Enhanced photocatalytic degradation of dimethyl phthalate by magnetic dual Z-scheme iron oxide/mpg-C3N4/BiOBr/polythiophene heterostructure photocatalyst under visible light

Article abstract of DOI:10.1016/j.molliq.2021.116947

A new magnetic dual Z-scheme heterostructure photocatalyst, consisting of mesoporous graphitic carbon nitride (mpg-C₃N₄), BiOBr, polythiophene (PTh), and magnetic iron oxide, was developed, and its structural, optical, and electrochemical properties were analyzed. The photocatalyst was employed for the degradation of dimethyl phthalate (DMP) in water under visible light to evaluate its photocatalytic activity. The results revealed that the photocatalytic activity of the new heterostructure photocatalyst was superior to that of its single-component counterparts. The reaction kinetics of photocatalytic DMP degradation followed pseudo first-order kinetics and the reaction rate constant was 3.7- and 4.5-fold higher than those of pristine g-C₃N₄ and BiOBr, respectively. The enhanced photocatalytic performance was chiefly attributed to the dual Z-scheme heterostructure generated between mpg-C₃N₄, PTh, and BiOBr, which effectively hindered the recombination of photogenerated electron and hole pairs in mpg-C₃N₄ and BiOBr, improving the photogenerated carrier transfer efficiency. Radical trapping test results indicated that the active species, h⁺, ·O₂^–, and ·OH, coexisted during the photocatalytic reaction process, and that h⁺ played a major role in the destruction of DMP molecules. DMP was degraded into intermediate small-molecule products, such as acids and alcohols, which were ultimately mineralized to CO₂ and H₂O. The magnetic property of the new heterostructure photocatalyst enabled its straightforward separation from water using magnetic separation technology for further reuse. This study provides a new material and method for the effective removal of DMP from aqueous solution.

Full text of DOI:10.1016/j.molliq.2021.116947

Journal of Molecular Liquids xxx (xxxx) xxx
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Enhanced photocatalytic degradation of dimethyl phthalate by magnetic
dual Z-scheme iron oxide/mpg-C3N4/BiOBr/polythiophene
heterostructure photocatalyst under visible light
⇑
Shaoxiu Li , Chan Lai, Changhui Li, Jintang Zhong, Zhifeng He, Qiuyi Peng, Xi Liu, Bohan Ke
School of Civil and Transportation Engineering, Guangdong University of Technology, No100, Waihuan Xi Guangzhou Higher Education Mega Center, Panyu District,
Guangzhou 510006, Guangdong, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new magnetic dual Z-scheme heterostructure photocatalyst, consisting of mesoporous graphitic carbon
nitride (mpg-C3N4), BiOBr, polythiophene (PTh), and magnetic iron oxide, was developed, and its struc-
tural, optical, and electrochemical properties were analyzed. The photocatalyst was employed for the
degradation of dimethyl phthalate (DMP) in water under visible light to evaluate its photocatalytic activ-
ity. The results revealed that the photocatalytic activity of the new heterostructure photocatalyst was
superior to that of its single-component counterparts. The reaction kinetics of photocatalytic DMP degra-
dation followed pseudo first-order kinetics and the reaction rate constant was 3.7- and 4.5-fold higher
than those of pristine g-C3N4 and BiOBr, respectively. The enhanced photocatalytic performance was
chiefly attributed to the dual Z-scheme heterostructure generated between mpg-C3N4, PTh, and BiOBr,
which effectively hindered the recombination of photogenerated electron and hole pairs in mpg-C3N4
and BiOBr, improving the photogenerated carrier transfer efficiency. Radical trapping test results indi-
cated that the active species, h+, ꢀO2–, and ꢀOH, coexisted during the photocatalytic reaction process,
and that h+ played a major role in the destruction of DMP molecules. DMP was degraded into interme-
diate small-molecule products, such as acids and alcohols, which were ultimately mineralized to CO2
and H2O. The magnetic property of the new heterostructure photocatalyst enabled its straightforward
separation from water using magnetic separation technology for further reuse. This study provides a
new material and method for the effective removal of DMP from aqueous solution.
Received 25 February 2021
Revised 30 June 2021
Accepted 5 July 2021
Available online xxxx
Keywords:
Photocatalysis
g-C3N4
BiOBr
Z-scheme heterostructure
Dimethyl phthalate
Polythiophene
Ó 2021 Elsevier B.V. All rights reserved.
1. Introduction
eral advantages, such as high chemical stability, a suitable band
gap, and favorable optical properties [10–14]. Among the
Dimethyl phthalate (DMP) is a plasticizer widely used in the
manufacture of plastic packaging, toys, decorative materials, and
various household necessities. It is also an endocrine disruptor
with carcinogenic, teratogenic, and mutagenic properties, and is
therefore detrimental to human health [1]. DMP has been detected
in the atmosphere, water, soil, and even in biological samples.
Moreover, it is not readily biodegradable and its removal via con-
ventional water treatment is challenging.
Photocatalytic technology has been shown to be effective for
the degradation of organic pollutants [2–6]. In this arena, the con-
ventional TiO2 photocatalyst is gradually being replaced by new
photocatalysts as TiO2 is active only under ultraviolet light [7–9].
