Design, preparation and properties of high-performance Z-scheme Bi2MoO6/g-C3N4-x composite photocatalyst

Article abstract of DOI:10.1016/j.cplett.2020.137381

g-C₃N_4-x with nitrogen-deficient structure was prepared by raising the thermal etching temperature to 625 °C, and Bi₂MoO₆ was prepared through hydrothermal synthesis. The Bi₂MoO₆/g-C_3<

Full text of DOI:10.1016/j.cplett.2020.137381

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Research paper
Design, Preparation and Properties of High-performance Z-scheme
Bi2MoO6/g-C3N4-x Composite Photocatalyst
Boxun Zhou, Qi Liu, Shici Zhang, Manzoor Sanjrani, Hang Yang, Shibin Xia
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S0009-2614(20)30296-7
DOI:
https://doi.org/10.1016/j.cplett.2020.137381
CPLETT 137381
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18 March 2020
18 March 2020
Please cite this article as: B. Zhou, Q. Liu, S. Zhang, M. Sanjrani, H. Yang, S. Xia, Design, Preparation and
Properties of High-performance Z-scheme Bi2MoO6/g-C3N4-x Composite Photocatalyst, Chemical Physics Letters
(2020), doi: https://doi.org/10.1016/j.cplett.2020.137381
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Design, Preparation and Properties of High-performance Z-
scheme Bi2MoO6/g-C3N4-x Composite Photocatalyst
Boxun Zhou, Qi Liu, Shici Zhang, MANZOOR SANJRANI, Hang Yang, Shibin Xia*
School of Resources and Environmental Engineering, Wuhan University of
Technology, Wuhan, 430070, People’s Republic of China
* Corresponding author, e-mail: xiashibin@126.com
Abstract
g-C3N4-x with nitrogen-deficient structure was prepared by raising the thermal etching
temperature to 625 °C, and Bi2MoO6 was prepared through hydrothermal synthesis.
The Bi2MoO6/g-C3N4-x composite photocatalyst with electron transfer mechanism as
Z-scheme was then synthesized through joint heating at 400 °C. Mechanism analysis
indicated that the formation of the nitrogen deficiency and Z-scheme carrier transport
path in Bi2MoO6/g-C3N4-x composite photocatalyst can reduce the recombination rate
of photogenerated electron-holes and prolong the lifetime of photogenerated carriers.
0.50 Bi2MoO6/g-C3N4-x had the highest photocatalytic activity, whose degradation rates
reached 95% after 1 h irradiation, more than twice the photocatalysis effect of original
g-C3N4.
Keywords: Graphitic carbon nitride, Nitrogen deficiency, Bi2MoO6, Photocatalytic
reduction, Z-scheme heterojunction

1. Introduction
Since the discovery of the photocatalytic properties of TiO2 by Japanese scientists
Fujishima and Honda for the first time in 1972[1], many researches have been
conducted on improving the efficiency of semiconductor photocatalysts[2]. A major
challenge is to suppress the recombination of photogenerated electron-hole pairs, which
are easily recombined due to the strong coulomb force[3,4]. In addition, for single-
component photocatalysts, the dilemma of their application is the lack of good
compatibility between strong redox capability and light response range[5–7]. That is,
in order to obtain a sufficient redox capability for a specific reaction to occur on a single
photocatalyst, a larger band gap is required. However, the higher utilization of solar
energy, the smaller band gap is required to increase the ability of absorbing light[8,9].
Therefore, it is of great significance to modify the photocatalyst against these
contradictions[10–13].
Researchers have found that Z-scheme photocatalysts inspired by natural
photosynthesis have great potential for achieving high photocatalytic performance[14].
Z-scheme photocatalysts have both strong oxidation and reduction capabilities and are
typically composed of two interconnected narrow band gap semiconductor
photocatalysts: one is an oxidized photocatalyst with a lower valence band position,
and the other is a reduced photocatalyst with a higher conduction band position[15].
Due to the unique charge transfer mechanism, Z-scheme photocatalyst can separate
photo-generated charge effectively, therefore it displays strong reduction and oxidation
ability at the same time[16,17].
As a unique 2D layered non-metallic material, the energy band structure of graphite
carbon nitride (g-C3N4) is very suitable for photocatalytic degradation of organic
pollutants, with simple synthesis method, good thermal and chemistry stability and no
secondary pollution. Therefore, it is regarded as a photocatalyst with broad application

