chem. Pap., 2018, 72, 235–250.
46 L. Nirumanda, S. Farhadia, A. Zabardastia and A. Khataeeb,
Ultrason. Sonochem., 2018, 42, 647–658.
47 X. H. Li, W. L. Guo, Z. H. Liu, R. Q. Wang and H. Liu, J.
Hazard. Mater., 2017, 324, 665–672.
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
2
9 D. K. Wang, M. T. Wang and Z. H. Li, ACS Catal., 2015, 5,
6852–6857.
0 X. H. Li, W. L. Guo, Z. H. Liu, R. Q. Wang and H. Liu, J.
Hazard. Mater., 2017, 324, 665–672.
1 L. Chi, Q. Xu, X. Y. Liang, J. D. Wang and X. T. Su, Small,
2016, 12, 1351–1358.
2 J. D. Xiao, L. L. Han, J. Luo, S. H. Yu and H. L. Jiang, Angew.
Chem., Int. Ed., 2018, 130, 1115–1119.
3 X. Y. Li, Y. H. Pi, L. Q. Wu, Q. B. Xia, J. L. Wu, Z. Li and J. Xiao,
Appl. Catal., B, 2017, 202, 653–663.
4 J. D. Hong, C. P. Chen, F. E. Bedoy, G. H. Kelsall, D. O'Hare
and C. Petit, Catal. Sci. Technol., 2016, 6, 5042–5051.
5 L. Shi, T. Wang, H. B. Zhang, K. Chang and J. H. Ye, Adv.
Funct. Mater., 2015, 25, 5360–5367.
6 W. Y. Huang, N. Liu, X. D. Zhang, M. H. Wu and L. Tang,
Appl. Surf. Sci., 2017, 425, 107–116.
7 C. J. Liang, C. F. Huang, N. Mohanty and R. M. Kurakalva,
Chemosphere, 2008, 73, 1540–1543.
8 K. M. L. Taylor-Pashow, J. D. Rocca, Z. G. Xie, S. Tran and
W. B. Lin, J. Am. Chem. Soc., 2009, 131, 14261–14263.
9 D. M. Chen, K. W. Wang, W. Z. Hong, R. L. Zong, W. Q. Yao
and Y. F. Zhu, Appl. Catal., B, 2015, 166–167, 366–373.
0 M. Y. Ye, Z. H. Zhao, Z. F. Hu, L. Q. Liu, H. M. Ji, Z. R. Shen
and T. Y. Ma, Angew. Chem., Int. Ed., 2017, 29, 8407–8411.
1 Y. J. Qian, X. Guo, Y. L. Zhang, Y. Peng, P. Z. Sun,
C. H. Huang, J. F. Niu, X. F. Zhou and J. C. Crittenden, 48 C. J. Liang and Y. Y. Guo, Environ. Sci. Technol., 2010, 44,
Environ. Sci. Technol., 2016, 50, 772–781. 8203–8208.
This journal is © The Royal Society of Chemistry 2018
J. Mater. Chem. A, 2018, 6, 23703–23711 | 23711