Semiconductor materials, bismuth oxyhalides (BiOX, X = Cl, Br,
I), have been extensively investigated as photocatalysts due to sev-
bismuth-based semiconductors, bismuth oxybromide (BiOBr) is a
typical photocatalyst with a unique layered structure consisting
of [Bi2O2]2+ layers interleaved by halogen ions. The narrow band
gap energy (2.54–2.91 eV) and stable photocatalytic activity of
BiOBr [15–18] has prompted considerable research into its applica-
bility for the photocatalytic degradation of organic pollutants and
hydrogen generation via water splitting under visible light [19–
23]. However, the prompt recombination of photogenerated
electron-hole pairs reduces its photocatalytic performance. There-
fore, numerous approaches for improving charge carrier migration
have been developed to date. Among them, the construction of a
heterojunction between BiOBr and other materials has been exten-
sively studied [24–29].
In recent years, the non-metallic graphitic carbon nitride (g-
C3N4) photocatalyst has been investigated as an alternative to the
conventional TiO2 photocatalyst for the degradation of organic pol-
lutants and hydrogen production due to important advantages,
⇑
Corresponding author.
E-mail address: water@gdut.edu.cn (S. Li).
https://doi.org/10.1016/j.molliq.2021.116947
0167-7322/Ó 2021 Elsevier B.V. All rights reserved.
Please cite this article as: S. Li, C. Lai, C. Li et al., Enhanced photocatalytic degradation of dimethyl phthalate by magnetic dual Z-scheme iron oxide/mpg-
C3N4/BiOBr/polythiophene heterostructure photocatalyst under visible light, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2021.116947

S. Li, C. Lai, C. Li et al.
Journal of Molecular Liquids xxx (xxxx) xxx
such as responsiveness to visible light, a narrow band gap, favor-
able thermal stability, and cost-efficient and straightforward
preparation [30–34]. However, the photocatalytic activity of pris-
tine g-C3N4 is restricted by its small specific surface area (SSA)
and low separation efficiency of photogenerated electron-hole
pairs. Furthermore, its narrow absorption range in the visible
region results in the inadequate utilization of visible light. Thus,
a variety of methods, such as surface modification, precious metal
doping, and heterojunction construction, have been implemented
to improve the photocatalytic activity of g-C3N4 [35–40]. Meso-
porous graphitic carbon nitride (mpg-C3N4) is often prepared via
templating and acid etching of g-C3N4 to enlarge its SSA [41,42].
The combination of BiOBr and g-C3N4 is considered to be a favor-
able heterojunction as these two materials have a well-matched
band, which promotes charge carrier transfer [43–48].
a muffle furnace for 4 h at a heating rate of 2.2 °C/min. After cool-
ing to room temperature, a yellow powder was obtained, which
was then ground, sieved, and stirred in 4 M NH4HF2 at room tem-
perature for 48 h to remove SiO2. Next, the treated powder was
washed with distilled water followed by ethanol and dried at
70 °C overnight to obtain templated g-C3N4.
A certain amount of templated g-C3N4 was stirred for 30 min in
40 ml of 0.5 M nitric acid. Subsequently, the mixture was kept in a
reaction kettle at 160℃ for 6 h. After cooling to room temperature,
the resulting powder was successively washed with distilled water
and ethanol, and dried at 70 °C overnight to afford the acid-etched
product, mpg-C3N4.
2.3. Magnetization of mpg-C3N4
On the other hand, although nanoparticle photocatalysts are
efficient and prolific, their separation from water is challenging,
limiting their practical application. This drawback can be
addressed by magnetizing the nanoparticles. Magnetic nanoparti-
cles can be separated from water using magnetic separation tech-
nology, and reused [49–53]. This approach enables the use of
nanoparticles for water treatment.
In this study, mpg-C3N4 was prepared and subsequently magne-
tized to generate magnetic mpg-C3N4 (MꢁmpgꢁC3N4), which was
used to generate a composite with BiOBr. To expand the absorption
range of visible light and further improve the photocatalytic activ-
ity, polythiophene (PTh), a conductive polymer exhibiting an excel-
lent capacitance performance, good conductivity, and light
absorption over the full wavelength band [54] was doped into
the MꢁmpgꢁC3N4/BiOBr composite. Thus, the MꢁmpgꢁC3N4/
BiOBr/PTh photocatalyst was prepared, and was then used to
degrade DMP under visible light. In this study, MꢁmpgꢁC3N4/
BiOBr/PTh was characterized, and its photocatalytic performance
for the degradation of DMP was investigated. Additionally, the
photocatalytic degradation mechanism was discussed. This study
provides an effective material and method for the removal of
DMP from water.
mpg-C3N4, (NH4)2Fe(SO4)2ꢀ6H2O and NH4Fe(SO4)2ꢀ12H2O were
prepared in a 2.5:1 mass ratio of mpg-C3N4 to theoretically gener-
ated Fe3O4, and a 1:1.43 molar ratio of Fe2+ to Fe3+. NH4Fe(SO4)2-
ꢀ12H2O was dissolved in 75 ml of distilled water and heated to
60℃ in a constant temperature water bath. Meanwhile, mpg-
C3N4 was dispersed in 125 ml distilled water via ultrasonic treat-
ment for 15 min, followed by heating and stirring. When the
mpg-C3N4 suspension reached 60℃, (NH4)2Fe(SO4)2ꢀ6H2O and the
heated NH4Fe(SO4)2ꢀ12H2O solution were added to the mpg-C3N4
suspension. An alkaline solution comprising Na2CO3 and NaOH in
a molar ratio of 5:3 was slowly dropped into the mpg-C3N4
suspension until the pH was above 11.0 and stirring was continued
at 60 ℃ for 30 min, followed by aging at the same temperature for
30 min. Finally, the product was washed several times with dis-
tilled water to remove the bases, dried at 70 °C for 8 h, and then
ground and sieved to afford magnetic mpg-C3N4 powder, denoted
as MꢁmpgꢁC3N4.
