Facile synthesis of iron and cerium co-doped g-C 3 N 4 with synergistic effect to enhance visible-light photocatalytic performance

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Journal of Hazardous Materials
journal homepage: www.elsevier.com/locate/jhazmat
Anionic polyacrylamide-assisted construction of thin 2D-2D WO
3
/g-C
3
N
4
Step-scheme heterojunction for enhanced tetracycline degradation under
visible light irradiation
Tao Pan
a,1
, Dongdong Chen
a,1
, Weicheng Xu
d
, Jianzhang Fang
a,b,c,
*, Shuxing Wu
a
, Zhang Liu
a
,
Kun Wu
a
, Zhanqiang Fang
c
a
School of Environment, South China Normal University, Guangzhou, 510006, China
b
Guangdong Provincial Key Laboratory of Chemical Pollution and Environmental Safety & MOE Key Laboratory of Theoretical Chemistry of Environment, South China
Normal University, Guangzhou, 510006, China
c
China Guangdong Technology Research Center for Ecological Management and Remediation of Urban Water System, Guangzhou, 510006, China
d
School of Environmental and Chemical Engineering, Foshan University, Foshan 528000, China
GRAPHICAL ABSTRACT
ARTICLE INFO
Editor: R Teresa
Keywords:
Anionic polyacrylamide
S-scheme heterojunction
C-W/N
Photocatalyst
Tetracycline degradation
ABSTRACT
Thin 2D/2D WO
3
/g-C
3
N
4
Step-scheme (S-scheme) heterojunction with carbon doping and bridge (C–W/N) was
constructed with anionic polyacrylamide (APAM), in which APAM functioned as an assistant templet and a
carbon source. APAM and WO
3
were inserted into g-C
3
N
4
nanosheet. The carbon, thin planar structure and WO
3
with oxygen vacancies result in fast charge transfer, high quantum efficiency and strong driving force for
photocatalytic reaction. Consequently, as-prepared C–W/N ternary composite photocatalyst exhibited sig-
nificantly enhanced photocatalytic performance for tetracycline (TC) degradation under visible light compared
to pure g-C
3
N
4
,WO
3
and other binary composites. Moreover, the material showed high stability and reusability
in cyclic TC degradation. The principal intermediate products over C–W/N photocatalyst were revealed by
HPLC-MS analysis. Corresponding degradation pathway of TC was also presented in this work. According to the
trapping experiments, analysis of electron spin resource (ESR) and band gap, possible charge transfer pathways
https://doi.org/10.1016/j.jhazmat.2020.122366
Received 15 January 2020; Received in revised form 8 February 2020; Accepted 20 February 2020
⁎
Corresponding author at: School of Environment, South China Normal University, Guangzhou, 510006, China.
E-mail address: fangjzh@scnu.edu.cn (J. Fang).
1
These authors contributed equally.
Journal of Hazardous Materials 393 (2020) 122366
Available online 21 February 2020
0304-3894/ © 2020 Elsevier B.V. All rights reserved.
T

of C–W/N are proposed and discussed in detail. Based on the results, carbon derived from APAM works not only
as electron mediator but also as acceptor for photocatalytic degradation reaction. It is a promising way to further
modulate heterojunction for varies applications.
1. Introduction
To obtain clean sustainable energy and remedy environmental
problem, semiconductor photocatalyst raises full attention and has been
regarded as the most promising way to convert solar energy (Qi et al.,
2018). So far various of organic, oxides and sulfide semiconductor
photocatalytic materials have been investigated (Chen et al., 2014;
Yuan et al., 2015). Graphitic carbon nitride (g-C
3
N
4
), as a metal free
semiconductor, is an attractive nontoxic polymeric photocatalyst with
excellent stability and suitable band gap, which can adsorb visible light
energy (Jiang et al., 2017b). However, photocatalytic activity of single-
phase g-C
3
N
4
is greatly limited by the low specific surface area, fast
recombination rate of photogenerated charge carriers and insufficient
light absorption (Dong et al., 2015;Ye and Chen, 2016). In order to
improve quantum efficiency and photocatalytic activity, various effort
have been applied, such as doping (Qiu et al., 2017;Wang et al., 2017),
exfoliation (Yang et al., 2017), heterojunction construction (Wang
et al., 2018b;Zhu et al., 2017), surface sensitization (Wu et al., 2017b;
Zhang et al., 2017a) and defect engineering (Shi et al., 2018;Zhang
et al., 2018). Constructing suitable heterojunction systems is known as
an effective way to extend light absorption and promote charge se-
paration simultaneously (Low et al., 2017).
Tungsten oxide (WO
3
) with good stability, visible-light absorption
and electron mobility is considered as an ideal material to achieve in-
tegration with g-C
3
N
4
(Kondofersky et al., 2015). With suitable band
position between these materials, WO
3
could improve charge transfer
rate and reduce recombination rate, which will significantly enhances
photocatalytic activity (Yu et al., 2017). Moreover, generation of
oxygen vacancy in the surface of WO
3
is a valid way to improve the
adsorption capacity of O
2
and accelerate the transfer rate of charge
carriers to produce superoxide radicals (Zhang et al., 2017b). Up to
now, lots of studies have been published about heterojunction systems
with WO
3
and g-C
3
N
4
(He et al., 2017;Pu et al., 2017;Zhao et al.,
2017). Fu et al (Fu et al., 2019). developed 2D/2D WO
3
/g-C
3
N
4
via
electrostatic self-assembly process which exhibited notable enhance-
ment in H
2
production. Compared to other 0D/2D or 1D/2D systems,
2D/2D heterojunction which has a large intimate interfacial contact is
more efficient and stable in catalytical reaction (Kumar et al., 2018;
Wei et al., 2018). In Jiang’s work (Jiang et al., 2018b), dual Z-scheme
WO
3
/g-C
3
N
4
/Bi
2
O
3
which was synthesized in one-step method using
melamine, H
2
WO
3
and Bi(NO
3
)
3
as precursors, showed efficient pho-
tocatalytic performance for refractory pollutants. Although, con-
structing heterostructure is an effective approach to promote photo-
catalytic performance, their practical application is still limited by the
relatively low visible light response, interfacial charge transfer rate and
surficial active sites (Yang et al., 2013).