prospects which has important research and application value in the fields of
photocatalytic decomposition of hydrogen production, degradation of organic
pollutants and reduction of carbon dioxide[18–22]. However, in recent research and
application of g-C3N4 as a photocatalyst, there are still certain challenges: common g-
C3N4 experimentally synthesized usually has a wide band gap (~2.7 eV) and can only
absorb a small fraction of short-wavelength light in visible light (λ < 460 nm).
Improving the absorption capacity of visible light has become one of the research
hotspots in this field[9,23–27]. Recently, some researchers found that the introduction
of nitrogen defects to the s-triazine structural unit of g-C3N4 is an effective way to solve
this problem[28,29]. The introduction of nitrogen defects can effectively improve the
separation efficiency of photogenerated electron-hole pairs. However, the reported
methods of introducing nitrogen defects generally involve multi-step operations and
harsh reaction conditions (such as high temperature treatment in a reducing atmosphere),
and the preparation process is complicated, so it is difficult to perform large-scale
applications in practical engineering[29–33].
Bi2MoO6 belongs to the Aurivillius phase perovskite and its structure is composed of
yttrium oxide layer (Bi2O2) mixed with perovskites (Am−1BmO3m+1) [34]. Due to its
conductive and photocatalytic properties, Bi2MoO6 has become a hot subject of current
research. Bi et al[35] synthesized nano-Bi2MoO6 photocatalyst with visible light
response capability by conventional solvothermal method and microwave-solvent
method, and the nano-Bi2MoO6 samples obtained under different conditions exhibited
superior photocatalytic properties. Liang et al[36]prepared Bi2MoO6/g-C3N4 composite
by ultrasonic assisted impregnation method, which exhibited the best photocatalytic
performance at pH = 7. Lin et al[37] successfully synthesized a high-efficiency visible
light-driven Ag/Ag3PO4/Bi2MoO6 Z-scheme photocatalyst through in-situ precipitation
method, which proved that the catalyst has superior photocatalytic effect, and the Z-
scheme system helped to separate photogenerated electrons and hole. However, those
existing methods to prepare Bi2MoO6/g-C3N4 composite cost a lot of raw materials and
time, and a more concise way is needed for large-scale production.

In this paper, Z-scheme Bi2MoO6/g-C3N4-x photocatalyst was prepared by a facile joint
heating method. The structure, morphology and composition of the prepared
photocatalyst was characterized by SEM, TEM, XRD and XPS. The photocatalytic
properties of the prepared materials were tested by photocatalytic degradation test.
Finally, combined with the mechanism and experiment of free radical trapping
experiments, the photocatalytic reaction mechanism of Bi2MoO6/g-C3N4-x composite
photocatalyst under visible light was studied and discussed.
2 Materials and Methods
2.1 Material preparation
The bulk g-C3N4-x was prepared by heating dicyanamide (C2H4N4, AR) to 625 °C for 4
h with a heating rate of 5 °C according to previous report[38], then it was ground to
powder and sonicated at room temperature for 4 hours to obtain g-C3N4-x nanoparticles.
The bulk g-C3N4 was prepared by heating dicyanamide to 550 °C for 4 h, while other
steps were the same as the preparation of g-C3N4-x.
1g mixture of ammonium molybdate ((NH4)2MoO4, AR) and bismuth nitrate
(Bi(NO3)3·5H2O, AR) in molar ratio of 1:4 was added to a solution of 50 mL double
distilled water and 50 mL ethanol (AR), and then ultrasonically dispersed at room
temperature for 4 h. After that, the solution was placed in a reaction vessel of
polytetrafluoroethylene substrate, stored at 180 °C for 12 hours. After cooling to room
temperature, the precipitate was separated by centrifugation at 5000 r/min for 10 min.
Then, the particles were washed with double distilled water and then dried at 60 °C to
obtain Bi2MoO6.
g-C3N4-x and Bi2MoO6 were ultrasonically dispersed in 100ml mixture of double
distilled water and ethanol (volume ratio of 1:1) by a mass ratio of m:1 (m = 0.25, 0.5,