2.4. Preparation of MꢁmpgꢁC3N4/BiOBr/PTh composite material
The MꢁmpgꢁC3N4, Bi(NO3)3ꢀ5H2O, and PTh dispersion was
weighed with a 1:5 mass ratio of MꢁmpgꢁC3N4 to theoretically
produced BiOBr, the PTh dispersion accounted for 0.25% of the total
weight of MꢁmpgꢁC3N4/BiOBr/PTh. The ratio of the MꢁmpgꢁC3-
N4, Bi(NO3)3ꢀ5H2O, and PTh was determinated by single factor
experiments and orthogonal experiment. The Bi(NO3)3ꢀ5H2O was
completely dissolved in 20 ml of ethylene glycol, followed by the
addition of the PTh dispersion and stirring for 15 min. MꢁmpgꢁC3-
N4 was then added, followed by stirring for 15 min and ultrasoni-
cation for another 15 min to obtain solution A. KBr was weighed
according to the stoichiometric relationship between Bi(NO3)3-
ꢀ5H2O and KBr in the reaction, dissolved in 20 ml of distilled water
and slowly dropped into solution A, which was subsequently stir-
red at 400 r/min for 30 min at room temperature, and then kept in
a reaction kettle at 140℃ for 18 h. Finally, the product was washed
successively with distilled water and ethanol, and dried at 70 °C
overnight to obtain the MꢁmpgꢁC3N4/BiOBr/PTh composite.
2. Experimental section
2.1. Materials
Urea (purity ꢂ 99%), tertiary butyl alcohol, and Na2-EDTA were
sourced from Tianjin Fuchen Chemical Reagent Co., Ltd. SiO2 sol
(30% solid) was purchased from LUDOX, USA. NH4HF2 and 1, 4-
benzoquinone (purity ꢂ 97%) were provided by MACKLIN, Shang-
hai, China. Bi(NO3)3ꢀ5H2O was sourced from Tianjin Kemiou Chem-
ical Reagent Co., Ltd., KBr was purchased from Tianjin Baishi
Chemical Industry Co., Ltd., Dimethyl phthalate (purity ꢂ 99.5%)
was sourced from the Tianjin Damao Chemical Reagent Factory.
Ethanol was obtained from Tianjin Fuyu Chemical Co., Ltd.
Polythiophene was purchased from Guangdong Wengjiang
Reagent Co., Ltd. (NH4)2Fe(SO4)2ꢀ6H2O was obtained from Tianjin
Yongda Chemical Reagent Co., Ltd., and NH4Fe(SO4)2ꢀ12H2O from
Shanghai Aladdin Reagent Co., Ltd. The reagents used in the study
were of analytical grade, except where mentioned above.
2.5. Characterization
Field emission scanning electron microscopy and transmission
electron microscopy (SEM/TEM) measurements were conducted
using an SU8220 field emission scanning electron microscope
(Hitachi, Japan) and a Talos F200S transmission electron micro-
scope (FEI, Czech Republic), respectively. Energy dispersive X-ray
spectroscopy (EDS) was detected using energy dispersive X-ray
spectrometry (Oxford X-Max50, UK). X-ray diffraction (XRD) spec-
troscopy was conducted using a D8 Advance X-ray diffractometer
(Bruker, Germany). X-ray photoelectron spectra (XPS) were col-
lected using an Escalab 250Xi X-ray photoelectron spectrometer
(Thermo Fisher Scientific, USA). Fourier transform infrared (FTIR)
2.2. mpg-C3N4 preparation
SiO2 sol was added to urea in a 1:3 mass ratio, followed by the
addition of distilled water until dissolution of urea to obtain a
mixed SiO2 and urea solution. The mixed solution was stirred at
90 °C to remove the water and obtain a white solid, which was sub-
sequently ground into powder, placed in a crucible, completely
wrapped and sealed with aluminum foil, and heated at 550 °C in
2

S. Li, C. Lai, C. Li et al.
Journal of Molecular Liquids xxx (xxxx) xxx
(f)
(a)
(b)
(e)
(d)
(c)
(d)
(e)
(c)
(b)
(a)
(f)
10 20 30 40 50 60 70 80
4000 3500 3000 2500 2000 1500 1000 500
-1
Wavenumber(cm )
2 (degree)
Fig. 2. FTIR spectra of (a) g-C3N4, (b) mpg-C3N4, (c) MꢁmpgꢁC3N4/BiOBr, (d)
Fig. 1. XRD patterns of (a) g-C3N4, (b) mpg-C3N4, (c) MꢁmpgꢁC3N4/BiOBr, (d)
MꢁmpgꢁC3N4/BiOBr/PTh, (e) BiOBr and (f) PTh.
MꢁmpgꢁC3N4/BiOBr/PTh, (e) PTh and (f) BiOBr.
spectra were recorded on a Nicolet IS50 Fourier transform infrared
spectrometer (Thermo Fisher Scientific, USA). The SSA was ana-
lyzed using an ASAP 2460 specific surface area and pore analyzer
(Micromeritics, USA). Magnetism was measured using a 7404
vibrating-sample magnetometer (VSM) (LakeShore, USA). UV–vis
diffuse reflectance spectra (UV–vis DRS) were obtained using an
UV-3600 Plus ultraviolet–visible spectrophotometer (Hong Kong,
China). Photoluminescence (PL) was measured using a Fluorolog-
3 fluorescence spectrophotometer (HORIBA Instruments Incorpo-
rated, USA). Electrochemical impedance spectroscopy (EIS) was
conducted using a 1470E electrochemical workstation (AMETEK
GB, Ltd., UK). The degradation products of DMP were detected
using ultra performance liquid chromatography (UPLC) coupled
with Q Exactive orbitrap mass spectrometry (Thermo Fisher Scien-
tific, USA).
loaded onto the composites. Nevertheless, the characteristic peaks
of mpg-C3N4 were barely visible in the XRD pattern of MꢁmpgꢁC3-
N4/BiOBr/PTh due to the low amount of mpg-C3N4, as was previ-
ously reported [43]. Similarly, the characteristic peaks of PTh
were absent, which is in agreement with Mahmoudian’s findings
[56].