The insertion of good conductivity material as electron-transfer
mediator between two photocatalytic materials can decrease the elec-
tron transfer distance, which can further reserve photogenerated holes
and electrons for redox reaction (Zhou et al., 2014). Chen et al (Chen
et al., 2019). Successfully synthetize WO
3
/Ag/ g-C
3
N
4
and reported that
the photocatalytic activity is remarkably enhanced with the adding of
Ag nanoparticles. However, it is well known that existence of noble
metal (Ag, Au) will cause relative high cost for practical application
(Wang et al., 2015;Yuan et al., 2017). Uncovering an accessible and
cheap material as mediator becomes a top priority, which can poten-
tially improve the activity of heterojunction in environmental re-
mediation.
Anionic polyacrylamide (APAM) is a hydrophilic polymer which is
easy to be carbonized (Choi et al., 2004). The amide group (−CONH
3
)
on APAM can form hydrogen bond with the uncondensed amine groups
on surface of g-C
3
N
4
(Schwinghammer et al., 2014). In this work, APAM
is used as a template to prepare carbon decorated 2D/2D WO
3
/g-C
3
N
4
Step-scheme (S-scheme) heterostructure (denoted as C–W/N) on the
basis of Z-scheme photocatalysts. Compared to that of WO
3
, g-C
3
N
4
and
their binary composites, the as-prepared C–W/N composite exhibited
significantly enhanced activity in degradation. Combined with in-
vestigation of properties and morphology it is revealed that the con-
siderable enhancement of photoactivity for tetracycline under visible
light irradiation which was due to the reduced bandgap, fast charge
transfer rate and enhanced quantum efficiency. Also, we developed a
strategy to ameliorate heterostructure system by combing electronic
and surface modulation at the same time.
Scheme 1. Synthesis process of C–W/N.
T. Pan, et al. Journal of Hazardous Materials 393 (2020) 122366
2

2. Experimental section
2.1. Sample preparation
2.1.1. Synthesis of pristine g-C
3
N
4
5g urea was placed in muffle furnace after recrystallization and
heated at 550 °C for 2 h. The faint yellow product was grinded in mortar
at room temperature. Finally, the pure powder g-C
3
N
4
was obtained
and preserved for further used.
2.1.2. Synthesis of pristine WO
3
Pure WO
3
was also prepared by the calcination method. Na
2
WO
3
,
the tungsten source, was completely dissolved in the deionized water.
Then, a certain concentrated HCl (12 mol/L) was added dropwisely
with stir to get H
2
O·WO
2
. The mixed solution was dried overnight at
100 °C and heated at 460 °C for 2 h and got green product WO
3
.
2.1.3. Synthesis of WO
3
/g-C
3
N
4
with APAM template assist
The specific synthesis of C–W/N photocatalyst was as follow (shown
in Scheme 1): 0.3 g g-C
3
N
4
and 0.03 g APAM were added in a beaker
containing 30 ml deionized water and ultrasonicated for 1 h to get
homogeneous solution. The mixture solution of 0.043 g Na
2
WO
4
and 2
ml HCl mentioned above was then transferred into a beaker and stirred
for 30 min. Acquired solution was stored in an air-dry oven at 100 °C
overnight to evaporate the water and get dry particulate matter. The
product was obtained after process of heating at 460 °C with heating
rate of 3 °C/min for 2 h. Finally, it was grinded into powder and labeled
as C–W/N. Binary composites WO
3
/g-C
3
N
4
, g-C
3
N
4
and WO
3
with
template-assisted, labeled as W/N, C-N and C–W, were synthesized by
same procedure without mixture of APAM, H
2
O·WO
3
and g-C
3
N
4
re-
spectively.
2.2. Characterization
Powder XRD analysis was performed by Ultima Ⅳadvance dif-
fractometer under Cu-Kαradiation at 40 kV and 40 mA. The scan rate
was 10°/min. The surface electronic state was studied by X-ray photo-
electron (XPS) excited by Al-Kαradiation spectra (ESCALAB 250, USA).
In order to confirm the surface morphology of samples, scanning elec-
tron microscopy (SEM) (ZEISS Ultra 55, Germany) and transmission
electron microscopy (TEM) (JECL, Japan) analysis were conducted. The
special surface area was measured by nitrogen adsorption/desorption
measurement using Micromeritics ASAP 2020 apparatus.
Photoluminescence (PL) emission spectra were recorded on a spectro-
fluorometer under excitation of 367 nm (F-2700, Hitachi, Japan). The
time-resolved transient photoluminescence (TRPL) decay spectra were
acquired by FLS920 transient fluorescence spectrophotometer
(Edinburgh Instruments, UK). FT-IR spectra of all samples were mea-
sured by Nicolet 6700 spectrometer in range of 4000-400cm
−1
(Thermo
scientific, USA). Raman spectroscopic measurements were executed on
a LabRAM HR800 spectrometer using a 532 nm excitation laser. UV–vis
diffuse-reflection spectra (DRS) were collected by a UV-3010 spectro-
photometer using BaSO
4
as the reflectance standard (Hitachi, Japan).