0.75) for 2 h. The mixture was then dried and heated to 400 °C for 4 hours at a
temperature increase rate of 10 °C/min. After naturally cooling to room temperature,
the mixture was again added to 100 ml 1:1 water-ethanol solution, and ultrasonically
dispersed at room temperature for 4 hours, followed by drying for use. The sample
obtained was recorded as mBi2MoO6/g-C3N4-x. In addition, mBi2MoO6/g-C3N4 is
synthesized by the same steps using g-C3N4 and Bi2MoO6.
2.2 Characterization
In this test, D8 Advance X-ray diffractometer (XRD) produced by AxS, Germany was
used to analyze the structure and crystal phase, and the Cu-Kα ray was used with the
scanning range of 10°~70°. The S-4800 scanning electron microscope (SEM)
manufactured by Hitachi was used to study the structure of the samples with the
accelerate voltage of 0.3 to 30 kV. In addition, the test can also quantitatively analyze
the elemental composition in the prepared samples by energy dispersive spectrometer
(EDS). Titan G2 60-300 with images corrector field emission high-resolution
transmission electron microscope (TEM) was used to observe the inside micro structure
of the samples, and the acceleration voltage was 200 kV and the magnification was 50
to 1.1 million times. Elemental composition and valence were characterized by
ESCALAB 250Xi X-ray photoelectron spectroscopy (XPS), and the value band
position of the samples were performed using UPS test. The elemental analysis was
conducted by a Vario EL cube type CHNS/O elemental analyzer (German Elementar
Analysis System Company). The light absorption capacity of the sample was measured
using a PerkinElmer Labmda 750 S UV-Vis spectrophotometer (UV-Vis DRS) with a
scan range of 200 to 700 nm. The specific surface area of the samples was analyzed
using the ASAP 2020M automatic fast surface area and porosity analyzer manufactured
by McMurray (Shanghai) Instrument Co. Ltd. to determine the specific surface areas
and porosities. The photocurrent response capability of the material was tested using a
PP211 & CHI1030B photoelectrochemical workstation. Platinum was used as the
electrode and Ag/AgCl was used as the reference electrode, with 0.1 mol/L KCl as the

electrolyte, frequency of 1000 Hz, amplitude of 5 mA, and initial voltage of zero. The
photoactive free radicals of the photocatalyst were measured using Bruker A300
Electron Spin-resonance Spectroscopy (ESR) under a 300 W xenon lamp. The
photoluminescence spectroscopy (PL) of the catalyst was measured by the Edinburgh-
steady-state/transient fluorescence spectrometer FLS1000. The excitation source was a
450 W ozone-free xenon lamp with an excitation wavelength of 365 nm.
2.3 Photocatalytic activity test
Rhodamine B (RhB) was used as target pollutants to evaluate the photocatalytic activity
of the catalyst. The specific test procedure was as follows: Firstly, 10 mg of the catalysts
was placed in a double-layer beaker, and 100 mL of RhB aqueous solution (10 mg/L)
was added. The solution was then stirred under dark conditions for 30 min to bring the
solution to an adsorption-desorption equilibrium. Then, a 300 W xenon lamp was
turned on as a light source, and the solution was reacted under light for 60 min, and the
distance between the light source and the solution was 15 cm. From the start of the dark
reaction, 3 mL of water sample was taken every 15 min and then centrifuged at 10,000
r/min for 10 min. The supernatant was taken and the absorbance was measured by a
spectrophotometer with wavelength of 556 nm. The concentration of the RhB solution
was calculated according to the following formula:
A
C = C0
A0
Where: C- The immediate concentration of RhB solution;
C0-The initial concentration of RhB solution;
A-Instant absorbance of RhB solution;
A0-The initial absorbance of RhB solution.
In general, in the process of photocatalytic degradation of organic pollutants, mainly
due to photogenerated electron-hole pairs migrate to the surface of the photocatalyst,
-
and hydroxyl radicals(·OH), superoxide radical(·O2 ) and hole(h+) formed by reaction
with oxygen and hydroxyl groups participate in the reaction. In order to investigate