The FTIR spectra of g-C3N4, mpg-C3N4, MꢁmpgꢁC3N4/BiOBr,
and MꢁmpgꢁC3N4/BiOBr/PTh are presented in Fig. 2. Several char-
acteristic g-C3N4 peaks are visible in Fig. 2a; the peak at 813 cmꢁ1
was ascribed to the characteristic bending vibration of the triazine
ring [57], the peaks at 1240 and 1317 cmꢁ1 were attributed to the
stretching vibrations of the C—NH—C connecting unit. The peaks at
1410, 1462, and 1632 cmꢁ1 were assigned to the C—N heterocyclic
skeletal stretching vibrations of the aromatic rings in g-C3N4. The
FTIR spectrum of mpg-C3N4 (Fig. 2b) was identical to that of g-
C3N4, indicating that the structure of g-C3N4 remained unchanged
upon SiO2 templating and acid etching. In the spectrum depicted in
Fig. 2c, new bands appeared at approximately 519 and 578 cmꢁ1
and were attributed to Bi—O and Fe—O stretching vibrations,
respectively, confirming that the composite contained iron oxide
and BiOBr. In addition, the characteristic PTh peaks can be seen
in Fig. 2e; the peak at 1638 cmꢁ1 was attributed to the asymmetric
stretching vibration of C = C of thiophene ring [58], the bending
vibration peak of the C—S bond in thiophene ring appeared at
651 cmꢁ1. But the C—S characteristic peak was not present in the
spectrum of [58]Fig. 2d, which could be ascribed to the low
amount of PTh in the composite, the observation was consistent
with the literature [58].
2.6. Photocatalytic experiments
A certain amount of photocatalyst was added to 50 ml DMP
solution of a certain concentration. The mixture was shaken under
dark conditions for 30 min. Subsequently, photodegradation was
induced under illumination with a 500 W GXZ500 xenon lamp.
The solution was collected and filtered at regular intervals, and
the filtrate was analyzed using a LC-210 high-performance liquid
chromatography (HPLC) instrument (INESA, China).
3. Results and discussion
3.1. Characterization of materials
Fig. 3 illustrates the SEM and TEM images of g-C3N4 and the
other photocatalysts, as well as the EDS spectrum of MꢁmpgꢁC3-
N4/BiOBr/PTh. As shown in Fig. 3A, pure g-C3N4 presented a layered
graphite-like structure with interlayer stacking of the conjugated
carbocyclic plane. Fig. 3B shows the significantly etched surface
of mpg-C3N4 particles, which increase the SSA to provide more
active reaction sites on the particle surfaces. Fig. 3C revealed the
typical layered nanosheet structure of BiOBr dispersed on the sur-
face of mpg-C3N4 forming a heterojunction. Furthermore, as seen
in Fig. 3D, the degree of BiOBr stacking increased after adding
PTh to MꢁmpgꢁC3N4/BiOBr compared to that in MꢁmpgꢁC3N4/
BiOBr (Fig. 3C) and BiOBr (Fig. 3E). The SEM images indicated that
MꢁmpgꢁC3N4, BiOBr, and PTh were successfully combined using
the hydrothermal method to form a heterojunction. The TEM
image in Fig. 3G shows the presence of numerous holes shaped
3.1.1. Microstructure characterization
The XRD patterns of g-C3N4 and its modified materials are
shown in Fig. 1. In the XRD patterns of g-C3N4 and mpg-C3N4,
two obvious diffraction peaks were observed at 12.94 and 27.63°,
which were characteristic diffraction peaks of g-C3N4, indicating
that the mpg-C3N4 remained the component of g-C3N4. The XRD
patterns shown in Fig. 1c and 1d contained the characteristic peaks
of BiOBr at 10.87, 21.96, 25.18, 31.77, 32.16, 39.34, 44.84, 46.21,
50.10, 53.37, 56.30, 57.16, and 67.43°, conforming to the existence
of BiOBr in the composites of MꢁmpgꢁC3N4/BiOBr and
MꢁmpgꢁC3N4/BiOBr/PTh. The diffraction peaks at 2h = 35.73,
43.06, 53.37, and 57.16° were attributed to Fe3O4 and
[55], suggesting that the magnetic substances were successfully
c-Fe2O3
3

S. Li, C. Lai, C. Li et al.
Journal of Molecular Liquids xxx (xxxx) xxx
Fig. 3. SEM images of (A) g-C3N4, (B) mpg-C3N4, (C) MꢁmpgꢁC3N4/BiOBr, (D) MꢁmpgꢁC3N4/BiOBr/PTh and (E) BiOBr; TEM images of (F) g-C3N4, (G) mpg-C3N4, (H)
MꢁmpgꢁC3N4/BiOBr and (I) MꢁmpgꢁC3N4/BiOBr/PTh; (J) EDS spectrum of MꢁmpgꢁC3N4/BiOBr/PTh.
according to the SiO2 template, created upon template removal by
NH4HF2. BiOBr flakes were deposited and combined with mpg-
C3N4 to generate the heterojunction evident in Fig. 3H, and the
heterojunction of MꢁmpgꢁC3N4/BiOBr/PTh is depicted in Fig. 3I.