TOC 5000 analyzer (Shimadzu, Japan) was applied to measure the total
organic carbon (TOC). The intermediates analysis was conducted by
HRAM LC-MC/MC system connected to Q Exactive orbitrap and
Thermo Scientific Ultimates 3000 RSLC equipped with an Hypersil
GOLD C18(100 × 2.1 mm, 1.9 μm). The mobile phase was 0.1 % formic
acid vs methanol (98:2 V/V) at a flow rate of 2.5 ml/min with an in-
jection of 1 μL. The column temperature was 40 °C. The electron spin
resource (ESR) signals were conducted to measure the active radicals on
a Bruker JES-FA300 spectrometer with spin-trapped reagent 5,5-di-
methyl-L-pyrroline N-oxide (DMPO).
2.3. Photoelectrochemical measurement
Photo-electrochemical analysis was carried out using a conventional
three-electrode configuration on a CHI 660E electrochemical work-
station (Ch Instruments, shanghai, China) with 0.1 M Na
2
SO
4
solution
used as electrolyte. Pt wire and prepared Ag/AgCl electrode were em-
ployed as counter and reference electrode respectively. The specific
preparation of working electrode was as follow: 50 mg of samples and
20 μlNafion aqueous were added to 2 ml ethanol and sonicated for 1 h
to get slurry mixture. A part of the mixture (50 μl) was spin-coated onto
the 1cm
2
FTO glass and dried at 60 °C in vacuum drying oven for 4 h.
2.4. Photocatalytic experiment
The photocatalytic degradation tests were performed in a glass re-
actor with recirculating cooling water under irradiation of 300 W
Xenon lamp equipped with 420 nm filter. Tetracycline (TC) was used as
probe pollutants. Briefly, 50 mg sample dispersed into 50 ml of certain
concentration TC solution (10−80 mg/L) with vigorous mechanical
stirring in the dark for 30 min to get the adsorption-desorption equili-
brium. At certain internal irradiation time, 3 ml solution was taken by
0.45 μmfilter and monitored by UV spectrophotometer at 357 nm to
record the changes in the concentration.
3. Results and discussion
3.1. XRD analysis
In order to characterize crystalline nature of all sample, XRD pattern
was obtained and presented in Fig. 1. For pure g-C
3
N
4
, the stronger
peak (0 0 2) at 27.4° and weaker peak (1 0 0) at 13.2° corresponding to
in-plane structural packing motif and interplanar layered stacking of
aromatic system respectively (Mohamed et al., 2018a,b). It is clear that
pure WO
3
has the triplet peaks of (0 0 2), (0 2 0) and (2 0 0) facets
Fig. 1. (a) XRD patterns of the as-prepared samples; (b) enlarge part of (0 0 2) peaks.
T. Pan, et al. Journal of Hazardous Materials 393 (2020) 122366
3

(JCPDS 83-0951) as shown in Fig. S1 (Parthibavarman et al., 2018).
The high intensity of these peaks indicated that WO
3
had highly crys-
talline nature. Additionally, no characteristic peaks of carbon could be
observed in all the patterns, illustrating that the formation of generated
carbon is amorphous (Wang et al., 2016). From C–W/N XRD pattern,
the characteristic diffraction peaks were attributed to g-C
3
N
4
and WO
3
which indicates that the phase purity of as-prepared sample was high
and the structure of g-C
3
N
4
and WO
3
kept unspoiled in the process of
fabrication. Besides, two weak peaks at (26.6°) and (28.7°) of WO
3
could be seen in the C–W/N patterns, supporting the well dispersion of
WO
3
(Zhao et al., 2017). Also, the intensities of g-C
3
N
4
and WO
3
(0 0 2)
plane are both decreased compared to pristine material in the pattern.
We postulated that the recombination between two (0 0 2) facets might
happen and cover g-C
3
N
4
(Cui et al., 2017). Compared to g-C
3
N
4
, the (0
0 2) peaks of C-N, W/N and C–W/N were slightly downshifted as shown
in Fig. 1b, indicating that WO
3
and C could be inserted into interlayers
of g-C
3
N
4
and cause interlayer expansion (Ding et al., 2015).
3.2. Chemical states analysis
High-resolution XPS measurements were performed to obtain sur-
face chemical states of g-C
3
N
4
and C–W/N. The survey spectra were
shown in Fig. 2a. From the results, the spectrum of C–W/N presented
the existence of C, N, O and W elements and all the spectra showed the
dominant peaks of C 1s and N 1s at around 288 and 398eV. It ex-
emplifies that the composition of C–W/N is analogous to that of g-C
3
N
4
.
As shown in Fig. 2b, the O 1s spectrum of g-C
3
N
4
could be fitted into
peaks at 531.5 and 532.6 eV corresponding to terminal hydroxyl groups
and absorbed H
2
O(Shaner et al., 2014). Compared with g-C
3
N
4
, the
new O 1s peaks in the spectrum of C–W/N located at 529.7eV is as-
signed to lattice O atoms of WO
3
.Fig. 2c exhibited the N1 s spectrum of
samples. These peaks could be deconvoluted into four peaks at 398.8,
400.0, 401.0 and 404.7 eV, which were ascribed to two-coordinate N
species (C = N–C) (Shan et al., 2016), three-coordinate N-(C)
3
(Liu
et al., 2017), surface amino group (−NH
2
) and π-π* excitations (Zheng
Fig. 2. (a) XPS survey and high-resolution spectra of (b) O1 s, (c, d) N1 s, (e) C1 s and (f) W4f of g-C
3
N
4
and C–W/N.