which reactive groups are involved in the degradation of RhB during photocatalytic
degradation, this study used masking agents to perform a degradation test again. In this
photocatalytic degradation test, 2 mmol/L of iso-propyl alcohol (IPA), benzoquinone
(BQ), and disodium edetate (EDTA-2Na) were added, respectively. The effects of
-
adding these three substances are capturing ·OH, ·O2 and h+. Comparing the
photocatalytic degradation effects of RhB after adding various masking agents, the
active substances participating in the reaction during the reaction were determined, and
the reaction mechanism of the photocatalytic reaction process were analyzed.
In practical application, the stability of photocatalyst affects the recovery performance
of photocatalyst, which indirectly affects the convenience of photocatalyst in practical
application. The test method for the stability of the photocatalyst is as follows: after the
end of the degradation experiment, the magnetic stirrer and the light source were turned
off, and the remaining solution and the photocatalyst were rotated at 10,000 rpm for 10
min. The photocatalyst after separation and recovery was washed by distilled water,
and then dried in an oven at 60 °C. The photocatalytic degradation test was again carried
out using the dried catalyst, and the performance of the photocatalyst was measured
again. This operation was repeated six times. The measurement of the photocatalytic
degradation ability was evaluated using the change of C/C0 of the RhB solution.
3. Result and discussion
3.1. Structural, morphological and chemical bonds analysis
XRD patterns of g-C3N4, g-C3N4-x, Bi2MoO6 and Bi2MoO6/g-C3N4-x at different
mixing ratios are shown in Fig. 1. The characteristic peaks of Bi2MoO6 correspond to
JCPDS 21-0102[39]. The pattern of g-C3N4-x showed two peaks at 2θ=27.70° and
13.00°, while in g-C3N4 they are 27.00° and 12.5°, corresponding to (002) and (100)
planes. The 0.326 nm and 0.681 nm interlayer distance may result from interlayer
accumulation[40]. The characteristic peaks of g-C3N4-x and Bi2MoO6 can be observed

in both three composite samples. SEM and TEM analysis also indicated that elements
Bi and Mo are evenly dispersed in 0.50Bi2MoO6/g-C3N4-x (Fig. 1S, 2S).
Fig. 1. XRD pattern of Bi2MoO6/g-C3N4-x, g-C3N4-x and g-C3N4.
XPS was conducted to investigate the element valence and elemental composition of
the surface of composite photocatalyst. The full spectrum of g-C3N4 and g-C3N4-x (Fig.
3S a) shows that these two catalysts still have the same elemental composition, and the
introduction of nitrogen defects does not introduce any other elements into the catalyst.
However, nitrogen defects extended or compressed the C-N bond to different degrees,
causing a slight shift in the binding energy (Fig. 3S b). The binding energy of N 1s
differentiates into two peaks (Fig. 3S c). The peak at 396.73 eV can prove the existence
of C-N-C bonds, and the peak at 398.10 eV is attributed to the presence of N-C3-x and
N-H[41]. Due to the introduction of nitrogen defects on the C-N ring, the binding
energy of the C-N bond in the XPS pattern of g- g-C3N4-x changed slightly. In addition,
from the UPS image analysis (Fig. 3S d), it is known that the top position of valence
band (VB) of g-C3N4 and g-C3N4-x is 1.96 eV, 1.64 eV, respectively, and their
conduction band (CB) can be easily calculated as -0.72eV and -0.89eV. The changes of
CB and VB prove that the position of the graphite phase valence band has changed
significantly after the introduction of nitrogen defects.