EDS spectrum of MꢁmpgꢁC3N4/BiOBr/PTh in Fig. 3J demonstrated
that MꢁmpgꢁC3N4/BiOBr/PTh contained elements of C, O, N, Bi, Br,
and Fe. S is the representative element in PTh composition, how-
ever, as PTh was present in a low amount in the MꢁmpgꢁC3N4/
BiOBr/PTh composite, the absence of S can be expected.
The XPS spectra of MꢁmpgꢁC3N4/BiOBr/PTh are displayed in
Fig. 4. It is evident from the MꢁmpgꢁC3N4/BiOBr/PTh survey spec-
trum (Fig. 4a) that the composite consisted primarily of C, O, N, Bi,
4

S. Li, C. Lai, C. Li et al.
Journal of Molecular Liquids xxx (xxxx) xxx
Br, and Fe elements. The C 1 s spectrum shown in Fig. 4b contained
three peaks at 284.59, 285.73, and 288.23 eV, which were attribu-
ted to C—C in graphitic carbon, sp2- hybridized C, and the N—C = N
group [43], respectively, suggesting the existence of g-C3N4 in the
MꢁmpgꢁC3N4/BiOBr/PTh composite. The N1s spectrum (Fig. 4c)
was deconvoluted into three distinct peaks located at 401.28,
Fig. 4. XPS spectra of MꢁmpgꢁC3N4/BiOBr/PTh, (a) survey spectrum , (b) C1s, (c) N1s, (d) Bi 4f, (e) O1s, (f) Br3d, (g) Fe2p and (h) S2s.
5

S. Li, C. Lai, C. Li et al.
Journal of Molecular Liquids xxx (xxxx) xxx
Fig. 4 (continued)
400, and 398.63 eV, as well as one weak peak at 404.87 eV; the
three distinct peaks were assigned to N—H, N—(C)3 and C—N = C
groups, respectively, while the weak peak may have arisen from
the electron localization of the heptazine ring [59]. The Bi 4f spec-
trum (Fig. 4d) displayed peaks at 164.15 and 158.85 eV, ascribed to
Bi 4f5/2 and Bi 4f7/2 of Bi3+ of the Bi–O bond, respectively. The XPS
spectrum of O 1 s contained two peaks at 529.63 and 531.49 eV
(Fig. 3e), assigned to the oxygen atoms in the Bi–O crystal lattice
and to those of surface hydroxyl groups [57], respectively. The Br
3d peaks at 67.92 and 68.96 eV (Fig. 3f) corresponded to Br 3d5/2
and Br 3d3/2, respectively. The Fe 2p spectrum (Fig. 3g) displayed
typical peaks at 711.5 and 723.68 eV, ascribed to Fe 2p3/2 and Fe
2p1/2 of oxidized iron species [60,61], respectively. Two peaks, at
228.34 and 231.77 eV, were observed in the S 2s spectrum, but
they were so inconspicuous that they were absent in the survey
spectrum. This is in accordance with the XRD, FTIR and EDS analy-
sis results.
Fig. 5 displays the nitrogen adsorption–desorption isotherms
and the pore size distribution of the photocatalysts. As shown in
Fig. 5A, type H3 hysteresis loops existed in the adsorption–desorp-
tion isotherms, it was caused by the flaky and granular particles or
particles with slit voids, indicating that the shapes of the photocat-
alysts was flaky and granular, this was in agreement with the
results of SEM and TEM analysis. In addition, it is evident that
MꢁmpgꢁC3N4/BiOBr/PTh is a mesoporous material seen from
Fig. 5B. The SSAs of g-C3N4, mpg-C3N4, and MꢁmpgꢁC3N4/BiOBr/
PTh, were determined to be 57.382, 90.129, and 21.391 m2/g,
respectively. The SSA of g-C3N4 increased after SiO2 templating
and acid etching, but decreased after it was combined with iron
oxide, BiOBr, and PTh. It is possible that BiOBr covered the surface
of mpg-C3N4 and iron oxide particles filled the holes of mpg-C3N4.
Nonetheless, the photocatalytic activity of MꢁmpgꢁC3N4/BiOBr/
PTh was superior to that of mpg-C3N4, principally as a result of
the curtailed recombination of photogenerated electron-hole pairs
and enhanced photogenerated carrier transfer efficiency in the
heterojunction of MꢁmpgꢁC3N4/BiOBr/PTh.
The ferromagnetic hysteresis loops of magnetic g-C3N4 and
MꢁmpgꢁC3N4/BiOBr/PTh are presented in Fig. 6. The large curva-
ture of the two hysteresis loops suggests that the two materials
can be readily magnetized and are strongly ferromagnetic, which
is beneficial for their separation from the aqueous phase and
recovery for recycling in practical applications. Moreover, the mag-
netization curve does not display remanence or coercivity, indicat-
ing that magnetic g-C3N4 and MꢁmpgꢁC3N4/BiOBr/PTh are
paramagnetic. The saturation magnetization of magnetic g-C3N4
and MꢁmpgꢁC3N4/BiOBr/PTh were 18.70 and 11.31 emu/g,
respectively. The finding that the former was higher than the latter
was attributed to the lower amount of magnetic iron oxides per
unit mass of MꢁmpgꢁC3N4/BiOBr/PTh composite compared to that
in magnetic g-C3N4. The magnetic measurement results verified
that magnetic iron oxides were incorporated into the MꢁmpgꢁC3-
N4/BiOBr/PTh composite.
