Ultrasound-assisted synthesis of BiVO4/C-dots/g-C3N4: Z-scheme heterojunction photocatalysts for degradation of minocycline hydrochloride and Rhodamine B: Optimization and mechanism investigation

Article abstract of DOI:10.1039/d0nj03375h

As a solid-state electron medium of composite photocatalysts, carbon dots (C-dots) increase the transition process of photogenerated carriers and inhibit their rapid recombination. In this work, novel and efficient Z-scheme heterojunction BiVO4/C-dots/g-C3N4 photocatalysts with C-dots as the photoelectron transfer center were designed. FTIR, HRTEM, TEM, XRD and PL were used to investigate the physical and chemical properties of the photocatalysts. Due to the up-conversion photoluminescence characteristics of the carbon dots, the BiVO4/C-dots/g-C3N4 photocatalysts exhibit significant interfacial charge transfer capability and a broader visible absorption range. In addition, the photocatalytic activity of the photocatalysts was improved by changing the doping amount and optimizing the synthesis conditions. As a result, the BiVO4/C-dots/g-C3N4 composites showed remarkable degradation efficiency of minocycline hydrochloride (Mino-HCl) and Rhodamine B (RhB) under near-visible LED light. Furthermore, the photodegradation efficiency of the BiVO4/C-dots/g-C3N4-40 photocatalyst was conspicuously improved over those of pure BiVO4, g-C3N4 and the composites with other content ratios. The comparative experiment also proved that the introduction of carbon dots can enhance the performance of the photocatalyst. Finally, the possible catalytic mechanism was explored via a free radical capture experiment for the active substances, and the excellent photostability and reusability of the BiVO4/C-dots/g-C3N4 composites were proved through three cycles of experiments.

Full text of DOI:10.1039/d0nj03375h

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Chechia Hu, Tzu-Jen Lin et al.
Yellowish and blue luminescent graphene oxide quantum dots
prepared via a microwave-assisted hydrothermal route using
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Ultrasound assisted synthesis of BiVO4/C-Dots/g-C3N4 Z-scheme heterojunction photocatalyst
for degradation of minocycline hydrochloride and Rhodamine B: Optimization and mechanism
investigation
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Qiang Yang1, Siqi Wei1, Limei Zhang1, and Rui Yang1,*
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Abstract
As solid-state electron medium of the composite photocatalyst, Carbon dots (C-dots) possess the
property of increasing the transition process of photogenerated carriers and inhibit their rapid
recombination. In this work, a novel and efficient Z-scheme heterojunction BiVO4/C-dots/g-C3N4
photocatalyst with C-Dots as the photoelectron transfer center has been designed. FTIR, HRTEM,
TEM, XRD and PL were used to investigate the physical and chemical properties of the photocatalysts.
Due of the up-conversion photoluminescence characteristics of carbon dots, the BiVO4/C-dots/g-C3N4
photocatalyst exhibits a significant interfacial charge transfer capability and a broader visible
absorption range. In addition, the photocatalytic activity of the photocatalysts was improved by
changing the doping amount and optimizing the synthesis conditions. As a result, BiVO4/C-dots/g-
C3N4 composites showed remarkable degradation efficiency of minocycline hydrochloride (Mino-HCl)
and Rhodamine B (RhB) under a near-visible LED light. Furthermore, the photodegradation efficiency
of BiVO4/C-dots/g-C3N4-40 photocatalyst was conspicuously improved than that of pure BiVO4, g-
C3N4 and other content ratio composites. The comparative experiment also proves that the introduction
of carbon dots can enhance the performance of photocatalyst. Finally, the possible catalytic mechanism
was explored via free radical capture experiment for active substances, and the excellent photostability
and reusability of BiVO4/C-dots/g-C3N4 composites were proved through three cycle of experiments.
Keywords Z-scheme heterojunction, photocatalysis, g-C3N4, carbon dots, electron medium
serious water pollution4. To solve this environmental
1. Introduction
problem, researchers have focused on the development
Over recent years, improper treatments of
industrial, medical and other types of wastewater have
caused serious water pollution, such as organic
pollutants and antibiotic wastewater1, 2.These pollutants
severely threaten the ecological environment of the
natural water as well as human health. For example, an
emerging broad-spectrum antibiotic, minocycline
hydrochloride (Mino-HCl), due to it cannot be
completely absorbed by organisms, and its metabolites
may be discharged into the environment in their original
state, resulting in antibiotic wastewater pollution3. In
addition, as an important chemical product, dye has
great economic value, but its wastewater is highly toxic,
carcinogenic, difficult to degrade and easy to cause
of novel photocatalysts. In recent years, many
environmentally friendly photocatalysts have been
developed and applied to the production of clean fuels
and environmental restoration5-8. Using solar energy
and reusable semiconductor materials to photodegrade
pollutants is a promising method of environmental
remediation9, 10
.