As for the XPS analysis of 0.50Bi2MoO6/g-C3N4-x, the peaks of the C 1s and N 1s did
not change significantly compared to g-C3N4-x (Fig. 3S f, g). The peaks at the binding
energies of 157.24 and 162.58 eV in Fig. 3S h represent Bi 4f7/2 and Bi 4f5/2,
respectively, and the binding energy around 230.48 eV and 233.65 eV can be attributed
to Mo 3d (Fig. 3S i)[39]. After Bi2MoO6 was compounded into graphite phase carbon
nitride containing nitrogen-defective structure, the crystal structure of Bi2MoO6 was
changed, and therefore the binding energy peak of Bi 4f and Mo 3d in Bi2MoO6/g-C3N4-
x composite photocatalyst can not perfectly match with pure Bi2MoO6. The broad and
asymmetrical peaks of O 1s (Fig. 3S j) indicated the existence of multiple chemical
states, and the peaks at 528.38 and 530.10 eV correspond to Bi-O and surface hydroxyl
groups (O-H), respectively[35,36,42]. In addition, the valence band positions of pure
Bi2MoO6 and g-C3N4-x were tested by UPS (Fig. 3S k), which were 2.15 and 1.64 eV
vs. NHE, respectively.
3.2. Optical property analysis
In order to characterize the optical properties of the prepared materials, UV-visible
diffuse reflection tests were performed on g-C3N4, g-C3N4-x (Fig. 2a) and Bi2MoO6/g-
C3N4-x composites (Fig. 2c). Light absorption edge of g-C3N4 and g-C3N4-x was 466 and
495 nm, respectively, proving that both had a certain visible light absorption capacity.
After introducing nitrogen defects, the light absorption region of g-C3N4-x was red-
shifted, because g-C3N4-x had larger specific surface area and porosity, and the light had
a sufficiently effective diffuse reflection inside the material. Therefore, the
photocatalyst's absorption and utilization of light was enhanced. The light absorption
edge of Bi2MoO6 was about 575 nm, showing superior absorption of visible light. With
the recombination of Bi2MoO6, the light absorption edge of Bi2MoO6/g-C3N4-x
composites showed obvious red shift. When the compounding amount was 25% and
75%, the light absorption edge was about 515 nm and 520 nm, respectively, and when

the compounding amount was 50%, the light absorption edge of the composite material
reached 528 nm, showing the most obvious red shift. Furthermore, the band gaps of g-
C3N4, g-C3N4-x and Bi2MoO6 were calculated using Kubelka-Munk equation and Tauc
Plot equation:
(hν ― E )2/n
αhν = A
g
Where: α- absorption coefficient
hν- optical frequency
A- characteristic constant of semiconductor
Eg- band gap
n- a constant, whose value depends on the type of transition of the
semiconductor, takes 1 for both g-C3N4, g-C3N4-x and Bi2MoO6.
According to Lambert-Beer's law, the absorption value A is proportional to the
absorption coefficient α. Therefore, it is possible to curve hν with (αhν)2, and the
intercept on the horizontal axis after the curve is tangent is the corresponding Eg. The
calculation results are shown in Fig. 2b, d. The band gaps of g-C3N4, g-C3N4-x and
Bi2MoO6 were 2.68, 2.53 and 2.58 eV, respectively. Combined with the UPS results,
the band gap positions of g-C3N4-x and Bi2MoO6 were 1.64~-0.89 eV and 2.15~-0.43
eV, respectively.

Fig. 2. Ultraviolet-visible diffuse reflectance spectroscopy ofg-C3N4, g-C3N4-x (a) and
Bi2MoO6/g-C3N4-x (c); band gap calculation of g-C3N4-x and Bi2MoO6 (b, d).
3.3. Photocatalytic performance evaluation
3.3.1. Photocatalytic activity test
In this study, RhB was used as a target pollutant to evaluate the photocatalytic
performance of Bi2MoO6/g-C3N4-x. After the catalysts were added to the test group for
30 min in the dark, the solution reached the adsorption-desorption equilibrium, and then
the light source was turned on for photocatalytic degradation test. After 60 min of light
irradiation, the degradation of RhB by pure g-C3N4-x and Bi2MoO6 reached about 50%,
and with the increase of Bi2MoO6 content, the degradation of RhB by Bi2MoO6/g-C3N4-
composite photocatalyst reached about 93%, 95% and 94%, respectively (Fig. 3a).
x
The reason for the increase of degradation rate was the introduction of nitrogen defects
and Bi2MoO6, which promoted the separation of photogenerated electron-hole pairs and
also inhibits the recombination of electron-hole pairs. However, with the increase of
Bi2MoO6 content, the excess Bi2MoO6 was beneficial to the recombination of
photogenerated carriers, which leads to the low efficiency of photo-generated carriers
participating in the reaction[43]. In addition, the degradation rate of 0.50Bi2MoO6/g-
C3N4 after 60 min light irradiation was only 72.5%, much lower than 0.50Bi2MoO6/g-
C3N4-x (95%), indicating that the introduction of nitrogen defects had a considerable
improvement in the photocatalytic degradation performance of the composite.