T. Pan, et al. Journal of Hazardous Materials 393 (2020) 122366
4

et al., 2016). It should be noted that an upshift was observed in N1 s
peak of C–W/N compared to that of g-C
3
N
4
(Fig. 2d). The possible
reason could be the chemical environment change around N when
doped carbon or interfacial charge transfers between g-C
3
N
4
and WO
3
(Bao et al., 2017;Fu et al., 2019). Two identical peaks at 284.8 and
288.3 eV were founded for C1 s state of both samples (Fig. 2e). The
peaks at 284.8 and 288.3 eV were assigned to C–C adventitious carbon
species and sp3 N = C–N
2
coordination (Yu et al., 2017). Owing to the
π-π* excitations through graphitic layer, there were broad region be-
tween 292.0 and 294.3 eV (Quanjun et al., 2015). According to Table
S1, we can observe that the C/N ration were increased from 0.74 (g-
C
3
N
4
) to 0.83 (C–W/N), suggesting that decorative carbon was formed.
In brief, C was successfully dispersed on the heterojunction formed by
g-C
3
N
4
and WO
3
. The W 4f spectrum (Fig. 2f) exhibits two peaks at 34.9
eV (W 4f
1/2
) and 37.0 eV (W 4f
5/2
). Based on previous literature, the
observation that two peaks of W4f
1/2
and W4f
5/2
were moved toward
lower energy by around 0.7 eV than those of pure WO
3
, which could be
result of decreasing in oxidation state about W (Cui et al., 2017;Yu
et al., 2017).
The ESR spectrum of C–W/N composites was investigated for re-
confirming the existence of oxygen vacancy in Fig. S2. It can be clearly
observed that C–W/N showed significant signal at g = 2.002 owing to
the electron trapping at oxygen vacancies (Zhang et al., 2016). The
results indicate that there are some oxygen vacancies distributed in the
surface of WO
3
which is in good agreement of XPS analysis of W.
3.3. Morphological, structural and optical properties analysis
To demonstrate microscopic structure and morphology of samples,
SEM analysis was performed as seen in Fig. 3 and Fig. S3. It can be
clearly seen that g-C
3
N
4
exhibited aggregated and thick planar structure
(Fig. S3a). SEM image of pure WO
3
reveals that WO
3
was mainly
composed of nanoplates structure (Fig. S3b). As shown in Fig. S3c and
d, bigger poly-porous structure appeared in the C–N to improve the
intimate contact with pollutants which is benefit for photocatalysis. As
for C–W/N, it could be looser and more porous compared to g-C
3
N
4
after integration with WO
3
(Fig. 3). The corresponding energy dis-
persive spectrum (EDS) of the prepared C–W/N was also performed to
reveal the presence of element as shown in Fig. S4. The result illustrates
that the sample is composed of C, N, O and W elements. To further
investigate the detailed morphology and crystal structure, TEM was
employed. As depicted in Fig. 4, image of C–W/N exhibits nearly
transparent feature in contrast to that of pure g-C
3
N
4
indicating ex-
foliation during the process of forming C–W/N. The higher contrast
parts circled by red dashed can be assigned to the integration of WO
3
nanoplates. We can evidently see the lattice fringes by high-resolution
TEM in Fig. 4e. The lattice spacing of 0.375 nm corresponding to (0 2 0)
facet of WO
3
confirmed the coexistence of g-C
3
N
4
and WO
3
phases(Yu
et al., 2017). As seen in Fig. 4f, EDS elemental mapping images of C–W/
N was carried out to confirm the successful integration of g-C
3
N
4
and
WO
3
. Based on above analysis, WO
3
nanoplates were closely contacted
with g-C
3
N
4
with high dispersion which could be cause of strong in-
terfacial electrostatic force (Fu et al., 2019). It is beneficial for im-
proving the transfer of charges between WO
3
and g-C
3
N
4
in the process
of photodegradation. From the TEM image of pure WO
3
(Fig. 4b), WO
3
exhibited sheet-like structure which is consistent with SEM images.
These results further demonstrate that the WO
3
contacted with g-C
3
N
4
was nanoplates and the heterojunction was 2D/2D system which had
large intimate interfacial contact.
The fitting N
2
adsorption-desorption isotherms and pore size dis-
tributions corresponding to all as-prepared samples are displayed in
Fig. 5. Obviously, all the curves show type Ⅳisotherms which possess
type H3 hysteresis loop indicating that these samples are mesoporous
material. BET specific surface areas, pore volumes and pore sizes cal-
culated by BJH methods are tabulated in Table (inset in Fig. 5). Cor-
responding pore size distribution curves are displayed in Fig. S5.
Compared to g-C
3
N
4
, W/N presented lower BET surface area. It could
probably result from the massive amount of the introduced WO
3
na-
noplates which had low specific surface area (Chen et al., 2019). After
adding APAM as adjustment, C–W/N, C–W and C–N exhibit increased
specific surface areas compared to W/N, WO
3
and g-C
3
N
4
, respectively,
which can provide more active sites and enhance adsorption abilities
(Di et al., 2016;Jiang et al., 2017a;Liu et al., 2016a). It should be noted
that although APAM shows a little effect on increasing the specific
surface of g-C
3
N
4
, the increased pore volume and pore size of C–N were
well suited to improve diffusion and reduce blocking. Therefore, it can
be concluded that APAM can promote the exfoliation of g-C
3
N
4
and
WO
3
to enhance surface area, pore volume and pore size, which is
consistent with results of SEM and TEM.
In order to investigate migration and photo-generated electron-hole
pair recombination of photocatalysts, photoluminescence (PL) spec-
troscopy was conducted under 367 nm light excitation (Fig. 6). Except
for pure WO
3
and C–W, all samples show a broad luminescence peak at
approximately 460 nm. Owing to rigid C–N plan and π-conjugated
eletronic structure of g-C
3
N
4
, the photocatalysts dominated by g-C
3
N
4
exhibited strong fluorescence (Lu et al., 2019). Obviously, C–W/N
heterostructures presented notably dropped PL intensity compared with
g-C
3
N
4
, C/N and W/N indicating that electron-hole pair recombination
was getting restrained. This is mainly attributed to coupling of WO
3
and
forming Z scheme photocatalytic system. Also, the decorative carbon
not only improved transport of photogenerated charge carries between
WO
3
and g-C
3
N
4
, but can also acted as electron-acceptor material to
enhance quantum efficiency (Wang et al., 2016). Moreover, the PL
lifetime of WO
3
, g-C
3
N
4
, W/N and C–W/N was calculated by time-re-
solved fluorescence spectra. As shown in Fig. 7, pure g-C
3
N
4
showed
longest PL life (τ
a
= 7.70 ns) measured from double-exponential
function. The shorter life time of C–W/N (τ
a
= 6.33 ns) and lower PL
intensity implied the faster charge transfer and separation compared to
Fig. 3. SEM images of C–W/N.