Graphite-phase carbon nitride (g-C3N4) has caught
widespread attentions of researchers because of its
narrow band gap and suitable conduction band
position11-14, and the irradiation of visible light can
stimulate g-C3N4 to generate photoelectrons and oxidize
-
O2 to form superoxide radicals (·O2 ) 15, 16. However, the
weak interaction (van der Waals force) among adjacent
1School of Chemistry and Chemical Engineering, Southwest University, No.2 Tiansheng Road, Chongqing 400715, People’s Republic of China
*Corresponding author. E-mail address: ruiyang@swu.edu.cn

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C-N layers in g-C3N4 will inhibit the separation of
photogenerated carrier17. Accordingly, a lot of efforts
has been put into improving the photocatalytic
performance of g-C3N4, such as the construction of
nanostructures18 and coupling with metal19 and
nonmetallic semiconductors20. Carbon dots (C-dots) as
a novel nanomaterials, has unique photogenerated
electron transfer and up-conversion photoluminescence
(PL) properties 21, which makes the carbon dots
modified g-C3N4 has a good degradation effect on
photocatalytic dyes and phenols22-24. Besides, the
conjugated π structure of C-Dots can interact C-Dots
with semiconductor to establish stable composite
materials25. In recent years, many composite catalysts
synthesized by doping carbon dots have been reported
using C-dots as an electron-hole bridge and research on
its degradation efficiency of minocycline hydrochloride.
Herein, a novel Z-scheme BiVO4/C-Dots/g-C3N4
heterojunction photocatalyst was fabricated by
sonication. The BiVO4/C-Dots/g-C3N4 photocatalysts
showed obvious photoactivity to degrade Rhodamine B
(RhB) and minocycline hydrochloride (Mino-HCl)
under LED light irradiation, and after introducing C-
Dots, the photocatalytic performance was significantly
improved compared with that of the binary catalyst.
Simultaneously, the impacts of different conditions on
the photocatalytic properties of the composites were
investigated through comparative experiments. The
stability of photocatalyst was evaluated by three cyclic
tests. And the degradation mechanism of contaminants
by BiVO4/C-Dots/g-C3N4 was investigated by free
radical capture experiment.
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to degrade wastewater26, 27
.
Bismuth vanadate (BiVO4) is a green, non-toxic
semiconductor photocatalyst, which has been widely
studied due to its narrow band gap and responses under
visible light 28. BiVO4 generally has three crystal form,
namely orthogonal crystal, tetragonal zircon and
monocline scheelite type, respectively29, 30. The
photocatalytic activity of monoclinic scheelite BiVO4 is
higher than that of other crystal forms, because its
degree of distortion around Bi3+ and V5+ is very high,
which increases the degree of local polarization and is
conducive to separate the photogenerated electrons and
holes. However, pure BiVO4 still has a low utilization
rate of visible light and a high photogenerated carrier
2. Experimental
2.1. Materials and reagents
Ammonium metavanadate (NH4VO3), ammonia
(NH3·H 2O), Bismuth nitrate (Bi(NO3)3·5H2O),
melamine (C3N3(NH2)3), nitric acid (HNO3),acetic acid
(CH3COOH), citric acid (CA), formamide (CH3NO),
EDTA-2Na, 4-benzoquinone (BQ), isopropanol (IPA),
rhodamine B and minocycline hydrochloride were
obtained from the Shanghai Macklin Biochemical Co.,
LTD, Shanghai, China.
binding rate after hybrid with different semiconductor
31
materials, such as BiVO4/Bi2O3 29and WO3/BiVO4
.
2.2. Preparation
To settle these problems,
a
novel Z-scheme
heterojunction photocatalyst was constructed by
combining BiVO4 and C-Dots with g-C3N4. Notably,
these BiVO4/C-Dots/g-C3N4 composites can not only
retain the reduction capacity of g-C3N4 at the negative
CB position, but also improve the oxidation capacity of
BiVO4 at the positive VB position. Moreover, Z-scheme
heterojunction photocatalyst can also improve the
photocatalytic activity owing to its excellent separation
efficiency of photogenerated electrons-holes and a wide
absorption spectrum. To our knowledge, there is
currently no research on constructing Z-scheme
BiVO4/C-Dots/g-C3N4 heterojunction photocatalysts
2.2.1 Synthesis of C-dots
The C-Dots were obtained by a hydrothermal
method in previous work32. Firstly, 1 g of citric acid and
10 mL formamide were mixed and stirred, then
transferred to the autoclave reactor and heated for 4 h at
180 ℃. The solution was centrifuged (8000 rpm, 10min)
3 times to remove insoluble particles. Subsequently, the
precipitation was collected by adding huge amount of
acetone (eluent) into the solution, then centrifuged and
dried it. At last, the C-Dots powders were prepared into
solutions with concentrations of 0.2, 0.5, 1, 2, 5, 10 and
20 mg/mL, respectively.
2

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on the JEM-2100. X-ray difDfOraI:c1t0i.o10n3V9ie/wD(0AXNrtiJRc0leD3O3,n7l5inHe
2.2.2 Preparation of g-C3N4/C-Dots and BiVO4
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SHIMADZU 7000) was used to characterize the
crystalline structure of all the prepared photocatalysts.