6

S. Li, C. Lai, C. Li et al.
Journal of Molecular Liquids xxx (xxxx) xxx
Fig. 5. (A) Nitrogen adsorption–desorption isotherms and (B) the pore size distribution of photocatalysts.
to that of pristine g-C3N4, which is beneficial for enhancing the
photocatalytic performance.
The fluorescence spectra of g-C3N4, mpg-C3N4, MꢁmpgꢁC3N4/
BiOBr, and MꢁmpgꢁC3N4/BiOBr/PTh are depicted in Fig. 8A. Evi-
dently, the PL intensity decreased considerably after MꢁmpgꢁC3-
N4 was combined with BiOBr, and decreased slightly after the
introduction of PTh, indicating that the heterojunction structure
of MꢁmpgꢁC3N4/BiOBr/PTh improved photogenerated charge sep-
aration and inhibited the recombination of photogenerated
electron-hole pairs.
EIS analysis was used to illustrate the separation of photogener-
ated charges in the photocatalysts. The EIS measurement results of
g-C3N4, mpg-C3N4, MꢁmpgꢁC3N4/BiOBr, and MꢁmpgꢁC3N4/
BiOBr/PTh are shown in Fig. 8B, wherein the order of the photogen-
erated charge transmission impedance values was determined as
g-C3N4 > mpg-C3N4 > M-mpg-C3N4/BiOBr > M-mpg-C3N4/BiOBr/
PTh. Among them, MꢁmpgꢁC3N4/BiOBr/PTh presented the small-
est arc radius and the lowest charge transfer resistance, suggesting
that this composite will have the highest separation efficiency of
photogenerated charge carriers and the fastest interface charge
transfer of the four photocatalysts. This is consistent with the
results of fluorescence spectroscopy measurements.
Fig. 6. Ferromagnetic hysteresis loops of (a) magnetic g-C3N4 and (b) MꢁmpgꢁC3-
N4/BiOBr/PTh.
3.1.2. Optical properties
Fig. 7A illustrates the UV–vis DRS of g-C3N4, mpg-C3N4,
MꢁmpgꢁC3N4/BiOBr, and MꢁmpgꢁC3N4/BiOBr/PTh. Absorption
edges were observed at 412, 421, 427, and 443 nm for g-C3N4,
mpg-C3N4, MꢁmpgꢁC3N4/BiOBr, and MꢁmpgꢁC3N4/BiOBr/PTh,
respectively, which indicated that the visible light absorption
range broadened with the formation of the composites. According
3.2. Photocatalytic activity
The photocatalytic DMP degradation efficiency of the photocat-
alysts is illustrated in Fig. 9A. It is evident that the adsorptive DMP
removal rate of mpg-C3N4 was higher than that of pristine g-C3N4
after 30 min without light absorption, as a result of the increase in
the SSA of g-C3N4 after SiO2 templating and acid etching. On the
other hand, the photocatalytic DMP degradation efficiency of
MꢁmpgꢁC3N4/BiOBr was significantly higher than that of g-C3N4,
to the UV–Vis DRS and the Kubelka-Munk transformation [62], the
1/2
plot of (
a
ht)
vs. photon energy was constructed as shown in
Fig. 7B, and the band gap energy of g-C3N4, mpg-C3N4,
MꢁmpgꢁC3N4/BiOBr, and MꢁmpgꢁC3N4/BiOBr/PTh were esti-
mated as 2.72, 2.68, 2.55 eV, and 2.46 eV, respectively, indicating
that the band gap width in the composites was reduced compared
Fig. 7. UV–vis diffuse reflective spectra of (a) g-C3N4, (b) mpg-C3N4, (c) MꢁmpgꢁC3N4/BiOBr and (d) MꢁmpgꢁC3N4/BiOBr/PTh.
7

S. Li, C. Lai, C. Li et al.
Journal of Molecular Liquids xxx (xxxx) xxx
Fig. 8. (A) Photoluminescence spectra and (B) electrochemical impedance spectroscopyof (a) g-C3N4, (b) mpg-C3N4, (c) MꢁmpgꢁC3N4/BiOBr and (d) MꢁmpgꢁC3N4/BiOBr/
PTh.
Fig. 9. (A) Photocatalytic degradation of DMP over (a) BiOBr, (b) g-C3N4, (c) mpg-C3N4, (d) MꢁmpgꢁC3N4/BiOBr, (e) mpg-C3N4/BiOBr/PTh and (f) MꢁmpgꢁC3N4/BiOBr/PTh
under visible light irradiation; (B) The kinetic curves of DMP degradation over different photocatalysts; (C) Cycling times of DMP degradation using MꢁmpgꢁC3N4/BiOBr/PTh
under visible light (photocatalyst:1.0 g/L, DMP: 20 mg/L, pH:7).
dCt=dt ¼ ꢁkC0
ð1Þ
mpg-C3N4, and BiOBr alone, demonstrating that the photocatalytic
performance was enhanced due to the conjunction of mpg-C3N4
and BiOBr. Moreover, the photocatalytic DMP degradation effi-
ciency increased after PTh was incorporated into the MꢁmpgꢁC3-
N4/BiOBr composite, which coincides with the UV–vis DRS and PL
characterization results, indirectly confirming the existence of PTh.
The photocatalytic DMP degradation efficiency of MꢁmpgꢁC3N4/
BiOBr/PTh was 40% and 50% higher than that of pristine g-C3N4
and BiOBr, respectively. The pseudo first-order kinetic reaction
model was used to explore the mechanism of photocatalytic
DMP degradation by MꢁmpgꢁC3N4/BiOBr/PTh, expressed as Eqs.