T. Pan, et al. Journal of Hazardous Materials 393 (2020) 122366
5

W/N (τ
a
= 6.80 ns), further supporting that the adjustment of APAM is
beneficial for transferring electrons.
The properties of samples can be further analyzed by Fourier
transform infrared (FT-IR) spectroscopy (Fig. 8). C–W/N, W/N, C–N
and g-C
3
N
4
exhibit aromatic C–N stretching vibration at 1248, 1325
and 1412 cm
−1
. The sharp bonds located at 810 cm
−1
were originated
from bending vibration of tri-s-triazine unites (Fu et al., 2017;Xia et al.,
2017). Meanwhile, the peaks belonging to C–N streching vibration at
1572 cm
−1
and 1639 cm
−1
could be observed (Yu et al., 2017). From
above analysis, the conclusion can be draw that the main CN structure
was not destroyed during the process of calcination to synthetize C–N,
W/N and C–W/N. As for WO
3
and C–W, the broad adsorption bands at
450−900 cm
−1
were due to the stretching virbration of W–O–W
(Boyadjiev et al., 2017). However, the W–O–W characterisitic peak
couldnot be observed in C–W/N because of the overtapping peaks
(Zhao et al., 2017). Additionally, the broad absorption bands from 3179
to 3447 cm
−1
can be attributed to adsorbed H
2
O and N–H vibration of
surface hydroxyl groups. Compared with g-C
3
N
4
, the peaks from 3179
to 3447 cm
−1
of C–N and C–W/N showed in lower wavenumbers and
broader corresponding bond (Fig. 8b). In consequent, interaction be-
tween APAM and g-C
3
N
4
happened via the intermolecular hydrogen
Fig. 4. TEM images of (a) g-C
3
N
4
, (b) WO
3
and (c, d) C–W/N; (e) HRTEM images of C–W/N; (f) TEM and its EDS mapping images of C–W/N.
Fig. 5. Nitrogen adsorption-desorption isotherms, BET surface areas, pore vo-
lumes and pore sizes of the as-prepared samples.
Fig. 6. Steady PL spectra of all the as-prepared samples.
Fig. 7. Time-resolved PL decay profiles of g-C
3
N
4
,WO
3
, W/N and C–W/N.
T. Pan, et al. Journal of Hazardous Materials 393 (2020) 122366
6

bonds (Bao et al., 2017;Mariana et al., 2013;Salavagione et al., 2009).
Raman spectra (Fig. S6) were conducted to reconfirm the molecular
structure of samples. As for WO
3
,W–O–W bending vibration was ver-
ified by the peaks from 275 and 328 cm
−1
. The peaks at 712 and 810
cm
−1
corresponded to dominant W–O stretching vibration can be ob-
served (Fu et al., 2019). After modification of PAM, C–W exhibits D and
G band located at 1350 and 1593cm-1, indicating the successful doping
of carbon on the surface of WO
3
(Wang et al., 2018a). The peaks at 275,
328, 712 and 810 cm
−1
become weaker or even disappear. It might be
caused by phonon softening and improved electron-phonon coupling
with relatively thin layer structure (Liu et al., 2016b). While, g-C
3
N
4
and C–W/N show no detectable peaks attributed to strong fluorescence
effect of g-C
3
N
4
(Chen et al., 2017b). Furthermore, interactions be-
tween g-C
3
N
4
and carbon cause the enhancement of peaks, from 1200
to 1700, 912 and 1179 cm
−1
,inC–W/N and C–N, which are ascribed to
C–N stretching vibration and aromatic C–H bending modes (Ma et al.,
2018;Zheng et al., 2020).
The light harvesting properties of C–W, C–N, W/N and C–W/N, as
well as the single g-C
3
N
4
and WO
3
, were characterized using UV–vis
DRS with results depicted in Fig. 9. As for g-C
3
N
4
,WO
3
and W/N, it
should be noted that their absorption intensity was increased in the
range of 400−800 nm after adding carbon, which was benefit for
narrowing the band gap and harvesting more visible-light (Ho et al.,
2015). It is consistent with the color change of the samples. From g-
C
3
N
4
and W/N’s faint yellow to C–N and C–W/N’s brown yellow (inset
of Fig. 10). The corresponding bandgaps of all photocatalysts can be
estimated by Kubelka-Munk theory (Fig. 10). From hυaxis intercept of
tangent lines, the bandgaps of g-C
3
N
4
,WO
3
,C–W, C–N, W/N and C–W/
N were determined to be 2.65, 2.39, 2.34, 2.56, 2.59 and 2.43 eV,
which was in agreement with light absorption.
Transient photocurrent responses were conducted to reveal light-
induced electrons migration property and ascertain separation effi-
ciency by repeating cycles switch light. As shown in Fig. 11a, the result
showed that C–W/N exhibited the highest photocurrent intensity under
light illumination. The enhanced photocurrent exemplified the higher
charge transfer efficiency, which also gave evidence that both the em-
bedded WO
3
nanoplates and carbon carbonized from APAM promoted
charge separation. Besides, electrochemical impedance spectra (EIS)
measurement was utilized to investigate charge transfer resistance
(Fig. 11b). The smallest arc radius of C–W/N manifested more effective
separation of electron-hole pairs and faster interfacial charge transport.
It signifies that W/N with doping and bridging of carbon could improve
the charge transfer properties and separation efficiency, which was
consistent with the above PL and photocurrent results.
3.4. Evaluation of photocatalytic activity
The photocatalytic performance of single, binary and ternary sam-
ples was investigated by photodegradation TC in aqueous solution. 300
W Xenon lamp with filter was used as visible-light source. The distance
between lamp and reactor were kept 10 cm all the time. The effect of
other parameters, initial concentration of pollutant and solution pH
value, were also taken into consideration. As illustrated in Fig. 12a, TC
could be rarely degraded under visible light, which provided backing
that TC is stable. For all as-prepared samples, TC can be removed
gradually with continuous irradiation. C–W/N composite exhibited best
degradation efficiency (90.54 %) as against that of other photocatalysts
after one-hour illumination. The removal rate of TC was in the order of
C–W/N > C–N > W/N > g-C
3
N
4
>C–W>WO
3
. Owing to the fast re-
combination of photogenerated electron-hole pairs, WO
3
exhibited the
lowest TC removal rate (12.14 %), even if it had more narrowed band
gap. Significantly, the photocatalytic performance of WO
3
and g-C
3
N
4
were both promoted by the adjustment of APAM. C–N and C–W had
presented the removal rate of about 77.97 % and 17.05 % respectively.
As expected, when WO
3
was combined with g-C
3
N
4
, W/N showed
better photocatalytic activities than g-C
3
N
4
. However, the rise of de-
gradation efficiency was slight (from 50.91 % to 70.69 %), which may
because of the decreasing utilization of photogenerated electrons and
holes in traditional Z-scheme heterogeneous system (Peng et al., 2014).
Furthermore, the C–W/N also shows relatively well photocatalytic
performance for TC compared with the latest works about similar Z-
scheme photocatalyst (Table 1).
The reaction kinetic study of photocatalytic TC was conducted by
fitting all the degradation date with pseudo-first-order correlation: ln
(C/C
0
) =kt, where C and C
0
represent initial concentration and
Fig. 8. (a) FT-IR spectra of all the samples; (b) Comparison of the peaks of g-C
3
N
4
, W/N and C–W/N.
Fig. 9. UV–vis spectrum of the as-prepared samples.
T. Pan, et al. Journal of Hazardous Materials 393 (2020) 122366
7

instantaneous concentration in reaction time t. k is the first-order re-
action rate constants. As shown in Fig. 12b, k of C–W/N is the highest
value (0.0378), which is 2.25 time higher than that of g-C
3
N
4
(Fig. S7).
It can be concluded that an optimum amount of decorating carbon onto
W/N heterojunction can exhibit significant improved photocatalytic
performance in TC degradation. The reusability and stability of pho-
tocatalyst also play significant roles in practical applications. To eval-
uate the photochemical stability of C–W/N, cycling TC photocatalytic
degradation experiments were executed under the same condition for 5
runs (Fig. 12c). After each experiment, we recycled the photocatalyst,
washed and dried for the next runs. As stated in the result, the C–W/N
photocatalyst had no apparent deactivation and still exhibited high
photocatalytic performance after 5 cycles. Furthermore, the XRD pat-
terns of used C–W/N was almost the same as before degradation, in-
dicating that crystal structure was intact (Fig. 12d).
TOC analysis was performed to measure the TC mineralization rate
during the photocatalytic process (Fig. S8). The removal rate of TOC by
C–W/N reached 44.8 % after 1 h, which was much higher than that of
WO
3
(4.4 %), C–W (6.3 %), g-C
3
N
4
(14.1 %), W/N (28.0 %) and C-N
(35.2 %). It indicated that C–W/N presented efficient mineralization for
TC.
Due to the fact that initial concentration of TC can influence the
final photocatalytic performance (Chen et al., 2017a), further study had
been proposed to investigate the effect of initial concentration on
photocatalytic activity. A series of experiments were conducted using
different initial TC concentrations over g-C
3
N
4
and C–W/N photo-
catalysts (Fig. 13a). It could be clearly observed that increasing TC
concentration had a negative impact on photocatalytic process for g-
C
3
N
4
where 57.33 %, 50.91 %, 38.14 % and 27.23 % were removed at
10, 20, 40 and 80 mg/l initial TC concentration. Presumably increasing
macromolecular intermediate products will reduce the catalytic sites
and adsorption for TC. Besides, it is worth noting that change of initial
TC concentration only had little impact on photodegradation for C–W/
N with 83.93 %, 90.54 %, 82.05 % and 82.99 % photocatalytic effi-
ciency at 10, 20, 40 and 80 mg/l TC concentration. The initial pH va-
lues of TC solution might influence on the electron transfer and surface
charge characteristics of prepared photocatalyst, which can affect their
photocatalytic degradation process. To explore the effect of pH, batch
experiments were carried out at the different pH in the range of 3-9.
The result supports that initial pH only had slight impact on photo-
activity for C–W/N, where 82.96 %, 84.69 %, 87.52 % and 86.33 % of
20 mg/l TC were removed in one-hour at pH value of 3, 5, 7 and 9
(Fig. 13b). It gives credence that C–W/N can remain relatively high
photocatalytic performance over a wide pH range.
In order to confirm the intermediates during the TC degradation
process with the photocatalysis of C–W/N, HPLC-MS was applied.
Spectrums of products after 0, 30 and 60 min were collected and shown
in Fig. S9. Based on the analysis of HPLC-MS, possible intermediator
and degradation process of TC could be inferred as illustrated in Fig. 14.