X-ray photoelectron spectroscopy (XPS, Perkin-Elmer
PHI 5000) was used to measure the chemical state of
element and element composition. UV–vis absorption
spectra were measured on a Shimadzu UV-2700
spectrometer, and the Fourier transform infrared (FT-IR,
FTIR-8400S) spectra were also recorded. PL spectra
dates were obtained on F-7000 spectrophotometer
(Hitachi, Japan). The electron spin resonance (ESR)
was measured on a Bruker model ESR ELEXSYS II
spectrometer. Specific surface areas and pore structures
are probed by measuring volumetric N2 adsorption-
desorption isotherms at liquid nitrogen temperature,
using an ASAP 2460 instrument (Micromeritics, USA).
Raman spectra were also recorded on HORIBA
LarRAM HR 800.
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g-C3N4/C-Dots was prepared by adding C-Dots to
the melamine and calcining. Briefly, melamine (2 g) and
C-Dots solution (2 mL) were dispersed in DI water (10
mL), and the precipitation was dried at 80 ℃ for 1 h.
The resulting powder was calcined in a muffle furnace
at 550℃ for 4 h (the heating rate was 5 ℃/min). Finally,
the light-yellow solid was ground into fine powder,
which was named GCDs. Pure g-C3N4 (GCN) was
synthesized through direct calcination of melamine
without C-Dots, and the procedures as same as
described above.
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BiVO4 was prepared through a hydrothermal route.
Firstly, 5 mmol Bi(NO3)3·5H2O was dissolved in 10 mL
acetic acid to form solution A, and 5 mmol NH4VO3 was
added into 50 mL deionized water to form solution B.
Then the solution B was dropped into solution A, and
ultrasonic treatment for 30 min to form yellow
precipitation. Subsequently, the pH of the suspension
was adjusted to 9 with NH3•H2O. After stirring for 2 h,
the solution was heated at 140℃ for 20 hours in
autoclave. After the reaction, it was cooled to ambient
temperature, centrifuged, washed and dried at 80 ℃
overnight and the obtained powder was calcined at 450°
for 2h.
2.4. Photoelectrochemical analysis
The
photoelectric
current
(PC)
and
electrochemical impedance spectroscopy (EIS) of the
photocatalyst were recorded through a CHI660E
electrochemical workstation using a standard three-
electrode system with a working electrode, a Pt wire
counter electrode and an Ag/AgCl reference electrode.
The preparation method of electrodes would be
presented in detail as follow: 5 mg of samples was
added into the mixed solution of 0.5 mL of deionized
water, 0.5 mL of alcohol and 50 μL of Nafion and then
uniformly dispersed by ultrasound to obtain a
suspension. Then, 2 μL of suspension was spread onto
a 1 cm2 ITO glass and dried under ambient condition. A
300W LED was used as the visible light source. And a
0.5 M Na2SO4 aqueous solution was used as the
electrolyte.
2.2.3 Synthesis of BiVO4/C-Dots/g-C3N4
BiVO4/C-Dots/g-C3N4 (BCG) was construct
through ultrasonic treatment and calcination. 1 g of
above GCDs was mixed with a certain amount of BiVO4,
and transferred into 50 mL deionized water to form a
suspension. After ultrasonic treatment for 30 min, the
suspension continued stirring for 10 h. The resulting
powder was washed several times and dried overnight
in a vacuum drying oven, then heated to 450℃ for 2 h
(5 ℃/min). Following the above procedure, BCG
composites with different mass ratios were synthesis by
adding 20, 30, 40, 50 % BiVO4 (labeled as BCG-20,
BCG-30, BCG-40, BCG-50, respectively).
2.5. Photocatalytic activity and photostability
The photocatalytic performance of all the
photocatalyst was investigated by degradation of
Rhodamine B (RhB, 20 mg/L) and minocycline
hydrochloride (Mino-HCl, 20 mg/L) under the 350W
LED (460 nm), and the simple photocatalytic device is
shown in Fig. S1. Briefly, 50 mg of the prepared
2.3. Characteristics
A transmission electron microscope (TEM) and
high resolution TEM (HRTEM) date was obtained with
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photocatalyst was dispersed into 50 mL of 20 mg/L RhB
(Mino-HCl) solution, stirred in the dark and kept for 30
minutes before irradiation to reach the adsorption-
desorption equilibrium. The suspension samples (3 mL)
were collected and filtered to remove photocatalyst, and
using the UV-vis absorption spectrophotometer to
measure the concentration of the solution. The stability
of BCG composites was evaluated by three cyclic tests.