In order to evaluate the kinetic parameters of the degradation reaction, the first-order
reaction model was used to fit the degradation process of RhB by g-C3N4-x. The
calculation results of the degradation rate constant of RhB by different photocatalysts
are shown in Fig. 3b by fitting the reaction process. The degradation rate constants of
0.25, 0.50, 0.75Bi2MoO6/g-C3N4-x for RhB were 0.04252 min-1, 0.04789 min-1 and
0.04000 min-1, respectively. The degradation rate constant of 0.50Bi2MoO6/g-C3N4 was
only 0.02189 min-1, significantly lower than the three above, suggesting that the
introduction of nitrogen deficiency also improved the degradation rate constant.
Fig. 3. Degradation of RhB by different photocatalysts under visible light (a) and the
degradation kinetic curve (b).
3.3.2. Effect of reaction conditions on photocatalyst
performance
In the process of photocatalytic reaction, the pH of the solution, the initial concentration
of the contaminant and the dosage of the catalyst can significantly affect the progress
of the photocatalytic reaction. In response to these influencing factors, single factor
experiments were conducted on various influencing factors in this study to explore the
effects of different reaction conditions on the efficiency of photocatalytic reactions. Fig.
4a shows the photocatalytic degradation curves of 0.50Bi2MoO6/g-C3N4-x versus 10

mg/L RhB solution at dosages of 10, 20, 50, and 100 mg, respectively. With the increase
of catalyst, the degradation rate of RhB was also significantly improved, because more
photocatalysts can provide more reaction sites. Fig. 4b shows the photocatalytic
degradation curves of 0.50Bi2MoO6/g-C3N4-x versus 5 mg/L RhB solution at the same
dosage gradients. The removal efficiency of RhB was also significantly increased as
the dosage increased. In addition, the efficiency of the photocatalytic reaction was
significantly improved relative to the (a) group test (10 mg/L) after the concentration
of the RhB solution was lowered to 5 mg/L. The main reason for the improvement is
that the high concentration of RhB solution is darker, which affects the absorption of
light by photocatalyst.
The pH of solution can affect the surface chemistry and energy level of the
semiconductor photocatalyst. These properties directly affect the separation of
photogenerated electrons and holes. The rate of migration, which affects the progress
of the photocatalytic reaction. Fig. 4c shows the photocatalytic degradation curves of
50 mg 0.50Bi2MoO6/g-C3N4-x versus 10 mg/L RhB solution at pH 1, 1, 7, 11, and 14,
respectively. When the solution was alkaline, the degradation efficiency of g-C3N4-x to
RhB was lower than that under neutral condition, because under alkaline conditions, a
large amount of OH- would adhere to the surface of the catalyst, occupying the active
site of the reaction of the catalyst with RhB, thereby inhibit the photocatalytic
reaction[44,45]. When the solution was acidic, the concentration of OH- is lower than
the neutral condition, so the degradation efficiency of 0.50Bi2MoO6/g-C3N4-x to RhB
was slightly improved.

Fig. 4. Degradation of RhB using 0.50Bi2MoO6/g-C3N4-x under different conditions.
3.3.3. Photocatalyst stability test
In order to explore the stability of the photocatalyst prepared in this study, 50 mg
0.50Bi2MoO6/g-C3N4-x was used for multiple photocatalytic degradation of RhB cycle
test. After the end of each cycle test, the photocatalyst was washed by distilled water,
placed in an oven to dry, then subjected to the next photocatalytic degradation test.
After 6 cycles of experiments, the degradation ability of 0.50Bi2MoO6/g-C3N4-x to RhB
did not decrease significantly (Fig. 5S). In addition, XRD and XPS were characterized
by cyclic reaction of Bi2MoO6/g-C3N4-x. No peaks of other elements were found in the
XPS characterization results, and no shift in the peaks of the existing elements was
observed, indicating that the chemical bonds in the catalyst did not change. The shift of
the characteristic peak and the formation of new characteristic peak were not found in
the XRD characterization results. Hence, the Bi2MoO6/g-C3N4-x photocatalyst exhibited