Intermediate with m/z of 431 is resulted from loss of one N-methyl
group (Li et al., 2020). Intermediates with m/z of 304, 301, 150 and
118 were generated through ring cleavage and losing of hydroxyl,
amino or methyl groups (Li et al., 2020;Wang et al., 2020;Wu et al.,
2020a,b). The products at m/z of 301 and 165 can be generated from
breaking cyclic hydrocarbon structure of TC (Li et al., 2020;Zhang
et al., 2020). Furthermore, intermediates with m/z 279 and 223 were
produced by the oxidation products with m/z of 301 and 304 (Wu et al.,
2020a).With further photocatalytic degradation, big molecular sub-
stances were successfully converted to relatively small molecules with
m/z of 175, 149, 137 and 133 (Li et al., 2019;Wu et al., 2020a;Xu
et al., 2018). Finally, these intermediates degraded into CO
2
and H
2
O.
The dramatically decreasing of m/z of 455 contributed by TC and H
+
indicating that TC was effectively removed.
Additionally, the universal applicability of C–W/N was also in-
vestigated by degradation of Methylene Blue (MB). As shown in Fig.
S10, the degradation rate for C–W/N in one hour was 92.4 %, which
was almost 2 time higher than g-C
3
N
4
(46.7 %). These results illustrated
that C–W/N had better photocatalytic performance for treatment of
variety contaminants in wastewater.
3.5. Photocatalytic mechanism
As is known to all, predominant active species play a vital role in
photodegradation. In order to explore the mechanistic information of
TC degradation, a series of trapping experiments for C–W/N were
performed. Three 1 mM typical chemicals,1-4-benzoquinone (BQ) (Liu
et al., 2015), triethanolamine (TEA) (Jiang et al., 2017a) and iso-
propanol (IPA) (Wu et al., 2017a) can serve as scavengers of ·O
2
−
,h
+
and ·OH radical respectively. As depicted in Fig. 15, after adding IPA,
Fig. 10. Evaluation of bandgap by Kubelka-Munk equations for all samples.
Fig. 11. (a) Transient photocurrent responses and (b) EIS Nyquist plots of the as-prepared samples.
T. Pan, et al. Journal of Hazardous Materials 393 (2020) 122366
8

Fig. 12. (a) Photocatalytic rate of TC under visible light irradiation on as-prepared samples (initial concentration = 20 mg/L, solution pH = 6, photocatalyst dosage
= 0.05 g), (b) Pseudo-first-order kinetic curves of all samples, (c) cycling runs of C–W/N for the degradation of TC and (d) XRD patterns of C–W/N sample before and
after successive photocatalytic experiment.
Table 1
Comparation of TC photocatalytic activity with other photocatalysts.
Photocatalyst Amount (mg) Solution/Volume(mL) /concentration(ppm) Time/ min k (min
−1
) Reference
C–W/N 50 Tetracycline/50/20 120 0.0378 this work
WO
3
/ g-C
3
N
4
/Bi
2
O
3
100 Tetracycline/100/10 120 0.0240 (Jiang et al., 2018b)
WO
3
/Ag/CN 100 Tetracycline/300/10 140 0.0538 (Chen et al., 2019)
Ag
2
CO
3
/Ag/WO
3
50 Tetracycline/100/10 90 0.0179 (Yuan et al., 2017)
WO
3
/g-C
3
N
4
100 Tetracycline/100/20 100 0.0210 (Zhang et al., 2019)
W–CN–5 100 Tetracycline hydrochloride/100/10 120 0.0213 (Yan et al., 2019)
Fig. 13. The effect of (a) initial concentration and (b) pH value on TC degradation over g-C
3
N
4
and C–W/N.
T. Pan, et al. Journal of Hazardous Materials 393 (2020) 122366
9

the degradation efficiency of TC was slightly decreased, implying that
just few ·OH took part in the photocatalytic procedure. In contrast, the
photocatalytic activity of removing TC decreases dramatically with the
addition of BQ and TEA, inhibited from 90.54 % to 31.73 % and 41.83
% respectively. Thus, it reveals that both ·O
2
−
and h
+
are the mainly
active participators for the TC degradation.
The active species generated in photodegradation was also identi-
fied by room-temperature ESR under visible light irradiation. As shown
in Fig. 16, no obvious peak appeared in the ESR spectra of C–W/N
sample under darkness. However, the characteristic peaks of DMPO-
·O
2
−
was clearly observed when methanol dispersion of C–W/N pho-
tocatalyst was exposed to visible light and the signal intensity increased
with irradiation time. It indicates the generation of ·O
2
−
in reaction
system. Moreover, the peaks corresponding to DMPO-·OH can also be
discovered under irradiation, suggesting ·OH was generated too.
To further explain the photocatalytic mechanism the band-gap
potentials of the samples should be confirmed. The conduction band
(CB) and valence band (VB) positions can be calculated by following
equations:
=− −
E
XE E
1
2
CB e g (1)
=+
E
EE
VB CB g (2)
Where C and E
e
represent the electronegativity of the semiconductor
and energy of free electrons on the hydrogen scale approximating to 4.5
eV. The electronegativity of WO
3
and g-C
3
N
4
are 6.58 eV and 4.67 eV
respectively (Chen et al., 2019). Herein, the E
CB
and E
VB
of prepared
samples are determined as shown in Table 2. The E
CB
and E
VB
can also
be determined by Motty-Schottky plots. As illustrated in Fig. 17, the
Mott-Schottky curves of both g-C
3
N
4
and WO
3
sheets show positive
slope at 1, 1.5 and 2 kHz frequencies. It is the feature of n-type
Fig. 14. The proposed photocatalytic pathway of TC degradation by C–W/N treatment.
T. Pan, et al. Journal of Hazardous Materials 393 (2020) 122366
10

semiconductor. The flat band potentials of g-C
3
N
4
and WO
3
can be
speculated to be -1.30 and 0.53 eV vs. Ag/AgCl, equal to -1.10 and 0.73
eV vs. NHE. The CB position is closer to that of flat band in n-type
semiconductor (Fu et al., 2019). Thus, E
CB
of g-C
3
N
4
(-1.10 eV) and
WO
3
(0.73 eV) are easily extrapolated. Combined with UV–vis analysis
and Eq. (2),E
VB
of g-C
3
N
4
and WO
3
are also calculated to be 1.55 and
3.07 eV which is similar to above results of theoretical calculation.
Based on the band gap of WO
3
and g-C
3
N
4
, two different kind of
charge separation and transfer ways have been proposed for C–W/N
photocatalyst. According to the traditional charge transfer, a possible
pathway has been discussed in Fig. 18a. The photogenerated electrons
will migrate to WO
3
while holes will transfer to g-C
3
N
4
. Finally, elec-
trons and holes will be accumulated at CB of WO
3
(from 0.73 eV) and
VB of g-C
3
N
4
(from 1.55 eV) respectively. However, the electrons on
WO
3
cannot react with O
2
to generate ·O
2
−
(O
2
/·O
2
−
=-0.33 eV) and
holes on g-C
3
N
4
cannot produce ·OH (·OH/OH
−
= 2.4 eV) owing to the
potential. It is not consistent with the results of trapping experiments
and ESR. ·O
2
−
is the predominant active participator for C–W/N in the
process of TC degradation. Therefore, other S-scheme transfer me-
chanism based on Z-scheme photocatalyst with doping and bridge of
carbon has been present as illustrated in Fig. 18b. In detail, g-C
3
N
4
and
WO
3
can both produce photoinduced electron excited from VB to CB
under visible-light irradiation. Then photogenerated electrons on the
CB of WO
3
and holes on the VB of g-C
3
N
4
would migrate and recombine
at carbon between WO
3
and g-C
3
N
4
which can reduce transfer distance.
The presence of carbon as electron mediator and 2D-2D close interfacial
contact could promote charge transfer and recombination. However,
the useful electrons (from -1.10 eV) and holes (from 3.07 eV) of strong
redox ability could be remined. Namely, photoexcited electron can
yield ·O
2
−
and holes which own higher energy level to directly degrade
TC or form active ·OH radicals. What’s more, the carbon and oxygen
vacancies dispersed on the surface of g-C
3
N
4
or WO
3
act as acceptor to
accelerate the interfacial charge transfer. Meanwhile, the adsorption of
O
2
can be improved owing to the chemisorption at oxygen vacancy sites
which is benefit for activation of O
2
molecules. As a result, C–W/N
photocatalyst follows a S-scheme mechanism and shows high removal
rate of TC under visible light (Fu et al., 2019;Jiang et al., 2018a;Tang
et al., 2019).
4. Conclusion
In summary, the novel S-scheme WO
3
/g-C
3
N
4
heterojunction with
carbon doping and bridge was successfully prepared by insertion of
APAM and WO
3
. The obtained C–W/N ternary composite shows im-
proved photocatalytic activity for TC degradation compared with pure
g-C
3
N
4
,WO
3
and other binary composites (C–W, C–N and W/N).
Importantly, C–W/N also exhibits excellent photocatalytic activity for
TC degradation in different initial concentration and pH value. It can be
ascribed to synergistic effect between carbon doping and bridge, large
BET surface area, strong visible light absorption and oxygen vacancies.
The S-scheme heterostructure with carbon decoration not only remains
high redox ability, but also shortens the distance of electron transfer
and improves the utilization of visible light. Moreover, high stability of
C–W/N could be attested by cycling experiment analysis. This work is
expected to offer a straightforward strategy to further modify hetero-
junction to fabricate more efficient and stable photocatalyst for en-
vironmental applications.
Credit author statement
I have made substantial contributions to the conception or design of
the work; I have revised the work critically for important intellectual
content.
I agree to be accountable for all aspects of the work in ensuring that
questions related to the accuracy or integrity of any part of the work are
appropriately investigated and resolved.
Fig. 15. The species trapping experiments of C–W/N in degradation of TC
under visible-light irradiation.
Fig. 16. DMPO spin-trapping ESR spectra with C-W/N sample of (a) DMPO-·O
2
−
and (b) DMPO-·OH adducts recorded under visible light irradiation.
Table 2
Band gap, conduction band and value band of as-prepared samples.
E
g
E
CB
E
VB
C–W/N 2.43 eV −1.04 eV 1.39 eV
C–N 2.56 eV −1.11eV 1.45 eV
W/N 2.59 eV −1.12eV 1.47 eV
g-C
3
N
4
2.65 eV −1.15 eV 1.50 eV
WO
3
2.34 eV 0.91eV 3.25 eV
C–W 2.39 eV 0.88eV 3.27 eV
T. Pan, et al. Journal of Hazardous Materials 393 (2020) 122366
11

All persons who have made substantial contributions to the work
reported in the manuscript, including those who provided editing and
writing assistance but who are not authors, are named in the
Acknowledgments section of the manuscript and have given their
written permission to be named. If the manuscript does not include
Acknowledgments, it is because the authors have not received sub-
stantial contributions from non authors.
Declaration of Interest Statement
We declare that we have no financial and personal relationships
with other people or organizations that can inappropriately influence
our work. There is no professional or other personal interest of any
nature or kind in any product, service and company that could be
construed as influencing the position presented in or the review of the
manuscript entitled “Anionic polyacrylamide-assisted construction of
thin 2D-2D WO3/g-C3N4 Step-scheme heterojunction for enhanced
tetracycline degradation under visible light irradiation”.
Acknowledgement
The authors greatly appreciate the support provided by South China
Normal University (No. 16 KJ20).
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.jhazmat.2020.122366.
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