the synthesized BiVO4 had high purity and single crystal
type. Fig. 1d-g demonstrate the XRD analysis of BCG
photocatalyst with different BiVO4 amount. The peak of
2θ = 27.5 ° in BCG-20 is ascribed to the (002) crystal
plane of GCN, which indicates that BiVO4 successfully
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3. Result and discussion
3.1. Characterization of the photocatalyst
3.1.1. XRD analysis
In order to investigate the crystal lattice structure,
the XRD analysis of different photocatalysts as shown
in Fig. 1. According to the reported33, the XRD pattern
of GCN exhibit primary diffraction peak near 27.5°
corresponds to the (002) planes 34, 35. Pure GCN and
GCDs show similar and obviously peaks at 27.5°,
probably because the concentration of C-Dots is limited,
the crystallinity is low, and the dispersion is high. The
pure BiVO4 has obvious diffraction peaks at 2θ =18.7°,
19.0°, 28.9°, 30.5°, 34.6°, 35.2°, 39.9°, 42.5°, 46.8°,
47.3°, 53.3°, 58.4° and 59.3° are ascribed to the crystal
planes of monoclinic BiVO4, which is (110), (011),
(121), (040), (200), (002), (211), (150), (060), (240),
(161), (321) and (123), respectively36, 37. Moreover,
there were no other characteristic peaks, indicating that
Fig. 1. XRD patterns of (a) GCN, (b) GCDs, (c) BiVO4, (d) BCG-
20, (e) BCG-30, (f) BCG-40, and (g) BCG-50.
binds to GCDs. As the BiVO4 content increases, the
diffraction peaks intensity of BiVO4 increase, while the
peaks intensity of GCDs decrease. The results show that
we successfully prepared BCG composites.
3.1.2. Morphology of catalysts
The morphology and microstructure of the prepared
photocatalyst were characterized by transmission
electron microscopy and high-resolution transmission
Fig. 2. TEM images of (a) BiVO4, (b) GCN, (c) BCG-40, and HRTEM images of (d) C-Dots, (e) GCDs and (f) BCG-40.
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Fig. 3. (a) FT-IR spectra and (b) Raman spectra of as-prepared photocatalysts.
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electron microscopy, and the results are shown in Fig. 2.
From the Fig. 2a, it can be observed that the pristine
BiVO4 exhibit clearly regular crystals. Fig. 2b exhibits
the TEM image of pure GCN，which can clearly see the
irregular layered sheet structure. Fig. 2d and 2e show
HRTEM images of C-Dots solution and GCDs, and it
can be clearly seen that the C-Dots adhere uniformly on
the surface of GCN. Fig. 2c and Fig. 2f are the TEM and
HRTEM images of BCG-40. It can be obviously
observed BiVO4 coat on the surface of GCDs, and the
binding of carbon points and nanoparticles can also be
observed between the interfaces of the two.
cm-1 are attracted by the C-N stretching vibration of sp3
hybrid, the 1638 cm-1 is caused by the C=N stretching
vibration of sp2 hybrid. The broad band ranged from
3080-3300 cm-1 is usually attributed to the N-H
stretching vibration of GCN, which is caused by the
occurrence of several hydrogenated nitrogen atoms on
the prepared GCN nanosheets, and the N-H stretching
vibration at the defect of the aromatic ring. For BCG-40
composites, the main peaks of GCN and BiVO4 still
existed which indicated that the combination of BiVO4
does not affect the structure of the GCDs. Although the
characteristic peaks of BiVO4 are weak, it can still prove
that BiVO4 successfully incorporated into GCDs and
prepared BCG. Raman spectral analysis was performed
on samples BiVO4 and GCDs, and the results were
shown in Fig. 3b. The vibrational bands of pure BiVO4
at approximate 121 cm-1, 210 cm-1, 323 cm-1, 829 cm-1
are measured, which are consistent with the previous
report38, 42, 43. The typical D and G peaks in Raman
spectrum are not observed in the figure, which may be
due to the fluorescence effect of GCDs itself, which
produces a strong background value under the action of
excitation light44. However, it can be seen that the
characteristic peak at 2200 cm-1 may be due to a small
amount of C≡N bonds produced by high temperature
processing.
3.1.3. FT-IR and Raman analysis
The FT-IR spectra of as-prepared composites are
showed in Fig. 3a, which was used to investigate the
chemical function groups of it. For the BiVO4, the peak
at 731 cm-1, 517 cm-1 and 475 cm-1 are deem to be the
3-
VO4 , Bi-O and V-O characteristic peak of BiVO4,
respectively38, 39. While the peak at 1619 cm-1 and 3415
cm-1 correspond to bending and stretching vibrations in
H2O molecules. The characteristic peak of pure GCN at
808 cm-1 is the typical stretching vibration peak of the
3-s-triazine ring units in GCN 40, and the peaks between
1200-1700 cm-1 was associated with two vibration
modes C=N and C-N, which is a unique skeleton
vibration of the aromatic ring41. Among them, the
several weak peaks at 1240 cm-1, 1322 cm-1, and 1410
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Fig. 4. XPS analysis of as-prepared photocatalysts: (a) survey spectrum (b) C1s, (c) N1s, (d) V2p, (e) O 1s, and (f) Bi4f.