good photocatalytic stability.
3.4. Mechanism of photocatalytic degradation
In order to determine the role of the three reactive groups in the Bi2MoO6/g-C3N4-x
visible light catalytic degradation system, and to investigate the formation, migration
and reaction mechanism of photogenerated electrons during the reaction, three different
radical scavengers were added to the photocatalytic degradation system[46–48]. The
test results are shown in Figure 5. The introduction of isopropanol (IPA), benzoquinone
(BQ) and EDTA-2Na in the catalytic system inhibits the photocatalytic reaction to
-
varying degrees, indicating that ·OH, ·O2 , h+ are both involved in photocatalytic
degradation of RhB, and their influence on the catalytic process from large to small
-
is ·OH> h+>·O2 .
Fig. 5. Effect of different radical scavengers on degradation of RhB by
0.50Bi2MoO6/g-C3N4-x.
-
Electron spin resonance (ESR) was also employed to test the signal of ·OH and ·O2
during the degradation proves (Fig. 6S). After receiving light, 0.50Bi2MoO6/g-C3N4-x
-
and Bi2MoO6 both produced obvious signal of ·O2 and ·OH, suggesting that the

Bi2MoO6/g-C3N4-x composites generated superoxide radicals and hydroxyl radicals
during the photocatalytic reaction. Photoluminescence spectroscopy (PL) results show
that the maximum excitation wavelengths of g-C3N4 and g-C3N4-x were 465 nm and
461 nm, respectively (Fig. 7S). The maximum excitation wavelength of g-C3N4-x red-
shifted and the fluorescence intensity decreased, indicating that the introduction of
nitrogen defects inhibited the photo-electron-hole pair binding[49]. This means g-C3N4-
has stronger visible light response and narrower band gap than g-C3N4, which is
x
consistent with the previous degradation experiments and UV-Vis DRS test results.
Furthermore, the photocurrent analysis of the sample was carried out to test the
instantaneous photocurrent response capability and electrochemical impedance of the
sample. The photocurrent mainly comes from the transfer of photogenerated electrons
by the catalyst after being irradiated with light to the counter electrode. The
photocurrent signals of g-C3N4-x, 0.50Bi2MoO6/g-C3N4-x and Bi2MoO6 were tested
under visible light irradiation, and they both produced significant photocurrent (Fig. 6a),
the order of which was 0.50Bi2MoO6/g-C3N4-x > g-C3N4-x > Bi2MoO6. The EIS Nyquist
curve of three samples (Fig. 6b) shows that the curvature of 0.50Bi2MoO6/g-C3N4-x was
smaller than that of g-C3N4-x and Bi2MoO6, indicating that the charge transfer rate in
0.50Bi2MoO6/g-C3N4-x was the highest.
Fig. 6. The photocurrent response curves (a) and EIS Nyquist plots (b) of g-C3N4-x、
0.50Bi2MoO6/g-C3N4-x and Bi2MoO6.
Based on the experimental data and results above, we conclude the photocatalytic