(1) and (2):
ꢁlnðCt=C0Þ ¼ kt
ð2Þ
where, C0 and Ct are the DMP concentration at time 0 and t,
respectively, mg/L, and k is the pseudo first-order kinetic reaction
rate constant, hꢁ1. Using the photocatalytic DMP degradation
results, k could be calculated employing equation (2). Fig. 9B indi-
cates that the photocatalytic DMP degradation followed pseudo
first-order kinetics, and the reaction rate constants for g-C3N4,
BiOBr, and MꢁmpgꢁC3N4/BiOBr/PTh catalysis were 0.052, 0.043,
and 0.193 hꢁ1, respectively; thus, that of MꢁmpgꢁC3N4/BiOBr/
8

S. Li, C. Lai, C. Li et al.
Journal of Molecular Liquids xxx (xxxx) xxx
PTh was 3.7- and 4.5-fold higher than that of g-C3N4 and BiOBr,
respectively. The enhanced photocatalytic performance exhibited
by MꢁmpgꢁC3N4/BiOBr/PTh was principally attributed to the for-
mation of the heterojunction.
The stability and reusability of MꢁmpgꢁC3N4/BiOBr/PTh was
investigated, and the results are shown in Fig. 9C. The photocat-
alytic DMP degradation efficiency decreased by 3.57% and 10.95%
after the second and the third run, respectively, which illustrated
that MꢁmpgꢁC3N4/BiOBr/PTh was stable and reusable for DMP
removal from water.
3.3. Effect of pH, dosage, and initial DMP concentration
The effect of pH on the photocatalytic DMP degradation by
MꢁmpgꢁC3N4/BiOBr/PTh was investigated, and the results are dis-
played in Fig. 10A. DMP could not be detected at pH 11.17 as a
result of its decomposition in strong alkaline media. Moreover,
the degradation efficiency was the highest under neutral condi-
tions (pH = 7.36). According to a previous report [63], DMP
becomes protonated, and thus positively charged, under acidic
conditions, as does MꢁmpgꢁC3N4/BiOBr/PTh; therefore, the elec-
trostatic repulsion between the protons on the photocatalyst sur-
face and the cationic form of DMP possibly leads to deterioration
of the photocatalytic efficiency. Analogously, DMP molecules and
the photocatalyst surface may become negatively charged under
weakly alkaline conditions, likewise reducing the photocatalytic
efficiency due to electrostatic repulsion. However, the photocat-
alytic degradation efficiency was superior under weakly alkaline
conditions to that under weakly acidic conditions, which may be
attributed to the increased generation of hydroxyl radicals (ꢀOH)
Fig. 11. Effect of different scavengers on photocatalytic degradation of DMP by
MꢁmpgꢁC3N4/BiOBr/PTh (photocatalyst: 1.0 g/L, DMP: 20 mg/L, pH: 7).
from the high amount of OH– formed under weakly alkaline
conditions.
The effect of photocatalyst concentration on photocatalytic
DMP degradation is illustrated in Fig. 10B. Evidently, exceedingly
low or high amounts of photocatalyst were detrimental to photo-
catalytic DMP degradation. An insufficient dosage could not pro-
vide sufficient active sites to generate oxidative radicals, while at
very high dosages, the photocatalyst powder increased the turbid-
ity of the solution, limiting light penetration and consequently
impeding photocatalyst activation and performance [64].
The effect of initial DMP concentration on the photocatalytic
degradation is presented in Fig. 10C. It can be seen that DMP pho-
Fig. 10. Photocatalytic degradation of DMP (A) at different pH (photocatalyst: 1.0 g/L, DMP: 20 mg/L), (B) under different dosages of MꢁmpgꢁC3N4/BiOBr/PTh (DMP: 20 mg/L,
pH:7) and (C) different initial concentration of DMP (photocatalyst: 1.0 g/L, pH:7).
9

S. Li, C. Lai, C. Li et al.
Journal of Molecular Liquids xxx (xxxx) xxx
Fig. 12. The schematic of mechanism for DMP photocatalytic degradation over MꢁmpgꢁC3N4/BiOBr/PTh under visible-light irradiation.
tocatalytic degradation efficiency increased with the increase in
initial DMP concentration from 5 to 20 mg/L. A high DMP initial
concentration promoted its photocatalytic oxidation, because the
driving force of reaction increases with the increase of the number
of DMP molecules in high DMP initial concentration.
A
plausible MꢁmpgꢁC3N4/BiOBr/PTh photocatalytic DMP
degradation mechanism is schematically described in Fig. 12.
When the heterojunction photocatalyst was irradiated by visible
light, mpg-C3N4, BiOBr, and PTh were photoexcited to generate
electrons and holes. Due to the band structure configuration, a dual
Z-scheme electron transfer pathway formed, whereby the photo-
generated electrons in the CB of mpg-C3N4 and BiOBr tended to
migrate to the h + in the HOMO of PTh, and subsequently transfer
to the LUMO, the potential of which was more negative than the
standard O2/ ꢀO–2 redox potential, resulting the O2 in water captur-
ing electrons to produce ꢀO2–. Meanwhile, because the h+ on the VB
of BiOBr was more positive than the ꢀOH/OH– potential, the h+
could oxidize OH– (H2O) to ꢀOH. Additionally, the Fe3O4 nanoparti-
cles on MꢁmpgꢁC3N4/BiOBr/PTh acted as an electron transmitter
in the process of electron transfer [68]. Thus, the dual Z-scheme
electron migration mechanism inhibited the recombination of
photogenerated electron-hole pairs in mpg-C3N4 and BiOBr, result-
ing in enhanced photocatalytic activity. Previous EIS measure-
ments (Fig. 8B) supported the inference of effective
photogenerated electron-hole pair separation in mpg-C3N4 and
BiOBr.
3.4. Photocatalytic mechanism
In order to elucidate the photocatalytic degradation mecha-
nism, the reactive species were investigated by conducting radical
trapping experiments, wherein disodium ethylenediaminetetraac-
etate (Na2-EDTA), tertiary butyl alcohol (TBA), and 1,4-
benzoquinone (p-BQ) were used as photogenerated hole (h+),
hydroxyl radical (ꢀOH), and superoxide radical (ꢀOꢁ2 ) scavengers,
respectively. The results are demonstrated in Fig. 11. DMP degra-
dation efficiency decreased to 40.54%, 63.58%, and 44.27% with
the addition of Na2-EDTA, TBA, and p-BQ, respectively, indicating
the presence of h+, ꢀOH, and ꢀO2– active species in the photocatalytic
reaction system, and the major role played by h+ in the degradation
of DMP.
Furthermore, to explore the photocatalytic degradation mecha-
nism, the value band (VB) and conduction band (CB) energies of the
photocatalysts need to be determined. The VB and CB of mpg-C3N4
and BiOBr could be estimated based on Eqs. (3) and (4) as follows
[65]:
EVB ¼ X ꢁ Ee þ 1=2Eg
ð3Þ
ECB ¼ EVB ꢁ Eg
ð4Þ
where X is the absolute electronegativity of the semiconductor,
Ee is the energy of free electrons on the hydrogen scale (4.5 eV),
and Eg is the band gap energy of the photocatalyst. According to
Fig. 7B and an Eg of 2.79 eV for BiOBr [66], the EVB and ECB of
mpg-C3N4 were calculated to be 1.56 and ꢁ1.12 eV, respectively,
while the EVB and ECB of BiOBr were 3.07 and 0.28 eV, respectively.
PTh is a conductive polymer, capable of absorbing photons to
excite electrons, which transfer from the ground state of the HOMO
to the excited state of the LUMO on account of its p–p* transition
properties under visible light irradiation [57], producing holes and
electrons in the HOMO and LUMO, respectively. The band energies
of its HOMO and LUMO are 0.88 and ꢁ1.22 eV [67], respectively.
Fig. 13. The intermediate products of photocatalytic DMP degradation.
10

S. Li, C. Lai, C. Li et al.
Journal of Molecular Liquids xxx (xxxx) xxx
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The intermediate products of the DMP degradation were ana-
lyzed using liquid chromatography coupled with mass spectrome-
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speculated as shown in Fig. 13. DMP molecules were oxidized to
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tion reaction could be expressed as follows:
MꢁmpgꢁC3N4/BiOBr/PTh + h
hþ + H2O !ꢀOH + Hþ
t
!MꢁmpgꢁC3N4/BiOBr/PTh + (eꢁ/hþ) ð5Þ
ð6Þ
ð7Þ
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—
eꢁ + O2 !ꢀO2
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BiOX (X = Cl, Br, and I), Rare Met. 27 (2008) 243–250.
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hþ + OH— !ꢀOH
ð8Þ
ð9Þ
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4. Conclusion
A
magnetic
dual
Z-scheme
MꢁmpgꢁC3N4/BiOBr/PTh
heterostructure photocatalyst was successfully synthesized via
the hydrothermal method, wherein the SSA of g-C3N4 was
increased using SiO2 templating and acid etching to provide meso-
porous g-C3N4. The formation of the heterostructure effectively
enhanced the photocatalytic activity. The photocatalytic DMP
degradation efficiency of MꢁmpgꢁC3N4/BiOBr/PTh was the high-
est at a photocatalyst dosage of 1.0 g/L in neutral media containing
20 mg/L DMP under visible light; furthermore, it was 40% and 50%
higher than that of pristine g-C3N4 and BiOBr, respectively. The
photocatalytic DMP degradation mechanism hinged on the dual
Z-scheme electron migration pathway, which inhibited the recom-
bination of photogenerated electron-hole pairs in mpg-C3N4 and
BiOBr. The saturation magnetization of MꢁmpgꢁC3N4/BiOBr/PTh
was 11.31 emu/g. Thus, MꢁmpgꢁC3N4/BiOBr/PTh could be sepa-
rated from water in a convenient manner, and reused. The
MꢁmpgꢁC3N4/BiOBr/PTh photocatalyst is an excellent material
for the removal of DMP from water.
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Declaration of Competing Interest
[22] J. Xu, W. Meng, Y. Zhang, L. Li, C. Guo, Photocatalytic degradation of
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
tetrabromobisphenol
A
by mesoporous BiOBr:Efficacy, products and
pathway, Appl. Catal. B: Environ. 107 (2011) 355–362.
[23] J. Xu, L. Li, C. Guo, Y. Zhang, S. Wang, Removal of benzotriazole from solution
by BiOBr photocatalysis under simulated solar irradiation, Chem. Eng. J. 221
(2013) 230–237.
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photocatalyst: A novel nanocomposite with p-n-n heterojunctions for highly
effective photocatalytic removal of organic contaminants, J. Photochem.
Photobiol. A: Chem. 379 (2019) 11–23.
[25] P. Singh, P. Sonu, A. Raizada, P. Sudhaik, P. Shandilya, S. Thakur, V.K. Agarwal,
Gupta, Enhanced photocatalytic activity and stability of AgBr/BiOBr/graphene
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Acknowledgements
This research is supported by the Science and Technology Plan-
ning Project of Guangdong Province of China (2013B020800008).
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