3.1.4 XPS analysis
V2p1/2 in V-O, respectively, which is consistent with the
binding energy of V5+ in crystal structure50. Fig. 4e
shows the XPS spectra of O1s. The O1s peak can be
split into two peaks at 530.50 and 531.44 eV, meaning
the presence of two forms of oxygen on surface of the
samples, which are attributed to Bi-O in BiVO4 and H2O,
respectively26, 51. In the XPS spectra of Bi4f (Fig. 4f),
the peaks are mainly concentered at 159.26 and 164.48
eV, which are attributed to the characteristic peaks of
Bi4f5/2 and Bi4f7/2 binding energies ,respectively52. It is
demonstrated that the Bi element of BiVO4 is Bi3+.
Finally, the FT-IR and XPS analysis proved that the
synthesis of BCG composites is successful.
XPS analysis was performed to investigate the
elemental composition and the chemical states of the as-
prepared catalysts. Shown in Fig. 4a, the survey XPS
spectrum shows that Bi, O, V, N, and C elements exist
in BCG-40 composite, as well as the characteristic
peaks generated by Bi4f, O1s, V2p, N1s, C1s. For the
pure GCN in the Fig. 4b, the peaks of C1s around
284.73 and 288.24 eV can be assigned to the C-C and
45,
C-(N)3 46. And Fig. 4c shows that N1s spectrum
features with three peaks at 398.71 eV, 400.64 eV,
404.45 eV14. The peak at 398.71 eV can be assigned to
the C=N-C functional group formed by the
hybridization of the sp2 orbital of N atom and two C
atoms. While the peak at 400.64 eV can be attributed to
3.1.5. PL analysis
the N-(C)3 group and the weak peak at 404.45 eV is the
Photoluminescence emission primarily comes from
the recombination of photogenerated electrons and
holes in semiconductors, and its spectrum can indicate
the recombination rate of photo-excited electrons and
holes, the low emission peak intensity represents the
low recombination rate 53. Fig. 5 shows the steady-state
fluorescence spectra of pure GCN, GCN/BiVO4, GCDs,
BCG-40 and pure BiVO4 under 380 nm excitation. It
can be clearly observed that the pure GCN has a strong
48
characteristic peak of graphitized species47,
.
Compared with pure GCN and GCDs, the
disappearance of BCG-40 composites peak at 404.45
eV proves that BiVO4 covers the graphitic species, and
π-electrons from CN heterocycles used to couple with
GCDs through an attractive force instead of binding
with C and N atoms in GCDs49. For BiVO4 in Fig. 4d,
the characteristic peaks of V2p in XPS spectrum at
517.08 eV and 524.71 eV, corresponding to V2p3/2 and
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emission peak near 470 nm, which coincides with the
separation of photogenerated carriersD.OTI:h1e0s.1e03P9L/D0reNsJu0l3t3s75H
of the prepared catalyst are also consistent with the
photocatalytic activity of contaminant degradation,
which will be confirmed in the following photocatalytic
activity tests.
visible light absorption band of the GCN photocatalyst53.
The PL spectra shows that the photoluminescence
intensity of GCN, GCDs and BCG-40 decreases in turn
and the relative photoluminescence intensity of BiVO4
is too low to be detected. Among these, pure GCN has
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3.1.6. Photocurrent response and EIS analysis
A high charge separation rate and quick migration
of photoinduced charges are two key factors for
increasing the photoactivity of a photocatalyst54, in
order to demonstrate the separation and transfer feature
of the photogenerated electron-hole pairs, the
photocurrent response (PC) tests and the
electrochemical impedance spectroscopy (EIS) of as-
prepared BiVO4, GCN, GCDs and BCG-40 were
measured. As shown in Fig. 6a, pure BiVO4 and GCN
have weak photocurrent intensity because of the low
separation efficiency of the photogenerated carriers.
However, the photocurrent intensity of GCDs was
higher as compared with that of BiVO4 and GCN, and
BCG-40 exhibits the highest photocurrent intensity,
implying that the introduction of C-Dots can help to
accelerate the transfer and separation of photogenerated
electron-holes. Moreover, to further investigate the
charge transfer resistance and separation efficiency of
the carriers, EIS measurement of as-prepared
photocatalysts were also carried out and displayed in
Fig. 6b. The diameter of the semicircle of the EIS
Nyquist plots is related to the transfer efficiency of
Fig. 5. PL spectra of as-prepared GCN, GCDs, GCN/BiVO4,
BCG-40 and BiVO4.
the highest emission peak intensity, which means that
the charge recombination rate of GCN is high, resulting
in poor photocatalytic efficiency. The BCG-40
composite has the lowest emission peak intensity, which
represents the best photocatalytic activity. Moreover,
the photoluminescence intensity of BCG-40 is lower
than that of GCN/BiVO4, which further proves that the
carbon dot as an electron transfer center enhances the
Fig. 6. (a) Photocurrent response and (b) EIS Nyquist plot of the pure BiVO4, GCN, GCDs, and BCG-40.
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photoinduced carriers, usually, the smaller the radius of
EIS Nyquist plots is, the higher the separation efficiency
of charge carriers is. It can be seen from the Fig. 6b that
the arc radius of BCG-40 was smaller than that of pure
BiVO4, GCN and GCDs, indicating that BCG-40
exhibited the highest photoactivity among those
photocatalysts. And the above results are well consistent
with the date of the PC and PL experiment.
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3.1.7 BET analysis
BET surface area and Barrett-Joiner-Haled pore
structure of the photocatalysts are investigated by
nitrogen-adsorption isotherms. As shown in Fig. 7a, the
adsorption-desorption curve of the catalysts belongs to
the type IV isotherm, which corresponds to the
mesoporous character of all investigated materials with
a negligible amount of micropores. The specific surface
areas of BiVO4, GCN, GCDs and BCG-40 composite
catalysts calculated by BET method were 2.7186 m2/g,
23.7486 m2/g, 24.1931 m2/g, and 19.7876 m2/g,
respectively. It can be clearly observed that the addition
of GCDs greatly increases the specific surface area of
the BiVO4. Although the specific surface area of the Z-
scheme heterostructure photocatalyst is slightly lower
than that of GCN, it cannot be ignored that the Z-scheme
heterostructure can enhance the photocatalytic
performance, and effective separation of photo-
generated carriers is the dominating factor of
photodegradation. From the pore size distribution in Fig.
7b, most of the pores were concentrated between 5-50
nm indicating the mesoporous structure, which is
consistent with the characteristics of the IV isotherm.
The bigger size of the photocatalyst pore structure will
enhance the adsorption rate of the contaminant, and
more detail date are summarized in Table 1.
Fig. 7. (a) N2 adsorption-desorption isotherms and (b) Barrett-
Joiner-Helenda pore size distribution curves of the as-prepared
photocatalysts.
3.2. Photocatalytic activity
Photodegradation efficiency of as-prepared
photocatalysts was examined through the method
described in Section 2.5 above. And the specific
situation of the concentration of RhB solution changing
with the illumination time shown in Fig. 8a. Firstly, the
concentration of RhB solution of the blank group has
slight change, indicating that the photodecomposition of
RhB solution itself was almost negligible. For pure
BiVO4 and GCN, the photocatalytic activity is also low,
only 9.6 % and 60.48 % of RhB solution was degraded
within 30 min. It is obvious that GCN/BiVO4 can
improve the photocatalytic efficiency, but the effect is
still very limited. The BCG composite material obtained
by introducing carbon dots can further inhibit the
recombination of photogenerated carriers and enhance
Table 1
The BET specific surface area, pore volume and pore
size of photocatalysts.
Sample
SBET
Pore Volume
(cm3/g)
Pore Diameter
(m2/g)
(nm)
BiVO4
GCN
2.7186
23.7486
24.1931
19.7876
0.0078
11.28
0.1637
27.58
GCDs
0.1564
25.85
BCG-40
0.1026
20.78
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the photocatalytic activity, which is also consistent with
dark adsorption, the probability of sDeOlfI-:d10e.g10r3a9d/aDt0iNoJn03o3f75H
minocycline hydrochloride under the light source is
very low (16 %). However, after 140 min of light
treatment, pure GCN, GCDs and pure BiVO4 only
degraded minocycline hydrochloride by 18 %, 22 % and
31 %, respectively. Unsurprisingly, the degradation rate
of GCN/BiVO4 is higher than that of GCN and BiVO4
while much lower than that of BCG. The BCG-40
showed the highest photocatalytic degradation
efficiency, and the removal rate of target pollutants
reached 65.3% within 140 minutes. Combining the
photodegradation process with a pseudo-first-order
model and drawing a slope graph in Fig. 9b, the k value
of BCG-40 is the largest, representing the best reaction
rate in the process of photocatalysis. The above
conclusions indicate that BCG-40 composite material
also has great potential in removing antibiotics from
wastewater.
PL spectroscopic analysis. Moreover, the research
shows that the loading of BiVO4 will affect the
photocatalytic efficiency of BCG composites. When the
proportion of BiVO4 is 40%, the photocatalytic
performance reaches the peak, and the degradation of
RhB is the largest (85.1%), which is 1.4 and 8.8 times
that GCN of and pure BiVO4, respectively. And the
more specific details are shown in Fig. S2.
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The kinetic behavior of photocatalyst can been
calculated as follows:
ln（퐶퐶0）= kt，
(1)
푡
where C0 and Ct are the initial and final concentrations
of targeted contaminants in the photocatalytic process,
and k (min-1) is the constant of equation 55. As shown in
the Fig. 8b, the k value of BCG-40 was 0.06317 min-1,
which is 18.7, 2.04, 1.87, 1.68, 1.54, 1.12 and 1.46 times
higher than for BiVO4, GCN, GCDs, GCN/BiVO4,
BCG-20, BCG-30 and BCG-50, respectively. This
result further proves that the carbon dots coupling with
BiVO4 and GCDs can improve the photoactivity of the
composite. And compared with previous reports, the
degradation of RhB by BCG-40 photocatalyst under
visible light was significantly improved in this work (as
shown in Table 2).
Table 2
Comparison on the rates of photocatalytic degradation
of RhB with different catalysts.
Catalyst
C
Time
(min)
120
120
150
80
Degrada
tion rate
85 %
Ref.
(mg/L)
15
CN/BiVO4
CDs/BiVO4
CN/SnO2
BiOCl/CN
BCG-40
Jiang56
Wang57
10
95 %
10
93 %
Huang58
Zhang59
This work
Minocycline hydrochloride was used to simulate the
degradation of antibiotic wastewater. As can be seen
from Fig. 9a, under the same conditions, after 30 min of
10
96 %
20
30
85 %
Fig. 8. (a) Photodegradation performance of as-prepared photocatalyst in removing RhB and (b) the kinetic behavior of RhB
photodegradation over pure BiVO4, GCN, GCN/BiVO4 and BCG-40 composite.
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Fig. 9. (a) Photodegradation performance of as-prepared photocatalyst in removing Mino HCl and (b) the kinetic behavior of Mino-HCl
photodegradation over pure GCN, BiVO4, GCN, GCDs, GCN/BiVO4 and BCG-40 composite.
Fig. 10. Effect of different concentrations of C-Dots on the degradation of RhB.
Fig. 11. (a-b) The degradation of RhB by BCG photocatalyst and (c) XRD patterns of BiVO4 before and after calcination.
concentration of C-Dots was gradually increased
3.3 Effect of the doping amount of C-Dots
without changing the amount of BiVO4 and GCN. From
As we know, C-dots as the electron transfer center
the Fig. 10, as the concentration of C-Dots increased
of Z-scheme heterojunction photocatalyst, can play a
from 0.2 mg/mL to 20 mg/mL, the photocatalytic
vital role in broadening the optical absorption region
degradation efficiency of the photocatalyst first
and decreasing electron-hole recombination events. In
increases and then decreases and reaches a peak at a
order to investigate the effect of the C-Dots doping
concentration of 1 mg/mL. This indicates that an
amount on the photocatalytic activity of BCG, the
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excessively large concentration of C-Dots will inhibit
more closely, which creates better conditions for the
4
the photocatalytic performance. This phenomenon may
be attributed to GCN itself can absorb visible light, and
the high content of C-Dots forms a competitive
relationship with GCN in the absorption of visible light,
thereby reducing the efficiency of visible
light degradation of RhB60.
electron transfer of the entire system, thereby promoting
the system redox process. In addition, BiVO4 has been
proved to exist in three crystal forms, and the three
crystal forms can be converted to each other at different
temperatures61. When the temperature is maintained at
450 ℃, BiVO4 is mainly composed of a monoclinic
crystal form, and the ionic structure around this crystal
form has a high degree of distortion and a very poor
symmetry, which can greatly increase the degree of
polarization and is conductive to electron-holes
separation. As shown in Fig. 11c, the XRD pattern of
BiVO4 before and after calcination was determined.
BiVO4 prepared by hydrothermal method shows an
amorphous structure, while the BiVO4 after calcination
has higher degree of monoclinic phase, narrower half
peak width and greater peak intensity, and there is no
other crystal phase diffraction peak. Therefore, this
temperature-induced change in crystal form may also be
one of the reasons for better photocatalytic activity after
calcination.
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3.4 Effect of calcination on the synthesis of
materials
As shown in Fig. 11a-b, the BCG-40 composite was
calcined at 450 ℃ and used it for photocatalytic
degradation of target pollutants (10 mg/mL RhB) under
the same conditions. Obviously, the degradation rate of
the photocatalyst after calcination was observed as high
as 94.4%, the degradation rate of the catalyst without
calcination is only 75.8%. It shows that the
photocatalytic material after calcination shows more
excellent photocatalytic performance. The reason for
this phenomenon may be that the specific surface area
of the composite material is increased by heat treatment,
and the phases of the composite material are formed
Fig. 12. (a) The recyclability tests of BCG composites under light irradiation before and after 3 recycles., and (b)FTIR, (c) XRD patterns
of BCG composites before and after degradation.
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conducted a free radical trapping experiment. Fig.13a
3.5 Photocatalytic recyclability and stability
Photocatalyst recycling and stability are important
steps in evaluating environmentally friendly
photocatalytic systems. Therefore, after centrifuging,
washing and drying, the used photocatalyst was
repeatedly tested under the same conditions to evaluate
the stability of the BCG-40. As shown in Fig.12a, after
3 cycles of recycling, the degradation performance of
BCG-40 photocatalyst is still stable, which indicated
that the composite has a good chemical stability. The
XRD and FTIR patterns of the BCG-40 before and after
degradation have shown in Fig. 12b and Fig. 12c,
respectively. No detectable changes were observed in
the FTIR and XRD of the recovered and after adsorbed
samples, indicating that the BCG composite has a small
adsorption effect during the degradation, and has good
photocatalytic activity and photostability.
illustrated the degradation ratio of the contaminant after
adding different scavengers, and EDTA-2Na,
isopropanol (IPA), 4-benzoquinone (BQ) were used as
scavengers of hV+B, ·OH and ·O-2, respectively. When a
certain amount of hV+B scavenger (EDTA-2Na) is added,
the removal rate of pollutants is only slightly changed
compared to the system without scavenger, indicating
that hV+B radical does not dominate the photocatalytic
process. In the case of ·O -2 scavenger (BQ), the
photocatalytic degradation performance was greatly
inhibited, and the removal rate decreased from 89 % to
31 %, this result indicates that the ·O-2 radical played a
vital role in the photodegradation of contaminants.
However, the photocatalytic performance also
decreased after the ·OH scavenger (IPA) was added,
indicating that ·OH free radicals also participate in
photocatalytic activities. Therefore, it can be judged that
both ·O-2 free radicals and ·OH free radicals are the
main active substances in photocatalytic degradation.
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3.6 Mechanism route analysis
In order to determine the active substances that
appeared in the photodegradation of contaminants, we
Fig. 13. (a) Trapping experiments of active species during the photocatalytic degradation. (b) DMPO spin-trapping ESR spectra for BCG-40
-
(b) in methanol (for DMPO- ·O 2 ) and (c) in aqueous dispersion (for DMPO- ·OH) under visible light irradiation.
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Fig. 14. The possible photocatalytic mechanism for the contamination.
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To prove the Z-scheme mechanism and further
confirm the main active species produced during this
photodegradation system, ESR spectra were measured.
As displayed in Fig. 13b and c, the typical peaks of
eV, and X of GCN and BiVO4 are 4.64 eV and 6.04 eV,
respectively. Therefore, it can be calculated that the ECB
and EVB of the GCN photocatalyst are −1.285 and 1.565
eV, while ECB and EVB of the BiVO4 are 0.34 and 2.74
-
DMPO- ·O 2 and DMPO- ·OH cannot be detected under
eV, respectively 35, 62, 63
.
dark conditions, while the obvious characteristic peaks
Combined with the free radical trapping
experiments, Fig. 14 illustrate the possible mechanism
of BCG-40 to photodegrade pollutants. Under the LED
light irradiation, the generated photoelectrons and holes
are allocated to their CB and VB, respectively.
Moreover, C-Dots becomes the charge transmission
bridge between GCN and BiVO4. Therefore, the
photoexcited electrons on the conduction band of
BiVO4 and the holes generated on the valence band of
GCN are transferred to C-Dots, resulting in the electrons
mainly concentrated on the conduction band of GCN,
and the holes are mainly accumulated on the valence
band of BiVO4. This phenomenon improves the
separation efficiency of photo-electrons and holes and
inhibits its recombination, and showing the good
photocatalytic performance of BCG. Moreover, the
GCN has a more negative CB reduction potential than
Eo (O2/·O -2 ), and BiVO4 has a more positive VB
potential than Eo (·OH /H2O), then the photogenerated
electrons generated by CB of GCN can be The O2
reaction produces ·O− , and holes can participate in the
formation of ·OH. Therefore, in this Z-scheme
-
of DMPO- ·O 2 and DMPO- ·OH species appeared
under visible light irradiation. It is noteworthy that the
-
signal intensity of DMPO- ·O 2 increases more than that
-
of DMPO- ·OH from 5 to 10 min. Therefore, ·O2 plays
a more important role in the photodegradation process,
which corresponds to that recorded in free radical
trapping experiments.
As we all know, the band gap structure of the
semiconductor affects the photocatalytic performance,
which is related to the conduction band and valence
band of the photocatalyst. The conduction band and
valence band of the photocatalyst can be calculated by
the following equations:
ECB=X− Ec −Eg/2 ,
EVB=ECB + Eg ,
(2)
(3)
where X, ECB, EVB and Eg represent absolute
electronegativity, conduction band potential and
valence band potential and forbidden band width of the
semiconductor, while Ec is the energy of the free
electron on the hydrogen standard (about 4.5 eV). In
general, the Eg of GCN and BiVO4 are 2.85 eV and 2.40
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heterojunction
photocatalyst
degradation
Acknowledgment
+
process, ·O − , ·OH and h VB co-participate in the

6
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9
mineralization of RhB and minocycline hydrochloride,
This research did not receive any specific grant from
funding agencies in the public, commercial, or not-for-
profit sectors. Special thanks to eceshi (www.eceshi.cn)
for the help of XPS and TEM analysis.
and finally achieve photocatalytic degradation.
BCG + hv → BCG + e- + hV+B
(4)
(5)
(6)
(7)
(8)
e- + O2 → ·O−
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·O− + H2O → ·OH + ·HO2

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hV+B + OH- → ·OH
·O− /·OH/ hV+B + Pollutant → CO2 + H2O / product (9)
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This research was highlighted as follows:
 BiVO4/C-Dots/g-C3N4 Z-scheme photocatalyst with C-Dots as
effective electrons transfer center were designed.
 Minocycline hydrochloride was effectively degraded by BCG-40
composites under LED irradiation
 C-Dots broaden visible light harvesting and inhibit the recombination
of photoelectrons and holes.
 The optimization of photocatalytic performance was investigated by
changing synthesis conditions and ratio