reaction mechanism of 0.50Bi2MoO6/g-C3N4-x. The band gaps of g-C3N4-x and
Bi2MoO6 are 1.64 ~ -0.89 eV and 2.15 ~ -0.43 eV, which can both be excited to generate
electrons and holes under visible light irradiation. Assuming that the electrons between
Bi2MoO6 and g-C3N4-x are transferred by the Z-scheme mechanism, the photogenerated
electrons in the conduction band (CB) of Bi2MoO6 are transferred to the valence band
(VB) of g-C3N4-x, and the holes generated on the VB of g-C3N4-x (h+) annihilation.
Therefore, photogenerated electrons are accumulated on the CB of g-C3N4-x, and react
with O2 to form superoxide radicals. On the VB of Bi2MoO6, holes are accumulated
and react with H2O or OH- to form hydroxyl radicals. Both three reactive groups
participate in the photocatalytic reaction process, degrading the target organic matter
until mineralization (Fig. 8S). This assumption is consistent with ESR testing and free
radical capture experiments.
However, if the charge transfer mechanism is a type II heterojunction, the holes in the
VB of Bi2MoO6 are transferred to the VB of g-C3N4-x, and the electrons in the CB of g-
C3N4-x are transferred to the CB of Bi2MoO6, which means the photogenerated electrons
-
accumulated on the CB of Bi2MoO6 can not reduce O2 to ·O2 (-0.33 eV vs. NHE),
contradicting the ESR test and the free radical trapation experiment. According to the
above discussion results, in the degradation process of RhB by 0.50Bi2MoO6/g-C3N4-x
composite photocatalyst, the electron transfer mode is Z type (Fig. 7b).

Fig. 7. Schematic mechanism of RhB degradation by g-C3N4, g-C3N4-x (a) and
0.50Bi2MoO6/g-C3N4-x (b).
4. Conclusion
The structure, morphology and composition of Bi2MoO6/g-C3N4-x composite
photocatalyst were characterized by SEM, TEM, XRD and XPS. The photocatalytic
properties of the prepared materials were tested by photodegradation test of RhB and
solution. Finally, the photocatalytic reaction mechanism of Z-scheme Bi2MoO6/g-C3N4-
x composite photocatalyst under visible light was studied and discussed in combination
with the mechanism test of free radical trapping test. The main conclusions are as
follows:
(1) The structure, morphology and elemental valence composition of the sample were
characterized by XRD, TEM, XPS, etc., which proved that Bi2MoO6 and g-C3N4-x were
successfully compounded without changing their original structures;
(2) The optimal mass ratio of Bi2MoO6 and g-C3N4-x in the synthesized composite
photocatalyst was determined by photocatalytic degradation test for different
proportions of Bi2MoO6/g-C3N4-x composite photocatalyst. Compared with Bi2MoO6
and g-C3N4-x, Bi2MoO6/g-C3N4-x composites showed higher photocatalytic degradation

rate under visible light, and the photocatalytic activity did not decrease significantly
after repeated tests.
(3) According to the results of ESR and free radical trap experiments, it was proved that
-
·OH, ·O2 , h+ were involved in the reaction process of 0.50Bi2MoO6/g-C3N4-x
degradation of target organic matter. And the effect of three reactive groups on the
-
photocatalytic reaction is ·OH> h+>·O2 .
(4) The electron transfer mechanism between Bi2MoO6 and g-C3N4-x is Z-scheme. The
loading of Bi2MoO6 enhances the separation efficiency of photogenerated electron-hole
pairs in the material and improves the photocatalytic performance of the material.
Acknowledgement
This research was supported by the Fuling Shale Gas Environmental Exploration
Technology of National Science and Technology Special Project (Grant
No.:2016ZX05060) and the Demonstration of Integrated Management of Rocky
Desertification and Enhancement of Ecological Service Function in Karst Peak-cluster
Depression (Grant No.: 2016YFC0502400).
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1. Increasing the photocatalysis efficiency of traditional carbon nitrogen by
introducing nitro deficiency and composed with BiMoO6.
2. Successfully synthesized high-efficiency BiMoO6/g-C3N4 photocatalyst using a
facile joint heating method.
3. Finding a convenient thermal etching method to control the nitro deficiency in
carbon nitrogen.
4. Using ESR and free radical trap experiments to determine the roles of ·OH, ·O2-,
h+ in the photocatalytic reaction.

Author contributions:
Boxun Zhou: Methodology, experiments, writing - original draft.
Qi Liu: Software, experiments, writing-revision.
Shici Zhang: Experiments, data analysis, writing-supervision.
MANZOOR SANJRANI: Writing-supervision, spell check.
Hang Yang: Resources, review-editing.
Shibin Xia: Conceptualization, reviewing-editing.
Declaration of interests
☒ The authors declare that they have no known competing financial interests or
personal relationships that could have appeared to influence the work reported in this
paper.
☐The authors declare the following financial interests/personal relationships which may
be considered as potential competing interests: